Biology

BIOLOGY Seventh Edition

Neil A. Campbell University of California, Riverside

Jane B. Reece Berkeley, California

C O N T R I B U T O R S AND ADVISORS

Lisa Urty Manuel Molles Carl Zimmer Christopher Wills

Mills College, Oakland, California University of New Mexico, Albuquerque Science writer, Guilford, Connecticut University of California, San Diego

P e t e r M i n o r s k y Journal of Plant Physiology and Mercy College, Dobbs Ferry, New York Mary Jane Niles

University of San Francisco, California

Antony Stretton

University of Wisconsin-Madison

Benjamin Cumminas San Francisco Boston New York Cape Town Hong Kong London Madrid Mexico City Montreal Munich Paris Singapore Sydney Tokyo Toronto

Editor-in-Chief: Beth Wilbur Senior Supervising Editor: Deborah Gale Supervising Editors: Pat Burner and Beth N, Winkkoff Managing Editor: Erin Gregg Art Director: Russell Chun Photo Editor: Travis Amos Marketing Managers: Josh Frost and Jeff Hester Developmental Editors: John Burner, Alice E Fugate, Sarah C. G. Jensen, Matt lee, Suzanne Olivier, Ruth Steyn, and Susan Wtisberg Developmental Artists: Hilair Chism, Blakelcv Kim, Kenneth Probst, Carlo. Simmons, and Laura Southworth Biology Media Producer: Christopher Delgado Media Project Manager: Bnenn Buchanan Project Editor: Amy C. Austin Photo Coordinator: Donna Kaltd Permissions Editors: Sue Hiving and Marcy I uncUa Publishing Assistants: Trinh Bui and Julia Khait Illustrations: Precision Graphics, Russell Chun, Phil Guzy, and Steve McEntee

PIE ISBN 0-321-27045-2 Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cumrnings, 1301 Sansome St., San Francisco, CA 94111. Alt rights reserved. Manufactured in the I'nued States ol America. This publication is protected by copyright and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. To obtain permission(s) to use material from this work, please submit a written request to Pearson Education, Inc., Permissions Department, 1900 E. Lake Ave., Glenview, IL 60025. For information regarding permissions, call (847) 486-2635. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and the publisher was aware of a trademark claim, the designations have been printed in initial caps or all caps. If you purchased this book within the United States or Canada you should be aware that it has been wrongfully imported without (he approval of the Publisher or the Author. 12 3 4 5 6 7 8 9 10—VHC—09 08 07 06 05

Text and Cover Designer: Mark Ong Photo Researchers: Brian Donnelly, Donna Kalal, Ira Klnnbcrg, Robin Samper, and Maureen Spuhltr Copy Editor: Janet Greenblatt Production Management, Art Coordination, and Design Support: Morgan E. Floyd, Robert R. Hansen, Sherrill Redd, S. Brendan Short, and Kirsten Sims at GTS Companies

Compositor and Prepress: GTS Companies Manufacturing Manager: Pam Augspurger Cover Printer: Phoenix Color Printer: Von Hoffmann Press, Inc. On the cover: Photograph of bird's nest-em. ^spic'iium nidus. Linda Broadfoot. Special (hanks to Dennis High, Center for Photographic Art, for his advice and assistance with cover research. Credits continue following the appendices.

Benjamin Cumminas San Francisco Boston New York Cape Town Hong Kong London Madrid Mexico City Montreal Munich Paris Singapore Sydney Tokyo Toronto

bout the Authors

Neil A. Campbell combined the investigative nature of a research scientist with the soul of an experienced and caring teacher. He earned his M.A. in Zoology from UCLA and his Ph.D. in Plant Biolog)' from the University of California, Riverside, where he received the Distinguished Alumnus Award in 2001. Dr. Campbell published numerous research articles on how certain desert and coastal plants thrive in salty soil and how the sensitive plant (Mimosa) and other legumes move their leaves. His 30 years of teaching in diverse environments included general biology courses at Cornell University, Pomona College, and San Bernardino Valley College, where he received the college's first Outstanding Professor Award in 1986. Most recently Dr. Campbell was a visiLing scholar in the Department of Botany and Plant Sciences at the University of California, Riverside. In addition to his authorship of this book, he coauthored Biology: Concepts and Connections and Essential Biology with Jane Reece. Each year, over 600,000 students worldwide use Campbell/Reece biology textbooks.

j a n e B. Reece has worked in biology publishing since 1978, when she joined the editorial staff of Benjamin Cummings. Her education includes an A.B. in Biology from Harvard University, an M.S. in Microbiology from Rutgers University and a Ph.D. in Bacteriology from the University of California, Berkeley. At UC Berkeley, and later as a postdoctoral fellow in genetics at Stanford University her research focused on genetic recombination in bacteria. She taught biology at Middlesex County College (New Jersey) and Queensborough Community College (New York). As an editor at Benjamin Cummings, Dr. Reece played major roles in a number of successful textbooks. In addition to being a coauthor with Neil Campbell on BIOLOGY, Biolog}'; Concepts and Connections, and Essential Biology, she coauthored The World oj the Cell Third Edition, with W M. Becker and M. E Poenie.

To Rochelle and Allison, with love

To Paul and Daniel, with love

N.A.C.

—J.B.R.

Preface

harles Darwin described evolution as a process of "descent with modification." It is a phrase that also fits the continuing evolution of BIOLOGY. This Seventh Edition is our most ambitious revision of the book since its origin—a new textbook "species" with several evolutionary adaptations shaped by the changing environment of biology courses and by the astonishing progress of biological research. But these adaptive modifications are still true to the two complementary teaching values at the core of every edition of BIOLOGY. First, we are dedicated to crafting each chapter from a framework of key concepts that will help students keep the details in placeSecond, we are committed to engaging students in scientific inquiry through a combination of diverse examples of biologists' research and opportunities for students to practice inquiry themselves. These dual emphases on concept building and scientific inquiry emerged from our decades of classroom experience. It is obviously gratifying that our approach has had such broad appeal to the thousands of instructors and millions of students who have made BIOLOGY the most widely used college science textbook. But with this privilege of sharing biology with so many students comes the responsibility to continue improving the book to serve the biology community even better. As we planned this new edition, we visited dozens of campuses to hear what students and their instructors had to say about their biology courses and textbooks. What we learned from those conversations about new directions in biology courses and the changing needs of students informed the many improvements you'll find in this Seventh Edition of BIOLOGY.

C

We have restructured each chapter to bring its key concepts into even sharper focus The discovery explosion that makes modern biology so exciting also threatens to suffocate students under an avalanche of information. The past few editions of BIOLOGY set the details in a context of key concepts, typically ten to twenty per chapter. In this new edition, we have taken the next evolutionary step of restructuring each chapter to help students focus on fewer, even bigger ideas—typically just five or six key concepts per chapter. A new Overview section at the beginning of each chapter sets an even broader context for the key concepts that follow. And at the end of each of the concept sections, a Concept Check with two or three questions enables students to assess whether they understand that concept before going on to the next. Answers to the Concept Check questions are located in Appendix A, as are the answers to the Self-Quizzes from the Chapter Review at the end of each chapter. In our ongoing interactions with students and instructors, they have responded enthusiastically to our new organization and pedagogy. Compared to other textbooks, including earlier editions of our own, students have found the new chapter structure and design of BIOLOGY, Seventh Edition, to be more inviting, more accessible, and much more efficient to use. But in achieving these goals, we have not compromised the depth and scientific accuracy the biology community has come to expect from us.

Key Concepts keep the supporting details in context.

54.1 Ecosystem g and chemical cycling 54.2 Physical and chemical factors limit primary

!•"' 'I". L;:.'!. i "i

•-" _-i" il ' "-"i'-i '•' . •" ti" 1 .' ![ ^i ! '••• ULt [llEISI 11 •'.c

usually 54.4 Biologna! and geochemical process parts of the ecosystem 54.5 Thee human human population population is is disr disrupting 'miurtl cycles ;iiroii«hom the biosphc

L'l

' '

The Overview sets sun. Energy tinv-i -.i'-i-.t.^K cccsviteras, while matier cycles -he stage for the rest of the chapter.

Figure references in color help

Resources critical to human iur\'iva] anc vielfce, ranging , ' 'j , '!?

Ecosystems, Energy, and Matter

A

..

•!!

•

. , 1

I

!

I

I

.I

I

• , • !

•

• ! •

•• •

•' ' ;)

How

=.:id Lheinital ._>:. I I - J ,.i t-coiysti'ivji

Each numbered < OBeepl H e a d

students move easily

announces the

between text and

:oln»ists regard the entire biosphere as a ;

figures.

Ecosystem ecology emphasizes

beginning of a new

energy flow and chemical cychng

concept section.

Concept Check Questions at the end of each concept section encourage students to assess their mastery of the concept.

Physical and chemical factors Umit primary production in ecosystems The a m o u n t of ligh; enetgv c o n v e n e d to criKmital e n e r g y ( e r -

.•"-"•-

s p r i m a r y pvodnGtion

.:•:":"•

- . •' ^

•

"i"^di_"_i i: .ri." :ia." ii.o .10 "i " svmi"c-ize .••"•:r^n n . i i i.r^ar •. r.ici^^i ies ".•..•.i^.1 e.1"

a forest, tor example "rv.rcis lave been feeding on leaf H Figure 1.10 A systems map of interactions between proteins in a cell. This diagram maps about 3,500 proteins (dots) and their network of interactions (lines connecting the proteins) in a fruit fiy cell. Systems biologists develop such models from huge databases of information about molecules and their interactions in the cell. A major goal of this systems approach is to use such models to predict how one change, such as an increase in the activity of a particular protein, can ripple through the cell's molecular circuitry to cause other changes. One of the applications will be a more accurate prediction of the side effects of various drugs.

10

CHAPTER i

Exploring Life

(reductionism). Then it is necessary to investigate how each part behaves in relation to others in the working system—all the protein-protein interactions, in the case of our fly cell example. Finally, with the help of computers and innovative software, it is possible to pool all the data from many research teams into the kind ol system network modeled in Figure 1.10. Though the basic idea of systems biology is simple, the practice is not, as you would expect from the complexity of biological systems. It has taken three key research developments to bring systems biology within reach: >* High-throughput technology. Systems biology depends on methods that can analyze biological materials very rapidly and produce enormous volumes of data. Such mega-data-collection methods are said to be "highthroughput." The automatic DNA-sequencing machines that made the Human Genome Project possible are examples of high-throughput devices (see Figure ] .9). I- Bioinformatics. The huge databases that result from highthroughput methods would be chaotic without the computing power, software, and mathematical models to process and integrate all this biological information. The new field of bioinformatics is extracting useful biological information from the enormous, ever-expanding data sets, such as DNA sequences and lists of protein interactions. The Internet is nurturing systems biology through dissemination of the digital data that feed bioinformatics. I* Interdisciplinary research teams. In 2003, Harvard Medical School formed a department of systems biology, its first new department in two decades. Nearby MIT is busy organizing over 80 faculty members from many departments Lnto a new program for computational and systems biology These and other systems biology start-ups are melting pots of diverse specialists, including engineers, medical scientists, chemists, physicists, mathematicians, computer scientists, and, of course, biologists from a variety of fields.

muscle cells require more energy during exercise, they increase their consumption of the sugar molecules that provide fuel. In contrast, when you rest, a different set of chemical reactions converts surplus sugar to substances that store the fuel. Like most of the cell's chemical processes, those that decompose or store sugar are accelerated, or catalyzed, by the specialized proteins called enzymes. Each type of enzyme catalyzes a specific chemical reaction. In many cases, these reactions are linked into chemical pathways, each step with its own enzyme. How does the cell coordinate its various chemical pathways? In our specific example of sugar management, how does the cell match fuel supply to demand by regulating its opposing pathways of sugar consumption and storage? The key is the ability of many biological processes to self-regulate by a mechanism called feedback. In leedback regulation, the output, or product, of a process regulates that very process. In life, the most common form of regulation is negative feedback, in which accumulation of an end product of a process slows that process (Figure 1.11). For example, the cell's breakdown of sugar generates chemical energy in the form of a substance called ATP. An excess accumulation of ATP "feeds back" and inhibits an enzyme near the beginning of the pathway. Though less common than negative feedback, there are also many biological processes regulated by positive feedback, in which an end product speech up its production. The clotting of your blood in response to injury is an example. "When a blood vessel is damaged, structures in the blood called platelets begin to aggregate at the site. Positive feedback occurs as chemicals released by the platelets attract more platelets. The platelet pile then initiates a complex process that seals the

A number of prominent scientists are promoting systems biology with missionary zeal, but so far, the excitement exceeds the achievements. However, as systems biology gathers momentum, ii is certain to have a growing impact on the questions biologists ask and the research they design. After all, scientists aspired to reach beyond reductionism to grasp how whole biological systems work long before new technology made modern systems biology possible. In fact, decades ago, biologists had already identified some of the key mechanisms that regulate the behavior of complex systems such as cells, organisms, and ecosystems.

Feedback Regulation in Biological Systems A kind of supply-and-demand economy applies to some of the dynamics of biological systems. For example, when your

A Figure 1.11 Negative feedback. This three-step chemical pathway converts substance A to substance D. A specific enzyme catalyzes each chemical reaction. Accumulation of the final product (D) inhibits the first enzyme in the sequence, thus slowing down production of more D.

CHAPTER 1 Exploring Life

n

A Figure 1.12 Positive feedback. In positive feedback, a product stimulates an enzyme in the reaction sequence, increasing the rate of production of the product. Positive feedback is less common than negative feedback in living systems.

wound with a clot. Figure 1.12 shows a simple model of positive feedback. Feedback is a regulatory motif common to life at all levels, from the molecular level to the biosphere. Such regulation is an example of the Integration that makes living systems much greater than the sum ol their parts.

Concept Check 1. Apply the principle of emergent properties to explain the relationship of a sentence to the alphabet of letters from which that sentence is constructed. 2. How does high-throughput technology complement bio informatics? 3. When you flush a toilet, water begins to fill the tank and lift a float attached to a lever. When the water level reaches a certain height, the lever shuts the water valve and prevents the tank from overflowing. What type of regulatory mechanism is at work in this nonliving system? For suggested answers, see Appendix A.

Concept

Biologists explore life across its g^eat diversity of species We can think of biology's enormous scope as having two dimensions. The "vertical" dimension, which we examined in this chapter's first two concepts, is the size scale that reaches all the way from molecules to the biosphere. But biology's 12

CHAPTER 1

Exploring Life

A Figure 1.13 Drawers of diversity. This is just a small sample of the tens of thousands of species in the moth and butterfly collection at the National Museum of Natural History in Washington, D.C.

scope also has a "horizontal" dimension stretching across the great diversity of species, now and over life's long history. Diversity is a hallmark of life. Biologists have so far identified and named about 1.8 million species. This enormous diversity of life includes approximately 5,200 known species of prokaryotes, 100,000 fungi, 290,000 plants, 52,000 vertebrates (animals with backbones), and 1,000,000 insects (more than half of all known forms of life). Researchers identify thousands of additional species each year. Estimates of the total species count range from about 10 million to over 200 million. Whatever the actual number, the vast variety of life makes biology's scope very wide (Figure 1.13).

Grouping Species: The Basic Idea There seems to be a human tendency to group diverse items according to similarities. For instance, perhaps you organize your music collection according to artist. And then maybe you group the various artists into broader categories, such as dance music, party music, exercise music, and study-time music. In the same way, grouping species that are similar is natural for us. We may speak of squirrels and butterflies, though we recognize that many different species belong to each group. We may even sort groups into broader categories, such as rodents (which include squirrels) and insects (which include butterflies). Taxonomy, the branch of biology7 that names and classifies species, formalizes this ordering of species into a series of groups of increasing breadth (Figure 1.14). You will leam more about this taxonomic scheme in Chapter 25. For now, we will focus on kingdoms and domains, the broadest units of classification.

Species

Genus

Class

Family

"Ft!

Phylum

Kingdom

Domain

JUPv

•K A A A ---

• "wm-:'-'>

Animalia

• Figure 1.14 Classifying l i f e . The taxonomic scheme classifies species into groups that are then combined into even broader groups. Species that are very closely related, such as polar bears and brown bears, are placed in the same genus, genera (plural) are grouped into families, and so on. This example classifies the species Ursus americanus, the American black bear.

The Three Domains of Life Until the last decade, most biologists adopted a taxonomic scheme that divided the diversity of life into five kingdoms, including the plant and animal kingdoms. But new methods, such as comparing the DNA sequences of diverse species, have led to an ongoing reevaluation of the number and boundaries of kingdoms. Different researchers have proposed anywhere from six kingdoms to dozens of kingdoms. But as debate continues at the kingdom level, there is more of a consensus that

Eukary

the kingdoms of life can now be grouped into three even higher levels of classification called domains. The three domains are named Bacteria, Archaea, and Eukarya. The first two domains, domain Bacteria and domain Archaea, both consist of prokaryoi.es (organisms with prokaryotic cells). Most prokaryotes are unicellular and microscopic. In the five-kingdom system, bacteria and archaea were combined in a single kingdom, called kingdom Monera, because they shared the prokaryotic form of cell structure. But evidence now supports the view that bacteria and archaea represent two very CHAPTER 1 Exploring life

Figure 1.15

g Life's Three Domains DOMAIN BACTERIA

Bacteria are the most diverse and ' 4 ^ widespread prokaryotes and are now divided among multiple kingdoms. Each of the rod-shaped structures in this photo is a bacterial cell.

DOMAIN EUKARYA

Protists (multiple kingdoms) e r e 100p.m unicellular eukaryotes and their relatively simpfe multiceliufar relatives. Pictured here is an assortment of protists inhabiting pond water. Scientists are currency debating how to-split the protists into several kingdoms that better represent evolution and diversity.

K i n g d o m Plantae consists of multiceilular eukaryotes that carry out photosynthesis, the conversion of fight energy to food.

K i n d o m Fungi is defined in part by the nutritional mode of its members, such as thi* mushroom, which absorb nutrients after decomposing organic material.

Kirrdom A n i m a l i a consists of multicelluiar eukaryotes that ingest other organisms.

DOMAIN ARCHAEA

Many of the prokaryotes known 0.5 j as archaea live in Earth's extreme environments, such as salty iakes and boiling hot springs. Domain Archaea includes multiple kingdoms. The photo shows a colony composed of many cells.

distinct branches of prokaryotic life, different in key ways that you'll learn about in Chapter 27. There is also molecular evidence that archaea are at least as closely related to eukaryotic organisms as they are to bacteria. All the eukaryotes (organisms with eukaryotic cells) are now grouped into the various kingdoms of domain Eukarya (Figure 1.15). In the era of the five-kingdom scheme, most ol the single-celled eukaryoi.es, including the microorganisms known as protozoans, were placed in a single kingdom, the kingdom Protista. Many biologists extended the boundaries of the kingdom Protista to include some multicellular forms, such as seaweeds, that are closely related to certain unicellular protists. The recent taxonomic trend has been to split the protists into several kingdoms. In addition to these protistan kingdoms, the domain Eukaiya includes three kingdoms of multicellular eukaryotes: the kingdoms Plantae, Fungi, and Animalia. These three kingdoms are distinguished partly by their modes of nutrition. Plants produce their own sugars and other foods by !4

Exploring Life

photosynthesis. Fungi are mostly decomposers that absorb nutrients by breaking down dead organisms and organic wastes, such as leaf litter and animal feces. Animals obtain food by Digestion, which is the eating and digesting of other organisms. It is, of course, the kingdom to which we belong.

Unity in the Diversity of Life As diverse as life is, there is also evidence of remarkable unity, especially at the molecular and cellular levels. An example is the universal genetic language of DNA, which is common to organisms as different as bacteria and animals. And among eukaryotes, unity is evident in many features of cell structure (Figure 1.16)

How can we account lor life's dual nature of unity and diversity? The process of evolution, introduced in the next, concept, illuminates both the similarities and differences among Earths life.

1 5 |i171

Cilia of Paramecium, The cilia of Paramecium propel the cell through pond water.

Cross section of cilium, as viewed with an electron microscope • Figure 1.17 Digging into the past. Paleontologist Paul Sereno gingerly excavates the leg bones of a dinosaur fossil in Niger, Africa.

Concept

Evolution accounts for life's unity and diversity Cilia of windpipe cells. The cells that line the human windpipe are equipped with cilia that help keep the lungs clean by moving a film of debris-trapping mucus upward. A Figure 1.16 An example of unity underlying the diversity of life: the architecture of cilia in eukaryotes. Cilia (singular, cilium) are extensions of cells that function in locomotion. They occur in eukaryotes as diverse as the single-celled Paramecium and humans. But even organisms so different share a common architecture for their cilia, which have an elaborate system of tubules that is revealed in cross-sectional views.

Concept Check 1. How is a mailing address analogous to biology's hierarchical taxonomic system? 2. What is the key difference that distinguishes organisms of domain Eukarya from the other two domains? For suggested answers, see Appendix A.

The history of life, as documented by fossils and other evidence, is a saga of a changing Earth billions of years old, inhabited by an evolving cast of living forms (Figure 1.17). This evolutionary view of life came into sharp focus in November 1859, when Charles Robert Darwin published one of the most important and controversial books ever written. Entitled On the Origin of Species by Natural Selection, Darwin's

book was an immediate bestseller and soon made "Darwinism" almost synonymous with the concept of evolution (Figure 1.18). The Origin of Species articu-

lated two mam points. First, Darwin presented evidence to support his view that contemporary species arose from

A Figure 1.18 Charles Darwin in 1859, the year he published The Origin of

Species. Exploring Life

15

OBSERVATION: Individual variation. Individuals in a population of any species vary in many heritable traits. OBSERVATION: Overproduction and competition. A population of any species has the potential to produce far more offspring than will survive lo produce offspring of their own. With more individuals than the environment can support, competition is inevitable. INFERENCE: Unequal reproductive success. From the observable facts of heritable variation and overproduction of offspring, Darwin inferred that individuals are unequal in their likelihood of surviving and reproducing. Those individuals with heritable traits best suited to the local environment will generally produce a disproportionately large number of healthy, fertile offspring. INFERENCE: Evolutionary adaptation. This unequal reproductive success can adapt a population to its environment. Over the generations, heritable traits that enhance survival and reproductive success tend to increase in frequency among a populations individuals. The population evolves.

A. Figure 1.19 Unity and diversity in the orchid family. These three rain forest orchids are variations on a common floral theme. For example, each of these flowers has a liplike petal that helps attract pollinating insects and provides a landing platform for the pollinators.

Darwin called this mechanism of evolutionary adaptation "natural selection'7 because the natural environment ''selects" for the propagation of certain traits. Figure 1.20 summarizes Darwin's theory of natural selection. The example in Figure 1.21 illustrates the ability of natural selection to "'edit" a populations heritable variations. We see the products of natural selection in the exquisite adaptations of organisms to the special circumstances of their way of life and their environment (Figure 1.22).

a succession ol ancestors. (We will discuss the evidence for evolution in detail in Chapter 22.) Darwin called this evolutionary history ol species "descent with modification." It was an insightful phrase, as it captured the duality of life's unity and diversity—unity in the kinship among species that descended from common ancestors; diversity in the modifications that evolved as species branched from their common ancestors (Figure 1.19). Darwin's second main point was to propose a mechanism for descent with modification. He called this evolutionary mechanism natural selection.

Natural Selection Darwin synthesized his theory of natural selection from observations that by themselves were neither new nor profound. Others had the pieces of the puzzle, but Darwin saw how they fit together. He inferred natural selection by connecting two readily observable features of life: 16

CHAPTER 1

Exploring Life

Differences in reproductive success

Evolution of adaptations • in the population

A Figure 1.20 Summary of natural selection.

0 Population with varied inherited traits

A Figure 1.22 Form fits function. Bats, the only mammals capable of active flight, have wings with webbing between extended "fingers." In the Darwinian view of life, such adaptations are refined by natural selection.

The Tree of Life

Q Elimination of individuals with certain traits.

0 Increasing frequency of traits that enhance survival and reproductive success. A Figure 1.21 Natural selection. This imaginary beetle population has colonized a locale where the soil has been blackened by a recent brush fire. Initially, the population varies extensively in the inherited coloration of the individuals, from very light gray to charcoal. For hungry birds that prey on the beetles, it is easiest to spot the beetles that are lightest in color.

Take another look at the skeletal architecture of the bats wings in Figure 1.22. These forelimbs, though adapted for flight, actually have all the same bones, joinLs, nerves, and blood vessels found in other limbs as diverse as the human arm, the horse's foreleg, and the whale's flipper. Indeed, all mammalian forelimbs are anatomical variations of a common architecture, much as the flowers in Figure 1.19 are variations on an underlying "orchid" theme. Such examples of kinship connect life's "unity in diversity" to the Darwinian concept of "descent with modification." In this view, the unity of mammalian limb anatomy reflects inheritance of that structure from a common ancestor—the "prototype" mammal from which all other mammals descended, their diverse forelimbs modified by natural selection operating over millions of generations in different environmental contexts. Fossils and other evidence corroborate anatomical unity in supporting this view of mammalian descent from a common ancestor. Thus, Darwin proposed that natural selection, by its cumulative effects over vast spans of time, could enable an ancestral species to "split" into two or more descendant species. This would occur, for example, if one population fragmented into several subpopulations isolated In different environments. In these various arenas of natural selection, one species could gradually radiate into many species as the geographically isolated populations adapted over many generations to different sets of environmental factors. The "family tree" of 14 finches in Figure 1.23, on the next page, illustrates a famous example of adaptive radiation of new species from a common ancestor. Darwin collected specimens of these birds during his 1835 visit to the remote Galapagos Islands, 900 kilometers (km) off the Pacific coast of South America. These relatively young, volcanic islands are home to many species of plants and animals found nowhere else in the world, though Galapagos organisms are clearly related to species on the South American mainland. After volcanism built the Galapagos several million years ago, finches probably diversified on the various islands from an ancestral finch CHAPTER i Exploring Life

17

Large ground finch

Small

tree finch

Large cactus ground finch

Sharp-beaked ground finch jtfmfomr,

Geospsra fuliginosa Geospiza comrostns

*Smi}L~*"

Camarhynchus psittacuia

Green warbk finch

M e d j u m ground

finch

Geospiza _ • difficilis C a c t u s . . . . ground finch Geospiza forth Geospiza scandens

Mangrove "finch

Cactospiza pallida.

Gerthidea olivacea

Camarhynchus pauper Sma{[ tree finch

Camarhynchus parvulus

Ca'ctospiza heliobates

Platyspiza crassirostris

round finches

"imon ancestor from i American mainland

A Figure 1.23 Descent with modification: adaptive radiation of finches on the Galapagos Islands. Note the specialization of beaks, which are adapted to various food sou on the different islands.

species that by chance reached the archipelago from the mainland. Years after Darwin's collection of Galapagos finches, researchers began to sort out the relationships among the finch species, first from anatomical and geographic data and more recently with the help ol DNA sequence comparisons. Biologists' diagrams of evolutionary relationships generally take treelike forms, and for good reason. Just as an individual has a genealogy that can be diagrammed as a family tree, each 18

CHAPTER 1

Exploring Life

species is one twig of a branching tree of life extending back in time through ancestral species more and more remote. Species that are very similar, such as the Galapagos hnches, share a common ancestor at a relatively recent branch point on the tree of life. But through an ancestor that lived much farther back in time, finches are related to sparrows, hawks, penguins, and all other birds. And birds, mammals, and all other vertebrates (animals with backbones) share a common ancestor even more

ancient. We find evidence of still broader relationships in such similarities as the matching machinery of all eukaryotic cilia (see Figure 1.16). Trace life back far enough, and there are only fossils of the primeval prokaryotes that inhabited Earth over 3.5 billion years ago. We can recognize their vestiges in our own cells-—in the universal genetic code, for example. All of life is connected through its long evolutionary history.

Concept Check 1. Explain why "editing" is better than "creating" as a metaphor for how natural selection acts on a population's heritable variation. 2. The three domains you learned about in Concept 1.3 can be represented in the tree of life as the three main branches. On the eukaryotic branch, three of the subbranches are the kingdoms Plantae, Fungi, and Animalia. Evidence supports the view that fungi and animals are more closely related to each other than either of these kingdoms is to plants. Draw a simple branching pattern that symbolizes the relationship between these three eukaryotic kingdoms. For suggested answers, see Appendix A.

Biology blends two main processes of scientific inquiry: discovery science and hypothesis-based science. Discovery science is mostly about describing nature. Hypothesis-based science is mostly about explaining nature. Most scientific inquiries combine these two research approaches.

Discovery Science Sometimes called descriptive science, discovery science describes natural structures and processes as accurately as possible through careful observation and analysis of data. For example, discovery science gradually built our understanding of cell structure, and it is discovery science that is expanding our databases of genomes of diverse species. Types of Data Observation is the use of the senses to gather information, either directly or indirectly with the help of tools such as microscopes that extend our senses. Recorded observations are called data. Put another way, data are items of information on which scientific inquiry is based. The term daia implies numbers to many people. But some data are qualitative, often in the form of recorded descriptions rather than numerical measurements. For example, Jane Goodall spent decades recording her observations of chimpanzee behavior during field research in a Gambian jungle (Figure 1.24). She also documented her observations with

| Concept

Biologists use various forms of inquiry to explore life The word science is derived from a Latin verb meaning "to know." Science is a wray ol knowing. It developed out of our curiosity about ourselves, other life-forms, the world, and the universe. Striving to understand seems to be one of our basic urges. At the heart ol science is inquiry, a search for information and explanation, often focusing on specific questions. Inquiry drove Darwin to seek answers in nature for how species adapt to their environments. And inquiry is driving the analyses of genomes that are helping us understand biological unity and diversity at the molecular level. In fact, the inquisitive mind is the engine that drives all progress in biology. There is no formula for successful scientific inquiry, no single scientific method with a rule book that researchers must rigidly follow. As in all quests, science includes elements of challenge, adventure, and surprise, along with careful planning, reasoning, creativity, cooperation, competition, patience, and the persistence to overcome setbacks. Such diverse elements of inquiry make science far less structured than most people realize. That said, it is possible to distill certain characteristics that help to distinguish science from other ways of describing and explaining nature.

A Figure 1.24 Jane Goodall collecting qualitative data on chimpanzee behavior. Goodall recorded her observations in field notebooks, often with sketches of the animals' behavior. CHAPTER i Exploring Life

photographs and movies. Along with these qualitative data, Goodall also enriched the field of animal behavior with volumes of quantitative data, which are generally recorded as measurements. Skim through, any of trie scientific journals in your college library, and you'll see many examples of quantitative data organized into lables and graphs. Induction in Discovery Science Discovery science can lead to important conclusions based on a type of logic called induction, or inductive reasoning. Through induction, we derive generalizations based on a large number of specific observations. "The sun always rises in the east" is an example. And so is "All organisms are made of cells." That generalization, part of the so-called cell theory, was based on two centuries of biologists discovering cells in the diverse biological specimens they observed with microscopes. The careful observations and data analyses of discovery science, along with the inductive generalizations they sometimes produce, are fundamental to our understanding of nature.

Hypothesis-Based Science The observations and inductions of discovery science engage inquisitive minds to seek natural causes and explanations for those observations. What caused the diversification of finches on the Galapagos Islands? What causes the roots of a plant seedling to grow downward and the leaf-bearing shoot to grow upward? What explains the generalization that the sun always rises in the east? In science, such inquiry usually involves the proposing and testing of hypothetical explanations, or hypotheses. The Role of Hypotheses in Inquiry In science, a hypothesis is a tentative answer to a well-framed question—an explanation on trial. It is usually an educated postulate, based on past experience and the available data of discovery science. A scientific hypothesis makes predictions that can be tested by recording additional observations or by designing experiments. We all use hypotheses in solving everyday problems. Let's say, for example, that your flashlight fails during a camp-out. That's an observation. The question is obvious: Why doesn't the flashlight work? Two reasonable hypotheses based on past experience are that (1) the batteries in the flashlight are dead or (2) the bulb is burnt out. Each of these alternative hypotheses makes predictions you can test with experiments. For example, the dead-battery hypothesis predicts that replacing the batteries will fix the problem. Figure 1.25 diagrams this campground inquiry. Of course, we rarely dissect our thought processes this way when we are solving a problem using hypotheses, predictions, and experiments. But hypothesisbased science clearly has its origins in the human tendency to figure things out by tinkering. 20

Exploring Life

Prediction: Replacing batteries will fix problem

Test prediction

Test prediction

Test falsifies hypothesis

Test does not falsify hypothesis

A Figure 1.25 A campground example of hypothesisbased inquiry.

Deduction: The "If. . . then" Logic of Hypothesis-Based Science A type of logic called deduction is built into hypothesisbased science. Deduction contrasts with induction, which, remember, is reasoning from a set of specific observations to reach a general conclusion. In deductive reasoning, the logic flows in the opposite direction, from the general to the

specific. From general premises, we extrapolate to the specific results we should expect if the premises are true. If all organisms are made of cells (premise 1), and humans are organisms (premise 2), then humans are composed of cells (deductive prediction about a specific case). In hypothesis-based science, deduction usually takes the form of predictions about what outcomes of experiments or observations we should expect il a particular hypothesis (premise) is correct. We then test the hypothesis by performing the experiment to see whether or not the results are as predicted. This deductive testing takes the form of "1/ . . . then" logic. Tn the case of the flashlight example: If the dead-batiery hypothesis is correct, and you replace the batteries with new ones, then the flashlight should work. A Closer Look at Hypotheses in Scientific Inquiry The flashlight example illustrates two important qualities of scientific hypotheses. First, a hypothesis must be testable; there must be some way to check the validity of the idea. Second, a hypothesis musi. be ialsifiablc; there must be some observation or experiment that could reveal if such an idea is actually not true. The hypothesis that dead batteries are the sole cause of the broken flashlight could be falsified by replacing the old batteries with new ones- But try to devise a test to falsify the hypothesis that invisible campground ghosts are fooling with your flashlight. Does restoring flashlight function by replacing the bulb falsify the ghost hypothesis? Not if the playful ghosts are continuing their mischief. The flashlight inquiry illustrates another key point about hypothesis-based science. The ideal is to frame two or more alternative hypotheses and design experiments to falsify those candidate explanations. In addition to the two explanations tested in Figure 1.25, one of the many additional hypotheses is that both the batteries and the bulb are bad. What does this hypothesis predict about the outcome of the experiments m Figure 1.25? What additional experiment would you design to test this hypothesis of multiple malfunction? We can inine the flashlight scenario for still one more important lesson about hypothesis-based science. Although the burnt-out bulb hypothesis stands up as the most likely explanation, notice that the testing supports that hypothesis not by proving that it is correct, but by not eliminating it through falsification. Perhaps the bulb was simply loose and the new bulb was inserted correctly. We could attempt to falsify the burnt-out bulb hypothesis by trying another experiment— removing the bulb and carefully reinstalling it. But no amount of experimental testing can prove a hypothesis beyond a shadow of doubt, because it is impossible to exhaust the testing of all alternative hypotheses. A hypothesis gains credibility by surviving various attempts to falsify it while testing eliminates (falsifies) alternative hypotheses.

The Myth of the Scientific Method The steps in the flashlight example of Figure 1.25 trace an idealized process of inquiry called the scientific method. We can recognize the elements of this process in most of the research articles published by scientists, but rarely in such structured form. Very few scientific inquiries adhere rigidly to the sequence of steps prescribed by the "textbook" scientific method. For example, a scientist may start to design an experiment, but then backtrack upon realizing that more observations are necessary In other cases, puzzling observations simply don't prompt welldefined questions until other research projects place those ob-" servations in a new context. For example, Darwin collected specimens of the Galapagos finches, but it wasn't until years later, as the idea of natural selection began to gel, that biologists began asking key questions about the history of those birds. Moreover, scientists sometimes redirect their research when they realize they have been "barking up the wrong tree" by asking the wrong question. For example, in the early 20th century, much research on schizophrenia and manic-depressive disorder (now called bipolar disorder) got sidetracked by focusing too much on the question of how life experiences cause these serious maladies. Research on the causes and potential treatments became more productive when it was refocused on questions of how certain chemical imbalances in the brain contribute to mental illness. To be fair, we acknowledge that such twists and turns in scientific inquiry become more evident with the advantage of historical perspective. There is still another reason that good science need not conform exactly to any one method of inquiry: Discovery science has contributed much to our understanding of nature without most of the steps of the so-called scientific method. It is important for you to get some experience with the power of the scientific method—by using it for some of the laboratory inquiries in your biology course, for example. But it is also important to avoid stereotyping science as lock-step adherence to this method.

A Case Study in Scientific Inquiry: Investigating Mimicry in Snake Populations Now that we have highlighted the key features of discovery science and hypotiiesis-based science, you should be able to recognize these forms of inquiry in a case study of actual scientific research. The story begins with a set of observations and generalizations from discovery science. Many poisonous animals are brightly colored, often with distinctive patterns that stand out against the background. This is called warning coloration because it apparently signals "dangerous species" to potential predators. But there are also mimics. These imposters look like poisonous species, but are actually relatively harmless. An example is the flower fly, a nonstinging insect that mimics the appearance of a stinging honeybee (Figure 1.26 on the next page). CHAPTER 1

Exploring Life

21

J

The geographic distribution of the Carolina snakes made it possible to test the key prediction of the mimicry hypothesis. Mimicry should help protect king snakes from predators, but only in regions where coral snakes also live. The mimicry hypothesis predicts that predators in non-coral snake areas will attack king snakes more frequently than will predators that live where coral snakes are present.

(nonstinging)

Field Experiments with Artificial Snakes To test the mimicry hypothesis, Harcombe made hundreds of artificial snakes out of wire covered with a claylike substance called plasticine. He fashioned two versions of fake snakes: an experimental group with the red, black, and yellow ring pattern of king snakes; and a control group of plain brown artificial snakes as a basis of comparison. Scarlet king snake

Honeybee (stinging)

~j

Range of eastern coral snake

A Figure 1.26 A stinging honeybee and its nonstinging mimic, a flower fly.

What is the function of such mimicry? What advantage does it confer on the mimics? In 1862, British scientist Henry Bates proposed the reasonable hypothesis that mimics such as flower flies benefit when predators confuse them with the harmful species. In other words, the deception may be an evolutionary adaptation that evolved by reducing the mimics risk of being eaten. As intuitive as this hypothesis may be, it has been relatively difficult to test, especially with field experiments. But then, in 2001, biologists David and Karin Pfennig, along with William Harcombe, an undergraduate at the University of North Carolina, designed a simple but elegant set of field experiments to test Bates's mimicry hypothesis. The team investigated a case of mimicry among snakes that live in North and South Carolina. A poisonous snake called the eastern coral snake has warning coloration: bold, alternating rings of red, yellow, and black. Predators rarely attack these snakes. It is unlikely that predators learn this avoidance behavior, as a first strike by a coral snake is usually deadly. Natural selection may have increased the frequency of predators that have inherited an instinctive recognition and avoidance of the warning coloration of the coral snake. A nonpoisonous snake named the scarlet king snake mimics the ringed coloration of the coral snake. Both king snakes and coral snakes live in the Carolines, but the king snakes' geographic range extends farther north and west into regions where no coral snakes are found (Figure 1.27). 22

CHAPTER i

Exploring Life

Range of scarlet king snake

3/

Eastern coral snake

Scarlet king snake A Figure 1.27 Geographic ranges of Carolina coral snakes and king snakes. The scarlet king snake (Lampropeltis triangulum) mimics the warning coloration of the poisonous eastern coral snake (Micrurus fulvius). Though these two snake species cohabit many regions throughout North and South Carolina, the geographic range of the king snake extends north and west of the range of the coral snake.

The researchers placed equal numbers of the two types of artificial snakes in Held sites throughout North and South Carolina, including the region where coral snakes are absent (see Figure 1.27)- Alter four weeks, the scientists retrieved the fake snakes and recorded how many had been attacked by looking for bite or claw7 marks. The most common predators were foxes, coyotes, and raccoons, but black bears also attacked some of the artificial snakes (Figure 1,28). The data fit the key prediction of the mimicry hypothesis. Compared to the brown artificial snakes, the ringed snakes were attacked by predators less frequently only in field sites within the geographic range of the poisonous coral snakes. Figure 1.29 summarizes the field experiments. This figure also introduces an illustration iormat we will use throughout the book to feature other examples of biological inquiry. Designing Controlled Experiments The snake mimicry experiment provides an example of how scientists design experiments to test the effect of one variable by canceling out the effects of any unwanted variables, such as the number of predators in this case. The design is called a controlled experiment, where an experimental group (the artificial king snakes, in this case) is compared with a control

Figure 1.29

Does the presence of poisonous coral snakes affect predation rates on their mimics, king snakes? EXPERIMENT David Pfennig and his colleagues made artificial snakes to test a prediction of the mimicry hypothesis: that king snakes benefit from mimicking the warning coloration of coral snakes only in regions where poisonous coral snakes are present. The Xs on the map below are field sites where the researchers placed equal numbers of artificial king snakes (experimental group) and brown artificial snakes (control group). The researchers recovered the artificial snakes after four weeks and tabulated predation data based on teeth and claw marks on the snakes (see Figure 1.28). In field sites where coral snakes were present, predators attacked far fewer artificial king snakes than brown artificial snakes. The warning coloration of the "king snakes" afforded no such protection where coral snakes were absent. In fact, at those field sites, the artificial king snakes were more likely to be attacked than the brown artificial snakes, perhaps because the bright pattern is particularly easy to spot against the background.

™ •1 )(

% of attacks on artificial king snakes % o f attacks o n brown artificial snakes Field site with artificial snakes

In areas w h e r e coral snakes were absent, most attacks w e r e on artificial king snakes.

In areas w h e r e coral snakes w e r e present, most attacks w e r e on b r o w n artificial snakes.

(b) Brown artificial snake that has been attacked A Figure 1.28 Artificial snakes used in field experiments to test the mimicry hypothesis. You can see where a bear

The field experiments support the mimicry hypothesis by not falsifying the key prediction that imitation of coral snakes is only effective where coral snakes are present. The experiments also tested an alternative hypothesis that predators generally avoid all snakes with brightly colored rings, whether or not poisonous snakes with that coloration live in the environment. That alternative hypothesis was falsified by the data showing thai the ringed coloration failed to repel predators where coral snakes were absent.

chomped on the brown artificial snake in (b).

CHAPTER i

Exploring Life

23

group (the brown artificial snakes). Ideally, the experimental and control groups differ only in the one factor the experiment is designed to test—in our example, the effect of the snakes' coloration on the behavior of predators. What if the researchers had failed to control their experiment? Without the brown mock snakes as a control group, the number of attacks on the fake king snakes in different geographic regions would tell us nothing about the effect of snake coloration on predator behavior at the different field sites. Perhaps, for example, fewer predators attacked the artificial king snakes in the eastern and southern field sites simply because fewer predators live there. Or maybe warmer temperatures in those regions make predators less hungry; The brown artificial snakes enabled the scientists to rule out such variables as predator density and temperature because those factors would have had equal effects on the control group and experimental group. Yet predators in the eastern and southern field sites attacked more brown artificial snakes than "king snakes." The clever experimental design left coloration as the only factor that could account for the low predation rate on the artificial king snakes placed within the range of coral snakes. It was not the absolute number of attacks on the artificial king snakes that counted, but the difference between that number and the number of attacks on the brown snakes. A common misconception is that the term controlled experiment means that scientists control the experimental environment to keep everything constant except the one variable being tested. But that's impossible in field research and not realistic even in highly regulated laboratory environments. Researchers usually "control" unwanted variables noL by eliminating them through environmental regulation, but by ramming their effects by using control groups.

Limitations of Science Scientific inquiry is a powerful way to know nature, bui there are limitations to the kinds of questions it can answer. These limits are set by science's requirements that hypotheses be testable and falsifiable and that observations and experimental results be repeatable. Observations that can't be verified may be interesting or even entertaining, but they cannot count as evidence in scientific inquiry. The headlines of supermarket tabloids would have you believe that humans are occasionally born with the head of a dog and that some of your classmates are extraterrestrials. The unconfirmed eyewitness accounts and the computer-rigged photos are amusing but unconvincing. In science, evidence from observations and experiments is only convincing if it stands up to the criterion of repeatability. The scientists who investigated snake mimicry in the Carolinas obtained similar data when they repeated their experiments with different species of coral snakes and king snakes in Ari-

24

Exploring Life

zona. And you should be able to obtain similar results if you were to repeat the snake experiments. Ultimately, the limitations of science are imposed by its naturalism—its seeking of natural causes for natural phenomena. Science can neither support nor falsify hypotheses that angels, ghosts, or spirits, both benevolent and evil, cause storms, rainbows, illnesses, and cures. Such supernatural explanations are simply outside the bounds of science.

Theories in Science "It's just a theory!" Our everyday use of the term theory often implies an untested speculation. But the term theory has a very different meaning in science. What is a scientific theory, anc. how is it different from a hypothesis or from mere speculation? First, a scientific theory is much broader in scope than a hypothesis. This is a hypothesis: "Mimicking poisonous snakes is an adaptation that protects nonpoisonous snakes from predators."1 But this is a theory: "Evolutionary adaptations evolve by natural selection." Darwin's theory of natural selection accounts for an enormous diversity of adaptations, including mimicry Second, a theory is general enough to spin off many new.. specific hypotheses that can be tested. For example, Peter and. Rosemary Grant, of Princeton University; were motivated by the theory of natural selection LO test the specific hypothesis that the beaks of the Galapagos finches evolve in response to changes in the types of available food. And third, compared to any one hypothesis, a theory is generally supported by a much more massive body of evidence. Those theories that become widely adopted in science (such as the theory of natural selection) explain a great diversity of observations and are supported by an accumulation of evidence, in fact, scrutiny of general theories continues through testing of the specific, falsifiable hypotheses they spawn. In spite of the body of evidence supporting a widely accepted theory, scientists must sometimes modify or even reject theories when new research methods produce results that don't fit. For example, the five-kingdom theory of biological diversity began to erode when new methods for comparing cells and molecules made it possible to test some ol the hypothetical relationships between organisms that were based on the theory. If there is :'truth" in science, it is conditional, based on the preponderance of available evidence.

Model Building in Science You may work with many models in your biology course this year. Perhaps you'll model cell division by using pipe cleaners or other objects as chromosomes. Or maybe you'll practice using mathematical models LO predict the growth of a bacterial population. Scientists often construct models as less abstract representations of ideas such as theories or

^ Figure 1.30 Modeling the pattern of blood flow through the four chambers of a human heart.

From body

*• Figure 1.31 Science as a social process. In her New York University laboratory, plant biologist Gloria Coruzzi mentor5 one of her students in the methods of molecular biology.

To lungs

natural phenomena such as biological processes. Scientific models can take many forms, such as diagrams, graphs, three-dimensional objects, computer programs, or mathematical equations. The choice of a model type depends on how it will be used to help explain and communicate the object, idea, or process it represents. Some models are meant to be as lifelike as possible. Other models are more useful if they are symbolic schematics. For example, the simple diagram in Figure 1.30 does a good job of modeling blood flow through the chambers of a human heart without looking anything like a real heart. A heart model designed to help train a physician to perform heart surgery would look very different. Whatever its design, the test of a model is how well it fits the available data, how comfortably it accommodates new observations, low accurately it predicts the outcomes of new experiments, and how effectively it clarifies and communicates the idea or process it represents.

The Culture of Science Movies and cartoons sometimes portray scientists as loners working in isolated labs. In reality, science is an intensely social activity. Most scientists work in teams, which often include both graduate and undergraduate students (Figure 1.31). And to succeed in science, it helps to be a good communicator. Research results have no impact until shared with a community of peers through seminars, publications, and websites. Both cooperation and competition characterize the scientific culture. Scientists working in the same research field often check one another's claims by attempting to confirm observations or repeat experiments. And when several scientists converge on the same research question, there is all the excitement of a race. Scientists enjoy the challenge of "getting there first" with an important discovery or key experiment. The biology community is part of society at large, embedded in the cultural milieu of the times. For example, changing attitudes about career choices have increased the

proportion of women in biology, which has in turn affected the emphasis in certain research fields. A few decades ago, for instance, biologists who studied the mating behavior oi animals focused mostly on competition among males ior access to females. More recent research, however, emphasizes the important role that females play in choosing mates. For example, in many bird species, females prefer the bright coloration that ''advertises" a male's vigorous health, a behavior that enhances the female's probability of having healthy offspring. Some philosophers of science argue that scientists are so influenced by cultural and political values that science is no more objective than other ways of ''knowing nature." At the other extreme are people who speak of scientific theories as though they were natural laws instead of human interpretations of nature. The reality of science is probably somewhere in between—rarely perfectly objective, but continuously vetted through the expectation that observations and experiments be repeatable and hypotheses be testable and falsifiable.

Science, Technology, and Society The relationship of science to society becomes clearer when we add technology to the picture. Though science and technology sometimes employ similar inquiry patterns, their basic goals differ. The goal of science is to understand natural phenomena. In contrast, technology generally applies scientific knowledge for some specific purpose. Biologists and other scientists often speak of ''discoveries," while engineers and other technologists more often speak of "inventions." And the beneficiaries of those inventions include scientists, who put new technology LO work in their research; the impact of information technology on systems biology is just one example. Thus, science and technology are interdependent.

CHAPTER 1

Exploring Life

Concept Check 1. Contrast induction with deduction. 2. Critique this statement: ''Scientists design controlled experiments to study a single variable by preventing all other factors from changing." 3. In the snake mimicry experiments, why did the researchers place some of their artificial snakes beyond the geographic range of coral snakes? 4. Contrast "theory" with "hypothesis." For suggested answers, see Appendix A.

Concept A Figure 1.32 DNA technology and crime scene i n v e s t i g a t i o n . Forensic technicians can use traces of DNA extracted from a blood sample or other body tissue collected at a crime scene to produce molecular fingerprints. The stained bands you see in this photograph represent fragments of DNA, and the pattern of bands varies from person to person.

The potent combination of science and technology has dramatic effects on society. For example, discovery of the structure of DNA by Watson and Crick 50 years ago and subsequent achievements in DNA science led to the many technologies of DNA engineering that are transforming a diversity of fields, including medicine, agriculture, and forensics (Figure 1.32). Perhaps Watson and Crick envisioned that their discovery would someday produce important applications, but it is unlikely that they could have predicted exactly what those applications would be. The directions that technology takes depend less on the curiosity that drives basic science than it does on the current needs and wants of people and on the social environment of the times. Debates about technology center more on "should we do it" than "'can we do it." With advances in technology come difficult choices. For example, under what circumstances is it acceptable to use DNA technology to check if people have genes for hereditary diseases? Should such tests always be voluntary, or are there any circumstances when genetic testing should be mandatory? Should insurance companies or employers have access to the information, as they do for many other types of personal health data? Such ethical issues have as much to do with politics, economics, and cultural values as with science and technology But scientists and engineers have a responsibility to help educate politicians, bureaucrats, corporate leaders, and voters about how science works and about the potential benefits and risks of specific technologies. The crucial science-technologysociety relationship is a theme that increases the significance of any biology course. 26

CHAPTER 1

Exploring Life

A set of themes connects the concepts of biology In some ways, biology is the most demanding of all sciences, partly because living systems are so complex and partly because biology is an interdisciplinary science that requires knowledge of chemistry physics, and mathematics. Modern biology is the decathlon of natural science. And of all the sciences, biology is the most connected to the humanities and social sciences. As biology students, you are definitely in the right place at the right time! No matter what brings you to biology, you will find the study of life to be endlessly challenging and uplifting. But this ever-expanding subject can also be a bit intimidating, even to professional biologists. How, then, can beginning students develop a coherent view of life instead of hopelessly trying to memorize the details of a subject that is now far too big to memorize? One approach is to fit the many things you learn into a set of themes that pervade all of biology—ways of thinking about life that will still apply decades from now, when much of the specific information fossilized in any textbook will be obsolete. Table 1.1 outlines a number of broad themes you will recognize from this first chapter of Biology These unifying themes will reemerge throughout the book to provide touchstones as you explore life and begin asking important questions of your own.

Concept Check 1. Write a sentence relating the theme of "scientific inquiry" to the theme of "science, technology, and society." For suggested answers, see Appendix A.

Table 1.1 Eleven Themes that Unify Biology Theme

Description

Theme

Description

Cells are every organisms, basic units of Structure and function. The two main types of cells are prokaryotic cells (in bacteria and archaea) and eukaryotic cells (in protists, plants, fungi, and animals).

Unity and diversity

Biologists group the diversity of life into three domains: Bacteria, Archaea, and Eukarya. As diverse as life is, we can also find unity, such as a universal genetic code. The more closely related two species are, the more characteristics they share.

The continuity of life depends on the inheritance of biological information in the form of DNA molecules. This genetic information is encoded in the nucleotide sequences ot the DNA.

Evolution, biology's core theme, explains both the unity and diversity of life. The Darwinian theory of natural selection accounts for adaptation of populations to their environment through the differential reproduciive success of varying individuals.

Emergent properties of biological system

The living world has a hierarchical organization, extending from molecules to the biosphere. With each step upward in level, system properties emerge as a result of interactions among components at the lower levels.

Structure and function

Form and function are correlated at all levels of biological organization.

Regulation

Feedback mechanisms regulate biological systems. In some cases, the regulation mainiains a relatively steady state for internal factors such as body temperature.

Scientific inquiry

The process of science includes observationbased discovery and the testing of explanations through hypothesis-based inquiry. Scientific credibility depends on the repeatability of observations and experiments,

Interaction with the environment

Organisms are open systems that exchange materials and energy with their surroundings. An organism's environment includes other organisms as well as nonliving factors.

Science, technology, and society

Many technologies are goal-oriented applications of science. The relationships of science and technology to society are now more crucial to understand than ever before.

Energy and life

All organisms must perform work, which requires energy. Energy flows from sunlight to producers to consumers.

CHAPTER 1

Exploring Life

27

CHAPTER 1

Exploring Life

29

Chapter *| Review Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY CONCEPTS

• Unity in the Diversity of Life (pp. 14-15) As diverse as life is, there is also evidence of remarkable unity.

Evolution accounts for life's unity and diversity

• A Hierarchy of Biological Organization {pp. 3-6) The hierarchy of life unfolds as follows: biosphere > ecosystem > community > population > organism > organ system > organ > tissue > cell > organelle > molecule > atom. Activity The Levels of Life Card Game

• Natural Selection (pp. 16-17) Darwin called the evolutionary history of species "descent with modification." He proposed natural selection as the mechanism for evolutionary adaptation of populations to their environments. Natural selection is the evolutionary process that occurs when a population's heritable variations are exposed to environmental factors that favor the reproductive success of some individuals over others. Investigation How Do Environmental Changes Affect a Population? Activity Form Fits Function: Cells

• A Closer Look at Ecosystems (p. 6) Whereas chemical nutrients recycle within an ecosystem, energy flows through an ecosystem. Activity Energy Flow and Chemical Cycling

• The Tree of Life (pp. 17-19) Each species is one twig of a branching tree of life extending back in time through ancestral species more and more remote. All of life is connected through its long evolutionary history.

Biologists explore life from the microscopic to the global scale

• A Closer Look at Cells (pp. 6-8) The cell is the lowest level of organization that can perform all activities required for life. Cells contain DNA, the substance of genes, which program the cell's production of proteins and transmit information from parents to offspring. Eukaryotic cells contain membrane-enclosed organelles, including a DNA-containing nucleus. Prokaryotic cells lack such organelles. Activity Heritable Information: DNA Activity Comparing Prokaryotic and Eukaryotic Cells

Biological systems are much more than the sum of their parts • The Emergent Properties of Systems (p. 9) Due to increasing complexity new properties emerge with each step upward in the hierarchy of biological order. • The Power and Limitations of Reductionism (pp. 9-10) Reductionism involves reducing complex systems to simpler components that are more manageable to study • Systems Biology (pp. 10-11) Systems biology seeks to create models of the dynamic behavior of whole biological systems. With such models, scientists will be able to predict how a change in one part of the system will affect the rest of the system. • Feedback Regulation in Biological Systems (pp. 11-12) In negative feedback, accumulation of an end product slows the process that produces that product. In positive feedback, the end product speeds up its production. Activity Regulation: Negative and Positive Feedback

Biologists explore life across its great diversity of species • Grouping Species: The Basic Idea (pp. 12-13) Taxonomy is the branch of biology that names and classifies species according to a system of broader and broader groups. • The Three Domains of Life (pp. 13-14) Domain Bacteria and Domain Archaea consist of prokaryotes. Domain Eukarya, the eukaryotes, includes the various protist kingdoms and the kingdoms Plantae, Fungi, and Animalia. Activity Classification Schemes

28

Exploring Life

Biologists use various forms of inquiry to explore life • Discovery Science (pp. 19-20) In discovery science, scientists describe some aspect of the world and use inductive reasoning to draw general conclusions. Graph it An Introduction to Graphing • Hypothesis-Based Science (pp. 20-21) Based on observations, scientists propose hypotheses that lead to predictions and then test the hypotheses by seeing if the predictions come true. Deductive reasoning is used in testing hypotheses: I/a hypothesis is correct, and we test it, then we can expect a particular outcome. Hypotheses must be testable and falsifiable. investigation How Does Acid Precipitation Affect Trees? • A Case Study in Scientific Inquiry: Investigating Mimicry in Snake Populations (pp. 21-24) Experiments must be designed to test the effect of one variable by testing control groups and experimental groups that vary in only that one variable. • Limitations of Science (p. 24) Science cannot address supernatural phenomena because hypotheses must be testable and falsifiable and observations and experimental results must be repeatable. • Theories in Science (p. 24) A scientific theory is broad in scope, generates new hypotheses, and is supported by a large body of evidence. • Model Building in Science (pp. 24-25) Models of ideas, structures, and processes help us understand scientific phenomena and make predictions. • The Culture of Science (p. 25) Science is a social activity characterized by both cooperation and competition. • Science, Technology, and Society (pp. 25-26) Technology applies scientific knowledge for some specific purpose. Activity Science, Technology, and Society: DDT

A set of themes connects the concepts of biology • Underlying themes provide a framework for understanding biology (pp. 26-27).

TESTING YOUR KNOWLEDGE Evolution Connection typical prokaryotic cell has about 3,000 genes in its DNA, while a arnan cell has about 25,000 genes. About 1,000 of these genes are esent in both types of cells. Based on your understanding of evotion, explain how such different organisms could have the same bset of genes.

Science, Technology, and Society The fruits of wild species of tomato are tiny compared to the giant beefsteak tomatoes available today. This difference in fruit size is almost entirely due to the larger number of cells in the domesticated fruits. Plant molecular biologists have recently discovered genes that are responsible for controlling cell division in tomatoes. Why would such a discover}7 be important to producers of other kinds of fruits and vegetables? To the study of human development and disease? To our basic understanding of biology?

Scientific Inquiry used on the results- of the snake mimicry case study suggest aniier hypothesis researchers might investigate further. vestigation How Do Environmental Changes Affect a jpulation? vestigation How Does Acid Precipitation Affect Trees?

CHAPTER 1 Exploring Life

29

The Chemistry of Life AN INTERVIEW WITH

Lydia Makhubu Until her recent retirement, Lydia Makhubu was the Vice Chancellor (in American terms, President) of the University of Swaziland, where she was also Professor of Chemistry. She received her higher education in Lesotho and at the Universities of Alberta and Toronto, where she earned a Ph.D. in Medicinal Chemistry. Building

Swaziland is unusual these days because it is a kingdom, with a king who has executive authority. In the population of about 1 million, there is only one ethnic group, the Swazis, so we haven't had the conflicts that have afflicted some other African nations. Swaziland was a British colony, gaining independence in 1968. These days, the economy isn't as good as it used to be, in part because we've had a lot of drought and we're heavily dependent on agriculture.

on her study of chemistry, Dr. Makhubu has had a distinguished career as a scientist in the area of health and traditional medicine, as a leader in higher education, and as a commentator on science, technology, and development in Africa and other developing regions. Among the many instances of her international service, she has been a consultant to several United Nations agencies and to the American Association for the Advancement of Science and has chaired the Association of Commonwealth Universities. Jane Reece met with her in Paris, where Dr. Makhubu was attending a meeting of the Executive Board of UNESCO, the United Nations Educational, Scientific and Cultural Organization.

Please tell us a little about Swaziland and its people. Swaziland is a small landlocked country, only 17,400 square kilometers in area (smaller than New Jersey). It borders Mozambique on the east and South Africa on the west and south. The country ranges in altitude from high to low, and it has a great diversity of organisms, especially plants. The capital, Mbabane, is in the ecological zone called the highveld, where the altitude is close to 1,800 meters above sea level. The altitude drops to the middleveld, which has rich soils especially good for agriculture. As you go toward Mozambique and the sea, the land gets low and flat—the lowveld. The climate and the plant and animal species change as the altitude changes.

30

What influenced you to become a medicinal chemist? In the early days, my parents were teachers, but then my father took up a career in the Ministry of Health, becoming an orderly in a medical clinic. We lived at the clinic, and I could see him check people—since doctors were scarce, an orderly had a lot oi responsibility 1 warned m be a medical doctor at that time. I ended up studying chemistry at college in Lesotho. From there, 1 went to Canada, where I did a masters degree and doctorate. 1 liked chemistry because it seemed to make sense: You mix this and that, and a product appears. 1 became interested in organic chemistry, and then, probably influenced by the importance of medicine in my society, I chose medicinal chemistry. I wanted to study the effects of drugs on the body.

What do you mean by "medicine in my society*'? The traditional medicine of my people. At college in Lesotho, we students used to argue about m-xiiuonal medicine: Some believed H was absolute nonsense; others thought it worked. 1 was interested in this question. So when 1 came back from Canada, I immediately sought out traditional healers, including some of my relatives, and I was shown a few of the medicinal preparations they used. I started working in the laboratory to try to identify what was in those medicines.

In other countries of Southern Africa, traditional healers have organized themselves into associations and have even established some clinics. But in Swaziland, the British banned traditional medicine by the Witchcraft Act of 1901—and this law has not been repealed yet. Even people who had access to modern clinics, though, often continued to go to traditional healers, as well, and this continues today.

Tell us about the research on the plant Phytolacca dodecandra and its potential for preventing disease. This plant, also called edod or soapberry, is a common bush in Africa. One day in 1964, an Ethiopian scientist, the late Aklilu Lemma of Addis Ababa University, was walking near a small stream, where he saw women washing clothes. He noticed a large pile of dead snails, of the type that transmits the disease schistosomiasis. He asked the women, "What are you using for soap?" and learned that they were using the. berries of Phytolacca. He then took berries to the lab, along with some living snails, and lound that extracts of the berries killed the snails.

What is schistosomiasis? Also called bilharzia or snail fever, this is a debilitating disease that afflicts more than 200 million people worldwide, it is one of the greatest scourges in the developing world. The disease is caused by a parasitic flatworm fa fluke) that uses an aquatic snail as a host during part of its life cycle (see Figure 33.11). Fluke larvae released by the snails pierce the skin of people standing or swimming in the water, infecting them. You can control schistosomiasis u\ killing the parasite withadrug or by killing the snail with a synthetic molluscicide—but both are too expensive in Africa.

Is this where Phytolacca comes in? Yes, Phytolacca berries are a better control me!hod :or bci'iisu'^orninsis in Africa than

synthetic chemicals of any sort because people can easily grow the plant and harvest the berries. Chemists have isolated the Phytolacca chemical that is lethal to snails, although it is not yet known exactly how it acis. Researchers have discovered that the chemical also kills some other parasites that live in African rivers, as well as the larvae of mosquitoes (which transmit malaria). And there are no bad environmental effects because the chemical readily decomposes. At the University of Swaziland, we obtained seeds of Phytolacca from Ethiopia, grew the plants, and harvested the berries. The Ethiopians came to show us how to do everything; it was a true collaboration between Swazi and Hthiopian scientists, with help from some Zimbabwans and an American. Working in the lab, we discovered the concentration at which the berry extract killed the snails, and then we went into the field for further tests. Now we have selected an area in Swaziland where schistosomiasis is very prevalent, and we're working with [he people there, teaching them how to grow and use Phytolacca. We hope that, in another year, the communities will be able lo control the disease themselves.

What goes on at your university's institute of traditional medicine? At this institute, officially the "Swaziland Institute of Research in Tradiuonal Medicine, Medicinal and Indigenous Food Plants," multidisciplinary learns study all aspects of traditional medicine. Traditional healers are essential learn members because they know the healing plants and how to use them. We have had several workshops

with traditional healers, trying lo convince them of the importance of sharing their knowledge with us —because they are going to die, like all of us, and the knowledge may soon be lost. However, the healers—even my relatives!—are reluctant to help. They think, "You with your white coats are going to make loads of money from my knowledge." Their belief system is another obstacle. The healers believe that they are given the power of healing by their ancestors, and they are supposed to pass on this knowledge only to their children. But mostly it is suspicion. You know, for a long time they were called witches, and quite a few of the older ones are still sore about that; they ask me, "When is thai Witchcraft Act of yours going to be repealed?"—as if 1 had written it! But slowly we are managing ;o convince them. We want to involve them for the long lerm, not only to show us the plants and help us grow them but 10 come into the lab to teach us how they prepare the medicine, so that we can quantify everything. But its not easy. It is also important, I think, to study the spiritual beliefs of the healers because the whole system is based on those beliefs. They say they are shown the plants in a dream by their ancestors' spirits, and they make diagnoses by throwing bones and going into a trance, during which the spirits speak to them.

What is the state of the environment and biological diversity in Swaziland? Not very good. I think the underlying problem is overreliance on the natural environment, es-

"You can control schistosomiasis by killing the parasite with a drug or by killing the snail with a synthetic molluscicide—but both are too expensive in Africa.... Phytolacca berries are a better control method . .. because people can easily grow the plant."

peeially plant resources, in many rural areas, people have chopped down trees for wood until the land is completely bare; they do not know how to replant. Their grazing animals, such as cattle, often eat whatever plants remain. And the healers may overharvest medicinal plants from the wild. Many plants are disappearing. Preservation of diversity goes along with preservation of the environment. So, you find that, in pans of Swaziland, certain animals have disappeared because the plants they lived on are no longer there. Even the climate is affected. For instance, in the forested highveld of Swaziland, there used to be lots of rain. But as the plants are removed, the rainfall lessens. Another issue is damage that can result from projects associated with economic development, such as mining and dam construction. It is only recently that companies carrying out these big projects are being required to take care of the environment.

What are the challenges that science education faces in Africa? We dont have enough resources to build proper science facilities, and we don't have enough science teachers. Another serious problem is the underrepresentation of women in science; this is particularly bad in Africa. Women are left behind. Science, especially physical science, is not considered a field for women. Many people think that if women go too far, they won't get a husband. But the situation is starting to change.

You are the President of the Third World Organization for Women in Science (TWOWS). What does this organization do? We provide fellowships for postgraduate study, enlisting support from organizational benefactors. The fellowship recipients are usually sent to good universities in de\ eloping countries, such as South Africa or Pakistan, where ihe available money can go a long way TWOW'S also promotes collaboration among women from developing countries who are already established scientists. But it's crucial to start at the earliest level, primary school. Researchers have learned that once girls get started in science, they do well. But they need to be encouraged by iheir teachers. If there is equipment available, it is used by the boys; the girls' role may be simply recording the results! So we are working hard to encourage the involvement of women scientists at all levels of education, to show the teachers that girls can be scientists.

31

The Chemical Context of Life A Figure 2.1 The bombardier beetle uses chemistry to defend itself.

Key Concepts 2.1

Matter consists of chemical elements in pure form and in combinations called compounds 2.2 An element's properties depend on the structure of its atoms 2.3 The formation and function of molecules depend on chemical bonding between atoms 2.4 Chemical reactions make and break chemical bonds

many connections to the themes introduced in Chapter 1. One of those themes is the organization of life into a hierarch) of structural levels, with additional properties emerging at each successive level. In this unit, we will see how the theme of emergent properties applies to the lowest levels of biological organization—to the ordering of atoms into molecules and to the interactions of those molecules within cells. Somewhere in the transition from molecules to cells, we will cross the blurry boundary7 between nonlife and life. We begin by considering the chemical components that make up all matter. As Lydia Makhubu mentioned in the interview on pages 30 and 31, chemistry is an integral aspect of biology;

Chemical Foundations of Biology

L

ike other animals, beetles have evolved structures and mechanisms thai defend them from attack. The soildwelling bombardier beetle has a particularly effective mechanism for dealing with the ants that plague it- Upon detecting an ant on its body, this beetle ejects a spray of boiling hot liquid from glands in its abdomen, aiming the spray directly at the ant. (In Figure 2.1, the beetle aims its spray at a scientists forceps.) The spray contains irritating chemicals that are generated at the moment of ejection by the explosive reaction of two sets of chemicals stored separately in the glands. The reaction produces heat and an audible pop. Research on the bombardier beetle has involved chemistry physics, and engineering, as well as biology This is not surprising, for unlike a college catalog of courses, nature is not neatly packaged into the individual natural sciences. Biologists specialize in the study of life, but organisms and the world they live in are natural systems to which basic concepts of chemistry and physics apply. Biology is a multidisciplinary science. This unit of chapters introduces key concepts of chemistry that will apply throughout our study of life. We will make 32

Concept &•

Matter consists of chemical elements in pure form and in combinations called compounds Elements and Compounds Organisms are composed of matter, which is anything that takes up space and has mass.* Matter exists in many diverse torms, each with its own characteristics. Rocks, metals, oils, gases, and humans are just a few examples of what seems an endless assortment of matter. * Someiirnes we substitute the term weight for mass, although the two are not identical. Mass is the amount of matter in an object, whereas the weight of an object is how strongly thai mass is pulled by gravity. The weight of an astronaut walking on the moon is approximately 1/6 that on Earth, but his or her mass is the same. However, as long as we are earthbound, the weight of an object is a measure of its mass; in everyday language, therefore, we tend to use the terms interchangeably

Table 2.1 Naturally Occurring Elements in the Human Body

Sodium

Chlorine

Sodium chloride

A Figure 2.2 The emergent properties of a compound. The metal sodium combines with the poisonous gas chlorine to form the edible compound sodium chloride, or table salt.

Matter is made up of elements. An element is a substance that cannot be broken down to other suhstances by chemical reactions. Today, chemists recognize 92 elements occurring in nature; gold, copper, carbon, and oxygen are examples. Each element has a symbol, usually the first letter or two of its nameSome of the symbols are derived from Latin or German names; for instance, the symbol for sodium is Na, from the Latin word natrium. A compound is a substance consisting of two or more different elements combined in a fixed ratio. Table salt, for example, is sodium chloride (NaCl), a compound composed of the elements sodium (Na) and chlorine (Cl) in a 1:1 ratio. Pure sodium is a metal and pure chlorine is a poisonous gas. When chemically combined, however, sodium and chlorine form an edible compound. This is a simple example of organized matter having emergent properties: A compound has characteristics different from those of its elements (Figure 2.2).

Essential Elements of Life About 25 of the 92 natural elements are known to he essential to life. Just four of these-—carbon (C), oxygen (O), hydrogen (H), and nitrogen (N)—make up 96% of living matter. Fhos-

^ Figure 2.3 The effects of essentialelement deficiencies, (a) This photo shows the effect of nitrogen deficiency in corn. In this controlled experiment, the plants on the left are growing in soil that was fertilized with compounds containing nitrogen, while the soil on the right is deficient in nitrogen, (b) Goiter, an enlarged thyroid gland, is the result of a deficiency of the trace element iodine. The goiter of this Malaysian woman can probably be reversed by iodine supplements.

Atomic Number (See p. 34)

Percentage of Human Body Weight

Symbol

Element

o c

Oxygen

H

Hydrogen

1

9.5

N

Nitrogen

7

3.3

Ca

Calcium

20

1.5

P

Phosphorus

15

1.0

65.0 18.5

Carbon

K

Potassium

19

0.4

S

Sulfur

16

0.3

Na

Sodium

11

0.2

Cl

Chlorine

17

0.2

Mg

Magnesium

12

0.1

Trace elements (less ihan 0 01%): boron (B), chromium (Cr), cobalt (Co), c per (Cu). fluorine (I7), iodine (I), iron (Fe) manganese (Mil), molybdenurr l ,Moj, selenium ;Se). .silicon (SO. tm :SrO vanadium fV). and /inr (Zn).

phorus (P), sulfur (S), calcium (Ca), potassium (K), and a few other elements account for most of the remaining 4% of an organism's weight. Table 2.1 lists by percentage the elements that make up the human body; the percentages for other organisms are similar. Figure 2.3a illustrates the effect of a deficiency of nitrogen, an essential element, in a plant. Trace elements are those required by an organism in only minute quantities. Some trace elements, such as iron (Fe), are needed by all forms of life; others are required only by certain species. For example, in vertebrates (animals with backbones), the element iodine (1) is an essential ingredient of a hormone produced by the thyroid gland. A daily intake of only 0.15 milligram (mg) of iodine is adequate for normal

(a) Nitrogen deficiency

(b) Iodine deficiency CHAPTER 2

The Chemical Context oFLife

33

activity of the human thyroid. An iodine deficiency in the diet causes the thyroid gland to grow to abnormal size, a condition called goiter (Figure 2.3b). Where it is available, iodized salt has reduced the incidence of goiter.

Cloud of negative charge (2 electrons)

,. Nucleus.

Concept Check 1. Explain why table salt is a compound, while the oxygen we breathe is not. 2, What four chemical elements are most abundant in the food you ate yesterday? For suggested answers, see Appendix A.

Concept

An element's properties depend on the structure of its atoms Each element consists of a certain kind of atom that is different from the atoms of any other element. An atom is the smallest unit of matter that still retains the properties of an element. Atoms are so small that it would take about a million of them to stretch across the period printed at the end of this sentence. We symbolize atoms with the same abbreviation used for the element made up of those atoms; thus, C stands for both the element carbon and a single carbon atom.

Subatomic Particles Although the atom is the smallest unit having the properties of its element, these tiny bits of matter are composed of even smaller parts, called subatomic particles. Physicists have split the atom into more than a hundred types ol particles, but only three kinds of particles are stable enough to be of relevance here: neutrons, protons, and electrons. Neutrons and protons are packed together tightly to form a dense core, or atomic nucleus, at the center of the atom. The electrons, moving at nearly the speed of light, form a cloud around the nucleus. Figure 2.4 shows two models of the structure of the helium atom as an example. Electrons and protons are electrically charged. Each electron has one unit of negative charge, and each proton has one unit of positive charge. A neutron, as its name implies, is electrically neutral. Protons give the nucleus a positive charge, and it is the attraction between opposite charges that keeps the rapidly moving electrons in the vicinity of the nucleus. The neutron and proton are almost identical in mass, each about 1.7 X 10~ 24 gram (g). Grams and other conventional units are not very useful for describing the mass of objects so 34

UNIT O N E

The Chemistry of Life

(a) This model represents the electrons as a cloud of negative charge, as if we had taken many snapshots of the 2 electrons over time, with each dot representing an electron's position at one point in time.

(b) In this even more simplified model, the electrons are shown as t w o small blue spheres on a circle around the nucleus.

A Figure 2.4 Simplified models of a helium (He) atom. The helium nucleus consists of 2 neutrons (brown) and 2 protons (pink). Two electrons (blue) move rapidly around the nucleus. These models are not to scale; they greatly overestimate the size of the nucleus in relation to the electron cloud.

minuscule. Thus, for atoms and subatomic particles (and for molecules as well), we use a unit of measurement called the dalton, in honor of John Dalton, the British scientist who helped develop atomic theory around 1800. (The dalton is the same as the atomic mass unit, or amu, a unit you may have encountered elsewhere.) Neutrons and protons have masses close to 1 dalton. Because the mass of an electron is only about Y2.000 that of a neutron or proton, we can ignore electrons when computing the total mass of an atom.

Atomic Number and Atomic Mass Atoms of the various elements differ in their number of subatomic particles. All atoms ol a particular element have the same number of protons in their nuclei. This number of protons, which is unique to that element, is called the atomic number and is written as a subscript to the left of the symbol for the element. The abbreviation 2He, for example, tells us that an atom of the element helium has 2 protons in its nucleus. Unless otherwise indicated, an atom is neutral in electrical charge, which means that its protons must be balanced by an equal number of electrons. Therefore, the atomic number tells us the number of protons and also the number of electrons m an electrically neutral atom. We can deduce the number of neutrons from a second quantity, the mass number, which is the sum of protons plus neutrons in the nucleus of an atom. The mass number is written

as a superscript to the left of an element's symbol. For example, we can use this shorthand to write an atom of helium as 2He. Because the atomic number indicates how many protons there are, we can determine the number of neutrons by subtracting the atomic number from the mass number: A 2He atom has 2 neutrons. An atom of sodium, ^Na, has 11 protons, 11 electrons, and 12 neutrons. The simplest atom is hydrogen *H, which has no neutrons; it consists of a lone proton with a single electron moving around it. Almost all of an atoms mass is concentrated in its nucleus, because, as mentioned earlier, the contribution of electrons to mass is negligible. Because neutrons and protons each have a mass very close to 1 dalton, the mass number is an approximation of the total mass of an atom, called its atomic mass. So we might say that the atomic mass of sodium (JfNa) is 23 daltons, although more precisely it is 22.9898 dakons.

Isotopes All atoms of a given element have the same number of protons, but some atoms have more neutrons than other atoms of the same element and therefore have greater mass. These different atomic forms are called isotopes of the element. Tn nature, an element occurs as a mixture of its isotopes. For example, consider the three isotopes of the element carbon, which has the atomic number 6. The most common isotope is carbon-12, L|C, which accounts [or about 99% of the carbon in nature. It has 6 neutrons. Most of the remaining 1 % of carbon consists of atoms of the isotope ^C, with 7 neutrons. A third, even rarer isotope, *gC, has 8 neutrons. Notice that all three isotopes of carbon have 6 protons—otherwise, they would not be carbon. Although isotopes of an element have slightly different masses, they behave identically in chemical reactions. (The number usually given as the atomic mass of an element, such as 22.9898 daltons for sodium, is actually an average of the atomic masses of all the elements naturally occurring isotopes.) Both 12C and 13C are stable isotopes, meaning thai their nuclei do not have a tendency to lose particles. The isotope 14C, however, is unstable, or radioactive. A radioactive isotope is one in which the nucleus decays spontaneously, giving off particles and energy. When the decay leads to a change in the number of protons, it transforms the atom to an atom of a different element. For example, radioactive carbon decays to form nitrogen. Radioactive isotopes have many useful applications in biology. In Chapter 26, you will learn how researchers use measurements of radioactivity in fossils to date those relics of past life. Radioactive isotopes are also useful as tracers to follow atoms through metabolism, the chemical processes of an organism. Cells use the radioactive atoms as they would nonradioactive isotopes of the same element, but the radioactive tracers can be readily detected. Figure 2.5 presents an example of how biologists use radioactive tracers to monitor biological processes, in this case cells making copies of their DNA.

Radioactive Tracers APPLICATION

Scientists use radioactive isotopes to label certain chemical substances, creating tracers that can be used to follow a metabolic process or locate the substance within an organism. In this example, radioactive tracers are being used to determine the effect of temperature on the rate at which cells make copies of their DNA.

ingredients including radioactive tracer (bright blue) Human cells

O

Ingredients for making DNA are added to human cells. One ingredient is labeled with 3 H , a radioactive isotope of hydrogen. Nine dishes of cells are incubated at different temperatures, The cells make new DNA, incorporating the radioactive tracer with 3 H . i The cells are placed in test tubes, their DNA is isolated, and unused ingredients are removed.

/DNA (old and new)

© A solution called scintillation fluid is added to the test tubes and they are placed in a scintillation counter. As the 3H in the newly made DNA decays, it emits radiation that excites chemicals in the scintillation fluid, causing them to give off light. Flashes of light are recorded by the scintillation counter.

U H 3 i i 9 H I

The frequency of flashes, which is recorded as

counts per minute, is proportional to the amount of the radioactive tracer present, indicating the amount of new DNA. In this experiment, when the counts per minute are plotted against temperature, it is clear that temperature affects the rate of DNA synthesis—the most DNA was made at 35 °C.

CHAPTER 2

The Chemical Context of Life

35

the center ot the field. Moreover, the electrons would be like two tiny gnats buzzing around the stadium, in a space approximately a million times as large as the nucleus. Atoms are

mostly empty space.

A Figure 2.6 A PET scan, a medical use for radioactive isotopes. PET, an acronym for positron-emission tomography, detects locations of intense chemical activity in the body. The patient is first injected with a nutrient such as glucose labeled with a radioactive isotope that emits subatomic particles. These particles collide with electrons made available by chemical reactions in the body. A PET scanner detects the energy released in these collisions and maps "hot spots," the regions of an organ that are most chemically active at the time. The color of the image varies with the amount of the isotope present, with the bright yellow color here identifying a hot spot of cancerous throat tissue.

Radioactive tracers are important diagnostic tools in medicine. For example, certain kidney disorders can be diagnosed by injecting small doses of substances containing radioactive isotopes into the blood and then measuring the amount of tracer excreted in the urine. Radioactive tracers are also used in combination with sophisticated imaging instruments, such as PET scanners, which can monitor chemical processes, such as those involved in cancerous growth, as they actually occur in the body (Figure 2.6). Although radioactive isotopes are very useful in biological research and medicine, radiation from decaying isotopes also poses a hazard to life by damaging cellular molecules. The severity of this damage depends on the type and amount of radiation an organism absorbs. One of the most serious environmental threats is radioactive fallout from nuclear accidents. The doses of most isotopes used in medical diagnosis, however, are relatively safe.

The Energy Levels of Electrons The simplified models ot the atom in Figure 2.4 greatly exaggerate the size of the nucleus relative to the volume of the whole atom. If an atom of helium were the size of Yankee Stadium, the nucleus would be only the size of a pencil eraser in UNIT O N E

The Chemistry of Life

When two atoms approach each other during a chemical reaction, their nuclei do not come close enough to interact. Of the three kinds of subatomic particles we have discussed, only electrons are directly involved in the chemical reactions between atoms. An atoms electrons vary in the amount of energy they possess. Energy is defined as the capacity to cause change, for instance by doing work. Potential energy is the energy thai matter possesses because of its location or structure. For example, because of its altitude, water in a reservoir on a hill has potential energy When the gates of the reservoir's dam are opened and the water runs downhill, the energy can be used to do work, such as turning generators. Because energy has been expended, the water has less energy at the bottom of the hill than it did in the reservoir. Matter has a natural tendency to move to the lowest possible state of potential energy; m this example, water runs downhill. To restore the potential energy of a reservoir, work must be done to elevate the water against gravity. The electrons of an atom also have potential energy because of how they are arranged in relation to the nucleus. The negatively charged electrons are attracted to the positively charged nucleus. It takes work to move an electron farther away from the nucleus, so the more distant the electrons are from the nucleus, the greater their potential energy. Unlike the continuous flow of water downhill, changes in the potential energy of electrons can occur only m steps of fixed amounts. An electron having a certain discrete amount of energy is something like a ball on a staircase (Figure 2.7a). The ball can have different amounts of potential energy, depending on which step it is on, but it cannot spend much time between the steps. An electron cannot exist in between its fixed states of potential energy. The different states of potential energy that electrons have in an atom are called energy levels. An electron's energy7 level is correlated with its average distance from the nucleus; these average distances are represented symbolically by electron shells (Figure 2.7b). The first shell is closest to the nucleus, and electrons in this shell have the lowest potential energy. Electrons in the second shell have more energy, electrons in the third shell more energy still, and so on. An electron can change the shell it occupies, but only by absorbing or losing an amount o{ energy equal to the difference in potential energy between its position in the old shell and that in the new shell. When an electron absorbs energy, it moves to a shell farther out from the nucleus. For example, light energy can excite an electron to a higher energy level. (Indeed, this is the first step taken when plants harness the energy of sunlight for photosynthesis, the process that produces food from carbon dioxide and water.) When an electron loses energy, it "falls back'1 to a

shell closer to the nucleus, and the lost energy is usually released to the environment in the form of heat. For example, sunlight excites electrons in the paint of a dark car to higher energy levels. When the electrons fall back to their original levels, the surface of the car heats up. This thermal energy can be transferred to the air or to your hand if you touch Lhe car.

(a) A ball bouncing down a flight of stairs provides an analogy for energy levels of electrons, because the ball can only rest on each step, not between steps.

Third energy level (shell)

Electron Configuration and Chemical Properties Second energy level (shell)

First energy level (shell)

Atomic nucleus

(b) An electron can move from one level to another only if the energy it gains or loses is exactly equal to the difference in energy between the two levels. Arrows indicate some of the step-wise changes in potential energy that are possible.

A Figure 2.7 Energy levels of an atom's electrons. Electrons exist only at fixed levels of potential energy, which are also called electron shells.

The chemical behavior of an atom is determined by its electron configuration—that is, the distribution of electrons in the atoms electron shells. Beginning with hydrogen, the simplest atom, we can imagine building the atoms of the other elements by adding I proton and 1 electron at a time (along with an appropriate number of neutrons). Figure 2.8, an abbreviated version of what is called the periodic table of the elements, shows this distribution of electrons for the first 18 elements, from hydrogen (jH) to argon (18Ar). The elements are arranged in three rows, or periods, corresponding to the number of electron shells in their atoms. The left-to-right sequence of elements in each row corresponds to the sequential addition of electrons (and protons). Hydrogens 1 electron and helium's 2 electrons are located in the first shell. Electrons, like all matter, tend to exist in the

Hydrogen TH

First shell

Lithium

Beryllium

Carbon fit

Nitrogen 7N

Oxygen SO

Fluorine £

Second shell

Third shell

A Figure 2.8 Electron-shell diagrams of the first 18 elements in the periodic t a b l e . In a standard pperiodic table, information for each element is presented as shown for helium in the inset. In the diagrams in this

modified table, electrons are shown as blue dots and electron shells (representing energy levels) as concentric rings. We are using these electron-shell diagrams as a convenient way to picture the distribution of an atom's electrons

among its electron shells, but keep in mind that these are simplified models. The elements are arranged in rows, each representing the filling of an electron shell. As electrons are added, they occupy the lowest available shell.

CHAPTER 2

The Chemical Context of Life

37

lowest available state of potential energy, which they have in the first shell However, the first shell can hold no more than 2 electrons; thus, there are only two elements (hydrogen and helium) in the first row of the table. An atom with more than 2 electrons must use higher shells because the first shell is full. The next element, lithium, has 3 electrons. Two of these electrons fill the first shell while the third electron occupies the second shell. The second shell holds a maximum of 8 electrons. Neon, at ihe end of the second row, has 8 electrons in the second shell, giving it a total of 10 electrons. The chemical behavior of an atom depends mostly on the number of electrons in its outermost shell. We call those outer electrons valence electrons and the outermost electron shell the valence shell. In the case of lithium, there is only 1 valence electron, and the second shell is the valence shell. Atoms with the same number of electrons in their valence shells exhibit similar chemical behavior. For example, fluorine (F) and chlorine (Cl) both have 7 valence electrons, and both combine with the element sodium to form compounds (see Figure 2.2). An atom with a completed valence shell is unreactive; that is, it will not interact readily with other atoms it encounters. At the far right of the periodic table are helium, neon, and argon,

the only three elements shown in Figure 2.8 that have full valence shells. These elements are said to be inert, meaning chemically unreactive. All the other atoms in Figure 2.8 are chemically reactive because they have incomplete valence shells.

Electron Orbitals Early in the 20th century, the electron shells of an atom were •visualized as concentric paths of electrons orbiting the nucleus, somewhat like planets orbiting the sun. It is still convenient to use two-dimensional concentric-circle diagrams to symbolize electron shells, as in Figure 2.8, if we bear in mind that an electron shell represents the average distance of an electron from the nucleus. This is only a model, however, and it does not give a real picture of an atom. In reality, we can never know the exact path of an electron. What we can do instead is describe the space in which an electron spends most of its time. The three-dimensional space where an electron is found 90% of the time is called an orbital. Fach electron shell consists of a specific number of orbitals of distinctive shapes and orientations (Figure 2.9). You can

Electron orbitals. Each orbital holds up to two electrons.

Three 2p orbitals

Is, 2s, and 2p orbitals

Electron-shell diagrams. Each shell is shown with its maximum number of electrons, grouped in pairs.

(a) First shell (maximum 2 electrons)

A Figure 2.9 Electron orbitals. The three dimensional shapes in the top half of this figure represent electron orbitals—the volumes of space where the electrons of an atom are most likely to be found. Each orbital hoids a maximum of 2 electrons. The bottom half of the figure shows the corresponding electron-

38

UNIT O N E

The Chemistry of Life

(b) Second shell (maximum 8 electrons) shell diagrams, (a) The first electron shell has one spherical (5) orbital, designated 1s. (b)The second and all higher shells each have one larger s orbital (designated 25 for the second shell) plus three dumbbell-shaped orbitals called p orbitals (2p for the second shell). The three 2p orbitals lie at right angles to one another

(c) Neon, with two filled shells (10 electrons)

along imaginary x-, y- and z-axes of the atom. Each 2p orbital is outlined here in a different color, (c) To symbolize the electron orbitals of the element neon, which has a total of 10 electrons, we superimpose the 1s orbital of the first shell and the 2s and three 2p orbitals of the second shell.

think of an orbital as a component of an electron shell. (Recall that an electron shell corresponds to a particular energy level.) The first electron shell has only one spherical s orbital (called Is), but the second shell has four orbitals: one large spherical s orbital (called 2s) and three dumbbell-shaped p orbitals (called 2p orbitals). Each 1p orbital is oriented at right angles to the other two 2p orbitals (see Figure 2.9). (The third and higher electron shells also have s and p orbitals, as well as orbitals of more complex shapes.) No more than 2 electrons can occupy a single orbital. The first electron shell can therefore accommodate a maximum of 2 electrons in its s orbital. The lone electron of a hydrogen atom occupies the Ls orbital, as do the 2 electrons oi a helium atom. The four orbitals of the second electron shell can hold a maximum of 8 electrons. Electrons in each of the four orbitals tiave nearly the same energy but they move in different volumes of space. The reactivity of atoms arises from the presence of unpaired electrons in one or more orbitals of their valence shells. Notice that the electron configurations in Figure 2.8 build up with the addition of 1 electron at a time. For simplicity, we place 1 electron on each side of the outer shell until the shell is half full, and then pair up electrons until the shell is full. When atoms interact to complete their valence shells, it is the unpaired electrons that are involved.

The formation and function of molecules depend on chemical bonding between atoms Now that we have looked at the structure of atoms, we can move up the hierarchy of organization and see how atoms combine to form molecules and ionic compounds. Atoms with incomplete valence shells can interact with certain other atoms in such a way that each partner completes its valence shell: The atoms either share or transfer valence electrons. These interactions usually result in atoms staying close together, held by attractions called chemical bonds. The strongest kinds of chemical bonds are covalent bonds and ionic bonds.

Covalent Bonds A covalent bond is the sharing of a pair of valence electrons by two atoms. For example, lets consider what happens when two hydrogen atoms approach each other. Recall that hydrogen has 1 valence electron in the first shell, but the shell's capacity is 2 electrons. When the two hydrogen atoms come close enough for their Is orbitals to overlap, they share their electrons (Figure 2.10). Each hydrogen atom now has 2 electrons associated with it in what amounts to a completed valence shell, shown in an electron-shell diagram in Hydrogen atoms (2 H)

Concept Check 1. A lithium atom has 3 protons and 4 neutrons. What is its atomic mass in daltons? 2. A nitrogen atom has 7 protons, and the most common isotope of nitrogen has 7 neutrons. A radioactive isotope of nitrogen has 8 neutrons. What is the atomic number and mass number of this radioactive nitrogen? Write as a chemical symbol with a subscript and superscript. 3. Look at Figure 2.8, and determine the atomic number of magnesium. How many protons and electrons does it have? How many electron shells? How many valence electrons are in the valence shell? 4. In an electron-shell diagram oi phosphorus, in which shell do electrons have the most potential energy? In which shell do electrons have the least potential energy? 5. How many electrons does fluorine have? How many electron shells? Name the orbitals that are occupied. How many unpaired electrons does fluorine have?

O In each hydrogen atom, the single electron is held in its orbital by its attraction to the proton in the nucleus.

When two hydrogen atoms approach each other, the electron of each atom is also attracted to the proton in the other nucleus.

The two electrons become shared in a covalent bond, forming an H2 molecule.

For suggested answers, see Appendix A. A Figure 2.10 Formation of a covalent bond.

CHAPTER 2

The Chemica < •••• itexi o\ I.

Figure 2.11a. Two or more atoms held together by covalent bonds constitute a molecule. In this case, we have formed a hydrogen molecule. We can abbreviate the structure of this

Name (molecular formula)

Electronshelf diagram

Structural formula

Spacefilling model

molecule as H = H , where the line represents a single covalent bond, or simply a single bond—that is, a pair of shared electrons. This notation, which represents both atoms and bonding, is called a structural formula. We can abbreviate even further by writing H2, a molecular formula indicating simply that the molecule consists of two atoms of hydrogen. With 6 electrons in its second electron shell, oxygen needs 2 more electrons to complete its valence shell. Two oxygen atoms form a molecule by sharing two pairs of valence electrons (Figure 2.11b). The atoms are thus joined by what is called a double covalent bond, or simply a double bond. Each atom that can share valence electrons has a bonding capacity corresponding to the number of covalent bonds the atom can form. When the bonds form, they give the atom a full complement of electrons in the valence shell. The bonding capacity of oxygen, for example, is 2. This bonding capacity is called the atom's valence and usually equals the number of unpaired electrons in the atom's outermost (valence) shell. See if you can determine the valences of hydrogen, oxygen, nitrogen, and carbon by studying the electron configurations in Figure 2.8. By counting the unpaired electrons, you can see that the valence of hydrogen is 1; oxygen, 2; nitrogen, 3; and carbon, 4. A more complicated case is phosphorus (P), another element important to life. Phosphorus can have a valence of 3, as we would predict from its 3 unpaired electrons. In biologically important molecules, however, we can consider it to have a valence of 5, forming three single bonds and one double bond. The molecules H2 and O2 are pure elements, not compounds. (Recall that a compound is a combination of two or more different elements.) An example of a molecule that is a compound is water, with the molecular formula H2O. It takes two atoms of hydrogen to satisfy the valence of one oxygen atom. Figure 2.11c shows the structure of a water molecule. Water is so important to life that Chapter 3 is devoted entirely to its structure and behavior. Another molecule that is a compound is methane, the main component of natural gas, with the molecular formula CH4 (Figure 2.1 Id). It takes four hydrogen atoms, each with a valence of 1. to complement one atom of carbon, with its valence of 4. We will look at many other compounds of carbon in Chapter 4. The attraction of a particular kind of atom for the electrons of a covalent bond is called its electronegativity. The more electronegative an atom, the more strongly it pulls shared electrons toward itself. In a covalent bond between two atoms of the same element, the outcome of the tug-of-war for common electrons is a standoff; the two atoms are equally electronegative. Such a bond, in which the electrons are shared equally, is a nonpolar covalenl bond. For example, the covalent bond of H2 is nonpolar, as is the double bond of O2- In other compounds, however, where one atom is bonded to a more dec40

UNIT O N E

The Chemistry of Life

(a) Hydrogen (H 2 ). Two hydrogen atoms can form a single bond.

(lip)

(fa) Oxygen (O 2 ). Two oxygen atoms share two pairs of electrons to form a double bond.

(c) Water (H 2 O). Two hydrogen atoms and one oxygen atom are joined by covalent bonds to produce a molecule of water.

(d) Methane (CH 4 ). Four hydrogen atoms can satisfy the valence of / one carbon I atom, forming methane.

4

A Figure 2.11 Covalent bonding in four molecules. A single covalent bond consists of a pair of shared electrons. The number of electrons required to complete an atom's valence shell generally determines how many bonds that atom will form. Three ways of indicating bonds are shown; the space-filling model comes closest to representing the actual shape of the molecule (see also Figure 2.16).

tronegative atom, the electrons of the bond are not shared equally. This sort of bond is called a polar covalent bond. Such bonds vary in their polarity, depending on the relative electronegativity of the two atoms. For example, the individual bonds of methane (CH4) are slightly polar because carbon and hydrogen differ slightly in electronegativity. In a more extreme example, the bonds between the oxygen and hydrogen atoms of a water molecule are quite polar (Figure 2.12). Oxygen is one of the most electronegative of the 92 elements, attracting shared electrons much more strongly than hydrogen does. In a covalent bond between oxygen and hydrogen, the electrons spend more time near the oxygen nucleus than they do near the hydrogen nucleus. Because electrons have a negative charge, the unequal sharing of electrons in water causes the oxygen atom to have a partial negative charge (indicated by the Greek letter 8 before a minus sign, 8 —, or '"delta minus") and each hydrogen atom a partial positive charge (8 + , or "delta plus").

E ecause oxygen (O) is more electronegative than hydrogen (H), shared electrons are pulled more toward oxygen.

This results in a partial negative charge on the oxygen and a partial positive charge on the hydrogens.

A Figure 2.12 Polar covalent bonds in a water molecule.

Ionic Bonds In some cases, two atoms are so unequal in their attraction for valence electrons that the more electronegative atom strips an electron completely away from its partner. This is what happens when an atom of sodium ( n Na) encounters an atom of chlorine (-|7Cl) (Figure 2.13). A sodium atom has a total of 11 electrons, with its single valence electron in the third electron shell. A chlorine atom has a total of 17 electrons, with 7 electrons in its valence shell. When these two atoms meet, the lone valence electron of sodium is transferred to the chlorine atom, and both atoms end up with their valence shells complete. (Because sodium no longer has an electron in the third shell, the second shell is now the valence shell.)

charged ion. Because of their opposite charges, cations and anions attract each other; this attraction is called an ionic bond. The transfer of an electron is not the formation ol a bond; rather, it allows a bond to form because it results in two ions. Any two ions of opposite charge can form an ionic bond. The ions need not have acquired their charge by an electron transfer with each other. Compounds formed by ionic bonds are called ionic compounds, or salts. We know the ionic compound sodium chloride (NaCl) as table salt (Figure 2.14). Salts are often found in nature as crystals of various sizes and shapes, each an aggregate of vast numbers of cations and anions bonded by their electrical attraction and arranged in a three-dimensional lattice. A salt crystal does not consist of molecules in the sense that a covalent compound does, because a covalently bonded molecule has a definite size and number of atoms. The formula for an ionic compound, such as NaCl, indicates only the ratio of elements in a crystal of the salt. "NaCl" is not a molecule. < Figure 2.14 A sodium chloride crystal. The sodium ions (Na + ) and chloride ions (Cl~) are held together by ionic bonds. The formula NaCl tells us that the ratio of N a + to C I " is 1:1.

The electron transfer between the two atoms moves one unit of negative charge from sodium to chlorine. Sodium: now with 11 protons but only 10 electrons, has a net electrical charge of 1+. A charged atom (or molecule) is called an ion. When the charge is positive, the ion is specifically called a cation; the sodium atom has become a cation. Conversely, the chlorine atom, having gained an extra electron, now has 17 protons and 18 electrons, giving it a net electrical charge of 1—. it has become a chloride ion-—an anion, or negatively

O The lone valence electron of a sodium atom is transferred to join the 7 valence electrons of a chlorine atom.

> Figure 2.13 Electron transfer and ionic bonding. The attraction between oppositely charged atoms, or ions, is an ionic bond. An ionic bond can form between any two oppositely charged ions, even if they have not been formed by transfer of an electron from one to the other.

Q Each resulting ion has a completed valence shell. An ionic bond can form between the oppositely charged ions.

ciCh bride ion (an anion)

Sodium atom

Sodium chloride (NaCl)

CHAPTER 2

The Chemical Context ofLife

41

Not all salts have equal numbers of cations and anions. For example, the ionic compound magnesium chloride (MgCl2) has two chloride ions for each magnesium ion. Magnesium

(«Mg) must lose 2 outer electrons if the atom is to have a complete valence shell, so it tends to become a cation with a net charge of 2+ (Mg2+). One magnesium cation can therefore form ionic bonds with two chloride anions. The term ion also applies to entire molecules that are electrically charged. In the salt ammonium chloride (NH4C1), for instance, the anion is a single chloride ion (Cl ), but the cation is ammonium (NH4+), a nitrogen atom with four covalently bonded hydrogen atoms. The whole ammonium ion has an electrica] charge of 1+ because it is 1 electron short. Environment affects the strength of ionic bonds. In a dry salt crystal the bonds are so strong that it takes a hammer and chisel to break enough of them to crack the crystal in two. Place the same salt crystal in water, however, and the salt dissolves as the attractions between its ions decrease. In the next chapter, you will learn how water dissolves salts.

Weak Chemical Bonds In living organisms, most of the strongest chemical bonds are covalent ones, which link atoms to form a cell's molecules. But weaker bonding within and between molecules is also indispensable in the cell, where the properties of life emerge from such interactions. The most important large biological molecules are held in their functional form by weak bonds. In addition, when two molecules in the cell make contact, they may adhere temporarily by weak bonds. The reversibility of weak bonding can be an advantage: Two molecules can come together, respond to one another in some way, and then separate. Several types of weak chemical bonds are important in living organisms. One is the ionic bond, which we just discussed. Another type of weak bond, crucial to life, is known as a hydrogen bond. Hydrogen Bonds Among the various kinds of weak chemical bonds, hydrogen bonds are so important in the chemistry of life that they deserve special attention. A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom. In living cells, the electronegative partners involved are usually oxygen or nitrogen atoms. Refer to Figure 2.15 to examine the simple case of hydrogen bonding between water (H2O) and ammonia (NH3). In the next chapter, we'll see how hydrogen bonds between water molecules allow some insects to walk on water. Van der Waals Interactions Even a molecule with nonpolar covalent bonds may have positively and negatively charged regions. Because electrons are in 42

U N I T ONE

The Chemistry o(

A hydrogen bond results from the attraction between the partial positive charge on the hydrogen atom of water and the partial negative charge on the nitrogen atom of ammonia.

• Figure 2.15 A hydrogen bond.

constant motion, they are not always symmetrically distributed in the molecule; at any instant, they may accumulate by chance in one part of the molecule or another. The results are everchanging NUCLEUS of ribosomes; a nucleus has one or more nucleoli Chromatin: material consisting of DNA and proteins; visible as individual chromosomes a dividing cell

Centrosome: region where the cell's microtubules are initiated; in an animal cell, contains a pair of centrioles (function unknown)

Plasma membrane: membrane enclosing the cell

CYTOSKELETON: reinforces cell's shape, functions in cell movement; components are made of protein Microfi la merits Intermediate filaments I Ribosomes: nonmembranous organelles (small brown dots) that make proteins; free in cytoplasm or bound to rough ER or nuclear envelope

Microtubules

Microvil projections that increase the cell's surface area Golgi apparatus: organelle active in synthesis, modification, sorting, and secretion of cell products Peroxisome: organelle with various specialized metabolic functions; produces hydrogen peroxide

100

UNIT TWO

The Cell

Mitochondrion: organelle where cellular respiration occurs and most ATP is generated

Lysosome: digestive organelle where macromolecules are hydrolyzed

In animal cells but not plant cells: Lysosomes Centrioles Flagella (in some plant sperm)

This drawing of a generalized plant cell reveals the similarities and differences between an animal cell and a plant cell. In addition to most of the features seen in an animal cell, a plant cell has membrane-enclosed organelles called plastids. The most important

Nuclear envelope k MJCLEUS

Nucleolus k\ Chromatin I \

Centrosome: region where the cell's microtubules are iiitiated; lacks centrioles in plant cells

Rough endoplasmic retkulum

type of plastid is the chloroplast. which carries out photosynthesis. Many plant cells have a large central vacuole; some may have one or more smaller vacuoles. Outside a plant cell's plasma membrane is a thick cell wall, perforated by channels called plasmodesmata.

If you preview the rest of the chapter now, you'll see Figure 6.9 repeated in miniature as orientation diagrams. In each case, a particular organelle is highlighted, color-coded to its appearance in Figure 6.9. As we take a closer look at individual organelies, the orientation diagrams will help you place those structures in the context of the whole cell. Smooth endoplasmic reticulum

Ribosomes (small brown dots) Central vacuole: prominent organelle in older plant cells; functions include storage, breakdown of waste products, hydrolysis of macromolecules; enlargement of vacuole is a major mechanism of plant growth Tonoplast: membrane enclosing the central vacuole Microfilaments | Intermediate filaments

CYTOSKELETON

Microtubules

Mitochondrion Y Peroxisome Plasma membrane Y Cell wall: outer layer that maintains cell's shape and protects cell from mechanical damage; made of cellulose, other polysaccharides, and protein Wall of adjacent cell

Chloroplast: photosynthetic organelle; converts energy of sunlight to chemical energy stored in sugar molecules

Plasmodesmata: channels through cell walls that connect the cytoplasms of adjacent cells

In plant cells but not animal cells: Chloroplasts Central vacuole and tonoplast Cell wall Plasmodesmata

CHAPTER 6 A Tour of the Cell

101

Concept Check 1. After carefully reviewing Figure 6.9, briefly describe the structure and function of each of the following organelles: nucleus, mitochondrion, chloroplast, central vacuole, endoplasmic reticulum, and Golgi apparatus. For suggested answers, see Appendix A.

Concept

The eukaryotic cell's genetic instructions are housed in the nucleus and carried out by the ribosomes On the first stop ol our detailed tour of the cell, let's look at two organelles involved in the genetic control ol the cell: the nucleus, which houses most of the cell's DNA, and the ribosomes, which use information from the DNA to make proteins.

The Nucleus: Genetic Library of the Cell The nucleus contains most of the genes in the eukaryotic cell (some genes are located in mitochondria and chloroplasts). It is generally the most conspicuous organelle in a eukaryotic cell, averaging about 5 um in diameter. The nuclear envelope encloses the nucleus (Figure 6.10), separating its contents from the cytoplasm. The nuclear envelope is a double membrane. The two membranes, each a lipid bilayer with associated proteins, are separated by a space of 20-40 nm. The envelope is perforated by pores that are about 100 nm in diameter. At the lip of each pore, the inner and outer membranes of the nuclear envelope are continuous. An intricate protein structure called a pore complex lines each pore and regulates the entry and exit of certain large macromolecules and particles. Except at the pores, the nuclear side of the envelope is lined by the nuclear lamina, a netlike array of protein filaments that maintains the shape of the nucleus by mechanically supporting the nuclear envelope. There is also much evidence for a nuclear matrix, a framework of fibers extending throughout the nuclear interior. (In Chapter 19, we will examine possible functions of the nuclear lamina and matrix in organizing the genetic material) Within the nucleus, the DNA is organized Into discrete units called chromosomes, structures that carry the genetic information. Each chromosome is made up of a material called chromatin, a complex of proteins and DNA. Stained chromatin usually appears through both light microscopes and electron microscopes as a diffuse mass. As a cell prepares to 102

" TWO The Cell

divide, however, the thin chromaiin fibers coil up (condense), becoming thick enough to be distinguished as the familiar separate structures we know as chromosomes. Each eukaryotic species has a characteristic number of chromosomes. A typical human ceil, for example, has 46 chromosomes in its nucleus: the exceptions are the sex cells (eggs and sperm), which have only 23 chromosomes in humans. A fruit fly cell has 8 chromosomes in most cells, with 4 in the sex cells. A prominent structure within the nondividing nucleus is the nucleolus (plural, nudeoli)., which appears through the electron microscope as a mass of densely stained granules and fibers adjoining part of the chromatin. Here a special type of RNA called ribasomal RNA (rRNA) is synthesized from instructions in the DNA. Also, proteins imported from the cytoplasm are assembled with rRNA into large and small ribosomal subunits in the nucleolus. These subunits then exit the nucleus through the nuclear pores to the cytoplasm, where a large and a small subunit can assemble into a ribosome. Sometimes there are two or more nucleoli; the number depends on the species and the stage in the cell's reproductive cycle. Recent studies have suggested that the nucleolus may perform additional functions as well. As we saw in Figure 5.25, the nucleus directs protein synthesis by synthesizing messenger RNA (mRNA) according to instructions provided by the DNA. The mRNA is then transported to the cytoplasm via the nuclear pores. Once an mRNA molecule reaches the cytoplasm, ribosomes translate the mRNA's genetic message into the primary structure of a specific polypeptide. This process of transcribing and translating genetic information is described in detail in Chapter 17.

Ribosomes: Protein Factories in the Cell Ribosomes, particles made of ribosomal RNA and protein, are the organelles that carry out protein synthesis (Figure 6.11). Cells that have high rates of protein synthesis have a particularly large number of ribosomes. For example, a human pancreas cell has a few million ribosomes. Not surprisingly, cells active in protein synthesis also have prominent nudeoli. (Keep in mind that both nucleoli and ribosomes, unlike most other organelles, are not enclosed in membrane.) Ribosomes build proteins in two cytoplasmic locales (see Figure 6.11). Free ribosomes are suspended in the cytosol, while bound ribosomes are attached to the outside of the endoplasmic reticulurn or nuclear envelope. Most of the proteins made on free ribosomes function within the cytosol; examples are enzymes that catalyze the first steps of sugar breakdown. Bound ribosomes generally make proteins that are destined either for insertion into membranes, for packaging within certain organeiles such as lysosomes (see Figure 6.9), or for export from the cell (secretion). Cells that specialize in protein secretion—for instance, the cells of the pancreas that secrete digestive enzymes—frequently have a high proportion of bound ribosomes. Bound and free ribosomes are structurally"

Surface of nuclear envelope. TEM of a specimen prepared by a special technique known as freeze-fracture. 0.25 jx

Ribosome

Nuclear lamina (TEM). The netlike lamina lines the inner surface of the nuclear envelope.

Pore complexes (TEM). Each pore is ringed by protein particles.

it Figure 6.10 The nucleus a n d its e n v e l o p e . Within the nucleus are the chromosomes, which appear as a mass of chromatin (DNA and associated proteins), and one or more nucleoli (singular, nucleolus), which function in ribosome synthesis. The nuclear envelope, which consists of t w o membranes separated by a narrow space, is perforated with pores and lined by the nuclear lamina.

Ribosomes

Cytosol Endoplasmic reticulurn (ER)

ee ribosomes Bound ribosomes

< Figure 6.11 Ribosomes. This electron 1

^

'

TEM showing ER and ribosomes

Diagram of a ribosome

micrograph of part of a pancreas cell shows many ribosomes, both free (in the cytosol) and bound (to the endoplasmic reticulum). The simplified diagram of a ribosome shows its t w o subunits.

CHAPTER 6

A Tour of the Cell

103

identical and can alternate between the two roles; the cell adjusts the relative numbers of each as metabolic changes alter the types of proteins that must be synthesized. You will learn

more about ribosome structure and function in Chapter 17.

Concept Check 1. What role do the ribosomes play in carrying out the genetic instructions? 2. Describe the composition of chromatin and of nucleoli and the function(s) of each. For suggested answers, see Appendix A.

plasmk means "within the cytoplasm," and reticulum is Latin for 'little net.") The ER consists of a network of membranous tubules and sacs called cisternae (from the Latin cisterna, a reservoir for a liquid). The ER membrane separates the internal compartment of the ER, called the ER lumen (cavity) or cisternal space, from the cytosol. And because the ER membrane is continuous with the nuclear envelope, the space between the two membranes Q{ the envelope is continuous with the lumen of the ER (Figure 6.12). There are two distinct, though connected, regions of ER that differ in structure and function: smooth ER and rough ER. Smooth ER is so named because its outer surface lacks ribosomes. Rough ER has ribosomes that stud the outer surface of the membrane and thus appears rough through the electron microscope. As already mentioned, ribosomes are also attached to the cytoplasmic side of the nuclear envelopes outer membrane, which is continuous with rough ER.

Concept

The endomembrane system regulates protein traffic and performs metabolic functions in the cell Many of the different membranes of the eukaryotic cell are part of an endomembrane system, which carries out a variety of tasks in the cell. These tasks include synthesis of proteins and their transport into membranes and organelles or out of the cell, metabolism and movement of lipids, and detoxification of poisons. The membranes of this system are related either through direct physical continuity or by the transfer of membrane segments as tiny vesicles (sacs made of membrane). Despite these relationships, the various membranes are not identical in structure and function. Moreover, the thickness, molecular composition, and types of chemical reactions carried out by proteins in a given membrane are not fixed, but may be modified several times during the membrane's life. The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, various kinds of vacuoles, and the plasma membrane (not actually an endomembrane in physical location, but nevertheless related to the endoplasmic reticulum and other internal membranes). We have already discussed the nuclear envelope and will now focus on the endoplasmic reticulum and the other endomembranes to which the endoplasmic reticulum gives rise.

The Endoplasmic Reticulum: Biosynthetic Factory The endoplasmic reticulum (ER) is such an extensive network of membranes that it accounts for more than half the total membrane in many eukaryotic cells. (The word endo104

UNIT TWO

The Cell

Functions of Smooth ER The smooth ER of various cell types functions in diverse metabolic processes. These processes include synthesis of lipids, metabolism of carbohydrates, and detoxification of drugs and poisons. Enzymes of the smooth ER are important to the synthesis of lipids, including oils, phospholipids, and steroids. Among the steroids produced by the smooth ER in animal cells are the sex hormones of vertebrates and the various steroid hormones secreted by the adrenal glands. The cells that actually synthesize and secrete these hormones—in the testes and ovaries, for example—are rich in smooth ER, a structural feature that fits the function of these cells. In the smooth ER, other enzymes help detoxify drugs and poisons, especially in liver cells. Detoxification usually involves adding hydroxyl groups to drugs, making them more soluble and easier to flush from the body. The sedative phenobarbital and other barbiturates are examples of drugs metabolized in this manner by smooth ER in liver cells. In fact, barbiturates, alcohol, and many other drugs induce the proliferation of smooth ER and its associated detoxification enzymes, thus increasing the rate of detoxification. This, in turn, increases tolerance to the drugs, meaning that higher doses are required to achieve a particular effect, such as sedation. Also, because some of the detoxification enzymes have relatively broad action, the proliferation of smooth ER in response to one drug can increase tolerance to other drugs as well. Barbiturate abuse, for example, may decrease the effectiveness of certain antibiotics and other useful drugs. The smooth ER also stores calcium ions. In muscle cells, for example, a specialized smooth ER membrane pumps calcium ions from the cytosol into Lhe ER lumen. When a muscle cell is stimulated by a nerve impulse, calcium ions rush back across the ER membrane into the cytosol and trigger

contraction of the muscle cell. In other cell types, calcium ion release from the smooth ER can trigger different responses. Functions of Rough ER Many types of specialized cells secrete proteins produced by ribosomes attached to rough ER. For example, certain cells in the pancreas secrete the protein insulin, a hormone, into the

bloodstream (see Figure 6.11). As a polypeptide chain grows from a bound rihosome, it is threaded into the ER lumen through a pore formed by a protein complex m the ER membrane. As the new protein enters the ER lumen, it folds into its native conformation. Most secretory proteins are glycoproteins, proteins that have carbohydrates covalently bonded to them. The carbohydrate is attached to the protein in the ER by specialized molecules built into the ER membrane. Once secretory proteins are formed, the ER membrane keeps them separate from the proteins, produced by free ribosomes, that will remain in the cytosol. Secretory proteins depart from the ER wrapped in the membranes of vesicles that bud like bubbles from a specialized region called transitional ER (see Figure 6.12). Vesicles in transit from one part of the cell to another are called transport vesicles; we will learn their fate in the next section. In addition to making secretory proteins, rough ER is a membrane factory for the cell; it grows in place by adding membrane proteins and phosphofipids to its own membrane. As polypeptides destined to be membrane proteins grow from the ribosomes, they are inserted into the ER membrane itself and are anchored there by their hydrophobic portions. The rough ER also makes its own membrane phospholipids; enzymes built into the ER membrane assemble phospholipids from precursors in the cytosol. The ER membrane expands and is transferred in the form of transport vesicles to other components of the en do membrane system.

The Golgi Apparatus: Shipping and Receiving Center

•

•

>

•

-

.

.

A Figure 6.12 Endoplasmic reticulum (ER). A membranous system of interconnected tubules and flattened sacs called cisternae, the ER is also continuous with the nuclear envelope. (The drawing is a cutaway view.) The membrane of the ER encloses a continuous compartment called the ER lumen (or asternal space). Rough ER, which is studded on its outer surface with ribosomes, can be distinguished from smooth ER in the electron micrograph (TEM). Transport vesicles bud off from a region of the rough ER called transitional ER and travel to the Golgi apparatus and other destinations.

After leaving the ER, many transport vesicles travel to the Golgi apparatus. We can think of the Golgi as a center of manufacturing, warehousing, sorting, and shipping. Here, products of the ER are modified and stored and then sent to other destinations. Not surprisingly, the Golgi apparatus is especially extensive in cells specialized for secretion. The Golgi apparatus consists of flattened membranous sacs—cistemae—looking like a stack of pita bread (Figure 6.13, on the next page). A cell may have many or even hundreds of these stacks. The membrane of each cisterna in a stack separates its internal space from the cytosol. Vesicles concentrated in the vicinity of the Golgi apparatus are engaged in tbe transfer of material between the parts of the Golgi and other structures. A Golgi stack has a distinct polarity, with the membranes of cisternae on opposite sides of the stack differing in thickness and molecular composition. The two poles of a Golgi stack are referred to as the as face and the trans face; these act, respectively, as the receiving and shipping departments of the Golgi apparatus. The as face is usually located near the ER. Transport vesicles move material from the ER to the Golgi apparatus. A vesicle that buds from the ER can add its membrane and the contents of its lumen to the cis face by fusing with a CHAPTER 6

A Tour of [he Cell

105

@ Vesicles coalesce to f o r m n e w cis Golgi cisternae Cisternae^"---^^^

© Cisternal maturation: Golgi cisternae move in a cisto-trans direction Q Vesicles form and leave Golgi, carrying specific proteins to other locations or to the plasma membrane for secretion 0 Vesicles transport specific proteins backward to newer Golgi cisternae A Figure 6.13 The Golgi apparatus. The Golgi apparatus consists of stacks of flattened sacs, or cisternae, which, unlike ER cisternae, are not physically connected. (The drawing is a cutaway view.) A Gofgi stack receives and dispatches transport vesicles and the products

The Cel

:••..•

till TEM of Golgi apparatus

they contain. A Golgi stack has a structural and functional polarity, with a cis face that receives vesicles containing ER products and a trans face that dispatches vesicles. The cisternal maturation model suggests that the Golgi cisternae themselves appear to "mature,"

In addition to its finishing work, the Golgi apparatus manufactures certain macromolecules by itself. Many polysaccharides secreted by cells are Golgi products, including pectins and certain other non-cellulose polysaccharides made by plant cells and incorporated along with cellulose into their cell walls. (Cellulose is made by enzymes located within the plasma membrane, which directly deposit this polysaccharide UNIT TWO

':'••

-trans face ("shipping" side of Golgi apparatus)

Golgi membrane. The trans face gives rise to vesicles, which pinch off and travel to other sites. Products of the ER are usually modified during their transit from the els region to the trans region of the Golgi. Proteins and phospholipids of membranes may be altered. For example, various Golgi enzymes modify the carbohydrate portions of glycoproteins. Carbohydrates are first added to proteins in the rough ER, often during the process of polypeptide synthesis. The carbohydrate on the resulting glycoprotein is then modified as it passes through the rest of the ER and the Golgi. The Golgi removes some sugar monomers and substitutes others, producing a large variety of carbohydrates.

106

m*tm moving from the cis to the fransface while carrying some proteins along. In addition, some vesicles recycle enzymes that had been carried forward, in moving cisternae, "backward" to a newer region where their functions are needed (TEM).

on the outside surface.) Golgi products that will be secreted depart from the trans face of the Golgi inside transport vesicles that eventually fuse with the plasma membrane. The Golgi manufactures and refines its products in stages, with different cisternae between the cis and trans regions containing unique teams of enzymes. Until recently, we viewed the Golgi as a static structure, with products in various stages of processing transferred from one cisterna to the next by vesicles. While this may occur, recent research has given rise to a new model of the Golgi as a more dynamic structure. According to the model called the cisternal maturation model, the cisternae of the Golgi actually progress forward from the cis to the trans face of the Golgi, carrying and modifying their protein cargo as they move. Figure 6.13 shows the details of this model. Before a Golgi stack dispatches its products by budding vesicles from the trans face, it sorts these products and targets them for various parts of the cell. Molecular identification tags, such as phosphate groups that have been added to the Golgi

products, aid in sorting by acting like ZIP codes on mailing labels- Finally transport vesicles budded from the Golgi may have external molecules on their membranes that recognize "docking sites" on the surface ol specific organelles or on the plasma membrane, thus targeting them appropriately.

Lysosomes: Digestive Compartments A lysosome is a membranous sac of hydrolytic enzymes that an animal cell uses to digest all kinds of macromolecules. Lysosomal enzymes work best in the acidic environment found in lysosomes. If a lysosome breaks open or leaks its contents, the released enzymes are not very active, because the cytosol has a neutral pH. However, excessive leakage from a large number of lysosomes can destroy a cell by auiodigestion. Hydrolytic enzymes and lysosomal membrane are made by rough ER and then transferred to the Golgi apparatus for further processing. At least some lysosomes probably arise by budding from the trans face of the Golgi apparatus (see Figure 6.13).

Proteins of the inner surface of the lysosomal membrane and the digestive enzymes themselves are thought to be spared from destruction by having three-dimensional conformations that protect vulnerable bonds from enzymatic attack. Lysosomes carry out intracellular digestion in a variety of circumstances. Amoebas and many other protists eat by engulfing smaller organisms or other food particles, a process called phagocytosis (from the Greek phagdn, to eat, and kyios, vessel, referring here to the cell). The food vacuole formed in this way then fuses with, a lysosome, whose enzymes digest the food (Figure 6.14a). Digestion products, including simple sugars, amino acids, and other monomers, pass into the cytosol and become nutrients for the cell. Some human cells also carry out phagocytosis. Among them are macrophages, a type of white blood cell that helps defend the body by engulfing and destroying bacteria and other invaders (see Figure 6.32). Lysosomes also use their hydrolytic enzymes to recycle the cell's own organic material, a process called autophagy. During autophagy, a damaged organelle or small amount of cytosol

Nudeu.s

Lysosome containing two damaged organelles

Peroxisorne' fragment

\ Lysosome

Lysosome contains active hydrolytic enzymes

Food vacuole fuses with lysosome \ ^•Digestive enzymes

\ \

Hydrolytic enzymes digest food particles

Lysosome fuses with vesicle containing damaged organelle

m

\

Hydrolytic enzymes digest organelle components \

\ A

,'•>

1 I

Lysosome

Lysosome Plasma membrane j

\ ^

Digest ion Digestion

Food v a c *

(a) Phagocytosis: lysosome digesting food

A Figure 6.14 Lysosomes, Lysosomes digest (hydrolyze) materials taken into the cell and recycle intracellular materials, (a) Top In this macrophage (a type of white blood cell) from a rat, the lysosomes are very dark because of a specific stain that reacts with one of the

^Vesicle containing damaged mitochondrion (b) Autophagy: lysosome breaking down damaged organelle products of digestion within the lysosome (TEM). Macrophages ingest bacteria and viruses and destroy them using lysosomes. Bottom This diagram shows one lysosome fusing with a food vacuoie during the process of phagocytosis. (b) Top In the cytoplasm of this rat liver cell, a

lysosome has engulfed two disabled organelles, a mitochondrion and a peroxisome, in the process of autophagy (TEM). Bottom This diagram shows a lysosome fusing with a vesicle containing a damaged mitochondrion.

CHAPTER 6 A Tour of ilic G_\l

107

becomes surrounded by a membrane, and a lysosome fuses with this vesicle (Figure 6.14b). The lysosomal enzymes dismantle the enclosed material, and the organic monomers are returned to the cytosol for reuse. With the help of lysosomes, the cell continually renews itself. A human liver cell, for example, recycles half of its macromolecules each week. The cells of people with inherited lysosomal storage diseases lack a functioning hydrolytic enzyme normally present in lysosomes. The lysosomes become engorged with indigestible substrates, which begin to interfere with other cellular activities. In Tay-Sachs disease, for example, a lipid-digesting enzyme is missing or inactive, and the brain becomes impaired by an accumulation of lipids in the cells. Fortunately lysosomal storage diseases are rare in the general population.

Vacuoles: Diverse Maintenance Compartments A plant or fungal cell may have one or several vacuoles. While vacuoles carry out hydrolysis and are thus similar to lysosomes, they carry out other functions as well. Food vacuoles, formed by phagocytosis, have already been mentioned (see Figure 6.14a). Many freshwater protists have contractile vacuoles that pump excess water out of the cell, thereby maintaining the appropriate concentration of salts and other molecules (see Figure 7.14). Mature plant cells generally contain a large central vacuole (Figure 6.15) enclosed by a membrane called the tonoplast. The central vacuole develops by the

Central vacuole

coalescence of smaller vacuoles, themselves derived from the endoplasmic reticulum and Golgi apparatus. The vacuole is in this way an integral part of a plant cells endomembrane system. Like all cellular membranes, the tonoplast is selective in transporting solutes; as a result, the solution inside the vacuole, called cell sap, differs in composition horn the cytosol. The plant cell's central vacuole is a versatile compartment. It can hold reserves of important organic compounds, such as the proteins stockpiled in the vacuoles of storage cells in seeds. It is also the plant cells main repository of inorganic ions, such as potassium and chloride. Many plant cells use their vacuoles as disposal sites for metabolic by-products that would endanger the cell ii they accumulated in the cytosol. Some vacuoles contain pigments that color the cells, such as the red and blue pigments of petals that help attract pollinating insects to flowers. Vacuoles may also help protect the plant against predators by containing compounds that are poisonous or unpalatable to animals. The vacuole has a major role in the growth of plant cells, which enlarge as their vacuoles absorb water, enabling the cell to become larger with a minimal investment in new cytoplasm. And because of ihe large vacuole, the cytosol often occupies only a thin layer between the plasma membrane and the tonoplast, so the ratio of membrane surface to cytosolic volume is great, even for a large plant cell.

The Endomembrane System: A Review Figure 6.16 reviews the endomembrane system, showing the flow ol membrane lipids and proteins through the various organelles. As the membrane moves from the F.R to the Golgi and then elsewhere, its molecular composition and metabolic functions are modified, along with those of its contents. The endomembrane system is a complex and dynamic player in the cells compartmental organization. We'll continue our tour of the cell with some membranous organelles that are not closely related to the endomembrane system, but play crucial roles m the energy transformations carried out by cells. Concept Check

Chloroplast

A Figure 6.15 The plant cell vacuole. The central vacuole is usually the largest compartment in a plant cell; the rest of the cytoplasm is generally confined to a narrow zone between the vacuolar membrane (tonoplast) and the plasma membrane (TEM).

108

UNIT T W O

The Cell

1. Describe the structural and functional distinctions between rough and smooth ER. 2. Imagine a protein that functions in the ER, but requires modification in the Golgi apparatus before it can achieve that function. Describe the protein's path through the cell, starting with the mRNA molecule that specifies the protein. 3. How do transport vesicles serve to integrate the endomembrcrne svsti1:" ? For suggested answers, see Appendix A.

Nuclear envelope is connected to rough ER, which is also continuous with smooth ER

Q Membranes and proteins produced by the ER flow in the form of transport vesicles to the Golgi

0 Golgi pinches off transport jesicles and other vesicles that give rise to lysosomes and vacuoles

© Lysosome available for fusion with another vesicle for digestion

@ Transport vesicle carries proteins to plasma membrane for secretion

© Plasma membrane expands by fusion of vesicles; proteins are secreted from cell

A Figure 6.16 Review: relationships among organelles of the endomembrane system. The red arrows show some of the migration pathways for membranes and the materials they enclose.

Concept

Mitochondria and chloroplasts change energy from one form to another Organisms transform energy they acquire from their surroundings. In eukaryotic cells, mitochondria and chloroplasts are the organelles that convert energy to forms thai cells can use for work. Mitochondria (singular, mitochondrion) are the sites of cellular respiration, the metabolic process that generates ATP by extracting energy from sugars, fats, and other fuels with the help of oxygen. Chloroplasts, found only in plants and algae, are the sites of photosynthesis. They convert solar energy to chemical energy by absorbing sunlight and using it to drive the synthesis of organic compounds such as sugars from carbon dioxide and water. Although mitochondria and chloroplasts are enclosed by membranes, they are not pan of the endomembrane system. In contrast to organelles of the endomembrane system, each of these organelles has at least two membranes separating the innermost space from the cytosol. Their membrane proteins

are made not by the ER, but by free ribosomes in the cytosol and by ribosomes contained within these organelles themselves. Not only do these organelles have ribosomes, but they also contain a small amount of DNA. It is this DNA that programs the synthesis of the proteins made on the organelles own ribosomes. (Proteins imported from the cytosol—constituting most of the organelles proteins—are programmed by nuclear DNA.) Mitochondria and chloroplasts are semiauLonomous organelles that grow and reproduce within the cell. In Chapters 9 and 10, we will focus on how mitochondria and chloroplasts function. We will consider the evolution of these organelles in Chapter 28. Here we are concerned mainly with the structure of these energy transformers. In this section, we will also consider the peroxisome, an oxidative organelle that is not part of the endomembrane system. Like mitochondria and chloroplasts, the peroxisome imports its proteins primarily from the cytosol.

Mitochondria: Chemical Energy Conversion Mitochondria are found in nearly all eukaryotic cells, including those of plants, animals, fungi, and protists. Some cells have a single large mitochondrion, but more often a cell has CHAPTER 6 A Tour of the Cell

109

hundreds or even thousands of mitochondria; the number is correlated with the cell's level of metabolic activity. For example, motile or contractile cells have proportionally more mitochondria per volume than less active cells. Mitochondria are about 1—10 urn long. Time-lapse films ol living cells reveal mitochondria moving around, changing their shapes, and dividing in two, unlike the static cylinders seen in electron micrographs of dead cells. The mitochondrion is enclosed by two membranes, each a phospholipid bilayer with a unique collection of embedded proteins (Figure 6.17). The outer membrane is smooth, but the inner membrane is convoluted, with infoldings called cristae. The inner membrane divides the mitochondrion into two internal compartments. The first is the mtermembrane space, the narrow region between the inner and outer membranes. The second compartment, the mitochondrial matrix, is enclosed by the inner membrane. The matrix contains many different enzymes as well as the mitochondrial DNA and ribosomes. Some of the metabolic steps of cellular respiration are catalyzed by enzymes in the matrix. Other proteins that function in respiration, including the enzyme that makes ATT^ are built into the inner membrane. As highly folded surfaces, the cristae give the inner mitochondrial membrane a large surface area for these proteins, thus enhancing the productivity of cellular respiration. This is another example of structure fitting function.

Chloroplasts: Capture of Light Energy The chloroplasi is a specialized member of a family of closely related plant organelles called plastids. Amyloplasts are colorless plastids that store starch (amylose), particularly in roots and tubers. Oiromoplasts have pigments that give fruits and

flowers their orange and yellow hues. Chloroplasts contain the green pigment chlorophyll, along with enzymes and other molecules that function in the photosynthetic production of sugar. These lens-shaped organelles, measuring about 2 urn by 5 jim, are found in leaves and other green organs of plants and in algae (Figure 6.18). The contents of a chloroplast are partitioned from the cytosoi by an envelope consisting of two membranes separated by a very narrow intermembrane space. Inside the chloroplast is another membranous system in the form of flattened, interconnected sacs called thylakoids. In some regions, thylakoids are stacked like poker chips; each stack is called a granum (plural, grand). The fluid outside the thylakoids is the stroma, which contains the chloroplast DNA and ribosomes as well as many enzymes. The membranes of the chloroplast divide the chloroplast space into three compartments: the intermembrane space, the stroma, and the thylakoid space. In Chapter 10, you will learn how this compartmental organization enables the chloroplast to convert light energy to chemical energy during photosynthesis. As with mitochondria, the static and rigid appearance of chloroplasts in micrographs or schematic diagrams is not true to their dynamic behavior in the living cell. Their shapes are changeable, and they grow and occasionally pinch in two, reproducing themselves. They are mobile and move around the cell with mitochondria and other organelles along tracks of the cytoskeleton, a structural network we will consider later in this chapter.

Peroxisomes: Oxidation The peroxisome is a specialized metabolic compartment bounded by a single membrane (Figure 6.19). Peroxisomes

Mitochondrion termembrane space / Outer__ membrane

^ Figure 6.17 The mitochondrion, site of cellular respiration. The inner and outer membranes of the mitochondrion are evident in the drawing and micrograph (TEM). The cristae are infoldings of the inner membrane. The cutaway drawing shows the t w o compartments bounded by the membranes: the intermembrane space and the mitochondrial matrix. Free ribosomes are seen in the matrix, along with one to several copies of the mitochondrial genome (DNA). The DNA molecules are usually circular and are attached to the inner mitochondrial membrane.

110

UNIT TWO

The Cell

f Figure 6.18 The chloroplast, site of photosynthesis. A chloropiast is enclosed by two membranes separated by a narrow intermembrane space that constitutes an outer compartment. The inner membrane encloses a second compartment containing the fluid called stroma. Free ribosomes and copies of the chloroplast genome (DNA) are present in the stroma. The stroma surrounds a third compartment, the thylakoid space, delineated by the thylakoid membrane. Interconnected thylakoid sacs (thyiakoids) are stacked to form structures called grana (singular, granum), which are further connected by thin tubules between individual thyiakoids (TEM).

Thylakoid

contain enzymes that transfer hydrogen from various substrates to oxygen, producing hydrogen peroxide (H2O2) as a by-product, from which the organelle derives its name. These reactions may have many different functions. Some peroxisomes use oxygen to break fatty acids down into smaller molecules that can then be transported to mitochondria,

where they are used as fuel for cellular respiration. Peroxisomes in the liver detoxify alcohol and other harmful compounds by transferring hydrogen from the poisons to oxygen. The H 2 O 2 formed by peroxisome metabolism is itself toxic, but the organelle contains an enzyme that converts the H2O2 to water. Enclosing in the same space both the enzymes that produce hydrogen peroxide and those that dispose of this toxic compound is another example of how the cell's compartmental structure is crucial to its functions. Specialized peroxisomes called glyoxysom.es are found in the fat-storing tissues of plant seeds. These organelles contain enzymes that initiate the conversion of fatty acids to sugar, which the emerging seedling can use as a source of energy and carbon until it is able to produce its own sugar by photosynthesis. Unlike lysosomes, peroxisomes do not bud from the endomembrane system. They grow larger by incorporating proteins made primarily in the cytosol, lipids made in the ER, and lipids synthesized within the peroxisome itself. Peroxisomes may increase in number by splitting in two when they reach a certain size. Concent Check '

1 jim At> Figure 6.19 Peroxisomes. Peroxisomes are roughly spherical and often have a granular or crystalline core that is thought to be a dense collection of enzyme molecules. This peroxisome is in a leaf cell. Notice its proximity to t w o chloroplasts and a mitochondrion. These organelles cooperate with peroxisomes in certain metabolic functions (TEM).

1. Describe at least two common characteristics of chloroplasts and mitochondria. 2. Explain the characteristics of mitochondria and chloroplasts that place them in a separate category from organelles in the endomembrane system. For suggested answers, see Appendix A.

CHAPTER 6

A Tour of the Cell

111

Concept 13

The cytoskeleton is a network of fibers that organizes structures and activities in the cell In the early days of electron microscopy, biologists thought that the organelles of a eukaryotic cell floated freely in the cytosol. But improvements in both light microscopy and electron microscopy have revealed the cytoskeleton, a network of fibers extending throughout the cytoplasm (Figure 6.20). The cytoskeleton, which plays a major role in organizing the structures and activities of the cell, is composed of three types of molecular structures: microtubules, microfilaments, and intermediate filaments (Table 6.1).

Roles of the Cytoskeleton: Support, Motility, and Regulation The most obvious function of the cytoskeleton is to give mechanical support to the cell and maintain its shape. This is especially important for animal cells, which lack walls. The remarkable strength and resilience of the cytoskeleton as a whole is based on its architecture- Like a geodesic dome, the cytoskeleton is stabilized by a balance between opposing forces exerted by its elements. And just as the skeleton of an animal helps fix the positions of other body parts, the cytoskeleton provides anchorage for many organelles and even cytosolic enzyme molecules. The cytoskeleton is more dynamic than an animal skeleton, however. It can be quickly dismantled in one part of the cell and reassembled In a new location, changing the shape of the cell. The cytoskeleton is also involved in several types of cell motility (movement). The term cell motility encompasses both

changes in cell location and more limited movements of parts of the cell- Cell motility generally requires the interaction of the cytoskeleton with proteins called motor proteins. Examples of such cell motility abound.. Cytoskeletal elements and motor proteins work together with plasma membrane molecules to allow whole cells to move along fibers outside the cell. Motor proteins bring about the movements of cilia and flagella by gripping microtubules within those organelles and propelling them past each other. A similar mechanism involving microfilaments causes muscle cells to contract. Inside the cell, vesicles often travel to their destinations along "monorails" provided by the cytoskeleton. For example, this is how vesicles containing neurotransmitter molecules migrate to the tips of axons, the long extensions of nerve cells that release these molecules as chemical signals to adjacent nerve cells (Figure 6.21). The vesicles that bud off from the ER travel to the Golgi along tracks built of cytoskeletal elements. It is the cytoskeleton that manipulates the plasma membrane to form food vacuoles during phagocytosis. Finally, the streaming of cytoplasm that circulates materials within many large plant cells is yet another kind of cellular movement brought about by components of the cytoskeleton.

?ceptor for otor protein

Motor protein (ATP powered)

Microtubuie of cytoskefeton

(a) Motor proteins that attach to receptors on organelles can "walk" the organeiles aiong microtubules or, in some cases, microfilaments.

0,25 |im

M

0.25

m

•

Microfilaments

A Figure 6.20 The cytoskeleton. In this TEM, prepared by a method known as deep-etching, the thicker, hollow microtubules and the thinner, solid microfilaments are visible. A third component of the cytoskeleton, intermediate filaments, is not evident here.

112

UNIT TWO

The Cell

(b) Vesicles containing neurotransmitters migrate to the tips of nerve cell axons via the mechanism in (a). In this SEM of a squid giant axon, t w o vesicles can be seen moving along a microtubuie. (A separate part of the experiment provided the evidence that they were in fact moving.)

A Figure 6.21 Motor proteins and the cytoskeleton.

Table 6.1 The Structure and Function of the Cytoskeleton Microtubules (Tubulin Polymers)

Microfilaments (Actin Filaments)

Intermediate Filaments

Structure

Hollow tubes; wall consists of 13 columns of tubulin molecules

Two intertwined strands of actin, each a polymer of actin subunits

Fibrous proteins supercoiled into thicker cables

Diameter

25 nm with 15-nm lumen

7 nm

8-12 nm

Protein subunits

Tubulin, consisting of a-iuhulin and fi-tubulin

Actin

One of several different proteins of the keratin family, depending on cell type

Main i unctions

Maintenance of cell shape (compression-resisting "girders")

Maintenance of cell shape (tension-bearing elements)

Maintenance of cell shape (tension-bearing elements)

Cell motility (as in cilia or flagella) Chromosome movements in cell division Organdie movements

Changes in cell shape

Anchorage of nucleus and certain other organelles

Property

10 urn

Muscle contraction Cytoplasmic streaming

Formation of nuclear lamina

Cell motility (as in pseudopodia) Cell division (cleavage furrow formation) 10 jim

5 urn

Micrographs of fibroblasts, a favorite cell type for cell biology studies. Each has been experimental ly treated to fluorescently tag the structure of interest.

The most recent addition to the list of possible cytoskeletal functions is the regulation of biochemical activities in the cell. Mounting evidence suggests that the cytoskeleton can transmit mechanical forces exerted by extracellular molecules via surface proteins of the cell to its interior—and even into the nucleus. In one experiment, investigators used a micromanipulation device to pull on certain plasma membrane proteins attached to the cytoskeleton. A video microscope captured almost instantaneous rearrangements of nucleoli and other structures in the nucleus. In this way, the transmission of

naturally occurring mechanical signals by the cytoskeleton may help regulate cell function.

Components of the Cytoskeleton Now lets look more closely at the three main types of fibers that make up the cytoskeleton (see Table 6.1). Microtubules are the thickest of the three types; microfilaments (also called actin filaments) are the thinnest; and intermediate filaments are fibers with diameters in a middle range.

CHAPTER 6 A Tour of [he Cell

113

Microtubules Microtubules are found in the cytoplasm of all eukaryotic cells. They are hollow rods measuring about 25 nra in diameter and from 200 nm to 25 urn in length. The wall of the hollow tube is constructed from a globular protein called tubulin. Each tubulin molecule is a dimer consisting of two slightly different polypeptide subunits, a-tubulin and (3-iubulin. A microtubule grows in length by adding tubulin dimers to its ends. Microtubules can be disassembled and their tubulin used to build microtubules elsewhere in the cell. Microtubules shape and support the cell and also serve as tracks along which organelles equipped with motor proteins can move (see Figure 6.21). For example, microtubules guide secretory vesicles from the Golgi apparatus to the plasma membrane. Microtubules are also responsible for the separation of chromosomes during cell division (see Chapter 12). Centrosomes and Centrioles. In many cells, microtubules grow out from a centrosome, a region often located near the nucleus that is considered to be a "microtubule-organizing center." These microtubules function as compression-resisting girders of the cytoskeleton. Within the cenlrosome of an animal cell are a pair of centrioles, each composed oi nine sets of triplet microtubules arranged in a ring (Figure 6.22). Before a cell divides, the centrioles replicate. Although centrioles may help organize microtubule assembly, they are not essential for this function in all eukaryotes; centrosomes of most plants lack centrioles, but have well-organized microtubules. Cilia and Flagella. In eukaryotes, a specialized arrangement of microtubules is responsible for the beating of flagella (singular, flagelium) and cilia (singular, cilium), locomotor appendages that protrude from some cells. Many unicellular eukaryotic organisms are propelled through water by cilia or flagella, and the sperm of animals, algae, and some plants have flagella. When cilia or flagella extend from cells that are held in place as part, of a tissue layer, they can move fluid over the surface of the tissue. For example, the ciliated lining of the windpipe sweeps mucus containing trapped debris out of the lungs (see Figure 6.4). In a woman's reproductive tract, the cilia lining the oviducts (fallopian tubes) help move an egg toward the uterus. Cilia usually occur in large numbers on the cell surface. They are about 0.25 um in diameter and about 2-20 um in length. Flagella are the same diameter but longer than cilia, measuring 10-200 um in length. Also, flagella are usually limited to just one or a few per cell. Flagella and cilia differ in their beating patterns (Figure 6,23). A flagellum. has an undulating motion that generates force in the same direction as the flagellums axis. In contrast, cilia work more like oars, with alternating power and recovery strokes generating force in a direction perpendicular to the cilium's axis. Though different in length, number per cell, and beating pattern, cilia and flagella share a common ultrastructure. A 114

Longitudinal section of one centriole

Microtubules

Cross section of the other centriole

A Figure 6.22 Centrosome containing a pair of centrioles. An animal cell has a pair of centrioles within its centrosome, the regior near the nucleus where the cell's microtubules are initiated. The centrioles, each about 250 nm (0.25 um) in diameter, are found at right angles to each other, and each is made up of nine sets of three microtubules. The blue portions of the drawing represent nontubulin proteins that connect the microtubule triplets (TEM).

cilium or flagellum has a core of microLubules sheathed in an extension of the plasma membrane (Figure 6.24). Nine doublets of microtubules, the members of each pair sharing part o their walls, are arranged in a ring. In the center of the ring are two single microtubules. This arrangement, referred to as the "9 + 2" pattern, is found in nearly all eukaryotic flagella and cilia. (The flagella of motile prokaryotes, discussed in Chapter 27, do not contain microtubules.) Flexible "wagon wheels" oi cross-linking proteins, evenly spaced along the length of the cilium or flagellum, connect the outer doublets to each other (the wheel rim) and to the two central microtubules (the wheel spokes). Each outer doublet also has pairs of side-arms spaced along its length and reaching toward the neighboring doublet: these are motor proteins. The microtubule assembly ol a cilium or flagellum is anchored in the cell by a basal body, which is structurally identical to a centriole. In fact, in many animals (including humans), the basal body of the fertilizing sperms flagellum enters the egg and becomes a centriole. Each motor protein extending from one microtubule doublet to the next is a large protein called dynein, which is composed

-4 Figure 6.23 A comparison of the beating of flagella and cilia.

(a) M o t i o n of flagella. A flagellum usually undulates, its snakelike motion driving a cell in the same direction as the axis of the flagellum. Propulsion of a human sperm cell is an example of flagellate locomotion (LM).

(b) Motion of cilia. Cilia have a backand-forth motion that moves the cell in a direction perpendicular to the axis of the cilium. A dense nap of cilia, beating at a rate of about 40 to 60 strokes a second, covers this Colpidium, a freshwater protozoan (SEM).

(b) A cross section through the cilium shows the "9 + 2" arrangement of microtubules (TEM). The outer microtubule doublets and the two central microtubules are held together by cross-linking proteins (purple in art), including the radial spokes. The doublets also have attached motor proteins, the dynein arms (red in art).

0.5 urn (a) A longitudinal section of a cilium shows microtubules running the length of the structure (TEM).

0.1 ftm

-Triplets

(c) Basal body: The nine outer doublets of a cilium or flagellum extend into the basal body, where each doublet joins another microtubule to form a ring of nine triplets. Each triplet is connected to the next by nontubulin proteins (blue). The t w o central microtubules terminate above the basal body (TEM). Cross section of basal body

Figure 6.24 Ultrastructure of a eukaryotic flagellum or cilium.

CHAPTER 6

A Tour of the Cell

115

of several polypeptides. These dynein arms are responsible for the bending movements oi cilia and flagella. A dynein arm performs a complex cycle of movements caused by changes in the conformation of the protein, with ATP providing the energy for these changes (Figure 6.25). The mechanics of dynein "walking" are reminiscent of a cat climbing a tree by attaching its claws, moving its legs, releasing its front claws, and grabbing again farther up the tree. Similarly, the dynein arms of one doublet attach to an adjacent doublet and pull so that the doublets slide past each other in opposite directions. The arms then release from the other doublet and reattach a little farther along its length. Without any restraints on the movement of the microtubule doublets, one doublet would continue to end to begin at the base of the cilium or flagellum and move c jtward toward the tip. Many successive bends, such as the c les shown here to the left and uit in a1 wavelike motif n. In this diagram, the two central microtubuies and the cross-linking proteir are not shew-. * Figure 6.25 How dynein "walking" moves flagella and cilia.

Plasma membrane

•iiiilli Microfilarnents filaments)

Myosin Tiiament— Myosin arm ^

(a) Myosin motors in muscle cell contraction. The "walking" of myosin arms drives the parallel myosin and actin filaments past each other so that the actin filaments approach each other in the middle (red arrows). This shortens the muscle and the cell. Muscle contraction involves contraction of many muscie cells at the .same time.

Cortex (outer cytoplasm, ge! with actin network Inner cytoplasm: sol with actin subunits Extending pseudopodium

Intermediate filaments

0.25 [im

& Figure 6.26 A structural role of microfilaments. The surface area of this nutrient-absorbing intestinal cell is increased by its many microviili (singular, microvillus), cellular extensions reinforced by bundles of microfilaments. These actin filaments are anchored to a network of itermediate filaments (TEM).

(b) Amoeboid movement. Interaction of actin filaments with my< near the cell's trailing end fat right) squeezes the interior fluid forward (to the left) into the pseudopodium.

Nonmoving cytoplasm (gel) , Chloroplast

cells. Thousands of actin filaments are arranged parallel to one another along the length of a muscle cell, interdigitated with thicker filaments made of a protein called myosin IFigure 6.27a). Myosin acts as a motor protein by means of projections (arms) that "walk" along the actin filaments. Contraction of the muscle cell results from the actin and myosin filaments sliding past one another in this way, shortening the cell. In other kinds of cells, actin filaments are associated with myosin in miniature and less elaborate versions of the arrangement in muscle cells. These actin-myosin aggregates are responsible for localized contractions of cells. For example, a contracting belt of microfilaments forms a cleavage furrow that pinches a dividing animal cell into two daughter cells. Localized contraction brought about by actin and myosin also plays a role in amoeboid movement (Figure 6.27b), in which a cell, such as an amoeba, for example, crawls along a surface by extending and flowing into cellular extensions called pseudopodia (from the Greek pseudcs, false, and pod, foot)- Pseudopodia extend and contract through the reversible assembly of actin subunits into microfilaments and of microfilaments into networks that convert cytoplasm from sol to

Streaming.—cytoplasm

(sol) Parallel actin filaments

(c) Cytoplasmic streaming in plant cells. A layer of cytoplasm cycles around the cell, moving over a carpet of parallel actin filaments. Myosin motors attached to organelles in the fluid cytosol may drivethe streaming by interacting with the actin,

A Figure 6.27 Microfilaments and motility. In the three examples shown in this figure, cell nuclei and most other organelles have been omitted for clarity.

work has been weakened. The pseu

CHAPTER 6

A Tour of ihe Cel

117

the actm reassembles into a network. Amoebas are not the only cells that move by crawling; so do many cells in the animal body, including some white blood cells.

In plant cells, both actin-myosin interactions and sol-gel transformations brought about by actm may be involved in cytoplasmic streaming, a circular flow of cytoplasm within cells (Figure 6.27c). This movement, which is especially common in large plant cells, speeds the distribution of materials within the cell. Intermediate Filaments Intermediate filaments are named for their diameter, which, at 8-12 nm, is larger than the diameter of micro filaments but smaller than that of micro tubules (see Table 6.1, p. 113). Specialized for bearing tension (like micro filaments), intermediate filaments are a diverse class of cytoskeletal elements. Each type is constructed from a different molecular subunit belonging to a family of proteins whose members include the keratins. Microtubules and microfilaments, in contrast, are consistent in diameter and composition in all eukaryotic cells. Intermediate filaments are more permanent fixtures of cells than are microfilaments and microtubules, which are often disassembled and reassembled in various parts of a cell. Even after cells die, intermediate filament networks often persist; for example, the outer layer of our skin consists of dead skin cells full of keratin proteins. Chemical treatments that remove microfilaments and microtubules from the cytoplasm of living cells leave a web of intermediate filaments that retains its original shape. Such experiments suggest that intermediate filaments are especially important in reinforcing the shape of a cell and fixing the position of certain organelles. For example, the nucleus commonly sits within a cage made of intermediate filaments, fixed in location by branches of the filaments that extend into the cytoplasm. Other intermediate filaments make up the nuclear lamma that lines the interior of the nuclear envelope (see Figure 6.10). In cases where the shape of the entire cell is correlated with function, intermediate filaments support Lhat shape. For instance, the long extensions (axons) of nerve cells that transmit impulses are strengthened by one class of intermediate filament. Thus, the various kinds of intermediate filaments may function as the framework of the entire cytoskelelon. Concept Check 1. Describe how the properties of microtubules, microfilaments, and intermediate filaments allow them to determine cell shape. 2. How do cilia and fiagella bend? For suggested answers, see Appendix A.

118

UNIT T W O

The Cell

Extracellular components and connections between cells help coordinate cellular activities Having crisscrossed the interior of the cell to explore various organelles, we complete our tour of the cell by returning to the surface of this microscopic world, where there are additional structures with important functions. The plasma membrane is usually regarded as the boundary oi the living cell, but most cells synthesize and secrete materials of one kind or another that are external to the plasma membrane. Although they are outside the cell, the study of these extracellular structures is central to cell biology because they are involved in so many cellular functions.

Cell Walls of Plants The cell wall is an extracellular structure of plant cells that distinguishes them from animal cells. The wall protects the plant cell, maintains its shape, and prevents excessive uptake of water. On the level of the whole plant, the strong walls of specialized cells hold the plant up against the force of gravity. Prokaryotes, fungi, and some protists also have cell walls, but we will postpone discussion of them until Unit Five. Plant cell walls are much thicker than the plasma membrane, ranging from 0.1 urn to several micrometers. The exact chemical composition of the waif varies from species to species and even from one cell type to another in the same plant, but the basic design of the wall is consistent. Microfibrils made of the polysaccharide cellulose (see Figure 5.8) are embedded in a matrix of other polysaceharides and protein. This combination of materials, strong fibers in a "ground substance" (matrix), is the same basic architectural design found in steel-reinforced concrete and in fiberglass. A young plant cell first secretes a relatively thin and flexible wall called the primary cell wall (Figure 6.28). Between primary walls of adjacent cells is the middle lamella, a thin layer rich in sticky polysaceharides called pectins. The middle lamella glues adjacent cells together (pectin is used as a thickening agent in jams and jellies). When the cell matures anc stops growing, it strengthens its wall. Some plant cells do this simply by secreting hardening substances into the primary wall. Other cells add a secondary cell wall between the plasma membrane and the primary wall. The secondary wall, often deposited in several laminated layers, has a strong and durable matrix that affords the cell protection and support. Wood, for example, consists mainly of secondary walls. Plant cell walls are commonly perforated by channels between adjacent cells called plasmodesmata (see Figure 6.28), which will be discussed shortly.

The Extracellular Matrix (ECM) of Animal Cells

Plant cell walls

lasmodesmata iii Figure 6.28 Plant cell w a l l s . The orientation drawing shows several cells, each with a large vacuole, a nucleus, and several chloroplasts and mitochondria. The transmission electron micrograph (TEM) shows the cell walls where t w o cells come together. The rnultilayered partition between plant cells consists of adjoining walls individually secreted by the cells.

Collagen fibers are embedded in a web of proteoglycan complexes.

Although animal cells lack walls akin to those of plant cells, they do have an elaborate extracellular matrix (ECM) (Figure 6.29). The main ingredients of the ECM are glycoproteins secreted by the cells. (Recall that glycoproteins are proteins with covalently bonded carbohydrate, usually short chains of sugars.) The most abundant glycoprotein in the ECM of most animal cells is collagen, which forms strong fibers outside the cells. In fact, collagen accounts for about half of the total protein in the human body. The collagen fibers are embedded in a network woven from proteoglycans, which are glycoproteins of another class. A proteoglycan molecule consists of a small core protein with many carbohydrate chains covalently attached, so that it may be up to 95% carbohydrate. Large proteoglycan complexes can form when hundreds of proteoglycans become noncovalently attached to a single long polysaccharide molecule, as shown in Figure 6.29. Some cells are attached to the ECM by still other ECM glycoproteins, including fibronectin. Fibronectin and other ECM proteins bind to cell surface receptor proteins called integrins that are built into the plasma membrane. Integrins span the membrane and bind on their cytoplasmic side to associated proteins attached to microfilaments of the cytoskeleton. The name integrin is based on the word integrate: Integrins are in a position to transmit changes between the ECM and the cytoskeleton and thus to integrate changes occurring outside and inside the cell.

A proteoglycan complex consists of hundreds of proteoglycan molecules attached noncovalently to a single long polysaccharide molecule.

Integrins are membrane proteins that are bound to the ECM on one side and to associated proteins attached to microfilaments on the other. This linkage can transmit stimuli between the cell's external environment and its interior and can result in changes in cell behavior. A. Figure 6.29 Extracellular matrix (ECM) of an animal cell. The molecular composition and structure of the ECM varies from one cell type to another. In this example, three different types of glycoproteins are present: proteoglycans, collagen, and fibronectin.

CHAPTER 6

A Tour ol the Celi

119

Current research on fibronectin, other ECM molecules, and integrins is revealing the influential role of the extracellular matrix in the lives of cells. By communicating with a cell through integrins, the ECM can regulate a cell's behavior. For example, some cells in a developing embryo migrate along specific pathways by matching ihe orientation of their microfilaments to the "grain" of fibers in the extracellular matrix. Researchers are also learning that the extracellular matrix around a cell can influence the activity of genes in the nucleus. Information about the ECM probably reaches the nucleus by a combination of mechanical and chemical signaling pathways. Mechanical signaling involves fibronectin, integrins, and microfilaments of the cytoskeleton. Changes in the cytoskeleton may in turn trigger chemical signaling pathways inside the cell, leading to changes in the set of proteins being made by the cell and therefore changes in the cell's function. In this way, the extracellular matrix of a particular tissue may help coordinate the behavior of all the cells within that tissue. Direct connections between cells also function in this coordination, as we discuss next.

Intercellular Junctions The many cells of an animal or plant are organized into tissues, organs, and organ systems. Neighboring cells often adhere, interact, and communicate through special patches of direct physical contact. Plants: Plasmodesmata It might seem that the nonliving cell walls of plants would isolate cells from one another. But in fact, as shown in Figure 6.30, plant cell walls are perforated with channels called plasmodesmata (singular, plasmodesma; from the Greek desmos, to bind). Cytosol passes through the plasmodesmata and connects the chemical environments of adjacent cells. These connections unify most of the plant into one living continuum. The plasma membranes of adjacent cells line the channel of each plasmodesma and thus are continuous. Water and small solutes can pass freely from cell to cell, and recent experiments have shown that in certain circumstances, specific proteins and RNA molecules can also do this. The macromolecules to be

Plasmodesmata

0.5 ^

Plasma membra

A Figure 6.30 Plasmodesmata between plant cells. The cytoplasm of one plant cell is continuous with the cytoplasm of its neighbors via plasmodesmata, channels through the cell walls (T£M).

120

UNIT TWO

The Cell

transported to neighboring cells seem to reach the plasmodesmata by moving along fibers of the cytoskeleton.

Animals.' Tight ]unctions, Desmosomes, and Gap Junctions In animals, there are three main types of intercellular junctions: tight junctions, desmosomes, and gap junctions (which are

most like the plasmodesmata of plants). All three types are especially common in epithelial tissue, which lines the internal surfaces of the body. Figure 6.31 uses epithelial cells of the intestinal lining to illustrate these junctions: please study this figure before moving on. Concept Check 5 1. In what ways are the cells of multicellular plants and animals structurally different from single-celled plants or animals? 2. What characteristics of the plant cell wall and animal cell extracellular matrix allow the cells to fulfill their need to exchange matter and information with their external environment? For suggested answers, see Appendix A.

The Cell: A Living Unit Greater Than the Sum of Its Parts From our panoramic view of the cells overall compartmental organization to our close-up inspection of each organelles architecture, this tour of the cell has provided many opportunities to correlate structure with function. (This would be a good time to review cell structure by returning to Figure 6.9, pp. 100 and 101.) But even as we dissect the cell, remember that none of its organelles works alone. As an example of cellular integration, consider the microscopic scene in Figure 6.32. The large cell is a macrophage (see Figure 6.14a). It helps defend the body against infections by ingesting bacteria (the smaller cells) into phagocytic vesicles. The macrophage crawls along a surface and reaches out to the bacteria with thin pseudopodia (called filopodia). Actin filaments interact with other elements of the cytoskeleton in these movements. After the macrophage engulfs the bacteria, they are destroyed by lysosomes. The elaborate endomembrane system produces the lysosomes. The digestive enzymes of the lyso somes and the proteins of the cytoskeleton are all made on ribosomes. And the synthesis of these proteins is programmed by genetic messages dispatched from the DNA in the nucleus. All these processes require energy, which mitochondria supply in the form of ATP. Cellular functions arise from cellular order: The cell is a living unit greater than the sum of its parts.

Figure 6.31

Intercellular Junctions in Animal Tissues

fl Tight junction

TIGHT JUNCTIONS At tight junctions, the membranes of neighboring cells are very tightly pressed against each other, bound together by specific proteins (purple). Forming continuous seals around the cells, tight junctions prevent leakage of extracellular fluid across a layer of epithelial cells.

faiDrHfc

Tight junctions prevent fluid from moving across a layer of cells

DESMOSOMES ft Desmosomes (also called anchoring IE junctions) function like rivets, fastening cells U together into strong sheets. Intermediate jjf filaments made of sturdy keratin proteins anchor desmosomes in the cytoplasm.

GAP JUNCTIONS

A\ "•Extracellular matrix

between cells

Gap junction

Plasma membranes of adjacent cells O.I urn

Gap junctions (also called communicating junctions) provide cytoplasmic channels from one cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars, amino acids, and other small molecules may pass. Gap junctions are necessary for communication between cells in many types of tissues, including heart muscle and animal embryos.

>• Figure 6.32 The emergence of cellular functions from the cooperation of many organelles. The ability of this macrophage (brown) to recognize, apprehend, and destroy bacteria (yellow) is a coordinated activity of the whole cell. Its cytoskeleton, lysosomes, and plasma membrane are among the components that function in phagocytosis (colorized SEM).

PTER 6

A Tour of the Cell

121

Chapter QQ to the Campbell BiolQgy website (www.wmpbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY CONCEPTS To study cells, biologists use microscopes and the tools of biochemistry • Microscopy (pp. 95-97) Improvements in microscopy have catalyzed progress in the study oi cell structure. Activity Metric System Review Investigation What Is the Size and Scale of Our World? • Isolating Organelles by Cell Fractionation (p. 97) Cell biologists can obtain pellets enriched in specific organelles by centrifuging disrupted cells.

Eukaryotic cells have internal membranes that compartmentalize their functions • Comparing Prokaryotic and Eukaryotic Cells (pp. 98-99) All cells are bounded by a plasma membrane. Unlike eukaryotic cells, prokaryotic cells lack nuclei and other membrane-enclosed organelles. Activity Prokaryotic Cell Structure and Function Activity Comparing Prokaryotic and Eukaryotic Cells • A Panoramic View of the Eukaryotic Cell (pp. 99-101) Plant and animal cells have most of the same organdies. Activity Build an Animal Cell and a Plant Cell

The eukaryotic cell's genetic instructions are housed in the nucleus and carried out by the ribosomes • The Nucleus: Genetic Library of the Cell (pp. 102-103) The nucleus houses DNA and nucleoli, where ribosomal subunits are made. Materials pass through pores in the nuclear envelope. • Ribosomes: Protein Factories in the Cell (pp. 102-104) Free ribosomes in the cytosol and bound ribosomes on the outside of the ER and the nuclear envelope synthesize proteins. Activity Role of the Nucleus and Ribosomes in Protein Synthesis

The endomembrane system regulates protein traffic and performs metabolic functions in the cell • The membranes of the endomembrane system are connected by physical continuity or through transport vesicles (p. 104).

Review • Lysosomes: Digestive Compartments [pp. 107-108) Lysosomes are sacs of hydrolytic enzymes. They break down ingested substances and cell macromolecules for recycling. • Vacuoles: Diverse Maintenance Compartments (p. 108) A plant cell's central vacuole functions in digestion, storage, waste disposal, cell growth, and protection. • The Endomembrane System: A Review (pp. 108-109) Activity The Endomembrane System

Mitochondria and chloroplasts change energy from one form to another • Mitochondria: Chemical Energy Conversion (pp. 109-110) Mitochondria, the sites of cellular respiration, have an outer membrane and an inner membrane that is folded into cristae. • Chloroplasts: Capture of Light Energy (pp. 110-111) Chloroplasts contain pigments that function in photosynthesis. Two membranes surround the fluid stroma, which contains thylakoids stacked into grana. Activity Build a Chloroplast and a Mitochondrion • Peroxisomes: Oxidation (pp. 110-111) Peroxisomes produce hydrogen peroxide (H 2 O 2 ) and convert it to water.

The cytoskeleton is a network of fibers that organizes structures and activities in the cell • Roles of the Cytoskeleton: Support, Motility, and Regulation (pp. 112-113) The cytoskeleton functions in structural support, for the cell, motility, and signal transmission. • Components of the Cytoskeleton (pp. 113-118) tvlicrotubules shape the cell, guide movement of organelles, and help separate the chromosome copies in dividing cells. Cilia and flagella are motile appendages containing microtubules. Microfilaments are thin rods built from actin; they function in muscle contraction, amoeboid movement, cytoplasmic streaming, and support for microviUi. Intermediate filaments support cell shape and fix organelles in place. Activity Cilia and Flagella

Extracellular components and connections between cells help coordinate cellular activities • Cell Walls of Plants (pp. 118-119) Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein. • The Extracellular Matrix (ECM) of Animal Cells (pp. 119-120) Animal cells secrete glycoproteins that form the ECM, which functions in support, adhesion, movement, and regulation.

• The Endoplasmic Reticulum: Biosynthetic Factory (pp. 104-105) Smooth ER synthesizes lipids, metabolizes carbohydrates, stores calcium, and detoxifies poisons. Rough ER has bound ribosomes and produces proteins and membranes, which are distributed by transport vesicles from the ER.

• Intercellular Junctions (pp. 120-121) Plants have plasmodesmata that pass through adjoining cell walls. Animal cells have tight junctions, desmosomes, and gap junctions. Activity Cell Junctions

• The Golgi Apparatus: Shipping and Receiving Center (pp. 105-107) Proteins are transported from the ER to the Golgi, where they are modified, sorted, and released in transport vesicles.

• The Cell: A Living Unit Greater Than the Sum of Its Parts (pp. 120-121) Activity Review: Animal Cell Structure and Function Activity Review: Plant Cell Structure and Function

122

UNIT TWO

The Cell

G YOUR KNOWLEDGE Evolution Connection Although the similarities among cells reveal the evolutionary unity of life, cells can differ dramatically in structure. Which aspects of cell structure best reveal their evolutionary unity? What are some examples of specialized cellular modifications?

Scientific Inquiry Imagine protein X, destined to go to the plasma membrane oi a cell. Assume that the mRNA carrying the genetic message for protein X has already been translated by ribosomes in a cell culture. You collect the cells, break them open, and then fractionate their contents by differential centrifugation as shown in Figure 6.5. In the pellet of which fraction would you expect to find protein X? Explain your answer by describing the transit of protein X through the cell,

Science, Technology, and Society Doctors at a California university removed a man's spleen, standard treatment for a type of leukemia. The disease did not recur. Researchers kept some of the spleen cells alive in a nutrient medium. They found that some of the cells produced a blood protein that showed promise for treating cancer and AIDS. The researchers patented the cells. The patient sued, claiming a share in profits from any products derived from his cells. The California Supreme Court ruled against the patient, stating that his suit "threatens to destroy the economic incentive to conduct important medical research." The U.S. Supreme Court agreed. Do you think the patient was treated fairly? What else would you like to know about this case that might help you make up your mind?

investigation What Is the Size and Scale of Our World?

CHAPTER 6 A Tour of the Cell

123

Membrane Structure and Function A Figure 7.1 The plasma membrane.

Key Concepts 7.1 Cellular membranes are fluid mosaics of lipids and proteins 7.2 Membrane structure results in selective permeability 7.3 Passive transport is diffusion of a substance across a membrane with no energy investment 7.4 Active transport uses energy to move solutes against their gradients 7.5 Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

permeability was highlighted when the 2003 Nobel Prize in Chemistry was awarded to Peter Agre (see the interview on pp. 92-93) and Roderick MacKinnon, two scientists who worked out how water and specific ions are transported into and out of the cell We will concentrate on the plasma membrane, the outermost membrane of the cell, represented in Figure 7.1. However, the same general principles of membrane traffic also apply to the many varieties of internal mem branes that partition the eukaryotic cell. To understand how membranes work, we begin by examining their architecture.

Concept

Life at the Edge

T

he plasma membrane is the edge of life, the boundary that separates the living cell from its nonliving surroundings. A remarkable film only about 8 nm thick—it would take over 8,000 to equal the thickness of this page—the plasma membrane controls traffic into and out of the cell it surrounds. Like all biological membranes, the plasma membrane exhibits selective permeability; thai is, it allows some substances to cross it more easily than others. One of the earliest episodes in the evolution of life may have been the formation of a membrane that enclosed a solution different from the surrounding solution while still permitting the uptake of nutrients and elimination of waste products. This ability oi the cell to discriminate in its chemical exchanges with its environment is fundamental to life, and it is the plasma membrane and its component molecules that make this selectivity possible. in this chapter, you will learn how cellular membranes control the passage of substances. The importance of selective 124

Cellular membranes are fluid mosaics of lipids and proteins Lipids and proteins are the staple ingredients of membranes, although carbohydrates are also important. The most abundani lipids in most membranes are phospholipids. The ability ol phospholipids to form membranes is inherent in their molecular structure. A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic region and a hydrophobic region (see Figure 5.13). Other types of membrane lipids are also amphipathic. Furthermore, most of the proteins of membranes have both hydrophobic and hydrophilic regions. How are phospholipids and proteins arranged in the membranes of cells? You encountered the currently accepted model for the arrangement of these molecules in Chapter 6 (see Figure 6.8). In this fluid mosaic model, the membrane is a fluid structure with a "mosaic" of various proteins embedded in or attached to a double layer (bilayer) of phospholipids. Well discuss this model in detail, starting with the store of how it was developed.

Membrane Models: Scientific Inquiry Scientists began building molecular models of the membrane decades before membranes were first seen with the electron microscope in the 1950s. In 1915, membranes isolated from red blood cells were chemically analyzed and found to be composed of lipids and proteins- Ten years later, two Dutch scientists, E- Gorter and E Grendel, reasoned that cell membranes must actually be phospholipid bilayers. Such a double layer of molecules could exist as a stable boundary between two aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to water (Figure 7,2). With the conclusion thai a phospholipid bilayer was the main fabric of a membrane, the next question was where to place the proteins. Although the heads of phospholipids are hydrophilic, the surface of a membrane consisting of a pure phospholipid bilayer adheres less strongly to water than does the surface of a biological membrane. Given these data, in 1935, Hugh Davson and James Danielli suggested that this difference :ould be accounted for if the membrane were coated on both sides with hydrophilic proteins. They proposed a sandwich model: a phospholipid bilayer between two layers of proteins. When researchers first used electron microscopes to study cells in the 1950s, the pictures seemed to support the DavsonDanielli model. By the 1960s, the Davson-Danielli sandwich had become widely accepted as the structure not only of the plasma membrane, but of all the internal membranes of t he cell. By the end of that decade, however, many cell biologists recognized two problems with the model. First, the generalization that all membranes of the cell are identical was challenged. Whereas the plasma membrane is 7-8 run thick and has a three-layered structure in electron micrographs, the inner membrane of the mitochondrion is only 6 nm thick and looks like a row of beads. Mitochondria! membranes also have a substantiallygreater percentage of proteins than do plasma membranes, and there are differences in the specific kinds of phospholipids and other lipids. In short, membranes with different functions differ in chemical composition and structure.

not very soluble in water. Membrane proteins have hydrophobic regions as well as hydrophilic regions (that is, they are amphipathic). If such proteins were layered on the surface of the membrane, their hydrophobic parts would be in an aqueous environment. In 1972, S. J. Singer and G. Nicolson proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer, with only their hydrophilic regions protruding far enough from the bilayer to be exposed to water (Figure 7.3). This molecular arrangement would maximize contact of hydrophilic regions of proteins and phospholipids with water while providing their hydrophobic parts with a uonaqueous environment. According to this model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. A method of preparing cells for electron microscopy called freeze-fracture has demonstrated visually that proteins are indeed embedded in the phospholipid bilayer of the membrane. Freeze-fracture splits a membrane along the middle of the phospholipid bilayer. When the halves of the fractured membrane are viewed in the electron microscope, the interior of the bilayer appears cobblestoned, with protein particles interspersed in a smooth matrix, as in the fluid mosaic model (Figure 7.4, next page). Other kinds of evidence further support this arrangement. Models are proposed by scientists as hypotheses, ways of organizing and explaining existing information. Replacing one model of membrane structure with another does not imply that the original model was worthless. The acceptance or rejection of a model depends on how well it fits observations and explains experimentat results. A good model also makes predictions that shape future research. Models inspire experiments, and few models survive these tests without modification. New findings may make a model obsolete; even then, it may not be totally scrapped, but revised to incorporate the new observations. The fluid mosaic model is continually being refined and may one day undergo further revision. Now let's take a closer look at membrane structure, beginning with the ability of lipids and proteins to drift laterally within the membrane.

A second, more serious problem with the sandwich model was the placement ol the proteins. Unlike proteins dissolved in the cytosol, membrane proteins are

A Figure 7.2 Phospholipid bilayer (cross section).

A Figure 7.3 The fluid mosaic model for membranes. CHAPTER 7

Membrane Structure and Function

125

Figure 7.4

Research Method Freeze-Fracture APPLICATION

A cell membrane can be split into its t w o layers, revealing the ultra structure of the membrane's interior. A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers. Extracellular layer x

These SEMs show membrane proteins (the "bumps") in the t w o layers, demonstrating that proteins are embedded in the phospholipid bilayer.

(a) Movement of phospholipids. Lipids move laterally in a membrane, but flip-flopping across the membrane is quite rare.

(b) Membrane fluidity. Unsaturated hydrocarbon tails of phospholipids have kinks that keep the molecules from packing together, enhancing membrane fluidity.

Cytoplasmic layei

Extracellular layer

The Fluidity of Membranes Membranes are not static sheets of molecules locked rigidly in place. A membrane is held together primarily by hydrophobic interactions, which are much weaker than covalent bonds (see Figure 5.20). Most of the lipids and some of the proteins can drift about laterally—thai is, in the plane of the membrane (Figure 7.5a). It is quite rare, however, for a molecule to flip-flop transversely across the membrane, switching from one phospholipid layer to the other; to do so, the hydrophilic part of the molecule must cross the hydrophobic core of the membrane. The lateral movement of phospholipids within the membrane is rapid. Adjacent phospholipids switch positions about 107 times per second, which means that a phospholipid can travel about 2 um—the length of a typical bacteria] cell—in 1 second. Proteins are much larger than lipids and move more slowly, but some membrane proteins do, in fact, drift (Figure 7.6). And some membrane proteins seem to move in a highly directed manner, perhaps driven along cytoskeletal fibers by motor proteins connected to the membrane proteins' cytoplasmic regions. However, many other membrane proteins seem to be held virtually immobile by their attachment to the cytoskeleton. A membrane remains fluid as temperature decreases, until finally the phospholipids settle into a closely packed arrangement and the membrane solidifies, much as bacon grease 126

UNIT TWO

The Cell

Cholesterol (c) Cholesterol within the animal cell membrane. Cholesterol reduces membrane fluidity at moderate temperatures by reducing phospholipid movement, but at low temperatures it hinders solidification by disrupting the regular packing of phospholipids.

A Figure 7.5 The fluidity of membranes.

forms lard when it cools. The temperature at which a membrane solidifies depends on the types of lipids it is made of. The membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails (see Figures 5.12 and 5.13). Because of kinks in the tails where double bonds are located, unsaturated hydrocarbons cannot pack together as closely as saturated hydrocarbons, and this makes the membrane more fluid (Figure 7.5b). The steroid cholesterol, which is wedged between phospholipid molecules in the plasma membranes of animal cells, has different effects on membrane fluidity at different temperatures (Figure 7.5c). At relatively warm temperatures—at 37CC, the body temperature of humans, for example— cholesterol makes the membrane less fluid by restraining the movement of phospholipids. However, because cholesterol

also hinders the close packing of phospholipids, it lowers the temperature required for the membrane to solidify. Thus, cholesterol can be thought of as a "temperature buffer" for the membrane, resisting changes in membrane fluidity that can be caused by changes in temperature. Membranes must be fluid to work properly; they are usually about as fluid as salad oil. When a membrane solidifies, its permeability changes, and enzymatic proteins m the membrane may become inactive—-for example, if their activity requires them to be able to move laterally in the membrane. The lipid composition of cell membranes can change as an adjustment to changing temperature. For instance, in many plants that tolerate extreme cold, such as winter wheat, the percentage of unsaturated phospholipids increases in autumn, an adaptation that keeps the membranes from solidifying during winter.

Membrane Proteins and Their Functions

—^TT^—__..,_— membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane.

Now we come to the mosaic aspect of the fluid mosaic model. A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer (Figure 7.7). More than 50 kinds

'aceliulai :rix(ECM)

Microfi laments of cytoskeleton

Cholesterol

Periphera protein CYTOPLASMIC SIDE OF MEMBRANE

A Figure 7.7 The detailed structure of an animal cell's plasma membrane, in cross section. CHAPTER 7

Membrane Structure and Function

127

of proteins have been found so far in the plasma membrane of red blood cells, for example. Phospholipids form the main fabric of the membrane, but proteins determine most of the membrane's specific functions. Different types of cells contain different sets of membrane proteins, and the various membranes within a cell each have a unique collection of proteins. Notice in Figure 7.7 that there are two major populations of membrane proteins. Integral proteins penetrate the hydrophobic core of the lipid bilayer. Many are transmemhrane proteins, which completely span the membrane. The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar ammo acids (see Figure 5.17), usually coiled into a helices (Figure 7.8). The hydrophilic parts ol the molecule are exposed to the aqueous solutions on either side of the membrane. Peripheral proteins are not embedded m the lipid bilayer at all; they are appendages loosely bound to the surface of the membrane, olten to the exposed parts of integral proteins (see Figure 7.7). On the cytoplasmic side of the plasma membrane, some membrane proteins are held in place by attachment to the cytoskeleton. And on the exterior side, certain membrane proteins are attached to fibers of the extracellular matrix (see Figure 6.29; infirgnns are one type of integral protein). These attachments combine to give animal cells a stronger framework than the plasma membrane itself could provide. Figure 7.9 gives an overview of six major functions performed by proteins of the plasma membrane. A single cell may

(a) Transport, (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a

particular solute, (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane. (b) Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway.

(c) Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell.

(d) Cell-cell recognition. Some glycoproteins serve as identification tags that are specifically recognized by other cells.

(e) Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions (see Figure 6.31).

A Figure 7.8 The structure of a transmembrane protein. The protein shown here, bacteriorhodopsin (a bacterial transport protein), has a distinct orientation in the membrane, with the N-terminus outside the cell and the C-terminus inside. This ribbon model highlights the a-helical secondary structure of the hydrophobic parts of the protein, which lie mostly within the hydrophobic core of the membrane. The protein includes seven transmembrane helices (outlined with cylinders for emphasis). The nonhelical hydrophilic segments of the protein are in contact with the aqueous solutions on the extracellular and cytoplasmic sides of the membrane.

128

UNIT TWO

The Cell

(f) Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (see Figure 6.29).

A Figure 7.9 Some functions of membrane proteins. In many cases, a single protein performs some combination of these tasks.

have membrane proteins carrying out several of these functions, and a single protein may have multiple functions. Thus, the membrane is a functional mosaic as well as a structural one.

1 he Role of Membrane Carbohydrates ii Cell-Cell Recognition Cell-cell recognition, a cell's ability to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism. H is important, for example, in the sorting of cells into tissues and organs in an animal embryo. It is also the basis for the rejection of foreign cells (including those of transplanted organs) by the immune system, an important line of c.efense in vertebrate animals (see Chapter 43). The way cells recognize other cells is by binding to surface molecules, often carbohydrates, on the plasma membrane (see Figure 7.9d). Membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units. Some of these carbohydrates are covalently bonded to lipids, forming molecules called glycolipids. (Recall that glyco refers to the presence of carbohydrate.) Most, however, are covalently bonded to proteins, which are thereby glycoproteins (see Figure 7.7). The carbohydrates on the external side of the plasma membrane vary from species to species, among individuals of the same species, and even from one cell type to another in a single individual. The diversity of the molecules and their location on the cell's surface enable membrane carbohydrates to function as markers that distinguish one cell from another. For example, the four human blood types designated A, B, AB, and O reflect variation in the carbohydrates on the surface jf red blood cells.

Transmernbrane glycoproteins Secretory protein v

Glycolipid \ Golgi £ apparatus

Vesicle

*

y

Plasma m e m b r a n e : Cytoplasmic faces Extracellular face.

-Membrane glycolipid

A Figure 7.10 Synthesis of membrane components and their orientation on the resulting membrane. The plasma membrane has distinct cytoplasmic and extracellular sides, or faces, with the extracellular face arising from the inside face of ER, Golgi, and vesicle membranes,

Synthesis and Sidedness of Membranes Membranes have distinct inside and outside faces. The two lipid layers may differ in specific lipid composition, and each protein has directional orientation in the membrane (see Figure 7.8). When a vesicle fuses with the plasma membrane, the outside layer of the vesicle becomes continuous with the cytoplasmic layer of the plasma membrane. Therefore, molecules that start out on the inside face of the FR end up on the outside face of the plasma membrane. The process, shown in Figure 7.10, starts with 0 the synthesis of membrane proteins and lipids in the endoplasmic reticulum. Carbohydrates (green) are added to the proteins (purple), making them glycoproteins. The carbohydrate portions may then be modified. © Inside the Golgi apparatus, the glycoproteins undergo further carbohydrate modification, and lipids acquire carbohydrates, becoming glycolipids. © Transmembrane proteins (purple dumbbells), membrane glycolipids, and secretory proteins (purple spheres) are transported in vesicles to the plasma membrane. © There the vesicles fuse with the membrane, releasing secretory proteins from the cell. Vesicle fusion positions the carbohydrates of membrane

glycoproteins and glycolipids on the outside of the plasma membrane. Thus, the asymmetrical distribution of proteins, lipids, and their associated carbohydrates in the plasma membrane is determined as the membrane is being built by the ER and Golgi apparatus. Concept Check 1. How would you expect the saturation levels of membrane fatty acids to differ in plants adapted to cold environments and plants adapted to hot environments? 2. The carbohydrates attached to some of the proteins and lipids of the plasma membrane are added as the membrane is made and refined in the ER and Golgi apparatus; the new membrane then forms transport vesicles that travel to the cell surface. On which side of the vesicle membrane are the carbohydrates? For suggested answers, see Appendix A.

CHAPTER 7

Membrane Structure and Function

129

Concept

Membrane structure results in selective permeability The biological membrane is an exquisite example of a supramolecular structure—many molecules ordered into a higher level of organization—with emergent properties beyond those of the individual molecules. The remainder of this chapter focuses on one of the most important of those properties: the ability to regulate transport across cellular boundaries, a function essential to the cells existence. We will see once again that form fits function: The fluid mosaic model helps explain how membranes regulate the cell's molecular traffic. A steady traffic of small molecules and ions moves across the plasma membrane in both directions. Consider the chemical exchanges between a muscle cell and the extracellular fluid that bathes it. Sugars, amino acids, and other nutrients enter the cell, and metabolic waste products leave it. The cell takes in oxygen for cellular respiration and expels carbon dioxide. U also regulates its concentrations of inorganic ions, such as Na+, K+, Ca 2+ , and Cl , by shuttling them one way or the other across the plasma membrane. AlLhough traffic through the membrane is extensive, cell membranes are selectively permeable, and substances do not cross the barrier indiscriminately: The cell is able to take up many varieties of small molecules and ions and exclude others. Moreover, substances that move through the membrane do so at different rates.

The Permeability of the Lipid Bilayer Hydrophobic (nonpolar) molecules, such as hydrocarbons, carbon dioxide, and oxygen, can dissolve in the lipid bilayer of the membrane and cross it with ease, without the aid of membrane proteins. However, the hydrophobic core of the membrane impedes the direct passage of ions and polar molecules, which are hydrophilic, through the membrane. Polar molecules such as glucose and other sugars pass only slowly through a lipid bilayer, and even water, an extremely small polar molecule, does not cross very rapidly. A charged atom or molecule and ils surrounding shell of water (see Figure 3.6) find the hydrophobic layer of the membrane even more difficult to penetrate. Fortunately, the lipid bilayer is only part of the story of a membranes selective permeability. Proteins built into the membrane play key roles in regulating transport.

Transport Proteins Cell membranes are permeable to specific ions and a variety of polar molecules. These hydrophilic substances can avoid 130

UNIT T w o

The Cell

contact with the lipid bilayer by passing through transport proteins that span the membrane. Some transport protein?', called channel proteins, function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane (see Figure 7.9a, left). For example, the passage of water molecules through the membrane in certain cells is greatly facilitated by channel proteins known as aquaporins. (These were discovered in the laboratory of Peter Agre; see pp. 92-93.) Other transport proteins, called carrier proteins, hold onto their passengers and change shape in a way that shuttles them across the membrane (see Figure 7.9a, right). In both cases, the transport protein is specific for the substance it translocates (moves), allowing only a certain sub • stance (or substances) to cross the membrane. For example, glucose carried in blood and needed by red blood cells for cellular activities enters these cells rapidly through specific transport proteins in the plasma membrane. This "glucose transporter" is so selective as a carrier protein that it even rejects fructose, a structural isomer of glucose. Thus, the selective permeability of a membrane depends on both the discriminating barrier of the lipid bilayer anc the specific transport proteins built into the membrane. Bui what determines the direction of traffic across a membrane? At a given time, will a particular substance enter or leave the cell? And what mechanisms actually drive molecules across membranes? We will address these questions next as we explore two modes of membrane traffic: passive transport and active transport. Concept Check 1. Two molecules that can cross a lipid bilayer without help from membrane proteins are Q2 and CO2. What properties allow this to occur? 2. Why would water molecules need a transport protein (aquaporin) to move rapidly and in large quantities across a membrane? For suggested answers, see Appendix A.

Concept

Passive transport is diffusion of a substance across a membrane with no energy investment Molecules have a type of energy called thermal motion (heal). One result of thermal motion is diffusion, the tendency for molecules of any substance to spread out evenly into the available space. Each molecule moves randomly, yet diffusion o\ a

population of molecules may be directional. A good way to visualize this is to imagine a synthetic membrane separating pure water from a solution of a dye in water. Assume that this membrane has microscopic pores and is permeable to the dye molecules (Figure 7.11a). Each dye molecule wanders randomly, but there wilL be a net movement of the dye molecules across the membrane to the side that began as pure water. The dye molecules will continue to spread across the membrane until both solutions have equal concentrations of the dye. Once that point is reached, there will be a dynamic equilibrium, with as many dye molecules crossing the membrane each second in one direction as in the other. We can now slate a simple rule of diffusion: In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Put another way, any substance will diffuse down its concentration gradient. Mo work must be done LO make this happen; diffusion is a spontaneous process. Note that each substance diffuses down its own concentration gradient, unaffected by the concentration differences of other substances (Figure 7.11b). Much of the traffic across cell membranes occurs by diffusion. When a substance is more concentrated on one side of a membrane than on the other, there is a tendency for the substance to diffuse across the membrane down its concentration gradient (assuming that the membrane is permeable to that substance). One important example is the uptake of oxygen by a cell performing cellular respiration. Dissolved oxygen

diffuses into the cell across the plasma membrane. As long as cellular respiration consumes the O2 as it enters, diffusion into the cell will continue, because the concentration gradient iavors movement in that direction. The diffusion of a substance across a biological membrane is called passive transport because the cell does not have to expend energy to make it happen. The concentration gradient itself represents potential energy (see Chapter 2, p. 36) and drives diffusion. Remember, however, that membranes are selectively permeable and therefore have different effects on the rates of diffusion of various molecules. In the case of water, aquaporins allow water to diffuse very rapidly across the membranes of certain cells. The movement of water across the plasma membrane has important consequences for cells.

Effects of Osmosis on Water Balance To see how two solutions with different solute concentrations interact, picture a U-shaped glass tube with a selectively permeable membrane separating two sugar solutions (Figure 7.12). Pores in this synthetic membrane are too small for sugar molecules to pass through but large enough for water molecules. How does this affect the water concentration? It seems logical that the solution with the higher concentration of solute would have the lower concentration of water and that water would diffuse into it from the other side for that reason. However, for a dilute solution like most biological fluids,

(a) Diffusion of one solute. The membrane has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions.

(b) Diffusion of t w o solutes. Solutions of t w o different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concentration gradient. There will be a net diffusion of the purple dye toward the left, even though the total solute concentration was initially greater on the left side.

A Figure 7.11 The diffusion of solutes across a membrane. Each of the large arrows under the diagrams shows the net diffusion of the dye molecules of that color.

CHAPTER 7

Membrane Structure and Function

131

Lower concentration of solute (sugar'

Same concentration of sugar

Selectivel permeable merrv brane: sugar molecules cannot pass through pores, but water molecules ca

Water molecules cluster around sugar molecules

More free water molecules (higher concentration)

Fewer free water molecules (lower concentration)

Water moves from an area of higher free water concentration to an area of lower free water concentration A Figure 7.12 Osmosis. Two sugar solutions of different concentrations are separated by a selectively permeable membrane, which the solvent (water) can pass through but the solute (sugar) cannot. Water molecules move randomly and may cross through the pores in either direction, but overall, water diffuses from the solution with less concentrated solute to that with more concentrated solute. This transport of water, or osmosis, eventually egualizes the sugar concentrations on both sides of the membrane.

solutes do not affect the water concentration significantly. Instead, tight clustering of water molecules around the hydrophilic solute molecules makes some of the water unavailable to cross the membrane. It is the difference in free water concentration that is imporant. But the effect is the same: Water diffuses across the membrane from the region of lower solute concentration to that of higher solute concentration until the solute concentrations on both sides of the membrane are equal. The diffusion of water across a selectively permeable membrane is called osmosis. The movement of water across cell membranes and the balance of water between the cell and its environment are crucial to organisms. Lets now apply to living cells what we have learned about osmosis in artificial systems.

Water Balance of Cells Without Walls When considering the behavior of a cell in a solution, both solute concentration and membrane permeability must be considered. Both factors are taken into account in the concept of tonicity, the ability of a solution to cause a cell to gain or lose water. The tonicity of a solution depends in part on its 132

UNIT TWO

The Cell

concentration of solutes that cannot cross the membrane (nonpenetrating solutes), relative to that in the cell itself. If there are more nonpenetrating solutes in the surrounding solution, water will tend to leave the cell, and vice versa. If a cell without a wall, such as an animal cell, is immersec in an environment that is isotonic to the cell (iso means "same"), there will be no net movement of water across the plasma membrane. Water flows across the membrane, but ai the same rate in both directions. In an isotonic environment, the volume of an animal cell is stable (Figure 7.13a). Now" let's transfer the cell to a solution that is hypertonic to the cell (hyper means "more," in this case more nonpenetrating solutes). The cell will lose water to its environment, shrivel, and probably die. This is one reason why an increase in the salinity (saltiness) of a lake can kill the animals there— if the lake water becomes hypertonic to the animals' cells, the cells might shrivel and die. However, taking up too much water can be just as hazardous to an animal cell as losing water. If we place the cell in a solution that is hypotonic to the cell (hypo means "less"), water will enter the cell faster than it leaves, and the cell will swell and lyse (burst) like an overfilled water balloon. A cell without rigid walls can tolerate neither excessive uptake nor excessive loss of water. This problem of water balance is automatically solved if such a cell lives in isotonic surroundings. Seawater is isotonic to many marine invertebrates. The cells of most terrestrial (land-dwelling) animals are bathed in an extracellular fluid that is isotonic to the cells. Animals and other organisms without rigid cell walls living in hypertonic or hypotonic environments must have special adaptations for osmoregulation, the control of water balance. For example, the protist Paramecium lives in pond water, which is hypotonic to the cell. Paramecium has a plasma membrane that is much less permeable to water than the membranes of most other cells, but this only slows the uptake of water, which continually enters the cell. Paramecium doesn't burst because it is also equipped with a contractile vacuole, an organelle that functions as a bilge pump to force water out of the cell as fast as it enters by osmosis (Figure 7.14). We will examine other evolutionary adaptations for osmoregulation m Chapter 44.

Water Balance of Cells with Walls The cells of plants, prokaryotes, fungi, and some protists have walls. When such a cell is immersed in a hypotonic solution— bathed in rainwater, for example-—the wall helps maintain the cells water balance. Consider a plant cell. Like an animal cell, the plant cell swells as water enters by osmosis (Figure 7.13b). However, the elastic wall will expand only so much before it exerts a back pressure on the cell that opposes further water uptake. At this point, the cell is turgid (very firm), which is the healthy state for most plant cells. Plants that are not

Hypotonic solution

fr* Figure 7.13 The water balance of living cells. How living cells react to changes in the solute concentration of their environment ck-pends on whether or not they have cell walls. (a) Animal cells such as this red blood cell do not have cell walls, (b) Plant cells do. (Arrows indicate net water movement since the cells were first placed in these solutions.)

Isotonic solution

Hypertonic solution

(a) Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.

(b) Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell.

woody, such as most houseplants, depend for mechanical support on cells kept turgid by a surrounding hypotonic solution. If a plant's cells and their surroundings are isotonic, there i.s no net tendency for water to enter, and the cells become fLaccid (limp). However, a wall is of no advantage if the cell is immersed in a hypertonic environment. In this case, a plant cell, like an an-

Filling vacuole

50 (irn

la) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. 50 |im Contracting vacuole

(b) When full, the vacuole and canals contract, expelling fluid from the cell.

A Figure 7.14 The contractile vacuole of Paramecium: an evolutionary adaptation for osmoregulation. The contractile vacuole of this freshwater protist offsets osmosis by bailing water out of the cell.

Plasmolyzed

imal cell, will lose water to its surroundings and shrink. As the plant cell shrivels, its plasma membrane pulls away from the wall. This phenomenon, called plasmolysis, causes the plant to wilt and can be lethal. The walled cells of bacteria and iungi also plasrnolyze in hypertonic environments-

Facilitated Diffusion: Passive Transport Aided by Proteins Lets look more closely at how water and certain hydrophilic solutes cross a membrane. As mentioned earlier, many polar molecules and ions impeded by the lipid bilayer oi the membrane diffuse passively with the help of transport proteins that span the membrane. This phenomenon, is called facilitated diffusion. Cell biologists are still trying to learn exactly how various transport proteins facilitate diffusion. Most transport proteins are very specific: They transport only particular substances but not others. As described earlier, the two types of transport proteins are channel proteins and carrier proteins. Channel proteins simply provide corridors that allow a specific molecule or ion to cross the membrane (Figure 7.15a). The hydrophilic passageways provided by these proteins allow water molecules or small ions to flow very quickly from one side of the membrane to the other. While water molecules are small enough to cross through the phospholipid bilayer, the rate of water movement by this route is relatively slow because of their polarity Aquaporins, the water channel proteins, facilitate the massive amounts of diffusion that occur in plant cells and in animal cells such as red blood cells (see Figure 7.13). Another group of channels are ion channels, many of which function as gated channels; a stimulus causes them to open or close. The stimulus may be electrical or chemical; if chemical, the stimulus is a substance other than the one to be transported. For example, stimulation of a nerve cell by CHAPTER 7

Membrane Sirucnirc and Function

133

EXTRACELLULAR FLUID

Active transport uses energy to move solutes against their gradients Channel protein

(a) A channel protein (purple) has a channel through which water molecules or a specific solute can pass.

Solute

(b) A carrier protein alternates between t w o conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.

A Figure 7.15 Two types of transport proteins that carry out facilitated diffusion. In both cases, the protein transports the solute down its concentration gradient.

certain neuretransmitter molecules opens gated channels that allow sodium ions into the cell. Carrier proteins seem to undergo a subtle change in shape that somehow translocates the solute-binding site across the membrane (Figure 7.15b). These changes in shape may be triggered by the binding and release of the transported molecule. Tn certain inherited diseases, specific transport systems are either defective or missing altogether. An example is cystinuria, a human disease characterized by the absence of a protein that transports cysteine and some other amino acids across the membranes of kidney cells. Kidney cells normally reabsorb these amino acids from the urine and return them to the blood, but an individual afflicted with cystinuria develops painful stones from ammo acids that accumulate and crystallize in the kidneys.

Concept Check i s 1. If a Paramecium were to swim from a hypotonic environment to an isotonic one, would the activity of its contractile vacuole increase or decrease? Why? For suggested answers, see Appendix A.

134

UNIT TWO

The Cell

Despite the help of transport proteins, facilitated diffusion is still considered passive transport because the solute being transported is moving down its concentration gradient. Facilitated diffusion speeds the transport of a solute by providing an efficient passage through the membrane, but it does not alter Lhi_s direction of transport. Some transport proteins, however, can move solutes against their concentration gradients, across the plasma membrane from the side where they are less concentrated to the side where they are more concentrated.

The Need for Energy in Active Transport To pump a molecule across a membrane against its gradient requires work; the cell must expend energy. Therefore, this type of membrane traffic is called active transport. The transport proteins that move solutes against a concentration gradient are all carrier proteins, rather than channel proteins. This makes sense, because when channel proteins art: open, they merely allow molecules to flow down their concentration gradient, rather than picking them up and transporting them against their gradientActive transport enables a cell to maintain internal concentrations of small molecules that differ from concentration.1-, in its environment. For example, compared to its surroundings, an animal cell has a much higher concentration of potassium ions and a much lower concentration of sodium ions. The plasma membrane helps maintain these steep gradients by pumping sodium out of the cell and potassium intc the cell, As in other types of cellular work, ATP supplies the energ} for most active transport. One way ATP can power active transport is by transferring its terminal phosphate group directly to the transport protein. This may induce the protein to change its conformation in a manner that translocates a solute bound to the protein across the membrane. One transport system that works this way is the sodium-potassium pump, which exchanges sodium (Na+) for potassium (K+) across the plasma membrane of animal cells (Figure 7.16). Figure 7.17 reviews the distinction between passive transport and active transport.

Maintenance of Membrane Potential by Ion Pumps All cells have voltages across their plasma membranes. Voltage is electrical potential energy—a separation of opposite charges.

XTOCELWIAR

Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane.

;Na*; Snoh

{Mi CYTOPLASM

\ S"'

[K*] high.



-

cr

becomes reduced — (gains electron)

We could generalize a redox reaction this way: -becomes oxidized

^

+

X

Y '

>

H

Y• 2 Pyruvate + 2 H?O — — — > • 2 ATP _

*

- 2 NADH + 2 H +

A Figure 9.8 The energy input and output of glycolysis.

As summarized in Figure 9.8 and described in detail in Figure 9.9, on the next two pages, the pathway of glycolysis consists of ten steps, which can be divided into two phases. During the energy investment phase, the cell actually spends ATP This investment is repaid with dividends during the energy payoff phase, when ATP is produced by substrate-level phosphorylation and NAD is reduced to NADH by electrons released from the oxidation of the food (glucose in this example). The net energy yield from glycolysis, per glucose molecule, is 2 ATP plus 2 NADH. Notice in Figure 9.9 that all of the carbon originally present in glucose is accounted for in the two molecules of pyruvate; no CO2 is released during glycolysis. Glycolysis occurs whether or not O2 is present. However, if O2 is present, the chemical energy stored in pyruvate and NADH can be extracted by the citric acid cycle and oxidative phosphorylation., respectively Concept Check 1. During the redox reaction in glycolysis, which molecule acts as the oxidizing agent? The reducing agent? For suggested answers, see Appendix A. CHAPTER 9 Cellular Respiration: Harvesting Chemical Energy

165

V Figure 9.9 A closer look at glycolysis. The orientation diagram at the right relates glycolysis to the whole process of respiration. Do not let the chemical detail in the main diagram block your view of glycolysis as a source of ATP and NADH.

ENERGY INVESTMENT PHASE Q Glucose enters the cell and is phosphorylated by the enzyme hexokinase, which transfers a phosphate group from ATP to the sugE r. The charge of the phosphate group traps the sugar in the cell because the plasma membrane is impermeable to ions. Phosphorylation also makes glucose more chemically reactive. In this diagram, the transfer of a phosphate group or pair of electrons from one reactant to another is indicated by coupled arrows: ^[1

H

OH

Glucose-6-phosphate

n© I Phosphogiucoisomerase «

J 0 Glucose-6-phosphate is rearranged to convert it to its isomer, I fructose-6-phosphate.

CH2O—® O

© This enzyme transfers a phosphate group from ATP to the sugar, investing another molecule of ATP in glycolysis. So far, 2 ATP have been used. With phosphate groups on its opposite ends, the sugar is now ready to be split in half. This is a key step for regulation of glycolysis; phosphofructokinase is allosterically regulated by ATP and its products.

© This is the reaction from which glycolysis gets its name. The enzyme cleaves the sugar molecule into two different threecarbon sugars: dihydroxyacetone phosphate and glyceraldehyde3-phosphate. These two sugars are isomers of each other.

C H 2 — 0 —(P) Glyceraldehyde3-phosphate

166

UNIT T W O

The Cell

© Isomerase catalyzes the reversible conversion between the two three-carbon sugars. This reaction never reaches equilibrium in the cell because the next enzyme in glycolysis uses only glyceraldehyde-3-phosphate as its substrate (and not dihydroxyacetone phosphate). This pulls the equilibrium in the direction of glyceraldehyde-3-phosphate, which is removed as fast as it forms. Thus, the net result of steps 4 and 5 is cleavage of a six-carbon sugar into two molecules of glyceraldehyde-3-phosphate; each will progress through the remaining steps of glycolysis.

ENERGY PAYOFF PHASE

CH2— 0— ® 1, 3-Bisphosphoglycerate

0 This enzyme catalyzes two sequential reactions while it holds glyceraldehyde-3-phosphate in its active site. First, the sugar is oxidized by the transfer of electrons and H+ to NAD+, forming NADH (a redox reaction). This reaction is very exergonic, and the enzyme uses the released energy to attach a phosphate group to the oxidized substrate, making a product of very high potential energy. The source of the phosphates is the pool of inorganic phosphate ions that are always present in the cytosol. Notice that the coefficient 2 precedes all molecules in the energy payoff phase; these steps occur after glucose is split into two three-carbon sugars (step 4).

2 ADP —s, M '] I Phosphoglycerokinase «

JI

0 Glycolysis produces some ATP by substrate-level phosphorylation. The phosphate group added in the previous step is transferred to AC ADP in an exergonic reaction. For each glucose molecule that began glycolysis, step 7 produces 2 ATP, since every product after the sugarsplitting step (step 4) is doubled. Recall that 2 ATP were invested to get sugar ready for splitting; this ATP debt has now been repaid. Glucose iich is U* - * * -

L%rt^rt

y-^vrM ir\\+r\r\

+ j^\

+ \ * IY-\

ryi

rtlnf

i 11 r\f

y^"p

^ _ tr\ \r\ />,r t-\ V^ /~\ n \ \ If r\r ^+£\

IA/I^I^I1^

^c

CHOH CH, — 0 — (J 3-Phosphoglycerate

Phosphoglyceromutase •

2

Lj Q Next, this enzyme relocates the remaining phosphate group. This \ \ step prepares the substrate for the next reaction.

0C—0 H —C — 0 — ®

CH2OH 2-Phosphoglycerate Q This enzyme causes a double bond to form in the substrate by extracting a water molecule, yielding phosphoenolpyruvate (PEP). •j The electrons of the substrate are rearranged in such a way that the remaining phosphate bond becomes very unstable, preparing the substrate for the next reaction.

2 H20 -*^4 7 I 2

0" C=0 C— 0 — (?)

Phosphoenolpyruvate 2 ADP —

Pyruvate kinase ••

c=o Pyruvate

(E) The last reaction of glycolysis produces more ATP by transferring the phosphate group from PEP to ADP, a second example of substratelevel phosphorylation. Since this step occurs twice for each glucose molecule, 2 ATP are produced. Overall, glycolysis has used 2 ATP in the energy investment phase (steps 1 and 3} and produced 4 ATP in the energy payoff phase (steps 7 and 10), for a net gain of 2 ATP. Glycolysis has repaid the ATP investment with 100% interest. Additional energy was stored by step 6 in NADH, which can be used to make ATP by oxidative phosphorylation if oxygen is present. Glucose has been broken down and oxidized to two molecules of pyruvate, the end product of the glycolytic pathway. If oxygen is present, the chemical energy in pyruvate can be extracted by the citric acid cycle.

Cellular K.espiration: Harvesting Chemical Energy

167

Concept

The citric acid cycle completes the energy-yielding oxidation of organic molecules Glycolysis releases less than a quarter of the chemical energy stored in glucose; most of the energy remains stockpiled in the two molecules of pyruvate. If molecular oxygen is present, the pyruvate enters the mitochondrion, where the enzymes of the citric acid cycle complete the oxidation of the organic fuel. Upon entering the mitochondrion via active transport, pyruvate is first converted to a compound called acetyl coenzyme A, or acetyl CoA (Figure 9.10). This step, the junction between glycolysis and the citric acid cycle, is accomplished hy a muluenzyme complex that catalyzes three reactions: 0 Pyruvates carboxyl group (—COO ), which is already fully oxidized and thus has little chemical energy, is removed and given off as a molecule of CO2. (This is the first step in which CO2 is released during respiration.) @ The remaining two-carbon fragment is oxidized, forming a compound named acetate (the ionized form of acetic acid). An enzyme transfers the extracted electrons to NAD+, storing energy in the form of NADH. ® Finally, coenzyme A, a sulfur-containing compound derived irom a B vitamin, is attached to the acetate by an unstable bond that makes the acetyl group (the attached acetate) very reactive. The product of this chemical grooming, acetyl CoA, is now ready to feed its acetyl group into the citric acid cycle for further oxidation. The citric acid cycle is also called the tricarboxylic acid cycle or the Krebs cycle, the latter honoring Hans Krebs, the GermanBritish scientist who was largely responsible for elucidating the pathway in the 1930s. The cycle functions as a metabolic furnace

that oxidizes organic iuel derived from pyruvate. Figure 9.11 summarizes the inputs and outputs as pyruvate is broken down to 3 CO2 molecules, including the molecule of CO2 released during the conversion of pyruvate to acetyl CoA. The cycle generates 1 ATP per turn by substrate-level phosphorylation, but most of the chemical energy is transferred lo NADH" and the related coenzyme FAD during the redox reactions. The reduced coenzymes, NADH and FADH2, shuttle their cargo of high-energy electrons to the electron transport chain. Now lets look at the citric acid cycle in more detail. The cycle has eight steps, each catalyzed by a specific enzyme. You can see in Figure 9.12 that for each turn of the citric acid cycle, two carbons (red) enter in the relatively reduced form of an acetyl group (step 1), and two different carbons (blue) leave in the completely oxidized form of CO2 (steps 3 and 4). The acetyl group of acetyl CoA joins the cycle by combining with the compound oxaloacetate, forming citrate (step 1).

\ Pyruvate from glycolysis, 2 molecules per glucose)

TO

p

"

' H I

.

} \ \ \ .i „

,:••„!•,

!,

1

1 n

MITOCHONDRION

Acetyl CoA

Pyruvate Transport protein

A Figure 9.10 Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the citric acid cycle. Because pyruvate is a charged molecule, it must enter the mitochondrion via active transport, with the help of a transport protein. Next, a complex of several enzymes (the pyruvate dehydrogenase complex) catalyzes the three numbered steps, which are described in the text. The acetyl group of acetyl CoA will enter the citric acid cycle. The CO2 molecule will diffuse out of the cell.

168

UNIT TWO

The Cell

A Figure 9.11 An overview of the citric acid cycle. To calculate the inputs and outputs on a per-glucose basis, multiply by 2, because each glucose molecule is split during glycolysis into two pyruvate molecules,

0 Acetyl CoA adds its two-carbon acetyl group to oxaloacetate, producing citrate.

i$ The substrate is oxidized, reducing NAD+ to NADH and regenerating oxaloacetate.

Q Citrate is converted to its isomer, isocitrate, by removal of one water molecule and addition of another.

O=C—COO"

j Addition of a water molecule rearranges bonds in the substrate.

C 00

Succinate I GTP

GDP

A J

Succinyl

C0A

+ H+

^

ADP J. « v

© Another CO2 is lost, and the resulting compound is oxidized, reducing NAD+ to NADH. The remaining molecule is then attached to coenzyme A by an unstable bond.

Q CoA is displaced by a phosphate group, which is transferred to GDP, forming GTP, and then to ADP, forming ATP (substrate-level phosphorylation). 4 Figure 9.12 A closer look at the citric acid cycle. In the chemical structures, red type traces the fate of the t w o carbon atoms that enter the cycle via acetyl CoA (step 1), and blue type indicates the t w o carbons that exit the cycle as CO 2 in steps 3 and 4. (The red labeling only goes through step 5, but you can continue to trace the fate of those carbons.) Notice that

the carbon atoms that enter the cycle from acetyl CoA do not leave the cycle in the same turn. They remain in the cycle, occupying a different location in the molecules on their next turn after another acetyl group is added. As a consequence, the oxaloacetate that is regenerated at step 8 is composed of different carbon atoms each time around. All the citric

CHAPTER 9

acid cycle enzymes are located in the mitochondria I matrix except for the enzyme that catalyzes step 6, which resides in the inner mitochondria! membrane. Carboxylic acids are represented in their ionized forms, as —-COO", because the ionized forms prevail at the pH within the mitochondrion. For example, citrate is the ionized form of citric acid.

Cellular Respiration: Harvesting Chemical Energy

169

(Citrate is the ionized form of citric acid, for which the citric acid cycle is named.) The next seven steps decompose the citrate back to oxaloacetate. It is this regeneration of oxaloacetate thai makes this process a cycle. For each acetyl group that enters the cycle, 3 NAD are reduced to NADH (steps 3, 4, and 8). In step 6, electrons are transferred not to NAD , but to a different electron acceptor, FAD (flavin adenine dinucleotide, derived from riboflavin, a B vitamin). Step 5 in the citric acid cycle forms a GTP molecule directly by substrate-level phosphorylation, similar to the ATP-generating steps of glycolysis. This GTP is then used to synthesize an ATP, the only ATP generated directly by the citric acid cycle. Most of the ATP output of respiration results from oxidative phosphorylation, when the NADH and FADH2 produced by the citric acid cycle relay the electrons extracted from food to the electron transport chain. In the process, they supply the necessary energy for the phosphorylation of ADP to ATP We will explore this process in the next section.

Concept Check 1. In which molecules is most of the energy from the citric acid cycles redox reactions conserved? How will these molecules convert their energy to a form that can be used to make ATP? 2. What cellular processes produce the carbon dioxide that you exhale? For suggested answers, see Appendix A.

Concept

During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis Our main objective in this chapter is to learn howr cells harvest the energy of food to make ATP But the metabolic components of respiration we have dissected so far, glycolysis and the citric acid cycle, produce only 4 ATP molecules per glucose molecule, all by substrate-level phosphorylation: 2 net ATP from glycolysis and 2 ATP from the citric acid cycle. At this point, molecules of NADH (and FADH2) account for most of the energy extracted from the food. These electron escorts link glycolysis and the citric acid cycle to the machinery of oxidative phosphorylation, which uses energy released, by the electron transport chain to power ATP synthesis. In this section, you will learn first how the electron transport chain works, then how the inner membrane of the mitochondrion couples electron flow down the chain to ATP synthesis. 170

UNIT T W O

The Cell

The Pathway of Electron Transport The electron transport chain is a collection of molecules e nbedded in the inner membrane of the mitochondrion. The folding of the inner membrane to form cristae increases its surface area, providing space for thousands of copies ol the chain in each mitochondrion. (Once again, we see thai structure fits function.) Most components of the chain are proteins, which exist in multiprotein complexes numbered I through IV Tightly bound to these proteins are prosthetic groups, nonprotein components essential for the catalytic functions of certain enzymes. Figure 9.13 shows the sequence ot electron carriers in tie electron transport chain and the drop in free energy as electrons travel down the chain. During electron transport along the chain, electron carriers alternate between reduced and oxidized states as they accept and donate electrons. Each component of the chain becomes reduced when it accepts electrons from its "uphill" neighbor, which has a lower affinity for electrons (is less electronegative). It then returns to its oxidized form as it passes electrons to its "downhill," more electronegative neighbor. Now let's take a closer look at the electron transport chain in Figure 9.13. Electrons removed from food by NAD+, durir g glycolysis and the citric acid cycle, are transferred from NADH to the first molecule of the electron transport chain. This molecule is a flavoprotein, so named because it has a prosthetic group called flavin mononucleotide (FMN in complex I). In the next redox reaction, the flavoprotein returns to its oxidized torm as it passes electrons to an iron-sulfur protein (Fe*S in complex I), one of a family of proteins with both iro.i and sulfur tightly bound. The iron-sulfur protein then passes the electrons to a compound called ubiquinone (Q in Figure Q.I3). This electron carrier is a small hydrophobic molecule, the only member of the electron transport chain that is not a protein. Ubiquinone is mobile within the membrane rather than residing in a particular complex. Most of the remaining electron carriers between ubiquinone and oxygen are proteins called cytochromes. Their prosthetic group, called a heme group, has an iron atom that accepts and donates electrons. (It is similar to the heme group in hemoglobin, the protein of red blood cells, except that the iron in hemoglobin carries oxygen, not electrons.) The electron transport chain has several types of cytochromes, each £ different protein with a slightly different- electron-carrying heme group. The last cytochrome of the chain, cyt a3, passes its electrons to oxygen, which is very electronegative. Each oxygen atom also picks up a pair of hydrogen ions from the aqueous solution, forming water. Another source of electrons for the transport chain is FADH2, the other reduced product of the citric acid cycle. Notice in Figure 9.13 that FADH2 adds its electrons to the electron transport chain at complex II, at a lower energy level than NADH does. Consequently, the electron transport chain

provides about one-third less energy for ATP synthesis when the electron donor is FADH2 rather than NADH. The electron transport chain makes no ATP directly. Its function is to ease the fall of electrons from food to oxygen, breaking a large free-energy drop into a series of smaller steps that release energy in manageable amounts. How does the mito:hondrion couple this electron transport and energy release to ATP synthesis? The answer is a mechanism called chemiosmosis.

Chemiosmosis: The Energy-Coupling Mechanism Populating the inner membrane of the mitochondrion are many copies of a protein complex called ATP synthase, the enzyme that actually makes ATP from ADP and inorganic phosphate (Figure 9.14). ATP synthase works like an ion pump running in reverse. Recall from Chapter 7 that ion pumps use ATP as an energy source to transport ions against their gradients. In the reverse of that process, ATP synthase uses the energy of an existing ion gradient to power ATP synthesis. The ion gradient that drives phosphorylation is a proton (hydrogen ion) gradient; that is, the power source for the ATP synthase is a difference in the concentration of H on opposite sides of the inner mitochondrial membrane. (We can also think of this gradient as a difference in pH, since pH is a measure of H+ concentration.) This process, in which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work such as the synthesis of ATP, is called chemiosmosis (from the Greek osmos, push). We have previously used the word osmosis in discussing water transport, but here it refers to the flow of H across a membrane. From studying the structure of ATP synthase, scientists have learned how the flow of H+ through this large enzyme powers ATP generation. ATP synthase is a multisubunit complex with

A rotor within the membrane spins clockwise when H + flows past it down the H + gradient. Astator anchored in the membrane holds the knob stationary. A rod (or "stalk") extending into the knob also spins, activating catalytic sites in the knob.

MITOCHONDRIAL MATRIX

Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP.

A Figure 9.13 Free-energy change during electron t r a n s p o r t . The overall energy drop (AG) for electrons traveling from NADH to oxygen is 53 kcal/mol, but this "fall" is broken up into a series of smaller steps by the electron transport chain. (An oxygen atom is represented here as V2 O 2 to emphasize that the electron transport chain reduces molecular oxygen, O 2 , not individual oxygen atoms. For every 2 NADH molecules, 1 O2 molecule is reduced to 2 H2O.)

A Figure 9.14 ATP synthase, a molecular mill. The ATP synthase protein complex functions as a mill, powered by the flow of hydrogen ions. This complex resides in mitochondrial and chloroplast membranes of eukaryotes and in the plasma membranes of prokaryotes. Each of the four parts of ATP synthase consists of a number of polypeptide subunits.

CHAPTER 9 Cellular Respiration; Harvesting Chemical Energy

171

four main parts, each made up of multiple polypeptides (see Figure 9.14): a rotor in the inner mitochondrial membrane; a knob that protrudes into the mitochondrial matrix; an internal rod extending from the rotor into the knob; and a stator, anchored next to the rotor, that holds the knob stationary: Hydrogen ions flow down a narrow space between the stator and rotor, causing the rotor and its attached rod to rotate, much as a rushing stream turns a waterwheef. The spinning rod causes conformational changes in the stationary knob, activating three catalytic sites in the subunits that make up the knob, such that ADP and inorganic phosphate combine to make ATE So how does the inner mitochondrial membrane generate and maintain the H gradient that drives ATP synthesis in the ATP synthase protein complex? Creating the H+ gradient is the function of the electron transport chain, which is shown

Inner mitochondrial membrane

in its mitochondrial location in Figure 9.15. The chain is an energy converter that uses the exergonic flow of electrons to pump H across the membrane, from the mitochondrial matrix into the intermembrane space. The H+ has a tendency to move back across the membrane, diffusing down its gradient. And the ATP synthases are the only sites on the membrane that are freely permeable to H + . The ions pass through a channel in ATP synthase, which uses the exergonic flow of H + to drive the phosphorylation of ADP (see Figure 9.14). Thus, the energy stored in an H gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis, an example of chemiosmosis. At this point, you may be wondering how the electrcn transport chain pumps hydrogen ions. Researchers have found that certain members of the electron transport chain


[CH20]). This hypothesis predicted that the O2 released during photosynthesis came from CO2. This idea was challenged in the 1930s by C. B. van Niel of Stanford University. Van Niel was investigating photosynthesis in bacteria that make their carbohydrate from CO2 but do not release O2. Van Niel concluded that, at least in these bacteria, CO2 is not spirt into carbon and oxygen. One group of bacteria used hydrogen sulfide (H2S) rather than water for photosynthesis, forming yellow globules of sulfur as a waste product (these globules are visible in Figure 10.2e). Here is the chemical equation for photosynthesis in these sulfur bacteria: CO2 + 2 H 2 S ^ iCH2O] + H2O + 25 Van Niel reasoned that the bacteria split H2S and used the hydrogen atoms to make sugar. He then generalized that idea, proposing that all photosynthetic organisms require a hydrogen source but that the source varies: Sulfur bacteria: CO2 + 2 H2S -» [CH2O] + H2O + 2 S Plants: CO2 + 2 H2O -> [CH2O) + H2O + O2 General: CO2 + 2 H2X -> [CH2O] + H2O + 2 X Thus, van Niel hypothesized that plants split water as a source of electrons from hydrogen atoms, releasing oxygen as a byproduct. Nearly 20 years later, scientists confirmed van Niels hypothesis by using oxygen-18 (18O), a heavy isotope, as a radioactive tracer to follow the fate of oxygen atoms during photosynthesis. The experiments showed that the O2 from plants was labeled with 18O only if water was the source of the tracer (experiment 1). if the ly O was introduced to the plant in the form of CO2, the label did not turn up in the released O2 (experiment 2). In the following summary red denotes labeled atoms of oxygen (18O): Experiment 1: CO2 + 2 H2O -* [CH2O| + H2O + O2 Experiment 2: CO2 + 2 H2O —* [CH2O] + H2O + O2 A significant result of the shuffling of atoms during photosynthesis is the extraction of hydrogen from water and its incorporation into sugar. The waste product of photosynthesis, 184

UNIT TWO

The Cell

A Figure 10.4 Tracking atoms through photosynthesis.

O2, is released to the atmosphere. Figure 10.4 shows the fates of all atoms in photosynthesis. Photosynthesis as a Redox Process Lets briefly compare photosynthesis with cellular respiration. Both processes involve redox reactions. During cellular respiration, energy is released from sugar when electrons associated with hydrogen are transported by carriers to oxygen, forming water as a by-product. The electrons lose potential energy as they "fall" down the electron transport chain toward electronegative oxygen, and the mitochondrion harnesses that energy to synthesize ATP (see Figure 9.15). Photosynthesis reverses the direction of electron flow. Water is split, and electrons are transferred along with hydrogen ions from t ne water to carbon dioxide, reducing it to sugar. Because the electrons increase in potential energy as they move from water to sugar, this process requires energy. This energy boost is provided by light.

The Two Stages of Photosynthesis: A Preview The equation for photosynthesis is a deceptively simple summary of a very complex process. Actually, photosynthesis is not a single process, but two processes, each with multiple steps. These two stages of photosynthesis are known as the light reactions (the photo part of photosynthesis) and the Calvin cycle (the synthesis part) (Figure 10.5). The light reactions are the steps of photosynthesis that convert solar energy to chemical energy. Light absorbed ty chlorophyll drives a transfer of electrons and hydrogen from water to an acceptor called NADP + (nicotinamide adenine dinucleotide phosphate), which temporarily stores the energized electrons. Water is split in the process, and thus it is the light reactions of photosynthesis that give olf O2 as a byproduct. The electron acceptor of the light reactions, NADP , is first cousin to NAD , which functions as an electron carrier in cellular respiration; the two molecules differ only by the presence of an extra phosphate group in the NADPT molecule. The light reactions use solar power to reduce NADP h to NADPH by adding a pair of electrons along with a hydrogen nucleus, or H+. The light reactions also generate ATP, using chemiosmosis to power the addition of a phosphate

• -igure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. In the chloroplast, the thylakoid membranes are the sites of the light reactions, whereas the Ca vin cycle occurs in the stroma. The light reactions use solar energy to make ATP and NADPH, which function as chemical energy and reducing power, respectively, in the Calvin cycle. The Calvin cycle incorporates C 0 2 into organic molecules, which are converted to sugar (Recall frcm Chapter 5 that most simple sugars have foi mutes that are some multiple of [CH 2 0].)

A smaller version of this diagram will reappear in several subsequent figures as a reminder of whether the events being described occur in the light reactions or in the Calvin cycle.

Chloroplast

group to ADP, a process called photophosphorylation. Thus, light energy is initially converted to chemical energy in the form of two compounds: NADPH, a source of energized electrons ("reducing power"), and ATP, the versatile energy currency of cells. Notice that the light reactions produce no sugar; that happens in the second stage of photosynthesis, the Calvin cycle. The Calvin cycle is named for Melvin Calvin, who, along with his colleagues, began to elucidate its steps in the late 1940s, The cycle begins by incorporating CO2 from the air into organic molecules already present in the chloroplast. This initial incorporation of carbon into organic compounds is known as carbon fixation. The Calvin cycle then reduces the fixed carbon to carbohydrate by the addition of electrons. The reducing power is provided by NADPH, which acquired energized electrons in the light reactions. To convert CO2 to carbohydrate, the Calvin cycle also requires chemical energy in the form of ATP, which is also generated by the light reactions. Thus, it is the Calvin cycle that makes sugar, but it can do so only with the help of the NADPH and ATP produced by the light reactions. The metabolic steps of the Calvin cycle are sometimes referred to as the dark reactions, or light-independent reactions, because none of the steps requires light directly. Nevertheless, the Calvin cycle in most plants occurs during daylight, ior only then can the light reactions provide the NADPH and

ATP thai the Calvin cycle requires. In essence, the chloroplast uses light energy to make sugar by coordinating the two stages of photosynthesis. As Figure 10.5 indicates, the thylakoids of the chloroplast are the sites of the light reactions, while the Calvin cycle occurs in the stroma. In the thylakoids, molecules of NADP+ and ADP pick up electrons and phosphate, respectively, and then are released to the stroma, where they transfer their high-energy cargo to the Calvin cycle. The two stages of photosynthesis are treated in this figure as metabolic modules that lake in ingredients and crank out products. Our next step toward understanding photosynthesis is to look more closely at how the two stages work, beginning with the light reactions.

Concept Check 1. How do the reactant molecules of photosynthesis reach the chloroplasts in leaves? 2. How did the use of an oxygen isotope help elucidate the chemistry of photosynthesis? 3. Describe how the two stages of photosynthesis are dependent on each other. For suggested answers, sec Appendix A.

CHAPTER 10

Photosynthesis

185

Concept

The light reactions convert solar energy to the chemical energy of ATP and NADPH Chloroplasts are chemical factories powered by the sun. Their thylakoids transform light energy into the chemical energy of ATP and NADPH. To understand this conversion better, we need to know about some important properties of light.

The Nature of Sunlight Light is a form of energy known as electromagnetic energy also called electromagnetic radiation. Electromagnetic energy travels in rhythmic waves analogous to those created by dropping a pebble into a pond. Electromagnetic waves, however, are disturbances of electrical and magnetic fields rather than disturbances of a material medium such as water. The distance between the crests of electromagnetic waves is called the wavelength. Wavelengths range from less than a nanometer (for gamma rays) to more than a kilometer (for radio waves)- This entire range of radiation is known as the electromagnetic spectrum (Figure 10.6). The segment most important to life is the narrow band from about 380 nm to 750 nm in wavelength. This radiation is known as visible light because it is detected as various colors by the human eye. The model of light as waves explains many of lights properties, but in certain respects light behaves as though it consists of discrete particles, called photons. Photons are not tangible objects, but they act like objects in that each of them

has a fixed quantity of energy. The amount of energy is inversely related to the wavelength of the light; the shorter t ic wavelength, the greater the energy of each photon of that light. Thus, a photon of violet light packs nearly twice as much energy as a photon of red light. Although the sun radiates the full spectrum of electromagnetic energy the atmosphere acts like a selective window, allowing visible light to pass through while screening out a substantial fraction of other radiation. The part of the spectrum we can see—visible light—is also the radiation that drives photosynthesis.

Photosynthetic Pigments: The Light Receptors When light meets matter, it may be reflected, transmitted, or absorbed. Substances that absorb visible light are known as pigments. Different pigments absorb light of different wavelengths, and the wavelengths that are absorbed disappear. If a pigment is illuminated with white light, the color we see is the color most reflected or transmitted by the pigment. (If a pigment absorbs all wavelengths, it appears black.) We see green when we look at a leaf because chlorophyll absorbs violet-blue and red light while transmitting and reflecting green light (Figure 10.7). The ability of a pigment to absorb various wavelengths of light can be measured with an instrument called a spectrophotometer. This machine direct; beams of light of different wavelengths through a solution of the pigment and measures the fraction of the light transmitted

Light

Absorbed light

Transmitted '. light A Figure 10.6 The electromagnetic spectrum. White light is a mixture of all wavelengths of visible light. A prism can sort white light into its component colors by bending light of different wavelengths at different angles. (Droplets of water in the atmosphere can act as prisms, forming a rainbow; see Figure 10.1.) Visible light drives photosynthesis.

186

UNIT TWO

The Cell

A Figure 10.7 Why leaves are green: interaction of light with chloroplasts. The chlorophyll molecules of chloroplasts absorb violet-blue and red light (the colors most effective in driving photosynthesis) and reflect or transmit green iight. This is why leaves appear green.

al each wavelength (Figure 10.8). A graph plotting a pigment's light absorption versus wavelength is called an absorption spectrum. The absorption spectra of chloroplast pigments provide clues to the relative effectiveness of different wavelengths for driving photosynthesis, since light can perform work in chloroplasts only if it is absorbed. Figure 10.9a shows the absorption spectra of three types of pigments in chloroplasts. If we look first at the absorption spectrum of chlorophyll a, it suggests that violetblue and red light work best for photosynthesis, since they are

Figure 10.9

aquiry Which wavelengths of light are most effective in driving photosynthesis? Three different experiments helped reveal I which wavelengths of light are photosynthetically important. The results are shown below.

Chlorophyll a

< Determining an Absorption pectrum i absorption spectrum is a visual representation of how well a particular pigment absorbs different wavelengths of visible light. Absorption spectra of various chloroplast pigments help scientists decipher each pigment's role in a plant. 600 A spectra photo meter measures the relative amounts of light of different wavelengths absorbed and transmitted by a pigment solution. O White light is separated into colors (wavelengths) by a prism. © One by one, the different colors of light are passed through the sample (chlorophyll in this example). Green light and blue light are shown here. © The transmitted light strikes a photoelectric tube, which converts the light energy to electricity. © The electrical current is measured by a galvanometer. The meter indicates the fraction of light transmitted through the sample, from which we can determine the amount of light absorbed. White light

Refracting prism

Slit moves to pass light of selected wavelength

Chlorophyll Photoelectric solution tube

Green light

Wavelength of light (nm) (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.

(b) Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.

The hiqh transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light.

400

The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light. See Figure 10.9a for absorption spectra of three types of chloroplast pigments.

500

(c) Engelmann's experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b. Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis. CHAPTER 10

Photosynthesis

187

absorbed, while green is the least effective color. This is confirmed by an action spectrum for photosynthesis (Figure 10.9b), which profiles the relative effectiveness of different wavelengths of radiation in driving the process. An action spectrum is prepared by illuminating chloroplasts with light of different colors and then plotting wavelength against some measure of photosynthetic rate, such as CO2 consumption or O2 release. The action spectrum for photosynthesis was first demonstrated in 1883 in an elegant experiment performed by German botanist Theodor W Engelmann, who used bacteria to measure rates of photosynthesis in filamentous algae (Figure 10.9c). Notice by comparing Figures 10.9a and 10.9b that the action spectrum for photosynthesis does not exactly match ihe absorption spectrum of chlorophyll a. The absorption spectrum of chlorophyll a alone underestimates the effectiveness of certain wavelengths in driving photosynthesis. This is partly because accessory pigments with different absorption spectra are also photosynthetically important in chloroplasts and broaden the spectrum of colors that can be used for photosynthesis. One of these accessory pigments is another form of chlorophyll, chlorophyll b. Chlorophyll b is almost identical to chlorophyll a, but a slight structural difference between them (Figure 10.10) is enough to give the two pigments slightly different absorption spectra (see Figure 10.9a). As a result, they have different colors—chlorophyll a is blue-green, whereas chlorophyll b is yellow-green. Other accessory pigments include carotenoids, hydrocarbons that are various shades of yellow and orange because they absorb violet and blue-green light (see Figure 10.9a). Carotenoids may broaden the spectrum of colors that can drive photosynthesis. However, a more important function of at least some carotenoids seems to be photoprotection: These compounds absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen, forming reactive oxidative molecules that are dangerous to the cell. Interestingly, carotenoids similar to the photoprotective ones in chloroplasts have a photoprotective role in the human eye. These and other related molecules are highlighted in health food products as "phytochemicals" (from the Greek phyton, plant) that have antioxidant powers. Plants can synthesize all the antioxidants they require, whereas humans and other animals must obtain some of them from their diets.

Excitation of Chlorophyll by Light What exactly happens when chlorophyll and other pigments absorb light? The colors corresponding to the absorbed wavelengths disappear from the spectrum of the transmitted and reflected light, but energy cannot disappear. When a molecule absorbs a photon of light, one of the molecules electrons is elevated to an orbital where it has more potential energy. When the electron is in its normal orbital, the pigment molecule is said to be in its ground state. Absorption of a photon boosts 188

UNIT TWO

The Cell

in chlorophyll a in chlorophyll b

Porphyrin ring: light-absorbing "head" of molecule; note magnesium atom at center

Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown A Figure 10.10 Structure of chlorophyll molecules in chloroplasts of plants. Chlorophyll a and chlorophyll b differ only in one of the functional groups bonded to the porphyrin ring.

an electron to an orbital of higher energy, and the pigmen': molecule is then said to be in an excited state. The only photons absorbed are those whose energy is exactly equal to thtenergy difference between the ground state and an excited state, and this energy difference varies from one kind of atom or molecule to another. Thus, a particular compound absorb? only photons corresponding to specific wavelengths, which is why each pigment has a unique absorption spectrum. Once absorption of a photon raises an electron trom the ground state to an excited state, the electron cannot remain there long. The excited state, like all high-energy states, is unstable. Generally, when isolated pigment molecules absorb light, their excited electrons drop back down to the ground-state orbital in a billionth of a second, releasing their excess energy as heat. This conversion of light energy to heat is what makes the top of an automobile so hot on a sunny day. (White cars are coolest because their paint reflects all wavelengths of visible light, although it may absorb ultraviolet and other invisible radiation.) In isolation, some pigments, including chlorophyll, emit light as well as heat after absorbing photons. As excited electrons fall back to the ground state, photons are given off. This afterglow is called fluorescence. If a solution of chlorophyll isolated from chloroplasts is illuminated, it will fluoresce in the red-orange part of the spectrum and also give off heat (Figure 10.11).

• ! igure 10.11 Excitation of isolated chlorophyll by light, (a) Absorption of a photon causes a transition of the chlorophyll mo ecule from its ground state to its excited stale. The photon boosts an electron to an orbital where it has more potential energy. If the illuminated molecule exists in isolation, its excited electron immediately drops back down to ihe ground-state orbital, and its excess energy is given off as heat and fluorescence (light), (b) A chlorophyll solution excited with ulti aviolet light fluoresces with a red-orange glow.

(a) Excitation of isolated chlorophyll molecule

A Photosystem: A Reaction Center Associated vith Light-Harvesting Complexes Chlorophyll molecules excited by the absorption of light energy produce very different results in an intact chloroplast than they do in isolation (see Figure 10.11). In their native environment of the thylakoid membrane, chlorophyll molecules are organized along with other small organic molecules and proteins into photosystems. A photosystem is composed of a reaction center surrounded by a number of light-harvesting complexes (Figure 10.12). Each light-harvesting complex consists of pigment molecules (which may include chlorophyll a, chlorophyll b, end carotenoids) bound to particular proteins. The number and variety of pigment molecules enable a photosystem to harvest light over a larger surface and a larger portion oi the spectrum than any single pigment molecule alone could. Together, these light-harvesting complexes act as an antenna for the reaction center. When a pigment molecule absorbs a phoLon, the energy is transferred from pigment molecule to pigment molecule within a light-harvesting complex until it is funneled into :he reaction center. The reaction center is a protein complex chat includes two special chlorophyll a molecules and a molecule called the primary electron acceptor. These chlorophyll a molecules are special because their molecular environment—their location and the other molecules with which they are associated—enables them to use the energy from light to boost one of their electrons to a higher energy level. The solar-powered transfer of an electron from a special chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions. As soon as the chlorophyll electron is excited to a higher energy level, the primary electron acceptor captures it; this is a redox reaction. Isolated chlorophyll fluoresces because there is no electron acceptor, so electrons of photoexcited chlorophyll drop right back to the ground state.

(b) Fluorescence

Thylakoid

THYLAKOID SPACE (INTERIOR OF THYLAKOID)

k Figure 10.12 How a photosystem harvests light. When a photon strikes a pigment molecule in a light-harvesting complex, the energy is passed from molecule to molecule until it reaches the reaction center. At the reaction center, an excited electron from one of the t w o special chlorophyll a molecules is captured by the primary electron acceptor.

In a chloroplast, this immediate plunge of high-energy electrons back to the ground state is prevented. Thus, each photosystem—a reaction center surrounded by light-harvesting complexes—functions in the chloroplast as a unit. It converts CHAPTER 1 o

Phmosyn thesis

189

light energy to chemical energy, which will ultimately be used for rhe synthesis of sugar. The thylakoid membrane is populated by two types of photosystems that cooperate in the light reactions of photosynthesis. They are called photosystem II (PS II) and photosystem I (PS I). (They were named in order of their discovery, but the two function sequentially, with photosystem II functioning first.) Each has a characteristic reaction center—a particular kind of primary electron acceptor next to a pair of special chlorophyll a molecules associated with specific proteins. The reaction-center chlorophyll a of photosystem II is known as P680 because this pigment is best at absorbing light having a wavelength of 680 nm (in the red part of the spectrum). The chlorophyll a at the reaction center of photosystem I is called P700 because it most effectively absorbs light of wavelength 700 nm (in the far red part of the spectrum). These two pigments, P680 and P700, are actually identical chlorophyll a molecules. However, their association with different proteins in the thylakoid membrane affects the electron distribution in the chlorophyll molecules and accounts for the slight differences in light-absorbing properties. Now let's see how the two photosystems work together in using light energy to generate ATP and NADPH, the two main products of the light reactions.

Noncyclic Electron Flow Light drives the synthesis of NADPH and ATP by energizing the

two photosystems embedded in the thylakoid membranes of chloroplasts. The key to this energy transformation is a flow of electrons through the photosystems and other molecular components built into the thylakoid membrane. During the light reactions of photosynthesis, there are two possible routes for electron flow: cyclic and noncyclic. Noncyclic electron flow, the predominant route, is shown in Figure 10.13. The numbers in tie text description correspond to the numbered steps in the figure. O A photon of light strikes a pigment molecule in a lightharvesting complex and is relayed to other pigment molecules until it reaches one of the two P680 chlorophyll a molecules in the PS II reaction center. It excites one of the P680 electrons to a higher energy state. © This electron is captured by the primary electron acceptor. © An enzyme splits a water molecule into two electrons, two hydrogen ions, and an oxygen atom. The electrons are supplied one by one to the P680 molecules, each replacing an electron lost to the primary electron acceptor. (Missing an electron, P680 is the strongest biological oxidizing agent known; its electron hole must be filled.)

f Figure 10.13 How noncyclic electron flow during the light reactions generates ATP and NADPH. The gold arrows trace the current of light-driven electrons from water to NADPH.

_

Photosystem I (PS I)

190

UNIT TWO

The Cell

NADP* + 2 H*

The oxygen atom immediately combines with another oxygen atom, forming O2. () Each photoexcited electron passes from the primary electron acceptor of PS II to PS I via an electron transport chain (similar to the electron transport chain that functions m cellular respiration). The electron transport chain between PS II and PS I is made up of the electron carrier plastoquinone (Pq), a cytochrome complex, and a protein called plastocyanin (Pc). f§) The exergonic "fall" of electrons to a lower energy level provides energy for the synthesis of ATP. rotein kinase 3.

0 Enzymes called protein phosphatases (PP) catalyze the removal of the phosphate groups from the proteins, making them inactive and available for reuse.

Q Finally, active protein kinase 3 phosphorylates a protein (pink) that brings about the cell's response to the signal.

Cellular response

A. Figure 11.8 A phosphorylation cascade. In a phosphorylation cascade, a series of different molecules in a pathway are phosphorylated in turn, each molecule adding a phosphate group to the next one in line. The active and inactive forms of each protein are represented by different shapes to remind you that activation is usually associated with a change in molecular conformation. Cell Communication

209

The importance of prolein kinases can hardly be overstated. About 2% of our own genes are thought to code for protein kinases. A single cell may have hundreds of different kinds, each specific for a different substrate protein. Together, they probably regulate a large proportion of the thousands of proteins in a cell. Among these are most of the proteins that, in turn, regulate cell reproduction. Abnormal activity of such a kinase can cause abnormal cell growth and contribute to the development of cancer.

and receptor tyrosine kinases. The two most widely used second messengers are cyclic AMP and calcium ions, Ca2+. A large variety of relay proteins are sensitive to the cytosolic concentration of one or the other of these second messengers. Cyclic AMP Once Earl Sutherland had established that epinephrine somehow causes glycogen breakdown without passing through the plasma membrane, the search began for the second messenger (he coined the term) that transmits the signal from the plasma membrane to the metabolic machinery in the cytoplasm. Sutherland found that the binding of epinephrine to the plasma membrane of a liver cell elevates the cytosolic concentration of a compound called cyclic adenosine mono phosphate, abbreviated cyclic AMP or cAMP (Figure 11.9). An enzyme embedded in the plasma membrane, adenylyl cyclase, converts ATP to cAMP in response to an extracellular signal—in this case, epinephrine. But the epinephrine doesn't stimulaie the adenylyl cyclase directly When epinephrine outside tie cell binds to a specific receptor protein, the protein activates adenylyl cyclase, which in turn can catalyze the synthesis of many molecules of cAMP In this way, the normal cellular concentration of cAMP can be boosted twentyfold in a matter ol seconds. The cAMP broadcasts the signal to the cytoplasm. It does not persist for long in the absence of the hormone, because another enzyme, called phosphodiesterase, converts the cAMP to AMP Another surge of epinephrine is needed to boost the cytosolic concentration of cAMP again. Subsequent research has revealed that epinephrine is onl/ one of many hormones and other signal molecules that trigger the formation of cAMP. It has also brought to light th; other components of cAMP pathways, including G proteins, G-protein-linked receptors, and protein kinases (Figur; 11.10). The immediate effect of cAMP is usually the activation of a senne/threonine kinase called protein kinase A. The activated kinase then phosphorylates various other proteins, depending on the cell type. (The complete pathway for

Equally important in the phosphorylation cascade are the protein phosphatases, enzymes that can rapidly remove phosphate groups from proteins, a process called dephosphorylation. By dephosphorylating and thus inactivating protein kinases, phosphatases provide the mechanism for turning off the signal transduction pathway when the initial signal is no longer present. Phosphatases also make the protein kinases available for reuse, enabling the cell to respond again to an extracellular signal. At any given moment, the activity of a protein regulated by phosphorylation depends on the balance in the cell between active kinase molecules and active phosphatase molecules. The phosphorylation/dephosphorylation system acts as a molecular switch in the cell, turning activities on or off as required.

Small Molecules and Ions as Second Messengers Not all components of signal transduction pathways are proteins. Many signaling pathways also involve small, nonprotein, water-soluble molecules or ions called second messengers. (The extracellular signal molecule that binds to the membrane receptor is a pathway's "first messenger.") Because second messengers are both small and water-soluble, they can readily spread throughout the cell by diffusion. For example, as we'll see shortly, it is a second messenger called cyclic AMP that carries the signal initiated by epinephrine from the plasma membrane of a liver or muscle cell into the cell's interior, where it brings about glycogen breakdown. Second messengers participate in pathways initiated by both G-protein-linked receptors

Adenylyl cyciase 0"

0"

Phosphodiesterase

0Pyrophosphate

(EKE), ATP

P

•

cr

x

o

Cyclic AMP

A Figure 11.9 Cyclic AMP. The second messenger cyclic AMP (cAMP) is made from ATP by adenylyl cyclase, an enzyme embedded in the plasma membrane. Cyclic AMP is inactivated by phosphodiesterase, an enzyme that converts it to AMP.

210

UNIT TWO

The Cell

- First messenger (signal molecule

blood flow to the penis, optimizing physiological conditions for penile erections. Calcium Ions and Inositol Trisphosphate (IP3)

ii Figure 11.10 cAMP as a second messenger in a Gprotein-signaling pathway. The first messenger activates a G-protein-linked receptor, which activates a specific G protein. In turn, the G protein activates iidenylyl cyclase, which catalyzes 1 he conversion of ATP to cAMP. The cAMP then activates another protein, usually protein kinase A.

epinephrine's stimulation of glycogen breakdown is shown la.er, in Figure 11.13.) "Further regulation of cell metabolism is provided by other G-protein systems that inhibit adenylyl cyclase. In these systems, a different signal molecule activates a different receptor, which activates an inhibitory G protein. Now that we know about the role of cAMP in G-proteinsignaling pathways, we can explain in molecular detail how certain microbes cause disease. Consider cholera, a disease that is frequently epidemic in places where the water supply is contaminated with human feces. People acquire the cholera bacterium, Vibrio cholerae, by drinking contaminated water. 1 he bacteria colonize the lining of the small intestine and produce a toxin. The cholera toxin is an enzyme that chemically modifies a G protein involved in regulating salt and water secretion. Because the modified G protein is unable to hydrolyze GTP to GDP, it remains stuck in its active form, continuously stimulating adenylyl cyclase to make cAMP The resulting high concentration of cAMP causes the intestinal cells to secrete large amounts of water and salts into the intestines. An infected person quickly develops profuse diarrhea and if left untreated can soon die from the loss of water and salts. Our understanding of signaling pathways involving cyclic AMP or related messengers has allowed us to develop treatments for certain conditions in humans. One such pathway uses cyclic GMP, or cGMP, as a signaling molecule; its effects include relaxation of smooth muscle cells in artery walls. A compound that inhibits the hydrolysis of cGMP to GMP, thus prolonging the signal, was originally prescribed for chest pains because it increased blood flow to the heart muscle. Under the trade name Viagra (see Figure 11.1), this compound is now widely used as a treatment for erectile dysfunction. Viagra causes dilation of blood vessels, which allows increased

Many signal molecules in animals, including neurotransmitters, growth factors, and some hormones, induce responses in their target cells via signal transduction pathways that increase the cytosolic concentration of calcium ions (Ca2+). Calcium is even more widely used than cAMP as a second messenger. Increasing the cytosolic concentration of Ca2+ causes many responses in animal cells, including muscle cell contraction, secretion of certain substances, and cell division. In plant cells, a wide range of hormonal and environmental stimuli can cause brief increases in cytosolic Ca + concentration, triggering various signaling pathways, such as the pathway for greening in response to light (see Figure 39.4). Cells use Ca as a second messenger in both G-protein and receptor tyrosine kinase pathways. Although cells always contain some Ca2+, this ion can function as a second messenger because its concentration in the cytosol is normally much lower than the concentration outside the cell (Figure 11.11). In fact, the level of Ca 2+ in

CELLULAR

generation (second filial generation). Mendel usually followed traits for at least the P, Fi, and F2 generations. Had Mendel stopped his experiments with the F: generation, the basic patterns of inheritance would have eluded him. It was mainly Mendel's quantitative analysis of F2 plants that revealed the two fundamental principles of heredity that are now known as the law of segregation and the law of independent assortment.

I he Law of Segregation II the blending model of inheritance were correct, the ¥l hybrids from a cross between purple-flowered and whiteflowered pea plants would have pale purple flowers, intermediate between the two varieties of the P generation. Notice in Figure 14.2 that the experiment produced a very different result: All the F-[ offspring had flowers just as purple as the purplef owered parents. What happened to the white-flowered plants' genetic contribution to the hybrids? If it were lost, then theFj plants could produce only purple-flowered offspring in the F2 generation. But when Mendel allowed the F-, plants to self-pollinate and planted their seeds, the white-flower trait reappeared in the F2 generation. Mendel used very large sample sizes and kept accurate lecords of his results: 705 of the F2 plants had purple flowers, and 224 had white flowers. These data fit a ratio of approximately three purple to one white (Figure 14.3). Mendel reasoned that the heritable factor for white flowers did not disappear in the Fi plants, but only the purple-flower factor was affecting flower color in these hybrids. In Mendel's terminology, purple flower color is a dominant trait and white flower color is a recessive trait. The reappearance of white-flowered plants in the F2 generation was evidence that the heritable fac.or causing that recessive trait had not been diluted by coexist\ng with the purple-flower factor in the Fx hybrids. Mendel observed the same pattern of inheritance in six other characters, each represented by two different traits Table 14.1, on the next page). For example, the parental pea seeds either had a smooth, round shape or were wrinkled. When Mendel crossed his two true-breeding varieties, all the Fi hybrids produced round seeds; this is the dominant trait. In the F^ generation, 75% of the seeds were

round and 2D% were wrinkled—a 3:1 ratio, as in Figure 14.3. Now let's see how Mendel deduced the law of segregation from his experimental results. In our discussion, we will use modern terms instead of some of the terms used by Mendel (for example, we'll use Are the genes for body color and wing size in fruit flies located on the same chromosome or different chromosomes? Morgan first mated true-breeding wild-type flies with black, vestigialwinged flies to produce heterozygous F1 dihybrids, all of which are wild-type in appearance. He tnen mated wild-type F, dihybrid females with black, vestigial-winged males, producing 2,300 F 2 offspring, which he classified according to phenotype.

somes to gametes. Figure 15.6 (p. 280) shows how crossing over in a dihybrid female fly resulted in recombinant ova and ultimately recombinant offspring in Morgan's testcross. Most of the ova had a chromosome with either the b+ vg+ or b vg parental genotype for body color and wing size, but some ova had a recombinant chromosome (b + vg or b vg"1"). Fertilization of these various classes of ova by homozygous recessive sperm (b vg) produced an offspring population in which 17% exhibited a nonparental, recombinant phenotype (see Figure 15.6). As we discuss next, the percentage of recombinant offspring, the recombination frequency, is related to the dis-

tance between linked genes.

Linkage Mapping Using Recombination Data: Scientific Inquiry The discovery of linked genes and recombination due to crossing over led one of Morgan's students, Alfred H. Sturtevant, to a method for constructing a genetic map, an ordered List of the genetic loci along a particular chromosome. Sturtevant hypothesized that recombination frequencies calculated from experiments like the one in Figures 15.5 and 15.6 depend on the distances between genes on a chromosome. He assumed that crossing over is a random event, and thus the chance of crossing over is approximately equal at all points along a chromosome. Based on these assumptions, SturteIf these t w o genes were on different chromosomes, the alleles from the F, dihybrid would sort into gametes independently, and we would expect to see equal numbers of the four types of offspring. If these t w o genes were on the same chromosome, we would expect each allele combination, b+ vg+ and b vg, to stay together as gametes formed. In this case, only offspring with parental phenotypes would be produced. Since most offspring had a parental phenotype, Morgan concluded that the genes for body color and wing size are located on the same chromosome. However, the production of a small number of offspring with nonparental phenotypes indicated that some mechanism occasionally breaks the linkage between genes on the same chromosome.

md one paternal chromatic! break at corresponding points and then are rejoined to each other (see Figure 13.11). In effect, the end portions of two nonsister chromatids trade places each time a crossover occurs. The recombinant chromosomes resulting from crossing over may bring alleles together in new combinations, and the subsequent events of meiosis distribute the recombinant chromo-

vant predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency.

His reasoning was simple: The greater the distance between two genes, the more points there are between them where crossing over can occur. Using recombination data from various fruit fly crosses, Sturtevant proceeded to assign relative positions to genes on the same chromosomes—that is, to map genes.

A genetic map based on recombination frequencies is specifically called a linkage map. Figure 15.7 (p. 281) shows Sturtevants linkage map of three genes: the body-color (b) and wing-size (vg) genes depicted in Figure 15.6 and a third gene, called cinnabar (en). Cinnabar is one of many Drosophila genes CHAPTER 15

The Chromosoma1 Basis of Inheritance

279

affecting eye color. Cinnabar eyes, a mutant phenotype, are a brighter red than the wild-type color. The recombination frequency between en and b is 9%; that between en and vg, 9.5%; and that between b and vg, 17%. In other words, crossovers between en and b and between en and vg are about half as frequent as crossovers between b and vg. Only a map that locates en about midway between b and vg is consistent with these data, as you can prove to yourself by drawing alternative maps.

Sturtevant expressed the distances between genes in map units, defining one map unit as equivalent to a 1% recombination frequency. Today map units often are called centimorgans in honor of Morgan. In practice, the interpretation of recombination data is more complicated than this example suggests. For example, some genes on a chromosome are so far from each other that a crossover between them is virtually certain. The observed

Testcross parents

b vg ^*MM» Ti""'1'" b vg

Black body, vestigial wings (double mutant)

Replication of chromosomes

vg

b vg

Meiosis I: Crossing over between b and vg loci produces new allele combinations.

Meiosis 1 and II: No new allele combinations are produced. Meiosis I I : Separation of chromatids produces recombinant gametes with the new allele combinations. Ova

b vq\ { / b* vcKY

Sperm Recombination _ frequency b vg

b vg

Parental-type offspring

A Figure 15.6 Chromosomal basis for recombination of linked genes. In these diagrams re-creating the testcross in Figure 15.5, we track chromosomes as well as genes. The maternal chromosomes are color-coded to

280

U N I T THREE

Genetics

391 recombinants , 2,300 total offspring '

b vg Recombinant offspring

distinguish one homologue from the other. Because crossing over between the b and vg loci occurs in some, but not all, ovum-producing cells, more ova with parental-type chromosomes than with recombinant ones are produced in the

mating females. Fertilization of the ova by sperm of genotype b vg gives rise to some recombinant offspring. The recombination frequency is the percentage of recombinant flies in the total pool of offspring.

A linkage map is based on the assumption that the probability of a crossover between t w o genetic loci is proportional to the distance separating the loci. The recombination irequencies used to construct a linkage map for a particular chromosome are obtained from experimental crosses, such as the cross depicted in Figure 15.6. The distances between genes are expressed as map units (centimorgans), with one map unit equivalent to a 1 % recombination frequency. Genes are arranged in the chromosome in the order that best fits the data.

In this example, the observed recombination frequencies between three Drosopbila gene pairs (b-cn 9%, cn-vg 9.5%, and b-vg 17%) best fit a linear order in which en is positioned about halfway between the other t w o genes:

Recombination frequencies

Long aristae (appendages on head)

Gray body

Chromosome The b-vg recombination frequency is slightly less than the sum of the b-cn and cn-vg frequencies because double crossovers are fairly likely to occur between b and vg in matings tracking these t w o genes. A second crossover would "cancel out" the first and thus reduce the observed b-vg recombination frequency.

frequency of recombination in crosses involving two such genes can have a maximum value of 50%, a result indistinguishable from that for genes on different chromosomes. In this case, the physical connection between genes on the same chromosome is not reflected in the results of genetic crosses. Despite being on the same chromosome and thus being physically linked, the genes are genetically unlinked; alleles of such genes assort independently as if they were on different chromosomes. In fact, the genes for two of the pea characters that Mendel studied—seed color and flower color—-are now known to be on the same chromosome, but the distance between them is so great that linkage is not observed in geletic crosses. Genes located far apart on a chromosome are mapped by adding the recombination frequencies from crosses involving each of the distant genes and a number of genes lying between them. Using recombination data, Sturtevant and his colleagues were able to map numerous Drosophila genes in linear arrays. They found that the genes clustered into four groups of

Normal wings

Red eyes

Wild-type phenotypes

-9% > 17%-

Red eyes

A Figure 15.8 A partial genetic (linkage) map of a Drosophila c h r o m o s o m e . This simplified map shows just a few of the genes that have been mapped on Drosophila chromosome II. The number at each gene locus indicates the number of map units between that locus and the locus for aristae length (left). Notice that more than one gene can affect a given phenotypic characteristic, such as eye color. Also, note that in contrast to the homologous autosomes (ll-IV), the X and Y sex chromosomes (I) have distinct shapes.

linked genes. Because microscopists had found four pairs of chromosomes in Drosophila cells, this clustering of genes was additional evidence that genes are located on chromosomes. Each chromosome has a linear array of specific gene loci (Figure 15.8). Because a linkage map is based on recombination frequencies, it gives only an approximate picture of a chromosome. The frequency of crossing over is not actually uniform over the length of a chromosome, as Sturtevant assumed, and therefore map units do not correspond to actual physical distances (in nanometers, for instance). A linkage map does portray the order of genes along a chromosome, but it does not accurately portray the precise locations of those genes. Other methods enable geneticists to construct cytogenetic maps of chromosomes, which locate genes with respect to chromosomal features, such as sLained bands, that can be seen in the microscope. The ultimate maps, which we will discuss in Chapter 20, show the physical distances between gene loci in DNA nucleotides. Comparing a linkage map with such a physical map or with a cytogenetic map of the same chromosome, we find that the linear order of genes is identical in all the maps; but the spacing between genes is not.

CHAPTER is

The Cbroino;!.•.rial Basis oI [nhtTitance

281

Concept Check 1. When two genes are located on the same chromosome, what is the physical basis for the production of recombinant offspring m a testcross between a dihybnd parent and a double-mutant parent? 2. For each type of offspring in Figure 15.5, explain the relationship between its phenotype and the alkies contributed by the female parent. 3. Genes A, B, and C are located on the same chromosome. Testcrosses show that the recombination frequency between A and B is 28% and between A and C is 12%. Can you determine the linear order of these genes?

(a) The X-Y system. In mammals, the sex of an offspring depend on whether the sperm ceii contains an X chromosome or a Y.

For suggested answers, see Appendix A. •

*

&

9

Concept

Sex-linked genes exhibit unique patterns of inheritance

c?

(b) The X-0 system. shoppers, roaches, snd some other *.-... insects, there is 01 y one type of sex chromosome, the X. - Females are XX; r ales h ve only one sex chromosome (XO). jffspring is determined d by whether the sperm cell X chromosome or no sex chromosome.

As you learned earlier, Morgan's discovery of a trait (white eyes) that correlated with the sex of flies was a key episode in the development of the chromosome theory of inheritance. In this section, we consider the role of sex chromosomes in inheritance in more detail. We begin by reviewing the chromosomal basis of sex determination in humans and some other animals. ) The Z-W system. In I 'ds, some fishes, and some insects,'

The Chromosomal Basis of Sex Whether we are male or female is one of our more obvious phenotypic characters. Although the anatomical and physiological differences between women and men are numerous, the chromosomal basis for determining sex is rather simple. In humans and other mammals, there are two varieties of sex chromosomes, designated X and Y. A person who inherits two X chromosomes, one from each parent, usually develops as a female. A male develops from a zygote containing one X chromosome and one Y chromosome (Figure 15.9a). The Y chromosome is much smaller than the X chromosome (see the micrograph to the left), and only relatively short segments at either end of the Y chromosome are homologous with corresponding regions of the X. These homologous regions allow the X and Y chromosomes in males to pair and behave like homologous chromosomes during meiosis in the testes.

282

U N I T THREE

Gentrr.es

sex chromosome pres it in the ovum (not the sperm) detei '• mines the sex of offsp rtg. The sex chromosomes-are Females are ZW and mafes are ZZ. designated Z and

((Diploid) I

?

" i (HaploidH

d

[) The haplo-diploid system. There are no sex chromosomes in most species of bees and ants. Females develop from fertilized ova and are thus diploid, Males develop from unfertilized egg and 3f& hapioid; they hove no fathers.

A Figure 15.9 Some chromosomal systems of sex d e t e r m i n a t i o n . Numerals indicate the number of autosomes. In Drosophila, males are XY, but sex depends on the ratio between the number of X chromosomes and the number of autosome sets, not simply on the presence of a Y chromosome.

In both testes and ovaries, the two sex chromosomes segregate during meiosis, and each gamete receives one. Each ovum contains one X chromosome. In contrast, sperm fall into two categories: Half the sperm cells a male produces contain an X chromosome, and half contain a Y chromosome. We

characters unrelated to sex. A gene located on either sex chromosome is called a sex-linked gene, although in humans the term has historically referred specifically to a gene on the X chromosome. (Note the distinction between the terms sexlinked gene, referring to a gene on a sex chromosome, and linked genes, referring to genes on the same chromosome that tend to be inherited together.) Sex-linked genes in humans follow the same pattern of inheritance that Morgan observed for the eye-color locus in Dwsophila (see Figure 15.4). Fathers pass sex-linked alleles to all of their daughters bui to none of their sons. In contrast, mothers can pass sex-linked alleles to both sons and daughters (Figure 15.10).

can trace the sex of each offspring to the moment of conception: If a sperm cell bearing an X chromosome happens to fertilize an ovum, the zygote is XX, a female; if a sperm cell containing a Y chromosome fertilizes an ovum, the zygote is XY, a male (see Figure 15.9a). Thus sex determination is a matter of chance—a fifty-fifty chance. Besides the mammalian X-Y system, three other chromosomal systems for determining sex are shown in Figure 15.9, in parts b-d. In humans, the anatomical signs of sex begin to emerge when the embryo is about two months old. Before then, the rudiments of the gonads are generic—they can develop into either ovaries or testes, depending on hormonal conditions within the embryo. Which of these two possibilities occurs depends on whether or not a Y chromosome is present. In 1990, a British research team identified a gene on the Y chromosome required for the development of testes. They named the gene S'RY, for sex-determining region of Y. In the absence of 5RY. the gonads develop into ovaries. The researchers emphasized that the presence (or absence) of SRYis just a trigger. The biochemical, physiological, and anatomical features that distinguish males and females are complex, and many genes are involved in their development. SKY codes for a protein that regulates other genes. Researchers have subsequently identified a number of additional genes on the Y chromosome that are required ior normal testis functioning. In the absence of these genes, an XY individual is male but does not produce normal sperm.

If a sex-linked trait is due to a recessive allele, a female will express the phenotype only if she is a hornozygote. Because males have only one locus, the terms homozygous and heterozygous lack meaning for describing their sex-linked genes (the term he miaous is used in such cases). Am male receiving the recessive allele from his mother will express the trait. For this reason, far more males than females have sex-linked recessive disorders. However, even though the chance of a female inheriting a double dose of the mutant allele is much less than the probability of a male inheriting a single dose, there are females with sex-linked disorders. For instance, color blindness is a mild disorder inherited as a sex-linked trait. A color-blind daughter may be born to a color-blind father whose mate is a carrier (see Figure 15.10c). However, because the sex-linked allele for color blindness is relatively rare, the probability that such a man and woman will mate is low.

Inheritance of Sex-Linked Genes

A number of human sex-linked disorders are much more serious than color blindness. An example is Duchenne muscular dystrophy, which affects about one out of every 3,500 males

In addition to their role in determining sex, the sex chromosomes, especially X chromosomes, have genes for many

Sperm

Sperm A

Ova (p) X X

A

(a) A father with the disorder will transmit 1 the mutant aliele to ail daughters but to no sons. When the mother is a dominant hornozygote, the daughters will have the normal phenotype but will be carriers of the mutation.

5perm

X'

li a carrier mates with a male of normal phenotype, there is a 5 0 % chance.that each daughter wii! be a carrier like her mother, and a 5 0 % chance that each son will have the disorder

(c) ff a carrier mates w i t h a male who has the disorder, there is a 5 0 % chance thateach child born to them will havp the disorder, regardless of sex. Daughters who do not have the disorder will be carriers, whereas males without the . . disorder wit! be completely free of the • recessive aliele.

& Figure 15.10 The transmission of sex-linked recessive traits. In this diagram, the superscript A represents a dominant allele carried on the X chromosome, and the superscript a ^presents a recessive allele. imagine that this recessive allele is a mutation that causes a sex-linked disorder, such as color blindness. White boxes indicate unaffected individuals, light-colored boxes indicate carriers, and dark-colored boxes indicate individuals with the sex-linked disorder.

CHAPTER is

The Chromosomal Kssis oi Inheritance

283

born in the United States. The disease is characterized by a progressive weakening of the muscles and loss ol coordination. Affected individuals rarely live past their early 20s. Researchers have traced the disorder to the absence of a key muscle protein called dystrophin and have mapped the gene for this protein to a specific locus on ihe X chromosome. Hemophilia is a sex-linked recessive disorder denned by the absence of one or more of the proteins required for blood clotting. When a person with hemophilia is injured, bleeding is prolonged because a firm clot is slow to form. Small cuts in the skin are usually not a problem, but bleeding in the muscles or joints can be painful and can lead to serious damage. Today, people with hemophilia are treated as needed with intravenous injections of the missing protein.

X Inactivation in Female Mammals Although female mammals, including humans, inherit two X chromosomes, one X chromosome in each cell becomes almost completely inactivated during embryonic development. As a result, the cells of females and males have the same effective dose (one copy) of genes with loci on the X chromosome. The inactive X in each cell of a female condenses into a compact object called a Barr body, which lies along the inside of the nuclear envelope. Most of the genes of the X chromosome that forms the Barr body are not expressed. In the ovaries, Barr-body chromosomes are reactivated in the cells that give rise to ova, so every female gamete has an active X. British geneticist Mary Tyon demonstrated that selection of which X chromosome will form the Barr body occurs randomly and independently in each embryonic cell present at the time of X inactivation. As a consequence, females consist of a mosaic of two types of cells: those with the active X derived from the father and those with the active X derived from the mother. After an X chromosome is inactivated in a

284

U N I T THREE

GeiieUCS

Inactivation of an X chromosome involves modification of the DNA, such as attachment of methyl groups (—-CH^) io one ol the nitrogenous bases of DNA nucleotides. (The regulatory role of DNA metbylation is discussed further in Chapter 19.) Researchers also have discovered a gene called XIST (for X-inactive specific transcript) that is active only on the Barr-body chromosome. Multiple copies of the RNA molecu.e produced from this gene apparently attach to the X chromosome on which they are made, eventually almost covering i:. Interaction of this RNA with the chromosome seems to init ate X inactivation. Our understanding of X inactivation is still rudimentary; however.

Concept Check 1. A white-eyed female Drosophila is mated with a redeyed (wild-type) male, the reciprocal cross of that shown in Figure 15.4. What phenotypes and genotypes do you predict for the offspring? 2. Neither Tim nor Rhoda has Duchenne muscular dystrophy, but their firstborn son does have it. What is the probability that a second child of this couple will have the disease? For suggested answers, see Appendix A.

T w o cell populations in adult cat:

• Figure 15.11 X inactivation and the t o r t o i s e s h e l l cat. The tortoiseshell gene is on the X chromosome, and the tortoiseshell phenotype requires the presence of t w o different alleles, one for orange fur and one for black fur. Normally, only females can have both alleles, because only they have t w o X chromosomes. If a female is heterozygous for the tortoiseshell gene, she is tortoiseshell. Orange patches are formed by populations of cells in which the X chromosome with the orange allele is active; black patches have cells in which the X chromosome with the black allele is active. ("Calico" cats also have white areas, which are determined by yet another gene.)

particular cell, all mitotic descendants of that cell have the same inactive X. Thus, if a female is heterozygous for a seKlinked trait, about half her cells will express one allele, while the others will express the alternate allele. Figure 15.11 shows how this mosaicism results in the mottled coloration of a tortoiseshell cat. In humans, mosaicism can be observed in a recessive X-linked mutation that prevents the development of sweat glands. A woman who is heterozygous for this trait has patches of normal skin and patches of skin lacking sweat glands.

Early embryo: X chromosomes

Concept

Alterations of chromosome number or structure cause some genetic disorders Sex-linked traits are not the only notable deviation from the inaeritance patterns observed by Mendel, and the gene mutaions that generate new alleles are not the only kind of changes to the genome that can affect phenotype. Physical ard chemical disturbances, as well as errors during meiosis, can damage chromosomes in major ways or alter their number in a cell. Large-scale chromosomal alterations often lead to spontaneous abortion (miscarriage) of a fetus, and individuals born with these types of genetic defects commonly exhibit various developmental disorders. In plants, such genetic defects may be tolerated to a greater extent than in animals.

Abnormal Chromosome Number Ideally, the meiotic spindle distributes chromosomes to daughter cells without error. But there is an occasional mishap, called a nondisjunction, in which the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II. In these cases, one gamete receives two of the same type of chromosome and another gamete receives no copy (Figure 15.12). The other chromosomes are usually distributed normally. If either of the aberrant gametes unites with a normal one at fertilization, the

offspring will have an abnormal number of a particular chromosome, a condition known as aneuploidy. If a chromosome is present in triplicate in the fertilized egg (so that the cell has a total of In 4- 1 chromosomes), the aneuploid cell is said to be trisomic for that chromosome. If a chromosome is missing (so that the cell has In — 1 chromosomes), the aneuploid cell is monosomic for that chromosome. Mitosis will subsequently transmit the anomaly to all embryonic cells. If the organism survives, it usually has a set of symptoms caused by the abnormal dose of the genes associated with the extra or missing chromosome. Nondisj unction can also occur during mitosis. If such an error takes place early in embryonic development, then the aneuploid condition is passed along by mitosis to a large number of cells and is likely to have a substantial effect on the organism. Some organisms have more than two complete chromosome sets. The general term for this chromosomal alteration is polyploidy, with the specific terms triploidy (3n) and tetraploidy (4n) indicating three or four chromosomal sets, respectively. One way a triploid cell may be produced is by the fertilization of an abnormal diploid egg produced by nondisjunction of all its chromosomes. An example of an accident that would result in tetraploidy is the failure of a In zygote to divide after replicating its chromosomes. Subsequent normal mitotic divisions would then produce a 4n embryo. Polyploidy is fairly common in the plant kingdom. As we will see in Chapter 24, the spontaneous origin of polyploid individuals plays an important role in the evolution of plants. In the animal kingdom, polyploid species are much less common, although they are known to occur among the fishes and amphibians. Researchers in Chile were the first to identify a

Meiosis I

"-N on disjunction .

/\' /

\

/"V /

Nondisjunction

\

Gamee .s

n -1 • Figure 15.12 Meiotic nondisjunction. Gametes with an abnormal chromosome number can arise by nondisjunction in either •Tieiosis I or meiosis II.

/

Meiosts II

ft

»-*• /

\

\

Vx "' /

\

+1

Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I

(b) Nondisjunction of sister chromatids in meiosis II

CHAPTER 15 The Chromosomal Basis of Inheritance

285

A Figure15.13 A tetraploid mammal. The somatic cells of this burrowing rodent, Tympanoctomys barrerae, have about twice as many chromosomes as those of closely related species. Interestingly, its sperm's head is unusually large, presumably a necessity for holding all that genetic material. Scientists think that this tetraploid species may have arisen when an ancestor doubled its chromosome number, presumably by errors in mitosis or meiosis within the animal's reproductive organs.

polyploid mammal, a rodent whose cells are tetraploid (Figure 15.13). Additional research has found that a closely related species also appears to be tetraploid. In general, polyploids are more nearly normal in appearance than aneuploids. One extra (or missing) chromosome apparently disrupts genetic balance more than does an entire extra set of chromosomes.

Alterations of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure, depicted in Figure 15.14. A deletion occurs when a chromosomal fragment lacking a centromere is

(a) A deletion removes a chromosomal segment.

lost. The affected chromosome is then missing certain genes. In some cases, if meiosis is in progress, such a "deleted" fragment may become attached as an extra segment to a sis,er chromatid, producing a duplication. Alternatively, a detached fragment could attach to a nonsister chromatid of a homologous chromosome. In that case, though, the "duplicated" segments might not be identical because the homologues coi Id carry different alleles of certain genes. A chromosomal fragment may also reattach to the original chromosome but in tie reverse orientation, producing an inversion. A fourth possible result of chromosomal breakage is for the fragment to join a nonhomologous chromosome, a rearrangement called a translocation. Deletions and duplications are especially likely to occur dvring meiosis. In crossing over, nonsister chromatids sometimes break and rejoin at "incorrect" places, so that one partner gives up more genes than it receives. The products of such a nonreciprocal crossover are one chromosome with a deletion and one chromosome with a duplication. A diploid embryo that is homozygous for a large deletion (or has a single X chromosome with a large deletion, in a male) is usually missing a number of essential genes, a condition that is ordinarily lethal. Duplications and trans locations also tend to have harmful effects. In reciprocal translations, in which segments are exchanged between nonhomologous chromosomes, and in inversions, the balance of genes is not abnormal—all genes are present m their normal doses. Never-

/

A Figure 15.14 Alterations of chromosome structure. Vertical arrows indicate breakage points. Dark purple highlights the chromosomal parts affected by the rearrangements.

286

U N I T THREE

Genetics

theless, translocations and inversions can alter phenotype because a gene's expression can be influenced by its location among neighboring genes.

Human Disorders Due to Chromosomal Alterations Alterations of chromosome number and structure are associated with a number of serious human disorders. Nondisjunction in meiosis results in aneuploid gametes. If an aneuploid gamete combines with a normal haploid gamete during fertilization, the result is an aneuploid zygote. Although the frequency of aneuploid zygotes may be quite high in humans, most of these chromosomal alterations are so disastrous to development that the embryos are spontaneously aborted long before birth. However, some types of ar.euploidy appear to upset the genetic balance less than others, with the result that individuals with certain aneuploid conditions can survive to birth and beyond. These individuals have a set of symptoms—a syndrome—• characteristic of the type of aneuploicly Genetic disorders caused by aneuploidy can be diagnosed before birth by fetal testing (see Figure 14.17). E own Syndrome (Trisomy 21) One aneuploid condition, Down syndrome, affects approximately one out of every 700 children born in the United States (Figure 15.15). Down syndrome is usually the result of an extra chro-

. Figure 15.15 Down syndrome. The child exhibits the facial •eatures characteristic of Down syndrome. The karyotype shows trisomy 21, the most common cause of this disorder.

mosome 21, so that each body cell has a total of 47 chromosomes. Because the cells are trisomic for chromosome 21, Down syndrome is often called trisomy 21. Down syndrome includes characteristic facial features, short stature, heart defects, susceptibility to respiratory infection, and mental retardation. Furthermore, individuals with Down syndrome are prone to developing leukemia and Alzheimer's disease. Although people with Down syndrome, on average, have a life span shorter than normal, some live to middle age or beyond. Most are sexually underdeveloped and sterile. The frequency of Down syndrome increases with the age of the mother. While the disorder occurs in just 0.04% of children born to women under age 30, the risk climbs to 1.25% for mothers in their early 30s and is even higher for older mothers. Because of this relatively high risk, pregnant women over 3 5 are candidates for fetal testing to check for trisomy 21 in the embryo. The correlation of Down syndrome with maternal age has not yet been explained. Most cases result from nondisjunction during meiosis I, and some research points to an age-dependent abnormality in a meiosis checkpoint that normally delays anaphase until all the kinetochores are attached to the spindle (like the M phase checkpoint of the mitotic cell cycle; see Chapter 12). Trisomies of some other chromosomes also increase in incidence with maternal age, although infants with these autosomal trisomies rarely survive for long.

Aneuploidy of Sex Chromosomes Nondisjunction ol sex chromosomes produces a variety of aneuploid conditions. Most of these conditions appear to upset genetic balance less than aneuploid conditions involving autosomes. This may be because the Y chromosome carries relatively few genes and because extra copies of the X chromosome become inactivated as Barr bodies in somatic cells. An extra X chromosome in a male, producing XXY, occurs approximately once in every 2,000 live births. People with this disorder, called Klinefelier syndrome, have male sex organs, but the testes are abnormally small and the man is sterile. "Even though the extra X is inactivated, some breast enlargement and other female body characteristics are common. The affected individual is usually of normal intelligence. Males with an extra Y chromosome (XYY) do not exhibit any well-defined syndrome, but they tend to be somewhat taller than average. Females with trisomy X (XXX), which occurs once in approximately 1,000 live births, are healthy and cannot be distinguished from XX females except by karyotype. Monosomy X, called Turner syndrome, occurs about once in every 5,000 births and is the only known viable monosomy in humans. Although these X0 individuals are phenotypically female, they are sterile because their sex organs do not mature. When provided with estrogen replacement therapy, girls with Turner syndrome do develop secondary sex characteristics. Most have normal intelligence. The Chromosomal Basis of Inheritance

287

• Figure 15.16 Translocation associated with chronic myelogenous leukemia (CML). The cancerous cells in nearly all CML patients contain an abnormally short chromosome 22, the so-called Philadelphia chromosome, and an abnormally long chromosome 9. These altered chromosomes result from the translocation shown here.

Normal chromosome 9

f

Reciprocal

Philadelphia chromosome Normal chromosome 22

Disorders Caused by Structurally Altered Chromosomes Many deletions in human chromosomes, even in a heterozygous state, cause severe problems. One such syndrome, known as cri du chat ("cry of the cat"), results from a specific deletion in chromosome 5. A child born with this deletion is mentally retarded, has a small head with unusual facial features, and has a cry that sounds like the mewing of a distressed cat. Such individuals usually die in infancy or early childhood. Another type ol chromosomal structural alteration associated with human disorders is translocation, the attachment of a fragment from one chromosome to another, nonhomologous chromosome. Chromosomal transl oca tions have been implicated in certain cancers, including chronic myelogenous leukemia (CML). Leukemia is a cancer affecting the cells that give rise to white blood cells, and in the cancerous cells of CML patients, a reciprocal translocation has occurred. In these cells, the exchange of a large portion of chromosome 22 with a small fragment from a tip of chromosome 9 produces a much shortened, easily recognized chromosome 22, called the Philadelphia chromosome (Figure 15.16). We will discuss how such an exchange might cause cancer in Chapter 19.

Concept Check 1. More common than completely poiyploid animals are mosaic polyploids, animals that are diploid except for patches of poiyploid cells. How might a mosaic telraploid—-an animal with some cells containing four sets of chromosomes—arise? 2. About 5% of individuals with Down syndrome have a chromosomal translocation in which one copy of chromosome 21 is attached to chromosome 14. How could this translocation in a parent's gonad lead to Down syndrome in a child? 3. Explain how a male cat could have the tortoiseshell phenotype. For suggested answers, see Appendix A.

288

U N I T THREE

Genetics

Translocated chromosome 9

translocation

Translocated chromosome 22

Concept

Some inheritance patterns are exceptions to the standard chromosome theory In the previous section, you learned about abnormal deviations from the usual patterns of chromosomal inheritance. We conclude this chapter by describing two normal exceptions lo Mendelian genetics, one involving genes located in the nucleus and the other involving genes located outside of the nucleus

Genomic Imprinting Throughout our discussions of Mendelian genetics and the chromosomal basis of inheritance, we have assumed that a specific allele will have the same effect regardless of whether it was inherited from the mother or the father. This is probably a safe assumption most of the time. For example, when Mendel crossed purple-flowered pea plants with whiteflowered pea plants, he observed the same results regardless of whether the purple-flowered parent supplied the ova or the pollen. In recent years, however, geneticists have identified two to three dozen traits in mammals that depend on whici parent passed along the alleles for those traits. Such variatio.i in phenotype depending on whether an allele is inherited from the male or female parent is called genomic imprinting. (Note that the issue here is not sex linkage; most imprinted genes are on autosomes.) Genomic imprinting occurs during the formation of gametes and results in the silencing of one allele of certain genes. Because these genes are imprinted differently in sperm and ova, a zygote expresses only one allele of an imprinted gene, either the allele inherited from the female parent or the alkie inherited from the male parent. The imprints are transmitted to all the body cells during development, so the same allele of a given gene—either the maternally inherited allele or the paternally inherited allele-—is expressed in all cells of that organism. In each generation, the old imprints are "erased"' in gamete-producing cells, and the chromosomes ol the

developing gametes are newiy imprinted according to the sex of :he individual. In a given species, the imprinted genes are always imprinted in the same way. For instance, a gene imprinted for maternal allele expression is always imprinted for maternal allele expression, generation after generation. Consider, for example, the gene for insulin-like growth factor 2 (Ig/2), one of the first imprinted genes to be identified. Although this growth factor is required for normal prenatal growth, only the paternal allele is expressed (Figure 15.17a). Evidence that the Ig/2 gene is imprinted initially came from crosses between wild-type mice and dwarf mice homozygous fot a recessive mutation in the Igj2 gene- The phenotypes of heterozygous offspring (one normal allele and one mutant) differed, depending on whether the mutant allele came from the mother or the father (Figure 15.17b).

Normal !gf2 allele (expressed) Paternal chromosomeMaternal

What exactly is a genomic imprint? In many cases, it seems to consist of methyl (—CH3) groups that are added to cytosine nucleotides of one of the alleles. Such methylation may directly silence the allele, an effect consistent with evidence that heavily methylated genes are usually inactive (see Chapter 19). However, for a few genes, methylation has been shown to activate expression of the allele. This is the case for the Jgj2 gene: Methylation of a certain DMA sequence on the paternal chromosome leads to expression of the paternal Ig/2 allele. Genomic imprinting is thought to affect only a small fraction of the genes in mammalian genomes, but most of the known imprinted genes are critical for embryonic development. In experiments with mice, for example, embryos engineered to inherit both copies of certain chromosomes from the same parent inevitably die before birth, whether that parent is male or female. Normal development apparently requires that embryonic cells have exactly one active copy-—not zero, not two-—of certain genes. The association of aberrant imprinting with abnormal development and certain cancers is stimulating numerous studies on how different genes are imprinted.

chromosomeNormal Igf2 allele (not expressed)

Wild-type mouse (normal size)

(a) A wild-type mouse is homozygous for the normal Igf2 allele. Normal Igf2 allele (expressed)

Maternal Mutant/g/2 allele (not expressed)

Normal size mouse

Mutant Igf2 allele (expressed) I

Paternal ^ S S S

Normal Igf2 allele (not expressed)

Dwarf mouse

(b) When a normal Igf2 allele is inherited from the father, heterozygous mice grow to normal size. But when a mutant allele is inherited from the father, heterozygous mice have the dwarf phenotype.

L Figure 15.17 Genomic imprinting of the mouse igf2 t e n e . (a) In mice, the paternal igf2 allele is expressed and the maternal allele is not. (b) Matings between wild-type mice and those homozygous for the recessive mutant Igf2 allele produce heterozygous offspring that can be either normal size or dwarf, depending on which parent passes on the mutant allele.

Inheritance of Organelle Genes Although our focus in this chapter has been on the chromosomal basis of inheritance, we end with an important amendment: Not all of a eukaryotic cell's genes are located on nuclear chromosomes, or even in the nucleus. Some genes are located in organelles in the cytoplasm; these genes are sometimes called extranuchargenes. Mitochondria, as well as chloroplasts and other plant plastids, contain small circular DNA molecules that carry genes coding for proteins and RNA. These organelles reproduce themselves and transmit their genes to daughter organelles. Because organelle genes are not distributed to offspring according to the same rules that direct the distribution of nuclear chromosomes during meiosis, they do not display Mendelian inheritance. The first hint that extranuclear genes exist came from studies by Karl Correns on the inheritance of yellow or white patches on the leaves of an otherwise green plant. In 1909, he observed that the coloration of the offspring was determined only by the maternal parent (the source of seeds that germinate to give rise to the offspring) and not by the paternal parent (the pollen source). Subsequent research showed that such coloration patterns, or variegation, are due to mutations in plastid genes that control pigmentation. In most plants, a zygote receives all its plastids from the cytoplasm of the egg and none from pollen, which contributes little more than a haploid set of chromosomes. As the zygote develops, plastids containing wild-type or mutant pigment genes are distributed randomly to daughter cells. The pattern of leaf coloration exhibited by a

CHAPTER 1 5

The Chromosomal BI;SLS of Inheritance

289

• Figure 15.18 Variegated leaves from Croton

from a person's mother may contribute to at least some cases of diabetes and heart disease, as well as to other disorders that commonly debilitate the elderly, such as Alzheimer's disease, hi the course of a lifetime, new mutations gradually accumulate in our mitochondrial DNA, and some lesearchers think that these mutations play a role tn the normal aging process. Wherever genes are located in the cell—in the nucleus DY in cytoplasmic organelles—their inheritance depends on ihe precise replication of DNA, the genetic material, in the next chapter, you will learn how this molecular reproduction occurs.

dioicus. Variegated (striped or spotted) leaves result from

mutations in pigment genes located in plastids, which generally are inherited from the maternal parent.

plant depends on the ratio of wild-type to mutant plastids in its various tissues (Figure 15.18). Similar maternal inheritance is also the rule for mitochondrial genes in most animals and plants, because almost all the mitochondria passed on to a zygote come from the cytoplasm of the egg. The products of most mitochondrial genes help make up the protein complexes of the electron transport chain and ATP synthase (see Chapter 9). Defects in one or more of these proteins, therefore, reduce the amount of ATP the cell can make and have been shown to cause a number of rare human disorders. Because the parts of the body most susceptible to energy deprivation are the nervous system and the muscles, most mitochondrial diseases primarily affect these systems. For example, a person with the disease called mitochondrial myopathy suffers weakness, intolerance of exercise, and muscle deterioration. In addition to the rare diseases clearly caused by defects in mitochondrial DNA, mitochondrial mutations inherited

Concept Check 1. Gene dosage, the number of active copies of a gene, is important to proper development. Identify and describe two processes that help establish the proper dosage of certain genes. 2. Reciprocal crosses between two primrose varieties, A and B, produced the following results; A female X B male —> offspring with all green (nonvariegated) leaves. B female X A male —» offspring with spotted (variegated) leaves. "Explain these results. 3. Mitochondrial genes are critical to the energy metabolism of cells, but mitochondrial disorders caused by mutations in these genes are generally not lethal. Why not? For suggested answers, see Appendix A.

Chapter 1 5 Review Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY Mendelian inheritance has its physical basis in the behavior of chromosomes • In the early 1900s, several researchers proposed that genes are located on chromosomes and that the behavior of chromosomes during meiosis accounts for Mendel's laws of segregation and independent assortment (pp. 274-275). • Morgan's Experimental Evidence: Scientific Inquiry (pp. 276-277) Morgan's discovery that transmission of the X chromosome in Drosophila correlates with inheritance of the eye-color trait was the first solid evidence indicating that a specific gene is associated with a specific chromosome. 290

U N I T THREE

GeneLic

Linked genes tend to be inherited together because the/ are located near each other on the same chromosome • How Linkage Affects Inheritance: Scientific Inquiry (pp. 277-278) Each chromosome has hundreds or thousands of genes. Genes on the same chromosome whose alleles are so close together that they do not assort independently are said to be linked. The alleles of unlinked genes are either on separate chromosomes or so tar apart on the same chromosome that they assort independently. • Genetic Recombination and Linkage (pp. 278-280) Recombinant offspring exhibit new combinations of traits inherited from two parents. Because of the independent assortment of chromosomes and random fertilization, unlinked genes exhibit a 50% frequency of recombination. Even with crossing over between nonsister chromatids during the first meiotic division, linked genes exhibit recombination frequencies less than 50%.

• Linkage Mapping Using Recombination Data: Scientific Inquiry (pp. 279-281) Geneticists can deduce the order of genes on a chromosome and the relative distances between them from recombination frequencies observed in genetic crosses. In general, the farther apart genes are on a chromosome, the more likely they are to be separated during crossing over. Activity Linked Genes and Crossing Over

S«:x-Iinked genes exhibit unique patterns of inheritance • The Chromosomal Basis of Sex (pp. 282-283) An organism's sex is an inherited phenotypic character usually determined by the presence or absence of certain chromosomes. Humans and other mammals have an X-Y system in which sex normally is determined by trie presence or absence of a Y chromosome. Different systems of sex determination are found in birds, fishes, and insects. • Inheritance of Sex-Linked Genes (pp. 283-284) The sex chromosomes carry certain genes for traits that are unrelated to maleness or femaleness. For instance, recessive alleles causing color blindness, hemophilia, and Duchenne muscular dystrophy are carried on the X chromosome. Fathers transmit such sexlinked alleles LO all daughters but to no sons. Any male who inherits a single sex-linked recessive allele from his mother will express the trait. Activity Sex-Linked Genes Investigation What Can Fruit Flies Reveal About Inheritance? Biology Labs On-Line FlyLab Biology Labs On-Line PedigreeLab

Some inheritance patterns are exceptions to the standard chromosome theory • Genomic Imprinting (pp. 288-289) In mammals, the phenotypic effects of certain genes depend on which allele is inherited from the mother and which is inherited from the father. Imprints are formed during gamete production, with the result that one allele (either maternal or paternal) is not expressed in offspring. Most imprinted genes now known play a role in embryonic development. • Inheritance of Organelle Genes (pp. 289-290) The inheri tance of traits controlled by the genes present in mitochondria and chloroplasts depends solely on the maternal parent because the zygote's cytoplasm comes from the egg. Some diseases affecting the nervous and muscular systems are caused by defects in mitochondrial genes that prevent cells from making enough ATP.

TESTING YOUR KNOWLEDGE Genetics Problems 1. A man with hemophilia (a recessive, sex-linked condition) has a daughter of normal phenotype. She marries a man who is normal for the trait. What is the probability that a daughter of this mating will be a hemophiliac? That a son will be a hemophiliac? If the couple has four sons, what is the probability that all four will be born with hemophilia?

• X Inactivation in Female Mammals (p. 284) In mam malian females, one of the two X chromosomes in each cell is randomly inactivated during early embryonic development. If a female is heterozygous for a particular gene located on the X chromosome, she will be mosaic for that character, with about half her cells expressing the maternal allele and about half expressing the paternal allele.

2. Pseudohypertrophic muscular dystrophy is an inherited disorder that causes gradual deterioration of the muscles. It is seen almost exclusively in boys born to apparently normal parents and usually results in death in the early teens. Is this disorder caused by a dominant or a recessive allele? Is its inheritance sex-linked or autosomal? How do you know? Explain why this disorder is almost never seen in girls.

/iterations of chromosome number or structure cause some genetic disorders I* Abnormal Chromosome Number (pp. 285-286)

3. Red-green color blindness is caused by a sex-linked recessive allele. A color-blind man marries a woman with normal vision whose father wTas color-blind. What is the probability that they will have a coLor-blind daughter? What is the probability that their first son will be color-blind? (Note: The two questions are worded a bit differently)

Aneuploidy can arise when a normal gamete unites with one containing two copies or no copies of a particular chromosome as a result of nondisjunction during meiosis. The cells of the resulting zygote have either one extra copy of that chromosome (trisomy) or are missing a copy (monosomy). Polyploidy in which there are more than two complete sets of chromosomes, can result from complete nondisj unction during gamete formation. Activity Polyploid Plants I* Alterations of Chromosome Structure (pp. 286-287) Chromosome breakage can result in various rearrangements. A lost fragment leaves one chromosome with a deletion; the deleted fragment may reattach to the same chromosome in a different orientation, producing an inversion. Or the fragment may attach to a homologous chromosome, producing a duplication, or to a nonhomologous chromosome, producing a translocation. !•• Human Disorders Due to Chromosomal Alterations (pp. 287-288) Changes in the number of chromosomes per cell or in the structure of individual chromosomes can affect phenotype. Such alterations cause Down syndrome (usually due to trisomy of chromosome 21), certain cancers associated with chromosomal translations, and various other human disorders.

4. A wild-type fruit fly (heterozygous for gray body color and normal wings) is mated with a black fly with vestigial wings. The offspring have the following phenotypic distribution: wild type, 778; black-vestigial, 785; black-normal, 158; gray-vestigial, 162. What is the recombination frequency between these genes for body color and wing size? 5. In another cross, a wild-type fruit fly (heterozygous for gray body color and red eyes) is mated with a black fruit fly with purple eyes. The offspring are as follows: wild type. 721; blackpurple, 751; gray-purple, 49; black-red, 45. What is the recombination frequency between these genes for body color and eye color? Using information from problem 4, what fruit flies (genotypes and phenotypes) would you mate to determine the sequence of the body-color, wing-size, and eye-color genes on the chromosome? CHAPTER 15

The Chromosomal Basis of Inheritance

291

6. What pattern of inheritance would lead a geneticist to suspect that an inherited disorder of cell metabolism is due to a defective mitochondrial gene?

7. Women born with an extra X chromosome (XXX) are healthy and phenotypically indistinguishable from normal XX women. What is a likely explanation for this finding? How could you test this explanation? 8. Determine the sequence of genes along a chromosome based on the following recombination frequencies: A—B, 8 map units; A-C, 28 map units; A-D. 25 map units; B-C, 20 map units; B-D, 33 map units. 9. Assume that genes A and B are linked and are 50 map units apart. An animal heterozygous at both loci is crossed with one that is homozygous recessive at both loci. What percentage of the offspring will show phenotypes resulting from crossovers? If you did not know that genes A and B were linked, how would you interpret the results of this cross? 10. A space probe discovers a planet inhabited by creatures who reproduce with the same hereditary patterns seen in humans. Three phenotypic characters are height (T = tall, t = dwarf), head appendages (A — antennae, a — no antennae), and nose morphology (5 = upturned snout, s = downturned snout). Since the creatures are not "intelligent/' Earth scientists are able to do some controlled breeding experiments, using various heterozygotes in testcrosses. For tall heterozygotes with, antennae, the offspring are: tall-antennae, 46; dwarf-antennae, 7; dwarfno antennae, 42; tall-no antennae, 5. For heterozygotes with antennae and an upturned snout, the offspring are: antennaeupturned snout, 47; antennae-down turned snout. 2; no antennae-downturned snout, 48; no antennae-upturned snout, 3. Calculate the recombination frequencies for both experiments. 11. Using the information from problem 10, a further testcross is done using a heterozygote for height and nose morphology. The offspring are: tall-upturned snout, 40; dwarf-upturned snout, 9; dwarf-downtumed snout, 42; tall-downturned snout, 9. Calculate the recombination frequency from these data; then use your answer from problem 10 to determine the correct sequence of the three linked genes. 12. The ABO blood type locus has been mapped on chromosome 9. A father who has blood type AB and a mother who has blood type O have a child with trisomy 9 and blood type A. Using this information, can you tell in wThich parent the nondisjunction occurred? Explain your answer. 13. Two genes of a fkvwer, one controlling blue (B) versus white (b) petals and the other controlling round (R) versus oval (r) stamens, are linked and are 10 map units apart. You cross a homozygous blue-oval plant with a homozygous white-round plant. The resulting Fx progeny are crossed with homozygous white-oval plants, and 1,000 progeny are obtained. How many plants of each of the four phenotypes do you expect?

292

UNIT THREE

Genercs

14. You design Drosophila crosses to provide recombination data for gene a, which is located on the same chromosome shown in Figure 15.8. Gene a has recombination frequencies of 14%

with the vestigial-wing locus and 26% with the brown-eye locus. Where is a located on the chromosome? For Genetics Problems answers, see Appendix A. Go to the website or CD-ROM for more quiz questions.

Evolution Connection You have seen that crossing over, or recombination, is thought to be evolutionarily advantageous because this process continually shuffles genetic alleles into novel combinations. Some organisms, however, have apparently lost the recombination mechanism, while in others, certain chromosomes do not recombine. What factors do you think may favor reduced levels of recombination?

Scientific Inquiry Consider Figure 15.5, in which the F± dihybrid females resulted from a cross between parental (P) flies with genotypes b+ b+ vg+ vg+ and b b vg vg. Now, imagine you make FL females by crossing two different P generation flies: b+ b+ vg vg X b b vg+ vg'. a. What will be the genotype of your FT females? Is this the same as that for the Fj females in Figure 15.5? b. Draw the chromosomes for the F]. females, indicating the position of each alkie, Are these the same as for the Fx females in Figure 15.5? c. Knowing that the distance between these two genes is 17 map units, predict the phenotypic ratios you will get from a cross. Will they be the same as in Figure 15.5? d. Draw the chromosomes of the P, F^, and F2 generations (as is done m Figure 15.6 lor the cross in Figure 15.5), showing how this arrangement of alleles in the P generation leads, via F1 gametes, to the phenotypic ratios seen in the F2 flies. Investigation What Can Fruit Flies Reveal About Inheritance? Biology Labs On-Line FlyLab Biology Labs On-Line PedigreeLab

Science, Technology, and Society About one in every 1,500 boys and one in every 2,500 girls are born with a fragile X chromosome, the tip of which hangs on to the rest of the chromosome by a thin thread of DNA. This abnormality causes mental retardation. Opinions differ about whether children with learning disorders should be tested by karyotyping for the presence of a fragile X chromosome. Some argue that it's always better to know the cause of the problem so that education specialized for that disorder can be prescribed. Others counter that attaching a specific biological cause to a learning disability stigmatizes a child and limits his or her opportunities. What is your evaluation of these arguments?

A Figure 16.1 Watson and Crick with their DNA model.

[Key Concepts 16.1 DNA is the genetic material 16,2 Many proteins work together in DNA replication and repair

lifefe Operating Instructions * n April 1953, James Watson and Francis Crick shook the scientific world with an elegant double-helical model for - the structure of deoxyribonucleic acid, or DNA. Figure 16.1 shows Watson and Crick admiring their DNA model, which they built from tin and wire. Over the past 50 years, their model has evolved from a novel proposition to an icon of modern biology. DNA, the substance of inheritance, is the most celebrated molecule of our time. Mendel's heritable factors and Morgans genes on chromosomes are, in fact, composed of DNA. Chemically speaking, your genetic endowment is the DNA contained in the 46 chromosomes you inherited from your parents. Of all nature's molecules, nucleic acids are unique in their ability to direct their own replication from monomers. Indeed, the resemblance of offspring to their parents has its basis in the precise replication of DNA and its transmission trom one generation to the next. Hereditary information is encoded in the chemical language of DNA and reproduced in ill the cells of your body. It is this DNA program that directs the development of your biochemical, anatomical, physiological, and, to some extent, behavioral traits. In this chapter, you will learn how biologists deduced that DNA is the genetic material, how Watson and Crick discovered its structure, and how cells replicate and repair their DNA—the molecular basis of inheritance.

Concept

DNA is the genetic material Today even schoolchildren have heard of DNA, and scientists routinely manipulate DNA in the laboratory and use it to change the heritable characteristics of cells. Early in the 20th century, however, the identification of the molecules of inheritance loomed as a major challenge to biologists.

The Search for the Genetic Material: Scientific Inquiry Once T. H. Morgans group showed that genes are located on chromosomes (described in Chapter 15), the two chemical components of chromosomes-—-DNA and protein-—-became the candidates for the genetic material. Until the 1940s, the case for proteins seemed stronger, especially since biochemists had identified them as a class of macromolecules with great heterogeneity and specificity of function, essential requirements for the hereditary material. Moreover, little was known about nucleic acids, whose physical and chemical properties seemed far too uniform to account for the multitude of specific inherited traits exhibited by every organism. This view gradually changed as experiments with microorganisms yielded unexpected results. As with the work of Mendel and Morgan, a key factor in determining the identity of the genetic material was the choice of appropriate experimental organisms. The role of DNA in heredity was first worked out by studying bacteria and the viruses that infect them, which are far simpler than pea plants, fruit flies, or humans. In this section, we will trace the search for the genetic material in some detail as a case study in scientific inquiry.

293

Evidence That DNA Can Transform Bacteria We can trace the discovery of the genetic role of DNA back to 1928. Frederick Griffith, a British medical officer, was studying Streptococcus pnewnoniae, a bacterium that

causes pneumonia in mammals. Griffith had two strains (varieties) of the bacterium, a pathogenic (disease-causing) one and a nonpathogenic (harmless) strain. He was surprised to find that when he killed the pathogenic bacteria with heat and then mixed the cell remains with living bacteria of the nonpathogenic strain, some of the living cells became pathogenic (Figure 16.2). Furthermore, this new trait of pathogenicity was inherited by all tbe descendants of the transformed bacteria. Clearly some chemical component of the dead pathogenic cells caused this heritable change, although the identity of the substance was not known. Griffith called the phenomenon transformation, now defined as a change in genotype and phenotype due to the assimilation of external DNA by a cell. (This use of the word transformation should not be confused with the conversion of a normal animal cell to a cancerous one, discussed in Chapter 12.)

Figure 16.2

iquiry Can the genetic trait of pathogenicity be transferred between bacteria? Bacteria of the "S" (smooth) strain of Streptococcus pneumoniae are pathogenic because they have a capsule that protects them from an animal's defense system. Bacteria of the "R" (rough) strain lack a capsule and are nonpathogenic. Frederick Griffith injected mice with the t w o strains as shown below:

Griffith concluded that the living R bacteria had been transformed into

Griffith's work set the stage for a pathogenic S bacteria by an unknown, heritable substance from the dead S celts. 14-year search for the identity of the transforming substance by American bacteriologist Oswald Avery. Avery purified various types of molecules Viruses that infect bacteria are widely used as tools by from the' heat-killed pathogenic bacteria, then tried to transresearchers in molecular genetics. These viruses are calleC form live nonpathogenic bacteria with each type. Only DNA bacteriophages (meaning "bacteria-eaters"), or just phages worked. Finally, in 1944, Avery and his colleagues Maclyn (Figure 16.3). In 1952, Alfred Hershey and Martha Chase perMcCarty and Colin MacLeod announced that the transforming formed experiments showing that DNA is the genetic materia agent was DNA. Their discovery was greeted with interest but of a phage known as T2. This is one of many phages that infect considerable skepticism, in part because of the lingering belief Esch.trich.ia coll (E. coli), a bacterium that normally lives in the that proteins were better candidates for the genetic material. intestines of mammals. At that time, biologists already knew Moreover, many biologists were not convinced that the genes that T2, like many other viruses, was composed almost entirel; of bacteria would be similar in composition and function to of DNA and protein. They also knew that the T2 phage could those of more complex organisms. But the major reason for the quickly turn an E. coli cell into a T2-producing factory that recontinued doubt was that so little was known about DNA. leased many copies when the cell ruptured. Somehow, T2 could reprogram its host cell to produce viruses. But which viEvidence That Viral DNA Can Program Cells ral component—protein or DNA—was responsible? Additional evidence for DNA as the genetic material came from studies of a virus that infects bacteria. Viruses are much simpler than cells. A virus is little more than DNA (or sometimes RNA) enclosed by a protective coat, which is often simply protein. To reproduce, a virus must infect a cell and take ovtr the cell's metabolic machinery. 294

UNIT

THREE

Genetics

Hershey and Chase answered this question by devising an experiment showing that only one of the two components oi T2 actually enters the E. coli cell during infection (Figure 16.4). In preparation for their experiment, they used different radioactive isotopes to tag phage DNA and protein. First, they grew T2 with E. coli in the presence of radioactive sulfur.

A Figure 16.3 Viruses infecting a bacterial cell. T2 and related phages attach to the host cell and inject their genetic material (colorized TEM).

Because protein, but not DNA, contains sulfur, the radioactive atoms were incorporated only into the protein of the phage. Next, in a similar way, the DNA of a separate batch of T2 was labeled with atoms of radioactive phosphorus; because nearly all the phages phosphorus is in its DNA, this procedure left the phage protein unlabeled. In the experiment, the proteinlabeled and DNA-labeled batches of T2 were each allowed to infect separate samples of nonradioactive E. coli cells. Shortly after the onset of infection, the cultures were whirled in a blender to shake loose any parts of the phages that remained outside the bacterial cells. The mixtures were then spun in a centrifuge, forcing the bacterial cells to form a pellet at the bottom of the centrifuge tubes, but allowing free phages and parts of phages, which are lighter, to remain suspended in the liquid, or supernatant. The scientists then measured the radioactivity in the pellet and in the supernatant.

Is DNA or protein the genetic material of phage T2? In their famous 1952 experiment, Alfred Hershey and Martha Chase used radioactive sulfur and phosphorus to trace the fates of the protein and DNA, respectively, of T2 phages that infected bacterial cells. Mixed radioactively labeled phages with bacteria. The phages infected the bacterial ceils. Phage
3' direction.

One new strand, the eading strand, can elongate continuously 5' -> 3' as the replication fork progresses. The other new strand, the lagging strand, must grow in an overall 3'-* 5' direction by addition of short segments, Okazaki fragments, that grow 5' —» 3' (numbered here in the order they were made).

DNA ligase

© DNA ligase joins Okazaki fragments by forming a bond between their free ends. This results in a continuous strand.

• Overall direction of replication A Figure 16.14 Synthesis of leading and lagging strands *Synthesis ol the leading strand and synthesis of the lagging strand occur concurrently and at the same rate. The lagging si rand is so named because its synthesis is slightly delayed relative to synthesis of the leading strand; each new fragment cannot be started until enough template has been exposed at the replication fork.

302

UNIT THREE

Genetics

d u r i n g D N A r e p l i c a t i o n . DNA poiymerase III (DNA pol 111) is closely associated with a protein that encircles the newly synthesized double helix like a doughnut. Note that Okazaki fragments are actually much longer than the ones shown here. In this figure, we depict only five bases per fragment for simplicity.

discovered them. The fragments are about 1,000 to 2,000 nucleotides long in E. coli and 100 to 200 nucleotides long in eukaryotes. Another enzyme, DNA ligase, eventually joins (ligates) the sugar-phosphate backbones of the Okazaki fragments, forming a single new DNA strand.

0 Primase joins RNA nucleotides into a primer.

Template strand

© DNA pol III adds DNA nucleotides to the primer, forming an Okazaki fragment.

Piiming DNA Synthesis DNA polymerases cannot initiate the synthesis of a polynucleotide; they can only add nucleotides to the 3' end of an already existing chain that is base-paired with the template strand (see Figure 16.13). The initial nucleotide chain is a short one called a primer. Primers may consist of either DNA or RNA (the oiher class of nucleic acid), and in initiating the replication of cellular DNA, the primer is a short stretch of RNA with an available 3' end. An enzyme called primase can start an RNA chain from scratch. Primase joins RNA nucleotides together one at a time, making a primer complementary to the template strand at the location where initiation of the new DNA strand will occur. (Primers are generally 5 to 10 nucleotides long.) DNA pol III then adds a DNA nucleotide to the 3' end of the RNA primer a id continues adding DNA nucleotides to the growing DNA strand according to the base-pairing rules. Only one primer is required for DNA pol III to begin synthesizing the leading strand. For synthesis of the lagging strand, however, each Okazaki fragment must be primed separately (figure 16.15). Another DNA polymerase, DNA polymerase I (DNA pol I), replaces the RNA nucleotides of the primers with F)NA versions, adding them one by one onto the 3' end of the adjacent Okazaki fragment (fragment 2 in Figure 16.15). But DNA pol I cannot join the final nucleotide of this replacement DNA segment to the first DNA nucleotide of the Okazaki fragrient wThose primer was JUSL replaced (fragment 1 in Figure 16.15). DNA ligase accomplishes this task, joining the sugarphosphate backbones of all the Okazaki fragments into a continuous DNA strand.

Q After reaching the next RNA primer (not shown), DNA pol ID falls off.

© After the second fragment is primed, DNA pol III adds DNA nucleotides until it reaches the first primer and falls off.

0 DNA pol I replaces the RNA with DNA, adding to the 3' end

Other ProteiiK That Assist DNA Replication You have learned about three kinds of proteins that function in DNA synthesis: DNA polymerases, ligase, and primase. Other kinds of proteins also participate, including helicase, topoisomerase, and single-strand binding proteins. Helicase is an enzyme that untwists the double helix at the replication lorks, separating the two parental strands and making them available as template strands. This untwisting causes tighter twisting and strain ahead of the replication fork, and topoisomerase helps relieve this strain. After helicase separates 1 he two parental strands, molecules of single-strand binding protein then bind to the unpaired DNA strands, stabilizing them until they serve as templates for the synthesis of new complementary strands. Table 16.1 and Figure 16.16, on the next page, summarize DNA replication. Study them carefully before proceeding.

© DNA ligase forms a y bond between the newest DNA and the adjacent DNA of fragment 1.

Q The lagging strand in this region is now complete. 3'

• Overall direction of replication • A. Figure 16.15 Synthesis of the lagging strand. CHAPTER is

The Molecular Basis of Inheritance

303

Table 16.1 Bacterial DNA replication proteins and their functions Function for Leading and Lagging Strands

Protein Helicase

Unwinds parental double helix at replication forks

Single-strand binding protein

Binds to and stabilizes single-stranded DNA until it can be used as a template

Topoisomerase

Corrects "overwinding" ahead of replication forks by breaking, swiveling, and rejoining DNA strands

Function for Leading Strand

Function for Lagging Strand

Primase

Synthesizes a single RNA primer at the 5' end of the leading strand

Synthesizes an RNA primer at the V end of each Okazaki fragment

DNA pol III

Continuously synthesizes the leading strand, adding on to the primer

Elongates each Okazaki fragment, adding on to its primer

DNA pol I

Removes primer from the 5' end of leading strand and replaces it with DNA, adding on to the adjacent 3'end

Removes the primer from the 5' end of each fragment and replaces it with DNA, adding on to [he 3' end of the adjacent fragment

DNA Ligase

joins the 3' end of the DNA that replaces the primer to the rest of the leading strand

Joins the Okazaki fragments

- Overall direction of replication • Origin of replication

Lagging strand

OVERVIEW

Leading strand

Q Helicase unwinds the parental double helix. Q Molecules of singlestrand binding protein stabilize the unwound template strands.

$ The leading strand is synthesized continuously in the 5' -» 3' direction by DNA pol I I I .

© Primase begins synthesis of the RNA primer for the fifth Okazaki fragment.

@ DNA pol I I I is completing synthesis of the f o u r t h fragment. When it reaches the RNA primer on the third fragment, it will dissociate, move to the replication fork, and add DNA nucleotides to the 3' end of the f i f t h fragment primer. A Figure 16.16 A summary of bacterial DNA replication. The detailed diagram shows one replication fork, but as indicated in the overview diagram, replication usually occurs simultaneously at two forks, one at

304

UNIT THREE

Genetics

Q DNA pol I removes the primer from the 5' end of the second fragment, replacing it w i t h DNA nucleotides that it adds one by one to the 3' end of the third fragment. The replacement of the last RNA nucleotide w i t h DNA leaves the sugarphosphate backbone with a free 3' end.

either end of a replication bubble. Notice in the overview diagram that a leading strand is initiated by an RNA primer (red), as is each Okazaki fragment in a lagging strandViewing each daughter strand in its entirety

O DNA ligase bonds the 3' end of the second fragment to the 5' end of the first fragment.

in the overview, you can see that half of it is made continuously as a leading strand, while the other half (on the other side of the origin) is synthesized in fragments as a lagging strand.

The DNA Replication Machine as a Stationary Complex It is traditional—and convenient-—to represent DNA polymerase molecules as locomotives moving along a DNA ''railread track," but such a model is inaccurate in two important ways. First, the various proteins that participate in DNA replication actually form a single large complex, a DNA replication "machine." Many protein-protein interactions facilitate the efficiency of this machine; for example, helicase works much more rapidly when it is in contact with primase. Second, the DNA replication machine is probably stationary during the replication process. In eukaryotic cells, multiple copies of tr e machine, perhaps grouped into "factories," may anchor to the nuclear matrix, a framework of fibers extending through the interior of the nucleus. Recent studies support a model in which DNA polymerase molecules "reel in" the parental DNA and extrude newly made daughter DNA molecules. Additional evidence suggests that the lagging strand is looped through the complex, so that when a DNA polymerase completes synthesis of an Okazaki fragment and dissociates, it doesn't have far to travel to reach the primer for the next fragment, near the replication fork. This looping of the lagging strand enables more Okazaki fragments to be synthesized in less time.

naturally in cells), radioactive emissions, X-rays, and ultraviolet light can change nucleotides in ways that can affect encoded genetic information, usually adversely. In addition, DNA bases often undergo spontaneous chemical changes under normal cellular conditions. Fortunately changes in DNA are usually corrected before they become self-perpetuating mutations. Each cell continuously monitors and repairs its genetic material . Because repair of damaged DNA is so important to the survival of an organism, it is no surprise that many different DNA repair enzymes have evolved. Almost 100 are known in E. coll, and about 130 have been identified so far in humans. Most mechanisms for repairing DNA damage take advantage of the base-paired structure of DNA. Usually, a segment of the strand containing the damage is cut out (excised) by a DNA-cutting enzyme—a nuclease—and the resulting gap is filled in with nucleotides properly paired with the nucleotides in the undamaged strand. The enzymes involved in filling the gap are a DNA polymerase and ligase. DNA repair of this type is called nucleotide excision repair (Figure 16.17).

Q A thymine dimer distorts the DNA molecule.

Proofreading and Repairing DNA

0 A nuclease enzyme cuts the damaged DNA strand at two points and the damaged section is removed.

We cannot attribute the accuracy of DNA replication solely to the specificity of base pairing. Although errors in the completed DNA molecule amount to only one in 10 billion nucleotides, initial pairing errors between incoming nucleotides and those in the template strand are 100,000 times more common—an error rate of one in 100,000 base pairs. During DNA replication, DNA polymerases proofread each nucleotide against its template as soon as it is added to the growing strand. Upon finding an incorrectly paired nucleotide, the polymerase removes the nucleotide and then resumes synthesis. (This action is similar to fixing a typing error by using the "delete" key and then entering the correct letter.) Mismatched nucleotides sometimes evade proofreading by a DNA polymerase or arise after DNA synthesis is completed— by damage to an existing nucleotide base, for instance. In mismatch repair, cells use special enzymes to fix incorrectly paired nucleotides. Researchers spotlighted the importance of such enzymes when they found that a hereditary defect in one of them is associated with a form of colon cancer. Apparently, this defect allows cancer-causing errors to accumulate in the DNA at a faster rate than normal. Maintenance of the genetic information encoded in DNA requires frequent repair of various kinds of damage to existing DNA. DNA molecules are constantly subjected to potentially harmful chemical and physical agents, as we'll discuss in Chapter 17. Reactive chemicals (in the environment and occurring

Nuclease

Q Repair synthesis by a DNA polymerase fills in the missing nucleotides.

0 DNA ligase seals the free end of the new DNA to the old DNA, making the strand complete.

A. Figure 16.17 Nucleotide excision repair of DNA d a m a g e . A team of enzymes detects and repairs damaged DNA. This figure shows DNA containing a thymine dimer, a type of damage often caused by ultraviolet radiation. A nuclease enzyme cuts out the damaged region of DNA, and a DNA polymerase (in bacteria, DNA pol I) replaces it with a normal DNA segment. Ligase completes the process by closing the remaining break in the sugar-phosphate backbone.

CHAPTER 16

The Mnkxulsr basis oflnheritance

305

One function of the DNA repair enzymes in our skin cells is to repair genetic damage caused by the ultraviolet rays of sunlight. One type of damage, the type shown in Figure 16.17, is the covalent linking of thymine bases that are adjacent on a DNA strand. Such thymine dimers cause the DNA to buckle and interfere with DNA replication. The importance of repairing this kind of damage is underscored by the disorder xeroderma pigmentosum; which in most cases is caused by an inherited defect in a nucleotide excision repair enzyme, individuals with this disorder are hypersensitive to sunlight; mutations in their skin cells caused by ultraviolet light are left uncorrected and cause skin cancer.

Replicating the Ends of DNA Molecules In spite of the major role played by DNA polymerases in DNA replication and repair, it turns out that there is a small portion of the cell's DNA that DNA polymerases cannot replicate or repair. For linear DNA, such as the DNA of eukaryotic chromosomes, the fact that a DNA polymerase can only add nucleotides to the 3' end of a preexisting polynucleotide leads to a problem. The usual replication machinery provides no way to complete the 5' ends of daughter DNA strands. Even it an Okazaki fragment can be started with an RNA primer bound to the very end of the template strand, once that primer is removed, it cannot be replaced with DNA, because there is no 3' end onto which DNA polymerase can add DNA nucleotides (Figure 16.18). As a result, repeated rounds of replication produce shorter and shorter DNA molecules. Prokaryotes do not have this problem because their DNA is circular (with no ends), but what about eukaryot.es? Eukaryotic chromosomal DNA molecules have nucleotide sequences called telomeres at their ends (Figure 16.19). Telomeres do not contain genes; instead, the DNA typically consists of multiple repetitions of one short nucleotide sequence. The repeated unit in human telomeres, for example, is the six-nucleotide sequence TTAGGG. The number of repetitions in a telomere varies from about 100 to 1,000. Telomenc DNA protects the organism's genes from being eroded through successive rounds of DNA replication. In addition, telomeric DNA and specific proteins associated with it somehow prevent the staggered ends of the daughter molecule from activating the cell's systems for monitoring DNA damage. (The end of a DNA molecule that is "seen" as a double-strand break may otherwise trigger signal transduction pathways leading to cell cycle arrest or cell death.) Telomeres do not prevent the shortening of DNA molecules due to successive rounds of replication; they just postpone the erosion of genes near the ends of DNA molecules. As shown in Figure 16.18, telomeres become shorter during every round of replication. As we would expect, telomeric DNA does tend to be shorter in dividing somatic cells of older individuals and in cultured cells that have divided many times. It has been proposed that shortening of telomeres is somehow connected 306

U N I T THREE

Last fragment A

f

Previous fragmen t "V

\

RNA primer ginq strand

5'

Removal of primers and replacement with DNA where a 3' end is available

Primer removed but cannot be replaced with DNA because no 3'end available for DNA polymerase

rrrrrrrrrrrr Second round of replication New leading strand 3' TiirmiiirfTi New lagging strand 5'8 Further rounds of replication Shorter and shorter daughter molecules A Figure 16.18 Shortening of the ends of linear DNA molecules. Here we follow the end of one strand of a DNA molecule through two rounds of replication. After the first round, the new lagging strand is shorter than its template. After a second round, both the leading and lagging strands have become shorter than the original parental DNA. Although not shown here, the other ends of these DNA molecules also become shorter.

1 urn A Figure 16.19 Telomeres. Eukaryotes have repetitive, noncoding sequences called telomeres at the ends of their DNA, marked in these mouse chromosomes by a bright orange stain (LM).

to the aging process of certain tissues and even to aging of the organism, as a whole. But what about the cells whose genomes persist unchanged from an organism to its offspring over many generations? If the chromosomes of germ cells (which give rise to gametes) became shorter in every cell cycle, essential genes would eventually be missing from the gametes they produce. Fortunately, this does not occur: An enzyme called telomerase catalyzes the lengthening of telomeres in eukaryotic germ cells, thus restoring their original length and compensating for the shortening thai occurs during DNA replication. The lengthening process is made possible by the presence, in telonierase, of a short molecule of RNA that serves as a template for new telomere segments. Telomerase is not active in most somatic cells, but its activity in germ cells results in telomeres of maximum length in the zygote. Normal shortening of telomeres may protect organisms from cancer by limiting the number of divisions that somatic cells can undergo. Cells from large tumors often have unusually short telomeres, as one would expect for cells that have undergone many cell divisions. Further shortening would presumably lead to self-destruction of the cancer. Intriguingly, researchers have found telomerase activity in cancerous soiratic cells, suggesting that its ability to stabilize telomere length may allow these cancer cells to persist. Many cancer

Chapter Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, investigations, and other interactive study aids.

SUMMARY OF KEY CONCEPTS D N A is the genetic material • The Search for the Genetic Material: Scientific Inquiry (pp. 293-296) Experiments with bacteria and with phages provided the first strong evidence that the genetic material is DNA, Activity The Hershey-Chase Experiment • Building a Structural Model of DNA: Scientific Inquiry (pp. 296-298) Watson and Crick deduced that DNA is a double helix. Two antiparallel sugar-phosphate chains wind around the outside of the molecufe; the nitrogenous bases project into the interior, where they hydrogen-bond in specific pairs, A with T and G with C. Activity DNA and RNA Structure Activity DNA Double Helix

Many proteins work together in DNA replication and repair •- The Basic Principle: Base Pairing to a Template Strand (pp. 299-300) DNA replication is semiconservative: The parent molecule unwinds, and each strand then serves as a template

cells do seem capable of unlimited cell division, as do immortal strains of cultured cells (see Chapter 12). If telomerase is indeed an important factor in many cancers, it may provide a useful target for both cancer diagnosis and chemotherapy. In this chapter, you have learned how DNA replication provides the copies of genes that parents pass to offspring. However, it is not enough that genes be copied and transmitted; they must also be expressed. In the next chapter, we will examine how the cell translates genetic information encoded in DNA.

Concept Check 1. What role does complementary base pairing play in the replication of DNA? 2. Identify two major functions of DNA pol ill in DNA replication. 3. Why is DNA pol I necessary to complete synthesis of a leading strand? Point out in the overview box in Figure 16.16 where DNA pol I would function on the top leading strand. 4. How are telomeres important for preserving eukaryotic genes? For suggested answers, see Appendix A.

Review for the synthesis of a new strand according to base-pairing rules. Activity DNA Replication: An Overview Investigation What Is the Correct Model jor DNA Replication? • DNA Replication: A Closer Look (pp. 300-305) DNA replication begins at origins of replication. Y-shaped replication forks form at opposite ends of a replication bubble, where the two DNA strands separate. DNA synthesis starts at the 3' end of an RNA primer, a short polynucleotide complementary to the template strand. DNA polymerases catalyze the synthesis of new DNA strands, working in the 5'—*3' direction. The leading strand is synthesized continuously, and the lagging strand is synthesized in short segments, called Okazaki fragments. The fragments are joined together by DNA ligase. Activity DNA Replication: A Closer Look Activity DNA Replication Review • Proofreading and Repairing DNA (pp. 305-306) DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides. In mismatch repair of DNA, repair enzymes correct errors in base pairing. In nucleotide excision repair, enzymes cut out and replace damaged stretches of DNA. • Replicating the Ends of DNA Molecules (pp. 306-307) The ends of eukaryotic chromosomal DNA get shorter with each round of replication. The presence of telomeres, repetitive sequences at the ends of linear DNA molecules, postpones the erosion of genes. Telomerase catalyzes the lengthening of • telomeres in germ cells.

CHAPTER 16 "Hie Molecular Basis oi Inheritance

307

TESTING YOUR KNOWLEDGE Evolution Connection Many bacteria may be able to respond to environmental stress by increasing the rate at which mutations occur during cell division. How might this be accomplished, and what might be an evolutionary advantage of this ability?

Scientific Inquiry Demonstrate your understanding of the Meselson-Stahl experiment by answering the following questions. a. Describe in your own words exactly what each of the centrifugation bands pictured in Figure 16.11 represents.

308

THREE

Geneucs

b. Imagine that the experiment is done as follows: Bacteria are first grown for several generations in a medium containing the lighter isotope of nitrogen, 14 N, then switched into a medium containing N. The rest of the experiment is identical. Redraw Figure 16.11 to reflect this experiment, predicting what band positions you would expect after one generation and after two generations if each of the three models shown in Figure 16.10 were true. Investigation What Is the Correct Model for DNA Replication?

Science, Technology, and Society Cooperation and competition are both common in science. What roles did these two social behaviors play in Watson and Crick's discovery of the double helix? How might competition between scientists accelerate progress? How might it slow progress?

From Gene to Protein A Figure 17.1 A ribosome, part of the protein synthesis machinery.

17.1 Genes specify proteins via transcription and translation 17.2 Transcription is the DNA-directed synthesis of RNA: a closer look 1 73 Eukaryotic cells modify RNA after transcription 1 ?A Translation is the RNA-directed synthesis of a polypeptide: a closer look 17.5 RNA plays multiple roles in the cell: a review 17.6 Comparing gene expression in prokaryotes and eukaryotes reveals key differences 17.7 Point mutations can affect protein structure and function

The Flow of Genetic Information ~^he information content of DNA, the genetic material, is in the form of specific sequences of nucleotides along •JL the DNA strands. But how does this information determine an organism's traits? Put another way, what does a gene actually say? And how is its message translated by cells into a specific trait, such as brown hair or type A blood? Consider, once again, Mendel's peas. One of the characters Mendel studied was stem length (see Table 14.1). Mendel did not know the physiological basis for the difference between the tall and dwarf varieties of pea plants, but plant scientists have since worked out the explanation: Dwarf peas lack growth hormones called gibberellins, which stimulate the normal elongation of stems. A dwarf plant treated with gibberellins from an external source grows to normal height.

Why do dwarf peas fail to make their own gibberellins? They are missing a key protein, an enzyme required for gibberellin synthesis. And they are missing that protein because they do not have a properly functioning gene for that protein. This example illustrates the main point of this chapter: The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins. In other words, proteins are the links between genotype and phenotype. The process by which DNA directs protein synthesis, gene expression, includes two stages, called transcription and translation. In Figure 17.1, you can see a computer model of a ribosome, which is part of the cellular machinery for translation—polypeptide synthesis. This chapter describes the flow of information from gene to protein in detail. By the end, you will understand how genetic mutations, such as the one causing the dwarf trait in pea plants, affect organisms through their proteins.

Concept

Genes specify proteins via transcription and translation Before going into the details of how genes direct protein synthesis, let's step back and examine how the fundamental relationship between genes and proteins was discovered.

Evidence from the Study of Metabolic Defects In 1909, British physician Archibald Garrod was the first to suggest that genes dictate phenotypes through enzymes that catalyze specific chemical reactions in the cell. Garrod postulated that the symptoms of an inherited disease reflect a person's inability Lo make a particular enzyme. He referred to

such diseases as "inborn errors of metabolism." Garrod gave as one example the hereditary condition called alkaptonuria, in which the urine is black because it contains the chemical alkapton, which darkens upon exposure to air. Garrod reasoned that most people have an enzyme that breaks down alkapton, whereas people with alkaptonuria have inherited an inability to make the enzyme that metabolizes alkapton. Garrod's idea was ahead of its time, but research conducted several decades later supported his hypothesis that a gene dictates the production of a specific enzyme. Biochemists accumulated much evidence that cells synthesize and degrade most organic molecules via metabolic pathways, in which each chemical reaction in a sequence is catalyzed by a specific enzyme. Such metabolic pathways lead, for instance, to the synthesis of the pigments that give fruit flies (Drosophila) their eye color (see Figure 15.3). In the 1930s, George Beadle and Boris Ephrussi speculated that in Drosophila, each of the various mutations affecting eye color blocks pigment synthesis at a specific step by preventing production of the enzyme that catalyzes that step. However, neither the chemical reactions nor the enzymes that catalyze them were known at the time. Nutritional Mutants in Neurospora: Scientific Inquiry A breakthrough in demonstrating the relationship between genes and enzymes came a few years later, when Beadle and Edward Tatum began working with a bread mold, Neurospora crassa. They bombarded Neurospora with X-rays and then looked among the survivors for mutants that differed in their nutritional needs from the wild-type mold. Wild-type Neurospora has modest food requirements. It can survive in the laboratory on agar (a moist support medium) mixed only with inorganic salts, glucose, and the vitamin biotin. From this minimal medium, the mold uses its metabolic pathways to produce all the other molecules it needs. Beadle and Tatum identified mutants that could not survive on minimal medium, apparently because they were unable to synthesize certain essential molecules from the minimal ingredients. However, most such nutritional mutants can survive on a complete growth medium, minimal medium supplemented with all 20 amino acids and a few other nutrients. To characterize the metabolic defect in each nutritional mutant, Beadle and Tatum took samples from the mutant growing on complete medium and distributed them to a number of different vials. Each vial contained minimal medium plus a single additional nutrient. The particular supplement that allowed growth indicated the metabolic defect. For example, if the only supplemented vial that supported growth of the mutant was the one fortified with the amino acid arginine, the researchers could conclude that the mutant was defective in the biochemical pathway that wild-type cells use to synthesize arginine. 310

UNIT THREE

Genetics

Beadle and Tatum went on to pin down each mutant's defect more specifically. Their work with arginine-requiring mutants was especially instructive. Using genetic crosses, they determined that their mutants fell into three classes, each mutated in a different gene. The researchers then showed that they could distinguish among the classes of mutants nutritionally by additional tests of their growth requirements (Figure 17.2). In the synthetic pathway leading to arginine, they suspected, a precursor nutrient is converted to ornithine, which is converted to citrulline, which is converted to arginine. When they tested their arginine mutants for growth on ornithine and citrulline, they found that one class could grow on either compound (or arginine), the second class only on citrulline (or arginine), and the third on neither—it absolutely required arginine. The three classes of mutants, the researchers reasoned, must be blocked at different steps in the pathway that synthesizes arginine, with each mutant class lacking the enzyme that catalyzes the blocked step. Because each mutant was defective in a single gene, Beadle and Tatum's results provided strong support for the one gene-one enzyme hypothesis, as they dubbed it, which stales that the function of a gene is to dictate the production of a specific enzyme. The researchers also showed how a combination of genetics and biochemistry could be used to work out the steps in a metabolic pathway. Further support for the one gene-one enzyme hypothesis came with experiments that identified the specific enzymes lacking in the mutants. The Products of Gene Expression: A Developing Story As researchers learned more about proteins, they made minor revisions to the one gene-one enzyme hypothesis. First of all, not all proteins are enzymes. Keratin, the structural protein of animal hair, and the hormone insulin are two examples of nonenzyme proteins. Because proteins that are not enzymes are nevertheless gene products, molecular biologists began to think in terms of one gene-one protein. However, many proteins are constructed from two or more different polypeptide chains, and each polypeptide is specified by its own gene. For example, hemoglobin, the oxygen-transporting protein of vertebrate red blood cells, is built from two kinds of polypeptides, and thus two genes code for this protein (see Figure 5.20). Beadle and Tatum's idea has therefore been restated as the one gene-one polypeptide hypothesis. Even this statement is not entirely accurate, though. As you will learn later in this chapter, some genes code for RNA molecules that have important functions in cells even though they are never translated into protein. But for now, we will focus on genes that code for polypeptides. (Note that it is common to refer to proteins, rather than polypeptides, as the gene products, a practice you will encounter in this book.)

Do individual genes specify different enzymes in arginine biosynthesis? /orking with the mold Neurospora crassa, George Beadie and Edward Tatum had isolated mutants requiring aiginine in their growth medium and had shown genetically that these mutants fell into three classes, each defective in a different giine. From other considerations, they suspected that the metabolic pathway of arginine biosynthesis included the precursors ornithine and citruiiine. Their most famous experiment, shown here, tested both their one gene-one enzyme hypothesis and their postulated ai ginine pathway. In this experiment, they grew their three classes 0" mutants under the four different conditions shown in the Results section below. The wild-type strain required only the minimal medium for growth. The three classes of mutants had different g'owth requirements. Wild type

Class I Mutants

Class II Mutants

Class I I I Mutants

Minimal medium (MM) (control) MM + Ornithine

u MM + Citruiiine

U

MM + Arginine (control)

y

P

u

i

From the growth patterns of the mutants, Beadle and Tatum deduced that each mutant was unable to carry out cne step in the pathway for synthesizing arginine, presumably because it lacked the necessary enzyme. Because each of their mutants was mutated in a single gene, they concluded that each nutated gene must normally dictate the production of one enzyme. Their results supported the one gene-one enzyme hypothesis and also confirmed the arginine pathway. (Notice that a mutant can grow only if supplied with a compound made after the defective step.) Class I Mutants (mutation in gene A)

Class II Mutants (mutation in gene 6)

Class I I I Mutants (mutation in gene C)

Precursor tmme

Precursor

Precursor

Precursor

Ornithine

Ornithine

Wild type

X

Ornithine

Ornithine

Citruiiine

Citruiiine

Citruiiine

^Enzyme

B

Citrufitne

B

A

»x

Enzyme

c Arginine

Arginine

c

Arginjne

A

B

X

Arginine

Basic Principles of Transcription and Translation Genes provide the instructions for making specific proteins. But a gene does not build a protein directly. The bridge between DNA and protein synthesis is the nucleic acid RNA. You learned in Chapter 5 that RNA is chemically similar to DNA, except that it contains ribose instead of deoxyribose as its sugar and has the nitrogenous base uracil rather than thymine (see Figure 5.26). Thus, each nucleoLide along a DNA strand has A, G, C, or T as its base, and each nucleotide along an RNA strand has A, G, C, or U as its base. An RNA molecule usually consists ol a single strand. it is customary to describe the flow of information from, gene to protein in linguistic terms because both nucleic acids and proteins are polymers with specific sequences of monomers that convey information, much as specific sequences of letters communicate information in a language like English. In DNA or RNA, the monomers are the four types of nucleotides, which differ in their nitrogenous bases. Genes are typically hundreds or thousands of nucleotides long, each gene having a specific sequence of bases. Each polypeptide of a protein also has monomers arranged in a particular linear order (the proteins primary structure), but its monomers are the 20 amino acids. Thus, nucleic acids and proteins contain information written in two different chemical languages. Getting from DNA to protein requires two major stages, transcription and translation. Transcription is the synthesis of RNA under the direction of DNA. Both nucleic acids use the same language, and the information is simply transcribed, or copied, from one molecule to the other Just as a DNA strand provides a template for the synthesis of a new complementary strand during DNA replication, it provides a template for assembling a sequence of RNA nucleotides. The resulting RNA molecule is a faithful transcript of the gene's protein-building instructions. In discussing proteincoding genes, tbis type of RNA molecule is called messenger RNA (mRNA), because it carries a genetic message from the DNA to the protein-synthesizing machinery of the cell. (Transcription is the general term for the synthesis of any kind of RNA on a DNA template. Later in this chapter, you will learn about other types of RNA produced by transcription.) Translation is the actual synthesis of a polypeptide, which occurs under the direction of mRNA. During this stage, there is a change in language: The cell must translate the base sequence of an mRNA molecule into the amino acid sequence of a polypeptide. The sites of translation are ribosomes, complex particles that facilitate the orderly linking of amino acids into polypeptide chains. You might wonder why proteins couldn't simply be translated directly from DNA. There are evolutionary reasons for using an RNA intermediate. First, it provides protection for the DNA and its genetic information. As an analogy, when an architect designs a house, the original specifications (analogous CHAPTER 17

From Gene to Protein

311

to DNA) are not what the construction workers use at the site. Instead they use copies of the originals (analogous to mRNA), keeping the originals pristine and undamaged. Second, using an RNA intermediate allows more copies of a protein to be made simultaneously, since many RNA transcripts can be made from one gene. Also, each RNA transcript can be translated repeatedly. Although the basic mechanics of transcription and translation are similar for prokaryotes and eukaryotes, there is an important difference in the flow of genetic information within the cells. Because bacteria lack nuclei, their DNA is not segregated from ribosomes and the other protein-synthesizing equipment (Figure 17.3a). As you will see later, this allows translation of an mRNA to begin while its transcription is still in progress (see Figure 17.22). In a eukaryotic cell, by contrast, the nuclear envelope separates transcription from translation in space and time (Figure 17.3b). Transcription occurs in the nucleus, and mRNA is transported to the cytoplasm, where translation occurs. But before they can leave the nucleus, eukaryotic RNA transcripts are modified in various ways to produce the final, functional mRNA. The transcription of a protein-coding eukaryotic gene results in pre-mRNA, and RNA processing yields the finished mRNA. The initial RNA transcript from any gene, including those coding lor RNA that is not translated into protein, is more generally called a primary transcript. Lets summarize: Genes program protein synthesis via genetic messages in the form of messenger RNA. Put another way, cells are governed by a molecular chain of command: DNA —* RNA —* protein. In the next section, we discuss how the instructions for assembling amino acids into a specific order are encoded in nucleic acids.

The Genetic Code

Polypeptide

(a) Prokaryotic cell. In a cell lacking a nucleus, mRNA produced by transcription is immediately translated without additional processing.

TRANSCRIPTION

RNA PROCESSING

TRANSLATION Polypeptide

When biologists began to suspect that the instructions for protein synthesis were encoded in DNA, they recognized a problem: There are only four nucleotide bases to specify 20 amino acids. Thus, the genetic code cannot be a language like Chinese, where each written symbol corresponds to a single word. How many bases, then, correspond to an amino acid? Codons: Triplets of Bases If each nucleotide base were translated into an amino acid. only 4 of the 20 amino acids could be specified. Would a language of two-letter code words suffice? The base sequence AG, for example, could specify one amino acid, and GT could specify another. Since there are four bases, this would give us 16 (that is, 42) possible arrangements—still not enough to code for all 20 amino acids. Triplets of nucleotide bases are the smallest units of uniform length that can code for all the ammo acids. If each arrangement of three consecutive bases specifies an amino acid, there can be 64 (that is, 43) possible code words-—more 312

U N I T THREE

GfllCUCS

(b) Eukaryotic cell. The nucleus provides a separate compartment for transcription. The original RNA transcript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA.

A Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information. In a cell, inherited information flows from DNA to RNA to protein. The two main stages of information flow are transcription and translation. A miniature version of part (a) or (b) accompanies several figures later in the chapter as an orientation diagram to help you see where a particular figure fits into the overall scheme.

than enough to specify all the amino acids. Experiments have verified that the flow of information from gene to protein is based on a triplet code: The genetic instructions for a polypeptide chain are written in the DNA as a series D! n on overlapping, three-nucleotide words. For example, the

base triplet AGT at a particular position along a DNA strand results in the placement of the amino acid serine at the corresponding position of the polypeptide to be produced. During transcription, the gene determines the sequence of bases along the length of an mRNA molecule (Figure 17.4). For each gene, only one of the two DNA strands is transcribed. This strand is called the template strand because it provides the template for ordering the sequence of nucleotides in an RNA transcript. A given DNA strand can be the template strand for some genes along a DNA molecule, while for other genes in otn.er regions, the complementary strand may function as the template. Note, however, that for a given gene, the same strand is used as the template every time it is transcribed. An mRNA molecule is complementary rather than identical to its DNA template because RNA bases are assembled on the template according to base-pairing rules. The pairs are similar to those that form during DNA replication, except that U, the RNA substitute for T, pairs with A and the mRNA nucleotides contain ribose instead of deoxyribose. Like a new strand of D N A, the RNA molecule is synthesized in an antiparallel direction to ihe template strand of DNA- (To review what is meant by "antiparallel" and the 5' and 3' ends of a nucleic acid chain, see Figure 16.7.) For example, the base triplet ACC along the DNA (written as 3'-ACC-5') provides a template for 5'-UGG-3' in

mRNA

5' v

TRANSLATION |

v ' ^ Codon I

v

I

J

V

v

\

J

V

v-

'

\

^rotein Amino acid A Figure 17.4 The t r i p l e t code. For each gene, one DNA strand functions as a template for transcription. The base-pairing rules for CNA synthesis also guide transcription, but uraci! (U) takes the place of thymine (T) in RNA. During translation, the mRNA is read as a sequence of base triplets, called codons. Each codon specifies an amino acid to be added to the growing polypeptide chain. The mRNA is read in the 5' -» 3' direction.

ihe mRNA molecule. The mRNA base triplets are called codons, and they are customarily written in the 5' —* 3' direction. In our example, UGG is the codon for the ammo acid tryptophan (abbreviated Tip). The term codon is also sometimes used for the DNA base triplets along the nontemplate strand. These codons are complementary to the template strand and thus identical in sequence to the mRNA except that they have T instead of U. (For this reason, the nontemplate DNA strand is sometimes called the "coding strand.") During translation, the sequence of codons along an mRNA molecule is decoded, or translated, into a sequence of amino acids making up a polypeptide chain. The codons are read by the translation machinery in the 5' —•* 3' direction along the mRNA. Each codon specifies which one of the 20 amino acids will be incorporated at the corresponding position along a polypeptide. Because codons are base triplets, the number of nucleotides making up a genetic message must be three times the number of amino acids making up the protein product. For example, it takes 300 nucleotides along an mRNA strand to code for a polypeptide that is f 00 amino acids long. Cracking ihe Code Molecular biologists cracked the code of life in the early 1960s, when a series of elegant experiments disclosed the amino acid translations of each of the RNA codons. The first codon was deciphered in 1961 by Marshall Nirenberg, of the National Institutes of Health, and his colleagues. Nirenberg synthesized an artificial mRNA by linking identical RNA nucleotides containing uracil as their base. No matter where this message started or stopped, it could contain only one codon in repetition: UUU. Nirenberg added this "poly-U" to a test-tube mixture containing amino acids, ribosomes, and the other components required for protein synthesis. His artificial system translated the poly-U into a polypeptide containing a single amino acid, phenylalanine (Phe), strung together as a long polyphenylalanine chain. Thus, Nirenberg determined that the mRNA codon UUU specifies the amino acid phenylalanine. Soon, the amino acids specified by the codons AAA, GGG, and CCC were also determined. Although more elaborate techniques were required to decode mixed triplets such as AUA and CGA, all 64 codons were deciphered by the mid-1960s. As Figure 17.5 on the next page shows, 61 of the 64 triplets code for amino acids. The three codons thai do not designate amino acids are <ustop" signals, or termination codons, marking the end of translation. Notice that the codon AUG has a dual function: Tt codes for the arnino acid methionine (Met) and also functions as a "start" signal, or initiation codon. Genetic messages begin with the mRNA codon AUG, which signals the protein-synthesizing machinery to begin translating the mRNA at that location. (Because AUG also stands for methionine, polypeptide chains begin with methionine when they are synthesized. However, an enzyme may subsequently remove this starter amino acid from the chain.)

C H A P T E R 17

From Gene to Protein

313

>• Figure 17.6 A tobacco plant expressing a firefly

Second mRNA base

•

uuu i uuc UUA

ucu "1 Phe

J Leu

UUG CUU

-

cue CUG

AUC

He

AUA

GUU

GUG

UCG _

UAG

ecu

CAU

1

CAC Pro CAA

CCG

J

ACU

-1

ACC

Thr

Met of

]

ACG GCU

J

-

AAU

AAA

GAU

-

UGC

Cys

UGA Stop

Trp

Stop.

1

CGU "

S

J'

G!n

1

CGC CGA

Arg

m • •

3

CGG_

J 1

Lys

1 Asp

AGC

Ser

m

A,

•

AGA" AGG^ GGU GGC

GAA

"j

GGA

GAG

J

GGG

Gly

1 I 1

A Figure 17.5 The dictionary of the genetic code. The three bases of an mRNA codon are designated here as the first, second, and third bases, reading in the 5' —* 3' direction along the mRNA. (Practice using this dictionary by finding the codons in Figure 17.4.) The codon AUG not only stands for the amino acid methionine (Met) but also functions as a "start" signal for ribosornes to begin translating the mRNA at that point. Three of the 64 codons function as "stop" signals, marking the end of a genetic message.

Notice in Figure 17.5 that there is redundancy in the genetic code, but no ambiguity For example, although codons GAA and GAG both specify glutamic acid (redundancy), neither of them ever specifies any other amino acid (no ambiguity). The redundancy in the code is not altogether random. In many cases, codons that are synonyms for a particular amino acid differ only m the third base of the triplet. We will consider a possible benefit for this redundancy later in the chapter. Our ability to extract the intended message from a written language depends on reading the symbols in the correct groupings—that is, in the correct reading frame. Consider this statement: "The red dog ate the cat." Group the letters incorrectly by starting at the wrong point, and the result will probably be gibberish: for example, "her edd oga tet hec at." The reading frame is also important in the molecular language of cells. The short stretch of polypeptide shown in Figure f7.4, for instance, will only be made correctly if the mRNA nucleotides are read from left to right (5' —* 3') in the groups ol three shown in the figure: UGG UUU GGC UCA. Although a genetic message is written with no spaces between the codons, the cell's protein-synthesizing machinery reads the message as a series of nonoverlapping three-letter words. The message is not read as a series of overlapping 314

* THREE

Genetics

g e n e . Because diverse forms of life share a common genetic code, it is possible to program one species to produce proteins characteristic of another species by transplanting DNA. In this experiment, researchers were able to incorporate a gene from a firefly into the DNA of a tobacco plant. The firefly gene codes for an enzyme that catalyzes a chemical reaction that releases light energy.

AGU"

Asn

GAC

Ala

GCA GCG

J

AAG

GCC Val

Tyr

CAG

AAC

ACA

GUC GUA

UAA

CCA

AUU

AUG

Ser

UCA

Leu

j

UAU UAC

ucc

CCC

CUA

m1

UK UGU" •

words—UGGUUU, and so on-—which would convey a ver different message. Evolution of the Genetic Code The genetic code is nearly universal, shared by organisms froi the simplest bacteria to the most complex animals. The RNA codon CCG, for instance, is translated as the amino acid prc line in all organisms whose genetic code has been examined. I laboratory experiments, genes can be transcribed and tran: lated after being transplanted from one species to another (Figure 17.6). Bacteria can be programmed by the insertion of human genes to synthesize certain human proteins for medical use. Such applications have produced many exciting developments in biotechnology (see Chapter 20). Exceptions to the universality of the genetic code indue e translation systems where a few codons differ from the standard ones. Slight variations in the genetic code exist in certai n unicellular eukaryotes and in the organelle genes of some species. Some prokaryotes can translate stop codons into one of two amino acids not found in most organisms. Despite these exceptions, the evolutionary significance of the code's near universality is clear. A language shared by all living things must have been operating very early in the history of life—j— early enough to be present in the common ancestors of aill modern organisms. A shared genetic vocabulary is a reminder of the kinship that bonds all life on Earth. Concept Check 1. Draw the nontemplate strand of DNA for the template shown in Figure 17.4. Compare and contrast its base sequence with the mRNA molecule. 2. What protein product would you expect from a poly-G mRNA that is 30 nucleotides long? For suggested answers, see Appendix A.

Concept

Transcription is the DNA-directed synthesis of RNA: a closer look NJow that we have considered the linguistic logic and evolutionary significance of the genetic code, we are ready to reexamine tr inscription, the first stage of gene expression, in more detail.

Molecular Components of Transcription Messenger RNA, the carrier of information from DNA to the cell's protein-synthesizing machinery, is transcribed from the

Transcription unit

'-"• T

Start point

RNA polymerase 0 Initiation. After RNA polymerase binds to the promoter, the DNA strands unwind, and the polymerase initiates RNA synthesis at the start point on the template strand.

template strand of a gene. An enzyme called an RNA polymerase pries the two strands of DNA apart and hooks together the RNA nucleotides as they base-pair along the DNA template (Figure 17.7). Like the DNA polymerases that function in DNA replication, RNA polymerases can only assemble a polynucleotide in its 5' —* 3' direction. Unlike DNA polymerases, however, RNA polymerases are able to start a chain from scratch; they don't need a primer. Specific sequences of nucleotides along the DNA mark where transcription of a gene begins and ends. The DNA sequence where RNA polymerase attaches and initiates transcription is known as the promoter; in prokaryotes, the sequence that signals the end of transcription is called the terminator. (The termination mechanism is different in eukaryotes, which we'll describe later.) Molecular biologists refer to the direction of transcription as "downstream" and the other direction as "upstream." These terms are also used to describe the positions of nucleotide sequences within the DNA or RNA. Thus, the promoter sequence in DNA is said to be upstream from the terminator. The stretch of DNA that is transcribed into an RNA molecule is called a transcription unit. Bacteria have a single type of RNA polymerase that synthesizes not only mRNA but also other types of RNA that function in

Elongation 5' : — 3' — Unwound >NA

RNA nucleotides transcript

Template strand of DNA

Elongation. The polymerase moves downstream, unwinding the DNA and elongating the RNA transcript 5' -* 3'. In the wake of transcription, the DNA strands re-form a double helix.

Termination. Eventually, the RNA transcript is released, and the polymerase detaches from the DNA.

Direction o f transcription ("downstream") ^ ^^ N

^ < \ Template strand o f DNA

Newly made RNA

^ Figure 17.7 The stages of transcription: initiation, elongation, and termination. This general depiction of Completed RNA transcript

transcription applies to both prokaryotes and eukaryotes, but the details of termination vary for prokaryotes and eukaryotes, as described in the text. Also, in a prokaryote, the RNA transcript is immediately usable as mRNA, whereas in a eukaryote, it must first undergo processing to become mRNA.

CHAPTER 17

From Gene to Protein

315

protein synthesis. In contrast, eukaryotes have three types of RNA polymerase in their nuclei, numbered I, II, and III. The one used for mRNA synthesis is RNA polymerase II. The other two RNA polymerases transcribe RNA molecules that are not translated into protein. In the discussion of transcription that follows, we start with the features of mRNA synthesis common to both prokaryotes and eukaryotes and then describe some key differences.

Synthesis of an RNA Transcript

A single gene can be transcribed simultaneously by sever; molecules of RNA polymerase following each other like truck in a convoy. A growing strand of RNA trails off from each polymerase, with the length of each new strand reflecting how far along the template the enzyme has traveled from the sta t point (see Figure 17.22). The congregation of many polymerase molecules simultaneously transcribing a single gene increases the amount of mRNA transcribed from it, which helps the cell make the encoded protein in large amounts.

The three stages of transcription, as shown in Figure 17.7 and described next, are initiation, elongation, and termination of the RNA chain. Study Figure 17.7 to familiarize yourself with the stages and the terms used to describe them.

0 Eukaryotic promoters commonly include a TATA box, a nudeotide sequence containing TATA, about 25 nucleotides upstream from the transcriptional start point. (By convention, nucleotide sequences are given as they occur on the non-template strand.)

RNA Polymerase Binding and Initiation of Transcription The promoter of a gene includes within it the transcription start point (the nucleotide where RNA synthesis actually begins) and typically extends several dozen nucleotide pairs "upstream" from the start point. In addition to serving as a binding site for RNA polymerase and determining where transcription starts, the promoter determines which of the two strands of the DNA helix is used as the template. Certain sections of a promoter are especially important for binding RNA polymerase. In prokaryotes, the RNA polymerase itself specifically recognizes and binds to the promoter. In eukaryotes, a collection of proteins called transcription factors mediate the binding of RNA polymerase and the initiation of transcription. Only after certain transcription factors are attached to the promoter does RNA polymerase II bind to it. The completed assembly of transcription factors and RNA polymerase II bound to the promoter is called a transcription initiation complex. Figure 17.8 shows the role of transcription factors and a crucial promoter DNA sequence called a TATA box in forming the initiation complex in eukaryotes. The interaction between eukaryotic RNA polymerase II and transcription factors is an example of the importance of proteinprotein interactions in controlling eukaryotic transcription (as we will discuss further in Chapter 19). Once the polymerase is firmly attached to the promoter DNA, the two DNA strands unwind there, and the enzyme starts transcribing die template strand.

Transcription factors

Several transcription factors, one recognizing the TATA box, must bind to the DNA before RNA polymerase II can do so.

© Additional transcription factors (purple) bind to the DNA along with RNA polymerase II, forming the transcription initiation complex. The DNA double helix then unwinds, and RNA synthesis begins at the start point on the template strand. RNA polymerase II -.Transcription factor

Elongation of the RNA Strand As RNA polymerase moves along the DNA, it continues to untwist the double helix, exposing about 10 to 20 DNA bases at a time for pairing with RNA nucleotides (see Figure 17.7). The enzyme adds nucleotides to the 3' end of the growing RNA molecule as it continues along the double helix. In the wake of this advancing wave of RNA synthesis, the new RNA molecule peels away from its DNA template and the DNA double helix re-forms. Transcription progresses at a rate of about 60 nucleotides per second in eukaryotes. 316

UNIT THREE

Genetics

X

RNA transcript

Transcription initiation complex A. Figure 17.8 The initiation of transcription at a eukaryot ic promoter. In eukaryotic cells, proteins called transcription factors mediate the initiation of transcription by RNA polymerase II

ermination of Transcription The mechanism of termination differs between prokaryotes snd eukaryotes. In prokaryotes, transcription proceeds tirough a terminator sequence in the DNA. The transcribed terminator (an RNA sequence) functions as the termination signal, causing the polymerase to detach from the DNA and release the transcript, which is available for immediate use as niRNA. In eukaryoles, however, the pre-mRNA is cleaved f "om the growing RNA chain while RNA polymerase II continues to transcribe the DNA. Specifically, the polymerase transcribes a sequence on the DNA called the polyadenylation signal sequence, which codes for a polyadenylation signal C^UAAA) in the pre-mRNA. Then, at a point about iO to 35 r ucleotides downstream from the AAUAAA signal, proteins associated with the growing RNA transcript cut it free from tjne polymerase, releasing the pre-mRNA. The polymerase continues transcribing for hundreds of nucleotides past the site where the pre-mRNA was released. Transcription is termimated when the polymerase eventually falls off the DNA (by a mechanism that is not fully understood). Once the pre-mRNA .as been made, it is modified during RNA processing, the ipic of our next section. Concept Check 1. Compare and contrast the functioning of DNA polymerase and RNA polymerase. 2. Is the promoter at the upstream or downstream end of a transcription unit? 3. In a prokaryote, how does RNA polymerase "know" where to start transcribing a gene? In a eukaryote? 4. How is the primary transcript produced by a prokaryotic cell different from that produced by a eukaryotic cell? For suggested answers, see Appendix A.

Concept

Eukaryotic cells modify RNA after transcription Enzymes in the eukaryotic nucleus modify pre-mRNA in specific ways before the genetic messages are dispatched to the cytoplasm. During this RNA processing, both ends of the primary transcript are usually altered. Also, in most cases, certain interior sections of the molecule are cut out and the remaining parts spliced together. These modifications help form an mRNA molecule that is ready to be translated.

Alteration of mRNA Ends Each end of a pre-mRNA molecule is modified in a particular way (Figure 17.9). The 5' end, the end transcribed first, is capped off with a modified form of a guanine (G) nucleotide after transcription of the first 20 to 40 nucleotides, forming a 5' cap. The 3' end of the pre-mRNA molecule is also modified before the mRNA exits the nucleus. Recall that the premRNA is released soon after the polyadenylation signal, AAUAAA, is transcribed. At the 3' end, an enzyme adds 50 to 250 adenine (A) nucleotides, forming a poly-A tail. The 5' cap and poly-A tail share several important functions. First, they seem to facilitate the export of ihe mature mRNA from the nucleus. Second, they help protect the mRNA from degradation by hydrolytic enzymes. And third, once the mRNA reaches the cytoplasm, both structures help ribosomes attach to the 5' end of the mRNA. Figure 17.9 shows a diagram of a eukaryotic mRNA molecule with cap and tail. The figure also shows the untranslated regions (UTRs) at the 5' and 3' ends of the mRNA (referred to as the 5' UTR and 3' UTR). The UTRs are parts of the mRNA that will not be translated into protein, but they have other functions, such as ribosome binding.

A modified guanine nucleotide added to the 5' end

50 to 250 adenine nucleotides added to the 3' end Protein-coding segment

5' Cap

A Figure 17.9 RNA processing: addition of the 5' cap and poly-A tail. Enzymes modify the t w o ends of a eukaryotic pre-mRNA molecule. The modified ends may promote ie export of mRNA from the nucleus and

5' UTR

""-Start codon Stop codony

help protect the mRNA from degradation. When the mRNA reaches the cytoplasm, the modified ends, in conjunction with certain cytoplasmic proteins, facilitate ribosome attachment. The 5' cap and poly-A tail are not

Polyadenyiation signal

\

Poly-A tail

translated into protein, nor are the regions called the 5' untranslated region (5' UTR) and 3' untranslated region (3' UTR).

CHAPTER w From Gene to Protein

317

Split Genes and RNA Splicing

sequence at each end of an iniron. Particles called small nuclear ribonuclmpmteins, abbreviated snRNPs (pronounced "snurps") recognize these splice sites. As the name implies. snRNPs arJ located in the cell nucleus and are composed of RNA and pro] tein molecules. The RNA in a snRNP particle is called a small nuclear RNA (snRNA); each molecule is about 150 nucleotides long. Several different snRNPs join with additional proteins to form an even larger assembly called a spliceosome, which ii almost as big as a ribosome. The spliceosome interacts with certain sites along an intron, releasing the intron and joining together the two exons that flanked the intron (Figure 17.11) There is strong evidence that snRNAs play a major role ii these catalytic processes, as well as in spliceosome assembl" and splice site recognition.

The most remarkable stage of RNA processing in the eukaryotic nucleus is the removal of a large portion of the RNA molecule that is initially synthesized—a cut-and-paste job called RNA splicing (Figure 17.10). The average length of a transcription unit along a eukaryotic DNA molecule is about 8,000 nucleotides, so the primary RNA transcript is also that long. But it takes only about 1,200 nucleotides to code for an average-sized protein of 400 amino acids. (Remember, each amino acid is encoded by a triplet of nucleotides.) This means that most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides, regions that are not translated. Even more surprising is that most of these noncoding sequences are interspersed between coding segments of the gene and thus between coding segments of the pre-mRNA. In other words, the sequence of DNA nucleotides that codes for a eukaryotic polypeptide is usually not continuous; it is split into segments. The noncoding segments ol nucleic acid that lie between coding regions are called intervening sequences, or introns for short. The other regions are called exons, because they are eventually expressed, usually by being translated into amino acid sequences. (Exceptions include the UTRs of the exons at the ends of the RNA, which make up part of the mRNA but are not translated into protein. Because of these exceptions, you may find it helpful to think of exons as sequences of RNA that exit the nucleus.) The terms intron and exon are used for both RNA sequences and the DNA sequences that encode them. In making a primary transcript from a gene, RNA polymerase II transcribes both introns and exons from the DNA, but the mRNA molecule that enters the cytoplasm is an abridged version. The introns are cut out from the molecule and the exons joined together, forming an mRNA molecule with a continuous coding sequence. This is the process of RNA splicing. How is pre-mRNA splicing carried out? Researchers have learned that the signal for RNA splicing is a short nucleotide

5' Exon

Ribozymes The idea of a catalytic role for snRNA arose from the discover of ribozymes, RNA molecules that function as enzymes. IT some organisms, RNA splicing can occur without proteins o" additional RNA molecules; The intron RNA functions as i ribozyme and catalyzes its own excision! For example, in the protozoan Tetrahymena, self-splicing occurs in the production of ribosomal RNA (rRNA), a component of the organisms ribosomes. The pre-rRNA actually removes its own introns. The fact that RNA is single-stranded plays an important role in allowing certain RNA molecules to function as ribozymes, A region of an RNA molecule may base-pair with k complementary region elsewhere in the same molecule, thui imparting specific structure to the RNA molecule as a whole. Also, some of the bases contain functional groups that may participate in catalysis. Just as the specific shape of an enzymatic protein and the functional groups on its amino acid side chains allow the protein to function as a catalyst, the structure of some RNA molecules allows them to function as catalysts, too. The discovery of ribozymes rendered obsolete the belief that all biological catalysts were proteins.

Intron

Exon

Codi n9 segmient

I Introns cut out anid I exons spliced togeether jf

5'UTR

A Figure 17.10 RNA processing: RNA splicing. The RNA molecule shown here codes for p-globin, one of the polypeptides of hemoglobin. The numbers under the RNA refer

318

UNIT THREE

to codons; (3-globin is 146 amino acids long. The p-giobin gene and its pre-mRNA transcript have three exons, corresponding to sequences that will leave the nucleus as mRNA. (The 5' UTR and 3'

UTR are parts of exons because they are includec in the mRNA; however, they do not code for protein.) During RNA processing, the introns are cut out and the exons spliced together.

Termination of Transcription Thi mechanism of termination differs between prokaryotes and eukaryotes. In prokaryotes, transcription proceeds through a terminator sequence in the DNA. The transcribed terminator (an RNA sequence) functions as the termination signal, causing the polymerase to detach from the DNA and release the transcript, which is available for immediate use as mRNA. In eukaryotes, however, the pre-rnRNA is cleaved from the growing RNA chain while RNA polymerase II conlirues to transcribe the DNA. Specifically, the polymerase transcribes a sequence on the DNA called the polyadenylation signal sequence, which codes for a polyadenylation signal (AAUAAA) in the pre-mRNA. Then, at a point about 10 to 35 nt.cleotides downstream from the AAUAAA signal, proteins associated with the growing RNA transcript cut it free from the polymerase, releasing the pre-mRNA. The polymerase continues transcribing for hundreds of nucleotides past the site where the pre-mRNA was released. Transcription is terminated when the polymerase eventually falls off the DNA (by a mechanism that is not fully understood). Once the pre-mRNA has been made, it is modified during RNA processing, the topic of our next section.

Concept Check 1. Compare and contrast the functioning of DNA polymerase and RNA polymerase. 2. Is the promoter at the upstream or downstream end of a transcription unit? 3. In a prokaryote, how does RNA polymerase "know" where to start transcribing a gene? In a eukaryote? 4. How is the primary transcript produced by a prokaryotic cell different from that produced by a eukaryotic cell? For suggested answers, see Appendix A-

Concept

Eukaryotic cells modify RNA after transcription Enzymes m the eukaryotic nucleus modify pre-mRNA in specific ways before the genetic messages are dispatched to the cytoplasm. During this RNA processing, both ends of the primary transcript are usually altered. Also, in most cases, certain interior sections of the molecule are cut out and the remaining parts spliced together. These modifications help form an mRNA molecule that is ready to be translated.

Alteration of mRNA Ends Each end ot a pre-mRNA molecule is modified in a particular way (Figure 17.9). The 5' end, the end transcribed first, is capped off with a modified form of a guanine (G) nucleotide after transcription of the first 20 to 40 nucleotides, forming a 5' cap. The 3' end of the pre-mRNA molecule is also modified before the mRNA exits the nucleus. Recall that the premRNA is released soon after the polyadenylation signal, AAUAAA, is transcribed. At the 3' end, an enzyme adds 50 to 250 adenine (A) nucleotides, forming a poly-A tail. The 5' cap and poly-A tail share several important [unctions. First, they seem to facilitate the export of the mature mRNA from the nucleus. Second, they help protect the mRNA from degradation by hydrolytic enzymes. And third, once the mRNA reaches the cytoplasm, both structures help ribosomes attach to the 5' end of the mRNA. Figure 17.9 shows a diagram of a eukaryotic mRNA molecule with cap and tail. The figure also shows the untranslated regions (UTRs) at the 5' and 3f ends of the mRNA (referred to as the 5' l/TR and 3' UTR). The UTRs are parts of the mRNA that will not be translated into protein, but they have other functions, such as ribosome binding.

A modified guanine nucleotide added to the 5' end

50 to 250 adenine nucleotides added to the 3' end Protein-coding segment

Start codon

* Figure 17.9 RNA processing: addition of the 5' cap and poly-A tail. Enzymes modify the t w o ends of a eukaryotic pre-mRNA molecule. The modified ends may promote the export of mRNA from the nucleus and

Poiyadenyiation signal

Stop codon^

help protect the mRNA from degradation. When the mRNA reaches the cytoplasm, the modified ends, in conjunction with certain cytoplasmic proteins, facilitate ribosome attachment. The 5' cap and poly-A tail are not

Poly-A tail

translated into protein, nor are the regions called the 5' untranslated region (5' UTR) and 3' untranslated region (3' UTR}.

CHAPTER 17

From Gene to Protein

317

Split Genes and RNA Splicing

sequence at each end of an intron. Particles called small nuclear ribonucleoproteins, abbreviated snRNPs (pronounced "snurps"), recognize these splice sites. As the name implies, snRNPs are located in the cell nucleus and are composed of RNA and protein molecules. The RNA in a snRNP particle is called a small nuclear RNA (snRNA); each molecule is about 150 nucleotides long. Several different snRNPs join with additional proteins to form an even larger assembly called a spliceosome, which is almost as big as a ribosome. The spliceosome interacts with certain sites along an intron, releasing the intron and joining together the two exons that flanked the intron (Figure 17.11). There is strong evidence that snRNAs play a major role in these catalytic processes, as well as in spliceosome assembly and splice site recognition.

The most remarkable stage of RNA processing in the eukaryotic nucleus is the removal of a large portion of the RNA molecule that is initially synthesized—a cut-and-paste job called RNA splicing (Figure 17.10). The average length of a transcription unit along a eukaryotic DNA molecule is about 8,000 nucleotides, so the primary RNA transcript is also that long. But it takes only about 1,200 nucleotides to code for an average-sized protein of 400 amino acids. (Remember, each amino acid is encoded by a triplet of nucleotides.) This means that most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides, regions that are not translated. Even more surprising is that most of these noncoding sequences are interspersed between coding segments of the gene and thus between coding segments of the pre-mRNA. In other words, the sequence of DNA nucleotides that codes for a eukaryotic polypeptide is usually not continuous; it is split into segments. The noncoding segments of nucleic acid that lie between coding regions are called intervening sequences, or introns for short. The other regions are called exons, because they are eventually expressed, usually by being translated into amino acid sequences. (Exceptions include the UTRs of the exons at the ends of the RNA, which make up part of the mRNA but are not translated into protein. Because of these exceptions, you may find it helpful to think of exons as sequences of RNA that exit the nucleus.) The terms intron and exon are used for both RNA sequences and the DNA sequences that encode them.

Ribozymes The idea of a catalytic role for snRNA arose from the discovery of ribozymes, RNA molecules that function as enzymes. In some organisms, RNA splicing can occur without proteins or additional RNA molecules: The intron RNA functions as a ribozyme and catalyzes its own excision! For example, in the protozoan Tetrahymena, self-splicing occurs in the production of nbosomal RNA (rRNA), a component ot the organism's ribosomes. The pre-rRNA actually removes its own introns. The fact that RNA is single-stranded plays an important role in allowing certain RNA molecules to function as ribozymes. A region of an RNA molecule may base-pair with a complementary region elsewhere in the same molecule, thus imparting specific structure to the RNA molecule as a whole. Also, some of the bases contain functional groups that may participate in catalysis, just as the specific shape of an enzvmatic protein and the functional groups on its amino acid sice chains allow the protein to function as a catalyst, the structure of some RNA molecules allows them to function as catalysts, too. The discovery of ribozymes rendered obsolete the belief that all biological catalysts were proteins.

In making a primary transcript from a gene, RNA polymerase 11 transcribes both introns and exons from the DNA, but the mRNA molecule that enters the cytoplasm is an abridged version. The introns are cut out from the molecule and the exons joined together, forming an mRNA molecule with a continuous coding sequence. This is the process of RNA splicing. How is pre-mRNA splicing carried out? Researchers have learned that the signal for RNA splicing is a short nucleotide

5' Exon 1

fntron 30

Exon

31

Coding .egment

I Introns cut out anid I exons spliced togeether W

/I 5'UTR

A Figure 17.10 RNA processing: RNA splicing. The RNA molecule shown here codes for p-globin, one of the polypeptides of hemoglobin. The numbers under the RNA refer

318

U N I T THREE

G e n t ' l ic

to codons; p-globin is 146 amino acids long. The |3-globin gene and its pre-mRNA transcript have three exons, corresponding to sequences that will leave the nucleus as mRNA. (The 5' UTR and 3'

146\ 3' UTR UTR are parts of exons because they are included in the mRNA; however, they do not code for protein.) During RNA processing, the introns are cut out and the exons spliced together.

RNA transcript (pre-mRNA)

Transcription RNA processing Translation Spliceosome

Polypeptide A Figure 17.12 Correspondence between exons and p r o t e i n d o m a i n s . In a large number of genes, different exons encode separate domains of the protein product.

Exon 1

Exon 2

A Figure 17.11 The roles of snRNPs and spliceosomes in p r e - m R N A splicing. The diagram shows only a portion of the prerrRNA transcript; additional introns and exons lie downstream from the ones pictured here. © Small nuclear ribonucleoproteins (snRNPs) and other proteins form a molecular complex called a spliceosome on a pre-mRNA containing exons and introns. © Within the spliceosome, snRNA base-pairs with nucleotides at specific sites along the intron. €& The RNA transcript is cut, releasing the intron and at the same time splicing the exons together. The spliceosome then comes apart, releasing spliced mRNA, which now contains only exons.

The Functional and Evolutionary Importance of Introns What are the biological functions of introns and RNA splici lg? One idea is that introns play regulator}1" roles in the cell; at least some introns contain sequences that control gene activity in some way. And the splicing process itself is necessary for the passage of mRNA from the nucleus to the cytoplasm. One established benefit of the presence of exons and introns in genes is to enable a single gene to encode more than one kind of polypeptide. A number of genes are known to give rise to two or more different polypeptides, depending on which segments are treated as exons during RNA processing; this is called alternative RNA splicing (see Figure 19.8). For example, sex differences in fruit flies are largely due to differences in how males and females splice the RNA transcribed from certain genes. Early results from the Human Genome Project (discussed in Chapter 20) suggest that alternative "RNA splicing may be one reason humans can get along with a

relatively small number of genes—not quite twice as many as a fruit fly. Because of alternative splicing, the number of different protein products an organism can produce is much greater than its number of genes. Proteins often have a modular architecture consisting of discrete structural and functional regions called domains. One domain of an enzymatic protein, for instance, might include the active site, while another might attach the protein to a cellular membrane. In many cases, different exons code for the different domains of a protein (Figure 17.12). The presence of introns in a gene may facilitate the evolution of new and potentially useful proteins as a result of a process known as exon shuffling. Introns increase the probability of potentially beneficial crossing over between the exons of alleles—simply by providing more terrain for crossovers without interrupting coding sequences. We can also imagine the occasional mixing and matching of exons between completely different (nonallelic) genes. Exon shuffling of either sort could lead to new proteins with novel combinations of functions. While most of the shuffling would result in nonbeneficial changes, occasionally a beneficial variant might arise. Concept Check 1. How does alteration of the 5' and 3' ends of premRNA affect the mRNA that exits the nucleus? 2. Describe the role of snRNPs in RNA splicing. 3. How can alternative RNA splicing generate a greater number of polypeptide products than there are genes? For suggested answers, see Appendix A.

C H A P T E R 17

From Gene to Protein

319

Concept

Translation is the RNA-directed synthesis of a polypeptide: a closer look We will now examine in greater detail how genetic information flows from mRNA to protein—the process of translation. As we did for transcription, we'll concentrate on the basic steps of translation that occur in both prokaryotes and eukaryoles while pointing out key differences.

Molecular Components of Translation In the process of translation, a cell interprets a genetic message and builds a polypeptide accordingly. The message LS a series of codons along an mRNA molecule, and the interpreter is called transfer RNA (tRNA). The function of tRNA is to transfer amino acids from the cytoplasmic pool of amino acids to a ribosome. A cell keeps its cytoplasm stocked with all 20 amino acids, either by synthesizing them from other compounds or by taking them up from the surrounding solution. The ribosome adds each amino acid brought to it by tRNA to the growing end of a polypeptide chain (Figure 17.13). Molecules of tRNA are not all identical. The key to translating a genetic message into a specific amino acid sequence is that each type of tRNA molecule translates a particular mRNA codon into a particular amino acid. As a tRNA molecule arrives at a ribosome, it bears a specific amino acid at one end. At the other end of the tRNA is a nucleotide triplet called an anticodon, which base-pairs with a complementary codon on mRNA. For example, consider the mRNA codon UUU, which is translated as the amino acid phenylalanine. The tRNA that base-pairs with this codon by hydrogen bonding has AAA as its anticodon and carries phenylalanine at its other end (see the middle tRNA in Figure 17.13). As an mRNA molecule is moved through a ribosome, phenylalanine will be added to the polypeptide chain whenever the codon UUU is presented for translation. Codon by codon, the genetic message is translated as tRNAs deposit amino acids in the order prescribed, and the ribosome joins the ammo acids into a chain. The tRNA molecule is a translator because it can read a nucleic acid word (the mRNA codon) and interpret it as a protein word (the amino acid). Translation is simple in principle but complex in its biochemistry and mechanics, especially in the eukaryotic cell. In dissecting translation, we'll concentrate on the slightly less complicated version of the process that occurs in prokaryotes. Lets first look at some of the major players in this cellular drama, then see how they act together to make a polypeptide. 320

UNIT THREE

mRNA

& Figure 17.13 Translation: the basic concept. As a molecule of mRNA is moved through a ribosome, codons are translated into amino acids, one by one. The interpreters are tRNA molecules, each type with a specific anticodon at one end and a corresponding amino acid at the other end. A tRNA adds its amino acid cargo to a growing polypeptide chain when the anticodon bonds to a complementary codon on the mRNA. The figures that follow show some of the details of translation in the prokaryotic cell.

The Structure and Function of Transfer RNA like mRNA and other types of cellular RNA, transfer RNA molecules are transcribed from DNA templates. In a eukar> otic cell, tRNA, like mRNA, is made m the nucleus and must travel from the nucleus to the cytoplasm, where translation occurs. In both prokaryotic and eukaryotic cells, each tKNA molecule is used repeatedly, picking up its designated amino acid in the cytosol, depositing this cargo at the ribosome, and then leaving the ribosome to pick up another amino acid. As illustrated in Figure 17.14, a tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long (compared to hundreds of nucleotides for most mRNA mol • ecules). Because of the presence of complementary stretches of bases that can hydrogen-bond to each other, this single strand can fold back upon itself, forming a molecule with ;. three-dimensional structure. Flattened into one plane to reveal this base pairing, a tRNA molecule looks like a cloverlea" (Figure 17.14a). The tRNA actually twists and folds into i

^Aminoacyl-tRNA synthetase (enzyme)

Amino acid attachment site

Q Active site binds the amino acid and ATP.

Q ATP loses t w o (P) groups A and joins amino acid as AMP.

Anticodon a) Two-dimensional structure. The four base-paired regions and three loops are characteristic of all tRNAs, as is the base sequence of the amino acid attachment site at the 3' end. The anticodon triplet is unique to each tRNA type. (The asterisks mark bases that have been chemically modified, a characteristic of tRNA.)

TO

^ A f n i n o acid attachment site

^Hydrogen bonds

H 3, codon

(b) Three-dimensional structure

88i 5 ,

O Activated amino acid is released by the enzyme.

Aminoacyl tRNA j (an "activated amino acid")

A Figure 17.15 An aminoacyl-tRNA synthetase joins a specific a m i n o acid to a t R N A . Linkage of the tRNA and amino acid is an endergonic process that occurs at the expense of ATP The ATP loses t w o phosphate groups, becoming AMP (adenosine monophosphate).

Anticodon (c) Symbol used n this book

ik Figure 17.14 The structure of transfer RNA (tRNA). Anticodons are conventionally written 3' —* 5' to align properly with codorts written 5' —> 3' (see Figure 17.13). For base pairing, RNA strands must be antiparallel, like DNA. For example, anticodon :-: r -AAG-5' pairs with mRNA codon 5'-UUC-3'.

compact three-dimensional structure that is roughly L-shaped (Figure 17.14b). The loop protruding from one end of the L includes the anticodon, the special base triplet that binds to a specific mRNA codon. From the other end of the L-shaped i RNA molecule protrudes its 3' end, which is the attachment site for an amino acid. Thus, the structure of a tRNA molecule fits its function.

The accurate translation of a genetic message requires two recognition steps. First, there must be a correct match between a tRNA and an amino acid, A tRNA that binds to an mRNA codon specifying a particular amino acid must carry only that amino acid to the ribosome. Fach amino acid is joined to the correct tRNA by a specific enzyme called an aminoacyl-tRNA synthetase (Figure 17.15). The active site of each type of aminoacyl-tRNA synthetase fits only a specific combination of amino acid and tRNA. There ate 20 different synthetases, one for each amino acid; each synthetase is able to bind all the different tRNAs that code for its specific amino acid. The synthetase catalyzes the covalent attachment of the amino acid to its tRNA in a process driven by the hydrolysis of ATP. The resulting aminoacyl tRNA, also called an activated amino acid, is released from the enzyme CHAPTER 17

From Gene to Protein

321

and delivers its amino acid to a growing polypeptide chain on a ribosome. The second recognition step involves a correct match between the tRNA anticodon and an mRNA codon. If one tRNA variety existed for each of the mRNA codons that specifies an amino acid, there would be 61 tRNAs (see Figure 17.5). In fact, there are only about 45, signifying that some tRNAs must be able to bind to more than one codon. Such versatility is possible because the rules for base pairing between the third base of a codon and the corresponding base of a tRNA anticodon are not as strict as those for DNA and mRNA codons. For example, the base U at the 5' end of a tRNA anticodon can pair with either A or G in the third position (at the 3' end) of an mRNA codon. This relaxation of the base-pairing rules is called wobble. Wobble explains why the synonymous codons for a given amino acid can differ in their third base, but usually not in their other bases.

Ribosomes Ribosomes facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis. A ribosome, which is large enough to be seen with an electron microscope, is made up of two subunits, called the large and small subunits (Figure 17.16). The ribosomal subunits are constructed of proteins and RNA molecules named ribosomal RNA, or rRNA. In eukaryotes, the subunits are made in the nucleolus. Ribosomal RNA genes on the chromosomal DNA are transcribed, and the RNA is processed and assembled with proteins imported from the cytoplasm. The resulting ribosomal subunits are then exported via nuclear pores to the cytoplasm. In both prokaryotes and eukaryotes, large and small subunits join to form a functional ribosome only when they attach to an mRNA molecule. About two-thirds of the mass of a ribosome is rRNA. Because most cells contain thousands of ribosomes, rRNA is the most abundant type of RNA. Although the ribosomes of prokaryotes and eukaryotes are very similar in structure and function, those of eukaryotes are slightly larger and differ somewhat from prokaryotic ribosomes in their molecular composition. The differences are medically significant. Certain antibiotic drugs can inactivate prokaryotic ribosomes without inhibiting the ability of eukaryotic ribosomes to make proteins. These drugs, including tetracycline and streptomycin, are used to combat bacterial infections. The structure of a ribosome reflects its function of bringing mRNA together with amino acid-bearing tRNAs. In addition to a binding site for mRNA, each ribosome has three binding sites lor tRNA (see Figure 17.16). The P site (peptidyl-tRNA site) holds the tRNA carrying the growing polypeptide chain, while the A site (aminoacyl-tRNA site) holds the tRNA carrying the next amino acid to be added to the chain. Discharged tRNAs leave the ribosome from the E site (exit site). The ribosome holds the tRNA and mRNA in close proximity and 322

UNIT THREE

(a) Computer model of functioning ribosome. This is a model of a bacterial ribosome, showing its overall shape. The eukaryotic ribosome is roughly similar. A ribosomal subunit is an aggregate of ribosomal RNA molecules and proteins. P site (Peptidyl-tRNA v ^ j J J binding site) make many copies of a polypeptide very quickly.

Completing and Targeting the Functional Protein The process of translation is often not sufficient to make a functional protein. In this section, you will learn about modifications that polypeptide chains undergo after the translation process as well as some of the mechanisms used to target completed proteins to specific sites in the cell.

Stop codon (UAG, UAA, or UGA) H When a ribosome reaches a stop codon on mRNA, the A site of the ribosome accepts a protein called a release factor instead of tRNA.

^ The release factor hydroiyzes the bond between the tRNA in the P site and the last amino acid of the polypeptide chain. The polypeptide is thus freed from the ribosome.

The t w o ribosomal subunits and the other components of the assembly dissociate.

A Figure 17.19 The termination of translation.

Piotein Folding and Post-Translational Modifications During its synthesis, a polypeptide chain begins to coil and fold spontaneously, forming a functional protein of specific conformation: a three-dimensional molecule with secondary aid tertiary structure (see Figure 5.20). A gene determines primary structure, and primary structure in turn determines conformation. In many cases, a chaperone protein (chapercnin) helps the polypeptide fold correctly (see Figure 5.23). Additional steps—posl-translational modifications—may be required before the protein can begin doing its particular job in the cell. Certain amino acids may be chemically modified by the attachment of sugars, lipids, phosphate groups, or other additions. Enzymes may remove one or more amino acids from the leading (amino) end of the polypeptide chain. In some cases, a single polypeptide chain may be enzymatically cleaved into two or more pieces. For example, the protein insulin is first synthesized as a single polypeptide chain but becomes active only after an enzyme cuts out a central part of the chain, leaving a protein made up of two polypeptide chains connected by disulfide bridges. In other cases, two or more polypeptides that are synthesized separately may c ome together, becoming the subunits of a protein that, has quaternary structure.

ribosomes make proteins of the endomembrane system (the nuclear envelope, ER, Golgi apparatus, lysosomes; vacuoles, and plasma membrane) as well as proteins secreted from the cell, such as insulin. The ribosomes themselves are identical and can switch their status from free to bound.

Growing polypeptides

Completed polypeptide

End of mRNA (3' end)

^ ^ ^®

(a) An rnRNA molecule is generally translated simultaneously by several ribosomes in clusters called polyribosomes. • • * •

Targeting Polypeptides to Specific Locations In electron micrographs of eukaryotic cells active in protein synthesis, two populations of ribosomes (and polyribosomes) are evident: free and bound (see Figure 6.11). Free ribosomes ire suspended in the cytosol and mostly synthesize proteins .hat dissolve in the cylosol and function there. In contrast, bound ribosomes are attached to the cytosolic side of the endoplasmic reticulum (ER), or to the nuclear envelope. Bound

0.1 pm (b) This micrograph shows a large polyribosome in a prokaryotic cell (TEM).

A Figure 17.20 Polyribosomes. C H A P T E R 17

From Gene to Protein

325

What determines whether a ribosome will be free in the cytosol or bound to rough ER at any particular time? Polypeptide synthesis always begins in the cytosol, when a free ribosome starts to translate an mRNA molecule. There the process continues to completion—unless the growing polypeptide itself cues the ribosome to attach to the ER. The polypeptides of proteins destined for the endomembrane system or for secretion are marked by a signal peptide, which targets the protein to the ER (Figure 17.21). The signal peptide, a sequence of about 20 amino acids at or near the leading (amino) end of the polypeptide, is recognized as it emerges from the ribosome by a proteinRNA complex called a signal-recognition particle (SRP). This particle functions as an adapter that brings the ribosome to a receptor protein built into the ER membrane. This receptor is part of a multiprotein translocation complex. Polypeptide synthesis continues there, and the growing polypeptide snakes across the membrane into the ER lumen via a protein pore. The signal peptide is usually removed by an enzyme. The rest of the completed polypeptide, if it is to be a secretory protein, is released into solution within the ER lumen (as in Figure 17.21). Alternatively, if the polypeptide is to be a membrane protein, it remains partially embedded in the ER membrane.

Other kinds of signal peptides are used to target polypeptides to mitochondria, chloroplasts, the interior of the nucleus, and other organelles that are not part of the endomembrane system. The critical difference in these cases is that translation is completed in the cytosol before the polypeptide is imported into the organelle. The mechanisms of translocation also vary, but in all cases studied to date, the "zip codes" that address proteins for secretion or to cellular locations are signal peptides of some sort. Prokaryotes also employ signal sequences to target proteins for secretion.

O Polypeptide synthesis begins on a free ribosome in the cytosol.

© The SRP leaves, and the polypeptide resumes growing, meanwhile translocating across the membrane. (The signal peptide stays attached to the membrane.)

Q An SRP binds to the signal peptide, halting synthesis momentarily.

0 The SRP binds to a receptor protein in the ER membrane. This receptor is part of a protein complex (a translocation complex) that has a membrane pore and a signal-cleaving enzyme,

A Figure 17.21 The signal mechanism for targeting proteins to the ER. A polypeptide destined for the endomembrane system or for secretion from the cell begins

326

U N I T THREE

Genetics

Concept Check 1. Which two processes ensure that the correct amino acid is added to a growing polypeptide chain? 2. Describe how the formation of polyribosomes can benefit the cell. 3. Describe how a polypeptide to be secreted is transported to the endomembrane system. For suggested answers, see Appendix A.

with a signal peptide, a series of amino acids that targets it for the ER. This figure shows the synthesis of a secretory protein and its simultaneous import into the ER. In the ER

0 The signalcleaving enzyme cuts off the signal peptide.

0 The rest of the completed polypeptide leavcis the ribosome anc folds into its final conformation.

and then in the Golgi, the protein is further processed. Finally, a transport vesicle conveys it to the plasma membrane for release from the cell (see Figure 7.9).

Ctoncept

RNA plays multiple roles in the cell: a review As we have seen, the cellular machinery of protein synthesis (and ER targeting) is dominated by RNA of various kinds. In addition lo mRNA, these include tRNA, rRNA, and, in eukaryoles, snRNA and SRP RNA (Table 17.1). A type of RNA called small nudeolar RNA (snoRNA) aids in processing prerRNA transcripts in the nucleolus, a process necessary for ribosome formation. The diverse functions of these small RNA molecules range from structural to informational to catalytic. Recent research has also revealed the presence of small, singleard double-stranded RNA molecules that play unexpectedly important roles in regulating which genes get expressed. These types of RNA are called small interfering RNA (siRNA) and microRNA (miRNA) (see Chapter 19).

bonds between bases in different parts of its own polynucleotide chain (as seen in tRNA; see Figure 17.14). Third, it has functional groups that allow it to act as a cataLyst (ribozyme). These three properties make RNA quite multifunctional. DNA may be the genetic material of all living cells, but RNA is much more versatile. You will learn in Chapter 18 that many viruses use RNA rather than DNA as their genetic material. In the past few years, scientists have begun to appreciate the diverse functions carried out by RNA molecules, in fact, the journal Science bestowed its 2002 "Breakthrough oi the Year" award on the discover}' of the small regulatory RNA molecules siRNA and miRNA. Concept Check 1. Describe three properties of RNA that allow it to perform diverse roles in the cell. for suggested answers, see Appendix A.

The ability of RNA to perform so many different functions i s based on three properties. First, RNA can hydrogen-bond to 01 her nucleic acid molecules (DNA or RNA). Second, it can assume a specific three-dimensional shape by forming hydrogen

,.able 17.1 Types of RNA in a Eukaryotic Cell Type of RNA

Functions

Messenger RNA (mRNA)

Carries information specifying amino acid sequences of proteins from DNA to rihosomes.

Transfer RNA (tRNA)

Serves as adapter molecule in protein synthesis; translates mRNA cottons into amino acids.

Rjbosomal RNA (rRNA)

Flays caialylic (nbozyme) roles and structural roles in ribosomes.

Primary transcript

Serves as a precursor to mRNA, rRNA, or tRNA, before being processed by splicing or cleavageSome intron RNA acts as a ribozyr.it:, catalyzing us own splicing.

Small nuclear RNA (snRNA)

Plays structural and catalytic roles m spliceosornes, the complexes ol protein and RNA that splice pre-mRNA.

Small nucleolar RNA (snoRNA) Small interfering "RNA (siRNA) and microRNA (miRNA)

Is a component of the signalrecognition particle (SRP), i.he proiein-RNA complex thai recognizes the signal pepiides of polypeptides targeted lo the ER. Aids in processing of pre-rRNA transcripts for ribosome subunii formation in the nucleolus. An: in\ olvc-d in regulation ui gene expression.

Comparing gene expression in prokaryotes and eukaryotes reveals key differences Although prokaryotes and eukaryotes carry out transcription and translation in very similar ways, we have noted certain differences in cellular machinery and in details of the processes. Prokaryotic and eukaryotic RNA polymerases are different, and those of eukaryotes depend on a complex set of transcription, factors. Transcription is terminated differently in the two kinds of cells. Also, prokaryotic and eukaryotic ribosomes are slightly different. The most important dilferences, however, arise from the eukaryotic cell's compartmental organization. Like a one-room workshop, a prokaryotic cell ensures a streamlined operation. In the absence of a nucleus, it can simultaneously transcribe and translate the same gene (Figure 17.22, on the next page), and the newly made protein can quickly diffuse to its site of function. In contrast, the eukaryotic cell's nuclear envelope segregates transcription from translation and provides a compartment for extensive RNA processing. This processing stage provides additional steps whose regulation can help coordinate the eukaryotic cell's elaborate activities (see Chapter 19). Finally, eukaryotic cells have complicated mechanisms for targeti rig proteins to the appropriate cellular compartment (organelle). Where did eukaryotes and prokaryotes get the genes that encode the huge diversity of proteins they synthesize? For the past few billion years, the ultimate source of new genes has been the mutation of preexisting genes, the topic of the next section. CHAPTER 17 From Gene to Proiein

327

RNA polymerase' /

'"

- •

5 ' -"••

.

•

fect on the phenotype of an organism, the mutant condition is referred to as a genetic disorder, or hereditary disease. For example, we can trace the genetic basis of sickle-cell disease to a mutation of a single base pair in the gene that codes for one of the polypeptides of hemoglobin. The change of a single nucleotide in the DNA's template strand leads to the production of an abnormal protein (Figure 17-23, and see Figure 5.2 I). In individuals who are homozygous for the mutant allele, the sickling of red blood cells caused by the altered hemoglobin produces the multiple symptoms associated with sickle-cell disease (see Chapter 14). Let's see how different types of point mutations translate into altered proteins.

Types of Point Mutations Point mutations within a gene can be divided into two general categories: base-pair substitutions and base-pair insertions or deletions. While reading about how these mutations affect proteins, refer to Figures 17.24 and 17.25, on the next two pages. A Figure 17.22 Coupled transcription and translation in b a c t e r i a . In prokaryotic cells, the translation of rnRNA can begin as soon as the leading (5') end of the mRNA molecule peels away from the DNA template. The micrograph (TEM) shows a strand of f. co//DNA being transcribed by RNA polymerase molecules. Attached to each RNA polymerase molecule is a growing strand of mRNA, which is already being translated by ribosomes. The newly synthesized polypeptides are not visible in the micrograph but are shown in the diagram.

Concept Check 1. In Figure 17.22, number the RNA polymerases in order of their initiation of transcription. Then number each rnRNA's ribosomes in order of their initiation of translation. 2. Would the arrangement shown in Figure 17.22 be found in a eukaryotic cell? Explain. For suggested answers, see Appendix A.

Concept

Point mutations can affect protein structure and function Mutations are changes in the genetic material of a cell (or virus). In Figure 15.14, we considered large-scale mutations, chromosomal rearrangements that affect long segments of DNA. Now we can examine point mutations, chemical changes in just one base pair of a gene. II a point mutation occurs in a gamete or in a cell that gives rise to gamei.es, it may be transmitted to offspring and to a succession of future generations. If the mutation has an adverse ef328

UNIT

THREE

Genet:;:?

Substitutions A base-pair substitution is the replacement of one nucleotide and its partner with another pair of nucleotides. Some substitutions are called silent mutations because, owing to the redundancy of the genetic code, they have no effect on the encoded protein. In other words, a change in a base pair may transform one codon into another that is translated into the same amino acid. For example, if 3'-CCG-5' on the template strand mutated to 3'-CCA-5', the mRNA codon that used to be GGC would become GGU, and a glycine would still be inserted at the proper location in the protein (see Figure 17.5). Other substitutions may change an amino acid but have little effect on the protein. The new amino acid may have properties similar to those of the amino acid it replaces, or it may be in a region of the protein where the exact sequence of amino acids is not essential to the proteins function. However, the base-pair substitutions of greatest interest are those that cause a readily detectable change in a protein. The alteration of a single amino acid in a crucial area of a protein— in the active site of an enzyme, for example—will significantly alter protein activity Occasionally, such a mutation leads to an improved protein or one with novel capabilities, but much more often such mutations are detrimental, leading to a useless or less active protein that impairs cellular function. Substitution mutations are usually missense mutations; that is, the altered codon still codes for an amino acid and thus makes sense, although not necessarily the light sense. But a point mutation can also change a codon for an amino acid into a stop codon. This is called a nonsense mutation, and it causes translation to be terminated prematurely; the resulting polypeptide will be shorter than the polypeptide encoded by the normal gene (see Figure 17.24). Nearly all nonsense mutations lead to nonfunctional proteins.

• Figure 17.23 The molecular basis of si :kle-cell disease: a point mutation.

Wild-type hemoglobin DNA

The allele that causes sickle-cell disease differs from the wild-type (normal) allele by a single DNA base pair.

Mutant hemoglobin DNA

5

'mTFTWiil II I! | L J L J U L J L J L J

nnnBQfinnn

QMIMJ m mRNA

Normal hemoglobin

^

^

Sickle-cell hemoglobin

ZOOZI

In the DNA, the mutant template strand has an A where the wild-type template has a T.

The mutant mRNA has a U instead of an A in one codon.

The mutant (sicklecell) hemoglobin has a valine (Val) instead of a glutamic acid (Glu).

Insertions and Deletions

V ild type

mRNA 5' Piotein

\

Met

\\ Carboxyl end

i

Base-pair substitution No effect on amino acid sequence

i ss A nstead of G

IP

Hi ,f A |u| C Met

rA n , J

Ml

Ly

U

H Phe

LI|J3_G u

••

C -

Pi f* lu| |A| A Stop

Stop Ik Figure 17.24 Base-pair substitution. Mutations are changes n DNA, but they are represented here as they are reflected in mRNA and its protein product. Base-pair substitutions may lead to silent, nissense, or nonsense mutations.

Insertions and deletions are additions or losses of nucleotide pairs in a gene. These mutations have a disastrous effect on the resulting protein more often than substitutions do. Because mRNA is read as a series of nucleotide triplets during translation, the insertion or deletion of nucleotides may alter the reading frame (triplet grouping) of the genetic message. Such a mutation, called a frameshift mutation, will occur whenever the number of nucleotides inserted or deleted is not a multiple of three (see Figure 17.25, on the next page). All the nucieotides that are downstream of the deletion or insertion will be improperly grouped into codons, and the result will be extensive missense probably ending sooner or later in nonsense and premature termination. Unless the frameshift is very near the end of the gene, it will produce a protein that is almost certain to be nonfunctional.

Mutagens Mutations can arise in a number of ways. Errors during DNA replication, repair, or recombination can lead to base-pair substitutions, insertions, or deletions, as well as to mutations affecting longer stretches of DNA. Mutations resulting from such errors are called spontaneous mutations. It is difficult to calculate the rate at which such mutations occur. Rough estimates have been made of the rate of mutation during DNA replication for both E. coll and eukaryotes, and the numbers are similar: About 1 nucleotide in every 1010 is altered and passed on to the next generation oi cells. A number of physical and chemical agents, called mutagens, interact with DNA in ways that cause mutations. In the 1920s, Hermann Muller discovered that X-rays caused genetic changes in fruit flies. With X-rays, he was able to make Drosophila mutants that he could use in his genetic studies. But he also recognized an alarming implication of his discovery: X-rays and other forms of high-energy radiation pose hazards to the genetic material of people as well as laboratory CHAPTER 17 From Gene to Protein

329

Wild type

Concept Check

mRNA 5' Protein

J

Met Carboxyl end

Amino end

Base-pair insertion or deletion Frarneshift causing immediate nonsense Extra U

1. What happens when one nucleotide pair is lost from the middle of the coding sequence of a gene? 2. The template strand of a gene contains the sequence 3'-TACTTGTCCGATATC-5'. Draw the double strand of DNA and the resulting mRNA, labeling all 5' and 3' ends. Determine the arnino acid sequence. Then show the same alter a mutation changes the template DNA sequence to 3'-TACTTGTCCAATATC-5'. What is the effect on the amino acid sequence? For suggested answers, see Appendix A.

What is a gene? revisiting the question Frarneshift causing extensive missense

Insertion or deletion of 3 nudeotides: no frarneshift but extra or missing amino acid

J

Stop

A Figure 17.25 Base-pair insertion or deletion. Strictly speaking, the example at the bottom is not a point mutation because it involves insertion or deletion of more than one nucleotide.

organisms. Mutagenic radiation, a physical mutagen, includes ultraviolet (UV) light, which can cause disruptive thymine dimers in DNA (see Figure 1.6.17). Chemical muiagens fall into several categories. Base analogs are chemicals that are similar to normal DNA bases but that pair incorrectly during DNA replication. Some other chemical mutagens interfere with correct DNA replication by inserting themselves into the DNA and distorting the double helix. Stil] other mutagens cause chemical changes in bases that change their pairing properties. Researchers have developed various methods to test the mutagenic activity of different chemicals. A major application of these tests is the preliminary screening of chemicals to iden tify those that may cause cancer. This approach makes sense because most carcinogens (cancer-causing chemicals) are mutagenic and, conversely, most mutagens are carcinogenic. 330

UNIT THREE

Geriencs

Our definition of a gene has evolved over the past few chapters, as it has through the history of genetics. We began with the Mendelian concept of a gene as a discrete unit of inheritance that affects a phenotypic character (Chapter 14). We saw that Morgan and his colleagues assigned such genes to specific loci on chromosomes (Chapter 15). We went on to view a gene as i region of specific nucleotide sequence along the length of a DNA molecule (Chapter 16). Finally, in this chapter, we have considered a functional definition of a gene as a DNA sequence coding for a specific polypeptide chain. All these definitions are useful, depending on the context in which genes are being studied. (Figure 17.26 summarizes the path from gene to polypeptide in a eukaryotic cell) Even the one gene-one polypeptide model must be refined and applied selectively. Most eukaryotic genes contain noncoding segments (introns), so large portions of these genes have no corresponding segments in polypeptides. Molecular biologists also often include promoters and certain other regulatory regions ofDNA within the boundaries of a gene. These DNA sequences are not transcribed, but they can be considered part of the functional gene because they must be present for transcription to occur. Our molecular definition of a gene must also be broad enough to include the DNA that is transcribed into rRNA, tRNA, and other RNAs that are not translated. These genes have no polypeptide products. Thus, we arrive at the following definition: A gene is a region of DNA whose final product is either a polypepiidr or cm RNA molecule.

For most genes, however, it is still useful to retain the one gene-one polypeptide idea. In this chapter, you have learned in molecular terms how a typical gene is expressed—by transcription into RNA and then translation into a polypeptide that forms a protein of specific structure and function. Proteins, in turn, bring about an organism's observable phenotype. Genes are regulated. We will explore the regulation of gene expression in eukaryotes in Chapters 19 and 21. In the next chapter, we begin our discussion of gene regulation by focusing on the simpler molecular biology of bacteria and viruses.

A Figure 17.26 A summary of 'transcription and translation in a sukaryotic cell. This diagram shows the path "from one gene to one poiypeptide. Keep in mind that each gene in the DNA can be transcribed repeatedly into many RNA molecules, and that each mRNA can be

translated repeatedly to yield many poiypeptide molecules. (Also, remember that the final products of some genes are not polypeptides but RNA molecules, including tRNA and rRNA.) In general, the steps of transcription and translation are similar in prokaryotic and eukaryotic cells. The major difference is the

occurrence of RNA processing in the eukaryotic nucleus. Other significant differences are found in the initiation stages of both transcription and translation and in the termination of transcription.

CHAPTER 17

From Gene to Protein

331

Chapter Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY CONCEPTS Genes specify proteins via transcription and translation • Evidence from the Study of Metabolic Defects (pp. 309-311) DNA controls metabolism by directing cells to make specific enzymes and other proteins. Beadle and Tatum's experiments with mutant strains of Neurospom supported the one gene—one enzyme hypothesis. Genes code for polypeptide chains or for RNA molecules. Investigation How Is a Metabolic Pathway Analyzed? • Basic Principles of Transcription and Translation (pp. 311-312) Transcription is the nucleotide-to-nucleotide transfer of information from DNA to RNA, while translation is the informational transfer from nucleotide sequence in RNA to amino acid sequence in a polypeptide. Activity Overview of Protein Synthesis • The Genetic Code (pp. 312-314) Genetic information is encoded as a sequence of nonoverlapping base triplets, or codons. A codon in messenger RNA (mRNA) either is translated into an amino acid (61 codons) or serves as a translational stop signal (3 codons). Codons must be read in the correct reading frame for the specified polypeptide to be produced.

Transcription is the DNA-directed synthesis of RNA: a closer look • Molecular Components of Transcription (pp. 315-316) RNA synthesis is catalyzed by RNA polymerase. It follows the same base-pairing rules as DNA replication, except that in RNA, uracil substitutes for thymine. Activity Transcription • Synthesis of an RNA Transcript (pp. 316-317) The three stages of transcription are initiation, elongation, and termination. Promoters signal the initiation of RNA synthesis. Transcription factors help eukaryotic RNA polymerase recognize promoter sequences. The mechanisms of termination are different in prokaryotes and eukaryotes.

Eukaryotic cells modify RNA after transcription • Alteration of mRNA Ends (p. 317) Eukaryotic mRNA molecules are processed before leaving the nucleus by modification of their ends and by RNA splicing. The 5' end receives a modified nucleotide cap, and the 3' end a poly-A tail. Activity RNA Processing • Split Genes and RNA Splicing (pp. 318-319) Most eukaryotic genes have introns interspersed among the coding regions, the exons. In RNA splicing, introns are removed and exons joined. RNA splicing is carried out by spliceosomes, but

332

UNIT THREE

Geneiics

Review in some cases, RNA alone catalyzes splicing. Catalytic RNA molecules are called ribozymes. The presence of introns allows for alternative RNA splicing.

Translation is the RNA-directed synthesis of a polypeptide: a closer look • Molecular Components of Translation (pp. 320-323) A cell translates an mRNA message into protein with the help of transfer RNA (tRNA). After binding specific amino acids, tRN/1 molecules line up by means of their anticodons at complementary codons on mRNA. Ribosomes help facilitate this coupling with binding sites for mRNA and tRNA. • Building a Polypeptide {pp. 323-325) Ribosomes coordinate the three stages of translation: initiation, elongation, and termination. The formation of peptide bonds between amino acids is catalyzed by rRNA. A number of ribosomes can translate a single mRNA molecule simultaneously, forming a polyribosome. Activity Translation Biology tabs On-line TranslationLab • Completing and Targeting the Functional Protein (pp. 324-326) Alter translation, proteins may be modified in ways that affect their three-dimensional shape. Free ribosomes in the cytosol initiate the synthesis of all proteins, but proteins destined for the endomembrane system or for secretion must be transported into the ER. Such proteins have signal peptides to which a signal-recognition particle (SRP) binds, enabling the translating ribosome to bind to the ER.

RNA plays multiple roles in the cell; a review • RNA can hydrogen-bond to other nucleic acid molecules (DNA or RNA). it can assume a specific three-dimensional shape. And it has functional groups that allow it to act as a catalyst, a ribozyme (p. 327).

Comparing gene expression in prokaryotes and eukaryotes reveals key differences • Because prokaryotic cells lack a nuclear envelope, translation can begin while transcription is still in progress. In a eukaryotic cell, the nuclear envelope separates transcription from translation, and extensive RNA processing occurs in the nucleus (pp. 327-328).

Point mutations can affect protein structure and function • Types of Point Mutations (pp. 328-330) A point mutation is a change in one DNA base pair, which may lead to production of a nonfunctional protein or no protein at all. Base-pair substitutions can cause missense or nonsense mutations. Base-pair insertions or deletions may produce frameshift mutations. • Mutagens (pp. 329-330) Spontaneous mutations can occur during DNA replication, recombination, or repair Chemical and physical mutagens can also alter genes.

Science, Technology, and Society Evolution Connection The genetic code (see Figure 17.5) is rich with evolutionary implications. For instance, notice that the 20 amino acids are not randomly scattered; most amino acids are coded for by a similar set of codons. What evolutionary explanations can be given for this pattern? (Hint: There is one explanation relating to historical ancestry anc some less obvious ones of a "form-fiLs-function" type.)

Our civilization generates many potentially mutagenic chemicals (pesticides, for example) and modifies the environment in ways that increase exposure to other mutagens, notably UV radiation. What role should government play in identifying mutagens and regulating their release to the environment?

Scientific Inquiry A biologist inserts a gene from a human liver ceil into the chrome some of a bacterium. The bacterium then transcribes and translates this gene. The protein produced is useless and is found to contain many more amino acids than does the protein made by the eukaryotic cell. Explain why. Investigation How Is a Metabolic Pathway Analyzed? Biology Labs On-Line JranslationLab

CHAPTER 17 From Gene to Protein

333

A Figure 18.1 T4 bacteriophage infecting an E coli cell.

Key Concepts 18.1 A virus has a genome but can reproduce only within a host cell 18.2 Viruses, viroids, and prions are formidable pathogens in animals and plants 18.3 Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria 18.4 Individual bacteria respond to environmental change by regulating their gene expression

Microbial Model Systems

T

he photo in Figure 18.1 shows a remarkable event: the attack of a bacterial cell by numerous structures that resemble miniature lollipops. These structures, a type of virus called T4 bacteriophage, are seen infecting the bacterium Escherichia coli in this colorized SEM. By injecting its DNA into the cell, the virus sets in motion a genetic takeover of the bacterium. Molecular biology was born in the laboratories of microbiologists studying viruses and bacteria. Microbes such as E coli and its viruses are called model systems because of their frequent use by researchers in studies that reveal broad biological principles. Experiments with viruses and bacteria provided most of the evidence that genes are made of DNA, and they were critical in working out the molecular mechanisms of the fundamental processes of DNA replication, transcription, and translation. Beyond their value as model systems, viruses and bacteria have unique genetic mechanisms that are interesting in their own right. These specialized mechanisms have important applications for understanding how viruses and bacteria cause

334

disease. In addition, techniques enabling scientists to manipulate genes and transfer them from one organism to another have emerged from the study of microbes. These techniques are having an important impact on both basic research and biotechnology (see Chapter 20). In this chapter, we explore the genetics of viruses ar d bacteria. Recall that bacteria are prokaryotes, with cells much smaller and more simply organized than those of eukaryotes, such as plants and animals. Viruses are smaller and simpler still (Figure 18.2). Lacking the structures and metabolic machinery found in cells, most viruses are little more than genes packaged in protein coats. We will begin with the structure of these simplest of all genetic systems and their role as disease-causing agents, or pathogens. Then we will discuss the genetics of bacteria and regulation of their gene expression.

Concept

A virus has a genome but can reproduce only within a host cell Scientists were able to detect viruses indirectly long before they were actually able to see them. The story of how viruses were discovered begins near the end of the 19th century

The Discovery of Viruses: Scientific Inquiry Tobacco mosaic disease stunts the growth of tobacco plants and gives their leaves a mottled, or mosaic, coloration (Figure 18.3). In 1883, Adolf Mayer, a German scientist, discovered that he could transmit the disease from plant to plant by Ribbing sap extracted from diseased leaves onto healthy plants. Aftei

• Figure 18.3 Infection by tobacco mosaic virus (TMV). A healthy, uninfected tobacco leaf (left) compared with a leaf experimentally infected with TMV (right).

tobacco mosaic virus (TMV). Subsequently TMV and many other viruses were actually seen with the help of the electron microscope. Animal cell nucleus—C.25 urn A Figure 18.2 Comparing the size of a virus, a bacterium, and an animal celt. Only a portion of a typical animal cell is shown. Its diameter is about ten times greater than the length of B. coli.

an unsuccessful search for an infectious microhe in the sap, Mayer concluded that the disease was caused by unusually small bacteria that could not be seen with the microscope. This hypothesis was tested a decade later by Dimitri Ivanowsky, a Russian who passed sap from infected tobacco leaves through a filter designed to remove bacteria. After filtering, the sap still produced mosaic disease. Ivanowsky clung to the hypothesis that bacteria caused tobacco mosaic disease. Perhaps, he reasoned, the bacteria were so small that they passed through the filter or made a filterable toxin that caused the disease. This latter possibility was ruled oat when the Dutch botanist Martinus Beijerinck discovered that the infectious agent in the filtered sap could reproduce, tie rubbed plants with filtered sap, and after these plants developed mosaic disease, he used their sap to infect more plants, continuing this process through a series of infections. The pathogen must have been reproducing, for its ability to cause disease was undiluted after several transfers from plant to plant. In fact, the pathogen reproduced only within the host it infected. Unlike bacteria, the mysterious agent of mosaic disease could not be cultivated on nutrient media in test tubes or petri dishes. Beijerinck imagined a reproducing particle much smaller and simpler than bacteria. His suspicions were confirmed in 1935 when the American scientist Wendell Stanley crystallized the infectious particle, now known as

Structure of Viruses The tiniest viruses are only 20 nm in diameter—smaller than a ribosome. Millions could easily fit on a pinhead. Even the largest viruses are barely visible in the light microscope. Stanley's discovery that some viruses could be crystallized was exciting and puzzling news. Not even the simplest of cells can aggregate into regular crystals. But if viruses are not cells, then what are they? They are infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope. Let's examine the structure of viruses more closely and then how they reproduce. Viral Genomes We usually think of genes as being made of double-stranded DNA—the conventional double helix—but many viruses defy this convention. Their genomes may consist of doublestranded DNA, single-stranded DNA, double-stranded RNA., or single-stranded RNA, depending on the kind of virus. A virus is called a DNA virus or an RNA virus, according to the kind of nucleic acid that makes up its genome, in either case, the genome is usually organized as a single linear or circular molecule of nucleic acid. The smallest viruses have only four genes, while the largest have several hundred. Capsitk and Envelopes The protein shell enclosing the viral genome is called a capsid. Depending on the type of virus, the capsid maybe rod-shaped, polyhedral, or more complex in shape (like T4). Capsids are built from a large number ol protein subunits called capsomeres, but the number of different kinds of proteins is usually small. Tobacco mosaic virus has a rigid, rod-shaped capsid CHAPTER is

The Genetics of Viruses and Bacteria

335

Membranous Capsomere

Glycoprotein 70-90 nirf (diameter)

20 nm (a) Tobacco mosaic virus has a helical capsid with the overall shape of a rigid rod.

50 nm (b) Adenoviruses have a polyhedral capsid with a glycoprotein spike at each vertex.

50 nm (c) Influenza viruses have an outer envelope studded with glycoprotein spikes. The genome consists of eight different RNA molecules, each wrapped in a helical capsid.

50 nm (d) Bacteriophage T4, like other "T-even" phages, has a complex capsid consisting of a polyhedral head and a tail apparatus.

A Figure 18.4 Viral structure. Viruses are made up of nucleic acid (DNA or RNA) enclosed in a protein coat (the capsid) and sometimes further wrapped in a membranous envelope. The individual protein subunits making up the capsid are called capsomeres. Although diverse in size and shape, viruses have common structural features, most of which appear in the four examples shown here. (All the micrographs are colorized TEMs.)

made from over a thousand molecules of a single type of protein arranged in a helix (Figure 18.4a). Adenoviruses, which infect the respiratory tracts of animals, have 252 identical protein molecules arranged in a polyhedral capsid with 20 triangular tacets—an icosahedron (Figure 18.4b). Some viruses have accessory structures that help them infect their hosts. For instance, a membranous envelope surrounds the capsids of influenza viruses and many other viruses found in animals (Figure 18.4c). These viral envelopes, which are derived from the membrane of the host cell, contain host cell phospholipids and membrane proteins. They also contain proteins and glycoproteins of viral origin (glycoproteins are proteins with carbohydrate covalently attached). Some viruses carry a few viral enzyme molecules within their capsids. The most complex capsids are found among viruses that infect bacteria, called bacteriophages, or simply phages. The first phages studied included seven that infect E. coli. These 336

U N I T THREE

Genetics

seven phages were named type 1 (Tl), type 2 (T2), and so forth, in the order of their discovery The three T-even phages (T2, T4, and T6) turned out to be very similar in structure. Their capsids have elongated icosahedral heads enclosing their DNA. Attached to the head is a protein tail piece with fibers that the phages use to attach to a bacterium (Figure 18.4d).

General Features of Viral Reproductive Cycles Viruses are obligate mtracellular parasites; They can reproduce only within a host cell. An isolated virus is unable to reproduce or do anything else except infect an appropriate hos: cell Viruses lack metabolic enzymes, iibosom.es, and othe" equipment for making proteins. Thus, isolated viruses are merely packaged sets of genes in transit from one host cell to another.

Each type of virus can infect only a limited range of host cells, called its host range. This host specificity results from the evolution of recognition systems by the virus. Viruses identify their host cells by a "lock-and-key" fit between protein 5 on the outside of the virus and specific receptor molecules on • he surface of cells. (Presumably, the receptors first evolved because they carried out functions of benefit to the organism.) Sorie viruses have broad host ranges. West Nile virus, for exam pie, can infect mosquitoes, birds, and humans, and equine encephalitis virus can infect mosquitoes, birds, horses, arid humans. Other viruses have host ranges so narrow that they infect only a single species. Measles virus and poliovirus, for rns:ance, can infect only humans. Furthermore, infection by viruses of multicellular eukaryotes is usually limited to particular tissues. Human cold viruses infect only the cells lining the upper respiratory tract, and the AIDS virus binds to specific receptors on certain types of white blood cells. A viral infection begins when the genome of a virus makes its way into a host cell (Figure 18.5). The mechanism by which this nucleic acid enters the cell varies, depending on the type of virus and the type oi host cell. For example, the T-even phages use their elaborate tail apparatus to inject DNA into a bacterium (see Figure 18.4d). Once inside, the viral genome can commandeer its host, reprogramming the cell to copy the viral nucleic acid and manufacture viral proteins. The host provides the nucleotides for making viral nucleic acids: as well as enzymes, ribosomes, tRNAs, arnino acids, ATP, and other components needed for making the viral prote ns dictated by viral genes. Most DNA viruses use the DNA polymerases of the host cell to synthesize new genomes along the templates provided by the viral DNA. In contrast, to replicate their genomes, RNA viruses use special virus-encoded polymerases that can use RNA as a template. (Uninfected cells generally make no enzymes for carrying out this latter process.) After the viral nucleic acid molecules and capsomeres are produced, their assembly into new viruses is often a spontaneous process of self-assembly, in fact, the RNA and capsomeres of TMV can be separated in the laboratory and then reassembled to form complete viruses simply by mixing the components together under the right conditions. The simplest type of viral reproductive cycle ends with the exit of hundreds o * thousands of viruses from the infected host cell, a process that often damages or destroys the cell. Such cellular damage and death, as well as the body's responses to this destruction, cause some of the symptoms associated with viral infections. The viral progeny that exit a cell have the potential To infect additional cells, spreading the viral infection, There are many variations on the simplified viral reproductive cycle we have traced in this overview. We will now take a closer look at some of these variations m bacterial viruses (phages) and animal viruses; later in the chapter, we will consider plant viruses,

Q Virus enters cell and is uncoated, releasing viral DNA and capsid proteins. Q Meanwhile, host enzymes transcribe the viral genome into viral mRNA, which other host enzymes use to make more viral proteins.

9» 9 . Viral Diseases in Animals (pp. 343-344) Symptoms may be caused by direct viral harm to cells or by the body's immune response. Vaccines stimulate the immune system to defend the host against specific viruses. *• Emerging Viruses (p. 344) Outbreaks of "new" viral diseases in humans are usually caused by existing viruses that expand their host territory. Investigation What Causes Infections in AIDS Patients? Investigation Why Do AIDS Rates Differ Across the U.S.? • Viral Diseases in Plants (p. 345) Viruses enter plant cells through damaged cell walls (horizontal transmission) or are inherited from a parent (vertical transmission).

Review Viroids and Prions: The Simplest Infectious Agents (pp. 345-346) Viroids are naked RNA molecules that infect plants and disrupt their growth. Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals.

Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria • The Bacterial Genome and Its Replication (p. 346) The bacterial chromosome is usually a circular DNA molecule with few associated proteins. Plasmids are smaller circular DNA molecules that can replicate independently of the chromosome. • Mutation and Genetic Recombination as Sources of Genetic Variation (pp. 346-348) Because bacteria can proliferate rapidly, new mutations can quickly increase a population's genetic variation. Further diversity can arise by recombination of the DNA from two different bacterial cells. • Mechanisms of Gene Transfer and Genetic Recombination in Bacteria (pp. 348-351) New bacterial strains can arise by the transfer of DNA from one cell to another cell. In transformation, naked DNA enters the cell from the surroundings. In transduction, bacterial DNA is carried from one cell to another by phages. In conjugation, an F donor cell, which contains the F plasmid, transfers plasmid DNA to an F~ recipient cell. The F factor of an Hfr cell which is integrated into the bacterial chromosome, brings some chromosomal DNA along with it when it is transferred to an F~" cell R plasmids confer resistance to various antibiotics. investigation What Are the Patterns of Antibiotic Resistance? • Transposition of Genetic Elements (pp. 351-352) DNA segments that can insert at multiple sites in a cell's DNA contribute to genetic shuffling in bacteria. Insertion sequences, the simplest bacterial transposable elements, consist of inverted repeats of DNA flanking a gene for transposase. Bacterial transposons have additional genes, such as those for antibiotic resistance.

Individual bacteria respond to environmental change by regulating their gene expression • Operons: The Basic Concept (pp. 353-354) Cells control metabolism by regulating enzyme activity or the expression of genes coding for enzymes. In bacteria, genes are often clustered into operons, with one promoter serving several adjacent genes. An operator site on the DNA switches the operon on or off. • Repressible and Inducible Operons: Two Types of Negative Gene Regulation (pp. 354-356) In a repressible operon, binding of a specific repressor protein to the operator shuts off transcription. The repressor is active when bound to a corepressor, usually the end product of an anabolic pathway In an inducible operon, binding of an inducer to an innately active repressor inactivates the repressor and turns on transcription. Inducible enzymes usually function in catabolic pathways. •• Positive Gene Regulation (p. 356) Some operons are also subject to positive control via a stimulatory activator protein, such as catabolite activator protein (CAP), which promotes transcription when bound to a site within the promoter. Activity The lac Operon in E. coli CHAPTER 18

The Genetics of Viruses and Bacteria

357

TESTING YOUR KNOWLEDGE Evolution Connection The success of some viruses lies in their ability to evolve within the host. Such a virus evades the host's defenses by rapidly mutating and producing many altered progeny viruses before the body can mount an attack. Thus, the viruses present late in infection differ from those that initially infected the body. Discuss this as an example of evolution in microcosm. "Which viral lineages tend to survive?

Scientific Inquiry When bacteria infect an animal, the number of bacteria in the body increases in an exponential fashion (graph A). After infection by a virulent animal virus with a lytic reproductive cycle, there is no evidence of infection for a while. Then, the number of viruses rises suddenly and subsequently increases in a series of steps (graph B). Explain the difference in the growth curves.

Time

Time •

Biological Inquiry: A Workbook of Investigative Cases Explore West Nile virus in the case "The Donor's Dilemma." Investigation What Causes Infections in AIDS Patients? Investigation Why Do AIDS Rates Differ Across the U.S.? Investigation What Are the Patterns of Antibiotic Resistance?

Science, Technology, and Society Explain how the excessive or inappropriate use of antibiotics poses a health hazard for a human population.

358

UNIT THREE

tifnetlCS

Aikaryotic Genomes Organization, Regulation, and Evolution A Figure 19.1 DNA in a eukaryotic chromosome from a developing salamander egg.

1 19.1 Chromatin structure is based on successive levels of DNA packing 19.2 Gene expression can be regulated at any stage, but the key step is transcription 1 9 3 Cancer results from genetic changes that affect cell cycle control 19.4 Eukaryotic genomes can have many noncoding DNA sequences in addition to genes 19.5 Duplications, rearrangements, and mutations of DNA contribute to genome evolution

How Eukaryotic Genomes Work and Evolve

E

ukaryotic cells face the same challenges as prokaryotic cells in expressing their genes. However, the typical eukaryotic genome is much larger, and, in multicellular eukaryotes, cell specialization is crucial. These two features present a formidable information-processing task for the eukaryotic cell. The human genome, for instance, has an estimated 25,000 genes—more than five times that of a typical prokaryote. It also includes an enormous amount of DNA that does not code far RNA or protein. Managing so much DNA requires that the genome be elaborately organized. In all organisms, DNA associates with proteins that condense it. In eukaryotes, the DNA-protein complex, called chromatin, is ordered into higher structural levels than the DNA-protein complex in prokaryoies. The fluorescence micrograph in Figure 19.1

gives a sense of the complex organization of the chromaiin in a eukaryotic chromosome. Part of the chromaiin is packed into the main axis (white) of the chromosome, while those parts that are being actively transcribed are spread out in loops (red). Both prokaryotes and eukaryotes must alter their patterns of gene expression in response to changes in environmental conditions. Multicellular eukaryotes, in addition, must develop and maintain multiple cell types. Each of these cell types contains the same genome but expresses a different subset of genes, a significant challenge in gene regulation. in this chapter, you will first learn about the structure of chromatin and how changes in chromatin structure affect gene expression. Then we will discuss other mechanisms for regulating expression of eukaryotic genes. As in prokaryotes, eukaryotic gene expression is most often regulated at the stage of transcription. Next, we will describe how disruptions in gene regulation can lead lo cancer. In the remainder of the chapter, we will consider the various types of nucleotide sequences m eukaryotic genomes and explain how they arose and changed during genome evolution. Being aware of the forces that have shaped—-and continue to shape—genomes will help you understand how biological diversity has evolved.

Concept

Chromatin structure is based on successive levels of DNA packing Eukaryotic DNA is precisely combined with a large amount of protein, and the resulting chromatin undergoes striking changes in the course of the cell cycle (see Figure 12.6). In interphase cells stained for light microscopy, the chromatin 359

usually appears as a diffuse mass within the nucleus, suggesting that the chromatin is highly extended. As a cell prepares for mitosis, its chromatin coils and folds up (condenses), eventually forming a characteristic number of short, thick chromosomes that are distinguishable from each other with the light microscope. Eukaryotic chromosomes contain an enormous amount of DNA relative to their condensed length. Each chromosome contains a single linear DNA double helix that, in humans, averages about 1.5 x 108 nucleotide pairs. If completely stretched out, such a DNA molecule would be about 4 cm long, thousands of times longer than the diameter of a cell nucleus. All this DNA—as well as the DNA of the other 45 human chromosomes—fits into the nucleus through the elaborate, multilevel system of DNA packing outlined in Figure 19.2

Nucleosomes, or "Beads on a String" Proteins called histories are responsible for the first level of DNA packing in chromatin. The mass of histone in chromatin is approximately equal to the mass of DNA- Histones have a high proportion of positively charged amino acids (lysine and arginine), and they bind tightly to the negatively charged DNA. (Recall that the phosphate groups of DNA give it a negative charge all along its length; see Figure 16.7.) Histones are very similar from one eukaryote to another, and similar proteins are found even in prokaryotes. The apparent conservation of histone genes during evolution probably reflects the pivotal role of histones in organizing DNA within cells. Tn electron micrographs, unfolded chromatin has the appearance of heads on a string, as shown in Figure 19.2a. In this configuration, a chromatin fiber is 10 nm in diameter (the lO-nm fiber). Each "bead" is a nucleosome, the basic unit of DNA packing; the "string" between the beads is called linker DNA- A nucleosome consists of DNA wound around a protein core composed of two molecules each of four types of histone; H2A, H2B, H.3, and H4. The amino end (N-terminus) of each histone protein (the histone tail) extends outward from the nucleosome. A molecule of a fifth histone, called HI, attaches to the DNA near the nucleosome when a 10-nm chromatin fiber undergoes the next level of packing. The association of DNA and histones in nucleosomes seems to remain essentially intact throughout the cell cycle. The histones leave the DNA only transiently during DNA replication, and, with very few exceptions, they stay with the DNA during transcription. How can DNA be transcribed when it is wrapped around histones in a nucleosome? Researchers have learned that changes in the shapes and positions of nucleosomes can allow RNA-synthesizing polymerases lo move along the DNA. Later in this chapter, well discuss some recent discoveries about the roles of histone tails and nucleosomes in the regulation of gene expression. 360

UNIT THREE

Higher Levels of DNA Packing The next level of packing is due LO interactions between the histone tails oi one nucleosome and the linker DNA and nucleosomes to either side. With the aid of histone HI, these interactions cause the extended 10-nm fiber to coil or fold, forming a chromatin fiber roughly 30 nm in thickness, the 30-rm fiber (Figure 19.2b). The 30-nm fiber, in turn, forms loops called looped domains attached to a chromosome scaffold made of nonhistone proteins, thus making up a 300-nm fiber (Figure 19.2c). In a mitotie chromosome, the looped domains themselves coil and fold, further compacting all the chromatin to produce the characteristic metaphase chromosome shown in the micrograph at the bottom of Figure 19.2d Particular genes always end up located at the same places in metaphase chromosomes, indicating that the packing steps are highly specific and precise. Though interphase chromatin is generally much less condensed than the chromatin of mitotie chromosomes, it shows several of the same levels ot higher-order packing. Much of the chromatin comprising a chromosome is present as a 10-nm fiber, but some is compacted into a 30-nm fiber, which in some regions is further folded into looped domains. Although an interphase chromosome lacks an obvious scaffold, its looped domains seem to be attached to the nuclear lamina, on the inside of the nuclear envelope, and perhaps also to fibers of the nuclear matrix. These attachments may help organize regions of active transcription. The chromatin of each chromosome occupies a specific restricted area within the interphase nucleus, and the chromatin fibers of different chromosomes do not become entangled. Even during interphase, the centromeres and telomeres cf chromosomes, as well as other chromosomal regions in som> cells, exist in the highly condensed state represented in Figure 19.2d. This type of interphase chromatin, visible as irregular clumps with a light microscope, is called heterochromatin, to distinguish it Irom the less compacted euchromatin ("true chromatin"). Because of its compaction, heterochromatin DNA is largely inaccessible to transcription enzymes and thus generally is not transcribed. In contrast, the looser packing oi euchromatin makes its DNA accessible to enzymes and available for transcription. In the next section, we will consider how changes in chromatin and other mechanisms allow a eel to regulate which of its genes are expressed. Concept Check 1. Describe the structure of a nucleosome, the basic unit of DNA packing in eukaryotic cells. 2. What chemical properties of histones and DNA enable these molecules to bind tightly together? 3. In general, how does dense packing of DNA in chromosomes prevent gene expression? For suggested answers, see Appendix A.

•

,

. , .

. - • • •

•

- ,

-

. . .

.

.-

(a) Nucleosomes (10-nm fiber). DNA and histone molecules form "beads on a string," the extended chromatin fiber seen during interphase. A nudeosome has eight histone molecules with the amino end (tail) of each projecting outward. A different type of histone, H I , can bind to DNA next to a nudeosome, where it helps to further compact the 10-nm fiber.

(b) 30-nm fiber. The string of nucleosomes coiis to form a chromatin fiber that is 30 nrn in diameter (tails not shown). This form is also seen during interphase.

(c) Looped domains (300-nm fiber). During prophase, further folding of the 30-nm fiber into looped domains forms a 300-nm fiber. The loops are attached to a scaffold of nonhistone proteins.

1,400 nm

(d) Metaphase chromosome. The chromatin folds further, resulting in the maximally compacted chromosome seen at metaphase. Each metaphase chromosome consists of t w o chromatids.

A Figure 19.2 Levels of c h r o m a t i n p a c k i n g . This series of diagrams and transmission electron micrographs depicts a current model for the progressive stages of DNA coiling and folding.

CHAPTER 19

Eukaryotic Genomes: Organization, Regulation, and Hvolution

361

Signal

Concept

Gene expression can be regulated at any stage, but the key step is transcription

NUCLEUS

All organisms must regulate which genes are expressed at any given time. Both unicellular organisms and the cells of multicellular organisms must continually turn genes on and off in response to signals from their external and internal environments. The cells of a multicellular organism must also regulate their gene expression on a more long-term basis. During development of a multicellular organism, its cells undergo a process of specialization in form and function called cell differentiation, resulting in several or many differentiated cell types. The mature human body, for instance, is composed of about 200 different cell types. Examples are muscle cells and nerve cells.

Differential Gene Expression A typical human cell probably expresses about 20% of its genes at any given time. Highly diflerenliated cells, such as muscle cells, express an even smaller fraction of their genes. Although almost all the cells in an organism contain an identical genome,* the subset of genes expressed in the cells of each type is unique, allowing these cells to carry out their specific function. The differences between cell types, therefore, are due not to different genes being present, but to differential gene expression, the expression of different genes by cells with the same genome. The genomes of eukaryot.es may contain tens of thousands of genes, but for quite a few species, only a small amount of the DNA—about 1.5% in humans—codes for protein. Of the remaining DNA, a very small fraction consists of genes for RNA products such as ribosomal RNA and transfer RNA. Most of the rest of the DNA seems to be noncoding, although recently researchers have learned that a significant amount of it may be transcribed inLo RNAs of unknown function. In any case, the transcription proteins of a cell must locate the right genes at the right time, a task on a par with finding a needle in a haystack. When gene expression goes awry, serious imbalances and diseases, including cancer, can arise. Figure 19.3 summarizes the entire process of gene expression in a eukaryotic cell, highlighting key stages in the expression of a protein-coding gene. Each stage depicted in Figure 19.3 is a potential control point at which gene expression can be turned on or off, accelerated, or slowed down.

*Cells of the immune system are an exception. During their differentia do rearrangement of the immunoglobulin genes results in a change in the genome, which will be discussed in Chapter 43.

362

UNIT THREE

Genetics

CYTOPLASM mRNA in cytoplasm

A Figure 19.3 Stages in gene expression that can be regulated in eukaryotic cells. In this diagram, the colored boxes indicate the processes most often regulated. The nuclear envelope separating transcription from translation in eukaryotic cells offers an opportunity for post-transcriptional control in the form of RNA processing that is absent in prokaryotes. In addition, eukaryotes have a greater variety of control mechanisms operating before transcription and after translation. The expression of any given gene, however, does not necessarily involve every stage shown; for example, not every polypeptide is cleaved. As in prokaryotes, transcription initiation is the most important control point.

Only 40 years ago, an understanding of the mechanisms thai control gene expression in eukaryotes seemed almost hopelessly out of reach. Since then, new research methods, including advances in DNA technology (see Chapter 20), have empowered molecular biologists to uncover many of the details of eukaryotic gene regulation. In all organisms, the expression of specific genes is most commonly regulated at transcription, often in response to signals coming from outside the cell For that reason, the term gene expression is often equated with transcription for both prokaryotes and eukaryotes. However, the greater complexity of eukaryotic cell structure and function provides opportunities for regulating gene expression at additional stages. In the following three sections, we'll examine some of the important control points of eukaryotic gene expression more closely.

acetylated region. Researchers have shown that some enzymes that acetylate or deacetylate histones are closely associated with or even components of the transcription factors that bind to promoters (see Figure 17.8). In other words, histone acetylation enzymes may promote the initiation of transcription not only by modifying chromatin structure, but also by binding to, and thus "recruiting," components of the transcription machinery. Several other chemical groups can be reversibly attached to amino acids in histone tails. For example, the addition of methyl groups (—CH3) to histone tails (methylation) can lead

Regulation of Chromatin Structure As mentioned earlier, the structural organization of cbrom itin not only packs a cells DNA into a compact form that fits inside the nucleus but also is important in helping regulate gene expression. Genes within heterocbromatin, which is highly condensed, are usually not expressed. The repressive effect of heterochromatin has been seen in experiments in which a transcriptionally active gene was inserted into a region of heterochromatin in yeast cells; the inserted gene was no longer expressed. In addition, the location of a gene's pron oter relative to nucleosomes and to the sites where the DNA attaches to the chromosome scaffold or nuclear lamina can also affect whether it is transcribed. A flurry of recent research indicates that certain chemical modifications to the histones and DNA of chromatin influence both chromatin structure and gene expression. Here we examine the effects of these modifications, which are catalyzed by specific enzymes.

(a) Histone tails protrude outward from a nucleosome. In this end view of a nucleosome, each type of histone is shown in a different color. The amino acids in the N-terminus tails are accessible for chemical modification.

Histone Modifications There is mounting evidence that chemical modifications to histones play a direct role in the regulation of gene transcription. The N-terminus of each histone molecule in a nucleosome protrudes outward from the nucleosome (Figure 19.4a). These histone tails are accessible to various modifying enzymes, which catalyze the addition or removal of specific chemical groups. In histone acetylation, acetyl groups (—COCH3) are attached to positively charged lysines in histone tails; deacetylaLion is the removal of acetyl groups. When the histone tails of a nucleosome are acetylated, their positive charges are neutralized and they no longer bind to neighboring nucleosomes (Figure 19.4b). Recall that such binding promotes the folding of chromatin into a more compact structure; when this binding does not occur, chromatin has a looser structure. As a result, transcription proteins have easier access to genes in an CHAPTER 19

Unacetylated histones

Acetylated histones

(b) Acetylation of histone tails promotes loose chromatin structure that permits transcription. A region of chromatin in which nucleosomes are unacetylated forms a compact structure (left) in which the DNA is not transcribed. When nucleosomes are highly acetylated (right), the chromatin becomes less compact, and the DNA is accessible for transcription.

.4 Figure 19.4 A simple model of histone tails and the e f f e c t of histone a c e t y l a t i o n . In addition to acetylation, histones can undergo several other types of modifications that also help determine the chromatin configuration in a region.

Eukaryotic Genomes: Organization, Regulation, and Evolution

363

to condensation of the chromatin. The recent discovery that this and many other modifications to histone tails can affect chromatin structure and gene expression has led to the histone code hypothesis. According to this model, specific combinations of modifications, rather than the overall level of histone acetylation, help determine the chromatin configuration, which in turn influences transcription. DNA Meihylation Distinct from methylation of histone tails is the addition of methyl groups to certain bases in DNA after DNA is synthesized. In fact, the DNA of most plants and animals has methylated bases, usually cytosine. Inactive DNA, such as that of inactivated mammalian X chromosomes (see Figure 15.11), is generally highly methylated compared with DNA that is actively transcribed, although there are exceptions. Comparison of the same genes in different tissues shows that the genes are usually more heavily methylated in cells in which they are not expressed. Removal of the extra methyl groups can turn on certain of these genes. Moreover, researchers have discovered that certain proteins that bind to methylated DNA recruit histone deacetylation enzymes. Thus, a dual mechanism, involving both DNA methylation and histone deacetylation, can repress transcription. At least in some species, DNA methylation seems to be essential for the long-term inactivation of certain genes that occurs during normal cell differentiation in the embryo. For instance, experiments have shown that deficient DNA methylation due to lack of a methylating enzyme leads to abnormal embryonic development in organisms as different as mice and Ambidopsis (a plant). Once methylated, genes usually stay that way through successive cell divisions. At DNA sites where one strand is already methylated, methylation enzymes correctly methylate the daughter strand after each round of DNA replication. Methylation patterns are thus passed on, and cells forming specialized tissues keep a chemical record of what occurred during embryonic development. A methylation pattern maintained in this way also accounts for genomic imprinting in mammals, where methylation permanently regulates expression of either the maternal or paternal allele of certain genes at the start of development (see Chapter 15). Epigenetic Inheritance The chromatin modifications that we have just discussed do not involve a change in the DNA sequence, and yet they may be passed along to future generations of cells. Inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance. Researchers are amassing more and more evidence for the importance of epigenetic information in regulation of gene expression. Clearly, enzymes that modify chromatin structure appear to be integral parts of the cell's machinery for regulating transcription. 364

U N I T THREE

Genetxs

Regulation of Transcription Initiation Chromatin-modifying enzymes provide initial control of gene expression by making a region of DNA either more or less able to bind the transcription machinery. Once a gene is optimally modified for expression, [he initiation of transcription is the most important and universally used stage at which gene expression is regulated. Before looking at how cells control their iranscription, let's review the structure of a typical eukaryotic gene and its transcript. Organization of a Typical Eukaryotic Gene A eukaryotic gene and the DNA elements (segments) that control it are typically organized as shown in Figure 19.5, which extends what you learned about eukaryotic genes in Chapter 1 T. Recall that a cluster of proteins called a transcription initiation complex assembles on the promoter sequence at the "upstream" end of the gene. One of these proteins, RNA polymerase II, then proceeds to transcribe the gene, synthesizing a primary RNA transcript (pre-mRNA). RNA processing includes enzymatic addition of a 5' cap and a poly-A tail, as well as splicing out of introns, to yield a mature mRNA. Associated with most eukaryotic genes are multiple control elements, segments of noncoding DNA that help regulate transcription by binding certain proteins. These control elements and the proteins they bind are critical to the precise regulation of gene expression seen in different cell types. The Roles of Transcription Factors To initiate transcription, eukaryotic RNA polymerase requires the assistance of proteins called transcription factors (see Figure 17.8). Because the transcription factors mentioned in Chapter 17 are essential for the transcription of all proteincoding genes, they are sometimes called general transcription factors. Only a few general transcription factors independently bind a DNA sequence, such as the TATA box within the promoter; the others primarily bind proteins, including each other and RNA polymerase II. Protein-protein interactions are crucial to the initiation of eukaryotic transcription. Onl; when the complete initiation complex has assembled can the polymerase begin to move along the DNA template strand, producing a complementary strand of RNA. The interaction of general transcription factors and RNA polymerase II with a promoter usually leads to only a low rate of initiation and production of few RNA transcripts. In eukaryotes, high levels of transcription of particular genes at the appropriate time and place depend on the interaction of control elements with other proteins that can be thought of as specific transcription factors. Enhancers and Specific Transcription Factors. As you can see in Figure 19.5, some control elements, named proximal

Enhancer (distal control elements)

Poly-A signal sequence

Proximal control elements Intron

Exon

Intron

Exon

Intron

Exon

Intron

Exon

Termination region

Upstream

Primary RNA transcript (pre-mRNA)

Cleaved 3' end

5'

of primary transcript RNA processing: Cap and tail added; introns excised and exons spliced together

Coding segment

5' Cap

A Figure 19.5 A eukaryotic gene and its. t r a n s c r i p t . Each eukaryotic gene has a promoter, a DNA sequence where RNA pclymerase binds and starts transcription, proceeding "downstream." A number of control elements (gold) are involved in regulating the initiation of transcription; these are DNA sequences located near (proximal to)

5' UTR (untranslated region)

codon

or far from (distal to) the promoter. Distal control elements can be grouped together as enhancers. A polyadenylation (poly-A) signal in the last exon of the gene is transcribed into an RNA sequence that determines where the transcript is cleaved and the poly-A tail added. Transcription may continue for hundreds of nucleotides beyond the poly-A signal before

control elements, are located close to the promoter. (Although some biologists consider proximal control elements part of the promoter, we do not.) The more distant distal control elements, groups of which are called enhancers, may be thousands ot nucleotides upstream or downstream of a gene or even within an intron. A given gene may have multiple enhancers, each active at a different time or in a different cell type or location in the organism. The interactions between enhancers and specific transcription factors called activators or repressors are particularly important in controlling gene expression. An activator is a protein that binds to an enhancer and stimulates transcription of a gene. Figure 19.6, on the next page, shows a current model ior how binding of activators to enhancers located far from the promoter can influence transcription. Protein-mediated bending of the DNA is thought to bring the bound activators in con.act with a group of so-called mediator proteins, which in turn nteract with proteins at the promoter. These multiple proteinprotein interactions help assemble and position the initiation complex on the promoter. Hundreds of transcription activators have been discovered in eukaryotes. Researchers have identified two common structural elements in a large number of activator proteins: a DNA-binding domain—-a part of the protein's three-dimensional structure that binds to DNA—and one or more activation domains. Activation CHAPTER 19

codon

3' UTR (untranslated region)

Poly-A tail

terminating. RNA processing of the primary transcript into a functional mRNA involves three steps: addition of the 5' cap, addition of the poly-A tail, and splicing. In the cell, the 5' cap is added soon after transcription is initiated; splicing and poly-A tail addition may also occur while transcription is still under way (see Figure 17.9).

domains bind other regulatory proteins or components of the transcription machinery, allowing a sequence of protein-protein interactions that result in transcription of a given gene. Some specific transcription factors function as repressors to inhibit expression of a particular gene. Eukaryotic repressors can cause inhibition of gene expression in several different ways. Certain repressors block the binding of activators etther to their control elements or to components of the transcription machinery. Other repressors bind directly to their own control elements in an enhancer and act to turn off transcription even in the presence of activators. In addition to affecting assembly of the transcription machinery directly, some activators and repressors act indirectly by influencing chromatin structure. Recall that a gene present in a region of chromatin with high levels of histone acetylation is able to bind the transcription machinery, whereas a gene in a region of chromatin with low levels of histone acetylation is not (see Figure 19.4). Studies in yeast and mammals show that some activators recruit proteins that acetylate histones near the promoters of specific genes, thus promoting transcription. In contrast, other repressors recruit proteins that deacetylate histones, leading to reduced transcription, a phenomenon referred to as silencing, fndeed, recruitment of chromatinmodifying proteins seems to be the most common mechanism of repression in eukaryotes. Eukaryoiic Genomes: Organization, Regulation, and Evolution

365

Gene

0 Activator proteins bind to distal control elements grouped as an enhancer in the DNA. This enhancer has three binding sites.

;

0 A DNA-bending protein brings the bound activators closer to the promoter. Other transcription factors, mediator proteins, and RNA polymerase are nearby.

© The activators bind to ; certain general transcription factors and mediator proteins, helping them form an active transcription initiation complex on the promoter.

x

--/

--

Transcription v~ initiation complex

RNA synthesis

A Figure 19.6 A model for the action of enhancers and transcription activators. Bending of the DNA by a protein enables enhancers to influence a promoter hundreds or even thousands of nucleotides away. Specific transcription factors called activators bind to the enhancer DNA sequences and then to a group of mediator proteins, which in turn bind to general transcription factors assembling the transcription initiation complex. These protein-protein interactions facilitate the correct positioning of the complex on the promoter and the initiation of RNA synthesis. Only one enhancer is shown in this figure, but a gene may have several that act at different times or in different cell types.

Combinatorial Control of Gene Activation. In eukaryotes, the precise control of transcription depends largely on the binding of activators to DNA control elements. Considering the very large number of genes that must be regulated in a typical animal or plant cell, the number of completely different nucleotide sequences found in control elements is surprisingly small. A dozen or so short nucleotide sequences appear again and again in the control elements for different genes. On average, each enhancer is composed of about ten control elements, each of which can bind only one or two specific transcription factors- The particular combination of control elements in an enhancer associated

366

UNIT THREE

Gen;-iii:s

with a gene turns out to be more important than the presence of a single unique control element in regulating transcription of the gene. Even with only a dozen control element sequences available, a large number of combinations are possible. A particular combination of control elements will be able to activate transcription only when the appropriate activator proteins are present, such as at a precise time during development or in a particular cell type. The example in Figure 19,7 illustrates how the use of different combinations of control elements to activate transcription allows exquisite regulation of transcription with a small set of control elements.

Enhancer

Coordinately Controlled Genes

Promoter

Lens cell nucleus

Available actr /ators

1//

v*

Albumin gene not expressed

(C Crystaliin gene not expressed

(a) Liver cell. The albumin gene is expressed, and the crystallin gene is not.

Crystallin gene expressed (b) Lens cell. The crystallin gene is expressed, and the albumin gene Is not.

A Figure 19.7 Cell type-specific transcription. Both liver cells and lens cells have the genes for making the proteins albumin and crystallin, but only liver cells make albumin (a blood protein) and only lens cells make crystallin (the main component of the lens of the eye). " h e specific transcription factors (activators and repressors) made in a particular type of cell determine which genes are expressed. In this example, the genes for albumin and crystallin are shown at the top, ±ach with an enhancer made up of three different control elements. Although the enhancers for the two genes share one control element, each enhancer has a unique combination of elements. All the activators required for high-level expression of the albumin gene are present only in liver cells (a), whereas the activators needed for expression of the crystallin gene are present only in lens cells (b). For simplicity, we consider only the role of activators here, although the presence or absence of repressors may also influence transcription in certain cell types.

CHAPTER 19

How does the eukaryotic cell deal with genes of related function that need LO be turned on or off at the same time? In Chapter 18, you learned that in prokaryotes, such coordinately controlled genes are often clustered into an operon, which is regulated by a single promoter and transcribed into a single mRNA molecule. Thus, the genes are expressed together, and the encoded proteins are produced concurrently. With rare exceptions, operons that work in this way have not been found in eukaryotic celLs. Recent studies of the genomes of several eukaryotic species have found thai some co-expressed genes are clustered near one another on the same chromosome. Examples include certain genes in the testis of the fruit fly and muscle-related genes in a small worm called a nematode. Unlike genes in prokaryotic operons, however, each eukaryotic gene in these clusters has its own promoter and is individually transcribed. The coordinate regulation of clustered genes in eukaryotic cells is thought to involve changes in the chromatin structure that make the entire group of genes either available or unavailable for transcription. More commonly, co-expressed eukaryotic genes, such as genes coding for the enzymes of a metabolic pathway, are found scattered over different chromosomes. In these cases, coordinate gene expression seems to depend on the association of a specific control element or combination of elements with every gene of a dispersed group. Copies of the activators that recognize these control elements bind to them, promoting simultaneous transcription ol the genes, no matter where they are in the genome. Coordinate control of dispersed genes in a eukaryotic cell often occurs in response to external chemical signals. A steroid hormone, for example, enters a cell and binds to a specific intraeellular receptor protein, forming a hormone-receptor complex that serves as a transcription activator (see Figure 11.6). Every gene whose transcription is stimulated by a particular steroid hormone, regardless of its chromosomal location, has a control element recognized by that hormonereceptor complex. Many signal molecules, such as nonsteroid hormones and growth factors, bind to receptors on a cells surface and never actually enter the cell. This kind of signal can control gene expression indirectly by triggering signal transduction pathways that lead to activation of particular transcription activators or repressors (see Figure 11.14). The principle of coordinate regulation is the same as in the case of steroid hormones: Genes with the same control elements are activated by the same chemical signals. Systems for coordinating gene regulation probably arose early in evolutionary history and evolved by the duplication and distribution of control elements within the genome.

"Eukaryotic Genomes: Organization, Regulation, and Evolution

367

Mechanisms of Post-Transcriptional Regulation Transcription alone does not constitute gene expression. The expression of a protein-coding gene is ultimately measured by the amount of functional protein a cell makes, and much happens between the synthesis of the RNA transcript and the activity of the protein in the cell. An increasing number of examples are being found of regulator}' mechanisms that operate at various stages after transcription (see Figure 19.3). These mechanisms allow a cell to fine-tune gene expression rapidly in response to environmental changes without altering its transcription patterns. Here we discuss how cells can regulate gene expression once a gene has been transcribed. RNA Processing RNA processing in the nucleus and the export of mature RNA to the cytoplasm provide several opportunities for regulating gene expression that are not available in prokaryotes. One example of regulation at the RNA-processing level is alternative RNA splicing, in which different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns (Figure 19.8). Regulatory proteins specific to a cell type control intron-exon choices by binding to regulatory sequences within the primary transcnpt.

RNA splicing

/ /

or

\ \

mRNA

A Figure 19.8 Alternative RNA splicing. The primary transcripts of some genes can be spliced in more than one way, generating different mRNA molecules. Notice in this example that one mRNA molecule has ended up with the green exon and the other with the purple exon. With alternative splicing, an organism can produce more than one type of polypeptide from a single gene.

368

UNIT THREE

mRNA Degradation The life span of mRNA molecules in the cytoplasm is an important factor in determining the pattern of protein synthesis in a ceil. Prokaryotic mRNA molecules [ypically are degraded by enzymes within a few minutes of their synthesis. This short life span of prokaryotic mRNAs is one reason prokaryotes can vary their patterns of protein synthesis so quickly in response to environmental changes. In contrast, mRNAs in multicellular eukaryotes typically survive for hours, days, or even weeks. For instance, the mRNAs for the hemoglobin polypeptides (a-globin and (3-globin) in developing red blood cells are unusually stable, and these long-lived mRNAs are translated repeatedly in these cells. Research on yeasts suggests that a common pathway of mRNA breakdown begins with the enzymatic shortening of the poly-A tail (see Figure 19.5). This helps trigger the action of enzymes that remove the 5' cap (the two ends of the mRNA may be briefly held together by the proteins involved). Removal of the cap, a critical step, is also regulated by particu ar nucleotide sequences in the mRNA. Once the cap is remove d, nuclease enzymes rapidly chew up the mRNA. Nucleotide sequences that affect the length of time an mRNA remains intact are often found in the untranslated region £UTR) at the 3' end of the molecule (see Figure 19.5). In one experiment, researchers transferred such a sequence from the shortlived mRNA for a growth factor to the 3' end of a normally stable globin mRNA. The globin mRNA was quickly degraded. During the past few years, another mechanism that blocks expression of specific mRNA molecules has come to light. Researchers have found small single-stranded RNA molecule?, called microRNAs (miRNAs), that can bind to complementary sequences in mRNA molecules. The miRNAs are formed from longer RNA precursors that fold back on themselves, forming a long, double-stranded hairpin structure held together by hydrogen bonds (Figure 19.9). An enzyme, fittingly called Dicer, then cuts the double-stranded RNA molecule into short fragments. One of the two strands is degraded, and the other strand (miRNA) associates with a large protein complex and acts as a homing device, directing the complex to any mRNA molecules that have the complementary sequence. Depending on various factors, the miRNA-protein complex then either degrades the target mRNA or blocks its translation. Inhibition of gene expression by RNA molecules was firs: observed by biologists who noticed that injecting doublestranded RNA molecules into a cell somehow turned off a gene with the same sequence. They called this experimental phenomenon RNA interference (or RNAi). It was later shown to be due to small interfering RNAs (siRNAs), RNAs of similar size and function as miRNAs. In fact, subsequent research showed that the cellular machineiy that generates siRNAs is the very same as that responsible for producing miRNAs naturally in the cell. The mechanisms by which these small RNAs function appear to be similar as well.

0 The microRNA (miRNA) precursor folds back on itself, held together by hydrogen bonds.

Q An enzyme called Dicer moves along the doublestranded RNA, cutting it into shorter segments.

Q One strand of each short doublestranded RNA is degraded; the other strand (miRNA} then associates with a complex of proteins.

0 The bound miRNA can base-pair with any target mRNAthat contains the complementary sequence.

0 The miRNA-protein complex prevents gene expression either by degrading the target mRNA or by blocking its translation.

Target mRNA

Blockage of translation

k Figure 19.9 Regulation of gene expression by microRNAs (miRNAs). RNA transcripts from miRNA-encoding genes are processed into miRNAs, which prevent expression of complementary mRNAs (siRNAs are believed to be generated and to function in a similar way).

Because the cellular RNAi pathway can lead to the destruction of RNAs with sequences complementary to those found in double-stranded RNA molecules, it is commonly believed to have originated as a natural defense against infection by RNA viruses. However, the fact that the RNAi pathway can affect expression of cellular genes also supports alternative models. In any case, it is clear that RNAi plays an important role in regulating gene expression in the cell. Initiation of Translation Translation presents another opportunity for regulating gene expression; such regulation occurs most commonly at the initiation stage (see Figure ] 7.17). The initiation of translation of selected mRNAs can be blocked by regulatory proteins that bind to specific sequences or structures within the untranslated region at the 5' end (5 ; UTR) of the mRNA, preventing the attachment of ribosomes. A different mechanism for blocking translation is seen in a variety of mRNAs present in the egg cells of many organisms: These stored mRNAs lack poly-A tails of sufficient size to allow translation initiation. At the appropriate time during embryonic development, a cytoplasmic enzyme adds more A residues, allowing translation to begin. Alternatively, translation of all the mRNAs in a cell may be regulated simultaneously. In a eukaryotic cell, such "global" control usually involves the activation or inactivation of one or more of the protein factors required to initiate translation. This mechanism also plays a role in starting translation of mRNAs that are stored in egg cells. Just after fertilization, CHAPTER 19

translation is triggered by the sudden activation of translation initiation factors. The response is a burst of synthesis of the proteins encoded by the stored mRNAs. Some plants and algae store mRNAs during periods of darkness; light then triggers the reactivation of the translational apparatus. Protein Processing and Degradation The final opportunities for controlling gene expression occur after translation. Often, eukaryotic polypeptides must be processed to yield functional protein molecules. For instance, cleavage of the initial insulin polypeptide (pro-insulin) forms the active hormone. In addition, many proteins undergo chemical modifications that make them functional. Regulatory proteins are commonly activated or inactivated by the reversible addition of phosphate groups, and proteins destined for the surface of animal cells acquire sugars. Cell-surface proteins and many others must also be transported to target destinations in the cell in order to function. Regulation might occur at any of the steps involved in modifying or transporting a protein. Finally, the length of time each protein functions in the cell is strictly regulated by means of selective degradation. Many proteins, such as the cyclins involved in regulating the cell cycle, must be relatively short-lived if the cell is to function appropriately (see Figure 12.16). To mark a particular protein for destruction, the cell commonly attaches molecules of a small protein called ubiquitin to the protein. Giani protein complexes called proteasomes then recognize the ubiquitin-tagged Eukaryotic Genome;: Organization, Regulation, and Evolution

369

Q Multiple ubiquitin molecules are attached to a protein by enzymes in the cytosol.

© Enzymatic components of the Q The ubiquitin-tagged protein proteasome cut the protein into is recognized by a proteasome, small peptides, which can be which unfolds the protein and further degraded by other sequesters it within a central cavity, enzymes in the cytosol.

Protein entering a proteasome A Figure 19.10 Degradation of a protein by a proteasome. A proteasome, an enormous protein complex with a shape suggesting a trash can, chops up unneeded proteins in the cell. In most cases, the proteins

attacked by a proteasome have been tagged with short chains of ubiquitin, a small protein. Steps 1 and 3 require ATP. Eukaryotic proteasomes are as massive as ribosomal subunits and are distributed throughout the

protein molecules and degrade them (Figure 19.10). The importance of proteasomes is underscored by the finding that mutations making cell cycle proteins impervious to proteasome degradation can lead to cancer. Concept Check 1. In genera], what is the eflect of histone acetylation and DNA methylarion on gene expression? 2. Compare the roles of general and specific transcription factors in regulating gene expression. 3. If you compared the nucleotide sequences of the distal control elements in the enhancers of three coordmately regulated genes, what would you expect to find? Why? 4. Once mRNA encoding a particular protein reaches the cytoplasm, what are four mechanisms that can regulate the amount of the active protein in the cell? For suggested answers, see Appendix A.

Concept

Cancer results from genetic changes that affect cell cycle control In Chapter 12, we considered cancer as a set of diseases in which cells escape from the control mechanisms normally limiting their growth. Now that we have discussed the molecular 370

U N I T THREE

Genelics

cell. Their barrel-like shape somewhat resembles that of chaperone proteins, which protect protein structure rather than destroy i! (see Figure 5.23).

basis of gene expression and its regulation, we are ready to look at cancer more closely. The gene regulation systems that go wrong during cancer turn out to be the very same systems that play important roles in embryonic development, the immune response, and many other biological processes. Thus, research into the molecular basis of cancer has both benefited from and informed many other fields of biology.

Types of Genes Associated with Cancer The genes that normally regulate cell growth and division during the cell cycle include genes for growth factors, their receptors, and the intraceilular molecules of signaling pathways. (To review the cell cycle, see Chapter 12.) Mutations that alter any of these genes in somatic cells can lead to cancer. The agent of such change can be random spontaneous mutation. However, it is likely that many cancer-causing mutations result from environmental influences, such as chemical carcinc gens, X-rays, and certain viruses. An early breakthrough in understanding cancer came i:i 1911, when Peyton Rous discovered a virus that causes cancer in chickens. Since then, scientists have recognized a number of tumor viruses that cause cancer in various animals, including humans (see Table 18.1). The Epstein-Barr vims, a herpesvirus that causes infectious mononucleosis, has been linked to several types of cancer, notably Burkitt's lymphoma. Papilloma viruses (of the papovavirus group) are associated with cancer ol the cervix. Among the retro viruses, one called HTLV-1 causes a type of adult leukemia. All tumor viruses transform cells into cancer cells through the integration of viral nucleic acid into host cell DNA.

Ontogenes and Proto-Oncogenes

Tumor-Suppressor Genes

Research on tumor viruses led to the discovery of cancer-causing genes called oncogenes (from the Greek onco, tumor) in certain retroviruses. Subsequently, close counterparts of these oncogenes were found in the genomes of humans and other animals. The normal cellular genes, called proto-oncogenes, code for proteins thai stimulate normal cell growth and division. How might a proto-oncogene—a gene that has an essential function in normal cells—become an oncogene, a cancercausing gene? In general, an oncogene arises from a genetic change that leads to an increase either in the amount of the proto-oncogenes protein product or in the intrinsic activity of each protein molecule. The genetic changes that convert proto-oncogenes to oncogenes fall into three main categories: movement of DNA within the genome, amplification of a proto-oncogene, and point mutations in a control element or in [he proto-oncogene itself (Figure 19.11).

In addition to genes whose products normally promote cell division, cells contain genes whose normal products inhibit cell division. Such genes are called tumor-suppressor genes because the proteins they encode help prevent uncontrolled cell growth. Any mutation that decreases the normal activity of a tumorsuppressor protein may contribute to the onset of cancer, in effect stimulating growth through the absence of suppression. The protein products of tumor-suppressor genes have various functions. Some tumor-suppressor proteins normally repair damaged DNA, a function that prevents the cell from accumulating cancer-causing mutations. Other tumorsuppressor proteins control the adhesion of cells to each other or to the extracellular matrix; proper cell anchorage is crucial in normal tissues—and often absent in cancers. Still other tumor-suppressor proteins are components of cellsignaling pathways that inhibit the cell cycle.

Cancer cells are frequently found to contain chromosomes that have broken and rejoined incorrectly, translocating fragments from one chromosome to another (see Figure 15.14). If a translocated proto-oncogene ends up near an especially active promoter (or other control element), its transcription may increase, making it an oncogene. Movement of transposable elements also may place a more active promoter near a protooncogene, increasing its expression, (Eukaryotic transposable elements are described later in the chapter.) The second main type of genetic change, amplification, increases the number oi ccpies of the proto-oncogene in the cell. The third possibility is a point mutation either (1) in the promoter or an enhancer that controls a proto-oncogene, causing an increase in its expression, or (2) in the coding sequence, changing the gene's product to a protein that is more active or more resistant to degradation than the normal protein. All these mechanisms can lead to abnormal stimulation of the cell cycle and put the cell on the path to malignancy.

V Figure 19,11 Genetic changes that can turn proto-oncogenes into oncogenes.

Interference with Normal Cell-Signaling Pathways The proteins encoded by many proto-oncogenes and tumorsuppressor genes are components of cell-signaling pathways. Let's take a closer look at how such proteins function in normal cells and what goes wrong with their function in cancer cells. We will focus on the products of two key genes, the ras proto-oncogene and the p53 tumor-suppressor gene. Mutations in ras occur in about 30% of human cancers; mutations in p53 in more than 50%. The Ras protein, encoded by the ras gene, is a G protein that relays a signal from a growth factor receptor on the plasma membrane to a cascade of protein kinases. The cellular response at the end of the pathway is the synthesis of a protein that stimulates the cell cycle (Figure 19.12a, on the next page). Normally, such a pathway will not operate unless triggered by

Proto-oncogene

Translocation or transposition: gene moved to new locus, under new controls

Newpromoter

Normal growth-stimulating protein in excess

\jy

Oncogene

Normal growth-stimulating protein in excess

CHAPTER 19

Normal growth-stimulating protein in excess

Oncogene

Hyperactive or degradationresistant protein

Eukaryotic Genomes: Organization, Regulation, and Evolution

371

^ - © Growth **\ factor © G protein (a) Cell cycle-stimulating pathway. This pathway is triggered by © a growth factor that binds to © its receptor in the plasma membrane. The signal is relayed to © a G protein cailed Ras. Like all G proteins, Ras is active when GTP is bound to it. Ras passes the signal to © a series of protein kinases. The last kinase activates 0 a transcription activator that turns on one or more genes for proteins that stimulate the cell cycle. If a mutation makes Ras or any other pathway component abnormally active, excessive cell division and cancer may result.

© Receptor

© Protein kinases-— (phosphorylation cascade)

V 0 Transcription--^^ factor (activator)

Gene expression Protein that stimulates the cell cycle

(b) Cell cycle-inhibiting pathway. In this pathway, © DNA damage is an intracellular signal that is passed via © protein kinases and leads to activation o f © p53. Activated p53 promotes transcription of the gene for a protein that inhibits the cell cycle. The resulting suppression of cell division ensures that the damaged DNA is not replicated. Mutations causing deficiencies in any pathway component can contribute to the development of cancer.

© Protein kinases A "

MUTATION

^

:

.

i

Y : O DNA damage in genome

.© Active — form ofp53

s

1

DNA > ^ ?

I Protein that inhibits the cell cycle

(c) Effects of mutations. Increased cell division, possibly leading to cancer, can result if the cell cycle is overstirnulated, as in (a), or not inhibited when it normally would be, as in (b).

EFFECTS OF MUTATIONS Protein overexpr

Cell cycle oversti mutated

A Figure 19.12 Signaling pathways that regulate cell division. Both stimulatory and inhibitory pathways regulate the cell cycle, commonly by influencing transcription. Cancer can result from aberrations in such pathways, in combination with other factors.

372

U N I T

T H R E E

{

Defective or missing transcriptior factor, such as p53, cannot activate transcription

the ; .ppropriate growth factor. But certain mutations in the ras gene can lead to production of a hyperactive Ras protein that triggers the kinase cascade, resulting in increased cell division even in the absence of growth factor. In fact, hyperactive versior s or excess amounts of any of the pathway's components can have the same outcome: excessive cell division. Figure 19.12b shows a pathway in which a signal leads to the synthesis of a protein that suppresses the cell cycle. In this case, the signaL is damage to the cell's DNA, perhaps as the result of exposure to ultraviolet light. Operation of this signaling pathway blocks the cell cycle until the damage has been repaired. Otherwise, the damage might contribute to tumor formation by causing mutations or chromosomal abnormalities. Thus, the genes for the components of the pathway act as tur.10r-suppressor genes. The tumor-suppressor protein encoded by the wild-type p53 gene is a specific transcription factor that promotes the synthesis of cell cycle-inhibiting proteins. That is why a mutation knocking out the p53 gene, like a mutation that leads to a hyperactive Ras protein, can lead to excessive cell growth and cancer (Figure 19.12c). The p53 gene, named for the 53,000-dalton molecular weight of its protein product, has been called the "guardian angel of the genome." Once activated, for example by DNA damage, the p53 protein functions as an activator for several genes. Often it activates a gene called p21, whose product halts the cell cycle by binding to cyclin-dependent kinases, allowing time for the cell to repair the DNA; the p53 protein can also turn on genes directly involved in DNA repair. When DMA damage is irreparable, p53 activates "suicide" genes, whose protein products cause cell death by a process called apoptosis (see Figure 21.18). Thus, in at least three ways, p53 prevents a cell from passing on mutations due to DNA damage. If mutations do accumulate and the cell survives through rr.any divisions—-as is more likely if the p53 tumor-suppressor gene is defective or missing—cancer may ensue.

The Multistep Model of Cancer Development More than one somatic mutation is generally needed to produce all the changes characteristic of a full-fledged cancer cell. This may help explain why the incidence of cancer increases greatly with age. If cancer results from an accumulation of mutations and if mutations occur throughout life, then the longer we live, the more likely we are to develop cancer. The model oi a multistep path to cancer is well supported by studies of one of the best understood types of human cancer, colorectal cancer. About 135,000 new cases of colorectal cancer are diagnosed each year in the United States, and the disease causes 60,000 deaths per year. Like most cancers, colorectal cancer develops gradually (Figure 19.13). The first sign is often a polyp, a small, benign growth in the colon lining. The cells of the polyp look normal, although they divide unusually frequently The tumor grows and may eventually become malignant, invading other tissues. The development of a malignant tumor is paralleled by a gradual accumulation of mutations that convert proto-oncogenes to oncogenes and knock out tumor-suppressor genes. A ras oncogene and a mutated j)53 tumor-suppressor gene are often involved. About a hali dozen changes must occur al the DNA level for a cell to become fully cancerous. These usually include the appearance of at least one active oncogene and the mutation or loss of several tumor-suppressor genes. Furthermore, since mutant tumor-suppressor alleles are usually recessive, in most cases mutations must knock out both alleles in a cell's genome to block tumor suppression. (Most oncogenes, on the other hand, behave as dominant alleles.) Finally, in many malignant tumors, the gene for telomerase is activated. This enzyme prevents the shortening of chromosome ends during DNA replication (see Figure 16.19). Production of telomerase in cancer cells removes a natural limit on the number of times the cells can divide.

• Figure 19.13 A multistep model for the development of colorectal cancer. Affecting the colon and/or rectum, this type of cancer is one of the best understood. Changes in a tumor parallel a series of genetic changes, including mutations affecting several tumor-suppressor genes (such as p53) and the ras proto-oncogene. Mutations of tumor-suppressor genes often entail loss (deletion) of the gene. APC stands for "adenornatous polyposis coli," and OCC stands for "deleted in colorectal cancer." Other mutation sequences can also lead to cancer.

0 Loss of tumorsuppressor gene APC (or other)

Normal colon epithelial cells

Q Loss of tumor-suppressor gene p53

© Activation of ras oncogene

Small benign growth (polyp)

, Q Loss of • tumor-suppressor gene DCC

CHAPTER 19

• 0 Additional • mutations Larger benign growth (adenoma)

Malignant tumor (carcinoma)

Eukaryotic Genomes: Organization, Regulation, and Evolution

373

Viruses seem to play a role in about 15% of human cancer cases worldwide. Viruses contribute to cancer development by integrating their genetic material into the DNA of infected cells. By this process, a retrovirus may donate an oncogene to the cell. Alternatively, integrated viral DNA may disrupt a tumor-suppressor gene or convert a proto-oncogene to an oncogene. Finally, some viruses produce proteins that inactivate p53 and other tumor-suppressor proteins, thus making the cell more prone to becoming cancerous.

Concept Check 1. Compare the usual iunctions of proteins encoded by proto-oncogenes with those encoded by tumorsuppressor genes. 2. Explain how the types of mutations that lead to cancer are different for a proto-oncogene and a tumor-suppressor gene. 3. Under what circumstances do we consider cancer to have a hereditary component?

Inherited Predisposition to Cancer The fact that multiple genetic changes are required to produce a cancer cell helps explain the observation that certain cancers run in some families. An individual inheriting an oncogene or a mutant allele of a lumor-suppressor gene is one step closer to accumulating the necessary mutations for cancer to develop than is an individual without any such mutations. Geneticists are devoting much effort to identifying inherited cancer alleles so that predisposition to certain cancers can be detected early in life. About 15% of colorectal cancers, for example, involve inherited mutations. Many of these affect the tumor-suppressor gene called adenomatous polyposis coli, or APC (see Figure 19.13). This gene has multiple functions in the cell, including regulation of cell migration and adhesion. Even in patients with no family history of the disease, the APC gene is mutated in 60% of colorectal cancers. In these individuals, new mutations must occur in both APC alleles before the gene's function is lost. Since only 15% of colorectal cancers are associated with known inherited mutations, researchers continue in their efforts to identify "markers" that could predict the risk of developing this type of cancer. There is evidence of a strong inherited predisposition m 5-10% of patients with breast cancer. This is the second most •common type oi cancer m the United States, striking over 180,000 women (and some men) annually and killing 40,000 each year. Mutations in the BRCA1 or BRCA2 gene are found in at least half of inherited breast cancers (BRCA stands for BReast CAncer). A woman who inherits one mutant BRCA1 allele has a 60% probability of developing breast cancer before the age ot 50, compared with only a 2% probability for an individual homozygous for the normal allele. Both BRCA I and BRCA2 are considered to be tumor-suppressor genes because their wildtype alleles protect against breast cancer and their mutant alleles are recessive. Researchers have not yet determined what the normal products of these genes actually do in the cell. However, recent evidence suggests that the BRCA2 protein is directly involved in repairing breaks that occur in both strands of DNA. The study of these and other genes associated with inherited cancer may lead to new methods for early diagnosis and treatment of all cancers. Studying these genes also increases our understanding of the normal processes of regulation of the genome. 374

UNIT THREE

Genetics

For suggested answers, see Appendix A.

Concept

Eukaryotic genomes can have many noncoding DNA sequences in addition to genes We have spent most of this chapter, and indeed this unit, focusing on genes that code for proteins. Yet the coding regions of these genes and the genes for RNA products such as rRNA and tRNA make up only a tiny portion of the genomes of most multicellular eukaryotes. The bulk of most eukaryotic genomes consists of noncoding DNA sequences, often described in the past as "junk DNA." However, much evidence is accumulating that noncoding DNA plays important roles n the cell, an idea supported by its persistence in diverse genomes over many hundreds of generations. In this section, we examine how genes and noncoding DNA sequences a:-e organized within eukaryotic genomes, using the human genome as our prime example. Genome organization tells us much about how genomes have evolved and continue io evolve, the subject of the final section of the chapter.

The Relationship Between Genomic Composition and Organismal Complexity Several trends are evident when we compare the genomes of prokaryotes and those of eukaryotes, including more complex groups such as mammals. Although there are exceptions, we find a general progression from smaller to larger genomes, but with fewer genes in a given length of DNA. For example, humans have 500 to 1,500 times as many base pairs in their genome as most prokaryotes, but on average only 5 to 15 times as many genes-—thus many fewer genes in any given length of DNA. In prokaryotic genomes, most of the DNA codes for protein, tRNA, or rRNA; the small amount of noncoding DNA consists mainly of regulatory sequences, such as promoters. The coding sequence of nucleotides along a prokaryotic gene

proceeds from start to finish without interruption by noncoding sequences (introns). In eukaryotic genomes, by contrast, most of the DNA does not encode protein or RNA, and it includes more complex regulatory sequences. In fact, humans have 10,000 times as much noncoding DNA as prokaryotes. Some of the noncoding DNA in multicellnlar eukaryotes is present as introns within genes. Indeed, introns account for most of the difference in average length between human genes (27,000 base pairs) and prokaryotic genes (1,000 base pairs). Now that the complete sequence of the human genome is available, we know what makes up most of the 98.5% that does not code for proteins, rRNAs, or tRNAs (Figure 19.14). Gene-related regulatory sequences and introns account for 24% of the human genome. The remaining sequences, located between functional genes, include some unique noncoding DIN A, such as gene fragments and mutated genes that are nonfunctional. Most intergenic DNA. however, is repetitive DNA, sequences that are present in multiple copies in the genome. Somewhat surprisingly, about three-fourths of this

repetitive DNA (44% of the entire human genome) is made up of transposable elements and sequences related to them.

Transposable Elements and Related Sequences All organisms seem to have stretches of DNA that can move from one location to another within the genome. In Chapter 18, we described transposable elements in prokaryotes, which may have been an evolutionary source of viruses.. However, the first evidence for wandering DNA segments did not come from experiments with prokaryotes, but from American geneticist Barbara McClintock's breeding experiments with Indian corn (maize) in the 1940s and 1950s (Figure 19.15). McClintock identified changes in the color ol corn kernels that made sense only if she postulated the existence of genetic elements capable oi moving from other locations in the genome into the genes for kernel color. McClintock's discovery received little attention until transposable elements were discovered in bacteria many years later and microbial geneticists learned more about the molecular basis of transposition.

Movement of Transposons and Retrotransposons

Exons (regions of genes coding for protein, rRNA, or tRNA) (1.5%)

Eukaryotic transposable elements are of two types; transposons, which move within a genome by means oi a DNA intermediate, and retrotransposons, which move by means of an RNA intermediate, a transcript of the retrotransposon DNA. Transposons can move by a "cut-and-paste" mechanism, which removes the element from the original site, or by

Large-segment duplications (5-6%)

A. Figure 19.14 Types of DNA sequences in the human g e n o m e . The coding sequences in genes (dark purple) make up only about 1,5% of the human genome, while introns and regulatory sequences associated with genes flight purple) make up about a quarter. The vast majority of the human genome does not code for human proteins or RNAs, and much of it is repetitive DNA (dark and light green). Because repetitive DNA is the most difficult to sequence and analyze, classification of some portions is tentative, and the percentages given here may shift slightly as genome analysis continues.

CHAPTER 19

A. Figure 19.15 The effect of transposable elements on corn k e r n e l color. Barbara McClintock first proposed the idea of mobile genetic elements after observing variegations in corn kernel color. Although her idea was met with skepticism when she proposed it in the 1940s, it was later validated. She received the Nobel Prize in 1983, at the age of 8 1 , for her pioneering research,

Eukaryotic Genomes: Organization, Regulation, and Evolution

375

a "copy-and-paste" mechanism, which leaves a copy behind (Figure 19.16a). Retrotransposons always leave a copy at the original site during transposiiion, since they are initially transcribed into an RNA intermediate (Figure 19.16b). To insert at another site, the RNA intermediate is first converted back to DNA by reverse transcriptase, an enzyme encoded in the retrotransposon itself. Thus, reverse transcriptase can be present in cells not infected with retroviruses. (In fact, retroviruses may have evolved from retrotransposons.) A cellular enzyme catalyzes insertion of the reverse-transcribed DNA at a new site. Most transposable elements in eukaryotic genomes are retrotransposons.

New copy of transposon

Transposon DNA of genome Transposon is copied

Mobile transposon (a) Transposon movement ("copy-and-paste" mechanism)

Retrotransposon

New copy of retrotransposon

DNA of genome

Sequences Related to Transposable Elements Reverse

•.

Multiple copies of transposable elements transcriptase and sequences related to them are scattered throughout eukaryotic genomes. A single unit is usually hundreds to thousands of base pairs long, and the dispersed "copies" are similar but usually (b) Retrotransposon movement not identical to each other. Some of these are transposable elements that move us- A Figure 19.16 Movement of eukaryotic transposable elements, (a) Movement of ing enzymes encoded either by t h e m - transposons by either the cut-and-paste mechanism or the copy-and-paste mechanism (shown here) selves or by other transposable elements involves a double-stranded DNA intermediate that is inserted into the genome, (b) Movement of .

retrotransposons begins with formation of a single-stranded RNA intermediate. The remaining steps

and some are related sequences that have a r e essentia ||y identical to part of the retrovirus reproductive cycle (see Figure 18.10). In movement lost the ability to move altogether. Trans- of transposons by the copy-and-paste mechanism and in movement of retrotransposons, the DNA posable elements and related sequences sequence remains in the original site as well as appearing in a new site, make up 25-50% of most mammalian replication or recombination. It accounts for about 15% of genomes and even higher percentages in amphibians and the human genome (see Figure 19.14), About a third of this higher plants (see Figure 19.14). (5% of the human genome) consists of large-segment dupliIn humans and other primates, a large portion of transposcations, in which a long stretch of DNA, between 10,000 able element-related DNA consists of a family of similar seand 300,000 nucleotide pairs, seems to have been copied quences called Alu elements. These sequences alone account from one chromosomal location to another, on the same or for approximately 10% of the human genome. Alu elements a different chromosome. are about 300 nucleotides long, much shorter than most funcIn contrast to single duplications of long sequences, simple tional transposable elements, and they do not code for any sequence DNA contains many copies of tandemly repeated protein. However, many Alu elements are transcribed into short sequences, as in the following example (showing one RNA molecules; their cellular function, if any, is unknown. Although many transposable elements encode proteins, DNA strand only): these proteins do not carry out normal cellular functions. . . . GTTACGTTACGTTACGTTACGTTACGTTAC . . . Thus, these elements often are described as "noncoding" DNA, along with other repetitive sequences. In this case, the repeated unit consists ol five nucleotides

Other Repetitive DNA, Including Simple Sequence DNA Repetitive DNA that is not related to transposable elements probably arose by mistakes that occurred during DNA 376

UNIT THREE

Genetics

(GTTAC). Repeated units often contain fewer than 15 nucleotides, but they may include as many as 500 nucleotides. The number of repeated units at a particular site in the genome varies as well. For example, there could be as many as several hundred thousand repetitions of the GTTAC unit at

one site. Altogether, simple sequence DNA makes up 3% of the .iuman genome. The nucleotide composition of simple sequence DNA is often different enough from the rest of the cell's DNA to have an intrinsically different density. If genomic DNA is cut into pieces and centrifuged at high speed, segments of different density migrate to different positions in the centrifuge tube. Repetitive DNA isolated in this way was originally called satdhie DhTA because it appeared as a "satellite" band in the centrifuge tube, separate from the rest of the DNA. Now the term is often used interchangeably with simple sequence DNA. Much of a genome's simple sequence DNA is located an chromosomal telomeres and centromeres, suggesting that this DNA plays a structural role for chromosomes. The DNA at centromeres is essential for the separation of chromatids in cell division (see Chapter 12). Centromeric DNA, along with simple sequence DNA located elsewhere, also may help organize the chromatin within the interphase nucleus. The simple sequence DNA located at telomeres, at the tips of chromosomes, prevents genes from being lost as the DNA shortens with each round of replication (see Chapter 16). Te.omeric DNA also binds proteins that protect the ends of a chromosome from degradation and from joining to other chromosomes.

Genes and Multigene Families We finish our discussion of the various types of DNA sequences in eukaryotic genomes with a closer look at genes. Recall that sequences coding for proteins and structural RNAs compose a mere 1.5% of the human genome (see Figure 19-14). If we include introns and regulatory sequences associated with genes, the total amount of gene-related DNA—coding and noncodirg—constitutes about 25% of the human genome. As in prokaryotes, most eukaryotic genes are present as unique sequences, with only one copy per haploid set of chromosomes. But in the human genome, such solitary genes make up only about half of the total coding DNA. The rest occurs in multigene families, collections of identical or very similar genes. Some multigene tamilies consist of identical DNA sequences, usually clustered tandemly. With the notable exception of the genes for histone proteins, multigene families of identical genes code for RNA products. An example is the family of identical sequences encoding the three largest ribosomal RNA (rRNA) molecules (Figure 19.17a). These rRNA molecules are encoded in a single transcription unit that is repeated tandemly hundreds to thousands of times in one or several clusters in the genome of a multicellular eukaryote. The many copies of this rRNA transcription unit help cells to quickly make the millions of ribosomes needed for active protein synthesis. The primary transcript is cleaved to yield the three rRNA molecules. These are then CHAPTER 19

(a) Part of the ribosomal RNA gene family. Three of the hundreds of copies of rRNA transcription units in a salamander genome are shown at the top (TEM), Each "feather" corresponds to a single transcription unit being transcribed by about 100 molecules of RNA polymerase (the dark dots along the DNA), moving left to right. The growing RNA transcripts extend out from the DNA. In the diagram below the TEM, one transcription unit is shown. It includes the genes for three types of rRNA (blue), adjacent to regions that are transcribed but later removed (yellow). A single transcript is made and then processed to yield one molecule of each of the three rRNAs, which make up part of a ribosome. A fourth rRNA (55 rRNA) is also found in the ribosome, but the gene encoding it is not part of this transcription unit.

Embryo

and adult

Embryo

Fetus

Adult

(b) The human a-giobin and j3-globin gene families. Hemoglobin is composed of t w o a-globin and t w o p-globin polypeptide subunits. The genes (dark blue) encoding a- and (i-globins are found in two families, organized as shown here. The noncoding DNA separating the functional genes within each family cluster includes pseudogenes (green), nonfunctional versions of the functional genes. Genes and pseudogenes are named with Greek letters.

A Figure 19.17 Gene families. Eukaryotic Genomes: Organization, Regulation, and Evolution

377

combined with proleins and one other kind of rRNA (5S rRNA) to form ribosomal subunits. The classic examples of multigene families of nonidenticai genes are two related families of genes that encode globins, a group of proteins that include the a and |3 polypeptide subunits of hemoglobin. One family, located on chromosome 16 in humans, encodes various forms of a-globin; the other, on chromosome 11, encodes forms of |3-globin (Figure 19.17b). The different forms of each globin subunit are expressed at different times in development, allowing hemoglobin to function effectively in the changing environment of the developing animal In humans, for example, the embryonic and fetal forms of hemoglobin have a higher affinity for oxygen than the adult forms, ensuring the efficient transfer of oxygen from mother to developing fetus. Also found in the globin gene family clusters are several pseudogenes, nonfunctional nucleotide sequences quite similar to the functional genes. The arrangement of the genes in gene families has provided insight into the evolution of genomes. We will consider some of the processes that have shaped the genomes of different species over evolutionary time in the next section.

Concept Check 1. Discuss the characteristics that make mammalian genomes larger than prokaryotic genomes. 2. How do introns, transposable elements, and simple sequence DNA differ in their distribution in the genome? 3. Discuss the clilierences in the organization of [he rRNA gene family and the globin gene families. How do these gene families benefit the organism?

can lead to the evolution of proteins (or RNA products) with related or entirely new functions.

Duplication of Chromosome Sets An accident in meiosis can result in one or more extra sets of chromosomes, a condition known as polyploidy In a pclyploid organism, one complete set of genes can provide essential functions for the organism. The genes in the one or more extra sets can diverge by accumulating mutations; these variations may persist if the organism carrying them survives and reproduces. In this way, genes with novel functions can evolve. As long as one copy of a crucial gene is expressed, i he divergence of another copy can lead to its encoded protein acting in a novel way, thereby changing the organism's phenotype. The accumulation of mutations in many (or even a few) genes may lead to the branching off of a new species, as happens often in plants (see Chapter 24). Although polyplcid animals exist, they are rare.

Duplication and Divergence of DNA Segments Errors during meiosis can also lead to the duplication of individual genes. Unequal crossing over during prophase 1 of meiosis, for instance, can result in one chromosome with a deletion and another with a duplication of a particular region. As illustrated in Figure 19.18, transposable elements in the

Transposable elemenK

Gene

For suggested answers, see Appendix A.

Concept

Duplications, rearrangements, and mutations of DNA contribute to genome evolution The basis of change at the genomic level is mutation, which underlies much of genome evolution. It seems likely that the earliest forms of life had a minimal number of genes—those necessary for survival and reproduction. If this was indeed the case, one aspect of evolution must have been an increase in the size of the genome, with the extra genetic material providing the raw material for gene diversification. In this section, we will first describe how extra copies of all or part of a genome can arise and then consider subsequent processes that 378

UNIT THREE

Genetic?

A Figure 19.18 Gene duplication due to unequal crossing over. One mechanism by which a gene (or other DNA segment) can be duplicated is recombination during meiosis between copies of a transposable element flanking the gene. Such recombination between misaligned nonsister chromatids of homologous chromosomes produces one chromatid with two copies of the gene and one chromatid with no copy.

genome can provide sites where nonsister chromatids can cross over, even when their homologous gene sequences are not correctly aligned. Also, slippage can occur during DNA replication, such that the template shifts with respect to the new complementary stnmd, and one region of the template Strand is either not copied or copied twi.:e. As a result, a region of DNA is deleted or duplicated. It is easy to imagine how such errors could occur in regions of repeats, such as the simple sec uence DNA described previously. The variability in numbers of repeated units of simple sequence DNA at the same site is probably due to errors like these. Evidence that molecular events such as unequal crossing over and slippage lead to duplication of genes is found in the existence of multigene families.

a-Globin gene family on chromosome 16 A

[3'Giobin gene family on chromosome 11

Figure 19.19 Evolution of the human a-globin and p-globin gene families.

Shown here is a model for evolution of the modern a-globin and p-globin gene families from a single ancestral globin gene,

Evolution of Genes with Related Functions: The Human G\obin Genes

ism and some may have had no effect, but some mutations may have altered the function of the protein product in a way that was advantageous to the organism at a particular life stage without substantially changing its oxygen-carrying function. Presumably, natural selection acted on these altered genes to maintain them in the population, leading to production of alternative forms of a-globin protein. The similarity in the ammo acid sequences of the various a-globin and p-globin proteins supports this model of gene duplication and mutation (Table 19.1). The amino acid sequences of the (3-globins, for instance, are much more similar to each other than to the a-globin sequences. The existence of several pseudogenes among the functional globin genes provides additional evidence for this model (see Figure 19.17b). That is, random mutations in these "genes" over evolutionary time have destroyed their function.

Duplication events can lead to the evolution of genes with, related functions, such as those of the a-globin and P-globin gene families (see Figure 19.17b). A comparison of gene sequences within a multigene family can suggest the order in which the genes arose. This approach to re-creating the evolutionary history of the various globin genes indicates that they all evolved from one common ancestral glohin gene, which Vvas duplicated and diverged into a-globin and p-globin ancestral genes about 450-500 million years ago (Figure 19.19). Each of these genes was later duplicated several times, and the copies then diverged from each other in sequence, yielding the current family members. In fact, the common ancestral globin gene also gave rise to the oxygen-binding muscle protein myoglobin and to the plant protein leghemoglobin. The latter two proteins function as monomers, and their genes are included in a "globin Table 19.1 Percentage of Similarity in Amino Acid Sequence superfamily." Between Human Globin Proteins After the duplication events, the differp-Globins u-Globins ences between the genes in the globin families undoubtedly arose from mutations that accumulated in the gene copies over many generations. The current 100 58 42 39 a-Globin: model is that the necessary function pro58 38 100 34 vided by an a-globin protein, for example, 42 73 34 100 was fulfilled by one gene, while other 100 (3-Globins y 39 38 73 copies of the a-globin gene accumulated 75 80 random mutations. Some mutations may 37 37 iave had an adverse affect on the organCHAPTER 19

Cukaryotic Genomes: Organization, Regulation, and Evolution

37 37 75 80 100

379

Evolution of Genes with Novel Functions In the evolution of the globin gene families, gene duplication and subsequent divergence produced family members whose protein products performed related functions. Alternatively, one copy of a duplicated gene can undergo alterations that lead to a completely new function for the protein product. The genes for lysozyme and a-lactalbumin are good examples. Lysozyme is an enzyme that helps prevent infection by hydrolyzing the cell walls of bacteria; a-lactalbumin is a nonenzymatic protein that plays a role in milk production in mammals. The two proteins are quite similar in their amino acid sequences and three-dimensional structures. Both genes are found in mammals, whereas only the lysozyme gene is present in birds. These findings suggest that at some time after the lineages leading to mammals and birds had separated, the lysozyme gene underwent a duplication event in the mammalian lineage but not in the avian lineage. Subsequently, one copy of the duplicated lysozyme gene evolved into a gene encoding a-lactalbumin, a protein with a completely different function.

Rearrangements of Parts of Genes: Exon Duplication and Exon Shuffling Rearrangement of existing DNA sequences has also contributed to genome evolution. The presence of introns in most eukaryotic genes may have promoted the evolution of new and potentially useful proteins by facilitating the duplication or repositioning of exons in the genome. Recall from Chapter 17 that an exon often codes for a domain, a distinct structural or functional region of a protein. We've already seen that unequal crossing over during meiosis can lead to duplication of a gene on one chromosome and its loss from the homologous chromosome (see Figure 19.18). By a similar process, a particular exon within a gene could be duplicated on one chromosome and deleted irom the homologous chromosome. The gene with the duplicated exon would code for a protein containing a second copy of the encoded domain. This change in the proteins structure could augment its function by increasing its stability, enhancing its ability to bind a particular ligand, or altering some other property. Quite a few protein-coding genes have multiple copies of related exons, which presumably arose by duplication and then diverged. The gene encoding the extracellular matrix protein collagen is a good example. Collagen is a structural protein with a highly repetitive amino acid sequence, which is reflected in the repetitive pattern of exons in the collagen gene. Alternatively, we can imagine the occasional mixing and matching of different exons either within a gene or between two nonallelic genes owing to errors in meiotic recombination. This process, termed exon ihujjling, could lead lo new 380

U N I T

T H R E E

proteins with novel combinations of functions. As an example, let's consider the gene for tissue plasminogen activator (TPA). The TPA protein is an extracellular protein involved in limiting blood clotting. It has four domains of three types, each encoded by an exon; one exon is present in two copies. Because each type of exon is also found in other proteiis, the gene for TPA is believed to have arisen by several instances of exon shuffling and duplication (Figure 19.2D). The TPA protein slows the clotting reaction and therefore limits the damage that can result from heart attacks and some types of stroke, as long as it is administered immediately to victims.

How Transposable Elements Contribute to Genome Evolution The persistence of transposable elements as a large fraction of some eukaryotic genomes is consistent with the idea that they can play an important role in shaping a genome over evolutionary time. These elements can contribute to the evolution of the genome in several ways. They can promote recombination, disrupt cellular genes or control elements, and carry entire genes or individual exons to new locations. The presence of homologous transposable element sequences scattered throughout the genome allows recombination to take place between different chromosomes. Most such alterations are probably detrimental, causing chromosomal translocations and other changes in the genome that may be lethal to the organism. But over the course of evolutionary time, an occasional recombination like this may be advantageous to the organism.

Epidermal growth factor gene with multiple EGF exons (green)

Fibronectin gene with mi "finger" exons (orange)

Plasminogen gene with a "kringle" exon (blue) Portions of ancestral genes

Exon shuffling TPA gene as it exists today

A Figure 19.20 Evolution of a new gene by exon shuffling Exon shuffling could have moved exons from ancestral forms of the genes for epidermal growth factor, fibronectin, and plasminogen (left) into the evolving gene for tissue plasminogen activator, TPA (right). The order in which these events might have occurred is unknown. Duplication of the "kringle" exon from plasminogen after its movemen" could account for the two copies of this exon in the TPA gene. Each type of exon encodes a particular domain in the TPA protein.

more often, so that the original protein is made. On occasion, however, splicing occurs at the new weak site, with the result that some of the Alu element ends up in the mRNA, coding for a new portion of the protein. In this way, alternative genetic combinations can be "tried out" while the function of the original gene product is retained. Clearly, these processes produce either no effect or harmful effects in most individual cases. However, over long periods of time, the generation of genetic diversity provides more raw material for natural selection to work on during evolution. Recent advances in DNA technology have allowed researchers to sequence and compare the genomes of many different species, increasing our understanding of how genomes evolve. You will learn more about these topics in the next chapter.

The movement of transposable elements around the genome can have several direct consequences. For instance, if a t:ansposable element "jumps'1 into the middle of a coding secuence of a protein-coding gene, it prevents the normal functioning of the interrupted gene. If a transposable element inserts within a regulatory sequence, the transposition may lead to increased or decreased production of one or more protens. Transposition caused both types of effects on the genes coding for pigment-synthesizing enzymes in McClintock's corn kernels. Again, while such changes may usually be harmful , in the long run some may prove beneficial. During transposition, a transposable element may carry along a gene or group of genes to a new position in the genome. This mechanism probably accounts for the location of the a-globin and fS-globin gene families on different human chromosomes, as well as the dispersion of the genes of certain other gene families. By a similar tag-along process, an exon from one gene may be inserted into another gene in a mechanism similar to that of exon shuffling during recombination. For example, an exon may be inserted by transposition into the intron of a protein-coding gene. If the inserted exon is retained in the RHA transcript during RNA splicing, the protein that is synthesized will have an additional domain, which may confer a new function on the protein.

Concept Check 1. Describe three examples of errors in cellular processes that lead to DNA duplications. 2. "What processes are thought to have led to the evolution of the globin gene families? 3. Look at the portions of the ftbroneciin and EGF genes shown in Figure 19.20 (left). How might they have arisen? 4. What are three ways transposable elements are thought to contribute to the evolution of the genome?

Recent research reveals yet another way that transposable elements can lead to new coding sequences. This work shows that an Alu element may hop into introns in a way that creates a weak alternative splice site in the RNA transcript. During processing of the transcript, the regular splice sites are used

For suggested answers, see Appendix A.

Chapter ^ €| Review Go to the Campbell Biology website (www.campbellbiology.com) or CDF.OM to explore Activities, Investigations, and other interactive study aids.

Gene expression can be regulated at any stage, but the key step is transcription • Differential Gene Expression (pp. 362-363) Each cell of a

SUMMARY OF KEY CONCEPTS

multieclluiar eukaryoie expresses only a fraction of its genes. In each type of differentiated cell, a unique subset of genes is expressed. Key stages at which gene expression may be regulated include changes in chromatin structure, initiation of transcription, RNA processing, mRNA degradation, translation, and protein processing and degradation. Activity Overview: Control of Gene Expression • Regulation of Chromatin Structure (pp. 363-364) Genes in highly compacted chromatin are generally not transcribed. Chemical modification of histone tails can affect the configuration of chromatin and thus gene expression. Histone acetylation seems to loosen chromatin structure and thereby enhance transcription. DNA methylation is associated with reduced transcription.

i Chromatin structure is based on successive levels of DNA packing • Nucleosomes, or "Beads on a String" (pp. 360-361) Eukaryotic chromatin is composed mostly of DNA and histone proteins that bind to each other and to the DNA to form nucleosomes, the most basic units of DNA packing. Histone tails extend outward from each bead-like nucleosome core.

> Higher Levels of DNA Packing (pp. 360-361) Additional folding leads ultimately to highly compacted heterochromatin., the form of chromatin in a metaphase chromosome. In interphase cells, most chromatin is in the highly extended form called euchromatin. Activity DNA Packing

• Regulation of Transcription Initiation (pp. 364-367) Multiple DNA control elements distant from the promoter (in one or more enhancers) bind specific transcription factors i9

Eukaryotic Genomes: Organization, Regulation, and Evolution

381

(activators or repressors) that regulate transcription initiation for specific genes within the genome. Bending of DNA enables activators bound to enhancers to contact proteins at the promoter. Unlike the genes of a prokaryotic operon, coordinately controlled eukaryotic genes each have a promoter and control elements. The same regulatory sequences are common to all the genes of a group, enabling recognition by the same specific transcription factors. Activity Control ofTranscription Investigation How Do You Design a Gene Expression System?

• Mechanisms of Post-Transcriptional Regulation (pp. 368-370) Regulation at the RNA-processing level is exemplified by alternative RNA splicing. Also, each mRNA has a characteristic life span, determined in part by sequences in the leader and trailer regions. RNA interference by single-stranded micro-RNAs can lead to degradation of an mRNA or block its translation. The initiation of translation can be controlled via regulation of initiation factors. After translation, various types of protein processing (such as cleavage and the addition of chemical groups) are subject to control, as is the degradation of proteins by proteasomes. Activity Post-Transcriptional Control Mechanisms Activity Review: Control of Gene Expression

Cancer results from genetic changes that affect cell cycle control • Types of Genes Associated with Cancer (pp. 370-371) The products of proto-oncogenes and tumor-suppressor genes control cell division. A DNA change that makes a proto-oncogene excessively active converts it to an oncogene, which may promote excessive cell division and cancer. A tumor-suppressor gene encodes a protein that inhibits abnormal cell division. A mutation in such a gene that reduces the activity of its protein product may also lead to excessive cell division and possibly to cancer. • Interference with Normal Cell-Signaling Pathways (pp. 371-373) Many proto-oncogenes and tumor-suppressor genes encode components of growth-stimulating and growthinhibiting signaling pathways, respectively. A hyperactive version of a protein in a stimulatory pathway, such as Ras (a G protein), functions as an oncogene protein. A defective version of a protein in an inhibitory pathway, such as p53 (a transcription activator), fails to function as a tumor suppressor. • The Multistep Model of Cancer Development (pp. 373—374) Normal cells are converted to cancer cells by the accumulation of multiple mutations affecting proto-oncogenes and tumor-suppressor genes. Certain viruses promote cancer by integration of viral DNA into a cell's genome. • Inherited Predisposition to Cancer (p. 374) Individuals who inherit a mutant oncogene or tumor-suppressor allele have an increased risk of developing certain types of cancer. Activity Causes oj Cancer

Eukaryotic genomes can have many noneoding DNA sequences in addition to genes • The Relationship Between Genomic Composition and Organisntal Complexity (pp. 374-375) Compared with prokaryotic genomes, the genomes of eukaryotes generally are larger, have longer genes, and contain a much greater amount of noneoding DNA both associated with genes (introns, regulatory sequences) and between genes (much of it repetitive sequences).

382

U N I T THREE

Genetics

• Transposable Elements and Related Sequences (pp. 375-376) The most abundant type of repetitive DNA in higher eukaryotes consists of transposable elements and related sequences. Two types of transposable elements occur in eukaryotes: transposons, which move via a DNA intermediate, and retrotransposons, which are the most prevalent and move via m RNA intermediate. Each element may be hundreds or thousands of base pairs long, and similar but usually not identical copies are dispersed throughout the genome. • Other Repetitive DNA, Including Simple Sequence DN \ (pp. 376-377) Short noneoding sequences that are tandemly repeated thousands of times (simple sequence DNA) are especially prominent in centromeres and telomeres, where they probably play structural roles in the chromosome. • Genes and Multigene Families (pp. 377-378) Most eukaryotic genes are present in one copy per haploid set of chromosomes. However, the transcription unit encoding the three largest rRNAs is tandemly repeated hundreds to thousands of times at one or several chromosomal sites, enabling the cell to make the rRNA for millions of ribosomes quickly. The multiple, slightly different genes in the two globin gene families encode polypeptides used at different developmental stages of an animal.

Duplications, rearrangements, and mutations of DNA contribute to genome evolution • Duplication of Chromosome Sets (p. 378) Accidents in cell division can lead to extra copies of all or part of a genome, which may then diverge if one set accumulates sequence changes. • Duplication and Divergence of DNA Segments (pp. 378-380) The genes encoding the various globin proteins evolved from one common ancestral globin gene, which duplicated and diverged into a-globin and [3-globin ancestral genes. Subsequent duplications of these genes and random mutations gave rise to the present globin genes, all of which code for oxygen-binding proteins. The copies of some duplicated genes have diverged so much during evolutionary time that the functions of their encoded proteins are now substantially different. • Rearrangements of Parts of Genes: Exon Duplication and Exon Shuffling (p. 380) Rearrangement of exons within and between genes during evolution has led to genes containing multiple copies of similar exons and/or several different exons derived from other genes. • How Transposable Elements Contribute to Genome Evolution (pp. 380-381) Movement of transposable elements or recombination between copies of the same element occasionally generates new sequence combinations that are beneficial to the organism. Such mechanisms can alter the functions of genes or their patterns of expression and regulation.

YOUR KNOWLEDGE Evolution Connection One of the revelations of the human genome sequence was the presence of relict prokaryotic sequences—genes of prokaryotes incorporated into our genome but now defunct molecular fossils. What may have occurred to maroon prokaryotic genes in our genome?

Scientific Inquiry Prc state cells usually require testosterone and other androgens to survive. But some prostate cancer cells thrive despite treatments that eliminate androgens. One hypothesis is that estrogen, often considered a female hormone, may be activating genes normally controlled by an androgen in these cancer cells. Describe one or more experiments to test this hypothesis. (See Figure 11.6 to review the action of these steroid hormones.) Investigation How Do You Design a Gene Expression System?

Science, Technology, and Society Trice amounts of dioxin were present in Agent Orange, a defoliant sprayed on vegetation during the Vietnam War. Animal tests suggest that dioxin can cause birth defects, cancer, liver and thymus damage, and immune system suppression, sometimes leading to death. But the animal tests are equivocal; a hamster is not affected hj a dose thai can kill a guinea pig. Dioxin acts somewhat like a steroid hormone, entering a cell and binding to a receptor protein tr at then attaches to the cell's DNA. How might this mechanism help explain the variety of dioxin's effects on different body systems and in different animals? How might you determine whether a type o" illness is related to dioxin exposure? How might you determine whether a particular individual became ill as a result of exposure to dioxin? Which would be more difficult to demonstrate? Why?

CHAPTER 19 Eukaryotic Genomes: Organization, Regulation, and Evolution

383

A Figure 20.1 DNA microarray that reveals expression levels of 2,400 human genes (enlarged photo).

Key Concepts 20.1 DNA cloning permits production of multiple copies of a specific gene or other DNA segment 20.2 Restriction fragment analysis detects DNA differences that affect restriction sites 20.3 Entire genomes can be mapped at the DNA level 20.4 Genome sequences provide clues to important biological questions 20.5 The practical applications of DNA technology affect our lives in many ways

Understanding and Manipulating Genomes

O

ne of the great achievements of modern science has been the sequencing of the human genome, which was largely completed by 2003. The sequencing of the first complete genome, that of a bacterium, had been carried out a mere eight years previously During the intervening years, researchers accelerated the pace of DNA sequencing, while working on other genomes, aided by the development of faster and faster sequencing machines. These sequencing accomplishments have all depended on advances in DNA technology, starting with the invention of methods for making recombinant DNA. This is DNA in which nucleotide sequences from two different sources—-often different species— are combined in vitro into the same DNA molecule. The methods for making recombinant DNA are central to genetic engineering, the direct manipulation of genes for practical purposes. Applications of genetic engineering include 384

the manufacture of hundreds of protein products, such as hormones and blood-clotting factors. Using DNA technology, scientists can make recombinant DNA and then introduce t into cultured cells that replicate the DNA and express its genes, yielding a desired protein. DNA technology has launched a revolution in the area of biotechnology, the manipulation of organisms or their components to make useful products. Practices that go back centuries are forms of biotechnology: for example, the use of microbes to make wine and cheese and the selective breeding of livestock, which exploits naturally occurring mutations and genetic recombination. Modern biotechnology based on the manipulation of DNA in vitro differs from earlier practices by enabling scientists to modify specific genes and move them between organisms as distinct as bacteria, plants, and animals. DNA technology is now applied in areas ranging from agriculture to criminal law. More important, its use allows researchers in virtually all fields of biology to tackle age-olc. questions in a more comprehensive way. For instance, the: level of expression of thousands of different genes can now be measured at the same time, as shown in the DNA microarray in Figure 20.1. In the photograph, the color of each spot represents the relative expression of one of 2,400 human genes in a particular tissue. With this technique, researchers can compare gene expression in particular tissues or under different conditions. The knowledge gained from such global expression studies was largely inaccessible only a few decades ago. In this chapter, we first describe the main techniques for manipulating DNA and then discuss how genomes are analyzed and compared at the DNA level. In the last section, we survey the practical applications of DNA technology, concluding the chapter by considering some of the social and ethical issues that arise as DNA technology becomes more pervasive in our lives.

ENA cloning permits production of multiple copies of a specific gene or other DNA segment Trie molecular biologist studying a particular gene faces a challenge. Naturally occurring DNA molecules are very long, and a single molecule usually carries many genes. Moreover, genes may occupy only a small proportion of the chromosomal DNA, the rest being noncoding nucleotide sequences. A single human gene, for example, might constitute only Vioo.ooo of a chromosomal DNA molecule. As a further complication, the distinctions between a gene and the surrounding DNA are subtle, consisting only of differences in nucleotide sequence. To work directly with specific genes, scientists have developed methods for preparing well-defined, gene-sized pieces of DNA in multiple identical copies, a process called gene c toning.

Cloned genes are useful for two basic purposes: to make many copies of a particular gene and to produce a protein product. Researchers can isolate copies of a cloned gene from bacteria for use in basic research or to endow an organism with a new metabolic capability, such as pest resistance. For example, a resistance gene present in one crop species might be cloned and transferred into plants of another species. Alternatively, a protein with medical uses, such as human growth hormone, can be harvested in large quantities from bacterial cultures carrying the cloned gene for the protein. Cell containing gene of interest ,„-• -^

DNA Cloning and Its Applications: A Preview Most methods for cloning pieces of DNA in the laboratory share certain general features. One common approach uses bacteria (most often, Eschericftia coli) and iheir plasmids. Recall from Chapter 18 i hat bacterial plasmids are relatively small, circular DNA molecules that replicate separate from a bacterial chromosome. For cloning genes or other pieces of DNA in the laboratory, a plasmid is first isolated from a bacterial cell, and then the foreign DNA is inserted into it (Figure 20.2). The resulting plasmid is now a recombinant DNA molecule, combining DNA from two sources. The plasmid is returned to a bacterial cell, producing a recombinant bacterium, which reproduces to form a clone of identical Gene for pest Gene used to alter Protein dissolves Human growth horcells. Because the dividing bacteria repliresistance inserted bacteria for cleaning blood clots in heart mone treats stunted into plants up toxic waste attack therapy growth cate the recombinant plasmid and pass it on to their descendants, the foreign gene • Figure 20.2 Overview of gene cloning with a bacterial plasmid, showing is "cloned" at the same time; that is, the v a r i o u s uses of c l o n e d g e n e s . In this simplified diagram of gene cloning in the laboratory, we clone of cells contains multiple copies of start with a plasmid isolated from a bacterial cell and a gene of interest from another organism. Only one copy of the plasmid and one copy of the gene of interest are shown at the top of the the gene. figure, but the starting materials would include many copies of each.

CHAPTER 2 0

DNA Technology and Genomics

385

Most protein-coding genes exist in only one copy per genome—something on the order of one part per million of DNA—so the ability to clone such rare DNA fragments is extremely valuable. In the remainder of this chapter, you will learn more about the techniques outlined in Figure 20.2 and related methods.

Restriction enzyme cuts the sugar-phosphate backbones at each arrow

Using Restriction Enzymes to Make Recombinant DNA Gene cloning and genetic engineering were made possible by the discovery of enzymes that cut DNA molecules at a limited number of specific locations. These enzymes, called restriction endonucleases, or restriction enzymes, were discovered in the late 1960s by researchers studying bacteria. In nature, these enzymes protect the bacterial cell against intruding DNA [rom other organisms, such as phages or other species of bacteria. They work by cutting up the foreign DNA, a process called restriction.

Hundreds of different restriction enzymes have been identified and isolated. Each restriction enzyme is very specific, recognizing a particular short DNA sequence, or restriction site, and cutting both DNA strands at specific points within this restriction site. The DNA of a bacterial cell is protected from the cells own restriction enzymes by the addition of methyl groups (—CH3) to adenines or cytosines within the sequences recognized by the enzymes. The top of Figure 20.3 illustrates a restriction site recognized by a particular restriction enzyme from E. colt. As shown in this example, most restriction sites are symmetrical: That is, the sequence of nucleotides is the same on both strands when read in the 5' —» 3' direction. Most restriction enzymes recognize sequences containing four to eight nucleotides. Because any sequence this short usually occurs (by chance) many times in a long DNA molecule, a restriction enzyme will make many cuts in a DNA molecule, yielding a set of restriction fragments. All copies of a particular DNA molecule always yield the same set of restriction fragments when exposed to the same restriction enzyme. Tn other words, a restriction enzyme cuts a DNA molecule in a reproducible way. (Later you will learn how the different fragments can be separated.) The most useful restriction enzymes cleave the sugarphosphate backbones in both DNA strands in a staggered way, as indicated in Figure 20.3. The resulting doublestranded restriction fragments have at least one singlestranded end, called a sticky end. These short extensions can form hydrogen-bonded base pairs with complementary sticky ends on any other DNA molecules cut with the same enzyme. The associations formed in this way are only temporary, but the associations between fragments can be made permanent by the enzyme DNA ligase. This enzyme catalyzes the formation of covalent bonds that close up the sugar-phosphate 386

UNIT THREE

Sticky end

0 DNA fragment from another source is added. Base pairing of sticky ends produces various combinations,

Fragment from different DNA molecule cut by the same restriction enzyme

site, and makes staggered cuts in the sugar-phosphate backbones within this sequence, producing fragments with sticky ends. Any fragments with complementary sticky ends can base-pair; if they come from different DNA molecules, recombinant DNA is the product.

backbones. As you can see at the bottom of Figure 20.3, the ligase-catalyzed joining of DNA from two different sources produces a stable recombinant DNA molecule.

Cloning a Eukaryotic Gene in a Bacterial Plasmid Now that you've learned about restriction enzymes and DNA ligase, we can take a closer look at how genes are cloned in plasmids. The original plasmid is called a cloning vector, defined as a DNA molecule that can carry7 foreign DNA into a cell and replicate there. Bacterial plasmids are widely used as cloning vectors for several reasons. They can be easily isolated Irom bacteria, manipulated to form recombinant plasmids by insertion of foreign DNA in vitro, and then reintroduced into bacterial cells. Moreover, bacterial cells reproduce rapidly and in the process multiply any foreign DNA they carry.

Producing Clones of Cells Figure 20.4 details one method for cloning a particular gene from humans or other eukaiyotic species using a bacterial plasmid as :he cloning vector. The step numbers in the text that follows correspond to those in the figure. O We begin by isolating the bacterial plasmid from E. coli cells and DNA containing the gene of interest from human cells grown in laboratory culture. The plasmid has been engineered to carry two genes that will later prove useful: ampR, which makes E. coli cells resistant to the antibiotic ampicillin, and lacZ, which encodes (5-galactosidase. This enzyme hydrolyzes the sugar lactose, as well as a synthetic molecular mimic called X-gal. Within the lacZ gene is a single copy of the restriction site recognized by the restriction enzyme used in the next step. O Both the plasmid and the human DNA are digested with the same restriction enzyme, one that produces sticky ends. The. enzyme cuts the plasmid DNA at its single restriction site within the lacZ gene, but cuts the human DNA at multiple sites, generating many thousands of fragments. One of the human DNA fragments carries the gene of interest. 3' direction. If a standard DNA polymerase were used, the protein would be denatured along with the DNA during the first heating step and would have to be replaced after each cycle. The key to automating PCR was the discovery of an unusual heat-stable DNA polymerase, first isolated from prokaryotes living in hot springs, that could withstand the heat at the start of each cycle. Just as impressive as the speed of PCR is its specificity. Only minute amounts of DNA need be present in the starting material, and this DNA can be in a partially degraded state. The key to this high

During each PCR cycte, the target DNA sequence is doubled. By the end of the third cycle, one-fourth of the molecules correspond exactly to the target sequence, with both strands of the correct length (see white boxes above). After 20 or so cycles, the target sequence molecules outnumber all others by a billionfold or more. •

CHAPTER 20

DNA Technology and Genomics

391

specificity is the primers, which hydrogen-bond only to sequences at opposite ends of the target segment. By the end of the third cycle, one-fourth of the molecules are identical to the target segment, with both strands the appropriate length. With each successive cycle, the number of target segment molecules of the correct length doubles, soon greatly outnumbering all other DNA molecules in the reaction.

their functions. Does a particular gene differ from person to person, and are there certain alkies associated with a hereditary disorder? Where in the body and when is the gene expressed? Where is the gene located within the genome? Is expression of the gene related to expression of other genes? We can also ask how the gene differs from species to species and begin to unravel its evolutionary history

Despite its speed and specificity, PCR amplification cannot substitute for gene cloning in cells when large amounts of a gene are desired. Occasional errors during PCR replication impose limits on the number of good copies that can be made by this method. Increasingly, however, PCR is used to make enough of a specific DNA fragment to clone it merely by inserting it into a vector. Devised in 1985, PCR has had a major impact on biological research and biotechnology PCR has been used to amplify DNA from a wide variety of sources: fragments of ancient DNA from a 40,000-year-old frozen woolly mammoth; DNA from fingerprints or from tiny amounts of blood, tissue, or semen found at crime scenes; DNA from single embryonic cells for rapid prenatal diagnosis of genetic disorders; and DNA of viral genes from cells infected with viruses that are difficult to detect, such as HIV We'll return to applications of PCR later in the chapter.

To answer such questions, we need to know the complete nucleotide sequence of the gene and its counterparts in various individuals and species, as well as its expression pattern. These topics are covered in the next two sections. In this section, we discuss a more indirect approach, called restriction fragment analysis, that detects certain differences in the IIJcleotide sequences of DNA molecules. This type of analysis can rapidly provide useful comparative information about DNA sequences.

Concept Check 1. If the medium used for plating cells in step 5 of Figure 20.4 did not contain ampicillin, cells containing no plasmid would be able to grow into colonies. What color would those colonies be and why? 2. Imagine you want to study human (3-globin, a protein present in red blood cells. To obtain sufficient amounts of the protein, you decide to clone the [3-globin gene. Would you construct a genomic library or a cDNA library? What material would you use as a source of DNA or RNA? 3. What are two potential difficulties in using plasmid vectors and bacterial host cells for production of large quantities of human proteins from cloned genes? For suggested answers, see Appendix A.

Concept

Restriction fragment analysis detects DNA differences that affect restriction sites With techniques available for making homogeneous preparations of large numbers of identical DNA segments, we can begin to tackle interesting questions about specific genes and 392

UNIT THREE

GcilC: ii'S

Gel Electrophoresis and Southern Blotting Many approaches for studying DNA molecules make use §1 gel electrophoresis. This technique uses a gel as a molecular sieve to separate nucleic acids or proteins on the basis of size, electrical charge, and other physical properties (Figure 20.8). Because nucleic acid molecules carry negative charges en their phosphate groups, they all travel toward the positive electrode in an electric field. As they move, the thicket of polymer fibers impedes longer molecules more than it does shorter ones, separating them by length. Thus, gel electrophoresis separates a mixture of linear DNA molecules into bands, each consisting of DNA molecules of the same length. In restriction fragment analysis, the DNA fragments produced by restriction enzyme digestion of a DNA molecule are sorted by gel electrophoresis. When the mixture of restriction fragments from a particular DNA molecule undergoes electrophoresis, it yields a band pattern characteristic of the starting molecule and the restriction enzyme used. In fact, the relatively small DNA molecules of viruses and plasmids can b: identified simply by their restriction fragment patterns. (Larger DNA molecules, such as those of eukaryotic chromosomes, yield so many fragments that they appear as a smear, rather than as distinct bands.) Because DNA can be recovered undamaged from gels, the procedure also provides a way to prepare pure samples of individual fragments. Restriction fragment analysis is also useful for comparing two different DNA molecules—for example, two alleles of a gene. A restriction enzyme recognizes a specific sequence of nucleotides, and a change in even one base pair will prevent it from cutting at a particular site. Thus, if the nucleotide differences between the alleles occur within a restriction enzyme recognition sequence, digestion with that enzyme will produce a different mixture of fragments from each alkie. And each mixture will give its own band pattern in gel electrophoresis. For example, sickle-cell disease is caused by a mutation in a

Gel Electrophoresis Gel electrophoresis is used for separating leic acids or proteins that differ in size, electrical charge, or other physical properties. DNA molecules are separated by gel electrophoresis in restriction fragment analysis of both cloned (see Figure 20.9) and genomic DNA (see Figure 20.10). Gel electrophoresis separates macromolecules on the basis of their rate of movement through a gel in an electric field. How far a DNA molecule travels while the current is on is inversely proportional to its length. A mixture of DNA molecules, usually fragments P'oduced by restriction enzyme digestion, is separated into "bands"; each band contains thousands of molecules of the same length.

molecules, i s placed in a separate well near one end of a thin slab of gel. The gel is supported by glass plates, bathed in an aqueous solution, and has electrodes attached to each end.

S

single nuckotide that is located within a restriction sequence in the P-globin gene (see Figure 17-23 and p. 267). As shown in Figure 20.9, restriction fragment analysis by electrophoresis can distinguish the normal and sickle-cell alleles of the fi-globin gene. The starting materials in Figure 20.9 are samples of the cloned and purified (3-globin alleles. Suppose, however, we wanted to compare genomic DNA samples from three individuals; a person homozygous for the normal (3-globin allele; a person with sicklecell disease, homozygous for the mutant allele; and a heterozygous carrier. Electrophoresis of genomic DNA digested with a restriction enzyme yields too many bands to distinguish them

-" ^Mixture

j |

f

I

W

of DNA

rrK^ojIes

Normal (i-globin allele

of dlffer " _ent sizes

— ^ A ,

|

j*-175 bp-4*—-201 bp' -H< -• Large fragmentDdel

f •

Ddel

Sickle-cell mutant (3-globin allele

,-Gel

_arge fragrnent-

-376 b p -

plates Ddel

When the current is turned on, the negatively charged DNA molecules move toward the positive electrode, with shorter molecules moving faster than longer ones. Bands are shown here in blue, but on an actual gel, DNA bands are not visible until a DNAbinding dye is added. The shortest molecules, having traveled farthest, end up in bands at the bottom of the gel.

Ddel

Ddel

Ddel

(a) Ddel restriction sites in normal and sickle-cell alleles of j}-globin gene. Shown here are the cloned alleles, separated from the vector DNA but including some DNA next to the coding sequence. The normal aliele contains t w o sites within the coding sequence recognized by the Ddel restriction enzyme. The sicklecell allele lacks one of these sites. Longer molecules

t,-.

After the current is turned off, a DNA-binding dye is added. This dye fluoresces pink in ultraviolet light, revealing the separated bands to which it binds. In this actual gel, the pink bands correspond to DNA fragments of different lengths separated by electrophoresis. If all the samples were initially cut with the same restriction enzyme, then the different band patterns indicate that they came from different sources.

Large fragment

376 bp 201 bp 175 bp

(b) Electrophoresis of restriction fragments from normal and sickle-cell alleles. Samples of each purified allele were cut with the Ddel enzyme and then subjected to gel electrophoresis, resulting in three bands for the normal allele and two bands for the sickle-cell allele. (The tiny fragments on the ends of both initial DNA molecules are identical and are not seen here.)

A Figure 20.9 Using restriction fragment analysis to distinguish the normal and sickle-cell alleles of the p-giobin g e n e , (a) The sickle-cell mutation destroys one of the Ddel restriction sites within the p-globin gene, (b) As a result, digestion with the Ddel enzyme generates different fragments from the normal and sickle-cell alleles. CHAPTER 20

DNA Technology and. Genomics

393

individually. Bui with a method called Southern blotting, which combines gel electrophoresis and nucleic acid hybridization, we can detect just those bands that include parts of the p-globin gene. The principle is the same as in nucleic acid hybridization for screening bacterial clones (see Figure 20.5). In this case, the probe is a radioactive single-stranded DNA molecule that is complementary to the p-globin gene. Figure 20.10 outlines the entire procedure and demonstrates how it can be used to compare DNA samples from the three individuals mentioned above. Southern blotting reveals not only whether a particular sequence is present in a sample of DNA but also the size of the restriction fragments thai contain the sequence. One of its many applications, as in our (3-globin example, is to identify heterozygote carriers of mutant alleles associated with genetic diseases.

together is a measure of the closeness ol the two loci on a chromosome. The discovery of RFLPs greatly increased the number of markers available for mapping the human genome. No longer were geneticists limited to genetic variations that lead to obvious phenotypic differences (such as genetic diseases) or even to differences in protein products.

Concept Check 1. Suppose you carry out electrophoresis on a sample of genomic DNA isolated from an individual and treated with a restriction enzyme. Alter staining the gel with a DNA-binding dye, what would you see? Explain. 2. Explain why restriction fragment length polymorphisms (RFLPs) can serve as genetic markers even though they produce no visible phenotypic differences.

Restriction Fragment Length Differences as Genetic Markers Restriction fragment analysis proved invaluable when biologists turned their attention to noncoding DNA, which comprises most of the DNA of animal and plant genomes (see Figure 19.14). When researchers subjected cloned segments of noncoding DNA from different individuals to procedures like that shown in Figure 20.8, they were excited to discover many differences in hand patterns. Like different alleles ol a gene, noncoding DNA sequences on homologous chromosomes may exhibit small nucleotide differences. Differences in the restriciion sites on homologous chromosomes that result in different restriction fragment patterns are called restriction fragment length polymorphisms (RFLPs, pronounced "Rif-lips"). RFLPs are scattered abundantly throughout genomes, including the human genome. This type of sequence difference in noncoding DNA is conceptually the same as a difference in coding sequence. Analogous to the single base-pair difference that identifies the sickle-cell allele, a RFLP can serve as a genetic marker for a particular location (locus) in the genome. A given RFLP may occur in numerous variants in a population. (The word polymorphisms comes from the Greek for "many forms/') RFLPs are detected and analyzed by Southern blotting, with the probe complementary to the sequence under consideration. The example shown in Figure 20.10 could as easily represent the detection of a RFLP in noncoding DNA as one in the coding sequences of two alleles. Because of the sensitivity of DNA hybridization, the entire genome can be used as the DNA starting material. (Samples of human DNA are typically obtained from white blood cells.) Because RFLP markers are inherited in a Mendelian fashion, they can serve as genetic markers for making linkage maps. The geneticist uses the same reasoning illustrated in Figure 15.6: The frequency with which two RFLP markers— or a RFLP marker and a certain allele for a gene—are inherited 394

UNIT

THREE

For suggested answers, see Appendix A.

Concept

Entire genomes can be mapped at the DNA level As early as 1980, molecular biologist David Botstein and colleagues proposed that the DNA variations reflected in RFLPs could serve as the basis for an extremely detailed map of the entire human genome. Since then, researchers have used sue i markers in conjunction, with the tools and techniques of DNA technology to develop more and more detailed maps of the genomes of a number of species. The most ambitious mapping project to date has been sequencing ot the human genome, officially begun as the Human Genome Project in 1990. This effort was largely completed in 2003 when the nucleotide sequence of the vast majority of DNA in each human chromosome (the 22 autosomes and the pair of sex chromosomes) was obtained. Organized by an international, publicly funded consortium of researchers at universities and research institutes, the project proceeded through three stages that provided progressively more detailed views of the human genome: genetic (or linkage) mapping, physical mapping, and DNA sequencing. (The interview with Eric Lander on pp. 236-237 gives a persona view of the project.) In addition to mapping human DNA, researchers with the Human Genome Project are also working on the genomes of other species important in biological research. They have completed sequences for E. coli and numerous other prokaryotes, Saccharomyces cerevisiae (yeast), Cacnorhabditis elegans (nematode), Drosophila melanogaster (fruit fly), Mus musculus

(mouse), and quite a few others. These genomes are of great

Southern Blotting of DNA Fragments APPLICATION

TOscaiJUKI'S can detect specific nucleotide sequences within a DNA I idinple with this method. In particular, Southern blotting is useful for comparing the restriction fragments produced from different samples of genomic DNA. In this example, we compare genomic DNA samples from three individuals: a homozygote for the normal p-g!obin alleie (I), a homozygote for the mutant sickle-cell alleie (II), and a heterozygote (111).

DNA + restriction enzyme

Restriction fragments

I

II

III

Nitrocellulose paper (blot) \ Gel

] Normal [J-globin

II Sickle-cell alleie

I I I Heterozygote

alfele { ) Preparation of restriction fragments. Each DNA sample is mixed with the same restriction enzyme, in this case Ddel. Digestion of each sample yields a mixture of thousands of restriction fragments.

. Gel electrophoresis. The restriction I fragments in each sample are separated by electrophoresis, forming a characteristic pattern of bands. (In reality, there would be many more bands than shown here, and they would be invisible unless stained.)

Blotting. With the gel arranged as shown above, capillary action pulls the alkaline solution upward through the gel, transferring the DNA to a sheet of nitrocellulose paper (the blot) and denaturing it in the process. The single strands of DNA stuck to the paper blot are positioned in bands corresponding to those on the gel.

Probe hydrogenbonds to fragments containing normal or mutant p-globin

I

Fragment from sickle-cell P-globin alleie

II

III

x

Film over paper blot

t Paper blot

Fragment from normal p-globin alleie

0 Hybridization with radioactive probe. The paper blot is exposed to a solution containing a radioactively labeled probe. In this example, the probe is single-stranded DNA complementary to the p-globin gene. Probe molecules attach by base-pairing to any restriction fragments containing a part of the p-globin gene. (The bands would not be visible yet.)

0 Autoradiography. A sheet of photographic film is laid over the paper blot. The radioactivity in the bound probe exposes the film to form an image corresponding to those bands containing DNA that base-pairs with the probe.

Because the band patterns for the three samples are clearly different, this method can be used to identify heterozygous carriers of the sickle-cell alleie (III), as well as those with the disease, w h o have two mutant alleles (II), and unaffected individuals, who have t w o normal alleles (I). The band patterns for samples 1 and II resemble those observed for the purified normal and mutant alleles, respectively, seen in Figure 20.9b. The band pattern for the sample from the heterozygote (III) is a combination of the patterns for the t w o homozygotes (I and II).

20

DNA Technology and. Genomics

395

interest in their own right and are also providing important insights of general biological significance, as we'll discuss later. In addition, the early mapping efforts on these genomes were useful for developing the strategies, methods, and new technologies necessary for deciphering the human genome, which is much larger.

Genetic (Linkage) Mapping: Relative Ordering of Markers Even before the Human Genome Project began, earlier research had painted a rough picture of the organization of the genomes of many organisms. For instance, the karyotype of a species reveals the number of chromosomes and their overall banding pattern (see Figure 13.3). And some genes had already been located on a particular region of a whole chromosome by fluorescence in situ hybridization (FISH), a method in which fluorescently labeled probes are allowed to hybridize to an immobilized array of whole chromosomes (see Figure 15,1). Cytogenetic maps based on this type of information provided the starting point for more detailed mapping. With cytogenetic maps of the chromosomes in hand, the initial stage in mapping a large genome is to construct a linkage map of several thousand genetic markers spaced throughout each of the chromosomes (Figure 20.11, stage Q). The order of the markers and the relative distances between them on such a map are based on recombination frequencies (see Chapter 15). The markers can be genes or any other identifiable sequences in the DNA, such as RFLPs or the simple sequence DNA discussed in Chapter 19. Relying primarily on simple sequence DNA, which is abundant in the human genome and has various "alleles" differing in length, researchers compiled a human genetic map with some 5,000 markers. Such a map enabled them to locate other markers, including genes, by testing for genetic linkage to the known markers. It was also valuable as a framework for organizing more detailed maps of particular regions.

Physical Mapping: Ordering DNA Fragments In a physical map, the distances between markers are expressed in some physical measure, usually the number of base pairs along the DNA. For whole-genome mapping, a physical map is made by cutting the DNA of each chromosome into a number of restriction fragments and then determining the original order of the fragments in the chromosomal DNA. The key is to make fragments that overlap and then use probes or automated nucleotide sequencing of the ends to find the overlaps (Figure 20.11, stage ®). In this way, more and more fragments can be assigned to a sequential order that corresponds to their order in a chromosome. Supplies of the DNA fragments used for physical mapping are prepared by cloning. In working with large genomes, 396

UNIT THREE

CuLTiiT'iCS

Cytogenetic map Chromosome banding pattern and location of

specific genes by fluorescence in situ hybridization (FISH)

0 Genetic (linkage) mapping Ordering of genetic markers such as RFLPs, simple sequence DNA, and other polymorphisms (about 200 per chromosome)

Q Physical mapping Ordering of large overlapping fragments cloned in YAC and BAC vectors, followed by ordering of smaller fragments cloned in phage and plasmid vectors

© DNA sequencing Determination of nucleotide sequence of each small fragment and assembly of the partial sequences into the complete genome sequence A Figure 20.11 Three-stage approach to mapping an entire genome. Starting with a cytogenetic map of each chromosome, researchers with the Human Genome Project proceeded through three stages of mapping to reach the ultimate goal, the nearly complete nucleotide sequence of every chromosome.

researchers carry out several rounds of DNA cutting, cloning, and physical mapping. The first cloning vector is often a yeast artificial chromosome (YAC), which can carry inserted fragments a million base pairs long, or a bacterial artificial chromosome (BAC), an artificial version of a bacterial chrcmosome that can carry inserts of 100,000 to 500,000 base pairs. After such long fragments are ordered, each fragment is cut into smaller pieces, which are cloned in plasmids or phages, ordered in turn, and finally sequenced.

DNA Sequencing The ultimate goal in mapping a genome is to determine the complete nucleotide sequence of each chromosome (Figure 20.11, stage ©). If a pure preparation of many copies of a DNA fragment up to about 800 base pairs in length is available, the sequence of the fragment can be determined by a sequencing machine. The usual sequencing technique, described in Figur • 20.12, was developed by British scientist Frederick Sanger; it is

search Met*

I Dideoxy Chain-Termination Method for Sequencing DNA

The sequence of nucleotides in any cloned DNA fragment up to about 800 pairs in length can be determined rapidly with specialized machines that carry out sequencing reactions and separate the labeled reaction products by length. I This method synthesizes a nested set of DNA strands complementary to the original DNA fragment. Each strand starts with the same primer and ends with a dkeoxyribonucleotide (ddNTP), a modified nudeotide. Incorporation of a ddNTP terminates a growing DNA strand because it lacks a 3' — O H group, the site for attachment of the next nudeotide (see Figure 16.13). In the set of strands synthesized, each nudeotide position along the original sequence is represented by strands ending at that point with the complementary ddNTP. Because each type of ddNTP is tagged with a distinct fluorescent label, the identity of the ending nucleotides of the new strands, and ultimately the entire original sequence, can be determined. { } The fragment of DNA to be sequenced is denatured into single strands and incubated in a test tube with the necessary ingredients for DNA synthesis; a primer designed to base pair with the known 3' end of the template strand, DNA polymerase, the four deoxyribonudeotides, and the four dideoxyribonucleotides, each tagged with a specific fluorescent molecule.

DNA (template strand) 5'

r' 3' & Synthesis of each new strand starts at the 3' end of the primer and continues until a dideoxyribonucleotide is inserted, at random, instead of the normal equivalent deoxyribonucleotide. This prevents further elongation of the strand. Eventually, a set of labeled strands of various lengths is generated, with the color of the tag representing the last nudeotide in the sequence.

Primer

Deoxyribonudeotides

T-fp'

-c -T -G -A -C -T -T -C -G -A -C -A -A

Dideoxyribonudeotides (fluorescently tagged)

H H r DNA

ddCTP

153

ddTTP

ES3

polymerase \

ddGTP

; H

OH

Labeled strands ddA-

cTG-

AAGCTG-

TT-

The labeled strands in the mixture are separated by passage through a polyacrylamide gel in a capillary tube, with shorter strands moving through faster. A fluorescence detector senses the color of each fluorescent tag as the strands come through. Strands differing by as little as one nudeotide in length can be distinguished.

' lh.c !ici|;ieui genome of an organism. IE likely to be revised as genome analysis continues. Mb = million base pair: best sludied of research organisms, are new to us. poses the opposite problem, determining the phenotype from the genotype. Starting with a long DNA sequence, how does one recognize genes and determine their function?

CHAPTER 20

DMA Technology and Genomics

399

Determining Gene Function So how do scientists determine the function of a new gene identified by genome sequencing and comparative analysis? Perhaps the most common approach is lo disable the gene and then observe the consequences in the cell or organism. In one application of this approach, called in vitro mutagenesis, specific mutations are introduced into the sequence of a cloned gene, after which the mutated gene is returned to a cell. If the introduced mutations alter or destroy the function of the gene product, the phenotype of the mutant cell may help reveal the function of the missing normal protein. Researchers can even put such a mutated gene into cells from the early embryo of a multicellular organism (such as a mouse) to study the role of the gene in the development and functioning of the whole organism. A simpler and faster method for silencing expression of selected genes exploits the phenomenon of RNA interference (RNAi), described in Chapter 19. This experimental approach uses synthetic double-stranded RNA molecules matching the sequence of a particular gene to trigger breakdown or to block translation of the gene's messenger RNA. To date, the RNAi technique has had some limited success in mammalian cells, including human cells in culture. But in other organisms, such as the nematode and the fruit fly, RNAi is already proving valuable for analyzing the functions of genes on a large scale. In one study, RNAi was used to prevent expression of 86% of the genes in early nematode embryos, one gene at a time. Analysis of the phenotypes of the worms that developed from these embryos allowed the researchers to group most of the genes into a small number of functional groups. This type of genome-wide analysis of gene function is sure to become more common as research focuses on the importance of interactions between genes in the system as a whole—the basis of systems biology (see Chapter 1).

The basic strategy in global expression studies is to isclate the mRNAs made in particular cells, use these molecules as templates for making the corresponding cDNAs by reverse transcription, and then compare this set of cDNAs with collections oi genomic DNA fragments. DNA technology mz kes such studies possible; with automation, they are easily performed on a large scale. Scientists can now measure the expression of thousands of genes at one time. Currently, the main approach for genome-wide expression studies uses DNA microarray assays. A DNA microarray consists of tiny amounts of a large number of single-stranded DNA fragments representing different genes fixed to a glass slide n a tightly spaced array (grid). (The array is also called a DNA chip by analog}' to a computer chip.) Ideally, these fragments represent all the genes of an organism, as is already possible for organisms whose genomes have been completely sequenced. Figure 20.14 outlines how the DNA fragments on a microarray are tested for hybridization with samples of cDNA molecules prepared from the mRNAs in particular cells of interest 2nd labeled with fluorescent dyes. For example, in one study, researchers performed microarray assays of more than 90% of the genes of C elegans during ever}' stage of its life cycle. The results showed that expression of nearly 60% of the genes changed dramatically during development, and many were expressed in a sex-specific pattern. Such studies illustrate the value of DNA microarrays to reveal general profiles of gene expression over the lifetime of an organism. In addition to uncovering gene interactions and providing clues lo gene function, DNA microarray assays may contribute to a better understanding of certain diseases and suggest new diagnostic techniques or therapies. For example, comparing patterns of gene expression in breast cancer tumors and noncancerous breast tissue has already resulted in more informed and effective treatment protocols. Ultimately, information from DNA microarray assays should provide us a grander view—of how ensembles of genes interact to form a living organism.

Studying Expression of Interacting Groups of Genes

Comparing Genomes of Different Species

A major goal of genomics is to learn how genes act together to produce and maintain a functioning organism. As mentioned earlier, part of the explanation for how humans get along with so few genes probably lies in the complexity of networks of interactions among genes and their products. Once the sequences of entire genomes of several organisms neared completion, some researchers began using these sequences to investigate which genes are transcribed in different situations, such as in different tissues or at different stages of development. They also consider whether groups of genes are expressed in a coordinated manner, with the aim of identifying global patterns or networks of expression. The results of such studies will begin to reveal how genes act together as a functional network in an organism.

The genomes of about 150 species had been completely or almost completely sequenced by the spring of 2004, with many more in progress. Of these, the vast majority are genomes jf prokaryotes, including about 20 archaean genomes. Among the 20 or so eukaryotic species in the group are vertebrates, invertebrates, and plants. The first eukaryotic genome to be completed was that of the yeast Saccharomyces cerevisiae, a single-celled organism; the nematode Caenorhabditis elegans, a simple worm, was the first multicellular organism whose genome was sequenced. The plant Arabidopsis thaliana, another important research organism, has also been completed. Other species whose genomes have been or are currently being sequenced include the honey bee, the dog, the rat, the chicken, and the frog.

400

UNIT THREE

DNA Microarray Assay of Gene Expression Levels With this method, researchers can test thoasands of genes simultaneously to determine which ones are expressed in a particular tissue, under different environmental conditions in various disease states, or at different developmental stages. They can also look for coordinated gene expression.

Tissue sample 0 Isolate mRNA.

Make cDNA by reverse transcription, using fiuorescently labeled nucleotides.

Labeled cDNA molecules (single strands) ۥ* Apply the cDNA mixture to a microarray, a microscope slide on which copies of single-stranded DNA fragments from the organism's genes are fixed, a different gene in each spot. The cDNA hybridizes with any complementary DNA on the microarray. C) Rinse off excess cDNA; scan microarray for fluorescence. Each fluorescent spot (yellow) represents a gene expressed in the tissue sample.

7 DNA microarray

_y

Size of an actual DNA microarray with all the genes of yeast (6,400 spots) •

ttiiiifi^^B^B The intensity of fluorescence at each spot is a measure of the expression of the gene represented by that spot in the tissue sample. Commonly, t w o different samples are tested together by labeling the cDNAs prepared from each sample with a | differently colored fluorescence label. The resulting color at a spot reveals the relative levels of expression of a particular gene in the t w o samples, which may be from different tissues or the same lissue under different conditions.

Comparisons of genome sequences from different species allow us to determine evolutionary relationships between those species. The more similar in sequence a gene is in two species, the more closely related those species are in their evolutionary history. Likewise, comparing multiple genes between species can shed light, on higher groupings of species, which reflect their evolutionary history. Indeed, comparisons of the complete genome sequences of bacteria, archaea, and eukarya strongly support the theory that these are the three fundamental domains of life. In addition to their value in evolutionary biology, comparative genome studies confirm the relevance of research on simpler organisms to our understanding of biology in general and human biology in particular. The similarities between genes of disparate organisms can be surprising, to the point that one researcher now views fruit flies as "little people with wings,'" The yeast genome also is proving quite useful in helping us to understand the human genome. For example, the large amount oi noncoding DNA in the human genome initially hindered the search for regulatory control elements. But comparisons of noncoding sequences in the human genome with those in the much smaller yeast genome revealed regions with highly conserved sequences; these turned out to be important regulatory sequences in both organisms. In another example, several yeast protein-coding genes are so similar to certain human disease genes that researchers have figured out the functions of the disease genes by studying their normal yeast counterparts. Comparing the genomes of two closely related species can also be quite useful, because their genomes are likely to be organized similarly. Once the sequence and organization of one genome is known, it can serve as a scaffold for organizing the DNA sequences from a closely related species as they are determined, greatly accelerating mapping of the second genome. For instance, the mouse genome, which is similar in size to the human genome, was mapped at a rapid pace, with the human genome sequence serving as a guide. This approach is particularly helpful when one of two related species has a much shorter genome than the other. An example is the tsetse fly, Glossina palpalis, which transmits the parasite causing African sleeping sickness. The tsetse fly genome contains 7 X 109 base pairs (more than twice the size of the human genome), but the genome of a closely related fly is only one-tenth as large. Researchers are sequencing the smaller genome first. Then they will set their sights on the much larger tsetse fly genome, focusing on the coding sequences the two species are expected to share. The small number of gene differences between closely related species also makes it is easier to correlate phenotypic differences between the species with particular genetic differences. For example, one gene that is clearly different in humans and chimpanzees appears to function in speech, a characteristic that obviously distinguishes the two species. And the genetic similarity between mice and humans, which share 80% of their genes, can CHAPTER 20

DNA Technology and Genomics

401

be exploited in studying certain human genetic diseases. If researchers know or can hypothesize the organ or tissue in which a defective gene causes a particular disease, they can look for genes that are expressed in these locations in experiments with mice. This approach has revealed several human genes of interest, including one that may be involved in Down syndrome. Other research efforts are under way to extend genomic studies to many more microbial species and to neglected species from diverse branches of the tree of life. These studies will advance our understanding of all aspects of biology, including health, ecology, and evolution.

Scientists are already well on their way to identifying the locations of the several million SNP sites in the hu.nan genome. These will be useful genetic markers for studying human evolution, the differences between human populations, and the migratory routes of human populations throughout history. SNPs and other polymorphisms in noncoding (and coding) DNA will also be valuable markers for identifying disease genes and genes [hat affect our health in more subtle ways. This is likely to change the practice of medicine later in the 21st century. However, applications of DNA research and technology are already affecting our lives in many ways, as we discuss in the final section of the chapter.

Future Directions in Genomics The success in sequencing genomes and studying entire sets of genes is encouraging scientists to attempt similar systematic study of the full protein sets (proteomes) encoded by genomes, an approach called proteomics. For reasons already mentioned, the number of proteins in humans and our close relatives undoubtedly exceeds the number of genes. Because proteins, not genes, actually carry out the activities of ibe cell, we must study when and where they are produced in an organism, and also how they interact, ii we are to understand the functioning of cells and organisms. Assembling and analyzing proteomes pose many experimental challenges, but ongoing technical advances are providing the necessary tools to meet those challenges. Genomics and proteomics are enabling biologists to approach the study of life from an increasingly global perspective. Biologists are now in a position to compile catalogs of genes and proteins—a listing of all the "parts" that contribute to the operation of cells, tissues, and organisms. With such catalogs in hand, researchers are shifting their attention from the individual parts to their functional integration in biological systems. A first step in this systems biology approach is defining gene circuits and protein interaction networks (see Figure 1.10). With the use of computer science and mathematics to process and integrate vast amounts of biological data, researchers can detect and quantify the many combinations of interactions. Another exciting prospect is our increasing understanding of the spectrum of genetic variation in humans. Because the history of the human species is so short, the amount of DNA variaiion among humans is small compared to that of many other species. Most of our diversity seems to be in the form of single nucleotide polymorphisms (SNPs, pronounced "snips"), which are single base-pair variations in the genome, usually detected by sequencing. In the human genome, SNPs occur on average about once in 1,000 base pairs. In other words, if you could compare your personal DNA sequence with that of a person of the same gender—either sitting next to you or on the other side of the world—you would find them to be 99.9% identical.

402

UNIT THREE

Genetics

Concept Check 1. Current estimates are that the human genome contains about 25,000 genes, but there is evidence for many more different human polypeptides. What processes might explain this discrepancy? 2. What is the major value of DNA microarray analysis for studying gene expression? 3. Why is the genetic variation among people so much less than it is among individuals of many other species? For suggested answers, see Appendix A.

Concept

The practical applications of DNA technology affect our lives in many ways DNA technology is in the news almost ever)- day. Most often, the topic of the story is a new and promising application in medicine, but this is just one of numerous fields benefiting from DNA technology and genetic engineering.

Medical Applications One obvious benefit of DNA technology is the identification of human genes whose mutation plays a role in genetic diseases. These discoveries may lead to ways of diagnosing, treating, and even preventing such conditions. DNA technology is also contributing to our understanding of "nongenetic" diseases, from arthritis to AIDS, since a persons genes influence susceptibility to these diseases. Furthermore, diseases of all sorts involve changes in gene expression within the affected cells and often within the patient's immune system. By using DNA microarray assays or other techniques to compare gene expression in healthy and diseased tissues, researchers hope

to find many of the genes that are turned on or off in particular diseases. These genes and their products are potential targets for prevention or therapy.

occur during gamete formation. Therefore, the marker and gene will almost always be inherited together, even though the RFLP marker is not part of the gene. The same principle applies to all kinds of markers, including SNPs.

Diagnosis of Diseases A new chapter in the diagnosis of infectious diseases has been opened by DNA technology, in particular the use of PCR and labeled nucleic acid probes to track down certain pathogens. For example, because the sequence of the HIV genetic material (RNA) is known, PCR can be used to amplify, and thus detect, HIV RNA in blood or tissue samples. RNA cannot be directly amplified by PCR, but the RNA genome is first converted to double-stranded cDNA with reverse transcriptase (RT). PCR is then performed on the cDNA, using a probe specific for one of the HIV genes. This technique, called RT-PCR, is often the best way to detect an otherwise elusive infection.

Human Gene Therapy Gene therapy—the alteration oi an afflicted individual's genes—holds great potential for treating disorders traceable to a single defective gene. In theory, a normal allele of the defective gene could be inserted into the somatic cells of the tissue affected by the disorder. For gene therapy of somatic cells to be permanent, the cells that receive the normal allele must be ones that multiply throughout the patient's life. Bone marrow cells, which include the stem cells that give rise to all the cells of the blood and immune system, are prime candidates. Figure 20.16 outlines one possible procedure for gene therapy of an individual

Medical scientists can now diagnose hundreds of human genetic disorders by using PCR and primers corresponding to cloned disease genes, then sequencing the amplified product to ook for the disease-causing mutation. Among the genes for human diseases that have been cloned are those for sicklecell disease, hemophilia, cystic fibrosis, Huntingtons disease, and Duchenne muscular dystrophy. Affected individuals with suc:h diseases often can be identified before the onset of symptoms, even before birth. It is also possible to identify symptomless carriers of potentially harmful recessive alleles (see Figure 20.10),

Insert RNA version of normal allele into retrovirus.

Let retrovirus infect bone marrow cells that have been removed from the patient and cultured.

Even when a disease gene has not yet been cloned, the presence of an abnormal allele can be diagnosed with reasonable accuracy if a closely linked RFLP marker has been found (Fi jure 20.15). Alleles for Huntington's disease and a number of other genetic diseases were first detected in this indirect way. If the marker and the gene itself are close enough, crossing over between the marker and the gene is very unlikely to

Restriction^ sites

Viral DNA carrying the normal allele inserts into chromosome.

Disease-causing allele

0 Normal allele

Inject engineered cells into patient.

A Figure 20.15 RFLPs as markers for disease-causing

A Figure 20.16 Gene therapy using a retrovirai vector. A

alieles. This diagram depicts homologous segments of DNA from a family in which some members have a genetic disease. In this family, different versions of a RFLP marker are found in unaffected family members and in those w h o exhibit the disease. If a family member has inherited the version of the RFLP marker with t w o restriction sites near the gene (rather than one), there is a high probability that the individual has also inherited the disease-causing allele.

retrovirus that has been rendered harmless is used as a vector in this procedure, which exploits the ability of a retrovirus to insert a DNA transcript of its RNA genome into the chromosomal DNA of its host cell (see Figure 18.10). If the foreign gene carried by the retroviral vector is expressed, the cell and its descendants will possess the gene product, and the patient may be cured. Cells that reproduce throughout life, such as bone marrow cells, are ideal candidates for gene therapy.

CHAPTER 2 o

DNA Technology and Genomics

403

whose bone marrow cells do not produce a vital enzyme because of a single defective gene. One type of severe combined immunodeficiency (SCID) is caused by just this kind of defect. If the treatment is successful, the patients bone marrow cells will begin producing the missing protein, and the patient will be cured.

risk making genetic changes that could be detrimental to the survival of our species in the future? We may have to face this question soon.

The procedure shown in Figure 20.16 was used in the first gene therapy trial for SCID, which began in 1990- But the clinical results from this and succeeding studies during the 1990s did not convincingly demonstrate the effectiveness of the treatment. In another trial that started in 2000, ten young children with SCID were treated by the same procedure. Nine of these patients showed significant, definitive improvement after two years, the first indisputable success of gene therapy. However, two of the patients subsequently developed leukemia, a type of blood cell cancer. Researchers discovered that in both cases, the retroviral vector used to carry the normal allele into bone marrow cells had inserted near a gene involved in the proliferation and development of blood cells, causing leukemia. This and other trials of retrovirus-based gene therapy were temporarily suspended in several countries. By learning more about the behavior of retroviruses, researchers may be able to control the insertion of retroviral vectors to a location that might avoid such problems.

You learned earlier in the chapter about DNA cloning and expression systems for producing large quantities of proteins that are present naturally in only minute amounts. The host cells used in such expression systems can even be engineered to secrete a protein as it is made, thereby simplifying the task of purifying it by traditional biochemical methods. Among the first pharmaceutical products "manufactured" in this way were human insulin and human growth hormone (HGH). Some 2 million people with diabetes in the United States depend on insulin treatment to control their disease. Human growth hormone has been a boon to children born with a form of dwarfism caused by inadequate amounts of HGH. Another important pharmaceutical product produced by genetic engineering is tissue plasminogen activator (TPA). If administered shortly after a heart attack, this protein helps dissolve blood clots and reduces the risk of subsequent heart attacks. The most recent developments in pharmaceutical prodi cts involve truly novel ways to fight certain diseases that do not respond to traditional drug treatments. One approach is the use of genetically engineered proteins that either block or mimic surface receptors on cell membranes. One such experimental drug mimics a receptor protein that HIV binds to while entering white blood cells. The HIV binds to the drug molecules instead and fails to enter the blood cells. DNA technology also can be used to produce vaccines, which stimulate the immune system to defend against specific pathogens (see Chapter 43). Traditional vaccines are of two types: inactivated (killed) microbes and viable but weakened (attenuated) microbes that generally do not cause illness. Most pathogens have one or more specific proteins on their surface that trigger an immune response against it. This type of protein, produced by recombinant DNA techniques, can be used as a vaccine against the pathogen. Alternatively genetic engineering methods can be used to modify the genome of the pathogen to attenuate it.

Several other technical questions are also posed by gene therapy For example, howT can the activity of the transferred gene be controlled so that cells make appropriate amounts of the gene product at the right time and in the right place? How can we be sure that the insertion of the therapeutic gene does not harm some other necessary cell function? As more is learned about DNA control elements and gene interactions, researchers may be able to answer such questions. In addition to technical challenges, gene therapy raises difficult ethical questions. Some critics suggest that tampering with human genes in any way will inevitably lead to the practice of eugenics, a deliberate effort Lo control the genetic makeup of human populations. Other observers see no iundamental difference between the transplantation of genes into somatic cells and the transplantation of organs. Treatment of human germ-line cells in the hope of correcting a defect in future generations raises further ethical issues. Such genetic engineering is routinely done in. laboratory mice, and the technical problems relating to simi]ar genetic engineering in humans will eventually be solved. Under what circumstances, if any should we alter the genomes of human germ lines or embryos? In a way, this could be thought ol as interfering with evolution. From a biological perspective, the elimination of unwanted alleles from the gene pool could backfire. Genetic variation is a necessary ingredient for the survival of a species as environmental conditions change with time. Genes that are damaging under some conditions may be advantageous under other conditions (one example is the sickle-cell allele, discussed in Chapter 14). Are we willing to 404

UNIT THREE

Ceneucs

Pharmaceutical Products

Forensic Evidence In violent crimes, body fluids or small pieces of tissue may be left at the scene or on the clothes or other possessions of the victim or assailant. If enough blood, semen, or tissue is available, forensic laboratories can determine the blood type or tissue type by using antibodies to detect specific cell-surface proteins. However, such tests require fairly fresh samples in relatively large amounts. Also, because many people have the same blood or tissue type, this approach can only exclude a suspect; it cannot provide strong evidence of guilt.

DNA testing, on the other hand, can identify the guilty individual with a high degree of certainty, because the DNA sequence of every person is unique (except for identical twins). RFLP analysis by Southern blotting is a powerful method for detecting similarities and differences in DNA samples and requires only tiny amounts of blood or other tissue (about 1,000 cells). In a murder case, for example, this method can be used to Compare DNA samples from the suspect, the victim, and a small amount of blood found at the crime scene. The forensic scientist usually tests for about five RFLP markers; in other words, only a few selected portions of the DNA are tested. However, even such a small set of markers from an individual car_ provide a DNA fingerprint, or specific pattern of bands, that is of forensic use, because the probability that two people (who are not identical twins) would have the exact same set of RFLP markers is very small. The autoradiograph in Figure 20,17 resembles the type of evidence presented to juries in murder trials. DNA fingerprinting can also be used to establish paternity. A comparison of the DNA of a mother, her child, and the purported father can conclusively settle a question of paternity. Sometimes paternity is of historical interest: DNA fingerprinting has provided strong evidence that Thomas Jefferson or one of his close male relatives fathered at least one of the children of his slave Sally Flemings. Today, instead of RFLPs, variations in the lengths of certain repeated base sequences in simple sequence DNA within the genome are increasingly used as markers for DNA fingerprinting. These repetitive DNA sequences are highly variable irom person to person, providing even more markers than RFLPs. For example, one individual may have the repeat unit ACA repeated 65 times at one genome locus, 118 times at a second locus, and so on, whereas another individual is likely to have different numbers of repeats at these loci. Such polymorphic genetic loci are sometimes called simple tandem repeats (STRs). The greater the number of markers examined in a DNA sample, the more likely it is that the DNA fingerprint is unique to one individual. PCR is often used to amplify particular STRs 01 other markers before electrophoresis. PCR is especially valuable when the DNA is in poor condition or available only ir. minute quantities. A tissue sample as small as 20 cells can be sufficient for PCR amplification. Just how reliable is DNA fingerprinting? In most forensic cases, the probability of two people having identical DNA fingerprints is between one chance in 100,000 and one in a billion. The exact figure depends on the number of markers compared and on the frequency of those markers in the general population. Information on how common various markers are in different ethnic groups is critical because these marker frequencies may vary considerably among ethnic groups and between a particular ethnic group and the population as a whole. With the increasing availability of frequency data, forensic scientists can make extremely accurate statistical

Defendant's blood (D)

Blood from defendant's clothes

Victim's blood (V)

0

=

28

A Figure 20.17 DNA fingerprints from a murder case. This autoradiograph shows that DNA in blood from the defendant's clothes matches the DNA fingerprint of the victim but differs from the DNA fingerprint of the defendant. This is evidence that the blood on the defendant's clothes came from the victim, not the defendant. The three DNA samples were subjected to Southern blotting using radioactive probes (see Figure 20.10). The DNA bands resulting from electrophoresis were exposed to probes for several different RFLP markers in succession, with the previous probe washed off before the next one was applied.

calculations. Thus, despite problems that can still arise from insufficient data, human error, or flawed evidence, DNA fingerprints are now accepted as compelling evidence by legal experts and scientists alike. In fact, DNA analysis on stored forensic samples has provided the evidence needed to solve many "cold cases" in recent years.

Environmental Cleanup Increasingly, the remarkable ability of certain microorganisms to transform chemicals is being exploited for environmental cleanup. Scientists are now engineering these metabolic capabilities into other microorganisms, which are then used to help treat some environmental problems. For example, many bacteria can extract heavy metals, such as copper, lead, and nickel, from their environments and incorporate the metals into compounds such as copper sulfate or lead sulfate, which are readily recoverable. Genetically engineered microbes may become important in both mining minerals (especially as ore reserves are depleted) and cleaning up highly Loxic mining wastes. Biotechnologists are also trying to engineer microbes thai can degrade chlorinated hydrocarbons and other harmful compounds. These microbes could be used in wastewaler treatment plants or by manufacturers before the compounds are ever released into the environment. CHAPTER 20

DNA Technology and Genomics

405

A related research area is the identification and engineering of microbes capable of detoxifying specific toxic wastes found in spills and waste dumps. For example, bacterial strains have been developed that can degrade some of the chemicals released during oil spills. By moving the genes responsible for these transformations into different organisms, bioengineers maybe able to develop strains that can survive the harsh conditions of environmental disasters and help detoxify the wastes.

Agricultural Applications Scientists are working to learn more about the genomes of agriculturally important plants and animals, and for a number of years they have been using DNA technology in an effort to improve agricultural productivity: Animal Husbandry and "Pharm" Animals DNA technology is now routinely used to make vaccines and growth hormones for treating farm animals. On a still largely experimental basis, scientists can also introduce a gene from one animal into the genome of another animal, which is then called a transgenic animal. To do this, they first remove egg cells from a female and fertilize them in vitro. Meanwhile, they have cloned the desired gene from another organism. They then inject the cloned DNA directly into the nuclei of the fertilized eggs. Some of the cells integrate the foreign DNA, the transgene, into their genomes and are able to express the foreign gene. The engineered embryos are then surgically implanted in a surrogate mother. If an embryo develops successfully, the result is a transgenic animal, containing a gene from a third "parent" that may even be of another species. The goals of creating a transgenic animal are often the same as the goals of traditional breeding—for instance, to make a sheep with better quality wool, a pig with leaner meat, or a cow that will mature in a shorter time. Scientists might, for example, identify and clone a gene that causes the development of larger muscles (muscles make up most of the meat we eat) in one variety of cattle and transfer it to other cattle or even to sheep. Transgenic animals also have been engineered to be pharmaceutical "factories"—producers of a large amount of an otherwise rare biological substance for medical use. For example, a transgene for a desired human protein, such as a hormone or blood-clotting factor, can be inserted into the genome of a farm mammal in such a way that the transgene's product is secreted in the animal's milk (Figure 20.18). The protein can then be purified from the milk, usually more easily than from a cell culture. Recently, researchers have engineered transgenic chickens that express large amounts of the transgene's product in eggs. Their success suggests that transgenic chickens may emerge as relatively inexpensive pharmaceutical factories in the near future. 406

UNIT THREE

& Figure 20.18 "Pharm" animals. These transgenic sheep carry a gene for a human blood protein, which they secrete in their milk. This protein inhibits an enzyme that contributes to lung damage in patients with cystic fibrosis and some other chronic respiratory diseases. Easily purified from the sheep's milk, the protein is currently under evaluation as a treatment for cystic fibrosis.

Human proteins produced by farm animals may differ in some ways from the corresponding natural human proteins. Thus, these proteins must be tested very carefully to ensure that they will not cause allergic reactions or other adverse effects in patients receiving them. Also, the health and welfare of farm animals carrying genes from humans and other foreign species are important issues; problems such as low fertility or increased susceptibility to disease are not uncommon. Genetic Engineering in Plants Agricultural scientists have already endowed a number of crop plants with genes for desirable traits, such as delayed ripening and resistance to spoilage and disease. In one striding way, plants are easier to genetically engineer than mcst animals. For many plant species, a single tissue cell grown in culture can give rise to an adult plant (see Figure 21.5). Thus, genetic manipulations can be performed on a single cell and the cell then used to generate an organism with new traits. The most commonly used vector for introducing new genes into plant cells is a plasmid, called the Ti plasmid, from the soil bacterium Agrobacterium tumejacicns. This plasmid integrates a segment of its DNA, known as T DNA, into the chromosomal DNA of its host plant cells. For vector purposes, researchers work with a version of the plasmid that does not cause disease, as the wild-type version does. Figure 20.19 outlines one method for using the Ti plasmid to produce transgenic plants. Scientists can introduce recombmant Ti plasmids into plant cells by electroporation. Alternatively, the recomblnant plasmid can be put back into Agrobacterium; susceptible plants or plant cells growing in culture are then infected with bacteria that contain the recombinant plasmid.

Using the Ti Plasmid to oduce Transgenic Plants Genes conferring useful traits, such as pest resistance, herbicide resistance, delayed ripening, and increased nutritional value, can be transferred from one plant variety or to another using the Ti plasmid as a vector.

Agrobacterium

tumefaciens

The Ti plasmid is isolated from the bacterium Agrobacterium tumefaciens. The segment of the plasmid that integrates into the genome of host cells is called T DNA.

Isolated plasmids and foreign DNA containing a gene of interest are incubated with a restriction enzyme that cuts in the middle of T DNA. After base pairing occurs between the sticky ends of the plasmids and foreign DNA fragments, DNA ligase is added. Some of the resulting stable recombinant plasmids contain the gene of interest.

destructive microbes and insects have reduced the need for chemical insecticides. Genetic engineering also has great potential for improving the nutritional value of crop plants. For instance, scientists have developed transgenic rice plants that produce yellow rice grains containing beta-carotene, which our body uses to make vitamin A (see Figure 38.16). This "golden" rice could help prevent vitamin A deficiency in the half of the world's population that depends on rice as a staple food. Currently, large numbers of young children in Southeast Asia suffer from vitamin A deficiency, which leads to vision impairment and increases susceptibility to disease. In a novel twist, the pharmaceutical industry is beginning to develop "pharm" plants, analogous to ;'pharm" animals. Although natural plants have long been sources of drugs, researchers are now creating plants that make human proteins for medical use and viral proteins for use as vaccines. Several such products are being tested in clinical trials, including vaccines for hepatitis B and an antibody produced in transgenic tobacco plants that interferes with the bacteria that cause tooth decay. Large amounts of these proteins might be produced more economically by plants than by cultured cells.

Safety and Ethical Questions Raised by DNA Technology

Recombinant plasmids can be introduced into cultured plant cells by electroporation. Or plasmids can be returned to Agrobacterium, which is then applied as a liquid suspension to the leaves of susceptible plants, infecting them. Once a plasmid is taken into a plant cell, its T DNA integrates into the cell's chromosomal DNA.

iransformed cells carrying the transgene of interest can regenerate complete plants mat exhibit the new trait conferred by the transgene. Plant with new trait

Genetic engineering is rapidly replacing traditional plantbreeding programs, especially for useful traits, such as herbicide or pest resistance, determined by one or a few genes. For example, crops engineered with a bacterial gene making the plants resistant to herbicides can grow while weeds are destroyed. Similarly, genetically engineered crops that can resist

Early concerns about potential dangers associated with recombinant DNA technology focused on the possibility thai hazardous new pathogens might be created. What might happen, for instance, if cancer cell genes were transferred into bacteria or viruses? To guard against such rogue microbes, scientists developed a set of guidelines that were adopted as formal government regulations in the United States and some other countries. One safety measure is a set of strict laboratory procedures designed to protect researchers from infection by engineered microbes and to prevent the microbes from accidentally leaving the laboratory. In addition, strains of microorganisms to be used in recombinant DNA experiments are genetically crippled to ensure thaL they cannot survive outside the laboratory. Finally, certain obviously dangerous experiments have been banned. Today, most public concern about possible hazards centers not on recombinant microbes but on genetically modified (GM) organisms used as food. In common language, a GM organism is one that has acquired by artificial means one or more genes from the same or another species. Salmon, for example, have been genetically modified by addition of a more active salmon growth hormone gene. However, the majority oi the GM organisms that contribute to our food supply are not animals, but crop plants. Some countries have been war}7 of the GM revolution, with, the safety of GM foods and possible environmental consequences of growing GM plants being the major concerns. In CHAPTER 20

DNA Technology and Genomics

407

1999, for instance, the European Union suspended the introduction of new GM crops pending new legislation. Early in 2000, negotiators from 130 countries (including the United

States) agreed on a Biosafety Protocol that requires exporters to identify GM organisms present in bulk food shipments and allows importing countries to decide whether the products pose environmental or health risks. Advocates of a cautious approach toward GM crops fear that transgenic plants might pass their new genes to close relatives in nearby wild areas. We know that lawn and crop grasses, for example, commonly exchange genes with wild relatives via pollen transfer. If crop plants carrying genes for resistance to herbicides, diseases, or insect pests pollinated wild ones, the offspring might become "super weeds" that are very difficult to control. Another possible hazard, suggested by one laboratory-based study, is that a transgene encoding a pesticide-type protein might cause plants to produce pollen toxic to butterflies. However, scientists with the Agricultural Research Service concluded from a two-year study that butterflies were unlikely to be exposed to toxic levels of pollen. As for the risks to human health from GM foods, some people fear that the protein products of transgenes might lead to allergic reactions. Although there is some evidence that this could happen, advocates claim lhat these proteins could be tested for their ability to cause allergic reactions. Today, governments and regulatory agencies throughout the world are grappling with how to facilitate the use of biotechnology in agriculture, industry, and medicine while ensuring that new products and procedures are safe. In the United States, such applications of biotechnology are eval-

Chapter Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

uated for potential risks by various regulatory agencies, including the Food and Drug Administration, Environmental Protection Agency, National Institutes of Health, and Department of Agriculture. These agencies are under increasing pressure from some consumer groups. Meanwhile, these same agencies and the public must consider the ethical implications of biotechnology Completion of the mapping of the human genome, for instance, raises significant ethical questions. Who should have the right to examine someone else's genes? How should that information be used? Should a person's genome be a factoi in suitability for a job or eligibility for insurance? Ethical considerations, as well as concerns about potential environmental and health hazards, will likely slow some applications of biotechnology There is always a danger that too much regulation will stifle basic research and its potential benefits. However, the power of DNA technology and genetic engineering— our ability to profoundly and rapidly alter species that have been evolving for millennia—demands that we proceed with humility and caution.

Concept Check 1. What is the advantage oi using stem cells for gene therapy? 2. List at least three different properties that have been acquired by crop plants via genetic engineering. For suggested answers, see Appendix A.

Review other DNA molecules; sealing of the base-paired fragments with DNA ligase produces recombinant DNA molecules. Activity Restriction Enzytnes

(pp. 385-386) DNA cloning and other techniques, collectively termed DNA technology, can be used to manipulate and analyze DNA and to produce useful new products and organisms. Activity Applications of DNA Technology

• Cloning a Eukaryotic Gene in a Bacterial Plasmid (pp. 386-388) A recombinant plasmid is made by inserting restriction fragments from DNA containing a gene of interest into a plasmid vector thai has been cut open by the same enzyme. Gene cloning results when the recombinant plasmid is introduced into a host bacterial cell and the foreign genes are replicated along with the bacterial chromosome as the host cell reproduces. A clone of cells carrying the gene of interest can be identified with a radioactively labeled nucleic acid probe that has a sequence complementary to the gene. Activity Cloning a Gene in Bacteria Investigation How Can Antibiotic-Resistant Plasmicts Transform E. coli?

• Using Restriction Enzymes to Make Recombinant DNA (p. 386) Bacterial restriction enzymes cut DNA molecules within short, specific nucleotide sequences to yield a set of double-stranded DNA fragments with single-stranded sticky ends. The sticky ends on fragments from one DNA source can base-pair with complementary sticky ends on fragments from

• Storing Cloned Genes in DNA Libraries (pp. 388-390) A genomic library is the collection of recombinant vector clones produced by cloning DNA fragments derived from an entire genome. A cDNA (complementary DNA) library is made by cloning DNA made in vitro by reverse transcription of all the mRNA produced by a particular kind of cell.

SUMMARY OF KEY CONCEPTS

DNA cloning permits production of multiple copies of a specific gene or other DNA segment • DNA Cloning and Its Applications: A Preview

408

U N I T

T H R E E

390-391) • Cloning and Expressing Eukaryotic Genes (pp. 39C Several technical difficulties hinder the expression, of clone eukaryotic genes in bacterial host cells. The use of cultured eukaryotic cells as host cells and yeast artificial chromosomes (YACs) as vectors helps avoid these problems. • Amplifying DNA in Vitro: The Polymerase Chain Reaction (PCR) (pp. 391-392) PCR can produce many copies of a specific target segment of DNA, using primers that bracket the desired sequence and a heat-resistant DNA pofyrnerase.

Restriction fragment analysis detects DNA differences thsit affect restriction sites • Gel Electrophoresis and Southern Blotting ,pp. 392-394) DNA restriction fragments of different lengths zan be separated by gel electrophoresis. Specific fragments can be identified by Southern blotting, using labeled probes that hybridize to the DNA immobilized on a "blot" of the gel. Activity Gel Electrophoresis of DNA • Restriction Fragment Length Differences as Genetic Markers (p. 394) Restriction fragment length polymorphisms (RFLPs) are differences in DNA sequence on homologous chromosomes that result in restriction fragments of different lengths, which can be detected by Southern blotting. The thousands of RFLPs present throughout eukaryotic DNA can serve as genetic markers. Activity Analyzing DNA Fragments Using Gel Electrophoresis Investigation How Can Gel Electrophoresis Be Used to Analyze DNA?

Entire genomes can be mapped at the DNA level • Genetic (Linkage) Mapping: Relative Ordering of Markers (p. 396) The order of genes and other inherited markers in the genome and the relative distances between them can be determined from recombination frequencies. • Physical Mapping: Ordering DNA Fragments (p. 396) A physical map is constructed by cutting a DNA molecule into many short fragments and arranging them in order by identifying overlaps. A physical map gives the actual distance in base pairs between markers. • DNA Sequencing (pp. 396-398) Relatively short DNA fragments can be sequenced by the dideoxy chain-termination method, which can be performed in automated sequencing machines. Activity The Human Genome Project: Genes on Human Chromosome 17

Genome sequences provide clues to important biological questions • Identifying Protein-Coding Genes in DNA Sequences (p. 399) Computer analysis of genome sequences helps researchers identify sequences that are likely to encode proteins. Current estimates are thai, the human genome contains about 25,000 genes, but the number of human proteins is much larger. Comparison of the sequences of "new" genes with those of known genes in other species may help identify new genes. • Determining Gene Function (p. 400) For a gene of unknown function, expenmental inactivation of the gene and observation of die resulting phenotypic effects can provide clues to its function.

• Studying Expression of Interacting Groups of Genes (p. 400) DXA nucroa::".;\ assays allow researchers to compare patterns of gene expression in different tissues, at different times, or under different conditions. • Comparing Genomes of Different Species (pp. 400-402) Comparative studies of genomes from related and widely divergent species are providing valuable information in many fields of biology. • Future Directions in Genomics (p. 402) Genomics is the systematic study of entire genomes; proteomics is the systematic study of all the proteins encoded by a genome. Single nucleotide polymorphisms (SNPs) provide useful markers for studying human genetic variation.

The practical applications of DNA technology affect our lives in many ways • Medical Applications (pp. 402-404) DNA technology is increasingly being used in the diagnosis of genetic and other diseases and offers potential for better treatment of certain genetic disorders or even permanent cures. • Pharmaceutical Products (p. 404) Large-scale production of human hormones and other proteins with therapeutic uses, including safer vaccines, are possible with DNA technology. • Forensic Evidence (pp. 404-405) DNA "fingerprints" obtained by analysis of tissue or body fluids found at crime scenes can provide definitive evidence that a suspect is guilty or not. Such fingerprints are also useful in parenthood disputes. Activity DNA Fingerprinting • Environmental Cleanup (pp. 405-406) Genetic engineering can be used to modify the metabolism of microorganisms so that they can be used to extract minerals from die environment or degrade various types of potentially toxic waste materials. • Agricultural Applications (pp. 406-407) The aim of developing transgenic plants and animals is to improve agricultural productivity and food quality. • Safety and Ethical Questions Raised by DNA Technology (pp. 407—408) The potential benefits of genetic engineering must be carefully weighed against the potential hazards of creating products or developing procedures that are harmful to humans or the environment. Activity Making Decisions About DNA Technology: Golden Rice

TESTING YOUR KNOWLEDGE Evolution Connection If DNA-based technologies become widely used, how might they change the way evolution proceeds, as compared with the natural evolutionary mechanisms of the past 4 billion years?

Scientific Inquiry You hope to study a gene that codes for a neurotransmitter protein in human brain cells. You know the arruno acid sequence of the protein. Explain how you might (a) identify the genes expressed in CHAPTER 20

DNA Technology and Genomics

409

a specific type of brain cell, (b) identify the gene for the neurotransmitter, (c) produce multiple copies of the gene for study, and (d) produce a quantity of the neurotransmitter for evaluation as a potential medication. Investigation How Can Antibiotic-Resistant Plasmids Transform E. coli? Investigation How Can Gel Electrophoresis Be Used to Analyze DNA?

410

U M IT

THREE

Science, Technology, and 5ociety Is there danger of discrimination based on testing for "harmful" genes?

What policies can you suggest that would prevent such abuses?

'

Genetic Basis of Development A Figure 21.1 Mutant Drosophila with an extra small eye on its antenna.

21.1 Embryonic development involves cell division, cell differentiation, and morphogenesis 21*2 Different cell types result from differential gene expression in cells with the same DNA 21.3 Pattern formation in animals and plants results from similar genetic and cellular mechanisms 21,4 Comparative studies help explain how the evolution of development leads to morphological diversity

From Single Cell to Multicellidar Organism

T

his chapter applies much of what you've learned about molecules, cells, and genes to one of biology's mosl important questions—how a complex multicellular organism develops from a single cell. The application of genetic analysis and DNA technology to the study of development has revolutionized the field. In much the same way that researchers have used mutations to deduce pathways of cellular metabolism, they now use mutations to dissect developmental pathways. In one striking example, Swiss researchers demonstrated in 1995 that a particular gene functions as a master switch that triggers development cf the eye in Drosophila. The scanning electron micrograph in Figure 21.1 shows part of the head of an abnormal fly that has a small extra eye on each antenna. In this fly, the

master gene triggering eye development was expressed in an abnormal body location, causing extra eyes. A similar gene activates eye development in mice and other mammals. In fact, developmental biologists are discovering remarkable similarities in the mechanisms that form diverse organisms. The scientific study of development got under way about 130 years ago, around the same time as genetics. But for decades, the two disciplines proceeded along mostly separate paths. Developmental biologists focused on embryology the study of the stages of development leading from a fertilized egg to a fully formed organism. They studied animals that lay their eggs in water, including marine invertebrates and freshwater vertebrate amphibians, such as frogs. By studying these and other animals, as well as plants, biologists worked out a description of animal development (see Chapter 47) and plant development (see Chapter 35) at the macroscopic and microscopic levels. In recent years, scientists have applied the concepts and tools of molecular genetics to the study of developmental biology with remarkably fruitful results. In this chapter, we introduce some of the basic mechanisms that control development in animals and plants, focusing on. what has been learned from molecular and genetic studies. After introducing the basic cellular processes underlying development, we consider how cells become different from each other and the factors that establish the spatial pattern of these different types of cells in the embryo. Next, we examine in more detail the molecular basis of several specific developmental phenomena as examples of some general principles of development. Finally, we discuss what researchers can learn about evolution from comparing developmental processes in different species.

411

Model Organisms for Genetic Studies of Development DROSOPHILA MELANOGASTER (FRUIT FLY)

Researchers can draw on a wealth of information, about the fruit fly Drospplu n referred to simply as Dw$op Figure 21.18 Molecular basis of apoptosis in C. elegans. Three proteins, Ced-3, Ced-4, and Ced-9, are critical to apoptosis and its regulation in the nematode. Apoptosis is more complicated in mammals but involves proteins similar to those in the nematode.

Interdigital tissue

A I igure 21.19 Effect of apoptosis during paw development in the mouse. In mice, humans, and other mammals, as v.'ell as land birds, the embryonic region that develops into feet or hands initially has a solid,

platelike structure. Apoptosis eliminates the cells in the interdigital regions, thus forming the digits. The embryonic mouse paws shown here are stained so that cells undergoing apoptosis appear bright green. Apoptosis of cells begins

cancers result from a failure of cell suicide. Cells that have suffered irreparable damage, including DNA damage that could lea.I to cancer, normally generate internal signals that trigger api >ptosis. Studies on the roles of induction and apoptosis during development of C. elegans were begun less than 30 years ago by Sydney Brenner, John E. Sulston, and H. Robert Horvitz. The importance of their studies was highlighted in 2003, when the Nobel Prize for Medicine was awarded to these researchers for significantly advancing our understanding of how genes regulate organ growth (.such as the nematode vulva) and the process of programmed cell death.

at the margin of each interdigital region (left), peaks as the tissue in these regions is reduced (middle), and is no longer visible when the interdigital tissue has been eliminated.

mechanisms regulating development are cell-signaling (indue Lion) and transcriptional regulation. The embryonic development of most plants occurs inside the seed and thus is relatively inaccessible to study (a mature seed already contains a fully formed embryo). However, other important aspects of plant development are observable throughout a plant's life in its meristems, particularly the apical meristems at the tips of shoots. It is there that cell division, morphogenesis, and differentiation give rise to new organs, such as leaves or the petals of flowers. We'll discuss two aspects of pattern formation in floral meristems, the apical meristems that produce flowers. Pattern Formation in Flowers

Plant Development: Cell Signaling amd Transcriptional Regulation The genetic analysis of plant development, using model organisms such as Arabidopsis (see Figure 21.2), has lagged behind that of animal models simply because there are fewer researchers working on plants. For example, in 2000, when the Arabidopsis DNA sequence was completed, fewer than 5% o its genes had been defined by mutational analysis, whereas over 25% of the genes in both Drosophila and C. elegans had been identified in that v, ay We are just beginning to understand the molecular basis of plant development in detail. Thanks to DNA technology and c ues from animal research, plant research is now progressing rapidly.

Environmental signals, such as day length and temperature, trigger signal transduction pathways that convert ordinary shoot meristems to floral meristems, causing a plant to flower. Researchers have combined a genetic approach with tissue transplantation to study induction in the development of tomato flowers. As shown in Figure 21.20, a floral meristem is a bump consisting of three layers of cells (LI—13) - All three layers participate in the formation of a flower, a reproductive

S lechanisms of Plant Development In general, cell lineage is much less important for pattern formation in plants than in animals. As mentioned previously, many plant cells are totipotent, and their fates depend more on positional information than on cell lineage. Therefore, the major

Floral meristem

Tomato flower

• Figure 21.20 Flower development. A flower develops from three cell layers (L1-L3) in a floral meristem. A specific pattern of cell division, differentiation, and enlargement produces a flower. The four types of organs (carpels, stamens, petals, and sepals) that make up a flower are arranged in concentric circles (whorls). Each species has a characteristic number of organs in each whorl. The tomato has six sepals, six petals, six stamens, and four carpels. CHAPTER 2i

The Genetic Basis of Development

429

structure with four types of organs: carpels (containing egg cells), stamens (containing sperm-bearing pollen), petals, and sepals (leaflike structures outside the petals). In a mature plant, the four types of organs are arranged radially, rather than linearly like the body structures in Drosophila. Tomato plants homozygous for a mutant allele called fasciated (j) produce flowers with an abnormally large number of organs. To study what controls the number of organs, researchers performed the grafting experiment outlined in Figure 21.21. They grafted stems from fasciated plants onto those of wild-type plants (F£ homozygous for normal allele) and then grew new plants from the shoots that sprouted near the graft sites. Many of the new plants were chimeras, organisms with a mixture of genetically different cells. Some chimeras produced floral meristems in which the three cell layers did not all come from the same "parent." The researchers identified the parental sources of the meristem layers by monitoring other genetic markers, such as an unrelated mutation causing yellow leaves. The results showed that whether the number of floral organs was normal or abnormally high depended on whether the L3 layer arose from wild-type or mutant cells. Thus, the 13 cell layer induces the overlying L2 and LI layers to form a particular number of organs. The mechanism of cell-cell signaling leading to this induction is not yet known, but is currently under study. In contrast to genes controlling organ number in flowers are genes controlling organ identity. An organ identity gene determines the type of structure that will grow from a meristem—for instance, whether a particular outgrowth from a floral meristem becomes a petal or a stamen. Most of what we know about such organ identity genes comes from research on flower development in Ambidopsis. Organ identity genes are analogous to homeotic genes in animals and are often referred to as plant homeotic genes. Just as a mutation in a fruit fly homeotic gene can cause legs to grow in place of antennae, a mutation in an organ identity gene can cause carpels to grow in place of sepals. By collecting and studying mutants with abnormal flowers, researchers have been able to identify and clone a number of floral organ identity genes. In plants with a "homeotic" mutation, specific organs are missing or repeated (Figure 21.22). Some of these mutant phenotypes are reminiscent of those caused by hicoid or other pattern formation mutations in Drosophila. Like the homeotic genes of animals, the organ identity genes of plants encode transcription factors that regulate specific target genes by binding to their enhancers in the DNA. In Chapter 35, you will learn about a current model of how these genes control organ development. Clearly, the developmental mechanisms used by plants are similar to those used by the two animal species we discussed earlier. In the next section, we will see what can be learned from comparing developmental strategies and molecular mechanisms across all multicellular organisms. 430

UNIT THREE

Genetics

Figure 21.21

Inquiry Which cell layers in the floral meristem determine the number of floral organs' EXPERIMENT Tomato plants with the fasciated (ff) mutation develop extra floral organs.

Fasciated (ff), extra organs Researchers grafted stems from mutant plants onto wild-type plants. They then planted the shoots that emerged near the graft site, many of which were chimeras.

For each chimera, researchers recorded the flower phenotype: wild-type or fasciated. Analysis using other genetic markers identified the parental source for each of the three cell layers of the floral meristem (L1-L3) in the chimeras. The flowers of the chimeric plants had the fasciated phenotype only when the L3 layer came from the fasciated parent. ______ L1

L__ - _ —J HI

Wild-type (FF) Fasciated (ff)

Plant

Flower

Phenotype

Wild-type parent

Wild-type

Fasciated (ff) parent

Fasciated

Floral Meristem

Fasciated Fasciated

• «.»MHMI.h'«

layer induce the L1 and L2 layers to form flowers with a particular number of organs. (The nature of the inductive signal from L3 is not entirely understood.)

Widespread Conservation of Developmental Genes Among Animals

Wild type

Mutant

Molecular analysis of the homeotic genes in Drosophila has shown that they all include a 180-nucleotide sequence called a homeobox, which specifies a 60-amino-acid homeodomain in the protein. An identical or very similar nucleotide sequence has been discovered in the homeotic genes of many invertebrates and vertebrates. In fact, the vertebrate genes homologous to the homeotic genes of fruit flies have even kept their chromosomal arrangement (Figure 21.23). (Homeotic genes in

• Figure 21.22 Mutations in floral organ identity genes. Wild-type Arabidopsis has four sepals, four petals, six stamens, and two carpels. If an organ identity gene called apetala2 is mutated, the identities of the organs in the four whorls are carpels, stamens, stamens, and carpels (there are no petals or sepals).

Adult fruit fly

Concept Check 1. Why are fruit ily maternal effect genes also called egg-polarity genes? 2. li a researcher removes the anchor cell from a C. elegdtxs embryo, the vulva does not form, even though all the cells that would have made the vulva are present. Explain why. 3. Explain why cutting and rooting a shoot from a plant, then planting it successfully, provides evidence that plant cells are totipotent.

Fruit fly embryo (10 hours)

Fly chromosome

Mouse chromosomes

For suggested answers, see Appendix A.

Mouse embryo (12 days)

Comparative studies help explain how the evolution of development leads to morphological diversity Biologists in the field of evolutionary developmental biology or "evo-devo" as it is often called, compare developmental processes of different multicellular organisms. Their aim is to understand how developmental processes have evolved and how changes in these processes can modify existing organismal features or lead to new ones. With the advent of molecular techniques and the recent flood of genomic information, we are beginning to realize that the genomes of related species With strikingly different forms may have only minor differences in gene sequence or regulation- Discovering the molecular basis underlying these differences, in turn, helps us understand how the myriad of diverse forms that cohabit this planet have arisen, thus informing the study of evolution.

A Figure 21.23 Conservation of homeotic genes in a fruit f l y a n d a m o u s e . Homeotic genes that control the form of anterior and posterior structures of the body occur in the same linear sequence on chromosomes in Drosophiia and mice. Each colored band on the chromosomes shown here represents a homeotic gene. In fruit flies, all homeotic genes are found on one chromosome. The mouse and other mammals have the same or similar sets of genes on four chromosomes. The color code indicates the parts of the embryos in which these genes are expressed and the adult body regions that result. All of these genes are essentially identical in flies and mice, except for those represented by blark bands, which are less similar in the two animals.

CHAPTER 21

The Genetic Basis of Development

431

animals are often called Hox genes.) Furthermore, related sequences have been found in regulatory genes of much more distantly related eukaryotes, including plants and yeasts, and even in prokaryotes. From these similarities, we can deduce that the homeobox DNA sequence evolved very early in the history of life and was sufficiently valuable to organisms to have been conserved in animals and plants virtually unchanged for hundreds of millions of years. Not all homeobox-containing genes are homeotic genes; that is, some do not directly control the identity of body parts. However, most of these genes, in animals at least, are associated with development, suggesting their ancient and fundamental importance in lhat process. In Drosophila, for example, homeoboxes are present not only in the homeotic genes but also in the egg-polarity gene hicoid, in several of the segmentation genes, and in the master regulatory gene for eye development. Researchers have found that the homeobox-encoded homeodomain is the part of a protein that binds to DNA when the protein functions as a transcriptional regulator. However, the shape of the homeodomain allows it to bind to any DNA segment; by itself it cannot select a specific sequence. Rather, more variable domains in a homeodomain-contaiuing protein determine which genes the protein regulates. Interaction of these latter domains with still other transcription factors helps a homeodomain-containing protein recognize specific enhancers in the DNA. Proteins with homeodomains probably regulate development by coordinating the transcription oi batteries of developmental genes, switching them on or off. In embryos of Drosophila and other animal species, different combinations of homeobox genes are active in different parts

* Figure 21.24 Effect of differences in Hox gene expression during development in crustaceans and insects. Changes in the expression patterns of four Hox genes have occurred over evolutionary time. These changes account in part for the different body plans of the brine shrimp Artemia, a crustacean (top), and the grasshopper, an insect. Shown here are regions of the adult body color-coded for expression of the Hox genes that determine formation of particular body parts during embryonic development.

432

U N I T THREE

Genetics

of the embryo. This selective expression of regulatory genes, varying over time and space, is central to pattern formation. Developmental biologists have found that in addition to homeotic genes, many other genes involved in development are highly conserved from species to species. These include numerous genes encoding components of signaling pathways. The extraordinary similarity among particular developmental genes in different animal species raises the question, How can the same genes be involved in the development of animals whose forms are so very different from each other? Current studies are suggesting likely answers to this question. In some cases, small changes in regulatory sequences of particular genes can lead to major changes in body form. For example, the differing patterns of expression of the Hox genes along the body axis in insects and crustaceans can explain the different number of leg-bearing segments among these segmented animals (Figure 21.24). In other cases, similar genes direct different developmental processes in different organisms, resulting in different body shapes. Several Hox genes, for instance, are expressed in the embryonic and larval stages of the sea urchin, nonsegmented animals that have a body plan quite different from insects and mice. Sea urchin adults make the pincushion-shaped shells you may have seen on the beach. They are among the organisms long used in classical embryological studies (see Chapter 47). Sequencing of the Arabidopsis genome has revealed that plants do have some homeobox-containing genes. However, these apparently do not lunction as master regulatory switches as do the homeobox-containing homeotic genes in animals. Other genes appear to carry out basic processes oi pattern formation in plants.

Abdomen

Comparison of Animal and Plant Development The last common ancestor of plants and animals was probably a single-celled microbe living hundreds of millions of years ago, so ihe processes of development must have evolved independently in the two lineages of organisms. Plants evolved with rigid cell walls that make the movement of cells and tissue layers virtually impossible, ruling out the morphogenetic movements of cells and tissues that are important in animals. Instead, morphogenesis in plants relies more heavily on differing planes of cell division and on selective cell enlargement. (You will learn about these processes in Chapter 35.) But despite the differences between plants and animals, there are some basic similarities in the actual mechanisms of development—legacies ol their shared cellular origins. In both plants and animals, development relies on a cascade of transcriptional regulators turning on or turning off genes in a finely tuned series—for example, setting up the head-to-tail axis in Drosophila and establishing the organ iduitLnee- in a radial pattern in the Arabidopi,is, 'lower. But the genes that direct these processes differ considerably in plants and animals. While quite a few of the master regulatory switches in Drosophila are homeobox-containing Hox genes, those in Arabidopsis belong to a completely different family of genes, called the Mads~box genes. And although

Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY

Embryonic development involves cell division, cell differentiation, and morphogenesis • In addition to mitosis, embryonic cells undergo differentiation, becoming specialized in structure and function. Morphogenesis encompasses the processes that give shape to the organism and its various parts. Several model organisms are commonly used to study different aspects of the genetic basis of development (pp. 412-415).

Different cell types result from differential gene expression in cells with the same DNA * Evidence for Genomic Equivalence (pp. 415-418) Cells differ in structure and function not because they contain different genes but because they express different portions of a common genome; they have genomic equivalence. Differentiated cells from mature plants are often totipotent, capable of generating a complete new plant. The nucleus from a differentiated animal cell can sometimes give rise to a new animal if transplanted

homeobox-containing genes can be found in plants and Mads-box genes in animals, in neither case do they perform the same major roles in development that they do in the other group. In this final chapter of the genetics unit, you have learned how genetic studies can reveal much about the molecular and cellular mechanisms underlying development. The unity of life is reflected in the similarity of biological mechanisms used to establish body pattern, although the genes directing development may differ among organisms. The similarities reflect the common ancestry of life on Earth. But the differences are also crucial, for they have created the huge diversity of organisms that have evolved. In the remainder of the book, we expand our perspective beyond the level of molecules, cells, and genes to explore this diversity on the organismal level.

Concept Check i 1. The DNA sequences called homeoboxes, which help homeotic genes in animals direct development, are common to flies and mice. Given this similarity, explain why these animals are so different. For suggested answers, see Appendix A.

to an enucleated egg cell. Pluripotent stem cells from animal embryos or adult tissues can reproduce and differentiate in vitro as well as in vivo, offering the potential for medical use. • Transcriptional Regulation of Gene Expression During Development (pp. 418-420) Differentiation is heralded by the appearance of tissue-specific proteins. These proteins enable differentiated cells to carry out their specialized roles. • Cytoplasmic Determinants and Cell-Cell Signals in Cell Differentiation (p. 420) Cytoplasmic determinants in the cytoplasm of the unfertilized egg regulate the expression of genes in the zygote that affect the developmental fate of embryonic cells. In the process called induction, signal molecules from embryonic cells cause transcriptional changes in nearby target cells. Activity Signal Transduction Pathways

Pattern formation in animals and plants results from similar genetic and cellular mechanisms • Pattern formation, the development of a spatial organization of tissues and organs, occurs continually in plants, but is mostly limited to embryos and juveniles in animals. Positional information, the molecular cues that control pattern formation, tell a cell its location relative to the body's axes and to other cells (p. 42 L). • Drosophila Development: A Cascade of Gene Activations (pp. 421—425) After fertilization, positional information on an increasingly fine scale specifies the segments in Drosophila and CHAPTER 21

The Genetic Basis oi Development

433

finally triggers the formation of each segments characteristic structures. Gradients of morphogens encoded by maternal effect genes, such as bicoid, produce regional differences in the sequential expression of three sets of segmentation genes, the products of which direct the actual formation of segments. Finally, master regulator)' genes, called homeotic genes, specify the type of appendages and other structures that form on each segment. Transcription factors encoded by the homeotic genes are regulatory proteins that control the expression of genes responsible for specific anatomical structures. Activity Role o/bicoid Gene in Drosophila Development Investigation How Do bicoid Mutations Alter Development? • C. elegans: The Role of Cell Signaling (pp. 425-429) The complete lineage of each ceil in C elegans is known. Cell signaling and induction are critical in determining worm cell fates, including apoptosis (programmed cell death). An inducing signal produced by one cell in the embryo can inidate a chain of inductions that results in the formation of a particular organ, such as the intestine or vulva. In apoptosis, precisely timed signals trigger the activation of a cascade of "suicide" proteins in the cells destined to die. • Plant Development: Cell Signaling and Transcriptional Regulation (pp. 429^-31) Induction by cell-cell signaling helps determine the numbers of floral organs that develop from a floral meristem. Organ identity genes determine the type of structure (stamen, carpal, sepal, or petal) that grows from each whorl of a floral meristem. The organ identity genes apparently act as master regulatory genes, each controlling the activity of other genes that more directly bring about an organs structure and function.

Comparative studies help explain how the evolution of development leads to morphological diversity • Widespread Conservation of Developmental Genes Among Animals (pp. 431-432) Homeotic genes and some other genes associated with animal development contain a homeobox region, whose sequence is identical or similar in diverse species. Related sequences are present in the genes of yeasts, plants, and even prokaryotes. Other developmental genes also are highly conserved among animal species. In many cases, genes with conserved sequences play different roles in the development of different species. In plants, for instance, homeobox-containing genes do not function in pattern formation as they do in many animals. • Comparison of Animal and Plant Development (p. 433) During embryonic development in both plants and animals, a cascade of transcription regulators turns genes on or off in a carefully regulated sequence. But the genes that direct analogous developmental processes differ considerably in sequence in plants and animals, as a result of their remote ancestry.

TESTING YOUR KNOWLEDGE Evolution Connection Genes important in the embryonic development of animals, such as homeobox-containing genes, have been relatively well conserved during evolution; that is, they are more similar among different species than are many other genes. Why is this?

434

• THREE

Genetics

Scientific Inquiry Stem cells in an adult organism can divide to form two daughter siem

cells, thus maintaining a population of relatively undifferennated eelk Alternatively, a given mitouc division may yield one daughter cell that remains a stem cell and a second daughter cell that initiates a differentiation pathway Propose one or more hypotheses to explain how tliis can happen. (Note: There is no easy answer to this question, but it is worth considering. For a hint, look at Figure 21.16a.) Investigation How Do bicoid Mutations Alter Development?

Science, Technology, and Society Government funding of embryonic stem cell research has been a contentious political issue. Why has this debate been so heated? Summarize the arguments for and against embryonic stem cell research, and explain your own position on the issue.

AN INTERVIEW WITH

Kenneth Kaneshiro The Hawaiian Islands are one of Earth's greatest natural laboratories for understanding mechanisms of evolution. Dr. Kenneth Kaneshiro has contributed much to that understanding through his research on the diverse species of Hawaiian Drosophila flies. Professor Kaneshiro is the director of the Hawaiian Evolutionary Biology Program at the University of Hawaii, Manoa, where he is also the director of the Center for Conservation Research and Training, i first met Dr. Kaneshiro in 2003, when I was a visiting scholar at lolani School in Honolulu, where Kaneshiro began his education. It was a joy returning to Hawaii a year later to conduct this interview. I love this job!

How did the Hawaiian Islands form? Geologically the islands are very young, with the oldest island, Kauai, being only about 5 to 6 million years old. The islands popped up in chronological fashion as die Pacific Plate moved northwest over a volcanic hotspoi on the seafloor. The youngest island, the Big Island of Hawaii, is over the hotspot now, and there is a new island, Loihi, beginning to form as an undersea mountain to the southeast of the Big Island.

And what makes the Hawaiian Islands such a compelling place to study evolution? First, they're the most isolated landmass in the world, sitting out in the middle of the Pacific nearly 2,000 miles away from continents in any direction. So for any species ol organism that arrived here—by blowing across the ocean on the wind, for example—the lounding population would have then been isolated from gene flow from other populations of that species.

436

And some of the organisms that made it here radiated profusely into new species by colonizing other islands. Since the islands formed in single file, from Kauai to the Big Island, you ::lso have a chronological sequence in the origin of species as founders went from older islands to newer ones. In the case of Dro>ophila. the evidence points to a single founder (a fertilized female) that arrived several million years ago and whose progeny eventually radiated Into the more than 500 described species of Hawaiian Drosophila flies. Thats about a quarter of all know i' Drasophila species in the world.

Obviously, that's far more Drosophila species than there are islands. Did environmental variation on each island contribute to that radiation of species? Yes, differences in elevation, rainfall, and other factors make each island very environmentally diverse. And each island also has what are called kipukas, 'Islands"' o! vegetation surrounded by lava. So, there are islands within islands, with the lava forming barriers between the kipukas. For example, we have studied two kipukas on the Big Island that used to be connecied before a lava flow separated them just 100 years ago. And we're already detecting significant genetic differences between Drosophila populations that live in these two kipukas. They're still the same species, but they've started to diverge. Speciation is still a very dynamic process on the Hawaiian Islands.

How did your interest in biology develop? It probably started when I was a child growing up here in Hawaii. When my dad took us fishing around Oahu, it was very scientific, though it didn't seem that way to me at the time. My dad considered the tides and wind, and he matched the colors of his fishing lures to the kinds of baitfish that might be out there. When I fish today, 1

still apply those lessons from my dad. 1 think it was that sort of scientific approach to fishing that first got me interested in biology.

When did that interest turn to evolutionary biology? When I was a freshman here at the University of Hawaii, I wanted to go into marine biology But because of family pressures, 1 enrolled in the premed program instead. To help pay my way, I took ajob with The Hawaiian Drosophna Project, starting as a dishwasher in the lab and learning how to prepare the nutrient media for breeding the flies. Within a few months, I was involved in actual research, dissecting genitalia and looking at other morphological characters that give us clues about the evolutionary history of the Hawaiian Drosophila species. I also had the opportunity to go into die field. So ecology also became part of my undergraduate education, not so much iron the classroom, but by being involved in the Drosophila research. Every summer, 10 or 12 top scientists with different research specialties wotild visit to work on various Drosophila projects. Looking over the shoulders of these eminent sc entists when I was an undergraduate really got me hooked. After I graduated, I switched to entomology for my graduate work so I could continue studying the Hawaiian Drosophila.

And that work included your research on mating behavior in Drosophila. I think many students will be surprised to learn that these flies court their mates. The courtship between males and females is very elaborate. And the mating behavior also includes competition between males. A male will defend a territory to which females are attracted for mating. In one Drosophila species, for example, the males have very wide heads, and two males will butt their heads together—like rains—and then push each other back and fon h as they joust for territory In another species, the

corr petition is more like sumo wrestling; the mates get up "tippy-toe" on their hind legs, grappling with their middle and Forelegs, and loci heads- But being a good fighter doesn't mef.n that you're also a good lover. For a male fly, oeing able to fight off other males and defend a territory gives him the most opportunity to encounter females, but he still has to be able to perform the very complex behavior that satisfies the courtship requirements of a female.

Su ch as? In ime species, the male hoists his abdomen up and over his head, in a scorpion-like pose. This displays to the female a row of specialized bristles that are on the underside of the abdomen. Each bristle is flattened like a fan. Then the male vibrates his abdomen, and the bristles waft a sex-attractant vapor called a pheromone secreted from an abdominal gland. At the same time, the male spreads his wings and rocks back and forth, emitting a sound. While dancing and singing, the male extends his mouthparts from a ve" y white lace. In response, the female actually kisses the male. Mating only occurs if the male can perform this elaborate display.

According to a model now known as the "Kaneshiro Hypothesis," changes in such mating behavior p] ayed a key role in the origin of Hawaiian Drosophila species, especially in the early stages of speciation. What's the basic idea? Shifts in mating behavior can occur in a small population after a founding event. Say you have

a population of flies on Kauai, the oldest island. Then Oahu pops up, and a fertilized female happens to make it there. She may found a new population on Oahu by producing a few hundred offspring. The males will vary in their ability to perform the species' original courtship rituals. But in such a small population, females who are very choosy will have less opportunity to reproduce than less choosy females, who will encounter more mates they are willing to accept. So selection favors fresh combinations of genes that combine adaptations to the new environment along with less rigid mating behavior than in the "parent" species back on Kauai. That would explain why mating behavior is typically the most complex in the oldest Drosophila species. I think such shifts in mating behavior have been very important in the evolution of the Hawaiian Drosophila, and probably in many other groups of organisms as well.

At the same time that biologists are studying the evolution of such a diversity of species on the Hawaiian islands, the islands have been designated a biodiversity hotspot, meaning that many species are endangered. What are the biggest threats to biodiversity in Hawaii? There is the destruction of habitat, but the impact of invasive species—normative species that are accidentally or purposefully brought to the islands—is probably the biggest threat. And ants may be the number one problem. Hawaii doesn't have any native ants; they're all alien.

In the case of Drosophila, the. evidence points to a single founder (a fertilized female) that arrived several million years igo and whose progeny eventually radiated into the more than pOO described species of Hawaiian Drosophila flies.

Their foraging in native forest ecosystems has severely impacted the native arthropod fauna. Invasive rats threaten the native birds by getting into nests and eating the eggs and young. Wild pigs are also a serious problem. They7 descended from hybridization between pigs the native Hawaitans brought to the islands and the pigs brought by Europeans. The wild pigs root in the forests, which creates puddles in which mosquitoes breed. The mosquitoes carrypathogens that cause malaria in birds. Also, invasive plants are crowding out many of the native plant species. Unfortunately, in spite of its small size, Hawaii is the extinction capital of the United States, if not the world.

One of your many hats is your role as director of the Center for Conservation Research and Training here at the University of Hawaii. What kind of work does the Center conduct? Our research interest in conservation biology mainly takes an ecos\ stem and ecortgional approach: What happens on the mountaintops affects ecosystems all the way down to the coral reefs and beyond. 1 think we have to understand such relationships between ecosystems in order to protect the islands' biodiversity. The Center also has a major commitment to education. We have a National Science Foundation grant that supports outreach programs where our graduate students work with schoolchildren and mentor their teachers. We get these K-12 students out into the field participating in research: They are collecting valid scientific data, discovering new species, and helping us understand how to eradicate some of the alien species. The sheer numbers of these young scientists enable us to make certain kinds of measurements that would be impossible otherwise. For example, researchers would usually monitor contaminants in a stream by collecting water samples at a few points along the stream. But our graduate students worked with 320 seventh graders to sample the water all the way from a mountain waterfall to where the stream drains into the ocean. To me, this kind of environmental education, beginning ai a young a£e\ will eventually increase public awareness and make us all more effective in protecting our water resources and our native ecosystems. Alohaand mahaio, Dr Kaneshiro.'

.

.

'

*

£

&

-

•

437

•HI

^1

Descent with View of Life

A Figure 22.1 A marine iguana, well-suited to its rocky habitat in the Galapagos Islands.

Key Concepts 22.1 The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species 22.2 In The Origin of Species, Darwin proposed that species change through natural selection 22.3 Darwin's theory explains a wide range of observations

Darwin Introduces a Revolutionary Theory

A

new era of biology began on November 24, 1859, the day Charles Darwin published On the Origin oj Species by Means oj Natural Selection. Darwin's book drew a cohesive picture of life by connecting the dots among what had once seemed a bewildering array of unrelated observations. The Origin oj Species focused biologists' attention on the great diversity of organisms—their origins and relationships, their similarities and differences, their geographic distribution, and their adaptations to surrounding environments (Figure 22.1).

Darwin made two major points in The Origin oj Species. First, he presented evidence that the many species of organisms presently inhabiting Earth are descendants of ancestral species that were different from the modern species. Second, he proposed a mechanism for this evolutionary process, which he termed natural selection. The basic idea of natural selection is that a population can change over generations if individuals that possess certain heritable traits leave more offspring than other individuals. The result of natural selection is

438

evolutionary adaptation, an accumulation of inherited characteristics that enhance organisms' ability to survive and reproduce in specific environments. In modern terms, we can define evolution as a change over time in the genetic composition of a population. Eventually, a population may accumulate enough change that it constitutes a new species—a new life-form. Thus we can also use the term evolution on a grand scale to mean the gradual appearance of all of biological diversity, from the earliest microbes to the enormous variety of organisms alive today Evolution is such a fundamental concept that, its study illuminates biology at every level from molecules to ecosystems, and it continues to transform medicine, agriculture, biotechnology and conservation biology You have already encountered evolution as the main thematic thread woven throughout this book. In this chapter, you will learn about the historical development of the Darwinian view of life.

Concept

The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species The impact of an intellectual revolution such as Darwinism depends on timing as well as logic. To understand why Darwin's ideas were revolutionary, we need to examine his views in the context of other Western ideas about Earth and its li::e (Figure 22.2)

Lamarck (species can change) Malthus (population limits)

American Revolution

• i

French Revolution

Wallace (evolution, natural selection) U.S Civil War

1 7 9 D I Hutton pr< ' 798 [ 1809 j Lamarck publishes his theory of evolution. 1830 |%yeil publishes Principles of Geology. 1831-1836 ["Darwin travels around the world on HMS Beagle 1837 [ Darwin begins his notebooks on the origin of species. 18441 Darwin writes his essay on the origin ot species. "•• 8581 Wallace senas his theory' to Darwin, 18591 The Origin or Species is oablished. 18651 Me^aei publishes inheritance papers.

A Figure 22.2 The historical context of Darwin's life and ideas. The dark blue bars above the timeline represent the lives of some individuals whose ideas have contributed to our modern understanding of evolution.

Resistance to the Idea of Evolution The Origin of Specks not only challenged prevailing scientific views but also shook the deepest roots of Western culture. Darwin's view of life contrasted sharply with traditional beliefs oi an Earth only a few thousand years old, populated by forms of life that had been created at the beginning and remained unchanged ever since. Darwin's book challenged a worldview that had been prevalent for centuries. Tlie Scale of Nature and Classification o) Species Although several Greek philosophers suggested that life might have evolved gradually, one philosopher who greatly influenced early Western science, Aristotle (384-322 B.C.), viewed species as fixed (unchanging). Through his observations of nature, Aristotle recognized certain "affinities" among living things, leading him to conclude that life-forms could be arranged on a ladder, or scale, of increasing complexity later called the scald naturae ("scale of nature")- Each form of life, perfect and permanent, had its allotted rung on this ladder. These ideas coincided with the Old Testament account of creation, which holds that species were individually designed bv God and therefore perfect. In the 1700s, many scientists interpreted the superb adaptations of organisms to their environments as evidence that the Creator had designed each species for a particular purpose.

One such scientist was Carolus Linnaeus (1707-1778), a Swedish physician and botanist who sought to classify life's diversity "for the greater glory of God." Linnaeus was a founder of taxonomy, the branch of biology concerned with naming and classifying organisms. He developed the twopart, or binomial, system of naming organisms according to genus and species that is still used today. In contrast to the linear hierarchy of the scala naturae, Linnaeus adopted a nested classification system, grouping similar species into increasingly general categories. For example, similar species are grouped in the same genus, similar genera (plural of genus) are grouped in the same family, and so on (see Figure 1.14). To Linnaeus, the observation that some species resemble each other did not imply evolutionary kinship, but rather the pattern of their creation. However, a century later his taxonomic system would play a role in Darwin's arguments for evolution. Fossils, Cuxier, and Catastrophism The study of lossils also helped to lay the groundwork for Darwin's ideas. Fossils are remains or traces of organisms from the past. Most fossils are found in sedimentary rocks formed from the sand and mud that settle to the bottom of seas, lakes, and marshes. New layers of sediment cover older ones and compress them into superimposed layers of rock called strata (singular, stratum). Later, erosion may carve

CHAPTER 22 Descent with Modification: A Darwinian View of Life

439

• Figure 22.3 Fossils from strata of s e d i m e n t a r y rock. The Colorado River carved the Grand Canyon through more than 2,000 m of rock, exposing sedimentary layers that are like huge pages from the book of life. Each stratum entombs fossils that represent some of the organisms from that period of Earth's history. The seed fern leaf fossil (top) is from the shallower Hermit Shale layer (265 million years old), and the trilobite fossil (bottom) is from the deeper Bright Angel Shale layer (530 million years old).

through upper (younger) strata and reveal older strata that had been buried. Fossils in each layer provide a glimpse of some of the organisms that populated Earth at the time that layer formed (Figure 22.3)

Paleontology, the study of fossils, was largely developed by French scientist Georges Cuvier (1769-1832). In examining rock layers in the region around Paris, Cuvier noted that the deeper (older) the strata, the more dissimilar the fossils are from current life. He also observed that from one stratum to the next, some new species appear while others disappear. He inferred that extinctions must have been a common occurrence in the history of life. Yet Cuvier staunchly opposed the idea of gradual evolutionary change. Instead, he advocated cata5trophism, speculating that each boundary between strata represent a catastropbe, such as a flood or drought, that destroyed many of the species living at that time. He proposed that these periodic catastrophes were usually confined to local geographic regions, which were repopulated by species immigrating from other areas.

Theories of Gradualism In contrast to catastrophism, the work of other scientists promoted the concept of gradualism—the idea that profound change can take place through the cumulative effect of slow but continuous processes. In 1795, Scottish geologist James Hutton (1726-1797) proposed that Earths geologic features could be explained by gradual mechanisms currently operating in the world. He suggested that valleys were formed by rivers wearing through rocks and that sedimentary rocks containing marine fossils were formed from particles that had eroded Irom the land and been carried by rivers to the sea.

440

UNIT FOUR

Mechanisms of Evolution

The leading geologist of Darwin's time, Charles L\ell (1797-1875), incorporated Hutton's thinking into a more comprehensive theory known as uniformitarianism. Lyell proposed that the same geologic processes are operating today as in the past, and at the same rate. Hutton and Lyells ideas exerted a strong influence on Darwin's thinking. Darwin agreed that if geologic change results from slow, continuous actions rather than sudd;n events, then Earth must be much older than the 6,000 years that theologians estimated. He later reasoned that perhaps similarly slow and subtle processes could act on living organisms over a long period of time, producing substantial change. Darwin was not the first to apply the principle of gradualism to biological evolution, however.

Lamarck's Theory of Evolution During the 18th century, several naturalists (including Darwin's grandfather, Erasmus Darwin) suggested that life evolves as environments change. But only one of Charles Darwin's predecessors developed a comprehensive model for how life evolves: French biologist Jean-Baptiste de Lamarck (1744-1829). Alas, Lamarck is primarily remembered today not for his visionary recognition that evolutionary change explains the fossil reco.:d and organisms' adaptations to their environments, but for the incorrect mechanism be proposed to explain how evolution occurs. Lamarck published his theory in 1809, the year Darwin was born. By comparing current species with fossil forms, Lamarck had found what appeared to be several lines oi descent, each a chronological series of older to younger fossils leading to a living species. He explained this with two principles th at

Concept

In The Origin of Species, Darwin proposed that species change through natural selection As the 19th century dawned, it was generally believed that species had remained unchanged since their creation. A few clouds of doubt about the permanence of species were beginning to gather, but no one could have forecast the thundering storm just over the horizon.

Darwin's Research Jk Figure 22.4 Acquired traits cannot be inherited. This bonsai tree was "trained" to grow as a dwarf by pruning and shaping. However, seeds from this tree would produce offspring of normal size.

were commonly accepted at that time. The first was use and disuse, the idea that parts of the body that are used extensively become larger and stronger, while those that are not used deteriorate. As an example, he cited a giraffe stretching its neck to reach leaves on high branches. The second principle, inheritance oj acquired characteristics, stated that an organism

could pass these modifications to its offspring. Lamarck reasoned that the long, muscular neck of the living giraffe had evolved over many generations as giraffes stretched their necks ever higher. Lamarck also thought that evolution happens because organisms have an innate drive to become more complex. Darwin rejected this idea in favor of natural selection, but he too thought that variation was introduced into the evolutionary process through inheritance of acquired characteristics. However, our modern understanding of genetics refutes this principle; there is no evidence that acquired characteristics can be inherited (Figure 22.4). Lamarck was vilified in his own time, especially by Cuvier, who denied that species ever evolve- In retrospect, however, Lamarck deserves credit for his insightful observations of nature and recognition of gradual evolutionary change as the best explanation for those observations.

Concept Check 1. Which of the individuals discussed in this section viewed species as fixed, and which viewed species as being able to change over time? 2. What was Lamarck's theory of evolution? "Explain its significance. For suggested answers, see Appendix A.

Charles Darwin (1809-1882) was born in Shrewsbury in western "England. Even as a boy, he had a consuming interest in nature. When he was not reading nature books, he was fishing, hunting, and collecting insects. Darwin's father, an eminent physician, could see no future for his 16-year-old son as a naturalist and sent him to the University of Edinburgh to study medicine. But Charles found medical school boring and surgery before the days of anesthesia horrifying. He left Edinburgh without a degree and enrolled at Cambridge University with the intention of becoming a clergyman. At that time in England, many scholars of science belonged to the clergy. At Cambridge, Darwin became the protege of the Reverend John Henslow, a professor of botany. Soon after Darwin received his B.A. degree, Henslow recommended the young graduate to Captain Robert FitzRoy, who was preparing the survey ship HMS Beagle for a voyage around the world. FitzRoy accepted Darwin on board because of his education and because of his age and social class, which were similar to FitzRoy's. The Voyage of the Beagle In 1831, the 22-year-old Darwin left England aboard the Beagle. The primary mission of the voyage was to chart poorly known stretches of the South American coastline. While the ship's crew surveyed the coast, Darwin spent most of his time on shore, observing and collecting thousands of South American plants and animals. He observed the various adaptations of plants and animals that inhabited such diverse environments as the Brazilian jungles, the expansive grasslands of the Argentine pampas, the desolate lands of Tierra del Fuego near Antarctica, and the towering heights of the Andes Mountains. Darwin noted that the plants and animals in temperate regions of South America more closely resembled species living in the South American tropics than species in temperate regions of Europe. Furthermore, the fossils he found, though clearly different from living species, were distinctly South American in their resemblance to the living organisms of that continent.

CHAPTER 22 Descent with Modification: A Darwinian View of Life

441

NORTH AMERICA PACIFIC OCEAN

Galapagos islands

I New Zealano A Figure 22.5 The voyage of HMS Beagle.

Geologic observations also impressed Darwin during the voyage. Despite bouts of seasickness, he read Lyell's Principles of Geology while aboard the Beagle. He experienced geologic change firsthand when a violent earthquake rocked the coast ol Chile, and he observed afterward that the coastline had risen by several feet. Finding fossils of ocean organisms high in the Andes Mountains, he inferred that the rocks containing the fossils must have been raised there by a long series of similar earthquakes. These observations reinforced what he had learned from Lyell—the physical evidence did not support the traditional view of a static Earth only a few thousand years old. Darwin's interest in the geographic distribution of species was further kindled by the Beagle's stop at the Galapagos, a group of geologically young volcanic islands located near the equator about 900 km (540 miles) west of South America (Figure 22.5). Darwin was fascinated by the unusual organisms he found there. Among the birds he collected on the Galapagos were several kinds of finches that, although quite similar, seemed to be different species. Some were unique to individual islands, while others were distributed on two or more adjacent islands. But Darwin did not fully grasp the significance of these observations until after his return to England in 1836. He and others found that although the animals on the Galapagos resemble species living on the South American mainland, most of the Galapagos animals live nowhere else in the world. He hypothesized that the (iaMpagos had been colonized by organ442

UNIT FOUR

Mechanism; of Evolution

isms that had strayed from South America and had then diversified on the various islands. Darwin's Focus on Adaptation As Darwin reassessed all that he had observed during the vo\ age, he began to perceive adaptation to the environment and the origin of new species as closely related processes. Could a new species arise from an ancestral form by the gradual accumulation of adaptations to a different environment? From studies made years after Darwin's voyage, biologists have concluded Lhat this is indeed what happened to the Galapagos finches. Their beaks and behaviors are adapted to the specific foods available on their home islands (Figure 22.6). Darwin realized that an explanation for such adaptations was essential to understanding evolution. By the early 1840s, Darwin had worked out the major features of his theory of natural selection as the mechanism of evolution. However, he had not yet published his ideas. He was in poor health, and he rarely left his home near London. Despite his reclusiveness, Darwin was not isolated. Already famous as a naturalist because of the letters and specimens he had sent to England during the voyage of the Beagle, Darwin corresponded extensively with scientists around tht world and was often visited by Lyell, Henslow, and others. In 1844, Darwin wrote a long essay on the origin of specie; and natural selection. However, he was reluctant to introduce

> Figure 22.6 Beak variation in Gal i p a g o s f i n c h e s . The Galapagos Islands are home to more than a dozen species of closely related finches, some found only on a single island. The most striking differences among them are their beaks, which are ada jted for specific diets.

(c) Seed eater. The large ground finch {Geospiza magnirostris) has a large beak adapted for cracking seeds that fall from plants to the ground.

(a) Cactus eater. The torn sharp beak of the cactus ground finch {Geospiza scandens) helps it tear and eat cactus flowers and pulp.

(b) Insect eater. The green warbler finch (Certhidea olivacea) uses its narrow, pointed beak to grasp insects,

hi;, theory publicly, apparently because he anticipated the uproar it would cause. Darwin asked his wife to publish his essay if he were to die before finishing a more thorough dissertation. Even as he procrastinated, he continued to compile evidence in support of his theory Lyell, not yet himself convinced of evolution, nevertheless urged Darwin to publish on the subject before someone else came to the same conclusions ard published first. In June 1858, LyelTs prediction came true. Darwin received a manuscript from Alfred Russel Wallace (1823-1913), a young British naturalist working in the East Indies who had developed a theory of natural selection similar to Darwin's. Wallace asked Darwin to evaluate his paper and forward it to b ell if it merited publication. Darwin complied, writing to b ell: "Your words have come true with a vengeance. . . . 1 never saw a more striking coincidence . . . so all my originalit;, whatever it may amount to, will be smashed." Lyell and a colleague then presented Wallace's paper, along with extracts from Darwin's unpublished 1844 essay, to the Linnean Society of London onjuly 1, 1858. Darwin quickly finished The Origin of Species and published it the next year. Although Wallace had written up his ideas for publication first, he was a great admirer of Darwin and agreed that Darwin had developed the theory of natural selection so extensively that he should be known as its main architect. Within a decade, Darwin's book and its proponents had convinced most biologists that biological diversity was the product of evolution. Darwin succeeded where previous evolutionists had failed, mainly because he presented his reasoning with immaculate logic and an avalanche of sup-

porting evidence. He soon followed up his first book with other pioneering work, in particular an exploration of the type of natural selection known as sexual selection (see Chapter 23).

The Origin of Species In publishing his theory, Darwin developed two main ideas: that evolution explains life's unity and diversity and that natural selection is a cause of adaptive evolution. Descent with Modification In the first edition of The Origin of Species, Darwin did not use the word evolution until the very end. Instead, he referred to descent with modification, a phrase that summarized his view of life. Darwin perceived unity in life, with all organisms related through descent from an ancestor that lived in the remote past. As the descendants of that ancestral organism spilled into various habitats over millions of years, they accumulated diverse modifications, or adaptations, that fit them to specific ways of life. In the Darwinian view, the history of life is like a tree, with multiple branchings from a common trunk to the tips of the youngest twigs that represent the diversity of living organisms. Each fork of the tree represents an ancestor of all the lines of evolution that subsequently branched from that point. Closely related species, such as the Asian elephant and African elephants, are very similar because they shared the same line of descent until a relatively recent divergence CHAPTER 22

Descent w i;h Modification A Darwinian View of Life

443

A Figure 22.7 Descent w i t h m o d i f i c a t i o n . This evolutionary tree of the elephants is based mainly on fossils—their anatomy, order of appearance in strata, and geographic distribution. Note that most branches of descent ended in extinction. Despite their very different appearances, the manatees and hyraxes are the elephants' closest living relatives. (Timeline not to scale.)

from their common ancestor (Figure 22.7). Most branches of evolution, even some major ones, are dead ends; about 99% of all species that have ever lived are now extinct. Thus, there are no living animals that fill the gap between the elephants and their nearest relatives today, the manatees and hyraxes, though some fossils have been found. Linnaeus had realized that some organisms resemble each other more closely than others, but he had not linked these resemblances to evolution- Nonetheless, because he had recognized that the great diversity of organisms could be organized into ''groups subordinate to groups" (Darwin's phrase), his taxonomic scheme largely fit with Darwin's theory To Darwin, the Linnaean hierarchy reflected the branching history of the tree of life, with organisms at the various taxonomic levels related through descent from common ancestors. Natural Selection and Adaptation How does natural selection work, and how does it explain adaptation? Evolutionary biologist Ernst Mayr has dissected the logic of Darwin's theory of natural selection into three inferences based on five observations:* * Adapted from E. Mayr, The Growth oj B:oio^\a:l Tkov.ghi: Diwr^iy. [ivolution (Old !nki'i:ui-v-c (Cambridge, MA: Harvard University Press, 1982). 444

UNIT FOUR

Mechanisms oi Evolution

OBSERVATION #1: For any species, population sizes would increase exponentially if all individuals that are born reproduced successfully (Figure 22.8). OBSERVATION #2: Nonetheless, populations tend to remain stable in size, except for seasonal fluctuations. OBSERVATION #3: Resources are limited. INFERENCE #1: Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving each generation. OBSERVATION #4: Members of a population vary extensively in their characteristics; no two individuals are exactly alike (Figure 22.9). OBSERVATION #5: Much of this variation is heritable. INFERENCE #2: Survival depends in part on inherited traits. Individuals whose inherited traits give them a high probability of surviving and reproducing in a given environment have higher fitness and are likely to leave more offspring than less fit individuals. INFERENCE #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over generations.

productive success results in favored traits being disproportionately represented in the next generation. Increases in the frequencies of favored traits in a population, which are always taking place regardless of whether the environment is changing, are an important source of evolutionary modification.

c Figure 22.8 Ovi rproduction of offspring. Just one branch of a maple tree bea's dozens of winged seeds. If all the tree's offspring survived, we would quickly be overwhelmed by maple forests.

•: * ~ ' 'A."

Artificial Selection. Darwin derived another piece of his theory from the many familiar examples of selective breeding of domesticated plants and animals. Humans have modified other species over many generations by selecting and breeding individuals that possess desired traits—a process called artificial selection. As a result of artificial selection, crop plants and animals bred as livestock or pets often bear little resemblance to their wild ancestors (Figure 22.10). If artificial selection can achieve so much change in a relatively short period of time, Darwin reasoned, then what he termed "natural selection" should be capable of considerable modification of species over hundreds or thousands of generations. Even if the advantages of some heritable traits over others are slight, the advantageous variations will gradually accumulate in the population, and less favorable variations will diminish.

Si*.''

Summary of Natural Selection. Lets once more state the main ideas of natural selection: A Figure 22.9 Variation in a population. To the extent that the

• Natural selection is the differential success in reproduction (the unequal ability of individuals to survive and reproduce) that results from the interaction between individuals that vary in heritable traits and their environment.

variation in color and dot patterns among the members of this population of ladybird beetles is heritable, it can be acted on by natural selection.

Darwin perceived an important connection between natural selection, which results from what he called the struggle for existence, and the capacity of organisms to lk overreproduce." Apparently, this insight came after he read a 1798 essay on population growth by Thomas Malthus. Malthus contended that much of human suffering—disease, famine, homelessness, end war—was the inescapable consequence of the human populations potential to increase faster than food and other resources. The capacity to overreproduce

(a) The seaweed is grown on nets in shallow coastal waters.

:.-.—•—=—Biade

Stipe (b) A worker spreads the harvested seaweed on bamboo screens to dry. •Holdfast

• JFigure 28.18 Seaweeds: adapted to life at the ocean's

(c) Paper-thin, glossy sheets of nori make a mineralrich wrap for rice, seafood, and vegetables in sushi.

margins. The sea palm (Postelsia) lives on rocks along the coast of the northwestern United States and western Canada. The thallus of thii brown alga is well adapted to maintaining a firm foothold despite the crashing surf.

& Figure 28.20 Edible seaweed. Nori is a traditional Japanese food made from the seaweed Porphyra (a red alga, which we discuss in more detail later in the chapter).

known as kelps (Figure 28.19). The stipes of these brown algae may be as long as 60 m. Seaweeds that inhabit the intertidal zone must cope with water churned by waves and wind, along with twice-daily low tides that expose the algae to the drying atmosphere and intense rays of the sun. Unique adaptations enable these seaweeds to survive. For example, their cell walls are composed of cellulose and gel-forming polysaccharides that help cushion the thalli from waves and reduce drying when the algae are exposed. Human Uses of Seaweeds Seaweeds—brown algae as well as large, multicellular red and green algae, which we discuss later in the chapter—are important commodities for humans. Many seaweeds are harvested for food. For example, the brown alga Laminaria (Japanese "kombu") is used in soups, and the red alga Porphyra (Japanese "nori") is eaten as crispy sheets or used to wrap sushi (Figure 28.20). The gel-forming substances in the cell walls of algae (algin in brown algae, agar and carrageenan in red algae) are also used to thicken many processed foods, such as pudding, ice cream, and salad dressing. Alternation of Generations A Figure 28.19 A kelp forest. The great kelp beds of temperate coastal waters provide habitat and food for a variety of organisms, including many fish species caught by humans. Macrocystis, a kelp common along the Pacific coast of the United States, can grow more than 60 m in a single season, the fastest linear growth recorded in any organism.

A variety of life cycles have evolved among the multicellular algae. The most complex life cycles include an alternation of generations, the alternation of multicellular haploid and diploid forms. Although haploid and diploid conditions CHAPTER 28

Pro!I

561

0 The sporophytes of this seaweed are usually found in water just below the line of the lowest tides, attached to rocks by branching holdfasts.

Q In early spring, at the end of the main growing season, cells on the surface of the blade develop into sporangia.

Q Sporangia produce zoospores by meiosis.

Q The zoospores are a structurally alike, but about half of them develop into male gametophytes and half into female gametophytes. The gametophytes look nothing like the sporophytes, being short, branched filaments that grow on the surface of subtidal rocks.

© Sperm fertilize the eggs.

0 Male gametophytes release sperm, and female gametophytes produce eggs, which remain attached to the female gametophyte. Eggs secrete a chemical signal that attracts sperm of the same species, thereby increasing the probability of fertilization in the ocean.

A Figure 28.21 The life cycle of Laminaria: an example of alternation of generations.

alternate in all sexual life cycles—human gametes, for example, are haploid-—the term alternation of generations applies only to life cycles in which both haploid and diploid stages are multicellular. As you will read in Chapter 29, alternation of generations also evolved in the life cycles of all plants. The complex life cycle of the brown alga Laminaria is an example of alternation of generations (Figure 28.21). The diploid individual is called the sporophyte because it produces reproductive cells called zoospores. The zoospores develop into haploid male and female gametophytes, which produce gametes. The union of two gametes (fertilization, orsyngamy) results in a diploid zygote, which gives rise to a new sporophyte. In Laminaria, the two generations are heteromorphic, meaning that the sporophytes and gametophytes are structurally different. Other algal life cycles have an alternation of 562

UNIT FIVE

The Evolutionary History of Biological Diversity

isomorphic generations, in which the sporophytes and gametophytes look similar to each other, although they differ ; chromosome number.

Concept Check 1. What unique cellular feature is common to all stramenopiles? 2. Compare the nutrition of oomycetes with that of golden algae. 3. How is the structure of a brown alga such as Laminaria well suited to its intertidal zone habitat? For suggested answers, see Appendix A.

Concept

Cercozoans and radiolarians have threadlike pseudopodia A nevj/ly recognized clade, Cercozoa (the cercozoans), contains a diversity of species that are among the organisms referred to as amoebas. Amoebas were formerly defined as protists that move and feed by means of pseudopodia, extensions that may bulge fromi virtually anywhere on the cell surface. When an amoeba moves, it extends a pseudopodium and anchors the tip, and then more cytoplasm streams into the pseudopodium. However, baseji on molecular systematics, it is now clear that amoebas do not constitute a monophyletic group but are dispersed across many distantly related eukaryotic taxa. Those that belong to the clade Cercozoa are distinguished morphologically from most othe r amoebas by their threadlike pseudopodia. Cercozoans include chlorarachniophytes (mentioned earlier in this chapter in the discussion of secondary endosymbiosis) and foraminiferans. Protists in another clade, Radiolaria (the radiolarians), also have threadlike pseudopodia and are closely related to cercozoans.

Foraminiferans (Forams) Fo :aminiferans (from the Latin foramen, little hole, and/erre, to bear), or forams, are named for their porous shells, called tests (Figure 28,22). Foram tests are generally multichambered and consist of organic material hardened with calcium carbonate . The pseudopodia that extend through the pores function in swimming, test formation, and feeding. Many forams also

20 urn

A. Figure 28.23 A radiolarian. Numerous threadlike axopodia radiate from the central body of this radiolarian, which is found in the Red Sea (LM).

derive nourishment from the photosynthesis of symbiotic algae that live within the tests. Forams are found in both the ocean and fresh water. Most species live in the sand or attach themselves to rocks or algae, but some are abundant in plankton. The largest forams, though single-celled, grow to a diameter of several centimeters. Ninety percent of all identified species of forams are known from fossils. Along with the calcareous remains of other protists, the fossilized tests of forams are components of marine sediments, including sedimentary rocks that are now land formations. Foram fossils are excellent markers for correlating the ages of sedimentary rocks in different parts of the world,

Radiolarians Radiolarians are mostly marine protists whose tests are fused into one delicate piece, which is generally made of silica. The pseudopodia of radiolarians, known as axopodia, radiate from the central body and are reinforced by bundles of microtubules (Figure 28.23). The microtubules are covered by a thin layer of cytoplasm, which surrounds through phagocytosis smaller microorganisms that become attached to the axopodia, Cytoplasmic streaming then carries the engulfed prey into the main part of the cell. After radiolarians die, their tests settle to the seafloor, where they have accumulated as an ooze that is hundreds of meters thick in some locations.

Concept Check 1. Why do forams have such a well-preserved fossil record? 2. Compare feeding in forams and radiolarians. 4 Figure 28.22 Globigerina, a foram with a snail-like test. Threadlike pseudopodia extend through pores in the test (LM). The inset SEM shows a calcareous foram test.

For suggested answers, see Appendix A.

CHAPTER 28

Proust;

563

Entamoebas Concept

Amoebozoans have lobe-shaped pseudopodia Many species of amoebas that have lobe-shaped, rather than threadlike, pseudopodia belong to the clade Amoebozoa (the amoebozoans). Amoebozoans include gymriamoebas, entamoebas, and slime molds.

Whereas most amoebozoans are free-living, those that b ?long to the genus Entamoeba are parasites that infect all classes of vertebrates as well as some invertebrates. Humans host at least six species of Entamoeba, but only one, E. histolytica, is kriown to be pathogenic. £ histolytica causes amebic dysentery and is spread via contaminated drinking water, food, or eating utensils. Responsible for up to 100,000 deaths worldwide every year, the disease is the third-leading cause of death due to asites, after malaria and schistosomiasis (see Chapter 33)

Gymnamoebas Gymnamoebas constitute a large and varied group of amoebozoans. These unicellular protists are ubiquitous in soil as well as freshwater and marine environments. Most are heterotrophs that actively seek and consume bacteria and other protists (Figure 28.24). Some gymnamoebas also feed on detritus (nonliving organic matter).

Slime Molds Slime molds, or mycetozoans (from the Latin meaning "fungus animals"), were once thought to be fungi because, like fungi, they produce fruiting bodies that aid in spore dispersal. However, the resemblance between mycetozoans and fungi appears to be another example of evolutionary convergence. Molecular systematics places slime molds in the clade Amoebozoa and suggests that they descended from unicellular, gymnamoebalike ancestors. Slime molds have diverged into two main branches, plasmodial slime molds and cellular slime molds, which are distinguished in part by their distinctive life cycles. Plasmodial Slime Molds Many species of plasmodial slime molds are brightly pigmented, usually yellow or orange (Figure 28.25). At one stage in their life cycle, they form a mass called a plasmodium, which may grow to a diameter of many centimeters (Figure 28.26), (Don't confuse a slime mold!? plasmodium with the apicomplexan parasite Plasmodium.) Despite its size, the plasmodium is not multicellular; it is a single mass of cytoplasm that is undivided by membranes and that contains many diploid nuclei. This "superceH" is the product of mitotic nuclear divisions that are not followed by cytokinesis, the division of cytoplasm. In most species, the mitotic divisions are synchronous, meaning that

• Figure 28.24 A gymnamoeba feeding. In this series of images selected from a video, a gymnamoeba (Amoeba sp.) uses its lobe-shaped pseudopodia to engulf its prey, a effete.

564

U N I T FIVE

The Evolutionary History of Biological Diversity

A Figure 28.25 Physarum pofycephalum, a plasmodial slime mold.

0 The feeding stage is a multinucleate plasmodium that lives on organic refuse.

Q The plasmodium takes a weblike form.

© Repeated mitotic divisions of the zygote's nucleus, without cytoplasmic division, form a feeding plasmodium ! and complete the life cycle.

J> The cells unite in pairs (flagellated with flagellated and amoeboid with amoeboid), forming diploid zygotes.

@ These cells are either amoeboid or flagellated; the two forms readily convert from one to the other.

0 The resistant spores disperse through the air to new locations and germinate, becoming active haploid cells when conditions are favorable.

Q The plasmodium erects stalked fruiting bodies (sporangia) when conditions become harsh.

© Within the bulbous tips of the sporangia, meiosis produces haploid spores.

Figure 28.26 The life cycle of a plasmodial slime mold.

thousands of nuclei go through each phase of mitosis at the same time. "Because of this characteristic, plasmodial slime molds have been used to study the molecular details of the cell cycle. Within the line channels of the plasmodium, cytoplasm streams first one way, then the other, in pulsing flows that are beautiful to watch through a microscope. This cytoplasmic Streaming apparently helps distribute nutrients and oxygen. The plasmodium extends pseudopodia through moist soil, leaf mulch, or rotting logs, engulfing food particles by phagocytosis as it grows. If the habitat begins to dry up or there is no iood [left, the plasmodium stops growing and differentiates into a stage of the life cycle that produces fruiting bodies, which function in sexual reproduction. In most plasmodial slime molds; the diploid condition is the predominant part of the life cycle.

Cellular Slime Molds Cellular slime molds pose a semantic question about what it means to be an individual organism. The feeding stage of the life cycle consists of solitary cells that function individually but when food is depleted the cells form an aggregate that functions as a unit (Figure 28.27, on the next page). Although this mass of cells superficially resembles a plasmodial slime mold, the cells remain separated by their membranes. Cellular slime molds also differ from plasmodial slime molds in being haploid organisms (only the zygote is diploid) and in having fruiting bodies that function in asexual rather than sexual reproduction. Furthermore, cellular slime molds have no flagellated stages. CHAPTER 28

i.Jl"Oii:-.tS

565

0 in a favorable environment, amoebas emerge from the spore coats and begin feeding.

O In the feeding stage of the life cycle, solitary haploid amoebas enguif bacteria.

Q During sexual reproduction, two haploid amoebas fuse and form a zygote.

© Spores | l d , i

SEXUAL REPRODUCTION

Zygote (In)

MEIOSIS

© The zygote becomes a gian cell (not shown) by consuming haploid amoebas. After developing a resistant wall, the giant cell undergoes meiosis followed by several mitotic divisions.

-"

O The resistant wall ruptures, releasing new haploid amoebas.

0 When food is depleted, hundreds of amoebas congregate in response to a chemical attractant and form a sluglike aggregate (photo below left). Aggregate formation is the beginning of asexual reproduction. © The aggregate migrates for a while and then stops. Some of the cells dry up after forming a stalk that supports an asexual fruiting body.

Key

200 ,um

Haploid (m Diploid (2r)

A Figure 28.27 The life cycle of Dictyostelium, a cellular slime mold.

Dictyostelium discoidcum, a common cellular slime mold on forest floors, has become a model organism for addressing the evolution of multicellularity One line of research has focused on the fruiting body stage. During this stage, the cells that form the stalk die as they dry out, while the spore cells at the top survive and have the potential to reproduce. Scientists have found that mutations to a single gene can turn individual Dictyostelium cells into "cheaters" that never become part of the stalk. Since these mutants gain a strong reproductive advantage over noncheaters, it is puzzling that all Dictyostelium cells are not cheaters. In 2003, scientists at Rice University and the University of Turin, Italy discovered why most of the cells do not cheat. Cheating mutants lack a protein on their cell surface, and noncheating cells can recognize this difference. Noncheaters preferentially aggregate with other noncheaters, thus depriving 566

UNIT FIVE

The Evolutionary History of Biological Diversity

cheaters of the opportunity to exploit them. Such a recognition system may have been important in the evolution of multicel lular eukaryotes such as animals and plants. Concept Check i 1. Contrast the pseudopodia of amoebozoans and forams. 2. In what sense is "fungus animal" a fitting description of a slime mold? In what sense is it not a fitting description? 3. Does cooperation between cells exist in amoebozoans? Explain. For suggested answers, see Appendix A.

Comcept

Red algae and g^een algae are the closest relatives of land plants As we described in Chapter 26, molecular systematics and studies of cell structure support this phylogenetic scenario: More than a billion years ago, a heterotrophic protist acquired a cyatiobacterial endosymbionl, and the photosynthetic descendants of this ancient protist evolved into red algae and green] algae (see Figure 28.3). At least 475 million years ago, the lineage that produced green algae gave rise to land plants. We wall examine land plants in Chapters 29 and 30; here we will look at the diversity of their closest algal relatives, red algae and green algae.

Red Algae Many of the 6,000 known species of red algae (rhodophytes, frorr. the Greek rhodos, red) are reddish, owing to an accessory pigment called phycoerythrin, which masks the green of chlorophyll. However, species adapted to shallower water have less phyeoerythrin. As a result, red algae species may be almost black in deep water, bright red at more moderate depths, and greenish in very shallow water. Some species lack pigmentation altogether and function heterotrophically as parasites on other red algae. Red algae are the most abundant large algae in the warm coastal waters of tropical oceans. Their accessory pigments allow them to absorb blue and green light, which penetrate relatively far into the water. A species of red alga has recently been

discovered living near the Bahamas at a depth of more Lhan 260 m. There ate also some freshwater and terrestrial species. Most red algae are multicellular, and the largest are included in the informal designation "seaweeds" (Figure 28.28), although no red algae are as big as the giant brown kelps. The thalli of many red algae are filamentous, often branched and interwoven in lacy patterns. The base of the thallus is usually differentiated as a simple holdfast. Red algae have especially diverse life cycles, and alternation of generations is common. But unlike other algae, they have no flagellated stages in their life cycle and depend on water currents to bring gametes together for fertilization.

Green Algae Green algae are named for their grass-green chloroplasts. In their ultrastructure and pigment composition, these chloroplasts are much like those of organisms we traditionally call plants. Molecular systematics and cellular morphology leave little doubt that green algae and land plants are closely related. In fact, some systematists now advocate the inclusion of green algae in an expanded "plant" kingdom, Viridiplantae (from the Latin viridis, green). Green algae are divided into two main groups, chlorophytes (from the Greek chloros, green) and charophyceans (see Figure 28.4). More than 7,000 species of chlorophytes have been identified. Most live in fresh water, but there are also many marine species. The simplest chlorophytes are biflagellated unicellular organisms such as Chlamydomonas, which resemble the gametes and zoospores of more complex chlorophytes. Various species of unicellular chlorophytes exist as

(b) Dulse (Palmaria palmata). This edible species has a "leafy" form.

(c) A coralline alga. The cell walls of coralline algae are hardened by calcium carbonate. Some coralline algae are members of the biological communities around coral reefs,

(a) Bonnemaisonia hamifera. T has a filamentous form.

CHAPTER 28

567

plankton or inhabit damp soil; some live symbiotically within other eukaryotes, contributing part of their photosynthetic output to the iood supply of their hosts. Chlorophyt.es are among the algae thai live symbiotically with fungi in the associations called lichens (see Figure 31.24). Some chlorophytes have even adapted to one of the last places on Earth you might expect to find them; snow. For example, Chlamydomonas nivalis can form dense alga] blooms on high-altitude glaciers and snowiields, where its reddish pigments produce an effect known as "watermelon snow" (Figure 28.29). These chlorophytes carry out photosynthesis despite subfreezing temperatures and intense visible and ultraviolet radiation. They are protected by radiation-blocking compounds in their cytoplasm and by the snow itself, which acts as a shield.

• Figure 28.29 Watermelon snow. Carotenoids in some snowdwelling chlorophytes, such as Chlamydomonas nivalis, turn the snow red.

Larger size and greater complexity evolved in chlorophytes by three different mechanisms: (1) the formation of colonies of individual cells, as seen in Volvox (Figure 28.30a) and in filamentous forms that contribute to the stringy masses known as pond scam; (2) the repeated division of nuclei with no

20 ^m

f Figure 28.30 Colonial and multicellular chlorophytes.

50 \im (a) Volvox. a colonial freshwater chlorophyte. The colony is a hollo' ball whose wall is composed of hundreds or thousands of biflagellated cells (see inset LM) embedded in a gelatinous matrix. The cells are usually connected by strands of cytoplasm; if isolated, these cells cannot reproduce. The large colonies seen here will eventually release the small "daughter" colonies within them (LM).

(b) Caulerpa, an intertidal chlorophyte. The branched filaments lack cross-walls and thus are multinucleate. In effect the thallus is one huge "supercell."

(c) Ulva, or sea lettuce. This edible seaweed has a multicellular thallus differentiated into leaflike blades and a rootlike holdfast that anchors the alga against turbulent waves and tides.

568

UNIT FIVE

The Evolutionary History of Biological Diversity

O In Chlamydomonas, mature cells are haploid and contain a single cup-shaped chloroplast (see TEM at left).

Flagella-

Cell wail-

© In response to a shortage of nutrients, drying of the pond, or some other stress, cells develop into gametes. Q Gametes of opposite mating types (designated + and -) pair off and cling together. Fusion of the gametes (syngamy) forms a diploid zygote.

Regionsof single chloropla

Q These daughter cells develop flagella and cell walls and then emerge as swimming zoospores from the wall of the parent cell that had enclosed them. The zoospores grow into mature haploid cells, completing the asexual life cycle.

© The zygote secretes a durable coat that protects the cell against harsh conditions.

@ When a mature cell reproduces asexually, it resorbs its flagella and then undergoes two rounds of mitosis, forming four cells (more in some species). Key Haploid (n) Diploid (2n)

Q After a dormant period, meiosis produces four haploid individuals (two of each mating type) that emerge from the coat and develop into mature cells.

Figure 28.31 The life cycle of Chlamydomonas, a unicellular chlorophyte.

cytoplasmic division, as seen in the multinucleate filaments of CaulerpcL (Figure 28,30b); and (3) the formation of true multicellular forms by cell division and cell differentiation, as in Iflva (Figure 28.30c). Some multicellular marine chlorophytes ;iire large and complex enough to qualify as seaweeds. Most chlorophytes have complex life cycles, with both sexual and asexual reproductive stages. Nearly all reproduce sexually by means of biflagellated gametes that have cup-shaped .chloroplasts (Figure 28.31). The exceptions are the conjugating algae, such as Spirogyra (see Figure 28.2d), which produce amoeboid gametes. Alternation of generations evolved in the life cycles of some green algae, including Ulva, in which alternate generations are isomorphic.

The other main group of green algae, the charophyceans, are the most closely related to land plants. For that reason, we will discuss them along with plants in Chapter 29.

Concept Check i 1. identify two ways in which red algae are different from brown algae. 2. Why is it accurate to say that Ulva has true multicellulanty bm CMukrpa docs not? For suggested answers, see Appendix A.

CHAPTER 28

569

Chapter 28 Review Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY CONCEPTS Protists are an extremely diverse assortment of eukaryotes • Protists are more diverse than all other eukaryotes and are no longer classified in a single kingdom. Most protists are unicellular; some are colonial or multicellular. Protists include photoautotraphs, heierotrophs, and mixotrophs. Most species are aquatic, but others are found in moist terrestrial habitats. Some species are exclusively asexual; others reproduce sexually Table 28.1 reviews the groups of protists surveyed in this chapter (pp. 549-550). • Endosvmbiosis in Eukaryotic Evolution (pp. 550-551) Some biologists hypothesize that mitochondria and plastids are descendants of alpha proteobacteria and cyanobacteria, respectively that were engulfed by eukaryotic cells and became endosymbionts. The plastid-bearing lineage eventually evolved into red algae and green algae. Other protist groups evolved from secondary endosymbiotic events in which red algae or green algae were themselves engulfed. Activity Tentative Phytogeny of Eukaryotes

Diplomonads and parabasalids have modified mitochondria • Diplomonads and parabasalids are adapted to anaerobic environments. They lack plastids, and their mitochondria do not contain DNA, an electron transport chain, or citric-acid cycle enzymes (p. 552). • Diplomonads (p. 552) Diplomonads have multiple ftagella and two nuclei. • Parabasalids (p. 553) Parabasalids include trichomonads, which move by means of flagella and an undulating part of the plasma membrane.

Euglenozoans have flagella with a unique internal structure • Euglenozoans have a spiral or crystalline rod inside their flagella, and most have disk-shaped mitochondria! cristae. They include autotrophs, predatory heterotrophs, and parasites (p. 553). • Kinetoplastids (pp. 553-554) Kinetoplastids have a kinetoplast, an organized mass of DNA within a large mitochondrion. Parasitic kinetoplasiids cause sleeping sickness and Chagas' disease.

Alveolates have sacs beneath the plasma membran : • Alveoli, membrane-bounded sacs beneath the plasma membrane, distinguish alveolates from other protists (p. 555). • Dinoflagellates (p. 555) Dinoflagellates are a diverse group of aquatic photoautotrophs and heterotrophs. Their characterisi ic spinning movements are produced by two flagella that lie in perpendicular grooves in their cell surface. Rapid growth of some dinoflagellate populations causes red tides. • Apicomplexans (pp. 555—556) Apicomplexans are parasites that have an apical complex of organdies specialized for invading host cells. They also have a nonphotosynthetic plastid, th: apicoplast. The apicomplexan Plasmodium causes malaria. • Ciliates (pp. 556-558) Ciliates use cilia to move and feed. They have large macronuclei and small micronuclei. The micronuclei function during conjugation, a sexual process that produces genetic variation. Conjugation is separate from reproduction, which generally occurs by binary fission.

Stramenopiles have "hairy" and smooth flagella • Oomycetes (Water Molds and Their Relatives) (pp. 558-559) Most oomycetes are decomposers or parasites and have filaments (hyphae) that facilitate nutrient uptake. Th oomycete Phytophthora infestans causes potato late blight. • Diatoms (pp. 559-560) Diatoms are surrounded by a twopart glass-like wall and are a major component of phytoplankton. Accumulations of fossilized diatom walls comprise much if the sediments known as diatomaceous earth. • Golden Algae (p. 560) Golden algae typically have two flagella attached near one end of the cell. Many species are plank tonic. Their color results from the carotenoids they contain. • Brown Algae {pp. 560-562) Brown algae are multicellular, mostly marine protists. They include some of the most comple algae commonly known as seaweeds, many of which are commercially important to humans. Like some red algae and green algae and all plants, some brown algae have a life cycle that features alternation of generations: A multicellular diploid form al-j ternates with a multicellular haploid form.

Cercozoans and radiolarians have threadlike pseudopodia • Foraminiferans (Forams) (p. 563) Forams are marine and freshwater amoebas with porous, generally multichambered shells (tests) made of organic material and calcium carbonate. Pseudopodia extend through the pores. Foram tests in marine sediments form an extensive fossil record. • Radiolarians (p. 563) Radiolarians have fused tests usually made of silica. They ingest microorganisms by phagocytosis using their pseudopodia, which radiate from their central body.

570

UNIT FIVE

The Evolutionary History of Biological Diversity

Table 28.1 A Sample of Protist Diversity Examples from Chapter

Major Clade

Key Characteristics

Diplomonadida (diplomonads)

Two equal-sized nuclei; modified mitochondria

Pa~~abasala (parabasalids)

Undulating membrane; modified mitochondria

Euglenozoa (euglenozoans)

Spiral or crystalline rod inside flagella

Trichomonas

Kinetoplastida (kinetoplastids)

Kineioplasi (DNA in mitochondrion)

Trypanosoma

Euglenophyta (euglenids)

Paramylon as storage snolvcule.

Euglena

AWeolata (alveolates)

Alveoli beneath plasma membrane Ceratlum, Pfiesteria

Dlnoflagellata (dinoflagellates)

Armor of cellulose plates

Apicomplexa (apicomplexans)

Apical complex of organelles

Plasmodium

Ciliophora (ciliates)

Cilia used in movement and feeding; macro- and micronuclei

Paramecium, Stenior

Stramenopila (stramenopiles)

Hairy and smooth, flagella

Oomycota (oomycetes)

Hyphae that absorb nutrients

Bacillariophyta (diatoms)

Glassy, two-part wall

Water molds, white rusts, downy mildews

Chrysophyta (golden algae)

Flagella attached near one end of cell

Dinobryon

Phaeophyta (brown algae)

All mulucellular, some with alternation of generations

Laminana, Macrocystis, Postdsia

Cercozoa (cercozoans) and Radiolaria (radiolarians)

Amoebas with threadlike pseudopodia Globigerina

Foraminfera (forams)

Porous shell

Radiolaria (radiolarians)

Pseudopodia radiating from ceniral body

Amoebozoa (amoebozoans)

Arnoebas with lobe-shaped pseudopodia

Gymnamoeba (gymnamoebas)

Soil-dwelling, freshwater, or marine

Amoeba

Entamoeba (entamoebas)

Parasites

Entamoeba

Multinucleate plasmodium; [rutting bodies that function in sexual reproduction

Physarum

iida vpiasmodial slime molds) Dictyostelida i,cellular slime molds)

Multicellular aggregate ;bat brins asexual fruiting bodies

Rhodophyta (red algae)

Phycoerythrin (accessory pigment); no flagellated stages

Chlorophyta (one group of green algae)

Plant-type chloroplasts

ilmoebozoans have lobe-shaped pseudopodia Gymnamoebas (p. 564) Gymnamoebas are common unicellular amoebozoans in soil as well as freshwater and marine environments. Most are heterotrophs. Entamoebas (p. 564) Entamoebas are parasites of vertebrates and some invertebrates. Uniamoeba hlsiohtka causes amebic dysentery in humans, • Slime Molds (pp. 564-566) Plasmodial slime molds aggregate into a plasmodium, a multinucleate mass of cytoplasm undivided by membranes. The plasmodium extends pseudopodia through decomposing material, engulfing food by phagocytosis. Cellular slime molds form multicellular aggregates in which the cells remain separated by their membranes. The cellular slime mold Dictyostelium discoidmm has become an experimental model for studying the evolution of multicellularity.

iia, Dek^cna, i'almari Caulcrpa, ChLnnvdomonas, Spirogym, Ulva, Volvox

Red algae and green algae are the closest relatives of land plants • Red Algae (p. 567) lied algae have colors ranging from green to black, owing to varying amounts of the accessory pigment phycoerythrin. Most red algae are multicellular; the largest are seaweeds. They are the most abundant large algae in coastal waters of the tropics. • Green Algae (pp. 567—569) Green algae (crilorophyi.es and charophyceans) are closely related to land plants. Most chlorophytes live in fresh water, although many are marine; others live in damp soil, in snow, or as symbionts in lichens. Chlorophytes include unicellular, colonial, and multicellular Forms. Most have complex life cycles. investigation What Kinds of Prolists Do Various Habitats Support?

CHAPTER 28

571

TESTING YOUR KNOWLEDGE Evolution Connection Explain why systematists consider the kingdom Protista to be an obsolete taxon.

Scientific Inquiry Applying the "If. . . then" logic of science (see Chapter 1), what are a few of the predictions that arise from the hypothesis that plants evolved from green algae? Put another way, how could you test this hypothesis? Investigation What Kinds oj Protists Do Various Habitats Support?

572

U N I T FIVE

The Evolutionary History of Biological Diversity

Science, Technology, and Society The ability of the pathogen Plasmodium to evade the human immune system is one reason developing a malaria vaccine is so difficult. Another reason is that less money is spent on malaria research than on research into diseases that affect far fewer people, such as cystic fibrosis. What are the possible reasons for this imbalance research effort?

A Figure 29.1 Tree ferns and a moss-covered log.

Hey Concepts 29.1 Land plants evolved from green algae 29.2 Land plants possess a set of derived terrestrial adaptations 2 9 3 The life cycles of mosses and other bryophytes are dominated by the gametopbyte stage 2$.4 Ferns and other seedless vascular plants formed the first forests

1 he Greening of Earth

I

ooking at a lush landscape, such as the forest scene in Figure 29.1, it is difficult to imagine the land without L—rfany plants or other organisms- Yet for more than the first 3 billion years of Earth's history, the terrestrial surface was lifeless. Geochemical evidence suggests that thin coatings of cyanobacteria existed on land about 1.2 billion years ago. But ill was only about 500 million years ago that plants, fungi, and irnals joined them ashore. In this chapter, we will focus on land plants and how they jvolved from aquatic green algae. Although some plant species, such as sea grasses, returned to aquatic habitats during their evolution, most plants live in terrestrial environments. Therefore, we will refer to all plants as land plants, even those that are now aquatic, to distinguish them from algae, which are photosynthetic protists. Since colonizing land, plants have diversified into roughly 290,000 living species inhabiting all but the harshest environments, such as some mountaintops and some desert and polar regions. The presence of plants has enabled other life-forms—including humans—to survive on land. Plant roots have created habitats for other organisms by stabilizing landscapes. More

importantly, plants are the source of oxygen and the ultimate provider of food for land animals. In this chapter, we will trace the first 100 million years of plant evolution, including the emergence of seedless plants such as mosses and ferns. In Chapter 30, we will examine the later evolution of seed plants.

Concept

Land plants evolved from green algae As you read in Chapter 28, researchers have identified green algae called charophyceans as the closest relatives of land plants. Here we will examine the evidence of this relationship and consider what it suggests about how the algal ancestors of land plants might have been well suited to make the move to land.

Morphological and Biochemical Evidence Many key characteristics of land plants also appear in a variety of protists, primarily algae. For example, plants are multicellular, eukaryotic, photosynthetic autotrophs, as are brown, red, and certain green algae (see Chapter 28). Plants have cell walls made of cellulose, and so do green algae, di no flagellates, and brown algae. And chloroplasts with chlorophylls a and h are present in green algae, euglenids, and a few dinoflagellates, as well as in plants. 573

• Figure 29.2 Rosette cellulose-synthesizing complexes. These distinctively rose-shaped arrays of proteins are found only in land plants and charophycean algae, suggesting their close kinship (SEM).

There are four key traits, however, that land plants share only with the charophyceans, strongly suggesting a close relationship between the two groups: • Rose-shaped complexes for cellulose synthesis. The cells of both land plants and charophyceans have rosette cellulose-synthesizing complexes. These are rose-shaped arrays of proteins in the plasma membrane that synthesize the cellulose microfibrils of the cell walls (Figure 29.2). Jn contrast, linear arrays of proteins synthesize cellulose in noncharophycean algae. Also, the cell walls of plants and charophyceans contain a higher percentage of cellulose than the cell walls ol noncharophycean algae. These differences indicate that the cellulose walls in plants and charophyceans evolved independently of those in other algae. • Peroxisome enzymes. The peroxisomes (see Figure 6.19) of both land plants and charophyceans contain enzymes that help minimize the loss of organic products as a result of photorespiration (see Chapter 10). The peroxisomes of other algae lack these enzymes. • Structure of flagellated sperm. In species of land plants that have flagellated sperm, the structure of the sperm closely resembles that of charophycean sperm. • Formation of a phragmoplast. Certain details of cell division occur only in land plants and certain charophyceans, including the genera Chora and Coleochaete. For example, the synthesis of new cross-walls (cell plates) during cell division involves the formation of a phragmoplast, an alignment of cytoskeletal elements and Golgi-derived vesicles across the midline of the dividing cell (see Figure 12.10).

Genetic Evidence Over the past decade, researchers involved in an international initiative called "Deep Green" have conducted a large-scale study of major transitions in plant evolution, analyzing genes from a wide range of plant and algal species. Comparisons of both nuclear and chloroplast genes agree with the morphological and biochemical data in pointing to charophyceans— particularly Chara and Coleochaete—as the closest living relatives of land plants (Figure 29.3). Note that this does not mean that these living algae are the ancestors of plants; however, they do offer a glimpse of what those ancestors might have been like. 574

UNIT FIVE

The Evolutionary History of Biological Diversity

40 Lim (b) Coleochaete orbicubris, a diskshaped charophycean (Llvl)

A Figure 29.3 Examples of charophyceans, the closest algal relatives of land plants.

Adaptations Enabling the Move to Land Many species of charophycean algae inhabit shallow waters around the edges of ponds and lakes, where they are subject to occasional drying. In such environments, natural selection favors individual algae that can survive periods when they a^e not submerged in water. In charophyceans, a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out. An ancestral form of this chemical adaptation may have also been the precursor to the tough sporopollenin walls that encase plant spores. It is likely that rhe accumulation of such traits by at least one population of charophycean ancestors enabled their descendants—-the first land plants—to live permanently above the waterline. These evolutionary novelties opened an expanse of terrestrial habitat, a new frontier that offered enormous benefits. The bright sunlight was unfiltered by water and plankton; the atmosphere had an abundance of CO2\ the soil was rich in mineral nutrients; and initially there were relatively few herbivores and pathogens. Benefiting from these environmental opportunities became possible as adaptations evolved in plants that allowed them to survive and reproduce on land.

Concept Check 1. Describe the evidence linking plants to a charophycean ancestry. For suggested answers, see Appendix A.

Viridiplantae

Concept

Streptophyta

Land plants possess a set of derived terrestrial adaptations Many of the adaptations that emerged after land plants diverged from iheir chatophycean relatives facilitated survival and reproduction on dry land. We will explore the most important derived traits of plants. We will then examine some fossil evidence of the divergence of land plants from charophyceans and survey main groups within the plant kingdom.

Chlorophytes

Charophyceans

Embryophytes

Defining the Plant Kingdom Where exactly is the line dividing land plants from algae? Systematists are currently debating the boundaries of the plant kingdom (Figure 29.4). The traditional scheme equates the kingdom Plantae with embryophytes (plants with embryos; phyte is from the Greek word for plant). Some plant biologists now propose that the boundaries of the plant kingdom should be expanded to include the green algae most closely related to plants—the charophyceans and a few related groups—and named kingdom Streptophyta. Others suggest an even broader definition of plants that also includes chlorophytes (the noncharophycean green algae) in a kingdom Viridiplantae (see Chapter 28). Since the debate is ongoing, this text retains the embryophyte definition ol the plant kingdom and uses kingdom Plantae as the formal name for the taxon.

Derived Traits of Plants five key traits appear in nearly all land plants but are absent in the charophyceans: apical meristems; alternation of generations; walled spores produced in sporangia; multicellular gatnetangia; and multicellular, dependent embryos. Figure 29.5, on the next two pages, depicts these traits. We can infer that these traits ^iVere absent in the ancestor common to land plants and charophyceans but instead evolved independently as derived traits of land plants. Some of these traits are not unique to plants, having evolved separately in other lineages, and some traits have been lost in certain lineages of plants. However, these key traits set the first land plants apart from their closest algal relatives. Additional derived traits that relate to terrestrial life have evolved in many plant species. Permanently exposed to the air, land plants run a Ear greater risk of desiccation (drying out) than their algal ancestors. The epidermis in many species has a covering, known as a cuticle, that consists of polymers called polyesters and waxes. The cuticle acts as waterproofing, helping prevent excessive water loss from the above-ground plant organs, while also providing some protection from microbial attack. Many land plants produce molecules called secondary com-pounds, so named because they are products of secondary metabolic pathways—side branches off the primary metabolic

Ancestral alga

• Figure 29.4 Three clades that are candidates for designation as the plant kingdom. This textbook adopts the embryophyte definition of plants and uses the name Plantae for the kingdom.

pathways that produce the lipids, carbohydrates, amino acids, and other compounds common to all organisms. Secondary compounds include alkaloids, terpenes, tannins, and phenolics such as flavonoids. Various alkaloids, terpenes, and tannins have a bitter taste, strong odor, or toxic effect that helps defend against herbivores and parasites. Flavonoids absorb harmful UV radiation and may act as signals in symbiotic relationships with beneficial soil microbes. Some phenolics deter attack by pathogenic microbes. As you survey the major groups of land plants, note how secondary compounds aid their survival. Humans also benefit from secondary compounds; just one example is use of the alkaloid quinines antimicrobial properties to fight malaria.

The Origin and Diversification of Plants Paleobotanists seeking the evolutionary origin of plants have long debated what constitutes the oldest fossil evidence of land plants. In the 1970s, researchers found fossil spores dating to the Ordovician period, up to 475 million years old. Although the fossil spores resemble those of living plants, they also have some striking differences. For example, spores of living plants are typically dispersed as single grains, but the fossil spores are fused together in groups of two or four. This difference raises the possibility that the fossil spores were not produced by pfants, but by some extinct algal relative. Furthermore, the oldest known fragments of plant tissue are 50 million years younger than the. puzzling spores. CHAPTER 29 Plant Diversity I: How Plants Colonized Land

575

Figure 29.5

Derived Traits of Land Plants APICAL MERISTEMS In terrestrial habitats, a photosynthetic organism finds essential resources in two very different places. Lighr and CO^ are mainly available above-ground; water and mineral nutrients are found mainly in the soil. Though plants cannot move from place to place, their roots and shoots can elongate, increasing exposure to environmental resources. This growth in length is sustained throughout the plant's life by the activity of apical meristems, localized regions of ceil division at the tips of shoots and roots. Cells produced by apical meristems differentiate into various tissues, including a surface epidermis that protects the body and several types of internal tissues. Shoot apical meristems also generate leaves in most plants. Thus, the complex bodies of plants show structural specialization tor subterranean arid aerial organs—roots and leaf-bearing shoots, respectively, in most plants.

Apical meristems of plant shoots and roots. The light micrographs are longitudinal sections at the t ps of a shoot and root.

Apical meristenv of root 100,urn

100nm

ALTERNATION OF GENERATIONS The life cycles of all land plants alternate between two different multicellular bodies, with each form producing the other. This type of reproductive cycle, called alternation of generations, also evolved in various groups of algae but does not occur in the charophyceans, the algae most closely related to land plants. We can infer that alternation of generations is a derived characteristic of land plants—-it was not present in the ancestor common to land plants and charophyceans. Haploid multicellular organism (gametophyte)

Spores

! FERTILIZATION :

2n

Zygote

Diploid multicellular organism (sporophyte) Alternation of generations: a generalized scheme

576

UNIT FIVE

The Evolutionary History of Biological Diversity

Take care not to confuse the alternation of generations in plan-s with the haploid and diploid stages in the life cycles of all sexually reproducing organisms (see Figure 13.6). In humans, for example, meiosis in the gonads (ovaries and testesj produces haploid gamen s that unite, forming diploid zygotes that divide and become multice]lular. The haploid stage is represented only by single-celled gametes. In contrast, alternation of generations is distinguished by the fact that there are both multicellular haploid and multicellular diploid stages in the life cycle. The two multicellular body forms that alternate in the life cycles of land plants are the gametophyte and sporophyte generations. Tht cells of the gameiophyte are haploid, meaning they have a single sei of chromosomes. The gametophyte is named for its production by mitosis of haploid gametes—eggs and sperm—that fuse during fertilization, forming diploid zygotes. Mitotic division of the zygote produces the multicellular sporophyte, the spore-producing generation. Thus, the cells of the sporophyte are diploid, having two sets of chromosomes—one from each gamete. Meiosis in a mature sporophyte produces haploid spores, reproductive cells that can develop into a new organism without fusing with another cell. In contrast, gametes cannot develop directly into a multicellular organism but instead must fuse and forma zygote. Mitotic division of a plant spore produces a new multicellular gametophyte. And so the alternation of generations continues, with sporophytes producing spores that develop into gametophytes, and gametophytes producing gametes that unite, forming the zygotes that develop into sporophytes.

Plant spores are haploid reproductive cells that have the potential to grow into multicellular, haploid gametophytes by mitosis. The polymer sporopollenin makes the walls of plant spores very tough and resistant to harsh environments. This chemical adaptation makes it possible for spores to be dispersed through dry air without harm. The sporophyte has multicellular organs called sporangia (singular, sporangium) that produce plant spores. Within a sporangium, diploid cells called sporocytes, also known as spore mother cells, undergo rneiosis and generate the haploid spores. The outer tissues of tr e sporangium protect the developing spores until they are released into the air. "Multicellular sporangia that produce spores with sporopolleninenriched walls are key terrestrial adaptations of land plants. Although charophyceans produce spores, these algae lack multicellular sporangia, and their flagellated, water-dispersed spores lack sporopollenin.

i Longitudinal section of Sphagnum sporangium (LM)

Sporophyte and sporangium of Sphagnum (a moss) MULTICELLULAR GAMETANG1A Another feature distinguishing early land plants from their algal ancestors was the production of gametes within multicellular organs called gametangia. The female gametangia are called archegonia (singular, archegonium), each of which is a vase-shaped organ that produces a single egg retained within the base of the organ. Male gametangia, called antVieridia (singular, antheridium), produce and release sperm into the environment. In many major groups of living plants, the sperm, have flagella and swim to the eggs through water droplets or water films. Each egg is fertilized within, an archegonium, where the zygote develops into an embryo. As you will see in Chapter 30, the gametophytes of seed plants are so reduced in size that the archegonia and antheridia have been lost in some lineages.

Female gametophyte

Male gametophyte Archegonia and antheridia of Marchantia (a liverwort) MULTICELLULAR. DEPENDENT EMBRYOS Multicellular plant embryos develop from zygotes that are retained Avithin tissues of the female parent. The parental tissues provide the developing embryo with nutrients, such as sugars and amino acids. The embryo has specialized placental transfer cells, sometimes present in the adjacent maternal tissue as well, which enhance the transfer of nutrients from parent to embryo through elaborate ingrowths of the wall surface (plasma membrane and cell wall). This interface is analogous to the nutrient-transferring embryo-mother interface of eutherian (placental) mammals. The multicellular, dependent embryo of land plants is such a significant derived trait that land plants are also known as embryophytes.

- , • • •

Embryo and placental transfer cell of Marchantia CHAPTER 29 Plant Diversity 1: How Plants Colonized Land

577

(a) Fossilized spores. Unlike the spores of most living plants, which are single grains, these spores found in Oman are in groups of four (left; one hidden) and two (right).

[Table 29.1 Ten Phyla of Extant Plants Common Name

Approximate Number of Extant Species

Bryophytes (nonvascular plants) Phylum Hepatophyta

Liverworts

Phylum Anthocerophyta

Hornworts

Phylum Bryophyia

Mosses

9,000

•

100 15,000

Vascular Plants (b) Fossilized sporophyte tissue. The spores were embedded in tissue that appears to be from plants.

Seedless Vascular Plants

A Figure 29.6 Ancient plant spores and tissue. In 2003, scientists from Britain and Oman shed some light on this mystery when they extracted spores from 475-millionyear-old rocks from the Middle Eastern country of Oman (Figure 29.6a). Unlike previously discovered spores of this age, these were embedded in plant cuticle material that is similar to spore-bearing tissue in living plants (Figure 29.6b). After finding other small fragments of tissue that clearly belonged to plants, the scientists concluded that the Oman spores represent fossil plants rather than algae. A 2001 "molecular clock" study (see Chapter 25) of plants suggested that the common ancestor of living plants existed 700 million years ago. If this is true, then the fossil record is missing the first 225 million years of plant evolution. In 2003, however, Michael Sanderson of the University of California reported an estimate based on molecular data of 490 to 425 million years, roughly the age of the spores discovered in Oman. Whatever the precise age of the first land plants, those ancestral species gave rise to the vast diversity of living plants. Table 29.1 summarizes the 10 phyla in the taxonomic scheme used in this chapter and in Chapter 30. As you read the following overview of land plants, look at Table 29.1 in conjunction with Figure 29.7, which reflects a view of plant phylogeny that is based on plant morphology, biochemistry, and genetics. Land plants can be informally grouped based on the presence or absence of an extensive system of vascular tissue, cells joined into tubes that transport water and nutrients throughout the plant body. Most plants have a complex vascular tissue system and are therefore called vascular plants. Plants that do not have an extensive transport system—-liverworts, hornworts, and mosses—are described as "nonvascular" plants, even though some mosses do have simple vascular tissue. Nonvascular plants are often informally called bryophytes (from the Greek bryon, moss, and phyton, plant). 578

UNIT FIVE

The Evolutionary History of Biological Diversity

Phylum Lycophyla

Lycophytes (club mosses, spike mosses. and quillworts)

Phylum Pterophyta

Pterophyles (ferns, horsetails, and whisk ferns)

1,200

12,000

Seed Plants Gymnosperms Phylum Ginkgophyia

Ginkgo

1

Phylum Cycadophyia

Cycads

130

Phylum Gnetophyta

Gneiophytes (Gnciu?n, Ephcdra, and Welwiischia)

Phylum ConiferophyLa

Conifers

75

600

Attgiosperms Phylum Anthophyta

Flowering plants

250,000

Although the term bryophyte is still commonly used to refer to all nonvascular plants, debate continues over the relationships of liverworts, hornworts, and mosses to each other andl to vascular plants. Whereas some molecular studies have concluded that bryophytes are not monophyletic, a recent analysis of amino acid sequences in chloroplasts asserts tha" bryophytes form a clade. The broken lines in Figure 29.7 reflect the current uncertainty regarding bryophyte phytogeny' Whether or not bryophytes are monophyletic, they share some derived trails with vascular plants, such as multicellular embryos and apical meristems, while lacking many innovations of vascular plants, such as roots and true leaves. Vascular plants form a clade, consisting of about 93% of all plant species. Vascular plants can be categorized further into three smaller clades. Two of these clades are the lycophytes (club mosses and their relatives) and the pterophytes (ferns and their relatives). Each of these clades lacks seeds, which is why collectively the two clades are often informally called seedless vascular plants. However, notice in Figure 29.7 that seedless vascular plants are not monophyletic. The third clade

Land plants

Bryophytes (nonvascuiar plants)

Origin of land plants (about 475 mya)

£ vascular plants consists of seed plants, the vast majority of ' iving plant species. A seed is an embryo packaged with a sup>ly of nutrients inside a protective coat. Seed plants can be diided into two groups, gymnosperms and angiosperms, based >n the absence or presence of enclosed chambers in which seeds mature. Gymnosperms (from the Greek gymnos, naked, and sperm, seed) are grouped together as "naked seed" plants because their seeds are not enclosed in chambers. Surviving gymnosperm species, which consist primarily of conifers, probably form a clade. Angiosperms (from the Greek an$.on, container) are a huge clade consisting of all flowering plants. Angiosperm seeds develop inside chambers called ovaries, which originate within flowers and mature into fruits.

i

Note that the phytogeny depicted in Figure 29.7 focuses only on the relationships between extant plant lineages-—•

•*§ Figure 29.7 Highlights of plant evolution. This diagram reflects a hypothesis about the general relationships between plant groups. The broken tines indicate that the phylogeny of bryophytes is uncertain. Table 29.1 provides formal names of the plant phyla we will examin in this chapter and the next.

those that have surviving members in addition to extinct members. Paleobotanists have also discovered fossils belonging to extinct plant lineages. Many of these fossils reveal the intermediate steps leading to the distinctive plant groups found on Earth today. Concept Check 1. Identify three derived traits that distinguish plants from charophyceans and facilitate life on land. Explain. 2. Identify each structure as either haploid or diploid; (a) sporophyte; (b) spore; (c) gametophyte; (d) zygote; (e) sperm; (f) egg. For suggested answers, see Appendix A.

CHAPTER 29 Planl Diversity 1: How Plants Colonized Land

579

Concept

The life cycles of mosses and other bryophytes are dominated by the gametophyte stage Bryophytes are represented today by three phyia of small herbaceous (nonwoody) plants: liverworts (phylum Hepatophyta), hornworts (phylum Anthocerophyta), and mosses (phylum Bryophyta). Liverworts and hornworts are named for their shapes, plus the suffix wort (from the Anglo-Saxon, herb). Mosses are the most familiar bryophytes, although some plants commonly called "mosses" are not really mosses at all. These include Irish moss (a red seaweed), reindeer moss (a lichen), club mosses (seedless vascular plants), and Spanish mosses (lichens in some regions and flowering plants in others). Note that the terms Bryophyta and bryophyte are not synonymous. Bryophyta is the formal taxonomic name for the phylum that consists solely of mosses. The term bryophyte is used informally to refer to all nonvascular plants—liverworts, hornworts, and mosses. As noted previously, it has not been established whether these three groups form a clade. Systematists also continue to debate the sequence in which the three phyla of bryophytes evolved. Bryophytes acquired many unique adaptations after their evolutionary split from the ancestors they share with living vascular plants. Nevertheless, living bryophytes likely reflect some traits of the earliest plants. The oldest known fossils of plant fragments, for example, include tissues that look very much like the interior of liverworts. Researchers are eager to discover more parts of these ancient plants to see if this resemblance is reflected more broadly

Bryophyte Gametophytes Unlike vascular plants, in all three bryophyte phyla the gametophytes are larger and longer-living than sporophytes, as shown in the moss life cycle in Figure 29.8. Sporophytes are typically present only part of the time. If bryophyte spores are dispersed to a favorable habitat, such as moist soil or tree bark, they may germinate and grow into gametophytes. Germinating moss spores, for example, characteristically produce a mass of green, branched, one-cellthick filaments known as a protonema (plural, protonemata; from the Greek proto, first, and nema, threads). A protonema 580

UNIT FIVE The Evolutionary History of Biological Diversity

has a large surface area that enhances absorption of water and minerals. In favorable conditions, a protonema produces one or more "buds," each with an apical meristem that generates a gamete-producing structure known as a gametophore (''gamete bearer"). Together, a protonema and a gametophore make up the body of a moss gametophyte. Bryophyte gametophytes generally form ground-hugging carpets and are at most only a few cells thick. Such thin body parts could not support a tall plant. A second constraint on the height of bryophytes is the absence, in mosE species of vascular tissue, which is required for long-distance transport of water and nutrients. The thin structure of bryophyte organs makes it possible to distribute materials without specialized vascular tissue. Some mosses, however, including the gerius Volytrichum, have conducting tissues in the center of their "stems," and a few of these mosses can grow as tall as 2 m as a result. Plant biologists are trying to determine whether these conducting tissues are homologous with vascular plant tissues or are the result of convergent evolution. The gametophytes are anchored by delicate rhizoids, which are long, tubular single cells (in liverworts and hornworts) or filaments of cells (in mosses). Unlike roots, which are characteristic of vascular plants, rhizoids are not composed of tissues, lack specialized conducting cells, and do not play a primary role in water and mineral absorption. Mature gametophytes produce gametes in gametangia covered by protective tissue. A gametophyte may have multiple gametangia. Eggs are produced singly in vase-shaped archegonia, whereas antheridia each produce many sperm. Sorrie bryophyte gametophytes are bisexual, but in mosses the archegonia and antheridia are typically carried on separale female and male gametophytes. Flagellated sperm swim through a film of water toward eggs, entering the archegonia in response to chemical attractants. Eggs are not released but instead remain within the bases of archegonia. After fertilization, embryos are retained within the archegonia. Layers of placental transfer cells help transport nutrients to the embryos as they develop into sporophytes.

Bryophyte Sporophytes Although bryophyte sporophytes are usually green and pho tosynthetic when young, they cannot live independently They remain attached to their parental gametophytes, from which they absorb sugars, amino acids, minerals, and water. Bryophytes have the smallest and simplest sporophytes of all extant plant groups, consistent with the hypothesis that larger and more complex sporophytes evolved only later in vascular plants. A typical sporophyte consists of a foot, a seta, and a sporangium. Embedded in the archegonium, the foot absorbs nutrients from the gametophyte. The seta (plural, setae), or stalk, conducts these materials to the sporangium, also called a capsule, which uses them to produce spores by meiosis. One capsule can generate up to 50 million spores.

T Figure 29.8 The life cycle of a Polytrichum moss. Male gametophyt

O Spores develop into threadlike protonemata

The haploid protonemata produce "buds' that grow into gametophytes.

A sperm swims through a film of moisture to an archegonium and fertilizes the e

Most mosses have separate male and female gametophytes, with antheridia and archegonia, respectively.

Female gametophyte

Archegonia

^J Meiosis occurs and haploid spores develop in the sporangium of the sporophyte. When the sporangium lid pops off, the peristome "teeth" regulate gradual release of the spores. © The sporophyte grows a long stalk, or seta, that emerges from the archegonium. Calyptra

N

Archegoniurn

Q Attached by its foot, the sporophyte remains nutritionally dependent on the gametophyte.

In most mosses, the seta becomes elongated, enhancing spore dispersal by elevating the capsule. An immature capsule has a protective cap of gametophyte tissue called the calyptra, which is shed when the capsule is mature. In most moss species, the upper part of the capsule features a ring of toothlike structures known as the peristome (see Figure 29.8). The peristome is specialized for gradual spore discharge, taking advantage of penodic wind gusts that can carry spores long distances. Hornwort and moss sporophytes are larger and more complex than those of liverworts. Both hornwort and moss sporo-

0 The diplotd zygote develops into a sporophyte embryo within the archegonium.

phytes also have specialized pores called stomata (singular, stoma), which are also found in all vascular plants. These pores support photosynthesis by allowing the exchange of CO2 and O^ between the outside air and the sporophyte interior (see Figure 10.3). Stomata are the main avenues by which water evaporates from the sporophyte. In hot, dry conditions, the stomata can close, minimizing water loss. The fact that stomata are present in mosses and homworts but absent in liverworts suggests three possible hypotheses for their evolution. If liverworts are the deepest-branching lineage CHAPTER 29 Plant Diversity I: How Plants Colonized Land

581

Figure 29.9

I Bryophyte Diversity LIVERWORTS (PHYLUM HEPATOPHYTA) The common name and scientific name (from the Latin hepaticus, liver) refer to the liver-shaped gametophytes of Marchantia, a genus common in the Northern Hemisphere. In medieval times, their shape was thought to be a sign that the plants could help treat liver diseases. Some

Gametophore of female gametophyte

liverworts, such as Marchantia, are described as "thalloid" because of the flattened shape of their gametophytes. (Recall from Chapter 28 that the body of a multicellular alga is called a thallus.) Marchantia gametangia are elevated on gametophores that look like miniature trees (see also Figure 29.5). You would need a magnifying glais to see the sporophytes, which have a short seta (stalk) with a round sporangium. Some liverworts are called "leafy" because their gametophytes have stemlike structures with many leaflike appendages. Typically found in tropical and subtropical regions, leafy liverworts are much more common than thalloid species.

Sporangium Marchantia pofymorpha, a "thalloid" liverwort

Marchantia sporophyte (LM)

HORNWORTS (PHYLUM ANTHOCEROPHYTA)

MOSSES (PHYLUM BRYOPHYTA)

The common name and scientific name (from the Greek kerns, horn) refer to the shape of the sporophyte, which also resembles a small grass blade. A typical sporophyte can grow up to about 5 cm high. A sporangium extends along its length and splits open to release mature spores, starting at the tip of the horn. Gametophytes, which are usually 1 to 2 cm in diameter, grow mostly horizontally and often have multiple sporophytes attached.

In contrast with liverworts and hornworts, moss ganiefophyi.es typically grow more vertically than horizontally Heights range from less than a millimeter to more than 50 cm but are less than 15 cm in most species. The gametophytes are the structures that primarily comprise a carpet of moss. Their "leaves" are usually only one cell thick, bui more complex "leaves" with ridges coated with cuticle can be found on the common hairy-cap moss and its close relatives. Moss sporophytes are typically elongated and visible to the naked eye, with si^es ranging up to about 20 cm. Though green and photosynthetic when young, they turn tan or brownish red when ready to release spores.

An Anthoceros hornwort species

Polytrichum commune, hairy-cap moss

Sporophyte Sporophyte

SB—Game*?;ophyte

Gametophyte

582

UNIT FIVE

The Evolutionary History of Biological Diversity

of land plants, then stomata evolved once in the ancestor of homworts, mosses, and vascular plants. If hornworts are the deepest-branching lineage, then stomata may have evolved once and then been lost in the liverwort lineage. Or perhaps homworts acquired stomata independently of mosses and vascular plants. This question is important to understanding plant evolution because stomata play a crucial role in the success of vascular plants, as you will see in Chapter 36. Figure 29.9, on the facing page, illustrates exampfes of gametophytes and sporophytes in the three bryophyte phyla.

Ecological and Economic Importance of Mosses Wind dispersal of lightweight spores has distributed mosses around the world. These plants are particularly common and diverse in moist forests and wetlands, where they form habitats for tiny animals. Some moss species even inhabit such extreme environments as mountaintops, tundra, and deserts (see Chapter 50). Many mosses exist in very cold or dry habitats because they can survive the loss of most of their body water, then rehydrate when moisture is available. Few vascular plants can survive the same degree of desiccation. Moreover, phenolic compounds in moss cell walls absorb damaging levels of radiation present in deserts or at high altitudes and latitudes. One wetland moss genus, Sphagnum, or "peat moss," is especially widespread, forming extensive deposits of partially decayed organic material known as peat (Figure 29.10). Boggy regions dominated by this moss are called peat bogs. Because of the resistant phenolic compounds embedded in its cell walls, Sphagnum does not decay readily In addition, it secretes compounds that may reduce bacterial activity. Low temperatures and nutrient levels in peat bogs also inhibit decay. As a result, peat bogs can preserve mummified corpses for thousands of years. Worldwide, an estimated 400 billion tons of organic carbon are stored in peat. These carbon reservoirs help stabilize global atmospheric CO 2 concentrations (see Chapter 54). Peat has long been a fuel source in Europe and Asia, and it is still harvested for fuel today, notably in Ireland and Canada. Because peat moss has large dead cells Lhat can absorb some 20 times the moss's weight in water, it also serves as a soil conditioner and is used for packing plant roots during shipment. Current overharvesting of Sphagnum may reduce its beneficial ecological effects.

(a) Peat being harvested from a peat bog Gametophyte

Sporangium at tip of sporophyte Living photosynthetic Dead watercells storing cells 100 urn

(b) Close up of Sphagnum. Note the "leafy" gametophytes and their offspring, the sporophytes.

;

•

-

(c) Sphagnum "leaf" (LM). The combination of living photosynthetic cells and dead water-storing cells gives the moss its spongy quality.

Concept Check •#; 1. How do bryophytes differ from other plants? 2. Give three examples of how structure fits function in bryophytes. For suggested answers, see Appendix A.

(d) "Tolland Man," a bog mummy dating from 405-100 B.C. The acidic, oxygen-poor conditions produced by Sphagnum can preserve human or other animal bodies for thousands of years.

A Figure 29.10 Sphagnum, or peat moss: a bryophyte with economic, ecological, and archaeological significance.

CHAPTER 29 Plant Diversity I: How Plants Colonized Land

583

Concept

Ferns and other seedless vascular plants formed the first forests Whereas bryophytes or bryophytelike plants were the prevalent vegetation during the first 100 million years of plant evolution, vascular plants dominate most landscapes today. Living seedless vascular plants provide insights into plant evolution during the Carboniferous period, when vascular plants began to diversify but most groups of seed plants had not yet evolved. The sperm of ferns and all other seedless vascular plants are flagellated and must swim through a film of water to reach eggs, as in bryophytes. Due to these swimming sperm, as well as their fragile gametophytes, living seedless vascular plants are most common in damp environments. Thus it is likely that before the emergence of seed plants, most plant life on Earth was limited to relatively damp habitats.

Origins and Traits of Vascular Plants Fossils of the forerunners of today's vascular plants date back about 420 million years. Unlike bryophytes, these species had branched sporophytes that were not dependent on gametophytes for growth (Figure 29.11). Although these plants grew no taller than about 50 cm, their branching made possible more complex bodies with multiple sporangia. This evolutionary development facilitated greater production of spores and increased survival despite herbivory, for even if some sporangia were eaten, others might survive. The ancestors of vascular plants already displayed some derived traits of living vascular plants, but they lacked other key adaptations that evolved later. This section describes the main traits that characterize living vascular plants: life cycles with dominant sporophytes, transport in vascular tissues called xylem and phloem, and the presence of roots and leaves, including spore-bearing leaves caLled sporophylls. Life Cycles with Dominant Sporophytes Fossils suggest that the ancestors of vascular plants had life cycles characterized by gametophytes and sporophytes that were about equal in size. Among extant vascular plants, however, the sporophyte (diploid) generation is the larger and more complex plant in the alternation of generations. In ferns, for example, the familiar leafy plants are the sporophytes. You would have to get down on your hands and knees and search the ground carefully to Find fern gametophytes, which are tiny strtictures that grow 584

UNIT FIVE The Evolutionary History of Biological Diversity

*- Figure 29.11 Aglaophyton major, an ancient relative of modern vascular plants. This reconstruction from fossils dating to about 420 million years ago exhibits dichotomous (Yshaped) branching and terminal sporangia. These traits characterize living vascular plants but are lacking in bryophytes (nonva5Ciilar plants).

on or just below the soil surface. UnLil you have a chance to do that, you can study the sporophyte-dominant life cycle of seedless vascular plants in Figure 29.12, which uses a fern as an example. Then, for review, compare this life cycle with Figure 29.8, which represents a typical gametophyte-dominated life cycle of mosses and other nonvascular plants. In Chapter 30, you will see that gametophytes became even more reduced during the evolution of seed plants. Transport in Xylem and Phloem Vascular plants have two types of vascular tissue: xylem and phloem. Xylem conducts most of the water and minerals. The xylem of all vascular plants includes tracheids, tube-shaped cells that carry water and minerals up from roots (see Figure 35.9). Because nonvascular plants lack tracheids, vascular plants are sometimes referred to as tracheophytes. Tracheids are actually dead cells—only their walls remain and provide a system of microscopic water pipes. The water-conducting cells in vascular plants are lignified; that is, their cell walls are strengthened by the phenolic polymer lignin. The tissue called phloem includes living sugar-conducting cells arranged into tubes that distribute sugars, amino acids, and other organic products (see Figure 35.9). Lignified vascular tissue permitted vascular plants to grow to greater heights than bryophytes. Their stems became strong enough to withstand drooping, and they could transport water and mineral nutrients high above the ground, Evolution of Roots Lignified vascular tissue also provides benefits below-ground. Instead of the rhizoids seen in bryophytes, roots evolved in almost all vascular plants. Roots are organs that anchor vascular plants and enable them to absorb water and nutrients from the soil. Roots also allow the shoot system to grow taller.

Q The fern spore develops into a small, photosynthetic gametophyte.

0 Sporangia release spores. Most fern species produce a single type of spore that gives rise to a bisexual gametophyte.

Key Haploid (n) Diploid (2n)

Q Although this illustration shows an egg and sperm from the same gametophyte, a variety of mechanisms promote cross-fertilization between gametophytes.

iHni Spore

Young gametophyte

Antheridium

ME1OSIS

New sporophyte

Zygote

0 Fern sperm use flagella to swim from the antheridia to eggs in the archegonia.

0 On the underside of the sporophyte's reproductive leaves are spots called sori. Each sorus is a cluster of sporangia. Fiddlehead

m A zygote develops into a new sporophyte, and the young plant grows out from an archegonium of its parent, the gametophyte.

A Figure 29.12 The life cycle of a f e r n .

Root tissues of living plants closely resemble stem tissues of early vascular plants preserved in fossils. This suggests that roots may have evolved from the lowermost, subterranean portions of stems in ancient vascular plants. It is not clear whether roots evolved only once in the common ancestor of all vascular plants or independently in different lineages. Although the roots of living members of Lhese lineages of vascular plants share many similarities, the fossil evidence hints at convergent evolution. The oldest lycophyte fossils, for example, already displayed simple roots 400 million years ago, when the ancestors of ferns and seed plants still had none. Studying genes that control root development in different vascular plant species may help resolve this controversy. Evolution of Leaves Leaves are organs that increase the surface area of vascular plants, thereby capturing more solar energy for photosynthesis. In terms of size and complexity, leaves can be classified as

either microphylls or megaphylls. All lycophytes (the oldest lineage of modern vascular plants) have microphylls, small, usually spine-shaped leaves with a single vein. Almost all other vascular plants have megaphylls, leaves with a highly branched vascular system. So named because they are typically larger than microphylis, megaphylls support greater photosynthetic productivity than microphylls as a result of their greater surface area served by a network of veins. Microphylls first appear in the fossil record 410 million years ago, but megaphylls do not emerge until about 370 million years ago, near the end of the Devonian period. According to one model of leaf evolution, microphylls originated as small outgrowths of stems. These outgrowths were supported by single strands of vascular tissue (Figure 29.13a). Megaphylls, by contrast, may have evolved from a series of branches lying close together on a stem, which became flattened and developed webbing that joined branches together (Figure 29.13b). To better understand the origin of leaves, scientists are exploring genetic control of leaf development. CHAPTER

29 Plant Diversity I: How Plants Colonized Land

585

crophylls, such as those of lycophytes, may have originated as small stem outgrowths supported by single, unbranched strands of vascular tissue

(b) Megaphyiis, which have branched vascular systems, may have evolved by the fusion of branched stems.

A Figure 29.13 Hypotheses for the evolution of leaves.

Sporophylls and Spore Variations

Classification of Seedless Vascular Plants

One evolutionary milestone was the emergence of sporophylls, modified leaves that bear sporangia. Sporophylls vary greatly in structure. For example, fern sporophylls produce clusters of sporangia known as sori (singular, sorus), usually on the undersides of the sporophylls (see Figure 29.12). In many lycophytes and in most gymnosperms, groups of sporophylls form cones, technically called strobili (from the Greek strobilos, cone). In Chapter 30, you will see how sporophylls form gymnosperm strobili and parts of flowers. Most seedless vascular plant species are homosporous, having one type of sporophyll producing one type of spore that typically develops into a bisexual gametophyte, as in most ferns. In contrast, a heterosporous species has two types of sporophylls and produces two kinds of spores. Megasporangia in megasporophylls produce, megaspores, which develop into female gametophytes. Microsporangia in microsporophylls produce microspor.es, which develop into male gametophytes. All seed plants and a few seedless vascular plants are heterosporous. The following diagram compares the two conditions:

As noted earlier, living seedless vascular plants form two clades: lycophytes and pterophytes. Lycophytes (phylum Lycophyta) include club mosses, spike mosses, and quillworts. Pterophytes (phylum Pterophyta) include ferns, horsetails, and whisk ferns and their relatives. Because they differ greatly in appearance, ferns, horsetails, and whisk ferns have long been considered separate phyla: phylum Pterophyta (ferns), phylum Sphenophyta (horsetails, also called sphenophytes), and phylum Psilophyta (whisk ferns and a related genus, together known as psilophytes). However, recent molecular comparisons provide convincing evidence that all three groups form a clade. Accordingly many systematists now classify them together as the phylum Pterophyta, as we do in this chapter. Others refer to them as three separate phyla within a clade. Figure 29.14, on the facing page, describes the general groups of seedless vascular plants.

Homosporous spore production Single type of spore—>

Sporangium in sporophyll

Typically bisexual

Female gametophyte Male —^ sperm gametophyte

The Evolutionary History of Biological Diversity

Phylum Lycophyta: Club Mosses, Spike Mosses, and Quillworts Modern species of lycophytes—the most primitive group of vascular plants—are relicts of a far more eminent past. By the Carboniferous period, there were two evolutionary lines— one composed of small herbaceous plants and another line of giant woody trees with diameters of more than 2 m and heights of more than 40 m. The giant lycophytes thrived for millions of years in warm, moist swamps, but they became extinct when the climate became cooler and drier at the end of the Carboniferous period. The small lycophytes survived, represented today by about 1,200 species. Though some are commonly called club mosses and spike mosses, they are not

true mosses, which are bryophytes.

Figure 29.14

Seedless Vascular Plant Diversity LYCOPHYTES (PHYLUM LYCOPHYTA) Many lycophytes grow on tropical trees as epiphytes, plants that use oilier plants as a substrate but are not parasites. Other species grow on temperate forest floors. In some species, the tiny gametophytes live aboveground and are photosynthetic. Others live underground, nurtured by symbiotic fungi. Sporophytes have upright stems with many small leaves, as well as ground-hugging stems that produce dichotomously branching roots. In club mosses, sporophylls are clustered into dub-shaped cones (strobili). Spike mosses are usually smaller and often grow horizontally. Quillworts, named for their leaf shape, are a single genus that lives in marshy areas. Club mosses are all homosporous, "whereas spike mosses and quillworts are all heterosporous. Spores are released in clouds and are so rich in oil that magicians and photographers once ignited them to create smoke or flashes of light.

Strobili (clusters o f = ^ = =

Isoetes gunnii, a quillwort Selaginella apoda, a spike moss

Diphasiastrum tristachyum, a club moss

PTEROPHYTES (PHYLUM PTEROPHYTA) Psilotum nudum, a whisk fern

Like primitive vascular plant fossils, the sporophytes of whisk ferns (Psihtum) have dichotomously branching stems but no rootsStems have scale-like outgrowths that lack vascular tissue and may have evolved as very reduced leaves. Each yellow knob on a stem consists of three fused sporangia. Species of tine genus Tmesipteris, closely related to whisk ferns and found only in the South Pacific, also lack roots but have small, leaflike outgrowths in their stems, giving them a vinelike appearance. Both genera are homosporous, with spores giving rise to bisexual gametophytes that grow underground and are only about a centimeter long.

Equisetum arvense, field horsetail

The name refers to the brushy appearance of the stems, which have a gritty texture that made them historically useful as "scouring rushes" to scrub pots and pans. Some species have separate fertile (cone-bearing) and vegetative stems. Horsetails are homosporous. with cones releasing spores that give rise to tiny male or bisexual gametophytes. Horsetails are also called arthrophytes ("jointed plants") because their stems have joints- Rings of small leaves or branches emerge from each joint, but the stem is the main photosynthetic organ. Large air canals carry oxygen to the roots, which often grow in waterlogged soil.

Athyrium filix-femina, lady fern

Unlike most other seedless vascular plants, ferns have megaphylls (see Figure 29.13b). Sporophytes typically have horizontal stems that give rise to large leaves called fronds, often divided into leaflets. A frond grows as its coiled tip, the fiddlehead, unfurls (see the cover of this textbook). Almost all species are homosporous- The gametophyte shrivels and dies after the young sporophyte detaches itself. In most species, sporophytes have stalked sporangia with spring-like devices that catapult spores several meters. Airborne spores can be carried far from their origin. Some species produce more than a trillion spores in a plant's lifetime.

CHAPTER 29 Plant Diversity T: How Plants Colonized Land

587

A Figure 29.15 Artist's conception of a Carboniferous forest based on fossil evidence. Most of the large trees with straight trunks are lycophytes. The tree on the left with feathery branches is a horsetail. Tree ferns, not shown here, were also abundant in the "coal forests" of the Carboniferous. Animals, including giant dragonflies, also thrived.

Phylum Pterophyta: Ferns, Horsetails, and WTiisfe Ferns and Relatives Ferns radiated extensively from their Devonian origins and grew alongside tree lycophytes and horsetails in the great Carboniferous swamp forests. Today ferns are by far the most widespread seedless vascular plants, numbering more than 12,000 species. Though most diverse in the tropics, many ferns thrive in temperate forests, and some species are even adapted to arid habitats. Horsetails were very diverse during the Carboniferous period, some growing as tall as 15 m. However, today only about 15 species survive as a single, widely distributed genus, Equisetum, found in marshy places or along streams. Psilotum (whisk ferns) and a closely related genus, Tmesipteris, form a clade consisting mainly of tropical epiphytes. Whisk ferns, the only vascular plants lacking true roots and leaves, are called "living fossils" because of their striking resemblance to fossils of ancient relatives of living vascular plants (see Figures 29.11 and 29.14). However, much evidence, including analyses of DNA sequences and sperm, structure, indicates that Psilotum and Tmesipteris are closely related to ferns. This hypothesis suggests that their ancestor's true roots and leaves were lost during evolution.

The Significance of Seedless Vascular Plants The ancestors of living lycophytes, horsetails, and ferns, along with their seedless vascular relatives, grew to great heights during the Carboniferous, forming the first forests (Figure 29.15).

With the evolution of vascular tissue, toots, and leaves, these plants accelerated their rate of photosynthesis, dramatically 588

UNIT FIVE

The Evolutionary History of Biological Diversity

increasing the removal of CO 2 from the atmosphere. Scientists estimate that CO 2 levels dropped by as much as a factor of five during the Carboniferous, causing global cooling that resulted in widespread glacier formation. The seedless vascular plants that formed the first forests eventually became coal. In the stagnant waters of Carboniferous swamps, dead plants did not completely decay. This organic material turned to thick layers of peat, later covered by the sea. Marine sediments piled on top, and over millions of years, heat and pressure converted the peat to coal. In fact, Carboniferous coal deposits are the most extensive ever formed, Coal was crucial to the Industrial Revolution, and people worldwide still burn 6 billion tons a year. It is ironic that coal, formed from plants that contributed to a global cooling, now contributes to global warming by returning carbon to the atmosphere (see Chapter 54). Growing along with the seedless plants in Carboniferous swamps were primitive seed plants. Though seed plants were not dominant at that time, they rose to prominence after the swamps began to dry up at the end of the Carboniferous period. The next chapter traces the origin and diversification of seed plants, continuing our theme of terrestrial adaptation.

Concept Check 1. What are a few key differences between seedless vascular plants and bryophytes? 2. What is the major difference between most lycophytes and most ferns and their relatives?

For suggested answers, see Appendix A.

Chapter 29 Review Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

anchor gametophytes to the surface. The flagellated sperm produced by antheridia require a film of water to travel to the eggs in archegonia.

SUMMARY OF KEY CONCEPTS

• Bryophyte Sporophytes (pp. 580-583) Sporophytes grow out of archegonia and are dependent on the haploid gametophytes for nourishment. Smaller and simpler than vascular plant sporophytes, they typically consist of a foot, seta (stalk), and sporangium. Hornwort and moss sporophytes have stomata. Activity Moss Life Cycle

Land plants evolved from green algae • Morphological and Biochemical Evidence (pp. 573-574) The traits shared by plants and charophyceans include rosette cellulose-synthesizing complexes, peroxisome enzymes, similarities in flagellated sperm structure, and formation of phragmoplasts during cell division. • Genetic Evidence (p. 574) Similarities in nuclear and chloroplast genes suggest that charophyceans are the closest living relatives ofland plants. • Adaptations Enabling the Move to Land (p. 574) Traits such as sporopolknin allow charophyceans to withstand occasional drying along the edges of ponds and lakes. These traits may have enabled the algal ancestors of plants to survive in terrestrial conditions, opening the way to the colonization of dryland.

Land plants possess a set of derived terrestrial adaptations • Defining the Plant Kingdom (p. 575) Some biologists think that the plant kingdom should be expanded to include some or all green algae. Until this phylogenetic debate is resolved, this textbook retains the embryophyte definition of kingdom Plantae, • Derived Traits of Plants (pp. 575-577) Derived traits that distinguish the clade ofland plants from charophyceans, their closest algal relatives, include apical meristems, alternation of generations, walled spores in sporangia, mukicellular gametangia, and multicellular, dependent embryos. Additional derived traits, such as the cuticle and secondary compounds, evolved in many plant species. Activity Terrestrial Adaptations of Plants • The Origin and Diversification of Plants (pp. 575, 578-579) Fossil evidence indicates that plants were on land at least 475 million years ago. Subsequently, plants diverged into several major groups, including bryophytes (nonvascular plants); seedless vascular plants, such as lycophytes and ferns; and the two groups of seed plants: gymnosperms and angiosperms (flowering plants). Most systematists divide plants into ten phyla. Activity Highlights of Plant Phytogeny

The life cycles of mosses and other bryophytes are dominated by the gametophyte stage • The three phyla of bryophytes—liverworts, homworts, and mosses—may not form a clade. Debate continues over the sequence of their evolution (p. 580). • Bryophyte Gametophytes (p. 580) Liverwort and hornwort gametophytes grow more horizontally, whereas those of mosses are more vertical. Gametophytes are the dominant generation and are typically most visible, such as a mat of moss. Rhizoids

• Ecological and Economic Importance of Mosses (p. 583) Sphagnum covers great expanses of land as peat bogs and peat lands, playing an important role in the carbon cycle. Concept

Ferns and other seedless vascular plants formed the first forests • Origins and Traits of Vascular Plants (pp. 584-586) Fossils of the forerunners of today's vascular plants date back about 420 million years ago and show that these tiny plants had independent, branching sporophytes but lacked other derived traits of vascular plants, such as xylem and phloem, roots, and leaves. Life Cycles with Dominant Sporophytes. In contrast with bryophytes, sporophytes of seedless vascular plants are the larger generation, as in the example of the familiar leafy fern plant. The gametophytes are tiny plants that grow on or below the surface. Activity Fern Life Cycle Investigation What Are the Different Stages of a Fern Life Cycle? Transport in Xylem and Phloem. Vascular plants have two vascular tissues: xylem and phloem. Xylem conducts most of the water and minerals. Xylem of all vascular plants includes dead cells called tracheids. The lignin in xylem enables most vascular plants to grow taller than bryophytes. Phloem, a living tissue, conducts sugars and other organic nutrients. Evolution of Roots. Unlike the rhizoids of bryophytes, roots play an important role in absorbing water and nutrients. Roots may have evolved from subterranean stems. It is unclear whether roots evolved independently in different lineages. Evolution of Leaves. In terms of evolution, leaves are categorized into two types: microphylls and megaphylls. Microphylls, leaves with a single vein, evolved first and are typical of lycophytes. Almost all other vascular plants have megaphylls, leaves with a highly branched vascular system. Megaphylls are usually larger, widi more photosynthetic productivity. Sporophylls and Spore Variations. Sporophylls are modified leaves with sporangia. Most seedless vascular plant species are homosporous, producing one type of spore, which usually develops into a bisexual gametophyte. All seed plants and some seedless vascular plant species are heterosporous, having two types of spores that give rise to male and female gametophytes. • Classification of Seedless Vascular Plants (pp. 586-588) Seedless vascular plants include the phylum Lycophyta (club mosses, spike mosses, and quillworts) and the phylum Pterophyta (ferns, horsetails, and whisk ferns and relatives). Ancient lycophytes included woody and herbaceous plants that dominated the first forests. Modern lycophytes are small herbaceous plants. Lycophyte sporophytes have upright stems bearing many microphylls and horizontal stems that grow along the surface. Ferns are the most diverse seedless vascular plants. Most fern species are homosporous and produce clusters of sporangia known as son. Horsetails and whisk ferns are actually close relatives of ferns. CHAPTER 29 Plant Diversity 1: How Plants Colonized Land

589

• The Significance of Seedless Vascular Plants (p. 588) Seedless vascular plants dominated the earliest forests- Their growth may have helped produce the major global cooling that characterized the end of the Carboniferous period. The decaying remnants of the first forests eventually became coal

TESTING YOUR KNOWLEDGE Evolution Connection Draw a cladogram that includes a moss, a fern, and a gymnosperm. Use a charophycean alga as the outgroup. (See Chapter 25 to review cladistics.) Label each branch of the cladogram with at. least one derived characteristic unique to that clade.

Scientific Inquiry In April 1986, an accident at a nuclear power plant in Chernobyl, Ukraine, scattered radioactive fallout for hundreds of miles. In assessing the biological effects of the radiation, researchers found mosses to be especially valuable as organisms for monitoring the damage. Radiation damages organisms by causing mutations. Explain why the genetic effects of radiation can be observed sooner in bryophytes than in plants from other groups. Imagine that you are conducting tests shortly after a nuclear accident. Using potted moss plants as your experimental organisms, design an experiment to test the hypothesis that the frequency of mutations decreases with the organisms distance from the source of radiation. Investigation What Are the Different Stages of a Fern Life Cycle?

Science, Technology, and Society Bryophytes (nonvascular plants) and seedless vascular plants are common, and several of them have important economic and ecological uses. Nevertheless, very few are important agriculturally. Why? What attributes do they lack that would make a plant useful in agriculture? What attributes limit their agricultural utility?

590

UNIT FIVE

The Evolutionary History of Biological Diversity

I 'ersity II

I

The Evolution of Seed Plants A Figure 30.1 An ancient squash seed.

[Key Concepts 30.1 30.2 30.3 0.4

The reduced gametophytes of seed plants are protected in ovules and pollen grains Gymnosperms bear "naked" seeds, typically on cones The reproductive adaptations of angiosperms include flowers and fruits Human welfare depends greatly on seed plants

human history, transforming most human societies from roving bands of hunter-gatherers to permanent settlements anchored by agriculture. In this chapter, we will first examine the general characteristics of seed plants. Then we will look at the distinguishing features and evolution of gymnosperms and angiosperms.

Concept

Feeding the World ontinuing the saga of how plants transformed Earth, this chapter follows the emergence and diversification __- of seed plants. Fossils and comparative studies of living plants offer clues about the origin of seed plants some 360 million years ago. Seeds changed the course of plant evolution, enabling their bearers to become the dominant producrs in most of the terrestrial ecosystems and to make up the 'ast majority of plant biodiversitySeed plants have also had an enormous impact on human : ociety. Starting about 13,000 years ago, humans began to domesticate wheat, maize (commonly called corn in the United States), bananas, and other wild seed plants. This practice emerged separately in various regions of the world, including he Near East, East Asia, Africa, and the Americas. One piece sf evidence, the well-preserved squash seed in Figure 30.1, was found in a cave in Mexico and dates from between 8,000 ind 10,000 years ago. This seed differs from wild squash ;eeds, suggesting that squash was being cultivated by that Ame. The domestication of seed plants, particularly angiosperms, produced the most important cultural change in

The reduced gametophytes of seed plants are protected in ovules and pollen grains We begin with an overview of key terrestrial adaptations that seed plants added to those already present in bryophytes and seedless vascular plants (see Chapter 29). In addition to seeds, the following are common to all seed plants: reduced gametophytes, heterospory, ovules, and pollen. Seedless plants lack these adaptations, except for a few species that exhibit heterospory.

Advantages of Reduced Gametophytes Mosses and other bryophytes have life cycles dominated by gametophytes, whereas ferns and other seedless vascular plants have sporophyte-dominated life cycles. The evolutionary trend of gametophyte reduction continued further in the vascular plant lineage that led to seed plants. While the gameLophytes of seedless vascular plants are visible to the naked eye, the gametophytes of seed plants are mostly microscopic. This miniaturization allowed for an important evolutionary innovation in seed plants: Their tiny gametophytes can develop from spores retained within the sporangia of the parental sporophyte. This arrangement protects the delicate female 591

(egg-containing) gametophytes from environmental stresses. The moist reproductive tissues of the sporophyte shield the gametophytes from drought conditions and from UV radiation. This relationship also enables the dependent gametophytes to obtain nutrients from the sporophyte. In contrast, the freeliving gametophytes of seedless plants must fend for themselves. Figure 30.2 contrasts the sporophyte -gam etophyte relationships in bryophytes, seedless vascular plants, and seed plants.

i

I %—Sporophyte (2r»

Gametophyte (n)

Heterospory: The Rule Among Seed Plants You read in Chapter 29 that nearly all seedless plants ai homosporous—they produce one kind of spore, which usually gives rise to a bisexual gametophyte. The closest relatives of seed plants are all homosporous, suggesting that seed plants also had homosporous ancestors. At some point, seed plants cjr their ancestors became heterosporous: Megasporangia in megasporophylls produce megaspores that give rise to female gametophytes, and microsporangia in microsporophylls prciduce microspores that give rise to male gametophytes. Each megasporangium has a single functional megaspore, whereas each microsporangium contains vast numbers of microspores. As we noted previously, the miniaturization of seed plant gametophytes contributed to the great success of this clade. Next we will look at the development of the female gameiophyie within an ovule and the development of the male gametojphyte in a pollen grain. Then we will follow the transformation of the ovule into a seed after fertilization.

Ovules and Production of Eggs (a) Sporophyte dependent on gametophyte (mosses and other bryophytes). In the gametophyte-dominant life cycle of mosses and other bryophytes, the dependent sporophyte is nourished by the gametophyte as it grows out of the archegoniurn.

Microscopic female gametophytes (n) in ovulate cones ^ (dependent)

(b) Large sporophyte and small, independent gametophyte (ferns and other seedless vascular plants). The sporophyte is the dominant generation in ai! vascular plants. The gametophytes of most ferns, though small, are photosynthetic and free-living (not dependent on • the sporophyte for nutrition).

Sporophyte (2n), the flowering plant (independent) Microscopic male gametophytes (n) inside these parts, of flowers (dependent)

Microscopic female gametophytes (n) inside these parts of flowers (dependent) (c) Reduced gametophyte dependent on sporophyte (seed plants: gymnosperms and angiosperms). Gametophytes of seed plants are :''•' surrounded by sporophyte tissue from which the gametophyte derives its nutrition. Unlike bryophytes and most seedless vascular plants, seed • : plants typically have microscopic gametophytes,

A Figure 30.2 Gametophyte/sporophyte relationships. 592

UNIT FIVE

The Evolutionary History of Biological Diversity

Although a few species of seedless plants are heterosporous, seed plants are unique in retaining the megaspore within the parenit sporophyte (see Figure 30.2c). Layers of sporophyte tissue called integuments envelop and protect the megasporangium. Gymjnosperm megaspores are surrounded by one integument, whereas those in angiosperms usually have two integument! The whole structure—megasporangium, megaspore, and their integument(s)—is called an ovule (Figure 30.3a). Inside each ovule (from the Latin ovulum, little egg), a female gametophyte develops from a megaspore and produces one or more egg cells

Pollen and Production of Sperm Microspores develop into pollen grains, which contain ths male gametophytes of seed plants. Protected by a tough coa. containing the polymer sporopollenin, pollen grains can be carried away from their parent plant by wind or by hitchhiking on the body of an animal that visits the plant Lo feed. Thi transfer of pollen to the part of a seed plant containing the ovules is called pollination. If a pollen grain germinates (begins growing), it gives rise to a pollen tube that discharges tw( sperm into the female gametophyte within the ovule, a: shown in Figure 30.3b. Recall that in bryophytes and seedless vascular plants sucr as ferns, free-living gametophytes release flagellated spern that must swim through a film of water to reach egg cells. Thi distance for this sperm transport rarely exceeds a few cen timeters. In seed plants, by contrast, the female gametophyte never leaves the sporophyte ovule, and the male gametophytes in pollen grains are durable travelers that can be carried long distances by the wind or by pollinators, depending on the species. Living gymnosperms provide evidence of th evolutionary transition. The sperm of some gymnosperm

Embryo (2n) 'new sporophyte) (a) Unfertilized ovule, in this sectional view through the ovule of a pine (a gymnosperm), a fleshy megasporangium is surrounded by a protective layer of tissue called an integument. (Angiosperms have t w o integuments.) A

(b) Fertilized ovule. A megaspore develops into a multicellular female gametophyte. The micropyle, the only opening through the integument, allows entry of a pollen grain. The pollen grain contains a male gametophyte, which develops a pollen tube that discharges sperm.

Figure 30.3 F r o m o v u l e to s e e d .

species retain the ancient flagellated condition, but flagella have been lost in the sperm of most gymnosperms and all angiosperms. The sperm of seed plants do not require water or naotility because the pollen is carried to the pollen tube, wjhich then conveys the sperm to the ovule.

T he Evolutionary Advantage of Seeds \Ye have been discussing characteristics of seed plants, but what exactly is a seed? if a sperm fertilizes an egg of a seed plant, the zygote grows into a sporophyte embryo. As shown in Figure 30.3c, the whole ovule develops into a seed, which consists of the embryo, along with a food supply, packaged within a protective coat derived from the integument(s). The evolution of seeds enabled plants bearing them to better resist harsh environments and to disperse offspring more widely. Until the advent of seeds, the spore was the only protective stage in any plant life cycle. For example, moss spores may survive even if the local environment becomes too cold, too hot, or too dry for the mosses themselves to live. Their tilny size enables the moss spores to be dispersed in a dormant slate to a new area, where they can germinate and give rise to new moss gametophytes if and when the environment is favorable enough for them to break dormancy. Spores were the njiain way that mosses and other seedless plants spread over

S

(c) Gymnosperm seed. Fertilization initiates the transformation of the ovule into a seed, which consists of a sporophyte embryo, a food supply, and a protective seed coat derived from the integument.

irth for the first 100 million years of plant life on land, in contrast to a spore, which is single-celled, a seed is a ulticellular structure that is much more resistant and cornex. Its protective coat is derived from the integument(s) of tike ovule. After being released from the parent plant, a seed ritay remain dormant for days, months, or even years. Under favorable conditions, it can then germinate, with the spororjhyte embryo emerging as a seedling. Some seeds drop close to their parent sporophyte plant; others are carried far by the wind or animals.

Concept Check 1. Contrast sperm delivery in seedless plants with sperm delivery in seed plants. 2. What additional features of seed plants, not present in seedless plants, contributed to their enormous success on land? For suggested answers, see Appendix A.

Gymnosperms bear "naked" seeds, typically on cones Recall from Chapter 29 that gymnosperms are plants that have "naked" seeds that are not enclosed in ovaries. Their seeds are exposed on modified leaves that usually form cones (strobili). In contrast, angiosperm seeds are enclosed in fruits, which are mature ovaries. Among the gymnosperms are many well-known varieties of conifers, or conebearing trees, including pine, fir, and redwood. Of the ten plant phyla in the taxonomic scheme adopted by this textbook (see Table 29.1), four are gymnosperms: Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta. The relationships of these four phyla to each other are uncertain. Figure 30.4, on the next two pages, surveys the diversity of extant gymnosperms. Plant Diversity II: The Evolution of Seed Plants

593

Gymnosperm Diversity PHYLUM CYCADOPHYTA

PHYLUM GINK60PHYTA

Cycads are the next largest group of gymnosperms after the conifers. They have large cones and palmlike leaves (true palm species are angiosperms). Only about 130 species survive today, but cycads thrived during the Mesozoic era, known as the "Age of Cycads" as well as the "Age of Dinosaurs."

Ginkgo biloba is the only extant species of this phylum. Also known as the maidenhair tree, it has deciduous fanlike leaves that turn gold in autumn. It is a popular ornamental tree in cities because it tolerates air pollution well. Landscapers usually plant only pollen-producing trees because the fleshy seeds smell rancid as they decay.

Cycas revoluta

PHYLUM GNETOPHYTA Plants in the phylum Gnetophyta, called gnetophytes, consist of three genera: Gnetum, Ephedm, and Wdwitschia. Some species are tropical, whereas others live in deserts. Although very different in appearance, the genera are grouped together based on molecular data. Welwitschia. This genus consists of one species, Welwitschia mirabilis, a plant that lives only in the deserts of southwestern Africa, its straplike leaves are among the largest leaves known.

Gnetum. This genus includes about 35 species of tropical trees, shrubs, and vines, mainly native to Africa and Asia. Their leaves look similar to those of flowering plants, and their seeds took somewhat like fruits.

Ephedra. This genus includes about 40 species that inhabit arid regions worldwide. These desert shrubs, commonly called "Mormon tea," produce the compound ephedrine, which is used medicinally as a decongestant.

594

UNIT FIVE

The Evolutionary History of Biological Diversity

Phylum Coniferophyta is by far the largest of the gymnosperm phyla,

Most conifers are evergreens; they retain their leaves throughout

consisting of about 600 species of conifers (from the Latin conus,

the year. Even during winter, a limited amount of photosynthesis oc-

ccne, and jerre, to carry)- Many are large trees, such as cypresses and

curs on sunny days. When spring comes, conifers already have fully

redwoods. A few conifer species dominate vast forested regions of

developed leaves that can take advantage of the sunnier, warmer

the Northern Hemisphere, where the growing season is relatively

days. Some conifers, such as the dawn redwood, tamarack, and

st ort because of latitude or altitude.

larch, are deciduous trees that lose leaves each autumn. Douglas fir. "Doug fir" (Pseudotsuga menziesii) provides more timber than any other North American tree species. Some uses include house framing, plywood, pulpwood for paper, railroad ties, and boxes and crates.

Common juniper. The "berries" of the common juniper (Juniperus communis) are actually ovuleproducing cones consisting of fleshy sporophylls.

Wollemia pine. Survivors of a conifer group once known only from fossils, living Wollemia pines (Wollemid nobilis) were discovered in 1994 in a national park only 150 kilometers from Sydney, Australia. The species consists of just 40 known individuals in two small groves. The inset photo compares the leaves of this "living fossil" with actual fossils.

Pacific yew. The bark of Pacific yew f"J3xa brevifolia) is a source of taxol, a compound used to treat women with ovarian cancer. The leaves of a E-iropean yew species produce a s&milar compound, which can be harvested without destroying the plants. Pharmaceutical companies are continuing to develop techniques for synthesizing drugs with taxol-like properties.

Bristlecone pine. This species (Pinus longaevs), which is found in the White Mountains of California, includes some of the oldest living organisms, reaching ages of more than 4,600 years. One tree (not shown here) is called Methuselah because it may be the world's oldest living tree, in order to protect the tree, scientists keep its location a secret.

Sequoia. This giant sequoia (Sequoiadendron giganteum) in California's Sequoia National Park weighs about 2,500 metric tons, equivalent to about 24 blue whales (the largest animals), or 40,000 people. The giant sequoia is one of the largest living organisms and also among the most ancient, with some individuals estimated to be between 1,800 and 2,700 years old. Their cousins, the coast redwoods {Sequoia sempervirens), grow to heights of more than 110 meters (taller than the Statue of Liberty) and are found only in a narrow coastal strip of northern California and southern Oregon.

30

Plant Diversity II: The Evolution of Seed Plants

595

Gymnosperm Evolution Fossil evidence reveals that by the late Devonian period, some plants had begun to acquire some adaptations that characterize seed plants. For example, Archaeopteris was a heterosporous tree that produced wood (Figure 30.5). It did not, however, bear seeds. Such transitional species of seedless vascular plants are sometimes called progymnosperms. The first seed-bearing plants to appear in the fossil record were gymnosperms dating from around 360 million years ago, more than 200 million years before the first angiosperm fossils. These early gymnosperm species became extinct, along with several later gymnosperm lineages. Although the relationships between extinct and surviving lineages of seed plants remain uncertain, morphological and molecular evidence places surviving lineages of seed plants into two clades: the gymnosperms and the angiosperms (see Figure 29.7). Early gymnosperms lived in Carboniferous ecosystems still dominated by lycophytes, horsetails, ferns, and other seedless vascular plants. As the Carboniferous period gave way to the Permian, markedly warmer and drier climatic conditions favored the spread of gymnosperms. The flora and fauna changed dramatically, as many groups of organisms disappeared and others became prominent (see Chapter 26). Though most pronounced in the seas, the changeover also affected terrestrial liie. For example, in the animal kingdom, amphibians decreased in diversity and were replaced by reptiles, which were especially well adapted to the and conditions. Similarly, the lycophytes, horsetails, and ferns that dominated the Carboniferous swamps were largely replaced by gymnosperms, which were more suited to the drier climate. In such gymnosperms as pines and firs, among the adaptations to arid conditions are their needle-shaped leaves, which have thick cuticles and relatively small surface areas.

• Figure 30,5 A progymnosperm. Archaeopteris, which lived 360 million years ago, produced wood and was heterosporous, but did not produce seeds. Growing up to 20 m tall, it had fernlike leaves.

Geologists consider the end of the Permian period, about 251 million years ago, to be the boundary between the Paleozoic ("old life") and Mesozoic ("new life") eras. Liie changed profoundly as gymnosperms dominated terrestrial ecosystems throughout the Mesozoic, serving as the food supply for giant herbivorous dinosaurs. The Mesozoic era ended with mass extinctions of the dinosaurs and many other groups, and the planet gradually cooled. Although angiosperms now dominate most terrestrial ecosystems, many gymnosperms remai an important part of Earths flora.

A Closer Look at the Life Cycle of a Pine As you read earlier in the chapter, the evolution of seed plan! included three key reproductive adaptations: the increasing dominance of the sporophyte generation; the advent of the seed as a resistant, dispersible stage in the life cycle; and the evolution of pollen as an airborne agent that brings gametes together. To reinforce your understanding of these adaptations, study Figure 30.6, which shows the life cycle of a pi one of the most familiar gymnosperms. The pine tree is the sporophyte; its sporangia are located o scalelike structures packed densely in cones. Like all seed plants, conifers are heterosporous. In conifers, the two types of spores are produced by separate cones: small pollen cones and large ovulate cones. In most pine species, each tree ha^ both types of cones. In pollen cones, microsporocytes (mifcrospore mother cells) undergo meiosis and produce haploid microspores. Each microspore develops into a pollen grain containing a male gametophyte. In pines and other conifers, the yellow pollen is released in large amounts and carried bv the wind, dusting everything in its path. Meanwhile, in ovulate cones, megasporocytes (megaspore mother cells) undergo meiosis and produce haploid megaspores. Surviving megaspores develop into female gametophytes, which are retained within the sporangia. From the time young pollen and ovulate cones appear on the tree, it takes nearly three years for the male and female gametophytes to be produced and brought together, and for mature seeds to form from the fertilized ovules. The scales of each ovulate cone then separate, and die seeds travel on the wind. A seed that lands in a habitable place germinates, its embryo emerging as a pine seedling.

1. Based on Figure 30.4, explain why the various types of gymnosperms can be described as being similar yet distinctive. 2. Explain how the pine life cycle (see Figure 30.6) reflects basic characteristics of seed plants. For suggested answers, see Appendix A.

596

U N I T FIVE

The Evolutionary History of Biological Diversity

• Figure 30.6 The life cycle of a pine. Q An ovulate cone scale has two ovules, each containing a megasporangium. Only one ovule is shown.

Key Haploid (n) Diploid (2n)

0 A pollen grain enters through the micropyle and germinates, forming a pollen tube that slowly digests through the megasporangium. •Ivlegasporangiurn

© W h i l e the pollen tube develops, the megasporocyte (megaspore mother cell) undergoes meiosis, producing four haploid cells. One survives as a megaspore.

Sporophyll Microsporangium © A pollen cone contains many microsporangia held in sporophylls. Each microsporangium contains microsporocytes (microspore mother cells). These undergo meiosis, giving rise to haploid microspores that develop into pollen grains.

Seedling

Seeds on surface of ovulate scale

Food reserves (gametophyte tissue) (n)

0 The female gametophyte develops within the megaspore and contains two or three archegonia, each with an egg.

Seed coat (derived from parent sporophyte) (2n)

Embryo (new sporophyte) (2n)

Egg nucleus (n)

0 Fertilization usually occurs more than a year after pollination. All eggs may be fertilized, but usually only one zygote develops into an embryo. The ovule becomes a seed, consisting of an embryo, food supply, and seed coat.

CHAPTER 30

© By the time the eggs are mature, two sperm cells have developed in the pollen tube, which extends to the female gametophyte. Fertilization occurs when sperm and egg nuclei unite.

Plant Diversity II: The Evolution of Seed Plants

597

Concept

The reproductive adaptations of angiosperms include flowers and fruits Commonly known as flowering plants, angiosperms are seed plants that produce the reproductive structures called (lowers and fruits. The name angiosperm (from the Greek angion, container) refers to seeds contained in fruits, the mature ovaries. Today, angiosperms are the most diverse and widespread of all plants, with more than 250,000 species (about 90% of all plant species).

Characteristics of Angiosperms All angiosperms are classified in a single phylum, Anthophyta (from the Greek anthos, flower). Before considering the evolution of angiosperms, we will examine their key adaptations—flowers and fruits—and the roles they play in the angiosperm life cycle. Flowers The flower is an angiosperm structure specialized for sexual reproduction. In many angiosperm species, insects or other animals transfer pollen from one flower to the female sex organs on another flower, which makes pollination more directed than the wind-dependent pollination of most gymnosperms. However, some angiosperms are wind-pollinated, particularly those species that occur in dense populations, such as grasses and tree species in temperate forests. A flower is a specialized shoot that can have up to four rings of modified leaves called floral organs: sepals, petals, stamens, and carpels (Figure 30.7). Starting at the base of the flower are the sepals, which are usually green and enclose the flower before it opens (think of a rosebud). Above the sepals are the petals, which are brightly colored in most flowTers and aid in attracting pollinators. Flowers that are wind-pollinated, however, generally lack brightly colored parts. The sepals and petals are sterile floral organs, meaning that they are not directly involved in reproduction. Within the whorl of petals are the fertile sporophylls, floral organs that produce spores. The two whorls of sporophylls are the stamens and carpels. Stamens, the microsporophylls, produce microspores that give rise to pollen grains containing male gametophytes. A stamen consists ol a stalk called the filament and a terminal sac, the anther, where pollen is produced. Carpels are the megasporophylls, which make 598

UNIT FIVE

The Evolutionary History of Biological Diversity

Sepal

I \^ \ j ^

Ovule

Receptacle & Figure 30.7 The structure of an idealized flower.

megaspores and their products, female gametophytes. Many angiosperms have flowers with multiple carpels. At the tip of the carpel is a sticky stigma that receives pollen. A style leads to the ovary at the base of the carpel, which contains one or more ovules. II fertilized, an ovule develops into a seed. The whorls ody plan emerged, scientists are studying newly discovered pler, resembling the "primitive" flowers of Archdefructus. Such ossils and developmental genes that underlie flowers and debates, typical whenever a seemingly transitional fossil is ther angiosperm innovations (see Chapter 35). discovered, may be resolved when more fossils and other types of evidence emerge. J :ossil Angiosperms An "Evo-Devo" Hypothesis of Flower Origins !n the late 1990s, scientists in China discovered some intriguing lossils of 125-rmllion-year-old angiosperms. These fossils, Chapter 24 discussed how studying developmental genes informs LOW named Archaefructus liaoningensis and Archaefructus sinensis hypotheses about animal evolution. The same holds true for flowFigure 30.11), display both derived and primitive traits. A. ering plants. Botanist Michael Frohlich of the Natural History 'nensis, for example, has anthers and has seeds inside closed Museum in London has used the "evo-devo" approach—the synarpels but lacks petals and sepals. In 2002, scientists comthesis of evolutionary and developmental biology—in hypothepleted a phylogenetic comparison of A. sinensis with 173 livsizing how pollen-producing and ovule-producing structures ng plants. (A. liaoningensis was not included because its fossils were combined into a single flower. He proposes that the ancestor re not as well preserved.) The researchers concluded that of angiosperms had separate pollen-producing and ovulejimong all known plant fossils, Archaefructus is the most producing structures. Then, as a result of a mutation, ovules declosely related to all living angiosperms. veloped on some microsporophylls, which evolved into carpels. Because Frohlich asserts that the flower evolved mainly If Archaefructus is indeed a "proto-angiosperm," that would from the pollen-producing ("male") reproductive structure of a suggest that the ancestors of flowering plants were herbaceous gymnosperm ancestor, his view is known as the "mostly male"" •ather than woody. Archaefructus was discovered along with hypothesis. Supporting evidence includes comparisons of fish fossils and has bulbous structures that might be aquatic genes that give rise to flowers and cones. Flower-development adaptations, implying that perhaps angiosperms originated as genes are usually related to pollen-producing gymnosperm aquatic plants. The discoverers of Archaefructus have suggested genes. Also, certain mutations cause flowering plants to grow .hat fast-growing herbaceous plants could have returned to CHAPTER 30 Plant Diversity II: The Evolution of Seed Plants

601

ovules on sepals and petals, which demonstrates that the position of ovules can easily be changed- By comparing angiosperm and gymnosperm genes, botanists are testing Frohlichs hypothesis and other evo-devo models for the origin of flowers.

Angiosperm Diversity From humble beginnings in the Mesozoic, angiosperms diversified into more than 250,000 species that now dominate most terrestrial ecosystems. Until the late 1990s, most systematists divided flowering plants into two groups, based partly on the number of cotyledons, or seed leaves, in the embryo. Species with one cotyledon were called monocois, and those with two were called dicots. Other features, such as flower and leaf structure, were also used to define the two groups. For example, monocots typically have parallel leaf veins (think of a grass

blade), while the veins of most dicots have a netlike pattern (think of an oak leaf}- Some examples of monocots are orchids, palms, and grain crops such as maize, wheat, and rice. Some examples of dicots are roses, peas, sunflowers, and maples. Recent DNA studies, however, indicate that the monocoitdicot distinction likely does not completely reflect evolutionary relationships. Current research supports the view that monocots form a clade but reveals that the remaining amgiospemi species are not monophyletic. The vast majority of species traditionally called dicots do form a clade, now knowm as eudicots ("true" dicots), but the others are now divided into several small lineages. Three of these lineages are informally called basal angiosperms because they appear to iriclude the flowering plants belonging to the oldest lineages Another lineage, known as the magnoliids, evolved late" Figure 30.12 provides an overview of angiosperm diversity

Figure 30.12

Angiosperm Diversity BASAL ANGIOSPERMS

HYPOTHETICAL TREE OF FLOWERING PLANTS

Surviving basal angiosperms are currently though! to consist of three small lineages, including only about 100 species. The oldest lineage seems to be represented by a single species, Amhorclia tr'a hopocia. Two other surviving lineages diverged later: a clade that includes water lilies and then a clade consisting of the star anise and its relatives.

Amborella trichopoda. This small shrub, found only on the South Pacific island of New Caledonia, may be the sole survivor of a branch at the very base of the angiosperm tree. Amborells lacks vessels, which are present in more derived angiosperms. Consisting of xylem cells arranged in continuous tubes, vessels transport water more efficiently than tracheids. Their absence in Amborella indicates they evolved after the lineage that gave rise to Amborella branched off.

Water lily {Nymphaea "Rene Gerard"). Water lilies are living members of a clade that may be predated only by the Amborella lineage.

Star anise (Illicium floridanum). This species represents a third surviving lineage of basal angiosperms.

602

U N I T FIVE

T h e E v o l u t i o n a r y H i s t o i \ ;>l l-'ioiosr_eal Diversity

MAGNOLIIDS Magnoliids consist of about 8,000 species, most notably magnolias', laurels, and black pepper plants. They include both woody and herbaceous species. Although they share some primitive traits witli basal angiosperms, such, as a typically spiral rather than whorleql arrangement of flower organs, magnoliids are actually more closely related to monocots and eudicots.

Southern magnolia (Magnolia grand/flora). This member of the magnolia family is a woody magnoliid. This variety of southern magnolia, called "Goliath," has flowers that measure up to about a foot across.

More than one-quarter of angiosperm species are monocots—-about 70,000 species. These examples represent some of the largest families.

More than two-thirds of angiosperm species are eudicots—roughly 170,000 species. Below is a sampling of flora] diversity

Characteristics

\l™cholzi californtca)

Fygmy date palm {Phoenix roebelenii)

Zucchini {Cucurbits pepo), female (left) and male flowers

CHAPTER 30 Plant Diversity II: The Evolution of Seed Plants

603

Evolutionary Links Between Angiosperms and Animals Ever since they colonized land, animals have influenced the evolution of terrestrial plants, and vice versa. Animals crawling on the forest floor created a selective pressure favoring plants that kept their spores and gametophytes off the ground, out of easy reach of most animals. This trend in plants may in turn have favored the evolution of flight in insects. This type ot mutual evolutionary influence between species is often advantageous for both partners. For example, as flowers and fruits evolved in angiosperms, some animals helped plants by dispersing their pollen and seeds. Meanwhile, the animals received a benefit as they ate the plants' nectars, seeds, and fruits. Plant-pollinator relationships have likely played a role in increasing angiosperm and animal diversity (Figure 30.13). The most extreme of these relationships is between an individual species of flower that can only be pollinated by a single animal species. In Madagascar, for example, one species of orchid has an If-inch-long nectary, so the nectar can only be consumed by a moth with an If-inch-long proboscis! Such linked adaptations, involving reciprocal genetic change in two species, are described as coevolution, as we will discuss farther in Chapter 53. Most relationships are less specific, however. For example, the flowers of a particular species may attract insects rather than birds, but many different insect species may serve as pollinators. Conversely, a single animal species—a honeybee species, for

(a) A flower pollinated by honeybees. This honeybee is harvesting pollen and nectar (a sugary solution secreted by flower glands) from a Scottish broom flower. The flower has a tripping mechanism that arches the stamens over the bee and dusts it with pollen, some of which will rub off onto the stigma of the next flower the bee visits.

UNIT FIVE

pollinators, such as diverse species of bees or hummingbirds1. Other influences, such as animals that eat flowers while providing no pollination, can also help to shape flower diversity. Angiosperm evolution has contributed particularly to the variety of insect species. But the expansion of grasslands during the past 65 million years has also increased the diversity of grazing mammals such as horses. Grasses that are C4 photcsynthesizers spread because a decline in the atmospheric COn levels gave a selective advantage to C4 photosynthesis (see Chap ter 10). And as you will read in Chapter 34, the shift fron forests to grasslands in Africa between 10 and 2 million year ago was a crucial factor in the evolution of our own lineage.

Concept Check 1. It has been said that an oak is an acorn's way of making more acorns. Write an explanation that includes these terms: sporophyte, gametophyte, ovule, seed, ovary, and fruit. 2. Compare and contrast a pine cone and a flower in terms of structure and function. 3. Explain the use of the terms monocot, dicot, and eudicot.

(b) A ffower pollinated by hummingbirds. The long, thin beak and tongue of this rufous hummingbird enable the animal to probe flowers that secrete nectar deep within floral tubes. Before the hummer leaves, anthers will dust its beak and head feathers with pollen. Many flowers that are pollinated by birds are red or pink, colors to which bird eyes are especially sensitive.

A Figure 30.13 Flower-pollinator relationships.

604

example—may pollinate many plant species. But even in thess less specific relationships, flower color, fragrance, and structure often relied spectalization for a particular taxonomic group of

The Evolutionary History of Biological Diversity

For suggested answers, see Appendix A.

(c) A flower pollinated by nocturnal animals. Some angiosperms, such as this cactus, depend mainly on nocturnal pollinators, including bats. Common adaptations of such plants include large, light-colored, highly fragrant flowers that nighttime pollinators can locate.

Human welfare depends greatly on seed plants Throughout this unit, we have been highlighting the important vijays in which humans depend on various groups of organisms. No group is more important to our survival than seed plants. In fc rests and on [arms, seed plants are key sources of food, fuel, ood products, and medicine. Our reliance on them makes the •eservation of plant diversity critical.

Products from Seed Plants Most of our food comes from angiosperms. Just six crops— wfheat, rice, maize, potatoes, cassava, and sweet potatoes-— yield 80% of all the calories consumed by humans. We also depend on angiosperms to feed livestock raised for meat: It takes A .8 pounds of grain to produce a pound of grain-fed beef. Modern crops are the products of a relatively recent burst af genetic change, resulting from artificial selection after humans began domesticating plants approximately 13,000 yjears ago. To appreciate the scale of the transformation, contrast the sizes of domesticated plants and their smaller wild relatives, as in the case of maize and teosinte (see Figure 8.14). At the genetic level, scientists can also glean informaon about domestication by comparing the genes of crops dth those of wild relatives. With maize, dramatic changes, ich as increased cob size and removal of the hard coating round teosinte kernels, may have been initiated by as few as ve gene mutations. How did wild plants change so dramatically in such relavely little time? For thousands of years, farmers have selected seeds of plants with desirable traits (large fruits, for examle) to plant for the next year's crops. But humans may also be omesticating certain other plants unconsciously, as in the case if wild almonds. Almonds contain a bitter compound called mygdalin that repels birds and other animals. Amygdalin Teaks down into cyanide, so eating a large number of wild atLonds can be fatal. But mutations can reduce the level of amygdalin, making almonds sweet rather than bitter. Wild birds eat almonds from trees with such mutations. According to one hypothesis, humans may have observed birds eating the almonds and then eaten the almonds themselves, passing some seeds in heir feces. The seeds germinated around human settlements md grew into plants with sweeter, less dangerous almonds. In addition to basic crops, flowering plants provide nontaple foods. Two of the world's most popular beverages come rom tea leaves and coffee beans, and you can thank the tropcal cacao tree for cocoa and chocolate. Spices are derived rom various plant parts, such as flowers (cloves, saffron), ruits and seeds (vanilla, black pepper, mustard, cumin), eaves (basil, mint, sage), and even bark (cinnamon).

Many seed plants, both gymnosperms and angiosperms; are sources of wood, which is absent in all living seedless plants. Wood consists of an accumulation of tough-walled xylem cells (see Chapter 35). Wood is the primary source of fuel for much of the world, and wood pulp, typically derived from conifers such as fir and pine, is used to make paper. Worldwide, wood also remains the most widely used construction material. For centuries, humans have also depended on seed plants for medicines. Many cultures have a long tradition of using herbal remedies, and scientific research has identified the relevant secondary compounds in many of these plants, leading to the synthesis of medicines. Willow leaves and bark, for instance, have been used since ancient times in pain-relieving remedies, including prescriptions by the Greek physician Hippocrates. In the 1800s, scientists traced the willow's medicinal property to the chemical salicin. A synthesized derivative, acetylsalicylic acid, is what we call aspirin. Although modern chemistry facilitates laboratory synthesis, plants remain an important direct source of medicinal compounds. In the United States, for example, about 25% of prescription drugs contain one or more active ingredients extracted or derived from plants, typically from seed plants. Other ingredients were first discovered in seed plants and then synthesized artificially. Table 30,1 lists some medicinal uses of secondary compounds of seed plants.

Table 30.1 A Sampling of Medicines Derived from Seed Plants Compound

Example of Source

Example of Use

Atropine

Belladonna plant

Digitalin

Foxglove

Pupil dilator in eye exams Heart medication

Menthol

Eucalyptus tree

Ingredient in cough medicines

Morphine

Opium poppy

Pain reliever

Quinine Taxoi

Cinchona tree (see below) Pacific yew

Malaria preventative Ovarian cancer drug

Tui'bocurarine

Curare tree

Muscle relaxant during surgery

Vinblastine

Periwinkle

Leukemia drug

Cinchona bark, source of quinine

CHAPTER 30 Plant Diversity TI: The Evolution of Seed Plants

605

Threats to Plant Diversity Although plants may be a renewable resource, plant diversity is

not. The exploding human population and its demand tor space and resources are extinguishing plant species at an unprecedented rate. The problem is especially severe in the tropics, where more than half the human population lives and where population growth is fastest. Fifty million acres of tropical rain forest, an area about the size of the state of Washington, are cleared each year, a rate that would completely eliminate Earth's tropical forests within 25 years. The most common cause of this destruction is slash-and-burn clearing of forests for agricultural use (see Chapter 55). As forests disappear, so do thousands of plant species. Once a species becomes extinct, it can never return. The loss of plant species is often accompanied by the loss of insects and other rain forest animals. Researchers estimate that habitat destruction in rain forests and other ecosystems is claiming hundreds of species each year. This rate is faster than in any other period, even during the extensive Permian and Cretaceous extinctions. While the toll is greatest in the tropics, the threat is global.

Chapter

Many people have ethical concerns about contributing i o the extinction of living forms. But there are also practical reasons to be concerned about the loss of plant diversity. So far, we have explored the potential uses of only a tiny fraction J>f the more than 290,000 known plant species. For example, almost all our food is based on the cultivation of only about two dozen species of seed plants. And fewer than 5,000 plarlit species have been studied as potential sources of medicinek The tropical rain forest may be a medicine chest of healing plants that could be extinct before we even know they exist. If we begin LO view rain forests and other ecosystems as living treasures that can regenerate only slowly, we may learn to hairvest their products at sustainable rates. What else can we do 1 preserve plant diversity? Few questions are as important.

Concept Check i 1. Explain why it is accurate to consider plant diversity to be a nonrenewable resource. For suggested answers, see Appendix A.

Review

Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

Gymnosperms bear "naked" seeds, typically on cones

SUMMARY OF KEY

The reduced gametophytes of seed plants are protected in ovules and pollen grains • Advantages of Reduced Gametophytes (pp. 591-592) The gametophytes of seed plants develop within the walls of spores retained within tissues of the parent sporophyte, which provides proiection and nutrients. • Heterospory; The Rule Among Seed Plants (p. 592) Seed plants evolved from plants that had megasporangia (which produce megaspores that give rise to female gametophytes) and inicrosporangia (which produce microspores that give rise LO male gametophytes). • Ovules and Production of Eggs (p. 592) An ovule consists of a megasporangium, megaspore, and protective integument(s). The female gametophyte develops from the megaspore and produces one or more eggs. • Pollen and Production of Sperm (pp. 592-593) Pollen, which can be dispersed by air or animals, eliminated the water requirement for fertilization. • The Evolutionary Advantage of Seeds (p. 593) A seed is a sporophyte embryo, along with its food supply, packaged in a protective coat. Seeds are more resistant than spores and can be distributed widely by wind or animals. 606

UNIT FIVE

The Evolutionary History of Biological Diversity

• Extant gymnosperms include cycads, Ginkgo biloba, gnetophytes: and conifers (pp. 593-595). • Gymnosperm Evolution (p. 596) Gymnosperms appear early in the plant fossil record and dominated Mesozoic terrestrial ecosystems. Living seed plants can be divided into two groups: gymnosperms and angiosperms. • A Closer Look at the Ufe Cycle of a Pine (pp. 596-597) Dominance of the sporophyte generation, the development of seed: from fertilized ovules, and the role of pollen in transferring sperm to ovules are key features oi a typical gymnosperm life cycle. Activity Pine Life Cycle

The reproductive adaptations of angiosperms include flowers and fruits • Characteristics of Angiosperms (pp. 598-601) Flowers generally consist of four whorls of modified leaves: sepals, petals, stamens (which produce pollen), and carpels (which pro duce ovules). Ovaries ripen into fruits, which are often carried by wind, water, or animals to new locations. In the angiosperm life cycle, double fertilization occurs when a pollen tube discharges two sperm into the female gametophyte (embryo sac) within an ovule. One sperm fertilizes the egg, while the other combines with two nuclei in the center cell of the female gametophyte and initiates development of food-storing endosperm tissue. The endosperm nourishes the developing embryo. Activity Angiosperm Life Cycle

Angiosperm Evolution (pp. 601-602) An adaptive radiation of angiosperms occurred during the late Mesozoic period. Fossils and evo-devo studies offer insights into the origin of flowers. Angiosperm Diversity (pp. 602-603) The two main groups of angiosperms are monocots and eudicots. Basal angiosperms are less derived. Magnoliids share some traits with basal angiosperms but are more closely related to monocots and eudicots. Investigation How Are Trees Identified by Their Leaves? Evolutionary Links Between Angiosperms and Animals (p. 604) Pollination of flowers by animals and transport of seeds by animals are two important relationships in terrestrial ecosystems.

uman welfare depends greatly on seed plants Products from Seed Plants (p. 605) Humans depend on seed pfants for food, wood, and many medicines. Threats to Plant Diversity (p. 606) Destruction of habitat is causing extinction of many plant species and the animal species they support.

TESTING YOUR KNOWLEDGE Evolution Connection The history of life has been punctuated by several mass extinctions. For example, the impact of a meteorite may have wiped out the dinosaurs and many forms of marine life at the end of the Cretaeous period (see Chapter 26). Fossils indicate thai plants were luch less severely affected by this and other mass extinctions. Vhat adaptations may have enabled plants to withstand these dissters belter than animals?

Scientific Inquiry uggest a way to test the hypothesis that a particular angiosperm pecies is pollinated exclusively by beetles. nvestigation How Are Trees Identified by Their Leaves?

Science, Technology, and Society Why are tropical rain forests being destroyed at such an alarming ate? What kinds of social, technological, and economic factors are esponsible? Most forests in developed Northern Hemisphere counries have already been cut. Do the developed nations have a right o ask the developing nations in the Southern Hemisphere to slow r stop the destruction of their forests? Defend your answer. What inds of benefits, incentives, or programs might slow the assault on le rain forests?

CHAPTER 30 Plant Diversity II: The Evolution ol Seed Plants

607

Fungi

li Figure 31.1 Two species of fungi decomposing a log.

Key Concepts 31.1 Fungi are heterotrophs that feed by absorption

31.2 Fungi produce spores through sexual or asexual life cycles 31.3 Fungi descended from an aquatic, singlecelled, flagellated protist

31.4 Fungi have radiated into a diverse set of lineages

31.5 Fungi have a powerful impact on ecosystems and human welfare

Mighty Mushrooms

I

f you were to walk through the Malheur National Forest in eastern Oregon, you might notice a few clusters of honey mushrooms (Armillaria ostoyae) scattered here and there beneath the towering trees. Though the trees might appear to dwarf the mushrooms, as strange as it sounds, the reverse is actually true. These mushrooms are only the above-ground portion of a single enormous fungus. Its subterranean network ot filaments spreads through 890 hectares (2,200 acres) of the forest—more than the area ol 1,600 football fields. Based on its current growth rate, scientists estimate that this fungus, which weighs hundreds of tons, has been growing for 2,600 years.

The innocuous honey mushrooms on the forest floor are a fitting symbol ol the neglected grandeur of the kingdom Fungi. Most of us are barely aware of these eukaryotes beyond the occasional brush with athlete's foot or spoiled food. Yet fungi are a huge and important component of the biosphere. Their diversity alone is staggering: Although 100,000 species 608

have been described, it is estimated that there are actually a: many as 1.5 million species ol fungi. Some fungi are single^ celled, but most form complex multicellular bodies, which ir many cases include the above-ground structures we know &\ mushrooms. This diversity has enabled fungi to colonize jus about every imaginable terrestrial habitat; airborne spore: have even been found 160 km above the ground. Fungi are not only diverse and widespread, but they are alsc essential for the well-being of most terrestrial ecosystems. The) break down organic material and recycle nutrients (Figure 31.1) allowing other organisms to assimilate essential chemical elements. Almost all plants depend on a symbiotic relationship with tungi that helps their roots absorb minerals from the soil Humans also benefit from fungi's services to agriculture anc forestry as well as their essential role in making products ranging from bread to antibiotics. But it is also true that a tiny fraction of fungi causes diseases in plants and animals. In this chapter, we will investigate the structure of fungi survey the members of the kingdom Fungi, and discuss theii ecological and commercial significance.

Concept

Fungi are heterotrophs that feed by absorption Despite their vast diversity, fungi share some key traits, most importantly the way in which they derive nutrition-

Nutrition and Fungal Lifestyles Like animals, fungi are heterotrophs-—they cannot make their own food as plants and algae can. But unlike animals, fungi do not ingest (eat) their food. Instead, they digest their food whiL

Fungal mycelia are nonmotile; they cannot run, swim, or Ely in search of food or mates. But the mycelium makes up for its lack of mobility by swiftly extending the tips of its hyphae into new territory In most fungi, the hyphae are divided into cells by crosswalls, or septa (singular, septum). Septa generally have pores large enough to allow ribosomes, mitochondria, and even nuclei to flow from cell to cell (Figure 31.3a). Some fungi lack septa. Known as coenocytic fungi, these organisms consist of a continuous cytoplasmic mass containing hundreds or thousands of nuclei (Figure 31,3b). The coenocytic condition results from the repeated division of nuclei without cytoplasmic division. This description may remind you of the plasmodial slime molds you read about in Chapter 28, which also consist of cytoplasmic masses containing many nuclei. This similarity was one reason that slime molds were formerly

it is still in the environment by secreting powerful hydrolytic enzymes, called exoenzymes, into their surroundings. Exoe1izym.es break down complex molecules to smaller organic compounds that the fungi can absorb into their bodies and use. This absorptive mode of nutrition is related to the diverse . h estyles exhibited by fungi. Among the fungi are species that live as decomposers (also called saprobes), parasites, and mut alistic symbionts. Saprobic fungi break down and absorb nutrients from nonliving organic material, such as fallen logs, aiimal corpses, and the wastes of living organisms. Parasitic fingi absorb nutrients from the cells of living hosts. Some parasitic fungi are pathogenic, including species that infect human lungs and other species that are responsible for about £ 0% of plant diseases. Mutualistic fungi also absorb nutrients f"om a host organism, but the mutualistic fungi reciprocate with functions beneficial to the host in some way such as aiding a plant in the uptake of minerals from the soil.

Body Structure Some fungi exist as single cells and are known as yeasts. However, most species £re multicellular. The morphology of multicellular fungi enhances their ability DO absorb nutrients from their surroundings (Figure 31.2). The bodies of these ilungi typically form a network of tiny filaments called hyphae (singular, hypha). Hyphae are composed of tubular cell walls surrounding the plasma membrane and cytoplasm of the cells. Unlike the cellulose walls of plants, fungal cell walls Contain chitin, a strong but flexible nitrogencontaining polysaccharide that is also

Reproductive structure. The mushroom produces tiny cells called spores.

Hyphae. The mushroom and its subterranean mycelium are a c _^!. nuo

ound in the external skeletons of insects md other arthropods. Fungal hyphae form an interwoven nass called a mycelium (plural, mycelia) hat surrounds and infiltrates the material f a mycelium maximizes the ratio of its

Mycelium

A. Figure 31.2 Structure of a multicellular fungus. The top photograph shows the sexual isurface area to its volume, making feeding structures, called fruiting bodies, of the penny bun fungus (Boletus edulis). The bottom left

more efficient. Just 1 cm of rich organic lilt o:ill may contain as much as 1 km of hyphae with a total surface area of more than 300 crn interfacing with the soil. A fungal mycelium grows rapidly, as proteins and other materials synthesized by the fungus are channeled through cytoplasmic streaming to the tips of the extending hy. The fungus concentrates its energy and resources on adding hyphal length and thus overall absorptive surface area, rather than on increasing hyphal girth.

photograph shows a macroscopic view of a mycelium growing on fallen conifer needles, and the bottom right photograph shows a microscopic view of a mycelium (LM).

x

Pore

^Septum

• Figure 31.3 Structure of hyphae.

(a) Septate hypha

(b) Coenocytic hypha CHAPTER 31

Fungi

609

classified as fungi; molecular comparisons have since confirmed that in fact the two clades are not closely related. Some fungi have specialized hyphae that allow them to feed on living animals (Figure 31.4a). Other species have specialized hyphae called haustoria that enable them to penetrate the tissue of their hosts (Figure 31.4b). Mutually beneficial relationships between such fungi and plant roots are called mycorrhizae. (Mycorrhizae means "fungus roots.") Mycorrhizal fungi (fungi that form mycorrhizae) can deliver phosphate ions and other minerals to plants, which the plants themselves cannot acquire on their own. In exchange, the plants supply the fungi with organic nutrients. There are several different types of mycorrhizal fungi. Ectomycorrhizal fungi (from the Greek ektos, out) form sheaths of hyphae over the surface of a root and also grow into the extracellular spaces of the root cortex (see Figure 37.12a). Endomycorrhizal fungi (from

the Greek entos, in) extend their hyphae through the root ce. wall and into tubes formed by invagination (pushing inward) oj the root cell membrane (see Figure 37.12b). You will read moi about these types of mycorrhizal fungi later in this chapter. Concept Check 1. Compare and contrast the nutritional mode of a fungus with your own nutritional mode. 2. Describe how the structure of a fungus is adapted to its nutritional mode. For suggested answers, see Appendix A.

Concept

Fungi produce spores through sexual or asexual life cycles

(a) Hyphae adapted for trapping and killing prey. In Arthrobotrys, a soil fungus, portions of the hyphae are modified as hoops that can constrict around a nematode (roundworm) in less than a second. The fungus then penetrates its prey with hyphae and digests the prey's inner tissues (SEM). Plant Fungal hypha*

Haustorium (b) Haustoria. Mutualistic and parasitic fungi grow specialized hyphae called haustoria that can penetrate the cell walls of plants. Haustoria remain separated from a plant cell's cytoplasm by the plasma membrane of the plant cell (dark gold).

A. Figure 31.4 Specialized hyphae.

610

LJNiTFiVE

The Evolutionary History of Biological Diversity

Fungi propagate themselves by producing vast numbers o spores, either sexually or asexually. For example, puffballs, the reproductive structures of certain fungi, may release trillions oil spores in cloud-like bursts (see Figure 31.18d). Spores can be carried long distances by wind or water, and it they land in a moist place where there is food, they germinate, producing new mycelia. To appreciate how effective spores are at dispers ing fungi, try leaving a slice of melon exposed to the air. Within a week or so, you should see fuzzy mycelia growing from the microscopic spores constantly falling onto the melon slice. Figure 31.5 generalizes the many different life cycles thai can produce fungal spores. In this section, we will survey general aspects of the sexual and asexual life cycles of fungi. Later in the chapter we will examine more closely the life cycles of specific phyla of fungi.

Sexual Reproduction The nuclei of fungal hyphae and spores of most species are haploid, except for transient diploid stages that form during sexual life cycles. Generally, sexual reproduction in fungi begins when hyphae from two distinct mycelia release sexual signaling molecules called pheromones. If the mycelia are of different mating types, the pheromones from each partner bind to receptors on the surface of the other, and the hyphae extend toward the source of the pheromones. When the hyphae meet, they fuse. This "compatibility test" contributes to genetic variation by preventing hyphae from fusing with other hyphae from the same mycelium or another genetically identical mycelium. The union of the cytoplasm of tbe two parent mycelia is known as plasmogamy. In many fungi, the haploid nuclei contributed by each parent do not fuse right away. Instead, parts of the mycelia contain coexisting, genetically different

Key Haploid (n) Heterokaryotic (unfused nuclei from different parents) Diploid (In)

Heterokaryotic P LAS MO GAMY (fusion of cytoplasm) I KARYOGAMY (fusion of nuclei)

Spore-producing' structures

Zygote SEXUAL REPRODUCTION

Spores •ASEXUAL REPRODUCTION

GERMINATION "ION

I Spore-producing structures

Spores

Figure 31.5 Generalized life cycle of fungi. Many—but not all—fungi reproduce both xually and asexually. Some reproduce only sexually, others only asexually.

uclei. Such a mycelium is said to be a helerokaryon (meanrig "different nuclei")- In some species, heterokaryotic nycelia become mosaics, with different nuclei restricted to afferent parts of the network, in other species, the different mclei mingle and may even exchange chromosomes and genes in a process similar to crossing over (see Chapter 13). In some fungi, the haploid nuclei pair off two to a cell, one rom each parent. Such a mycelium is said to be dikaryotic meaning "two nuclei"). As a dikaryotic mycelium grows, the wo nuclei in each cell divide in tandem without fusing. Hours, days, or even centuries may pass between plas"nogarny and the next stage in the sexual cycle, karyogamy. " During karyogamy, the haploid nuclei contributed by the two parents fuse, producing diploid cells. Zygotes and other tranient structures form during karyogamy, the only stage in vhich diploidy exists in most fungi. Meiosis restores the hapoid condition, and the mycelium then produces specialized eproductive structures that produce and disperse the spores. The sexual processes of karyogamy and meiosis generate xtensive genetic variation, an important prerequisite for daptive evolution. (See Chapters 13 and 23 to review how pex can increase genetic diversity within a population.) The leterokaryotic condition also offers some of the advantages of iiploidy, in that one haploid genome may be able to compensate for harmful mutations in the other.

only reproduce asexually. As with sexual reproduction, the processes of asexual reproduction vary widely among fungi. Some fungi that can reproduce asexually grow as mold. Mold is a familiar sight in the kitchen, where it forms furry carpets on fruit, bread, and other foods (Figure 31.6). Molds grow rapidly as mycelia and produce spores. Many species that can grow as molds can also reproduce sexually il they come in contact with other mating types. Other asexual fungi are yeasts. Yeasts inhabit liquid or moist habitats, including plant sap and animal tissues. Instead of producing spores, yeasts reproduce asexually by simple cell division or by the pinching of small "bud cells" off a parent cell (Figure 31.7). Some yeast species can also grow as filamentous mycelia, depending on the availability of nutrients. Yeasts sometimes can also reproduce sexually

A Figure 31.6 Penicillium, a mold commonly encountered as a saprobe on f o o d . The clusters of bead-like structures in the SEM are conidia, involved in asexual reproduction.

lOjirn

Asexual Reproduction In addition to sexual reproduction, many fungi can reproduce asexually. Clones are produced by mitotic production of spores, which can be spread by air or water. Some species

•> Figure 31.7 The yeast Saccharomyces cerevisiae in various stages of budding (SEM).

CHAPTER 31

Fungi

611

Many molds and yeasts have no known sexual stage. Biologists who study fungi, or mycologists, have traditionally called such fungi deuteromycetes (the suffix -mycete means "fungus") or imperfect fungi (from the botanical use of the term perfect to refer to the sexual stages of life cycles). Whenever a sexual stage of a so-called deuteromycete is discovered, the species is classified in a particular phylum, depending on the type of sexual structures it forms. In addition LO searching for little-known sexual stages of such unassigned fungi, mycologists also can now avail themselves of genetic techniques to identify taxonomic status. Concept Check 1. In terms of haploidy versus diploidy, how do the life cycles of humans and lungi differ? 2. Suppose that you sample the DNA of two mushrooms on opposite sides of your yard and find that they are identical. What are two hypotheses that could reasonably account for this result? For suggested answers, see Appendix A.

Concept

Fungi descended from an aquatic, single-celled, flagellated protist

The Origin of Fungi Phylogenetic systematics suggests that fungi evolved from a flagellated ancestor. While the majority of fungi lack flagella, the lineages of fungi thought to be the earliest to diverge (the chytrids, discussed later in this chapter) do have flagella. Moreover, most of the protists that share a close common ancestor with animals and fungi also have flagella. These three groups of eukaryotes—the fungi, animals, and their protistan relatives—are called opisthokonts, members of the clade Opisthokonta. This name refers to the posterior (opistho) location of the flagellum in these organisms. Phylogenetic evidence also suggests that the ancestor of fungi was unicellular. The fact that other evidence indicates that animals are more closely related to some unicellular opisthokonts than to fungi suggests that animals and fungi must have evolved multicellularity independently from different single-celled ancestors. 612

UNIT FIVE

The Evolutionary History of Biological Diversity

>• Figure 31.8 Fossil fungal hyphae and spores from the Ordovician period (about 460 million years ago; LM).

Based on molecular clock analysis (see Chapter 25), scientists have estimated that the ancestors of animals and fungi diverged into separate lineages 1.5 billion years ago. However, the oldest undisputed fossils of fungi are only about 460 im> lion years old (Figure 31.8). One possible explanation for this discrepancy is that the microscopic ancestors of terrestrial fungi fossilized poorly.

The Move to Land Much of the fungal diversity we observe today may have hac. its phylogenetic origin in an adaptive radiation when life began to colonize land. Fossils of the earliest known vascular plants from the late Silurian period contain evidence of myco.rci hizae, the symbiotic relationships between plants and subterL ranean fungi we discussed earlier. Plants probably existed irl this symbiotic relationship from the earliest periods of colonization of land.

1. Why are fungi classified as opisthokonts when most fungi lack flagella? 2. Explain the evolutionary significance oi the presence of mycorrhizae in the earliest vascular plants. For suggested answers, see Appendix A.

Concept

Fungi have radiated into a diverse set of lineages The phylogeny of fungi is currently the subject of much research. In the past decade, molecular analysis has helpec clarify the evolutionary relationships between fungal groups although there are still areas of uncertainty Figure 31.9 presents a simplified version of a current hypothesis of fungal phylogeny. In this section, we will survey each of the major funga groups identified in this phylogenetic tree.

hytrids

Zygote fungi

Hyphae

A Figure 31.10 Chytrids. The globular fruiting body of Chytridium sprouts branched hyphae (LM). Inset: Chytrids have a flagellated stage called a zoospore (TEM).

Zygomycetes and other chytrids

Gtomeromycetes, ascomycetes, and i basidiomycetes

A Figure 31.9 Phylogeny of fungi. Most mycologists currently 'ecognize five phyla of fungi. Molecular systematics is providing information on the relationships between these phyla; the dashed branches indicate groups thought to be paraphyletic (see Figure 31.11).

ytrids Fungi classified in the phylum Chytridiomycota, called chytrids, are ubiquitous in lakes and soil. Some are saprobes; others parasitize protists, plants, or animals. Molecular evidence supports the hypothesis that chytrids diverged earliest in fungal evolution. Like other fungi, chytrids have cell walls made of chitin, and they also share certain key enzymes and metabolic pathways with other fungal groups. Some chytrids form colonies with hyphae, while others exist as single spherical cells. But chytrids are unique among fungi in having flagellated spores, called zoospores (Figure 31.10). Until recently systematists thought that fungi lost flagella only once in their history, after chytrids had diverged from other lineages. However, molecular data indicate that some "chytrids" are actually more closely related to another fungal group, the zygomycetes. If this is true, then flagella were lost on more than one occasion during the evolution of fungi (Figure 31.11). For this reason, many systematists consider the phyla Chytridiomycota and Zygomycota to be paraphyletic (compare Figures 31.9 and 31.11).

A Figure 31.11 Multiple evolutionary losses of flagella. Phylogenetic studies indicate that the common ancestor of fungi had flagella, which have been lost independently in several lineages.

Zygomycetes The approximately 1,000 known species of zygomycetes (fungi in the phylum Zygomycota) exhibit a considerable diversity of life histories. This phylum includes fastgrowing molds responsible for rotting produce such as peaches, strawberries, and sweet potatoes during storage. Other zygomycetes live as parasites or as commensal (neutral) symbionts of animals. CHAPTER 31

Fungi

613

The life cycle of Rhizopiis stolonijer (black bread mold) is fairly typical of zygomycetes (Figure 31.12). Horizontal hyphae spread out over the food, penetrate it, and absorb nutrients. The hyphae are coenocytic, with septa found only where reproductive cells are formed. In the asexual phase, bulbous black sporangia develop at the tips of upright hyphae. Within each sporangium, hundreds of haploid spores develop and are dispersed through the air. Spores that happen to land on moist food germinate, growing into new mycelia. Some zygomycetes, such as Pilobohs, can actually "aim" their sporangia toward conditions associated with good food sources (Figure 31.13).

If environmental conditions deteriorate—-for instance, f the mold consumes all its food—Rhizopus may reproduce se: ually The parents in a sexual union are mycelia of differed mating types that possess different chemical markers although they may appear identical. Plasmogamy produces a sturdy structure called a sygosporangium, in which karyojgamy and then meiosis occur. Note that while a zygospo!rangium represents the zygote (2n) stage in the life cycle, it i.s not a zygote in the usual sense (that is, a cell with one diploid nucleus). Rather, a zygosporangium is a multinucleate structure, first heterokaryotic with many haploid nuclei from thi two parents, then with many diploid nuclei after karyogamy.

Key O Mycelia have various mating types {here designated +, with red nuclei, and -, with blue nuclei).

© Neighboring mycelia of different mating types form hyphal extensions called gametangia, each walled off around several haploid nuclei by a septum.

Haploid (n) Heterokaryotic (n + n Diploid (2n)

© A heterokaryoti zygosporangium forms, containing multiple haploid nuclei from the two parents.

© Mycelia can also reproduce asexually by forming sporangia that produce genetically identical haploid spores. © The sporangium disperses genetically diverse, haploid spores. -Sporangium

Dispersal and germination

© When conditions are favorable, karyogamy occurs, followed by meiosis.

Mycelium

A Figure 31.12 The life cycle of the zygomycete Rhizopus stolonifer (black bread mold). 614

UNIT FIVE

The Evolutionary History of Biological Diversity

© This cell develops a rough, thick-walled coating that can resist dry environments and other harsh conditions for months.

t The zygosporangium then breaks dormancy, germinating into a short sporangium.

years, however, it has become clear that microsporidia are not primitive eukaryotes, but rather are highly derived parasites. In 2002, Bryony Williams and colleagues at the Natural History Museum in London discovered that microsporidia actually have tiny organdies derived from mitochondria. Meanwhile, molecular comparisons have revealed that microsporidia are more closely related to fungi than to any other eukaryotes. A 2003 analysis provided some data suggesting that microsporidia be classified as zygomycetes. Microsporidia demonstrate the extraordinary changes that can occur as organisms adapt to a parasitic lifestyle. In their case, they lost almost all resemblance to their fungal relatives. 0.5 mm

Figure 31.13 PUobolus aiming its sporangia. This i:ygomycete decomposes animal dung. The mycelium bends its sporebearing hyphae toward bright light, where grass is likely to be growing. "Jrhe fungus then shoots its sporangia like cannonballs as far as 2 m. Grazing animals such as cows ingest the fungi with the grass and then ;catter the spores in feces.

Zygosporangia are resistant to freezing and drying and are netabolically inactive. When conditions improve, a zygospo'rangium undergoes meiosis, germinates into a sporangium, and releases genetically diverse haploid spores that may colonize a new substrate. Microsporidia Microsporidia are unicellular parasites of animals and protists {Figure 31.14). They are often used in the biological control of insect pests. While microsporidia do not normally infect humans, they do pose a risk to people with HIV and other immune-compromised conditions. In many ways, microsporidia are unlike most other eukaryotes. They do not have conventional mitochondria, for example. As a result, microsporidia have been something of a taxonomic mystery, thought by some researchers to be an ancient, deep-branching lineage of eukaryotes. In recent

Glomeromycetes The glomeromycetes, fungi assigned to the phylum Glomeromycota, were formerly considered zygomycetes. But analyses of hundreds of fungal genomes indicate that the glomeromycetes form a separate clade (monophyletic group). Despite their lack of numbers—only 160 species have been identified to date—the glomeromycetes are an ecologically significant group. All glomeromycetes form a distinct type of endomycorrhizae called arbuscular mycorrhizae (Figure 31.15). The tips of the hyphae that push into plant root cells branch into tiny treelike structures known as arbuscules. About 90% of all plants have such symbiotic partnerships with glomeromycetes.

Figure 31.14 A eukaryotic cell infected by microsporidia. A large vacuole nside this host eukaryotic cell contains spores and developing : orms of the parasite Encephalitozoon ntestinalis (TEM).

Spore

A Figure 31.15 Arbuscular mycorrhizae. Glomeromycetes form endomycorrhizae with plant roots, supplying minerals and other nutrients to the roots. This SEM depicts the branched hyphae—an arbuscule—of Giomus mosseae bulging into a root cell by pushing in the membrane (the root has been treated to remove the cytoplasm).

CHAPTER 31

Fungi

615

Ascomycetes Mycologists have described more

than 32,000 species of ascomycetes (fungi in the phylum Ascomycota) from a variety of marine, freshwater, and terrestrial habitats. The defining feature of ascomycetes is the production of sexual spores in saclike asci (singular, ascus); thus, they are commonly called sac fungi. Unlike zygomycetes, most ascomycetes bear their sexual stages in fruiting bodies, or ascocarps, which range in size from microscopic to macroscopic. The spore-forming asci are found in the ascocarps.

system that prevents "junk DNA" from accumulating tc harmful levels. Although the life cycles of various ascomycete groups differ in the details of their reproductive structures and processes there are some common elements. Ascomycetes reproduce: asexually by producing enormous numbers of asexual spore: called conidia (Figure 31.17). Conidia are not formed inside sporangia, as are the asexual spores of most zygomycetes Rather, they are produced externally at the tips of specializec hyphae called conidiophores, often in clusters or long chains from which they may be dispersed by the wind. Conidia ma) also be involved in sexual reproduction, fusing with hyphae from a mycelium of a different mating type, as occurs in Neurospora (see Figure 31.17). Following tusion of two diflerent mating types, plasmogam) occurs. The nonseptated bulge, or ascogonium, is now

Ascomycetes vary in size and complexity from unicellular yeasts to elaborate cup fungi and morels (Figure 31.16). They include some of the most devastating plant pathogens, which will be discussed later in the chapter. However, many ascomycetes are important saprobes, particularly of plant material. More than 40% of all ascomycete species live with green algae or cyanobacteria in symbiotic associations called lichens. Some ascomycetes form mycorrhizae with plants. Still others live between mesophyll cells in leaves, where they release toxic compounds that apparently help protect the plant from insects. One of the best-studied ascomycetes is Neurospora crassa, a bread mold (see

Figure 31.1 fid). Though Neurospora grows readily on bread, in the wild it is found growing on burned vegetation. As discussed in Chapter 17, biologists in the 1930s used Neurospora to formulate the one gene-one enzyme hypothesis. Today, this ascomycete continues to serve as a model organism; in 2003 its entire genome was published. With 10,000 genes, the genome of this tiny fungus is three-fourths the size of the Drosophila genome and one-third the size of the human genome! However, the Neurospora genome is relatively compact, having few of the stretches of noncoding DNA that occupy much space in the genomes oi humans and many other eukaryotes. Biologists have found evidence suggesting that Neurospora has a genomic defense

616

UNIT FIVE

(a) The cup-shaped ascocarps (fruiting bodies) of Aleuria aurantia give this species its common name: orange peel fungus. (b) The edible ascocarp of Morchella esculenta, the succulent morel, is often found under trees in orchards. lOyrn

Tuber melanosporum is a truffle, an ascocarp that grows underground and emits strong odors. These ascocarps have been dug up and the middle one sliced open.

A Figure 31.16 Ascomycetes (sac fungi).

The Evolutionary History of Biological Diversify

(d) Neurospora crassa feeds as a mold on bread and other food (SEM).

b'terokaryotic. The coenocytic ascogonium extends hyphae that are partitioned by septa, forming dikaryotic cells, each with two haploid nuclei representing the two parents. The cells at trie tips of these dikaryotic hyphae develop into asci. Within an aicus, karyogamy combines the two parental genomes, and then meiosis forms four genetically different nuclei. This is folk wed by a mitotic division, forming eight ascospores. In many ~ ci, the eight ascospores are lined up in a row in the order in

O Ascomycete mycelia can also reproduce asexually by producing haploid conidia.

which they formed from a single zygotic nucleus. The ascospores develop in and are eventually discharged from the ascocarp. In contrast to the life cycle of zygomycetes, the extended dikaryotic stage of ascomycetes (and also basidiomycetes) provides increased opportunity for genetic recombination. For example, in some ascomycetes, certain dikaryotic cells fuse repeatedly, recombining genomes and resulting in a multitude of genetically different offspring from one mating event.

K e

O Neurospora can reproduce sexually by producing specialized hyphae. Conidia of the opposite mating type fuse to these hyphae.

Haploid (n) Dikaryotic (n + n) Diploid(2n)

ASEXUAL §g- ' REPRODUCTION Myceln © A dikaryotic ascus develops.

SEXUAL REPRODUCTION © Karyogamy occurs within the ascus, producing a dipioid nucleus.

The developing asci are contained in an scocarp. The ascospores are discharged forcibly "om the asci through an opening in the ascocarp. Germinating ascospores ve rise to new mycelia.

© The dipioid nucleus divides by meiosis, yielding four haploid nuclei.

0 Each haploid nucleus divides once by mitosis, yielding eight nuclei. Cell walls develop around the nuclei, forming ascospores (LM). Figure 31.17 The life cycle of Neurospora era:

omycete.

CHAPTER

31

Fungi

617

Basidiomycetes

(b) Maiden veil fungus (Dictyphora), a fung with an odor like rotting meat

Approximately 30,000 fungi, including mushrooms and shelf fungi, are called basidiomycetes and are classified in the phylum Basidiomycota (Figure 31.18). The phylum also includes molds, mutualists that form mycorrhizae, and two groups of particularly destructive plant parasites, the rusts and smuts. The name oi the phylum derives from the basidium (Latin for "little pedestal"), a cell in which a transient diploid stage occurs during the fungal life cycle. The club-like shape of the basidium also gives rise to the common name club fungus. Basidiomycetes are important decomposers of wood and other plant material. Of all the fungi, the saprobic basidiomycetes are the best at decomposing the complex polymer lignin, an abundant component of wood. Many shelf fungi break down the wood of weak or damaged trees and continue to decompose the wood after the tree dies.

(a) Fly agaric (Amanita muscaria), a common species in conifer forests ir the northern hemisphere

(d) Puffballs emitting spore? (c) Shelf fungi, important decomposers of wood

The life cycle of a basidiomycete usuA. Figure 31.18 Basidiomycetes (club fungi). ally includes a long-lived dikaryotic mycelium. As m ascomycetes, this extended dikaryotic stage provides opportunities for many genetic recombinations to occur, in effect multiplying the result of a single mating. Periodically, in response to environmental stimuli, the mycelium reproduces sexually by producing elaborate fruiting bodies called basidiocarps (see Figure 31.20). A mushroom is a familiar example of a basidiocarp. By concentrating growth in the hypbae of mushrooms, a basidiomycete mycelium can quickly erect its fruiting structures in just a few hours; a mushroom pops up as it absorbs water and as cytoplasm streams in from the dikaryotic mycelium. By this process, a ring of mushrooms, popularly A Figure 31.19 A fairy ring. According to legend, these called a "fairy ring," may appear on a lawn literally overnight mushrooms spring up where fairies have danced in a ring on a (Figure 31.19). The mycelium below the fairy ring expands moonlit night. (The text provides a biological explanation of how fairy outward at a rate of about 30 cm per year, decomposing rings form.) 618

UNIT FIVE

The Evolutionary Hislnry oi Biological Diversiiy

O Two haploid mycelia of different mating types undergo plasmogamy.

© A dikaryotic mycelium forms, growing faster than, and ultimately crowding out, the haploid parental mycelia.

© Environmental cues such as rain or temperature changes induce the dikaryotic mycelium to form compact masses that develop into basidiocarps (mushrooms, in this case).

PLASMOGAMY

© In a suitable nvironment, the basidiospores germinate and grow into short-lived haploid mycelia.

Mating type {-)

Mating type (+;

SEXUAL REPRODUCTION

Basidiocarp (dikaryotic)

When mature, the basidiospores are ejected, fall from the cap, and are dispersed by the wind.

Basidium with four appendages Basidium containing four haploid nuc

1 fim

Basidiospore

© Each diploid nucleus yields four haploid nuclei. Each basidium grows four appendages, and one haploid nucleus enters each appendage and develops into a basidiospore (SEM).

© The basidiocarp gills are lined with terminal dikaryotic cells called basidia.

Key © Karyogamy in the basidia produces diploid nuclei, which then undergo meiosis.

Haploid (n) Dikaryotic (n H Diploid (2n)

it Figure 31.20 The life cycle of a mushroom-forming basidiomycete.

rganic matter in the soil as it grows. Some giant fairy rings lay be centuries old. The numerous basidia in a basidiocarp are the sources of sexual spores called basidiospores (Figure 31.20)- The mushroom's cap supports and protects a large surface area of basidia On gills. A common, store-bought white mushroom has a gill surface area of about 200 cm2 and may release a billion basidiospores, which drop from the bottom of the cap and are llown away. Asexual reproduction is much less common in basidiomycetes than in ascomycetes.

Concept Check 1. What feature of chytrids supports the hypothesis that they represent the most primitive fungal lineage? 2. Why are glomeromycetes so ecologically significant? 3. Give different examples of how form fits function in zygomycetes, ascomycetes, and basidiomycetes. For suggested answers, see Appendix A.

CHAPTER 31

619

Figure 31.21

Concept

Fungi have a powerful impact on ecosystems and human welfare In our survey of fungal classification, we've touched on some of the ways fungi influence other organisms, including ourselves. We will now look more closely at these impacts.

Decomposers Fungi are well adapted as decomposers of organic material, including the cellulose and lignin of plant cell walls. However, almost any carbon-containing substrate—-including jet fuel and wall paint—can be consumed by at least some fungi. Fungi and bacteria are primarily responsible for keeping ecosystems stocked with the inorganic nutrients essential for plant growth. Without these decomposers, carbon, nitrogen, and other elements would become tied up in organic matter. Plants and the animals that eat them could not exist because elements taken from the soil would not be returned (see Chapter 54).

Does having mycorrhizae benefit a plant? Researchers grew soybean plants in soil treated with fungicide (poison that kills fungi) to prevent the formation of mycorrhizae in the experimental group. A control group was exposed to fungi that formed mycorrhizae in the soybean plants' roots.

The soybean plant on the left is typical of the experimental group. Its stunted growth is probably due to a phosphorus deficiency. The taller, healthier plant on the right is typical of the control group and has mycorrhizae.

Symbionts Fungi form symbiotic relationships with plants, algae, cyanobacteria, and animals. All of these relationships have profound ecological effects. Mycorrhizae Mycorrhizae are enormously important in natural ecosystems and agriculture. As you have read, almost all vascular plants have mycorrhizae and rely on their fungal partners for essential nutrients. It is relatively easy to demonstrate the significance of mycorrhizae by comparing the growth of plants with and without mycorrhizae (Figure 31.21). Foresters commonly inoculate pine seedlings with ecLomycorrhizal fungi to promote vigorous growth in the trees.

These results indicate that the presence of mycorrhizae benefits a soybean plant and support the hypothesis that mycorrhizae enhance the plant's ability to take up phosphate and other needed minerals.

Fungus-Animal Symbiosis Some fungi share their digestive services with animals, helping break down plant material in the guts of cattle and other grazing mammals. Many species of ants and termites take advantage of the digestive power of fungi by raising them in "farms." The insects scour tropical forests in search of leaves, which they carry back to their nests and feed to the fungi (Figure 31.22). The fungi break down the leaves into a substance the insects can digest. In some tropical forests, the fungi have helped these insects become the major consumers of leaves. The evolution of such farmer insects and that of their fungal "crops" have been tightly linked for well over 50 million years. The fungi have become so dependent on their caretakers that in many cases they can no longer survive without the insects. 620

UNIT FIVE

The Evolutionary History of Biological Diversity

A Figure 31.22 Fungus-gardening insects. These leaf-cutting ants depend on fungi to convert plant material to a form the insects can digest. The fungi, in turn, depend on the nutrients from the leave' the ants feed them.

uptake of minerals. Fungal pigments help shade the algae or cyanobacteria from intense sunlight. Some fungal compounds are toxic and prevent lichens from being eaten by animals. The fungi of many lichens reproduce sexually by forming ascocarps or basidiocarps. Lichen algae reproduce independently of the fungus by asexual cell division. As might be expected of "dual organisms," asexual reproduction as a symbiotic unit also occurs commonly, either by fragmentation of the parental lichen or by the formation of soredia, small clusters of hyphae with embedded algae (see Figure 31.24).

(b) A foliose (leaf-like) lichen

(c) Crustose (crust-like) lichens

Phylogenetic studies of lichen DNA have helped illuminate the evolution of this symbiosis. Molecular studies published in 2001 support the hypothesis that all living lichens can be traced to three original associations involving a fungus and a photosynthettc symbiont. The same studies also indicate that numerous free-living fungi descended from lichen-forming ancestors. Penicillium, the

Figure 31.23 Variation in lichen growth forms. Ascocarp of fungus

Lichens ichens are a symbiotic association of millions of photosynthetic nicroorganisms held in a mass of fungal hyphae. They form a urface-hugging carpet that can be found growing on rocks, rating logs, trees, and roofs in various shrub-like, leaf-like, or en:rusting forms (Figure 31.23). The photosynthetic partners are ypically unicellular or filamentous green algae or cyanobacteria. Che fungal component is most often an ascomycete, but several ^asidiomycete lichens are known. The fungus usually gives a ichen its overall shape and structure, and tissues formed by hy3hae account for most of the lichens mass. The algae or cyanobaceria generally occupy an inner layer below the lichen surface Figure 31.24). The merger of fungus and alga or cyanobacterium so complete that lichens are actually given scientific names as hough they were single organisms. More than 13,500 species lave been described, accounting for a fifth of all known fungi. In most lichens that have been studied, each partner provides Something the other could not obtain on its own. The algae provide carbon compounds; the cyanobacteria also fix nitrogen (see Chapter 27) and provide organic nitrogen. The fungi provide .heir photosynthetic partners with a suitable environment for growth. The physical arrangement of hyphae allows for gas exchange, protects the photosynthetic partner, and retains water and minerals, most of which are absorbed either from airborne Just or from rain. The fungi also secrete acids, which aid in the

A Figure 31.24 Anatomy of an ascomycete lichen (colorized SEM). CHAPTER 31

Fungi

621

free-living fungus that provides the antibiotic penicillin, for example, is thought to have descended from a lichen fungus. Lichens are important pioneers on newly cleared rock and soil surfaces, such as burned forests and volcanic flows. They break down ihe surface by physically penetrating and chemically attacking it, and they trap windblown soil. Nitrogen-fixing lichens also add organic nitrogen to some ecosystems. These processes make it possible for a succession of plants to grow. As tough as lichens are, many do not stand up very well to air pollution. Their passive mode of mineral uptake from rain and moist air makes them particularly sensitive to sulfur dioxide and other aerial poisons. The death of sensitive lichens and an increase in the number of hardier species in an area can be an early warning that air quality is deteriorating.

from fungi such as the basidiomycete Pucdnia graminis, which causes black stem rust on wheat. Some of the fungi that attack food crops are toxic to humans. For example, certain species of the ascomycete mole Aspergillus contaminate improperly stored grain and peanut? by secreting carcinogenic compounds called aflatoxins. In another example, the ascomycete Claviceps purpurea forrm purple structures called ergots on rye. If diseased rye is inadvertently milled into flour and then consumed, poisons from the ergots can cause ergotism, a condition characterized b> gangrene, nervous spasms, burning sensations, hallucinations and temporary insanity. An epidemic of ergotism arounc A.D. 944 killed more than 40,000 people in France. One of the compounds that has been isolated from ergots is lysergic acid, the raw material from which the hallucinogen LSD is made. Animals are much less susceptible to parasitic fungi than are plants. Only about 50 species of fungi are known to parasitize humans and other animals, but these relatively few species do considerable damage. The general term for such a fungal infection is mycosis. Skin mycoses include the disease ringworm, so named because it appears as circular red areas on the skin. The ascomycetes that cause ringworm can infect almost any skin surface. Most commonly, they grow on the feet, causing ths intense itching and blisters known as athletes foot. Thougl highly contagious, athlete's foot and other ringworm infections can be treated with fungicidal lotions and powders. Systemic mycoses, by contrast, spread through the body and usually cause very serious illnesses. They are typically causec

Pathogens Of the 100,000 known species of fungi, about 30% make their living as parasites, mostly on or in plants (Figure 31.25). For example, Ophiostoma ulmi, the ascomycete that causes Dutch elm disease, has drastically changed the landscape of the northeastern United States. Accidentally introduced to the United States on logs that were sent from Europe to help pay World War 1 debts, the fungus is carried from tree to tree by bark beetles. Another ascomycete, Cryphonectria parasitica, has killed 4 billion native American chestnut trees in the eastern United States. Fungi are also serious agricultural pests. Between 10% and 50% of the world's fruit harvest is lost each year to fungal attack. Grain crops suffer major losses each year

(a) Corn smut on corn

(b) Tar spot fungus on maple leaves

A Figure 31.25 Examples of fungal diseases of plants. 622

UNIT FIVE

The Evolutionary History of Biological Diversity

-.

(c) Ergots on rye

by inhaled spores. Coccidioidomycosis is a systemic mycosis tiiat produces tuberculosis-like symptoms in the lungs. It is so leadly that it is now considered a potential biological weapon. Some mycoses are opportunistic, occurring only when a Lange in the body's microbiology, chemistry, or immunology dllows fungi to grow unchecked. Candida alhicans, for example, is one of the normal inhabitants of moist epithelia, such as the vaginal lining. Under certain circumstances, Candida can grow too rapidly and become pathogenic, leading to socalled "yeast infections." Many other opportunistic mycoses in rjiumans have become more common in recent decades, due . part to AIDS, which compromises the immune system. Over the past few years, the alleged dangers of black mold '>tachybotrys chartarum, an ascomycete) have been the subject f sensalionalistic news reports. Black mold thrives in damp uildings and has been implicated in some studies as the cause f a wide variety of diseases. As a result, entire "sick buildings" ave been abandoned. Yet there is no well-substantiated evidence linking the presence of 5. chartarum to these diseases. 4any other factors—including bacteria, human-made chemials, and other fungi—may cause some of the symptoms ttributed to black mold.

Practical Uses of Fungi he dangers posed by fungi should not overshadow the imaense benefits we derive from these remarkable eukaryotes. Ve depend on their ecological sendees as decomposers and ecyclers of organic matter. Without mycorrhizae, our agriculture would be far less productive. Mushrooms are a popular food, but they are not the only ungi we eat. The distinctive flavors of certain kinds of cheeses, including Roquefort and blue cheese, come from the fungi ;ed to ripen them. The soft drink industry uses a species of iSpergillus to produce citric acid for colas. Morels and truffles, |he edible fruiting bodies of various ascomycetes, are highly 'rized for their complex flavors (see Figure 31.16b and c). 'hese fungi can fetch several hundred dollars a pound. Truf£s release strong odors that attract mammals and insects, which feed on them and disperse their spores. In some cases, i he odors mimic the sex attractants of certain mammals. Humans have used yeasts to produce alcoholic beverages and raise bread for thousands of years. Under anaerobic con• litions, yeasts ferment sugars to alcohol and CO2, which leavens dough. Only relatively recently have the yeasts involved >een separated into pure cultures for more controlled use. The yeast Saccharomyces cerevisiae is the most important of all cultured lungi (see Figure 31.7). It is available as many strains of baker's yeast and brewer's yeast. Many fungi have great medical value as well. For example, compound extracted from ergots is used to reduce high ilood pressure and to stop maternal bleeding after childbirth. >ome fungi produce antibiotics that are essential in treating lacterial infections. In fact, the first antibiotic discovered

A Figure 31.26 Fungal production of an antibiotic. The mold Penicillium produces an antibiotic that inhibits the growth of Staphylococcus bacteria, resulting in the clear area between the mold and the bacteria.

was penicillin, made by the ascomycete mold Penicillium (Figure 31.26) Fungi also figure prominently in research in molecular biology and biotechnology. Researchers use Saccharomyces to study the molecular genetics of eukaryotes because its cells are easy to culture and manipulate (see Chapter 19). Scientists are gaining insight into the genes involved in human diseases such as Parkinson's disease and Huntington's disease by examining the interactions of homologous genes in Saccharomyces. Genetically modified fungi hold much promise. Although bacteria such as Escherichia coli can produce some useful proteins, they cannot synthesize glycoproteins because they lack enzymes that can attach carbohydrates to proteins. Fungi, on the other hand, contain such enzymes. In 2003, scientists succeeded in engineering a strain of 5. cerevisiae that produces human glycoproteins, such as insulin. Such fungus-produced glycoproteins could treat people with medical conditions that make them unabie to produce these compounds. Meanwhile, other researchers are sequencing the genome of the wooddigesting basidiomycete Phanerochaete chrysosporium, also known as white rot. They hope to decipher the metabolic pathways by which white rot breaks down wood, with the goal of harnessing these pathways to produce paper pulp. Having now completed our survey of the kingdom Fungi, we will turn in the remaining chapters of this unit to its sister kingdom, Animalia, to which we humans belong.

Concept Check 1. What are some of the benefits that algae in lichens can derive from their relationship with fungi? 2. What characteristics of pathogenic fungi result in their being efficiently transmitted? For suggested answers, see Appendix A.

CHAPTER 31

Fung]

623

Chapter Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY CONCEPTS Fungi are heterotrophs that feed by absorption

Review Table 31.1 Review of Fungal Phyla Phylum

Distinguishing Feature

Chytridiomycota (chytrids)

Motile spores with flagella

Zygomycota zygosporangium as sexual stage

• Nutrition and Fungal Lifestyles (pp. 608-609) All fungi are heterotrophs (including decomposers and syrabionts) that acquire nutrients by absorption. They secrete enzymes that break down complex molecules in food to smaller molecules that can be absorbed. Activity Fungal Reproduction and Nutrition • Body Structure (pp. 609-610) Fungi consist of mycelia, networks of branched hyphae adapted for absorption. Most fungi have cell walls made of chitin. Some fungi have hyphae partitioned into cells by septa, with pores allowing cell-to-cell movement of materials. Coenocytic fungi lack septa. Mycorrhizal fungi have a symbiotic relationship with plants.

Glomeromycota

Arbuscular mycorrhizae

Ascomycota (sac fungi)

Sexual spores borne internally in sacs called asci

Basidiomycota (club fungi)

Elaborate fruiting body called basidiocarp

Fungi produce spores through sexual or asexual life cycles • Sexual Reproduction (pp. 610—611) The sexual cycle involves cytoplasmic fusion (plasmogamy) and nuclear fusion (karyogamy), with an intervening heterokaryotic stage in which cells have haploid nuclei from two parents. The diploid phase resulting from karyogamy is short-lived and undergoes meiosis, producing haploid spores. • Asexual Reproduction (pp. 611-612) Molds are rapidly growing, asexually reproducing fungi. Yeasts are unicellular fungi adapted to life in liquids such as plant saps. Fungi with no known sexual stage have traditionally been called deuteromycetes, but mycologists are using genetic techniques to assign many of these fungi to particular phyla.

Fungi descended from an aquatic, single-celled, flagellated protist • The Origin of Fungi (p. 612) M.olecular evidence supports the hypothesis that fungi and animals diverged from a common ancestor that was unicellular and bore flagella. • The Move to Land (p. 612) Fungi were among the earliest colonizers of land, probably as symbionts with early land plants.

• Glomeromycetes (p. 615) The vast majority of plants have symbiotic relationships with glomeromycetes in the form of arbuscular mycorrhizae. • Ascomycetes (pp. 616-617) Ascomycetes (sac fungi) reproduce asexually by producing vast numbers of asexual spores called conidia. Sexual reproduction involves the formation of spores in sacs, or asci, at the ends of dikaryotic hyphae, which are usually contained in fruiting bodies called ascocarps. • Basidiomycetes (pp. 618-619) Important decomposers of wood, the mycelia of basidomycetes (club fungi) can grow for years in the heterokaryotic stage. Sexual reproduction involves the formation of fruiting bodies called basidiocarps, which produce spores on club-shaped basidia at the ends of dikaryotic hyphae. Activity Fungal Life Cycles investigation How Docs the Fungus Pilobolus Succeed as a Decomposer?

Fungi have radiated into a diverse set of lineages • Table 31.1, in the next column, summarizes the fungal phyla and some of their distinguishing characteristics. • Chytrids (p. 613) Chytrids are saprobic or parasitic fungi found in freshwater and terrestrial habitats. They are the only fungi that produce flagellated spores. • Zygomycetes (pp. 613-615) Zygomycetes, such as black bread mold, are named for their sexually produced zygosporangia, which are heterokaryotic structures capable of persisting through unfavorable conditions. Unicellular parasites called microsporidia are now thought to be zygomycetes.

624

UNIT

FIVE

The Evolutionary History of Biological Diversity

Fungi have a powerful impact on ecosystems and human welfare • Decomposers (p. 620) Fungi perform essential recycling of chemical elements between the living and nonliving world. • Symbionts (pp. 620-622) Mycorrhizae increase plant productivity. Fungi enable animals such as cattle, ants, and termites to digest plant tissue. Lichens are highly integrated symbiotic associations of fungi and algae or cyanobacteria.

*• Pathogens (pp. 622-623) About 30% of all known fungal species are parasites, mostly of plants. Some fungi also cause human diseases. • Practical Uses of Fungi (p. 623) Humans eat many fungi and use others to make cheeses, alcoholic beverages, and bread. Antibiotics produced by fungi treat bacterial infections. Genetic research on fungi is leading to applications in biotechnology

TESTING YOUR KNOWLEDGE Evolution Connection The fungus-alga symbiosis that makes up a lichen is thought to have evolved several times independently in different fungal groups. However, lichens fall into three well-defined growth forms (see Figure 31.23). What research could you perform LO test the following hypotheses? Hypothesis 1: Crustose, foliose, aid fruticose lichens each represent a monophyletic group. Hypothesis 2: Each lichen growth form represents convergent •olution by taxonomically diverse fungi.

Scientific Inquiry ichens colonize gravestones, such as the ne in this photo, almost as soon as the .ones are placed and then continue to grow for decades or even centuries, xplain how you could calculate the g rowth rate of a particular lichen species t y collecting data at an old cemetery. tvestigation How Does the Fungus lobolus Succeed as a Decomposer?

Science, Technology, and Society American chestnut trees once made up more than 25% of the hardwood forests of the eastern United States. These trees were wiped out by a fungus accidentally introduced on imported Asian chestnuts, which are not affected. More recently, a fungus has killed many eastern dogwood trees; some experts suspect that the parasite as accidentally introduced from somewhere else. Why are plants p articularly vulnerable to fungi imported from other regions? What kinds of human activities might contribute to the spread of plant iseases? Do you think introductions of plant pathogens such as lestnut blight are more or less likely to occur in the future? Why?

CHAPTER 31

Fungi

625

An Introduction ^ >A to Animal J* Diversity • :

A Figure 32.1 An underwater glimpse of animal diversity on and around a coral reef.

32.1 Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers 32.2 The history of animals may span more than a billion years 32.3 Animals can be characterized by "body plans" 32.4 Leading hypotheses agree on major features of the animal phylogenetic tree

Welcome to Your Kingdom

R

eading the last few chapters, you may have felt a little like a tourist among some rather unfamiliar organisms, such as slime molds, whisk ferns, and sac fungi. You probably are more at home with the topic introduced in this chapter—the animal kingdom, which of course includes yourself. But as Figure 32.1 suggests, animal diversity extends far beyond humans or even the dogs, cats, birds, and other animals we humans regularly encounter. Biologists have identified 1.3 million living species of animals, and estimates of the total number of animal species run far higher, from 10 to 20 million to as many as 100 to 200 million. This vast diversity encompasses a spectacular range of morphological variation, from corals to cockroaches to crocodiles.

In this chapter, we embark on a tour of the animal kingdom, which will continue in the next two chapters. We will consider the characteristics that all animals share as well as those that distinguish various taxonomic groups. This information is central to understanding why animal phytogeny is currently one of the liveliest arenas of biological research and debate, as you will read later in the chapter. 626

Concept

Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers Constructing a good definition of an animal is not straight forward, as there are exceptions to nearly every criterion fo distinguishing animals from other life-forms. However, several characteristics of animals, when taken together, sufficientl define the group for our discussion.

Nutritional Mode Animals differ from both plants and fungi in their mode of nu trition. Recall that plants are autotrophic eukaryotes capable of generating organic molecules through photosynthesis1; fungi are heterotrophs that grow on or near their food, releasing exoenzymes that digest the food outside their bodies. Unlike plants, animals cannot construct all of their own organic molecules and so, in most cases, they ingest them—either by eating other living organisms or by eating nonliving organic material. But unlike fungi, most animals use enzymes to dige< their food only after they have ingested it.

Cell Structure and Specialization Animals are eukaryotes, and like plants and fungi (but unlike most protists), animals are multicellular. In contrast to plants and fungi, however, animals lack the structural support of ce|ll walls. Instead, animal bodies are held together by structural proteins, the most abundant being collagen (see Figure 6,29|).

have a different habitat than the adult, as in the case of the aquatic tadpole (larva) of a terrestrial frog. Animal larvae eventually undergo metamorphosis, a resurgence of development that transforms the animal into an adult. Desptte the extraordinary diversity of morphology exhibited by adult animals, the underlying genetic network that controls animal development has been relatively conserved. All eukaryotes have genes that regulate the expression of other genes, and many of these regulatory genes contain common "modules" of DNA sequences called homeoboxes (see Chapter 21). Animals share a unique homeobox-containing family of genes, known as Hox genes, suggesting that this gene family evolved in the eukaryote lineage that gave rise to animals. Hox genes play important roles in the development of animal embryos, controlling the expression of dozens or even hundreds of other genes. Hox genes can thus control cell division and differentiation, producing different morphological features of animals.

in addition to collagen, which is found mainly in extracellular lattices, animals have three unique types of intercellular .motions—tight junctions, desmosomes, and gap junctions— that consist of other structural proteins (see Figure 6.31). Among animal cells are two specialized forms not seen in c ther multicellular organisms: muscle cells and nerve cells. In most animals, these specialized cells are organized into muscle tissue and nervous tissue, respectively, and are responsible for movement and impulse conduction.

Reproduction and Development Most animals reproduce sexually, and the diploid stage usually dominates the life cycle. In most species, a small, flagellated sperm fertilizes a larger, nonmotile egg, forming a diploid zygote. The zygote then undergoes cleavage, a succession of mitotic cell divisions without cell growth between division cycles. During the development of most animals, cleavage leads to the formation of a multicellular stage called 2 blastula, which in many animals takes the form of a hollow ball (Figure 32.2). Following the blastula stage is the process of gastrulation, during which layers of embryonic tissues that will develop into adult body parts are produced. The resulting developmental stage is called a gastrula.

The sponges, which represent the lineage of simplest extant (living) animals, have Hox genes that regulate the formation of water channels in the body wall, the primary feature of sponge morphology (see Chapter 33). In more complex animals, the Hox gene family underwent further duplications, yielding a more versatile "toolkit" for regulating development. In bilaterians (a grouping that includes vertebrates, insects, and most other animals), Hox genes regulate patterning of the anteriorposterior axis, as well as other aspects of development. The same conserved genetic network governs the development of both a fly and a human, despite their obvious differences and hundreds of millions of years of divergent evolution.

Some animals develop directly through transient stages of ijna.turation into adults, but the life cycles of many animals also include at least one larval stage. A larva is a sexually immature form of an animal that is morphologically distinct from the adult stage, usually eats different food, and may even

0 The zygote of an animal undergoes a succession of mitotic cell divisions called cleavage.

© Only one cleavage stage—the eight-cell embryo—is shown here.

Cleavage

@ The endoderm of the archenteron develops into the tissue lining the animal's digestive tract. 0 The blind pouch formed by gastrulation, called the archenteron, opens to the outside via the blastopore.

Zygote

0 In most animals, cleavage results in the formation of a multicellular stage called a blastula. The blastula of many animals is a hollow ball of cells.

Cleavage

Eight-cell stage

Cross section of blastula

Blastopore

i* Figure 32.2 Early embryonic development in animals.

0 Most animals also undergo gastrulation, a rearrangement of the embryo in which one end of the embryo folds inward, expands, and eventually fills the blastocoel, producing layers of embryonic tissues: the ectoderm (outer layer) and the endoderm (inner layer). CHAPTER 32 An Introduction to Animal Diversity

627

Neoproterozoic Era (1 Billion-542 Million Years Ago)

Concept Check 1. Both plants and animals are multicellular eukaryotes. identify four ways in which plants and animals differ. 2. Complex early developmental patterns such as the formation of a blastula and a gastrula are shared by diverse animals ranging from grasshoppers to clams to humans. What does this observation imply about the timing of the origins of these processes in animal evolution?

Despite the molecular data inditing a much earlier origffi Q [ animals, the first generally accepted fossils of animals are only 575 million years old. These fossils are known collectively as the Ediacaran fauna, named for the Ediacara Hills of Australia! where they were first discovered (Figure 32.5). Similar fossils have since been found on other continents. Some appear to be related to living cnidanans such as corals (see Chapter 33). Other fossils may represent soft-bodied molluscs, and numerous fossilized tunnels and tracks indicate the presence ol several forms of worms. In addition to these macroscopic fossils, Neoproterozoic rocks have also yielded microscopic signs of early animals. As we discussed in Chapter 26, 570-million-year-old embryos discovered in China clearly exhibit the basic structural organj ization of present-day animal embryos, though paleontologists are still uncertain which animal clade the embryos represent Though older fossils of animals will likely be discovered in the future, the fossil record as it is known today strongly suggests

For suggested answers, see Appendix A.

Concept

The history of animals may span more than a hillion years The animal kingdom includes not only the great diversity of living species, but the even greater diversity of extinct ones as well. (Some paleontologists have estimated that 99% of all animal species are extinct.) Various studies suggest that animal diversification began more than a billion years ago. For example, some calculations based on molecular clocks estimate that the ancestors of animals diverged from the ancestors of fungi as far back as 1.5 billion years ago. Similar studies on the common ancestor of living animals suggest that it lived 1.2 billion— 800 million years ago. This common ancestor may have resembled modern choanoflagellates (Figure 32.3), protists that are the closest living relatives of animals, and was probably itself a colonial, flagellated protist (Figure 32.4). In this section, we will survey fossil evidence of animal evolution over four geologic eras.

T Figure 32.5 Ediacaran fossils. Fossils dating from 575 million years ago include animals (a) with simple, radial forms and (b) with many body segments and legs.

Digestsv cavtty

Colonial protist, an aggregate of identical cells

Hollow sphere of unspeciaiized ceils (shown in cross section)

Beginning of cell specialization

"protoanimal"

A. Figure 32.3 A choanoflagellate

A Figure 32.4 One hypothesis for the origin of animals from a flagellated protist.

colony. Such a colony is about 0.02 mm high.

(The arrows symbolize evolutionary time.)

628

U N I T FIVE

The Evolutionary History of Biological Diversity

r.hai the end of the Neoproterozoic Era was a time of rising levels of animal diversity.

Paleozoic Era (542-251 Million Years Ago) Animal diversification appears to have accelerated dramatically between 542 and 525 million years ago, early in the Cambrian period of the Paleozoic Era—-a phenomenon often referred to as the Cambrian explosion. In strata formed before the Cambrian explosion, only a handful of animal phyla can be recognized. But in strata that are between 542 and 525 million years old, paleontologists have found the oldest fossils of about half o f all extant phyla. Many of these distinctive fossils—which include the first animals with hard mineralized skeletons—look quite different from most living animals (Figure 32.6). But for the most part, paleontologists have established that these various Cambrian fossils are members of extant animal phyla—or at least are close relatives. There are several current hypotheses regarding the cause of the Cambrian explosion. Some evidence suggests that new predator-prey relationships that emerged in the Cambrian period generated diversity through natural selection. Predators acquired adaptations, such as new forms of locomotion that helped them catch prey, while prey species acquired new defenses, such as protective shells. Another hypothesis focuses on the rise in atmospheric oxygen that preceded the Cambrian explosion. With more oxygen available, the opportunity arose for the success of animals with higher metabolic rates and larger body size. A third hypothesis holds that the evolution of the Hox gene complex provided the developmental flexibility that resulted in variations in morphology. These h/potheses are not mutually exclusive, however; predatorprey relationships, atmospheric changes, and developmental flexibility may each have played a role. The Cambrian period was followed by the Ordovician, Silurian, and Devonian periods, when animal diversity continued to increase, although punctuated by episodes of mass extinctions. Vertebrates (fishes) emerged as the top predators of tljie marine food web. By 460 million years ago, the "innovations" that emerged during the Cambrian period were making an impact on land. Arthropods began to adapt to terrestrial habitats, as indicated by the appearance of millipedes and centipedes. Fern galls—enlarged cavities that resident insects stimulate fern plants to form, providing protection for the insects—date back to at least 302 million years ago, suggesting that insects and plants were influencing each other's evolution by that time. Vertebrates made the transition to land around 360 million years ago and diversified into numerous terrestrial lineages. Two of these survive today: amphibians (such as frogs and salamanders) and amniotes (such as reptiles and mammals). We will explore these groups, known collectively as tetrapods, ir. more detail in Chapter 34.

A Figure 32.6 A Cambrian seascape. This artist's reconstruction depicts a diverse array of organisms represented in fossils from the Burgess Shale site in British Columbia, Canada. The animals include Pikaia (swimming eel-like chordate), Hallutigenia (animal with toothpick-like spikes on seafloor), Anomalocaris (large animal with hooked claws), and Marella (arthropod swimming at left).

Mesozoic Era (251—65.5 Million Years Ago) Few fundamentally new body plans emerged among animals during the Mesozoic era. But the animal phyla that had evolved during the Paleozoic now began to spread into new ecological niches. In the oceans, the first coral reefs formed, providing other animals with new marine habitats. Some reptiles returned to the water and succeeded as large aquatic predators. On land, modification of the tetrapod body plan included wings and other flight equipment in pterosaurs and birds. Large dinosaurs emerged, both as predators and herbivores. At the same time, the first mammals—tiny nocturnal insect-eaters—appeared on the scene.

Cenozoic Era (65.5 Million Years Ago to the Present) As you read in Chapter 30, insects and flowering plants both underwent a dramatic diversification during the Cenozoic era. The beginning of this era followed mass extinctions of both terrestrial and marine animals. Among the groups of species that disappeared were the large, nonflying dinosaurs and the marine reptiles. The fossil record of the early Cenozoic documents the rise of large mammalian herbivores and carnivores as mammals began to exploit the vacated ecological niches. The global climate gradually cooled throughout the Cenozoic, triggering significant shifts in many animal lineages. Among primates, for example, some species in Africa adapted to the CHAPTER 32 An Introduction to Animal Diversity

629

open woodlands and savannas that replaced the former dense forests. The ancestors of our own species were among those grassland apes.

Concept Check 1. Put the following milestones in animal evolution in chronological order from least recent to most recent: (a) origin of mammals, (b) earliest evidence of terrestrial arthropods, (c) Ediacaran fauna, (d) extinction of large, nonflying dinosaurs. 2. Explain how the relatively rapid Cambrian radiation of animal phyla could have been the product of causes both external and internal to organisms.

(a) Radial symmetry. The parts of a radial animal, such as a sea anemone (phylum Cnidaria), radiate from the center. Any imaginary slice through the central axis divides the animal into mirror images.

For suggested answers, see Appendix A.

(b) Bilateral symmetry. A bilateral animal, such as a lobster (phylum Arthropoda), has a left side and a right side. Only one imaginary cut. divides the animal into mirror-image halves.

Concept

Animals can be characterized by "body plans" One way in which zoologists categorize the diversity of animals is according to general features of morphology and development. A group of animal species that share the same level of organizational complexity is known as a grade. Grades, it's important to remember, are not necessarily equivalent to cladcs. Consider the case of slugs: A class of animals called gastropods includes many species that lack shells and are referred to as slugs, along with many shelled species such as snails. You may have encountered terrestrial slugs in a garden; many other slug species live in aquatic habitats. But these species do not all descend from a common ancestor and thus do not form a monophyletic clade (see Figure 25.10). Rather, phylogenetic studies show that several gastropod lineages independently lost their shells and became "slugs." The slug grade, in other words, is polyphyletic. The set of morphological and developmental traits that define a grade are generally integrated into a functional whole referred to as a body plan. Lets now explore some of the major features of animal body plans.

Symmetry Animals can be categorized according to the symmetry of their bodies (or its absence). Most sponges, for example, lack symmetry altogether. Among the animals that do have symmetrical bodies, symmetry can take different forms. Some animals exhibit radial symmetry, the form found in a flowerpot (Figure 32.7a). Sea anemones, for example, have a top (oral, 630

UNIT FIVE

The Evolutionary History of Biological Diversity

A Figure 32.7 Body s y m m e t r y . The flowerpot and shovel are included to help you remember the radial-bilateral distinction.

or mouth) side and a bottom (aboral) side. But they have n head and rear end, and no left and right side. The two-sided symmetry seen in a shovel is an example of bilateral symmetry (Figure 32.7b). A bilateral animal has a dorsal (top) side and a ventral (bottom) side, as well as a left and right side and an anterior (head) end with a mouth and a posterior (tail) end. Many animals with a bilaterally symmetrical body plan (such as arthropods and mammals) have sensory equipment concentrated at the anterior end, along with a central nervous system ("brain") in the head—an evolutionary trend known as cephalization (from the Greek keyhah, head). The symmetry of an animal generally fits its lifestyle. Many radial animals are sessile (living attached to a substrate) or planktonic (drifting or weakly swimming, such as jellies, or jellyfishes). Their symmetry equips them to meet the environment equally well from all sides. In contrast, bilateral animals generally move actively from place to place. Their central nervous system enables them to coordinate complex movements involved in crawling, burrowing, Hying, or swimming. These two fundamentally different kinds of symmetry probably arose very early in the history of animal life (see Figure 32.5).

Tissues Animal body plans also vary according to the organization of the animal's tissues. True tissues are collections of specialized cells isolated from other tissues by membranous layefs. Sponges lack true tissues. In all other animals, the embr) o

becomes Layered through the process of gastrulation, as you read earlier in this chapter (see Figure 32.2). As development progresses, these concentric layers, called germ layers, form ihe various tissues and organs of the body. Ectoderm, the germ layer covering the surface of the embryo, gives rise to the outer covering of the animal and, in some phyla, to the central nervous system. Endoderm, the innermost germ layer, lines llie developing digesiive lube, or archenteron, and gives rise to the lining of the digestive tract and organs derived from it, such as the liver and lungs of vertebrates. Animals that have only these two germ layers are said to be diploblastic. Diploblasts include the animals called cnidarians (jellies and corals, for example) as well as comb jellies (see Chapter 33). Other animals have a third germ layer, called the mesoderm, between the ectoderm and endoderm. These animals are said to be triploblastic (having three germ layers). In Lriploblasts, the mesoderm forms the muscles and most other organs between the digestive tube and the outer covering of the animal. Triploblasts include all bilaterally symmetrical animals, which range from flatworms to arthropods to vertebrates. (Although some diploblasts actually do have a third germ layer, it is not nearly as well developed as the mesoderm of animals considered to be triploblastic.)

(a) Coelomate. Coelomates such as annelids have a true coelom, a body cavity completely lined by tissue derived from mesoderm.

Digestive tract (from endoderm) (b) Pseudocoelomate. Pseudocoelomates such as nematodes have a body cavity only partially lined by tissue derived from mesoderm.

Body Cavities Some triploblastic animals possess a body cavily, a fluid-filled space separating the digestive tract from the outer body wall. This body cavity is also known as a coelom (from the Greek knilos, hollow). A so-called "true" coelom forms from tissue derived from mesoderm. The inner and outer layers of tissue that surround the cavity connect dorsally and ventrally and form structures called mesenteries that suspend the internal organs. Animals that possess a true coelom are known as coelomates (Figure 32.8a). Some triploblastic animals have a cavity formed from the blastocoel, rather than from mesoderm (Figure 32.8b). Such a cavity is called a "pseudocoelom" (from the Greek pseudo, false), and animals that have one are pseudocoelomales. Despite its name, a pseudocoelom is not false; it is a fully functional cavity. Finally, some triplobastic animals lack a coelom altogether (Figure 32.8c). They are known collectively as acoelomates (firom the Greek a, without). A body cavity has many functions. Its fluid cushions the suspended organs, helping to prevent internal injury. In softbodied coelomates, such as earthworms, the coelom contains noncompressible fluid thai, acts like a skeleton against which muscles can work. The cavity also enables the internal organs tq grow and move independently of the outer body wall. If it were not for your coelom, for example, every beat of your heart or ripple of your intestine could warp your body's surface.

(c) Acoelomate. Acoelomates such as flatworms lack a body cavity between the digestive tract and outer body wall.

A Figure 32.8 Body plans of triploblastic animals. The various organ systems of an animal develop from the three germ layers that form in the embryo. Blue represents tissue derived from ectoderm, red from mesoderm, and yellow from endoderm.

Current phylogenetic research suggests that true coeloms and pseudocoeloms have been gained or lost multiple times in the course of animal evolution. Thus, the terms coelomates and pseudocoelomates refer to grades, not clades.

Protostome and Deuterostome Development Based on certain features of early development, many animals can be categorized as having one of two developmental modes: protostome development or deuterostome development. Three features often distinguish these modes. Cleavage A pattern in many animals with protostome development is spiral cleavage, in which the planes of cell division are diagonal to the vertical axis of the embryo. As seen in the CHAPTER 32

An Inuoduciion lo Animal Diversity

631

• Figure 32,9 A comparison of protostome and deuterostome development. These are usefu! genera! distinctions, though there are many variations and exceptions to these patterns. (Blue = ectoderm; red = mesoderm; yellow = endoderm.)

Protostome development {examples: molluscs, annelids, arthropods)

Deuterostome development (examples: echinoderms, chordates) Eight-cef! 'stage

Eight-cell stage

Schizocoelous: solid masses of mesoderm split and- form coelom

Enterocoelous: folds of archenteron form coelom

/Anus

/ Mouth

Digestive tube

^ - Mouth Mouth develops from blastopore

eight-cell stage resulting from spiral cleavage, smaller cells lie in the grooves between larger, underlying cells (Figure 32.9a) Furthermore, the so-called determinate cleavage of some animals with this development pattern rigidly casts ("determines") the developmental fate of each embryonic cell very early. A cell isolated at the four-ceil stage from a snail, for example, forms an inviable embryo that lacks many parts. In contrast to the spiral cleavage pattern, deuterosiome development is predominantly characterized by radial cleavage. The cleavage planes are either parallel or perpendicular to the vertical axis of the egg; as seen in the eight-cell stage, the tiers of cells are aligned, one directly above the other. Most animals with deuterostome development are further characterized by indeterminate cleavage, meaning that each cell produced by early cleavage divisions retains the capacity to develop into a complete embryo. For example, if the cells of a sea star embryo are isolated at the four-cell stage, each will form a larva. It is the indeterminate cleavage of the human zygote that makes identical twins possible. This characteristic also explains the developmental versatility of the embryonic "stem cells" that may provide new ways to overcome a variety of dis632

UNIT FIVE The Evolutionary History of Biological Diversity

(a) Cleavage. In general, protostome development begins with spiral, determinate cleavage. Deuterostome development is characterized by radial, indeterminate cleavage.

(b) Coelom formation. Coelom formation begins in the gastrula stage. In protostome development, the coelom forms from splits in the mesoderm (schizocoelous development). In deuterostome development, the coelom forms from mesodermal outpocketings of the archenteron (enterocoelous development).

(c) Fate of the blastopore. In protostome development, the mouth forms from the blastopore. In deuterostome development, the mouth forms from a secondary opening.

•"Anus Anus develops from btastoppre

eases, including juvenile diabetes, Parkinson's disease, and Alzheimer's disease (see Chapter 21). Coelom Formation Another difference between protostome and deuterostome development is apparent later in the process, in gastrulation, the developing digestive tube of an embryo initially forms as a blir d pouch, the archenteron (Figure 32.9b). As the archenteron forms in protostome development, initially solid masses of mesoderm split and form the coelomic cavity; this pattern is called schizocoelous development (from the Greek schizein, to spl it). In contrast, formation of the body cavity in deuterostome development is described as enterocoelous: The mesoderm buds from the wall of the archenteron, and its cavity becom the coelom (see Figure 32.9b). Fate oj the Blastopore The fundamental character that distinguishes the two developmental modes is the Sate of the blastopore, the indentation that during gastrulation leads to the formation of the archenteron (Figure 32.9c). After the archenteron develops, a secoi d

opening forms at the opposite end of the gastrula. Ultimately, the blastopore and this second opening become the two openings of the digestive tube (the mouth and the anus), in protostome development, the mouth generally develops from lhe first opening, the blastopore, and it is for this characteristic i hat the term protostume derives (from the Greek protos, first, and stoma, mouth). In deuterostome (from the Greek deuteros, second) development, the mouth is derived from the secondary opening, and the blastopore usually forms the anus.

Concept Check 1. Why is it important to distinguish grade -level characteristics from characteristics that unite clades? 1. Compare three features of the early development of a snail (a mollusc) and a human (a chordate). For suggested answers, see Appendix A.

Concept

Leading hypotheses agree on major features of the animal phylogenetic tree Zoologists currently recognize about 35 animal phyla. But the /elationships between these phyla continue to be debated- Although many biology students must find it frustrating that the phylogenetic trees depicted in textbooks cannot be memorized as set-in-stone truths, the uncertainty inherent in these diagrams is a healthy reminder that science is a process of inquiry and as such is dynamic. Researchers have long tested their hypotheses about animal ihylogeny through morphological studies. In the mid-1990s, zoologists also began Lo study the molecular systematics of an.mals. Additional clues to animal phytogeny have come from new studies of lesser-known phyla, along with fossil analyses hat can help clarify which traits are primitive in various animal groups and which are derived. Recall that science is partly distinguished from other ways of knowing because iLs ideas :an be falsified through testing with experiments, observaions, and new analytical methods. Modern phylogenetic systematics is based on the identtficaion of clades, which are monophyletic sets of taxa as defined by shared derived characters unique to those taxa and their common ancestor. (See Chapter 25 to review cladistic analysis and phylogenetic systematics.) Based on cladistic methods, a phylogenetic tree takes shape as a hierarchy of clades nested within larger clades-—-the finer branches and thicker branches of the tree, respectively. Defining the shared derived characters

is the key to a particular hypothesis. A clade might be defined by key anatomical and embryological similarities that researchers conclude are homologous. "More recently, comparisons of monomer sequences in proteins or nucleic acids across species have provided another data source lor inferring common ancestry and defining clades. But whether the daia are "traditional* morphological characters or ''new" molecular sequences or a combination of both, the assumptions and inferences inherent in the resulting trees are the same. To get a sense ol the current debate in animal systematics, let's examine two current phylogenetic hypotheses. These hypotheses are presented on the next two pages as phylogenetic trees. The tree in Figure 32.10 is based on systematic analyses of morphological characters. Figure 32.11 is a simplified compilation of results from recent molecular studies.

Points of Agreement The hypotheses agree on a number of major features of animal phylogeny. After reading each point, see how the statement is reflected by the phylogenetic trees in Figures 32.10 and 32.11. 1. All animals share a common ancestor. Both trees indicate that the animal kingdom is monophyletic, representing a clade called Metazoa. In other words, if we could trace all extant and extinct animal lineages back to their origin, the lineages would converge on a common ancestor. 2. Sponges are basal animals. Among the extant phyla, sponges (phylum Porilera) branch from the base of both animal trees. They exhibit a parazoan (meaning "beside the animals") grade of organization. Tissues evolved only after sponges diverged from other animals. Note that recent molecular analyses suggest that sponges are paraphyletic, as indicated in Figure 32.11. 3. Eumetazoa is a clade of animals with true tissues. All animals except lor sponges belong to a clade of eumetazoans ("true animals"). The common ancestor of living eumetazoans acquired true tissues. Basal members of the eumetazoan clade include the phyla Cnidaria (which includes jellies) and Ctenophora (comb jellies). These basal eumetazoans are diploblastic and generally feature radial symmetry. As a result, they are often placed in the informal grade called Radiata. 4. Most animal phyla belong lo the clade Bilateria. Bilateral symmetry is a shared derived character that helps to define a clade (and grade) containing the majority of animal phyla, called the bilaterians. The Cambrian explosion was primarily a rapid diversification of the bilaterians. 5. Vertebrates and some other phyla belong to the clade Deuterostomia. The name deuterostome refers not only to an animal development grade, but also to a clade that includes vertebrates. (Note, however, ihat the two hypotheses depicted disagree as to which other phyla are also deuterostomes.) CHAPTER 32 An Introduction to Animal Diversity

633

Ancestral colonial flagellate • Figure 32.10 One hypothesis of animal phytogeny based mainly on morphological and developmental comparisons. The bilaterians are divided into protostomes and deuterostomes.

Disagreement over the Bilaterians While these two phylogenetic hypotheses agree on the overall structure of the animal tree, they also disagree on some significant points. The most important of these is the relationships among the bilaterians. The morphology-based tree in Figure 32.10 divides the bilaterians into two clades: deutero634

UNIT FIVE

The EvoiuLionary History of Biological Diversity

stomes and protostomes. In other words, this hypothesis assumes that these two modes of development reflect a phylogenetic pattern. Such a division has long been recognized by zoologists. Within the protostomes, Figure 32.10 indicates that arthropods (which include insects and crustaceans such as lobsters) are grouped with annelids (which include earthworms). Both groups have segmented bodies (think of an earthworm, which is an annelid, and the underside of the tail of a lobster, which is an arthropod). In contrast, several recent molecular studies, as shown in Figure 32.11, generally assign two sister taxa to the protostomes rather than one: the ecdysozoans and the lophotrochozoans. The name Ecdysozoa refers to a characteristic shared by nematodes, arthropods, and some oi the other ecdysozoan phyla (which are not included in our survey). These animals secrete external skeletons (exoskeletons); the stiff covering of a cricket is an example. As the animal grows, it molts, squirming out of its old exoskeleton and secreting a new, larger one. The shedding of the old exoskeleton is called ecdysis, the process for which the ecdysozoans are named (Figure 32.12). Though named for this! characteristic, the clade is actually defined mainly by molecular, evidence supporting the common ancestry of its members. Furthermore, some taxa excluded from this clade by their molecular data do in fact molt, such as certain leeches. The name Lophotwdiozoci refers to two different structures observed in animals belonging to this clade. Some animals, such

• Figure 32.12 Ecdysis. This molting cicada is in the process of emerging from its old exoskeleton. The animal now secretes a new, larger exoskeleton.

Ancestral colonial flagellate A Figure 32.11 One hypothesis of animal phytogeny based mainly on molecular data. The bilaterians are divided into deuterostomes, lophotrochozoans, and ecdysozoans, plus rotifers.

as. ectoprocts, develop a structure called a lophophore (from the Greek lophos, crest, and pherein, to carry), a crown of ciliated tentacles that function in feeding (Figure 32.13a). Other phyla, including annelids and molluscs, go through a distinctive larval stage called the trochophore larva (Figure 32.13b)—hence the name lophotrochozoan.

(a) An ectoproct, a lophophorate

(b) Structure of trochophore larva

A Figure 32.13 Characteristics of lophotrochozoans. CHAPTER 32

An Introduction to Animal Diversity

635

Future Directions in Animal Systematics like any area of scientific inquiry, animal systetnatics is constantly evolving. New sources of information emerge, offering the opportunity to test current hypotheses, which can be modified or replaced by new ones. Systematists are now conducting large-scale analyses of multiple genes across a wide sample of animal phyla. A better understanding of the relationships between these phyla will enable scientists to have a clearer picture of how the diversity of animal body plans arose. In Chapter 33 and 34, we will take a closer look at extant animal phyla and their evolutionary history.

Chapter

Concept Check 1. What evidence indicates that cnidarians share a more recent common ancestor with other1 animak than with sponges? 2. How do the phylogenetic hypotheses presented in Figures 32.10 and 32.11 differ in structuring the major branches within the clade Bilateria? 3. Why is it valuable to collect multiple types of data to evaluate evolutionary relationships among animal phyla? For suggested answers, see Appendix A.

Review

Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

Animals can be characterized by "body plans"

SUMMARY OF KEY CONCEPTS Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers • Nutritional Mode (p. 626) Animals are heterotrophs that ingest their food. • Cell Structure and Specialization (pp. 626-627) Animals are multicellular eukaryotes- Their cells lack cell walls; their bodies are held together instead by structural proteins such as collagen. Nervous tissue and muscle tissue are unique to animals. • Reproduction and Development (pp. 627-628) Gastrulation follows the formation of the blastula, resulting in formation of embryonic tissue layers. All animals, and only animals, have Hox genes that regulate die development of body form. Although the Hox family of genes has been highly conserved, it can produce a wide diversity of animal morphology.

• Symmetry (p. 630) Animals may lack any symmetry or have radial or bilateral symmetry. Bilaterally symmetrical animals have dorsal and ventral sides, as well as anterior and posterior ends. Many bilaterally symmetrical animals have undergone cephalization. • Tissues (pp. 630-631) Animal embryos form germ layers and may be diploblastic (having two germ layers) or triploblastic (having three germ layers). • Body Cavities (p. 631) In triploblastic animals, a body cavity may be present or absent. A body cavity can be a pseudocoelom (derived from blastocoel) or a true coelom (derived from mesoderm). • Protostome and Deuterostome Development (pp. 6 3 1 633) These two modes of development differ in characteristics of cleavage, coelom formation, and blastopore fate.

Leading hypotheses agree on major features of the animal phylogenetic tree

• Neoproterozoic Era (1 Billion-542 Million Years Ago) (pp. 628-629) Early members of the animal fossil record include the Ediacaran fauna.

• Points of Agreement (p. 633) The animal kingdom is monc phyletic. The common ancestor of animals was probably a colonial choanoflagellate. Sponges exhibit a parazoan grade of organization. "True animals" possess true tissues and include all animals except sponges. Cnidaria and Ctenophora are sometimes placed in the grade Radiata. Bilateral symmetry is a trait shared by a clade containing the other phyla of animals.

• Paleozoic Era ( 5 4 2 - 2 5 1 Million Years Ago) (p. 629) The Cambrian explosion marks the earliest fossil appearance of many major groups of living animals.

• Disagreement over the Bilaterians (pp. 634-635) Different analyses of animal systematics support different bilaterian ciades.

• Mesozoic Era (251-65.5 Million Years Ago) (p. 629) During the Mesozoic era, dinosaurs were dominant terrestrial vertebrates. Coral reefs emerged, becoming important marine ecological niches for other organisms.

• Future Directions in Animal Systematics (p. 636) Phylogenetic studies based on larger databases will likely provide further insights into animal evolutionary history. Activity Animal Phylogenetic Tree Investigation Hoiv Do Molecular Data Fit Traditional Phylogenies?

The history of animals may span more than a billion years

• Cenozoic Era (65.5 Million Years Ago to the Present) (pp. 629-630) The present-day mammal orders diversified during the Cenozoic. Insects diversified.

636

UNIT FIVE

The Evolutionary History of Biological Diversity

Science, Technology, and Society Evolution Connection 'Some scientists suggest that the phrase "Cambrian fizzle" might be more appropriate than "Cambrian explosion" to describe the diversification of animals during that geologic period. In a similar vein, Lynn Margulis, of the University of Massachusetts, has compared observing an explosion of animal diversity in Cambrian strata to monitoring Earth from a satellite and noticing the emergence of cities only when they are large enough to be visible from that distance. What do these statements imply about the evolutionary history of animals during that time?

The study of animal phylogeny is viewed by some people as "science for science's sake," and some organizations that fund scientific research tend to favor projects that have more apparent applications to human needs. On the other hand, general interest in the history of life seems to remain high, with articles on new discoveries frequentlyfeatured in popular magazines. Suppose you had the opportunity to join a research team studying animal pbylogeny. Write a shori letter to a nonbiologist explaining why this research might be worth funding.

Scientific Inquiry If you were constructing a phylogenetic analysis of the animal kingdom, based on comparing morphological characteristics, why would the presence of flagella be a poor choice of a characteristic to use as a basis for grouping phyla into clades? Investigation How Do Molecular Data Fit Traditional Fhylogenies?

An Introduction to Animal Diversity

637

A Figure 33.1 A Christmas tree worm, a marine invertebrate.

Key Concepts 33.1 Sponges are sessile and have a porous body and choanocytes 33.2 Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes 3 3 3 Most animals have bilateral symmetry 33 A Molluscs have a muscular foot, a visceral mass, and a mantle 33.5 Annelids are segmented worms 33.6 Nematodes are nonsegmented pseudocoelomates covered by a tough cuticle 33.7 Arthropods are segmented coelomates that have an exoskeleton and jointed appendages 33.8 Echinoderms and chordates are deuterostomes

that have been described. Invertebrates occupy almost every habitat on Earth, from the scalding water released by deep-sea hydrothermal vents to the rocky, frozen ground of Antarctica. In this chapter, we'll take a brief tour of the invertebrate world, using the phylogenetic tree in Figure 33.2 as a guide. Figure 33.3, on the next three pages, explores two dozen in} vertebrate phyla. We'll examine 14 of those phyla in more dejtail in this chapter.

Life Without a Backbone

A

t first glance, you might mistake the organism shown in Figure 33.1 for some type of seaweed. But this colorful inhabitant of coral reefs is actually an animal, not an alga. Specifically, it is a kind of segmented worm commonly called a Christmas tree worm. The two whorls are tentacles, which the worm uses for gas exchange and for filtering small organisms from the water. The tentacles emerge from a tube of calcium carbonate secreted by the worm that protects and supports its soft body Light-sensitive structures on the tentacles can detect the shadow cast by a predator, triggering muscular contractions that rapidly withdraw the tentacles into the tube. Christmas tree worms are invertebrates—animals that lack a backbone. Invertebrates account for 95% of known animal species and all but one of the roughly 35 animal phyla 638

Deuterostornia

A Figure 33.2 Review of animal phytogeny.

Figure 33.3

Invertebrate Diversity The animal kingdom is divided into about 3^ phyla encompassing 1.3 million known species, but estimates of the total number of species range from 10 million to 200 million. Here we explore 24

animal phyla, all of which include invertebrates. Of the phyla surveyed in this figure, those that are illustrated with smaller-sized "preview" photographs are discussed more fully later in this chapter.

CNIDARIA (10,000 species) Sponges are simple, sessile animals that lack true tissues. They live as suspension feeders, trapping particles chat pass through the internal channels of their bodies (see Concept 33.1).

Cnidarians include corals, jellies, and hydras. These animals share a distinctive body plan that includes a gastrovascular cavity with a single opening that serves as both mouth and anus (see Concept 33.2). A jelly

A sponge

KINORHYNCHA (150 species)

PLACOZOA (1 species) On first inspection, the single known species in this phylum, Tichoplax adhaarus, does not even look like an animal. It consists of a few thousand cells arranged in a double-layered plate 2 mm across. Feeding on organic detritus, Trichoplax can reproduce by dividing into two individuals or by budding off many multicellular individuals.

250 urn A placozoan (LM)

Almost all kinorhynchs are less than 1 mm long. They live in sand and mud in oceans around the world, from the imertidal zone to depths of 8,000 m. A kinorhynch's body consists of 13 segments covered in plates. The mouth is tipped with a ring of spines and can be retracted into the body.

A kinorhynch (LM)

;pite their microscopic size, rotifers have •cialized organ systems, including an alimentary canal (digestive tract). They feed on microorganisms suspended in water (see Concept 33.3). A rotifer (LM)

A marine flatworm

"Ectoprocts (also known as bryozoans) live as sessile colonies and are covered by a tough exoskeleton (see Concept 33.3).

Phoronids are marine worms. They live in tunnels in the seafloor, extending tentacles out of the tunnel opening to trap food panicles (see Concept 33.3).

Ectoprocts

Phoronids continued on the next page

CHAPTER

33

639

Figure 33.3 (continued)

Invertebrate Diversity NEMERTEA (900 species)

BRACHIOPODA (335 species) Brachiopods, or lamp shells, may be easily mistaken for clams or other molluscs. However, most brachiopods have a unique stalk that anchors them to iheir substrate (see Concept 33.3).

Proboscis worms, or ribbon worms, swim through water or burrow in sand, extending a unique proboscis to capture prey. Like flatworms, they fack a true coelom, but they have an alimentary canal (see Concept 33.3). A ribbon worm

A brachiopod

CTENOPHORA (100 species) Acanthocephalans (from the Greek acanihias, prickly and cephalo, head) are commonly known as thorny-headed worms because of the curved hooks located on the proboscis at the anterior end of their body. All species are parasites. The larvae develop in arthropods, and the adults live in vertebrates. Some acanthocephalans manipulate An acanthocephalan their intermediate hosts in ways that increase their chances d reaching their final hosts. For example, acanthocephalans that infect New Zealand mud crabs force their hosts to move to more visible locations on the beach, where the crabs are more likely to be eaten by birds, the worms' final hosts.

m

Ctenophores (comb jellies) are diploblastic like cnidarians, suggesting that both phyla diverged from other animals very early in evolution. Although they superficially resemble some cnidarian;, comb jellies possess a number of distinctive traits, including a set of eight "combs" of cilia that propel die animals through th; Actenophore, or comb jelly water. They also have a unique method for catching prey: When a small animal contacts one of their two tentacles, specialized cells burst open, covering the prey with sticky threads. Comb jellies make up a major portion of the ocean's planktonic biomass.

ANNELIDA (16,500 species) Annelids, or segmented worms, are distinguished from other worms by their body segmentation. Earthworms are the most familiar annelids, but the phylum also includes marine and freshwater species (see Concept 33.5).

Molluscs (including snails, clams, squids, and octopuses) have a soft body that in many species is protected by a hard shell (see Concept 33.4).

An octopus

A marine annelid

LORICIFERA (10 species) Loriciferans (from the Latin lorica, corset, and ferre, to bear) are animals measuring only 0.1-0.4 mm in length that inhabit the deep-sea bottom. A loriciferan can telescope its head, neck, and thorax in and out of the lorica, a pocket formed by six plates surrounding the abdomen. Though the natural history of loriciferans is mostly a mystery, at least some species likely eat bacteria. 640

UNIT FIVE

A loriciferan (LM)

The Evolutionary History of Biological Diversity

PRIAPULA (16 species)

A priapulan

Priapulans are worms with a large, rounded proboscis at the anterior end. (They are named after Priapos, the Greek god of fertility, who was symbolized by a giant penis.) Ranging from 0.5 mm to 20 cm in length, most species burrow through seafloor sediments. Fossil evidence suggests that priapulans were among the major predators during the Cambrian period.

NEMATODA (25.000 species) Roundworms are enormously abundant and diverse in the soil and in aquatic habitats; many species parasitize plants and animals. The most distinctive feature of roundworms is a tough, cuticle that coats the body (see Concept 33.6).

ARTHROPODA (1,000,000+ species) The vast majority of known animal species, including insects, crustaceans, and arachnids, are arthropods. All arthropods have a segmented exoskeleton and jointed appendages (see Concept 33.7).

A roundworm

A scorpion (an arachnid) CYCLIOPHORA (1 species)

TARDIGRADA (800 species)

The only known species of cycliophoran. Svmhum pandora, was discovered in 1995 on the mouthparts of a lobster. This tiny vase-shaped creature has a i.nique body plan and a particularly bizarre life cycle. Males impregnate females that are still developing in their mothers' bodies. The fertilized females taen escape, settle elsewhere on A cycliophoran (colorized SEM) the lobster, and release their offspring. The offspring apparently leave that lobster and search for another one to which they attach.

Tardigrades (from the Latin tardus, slow, and gradus, step) are sometimes called water bears for their rounded shape, stubby appendages, and lumbering, beatiike gait. Most tardigrades are less than 0.5 mm in length. Some live in oceans or fresh water, while others live on plants or animals. As many as 2 milTardigrades (colorized SEM) lion tardigrades can be found on a square meter of moss. Harsh conditions may cause tardigrades to enter a state of dormancy; while dormant, they can survive temperatures as low as —272°C, close to absolute zero!

ONYCHOPHORA (110 species) Onychophorans. also called velvet worms, originated during t he Cambrian explosion (see Chapter 32). Originally, they thrived in the ocean, but at some point they succeeded in colonizing land. Today they live only in humid forests. Onychophorans have fleshy antennae and several dozen pairs of saclike legs.

An onychophoran

An acorn worm

Like echinoderms and chordates, hemichordates are deuterostomes (see Chapter 32). Hemichordates also share other traits with chordates, such as gill slits and a dorsal nerve cord. Most hemichordates are known as enteropneusts, or acorn worms. Acorn worms are marine and generally live buried in mud or under rocks; they may reach a length greater than 2 m.

CHORDATA (52,000 species) Echinoderms, such as sand dollars, sea stars, and sea urchins, are aquatic animals that display radial symmetry as adults. They move and feed by using a network of internal canals to pump water to different parts of the body (see Concept 33.8).

More than 90% of all chordate species i animals with backbones (vertebrates). However, the phylum Chordata also includes three groups of invertebrates: tunicates, lancelels, and hagfishes. See Chapter 34 for a full discussion of this phylum.

CHAPTER

33

641

Sponges are §essile and have a porous body and choanocytes Sponges (phylum Porifera) are so sedentary that they were mistaken for plants by the ancient Greeks. Living in both fresh and marine waters, sponges are suspension feeders: They capture food particles suspended in the water that passes through their body, which typically resembles a sac perforated with pores (the name Porifera means "pore bearer'1)- Water is drawn through the pores into a central cavity the spongocoel, and then flows out of the sponge through a larger opening called the osculum (Figure 33.4). More complex sponges have folded body walls, and many contain branched water canals and several oscula. Under certain conditions, the cells around the pores and osculum contract, closing the openings. Sponges range in size from a few millimeters to a few meters. Unlike eumetazoans, sponges lack true tissues, groups of similar cells that act as a functional unit and are isolated from other tissues by membranous layers. However, the sponge body does contain several different cell types. Lining the interior of the spongocoel or internal water chambers are flagellated

choanocytes, or collar cells (named for the membranous collar around the base of the flagellum). The flagella generate a water current, and the collars trap food particles that the choanocytes then ingest by phagocytosis. The similarity between choanocytes and the cells of cboanoflagellates supports the molecular evidence suggesting that animals evolved from a choanoflagellate-like ancestor (see Chapter 32). The body of a sponge consists of two layers of cells separated by a gelatinous region, the mesohyl. Wandering through the mesohyl are cells called amoebocytes, named for their use of pseudopodia. Amoebocytes have many functions. They take up food from the water and from choanocytes, digest it. and carry nutrients to other cells. They also manufacture tough skeletal fibers within the mesohyl. In some groups of sponges, these fibers are sharp spicules made from calcium carbonate or silica; other sponges produce more flexible fibers composed 01 a collagen protein called spongin. (These pliant skeletons are used as bath sponges.) Most sponges are hermaphrodites (named for the Greek god Hermes and goddess Aphrodite), meaning that each individual functions as both male and female in sexual reproduc| tion by producing sperm and eggs. Almost all sponges exhibit sequential hermaphroditism, iunctioning first as one sex and then as the other. Gametes arise from choanocytes or amoebocytes. Eggs reside in the mesohyl, but sperm are carried out of the sponge by the water current. Cross-fertilization results from some of the sperm

0 Choanocytes. The spongocoel is lined with feeding cells called choanocytes. By beating flagella, the choanocytes create a current that draws water in through the porocytes.

Flagellum

Food particles in mucus

Choanocyte

Azure vase sponge {Callyspongia plitifera) Q Spongocoel. Water passing through porocytes enters a cavity called the spongocoel. Q Porocytes. Water enters the epidermis through channels formed by porocytes, doughnut-shaped ceils that span the body wall. Q Epidermis. The outer layer consists of tightly packed epidermal cells. O Mesohyl. The wall of this simple sponge consists of two layers of cells separated by a gelatinous matrix, the mesohyl ("middle matter"). A Figure 33.4 Anatomy of a sponge.

642

The Evolutionary History of Biological Diversity

Phagocytosis of food particles

Arnoebocyte

© The movement of the choanocyte flagella also draws water through its collar of fingerlike projections. Food particles are trapped in the mucus coating the projections, engulfed by phagocytosis, and either digested or transferred to amoebocytes. 0 Amoebocyte. Amoebocytes transport nutrients to other cells of the sponge body and also produce materials for skeletal fibers (spicules)

being drawn into neighboring individuals. Fertilization occurs in the mesohyl, where the zygotes develop into flagellated, swimming larvae that disperse from the parent sponge. Upon settling on a suitable substrate, a larva develops into the sessile adult. Sponges produce a variety of antibiotics and other defen•ive compounds. Researchers are now isolating these compounds, which hold promise for fighting human diseases. For example, Robert Pettit and his colleagues at Arizona State University have found a compound called cribrostatin in marine sponges that can kill penicillin-resistant strains of the bacterium Streptococcus. The team is studying other spongederived compounds as anti-cancer agents.

anemones. A medusa is a flattened, mouth-down version oi the polyp. It moves freely in the water by a combination of passive drifting and contractions of its bell-shaped body. Medusae include free-swimming jellies. The tentacles of a jelly dangle from the oral surface, which points downward. Some cnidarians exist only as polyps or only as medusae; others have both a medusa stage and a polyp stage in their life cycle. Cnidarians are carnivores that use tentacles arranged in a ring around their mouth to capture prey and to push the food into their gastrovascular cavity, where digestion begins. The undigested remains are egested through the mouth/anus. The tentacles are armed with batteries of cnidocytes, unique cells that function in defense and the capture of prey (Figure 33.6).

Concept Check . 1. Describe how sponges feed. 2. Explain how changes in water currents can alfect sponge reproduction.

Polyp

For suggested answers, see Appendix A.

Concept

Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes All animals except sponges belong to the clade Eumetazoa, the animals with true tissues (see Chapter 32). One of the oldest animal groups in this clade is the phylum Cnidaria. Cnidarians have diversified into a wide range of both sessile and floating | forms, including jellies (commonly called "jellyfish"), corals, and hydras. Yet cnidarians continue to exhibit a relatively simple, diploblastic, radial body plan that existed some 570 million years ago. The basic body plan of a cnidarian is a sac with a central digestive compartment, the gastrovascular cavity. A single opening to this cavity functions as both mouth and anus. There are two variations on this body plan: the sessile polyp and the floatling medusa (Figure 33.5). Polyps are cylindrical forms that adhere to the substrate by the aboral end of the body (the end opposite the mouth) and extend their

Mouth/anus A Figure 33.5 Polyp and medusa forms of cnidarians. The body wall of a cnidarian has t w o layers of cells: an outer layer of epidermis (from ectoderm) and an inner layer of gastrodermis (from endoderm). Digestion begins in the gastrovascular cavity and is completed inside food vacuoles in the gastrodermal cells. Flagella on the gastrodermal cells keep the contents of the gastrovascular cavity agitated and help distribute nutrients. Sandwiched between the epidermis and gastrodermis is a gelatinous layer, the mesoglea.

"Tpqger'

Cnidocyte

,

1v & Figure 33.6 A cnidocyte of a hydra. This type of cnidocyte contains a stinging capsule, tentacles, waiting for prey. Examples of the nematocyst, which itself contains an inverted thread. When a "trigger" is stimulated by touch the polyp form include hydras and sea or by certain chemicals, the thread shoots out, puncturing and injecting poison into prey.

CHAPTER 33

643

Cnidocytes contain cnidae (from the Greek cnide, nettle), capsule-like organelles that are capable of everting and that give phylum Cnidaria its name. Cnidae called nematocysts are stinging capsules. Other cnidae have very long threads that stick to or entangle small prey that bump into the tentacles. Contractile tissues and nerves occur in their simplest forms in cnidarians. Cells of the epidermis (outer layer) and gastrodennis (inner layer) have bundles of microfilamenis arranged into contractile fibers (see Chapter 6). The gastrovascular cavity acts as a hydrostatic skeleton against which the contractile cells can work. When a cnidarian closes its mouth, the volume of the cavity is fixed, and contraction of selected cells causes the animal to change shape. Movements are coordinated by a nerve net. Cnidarians have no brain, and the noncentralized nerve net is associated with simple sensory structures that are distributed radially around the body Thus, the animal can detect and respond to stimuli equally from all directions. As summarized in Table 33.1, the phylum Cnidaria is divided into four major classes: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa (Figure 33.7).

Hydrozoans Most hydrozoans alternate between polyp and medusa forms, as in the life cycle of Obelia (Figure 33.8). The polyp stage, a colony of interconnected polyps in the case of Obelia, is more conspicuous than the medusae. Hydras, among the few cnidarians found in fresh water, are unusual hydrozoans in that they exist only in the polyp form. When environmental conditions are favorable, a hydra reproduces asexually by

(b) Many species of jellies (class Scyphozoa), including the species pictured here, are biolumtnescent. The largest scyphozoans have tentacles more than 100 rn long dangling from a bell-shaped body up to 2 m in diameter. (a) These colonial polyps are members of class Hydrozoa.

A Figure 33.7 Cnidarians. 644

UNIT FIVE

The Evolutionary History of Biological Diversity

budding, the formation of outgrowths that pinch off from the parent and live independently (see Figure 13.2). When environmental conditions deteriorate, hydras can reproduce sexually, forming resistant zygotes that remain dormant until the conditions improve.

Scyphozoans The medusa generally is the predominant stage in the life cycle of the class Scyphozoa. The medusae of most species live among the plankton as jellies. Most coastal scyphozoans go through a small polyp stage during their life cycle, whereas those that live in the open ocean generally lack the polyp stage completely.

Table 33.1 Classes of Phylum Cnidaria Class and Examples

Main Characteristics

Hydrozoa (Portuguese manof-war, hydras, Ohdia, some corals; see Figures 33.7a and 33.8)

Most marine, a few freshwater; both polyp and medusa stages in most species; polyp stage often colonial

Scyphozoa (jellies, sea nettles; see Figure 33.7b)

All marine; polyp stage reduced: free-swimming; medusae up to 2 m in diameter

Cubozoa (box jellies, sea wasps; see Figure 33.7c)

All marine; box-shaped medusae; complex eyes All marine; medusa stage completely absent; most sessile; many colonial

Anihozoa (sea anemones, most corals, sea fans; see Figure 33.7d)

(c) The sea wasp (Chironex flecken) is a member of class Cubozoa. Its poison, which can subdue fish and other large prey, is more potent than cobra venom.

(d) Sea anemones and other members of class Anthozoa exist only as polyps.

© Some of the colony's polyps, equipped with tentacles, are specialized for feeding.

© Other polyps, specialized for reproduction, lack tentacles and produce tiny medusae by asexual budding.

Q The medusae swim off, grow, and reproduce sexually.

I Gonad

MEIOSIS "^Hl

Medusa

© The planula eventually settles and develops into a new polyp.

i / '

© The zygote develops into a solid ciliated larva called a planula.

Haploid (n) Diploid (2n)

Figure 33.8 The life cycle of the hydrozoan Obelia. The polyp stage is asexual, and :he medusa stage is sexual; these two stages alternate, one producing the other. Do not confuse his with the alternation of generations that occurs in plants and some algae. Both the polyp and he medusa are diploid organisms. (Typical of animals, only the gametes of Obelia are haploid.) By :ontrast, one plant generation is haploid, and the other is diploid.

Cubozoans

Anthozoans

\s their name (which means "cube animals") suggests, cuboloans have a box-shaped medusa stage. They can be distinguished from scyphozoans in other significant ways, such as having complex eyes embedded in the fringe of their medusae. Cubozoans, which generally live in tropical oceans, are often equipped with highly toxic cnidocytes. The sea wasp Chironexjleckeri), a cubozoan that lives off the coast of northem Australia, is one of the deadliest organisms on Earth: Its sting causes intense pain and can lead to respirator}' failure, cardiac arrest, and death within minutes. The amount of poison in Tie sea wasp is enough to kill 60 people. The poison of sea /asps isn't universally fatal however; sea turtles have defenses against it, allowing them to eat the cubozoan in great quantities.

Sea anemones and corals belong to the class Anthozoa ("flower animals")- They occur only as polyps. Corals live as solitary or colonial forms and secrete a hard external skeleton of calcium carbonate. Each polyp generation builds on the skeletal remains of earlier generations, constructing "rocks" with shapes characteristic of their species. It is these skeletons that we call coral. Coral reefs are to tropical seas what ram forests are to tropical land: They provide habitat for a wealth of other species. Unfortunately, like rain forests, coral reefs are being destroyed at an alarming rate by human activity. Pollution and overfishing are major threats, and global warming may also be contributing to their demise. Well examine this problem in more detail in Chapter 54. CHAPTER 33

Invertebrates

645

Concept Check 1. Compare and contrast the polyp and medusa forms of cnidarians. 2. Describe the structure and function of the stinging cells for which cnidarians are named. For suggested answers, see Appendix A.

Concept

Most animals have bilateral symmetry The vast majority of animal species belong to the clade Bilateria, which consists of animals with bilateral symmetry and triploblastic development (see Chapter 32). Most bilaterians are also coelomates. While the sequence of bilaterian evolution is still a subject of active investigation, researchers generally agree that the most recent common ancestor of Living bilaterians probably existed in the late Proterozoic. During the Cambrian explosion, most major groups of bilaterians emerged. This section will focus on just six bilaterian phyla; Concepts 33.4-33.8 will explore six other major bilaterian phyla.

Flatworms FlaLworms (phylum Platyhelminthes) live in marine, freshwater, and damp terrestrial habitats. In addition to many freeliving forms, flatworms include many parasitic species, such as flukes and tapeworms. Flatworms are so named because their bodies are thin between the dorsal and ventral surfaces (flattened dorsoventrally; platyhelminth means "flat worm")The smallest are nearly microscopic free-living species, while some tapeworms can be over 20 m long. (Note that worm is not a formal taxonomic name but a general term for animals with long, thin bodies.) Although flatworms undergo triploblastic development, they are acoelomates (animals that lack a body cavity). Their flat shape places all cells close to the surrounding water, enabling gas exchange and the elimination of nitrogenous waste (ammonia) to occur by diffusion across the body surface. Flatworms have no organs specialized for gas exchange or circulation, and their relatively simple excretory apparatus functions mainly to maintain osmotic balance with their surroundings. This apparatus consists of ciliated cells called flame bulbs that waft fluid through branched ducts opening to the outside (see Figure 44.10). Most flatworms have a gastrovascular cavity with only one opening. The fine branches of the gastrovascular cavity distribute food throughout the animal. 646

UNIT FIVE

The Evolutionary History of Biological Diversity

Flatworms are divided into four classes (Table 33.2) Turbellaria (mostly free-living flatworms), Monogenea (mono geneans), Trematoda (trematodes, or flukes), and Cestodi (tapeworms). Turhellarians Turbellarians are nearly all free-living and mostly marint (Figure 33.9). The best-known turbellarians are members a the genus Dugesia, commonly called planarians. Abundant ir unpolluted ponds and streams, planarians prey on smaller an imals or feed on dead animals. Planarians move by using cilia on their ventral epidermis gliding along a film of mucus they secrete. Some other turbel

Table 33.2 Classes of Phylum Platyhelminthes Class and Examples

Main Characteristics

Turbellaria (mostly free-living flaiworms, such as Dugesia; see Figures 33.9 and 33.10)

Most marine, some freshwater, a few terrestrial; predators and scavengers; body surface ciliated

Monogenea (monogeneans)

Marine and freshwater parasites; most infect external surfaces of fishes; life history simple; ciliated larva slarts infection on host

Trematoda (trematodes. also called flukes; see Figure 33.11)

Parasites, almost always of vertebrates; two suckers attach to host; most life cycles include intermediate hosts Parasites of vertebrates; scolex attaches to host; progloitids produce eggs and break off after fertilization; no head or digestive system; life cycle with one or more intermediate hosts

Cestoda (laneworms; see Figure 33.12)

A Figure 33.9 A marine flatworm (class Turbellaria).

Brians also use their muscles to swim through water with an ndulating motion. A planarians head is equipped with a pair of light-sensitive yespots and lateral flaps that function mainly to detect specific hemicals. The planarian nervous system is more complex and entralized than the nerve nets of cnidarians (Figure 33.10). 'lanarians can learn to modify their responses to stimuli. Planarians can reproduce asexually through regeneration, he parent constricts in the middle, and each half regenerates ie missing end. Sexual reproduction also occurs. Although planarians are hermaphrodites, copulating mates cross-fertilize.

Pharynx. The mouth is at the tip of a muscular pharynx that extends from the animal's ventral side. Digestive juices are spilled onto prey, and the pharynx sucks small pieces of food into the gastrovascular cavity, where digestion continues.

Digestion is completed within the cells lining the gastrovascular cavity, which has three branches, each with fine subbranches that provide an extensive surface area.

/

Undigested wastes are egested through the mouth.

Monogeneans and Trematodes Aonogeneans and trematodes live as parasites in or on other nimals. Many have suckers for attaching to internal organs r to the outer surfaces of the host- A tough covering helps rotect the parasites within their hosts. Reproductive organs ccupy nearly the entire interior of these worms. As a group, trematodes parasitize a wide range of hosts, Ganglia. Located at the anterior end Ventral nerve cords. From nd most species have complex life cycles with alternating the ganglia, a pair of of the worm, near the main sources exual and asexual stages. Many trematodes require an interventral nerve cords runs of sensory input, is a pair of ganglia, necliate host in which larvae develop before infecting the final the length of the body. dense clusters of nerve cells. .ost (usually a vertebrate), where the adult worms live. For xample, trematodes that parasitize h u m a n s spend part of A Figure 33.10 Anatomy of a planarian, a turbellarian. leir lives in snail hosts (Figure 33.11). "he 200 million people around the world Mature flukes live in the blood vessels of the human /ho are infected with blood llukes intestine. A female fluke fits into a groove running Schistosoma) suffer from schistosomiasis, the length of the larger male's body, as shown in the light micrograph at right. disease whose symptoms include pain, nemia, and dysentery Living within different hosts p u t s .emands on trematodes that free-living nimals don't face. A blood fluke, for fntance, must evade the immune systems f both snails and humans. By mimicking ie surface proteins of its hosts, the blood uke creates a partial irnmunologicai amouflage for itself. It also releases molcules that manipulate the hosts' immune ystems into tolerating the parasite's exisence. These defenses are so effective that ndividual flukes can survive in humans or more than 40 years. Most monogeneans are external parates of fish. The monogenean life cycle is elatively simple, with a ciliated, freewimming larva initiating the infection on host. Although monogeneans have been raditionally aligned with the trematodes, ;ime structural and chemical evidence uggests they are more closely related to

. • -These | the si veS5 \ ^ I fieJds • with i

The eggs develop in water into ciliated larvae. These larvae infect snails, the intermediate hosts.

I

" I

.... anotr larva, The sr

-a Figure 33.11 The life cycle of a btood fluke (Schistosoma mansoni), a trematode.

ipevorms. 647

Tapeworms Tapeworms (class Cestoidea) are also parasitic (Figure 33.12). The adults live mostly inside vertebrates, including humans. In many tapeworms, the anterior end, or scolex, is armed with suckers and often hooks that lock the worm to the intestinal lining of the host. Tapeworms lack a gastrovascular cavity; they absorb nutrients released by digestion in the hosts intestine. Absorption occurs across the tapeworms body surface. Posterior to the scolex is a long ribbon of units called proglottids, which are little more than sacs of sex organs. Mature proglottids, loaded with thousands of eggs, are released from the posterior end of a mature tapeworm and leave the host's body in feces. In one type ol life cycle, human feces contaminate the food or water of intermediate hosts, such as pigs or cattle, and the tapeworm eggs develop into larvae that encyst in muscles of these animals. Humans acquire the larvae by eating undercooked meat contaminated with cysts, and the worms develop into mature adults within the human. Large tapeworms can block the intestines and rob enough nutrients from the human host to cause nutritional deficiencies. An orally administered drug named niclosamide kills the adult worms.

Rotifers Rotifers (phylum Rotifera) are tiny animals that inhabit fresh water, the ocean, and damp soil. Ranging in size from about 50 urn to 2 mm, rotifers are smaller than many protests but nevertheless are truly multicellular

and have specialized organ systems (Figure 33.13). In con trast to cnidarians and flatworms, which have a gastrovas cular cavity, rotifers have an alimentary canal, a digestivt tube with a separate mouth and anus. Internal organs lit within the pseudocoelom, a body cavity that is not com pletely lined by mesoderm (see Figure 32.8b). Fluid in the pseudocoelom serves as a hydrostatic skeleton (see Chapte" 49). Movement of a rotifers body distributes the Quic throughout the body, circulating nutrients and wastes ir these tiny animals. The word rotifer derived from Latin, means "wheel-bearer,' a reference to the crown of cilia that draws a vortex of wate" into the mouth. Posterior to the mouth, a region of the diges tive tract called the pharynx bears jaws (trophi) that grind up food, mostly microorganisms suspended in the water. Rotifers undergo unusual forms of reproduction. Some species consist only of females that produce more female: from unfertilized eggs, a type of reproduction called parthenogenesis. Other species produce two types of eggs that develop by parthenogenesis, one type forming females and the other type developing into simplified males that can not even feed themselves. These males survive long enough tc produce sperm that fertilize eggs, which form resistant zygotes that can survive when a pond dries up. When conditions are favorable, the zygotes break dormancy and develop into a new female generation that reproduces by parthenogenesis until conditions become unfavorable again. It is puzzling that so many rotifer species survive withou males. The vast majority of animals and plants reproduce sexually at least some of the time, and sexual reproduction has certain advantages over asexual reproduction. For example species that reproduce asexually tend to accumulate harmful mutations in their genomes faster than sexually reproducing species. As a result, asexual species should experience higher rates of extinction and lower rates of speciation.

:-Proglottids with reproductive structures 200 pin

A Figure 33.12 Anatomy of a tapeworm. The inset shows a closeup of the scolex (colorized SEM). 648

UNIT FIVE

The Evolutionary History of Biological Diversity

A Figure 33.13 A rotifer. These pseudocoelomates, smaller than many protists, are generally more anatomically complex than flatworms (LM).

Nobel prize-winning biologist Matthew Meselson, of Harrd University, has been studying a class of asexual rotifers lied Bdelloidea. Some 360 species of bdelloid rotifers are own, and all of them reproduce by parthenogenesis without / males. Paleontologists have discovered bdelloid rotifers eserved in 35-million-year-old amber, and the morphology of ese fossils resembles only the female form, with no evidence males. By comparing the DNA of bdelloids with that of their Dsest sexually reproducing rotifer relatives, Meselson and his illeagues concluded that bdelloids have likely been asexual for iich longer than 35 million years. How these animals anage to flout the general rule against long-lived asexuality i puzzle.

ophophorates: Ectoprocts, Phoronids, and racliiopods

length. Some species live buried in the sand within tubes made of chitin, extending their lophophore from the opening of the tube and withdrawing it into the tube when threatened (Figure 33.14b)

Brachiopods, or lamp shells, superficially resemble clams and other hinge-shelled molluscs, but the two halves of the brachiopod shell are dorsal and ventral rather than lateral, as in clams (Figure 33.14c). All brachiopods are marine. Most live attached to the seafloor by a stalk, opening their shell slightly to allow water to flow over the lophophore. The living brachiopods are remnants of a much richer past that included 30,000 Paleozoic and Mesozoic species. Lingula, a living brachiopod genus, is nearly identical to brachiopods that lived 400 million years ago.

Nemerteans

Members of the phylum Nemertea are commonly called proaterians in three phyla—Ectoprocta, Phoronida, and boscis worms or ribbon worms (Figure 33.15). A nemerteans achiopoda—are traditionally called lopbophorates because ey all have a lophophore, a horseshoeaped or circular crown of ciliated tentas that surround the mouth (see Figure ,13a). As the cilia draw water toward . mouth, the tentacles trap suspended id particles. The common occurrence of s complex apparatus in lophophorates ggesLs that these three phyla are related, her similarities, such as a U-shaped nentary canal and the absence of a disct head, are adaptations to a sessile stence. In contrast to flatworms, which k a body cavity, and rotifers, which v?e a pseudocoelom, lophophorates ?e a true coelom completely lined by soderm (see Figure 32.8a). (a) Ectoprocts, such as this sea (b) In phoronids such as (c) Brachiopods have a hinged shell, Ectoprocts (from the Greek ecto, outmat {Membranipora Phoronis hippocrepia, the The two parts of the shell are e, and procta, anus) are colonial animembranacea), are colonial lophophore and mouth dorsal and ventral. .s that superficially resemble plants, leir common name, bryozoans, means oss animals.") In most species, the ony is encased in a hard exoskeleton th pores through which the lophoores extend (Figure 33.14a). Most ec>roct species live in the sea, where they among the most widespread and nurous sessile animals. Several species important reef builders. Ectoprocts o live in lakes and rivers. Colonies of freshwater ectoproct Pectinatella magca form on submerged sticks or rocks d can grow into a gelatinous, ball-shaped ss more than 10 cm across. Phoronids are tube-dwelling marine rms ranging from 1 mm to 50 cm in

lophophorates.

are at one end of an elongated trunk.

A Figure 33.14 Lophophorates.

M Figure 33.15 A ribbon worm, phylum Nemertea. C H A P T E R 33

Invertebrates

649

body is structurally acoelomate, like that of a flatworm, but it contains a small, fluid-filled sac that may be a reduced version of a coelom. The sac and fluid hydraulically operate an extensible proboscis, which rapidly shoots out of the worm's body, in many cases delivering a toxin to its prey. Nemerteans range in length from less than 1 mm to several meters. Nearly all members of this phylum are marine, but a few species inhabit fresh water or damp soil. Some are active swimmers; others burrow in the sand. Nemerteans and flatworms have similar excretory, sensory, and nervous systems. But in addition to the unique proboscis apparatus, two anatomical features not found in flatworms have evolved in the phylum Nemertea: an alimentary canal and a closed circulatory system, in which the blood is contained in vessels and is therefore distinct from fluid in the body cavity. Nemerteans have no heart; their blood is propelled by muscles squeezing the vessels.

Concept Check 1. Explain how tapeworms can survive without a coelom, a mouth, a digestive system, or an excretory system. 2. Is the presence or absence of an alimentary canal related to the size of an animal? Support your answer with two examples. 3. Explain how, in terms of function, ectoprocts have more in common with nonbilaterian corals than with their closer bilaterian relatives. For suggested answers, see Appendix A.

T Figure 33.16 The basic body plan of a mollusc. Nephridium. Excretory organs called nephridia remove metabolic wastes from the hemolymph.

Molluscs have a muscular foot, a visceral mass, and a mantle Snails and slugs, oysters and clams, and octopuses and squi are all molluscs (phylum Mollusca). Most molluscs are m rine, though some inhabit fresh water, and there are snails a slugs that live on land. Molluscs are soft-bodied animals (fro the Latin molluscus, soft), but most are protected by a ha shell made of calcium carbonate. Slugs, squids, and octopus have a reduced internal shell or have lost their shell con pletely during their evolution. Despite their apparent differences, all molluscs have a sin ilar body plan (Figure 33-16). The body has three main part a muscular foot, usually used for movement; a visceral ma containing most of the internal organs; and a mantle, a fold tissue that drapes over the visceral mass and secretes a shell one is present). In many molluscs, the mantle extends beyon the visceral mass, producing a water-filled chamber, tr mantle cavity, which houses the gills, anus, and excreto pores. Many molluscs feed by using a straplike rasping orga called a radula to scrape up food. Most molluscs have separate sexes, with gonads (ovaries testes) located in the visceral mass. Many snails, however, a hermaphrodites. The life cycle oi many marine molluscs i eludes a ciliated larval stage, the trochophore, which is al characteristic of marine annelids (segmented worms) an some other invertebrates (see Figure 32.13b). The basic body plan of molluscs has evolved in varioi ways in the eight classes of the phylum. We examine four Heart. Most molluscs have an open circulatory system. The dorsally located heart pumps circulatory fluid called hemolymph through arteries into sinuses (body spaces). The organs of the mollusc are thus continually bathed in hemolymph.

Radula. The mouth region in many mollusc species contains a rasp-like feeding organ called a radula. Th belt of backwardcurved teeth slides back and forth, scraping and scooping like a backhoe.

The nervous system consists of a nerve ring around the esophagus, from which nerve cords extend.

650

Concept

The Evolutionary History of Biological Diversity

ose classes here (Table 33.3): Polyplacophora (chitons), Gasopoda (snails and slugs), Bivalvia (clams, oysters, and other b valves), and Cephalopoda (squids, octopuses, cuttlefish, id chambered nautiluses).

Chitons hitons have an oval-shaped body and a shell divided into ght dorsal plates (Figure 33.17); the body itself, however, is lsegmented. You can find these marine animals clinging to cks along the shore during low tide. Try to dislodge a chiton t f hand, and you will be surprised at how well its loot, acting a suction cup, grips the rock. A chiton can also use its foot

Table 33.3 Major Classes of Phylum Mollusca Class and Examples

Main Characteristics

Polypiacophora (chitons; see Figure 33.17)

Marine; shell with eight plates; fool used for locomotion; radula; no head Marine, freshwater, or terrestrial; asymmetrical body, usually with a coiled shell; shell reduced or absent in some; fool for locomotion; radula

Gastropoda (snails, slugs; see Figures 33.18 and 33.19)

Bivalvia (clams, mussels, scallops, oysters; see Figures 33.20 and 33.21)

Marine and freshwater; flattened shell with two valves; head reduced; paired gills; no radula; most are suspension feeders; mantle forms siphons

Cephalopoda (squids, octopuses, cuttlefish, chambered nautiluses; see Figure 33.22)

Marine; head surrounded by grasping tentacles, usually with suckers; shell external, internal, or absent; mouth with or without radula; locomotion by jet propulsion using siphon made Irom foot

to creep slowly over the rock surface. Chitons use their radula to cut and. ingest algae.

Gastropods About three-quarters of all living species of molluscs are gastropods (Figure 33.18). Most gastropods are marine, but there are also many freshwater species; garden snails and slugs are among the gastropods that have adapted to land. The most distinctive characteristic of the class Gastropoda is a developmental process known as torsion. As a gastropod embryo develops, its visceral mass rotates up to 180°, causing the animal's anus and mantle cavity to wind up above its head (Figure 33.19). After torsion, some of the organs that were bilateral are reduced in size or are lost on one side of the body. Torsion should not be contused with the formation of a coiled shell, which is an independent developmental process. Most gastropods have a single, spiraled shell into which the animal can retreat when threatened. The shell is often conical but is somewhat flattened in abalones and limpets. Many gastropods have a distinct head with eyes at the tips of tentacles. Gastropods move literally at a snail's pace by a rippling motion of their foot or by means of cilia. Most gastropods use their

A Figure 3 3 . 1 8 Gastropods.

(b) A sea slug. Nudibranchs, or sea slugs, lost their shell during their evolution.

Mantle cavity

Stomach

• Figure 33.19 The results of torsion in a gastropod. A Figure 33.17 A chiton. Clinging tenaciously to rocks in the ntertidal zone, this chiton displays the eight-plate shell characteristic of molluscs in the class Polyplacophora.

Because of torsion (twisting of the visceral mass) during embryonic development, the digestive tract is coiled and the anus is near the anterior end of the animal. CHAPTER 33

Invendiratts

6iI

radula to graze on algae or plants. Several groups, however, are predators, and their radula has become modified for boring holes in the shells of other molluscs or for tearing apart prey. In the cone snails, the teeth of the radula form poison darts that are used to subdue prey. Terrestrial snails lack the gills typical of most aquatic gastropods; instead, the lining of the mantle cavity functions as a lung, exchanging respiratory gases with the air.

Bivalves The molluscs of class Bivalvia include many species of clams, oysters, mussels, and scallops. Bivalves have a shell divided into two halves (Figure 33.20). The halves are hinged at the mid-

dorsal line, and powerful adductor muscles draw them tightl together to protect the soft-bodied animal. Bivalves have no dis tinct head, and the radula has been lost. Some bivalves hav eyes and sensory tentacles along the outer edge 01 their mantle The mantle cavity of a bivalve contains gills that are use for feeding as well as gas exchange (Figure 33.21). Most b valves are suspension feeders. They trap fine food particles i mucus that coats their gills, and cilia then convey the particle to their mouth. Water enters the mantle cavity through a incurrent siphon, passes over the gills, and then exits th mantle cavity through an excurrent siphon. Most bivalves lead rather sedentary lives, a characteristi suited to suspension feeding. Sessile mussels secrete stron threads that tether them to rocks, docks, boats, and the she! of other animals. However, clams can pull themselves into th sand or mud, using their muscular foot for an anchor, an scallops can skitter along the seafloor by flapping their shell? rather like the mechanical false teeth sold in novelty shops.

Cephalopods

A Figure 33.20 A b i v a l v e . This scallop has many eyes (dark blue spots) peering out from each half of its hinged shell.

A Figure 33.21 Anatomy of a clam. The left half of the clam's shell has been removed. Food particles suspended in water that enters through the incurrent siphon are collected by the gills and passed via cilia and elongated flaps called palps to the mouth. 652

UNIT FIVE

The Evolutionary History of Biological Diversity

Cephalopods are active predators. They use their tentacles t grasp prey and their beak-like jaws to inject an immobilizin poison. The foot of a cephalopod has become modified in; to a muscular excurrent siphon and parts of the tentacle and head. (Cephalopod means "head foot.") Most octopuse, creep along the seafloor in search of crabs and other foot (Figure 33.22a). Squids dart about by drawing water into th mantle cavity and then firing a jet of water through the excur rent siphon (Figure 33.22b). They steer by pointing the siphor in different directions. A mantle covers the visceral mass of cephalopods, but th shell is reduced and internal (in squids and cuttlefish) o missing altogether (in many octopuses). One small group o shelled cephalopods, the chambered nautiluses, survive! today (Figure 33.22c) Cephalopods are the only molluscs with a closed circula tory system. They also have well-developed sense organs an> a complex brain. The ability to learn and behave in a complex manner is probably more critical to fast-moving predator than to sedentary animals such as clams. The ancestors of octopuses and squids were probably shellec molluscs that took up a predatory lifestyle; the shell was lost in later evolution. Shelled cephalopods called ammonites, somi of them as large as truck tires, were the dominant invertebrat> predators of the seas for hundreds of millions of years unti their disappearance during the mass extinctions at the end o the Cretaceous period (see Chapter 26). Most species of squid are less than 75 cm long, but some are considerably larger. The giant squid (Architeuthis dux) was for a long time the largest squid known, with a mantle up to 2.25 m long and a total length of 18 m. In 2003, however, a specimen of the rare species Mesonychoteuthis hamiltoni with a mantle length of 2.5 m was caught near Antarctica. Some bi-

(c) Chambered nautiluses are the only living cephalopods with an external shell. j) Octopuses are considered among the most intelligent invertebrates.

Figure 33.22 Cephalopods.

legists think this specimen was a juvenile and estimate that dults oi its species could be twice as large! Unlike A. dux, /hich has large suckers and small teeth on its tentacles, M. imiltoni has rotating bars at the ends of its tentacles that can eliver deadly lacerations. It is likely that A. dux and M. hamiltoni spend most of their me m the deep ocean, where they may feed on large fishes, emains of both species have been found in the stomachs of )erm whales, which are probably their only natural predator, cientists have never observed either squid species in its natral habitat. As a result, these marine giants remain among the reat mysteries of invertebrate life.

from less than 1 mm to 3 m, the length of a giant Australian earthworm. The phylum Annelida is divided into three classes (Table 33.4): Oligochaeta (earthworms and their relatives), Polychaeta (polychaetes), and Hirudinea (leeches).

Oligochaetes Oligochaetes (from the Greek oligos, few, and chaite, long hair) are named for their relatively sparse chaetae, or bristles made of chitin. This class of segmented worms includes the earthworms and a variety of aquatic species. Earthworms eat their way through the soil, extracting nutrients as the soil passes through the alimentary canal. Undigested material, mixed with mucus secreted into the canal, is egested as castings through the anus. Farmers value earthworms because

Concept Check 1. "Explain how the modification of the molluscan foot in gastropods and cephalopods relates to their respective lifestyles. 2. How have bivalves diverged from the basic molluscan body plan? For suggested answers, see Appendix A.

Concept

Annelids are segmented worms Annelida means "little rings," referring to the annelid body's resemblance to a series of fused rings. Annelids live in the sea, most freshwater habitats, and damp soil. They range in length

Table 33.4 Classes of Phylum Annelida Class and Examples

Main Characteristics

Oligochaeta (freshwater, marine, and terrestrial segmented worms, such as earthworms; see Figure 33.23) Polychaeia (mostly marine segmented worms; see Figure 33.24)

Reduced head; no parapodia, but chaetae

Hirudinea (leeches; see Figure 33.25)

Well-developed head; each segment usually has parapodia with chaetae; tube-dwelling and free-living Body usually flattened, with reduced coelom and segmentation; chaetae absent; suckers at anterior and posterior ends; parasites, predators, and scavengers

CHAPTER 33

Invertebrates

653

the animafs tifl the earth, and their castings improve the texture of the soil. (Darwin estimated that 1 acre of British larmland contained about 50,000 earthworms that produced 18 tons of castings per year.) Figure 33.23 provides a guided tour of the anatomy of an earthworm, which is representative of annelids. Earthworms are hermaphrodites, but they cross-fertilize. Two earthworms mate by aligning themselves in such a way

Each segment is surrounded by longitudinal muscle, which in turn is surrounded by circular muscle. Earthworms coordinate the contraction of these two sets of muscles to move (see Figure 49.25). These muscles work against the noncompressible coelomic fluid, which acts as a hydrostatic skeleton.

that they exchange sperm, and then they separate. Th received sperm are stored temporarily while an organ calle the clitellum secretes a mucous cocoon. The cocoon slide along trie worm, picking up the eggs and then the ston sperm. The cocoon then slips off the worm's head and remain in the soil while the embryos develop. Some earthwom can also reproduce asexually by fragmentation followed t regeneration.

Coelom. The coelom of the earthworm is partitioned by septa.

Many of the internal structures are repeated within each segment of the earthworm.

Chaetae. Each segment has four pairs of chaetae, bristles that provide traction for burrowing.

Metanephridium. Each segment of the worm contains a pair of excretory tubes, called metanephridia, with ciliated funnels, called nephrostomes. The metanephridia remove wastes from the blood and coelomic fluid through exterior pores.

Tiny blood vessels are abundant in the earthworm's skin, which functions as its respiratory organ. The blood contains oxygen carrying hemoglobin.

Metanephridium ntestine Giant Australian earthworm Cerebral ganglia. The earthworm nervous system features a brainlike pair of cerebral ganglia above and in front of the pharynx. A ring of nerves around the pharynx connects to a subpharyngeal ganglion, from which a fused pair of nerve cords runs posteriorly.

A Figure 33.23 Anatomy of an earthworm.

654

UNIT FIVE

The circulatory system, a network of vessels, is closed. The dorsal and ventral vessels are linked by segmental pairs of vessels. The dorsal vessel and five pairs of vessels that circle the esophagus of an earthworm are muscular and pump blood through the circulatory system.

The Evolutionary History of Biological Diversity

Ventral nerve cords with segmental ganglia. The nerve cords penetrate the septa and run the length of the anima as do the digestive tract and longitudinal blood vessels.

< Figure 33.25 A leech. A

Polychaetes Ilach segment of a polychaete has a pair of paddle-like or tdge-like structures called parapodia ("almost feet") that f mction in locomotion (Figure 33.24). Each parapodium has iveral chaetae, which are more numerous than those in igochaetes. In many polychaetes, the parapodia are richly rpplied with blood vessels and function as gills. Polychaetes make up a large and diverse class, most of hose members are marine. A few species drift and swim mong the plankton, many crawl on or burrow in the seafloor, nd many others live in tubes. Some tube-dwellers, such as t ic fan worms and feather-duster worms, build their tubes by ixing mucus with bits of sand and broken shells. Others, ich as Christmas tree worms (see Figure 33.1), construct ibes using only their own secretions. < Figure 33.24 A polychaete. Hesiolyra bergi lives on the seafloor around deep-sea hydrothermal vents.

nurse applied this medicinal leech (Hirudo medicinalis) to a patient's sore thumb to drain blood from a hematoma fan abnormal accumulation of blood around an internal injury).

Concept Check 1. Annelid anatomy can be described as "a tube within a tube."' Explain. 2. Explain how an earthworm uses its segmental muscles and coelom in movement. "

..-••.••"•-"

i n c r s , see Appendix A.

Concept

Nematodes are nonsegmented pseudocoelomates covered by a tough cuticle majority of leeches inhabit fresh water, but there are also larine species as well as terrestrial leeches found in moist vegtation. Leeches range in length from about 1 to 30 cm. Many re predators that feed on other invertebrates, but some are parasites that suck blood by attaching temporarily to other animals, ncludmg humans (Figure 33.25). Some parasitic species use lladelike ]aws to slit the skin of the host, whereas others secrete jnzymes that digest a hole through the skin. The host is usually iblivious to this attack because the leech secretes an anesthetic, klter making the incision, the leech secretes another chemical, lirudin, which keeps the blood of the host from coagulating tear the incision. The parasite then sucks as much blood as it :an hold, often more than ten times its own weight. After this gorging, a leech can last for months without another meal. Until this century, leeches w^ere frequently used for blood"etting. Today they are used to drain blood that accumulates in lissues following certain injuries or surgeries. Researchers are ilso investigating the potential use of hirudin to dissolve unwanted blood clots that form during surgery or as a result of eart disease. A recombinant form of hirudin has been develped and is in clinical trials.

Among the most widespread of all animals, nematodes, or roundworms, are found in most aquatic habitats, in the soil, in the moist tissues of plants, and in the body fluids and tissues of animals. In contrast to annelids, nematodes do not have a segmented body. The cylindrical bodies of nematodes (phylum Nematoda) range from less than 1 mm to more than a meter in length, often tapering to a fine tip at uhe posterior end and to a more blunt tip at the anterior end (Figure 33.26). The body is covered by a tough coat called a cuticle; as the worm grows, it

• Figure 33.26 A free-living nematode (colorized SEM).

CHAPTER 33

Invertebrates

655

periodically sheds its old cuticle and secretes a new, larger one. Nematodes have an alimentary canal, though they lack a circulatory system. Nutrients are transported throughout the body via fluid in the pseudocoelom. The muscles of nematodes are all longitudinal, and their contraction produces a thrashing motion. Nematode reproduction is usually sexual and involves internal fertilization. In most species, the sexes are separate and females are larger than males. A female may deposit 100,000 or more fertilized eggs per day. The zygotes of most species are resistant cells tbat can survive harsh conditions. Great numbers of nematodes live in moist soil and in decomposing organic matter on the bottoms of lakes and oceans. While 25,000 species are known, perhaps 20 times that number actually exist. If nothing but nematodes remained, it has been said, they would still preserve the outline of the planet and many of its features. These extremely numerous free-living worms play an important role in decomposition and nutrient cycling, but little is known about most species. One species of soil nematode, Caenorhabditis elegans, is well studied and has become a model research organism in developmental biology (see Chapter 21). Ongoing studies on C. dedans are revealing some of the mechanisms involved in aging in humans, among other findings. Phylum Nematoda includes many important agricultural pests that attack the roots of plants. Other species of nematodes parasitize animals. Humans host at least 50 nematode species, including various pinworms and hookworms. One notorious nematode is Trichinella spiralis., the worm that causes trichinosis (Figure 33.27). Humans acquire this nematode by eating undercooked infected pork or other meat with juvenile worms encysted in the muscle tissue. Within the human intestine, the juveniles develop into sexually mature adults. Females burrow into the intestinal muscles and produce

Encysted juveniles

50 iim

more juveniles, which bore through the body or travel in lym phatic vessels to other organs, including skeletal muscles where they encyst. Parasitic nematodes have an extraordinary molecula toolkit that enables them to redirect some of the cellular func tions of their hosts. Plant-parasitic nematodes inject mol ecules that induce the development of root cells, which ther supply nutrients to the parasites. Trichinella invades individua muscle cells and controls the expression of specific musclt genes, which code for proteins that make the cell elastii enough to house the nematode. Additionally, the cell release signals that attract blood vessels, which then supply the nema tode with nutrients. These extraordinary parasites have beei dubbed "animals that act like viruses."

1. Why would it be risky to order pork chops "rare" in a restaurant? 2. How does the nematode body plan differ from that of annelids? For suggested answers, see Appendix A.

Arthropods are segmented coelomates that have an exoskeleton and jointed appendages Zoologists estimate that the arthropod population of th world, including crustaceans, spiders, and insects, number about a billion billion (1018) individuals. More than 1 millio arthropod species have been described, most of which ar insects. In fact, two out of every three species known ai arthropods, and members of the phylum Arthropoda can b found in nearly all habitats of the biosphere. On the criteria t species diversity, distribution, and sheer numbers, anhropoc must be regarded as the most successful of all animal phyla.

General Characteristics of Arthropods

A Figure 33.27 Juveniles of the parasitic nematode Trichinella spiralis encysted in human muscle tissue (LM).

U N I T FIVE

The Evolutionary History of Biological Diversity

The diversity and success of arthropods are largely related t their segmentation, hard exoskeleton, and jointed appendagt (arthropod means "jointed feet"). Early arthropods, such as th trilobites, had pronounced segmentation, but their ap pendages showed little variation from segment to segmen (Figure 33.28). As arthropods continued to evolve, the seg ments tended to fuse and become fewer in number, and th appendages became specialized for a variety of function:

These evolutionary changes resulted not only in great diversification but also in an efficient body plan that permits the division of labor among different regions. For example, the various appendages of some living arthropods are modified ft r walking, feeding, sensory reception, copulation, and dense. Figure 33.29 illustrates the diverse appendages and her arthropod characteristics of a lobster.

The body of an arthropod is completely covered by the cuticle, an exoskeleton (external skeleton) constructed from layers of protein and the polysaccharide chitin. The cuticle can be thick and hard over some parts of the body and paperthin and flexible over others, such as Lhe joints. The rigid exoskeleton protects the animal and provides points of attachment for the muscles that move the appendages. But it also means that an arthropod cannot grow without occasionally shedding its exoskeleton and producing a larger one. This process, called molting or ecdysis, is energetically expensive. A recently molted arthropod is also vulnerable to predation and other dangers until its new, soft exoskeleton hardens. When the arthropod exoskeleton first evolved in the seas, its main functions were probably protection and anchorage for muscles, but it later additionally enabled certain arthropods to live on land. The exoskeleton;s relative impermeability to water helped prevent desiccation, and its strength solved the problem of support when arthropods left the buoyancy of water. Arthropods began to diversify on land following the colonization of land by plants in the early Paleozoic. In 2004, an amateur fossil hunter in Scotland found a 428million-year-old fossil of a millipede. Fossilized tracks of other terrestrial arthropods date from about 450 million years ago.

Figure 33.28 A trilobite fossil. Trilobites were common snizens of the shallow seas throughout the Paleozoic era but sappeared with the great Permian extinctions about 250 million years jo. Paleontologists have described about 4,000 trilobite species.

Cephalothora:

Antennae (sensory reception)

Pincer (defense)

Arthropods have well-developed sensory organs, including eyes, olfactory (smell) receptors, and antennae that function in both touch and smell. Most sensory organs are concentrated at the anterior end of the animal. Like many molluscs, arthropods have j an open circulatory system in which fluid called hemolymph is propelled by a heart through short arteries and then into spaces called sinuses surrounding the tissues and organs. (The term blood is best reserved for fluid in a closed circulatory system.) Hemolymph reenters the arthropod heart through pores that are usually equipped with valves. The body sinuses are collectively called the hemocoel, which is not part oi the coelom. In most arthropods, the coelorn that forms in the embryo becomes much reduced as development progresses, and the hemocoel becomes the main body cavity in adults. Despite their similarity, the open circulatory systems of molluscs and arthropods probably arose independently.

Mouthparts (feeding)

Figure 33.29 External anatomy of an arthropod. Many of the distinctive features of thropods are apparent in this dorsal view of a lobster, along with some uniquely crustacean aracteristics. The body is segmented, but this characteristic is obvious only in the abdomen. The pendages (including antennae, pincers, mouthparts, walking legs, and swimming appendages), e jointed. The head bears a pair of compound (multilens) eyes, each situated on a movable stalk e whole body, including appendages, is covered by an exoskeleton.

A variety of organs specialized for gas exchange have evolved in arthropods. These organs allow the diffusion of respiratory gases in spite of the exoskeleton. Most aquatic species have gills with thin, feathery extensions that place an extensive surface area in contact with the C H A P T E R 33

Tnvertebraies

657

surrounding water. Terrestrial arthropods generally have internal surfaces specialized for gas exchange. Most insects, lor instance, have tracheal systems, branched air ducts leading into the interior from pores m trie cuticle. Findings from molecular systematics are leading biologists to develop new hypotheses about arthropod evolutionary relationships, just as they are for other branches of the tree of Hie. Evidence now suggests that living arthropods consist of four major lineages that diverged early in the evolution of the phylum (Table 33.5): cheliceriforms (sea spiders, horseshoe crabs, scorpions, ticks, mites, and spiders); myriapods (centipedes and millipedes); hexapods (insects and their wingless, sixlegged relatives); and crustaceans (crabs, lobsters, shrimps, barnacles, and many others).

A Figure 33.30 Horseshoe crabs (Limulus polyphemus).

Cheliceriforms

Common on the Atlantic and Gulf coasts of the United States, these "living fossils" have changed little in hundreds of millions of years. They have survived from a rich diversity of cheliceriforms that once filled the seas.

Cheficeriforms (subphylum Cheliceriformes; from the Greek cheilos, lips, and cheir, arm) are named for clawlike feeding appendages called chelicerae, which serve as pincers or fangs. Cheliceriforms have an anterior cephalothorax and a posterior abdomen. They lack antennae, and most have simple eyes (eyes with a single lens).

Table 33.5 Subphyla of Phylum Arthropoda Subphyium and Examples

Main Characteristics

Cheliceriformes (horseshoe crabs, spiders, scorpions, ticks, mites; see Figures 33.30-33.32)

Body having one or two main parts; six pairs ol appendages (chelicerae, pedipalps, and four pairs of walking legs); mostly terrestrial or marine

Myriapoda (millipedes and centipedes; see Figures 33.33 and 33.34)

Distinct head bearing antennae and chewing mouthparls: terrestrial; millipedes are herbivorous and have iwo pairs of walking legs per trunk segment; centipedes are carnivorous and have one pair of walking legs per trunk segment and poison claws on first body segment Body divided into head, thorax, and abdomen; antennae present; mouthparls modified for chewing, sucking, or lapping; three pairs of iegs and usually two pairs ol wings; mostly terrestrial

Hexapoda (insects, springtails; see Figures 33.35-33.37)

Crustacea (crabs, lobsters, crayfish., shrimp; see Figures 33.29 and 33.38)

658

UNIT FIVE

Body oi two or three parts; antennae present; chewing mouthparts; three or more pairs of legs; mostly marine and freshwater

The Evolutionary History of Biological Diversity

The earliest cheliceriforms were eurypterids, or wate scorpions. These mainly marine and freshwater predator grew up to 3 m long. Most ol the marine cheliceriforms, in eluding all of the eurypterids, are extinct; among the marin species that survive today are the sea spiders (pyenogonids and the horseshoe crabs (Figure 33.30). The bulk of modem cheliceriforms are arachnids, a group that includes scorpions, spiders, ticks, and mites (Figure 33.31)! Ticks and many mites are among a large group of parasiti arthropods. Nearly all ticks are bloodsucking parasites on th body surfaces of reptiles or mammals. Parasitic mites live on o in a wide variety of vertebrates, invertebrates, and plants. Arachnids have a cephalothorax that has six pairs c appendages: the chelicerae, a pair of pedipalps thai usuall function in sensing or feeding, and four pairs of walkin legs (Figure 33.32). Spiders use their fang-like chelicerae which are equipped with poison glands, to attack prey. As th chelicerae masticate (chew) the prey, the spider spills digestiv juices onto the torn tissues. The food softens, and the spide sucks up the liquid meal. In most spiders, gas exchange is carried out by book lungs stacked plates contained in an internal chamber (see Figur 33.32). The extensive surface area of these respiratory organ is a structural adaptation that enhances the exchange of O and CO2 between the hemolymph and air. A unique adaptation of many spiders is the ability to catct insects by constructing webs of silk, a liquid protein producec by specialized abdominal glands. The silk is spun by organ called spinnerets into fibers that solidify. Each spider engi neers a style of web characteristic of its species and build it perfectly on the first try. This complex behavior is appar ently inherited. Various spiders also use silk in other ways: a droplines for rapid escape, as a cover for eggs, and even a. -'gift wrapv for food that males offer females during courtship

(a) Scorpions have pedipalps that are pincers specialized for defense and the capture of food. The tip of the tail bears a poisonous stinger.

(b) Dust mites are ubiquitous scavengers in human dwellings but are harmless except to those people who are allergic to them (colorized SEM).

Figure 33.31 Arachnids.

(c) Web-building spiders are generally most active during the daytime.

Stomach

Figure 33.32 natomy of a spider.

Spinnerets

Chelicera (exit for eggs)

Silk gland

Pedipalp

Sperm receptacle

yriapods illipedes and centipedes belong to the subphylum Myriapoda, e myriapods. All living myriapods are terrestrial. Their head s a pair of antennae and three pairs of appendages modified mouthparts, including the jaw-like mandibles. Millipedes (class Diplopoda) have a large number of legs, ough fewer than the thousand their name implies (Figure .33). Each trunk segment is formed from two fused segments d has two pairs of legs. Millipedes eat decaying leaves and ler plant matter. They may have been among the earliest anlals on land, living on mosses and primitive vascular plants. Unlike millipedes, centipedes (class Chilopoda) are carnires. Each segment of a centipede's trunk region has one pair

A Figure 33.33 A millipede. CHAPTER 33

Invertebrates

659

A Figure 33.34 A centipede.

of legs (Figure 33.34). Centipedes have poison claws on their foremost trunk segment that paralyze prey and aid in defense.

Insects Insects and their relatives (subphylum Hexapoda) are more species-rich than all other forms of life combined. They live in almost every terrestrial habitat and in fresh water, and flying

The insect body has three regions: head, thorax, and abdomen. The segmentation of the thorax and abdomen are obvious, but the segments that form the head are fused. Abdomen

Thorax

Head /Compound eye

insects fill the air. Insects are rare, though not absent, in ih seas, where crustaceans are the dominant arthropods. The ir ternal anatomy of an insect includes several complex or systems, which are highlighted In Figure 33.35. The oldest insect fossils date from the Devonian perio: which began about 416 million years ago. However, whe. flight evolved during the Carboniferous and Permian period it spurred an explosion in insect variety A fossil record c diverse insect mouthparts indicates that specialized feedir on gymnosperms and other Carboniferous plants also cor tributed to the adaptive radiation of insects. A widely hel 1 hypothesis is that the greatest diversification of insects para leled the evolutionary radiation of flowering plants during tt Cretaceous and early Tertiary periods about 65-60 millio years ago. This view is challenged by new research suggestir that insects diversified extensively before the angiosperm ra diation. Thus, during the evolution of flowering plants an the herbivorous insects that pollinated them, insect diversit may have been as much a cause of angiosperm radiation an effect. Flight is obviously one key to the great success of insect: An animal that can fly can escape many predators, find foo and mates, and disperse to new habitats much faster than a animal that must crawl about on the ground. Many insec have one or two pairs of wings that emerge from the dorsal sic

Figure 33.35 Anatomy of a grasshopper, an insect.

Heart. The insect heart drives hemolymph through an open circulatory system.

Cerebral ganglion. The two nerve cords meet in the head, where the ganglia of several anterior segments are fused into a cerebral ganglion (brain). The antennae, eyes, and other sense organs are concentrated on the head.

Malpighian tubules. Metabolic wastes are removed from the hemolymph by excretory organs called Malpighian tubules, which are outpocketings of the digestive tract.

Tracheal tubes. Gas exchange in insects is accomplished by a tracheal system of branched, chitin-lined tubes that infiltrate the body and carry oxygen directly to cells. The tracheal system opens to the outside of the body through spiracles, pores that can control air flow and water loss by opening or closing.

660

UNIT FIVE

The Evolutionary History of Biological Diversity

Nerve cords. The insect nervous system consists of a pair of ventral nerve cords with several segmental ganglia.

Insect mouthparts are formed from several pairs of modified appendages The mouthparts include mandibles, which grasshoppers use for chewing. In other insects, mouthparts are specialized for lapping, piercing, or sucking.

of the thorax. Because the wings are extensions of the cuticle ard not true appendages, insects can fly without sacrificing any walking legs. By contrast, the Eying vertebrates-—birds and b; ts—have one of their two pairs of walking legs modified into w ngs and are generally quite clumsy on the ground. Insect wings may have first evolved as extensions of the ci tide that helped the insect body absorb heat, only later hi coming organs for flight. Other views suggest that wings allowed insects to glide from vegetation to the ground, or even that they served as gills in aquatic insects. Still another hypothesis is that insect wings functioned for swimming before trey functioned for flight. Morphological and molecular data indicate that wings e\ olved only once in insects. Dragonflies, which have two si niiar pairs of wings, were among the first insects to fly. Seveial insect orders that evolved later than dragonflies have modified flight equipment. The wings of bees and wasps, for instance, are hooked together and move as a single pair. Butrfly wings operate in a similar fashion because the anterior lir overlaps the posterior wings. In beetles, the posterior .ngs function in flight, while the anterior ones are modified covers that protect the flight wings when the beetle is on tf e ground or is burrowing. Many insects undergo metamorphosis during their developent. In the incomplete metamorphosis of grasshoppers and me other orders, the young (called nymphs) resemble adults

but are smaller, have different body proportions, and lack wings. The nymph goes through a series of molts, each time looking more like an adult. With the final molt, the insect reaches full size, acquires wings, and becomes sexually mature. Insects with complete metamorphosis have larval stages specialized for eating and growing that are known by such names as maggot, grub, or caterpillar. The larval stage looks entirely different from the adult stage, which is specialized for dispersal and reproduction. Metamorphosis from the larval stage to the adult occurs during a pupal stage (Figure 33.36). Reproduction in insects is usually sexual, with separate male and female individuals. Adults come together and recognize each other as members of the same species by advertising with bright colors (as in butterflies), sound (as in crickets), or odors (as in moths)- Fertilization is generally internal. In most species, sperm are deposited directly into the female's vagina at the Lime of copulation, though in some species the male deposits a sperm packet outside the female, and the female picks it up, An internal structure in the female called the spermatheca stores the sperm, usually enough to fertilize more than one batch of eggs. Many insects mate only once in a lifetime. After mating, a female often lays her eggs on an appropriate food source where the next generation can begin eating as soon as it hatches. Insects are classified in about 26 orders, 15 of which are explored in Figure 33,37, on the next two pages.

(c) Pupa

Figure 33.36 Metamorphosis of a butterfly, (a) The larva (caterpillar) spends its time ating and growing, molting as it grows, (b) After several molts, the larva develops into a pupa. ) Within the pupa, the larval tissues are broken down, and the adult is built by the division and d fferentiation of cells that were quiescent in the larva, (d) Eventually, the adult begins to emerge fi Dm the pupal cuticle, (e) Hemolymph is pumped into veins of the wings and then withdrawn, leaving the hardened veins as struts supporting the wings. The insect will fly off and reproduce, c eriving much of its nourishment from the calories stored by the feeding larva.

(e) Adult CHAPTER 33

Invertebrates

661

Insect Diversity APPROXIMATE NUMBER OF SPECIES Blattodea

4,000

1,200

Hemiptera

Hymenoptera

Isoptera

Hemipterans are so-called "true bugs," including bed bugs, assassin bugs, and chinch bugs. ;. insects in other orders are sometimes erroneously called bugs.) Hemipterans have two pairs of wings, one pair parih leathery, the other membranous. They have piercing or sucking mouthparts and undergo incomplete metamorphosis.

Ants, bees, and wasps are generally highly social insects. They have two pairs of membranous wings, a mobile head, and chewing or sucking mouthparts. The females of many species have a posterior .stinging organ. Flvnienopierans undergo complete metamorphosis. Termites are widespread social insects that produce enormous colonies. It has been estimated that there are 700 kg of termites for every person on Earth! Some termites have two pairs ot membranous wings, while others are wingless. They feed on wood with the aid o! rmcrobial symptoms carried in specialized chambers in their hindgut.

UNIT FIVE

German cockroach

The Evolutionary History of Biological Diversity

jfara

Japanese beetle

Earwigs are generally nocturnal scavengers. While some species are wingless, others have two pairs of wings, one of which is thick and leathery, the other membranous. Earwigs have biting mouthparts and large posterior pincers. They undergo incomplete metamorphosis.

Dipterans have one pair of wings; the second pair has become modified into balancing organs called halteres. Their head is large and mobile; their mouthparts are adapted for sucking, piercing, or lapping. Dipterans undergo complete metamorphosis. Flies and mosquitoes are among die best-known dipterans, which live as scavengers, predators, and parasites.

Diptera

662

Cockroaches have a dorsoventrally flattened body, with legs modified for rapid running. Forew ings, when present, are leather)*, whereas hind \\ ings are fan like K:\ver ihan 40 cockroach species live in houses; the rest exploit, habitats ranging from tropical forest floors LO caves.and deserts. Beetles comprise the most species-rich order of insects. They have two pairs of wings, one of which is thick and leathery, the other membranous. They have an armored exoskdeum and mouthparts adapted for biting and chewing. Beetles undergo complete metamorphosis.

Coleoptera

Dermaptera

MAIN CHARACTERISTICS

Earwig

Horsefly

Leaffooted bug

Cicada-killer wasp

APPROXIMATE NUMBER OF SPECIES Lepidoptera

Odonata

120,000

5,000

MAIN CHARACTERISTICS

Butterflies and nioiii? are among nie best-known insect's. They have two pairs of wings covered with tiny scales. To feed, they uncoil a long proboscis. Most feed on nectar, but some species feed on other substances, including animal blood or tears.

Dragonflies and damselfiies have two pairs of large, membranous wings. They have an elongated abdomen, large, compound eyes, and chewing mouthparts. They undergo incomplete metamorphosis and are active predators. Dragonfly

Or diopter a

13,000

Phasmida

are mostly herbifor jumping, two ous), and biting or ke courtship sounds idge on their hind .amorphosis.

Katydid

Stick insects and leaf insects are exquisite mimics of plants. The eggs of some species even mimic seeds of the plants on which the insects live. Their body is cylindrical or flattened dorsoventrally. They lack forewings but have fanlike hind wings. Their mouthparts are adapted !• •; bknig or chewing.

Phthiraptera

Siphonaptera

Grasshoppers, crickets, and their rel vorous. They have large hind legs ac pairs of wings (one leathery, one me chewing mouthparts. Males commo: by rubbing together body parts, sue" leg. Orthopterans undergo incompk

•tick insect

Commonly called sucking lice, these insects spend their entire life as an ectoparasite Seeding on the hair or feathers of a single host. Their legs, equipped with clawlike tarsi, are adapted for clinging to their hosts. They lack wings and have reduced eyes. Sucking lice undergo incomplete metamorphosis.

2,400

Thysanura

V^

^

Human body louse

Fleas are bloodsucking ectoparasites on birds and mammals. Their body is wingless and laterally compressed. Their legs are modified for clinging to their hosts and for long-distance jumping. They undergo complete metamorphosis.

Silverfish are small, wingless insects with a flattened body and reduced eyes. They live in leal litter or under bark. They can also infest buildings, where they can become pests. Silverfish

Trichoptera

7,100

The larvae of caddisflies live in streams, where they make houses from sand grains, wood fragments, or other material held together by silk. Adults have two pairs of hairy wings and chewing or lapping mouthparts. They undergo complete metamorphosis.

CHAPTER

Caddisfly

33

Invertebrates

663

Animals as numerous, diverse, and widespread as insects are bound to affect the lives of all other terrestrial organisms, including humans. On the one hand, we depend on bees,

rine. Crayfish, however, live in fresh water, and some tropic crabs live on landMany small crustaceans are important members of mail

flies, and many other insects to pollinate our crops and

and freshwater plankton communities. Planktonic en

orchards. On the other hand, insects are carriers for many diseases, including African sleeping sickness (spread by tsetse flies that carry Trypanosoma; see Figure 28.7) and malaria (spread by mosquitoes that carry Plasmodium; see Figure 28.11). Furthermore, insects compete with humans for food. In parts of Africa, for instance, insects claim about 75% of the crops. Trying to minimize their losses, farmers in the United States spend billions of dollars each year on pesticides, spraying crops with massive doses of some of the deadliest poisons ever invented. Try as they may, not even humans have challenged the preeminence of insects and their arthropod kin. As Cornell University entomologist Thomas Eisner puts it: "Bugs are not going to inherit the Earth. They own it now. So we might as well make peace with the landlord.'1

taceans include many species of copepods, which are amoi the most numerous of all animals, and the shrimplike kn which grow to about 3 cm long (Figure 33.38b). A major food source for baleen whales (including blue whales and rig it whales), krill are now7 being harvested in great numbers by humans for food and agricultural fertilizer. The larvae of ma iy larger-bodied crustaceans are also planktonic. Barnacles are a group of mostly sessile crustaceans wha >e cuticle is hardened into a shell containing calcium carbonate (Figure 33.38c). Most barnacles anchor themselves to rocljs, boat hulls, pilings, and other submerged surfaces. The adr t-

Crustaceans While arachnids and insects thrive on land, crustaceans, for the most part, have remained in marine and freshwater environments. Crustaceans (subphylum Crustacea) typically have biramous (branched) appendages that are extensively specialized. Lobsters and crayfish, ior instance, have a toolkit of 19 pairs of appendages (see Figure 33.29). The anterior-most appendages are antennae; crustaceans are the only arthropods with two pairs. Three or more pairs of appendages are modified as mouthparts, including the hard mandibles. Walking legs are present on the thorax, and, unlike insects, crustaceans have appendages on the abdomen. A lost appendage can be regenerated. Small crustaceans exchange gases across thin areas of the cuticle; larger species have gills. Nitrogenous wastes also diffuse through thin areas of the cuticle, but a pair of glands regulates the salt balance of the hemolymph. Sexes are separate in most crustaceans. In the case of lobsters and crayfish, the male uses a specialized pair of abdominal appendages to transfer sperm to the reproductive pore of the female during copulation. Most aquatic crustaceans go through one or more swimming larval stages. One of the largest groups of crustaceans (numbering about 10,000 species) is the isopods, which include terrestrial, freshwater, and marine species. Some isopod species are abundant in habitats at the bottom of the deep ocean. Among the terrestrial isopods are the pill bugs, or wood lice, common on the undersides of moist logs and leaves. Lobsters, crayfish, crabs, and shrimp are all relatively large crustaceans called decapods (Figure 33.38a). The cuticle of decapods is hardened by calcium carbonate; the portion that covers the dorsal side of the cep halo thorax forms a shield called the carapace. Most decapod species are ma-

664

U N I T FIVE

The Evolutionary History of Biological Diversity

(b) Planktonic crustaceans known as krill are consumed in vast quantities by whales.

A Figure 33.38 Crustaceans.

(c) The jointed appendages projecting from the shells of these barnacles capture organisms and organic particles suspended in the water.

f e they use is as strong as any synthetic glue. To feed, they tend appendages from their shell to strain food from the XEcr. Other barnacles live as parasites mside hosts such as abs, where their bodies resemble the roots of a plant. Barcles were not recognized as crustaceans until the 1800s, When naturalists discovered that barnacle larvae resemble those of other crustaceans. The remarkable mix of unique tiaits and crustacean homologies found in barnacles was a major inspiration to Charles Darwin as he developed his theory of evolution.

Concept Check • 1. In contrast to our jaws, which move up and down, the mouthparts of arthropods move side to side. Explain this feature of arthropods m terms of the origin of their mouthparts. 2. Would it be reasonable to call phylum Arthropoda the most successful animal phylum? Explain your answer. 3. Describe two adaptations that enabled insects to thrive on land. For suggested answers, see Appendix A.

Figure 33.39 Anatomy of a sea star, i echinoderm.

Central disk. The central disk has a nerve ring and nerve cords radiating from the ring into the arms.

Concept

Echinoderms and chordates are deuterostomes Sea stars and other echinoderms (phylum Echinodermata) may seem to have little in common with phylum Chordala, which includes the vertebrates—animals that have a backbone. In fact, all these animals share features characteristic of deuterostomes: radial cleavage, development of the coelom from the archenteron, and formation of the mouth at the end of the embryo opposite the blastopore (see Figure 32.9). Molecular systematics has reinforced Deuterostomia as a clade of bilaterian animals.

Echinoderms Sea stars and most other echinoderms (from the Greek echin, spiny, and derma, skin) are slow-moving or sessile marine animals. A thin skin covers an endoskeleton oi hard calcareous plates. Most echinoderms are prickly from skeletal bumps and spines. Unique to echinoderms is the water vascular system, a network of hydraulic canals branching into extensions called tube feet that function in locomotion, feeding, and gas exchange (Figure 33.39). Sexual reproduction of echinoderms

A short digestive tract runs from the mouth on the bottom of the central disk to the anus on top of the disk. The surface of a sea star is covered by spines that help defend against predators, as well as by small gills that provide gas exchange.

Madreporite. Water can flow in or out of the water vascuiar system into the surrounding water through the madreporite.

Digestive glands secrete digestive juices and aid in the absorption and storage of nutrients.

Radial canal. The water vascular system consists of a ring canal in the central disk and five radial canals, each running in a groove down the entire length of an arm.

Branching from each radial canal are hundreds of hollow, muscular tube feet filled with fluid. Each tube foot consists of a bulb-like ampulla and suckered podium (foot portion). When the ampulla squeezes, it forces water into the podium and makes it expand. The podium then contacts the substrate. When the muscles in the wall of the podium contract, they force water back into the ampulla, making the podium shorten and bend.

C H A P T E R 33

Invertebrai.es

665

usually involves separate male and female individuals that release their gametes into the water. The internal and external parts of most echinoderms radiate from the center, often as five spokes. HoWeVer, the radial anatomy oi adult echinoderms is a secondary adaptation, as eehinoderm larvae have bilateral symmetry. Furthermore, the symmetry of adult echinoderms is not perfectly radial. For example, the opening (madreporite) of a sea stars water vascular system is not central but shifted to one side. Living echinoderms are divided into six classes (Table 33.6; Figure 33.40): Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lilies and feather stars), Holothuroidea (sea cucumbers), and Concentricycloidea (sea daisies).

star secretes juices that begin digesting the soft body of th mollusc within its own shell. Sea stars and some other echinoderms have considerabl

Sea Stars

Sea urchins and sand dollars have no arms, but they do hav five rows of tube feet that function in slow movement. Se urchins also have muscles that pivot their long spines, whic aids in locomotion. The mouth of a sea urchin is ringed b complex, jaw-like structures adapted lor eating seaweeds an other food. Sea urchins are roughly spherical, whereas san dollars are flattened and disk-shaped.

Sea stars have multiple arms radiating from a central disk. The undersurfaces of the arms bear tube feet, each of which can act like a suction disk. By a complex set of hydraulic and muscular actions, the suction can be created or released (see Figure 33.39). The sea star adheres firmly to rocks or creeps along slowly as the tube feet extend, grip, contract, release, extend, and grip again. Sea stars also use their tube feet to grasp prey, such as clams and oysters. The arms of the sea star embrace the closed bivalve, hanging on tightly by the tube feet. The sea star then turns its stomach inside out, everting it through its mouth and into the narrow opening between the halves of the bivalves shell. The digestive system of the sea

Table 33.6 Classes of Phylum Echinodermata Class and Examples

Main Characteristics

Asteroidea (sea stars; see Figures 33.39 and 33.40a)

Star-shaped bod) wish multiple arms; moulh directed to substrate

Ophiuroidea (brittle stars; see Figure 33.40b)

Distinct central disk; long, flexible arms; tube feet lack suckers

Echinoidea (sea urchins, sand dollars; see Figure 33.40c)

Roughly spherical or diskshaped; no arms; five rows of tube feet enable slow movement; mouth ringed by complex, jawlike structure

Crinoidea (sea lilies, feather stars; see Figure 33.40d)

Feathered arms surrounding upward-pointing mouth

Holothuroidea (sea cucumbers; see Figure 33.40e)

Cucumber-shaped body; five rows of tube feet; additional tube leei modified as feeding tentacles; reduced skeleton; no spines

Concentricycloidea (sea daisies; see Figure 33.40f)

Disk-shaped body ringed with small spines; incomplete digestive system; live on submerged wood

666

U N I T FIVE

T h e E v o l u t i o n a r y H i s r o n o i Biological !)i\; j r-iiy

powers of regeneration. Sea stars can regrow lost arms, an members of one genus can even regrow an entire body from single arm. Brittle Stars Brittle stars have a distinct central disk and long, flexible arm; They move by serpentine lashing of their arms, as their tube fee lack suckers and thus cannot be used for gripping. Some speci are suspension teeders; others are predators or scavengers. Sea Urchins and Sand Dollars

Sea Lilies and Feather Stars Sea lilies live attached to the substrate by a stalk; feather stai crawl about by using their long, flexible arms. Both use the arms in suspension feeding. The arms encircle the moutl which is directed upward, away from the substrate. Crinoide is an ancient class whose evolution has been very conserva tive; fossilized sea lilies some 500 million years old are e; tremely similar to present-day members of the class. Sea Cucumbers On casual inspection, sea cucumbers do not look much lik other echinoderms. They lack spines, and their endoskeleto is much reduced. They are also elongated in their oral-abora axis, giving them the shape for which they are named and fur ther disguising their relationship to sea stars and sea urchins. Closer examination, however, reveals that sea cucumbers hav five rows of tube leet. Some of the tube feet around themout are developed as feeding tentacles. Sea Daisies Sea daisies were discovered in 1986, and only two species ar known. Both live on submerged wood off the coast of Ne\ Zealand and the Bahamas. Their armless body is typically disk-shaped, has a five-fold symmetry, and measures less thai a centimeter in diameter. The edge of the body is ringed witlj small spines. Sea daisies absorb nutrients through the mem brane surrounding their body. The relationship of sea daisie to other echinoderms remains unclear; some taxonomist: consider sea daisies to be highly derived sea stars.

(a) A sea star (class Asteroidea)

(fa) A brittle star (class Ophiuroidea)

(d) A feather star (class Crinoidea)

i) A sea cucumber (class Hoiothuroidea)

(f) A sea daisy (class Concentricycloidea)

Figure 33.40 Echinoderms.

Zhordates aylum Chordata consists of two subphyla of invertebrates as veil as the hagfishes and the vertebrates. The close relationhip between echinoderms and chordates does not mean that >ne phylum evolved from the other. Echinoderms and chorates have existed as distinct phyla for at least half a billion ears. We will trace the phylogeny of chordates in Chapter 34, ocusing on the history of vertebrates.

Concept Check 1. Explain how the symmetry of echinoderms and cnidarians exemplifies convergent evolution. 2. Describe the hydraulic and muscular actions by which a sea star moves its tube feet. For suggested answers, see Appendix A.

CHAPTER 33

Invertebrates

667

Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY Table 33.7 summarizes the groups of animals surveyed in this chapter.

Sponges are sessile and have a porous body and choanocytes • Sponges lack true tissues and organs. They suspension feed by drawing water through pores; choanocytes (flagellated collar cells) ingest suspended food (pp. 642—643).

Table 33.7 Selected Animal Phyla

668

UNIT FIVE

Phylum

Description

Porifera (sponges)

Lack true tissues; have choanocytes (collar cells—• unique flagellated cells that ingest bacteria and tiny food panicles)

Cnidaria (hydras, jellies, sea anemones, corals)

Unique stinging structures (cnidae), each housed in a specialized cell (cnidocyte); gastrovascular cavity (digestive compartment with a single opening)

Platyhelminthes (fistworms)

Dorsoventrally flattened, unsegmented acoelomates; gastrovascular cavity or no digestive tract

Rotifera (rotifers)

Pseudocoelomates with alimentary canal (digestive tube with mouth and anus); jaws (trophi) in pharynx; head with ciliated crown

Lophophorates: Ectoprocta, Phoronida, Brachiopoda

Coelomates with lophophore? (feeding structures bearing ciliated tentacles)

Nemertea (proboscis worms)

Unique anterior proboscis surrounded by fluid-filled sac; alimentary canal; closed circulatory system

Mollusca (clams, snails, squids)

Coelomaies with three main body parts (muscular foot, visceral mass, mantle); coelom reduced; most have hard shell made of calcium carbonate

Annelida (segmented worms)

Coelomates with body wall and internal organs (except digestive tract) segmented

Nematoda (roundworms)

Cylindrical, unsegmented pseudocoelomates with tapered ends; no circulatory system

Arthropoda (crustaceans, insects, spiders)

Coelomates with segmented body, jointed appendages, and exoskeleton made of protein and chitin

Echinodermata (se; stars, sea urchins)

Coelomates with secondary radial anatomy (larvae bilateral; adults radial); unique water vascular system; endoskeleton

Chordata (lancelets, tunicates, vertebrates)

Coelomates wilh notochord; dorsal, hollow nerve cord; pharyngeal slits; muscular, post-anal tail

The Evolutionary History of Biological Diversity

Cnidarians have radial symmetry, a gastrovascular tvity, and cnidocytes Cnidarians are mainly marine carnivores possessing tentacles armed with cnidocytes that aid in. defense and the capture of prey. Two body forms are sessile polyps and floating medusae (pp. 643-644). • Hydrozoans (pp. 644-645) Class Hydrozoa usually alternates polyp and medusa forms, although the polyp is more conspicuous. Scyphozoans (p. 644) In class Scyphozoa, jellies (medusae) are the prevalent form of the life cycle. Cubozoans (p. 645) In class Cubozoa (box jellies and sea wasps), the medusa is box-shaped and has complex eyes. Anthozoans (pp. 645-646) Class Anthozoa contains the sea anemones and corals, which occur only as polyps.

Most animals have bilateral symmetry • • Flatworms (pp. 646-648) Flatworms are dorsoventrally flattened animals with a gastrovascular cavity. Class Turbellaria ts made up of mostly free-living, primarily marine species. Members of the classes Trematoda and Monogenea live as parasites in or on animals. Class Cestoda consists of tapeworms, all of which are parasites and lack a digestive tract. Rotifers (pp. 648-649) Found mainly in fresh water, many rotifer species are parthenogenetic. Lophophorates: Ectoprocts, Phoronids, and Brachiopods (p. 649) Lophophorates are coelomates that have a lophophore, a horseshoe-shaped, suspension-feeding organ bearing ciliated tentacles. Nemerteans (pp. 649-650) Nemerteans have a unique retractable tube (proboscis) used for defense and prey capture. A fluid-filled sac is used to extend die proboscis.

lolluscs have a muscular foot, a visceral mass, and a antle • Chitons (p. 651) Class Polyplacophora is composed of the chitons, oval-shaped marine animals encased in an armor of eight dorsal plates. Gastropods (pp. 651-652) Most members of class Gastropoda, the snails and their relatives, possess a single, spiraled shell. Embryonic torsion of the body is a distinctive characteristic. Many slugs lack a shell or have a reduced shell. • Bivalves (p. 652) Class Bivalvia (clams and their relatives) have a hinged shell divided into two halves. • Cephalopods (pp. 652-653) Class Cephalopoda includes squids and octopuses, carnivores with beak-like jaws surrounded by tentacles of their modified foot.

• Leeches (p. 655) Many members of class Hirudinea are bloodsucking parasites.

Nematodes are nonsegmented pseudocoelomates covered by a tough cuticle • Among the most widespread and numerous animals, nematodes inhabit the soil and most aquatic habitats. Some species are important parasites of animals and plants (pp. 655-656).

Arthropods are segmented coelomates that have an exoskeleton and jointed appendages • General Characteristics of Arthropods (pp. 656-658) Variation in arthropod morphology consists mainly in specializations of groups of segments and in appendages. The arthropod exoskeleton, made of protein and chitin, undergoes regular ecdysis (molting). • Cheliceriforms (pp. 658-659) Cheliceriforms include spiders, ticks, and mites. They have an anterior cephalothorax and a posterior abdomen. The most anterior appendages are modified as chelicerae (either pincers or fangs). • Myriapods (pp. 659-660) Millipedes are wormlike, with a large number of walking legs. They were among the first animals to live on land. Centipedes are terrestrial carnivores with poison claws. • Insects (pp. 660-664) Insects exceed all other animals combined in species diversity. Flight has been an important factor in the success of insects. Investigation How Are Insect Species Identified? • Crustaceans (pp. 664-665) Crustaceans, which include lobsters, crabs, shrimp, and barnacles, are primarily aquatic. They have numerous appendages, many of which are specialized for feeding and locomotion.

Echinoderms and chordates are deuterostomes • Echinoderms (pp. 665-667) Echinoderms (sea stars and their relatives) have a water vascular system ending in tube feet used for locomotion and feeding. The radial anatomy of many species evolved secondarily from the bilateral symmetry of ancestors. A thin, bumpy, or spiny skin covers a calcareous ertdo5keleton. Activity Characteristics of Invertebrates • Chordates (p. 667) Chordates include two invertebrate subphyla and all vertebrates. Chordates share many features of embryonic development with echinoderms.

TESTING YOUR KNOWLEDGE Evolution Connection

\nnelids are segmented worms Oligochaetes (pp. 653-654) Class Oligochaeta includes earthworms and various aquatic species. Polychaetes (p. 655) Members of class Polychaeta posses; paddle-like parapodia that function as gills and aid in locomotion.

Horseshoe crabs are called "living fossils" because the fossil record shows that they have remained essentially unchanged in morphology for many millions of years. Why might these organisms have retained the same morphology for such a long time? What other aspects of their biology, less obvious than structure, do you think may have evolved over that time?

CHAPTER 33

Invertebrates

669

Scientific Inquiry A marine biologist has dredged up an unknown animal from the

seafloor, Describe some of the characteristics she should look at to determine the phylum to which the animal should be assigned. Investigation How Are Insect Species Identified?

Science, Technology, and Society Construction of a dam and irrigation canals in an African country has enabled farmers to increase the amount of food they can grow. In the past, crops were planted only after spring floods; the fields were too dry the rest of the year. Now fields can be watered year-round. Improvement in crop yield has had an unexpected cost—a tremendous increase in the incidence of schistosomiasis. Look at the blood fluke life cycle in Figure 33.11 and imagine that your Peace Corps assignment is to help local health officials control the disease. Why do you think the irrigation project increased the incidence of schistosomiasis? It is difficult and expensive to control the disease with drugs. Suggest three other methods that could be tried to prevent people from becoming infected.

670

UNIT FIVE

The Evolutionary History of Biological Diversity

Vertebrates A Figure 34.1 The vertebrae and skull of a snake, a terrestrial vertebrate.

Key Concepts 4.1 Chordates have a notochord and a dorsal, hollow nerve cord 4.2 Craniates are chordates that have a head 4.3 Vertebrates are craniates that have a backbone 4.4 Gnathostomes are vertebrates that have jaws 4.5 Tetrapods are gnathostomes that have limbs and feet 4.6 Amniotes are tetrapods that have a terrestrially adapted egg 4.7 Mammals are amniotes that have hair and produce milk 4.8 Humans are bipedal hominoids with a large brain

ialf a Billion Years of Backbones

B

y the dawn of the Cambrian period, some 540 million years ago, an astonishing variety of animals inhabited Earth's oceans. Predators used claws and mandibles to kewer their prey. Many animals had protective spikes and arnor as well as complex mouthparts that enabled their bearers filter food from the water. Worms slithered into the muck feed on organic matter. Amidst all this bustle, it would have een easy to overlook certain slender, 2-cm-long creatures iding through the water. Although they lacked armor, eyes, nd appendages, they would leave behind a remarkable legacy, hese animals gave rise to one of the most successful groups of nimals ever to swim, walk, or fly: the vertebrates, which erive their name from vertebrae, the series of bones that make p the vertebral column, or backbone (Figure 34.1). For nearly

200 million years, vertebrates were restricted lo the oceans, but about 360 million years ago the evolution of legs and feet in one lineage of vertebrates accompanied these vertebrates' move to land. There they diversified into amphibians, reptiles (including birds), and mammals. There are approximately 52,000 species of vertebrates, a relatively small number compared to the 1 million insect species on Earth. But what vertebrates lack in species diversity they have made up for with other statistics. Plani-eating dinosaurs as massive as 40,000 kg were the heaviest animals ever to walk on land. The biggest animal ever to exist on Earth is the blue whale (a mammal), which can exceed a mass of 100,000 kg. Vertebrates are capable of global journeys: For example, birds called Arctic terns, which breed mainly around the shores of the Arctic Ocean, travel to Antarctica to spend the rest of the year before making a return trip. Vertebrates also include the only species capable of full-blown language, complex toolmaking, and symbolic art—humans. In this chapter, you will learn about current hypotheses regarding the origins of vertebrates from invertebrate ancestors. We will track the stepwise evolution of the vertebrate body plan, from a notochord to a bead to a mineralized skeleton, and explore the major groups of vertebrates (both living and extinct) as well as the evolutionary history of our own species.

Concept

Chordates have a notochord and a dorsal, hollow nerve cord Vertebrates are a subphylum of the phylum Chordata (the chordates). Chordates are bilaterian (bilaterally symmetrical) animals, and within Bilateria, they belong to the clade of 671

Osteichthvans

, .

'

.mniotic egg

Legs

Lobed fins

Lungs or lung derivatives

jaws, mineralized skeleton

Vertebral column

Ancestral deuterostome

672

UNIT FIVE

.4 Figure 34,2 Hypothetical phylogeny of chordates. This diagram shows the major ciades of chordates in relation to the-other main deuterostome clade, Echinodennata (see Chapter 33). Some of the derived characters of certain ciades are listed; for example, all chordates, and only chordates, have a notochord, •

The Evoluiionary History of Biological Diversity

imals known as Deuterostomia (see Chapter 32). The bestlown deuterostomes, aside from vertebrates, are the echinorms, members of the phylum that includes sea stars and sea chins. However, as shown in Figure 34.2, on the facing ge, two groups of invertebrate deuterostom.es, the urochortes and the cephalochordates, are more closely related to rtebrates than to other invertebrates. Along with the haghes and the vertebrates, they make up the chordate phylum.

derived Characters of Chordates L chordates share a set of derived characters, though many s secies possess some of these traits only during embryonic c evelopment. Figure 34.3 illustrates the four key characters of chordates: a notochord; a dorsal, hollow nerve cord; pharyn£ eal slits or clefts; and a muscular, post-anal tail. otochord Chordates are named for a skeletal structure, the notochord, resent in all chordate embryos as well as in some adult chorates. The notochord is a longitudinal, flexible rod located letween the digestive tube and the nerve cord. It is composed f large, fluid-filled cells encased in fairly stiff, fibrous tissue, he notochord provides skeletal support throughout most of ie length of a chordate, and in larvae or adults that retain it, also provides a firm but flexible structure against which •mscles can work during swimming. In most vertebrates, a Lore complex, jointed skeleton develops, and the adult reains only remnants of the embryonic notochord. In humans, ie notochord is reduced to gelatinous disks sandwiched between the vertebrae.

chordates- Other animal phyla have solid nerve cords, and in most cases they are ventrally located. The nerve cord of a chordate embryo develops into the central nervous system: the brain and spinal cord. Pharyngeal Slits or Clefts The digestive tube of chordates extends from the mouth to the anus. The region just posterior to the mouth is the pharynx. In all chordate embryos, a series of pouches separated by grooves forms along the sides of the pharynx, in most chordates, these grooves (known as pharyngeal clefts) develop into slits that open to the outside of the body. These pharyngeal slits allow water entering the mouth to exit the body without passing through the entire digestive tract. Pharyngeal slits function as suspension-feeding devices in many invertebrate chordates. In vertebrates (with the exception of terrestrial vertebrates, the tetrapods), these slits and the structures that support them have been modified for gas exchange and are known as gill slits. In tetrapods, the pharyngeal clefts do not develop into slits. Instead, they play an important role in the development of parts of the ear and other structures in the head and neck. Muscular, Post-Anal Tail Chordates have a tail extending posterior to the anus, although in many species it is lost during embryonic development. In contrast, most nonchordates have a digestive tract that extends nearly the whole length of the body. The chordate tail contains skeletal elements and muscles, and it provides much of the propelling force in many aquatic species.

Tunicates

1

I

Dip

phi

=

LJ

ChOf

ata

g 5 £ |

My

he nerve cord of a chordate embryo develops from a plate of ctoderm that rolls into a tube located dorsal to the notolord. The resulting dorsal, hollow nerve cord is unique to

% tjlji %


#

Lepidosau

Synapsids

Reptiles

A Figure 34.23 A phylogeny of amniotes. Extant groups are named at the top in boldface type. The dashed lines in the turtle branch indicate the uncertainty around the relationship of turtles to other reptiles.

687

Extraembryonic membranes

Derived Characters of Amniotes

Allantois. The allantois is a disposal

Amniotes are named for the major derived sac ffor certain metabolic l wastes prod character of the clade, the amniotic egg.

" « d by the embryo. The membrane

of the allantois also functions with

which contains specialized membranes that protect the embryo {Figure 34.24). Called extraembryonic membranes be1

the chorion as a respiratory organ, ^~ . . „ \ Amnion. The amnion protects \ cavity that cushions against the embryo in a fluid-filled mechanical shock.

Chorion. The chorion and the membrane of

the allantois exchange gases between the embryo and the air. Oxygen and carbon dioxide diffuse freely across the shell. Yolk sac. The yolk sac contains the yolk, a stockpile of nutrients. Eloof vessels in the yolk sac membrane transport nutrients from the yolk nto the embryo. Other nutrients a stored in the albumen ("egg white

cause they are not part of the body oi the embryo itself, these membranes develop embryo. They function in gas from tissue layers that grow outexchange, from the waste storage, and the transfer of stored nutrients to the embryo. The amniotic egg is named for one of these membranes, the amnion, which encloses a compartment of •Yolk (nutrients) fluid that bathes the embryo and acts as a hydraulic shock absorber. In contrast to the shell-less eggs of amphibians, the amniotic eggs of most reptiles and some mammals have a shell. The shells of bird eggs are calcareous (made of calcium carbonate) and inflexiA F j g u r e MM T h f i a m n i o t k e g g T h e e m b r y o s of rept|les a n d m a m m a j s f o r m four ble, while the shells of many nonbird extraembryonic membranes: the amnion, allantois, chorion, and yolk sac. This drawing shows reptile eggs are leathery and flexible. The these extraembryonic membranes in the shelled egg of a reptile. shell significantly slows dehydration of the egg in air, an adaptation that allowed amniotes to occupy a wider range of terrestrial habitats than Reptiles amphibians, their closest living relatives. (Seeds played a simThe reptile clade includ ilar role in the evolution of land plants, as you learned in the tuatara, lizards, snake Chapter 29.) Most mammals have dispensed with the shell, turtles, crocodilians, an and the embryo avoids desiccation by developing within the birds, along with a numb mother. of extincL groups such Amniotes also acquired other adaptations to terrestrial life, the large nonflying dim including less permeable skin and the ability to use the rib saurs. Because all of tr cage to ventilate the lungs. Whereas earlier tetrapods and livliving reptile lineages a ing amphibians generally sprawl their legs out, living amniotes highly derived, none ca display, to varying degrees, a more elevated stance. serve as a straightforwar i model for the earliest re] Early Amniotes tiles that lived some 32) million years ago. Neve The most recent common ancestor of living amphibians and theless, comparative stu> amniotes lived about 340 million years ago, in the Carboniferies allow us to infer som ous period. No fossils of amniotic eggs have been found from of the derived characte that time, which is not surprising given how delicate they are. that likely distinguished early reptiles from other tetrapods. Thus, it is not yet possible to say when the amniotic egg Unlike amphibians, reptiles have scales that contain the prc evolved, although it must have existed in the last common antein keratin. Scales create a waterproof barrier that helps prever cestor of living amniotes, which all have amniotic eggs. dehydration in dry air. (In crocodiles, which have adapted t) What is evident from fossils of early amniotes and their closwater, more permeable scales, called scutes, have evolved.) est relatives is that they lived in drier environments than did Scales prevent reptiles from breathing through their skin lik earlier tetrapods. Some of them were herbivores, as evidenced amphibians; most reptiles rely on their lungs alone for gas a by their grinding teeth and other features. Herbivorous amchange. Turtles are the exception to this rule; many turtles al; niotes began consuming large amounts of plant matter. They, use the moist surfaces of their cloaca for gas exchange. "in turn, became prey for large predatory amniotes. 688

U N I T FIVE

The Evolutionary History of Biological Diversity

As parareptiles were dwindling, an equally ancient clade of reptiles, the diapsids, was diversifying. One of the most obvious derived characters of diapsids is a pair of holes on each side of the skull, behind the eye socket. The diapsids are composed of two main lineages. One lineage gave rise to the lepidosaurs, which include tuataras, lizards, and snakes. This lineage also produced a number of marine reptiles, including plesiosaurs and ichthyosaurs (see Figure 34.23), some of which rivaled today's whales in length; all of them are extinct. (We'll say more about living lepidosaurs shortly.)

A Figure 34.25 A hatching reptile. This Komodo dragon is brjeaking out of a parchment-like shell, a common type of shell among ing reptiles other than birds.

Most reptiles lay shelled eggs on land (Figure 34.25). Fertil; ation must occur internally, before the shell is secreted. any species of snakes and lizards are viviparous; their extraLbryonic membranes form a placenta that enables the nbryo to obtain nutrients from its mother. Many reptiles are sometimes said to be "cold-blooded" beuse they do not use their metabolism extensively to control their body temperature. However, they do regulate their body imperature by using behavioral adaptations. For example, .any lizards bask in the sun when the air is cool and seek lade when the air is too warm. A more accurate description of lese reptiles is to say that they are ectothermic, which refers the absorption of external heat as the main source of body eat. (This topic is discussed in more detail in Chapter 40.) By eating directly with solar energy rather than through the letabolic breakdown of food, an ectothermic reptile can sur.ve on less than 10% of the food energy required by a mammal f the same size. The reptile clade is not entirely ectothermic; irds are endothermic, capable of keeping the body warm .rough metabolism. he Origin and Evolutionary adiation of Reptiles be oldest reptilian fossils, found in rocks from Kansas, date om the end of the Carboniferous period, about 300 million ears ago. The first major group of reptiles to emerge were the arareptiles, which were mostly large, stocky, quadrupedal erbivores. Some parareptiles had dermal plates on their skin lat may have provided them with defense against predators nainly synapsids, the ancestors of mammals, which you will arn about later in this chapter). Parareptiles died out by >out 200 million years ago, at the end of the Triassic period, ome systematists have proposed that parareptiles gave rise to irtles, pointing to the possible homology of parareptile der.al plates and turtle shells. But molecular studies point to iferent reptilian origins for turtles. The subject remains conoversial among experts.

The other diapsid lineage, the archosaurs, produced the crocodilians (which we'll discuss later) and many extinct groups, including the pterosaurs and some dinosaurs. Pterosaurs, which originated in the late Triassic, were the first tetrapods to take to the air. The pterosaur wing was completely different from the wings of birds and bats. It consisted of a bristlecovered membrane stretched between the trunk or hind leg and a very long digit on the foreleg. Well-preserved fossils show the presence of muscles, blood vessels, and nerves in the wing membranes, suggesting that pterosaurs could dynamically adjust their membranes to assist their night. The smallest pterosaurs were no bigger than a gull, and the largest had a wingspan of nearly l l m . They appear to have converged on many of the ecological roles later played by birds; some were insect-eaters, others grabbed fish out of the ocean, and still others filtered small animals through a long beak. By the end of the Cretaceous period 65 million years ago, pterosaurs had become extinct. On land, the dinosaurs diversified into a vast range of shapes and sizes, from bipeds the size of a pigeon to 45-m-long quadrupeds with a neck long enough to let them browse the tops of trees. One branch of dinosaurs, the ornithischians, were herbivores; they included many species with elaborate defenses against predators, such as tail clubs and horned crests. The other main branch of dinosaurs, the saurischians, included the long-necked giants and a group called the theropods, which were bipedal carnivores. Theropods included the famous Tyrannosaurus rex as well as the ancestors of birds. There is continuing debate about the metabolism of dinosaurs. Some researchers have pointed out that the Mesozoic climate over much of the dinosaurs' range was relatively warm and consistent, and they have suggested that the low surface-to-volume ratios of large dinosaurs combined with behavioral adaptations such as basking may have been sufficient for an ectotherm to maintain a suitable body temperature. However, some anatomical evidence supports the hypothesis that at least some dinosaurs were endotherms. Furthermore, paleontologists have found fossils of dinosaurs in both Antarctica and the Arctic; although the climate in these areas was milder when dinosaurs existed than it is today, it was cool enough that small dinosaurs may have had difficulty maintaining a high body temperature through ectothermy. The dinosaur that gave rise to birds was certainly endothermic, as are all birds.

CHAPTER 34

Vertebrates

689

A Figure 34.26 Dinosaur parental care. A fossil of a nesting dinosaur and her eggs found in Mongolia sheds light on how dinosaurs cared for their eggs and hatchlings. Known as Oviraptor, this dinosaur, which lived 80 million years ago, covered her eggs with her limbs, which may have been feathered.

Traditionally, dinosaurs were considered slow, sluggish creatures. Since the early 1970s, however, fossil discoveries and research have led to the conclusion that many dinosaurs were agile, fast moving, and, in some cases, social. Paleontologists have also discovered signs of parental care among dinosaurs (Figure 34.26). All dinosaurs (except birds) became extinct by the end of the Cretaceous period. However, the fossil record indicates that the number of dinosaur species had begun to decline several million years before the Cretaceous ended. It is unclear whether the extinction of dinosaurs was hastened by the asteroid or comet impact you read about in Chapter 26, as many other terrestrial vertebrates, including most mammals, survived. Lepidosaurs

Most lizards are small; the jaragua lizaril, discovered in the Dominican Republic 2001, is only 16 mm long—small enou| to fit comfortably on a dime. In contra the Komodo dragon of Indonesia c reach a length of 3 m. It hunts deer ar d other large prey, delivering deadly bacteria with its bile. As its wounded prey d of the infection, the lizard slowly stalks I Snakes are legless lepidosaurs that evolved from lizards closely related to tile Komodo dragon (Figure 34.27c). T o d i some species of snakes retain vestigial pelvic and limb bones, which provide e? idence of their ancestry Despite their lack of eggs, snakes a quite proficient at. moving on land, most c ten by producing waves of lateral bendir that pass from head to tail. Force exerted b y the bends against solid objects pushes the snake forward. Snakes can also move p^ gripping the ground with their belly scales at several pom along the body, while the scales at intervening points are lifte slightly off the ground and pulled forward. Snakes are carnivorous, and a number of adaptations ai them in hunting and eating prey. They have acute chemic; sensors, and though they lack eardrums, they are sensitive t ground vibrations, which helps them detect the movemeni of prey. Heat-detecting organs between the eyes and nostrils c pit vipers, including rattlesnakes, are sensitive to minute tei perature changes, enabling these night hunters to locate wai animals. Poisonous snakes inject their toxin through a pair c sharp, hollow or grooved teeth. The flicking tongue is not po sonous but helps fan odors toward olfactory (smell) organs o the roof of the mouth. Loosely articulated jaws and elasti skin enable most snakes to swallow prey larger than the d ameter of the snake itself.

One surviving lineage of lepidosaurs is represented by two species of lizard-like reptiles called tuatara (Figure 34.27a). Tuatara relatives lived at least 220 million years ago, and they thrived on many continents well into the Cretaceous period, reaching up to a meter in length. Today, however, tuatara are found only on 30 islands oil the coast of New Zealand. When humans arrived in New Zealand 750 years ago, the rats that accompanied them devoured tuatara eggs, eventually eliminating the reptiles on the main islands. The tuatara that remain on the outlying islands are about 50 cm long and feed on insects, small lizards, and bird eggs and chicks. They can live to be over 1 00 years old. Their survival depends on whether their remaining habitats are kept rat-free.

Turtles are the most distinctive group of reptiles alive toda All turtles have a boxlike shell made of upper and lowye shields that are fused to the vertebrae, clavicles (collarbones' and ribs (Figure 34.27d). In most species, the shell is hare providing excellent defense against predators. The earliest fos sils of turtles, dating from about 220 million years ago, hav fully developed shells. Without transitional fossils to analyze the origin of the turtle shell remains a puzzle. As mentione earlier, some paleontologists have suggested that turtle shel evolved from the dermal plates of parareptiles. But other stuc ies have linked turtles to archosaurs or lepidosaurs.

The other major living lineage of lepidosaurs are the squamates (lizards and snakes). Lizards are the most numerous and diverse reptiles (apart from birds) alive today (Figure 34.27b).

The earliest turtles could not retract their head into thei shell, but mechanisms for doing so evolved independently i twTo separate branches of turtles. The side-necked turtle

690

U N I T FIVE

The Rvoliuionary HisLory oi Biological Diversity

Turtles

(b) Australian thorny devil lizard (Moloch horridus)

er's pit viper (Tropidolaemus wagleri), a snake

(d) Eastern box turtle (Terrapene Carolina Carolina)

(e) American alligator (Alligator mississipiensis)

k Figure 34.27 Extant reptiles (other than birds).

(pleurodires) fold their neck horizontally, while the verticalcked turtles (cryptodires) fold their neck vertically. Some turtles have adapted to deserts, and others live almost tirely in ponds and rivers. Still others have returned to the a. Sea turtles have a reduced shell and enlarged forelimbs that fi nction as flippers. This radiation has produced the largest livg turtles, the leatherbacks, which can weigh over 1,500 kg. F ceding on jellies, they dive as deep as 60 m. Leatherbacks and her sea turtles are endangered by fishing boats that accidents lly catch them in their nets, as well as by the human developrnt of the beaches where the turtles lay their eggs.

the southeastern United States. Alligators in the United States have made a strong comeback after spending years on the endangered species list.

Birds There are 8,600 species of birds in the world. Like crocodilians, birds are archosaurs, but almost every feature of their reptilian anatomy has undergone modification in their adaptation to flight. Derived Characters of Birds

Alligators and Crocodiles A ligators and crocodiles (collectively called crocodilians) belong an archosaur lineage that reaches back to the late Triassic (I igure 34.27e). The earliest members of this lineage were small Testrial quadrupeds with long, slender legs. Later species became larger and adapted to aquatic habitats, breathing air through their upturned nostrils. Some Mesozoic crocodilians grew as long as 10 m and may have attacked large dinosaurs. Living crocodilians are confined to the warm regions of rica, China, Indonesia, India, Australia, South America, and

Many of the characters of birds are adaptations that facilitate flight, including weight-saving modifications that make flying more efficient. For example, birds lack a urinary bladder, and the females of most species have only one ovary. The gonads of both females and males are usually small, except duringthe breeding season, when they increase in size. Living birds are also toothless, an adaptation that trims the weight of the head. The skull is especially light, although a bird's skeleton as a whole is no lighter in relation to body weight than the skeleton of a mammal of similar size. CHAPTER 34

Vertebrates

691

-=i Figure 34.28 Form fits function: the avian wing and feather, (a) A wing is a

(c) Feather structure

A bird's most obvious adaptations for flight are its wings and feathers (Figure 34.28). Feathers are made of a protein called P-keratin that is also found in the scales of other reptiles. The shape and arrangement of the feathers form the wings into airfoils, which illustrate some of the same principles of aerodynamics as the wings of an airplane. Power for flapping the wings comes from contractions of large pectoral (breast) muscles anchored to a keel on the sternum (breastbone). Some birds, such as eagles and hawks, have wings adapted for soaring on air currents and flap their wings only occasionally (see Figure 34.28); other birds, including hummingbirds, must flap continuously to stay aloft. The fastest birds are the appropriately named swifts, which can fly 170 km/hr. The evolution of flight provides numerous benefits. It enhances hunting and scavenging; many birds consume flying insects, an abundant, highly nutritious food resource. Flight also provides ready escape from earthbound predators and enables some birds to migrate great distances to exploit different food resources and seasonal breeding areas. Flying requires a great expenditure of energy from an active metabolism. Birds are endothermic; they use their own metabolic heat to maintain a high, constant body temperature. Feathers and in some species a layer of fat provide insulation that enables birds to retain body heat. An efficient respiratory system and a circulatory system with a four-chambered heart keep tissues well supplied with oxygen and nutrients, supporting a high rate of metabolism. The lungs have tiny tubes leading to and from elastic air sacs that help dissipate heat and reduce the density of the body. 692

UNIT FIVE

The Evolutionary History of Biological Diversity

remodeled version of the tetrapod foreiimb. (b) The bones of many birds have a honeycombed internal structure and are filled with air. (c) A feather consists of a central air-filled shaft, from which radiate the vanes. The vanes are made up of barbs, which bear small branches called barbules. Birds have contour feathers and downy feathers. Contour feathers are tr stiff ones that contribute to the aerodynamic shapes the wings and body. Their barbules have hooks that cling to barbules on neighboring barbs. When a bird preens, it runs the length of each contour feather through its bill, engaging the hooks and uniting the barbs into a precise shape. Downy feathers lack h o o k , and the free-form arrangement of their barbs produq ;s a fluffiness that provides insulation by trapping air.

Flight also requires both acute vision and fine muscle CO" trol. Birds have excellent eyesight, perhaps the best of all tl vertebrates. The visual areas of the brain are well developed as are the motor areas. The avian brain is proportionate larger than those of amphibians and nonbird reptiles. Birds generally display very complex behaviors, particulai during breeding season, when they engage in elabora courtship rituals. Because eggs are shelled when laid, fertiliz tion must be internal. Copulation usually involves contact b tween the mates' vents, the openings to their cloacas. After eg are laid, the avian embryo must be kept warm through broo> ing by the mother, father, or both, depending on the species. The Origin of Birds Cladistic analyses of birds and of reptilian fossils strong y suggest that birds belong to the group of bipedal saurischiar called theropods. Since the late 1990s, Chinese paleontoL gists have unearthed a spectacular trove of feathered then pod fossils that are shedding light on the first birds. Sever il species of dinosaurs closely related to birds had feathers with vanes, and a wider range of species had filamentous feather Such findings imply that feathers evolved long before pov ered flight. Among the possible functions of these early feat] ers were insulation, camouflage, and courtship display. Scientists have investigated two possible ways in whici powered flight may have evolved in theropods. In one so nario, feathers may have enabled small, ground-running d nosaurs chasing prey or escaping predators to gain extra lift i s

Wing claw

V Figure 34.30 A small sample of living birds.

Figure 34.29 Artist's reconstruction of Archaeopteryx, the earliest known bird. Fossil evidence •ndicates that Archaeopteryx was capable of powered flight but tained many characters of nonbird dinosaurs.

ey jumped into the air. Ln another scenario, some dinosaurs uld climb trees and glide, aided by feathers. By 150 million years ago, feathered theropods had evolved to birds. Archaeopteryx, which was discovered in a German mestone quarry in 1861, remains the earliest known bird igure 34.29). It had feathered wings but retained primitive laracters such as teeth, clawed digits in its wings, and a long J. Most experts agree that Archaeopleryx was a weak flyer at st. Fossils of a number of other birds from the Cretaceous -nod show a gradual loss of certain features of dinosaur atomy, such as teeth, as well as the acquisition of innovaons that are shared by all birds today, including a short tail vered by a fan of feathers.

(b) Mallards. Like many bird species, the mallard exhibits pronounced color differences between the sexes.

ving Birds ear evidence of Neomithes, the clade that includes the 28 or:rs of living birds, can be found after the Cretaceousleogene boundary 65.5 million years ago. Most birds can fly, V t there are several orders that include one or more flightless eaes. The ratites (order Struthioniformes), which consist of ostrich, rhea, kiwi, cassowary, and emu, are all flightless. In itites (from the Latin ratitus, flat-bottomed), the sternal keel is bsenL, and the pectoral muscles are not greatly enlarged : igure 34.30a). Penguins make up the flightless order phenisciformes, but, like flying birds, they have powerful ectoral muscles, which they use in swimming. Certain species frails, ducks, and pigeons are also flightless. The demands of flight have rendered the general body form f many flying birds similar to one another, yet experienced

(c) Laysan albatrosses. Like most birds, Laysan albatrosses have specific mating behaviors, such as this courtship ritual.

(d) Barn swallows. The barn swallow is a member of the order Passeriformes. Species in this order are called perching birds because the toes of their feet can lock around a branch or wire, enabling the bird to rest in place for long periods.

CHAPTER 34

Vertebrates

693

*•«,

malian character. All mammalian mothe nourish their young with milk, a balance I diet rich in fats, sugars, proteins, mineral

and vitamins. Hair, another mammalian

Raptor (such as a bald eagle)

Perching bird (such as a cardinal)

A Figure 34.31 Diversity of form and function in bird feet.

bird-watchers can distinguish different species by their body profile, flying style, behavior, feather colors, and beak shape (Figure 34.30b, c, and d). The beak has proved to be very adaptable during avian evolution, taking on a great variety of shapes suitable for different diets. Foot structure, too, shows considerable variation (Figure 34.31). Various birds use their feet for perching on branches, grasping food, defense, swimming or walking, and even courtship (see Figure 24.4e).

Concept Check i 1. Defend or refute the following statement: The amniotic egg of a reptile is a closed system in which the embryo develops in isolation from the outside environment. 2. Identify four avian adaptations for flight. For suggested answers, see Appendix A.

Concept

Mammals are amniotes that have hair and produce milk Reptiles (including birds) represent one of two great lineages of amniotes. The other lineage is our own, the mammals (class Mammalia). Today, there are more than 5,000 species of mammals on Earth.

Derived Characters of Mammals Mammary glands, which produce milk for offspring, are a distinctively mam-

694

UNIT FIVE

The Evolutionary History ol Biological Diversity

characteristic, and a fat layer under the sk help the body retain heat. Like birds, man mals are endothermic, and most have high metabolic rate. Efficient respirato: and circulatory systems (including a fouchambered heart) support a mamma metabolism. A sheet of muscle called tl" diaphragm helps ventilate the lungs. Mammals generally have a larger brain than other vert brates of equivalent size, and many species are capable learner; The relatively long duration of parental care extends the tin for offspring to learn important survival skills by observin their parents. Differentiation of teeth is another important mammal trait. Whereas the teeth of reptiles are generally conical anil uniform in size, the teeth of mammals come in a variety cf sizes and shapes adapted for chewing many kinds of food Humans, for example, have teeth modihed for shearing (inc sors and canine teeth) and for crushing and grinding (premi lars and molars).

Early Evolution of Mammals Mammals belong to a group of amniotes known as synapsid Nonmammalian synapsids lacked hair, had a sprawling gai and laid eggs. A distinctive characteristic of synapsids is th temporal fenestra, a hole behind the eye socket on each side the skull. Humans retain this feature; your jaw muscles pas through the temporal fenestra and anchor on your tempi The jaw was remodeled during the evolution of mamma from nonmammalian synapsids, and two of the bones th; formerly made up the jaw joint were incorporated into th mammalian middle ear (Figure 34.32). Synapsids evolved into large herbivores and carnivore during the Permian period, and for a time they were the dorr inant tetrapods. However, the Permian-Triassic extinction took a heavy toll on them, and their diversity fell during th Triassic. Mammal-like synapsids emerged by the end of th Triassic 200 million years ago. While not true mammals, thes synapsids had acquired a number of the derived character: that distinguish mammals from other amnioi.es. They wer small and probably hairy, and they likely fed on insects 2 night. They probably had a higher metabolism than othe synapsids, but they still laid eggs. During the Jurassic period, the first true mammals arose an diversified into a number of lineages, many of which are extinc Yet throughout the Mesozoic era, nearly all mammals remaine* about the size of todays shrews. One possible explanation fo

Marsupials

Jaw joint

v joint

Dimetrodon

Morganucodon

The lower jaw of Dimetrodon is composed of several fused bones; t w o small bones, the quadrate and articular, form part of the jaw joint. In Morganucodon, the lower jaw is reduced to a single Done, the dentary, and the location of the jaw joint has shifted. Middle ear E ardrum

\

Stapes

Eardrum

Middle ear

Opossums, kangaroos, and koalas are examples oi marsupials. Both marsupials and eutherians share derived characters not found among monotremes. They have a higher metabolic rate and nipples that provide milk, and they give birth to live young. The embryo develops inside the uterus of the female's reproductive tract. The lining of the uterus and the extraembryonic membranes that arise from the embryo form a placenta, a structure in which nutrients diffuse into the embryo from the mother's blood.

A marsupial is born very early in its development and completes its embry-Stapes onic development while nursing. In most species, the nursing young are - r r Incus (evolved held within a maternal pouch called a • • from quadrate) marsupium (Figure 34.34a, on die next "•C Malleus (evolved page). A red kangaroo, for instance, is " from articular) about the size of a honeybee at its birth, just 33 days after fertilization. Its hind Dimetrodon Morganucodon legs are merely buds, but its forelegs are ) During the evolutionary remodeling of the mammalian skull, the quadrate and articular bones strong enough for it to crawl from the became incorporated into the middle ear as t w o of the three bones that transmit sound from the exit of its mother's reproductive tract to eardrum to the inner ear. The steps in this evolutionary remodeling are evident in a succession of fossils. a pouch that opens to the front of her body, a journey that, lasts a few minutes. Figure 34.32 The evolution of the mammalian jaw and ear bones. Dimetrodon In other species, the marsupium opens as an early synapsid, a reptile-like lineage that eventually gave rise to the mammals. Morganucodon is a marnmaliform, a close relative of living mammals that lived 200 million years ago. to the rear of the mother's body; in bandicoots this protects the young as •ows in the dirt (Figure 34.34b, on the next their mother burr r small size is that dinosaurs already occupied ecological page.) ches of large-bodied animals. During the Mesozoic, the three major lineages of living animals emerged: monotremes (egg-laying mammals), arsupials (mammals with a pouch), and eutherians (plantal mammals). After the extinction of large dinosaurs, erosaurs, and marine reptiles during the late Cretaceous riod, mammals underwent an adaptive radiation, giving ;e to large predators and herbivores, as well as flying and uatic species. .-'

i

hInner ear

onotremes onotremes are found only in Australia and New Guinea d are represented by one species of platypus and two ecies of echidnas (spiny anteaters). Monotremes lay eggs, a aracter that is primitive for amniotes and retained in most stiles (Figure 34.33). Like all mammals, monotremes have hair and produce milk, but they lack nipples. Milk is secreted )y glands on the belly of the mother. After hatching, the baby rucks the milk from the mothers fur.

A Figure 34.33 S h o r t - b e a k e d e c h i d n a {Tachyglossus aculeatus), an A u s t r a l i a n m o n o t r e m e . Monotremes have hair and produce milk, but they lack nipples. Monotremes are the only mammals that lay eggs (inset).

CHAPTER 34

Vertebrates

695

Eutherian mammals

Marsupial mammals

Deer mouse

Plantigal

Marsupial mole

(a) A young brushtail possum. The young of marsupials are bom very early in their development. They finish their growth while nursing from a nipple (in their mother's pouch in most species).

Sugar glider

Flying squirrel

!

(b) Long-nosed bandicoot. Most bandicoots are diggers and burrowers that eat mainly insects but also some small vertebrates and plant material. Their rear-opening pouch helps protect the young from dirt as the mother digs. Other marsupials, such as kangaroos, have a pouch that opens to the front.

, «

Tasmanian devi!

A Figure 34.34 Australian marsupials.

Marsupials existed worldwide during the Mesozoic era, but today they are found only in the Australian region and in North and South America. The biogeography of marsupials is an example of the interplay between biological and geologic evolution (see Chapter 26). Alter the breakup of Pangaea, South America and Australia became island continents, and their marsupials diversified in isolation from the eutherians that began an adaptive radiation on the northern continents. Australia has not been in contact with another continent since early in the Cenozoic era, about 65 million years ago. In Australia, convergent evolution has resulted in a diversity of marsupials that resemble eutherians in similar ecological niches in other parts of the world (Figure 34.35). Although South America had a diverse marsupial fauna throughout the Paleogene, it

696

UNIT

FIVE

The Evolutionary History of Biological Diversity

Kangaroo

Patagonian cavy

A Figure 34.35 Evolutionary convergence of marsupials and eutherians (placental mammals). (Drawings are not to scale.)

S experienced several migrations of eutherians. One of the >st important migrations occurred about 3 million years ago, en North and South America j oined at the Panamanian isthis and extensive two-way traffic of animals took place over land bridge. Today, only three families of marsupials live tside the Australian region, and only one species, the oposm, is found in North America.

Eutherians (Placental Mammals) therians are commonly called placental mammals because ir placentas are more complex than those of marsupials, mpared to marsupials, eutherians have a longer period of gnancy Young eutherians complete their embryonic developnt within the uterus, joined to their mother by the placenta. 1 i£ eutherian placenta provides an intimate and long-lasting sociation between the mother and her developing young. The living orders of eutherians originated in the late Greta•us period. Figure 34.36, on the next two pages, explores .e major euLherian orders and their possible phylogenetic retionships with each other as well as with the monotremes id marsupials.

iimates mammalian order Primates includes the lemurs, the tarers, the monkeys, and the apes. Humans are members of the •e group. erived Characters of Primates. Most primates have hands lid feet adapted for grasping, and their digits have flat nails stead of the narrow claws of other mammals. There are her characteristic features of the hands and feet, such as .in ridges on the fingers (which account for human fingerrints). Relative to other mammals, primates have a large ain and short jaws, giving them a flat lace. Their torwardoking eyes are close together on the front of the face. Priates also have relatively well-developed parental care and mplex social behavior. The earliest primates were probably Lree-dwellers, and any of the characteristics of primates are adaptations to the jnands of living in the trees. Grasping hands and feet allow imates to hang onto tree branches. All modem primates, cept humans, have a big toe that is widely separated from e other toes, enabling them to grasp branches with their et. All primates have a thumb that is relatively mobile and parate from the fingers, but anthropoids (monkeys, apes, d humans) have a fully opposable thumb; that is, they n touch the ventral surface (fingerprint side) of the tip of four fingers with the ventral surface oi~ the thumb of the me hand. In monkeys and apes, the opposable thumb functions in a grasping "power grip," but in humans, a dis-

tinctive bone structure at the base ol the thumb allows it to be used for more precise manipulation. The unique dexterity of humans represents descent with modification from our tree-dwelling ancestors. Arboreal maneuvering also requires excellent eye-hand coordination. The overlapping visual fields of the two eyes enhance depth perception, an obvious advantage when brachiating (traveling by swinging from branch to branch in trees). Living Primates. There are three main groups of living primates: the lemurs of Madagascar (Figure 34.37) and the lorises and pottos of tropical Africa and southern Asia; the tarsiers, wTiich live in Southeast Asia; and the anthropoids, which include monkeys and hominoids and are found worldwide. Lemurs, lorises, and pottos probably resemble early arboreal primates. The oldest known anthropoid fossils, discovered in China in mid-Eocene strata about 45 million years old, indicate that tarsiers are more closely related to anthropoids (Figure 34.38, on page 700). You will see in Figure 34.38 that monkeys do not constitute a monophyletic group. The lossil record indicates that monkeys first appeared in the New World (in South America) during the Oligocene. By that time, South America and Africa had drifted apart. The first monkeys probably evolved in the Old World (in Africa and Asia), and they may have reached South America by rafting on logs or other debris from Africa. What is certain is that New World monkeys and Old World monkeys underwent separate adaptive radiations during their many millions of

M Figure 34.37 Coquerel's

sifakas (Propithecus vereauxi coqwere/i"). a

type of lemur.

697

Mammalian Diversity PHYLOGENETIC RELATIONSHIPS OF MAMMALS Evidence from numerous fossils and molecular analyses indicates that monotremes diverged from other mammals about 200 million years ago and that marsupials diverged from eutherians (placental mammals) about 180 million years ago. Molecular systematics has

This clade of eutherians evolved in Africa when the continent was isolated from other landmasses. it includes Earth's largest living land animal (the African elephant), as well as species that weigh less than 10 g.

All members of this clade, which underwent an adaptive radiation in South America, belong to the order Xenarthra. One species, the nine-banded armadillo, is found in the southern United States.

\

Monotremata

Marsupialia

Proboscidea Sirenia Tubulidentata Hyracoidea Afrosoricida (golden moles and tenrecs) Macroscelidea (elephant shrews)

Ancestral mammal

698

U N I T FIVE

The Evolutionary History of Biological Diversity

helped to clarify the evolutionary relationships between the euthfrrian orders, though there is still no broad consensus on a phylo^e netic tree. One current hypothesis, represented by the tree shoWi below, clusters the eutherian orders into four main clades.

This is the largest eutherian clade. It includes the rodents, which make up the largest mammalian order by far, with about 1,770 species. Humans belong to the order Primates.

Xenarthra

This diverse clade includes terrestrial and marine mammal as well as bats, the only flying mammals. A growing body of evidence, including Eocene fossils of whales with feet, supports putting whales in the same order (Cetartiodactyla) as pigs, cows, and hippos.

Rodentia Lagomorpha Primates Dermoptera (flying lemurs) Scandentia (tree shrews)

Carnivora Cetartiodactyla Perissodactyla Chiroptera Eufipotyphla Pholidota (pangolins)

Possible phylogenetic tree of mammafs. All 20 extant orders of mammals are listed at the top of the tree. Boldfaced orders are explored on the facing page.

ORDERS AND EXAMPLES Monotremata Platypuses, echidnas

MAIN CHARACTERISTICS

ORDERS AND EXAMPLES

MAIN CHARACTERISTICS

Lay eggs; no nipples; young suck milk from fur of mother

Marsupiali a Kangaroos, opossums, koalas

Embryo completes development in pouch on mother

Long, muscular trunk; thick, loose skin; upper incisors elongated as tusks

Tubulidentata Aardvark

Teeth consisting of many thin tubes cemented together; eats ants and termites

Aquatic; finlike forelimbs and no hind limbs; herbivorous

Hyracoidea Hyraxes

Echidna Proboscidea Elephants

African elephant Sirenia • Manatees, dugongs

.

Rock hyrax Xenarthra Sloths, I anteaters, armadillos

Reduced lee'th or no teeth; herbivorous (sloths) or carnivorous (anteaters, armadillos)

;Lagomorpha ; Rabbits, hares, picas

Carnivora Dogs, wolves, bears, cats, weasels, otters, seals, walruses Celaniodaclyla Artiodactyls Sheep, pigs, cattle, deer, giraffes

Rodentia Squirrels, beavers, rats. porcupines, mice

Chisel-like incisors; hind legs longer ;han forelegs and adapted for running and jumping

Primates Lemurs, monkeys, apes, humans

Sharp, pointed canine teeth and molars lor shearing: carnivorous

Perissodactyla Horses, zebras, tapirs, rhinoceroses

Coyote

n

Cetaceans Whales, dolphins, porpoises

Pacific whitesided porpoise

Chisel-like, continuously growing incisors worn down by gnawing, herbivorous Red squii

Golden tion tamarin

Opposable thumbs; forward-lacing eyes; \veli-de\eloped cerebral cortex: omnivorous

Hooves with an odd number of toes on each foot; herbivorous Indian rhinoceros

Hooves wuh an even number of toes on each foot; herbivorous

Chiroptera Bats

Frog-eating bat

Bighorn sheep

I

Shoit legs; stumpy tail; herbivorous; complex, multichambered stomach

Aquatic: streamlined body; paddle-like forelimbs and no hind limbs; thick layer of insulating blubber; carnivorous

Eulipotyphla "Core insectivores": some moles, some shrews

Adapted for flight; broad skinfold that extends from elongated fingers to body and legs; carnivorous or herbivorous

Diet consists mainly of insects and other small invertebrates Star-nosed mole

699

>• Figure 34.38 A phylogenetic tree of p r i m a t e s . The fossil record indicates that anthropoids began diverging from other primates about 50 million years ago. New

World monkeys, Old World monkeys, and hominoids (the clade that includes gibbons, orangutans, gorillas, chimpanzees, and humans) have been evolving as separate lineages for over 30 million years. The lineages leading to humans branched off from other hominoids somewhere between 5 and 7 million years ago.

years of separation (Figure 34.39). All species of New World monkeys are arboreal, whereas Old World monkeys include ground-dwelling as well as arboreal species. Most monkeys in both groups are diurnal (active during the day) and usually live in bands held together by social behavior. The other group of anthropoids, the hominoids, consists of primates informally called apes (Figure 34.40). This group includes the genera Hylohates (gibbons), Pongo (orangutans), Gorilla (gorillas), Pan (chimpanzees and bonobos), and Homo (humans). Hominoids diverged from Old World monkeys about 20-25 million years ago. Today, nonhuman hominoids are found exclusively in tropical regions of the Old World. With the exception of gibbons, living hominoids are larger than monkeys. All living hominoids have relatively long arms, short legs, and no tail. Although all nonhuman hominoids spend time in trees, only gibbons and orangutans are primarily arboreal. Social organization varies among the genera of hominoids; gorillas and chimpanzees are highly social. Hominoids have a larger brain in proportion to their body size than other primates, and hominoid behavior is more flexible.

• Figure 34.39 New World monkeys and Old World monkeys. 700

UNIT FIVE

Ancestral primate

(a) New World monkeys, such as spider monkeys (shown here), squirrel monkeys, and capuchins, have a prehensile tail and nostrils that open to the sides.

The Evolutionary History of Biological Diversity

(b) Old World monkeys lack a prehensile tail, and their nostrils open downward. This group includes macaques (shown here), mandrills, baboons, and rhesus monkeys.

(a) Gibbons, such as this Muller's gibbon, are found only in southeastern Asia. Their very long arms and fingers are adaptations (or brachiation.

(b) Orangutans are shy, solitary apes that live in the rain forests of Sumatra and Borneo. They spend most of their time in trees; note the foot adapted for grasping and the opposable thumb.

d) Chimpanzees live in tropical Africa. They feed and sle trees but also spend a great deal of time on the ground. Chimpanzees are intelligent communicative, and social.

(c) Gorillas are the largest apes: some males are almost 2 m tall and weigh about 200 kg, Found only in Africa, these herbivores usually live in groups of up to about 20 individuals.

(e) Bonobos are closely related to chimpanzees but are smaller. They survive today only in the African nation of Congo.

A Figure 34.40 Homirtoids (apes).

When you consider that life has existed on Earth for at least 3.5 billion years, we are clearly evolutionary newcomers. 1. Contrast monotremes, marsupials, and eutherians in terms of how they bear young. 2. Identify at least five derived traits of primates.

Derived Characters of Humans

Humans are bipedal hominoids with a large brain

A number of characters distinguish humans from other hominoids. Most obviously, humans stand upright and walk on two legs. Humans have a much larger brain than other hominoids and are capable of language, symbolic thought, and the manufacture and use of complex tools. Humans also have reduced jawbones and jaw muscles, along with a shorter digestive tract. The list of derived human characters at the molecular level is growing as scientists compare the genomes of humans and chimpanzees. Although the two genomes are 99% identical, a disparity of 1% can translate into a large number of differences in a genome that contains 3 billion base pairs.

In our tour of Earth's biodiversity, we come at last to our own pecies. Homo sapiens, which is about 160,000 years old.

Bear in mind that such genomic differences—-and whatever derived phenotypic characters they code for—separate humans

For suggested answers, see Appendix A.

CHAPTER 34

Vertebrates

701

from other living hominoids. But many of these new characters first emerged in our ancestors, long before our own species appeared. We will consider some of these ancestors to see how

canine teeth, and some fossils suggest that they had relatively flat faces. They also show signs ol having been more upright an \ bipedal than other hominoids. One clue to their upright stara

these traits originated.

can be found in the foramen magnum, the hole at the base of tl

The Earliest Hominids The study of human origins is known as paleoanthropology. Paleoanthropologists have unearthed fossils of approximately 20 species of extinct hominoids thai are more closely related to humans than to chimpanzees. These species are known as hominids (Figure 34.41). Since 1994, four hominid species dating from more than 4 million years ago have been discovered in the fossil record. The oldest of these hominids, Sahelanthropus tchadensis, lived about 7-6 million years ago; its fossils were discovered in 2002. Salielanthropus and other early hominids shared some of the derived characters ol humans. For example, they had reduced

skull through which the spinal cord exits. In chimpanzees, tl foramen magnimi is relatively far back on the skull, while in ear hominids (and in humans), it is located underneath the sku This derived position allows us to hold our head directly over OL body, and apparently early hominids did as well. Leg bones < f Australopithecus cinamemis, a hominid that lived 4,5-4 millio years ago, also suggest that early hominids were increasing bipedal. (We will return to the subject of bipedalism later.) Note that the characters that distinguish humans fro; other living hominoids did not all evolve in tight unisor While early hominids were showing signs of bipedalism, the brains remained small—about 400-450 cm3 in volume, corr pared with an average of 1,300 cm3 for Homo sapiens. Tl earliest hominids also were small (the 4.5-million-year-o]

Paranthropi

Homo neandei

0 Homo ergaster

Paranthropu boisei 0.5 1.0 A ustralopithecus africanus

1.5 2.0

,

. 2.5 Australopithecus garhi

, 3.0

'L

3.5 4.0 4.5

5.5 6.0

!

I

. Homo habilis

Australopithecus afarensis

Ardipithecus ramidus

5.0

Homo rudoifensis

Orrorin tugenensis

6.5 Sahelanthropus tch'adensis

7.0

A Figure 34.41 A timeline for some selected hominid species. Most of these fossils come from sites in eastern and southern Africa. Note that at most times in hominid history, two or more hominid species were contemporaries. The names of some of the species are controversial, reflecting phylogenetic debates about the interpretation of skeletal details and biogeography.

702

UNIT FIVE

The Evolutionary History of Biological Diversity

r

dipithecus ramidiis is estimated to have weighed only 40 kg) id had relatively large teeth and a lower jaw that projected yond the upper part ol the face. (Humans, in contrast, have relatively flat face. Compare your own face with that of the impanzee in Figure 34.40d.) Differing rates of evolution in fferent features is known as mosaic evolution. It's important LO avoid two common misconceptions when considering these early hominids. One is to think of them as himpanzees. Chimpanzees represent the tip of a separate ranch of hominord evolution, and they acquired derived laracters of their own after they diverged from their common ncestor with humans. Another misconception is to imagine that human evolun is a ladder leading directly from an ancestral hominoid to 'omo sapiens. This error is often illustrated as a parade of fos1 hominids that become progressively more like ourselves as ley march across the page. If human evolution rs a parade, it a very disorderly one, with many groups breaking away •ora the march to wander down dead-end alleyways. At mes, several hominid species coexisted, but all except one e—the one that gave rise to Homo sapiens-—-ended in xtinction. Some paleoanthropologists suggest that converence was common in early hominid evolution. Different liniges of hominids acquired different combinations of derived laracters and retained different sets of primitive characters, has even been suggested that some of the fossils claimed to e early hominids, such as Orrorin tugenensis (see Figure 4.41) are actually outside the hominid clade altogether. If that's true, they must have independently acquired some homfnid-like traits.

Australopithecus ajarcnsis (for the Afar region). Fossils discovered in the early 1990s show that A. ajarensis existed as a species for at least 1 million years. At the risk of oversimplifying, one could say that A. ajarensis had fewer derived characters of humans above the neck than below. Lucy's head was the size of a softball, indicating a brain size about the same as that of a chimpanzee of Lucy's body size. A. ajarensis skulls also have a long lower jaw. Skeletons of A. ajarensis suggest a capacity for arboreal locomotion, with arms relatively long in proportion to body size (compared to the proportions in humans). However, fragments of the pelvis and skull bones indicate that A. ajarensis walked on two legs. Fossilized footprints in Laetoli, Tanzania, corroborate the skeletal evidence that hominids living at the time of A. ajarensis were bipedal (Figure 34.42).

Australopiths The fossil record indicates that hominid diversity increased dramatically between 4 and 2 million years ago. Many of the hominids from this period are collectively called australopiths. Their phytogeny remains unresolved on many points, but as a ;>roup, they are almost certainly paraphylenc. Auslralopiil'icius anamensis, mentioned earlier, links the australopiths to older lominids such as Ardipithecus ramidus. Australopiths got their name from the 1924 discovery in South Africa of Australopithecus ajricanus ("southern ape of Africa"), which lived between 3 and 2.4 million years ago. With the discovery of more fossils, it became clear that A. ajricanus walked fully erect (was bipedal) and had human-like lands and teeth. However, its brain was only about one-third the size of the brain of a present-day human. In 1974, in the Afar region of Ethiopia, paleoanthropologists discovered a 3.24-million-year-old Australopithecus skeleton that was 40% complete. :iLucy," as the fossil was named, was short—only about 1 m tall. Lucy and similar fossils have been considered sufficiently different from Australopithecus ajricanus to be designated as a separate species,

(a) Lucy, a 3.24-million-yearold skeleton, represents the hominid species Australopithecus afarensis.

(b) The Laetoli footprints, more than 3.5 million years oid, confirm that upright posture evolved quite early in hominid history.

(c) An artist's reconstruction of what A, afarensis may have looked like.

A Figure 34.42 Upright posture predates an enlarged brain in human evolution.

CHAPTER 34

Vertebrates

703

Another lineage of australopiths consisted of the "robust" australopiths. These hominids, which included species such as Parcmthropus boisei, had sturdy skulls with powerful jaws and large teeth, adapted for grinding and chewing hard, tough foods. They contrast with the "gracile" (slender) australopiths, including A. ajarensis and A. africanus, which had lighter feeding equipment adapted for softer foods. Combining evidence from the earliest hominids with the much richer fossil record of later australopiths makes it possible to consider hypotheses about important trends in hominid evolution. Let's consider two of these trends: the emergence of bipedalism and tool use.

Bipedalism Our anthropoid ancestors of 30-35 million years ago were still tree-dwellers. But about 20 million years ago, the Indian Plate collided with Asia and thrust up the Himalayan range (see Figure 26.20). The climate became drier, and the forests of what are now Africa and Asia contracted. The result was an increased area oi savanna (grassland) habitat, with fewer trees. For decades, paleoanthropologists have seen a strong connection between the rise of savannas and the rise of bipedal hominids- According to one hypothesis, tree-dwelling hominids could no longer move through the canopy, so natural selection favored adaptations that made moving over open ground more efficient. Although some elements of this hypothesis survive, the picture now appears somewhat more complex. Although all recently discovered fossils of early hominids show some indications of bipedalism, none of these hominids lived in savannas. Instead, they lived in mixed habitats ranging from forests to open woodlands. An alternative hypothesis is that bipedalism evolved as an adaptation that allowed early hominids to reach low-hanging fruit on trees. Hominids did not become more bipedal in a simple, linear fashion. Australopiths seem to have had various locomotor styles, and some spent more time on the ground than others. Only about 1.9 million years ago did hominids begin to walk long distances on two legs. These hominids lived in more arid environments, where bipedal walking requires less energy than walking on all fours.

Tool Use As you read earlier, the manufacture and use of complex tools is a derived behavioral character of humans. Determining the origin of tool use in hominid evolution is one of paleoanthropology's great challenges today- Other hominoids are capable ol surprisingly sophisticated tool use. Orangutans, for example, can fashion probes from sticks lor retrieving insects from their nests. Chimpanzees are even more adept, using rocks to smash open food and putting leaves on their feet to walk over thorns. It's likely that early hominids were capable of this sort of simple Loo! use, but 704

UNIT FIVE The Evolutionary History of Biological Diversity

finding fossils of modified sticks and leaves that were used; shoes is practically impossible. The oldest generally accepted evidence ol tool use by hi minids are 2.5-million-year-old cut marks on animal bon found in Ethiopia. These marks suggest that hominids a flesh from the bones of animals using stone tools. Interes ingly, the hominids whose fossils were found near the si where the bones were discovered had a relatively small brail If these hominids, which have been named Australopithea garhi, were indeed the creators of the stone tools used on tr bones, that would suggest that stone tool use originated b fore the evolution of large hominid brains.

Early Homo The earliest fossils that paleoanthropologists place in oi. genus, Homo, are those of the species Homo habihs. These lo sils. ranging in age from about 2.4 to 1.6 million years, sho clear signs of certain derived hominid characters above th neck. Compared to the australopiths, H. habilis had a shorte jaw and a larger brain volume, about 600-750 cm3. Shar stone tools have also been found with some fossils oi H. habi (the name means "handy man"). Fossils from 1.9 to 1.6 million years ago mark a new stage hominid evolution. A number of paleoanthropologists recogniz these fossils as those ol a distinct species, Homo crgasier. Homo c gaster had a substantially larger brain than H. habilis (over 90 cm"), as well as long, slender legs with hip joints well adapted ft long-distance walking (Figure 34.43). The fingers were relative short and straight, suggesting that H. ergaster did not climb tres like earlier hominids. Homo ergaster has been found in far mm arid environments than earlier hominids and has been associate with more sophisticated stone tools. Its smaller teeth also suggesi that H. ergaster either ate different foods than australopiths (mor meat and less plant material) or prepared some of its food before chewing, perhaps by cooking or mashing the food. Homo ergasier marks an important shift in the relative sizes of the sexes. In primates, a size difference between males and females is a major component ol sexual dimorphism (see Chapter 23). On average, male gorillas and orangutans weigh about twice as much as females of their species. In chimpanzees and bonobos, males are only about 1.35 times as heavy as females, on average. In Australopithecus ajarensis, males were 1 -5 times as heavy as females. But in early Homo, sexual dimorphism was significantly reduced, and this trend continued through our own species: Human males average about 1.2 times the weight of females. The reduced sexual dimorphism may offer some clues to the social systems of extinct hominids. In living primates extreme sexual dimorphism is associated with intense malemale competition for multiple females. In species that undergo more pair-bonding (including our own), sexual dimorphism is less dramatic. Male and female H. ergaster may therefore

prominent brow. The hominid was given the name Homo ne ancle rthalensis, commonly called a Neanderthal. At one time, many paleoanthropologists considered Neanderthals to be a stage in the evolution of Homo erectus into Homo sapiens. Now most have abandoned this view Neanderthals appear to have descended from an earlier spec.es. Homo kdclclbcr^cnsis.. which originated in Africa about 600,000 years ago and then spread to Europe. Neanderthals, which appeared in Europe and the Near East by 200,000 years ago, had a brain as large as that of present-day humans and were capable of making hunting tools from stone and wood. But despite their adaptations, Neanderthals apparently became extinct 30,000 years ago, contributing nothing to the gene pool of living humans. Evidence of the extinction of Neanderthals can be found in their DNA. Scientists extracted fragments of DNA from fossils of four Neanderthals that lived at different times in different localities in Europe. They then compared the Neanderthal DNA with DNA of living humans from Europe, Africa, and Asia. If Neanderthals had given rise to European humans, then both groups should share a common ancestor, with other humans being more distantly related. Instead, the DNA analysis indicates that all of the Neanderthals form a clade, while living Europeans are more closely related to living Africans and Asians. < Figure 34.43 Fossil and artist's reconstruction of Homo ergraster. This 1.7-miIIion-year-old fossil from Kenya belongs to a young male Homo ergaster. This individual was tall, slender, and fully bipedal, and he had a relatively large brain.

lave engaged in more pair-bonding than earlier hominids did. This shift may have been associated with long-term biparental :are of babies. Human babies depend on their parents for food ind protection much longer than do the young of cbimjanzees and other hominoids. Fossils now recognized as Homo ergaster were originally considered early members of another species, Homo erectus, and ;ome paleoanthropologists still hold this position. Homo erectus originated in Africa and was the first hominid to migrate out of Vfrica. The oldest fossils of hominids outside Africa, dating back 1.8 million years, were discovered in 2000 in the former Soviet Republic of Georgia. Homo erectus eventually migrated as far as the Indonesian archipelago. Comparisons of H. erectus dossils with humans and studies of human DNA indicate that H. erectus became extinct sometime after 200,000 years ago.

Homo Sapiens Evidence from fossils, archaeological relics, and DNA studies is beginning to cohere into a compelling hypothesis about how our own species, Homo sapiens, emerged and spread around the world. It is now clear that the ancestors of humans originated in Africa. Older species (perhaps H. ergaster or II. erectus) gave rise to newer species, such as H. he\delbergensi$ and ultimately H. sapiens. In 2003, researchers working in Ethiopia reported the discovery of 160,000-year-old fossils of H. sapiens, the oldest known fossils of our own species (Figure 34.44). These

Neanderthals

< Figure 34.44 Oldest known fossil of Homo

In 1856, miners discovered some mysterious human fossils in i cave in the Neander Valley in Germany. The 40,000-year-old ifossils belonged to a thick-boned, heavy hominid with a

sapiens. This skull differs little from the skulls of living humans.

CHAPTER 34

Vertebrates

705

early humans lacked the heavy browridges of H. erectus and Neanderthals and were more slender than other hominids. The Ethiopian fossils support molecular evidence about the origin of humans. As we noted earlier, DNA analysis indicates thai all living humans are more closely related to one another than to Neanderthals. Other studies on human DNA show that Europeans and Asians share a relatively recent common ancestor and that many African lineages branched off more ancient positions on the human family tree. These findings strongly suggest that all living humans have ancestors that originated as Homo sapiens in Africa, which is further supported by analysis of mitochondrial DNA and Y chromosomes from members ot various human populations. The oldest fossils of Homo sapiens outside Africa date back about 50,000 years. Studies of the human Y chromosome suggest that humans spread beyond Africa in one or more waves, first into Asia and then to Europe and Australia. The date of the first arrival of humans in the New World is still a matter of debate, although the oldest generally accepted evidence puts that date at f 5,000 years ago, New findings continually update our understanding of the context of H. sapiens's evolution. For example, in October 2004, Peter Brown, of the University of New England in New South Wales, Australia, Thomas Sutikna, of the Indonesian Centre for Archaeology, and their colleagues reported an astonishing find unearthed in 2003: the skeleton of an adult hominid dating from just 18,000 years ago and representing a previously unknown species, now named Homo Jloresiensis. Discovered in a limestone cave on the Indonesian island of Flores, the individual was much shorter and had a much smaller brain volume than H. sapiens—more similar, in fact, to an australopith. However, the skeleton also displays many derived traits, including skull thickness and proportions and teeth shape, that suggest it is descended from the larger H. erectus. One intriguing explanation for this species' apparent "shrinkage" is that isolation on the island may have resulted in selection for greatly reduced size. Such dramatic size reduction is well studied in other dwarf mammalian species endemic to islands; these include primitive pygmy elephants found in the same vicinity as the H. floresiensis specimen. Compelling questions that may yet be answered from the cache of anthropological and archeological finds on Flores include more clues to the origin of H. jloresiensis, whether its members used tools, and whether they ever encountered H. sapiens, which coexisted in Indonesia during the late Pleistocene. The rapid expansion ol our species (and the replacement ot Neanderthals) may have been spurred by the evolution of human cognition as Homo sapiens evolved in Africa. Although Neanderthals and other hominids were able to produce sophisticated tools, they showed little creativity and not much capability for symbolic thought, as far as we can tell. In contrast, researchers are beginning to find evidence of more sophisticated thought as Homo sapiens evolved. For example, in 2002, 706

UNIT FIVE The Evolutionary History of Biological Diversity

• Figure 34.45 Art, a human hallmark. The engravings on this 77,000-year-old piece of ochre, discovered in South Africa's Blombos Cave, are among the earliest signs of symbolic thought in humans.

researchers reported the discovery in South Africa of 77,000 year-old art—geometric markings made on pieces of ochr (Figure 34.45). And in 2004, archaeologists working in south ern and east Africa found 75,000-year-old ostrich eggs am snail shells with holes neatly drilled through them. By 36,00 years ago, humans were producing spectacular cave paintings Symbolic thought may have emerged along with full-blowi human language. Both may have raised the survival and repro ductive fitness of humans by allowing them to construct ne tools and teach others how to build them. Long-range trade fo scarce resources also became possible. As the population ii Africa rose, population pressures may have driven humans U migrate into Asia and then Europe. Neanderthals may hav been driven to extinction by the combined stresses of the las ice age and competition from newly arrived humans. Clues to the cognitive transformation of humans can b found in the human genome as well as in archaeological sites, i gene known as FOXP2 was identified in 2001 as essential fo human language. People who inherit mutated versions of th gene suffer from a range of language impediments and have re duced activity in Broca's area in the brain (see Chapter 48). Ir 2002, geneticists compared the F0XP2 gene in humans with the homologous gene in other mammals. They concluded that the gene experienced intense natural selection after the ances tors of humans and chimpanzees diverged. By comparing mu tations in flanking regions of the gene, the researchers estimatei that this bout of natural selection occurred within the pas 200,000 years. Of course, the human capacity for language in volves many regions of the brain, and its almost certain tha many other genes are essential for language. But the evolution ary change in FOXP2 may be the first genetic clue as to how ou own species came to play its unique role in the world. Our discussion of humans brings this unit of chapters on bi ological diversity to an end. But this organisation isn't meant tc imply that life consists of a ladder leading from lowly microbe to lofty humanity. Biological diversity is the product of branch ing phylogeny, not ladderlike "progress," however we choose t

rr easure it, The fact thai there are more species of ray-finned fi ;hes alive today than all other vertebrates combined is a r ear indication that our finned relatives are not outmoded nderachievers that failed to get out of the water. The itrapods—amphibians, reptiles, and mammals—are derived

through adaptive evolution. Biology exalts life's diversity, past and present.

Concept Check 1. Contrast hominoids with hominids. 2. How do the characteristics of Homo ergaster illustrate mosaic evolution? For suggested answers, see Appendix A.

Chapter Go to the Campbell Biology website (www.campbellbiology.com) or CDRDM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF --_—

-

-1 •

Review • Fossils of Early Vertebrates (pp. 678-679) Conodonts were the first vertebrates with mineralized skeletal elements in their mouth and pharynx. Armored jawless vertebrates ("ostracoderms") had defensive plates of bone on their skin. • Origins of Bone and Teeth (p. 679) Mineralization appears to have originated with vertebrate mouthparts; the vertebrate endoskeleton became fully mineralized much later.

Chordates have a notochord and a dorsal, hollow i lerve cord (

Derived Characters of Chordates (p. 673) Chordate char acters include a notochord; a dorsal, hollow nerve cord; pharyngeal slits or clefts; and a muscular, post-anal tail.

I - Tunicates (pp. 673-674) Tunicates are marine suspension feeders commonly called sea squirts. They lose some of the derived characters of chordates as adults. I • Lancelets (p. 674) Lancelets are marine suspension feeders that retain the hallmarks of the chordate body plan as adults. I • Early Chordate Evolution (pp. 674-675) The current life history of tunicates probably does not reflect that of the ancestral chordate. Gene expression in lancelets holds clues to the evolution of the vertebrate brain.

raniates are chordates that have a head I • Derived Characters of Craniates (p. 676) Craniates have a head, including a skull, a brain, eyes, and other sensor}' organs. Many structures in craniates develop partly from a novel population of cells, the neural crest. The Origin of Craniates (p. 676) Craniates evolved at least 530 million years ago, during the Cambrian explosion. I - Hagfishes (pp. 676-677) Hagnshes are jawless marine craniates that have a cartilaginous skull and an axial rod of cartilage derived from the notochord. They lack vertebrae.

Vertebrates are craniates that have a backbone I • Derived Characters of Vertebrates (p. 678) Vertebrates have vertebrae, an elaborate skull, and, in aquatic forms, fin rays. I' Lampreys (p. 678) Lampreys are jawless vertebrates that have cartilaginous segments surrounding the notochord and arching partly over the nerve cord.

Gnathostomes are vertebrates that have jaws • Derived Characters of Gnathostomes (pp. 679-680) Gnathostomes have jaws, which evolved from skeletal supports of the pharyngeal slits; enhanced sensory systems, including the lateral line system; an extensively mineralized endoskeleton; and paired appendages. • Fossil Gnathostomes (p. 680) Placoderms were close relatives of living gnathostomes. Acanthodians were closely related to osteichthyans. • Chondrichthyans (Sharks, Rays, and Their Relatives) (pp. 680-682) Chondrichthyans, which include sharks and rays, have a cartilaginous skeleton that evolved secondarily from an ancestral mineralized skeleton. • Ray-Finned Fishes and Lobe-Fins (pp. 682-684) Osteichthyans have a skeleton reinforced by calcium phosphate. Aquatic forms have bony gill covers and a swim bladder; some also have lungs. Ray-finned fishes have maneuverable lins supported by long rays. Lobe-fins include coelacanths, lungfishes, and tetrapods. Aquatic lobe-fins have muscular pectoral and pelvic lins.

Tetrapods are gnathostomes that have limbs and feet • Derived Characters of Tetrapods (p. 684) Tetrapods have four limbs and feet with digits. They also have other adaptations for life on land, such as ears. • The Origin of Tetrapods (p. 684) Fossil evidence suggests that tetrapod limbs, now mainly used for walking on land, were originally used for paddling in water. • Amphibians (pp. 685-687) Amphibians include salamanders, frogs, and caecilians. Most have moist skin that complements the lungs in gas exchange. Most frogs and some salamanders undergo metamorphosis of an aquatic larva into a terrestrial adult.

CHAPTER 34

707

• Bipedalism (p. 704) Hominids began to walk long distances on two legs about 1.9 million years ago.

Amniotes are tetrapods that have a terrestrially adapted egg • Derived Characters of Amniotes (p. 688) The amniotic egg contains extraembryonic membranes that carry out a variety of functions, including gas exchange and protection. Amniotes also have other terrestrial adaptations, such as relatively impermeable skin. • Early Amniotes (p. 688) Early amniotes appeared in the Carboniferous period. They included large herbivores and predators. • Reptiles (pp. 688-691) Reptiks include tuatara, lizards, snakes, turtles, crocodilians, and birds. Extinct forms included parareptiles, dinosaurs, pterosaurs, and marine reptiles. Most reptiles are ectothermic, although birds are endothermic (as some dinosaurs may have been). • Birds (pp. 691-694) Birds probably descended from a group of small, carnivorous dinosaurs known as theropods. They have a variety of adaptations well suited to a lifestyle involving flight. Investigation How Does Bone Structure Shed Light on the Origin of Birds?

• Tool Use (p. 704) The oldest evidence of tool use—cut mark:

on animal bones—is 2.5 million years old. • Early Homo (pp. 704-706) Homo crgaster was the first fully bipedal, large-brained hominid. Homo erectus was the first hominid to leave Africa. • Neanderthals (p. 705) Neanderthals lived in Europe and the Near East from 200,000 to 30,000 years ago. They became exLinct a few thousand years after the arrival of Homo sapiens in Europe. • Homo Sapiens (pp. 705-706) Homo sapiens appeared in Afrit by at Least 160,000 years ago and spread to other continents about 50,000 years ago. This spread may have been preceded fj y changes to the brain that made symbolic thought and other coj nitive innovations possible. Research into the origins and contemporaries of /1 onto sapiens is a lively area of research and debate, including the recent discovery of a new species, Homo Jioresiensis, dating from the late Pleistocene. Activity Human Evolution

TESTING YOUR KNOWLEDGE Mammals are amniotes that have hair and produce milk • Derived Characters of Mammals (p. 694) Hair and mammary glands are two derived characteristics of mammals. • Early Evolution of Mammals (pp. 694-695) Mammals evolved from synapsids in the late Triassic period. Living lineages of mammals originated in the Jurassic but did not undergo a significant adaptive radiation until the beginning of the Paleogene. • Monotremes (p. 695) Monotremes are a small group of egglaying mammals consisting of echidnas and the platypus. • Marsupials (pp. 695-697) Marsupials include opossums, kangaroos, and koalas. Marsupial young begin their embryonic development attached to a placenta in the mother's uterus but complete development inside a maternal pouch. • Eutherians (Placental Mammals) (pp. 697-701) Eutherians have young that complete their embryonic development attached to a placenta- All primates have hands and (with the exception of humans) feet adapted for gripping. Living primates include lemurs and their relatives, tarsiers, and anthropoids- Anthropoids diverged early into New World and Old World monkeys. Hominoids (gibbons, orangutans, gorillas, chimpanzees, bonobos, and humans) evolved from Old World monkeys. Activity Characteristics of Chordates Activity Primate Diversity

Humans are bipedal hominoids with a large brain • Derived Characters of Humans (pp. 701-702) Humans have bipedal locomotion, a shortened jaw, and an enlarged brain. • The Earliest Hominids (pp. 702-703) Hominids originated in Africa at least 6-7 million years ago. Early hominids had a small brain but probably walked upright. • Australopiths (pp. 703-704) Australopiths are a paraphyletic assemblage of hominids that lived between 4 and 2 million years ago. Some species walked fully erect and had human-like hands and teeth.

708

UNIT FIVE

The Evolutionary History of Biological Diversity

Evolution Connection Identify one characteristic that qualifies humans for membership i each of the following clades: eukaryotes, animals, deuterostomes, chordates, vertebrates, gnathostomes, amniotes, mammals, primates

Scientific Inquiry Scientific inquiry often involves trying to make sense of an interes ing observation. One such observation concerns patterns of geneti versus morphological divergence in some vertebrate groups. Amphit ians, for example, show great similarity in morphology, yet they ar far more genetically diverse than the morphologically diverse bird; A related pattern is evident in chimpanzee-human comparisons: These two species are quite divergent morphologically but are much more similar genetically. Propose one or more hypotheses that might explain these puzzling patterns, investigation How Does Bone Structure Shed Light on the Origin of Birds?

Science, Technology, and Society While human biological evolution is Darwinian, the evolution of human culture could be described as Lamarckian. Explain this distinction after reviewing the ideas of Darwin and Lamarck in Chapter 22.

Plant Form and Function AN INTERVIEW WITH

Natasha Raikhel From her early work as an invertebrate zoologist at the University of Leningrad to her research today as one of the most prominent American plant biologists, Natasha Raikhel has traveled very far, both geographically and scientifically. Dr. Raikhel is now the Distinguished Professor of Plant Cell Biology at the University of California, Riverside (UCR), where she is also the director of the Center for Pfant Cell Biology. In 2003, the American Society for Cell Biology honored Professor Raikhel's achievements with the Women in Cell Biology Senior Career Recognition Award, and in 2004 she received the Stephen Hales Prize of the American Society of Plant Biologists; the award is given to

What were the circumstances of your emigration? This was in the iate '70s, and as Jews in the Soviet Union, our career opportunities in science were limited, but we were lortunate enough to be allowed to emigrate. After our first son was born in 1975, we wanted him to be exposed to greater opportunities, and that helped us finalize our decision to leave. It was difficult to emigrate from the Soviet Union during that period. But there was a joke at that time that the Carter administration in the United States had a grains-for-jews exchange policy—the more wheat the U.S. sent to the Soviet Union, the more Jews were allowed out. So we were very lucky that we were able to get out of the Soviet Union at a time when many people would have loved to leave.

a scientist who has served the science of plant biology in some noteworthy manner. One of Dr. Raikhel's current interests is making systems biology possible by building multidisciplinary research teams. Dr. Raikhel's service to the biology community and her generosity as a mentor to young researchers exemplify the collaborative spirit of the science culture.

How did you begin your science career in what was then the Soviet Union? Actually, I didn't go directly into science. 1 was a "musician and was supposed to become a professional pianist. But at some point, I realized that I could not be one of the best pianists, and I didn't like that limitation. I always liked nature, and I was able to study biology at the University of Leningrad. I met my husband when we were both zoology students in Leningrad (now Si. Petersburg), where we continued our work at the university through graduate school until we acquired faculty positions. And then we decided to leave the Soviet Union.

710

How did you find your first jobs here? It was difficult for scientists emigrating from the Soviet Union to find academic jobs because oi a lack of references. But again, we were lucky. The research institute (Institute of Cytology) where 1 worked had a keen interest in modern biology, and the professor in charge invited visiting professors to spend their sabbatical leaves doing research at the institute. A professor, Jerry Paulin, who came with his family from the University of Georgia, had his desk right next to mine in the lab. Our families became friends. At that time, it was very difficult for American visitors in the Soviet Union to get around because they were told, "You cannot go here, you cannot go there." So we helped the Paulins during their visit to Leningrad. Later, when we planned our immigration to the United States, jerry Paulin was the one professor in the U.S. who knew us and our work as scientists, and he helped us find jobs at the University of Georgia. And so we started all over again as postdoctoral researchers when we came to this country.

In Leningrad, you worked on protozoans—ciliates. But in Georgia, you began your work with plants. What motivated that change? My interest is at the basic cellular level, and sc the switch of organisms was not too difficult realized very soon after arriving in the U.S. that it would be hard to get grants for researd on free-living protozoans, which do not cause disease. At the time, plant research was really starting to take off in the U.S., and Georgia was one of the key places where plant biologj was very modern. 1 sensed this and wanted to be part of that environment, and so I learned lot about plants. I also began to reeducate myself to do molecular biology. Molecular approaches in plant biology were just beginning. I didn't have to catch up because I learned along with many people who were also just beginning in molecular biology. So, once again, I was lucky with good timing.

Speaking of good timing, why is this a good time to be in plant biology? One reason is that we now know a lot about a research organism, the plant Ambidopsis. Its genome has been sequenced, and many of its proteins have been identified. Once you understand the functions of these proteins, how the proteins interact, and how pathways in the cell interact, then you can really start to answer questions about how cells function and how th whole plant works. Systems biology becomes possible, and I think this makes plant biology much more exciting.

What will it take to make plant systems biology work? The essence of systems biology is to be able to model organisms-—to be able to predict how various pathways in the organism interact and

how a change in one pathway will affect the w'role organism. Tins requires our approaches to be more quantitative and much, more integrated. We need to infuse plan! biology \\ ',;h disciplines such as informatics and engineering, but this is difficult because people are coming from different scientific cultures. Right now, many of us are retraining to understand fields such as bioinformatics, but it will be much better when such multidisciplinary experience becomes the normal environment for [raining biologists. It is very important that the new generation of plant biologists be very quantitative and comfortable with bioinformatics, and that they are good biologists too. It'will take time to create this new generaiion of scientists. But many people are looking to tile community of Arabic! op v.'< researchers ;o make the systems approach work, because this community is exemplary in terms of openness and sharing among colleagues.

The number of protein-coding genes in Arabidopsis rivals that of humans and other complex animals. Did that discovery surprise you? Not really. Because plants are immobile, they have to be very versatile in their ability to respond to environmental stresses. Plants have to be exquisite to survive because they can't run. This requires a lot of different kinds of proteins, including many that are unique to plants.

Are there important agricultural applications from all this work on Arabidopsis? les, because Arabidopsis is such a convenient model organism lor studying processes that are nportant in all plants, including crop plants.

For example, resistance to cold and resistance to pathogens are two traits that are very useful in crop plants. But such traits are much easier to analyze in Arabidopsis. So, il we first identify certain genes and regulatory pathways that can h.c p Ajo.t-Kh-i'sis resist environmental stress, we can then transfer some of that knowledge to improve crop plants.

What is the focus of your own lab's research? For a long time, my lab group has worked on me trafficking ol molecules through the cell's vesicles and vacuoles. And in the last few years, we've added another project, the synthesis of the cell wall in plants. Vacuoles and cell walls have unique plant-specific functions and are essential for plant life. 1 therefore am very interested in fully understanding these mechanisms.

In recent years, your influence has extended beyond your own lab to a leadership role in the international community of plant cell biologists. Why do you think this expanded role is important at this stage in your career? Well, first of all, 1 want to make it clear that my research with my own lab group is still my number-one priority. But with time, you start to realize that you have experience that enables you to do more for the science community in your field. 1 got a lot from the community of plant biologists, and now 1 want to be constructive in giving back lo the community. In fact, at a certain point in your career, you have an obligation to give more than you take. For example, 1 was not looking to become editor-in-chief of the journal Plant Physiology, but colleagues convinced me to take the job. This takes a lot of

my time now, but the journal, is evolving beautifully, which is very rewarding.

As another example of "giving something back," you participate in a program that provides summer research opportunities at UCR for community college students. Is it fun for you to have these students in your lab? 1 love to be around these young people because they keep me young. At first, when these students join the lab, they are completely naive about research and just don't know how to get started. But once you put a fire under them and get them excited, it's the most rewarding thing in the world. For example, this past summer, we had a student from San Bernardino Valley College, a community college nearby. He just loved his project, and he will be on a paper that we are writing. He will also be transferring to UCR, and he still works in my lab for many hours each week, even though he's taking lots of courses. This student now wants to become a plant researcher—-he's a convert! And that gives me great satisfaction.

With two very busy scientists in the family, how do you and your husband balance the demands of your careers with the responsibilities and enjoyment of family life? 1 want young people beginning in science to understand that they will not have to shy away from having a family and having children because of their work. The most wonderful things I have in my life are my two sons. Balancing career and family is not easy, but it's all possible because I have a great husband, and we make decisions and do things together. You can't say, "You stay at home, and that's how we will share." Maybe that does work for some people, but in our case, we both wanted careers in science.

What other advice do you have for students who are interested in becoming scientists? Because of the way biology is changing, 1 think students should look for programs that offer interdisciplinary approaches. They should not lock themselves into certain subfields. And they should reach for the stars. You have to be ambitious and work hard, and you have to know what you want, and then don't let anything stop you. You can grow as a scientist here like nowhere else in the world. 1 think I'm a living example of that. It's a fantastic life!

711

^ cture, Growth, and Development . Figure 35.1 Fanwort (Cabomba caroliniana).

Key Concepts 35.1 The plant body has a hierarchy of organs, tissues, and cells 35.2 Meristems generate cells for new organs 3 5 3 Primary growth lengthens roots and shoots 35.4 Secondary growth adds girth to stems and roots in woody plants 35.5 Growth, morphogenesis, and differentiation produce the plant body

No Two Plants Are Alike 1

o some people, the plant in Figure 35.1 is an intrusive aquatic weed that clogs streams, rivers, and lakes. Others consider it an attractive addition to an aquarium. Whatever else the lanwort (Cabomba caroliniana) may be, it is a striking example of plasticity—an organism's ability to alter or "mold" itself in response to local environmental conditions. The underwater leaves have a feathery appearance, an adaptation that may provide protection from the stress of moving water. In contrast, the surface leaves are pads that aid in flotation. Both leaf types have genetically identical cells, but the dissimilar environments cause dilferent genes involved in leaf formation to be turned on or off- Such extreme developmental plasticity is much more common in plants than in animals and may help compensate for their lack of mobility. As Natasha Raikhel puts it in the interview preceding this chapter, "Plants have to be exquisite to survive because they can't run." Also, since the form of any plant is controlled by environmental as well as genetic factors, no two plants are exactly alike. In addition to plastic structural responses by individual plants to specific environments, entire species have by natu712

ral selection accumulated characteristics of morphology, s external form, that vary little among plants within the specie For example, some species of desert plants, such as cacti, hav leaves that are so highly reduced as spines that the stem is a< tually the primary photosynthetic organ. This reduction ij leaf size, and thus in surface area, results in reduced wate loss. These leaf adaptations have enhanced survival and n productive success in the desert environment. This chapter focuses on how the body of a plant is forme< setting the stage for the rest of this unit on plant biolog Chapters 29 and 30 described the evolution and characteris tics of bryophytes, seedless vascular plants, gymnosperm and angiosperms. This chapter and Unit Six in general focu mainly on vascular plants—especially angiosperms becaus flowering plants comprise about 90% of plant species and a the base of nearly every terrestrial food web. As the world'; population increases, the need for plants to supply food, fuel fiber, medicine, lumber, and paper has never been greater heightening the importance of understanding how plant; grow and develop.

Concept

The plant body has a hierarchy of organs, tissues, and cells Plants, like multicellular animals, have organs composed o different tissues, and these tissues are composed of cells. / tissue is a group of cells with a common function, structure, o. both. An organ consists of several types of tissues that togethe carry out particular functions. In looking at the hierarchy o plant organs, tissues, and cells, we will focus first on organs a: the most readily observable features of plant structure.

T tie Three Basic Plant Organs: Roots, Stems, and Leaves basic morphology of vascular planis reflects their evolunary history as terrestrial organisms that inhabit and draw reurees from two very different environments—below-ground d above-ground. Plants must absorb water and minerals from b low the ground and CO2 and light from above the ground. T le evolutionary solution to this separation of resources was e development of three basic organs: roots, stems, and leaves. hey are organized into a root system and a shoot system, e latter consisting of stems and leaves (Figure 35.2). With ft w exceptions, angiosperms and other vascular plants rely impletely on both systems for survival. Roots are typically mphotosynthetic and would starve without the organic nuients imported from the shoot system. Conversely, the shoot stern depends on the water and minerals that roots absorb om the soil. Later in the chapter, we will discuss the transition from getative shoots (shoots that are nonreproductive) to reproactive shoots. In angiosperms, the reproductive shoots are owers, which are composed of leaves that are highly modied for sexual reproduction.

Reproductive shoot (flower) Terminal budNodeIntemode

As we take a closer look at roots, stems, and leaves, try to view7 these organs from the evolutionary perspective of adaptations to living on land, fn identifying some variations in these organs, we will focus mainly on the two major groups of angiosperms: monocots and eudicots (see Figure 30.12). Roots A root is an organ that anchors a vascular plant (usually in the soil), absorbs minerals and water, and often stores organic nutrients. Most eudicots and gymnosperms have a taproot system, consisting of one main vertical root (the taproot) that develops from an embryonic root. The taproot gives rise to lateral roots, also called branch roots (see Figure 35.2). In angiosperms, the taproot often stores organic nutrients that the plant consumes during flowering and fruit production. For this reason, root crops such as carrots, turnips, and sugar beets are harvested before they flower. Taproot systems generally penetrate deeply into the ground. In seedless vascular plants and in most monocots, such as grasses, the embryonic root dies and does not give rise to a main root. Instead, many small roots grow from the stem, with each small root forming its own lateral roots. The result is a fibrous root system—-a mat ol generally thin roots spreading out below the soil surface, with no root standing out as the main one (see Figure 30.12). Roots arising from the stem are said to be adventitious (from the Latin adventidus, extraneous), a term describing any plant part that grows m an unusual location. A fibrous root system is usually shallower than a taproot system. Grass roots are particularly shallow, being concentrated in the upper few centimeters of the soil. Because grass roots hold the topsoil in place, they make excellent ground cover for preventing erosion. Large monocots, such as palms and bamboo, are mainly anchored by sturdy rhizomes, which are horizontal underground stems. The entire root system helps anchor a plant, but in most plants the absorption of water and minerals occurs primarily near the root tips, where vast numbers of tiny root hairs increase the surface area of the root enormously (Figure 35.3). A

•4 Figure 35.3 Root hairs and root tip. Growing by the thousands just behind each root tip, root hairs increase the surface area for the absorption of water and minerals by the roots.

Figure 35.2 An overview of a flowering plant. The plant ody is divided into a root system and a shoot system, connected by scular tissue (purple strands in this diagram) that is continuous roughout the plant. The plant shown is an idealized eudicot.

CHAPTER 35

Plan!. Structure, Growth, and Development

713

root hair is an extension of a root epidermal cell (protective cell on a plant surface). Root hairs are not to be confused with lateral roots, which are multicellular organs. Absorption is often enhanced by symbiotic relationships between plant roots and fungi and bacteria, as you will see in Chapters 36 and 37.

Many plants have modified roots. Some oi these arise froi roots, and others are adventitious, developing from sterr and, in rare cases, leaves. Some modified roots provide mo

support and anchorage, while others store water and nutrien or absorb oxygen or water from the air (Figure 35.4).

T Figure 35.4 Modified roots. Environmental adaptations may result in roots being modified for a variety of functions, Many modified roots are aerial roots that are above the ground during normal development.

(a) Prop roots. The aerial roots shown here in maize are examples of prop roots, so named because they support tall, topheavy plants. All roots of a mature maize plant are adventitious after the original roots die. The emerging roots shown here will eventually penetrate the soil.

(b) Storage roots. Many plants, such as sweet potatoes, store food and water in their roots.

(d) Buttress roots. Aerial roots that look like buttresses support the tall trunks of some tropical trees, such as this ceiba tree in Central America.

714

U N I T six

Plant h o r m and Function

(c) "Strangling" aerial roots. The seeds of this strangler fig germinate in the branches of tall trees and send numerous aerial roots to the ground. These snake-like roots gradually wrap around the hosts and objects such as this Cambodian temple ruin. Eventually, the host tree dies of strangulation and shading.

(e) Pneumatophores. Also known as air roots, pneumatophores are produced by trees such as mangroves that inhabit tidai swamps. By projecting above the surface, they enable the root system to obtain oxygen, which is lacking in the thick, waterlogged mud.

Si ems A stem is an organ consisting of an alternating system of nodes, ME points at which leaves are attached, and internodes, the st :m segments between nodes (see Figure 35.2). In the angle (axil) formed by each leaf and the stem is an axillary bud, a structure that has the potential to form a lateral shoot, comrr pnly called a branch. Most axillary buds of a young shoot are dormant (not growing). Thus, elongation of a young shoot is usually concentrated near the shoot apex (tip), which consists o: a terminal bud with developing leaves and a compact series of nodes and internodes. The proximity of the terminal bud is partly responsible lor irhibiting the growth of axillary buds, a phenomenon called a deal dominance. By concentrating resources toward elontion1 apical dominance is an evolutionary adaptation that creases the plant's exposure to light. But what if an animal its the end of the shoot? Or what if, because of obstructions, li jh,t is more intense to the side of a plant than directly above ? Under such conditions, axillary buds break dormancy; that , they start growing. A growing axillary bud gives rise to a It teral shoot, complete with its own terminal bud, leaves, and

Figure 35.5 Modified stems.

axillary buds. Removing the terminal bud usually stimulates the growth of axillary buds, resulting in more lateral shoots. That is why pruning trees and shrubs and "pinching back" houseplants will make them "bushier." Modified stems with diverse functions have evolved in many plants as environmental adaptations. These modified stems, which include stolons, rhizomes, tubers, and bulbs, are often mistaken for roots (Figure 35.5). Leaves The leaf is the main photosynthetic organ of most vascular plants, although green stems also perform photosynthesis. Leaves vary extensively in form but generally consist of a flattened blade and a stalk, the petiole, which joins the leaf to a node of the stem (see Figure 35.2). Among angiosperms, grasses and many other monocots lack petioles; instead, the base of the leaf forms a sheath that envelops the stem. Some monocots, including palm trees, do have petioles. Monocots and eudicots differ in the arrangement of veins, the vascular tissue of leaves. Most monocots have parallel major veins that run the length of the leaf blade. In contrast,

(a) Stolons. Shown here on a strawberry plant, stolons are horizontal stems that grow along the surface. These "runners" enable a plant to produce asexually, as

(d) Rhizomes. The edible base of this ginger plant is an example of a rhizome, a horizontal stem that grows just below the surface or emerges and grows along the surface.

Rhizome i) Bulbs. Bulbs are vertical, underground shoots consisting mostly of the enlarged bases of leaves that store food. You can see the many layers of modified leaves attached to the short stem by slicing an onion bulb lengthwise.

(c) Tubers. Tubers, such as these red potatoes, are enlarged ends of rhizomes specialized for storing food. The "eyes" arranged in a spiral pattern around a potato are clusters of axillary buds that mark the nodes.

CHAPTER 35

-'A

Plant Structure, Growth, and Development

715

eudicot leaves generally have a multibranched network of major veins (see Figure 30.12). In identifying and classifying angiosperms, taxcmomists rely mainly on floral morphology, but they also use variations in leaf morphology, such as leaf shape, spatial arrangement of leaves, and the pattern of a leaf's veins. Figure 35.6 illustrates a difference in leaf shape: simple versus compound leaves. Most very large leaves are compound or doubly compound. This structural adaptation may enable large leaves to withstand strong wind with less tearing and also confine some pathogens that invade the leaf to a single leaflet, rather than allowing them to spread to the entire leaf. Most leaves are specialized for photosynthesis. However, some plant species have leaves that have become adapted by evolution for other functions, such as support, protection, storage, or reproduction (Figure 35.7).

• Figure 35.7 Modified leaves (a) Tendrils. The tendrils by which this pea plant clings to a support are modified leaves. After it has "lassoed" a support, a tendril forms a coi that brings the plant closer to the support Tendrils are typical modified leaves, b some tendrils are modified stems, as in grapevines.

(a) Simple leaf. A simple leaf is a single, undivided blade. Some simple leaves are deeply lobed, as in an oak leaf.

Leaflet (b) Compound leaf, in a compound leaf, the blade consists of multiple leaflets. Notice that a leaflet has no axillary bud at its base.

(c) Doubly compound leaf. in a doubly compound leaf, each leaflet is divided into smaller leaflets.

Leaflet - Petiole -Axillary bud

A Figure 35.6 Simple versus compound leaves. You can distinguish simple leaves from compound leaves by looking for axillary buds. Each leaf has only one axillary bud, where the petiole attaches to the stem.

716

UNIT six

Plant Form and Function

(e) Reproductive leaves. The leaves of some succulents, such as Kalanchoe daigremontiana, produce adventitious plantlets, which fal off the leaf and take root in the soil.

The Three Tissue Systems: Dermal, Vascular, a tid Ground tch plant organ—root, stem, or leaf—has dermal, vascular, id ground tissues. A tissue system consists of one or more Ssues organized into a functional unit connecting the organs a plant. Each tissue system is continuous throughout the tire plant body, but specific characteristics of the tissues and eir spatial relationships to one another vary in different orins (Figure 35.8). The dermal tissue system is the outer protective covering, ke our skin, it forms the first line of defense against physical .mage and pathogenic organisms. In nonwoody plants, the .rmal tissue usually consists of a single layer of tightly packed 11s called the epidermis. In woody plants, protective tissues iown as periderm replace the epidermis in older regions of ems and roots by a process discussed later in the chapter. In dition to protecting the plant from water loss and disease, e epidermis has specialized characteristics in each organ. For cample, the root hairs so important in the absorption of water id minerals are extensions of epidermal cells near root tips. In e epidermis of leaves and most stems, a wax)' coating called e cuticle helps prevent water loss—an important adaptation

to living on land. Later we will look at specialized leaf cells that regulate CO2 exchange. Leaftrichom.es, which are outgrowths of the epidermis, are yet another example of specialization. For instance, trichomes in aromatic leaves such as mint secrete oils that protect plants from herbivores and disease. The vascular tissue system carries out long-distance transport of materials between roots and shoots. The two vascular tissues are xylem and phloem. Xylem conveys water and dissolved minerals upward from roots into the shoots. Phloem transports organic nutrients such as sugars from where they are made (usually the leaves) to where they are needed—-usually roots and sites of growth, such as developing leaves and fruits. The vascular tissue of a root or stem is collectively called the stele (the Greek word for "pillar"). The arrangement of the stele varies, depending on species and organ. In angiosperms, the stele of the root is in the form of a solid central vascular cylinder. In contrast, the stele of stems and leaves is divided into vascular bundles, strands consisting of xylem and phloem. Both xylem and phloem are composed of a variety of cell types, including cells highly specialized for transport. Tissues that are neither dermal nor vascular are part of the ground tissue system. Ground tissue that is internal to the vascular tissue is called pith, and ground tissue that is external to the vascular tissue is called cortex. The ground tissue system is more than just filler. It includes various cells specialized for functions such as storage, photosynthesis, and support.

Common Types of Plant Cells Like any multicellular organism, a plant is characterized by cellular differentiation, the specialization of cells in structure and function. In plant cells, differentiation is sometimes evident within the protoplast, the cell contents exclusive of the cell wall. For example, the protoplasts of some plant cells have chloroplasts, whereas other types of plant cells lack functional chloroplasts. Cell wall modifications also play a role in plant cell differentiation. Figure 35.9, on the next two pages, focuses on some major types of plant cells: parenchyma, collenchyma, sclerenchyma, the water-conducting cells of the xylem, and the sugarconducting cells of the phloem. Notice the structural adaptations that make specific functions possible. You may wish to review Figures 6.9 and 6.28, which show basic plant cell structure.

Concept Check Ground ' tissue

Vascular tissue

Figure 35.8 The three tissue systems. The dermal tissue ,tem (blue) covers the entire body of a plant. The vascular tissue system (purple) is also continuous throughout the plant, but is arranged differently in each organ. The ground tissue system (yellow), responsible for most of the plant's metabolic functions, is located between the dermal tissue and the vascular tissue in each organ.

1. How does the vascular tissue system enable leaves and roots to combine functions to support growth and development of the whole plant? 2. Describe at least three specializations in plant organs and plant cells that are adaptations to life on land. 3. Describe the role of each tissue system in a leaf. For suggested answers, see Appendix A.

CHAPTER 35

Plant Structure, Growth, and Development

717

Examples of Differentiated Plant Cells PARENCHYMA CELLS Mature parenchyma cells have primary walls that are relatively thin and flexible, and most lack secondary walls. (See Figure 6.28 to review primary and secondary layers of cell walls.) The protoplast generally has a large central vacuole. Parenchyma cells are often depicted as "typical" plant cells because they appear to be the least specialized stiTicturally Parenchyma cells perform most of the metabolic functions of the plant, synthesizing and storing various organic products. For example, photosynthesis occurs within the chloroplasts of parenchyma cells in the leaf. Some parenchyma cells in stems and roots have colorless plastids that store starch. The fleshy tissue of a typical fruit is composed mainly of parenchyma cells. Most parenchyma cells retain the ability to divide and differentiate into other types of plant cells under special conditions— during the repair and replacement of organs after injury to the plant, for example. It is even possible in the laboratory to regenerate an entire plant from a single parenchyma cell. Parenchyma cells in Eh

with chioroplasts

COLLENCHYMA CELLS Grouped in strands or cylinders, coUenchyma cells help support young parts of the plant shoot. CoUenchyma cells have thicker primary walls than parenchyma cells, though the walls are unevenly thickened. Young stems and petioles often have strands of coUenchyma cefls just below their epidermis (the "strings" of a celery stalk, for example). CoUenchyma cells lack secondary walls, and the hardening agent lignin is absent in their primary walls. Therefore, they provide flexible support without restraining growth. At functional maturity, coUenchyma cells are living and flexible, elongating with the stems and leaves they support—unlike sclerenchyma cells, which we discuss next.

80 urn

CoUenchyma cells (in cortex of Sambucus, elderberry; cell walls stained red)

SCLERENCHYMA CELLS Also functioning as supporting elements in the plant, but with thick secondary walls usually strengthened by lignin, sclerenchyma cells are much more rigid than coUenchyma cells. Mature sclerenchyma cells cannot elongate, and they occur in regions of the plant that have stopped growing in length. Sclerenchyma cells are so specialized for support that many are dead at functional maturity, but they produce secondary walls before the protoplast dies. The rigid walls remain as a "skeleton" that supports the plant, in some cases for hundreds of years. In parts of the plant that are still elongating, the secondary walls of immature sclerenchyma are deposited unevenly in spiral or ring patterns. These forms of cell wall thickenings enable the cell wall to stretch like a spring as the cell elongates.

Cell wa Two types of sclerenchyma ceUs called sclereids and fibers are specialized entirely for support and strengthening. Sclereids, which are shorter than fibers and irregular in shape, have very thick, lignified secondary walls. Sclereids impart the hardness to nutshells and seed coats and the gritty texture to pear fruits. Fibers, which are usually arranged in threads, are long, slender, and tapered. Some are used commercially, such as hemp fibers for making rope and flax fibers for weaving into linen.

718

UNIT six

Plant Form and Function

The two types of water-conducting cells, tracVieids and vessel elements, ars tubular, elongated cells that are dead at functional maturity. Tracheids are found in the xyleni of all vascular plants. In addition to tracheids, most angiosperms, as well as a few gymnosperms and a few seedless vascular plants, have vessel elements. When the protoplast of a traeheid or vessel element disintegrates, the cells thickened cell walls remain behind, forming a nonliving conduit through, which water can. flow. The se ^ondary walls of iracheids and vessel elements are often interrupted by pi s, thinner regions where only primary walls are present (see Figure 6. i8 to review primary and secondary walls). Water can migrate laterally between neighboring cells through pits. Tracheids are long, thin cells with tapered ends. Water moves from cell to cell mainly through the pits, where it does not have to cross thick secoadary walls. The secondary walls of tracheids are hardened with lignin, wkich prevents collapse under the tensions of water transport and also provides support. Vessel elements are generally wider, shorter, thinner walled, and less tapered than tracheids. They are aligned end to end, forming long micropipes known as vessels. The end walls of vessel elements have perforations, enabling water to flow freely through, the vessels.

Vessel elements with partially perforated end walls SUGAR-CONDUCTING CELLS OF THE PHLOEM Unlike the water-conducting cells of the xylem, the sugar-conducting cells of the phloem are alive at functional maturity. In seedless vascular rjlants and gymnosperms, sugars and other organic nutrients are transported through long, narrow cells called sieve cells. In the phloem of ansiosperms, these nutrients are transported through, sieve tubes, which cprisist of chains of cells called sieve-tube members. Though alive, sieve-tube members lack such organdies as the nucleus, ribosomes, and a distinct vacuole. "This reduction in cell contents enables nutrients to pass more easily through the cell. The end walls between sieve-tube members, called sieve plates, have pores that facilitate the flow of fluid from cell to cell along the sieve tjube. Alongside each sieve-tube member is a nonconducting cell called a companion cell, which is connected do the sieve-tube member by numerous channels, the flasmodesmata (see Figure 6.8). The nucleus and ribosomes of the companion cell may serve not only that cell Lself but also the adjacent sieve-tube member. In some lants, companion cells in leaves also help load sugars nto the sieve-tube members, which then transport the ugars to other parts of the plant.

Sieve-tube members: | longitudinal view

Sieve-tube members: longitudinal view

Sieve plate with pores

CHAPTER 35 Plant Structure, Growth, and Development

719

Concept

Meristems generate cells for new organs So far, we have looked at the structure and arrangement of plant tissues and cells in mature organs. But how does this organization arise? A major difference between plants and most animals is that plant growth is not limited to an embryonic or juvenile period. Tnstead, growth occurs throughout the plant's life, a condition known as indeterminate growth. At any given time, a typical plant consists of embryonic, developing, and mature organs. Except for periods of dormancy, most plants grow continuously In contrast, most animals and some plant organs, such as most leaves, undergo determinate growth; that is, they cease growing after reaching a certain size. Although plants continue to grow throughout their lives, they do die, of course. Based on the length of their life cycle, flowering plants can be categorized as annuals, biennials, or perennials. Annuals complete their life cycle—from germination to flowering to seed production to death—-in a single year or less. Many wildflowers are annuals, as are the most important food crops, including the cereal grains and legumes. Biennials generally live two years, often including an intervening cold period (winter) between vegetative growth (first spring/summer) and flowering (second spring/summer). Beets and carrots are bienni-

als but are rarely left in the ground long enough to flows Perennials live many years and include trees, shrubs, and son grasses. Some buffalo grass of the North American plains is I lieved to have been growing for 10,000 years From seeds th sprouted at the close of the last ice age. When a perennial dies is not usually from old age, but from an infection or some err ronmental trauma, such as fire or severe drought. Plants are capable of indeterminate growth because th have perpetually embryonic tissues called meristems. The are two main types; apical meristems and lateral meriste Apical meristems, located at the tips ol roots and in the bu of shoots, provide additional cells that enable the plant to grc in length, a process known as primary growth. Primary grow allows roots to extend throughout the soil and shoots to i crease exposure to light and CO2. In herbaceous (nonwooc plants, primary growth produces all, or almost all, of the pla body Woody plants, however, grow in girth in the parts stems and roots where primary growth has ceased. This grow in thickness, known as secondary growth, is caused by the z tivity of lateral meristems called the vascular cambium ai cork cambium. These cylinders of dividing cells extend alo" the length of roots and stems (Figure 35.10). The vascul cambium adds layers of vascular tissue called secondary xyle (wood) and secondary phloem. The cork cambium replac the epidermis with periderm, which is thicker and tougher. The ceils within meristems divide relatively frequent generating additional cells. Some products of this divisio

Primary growth in stems /Epidermis

Secondary growth in stems /Periderm

The vascular cambium adds secondary xylem and phloem. Vascular cambium A Figure 35.10 An overview of primary and secondary growth. 720

U N I T six

Plant Form and Function

lain in the meristem and produce more cells, while others erentiate and are incorporated into tissues and organs of growing plant. Cells that remain as sources of new cells are called initials. The new cells displaced from the meristem, i Led derivatives, continue to divide until the cells they prodi ce become specialized within developing tissues. In woody plants, primary and secondary growth occur nultaneously but in different locations. Each growing sean, primary growth near the apical meristenis produces iung extensions of roots and shoots, while lateral merims produce secondary growth that thickens and strengthens olier parts of the plant (Figure 35.11). The oldest regions, i: :h as a tree trunk base, have the most accumulation of sues produced by secondary growth.

< Figure 35.11 Three years' past growth evident in a winter twig. This year's growth (one year old)

Scars left by terminal bud scales of previous winters

Concept Check 1

1. Cells in lower layers of your skin divide and replace dead cells sloughed from the surface. Why is it inaccurate to compare such regions of cell division to a plant meristem? 2. Contrast the types of growth arising from apical and lateral meristems. For suggested answers, see Appendix A.

Vascular cylinder

Concept

I*rimary growth lengthens oots and shoots Primary growth produces the primary plant body, the parts of the root and shoot systems produced by ttical meristems. in herbaceous plants, the primary P ant body is usually the entire plant. In woody plants, it consists only of the youngest parts, which have not yet become woody. Although apical merisms lengthen both roots and shoots, there are differences in the primary growth of these two systems.

I 'rimary Growth of Roots ie root tip is covered by a thimble-like root cap, hich protects the delicate apical merisLem as the ot pushes through the abrasive soil during priary growth. The root cap also secretes a polysaclaride slime that lubricates the soil around the root i. Growth occurs just behind the root tip, in three mes of cells at successive stages of primary owth. Moving away from the root tip, they are the nes of cell division, elongation, and maturation igure 35.12).

Root cap -

100 urn

W

A Figure 35.12 Primary growth of a root. The diagram and light micrograph take us into the tip of an onion root. Mitosis is concentrated in the zone of cell division, where the apical meristem and its immediate products are located. The apical meristem also maintains the root cap by generating new cells that replace those that are sloughed off. Most lengthening of the root is concentrated in the zone of elongation. Cells become functionally mature in the zone of maturation. The zones grade into one another without sharp boundaries.

CHAPTER 35 Plant Structure, Growth, and Development

721

The three zones of cells grade together, with no sharp boundaries. The zone of cell division includes the root apical menstem and its derivatives. New root cells are produced in this region, including the cells of the root cap. In the zone of elongation, root cells elongate, sometimes to more than ten times their original length. Cell elongation is mainly responsible for pushing the root tip farther into the soil. Meanwhile, the root apical meristem keeps adding cells to the younger end of the zone of elongation. Even before the root cells finish lengthening, many begin specializing in structure and function. In the zone of maturation, cells complete their differentiation and become functionally mature. The primary growth of roots produces the epidermis, ground tissue, and vascular tissue. The light micrographs in Figure 35.13 show in transverse (cross) section the three primary tissue

systems in the young roots of a eudicot (Ranunculus, butterct and a monocot (Zea, maize). Water and minerals absorbed fr< the soil must enter through the epidermis, a single layer of ce covering the root. Root hairs enhance this process by greatly creasing the surface area of epidermal cells. In most roots, the stele is a vascular cylinder, a solid core xylem and phloem (see Figure 35.13a). The xylem radia from the center in two or more spokes, with phloem devek ing in the wedges between the spokes. In many monocot roc the vascular tissue consists of a central core of parenchyi cells surrounded by alternating rings of xylem and phloem (< Figure 35.13b). The central region is often called pith b should not be confused with stem pith, which is ground tissi The ground tissue of roots, consisting mostly of parenchyi cells, fills the cortex, the region between the vascular cylinc

WMi

(a) Transverse section of a typical root. In the roots of typical gymnosperms and eudicots, as well as some monocots, the stele is a vascular cylinder consisting of a lobed core of xylem with phloem between the lobes.

(b) Transverse section of a root with parenchyma in the center. The stele of many monocot roots is a vascular cylinder with a core of parenchyma surrounded by a ring of alternating xylem and phloem

A Figure 35.13 Organization of primary tissues in young roots.

50 urn

722

U N I T six

Plant Form and Function

Parts (a) and (b) show the three primary tissue systems in the roots of Ranunculus (buttercup) and Zea (maize), respectively. Note that the buttercup root has a central core of xylem and phloem, whereas the maize root has a core of parenchyma cells. These are two basic patterns of root organization, of which there are many variations, depending on the plant species. (All LMs)

Figure 35.14 The formation of a lateral r o o t . A lateral root originates in ithe pericycle, the outermost er of the vascular cylinder of oot, and grows out through i cortex and epidermis, in this ies of micrographs, the view the original root is a nsverse section, while the w of the lateral root is a igitudinal section.

d epidermis. Cells within e ground tissue store ornic nutrients, and their asma membranes absorb nerals from the soil solun. The innermost layer of cortex is called the dodermis, a cylinder one 1 thick that forms the undary with the vascular inder. You will learn in Ciapter 36 how the endo:rmis is a selective barrier gulating passage of subnces from the soil solution 0 the vascular cylinder. Lateral roots arise from within the pericycle, the outermost 1 layer in the vascular cylinder (see Figure 35.13). A lateral it elongates and pushes through the cortex and epidermis til it emerges from the established root (Figure 35.14). It inot originate near the root's surface because it must remain nnected with the vascular cylinder of the established root as t ol the continuous vascular tissue system.

Apical meristem

Leaf primordia

P rimary Growth of Shoots shoot apical meristem is a dome-shaped mass of dividing Is at the tip of the terminal bud (Figure 35.15). Leaves arise leaf primordia (singular, primordium), finger-like projecns along the Hanks of the apical meristem. Axillary buds velop from islands of meristematic cells left by the apical ristem at the bases of the leaf primordia. Axillary buds can m lateral shoots at some later time (see Figure 35.2). Within a bud, leaf primordia are crowded close together cause internodes are very short. Most of the actual elongan of the shoot occurs by the growth in length of slightly er internodes below the shoot apex. This growth is due to ;h cell division and cell elongation within the internode. Tie plants, including grasses, elongate all along the shoot ;ause there are meristematic regions called intercalary ristems at the base of each leaf. That is why grass continues ;row after being mowed.

0.25 mm

A Figure 35.15 The terminal bud and primary growth of a s h o o t . Leaf primordia arise from the flanks of the apical dome. This is a longitudinal section of the shoot tip of Coleus (LM).

"e, Growth, and Development

723

Tissue Organization of Stems The epidermis covers stems as part of the continuous dermal tissue system. Vascular tissue runs the length of a stem in vascular bundles. Unlike lateral roots, which arise from vascular tissue deep within a root (see Figure 35.14), lateral shoots arise from preexisting axillary buds on the surface of a stem (see Figure 35.15). The vascular bundles of the stem converge with the roots vascular cylinder in a zone of transition located near the soil surface. In gymnosperms and most eudicots, the vascular tissue consists of vascular bundles arranged in a ring (Figure 35.16a). The xylem in each vascular bundle faces the pith, and the phloem faces the cortex. In most monocot stems, the bundles are scattered throughout the ground tissue, rather than forming a ring (Figure 35.16b). In both monocot and eudicot stems, ground tissue consists mostly of parenchyma, but collenchyma cells just beneath the epidermis strengthen many stems. Sclerenchyma cells, specifically fiber cells within vascular bundles, also provide support. Tissue Organization of Leaves Figure 35,17 provides an overview of leaf structure. The epidermal barrier is interrupted by the stomata (singular, sLoma), which allow CO2 exchange between the surrounding air and the photosynthetic cells inside the leaf. The term stoma can refer to the stomatal pore or to the entire stomatal complex consisting of a pore flanked by two guard cells, which regulate the opening and closing of the pore. In addition to regulating

C0 2 exchange, stomata are major avenues for the evaporam t loss of water, as you will see in Chapter 36. The ground tissue is sandwiched between the upper an 1

lower epidermis, a region called the mesophyll (from th: Greek mesos, middle, and phyll, leaf). Mesophyll consists mainly of parenchyma cells specialized for photosynthes The leaves of many eudicots have two distinct areas: palisa* mesophyll and spongy mesophyll. The palisade mesophy or palisade parenchyma, consists of one or more layers elongated cells on the upper part of the leaf. The spongy me ophyll, also called spongy parenchyma, is below the palisa< mesophyll. These cells are more loosely arranged, with labyrinth of air spaces through which CO2 and oxygen circ late around the cells and up to the palisade region. The a spaces are particularly large in the vicinity of stomata, whe gas exchange with the outside air occurs. The vascular tissue of each leaf is continuous with the va cular tissue of the stem. Leaf traces, connections from vase lar bundles in the stem, pass through petioles and into leave Veins are the leaf's vascular bundles, which subdivide repea edly and branch throughout the mesophyll. This netwo. brings xylem and phloem into close contact with the phot< synthetic tissue, which obtains water and minerals from th xylem and loads its sugars and other organic products int ) the phloem for shipment to other parts of the plant. The va? cular structure also functions as a skeleton that reinforces tl shape of the leaf. Each vein is enclosed by a protects bundle sheath, consisting of one or more cell layers, usual consisting of parenchyma.

Epidermis bundle 1 mm (a) A eudicot stem. A eudicot stern (sunflower), with vascular bundles forming a ring. Ground tissue toward the inside is called pith, and ground tissue toward the outside is called cortex. (LM of transverse section)

A Figure 35.16 Organization of primary tissues in young stems. 724

U N I T six

Plant Form and Function

1 mm (b) A monocot stem. A monocot stem (maize) with vascular bundles scattered throughout the ground tissue. In such an arrangement, ground tissue is not partitioned into pith and cortex. (LM of transverse section)

(b) Surface view of a spiderwort {Tradescantia) leaf (LM)

(c) Transverse section of a lilac (Syringa) leaf (LM)

Concept Check 1. Describe how roots and shoots differ in their branching. 2. Contrast primary growth in roots and shoots. j 3. Describe the functions ol leaf veins. For suggested answers, see Appendix A,

Primary and secondary growth occur simultaneously but in different regions. While an apical meristem elongates a stem or root, secondary growth commences where primary growth has stopped, occurring in older regions of all gymnosperm species and many eudicots, but rarely in monocots. The process is similar in stems and roots, which look much the same after extensive secondary growth. Figure 35.18, on the next page, provides an overview of growth in a woody stem.

The Vascular Cambium and Secondary Vascular Tissue

Concept

J Secondary growth adds girth to terns and roots in woody plants condary growth, the growth in thickness produced by latal meristems, occurs in stems and roots of woody plants, but irely in leaves. The secondary plant body consists of the tisles produced by the vascular cambium and cork cambium. The vascular cambium adds secondary xylem (wood) and secndary phloem. Cork cambium produces a Lough, thick coving consisting mainly of cork cells.

The vascular cambium is a cylinder of meristematic cells one cell thick. It increases in circumference and also lays down successive layers of secondary xylem to its interior and secondary phloem to its exterior, each layer with a larger diameter than the previous layer (see Figure 35.18). In this way, it is primarily responsible for the thickening of a root or stem. The vascular cambium develops from undifferentiated cells and parenchyma cells that regain the capacity to divide. In a typical gymnosperm or woody eudicot stem, the vascular cambium forms in a layer between the primary xylem and primary phloem of each vascular bundle and in the ground tissue between the bundles. The meristematic bands within and between the CHAPTER

35

Plant Structure, Growth, and Development

725

(a) Primary and secondary growth in a two-year-old stem

Q In the youngest part of the stem, you can see the primary plant body, as formed by the apical meristem during primary growth. The vascular cambium

Epidermis-

Q As primary growth continues to elongate the stem, the portion of the stem formec earlier the same year has already started secondary growth. This portion increases in girth as fusiform initials of the vascula cambium form secondary xylem to the inside and secondary phloem to the outside.

i5 beginning to telop. Pith/

Cortex •

Primary xylem Vascular cambium Primary phloem Corte: Epidermis Phloem ray

0 The ray initials of the vascular cambium give rise to the xylem and phloem rays. 0 As the diameter of the vascular cambiurr increases, the secondary phloem and oth tissues external to the cambium cannot keep pace with the expansion because th cells no longer divide. As a result, these tissues, including the epidermis, rupture. A second lateral meristem, the cork cairn biurn, develops from parenchyma cells in the cortex. The cork cambium produces cork cells, which replace the epidermis.

Primary xylem Secondary x Vascular car Secondary lary phi' Primal ary phli O First cork cambium'

0 In year 2 of secondary growth, the vascular cambium adds to the secondary xylem and phloem, and the cork cambium produces cork. 0 As the diameter of the stem continues to increase, the outermost tissues exterior to the cork cambium rupture and slough off from the stem. 0 Cork cambium re-forms in progressively deeper layers of the cortex. When none of the original cortex is left, the cork cambium develops from parenchyma cells in the secondary phloem.

Secondary xylem (two years of production^

0 Each cork cambium and the tissues it produces form a layer of periderm.

Vascular cambium Secondary phloem 0

0 Bark consists of all tissues exterior to the vascular cambium.

Most recenr cork cambium O

0 Cork

Layers of periderrn

Secondary phloem Vascular cambiurr Secondary xylem

/ l a t e wood

A Figure 35,18 Primary and secondary g r o w t h of a stem. You can track the progress of secondary growth by examining the sections through sequentially older parts of the stem. (You would observe the same changes if you could follow the youngest region, near the apex, for the next three years.)

726

UNIT six

Plant Form and Function

(b) Transverse section of a three-yearold stem (LM)

Types of cell division. An initial can divide transversely to form two cambia! initials (C) or radially to form an initial and either a xylem (X) or phloem (P) cell.

Accumulation of secondary growth. Although shown here as alternately adding xylem and phloem, a cambial initial usually produces much more xylem.

Figure 35.19 Cell division in the vascular cambium.

scular bundles unite to become a continuous cylinder of diding cells. In a typical gymnosperm or woody eudicot root, e vascular cambium forms in segments between the primary loem, the lobes o) primary xylem, and the pericycle, evenally becoming a cylinderViewed in transverse section, the vascular cambium apars as a ring, with interspersed regions of cells called aform initials and ray initials. When these initials divide, ey increase the circumference of the cambium itself and add econdary xylem to the inside of the cambium and secondary hloem to the outside (Figure 35.19). Fusiform initials pro.uce elongated cells such as the tracheids, vessel elements, and bers of the xylem, as well as the sieve-tube members, companm. cells, parenchyma, and fibers of the phloem. They have taered (fusiform) ends and are oriented parallel to the axis of a lem or root. Ray initials, which are shorter and oriented perpendicular to the stem or root axis, produce vascular rays— idial files consisting mainly of parenchyma cells. Vascular rays re living avenues that move water and nutrients between the econdary xylem and secondary phloem. They also store starch nd other organic nutrients. The portion of a vascular ray tested in the secondary xylem is known as a xylem ray The poion located in the secondary phloem is called a phloem ray. As secondary growth continues over the years, layers of econdary xylem (wood) accumulate, consisting mainly of racheids, vessel elements, and fibers (see Figure 35.9). GymQsperms have tracheids, whereas angiosperms have both traleids and vessel elements. Dead at functional maturity, both ypes of cells have thick, lignifled walls that give wood its hardess and strength. Tracheids and vessel elements that develop

early in the growing season, typically in early spring, are known as early wood and usually have relatively large diameters and thin cell walls (see Figure 35.18b). This structure maximizes deliver)' of water to new, expanding leaves. Tracheids and vessel elements produced later in the growing season, during late summer or early fall, are known as late wood. They are thickwalled cells thai do not transport as much water but do add more support than do the thinner-walled cells of early wood. In temperate regions, secondary growth in perennial plants is interrupted each year when the vascular cambium becomes dormant during winter. When growth resumes the next spring, the boundary between the large cells of the new early wood and the smaller cells of the late wood produced during the previous growing season is usually a distinct ring in the transverse sections of most tree trunks and roots. Therefore, a tree's age can be estimated by counting its annual rings. The rings can have varying thicknesses, reflecting the amount of seasonal growth. As a tree or woody shrub ages, the older layers of secondary xylem no longer transport water and minerals (xylem sap). These layers are called heartwood because they are closer to the center of a stem or root (Figure 35.20). The outer layers still transport xylem sap and are therefore known as sapwood. That is why a large tree can still survive even if the center of its trunk is hollow. Because each new layer of secondary xylem has a larger circumference, secondary growth enables the xylem to transport more sap each year, supplying an increasing number of leaves. Heartwood is generally darker than sapwood because of resins and other compounds that clog the cell cavities and help protect the core of the tree from fungi and wood-boring insects. Only the youngest secondary phloem, closest to the vascular cambium, functions in sugar transport. As a stem or root increases in circumference, the older secondary phloem is sloughed oft, which is why secondary phloem does not accumulate as extensively as does secondary xylem.

Growth ring

rHeartwood^-%—•>— Secondary < x

y|em

[sapwooc

—--

Vascular cambium Secondary phloem Bark Layers of peri derm

A Figure 35.20 Anatomy of a tree trunk. CHAPTER 35

Plant Structure, Growth, and Development

727

Cork Cambia and the Production of Periderm During the early stages of secondary growth, the epidermis is pushed outward, causing it to split, dvy, and fall off the stem or root. It is replaced by two tissues produced by the first cork cambium, which arises in the outer cortex in stems (see Figure 35.18a) and in the outer layer of the pericycle in roots. One tissue, called phelloderm, is a thin layer of parenchyma cells that forms to the interior of the cork cambium. The other tissue consists of cork cells that accumulate to the exterior of the cork cambium. As cork cells mature, they deposit a waxy material called suberin m their walls and then die. The cork tissue then functions as a barrier that helps protect the stem or root from water loss, physical damage, and pathogens. A cork cambium and the tissues it produces comprise a layer of periderm. Because cork cells have suberin and are usually compacted together, most of the periderm is impermeable to water and gases, unlike the epidermis. Therefore, in most plants, absorption of water and minerals takes place primarily in the youngest parts of roots, mainly the root hairs. The older parts anchor the plant and transport water and solutes between roots and shoots. Dotting the periderm are small, raised areas called lenticels, in which there is more space between the cork cells, enabling living cells within a woody stem or root to exchange gases with the outside air. Unlike the vascular cambium, cells of the cork cambium do not continue Lo divide; thus, there is no increase in its circumference. Instead, the thickening of a stem or root splits the first cork cambium, which loses its meristematic activity and differentiates into cork cells. A new cork cambium forms to the inside, resulting in another layer of periderm. As this process continues, older layers of periderm are sloughed off, as is evident in the cracked, peeling bark of many tree trunks. Many people think that bark is only the protective outer covering of a woody stem or root. Actually bark includes all tissues external to the vascular cambium. In an outward direction, its main components are the secondary phloem (produced by the vascular cambium), the most recent periderm, and all the older layers of periderm (see Figure 35.20).

Concept Check 1. A sign is hammered into a tree 2 m from the trees base. If the tree is 10 m tall and elongates 1 m each year, how high will the sign be after 10 years? 2. A tree can survive even if a tunnel is cut through its center. However, removing a complete ring of bark around the trunk (a process called girdling) will kill the tree. Explain why For suggested answers, see Appendix A.

728

UNIT six

PlariL Form and Function

Concept

Growth, morphogenesis, and differentiation produce the plant body So far, we have described the development of the plant boc y from meristems. Here we will move from a description if plant growth and development to the mechanisms that u: derlie these processes. Consider a typical annual weed. It may consist of billioi of cells—some large, some small, some highly specialized and others not, but all derived from a single fertilized egg. The it crease in mass, or growth, that occurs during the life of the plai results from both cell division and cell expansion, but what co: trols these processes? Why do leaves stop growing upon reac. ing a certain size, whereas apical meristems divide perpetuall Also, note that the billions of cells in our hypothetical weed a not an undifferentiated clump of cells. They are organized in recognizable tissues and organs. Leaves arise from nodes; roo (unless adventitious) do not. Epidermis forms on the exterior (if the leaf, and vascular tissue in the interior—never wee versa The development of body form and organization is call© i morphogenesis. Each cell in the plant body contains tr. same set of genes, exact copies of the genome present in tr. fertilized egg. Different patterns of gene expression amon cells cause the cellular differentiation that creates a diversi of cell types (see Chapter 21). The three development; 1 processes of growth, morphogenesis, and cellular different^ tion act in concert to transform the fertilized egg into a plan

Molecular Biology: Revolutionizing the Study of Plants Modern molecular techniques are helping plant biologists e> plore how growth, morphogenesis, and cellular differentiatioj give rise to a plant. In the current renaissance in plant biolog new laboratory methods coupled with clever choices of e; perimental organisms have catalyzed a research explosior One research focus is Arabidopsis thaliana, a little weed of th' mustard family that is small enough to allow researchers t cultivate thousands of plants in a few square meters of la space (Figure 35.21). Us short generation span, about si weeks from germination to flowering, makes Arabidopsis a excellent model for genetic studies. Plant biologists also favc the tiny amount of DNA per cell, among the smallest genomt of all known plants. As a result of these attributes, Arabidops was the first plant to have its entire genome sequenced—a six year, multinational effort. Ai'abidopsis possesses about 26.000 genes, but many o" these are duplicates. There are probably fewer than 15,00* different types of genes, a level of complexity similar to tha

foand in the fly Drosophila. Knowing what some of the Arabid jpsis genes do has already expanded our understanding of plant development (see Figure 35.21). To fill the gaps in our owledge, plant biologists have launched an ambitious quest determine the function of every one of the plant's genes by tr e year 2010. Toward this end, they are attempting to create t utants for every gene in the plant's genome. We will discuss Cell organization and biogenesis (1.7%) v

DNA metabolism (1.8%)

some of these mutants shortly as we take a closer look at the molecular mechanisms that underlie growth, morphogenesis, and cellular differentiation. By identifying each gene's function and tracking every chemical pathway, researchers aim to establish a blueprint for how plants are built, a major goal of systems biology, as discussed by Natasha Raikhel in the interview on pages 710-711. It may one day be possible to create a computer-generated "virtual plant" that enables researchers to visualize which plant genes are activated in different parts of the plant during the entire course of development.

Carbohydrate metabolism (2.4%) known

iA

Signal transduction (2.6%) Protein biosynthesis (2.7%) Electron transport (3%) Protein modification (3.7%) Protein metabolism (5.7%)

Growth: Cell Division and Cell Expansion By increasing cell number, cell division in meristems increases the potential for growth. However, it is cell expansion that accounts for the actual increase in plant mass. The process of plant cell division is described more fully in Chapter 12 (see Figure 12.10), and Chapter 39 discusses the process of cell elongation (see Figure 39.8). Here we are more concerned with how these processes contribute to plant form.

Transcription (6.1 %) Other metabolism (6.6%) Transport (8.5%)

Figure 35.21 Arabidopsis thaliana. Owing to its small size, pid life cycle, and small genome, Arabidopsis was the first plant to h ave its entire genome sequenced (about 26,000 genes). The pie chart presents the proportion of Arabidopsis genes in different functional tegories. (Data from TAIR, The Arabidopsis Information Resource, 2004)

The Plane and Symmetry of Cell Division The plane (direction) and symmetry of cell division are immensely important in determining plant form. Imagine a single cell poised to undergo mitosis. If the planes of division of its descendants are parallel to the plane of the first cell division, a single file of cells will be produced (Figure 35.22a). At the other extreme, if the planes vary randomly, a

Division in three planes

(a) Cell divisions in the same plane produce a single file of cells, whereas cell divisions in three planes give rise to a cube.

^Developing guard cells

Asymmetricaj cell division Unspecialized epidermal cell

Unspecialized epidermal cell

Guard cei! "mother cell"

Unspecialized epidermal cell

(b) An asymmetrical cell division precedes the development of epidermal guard cells, the cells that border stomata (see Figure 35.17).

Figure 35.22 The plane and symmetry of cell division influence development of form. CHAPTER 35

Plani Structure, Growth, and Development

729

disorganized clump of cells results. Meanwhile, even though chromosomes are allocated to daughter cells equally during mitosis, the cytoplasm may divide asymmetrically. Asymmetrical cell division, in which one daughter cell receives more cytoplasm than the other during mitosis, is fairly common in plant cells and usually signals a key event in development. For example, the formation of guard cells typically involves both an asymmetrical cell division and a change in the plane of cell division. An epidermal cell divides asymmetrically to form a large cell thai remains an unspecialized epidermal cell and a small cell that becomes the guard cell "mother cell" Guard cells form when this small mother cell divides in a plane perpendicular to the first cell division (Figure 35.22b).

The plane in which a cell divides is determined during late interphase. The first sign of this spatial orientation is a rearrangement of the cytoskeleton. Microtubules in the cytoplasm become concentrated into a ring called Lhe preprophase band (Figure 35.23). The band disappears before metaphase, but it predicts the future plane of cell division. The "imprint" consists of an ordered array of actin micronlaments that remain after the microtubules disperse.

Orientation of Cell Expansion Before discussing how cell expansion contributes to plar form, it is useful to consider the difference in cell expansio between plants and animals. Animal cells grow mainly by syi thesizing protein-rich cytoplasm, a metabolically expense process. Growing plant cells also produce additional protei: rich material in their cytoplasm, but water uptake typically a counts for about 90% of expansion. Most of this water packaged in the large central vacuole formed by the coale. cence of many smaller vacuoles as a cell grows. A plant c grow rapidly and economically because water uptake enabl a small amount of cytoplasm to go a long way. Bamboo shoot for instance, can elongate more than 2 m per week. Rapid e> tension of shoots and roots increases the exposure to light an i soil, an important evolutionary adaptation to the immobi lifestyle of plants. Plant cells rarely expand equally in all directions. The greatest expansion is usually oriented along the plants mat axis. For example, cells near the root tip may elongate up t ) 20 times their original length, with relatively little increase i width. The orientation of cellulose microfibrils in the inne most layers of the cell wall causes this differential growth. Th microfibrils cannot stretch much, so the cell expands main, perpendicular to the "grain" of the microfibrils, as shown i Figure 35,24.

As with the plane of cell division, microtubules play a ke role in regulating the plane of cell expansion. It is the orient; tion of microtubules in the cell's outermost cytoplasm that termines the orientation of cellulose microfibrils that are de posited in the cell wall. Microtubules and Plant Growth

Preprophase bands , of microtubules ,

10 \m\

-Cell plates v

A Figure 35.23 The preprophase band and the plane of cell d i v i s i o n . The location of the preprophase band predicts the plane of cell division. Although the cells shown on the left and right are similar in shape, they will divide in different planes. Each cell is represented by t w o light micrographs, one (top) unstained and the other (bottom) stained with a fluorescent dye that binds specifically to microtubules. The stained microtubules form a "halo" (preprophase band) around the nucleus in the outer cytoplasm.

730

U N I T six

Plant Form and function

Studies of/ass mutants of Arabidopsis have confirmed the irr portance of cytoplasmic microtubules in cell division and pansion. The \ass mutants have unusually squat cells wit seemingly random planes of cell division. Their roots ami stems have no ordered cell files and layers. Despite these at normalities, jass mutants develop into tiny adult plants, bu their organs are compressed longitudinally (Figure 35.25) The stubby form and disorganized tissue arrangement ca be traced to abnormal organization of microtubules. Dunn interphase, the microtubules are randomly positioned, an the preprophase bands do not form prior to mitosis (se : Figure 35.23). As a result, there is no orderly "grain" of edit lose microfibrils in the cell wall to determine the direction elongation (see Figure 35.24). This defect gives rise to cell that expand in all directions and divide haphazardly.

Morphogenesis and Pattern Formation A plants body is more than a collection of dividing and ex panding cells. Morphogenesis must occur for development t

Figure 35.24 The orientation of plant If e x p a n s i o n . Growing plant cells expand ainly through water uptake. In a growing cell, izymes weaken cross-links in the cell wall, owing it to expand as water diffuses into the vacuole by osmosis. As shown in these two examples of different orientations, the orientation off cell growth is mainly in the plane perpendicular t(p the orientation of cellulose microfibrils in the 'all. The microfibrils are embedded in a matrix of her (non-cellulose) polysaccharides, some of 'hich form the cross-links visible in the micrograph (" EM). Loosening of the wall occurs when hydrogen ins secreted by the cell activate cell wall enzymes at break the cross-links between polymers in the all. This reduces restraint on the turgid cell, which n take up more water and expand. Small cuoles, which accumulate most of this water, lalesce and form the cell's central vacuole.

proceed properly; that is, cells must be organized into multicellular arrangements such as tissues and organs. The development of specific structures in specific locations is called pattern formation. Many developmental biologists postulate that pattern formation is determined by positional information in the form of signals that continuously indicate to each cell its location within a developing structure. According to this idea, each cell within a developing organ responds to positional information from neighboring cells by differentiating into a particular cell type. Developmental biologists are accumulating evidence that gradients of specific molecules, generally proteins or mRNAs, provide positional information. For example, a substance diffusing from a shoot apical meristem may "inform" the ceils below of their distance from the shoot apex. Cells possibly gauge their radial positions within the developing organ by detecting a second chemical signal that emanates from the outermost cells. The gradients of these two substances would be sufficient for each cell to ''get a fix" on its position relative to the longitudinal and radial axes of the developing organ. This idea of diffusible chemical signals is one of the hypotheses that developmental biologists are testing.

) Wild-type seedling

(c) Mature fass mutant

Figure 35.25 The fass m u t a n t of Arabidopsis c o n f i r m s t te i m p o r t a n c e of cytoplasmic m i c r o t u b u l e s to p l a n t g r o w t h . The squat body of the fass mutant results from cell division id cell elongation being randomly oriented instead of orienting in the rection of the normal plant axis.

One type of positional information is associated with polarity, the condition of having structural differences at opposite ends of an organism. Plants typically have an axis, with a root end and a shoot end. Such polarity is most obvious in morphological differences, but it is also manifest in physiological properties, including the unidirectional movement of the hormone auxin (see Chapter 39) and emergence of adventitious roots and shoots from "cuttings." Adventitious roots form within the root end of a stem cutting, and adventitious shoots arise from the shoot end of a root cutting. CHAPTER 35

Plant Structure, Growth, and Development

731

* Figure 35.26 Establishment of axial p o l a r i t y . The normal Arabidopsis seedling (left) has a shoot end and a root end. In the gnom mutant (right), the first division of the zygote was not asymmetrical: as a result, the plant is ball-shaped and lacks leaves and roots. The defect in gnom mutants has been traced to an inability to transport the hormone auxin in a polar manner.

The first division of a plant zygote is normally asymmetrical, initiating polarization of the plant body into shoot and root. Once this polarity has been induced, it becomes exceedingly difficult to reverse experimentally Therefore, the proper establishment of axial polarity is a critical step in a plant's morphogenesis. In the gnom mutant of Arabidopsis, the establishment of polarity is defective. The first cell division of the zygote is abnormal in being symmetrical, and the resulting ball-shaped plant has neither roots nor leaves (Figure 35.26). Morphogenesis in plants, as in other multicellular organisms, is often under the control of master regulatory genes called homeotic genes that mediate many of the major events in an individual's development, such as the initiation of an organ (see Chapter 21). For example, the protein product of the KNOTTED-1 homeotic gene, found in many plant species, is important in the development of leaf morphology including the production of compound leaves. If the KNOTTED-1 gene is overexpressed in tomato plants, the normally compound leaves become "super-compound" (Figure 35.27).

Gene Expression and Control of Cellular Differentiation What makes cellular differentiation so fascinating is that the cells of a developing organism synthesize different proteins

and diverge in structure and function even though they sha a common genome. The cloning of whole plants from soma cells supports the conclusion that the genome of a differen

ated cell remains intact (see Figure 21.5). If a mature c removed from a root or leaf can dedifferentiate in tissue ex ture and give rise to the diverse cell types of a plant, then must possess all the genes necessary to make any kind of pla cell. This means that cellular differentiation depends, to large extent, on the control of gene expression—the regu tion of transcription and translation leading to specific pi teins. Cells selectively express certain genes at specific tim during their differentiation. A guard cell has the genes th program the self-destruction of a vessel element protopla but it does not express those genes. A xylem vessel eleme does express those genes, but only does so at a specific time its differentiation, after the cell has elongated and has pr duced its secondary wall. Researchers are beginning to u ravel the molecular mechanisms that switch specific genes and off at critical times during a cell's development (see Cha ters 19 and 21). Cellular differentiation to a large extent depends on po tional information—where a particular cell is located relati to other cells. For example, two distinct cell types are form in the root epidermis of Arabidopsis: root hair cells and hairle epidermal cells. Cell fate is associated with the position oft epidermal cells. The immature epidermal cells that areinconta with two underlying cells of the root cortex differentiate in root hair cells, whereas the immature epidermal cells in conta with only one cortical cell differentiate into mature hairless ce" Differential expression of a homeotic gene called GLABRA (from the Latin glaber, bald) is required for appropriate ro hair distribution. Researchers have demonstrated this by co pling the GLABRA-2 gene to a "reporter gene" that caus every cell expressing GLABRA-2 in the root to turn pale bl following a certain treatment (Figure 35.28). The GLABRA gene is normally expressed only in epidermal cells that w not develop root hairs. If GLABRA-2 is rendered dysfunction by mutation, every root epidermal cell develops a root hair.

Location and a Cell's Developmental Fate

A Figure 35.27 Overexpression of a homeotic gene in leaf f o r m a t i o n . KNOTTED-1 is a homeotic gene involved in leaf and leaflet formation. Its overexpression in tomato plants results in leaves that are "super-compound" (right) compared with normal leaves (left).

732

UNIT six

Plant Form and Function

In the process oi shaping a rudimentary organ, patterns of o division and cell expansion affect the differentiation of cells 1 placing them in specific locations relative to other cells. Thu positional information underlies all the processes of develo ment: growth, morphogenesis, and differentiation. One a proach to studying the relationships among these processes clonal analysis, in which the cell lineages (clones) derived fro each cell in an apical meristem are mapped as organs develo Researchers can do this by using radiation or chemicals in o der to induce somatic mutations that will alter the chromosorr number or otherwise tag a cell in some way that distinguishes from the neighboring cells in the shoot apex. The lineage

n epidermal cells border a single cortical II, the homeotic gene (3LABRA-2 is selectively pressed, and these cells will remain hairless. ie blue color in this light micrograph indites cells in which GLABRA-2 is expressed.) Here an epidermal cell borders two cortical cells. GLABRA-2 is not expressed, and the cell will develop a root hair.

Shifts in Development: Phase Changes Multicellular organisms generally pass through developmental phases. In humans, these are infancy, childhood, adolescence, and adulthood, with puberty as the dividing line between the nonreproductive and reproductive phases. Plants also pass through phases, developing from a juvenile phase to an adult vegetative phase to an adult reproductive phase. In animals, these developmental changes take place throughout the entire organism, such as when a larva develops into an adult animal. In plants, in marked contrast, they occur within a single region, the shoot apical meristem. The morphological changes that arise from these transitions in shoot apical meristem activity are called phase changes. During the transition from a juvenile phase to an adult phase, the most obvious morphological changes typically occur in leaf size and shape (Figure 35.29). Once the apical meristem has laid down juvenile

ne ring of cells external to the epiermal layer is composed of root ap cells that will be sloughed off as ie root hairs start to differentiate. Figure 35.28 Control of root hair differentiation by a homeotic gene.

cills derived by mitosis and cell division from the mutant nieristematic cell will also be "marked." For example, a single cell in the shoot apical meristem may undergo a n utation that prevents chlorophyll from being produced. This cell and all of its descendants will be "albino," and they « ill appear as a linear file of colorless cells running down the I( ng axis of the otherwise green shoot. How early is a cell's developmental fate determined by its pasition in an embryonic structure? To some extent, the developmental fates of cells in the shoot apex are predictable, example, clonal mapping has shown that almost all the :11s derived from the outermost meristematic cells become p lit of the dermal tissue. However, we cannot pinpoint which n eristematic cells will give rise to specific tissues and organs, pparently random changes in rates and planes of cell divi3n can reorganize the meristem. For example, the outermost Us usually divide perpendicular to the surface of the shoot >ex, adding cells 10 the surface layer. Occasionally, however, K of the outermost cells divides parallel to the surface of the loot apex, placing one daughter cell beneath the surface, nong cells derived from different lineages. Such exceptions indicate that meristematic cells are not dedicated early to rming specific tissues and organs. Instead, the cell's final potion in an emerging organ determines what kind of cell it ill become, possibly as a result of positional information.

Leaves produced by adult phase of apical meristem

Leaves produced by juvenile phase of apical meristem

A Figure 35.29 Phase change in the shoot system of Acacia koa. This native of Hawaii has compound juvenile leaves, consisting of many small leaflets, and mature, sickle-shaped "leaves" (technically, highly modified leaf stalks). This dual foliage reflects a phase change in the development of the apical meristem of each shoot. In the juvenile vegetative phase of an apical meristem, compound leaves develop at each node. In the adult vegetative phase of an apical meristem, sickle-shaped leaves are produced. Once a node forms, the developmental phase—juvenile or adult—is fixed; that is, compound leaves do not mature into sickle-shaped "leaves."

CHAPTER 35

Plant Structure, Growth, and Development

733

nodes and internodes, they retain that status even after the shoot continues to elongate and the shoot apical meristem has changed to the adult phase. Therefore, any new leaves that develop on branches that emerge From axillary buds at juvenile nodes will also be juvenile, even though the apical meristem may have been laying down mature nodes for years. Phase changes are examples of plasticity in plant development. The transition from juvenile to adult leaves is only one type of phase change. The transition in the fanwort (see Figure 35.1) from feathery underwater leaves to fan-shaped floating leaves is another example. Next, we will examine a common but nevertheless remarkable phase change—the transition of a vegetative shoot apical meristem into a floral meristem.

Genetic Control of Flowering Flower formation involves a phase change from vegetative growth to reproductive growth. This transition is triggered by a combination of environmental cues, such as day length, and internal signals, such as hormones. (You will learn more about the control of flowering in Chapter 39.) Unlike vegetative growth, which is indeterminate, reproductive growth is determinate: The production of a flower by a shoot apical meristem stops the primary growth of that shoot. The transition from vegetative growth to flowering is associated with the switching-on of floral meristem identity genes. The protein products of these genes are transcription factors that regulate the genes required for the conversion of the indeterminate vegetative meristems into determinate floral meristems. When a shoot apical meristem is induced to flower, each leaf primordium that forms will develop into a specific type of floral organ based on its relative position—a stamen, carpel, sepal, or petal (see Figure 30.7 to review basic flower structure). Viewed from above, the floral organs develop in four concentric circles, or whorls: Sepals form the fourth (outermost) whorl; petals form the third; stamens form the second; and carpels form the first (innermost) whorl. Plant biologists have identified several organ identity genes that regulate the development of this characteristic floral pattern. Organ identity genes, also called plant homeotic genes, code for transcription factors. Positional information determines which organ identity genes are expressed in a particular floral organ primordium. The result is development of an emerging leaf into a specific floral organ, such

• Figure 35.30 Organ identity genes and pattern formation in flower development.

734

U N I T six

Plant Form and Function

as a petal or a stamen. Just as a mutation in a fruit fly home' gene can cause legs to grow in place of antennae, a mutati in a plant organ identity gene can cause abnormal floral c velopment, such as petals growing in place of stamens, shown in Figure 35.30. By collecting and studying mutants with abnormal flo ers, researchers have identified and cloned three classes! of floral organ identity genes, and they are beginning to c termine how these genes act. The ABC model of flccv formation, diagrammed in Figure 35.31a, identifies h these genes direct the formation of the four types of flos al organs. The ABC model proposes that each class of org identity genes is switched "on" in two specific whorls of t floral meristem. Normally, A genes are switched on in t two outer whorls (sepals and petals); B genes are switch d on in the two middle whorls (petals and stamens); and C genes are switched on in the two inner whorls (stameis and carpels). Sepals arise from those parts of the flo al meristems in which only A genes are active; petals ar se where A and B genes are active; stamens where B and C genes are active; and carpels where only C genes are acti The ABC model can account for the phenotypes of mutar lacking A, B, or C gene activity, with one addition: Wh< gene A activity is present, it inhibits C, and vice versa. If ther A or C is missing, the other takes its place. Figure 35.31b shows the floral patterns of mutants lacking each!

(a) Normal Arabidopsis flower. Arabidopsis normally has four whorls of flower parts: sepals (Se), petals (Pe), stamens (St), and carpels fCa).

(b) Abnormal Arabidopsis flower. Reseachers have identified several mutations of organ identity genes that cause abnormal flowers to develop. This flower has an extra set of petals in place of stamens and an internal flower where normal plants have carpels.

T Figure 35.31 The ABC hypothesis for the functioning of organ identity genes in flower development.

(a) A schematic diagram of the ABC

^*

Active genes:

A

A

C

C

C

C

A

66 B B cccccccc

A

A A C CC C A,

hypothesis. Studies of plant mutations reveal that three classes of organ identity genes are responsible for the spatial pattern of floral parts. These genes are designated A, B, and C in this schematic diagram of a floral meristem in transverse view. These genes regulate expression of other genes responsible for development of sepals, petals, stamens, and carpels. Sepals develop from the meristematic region where only A genes are active. Petals develop where both A and B genes are expressed. Stamens arise where B and C genes are active. Carpels arise where only C genes are expressed.

A A ABBAABBA

A A

Whorls:

Stamen

Carpel / Petal

Sepal 'ild type

( I ) Side view of organ identity mutant flowers. Combining the model shown in part (a) with the rule that if A gene or C gene activity is

e three classes of organ identity genes and depicts how e model accounts for the iloral phenotypes. By conructing such hypotheses and designing experiments to st them, researchers are tracing the genetic basis of plant velopment. In dissecting the plant to examine its parts, as we have me in this chapter, we must remember that the whole plant fi notions as an integrated organism. In the following chaprs, you will learn more about how materials are transported thin vascular plants (Chapter 36), how plants obtain nutri.ts (Chapter 37), how plants reproduce (Chapter 38, focusg on flowering plants), and how the various functions of the ant are coordinated (Chapter 39). Remembering that strucre fits function and that plant anatomy and physiology

Mutant lacking 6

Mutant lacking C

missing, the other activity spreads through all four whorls, we can explain the phenotypes of mutants lacking a functional A, B, or C organ identity gene-

reflect evolutionary adaptations to the problems of living on land will enhance your understanding of plants.

Concept Check 1. How can two cells in a plant have such vastly different structure even though they possess the same genome? 2. Explain how a mutation such as/ass in Arabidopsis results in a stubby plant rather than a normal elongated one. For suggested answers, see Appendix A.

CHAPTER 35

Plant Structure, Growth, and Development

735

Chapter 3 5 Review Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, investigations, and other interactive study aids.

SUMMARY OF KEY CONCEPTS The plant body has a hierarchy of organs, tissues, and cells • The Three Basic Organs: Roots, Stems, and Leaves (pp. 713-716) Roots anchor the plant, absorb and conduct water and minerals, and store food. The shoot system consists of stems, leaves, and (in angiosperms) flowers. Leaves are attached by their petioles to the nodes of the stem, with internodes of the stem separating the nodes. Axillary buds, located in the axils of petioles and stems, have the potential to extend as vegetative or floral shoots - The two main groups of angiosperms, monocots and eudicots, differ in anatomical details. Activity Root, Stem, and Leaf Sections • The Three Tissue Systems: Dermal, Vascular, and Ground (p. 717) Dermal tissue (epidermis and periderm), vascular ttssue (xylem and phloem), and ground tissue are continuous throughout the plant, although in the various plant organs the three tissues differ in arrangement and in some specialized functions. Vascular tissues integrate the parts of the plant. Water and minerals move up from roots in the xyiem. Sugar is exported from leaves or storage organs to the phloem. • Common Types of Plant Cells {pp. 717-719) Parenchyma cells, relatively unspecialized cells that retain the ability to divide, perform most of the plant's metabolic functions of synthesis and storage. Collenchyma cells, which have unevenly thickened walls, support young, growing parts of the plant. Sclerenchyma cells—fibers and sclereids—have thick, lignified walls that help support mature, nongrowing parts of the plant. Tracheids and vessel elements, the water-conducting cells of xylem, have thick walls and are dead at functional maturity. Sieve-tube members are the sugar-transporting cells of phloem in angiosperms. Though alive at functional maturity, sieve-tube members depend on the services of neighboring companion cells.

Meristems generate cells for new organs • Apical meristems elongate shoots and roots through primary growth. Lateral meristems add girth to woody plants through secondary growth (pp. 720—721). Concept -S

Primary growth lengthens roots and shoots • Primary Growth of Roots (pp. 721-723) Apical meristems produce cells that continue to divide as meristematic cells. In roots, the apical meristem is located near the tip, where it regenerates the root cap. • Primary Growth of Shoots (pp. 723-725) The apical meris tern of a shoot is located in the terminal bud, where it gives rise to a repetition of internodes and leaf-bearing nodes. investigation What Are Functions ojMonocot Tissues?

736

UNIT six

Plant Form and Function

Concept

Secondary growth adds girth to stems and roots in woody plants • The Vascular Cambium and Secondary Vascular Tissue (pp. 725-727) The vascular cambium develops from parenchyma cells into a meristematic cylinder that produces secondary xylem and secondary phloem. Older layers of secondary xylem. (heartwood) become inactive, while younger layers (sapwood) still conduct water. Only the youngest secondary phloem is active in conducting organic nutrients. • Cork Cambia and the Production of Periderm (p. 728) The cork cambium gives rise to the secondary plant body's protective covering, or periderm, which consists of the cork cambium plus the layers of cork cells it produces. Bark consists of all the tissues external to the vascular cambium: secondary phloem and periderm. Activity Primary and Secondary Growth

Growth, morphogenesis, and differentiation produce the plant body • Molecular Biology: Revolutionizing the Study of Plants (pp. 728-729) New techniques and model systems, including ArahiAopsxs, are catalyzing explosive progress in our understanding of plants. Arabidopsis is the Erst plant to have had its entire genome sequenced. • Growth: Cell Division and Cell Expansion (pp. 729-730 Cell division and cell expansion are primary determinants of growth and form. A preprophase band determines where a cell plate will form in a dividing cell- The orientation of the cytoskeleton also affects the direction of cell elongation by controlling the orientation of cellulose microfibrils within the cell wall. • Morphogenesis and Pattern Formation (pp. 730-732) Development of tissues and organs in specific locations depends on the ability of cells to detect and respond to positional information, such as information associated with polarity. Homeotic genes often control morphogenesis. • Gene Expression and Control of Cellular Differentiation (p. 732) The challenge of understanding cellular differentiation is explaining how cells with matching genomes diverge into various cell types. • Location and a Cell's Developmental Fate (pp. 732-733) A cells position in a developing organ determines its pathway of differentiation. • Shifts in Development: Phase Changes (pp. 733-734) Internal or environmental cues may cause a plant to swiich from one developmental phase to another—for example, from development of juvenile leaves to development of mature leaves. Such morphological changes are called phase changes. • Genetic Control of Flowering (pp. 734-735) Research on organ identity genes m developing flowers provides an important model system for the study of pattern formation. The ABC model of flower formation identifies how three classes of organ identity genes control formation of sepals, petals, stamens, and carpels

scientific Inquiry

TESTING YOUR KNOWLEDGE

Evolution Connection The evolution of plant structure can be explored by looking at alternative growth strategies of related plants that occur in different environments. In this respect, Darwin was one of the earliest observers to note that many herbaceous plant species on continental mainlands have woody, tree-like relatives on remote oceanic islands. In the Hawaiian Islands, tor example, one can Find tree lobelias and tall, woody violets, groups that occur as small herbs in North America. Suggest an evolutionary hypothesis for this trend: Why is it so common for woody, tree-like forms to evolve from herbaceous ancestors that colonize isolated islands?

Write a paragraph explaining why certain mutants are so useful to researchers who investigate the regulation of plant development. Your paragraph should include at least one specific example. Investigation What Are Functions ofMonocot Tissues?

Science, Technology, and Society Make a list of the plants and plant products you use in a typical day How do you use these various plant products? Do you think the number of plants and plant products used in everyday life has increased or decreased m the last century? Do you think the number is likely to increase or decrease in the. future7 Why?

CHAPTER 35

Plant Structure, Growth, and Development

737

. Transport in Vascular Plants A Figure 36.1 Coast redwoods (Sequoia sempervirens).

Key Concepts 36.1 Physical processes drive the transport of materials in plants over a range of distances 36.2 Roots absorb water and minerals from the soil 36.3 Water and minerals ascend from roots to shoots through the xylem 36.4 Stomata help regulate the rate of transpiration 36.5 Organic nutrients are translocated through the phloem

Pathways for Survival The algal ancestors of plants absorbed water, minerals, and CO-Z directly from the water in which they were immersed; none of their cells were far from these ingredients. Bryophytes also lack an extensive transport system and are confined to living in very moist environments. For vascular plants, in contrast, the evolutionary journey onto land involved the differentiation of the plant body into roots and shoots. Roots absorb water and minerals from the soil, and shoots absorb light and atmospheric CO2 for photosynthesis. Xylem transports water and minerals from roots to shoots. Phloem transports sugars from where they are produced or stored to where they are needed for growth and metabolism. Such transport, which is necessary for a plant to function as a whole, may occur over long distances. "For example, the highest leaves of some coast redwoods are more than 100 m (over 300 feet) from the roots (Figure 36.1). What enables a vascular plant to conduct water, minerals, and organic nutrients over such long distances? The mechanisms responsible for internal transport are the subject of this chapter. 738

Concept

Physical forces drive the transport of materials in plants over a range of distances Transport in vascular plants occurs on three scales: (1) tran port of water and solutes by individual cells, such as ro hairs; (2) short-distance transport of substances from cell cell at the levels of tissues and organs, such as the loading sugar from photosynthetic leaf cells into the sieve tubes of t phloem; and (3) long-distance transport within xylem ar phloem at the level of the whole plant. A variety of physic processes are involved in these different types of transpo Figure 36.2 provides an overview of long-distance transport a vascular plant.

Selective Permeability of Membranes: A Review We covered the transport ot solutes and water across biolog cal membranes in detail in Chapter 7. Here we reexamine few of these transport processes in the specific context of pla cells. The selective permeability of a plant cell's plasma men brane controls the movement of solutes into and out of th cell. Recall from Chapter 7 that solutes tend to diffuse dow their gradients and that diffusion across a membrane is call passive transport because it happens without the cell direct using metabolic energy. Active transport i.s the pumping solutes across membranes against their electrochemical grad ents, the combined effects of the concentration gradient of tl solute and the voltage (charge difference) across the mer brane. It is called "active" because the cell must expend e

0 Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon for photosynthesis. Some O2 produced by photosynthesis is used in cellular respiration.

0 Sugars are produced by photosynthesis in the leaves.

Transpiration, the loss of water from leaves (mostly through stomata), creates a force within aves that pulls xylem sap upward.

0 Sugars are transported as phloem sap to roots and other parts of the plant. 0 Water and minerals a, transported upward fro roots to shoots as xylem sa

0 Roots absorb water and dissolved minerals from the soil.

0 Roots exchange gases with the air spaces of soil, taking in O2 and discharging CO2. In cellular respiration, O2 supports the breakdown of sugars.

Figure 36.2 An overview of transport in a vascular plant.

yy, usually in the form of ATP, to transport a solute "uphill"— iunter to the net direction in which the solute diffuses. Most solutes cannot cross the lipid bilayer of the memane; they must pass through transport proteins embedded ' the membrane. Some transport proteins bind selectively to solute on one side of the membrane and release the solute on tite opposite side. Other transport proteins provide selective tannels across the membrane. For example, membranes of pst plant cells have potassium channels that allow potasjm ions (K ) to pass, but not other ions, such as sodium Ia + ). Later in this chapter, we will discuss how K+ channels guard cells function in the opening and closing of stomata. ime channels are gated, opening or closing in response to rtain stimuli.

pump moves positive charge, in the form of H +, out of the cell, the pump also contributes to a voltage known as a membrane potential, a separation of opposite charges across a membrane. Proton pumping makes the inside of a plant cell negative in charge relative to the outside. This voltage is called a membrane potential because the charge separation is a form of potential energy that can be harnessed to perform cellular work. Plant cells use energy stored in the proton gradient and membrane potential to drive the transport of many different solutes. For example, the membrane potential generated by proton pumps contributes to the uptake of K+ by root cells

•Qton purnp aerates mem•ane potential

I he Central Role of Proton Pumps e most important active transport protein in the plasma smbranes of plant cells is the proton pump, which uses en;y from ATP to pump hydrogen ions (H + ) out of the cell. is results in a proton gradient with a higher H+ concentran outside the cell than inside (Figure 36.3). The gradient is arm of potential (stored) energy because the hydrogen ions id to diffuse "downhill" back into the cell, and this "flow" of can be harnessed to do wTork. And because the proton

A Figure 36.3 Proton pumps provide energy for solute t r a n s p o r t . By pumping H + out of the cell, proton pumps produce an H + gradient and a charge separation called a membrane potential. These t w o forms of potential energy can be used to drive the transport of solutes.

CHAPTER 36

Transport in Vascular Plants

739

•RACELLULAR FLUID

y the membrane otential. ; \is

Transport protein

(a) Membrane potential and cation uptake

(b) Cotransport of anions

pumps normally run in reverse, using ATP energy to pum H+ against its gradient. In both cases, proton gradients enabl one process to drive another.

Effects of Differences in Water Potential To survive, plants must balance water uptake and loss. The m uptake or loss of water by a cell occurs by osmosis, the passive transport of water across a membrane (see Figure 7,12). Hoi * can we predict the direction of osmosis when a cell is s rounded by a particular solution? In the case of an animal c if the plasma membrane is impermeable to the solutes, ii enough to know whether the extracellular solution has a Iowa r or higher solute concentration relative to the cell. Water wi.l move by osmosis from the solution with the lower solute a centration to the solution with the higher solute concentrati But a plant cell has a cell wall, which adds another factor fecting osmosis: physical pressure. The combined effects of solute concentration and physical pressure are incorporated into a measurement that is called water potential, abbrevia d by the Greek letter psi (\|/). Water potential determines the direction of movement of water. The most important thing to remember is that free \ ter, water that is not bound to solutes or surfaces, moves/T regions of higher water potential to regions of lower water potenti il

(c) Cotransport of a neutral solute A Figure 36.4 Solute transport in plant cells.

(Figure 36.4a). In the mechanism called cotransport, a transport protein couples the downhill passage of one solute (H + ) to the uphill passage of another (NO3~, in the case of Figure 36.4b). The "coattail" effect of cotransport is also responsible for the uptake of the sugar sucrose by plant cells (Figure 36.4c). A membrane protein cotransports sucrose with the H~*~ that Is moving down its gradient through the protein. The role of proton pumps in transport is an application of chemiosmosis (see Figure 9.15). The key feature of chemiosmosis is a transmembrane proton gradient, which links energy-releasing processes to energy-consuming processes in cells. For example, you learned in Chapters 9 and 10 that mitochondria and chloroplasts use proton gradients generated by electron transport chains (which release energy) to drive ATP synthesis (which consumes energy). The ATP synthases that couple H+ diffusion to ATP synthesis during cellular respiration and photosynthesis work somewhat like the proton pumps in plant cells. But in. contrast to ATP synthases, proton 740

U N I T six

Plant Form and Function

if there is no barrier to its free flow. For example, if a plant cell is immersed in a solution having a higher water potential th the cell, uptake of water will cause the cell to swell. By mi ing, water can perform work, such as expanding a cell. T "potential" in water potential is water's potential energy water's capacity to perform work when it moves from a regie of higher \|/ to a region of lower \|i. Water potential is a speci il case of the general tendency of systems to change spon neously to a state of lowest free energy (see Figure 8.5). Plant biologists measure \j/ in units of pressure cal megapascals (abbreviated MPa). Physicists have assigned tl value of zero to the water potential of pure water in a contain open to the atmosphere (\|/ — 0 MPa) under standard con tions (at sea level and at room temperature). One MPa is eqi to about 10 atmospheres of pressure. (An atmosphere is t pressure exerted at sea level by an imaginary column of air tending through the entire height of the atmosphere—abo 1 kg of pressure per square centimeter.) A few examples w 11 give you an idea of the magnitude of a megapascal: Your lun exert less than 0.1 MPa. A car tire is usually inflated to abo 0.2 MPa. Water pressure in home plumbing is about 0.25 M' In contrast, most plant cells exist at approximately 1 MPa. How Solutes and Pressure Affect Water Potential Both pressure and solute concentration affect water potenti as expressed in the following water potential equation:

membrane separating the two arms of the tube. The membrane is permeable to water but not to solutes. What happens if we fill the right arm of the tube with a 0.1 M solution (\j/s = -0.23 MPa) and fill the left arm with pure water (\|/5 = 0)? In the absence of any physical pressure (that is, when \j/ P = 0), the water potential \|f will be equal to y s . Thus, the tf of the left arm of the tube (pure water) will be 0, whereas the \j/ of the right arm will be —0.23 MPa. Because water always moves from regions of higher water potential to regions of lower water potential, the net water movement in this case will be from the left arm of the tube to the right arm (Figure 36.5a). But if we now apply a physical pressure of +0.23 MPa to the solution in the right arm, we raise its water potential from a negative value to 0 MPa (y = —0.23 + 0.23). There will now be no net flow of water between this pressurized solution and the compartment of pure water (Figure 36.5b). In fact, if we increase \yP to +0.30 MPa, then the solution has a water potential of +0.07 MPa (\|/ = -0.23 + 0.30), and this solution will actually lose water to a compartment containing pure water (Figure 36.5c). Finally, imagine using a plunger to pull upward on the pure water instead of pushing downward on the solution. A negative pressure (tension) of —0.30 MPa on the water compartment would be sufficient to draw water from the solution that has a water potential of —0.23 MPa (Figure 36.5d). Again, keep in mind the key point; Water moves

here y is the water potential, \|/s is the solute potential Osmotic potential), and y? is the pressure potential. The solute potential (\|%) of a solution is proportional to le number of dissolved solute molecules. Solute potential is so called osmotic potential because solutes affect the direcon of osmosis. Solutes may be any type of dissolved chemial. By definition, the \|/s of P u r e water is 0. But what happens when solutes are added to this pure water? The solutes bind ater molecules, reducing the number of free water molecules nd lowering the capacity of the water to do work. Thus, dding solutes always lowers the water potential, and the \|/s a solution is always negative. A 0.1-molar (0.1 M) solution ' a sugar, for example, has a \j/ s of —0-23 MPa, Pressure potential (\|/P) is the physical pressure on a soluion. Unlike \|%, \|/P can be positive or negative. For example, he water in the dead xylem cells of a transpiring plant is often mder a negative pressure (tension) of less than —2 MPa. Conersely, much like the air in a balloon, the water in living cells usually under positive pressure. The cell contents press the ilasma membrane against the cell wall, producing what is < ailed turgor pressure. ! Quantitative Analysis of Water Potential ow that we have introduced the water potential equation nd its components, let's put it to use. First we will look at ''ater movement in an artificial system. Then we will apply the equation to a living plant cell. The artificial model represented in Figure 36.5 shows the movement of water within a U-shaped tube that has a

in the direction of higher to lower water potential.

Now let us consider how water potential affects uptake and loss of water by plant cells. First, imagine a flaccid (limp) cell that has a \|/P of 0. Suppose this flaccid cell is bathed in a solution of higher solute concentration (more negative solute

(d!

(c)

w -H2O

y P .= 0 Vs = -0.23 y = -0.23 MPa

\jf = 0 MPa

Figure 36.5 Water potential and ater movement: an artificial model. this U-shaped apparatus, a selectively rmeable membrane separates pure water om a 0.1 M solution containing a particular lute that cannot pass freely across the embrane. Water moves across a selectively

¥ F - 0,23 tys = -0.23 y = 0 MPa

= 0 MPa

VP = 0.30 y s = -0.23 y = 0-07 MPa

permeable membrane from where water potential is higher to where it is lower. The water potential (\\i) of pure water at atmospheric pressure is 0 MPa. If we know the values of pressure potential environmental \]/. The cell loses water and plasmolyzes. After plasmoiysis is complete, the water potentials of the cell and its surroundings are the same.

Turgid cell at osmotic equilibriu with its surrounding 0.7 Vp= Vs = -0.7 y = 0 MPa

(b) Initial conditions: cellular \|/ < environmental \|f. There is a net uptake of water by osmosis, causing the cell to become turgid. When this tendency for water to enter is offset by the back pressure of the elastic wall, water potentials are equal for the cell and its surroundings. (The volume change of the cell is exaggerated in this diagram.)

A Figure 36.6 Water relations in plant cells. In these experiments, identical cells, initially flaccid, are placed in two different environments. (The protoplasts of flaccid cells are in contact with their walls but lack turgor pressure.) The blue arrows indicate the initial direction of net water movement.

potential) than the cell itself (Figure 36.6a). Since the external solution has the lower (more negative) water potential, water will leave the cell by osmosis, and the cell's protoplast will plasmolyze, or shrink and pull away from its wall. Now let's place the same flaccid cell in pure water (\j/ = 0) (Figure 36.6b). The cell has a lower water potential than pure water because of the presence of solutes, and water enters the cell by osmosis. The contents of the cell begin to swell and press the plasma membrane against the cell wall, producing a turgor pressure. The partially elastic wall pushes back against the pressurized cell. When this wall pressure is enough to offset the tendency for water to enter because of the solutes in the cell, then \|/P and \j/ s are equal and \|/ = 0. This matches the water potential of the extracellular environment—in this example, 0 MPa. A dynamic equilibrium has been reached, and there is no further net movement of water.

specific and too rapid to be explained entirely by diffusio through the lipid bilayer. Indeed, water typically crosses va< uolar and plasma membranes through transport protein called aquaporins (see Chapter 7). These selective channe. do not affect the water potential gradient or the direction water flow, but rather the rate at which water diffuses down it; water potential gradient. Evidence is accumulating that th rate of water movement through these proteins is regulated b phosphorylation of the aquaporin proteins induced b changes in second messengers such as calcium ions (Ca ).

In contrast to a flaccid cell, a walled cell that has a greater solute concentration than its surroundings is turgid, or very firm. Healthy plant cells are turgid most of the time. Their turgor contributes to support in nonwoody parts of the plant. You can see the effects of turgor loss in wilting, the drooping of leaves and stems as a result of cells becoming flaccid (Figure 36.7). Aquaporin Proteins and Water Transport Water potential is the force that moves water across the membranes of plant cells, but how do the water molecules actually cross the membranes? Because water molecules are so small, they move relatively freely across the lipid bilayer, even though the middle zone is hydrophobic (see Figure 7.2). Water transport across biological membranes, however, is too 742

UNIT six

Plant Form and Function

A Figure 36.7 A watered Impatiens plant regains its turgor

Three Major Compartments of Vacuolated lant Cells ransport is also regulated by the compartmental structure of lant cells. Outside the protoplast is a cell wall that helps r laintain the cell's shape (see Figure 6.9). However, it is the sectively permeable plasma membrane that directly controls the affic of molecules into and out of the protoplast. The plasma riembrane is a barrier between two major compartments: the ill wall and the cytosol (the part of the cytoplasm contained ithin. the plasma membrane but outside the intracellular orknelles). Most mature plant cells have a third major compartlent, the vacuole, a large organelle that can occupy as much as J% or more of the protoplasts volume (Figure 36.8a). The acuolar membrane, or tonoplast, regulates molecular traffic stween the cytosol and the vacuolar contents, called cell sap. proton pumps expel H+ from the cytosol into the vacuole. t resulting pH gradient is used to move other ions across the icuolar membrane by chemiosmosis. In most plant tissues, the cell walls and cytosol are continous from cell to cell. Plasmodesmata connect the cytosolic tmpartments of neighboring cells, thereby forming a continous pathway for transport of certain molecules between ells. This cytoplasmic continuum is called the symplast igure 36.8b). The continuum of cell walls plus the extracellar spaces is called the apoplast. The third cellular cornartment, the vacuole, is not shared with neighboring cells.

unctions of the Symplast and l t i n Transport ow do water and solutes move from one cation to another within plant tissues ad organs? For example, what mechasms transport water and minerals from Le root hairs to the vascular cylinder of .e root? Such short-distance transport is imetimes called lateral transport because s usual direction is along the radial axis plant organs, rather than up and down ong the length of the plant. Three routes are available for this ansport (see Figure 36.8b). By the first ute, substances move out of one cell, .ross the cell wall, and into the neighoring cell, which may then pass the bstances along to the next cell in the p athway by the same mechanism. This ansmembrane route requires repeated ossings of plasma membranes as the lutes exit one cell and enter the next. The second route, via the symplast, the mtinuum of cytosol within a plant tis-

Transport proteins in the plasma membrane regulate traffic of molecules between the cytosol and the cell wall.

sue, requires only one crossing of a plasma membrane. After entering one cell, solutes and water can then move from cell to cell via plasmodesmata. The third route for short-distance transport within a plant tissue or organ is along the apoplast, the pathway consisting of cell walls and extracellular spaces. Without entering a protoplast, water and solutes can move from one location to another within a root or other organ along the byways provided by the continuum of cell walls.

Bulk Flow in Long-Distance Transport Diffusion in a solution is fairly efficient for transport over distances of cellular dimensions (less than 100 urn), but it is much too slow to function in long-distance transport within a plant. For example, diffusion from one end of a cell to the other takes seconds, but diffusion from the roots to the top of a giant redwood would take decades or more. Long-distance transport occurs through bulk flow, the movement of a fluid driven by pressure. In bulk flow, water and solutes move through the tracheids and vessels of the xylem and through the sieve tubes of the phloem. In the phloem, for example, the loading of sugar generates a high positive pressure at one end of a sieve tube, forcing sap to the opposite end of the tube. In xylem, it is actually tension (negative pressure) that drives

Cell wail Cytosol

Transport proteins in the vacuolar membrane regulate traffic of molecules between the cytosol and the vacuole.

Plasmodesrna Vacuolar membrane Plasma membrane (tonoplast) (a) Cell compartments. The cell wall, cytosol, and vacuole are the three main compartments of most mature plant cells. Key Symplast Transmembrane route x

Apoplast Apoplast

The symplast is the I continuum of cytosol connected by plasmodesmata. I

Symplast

The apoplast is the continuum of cell wails and extracellular spaces.

Symplastic route^ Apoplastic route

(b) Transport routes between cells. At the tissue level, there are three passages: the transmembrane, symplastic, and apoplastic routes. Substances may transfer from one route to another.

A Figure 36.8 Cell compartments and routes for short-distance transport. Transport in Vascular Plants

743

long-distance transport. Transpiration, the evaporation of water from a leaf, reduces pressure in the leaf xylem. This creates a tension that pulls xylem sap upward from the roots.

shoot system. This section focuses on short-distance transpo between cells in the soil-to-xylem pathway As you read, u Figure 36.9 to reinforce your understanding.

If you have ever dealt with a partially clogged drain, you

know that the volume of flow through a pipe depends on the pipe's internal diameter. Clogs reduce flow because they reduce the effective diameter of the drainpipe. Such household experiences help us understand how the unusual structures of plant cells specialized for bulk flow—the sieve-tube members of the phloem and the tracheids and vessel elements of the xylem—fit their function. Recall from Chapter 35 that the cytoplasm of sieve-tube members is almost devoid of internal organelles and that mature tracheids and vessel elements, being dead cells, have no cytoplasm. Like unplugging a kitchen drain, loss of cytoplasm in a plants "plumbing" allows for efficient bulk flow through the xylem and phloem. Bulk flow is also enhanced by the perforated end walls of vessel elements and the porous plates connecting sieve-tube members (see Figure 35.9). Now that we have an overview of the basic mechanisms of transport at the cellular, tissue, and whole-plant levels, we will look more closely at how these mechanisms work together. For example, bulk flow due to a pressure difference is the mechanism of long-distance transport of phloem sap, but it is active transport of sugar at the cellular level that maintains this pressure difference. The four transport functions we will examine in more detail are the absorption of water and minerals by roots, the ascent of xylem sap, the control of transpiration, and the transport of organic nutrients within phloem. Concept Check i 1. Some farmers throughout the world irrigate crops using groundwater, which has a relatively high content of dissolved salts. How might this practice affect water potential in crops? 2. If a plant cell immersed in distilled water has a \|% of -0.7 MPa and a \|/ of 0 MPa, what is the cell's \\f?l If you put the same cell in an open beaker of solution that has a \jr of -0.4 MPa, what would be the cells \|/p at equilibrium? For suggested answers, see Appendix A.

Concept

Roots absorb water and minerals from the soil Water and mineral salts from the soil enter the plant through the epidermis of roots, cross the root cortex, pass into the vascular cylinder, and then flow up tracheids and vessels to the 744

UNIT six Plant Form and Function

The Roles of Root Hairs, Mycorrhizae, and Cortical Cells Much oi the absorption of water and minerals occurs ne root tips, where the epidermis is permeable to water an where root hairs are located. Root hairs, which are extensio: of epidermal cells, account for much of the surface area roots (see Figure 35.12). Soil particles, usually coated wi water and dissolved minerals, adhere tightly to the root hair The soil solution flows into the hydrophilic walls of epiderrr cells and passes freely along the apoplast into the root corte This exposes the symplast of all the cells of the cortex to tl soil solution, providing a much greater surface area of me brane than the surface area of the epidermis alone. As the soil solution moves along the apoplast into the root cells of the epidermis and cortex take up water and certa solutes into the symplast. Although the soil solution is usua very dilute, active transport enables roots to accumulate e sential minerals, such as K+, to concentrations hundreds times higher than in the soil. Most plants form mutually beneficial relationships wi fungi, which facilitate the absorption of water and minera from the soil Roots and fungi form mycorrhizae, symbiot structures consisting of plant roots united with fungal hyph. (filaments) (Figure 36.10). The hyphae absorb water and s lected minerals, transferring much of these resources to th host plant. Chapter 37 highlights the role of mycorrhizae plant nutrition, and Chapter 31 features the fungal partners these mutualistic relationships. What is important to unde stand here is that the mycelium (network of hyphae) of tr fungus endows mycorrhizae, and thus the plant roots, with a enormous surface area for absorption. As much as 3 m of h phae can extend from each centimeter along a root's lengt reaching a far greater volume of soil than the root alone cou* penetrate. Mycorrhizae enable even older regions of roots: k from the root tips, to supply water and minerals to the planl

The Endodermis: A Selective Sentry Water and minerals that pass from the soil into the root corte cannot be transported to the rest of the plant until they ent the xylem of the vascular cylinder. The endodermis, the ir nermost layer of cells in the root cortex, surrounds the vasct lar cylinder and functions as a last checkpoint for the selectiv passage of minerals from the cortex into the vascular tissu (see Figure 36.9). Minerals already in the symplast when the reach the endodermis continue through the plasmodesmata endodermal cells and pass into the vascular cylinder. Thes minerals were already screened by the selective membrar they had to cross to enter the symplast in the epidermis c

Figure 36.9 Lateral transport of nerals and water in roots.

Casparian strip x Endoderma! cell

Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals can then soak into the cortex along this matrix of walls.

Minerals and water that cross the plasma membranes of root hairs enter the symplast.

As soil solution moves along the apoplast, some water and minerals are transported into the protoplasts of cells of the epidermis and cortex and then move inward via the symplast.

Syrr route

Epiderrr

Within the transverse and radial walls of each endodermal cell is the Casparian strip, a belt of waxy material (purple band) that blocks the passage of water and dissolved minerals. Only minerals already in the sympiast or entering that pathway by crossing the plasma membrane of an endodermal cell can detour around the Casparian strip and pass into the vascular cylinder.

rtex- Those minerals that reach the endodermis via the oplast encounter a dead end that, blocks their passage into e vascular cylinder: In the transverse and radial walls of ch endodermal cell is the Casparian strip, a belt made of berin, a waxy material impervious to water and dissolved .nerals. Thus, water and minerals cannot cross the endoderis and enter vascular tissue via the apoplast. The only way st this barrier is for the water and minerals to cross the asma membrane of an endodermal cell and enter the vascu" cylinder via the symplast. The endodermis, with its Casparian strip, ensures that no merals can reach the vascular tissue of the root without ossing a selectively permeable plasma membrane. If miners do not enter the symplast of cells in the epidermis or corx, they must enter endodermal cells or be excluded from the scular tissue. The endodermis also prevents solutes that ve been accumulated in the xylem sap from leaking back o the soil solution. The structure of the endodermis and its ategic location in the root fit its function as sentry of the borr between the cortex and the vascular cylinder, a function

Cortex

Endodermis

Vascular cylinder

0 Endodermal cells and also parenchyma cells within the vascular cylinder discharge water and minerals into their walls (apoplast). The xylem vessels transport the water and minerals upward into the shoot system.

A Figure 36.10 Mycorrhizae, symbiotic associations of f u n g i a n d r o o t s . The white mycelium of the fungus ensheathes these roots of a pine tree. The fungal hyphae provide an extensive surface area for the absorption of water and minerals.

CHAPTER 36

Transport in Vascular Plants

745

that contributes to the ability of roots to transport certain minerals preferentially from the soil into the xylem. The last segment in the soil-to-xylem pathway is the passage

of water and minerals into the tracheids and vessel elements of the xylem. These water-conducting cells lack protoplasts when mature and are therefore part of the apoplast. Endodermal cells, as well as parenchyma cells within the vascular cylinder, discharge minerals from their protoplasts into their walls. Both diffusion and active transport are involved in this transfer of solutes from symplast to apoplast, and the water and minerals are now free to enter the tracheids and vessels. The water and mineral nutrients we have tracked from the soil to the root xylem can now be transported upward as xylem sap to the shoot system. A Figure 36.11 Guttation. Root pressure is forcing excess water from this strawberry leaf.

Concept Check 1. Why might a crop develop a severe phosphate deficiency after being sprayed with a fungicide? 2. A scientist adds a water-soluble inhibitor of photosynthesis to the roots of a plant. However, photosynthesis is not affected by addition of the inhibitor in this manner. Why? For suggested answers, see Appendix A.

Concept

Water and minerals ascend from roots to shoots through the xylem Here we will focus on the long-distance transport of xylem sap. The sap flows upward from roots throughout the shoot system to veins that branch throughout each leaf. Leaves depend on this efficient delivery system for their supply of wTater. Plants lose an astonishing amount of water by transpiration, the loss of water vapor from leaves and other aerial parts of the plant. Consider the example of maize (commonly called corn in the U.S.). A single plant transpires 125 L of water during a growing season. A maize crop growing at a typical density of 75,000 plants per hectare transpires almost 10 million L (10 million kg) of water per hectare every growing season (equivalent to about 1.25 million gallons of water per acre per growing season). Unless the transpired water is replaced by water transported up from the roots, the leaves will wilt and the plants will eventually die. The upward flow of xylem sap also brings mineral nutrients to the shoot system.

Factors Affecting the Ascent of Xylem Sap Xylem sap rises to heights of more than 100 m in the tallest trees. Is the sap pushed upward from the roots, or is it pulled 746

U N I T six

Plani Form and Function

upward by the leaves? Lets evaluate the relative contributioi of these two possible mechanisms. Pushing Xylem Sap: Root Pressure At night, when transpiration is very low or zero, root cells cor tinue pumping mineral ions into the xylem of the vascult cylinder. Meanwhile, the endodermis helps prevent the ior from leaking out. The resulting accumulation of minerals lower the water potential within the vascular cylinder. Water flows i from the root cortex, generating root pressure, an upward pus of xylem sap. The root pressure sometimes causes more water t enter the leaves than is transpired, resulting in guttation, th • exudation of water droplets that can be seen in the morning o tips of grass blades or the leaf margins of some small, herb; ceous eudicots (Figure 36.11). Guttation fluid differs from de1 which is condensed moisture produced during transpiration In most plants, root pressure is a minor mechanism drivin ; the ascent of xylem sap, at most forcing water upward only few meters. Many plants do not generate any root pressurt Even in plants that display guttation, root pressure cannc keep pace with transpiration after sunrise. For the most par xylem sap is not pushed from below by root pressure bi pulled upward by the leaves themselves. Pulling Xylem Sap: The Transpiration-CohesionTension Mechanism To move material upward, we can apply positive pressur from below or negative pressure from above (as when suckin liquid through a straw). Here we will focus on the process b which water is pulled upward by negative pressure in th xylem. As we investigate this mechanism of transport, we wi see that transpiration provides the pull, and the cohesion c water due to hydrogen bonding transmits the upward pu! along the entire length of the xylem to the roots.

Transpirational Pull. Stomata, the microscopic pores on the "face ol a leaf, lead to a maze of internal airspaces that exse the mesophyll cells to the carbon dioxide they need for photosynthesis. The.air in these spaces is saturated with water [por because it is in contact with the moist walls of the cells. Op. most days, the air outside the leaf is drier; that is, it has a wer water potential than the air inside the leaf. Therefore, kter vapor in the airspaces of a leaf diffuses down its water •tential gradient and exits the leaf via the stomata. It is this ss of water vapor from the leaf by diffusion and evaporation at we call transpiration. But how does loss of water vapor from the leaf translate into )ulling force for upward movement of water through a plant? le leading hypothesis' is that negative pressure that causes iter to move up through the xylem develops at the air-water terface in mesophyll cell walls. Water is brought to the leaves the xylem in leaf veins and then is drawn into the mesop iyll cells and into their cell walls. This movement depends adhesion of water to cellulose microfibrils and other hy•ophilic components in plant cell walls. At first, water evapotes from a thin water film lining the airspaces surrounding

mesophyll cells. As more water is lost to the airs the air-water interface retreats deeper into the cell wall and becomes more curved (Figure 36.12). As even more molecules evaporate, the degree of curvature and the surface tension of the water molecules increase, and the pressure at the air-water interface becomes increasingly negative. Water molecules from the more hydrated parts of the leaf are then pulled toward this area, reducing the tension. These pulling forces are transferred to the xylem because each water molecule is cohesively bound to the next by hydrogen bonds. Thus, transpirational pull depends on some of the special properties of water that were discussed in Chapter 3: adhesion, cohesion, and surface tension. The role of negative pressure fits with what you learned earlier about the water potential equation because negative pressure (tension) lowers water potential. Since water moves from where its potential is higher to where it is lower, the increasingly negative pressure at the air-water interface causes xylem cells to lose water to mesophyll cells, which lose water to the airspaces, where it diffuses out through stomata. In short, the negative water potential of leaves provides the "pull" in transpirational pull.

0 Evaporation causes the air-water interface to retreat farther into the cell wall and become more curved as the rate of transpiration increases. As the interface becomes more curved, the water film's pressure becomes more negative. This negative pressure, or tension, pulls water from the xylem, where the pressure is greater. \

I In transpiration, water vapor (shown as biue dots) diffuses from the moist air spaces of the leaf to the drier air outside via stomata.

* . _ , 0.00 MPa

Q At first, the water vapor lost by transpiration is replaced by evaporation from the water film that coats mesophyll cells.

Figure 36.12 The generation of transpirational pull in a leaf. The negative pressure he air-water interface in the leaf is the physical basis of transpirational pull, which draws water of the xylem.

CHAPTER 36 Transport in Vascular Plants

747

Cohesion and Adhesion in the Ascent of Xylem Sap. The Xylem Sap Ascent by Bulk Flow: A Review transpirational pull on xylem sap is transmitted all the way The transpiration-cohesion-tension mechanism that transport from the leaves to the root tips and even into the soil solution xylem sap against gravity is an excellent example of how phy^ (Figure 36.13). Cohesion and adhesion facilitate this longcal principles apply to biological processes. In the long-distan distance transport. The cohesion of water due to hydrogen transport of water from roots to leaves by bulk flow, the mov bonding makes it possible to pull a column of sap from above ment of fluid is driven by a water potential difference at opp without the water molecules separating. Water molecules exitsite ends of a conduit. In a plant, the conduits are vessels ing the xylem in the leaf tug on adjacent water molecules, and chains of tracheids. The water potential difference is generate I this pull is relayed, molecule by molecule, down the entire at the leaf end by transpirational pull, which lowers the wat column of water in the xylem. The strong adhesion of water potential (increases tension) at the "upstream" end of the xylen molecules (again by hydrogen bonds) to the hydrophilic walls On a smaller scale, water potential gradients drive the o of xylem cells aids in offsetting the downward pull of gravity. motic movement of water from cell to cell within root and le f The upward pull on the sap creates tension within the xylem. tissue (see Figure 36.13). Differences in both solute concei Pressure will cause an elastic pipe to swell, but tension will pull tration and turgor pressure contribute to this short-distam the walls of the pipe inward. You can actually measure a decrease in the diameter of a tree trunk on a warm day, when transpira-Xylem tional pull puts the xylem under tension. sap The thick secondary walls prevent vessels Mesophy Outside from collapsing, much as wire rings maincells = -100.0 MPa tain the shape of a vacuum hose. The tension produced by transpirational pull lowers water potential in the root xylem to such an extent that water flows passively from the soil, across the root cortex, and into the vascular cylinder. Transpirational pull can extend down to the roots only through an unbroken chain of water molecules. Cavitation, the formation of a water vapor pocket in a vessel, such as when xylem sap freezes in winter, breaks the chain. The air bubbles resulting from cavitation expand and become embolisms, blockages of the water channels of the xylem. The rapid expansion of air bubbles during cavitation produces clicking noises that can be heard by placing sensitive microphones at the surface of the stem. Root pressure enables small plants to refill embolized vessels in spring. In trees, though, root pressure cannot push water to the top, so a vessel with a water vapor Root xylem pocket usually cannot function as a water = - 0 . 6 MPa Soil pipe again. However, the chain of moleparticle Soil*? cules can detour around the pocket = - 0 . 3 MPa through pits between adjacent tracheids Water uptake or vessels, and secondary growth adds a from soil layer of new xylem each year. Only the A Figure 36.13 Ascent of x y l e m sap. Hydrogen bonding forms an unbroken chain of wate youngest, outermost secondary xylem molecules extending from leaves all the way to the soil. The force that drives the ascent of xylem transports water. Although no longer sap is a gradient of water potential (V). For the bulk flow over long distance, the ¥ gradient is due mainly to a gradient of the pressure potential ( ¥ P ). Transpiration results in the ¥ P at the leaf end c functional in water transport, the older the xylem being lower than the ¥ P at the root end. The ¥ values shown at the left are a secondary xylem does provide support "snapshot." During daylight, these specific values may vary, but the direction of the water potential gradient remains the same. for the tree (see Figure 35.20). 748

U N I T six

Plant F o r m a n d Function

msport. In contrast, bulk flow depends only on pressure. LOther contrast with osmosis, which moves only water, is tl at bulk flow moves the whole solution, water plus minerals d any other solutes dissolved in the water. The plant expends no energy to lift xylem sap by bulk flow, stead, the absorption of sunlight drives transpiration by causwater to evaporate from the moist walls of mesophyll cells Ld by lowering the water potential in the air spaces within a kaf. Thus, the ascent of xylem sap is ultimately solar powered. Concept Check . What would be the effect of fertilizing a plant during a drought? . Plants called epiphytes, including many orchid species, live in the very humid tropics and grow on tree trunks. Epiphytes have no contact with the soil but can absorb water from the air. How is this possible? . A tip for helping cut flowers last longer without wilting is to cut off the ends of the stems underwater and then transfer the flowers to a vase while water droplets are still present on the cut ends of the stems. Explain why this works. For suggested answers, see Appendix A.

Concept

Stomata help regulate the rate of transpiration I eaves generally have broad surface areas and high surface areaD-volume ratios. The broad surface area is a morphological daptation that enhances the absorption of light needed to drive photosynthesis. The high surface area-to-volume ratio aids in uptake of carbon dioxide during photosynthesis as well as in

the release of oxygen produced as a by-product of photosynthesis. Upon diffusing through the stomata, CO2 enters a honeycomb of air spaces formed by the spongy parenchyma cells (see Figure 35.17). Because of the irregular shape of these cells, the internal surface area of the leaf may be 10 to 30 times greater than the external surface area we see when we look at the leaf. Although broad surface areas and high surface area-tovolume ratios increase photosynthesis, they also have the serious drawback of increasing water loss by way of the stomata. Thus, a plants tremendous requirement for water is part of the cost of making food by photosynthesis. By opening and closing the stomata, guard cells help balance the plants requirement to conserve water with its requirement for photosynthesis (Figure 36.14).

Effects of Transpiration on Wilting and Leaf Temperature A leaf may transpire more than its weight in water each day, and water may move through the xylem at a rate as fast as 75 cm/min, about the speed of the tip of a second hand sweeping around a wall clock. If transpiration continues to pull sufficient water upward to the leaves, they will not wilt. But the rate of transpiration is greatest on a day that is sunny, warm, dry, and windy because these environmental factors increase evaporation. Although plants can adjust to such conditions by regulating the size of the stomatal openings, some evaporative water loss does occur even when the stomata are closed. Under these prolonged drought conditions, the leaves begin to wilt as their cells lose turgor pressure (see Figure 36.7). Transpiration also results in evaporative cooling, which can lower the temperature of a leaf by as much as 10-15°C compared with the surrounding air. This prevents the leaf from reaching temperatures that could denature various enzymes involved in photosynthesis and other metabolic processes. Cacti and other desert succulents, which have low rates of transpiration, can tolerate high leaf temperatures; in this case, the loss of water due to transpiration is a greater threat than overheating. Evolution of the cactus's biochemistry has facilitated survival in high temperatures.

•« Figure 36.14 Open stomata (left) and closed stomata (colorized SEM).

CHAPTER 36

Transport in Vascular Plants

749

Stomata: Major Pathways for Water Loss

Cells t u r g i d / S t o m a open " Radially oriented cellulose microfibrils

About 90% of the water a plant loses escapes through stomata, though these pores account for only 1-2% of the external leaf surface. The waxy cuticle limits water loss through the remaining surface of the leaf. Each stoma is flanked by a pair of guard cells, which are kidney-shaped in eudicots and dumbbell-shaped in many monocots. Guard cells control the diameter of the stoma by changing shape, thereby widening or narrowing the gap between the two cells (Figure 36,15a). The amount of water lost by a leaf depends on the number of stomata and the average size of their apertures. The stomatal density of a leaf, which may be as high as 20,000 per square centimeter, is under both genetic and environmental control. For example, as a result of evolution by natural selection, desert plants have lower stomatal densities than do marsh plants. Stomatal density, however, is also a developmentally plastic feature of many plants. High light intensities and low carbon dioxide levels during leaf development tend to increase stomatal density in many plant species. By measuring the stomatal density of leaf fossils, scientists have been able to gain insight into the levels of atmospheric CO2 in past climates. A recent British survey found that the stomatal density of many woodland species has decreased since 1927, when a similar survey was made. This survey is consistent with the finding that atmospheric CO2 levels increased dramatically during the 20th century as a result of the increased burning of fossil fuels. When guard cells take in water from neighboring cells by osmosis, they become more turgid and bowed. In most angiosperm species, the cell walls of guard cells are uneven in thickness, and the cellulose microfibrils are oriented in a direction that causes the guard cells to buckle outward when they are turgid, as you can see in Figure 36.15a. This buckling increases the size of the pore between the guard cells. When the cells lose water and become flaccid, they become less bowed and close the pore. The changes in turgor pressure that open and close stomata result primarily from the reversible uptake and loss of potassium ions (K+) by the guard cells. Stomata open when guard cells actively accumulate K+ from neighboring epidermal cells (Figure 36.15b). This uptake of solute causes the water potential to become more negative within the guard cells, and the cells become more turgid as water enters by osmosis. Most of the K+ and water are stored in the vacuole, and thus the vacuolar membrane also plays a role in regulating the water potential of guard cells. Stomatal closing results from an exit of K+ from guard cells to neighboring cells, which leads to an osmotic loss of water. Regulation of aquaporins may also be involved in the swelling and shrinking of guard cells by varying the permeability of the membranes to water. The K+ fluxes across the guard cell membrane are coupled to the generation of membrane potentials by proton pumps. 750

UNIT six

Plant Form and Function

Cells flaccid/Stoma closed

.Guard cell (a) Changes in guard cell shape and stomatal opening and closin (surface view). Guard cells of a typical angiosperm are illustrated i their turgid (stoma open) and flaccid (stoma closed) states. The pair of guard cells buckle outward when turgid. Cellulose microfibrils in the walls resist stretching and compression in the direction parallel t the microfibrils. Thus, the radial orientation of the microfibrils cause the cells to increase in length more than width when turgor increases. The t w o guard cells are attached at their tips, so the increase in length causes buckling.

H2O - ' • H , 0 •

K

V.,

'VI

'•«• I 1

(b) Role of potassium in stomatal o p e n i n g and closing. The transport of K+ (potassium ions, symbolized here as red dots) across the plasma membrane and vacuolar membrane causes the turgor changes of guard cells.

A Figure 36.15 The mechanism of stomatal opening and closing.

Stomatal opening correlates with active transport of H out o the guard cell. The resulting voltage (membrane potential drives K into the cell through specific membrane channel (see Figure 36.4a). In general, stomata are open during the day and closed a night. This prevents the plant from losing water when it is to dark for photosynthesis. At least three cues contribute I stomatal opening at dawn. First, light itself stimulates guan cells to accumulate K+ and become turgid. This response i triggered by the illumination of a blue-light receptor in th plasma membrane of guard cells. Activation of these blue light receptors stimulates the activity of ATP-powered protor pumps in the plasma membrane of the guard cells, in turr promoting the uptake of K+.

A second stimulus causing stomata to open is depletion of CQ>2 within air spaces of the leaf, which occurs when photosy: ithesis begins in the mesophyll. A plant will actually open iis mata at night if it \s placed in a chamber devoid of CO2. A third cue causing stomatal opening is an internal "clock" the guard cells. Even if you keep a plant in a dark closet, mata will continue their daily rhythm of opening and clos*. All eukaryotic organisms have internal clocks that regue cyclic processes. Cycles that have intervals of appropriately 24 hours are called circadian rhythms. You will learn ore about circadian rhythms and the biological clocks that ntrol them in Chapter 39. Environmental stresses can cause stomata to close during e daytime. When the plant suffers a water deficiency, guard ils may lose turgor and close stomata. In addition, a horone called abscisic acid, which is produced in the roots in sponse to water deficiency, signals guard cells to close stomThis response reduces further wilting but also restricts 3take of CO2 and thereby slows photosynthesis. This is one ason why droughts reduce crop yields. Guard cells arbitrate the photosynthesis-transpiration comomise on a moment-to-moment basis by integrating a variy of internal and external stimuli. Even the passage of a .oud or a transient shaft of sunlight through a forest canopy an affect the rate of transpiration.

[erophyte Adaptations That Reduce Transpiration .ants adapted to arid climates, called xerophytes, have varius leaf modifications that reduce the rate of transpiration, /[any xerophytes have small, thick leaves, an adaptation that mits water loss by reducing surface area relative to leaf volume. A thick cuticle gives some of these leaves a leathery consistency. Some other xerophyte adaptations are highly reflective eaves and hairy leaves that trap a boundary layer of water. The komata of xerophytes are concentrated on the lower (shady) leaf surface, and they are often located in depressions that ihelter the pores from the dry wind (Figure 36.16). During the .riest months, some desert plants shed their leaves. Others, ,uch as cacti, subsist on water the plant stores in fleshy stems luring the rainy season. An elegant adaptation to arid habitats is found in succulents of the family Crassulaceae, in ice plants, and in many other plant families. These plants assimilate CO2 by an alternative photosynthetic pathway known as CAM, for crassulacean acid metabolism (see Figure 10.20). Mesophyll cells in a CAM plant have enzymes that can incorporate CO2 into organic acids during the night. During the daytime, the organic acids are broken down to release CO2 in the same cells, and sugars are synthesized by the conventional (C3) photosynthetic pathway Because the leaf takes in CO2 at night, the stomata can close during the day, when transpiration would be greatest.

Lower epiderrna tissue

A Figure 36.16 Structural adaptations of a xerophyte leaf. Oleander (Nerium oleander), shown in the inset, is commonly found in arid climates. The leaves have a thick cuticle and multiple-layered epidermal tissue that reduce water loss. Stomata are recessed in "crypts," an adaptation that reduces the rate of transpiration by protecting the stomata from hot, dry wind. The trichomes ("hairs") also help minimize transpiration by breaking up the flow of air, allowing the chamber of the crypt to have a higher humidity than the surrounding atmosphere (LM).

Concept Check 1. Some leaf molds, which are fungi that parasitize plants, secrete a chemical that causes guard cells to accumulate potassium ions. How does this adaptation enable the leaf mold to infect the plant? 2. Describe the environmental conditions that would minimize the transpiration-to-photosynthesis ratio for a C3 plant, such as an oak tree. For suggested answers, see Appendix A.

Concept

Organic nutrients are translocated through the phloem Xylem sap flows from roots to leaves, in a direction opposite to that necessary to transport sugars from leaves to other parts of the plant. It is a second vascular tissue, the phloem, that CHAPTER 36

Transport in Vascular Plants

751

transports the products of photosynthesis. This transport of organic nutrients in the plant is called translocation.

Movement from Sugar Sources to Sugar Sinks In angiosperms, the specialized cells of phloem that function as the conduits for translocation are the sieve-tube members, arranged end to end to form long sieve tubes. Between the cells are sieve plates, structures that allow the flow of sap along the sieve tube (see Figure 35.9). Phloem sap is an aqueous solution that differs markedly in composition from xylem sap. By far the most prevalent solute m phloem sap is sugar, primarily the disaccharide sucrose in most species. The sucrose concentration may be as high as 30% by weight, giving the sap a syrupy thickness. Phloem sap may also contain minerals, amino acids, and hormones. In contrast to the unidirectional transport of xylem sap from roots to leaves, the direction that phloem sap travels is variable. However, sieve tubes always carry sugars from a sugar source to a sugar sink. A sugar source is a plant organ that is a net producer of sugar, by photosynthesis or by breakdown of starch. Mature leaves are the primary sugar sources. A sugar sink is an organ that is a net consumer or storer of sugar. Growing roots, buds, stems, and fruits are sugar sinks. A storage organ, such as a tuber or a bulb, may be a source or a sink, depending on the season. When stockpiling carbohydrates in the summer, it is a sugar sink. After breaking dormancy in Lhe spring, it is a source as its starch is broken down to sugar, which is carried to the growing tips of the plant.

A sugar sink usually receives sugar from the nearest sourc Upper leaves on a branch may send sugar to the growing sho tip, whereas lower leaves export sugar to roots. A growing fh may monopolize sugar sources around it. For each sieve tul the direction of transport depends on the locations of t source and sink connected by that tube. Therefore, neighbori tubes may carry sap in opposite directions. Direction of flc may also vary by season or developmental stage of the plant. Sugar must be loaded into sieve-tube members before bei exported to sinks. In some species, it moves from mesophyll ce to sieve-tube members via the symplast, passing through pis modesmata. In other species, it moves by symplastic a] apoplastic pathways (Figure 36.17a). In maize leaves, for exar pie, sucrose diffuses through the symplast from chloropla containing mesophyll cells into small veins. Much of it th moves into the apoplast and is accumulated by nearby sieve-tu members, either directly or through companion cells, in son plants, companion cells have many ingrowths of their walls, e hancing transfer of solutes between apoplast and sympla Such modified cells are called transfer cells (see Figure 29.5 In maize and many other plants, phloem loading requir active transport because sucrose concentrations in sieve-tu members are two to three times higher than in mesophy Proton pumping and cotransport of sucrose and H+ enab the cells to accumulate sucrose (Figure 36.17b). Phloem unloads sucrose at the sink end of a sieve tube. T process varies by plant species and type of organ. However, t concentration of free sugar in the sink is always lower than the sieve tube because the unloaded sugar is either consume during growth and metabolism of the sink cells or convert into insoluble polymers such as starch. As a result of this sug concentration gradient, sugar molecules diffuse from L phloem into the sink tissues, and water follows by osmosis.

Cotransporter

/ Mesophyll cell

Bundlesheath cell

Phloem parenchyma cel

(a) Sucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sieve-tube members. In some species, sucrose exits the symplast (red arrow) near sieve tubes and is actively accumulated from the apoplast by sieve-tube members and their companion cells.

A Figure 36.17 Loading of sucrose into phloem.

752

UNIT six

Plant Form and Function

(b) A chemiosmotic mechanism is responsible for the active transpi of sucrose into companion cells and sieve-tube members. Proto pumps generate an H + gradient, which drives sucrose accumulation with the help of a cotransport protein that coupl sucrose transport to the diffusion of H + back into the cell.

Pressure Flow: The Mechanism of Tr dislocation in Angiosperms Phoem sap flows from source to sink at rates as great as 1 m/hr, ch too fast LO be accounted for by either diffusion or cyto;mic streaming. In studying angiosperms, researchers have eluded that sap moves through a sieve tube by bulk flow /en by positive pressure (thus the synonym pressure flow), shown in Figure 36.18. The building of pressure at the rce end and reduction of that pressure at the sink end se water to flow from source to sink, carrying the sugar ng. Xylem recycles the water from sink to source. The pressure flow hypothesis explains why phloem sap alys flows from source to sink. Experiments such as the one described in Figure 36.19 build a strong case for pressure flow as the mechanism of translocation in angiosperms. It is not yet krown if this model applies to other vascular plants. We have seen examples of sugar transport on three levels: at th: cellular level across plasma membranes (sucrose accumulal by active transport in phloem); short-distance transport n organs (sucrose migration from mesophyll to phloem the symplast and apoplast); and long-distance transport beeen organs (bulk flow in sieve tubes). Understanding these

v-ess* >ssel XV16 IT]':

• 'Sieve tube!,: '(phloem)

Source ceil

0 Loading of sugar (green dots) into the sieve tube at the source reduces water potential inside the sieve-tube members. This causes the tube to take up water by osmosis.

Figure 36.19

jcjyiry What causes phloem sap to flow from source to sink? test the pressure flow hypothesis, researchers used aphids that feed on phloem sap. An aphid probes with a hypodermic-like mouthpart called a stylet that penetrates a sieve-tube member. As sieve-tube pressure force-feeds aphids, they can be severed from their stylets, which serve as taps exuding sap for hours. Researchers measured the flow and sugar concentration of sap from stylets at different points between a source and sink.

25 jam

Aphid feeding

Stylet in sieve-tube member

Severed stylet exuding sap

The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration. The results of such experiments support the pressure flow hypothesis.

O This uptake of water generates a positive pressure that forces the sap to flow along the tube.

!

Sink cell (storage root)

(§) The pressure is relieved by the unloading of sugar and the consequent loss of water from the tube at the sink. © In the case of leaf-to-root translocation, xylem recycles water from sink to source.

Concent Check 1. Compare and contrast the forces that move phloem sap versus the forces that move xylem sap over long distance. 2. Potatoes break down starch into sugar at low temperature. (This is a problem for the potato chip industry because the sugar in chilled potatoes turns dark brown during processing.) What effect would cooling the soil around an expanding potato tuber have on sugar import into it? For suggested answers, see Appendix A.

Figure 36.18 Pressure flow in a sieve tube.

CHAPTER 3S

Transport in Vascular Plants

753

Chapter Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

Review • Xylem Sap Ascent by Bulk Flow: A Review (pp. 748-74*) The movement of xylem sap against gravity is maintained by transpiration. Activity Transport of Xylem Sap

SUMMARY OF KEY CONCEPTS Stomata help regulate the rate of transpiration Physical processes drive the transport of materials in plants over a range of distances • Selective Permeability of Membranes: A Review {pp. 738-739) Specific transport proteins enable plane cells to maintain an internal environment different from their surroundings. • The Central Role of Proton Pumps (pp. 739-740) The membrane potential and H gradient generated by proton pumps are harnessed to drive the transport of a variety of solutes. • Effects of Differences in Water Potential (pp. 740-742) Solutes decrease water potential, while pressure increases water potential. Water flows by osmosis from a region with higher water potential to a region with a lower potential. • Three Major Compartments of Vacuolated Plant Cells (p. 743) The plasma membrane regulates transport between the cytosol and cell wall, while the vacuolar membrane regulates transport between the cytosol and vacuole. • Functions of the Symplast and Apoplast in Transport (p. 743) The symplast is the continuum of cytosol linked by plasmodesmata. The apoplast is the continuum of cell walls and the extracellular spaces. • Bulk Flow in Long-Distance Transport (pp. 743-744) Transport of xylem and phloem sap is due to pressure differences at opposite ends of conduits—xylem vessels and sieve tubes.

Roots absorb water and minerals from the soil • The Roles of Root Hairs, Mycorrhizae, and Cortical Cells (p. 744) Root hairs are the most important avenues of absorption near root tips, but mycorrhizae, symbiotic associations of fungi and roots, are responsible for most absorption by the whole root system. Once soil solution enters the root, the extensive surface area of cortical cell membranes enhances uptake of water and selected minerals. • The Endodermis: A Selective Sentry (pp. 744-746) Water can cross the cortex via the symplast or apoplast, but minerals that reach the endodermis via the apoplast must finally cross the selective membranes of endodermal cells. The waxy Casparian strip of the endodermal wall blocks apoplastic transfer of minerals from the cortex to the vascular cylinder.

Water and minerals ascend from roots to shoots through the xylem • Factors Affecting the Ascent of Xylem Sap (pp. 746-748) Loss oi water vapor (transpiration) lowers water potential in the leaf by producing a negative pressure (tension). This low water potential draws water from the xylem. Cohesion and adhesion of the water transmit the pulling force down to the roots.

754

UNIT six

Plant Form and Function

• Effects of Transpiration on Wilting and Leaf Temperatu re (p. 749) Plants lose an astonishing amount of water as a result of transpiration. If this lost water is not replaced by water absorption from the roots, the plant will gradually lose water and wilt. Because evaporative cooling is reduced, wilted plants may become overheated. • Stomata: Major Pathways for Water Loss (pp. 750-751 Stomata support photosynthesis by allowing CO 2 and O2 exchange between the leaf and atmosphere, but these pores are also die main avenues for transpi rational loss of water from the plant. Turgor changes in guard cells, which depend on K+ and water transport into and out of the cells, regulate the size of thi siomatal openings. Investigation How Is the Rate of Transpiration Calculated? • Xerophyte Adaptations That Reduce Transpiration (p. 751) Protection of stomata within leaf indentations and other structural adaptations enable certain plants to survive in arid environments.

Organic nutrients are translocated through the phloei • Movement from Sugar Sources to Sugar Sinks (p. 752) Mature leaves are the main sugar sources, though storage organ such as bulbs can be sugar sources during certain seasons. Developing roots and shoot tips are some examples of sugar sinks. Phloem loading and unloading depend on the active transport of sucrose. The sucrose is cotransported along with H~\ which is diffusing down a gradient that is generated by proton pumps. Activity Translocation of Phloem Sap • Pressure Flow: The Mechanism of Translocation in An giosperms (p. 753) Loading of sugar at the source end of a sieve tube and unloading at the sink end maintain a pressure difference that keeps the sap flowing through the tube.

TESTING YOUR KNOWLEDGE

Evolution Connection Analysis of preserved leaves shows that the density of stomata per unit area of leaf has decreased over the past 200 years. Suggest a hypothesis that connects this evolutionary trend to environmental change.

Scientific Inquiry Barrel cacti (Ferocactus) of the Sonoran Desert do not grow straight up, but tilt southward at about a 45° angle. Suggest a hypothesis for the function of this evolutionary adaptation. How coipld you test your hypothesis? in\ estigation How Is the Rate of Transpiration Calculated?

Science, Technology, and Society W; ter use is a serious social and environmental issue in the arid southwestern United States, In recent years, there has been growing criticism of water-intensive ornamental landscapes like lawns and golf greens. These areas are maintained artificially by diverting water from rivers and streams or pumping from ancient subterranean lifers. Is this form of water use something society should limit or :n eliminate in such areas? Or should property owners be free to dscape as they choose? Defend your side in this debate.

CHAPTER 36

Transport in Vascular Plants

755

A Figure 37.1 Root and shoot systems of a pea seedling.

37.1 Plants require certain chemical elements to complete their life cycle 37.2 Soil quality is a major determinant of plant distribution and growth 37.3 Nitrogen is often the mineral that has the greatest effect on plant growth 37.4 Plant nutritional adaptations often involve relationships with other organisms

A Nutritional Network

E

very organism continuously exchanges energy and materials with its environment. At the level of the ecosystem, plants and other photosynthetic autotrophs perform the key step of transforming inorganic compounds into organic compounds. Autotrophic, however, does not mean autonomous. Plants need light as the energy source for photosynthesis. In order to synthesize organic matter, plants also require raw materials in the form of inorganic nutrients: water, minerals, and carbon dioxide. For a typical plant, water and minerals come from the soil, while carbon dioxide comes from the air. The branching root system and the shoot system of a vascular plant (Figure 37.1) ensure extensive networking with both of these reservoirs of inorganic nutrients.

In Chapter 36, you studied the mechanisms by which vascular plants transport water, minerals, and organic nutrients. Here you will learn more about nutritional requirements and examine some nutritional adaptations that have evolved in plants, often in relationship with other organisms. 756

Concept

Plants require certain chemical elements to complete their life cycle Watch a large plant grow from a tiny seed, and you cannot help wondering where all the mass comes from. Aristotle thought that soil provided the substance for plant growth because plants seemed to spring from the ground. He believed that leaves of flowering plants simply shaded developing fru t. In the 17th century, Jan Baptista van Helmont performed an experiment to test the hypothesis that plants grow by consur iing soil. He planted a small willow in a pot that containi 90.9 kg of soil. After five years, the willow had grown into tree weighing 76.8 kg, but only 0.06 kg of soil had disappeared from the pot. He concluded that the willow had grown mainlly from the water he had added regularly. A century later, i n English physiologist named Stephen Hales postulated thit plants are nourished mostly by air. There is some truth to all three hypotheses because so 1, water, and air all contribute to plant growth (Figure 37.2). Plants extract mineral nutrients, essential chemical elements, from the soil in the form of inorganic ions. Plants acquire nitrogen, for example, in the form of nitrate ions (NO3~). Howevc r, mineral nutrients add little to the plants overall mass. Typical y, 80-90% of a plant is water, and plants grow mainly by accum ilating water in the central vacuoles of their cells. Water is also a nutrient that supplies most of the hydrogen atoms and some pf the oxygen atoms incorporated into organic compounds I y photosynthesis (see Figure 10.4). Still, only a small fraction pf the water that enters a plant contributes atoms to organic mol :cules. For example, it has been estimated that more than 90%

Roots take in 2 and expel CO2. The plant uses O 2 for cellular respiration but is a ne producer.

Plants growing on mine tailings, for instance, may contain gold or silver, but these minerals have no nutritional function. To determine which chemical elements are essential elements, researchers use hydroponic culture, in which plants are grown without soil by using mineral soluiions (Figure 37.3). Such studies have helped identify 17 essential elements that are needed by all plants (Table 37.1, on the next page). Nine of the essential elements are called macronutrients because plants require them in relatively large amounts. Six of these are the major components of organic compounds forming the structure of a plant: carbon, oxygen, hydrogen, nitrogen, phosphorus, and sulfur. The other three macronutrients are potassium, calcium, and magnesium. The remaining eight essential elements are known as micronutrients because plants need them in very small amounts. They are chlorine, iron, manganese, boron, zinc, copper, nickel, and molybdenum. Micronutrients function in plants mainly as cofactors, non-protein helpers in enzymatic reactions (see Chapter 8). Iron, for example, is a metallic component of cytochromes, the proteins in the electron transport

A igure 37.2 The uptake of nutrients by a plant: a r e v i e w . From C 0 2 , 0 2 , H 2 0, and minerals, the plant produces all iof its own organic material. See also Figure 36.2.

water absorbed by maize plants is lost by transpiration. The water retained by a plant serves three main functions: It acts as a so vent, provides most of the volume for cell elongation, and helps maintain the form of soft tissue by keeping cells turgid. By weight, the bulk of a plant's organic material is derived not from water or soil minerals, but from the CO2 that is assimilated from the air. We can measure water content by comparing the weight of plant material before and after it is dried. We can then analyze the chemical composition of the dry residue. Organic substances account for about 96% of the dry weight, with inorganic substances making up the remaining 4%. Most of the orjganic material is carbohydrate, including the cellulose of cejll walls. Thus, the components of carbohydrates—carbon, oxygen, and hydrogen—are the most abundant elements in the dry weight of a plant. Because some organic molecules contain nitrogen, sulfur, or phosphorus, these elements are aho relatively abundant in plants.

Hydroponic Culture In hydroponic culture, plants are grown in mineral solutions without soil. One use of hydroponic culture is to identify essential elements in plants. Plant roots are bathed in aerated solutions of known mineral composition. Aerating the water provides the roots with oxygen for cellular respiration. A particular mineral, such as potassium, can be omitted to test whether it is essential.

Macronutrients and Micronutrients Wore than 50 chemical elements have been identified among tr e inorganic substances in plants, but not all of these elements are essential. A chemical element is considered an essential element if it is required for a plant to complete a life cycle and produce another generation. In studying the chemical composition of plants, we must distinguish elements that are essential ft 3m those that are merely present in the plant. To some extent, tl e chemical elements in a plant reflect the soil composition.

Control: Solution containing all minerals

Experimental: Solution without potassium

If the omitted mineral is essential, mineral deficiency symptoms occur, such as stunted growth and discolored leaves. Deficiencies of different elements may have different symptoms, which can aid in diagnosing mineral deficiencies in soil.

CHAPTER 37

Plant Nutrition

757

Table 37.1 Essential Elements in Plants

Element Macronutrients Carbon

Form Available

% Mass in

to Plants

Dry Tissue

Major Functions

co2 co2

45%

Oxygen

45%

Major component of plant's organic compounds

Hydrogen

H2O

6%

Major component of plant's organic compounds

Nitrogen

NO 3 ", NH,*

1.5%

Potassium

K*

1.0%

Component of nucleic acids, proteins, hormones, chlorophyll, coehzymes Cofactor that functions in protein synthesis; major solute functioning in water balance; operation of stomata

Calcium

2

Major component of plant's organic compounds

Ca *

0.5%

Magnesium

Mg2+

0.2%

Component of chlorophyll; activates many enzymes

Phosphorus

H 2 PO 4 "",HPO 4 2 "

0.2%

Component of nucleic acids, phospholipids, ATP, several coenzymes

Sulfur Micronutnents Chlorine

SO 4 2 "

Iron

Fe , Fe

Manganese

Component of proteins, coenzymes

cr 3+

Important in formation and stability of cell walls and in maintenance of membrane structure and permeability; activates some enzymes; regulates many responses of cells to stimuli

Required for water-splitting step of photosynthesis; functions in water balance 2+

Mn2+

0.01% 0.005%

Component of cyiochromes; activates some enzymes Active in formation of amino acids; activates some enzymes; required for water-splitting step of photosynthesis Cofactor in chlorophyll synthesis; may be involved in carbohydrate transport and nucleic acid synthesis; role in cell wall function

Zinc

0.002%

Active in formation of chlorophyll; activates some enzymes

Copper

Cu + , Cu" +

< 0.001%

Component of many redox and lignin-biosynthetic enzymes

Nickel

Ni 2 +

< 0.001%

Cofactor for an enzyme functioning in nitrogen metabolism

< 0.0001%

Essential for symbiotic relationship with nitrogen-fixing bacteria; cofactor that functions in nitrate reduction

Molybdenum

MoO4

chains of chloroplasts and mitochondria. It is because micronutrients generally play catalytic roles that plants need only minute quantities. The requirement for molybdenum, for instance, is so modest that there is only one atom of this rare element for every 60 million atoms of hydrogen m dried plant material. Yet a deficiency of molybdenum or any other micronutrient can weaken or kill a plant.

Symptoms of Mineral Deficiency The symptoms of a mineral deficiency depend partly on the nutrients function. For example, a deficiency of magnesium, a component of chlorophyll, causes yellowing of the leaves, known as chlorosis. In some cases, the relationship between a mineral deficiency and its symptoms is less direct. For instance, iron deficiency can cause chlorosis even though chloro758

UNIT six

Plant Form and Function

phyll contains no iron, because iron ions are required as a c factor in one of the enzymatic steps of chlorophyll synthesis Mineral deficiency symptoms depend not only on the role pf the nutrient but also on its mobility within the plant. If a nutrient moves about freely, symptoms will show up first in older organs because young, growing tissues have more "drawing power" for nutrients in short supply For example, magnesium is relatively mobile and is shunted preferentially to young leaves. Therefore, a plant starved for magnesium will show signs of chlorosis first in its older leaves. The mechanism for preferential routing is the source-to-sink translocation in phloem as minerals move along with the sugars to the growing tissues (see Figure 36.18). In contrast, a deficiency of a mineral that is relatively immobile will affect young parts of the plant first. Older tissuks may have adequate amounts, which they are able to retain duling periods of short supply For example, iron does not mole

Concept

Soil quality is a major determinant of plant distribution and growth

• Figure 37.4 The most common mineral deficiencies, as se n in m a i z e leaves. Phosphate-deficient plants have reddish purple margins, particularly in young leaves. Potassium-deficient plants exhibit "firing," or drying, along tips and margins of older leaves. Nitrogen deficiency is evident in a yellowing that starts at the tip and moves along the center (midrib) of older leaves.

freely within a plant, and an iron deficiency will cause yellowing of young leaves before any effect on older leaves is visible. Deficiencies of nitrogen, phosphorus, and potassium are mjst common. Shortages of micronutrients are less common arl d tend to occur in certain geographic regions because of differences in soil composition. The symptoms of a mineral deficiency are often distinctive enough for a plant physiologist or farmer to diagnose its cause (Figure 37.4). One way to confirm a diagnosis is to analyze the mineral content of the plant and sail. The amount of a micronutrient needed to correct a deficiency is usually quite small. For example, a zinc deficiency in fruit trees can usually be cured by hammering a few zinc nails into each tree trunk. Moderation is important because overdoses of many nutrients can be toxic to plants. Hydropomc culture can ensure optimal mineral nutrition by using nutrient solutions that can be precisely regulated. However, this method is not used widely in agriculture because it is relatively expensive compared with growing crops in soil.

IConcept Check 1. Explain how Table 37.1 can be used to support Hales' hypothesis, yet does not refute van Helmonts hypothesis. 2. Are some essential elements more important than others? Explain. 3. Can a single leaf be used to diagnose all of a plants mineral deficiencies? Explain. For suggested answers, see Appendix A.

Along with climate, the major factors determining whether particular plants can grow well in a certain location are the texture and composition of the soil Texture is the soil's general structure, referring to the relative amounts of various sizes of soil particles. Composition refers to the soil's organic and inorganic chemical components. Plants that grow naturally in a certain type of soil are adapted to its texture and composition and can absorb water and extract essential mineral nutrients. In turn, plants affect the soil, as you will soon see- The soil-plant interface is a critical part of the chemical cycles that sustain terrestrial ecosystems.

Texture and Composition of Soils Soil has its origin in the weathering of solid rock. The freezing of water that has seeped into crevices can mechanically fracture rocks. Acids dissolved in the water can also help break rocks down chemically When organisms are able to invade the rock, they accelerate breakdown by chemical and mechanical means. Some organisms, for example, secrete acids that dissolve the rock. Roots that grow in fissures lead to mechanical fracturing. The eventual result of all this activity is topsoil, a mixture of particles derived from rock, living organisms, and humus, the remains of partially decayed organic material. The topsoil and other distinct soil layers, or horizons, are olten visible in vertical profile where there is a road cut or deep hole (Figure 37.5, on the next page). The topsoil, also known as the A horizon, is the richest in organic material and is therefore most important for plant growth. The texture of topsoil depends on the sizes of its particles, which are classified in a range from coarse sand to microscopic clay particles. The most fertile soils are usually loams, made up of roughly equal amounts of sand, silt (particles of intermediate size), and clay. Loamy soils have enough fine particles to provide a large surface area tor retaining minerals and water, which adhere to the particles. But loams also have enough coarse particles to provide air spaces containing oxygen that can be used by roots for cellular respiration. II soil does not drain adequately, roots suffocate because the air spaces are replaced by water; the roots may also be attacked by molds that favor soaked soil- These are common hazards tor houseplants that are overwatered in pots with poor drainage. Soil composition includes organic components as well as minerals. Topsoil is home to an astonishing number and variety of organisms. A teaspoon of topsoil has about 5 billion CHAPTER 37 Plant Nutrition

759

The A horizon is the topsoil, a mixture of broken-down rock of various textures, living organisms, and decaying organic matter. The B horizon contains much less organic matter than the A horizon and is less weathered. The C horizon, composed mainly of partially broken-down rock, serves as the "parent" material for the upper layers of soil.

< Figure 37.5 Soil horizons. This researcher photographing a vertical profile of three soil layei or horizons.

bacteria that cohabit with various fungi, algae and other protists, insects, earthworms, nematodes, and roots of plants. The activities of all these organisms affect the soil's physical and chemical properties- Earthworms, for instance, turn over and aerate the soil by their burrowing and add mucus that holds fine soil particles together. Meanwhile, the metabolism of bacteria alters the mineral composition of the soil. Plant roots can also affect soil composition and texture. For instance, they can affect soil pH by releasing organic acids, and they reintorce the soil against erosion. Humus, an important component of topsoil, consists of decomposing organic material formed by the action of bacteria and fungi on dead organisms, feces, fallen leaves, and other organic refuse. Humus prevents clay from packing together and builds a crumbly soil that retains water but is still porous enough for the adequate aeration of roots. Humus is also a reservoir of mineral nutrients that are returned gradually to the soil as microorganisms decompose the organic matter. After a heavy rainfall, water drains away from the larger spaces of the soil, but smaller spaces retain water because of its attraction to the negatively charged surfaces of clay and other soil particles. Some water adheres so tightly to soil particles that it cannot be extracted by plants. The film of water bound less tightly to the particles is the water generally available to plants (Figure 37.6a). It is not pure water but a soil solution containing dissolved minerals in the form of ions. Roots absorb this soil solution. To be available to roots, mineral ions must be released from the soil particles into the soil solution. Negatively charged ions (anions)—such as nitrate (NO 3 ), phosphate (H2PO4 ), and sulfate (SO4 )—are not bound tightly to

760

U N I T six

Plant Form and Function

the negatively charged soil particles and are therefore easily released. However, during heavy rain or irrigation, they are leached (drained) quickly into the groundwater, making them less available for uptake by roots. Positively charged ions (cations)—such as potassium (K+), calcium (Ca2+), arid magnesium (Mg2+)—are less likely to be leached because they bind closely to soil particle surfaces. Mineral cations become available for absorption when they enter the soil solution after being displaced from soil particles by cations in the form of H+. This process, called cation exchange, is stimulated by the roots, which add H+ to the soil solution (Figure 37.6b).

Soil Conservation and Sustainable Agriculture It may take centuries for a soil to become fertile through tl breakdown of rock and the accumulation of organic materis 1, but human mismanagement can destroy that fertility within few years. Soil mismanagement has been a recurring problei in human history. For example, the Dust Bowl was an ecolog ical and human disaster that took place in the southwestern Great Plains region of the United States in the 1930s. Before the arrival of farmers, the region was covered by hardy grasses that held the soil in place in spite of the long recurrent droughts and torrential rains characteristic of the regiom. However, in the late 1800s and early 1900s, many homesteaders settled in the region, planting wheat and raising cattle. These land uses left the topsoil exposed to erosion bywinds that often sweep over the area. Bad luck in the form of a few years of drought made the problem worse. Much of tVe topsoil was blown away, rendering millions of hectares of

Soil particle surrounded by film of water

/

Soil particle

2*

ft

CO 2 —•H 2 CO 3 —"-HCO 3 -+ H-

t

f

'

- / / Root hair

•"Air space

(a) Soil water. A plant cannot extract all the water in the soil because some of it is tightly held by hydrophilic soil particles. Water bound less tightly to soil particles can be absorbed by the root.

(b) Cation exchange in soil. Hydrogen ions (H+) help make nutrientsavailable by displacing positively charged minerals (cations such as Ca 2+ ) that were bound tightly to the surface of negatively charged soil particles. Plants contribute H+ by secreting it from root hairs and also by cellular respiration, which releases CO 2 into the soil solution, where it reacts with H2O to form carbonic acid (H2CO3). Dissociation of this acid adds H + to the soil solution.

Figure 37.6 The availability of soil water and minerals.

farmland useless and forcing hundreds of thousands of people abandon homes and land, a plight immortalized In John Steinbeck's The Grapes of Wrath. Better soil conservation practices could have preserved soil fertility and sustained agricultural productivity To understand soil conservation, we must first remember that agriculture can only be sustained by human intervention. In forests, grasslands, and other natural ecosystems, mineral nutrients are usually recycled by the decomposition of dead organic material in the soil. In contrast, when farmers harvest a crop, essential elements are diverted from the c lemical cycles going on in that location. In general, agnc llture depletes the mineral content of the soil. To grow 1,000 kg of wheat grain, the soil gives up 20 kg of nitrogen, 4 kg of phosphorus, and 4.5 kg of potassium. Each year, the soil fertility diminishes unless fertilizers replace lost minerals such as nitrogen, phosphorus, and potassium. Many crops also use far more water than the vegetation that once grew naturally on that land, lorcing farmers to irrigate. Prudent fertilization, thoughtful irrigation, and the prevention df erosion are three of the most important goals of soil conservation. Complementing soil conservation is soil reclamation, the goal of returning agricultural productivity to exhausted or damaged soil. More than 30% of the worlds farmland suffers from low productivity stemming from poor

soil conditions such as chemical contamination, mineral deficiencies, acidity, salinity and poor drainage. Fertilisers Prehistoric farmers may have started fertilizing their fields after noticing that grass grew faster and greener where animals had defecated. In developed nations today, most farmers use commercially produced fertilizers containing minerals that are either mined or prepared by industrial processes. These fertilizers are usually enriched in nitrogen, phosphorus, and potassium, the macro nutrients most commonly deficient in farm and garden soils. Fertilizers are labeled with a three-number code called the N-P-K ratio, indicating the content of these minerals. A fertilizer marked "15-10-5," tor instance, is 15% nitrogen (as ammonium or nitrate), 10% phosphorus (as phosphoric acid), and 5% potassium (as the mineral potash). Manure, fishmeal, and compost are called "organic" fertilizers because they are of biological origin and contain decomposing organic material. Before plants can use organic material, however, it must be decomposed into the inorganic nutrients thai roots can absorb. Whether from organic fertilizer or a chemical factory, the minerals a plant extracts are in the same form, but organic fertilizers release minerals gradually whereas minerals in

C H A P T E R 37

Pianl Nutrition

761

Irrigation

No phosphorus deficiency

Beginning

phosphorus deficiency

A Figure 37.7 Deficiency warnings from "smart" plants. Some plants have been genetically modified to signal an impending nutrient deficiency before irreparable damage or stunting occurs. For example, after laboratory treatments, the research plant Arabidopsis develops a blue color in response to an imminent phosphate deficiency.

Water is most often the limiting factor in plant growth. Irrigation can transform a desert into a garden, but farming in arid regions is a huge drain on water resources. Many rivers in the southwestern United States have been reduced to trickles by diversion of water for irrigation. Another problem is that irrigation in an arid region can gradually make the soil so salty that it may become completely infertile. Salts dissolved in irrigation water accumulate in the soil as the water evaporates, making the water potential of the soil solution more negative, thereby reducing water uptake by lowering the water-potential gradient from soil to roots (see Chapter 36). As the world population grows, more and more acres of arid land are being cultivated. New irrigation methods mjiy reduce the risks of running out of water or losing tarmland to salinization (salt accumulation). Drip irrigation is used for many crops m the western United States, resulting in less evaporation. In another approach, plant breeders are develo ) ing plant varieties that require less water.

commercial fertilizers are immediately available but may not be retained by the soil ior long. Excess minerals not absorbed by roots are usually wasted because they are often leached from the soil by rainwater or irrigation. To make matters worse, mineral runoff may pollute groundwater, streams, and lakes. Erosion Agricultural researchers are developing ways to maintain crop yields while reducing fertilizer use. One approach is Lo Topsoil from thousands of acres of farmland is lost to water genetically engineer "smart" plants that inform the grower and wind erosion each year in the United States alone. Certain when a nutrient deficiency is imminent—but bejore damage precautions, such as planting rows of trees as windbreaks, terhas occurred (Figure 37.7). One type of smart plant takes adracing hillside crops, and cultivating in a contour pattern, can vantage of a promoter (a DNA sequence that indicates where prevent loss of topsoil (Figure 37.8). Crops such as alfalfa and to begin transcription) that more readily binds RNA polywheat provide good ground cover and protect the soil better merase (the enzyme that drives transcription) when the phosthan maize and other crops that are usually planted in mo"e phorus content of plant tissues begins to decline. This promoter widely spaced rows. is linked to a reporter gene that leads to the production of If managed properly, soil is a renewable resource in which a blue pigment in the leaf cells. When leaves of these smart farmers can grow food for many generations. The goal of scil plants develop a blue tinge, the farmer knows it is time to add management is sustainable agriculture, a commitment emphosphate-con tain ing fertilizer. bracing a variety of farming methods that are conservatioitTo fertilize judiciously, a farmer must note the soil pH beminded, environmentally sale, and profitable. cause it affects cation exchange and the chemical form of minerals. Although an essential element may be abundant, plants may be starving for that element because it is bound too tightly to clay particles or is in a chemical form [he plant cannot absorb. Managing soil pH is tricky; a change in H+ concentration may make one mineral more available but make another less available. At pH 8, for instance, the plant can absorb calcium, but iron is almost completely unavailable. The soil pH should be matched to a crop's mineral needs. If the soil is too alkaline, adding sulfate will lower the pH. Soil that is too acidic can be adjusted by liming (adding calcium carbonate or calcium hydroxide). A major problem with acidic soils, particularly in tropical areas, is that aluminum dissolves at low pH and becomes toxic to roots. Some plants cope with high aluFigure 37.8 C o n t o u r t i l l a g e . These crops in Wisconsin are planted in row mmum levels by secreting orgamc amons that bind the t h a t go around _ r a t h e r t h a n up a n d down ^ t h e N | |s C o n t o u r t i h a g e he , ps s b w t h e aluminum and render it harmless runoff of water and erosion of topsoil after heavy rains. 762

Plant Form and Function

-

Sod Reclamation Some areas are unfit for agriculture because of contamination of soil or groundwater with toxic heavy metals or organic pollutants. Traditionally, soil reclamation has focused on nonbiological technologies, such as removal and storage of contaminated soil in landfills, but these techniques are very costly and olten disrupt the landscape. A new method known as phytoremediation is a biological, nondestructive technology that seeks to reclaim contaminated areas cheaply by using the remarkable ability of some plants to extract soil pollutants and concentrate them in portions of the plant thai can be easily removed for safe disposal. For example, alpine pennycress (Thlaspi caerulescens) can accumulate zinc in its shoots at concentrations 300 times higher than most plants can tolerate. Such plants show promise for cleaning up areas contaminated by smelters, mining operations, or nuclear testing. Phytoremediation is part of the more general technology of bioremediation, which includes the use of prokaiyotes and sometimes protists to detoxify polluted sites (see Chapters 27 and 55).

Concept Check »9 1. "What are the general characteristics of good soil? 2. "Explain how the phrase "too much of a good thing" can apply to watering and fertilizing plants. For suggested answers, see. Appendix A.

Nitrogen is often the mineral that has the greatest effect on plant growth Of all the mineral nutrients, nitrogen contributes the most to plant growth and crop yields. Plants require nitrogen as a component of proteins, nucleic acids, chlorophyll, and other important organic molecules.

Soil Bacteria and Nitrogen Availability It is ironic that plants can suffer nitrogen deficiencies, for the atmosphere is nearly 80% nitrogen. However, this atmospheric nitrogen is gaseous N2, a lorm plants cannot use. For plants to absorb nitrogen, it must first be converted to ammonium (NH4+) or nitrate (NO3 ). In contrast to other minerals, the NH./ and NO3~"" in soil are not derived from the breakdown of rock. Over the short term, the main source of these minerals is the decomposition of humus by microbes, including ammonifying bacteria (Figure 37.9). Through decomposition, the nitrogen in proteins and other organic compound:-. \s repackaged in inorganic compounds that are recycled when absorbed as minerals by roots- Some nitrogen is lost when soil microbes called denitrifying bacteria convert NO^~ to N2, which diffuses from the soil into the atmosphere. However, other bacteria called

Atmosphere Atmosphere Soil

(from soil) \ V

3

(ammonia) ^"

0

(ammonium

y Ammonifying bacteria Orgamc material (humus)

4 Figure 37.9 The role of soil bacteria in the nitrogen nutrition of plants. Ammonium is made available to plants by two types of soil bacteria: those that fix atmospheric \t2 (nitrogen-fixing bacteria) and those that

decompose organic material (ammonifying bacteria). Although plants absorb some ammonium from the soil, they absorb mainly nitrate, which is produced from ammonium by nitrifying bacteria. Plants reduce nitrate back to

ammonium before incorporating the nitrogen into organic compounds. Xylem transports nitrogen from roots to shoots in the form of nitrate, amino acids, and various other organic compounds, depending on the species. CHAPTER 37 Plant Nutrition

763

nitrogen-fixing bacteria restock nitrogenous minerals in the soil by converting N2 from the atmosphere to NH3 (ammonia). in a metabolic process called nitrogen fixation. The complex cycling of nitrogen in ecosystems is traced in detail in Chapter 54. Here we focus on nitrogen fixation and the other steps that lead directiy to nitrogen assimilation by plants. Ail iife on Earth depends on nitrogen fixation, a process performed only by some bacterial species. Several of these species live freely in the soil, while others live in plant roots in symbiotic relationships. (You will learn more about these relationships in the next section.) The conversion of atmospheric nitrogen (N2) to ammonia (NH3) is a complicated, multistep process, but we can simplify nitrogen fixation by just indicating the reactants and products: N2 + 8e~ + 8H + + 16ATP~*2NH3 + H2 + 16ADP+ 16 ®t The enzyme complex nitrogenase catalyzes the entire reaction sequence, which reduces N2 to NH3 by adding electrons along with FT. Notice that nitrogen fixation is very expensive in terms of metabolic energy, costing the bacteria eight ATP molecules for each ammonia molecule synthesized. Nitrogenfixing bacteria are therefore most abundant in soils rich in organic material, which provides fuel for cellular respiration to produce the ATR In the soil solution, ammonia picks up another hydrogen ion to form an ammonium ion (NH4+), which plants can absorb. However, plants acquire their nitrogen mainly in the form of nitrate CNO3 ), which is produced in the soil by nitrifying bacteria that oxidize ammonium (see Figure 37.9). After nitrate is absorbed by the roots, a plant enzyme can reduce the nitrate back to ammonium, which other enzymes can then incorporate into amino acids and other organic compounds. Most plant species export nitrogen from roots to shoots, via the xylem, in the form of nitrate or organic compounds that have been synthesized in the roots.

Improving the Protein Yield of Crops The incorporation of fixed nitrogen by plants into proteins and other organic substances has a major impact on human welfare because the most common form of malnutrition in humans is protein deficiency. The majority of people in the world, particularly in developing countries, have a predominantly vegetarian diet and thus depend mainly on plants for protein. Unfortunately many plants have a low protein content, and those proteins may be deficient in one or more amino acids that humans need from their diet (see Figure 41.10). Improving the quality and quantity of proteins in crops is a major goal of agricultural research. Plant breeding has resulted in new varieties of maize, wheat, and rice that are enriched in protein. However, many of these "super" varieties have an extraordinary demand for nitrogen, usually supplied in commercial fertilizer. Like 764

UNIT s i x

Plant Form and Function

biological nitrogen fixation, industrial production of ammon a and nitrate from atmospheric nitrogen consumes a lot of energy. A chemical factory making fertilizer consumes large quantities of fossil fuels. The countries that most need high-protein crops are usually the ones least able to pay the fuel bill. In the future, new catalysts based on the mechanism by which nitrogenase fixes nitrogen may make commercial fertilizer production less costly. Several years ago, biochemists determined the structure of nitrogenase in Rhizobium, a genus of nitrogenfixing bacteria, providing a model for chemical engineers to design catalysts by imitating nature. Another strategy that could potentially increase protein yields of crops is to improve the productivity of symbiotic nitrogen fixation, a process examined in the next section.

Concept Check 1. Explain why nitrogen-fixing bacteria are essential to human welfare. For suggested answer, see Appendix A.

Concept

Plant nutritional adaptations often involve relationships with other organisms Many plants have nutritional adaptations that involve other organisms. Two of these relationships are mutualistic: symbiotic nitrogen fixation, involving roots and bacteria, and mycorrhizae, involving roots and fungi. After examining these mutually beneficial relationships, we will look at adaptations that are one-sided and relatively unusual: epiphytes, parasitii plants, and carnivorous plants.

The Role of Bacteria in Symbiotic Nitrogen Fixation Symbiotic relationships with nitrogen-fixing bacteria provide some plant species with a built-in source of hxed nitrogen for assimilation into organic compounds. From an agricultural perspective, the most important and efficient symbioses between nitrogen-hxing bacteria and plants occur in the legume famil) including peas, beans, soybeans, peanuts, alfalfa, and clover. Along a legume's roots are swellings called nodules com posed of plant cells that have been "infected" by nitrogen-fixing Rhizobium ("root living") bacteria (Figure 37.10a). Inside the nodule, Rhizobium bacteria assume a form called bacteroids, which are contained within vesicles formed by the root cell (Figure 37.10b). Rhizobium bacteria can fix atmospheric N2

and supply it as ammonium, a form readily used by the plant (see Figure 37.9). T_egume-Rhi?oMum symbioses generate more useful nitrogen for plants than all industrial fertilizers, and the symbiosis provides the right amounts of nitrogen at the right time at virtually no cost to the farmer. In addition to supplying the legume with nitrogen, symbiotic nitrogen fixation significantly reduces spending on fertilizers for subsequent crops. The location of the bacteroids inside living, nonphotosynthetic cells favors nitrogen fixation, which requires an anaerobic environment. Lignified external layers may also limit gas exchange. Some root nodules are reddish, owing to a molecule called leghemoglobin (leg- for "legume"), an iron-containing protein that binds reversibly to oxygen (like the hemogiobin of human red blood cells). This protein is an oxygen "buffer," keeping the concentration of free O2 low and regulating the oxygen supply for the intense respiration that the bacteria require to produce ATP for nitrogen fixation. Each legume is associated with a particular strain of Rhizobium. Figure 37.11 describes how a root nodule develops after bacteria enter through what is called an infection thread. The symbiotic relationship between a legume and nitrogen-fixing bacteria is mutualistic because the bacteria supply the plant

Bacteroids within vesicle 9 '

(a) Pea plant root. The bumps on this pea plant root are nodules containing Rhizobium bacteria. The bacteria fix nitrogen and obtain photosynthetic products supplied by the plant.

(b) Bacteroids in a soybean root nodule. In this TEM, a cell from a root nodule of soybean is filled with bacteroids in vesicles. The cells on the left are uninfected.

Figure 37.10 Root nodules on legumes.

I Roots emit chemical signals that attract Rhizobium bacteria. The bacteria then emit signals that stimulate root hairs to elongate and to form 3D infection thread by an imagination of the plasma membrane.

• The bacteria penetrate the cortex within the infection thread. Ceils of the cortex and pericyde begin dividing, and vesicles containing the bacteria bud into cortical cells from the branching infection thread. This process results in the formation of bacteroids.

Dtv mg cells £ .' in r & 3t cortex ;'•'';.. Infectedroot hair

:tei"Oi d

I i

•' •• .

- 0!Vi-ding cells pe? icycle

.Developing root nodule ~^.~-"J__ Bacteroid Growth continues in the affected regions "of the cortex and pericyde, and these two masses of dividing cells fuse, forming the nodule.

The nodule" develops vascuiar tissue that supplies nutrients to the nodule and carries • nitrogenous compounds into the vascular cylinder for distribution throughout the plant. Nodule vascular tissue

Figure 37.11 Development of a soybean root nodule. Plant Nutrition

765

with fixed nitrogen while the plant provides the bacteria with carbohydrates and other organic compounds. Most of the ammonium produced by symbiotic nitrogen fixation is used by the nodules to make amino acids, which are then transported to the shoot via the xylem.

nitrogen fixation. Rice farmers culture a free-floating aquatic fern called Azolla, which has symbiotic cyanobacteria that fix nitrogen and increase the fertility of the rice paddy. The growing riee eventually shades and kills the Azolh, and decomposition of this organic material adds more nitrogenoL compounds to the paddy.

The Molecular Biology of Root Nodule Formation How does a legume species recognize a certain strain of Rhizobium among the many bacterial strains in the soil? And how does an encounter with that specific Rhizobium lead to development of a nodule? These two questions have led researchers to uncover a chemical dialogue between the bacteria and the root. Each partner responds to the chemical signals from the other by expressing certain genes whose products contribute to nodule formation. The plant initiates the communication when its roots secrete molecules called Ilavonoids, which enter Rhizobium cells living near the roots. The signal's specificity arises from variations in flavonoid structure, with a particular legume species secreting a flavonoid that only a certain Rhizobium strain detects and absorbs. The plant's signal activates a generegulating protein in the bacterium, which switches on a cluster of bacterial genes called nod, for "nodulation" genes. These nod genes produce enzymes that catalyze production of species-specific molecules called Nod factors. Secreted by the bacterial cells, the Nod factors signal the root to initiate the infection process that enables the Rhizobium to enter the root and to begin forming the root nodule. The plant's responses require activation of genes called early nodulin genes by a signal transduction pathway involving Ca 2+ as second messengers (see Chapter 11). By understanding the molecular biology underlying the formation of root nodules, researchers hope to learn how to induce Rhizobium uptake and nodule formation in crop plants that do not normally form such nitrogen-fixing symbiotic relationships. Symbiotic Nitrogen Fixation and Agriculture The agricultural benefits of symbiotic nitrogen fixation underlie crop rotation. In this practice, a non-legume such as maize is planted one year, and the following year alfalfa or some other legume is planted to restore the concentration of fixed nitrogen in the soil. To ensure that the legume encounters its specific Rhizobium, ihe seeds are soaked in a culture of the bacteria or dusted with bacterial spores before sowing. Instead of being harvested, the legume crop is often plowed under so that it will decompose as "green manure," reducing the need for manufactured fertilizers. Many plant families besides legumes include species that benefit from symbiotic nitrogen fixation. For example, alder trees and certain tropical grasses host gram-positive bacteria of the actinomycetes group (see Figure 27.13). Rice, a crop of great commercial importance, benefits indirectly from symbiotic 766

UNIT six Plant Form and Function

Mycorrhizae and Plant Nutrition Mycorrhizae ("fungus roots") are modified roots consisting of mutualistic associations of fungi and roots (see Figures 31. lp and 36.10). The fungus benefits from a steady supply of sugar donated by the host plant. In return, the fungus increases tbe surface area for water uptake and selectively absorbs phosphate and other minerals from the soil and supplies them to the plant. The fungi of mycorrhizae also secrete growth factors that stimulate roots to grow and branch, as well as antibiotics that may help protect the plant from pathogenic bacteria and fungi, in the soil. Mycorrhizae are not oddities; they are formed by most plant species. In fact, this plant-fungus symbiosis might have been one of the evolutionary adaptations that made it possible for plants to colonize land in the first place. Indeed, fossilized roots from some of the earliest plants include mycorrhizae. When terrestrial ecosystems were young, the soil was probably not very rich in nutrients. The lungi of mycorrhizae, which are more efficient at absorbing minerals than the roots themselves, would have helped nourish the pioneering plants. Even today, the plants that first become established on nutrientpoor soils, such as abandoned farmland or eroded hillsides, are usually mycorrhizae-rich. The Two Main Types of Mycorrhizae The modified roots formed from the symbiosis of fungi ant. plants take two major forms: ectomycorrhizae and endomycorrhizae. In ectomycorrhizae, the mycelium (mass of branching hyphae; see Chapter 31) forms a dense sheath, or mantle, over the surface of the root (Figure 37.12a). Fungal hyphae extend from the mantle into the soil, greatly increasing the surface area lor water and mineral absorption. Hyphae also grow into the cortex of the root. These hyphae do not penetrate the root cell; but form a network in the apoplast, or extracellular space, that facilitates nutrient exchange between the lungus and the plant. Compared with "uninfected" roots, ectomycorrhizae are generally thicker, shorter, and more branched. They typically do not form root hairs, which would be superfluous given the extensive surface area of the fungal mycelium. About 10% of plant families have species that form ectomycorrhizae, and the vast majority of these species are woody, including members of the pine, spruce oak, walnut, birch, willow, and eucalyptus families. In contrast, endomycorrhizae do not have a dense mantle ensheathing the root (Figure 37.12b). A microscope is needed to see the fine fungal hyphae that extend from the soil into th

(3) Ectomycorrhizae. The mantle of the fungal mycelium ensheath the root, fungal hyphae extent from the mantle into the soil, absorbing water and minerals, esoeudiiy phosphate. Hyphafi also extend into the extracellu1 prov'iCj; !ci extensive surface are f o r nutrient exchange betweer the fungus and its host plant.

h) Endomycorrhizae. No mantle forms around the root, but rni-Lfoscopic fungai hyphae extend into the root Within the root cortex, the fungus makes extensive contact with the plant through branching of hyphae that •form arbuscules, providing an ei'omous surface area for nutrient swapping. The hyphae penetrate the celt wails, but not the plasma membranes, of cells within the cortex.

i. Figure 37.12 Mycorrhizae.

root. Hyphae also extend into the root cells (hence the term endomycorrhizae) by digesting small patches of the root cell walls. However, a fungal hypha does not actually pierce the plasma membrane and enter the root cell's cytoplasm. Instead, LL grows into a tube formed by imagination of the root cell's membrane. The action is analogous to poking a linger gently into a balloon; your finger is like the fungal hypha, and the balloon skin is like the root cell's membrane. Alter the fungal hyphae have penetrated in this way; some form densely branched structures called arbuscules ("little trees"), which are important sites of nutrient transfer between the fungus and the plant. Hyphae may also form oval vesicles, which possibly store food for the fungus. To the unaided eye, endomycorrhizae look like "normal"1 roots with root hairs, but a microscope reveals a symbiotic relationship of enormous importance to plant nutrition. Endomycorrhizae are much more common than ectomycorrhizae and are found in over 85% of riant species, including important crop plants such as maize, wheat, and legumes. Agricultural Importance of Mycorrhizae Roots can be transformed into mycorrhizae only if exposed to the appropriate species of fungus. In most natural ecosystems, these fungi are present in the soil, and seedlings develop mycorrhizae. But if seeds are collected in one environment and planted in foreign soil, the plants may show signs of malnutri-

tion (particularly phosphorus deficiency) resulting from absence of the plants' mycorrhizal partners. Researchers observe similar results in experiments in which soil fungi are poisoned. Farmers and foresters are already applying the lessons of such research. For example, inoculating pine seeds with spores of mycorrhizal fungi promotes formation of mycorrhizae by the seedlings. Infected pine seedlings grow more vigorously than trees without the fungal association.

Epiphytes, Parasitic Plants, and Carnivorous Plants Almost all plant species have mutualistic relationships with fungi, bacteria, or both. Though rarer, there are also plant species with nutritional adaptations that use other organisms in nonmutualistic ways. Figure 37.13, on the next page, provides an overview of three unusual adaptations: epiphytes, parasitic plants, and carnivorous plants.

Concept Check 1. Compare and.contrast root nodules and mycorrhizae. 2. Contrast epiphytes with parasitic plants. For suggested answers, see Appoidix A.

CHAPTER 37

Plant Nutrition

767

Figure 3 7 . 1 3

g Unusual Nutritional Adaptations in Plants An epiphyte (from the Greek epi, upon, and phyton, plant) nourishes itself but grows on another plant, usually anchored to branches or trunks of living trees. Epiphytes absorb water and minerals from rain, mostly through leaves rather than roots. Some examples are staghorn ferns and many orchids.

Staghorn fern, an epiphyte. This tropical fern (genus Platycerium) grows on large rocks, cliffs, and trees. It has t w o types of fronds: branched fronds resembling antlers and circular fronds that form a collar around the base of the fern.

PARASITIC PLANTS

Unlike epiphytes, parasitic plants absorb sugars and minerals from their living hosts, although some parasitic species are photosynthetic.

Mistletoe, a photosynthetic parasite. Tacked above doorways during the holiday season, mistletoe (genus Phoradendron) lives in nature as a parasite on oaks and other trees.

Many species have roots that function as haustoria, nutrient-absorbing projections that enter the host plant.

Dodder, a nonphotosynthetic parasite. Dodder (genus Cuscuta), the orange "strings' on this pickleweed, draws its nutrients from the host. The transverse section shows a haustorium tapping the host's phloem (LM).

Indian pipe, a nonphotosynthetic parasite. Also called the ghost flower, this species {Monotropa uniflora) absorbs nutrients from thfc fungal hyphae of mycorrhizae of green plants.

C A R N I V O R O U S PLANTS

Carnivorous plants are photosynthetic but obtain some nitrogen and minerals by killing and digesting insects and other small animals.

Venus' flytrap. Triggered by electrical impulses from sensory hairs, two leaf lobes close in half a second. Despite its common name, Dionaea muscipula usually catches ants and grasshoppers.

768

UNIT six

Plant Form and Function

Carnivorous plants live in acid bogs and other habitats where soils are poor in nitrogen and other minerals. Various insect traps consis: of modified leaves usually equipped with glands that secrete digestive enzymes. Fortunately for animals, such turnabouts are rare!

Pitcher plants. Nepenthes, Sarracenia, and other genera have water-filled funnels. The insects drown and are digested by enzymes.

Sundews. Sundews (genus Drosera) exude a sticky fluid that glitters like dew. Insects get stuck to the leaf hairs, which enfold the prey.

Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY CONCEPTS

Plants require certain chemical elements to complete their life cycle • Plants derive most of their organic mass from the C 0 2 of air, but they also depend on soil nutrients in the form of water and minerals. The branching of root and shoot systems helps plants come into contact with the resources that they need from the environment (pp. 756-757). Macronutrients and Micronutrients (pp. 757-758) Macronutrients, elements required in relatively large amounts. include carbon, oxygen, hydrogen, nitrogen, and other major ingredients of organic compounds. Micronutrients, elements required in very small amounts, typically have catalytic functions as cofactors of enzymes. Symptoms of Mineral Deficiency (pp. 758-759) Deficiency of a mobile nutrient u>uaLv aifects older organs more than younger ones; the reverse is true for nutrients that are less mobile within a plant. Macronutrient deficiencies are most common, particularly deficiencies of nitrogen, phosphorus, and potassium.

Soil quality is a major determinant of plant distribution and growth I*- Texture and Composition of Soils (pp. 759-760) Various sizes of particles derived from the breakdown of rock are found in soil, along with organic material (humus) in various stages of decomposition. Acids derived from roots contribute to a plant's uptake of minerals when H+ displaces mineral cations from clay particles. Activity How Plants Obtain Minerals from Soil Investigation How Does Acid Precipitation Affect Mineral Deficiency? • Soil Conservation and Sustainable Agriculture (pp. 760-763) In contrast to natural ecosystems, agriculture depletes the mineral content of soil, taxes water reserves, and encourages erosion. The goal of soil conservation strategies is to minimize this damage. A major goal of agricultural researchers is to reduce the amounts of fertilizer added to soils without sacrificing high crop yields, Graph It Global Soil Degradation

• Improving the Protein Yield of Crops (p. 764) Such research addresses the most widespread form of human malnutrition: protein deficiency

Plant nutritional adaptations often involve relationships with other organisms • The Role of Bacteria in Symbiotic Nitrogen Fixation (pp. 764-766) The development of nitrogen-fixing root nodules depends on chemical dialogue between Rhizobium bacteria and root cells of their specific plant hosts. The bacteria of a nodule obtain sugar from the plant and supply the plant with fixed nitrogen. • Mycorrhizae and Plant Nutrition (pp. 766-767) Mycorrhizae are modified roots consisting of mutualistic associations of fungi and roots. The fungal hyphae of both ectomycorrbizae and endomycorrhizae absorb water and minerals, which they supply to their plant hosts. • Epiphytes, Parasitic Plants, and Carnivorous Plants (pp. 767-768) "Epiphytes grow on the surfaces of other plants but acquire water and minerals from rain. Parasitic plants absorb nutrients from host plants. Carnivorous plants supplement their mineral nutrition by digesting animals.

TESTING YOUR KNOWLEDGE

Evolution Connection Imagine taking the plants out of the picture in Figure 37.9. Write a paragraph explaining how the soil bacteria could sustain the recycling of nitrogen before land plants evolved.

Scientific Inquiry Acid precipitation contains an abnormally high concentration of hydrogen ions (H+). One effect of acid precipitation is to deplete the soil of plant nutrients such as calcium (Ca~+), potassium 0 0 , and magnesium (Mg2+). Suggest a hypothesis to explain how acid precipitation washes these nutrients from the soil. How might you test your hypothesis? Investigation How Does Acid Precipitation Affect Mineral Deficiency?

Nitrogen is often the mineral that has the greatest effect on plant growth • Soil Bacteria and Nitrogen Availability (pp. 763-764) Nitrogen-fixing bacteria convert atmospheric N2 to nitrogenous minerals that plants can absorb as a nitrogen source for organic syn thesis. Activity The Nitrogen Cycle CHAPTER 37 Plant Nutrition

769

Science, Technology, and Society About 10% of U.S. cropland is irrigated. Agriculture is by far the biggest user of water in arid western states, including Colorado, Arizona, and California. The populations of these states are growing, and there is an ongoing conflict between cities and farm regions over water. To ensure adequate water supplies for urban growth, cities are purchasing water rights from farmers. This is often the least expensive way for a city to obtain more water, and it is possible for some farmers to make more money selling water rights than growing crops. Discuss the possible consequences of this trend. Is this the best way to allocate water for all concerned? Why or why not?

770

UNIT six

Plant Form and Function

c

osperm ^production and Biotechnology A Figure 38.1 Rafflesia arnoldii, "monster flower" of Indonesia.

(Key Concepts 38.1 Pollination enables gametes to come together within a flower 38.2 After fertilization, ovules develop into seeds and ovaries into fruits 3B3 Many flowering plants clone themselves by asexual reproduction 3i 8,4 Plant biotechnology is transforming agriculture

To Seed or Not to Seed

T

he parasitic plant Rajjlcsia arnoldii, found only in Southeast Asia, spends most of its life invisible to passersby, growing within the woody tissue of a host v ine. The plant makes its presence known in a spectacular iashion by producing a cabbage-sized floral bud that eventually develops into a gigantic (lower the size of an automobile tire (Figure 38.1). With an odor reminiscent of a decaying corpse, the ilower attracts carrion flies that shuttle the pollen irom one ilower to another. A few days after opening, the [lower collapses and shrivels, its function completed. A single lemale flower may produce up to 4 million seeds. However, sexual reproduction, as in Rajflesia, is not the only means by which flowering plants (angiosperms) reproduce. Many species also reproduce asexually, creating offspring that are genetically identical to the parent. The propagation of flowering plants by sexual and asexual reproduction forms the basis of agriculture. Since the dawn of agriculture about 10,000 years ago, plant breeders have genetically manipulated the traits of a few hundred wild angiosperm

species by artificial selection, transforming them into the crops we cultivate today. The speed and extent of plant modification have increased dramatically in recent decades wiih the advent of genetic engineering. In Chapters 29 and 30, we approached plant reproduction from an evolutionary perspective, tracing the descent of angiosperms and other land plants from their algal ancestors, in this chapter, we will explore the reproductive biology of flowering plants in much greater detail because they are the most important group of plants in most terrestrial ecosystems and in agriculture. After discussing the sexual and asexual reproduction of angiosperms, we will then examine modern plant biotechnology and the role ol humans in the genetic alteration of crop species.

Concept

Pollination enables gametes to come together within a flower Recall from Chapters 29 and 30 that the life cycles of plants are characterized by an alternation of generations, in which haploid (n) and diploid (2n) generations take turns producing each other (see Figures 29.5 and 30.10). The diploid plant, which is the sporophyte, produces haploid spores by meiosis. These spores divide by mitosis, giving rise to the gametophytes, the small male and female haploid plants that produce gametes (sperm and eggs). Fertilization results in diploid zygotes, which divide by mitosis and form new sporophytes. In angiosperms, the sporophyte is the dominant generation in the sense that it is the largest, most conspicuous, and longest-living plant we see. Over the course of seed plant 771

•Germinated pollen grain (n) (male gametophyte) on stigma of carpel

Ovary (base of carpel) Ovule Embryo sac (n) (female gametophyte)

Egg (n) ' Receptacle (a) An idealized flower.

Zygote{In)

Haploid (n) Diploid (2n) Embryo (2r) (sporophyt?) (b) Simplified angiosperm life cycle. See Figure 30.10 for a more detailed version of the life cycle, including meiosis.

Germinating seed

-•Simple fruit (develops from ovary)

A Figure 38.2 An overview of angiosperm reproduction.

evolution, gametophytes became reduced in size and wholly dependent on the sporophyte generation for nutrients. Angiosperm gametophytes are the most reduced of all plants, consisting of only a few cells. Angiosperm sporophytes develop a unique reproductive structure—the flower. Figure 38.2 reviews the angiosperm life cycle, which is shown in more detail in Figure 30.10. Male and female gametophytes develop within the anthers and ovules, respectively Pollination by wind, water, or animals brings a pollen grain containing a male gametophyte to the stigma of a flower. Pollen germination brings sperm produced by the male gametophyte to a female gametophyte contained in an ovule embedded in the ovary of a flower. Union of egg and sperm (fertilization) takes place within each ovule in the ovary. Ovules develop into seeds, while the ovary itself becomes a fruit (another unique structure of the angiosperms). In this section, we will focus on the role of the flower in gametophyte development and the process of pollination.

Flower Structure Flowers, the reproductive shoots of the angiosperm sporophyte, are typically composed of four whorls of highly modified leaves called floral organs, which are separated by very short internodes. Unlike vegetative shoots, which grow inde772

UNIT six Plant Form and Function

termmately, flowers are determinate shoots, meaning that they cease growing after the flower and fruit are formed. Floral organs—-sepals, petals, stamens, and carpels—aie attached to a part of the stem called the receptacle. Stamens and carpels are reproductive organs, whereas sepals and petals are sterile. Sepals, which enclose and protect the floral bud before it opens, are usually green and more leaflike in appearance than the other floral organs. In many species, petals are more brightly colored than sepals and advertise the flower to insects and other pollinators. A stamen consists of a stalk called the filament and a terrr inal structure called the anther; within the anther are chambers called pollen sacs, in which pollen is produced. A carpel has an ovary at its base and a long, slender neck called the style. At the top of the style is a sticky structure called the stigma that serves as a landing platform for pollen. Within the ovary are one or more ovules, with the number depending on the species. The flower shown in Figure 38.2 has a single carpel, but the flowers of many species have multiple carpels. In most species, two or more carpels are fused into a single structure; the result is an ovary with two or more chambers, each containing one or more ovules. The term pistil is sometimes used to refer to a single carpel or to a group of fused carpels. Figure 38.3 shows examples of variations in floral structure that have evolved during the 140 million years of angiosperm history.

Figure 38.3

>rinq Floral Variations Chapter 30 focused on general characteristics of angiosperms (see Figure 30.12). Here we look at some differences in floral traits. Depending on the species, flowers can vary in the presence or absence of sepals, petals, stamens, or carpels. Complete flowers have all iour basic floral organs (see Figure 38.2a). Incomplete flowers lack

one or more of these organs. For example, most grass flowers lack petals. Some incomplete flowers are sterile, lacking functional stamens and carpels. Flowers also vary in size, shape, color, odor, and arrangement of floral organs. Much of this diversity represents adaptation to specific groups of pollinators (see Figure 30.13). FLORAL DISTRIBUTION

O V A R Y LOCATION

Flowers can differ in symmetry In bilateral symmetry, the flower can be divided into two equal parts by a single imaginary line. In radial symmetry, the sepals, petals, stamens, And carpels radiate out from a center. Floral organs can also be either fused or separate. For instance, daffodil petals are fused into a I unnel.

The location, of the ovary may vary in relation to the stamens, petals, and sepals An ovary is called superior if those parts are attached below it, semi-interior if they are attached alongside it, and inferior if they are attached above it.

Floral distribution can also differ. Some species have individual flowers, while others have clusters called inflorescences. Lupine inflorescence

Bilateral symmetry (orchid) Sunflower inflorescence. A sunflower's centra! disk actually consists of hundreds of tiny incomplete flowers. What look like petals are actually sterile flowers.

Semi-inferior ovary

Inferior ovary

Radial symmetry (daffodil)

REPRODUCTIVE

Most species have a single type of flower that has functional stamens and carpels. All complete flowers and some incomplete flowers have functional stamens and carpels. In most incomplete flowers, stamens or carpels are e.ther absent or nonfunctional. Incomplete flowers that have only functional stamens are called staminate, and those with only functional carpels are called carpellate. If staminate and carpellate flowers are on the same plant, the species is said to be monoecious (irom the Greek word meaning "one house") A dioecious ("two houses") species has staminate flowers and carpellate flowers on separate plants.

VARIATIONS

L Maize, a monoecious species. A maize "ear" (left) consists of kernels (one-seeded fruits) that develop from an inflorescence of fertilized carpellate flowers. Each kernel is derived from a single flower. Each "silk" strand consists of a stigma and long style. The tassels (right) are staminate inflorescences.

CHAPTER 3a

Dioecious Sagittaria latifolia (common arrowhead). The staminate flower (left) lacks carpels, and the carpellate flower (right) lacks stamens. Having these two types of flowers on separate plants reduces inbreeding.

Angiospertn Reproduction and Biotechnology

773

Anther at

Gametophyte Development and Pollination Anthers and ovules bear sporangia, structures where spores are produced by meiosis and gametophytes develop. Pollen grains, each consisting of a mature male gametophyte surrounded by a spore wall, are formed within pollen sacs (microsporangia) of anthers. An egg-producing female gametophyte, or embryo sac, forms within each ovule. In angiosperms, pollination is the transfer of pollen from an anther to a stigma. It pollination is successful, a pollen grain produces a structure called a pollen tube, which grows and digests its way down into the ovary via the style and discharges sperm in the vicinity of the embryo sac, resulting in fertilization of the egg (see Figure 38.2b). The zygote gives rise to an embryo, and as the embryo grows, the ovule that contains it develops into a seed. The entire ovary,

.-Germinated pollen grain

meanwhile, develops into a fruit containing one or more seeds, depending on the species. Fruits, which disperse b; dropping to the ground or being carried by wind or animals, help spread seeds some distance from their source plants. When light, soil, and temperature conditions arc suitable, seeds germinate and the embryo carried in tht seed grows and develops into a seedling. We will now look more closely at the development of angiosperm gametophytes and the process of pollination. Keep in mind, however, that there are many variations in the details of these processes, depending on the species. Within the microsporangia (pollen sacs) of an anther are many diploid cells called microsporocytes, also known as mil crospore mother cells (Figure 38.4a). Each microsporocyte un dergoes meiosis, forming four haploid microspores, each of which can eventually give rise to a haploid male gametophyte

• Figure 38.4 The d e v e l o p m e n t of a n g i o s p e r m g a m e t o p h y t e s ( p o l l e n grains a n d e m b r y o sacs). (a) Development of a male g a m e t o p h y t e (pollen grain). Pollen grains develop within the microsporangia (pollen sacs) of anthers at the tips of the stamens.

(b) Development of a female g a m e t o p h y t e (embryo sac). The embryo sac develops within an ovule, itself enclosed by the ovary at the base of a carpel.

Pollen sac (microsporangium!

0 Each one of the microsporangia contains diploid microsporocytes (microspore mother cells).

Megasporangium

Each microsporocyte divides by meiosis to produce four haploid microspores, each of which develops into a pollen grain.

Q A pollen grain becomes a mature male gametophyte when its generative nucleus divides and forms t w o sperm. This usually occurs after a pollen grain lands on the stigma of a carpel and the pollen tube begins to grow. (See Figure 38.2b.)

774

UNIT six

Plant Form and Function

© W i t h i n the ovule's megasporangium is a large diploid cell called the megasporocyte (megaspore mother ceil). 0 The megasporocyte divides by meiosis and gives rise to four haploid cells, but in most species only one of these survives as the megaspore. © Three mitotic divisions of the megaspore form the embryo sac, a multicellular female gametophyte. The ovule now consists of the embryo sac along with the surrounding integuments (protective tissue)

A microspore undergoes mitosis and cytokinesis, producing two separate cells called the generative cell and tube cell. Toget.ier, these two cells and the spore wall constitute a pollen grain, which at this stage of its development is an immature mile gametophyte. The spore wall usually exhibits an elaborate pattern unique to the particular plant species. During maturation of the male gametophyte, the generative cell passes into the tube cell. The tube cell now has a completely free-standing cell inside it (the generative cell). The tube cell produces the pollen tube, a structure essential for sperm delivery to the egg. During elongation of the pollen tube, the generative cell usually divides arid produces two sperm cells, which remain inside the tube cell (see Figure 30.10). The pollen tube grows through the long style of the carpel and into the ovary, where it then releases the sflerm cells in the vicinity of an embryo sac. One or more ovules, each containing a megasporangium, form within the chambers of the ovary (Figure 38.4b). One call in the megasporangium of each ovule, the megasporocyte (or megaspore mother cell), grows and then goes through meiosis, producing four haploid megaspores. The details of the next steps vary extensively, depending on the species. In most angiosperm species, only one megaspore survives. This megaspore continues to grow, and its nucleus divdes by mitosis three times without cytokinesis, resulting in one large cell with eight haploid nuclei. Membranes then partition this mass into a multicellular female gametophyte—the embryo sac. At one end of the embryo sac are three cells: tine egg cell and two cells called synergids. The synergids flank the egg cell and function in the attraction and guidance of the pollen tube to the embryo sac. At the opposite end of the embryo sac are three antipodal cells of unknown function. The retaining two nuclei, called polar nuclei, are not partitioned ito separate cells but instead share the cytoplasm of the large entral cell of the embryo sac. The ovule, which will eventually ecome a seed, now consists of the embryo sac and two surDunding integuments (layers of protective sporophytic tissue that eventually develop into the seed coat). Pollination, the transfer of pollen from anther to stigma, is ie first step in a chain of events that can lead to fertilization. "This step is accomplished in various ways. In some angioperms, including grasses and many trees, wind is a pollinating gent. In such plants, the release of enormous quantities of iollen compensates for the randomness of this dispersal nechanism. At certain times of the yean the air is loaded with iollen grains, as anyone plagued with pollen allergies can atest. Some aquatic plants rely on water to disperse pollen, vlost angiosperms, however, depend on insects, birds, or other animals to transfer pollen directly to other flowers.

increasing the likelihood that at least some of the offspring will survive such challenges as environmental change and pathogens (see Chapter 23). Nevertheless, some flowers, such as garden peas, self-fertilize. This process, called "selfmg," can be a desirable attribute in some crop plants because it ensures that a seed will develop. Many angiosperm species, however, have mechanisms that make it difficult or impossible for a flower to fertilize itseli. The various barriers that prevent selffertilization contribute to genetic variety by ensuring that sperm and eggs come from different parents. In dioecious species, of course, plants cannot self-fertilize because they have either staminate or carpellate flowers (see Figure 38.3). And m some plants that have flowers with functional stamens and carpels, these floral organs mature at different times or are structurally arranged in such a way that it is unlikely that an animal pollinator could transfer pollen from an anther to a stigma of the same flower (Figure 38.5). However, the most common anti-selfing mechanism in flowering plants is known as self-incompatibility, the ability of a plant to reject its own pollen and sometimes the pollen of closely related individuals. If a pollen grain lands on a stigma ol a flower on the same plant, a biochemical block prevents the pollen from completing its development and fertilizing an egg. Researchers are unraveling the molecular mechanisms that are involved in self-incompatibility. This plant response is analogous to the immune response of animals in that both are based on the ability of organisms to distinguish the cells of "self from those of "nonself." The key difference is that the animal immune system rejects nonself, as when the system mounts a defense against a pathogen or attempts to reject a transplanted organ. Self-incompatibility in plants, in contrast, is a rejection of self.

Stigma

Pin flower

Thrum flower

A Figure 38.5 "Pin" and "thrum" flower types reduce

VIechanisms That Prevent Self-Fertilization Generally, one of the great advantages of sexual reproduction is that it increases the genetic diversity of offspring, thereby

s e l f - f e r t i l i z a t i o n . Some species produce t w o types of flowers: "pins," which have long styles and short stamens, and "thrums," which have short styles and long stamens. An insect foraging for nectar would collect pollen on different parts of its body; pin pollen would be deposited on thrum stigmas, and vice versa.

CHAPTER 38

Angiosperm Reproduction and Biotechnology

775

Recognition of "self pollen is based on genes for sellincompatibility, called 5-genes. In the gene pool of a plant population, there can be dozens of alleles of an S-gene. If a pollen

grain has an allele that matches an allele of the stigma on which it lands, the pollen tube fails to grow. Depending on the species, self-recognition blocks pollen tube growth by one of two molecular mechanisms: gametophytic self-incompatibility or sporophytic self-incompatibility. In gametophytic self-incompatibility, the S-allele in the pollen genome governs the blocking of fertilization. For example, an S] pollen grain from an S^Sj parental sporophyte will fail to fertilize eggs of an 5iS2 flower but will fertilize an 5253 flower. An S2 pollen grain would not fertilize either flower. Selfrecognition of this kind involves the enzymatic destruction of RNA within a rudimentary pollen tube. RNA-hydrolyzing enzymes in the style of the carpel can enter a pollen tube and attack its RNA only if the pollen is of a "self type. In sporophytic self-incompatibility, fertilization is blocked by S-allele gene products in tissues of the parental sporophyte that adhere to the pollen wall. For example, neither an S\ nor 52 pollen grain from an S^j parental sporophyte will fertilize eggs of an S\$2 flower or S2S3 flower. Sporophytic incompatibility involves a signal transduction pathway in epidermal cells of the stigma that prevents germination of the pollen grain. Some crops, such as nonhybrid cultivated varieties of peas, maize, and tomatoes, routinely self-pollinate with satisfactory results. However, plant breeders sometimes hybridize different varieties of a crop plant in order to combine the best traits of the varieties and counter the loss of vigor that can result from excessive inbreeding (see Chapter 14). To obtain hybrid seeds; plant breeders currently must prevent seli-lertilizanon either by laboriously removing the anthers from the parent plants that provide the seeds or by developing male sterile plants. The latter option is increasingly important. Eventually, it may also be possible to impose self-incompatibility on crop species that are normally self-compatible. Basic research on mechanisms of selfincompatibility may therefore lead to agricultural applications.

Concept Check , 1. Give some examples of how form fits function in flower structure. 2. Distinguish pollination from fertilization. 3. Given the seeming disadvantages of selfing as a reproductive "strategy" in nature, it is surprising that about 20% of angiosperm species primarily rely on selfing. Although fairly common in nature, self-fertilization has been called an "evolutionary dead end." Suggest a reason why selfing might be selected for in nature and yet still be an "evolutionary dead end/' For suggested answers, see Appendix A.

776

U N I T six

Plant Form and Function

Concept

After fertilization, ovules develop into seeds and ovaries into fruits We have traced the processes ol gametophyte developme and pollination. Now we will look at fertilization and its pro ucts: seeds and fruits.

Double Fertilization After landing on a receptive stigma, a pollen grain absorbs moisture and germinates; that is, it produces a pollen tube that extends down between the cells of the style toward the ovaly (Figure 38.6). The nucleus of the generative cell divides by mitfpsis and forms two sperm. Directed by a chemical attractant, possibly calcium, the tip of the pollen tube enters the ovary, prob( s

O 'f a pollen grain germinates, a pollen tube grows down the style toward the ovary.

s

~- Micropyle

-Ovule

© The pollen tube discharges two sperm into the female gametophyte (embryo sac) within an ovule.

Q One sperm fertilizes the egg, forming the zygote. The other sperm combines with the two polar nuclei of the embryo sac's large central cell, forming a triploid cell that develops into the nutritive tissue called endosperm.

-Egg

-Endosperm nucleus (3n) (2 polar nuclei plus sperm) - Zygote (2n) (egg plus sperm)

A Figure 38.6 Growth of the pollen tube and double fertilization.

through the micropyle (a gap in the integuments of the ovule), anjd discharges its two sperm near or within the embryo sac. The events that follow are a distinctive feature of the angic sperm life cycle. One sperm fertilizes the egg to form the zygote. The other sperm combines with the two polar nuclei toiform a triploid (3n) nucleus in the center of the large central cell of the embryo sac. This large cell will give rise to the erdosperm, a food-storing tissue of the seed. The union of two sperm cells with different nuclei of the embryo sac Is cailled double fertilization. Double fertilization ensures that tne endosperm will develop only in ovules where the egg has beten fertilized, thereby preventing angiosperms from squandering nutrients.

seedling after germination. In other eudicots (including bean seeds), the food reserves of the endosperm are completely exported to the cotyledons before the seed completes its development; consequently, the mature seed lacks endosperm. Embryo Development The first mitotic division of the zygote is transverse, splitting the fertilized egg into a basal cell and a terminal cell (Figure 38.7). The terminal cell eventually gives rise to most of the embryo. The basal cell continues to divide transversely, producing a

The tissues surrounding the embryo sac have prevented researchers from being able to directly observe fertilization in plants grown under normal conditions. Recently, however, scientists have isolated sperm from germinated pollen grains and ei^gs from embryo sacs and have observed the merging of plant g imetes in vitro (in an artificial environment). The first cellular event that takes place after gamete fusion is an increase m the cytoplasmic calcium (Ca2+) levels of the egg, as also occurs djuring animal gamete fusion (see Chapter 47). Another similarity to animals is the establishment of a block to polyspermy the fertilization of an egg by more than one sperm cell. Thus, rnaize {Zca mays) sperm cannot fuse with zygotes in vitro. In maize, this barrier to polyspermy is established as early as 45 seconds after the initial sperm fusion with the egg.

From Ovule to Seed iter double fertilization, each ovule develops into a seed, and ijhe ovary develops into a fruit enclosing the seed(s). As the embryo develops from the zygote, the seed stockpiles proteins, oils., and starch to varying extents, depending on the species. 1 This is why seeds are such major sugar sinks (see Chapter 36). nitially, these nutrients are stored in the endosperm, but later n seed development in many species, the storage function of .he endosperm is more or less taken over by the swelling ;otyledons of the embryo. Indosperm Development Endosperm development usually precedes embryo development. After double fertilization, the triploid nucleus of the ovule's central cell divides, forming a multinucleate "superceir ha-iving a milky consistency. This liquid mass, the endosperm, becomes multicellular when cytokinesis partitions the cytoplasm by forming membranes between the nuclei. Eventually, these "naked" cells produce cell walls, and the endosperm becomes solid. Coconut "milk" is an example of liquid endosperm; coconut "meat" is an example of solid endosperm. The white fluffy part of popcorn is also solid endosperm. In grains and most other monocots, as well as many eudicots, the endosperm stores nutrients that can be used by the

Shoot apex Root apex Suspensor

A Figure 38.7 The development of a eudicot plant embryo. By the time the ovule becomes a mature seed and the integuments harden and thicken to form the seed coat, the zygote has given rise to an embryonic plant with rudimentary organs. CHAPTER 38

Angiosperm Reproduction and Biotechnology

7/7

thread of cells called the suspensor, which anchors the embryo to its parent. The suspensor functions in the transfer of nutrients to the embryo from the parent plant and, in some plants,

from the endosperm. As the suspensor elongates, it also pushes the embryo deeper into the nutritive and protective tissues. Meanwhile, the terminal cell divides several times and forms a spherical proembryo attached to the suspensor. The cotyledons begin to form as bumps on the proembryo. A eudicot, with its two cotyledons, is heart-shaped at this stage. Only one cotyledon develops in monocots. Soon after the rudimentary cotyledons appear, the embryo elongates. Cradled between the cotyledons is the embryonic shoot apex, which includes the shoot apical meristem. At the opposite end of the embryo's axis, where the suspensor attaches, is the embryonic root apex, which includes the root apical meristem. After the seed germinates, the apical meristems at the tips of shoots and roots will sustain primary growth as long as the plant lives (see Figure 35.10).

I

Seed c o a t v

Cotyledons

(a) Common garden bean, a eudicot w i t h thick cotyledons. The fleshy cotyledons store food absorbed from the endosperm befor the seed germinates.

(b) Castor bean, a eudicot w i t h thin cotyledons. The narrow, membranous cotyledons (shown in edge and flat views) absorb food from the endosperm when the seed germinates.

Structure of the Mature Seed During the last stages of its maturation, the seed dehydrates until its water content is only about 5-15% of its weight. The embryo, surrounded by a food supply (cotyledons, endosperm, or both), becomes dormant; it stops growing and its metabolism nearly ceases. The embryo and its food supply are enclosed by a hard, protective seed coat formed from the integuments of the ovule. We can take a closer look at one type of eudicot seed by splitting open the seed of a common garden bean. At this stage, the embryo consists of an elongate structure, the embryonic axis, attached to fleshy cotyledons (Figure 38,8a). Below the point at which the cotyledons are attached, the embryonic axis is called the hypocotyl (from the Greek hypo, under). The hypocotyl terminates in the radicle, or embryonic root. The portion of the embryonic axis above where the cotyledons are attached is the epicotyl (from the Greek epi, on, over). It consists of the shoot tip with a pair of miniature leaves. The cotyledons of the common garden bean are packed with starch before the seed germinates because they absorbed carbohydrates from the endosperm when the seed developed. However, the seeds of some eudicots, such as castor beans (Ricinus communis), retain their food supply in the endosperm and have cotyledons that are very thin (Figure 38.8b). The cotyledons absorb nutrients from the endosperm and transfer them to the rest of the embryo when the seed germinates. The embryo of a monocot has a single cotyledon (Figure 38.8c). K4embers of the grass family, including maize and wheat, have a specialized type of cotyledon called a scutellum (from the Latin scutella, small shield, a reference to the scutellums shape). The scutellum is very thin, with a large surface area pressed against the endosperm, from which the scutellum absorbs nutrients during germination. The embryo of a grass seed 778

U N I T six

Plant Form and Function

•v—Pericarp fused S& with seed coat

(c) Maize, a monocot. Like all monocots, maize has only one cotyledon. Maize and other grasses have a large cotyledon called , 5cutellum. The rudimentary shoot is sheathed in a structure called the coleoptile, and the coleorhiza covers the young root.

A Figure 38.8 Seed structure.

is enclosed by two sheathes: a coleoptile, which covers th young shoot, and a coleorhiza, which covers the young root.

From Ovary to Fruit While the seeds are developing from ovules, the ovary of th flower is developing into a fruit, which protects the enclose seeds and, when mature, aids in their dispersal by wind or an imals. Fertilization triggers hormonal changes that cause th ovary to begin its transformation into a fruit. If a flower ha not been pollinated, fruit usually does not develop, and th entire flower withers and falls away. During fruit development, the ovary wall becomes the pen carp, the thickened wall of the fruit. As the ovary grows, the othe parts of the flower wither and are shed. For example, thepointe dp of a pea pod is the withered remains of the pea flower's stigma Fruits are classified into several types, depending on thei developmental origin (Figure 38.9). Most fruits are derivec from a single carpel or several fused carpels and are calle

simple fruits. Some simple fruits are fleshy, such as a peach, whereas others are dry, such as a pea pod or a nut (see Figure 30 8). An aggregate fruit results from a single flower chat has mire than one separate carpel, each forming a small fruit. These "fmitlets" are clustered together on a single receptacle, as in a raspberry. A multiple fruit develops from an inflorescence, a group of flowers tightly clustered together. When the walls of the many ovaries start to thicken, they fuse together arid become incorporated into one fruit, as in a pineapple. In some angiosperms, other floral parts in addition to ovaries contribute to what we commonly call the fruit. Such fruits are called accessory fruits. In apple flowers, for example, the ovary is embedded in the receptacle (see Figure 38.2b), and the fleshy part of this simple fruit is derived mainly from the enlarged receptacle; only the apple core develops from the ovary: 3[Other example is the strawberry, an aggregate fruit consisting an enlarged receptacle embedded with tiny one-seeded fruits. A fruit usually ripens about the same time that its seeds mplete their development. Whereas the ripening of a dry fruit, such as a soybean pod, involves the aging and drying out of fruit tissues, the process in a fleshy fruit is more elaborate. Gomplex interactions of hormones result in an edible fruit that serves as an enticement to animals that help spread the seeds- The fruit's "pulp" becomes softer as a result of enzymes digesting components of the cell walls. There is usually a color

change from green to another color, such as red, orange, or yellow. The fruit becomes sweeter as organic acids or starch molecules are converted to sugar, which may reach a concentration of as much as 20% in a ripe fruit.

Seed Germination As a seed matures, it dehydrates and enters a phase referred to as dormancy (from the Latin word meaning "to sleep"), a condition of extremely low metabolic rate and suspension of growth and development. Conditions required to break dormancy vary between plant species. Some seeds germinate as soon as they are in a suitable environment. Other seeds remain dormant even if sown in a favorable place until a specific environmental cue causes them to break dormancy. Seed Dormancy: Adaptation for Tough Times Seed dormancy increases the chances that germination will occur at a Lime and place most advantageous to the seedling. Breaking dormancy generally requires certain environmental conditions. Seeds of many desert plants, for instance, germinate only after a substantial rainfall. If they were to germinate after a mild drizzle, the soil might become too dry to support the seedlings. Where natural fires are common, many seeds require intense heat to break dormancy; seedlings are therefore

^Carpels

Raspberry flower

Raspberry fruit

Pea fruit (a) Simple fruit. A simple fruit develops from a single carpel (or several fused carpels) of one flower (examples: pea, .lemon, peanut).

(b) Aggregate fruit. An aggregate fruit develops from many separate carpels of one flower (examples: raspberry, blackberry, strawberry).

Pineapple inflorescence

Pineapple fruit (c) Multiple fruit. A multiple fruit develops from many carpels of r flowers (examples; pineapple, fig).

Figure 38.9 Developmental origin of fruits. CHAPTER 38

Angiosperm Reproduction and Biotechnology

779

most abundant after fire has cleared away competing vegetation. Where winters are harsh, seeds may require extended exposure to cold. Seeds sown during summer or fall do not germinate until the following spring, ensuring a long growth season before the next winter. Some small seeds, such as those of some varieties o( lettuce, require light for germination and will break dormancy only if they are buried shallow enough for the seedlings to poke through the soil surface. Some seeds have coats that must be weakened by chemical attack as they pass through an animal's digestive tract, and thus are likely to be carried some distance before germinating. The length of time a dormant seed remains viable and capable of germinating varies from a few days to decades or even longer, depending on the species and environmental conditions. Most seeds are durable enough to last a year or two until conditions are favorable for germinating. Thus, the soil has a bank of ungerminated seeds that may have accumulated for several years. This is one reason vegetation reappears so rapidly after a fire, drought, flood, or some other environmental disruption.

.-••"^ """\

Epicotyl

f\ - "-'v-

•A rp%\ i Cotyledon

Hypocotyl'

Seed coat

(a) Common garden bean. In common garden beans, straightening a hook in the hypocotyl pulls the cotyledons from the soil.

Foliage leaves N

From Seed to Seedling Coleoptile

Germination of seeds depends on the physical process called imbibition, the uptake of water due to the low water potential of the dry seed. Imbibing water causes the seed to expand and rupture its coat and also triggers metabolic changes in the embryo that enable it to resume growth. Following hyd ration, enzymes begin digesting the storage materials of the endosperm or cotyledons, and the nutrients are transferred to the growing regions of the embryo. The first organ to emerge from the germinating seed is the radicle, the embryonic root. Next, the shool Up must break through the soil surface. In garden beans and many other eudicots, a hook forms in the hypocotyl, and growth pushes the hook above ground (Figure 38.10a). Stimulated by light, the hypocotyl straightens, raising the cotyledons and epicotyl. Thus, the delicate shoot apex and bulky cotyledons are pulled upward rather than being pushed tip-first through the abrasive soil. The epicotyl now spreads its first foliage leaves (true leaves, so called to distinguish them from cotyledons, or "seed leaves"). The foliage leaves expand, become green, and begin making food by photosynthesis. The cotyledons shrivel and fall away from the seedling, their food reserves having been exhausted by the germinating embryo. Maize and other grasses, which are monocots, use a different method for breaking ground when they germinate (Figure 38.10b). The coleoptile, the sheath enclosing and protecting the embryonic shoot, pushes upward through the soil and into the air. The shoot tip then grows straight up through the tunnel provided by the tubular coleoptile and eventually breaks out through the coleoptiles tip. Seed germination is a precarious stage in a plant's life. The tough seed gives rise to a fragile seedling that will be exposed to predators, parasites, wind, and other hazards. In the wild, 780

U N I T six

Plant Form and Function

Radicle (b) Maize. In maize and other grasses, the shoot grows straight up through the tube of the coleoptile.

A Figure 38.10 Two common types of seed germination.

only a small fraction of seedlings endure long enough to become parents themselves. Production of enormous numbers of seeds compensates for the odds against individual survive and gives natural selection ample genetic variations to screei However, this is a very expensive means of reproduction terms of the resources consumed in flowering and fruiting Asexual reproduction, generally simpler and less hazardoir for offspring, is an alternative means of plant propagation. Concept Check i 1. Some dioecious species have the XY sexual genotype for male and XX for female. After double fertilization, what would be the genotypes of the endosperm nuclei and the embryos? 2. Explain how the basic structures of seeds and fruits facilitate reproduction. 3. What is the advantage of seed dormancy? For suggested answers, see Appendix A.

Concept

Many flowering plants clone themselves by asexual reproduction Imagine chopping off your finger and watching it develop into an exact copy of you. This would be an example of asexual re] >roduction, in which offspring are derived from a single parent without genetic recombination (which, of course, does ndt occur in humans). The result would be a clone, an asexualliy produced, genetically identical organism. Like many seedless plants and some gymnosperms, many an Jiosperm species reproduce both asexually and sexually. If reproducing asexually, a plant passes on all of its genes to its offspring. If reproducing sexually, a plant passes on only half of its genes. Each reproductive mode offers advantages in certam situations. If a plant is superbly suited to a stable environment, asexual reproduction has advantages. A plant can clone many ccjpies of itself, and if the environmental conditions remain stable, these clones, too, will be well suited for that environment. Also, the offspring are not as frail as seedlings produced by sexual reproduction in seed plants. The offspring are usually mature vegetative fragments from the parent plant, which is why asexual reproduction in plants is also known as vegetative re production. A sprawling clone of prairie grass may cover an area so thoroughly that seedlings of the same or other species have little chance of competing. In unstable environments, where evolving pathogens and other variables affect survival and reproductive success, sexual reproduction can be an advantage because it generates variation in offspring and populations. In contrast, the genotypic uniformity of asexually produced plants puts them at great risk of local extinction if there is a catastrophic environmental change, such as a new strain of disease. Moreover, seeds (which are almost always produced sexually) facilitate the dispersal of offspring to more distant, locations. And finally, seed dormancy allows growth to be suspended until hostile environmental conditions are more favorable.

Mechanisms of Asexual Reproduction Asexual reproduction in plants is typically an extension of the capacity for indeterminate growth. Plants, remember, ave meristematic tissues of dividing, undifferentiated cells that can sustain or renew growth indefinitely In addition, thi parenchyma cells throughout the plant can divide and differentiate into more specialized types of cells, enabling plants to regenerate lost parts. Detached vegetative fragments of some plants can develop into whole offspring; a severed stem, for i istance, may develop adventitious roots and become a whole

t

A Figure 38.11 Asexual reproduction in aspen trees. Some aspen groves, such as those shown here, actually consist of thousands of trees descended by asexual reproduction. Each grove of trees derives from the root system of one parent. Notice that genetic differences between groves descended from different parents result in different timing for the development of fall color and the loss of leaves.

plant. Fragmentation, the separation of a parent plant into parts that develop into whole plants, is one oi the most common modes of asexual reproduction. For example, in some species the root system of a single parent gives rise to many adventitious shoots that become separate shoot systems. The result is a clone formed by asexual reproduction from one parent (Figure 38.11). Such asexual propagation has produced the oldest of all known plant clones, a ring of creosote bushes in the Mojave Desert of California, believed to be at least 12,000 years old. An entirely different mechanism of asexual reproduction has evolved in dandelions and some other plants. These plants can sometimes produce seeds without pollination or fertilization. This asexual production of seeds is called apomixis (from the Greek words meaning "away from the act of mixing") because there is no joining of sperm and egg. Instead, a diploid cell in the ovule gives rise to the embryo, and the ovules mature into seeds, which in the dandelion are dispersed by windblown fruits. Thus, these plants clone themselves by an asexual process but have the advantage of seed dispersal, usually associated with sexual reproduction.

Vegetative Propagation and Agriculture With the objective of improving crops and ornamental plants, humans have devised various methods for asexual propagation of angiosperms. Most of these methods are based on the ability of plants to form adventitious roots or shoots. Clones from Cuttings Most houseplants, woody ornamentals, and orchard trees are asexually reproduced from plant fragments called cuttings. In some cases, shoot or stem cuttings are used. At the cut end of CHAPTER 38

Angiosperm Reproduction and Biotechnology

781

the shoot, a mass of dividing, undifferentiated cells called a callus forms, and then adventitious roots develop from the callus. If the shoot fragment includes a node, then adventitious roots form without a callus stage. Some plants, including African violets, can be propagated from single leaves rather than stems. For still other plants, cuttings are taken from specialized storage stems. For example, a potato can be cut up into several pieces, each with a vegetative bud, or "eye," that regenerates a whole plant.

Grafting In a modification of vegetative reproduction from cuttings, a twig or bud from one plant can be grafted onto a plant of a closely related species or a different variety of the same species. Grafting makes it possible to combine the best qualities of different species or varieties into a single plant. The graft is usually done when the plant is young. The plant that provides the root system is called the stock; the twig grafted onto the stock is referred to as the scion. For example, scions from French varieties of vines that produce superior wine grapes are grafted onto root stocks of American varieties, which are more resistant to certain soil pathogens. The genes of the scion determine the quality of the fruit, so it is not diminished by the genetic makeup of the stock. Tn some cases of grafting, however, the stock can alter the characteristics of the shoot system that develops from the scion. For example, dwarf fruit trees, which allow for more easy harvesting of the fruit, are made by grafting normal twigs onto stocks of dwarf varieties that retard the vegetative growth of the shoot system. Because seeds are produced by the part of the plant derived from the scion, they give rise to plants of the scion species when planted. Test-Tube Cloning and Related Techniques Plant biologists have adopted in vitro methods to create and clone novel plant varieties. It is possible to grow whole plants by culturing small explants (pieces of tissue cut from the parent) or even single parenchyma cells on an artificial medium containing nutrients and hormones (Figure 38.12a). The cultured cells divide and form an undifferentiated callus. When the hormonal balance is manipulated in the culture medium, the callus can sprout shoots and roots with fully differentiated cells (Figure 38.12b). The test-tube plantlets can then be transferred to soil, where they continue their growth. A single plant can be cloned into thousands of copies by subdividing calluses as they grow. This method is used for propagating orchids and for cloning pine trees that produce wood at unusually fast rates. Plant tissue culture also facilitates genetic engineering. Most techniques for the introduction of foreign genes into plants require small pieces of plant tissue or single plant cells as the starting material. In plant biology, the term transgenic is used to describe genetically modified (GM) organisms that 782

UNIT six Plant Form and Function

(a) Just a few parenchyma cells from a carrot gave rise to this callus, a mass of undifferentiated cells.

(b) The callus differentiates into entire plant, with leaves, stems, and roots.

A Figure 38.12 Test-tube cloning of carrots. (See also Figure 21.5.)

have been engineered to express a gene from another specks. Test-tube culture makes it possible to regenerate GM plants from a single plant cell into which the foreign DNA has been incorporated. The techniques of genetic engineering are discussed in more detail in Chapter 20. Some researchers are coupling a technique that is known as protoplast fusion with tissue culture methods to invent neiw plant varieties that can be cloned. Protoplasts are plant eels wilh their cell walls removed by treatment with enzymes (cellulases and pectinases) isolated from fungi (Figure 38.13). Before they are cultured, protoplasts can be screened for mutations that may improve the agricultural value of the plant. Ijn some cases, it Is possible to fuse two protoplasts from different plant species that would otherwise be reproductively incompatible, and then culture the hybrid protoplasts. Each protoplast can regenerate a wall and eventually form a hybrid plantlet. One success of this method is a hybrid between a potal and a wild relative called black nightshade. The nightshade resistant to an herbicide commonly used to kill weeds. Tf hybrids are also resistant, making it possible to "weed" a field with the herbicide without killing potato plants.

•4 Figure 38.13 Protoplasts

50 jim

These wall-less plant cells are prepared by treating cells or tissues with wall-degrading enzymes isolated from certain types of fungi. Researchers can fuse protoplasts from different species to make hybrids and can also culture the hybrid cells to produce a new plant.

The in vitro culturing of plant cells and tissues is fundamental to most types of plant biotechnology. The other basic prcjcess is the ability to produce transgenic plants through variouls methods of genetic engineering. In the next section, we takje a closer look at plant biotechnology.

loncept Check L. "Explain how both asexual and sexual reproduction contribute to the reproductive success of plants. 1. The seedless banana, the world's most popular fruit, is losing the battle against two fungal epidemics. Why do such epidemics generally pose a greater risk to asexually propagated crops? For suggested answers, see Appendix A.

Concept

Plant biotechnology is transforming agriculture Plant biotechnology has two meanings. In the general sense, it refers to innovations in the use of plants (or substances obtained from plants) to make products of use LO humans—an endeavor that began in prehistory. In a more specific sense, biotechnology refers to the use of GM organisms in agriculture and industry. Indeed, in the last two decades, genetic engineering has become such a powerful force that the terms genetic engineering and biotechnology have become synonymous in the media. In this last sejction, we expand on the discussion in Chapter 20 by examininjg how humans have altered plants to suit their purposes.

of our food are the result of natural hybridization between different species of grasses. Such hybridization is common in plants and has long been exploited by breeders to introduce additional genetic variation for artificial selection and crop improvement. One modern example is the selective breeding of maize. Maize is a staple in many developing countries, but the most common varieties are poor sources of protein, requiring that diets be supplemented with other sources, such as beans. The proteins in popular maize varieties are very low in lysine and tryptophan, two of the eight essential amino acids that humans cannot synthesize and must obtain from their diet (see Figure 41.10). Forty years ago, researchers discovered a mutant known as opaquc-2 that has much higher levels of lysine and tryptophan. This maize variety is more nutritious, as reflected in the fact that swine fed with opaque-2 maize gain weight three times faster than those fed with normal maize. However, as is often the case in plant breeding, a highly desirable trait was linked with undesirable ones. The opaque-2 maize kernels have a soft endosperm that makes them difficult to harvest and more vulnerable to attack by pests. Using the conventional methods of hybridization and artificial selection, plant breeders converted the soft opaque-2 endosperm into a more desirable hard endosperm type, a transition that took hundreds of scientists nearly 20 years to accomplish. If modern methods of genetic engineering had been available, one laboratory might have inserted genes for the high lysine and tryptophan condition into hardendosperm maize varieties over a period of only a few years. Unlike traditional plant breeders, modern plant biotechnologists, using techniques of genetic engineering, are not limited to the transfer of genes between closely related species or varieties of the same species. For example, traditional breeding techniques could not be used to insert a desired gene from daffodil into rice because the many intermediate species between rice and

rtificial Selection "Humans have intervened in the reproduction and genetic makeup of plants for thousands of years. It is no exaggeration to say that maize is an unnatural monster created by humans. Left on its own in nature, maize would soon become extilnct for the simple reason that it cannot spread its seeds. Maize kernels are not only permanently attached to the central axis (the ^'cob") but also permanently protected by tough, overlapping leaf sheathes (the "husk"). These attributes arose not by natural selection but by artificial selection by humans (Figure 38.14). (See Chapter 22 to review the basic concept of artificial selection.) Indeed, Neolithic (late Stone Age) h umans domesticated most of our crop species over a relatively short period about 10,000 years ago. But genetic modification ;an long before humans started altering crops by artificial s:leciion. For example, the wheat species we rely on for much

A Figure 38.14 Maize: a product of artificial selection. Modem maize (bottom) was derived from teosinte (top). Teosinte kernels are tiny, and each row has a husk that must be removed to get at the kernel. The seeds are loose at maturity, allowing dispersal, which probably made harvesting difficult for early farmers. Neolithic farmers selected for larger cob and kernel size as well as the permanent attachment of seeds to the cob and the encasing of the entire cob by a tough husk.

CHAPTER 38

Angiosperm Reproduction and Biotechnology

783

What causes polar movement of auxin from shoot tip to base? To investigate how auxin is transported Lnidirectionally, researchers designed an experiment to identify the location of the auxin transport protein. They used a a greenishyellow fluorescent molecule to label antibodies that bind to the auxin transport protein. They applied the antibodies to longitudinally sectioned Arabidopsis stems. The left micrograph shows that the auxin transport protein is not found in all tissues of the stem, but only in the xylem parenchyma. In the right micrograph, a higher magnification reveals that the auxin transport protein is primarily k calized to the basal end of the cells.

Lateral and Adventitious Root Formation. Auxins are used commercially in the vegetative propagation of plants by cuttings. Treating a detached leaf or stem with rooting powder containing auxin often causes adventitious roots to form near the cut surface. Auxin is also involved in the branching of roots. Researchers found that an Arabidopsis mutant that exhibits extreme proliferation of lateral roots has an auxm concentration 17-fold higher than normal. The results support the hypothesis that I concentration of the auxin transport protein at the basal ends of cells is responsible for polar transport of auxin.

Auxins as Herbicides. A number of synthetic auxins, such as 2,4-dichlorophenoxyacteic acid (2,4-D), are widely used as herbicides. Monocots, such as maize and turfgrass, can rapidly

© Wedge-shaped expansins, activated ^ Figure 39.8 Cell elongation m by low pH, separate cellulose microfibrils from response to auxin: the acid cross-linking polysaccharides. The exposed cross-linking growth hypothesis. polysaccharides are now more accessible to cell wall enzymes. Expansin CELL WALL © The enzymatic cleaving of the cross-linking polysaccharides allows the microfibrils to slide. The Cell extensibility of Plasma wal| the cell wall is membrane increased. Turgor causes the cell to expand.

Nucleus J Cytoplasm Vacuole 0 With the cellulose loosened, the cell can elongate.

CHAPTER 39 Plant Responses to Internal and External Signals

795

inactivate such synthetic auxins. However, eudicots cannot and therefore die from hormonal overdose. Spraying cereal fields or turf with 2,4-D eliminates eudicot (broadleaf) weeds such as dandelions. Other Effects of Auxin. In addition to stimulating cell elongation lor primary growth, auxin affects secondary growth. It induces cell division in the vascular cambium and influences differentiation of secondary xylem (see Figures 35.18 and 35.19). Developing seeds synthesize auxin, which promotes the growth of fruits in plants. Under greenhouse conditions, seed set is often poor because of the lack of insect pollinators; this makes for poorly developed tomato fruits. Synthetic auxins sprayed on greenhouse-grown tomato vines induce fruit development without a need for pollination. This makes it possible to grow seedless tomatoes by substituting synthetic auxin for the auxin normally synthesized by the developing seeds.

Cytokinins Trial-and-error attempts to find chemical additives that would enhance the growth and development of plant cells in tissue culture led to the discovery of cytokinins. In the 1940s, Johannes van Overbeek, working at the Cold Spring Harbor Laboratory in New York, found he could stimulate the growth of plant embryos by adding coconut milk, the liquid endosperm of a coconuts giant seed, to his culture medium. A decade later, Folke Skoog and Carlos O. Miller, at the University of Wisconsin-Madison, induced cultured tobacco cells to divide by adding degraded samples of DNA. The active ingredients of both experimental additives turned out to be some modified forms o( adenine, one of the components of nucleic acids. These growth regulators were named cytokinins because they stimulate cytokinesis, or cell division. Of the variety of cytokinins that occur naturally in plants, the most common is zeatin, so named because it was first discovered in maize (Zea mays). Although much remains to be learned about cytokinin synthesis and signal transduction, some of the major effects that cytokinins have on the physiology and development of plants are well documented.

sap. Acting in concert with auxin, cytokinins stimulate cell c 1visionand influence the pathway of differentiation. The effects of cytokinins on cells growing in tissue culture provide clues about how this class of hormones may function in an intact plant. When a piece of parenchyma tissue from a stem is cultured in the absence of cytokinins, the cells grow very large but do not divide. But if cytokinins are added along with auxin, the cells divide. Cytokinins alone have no effect. The ratio of cytokinin to auxin controls cell differentiation. When the concentrations of the two hormones are at the appropriate levels, the mass of cells continues to grow; but it remains' a cluster of undifferentiated cells called a callus (see Figure 38.12). If cytokinin levels are raised, shoot buds develop fro the callus. If auxin levels are raised, roots form. Control of Apical Dominance. Cytokinins, auxin, and oth^r factors interact in the control of apical dominance, the abili y of the terminal bud to suppress the development of axilla buds (Figure 39.9a). Until recently, the leading hypothesis explain the hormonal regulation of apical dominance—the direct inhibition hypothesis—proposed that auxin and cytokinin act antagonistically in regulating axillary bud growth. According to this view, auxin transported down the shoot from the terminal bud directly inhibits axillary buds from growing, causing a shoot to lengthen at the expense of lateral branching. Meanwhile, cytokinins entering the shoot system

Control of Cell Division and Differenti- (a) Intact plant (b) Plant with apical bud removed ation. Cytokinins are produced in actively growing tissues, particularly in A Figure 39.9 Apical dominance, (a) Auxin from the apical bud inhibits the growth of roots, embryos, and fruits. Cytokinins ax.llary buds. This favors elongation of the shoot's main ax.s. Cytokinins which are transported I , . upward from roots, counter auxin, stimulating the growth of axillary buds. This explains why, in produced in the root reach their target tis- m a n y p | an ts, axillary buds near the shoot tip are less likely to grow than those closer to the roots, sues by moving up the plant in the xylem (b) Removal of the apical bud from the same plant enabled lateral branches to grow. 796

UNIT six

Plant Form and Function

fro in roots counter the action of auxin by signaling axillary bu Is to begin growing. Thus, the ratio of auxin and cytokinin was viewed as the critical factor in controlling axillary bud inhibition. Many observations are consistent with the direct inhibition hypothesis. If the terminal bud, the primary source of au an, is removed, the inhibition of axillary buds is removed and the plant becomes bushier (Figure 39.9b). Application of aupn to the cut surface of the decapitated shoot resuppresses tin growth of the lateral buds. Mutants that overproduce cytokinins or plants treated with cytokinins also tend to be bushier than normal. One prediction of the direct inhibition hypothesis not borne out by experiment is that decapitation, by removing the primary source of auxin, should lead to a decrease in the auxin levels of axillary buds. Biochemical studies , however, have revealed the opposite: Auxin levels actually in< rease in the axillary buds of decapitated plants. The direct inhibition hypothesis, therefore, does not account for all experimental findings. It is likely that plant biologists have not uncovered all the pieces of this puzzle. A:iti-Aging Effects. Cytokinins retard the aging of some plant organs by inhibiting protein breakdown, by stimulating RNA and protein synthesis, and by mobilizing nutrients from surrounding tissues. If leaves removed from a plant are dipped in a cytokinin solution, they stay green much longer than otherwise. Cytokinins also slow the deterioration of leaves on in tact plants. Because of this anti-aging effect, florists use cytokrnn sprays to keep cut flowers fresh. G ibberellins A century ago, farmers in Asia noticed that some rice seedlings in their paddies grew so tall and spindly that they toppled over before they could mature and flower. In 1926, Ewiti Kurosawa, a Japanese plant pathologist, discovered that a fungus of the genus Glbberella caused this "foolish seedling disease." By the itoOs, Japanese scientists had determined that the fungus produced hyperelongation of rice stems by secreting a chemical, which was given the name gibberellin. In the 1950s, researchers discovered that plants also produce gibberellins. In die past 50 years, scientists have identified more than 100 cliffs rent gibberellins that occur naturally in plants, although a much smaller number occur in each plant species. "Foolish r ce" seedlings, it seems, suffer from an overdose of gibberellins normally found in plants in lower concentrations. Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination. Stem Elongation. Roots and young leaves are major sites of gibberellin production. Gibberellins stimulate growth of Both leaves and stems, but have little effect on root growth. m stems, gibberellins stimulate cell elongation and cell division. Like auxm, gibberellins cause cell wall loosening, but

not by acidifying the cell wall. One hypothesis proposes that gibberellins stimulate cell-wall-loosening enzymes that facilitate the penetration of expansin proteins into the cell wall. Thus, in a growing stem, auxin, by acidifying the cell wall and activating expansins, and gibberellins, by facilitating the penetration of expansins, act in concert to promote elongation. The effects of gibberellins in enhancing stem elongation are evident when certain dwarf (mutant) varieties of plants are treated with gibberellins. For instance, some dwarf pea plants (including the variety Mendel studied; see Chapter 14) grow to normal height if treated with gibberellins. If gibberellins are applied to plants of normal size, there is often no response. Apparently, these plants are already producing an optimal dose of the hormone. The most dramatic example of gibberellin-induced stem elongation is bolting, the rapid growth of the floral stalk. In their vegetative state, some plants, such as cabbage, develop in a rosette form, low to the ground with very short internodes. As the plant switches to reproductive growth, a surge of gibberellins induces internodes to elongate rapidly, elevating floral buds that develop at stem tips. Fruit Growth. In many plants, both auxin and gibberellins must be present for fruit to set. The most important commercial application of gibberellins is in the spraying of Thompson seedless grapes (Figure 39.10). The hormone makes the individual grapes grow larger, a trait prized by the consumer. The gibberellin sprays also make the internodes of the grape bunch elongate, allowing more space for the individual grapes, This increase in space, by enhancing air circulation between the grapes, also makes it harder for yeast and other microorganisms to infect the fruits.

A Figure 39.10 The effect of gibberellin treatment on Thompson seedless grapes. The grape bunch on the left is an untreated control. The bunch on the right is growing from a vine that was sprayed with gibberellin during fruit development. CHAPTER 39 Plant Responses to Internal and External. Signals

797

Germination. The embryo of a seed is a rich source of gibberellins. After water is imbibed, the release of gibberellins from the embryo signals the seeds to break dormancy and germinate. Some seeds thai require special environmental conditions to germinate, such as exposure to light or cold temperatures, break dormancy if they are treated with gibberellins. Gibberellins support the growth of cereal seedlings by stimulating the synthesis of digestive enzymes such as a-amylase that mobilize stored nutrients (Figure 39.11). Brassinosteroids Brassinosteroids are steroids that are chemically similar to cholesterol and the sex hormones of animals. They induce cell elongation and division in stem segments and seedlings at concentrations as low as 10~ ]2 M. They also retard leaf abscission and promote xylem differentiation. These effects are so qualitatively similar to those of auxin that it took several years for plant physiologists to determine that brassinosteroids were not types of auxins. Evidence from molecular biology established brassinosteroids as plant hormones. Joanne Chory and colleagues at the Salk Institute in San Diego were interested in an Arabidopsis mutant that had morphological features similar to light-grown plants even though the mutants were grown in the dark. They discovered thai the mutation affects a gene that normally codes for an enzyme similar to one involved in steroid synthesis in mammals. They also showed that mutant plants could be re-

O After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm.

Q The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is a-amylase, which hydrolyzes starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.)

stored to normal phenotype by experimental application of brassinosteroids. The mutant studied by Chory was brassinosteroid-deficient. Abscisic Acid In the 1960s, one research group studying the chemical changes that precede bud dormancy and leaf abscission in deciduous trees and another team investigating chemical changes preceding abscission of cotton fruits isolated ihe same compound, abscisic acid (ABA). Ironically ABA is no longer drought to play a primary role in bud dormancy or leaf abscission, but it is very important in other functions. Unlike the growth-stimulating hormones we have studied so far— auxin, cytokinins, gibberellins, and brassinosteroids—ABA slows growth. Often, ABA antagonizes the actions of the. growth hormones, and it is the ratio of ABA to one or more growth hormones that determines the final physiological outcome. We will consider two of ABAs many effects: seed dormancy and drought tolerance.

Seed Dormancy. Seed dormancy has great survival value because it ensures that the seed will germinate only when there are optimal conditions of light, temperature, and moisture (see Chapter 38). What prevents a seed dispersed in autumn from germinating immediately, only to be killed by winter? What mechanisms ensure that such seeds germinate m the spring? For that matter, what prevents seeds from germinating in the dark, moist interior of the fruit? The answer to these questions is ABA. The levels of ABA may increase 100-fold during seed maturation. The Q Sugars and other high levels of ABA in maturing seeds innutrients absorbed hibit germination and induce the profrom the endosperm by the scutellum duction of special proteins that help the (cotyledon) are consumed seeds withstand the extreme dehydraduring growth of the tion that accompanies maturation. embryo into a seedling.

A Figure 39.11 Gibberellins mobilize nutrients during the germination of grain seeds.

798

U N I T six

Plan: Form and Function

Many types of dormant seeds germinate when ABA is removed or inactivated in some way. The seeds of some desert plants break dormancy only when heavy rams wash ABA out of the seed. Other seeds require light or prolonged exposure to cold lo trigger the inactivation of ABA. Often, the ratio of ABA to gibberellins determines whether the seed remains dormant or germinates, and the addition of ABA to seeds that are primed to germinate returns them to the dormant condition. A maize mutant that has grains that germinate while still on the cob lacks a functional transcription factor required for ABA to induce expression of certain genes (Figure 39.12).

^Coleoptile

A Figure 39.12 Precocious germination of mutant m a i z e seeds. Abscisic acid induces dormancy in seeds. When its action is blocked—in this case, by a mutation affecting an abscisicacid-regulated transcription factor—precocious germination results.

Drought Tolerance. ABA is the primary internal signal thai enables plants to withstand drought. When, a plant begins to wilt, ABA accumulates in leaves and causes stomata to close rapidly, reducing transpiration and preventing further water loss. ABA, through its effects on second messengers such as calcium, causes an increase in the opening ol outwardly directed potassium channels in the plasma membrane of guard cells, leading to a massive loss of potassium from them. The accompanying osmotic loss of water leads to a reduction in guard cell turgor and a decrease in siomatal aperture (see Figure 36.15). Tn some cases, water shortage can stress the root system before the shoot system, and ABA transported from roots to leaves may function as an "early warning system." Mutants that are especially prone to wilting are in many cases deficient in the production of ABA.

The Triple Response to Mechanical Stress, imagine a pea seedling pushing upward through the soil, only to come up against a stone. As it pushes against the obstacle, the stress in its delicate tip induces the seedling to produce ethylene. The hormone then instigates a growth maneuver known as the triple response that enables the shoot to avoid the obstacle. The three parts of this response are a slowing of stem elongation, a thickening of the stem (which makes it stronger), and a curvature that causes the stem to start growing horizontally. As the stem continues to grow, its tip touches upward intermittently. If these movements continue to detect a solid object above, then another pulse of ethylene is generated and the stem continues its horizontal progress. If, however, the upward touch detects no solid object, then ethylene production decreases, and the stem, now clear of the obstacle, resumes its normal upward growth. It is ethylene that induces the stem to grow horizontally rather than the physical obstruction per se; when ethylene is applied to normal seedlings growing free of all physical impediments, they still undergo the triple response (Figure 39,13).

Figure 39.13

How does ethylene concentration affect the triple response in seedlings? EXPERIMENT I

Germinating pea seedlings were placed in the ark and exposed to varying ethylene concentrations. Their growth 'as compared with a control seedling not treated with ethylene.

All the treated seedlings exhibited the triple ^sponse. Response was greater with increased concentration.

Ethylene During the 1800s, when coal gas was used for street illumination, leakage from gas mains caused nearby trees to drop leaves prematurely. In 1901, the Russian scientist Dimitry Ndjubow demonstrated that the gas ethylene was the active factor in "illuminating gas." The idea that ethylene is a plant hormone only became widely accepted, however, when the advent of gas chromatography simplified measurement. Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection. Production also occurs in fruit ripening and programmed cell depth and in response to high concentrations of externally apolied auxin. Indeed, many effects previously ascribed to auxin, such as inhibition of root elongation, may be due to au^in-induced ethylene production. We will focus on four of ethylenes many effects; response to mechanical stress, programmed cell death, leaf abscission, and fruit ripening.

,

• • .

,.:

,

•••

,,••

.

•

i !

;

;

CHAPTER 39 Plant Responses to Internal and External Signals

799

Studies of Arabidopsis mutants with abnormal triple responses are an example of how biologists identify a signal transducti on pathway. Scientists isolated ethylene-insensitive (ein) mutants that fail to undergo the triple response after exposure to ethylene (Figure 39.14a). Some types of ein mutants are insensitive to ethylene because they lack a functional ethylene receptor. Other mutants undergo the triple response even out of soil, in the air, where there are no physical obstacles. Some mutants of this type have a regulatory defect that causes them to produce ethylene at rates 20 times normal The phenotype of such elhylene-overproducing (eto) mutants ein mutant. An ethylene-insensitive (ein) (b) ctr mutant. A constitutive triple-response can be restored to wild-type by treating (a) mutant fails to undergo the triple response in (ctr) mutant undergoes the triple response the seedlings with inhibitors of ethylene the presence of ethylene. even in the absence of ethylene. synthesis. Still other mutants, called constitutive triple-response (ctr) mu- A Figure 39.14 Ethylene triple-response Arabidopsis mutants. tants, undergo the triple response in air but do not respond to inhibitors of ethylene synthesis (Figure 39.14b). In this case, ethylene signal transduction is permanently turned on, even though there is no ethylene present. Figure 39.15 summarizes tlie responses of ein, eto, and ctr mutants to ethylene and ethylene synthesis inhibitors. The affected gene in ctr mutants turns out to encode for a Wild-type protein kinase. The fact that this mutation activates the ethylene response suggests that the normal kinase product of the wild-type allele is a negative regulator of ethylene signal transduction. Here is one hypothesis for how the pathway works in Ethylene insensitive wild-type plants: Binding of the hormone ethylene to the (ein) ethylene receptor leads to inactivation of the kinase; and inactivation of this negative regulator allows synthesis of the proteins required for the triple response. Apoptosis: Programmed Cell Death. Consider the shedding of a leaf in autumn or the death of an annual after flowering. Or think about the final step in differentiation of a vessel element, when its living contents are destroyed, leaving a hollow tube behind. Such events involve programmed death of certain cells or organs—or of the entire plant. Cells, organs, and plants genetically programmed to die on a schedule do not simply shut down cellular machinery and awyait death. Rather, the onset of programmed cell death, called apoptosis, is a very busy time in a cell's life, requiring new gene expression (see Figure 21.17). During apoptosis, newly formed enzymes break down many chemical components, including chlorophyll, DNA, RNA, proteins, and membrane lipids. The plant salvages many of the breakdown products. A burst of ethylene is almost always associated with this programmed destruction of cells, organs, or the whole plant. 800

U N I T six

Plant Form and Function

Ethylene overproducing (eto)

Constitutive triple response (ctr)

A Figure 39.15 Ethylene signal transduction mutants can be distinguished by their different responses to experimental treatments.

Leaf Abscission. The loss of leaves each autumn is an adaptation that keeps deciduous trees from desiccating during winter when the roots cannot absorb water from the frozen ground. Before leaves abscise, many essential elements are salvaged from the dying leaves and are stored in stem parenchyma cells. Thes;

nuLrients are recycled back to developing leaves the following spring. Fall color is a combination of new red pigments made during autumn and yellow and orange carotenoids (see Chapter 10) that were already present in the leaf but are rendered visib| e by the breakdown of the dark green chlorophyll in autumn. When an autumn leaf falls, the breaking point is an abscission layer that develops near the base of the petiole (Figure 39.16). The small parenchyma cells of this layer have very thin walls, and there are no fiber cells around the vascular tissue. The abscission layer is further weakened when enzymes hydrolyze polysaccharides in the cell walls. Finally, the weight of the leaf, with the help of wind, causes a separation within the abscission layer. Even before the leaf falls, a layer of cork forms a protective scar on the twig side of the abscission layer, preventing pathogens from invading the plant. A change in the balance of ethylene and auxin controls abscission. An aging leaf produces less and less auxin, and this renders the cells of the abscission layer more sensitive to ethyiene. As the influence of ethylene on the abscission layer

prevails, the cells produce enzymes that digest the cellulose and other components of cell walls. Fruit Ripening. Immature fleshy fruits that are tart, hard, and green help protect the developing seeds from herbivores. After ripening, the mature fruits help attract animals that disperse the seeds (see Figures 30.8 and 30.9). A burst of ethylene production in the fruit triggers the ripening process. The enzymatic breakdown of cell wall components softens the fruit, and the conversion of starches and acids to sugars makes the fruit sweet. The production of new scents and colors will help advertise ripeness to animals, which eat the fruits and disperse the seeds. A chain reaction occurs during ripening: Ethylene triggers ripening, and ripening then triggers even more ethylene production—one of the rare examples of positive feedback in physiology (see Figure 1.12). The resul t is a huge burst in ethylene production. Because ethylene is a gas, the signal to ripen even spreads from fruit to Fruit: One bad apple, in fact, does spoil the lot. If you pick or buy green fruit, you may be able to speed ripening by storing the fruit in a paper bag, allowing the ethylene gas to accumulate. On a commercial scale, many kinds of fruits are ripened in huge storage containers in which ethylene levels are enhanced. In other cases, measures are taken to retard ripening caused by natural ethylene. Apples, for instance, are stored in bins flushed with carbon dioxide. Circulating Lhe air prevents ethylene from accumulating, and carbon dioxide inhibits synthesis of new ethylene. Stored in this way, apples picked in autumn can still be shipped to grocery stores the following summer. Given the importance of ethylene in the post-harvest physiology of fruits, the genetic engineering of ethylene signal transduction pathways has potentially important commercial applications. For example, molecular biologists, by engineering a way to block the transcription of one of the genes required for ethylene synthesis, have created tomato fruits that ripen on demand. These fruits are picked while green and will not ripen unless ethylene gas is added. As such methods are refined, they will reduce spoilage of fruits and vegetables, a problem that currently ruins almost half the produce harvested in the United States.

Systems Biology and Hormone Interactions

Protective layer Stem

Abscission layer Petiole

Figure 39.16 Abscission of a maple leaf. Abscission is lontrolled by a change in the balance of ethylene and auxin. The abscission layer can be seen here as a vertical band at the base of the petiole. After the leaf falls, a protective layer of cork becomes the leaf scar that helps prevent pathogens from invading the plant (LM).

As we have discussed, plant responses often involve interactions of many hormones and their signal transduction pathways. The study of hormone interactions can be a complex problem. For example, flooding of deepwater rice leads to a 50-fold increase in internal ethylene and a rapid increase in stem elongation. But ethylene's role in this response is a small part of the story. Flooding also leads to an increase in sensitivity to GA that is mediated by a decrease in ABA levels. Thus, stem elongation is actually the result of an interaction among these three hormones and their signal transduction chains. CHAPTER 39 Plant Responses to Internal and External Signals

801

Imagine yourself as a molecular biologist assigned the task of genetically engineering a deepwater rice plant so that it grows even faster when submerged. What would be the best molecular

targets for genetic manipulation? An enzyme that inactivates ABA? An enzyme that produces more GA? An ethylene receptor? It is difficult to predict. And this is by no means an isolated problem. Virtually every plant response discussed in this chapter is of comparable complexity. Because of this pervasive and unavoidable problem of complex interactions, many plant biologists, including Natasha Raikhel, who is interviewed on pages 710-711, are promoting a new systems-based approach to plant biology. Chapter 1 provided a general description of systems biology, which attempts to discover and understand biological properties that emerge from the interactions of many system elements (for example, mRNAs, proteins, hormones, and metabolites). Using genomic techniques, biologists can now identify all the genes in a plant and have already sequenced two plant genomes— the research plant Ambidopsis and the crop plant rice (Oryza sativa). Moreover, using microarray and proteomic techniques (see Chapter 20), scientists can resolve which genes are activated or inactivated during development or in response to an environmental change. However, identifying all the genes and proteins (system elements) in an organism is comparable to listing all the parts of an airplane. Although such a list provides a catalog of components, it is not sufficient for understanding the complexity underlying the integrated system. What plant biologists really need to know is how all these system elements interact. A systems-based approach may greatly alter how plant biology is done. The dream is that laboratories will be equipped with fast-moving (high-throughput) robotic scanners that record which genes in a plants genome are activated in which cells and under what conditions. New hypotheses and avenues of research will emerge from analysis of these comprehensive data sets. Ultimately one goal of systems biology is to model a living plant predictably. Armed with such detailed knowledge, a molecular biologist attempting to genetically engineer faster stem elongation into rice could proceed much more efficiently. The ability to model a living plant would facilitate predicting the result of a genetic manipulation before even setting foot in the laboratory. Concept Check 1. Predict the triple-response phenotype of a plant with the double mutation ctr and tin. Explain. 2. In a diseased state known as "witch's broom," branches grow and proliferate excessively Suggest a hypothesis to explain how a pathogen might induce this growth pattern. 3. Fusicoccin is a fungal toxin that stimulates the plasma membrane H+ pump of plant cells. How might it affect growth of isolated stem sections? For suggested answers, see Appendix A.

802

UNIT six

Plant Form and Function

Concept

Responses to light are critical for plant success Light is an especially important environmental factor in tl e lives of plants. In addition to being required for photosynthesis, light cues many key events in plant growth and development. Effects of light on plant morphology are what plant biologists call photomorphogenesis. Light reception also allows plants to measure the passage of days and seasons. Plants can detect not only the presence of light but also its direction, intensity, and wavelength (color). A graph called an action spectrum depicts the relative effectiveness of different wavelengths of radiation in driving a particular process. For example, the action spectrum for photosynthesis has two peaks, one in the red and one in the blue (see Figure 10.9). This is because chlorophyll absorbs light primarily in the red and blue portions of the visible spectrum. Action spectra are useful in the study of any process that depends on light, such as phototropism (Figure 39.17). By comparing action spectia of different plant responses, researchers determine which responses are mediated by the same photoreceptor (pigment). They also compare action spectra with absorption spectra of pigments. A close correspondence suggests that the pigment may be the photoreceptor mediating the response. Action spectra reveal that red and blue light are the most important colors in regulating a plant's photomorphogenesis. These observations led researchers to two major classes of light receptors; blue-light photoreceptors and phytochromes, phojtoreceptors that absorb mostly red light.

Blue-Light Photoreceptors Blue light initiates diverse responses in plants, including phototropism, the light-induced opening of stomata (see Figure 36.14), and the light-induced slowing of hypocotyl elongation that occurs when a seedling breaks ground. The biochemical identity of the blue-light photoreceptor was so elusive that in the 1970s, plant physiologists began to call thi; putative receptor cryptochrome (from the Greek kryptos, hidden, and chrom, pigment). In the 1990s, molecular biologist•; analyzing Arabidopsis mutants found that plants use at least three different types of pigments to detect blue light: cryptochromes (for inhibition of hypocotyl elongation), phototropin (for phototropism), and a carotenoid-based photorecepto called zeaxanthin (for stomatal opening).

Phytochromes as Photoreceptors When we introduced signal transduction in plants earlier ir the chapter, we discussed the role of a family of plant pigments called phytochromes in the de-etiolation process

Figure 39.18

(

t What wavelengths stimulate hototropic bending toward light?

Researchers exposed maize (lea mays) o >!eoptiles to violet, blue, green, yellow, orange, and red light to test which wavelengths stimulate the phototropic bending toward light. The graph below shows phototropic effectiveness (curvature per photon) relative to effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only with wavelengths below 500 nm and was greatest with blue light.

fsHiussv How does the order of red and farred illumination affect seed germination? EXPERIMENT ^ j r i n g the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red light or far-red light to test the effects on germination. After the light exposure, the seeds were placed in the dark, and the results were compared with control seeds that were not exposed to light. i The bar below each photo indicates the sequence of red-light exposure, far-red light exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposed to red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right).

%®* Dark icon trol)

The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.

Phytochromes regulate many of a plant's responses Lo light throughout its life. Lets look at a couple of examples. Phytochromes and Seed Germination Studies of seed germination led to the discover)' of phytoChrornes. Because of their limited nutrient reserves, the successful sprouting of many types of small seeds, such as lettuce, requires germination only when conditions, especially the ight environment, are near optimal. Such seeds often remain

i Red light stimulated germination, and far-red light inhibited germination. The final exposure was the determining factor. The effects of red and far-red light were reversible.

dormant for many years until a change in light conditions occurs. For example, the death of a shading tree or the plowing of a field may create a favorable light environment. In the 1930s, scientists at the U.S. Department of Agriculture determined the action spectrum for light-induced germination of lettuce seeds (Figure 39.18). They exposed water-swollen CHAPTER 39 Plant Responses lo Internal and External Signals

803

A phytochrome consists of two identical proteins joined to form one functional molecule. Each of these proteins has two domains.

In its Pr isomer form, a phytochrome absorbs red light ma> mally, whereas in its Pj> isomer form, it absorbs far-red light:

Chromophore

Red light

Photoreceptor activity. One domain, which functions as the photoreceptor, is covalently bonded to a nonprotein pigment, or chromophore.

\

Far-red light Kinase activity. The other domain has protein kinase activity. The photoreceptor domains interact with the kinase domains to link light reception to cellular responses triggered by the kinase. A Figure 39.19 S t r u c t u r e of a p h y t o c h r o m e .

seeds to a few minutes of monochromatic (single-colored) light of various wavelengths and then stored the seeds in the dark. After two days, the researchers counted the number of seeds that had germinated under each light regimen. They found that red light of wavelength 660 nm increased the germination percentage of lettuce seeds maximally, whereas far-red light-—that is, light of wavelengths just near the edge of human visibility (730 nm)—-inhibited germination compared with dark controls. What happens when the lettuce seeds are subjected to a flash of red (R) light followed by a flash of far-red (FR) light or, conversely, to FR light followed by R light? The last flash of light determines the seeds' response. In oLher words, the effects of red and far-red light are reversible. The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome. It consists of a protein component covalently bonded to a nonprotein part that functions as a chromophore, the light-absorbing part of the molecule (Figure 39.19). So far, researchers have identified five phytochrornes in Arabidopsi-,. each with a slightly different protein component. The chromophore of a phytochrome is photoreversible, reverting back and forth between two isomeric forms, depending on the color of light provided (see Figure 4.7 to review isomers).

Pr^Pfr interconversion is a switching mechanism that controls various light-induced events in the life of the plait (Figure 39.20). Pfr is the form of phytochrome that triggers mai LV of a plant's developmental responses to light. For example, Pr n lettuce seeds that are exposed to red light is converted to Pfr, stimulating the cellular responses that lead to germination. When red-illuminated seeds are then exposed to far-red light, the Pfr is converted back to Pt, inhibiting the germination response. How does phytochrome switching explain light-induced germination in nature? Plants synthesize phytochrome as f r, and if seeds are kept in the dark, the pigment remains almost entirely in the Pr form (see Figure 39.20). But if the seeds are illuminated with sunlight, the phytochrome is exposed to red light (along with all the other wavelengths in sunlight), and much of the Pr is converted to Pfr. The appearance of Pfr is or e of the ways that plants detect sunlight. When seeds are exposed to adequate sunlight for the first time, it is the appearance of F!fr that triggers their germination. Phytochromes and Shade Avoidance The phytochrome system also provides the plant with info: mation about the quality of light. Sunlight includes both rei and far-red radiation. Thus, during the day, the P r ^ P ^ photoreversion reaches a dynamic equilibrium, with the ratio of the two phytochrome forms indicating the relative amounts of red and far-red light. This sensing mechanism enables plants LO adapt to changes in light conditions. Consider, for examplt, the "shade-avoidance" response of a tree that require; relatively high light intensity If other trees in a forest shade this tree, the phytochrome ratio shifts in favor of Pr because the forest canopy screens out more red light than far-red light

Red light Responses: seed germination, control of flowering, etc.

Synthesis-—*

< Figure 39.20 Phytochrome: a molecular

Slow conversion in darkness (some plants)

804

U N I T six

Planl Form and Function

A

•> Enzymatic destruction

s w i t c h i n g m e c h a n i s m . Absorption of red light causes the bluish Pr to change to the blue-greenish Pfr. Far-red light reverses this conversion. In most cases, it is the Pfr form of the pigment that switches on physiological and developmental responses in the plant.

Th.s is because the chlorophyll pigments in the leaves of the canopy absorb red light and allow far-red light to pass. This shift m the ratio of red to far-red light induces the tree to alloca e more of its resources to growing taller. In contrast, direct sunlight increases the proportion of Piv, which stimulates branching and inhibits vertical growth. In addition to helping plants detect light, phytochrome hejlps a plant to keep track of the passage of days and seasons. To understand phytochromes role in these timekeeping processes, we must first examine the clock itself.

B Lological Clocks and Orcadian Rhythms Many plant processes, such as transpiration and synthesis of certain enzymes, oscillate during the course of a day. Some of these cyclic variations are responses to the changes in light levels, temperature, and relative humidity that accompany the 2,4-hour cycle of day and night. However, one can eliminate these exogenous (external) factors by growing plants in growth chambers under rigidly maintained conditions of light, temperature, and humidity. Even under artificially constant conditions, many physiological processes in plants, such as the opening and closing of stomata and the production of photosynthetic enzymes, continue to oscillate with a frequency of about 24 hours. For example, many legumes lower their leaves in the evening and raise them in the morning (Figure 39.21). A bean plant continues these "sleep movements" even if kept in constant light or constant darkness; the leaves are mot simply responding to sunrise and sunset. Such cycles with a requency that is about 24 hours and not directly paced by any ;nown environmental variable are called circadian rhythms from the Latin circa, approximately, and dies, day), and they are ommon to all eukaryotic life. Your pulse, blood pressure, tem>erature, rate of cell division, blood cell count, alertness, urine omposition, metabolic rate, sex drive, and response to medicatons all fluctuate in a circadian manner. Current research indicates that the molecular "gears" of the circadian clock are endogenous (internal), rather than a daily

Noon

Midnight

A Figure 39.21 Sleep movements of a bean plant (Phaseolus vulgaris). The movements are caused by reversible changes in the turgor pressure of cells on opposing sides of the pulvini, motor organs of the leaf.

response to some subtle but pervasive environmental cycle, such as geomagnetism or cosmic radiation. Organisms, including plants and humans, will continue their rhythms when placed in the deepest mine shafts or when orbited in satellites, conditions that alter these subtle geophysical periodicities. The circadian clock, however, can be entrained (set) to a period of precisely 24 hours by daily signals from the environment. ff an organism is kept in a constant environment, its circadian rhythms deviate from a 24-hour period (a period is the duration of one cycle). These free-running periods, as they are called, vary from about 21 to 27 hours, depending on the particular rhythmic response. The sleep movements of bean plants, for instance, have a period of 26 hours when the plants are kept in free-running conditions of constant darkness. Deviation of the free-running period from exactly 24 hours does not mean that biological clocks drift erratically. Free-running clocks are still keeping perfect time, but they are not synchronized with the outside world. How do biological clocks work? In attempting to answer this question, we must distinguish between the clock and the rhythmic processes it controls. For example, the leaves of the bean plant in Figure 39.21 are the clock's "hands," but are not the essence of the clock itself. If bean leaves are restrained for several hours and then released, they will rush to the position appropriate lor the time of day. We can interfere with a biological rhythm, but the clockwork continues. Researchers are tracing the clock to a molecular mechanism that may be common to all eukaryotes. A leading hypothesis is that biological timekeeping may depend on the synthesis of a protein that regulates its own production through feedback control. This protein may be a transcription factor that inhibits transcription of the gene that encodes for the transcription factor itself. The concentration of this transcription factor may accumulate during the first half of the circadian cycle and then decline during the second half owing to self-inhibition of its own production. Researchers have recently used a novel technique to identify clock mutants of AroHdopsk, One prominent circadian rhythm in plants is the daily production of certain photosynthesisrelated proteins. Molecular biologists traced this rhythm to the promoter that regulates the transcription of the genes for these photosynthesis proteins. To identify clock mutants, scientists spliced the gene for an enzyme called luciferase to the promoter. Luciferase is the enzyme responsible for the bioluminescence of fireflies. When the biological clock turned on the promoter in the Arabidopsis genome, it also turned on the production of luciferase. The plants began to glow with a circadian periodicity. Clock mutants were then isolated by selecting specimens that glowed for a longer or shorter time than normal. The altered genes in some of these mutants affect proteins that normally bind photoreceptors. Perhaps these particular mutations disrupt a light-dependent mechanism that sets the biological clock. CHAPTER 39 Plant Responses to Internal and Externai Signals

803

The Effect of Light on the Biological Clock As we have discussed, the free-running period of the circadian rhythm of bean leaf movements is 26 hours. Consider a bean plant placed at dawn in a dark cabinet for 72 hours: Its leaves would not rise again until 2 hours after natural dawn on the second day, and 4 hours after natural dawn on the third day and so on. Shut off from environmental cues, the plant becomes desynchronized. Desynchronization also happens when we cross several time zones in an airplane; when we reach our destination, the clocks on the wall are not synchronized with our internal clocks. All eukaiyotes are probably prone to jet lag. The factor that entrains the biological clock to precisely 24 hours every day is light. Both phytochrome and blue-light photoreceptors can entrain circadian rhythms in plants, but our understanding of how phytochrome does this is more complete. The mechanism involves turning cellular responses on and off by means of the P^^Ffr switch. Consider again the photoreversible system in Figure 39.20. In darkness, the phytochrome ratio shifts gradually in favor of the Pr form, partly a result of turnover in the overall phytochrome pool. The pigment is synthesized in the Pr form, and enzymes destroy more Pfr than Pr. In some plant species, PJV present at sundown slowly converts to P,. In darkness, there is no means for the Pr to be reconverted to Pir. but upon illumination, the Pfc level suddenly increases again by rapid conversion from PT. This increase in Pfr each day at dawn resets the biological clock: Bean leaves reach their most extreme night position 16 hours after dawn. In nature, interactions between phytochrome and the biological clock enable plants to measure the passage of night and day. The relative lengths of night and day, however, change over the course of the year (except at the equator). Plants use this change to adjust activities in synchrony with the seasons.

Photoperiodism and Responses to Seasons Imagine the consequences if a plant produced flowers when pollinators were not present or if a deciduous tree produced leaves in the middle of winter. Seasonal events are of critical importance in the life cycles of most plants. Seed germination, [lowering, and the onset and breaking of bud dormancy are all stages that usually occur at specific times of the year. The environmental stimulus plants use most often to detect the time of year is the photoperiod, the relative lengths of night and day. A physiological response to photoperiod, such as flowering, is called photoperiodism. Photoperiodism and Control of Flowering An early clue to how plants detect seasons came from a mutant variety ol tobacco, Maryland Mammoth, which grew tall 806

U N I T six

Plant F o r m and Function

but failed to flower during summer. It finally bloomed in a greenhouse in December. After trying to induce earlier flowering by varying temperature, moisture, and mineral nutritioi, researchers learned that the shortening days of winter stim.ilated this variety to flower. If the plants were kept in lighttight boxes so that lamps could manipulate "day" and ':nighi," flowering occurred only if the day length was 14 hours or shorter. It did not flower during summer because at Marylands latitude, the days were too long during that season. The researchers called Maryland Mammoth a short-dny plani. because it apparently required a light period short \r than a critical length to flower. Chrysanthemums, poinsettias, and some soybean varieties are some other short-day plants, which generally flower in late summer, fall, or winter. Another group of plants will flower only when the light period is long&r than a certain number ol hours. These long-day plants will generally flower in late spring or early summer. Spinach, fqr example, flowers when days are 14 hours or longer. "Radish, lettuce, iris, and many cereal varieties are also long-day plants. Day-neutral plants are unaffected by photoperiod and flower when reaching a certain stage of maturity, regardless of day length. Tomatoes, rice, and dandelions are examples. Critical Night Length. In the 1940s, researchers discovered that flowering and other responses to photoperiod are actuall y controlled by night length, not day length (Figure 39.22). Many of these scientists worked with cocklebur (Xanthium strumdrium), a short-day plant that flowers only when days are 16 hours or shorter (and nights are at least 8 hours long)!. These researchers found that if the daytime portion of the photoperiod is broken by a brief exposure to darkness, there is no effect on flowering. However, if the nighttime part of the photoperiod is interrupted by even a few minutes of dim light, cocklebur will not flower, and this turned out to be true for other short-day plants as well (see Figure 39.22a). Cocklebuj is actually unresponsive to day length, but it requires at least 8 hours of continuous darkness to flower. Short-day plants are re ally long-night plants, but the older term is embedded firmly in the jargon of plant physiology. Similarly, long-day plantJ are actually short-night plants. A long-day plant grown on photoperiods ot long nights that would not normally induce flowering will flower if the period of continuous darkness is interrupted by a few minutes of light (see Figure 39.22b). Notice (hat we distinguish long-day from short-day plants not by an absolute night length but by whether the critical night length sets a maximum (long-day plants) or minimum (short-; day plants) number of hours of darkness required for flowering. In both cases, the actual number of hours in the critical night length is specific to each species of plant. Red light is the most effective color in interrupting the nighttime portion of the photoperiod. Action spectra and photoreversibility experiments show that phytochrome is the pigment that receives the red light (Figure 39.23). For example, if a flash

Figure 39.22

Figure 39.23

How does interrupting the dark Jeriod with a brief exposure to light affect

Is phytochrome the pigment that measures the interruption of dark periods in photoperiodic response?

MMfcJUH During the 1940s, researchers conducted ts in which periods of darkness were interrupted with brief 30 light to test how the light and dark portions of a photoI period affected flowering in "short-day" and "long-day" plants.

A unique characteristic of phytochrome is reversibility in response to red and far-red light. To test whether phytochrome is the pigment measuring interruption of dark periods, researchers observed how flashes of red light and far-red light affected flowering in "short-day" and "long-day" plants.

I y (a) "Short-day" plants flowered only if a period of continuous darkness was longer than a critical dark period for that particular species (13 hours in this example). A period of darkness can be ended by a brief exposure to light.

(b) "Long-day" plants flowered only if a period of continuous darkness was shorter than a critical dark period for that particular species (13 hours in this example).

The experiments indicated that flowering of each species was determined by a critical period of darkness "critical night length") for that species, not by a specific period of light. Therefore, "short-day" plants are more properly called 'long-night" plants, and "long-day" plants are really "short-night" olants.

of red light (R) during the dark period is followed by a flash of far-red (FR) light, then the plant detects no interruption, of Light length. As in the case of phytochrome-mediated seed germination, red/far-red photoreversibility occurs. Plants detect night length very precisely; some short-day plants will not flower if night is even 1 minute shorter than tie critical length. Some plant species always flower on the same day each year. It appears that plants use their biological clock, apparently entrained with the help of phytochrome, to Dell the season of the year by measuring night length. The floriculture (flower-growing) industry has applied this knowldge to produce flowers out of season. Chrysanthemums, for nstance, are short-day plants that normally bloom in fall, but

Sk

\g

-je

^f

Short-day (long-night) plant

Long-day (short-night) plant I A flash of red light shortened the dark period. A subsequent flash of far-red light canceled the red light's effect. If a red flash followed a far-red flash, the effect of the far-red light was canceled. This reversibility indicated that it is phytochrome that measures the interruption of dark periods.

their blooming can be stalled until Mother's Day in May by punctuating each long night with a flash of light, thus turning one long night into two short nights. Some plants bloom after a single exposure to the photoperiod required for flowering. Other species need several successive days of the appropriate photoperiod. Still other plants will respond to a photoperiod only if they have been previously exposed to some other environmental stimulus, such as a period of cold temperatures. Winter wheat, for example, will not flower unless it has been exposed to several weeks of temperatures below 10°C This use of pretreatment with cold to induce flowering is called vernalization (Latin for "spring"). Several weeks after winter wheat is vernalized, a photoperiod with long days (short nights) induces flowering. CHAPTER 39 Plant Responses to Internal and External Signals

807

A Flowering Hormone?

Meristem Transition and Flowering

Floral buds develop into flowers, but in many species it is leaves that detect photoperiod. When the photoperiodic requirement for flowering is met, signals from leaves cue buds to develop as flowers. To induce a short-day or long-day plant to flower, it is enough in many species to expose one leaf to the appropriate photoperiod. Indeed, if only one leaf is left attached to the plant, photoperiod is detected and floral buds are induced- If all leaves are removed, the plant is insensitive to photoperiod. The flowering signal, not yet chemically identified, is called florigen, and it may be a hormone or change in relative concentrations of multiple hormones (Figure 39.24). The flowering stimulus appears to be the same for short-day and long-day plants, despite differing photoperiodic conditions required for leaves to send this signal.

Whatever combination of environmental cues (such as photoperiod or vernalization) and internal signals (such as hormones) is necessary for flowering to occur, the outcome is the transition of a bud's meristem from a vegetative state to a flowering state. This transition requires changes in the expression of genes that regulate pattern formation. Meristem identity genes that induce the bud to form a flower instead of a vegetative shoot must first be switched on. Then the organ identity genes that specify the spatial organization of the floral organs-—sepals, petals, stamens, and carpels—-are activated in the appropriate regions of the meristem (see Figure 35.31). Research on flower development is progressing rapidly, and one goal is to identify the signal transduction pathways that link such cues as photoperiod and hormonal changes to the gene expression required for flowering.

Concept Check Figure 39.24

Inquiry Is there a flowering hormone? To test whether there is a flowering hormone, researchers conducted an experiment in which a plant that had been induced to flower by photoperiod was grafted to a plant that had not been induced.

Plant subjected to photoperiod that induces flowering

Plant subjected to photoperiod that does not induce flowering

1. A plant flowers in a controlled chamber with a daily cycle of 10 hours of light and 14 hours of darkness. Is it a short-day plant? Explain. 2. Assume that poinsettias grown in a greenhouse require at least 14 hours of darkness to flower. If work has to be done at night, what light source would not disrupt the flowering schedule? Explain. 3. Upon germination, some vine seedlings grow toward darkness until reaching an upright structure. This adaptation helps the vine "find" a shaded object to climb. Suggest an experiment to test whether this negative phototropism is mediated by blue light photoreceptors or by phytochrome. For suggested answers, see Appendix A,

Plants respond to a wide variety of stimuli other than light

Both plants flowered, indicating the transmission of a flower-inducing substance. In some cases, the transmission worked even if one was a short-day plant and the other was a long-day plant.

808

U N I T six

Plant Form and Function

Plants can neither migrate to a watering hole when water is scarce nor seek shelter when the weather is too windy. A seed, landing upside down in the soil, cannot maneuver itself into an upright position. Because of their immobility, plants must adjust to a wide range of environmental circumstances through developmental and physiological mechanisms. Natural selection has refined these responses. Light is so important in the life of a plant that we devoted the entire previous section to a plants reception of and response to this one environmental factor. In this section, we examine responses to some of the other environmental stimuli that a plant commonly faces in its "struggle for existence."

Gravity ^cause plants are solar-powered organisms, it is not surprisg that mechanisms for growing toward sunlight have olved. But what environmental cue does the shoot of a y0ung seedling use to grow upward when it is completely underground and there is no light for it to detect? Similarly, what environmental factor prompts the young root to grow downrd? The answer to both questions is gravity. Place a plant on its side, and it adjusts its growth so that the shoot bends upward and the root curves downward. In their r< sponses to gravity, or gravitropism, roots display positive gravitropism (Figure 39.25a) and shoots exhibit negative gravitropism. Gravitropism functions as soon as a seed germinates, ensuring that the root grows into the soil and the shoot reaches sunlight, regardless of how the seed is oriented when it lands. Auxin plays a key role in gravitropism.

t

Plants may detect gravity by the settling of statoliths, specialized plastids containing dense starch grains, to the lower portions of cells (Figure 39.25b). In roots, statoliths are located in certain cells of the root cap. According to one hypothesis, the aggregation of statoliths at the low points of these cells triggers redistribution of calcium, which causes lateral transport of auxin within the root. The calcium and auxin accumulate on the lower si!de of the root's zone of elongation. Because these chemicals are dissolved, they do not respond to gravity but must be actively

transported to one side of the root. At high concentration, auxin inhibits cell elongation, an effect that slows growth on the root's lower side. The more rapid elongation of cells on the upper side causes the root to curve as it grows. This tropism continues until the root grows straight down. Plant physiologists are refining the "falling statoiith" hypothesis of root gravitropism as they try new experiments. For example, mutants of Ambidopsis and tobacco that lack statoliths are still capable of gravitropism, though the response is slower than in wild-type plants. It could be that the entire cell helps the root sense gravity by mechanically pulling on proteins that tether the protoplast to the cell wall, stretching the proteins on the "up" side and compressing the proteins on the "down" side of the root cells. Dense organelles, in addition to starch granules, may also contribute by distorting the cytoskeleton as they are pulled by gravity. Statoliths, because of their density, may enhance gravitational sensing by a mechanism that works more slowly in their absence.

Mechanical Stimuli A tree growing on a windy mountain ridge usually has a shorter, stockier trunk than a tree of the same species growing in a more sheltered location. The advantage of this stunted morphology is that it enables the plant to hold its ground against strong gusts of wind. The term thigmomorphogenesis (from the Greek thigma, touch) refers to the changes in form that result from mechanical perturbation. Plants are very sensitive to mechanical stress: Even the act of measuring the length of a leaf with a ruler alters its subsequent growth. Rubbing the stems of a young plant a couple of times daily results in plants that are shorter than controls (Figure 39.26).

(a) M Figure 39,25 Positive gravitropism in roots: the s atolith hypothesis, (a) Over the course of hours, a horizontally oriented primary root of maize will bend gravitropically until its growing tip becomes vertically oriented, (b) Within minutes after the rc'Ot is placed horizontally, statoliths begin settling to the lowest sides of root cap cells. The statoiith hypothesis proposes that this settling rray be the gravity-sensing mechanism that leads to redistribution of auxin and differential rates of elongation by cells on opposite sides of the root. (LMs)

A Figure 39.26 Altering gene expression by touch in Arabidopsis. The shorter plant on the left was rubbed twice a day. The untouched plant (right) grew much taller.

CHAPTER 39

Plant Responses to Internal and External Signal:

809

Mechanical stimulation activates a signal transduction pathway involving an increase in cytosolic Ca2 + that mediates the activation of specific genes, some of which encode for proteins that affect cell wall properties. Some plant species have become, over the course of their evolution, "touch specialists." Acute responsiveness to mechanical stimuli is an integral part of these plants' life "strategies." Most vines and other climbing plants have tendrils that coil rapidly around supports (see Figure 35.7a). These grasping organs usually grow straight until they touch something; the contact stimulates a coiling response caused by differential growth of cells on opposite sides of the tendril. This directional growth in response to touch is called thigmotropism, and it allows the Pulvinus vine to take advantage of whatever me(motor chanical supports it comes across as it organ) climbs upward toward a forest canopy Other examples of touch specialists are plants that undergo rapid leaf movements in response to mechanical stimula(c) Motor organs 0.5 u.m tion. For example, when the compound leaf of the sensitive plant Mimosa pudica A Figure 39.27 Rapid turgor movements by the sensitive plant (Mimosa pudica) is touched, it collapses and its leaflets (a) In the unstimulated plant, leaflets are spread apart, (b) Within a second or two of being fold together (Figure 39.27). This re- touched, the leaflets have folded together, (c) In these light micrographs of a leaflet pair in the sponse, which takes only a second or two results from a rapid loss of tureor by cells within pulvmi, specialized motor

closed (stimulated)state, you can seethe motor cells in the sectioned pulvini (motor organs). The " ure of a pulvinus is caused by motor cells on one side losing water and becoming flaccid while cells on the opposite side retain their turgor.

curva t;

organs located at the joints of the leaf. The motor cells suddenly become flaccid after stimulation because they lose potassium, which causes water to leave the cells by osmosis. It takes about 10 minutes for the cells to regain their turgor and restore the ''unstimulated" form of the leaf. The function of the sensitive plant's behavior invites speculation. Perhaps by folding its leaves and reducing its surface area when jostled by strong winds, the plant conserves water. Or perhaps because the collapse of the leaves exposes thorns on the stem, the rapid response of the sensitive plant- discourages herbivores. A remarkable feature of rapid leaf movements is the mode of transmission of the stimulus through the plant. If one leaflet on a sensitive plant is touched, first that leaflet responds, then the adjacent leaflet responds, and so on, until all the leaflet pairs have folded together. From the point of stimulation, the signal that produces this response travels at a speed of about 1 cm/sec. An electrical impulse, traveling at the same rate, can be detected by attaching electrodes to the leaf. These impulses, called action potentials, resemble nerve 810

U N I T six

Plant Form and Function

impulses in animals, though the action potentials of plants a:e thousands of times slower. Action potentials, which have been discovered in many species of algae and plants, may be wide ly used as a form of internal communication. Another example is the Venus' flytrap (Dionaea musapida), in which action pote ltials are transmitted from sensory hairs in the trap to the cells that respond by closing the trap (see Figure 37.13). In the ca.;e of Mimosa pudica, more violent stimuli, such as touching a leaf with a hot needle, causes all the leaves and leaflets on a plant to droop, but this systemic response involves the spread of chemical signals released from the injured area to other parts of the shoot.

Environmental Stresses Occasionally, certain factors m the environment change s. verely enough to have a potentially adverse effect on a plan s survival, growth, and reproduction. Environmental stresses,

Flooding

such as flooding, drought, or extreme temperatures, can have a devastating impact on crop yields in agriculture. In natural ecosystems, plants that cannot tolerate an environmental stress will either succumb or be outcompeted by other plants, ami they will become locally extinct. Thus, environmental stresses are also important in determining the geographic rar ges of plants. Here, we will consider some of the more common abiotic (nonliving) stresses that, plants encounter. In thi last section of the chapter, we will examine the defensive responses of plants to common biotic (living) stresses, such as paihogens and herbivores.

An overwatered houseplant may suffocate because the soil lacks the air spaces that provide oxygen for cellular respiration in the roots. Some plants are structurally adapted to very wet habitats. For example, the submerged roots of trees called mangroves, which inhabit coastal marshes, are continuous with aerial roots that provide access to oxygen. But how do plants less specialized for aquatic environments cope with oxygen deprivation in waterlogged soils? Oxygen deprivation stimulates the production of the hormone ethylene, which causes some of the cells in the root cortex to undergo apoptosis (programmed cell death). Enzymatic destruction of cells creates air tubes that function as "snorkels," providing oxygen to the submerged roots (Figure 39.28).

Di ought On a bright, warm, dry day, a plant may be stressed by a watei deficiency because it is losing water by transpiration faster than the water can be restored by uptake from the so.l. Prolonged drought can stress crops and the plants ol natural ecosystems for weeks or months. Severe water deficit, of course, will kill a plant, as you may know from experience with neglected hnuseplants. But plants have control systems that enable them to cope with less extreme water deficits. Many of a plant's responses to water deficit help the plant conserve water by reducing the rate of transpiration. Water deficit in a leaf causes guard cells to lose turgor, a simple control mechanism that slows transpiration, by closing stomata (see Figure 36.15). Water deficit also stimulates increased synthesis and release of abscisic acid in the leaf, and this hormone helps keep stomata closed by acting on guard cell rrjembranes. Leaves respond to water deficit in several other ways. Because cell expansion is a turgor-dependent process, a water deficit will inhibit the growth of young leaves. This response minimizes the transpirational less of water by slowing the increase in leaf surface. When the leaves of many grasses and other plants wilt from a water deficit, they roll into a shape that reduces n anspiration by exposing less leaf surface to dry air and wind. While all of these res 5onses of leaves help the plant conserve

Salt Stress An excess of sodium chloride or other salts in the soil threatens plants for two reasons. First, by lowering the water potential of the soil solution, salt can cause a water deficit in plants even though the soil has plenty of water. As the water potential of the soil solution becomes more negative, the water potential gradient from soil to roots is lowered, thereby reducing water uptake (see Chapter 36). The second problem with saline soil is that sodium and certain other ions are toxic to plants when their concentrations are relatively high. The selectively permeable membranes of root cells impede the uptake of most harmful ions, but this only aggravates the problem of acquiring water from soil that is rich in solutes. Many plants can respond to moderate soil salinity by producing solutes that are well tolerated at high concentrations, mostly organic compounds that keep the water potential of cells more negative than that of the

ig a drought, the soil usually cries from the surface down. This inhibits tiie growth of shallow roots, partly because cells cannot maintain the turgor required for elongation. Deeper roots sur- (a) Control root (aerated) rounded by soil that is still moist continue

100 urn (b) Experimental root (nonaerated)

to grow Thus, the root system proliferates * F i g u r e 3 9 - 2 8 A developmental response of maize roots to flooding and oxygen , ' deprivation, (a) A transverse section of a control root grown in an aerated hydroponic medium. i a way that maximizes exposure to soil ( b ) An experimental root grown in a nonaerated hydroponic medium. Ethylene-stimulated -7ater. apoptosis (programmed cell death) creates the air tubes. (SEMs) CHAPTER 39 Plant Responses to Internal and External Signals

811

soil solution without admitting toxic quantities of salt. However, most plants cannot survive salt stress for long. The exceptions are halophytes, salt-tolerant plants with adaptations such as salt glands. These glands pump salts out across the leaf epidermis. Heat Stress Excessive heat can harm and eventually kill a plant by denaturing its enzymes and damaging its metabolism in other ways. One function of transpiration is evaporative cooling. On a warm day, for example, the temperature o[ a leaf may be 3-10°C below the ambient air temperature. Of course, hot, dry weather also tends to cause water deficiency in many plants; the closing of stomata in response to this stress conserves water but then sacrifices evaporative cooling. This dilemma is one of the reasons that very hot, dry days take such a toll on most plants. Most plants have a backup response that enables them to survive heat stress. Above a certain temperature—about 40°C lor most plants that inhabit temperate regions—plant cells begin synthesizing relatively large quantities of special proteins called heat-shock proteins. Researchers have also discovered this response in heat-stressed animals and microorganisms. Some heat-shock proteins are chaperone proteins, which function in unstressed cells as temporary scaffolds that help other proteins fold into their functional shapes (see Chapter 5). In their roles as heat-shock proteins, perhaps these molecules embrace enzymes and other proteins and help prevent denaiuration.

contains more solutes than the very dilute solution lound in the cell wall, and solutes depress the freezing point of a solution. The reduction in liquid water in the cell wall caused by ice formation lowers the extracellular water potential, causing water to leave the cytoplasm. The resulting increase in the concentration of ionic salts in the cytoplasm is harmful and can lead to cell death. Whether the cell survives depends to a large extent on how resistant it is to dehydration. Plants that are native to regions where winters are cold have special adaptations that enable them to cope with freezing stress. For example, before the onset of winter, the cells of many frosttolerant species increase their cytoplasmic levels of specific solutes, such as sugars, that are better tolerated at high concentrations and that help reduce the loss of water from the cell during extracellular freezing. Concept Check 1. Thermal images are photographs of the heat emitted by an object. Researchers have used thermal imaging of plants to isolate mutants that overproduce abscisic acid. Suggest a reason why these mutants are warmer than wild-type plants under conditions that are normally nonstressful. 2. A worker in a commercial greenhouse finds that the potted chrysanthemums nearest to the aisles are often shorter than those growing in the middle of the bench. Suggest an explanation. For suggested answers, see Appendix A.

Cold Stress One problem plants face when the temperature of the environment falls is a change in the fluidity of cell membranes. Recall from Chapter 7 that a biological membrane is a fluid mosaic, with proteins and lipids moving laterally in the plane of the membrane. When a membrane cools below a critical point, it loses its fluidity as the lipids become locked into crystalline structures. This alters solute transport across the membrane and also adversely affects the functions of membrane proteins. Plants respond to cold stress by altering the lipid composition of their membranes. For example, membrane lipids increase in their proportion of unsaturated fatty acids, which have shapes that help keep membranes fluid at lower temperatures by impeding crystal formation (see Figure 7.5b). Such molecular modification of the membrane requires from several hours to days, which is one reason rapid chilling is generally more stressful to plants than the more gradual drop in air temperature that occurs seasonally. Freezing is a more severe version of cold stress. At subfreezing temperatures, ice forms in the cell walls and intercellular spaces of most plants. The cytosol generally does not freeze at the cooling rates encountered in nature because it 812

U N I T six

Plani Form and Function

Concept

Plants defend themselves against herbivores and pathogens Plants do not exist in isolation but interact with many other species in their communities. Some of these interspecific interactions—for example, the associations of plants with fungi in mycorrhizae (see Figure 37.12) or with pollinators (see Figure 30.13)—are mutually beneficial. Most of the interactions that plants have with other organisms, however, are not beneficial to the plant. As primary producers, plants are at the base of most food webs and are subject to attack by a wide range of plant-eating (herbivorous) animals. A plant is also subject to infection by a diversity of pathogenic viruses, bacteria, and fungi that have the potential to damage tissues or even kill the plant. Plants counter these threats with defense systems that deter herbivory and prevent infection or combat pathogens that do manage to infect the plant.

Defenses Against Herbivores Herbivoiy—animals eating plants—is a stress that plants face in any ecosystem. Plants counter excessive herbivory with both physical defenses, such as thorns, and chemical defenses, such as the production of distasteful or toxic compounds. For example, some plants produce an unusual amino acid called cknavanine, which is named for one of its sources, the jackbean (^anavalia ensijormis). Canavanine resembles arginine, one of tie 20 amino acids organisms incorporate into their proteins. Tf an insect eats a plant containing canavanine, the molecule is incorporated into the insects proteins in place of arginine. Because canavanine is different enough from arginine to adversely a "feet the conformation and hence the function of the proteins, tljie insect dies. Some plants even "recruit" predatory animals that help defend the plant against specific herbivores. For example, insects cjalled parasitoid wasps inject their eggs into their prey, including caterpillars feeding on plants. The eggs hatch within the caterpillars, and the larvae eat through their organic containers f:om the inside out. The plant, which benefits from the deruction of the herbivorous caterpillars, has an active role in lis drama. A leaf damaged by caterpillars releases volatile impounds that attract parasitoid wasps. The stimulus for this :sponse is a combination of physical damage to the leaf :msed by the munching caterpillar and a specific compound the caterpillars saliva (Figure 39.29). The volatile molecules a plant releases in response to herbiv ore damage can also function as an "early warning system" for nearby plants of the same species. Lima bean plants infested with spider mites release volatile chemicals that signal "news"

Recruitment of parasitoid wasps that lay their eggs within caterpillars

v.

j

. Figure 39.29 A maize leaf "recruits" a parasitoid wasp £S a defensive response to an herbivore, an army-worm caterpillar.

of the attack to neighboring, noninfested lima bean plants. In response to these volatiles, noninfested lima bean leaves activate defense genes. Volatiles released from leaves mechanically wounded in experiments do not have the same effect. Genes activated by infestation-released volatiles overlap considerably with those produced by exposure to jasmonic acid, an important molecule in plant defense. As a result of this gene activation, noninfested neighbors become less susceptible to spider mites and more attractive to another species of mite that preys on spider mites.

Defenses Against Pathogens A plant's first line of defense against infection is the physical barrier of the plants "skin," the epidermis of the primary plant body and the periderm of the secondary plant body (see Figure 35.18). This first defense system, however, is not impenetrable. Viruses, bacteria, and the spores and hyphae of fungi can still enter the plant through injuries or through the natural openings in the epidermis, such as stomata. Once a pathogen invades, the plant mounts a chemical attack as a second line of defense that kills the pathogens and prevents their spread from the site of infection. This second defense system is enhanced by the plant's inherited ability to recognize certain pathogens. Gene-for-Gene Recognition Plants are generally resistant to most pathogens. This is because plants have the ability to recognize invading pathogens and then to mount successful defenses. In a converse manner, successful pathogens cause disease because they are able to evade recognition or suppress host defense mechanisms. Pathogens against which a plant has little specific defense are said to be virulent. They are the exceptions, for if they were not, then hosts and pathogens would soon perish together. A kind of "compromise" has coevolved between plants and most of their pathogens. In such cases, the pathogen gains enough access to its host to enable it to perpetuate itself without severely damaging or killing the plant. Strains of pathogens that only mildly harm but do not kill the host plant are termed avirulent. Gene-for-gene recognition is a widespread form of plant disease resistance that involves recognition of pathogenderived molecules by the protein products of specific plant disease resistance (R) genes. There are many pathogens, and plants have many R genes—Arabidopsis has at least several hundred. An R protein usually recognizes only a single corresponding pathogen molecule Lhat is encoded by an avirulence (Avr) gene. Many Avr proteins play an active role in pathogenesis and are thought to redirect the host metabolism to the advantage of the pathogen. The questions of how R proteins function and evolve are currently topics of intense research. In the simplest biochemical model (the receptor-ligand model), an CHAPTER 39 Plant Responses to Internal and External Signals

813

R protein functions as a plant receptor molecule that triggers resistance upon binding to the correct corresponding Avr prolein (Figure 39.30). If the plant hosi lacks the R gene that counteracts the pathogen's Avr gene, then the pathogen can invade and kill the plant. An alternative model of gene-forgene recognition, dubbed the "guard" hypothesis, proposes that R proteins function as a surveillance system of other plant

Receptor coded by R ailele

Signal molecule (ligand) from Avr gene product

Avrallele • Avirulent pathogen

Plant eel! is resistant

(a) ff an Avr alleie in the pathogen corresponds to an R alieie ; in the host plant, the host plant will have resistance, making the pathogen aviruient. R alieies probably code for receptors in the plasma membranes of host plant ceiis. Avr sllejes produce compounds that can act as ligands, binding to receptors in ho.

No Avr aileie; virulent pathogen Pfant cell becomes diseased

Virulent pathogen

No R aileie; plant cell becomes diseased

proteins that undergo activity or conformational changes induced by Avr proteins. Regardless of the exact mechanism

involved, recognition of pathogen-dewed molecules l^y R proteins triggers a signal transduction pathway leading tc a defense response in the infected plant tissue. This defense includes both an enhancement of the localized response at the site of infection and a more general systemic response of t whole plant. Plant Responses to Pathogen Invasions In contrast to the strain-specific Avr-R interactions that co 1trol plant disease resistance to very narrow groups pf pathogens (specifically, strains that contain the appropriste Avr aileie), molecules called elicitors induce a broader type of host defense response. Oligosaccharins, derived from cellulose fragments released by cell wall damage, are one pf the major classes of elicitors. Elicitors stimulate the production of antimicrobial compounds called phytoalexins. Imfection also activates genes that produce PR proteins (pathogenesis-related proteins). Some of these proteins a:e antimicrobial, attacking molecules in the cell wall of a ba> terium, for example. Others may function as signals that spread "news" of the infection to nearby cells. Infection also stimulates the cross-linking of molecules in the cell wall and the deposition of lignin, responses that set up a local barricade that slows spread of the pathogen to other parts of the plant. If the pathogen is avirulent based on an R-Avr match, thtn the localized defense response is even more vigorous and jis known as a hypersensitive response (abbreviated HR). There is an enhanced production of phytoalexins and PR proteins, and the "sealing" response that contains the infection is more effective. After the cells at the site of infection mouiiit their chemical defense and seal off the area, they destroy themselves. We can see the result of an HR as lesions on a leaf or other infected organ. As "sick" as such a leaf appears, it wTl still survive, and its defense response will then help protect the rest of the plant (Figure 39.31). Systemic Acquired Resistance

Virulent pathogen No P, aileie; plant cell becomes diseased (b) If there is no gene-for-gene recognition because of one of the above three conditions, the pathogen wiit be virulent, causing disease to develop. -

.

•

.

•>.

•

.

A Figure 39.30 Gene-for-gene resistance of plants to pathogens: the receptor-ligand model.

814

U N I T six

Plant Form and Function

The hypersensitive response, as you have learned, is localized and specific, a containment response based on gene-for-gene (R-Avr) recognition between host and pathogen. However, this defense response also includes production of chemical signals that ''sound the alarm" of infection to the whole plant. Released from the site of infection, the alarm hoimones are transported throughout the plant, stimulating production of phytoalexins and PR proteins. This response, called systemic acquired resistance (SAR), is nonspecific, providing protection against a diversity of pathogens fcr days (see Figure 39.31).

i In a hypersensitive response (HR), plant cells produce antimicrobial molecules, seal off infected areas by modifying thetr walls, and then destroy themselves. This localized response produces lesions and protects other parts of an infected leaf.

This identification step triggers a • signal transduction pathway. Specific resistance is based on the binding of ligands from the pathogen to receptors in plant cells.

Systemic acquired resistance

hypersensitive response

Systemic acquired resistance is activated: the , production of molecules that help protect the celi against a diversity of pathogens for several days.

A Figure 39,31 Defense responses against an avirulent pathogen.

A good candidate for one of the hormones responsible for activating SAR is salicylic acid. A modified form of this compound, acetylsalicylic acid, is the active ingredient in aspirin. Centuries before aspirin was sold as a pain reliever, si me cultures had learned that chewing the bark of a willow tree (Salix) would lessen the pain of a toothache or headache. With the discovery of systemic acquired resistance, biologists hive finally learned one function of salicylic acid in plants. Aspirin turns out to be a natural medicine in the plants that prodace it, but with effects entirely different from the medicinal action in humans who consume the drug. Plant biologists investigating disease resistance and other evolutionary adaptations of plants are getting to the heart of how a plant responds to internal and external signals. These scientists, along with thousands of other plant biologists working on other questions and millions of students experimenting with plants in biology courses, are all extending a

centuries-old tradition of curiosity about the green organisms that feed the biosphere.

Concept Check 1. Chewing insects mechanically damage plants and lessen the surface area of leaves for photosynthesis. In addition, these insects make plants more vulnerable to pathogen attack. Suggest a reason why. 2. A scientist finds that a population of plants growing in a breezy location is more prone to defoliation by insects than a population of the same species growing in a sheltered area. Suggest a hypothesis to account for this observation. For suggested answers, see Appendix A.

CHAPTER 39

Plant Responses to Internal and External Signals

815

Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY CONCEPTS Signal transduction pathways link signal reception to response • Reception (p. 789) Internal and external signals are detected by receptors, proteins that change in response to specific stimuli. • Transduction (pp. 789-790) Second messengers transfer and amplify signals from receptors to proteins that cause specific responses. • Response (pp. 790-791) Responses to stimulation typically involve activation of enzymes by stimulating transcription of mRNA for the enzyme (transcriptional regulation) or by activating existing enzyme molecules (post-translational modification of proteins).

Plant hormones help coordinate growth, development, and responses to stimuli • The Discovery of Plant Hormones (pp. 792-793) Researchers discovered auxin by identifying the compound responsible for transmitting a signal downward through coleoptiles, from the tips to the elongating regions, during phototropism, • A Survey of Plant Hormones (pp. 793-801) Auxin, produced primarily in the apical meristem of the shoot, stimulates cell elongation in different target tissues. Cytokinins, produced in actively growing tissues such as roots, embryos, and fruits, stimulate cell division. Gibberellins, produced in roots and young leaves, stimulate growth in leaves and stems. Brassinosteroids, chemically similar to the sex hormones of animals, induce cell elongation and division. Abscisic acid maintains dormancy in seeds. Ethylene helps control fruit ripening. Activity Leaf Abscission Investigation What Plant Hormones Affect Organ Formation? • Systems Biology and Hormone Interactions (pp. 801-802) Interactions between hormones and their signal transduction pathways makes it difficult to predict what effect a genetic manipulation will have on a plant. Systems biology seeks a comprehensive understanding of plants that will permit successful modeling of plant functions.

Responses to light are critical for plant success • Blue-Light Photoreceptors (p. 802) Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism. • Phytochromes as Photoreceptors (pp. 802-805) Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses. • Biological Clocks and Circadian Rhythms (p. 805) Free-running circadian cycles are approximately 24 hours long but are entrained to exactly 24 hours by the day/night cycle.

816

UNIT six

Plant Form and Function

• The Effect of Light on the Biological Clock (p. 806) Phytochrome conversion marks sunrise and sunset, providing the clock with environmental cues. • Photoperiodism and Responses to Seasons (pp. 806-808) Some developmental processes, including flowering in many plant species, require a certain photoperiod. For example, a critical night length sets a minimum (in short-day plants) or maximum (in long-day plants) number of hours of darkness required for flowering. Activity Flowering Lab

Plants respond to a wide variety of stimuli other than light • Gravity (p. 809) Response to gravity is known as gravitropism. Roots show positive gravitropism, and stems show negative gravitropism. Specialized plastids called statoliths may enable plants to detect gravity • Mechanical Stimuli (pp. 809-810) Growth in response to touch is called thigmotropism. Rapid leaf movements are transmissions of electrical impulses called action potentials. • Environmental Stresses (pp. 810-812) During drought, plants respond to water deficit by reducing transpiration. Enzymatic destruction of cells creates air tubes that help plants survive oxygen deprivation during flooding. Plants respond to salt stress by producing solutes tolerated at high concentrations, keeping the water potential of cells more negative than that of the soil solution. Heat-shock proteins help plants survive heat stress. Altering lipid composition of membranes is a response to cold stress.

Plants defend themselves against herbivores and pathogens • Defenses Against Herbivores (p. 813) Physical defenses include morphological adaptations such as thorns, chemical defenses such as distasteful or toxic compounds, and airborne attractants that bring animals that destroy herbivores. • Defenses Against Pathogens (pp. 813-815) A pathogen is avirulent if it has a specific Avr gene corresponding to a particular R allele in the host plant. A hypersensitive response against an avirulent pathogen seals off the infection and kills both pathogen and host cells in the region of the infection. Salicylic acid is a signal molecule that triggers systemic acquired resistance (SAR), generalized defense responses in organs distant from the original site of infection.

TESTING YOUR KNOWLEDGE Evolution Connection Coevolution is defined as reciprocal adaptations between two species, with each species adapting in how it interacts with the other. In this context of coevolution, write a paragraph explaining the relationship between a plant and an avirulent pathogen.

Scientific Inquiry A plant biologist observed a peculiar pattern when a tropical shrub was attacked by caterpillars. After a caterpillar ate a leaf, it would skip over nearby leaves and attack a leaf some distance away. The researcher found that when a leaf was eaten, nearby leaves started making a chemical that deterred caterpillars. Simply removing a leaf did not deter them from eating nearby leaves. The biologist suspected that a damaged leaf sent out a chemical that signaled other leaves. How could the researcher test this hypothesis? Investigation What Plant Honnones Affect Organ Formation?

Science, Technology, and Society Based on your study of this chapter, write a short essay explaining at least three examples of how knowledge about the control systems of plants is applied in agriculture or horticulture.

CHAPTER

39

Plant Responses to Internal and External Signals

817

AN INTERVIEW WITH

Erich Jarvis Erich Jarvis is an assistant professor at the Duke University Medical Center, where he teaches neuroscience and studies the brain, using vocal communication by zebra finches and other songbirds as his main model system. Growing up in New York City, he attended the High School of the Performing Arts, where he showed great promise as a dancer. However, he chose to go another route, completing a double major in biology and mathematics at Hunter College and then a Ph.D. in molecular neurobiology and animal behavior at Rockefeller University. Dr. Jarvis has won numerous awards, including the National Science Founda-

dancer, you need to have discipline. It's a very difficult field, there's a lot ol competition, and you have to practice and practice. As a scientist, you also need discipline, and you have to deal with competition. There are actually more opportunities in science, although science is very difficult. Nature doesn't give up its secrets without a struggle! On top of that, both fields require creativity. As a performing artist, you want to do something new and beautiful, it's the same for science: You're trying to do something new and creative. Also, in both fields, you have to adapt your schedule to what is required. There's so much overlap between being an artist and being a scientist that I'm amazed people don't see it more.

tion's highest honor for young researchers, the Alan T. Waterman Award. In addition to re-

Why did biology appeal to you?

search and teaching, Dr. Jarvis has long been ac-

I think it had something to do with my cultural background. Like many of us in this country, I have a mixed ancestry—mine is African, Native American, and "European. There was something about the cultural atmosphere in my family that seems especially connected to earthly things, nature, and so forth. Perhaps for that reason, I felt more a: home in biology than in chemistry or physics, though I liked those subjects too.

tive in recruiting minority students to science.

What were your educational inspirations and challenges growing up? Although my parents were separated and we were sometimes poor, I had a lot of support from both sides of my family, and my mother always encouraged me to aspire to something great. Most of my family were in the performing arts, and continuing in that tradition, 1 had the goal of becoming a dancer. I began with ballet and then went on to jazz and modem dance. But in my senior year of high school, I had a change of heart. I realized 1 wanted to have a bigger, positive impact in the world, and I thought 1 could do that best as a scientist. I was an okay student, but 1 didn't have a strong academic background. So when I got to college, I had to take remedial classes—1 had a lot of catching up to do.

Do you see any connection between dance and science? In terms of what's required for a successful career, I don't really see much of a difference. As a

818

Did you get to do research as an undergraduate? At Hunter College, I benefited from several programs that promoted minority access to careers in scientific research. For four years, 1 worked in the laboratory of Professor Rivka Rudner, who was studying bacterial genes for ribosomal RNA. It took me almost a year to start to understand things, but then the excitement of laboratory research overcame me like a passion. Dr. Rudner would have to throw me out of the laboratory sometimes and tell me to go home and sleep. But soon after graduation, I published six papers.

What led you to neuroscience? 1 was fascinated by the human brain. I wanted to know how the brain generates behavior, he w it perceives it, and how it leams it. Ol course, [ realized that neurobiologists mostly have to work with nonhuman animals. When I got to graduate school, I started reading lots of papers, and I was disappointed to learn how little wa= known about the behavior of most common model organisms, such as the rat. At the time, neuroscience consisted of anatomy, neurophyi-iology (the study of nerve impulses and so fort i), and an increasing amount of molecular biology—but very little behavior. But then 1 learned about Fernando Nottebohm's exciting researcl with songbirds, and I joined his laboratory The wonderful thing about the songbird system was that, when I began, there were already 40 years of research on the ecology and natural behavior of these animals. This research was li ke a box full of ideas for studying the brain mechanisms of songbird behavior, in particular their imitation of sounds. Very few animals have tht ability to imitate what they hear—only songbirds, parrots, hummingbirds, bats, dolphins, and humans. Usually the animal imitates soun is ol its own species, though some species can imitate other species and even mechanical sounds like a car. Fernando Nottebohm had identified nearly the complete brain pathway responsible for a songbirds production of a learned sound 1 wanted to combine my molecular biology trailing with Dr. Nottebohm's discoveries and his knowledge of behavior. Already, an assistant professor in the laboratory, David Clayton, was using molecular biology. I was fascinated by the idea of figuring out the molecular genetics of imitative behavior in songbirds.

How might genes be connected to this sort of behavior? After years of work on this question, I have now come to believe that what makes a vocal learner are genes that control the construction

of brain circuits that connect to vocal muscles. Ant in addition to studying how genes regulate her- amor, we're also studying how behavior regulaies genes! The act of producing the learned sounds causes huge, short-term changes in gen t expression in certain brain areas. Many of the genes involved encode transcription factors, wh.ch regulate the expression of a variety o[ other genes. When a songbird is singing, we see up :o a 60-fold change in the amount of specifl: mRNAs and proteins in these brain areas. vVe think this is happening lor most behavior; in most animals, although we don't yet km iw the full circuits in other cases—in the Ciis • o\ walking, lor example. But in songbirds, we can study every single group of neurons involved in the singing pathway and how their geres are regulated. So now we're trying to find out the relationshi os between the rapid changes in gene exprt ision in the brain as a result of the singing be! avior, the electrical impulses that are going through the neural circuits to regulate both the se genes and the behavior, and the behavior itst If. To do this, we have to measure the mRNA ant. protein synthesis of certain brain cells and record electrical Impulses from those same cells, both during the behavior.

How do you do these things? To detect the electrical impulses, we implant ha;r-thin microelectrodes in different brain area;- of a songbird, usually a zebra finch. The wres are connected to what's called a commutator, a mechanism originally used in helicopter-",. It allows the bird to fly around in the cage

without getting tangled in the wires. The bird can sing and communicate with other birds while we record the electrical impulses. To find out which genes are expressed in the nerve cells that are firing, we use DNA microarrays (see Figure 20.14). For this part of our work, we are sequencing the "transcriptome'' of the zebra finch brain—the DNA corresponding to all the different mRNAs made in brain tissue. We want to be able to put all the genes expressed in the songbird brain on microarrays. Then we'll be able to identify which genes are involved in any given behavior.

You have also studied birds in the wild. Why? I wanted to see if the singing-driven gene expression we find in the lab occurs in the real world. So my colleague Claudio Mello and I visited songbird sites in a station wagon loaded with lab equipment and dry ice. To attract wild birds, we played recorded songs, causing males in the vicinity to sing like crazy. After about half an hour, when the products of gene expression in the brain should have accumulated, we captured the birds with a net. Then we took the brains back to the laboratory, and we did find robust singing-driven gene expression in those brains. But, more exciting, we also found phenomena that we hadn't seen in the lab; Some brain regions showed different patterns of gene expression that, 1 later discovered, depended on the social context in which the birds had been singing—to another male, to a female, or something else. And there turns out to be evidence for contextdependent patterns of activity in comparable

brain areas in other animals, such as monkeys. We think there's a context-dependent brain padiway that is used for learned vocalization, speech, and other learned cognitive behaviors.

You're combining many approaches in your work! Yes, and in addition to molecular biology, anatomy, electroph) siology, and behavior, we're making a lot of use of bioinformatics, which involves math and computer science. Biologists routinely use the computer to analyze gene expression data from DNA microarrays, but we also include in our computer analyses information about anatomical connectivity, behavioral data from song sonograms, and patterns of neural impulses. Our inference algorithm software helps us identify causal relationships and even proposes models we can test experimentally. We are trying to look at what we study as an integrated system, not just a pattern of regulated genes or a combination of neural circuits. This is the way I approach science: If I'm going to study how something works, I've got to look at all aspects of it. I really encourage people to look at the big picture. That's probably the only way to reach the ultimate goal of understanding how the brain generates speech and language.

Are there potential medical benefits from this research? One of our projects relates to Parkinsons disease, which is caused by a aegenei ation ol neurons in certain parts of the brain. We're studying birds with similar defects. The songbird brain area that functions differently in different social contexts is the same area affected by Parkinson's disease in humans, and we see some similarities in vocal behavior associated with this area. For example, people with Parkinson's disease often stutter, and when damage occurs to this area of the bird brain, the affected bird sometimes does too. So we think we might be able to use these birds to study speech disorders.

What is your advice to students interested in biology? It's important for students, especially those corning from a disadvamaged background, to know that biological research can be a very rewarding career, if you have a passion for it. Like an artist, you have to find your work intrinsically interesting. Figuring out how nature works is not an easy job; there are many frustrations. But intelLectually, it can be one of the most rewarding and empowering of career choices.

819

I inciples of Animal Form and Function A Figure 40.1 A sphinx moth feeding on orchid nectar.

Key Concepts 40.1 Physical laws and the environment constrain animal size and shape 40.2 Animal form and function are correlated at all levels of organization 40.3 Animals use the chemical energy in food to sustain form and function 40.4 Many animals regulate their internal environment within relatively narrow limits 40.5 Thermoregulation contributes to homeostasis and involves anatomy, physiology, and behavior

Diverse Forms, Common Challenges

A

nimals inhabit almost every part of the biosphere. Despite their amazing diversity of habitat, form, and function, all animals mast solve a common set of problems: Animals as different as hydras, halibut, and humans must obtain oxygen, nourish themselves, excrete waste products, and move. How do animals of diverse evolutionary history and varying complexity solve these general challenges of life? As we explore this question throughout Unit 7, note the recurring theme of natural selection and adaptation. This chapter introduces the unit by presenting some unifying concepts that apply across the animal kingdom. For example, the comparative study of animals reveals that form and function are closely correlated. Consider the long, thin, tonguelike proboscis of the sphinx moth (Xanthopan morgani) in Figure 40.1. The extended proboscis is a structural adaptation for feeding, serving as a straw through which the moth can suck nectar 820

from deep within tube-shaped flowers. Analyzing a biological structure, such as a sphinx moth's proboscis, gives us clues about what it does and how it functions. Anatomy is me study of the structure of an organism; physiology is the study of the junctions an organism performs. Natural selection can fit structure to function by selecting, over many generations, what works best among the available variations in a population. I A feeding sphinx moth also illustrates an animals need for fuel in the form of chemical energy. We will apply concepts !of bioenergetics—-how organisms obtain, process, and use thihir energy resources—as another theme throughout our comparative study of animals. One use of an animal's energy resourc es is to regulate its internal environment. Tn this chapter, we w ill begin to explore the concept of homeostasis, using the example of body temperature regulation.

Concept

Physical laws and the environment constrain animal size and shape An animal's size and shape, features that biologists often call "body plans" or "designs/1 are fundamental aspects of forim and function that significantly affect the way an animal interacts with its environment. By using the terms plan and design here, we do not mean to imply that animal body forms are products of conscious invention. The body plan of an anirral results from a pattern of development programmed by tljie genome, itself the product of millions of years of evolution. And the possibilities are not infinite—physical laws and the need to exchange materials with the environment place a rtain limits on the range of animal forms.

Pfc ysical Laws and Animal Form Imagine seeing a winged serpent several meters long and weighing hundreds of kilograms flying through the air. Fortunately, you won't experience such a horrifying sight outside of a movie. Physical requirements constrain what natural selectioi can "invent," including the size and shape of flying animals. An animal the size and shape of a mythical dragon could not generate enough lift with its wings to get off the ground. Thjis is just one example of how physical laws—in this case, the pWsics of flight—limit the evolution of an organism's form. Consider another example: how the laws of hydrodynamics constrain the shapes that are possible for aquatic animals that swim very fast. Water is about a thousand times denser than air; this, any bump on the body surface that causes drag would impede a swimmer even more than it would a runner or a flyer.

Tuna and other fast ray-finned fishes can swim at speeds up lo 80 km/hr. Sharks, penguins (birds), and aquatic mammals such as dolphins, seals, and whales are also fast swimmers. These animals all have the same streamlined body form: a fusiform shape, which means tapered on both ends (Figure 40.2). The fact that these speedy swimmers have similar shapes is an example of convergent evolution (see Chapter 25). Convergence occurs because natural selection shapes similar adaptations when diverse organisms face the same environmental challenge, such as the resistance of water to fast travel.

Exchange with the Environment An animal's size and shape have a direct effect on how the animal exchanges energy and materials with its surroundings. An animal's body plan must allow all of its living cells to be bathed in an aqueous medium, a requirement for maintaining the fluid integrity of the plasma membranes. Exchange with the environment occurs as substances dissolved in the aqueous medium diffuse and are transported across the cells' plasma membranes. As shown in Figure 40.3a, a single-celled protist living in water has a sufficient surface area of plasma membrane to service its entire volume of cytoplasm. (Thus, surface-to-volume ratio is one of the physical constraints on the size of single-celled protists.) Multicellular animals are composed of numerous cells, each with its own plasma membrane that functions as a loading and unloading platform for a modest volume of cytoplasm. But this organization only works if all the cells of the animal have access to a suitable aqueous environment. A hydra, which has a saclike body plan, has a body wall only two cell layers thick (Figure 40.3b). Because its gastrovascular cavity opens to the exterior, both outer and inner layers of cells are

(a) Single c &. Figure 40.3 Contact with the environment, (a) In a (e)Seal A Figure 40.2 Evolutionary convergence in fast swimmers.

unicellular protist, such as this amoeba, the entire surface area contacts the environment, (b) A hydra's body consists of t w o layers of cells. Because the aqueous environment can circulate in and out of the hydra's mouth, virtually every one of its cells directly contacts the environment and exchanges materials with it.

CHAPTER 40 Basic Principles of Animal Form and Function

821

External environment Food

ratory

A microscopic view of the lung reveals that it is much more spongelike than balioonlike. This construction provides an expansive wet surface for gas exchange with the environment (SEM). 10 u.m

The lining of the small intestine, a digestive organ, is elaborated with fingeriike projections that expand the surface area for nutrient absorption (cross-section, SEM)

Unabsorbed matter (feces)

• Figure 40.4 Internal exchange surfaces of complex animals. This diagrammatic animal illustrates the logistics of chemicai exchange with the environment by a mammal. Most animals have surfaces that are specialized for exchanging certain chemicals

vith the surroundings. These exchange surfaces are usually internal, but are connected to the environment via openings on the body surface (the mouth, for example). The exchange surfaces are finely branched or folded, giving them a very large area. The digestive,

bathed in water. A flat body shape is another design that maximizes exposure to the surrounding medium. For instance, a parasitic tapeworm may be several meters long, but because it is very thin, most of its cells are bathed in the intestinal fluid of the worms vertebrate host, the source of nutrients. Two-layered sacs and flat shapes are designs that put a large surface area in contact with the environment, but these simple forms do not allow much complexity in internal organization. Most animals are more complex and made up of compact masses of cells; their outer surfaces are relatively small compared with their volumes. As an extreme comparison, the surface-to-volume ratio of a whale is hundreds of thousands of times smaller than that of a water flea (Daphnia), yet every cell in the whale must be bathed in fluid and have access to oxygen, nutrients, and other resources. Extensively folded or branched internal surfaces facilitate this exchange with the environment in whales and most other animals (Figure 40.4). Despite the greater challenges of exchange with the environment, complex body forms have distinct benefits. For example, 822

UNIT SEVEN

Metabolic waste products (urine)

Animal Form and Function

Inside a kidney is a mass of microscopic tubules that exhange chemicals with blood flowing through a web of tiny vessels called capillaries (SEM).

respiratory, and excretory systems all have sue exchange surfaces. Chemicals transported across these surfaces are carried throughout t body by the circulatory system.

a specialized outer covering can protect against predators; large muscles can enable rapid movement; and internal digestive organs can break down food gradually, controlling the release of stored energy. And because the cells' immediate environment is the internal body fluid, the animal's organ systems can control the composition of this solution, allowing the animal to maintain a relatively stable internal environment while living in a variable external environment. A complex body form is especially well suited for animals living on land, where the external environment may be highly variable.

Concept Check 1. How does a large surface area contribute to the functions oi the small intestine, the lungs, and the kidneys? For suggested answers, see Appendix A.

(Concept!

Animal form and function are correlated at all levels of organization As living things, animals exhibit hierarchical levels of organization, each with emergent properties (see Chapter 1). Most animals are composed of specialized cells organized into tissues ths t have different functions. Tissues are combined into functional units called organs, and groups of organs that work together form organ systems. For example, the digestive system consists of a stomach, small intestine, large intestine, and other organs, each composed of different kinds of tissues.

Tissue Structure and Function Tissues are groups of cells with a common structure and function. Different types of tissues have different structures that are su ted to their functions. For example, a tissue may be held together by a sticky extracellular matrix that coats the cells (see Figure 6.29) or weaves them together in a fabric of fibers. Indeed, the term tissue derives from a Latin word meaning "weave." Tissues are classified into four main categories—-epithelial tisisue, connective tissue, muscle tissue, and nervous tissue— explored in Figure 40.5, on the next three pages. Epithelial Tissue Occurring in sheets of tightly packed cells, epithelial tissue covers the outside of the body and lines organs and cavities within the body. The cells of an epithelial tissue, or epithelium (plural, epithelia), are closely joined, with little material between them. In many epithelia, the cells are riveted together by tidht junctions (see Figure 6.31). This tight packing enables the epithelium to function as a barrier against mechanical injury, microbes, and fluid loss. Some epithelia, called glandular epithelia, absorb or secrete chemical solutions. For example, the glandular epithelia that line the lumen (cavity) of the digestive and respiratory tracts form a mucous membrane; they secrete mucus that lubricates the surface and keeps it moist. Two criteria for classifying epithelia are the number of cell layers and the shape of the cells on the exposed surface (see Figure 40.5). A simple epithelium has a single layer of cells, whereas a stratified epithelium has multiple tiers of cells. A "pseudostratified" epithelium is single-layered but appears stratified because the cells vary in length. The shape of the cells at the exposed surface may be cuboidal (like dice), columnar (like bricks standing on end), or squamous (like floor tiles). Connective Tissue Connective tissue functions mainly to bind and support other tissues. In contrast to epithelia, with their tightly packed cells,

connective tissues have a sparse population of cells scattered through an extracellular matrix. The matrix generally consists of a web of fibers embedded in a uniform foundation that may be liquid, jellylike, or solid. In most cases, the substances that make up the matrix are secreted by the cells of the connective tissue. Connective tissue libers, which are made of protein, are of three kinds: collagenous fibers, elastic fibers, and reticular fibers. Collagenous fibers are made of collagen, probably the most abundant protein in the animal kingdom. Collagenous fibers are nonelastic and do not tear easily when pulled lengthwise. If you pinch and pull some skin on the back of your hand, it is mainly collagen that keeps the flesh irom tearing away from the bone. Elastic fibers are long threads made of a protein called elastin. Elastic fibers provide a rubbery quality that complements the nonelastic strength of collagenous fibers. When you release the skin on the back of your hand, elastic fibers quickly restore the skin to its original shape. Reticular fibers are very thin and branched. Composed of collagen and continuous with collagenous fibers, they form a tightly woven fabric that joins connective tissue to adjacent tissues. The major types of connective tissue in vertebrates are loose connective tissue, adipose tissue, fibrous connective tissue, cartilage, bone, and blood (see Figure 40.5). Among the cells scattered in loose connective tissue, two types predominate: fibroblasts and macrophages. Fibroblasts secrete the protein ingredients of the extracellular fibers. Macrophages are amoeboid cells that roam the maze of fibers, engulfing foreign particles and the debris of dead cells by phagocytosis (see Chapter 6). You will learn more about the specific functions of those cells and various connective tissues later in this unit. Muscle Tissue Muscle tissue is composed of long cells called muscle fibers that are capable of contracting, usually when stimulated by nerve signals. Arranged in parallel within the cytoplasm of muscle fibers are large numbers of contracting units called myofibrils made of the proteins actin and myosin. (You will learn more about muscle contraction in Chapter 49.) Muscle is the most abundant tissue in most animals, and muscle contraction accounts for much of the energy-consuming cellular work in an active animal. In the vertebrate body, there are three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle (see Figure 40.5). Nervous Tissue Nervous tissue senses stimuli and transmits signals in the form of nerve impulses from one part of the animal to another. The functional unit of nervous tissue is the neuron, or nerve cell, which is uniquely specialized to transmit nerve impulses, as we discuss in detail in Chapter 48. In many animals, nervous tissue is concentrated in the brain, which functions as the control center that coordinates many of the animal's activities. CHAPTER 40 Basic Principles of Animal Form and Function

823

Structure and Function in Animal Tissues EPITHELIAL TISSUE Columnar epithelia, which have cells with relatively large cytoplasmic volumes, are often located where secretion or active absorption of substances is an important function.

A simple columnar epithelium lines the intestines. This epithelium secretes digestive jukes and absorbs nutrients.

A stratified columnar epithelium lines the inner surface of the urethra, the tube through which urine exits the body.

Cuboidal epithelia, with cells specialized for secretion, make up the epithelia of kidney tubules and many glands, including the thyroid gland and salivary glands. Glandular epithelia lining tubules _..,. "•"'"&'>'""^.f ••' :'f mammalian bone consists of '£• ' •» ••" » Central repeating units called osteons Ji , • . . canal (or Haversian systems). Each *; ?& osteon has concentric layers of ^9^HB the mineralized matrix, which ^S3JJH Osteon are deposited around a central -- . # .

canal containing blood vessels and nerves that service the bone.

Plj^^Hl pSHHPS ' 700 Lim

Although blood functions differently from other connective tissues, it does meet the criterion of having an extensive extracellular matrix. In this case, the matrix is a liquid called plasma, consisting of water, salts, and a variety of dissolved proteins. Suspended in the plasma are two classes of blood cells, erythrocytes (red blood cells) and leukocytes (white blood cells), and cell fragments called platelets. Red cells carry oxygen; white cells function in defense against viruses, blood cells bacteria, and other invaders; and platelets aid in ite blood cell blood clotting. The liquid matrix enables rapid transport of blood cells, nutrients, and wastes through the body.

55 iim

continued on the next page CHAPTER 40 Basic Principles of Animal Form and Function

825

Figure 40.5 (continued)

Structure and Function in Animal Tissues MUSCLE TISSUE 100 jam , Attached to bones by tendons, skeletal muscle is responsible for voluntary movements of the body. Skeletal muscle consists of bundles of long cells called fibers; each fiber is a bundle of strands called myofibrils. The arrangement of contractile units, or sarcomeres, along the length of the fibers gives the cells a striped (striated) appearance under the microscope. For this reason, skeletal muscle is also called striated muscle. Adult mammals have a fixed number of muscle cells; building muscle does not increase the number of cells but rather enlarges those already present.

| | | | 3 j g j t j i ^ Multiple /^ nuclei ^Muscle fiber Sarcomere

Cardiac muscle forms the contractile wall of the heart. It is striated like skeletal muscle and has contractile properties similar to those of skeletal muscle. Unlike skeletal muscle, however, cardiac muscle carries out an unconscious task: contractior1 of the heart. Cardiac muscle fibers branch and interconnect via intercalated disks, which relay signals from cell to cell and help synchronize the heartbeat.

Smooth muscle, so named because it lacks striations, is found in the walls of the digestive tract, urinary bladder, arteries, and other internal organs. The cells are spindle-shaped. They contract more slowly than skeletal muscles but can remain contracted longer. Controlled by different kinds of nerves than those controlling skeletal muscles, smooth muscles are responsible for involuntary body activities, such as churning of the stomach or constriction of arteries. 25|iin NERVOUS TISSUE Nerve cells (neurons) are the basic units of the nervous system. A neuron consists of a cell body and two or more extensions, or processes, called dendrites and axons, which in certain neurons may be as long as a meter in some animals. Dendrites transmit impulses from their tips toward the rest of the neuron. Axons transmit impulses toward another neuron or toward an effector, a structure such as a muscle cell that carries out a body response. The long axons of some motor neurons enable quick responses by voluntary muscles. 826

U N I T SEVEN

Animal Form and function

Table 40,1 Organ Systems: Their Main Components and Functions in Mammals Organ System

Main Components

Main Functions

Digestive

Mouth, pharynx, esophagus, stomach, intestines, liver, pancreas, anus

rood processing (ingestion, digesiion, absorption, elimination)

I Circulatory

Heart, blood vessels, blood

Internal, distribution ol" materials

[Respiratory

Lungs, trachea, other breathing tubes

Gas exchange (uptake of oxygen; disposal of carbon dioxide)

i Immune and lymphatic

Bone marrow, lymph nodes, Lhymus, spleen, lymph vessels, white blood cells Kidneys, ureters, urinary bladder, urethra

Body defense (fighting infections and cancer)

Endocrine

Pituitary, thyroid, pancreas, olher hormone-secreting glands

Coordination of body activities (such as digestion, metabolism)

Excretory

Disposal of metabolic wastes; regulation of osmotic balance of blood

Reproductive

Ovaries, tesies, and associated organs

Reproduction

Nervous

Brain, spinal cord, nerves, sensory organs

Coordination ol body activities; detection ol stimuli and lormulation of responses to them

Integumentary

Skin and its derivatives (such as hair, claws, skin glands)

Protection against mechanical injury, infection, drying out; thermoregulation

Skeletal

Skeleton (bones, tendons, ligaments, cartilage)

Body support, protection of internal organs, movement

Muscular

Skeletal muscles

Movement, locomotion

C rgans and Organ Systems Im all but the simplest animals (sponges and some cnidariams), different tissues are organized into organs. In some organs, the tissues are arranged in layers. For example, the vertebrate stomach has four major tissue layers (Figure 40.6).

Mucosa. The mucosa is an epithelial layer that lines the lumen.

Submucosa. The submucosa is a matrix of connective tissue that contains blood vessels and nerves.

Muscularis. The muscularis consists mainly of smooth J muscle tissue. } Serosa. External to the muscularis is the serosa, a thin layer of connective and epithelial tissue.

k Figure 40.6 Tissue layers of the stomach, a digestive j r g a n . The wall of the stomach and other tubular organs of the digestive system has four main tissue layers (SEM).

A thick epithelium lines the lumen and secretes mucus and digestive juices into it. Outside this layer is a zone of connective tissue, surrounded by a thick layer of smooth muscle. Yet another layer of connective tissue encases the entire stomach. Many of the organs of vertebrates are suspended by sheets of connective tissue called mesenteries in moist or fluid-filled body cavities. Mammals have a thoracic cavity housing the lungs and heart that is separated Irom the lower abdominal cavity by a sheet of muscle called the diaphragm. Representing a level of organization higher than organs, organ systems carry out the major body functions of most animals (Table 40.1). Each organ system consists of several organs and has specific functions, but the efforts of all systems must be coordinated for the animal to survive, For instance, nutrients absorbed from the digestive tract are distributed throughout the body by the circulatory system. But the heart, which pumps blood through the circulatory system, depends on nutrients absorbed by the digestive tract as well as on oxygen (O2) obtained by the respiratory' system. Any organism, whether single-celled or an assembly of organ systems, is a coordinated living whole greater than the sum of its parts.

Concept Check 1. Describe how the epithelial tissue that lines the stomach lumen is well suited to its function. 2. Suggest why a disease that damages connective tissue is likely to threaten most ol the body's organs. 3. How are muscle and nervous tissue interdependent? For suggested answers, see Appendix A.

CHAPTER 4o Basic Principles of Animal Form and Function

827

Concept

Animals use the chemical energy in food to sustain form and function All organisms require chemical energy for growth, repair, physiological processes (including movement in the case of animals), regulation, and reproduction. As we have discussed in other chapters, organisms can be classified by how they obtain this energy Autotrophs such as plants use light energy to build energy-rich organic molecules and then use those organic molecules for fuel. In contrast, heterotrophs such as animals must obtain their chemical energy from food, which contains organic molecules synthesized by other organisms.

Bioenergetics The How of energy through an animal—-its bioenergetics— ultimately limits the animals behavior, growth, and reproduction and determines how much food it needs. Studying an animal's bioenergetics tells us a great deal about the animal's adaptations. Energy Sources and Allocation Animals harvest chemical energy from the food they eat. Food is digested by enzymatic hydrolysis (see Figure 5.2b), and energycontaining molecules are absorbed by body cells. Once absorbed, these energy-containing molecules have several possible fates. Most are used to generate ATP by the catabolic processes of cellular respiration and fermentation (see Chapter 9). The chemical energy of ATP powers cellular work, enabling cells, organs, and organ systems to perform the many functions that keep an animal alive. Because the production and use of ATP generate heat, an animal continuously gives off heat to its surroundings (heat balance is discussed in more detail later in this chapter). After the energetic needs of staying alive are met, any remaining molecules from food can be used in biosynthesis, including body growth and repair, synthesis of storage material such as fat, and production of gametes (Figure 40.7). Biosynthesis requires both carbon skeletons for new structures and ATP to power their assembly. In some situations, biosynthetic products (such as body fat) can be broken down into fuel molecules for production of additional ATP, depending on the needs of the animal (see Figure 9.19). Quantifying Energy Use Understanding the bioenergetics of animals depends on the ability to quantify their energy needs. How much of the total energy an animal obtains from food does it need just to stay alive? How much energy must be expended to walk, run, swim, or fly 828

UNIT SEVEN

Animal Form and Function

A Figure 40.7 Bioenergetics of an animal: an overview. from one place to another? What fraction of energy intake can be used for reproduction? Physiologists obtain answers to soon questions by measuring the rates at which animals use chemical energy and how these rates change in different circumstances!. The amount of energy an animal uses in a unit of time is called its metabolic rate—the sum of all the energy-requiring biochemical reactions occurring over a given time interval!. Energy is measured in calories (cal) or kilocalories (kcal). (A kilocalorie is 1,000 calories. The unit Calorie, with a capital as used by many nutritionists, is actually a kilocalorie.) Metabolic rate can be determined in several ways. Becaustt nearly all of the chemical energy used in cellular respiration eventually appears as heat, metabolic rate can be measured by monitoring an animal's rate of heat loss. Researchers can use a calorimeter, which is a closed, insulated chamber equipped with a device that records an animal's heat loss, to measure metabolic rate for small animals. A more indirect way to mea sure metabolic rate is to determine the amount of oxygen con sumed or carbon dioxide produced by an animals cellular respiration (Figure 40.8). Over long periods, the rate of food consumption and the energy content of the food (about 4.5-5 kcal per gram of protein or carbohydrate and about 9 kcal pen gram of fat) can be used to estimate metabolic rate. However] this method must account for the energy stored in food thai the animal cannot use (the energy lost in feces and urine).

course, uses far more calories than the whole mouse). The higher metabolic rate of a smaller animal's tissues demands a proportionately greater rate of oxygen delivery. Correlated with its higher metabolic rate per gram, the smaller animal also has a higher breathing rate, blood volume (relative to its size), and heart rate (pulse), and it must eat much more food per unit, of body mass. The explanation for the inverse relationship between metabolic rate and size is still unclear. One hypothesis is that for (b) Similarly, the metabolic rate of a man fitted with a breathing apparatus is endotherms, the smaller the animal, the being monitored while he works out greater the energy cost of maintaining a on a stationary bike. stable body temperature. In effect, the smaller an animal, the greater its surfaceFigure 40.8 Measuring metabolic rate. to-volume ratio and thus the greater the loss of heat to (or gain of heat from) the surroundings. Logical as this hypothesis seems, it cannot be Bioenergetic Strategies the complete explanation, as it fails to explain the inverse relationship between metabolism and size in ectoiherms, which An animal's metabolic rate is closely related to its bioenergetic do not use metabolic heat to maintain body temperature. Al"strategy" There are two basic bioenergetic strategies found in though this relationship has been extensively documented in animals. Birds and mammals are mainly endothermic, meanboth endotherms and ectotherms, researchers continue to ing that their bodies are warmed mostly by heat generated by search for its underlying causes. metabolism, and their body temperature is maintained within (al This photograph shows a ghost crab in a respirometer. Temperature is held constant in the chamber, with air of known O 2 concentration flowing through. The crab's metabolic rate is calculated from the difference between the amount of O 2 entering and the amount of O 2 leaving the respirometer. This crab is on a treadmill, running at a constant speed as measurements are made.

a relatively narrow range. Endotherrny is a high-energy strategy that permits intense, long-duration activity over a wide range of environmental temperatures. In contrast, most fishes, amphibians, reptiles other than birds, and invertebrates are ectothermic, meaning that they gain their heat mostly from external sources. The ectothermic strategy requires much less energy than is needed by endotherms because ot the high energy cost of heating (or cooling) an endothermic body In general, endotherms have higher metabolic rates than ectotherms. You will read more about endothermic and ectothermic strategies later in this chapter.

Influences on Metabolic Rate 'he metabolic rates of animals are affected by many factors iesides whether the animal is an endotherm or an ectotherm. 3ne of animal biology's most intriguing (but largely unanswered) questions has to do with the relationship between 3ody size and metabolic rate. izc and Metabolic Rate 3y measuring the metabolic rates of many species of vertebrates and invertebrates, physiologists have shown that the amount of energy it takes to maintain each gram of body weight is inversely related to body size. Each gram of a mouse, for instance, requires about 20 Limes more calories than a gram of an elephant (even though the whole elephant, of

Activity and Metabolic Rate Every animal experiences a range of metabolic rates. A minimum rate powers the basic functions that support life, such as cell maintenance, breathing, and heartbeat. The metabolic rate of a nongrowing endotherm that is at rest, has an empty stomach, and is not experiencing stress is called the basal metabolic rate (BMR). The BMR for humans averages about 1,600-1,800 kcal per day for adult males and about 1,300-1,500 kcal per day for adult females. These BMRs are about equivalent to the energy requirement of a 75-watt light bulb. In an ectotherm, body temperature changes with the temperature of the animal's surroundings, and so does metabolic rate. "Unlike BMRs, which can be determined within a range of environmental temperatures, the minimum metabolic rate of an ectotherm must be determined at a specific temperature. The metabolic rate of a resting, fasting, nonstressed ectotherm at a particular temperature is called its standard metabolic rate (SMR). For both ectotherms and endotherms, activity has a large effect on metabolic rate. Any behavior, even a person reading quietly at a desk or an insect extending its wings, consumes energy beyond the BMR or SMR. Maximum metabolic rates (the highest rates of ATP utilization) occur during peak activity, such as lifting heavy weights, sprinting, or high-speed swimming. CHAPTER 40 Basic Principles of Animal Form and Function

829

In general, an animals maximum possible metabolic rate is inversely related to the duration of activity. Figure 40.9 contrasts ectothermic and endothermic "strategies" for sustaining activity over different time intervals. Botk an alligator (ectotherm) and a human (endotherm) are capable of very intense exercise in short spurts of a minute or less. During these "sprints/' the ATP present in muscle cells and ATP generated anaerobically by glycolysis are sufficient to power the activity. Neither the ectotherm nor the endotherm can sustain their maximum metabolic rates and peak levels of activity over longer periods of exercise, though the endotherm has an advantage in such endurance tests. Sustained activity depends on the aerobic process of cellular respiration for ATP supply, and an endotherms respiration rate is about ten times greater than an ectotherm's. Only a few ectotherms, such as migratory monarch butterflies or salmon, are capable of long-duration activities. Between the extremes of BMR or SMR and maximum metabolic rate, many factors influence energy requirements, including age, sex, size, body and environmental temperatures, quality and quantity of food, activity level, oxygen availability, and hormonal balance. Time of day also plays a role. Birds, humans, and

A = 60-kg alligator

H = 60-kg human A

H

JKjf

n "ond

1 minute

1 hour

1 day

Time interval

Existing intracellular ATP 3

ATP from glycoiysfs ATP from aerobic respiration

A Figure 40.9 Maximum metabolic rates over different t i m e spans. The bars compare an ectotherm (alligator) and an endotherm (human) for their maximum potential metabolic rates and ATP sources over different durations of time. The human's basal metabolic rate (about 1.2 kcal/min) is much greater than the alligator's standard metabolic rate (about 0.04 kcal/min). The human's higher BMR partly contributes to his ability to sustain a higher maximum metabolic rate over a longer period.

830

UNIT SEVEN

Animal Form and Function

many insects are usually active (and therefore have their higher t metabolic rates) during daylight hours. In contrast, bats, mice, and many other mammals generally are active and have their kigkest metabolic rates at nigkt or during tke kours of dawn and dusk. Metabolic rates measured when animals are performing a variety of activities give a better idea of the energy costs of every day life. For most terrestrial animals (both endotherms and ec totherms), the average daily rate of energy consumption is 2 to times BMR or SMR. Humans in most developed countries hav an unusually low average daily metabolic rate of about 1.5 time? BMR—an indication of relatively sedentary lifestyles.

Energy Budgets Different species of animals use the energy and materials in food in different ways, depending on their environment, behavior, size, and basic strategy of endothermy or ectothermy. For most animals, the majority of food is devoted to the proiduction of ATP, and relatively little goes to growth or reproL duction. However, the amount of energy used for BMR (or SMR), activity, and body temperature control varies consider^ ably between species. For example, consider the typical enp ergy "budgets" of four terrestrial vertebrates: a 25-g femal Figure 41.12 The four stages of food processing.

CHAPTER 41

Animal Nutrition

853

gastrovascular cavity, functions in both digestion and distribution of nutrients throughout the body (hence the vascular part of the term). The cnidarians called hydras provide a good example of now a gastrovascular cavity works. Hydras are carnivores that sting prey with specialized organelles called nematocysts and then use tentacles to stuff the food through their mouth into their gastrovascular cavity (Figure 41.13). Specialized gland cells of the gastrodermis, the tissue layer that lines the cavity, then secrete digestive enzymes that break the soft tissues of the prey into tiny pieces. Other gastrodermal cells, called nutritive muscular cells, engulf these food particles, and most of the actual hydrolysis of macromolecules occurs intracellularly, as in sponges. After a hydra has digested its meal, undigested materials remaining in the gastrovascular cavity, such as the exoskeletons of small crustaceans, are eliminated through the single opening, which functions in the

Crop Esophagus

|

Gizzard /

Pharynx

Lumen of intestine (a) Earthworm. The digestive tract of an earthworm includes a muscular pharynx that sucks food in through the mouth. Food passes through the esophagus and is stored and moistened in ; the crop. The muscular gizzard, which contains small bits of sand and gravel, pulverizes the food. Digestion and absorption occur in the intestine, which has a dorsal fold, the typhiosole, that increases the surface area for nutrient absorption.

Mouth

Gastric ceca

(b) Grasshopper. A grasshopper has several digestive chambers grouped into three main regions: a foregut, with an esophagus and crop; a midgut; and a hindgut. Food is moistened and storec in the crop, but most digestion occurs in the midgut. Gastric ceca, pouches extending from the midgut, absorb nutrients.

Esophagi Nutritive musculai

Mesenchyme-

4 Figure 41.13 D i g e s t i o n in a h y d r a . The outer epidermis of the hydra has protective and sensory functions, whereas the inner gastrodermis is specialized for digestion. Digestion begins in the gastrovascular cavity and is completed intraceilularly after small food particles are engulfed by the gastrodermal cells.

UNIT SEVEN

Animal Form and Function

(c) Bird. Many birds have three separate chambers—the crop, stomach, and gizzard—where food is pulverized and churned before passing into the intestine. A bird's crop and gizzard function very much like those of an earthworm. In most birds, chemical digestion and absorption of nutrients occur in the intestine.

A Figure 41.14 Variation in alimentary canafs.

dua role of mouth and anus. Many flatworms also have a gastroVjascular cavity with a single opening (see Figure 33.10). In contrast to cnidarians and flatworms, most animals— including nematodes, annelids, molluscs, arthropods, echinoderms, and chordates—have a digestive tube extending between two openings, a mouth and an anus. Such a tube is called a complete digestive tract or an alimentary canal. Because food moves along the canal in a single direction, the tube can be organized into specialized regions that carry out digestion and nutrient absorption in a stepwise fashion (Fit ure 41.14). Another advantage of a complete digestive tract is the ability to ingest additional food before earlier meals are completely digested—which may be difficult or inefficient for;animals with gastrovascular cavities. CJoncept Check' 1. What is the main anatomical distinction between a gastrovascular cavity and an alimentary canal? 1. Why are nutrients from a recently ingested meal not really "inside" your body prior to the absorption stage of food processing? For suggested answers, see Appendix A.

Concept

Each organ of the mammalian digestive system has specialized food-processing functions The general principles of food processing are similar lor a diversity of animals, so we can use the digestive system of mammals as a representative example. The mammalian digestive system consists of the alimentary canal and various accessory glands that secrete digestive juices into the canal through ducts (Figure 41.15). Peristalsis, rhythmic waves of contraction by smooth muscles m the wall of the canal, pushes the food along the tract. At some of the junctions between specialized segments of the digestive tube, the muscular layer is modified into ringlike valves called sphincters, which close off the tube like drawstrings, regulating the passage of material between chambers of the canal. The accessory glands of the mammalian digestive system are three pairs of salivary glands, the pancreas, the liver, and the gallbladder, which stores a digestive juice. Using the human digestive system as a model, let's now follow a meal through the alimentary canal, examining in more

glands

Rectum -Anus A schematic diagram of the human digestive system A Figure 41.15 The h u m a n d i g e s t i v e s y s t e m . After food is chewed and swallowed, it takes only 5-10 seconds for it to pass down the esophagus and into the stomach, where it spends 2-6 hours being partially digested. Final digestion and nutrient absorption occur in the small intestine over a period of 5-6 hours. In 12-24 hours, any undigested material passes through the large intestine, and feces are expelled through the anus.

Animal Nutrition

855

detail what happens to the food in each of the processing stations along the way (see Figure 41.15).

The Oral Cavity, Pharynx, and Esophagus Both physical and chemical digestion of food begin in the mouth. During chewing, teeth of various shapes cut, smash, and grind food, making it easier to swallow and increasing its surface area. The presence of food in the oral cavity triggers a nervous reflex that causes the salivary glands to deliver saliva through ducts to the oral cavity. Even before food is actually in the mouth, salivation may occur in anticipation because of learned associations between eating and the time of day, cooking odors, or other stimuli. Humans secrete more than a liter of saliva each day Saliva contains a slippery glycoprotein (carbo hydrate protein complex) called rrmcin, which protects the lining of the mouth from abrasion and lubricates food for easier swallowing. Saliva also contains buffers that help prevent tooth decay by neutralizing acid in the mouth. Antibacterial agents in saliva kill many of the bacteria that enter the mouth with food. Chemical digestion of carbohydrates, a main source of chemical energy, begins in the oral cavity. Saliva contains salivary amylase, an enzyme that hydrolyzes starch (a glucose polymer from plants) and glycogen (a glucose polymer

from animals). The main products of this enzyme's action are smaller polysaccharides and the disaccharide maltose. The tongue tastes food, manipulates it during chewing, and helps shape the food into a ball called a bolus. During swallowing, the tongue pushes a bolus to the back of the oral cavity and into the pharynx. The region we call our throat is the pharynx, a junction that opens to both the esophagus and the windpipe (trachea). When we swallow, the top of the windpipe moves up so that its opening, the glottis, is blocked by a cartilaginous flap, the epiglottis. You can see this motion in the bobbing of the "Adam's apple" during swallowing. This tightly controlled mechanism normally ensures that a bolus is guided into the entrance of the esophagus (Figure 41.16, steps 1-4). Food or liquids may go "down the wrong pipe" because the swallowing reflex didn't close the opening of the windpipe in time. The resulting blockage of airflow (choking) stimulates vigorous coughing, which usually expels the material. If it is not expelled quickly, the lack of airflow to the lungs can be fatal. The esophagus conducts food from the pharynx down to the stomach by peristalsis (see Figure 41.16, step 6), The muscles at the very top of the esophagus are striated (voluntary). Thus, the act of swallowing begins voluntarily, but then the involuntary waves of contraction by smooth muscles in the rest of the esophagus take over.

© The esophageal sphincter relaxes, allowing the bolus to enter the esophagus.

O When a person is not swallowing, the esophageai sphincter muscle is contracted, the epiglottis is up, and the glottis is open, allowing air to flow through the trachea to the lungs.

Q The swallowing reflex is triggered when a bolus of food reaches the pharynx.

0 The larynx, the upper part of the respiratory tract, moves upward and tips the epiglottis over the glottis, © Waves of muscular preventing food contraction (peristalsis) from entering the move the bolus down the trachea. esophagus to the stomach. |

A Figure 41.16 From mouth to stomach: the swallowing reflex and esophageai peristalsis.

856

r SEVEN

Animal Form ar.d Funciion

0 After the food has entered the esophagus, the larynx moves downward and opens the breathing passage.

Stomach

THe Stomach Tht stomach stores food and performs preliminary steps of digestion. This large organ is located in the upper abdominal cavity, just below the diaphragm. With accordionlike folds anil a very elastic wall, the stomach can stretch to accommodate about 2 L of food and fluid. It is because the stomach can stare an entire meal that we do not need to eat constantly. Besides storing food, the stomach performs important digestive fumctions; It secretes a digestive fluid called gastric juice and mipces this secretion with the food by the churning action of the smooth muscles in the stomach wall. •Gastric juice is secreted by the epithelium lining numerous deep pits in the stomach wall. With a high concentration of hydrochloric acid, gastric juice has a pH of about 2—acidic enough to dissolve iron nails. One function of the acid is to disrupt the extracellular matrix that binds cells together in meat and plant material. The acid also kills most bacteria that are swallowed with food. Also present in gastric juice is pepsin, an enzyme that begins the hydrolysis of proteins. Pepsin breaks

peptide bonds adjacent to specific amino acids, cleaving proteins into smaller polypeptides, which are later digested completely to amino acids in the small intestine. Pepsin is one of the few enzymes that works best in a strongly acidic environment. The low pH of gastric juice denatures (unfolds) the proteins in food, increasing exposure of their peptide bonds to pepsin. What prevents pepsin from destroying the cells of the stomach wall? First, pepsin is secreted in an inactive form called pepsinogen by specialized cells called chief cells located in gastric pits (Figure 41.17). Other cells, called parietal cells, also in the pits, secrete hydrochloric acid. The acid converts pepsinogen to active pepsin by removing a small portion of the molecule and exposing its active site. Because different cells secrete the acid and pepsinogen, the two ingredients do not mix—and pepsinogen is not activated—until they enter the lumen (cavity) of the stomach. Activation of pepsinogen is an example of positive feedback: Once some pepsinogen is activated by acid, activation occurs at an increasingly rapid rate because pepsin itself can activate additional molecules of pepsinogen. Many other digestive enzymes are also secreted in inactive Esophagus

JS

Cardiac orifice—~^9^M

Pyloric sphincter'

Interior surface of stomach. The interior surface of the mach wall is highly folded and dotted with pits leading into tubular gastric glands. astric gland. The gastric glands have three types of cells that secrete different components of the gastric juice: mucus cells, chief cells, and parietal cells.

^--Epithelium

© Pepsinogen - > Pepsin © (active enzyme) HCI

0 HCI converts pepsinogen to pepsin.

Mucus cells secrete mucus, which lubricates and protects the cells lining the stomach.

© Pepsin then activates more pepsinogen, starting a chain reaction. Pepsin begins the chemical digestion of proteins.

Chief cells secrete pepsino-1 /[ gen, an inactive form of the digestive enzyme pepsin. Parietal cells secrete hydrochloric acid (HC1).

Q Pepsinogen and HCI are secreted into the lumen of the stomach.

Parietal cell

A Figure 41,17 The stomach and its secretions. The micrograph (colorized SEM) shows a gastric f it on the interior surface of the stomach, through which digestive juices are secreted. CHAPTER 41

Animal Nutrition

857

forms that become active within the lumen of the digestive tract The stomach's second defense against self-digestion is a coating of mucus, secreted by the epithelial cells of the stomach lining. Still, the epithelium is constantly eroded, and mitosis generates enough cells to completely replace the stomach lining every three days. Gastric ulcers, lesions in this lining, are caused mainly by the acid-tolerant bacterium Helicobacter pylori (Figure 41.18). Though treatable with antibiotics, gastric ulcers may worsen if pepsin and acid destroy the lining faster than it can regenerate. About every 20 seconds, the stomach contents are mixed by the churning action of smooth muscles. You may feel hunger pangs when your empty stomach churns. (Sensations of hunger are also associated with brain centers that monitor the blood's nutritional status and levels of the appetitecontrolling hormones discussed earlier in this chapter.) As a result of mixing and enzyme action, what begins in the stomach as a recently swallowed meal becomes a nutrient-rich broth known as acid chyme. Most of the time, the stomach is closed off at both ends (see Figure 41.15). The opening from the esophagus to the stomach, the cardiac orifice, normally dilates only when a bolus arrives. The occasional backflow of acid chyme from the stomach into the lower end of the esophagus causes heartburn. (If backflow is a persistent problem, an ulcer may develop in the esophagus.) At the opening from the stomach to the small intestine is the pyloric sphincter, which helps regulate the passage of chyme into the intestine, one squirt at a time. It takes about 2 to 6 hours after a meal for the stomach to empty in this way.

The Small Intestine With a length of more than 6 rn in humans, the sm, 11

intestine is the longest section of the alimentary canal (its name refers to its small diameter, compared with that of the large intestine). Most of the enzymatic hydrolysis of food macromolecules and most of the absorption of nutrients intc the blood occur in the small intestine. Enzymatic Action in the Small Intestine The first 25 cm or so of the small intestine is called the duodenum. It is here that acid chyme from the sloma mixes with digestive juices from the pancreas, liver, gallbladder, and gland cells of the intestinal wall itself (Figure 41.19). The pancreas produces several hydrolytic enzymes and an alkaline solution rich in bicarbonate. The bicarbonate acts as a buffer, offsetting the acidity of chyme from the stomach. Tfie pancreatic enzymes include protein-digesting enzymes (proteases) that are secreted into the duodenum in inactive form. In a chain reaction similar to the activation of pepsin in the stomach, the pancreatic proteases are activated once they a safely located in the extracellular space within the duodenu (Figure 41.20)

The liver performs a wide variety of functions in the body, including the production of bile, a mixture of substances that is stored in the gallbladder until needed. Bile contains no digestive enzymes, but it does contain bile salts, which act as detergents (emulsifiers) that aid in the digestion and absorptidn of fats (see Figure 41.24). Bile also contains pigments that are by-products of red blood cell destruction in the liver; these bile pigments are eliminated from the body with the feces. The epithelial lining of the duodenum, called the brus. border, is the source of several digestive enzymes. Some i)f these enzymes are secreted into the lumen of the duodenun ,

Gallbladder

A Figure 41.18 Ulcer-causing bacteria. The bacteria visible in this colorized SEM, Helicobacter pylori, initiate ulcers by destroying protective mucus and causing inflammation of the stomach lining. Then the acidic gastric juice can attack the stomach tissue. In severe ulcers, the erosion can produce a hole in the stomach wall and cause life-threatening internal bleeding and infection.

858

UNIT SEVEN

Animal Form and Function

A Figure 41.19 The duodenum. Hydrolytic enzymes from accessory glands mix with acid chyme in the duodenum, continuing the digestion process. Note that bile is produced in the liver but storec in the gallbladder, which releases bile into the duodenum as needed

Membrane-bound enteropeptidase

but olher digestive enzymes are actually bound to the surface of epithelial cells. Enzymatic digestion is completed as peristalsis moves the mixture of chyme and digestive juices along the small intestine (Figure 41.21). Most digestion is completed early in this journey, while the chyme is still in the duodenum. The remaining regions of the small intestine, called the jejunum and ileum, function mainly in the absorption of nutrients and water. Figure 41.22, on the next page, diagrams how hormones help coordinate the secretion of digestive juices into the alimentary canal. Absorption of Nutrients

Lumen of duodenum A Figure 41.20 Protease activation. The pancreas secretes inactive proteases into the duodenum. An enzyme called enteropeptidase, which is bound to the intestinal epithelium, converts trypsinogen to trypsin. Trypsin then activates other proteases. (© indicates activation.)

Polysaccharides (starch, glycogen)

Nucleic acid digestion

Protein digestion

Carbohydrate digestion Oral cavity, priarynx, esophagus

To enter the body nutrients in the lumen must cross the lining of the digestive tract. A few nutrients are absorbed in the stomach and large intestine, but most absorption occurs in the small intestine. This organ has a huge surface area—300 m , roughly the size of a tennis court. Large circular folds in the lining bear fmgerlike projections called villi, and each Fat digestion

Disaccharides (sucrose, lactose)

Salivary amylase Smaller polysaccharides, maltose

Small polypeptides Lumen of small intestine

Polysaccharides

DNA, RNA

Polype ptides

Pancreatic amylases

Pancreatic trypsin and chymotrypsin (These proteases cleave bonds adjacent to certain ainino acids.)

Maltose and other disaccharides

Pancreatic nucleases

Nucleotides

Smaller polypeptides |

Pancreatic carboxypeptidase

|

Giycerol, fatty acids, giycerides

Small peptides

Disaccharidases

Fat droplets (A coating of bile salts prevents small droplets from coalescing into larger globules, increasing exposure to lipase.) Pancreatic lipase

Amino acids

Epithelium cf small intestine (brush border)

Fat globules (insoluble i water, fats aggregate as globules.)

Nucleotidases

Dipeptidases, carboxypeptidase, and aminopeptidase (These proteases split off one amino acid at a time, working from opposite ends of a polypeptide.)

J

Monosaccharides

Nitrogenous bases, sugars, phosphates

t Figure 41.21 Flowchart of enzymatic digestion in the human digestive system. CHAPTER 4i

Animal Nutrition

359

Enterogastrone secretec the duodenum inhibits peristalsis and acid secre by the stomach, thereby slowing digestion when chyme rich in fats enters he duodenum.

Gastrin from the stomac recirculates via the bloodstream back to the stomach, where it stimu the production of gastri juices. Amino acids or fatty acids in the duodenum trigger the release of cholecystokinin (CCK), which stimulates the release of digestive enzymes from the pancreas and bile from the gallbladder. "*vOy*

Key Q £

Secreted by the duodenu secretin stimulates the pancreas to release sodiu bicarbonate, which neutralizes acid chyme fr the stomach.

Stimulation Inhibition

A Figure 41.22 Hormonal control of digestion. Many animals go for long intervals between meals and do not need their digestive systems to run continuously. Hormones released by the stomach and duodenum help ensure that digestive secretions are present only when needed. epithelial cell of a villus has many microscopic appendages called microvilli thai are exposed to the intestinal lumen (Figure 41.23). (The microvilli's shape is the basis of the term brush border for the intestinal epithelium.) This enormous microvillar surface is an adaptation that greatly increases the rate of nutrient absorption. Vein carrying blood to hepatic portal vessel

fr

Nutrient absorption

Intestinal wall

A Figure 41.23 The structure of the small intestine. 860

UNIT SEVEN

Animal Form and Function

Penetrating the core of each villus is a net of microscopic blood vessels (capillaries) and a small vessel of the lympha IC system called a lacteal. (In addition to their circulatory system, vertebrates have an associated network of vessels—-the lymphatic system—thai carries a clear fluid called lymph, discussed in Chapter 43.) Nutrients are absorbed across the

inlt stinal epithelium and then across the unicellular epithelium of the capillaries or lacteals. Only these two single layers of epithelial cells separate nutrients in the lumen of the intestine from the bloodstream. In some cases, transport of nutrients across the epithelial cells is passive. The simple sugar fructose, for example, apparently moves by diffusion down its concentration gradieni from the lumen of the intestine into the epithelial cells and

0 Large fat globules are emulsified by bile salts in the duodenum.

Q Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides, which then form micelles.

_ Fatty acids and monoglycerides leave micelles and enter epithelial cells by diffusion.

then into capillaries. Other nutrients, including amino acids, small peptides, vitamins, and glucose and several other simple sugars, are pumped against concentration gradients by the epithelial membranes. This active transport allows the intestine to absorb a much higher proportion of the nutrients in the intestine than would be possible with passive diffusion. Amino acids and sugars pass through the epithelium, enter capillaries, and are carried away from the intestine by the bloodstream. After glycerol and fatty acids are absorbed by epithelial cells, they are recombined into fats within those cells. The fats are then mixed with cholesterol and coated with proteins, forming small globules called chylomicrons, most of which are transported by exocytosis out of the epithelial cells and into lacteals (Figure 41.24). The lacteals converge into the larger vessels of the lymphatic system. Lymph, containing chylomicrons, eventually drains from the lymphatic system into large veins that return blood to the heart. In contrast to the lacteals, the capillaries and veins that drain nutrients away from the villi all converge into the hepatic portal vein, a blood vessel that leads directly to the liver. This ensures that the liver—which has the metabolic versatility to interconvert various organic molecules—has first access to amino acids and sugars absorbed after a meal is digested. Therefore, blood that leaves the liver may have a very different balance of these nutrients than the blood that entered via the hepatic portal vein. For example, the liver helps regulate the level of glucose molecules in the blood, and blood exiting the liver usually has a glucose concentration very close to 0.1%, regardless of the carbohydrate content of a meal (see Figure 41.3). "From the liver, blood travels to the heart, which pumps the blood and the nutrients it contains to all parts of the body.

The Large Intestine The large intestine, or colon (Figure 41.25), is connected to the small intestine at a T-shaped junction, where a sphincter

Epithelial cells of small intestine

Q Chylomicrons containing fatty substances are transported out of the epithelial cells and into lacteals, where they are carried away from the intestine by lymph.

£k Figure 41.24 Digestion and absorption of fats. Hydrolysis clif fats is a digestive challenge because fat molecules are insoluble in \yater. However, bile salts from the gallbladder secreted into the tiuodenum coat tiny fat droplets and keep them from coalescing, a arocess called emulsification. Because the droplets are small, a large surface area of fat is exposed to lipase. Once the fat molecules have ijjeen hydrolyzed, they form micelles, which enable the fatty substances 1O diffuse into the epithelial lining of the small intestine. From the pithelial cells, they can be absorbed into the circulatory system.

-* Figure 41.25 Digital image of a human colon. This image was produced by integrating twodimensional sectional views of the large intestine.

CHAPTER 41 Animal Nutrition

861

(a muscular valve) controls the movement of material. One arm of the T is a pouch called the cecum (see Figure 41.15). Compared to many other mammals, humans have a relatively small cecum. The human cecum has a hngerlike extension, the appendix, which is dispensable. (Lymphoid tissue in the appendix makes a minor contribution to body defense.) The main branch of the human colon is about 1.5 m long. A major function of the colon is to recover water that has entered the alimentary canal as the solvent of the various digestive juices. About 7 1_ of fluid are secreted into the lumen of the digestive tract each day, which is much more liquid than most people drink. Most of this water is reabsorbed when nutrients are absorbed in the small intestine. The colon reclaims much of the remaining water that was not absorbed in the small intestine. Together, the small intestine and colon reabsorb about 90% of the water that enters the alimentary canal. The wastes of the digestive tract, the feces, become more solid as they are moved along the colon by peristalsis. The movement is sluggish, and it generally takes about 12 to 24 hours for material to travel the length of the organ. If the lining of the colon is irritated—by a viral or bacterial infection, for instance—less water than normal may be reabsorbed, resulting in diarrhea. The opposite problem, constipation, occurs when peristalsis moves the feces along the colon too slowly. An excess of water is reabsorbed, and the feces become compacted. Living in the large intestine is a rich flora of mostly harmless bacteria. One of the common inhabitants of the human colon is Escherichia coli, a favorite research organism of molecular biologists (see Chapter 18). The presence of E. coli in lakes and streams is an indication of contamination by untreated sewage. Intestinal bacteria live on unabsorbed organic material. As by-products of their metabolism, many colon bacteria generate gases, including methane and hydrogen sulfide. Some of the bacteria produce vitamins, including biotin, folic acid, vitamin K, and several B vitamins. These vitamins, absorbed into the blood, supplement our dietary intake of vitamins. Feces contain masses of bacteria, as well as cellulose and other undigested materials. Although cellulose fibers have no caloric value to humans, their presence in the diet helps move food along the digestive tract. The terminal portion of the colon is called the rectum, where feces are stored until they can be eliminated. Between the rectum and the anus are two sphincters, one involuntary and the other voluntary. One or more times each day, strong contractions of the colon create an urge to defecate. We have now followed a meal from one opening (the mouth) of the alimentary canal to the other (the anus). In the last section of this chapter, we'll see how some of the digestive adaptations of animals may have evolved. 862

UNIT SEVEN

Animal Form and Function

Concept Check 1. In the weightless environment of space, how does food swallowed by an astronaut reach the stomach? 2. Describe two key digestive functions of the hydrochloric acid in gastric juice. 3. What materials are mixed within the duodenum during digestion of a meal? 4. How is the structure oi the brush border (epithelium) of the small intestine adapted to its function of nutrient absorption? 5. Explain why treatment of a chronic infection, with antibiotics lor an extended period of time may cause vitamin K deficiency. 6. After reviewing Figure 41.22, explain how the pancreas times its secretion of digestive juice to mix with a partially digested meal in the duodenum. For suggested answers, see Appendix A.

Evolutionary adaptations of vertebrate digestive systems are often associated with diet The digestive systems of mammals and other vertebrates are variations on a common plan, but there are many intriguing adaptations, often associated with the animal's diet. We will examine just a few.

Some Dental Adaptations Dentition, an animal's assortment of teeth, is one example of structural variation reflecting diet. Particularly in mammals, evolutionary adaptation of Leeth for processing different kinds of food is one of the major reasons this vertebrate class has been so successful. Compare the dentition oi carnivorous, herbivorous, and omnivorous mammals in Figure 41.26. Nonmammalian vertebrates generally have less specialized dentition, but there are interesting exceptions. For example, poisonous snakes, such as rattlesnakes, have fangs, modified teeth that inject venom into prey. Some fangs are hollow, like syringes, while others drip the poison along grooves on the surfaces of the teeth. All snakes have another important anatomical adaptation associated with feeding: They swallow their prey whole, with no chewing, and the lower jaw is loosely hinged to the skull by an elastic ligament that permits the mouth and throat to open very wide for swallowing impressively large prey (once again, witness the astonishing episode recorded in Figure 41.2).

Herbivore

Carnivore

A Figure 41.27 The digestive tracts of a carnivore (coyote) and herbivore (koala) compared. Although these two mammals

A Figure 41.26 Dentition and diet, (a) Carnivores, such as members of the dog and cat families, generally have pointed incisors and canines that can be used to kill prey and rip or cut away pieces of flesh. The jagged premoiars and molars crush and shred food, (b) In contrast, herbivorous mammals, such as horses and deer, usually have teeth with broad, ridged surfaces that grind tough plant material. The incisors and canines are generally modified for biting off pieces of vegetation. In some herbivorous mammals, canines are absent, (c) Humans, being omnivores adapted for eating both vegetation and meat, have a relatively unspecialized dentition. The permanent (adult) set of teeth is 32 in number. Beginning at the midline of the upper and lower jaw are two bladelike incisors for biting, a pointed canine for tearing, two premoiars for grinding, and three molars for crushing.

Stomach and Intestinal Adaptations Large, expandable stomachs are common in carnivores, which may go for a long time between meals and therefore must eat as much as they can when they do catch prey. For example, a 200-kg African lion can consume 40 kg of meat in one meal. The length of the vertebrate digestive system is also correlated with diet. In general, herbivores and omnivores have longer alimentary canals relative to their body size than carnivores (Figure 41.27). Vegetation is more difficult to digest than meat because it contains cell walls. A longer tract furnishes more time for digestion and more surface area for the absorption of nutrients.

are about the same size, the koala's intestines are much longer, an adaptation that enhances processing of fibrous, protein-poor eucalyptus (eaves from which it obtains virtually all its food and water. Extensive chewing chops the leaves into very small pieces, increasing exposure of the food to digestive juices. The koala's cecum—at 2 m, the longest of any animal of equivalent size—functions as a fermentation chamber where symbiotic bacteria convert the shredded leaves into a more nutritious diet.

Symbiotic Adaptations Herbivorous animals face a nutritional challenge: Much of the chemical energy in their diets is contained in the cellulose of plant cell walls, but animals do not produce enzymes that hydrolyze cellulose. Many vertebrates (as well as termites, whose wood diets are largely cellulose) solve this problem by housing large populations of symbiotic bacteria and protists in fermentation chambers in their alimentary canals. These microorganisms do have enzymes that can digest cellulose to simple sugars and other compounds that the animal can absorb. In many cases, the microorganisms can also use the sugars irom digested cellulose along with minerals to make a variety of nutrients essential to the animal, such as vitamins and amino acids. The location ol symbiotic microbes in the digestive tracts of herbivores varies, depending on the type of animal. The hoatzin, an herbivorous bird that lives in South American rain forests, has a large, muscular crop (an esophageal pouch) that houses symbiotic microorganisms. Hard ridges in the wall of the crop grind plant leaves into small fragments, and the microorganisms break down cellulose. Many herbivorous mammals, including horses, house CHAPTER 41 Animal Niunuon

863

O Rumen. When the cow first chews and swallows a mouthful of grass, boluses (green arrows) enter the rumen.

0 Reticulum. Some boluses also enter t reticulum. In both the rumen and the reticulum, symbiotic prokaryotes and protists (mainly ciliates) go to work on t cellulose-rich meal. As by-products of th metabolism, the microorganisms secrete fatty acids. The cow periodically regurgitates and rechews the cud (red arrows), which further breaks down the fibers, making them more accessible to further microbial action.

O Abomasum. The cud, containing great numbers of microorganisms, finally passes to the abomasum for digestion by the cow's own enzymes (black arrows).

© Omasum. The cow then reswallows the cud (blue arrows), which moves to the omasum, where water is removed.

A Figure 41.28 Ruminant digestion. The stomach of a ruminant has four chambers. Because of the microbial action in the chambers, the diet from which a ruminant actually absorbs its nutrients is much richer than the grass the animal originally ate. In fact, a ruminant eating grass or hay obtains many of its nutrients by digesting the symbiotic microorganisms, which reproduce rapidly enough in the rumen to maintain a stable population.

symbiotic microorganisms in a large cecum. the pouch where the small and large intestines connect. The symbiotic bacteria of rabbits and some rodents live in the large intestine as well as in the cecum. Since most nutrients are absorbed in the small intestine, nourishing by-products of fermentation by bacteria in the large intestine are initially lost with the feces. Rabbits and rodents recover these nutrients by eating some of their feces and passing the food through the alimentary canal a second time. (The familiar rabbit "pellets," which are not reingested, are the feces eliminated after food has passed through the digestive tract twice.) The koala, an Australian marsupial, also has an enlarged cecum, where symbiotic bacteria ferment finely shredded eucalyptus leaves (see Figure 41.27)- The most elaborate adaptations for an herbivorous diet have evolved in the animals called ruminants, which include deer, cattle, and sheep (Figure 41.28).

Provisioning the body also involves distributing nutrients cells throughout the body (circulation) and exchanging res ratory gases with the environment-

In the next chapter, we will see that obtaining food, digesting it, and absorbing nutrients are only parts of a larger story.

For suggested answers, see Appendix A.

864

U N I T SEVEN

Animal Form and Function

Concept Check' 1. Explain how human dentition is adapted for an omnivorous diet. 2. Compared with an adult frog, a tadpole (frog larva) has a much longer intestine relative to its body size. What does this suggest about the diets of these two stages in a frog's life history? 3. "Chewing its cud" is a common expression about cattle. What is the cud, and what role does it play in bovine nutrition?

Go '0 the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

The main stages of food processing are ingestion, digestion, absorption, and elimination

SUMMARY OF KEY CONCEPTS • kn animal's diet must supply chemical energy, organic raw materials, and essential nutrients. Herbivores mainly eat plants, carnivores mainly eat other animals, and omnivores regularly •eat both plant and animal matter. Many aquatic animals are suspension feeders, sifting small particles from the water. Substrate feeders tunnel through their food, eating as they go. Fluid feeders suck nutrient-rich fluids from a living host. Most animals are bulk feeders, eating relatively large pieces of food (pp. 844-845).

meostatic mechanisms manage an animal's xgy budget Glucose Regulation as an Example of Homeostasis (p. 846) Animals store excess calories as glycogen in the liver and muscles and as fat. These energy stores can be tapped when an animal is in need of ATP. Blood glucose level is maintained within a relatively narrow range by a negative feedback mechanism. Caloric Imbalance (pp. 846-848) Undernourished animals have diets deficient in calories. Overnourished (obese) animals consume more calories than they need. Obesity is a serious health problem worldwide and especially in the United States, where lack of exercise and fattening foods make an unhealthy combination. Obesity is also strongly influenced by genes. The problem of maintaining healthy weight partly stems from our evolutionary past, when fat hoarding was a means of survival.

Jui animal's diet most supply carbon skeletons nd essential nutrients Carbon skeletons are required in biosynthesis. Essential nutrients must be supplied in preassembled form. Malnourished animals are missing one or more essential nutrients in their diet, a condition more common than undernourishment in human populations (p. 849). • Essential Amino Acids (pp. 849-850) Animals require 20 amino acids and can synthesize about half of them from the other molecules they obtain from their diet. Essential amino acids are those an animal cannot make. An animal whose diet is lacking one or more essential amino acids will become malnourished and suffer protein deficiency. Essential Fatty Acids (p. 850) Essential fatty acids, which an animal cannot make, are unsaturated, meaning that they have double bonds. Deficiencies in essential fatty acids are rare. • Vitamins (pp. 850-851) Vitamins are organic molecules required in small amounts. They are either water-soluble or fat-soluble. • Minerals (pp. 851-852) Minerals are inorganic nutrients, usually required in small amounts.

• Food processing in animals involves ingestion (the act of eating), digestion (enzymatic breakdown of the macromolecules of food into their monomers), absorption (the uptake of nutrients by body cells), and elimination (the passage of undigested materials out of the body in feces) (p. 853). • Digestive Compartments (pp. 853-855) In intracellular digestion, food particles are engulfed by endocytosis and digested within food vacuoles. Most animals use extracellular digestion, with enzymatic hydrolysis occurring outside cells in a gastrovascular cavity or alimentary canal.

Each organ of the mammalian digestive system has specialized food-processing functions • The mammalian digestive system is composed of the alimentary canal and accessory glands that secrete digestive juices into the canal (pp. 855-856). • The Oral Cavity, Pharynx, and Esophagus (p. 856) Food is lubricated and digestion begins in the oral cavity, where teeth chew food into smaller particles that are exposed to salivary amylase, initiating the breakdown of glucose polymers. The pharynx is the intersection leading to the trachea and esophagus. The esophagus conducts food from the pharynx to the stomach by involuntary peristaltic waves. • The Stomach (pp. 857-858) The stomach stores food and secretes gastric juice, which converts a meal to acid chyme. Gastric juice includes hydrochloric acid and the enzyme pepsin. • The Small Intestine (pp. 838-861) The small intestine is the major organ of digestion and absorption. Acid chyme from the stomach mixes in the duodenum with intestinal juice, bile, and pancreatic juice. Diverse enzymes complete the hydrolysis of food molecules to monomers, which are absorbed into the blood across the lining of the small intestine. Hormones help regulate digestive juice secretions. • The Large Intestine (pp. 861-862) The large intestine (colon) aids the small intestine in reabsorbing water and bouses bacteria, some of which synthesize vitamins. Feces pass through the rectum and out the anus. Activity Digestive System Function Investigation What Role Does Amylase Play in Digestion? Activity Hormonal Control of Digestion

Evolutionary adaptations of vertebrate digestive systems are often associated with diet • Some Dental Adaptations (pp. 862-863) A mammals dentition is generally correlated with its diet. In particular, mammals have specialized dentition that best enables them to ingest their usual diet. • Stomach and Intestinal Adaptations (p. 863) Herbivores generally have longer alimentary canals than carnivores, reflecting the longer time needed to digest vegetation.

CHAPTER 41 Animal Nutrition

865

• Symbiotic Adaptations (pp. 863-864) Many herbivorous animals have fermentation chambers where symbiotic microorganisms digest cellulose.

Science, Technology, and Society The media report numerous claims and counterclaims about the

benefits and dangers of certain foods. Just a few examples are de TESTING YOUR KNOWLEDGE Evolution Connection The human esophagus and trachea share a common passage leading from the mouth and nasal passages, a "design" that occasionally contributes to death by choking. After reviewing vertebrate evolution in Chapter 34, explain the historical (evolutionary) basis for this "imperfect" anatomy.

Scientific Inquiry Design a controlled experiment to test the hypothesis that human salivary amylase digests starch faster at 37°C (human body temperature) than it does at either 20°C (approximate room temperature) or 43°C (110°F). Your only materials and equipment are a source of human saliva, distilled water, starch, an iodine reagent that stains starch dark purple, beakers, and several water baths that can be maintained at constant temperatures. How would you interpret the results if (a) the rate of enzyme activity were highest at 37°C; or (b) the rate of enzyme activity were highest at 43°C? Investigation What Role Does Amylase Play in Digestion? Biological Inquiry: A Workbook of Investigative Cases Explore several mammalian mechanisms for starch digestion in the case "Galloper's Gut."

866

UNIT SEVEN

Animal Form and Function

bates about vitamin doses, advocacy of diets enriched in certain food molecules such as carbohydrates or proteins, much discussi about low-carbohydrate diets, and publicity about new products such as cholesterol-lowering margarine. Have you modified your eating habits on the basis of nutritional information disseminatec by the media? Why or why not? How should a person evaluate whether such nutritional claims are valid?

Circulation and Gas Exchange A Figure 42.1 The gills of a salmon, which exchange gases between the blood and the environment.

IKey Concepts 42.1 Circulatory systems reflect phytogeny 42.2 Double circulation in mammals depends on the anatomy and pumping cycle of the heart 42.3 Physical principles govern blood circulation •2.4 Blood is a connective tissue with cells suspended in plasma 2.5 Gas exchange occurs across specialized respiratory surfaces i- 2.6 Breathing ventilates the lungs 2.7 Respiratory pigments bind and transport gases

across the thin epithelium covering the gills and into the blood, while carbon dioxide diffuses out into the water. Salmon and most other animals have organ systems specialized for exchanging materials with the environment, and most also have an internal transport system that conveys fluid (blood or interstitial fluid) throughout the body (see Figure 40.4). In this chapter, we will explore mechanisms of internal transport in animals. We will also examine a key example of chemical transfer between animals and their environment; the exchange of the gases oxygen (O2) and carbon dioxide (CO2), a process essential to cellular respiration and bioenergetics.

Concept

'Irading with the Environment

E

very organism must exchange materials and energy with its environment, and this exchange ultimately occurs at the cellular level. Cells live in aqueous sur-oundings; the resources they need, such as nutrients and oxygen, move across the plasma membrane into the cytoplasm, and metabolic wastes, such as carbon dioxide, move out of the cell. In unicellular organisms, these exchanges occur directly with the external environment. For most of the cells making up multicellular organisms, however, direct exchange with the environment is not possible. This constraint is associated with the evolution of physiological systems specialized for material transport and exchange. The feathery gills of a salmon (Figure 42.1) present an expansive surface area to the outside environment. Networks of tiny blood vessels (capillaries) lie close to the outside surfaces of the gills. Oxygen dissolved in the surrounding water diffuses

Circulatory systems reflect phylogeny Diffusion alone is not adequate for transporting substances over long distances in animals—for example, for moving glucose from the digestive tract and oxygen from the lungs to the brain of a mammal. Diffusion is inefficient over distances of more than a few millimeters, because the time it takes for a substance to diffuse from one place to another is proportional to the square of the distance. For example, if it takes 1 second for a given quantity of glucose to diffuse 100 um, it will take 100 seconds for the same quantity to diffuse 1 mm and almost 3 hours to diffuse 1 cm. The circulatory system solves this problem by ensuring that no substance must diffuse very far to enter or leave a cell. By rapidly transporting fluid in bulk throughout the body, the circulatory system functionally connects the aqueous environment of the body cells to the organs that exchange gases, absorb nutrients, and dispose of wastes. In the lungs of a mammal, for example, oxygen from inhaled air diffuses' 867

across a thin epithelium and into the blood, while carbon dioxide diffuses in the opposite direction. Bulk Quid movement in the circulatory system, powered by the heart, then quickly carries the oxygen-rich blood to all parts of trie body. As the blood streams through the body tissues within capillaries, chemicals are transported between the blood and the interstitial fluid that directly bathes the cells. Internal transport and gas exchange are functionally related in most animal phyla, and so in this chapter we will focus on both the circulatory and respiratory systems. We will also highlight the role of these two organ systems in maintaining homeostasis (see Chapter 40)—for example, in regulating the interstitial fluids content of nutrients and wastes. First, lets look at circulation in invertebrate animals.

access to nutrients, but the nutrients have to diffuse only short distance to reach the cells of the outer layer. Planarians and most other flatworms also have gastrowscular cavities that exchange materials with the environment through a single opening (see Figure 33.10). The flat shape of the body and the branching of the gastrovascular cavity throughout the animal ensure that all cells are bathed byi a suitable medium and that diffusion distances are short. Open and Closed Circulatory Systems

For animals with many cell layers, gastrovascular cavities a *e insufficient for internal transport because diffusion distances are too great for adequate exchange of nutrients and wastes. In such animals, two types of circulatory systems that overcome the limitations of diffusion have evolved: open and Invertebrate Circulation closed. Both have three basic components: a circulatory fluid The wide range of invertebrate body size and form is paralleled (blood), a set of tubes (blood vessels) through which tie by diversity in circulatory systems. The different selective presblood moves through the body, and a muscular pump (the sures of various environments have also led to evolutionary heart). The heart powers circulation by using metabolic modification of circulatory systems among invertebrates. energy to elevate the hydrostatic pressure ot the blood, which then flows down a pressure gradient through its cirGastrovascular Cavities cuit and back to the heart. This blood pressure is the motive force for fluid movement in the circulatory system. Owing to the simplicity of their body plan, hydras and other cnidarians do not require a true circulatory system. In these In insects, other arthropods, and most molluscs, blood animals, a body wall only two cells thick encloses a central bathes the organs directly in an open circulatory system gastrovascular cavity which serves both in digestion and in (Figure 42.3a). There is no distinction between blood and interdistribution of substances throughout the body (see Figure stitial fluid, and this general body fluid is more correctly termed 41.13). The fluid inside the cavity is continuous with the hemolymph. One or more hearts pump the hemolymph into an water outside through a single opening; thus, both the inner interconnected system of sinuses, which are spaces surroundand outer tissue layers are bathed by fluid. Thin branches of a ing the organs. Here, chemical exchange occurs between the hydras gastrovascular cavity extend into the animal's tentahemolymph and body cells. In insects and other arthropods, cles, and some cnidarians, such as jellies, have even more the heart is an elongated tube located dorsally. When the heart elaborate gastrovascular cavities (Figure 42.2). Since digestion contracts, it pumps hemolymph through vessels out into sit begins in the cavity, only the cells of the inner layer have direct nuses. When the heart relaxes, it draws hemolymph into the circulatory system through pores called ostia. Body movements that squeeze the sinuses help circulate the hemolymph. ' In a closed circulatory system, blood is confined to vessels and is distinct from the interstitial fluid (Figure 42.3b). OnJ or more hearts pump blood into large vessels that branch into smaller one;, coursing through the organs. Here, mateJ rials are exchanged by diffusion between the blood and the interstitial fluid bathing the cells. Earthworms, squids, octopuses and all vertebrates have closed circulatory systems. The fact that open and closed circulatory A Figure 42.2 Internal transport in the cnidarian Aurelia. The animal is viewed here systems are each widespread among anifrom its underside (oral surface). The mouth leads to an elaborate gastrovascular cavity that has mals suggests that both offer advantages. branches radiating to and from a circular canal. Ciliated cells lining the canals circulate fluid in th directions indicated by the arrows. For example, the lower hydrostatic 868

UNIT SEVEN

Animal Form and Function

Hemolymph in sinuses surrounding organs

% Anterior vessel

Lateral vessels A

Ostia

£

yK •-

-«:v Tubular heart

i) An open circulatory system. In an open circulatory system, such as that of a grasshopper, blood and interstitial fluid are the same, and this fluid is called hemolymph. The heart pumps hemolymph through vessels into sinuses, where materials are exchanged between the hemolymph and cells. Hemolymph returns to the heart through ostia, which are equipped with valves that close when the heart contracts. 4 Figure 42.3 Open and closed circulatory systems.

•ressures associated with open circulatory systems make them sss costly than closed systems in terms of energy expenditure, urthermore, because they lack an extensive system of blood essels, open systems require less energy to build and mainain. And in some invertebrates, open circulatory systems serve variety of other functions. For example, in molluscs and reshly molted aquatic arthropods, the open circulatory system unctions as a hydrostatic skeleton in supporting the body. What advantages might be associated with closed circulaory systems? Closed systems, with their higher blood pressure, are more effective at transporting circulatory fluids to meet the high metabolic demands of the tissues and cells of larger and more active animals. For instance, among the molluscs, only the large and active squids and octopuses have closed circulatory systems. And although all arthropods have open circulatory systems, the larger crustaceans, such as the lobsters and crabs, have a more developed system of arteries and veins as well as an accessory pumping organ that helps maintain blood pressure. Closed circulatory systems are most highly developed in the vertebrates.

Survey of Vertebrate Circulation Humans and other vertebrates have a closed circulatory system, often called the cardiovascular system. Generally, the

Auxiliary hearts

Ventral vessels

(b) A closed circulatory system. Closed circulatory systems circulate blood entirely within vessels, distinct from the interstitial fluid. Chemical exchange occurs between the blood and the Interstitial fluid, and between the interstitial fluid and body cells. In an earthworm, three major vessels branch into smaller vessels that supply blood to the various organs. The dorsal vessel functions as the main heart, pumping blood forward by peristalsis. Near the worm's anterior end, five pairs of vessels loop around the digestive tract and function as auxiliary hearts, propelling blood ventrally.

vertebrate heart has one or two atria (singular, atrium), the chambers that receive blood returning to the heart, and one or two ventricles, the chambers that pump blood out of the heart. Arteries, veins, and capillaries are the three main kinds of blood vessels, which in the human body have a total length of about 100,000 km. Arteries carry blood away from the heart to organs throughout the body. Within organs, arteries branch into arterioles, small vessels that convey blood to the capillaries. Capillaries are microscopic vessels with very thin, porous walls. Networks of these vessels, called capillary beds, infiltrate each tissue. Across the thin walls of capillaries, chemicals, including dissolved gases, are exchanged by diffusion between blood and the interstitial fluid around the tissue cells. At their "downstream" end, capillaries converge into venules, and venules converge into veins. Generally speaking, veins return blood to the heart. Notice that arteries and veins are distinguished by the direction in which they carry blood, not by the characteristics of the blood they contain. All arteries carry blood from the heart toward capillaries, and veins return blood to the heart from capillaries. A significant exception is the hepatic portal vein that carries blood from capillary beds in the digestive system to capillary beds in the liver. Blood flowing from the liver passes into the hepatic vein, which conducts blood to the heart. CHAPTER 42 Circulation and Gas Exchange

869

The cardiovascular systems of different vertebrate taxa are variations of this general scheme, modified by natural selection. Metabolic rate (see Chapter 40) is an important factor in the evolution of cardiovascular systems. In general, animals with higher metabolic rates have more complex circulatory systems and more powerful hearts than animals with lower metabolic rates. Similarly, within an animal, the complexity and number oi blood vessels in a particular organ are correlated with that organ's metabolic requirements. Perhaps

the most fundamental differences in cardiovascular adapations between animals are associated with gill breathing iin most aquatic vertebrates compared with lung breathing iin terrestrial vertebrates.

Fishes A fish heart has two main chambers, one ventricle and o atrium (Figure 42.4). Blood pumped from the ventricle trav

Vertebrate Circulatory Systems

Fishes have a two-chambered heart and a single circuit of blood flow.

Gill capillaries

AMPHIBIANS

REPTILES (EXCEPT BIRDS)

MAMMALS AND BIRDS

Amphibians have a threechambered heart and two circuits of blood flow: pulmocutaneous and systemic. Some mixing of oxygen-rich and oxygen-poor blood occurs in the single ventricle.

Reptiles other than birds have a three-chambered heart and two circuits of blood flow. However, a septum partially divides the single ventricle, further reducing mixing of oxygen-rich, and oxygen-poor blood.

Mammals and birds have a fourchambered heart that completely: segregates oxygen-rich and oxygen-poor blood. (The major • vessels near the heart are slightly different in birds, but the pattern of double circulation is essentially; the same a.s depicted here.)

Lung and skin capillaries

Systemic capillaries

Systemic capillaries

Lung capillaries

Systemic capillaries

Systemic circuits include all body tissues. Note that circulatory systems are depicted as if the animal is facing you: with the right side of the heart shown at the left and vice-versa.

870

UNIT SEVEN

Animal Form and Function

Lung capillaries

firs- to the gills (the gill circulation), where it picks up oxygen (Pi) and disposes of carbon dioxide (CO2) across capillary walls. The gill capillaries converge into a vessel that carries oxygen-rich blood to capillary beds throughout all other parts of the body (the systemic circulation). Blood then returns in veins to the atrium of the heart. Notice that in a fish, blood mist pass through two capillary beds during each circuit. When blood flows through a capillary bed, blood pressure— the motive force for circulation—drops substantially (for reasons we will explain shortly). Therefore, oxygen-rich blood lelving the gills flows to the systemic circulation quite slowly (although the process is aided by body movements during swimming). This constrains the delivery of O2 to body tissues and hence the maximum aerobic metabolic rate of fishes. A mphibxans ogs and other amphibians have a three-chambered heart, with /o atria and one ventricle (see Figure 42.4). The ventricle amps blood into a forked artery that splits the ventricles outut into the pulmocutaneous circuit and the systemic circuit. Tihe pulmocutaneous circuit leads to capillaries in the gas exchange organs (the lungs and skin in a frog), where the blood r. icks up O2 and releases CO2 before returning to the heart's left 2 trium. Most of the returning oxygen-rich blood is pumped into tie systemic circuit, which supplies all organs and then returns (|>xygen-poor blood to the right atrium via the veins. In the venricle of the frog, there is some mixing of oxygen-rich blood that Las returned from the lungs with oxygen-poor blood that has reurned from the rest of the body. However, a ridge within the entricle diverts most of the oxygen-rich blood from the left trium into the systemic circuit and most of the oxygen-poor >lood from the right atrium into the pulmocutaneous circuit. This organization, called double circulation, provides a gorous flow of blood to the brain, muscles, and other organs because the blood is pumped a second time after it loses pressure in the capillary beds of the lungs or skin. This contrasts sharply with single circulation in fishes, in which blood flows directly from the respiratory organs (gills) to other organs under reduced pressure.

Mammals and Birds In all mammals and birds, the ventricle is completely divided into separate right and left chambers (see Figure 42.4). The left side of the heart receives and pumps only oxygen-rich blood, while the right side receives and pumps only oxygen-poor blood. Oxygen delivery is enhanced because there is no mixing of oxygen-rich and oxygen-poor blood, and double circulation restores pressure to the systemic circuit after blood has passed through the lung capillaries. A powerful four-chambered heart was an essential adaptation in support of the endothermic way of life characteristic of mammals and birds. Endotherms use about ten times as much energy as equal-sized ectotherms; therefore, their circulatory systems need to deliver about ten times as much fuel and O2 to their tissues (and remove ten times as much CO2 and other wastes). This large traffic of substances is made possible by separate and independent systemic and pulmonary circulations and by large, powerful hearts that pump the necessary volume of blood. As we discussed in Chapter 25, mammals and birds descended from different reptilian ancestors, and their four-chambered hearts evolved independently—an example of convergent evolution.

1. What fundamental physical constraint necessitates a circulatory system in large organisms? 2. What is one advantage of a closed circulatory system? What is a disadvantage? 3. What are two physiological advantages of separate respiratory (pulmocutaneous or pulmonary) and systemic circuits over a single circuit as seen m fishes, which combines gill and systemic circulation? For suggested answers, see Appendix A.

Reptiles (Except Birds) Reptiles have double circulation with a pulmonary circuit (lungs) and a systemic circuit (see Figure 42.4). Turtles, snakes, and lizards have a three-chambered heart, although the ventricle is partially divided by a septum, which results in even less mixing of oxygen-rich and oxygen-poor blood than in amphibians. In the crocodilians, a septum divides the ventricle completely into separate right and left chambers. All reptiles except birds have two arteries leading from the heart to the systemic circuit, and arterial valves allow them to divert most of their blood from the pulmonary circuit to the systemic circuit.

Concept

Double circulation in mammals depends on the anatomy and pumping cycle of the heart Because heart disease is such a significant and widespread health problem in human populations, scientists have studied the human circulatory system in much greater detail than any other. Us structure and function can serve as a model for exploring mammalian circulation in general. CHAPTER 42

Circulation and Gas Exchange

871

Mammalian Circulation: The Pathway As you read this detailed discussion of blood flow through the mammalian cardiovascular system, refer to Figure 42.5, which has numbers keyed to the circled numbers in the text. Beginning our tour with the pulmonary (lung) circuit, O the right ventricle pumps blood to the lungs via © the pulmonary arteries. As the blood flows through © capillary beds in the left and right lungs, it loads O2 and unloads CO2. Oxygenrich blood returns from the lungs via the pulmonary veins to © the left atrium of the heart. Next, the oxygen-rich blood flows into © the left ventricle as the ventricle opens and the atrium contracts. The left ventricle pumps the oxygen-rich blood out to body tissues through the systemic circuit. Blood leaves the left ventricle via © the aorta, which conveys blood to arteries leading throughout the body. The first branches from the aorta are the coronary arteries (not shown), which supply blood to the heart muscle itself. Then come branches leading to capillary beds © in the head and arms (forelimbs). The aorta continues in a posterior direction, supplying oxygen-rich blood to arteries leading to © arterioles and capillary beds in the abdominal organs and legs. Within the capillaries, O2 and CO2 diffuse along their concentration gradients,

Anterio vena cava

with O2 moving from the blood to the tissues and CO2 p oduced by cellular respiration diffusing into the bloodstream. Capillaries rejoin, forming venules, which convey blood to veins. Oxygen-poor blood from the head, neck, and forelimbs is channeled into a large vein called © the anterior (or superior) vena cava. Another large vein called © the posterior (or inferior) vena cava drains blood from the trunk and hind limbs. The two venae cavae empty their blood into © the right atrium, from which the oxygen-poor blood flows into t I right ventricle.

The Mammalian Heart: A Closer Look A closer look at the mammalian heart (using the human heart as an example) provides a better understanding of how double circulation works (Figure 42.6). Located beneath the breastbone (sternum), the human heart is about the size of a clenched fist and consists mostly of cardiac muscle (see Figure 40.5). The two atria have relatively thin walls and serve as collection charrjibers for blood returning to the heart, most of which flows intjo the ventricles as they relax. Contraction of the atria completes filling of the ventricles. The ventricles have thicker walls and contract much more strongly than the atria—especially the left ventricle, which must pump blood to all body organs througi the systemic circuit. The heart contracts and relaxes m a rhythmic cycle. When it contracts, it pumps blood; when it relaxes, its chambers fill with blood. The cardiac cycle refers to one complet sequence of pumping and filling. The contraction phase of the cycle is called systole, and the relaxation phase is diastole (Figure 42.7). The volume of blood per minute that the lefi

Pulmonary artery

-Aorta •Pulmonary artery

Capillaries of abdominal organs and hind limbs

^Atrioventricular valve

A Figure 42.5 The mammalian cardiovascular system: an o v e r v i e w . Note that the dual circuits operate simultaneously, not in the serial fashion that the numbering in the diagram suggests. The t w o ventricles pump almost in unison; while some blood is traveling in the pulmonary circuit, the rest of the blood is flowing in the systemic circuit.

872

UNIT SEVEN

Animal Form and Function

A Figure 42.6 The mammalian heart: a closer look. Notice the valves, which prevent backflow of blood within the heart, and the relative thickness of the walls of the heart chambers.

gi

© Atrial systole; ventricular diastole

0 Atrial and ventricular diastole

©Ventricular systole; atrial diastole

A Figure 42.7 The cardiac cycle. For an adult human at rest with a pulse of about 75 beats per minute, one complete cardiac cycle takes about 0.8 second. O During a relaxation phase (atria and ventricles in diastole), blood returning from the large veins flows into the atria and ventricles. © A brief period of atrial systole then forces all remaining bllood out of the atria into the ventricles. © During the remainder of the cycle, ventricular systole pumps blood into the large arteries, Note mat seven-eighths of the time—all but 0.1 second of the cardiac cycle—the atria are relaxed and are filling with blood returning via the eins.

•entricle pumps into the systemic circuit is called cardiac iutput. Cardiac output depends on two factors: the rate of :ontraction, or heart rate (number of beats per minute); and ;troke volume, the amount of blood pumped by the left venjicle in each contraction. The average stroke volume in hunans is about 75 mL. A person with this stroke volume and a leart rate at rest of 70 beats per minute has a cardiac output of 5.25 L/min—about equivalent to the total volume of blood in the human body. Cardiac output can increase as much as about fivefold during heavy exercise. This is equivalent to pumping an amount of blood matching an average persons body mass every 2-3 minutes, Four valves in the heart, each consisting of flaps made of connective tissue, prevent backflow and keep blood moving in the correct direction (see Figure 42.6). Between each atrium and ventricle is an atrioventricular (AV) valve. The AV valves are anchored by strong fibers that prevent them from turning

inside out. Pressure generated by the powerful contraction of the ventricles closes the AV valves, keeping blood from flowing back into the atria. Semilunar valves are located at the two exits of the heart: where the aorta leaves the left ventricle and the pulmonary artery leaves the right ventricle. These valves are forced open by pressure generated by contraction of the ventricles. When the ventricles relax, blood starts to flow back toward the heart, closing the semilunar valves, which prevents blood from flowing back into the ventricles. The elastic walls of the arteries expand when they receive the blood expelled from the ventricles. By feeling your pulse— the rhythmic stretching of arteries caused by the pressure of blood driven by the powerful contractions of the ventricles— you can measure your heart rate. The heart sounds heard with a stethoscope are caused by the closing of the valves. (Even without a stethoscope, you can hear these sounds by pressing your ear tightly against the chest of a friend—a close friend.) The sound pattern is "lubdup, lub-dup, lub-dup." The first heart sound ("lub") is created by the recoil of blood against the closed AV valves. The second sound ("dup") is the recoil of blood against the semilunar valves. A defect in one or more valves causes a condition known as a heart murmur, which may be detectable as a hissing sound when a stream of blood squirts backward through a valve. Some people are born with heart murmurs; in others, the valves may be damaged by infection (from rheumatic fever, for instance). Most heart murmurs do not reduce the efficiency of blood flow enough to warrant surgery.

Maintaining the Heart's Rhythmic Beat The timely deliver}' of oxygen to the body's organs is critical. For example, brain cells die within a few minutes if their oxygen supply is interrupted. Thus, maintaining heart function is crucial for survival. Several mechanisms have evolved that ensure the continuity and control of heartbeat. Some cardiac muscle cells are self-excitable, meaning they contract without any signal from the nervous system, even if removed from the heart and placed in tissue culture. Each of these cells has its own intrinsic contraction rhythm. How are their contractions coordinated in the intact heart? A region of the heart called the sinoatrial (SA) node, or pacemaker, sets the rate and timing at which all cardiac muscle cells contract. Composed of specialized muscle tissue, the SA node is located in the wall of the right atrium, near the point where the superior vena cava enters the heart. Because the pacemaker of the human heart (and of other vertebrates) is made up of specialized muscle tissues and located within the heart itself, the vertebrate heart is referred to as a myogenic heart. In contrast, the pacemakers of most arthropod hearts originate in motor nerves arising from the outside, an arrangement called a neurogenic heart.

CHAPTER 42

Circulation and Gas Exchange

873

The SA node generates electrical impulses much like those produced by nerve cells. Because cardiac muscle cells are electrically coupled (by the intercalated disks between adjacent cells), impulses from the SA node spread rapidly through the walls of the atria, causing both atria to contract in unison (Figure 42.8). The impulses also pass to another region of specialized cardiac muscle tissue, a relay point called the atrioventricular (AV) node, located in the wall between the right atrium and right ventricle. Here the impulses are delayed for about 0.1 second before spreading to the walls of the ventricles. The delay ensures that the atria empty completely before the ventricles contract. Specialized muscle fibers called bundle branches and Purkinje fibers then conduct the signals to the apex of the heart and throughout the ventricular walls. The impulses that travel through cardiac muscle during the heart cycle produce electrical currents that are conducted through body fluids to the skin, where the currents can be detected by electrodes and recorded as an electrocardiogram (ECG or EKG). The SA node sets the tempo for the entire heart, but is influenced by a variety of physiological cues. Two sets of nerves affect heart rate: one set speeds up the pacemaker, and the other set slows it down. Heart rate is a compromise regulated by the opposing actions of these two sets of nerves. The pacemaker is also influenced by hormones secreted into the blood by glands. For example, epinephrine, the "fight-or-flight" hormone secreted by the adrenal glands, increases heart rate (see Chapter 45). Body temperature is another factor that affects the pacemaker. An increase of only 1°C raises the heart rate by about 10 beats per minute. This is the reason your pulse increases substantially when you have a fever. Heart rate

naker • crates wave of nals to contract.

© .Signals are-delayed " at AV node.

© Signals pass to heart apex.

also increases with exercise, an adaptation that enables the ci culatory system to provide the additional O^ needed by mu cles hard at work.

Concent Check 1. Some babies are born with a small hole between their left and right ventricles. Explain how, if not surgically corrected, this hole would affect the O2 content of the blood entering the systemic circuit from the heart. 2. Why is it important that the AV node of the heart slow or delay the electrical impulse moving from the SA node and the atrial walls to the ventricles? For suggested answers, see Appendix A.

Concept

Physical principles govern blood circulation Blood delivers nutrients and removes wastes throughout a animals body. These functions are made possible by the circu latory system, a branching network of vessels similar in some ways to the plumbing system that delivers fresh water to a city and removes the city's wastes. The same physical principle that govern the operation of such plumbing systems also in fluence the functioning of animal circulatory systems.

Signals spread

throughout ventricles

Bundle branches

Heart apex

Purkinje fibers

A Figure 42.8 The c o n t r o l of h e a r t r h y t h m . The gold portions of the graphs at the bottom indicate the components of an electrocardiogram (ECG) corresponding to the sequence of electrical events in the heart. In step 4, the black portion of the ECG to the right of the gold "spike" represents electrical activity after the ventricles contract; during this phase, the ventricles become electrically re-primed and thus able to conduct the next round of contraction signals.

874

U N I T SEVEN

Animal Form and Function

Blood Vessel Structure and Function The "infrastructure'5 of the circulatory sys tern consists of its network of blood vessels. All blood vessels are built of similai tissues. The walls of both arteries and veins, for instance, have three similar layers (Figure 42.9). On the outside, a layer of connective tissue with elastic fibers allows the vessel to stretch and recoil. A middle layer contains smooth muscle and more elastic fibers. Lining the lumen of all blood vessels, including capillaries, is an endothelium, a single layer of flattened cells that provides a smooth surface that minimizes resistance to the flow of blood. Structural differences correlate with the different functions of arteries, veins, and capillaries. Capillaries lack the two

Artery

Artery

Arteriole

Venule

4 Figure 42,9 The structure of blood vessels. This micrograph (SEM) shows an rtery next to a thinner-walled vein.

outer layers, and their very thin walls consist only of endohelium and its basement membrane. This facilitates the exhange of substances between the blood and the interstitial luid that bathes the cells. Arteries have thicker middle and )Uter layers than veins. Blood flows through the vessels of the irculatory system at uneven speeds and pressures. The thicker walls of arteries provide strength to accommodate blood lumped rapidly and at high pressure by the heart, and their lasticity helps maintain blood pressure even when the heart elaxes between contractions. The thinner-walled veins coney blood back to the heart at low velocity and pressure. 3lood flows through the veins mainly as a result of muscle action; whenever you move, your skeletal muscles squeeze your veins and push blood through them. Within large veins, flaps of tissue act as one-way valves that allow blood to flow only toward the heart (Figure 42.10).

segments of the pipe faster than it flows through wider segments. Since the volume of flow per second must be constant through the entire pipe, the fluid must flow faster as the crosssectional area of the pipe narrows (think of the velocity of water squirted by a hose with and without a nozzle). Based on the law of continuity, you might think that blood should travel faster through capillaries than through arteries, because the diameter of capillaries is very small. However, it is the total cross-sectional area of capillaries that determines flow rate. Each artery conveys blood to such an enormous number of capillaries that the total cross-sectional area is much greater in capillary beds than in any other part of the circulatory system. For this reason, the blood slows substantially as it enters the arterioles from arteries, and slows further still in the capillary beds. Capillaries are the only vessels with walls thin enough to permit the transfer of substances between the blood and interstitial fluid, and the slower flow of blood through these tiny vessels enhances this exchange. As blood leaves the capillaries and enters the venules and veins, it speeds up again as a result of the reduction in total cross-sectional area (Figure 42:11, on the next page).

Direction of blood flow in vein (toward heart)

•Valve (open)

Skeletal muscle

•Valve (closed)

Blood Flow Velocity Blood travels over a thousand times faster in the aorta (about 30 cm/sec on average) than in capillaries (about 0.026 cm/sec). This velocity change follows from the law oj continuity, which describes fluid movement through pipes. If a pipe's diameter changes over its length, a fluid flows through narrower

A Figure 42.10 Blood flow in veins. Contracting skeletal muscles squeeze the veins. Flaps of tissue within the veins act as oneway valves that keep blood moving only toward the heart. If we sit or stand too long, the lack of muscular activity causes our feet to swell with stranded blood unable to return to the heart.

CHAPTER 42

Circulation and Gas Exchange

875

Blood Pressure Fluids exert a force called hydrostatic pressure against the surfaces they coniact, and it is that pressure that drives fluids through pipes. Fluids flow from areas of higher pressure Lo areas of lower pressure. The hydrostatic pressure that blood exerts against the wall of a vessel and that propels the blood is called blood pressure. Blood pressure is much greater in arteries than in veins and is highest in arteries when the heart contracts during ventricular systole (systolic pressure; see Figure 42.11). When you take your pulse by placing your fingers on your wrist, you can feel an artery bulge with each heartbeat. The surge of pressure is partly due to the narrow openings of arterioles impeding the exit of blood from the arteries. Thus, when the heart contracts, blood enters the arteries faster than it can leave, and the vessels stretch from the pressure. The elastic walls of the arteries snap back during diastole, but the heart contracts again before enough blood has flowed into the arterioles to completely relieve pressure in the arteries. This

A Figure 42.11 The interrelationship of blood flow velocity, cross-sectional area of blood vessels, and blood pressure. Blood flow velocity decreases markedly in the arteriofes and is slowest in the capillaries, owing to an increase in total cross-sectional area. Blood pressure, the main force driving blood from the heart to the capillaries, is highest in the arteries.

876

UNIT SEVEN

Animal Form and Function

impedance by the arterioles is called peripheral resistance. As a consequence of the elastic arteries working against peripheral resistance, there is a substantial blood pressure even during diastole (diastolic pressure), and blood continuously flows into arterioles and capillaries (see Figure 42.11). As shown in Figure 42.12, the arterial blood pressure of a healthy human at rest oscillates between about 120 mm Hg (millimeters of mercury; a unit of pressure) at systole and less than 80 mm Hg at diastoleBlood pressure is determined partly by cardiac output ard partly by peripheral resistance. Contraction of smooth muscles in ihe walls of the arterioles reduces the diameter of tl e tiny vessels, increases peripheral resistance, and therefore increases blood pressure upstream in the arteries. When tr e smooth muscles relax, the arterioles dilate, which increases their diameter. Consequently, blood flow through the arteriolt s increases and pressure in the arteries falls. Nerve impulses, hormones, and other signals control these aneriole wall muscles. Stress, either physical or emotional, can raise blood pressure by triggering nervous and hormonal responses that constrict blood vessels. Cardiac output is adjusted in coordination with changes n peripheral resistance. This coordination of regulatory mechanisms maintains adequate blood flow as the body's demands oi the circulatory system change. Dunng heavy exercise, for exam pie, the arterioles in working muscles dilate. This response ad mils a greater flow of oxygen-rich blood to the muscles and als< decreases peripheral resistance. By itself, this would cause a droj in blood pressure (and therefore blood flow) in the body as whole. However, cardiac output increases, maintaining blooc pressure and supporting the necessary increase in blood flow. In large land animals, another factor that affects blood pres sure is gravity. Besides the force needed to overcome periphera resistance, additional pressure is necessary to push blooc above the level of the heart. In a standing human, blood musl rise about 0.35 m to get from the heart to the brain. This demands an extra 27 mm Hg of pressure, which requires thel heart to expend more energy in its contraction cycle. This pumping challenge is significantly greater for animals with long necks. A standing giraffe, for example, needs to pump blood as much as 2.5 m above the heart. That requires about 190 mm Hg of additional blood pressure in the left ventricle, and a giraffes normal systolic pressure near the heart is over 250 mm. Hg. (Systolic pressure that high would be extremely dangerous in a human.) Check valves and sinuses, along with feedback mechanisms that reduce cardiac output, prevent this high pressure from damaging the giraffes brain when it lowers its head to drink—a body position that causes blood to flow downhill almost 2 m from the heart, adding an extra 150 mm Hg of blood pressure in the arteries leading to the brain. Physiologists speculate about blood pressure and cardiovascular adaptations in dinosaurs—some of which had necks almost

O The cuff is loosened further until the blood flows freely through the artery and the sounds below the cuff disappear. The pressure at this point is the diastolic pressure remaining in the artery when the heart is relaxed.

0 A typical blood pressure reading for a 20-year-old is 1J20/70, The units for these numbers are mm of melrcury (Hg); a blood pressure of 120 is a force that carii support a column of mercury 120 mm high.

Blood pressure reading: 120/70

f& A sphygmomanometer, an inflatable cuff attached to a pressure gauge, measures blood pressure in an ajtery. The cuff is wrapped around the upper arm and inflated until the pressure closes the artery, so that no bilood flows past the cuff. When this occurs, the pressure ekerted by the cuff exceeds the pressure in the artery.

© A stethoscope is used to listen for sounds of blood flow below the cuff. If the artery is closed, there is no pulse below the cuff. The cuff is gradually deflated until blood begins to flow into the forearm, and sounds from blood pulsing into the artery below the cuff can be heard with the stethoscope. This occurs when the blood pressure is greater than the pressure exerted by the cuff. The pressure at this point is the systolic pressure.

il Figure 42.12 Measurement of blood pressure. Blood pressure is recorded as two numbers separated by a slash. The first number is the systolic pressure; the second is the diastolic pressure.

0 m long, which would have required a systolic pressure of early 760 mm Hg to pump blood to the brain when the head /as fully raised. But evidence indicates that dinosaurs proba1? ly did not have hearts powerful enough to generate such presures. Based on this analysis and on studies of neck-bone •tructure, some biologists have concluded that the louglecked dinosaurs fed close Lo the ground rather than raising heir head to feed on high foliage. By the time blood reaches the veins, its pressure is not afected much by the action of the heart. This is because the blood ncounters so much resistance as it passes through the millions f tiny arierioles and capillaries that the pressure generated by :he pumping heart has been dissipated and can no longer projpel the blood through the veins. How does blood return to the heart, especially when it must travel from the lower extremities jagainst gravity? Rhythmic contractions of smooth muscles in the walls of venules and veins account for some movement of the blood. More importantly, the activity of skeletal muscles during exercise squeezes blood through the veins (see Figure 42.10). Also, when we inhale, the change in pressure within the thoracic (chest) cavity causes the venae cavae and other large veins near the heart to expand and fill with blood.

Capillary Function At any given time, only about 5-10% of the body's capillaries have blood flowing through them. However, each tissue has many capillaries, so every part of the body is supplied with blood at all times. Capillaries in the brain, heart, kidneys, and liver are usually filled to capacity, but in many other sites, the blood supply varies over time as blood is diverted from one destination to another. After a meal, for instance, blood supply to the digestive traci increases. During strenuous exercise, blood is diverted from the digestive tract and supplied more generously to skeletal muscles and skin. This is one reason that exercising heavily immediately after eating a big meal may cause indigestion. Two mechanisms regulate the distribution of blood in capillary beds. Both depend on smooth muscles controlled by nerve signals and hormones. In one mechanism, contraction of the smooth muscle layer in the wall of an arteriole constricts the vessel, reducing its diameter and decreasing blood flow through it to a capillary bed. When the muscle layer relaxes, the arteriole dilates, allowing blood to enter the capillaries. In the other mechanism, rings of smooth muscle— called precapillary sphincters because they are located at the

CHAPTER 42 Circulation and Gas Exchange

877

entrance to capillary beds—control the flow of blood between arterioles and venules (Figure 42.13). As you have read, the critical exchange of substances between the blood and the interstitial fluid that bathes the cells takes place across the thin endothelial walls of the capillaries. Some substances may be carried across an endothelial cell in vesicles that form by endocytosis on one side of the cell and

Precapil

Arteriole (a) Sphincters relaxed

then release their contents by exocytosis on the opposite sid ;. Others simply diffuse between the blood and the interstitial fluid. Small molecules, such as O2 and CO2, diffuse down concentration gradients across the endothelial cells. Diffusion can also occur through the clefts between adjoining cells. However, transport through these clefts occurs mainly by bulk flow due to fluid pressure. Blood pressure within the capillary pushis fluid (consisting of water and small solutes such as sugars, salts, O2, and urea) through the capillary clefts. The outward movjment of this fluid causes a net loss of fluid from the upstreato end of the capillary near an arteriole. Blood cells suspended in blood and most proteins dissolved in the blood are too large to pass readily through the endothelium and remain in the capilaries. The blood proteins remaining in the capillaries, espt dally albumin, create approximately constant osmotic pressuie from the arteriole to the venule end of a capillary bed. In cor trast, blood pressure drops sharply This difference betweei blood pressure and osmotic pressure drives fluids out of capi laries at the arteriole end and into capillaries at the venule en I (Figure 42.14). About 85% of the fluid that leaves the blood it the arterial end of a capillary bed reenters from the interstitial fluid at the venous end, and the remaining 15% is eventual! returned to the blood by the vessels of the lymphatic system.

Fluid Return by the Lymphatic System

(b) Sphincters contracted

(c) Capillaries and larger vessels (SEM)

20 ^m

k Figure 42.13 Blood flow in capillary beds. Precapillary sphincters regulate the passage of blood into capillary beds. Some blood flows directly from arterioles to venules through capillaries called thoroughfare channels, which are always open. 878

UNIT SEVEN

Animal Form and Function

So much blood passes through the capillaries that the cumula tive loss of fluid adds up to about 4 L per day There is also som leakage of blood proteins, even though the capillary wall is not very permeable to large molecules. The lost fluid and proteins return to the blood via the lymphatic system. Fluid enters thii system by diffusing into tiny lymph capillaries intermingled among capillaries of the cardiovascular system. Once inside tfo lymphatic system, the fluid is called lymph; its composition i; about the same as that of interstitial fluid. The lymphatic systen drains into the circulatory system near the junction of the venae cavae with the right atrium (see Figure 43.5). Lymph vessels, like veins, have valves that prevent the backflow of fluid toward the capillaries. Rhythmic contractions o the vessel walls help draw fluid into lymphatic capillaries. Alsc like veins, lymph vessels depend mainly on the movement o skeletal muscles to squeeze fluid toward the heart. Along a lymph vessel are organs called lymph nodes. By filtering the lymph and attacking viruses and bacteria, lymph! nodes play an important role in the body's defense. Inside each lymph node is a honeycomb of connective tissue with spaces filled by white blood cells specialized for defense. When the body is fighting an infection, these cells multiply rapidly, and the lymph nodes become swollen and tender (which is why your doctor checks your neck for swollen lymph nodes when you feel sick). The lymphatic system helps defend against infection and maintains the volume and protein concentration of the blood.

"**SM 'JTERSTITIAL FitJID

•let fluid novement out

Net fluid movement

l

:u At the arterial end of a capillary, blood pressure is greater than osmotic pressure, and fluid flows out of the caoillary into the interstitial fluid.

At the venule end of a capillary, blood pressure is less than • osmotic pressure, and fluid flows from the interstitial fluid into the capillary.

Biood pressure Osmotic pressure inward flow

Outward flow

^ • Q

Arterial end of capillary Figure 42.14 Fluid exchange between capillaries and the interstitial fluid. The rograph at top left shows red blood cells traveling through a capillary (LM).

Recall from Chapter 41 that the lymphatic system also trans:orts fats from the digestive tract to the circulatory system.

in invertebrates with open circulation, blood (hemolymph) is not different from interstitial fluid. However, blood in the closed circulatory systems of vertebrates is a specialized connective tissue.

I Concent Check 1. What is the primary cause of the low velocity of blood flow through capillaries? 2. How does increasing blood pressure by increasing cardiac output combined with diverting most blood flow to the skeletal muscles prepare the body to contront or flee danger? 3. Explain how edema—the accumulation of fluid in body tissues—can result from a decrease in plasma protein due to severe protein deficiency in the diet. For suggested answers, see Appendix A.

Blood Composition and Function Blood consists of several kinds of cells suspended in a liquid matrix called plasma. After a blood sample is collected, the cells can be separated from plasma by spinning the whole blood in a centrifuge (an anticoagulant must be added to prevent the blood from clotting). The cellular elements (cells and cell fragments), which occupy about 45% of the volume of blood, settle to the bottom of the centrifuge tube, forming a dense red pellet. Above this cellular pellet is the transparent, straw-colored plasma. Plasma

Blood is a connective tissue with cells suspended in plasma We now shift our focus from the tubes and pumps of circulatory systems to the fluids being circulated. As explained earlier,

Blood plasma is about 90% water. Among its many solutes are inorganic salts in the form of dissolved ions, sometimes referred to as blood electrolytes (Figure 42.15, on the next page). The combined concentration of these ions is important in maintaining the osmotic balance of the blood. Some ions also help to buffer the blood, which in humans normally has a pH of 7.4. And the normal functioning of muscles and nerves depends on the concentration of key ions in the interstitial CHAPTER 42

Circulation and Gas Exchange

879

Solvent for carrying other substances

Water

Ions (blood electrolytes) Sodium Potassium Calcium Magnesium Chloride Bicarbonate Plasma proteins Albumin

ular elements 45%

Major f u n c t i o n s

Constituent

Cell t y p e

Number per (iL (mm 3 ) of blood

Erythrocytes (red blood cells) Osmotic balance, pH buffering, and regulation of membrane permeability

5-6 million

990 Leukocytes (white blood cells)

5,000-10,000

Functions

Transport oxygen and help transport carbon dioxide

Defense and immunity

Osmotic balance, pH buffering

Fibrinogen

Clotting

Immunoglobulins (antibodies)

Defense

Substances transported by blood Nutrients (such as glucose, fatty acids, vitamins) Waste products of metabolism Respiratory gases (O 2 and CO2) Hormones

' *••;-,>• Platelets

Neutrophil . ^k

Monocyt< 250,000-

Blood clotting

A Figure 42.15 The composition of mammalian blood.

fluid, which reflects their concentration in plasma. The kidney maintains plasma electrolytes at precise concentrations, an example of homeostasis we will explore in detail in Chapter 44. Another important class of solutes is the plasma proteins, which have many functions. Collectively, they act as buffers against pH changes, help maintain the osmotic balance between blood and interstitial fluid, and contribute to the blood's viscosity (thickness). The various types of plasma proteins also have specific functions. Some are escorts for lipids, which are insoluble in water and can travel in blood only when bound to proteins. Another class of proteins, the immunoglobulins, or antibodies, help combat viruses and other foreign agents that invade the body (see Chapter 43). And the plasma proteins called fibrinogens are clotting factors thai help plug leaks when blood vessels are injured. Blood plasma from which these clotting factors have been removed is called serum. Plasma also contains a wide variety of substances in transit from one part of the body to another, including nutrients, metabolic wastes, respiratory gases, and hormones. Blood plasma and interstitial fluid are similar in composition, except that plasma has a much higher protein concentration (capillary walls, remember, are not very permeable to proteins). 880

UNIT SEVEN

Animal Form and Function

Cellular Elements Suspended in blood plasma are two classes of cells (see Figu 42.15): red blood cells, which transport oxygen, and whi blood cells, which function in defense. A third cellular element, platelets, are fragments of cells that are involved in t clotting process. Erythrocytes. Red blood ceils, or erythrocytes, are by fat the most numerous blood cells. Each microliter (pL; or mmi) of human blood contains 5 to 6 million red cells, and there ar about 25 trillion of these cells in the body's 5 L of blood. Erythrocytes are an excellent example of the close relation ship between structure and function. Their main function o O2 transport depends on rapid diffusion of O2 across thei plasma membranes. Human erythrocytes are small disk (about 7-8.5 um in diameter) that are biconcave—thinner ir the center than at the edges. Their small size and biconcavt shape provide a large collective surface area for the total pop ulation of erythrocytes. and the greater the total area of eryth rocyte membrane in a given volume o[ blood, the more rapidly O2 can diffuse. Mammalian erythrocytes lack nuclei an unusual characteristic that leaves more space in these tiny cells for hemoglobin, the iron-containing protein thai transports oxygen (see Figure 5.20). Erythrocytes also lack mitochondria and generate their ATP exclusively by anaerobic

meitabolism. Oxygen transport by erythrocytes would be less efficient if their own metabolism were aerobic and consumed some of the oxygen they carry. Despite its small size, an erythrocyte contains about 250 millie i molecules of hemoglobin. Since each hemoglobin binds up to four molecules of O 2 , one erythrocyte can transport ab 3Ut a billion O2 molecules. Researchers have found that hemoglobin also binds the gaseous molecule nitric oxide (NO) asjwell as O2- As erythrocytes pass through the capillary beds of lungs, gills, or other respiratory organs, O2 diffuses into the er nhrocytes where hemoglobin binds O2 and NO. In the syste: nic capillaries, hemoglobin unloads O 2 , which then diffuses in.o body cells. The NO relaxes the capillary walls, allowing thiem to expand, which probably helps deliver O2 to the cells. Leukocytes. The blood contains live major types of white bllood cells, or leukocytes: monocytes, neutrophils, basophils, eosinophils, and lymphocytes (see Figure 42.15). Their collective function is to fight infections. For example, monocytes and ncutrophils are phagocytes, which engulf and digest bacteria amd debris from the body's own dead cells. As we will see in Chapter 43, lymphocytes develop into specialized B cells and T cells, which produce the immune response against foreign substances. White blood cells spend most of their time outside tike circulatory system, patrolling through interstitial fluid and tme lymphatic system, where most of the battles against piathogens are waged. Normally, a microliter of human blood has about 5,000 to 10,000 leukocytes, but their numbers inc :ease temporarily whenever the body is fighting an infection. Ilatelets. The third cellular element of blood, platelets, are f agments of cells about 2-3 um in diameter. They have no nuclei and originate as pinched-off cytoplasmic fragments of large cells in the bone marrow. Platelets then enter the blood snd function in the important process of blood clotting. tern Cells and the Replacement of Cellular Elements 'Tie cellular elements of blood (erythrocytes, leukocytes, and •>latelets) wear out and are replaced constantly throughout a person's life. Erythrocytes, for example, usually circulate for only three to iour months and then are destroyed by phago' rytic cells in the liver and spleen. Enzymes digest the old cell's nacromolecules, and biosynthetic processes construct new nacromolecules using many of the monomers, such as amino pcids, harvested from the old blood cells, as well as new maerials and energy from food. Many of the iron atoms derived rom the hemoglobin in old erythrocytes are incorporated into tew hemoglobin molecules. Erythrocytes, leukocytes, and platelets all develop from a :ommon source, a single population of cells called pfuripotent stem cells in the red marrow of bones, particularly the ribs, vertebrae, breastbone, and pelvis (Figure 42.16). "Pluripo-

t

Erythrocytes Platelets

Monocytes

& Figure 42.16 Differentiation of blood cells. Some of the pluripotent stem cells differentiate into lymphoid stem cells, which then develop into B cells and T cells, t w o types of lymphocytes that function in the immune response (see Chapter 43). All other blood cells differentiate from myeloid stem cells.

tent" means that these cells have the potential to differentiate into any type of blood cell or into cells that produce platelets. Pluripotent stem cells arise in the early embryo, and the population renews itself while replenishing the blood with cellular elements. (See Chapter 21 for more on stem cells.) A negative-feedback mechanism, sensitive to the amount of O2 reaching the body's tissues via the blood, controls erythrocyte production. If the tissues do not receive enough O2, the kidney synthesizes and secretes a hormone called erythropoietin (EPO), which stimulates production of erythrocytes. if blood is delivering more O2 than the tissues can use, the level of EPO falls and erythrocyte production slows. Physicians use synthetic erythropoietin to treat people with health problems such as anemia, a condition of lower-thannormal hemoglobin levels. However, some athletes abuse EPO by injecting themselves with the drug to increase their erythrocyte levels. This practice, known as blood doping, is banned by the International Olympic Committee and other sports federations. In 2002, several athletes competing in the winter Olympics in Salt Lake City tested positive for EPO-like chemicals and as a result were stripped of some of their medals. Recently researchers succeeded in isolating pluripotent stem cells and growing these cells in laboratory cultures. Purified

CHAPTER 42

Circulation and Gas Exchange

881

pluripotent stem cells may soon provide an effective treatment for a number of human diseases, including leukemia. A person with leukemia has a cancerous line of the stem cells that produce leukocytes. The cancerous stem cells crowd out cells that make erythrocyies and produce an unusually high number of leukocytes, many of which are abnormal. One experimental treatment for leukemia involves removing pluripoient stem cells from a patient, destroying the bone marrow, and restocking it with noncancerous stem cells. As few as 30 of these cells can completely repopulate the bone marrow.

clotting factors have been discovered, and the mechanism is still not fully understood. A genetic mutation that affects any step of the clotting process causes hemophilia, a disease characterized by excessive bleeding from even minor cuts and bruises. Anticlotting factors in the blood normally prevent spontineous clotting in the absence of injury Sometimes, howevt r, platelets clump and fibrin coagulates within a blood vesst 1, blocking the flow of blood. Such a clot is called a thrombus. These potentially dangerous clots are more likely to form individuals with cardiovascular disease.

Blood Clotting

Cardiovascular Disease

Most people get cuts and scrapes from time to time, yet we do not bleed to death because blood contains a self-sealing material that plugs leaks. The sealant is always present in an inactive form called fibrinogen. A clot forms only when this plasma protein is converted to its active form, fibrin, which aggregates into threads that form the framework of the clot. The clotting mechanism usually begins with the release of clotting factors from platelets and involves a complex chain of reactions that ultimately transforms fibrinogen to fibrin (Figure 42.17). More than a dozen

More than half the deaths in the United States are caused I y cardiovascular diseases, disorders of the heart and blocd vessels. The tendency to develop cardiovascular disease is inherited to some extent, but lifestyle plays a large role, too. Nongenetic factors that increase the risk of cardiovascular problems include smoking, lack of exercise, a diet rich in an inal fat, and high concentrations of cholesterol in the blood Cholesterol travels in the blood plasma mainly in the fon of particles consisting of thousands of cholesterol molecuk

O The clotting process begins when the endothelium of a vessel is damaged, exposing connective tissue in the vessel wall to blood. Platelets adhere to collagen fibers in the connective tissue and release a substance that makes nearby platelets sticky.

© The platelets form a plug that provides emergency protection against blood loss,

© This seal is reinforced by a clot of fibrin when vessel damage is severe. Fibrin is formed via a multistep process: Clotting factors released from the clumped platelets or damaged cells mix with clotting factors in the plasma, forming an activation cascade that converts a plasma protein called prothrombin to its active form, thrombin. Thrombin itself is an enzyme that catalyzes the final step of the clotting process, the conversion of fibrinogen to fibrin. The threads of fibrin become interwoven into a patch (see colorized SEM).

Fibrin clot

Clotting factors from: — * platelets • ^ Damaged cells —• Plasma (factors include calcium, vitamin K)

Prothrombin

*

»•

Fibrinogen

• Figure 42.17 Blood clotting. 882

UNIT SEVEN

Animal Form and Function

Throi Thrombin

^

P

Fibrin

J

Red blood ce

* *

5 Jim

s

: .

«

"• • * • ! * /

Trie final blow is usually a heart attack or a stroke. A heart attack is the death of cardiac muscle tissue resulting from pro|| ", longed blockage of one or more coronary arteries, the vessels that supply oxygenrich blood to the heart. Because they are small in diameter to begin with, the coronary arteries are particularly vulnerable. Such blockage can destroy cardiac muscle quickly, since the constantly beating heart muscle cannot survive long without oxygen. A stroke is the death of nervous tish i 50 \xrr\ (b) Partly clogged artery (a) Normal artery 250 Lirn sue in the bram, usually resulting from rupture or blockage of arteries in the head. A | Figure 4 2 . 1 8 Atherosclerosis. These light micrographs contrast a cross section of (a) a Heart attacks and strokes frequently normal (healthy) artery with (b) an artery partially blocked by an atherosclerotic plaque. Plaque consist mostly of fibrous connective tissue and smooth muscle cells infiltrated with lipids. result from a thrombus, or blood clot, that clogs an artery. A key process leading to the clogging of an artery by a thrombus is an inflammatory and other lipids bound to a protein. One type of particle— response triggered by the accumulation of LDLs in the artery's low-density lipoproteins (LDLs), often called the "bad cholesterol"—is associated with the deposition of cholesterol inner lining. Such an inflammation, which is analogous to the arterial plaques, growths that develop on the inner walls of body's response to a cut infected by bacteria (see Figure 43.6), teries. Another type—high-density lipoproteins (HDLs), can cause plaques to rupture, releasing fragments that form a ood cholesterol"—appears to reduce the deposition of thrombus. The thrombus may originate in a coronary artery or Lolesterol. Exercise increases HDL concentration, whereas an artery in the brain, ox it may develop elsewhere in the circunoking has the opposite effect on the LDL/HDL ratio. latory system and reach the heart or brain via the bloodstream. Healthy arteries have smooth inner linings that promote The transported clot, called an embolus, is swept along until it limpeded blood flow. The deposition of cholesterol thickens lodges in an artery too small for the clot to pass. An embolus is id roughens this smooth lining. A plaque forms at the site more likely to become trapped in a vessel that has been nard becomes infiltrated with fibrous connective tissue and rowed by plaques. The embolus blocks blood flow, and cardiac 11 more cholesterol. Such plaques narrow the bore of the or brain tissue downstream from the obstruction may die horn artery, leading to a chronic cardiovascular disease known as O2 deprivation. If damage in the heart interrupts the conducatherosclerosis (Figure 42.18). The rough lining of an athertion of electrical impulses through cardiac muscle, heart rate osclerotic artery seems to encourage the adhesion of platelets, may change drastically or the heart may stop beating altogether. t iggermg the clotting process and interfering with circulation. Still, the victim may survive if a heartbeat is restored by cardioHypertension (high blood pressure) promotes atheroscleropulmonary resuscitation (CPR) or some other emergency procesjis and increases the risk of heart attack and stroke. Atheroscledure within a few minutes of the attack. The effects of a stroke >sis tends to raise blood pressure by narrowing the vessels and and the individuals chance of survival depend on the extent and :ducing their elasticity. According to one hypothesis, chronic location of the damaged brain tissue. Igh blood pressure damages the endothelium that lines arteries, promoting plaque formation. Fortunately, hypertension is dimple to diagnose and can usually be controlled by diet, exerConcept Check cise, medication, or a combination of these. A diastolic pressure Connective tissue.

Smooth muscle

Plaque

above 90 may be cause for concern, and living with extreme hypertension—say, 200/120—is courting disaster. As atherosclerosis progresses, arteries become narrower, and he threat of heart attack or stroke increases. There may be warnng signs. For example, if a coronary artery is only partially >locked, the person may feel occasional chest pain, a condition mown as angina pectoris. The pain is most likely to appear when the heart is laboring hard as a result of physical or emoional stress, and it signals that part of the heart is not receiving nough O2- However, many people with atherosclerosis are ompletely unaware of their condition until catastrophe strikes.

1. About how many red blood cells does the bone marrow of a human produce per day, assuming a total red blood cell count of 25 trillion (2.5 X 10 u ) and an average longevity of 4 months for the cells? 2. F-xplain why a physician might order a white-cell count for a patient with symptoms of an infection. 3. How can a few dozen transplanted bone marrow stem cells replace the wide variety of cells that occur in bone marrow? For suggested answers, see Appendix A.

CHAPTER 42

Circulation and Gas Exchange

883

Concept

Gas exchange occurs across specialized respiratory surfaces In the remainder of this chapter, we will focus on the process of gas exchange. Although this process is often called respiration, it should not be confused with the energy transformations of cellular respiration. Gas exchange is the uptake of molecular oxygen (O2) from the environment and the discharge of carbon dioxide (CO2) to the environment (Figure 42.19). These exchanges are necessary to support the production of ATP in cellular respiration and usually involve both the respiratory system and the circulatory system of an animal. The source of O2, called the respiratory medium, is air for terrestrial animals and water for most aquatic animals. The atmosphere is Earth's main reservoir of O2 and is about 21% OT (by volume). Oceans, lakes, and other bodies of water contain O2 in dissolved form. The amount of O2 dissolved in a given volume of water varies considerably, but is always much less than in an equivalent volume of air. The part of an animal's body where gases are exchanged with the surrounding environment is called the respiratory surface. Animals do not move O2 and CO2 across membranes by active transport, so movement of these gases between the respiratory surface and the environment occurs entirely by diffusion. The rate of diffusion is proportional to the surface area across which diffusion occurs and inversely proportional to the square of the distance through which molecules must move. As a result, respiratory surfaces tend to be thin and have a large surface area, structural adaptations that maximize the rate of gas exchange. Additionally, all living cells must be bathed in water to maintain their plasma membranes. Thus,

the respiratory surfaces of terrestrial as well as aquatic anirru Is are moist, and O2 and CO2 diffuse across these surfaces after first dissolving in water. Trie respiratory surface must supply 02 and expel C0 2 for the entire body, and a variety of solutions to the problem of providing a large enough surface have evolved. The structure of a respiratory surface depends mainly on the size of the organism and whether it lives in water or on land, but it is also influenced by metabolic demands for gas exchange. Thus, i endotherm generally has a larger area of respiratory surf* than a similar-sized ectotherm. Gas exchange occurs over the entire surface area of most protists and other unicellular organisms. Similarly, for some relatively simple animals, such as sponges, cnidarians, and flatworms, the plasma membrane of every cell in the body close enough to the external environment for gases to diffu in and out. In many animals, however, the bulk of the boc does not have direct access to the respiratory medium. Tl respiratory surface in these animals is a thin, moist epith lium separating the respiratory medium from the blood ( capillaries, which transport gases to and from the rest of tr e body (see Figure 42.19). Some animals use their entire outer skin as a respiratory o gan. An earthworm, for example, has moist skin and e: changes gases by diffusion across its general body surface. Just below the earthworms skin is a dense net of capillaries. Because the respiratory surface must remain moist, earthworms and many other skin-breathers, including some amphibian?, must live in water or damp places. Animals whose only respiratory organ is moist skin are usually small and are either long and thin or flat, with a higk ratio of surface area to volume. For most other animals, thje general body surface lacks sufficient area to exchange gases for the whole body. The solution is a respiratory organ that is extensively folded or branched, thereby enlarging the available surface area for gas exchange. Gills, tracheae, and lungs ar: the three most common respiratory organs.

Gills in Aquatic Animals Gills are outfoldings of the body surface that are suspended in the water. In some invertebrates, such as sea stars, the gill have a simple shape and are distributed over much of th body (Figure 42.20a). Many segmented worms have flaplifa gills that extend from each segment of their body (Figure 42.20b) or long, feathery gills clustered at the head or tail. Tht gills of scallops (Figure 42.20c), crayfish (Figure 42.20d), anc many other animals are restricted to a local body region. Thf total surface area of the gills is often much greater than that o the rest of the body.

A. Figure 42.19 The role of gas exchange in bioenergetics. 884

UNIT SEVEN

Animal Form and Function

As a respiratory medium, water has both advantages anc disadvantages. There is no problem keeping the plasms membranes of the respiratory surface cells moist, since th
• EXHALATION Diaphragm relaxes H

A Figure 42.24 Negative pressure breathing. A mammal breathes by chai pressure within its lungs relative to the pressure of the outside atmosphere.

How a Bird Breathes Ventilation is much more complex in birds than in mammals. Besides lungs, }irds have eight or nine air sacs that penetrate the abdomen, neck, and wings. The

A

^ i 1 9Ure , 42 ; 25 TT a v i a " r e s P i r a t ° r i' system. Contraction and relaxation of the air sacs

ventilates the lungs, forcing air in one direction through tiny parallel tubes in the lungs called air sacs do not function directly in gas ex- parabronchi (inset, SEM). Gas exchange occurs across the walls of the parabronchi. During change, but act as bellows that keep air inhalation, both sets of air sacs expand. The posterior sacs fill with fresh air (blue) from the outside, w h i l e t h e a n t e r i o r sacs fil1 w i t n s t a l e air flowing through the lungs (Figure 42.25) (9 r a y) f r o m t n e lun 9 s - During exhalation, both sets of air , , ' sacs deflate, forcing air from the posterior sacs into the lungs, and air from the anterior sacs out of 1 he entire system—lungs and air sacs—is t h e s y s t e m v i a t h e t r a c h e a . Two cycles of inhalation and exhalation are required for the air to pass ventilated when the bird breathes. Air all the way through the system and out of the bird. CHAPTER 42

Circulation and. Gas Exchange

889

species of birds (notably the bar-headed goose) easily fly over the same mountains during migration.

Control of Breathing in Humans Humans can voluntarily hold their breath or breathe faster and deeper, but most of the time automatic mechanisms regulate breathing. This ensures that the work of the respiratory system is coordinated with that of the cardiovascular system and with metabolic demands for gas exchange. The main "breathing control centers are located in two regions of the brain, the medulla oblongata and the pons (Figure 42.26). Aided by the control center in the pons, the medullas center sets the basic breathing rhythm. Secondary control over breathing is exerted by sensors m the aorta and carotid arteries that monitor O2 and CO2 concentrations in the blood as well as blood pH. During deep breathing, a negative-feedback mechanism prevents the lungs from overexpanding; stretch sensors in the lung tissue send nerve impulses that inhibit the medullas control center. The medullas control center regulates breathing activity in response to changes in pH of the tissue fluid (cerebrospinal fluid) bathing the brain. The pH of the cerebrospinal fluid is determined mainly by CQ2 concentrations m the blood. Carbon dioxide diffuses from the blood to the cerebrospinal fluid, where it reacts with water and forms carbonic acid, which lowers the pH. When the medullas control center registers a slight drop in

O The control center in the medulla sets the basic rhythm, and a control center in the pons moderates it, smoothing out the transitions between inhalations and exhalations.

© Nerve impulses trigger muscle contraction. Nerves from a breathing control center in the medulla oblongata of the brain send impulses to the diaphragm and rib muscles, stimulating them to contract and causing inhalation.

© In a person at rest, these nerve impulses result in about 10 to 14 inhalations per minute. Between inhalations, the muscles relax and the person exhales.

A Figure 42.26 Automatic control of breathing.

890

U N I T SEVEN

Animal Form and Function

pH (increase in CO2) of the cerebrospinal fluid, it increases ft depth and rate of breathing, and the excess CO2 is eliminated i exhaled air. This occurs during exercise, for example.

Oxygen concentrations in the blood usually have little effect on the breathing control centers. However, when the O2 level severely depressed (at high altitudes, for instance), O2 senso in the aorta and carotid arteries in the neck send alarm signa to the breathing control centers, which respond by increasirg the breathing rate. Normally, a rise in CO2 concentration is a good indication of a fall in O2 concentration, because CO2 j-S produced by the same process that consumes O2—cellular respiration. However, it is possible to trick the breathing center qy hyperventilating. Excessively deep, rapid breathing purges the blood of so much CO2 that the breathing center temporarily ceases to send impulses to the rib muscles and diaphragm. Breathing stops until CO2 levels increase (or O2 levels decrease) enough to switch the breathing center back on. The breathing center responds to a variety of nervous and chemical signals and adjusts breathing rate and depth to met changing demands. However, breathing control is only effec rive if it is coordinated with control of the cardiovascular sys tern so that there is a good match between lung ventilatio and the amount of blood flowing through alveolar capillaries During exercise, for instance, increased cardiac output matched to the increased breathing rate, which enhances O uptake and CO2 removal as blood flows through the lungs.

O The medulla's control center also helps regulate blood CO2 level. Sensors in the medulla detect changes in the pH (reflecting CO, concentration) of the blood and cereorospinal fluid bathing the surface of the brain.

@ Nerve impulses relay changes in CO2 and Oj concentrations. Other sensors in the walls of the aorta and carotid arteries in the neck detect changes in blood pH and send nerve impulses to the medulla. In response, the medulla's breathing control center alters the rate and depth of breathing, increasing both to dispose of excess CO2 or decreasing both if CO2 levels are depressed. © The sensors in the aorta and carotid arteries also detect changes in O2 levels in the blood and signal the medulla to increase the breathing rate when levels become very low.

Concept Check 1. How does an increase in the CO2 concentration in the blood affect the pH of cerebrospinal fluid? 2. A slight decrease in blood pH causes the heart's pacemaker to speed up. What is ihe function of this control mechanism? 3. How does breathing differ in mammals and birds? For suggested answers, see Appendix A.

[Concept

diffuses from the blood to the air in the alveoli. Meanwhile, O2 in the air dissolves in the fluid that coats the epithelium and diffuses into the blood. © By the time the blood leaves the lungs in the pulmonary veins, its PO2 has been raised and its Pco?_ n a s been lowered. After returning to the heart, this blood is pumped through the systemic circuit. © In the tissue capillaries, gradients of partial pressure favor the diffusion of O2 out of the blood and CO2 into the blood. This is because cellular respiration removes O2 from and adds CO2 to the interstitial fluid (again, by diffusion, from mitochondria in nearby cells). © After the blood unloads O2 and loads CO2, it is returned to the heart and pumped to the lungs again, where it exchanges gases with air in the alveoli.

Respiratory pigments bind and transport gases

Exhaled air

T ae high metabolic demands of many organisms require that the b\ ood transport large quantities of O2 and CO2. It appears that milar solutions that address the transport requirements set high metabolic demands have evolved independently in fferent groups of animals. First, we'll see the role that O2 and C O2 gradients play, using the mammalian respiratory system an example. Then well examine blood molecules called piratory pigments that aid in the process of gas exchange.

1 he Role of Partial Pressure Gradients C iases diffuse down pressure gradients in the lungs and other t rgans. Diffusion of a gas, whether present in air or dissolved l water, depends on differences in a quantity called partial ressure. At sea level, the atmosphere exerts a total pressure of 60 mm Hg. This is a downward force equal to that exerted by column of mercury 760 mm high. Since the atmosphere is 1% O2 (by volume), the partial pressure of O2 (abbreviated o2) is 0.21 X 760, or about 160 mrnHg. This is the portion f atmospheric pressure contributed by O2; hence the term artial pressure. The partial pressure of CO2 (Pco2) a t - s e a l eve l only 0.23 mm Hg. When water is exposed to air, the amount )f a gas that dissolves in the water is proportional to its partial pressure in the air and its solubility in water. An equilibrium is eventually reached when gas molecules enter and leave the soution at the same rate. At this point, the gas is said to have the ame partial pressure in solution as it does in the air. Thus, in a glass of water exposed to air at sea-level air pressure, the PQ2 us 160 mm Hg and the Pco2 i-s 0-23 mm Hg. A gas always diffuses from a region of higher partial pres,ure to a region of lower partial pressure. © Blood arriving at the lungs via the pulmonary arteries has a lower Po_, and a higher Pco2 t n a n the a^r m the alveoli (Figure 42.27). Notice that the air in the alveoli has a lower ¥o, and a higher Pc;c,2 than air at sea level, since it is not completely replaced with fresh air during breathing. As blood enters the alveolar capillaries, CO2

Tissue cells R4QF45I °2 C 0 2 A Figure 42.27 Loading and unloading of respiratory gases. The colored bars indicate the partial pressures (in mm Hg) of 02 (PQJ and C02 (PcoJ in different locations. CHAPTER 42

Circulation and Gas Exchange

891

Respiratory Pigments The low solubility of O2 in water (and thus in blood) is a problem for animals that rely on the circulalory system to deliver O2. Suppose all our O2 were delivered in solution in Lhe blood. During intense exercise, a person can consume almost 2 L of O2 per minute, and all of it must be carried in the blood from the lungs to the active tissues. But at normal body temperature and air pressure, only 4.5 rnL of O2 can dissolve into a liter of blood in the lungs. If 80% of the dissolved O2 were delivered to the tissues (an unrealistically high percentage), the heart would need to pump 500 L of blood per minute. In fact, most animals transport most of their O2 bound to certain proteins called respiratory pigments instead of in dissolved form. Respiratory pigments circulate with the blood, often contained within specialized cells. The pigments greatly increase the amount of oxygen that can be carried in blood (to about 200 mL of O2 per liter in mammalian blood). In our example of an exercising human with a delivery rate of 80%, that greatly reduces the cardiac output necessary for O2 transport to a manageable 12.5 L of blood per minute. Oxygen Transport A diversity of respiratory pigments have evolved in various animal taxa. One example, hemocyanin, found in arthropods and many molluscs, has copper as its oxygen-binding component, coloring the blood bluish. The respiratory pigment of almost all vertebrates and a wide variety of invertebrates is the protein hemoglobin, contained in the erythrocytes of vertebrates. Hemoglobin consists of four subunits, each with a cofactor called a heme group that has an iron atom at its center. The iron binds the O2; thus, each hemoglobin molecule can carry lour molecules of O2 (see Figure 5.20). Like all respiratory pigments, hemoglobin must bind O2 reversibly, loading O2 in the lungs or gills and unloading it in other parts of the body (Figure 42.28). This process depends on cooperation between the subunits of the hemoglobin molecule (see Chapter 8). Binding of O2 to one subunit induces the others to change shape slightly, with the result that their affinity for O2 increases. And when one subunit unloads its O2, the other three quickly unload too as a shape change lowers their affinity for O2. Heme group

Iron atom

x

Polypeptide chain

A Figure 42.28 Hemoglobin loading and unloading O2.

892

UNIT SEVEN

Animal Form and Function

Cooperative O2 binding and release is evident in t dissociation curve for hemoglobin (Figure 42.29). Over t

Tissues during exercise

Tissues at rest P o {mm Hg)

'

(a) PQ and Hemoglobin Dissociation at 37°C and pH 7.4. The curve shows the relative amounts of O? bound to b.emo. giobin exposed to solutions varying in their PQ2- At a Po2 of 104 mm Hg, typical in the lungs, hemoglobin is about 9 8 % saturated with O z . At a PQ2 of 40 mm Hg, common in the vicinity of tissues, hemoglobin is only about 7 0 % saturated. Hemoglobin can release its O2 to metabolicatly very active tissues, such as muscle tissue during exercise.

P o (mm Hg) (b) pH and Hemoglobin Dissociation. Hydrogen ions affe the conformation of hemoglobin—-a drop in pH shifts the Odissociation curve toward the right. At a given PQV say 40 mm Hg, hemoglobin gives up more O,at pH 7.2 than at pH 7.4, the normal pH of human blood. The pH decreases (becomes more acidic) in very active tissues because the CO 2 produced by respiration reacts with water, forming carbonic acid. Hemoglobin then releases more O2, which supports the increased cellular respiration in the active tissues.

A Figure 42.29 Dissociation curves for hemoglobin.

range of O2 partial pressures (Po2) where th: dissociation curve has a steep slope, CTHI a slight change in ?Oi causes hemoglpbin to load or unload a substantial amount of O2. Notice that the steep part ofl the curve corresponds to the range of o|i partial pressures found in body tissues. When cells in a particular location begin working harder—during exercise, for inince—Po dips in their vicinity as the O2 is consumed in cellular respiration. Beuse of the effect of subunit cooperativity, slight drop in Po, is enough to cause a atively large increase in the amount of C^2 the blood unloads.

O Carbon dioxide produced by body tissues diffuses into the interstitial fluid and the plasma.

© Over 90% of the CO2 diffuses into red blood cells, leaving only 7% in the plasma as dissolved CO2. © Some CO2 is picked up and transported by hemoglobin. O However, most CO2 reacts with water in red blood cells, forming carbonic acid (H2CO3), a reaction catalyzed by carbonic anhydrase contained within red blood cells.

As with all proteins, hemoglobin's nformation is sensitive to a variety of f; ctors. For example, a drop in pH lowers the affinity of hemoglobin for O2, an effect called the Bohr shift (see Figure 42.29b). Because CO2 reacts with water, forming carbonic acid (H2CO3), an active tissue bwers the pH of its surroundings and i lduces hemoglobin to release more O2, which can then be used for cellular respiration.

0 Carbonic acid dissociates into a bicarbonate ion (HCO3') and a hydrogen ion (H+). 0 Hemoglobin binds most of the H+frorn H2CO3, preventing the H+ from acidifying the blood and thus preventing the Bohr shift. © Most of the HCO3~ diffuses into the plasma where it is carried in the bloodstream to the lungs.

Carbon Dioxide Transport In addition to its role in oxygen transport, hemoglobin also helps transport ~O2 and assists in buffering—-that is, ^eventing harmful changes in blood pH. Jnly about 7% of the CO2 released by espiring cells is transported in solution n blood plasma. Another 23% binds to he multiple amino groups of hemoglotin, and about 70% is transported in the ilood in the form of bicarbonate ions HCO 3 ^). Carbon dioxide from respiring ells diffuses into the blood plasma and hen into the erythrocytes (Figure 42.30). he CO2 Erst reacts with water (assisted by the enzyme carbonic anhydrase) and forms H2CO;3, which then dissociates into a hydrogen ion (H + ) and HCO3~tvlost of the H attaches to hemoglobin and other proteins, minimizing the hange in blood pH. The HCO3~ diffuses into the plasma. As blood flows through the lungs, the process is rapidly reversed as diffusion of CO-> out of the

© In the lungs, HCO3" diffuses from the plasma into red blood cells, combining with H+ released from hemoglobin and forming H2CO3.

0 Carbonic acid is converted back into CO2 and water.

) CO2 formed from H2CO3 is unloaded from hemoglobin and diffuses into the interstitial fluid. CO, CO2 diffuses into the alveolar space, from which it is expelled during exhalation. The reduction of CO2 concentration in the plasma drives the breakdown of H2CO3 into CO2 and water in the red blood cells (see step 9), a reversal of the reaction that occurs in the tissues {see step 4). A Figure 42.30 Carbon dioxide transport in the blood.

Circulation and G&b ExLhanj;*.'

893

blood shifts the chemical equilibrium in favor of the conversion of HCO3~ to CO2.

Elite Animal Athletes For some animals, such as long-distance runners and migrator)' birds and mammals, the O2 demands of daily activities would overwhelm the capacity of a typical respiratory system. Other animals, such as diving mammals, are capable of being active underwater for extended periods without breathing. What evolutionary adaptations enable these animals to perform such feats? The Ultimate Endurance Runner The elite animal marathon runner may be the pronghorn, an antelope-like mammal native to the grasslands of North America, where it has roamed for more than 4 million years. Pronghorns are capable of running as fast as 100 km/hr. Though their top speed does not reach that of the cheetah, pronghorns can sustain high speeds over long distances and have been timed sprinting 11 km in 10 minutes, maintaining an average speed of 65 km/hr. Stan Lindstedt and his colleagues at the University of Wyoming and University of Bern were curious about how pronghorns sustain their combination of great speed and endurance: through enhancements of normal physiological mechanisms that supply increased O2 to muscles or through greater energetic efficiency? The researchers exercised pronghorns on a treadmill to estimate their maximum rate of O2 consumption (Figure 42.31) and discovered something surprising: Pronghorns consume O2 at three times the rate predicted for an animal of their size. Normally, as animals increase in size, their rate of O2 consumption per gram declines. One gram of shrew tissue, for example, consumes as much O2 in a day as a gram of elephant tissue consumes in an entire month. But Lindstedt and his colleagues discovered that the rate of O2 consumption per gram of tissue by a pronghorn was the same as that of a 10-g mouse! To establish a more appropriate perspective on pronghorn performance, the research team compared various physiological characteristics of pronghorns with similar-sized domestic goats, which are adapted to climbing rather than running. They found that the maximum rate of O2 consumption by pronghorns is five times that of goats. Why? Pronghorns have a larger surface area for O2 diftusion in the lungs, nearly five times the cardiac output, much higher muscle mass, and a higher volume and density of mitochondria than goats. In addition, the pronghorns maintain higher muscle temperatures. The researchers concluded that the pronghorns extreme O2 consumption rate, which underlies their ability to run at high speeds over long distances, results from enhancements of the normal physiological mechanisms present in other mammals. We can see in these enhancements the results of natural selection, perhaps exerted by 894

UNIT SEVEN

Animal Form and Function

A Figure 42.31 Measuring rate of O2 consumption in a running pronghorn. Stan Lindstedt collects respiratory data from pronghorn running on a treadmill at 40 km/hr.

the predators that have chased pronghorns across the ope I plains of North America for millions of years. Diving Mammals The majority of animals can exchange gases continuously, but sometimes there is no access to the normal respiratory medium—for example, when an air-breather swims underwa ter. Whereas most humans, even well-trained divers, can hole their breath for 2 or 3 minutes and swim to depths of 20 m o so, the Weddell seal of Antarctica routinely plunges tc 200-500 m and remains there for about 20 minutes (some times for more than an hour). Some species of seals, sea turtles and whales make even more impressive dives. Elephant seal; can reach depths of 1,500 m—almost a mile—and stay sub merged for as much as 2 hours! One elephant seal carrying i recording device spent 40 days at sea diving almost continuously, with no surface period longer than 6 minutes. In contrast, humans need to carry extra air—-in the form of scuba tanks—to remain submerged for comparable periods. One adaptation of the Weddell seal (and other divin mammals) is an ability to store large amounts of O2. Compared to humans, the seal can store about twice as much O2 per kilogram of body mass, mostly in the blood and muscles. About 36% of our total O2 is in our lungs, and 51% is in our blood. In contrast, the Weddell seal holds only about 5% of its oxygen in its relatively small lungs (and may exhale before diving, which reduces buoyancy), stockpiling 70% in the blood. The seal has about twice the volume of blood per kilogram of body mass as a human. Another adaptation is the seal's huge spleen, which can store about 24 L of blood. The spleen probably contracts after a dive begins, fortifying the blood with erythrocytes loaded with O2. Diving mammals also have a high concentration of an oxygen-storing protein called myoglobin in their muscles. The Weddell seal

l store about 25% of its O2 in muscle, compared to only 12 % in humans. Diving mammals not only begin an underwater trip with a atively large O2 stockpile, but also have adaptations that conserve O2. They swim with little muscular effort and often use buoyancy changes to glide passively upward or downward. Their heart rate and O2 consumption rale decrease durirlg a dive, and regulatory mechanisms affecting peripheral resistance route most blood to the brain, spinal cord, eyes, adrenal glands, and placenta (in pregnant seals). Blood supply to tne muscles is restricted and is shut off altogether during the longest dives. During dives of more than about 20 minutes, a Weddell seal's muscles deplete the O2 stored in myoglobin akd then derive their ATP from fermentation instead of respi.tion (see Chapter 9). The unusual abilities of the Weddell seal and other airhreathing divers to power their bodies during long dives .owcase two related themes in our study of organisms—the sponse to environmental challenges over the short term by

Chapter Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY Or KEY Circulatory systems reflect phylogeny Transport systems functionally connect the organs of exchange with the body cells. Most complex animals 'nave internal transport systems that circulate fluid, providing a lifeline between the aqueous environment of living cells and the organs, such as lungs, that exchange chemicals with the outside environment (pp. 867-868). Invertebrate Circulation (pp. 868-869) Most invertebrates have a gastrovascular cavity or a circulatory system for internal transport.

physiological adjustments and over the long term as a result of natural selection.

Concept Check ; 1. What determines whether O2 and CO2 diffuse into or out of the capillaries in the tissues and near the alveolar spaces? Explain. 2. How does the Bohr shift help deliver O2 to very active tissues? 3. Carbon dioxide within red blood cells in the tissue capillaries combines with water, forming carbonic acid, What causes the reverse of this reaction in red blood cells in capillaries near the alveolar spaces? 4. Describe three adaptations that enable Weddell seals to stay underwater much longer than humans can. For suggested answers, see Appendix A.

Review tissues by the left ventricle. Blood returns to the heart through the right atrium. Activity Mammalian Cardiovascular System Structure

The Mammalian Heart: A Closer Look (pp. 872-873) The heart rate (pulse) is the number of times the heart beats each minute. The cardiac cycle, one complete sequence of the heart's pumping and filling, consists of periods of contraction, called systole, and periods of relaxation, called diastole. The cardiac output is the volume of blood pumped into the systemic circulation per minute.

Maintaining the Heart's Rhythmic Beat (pp. 873-874) Impulses originating at the sinoatrial (SA) node (.pacemaker) of the right atrium pass to the atrioventricular (AV) node. After a delay, they are conducted along the bundle branches and Purkinje fibers. The pacemaker is influenced by nerves, hormones, body temperature, and exercise.

Survey of Vertebrate Circulation (pp. 869-871) Adaptations of the cardiovascular system reflect vertebrate phylogeny. Blood flows in a closed cardiovascular system consisting of blood vessels and a two- to four-chambered heart. Arteries convey blood to capillaries, the sites of chemical exchange between blood and interstitial fluid. Veins return blood from capillaries to the heart.

Double circulation in mammals depends on the anatomy and pumping cycle of the heart Mammalian Circulation: The Pathway (p. 872) Heart valves dictate a one-way flow of blood through the heart, beginning with the right ventricle pumping blood to the lungs, where it loads 0 2 and unloads C 0 2 . Oxygen-rich blood from the lungs enters the heart at the left atrium and is pumped to the body

Physical principles govern blood circulation • Blood Vessel Structure and Function (pp. 874-875) Structural differences of arteries, veins, and capillaries correlate with their different functions. • Blood Flow Velocity (pp. 875-876) Physical laws governing the movement of fluids through pipes influence blood flow and blood pressure. The velocity of blood flow varies in. the circulatory system, being slowest in the capillary beds as a result of the high resistance and. large total cross-sectional area of the arterioles and capillaries. • Blood Pressure (pp. 876-877) Blood pressure, the hydrostatic force that bfood exerts against the wall of a vessel, is determined by the cardiac output and peripheral resistance due to variable constriction of the arterioles.

CHAPTER 42

Circulation and Gas Exchange

895

• Capillary Function (pp. 877-879) Transfer of substances between the blood and the interstitial fluid occurs across the thin walls of capillaries. Fluid Return by the Lymphatic System (pp. 878-879) The lymphatic system returns fluid to the blood and aids in body defense. Fluid reenters the circulation directly at the venous end of the capillary and indirectly through the lymphatic system. Activity Path of Blood Flow in Mammuls Biology Labs On-Line CardioLab Activity Mammalian Cardiovascular System Function

Blood is a connective tissue with cells suspended in plasma • Blood Composition and Function (pp. 879-882) Whole blood consists oi cellular elements (cells and fragments of cells called platelets) suspended in a liquid matrix called plasma. Plasma proteins influence blood pH, osmotic pressure, and viscosity and function in lipid transport, immunity (antibodies), and blood clotting (fibrinogens). Red blood cells, or erythrocytes, transport oxygen. Five types of white blood cells, or leukocytes, function in defense by phagocytosing bacteria and debris or by producing antibodies. Platelets function in blood clotting, a cascade of complex reactions that converts plasma fibrinogen to fibrin. • Cardiovascular Disease (pp. 882-883) Cardiovascular diseases are the leading cause of death in most developed nations, including the United States. Investigation How Is Cardiovascular Fitness Measured?

Gas exchange occurs across specialized respiratory surfaces • Gas exchange supplies oxygen for cellular respiration and disposes of carbon dioxide. Animals require large, moist respirator}' surfaces for the adequate diffusion of respiratory gases (O 2 and CO2) between their cells and the respiratory medium, either air or water (p. 884). • Gills in Aquatic Animals (pp. 884-886) Gills, the respiratory adaptations of most aquatic animals, are outfoldings of the body surface specialized for gas exchange. The effectiveness of gas exchange in some gills, including chose of fishes, is increased by ventilation and countercurrent flow of blood and water. • Tracheal Systems in Insects (pp. 886-887) The tracheae of insects are tiny branching tubes that penetrate the body, bringing O2 directly to cells. • Lungs (pp. 886-888) Spiders, land snails, and most terrestrial vertebrates have internal lungs. In mammals, air inhaled through the nostrils passes through the pharynx into the trachea, bronchi, bronchioles, and dead-end alveoli, where gas exchange occurs. Activity The Human Respiratory System

Breathing ventilates the l u n g s • How an Amphibian Breathes (p. 888) An amphibian ventilates its lungs by positive pressure breathing, which forces air down the trachea. • How a Mammal Breathes (pp. 888-889) Mammals ventilate their lungs by negative pressure breathing, which pulls air into the lungs. Lung volume increases as the rib muscles and diaphragm contract. 896

UNIT SEVEN

Animal Form and Function

• How a Bird Breathes (pp. 889-890) Besides lungs, birds have eight or nine air sacs that act as bellows, keeping air flowing through the lungs. Air passes through the lungs in one direction only Every exhalation completely renews the air in the lungs. • Control of Breathing in Humans (p. 890) Control centers in the medulla oblongata of the brain regulate the rate and depth of breathing. Sensors detect the pH of cerebrospmal fluid (reflecting CO 2 concentration in the blood), and the medulla adjusts breathing rate and depth to match metabolic demands. Secondary control over breathing is exerted by sensors in the aorta and carotid arteries that monitor blood levels of O2 and CO 2 and blood pi L.

Respiratory pigments bind and transport gases • The Role of Partial Pressure Gradients (p. 891) Gases diffuse down pressure gradients in the lungs and other organs. Oxygen and CO 2 diffuse from where their partial pressures are higher to where they are lower. • Respiratory Pigments (pp. 892-894) Respiratory pigments transport gases and help buffer the blood. Respiratory pigments greatly increase the amount of O2 that blood can carry. Many arthropods and molluscs have copper-containing hemocyanin; vertebrates and a wide variety of invertebrates have hemoglobin. • Elite Animal Athletes (pp. 894-895) The pronghoms extreme O2 consumption rate underlies its ability to run at high speeds over long distances. Deep-diving air-breathers stockpile O2 and deplete it slowly Activity Transport of Respiratory Gases Biology Labs On-Line HemoglobinLab

TESTING YOUR KNOWLEDGE Evolution Connection One of the many mutant opponents that the movie monster Godzilla contends with is Mothra, a giant mothlike creature with a wingspan of several dozen feet. Science fiction creatures like these can be critiqued on the grounds of biomechanical and physiological principles. Focusing on respiration and the principles of gas exchange you learned about in this chapter, what physiological problems would Mothra face? The largest insects that have ever lived are Paleozoic dragonflies with half-meter wingspans. Why do you think truly giant insects are improbable?

Scientific Inquiry The hemoglobin of a human fetus differs from adult hemoglobin. Compare the dissociation curves of the two hemoglobins in the graph below. Propose a hypothesis for the junction of this difference between these two versions of hemoglobin.

0 20 40 60 SO 1.0 Po (mm Hg)

Investigation Howls Cardiovascular Fitness Measured? Biology Labs On-Line CardioLab Biology Labs On-Line HemoglobinLab

Science, Technology, and Society Hundreds of studies have linked smoking with cardiovascular and lung disease. According to most health authorities, smoking is the leading cause of preventable, premature death in the United States. Antismoking and health groups have proposed that cigarette advertising in all media be banned entirely. What are some arguments in favor of a total ban on cigarette advertising? What are arguments in opposition? Do you favor or oppose such a ban? Defend your position.

CHAPTER 42

Circulation and Gas Exchange

897

A Figure 43.1 A macrophage (blue) ingesting a yeast cell (green).

Key Concepts 43.1 Innate immunity provides broad defenses against infection 43.2 In acquired immunity, lymphocytes provide specific defenses against infection 43.3 Humoral and cell-mediated immunity defend against different types of threats 43.4 The immune system's ability to distinguish self from nonself limits tissue transplantation 43.5 Exaggerated, self-directed, or diminished immune responses can cause disease

Reconnaissance, Recognition, and Response

A

n animal must defend itself against the many potentially dangerous viruses, bacteria, and other pathogens it encounters in the air, in food, and in water. It must also contend with abnormal body cells that may develop into cancer. Two major kinds of defense have evolved that counter these threats. The first, called innate immunity, is present before any exposure to pathogens and is effective from the time of birth. Innate defenses are largely nonspecific, quickly recognizing and responding to a broad range of microbes regardless of iheir precise identity. Innate immunity consists of external barriers formed by the skin and mucous membranes, plus a set of internal cellular and chemical defenses that combat infectious agents that breach the external barriers. Key players in these internal defenses are macrophages and other phagocytic cells, which ingest 898

and then destroy pathogens. For example. Figure 43-1 (< colorized SEM) shows a macrophage engulfing a yeast cell. The second major kind of defense is acquired immunity also called adaptive immunity. It develops only after exposure to inducing agents such as microbes, abnormal body cells, toxins, or other foreign substances. Acquired defenses are highly! specific—that is, they can distinguish one inducing agent fron: another, even those that differ only slightly. This recognition! is achieved by white blood cells called lymphocytes, which produce two general types of immune responses. In the humoral response, cells derived from B lymphocytes secrete defensive proteins called antibodies that bind to microbes and mark them for elimination. In the cell-mediated response, cytotoxic lymphocytes directly destroy infected body cells, cancer cells, or foreign tissue. Figure 43.2 summarizes innate and acquired immunity. In this chapter, you will learn how the various cellular and chemical components of these two kinds of defense together protect vertebrates from various threats. Later in the chapter, we will also look briefly at immunity in invertebrates, which are protected solely by innate, nonspecific mechanisms.

Concept

Innate immunity provides broad defenses against infection An invading microbe must penetrate the external barriers formed by an animals skin and mucous membranes, which cover the surface and line the openings of the body A pathogen that successfully breaks through these external defenses soon encounters several innate cellular and chemical mechanisms that impede its attack on the body.

*-• Figure 43.2 Overview of vertebrate defenses against bacteria, viruses, and other p a t h o g e n s . Defenses in vertebrates can be divided into inijiate and acquired immunity. If an invading pathogen breaches the body's external innate defenses, various internal innate defenses quiickly come into play. The defenses provided by acquired injimunity against specific prthogens develop more slowly. Sme components of innate irfimunity also function in a quired immunity.

INNATE IMMUNITY

ACQUIRED IMMUNITY

broad range of microbes

Slower responses to specific microbes

External defenses

Internal defenses $~ Phagocytic cells

Humors response

$*• Mucous membranes

& Antimicrobial proteins

(antibodies)

%>• Secretions •

p- Inflammatory response

•

Skin

8* Natural killer cells

' Cell-mediated response (cytotoxic lymphocytes)

I xternal Defenses I itact skin is a barrier that cannot normally be penetrated by ruses or bacteria, but even tiny abrasions may allow their passage. Likewise, the mucous membranes lining the digestive, respiratory, and genitourinary tracts bar the entry of potentially harmful microbes. Certain cells of these mucous membranes also produce mucus, a viscous fluid that traps ijnicrobes and other particles. In the trachea, for example, ciliated epithelial cells sweep mucus and any entrapped microbes upward, preventing the microbes from entering the iungs (Figure 43.3). Microbial colonization of the body is also inhibited by the washing action of the mucous secretions, saliva, and tears that constantly bathe the surfaces of various bxposed epithelia. leyond their physical role in inhibiting microbe entry; secretions of the skin and mucous membranes provide an environment that is often hostile to microbes. In humans, secretions from sebaceous (oil) glands and sweat glands give the skin a pH ranging from 3 to 5, which is acidic enough to prevent colonization by many microbes. (Bacteria that normally inhabit the skin are adapted to its acidic, relatively dry environment.) Similarly, microbes in food or water and those in swallowed mucus must contend with the acidic environment of the stomach, which destroys most pathogens before they can enter the intestines. But some pathogens, such as the aepatitis A virus, can survive gastric acidity and successfully enter the body via the digestive tract. Secretions from the skin and mucous membranes also contain antimicrobial proteins. One such protein is lysozyme, an enzyme that digests the cell walls of many bacteria. Present in saliva, tears, and mucous secretions, lysozyme can destroy susceptible bacteria as they enter the upper respiratory tract or the openings around the eyes.

Internal Cellular and Chemical Defenses Microbes that penetrate the body's external defenses, such as those that enter through a break in the skin, must contend

A Figure 43.3 External innate defense by mucous m e m b r a n e s . The lining of the trachea contains mucus-producing cells (orange) and cells with cilia (yellow). Synchronized beating of the cilia expels mucus and trapped microbes upward into the pharynx (colorized SEM).

with the body's internal mechanisms of innate defense. These defenses depend mainly on phagocytosis, the mgestion of invading microorganisms by certain types of white blood cells generically referred to as phagocytes. These cells produce certain antimicrobial proteins and help initiate inflammation, which can limit the spread of microbes in the body. Nonphagocytic white blood cells called natural killer cells also play a key role in innate defenses. The various nonspecific mechanisms help limit the spread of microbes before the body can mount acquired, specific immune responses. Phagocytic Cells Phagocytes attach to their prey via surface receptors that bind to structures found on many microorganisms but not on normal body cells. Among the structures bound by these receptors are certain polysaccharides on the surface of bacteria. CHAPTER 43

The Immune Svstem

899

After attaching to one or more microbes, a phagocyte engulfs the microbes, forming a vacuole that fuses with a lysosome (Figure 43.4). Microbes are destroyed by lysosomes in two ways. First, nitric oxide and other toxic forms of oxygen contained in the lysosomes may poison the engulfed microbes. Second, lysozyme and other enzymes degrade microbial components. Some microorganisms have adaptations that enable them to evade destruction by phagocytic cells. For example, the outer capsule that surrounds some bacterial cells hides their surface polysaccharides and prevents phagocytes from attaching to them. Other bacteria, such as Mycobacterium tuberculosis, which causes tuberculosis, are readily bound and engulfed by phagocytes but are resistant to destruction within lysosomes. Because such microbes can grow and reproduce within their host's cells, they are effectively hidden from the acquired defenses of the body Evolution of these and other mechanisms that prevent destruction by the immune system has increased the pathogenic threat of many microbes. Four types of white blood cells (leukocytes) are phagocytic. They differ in their abundance, average life span, and phagocytic ability. By far the most abundant are neutrophils, which constitute about 60-70% of all white blood cells. Neutrophils are attracted to and then enter infected tissue, engulfing and destroying the microbes there. However, neutrophils tend to self-destruct in the process of phagocytosis, and their average life span is only a few days.

Microbes

An even more effective phagocytic defense comes from macrophages ("big eaters11)- These large, long-lived cells develop from monocytes, which constitute about 5% of cirp culating white blood cells. New monocytes circulate in the blood for only a few hours and then migrate into tissues where they are transformed into macrophages. Carrying out phagocytosis sets off internal signaling pathways that activate the macrophages, increasing their defensive abilities in vari ous ways (described later in this chapter). Some macrophage.^ migrate throughout the body, while others reside permanently in various organs and tissues. Macrophages that are perma nent residents in the spleen, lymph nodes, and other tissues o the lymphatic system are particularly well positioned to comba infectious agents. Microbes that enter the blood become trapped in the netlike architecture of the spleen, whereas microbes ii interstitial fluid flow into lymph and are trapped in lympl nodes. In either location microbes soon encounter residen macrophages. Figure 43.5 shows the components of the lym phatic system and summarizes its role in the body's defenses. The other two types of phagocytes are less abundant anc play a more limited role in innate defense than neutrophils and macrophages. Eosinophils have low phagocytic activity but are critical to defense against multicellular parasitic invaders, such as the blood fluke Schlstosoma mansoni. Rathei than engulfing such a parasite, eosinophils position themselves against the parasites body and then discharge destructive enzymes that damage the invader. The fourth type of phagocyte dendritic cells, can ingest microbes like macrophages do However, as you will learn later in the chapter, their primary role is to stimulate the development of acquired immunity. Antimicrobial Proteins

0 Toxic compounds and lysosomal enzymes destroy microbes. 0 Microbial debris is released by exocytosis. A Figure 43.4 Phagocytosis. This schematic depicts events following attachment of one kind of phagocyte, a macrophage, to microbes via its surface receptors (not shown). The process is similar in other types of phagocytic cells.

900

U N I T SEVEN

Animal Form and Function

Numerous proteins function in innate defense by attacking microbes directly or by impeding their reproduction. You have already learned about the antimicrobial action of lysozyme. Other antimicrobial proteins include about 30 serum proteins that make up the complement system. In the absence of infection, these proteins are inactive. Substances on the surface of many microbes, however, can trigger a cascade of steps that activate the complement system, leading to lysis (bursting) of invading cells. Certain complement proteins also help trig inflammation or play a role in acquired defense. Two types of interferon (a and |3) provide innate defense against viral infections. These proteins are secreted by virusinfected body cells and induce neighboring uninfected cells to produce other substances that inhibit viral reproduction. In this way, mterferons limit the cell-to-cell spread of viruses in the body, helping control viral infections such as colds and influenza. This innate defense mechanism is not virus-specific; interferons produced in response to one virus may also confei short-term resistance to unrelated viruses. Certain lymphocytes secrete a third type of interferon (7) that helps activate

O Interstitial fluid bathing the tissues, along with the white blood cells in it, continually enters lymphatic capillaries.

©Fluid inside the lymphatic capillaries, called lymph, flows through lymphatic vessels throughout the body.

Lymphatic vessels r turn lymph to the ilood via two large jets that drain into veins near the shoulders.

© W i t h i n lymph nodes, microbes and foreign particles present in the circulating lymph encounter macrophages, dendritic cells, and lymphocytes, which carry out various defensive actions.

A Figure 43.5 The human lymphatic system. The lymphatic system consists of lymphatic vessels, through which lymph travels, and various structures that trap "foreign" molecules and articles. These structures include the adenoids, tonsils, lymph nodes, spleen, Peyer's patches, and ppendix. The flow of lymph is traced in steps 1-4.

nacrophages, enhancing their phagocytic ability. Interferons can iow be mass-produced by recombinant DNA technology and re being tested for the treatment of viral infections and cancer. Yet another group of antimicrobial proteins, fittingly named efcnsins, are secreted by activated macrophages. These small iroteins damage broad groups of pathogens by various mechanisms without harming body cells. Inflammatory Response 3amage to tissue by physical injury or the entry of pathogens eads to release of numerous chemical signals that trigger a ocalized inflammatory response. One of the most active chemicals is histamine, which is stored in mast cells found n connective tissues. When injured, mast cells release their histamine, triggering dilation and increased permeability of nearby capillaries. Activated macrophages and other cells discharge additional signals, such as prostaglandins, that further promote blood flow to the injured site. The resulting increased local blood supply causes the redness and heat typical of inflammation (from the Latin injlammare, to set on fire). The blood-engorged capillaries leak fluid into neighboring tissues, causing swelling, another sign of local inflammation.

Although heat and swelling are uncomfortable sensations, the enhanced blood How and vessel permeability that cause them are critical to innate defense. These vascular changes help deliver antimicrobial proteins and clotting elements to the injured area. Several activated complement proteins, for example, promote the release of histamine or attract phagocytes to the site. Blood clotting begins the repair process and helps block the spread of microbes to other parts of the body. In addition, increased local blood flow and vessel permeability allow more neutrophils and monocyte-macrophages to move from the blood into injured tissues. Small proteins called chemokines direct the migration of these phagocytes and signal them to increase production of microbe-killing compounds. Chemokines are secreted by many cell types, including blood vessel endothelial cells, near a site of injury or infection. Figure 43.G, on the next page, summarizes the major events in local inflammation resulting from an infected pinprick. A minor injury causes local inflammation, but the body may also mount a systemic (widespread) response to severe tissue damage or infection. Injured cells often put out a call for reinforcements, secreting chemicals that stimulate the release of additional.neutrophils from the bone marrow. In a severe infection, such as meningitis or appendicitis, the CHAPTER 43 The Immune System

901

0 Chemical signals released by activated macrophages and mast cells at the injury site cause nearby capillaries to widen and become more permeable.

Fluid, antimicrobial proteins, and clotting elements move from the blood to the site. Clotting begins.

Chemokines released by various kinds of cells attract more phagocytic cells from the blood to the injury site.

© Neutrophils and macrophages phagocytose pathogens and cell debris at the site, and the tissue heals.

A Figure 43.6 Major events in the local inflammatory response.

number of white blood cells in the blood may increase severalfold within a few hours of the initial inflammatory events. Another systemic response to infection is fever. Fever may occur when certain toxins produced by pathogens and substances released by activated macrophages set the body's thermostat at a higher temperature. A very high fever is dangerous, bat moderate fever may facilitate phagocytosis and, by speeding up body reactions, hasten the repair of tissues. Certain bacterial infections can induce an overwhelming systemic inflammatory response, leading to a condition known as septic shock. Characterized by very high fever and low blood pressure, septic shock is a common cause of death in hospital critical care units. Clearly, local inflammation is essential for healing, but systemic inflammation can be devastating.

Natural Killer Cells We wrap up our discussion of vertebrate innate defenses with natural killer (NK) cells. NK cells patrol the body and attack virus-infected body cells and cancer cells. Surface receptors on an NK cell recognize general features on the surface of its targets. Once it is attached to a virus-infected cell or cancer cell, the NK cell releases chemicals that lead to the death of the stricken cell by apoptosis, or programmed cell death (see Figure 21.18). Athough the defense provided by NK cells is not 100% effective, viral infections and cancer would occur much more often without these innate sentinels of the body

Invertebrate Immune Mechanisms Before we move on to examine acquired immunity in vertebrates, we should note that invertebrates also have highly 902

UNIT SEVEN Animal Form and Function

effective innate defenses. For example, sea stars possess amoe boid cells that ingest foreign matter by phagocytosis and se crete molecules that enhance the animal's defensive respons Recent studies of the fruit fly Drosophila melanogaster also havf revealed striking parallels between insect defenses and verte brate innate defenses. The exoskeleton of insects, like the skir and mucous membranes of vertebrates, provides an extema barrier that can prevent entry of intruders. If an insects ex oskeleton is damaged, pathogens that enter the insect's bod\ must face several internal innate defenses. The insect equivalent to blood, the hemolymph, contain; circulating cells called hemocytes. Some hemocytes ingest bac teria and other foreign substances by phagocytosis, whik other hemocytes form a cellular capsule around large para sites. The presence of pathogens signals still other hemocytes to make and secrete various antimicrobial peptides that bind to their pathogen targets, leading to death of the pathogens The internal signaling pathways that trigger hemocytes to produce antimicrobial peptides are comparable to those that activate vertebrate macrophages. In addition, certain hemocytes contain the enzyme phenoloxidase. Once activated, this enzyme converts phenols to reactive compounds that link together into large aggregates. These are deposited around parasites and wounded tissue, helping to prevent the spread o: parasites beyond the affected area. Activation of phenoloxidase in insects occurs by a cascade of steps similar to those that activate vertebrate complement proteins. Recent research indicates that invertebrates lack cells analogous to lymphocytes, the white blood cells responsible for acquired, specific immunity in vertebrates (see Figure 43.2). But even though they depend on innate, nonspecific mechanisms,

certain invertebrate defenses do exhibit some features characteristic of acquired immunity. For instance, acquired immunity usually is directed against nonself cells, not normal body (self) cells. The ability to distinguish self from nonself is seen in the most ancient invertebrate lineage, the sponges. If cells from two sponges of the same species are mixed, the cells from each sponge sort themselves and reaggregate, excluding cells from the other individual. Another hallmark of acquired immunity is immunological memory—the ability to respond more quickly to a particular invader or foreign tissue the second time it is encountered. Earthworms exhibit something like this: Phagocytic cells in a worm attack a second graft from the same donor worm much mjore rapidly than the first graft. Most invertebrates, however, exhibit no such irnmunological memory. Following this glimpse at invertebrate host defenses, we begin in the next section to examine the highly developed mechanisms of acquired immunity found in vertebrates. [Concept Check 1. Innate defenses are nonspecific. How, then, do macrophages recognize an infectious agent, such as a bacterium? 2. What causes the common signs of inflammationredness, swelling, and heat—and how do these changes help protect the body against infection? 3. State two ways in which the innate defenses of insects (invertebrates) and vertebrates are similar. For suggested answers, see Appendix A.

Concept

In acquired immunity, lymphocytes provide specific defenses against infection While invading pathogens are under assault by a vertebrate's innate defenses, the intruders inevitably come into contact with lymphocytes, the key cells of acquired immunity—the body's second major kind of defense (see Figure 43.2). Direct contact with microbes and signals from active innate defenses cause lymphocytes to join the fight. For example, as macro^hages and dendritic cells phagocytose microbes, the phagocytes begin to secrete cytokines, proteins that help activate ymphocytes and other cells of the immune system. This is just one example of how innate and acquired defenses interact. Any foreign molecule that is specifically recognized by lymphocytes and elicits a response from them is called an antigen. Most antigens are large molecules, either proteins or

Epitopes (antigenic determinants)

a -Antibody B

Antibody C-

.4 Figure 43.7 Epitopes (antigenic determinants). Only small, specific regions on antigens, called epitopes, are bound by the antigen receptors on lymphocytes and by secreted antibodies. In this example, three different secreted antibody molecules react with different epitopes on the same large antigen molecule.

polysaccharides. Some antigens, such as toxins secreted by bacteria, are dissolved in extracellular fluid, but many protrude from the surface of pathogens or transplanted cells. A lymphocyte actually recognizes and binds to just a small, accessible portion of an antigen, called an epitope or antigenic determinant. A single antigen usually has several different epitopes, each capable of inducing a response from lymphocytes that recognize that epitope. Antibodies, which are secreted by certain lymphocytes in response to antigens, likewise bind to specific epitopes (Figure 43.7). In this section, we first describe how lymphocytes recognize antigens, or more specifically the epitopes on antigens. We then trace how the vertebrate body becomes populated with a large set of lymphocytes that collectively can recognize and mount a pinpoint attack against any one of a multitude of antigens. Later in the chapter we will examine the various defensive actions of the different types of lymphocytes.

Antigen Recognition by Lymphocytes The vertebrate body is populated by two main types of lymphocytes: B lymphocytes (B cells) and T lymphocytes (T cells). Both types of lymphocytes circulate through the blood and lymph and are concentrated in the spleen, lymph nodes, and other lymphoid tissues (see Figure 43.5). B cells and T cells recognize antigens by means of antigen-specific receptors embedded in their plasma membranes. A single B or T cell bears about 100,000 of these antigen receptors, and all the receptors on a single cell are identical—that is, they all recognize the same epitope. In other words, each lymphocyte displays specificity for a particular epitope on an antigen and defends against that antigen or a small set of closely related antigens. B Cell Receptors for Antigens Each B cell receptor for an antigen is a Y-shaped molecule consisting of four polypeptide chains: two identical heavy chains and two identical light chains linked by disulfide bridges. A region in the tail portion of the molecule, the transmembrane CHAPTER 43 The Immune System

903

region, anchors the receptor in the cell's plasma membrane, and a short region at the end of the tail extends into the cytoplasm. At the tips of the Y are the light- and heavy-chain variable (V) regions (Figure 43.8a), so named because their amino acid sequences vary extensively from one B cell to another. The remainder of the molecule is made up of the constant (C) regions, whose amino acid sequences vary little from cell to cell. As shown in Figure 43.8a, each B cell receptor has two identical antigen-binding sites. The unique contour of each binding site is formed from part of a heavy-chain V region and part of a light-chain V region. The interaction between an antigen-binding site and its corresponding antigen is stabilized by multiple noncovalent bonds between chemical groups on the respective molecules. The antigens bound by B cell receptors in this way include molecules that are on the surface of, or are released from, all types of infectious agents. In other words, a B cell recognizes an intact antigen in its native state. Secreted antibodies, or immunoglobulins, are structurally similar to B cell receptors, but they lack the transmembrane regions that anchor receptors in the plasma membrane. Because of this structural similarity, B cell receptors are often called membrane antibodies or membrane immunoglobulins.

T Cell Receptors for Antigens and the Role of the MHC Each T cell receptor for an antigen consists of two different polypeptide chains, an a chain and a [3 chain, linked by a disulfide bridge (Figure 43.8b). Near the base of (.he molecule is a transmembrane region that anchors the molecule in the

cell's plasma membrane. At the outer tip of the molecule, the a and (3 chain variable (V) regions form a single antigenbinding site. The remainder of the molecule is made up of the constant (C) regions. T cell receptors recognize and bind with antigens just as specifically as B cell receptors. However, while the receptors on B cells recognize intact antigens, the receptors on T cells recognize small fragments of antigens that are bound to normal cellsurface proteins called MHC molecules. MHC molecules are so named because they are encoded by a family of genes called the major histocompatibility complex (MHC). As a newly synthesized MHC molecule is transported toward the plasma membrane, it binds with a fragment of protein (peptide) antigen within the cell and brings it to the cell surface, a process called antigen presentation. A nearby T cell can detect the antigen fragment thus displayed on the cell's surface (Figure 43.9). There are two ways in which foreign antigens can end up inside cells of the body. Depending on their source, these peptide antigens are handled by a different class of MHC molecule and recognized by a particular subgroup of T cells: •- Class I MHC molecules, found on almost all nucleated cells of the body, bind peptides derived from foreign antigens that have been synthesized within the cell. Any body cell that becomes infected or cancerous can display such peptide antigens by virtue of its class I MHC molecules. Class I MHC molecules displaying bound peptide antigens are recognized by a subgroup of T cells called cytotoxii T cells (see Figure 43.9a).

(a) A B cell receptor consists of t w o identical heavy chains and two identical light chains linked by several disulfide bridges.

(b) A T cell receptor consists of one a chain and one P chain linked by a disulfide bridge.

A Figure 43.8 Antigen receptors on lymphocytes. All the antigen receptors on a particular B cell or T cell bind the same antigen. The variable (V) regions vary extensively from cell to cell, accounting for the different binding specificities of individual lymphocytes; the constant (C) regions vary little or not at all.

904

UNIT SEVEN

Animal Form and Function

> Figure 43.9 The interaction of T cells w i t h MHC molecules. Class i and class II MHC molecules display peptide fragments of antigens to (a) cytotoxic T cells and (b) helper T cells, respectively. In each case, the T cell receptor binds with an MHC molecule-peptide antigen complex. Class I MHC molecules are made by most nucleated cells, whereas class II MHC molecules are made primarily by antigenpresenting cells (macrophages, dendritic cells, and B cells).

O A fragment of foreign protein (antigen) inside the cell associates with an MHC molecule and is transported to the cell surface.

Antigen-^ fragment

Class 1 MHC molecule

Hy W* The combination of MHC molecule and antigen is recognized by a T cell, alerting it to the infection.

Cytotoxic T cell

Helper T cell

+• Class II MHC molecules are made by just a few cell types, mainly dendritic cells, macrophages, and B cells, in these cells, class 11 MHC molecules bind peptides derived from foreign materials that have been internalized and fragmented through phagocytosis or endocytosis. Dendritic cells, macrophages, and B cells are known as antigen-presenting cells because of their key role in displaying such internalized antigens to another subgroup of T cells called helper T cells (see Figure 43.9b). Each vertebrate species possesses numerous different alleles for each class I and class II MHC gene; MHC proteins are the most polymorphic known. Because of the large number of different MHC alleles in the human population, most of us are heterozygous for every one of our MHC genes and produce a broad array of MHC molecules. Collectively these molecules are capable of binding to and presenting a large number of peptide antigens. Moreover, any two people, except identical twins, are very unlikely to have exactly the same set of MHC molecules. Thus the MHC provides a biochemical fingerprint, unique to virtually every individual, that marks body cells as "self." In fact, the discovery of the MHC occurred in the process of studying the phenomena of skin graft rejection and acceptance.

Lymphocyte Development Now that you have read how lymphocytes recognize antigens, let's examine how these cells develop and populate the vertebrate body Like all blood cells, lymphocytes originate from pluripotent stem cells in the bone marrow (see Chapter 42). Newly formed lymphocytes are all alike, but they later develop into either T cells or B cells, depending on where they continue their maturation (Figure 43.10). Lymphocytes that migrate from the bone marrow to the thymus, a gland in the thoracic cavity above the heart, develop into T cells ("T" for thymus).

-Thymus

-T cell

Blood, iymph, and lymphoid tissues (lymph nodes, spleen, and others) A Figure 43.10 Overview of lymphocyte development. Lymphocytes arise from stem cells in the bone marrow and differentiate without any contact with antigens. B cells develop entirely in the bone marrow, whereas T cells complete their development in the thymus. Mature lymphocytes, each specific for a particular epitope, circulate in the blood and lymph to various lymphoid tissues, where they encounter antigens.

Lymphocytes that remain in the bone marrow and complete their maturation there become B cells. (The "B" actually stands for the bursa of Fabricius, an organ unique to birds where avian B cells mature and the site where B cells were first discovered. But you can think of "B': as standing for bone marrow, since that is where B cells mature in other vertebrates.) CHAPTER 43

The Immune System

905

antigen-binding specificity Thus, our repertoire of lymphocytes can respond to an enormous number of different antigens. At the core of lymphocyte diversity are the unique genes that encode the antigen receptor chains. These genes consist of numerous coding gene segments that undergo random, permanent rearrangement, forming functional genes that can be expressed as receptor chains. In this discussion we will focus on the gene coding for the light chain of the B cell receptor (membrane immunoglobulin), but keep in mind that the genes for the heavy chain and for the a and |3 chains of the T cell receptor undergo similar rearrangements. The light and heavy chains of the B cell receptor and of secreted antibody are encoded by the same genes, called immunoglobulin (Ig) genes. The immunoglobulin light-chain gene contains a series of 40 variable (V) gene segments separated by a long stretch of DNAfrom 5 joining (J) gene segments (Figure 43.11). Beyond the J gene segments is an intron, followed by a single exon designated C because it codes for the constant region of the light chain (see Figure 17.10 to review introns and exons). In this state, the light-chain gene is not functional. However, early in B cell development, a set of enzymes collectively called recombinase links one V gene segment to one J gene segment, eliminating the long stretch of DNA between them and forming a single exon that is part V and part j. Recombinase acts randomly; that is, it can link any one of the 40 V" gene segments to any one of the 5 J gene segments. So,

There are three key events in the life of a lymphocyte. The first two events take place as a lymphocyte matures in the bone marrow (B cell) or thymus (T cell), well before the cell has contact with any antigen. The third event occurs when a mature lymphocyte encounters and binds a specific antigen, leading to its activation, proliferation, and differentiation, a process called clonal selection. Here we describe these three developmental events in the order of their occurrence. Generation of Lymphocyte Diversity by Gene Rearrangement Recall that B cells and T cells recognize specific epitopes on antigens via their antigen receptors. A single B cell or T cell bears about 100,000 of these receptors, all identical. However, if we were to compare one B cell to another (or one T cell to another), the chances that both would have the same B cell receptor (or T cell receptor) are very slim. The variable regions at the tip of each antigen receptor chain, which form the antigenbinding site, account for the diversity of lymphocytes (see Figure 43-8). The sequence of amino acids in these regions varies from cell to cell and determines the specificity of an antigen receptor. The variability of these regions, and therefore the possible antigen specificities, is enormous. It is estimated that each person has as many as 1 million different B cells and 10 million different T cells, each with a particular

V4-V39 DNA of undifferentiated Bcell

—/ I

1HE]

0 Deletion of DNA between a V segment and J segment and joining of the segments

• Figure 43.11 Immunoglobulin gene rearrangement. The random joining of V and J gene segments (V3 and Js in this example) results in a functional gene that encodes the light-chain polypeptide of a B cell receptor. In a cell that produces secreted antibody, the same premRNA is formed, but alternative processing yields an mRNA that lacks coding sequences for the transmembrane region that anchors receptors in the membrane. Gene rearrangement plays a major role in generating a diverse repertoire of lymphocytes and secreted antibodies.

906

UNIT SEVEN

DNA of differentiated Bcelt Functional gene @ Transcription of resulting permanently rearranged, functional gene pre-mRNA

© RNA processing (removal of intron; addition of cap and poly (A) tail) mRNA

Cap

Light-chain pofypepttde 1

Animal Form and Function

V

intron

for the light-chain gene, there are 200 possible gene products (40 V X 5 J). In any one cell, however, only one of the 200 possible light chains is made. Once V-j rearrangement has occurred, the gene can be transcribed. The resulting pre-mRNA is processed, forming mRNA that is translated into a light chain containing a variable region and a constant region. The light chains produced in this way combine randomly with the heavy chains that are similarly produced, forming the B cell's antigen receptors (see Figure43.il). Testing and Removal of Self-Reactive Lymphocytes Because the rearrangements of antigen receptor genes are random, a developing lymphocyte may end up with antigen receptors that are specific for some of the body's own molecules. As B cells and T cells are maturing in the bone marrow and thymus, respectively, their antigen receptors are tested for potential self-reactivity For example, the receptors of maturing T cells are tested against class I and class II MHC molecules, which are both expressed at high levels in the thymus. For the most part, lymphocytes bearing receptors specific for molecules already present in the body are either destroyed by apoptosis or rendered nonfunctional, leaving only lymphocytes that react to foreign (nonself) molecules. This capacity to distinguish self from nonself continues to develop even

as the cells migrate to lymphoid organs. Thus, the body normally has no mature lymphocytes that react against self components: The immune system exhibits the critical feature of self-tolerance. Failure of self-tolerance can lead to autoimmune diseases such as multiple sclerosis, as you will read later. Clonal Selection of Lymphocytes A soluble antigen or an antigen present on the surface of a microbe, infected body cell or cancer cell encounters a large repertoire of B cells and T cells m the body. However, a given antigen interacts only with the relatively few lymphocytes bearing receptors specific for epitopes on that antigen. The selection of a B cell or T cell by an antigen activates the lymphocyte, stimulating it to divide many times and to differentiate, forming two clones of daughter cells. One clone consists of a large number of short-lived effector cells that combat the same antigen. The nature and function of the effector cells depend on whether the lymphocyte selected is a helper T cell, a cytotoxic T cell, or a B cell. The other clone consists of memory cells, long-lived cells bearing receptors specific for the same inducing antigen. This antigen-driven cloning of lymphocytes is called clonal selection (Figure 43.12). The concept of clonal selection is so fundamental to understanding acquired immunity that it is worth restating: Each antigen, by binding to specific receptors,

Antigen molecules bind to the antigen receptors of only one of the three B cells shown.

The selected B cell proliferates, forming a clone of identical cells bearing receptors for the selecting antigen.

Some proliferating cells develop into long-lived memory cells that can respond rapidly upon subsequent exposure to the same antigen.

Cione of memory cells

Clone of plasma cells

Some proliferating cells develop into short-lived plasma cells that secrete antibodies specific for the antigen.

A Figure 43.12 Clonal selection of B cells. A B cell is selected by an antigen to proliferate and differentiate into memory B cells and antibody-secreting plasma cells. All the B cells whose receptors have a different specificity (indicated by different shapes and colors) do not respond to this antigen. T lymphocytes undergo a similar process, generating memory T cells and effector T cells. CHAPTER 43 The Immune System

907

selectively activates a tiny fraction of cells from the body's diverse pool of lymphocytes; this relatively small number of selected cells gives rise to clones of thousands of cells, all specific for and dedicated to eliminating that antigen. The selective proliferation and differentiation of lymphocytes that occurs the first time the body is exposed to a particular antigen represents the primary immune response. In the primary response, the maximum effector response does not occur until about 10 to 17 days after the initial exposure to the antigen. During this time, selected B cells generate antibodysecreting effector B cells, called plasma cells, and selected T cells are activated to their effector forms, which have distinct functions (discussed in the next section). While these effector cells are developing, a stricken individual may become ill. Eventually, symptoms of illness diminish and disappear as antibodies and effector T cells clear the antigen from the body. If an individual is exposed again to the same antigen, the response is faster (typically only 2 to 7 days), of greater magnitude, and more prolonged. This is the secondary immune response. Measures of antibody concentrations in the blood serum over time show clearly the difference between primary and secondary immune responses (Figure 43.13). In addition to being more numerous, antibodies produced in the secondary response tend to have greater affinity for the antigen than those secreted in the primary response. The immune systems capacity to generate secondary immune responses, called immunological memory, depends on the

Q Secondary response to antigen A produces antibodies to A; primary response to antigen B produces antibodies to E

O Day 1: First exposure to antigen A @ Primary response to

© Day 28: Second exposure to antigen A; first

clones of long-lived T and B memory cells generated following initial exposure to an antigen. These memory cells are poised to proliferate and differentiate rapidly when they later contact the same antigen. The long-term protection developed after exposure to a pathogen was recognized 2,400 years ago by Thucydides of Athens, who described how those sick and dying of plague were cared for by others who had recovered, "for no one was ever attacked a second time."

Concept Check 1. Draw a B cell receptor, and label the following: light chains, heavy chains, disulfide bridges, variable (V) regions, constant (C) regions, antigen-binding sites, transmembrane region, and cytoplasmic tails. How does the structure of a secreted antibody differ? 2. What is the major difference in the types of antigens bound by B cell receptors and T cell receptors? 3. Consider the process of clonal selection of B cells shown in Figure 43.12. How does this process demonstrate both the specificity and memory of acquired immunity? 4. A light-chain immunoglobulin gene consists of 40 V gene segments and 5 J gene segments, and a heavychain gene consists of 51 Vgene segments, 6Jgene segments, and another set of gene segments D, of which there are 27. How many different antigenbinding specificities can be generated, given random V-J and V-D-J rearrangements? For suggested answers, see Appendix A.

Concept

Humoral and cell-mediated immunity defend against different types of threats

A Figure 43.13 The specificity of immunological memory. Long-lived memory cells generated in the primary response to antigen A give rise to a heightened secondary response to the same antigen, but do not affect the primary response to a different antigen (B).

908

UNIT SEVEN

Animal Form and Function

Early evidence that substances in the blood plasma and lymph play a role in acquired immunity came from experiments performed near the end of the 19th century. Researchers transferred such fluids (long ago called humors) from an animal that had recovered from an infection by a particular microbe to another animal that had not been exposed to that microbe. Later, when the second animal was infected with the same microbe, it did not become ill. The investigators had transferred what we now know were secreted antibodies from one animal to the other. They also found that immunity to some infections could be passed along only if certain cells, later identified as cytotoxic T cells, were transferred.

• Figure 43.14 An overview of the acquired immune response. Stimulation

Cell-mediated immune response

Humoral immune response

of h e l p e r ! cells by an antigen generally requires direct contact between a dendritic cell and helper T cell in a primary response (shown here) or between a rnacrophage and memory T ceil in a secondary response (not shown). Once activated, a helper T cell stimulates the humoral response directly by contacting B cells and indirectly by secreting cytokines. An activated helper T cell stimulates the cellmediated response indirectly via cytokines.

First exposure to antigen

Antigens engulfed and displayed by dendritic cells

Secrete antibodies that defend against pathogens and toxins in extracellular fluid

These and many other studies led to the current understanding that acquired immunity includes two branches. The humoral immune response involves the activation and clonal selection of B cells, resulting in production of secreted antibodies that circulate in the blood and lymph. The cell-mediated immune response involves the activation and clonal selection of cytotoxic T cells, which directly destroy certain target cells. Figure 43.14 outlines the roles of the major participants in acquired immune responses. Central to this network of cellular interactions is the helper T cell, which responds to peptide antigens displayed on antigen-presenting cells and in turn stimulates the activation of nearby B cells and cytotoxic T cells. Now lets fill in the details of this road map and examine how each branch of the acquired immune response defends against particular types of assault.

Helper T Cells: A Response to Nearly All Antigens When a helper T cell encounters and recognizes a class II MHC molecule—antigen complex on an antigenpresenting cell, the helper T cell proliferates and differen-

§

Antigens displayed by infected cells

Defend against infected cells, cancer cells, and transplanted tissues

tiates into a clone of activated helper T cells and memory helper T cells. A surface protein called CD4, present on most helper T cells, binds the class II MHC molecule. This interaction helps keep the helper T cell and the antigen-presenting cell joined while activation of the helper T cell proceeds. Activated helper T cells secrete several different cytokines that stimulate other lymphocytes, thereby promoting cell-mediated and humoral responses. The helper T cell itself is also subject to regulation by cytokines. For instance, as a dendritic cell presents an antigen to a helper T cell, the dendritic cell is stimulated to secrete cytokines that, along with an antigen, stimulate the helper T cell to produce its own set of cytokines. As you've read, the class II MHC molecules recognized by helper T cells are found mainly on dendritic cells, macrophages, and B cells. Dendritic cells are particularly effective in presenting antigens to naive helper T cells, so called because they have not previously detected an antigen. In other words, dendritic cells are important in triggering a primary immune response. Dendritic cells are located in the epidermis and in many other tissues, where they efficiently capture antigens. They then migrate from the site of infection to various lymphoid tissues, where they present antigens, via class II MHC molecules, to helper T cells The Immune System

909

O After a dendritic cell engulfs and degrades a bacterium, it displays bacterial antigen fragments (peptides) complexed with a ciass II MHC molecule on the celi surface. A specific helper T cell binds to the displayed complex via its TCR with the aid of CD4. This interaction promotes secretion of cytokines by the dendritic cell. Cytotoxic T cell

Dendritic

Cell-mediated immunity (attack on infected cells)

Humoral immunity (secretion of antibodies by plasma cells)

Cytokines

Proliferation of the T cell, stimulated by cytokines from both the dendritic cell and the T cell itself, gives rise to a clone of activated helper T cells (not shown), all with receptors for the same MHC-antigen complex.

(Figure 43.15). Macrophages play the key role in initiating a secondary immune response by presenting antigens to memory helper T cells, while B cells primarily present antigens to helper T cells in the course of the humoral response.

Cytotoxic T Cells: A Response to Infected Cells and Cancer Cells Cytotoxic T cells, the effectors oi cell-mediated immunity, eliminate body cells infected by viruses or other intracellular pathogens as well as cancer cells and transplanted cells. Fragments of nonself proteins synthesized in such target cells associate with class I MHC molecules and are displayed on the cell surface, where they can be recognized by cytotoxic T cells. A surface protein called CD8, present on most cylotoxic T cells, greatly enhances the interaction between a target cell and a cytotoxic T cell. Binding of CD8 to the side of a class I MHC molecule helps keep the two cells in contact during activation of the cytotoxic T cell. Thus, the role of class I MHC molecules and CD8 is similar to that of class II MHC molecules and CD4, except that different cell types are involved. When a cytotoxic T cell is selected by binding to class 1 MHC molecule-antigen complexes on an infected body cell, the cytotoxic T cell is activated and differentiates into an active killer. Cytokines secreted from nearby helper T cells promote this activation. The activated cytotoxic T cell then secretes proteins that act on the bound infected cell, leading to its destruction (Figure 43.16). The death of the infected cell not only 910

UNIT SEVEN

Animal Form and Function

I The cells in this clone secrete other cytokines that help activate B cells and cytotoxic T cells.

deprives the pathogen of a place to reproduce but also exposes it to circulating antibodies, which mark it for disposal. After destroying an infected cell, the cytotoxic T cell may move on and kill other cells infected with the same pathogen. In the same way cytotoxic T cells defend against malignant tumors. Because tumor cells carry distinctive molecules (tumor antigens) not found on normal body cells, they are identified as foreign by the immune system. Class I MHC molecules on a tumor cell display fragments of tumor antigens to cytotoxic T cells. Interestingly, certain cancers and viruses (such as the Epstein-Barr virus) actively reduce the number of class I MHC molecules on affected cells, helping them escape detection by cytotoxic T cells. But the body has a backup defense: Recall that natural killer (NK) cells, part of the body's nonspecific innate defenses, can induce apoptosis in virus-infected and cancer cells.

B Cells: A Response to Extracellular Pathogens Antigens that elicit a humoral immune response are typicall} proteins and polysaccharides present on the surface of bacteria or incompatible transplanted tissue or transfused blood cells. In addition, for some people, proteins of foreign substances such as pollen or bee venom act as antigens that induce an allergic, or hypersensitive, humoral response. Figure 43.17 depicts the events in the humoral response to a typical protein antigen. The activation of B cells is aided by cytokines secreted from helper T cells activated by the same

O A specific cytotoxic T cell binds to a class i MHC-antigen complex on a target cell via its TCR with the aid of CD8. This interaction, along with Cytokines from helper T cells, leads to the activation of the cytotoxic cell.

© The granzymes initiate apoptosis within the target cells, leading to fragmentation of the nucleus, release of small apoptotic bodies, and eventual cell death. The released cytotoxic T cell can attack other target cells.

0 The activated T cell releases perforin molecules, which form pores in the target cell membrane, and proteolytic enzymes (granzymes), which enter the target cell by endocytosis.

Cytotoxic T cell

©

r

/Apoptotic / target cell

V*

• Figure 43.16 The killing action of cytotoxic T cells. After interacting with a target cell, such as an infected body cell or cancer cell, an activated cytotoxic T cell releases perforins and proteolytic enzymes (granzymes) that promote death of the target cell. The colorized SEM shows a cancer cell with a perforin-induced pore in the early stages of apoptosis. TCR = T cell receptor.

After a rnacrophage engulfs and degrades a bacterium, it displays a peptide antigen complexed with a class II MHC molecule. A helper T cell that recognizes the displayed complex is activated with the aid of cytokines secreted from the macrophage, forming a clone of activated helper T cells (not shown). Macrophage y

molecule

© A B cell that has taken up and degraded the same bacterium displays class II MHC-peptide antigen complexes. An activated helper T cell bearing receptors specific for the displayed antigen binds to the B cell. This interaction, with the aid of cytokines from the T ceil, activates the B cell.

© T h e activated B cell proliferates and differentiates into memory B cells and antibody-secreting plasma cells. The secreted antibodies are specific for the same bacterial antigen that initiated the response.

/Bacterium

\

A Figure 43.17 Humoral immune response. Most protein antigens require activated helper T cells to trigger a humoral response. Either a macrophage (shown here) or a dendritic cell can activate helper T cells. The TEM of a plasma cell reveals abundant endoplasmic reticulum, a common feature of cells dedicated to making proteins for secretion. TCR = T cell receptor. A 0 indicates stimulation, CHAPTER 43

The Immune System

911

antigen. Stimulated by both an antigen and cytokines, the B cell proliferates and differentiates into a clone ol antibody-secreting plasma cells and a clone of memory B cells. When an antigen first binds to receptors on trie surface of a B cell, the cell takes in a few of the foreign molecules by receptor-mediated endocyt'osis. In a process similar to antigen presentation by macrophages and dendritic cells, the B cell then presents antigen fragments to a helper T cell. This achieves the direct cell-cell contact critical to B cell activation (see step 2 in Figure 43.17). Note, however, that a macrophage or dendritic cell can present peptide fragments from a wide variety of antigens, whereas a B cell internalizes and presents only the antigen to which it specifically binds. Antigens that induce antibody production only with assistance from helper T cells, as shown in Figure 43.17, are known as T-dcpendent antigens. Some antigens, however, can evoke a B cell response without involvement of helper T cells. Such T-independent antigens include the polysaccharides of many bacterial capsules and the proteins that make up bacterial flagella. Apparently the repeated subunits of these molecules bind simultaneously to several antigen receptors on a single B cell, providing enough stimulus to activate the cell without the help of cytokines. The response to T-independent antigens is very important in defending against many bacteria. However, this response is generally weaker than the response to T-dependent antigens and generates no memory B cells. Most antigens recognized by B cells contain multiple epitopes. For this reason, exposure to a single antigen normally stimulates a variety of different B cells, each giving rise to a clone of thousands of plasma cells. All the plasma cells in one clone secrete antibodies specific for the epitope that provoked their production. Each plasma cell secretes an estimated 2,000 antibody molecules per second over the cell's 4- to 5-day life span. Next we look more closely at antibodies and how they mediate the disposal of antigens.

The power of antibody specificity and antigen-antibody binding has been harnessed in laboratory research, clinical diagnosis, and the treatment of diseases. Some of these antibody r I tools are polyclonal: They are the products of many different clones of B cells, each specific for a different epitope. Antibodies

• Only \g class that crosses placenta, thus conferring passive immunity on fetus • Promotes opsonization, neutralization, and agglutination of antigens; less effective in complement activation than IgM (see Figure 43.19) IgA (dimer)

> Provides localized defense of mucous 7 membranes by agglutination and 1 I chain neutralization of antigens {see Figure 43.19) Secretory—Cp>

component Vj r\

912

UNIT SEVEN

Animal Form and. Function

•

> Presence in breast milk confers passive immunity on nursing infant

•Hi

• • • H

• Triggers release from mast ceils sn basophils of histarnine and other chemicals that cause allergic react (see Figure 43.20)

Antibody Classes A secreted antibody has the same general Y-shaped structure as a B cell receptor (see Figure 43.8a) but lacks a transmembrane region that would anchor it to the plasma membrane. Although the antigen-binding sites on an antibody are responsible for its ability to identify a specific antigen, the tail of the Y-shaped molecule, formed by the constant (C) regions of the heavy chains, is responsible for the antibody's distribution in the body and for the mechanisms by which it mediates antigen disposal. There are five major types of heavy-chain constant regions, and these determine five major classes of antibodies. The class names are based on the alternate term for antibody-— imrrrunoglobulin (Ig). The structures and functions of these antibody classes are summarized in Figure 43.18. Note that two classes exist primarily as polymers of the basic antibody molecule: IgM as a pentamer and IgA as a dimer. The other three classes—IgG, IgE, and IgD—exist exclusively as monomers.

• Present in secretions such as tears, saliva, mucus, and breast milk

- Present primarily on surface of naive B celis that have not been exposed to antigens • Acts as antigen receptor in antigenstimulated proliferation and differentiation of B cells (dona! selection) Transmembrane region

A Figure 43.18 The five classes of immunoglobulins. All classes consist of similar Y-shaped molecules in which the tail region determines the distribution and functions characteristic of each class, IgM and IgA antibodies contain a J chain (unrelated to the J gene segment) that helps hold the monomeric subunits together. As an IgA antibody is secreted across a mucous membrane, it acquires a secretory component that protects it from cleavage by enzymes.

produced in the body following exposure to a microbial antigen are polycfonal. In contrast, other antibody tools are monoclonal They are prepared from a single clone of B cells grown in culture- All the monoclonal antibodies produced by such a culture are identical and specific for the same epitope on an antigen. In both basic research and medical applications, monoclonal antibodies are particularly useful for tagging specific molecules. For example, certain types of cancer are treated with tumor-specific monoclonal antibodies bound to toxin molecules. The toxin-linked antibodies carry out a precise search-and-destroy mission, selectively attaching to and killing tumor cells. Antibody-Mediated Disposal of Antigens The binding of antibodies to antigens is also the basis of several antigen disposal mechanisms (Figure 43.19). In the simplest of

these, viral neutralisation, antibodies bind to certain proteins on the surface of a virus, thereby blocking the virus's ability to infect a host cell. Similarly, antibodies may bind to a pathogenic bacterium, coating much of the bacterial surface. In a process called opsonization, the bound antibodies enhance macrophage attachment to the microbes, and thus increase phagocytosis. Antibody-media ted agglutination (clumping) of bacteria or viruses forms aggregates that can be readily phagocytosed by macrophages. Agglutination is possible because each antibody molecule has at least two antigen-binding sites that can bind to identical epitopes on separate bacterial cells or virus particles, linking them together. Because of their pentamenc structure, igM antibodies can link together five or more viruses or bacteria (as shown in Figure 43.19). In the similar process of precipitation, antibodies cross-link soluble antigen molecules dissolved in body fluids, forming immobile aggregates that are disposed of by phagocytes.

.* Figure 43.19 Antibody-mediated mechanisms of antigen disposal. The binding of antibodies to antigens marks microbes, foreign particles, and soluble antigens for inactivation or destruction. Following activation of the complement system, the membrane attack complex (MAC) forms pores in foreign cells. The pores allow ions and water to rush into the cell, leading to its swelling and eventual lysis.

CHAPTER 43

The Immune System

913

As you read earlier in the chapter, substances on many microbes activate the complement system as part of the body's innate defenses. The complement system also participates in the antibody-mediated disposal of microbes and transplanted body cells. In this case, binding of antigen-antibody complexes on a microbe or foreign cell to one of the complement proteins triggers a cascade in which each component activates the next. Ultimately, activated complement proteins generate a membrane attack complex (MAC) that forms a pore in the membrane. Ions and water rush into the cell, causing it to swell and lyse (see Figure 43.19, right). Whether activated as pan of innate or acquired defenses, the complement cascade results in lysis of microbes and produces activated complement proteins that also promote inflammation or stimulate phagocytosis. As shown in Figure 43.19, antibodies promote phagocytosis in several ways. Recall that phagocytosis enables macrophages and dendritic cells to present antigens to and stimulate helper T cells, which in turn stimulate the very B cells whose antibodies contribute to phagocytosis. This positive feedback between the innate and acquired immune systems contributes to a coordinated, effective response to infection.

Active and Passive Immunization Immunity conferred by natural exposure to an infectious agent is called active immunity because it depends on the action of a person's own lymphocytes and the resulting memory cells specific for the invading pathogen. Active immunity also can develop following immunization, often called vaccination (from the Latin vacca, cow). The first vaccine consisted of the virus causing cowpox, a mild disease usually seen in cows but also occasionally in humans. In the late 1700s, the English physician Edward Jenner observed that milkmaids who previously had contracted cowpox were resistant to subsequent smallpox infection, a disfiguring and potentially fatal disease. In 1796, in his now-famous experiment, Jenner scratched a farm boy with a needle bearing virus-containing fluid from a sore on a milkmaid who had cowpox. When the boy was later exposed to the smallpox virus, he did not become sick. Cowpox virus protects against smallpox virus because the two viruses are so similar that the immune system cannot distinguish them. Vaccination with ihe cowpox virus sensitizes the immune system., so that it reacts vigorously if later exposed to cowpox virus or, more importantly, smallpox virus. Modern vaccines include inactivated bacterial toxins, killed microbes, parts of microbes, viable but weakened microbes that generally do not cause illness, and even genes encoding microbial proteins. All these agents induce an immediate immune response and long-lasting immunological memory (thanks to memory cells). A vaccinated person who encounters the actual pathogen from which the vaccine was 914

UNIT SEVEN

Animal Form and Function

derived will have the same quick secondary response as a person previously infected with the pathogen. A worldwide vaccination campaign led to eradication of smallpox in the late 1970s. Routine active immunization of infants and children has dramatically reduced the incidence of other infectious diseases, such as polio, measles, and whooping cough, in developed countries. Unfortunately, not all infectious agents are easily managed by vaccination. For example, the emergence of new strains of pathogens with slightly altered surface antigens complicates development of vaccines against some microbes, such as the parasite that causes malaria. Immunity can also be conferred by transferring antibodies from an individual who is immune to a particular infectious agent to someone who is not. This is called passive immunity because it does not result from the action of the recipients own B and T cells. Instead, the transferred antibodies are poised to immediately help destroy any microbes for which they are specific. Passive immunity provides immediate protection, but it persists only as long as the transferred antibodies last (a few weeks to a few months). Passive immunization occurs naturally when IgG antibodies of a pregnant woman cross the placenta to her fetus. In addition, IgA antibodies are passed from a mother to her nursing infant in breast milk. These help protect the infant from infection while the baby's own immune system is maturing. In artificial passive immunization, antibodies from an immune animal are injected into a nonimrnune animal. For example, a person bitten by a rabid animal may be given antibodies isolated from other people who have been vaccinated against rabies. This measure is important because rabies may progress rapidly, and the response to active immunization may be tqo slow to save the victim. Most people who have been exposed to the rabies virus are both passively and actively immunized. The injected antibodies help keep the virus in check until the victim's own immune response, induced by active immunization and by the infection itself, takes over.

Coiicenl Check 1. Describe the main role of each of the following cell types, once it is activated by antigens and cytokines: helper T cell, cytotoxic T cell, and B cell. 2. What cells and functions would be deficient in a child born without a thymus? 3. Discuss how antibodies help protect us from infection or the effects of infection. 4. Explain why passive immunization provides shortterm protection from an infection, whereas active immunization provides long-term protection. For suggested answers, see Appendix A.

is that these antibodies arise in response to normal bacterial inhabitants of the body that have epitopes very similar to blood group antigens. For example, a person with type A blood makes antibodies against B-like bacterial epitopes; which the immune system considers foreign, but does not make antibodies against A-like bacterial epitopes, which the immune system recognizes as self. The preexisting anti-B antibodies in a person with type A blood will cause an immediate and devastating transfusion reaction if that person receives a transfusion of type B blood. This reaction involves lysis of the transfused red blood cells, which can lead to chills, fever, shock, and kidney malfunction. By the same token, anti-A antibodies in the donated type B blood can act against the recipient's type A red blood cells. This latter reaction can be minimized by transfusing packed cells instead of whole blood, so that donor antibodies in the fluid portion are not transferred. Table 43.1 lists the recipient and donor (packed cell) combinations that are safe and those that result in transfusion reactions. Note that the row shaded in blue indicates that a person with type AB blood can safely receive blood of any type (and thus is called a "universal recipient"). The column shaded in green indicates that a person with type O blood can safely donate blood to any recipient (and thus is called a "universal donor"). Blood group antigens and related bacterial epitopes are polysaccharides. Such polysaccharide antigens induce immune responses in which no memory cells are generated. As a result, anti-blood group antibodies are always IgM (generated from primary responses) rather than IgG (generated from secondary responses). This is fortunate in pregnancy because IgM does not cross the placenta, so no harm comes Lo a fetus with a blood type that is not compatible with its mother's. However, another red blood cell antigen, the Rh factor, can cause trouble for a fetus. As a protein antigen, the Rh factor induces immune responses in which memory cells are generated. Later exposure of these memory cells to the Rh factor leads to production of anti-Rh antibodies that are IgG.

Concept

The immune system's ability to distinguish self from nonself limits tissue transplantation In addition to distinguishing between the body's own cells and invading pathogens, the immune system can wage war against cells from other individuals. For example, skin transplanted from, one person to a genetically nonidentical person will look healthy for a week or so, but will then be destroyed (rejected) by the recipient's immune response. (Interestingly, however, a pregnant woman does not reject her fetus as nonself tissue. Apparently, the structure of the placenta—• described in Chapter 46—is the key to this tolerance.) Keep in mind that the body's hostile reaction to an incompatible blood transfusion or transplant of other tissues or whole organs is not a disorder of the immune system, but the normal reaction of a healthy immune system exposed to foreign antigens.

Blood Groups and Transfusions In Chapter 14, we discussed the genetics of ABO blood groups in humans. Recall that type A red blood cells have A antigen molecules on their surface. The A antigen may be recognized as foreign if placed in the body of another person. Similarly, the B antigen is found on type B red blood cells; both A and B antigens are found on type AB red blood cells; and neither antigen is found on type O red blood cells (see Table 14.2). Individuals with type A blood do not, of course, produce antibodies against the A antigen, because they are self-tolerant. However, these individuals do have circulating antibodies against the B antigen, even if they have never been exposed to type B blood! You may find it odd that antibodies Lo foreign blood group antigens exist in the body even in the absence of exposure to foreign blood cells. The explanation

Table 43.1 Blood Groups That Can and Cannot Be Safely Combined in Transfusion* Recipient's Blood Group

Antibodies in Recipient's Blood

Presence (+) or Absence (-) of Tranfusion Reaction: Donated Blood Group (Packed Cells) A

A B AB 0

B

AB

O

Anti-B Anti-A No anti-A or anti-B Anti-A and anti-B

* Individuals with type AB blood areunhersal recipient? (bine row)1 those wuh type 0 lilooc are universal donors (green column).

The Immune System

915

A potentially dangerous situation can arise when a mother who is Rh-negative (lacks the Rh factor) carries a fetus that is Rh-positive, having inherited the factor from its lather. If small amounts of fetal blood cross the placenta, which may happen late in pregnancy or during delivery, the mother mounts a humoral response against the Rh factor. The danger occurs in subsequent pregnancies with an Rh-positive fetus, when the mothers Rh-specific memory B cells are exposed to the Rh factor from the fetus. These B cells produce anti-Rh IgG antibodies, which can cross the placenta and destroy the red blood cells of the fetus- To prevent this, the mother is injected with anti-Rh antibodies around the seventh month of pregnancy and again just after delivering an Rh-positive baby. She is, in effect, passively immunized (artificially) to eliminate any fetal Rh-bearing red blood cells that cross the placenta before her own immune system responds to them and generates immunological memory that would endanger future Rh-positive babies.

Tissue and Organ Transplants Major histocompatibility complex (MHC) molecules are responsible for stimulating the immune response that leads to rejection of tissue and organ transplants, or grafts. As you read earlier, the polymorphism of the MHC almost guarantees that no two people, except identical twins, have exactly the same set of MHC molecules. Thus, a rejection reaction is mounted in the vast majority of graft and transplant recipients because at least some MHC molecules on the donated tissue are foreign to the recipient. There is no danger of rejection if the donor and recipient are identical twins or if tissue is grafted from one part of an individual's body to another part. To minimize the extent of rejection in nonidentical transplants, attempts are made to use donor tissue bearing MHC molecules that match those of the recipient as closely as possible. In addition, the recipient takes medicines that suppress immune responses. However, these medicines can leave the recipient more susceptible to infections and cancer during the course of treatment. In a bone marrow transplant, the graft itself, rather than the recipient, is the source of potential immune rejection. Bone marrow transplants are used to treat leukemia and other cancers as well as various hematological (blood cell) diseases. As in any transplant, the MHC of the donor and recipient are matched as closely as possible. Prior to receiving transplanted bone marrow, the recipient is typically treated with irradiation to eliminate his or her own bone marrow cells, including any abnormal cells. This treatment effectively obliterates the recipients immune system, leaving little chance of graft rejection. However, the great danger in bone marrow transplants is that lymphocytes in the donated marrow will react against the recipient. This graft versus host reaction is limited if the MHC molecules of the donor and recipient are well matched. Bone marrow donor programs around the world continually 916

UNIT SEVEN

Animal Form and Function

seek volunteers; because of the great variability of the MHC, a diverse pool of potential donors is essential.

Concept Check 1. Explain why a person who has type AB blood is considered a universal blood recipient. 2. Tn bone marrow transplantation, there is a danger of a graft versus host reaction. Why is this reaction particular to a bone marrow transplant? 3. Severely burned patients generally must receive numerous skin grafts. What is the advantage of using skin from an unburned part of a patients own body (an autograft) rather than from another person? For suggested answers, see Appendix A.

Concept

Exaggerated, self-directed, or diminished immune responses can cause disease The highly regulated interplay of lymphocytes with foreign substances, with each other, and with other body cells provides extraordinary protection against many pathogens. However, if this delicate balance is disrupted by an immune system malfunction, the effects on the individual can range from the minor inconvenience of some allergies to the serious and often fatal consequences of certain autoimmune and immunodeficiency diseases.

Allergies Allergies are exaggerated (hypersensitive) responses to certa antigens called allergens. One hypothesis to explain the origin of allergies is that they are evolutionary remnants of the immune system's response to parasitic worms. The humoral mechanism that combats worms is similar to the allergic response that causes such disorders as hay fever and allergic asthma. The most common allergies involve antibodies of the IgE class (see Figure 43.18). Hay fever, for instance, occurs when plasma eel Is secrete IgE antibodies specific for antigens on the surface of pollen grains. Some of these antibodies attach by their tails to mast cells present in connective tissues. Later, when pollen grains again enter the body, they attach to the antigen-binding sites of mast cell-associated IgE, cross-linking adjacent antibody molecules. This induces the mast cell to release histarnine and other inflammatory agents from their granules (vesicles), a process called degmnulation (Figure 43.20). Recall that histamine causes dilation and increased permeability of small blood vessels.

0 IgE antibodies produced in response to initial exposure to an allergen bind to receptors or mast cells.

© On subsequent exposure to the same allergen, IgE molecules attached to a mast cell recognize and bind the allergen.

0 Degranulation of the cell, triggered by cross-linking of adjacent IgE molecules, releases histamine and other chemicals, leading to allergy symptoms.

A Figure 43.20 Mast cells, IgE, and the allergic response. The colorized SEM shows a degranulated mast cell that has released granules containing histamine and other inflammatory agents.

Such vascular changes lead to typical allergy symptoms: sneezing, runny nose, tearing eyes, and smooth muscle contractions that can result in breathing difficulty. Antihistamines diminish allergy symptoms by blocking receptors for histamine. An acute allergic response sometimes leads to anaphylactic shock, a whole-body, life-threatening reaction that can occur within seconds of exposure to an allergen. Anaphylactic shock develops when widespread mast cell degranulation triggers abrupt dilation of peripheral blood vessels, causing a precipitous drop in blood pressure. Death may occur within a few minutes. Allergic responses to bee venom or peniciflin can lead to anaphylactic shock in people who are extremely allergic to these substances. Likewise, people very allergic to peanuts, hsh, or other foods have died from eating only tiny amounts ol these allergens. Some individuals with severe hypersensitivities carry syringes containing the hormone epmephrine, which counteracts this allergic response.

Autoimmune Diseases In some individuals, the immune system loses tolerance for self and turns against certain molecules of the body, causing one of the many autoimmune diseases. In systemic lupus erythematosus Oupus), the immune system generates antibodies (known as autoantibodies) against a wide range of self molecules, including histones and DNA released by the normal breakdown of body cells. Lupus is characterized by skin rashes, fever, arthritis, and kidney dysfunction. Another antibody-mediated autoimmune disease, rheumatoid arthritis, leads to damage and painful inflammation of the cartilage and bone of joints (Figure 43.21). In insulin-dependent diabetes mellitus, the insulin-producing beta :11s of the pancreas are the targets of autoimmune cytotoxic cells. Yet another example is multiple sclerosis, the most comnon chronic neurological disease in developed countries. In this

disease, T cells infiltrate the central nervous system and destroy the myelin sheath that surrounds some neurons (see Figure 48.5). This results in a number of serious neurological abnormalities. The mechanisms leading to autoimmunity are not fully understood. For a long time, it was thought that people with autoimmune diseases had self-reactive lymphocytes that had happened to escape A Figure 43.21 X-ray of elimination during their de- a hand deformed by velopment. We now know rheumatoid arthritis. that healthy people also have lymphocytes with the capacity to react against self. However, several regulatory mechanisms make these cells nonfunctional, so they cannot induce autoimmune reactions. Thus, autoimmune disease likely arises from some failure in immune system regulation.

Immunodeficiency Diseases The inability of the immune system to protect the body from pathogens or cancer cells that it should normally be able to defeat reflects some sort of deficiency in the system. An immunodeficiency disease caused by a genetic or developmental defect in the immune system is classified as an inborn or primary immunodeficiency. An immunodeficiency disease that develops later in life following exposure to various chemical and biological agents is classified as an acquired or secondary immunodeficiency. Whatever the cause and nature of the immunodeficiency, a person with this kind of disease CHAPTER 43

The immune System

917

is subject to frequent and recurrent infections and also is more susceptible to cancer.

Inborn (Primary) Immunodeficiencies Inborn immunodeficiencies result from defects in the development of various immune system cells or defects in the production of specific proteins, such as IgA antibodies or complement components. Depending on the specific genetic defect, innate defenses, acquired defenses, or both may be impaired. In severe combined immunodeficiency (SC1D), both the humoral and cellmediated branches of acquired immunity fail to function. The long-term survival of people with this genetic disease usually requires a bone marrow transplant that will continue to supply functional lymphocytes. In one type of SCID, a deficiency of the enzyme adenosine deaminase leads to accumulation of substances that are toxic to both B and T cells. Since the early 1990s, medical researchers have been testing a gene therapy for this disease in which the individual's own bone marrow cells are removed, engineered to contain a functional adenosine deaminase gene, and then returned to the body (see Figure 20.16). Recent successes include a child with SCID who received adenosine deaminase gene therapy when she was 2 years old. About two years after treatment, her T cells and B cells were still functioning normally. In fact, when a family member came down with chicken pox, the treated girl did not develop any sign of the disease, evidence that her immune system was working. Acquired (Secondary) Immunodeficiencies Immune dysfunction that develops later in life can be caused by exposure to a number of agents. Drugs used to fight autoimmune diseases or to prevent the rejection of a transplant suppress the immune system, leading to an immunodeficient state. In addition, the immune system is suppressed by certain cancers, especially Hodgkin's disease, which damages the lymphatic system. Acquired immunodeficiencies range from temporary states that may arise from physiological stress to the devastating acquired immunodeficiency syndrome, or AIDS, which is caused by a virus. Stress and the Immune System. Healthy immune function appears to depend on both the endocrine system and the nervous systems. Nearly 2,000 years ago, the Greek physician Galen recorded that people suffering from depression were more likely than others to develop cancer. In fact, there is growing evidence that physical and emotional stress can harm immunity. Hormones secreted by the adrenal glands during stress affect the numbers of white blood cells and may suppress the immune system in other ways. The association between emotional stress and immune function also involves the nervous system. Some neurotransmitters secreted when we are relaxed and happy may enhance 918

UNIT SEVEN

Animal Form a n d Function

immunity. In one study, college students were examined j^st after a vacation and again during final exams. Their immune systems were impaired in various ways during exam week; for example, interferon levels were lower. These and other observations indicate that general health and state of mind affect immunity. Physiological evidence also points to an immune system-nervous system link: Receptors Tor neurotransmitters have been discovered on the surface of lymphocytes, and a network of nerve fibers penetrates deep into the thymus. Acquired Immunodeficiency Syndrome (AIDS). People with AIDS are highly susceptible to opportunistic infections and cancers that take advantage of an immune system in collapse. For example, infection by Pneumocystis carinii, a ubiquitous fungus, can cause severe pneumonia in people with AIDS but is successfully rebuffed by individuals with a healthy immune system. Likewise, Kaposi's sarcoma is a rare cancer that occurs most commonly in AIDS patients. Such opportunistic diseases, as well as neurological damage and physiological wasting, can lead to death from AIDS. Because AIDS arises from the loss of helper T cells, both humoral and cell-mediated immune responses are impaired. This loss of helper T cells results from infection by the human immunodeficiency virus (HIV), a retrovirus (Figure 43.22). HIV gains entry into cells by making use of three proteins that participate in normal immune responses. The main receptor for HIV on helper T cells is the cells CD4 molecule. The virus also infects other cell types, such as macrophages and brain cells, that have low levels of CD4. In addition to CD4, HIV entry requires a second cell-surface protein, a co-receptor. One coreceptor, called fusin, is present on all the cell types infected by HIV, while a different co-receptor is present only on macrophages and helper T cells. Both of these HIV co-receptors function as chemokine receptors in uninfected cells. In fact, these proteins were first recognized as HIV co-receptors after it was discovered that chemokines can block HIV entry into cells. Once inside a cell, the HIV RNA genome is reversetranscribed, and the product DNA is integrated into the host cell's genome. In this form, the viral genome can direct the production of new virus particles (see Figure 18.10). The infected cell's machinery for transcription and translation thus are hijacked for the benefit of the virus. The death of helper T cells in HIV infection is thought to occur in two ways: Infected cells may succumb to the damaging effects of virus reproduction, and both infected and uninfected cells may undergo inappropriately timed apoptosis triggered by the virus. At this lime, HIV infection cannot be cured, although certain drugs can slow HIV reproduction and the progression to AIDS. However, these drugs are very expensive and not available to all infected people. In addition, the muLarional changes that occur in each round of virus reproduction can generate strains of HIV that are drug resistant. The impact o:'

most of the HIV infections in the United Slates. However, transmission of HIV among heterosexuals is rapidly increasing as a result of unprotected sex with infected partners. In Africa and Asia, transmission occurs primarily by heterosexual sex. In a December 2003 report, the joint United Nations Program on AIDS estimated that 40 million people worldwide are living with HIV/AIDS. The best approach for slowing the spread of HIV is to educate people about the practices that transmit the virus, such as using dirty needles and having sex without a condom. Although condoms do not completely eliminate the risk of transmitting HIV (or other similarly transmitted viruses, such as the hepatitis B virus), they do reduce it. Anyone who has sex—vaginal, oral, or anal—with a potentially infected partner risks exposure to the deadly virus.

Concept Check

A Figure 43.22 A T cell infected with HIV. Newly made virus particles (gray) are seen budding from the surface of the T cell in this colorized SEM.

1. How would a macrophage deficiency likely affect a persons innate and acquired defenses? 2. Many anti-allergy medications block responses by masi cells. Explain why these drugs are effective in treating allergies such as hay fever. 3. In myasLhenia gravis, antibodies bind Lo and block acetylcholine receptors at neuromuscular junctions, preventing muscle contraction. Is this disease best classified as an immunodeficiency disease, an autoimmune disease, or an allergic disease? Explain. 4. People who have nonfunctional chemokine receptors because of a genetic mutation are immune to HIV infection. Explain this finding.

viral drug resistance can be minimized by using a combination of drugs; viruses newly resistant to one drug can be defeated by another. But the appearance of strains resistant to multiple drugs reduces the effectiveness of multidrug "cocktails" in some patients. Frequent mutational changes in HIV surface antigens also have hampered efforts to develop an effective vaccine. Transmission of HIV requires the transfer of body fluids containing infected cells, such as semen or blood, from person to person. Unprotected sex (that is, without a condom) among male homosexuals and transmission via HlV-contatninated needles (typically among intravenous drug users) account for

Chapter Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY

For suggested answers, see Appendix A.

?

; Review act against invading microbes. In local inflammation, histamine and other chemicals released from injured cells promote changes in blood vessels that allow fluid, more phagocytes, and antimicrobial proteins to enter the tissues. Natural killer (NK) cells can induce the death of virus-infected or cancer cells via apoptosis.

Invertebrate Immune Mechanisms (pp. 902-903) Insects Innate immunity provides broad defenses against infection I* External Defenses (p. 899) Intact skin and mucous membranes form physical barriers that bar the entry of microorganisms and viruses. Mucus produced by cells in these membranes, the low pH of the skin and stomach, and degradation by lysozyme also deter infection by pathogens.

^ Internal Cellular and Chemical Defenses (pp. 899-902) Phagocytic cells ingest microbes that penetrate external innate defenses and help trigger an inflammatory response. Complement proteins, interferons, and other antimicrobial proteins also

defend themselves by mechanisms similar in many respects to vertebrate innate defenses.

In acquired immunity, lymphocytes provide specific defenses against infection • Antigen Recognition by Lymphocytes (pp. 903-905) Receptors on lymphocytes bind specifically to small regions of an antigen (epitopes). B cells recognize intact antigens, T cells recognize small antigen fragments (peptide antigens) that are complexed with cell-surface proteins called major histocompatibility (MHC) molecules. Class I MHC molecules, located on all CHAPTER 43

The Immune System

919

nucleated cells, display peptide antigens to cytotoxic T cells. Class II MHC molecules, located mainly on dendritic cells, macrophages, and B cells (antigen-presenting cells), display peptide antigens to helper T cells. • Lymphocyte Development (pp. 905-908) Lymphocytes arise from stem cells in the bone marrow and complete their maturation in the bone marrow (B cells) or in the thymus (T cells). Early in development, random, permanent gene rearrangement forms functional genes encoding the B or T cell antigen receptor chains. All the antigen receptors produced by a single lymphocyte are specific for the same antigen. Self-reactive lymphocytes, whose receptors bind to normal body components, are destroyed or inactivated. In a primary immune response, binding of antigen to a mature lymphocyte induces the lymphocyte's proliferation and differentiation (clonal selection), generating a clone of short-lived activated effector cells and a clone of long-lived memory cells. These memory cells account for the faster, more efficient secondary immune response.

Humoral and cell-mediated immunity defend against different types of threats • Helper T Cells: A Response to Nearly All Antigens (pp. 909-910) Helper T cells make CD4, a surface protein that enhances their binding to class II MHC molecule-antigen complexes on antigen-presenting cells. Activated helper T cells secrete several different cytokines that stimulate other lymphocytes. • Cytotoxic T Cells: A Response to Infected Cells and Cancer Cells (pp. 910-911) Cytotoxic T cells make CDS, a surface protein that enhances their binding to class I MHC molecule-antigen complexes on infected cells, cancer cells, and transplanted tissues. Activated cytotoxic T cells secrete proteins that initiate destruction of their target cells. • B Cells: A Response to Extracellular Pathogens (pp. 910-914) The clonal selection of B cells generates antibodysecreting plasma cells, the effector cells of humoral immunity. The five major antibody classes differ in their distributions and functions within the body. Binding ot antibodies to antigens on the surface of pathogens leads to elimination of the microbes by phagocytosis and complement-mediated lysis. • Active and Passive Immunization (p. 914) Active immunity develops naturally in response to an infection; it also develops artificially by immunization (vaccination). In immunization, a nonpathogenic form of a microbe or part of a microbe elicits an immune response to and immunological memory for that microbe. Passive immunity, which provides immediate, short-term protecLion, is conferred naturally when IgG crosses the placenta from mother to fetus or when IgA passes from mother to infant in breast milk. It also can be conferred artificially by injecting antibodies into a nonimmune person. Activity Immune Responses

The immune system's ability to distinguish self from nonself limits tissue transplantation • Blood Groups and Transfusions (pp. 915-916) Certain antigens on red blood cells determine whether a person has type A, B, AB, or O blood. Because antibodies to nonself blood antigens already exist in the body, transfusion with incompatible blood leads to destruction of the transfused cells. The Rh factor, another red blood cell antigen, creates difficulties when an Rh-negative mother carries successive Rh-positive fetuses.

920

UNIT SEVEN

Animal Form and Function

• Tissue and Organ Transplants (p. 916) MHC molecules are responsible for stimulating the rejection of tissue grafts and organ transplants. The chances of successful transplantation are increased if the donor and recipient MHC tissue types are well matched and if immunosuppressive drugs are given to the recipient. Lymphocytes in bone marrow transplants may cause a graft versus host reaction in recipients.

Exaggerated, self-directed, or diminished immune responses can cause disease • Allergies (pp. 916-917) In localized allergies such as hay fever, IgE antibodies produced after first exposure to an allergen attach to receptors on mast cells. The next time the sameallergen enters the body, it binds to mast cell-associated IgE molecules, inducing the cell to release histamine and other mediators that cause vascular changes and typical symptom. • Autoimmune Diseases (p. 917) Loss of the immune system's normal self-tolerance can lead to autoimmune diseases such as systemic lupus erythematosus (lupus), multiple sclerosis, rheumatoid arthritis, and insulin-dependent diabetes. • Immunodeficiency Diseases (pp. 917-919) Inborn (primary) immunodeficiencies result from hereditary or congenital defects that prevent proper functioning of innate, humoral, and/or cellmediated defenses. AIDS is an acquired (secondary) immunodeficiency caused by the human immunodeficiency virus (HIV). HIV infection leads to destruction of helper T cells, leaving the patient prone to opportunistic diseases owing to deficient humoral and cell-mediated immune responses. Activity HIV Reproductive Cycle Investigation What Causes Infections in AIDS Patients? Investigation Why Do AIDS Rates Differ Across the U.S.?

TESTING YOUR KNOWLEDGE Evolution Connection One reason for the success of invertebrates, which make up more than 90% of living animal species, is their effective defense against microbes. Describe one mechanism by which invertebrates combat such invaders, and discuss how this mechanism comprises an evolu tionary adaptation that is retained in the vertebrate immune system.

Scientific Inquiry One effect of interferon--y is to increase the number of class I MHC molecules on the cell surface. Suppose you want to test its effective' ness in treating viral infections and cancer. What effects would youi predict interferon-7 to have on the immune response of laboratory animals against (a) virus-infected cells and (b) cancer cells? Investigation What Causes Infections in AIDS Patients? Investigation Why Do AIDS Rates Differ Across the U.S.?

Science, Technology, and Society Both an injectable inactivated (killed) vaccine and an oral attenuated (live) vaccine are available for immunization against poliovirus, which can cause paralysis by destroying nerve cells in the brain and spinal cord. The oral vaccine is no longer recommended in western countries, where polio has been eradicated, because the live virus in this vaccine may mutate to a more virulent form, and be reintroduced into the population. However, the oral vaccine continues to be used in countries where polio persists because it is easy to administer (no needle!) and is highly effective. Moreover, the attenuated virus can spread to (and immunize) unvaccinated individuals. Do you feel this risk of mutation lo virulence (about 1 in 12 million) is acceptable when compared with the benefits of oral vaccination? How do you think public heaab decisions of niis type should be made?

CHAPTER 43

The Immune System

911

A Figure 44.1 Salvin's albatrosses {Diomeda cauta saivtni), birds that can drink seawater with no ill effects.

Key Concepts 44.1 Osmoregulation balances the uptake and loss of water and solutes 44.2 An animal's nitrogenous wastes reflect its phylogeny and habitat 4 4 . 3 Diverse excretory systems are variations on a tubular theme 4 4 . 4 Nephrons and associated blood vessels are the functional units of the mammalian kidney 4 4 . 5 The mammalian kidney's ability to conserve water is a key terrestrial adaptation 4 4 . 6 Diverse adaptations of the vertebrate kidney have evolved in different environments

At the same time, metabolism presents organisms with the problem of waste disposal. The breakdown of proteins and nucleic acids is particularly problematic, since the primary metabolic waste product produced, ammonia, is very toxic. Studies of how animals meet these physiological challenges provide some of the most prominent examples of homeostasis. This chapter focuses on two key homeostatic processes: osmoregulation, how animals regulate solute concentrations and balance the gain and loss of water; and excretion, how animals get rid oi the nitrogen-containing waste products of metabolism.

Concept

A Balancing Act

T

he physiological systems of animals, from cells and tissues to organs and organ systems, operate within a fluid environment. For such systems to function properly, this environment, particularly the relative concentrations of water and solutes, must be maintained within fairly narrow limits, often in the face of strong challenges from an animal's external environment. For example, freshwater animals, which live in an external environment that threatens to flood and dilute their body fluids, show adaptations that reduce water uptake, conserve solutes, and absorb salts from their surroundings. At the other extreme, desert and marine animals face desiccating environments with the potential to quickly deplete the animals' body warer (Figure 44.1). Success in such environments depends on conservation of water and elimination of excess salts. 922

Osmoregulation balances the uptake and loss of water and solutes Just as thermoregulation depends on balancing heat loss anc gain (see Chapter 40), an animal's ability to regulate the chemical composition of its body fluids depends on balancing the uptake and loss of water and solutes. This osmoregulation is based largely on controlled movement of solutes between internal fluids and the external environment. The process also regulates the movement of water, which follows solutes by osmosis. An animal must also remove various metabolic waste products before they accumulate to harmful levels.

Osmosis All animals—regardless of phylogeny, habitat, or type of waste produced—face the same central problem of osmoregulation: Over Lime, the rates of water uptake and loss must balance.

Lacking cell walls, animal cells swell and burst if there is a continuous net uptake of water or shrivel and die if there is a substantial net loss of water. Water enters and leaves cells by osmosis. Recall from Chapter 7 that osmosis, a special case of diffusion, is the movement of water across a selectively permeable membrane. It occurs whenever two solutions separated by tbe membrane differ in osmotic pressure, or osmolarity (total solute concentration expressed as molarity, or moles of solute per liter of solution; see Chapter 3). The unit of measurement for osmolarity used in this chapter is milliosmoles per liter (mosm/L); 1 mosm/L is equivalent to a total solute concentration of 10~3 M. The osmolarity of human blood is about 300 mosm/L, while seawater has an osmolarity of about 1,000 rnosm/L. If two solutions separated by a selectively permeable membrane have the same osmolarity, they are said to be isoosmotic. There is no net movement of water by osmosis between isoosmotic solutions; although water molecules are continually crossing the membrane, they do so at equal rates in both directions. When two solutions differ in osmolarity, the one with the greater concentration of solutes is said to be hyperosmotic and tbe more dilute solution is said to be hypoosmotic. Water flows by osmosis from a hypoosmotic solution to a hyperosmotic one.*

Osmotic Challenges There are two basic solutions to the problem of balancing water gam with water loss. One—available only to marine animals—• is to be isoosmotic to the surroundings. Such an animal, which does not actively adjust its internal osmolarity, is known as an osmoconformer. Because an osmoconformers internal osmolarity is the same as that of its environment, there is no tendency to gain or lose water. Osmoconformers often live in water that has a very stable composition and hence have a very constant internal osmolarity. In contrast, an osmoregulator is an animal that must control its internal osmolarity because its body fluids are not isoosmotic with the outside environment. An osmoregulator must discharge excess water if it lives in a hypoosmotic environment or take in water to offset osmotic loss if it inhabits a hyperosmotic environment. Osmoregulation enables animals to live in environments that are uninhabitable for osmoconformers, such as freshwater and terrestrial habitats; it also allows many marine animals to maintain internal osmolarities different from that of seawater.

concentrations in a system, osmoregulators must expend energy to maintain the osmotic gradients that allow water to move in or out. They do so by using active transport to manipulate solute concentrations in their body fluids. The energy cost of osmoregulation depends on how different an animal's osmolarity is from its surroundings, how easily water and solutes can move across the animals surface, and how much work is required to pump solutes across the membrane. Because of the difference in solute concentration between their body fluids (240-450 mosm/L), fresh water (0.5-15 rnosrn/L), and seawater (approximately 1,000 mosm/L), osmoregulation accounts for nearly 5% of the resting metabolic rate of many marine and freshwater bony fishes. For brine shrimp, small crustaceans that live in Utate Great Salt Lake and in other extremely salty lakes, the gradient between internal and external osmolarities is very large and the cost of osmoregulation is correspondingly high—as much as 30% of the resting metabolic rate. In contrast, marine osmoconformers, which are isoosmotic with seawater, expend little energy on osmoregulation. Most animals, whether osmoconformers or osmoregulators, cannot tolerate substantial changes in external osmolarity and are said to be stenohalirte (from the Greek stenos, narrow; haline refers to salt). In contrast, euryhaline animals (from the Greek curys, broad)—which include both certain osmoconformers and certain osmoregulators—can survive large fluctuations in external osmolarity. Familiar examples of euryhaline osmoregulators are the various species of salmon. A more extreme example is a fish called tilapia (a native ol Africa widely grown in fish "farms" for human food), which can adjust to any salt concentration between fresh water and 2,000 mosm/L, twice that of seawater (Figure 44.2). Next, well take a closer look at some of the adaptations for osmoregulation that have evolved in marine, freshwater, and terrestrial animals.

Whenever animals maintain an osmolarity difference between the body and the external environment, osmoregulatloti has an energy cost. Because diffusion tends to equalize

* In this chapter, we use the terms isoosimnic, kyp,)osmniic. and hyperoimniic. which refeT specifically to osmolarily, instead of the more familiar terms is™;?!!c, hyvotonic, and kypertomc. The Latter set oi terms applies to the response of animal Cells—whether they swell or shrink—in solutions of knoun si.'lute ivmcen;rations.

A Figure 44.2 Largemouth tilapia (Tilapia mossambica), an extreme euryhaline osmoregulator. CHAPTER 44

Osmoie^ulaion and txcretion

923

Marine Animals Animals first evolved in the sea, and more animal phyla are found there than in any other environment. Most marine invertebrates are osmoconformers. Their total osmolarity (the sum of the concentrations of all dissolved substances) is the same as that of seawater. However, they differ considerably from seawater in their concentrations of most specific solutes. Thus, even an animal that conforms to the osmolarity of its surroundings regulates its internal composition of solutes. Marine vertebrates and some marine invertebrates are osmoregulators. For most of these animals, the ocean is a strongly dehydrating environment because it is much saltier than internal fluids, and water tends to be lost from their body by osmosis. Marine bony fishes, such as cod, are hypoosmotic to seawater and constantly lose water by osmosis and gam salt both by diffusion and from the food they eat (Figure 44.3a). The fishes balance the water loss by drinking large amounts of seawater. Their gills and skin dispose of sodium chloride; in the gills, specialized chloride cells actively transport chloride ions (Cl ) out, and sodium ions (Na+) follow passively. The kidneys of marine fishes dispose of excess calcium, magnesium, and sulfate ions while excreting only small amounts of water. Marine sharks and most other chondrichthyans (cartilaginous animals; see Chapter 34) use a different osmoregulatory "strategy" Like bony fishes, their internal salt concentration is much less than that of seawater, so salt tends to diffuse into their body from the water, especially across their gills. The kidneys of sharks remove some of this salt load, and the rest is excreted by an organ called the rectal gland or is lost in feces. Unlike bony fishes, and despite relatively low internal salt concentration, marine sharks do not experience a large and continuous osmotic water loss. The explanation is that sharks

Freshwater Animals The osmoregulatory problems of freshwater animals are opposite those of marine animals. Freshwater animals constantly gain water by osmosis and lose salts by diffusion because the osmolarity of their internal fluids is much higher than that of their surroundings. However, the body fluids of most freshwater animals do have lower solute concentrations compared to their marine relatives, an adaptation to their low-salinity freshwater habitat. For instance, whereas marine molluscs have body fluids with a solute concentration of approximately 1,000 mosm/L, some freshwater mussels maintain the solute concentration of their body fluids at about 40 mosm/L. The reduced osmotic difference between body fluids and the surrounding freshwater environment reduces the energy the animal expends for osmoregulation. Many freshwater animals, including fishes such as perch, maintain water balance by excreting large amounts of very dilute urine. Salts lost by diffusion and in the urine are replenished by foods and by uptake across the gills; chloride cells in the gills actively transport Cl , and Na~ follows (Figure 44.3b}.

Osmotic water gain through gifis and other parts of body surface

Gain of water and salt ions from food and by drinking seawater

Excretion of salt ions from gills

maintain high concentrations of the nitrogenous waste urea (a product of protein and nucleic acid metabolism produced by many animals; see Figure 44.8). Another organic solute, trimethylamine oxide (TMAO), protects proteins from damage by urea. (If you have ever prepared shark meat, you know it should be soaked in fresh water to remove urea before cooking.) A shark's total solute concentration of body fluids (salts, urea, TMAO, and other compounds) is somewhat greater than 1,000 mosm/L and therefore slightly hyperosmotic to seawater. Consequently, water slowly enters the shark's body by osmosis and in food (sharks do not drink), and this small influx of water is disposed of in urine produced by the kidneys.

Osmotic water loss through gills and other parts of body surface

x

Excretion of large amounts of water in dilute urine from kidney;

» * t Excretion of salt ions a nd small amounts of water in scanty urine from kidneys

(a) Osmoregufation in a saltwater fish

(b) Osmoregulation in a freshwater fish

A Figure 44,3 Osmoregulation in marine and freshwater bony fishes: a comparison. 924

UNIT SEVEN

Animal Form and Function

ialmon and other euryhaline fishes that migrate between seawater and fresh water undergo dramatic and rapid changes in osmoregulatory status. While in the ocean, salmon osmoregulate like other marine fishes by drinking seawater and excreting excess salt from the gills. When they migrate to fresh water, salmon cease drinking and begin to produce large amounts of dilute urine, and their gills start taking up salt from the dilute environment—-just like fishes that spend their entire lives in fresh water. Animals That Live in Temporary Waters Dehydration is fatal for most animals, but some aquatic invertebrates living in temporary ponds and films of water around soil particles can lose almost all their body water and survive in a dormant state when their habitats dry up. This remarkable adaptation is called anhydrobiosis ("life without water"). Among the most striking examples are the tardigrades, or water bears, tiny invertebrates less than 1 mm long (Figure 44.4). In their active, hydrated state (see Rgure 44.4a), these animals contain about 85% water by weight, but they can dehydrate to less than 2% water and survive in an inactive state (see Figure 44.4b), dry as dust, for a decade or more, just add water, and within minutes the rehydrated tardigrades are moving about and feeding. Anhydrobiotic animals must have adaptations that keep eir cells' membranes intact. Researchers are just beginning to learn how tardigrades survive drying out, but studies of anhydrobiotic roundworms (phylum Nematoda) show that dehydrated individuals contain large amounts of sugars. In particular, a disaccharide called trehalose seems to protect ihe cells by replacing the water that is normally associated with membranes and proteins. Many insects that survive freezing in the winter also utilize trehalose as a membrane protectant.

Land Animals The threat of desiccation is a major regulatory problem for terrestrial plants and animals. Humans die if they lose about 12% of their body water; mammals that evolved in dry environments, such as camels, can withstand about twice that level of dehydration. Adaptations that reduce water loss are key to survival on land. Much as a waxy cuticle contributes to the success of land plants, most terrestrial animals have body coverings that help prevent dehydration. Examples are the waxy layers of insect exoskeletons, the shells of land snails, and the layers of dead, keratinized skin cells covering most terrestrial vertebrates. Many terrestrial animals, especially desert-dwellers, are nocturnal; this reduces evaporative water loss by taking advantage of the lower temperatures and higher relative humidity of night air. Despite these adaptations, most terrestrial animals lose considerable water from moist surfaces in their gas exchange organs, in urine and feces, and across their skin. Land animals bal ance their water budgets by drinking and eating moist foods and by using metabolic water (water produced during cellular respiration). Some animals, such as many insect-eating desert birds and other reptiles, are so well adapted for minimizing water loss that they can survive in deserts without drinking. Kangaroo rats lose so little water that they can recover 90% of the loss by using metabolic water (Figure 44.5), gaining the remaining 10% from the small amount of water in their diet of

Water gain

Water loss

Urine (o.45!

Evaporation (1.46) (a) Hydrated tardigrade

(b) Dehydrated tardigrade

A Figure 44.4 Anhydrobiosis. Tardigrades (water bears) inhabit temporary ponds and droplets of water in soil and on moist plants (SEMs).

A Figure 44.5 Water balance in two terrestrial mammals. Kangaroo rats, which live in the American southwest, eat mostly dry seeds and do not drink water. A kangaroo rat loses water mainly by evaporation during gas exchange and gains water mainly from cellular metabolism. In contrast, a human loses a large amount of water in urine and regains it mostly in food and drink. CHAPTER 14

Osmoregulation and Excretion

925

Figure 44.6

Nasal salt gland

What role does fur play in water conservation by camels? EXPERIMENT Knut and Bodil Schmidt-Nielsen and their colleagues from Duke University observed that the fur of camels exposed to full sun in the Sahara Desert could reach temperatures of over 70°C, while the animals' skin remained more than 30°C cooler. The Schmidt-Nielsens reasoned that insulation of the skin by fur may substantially reduce the need for evaporative cooling by sweating. To test this hypothesis, they compared the water loss rates of undipped and clipped camels. Removing the fur of a camel increased the rate of water loss through sweating by up to 5 0 % .

(a) An albatross's salt glands empty via a duct into the nostrils, and the salty solution either drips off the tip of the beak or is exhaled in a fine mist. Lumen of secretory tubule

Experimental group (Clipped fur) Blood flow

The fur of camels plays a critical role in their conserving water in the hot desert environments where they live,

seeds. In studying the adaptations of animals to desert environments, physiologists have discovered that major water savings can result from simple anatomical features, such as the fur of the camel (Figure 44.6).

Transport Epithelia The ultimate function of osmoregulation is to maintain the composition of cellular cytoplasm, but most animals do this indirectly by managing the composition of an internal body fluid that bathes the cells. In insects and other animals with an open circulatory system, this fluid is the hemolymph (see Chapter 42). In vertebrates and other animals with a closed circulatory system, the cells are bathed in an interstitial fluid that is controlled indirectly through the composition of blood. The maintenance of fluid composition depends on specialized structures ranging from cells that regulate solute movement to complex organs such as the vertebrate kidney In most animals, one or more different kinds of transport epithelium—a layer or layers of specialized epithelial cells that regulate solute movements—are essential components of osmotic regulation and metabolic waste disposal. Transport epithelia move specific solutes in controlled amounts in spe926

UNIT SEVEN Animal Form and Function

(b) One of several thousand secretory tubules in a saltexcreting gland. Each tubule is lined by a transport epithelium surrounded by capillaries, and drains into a central duct.

Secretory cell of transport epithelium

(c) The secretory cells actively transport salt from the blood into the tubules. Blood flows counter to the flow of salt secretion. By maintaining a concentration gradient of salt in the tubule (aqua), this countercurrent system enhances salt transfer from the blood to the lumen of the tubule.

A Figure 44.7 Salt-excreting glands in birds.

cific directions. Some transport epithelia face the outside environment directly, while others line channels connected to the outside by an opening on the body surface, joined by impermeable tight junctions (see Figure 6.31), the cells of the epithelium form a barrier at the tissue-environment boundary This arrangement ensures that any solutes moving between animal and environment must pass through a selectively permeable membrane. In most animals, transport epithelia are arranged into complex tubular networks with extensive surface areas. We find some of the best examples in the salt glands of marine birds, which remove excess sodium chloride from the blood (Figure 44.7). For example, the albatross, which spends months or

years at sea and needs to obtain both tood and water from the ocean, can drink seawater because its nasal salt glands secreLe fluid much saltier than the ocean. Thus, even though drinking seawater brings in a lot of salt, the bird achieves a net gain of water. Humans who drink seawater, by contrast, must use more water to excrete the salt load than was gained by drinking. The molecular structure of the plasma membrane determines the kinds and directions of solutes that move across a particular type of transport epithelium. In contrast to saltexcreting glands, transport epithelia in the gills of freshwater fishes use active transport to move salts from the dilute surrounding water into the blood. Transport epithelia in excretory organs olien have the dual functions of maintaining water balance and disposing of metabolic wastes.

I

1

Nitrogenous bases

—NH2 Amino groups

ISSEB-

Concept Check 1. The movement of salt from the surrounding water to the blood of a freshwater fish requires the expenditure of energy in the form of ATP Why? 2. Why are no freshwater animals osmoconformers? 3. How are sharks able to expend proportionately less energy for osmoregulation compared to marine bony fishes?

Most aquatic animals, including most bony fishes

Many reptiles (including birds), insects, land snails

Mammals, most amphibians, sharks, some bony fishes

For suggested answers, sec Appendix A. HN

\

II H

(Concept

An animal's nitrogenous wastes reflect its phytogeny and habitat Because most metabolic wastes must be dissolved in water when they are removed from the body, the type and quantity of an animal's waste products may have a large impact on its vater balance. In terms of their effect on osmoregulation, imong the most important waste products are the nitrogenous nitrogen-containing) breakdown products of proteins and nucleic acids (Figure 44.8). When these macromolecules are broken apart for energy or converted to carbohydrates or fats, enzymes remove nitrogen in the form of ammonia (NH3), a very toxic molecule. Some animals excrete ammonia directly, m many species first convert the ammonia to other compounds that are less toxic but that require energy in the form of ATP to produce.

Uric acid A Figure 44.8 Nitrogenous wastes.

Ammonia Because ammonia is very soluble but can only be tolerated at very low concentrations, animals that excrete nitrogenous wastes as ammonia need access to lots of water. Therefore, ammonia excretion ts most common in aquatic species. Ammonia molecules easily pass through membranes and are readily lost by diffusion to the surrounding water. In many invertebrates, ammonia release occurs across the whole body surface. In fishes, most of the ammonia is lost as ammonium ions (NH++) across the epithelium of the gills, with kidneys excreting only minor amounts of nitrogenous wastes. In freshwater fishes, the gill epithelium takes up Na+ from the water in exchange for NH4~\ which helps to maintain a much higher Na+ concentration in body fluids than in the surrounding water.

Forms of Nitrogenous Waste

Urea

Different animals excrete nitrogenous wastes in different forms—ammonia, urea, or uric acid-—which vary in their toxlCity and energy costs.

Although it works well in many aquatic species, ammonia excretion is much less suitable for land animals. Because ammonia is so toxic, it can be transported and excreted only in large CHAPTER 44 Osmoregulauon and Excretion

927

volumes of very dilute solutions, and most terrestrial animals and many marine species (which tend to lose water to their environment by osmosis) simply do not have access to sufficient water. Instead, mammals, most adult amphibians, sharks, and some marine bony fishes and turtles excrete mainly urea, a substance produced in the vertebrate liver by a metabolic cycle that combines ammonia with carbon dioxide. The circulatory system carries urea to the excretory organs, the kidneys. The main advantage of urea is its low toxicity, about 100,000 times less than that of ammonia. This permits animals to transport and store urea safely at high concentrations. Further, a urea-excreting animal requires much less water, because much less water is lost when a given quantity of nitrogen is excreted in a concentrated solution of urea rather than a dilute solution of ammonia. The main disadvantage of urea is that animals must expend energy to produce it from ammonia. From a bioenergetic standpoint, we would predict that animals that spend part of their lives in water and part on land would switch between excreting ammonia (thereby saving energy) and area (reducing excretory water loss). In fact, many amphibians excrete mainly ammonia when they are aquatic tadpoles and switch largely to urea when they are land-dwelling adults.

waste product conveyed a selective advantage because it precipitates out of solution and can be stored within the egg as a harmless solid left behind when the animal hatches. The type of nitrogenous waste produced by vertebrates depends on habitat as well as on evolutionary lineage. For example, terrestrial turtles (which often live in dry areas) excrete mainly uric acid, whereas aquatic turtles excrete both urea and ammonia. In some species, individuals can change forms of nitrogenous wastes when environmental conditions change. For example, certain tortoises that usually produce urea shift to uric acid when temperature increases and water becomes less available. This is another example of how response to the environment occurs on two levels: Over generations, evolution determines the limits of physiological responses for a species, but during their lives, individual organisms make physiological adjustments within these evolutionary constraints. The amount of nitrogenous waste produced is coupled to the energy budget, as it strongly depends on how much and what kind of food an animal eats. Because they use energy at high rates, endolherms eat more food-—and therefore produce more nitrogenous wastes—per unit volume than ectotherms. Predators, which derive much of their energy from dietary proteins, excrete more nitrogen than animals that rely mainly on lipids o carbohydrates as energy sources.

Uric Acid Insects, land snails, and many reptiles, including birds, excrete uric acid as their major nitrogenous waste. Like urea, uric acid is relatively nontoxic. But unlike either ammonia or urea: uric acid is largely insoluble in water and can be excreted as a semisolid paste with very little water loss. This is a great advantage for animals with little access to water, but there is a cost: Uric acid is even more energetically expensive to produce than urea, requiring considerable ATP for synthesis from ammonia.

Concept Check 1. Dragonfly larvae, which are aquatic, excrete ammonia, whereas adult dragonflies, which are terrestrial, excrete uric acid. Explain. 2. What role does the vertebrate liver play in the body's processing of nitrogenous waste? 3. What advantage does uric acid offer as a nitrogenous waste in arid environments?

The Influence of Evolution and Environment on Nitrogenous Wastes In general, the kinds of nitrogenous wastes excreted depend on an animal's evolutionary history and habitat—especially the availability of water (see Figure 44.8). For example, uric acid and urea represent different adaptations for excreting nitrogenous wastes with minimal water loss. One factor thai. seems to have been important in determining which of these alternatives evolved in a particular group of animals is the mode of reproduction. Soluble wastes can diffuse out of a shell-less amphibian egg or be carried away by the mother's blood in a mammalian embryo. However, the shelled eggs produced by birds and other reptiles are permeable to gases but not to liquids, which means that soluble nitrogenous wastes released by an embryo would be trapped within the egg and could accumulate to dangerous levels (although urea is much less harmful than ammonia, it does become toxic at very high concentrations). The evolution of uric acid as a 928

UNIT SEVEN

Animal Form and Function

For suggested answers, see Appendix A.

Concept

Diverse excretory systems are variations on a tubular theme Although the problems of water balance are very different on land, in salt water, and in fresh water, solving them all depends on the regulation of solute movement between internal fluids and the external environment. Much of this movement is handled by excretory systems, which are central to homeostasis because they dispose of metabolic wastes and control body fluid composition by adjusting rates of solute loss. Belore we describe particular excretory systems, let's consider the basic process of excretion.

Excretory Processes Although excretory systems are diverse, nearly all produce the fluid waste urine in a process that involves several steps (Figure 44.9). First, body fluid (blood, coelomic fluid, or hemolymph) is collected. The initial fluid collection usually involves filtration through selectively permeable membranes consisting of a single layer of transport epithelium. These membranes retain cells as well as proteins and other large molecules in the body fluid; hydrostatic pressure (blood pressure in many animals) forces water and small solutes, such as salts, sugars, amino acids, and nitrogenous wastes, into the excretory system. This fluid is called the filtrate. "Even when filtration occurs, fluid collection is largely nonselective; thus, it is important that essential small molecules are recovered from the filtrate and returned to the body fluids. In the second step of the process, selective reabsorption, excretory systems use active transport to reabsorb valuable solutes such as glucose, certain salts, and amino acids from the filtrate. Nonessential solutes and wastes (for example, excess salts and toxins) are left in the filtrate or are added to it by selective secretion, which also uses active transport. The pumping of various solutes also adjusts the osmotic movement

of water into or out of the filtrate. The processed filtrate is then excreted from the system and from the body as urine.

Survey of Excretory Systems The systems that perform basic excretory functions vary widely among animal groups. However, they are generally built on a complex network of tubules that provide a large surface area for the exchange of water and solutes, including nitrogenous wastes. Protonephridia: Flame-Bulb Systems Flatworms (phylum Platyhelminthes) have excretory systems called protonephridia (singular, protoncphridlum), A protonephridium is a network of dead-end tubules lacking internal openings. As shown in Figure 44.10, the tubules branch throughout the body, and the smallest branches are capped by a cellular unit called a flame bulb. The flame bulb has a tuft of cilia projecting into the tubule. (The beating cilia resemble a flickering flame, hence, the name flame bulb.) The beating of the cilia draws water and solutes lrom the interstitial fluid through the flame bulb (filtration) into the tubule system and then moves the urine outward through the tubules until they empty to the external environment through openings called

0 Filtration. The excretory tubule collects a filtrate from the blood. Water and solutes are forced by blood pressure across the selectively permeable membranes of a cluster of capillaries and into the excretory tubule.

Interstitial fluid filters through membrane where cap cell and tubule cell interdigitate (interlock)

© Reabsorption. The transport epithelium reclaims valuable substances from the filtrate and returns them to the body fluids.

Tubule cell -

0 Secretion. Other substances, such as toxins and excess ions, are extracted from body fluids and added to the contents of the excretory tubule.

O Excretion. The filtrate leaves the system and the body.

Figure 44.9 Key functions of excretory systems: an overview. Most excretory systems produce a filtrate by pressureiltering body fluids and then modify the filtrate's contents. This diagram is modeled after the vertebrate excretory system.

•Protonephridia (tubules)

Tubule

A Figure 44.10 Protonephridia: the flame-bulb system of a planarian. Protonephridia are branching internal tubules that function mainly in osmoregulation. CHAPTER 44

Osmoresuiaiion and Excretion

929

volumes of very dilute solutions, and most terrestrial animals and many marine species (which tend to lose water to their environment by osmosis) simply do not have access to sufficient^

waste product conveyed a selective advantage because it precipitates out of solution and can be stored within the egg as a harmless solid left behind when the animal hatches.

nephridiopores. Excreted urine is very dilute in freshwater flatworms, helping balance the osmotic uptake of water from the environment. Apparently, the tubules reabsorb most solutes before the urine exits the body. The flame-bulb systems of freshwater flatworms seem to function mainly in osmoregulation; most metabolic wastes diffuse out of the animal across the body surface or are excreted into the gastrovascular cavity and eliminated through the mouth (see Figure 33.10). However, in some parasitic flatworms, which are isoosmotic to the surrounding fluids of their host organisms, protonephridia mainly dispose of nitrogenous wastes. This difference in function shows how structures common to a group of organisms can be adapted in diverse ways by evolution in different environmenis. Protonephridia are also found in rotifers, some annelids, the larvae of molluscs, and lancelets, which are invertebrate chordates. (See Chapters 33 and 34 to review these animal phyla.)

a metanephridium is surrounded by a ciliated funnel, the nephrostome. Fluid enters the nephrostome and passes through a coiled collecting tubule, which includes a storage bladder that opens to the outside through the nephridiopore. An earthworm's metanephridia have both excretory and oismoregulatory functions. As urine moves along the tubule, the transport epithelium bordering the lumen reabsorbs most solutes and returns them to the blood in the capillaries. Nitrogenous wastes remain in the tubule and are excreted to the outside. Earthworms inhabit damp soil and usually experience a net uptake of water by osmosis through the skin. Their metanephridia balance the water influx by producing dilute urine (hypoosmotic to the body fluids).

Metanephridia Another type of tubular excretory system, metanephridia (singular, metanephridium), has internal openings that collect body fluids (Figure 44.11). Metanephridia are found in most annelids, including earthworms. Each segment of a worm has a pair of metanephridia, which are immersed in coelornic fluid and enveloped by a capillary network. The internal opening of

Malpighian Tubules Insects and other terrestrial arthropods have organs called Malpighian tubules that remove nitrogenous wastes and also function in osmoregulation (Figure 44.12). The malpighian tubules open into the digestive tract and dead-end at tips that are immersed in hemolymph (circulatory fluid). The transport epithelium lining the tubules secretes certain solutes, including nitrogenous wastes, from the hemolymph into the lumen of the tubule. Water follows the solutes into the tubule by osmosis, and the fluid then passes into the rectum, where most solutes are pumped back into the hemolymph. Water again follows the solutes, and the nitrogenous wastes—mainly insoluble uric acid—-are eliminated as nearly dry matter along

Digestive tract

Midgut (stomach)

f

Sait, water, and nitrogenous "^ £ wastes ^

Nephridiopore Nephrostome

Malpighian tubules Feces and urine

Anus

Reabsorption of H 2 O, ions, and valuable organic molecules

Metanephridium HEMOLYMPH

A Figure 44.11 Metanephridia of an earthworm. Each segment of the worm contains a pair of metanephridia, which collect coelomic fluid from the adjacent anterior segment. (Only one metanephridium of each pair is shown here.)

930

UNIT SEVEN

Animal Form and Function

A Figure 44.12 Malpighian tubules of insects. Malpighian tubules are outpocketings of the digestive tract that remove nitrogenous wastes and function in osmoregulation.

with the feces. Highly effective in conserving water, Lhe insect excretory system is one of several key adaptations contributing to the tremendous success of these animals on land. Vertebrate Kidneys The kidneys of vertebrates usually function in both osmoregulation and excretion. Like the excretory organs of most animal phyla, kidneys are built of tubules. Since the osmoconforming hagfishes, which are not vertebrates but are among the most primitive living chordates, have kidneys with segmentally arranged excretory tubules, it is likely that the excretory structures of vertebrate ancestors were segmented. The kidneys of most vertebrates, however, are compact, nonsegmented organs containing numerous tubules arranged in a highly organized manner. A dense network of capillaries intimately associated with the tubules is also an integral part of the vertebrate excretory system, as are the ducts and other structures that carry urine out of the tubules and kidney and eventually out of the body. In the next two sections, we will focus on the mammalian excretory' system, using humans as our primary example. We will then end the chapter by comparing the excretory organs of the various vertebrate classes to see how evolutionary modifications function in different environments.

Concept Check' 1. What are the fundamental processes involved in all excretory systems, regardless of their anatomical differences or evolutionary origins? 2. Describe some advantages of an excretory system built around a network of fine tubules.

urinary bladder. During urination, urine is expelled from the urinary bladder through a tube called the urethra, which empties to the outside near the vagina in females or through the penis in males. Sphincter muscles near the junction of the urethra and the bladder, which are under nervous system control, regulate urination.

Structure and Function of the Nephron and Associated Structures The mammalian kidney has two distinct regions, an outer renal cortex and an inner renal medulla (Figure 44.13b). Packing both regions are microscopic excretory tubules and their associated blood vessels. The nephron—the functional unit of the vertebrate kidney—consists of a single long tubule and a ball of capillaries called the glomemlus (Figure 44.13c and d). The blind end of the tubule forms a cup-shaped swelling, called Bowman's capsule, which surrounds the glomerulus. Each human kidney contains about a million nephrons, with a total tubule length of 80 km. Filtration of the Blood Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman's capsule (see Figure 44.13d). The porous capillaries, along with specialized cells of the capsule called podocytes, are permeable to water and small solutes but not to blood cells or large molecules such as plasma proteins. Filtration of small molecules is nonselective, and the nitrate in Bowman's capsule contains salts, glucose, amino acids, and vitamins; nitrogenous wastes such as urea; and other small molecules—a mixture that mirrors the concentrations of these substances in blood plasma.

for suggested answers, see Appendix A.

Pathway of the Filtrate

Nephrons and associated blood vessels are the functional units of the mammalian kidney The excretory system of mammals centers on the kidneys, which are also the principal site of water balance and salt regulation. Mammals have a pair of kidneys. Each kidney, beanshaped and about 10 cm long in humans, is supplied with blood by a renal artery and drained by a renal vein (Figure 44.13a, on the following page). Blood flow through the kidneys is voluminous. In humans, the kidneys account for less than 1% of body weight, but they receive about 20% of resting cardiac output. Urine exits each kidney through a duct called the ureter, and both ureters drain into a common

From Bowman's capsule, the nitrate passes through three regions of the nephron: the proximal tubule; the loop of Henle, a hairpin turn with a descending limb and an ascending limb; and the distal tubule. The distal tubule empties into a collecting duct, which receives processed nitrate from many nephrons. This nitrate flows from the many collecting ducts of the kidney into the renal pelvis, which is dramed by the ureter. In the human kidney, approximately 80% of the nephrons, the cortical nephrons, have reduced loops of Henle and are almost entirely confined to the renal cortex. The other 20%, the juxtamedullary nephrons, have well-developed loops that extend deeply into the renal medulla. Only mammals and birds have juxtamedullary nephrons; the nephrons of other vertebrates lack loops of Henle. It is the juxtamedullary nephrons that enable mammals to produce urine that is hyperosmotic to body fluids, an adaptation that is extremely important fox water conservation. CHAPTER 44 Osmoregulation and Excretion

931

(a) Excretory organs and major associated blood vessels

(c) Nephron

A Figure 44.13 The mammalian excretory system.

The nephron and the collecting duct are lined by a transport epithelium that processes the filtrate to form the urine. One of this epithelium's most important tasks is reabsorption of solutes and water. Between 1,100 and 2,000 L of blood flows through a pair of human kidneys each day a volume about 275 times the total volume of blood in the body From 932

UNIT SEVEN

Animal Form and Function

this enormous traffic of blood, the nephrons and collecting ducts process about 180 L of initial filtrate, equivalent to two or three times the body weight of an average person. Of this, nearly all of the sugar, vitamins, and other organic nutrients and about 99% of the water are reabsorbed into the blood, leaving only about 1.5 L of urine to be voided.

Blood Vessels Associated with the Nephrons Each nephron is supplied with blood by an afferent arteriole, a branch of the renal artery that subdivides into the capillaries of the glomerulus (see Figure 44.13d). The capillaries converge as they leave the glomerulus, forming an efferent arteriole. This vessel subdivides again, forming the peritubular capillaries, which surround the proximal and distal tubules. More capillaries extend downward and form the vasa recta, the capillaries that serve the loop of Henle. The vasa recta also form a loop, with descending and ascending vessels conveying blood in opposite directions. Although the excretory tubules and their surrounding capillaries are closely associated, they do not exchange materials directly. The tubules and capillaries are immersed in interstitial fluid, through which various substances diffuse between the plasma within capillaries and the nitrate within the nephron tubule. This exchange is facilitated by the relative direction of blood flow and nitrate flow in the nephrons.

From Blood Filtrate to Urine: A Closer Look In this section we will concentrate on how the filtrate becomes urine as it flows through the mammalian nephron and col-

Proximal tubule NaCI

Nutrients

HCO'-f HO 4 K"

lecting duct. The circled numbers correspond to the numbers in Figure 44.14. O Proximal tubule. Secretion and reabsorption in the proximal tubule substantially alter the volume and composition of filtrate. For example, the cells of the transport epithelium help maintain a relatively constant pH in body fluids by the controlled secretion of H + . The cells also synthesize and secrete ammonia, which neutralizes the acid and keeps the filtrate from becoming too acidic. The more acidic the filtrate, the more ammonia the cells produce and secrete, and the urine of a mammal usually contains some ammonia from this source (even though most nitrogenous waste is excreted as urea). The proximal tubules also reabsorb about 90% of the important buffer bicarbonate (HCO3~). Drugs and other poisons that have been processed in the liver pass from the peritubular capillaries into the interstitial fluid, and then are secreted across the epithelium of the proximal tubule into the nephron's lumen. Conversely, valuable nutrients, including glucose, amino acids, and potassium (K + ), are actively or passively transported from the filtrate to the interstitial fluid and then are moved into the peritubular capillaries.

0 Distal tubule H2O NaCI 4 HCO3"

=il trate Salts (NaC! and others)

H*

OUTER MEDULLA

Urea Glucose; amino acids Some drugs

r

Key

Active transport -»#• Passive transport *™*^-

A Figure 44.14 The nephron and collecting duct: regional functions ~\, of the transport epithelium. The numbered regions in this diagram are keyed to the circled numbers in the text discussion of kidney function

CHAPTER 44

Osmoregulation and Excretion

933

One of the most important functions of the proximal tubule is reabsorpnon of most of the NaCl (salt) and water from the huge initial filtrate volume. Salt in the filtrate diffuses into the cells of the transport epithelium, and the membranes of the cells actively transport Na1 into the interstitial fluid. This transfer of positive charge is balanced by the passive transport of C\~ out of the tubule. As salt moves from the filtrate to the interstitial fluid, water follows by osmosis. The exterior side of the epithelium has a much smaller surface area than the side facing the lumen, minimizing leakage of salt and water back into the tubule. Instead, the salt and water now diffuse from the interstitial fluid into the peritubular capillaries. © Descending limb of the loop of Henle. Reabsorption of water continues as the filtrate moves into the descending limb of the loop of Henle. Here the transport epithelium is freely permeable to water but not very permeable to salt and other small solutes. For water to move out of the tubule by osmosis, the interstitial fluid bathing the tubule must be hyperosmotic to the filtrate. The osmolarity of the interstitial fluid does in fact become progressively greater from the outer cortex to the inner medulla of the kidney, Thus, filtrate moving downward from the cortex to the medulla within the descending limb of the loop of Henle continues to lose water to interstitial fluid of greater and greater osmolarity, which increases the solute concentration of the filtrate. © Ascending limb of the loop of Henle. The filtrate reaches the tip of the loop, deep in the renal medulla in the case of juxtamedullary nephrons, then moves back to the cortex within the ascending limb. In contrast to the descending limb, the transport epithelium of the ascending limb is permeable to salt but not to water. The ascending limb has two specialized regions: a thin segment near the loop tip and a thick segment adjacent to the distal tubule. As filtrate ascends in the thin segment, NaCl, which became concentrated in the descending limb, diffuses out of the permeable tubule into the interstitial fluid. This movement increases the osmolarity of the interstitial fluid in the medulla. The exodus of salt from the filtrate continues m the thick segment of the ascending limb, but here the epithelium actively transports NaCl into the interstitial fluid. By losing salt without giving up water, the nitrate is progressively diluted as it moves up to the cortex in the ascending limb of the loop. 0 Distal tubule. The distal tubule plays a key role in regulating the KT^ and NaCl concentration of body fluids by varying the amount of the K4" that is secreted into the fil934

UNIT SEVEN

Animal Form and F u n d ion

trate and the amount of NaCl reabsorbed from the filtrate. Like the proximal tubule, the distal tubule also contributes to pH regulation by the controlled secretion o f H+ and reabsorption of bicarbonate (HCO3~). @ Collecting duct. The collecting duct carries the filtrate through the medulla to the renal pelvis. By actively reabsorbing NaCl, the transport epithelium of the collecting duct plays a large role in determining how much salt is actually excreted in the urine. Though its degree of permeability is under hormonal control, the epithelium is permeable to water. However, it is not permeable to salt or, in the renal cortex, to urea. Thus, as the collecting duct traverses the gradient of osmolarity in the kidney, the filtrate becomes increasingly concentrated as it loses more and more water by osmosis to the hyperosmotic interstitial fluid. In the inner medulla, the duct becomes permeable to urea. Because of the high urea concentration in the filtrate at this point, some urea diffuses out of the duct and into the interstitial fluid. Along with NaCl, this urea contributes to the high osmolarity of the interstitial fluid in the medulla. This high osmolarity enables the mammalian kidney to conserve water by excreting urine that is hyperosmotic to the general body fluids.

Concept Check

;

1. How would a decrease in blood pressure in the afferent arteriole leading to a glomerulus affect the rate of filtration of blood within Bowman's capsule? 2. A variety of drugs make the epithelium of the collecting duct less permeable to water. How would this affect kidney function? 3. List these parts of a nephron in the order in which filtrate moves through them: proximal tubule, Bowman's capsule, distal tubule, loop of Henle For suggested answers, see Appendix A.

Concept

The mammalian kidney's ability to conserve water is a key terrestrial adaptation The osmolarity of human blood is about 300 mosm/L, but the kidney can excrete urine up to four times as concentrated— about 1,200 mosm/L. Some mammals can do even better. For

example, Australian hopping mice, which live in dry desert regions, can produce urine concentrated to 9,300 mosm/L— 9 times as concentrated as seawater and 25 times as concentrated as the animal's body fluid.

Solute Gradients and Water Conservation In a mammalian kidney, the cooperative action and precise arrangement of the loops of Henle and the collecting ducts are largely responsible for the osmotic gradient that concentrates the urine. But even with this highly organized structure, the maintenance of osmotic differences and the production of hyperosmotic urine are possible only because considerable energy is expended for the active transport of solutes against concentration gradients. In essence, the nephrons—especially the loops of Henle—-can be thought of as tiny energyconsuming machines whose function is to produce a region of high osmolarity in the kidney, which can then be used to extract water from the filtrate in the collecting duct. The two primary solutes in this osmolarity gradient are NaCl, which is deposited in the renal medulla by the loop of Henle, and urea, which leaks across the epithelium of the collecting duct in the inner medulla (see Figure 44.14).

tubule as the nitrate rounds the curve and enters the ascending limb, which, remember, is permeable to salt but not to water. Thus, the two limbs of the loop of Henle cooperate in maintaining the gradient of osmolarity in the interstitial fluid of the kidney. The descending limb produces a progressively saltier nitrate, and then NaCl diffuses from the ascending limb to help maintain a high osmolarity in the interstitial fluid of the renal medulla. Notice that the loop of Henle has several qualities of a countercurrent system, similar in principle to the count ere urrent mechanisms that maximize oxygen absorption by fish gills (see Figure 42.21) or reduce heat loss in endotherms (see Figure 40.15). In these cases, the countercurrent mechanisms involve passive movement along either an oxygen concentration gradient or a heat gradient. In contrast, the countercurrent system involving the loop of Henle expends energy to

To better understand the physiology of the mammalian kidney as a waterconserving organ, let's retrace the flow of filtrate through the excretory tubule, this time focusing on how the juxtamedullary nephrons maintain an osmolarity gradient in the kidney and use that gradient to excrete a hyperosmotic urine (Figure 44,15). Filtrate passing from Bowman's capsule to the proximal tubule has an osmolarity of about 300 mosm/L, the same as blood. As the nitrate flows through the proximal tubule in the renal cortex, a large amount of water and salt is reabsorbed; thus, the volume of filtrate decreases substantially, but because of the salt loss, its osmolarity remains about the same. As the filtrate flows from cortex to medulla in the descending limb of the loop of Henle, water leaves the tubule by

A

Figure 44.15 How the human kidney concentrates urine: the two-solute

Ti

-i . r -i n model. Two solutes contribute to the osmolarity of the interstitial fluid' NaCl and urea The loop osmosis, i n e osmoianty ot me nitrate 0 f H enle maintains the interstitial gradient of NaCl, which increases in the descending limb and increases as solutes, including NaCl, be- decreases in the ascending limb. Urea diffuses into the interstitial fluid of the medulla from the come more concentrated The highest os- collecting duct (most of the urea in the filtrate remains in the collecting duct and is excreted). The molarity (about 1,200 mosm/L) occurs at f i l t r a t e ™, kes * r e e tri P s b e t « e e " *? rorte * a n d medulla, first down, then up, and then down again in the collecting duct. As the filtrate flows in the collecting duct past interstitial fluid of the eLbow ot the loop of Henle. This max- increasing osmolarity, more water moves out of the duct by osmosis, thereby concentrating the imizes the diffusion of salt out of the solutes, including urea, that are left behind in the filtrate.

Osmoregulation and Excretion

935

actively transport NaCl from the filtrate in the upper part of the ascending limb of the loop. Such countercurrent systems, which expend energy to create concentration gradients, are called countercurrent multiplier systems. The countercurrent multiplier system involving the loop of Henle maintains a high salt concentration in the interior of the kidney, enabling the kidney to form concentrated urine. What prevents the capillaries of the vasa recta from dissipating the gradient by carrying away the high concentration of NaCl in the medullas interstitial fluid? Notice in Figure 44.13 that the vasa recta is also a countercurrent system, with descending and ascending vessels carrying blood in opposite directions through the kidney's osmolarity gradient. As the descending vessel conveys blood toward the inner medulla, water is lost from the blood and NaCl diffuses into it- These fluxes are reversed as blood flows back toward the cortex in the ascending vessel, with water reentering the blood and salt diffusing out. Thus, the vasa recta can supply the kidney with nutrients and other important substances carried by blood without interfering with the osmolarity gradient that makes it possible for the kidney to excrete hyperosmotic urine. The countercurrent-like characteristics of the loop of Henle and the vasa recta make it easier to maintain the steep osmotic gradient between the medulla and cortex. However, any osmotic gradient in an animal will eventually be eliminated by diffusion unless energy is expended to preserve it. In the kidney, this expenditure largely occurs in the thick segment of the ascending limb of the loop of Henle, where NaCl is actively transported out of the tubule. Even with the benefits of countercurrent exchange, this process—along with other renal active transport systems—consumes considerable ATP, and for its size, the kidney has one of the highest metabolic rates of any organ. By the time the filtrate reaches the distal tubule, it is actually hypoosmotic to body fluids because of active transport of NaCl out of the thick segment of the ascending limb. Now the filtrate descends again toward the medulla, this time in the collecting duct, which is permeable to water but not to salt. Therefore, osmosis extracts water from the filtrate as it passes from cortex to medulla and encounters interstitial fluid of increasing osmolarity. This concentrates salt, urea, and other solutes in the filtrate. Some urea leaks out of the lower portion of the collecting duct and contributes to the high interstitial osmolarity of the inner medulla. (This urea is recycled by diffusion into the loop of Henle, but continual leakage from the collecting duct maintains a high interstitial urea concentration.) Before leaving the kidney, the urine may attain the osmolarity of the interstitial fluid in the inner medulla, which can be as high as 1,200 mosm/L. Although isoosmotic to the inner medullas interstitial fluid, the urine is hyperosmotic to blood and interstitial fluid elsewhere in the body This high osmolarity allows the solutes remaining in the urine to be excreted from the body with minimal water loss. 936

UNIT SEVEN Animal Form and Function

The juxtamedullary nephron, with its urine-concentrating features, is a key adaptation to terrestrial life, enabling mammals to get rid of salts and nitrogenous wastes without squandering water. As we have seen, the remarkable ability of the mammalian kidney to produce hyperosmotic urine is completely dependent on the precise arrangement of the tubules and collecting ducts in the renal cortex and medulla. In this respect, the kidney is one of the clearest examples of how the function of an organ is inseparably linked to its structure

Regulation of Kidney Function One of the most important aspects of the mammalian kidney is its ability to adjust both the volume and osmolarity of urine, depending on the animal's water and salt balance and the rate of urea production. In situations of high salt intake and low water availability, a mammal can excrete urea and salt with minimal water loss in small volumes of hyperosmotic urine. But if salt is scarce and fluid intake is high, the kidney can get nd of the excess water with little salt loss by producing large volumes of hypoosmotic urine (as dilute as 70 mosm/L, compared to about 300 mosm/L for human blood). This versatility in osmoregulatory function is managed with a combinatio: of nervous and hormonal controls. One hormone that is important in regulating water balance is antidiuretic hormone (ADH) (Figure 44.16a). ADH is produced in the hypothalamus of the brain and is stored in and released from the posterior pituitary gland, which is positioned just below the hypothalamus. Osmoreceptor cells in the hypothalamus monitor the osmolarity of blood; when it rises above a set point of 300 mosm/L (perhaps due to water loss from sweating or to ingestion of salty food), more ADH is released into the bloodstream and reaches the kidney. The main targets of ADH are the distal tubules and collecting ducts of the kidney, where the hormone increases the permeability of the epithelium to water. This amplifies water reabsorption. which reduces urine volume and helps prevent further increase of blood osmolarity above the set point. By negative feedback, the subsiding osmolarity of the blood reduces the activity of osmoreceptor cells in the hypothalamus, and less ADH is then secreted. But only the gain of additional water m food and drink can bring osmolarity all the way back down to 300 mosm/L. Conversely, if a large intake of water has reduced blood osmolarity below the set point, very little ADH is released. This decreases the permeability of the distal tubules and collecting ducts, so water reabsorption is reduced, resulting in increased discharge of dilute urine. (Increased urination is called diuresis, and it is because ADH opposes this state that it is called antidiuretic hormone.) Alcohol can disturb water balance by inhibiting the release of ADH, causing excessive urinary water loss and dehydration (which may cause some of the symptoms of a hangover). Normally, blood osmolarity, ADH

STIMULUS: The juxtaglomerular apparatus (JGA) responds to low blood volume or blood pressure (such as due to dehydration or loss of blood)

STIMULUS: The release of AOH is triggered when osmoreceptor cells in the hypothalamus detect an increase in the osmotarity of the blood

Collecting duct

5 (AngiotensinogeTi)

Homeostasis: Blood osmolarity a) Antidiuretic hormone (ADH) enhances fluid retention by making the kidneys reclaim more water.

(b) The renin-angiotensin-aldosterone system (RAAS) leads to an increase in blood volume and pressure.

A Figure 44.16 Hormonal control of the kidney by negative feedback circuits.

please, and water reabsorption in the kidney are all linked in feedback loop that contributes to homeostasis. A second regulatory mechanism involves a specialized tissue called the juxtaglomerular apparatus (JGA), located near the afferent arteriole that supplies blood to the glomerulus (Figure 44.16b). When blood pressure or blood volume in the afferent arteriole drops (for instance, as a result of reduced salt intake or loss of blood), the enzyme renin initiates chemical reactions that convert a plasma protein called angiotensinogen to a peptide called angiotensin II. Functioning as a hormone, angiotensin II raises blood pressure by constricting arterioles, decreasing blood flow to many capillaries, including those of the kidney Angiotensin II also stimulates the proximal tubules of the nephrons to reabsorb more NaCl and water. This reduces the amount of salt and water excreted in the urine and consequently raises blood volume and pressure. Another effect of angiotensin II is stimulation of the adrenal glands to release a hormone called aldosterone. This hormone acts on the

nephrons' distal tubules, making them reabsorb more sodium (Na+) and water and increasing blood volume and pressure. In summary, the renin-angiotensin-aldosterone system (RAAS) is part of a complex feedback circuit that functions in homeostasis. A drop in blood pressure and blood volume triggers renin release from the JGA. In turn, the rise in blood pressure and volume resulting from the various actions of angiotensin II and aldosterone reduce the release of renin. The functions of ADH and the RAAS may seem to be redundant, but this is not the case. Both increase water reabsorption, but they counter different osmoregulatory problems. The release of ADH is a response to an increase in the osmolarity of the blood, as when the body is dehydrated from excessive water loss or inadequate intake of water. However, a situation that causes an excessive loss of both salt and body fluids—an injury, for example, or severe diarrhea—will reduce blood volume without increasing osmolarity. This will not induce a change in ADH release, but the RAAS will respond to CHAPTER

44

ion and Excretion

937

the fall in blood volume and pressure by increasing water and Na+ reabsorption. ADH and the RAAS are partners in homeostasis; ADH alone would lower blood Na" concentration by

access to drinking water to dilute it. Instead, their kidneys shift to producing small quantities of highly concentrated urine (up to 4,600 mosm/L), an adjustment that disposes of

stimulating water reabsorption in the kidney, but the RAAS

the urea load while conserving as much water as possible. The

helps maintain balance by stimulating Na+ reabsorption. Still another hormone, a peptide called atrial natriuretic factor (ANF), opposes the RAAS. The walls of the atria of the heart release ANF in response to an increase in blood volume and pressure. ANF inhibits the release of renin from the JGA, inhibits NaCl reabsorption by the collecting ducts, and reduces aldosterone release from the adrenal glands. These actions lower blood volume and pressure. Thus, ADH, the RAAS, and ANF provide an elaborate system of checks and balances that regulate the kidney's ability to control the osmolarity, salt concentration, volume, and pressure of blood. The precise regulatory role of ANF is an area of active research. The flexibility of the mammalian kidney enables it to adjust rapidly to contrasting osmoregulatory and excretory problems. The South American vampire bat illustrates this versatility (Figure 44.17). Bats of this species feed on the blood of large birds and mammals. The bats use their sharp teeth to make a small incision in the victims skin and then lap up blood from the wound. Anticoagulants in the bat's saliva prevent the blood from clotting, but the prey animal is usually not seriously harmed. Because vampire bats often search for many hours and fly for long distances to locate a suitable victim, they benefit from consuming as much blood as possible when they do find prey—so much that after feeding, a bat could be too heavy to fly. However, the bat's kidneys offload much of the water absorbed from a blood meal by excreting large volumes of dilute urine as it feeds, up to 24% of body mass per hour. Having lost enough weight to take off, the bat can fly back to its roost in a cave or hollow tree, where it spends the day In the roost, the bat faces a very different regulatory problem. Its food is mostly protein, which generates large quantities of urea—but roosting bats don't have

vampire bat's ability to alternate rapidly between producing large amounts of dilute urine and small amounts of very hyperosmotic urine is an essential part of its adaptation to an unusual food source. Concept Check 1. How does alcohol affect regulation of water balance in the body? 2. How does eating salty food affect kidney function? 3. Identify a major functional consequence of the countercurrent-like characteristics of the loop of Henle. For suggested answers, see Appendix A.

Concept

Diverse adaptations of the vertebrate kidney have evolved in different environments Vertebrate animals occupy habitats ranging from rain forests to deserts and from some of the saltiest bodies of water to the dilute waters of high mountain lakes. Variations in nephron structure and function equip the kidneys of different vertebrates for osmoregulation in their various habitats. The adaptations of the vertebrate kidney are most clearly revealed by comparing species that inhabit a wide range of environments or by comparing the responses of different vertebrate groups to similar environmental conditions (Figure 44.18). In all animals, the intricate physiological machines we call organs work continuously, maintaining solute and water balance and excreting nitrogenous wastes. The details that we have reviewed in this chapter only hint at the great complexity of the neural and hormonal mechanisms involved in regulating these homeostatic processes. The next chapter furthe explores the hormonal control of homeostasis. Concept Check * 1. What do the number and length of nephrons indicate about the habitat of fishes? What do they indicate about rates of urine production?

A Figure 44.17 A vampire bat (Desmodus rotundas), a mammal with a unique excretory situation. 938

UNIT SEVEN

Animal Form and Function

For suggested answers, see Appendix A.

Figure 44.18

Environmental Adaptations of the Vertebrate Kidney BIRDS AND OTHER REPTILES Mammals that excrete the most hyperosmotic urine, such as Australian hopping mice, North American kangaroo rats, and other desert mammals, have exceptionally long loops of Henk. Long loops maintain steep Bannertail Kangaroo rat osmotic gradients in the kidney, {Dipodomys spectabilis) resulting in urine becoming very concentrated as it passes from cortex to medulla in the collecting ducts. In contrast, beavers, muskrats, and other aquatic mammals that spend much of their time in fresh water and rarely face problems of dehydration, have nephrons with very short loops, resulting in a much lower ability to concentrate urine. Terrestrial mammals living in moist conditions have loops of Henle of intermediate length and the capacity to produce urine intermediate in concentration to that produced by Beaver {Castor canadensis) freshwater and desert mammals.

Because they are hyperosmotic to their surroundings, freshwater fishes must excrete excess water continuously. In contrast to mammals and birds, freshwater fishes produce large volumes of very dilute urine. Their kidneys, which have a large number of Rainbow trout nephrons, produce filtrate at a {Oncorrhynchus mykiss) high rate. Freshwater fishes conserve salts by reabsorbing ions from the filtrate in their distal tubules, leaving water behind. Amphibian kidneys function much like those of freshwater fishes. When in fresh, water, the skin of the frog accumulates certain salts from the water by active transport, and the kidneys excrete dilute urine. On land, where dehydration is the most pressing problem of osmoregulation, frogs conserve body fluid by reabsorbing water across the epithelium of the unnary bladder. Frog {Rana temporaria)

HI

Birds, like mammals, have kidneys with juxtamedullary nephrons that specialize in conserving water. However, the nephrons of birds have much shorter loops of Henle; thus, bird kidneys cannot concentrate urine to the high Roadrunner osmolarities achieved by (Geococcyx californianus) mammalian kidneys. Although they can produce hyperosmotic urine, the main water conservation adaptation of birds is uric acid, which can be excreted as a paste, as the nitrogen waste molecule, thereby reducing urine volume. The kidneys of other reptiles, having only cortical nephrons, produce urine that is, at most, isoosmotic to body fluids. However, the epithelium of the cloaca (see Chapter 34) helps conserve fluid by reabsorbing some of the water present in urine and feces. Also like birds, most other terrestrial reptiles excrete nitrogenous wastes as uric Desert iguana acid. {Dipsosaurus dorsalis)

Northern bluefin tuna (Thunnus thynnus) Because they are hypoosmotic to seawater, marine bony fishes lose body water and gain excess salts from their surroundings, environmental challenges opposite those faced by their freshwater relatives. Compared to freshwater fishes, marine fishes have fewer and smaller nephrons, which lack a distal tubule. In addition, the kidneys of most marine fishes have small glomeruli, and some lack glomeruli entirely. Thus, the kidneys of marine fishes have low filtration rates and excrete very little urine. The main function of the kidneys is to get rid of double-charged ions such as calcium (Ca 2+ ), magnesium (Mg 2+ ), and sulfate (SO 4 2 ), which the fish takes in by its incessant drinking of seawater. Marine fishes rid themselves of these ions by secreting them into the proximal tubules of the nephrons and excreting them with the urine.

CHAPTER 44

Osmoregulation and Excretion

939

Chapter Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY CONCEPTS Osmoregulation balances the uptake and loss of water and solutes • Osmoregulation is based largely on the controlled movement of solutes between internal fluids and the external environment, and the movement of water, which follows by osmosis (p. 922). • Osmosis (pp. 922-923) Cells require a balance between osmotic gain and loss of water. Water uptake and loss are balanced by various mechanisms of osmoregulation in different environments. • Osmotic Challenges (pp. 923-926) Osmoconformers, which are only marine animals, are isoosmotic with their surroundings and do not regulate their osmolarity. Osmoregulators expend energy to control water uptake and loss in a hyperosmotic or hypoosmotic environment. Among marine animals, most invertebrates are osmoconformers. Sharks have an osmolarity slightly higher than seawater because they retain urea. Marine bony fishes lose water to their hyperosmotic environment and drink seawater. Marine vertebrates excrete excess salt through rectal glands, gills, salt-excreting glands, or kidneys. Freshwater animals, which constantly take in water from their hypoosmotic environment, excrete dilute urine. Lost salt is replaced by eating or by ion uptake by gills. Terrestrial animals combat desiccation through behavioral adaptations, water-conserving excretory organs, and drinking and eating food with high water content. Animals in temporary waters may be anhydrobiotic. • Transport Epithelia (pp. 926-927) Water balance and waste disposal depend on transport epithelium, layers of specialized epithelial cells that regulate the solute movements required for waste disposal and for tempering changes in body fluids.

An animars nitrogenous wastes reflect its phylogeny and habitat • Forms of Nitrogenous Waste (pp. 927-928) Protein and nucleic acid metabolism generates ammonia, a toxic waste product excreted in three forms. Most aquatic animals excrete ammonia across the body surface or gill epithelia into the surrounding water. The liver of mammals and most adult amphibians converts ammonia to the less toxic urea, which is carried to the kidneys, concentrated, and excreted with a minimal loss of water. Uric acid is an insoluble, paste-like precipitate excreted in the paste-like urine of land snails, insects, and many reptiles, including birds. • The Influence of Evolution and Environment on Nitrogenous Wastes (p. 928) The kinds of nitrogenous wastes excreted depend on an animal's evolutionary history and habitat. The amount of nitrogenous waste produced is coupled to the animal's energy budget.

Diverse excretory systems are variations on a tubular theme • Excretory Processes (p. 929) Most excretory systems produce urine by refining a filtrate derived from body fluids.

940

UNIT SEVEN

Animal Form and Function

•;• Review Key functions of most excretory systems are nitration (pressurefiltering of body fluids, producing a filtrate) and the production of urine from the filtrate by selective reabsorption, (reclaiming valuable solutes from the filtrate) and secretion (addition of toxins and other solutes from the body fluids to the filtrate). • Survey of Excretory Systems (pp. 929-931) Extracellular fluid is filtered into the protonephridia of the flame-bulb system in flatworms; these tubules excrete a dilute fluid and also function in osmoregulation. Each segment of an earthworm has a pair of open-ended metanephridia, tubules that collect coelomic fluid and produce dilute urine for excretion. In insects. Malpighian tubules function in osmoregulation and removal of nitrogenous wastes from the hemolymph. Insects produce a relatively dry waste matter, an important adaptation to terrestrial life. Kidneys, the excretory organs of vertebrates, function in both excretion and osmoregulation.

Nephrons and associated blood vessels are the functional units of the mammalian kidney • Structure and Function of the Nephron and Associate! Structures (pp. 931-933) Excretory tubules (consisting of nephrons and collecting ducts) and associated blood vessels pack the kidney Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowmans capsule. Filtration of small molecules is nonselective and the filtrate in Bowmans capsule contains a mixture of small molecules that mirrors the concentrations of these substances in blood plasma. Fluid from several nephrons flows into a collecting duct. A urete conveys urine from the renal pelvis to the urinary bladder. • From Blood Filtrate to Urine: A Closer Look (pp. 933-934) Nephrons control the composition of the blood by filtration, secretion, and reabsorption. Secretion and reabsorption in the proximal tubule substantially alter the volume and composition of filtrate. The descending limb of the loop of Henle is permeable to water but not to salt; water moves by osmosis into the hyperosmotic interstitial fluid. Salt diffuses out oi the concentrated filtrate as it moves through the salt-permeable ascending limb of the loop of Henle. The distal tubule plays a key role in regulating the K+ and NaCl concentration of body fluids. The collecting duct carries the filtrate through the medulla to the renal pelvis and reabsorbs NaCl. Activity Structure of the Human Excretory System Activity Nephron Function

The mammalian kidney's ability to conserve water is a key terrestrial adaptation • Solute Gradients and Water Conservation (pp. 935-936; In a mammalian kidney, the cooperative action and precise arrangement of the loops of Henle and the collecting ducts are largely responsible for the osmotic gradient that concentrates the urine. The countercurrent multiplier system involving the loop o Henle maintains a high salt concentration in the interior of die kidney, which enables the kidney to form concentrated urine. The collecting duct, permeable to water but not to salt, conducts the filtrate through the kidney's osmolarity gradient, and more water exits the filtrate by osmosis. Urea, which diffuses out of the collecting duct as it traverses the inner medulla, forms, along with NaCl, the osmotic gradient that enables the kidney to produce urine that is hyperosmotic to the blood.

• Regulation of Kidney Function (pp. 936-938) The osmo larity of the urine is regulated by nervous system and hormonal control of water and salt reabsorption in the kidneys. This regulation involves the actions of antidiuretic hormone (ADH), the renin-angiotensin-aldosterone system (RAAS), and atrial natriutetic factor (ANF). Activity Control of Water Reabsorption investigation What Affects Urine Production?

Diverse adaptations of the vertebrate kidney have evolved in different environments The form and function of nephrons in the various vertebrate classes are related primarily to the requirements for osmoregulation in the animal's habitat. Desert mammals, which excrete the most hyperosmotic urine, have exceptionally long loops of Henle, while animals living in moist or aquatic habitats have short loops and excrete less concentrated urine. Although birds can produce a hyperosmotic urine, the main water conservation adaptation of birds is removal of nitrogen as uric acid, which can be excreted as a paste. Most other terrestrial reptiles excrete uric acid. Freshwater fishes and amphibians produce large volumes of very dilute urine. The kidneys of marine bony fishes have low filtration rates and excrete very little urine, (pp. 938—939)

TESTING YOUR KNOWLEDGE Evolution Connection A large part of the evolutionary success of arthropods and vertebrates on land is attributable to their osmoregulatory capabilities. Compare and contrast the Malpighian tubule with the nephron in regard to anatomy, relationship to circulation, and physiological mechanisms for conserving body water.

Scientific Inquiry Merriam's kangaroo rats {Dipodomys mernami) are native to western North America, where they live in a wide range of habitats ranging from moist, cool woodlands to the hottest and driest places on the continent. Assuming that natural selection acting on local populations has resulted in differences in water conservation among Merriam's kangaroo rat populations, propose a hypothesis concerning the relative rates of evaporative water loss by populations that live in dry versus moist environments. Using a humidity ;nsor to detect evaporative water loss by kangaroo rats, how could you test your hypothesis? nvesttgation What Affects Urine Production?

Science, Technology, and Society Kidneys were the first organs to be successfully transplanted. A donor can live a normal life with a single kidney, making it possible or individuals to donate a kidney to an ailing relative or even an unrelated individual with a similar tissue type. In some countries, wor people sell kidneys to transplant recipients through organ brokers. What are some of the ethical issues associated with this organ commerce?

Uiion snd Excretion

941

lones and the Endocrine System A Figure 45.1 An anise swallowtail butterfly emerging from its chrysalis.

=Key Concepts 45.1 The endocrine system and the nervous system act individually and together in regulating an animal's physiology 45.2 Hormones and other chemical signals bind to target cell receptors, initiating pathways that culminate in specific cell responses 45.3 The hypothalamus and pituitary integrate many functions of the vertebrate endocrine system 45.4 Nonpituitary hormones help regulate metabolism, homeostasis, development, and behavior 4 S 3 Invertebrate regulatory systems also involve endocrine and nervous system interactions

The Body's Long-Distance Regulators r ^ | eople offer hormones as an explanation for the howling I—•'of alley cats and the moodiness of teenagers. In the JL United States more than a million people with diabetes take the hormone insulin, and other hormones are used in cosmetics to keep the skin smooth or in livestock feed to fatten cattle. These powerful substances are also involved in even more dramatic transformations. In becoming an adult, a butterfly like the one in Figure 45,1 undergoes a complete change of body form, a metamorphosis regulated by hormones. Internal communication involving those hormones enables different parts of the insects adult body to develop in :oncert.

An animal hormone (from the Greek horman, to excite) is a chemical signal that is secreted into the circulatory system (usually the blood) and communicates regulatory messages within the body. A hormone may reach all parts of the body, but only certain types of cells, the target cells, are equipped to respond. Thus, a given hormone traveling in the bloodstream elicits specific responses—such as a change in metabolism— from its target cells, while other cell types are unaffected by that particular hormone. In this chapter, we will describe how the basic concepts of biological control systems apply to hormonal pathways and how hormones act on target cells. Here we will focus on hormones that help maintain homeostasis; in Chapters 46 and 47 we will discuss the role of hormones in regulating growth, development, and reproduction. This chapter will examine the major types of hormones in vertebrates, including where they are formed in the body and their major effects. In addition, we will consider comparable regulatory mechanisms in invertebrates.

Concept

The endocrine system and the nervous system act individually and together in regulating an animal's physiology Animals have two systems of internal communication and regulation, the nervous system and the endocrine system. The nervous system, which we will discuss in Chapter 48, conveys high-speed electrical signals along specialized cells called neurons. These rapid messages control the movement of body 943

parts in response to sudden environmental changes, such as occur when you jerk your hand away from a hot pan or when your pupils dilate as you enter a dark room. Collectively, all of an animal's hormone-secreting cells constitute its endocrine system. Hormones coordinate slower but longer-acting responses to stimuli such as stress, dehydration, and low blood glucose levels. Hormones also regulate long-term developmental processes by informing different parts of the body how fast to grow or when to develop the characteristics that distinguish male from female or juvenile from adult. Hormone-secreting organs, called endocrine glands, are referred to as ductless glands because they secrete their chemical messengers directly into extracellular fluid. From there, the chemicals diffuse into the circulation.

Overlap Between Endocrine and Nervous Regulation While it is convenient to distinguish between the endocrine and nervous systems, in reality the lines between these two regulatory systems are blurred. In particular, certain specialized nerve cells known as neurosecretory cells release hormones into the blood. In animals as distinct as insects and vertebrates, a part of the brain called the hypothalamus contains neurosecretory cells. The hormones produced by neurosecretory cells are sometimes called neurohormones to distinguish them from the "classic" hormones released by endocrine glands. A few chemicals serve both as hormones in the endocrine system and as chemical signals in the nervous system. Epinephrine, for example, functions in the vertebrate body as the so-called "fight-or-flight" hormone (produced by the adrenal medulla, an endocrine gland) and as a neurotransmitter, a local chemical signal that conveys messages between neurons in the nervous system (see Chapter 48). In addition, the nervous system plays a role in certain sustained responses—for example, controlling day/night cycles and reproductive cycles in many animals—often by increasing or decreasing secretion from endocrine glands. Thus, although the endocrine and nervous systems are anatomically distinct, they interact functionally in regulating a number of physiological processes.

Control Pathways and Feedback Loops Let's review the fundamental concepts of biological control systems introduced in Chapter 40 and apply them to regulation by hormones. A receptor, or sensor detects a stimulus— for example, a change in blood calcium level—and sends the information to a control center. After comparing the incoming information to a set point, or "desired" value, the control center sends out a signal that directs an effector to respond. In endocrine and neuroendocrine pathways, this outgoing signal, 944

UNIT SEVEN

Animal Form and Function

called an efferent signal, is a hormone or neurohormone, which acts on particular effector tissues and elicits specific physiological or developmental changes. The three types of simple hormonal pathways depicted in Figure 45.2 include these basic functional components of a control system. Not depicted in this figure are complex neuroendocrine pathways in which a hormone secreted by one endocrine tissue acts on another endocrine tissue, controlling its release of a different hormone, which then acts on target tissues. Regulation by each of the 20 or so different hormones you will learn about in this and other chapters involves one of these general types of simple or complex pathways. Another common feature of control pathways is a feedback loop connecting the response to the initial stimulus. In negative feedback, the effector response reduces the initial stimulus, and eventually the response ceases. This feedback mechanism prevents overreaction by the system and wild fluctuations m the variable being regulated. Negative feedback operates in many endocrine and nervous pathways, especially those involved in maintaining homeostasis (see Chapter 40). Later in this chapter, we will examine how negative feedback contributes to the hormonal control of blood calcium and glucose levels. In contrast to negative feedback, which dampens the stimulus, positive feedback reinforces the stimulus and leads to an even greater response. The neurohormone pathway that regulates the release of milk by a nursing mother is an example of positive feedback (see Figure 45.2b). Suckling stimulates sensory nerve cells in the nipples, which send nervous signals that eventually reach the hypothalamus, the control center. An outgoing signal from the hypothalamus triggers the release of the neurohormone oxytocin from the posterior pituitary gland. Oxytocin then causes the mammary glands to secrete milk. The release of milk in turn leads to more suckling and stimulation of the pathway, until the baby is satisfied.

Concept Check• 1. How do neuro hormones differ from "classic" hormones? How are they similar? 2. Different biological control systems exhibit common features: a receptor/sensor, a control center, an efferent signal, and an effector. Draw two sketches showing how these components are arranged in a simple endocrine pathway and a simple neurohormone pathway 3. Explain why, unlike negative feedback, positive feedback is not a common feature of hormonal pathways that help maintain homeostasis. For suggested answers, see Appendix A.

(a) Simple endocrine pathway

(b) Simple neurohormone pathway

4 Figure 45.2 Basic patterns of simple hormonal control pathways. In each pathway, a receptor/sensor (blue) detects a change in some internal or external variable—the stimulus—and informs the control center (gold). The control center sends out an efferent signal, either a hormone (red circles) or neurohormone (red squares). An endocrine cell carries out both the receptor and control center functions.

I Concept

Hormones and other chemical signals bind to target cell receptors, initiating pathways that culminate in specific cell responses Hormones, the body's long-distance chemical regulators, convey information via the bloodstream to target cells throughout the body. Other chemical signals, called local regulators, transmit information to target cells near the secreting cells. Still other chemical signals, called pheromones, carry messages between different individuals of a species, as in mate at-

id

/ Target 1 effectors /

Mammary gland

Response

Miik production

(c) Simple neuroendocrine pathway

traction. In this chapter, as mentioned earlier, we will concentrate on hormones (and neurohormones) that are not directly involved in reproduction. Three major classes of molecules function as hormones in vertebrates: proteins and peptides (small polypeptides containing up to 30 arnino acids); amines derived from amino acids; and steroids. Most protein/peptide and arnine hormones are water-soluble, whereas steroid hormones are not. Regardless of their chemical nature, however, signaling by any oi these molecules involves three key events: reception, signal transduction, and response (see Chapter 11). Reception of the signal occurs when the signal molecule binds to a specific receptor protein in or on the target cell. Each signal molecule has a specific shape that can be recognized by that signal's receptors- Receptors may be located in the plasma membrane of a target cell or inside the cell. The binding of a signal molecule CHAPTER 45

Hormones and the Endocrine System

945

to a receptor protein triggers events within the target cell— signal transduction—that result in a response, a change in the

cell's behavior. Cells that lack receptors for a particular chemical signal are unresponsive to that signal. Let's take a closer look at signal transduction and the kinds of cell responses induced by different types of chemical signals.

Cell-Surface Receptors for Water-Soluble Hormones The receptors for most water-soluble hormones are embedded in the plasma membrane, projecting outward from the cell surface (Figure 45.3a). Binding of a hormone 10 its receptor initiates a signal transduction pathway, a series of changes in cellular proteins that converts an extracellular chemical signal to a specific intracellular responseDepending on the hormone and target cell, the response may be the activation of an enzyme, a change in the uptake or secretion of specific molecules, or rearrangement of the cytoskeleton. Signal transduction from some cell-surface receptors activates proteins in the cytoplasm that then move into the nucleus and directly or indirectly regulate transcription ol specific genes.

that interaction between the hormone and a surface receptor is required for hormone action. A particular hormone may cause diverse responses in target cells with different receptors for the hormone, different signal transduction pathways, and/or different proteins for carrying out the response. Consider the multiple effects of epinephrine in mediating the body's response to short-term stress (Figure 45.4). For example, liver cells and the smooth muscle of blood vessels supplying skeletal muscle contain (3-type epinephrine receptors, whereas the smooth muscle of intestinal blood vessels have a-type epinephrine receptors. These tissues respond differently to epinephrine, resulting in decreased blood flow to the digestive tract and increased delivery of glucose to major skeletal muscles. These effects help the body react quickly in emergencies.

Early evidence for the role of cellsurface receptors in triggering signal transduction pathways came from studies on how the hormone epinephrine stimulates breakdown of glycogen to glucose (see Chapter 11). Another demonstration of the role of cell-surface receptors involves changes in a frog's skin color, an adaptation that helps camouflage the frog in changing light- Skin cells called melanocytes contain the dark brown pigCytoplasmic ment melanin in cytoplasmic organelles response called melanosomes. The frog's skin appears light when melanosomes cluster tightly around the cell nuclei and darker when melanosomes spread throughout the cytoplasm. A peptide hormone called Nuclear response melanocyte-stimulating hormone controls the arrangement of melanosomes and thus the frog's skin color. Adding melanocyte-stimulating hormone to the interstitial fluid surrounding the pigment- (a) Receptor in plasma membrane (b) Receptor in cell nucleus containing cells causes the melanosoraes A Figure 45.3 Mechanisms of hormonal signaling: a review, (a) A water-soluble to disperse. However, direct microinjec- hormone binds to a receptor protein on the surface of a target cell. This interaction triggers a signal tion of melanocyte-stimulating hormone transduction pathway that leads to a change in a cytoplasmic function or a change in gene ° transcription in the nucleus, (b) A lipid-soluble hormone penetrates the target cell s plasma into individual melanocytes does not i n duce melanosome dispersion—evidence 946

UNIT SEVEN

membrane and binds to an intracellular receptor, either in the cytoplasm or in the nucleus (shown here). The signal-receptor complex acts as a transcription factor, typically activating gene expression.

Animal Form and Funciion

> Figure 45.4 One chemical signal, different effects. Epinephrine, the primary "fight-or-flight" hormone, produces different responses in different target cells. Responses of target cells may differ if they have different receptors for a hormone [compare (a) with (b)]. Target cells with the same receptor exhibit different responses if they have different signal transduction pathways and/or effector proteins [compare (b) with (c)].

|3 receptor - -^^Giycogen deposits

I '

, Vessel / constricts

[a) intestinal blood vessel

y Vessel / dilates

j if ^*?

(b) Skeletal muscle blood vessei

Glycogen z^*' breaks down / ^ * " a n d giucose ^ - ^ ^ is released from cell

(c) Liver cell

Different intracelluiar proteins — * - different cell responses

Intracellular Receptors for Lipid-Soluble Hormones The first indication that the receptors for some hormones are located inside target cells came from studying the vertebrate hormones estrogen and progesterone. For most mammals, including humans, these steroid hormones are necessary for the normal development and function of the female reproductive system. In the early 1960s, researchers demonstrated that estrogen and progesterone accumulate within the nuclei of cells in the reproductive tract of female rats. By contrast, no estrogen accumulated in the cells of tissues that do not respond to estrogen. These observations led to the hypothesis that cells sensitive to a steroid hormone contain internal receptor molecules that bind specifically to that hormone. Researchers later identified the intracellular proteins that function as receptors for steroid hormones, thyroid hormones, and the hormonal form of vitamin D. All of these hormones are small, mostly nonpolar (lipid-soluble) molecules thai diffuse easily through the phospholipid interior ot cellular membranes. intracellular receptors usually perform the entire task of transducing a signal within a target cell. The chemical signal activates the receptor, which then directly triggers the cell's response. In almost all cases, the intracellular receptor activated by a lipid-soluble hormone is a transcription factor, and the response is a change in gene expression. Most intracellular receptors are already located in the nucleus (Figure 45.3b, on page 946) when they bind hormone molecules, which have diffused in from the bloodstream. The resulting hormone-receptor complexes bind, in turn, to specific sites in the cells DNA and stimulate the transcription of specific genes. Some steroid hormone receptors, however, are trapped in the cytoplasm when no hormone is present. Binding of a steroid hormone to its cyLoplasmic receptor forms a

hormone-receptor complex that can move into the nucleus and stimulate transcription of specific genes (see Figure 11.6). In both cases, mRNA produced in response to hormone stimulation is translated into new protein in the cytoplasm. For example, estrogen induces cells in the reproductive system of a female bird to synthesize large amounts of ovalbumin, the main protein of egg white. As with hormones that bind to cell-surface receptors, hormones that bind to intracellular receptors may exert different effects on different target cells. The estrogen that stimulates a bird's reproductive system to make ovalbumin causes the bird's liver to make other proteins. The same hormone also may have different effects in different species. For instance, thyroxine produced by the thyroid gland regulates metabolism in humans and other vertebrates. But in frogs, thyroxine has additional effects: it triggers the metamorphosis of a tadpole into an adult, stimulating resorption of the tadpole's tail and other changes.

Paracrine Signaling by Local Regulators Before continuing our discussion of the endocrine system and hormonal regulation, let us briefly consider local regulators. In contrast to long-distance endocrine signaling by hormones, local regulators convey messages between neighboring cells— a process referred to as paracrine signaling (see Figure 11.4). Once secreted by the cells that make them, local regulators act on nearby target cells within seconds or even milliseconds, eliciting cell responses more quickly than hormones can. Some local regulators have cell-surface receptors; others have intracellular receptors. Binding of local regulators to their specific receptors triggers events within target cells similar to those elicited by hormones (see Figure 45.3). Several types of chemical compounds function as local regulators. Many neurotransmitters, the key local regulators in the CHAPTER 45

Hormones and the Endocrine Svsiem

947

nervous system, are amino acid derivatives. Among peptide/ protein local regulators are cytokines, which play a role in immune responses (see Chapter 43), and most growth factors, which stimulate cell proliferation and differentiation. Growth factors must be present in the extracellular environment in order for many types of cells to grow, divide, and develop normally. The functions of various growth factors in regulating cell division and tissue development are described in other chapters. Another important local regulator is the gas nitric oxide (NO). When the blood oxygen level falls, endothelial cells m blood vessel walls synthesize and release NO. Nitric oxide activates an enzyme that relaxes the neighboring smooth muscle cells, which in turn dilates the vessels and improves blood flow to tissues. Nitric oxide also plays a role in male sexual function by increasing blood flow into the penis and producing an erection. Highly reactive and potentially toxic, NO usually triggers changes in a target cell within a few seconds of contact and then breaks down. The drug Viagra (sildenafil citrate), a treatment for male erectile dysfunction, sustains an erection by interfering with this breakdown of NO. Nitric oxide has other functions as well: In the nervous system, it can function as a neurotransmitter, and NO secreted by certain white blood cells can kill bacteria and cancer cells in body fluids. A group of local regulators called prostaglandins (PGs) are modified fatty acids, often derived from lipids in the plasma membrane. They are so named because they were first discovered in prostate-gland secretions that contribute LO semen. Released from most types of cells into interstitial fluid, prostaglandins regulate nearby cells in various ways, depending on the tissue. In semen that reaches the reproductive tract of a female, prostaglandins stimulate smooth muscles of the females uterine wall to contract, helping sperm reach an egg. During childbirth, prostaglandins secreted by cells of the females placenta cause the nearby muscles of the uterus to become more excitable, helping to induce labor (see Figure 46.18). In the immune system, various prostaglandins help induce fever and inflammation and also intensify the sensation of pain. These responses contribute to the body's defense by sounding A Figure 45.5 Activated platelets aggregating, a an alarm that something process regulated in part by harmful is occurring. The prostaglandins. Following anti-inflammatory effects of injury to a blood vessel wail, aspirin and ibuprofen are platelets (pink and purple) develop . , , , , a sticky outer surface and adhere due to the drugs inhibition to each other, as shown in this of prostaglandin synthesis, colorized SEM. 948

UNIT SEVEN

Animal Form and Function

Prostaglandins also help regulate the aggregation of platelets, an early step in the formation of blood clots (Figure 45,5). This is why some physicians recommend that people who are at risk for a heart attack take aspirin on a regular basis. In the respiratory system, two prostaglandins with Very similar molecular structures have opposite effects on the smooth muscle cells in the walls of blood vessels serving the lungs. Prostaglandin E signals the muscle cells to relax, which dilates the blood vessels and promotes oxygenation of the blood. Prostaglandin F signals the muscle cells to contract, which constricts the vessels and reduces blood flow through the lungs. Shifts in the relative concentrations of these two antagonistic (opposing) signals help maintain homeostasis in changing circumstances. Later in the chapter, we will encounter other antagonistic signals that counterbalance each other.

Concept Check • 1. How do the mechanisms for inducing responses in target cells differ for water-soluble hormones and steroid hormones? 2. Explain how a single hormone, such as epinephrine, can cause different, responses in different tissues. 3. Why do local regulators, such as prostaglandins, generally elicit responses in their target cells more rapidly than hormones? For suggested answers, see Appendix A.

The hypothalamus and pituitary integrate many functions of the vertebrate endocrine system So far, we have looked at the basic components of hormonal regulatory pathways and how a hormonal signal is converted to a cellular response. Now we turn to the physiological dffects of the primary vertebrate hormones and the role ol the endocrine system in adjusting the body's activities to shifting environmental and developmental conditions. We begin with the hypothalamus and pituitary gland, which control much of the endocrine system. As you read, you may want to refer to Table 45.1, which summarizes the actions of major human hormones, and to Figure 45.6, on page 950, which illustrates the major endocrine glands in the human body. Small reference diagrams accompanying each section of the text will help you recall each gland's location.

Table 45,1 Major Human Endocrine Glands and Some of Their Hormones Chemical Class

Representative Actions

Gland

Hormone

Hypothalamus

Hormones released from the posterior pituitary and hormones that regulate the anterior pituitary (see below)

Pituitary gland Posterior pituitary (releases neurohormones made in hypothalamus) Anterior pituitary

Thyroid gland

Parathyroid glands

- •y

Pancreas K£}r

Regulated By

Oxytocin

Peptide

Stimulates contraction of uterus and mammary gland cells

Nervous system

Antidiuretic hormone (ADH)

Peptide

Promotes retention of water by kidneys

Water/salt balance

Growth hormone (GH)

Protein

Prolactin (PRL)

Protein

Stimulates growth (especially bones) and metabolic functions Stimulates milk production and secretion

Hypothalamic hormones Hypothalamic hormones

Follicle-stimulating hormone (FSH) Luteinizing hormone (LH) Thyroid-stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH)

Glycoprotein

Stimulates production of ova and sperm

Hypothalamic hormones

Stimulates ovaries and lestes

Hypothalamic hormones

Glycoprotein

Stimulates thyroid gland

Thyroxine in blood; hypothalamic hormones

Peptide

Stimulates adrenal cortex to secrete glucocorticoids

Glucocorticoids; hypothalamic hormones

Triiodothyronine (T3) and thyroxine (T4)

Amine

Stimulate and maintain metabolic processes

TSH

Calcitonin

Peptide

Lowers biood calcium k:\'cl

Calcium in blood

Parathyroid hormone (PTH)

Peptide

Raises blood calcium level

Calcium in blood

Glycoprotein

Insulin

Protein

Lowers blood glucose level

Glucose in blood

Glucagon

Protein

Raises blood glucose level

Glucose in blood

Epinephrine and norepinephrine

Amine

Nervous system

Glucocorticoids

Steroid

Raise blood glucose level; increase metabolic activities; constrict certain blood vessels Raise blood glucose level

Mineral ocorticoids

Steroid

Promote reabsorption ol Na+ and excretion of K+ in kidneys

K' in blood

Androgens

Steroid

Support sperm formation; promote development and maintenance of male secondary sex characteristics

FSH and LH

Estrogens

Steroid

Stimulate uterine lining growth; promote development and maintenance of female secondary sex characteristics

FSH and LH

Progesterone

Steroid

Promotes uterine lining growth

FSH and LH

Involved in biological rhythms

Light/dark cycles

Adrenal glands Adrenal medulla

Adrenal con ex

:y

X Figure 47.16 Adult derivatives of the three embryonic germ layers in vertebrates.

ENDODERM Epidermis of skin and its derivatives (including sweat glands, hair follicles) Epithelial lining of mouth and rectum Sense receptors in epidermis Cornea and lens of eye Nervous system Adrenal medulla Tooth enamel Epithelium or pineal and pituitary glands

mals. Within the shell or uterus, the embryos of these animals are surrounded by fluid within a sac formed by a membrane called the aninion. Reptiles (including birds) and mammals are therefore called amniotes (see Chapter 34). We have already seen that embryonic development of the chick, anamniole, is very similar to that of the frog, a vertebrate .hat lacks an ainnion. However, in the chick, development ilso includes the formation of extraembryonic membranes, membranes located outside the embryo. Notice in Figure 47.15a that only pan of each germ layer contributes to the embryo itself. The germ layers located outside the embryo proper develop into the amnion arid three other membranes—the yolk sac, the chorion, and the allantois. These four extraembryonic membranes provide a "liferupport system" for further embryonic development within the shelled egg or the uterus of an amniote. Each membrane is a sheet of cells derived from two germ layers. Figure 47.17 reviews the locations and functions of these membranes in birds and other reptiles. Mammalian extraembryonic membranes will be discussed next, as we describe early development in mammalian embryos. Formation of the placenta, a structure unique to marsupial and eutherian mammals, is an important part of this process.

Mammalian Development In most mammalian species, fertilization takes place in the oviduct, and the earliest stages of development occur while the embryo completes its journey down the oviduct to the uterus (see Figure 46.15). In contrast to the large, yolky eggs of birds, other reptiles, and monotremes, the eggs ol most mammals are quite small, storing little in the way of food reserves. As already mentioned, the mammalian egg cell and zygote have not yet been shown to exhibit obvious polarity with respect to the contents ol the cytoplasm, and cleavage of the zygote, which lacks yolk, is holoblastic. However, mammalian gastrulation and early organogenesis follow a pattern similar to that of birds and other reptiles. (Recall from Chapter 34 that mammals descended from reptilian stock during the Mesozoic era.)

• • • •

• Epithelial lining of digestive tract • Epithelial lining of respiratory system • Lining of urethra, urinary bladder, and reproductive system • Liver • Pancreas • Thymus • Thyroid and parathyroid glands

-Notoi:hord Skeieta! syste Muse ularsvs" Muse ular lay*

• Excre • Circulatory ai syste • Reprciductive p: germ • Derrr • Linin:3 of boc • Adreilat cortc

Amnion. The amnion protects the embryo in a fluid-filled cavity that prevents dehydration and cushions mechanical shock.

Chorion. The chorion and the membrane of the allantois exchange gases between the embryo and the surrounding air. Oxygen and carbon dioxide diffuse freely across the egg's shell.

Allantois. The allantois functions as a disposal sac for certain metabolic wastes produced by the embryo. The membrane of the allantois also functions with the chorion as a respiratory organ.

Yolk sac. The yolk sac expands over the yolk, a stockpile of nutrients stored in the egg. Blood vessels in the yolk sac membrane transport nutrients from the yolk into the embryo. Other nutrients are stored in the albumen (the "egg white").

A Figure 47,17 Extraembryonic membranes in birds and o t h e r reptiles. There are four extraembryonic membranes: the amnion, the allantois, the chorion, and the yolk sac. Each membrane is a sheet of cells that develops from epithelial sheets of t w o of the three germ layers external to the embryo proper (see Figure 47.15a).

Early cleavage is relatively slow in mammals. In the case of humans, the first division is complete about 36 hours after fertilization, the second division at about 60 hours, and the third division at about 72 hours. The blastomeres are equal in size. At the eight-cell stage, the blastomeres become tightly adhered lo one another, causing the outer surface of the CHAPTER 47

Animal Development

999

embryo to appear smooth. Figure 47.18 depicts development of the human embryo starting about 7 days after fertilization (see key numbers in the figure): O At the completion of cleavage, the embryo has over 100 cells arranged around a central cavity and has traveled down the oviduct to the uterus. This embryonic stage, the blastocyst, is the mammalian version of a blastula. Clustered at one end of the blastocyst cavity is a group of cells called the inner cell mass, which will subsequently develop into the embryo proper and form or contribute to all the extraembryomc membranes. © The trophoblast, the outer epithelium ol the blastocyst, initiates implantation by secreting enzymes that break down molecules of the endometrium, the lining of the uterus. This allows the blastocyst to invade the endometrium. As the trophoblast thickens through cell division, it extends fmgerlike projections into the surrounding maternal tissue, which is rich in blood vessels. Invasion by the trophoblast leads to erosion of capillaries in the endometrium, causing blood to spill out and bathe trophoblast tissues. Around the time of implantation, the inner cell mass of the blastocyst forms a flat disk with an upper layer of cells, the epiblast, and a lower layer, the hypoblasL, which are homologous to the epiblast and hypoblast of birds (see Figure 47.10, step 3). As in birds, the human embryo develops almost entirely from epiblast cells. © As implantation is completed, gastnilation begins. Cells move inward from the epiblast through a primitive streak to form mesoderm and endoderm, just as in the chick. At the same time, extraembryonic membranes begin to form. The trophoblast continues to expand into the endometrium. The invading trophoblast, mesodermal cells derived from the epiblast, and adjacent endometrial tissue all contribute to formation of the placenta (see Figure 46.16). Q By the end of gastnilation, the embryonic germ layers have formed. The three-layered embryo is now surrounded by proliferating extraembryonic mesoderm and the four extraembryonic membranes. The extraembryonic membranes in mammals are homologous to those of birds and other reptiles and develop in a similar way The chorion, which completely surrounds the embryo and the other extraembryonic membranes, functions in gas exchange. The amnion eventually encloses the embryo in a protective fluid-filled amniotic cavity (The fluid from this cavity is the "water" expelled from the vagina of the mother when the amnion breaks just prior to childbirth.) Below the developing embryo proper, the yolk sac encloses another fluid-filled cavity. Although this cavity contains no yolk, the membrane that surrounds it is given the same name as the homologous membrane in birds and other reptiles. The yolk sac membrane 1000

U N I T SEVEN

Animal Form a n d Function

Inner cell mass — Trophoblast

0

Blastocyst reaches uterus.

Expanding region of trophoblast

Maternal blood vessel

Epiblast Hypoblast Trophoblast

Blastocyst implants.

Extraembryonic membranes start to form and gastrulation begins.

^Extraembryonic mesoderm cells (from epiblast)

© Gastrulation has produced a threelayered embryo with four extraembryonic membranes.

A Figure 47.18 Four stages in earfy embryonic development of a h u m a n . The epiblast gives rise to the three germ layers, which form the embryo proper. See the text for a description of each stage.

of mammals is a site of early formation of blood cells, which later migrate into the embryo proper The fourth extraembryonic membrane, the allantois, is incorporated into the umbilical cord. Here it forms blood vessels that transport oxygen and nutrients from the placenta to the embryo and rid the embryo of carbon dioxide and nitrogenous wastes. Thus, the extraembryonic membranes of shelled eggs, where embryos are nourished with yolk, were conserved as mammals diverged from reptiles in the course of evolution, but with modifications adapted to development within the reproductive tract of the mother. The completion of gastmlation is followed by formation of the notochord, neural tube, and somites, the first events in organogenesis. By the end of the first trimester of human development, rudiments of all the major organs have developed from the three germ layers (see Figure 47.16).

Concept Check 1. Predict what would happen if you injected Ca2+ into an unfertilized sea urchin egg. 2. A frog zygote and frog blastula are nearly the same size. Explain this observation. 3. Although gastrulation looks different in sea urchin, frog, and chick embryos, what is its common result in these animals? 4. Compare cell movements during gastrulation and organogenesis in animals.

The Cytoskeleton, Cell Motility, and Convergent Extension Changes in the shape of a cell usually involve reorganization of the cytoskeleton (see Table 6.]). Consider, for example, how the cells of the neural plate form the neural tube (Figure 47.19). First, microtubules oriented parallel to the dorsalventral axis of the embryo apparently help lengthen the cells in that direction. At the dorsal end of each cell is a parallel array of actin filaments oriented crosswise. These contract, giving the cells a wedge shape that forces the ectoderm layer to bend inward. Similar changes in cell shape occur during other invaginations (inpocketings) and evaginations (outpocketings) of tissue layers throughout development. The cytoskeleton also drives cell migration, the active movement of cells from one place to another in developing animals. Cells "crawl" within the embryo by using cytoskeletal fibers to extend and retract cellular protrusions. This type of motility is akin to the amoeboid movement described in Figure 6.27b, but in contrast to the thick pseudopodia of amoeboid cells, the cellular protrusions of migrating embryonic cells are usually flat sheets (lamellipodia) or spikes (filopodia). During gastrulation in some organisms, imagination is initiated by the wedging of cells on the surface of the blastula, but the movement of cells deeper into the embryo involves the extension of filopodia by cells at the leading edge of the migrating tissue. The cells that first move through the blastopore

For suggested answers, see Appendix A.

O Microtubules help elongate the cells of the neural plate.

(Concept

Morphogenesis in animals involves specific changes in cell shape, position, and adhesion Now that you have learned about the main events of embryonic development in animals, we will address the cellular and molecular mechanisms by which they occur. Although biologists are far from fully understanding these mechanisms, several key principles have emerged as fundamental to the development of all animals. Morphogenesis is a major aspect of development in both animals and plants, but only in animals does it involve the movement of cells. Movement of parts of a cell can bring about changes in cell shape or enable a cell to migrate from one place to another within the embryo. Changes in both cell shape and cell position are involved in cleavage, gastrulation, and organogenesis. Here we consider some of the cellular components that contribute to these events.

Q Microfilaments at the dorsal end of the cells may then contract, deforming the cells into wedge shapes. Q Cell wedging in the opposite direction causes the ectoderm to form a "hinge." 0 Pinching off of the neural piate forms the neural tube. A Figure 47.19 Change in cellular shape during m o r p h o g e n e s i s . Reorganization of the cytoskeleton is associated with morphogenetic changes in embryonic tissues, as shown here for the formation of the neural tube in vertebrates,

CHAPTER 47

Animal Development

1001

and up along the inside of the blastocoel wall drag others behind them, thus helping direct movement of the entire sheet of cells from the embryo's surface into appropriate locations in the blastocoel. The involuted sheet then forms the endoderm A j , / Figure 47.20 Convergent extension of a sheet of cells. In this simplified diagram, c i and mesoderm oi the embryo (see figure c h a n g e s i n ce|| s h g p e a f | d p o s i t i o n c a y 5 e t h e |ayer o f ce||s ( Q b e c o m e n a r r o w e r a n d , o n g e r M o | e c u a r 4712). There are also many situations in signals are believed to cause the cells to elongate in a particular direction and to crawl between each which cells migrate individually, as when other (convergence), leading to extension of the cell sheet in a perpendicular direction the cells of a somite or the neural crest disperse to various parts of the embryo. Figure 47.21

Cell crawling is also involved in convergent extension, a type of morphogenetic movement in which the cells of a tissue layer rearrange themselves so that the sheet becomes narrower (converges) while it becomes longer (extends) (Figure 47.20)- When many cells wedge between one another, the tissue can extend dramatically. Convergent extension is important in early embryonic development, it occurs, for example, as the archenteron elongates in sea urchin embryos and during involution in the frog gastrula. In the latter case, convergent extension is responsible for the change infrog shape from the spherical gastrula to the submarine-shaped embryo

Does fibronectin promote cell migration? jarchers placed a strip of fibronectin on an artificial underlayer. After positioning migratory neural crest cells at one end of the strip, the researchers observed the movement of the cells in a light microscope. n this micrograph, the dashed lines indicate the edges of the fibronectin layer. Note that cells are migrating along the strip, not off of it.

seen in Figure 47.14c.

Roles of the Extracellular Matrix and Cell Adhesion Molecules Scientists are coming closer to understanding die signaling pathways that trigger and guide cell movement. The process appears to involve the extracellular matrix (ECM), the mixture of secreted glycoproteins lying outside the plasma membranes of cells (see Figure 6.29). The ECM is known to help guide cells in many types of morphogenetic movements. ECM fibers may function as tracks, directing migrating cells along particular routes. Several kinds of extracellular glycoproteins, including fibronectin, promote cell migration by providing specific molecular anchorage for moving cells. The experiment depicted in Figure 47.21 provides evidence for this function of fibronectin in guiding cell migration. Evidence also exists that fibronectin plays a similar role ai the leading edge of the involuting tissue during frog gastrulation. Fibronectin fibers line the roof of the blastocoel, and as the future mesoderm moves into the interior of the embryo, the cells at the free edge of the involuting sheet migrate along the fibers (see Figure 47.12). Researchers can prevent the attachment of cells to fibronectin by injecting embryos with antibodies against fibronectin. This treatment prevents the inward movement of the mesoderm. Other substances in the ECM keep cells on the correct paths by inhibiting migration in certain directions. Thus, depending on the substances they secrete, nonmigratory cells situated along migration pathways may promote or inhibit movement of other cells. 1002

UNIT SEVEN

Animal Form and Funciion

Direction of migration

50 urn

Fibronectin helps promote cell migration, possibly by providing anchorage for the migrating cells.

"

As migrating cells move along specific paths through the embryo, receptor proteins on their surfaces pick up directional cues from the immediate environment. Such signals from the FCM environment can direct the orientation ol cytoskeletai elements in a way that propels the cell in the proper direction. Also contributing to cell migration and stable tissue structure are glycoproteins called cell adhesion molecules (CAMs), which are located on the surfaces of cells and bind to CAK'ls on other cells. CAMs vary in amount, chemical identity or both from one type of cell to another, and these differences help regulate morphogenetic movements and tissue building. One important class of cell-to-cell adhesion molecule is the cadherins, which require calcium ions for proper function.

There are many different cadherins, and the gene for each cadherin is expressed in specific locations at specific times during embryonic development. Researchers have vividly demonstrated the importance of one particular cadherin in the formation of the frog blastula (Figure 47.22). Cadherins are also involved in the tight adhesion of cells in the mammalian embryo that first occurs at the eight-cell stage, when cadherin production begins. As we have seen, cell behavior and molecular mechanisms contribute to the morphogenesis of the embryo. The same basic cellular and genetic processes ensure that the various types of cells end up in the right places in each embryo.

Concept Check 1. During formation of the neural tube, cube-shaped. cells change to wedge-shaped cells. Describe the roles of micro tubules and microfilaments in this process. 2. In the frog embryo, convergent extension is thought to elongate the notochord along the anteriorposterior axis. How does this morphogenetic process work? For suggested answers, see Appendix A.

Concept 8 Figure 47.22

> Is cadherin required for development of the blastula? Researchers injected frog eggs with nucleic acid complementary to the mRNA encoding a cadherin known as EP cadherin. This "antisense" nucleic acid leads to destruction of the mRNA for normal EP cadherin, so no EP cadherin protein is produced. Frog sperm were then added to control (noninjected) eggs and to experimental (injected) eggs. The control and experimental embryos that developed were observed in a light microscope.

As shown in these micrographs, fertilized control eggs developed into normal blastulas, but fertilized experimental eggs did not. In the absence of EP cadherin, the blastocoel did not form properly, and the cells were arranged in a disorganized fashion.

The developmental fate of cells depends on their hist017 and on inductive signals Coupled with the morphogenetic changes that give an animal and its parts their characteristic shapes, development also requires the timely differentiation of many kinds of cells in specific locations. Two general principles integrate our current knowledge of the genetic and cellular mechanisms that underlie differentiation during embryonic development. First, during early cleavage divisions, embryonic cells must somehow become different from one another. In many animal

species, initial differences between cells result from the uneven distribution of cytoplasmic determinants in the unfertilized egg. By partitioning the heterogeneous cytoplasm of a polarized egg, cleavage parcels out different mRNAs, proteins, and other molecules to blastomeres in a type of asymmetrical cell division (see Figure 21.11a). The resulting differences in the cells' cytoplasmic composition help specify the body axes and influence the expression of genes that affect the developmental fate of the cells. In amniotes, local environmental differences play the major role in establishing early differences between embryonic cells. For example, cells of the inner cell mass are located internally in the early human embryo, whereas trophoblast cells are located on the outside surface of the blasto cyst. The different environments of these two groups of cells appear to determine their very different fates. Second, once initial cell asymmetries are set up, subsequent interactions among the embryonic cells influence their fate, usually by causing changes in gene expression. This mechanism, termed

Experimental embryo

Proper blastula formation in the frog requires EP cadherin.

induction, eventually brings about the differentiation of the many specialized cell types making up a new animal. Induction may be mediated by diffusible chemical signals or, if the cells are actually in contact, by cell-surface interactions. It will help to keep these two principles in mind as we delve into the molecular and cellular mechanisms of differentiation CHAPTER 47

Animal Development

1003

and pattern formation during embryonic development of the species we're focusing on in this chapter. To ask questions about how the fate of an early embryonic cell is determined, we need to know what that fate is. So let's first look at some historic experiments that provided early researchers with information about cell fates.

Epidermis

Fate Mapping As discussed in Chapter 21, researchers have worked out the developmental history of ever)7 cell in Cae.no rhabditis elegans, beginning with the first cleavage division of the zygote (see Figure 21.15). Although such a complete cell lineage has not yet been determined for any other animal, biologists have been making more general territorial diagrams of embryonic development, called fate maps, for many years. In classic studies performed in the 1920s, German embryologist W Vogt charted fate maps for different regions of early amphibian embryos. Vogt's results were among the earliest indications that the lineage of cells making up the three germ layers created by gastrulation is traceable to cells in the blastula, before gastrulation has begun (Figure 47.23a). Later researchers developed more sophisticated techniques that allowed them to mark an individual blastomere during cleavage and then follow the marker as it was distributed to all the mitotic descendants of that cell (Figure 47.23b). Developmental biologists have combined fate-mapping studies with experimental manipulation of parts of embryos in which they study whether a cell's fate can be changed by moving it elsewhere in the embryo. Two important conclusions have emerged. First, in most animals, specific tissues of the older embryo can be attributed to certain early "founder cells." Second, as development proceeds, a cell's developmental potential—the range of structures that it can give rise to—becomes restricted. (For review, see the discussion of cell fate determination in Chapter 21, pp. 418-420.) Starting with the normal embryo's fate map, researchers can examine how the differentiation of cells is altered in experimental situations or in mutant embryos.

Establishing Cellular Asymmetries To understand at the molecular level how embryonic cells acquire their fates, it is helpful to think first about how the basic axes of the embryo are established. This can often be traced to a specific event in early development that sets up a particular cellular asymmetry, thus beginning to lay out the body plan. The Axes of the Basic Body Plan As you have learned, a bilaterally symmetrical animal has an anterior-posterior axis, a dorsal-ventral axis, and left and right sides (see Figure 47.8a). Establishing this basic body plan is a I 004

U N I T SEVEN

Animal Form and Function

) Fate map of a frog embryo. The fates of groups of cells in a frog blastufa (left) were determined in part by marking different regions of the blastuia surface with nontoxic dyes of various colors. The embryos were sectioned at later stages of development, such as the neural tube stage shown on the right, and the locations of the dyed ceils determined.

b) Celt lineage analysis in a tunicate, in lineage analysis, "individual cell is injected with a dye during cleavage, as indicated in the drawings of 64-ceil embryos of a tunica-te, an invertebrate . chordate. The dark regions in the light micrographs of larvae correspond to the cells that developed from the two different blastomeres indicated in the drawings.

A Figure 47,23 Fate mapping for two chordates.

6 rst step in morphogenesis and is a prerequisite for the development of tissues and organs. In nonamniotic vertebrates, basic instructions for establishing the body axes are set down early, during oogenesis or fertilization. For example, in many frogs, the locations of melanin and yolk in the unfertilized egg define the animal and vegetal hemispheres, respectively The animal-vegetal axis indirectly determines the anterior-posterior body axis. Fertilization then triggers cortical rotation, which establishes the dorsal-ventral axis and at the same time leads to the appearance of the gray crescent, whose position marks the dorsal side (see Figure 47.8b). Once any two axes are established the third (in this case, the left-right) axis is specified by default. (Of course, specific molecular mechanisms must then: carry out the pattern associated with that axis.) In amniotes, the body axes are not fully established until later. In chicks, gravity is apparently involved in establishing

the anterior-posterior axis as the egg travels down the oviduct in the hen before bejng laid. Later, pH differences between the two sides of the blastoderm cells establish the dorsal-ventral axis (see Figure 47.10). If the pH is artificially reversed above and below the blastoderm, the part facing the egg white will turn into the belly (ventral side) and the side facing the yolk will turn into the back (dorsal side), opposite to their normal fates. In mammals, no polarity is obvious until after cleavage, although recent research suggests that the orientation of the egg and sperm nuclei before their fusion may play a role in determining the axes.

Figure 47.24

¥ How does distribution of the gray crescent at the first cleavage affect the potency of the two daughter cells?

Left (control): Fertilized salamander eggs were allowed to divide normally, resulting in the gray crescent being evenly divided between the two blastomeres.

Right (experimental): Fertilized eggs were constricted by a thread so that the first cleavage plane restricted the gray crescent to one blastomere.

Restriction of Cellular Potency In many species that have cytoplasmic deThe two blastomeres were terminants, only the zygote is totipotent— then separated and alllowed to develop. that is, capable of developing into all the cells types found in the adult. In these organisms, the first cleavage is asymmetrical, with the two blastomeres receiving different cytoplasmic determinants. However, even in species that have cytoplasmic determinants, the first cleavage may occur along an axis that produces two identical blastomeres, which will have equal developmental potential. This is what happens in amphibians, for instance, as demonstrated in 1938 in an experilastomeres that receive half or all of the gray crescent develop into ment by German zoologist Hans Spenormal embryos, but a blastomere that receives none of the gray crescent gives rise to an mann (Figure 47.24). Thus, the fates of piece." abnormal embryo without dorsal structures. Spemann called it aa "belly "t embryonic cells can be affected not only by the distribution of cytoplasmic determi- E X B S 9 B S I The totipotency of the two blastomeres norm mally formed during the first cleavage division depends on cytoplasmic determinants localized in the gray crescent. nants but also by how their distribution is affected by the zygotes characteristic pattern of cleavage. In contrast to many other animals, the cells of mamcapacity to give rise to more than one kind of cell, though malian embryos remain totipotent until the 16-cell stage, they have lost their totipotency. If left alone, the dorsal when they become arranged into the precursors of the troectoderm of an early amphibian gastrula will develop into a phoblast and inner cell mass of the blastocysl. At this time, neural plate above the notochord. If the dorsal ectoderm is their different locations determine their fates. The early experimentally replaced with ectoderm from some other loblastomeres of mammals seem to receive equivalent cation in the same gastrula, the transplanted tissue will amounts of cyLoplasmic components from the egg. Indeed, form a neural plate. If the same experiment is performed on up to the 8-cell stage, the blastomeres of a mammalian ema late-stage gastrula, however, the transplanted ectoderm bryo all look alike, and each can form a complete embryo if will not respond to its new location and will not form a neuisolated. ral plate. In general, the tissue-specific fates of cells in a late Regardless of how similar or different early embryonic gastrula are fixed. Even when they are manipulated expericells are in a particular species, the progressive restriction of mentally, these cells usually give rise to the same types of potency is a general feature of development in all animals. cell as m the normal embryo, indicating that their fate is In some species, the cells of the early gastrula retain the already sealed. CHAPTER 47 Animal Development

1005

Cell Fate Determination and Pattern Formation by Inductive Signals Once embryonic cell division creates cells that differ from each other, the cells begin to influence each other's fates by induction. At the molecular level, the effect of induction—the response to an inductive signal—is usually the switching on of a set of genes that make the receiving cells differentiate into a specific tissue. You have already learned about the rale of induction in vulval development in the nematode C. elegans (see Figure 21.16b). Here we examine two other examples of induction, an essential process in the development of many tissues in most animals.

Figure 47.2S

vi .V Can the dorsal lip of the blastopore induce cells in another part of the amphibian embryo to change their developmental fate? Spemann and Mangold transplanted a piece of the dorsal lip of a pigmented newt gastruia to the ventral side of the early gastruia of a nonpigmented newt. ,-Pigmented gastruia (donor embryo) Dorsal lip of blastopore

The "Organizer" ofSpemann and Mangold The importance of induction during development of amphibians was dramatically demonstrated in transplantation experiments performed by Hans Spemann and his student Hilde Mangold in the 1920s. Based on the results of their most famous experiment, summarized in Figure 47.25, they concluded that the dorsal lip of the blastopore in the early gastrula functions as an organizer of the embryo by initiating a chain of inductions that results in the formation of the notochord, the neural tube, and other organs. Developmental biologists are working intensively to identify the molecular basis of induction by Spemdnn's organizer (also called the gastruia organizer, or simply the organizer). An important clue has come from studies of a growth factor called bone morphogenetic protein 4 (BMP-4). (Bone morphogenetic proteins, a family of related proteins with a variety of developmental roles, derive their name from members of the family that are important in bone formation.) Amphibian BMP-4 is active exclusively in cells on the ventral side of the gastruia, inducing these cells to travel down the pathway toward formation of ventral structures. One major function of the cells of the organizer seems to be to inactivate BMP-4 on the dorsal side of the embryo by producing proteins that bind to BMP-4, rendering it unable to signal. This, in turn, allows formation of dorsal structures such as the notochord and neural tube. Proteins related to BMP-4 and its inhibitors are also found in other animals, including invertebrates such as the fruit fly. The ubiquity of these molecules suggests that they evolved long ago and may participate in the development of many different organisms. The induction that causes dorsal ectoderm to develop into the neural tube is only one of many cell-cell interactions that transform the three germ layers into organ systems. Many inductions seem to involve a sequence of inductive steps that progressively determine the fate of cells. For example, in the late gastruia of the frog, ectoderm cells destined to become the lenses of the eyes receive inductive signals from the ectodermal cells that will become the neural plate. Additional inductive signals probably come from endodermal cells and mesodermal cells. Finally, inductive signals from the optic cup, an outgrowth 1006

UNIT SEVEN

Animal Form and Function

Nonpigmented gastruia (recipient embryo) During subsequent development, the recipient embryo formed a second notochord and neural tube in the region of the transplant, and eventually most of a second embryo. Examination of the interior of the double embryo revealed that the secondary structures were formed in part from host tissue. Primary embryo

Secondary (induced) embryo Primary structures: Neural tube Notochord

x

Secondary

-.. • ..

structures:

N^"""- Notochord (pigmented cells) Neural tube (mostly nonpigmented cells! The transplanted dorsal lip was able to induce cells in a different region of the recipient to form structures different from their normal fate. In effect, the dorsal lip "organized" the later development of an entire embryo.

of the developing brain, complete the determination of the lensforming cells.

Formation of the Vertebrate Limb The action of the gastruia organizer is a classic example of in duction, and we can see that the organizer induces cells t

take on their fates in appropriate locations relative to each other. Thus, inductive signals play a major role in pattern formation—the development of an animal's spatial organization, the arrangement of organs and tissues in their characteristic places in three-dimensional space. The molecular cues that control pattern formation, called positional information, tell a cell where it is with respect to the animals body axes and help to determine how the cell and its descendants respond to future molecular signals. In Chapter 21, we discussed pattern formation in the development of the body segments of Drosoyhila. For understanding pattern formation in vertebrates, a classic model system has been limb development in the chick. The wings and legs of chicks, like all vertebrate limbs, begin as bumps of tissue called limb buds (Figure 47.26a). Each component of a chiek limb, such as a specific bone or muscle, develops with a precise location and orientation relative to three axes: the proximaldistal axis (the "shoulder-to-fmgertip" axis), the anteriorposterior axis (the "thumb-to-little finger" axis), and the dorsal-ventral axis (the "knuckle-to-palm" axis). The embryonic cells within a limb bud respond to positional inlormation indicating location along these three axes (Figure 47.26b). A limb bud consists of a core of mesodermal tissue covered by a layer of ectoderm. Two critical organizer regions in the limb bud have profound effects on ihe limb's development. These two organizer regions are present in all vertebrate limb buds, including those that will develop into forelimbs (such as wings or arms) and those destined to become hind limbs. The cells of these regions secrete proteins that provide key positional information to the other cells of the bud. One limb-bud organizer region is the apical ectodermal ridge (AER), a thickened area of ectoderm at the tip of the bud (see Figure 47.26a). The AF_R is required for the outgrowth of the limb along the proximal-distal axis and for patterning along this axis. The cells ol the AER produce several secreted protein signals, belonging to the fibroblast growth tactor (FGF) family, that promote limb-bud outgrowth. If the AER is surgically removed and beads soaked with FGF are put in its place, a nearly normal limb will develop. The AER and other ectoderm of the limb bud also appear to guide pattern formation along the limb's dorsal-ventral axis. In experiments where the ectoderm of a limb bud, including the AER, is detached from the mesoderm and then replaced with us orientation rotated 180° back-to-front, the limb elements that form have reversed dorsal-ventral orientation. (This is equivalent to reversing the palm and back of your hand.) The second major limb-bud organizer region is the zone of polarizing activity (ZPA), a block of mesodermal tissue located underneath the ectoderm where the posterior side of the bud is attached to the body (see Figure 47.26a). The ZPA is necessary for proper pattern formation along the anteriorposterior axis of the limb. Cells nearest the ZPA give rise to the posterior structures, such as the most posterior of the chick's

50p,m (a) Organizer regions. Vertebrate limbs develop from protrusions called limb buds, each consisting of mesoderm cells covered by a layer of ectoderm. Two regions, termed the apical ectodermal ridge (AER, shown in this SEM) and the zone of polarizing activity (ZPA), play key organizer roles in limb pattern formation.

Posterior (b) Wing of chick embryo. As the bud develops into a limb, a specific pattern of tissues emerges. In the chick wing, for example, the three digits are always present in the arrangement shown here. Pattern formation requires each embryonic cell to receive some kind of positional information indicating location along the three axes of the limb. The AER and ZPA secrete molecules that help provide this information.

A Figure 47.26 Vertebrate limb development.

CHAPTER 47

Animal Development

1007

Figure 47.27

What role does the zone of polarizing activity (ZPA) play in limb pattern formation in vertebrates? ZPA tissue from a donor chick embryo was transplanted under the ectoderm in the anterior margin of a recipient chick limb bud. Anterior •New ZPA Host limb bud

Posterior In the grafted host limb bud, extra digits developed from host tissue in a mirror-image arrangement to the normal digits, which also formed (see Figure 47.26b for a diagram of a normal chick wing).

The mirror-image duplication observed in this experiment suggests that ZPA cells secrete a signal that diffuses from its source and conveys positional information indicating "posterior." As the distance from the ZPA increases, the signal concentration decreases and hence more anterior digits develop.

three digiis (like our little finger); cells farthest from the ZPA form anterior structures, including the most anterior digit (like our thumb). The tissue transplantation experiment outlined in Figure 47.27 supports the hypothesis that the ZPA produces some sort of inductive signal that conveys positional information indicating "posterior." Indeed, researchers have discovered that the cells of the ZPA secrete an important protein growth factor called Sonic hedgehog. * If cells genetically engineered to produce large amounts of Sonic hedgehog are implanted in the anterior region of a normal limb bud, a mirror-image limb

* Sonic hedgehog gets its name from two sources, its similarity to a Dwsophila protein called Hedgehog, which is involved in segmentation of the fly embryo, and a video game character.

1008

UNIT SEVEN

Animal Form and Function

results-—-just as if a ZPA had been grafted there. Evidence from studying the mouse version of Sonic hedgehog suggests that extra toes in mice—and perhaps also in humans—can result from the production of this protein in the wrong part of the limb bud. We can conclude from experiments like these that pattern formation requires cells to receive and interpret environmental cues that vary from one location to another. These cues, acting together along three axes, tell cells where they are in the three-dimensional realm of a developing organ. In vertebrate limb development, for instance, we now know that specific proteins serve as some of these cues. In other words, organizer regions such as t_he AER and the ZPA function as signaling centers. Researchers have recently established that these two regions also interact with each other by way of signaling molecules and signaling pathways to influence each others developmental fates. Such mutual signaling interactions between mesoderm and ectoderm also occur during formation of the neural tube and during development of many other tissues and organs. What determines whether a limb bud develops into a forelimb or a hind limb? The cells receiving the signals from the AER and ZPA respond according to their developmental histories. Before the AER or ZPA issues its signals, earlier developmental signals have set up patterns of gene expression distinguishing the future forelimbs from the future hmd limbs. These differences cause the limb-bud cells of the forelimb and hind limb to react differently to the same positional cues. Thus, construction of the fully formed animal involves a sequence of events that include many steps of signaling and difjferentiation. Initial cell asymmetries allow different types of cells to influence each other to express specific sets of genes. The products of these genes then direct cells to differentiate into specific types. Coordinated with morphogenesis, various pathways of pattern formation occur in all the different parts of the developing embryo. These processes ultimately produce a complex arrangement of multiple tissues and organs, each functioning in the appropriate location to form a single coordinated organism.

Concept Check 1. Although there are three body axes, only two must be determined during early development. Why? 2. Predict what would happen if you transplanted tissue from the organizer region (above the dorsal lip) of a late-stage frog gastrula to its ventral side. 3. If the ventral cells of an early frog gastrula are experimentally induced to express a protein that inhibits BMP-4, could a second embryo develop? Explain. For suggested answers, see Appendix A.

Chapter Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY CONCEPTS After fertilization, embryonic development proceeds through cleavage, gastrulation, and organogenesis • Fertilization (pp. 988-991) Fertilization brings together the nuclei of sperm and egg, forming a diploid zygote, and activates the egg to initiate the onset of embryonic development. The acrosomal reaction, which is triggered when the sperm meets the egg, releases hydrolytic enzymes that digest material surrounding the egg. Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy in many animals. Sperm-egg fusion also initiates the cortical reaction, inducing a rise in Ca2"" that stimulates cortical granules to release their contents outside die egg. This forms a fertilization envelope that functions as a slow block to polyspermy In mammalian fertilization, the cortical reaction modifies the zona pellucida as a slow block to polyspermy. •• Cleavage (pp. 992-994) Fertilization is followed by cleavage, a period of rapid cell division without growth, which results in the production of a large number of cells called blastomeres. Holoblastic cleavage, or division of the entire egg, occurs in species whose eggs have little or moderate amounts of yolk, such as sea urchins, frogs, and mammals, Meroblastic cleavage, incomplete division of the egg, occurs in species with yolk-rich eggs, such as birds and other reptiles. Cleavage planes usually follow a specific pattern relative to the animal and vegetal poles of the zygote. In many species, cleavage creates a multicellular ball called the blastula, which contains a fluid-filled cavity, the blastocoel. Activity Sea Urchin Development I*- Gastrulation (pp. 994-997) Gastrulation transforms the biastula into a gastrula, which has a primitive digestive cavity (the archenterori) and three embryonic germ layers: the ectoderm, mesoderm, and endoderm. Investigation What Determines Cell Differentiation in the Sea Urchin? Organogenesis (pp. 997-999) The organs of the animal body develop from specific portions of the three embryonic gejm layers. Early events in organogenesis in vertebrates include formation of die notochord by condensation of dorsal mesoderm, development of the neural tube from infolding of the ectodermal neural plate, and formation of the coelom from splitting of lateral mesoderm. Activity Frog Development

!

Review proper develops from a single layer of cells, the epiblast, within the blastocyst. Extraembryonic membranes homologous to those of birds and other reptiles function in intrauterine development.

Morphogenesis in animals involves specific changes in cell shape, position, and adhesion • The Cytoskeleton, Cell Motility, and Convergent Extension (pp. 1001-1002) Cytoskeletal rearrangements are responsible for changes in both the shape and position of cells. Both kinds of changes are involved in tissue invaginations, as occurs in gastrulation, for example. In convergent extension, cell movements cause a sheet of cells to become narrower and longer. • Roles of the Extracellular Matrix and Cell Adhesion Molecules (pp. 1002-1003) Fibers of the extracellular matrix provide anchorage for cells and also help guide migrating cells toward their destinations. Fibronectin and other glycoproteins located on cell surfaces are important for cell migration and for holding cells together in tissues.

The developmental fate of cells depends on their history and on inductive signals • Eate Mapping (p. 1004) Experimentally derived fate maps of embryos have shown that specific regions of the zygote or blastula develop into specific parts of older embryos. • Establishing Cellular Asymmetries (pp. 1004-1005) In non-amniotic species, unevenly distributed cytoplasmic determinants in the egg cell are important in establishing the body axes and in setting up differences between the blastomeres resulting from cleavage of the zygote. Cells that receive different cytoplasmic determinants undergo different fates. In amniotes, local environmental differences play the major role in establishing initial differences between cells and later the body axes. As embryonic development proceeds, the potency of cells becomes progressively more limited in all species. • Cell Fate Determination and Pattern Formation by Inductive Signals (pp. 1006-1008) Cells in a developing embryo receive and interpret positional information that vanes with location. This information is often in the form of signal molecules secreted by cells in special "organizer" regions of the embryo, such as the dorsal lip of the blastopore in the amphibian gastrula and the AER and ZPA of the vertebrate limb bud. The signal molecules influence gene expression in the cells that receive them, leading to differentiation and the development of particular structures.

Developmental Adaptations of Amniotes (pp. 998-999) The embryos of birds, other reptiles, and mammals develop within a fluid-filled sac that is contained within a shell or the uterus. In these organisms, the three germ layers give rise not only to embryonic tissue but also to the four extraembryonic membranes: the arnnion, chorion, yolk sac, and allantois. Mammalian Development (pp. 999-1001) The eggs of placental mammals are small and store few nutrients. They exhibit holoblastic cleavage and show no obvious polarity Gastrulation and organogenesis, however, resemble the processes in birds and other reptiles. After fertilization and early cleavage in the oviduct, the blastocyst implants in the uterus. The trophoblast initiates formation of the fetal portion of the placenta, and the embryo

TESTING YOUR KNOWLEDGE Evolution Connection Evolution in insects and vertebrates has involved the repeated duplication of body segments, followed by fusion of some segments and specialization of their structure and function. What parts of vertebrate anatomy reflect the vertebrate segmentation pattern? Can you guess what vertebrate body part is a product of segment fusion and specialization?

CHAPTER 47

Animal Development

1009

Scientific Inquiry The "snout" of a frog tadpole bears a sucker. A salamander tadpole has a mustache-shaped structure called a balancer in the same area. You perform an experiment in which you transplant ectoderm from the side of a young salamander embryo lo the snout of a frog embryo. You find that the tadpole that develops has a balancer. When you transplant ectoderm from the side of a slightly older salamander embryo to the snout of a frog embryo, the Frog tadpole ends up with a patch of salamander skin on its snout. Suggest a hypothesis to explain the results of this experiment in terms of the mechanisms of development. How might you test your hypothesis?

Investigation What Determines Cell Differentiation in the Sea Urchin?

1010

UNIT SEVEN

Animal Form and Function

Science, Technology, and Society Many scientists believe that fetal tissue transplants offer great potential for treating Parkinson's disease, epilepsy, diabetes, Alzheimer's disease, and spinal cord injuries. Why might tissues from a fetus be particularly useful for replacing diseased or damaged cells in such conditions? Some people would allow only tissues from miscarriages to be used in fetal transplant research. But most researchers prefer to use tissues from surgically aborted fetuses. Why? What is your position on this controversial issue, and why?

Nervous Systems

if" A Figure 48.1 A functional magnetic resonance image of brain areas activated during language processing.

Key Concepls 4B.1 Nervous systems consist of circuits of neurons and supporting cells 48.2 Ion pumps and ion channels maintain the resting potential of a neuron 43.3 Action potentials are the signals conducted by axons 48.4 Neurons communicate with other cells at synapses 48.5 The vertebrate nervous system is regionally specialized 48.6 The cerebral cortex controls voluntary movement and cognitive functions 4 8 . ? CNS injuries and diseases are the focus of much research

Command and Control Center

W

hat happens in your brain when you picture something with your "mind's eye"? Until recently, scientists had little hope of answering that question. The human brain contains an estimated 101 ] (100 billion) nerve cells, or neurons. Each neuron may communicate with thousands of other neurons in complex information-processing circuits that make the most powerful electronic computers lbok primitive. An engineer who wants to learn how a computer functions can just open the computer's housing and tjrace the circuits. But except for rare glimpses (such as during Urain surgery), the circuitry of the living human brain has >een hidden from view.

That's no longer the case, however, thanks in part to recent technologies that can record brain activity from outside a person's skull. One technique is functional magnetic resonance imaging (fMRl). During fMRl, a subject lies with his or her head in the hole of a large, doughnut-shaped magnet that records increased blood flow in brain areas with active neurons. A computer ihen uses the data to construct a threedimensional map of the subject's brain activity, like the one shown in Figure 48.1. These recordings can be made while the subject is doing various tasks, such as speaking, moving a hand, looking at pictures, or forming a mental image of an object or a person's face. The results of brain imaging and other research methods, such as those described by Erich Jarvis in the interview on pages 818-819, reveal that groups of neurons function in specialized circuits dedicated to different tasks. These circuits are responsible for the feats of sensing and moving (see Chapter 49) and for the many types of animal behavior (see Chapter 51). The ability to sense and react originated billions of years ago with prokaryotes that could detect changes in their environment and respond in ways that enhanced their survival and reproductive success—for example, locating food sources by chemotaxis (see Chapter 27). Later modification of this simple process provided rnulti cellular organisms with a mechanism for communication between cells of the body. By the time of the Cambrian explosion more than 500 million years ago (see Chapter 32), systems of neurons that allowed animals to sense and move rapidly had evolved in essentially their present-day forms. in this chapter, we will discuss the organization ol animal nervous systems and the mechanisms by which neurons transmit information. We'll also investigate some of the functions performed by specific parts of the vertebrate

1011

brain. Finally, we'll examine several mental illnesses and neurological disorders that are the subject of intense research today.

Concept

Nervous systems consist of circuits of neurons and supporting cells All animals except the sponges have some type of nervous system. What distinguishes the nervous systems of different animal groups is not so much their basic building blocks— the neurons themselves—-but how the neurons are organized into circuits.

Organization of Nervous Systems The simplest animals with nervous systems, the cnidarians, have radially symmetrical bodies organized around a gastrovascular cavity (see Figure 33.5). In some cnidarians, such as the hydra shown in Figure 48.2a, the neurons controlling the contraction and expansion of the gastrovascular cavity are arranged in diffuse nerve nets. The nervous systems of more complex animals contain nerve nets as well as nerves, which are bundles of nberlike extensions of neurons. For example, sea stars have a nerve net in each arm, connected by radial nerves to a central nerve ring (Figure 48.2b); this organization

is better suited than a diffuse nerve net for controlling more complex movements. Greater complexity of nervous systems and more complex behavior evolved with cephalization, which included the clustering of neurons in a brain near the anterior (front) end in animals with elongated, bilaterally symmetrical bodies. In fiatworms, such as the planarian shown in Figure 48.2c, a small brain and longitudinal nerve cords constitute the simplest clearly defined central nervous system (CNS). In more complex invertebrates, such as annelids (Figure 48.2d) and arthropods (Figure 48.2e)} behavior is regulated by more complicated brains and ventral nerve cords containing segmentally arranged clusters of neurons called ganglia (singular ganglion). Nerves thai connect the CNS with the rest of an animal's body make up the peripheral nervous system (PNS). Molluscs are good examples of how nervous system organization correlates with an animals lifestyle. Sessile or slowmoving molluscs such as clams and chitons have little or no cephalization and relatively simple sense organs (Figure 48.2f) In contrast, cephalopod molluscs (squids and octopuses) have the most sophisticated nervous systems of any invertebrates, rivaling even those of some vertebrates. The large brain of cephalopods, accompanied by large, image-forming eyes and rapid signaling along nerves, supports the active, predatory life of these animals (Figure 48.2g). Researchers have demonstrated that octopuses can learn to discriminate among visual patterns and can perform complex tasks. In vertebrates, the CNS consists of the brain and the spinal cord, which runs along the dorsal (back) side of the body; nerves and ganglia

Eyespot Brain

Radia; nerve

Nerve cord

Ventral nerve cord

'/ %? •

5egmental-';/=¥;ganglion (a) Hydra (cnidarian)

Brain

(b) Sea star (echinoderm)

Antenonerve ring

(c) Planarian (flatworm)

(d) Leech (annelid)

—.-Gang -' A^T/A . Ganglia

TIM (e) Insect (arthropod)

(f) Chiton (mollusc)

A Figure 48.2 O r g a n i z a t i o n of s o m e n e r v o u s systems. 1012

U N I T SEVEN

Animal Form and Function

(g) Squid (mollusc)

(h) Salamander (chordate)

comprise the PNS (Figure 48.2h). We will examine the vertebrate nervous system in more detail later in this chapter.

Information Processing In general, there are three stages in the processing of information by nervous systems: sensory input, integration, and motor output (Figure 48.3). These three stages are handled by

Motor o u t p u t

Neuron Structure

Effector Peripheral nervous system (PNS)

Central nervous system (CNS)

A Figure 48.3 Overview of information processing by nervous systems.

© Sensors detect a sudden stretch in the quadriceps.

specialized populations of neurons. Sensory neurons transmit information from sensors that detect external stimuli (light, sound, touch, heat, smell, and taste) and internal conditions (such as blood pressure, blood CO2 level, and muscle tension). This information is sent to the CNS, where interneurons integrate (analyze and interpret) the sensory input, taking into account the immediate context as well as what has happened in the past. The greatest complexity in neural circuits exists in the connections between interneurons. Motor output leaves the CNS via motor neurons, which communicate with effector cells (muscle cells or endocrine cells). The stages of sensory input, integration, and motor output are easiest to study in the relatively simple nerve circuits that produce reflexes, the body's automatic responses to stimuli. Figure 48.4 diagrams the nerve circuit that underlies the knee-jerk reflex in humans.

Q Sensory neurons convey the information to the spinal cord.

As you read in Chapter 1, one of the major themes of biology is that form Tits function. This theme, which applies at all levels from molecules to organisms, is clearly illustrated at the cellular level by neurons. The ability oi neurons to receive and transmit information depends on their elaborate structure. Most oi a neuron's organdies, including its nucleus,

© The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward. Gray matter

0 T h e reflex is initiated by tapping the tendon connected to the quadriceps (extensor) muscle.

0 Sensory neurons from the quadriceps also communicate with interneurons in the spinal cord. 0 The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps.

k Figure 48.4 The knee-jerk reflex. Many neurons are involved in the reflex, but for simplicity, only one neuron of each type is shown. CHAPTER 48

Nervous Systems

1013

zDendrites

Presynaptic cell

A Figure 48.5 Structure of a vertebrate neuron.

are located in the cell body (Figure 48.5). Arising from the cell body are two types of extensions: numerous dendrites and a single axon. Dendrites (from the Greek dendron, tree) are highly branched extensions that receive signals from other neurons. The axon is typically a much longer extension that transmits signals to other cells, which may be neurons or effector cells. Some axons, such as the ones that reach from your spinal cord to muscle cells in your feet, may be over a meter long. The conical region of an axon where it joins the cell body is called the axon hillock; as we will see, this is typically the region where the signals that travel down the axon are generated. Many axons are enclosed by a layer called the myelin sheath, described later in this section. Near its end, an axon usually divides into A Figure 48.6 Structural diversity of vertebrate neurons. Cell bodies and dendrites are several branches each of which ends in a ^lack 'n ^ e s e diagrams; axons are red. In (a), the cell body is connected only to the axon, which , , ,, conveys siqnals from the dendrites to the axon's terminal branches (at the bottom). synaptic terminal. The site ol communication between a synaptic terminal and another cell is called a synapse. At most synapses, information Supporting Cells (Glia) is passed from the transmitting neuron (the presynaptic cell) Glia (from a Greek word meaning "glue") are supporting cells to the receiving cell (the postsynaptic cell) by means of that are essential for the structural integrity of the nervous syschemical messengers called neurotransmitters. tem and for the normal functioning of neurons. Ln the mamThe complexity of a neuron's shape reflects the number of malian brain, glia outnumber neurons 10-fold to 50-fold. synapses it has with other neurons (Figure 48.6). For examThere are several types of glia in the brain and spinal cord, inple, the interneuron shown in the lower part of Figure 48.6b cluding astrocytes, radial glia, oligodendrocytes, and Schwanr has about 100,000 synapses on its highly branched dencells. As a group, these cells do much more than just hold drites, whereas neurons with simpler dendrites have far fewer neurons together. synapses. 1014

UNIT SEVEN

Animal Form and Function

•4 Figure 48.7 Astrocytes. In this section through a mammalian cerebral cortex, astrocytes appear green after being labeled with a fluorescent antibody (LM). The blue dots are cell nuclei, labeled with a different antibody. The term "astrocyte" refers to the star-like shape of the cells.

during development when Schwann cells or oligodendrocytes grow around axons, wrapping them in many layers of membrane, somewhat like a jelly roll. These membranes are mostly lipid, which is a poor conductor of electrical currents. Thus, the myelin sheath provides electrical insulation of the axon, analogous to plastic insulation that covers many electrical wires. In the disease multiple sclerosis, myelin sheaths gradually deteriorate, resulting in a progressive loss of body function clue to the disruption of nerve signal transmission.

Concept Check In the CNS, astrocytes provide structural support for neurons and regulate the extracellular concentrations of ions and n euro transmitters (Figure 48.7). Some astrocytes respond to activity in neighboring neurons by facilitating information transfer at those neurons' synapses. Scientists hypothesize that this facilitation may be part of the cellular mechanism of learning and memory. Astrocytes adjacent to active neurons also cause nearby blood vessels to dilate, which increases blood flow to the area, enabling the neurons to obtain oxygen and glucose more quickly. During development, astrocytes induce the formation of tight junctions (see Figure 6.31) between cells that line the capillaries in the brain and spinal cord. The result is the blood-brain barrier, which restricts the passage of most substances into the CNS, allowing the extracellular chemical environment of the CNS to be tightly controlled. In an embryo, radial glia form tracks along which newly formed neurons migrate from the neural tube, the structure that gives rise to the CNS (see Figures 47.14 and 47.15). Both radial glia and astrocytes can also act as stem cells, generating neurons and other glia. Researchers view these multipotent precursors as a potential way to replace neurons and glia that are lost to injury or disease, a topic explored in Concept 48.7. Oligodendrocytes (in the CNS) and Schwarm cells (in the PNS) are glia that form the myelin sheaths around the axons of many vertebrate neurons. The structure of a myelinated PNS axon is shown in Figure 48.8. Neurons become myelinated

1. a. Arrange the following neurons in the correct sequence for information flow during the knee-jerk reflex: interneuron, sensory neuron, motor neuron. b. Which of the neuron types is located entirely within the CNS? 2. Would severing a neuron's axon stop the neuron from receiving or from transmitting information? Explain. 3. What would be the most obvious structural abnormality in the nervous system of a mouse lacking oligodendrocytes? For suggested answers, see Appendix A.

Concept

Ion pumps and ion channels maintain the resting potential of a neuron We noted in Chapter 7 that all cells have an electrical potential difference (voltage) across their plasma membrane. This voltage is called the membrane potential. In neurons, the membrane potential is typically between —60 and —80 mV (millivolts) when the cell is not transmitting signals. The minus

Node of Ranvier ""--. /

Layers of myelin

Sch wann cell

Axon/

Myelin s eath

Nodes of Ranvier

Nucleus o f - ^ ^ Schwann celt

A Figure 48.8 Schwann cells and the myelin sheath, in the PNS, glia called Schwann cells •"'• 'vrap themselves around axons, forming layers of myelin. Gaps between adjacent Schwann cells are re called nodes of Ranvier. The TEM shows a cross section through a myelinated axon.

CHAPTER 48

Nervous Systems

1015

EXTRACELLUL FLUID

CYTOSOL

[Na+]

Electrophysiologists use intracellular recording to measure the membrane potential of neurons and other cells. A microelectrode is made from a glass capillary tube filled with an electrically conductive salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 prn). While looking through a microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the microeiectrode tip inside the cell and a reference electrode placed in the solution outside the cell.

-

+ [Na+]

15 mM

150 m/V'

150 mM

5mM

[Cli 10mM

\R

ten -

+ 120 mM

[Ai 100 mM

A

-f-

'.•.:

membrane

A Figure 48.10 Ionic gradients across the plasma membrane of a mammalian neuron. The concentrations of Na Voltage recorder -Reference electrode

sign indicates that the inside of the cell is negative relative to the outside. Figure 48.9 explains how electrophysiologists measure a cells membrane potential.

The Resting Potential The membrane potential of a neuron that is not transmitting signals is called the resting potential. In ail neurons, the resting potential depends on the ionic gradients that exist across the plasma membrane (Figure 48.10). In mammals, for example, the extracellular fluid has a sodium ion (Na+) concentration of 150 millimolar (mM) and a potassium ion (K ) concentration of 5 mM (1 millimolar = 1 millimole/liLer = 1 0 '' mol/L). In the cytosol, the Na concentration is 15 mM and the K concentration is 150 mM. Thus, the Na concentration gradient, expressed as the ratio of outside concentration/inside concentration, is 150/15, or 10. The K concentration gradient is 5/150, or 1/30. (There are also anion gradients, but we will ignore them for the moment.) The Na + and K+ gradients are maintained by the sodium-potassium pump (see Figure 7.16). The fact that the gradients are responsible for the resting potential is shown by a simple experiment: If the pump is disabled by the addition of a specific poison, the gradients gradually disappear, and so does the resting potential. It is particularly challenging to understand how the resting potential arises from ionic gradients, but it is an essential step in learning how neurons, including our own, work. We start with a model of a mammalian neuron consisting of two chambers separated by an artificial membrane (Figure 48.11a). The membrane contains many ion channels (see Chapter 7) that allow only K+ to diffuse across the membrane. To produce a con1016

UNIT SEVEN

Animal Form and Function

and Cl are higher in the extracellular fluid than in the cytosol. The reverse is true for K+. The cytoso! also contains a variety of organic anions [A~], including charged amino acids.

centration gradient for K+ like that of a mammalian neuron, we add 150 mM potassium chloride (KCl) to the inner chamber and 5 mM KCl to the outer chamber. Like any solute, K+ tends to diffuse down its concentration gradient, from an area of higher concentration (inner chamber) to an area of lower concentration (outer chamber). But because the channels are selective for K , chloride ions (Cl ) cannot cross the membrane. As a result, a separation of charge (voltage) develops across the membrane, with an excess of negative charge on the side of the membrane facing the inner chamber. The developing membrane voltage opposes the efflux of K+ because the excess negative charges attract the positively charged K+. Thus, an electrical gradient builds up whose direction is opposite that of the concentration gradient. When the electrical gradient exactly balances the concentration gradient, an equilibrium is established. At equilibnum, there is no net diffusion of K4" across the membrane.* The magnitude of the membrane voltage at equilibrium, called the equilibrium potential (E ion ), is given by a formula called the Nernst equation. For an ion with a net charge of + 1, such as K+, at 37°C, the Nernst equation is

The Nernst equation applies to any membrane that is permeable to a single type of ion. In our model, the membrane is permeable only to K+, and the Nernst equation can be used to calculate EK, the equilibrium potential for K + : EK — 62 mV llo;

- -92mV(at 37°C)

* The charge separation needed to generate the resting potential is extreme .remely small: about 10~ 1 2 mol/cm2 of membrane. Thus, the ionic diffusion that leads to the resting potential does not appreciably change th" ;™ [rations on either side of the membrane.

Inner chamber 15mM NaCl

Outer chamber 150 rn/Vf NaCi

Na+ a Sodium -

(a) Membrane selectively permeable to K+

(b) Membrane selectively permeable to Na+

The minus sign indicates that with this K concentration gradient, K+ is at equilibrium when the inside of the membrane is 92 mV more negative than the outside. Now imagine that we change OUT model neuron by using a membrane that contains ion channels selective for Na' (Figure 48.11b). We also replace the contents of the chambers to produce a concentration gradient for Na like that ot a "mammalian neuron: 15 mM sodium chloride (NaCl) in the inner chamber and 1 50 mM NaCl m the outer chamber. Under these conditions, the Nernst equation can be used to calculate £Na, the equilibrium potential for Na+: ENa= 62 mV

-4 Figure 48.11 Modeling a mammalian neuron. Each beaker is divided into two chambers by an artificial membrane, (a) The membrane is selectively permeable to K+', and the inner chamber contains a 30-fold higher concentration of K+ than the outer chamber; at equilibrium, the inside of the membrane is -92 mV relative to the outside, (b) The membrane is selectively permeable to Na+, and the inner chamber contains a ten-fold lower concentration of Na^ than the outer chamber; at equilibrium, the inside of the membrane is +62 mV relative to the outside.

- +62 mV (ai 37°C)

'he plus sign indicates that with this Na^ concentration gra[ient, Na+ is at equilibrium when the inside of the membrane 62 mV more positive than the outside. How does a real mammalian neuron differ from these model neurons? The plasma membrane of a real neuron at rest has many potassium channels that are open, but it also has a relatively small number of open sodium channels. Consequently the resting potential is around —60 to —80 my between EK and ENa. Because neither K+ nor Na+ is at equilibrium, there is a net flow of each ion (a current) across the membrane at rest. The resting potential remains steady, which means that the K+ and Na+ currents are equal and opposite. The reason the resting potential is closer to EK than to ENa is that the membrane is more permeable to K4" than to Na+. If something causes the membrane's permeability to Na+ to increase, the membrane potential will move toward E^a and away from EK. This is the hasis of nearly all electrical signals in the nervous system: The membrane potential can change from its resting value when the membrane's permeability to particular ions changes. Sodium and

potassium ions play major roles, but there are also important roles for chloride (cr) and calcium (Ca2+) ions; they follow the same rules (as described by the Nernst equation).

Gated Ion Channels The resting potential results from the diffusion of K+ and Na+ through ion channels that are always open; these channels are said to be ungated. Neurons also have gated ion channels, which open or close in response to one oi three kinds of stimuli. Stretch-gated ion channels are found in cells that sense stretch (see Figure 48.4) and open when the membrane is mechanically deformed. Ligand-gated ion channels are found at synapses and open or close when a specific chemical, such as a neurolransmitter, binds to the channel. Voltage-gated ion channels are found in axons (and in the dendrites and cell body of some neurons, as well as in some other types of cells) and open or close when the membrane potential changes. As we'll explain in the next section, gated ion channels are responsible for generating the signals of the nervous system. Concept Check 1. What is the equilibrium potential (Ex) at 37°C for an ion X+ if [X + ] out , lde = 10 mM and [X+]inside = 100 mM? 2. Suppose a cell's membrane potential shifts from — 70 mV to —50 mV What changes in the cell's permeability to K or Na could cause such a shift? 3. Contrast ligand-gated and voltage-gated ion channels in terms of the stimuli that open them. For suggested answers, see Appendix A.

Concept

Action potentials are the signals conducted by axons If a cell has gated ion channels, its membrane potential may change in response to stimuli that open or close those CHAPTER 48 Nervous Systems

1017

channels. Some such stimuli trigger a hyperpolarization (Figure 48.12a), an increase in the magnitude of the membrane potential (the inside of the membrane becomes more negative). Hyperpolarizations may be caused by tbe opening of gated K+ channels, which increases the membrane's permeability to K+ and causes the potential to approach EK (~92 mV al 37°C). Other stimuli trigger a depolarization (Figure 48.12b), a reduction in the magnitude of the membrane potential (the inside of the membrane becomes less negative). Depolarizations may be due to the opening of gated Na+ channels, which increases the membranes permeability to Na~ and causes the potential to approach £Na (+62 mV at 37°C). These changes in membrane potential are called graded potentials because the magnitude of the hyperpolarization or depolarization varies with ihe strength of the stimulus: A larger stimulus causes a larger change in permeability and thus a larger change in the membrane potential.

Production of Action Potentials In most neurons, depolarizations are graded only up to a certain membrane voltage, called the threshold. A stimulus strong enough to produce a depolarization that reaches the threshold triggers a different type of response, called an action potential (Figure 48.12c). An action potential is an all-or-none phenomenon: Once triggered, it has a magnitude that is independent of the strength of the triggering stimulus. Action potentials are the signals that carry information along axons,

sometimes over great distances, such as from your toes to your spinal cord. The action potentials of most neurons are very brief—only about 1-2 milliseconds (msec) in duration. Having brief action potentials enables a neuron LO produce them at a high frequency This feature is significant because neurons encode information in their action potential frequency. For example, in the sensory neurons that function in the knee-jerk reflex, action potential frequency is related to the magnitude and suddenness of stretch in the quadriceps muscle. As Figure 48.13 illustrates, both voltage-gated Na+ channels and voltage-gated K^ channels are involved in the production of an action potential. Both types of channels are opened by depolarizing the membrane, but they respond independently and sequentially: Na+ channels open before K+ channels. Each voltage-gated Na+ channel has two gates, an activation gate and an inactivation gate, and both must be open for Na+ to diffuse through the channel. © At the resting potential, the activation gate is closed and the inactivation gate is open on most Na+ channels. Depolarization of the membrane rapidly opens the activation gate and slowly closes the inactiva tion gate. Each voltage-gated K+ channel has just one gate, an activa tion gate. At the resting potential, the activation gate on most K+ channels is closed. Depolarization of the membrane slowly opens the K+ channel's activation gate. How do these channel properties contribute to the production of an action potential? © When a stimulus depolarizes the

Stimuli

0-

ane poten

1

-50 Threshold

Resting potentia Hyperpolarizations 1000

1 2

3

4

5

Time (msec;

Time (msec) (a) Graded hyperpolarizations produced by t w o stimuli that increase membrane permeability to K+. The larger stimulus produces a larger hyperpolarization.

(b) Graded depolarizations produced by t w o stimuli that increase membrane permeability to Na + . The larger stimulus produces a larger depolarization.

A Figure 48.12 Graded potentials and an action potential in a neuron.

1018

UNIT SEVEN

Animal Form and Function

(c) Action potential triggered by a depolarization that reaches the threshold.

H

(Na

3] Rising phase of the action potential Depolarization opens the activation gates on most Na + channels, while the K+ channels' activation gates remain closed. A Na + influx makes the inside of the membrane positive with respect to the outside.

41 Falling phase of the action potential The inactivation gates on most Na4" channels close, blocking Na + influx. The activation gates on most K + channels open, permitting K+ efflux which again makes the inside of the cell negative.

A stimulus opens the activation gates on some Na + channels. Na + influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential.

Plasma membrane Cytosol

Sodium channel

K

EffiSSEG3l Both gates cf the Na+ channels are closed, but the activation gates on some K + channels are still open. As these gates close on most K+ channels, and the inactivation gates open on Na + channels, the membrane returns to its resting state.

Inactivation gate

© [ 2 2 3 E H B 3 9 ^e activation gates on the Na+ and K + channels are closed, and the membrane's resting potential is maintained.

A Figure 48.13 The role of voltage-gated ion channels in the generation of an a c t i o n p o t e n t i a l . The circled numbers on the graph in the center and the colors of the action potential phases correspond to the five diagrams showing voltage-gated Na + and K + channels in a neuron's plasma membrane. (Ungated ion channels are not illustrated.)

nembrane, the activation gates on some Na channels open, alowing more Na+ to diffuse into the cell. The Na+ influx causes urther depolarization, which opens the activation gates on still more Na+ channels, allowing even more Na + to diffuse into the cell, and so on. © Once the threshold is crossed, this positiveeedback cycle rapidly brings the membrane potential close to : Na during the risingphase. © However, two events prevent the membrane potential from actually reaching ENa: (a) The inacti•ation gates on most Na+ channels close, halting Na+ influx; and (b) the activation gates on most K+ channels open, causing rapid efflux of K+. Both events quickly bring the membrane

potential back toward £K during the/ailing phase. © In fact, in the final phase of an action potential, called the undershoot, the membrane's permeability to K+ is higher than at rest, so the membrane potential is closer to £K than it is at the resting potential. The K4" channels' activation gates eventually close, and the membrane potential returns to the resting potential. The Na+ channels' inactivation gates remain closed during the falling phase and the early part of the undershoot. As a result, if a second depolarizing stimulus occurs during this period, it will be unable to trigger an action potential. The "downtime" following an action potential when a second action CHAPTER 48

Ner\ ous Svsiems

1019

potential cannot be initiated is called the refractory period. This interval sets a limit on the maximum frequency at which action potentials can be generated.

Conduction of Action Potentials For an action potential to function as a long-distance signal, it must travel without diminishing from the cell body to the synaptic terminals. It does so by regenerating itself along the axon. At the site where an action potential is initiated (usually the axon hillock), Na+ influx during the rising phase creates an electrical current that depolarizes the neighboring region of the axon membrane (Figure 48.14). The depolarization in the neighboring region is large enough to reach the threshold, causing the action potential to be re-initiated there. This process is repeated many times as the action potential travels the length of the axon. Immediately behind the traveling zone of depolarization due to Na+ influx is a zone of repolarization due to K+ efflux. in the repolarized zone, the activation gates of Na+ channels are still closed. Consequently, the inward current that depolarizes the axon membrane ahead of the action potential cannot produce another action potential behind it. This prevents action potentials from traveling back toward the cell body. Thus, once an action potential starts, it normally moves in only one direction—toward the synaptic terminals.

Action potential

• An action potential is generated as Na + flows inward across the membrane at one location.

Action potential

Conduction Speed Several factors affect the speed at which action potentials are conducted. One factor is the diameter of the axon: The larger (wider) the axon's diameter, the faster the conduction. This is because resistance to the flow of electrical current is inversely proportional to the cross-sectional area of a conductor (such as a wire or an axon). As an analogy, think about how a wide hose offers less resistance to the flow of water than a narrow hose does. Similarly a wide axon provides less resistance to the depolarizing current associated with an action potential than a narrow axon does. Hence, the resulting depolarization can spread farther along the interior of a wide axon, bringing more distant regions of the membrane to the threshold sooner. In invertebrates, conduction speed varies from several centimeters per second in very narrow axons to about 100 m/sec in the giant axons of squids and some arthropods. These giant axons function in behavioral responses requiring great speed, such as the backward tail-flip that enables a lobster or crayfish to escape from a predator. A different means of increasing the conduction speed of action potentials has evolved in vertebrates. Recall that many vertebrate axons are surrounded by a myelin sheath (see Figure 48.8). Myelin increases the conduction speed of action potentials by insulating the axon membrane. Insulation has

1020

UNIT SEVEN

Animal Form and Function

The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K + flows outward.

^_K+_

i i

^

potential

The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.

A Figure 48.14 Conduction of an action potential. The three parts of this figure show events that occur in an axon at three successive times as an action potential passes from left to right. At each point along the axon, voltage-gated ion channels go through the sequence of changes described in Figure 48.13. The colors of membrane regions shown here correspond to the action potential phases in Figure 48.13.

••3 Figure 48.15 Saltatory conduction. In a myelinated axon, the depolarizing current during an action potential at one node of Ranvier spreads along the interior of the axon to the next node (blue arrows), where it re-initiates itself. Thus, the action potential jumps from node to node as it travels along the axon (red arrows).

the same effect as increasing the axon's diameter: It causes the depolarizing current associated with an action potential to spread farther along the interior of the axon, bringing more distant regions of the membrane to the threshold sooner. The great advantage of myelination is its space efficiency. A myelinated axon 20 um in diameter has about the same conduction speed as a squid giant axon (diameter 1 mm), but more than 2,000 of those myelinated axons could be packed into the space occupied by just one giant axon. In a myelinated axon, voltage-gated Na+ and K channels are concentrated at gaps in the myelin sheath called nodes of Ranvier (see Figure 48.8). The extracellular fluid is in contact with the axon membrane only at the nodes. As a result, action potentials are not generated in the regions between the nodes. Rather, the inward current produced during the rising phase of the action potential at a node travels all the way to the next node, where it depolarizes the membrane and generates a new action potential (Figure 48.15). This mechanism is called saltatory conduction (from the Latin saltare, to leap) because the action potential appears to jump along the axon from node to node. Saltatory conduction can transmit action potentials at speeds up to 120 m/sec in myelinated axons.

Concept

Neurons communicate with other cells at synapses When an action potential reaches the terminals of an axon, it generally stops there. In most cases, action potentials are not transmitted from neurons to other cells. However, information is transmitted, and this transmission occurs at the synapses. Some synapses, called electrical synapses, contain gap junctions (see Figure 6.31), which do allow electrical current to flow directly from cell to cell. In both vertebrates and invertebrates, electrical synapses synchronize the activity of neurons responsible for certain rapid, stereotypical behaviors. For example, electrical synapses associated with the giant axons of lobsters and other crustaceans facilitate the swift execution of their escape responses. The vast majority of synapses are chemical synapses, which involve the release of a chemical neurotransmitter by the presynaptic neuron. The presynaptic neuron synthesizes the neurotransmitter and packages it in synaptic vesicles, which are stored in the neurons synaptic terminals. Hundreds of synaptic terminals may interact with the cell body and dendrites of a postsynaptic neuron (Figure 48.16). When an action potential

1. How does an action potential differ from a graded potential? 2. Suppose a mutation caused the inactivation gates on Na+ channels to remain closed for a longer time following an action potential. How would that affect the maximum frequency at which action potentials could be generated? 3. Arrange the following from lowest to highest conduction speed: (a) myelinated, small-diameter axon; (b) myelinated, large-diameter axon; (c) unmyelinated, small-diameter axon. tor suggested answers, see Appendix A.

A Figure 48.16 Synaptic terminals on the cell body of a postsynaptic neuron (colorized SEM).

CHAPTER 48

Nervous Systems

1021

A Figure 48.17 A chemical synapse. © When an action potential depolarizes the plasma membrane of the synaptic terminal, it © opens voltage-gated Ca2 + channels in the membrane, triggering an influx of C a 2 + . © The elevated Ca 2 + concentration in the terminal causes synaptic vesicles to fuse with the

presynaptic membrane. © The vesicles release neurotransmitter into the synaptic cleft. © The neurotransmitter binds to the receptor portion of ligand-gated ion channels in the postsynaptic membrane, opening the channels. In the synapse illustrated here, both Na4" and K+ can diffuse through the channels. ® The

reaches a synaptic terminal, it depolarizes the terminal membrane, opening voltage-gated calcium channels in the membrane (Figure 48.17). Calcium ions (Ca2+) then diffuse into the terminal, and the rise in Ca2^ concentration in the terminal causes some of the synaptic vesicles to fuse with the terminal membrane, releasing the neurotransmitter by exocytosis (see Chapter 7). The neurotransmitter diffuses across the synaptic cleft, a narrow gap that separates the presynaptic neuron from the postsynaptic cell. The effect of the neurotransmitter on the postsynaptic cell may be either direct or indirect (described next). Information transfer is much more modifiable at chemical synapses than at electrical synapses. A variety of factors can affect the amount of neurotransmitter that is released or the responsiveness of the posisynaptic cell. Such modifications underlie an animal's ability to alter its behavior in response to change and form the basis for learning and memory.

Direct Synaptic Transmission At many chemical synapses, ligand-gated ion channels capable of binding to the neurotransmitter are clustered in the 1022

UNIT SEVEN Animal Form and Function

neurotransmitter releases from the receptors, and the channels close. Synaptic transmission ends when the neurotransmitter diffuses out o the synaptic cleft, is taken up by the synaptic terminal or another cell, or is degraded by an enzyme.

membrane of the postsynaptic cell, directly opposite th synaptic terminal (see Figure 48.17). Binding of the neuro transmitter to a particular part of the channel, the recepto opens the channel and allows specific ions to diffuse acros the postsynaptic membrane. This mechanism of informatio transfer is called direct synaptic transmission. The result i generally a postsynaptic potential, a change in the membran potential of the postsynaptic cell. At some synapses, fo example, the neurotransmitter binds to a type of channel throug which both Na+ and KT can diffuse. When those channeJ open, the postsynaptic membrane depolarizes as the mem brane potential approaches a value roughly midway betwee EK and EKa. Since these depolarizations bring the membran potential toward the threshold, they are called excitator postsynaptic potentials (EPSPs). At other synapses, a dii ferent neurotransmitter binds to channels that are selectiv for K+ only. When those channels open, the postsynapti membrane hyperpolarizes. Hyperpolarizations produced i this manner are called inhibitory postsynaptic potential (IPSPs) because they move the membrane potential farthe from the threshold.

Various mechanisms terminate the effect of neurotransmitters on postsynaptic cells. At many synapses, the neurotransmitter simply diffuses out of the syrvaptic cleft. At others, the neurotransmitter is taken up by the presynaptic neuron through active transport and repackaged into synaptic vesicles. Glia actively take up the neurotransmitter at some synapses and metabolize it as fuel. The n euro trans milter acetylcholine (discussed shortly) is degraded by an enzyme, acetylcholinesterase, which resides in the synaptic cleft. Summation of Postsynaptic Potentials Unlike action potentials, which are all-or-none, postsynaptic potentials are graded; their magnitude varies with a number of factors, including the amount oi neurotransmitter released by the presynaptic neuron. Another difference is that postsynaptic potentials usually do nol regenerate themselves as they spread along the membrane of a cell; they become smaller with distance from the synapse. Recall that most synapses on a neuron are located on its dendntes or cell body, whereas action potentials are generally initiated at the axon hillock. Therefore, a single "EPSP is usually too small to trigger an actiion potential in a postsynaptic neuron (Figure 48.18a). However, if two IiPSPs occur in rapid succession at a single smapse, the second EPSP may begin before the postsynaptic neuron's membrane potential has returned to the resting potential after the first EPSP. When that happens, the EPSPs add together, an effect called temporal summation (Figure 48.18b). Moreover, EPSPs produced nearly simultaneously by different

synapses on the same postsynaptic neuron can also add together, an effect called spatial summation (Figure 48,18c). Through spatial and temporal summation, several EPSPs can depolarize the membrane at the axon hillock to the threshold, causing the postsynaptic neuron to produce an action potential. Summation also applies to IPSPs: Two or more IPSPs occurring nearly simultaneously or in rapid succession have a larger hyperpolarizing effect than a single IPSP. Through summation, an IPSP can also counter the effect of an EPSP (Figure 48.18d). This interplay between multiple excitatory and inhibitory inputs is the essence of integration in the nervous system. The axon hillock is the neuron's integrating center, the region where the membrane potential at any instant represents the summed effect of all EPSPs and IPSPs. Whenever the membrane potential at the axon hillock reaches the threshold, an action potential is generated and travels along the axon to its synaptic terminals. After the refractor)7 period, the neuron may produce another action potential if the threshold is reached again at the axon hillock. On the other hand, the summed effect of EPSPs and IPSPs may hold the membrane potential below the threshold, preventing production of action potentials.

Indirect Synaptic Transmission So far, we have focused on direct synaptic transmission, in which a neurotransmitter binds directly to an ion channel, causing the channel to open. In indirect synoptic transmission, a neurotransmitter binds to a receptor that is not part of an ion channel. This activates a signal transduction pathway involving

Terminal branch of presynaptic neuron

Threshold of axon of postsynaptic neuron Resting potential_

(a) Subthreshold, no summation

(b) Temporal summation

(c) Spatial summation

(tf) Spatial summation of EPSP and IPSP

A Figure 48.18 Summation of postsynaptic potentials. These graphs trace changes in the membrane potential at a postsynaptic neuron's axon hillock. The arrows indicate times when postsynaptic potentials occur at two excitatory synapses £Ei and E2, green in the diagrams above the graphs) and at one inhibitory synapse (I, red). Like most EPSPs, those produced at E, or E2 do not reach the threshold at the axon hillock without summation.

CHAPTER 48

NclA'OUS Sv

1023

a second messenger in the postsynaptic cell (see Chapter 11). Compared to the postsynaptic potentials produced by direct synaptic transmission, the effects of indirect synaptic transmission have a slower onset but last longer (up to several minutes). A variety of signal transduction pathways play a role in indirect synaptic transmission. One of the best-studied pathways involves cyclic AMP (cAMP) as a second messenger. For example, when the neurotransmitter norepinephrine binds to its receptor, the neurotransmitter-receptor complex activates a G protein, which in turn activates adenylyl cyclase, the enzyme that converts ATP to cAMP (see Chapter 11). Cyclic AMP activates protein kinase A, which phosphorylates specific channel proteins in the postsynaptic membrane, causing them to open or, in some cases, to close. Because of the amplifying effect of the signal transduction pathway, the binding of a neurotransmitter molecule to a single receptor can open or close many channels.

Neurotransmitters Table 48.1 lists some of the major known neurotransmitters. Each neurotransmitter binds to its own group of receptors; some neurotransmitters have a dozen or more different recep-

tors, which can produce very different effects in postsynaptic cells. Drugs that target specific receptors are powerful tools in the treatment of nervous system diseases. Acetylcholine Acetylcholine is one of the most common neurotransmitters In both invertebrates and vertebrates. In the vertebrate CMS, it can be either inhibitory or excitatory, depending on the type of receptor. At the vertebrate neuromuscular junction, the synapse between a motor neuron and a skeletal muscle cell, acetylcholine released by the motor neuron binds to receptors on ligand-gated channels in the muscle cell, producing an EPSP via direct synaptic transmission. The drug nicotine binds to the same receptors, which are also found elsewhere in the PNS and at various sites in the CNS. Nicotines physiological and psychological effects result from its affinity for this type of acetylcholine receptor. In vertebrate cardiac (heart) muscle, acetylcholine released by parasympathetic neurons (discussed later) activates a signal transduction pathway whose G proteins have two effects: inhibition of adenylyl cyclase and opening of K channels ii the muscle cell membrane. Both effects reduce the strength an rate of contraction of cardiac muscle cells.

Table 48.1 Major Neurotransmitters Neurotransmitter

Structure

Functional Class

Secretion Sites

ri,C — C—0—CH-—CH—N"h-—[CH,]

Excitatory to vertebrate skeletal muscles; excitatory or inhibitory at other sites

CNS; PN5; vertebrate neuromuscular junction

Norepinephrine

Excitatory or inhibitory

CNS; PNS

Dopamine

Generally excitatory: may be inhibitory at some sites

CNS; PNS

Generally inhibitory

CNS

Inhibitory

CNS; invertebrate neuromuscular junction

Inhibitory

CNS

Excitatory

CNS; invertebrate neuromuscular junction

Excitatory

CNS

Acetylcholine

Biogenic Amines

Serotonin

H0_^\

H

Amino Acids GABA (gamma aminobutyric acid)

H2N — CH ? — CH.—CH — COOH

Glycine Glutamate

H 2 N — C H — CH — CH — COOH COOH

Aspartate

H7N—CH—CH,—COOH 7 j 2 COOH

Neuropeptides (a very diverse group, only two of which are shown) Substance P

Arg-—Pro—Lys—Pro— of muscle fiber

A Figure 49.32 The roles of the sarcoplasmic reticulum and T tubules in muscle fiber contraction. The synaptic terminal of a motor neuron releases acetylcholine, which depolarizes the plasma membrane of the muscle fiber. The depolarization causes action potentials (blue arrows) to sweep across the muscle fiber and deep into it along the transverse (T) tubules. The action potentials trigger the release of calcium (green dots) from the sarcoplasmic reticulum into the cytosol, Calcium initiates the sliding of filaments by allowing myosin to bind to actin.

atrophy. ALS is progressive and usually fatal within five years after symptoms appear; there is no cure or treatment at this time. Botulism results from the consumption of an exotoxin secreted by the bacterium Clostridium botulinum in improperly preserved foods (see Chapter 27). The toxin, which paralyzes muscles by blocking the release of acetylcholine from motor neurons, is also injected into certain facial muscles to eliminate frown lines in "Botox" treatments. Myasthenia gravis is an autoimmune disease in which a person produces antibodies to the acetylcholine receptors on skeletal muscle fibers. The number of these receptors decreases, and the synaptic transmission between motor neurons and the muscle fibers becomes less effective. Neural Control of Muscle Tension When an action potential in a motor neuron releases acetylcholine on a skeletal muscle fiber, the muscle fiber responds by producing a brief, all-or-none contraction, called a twitch. However, our everyday experience shows that the contraction of a whole muscle, such as the biceps, is graded; we can voluntarily alter the extent and strength of its contraction. Experimental studies confirm this observation. There are two basic mechanisms by which the nervous system produces graded contractions of whole muscles: (1) by varying the number of muscle CHAPTER 49

Sensorv and Motor Mechanisms

1069

PLASMA MEMBRANE

• Action potential is p: gated along plasma

-

Action potential triggers Ca 2 * release from sarcopiasmk; reticulum

(SR). — *a*

Tropomyosin blockage of myosinbinding sites is restored; contraction ends, and muscle fiber relaxes.

Ca 2 *

Calcium ions bind to troponin; troponin changes shape, removing blocking action of tropomyosin; myosin-bindinc sites exposed.

0 Cytosolic Ca 2+ is removed by active transport into SR after action potential ends.

0 Myosin cross-bridges alternately attach to actin and detach, pulling actin filaments toward center of sarcomere; ATP powers sliding of filaments.

A Figure 49.33 Review of contraction in a skeletal muscle fiber.

fibers that contract and (2) by varying the rate at which muscle fibers are stimulated. Let's consider each mechanism in turn. In a vertebrate skeletal muscle, each muscle fiber is innervated by only one motor neuron, but each branched motor neuron may synapse with many muscle fibers (Figure 49.34). There may be hundreds of motor neurons controlling a muscle, each with its own pool of muscle fibers scattered throughout the muscle. A motor unit consists of a single motor neuron and all the muscle fibers it controls. When a motor neuron produces an action potential, all the muscle fibers in its motor unit contract as a group. The strength of the resulting contraction therefore depends on how many muscle fibers the motor neuron controls. In most muscles, the number of muscle fibers in different motor units ranges from a few to 1070

UNIT SEVEN Animal Form and Function

hundreds. The nervous system can thus regulate the strength of contraction in a whole muscle by determining how mammotor units are activated at a given instant and by selecting large or small motor units to activate. The force (tension) developed by a muscle progressively increases as more and more of the motor neurons controlling the muscle are activated, J process called recruitment of motor neurons. Depending on the number of motor neurons your brain recruits and the size of their motor units, you can lift a fork or something much heavier, like your biology textbook. Some muscles, especially those that hold the body up and maintain posture, are almost always partially contracted. Prolonged contraction can result in muscle fatigue caused by the depletion of ATP, dissipation of ion gradients required for

Spinal cord Motor unit 2 Synaptic terminals

A Figure 49.35 Summation of twitches. This graph compares the tension developed in a muscle fiber in response to a single action potential in a motor neuron, a pair of action potentials, and a series of action potentials. The dashed lines show the tension that would have developed if only the first action potential had occurred.

i. Figure 49.34 Motor units in a vertebrate skeletal muscle. Each muscle fiber (cell) has a single synapse with one motor neuron, but each motor neuron typically synapses with several or many muscle muse fibers. A motor neuron and all the muscle fibers it controls constitute a motor unit.

normal electrical signaling, and accumulation of lactate (see Figure 9.17). Recent research suggests that although lactate may contribute to muscle fatigue, it may have a beneficial effect on muscle function. In a mechanism that reduces fatigue, the nervous system alternates activation among the motor units in a muscle, allowing different motor units to take turns maintaining the prolonged contraction. The second mechanism by which the nervous system produces graded whole-muscle contractions is by varying the rate of muscle fiber stimulation. A single action potential will produce a twitch lasting about 100 milliseconds (msec) or less. If a second action potential arrives before the muscle fiber has completely relaxed, the two twitches will sum, resulting in greater tension Figure 49.35). Further summation occurs as the rate of stimulation increases. When the rate is high enough that the muscle liber cannot relax at all between stimuli, the twitches fuse into one smooth, sustained contraction called tetanus (not to be confused with the disease of the same name). Motor neurons usually deliver their action potentials in rapid-fire volleys, and the resulting summation of tension results in the smooth contraction typical of tetanus rather than the jerky actions of individual twitches. The increase in tension during summation and tetanus oc.curs because muscle fibers are not directly attached to bones

but instead are connected via tendons and connective tissues. When a muscle fiber contracts, it stretches these elastic structures, which then transmit tension to the bones. In a single twitch, the muscle fiber begins to relax before the elastic structures are fully stretched. During summation, however, the high-frequency action potentials maintain an elevated concentration of calcium in the muscle fiber's cytosol, prolonging cross-bridge cycling and causing greater stretching of the elastic structures. During tetanus, the elastic structures are fully stretched, and all of the tension generated by the muscle fiber is transmitted to the bones. T^es of Muscle Fibers All skeletal muscle fibers contract when stimulated by an action potential in a motor neuron, but the speed at which they contract differs among muscle fibers. This difference is mainly due to the rate at which their myosin heads hydrolyze ATE Based on their speed of contraction, we can classify muscle fibers as either fast or slow. Fast fibers are used for brief, rapid, powerful contractions. In contrast, slow fibers, often found in muscles that maintain posture, can sustain long contractions. A slow fiber has less sarcoplasmic reticulum and slower calcium pumps than a fast fiber, so calcium remains in the cytosol longer. This causes a twitch in a slow fiber to last about five times as long as one in a fast fiber. Another criterion for classifying muscle fibers is the major metabolic pathway they use for producing ATP Fibers that rely mostly on aerobic respiration are called oxidative fibers, while those that primarily use glycolysis are called glycolytic fibers. Oxidative fibers are specialized to make use of a steady supply of energy: They have many mitochondria, a rich blood supply, and a large amount of an oxygen-storing protein called myoglobin. Myoglobin, the brownish-red pigment in the dark meat of poultry and fish, binds oxygen more tightly than does CHAPTER 49

Sensorv and Motor Mechanisms

1071

Table 49.1 Types of Skeletal Muscle Fibers Slow Oxidative

Fast Oxidative

Fast Glycolytic

Contraction speed

Slow

Fast

Fast

Myosin ATPase activity

Slow

Fast

Fast

Major pathway for ATP synthesis Race of fatigue

Aerobic respiration Slow

Aerobic respiration Intermediate

provide direct electrical coupling between the cells. Thus, an action potential generated by a cell in one part of the heart will

spread to all other cardiac muscle cells,

Fiber diameter

Small

Intermediate

Mitochondria

Many

Many

Capillaries

Many

Many

Myoglobin content

High

High

Color

Red

Red to pink

hemoglobin, so it can effectively extract oxygen from the blood. All glycolytic fibers are fast, but oxidative fibers can be either fast or slow. Therefore, considering both contraction speed and ATP synthesis, we can identify three main types of skeletal muscle fibers: slow oxidative, fast oxidative, and fast glycolytic. Table 49.1 compares some of their characteristics. Most human skeletal muscles contain all three fiber types, although the muscles of the eye and hand lack slow oxidative fibers. In muscles that have a mixture of fast and slow fibers, the relative proportions of each are genetically determined. However, if such a muscle is used repeatedly for activities requiring high endurance, some fast glycolytic fibers can develop into fast oxidative fibers. Since fast oxidative fibers fatigue more slowly than fast glycolytic fibers, the muscle as a whole will become more resistant to fatigue.

Other Types of Muscle There are many types of muscles in the animal kingdom. As noted previously, however, they share the same fundamental mechanism of contraction; the sliding of actin and myosin filaments past each other. In addition to skeletal muscle, vertebrates have cardiac muscle and smooth muscle (see Figure 40.5). Vertebrate cardiac muscle is found in only one place—the heart. Like skeletal muscle, cardiac muscle is striated. However, structural differences between skeletal and cardiac muscle fibers result in differences in their electrical and membrane properties. Whereas skeletal muscle fibers will not produce action potentials unless stimulated by a motor neuron, cardiac muscle cells have ion channels in their plasma membrane that cause rhythmic depolarizations, triggering action potentials without input from the nervous system. Action potentials of cardiac muscle cells last up to 20 times longer than those of skeletal muscle fibers and play a key role in controlling duration of contraction. Plasma membranes of adjacent cardiac muscle cells interlock at specialized regions called intercalated disks, where gap junctions (see Figure 6.30) 1072

UNIT SEVEN

Animal Form and Function

and the whole heart will contract.

Smooth muscle is found mainly in the walls of hollow organs, such as blood vessels and organs of the digestive tract. Fast Smooth muscle cells lack striations because Large their actin and myosin filaments are not Few regularly arrayed along the length of the Few cell. Instead, the thick filaments are scatLow tered throughout the cytoplasm, and the thin filaments are attached to structures White called dense bodies, some of which are tethered to the plasma membrane. There is less myosin than in striated muscle fibers, and the myosin is not associated with specific actin strands. Further, smooth muscle cells have no troponin complex or T tubules, and their SR is not well developed. During an action potential, calcium ions enter the cytosol mainly through the plasma membrane, and the amount reaching the filaments is rather small. Calcium ions cause contraction by binding to calmodulin (see Chapter 11), which activates an enzyme that phosphorylates the myosin head. Smooth muscles contract relatively slowly but over a much greater range of length than striated muscle. Some smooth muscle cells contract only when stimulated by neurons of the autonomic nervous system. Others can generate action potentials without neural input and are electrically coupled to one anothe". Invertebrates have muscle cells similar to vertebrate skeletal and smooth muscle cells. Arthropod skeletal muscles ar= nearly identical to vertebrate skeletal muscles. However, the flight muscles of insects are capable of independent, rhythmic contraction, so the wings of some insects can actually beat faster than action potentials can arrive from the central nervous system. Another interesting evolutionary adaptation has been discovered in the muscles that hold clam shells closed. The thick filaments in these muscles contain a protein called paramyosin that enables the muscles to remain contracted with a low rate of energy consumption for as long as a month. Glycolysis

Concept Check 1. Summarize the microscopic evidence indicating that thick and thin filaments slide past each other when a skeletal muscle fiber contracts. 2. How can the nervous system cause a skeletal muscle to produce the most forceful contraction it is capable of? 3. Contrast the role of calcium ions in the contraction of a skeletal muscle fiber and a smooth muscle cell. For suggested answers, see Appendix A.

Locomotion requires energy to overcome friction and gravity Movement is a hallmark of animals. To catch food, an animal must either move through its environment or move the surrounding water or air past itself. Although all sponges are sessile, they use beating flagella to generate water currents that draw and trap small food particles, and sessile cnidarians wave tentacles that capture prey (see Chapter 33). Most animals, however, are mobile and spend a considerable portion of their time and energy actively searching for food, as well as escaping from danger and looking for mates. Our focus here is locomotion, or active travel from place to place. The modes of animal locomotion are diverse. Most animal phyla include species that swim. On land and in the sediments on the floor of the sea and lakes, animals crawl, walk, run, or hop. Active flight (in contrast to gliding downward from a tree or elevated ground) has evolved in only a few animal groups: insects, reptiles (including birds), and, among the mammals, bats. A group of large flying reptiles died out millions of years ago, leaving birds and bats as the only flying vertebrates. In all its modes, locomotion requires that an animal expend energy to overcome two forces that tend to keep it stationary: friction and gravity. Exerting force requires energy-consuming cellular work.

Swimming Because most animals are reasonably buoyant in water, overcoming gravity is less of a problem for swimming animals than for species that move on land or through the air. On the c ther hand, water is a much denser and more viscous medium t.ian air, and thus drag (friction) is a major problem for aquatic animals. A sleek, fusiform (torpedo-like) shape is a common adaptation of fast swimmers (see Figure 40.2). Animals swim in diverse ways. For instance, many insects and four-legged vertebrates use their legs as oars to push against the water. Squids, scallops, and some cnidarians are jet-propelled, taking in water and squirting it out in bursts. Sharks and bony fishes swim by moving their body and tail Irom side to side, while whales and other aquatic mammals move by undulating their body and tail up and down.

from falling down. With each step, the animal's leg muscles must overcome inertia by accelerating a leg from a standing start. For moving on land, powerful muscles and strong skeletal support are more important than a streamlined shape. Diverse adaptations for traveling on land have evolved m various vertebrates. For example, kangaroos have large, powerful muscles m their hind legs, suitable for locomotion by hopping (Figure 49.36). When a kangaroo lands, tendons in its hind legs momentarily store energy. The farther the animal hops, the more energy the tendons store. Analogous to the energy in a compressed spring on a pogo stick, the energy stored in the tendons is available for the next jump and reduces the total amount of energy the animal must expend to travel. The pogo stick analogy applies to many land animals; for instance, the legs of an insect, a dog, or a human retain some energy during walking or running, although considerably less than those of a kangaroo. Maintaining balance is another prerequisite for walking, running, or hopping. A kangaroo's large tail helps balance its body during leaps and also forms a stable tripod with its hind legs when the animal sits or moves slowly. Illustrating the same principle, a walking cat, dog, or horse keeps three feet on the ground. Bipedal animals, such as humans and birds, keep part of at least one foot on the ground when walking. When an animal runs, all four feet (or both feet for bipeds) may be off the ground briefly, but at running speeds it is momentum more than foot contact that keeps the body upright. Crawling poses a very different situation. Because much of its body is in contact with the ground, a crawling animal must exert considerable effort to overcome friction. You have read how earthworms crawl by peristalsis. Many snakes crawl by undulating their entire body from side to side. Assisted by

Locomotion on Land In general, the problems of locomotion on land are the opposite of those in water. On land, a walking, running, hopping, or crawling animal must be able to support itself and move igainst gravity, but air poses relatively little resistance, at least at moderate speeds. When a land animal walks, runs, or hops, ,ts leg muscles expend energy both to propel it and to keep it

A Figure 49.36 Energy-efficient locomotion on land. Members of the kangaroo family travel from place to place mainly by leaping on their large hind legs. Kinetic energy momentarily stored in tendons after each leap provides a boost for the next leap. In fact, a large kangaroo hopping at 30 km/hr uses no more energy per minute than it does at 6 km/hr. The large tail helps balance the kangaroo when it leaps as well as when it sits,

CHAPTER 49

Sensory and Motor Mechanisms

1073

large, movable scales on the underside, a snake's body pushes against the ground, driving the animal forward. Boa constrictors and pythons creep straight forward, driven by muscles that lift belly scales off the ground, tilt the scales forward, and then push them backward against the ground.

Flying Gravity poses a major problem for flight because wings must develop enough lift to overcome gravity's downward force. The key to flight is wing shape. All types of wings, including those of airplanes, are airfoils—structures whose shape alters air currents in a way that helps them stay aloft. Flying animals are relatively light, with body masses ranging from less than a gram for some insects to about 20 kg for the largest flying birds. Many flying animals have structural adaptations that reduce body mass. Birds, for example, have hollow, air-filled bones and lack teeth (see Chapter 34). A fusiform shape helps reduce drag in air as it does in water.

Comparing Costs of Locomotion The study of locomotion returns us to the theme of animal bioenergetics, discussed in Chapter 40. The energy cost of locomotion depends on the mode of locomotion and the environment (Figure 49.37). Running animals generally will expend more energy per meter traveled than equivalently sized animals specialized for swimming, partly because running or walking requires energy to overcome gravity. Swimming is the most energy-efficient mode of locomotion (assuming that an animal is specialized for swimming). And if we were to compare the energy consumption per minute rather than per meter, we would find that flying animals use more energy than swimming or running animals with the same body mass. The downward slope of each line on the graph in Figure 49.37 also shows that a larger animal travels more efficiently than a smaller animal specialized for the same mode of transport. For example, a 450-kg horse expends less energy per kilogram of body mass than a 4-kg cat running the same distance. Of course, the total amount of energy expended in locomotion is greater for the larger animal. An animals use of energy to move determines how much energy in the food it consumes is available for other activities, such as growth and reproduction. Therefore, structural and behavioral adaptations that maximize the efficiency of locomotion increase an animal's evolutionary fitness. Although we have discussed sensory receptors and muscles separately in this chapter, they are part of a single integrated system linking together brain, body and the external world. An animal's behavior is the product of this system. Unit Eight discusses behavior within the broader context of ecology, the study of interactions between organisms and their environment. 1074

UNITSEVEN Animal Form and Function

Figure 49.37

:qsiin? What are the energy costs of locomotion? Physiologists typically determine an animal's rate of energy use during locomotion by measuring its oxygen consumption or carbon dioxide production while it swims in a water flume, runs on a treadmill, or flies in a wind tunnel. For example, the trained parakeet shown below is wearing a plastic face mask connected to a tube that collects the air the bird exhales as it flies.

This graph compares the energy cost, in joules per kilogram of body mass per meter traveled, for animals specialized for running, flying, and swimming (1 J = 0.24 cal). Notice that both axes are plotted on logarithmic scales.

For animals of a given body mass, swimming is the most energy-efficient and running the least energy-efficient mode of locomotion. In any mode, a small animal expends more energy per kilogram of body mass than a large animal.

Concept Check 1. Contrast swimming and flying in terms of the main problems they pose and the adaptations that allow animals to overcome those problems. 2. Based on Figure 49.37, which animal uses more energy per kilogram of body mass to move 1 m—a 1-g flyer or a 1-kg runner? For suggested answers, see Appendix A.

Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY CONCEPTS

tongue and in the mouth. Five taste perceptions—sweet, sour, salty, bitter, and umami (elicited by glutamate)—involve several different transduction mechanisms. • Smell in Humans (pp. 1056-1057) Olfactory recepior cells line the upper part of the nasal cavity and send their axons to the olfactory bulb of the brain. More than 1,000 genes code for membrane proteins that bind to specific classes of odorants, and each receptor cell expresses only one or at most a few of those genes.

Sensory receptors transduce stimulus energy and transmit signals to the central nervous system • Functions Performed by Sensory Receptors (pp. 10461048) Sensory receptors are usually specialized neurons or epithelial cells that detect environmental stimuli. Exteroreceptors detect external stimuli; interoreceptors detect internal stimuli. Sensory transduction is the conversion of stimulus energy into a change in membrane potential called a receptor potential. Signal transduction pathways in receptor cells often amplify the signal, which causes the receptor cell either to produce action potentials or to release neurotransmitter at a synapse with a sensory neuron. • Types of Sensory Receptors (pp. 1048-1049) Mechanoreceptors respond to stimuli such as pressure, touch, stretch, motion, and sound. Chemoreceptors detect either total solute concentrations or specific molecules. Electromagnetic receptors detect different forms of electromagnetic radiation. Various types of thermoreceptors signal surface and core temperatures of the body. Pain is detected by a group of diverse receptors that respond to excess heat, pressure, or specific classes of chemicals.

The mechanoreceptors involved with hearing and equilibrium detect settling particles or moving fluid • Sensing Gravity and Sound in Invertebrates (p. 1050) Most invertebrates sense their orientation with respect to gravity by means of statocysts. Many arthropods sense sounds with body hairs that vibrate and with localized "ears," consisting of a tympanic membrane and receptor cells. • Hearing and Equilibrium in Mammals (pp. 1050-1053) The tympanic membrane (eardrum) transmits sound waves to three small bones of the middle ear, which transmit the waves through the oval window to the fluid in the coiled cochlea of the inner ear. Pressure waves in the fluid vibrate the basilar membrane, depolarizing hair cells in the organ of Corti and triggering action potentials that travel via the auditory nerve to the brain. Each region of the basilar membrane vibrates most vigorously at a particular frequency and leads to excitation of a specific auditory area of the cerebral cortex. The utricle, saccule, and three semicircular canals in the inner ear function in balance and equilibrium. • Hearing and Equilibrium in Other Vertebrates (pp. 1053-1054) The detection of water movement in fishes and aquatic amphibians is accomplished by a lateral line system containing clustered hair cells.

The senses of taste and smell are closely related in most animals • Taste in Humans (pp. 1055-1056) Taste and smell both depend on the stimulation of chemoreceptors by small, dissolved molecules that bind to proteins on the plasma membrane. In humans, taste receptors are organized into taste buds on the

Similar mechanisms underlie \ision throughout the animal kingdom • Vision in Invertebrates (pp. 1057-1058) The light detectors of invertebrates include the simple, light-sensitive eyespot of planarians; the image-forming compound eyes of insects, crustaceans, and some polychaetes; and the single-lens eyes of some jellies, polychaetes, spiders, and many molluscs. • The Vertebrate Visual System (pp. 1058-1063) The main parts of the vertebrate eye are the sclera, which includes the cornea; the conjunctiva; the choroid, which includes the iris; the retina, which contains the photoreceptors; and the lens, which focuses light on the retina. Photoreceptors (rods and cones) contain a pigment, retinal, bonded to a protein (opsin). Absorption of light by retinal triggers a signal transduction pathway that hyperpolarizes the photoreceptors, causing them to release less neurotransmitter. Synapses transmit information from photoreceptors to bipolar cells and then to ganglion cells, whose axons in the optic nerve convey action potentials to the brain. Other neurons in the retina integrate information before it is sent to the brain. Most axons in the optic nerves go to the lateral geniculate nuclei of the thalamus, which relays information to the primary visual cortex. Several integrating centers in the cerebral cortex are active in creating visual perceptions. Activity Structure and Function of the Eve

Animal skeletons function in support, protection, and movement • Types of Skeletons (pp. 1063-1064) A hydrostatic skeleton, found in most cnidarians, flatworms, nematodes, and annelids, consists of fluid under pressure in a closed body compartment. Exoskdetons, found in most molluscs and arthropods, are hard coverings deposited on the surface of an animal. Endoskeletons, found in sponges, echinoderms, and chordates, are hard supporting elements embedded within an animal's body. Activity Human Skeleton • Physical Support on Land (pp. 1064-1066) In addition to the skeleton, muscles and tendons help support large land vertebrates.

Muscles move skeletal parts by contracting • Vertebrate Skeletal Muscle (pp. 1066-1072) Skeletal muscles, often present in antagonistic pairs, provide movement by contracting and pulling against the skeleton. Vertebrate skeletal muscle consists of a bundle of muscle cells (fibers), each of which contains myofibrils composed of thin filaments of actin and thick filaments of rnyosin. Myosin heads, energized by the hydrolysis of ATP, bind to the ihin filaments, forming crossbridges. Bending of the myosin heads exerts force on the thin filaments. When ATP binds to the myosin heads, they release, CHAPTER 49

Sensory and Motor Mechanist!

1075

ready to start a new cycle. Repeated cycles cause the thick and thin filaments to slide past each other, shortening the sarcomere and contracting the muscle fiber. A motor neuron initiates contraction by releasing acetylcholine, which depolarizes the muscle fiber. Action potentials travel to the interior of the muscle fiber along the T tubules, stimulating the release of Ca 2+ from the sarcoplasmic reticulum. The calcium ions reposition the tropomyosin-troponin complex on the thin filaments, exposing the myosin-binding sites on actin and allowing the cross-bridge cycle to proceed. A motor unit consists of a motor neuron and the muscle fibers it innervates. Recruitment of multiple motor units results in stronger contractions. A twitch results from a single action potential in a motor neuron. More rapidly delivered action potentials produce a graded contraction by summation. Tetanus is a state of smooth and sustained contraction produced when motor neurons deliver a volley of action potentials. Skeletal muscle fibers are classified as slow oxidative, fast oxidative, or fast glycolytic based on their contraction speed and major pathway for producing ATP Activity Skeletal Muscle Structure Activity Muscle Contraction Investigation How Do Electrical Stimuli Affect Muscle Contraction? • Other Types of Muscle (p. 1072) Cardiac muscle, found only in the heart, consists of striated cells that are electrically connected by intercalated disks and can generate action potentials without neural input. In smooth muscle, contractions are slow and may be initiated by the muscles themselves or by stimulation from neurons in the autonomic nervous system.

Locomotion requires energy to overcome friction and gravity • Swimming {p. 1073) Overcoming friction is a major problem for swimmers. Gravity is less of a problem for swimming animals than for those that move on land or fly. • Locomotion on Land (pp. 1073-1074) Walking, running, hopping, or crawling on land requires an animal to support itself and to move against gravity. • Flying (p. 1074) Flight requires that wings develop enough lift to overcome the downward force of gravity. • Comparing Costs of Locomotion {p. 1074) Animals that are specialized for swimming expend less energy per meter traveled than equivalently sized animals specialized for flying or running.

1076

UNIT SEVEN

Animal Form and Function

YOUR KNOWLEDGE Evolution Connection In general, locomotion on land requires more energy than locomotion in water. By integrating what you have learned throughout these chapters on animal functions, discuss some of the evolutionary adaptations of mammals that support the high energy requirements for moving on land.

Scientific Inquiry Although skeletal muscles generally fatigue fairly rapidly, clam shell muscles have a protein called paramyosin that allows them to sustain contraction for up to a month. From your knowledge of the cellular mechanism of contraction, propose a hypothesis to explain how paramyosin might work. How would you test your hypothesis experimentally? Investigation How Do Electrical Stimuli Affect Muscle Contraction?

Science, Technology, and Society You may know an older person who has broken a bone (often a hip) partly because of osteoporosis, a loss of bone density that affects many women after menopause. As a means of avoiding osteoporosis, researchers recommend exercise and maximum calcium intake when a person is between 10 and 30 years old. Is it realistic to expect young people to view themselves as future senior citizens? How would you suggest that they be encouraged to develop good health habits that might not pay off for 40 or 50 years?

AN INTERVIEW WITH

Gene E. Likens If scientists wore medals on their chests like military generals, ecologist Gene Likens would be one of biology's most decorated heroes. His recent awards include the 2001 National Medal of Science and the 2003 Blue Planet Prize, which honors "outstanding scientific research that helps solve environmental problems." Dr. Likens shared the latter award with his longtime collaborator, Herbert Bormann, with whom Likens pioneered the field of "long-term ecological research" at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. As President and Director of the Institute of Ecosystem Studies in upstate

sense of it. So, we started with the smallwatershed approach. The idea was very simple: We used a medical analogy. We figured we could measure the chemistry of stream water, much like a physician would measure the chemistry of blood, to analyze the health of the watershed, the patient. We wanted to measure all the inputs to the system, such as in precipitation, and all the outputs such as in the stream water draining the system, and from that, try to understand the health of this patient. In each watershed, we find a place where the bedrock is close to the surface, and then build a gauging station, also called a weir. All the water coming down the hill is forced to pass through the weir, so we can measure the amount and chemical composition of the stream water very accurately year round.

Hubbard Brook each summer to continue his inquiry about how forest and aquatic ecosystems work. We conducted this interview at one of Hubbard Brook's historic research sites.

How did the Hubbard Brook Experimental Forest become the focus of your ecosystem research? When I joined the Dartmouth Faculty in 1961,1 met Herb Bormann, who had this idea for a small-watershed approach for studying forest ecosystems. The Hubbard Brook Experimental Forest was ideal for such studies and was close by, And so it was one of those chance situations where it all just fell together. Much of my scientific career has been the result of such serendipity—that is, keeping your eyes, your ears, and your mind open, and when there's an opportunity, jump in and try to make something productive from it.

Why is Hubbard Brook so attractive as a field station for ecosystem research? Our challenge from the beginning was to confront the complexity of this ecosystem and make

1078

Hubbard Brook is also famous for experiments on the effects of deforestation. How do you do such large-scale experiments, arid what were some key results?

Our proposals for research grants mostly emphasize short-term questions that require longterm data. For example, long-term data from Hubbard Brook were critical in establishing that acid rain exists, and also provide the basis for shorter-term measurements of whether the problem of acid rain is getting worse or better. Long-term ecological research, has become a hallmark of Hubbard Brook studies.

An example of these experimental manipulations is when we cut down all the trees in one of the watersheds and left them where they fell. We then used herbicides to prevent regrowth "or three years. We compared the inputs and outputs for this experimental watershed with a reference watershed that was not altered. We expected more water to run off the experimental watershed, and that turned out to be the case. What we didn't expect was that the chemistry of the runoff would change so dramatically. For example, nitrate levels in the water shot up to a level that exceeded public health standards foi drinking. Cutting the trees totally disrupted the recycling of nitrogen in the soil, and the nitrate that accumulated moved downstream. The nitrogen loss slowed after the second year, but some soil nutrients, such as calcium, still haven't come back to the levels and patterns that existed before the trees were cut in 1965.

What was the public impact of evidence for acid rain in the Hubbard Brook Forest?

How do these tree-cutting experiments relate to issues of forest management?

Although Swedish scientists studied acid rain earlier, our research was the first documentation of acid rain in North America- When we published our first paper on acid rain in 1972, the press really picked up on it. The evidence that there was something wrong with rain was a powerful issue that the public could relate to. The long-term data from Hubbard Brook made it very

The research is fundamentally important to our basic understanding of how an ecosystem like this works, but the popular press picked up the story in terms of the clear-cutting controversy. Based on our long-term data, we proposed in a 1978 Science article that you can do clearcutting in these kinds of forests if you follow specific guidelines. Clear cutting shouldn't be

New York, Dr. Likens manages a large team of scientists and educators, but he returns to

difficult to argue that there was no such thing, as acid rain, and 1 think it's fair to say that the quality of those long-term data contributed to the 1990 amendments to the Clean Air Act that addressed the link between acid rain and combustion of fossil fuels.

This is your forty-second straight summer of such studies here at Hubbard Brook. It must be tough to get funding to support research over such a long term.

done on really steep slopes or in very large blocks, and certainly the stream channel should be protected. But most importantly, a site should not be clear cut very often. If a forest is harvested too frequently then the system will become depleted of critical nutrients. It's just like a garden; if you continue to take away and never put anything back, you're going to deplete the soil. The Forest Service has now adopted a 100-year rotation policy (a particular section of forest can be logged only once every 100 years), and we're very proud of that, That forest management protocol is based on science.

Which current research projects at Hubbard Brook do you think are most likely to capture the attention of both the ecology community arid the general public? Richard Holmes at Dartmouth College and his colleagues are conducting really elegant longterm studies of bird populations. I think one of their newsworthy findings is that the total number of birds of all species present in the forest is currently about a third of what it was in 1969. Some species have actually increased in abundance, while others have almost disappeared from here. The researchers are now focusing on how predation, food availability, and other factors may be affecting the bird populations. Another project that I think is quite newsworthy involves a beautiful little lake at the bottom of this valley called Mirror Lake. One of the things 1 started at the very beginning of my work here, about 1965, was to record the period of ice cover on the lake. The ice cover is related to the total beat budget

for the lake, so it's an indication of temperature and climate change. Well, as you might expect, the period of ice cover is highly variable over the short term because the weather vanes from year to year. But our long-term data show that there is a highly statistically significant decline in the period of ice cover—about 19 fewer days of ice cover on the lake now than in the 1960s. This is a clear indication of global warming.

do big projects in science today is with teams, particularly in fields that emphasize biocomplexity But such collaborative science requires trust in your colleagues. I think we should offer more training to graduate students to prepare them for working with research teams.

I was asked that question many times by my colleagues, and some thought I was crazy The job description for the IES position basically said that 1 could do anything I wanted as long as 1 did it well And I liked that a lot. I was the founding director of IES, and I'm extremely proud of how the Institute has built such an outstanding international reputation. We have a superb scientific staff, often working together on projects ranging from a study of invasive zebra mussels in the Hudson River to research on the ecology of infectious diseases, including Lyme disease.

students this year are doing research here at Hubbard Brook, and the rest of the group will come for a visit next week.

Speaking of training, IES complements its scientific mission with education Along with your work at Hubbard programs. Are there any opportunities there for Brook, you are President and undergrads? Director of the Institute of Ecosystem Studies (IES). What We have an NSF Research Experiences for Unmotivated you 20 years ago to dergraduates (REU) program. Each summer, trade a prestigious endowed chair about 10 to 12 REU students are selected. In at Cornell for the challenge of addition to participating in research, the REUs leading a not-for-profit research receive instruction on writing scientific papers, and education institute? finding jobs, and scientific ethics. Two of the

What do you find so attractive about the multidisciplinary, teamoriented research you direct at IES? Many brains can often make faster and better advances than a single brain. The only way you can

Much of my scientific career has been the result of such serendipity—that is, keeping your eyes, your ears, and your mind open, and when there's an opportunity, jump in and try to tonake something productive from it.

What was it about your own education, formal or otherwise, that made you so interested in nature? I grew up on a small farm in Indiana, and my early childhood was spent barefoot, out in the forests and particularly around lakes, When I went to college, I was planning to become an elementary school teacher and a baseball coach. But I had a professor who just kept insisting that I go to graduate school. So to get him off my back, I applied to graduate schools and went to the University of Wisconsin, where I studied lake systems. It was another wonderful serendipitous event for me.

How have your 42 years of experience here at Hubbard Brook affected your view of nature? Hubbard Brook has provided me with just wonderful questions to explore. I've gone from being a naive farm kid to using very sophisticated techniques to answer questions about this complex system and trying to help people understand why it's important to protect this kind of complexity in natural ecosystems. You must respect nature because it provides the basic life support system: clean air, clean water, clean soil, and nourishing food. And if you give these ecosystems a chance, they're resilient and will continue to provide the life support we need. Learning that here at Hubbard Brook changed my life. And can you believe someone pays me to work in such a beautiful place?

1079

itroduction to Ecology and the Biosphere & Figure 50.1 The richness of the biosphere evident in one area of a Panamanian forest.

50.1 Ecology is the study of interactions between organisms and the environment 50.2 Interactions between organisms and the environment limit the distribution of species 50.3 Abiotic and biotic factors influence the structure and dynamics of aquatic biomes 50.4 Climate largely determines the distribution and structure of terrestrial biomes

Hercules scarab beetle (Dynastes hercules) pictured in Figure 50.1 lives. Earth's tropical forests are home to millions of species, including an estimated 5-30 million still undescribed species of insects, spiders, and other arthropods. In fact, every part of the biosphere is inhabited by diverse life-forms, most of which, especially the microbial species, are unknown to science. This chapter introduces the science of ecology and describes some of the factors, both living and nonliving, that affect the distribution of organisms. It also surveys the major types of aquatic and terrestrial habitats where organisms live and where ecologists go to study them..

The Scope of Ecology , rganisms are open systems that interact continuously I with their environment-—a theme that has already surfaced many times in this book. The scientific study of the interactions between organisms and the environment is called ecology (from the Greek oikos, home, and logos, to study). These interactions determine both the distribution of organisms and their abundance, leading to three questions that ecologists often ask about organisms: Where do they live? Why do they live where they do? And how many are there? Ecologists also study how interactions between organisms and the environment affect phenomena such as the number of species living in a particular area, the cycling of nutrients in a forest or lake, and the growth of populations. Because of its great scope, ecology is an enormously complex and exciting area of biology, as well as one of critical importance. Ecology reveals the richness of the biosphere—-the entire portion of Earth inhabited by life—and can provide the basic understanding that will help us to conserve and sustain that richness, now threatened more than ever by human activity. The richness of the biosphere is particularly apparent in tropical forests, such as the Panamanian forest where the 1080

Concept

Ecology is the study of interactions between organisms and the environment Humans have always had an interest in the distribution ami abundance of other organisms. As hunters and gatherers, prehistoric people had to learn where game and edible plants could be found in abundance. With the development of agriculture and the domestication of animals, people learned more about how the environment affects the growth, survival, and reproduction of plants and animals. Later; naturalists from Aristotle to Darwin and beyond observed and described organisms in their natural habitats and systematically recorded their observations. Because extraordinary insight can still be gained through this descriptive approach to discovery science (see Chapter 1), natural history remains a fundamental part of the science of ecology. In addition to its long history as a descriptive science, ecology is also a rigorous experimental science. Despite the

difficulties of conducting experiments in natural environments, ecologists often test their hypotheses through field experiments. The long-term research by Gene Likens and his colleagues in the Hubbard Brook Experimental Forest is one example (see the interview on pages 1078-1079). Other examples of field experiments include studies in which researchers measure the impact of herbivory (plant-eating) on plant species diversity by comparing open control plots to experimental "exclosures" designed to keep out certain herbivore species. Perhaps because of ecology's focus on. complex systems that challenge the capacity of scientists to produce consistent results, ecologists have been innovators in the areas of experimental design and the application of statistical inference. You will encounter many examples of ecological experiments throughout this unit.

time (decades, centuries, millennia, and longer). For instance, hawks feeding on field mice have an immediate impact on the prey population by killing certain individuals, thereby reducing population size (an ecological effect) and altering the gene pool (an evolutionary effect). One long-term evolutionary effect of this predator-prey interaction may be selection for mice with fur coloration that camouflages the animals.

Organisms and the Environment

The environment of any organism includes abiotic, or nonliving, components-—chemical and physical factors such as temperature, light, water, and nutrients-—-and biotic, or living, components—all the organisms, or the biota, that are part of the individual's environment. Other organisms may compete with an individual for food and other resources, prey upon it, parasitize it, provide its food, or change its physical and Ecology and Evolutionary Biology chemical environment. Questions about the relative influence of various environEcology and evolutionary biology are closely related sciences. mental components, both abiotic and biotic, are frequently at Darwin's extensive observations of the distribution of organthe heart of ecological studies. For example, Figure 50.2 isms and their adaptation to specific environments led him shows the geographic range of the red kangaroo in Australia to propose that environmental factors interacting with variaand illustrates the basis for two common ecological questions: tion within populations could cause evolutionary change (see What environmental factors limit the geographic range, or Chapter 22). Today, we have ample evidence that events that ocdistribution, of a species such as the red kangaroo? And what cur in the framework of ecological time (minutes, months, and factors determine its abundance? Some of the climatic factors years) translate into effects over the longer scale of evolutionary that ecologists might consider in the case of the red kangaroo are indicated in Climate in northern Australia Figure 50.2. Notice that red kangaroos do is hot and wet, with seasonal not live in regions around the periphery drought. of Australia, where the climate varies from moist to wet- Notice also that red kangaroos are most abundant in a few areas, shaded dark brown to orange, where the Red kangaroos climate is drier and more variable from occur in most semiarid and arid year to year. Near the edges of their range, regions of the where the climate is wetter than in the ininterior, where precipitation is ^ ^ h d d { ior d h relativelyy low and ^ " tan, red kangaroos are scarce. variable from year to year. These patterns suggest that an abiotic factor, the amount and variability of precipitation, influences the distribution of red kangaroos in Australia. However, are the controls on red kangaroo distribution only abiotic? Perhaps climate influences red kangaroo populations indirectly through mtheastern Australia biotic factors, such as pathogens, parasites, has a wet, cool climate. . , r i .-, , ., 7 Southern Australia has competitors, predators, and food availabilcool, moist winters and ity. For instance, do wetter regions harbor warm, dry summers. pathogens and parasites deadly to red Tasmania kangaroos? Ecologists generally need to consider multiple factors and alternative A Figure 50.2 Distribution and abundance of the red kangaroo in Australia, hypotheses when attempting to explain based on aerial surveys. patterns of distribution and abundance.

XJ

CHAPTER so

An Introduction to Ecology and the Biosphere

1081

Subfields of Ecology Ecology can be divided into areas of study ranging from the ecology of individual organisms to the dynamics of ecosystems and landscapes (Figure 50.3). While each of these subfields has some unique terminology and scientific concerns, modern ecological studies increasingly cross the boundaries between traditionally separate areas. Organismal ecology, which may be subdivided into the disciplines of physiological ecology, evolutionary ecology, and behavioral ecology, concerns how an organisms structure, physiology, and (for animals) behavior meet the challenges posed by the environment. A population is a group of individuals of the same species living in a particular geographic area. Population ecology concentrates mainly on factors that affect how many individuals of a particular species live in an area. A community consists of all the organisms of all the species that inhabit a particular area; it is an assemblage of populations of many different species. Thus, community ecology deals

with the whole array of interacting species in a communi y This area of research focuses on how interactions such as predation, competition, and disease, as well as abiotic factors such

as disturbance, affect community structure and organisation An ecosystem consists of all the abiotic factors in addition to the entire community of species that exist in a certain area. An ecosystem—a lake, for example—may contain many different communities. In ecosystem ecology, the emphasis is on energy flow and chemical cycling among the various biotic and abiotic components. Landscape ecology deals with arrays of ecosystems—just two examples are a corridor of trees lining a river flowing through a sparsely vegetated plain, or patches of coral reef surrounded by turtle grass-—-and how they are arranged in a geographic region. Every landscape or seascape consists of a mosaic of different types of "patches," an environmental characteristic ecologists refer to as patchiness. Landscape ecological research focuses on the factors controlling exchanges of energy, materials, and organisms among the ecosystem patches making up a landscape or seascape.

(a) Organismal ecology. How do humpback whales select their calving areas?

(b) Population ecology. What enviromental factors affect the reproductive rate of deer mice?

(c) Community ecology. What factors influence the diversity of species that make up a particular forest? (e) Landscape ecology. To what extent do the trees lining the drainage channels in this landscape serve as corridors of dispersal for forest animals?

A Figure 50.3 Examples of questions in different subfields of ecology.

1082

UNIT EIGHT

Ecology

(d) Ecosystem ecology. What factors control photosynthetic productivity in a temperate grassland ecosystem?

The biosphere is the global ecosystem—the sum of all the planet's ecosystems. This broadest area of ecology includes the entire portion of Earth inhabited by life: the atmosphere to an altitude of several kilometers, the land down to and including water-bearing rocks at least 3 kilometers below-ground, lakes and streams, caves, and the oceans to a depth of several kilometers. An example of research at the biosphere level is the analysis of how changes in atmospheric CO2 concentration may affect Earth's climate and, in turn, all life.

E cology and Environmental Issues Because the term ecology is so often misused in popular writing to refer to environmental concerns, it is important to clarify the difference between ecology and environmentalism (advocating for the protection or preservation of the natural environment). To address environmental problems, we need to understand the often complicated and delicate relationships between organisms and the environment. The science of ecology provides that understanding. If we know that phosphate promotes the growth of algae in lakes, for instance, we may decide to limit the use of phosphate-rich fertilizers in surrounding areas to protect lakes from becoming clogged with algae. Much of society's current awareness of environmental issues had its beginnings in 1962 with Rachel Carson's Silent Spring (Figure 50.4). In that now-classic book, Carson warned that the widespread use of pesticides such as DDT was causing population declines in many nontarget organisms. Today, acid precipitation, localized famine aggravated by land misuse and population growth, the poisoning of soil and streams with toxic wastes, and the growing list of species extinct or endangered because of habitat destruction are just a few of the problems that threaten the home we share with millions of other forms of life. Many influential ecologists, including Gene Likens, recognize a responsibility to educate legislators and the general public about decisions that affect the environment. An important part of this responsibility is communicating the scientific complexity of environmental issues. Politicians and lawyers often want definitive answers to such environmental questions as how much old-growth forest is needed to "save

•* Figure 50.4 Rachel Carson. In Silent Spring, a book seminal to the modern environmental movement, Carson had a broad message; "The 'control of nature' is a phrase conceived in arrogance, born of the Neanderthal age of biology and philosophy, when it was supposed that nature exists for the convenience of man."

the spotted owls." While ecological studies can certainly provide essential information for making policy decisions on habitat preservation, responses to such questions often include further questions: How many owls must be saved? With what certainty must they be saved? How long can they survive in this amount of forest? Ecologists can help answer these questions so that the public and policymakers can make informed decisions about environmental issues. Although our ecological information is always incomplete, we cannot abstain from making decisions about environmental issues until all the answers are known. But given what we do know about the interconnectedness of the biosphere, it is probably wise to follow the precautionary principle, which can be expressed simply as "An ounce of prevention is worth a pound of cure." Aldo Leopold, the famous wildlife conservationist, expressed the precautionary principle well when he wrote, "To keep every cog and wheel is the first precaution to intelligent tinkering."

Concept Check . 1. Contrast the terms ecology and environmentalism. How does ecology relate to environmentalism? 2. How can an event that occurs on the ecological time scale affect events that occur on an evolutionary time scale? 3. Within which area of ecology is each of the following directly working: (a) an ecologist studying the distribution and abundance of red kangaroos in Australia; (b) an ecologist studying changes in which plant species are most abundant in a forest following a wildfire? For suggested answers, see Appendix A.

Concept

Interactions between organisms and the environment limit the distribution of species In Chapter 22, we discussed biogeography, the study of the past and present distribution of individual species, in the context of evolutionary theory. Ecologists have long recognized global and regional patterns in the distribution of organisms within the biosphere. Kangaroos occur in Australia but not in North America, wThereas pronghorn antelope occur in the western United States but not in Europe or Africa. More than a century ago, Darwin, Wallace, and other naturalists began to identify broad patterns of distribution by naming biogeographic CHAPTER so

An Introduction to Ecology and the Biosphere

1083

get there because a barrier to their dispersal existed; the area has been inaccessible to kangaroos since they first appeared. However, while land-bound kangaroos have not reached North

America under their own power, some other organisms that are adapted for long-distance dispersal, such as some birds, have. The dispersal of organisms is critical to understanding both geographic isolation in evolution (see Chapter 24) and the broad patterns of current geographic distributions of species. -Equati

Natural Range Expansions ~(23.5°S)Tropic of Capricorn

A Figure 50.5 B i o g e o g r a p h i c r e a l m s . Continental drift and barriers such as deserts and mountain ranges all contribute to the distinctive flora and fauna found in Earth's major biogeographic realms. The realms are not sharply delineated but grade together in zones where species from adjacent realms coexist.

realms (Figure 50.5), We now associate these biogeographic realms with patterns of continental drift that followed the breakup of the supercontinent Pangaea (see Chapter 26). Biogeography provides a good starting point for understanding what limits the geographic distribution of a species. To see how ecologists might arrive at such an understanding, let's work our way through the series of questions in the flowchart in Figure 50.6.

Dispersal and Distribution The movement of individuals away from centers of high population density or from their area of origin, called dispersal, contributes to the global distribution of organisms. For example, why are there no kangaroos in North America? A biogeographer might propose this simple hypothesis: They could not

The role of dispersal is demonstrated when organisms expand their range by moving into areas where they did not exist previously. For instance, the cattle egret (see Figure 53.10), a species widely distributed in Africa, Eurasia, and Australia, was not found in the Americas 200 years ago. But in the late 1800s, some of these strong-flying birds managed to fly across the Atlantic Ocean and colonize northeastern South America. From there, cattle egrets gradually spread northward through Central America and into North America, reaching Florida in the 1950s. They have since spread widely and today have breeding populations as far west as the Pacific Coast and as far north as southern Canada. The great-tailed grackle, a large, conspicuous bird related to blackbirds and orioles, is another species that ha:; expanded its range over time, moving northward from the coasi of the Gulf of Mexico and the Rio Grande Valley (Figure 50.7). Natural range expansions clearly show the influence of dispersal on distribution, but opportunities to observe such dispersal are rare. As a consequence, ecologists have often turned to experimental methods to better understand the role of dispersal in limiting the distribution of species. Species Transplants One direct way to determine if dispersal is a key factor limiting distribution is to observe the results of intentional or accidental transplants of a species to areas where it was previously

Predation, parasitism, competition, disease Water Oxygen Salinity PH Soil nutrients, etc

Abiotic factors limit distribution?

A Figure 50.6 Flowchart of factors limiting geographic d i s t r i b u t i o n . As ecologists study the factors limiting a species' distribution, they often consider a series of questions like these,

1084

UNIT EIGHT

Ecology

Physical factors

- Temperature Light soil structure Fire Moisture, etc.

Although habitat selection is one of the least understood ecological processes, some instances have been closely studied, including habitat selection by several insect species. Female insects often oviposit (deposit eggs) only in response to a very narrow set of stimuli, which may restrict distribution of the insects to certain host plants. Larvae of the European com borer, for example, can feed on a wide variety of plants but occur almost exclusively on corn because the ovipositing females are attracted by odors produced by the corn plant. And consider the ecology of anopheline mosquitoes. Important carriers of disease, these insects have been extensively studied as part of efforts to eradicate malaria in tropical areas. Each anopheline species is usually associated with a particular type of habitat. Surprisingly, large areas of apparently suitable tropical habitat are completely free of dangerous mosquitoes. Habitat selection for oviposition sites by female mosquitoes appears to restrict their distribution.

i Figure 50.7 Spread of breeding populations of the great-tailed grackle in the United States from 1974 to 1996. The grackle expanded its breeding range substantially in just 22 years.

absent. For a transplant to be considered successful, some of dpe organisms must not only survive in the new area but also reproduce there. Thus, a transplants success may not be determined until at least one life cycle is complete. If a transplant is successful, then we can conclude that the potential range of the species is larger than its actual range; in other words, the species could live in certain areas where it currently does not. Species introduced to new geographic locations often disrupt the communities and ecosystems to which they have been introduced and spread far beyond the area of intended introduction (see Chapter 55). Consequently, ecologists rarely conduct transplant experiments today. Instead, they document the outcome when a species has been transplanted for other purposes, such as to introduce game animals or sport fish, or when a species has been accidentally transplanted.

Behavior and Habitat Selection As transplant experiments show, some organisms do not occupy all of their potential range, even though they may be physically able to disperse into the unoccupied areas. To follow our line of questioning from Figure 50.6, does behavior play a role in limiting distribution in such cases? When individuals seem to avoid certain habitats, even when the habitats are suitable, their distribution may be limited by habitat selection behavior. While habitat selection is typically applied to animals, plants may also select their habitats—for instance, by producing seeds that germinate only under a restricted set of environmental conditions—even though individual plants cannot move once they are rooted.

It might seem that a tropical mosquito is deficient because she does not lay eggs in apparently suitable rice field habitats. But this behavior may simply reflect the fact that rice fields are an evolutionarily recent habitat, and the mosquito is not adapted to it. Adaptation is not instantaneous, nor does evolution produce organisms perfectly adapted to every environment (see Chapter 23).

Biotic Factors If behavior does not limit the distribution of a species, our next question is whether biotic factors—that is, other species—are responsible. In many cases, a species cannot complete its full life cycle if transplanted to a new area. This inability to survive and reproduce may be due to negative interactions with other organisms in the form of predation, parasitism, disease, or competition. Or survival and reproduction may be limited by the absence of other species on which the transplanted species depends. For instance, the absence of particular pollinator species could prevent reproduction by a transplanted plant species, though such specific dependence is rare. More common examples of biotic limitation on geographic distribution involve predators (organisms that kill their prey) and herbivores (organisms that eat plants or algae). Simply put, organisms that eat can limit the distribution of organisms that get eaten. Let's examine one specific case of an herbivore limiting the distribution of a food species. In certain marine ecosystems, there is often an inverse relationship between the abundance of sea urchins and the abundance of seaweeds (large marine algae, such as kelp). Where sea urchins that graze on seaweeds and other algae are common, large stands of seaweeds do not become established. Thus, the local distribution of seaweeds appears to be limited by sea urchins. This kind of interaction can be tested by "removal and addition'' experiments. In studies near Sydney, Australia, W. J. Fletcher, of the University of CHAPTER so An Introduction to Ecology and the Biosphere

1085

Sydney, tested the hypothesis that sea urchins are a biotic factor limiting seaweed distribution. Fletcher reasoned that if this hypothesis is correct, then more seaweeds should invade an area from which sea urchins have been removed. Conversely, if urchins are added to an area rich in seaweeds, the seaweeds should be diminished. One complication is that there are often several other herbivores in addition to sea urchins in the habitats where seaweeds may grow; thus, Fletcher needed to perform a series of manipulative field experiments to isolate the influence of sea urchins on seaweeds in his study area (Figure 50.8). In addition to predation and herbivory, the presence or absence of food resources, parasites, diseases, and competing organisms can act as biotic limitations on species distribution. Unfortunately, some of the most dramatic cases of limitation occur when humans introduce (either accidentally or intentionally) exotic predators or diseases into new areas and wipe out native species. You will encounter examples of these impacts in Chapter 55, which discusses conservation ecology

Abiotic Factors The last question in our flowchart considers abiotic factors as limits to distribution. The global distribution of organisms broadly reflects the influence of abiotic factors, such as regional differences in temperature, water, and sunlight. Throughout this discussion, keep in mind that the environment is characterized by both spatial heterogeneity and temporal heterogeneity.

In other words, it varies in both space and time. Although tvvo regions of Earth may experience different conditions at any given time, daily and annual fluctuations of abiotic factors may either blur or accentuate regional distinctions. Temperature

Environmental temperature is an important factor in the distribution of organisms because of its effect on biological processes. Cells may rupture if the water they contain freezes (at temperatures below 0°C), and the proteins of most organisms denature at temperatures above 45°C In addition, few organisms can maintain an appropriately active metabolism at very low or very high temperatures. Extraordinary adaptations enable some organisms, such as thermophilic prokaryotes (see Chapter 27), to live Figure 50.8 outside the temperature range habitable B«|ii;Vy Does feeding by sea urchins and limpets affect by other life. seaweed distribution? An organism's internal temperature is W. J. Fletcher tested the effects of two algae-eating animals, sea urchins affected by heat exchange with its environand limpets, on seaweed abundance near Sydney, Australia. In areas adjacent to a control site, ment, and most organisms cannot maineither the urchins, the limpets, or both were removed. tain tissue temperatures more than a few degrees above or below the ambient temFletcher observed a large difference in seaweed growth between areas with and without sea urchins. perature (see Chapter 40). As endotherms, mammals and birds are the major excepRemoving both tions, but even endothermic species func100 limpets and , "1 | § Sea urchins or tion best within various, quite narrow y^Both limpets Y |p urchin removing only environmental temperature ranges. ^r and urchins *S J urchins increased so f removed^X^

m

60-

seaweed cover dramatically.

^X^Only ^r urchins ^r removed

>/ Limpet

40-

Water

/Only limpets removed / /

20-

/

^

/

o August 1.9S2

February 1983

.Control (both /urchins and / limpets present) August 1983

/ /

Almost no seaweed grew in areas where both urchins and limpets were present, or where only limpets were removed.

February 1984

Removing both limpets and urchins resulted in the greatest increase of seaweed cover, indicating that both species have some influence on seaweed distribution. But since removing only urchins greatly increased seaweed growth while removing only limpets had little effect, Fletcher concluded that sea urchins have a much greater effect than limpets in limiting seaweed distribution.

1086

UNIT EIGHT

Ecology

The dramatic variation in water availability among habitats is another important factor in species distribution. Freshwater and marine organisms live submerged in aquatic environments, but most are restricted to either freshwater or saltwater habitats by their limited ability for osmoregulation. Terrestrial organisms face a nearly constant threat of desiccation, and their global distribution reflects their ability to obtain and conserve water. Desert organisms exhibit a variety of adaptations for water acquisition and conservation in a desiccating environment, as described in Chapter 44.

Sunlight Sunlight provides the energy that drives nearly all ecosystems, although only plants and other photosynthetic organisms use this energy source directly. Light intensity is not the most important factor limiting plant growth in many terrestrial environments, although shading by a forest canopy makes competition for light in the understory intense. In aquatic environments, however, the intensity and quality of light limit the distribution of photosynthetic organisms. Every meter of water depth selectively absorbs about 45% of the red light and about 2% of the blue light passing through it. As a result, most photosynthesis in aquatic environments occurs relatively near the surface. The photosynthetic organisms themselves absorb some of the light that penetrates, further reducing light levels in the waters below. Light is also important to the development and behavior of die many organisms that are sensitive to photoperiod, the relative lengths of daytime and nighttime. Photoperiod is a more reliable indicator than temperature for cuing seasonal events, such as flowering by plants (see Chapter 39) or migration by animals.

influences the resident organisms. In freshwater and marine environments., the structure of the substrate determines the organisms that can attach to or burrow in it. Now that we have surveyed the abiotic factors that affect the distribution of organisms, let's focus on how those factors vary with climate as we consider the major role that climate plays in determining species distribution.

Climate Four abiotic factors—temperature, water, sunlight, and wind— are the major components of climate, the prevailing weather conditions in a particular area. Climatic factors, particularly temperature and water, have a major influence on the distribution of organisms. We can describe climate patterns on two scales: macroclimate, patterns on the global, regional, and local level; and microclimate, very fine patterns, such as those encountered by the community of organisms underneath a fallen log. First, let's consider Earth's macroclimate. Global Climate Patterns

ind 'ind amplifies the effects of environmental temperature on organisms by increasing heat loss due to evaporation and convection (see Chapter 40). It also contributes to water loss in organisms by increasing the rate of evaporative cooling in animals and transpiration in plants. In addition, wind can have a substantial effect on the morphology of plants by inhibiting the growth of limbs on the windward side of trees, resulting in a "flagged" appearance (Figure 50.9). and Soil

Earths global climate patterns are determined largely by the input of solar energy and the planet's movement in space. The sun's warming effect on the atmosphere, land, and water establishes the temperature variations, cycles of air movement, and evaporation of water that are responsible for dramatic latitudinal variations in climate. Figure 50.10, on the next two pages, summarizes Earth's climate patterns and how they are formed. Regional, Local, and Seasonal Effects on Climate

The physical structure, pH, and mineral composition of rocks and soil limit the distribution of plants and thus of the animals that feed upon them, contributing to the patchiness of terrestrial ecosystems. In streams and rivers, the composition of the substrate (bottom surface) can affect water chemistry, which in turn

Proximity to bodies of water and topographic features such as mountain ranges create regional climatic variations, and smaller features of the landscape contribute to local climatic variation. These regional and local variations in climate contribute to the patchiness of the biosphere. Seasonal variation is another influence on climate.

• Figure 50.9 "Flagging" of tree limbs due to wind.

Bodies of Water. Ocean currents influence climate along the coasts of continents by heating or cooling overlying air masses, which may then pass across the land. Coastal regions are generally moister than inland areas at the same latitude. The cool, misty climate produced by the cold California current that flows southward along the western United States supports a temperate rain forest ecosystem dominated by large coniferous trees in the Pacific Northwest and large redwood groves farther south. Similarly, the warm Gulf Stream flowing northward from the Caribbean Sea and across the North Atlantic tempers the climate on the west coast of northern Europe. As a result, northwest Europe is warmer during winter than New England, which is actually farther south but is cooled by the Labrador Current flowing south from the coast of Greenland. CHAPTER 50 An Introduction to Ecology and the Biosphere

1087

Global Climate Patterns LATITUDINAL VARIATION IN SUNLIGHT INTENSITY Earth's curved shape causes latitudinal variation in the intensity of sunlight. Because sunlight strikes the equator perpendicularly, the most heat and light per unit of surface area are delivered there. At higher latitudes, sunlight strikes Earth at an oblique angle, and thus the light energy is more diffuse on Earth's surface,

.ow angle of incoming s

SEASONAL VARIATION IN SUNLIGHT INTENSITY Earth's tilt causes seasonal variation in the intensity of solar radiation. Because the planet is tilted on its axis by 23.5° relative to its plane of orbit around the sun, the tropics (those regions that lie between 23.5° north latitude and 23.5° south latitude) experience the greatest

annual input of solar radiation and the least seasonal variation. The seasonal variation of light and temperature increases steadily toward the poles.

March equinox: Equator faces sun directly; neither poie tilts toward sun; ail regions on Earth experience 12 hours of daylight and 12 hours of darkness. June solstice: Northern Hemisphere tilts toward sun; summer begins in Northern Hemisphere; winter begins in Southern Hemisphere.

0 c (equator

Constant tilt of 23.5*

September equinox: Equator faces sun directly; neither poie tilts toward sun; alt regions on Earth experience t2 hours of daylight and 12 hours of darkness.

1088

UNIT EIGHT

Ecology

December solstice: Northern Hemisphere tilts away from sun; winter begins in Northern Hemisphere; summer begins in Southern Hemisphere.

GLOBAL AIR CIRCULATION AND PRECIPITATION PATTERNS Intense solar radiation near the equator initiates a global pattern of air circulation and precipitation. High temperatures in the tropics evaporate water from Earth's surface and cause warm, wet air masses to rise (blue arrows) and flow toward the poles. The rising air masses release much of their water content, creating abundant precipitation in tropical regions. The high-altitude air masses, now dry, descend (brown arrows) toward Earth, absorbing moisture from the land and creating an arid climate conducive to the development of

the deserts that are common at latitudes around 30° north and south. Some of the descending air then flows toward the poles, depositing abundant precipitation (though less than in the tropics). The air masses again rise and release moisture in the vicinity of 60 c latitude. Some of the cold, dry rising air then flows to the poles, where it descends and flows back toward the equator, absorbing moisture and creating the comparatively rainless and bitterly cold climates of the polar regions,

""""%

Descending Jf*™"' dry air 1 absorbs I moisture J

\

9

> Arid zone

; {

\. Ascending' I 1 rndisfairi \ 1 y ; releases:: A S •• mcifsiLire1. '

Descending dry air absorbs moisture

x^,,..

0° Arid zone

Tropics 1

GLOBAL WIND PATTERNS Air flowing close to Earth's surface creates predictable global wind patterns. As Earth rotates on its axis, land near the equator moves faster than that at the poles, deflecting the winds from the vertical paths shown above and creating more easterly and westerly flows. Cooling trade winds blow from east to west in the tropics; prevailing westerlies blow from west to east in the temperate zones, the regions between the tropics and the Arctic Circle or the Antarctic Circle.

An Introduction to Ecology and the Biosphere

1089

0 Warm air over land rises.

paralleling the decline of temperature with increasing latitude. In the north temperate zone, for example, a 1,000-m increase in elevation produces a temperature change equivalent to that produced by an 880-km increase in latitude. This is one reason the biological communities of mountains are similar those at lower elevations but farther from the equator. When warm, moist air approaches a mountain, the air rises and cools, releasing moisture on the windward side of the peaK (Figure 50.12). On the leeward side of the mountain, cooler, dry air descends, absorbing moisture and producing a "rain shadow" Deserts commonly occur on the leeward side of mountain ranges, a phenomenon evident in the Great Basin and the Mojave Desert of western North America, the Gobi Desert of Asia, and the small deserts that characterize the southwest corners of some Caribbean islands.

A Figure 50.11 Moderating effects of large bodies of water on climate. This figure illustrates what happens on a warm summer day.

In general, oceans and large lakes moderate the climate of nearby terrestrial environments. During a warm summer day, when the land is hotter than a large lake or the ocean, air over the land heats up and rises, drawing a cool breeze from the water across the land (Figure 50.11). At night, air over the now warmer ocean or lake rises, reversing the circulation and drawing cooler air from the land out over the water, replacing it with warmer air from offshore. Proximity to water does not always moderate climate, however. In summer in certain regions, including the coast of central and southern California, cool, dry ocean breezes are warmed when they contact the land, absorbing moisture and creating a hot, rainless climate just a few miles inland. This climate pattern also occurs in the area around the Mediterranean Sea, which 0 As moist air moves in gives it the name Mediterranean climate. off the Pacific Ocean and Mountains. Mountains have a significant effect on the amount of sunlight reaching an area, as well as on local temperature and rainfall. South-facing slopes m the Northern Hemisphere receive more sunlight than nearby north-facing slopes and are therefore warmer and drier. These abiotic differences influence species distribution; for example, in many mountains of western North America, spruce and other conifers occupy the north-facing slopes, whereas shrubby, drought-resistant plants inhabit the south-facing slopes. In addition, at any particular latitude, air temperature declines approximately 6°C with every 1,000-m increase in elevation, 1090

U N I T EIGHT

Ecology

Seasonality. In addition to the global changes in day length, solar radiation, and temperature described earlier, the changing angle of the sun over the course of the year affects local environments. For example, the belts of wet and dry air or. either side of the equator move slightly northward and southward with the changing angle of the sun, producing marked wet and dry seasons around 20° latitude, where many tropical deciduous forests grow. In addition, seasonal changes in wind patterns produce variations in ocean currents, sometimes causing the upwelling of nutrient-rich, cold water from deep ocean layers, thus nourishing organisms that live near th< surface. Lakes are also sensitive to seasonal temperature changes. During the summer and winter, many lakes in temperate regions are thermally stratified—that is, layered vertically according to temperature. Such lakes undergo a semiannual mixing of their waters as a result of changing temperature

encounters the westernmost mountains, it flows upward, cools at higher altitudes, and drops a large amount of water. The world's tallest trees, the coastal redwoods,

Q Farther inland, precipitation increases again as the air moves up and over higher mountains. Some of the world's deepest snow packs occur here.

'

A Figure 50.12 How mountains affect rainfall.

Q On the eastern side of the Sierra Nevada, there is little precipitation. As a result of this rain shadow, much of central Nevada is desert.

O In winter, the coldest water in the lake (0°C) lies just below the surface ice; water is progressively warmer at deeper levels of the lake, typically 4-5°C at the bottom. Oj (mg/L)

!

o

* •

3

-1

8

1*

16

i

24LX^

In spring, as the sun melts the ice, the surface water warms to 4°C and sinks below the cooler layers immediately below, eliminating the thermal stratification. Spring winds mix the water to great depth, bringing oxygen (Oj) to the bottom waters (see graphs) and nutrients to the surface. Spring

Winter

12

O 2 (mg/L) 0

4

*• 8

12

Ol concentration H High 11 Medium I ] Low O2 (mg/L)

1

3

4

O2 (mg/L) -

•

3

12

N

Autumn *3 In autumn, as surface water cools rapidly, it sinks below the underlying layers, remixing the water until the surface begins to freeze and the winter temperature profile is reestablished.

:

Thermocline

7.1 s •

Summer

In summer, the lake regains a distinctive thermal profile, with warm surface water separated from cold bottom water by a narrow vertical zone of rapid temperature change, called a thermocline.

A Figure 50.13 Seasonal turnover in lakes with winter ice cover.

profiles (Figure 50.13). This turnover, as it is called, brings oxygenated water from a lake's surface to the bottom and nutrient-rich water from the bottom to the surface in both spring and autumn. These cyclic changes in the abiotic properties of lakes are essential for the survival and growth of organisms at all levels within this ecosystem. Microclimate Many features in the environment influence microclimates by casting shade, affecting evaporation from soil, and changing wind patterns. For example, forest trees frequently moderate the microclimate below them. Consequently, cleared areas generally experience greater temperature extremes than the forest interior because of greater solar radiation and wind currents that are established by the rapid heating and cooling of open land; evaporation is generally greater in clearings as well. Within a forest, low-lying ground is usually wetter than high ground and tends to be occupied by different species of trees. A log or large stone shelters organisms such as salamanders, worms, and insects, buffering them from the extremes of temperature and moisture. Every environment on Earth is similarly characterized by a mosaic of small-scale differences in the abiotic factors that influence the local distributions of organisms.

Long-Term Climate Change If temperature and moisture are the master factors limiting the geographic ranges of plants and animals, the global climate change currently under way will profoundly affect the biosphere (see Chapter 54). One way to predict the possible effects is to look back at the changes that have occurred in temperate regions since the last ice age ended. Until about 16,000 years ago, continental glaciers covered much of North America and Eurasia. As the climate warmed and the glaciers retreated, tree distribution expanded northward- A detailed record of these migrations is captured in fossil pollen deposited in lakes and ponds. (It may seem odd to think of trees "migrating," but recall from Chapter 38 that wind and animals can disperse seeds, sometimes over great distances.) If researchers can determine the climatic limits of current geographic distributions for organisms, they can make predictions about how distributions will change with climatic warming. A major question when applying this approach to plants is whether seed dispersal is rapid enough to sustain the migration of each species as climate changes. For example, fossils suggest that the eastern hemlock was delayed nearly 2,500 years in its movement north at the end of the last ice age partly because of relatively slow seed dispersal. CHAPTER so

An Introduction to Ecology and the Biosphere

1091

year. The unhappy conclusion is clear: Without human assistance in moving into new ranges where they can survive as the climate warms, migrating species such as the American, beech may become extinct.

Concept Check

^r

\)

(a)4.5°C warming over (b) 6.5°C warming over next century next century A Figure 50.14 Current range and predicted range for the American beech (Fagus grandifolia) under two scenarios of climate change.

Let's look at a specific case of how the fossil record of past tree migrations can inform predictions about the biological impact of the current global warming trend. Figure 50.14 shows the current and predicted geographic ranges of the American beech (Fagus grandifolia) under two different climate-change models. These models predict that the northern limit of the beech's range will move 700-900 km northward in the next century, and its southern range limit will move northward an even greater distance. If these predictions are even approximately correct, the beech must move 7-9 km per year northward to keep pace with the warming climate. However, since the end of the last ice age, the beech has migrated into its present range at a rate of only 0.2 km per

1. Give examples of human actions that could expand a species1 distribution by changing its (a) dispersal or (b) biotic interactions. 2. Explain how the sun's unequal heating of Earth's surface influences global climate patterns. For suggested answers, see Appendix A.

Concept

Abiotic and biotic factors influence the structure and dynamics of aquatic biomes We have seen how both biotic and abiotic factors influence the distribution of organisms on Earth. Varying combinations of these factors determine the nature of Earth's many biomes, major types of ecological associations that occupy broad geographic regions of land or water. We'll begin by examining Earths aquatic biomes (Figure 50.15).

• Figure 50.15 The distribution of major aquatic biomes. Key Lakes Coral reefs Rivers I '

I Oceanic pelagic ' zone Estuaries Intertidal zone Abyssal zone (below oceanic pelagic zone)

1092

UNIT EIGHT

Ecology

Aquatic biomes account for the largest part of the biosphere in terms of area, and all types are found around the globe. Ecologists distinguish between freshwater biomes and marine biomes on the basis of physical and chemical differences. For example, marine biomes generally have salt concentrations that average 3%, whereas freshwater biomes are usually characterized by a salt concentration of less than 1%. The largest marine biomes, the oceans, cover about 75% of Earth's surface and thus have an enormous impact on the biosphere. The evaporation of water from the oceans provides most of the planet's rainfall, and ocean temperatures have a major effect on world climate and wind patterns. In addition, marine algae and photosynthetic bacteria supply a substantial portion of the world's oxygen and consume huge amounts of atmospheric carbon dioxide. Freshwater biomes are closely linked to the soils and biotic components of the terrestrial biomes through which they pass or in which they are situated. The particular characteristics of a freshwater biome are also influenced by the patterns and speed of water flow and the climate to which the biome is exposed. Many aquatic biomes are physically and chemically stratified, as illustrated in Figure 50.16 for both a lake and a marine environment. Light is absorbed by both the water itself and the photosynthetic organisms in it, so its intensity decreases rapidly with depth, as mentioned earlier. Ecologists distinguish between the upper photic zone, where there is sufficient light for photosynthesis, and the lower aphotic zone, where little light penetrates. At the bottom of all aquatic biomes,

the substrate is called the benthic zone. Made up of sand and organic and inorganic sediments ("ooze"), the benthic zone is occupied by communities of organisms collectively called benthos. A major source of food for the benthos is dead organic matter called detritus, which "rains" down from the productive surface waters of the photic zone. The deepest regions of the ocean floor are known as the abyssal zone. Thermal energy from sunlight warms surface waters to whatever depth the sunlight penetrates, but the deeper waters remain quite cold. As a result, water temperature in lakes tends to be stratified, especially during summer and winter (see Figure 50.13). In the ocean and in most lakes, a narrow stratum of rapid temperature change called a thermocline separates the more uniformly warm upper layer from more uniformly cold deeper waters. In both freshwater and marine environments, communities are distributed according to depth of the water, degree of light penetration, distance from shore, and open water versus bottom. Marine communities illustrate the limitations on species distribution that result from these abiotic factors. Phytoplankton, zooplankton, and many fish species occur in the relatively shallow photic zone (see Figure 50.16b). Because water absorbs light so well and the ocean is so deep, most of the ocean volume is virtually devoid of light (the aphotic zone) and harbors relatively little life, except for microorganisms and relatively sparse populations of luminescent fishes and invertebrates. Figure 50.17, on the next four pages, surveys the major aquatic biomes. Intertidal zone Neritic zone

Oceanic zone

\Bentnk

-2,500-6,000 m Abyssal zone (deepest regions of ocean floor (a) Zonation in a lake. The lake environment is generally classified on the basis of three physical criteria: light penetration (photic and aphotic zones), distance from shore and water depth (littoral and limnetic zones), and whether it is open water (pelagic zone) or bottom (benthic zone).

(b) Marine zonation. Like lakes, the marine environment is generally classified on the basis of light penetration (photic and aphotic zones), distance from shore and water depth (intertidal, neritic, and oceanic zones), and whether it is open water (pelagic zone) or bottom (benthic and abyssal zones).

A. Figure 50.16 Zonation in aquatic environments.

CHAPTER so

An Introduction to Ecology and the Biosphere

1093

Physical Environment Standing bodies of water range from ponds a few square meters in area to lakes covering thousands of square kilometers. Light decreases with depth, creating stratification (see Figure 50.16a). Temperate lakes may have a seasonal thermocline (see Figure 50.13); tropical lowland lakes have a thermocline year-round.

An oligotrophic lake in Grand Teton, Wyoming

Chemical Environment The salinity (salt content), oxygen concentration, and nutrient content differ greatly among lakes and can vary substantially with season. Oligotrophic lakes are nutrient-poor and generally oxygen-rich; eutrophic lakes are nutrient-rich and often depleted of oxygen if ice-covered in winter and in the deepest zone during summer. The amount of decomposable organic matter in bottom sediments is low in oligotrophic lakes and high in ev.trophic lakes. Geologic Features Oligotrophic lakes tend to have less surface area relative to their depth than eutrophic lakes. Over long periods of time, an oligotrophic lake may become more eutrophic as runoT adds sediments and nutrients to the lake. Photosynthetic Organisms Rates of photosynthesis are higher in eutrophic lakes than in oligotrophic lakes. Rooted and floating aquatic plants live in the littoral zone, the shallow, well-lighted waters close to shore. Further away from shore, the limnetic zone, where water is too deep to support rooted aquatic plants, is inhabited by a variety of phytoplankton and cyaiiobacteria. Animals In the limnetic zone, small drifting animals, or zooplankton, graze on the phytoplankton. The benthic zone is inhabited by a variety of invertebrate animals, with the species composition depending partly on oxygen levels. Fishes live in all zones in lakes with sufficient oxygen. Human Impact Pollution by runoff from fertilized land and dumping of municipal wastes leads to nutrient enrichment, which car produce algal blooms, oxygen depletion, and fish kills.

A eutrophic lake in Okavango delta, Botswana

WETLANDS Physical Environment A wetland is an area covered with water for a long enough period to support aquatic plants. Wetlands range from those that are permanently inundated to those that flood infrequently Chemical Environment Because of the high organic production and decomposition in wetlands, both the water and the soils are periodically low in dissolved oxygen. Wetlands have a high capacity to filter dissolved nutrients and chemical pollutants. Geologic Features Basin wetlands develop in shallow basins, ranging from upland depressions to filled-in lakes and ponds. Riverine wetlands develop along shallow and periodically flooded banks of rivers and streams. Fringe wetlands occur along the coasts of large lakes and seas, where water flows back and forth because of rising lake levels or tidal action. Thus, fringe wetlands include both freshwater and marine biomes. Photosynthetic Organisms Wetlands are among the most productive biomes on Earth. Their water-saturated soils favor the growth of plants, such as floating pond lilies and emergent cattails, many sedges, tamarack, and black spruce, that have adaptations enabling them to grow in water or in soil that is periodically anaerobic owing to the presence of unaerated water. Woody plants dominate the vegetation of swamps, while bogs are dominated by sphagnum mosses. Animals Wetlands are home to a diverse community of invertebrates, which in turn support a wide variety of birds. Herbivores, 1094

UNIT EIGHT

Ecology

Okefenokee National Wetland Reserve in Georgia

from crustaceans and aquatic insect larvae to muskrats, consume algae, detritus, and plants. Carnivores are also varied and may include dragonflies, otters, alligators, and owls. Human Impact Draining and filling have destroyed up to 90% of wetlands in some regions.

Physical Environment The most prominent physical characteristic of streams and rivers is current. Headwater streams are generally cod, clear, turbulent, and swift. Farther downstream, where numerous tributaries may have joined, forming a river, the water is generally w&rmer and more turbid, since rivers generally carry more sediment than their headwaters. Streams and rivers are stratified into vertical zones, extending from surface water through groundwater. Chemical Environment The salt and nutrient content of streams and rivers increases from the headwaters to the mouth. In streams, headwaters are generally rich in oxygen. River water may also contain substantial oxygen, except where there has been organic enrichment from either natural or human sources. Geologjc Features Headwater stream channels are often narrow, with a rocky bottom alternating between riffles and pools. The downstream reaches of rivers are generally wide and meandering. River bottoms are often silty from sediments deposited over long periods of time. Photosynthetic Organisms Headwater streams that flow through grasslands or deserts may be rich in algae or rooted aquatic plants; but in streams flowing through temperate or tropical forests, leaves and other organic matter produced by terrestrial vegetation are the primary source of food for aquatic consumers. In rivers, a large fraction of the organic matter consists of dissolved and highly fragmented material that is earned by the current from forested headwater streams. Animals A great diversity of fishes and invertebrates inhabit unpolluted rivers and streams, distributed according to, and throughout, the vertical zones.

Human Impact Municipal, agricultural, and industrial pollution degrade water quality and kill aquatic organisms. Damming and flood control impair natural functioning of stream and river ecosystems and threaten migratory species such as salmon.

A headwater stream in the Great Smoky Mountains

The Mississippi River far from its headwaters

Physical Environment An estuary is a transition area between river and sea. Estuaries "have very complex flow patterns. During a rising tide, seawater flows up the estuary channel, flowing back down again during the falling tide. Often higher-density seawater occupies the bottom of an estuary channel, while lower-density river water forms a surface layer that mixes little with the salty bottom layer. Chemical Environment Salinity varies spatially within estuaries, from nearly that of fresh water to that of seawater. Salinity also varies with the rise and fall of the tides. Nutrients from the river make estuaries, like wetlands, among the most productive biomes. Geologic Features Estuarine flow patterns combined with the sediments carried by river and tidal water create a complex network of tidal channels, islands, natural levees, and mudflats. Photosynthetic Organisms Saltmarsh grasses and algae, including phytoplankton, are the major producers in estuaries. Animals Estuaries support an abundance of worms, oysters, crabs, and many of the fish species that humans consume. Because of the abundant food in estuaries, many marine invertebrates and fishes use them as a breeding ground; others migrate through estuaries to freshwater habitats upstream. Estuaries are also crucial feeding areas for many semi-aquatic vertebrates, particularly waterfowl. Human Impact Pollution from upstream, and also filling and dredging, have disrupted estuaries worldwide. An estuary in a low coastal plain of Georgia

continued on next page

CHAPTER so

An Introduction to Ecology and the Biosphere

1095

Figure 50.17 (continued)

Exploring Aquatic Biomes INTERTIDAL ZONES

---

Rocky intertidal zone on the Oregon coast Physical Environment An intertidal zone is periodically submerged and exposed by the tides, twice daily on most marine shores. Upper zones experience longer exposures to air and greater variations in physical environment. Among the physical challenges faced by intertidal organisms are variations in temperature and salinity and the mechanical forces of wave action. Changes in physical conditions from the upper intertidal zone to

the lower intertidal zone limit the distributions of many organisms to particular strata, as shown in the photograph. Chemical Environment Oxygen and nutrient levels are generally high and are renewed with each turn of the tides. Geologic Features The substrates of intertidal zones, which are generally either rocky or sandy, select for particular behavior and anatomy among intertidal organisms. The configuration of bays or coastlines influences the magnitude of tides and the relative exposure of intertidal organisms to wave action. Photosynthetic Organisms A high diversity and biomass of attached marine algae inhabit rocky intertidal zones, especially in the lower zone. Because of the instability of the substrate, sandy intertidal zones exposed to vigorous wave action generally lack attached plants or algae, while sandy intertidal zones in protected bays or lagoons often support rich beds of sea grass and algae. Animals Many of the animals of rocky intertidal environments have structural adaptations that enable them to attach to the hard substrate. The composition, density, and diversity of intertidal animals change markedly from the upper to lower intertidal zones. Many of the animals in sandy or muddy intertidal environments, such as suspensionfeeding worms and clams and predatory crustaceans, bury themselves in sand or mud, feeding as the tides bring sources of food. Other common animals are sponges, sea anemones, molluscs, echinoderms, and small fishes. Human Impact Oil pollution has disrupted many intertidal area^.. Recreational use has caused a severe decline in the numbers of beach-nesting birds and sea turtles.

OCEANIC PELAGIC BIOME plankton account for less than half of the photosynthetic activity on Physical Environment The oceanic pelagic biorne is a vast realm Earth. of open blue water, constantly mixed by wind-driven oceanic curAnimals The most abundant animals and other heterotrophs in this rents. The surface waters of temperate ocean areas turn over during biome are zooplankton. These protozoans, worms, copepods fall through spring. Because of higher water clarity, the photic zone shrimp-like krill, jellies, and the small larvae of invertebrates anci extends to greater depths than in coastal marine waters. fishes graze on photosynthetic plankton. The oceanic pelagic biome Chemical Environment Oxygen levels are generally high. Nutrient also includes free-swimming animals, such as large squids, fishes, concentrations are generally lower than in coastal waters. Because sea turtles, and marine mammals. they are thermally stratified year-round, some tropical areas of the oceanic pelagic biome have lower nutrient concentrations than Human Impact Overfishing has depleted fish stocks in all Earth's temperate oceans. Turnover during fall through spring renews nutrioceans, which have also been polluted by waste dumping and oil spills. ents in the photic zones of Lem Derate and high-latitude ocean areas. Geologic Features The most prominent geologic characteristic of the oceanic pelagic biome is its vastness and the great depth of Lhe ocean basins. This biome covers approximately 70% of Earths surface and has an average depth of nearly 4,000 m. The deepest point in the ocean is more than 10,000 m beneath the surface. Photosynthetic Organisms The dominant photosynthetic organisms are phytoplankton, including photosynthetic bacteria, that drift with the oceanic currents. Spring turnover and renewal of nutrients in temperate oceans produces a surge of phytoplankton growth. Despite the large extent of their biome, photosynthetic Open ocean off the island of Hawa

1096

U N I T EIGHT

Ecolo°

CORAL REEFS

Ihysical Environment Reef-building corals are limited to the photic zone of relatively stable tropical marine environments with bigh water clarity They are sensitive to temperatures below about 18D-20°C and above 30°C Chemical Environment Corals require high oxygen levels and are e xcluded by high inputs of fresh water and nutrients. (Geologic Features Corals require a solid substrate for attachment. i. coral reef, which is formed largely from the calcium carbonate skeletons of corals, develops over a long time on oceanic islands. It 1 egins as a fringing reef on young high islands, forming an offshore 1 arrier reef later in the history of the island and becoming a coral atol] as the older oceanic island submerges. ] 'hotosynthetic Organisms Dinoflagellate algae live within the tissues of the corals, forming a mutualistic, symbiotic relationship that provides the corals with organic molecules. Diverse red and green marine algae also contribute substantial amounts of photosynthesis on coral reefs. Animals Corals, a diverse group of cnidarians (see Chapter 33), are themselves the predominant animals on coral reefs. However, fish and invertebrate diversity is exceptionally high. Overall animal di'ersity on coral reefs rivals that of tropical forests. iuman Impact Collecting of coral skeletons, often using poisons md explosives, as well as overftshing for food and for the aquarium

A cora! reef in the Red Sea

trade, have reduced populations of corals and reef fishes. Global warming and pollution may be contributing to large-scale coral mortality.

MARINE BENTHIC ZONE Physical Environment The marine benthic zone consists of the ;eafloor below the surface waters of the coastal, or neritic, zone and _he offshore, pelagic zone (see Figure 50.16b). Although in shallow, lear-coastal waters the benthic zone receives enough sunlight to support photosynthetic organisms, most of the ocean's benthic zone receives no sunlight. Water temperatures decline with depth, while pressure increases. As a result, organisms in the very deep benthic, or abyssal, zone are adapted to continuous cold (about 3°C) and extremely high water pressure. Chemical Environment Except in some areas of organic enrichment, oxygen is present at sufficient concentrations to support a diversity of animals.

Geologic Features Soft sediments cover most of the benthic zone. However, there are areas of rocky substrate on reefs, submarine mountains, and new oceanic crust created by seafloor volcanoes. Food-Producing Organisms Photosynthetic organisms, mainly seaweeds and filamentous algae, are limited to shallow benthic areas with sufficient light to support them. Unique assemblages of organisms, such as those shown in the photo, are associated with deep-sea hydrothermal vents of volcanic origin on mid-ocean ridges. In this ibr ; ;. not. oxygen-deficient environment, the food producers are chernoautotrophic prokaryotes (see Chapter 27) that obtain energy by oxidizing H2S formed by a reaction of the hot water with dissolved sulfate (SO42~~). Animals Neritic benthic communities include numerous invertebrates and fishes. Beyond the photic zone, most consumers depend entirely on organic matter raining down from above. Among the animals of the deep-sea hydrothermal vent communities are giant tube-dwelling worms (pictured at left), some more than 1 m long, They are apparently nourished by chemosynthetic prokaryotes that live as symbionts within the worms. Many other invertebrates, including arthropods and echinoderms, are also abundant around the hydrothermal vents. Human Impact Overfishing has decimated important benthic fish populations such as the cod of the Grand Banks off Newfoundland. Dumping of organic wastes has created oxygen-deprived benthic areas.

A deep-sea hydrothermal vent community

CHAPTER so

An Introduction to Ecology and the Biosphere

1097

Concept Check The following questions refer to Figure 50.17. 1. Are stoneflies, benthic aquatic insects that require relatively high concentrations of oxygen, more likely to live in oligotrophic lakes or eutrophic lakes? Why? 2. Why are phytoplankton, and not benthic algae or rooted aquatic plants, the dominant photosynthetic organisms of the oceanic pelagic biome? For suggested answers, see Appendix A.

Concept

Climate largely determines the distribution and structure of terrestrial biomes All the abiotic factors discussed in this chapter, but especially climate, are important in determining why a particular terrestrial biome is found in a certain area. Because there are latitudinal patterns of climate over Earths surface (see Figure 50.10), there are also latitudinal patterns of biome distribution.

Climate and Terrestrial Biomes We can see the great impact of climate on the distribution of organisms by constructing a climograph, a plot of the temperature and precipitation in a particular region. For example, Figure 50.18 is a climograph denoting annual mean temperature and precipitation for some of the biomes found in North America. Notice that the range of precipitation in northern coniferous forests is similar to that in temperate forests, but the temperature ranges are different. Grasslands are generally drier than either kind of forest, and deserts are drier still. Annual means for temperature and rainfall are reasonably well correlated with the biomes that exist in different regions. However, it is important to distinguish correlation from causation. Although the climograph provides circumstantial evidence that temperature and rainfall are important to the distribution of biomes, it does not confirm that these variables govern the biomes' location. Only a detailed analysis of the water and temperature tolerances of individual species could establish the controlling effects of these variables. As Figure 50.18 shows, there are regions where biomes overlap. Thus factors other than mean temperature and precipitation must play a role m determining where biomes exist.. For example, certain areas in North America with a particular temperature and precipitation combination support a temperate broadleaf forest, but other areas with similar values for these variables support a coniferous forest. How do we explain this variation? First, remember that the climograph is based on annual averages. Often it is not only the mean or average climate 1098

U N I T EIGHT

Ecology

100 200 300 Annual mean precipitation (cm)

400

A. Figure 50.18 A climograph for some major types of biomes in North America. The areas plotted here encompass the range of annual mean temperature and precipitation in the biomes.

that is important but also the pattern of climatic variation. For example, some areas may receive regular precipitation throughout the year, whereas other areas with the same amount of annual precipitation have distinct wet and dry seasons. A similar phenomenon may occur with respect to temperature. Other factors, such as the bedrock in an area, may greatly affect mineral nutrient availability and soil structure, which in turn affect the kind of vegetation that will develop. The general distribution of the major terrestrial biomes is shown in Figure 50.19.

General Features of Terrestrial Biomes Most terrestrial biomes are named for major physical or climatic features and for their predominant vegetation. Temperate grasslands, for instance, are generally found in middle latitudes, where the climate is more moderate than in the tropics or polar regions, and are dominated by various grass species. Each biome is also characterized by microorganisms, fungi, and animals adapted to that particular environment. For example, temperate grasslands are more likely than forests to be populated by large grazing mammals. Vertical stratification is an important feature of terrestrial biomes, and the shapes and sizes of plants largely define the layering. For example, in many forests, the layers consist of the upper canopy, then the low-tree stratum, the shrub understory, the ground layer of herbaceous plants, the forest floor (litter layer), and finally the root layer. Nonforesl biomes have similar, though usually less pronounced, strata. Grasslands have an herbaceous layer of grasses and forbs (small broadleaf plants), a litter layer, and a root layer. Stratification of vegetation provides many different habitats for animals, which often occupy well-defined feeding groups, from the

Key |

Tropical forest

£ Savanna ]

Desert

Chaparral

I

Tundra

Temperate grassland

|

High mountains

Temperate broadleaf forest

~2 Polar ice

Coniferous forest

A Figure 50.19 The distribution of major terrestrial biomes. Although terrestrial [ iornes are mapped here with sharp boundaries, biomes actually grade into one another, sometimes over relatively large areas.

insectivorous and carnivorous birds and bats that feed above canopies to the small mammals, numerous worms, and arthropods that forage in the litter and root layers for food. Although Figure 50.19 shows distinct boundaries between ;he biomes, in actuality, terrestrial biomes usually grade into each other, without sharp boundaries. The area of intergradaion, called an ecotone, may be wide or narrow. The actual species composition of any one kind of biome /aries from one location to another. For instance, in the northern coniferous forest (taiga) of North America, red spruce is ;ornmon in the east but does not occur in most other areas, vvhere black spruce and white spruce are abundant. Although [he vegetation of African deserts superficially resembles that of North American deserts, the plants are actually in different Families. Such "ecological equivalents" can. arise because of convergent evolution (see Figure 25.5). Biomes are dynamic, and disturbance rather than stability tends to be the rule. For example, hurricanes create openings for new species in tropical and temperate forests. In northern coniferous forests, old trees die and fall over, or snowfall may break branches, producing gaps that allow deciduous species, such as aspen and birch, to grow. As a result, biomes usually

exhibit extensive patchiness, with several different communities represented in any particular area. In many biomes, the dominant plants depend on periodic disturbance. For example, natural wildfires are an integral component of grasslands, savannas, chaparral, and many coniferous forests. Before agricultural and urban development, much of the southeastern United States was dominated by a single conifer species, the longleaf pine. Without periodic burning, broadleaf trees tended to replace the pines. Forest managers now use fire as a tool to help maintain many coniferous forests. In many biomes today, extensive human activity has radically altered the natural patterns of periodic physical disturbance. Fires, which used to be part of life on the Great Plains, are now controlled for the sake of agricultural land use. Humans have altered much of Earth's surface, replacing original biomes with urban and agricultural ones. Most of the eastern United States, for example, is classified as temperate broadleaf forest, but human activity has eliminated all but a tiny percentage of the original forest. Figure 50.20, on the next four pages, surveys the major terrestrial biomes. CHAPTER so An Introduction to Ecology and the Biosphere

1099

Figure 50.20

Terrestrial Biomes TROPICAL FOREST

Distribution Equatorial and subequatorial regions. Precipitation In tropical rain forests, rainfall is relatively constant, about 200-400 cm annually In tropical dry forests, precipitation is highly seasonal, about ^ tropical rain forest n Borneo 150-200 cm annually with a six- to seven-month dry season. generally cover tropical forest trees, but are less abundant in dry forest?-. Temperature Air temperatures are warm year-round, averaging Thorny shrubs and succulents are common in tropical dry forests. 25-29°C with little seasonal variation. Animals Animal diversity is higher in tropical forests than in an;' Plants Tropical forests are stratified, and competition for light is inother terrestrial biome. The animals, including amphibians, birdi tense. Strata in rain forests include a layer of emergent trees that grow and other reptiles, mammals, and arthropods, are adapted to th: above a closed canopy the canopy trees, one or two layers of subthree-dimensional environment and are often inconspicuous. canopy trees, and shrub and herb layers. There are generally fewer Human Impact Humans long ago established thriving communi • strata in tropical dry forests. Broadleaf evergreen trees are dominant in ties in tropical forests. Rapid population growth leading to agricul • tropical rain forests, whereas tropical dry forest trees drop their leaves ture and development is now destroying tropical forests. during the dry season. Epiphytes such as bromeliads and orchids

Distribution Deserts occur in a band near 30° north and south latitude or at other latitudes in the interior of continents (for instance, the Gobi Desert of north central Asia). Precipitation Precipitation is low and highly variable, generally less than 30 cm per year. Temperature Temperature is variable seasonally and daily Maximum air temperature in hot deserts may exceed 50°C; in cold deserts air temperature may fall below -30°C. Plants Desert landscapes are dominated by low, widely scattered

vegetation; the proportion of bare ground is high compared witt other terrestrial biomes. The plants include succulents such as cacti, deeply rooted shrubs, and herbs that grow during the infrequen. moist periods. Desert plant adaptations include heat and desiccation tolerance, water storage, and reduced leaf surface area. Physical defenses, such as spines, and chemical defenses, such as toxins in the leaves of shrubs, are common. Many of the plants exhibit C4 or CAM photosynthesis (see Chapter 10). Animals Common desert animals include many kinds of snake? and lizards, scorpions, ants, beetles, migratory and residen! birds, and seed-eating rodents. Many species are nocturnal Water conservation is a common adaptation, with some water from metabolic breakdown of carImpact Long-distance transport water and deep groundwater wells have allowed humans to maintain substantial populations in deserts. Conversion to irrigated agriculture and urbanization have reduced the natural biodiversity of deserts.

The Sonoran Desert

1100

UNIT EIGHT

Ecology

A typical savanna in Kenya

I istribution Equatorial and subequatorial regions. Precipitation Rainfall, which is seasonal, averages 30—50 cm per year. The dry season can last up to eight or nine months. Temperature The savanna is warm year-round, averaging 24-29°C, 1: ut with somewhat more seasonal variation than in tropical forests. Hants The scattered trees found in the savanna are often thorny with reduced leaf surface area, an apparent adaptation to the relatively dry conditions. Fires are common in the dry season, and the cominant plant species are fire-adapted and tolerant of seasonal

] Hstribution This biome occurs in mtdlatitude coastal regions on i everal continents, and its many names reflect its far-flung distribution: (haparral in North America, matorral in Spain and Chile, garigue ; nd maquis in southern France, and fynbos in South Africa. 1'recipitation Precipitation is highly seasonal, with rainy winters {•nd long, dry summers. Annual precipitation generally falls within i he range of 30-50 cm. •Temperature Fall, winter, and spring are cool, with average temperatures ranging from 10-12°C. Average summer temperature can :each 30°C, and daytime maximum temperature can exceed 40°C. " 'lants The chaparral is dominated by shrubs and small trees, along with a high diversity of grasses and herbs. °lant diversity is high, with many species confined to a specific, relatively small geographic irea. Adaptations to drought nclude the tough evergreen eaves of woody plants hat reduce water loss. \daptations to fire are ilso prominent. Some of 1 he shrubs produce seeds .hat will germinate only

drought. Grasses and forbs, which make up most of the ground cover, grow rapidly in response to seasonal rains and are tolerant of grazing by large mammals and other herbivores. Animals Large herbivorous mammals, such as wildebeests and zebras, and their predators, including lions and hyenas, are common inhabitants. However, the dominant herbivores are actually insects, especially termites. During seasonal droughts, grazing mammals must migrate to other parts of the savanna with more forage and scattered watering holes. Human Impact The earliest humans appear to have lived on savannas. Fires set by humans may help maintain this biome. Cattle ranching and overhunting have led to declines in large-mammal populations.

after a hot fire; food reserves stored in their fire-resistant roots enable them to resprout quickly and use nutrients released by the fire. Animals Native mammals include browsers, such as deer and goats, that feed on twigs and buds of woody vegetation and a high diversity of small mammals. Chaparral areas also support a high diversity of amphibians, birds and other reptiles, and insects. Human Impact Chaparral areas have been heavily settled and reduced through conversion to agriculture and urbanization. Humans contribute to the fires that sweep across chaparral.

An area of chaparral in California continued on next page

CHAPTER so

An Introduction to Ecology and the Biosphere

1101

Figure 50,20 (continued)

'orincf Terrestrial Biomes TEMPERATE GRASSLAND

Distribution The veldts of South Africa, the puszta of Hungary, the pampas of Argentina and Uruguay, the steppes of Russia, and the Sheyenne National Grassland in North Dakota plains and prairies of central North America and to fire. The grasses of temperate grassland sprout quickly fol • are all temperate grasslands. lowing fire. Grazing by large mammals helps prevent establishmen: Precipitation Precipitation is highly seasona. .al, with relatively dry of woody shrubs and trees. winters and wet summers. Annual precipitation generally averages Animals Native mammals include large grazers such as bison am i between 30 and 100 cm. Periodic drought is common. wild horses. Temperate grasslands are also inhabited by a wide variTemperature Winters are generally cold, with average temperaety of burrowing mammals, such as prairie dogs in North America. tures frequently falling well below — 10°C. Summers, with average Human Impact Deep, fertile soils make temperate grasslands ideai temperatures frequently approaching 30°C, are hot. places for agriculture, especially for growing grains. As a consePlants The dominant plants are grasses and forbs, which vary in quence, most grassland in North America and much of Eurasia has height from a few centimeters to 2 meters in tallgrass prairie. Some been converted to farmland. of the main adaptations of plants are to periodic, protracted droughts CONIFEROUS FOREST Distribution Extending in a broad band across northern North America and Eurasia to the edge of the arctic tundra, the northern coniferous forest, or taiga, is the largest terrestrial biome on Earth. Precipitation Precipitation generally ranges from 30 to 70 cm, and periodic droughts are common. However, some coastal coniferous forests of the United States Pacific Northwest are temperate rain forests that may receive over 300 cm of annual precipitation. Temperature Winters are usually cold and long; summers may be hot. Some areas of coniferous forest in Siberia range in temperature from — 70°C in the winter to over 30°C in summer,

Rocky Mountain National Park in Colorado 1102

UNIT EIGHT

Ecology

Plants Cone-bearing trees, such as pine, spruce, fir, and hemlock dominate coniferous forests. The conical shape of many conifers pre vents too much snow from accumulating and breaking thei branches. The diversity of plants in the shrub and herb layers o' coniferous forests is lower than in temperate broadleaf forests. Animals While many migratory birds nest in coniferous forests many species reside there year-round. The mammals of coniferou: forests, which include moose, brown bears, and Siberian tigers, are di verse. Periodic outbreaks of insects that feed on the dominant tree? can kill vast tracts of trees. Human Impact Although they have not been heavily settled by human populations, coniferous forests are being logged at an alarming rate, and the old-growth stands of these trees ma; soon disappear.

TEMPERATE BROADLEAF FOREST

C reat Smoky Mountains National Park in North Carolina

Distribution Found mainly at midlatitudes in the Northern Hemis }here, with smaller areas in New Zealand and Australia. I recipitation Precipitation can average from about 70 to over 200 cm annually. Significant amounts fall during all seasons, including summer rain and winter snow. Temperature Winter temperatures average around 0°C. Summers, with maximum temperatures near 30°C, are hot and humid. I Ian is A mature temperate broadleaf forest has distinct, highly c iverse, vertical layers, including a closed canopy, one or two strata cf understory trees, a shrub layer, and an herbaceous stratum.

1 Hstribution Tundra covers expansive areas of the Arctic, amount i ng to 20% of Earths land surface. High winds and cold temperatares create similar plant communities, called alpine tundra, on very 1 igh mountaintops at all latitudes, including the tropics. ] "recipitation Precipitation averages from 20 to 60 cm annually in i rctic tundra but may exceed 100 cm in alpine tundra. '•temperature Winters are long and cold, with sverages in some areas below -30°C. Summers ; re short with cool temperatures, generally ; veraging less than ] 0°C. Plants The vegetation of tundra is mostly herbai eous, consisting of a mixture of lichens, mosses, grasses, and forbs, along with some dwarf shrubs

There are few epiphytes. The dominant plants in the Northern Hemisphere are deciduous trees, which drop their leaves before winter, when low temperatures would reduce photosynthesis and make water uptake from frozen soil difficult. In Australia, evergreen eucalyptus dominate these forests. Animals In the Northern Hemisphere, many mammals hibernate in winter, while many bird species migrate to warmer climates. The mammals, birds, and insects make use of all vertical layers of the forest. Human Impact Temperate broadleaf forest has been heavily settled on all continents. Logging and land clearing for agriculture and urban development destroyed virtually all the original deciduous forests in North America. However, owing to their capacity for recovery, these forests are returning over much of their former range.

and trees. A permanently frozen layer of soil called permafrost generally prevents water infiltration. Animals Large grazing musk ox are resident, while caribou and reindeer are migratory. Predators include bears, wolves, and foxes. Migratory birds use tundra intensely during the summer as nesting grounds. Human Impact Tundra is sparsely settled but has become the focus of significant mineral and oil extraction in recent years.

Denali National Park, Alaska, in autumn

CHAPTER so

An Introduction to Ecology and the Biosphere

1103

Throughout your exploration in this chapter of Earth's varied aquatic and terrestrial biomes, you have seen many examples of the considerable impact of the abiotic factors in an organism's environment. In die next chapter, we will look more closely at organisms themselves, examining how behavioral mechanisms and adaptations play key roles in the interactions between an organism and both the nonliving and living parts of its environment.

Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY

Ecology is the study of interactions between organisms and the environment • Ecology and Evolutionary Biology (p. 1081) Events that occur in ecological time affect life on the scale of evolutionary time. • Organisms and the Environment (p. 1081) Examples of questions that ecologists ask are, Who lives where? Why do they live there? and How many are there? Ecologists use observations and experiments to test explanations for the distribution and abundance of species and other ecological phenomena. The environment of any organism includes both abiotic and biotic components. • Subfields of Ecology (pp. 1082-1083) Ecology can be divided into several subfields of study, ranging from the ecology of organisms to the dynamics of ecosystems, landscapes, and the biosphere. Modem ecological studies cross boundaries between traditionally separaie areas. • Ecology and Environmental Issues (p. 1083) Ecology pro vides the scientific understanding underlying environmental issues. Many ecologists favor the precautionary principle of "Look before you leap." Activity Science, Technology, and Society: DDT

Interactions between organisms and the environment limit the distribution of species • Dispersal and Distribution (pp. 1084-1085) The dispersal of organisms results in broad patterns of geographic distribution. Natural range expansions and species transplants suggest hypotheses LO explain why species are found where they are. Transplanted species may disrupt the ecosystem at the new site. • Behavior and Habitat Selection (p. 1085) Some organisms do not occupy all of their potential range. Species distribution may be limited by habitat selection behavior. • Biotic Factors (pp. 1085-1086) Biotic factors that affect the distribution of organisms include interactions with otherspecies, such as predation and competition. 1104

UNIT EIGHT

Ecology

Concept Check 1. judging from the climograph in Figure 50.18, what mainly differentiates dry tundra and deserts'? 2. Identify the natural biome in which you live and summarize its abiotic and biotic characteristics. Do these reflect your actual surroundings? Explain. For suggested answers, see Appendix A.

• Abiotic Factors (pp. 1086-1087) Among important abiotic factors affecting species distribution are temperature, water, sunlight, wind, and rocks and soil. Activity Adaptations to Biotic and Abiotic Factors investigation How Do Abiotic Factors Affect Distribution of Organisms? • Climate (pp. 1087-1092) Global climate patterns are largely determined by the input of solar energy and Earth's rotation around the sun. Regional, local, and seasonal effects on climate are influenced by bodies of water, mountains, and the changing angle of the sun over the year. Fine-scale differences in abiotic factors determine microclimates.

Abiotic and biotic factors influence the structure and dynamics of aquatic biomes • Aquatic biomes account for the largest part of the biosphere in terms of area and are generally stratified with regard to light penetration, temperature, and community structure. Marine biomes have a higher salt concentration than freshwater biomes (pp. 1092-1098). Activity Aquatic Biomes

Climate largely determines the distribution and structure of terrestrial biomes • Climate and Terrestrial Biomes (p. 1098) Climographs show that temperature and precipitation are correlated with biomes, but because biomes overlap, other abiotic factors must play a role in biome location. • General Features of Terrestrial Biomes (pp. 1098-1103) Terrestrial biomes are often named for major physical or climatic factors and for their predominant vegetation. Stratification is an important feature of terrestrial biomes. Activity Terrestrial Biomes

TESTING YOUR KNOWLEDGE Evolution Connection Discuss how the concept of time applies to ecological situations and evolutionary changes. Do ecological time and evolutionary time ever correspond? If so, what are some examples?

Scientific Inquiry Hiking up a mountain, you notice a plant species that has one g-owth form at low elevations and a very different growth form at h:gh elevations. You wonder if these represent two genetically distinct populations of this species, each adapted to the prevailing conditions, or if this species has developmental flexibility and can a ,sume either growth form, depending on local conditions. What experiments could you design to test these two hypotheses?

Science, Technology, and Society In pet shops throughout North America, you can purchase a variety of nonnative fishes, birds, and reptiles. Describe some scenarios in which such pet trade could endanger native plants and animals. Should governments regulate the pet trade? Are there currently any restrictions on what species a pet shop can sell in your city? How would you balance such regulation against a person's individual rights?

Investigation How Do Abiotic Factors Affect Distribution of Organisms?

CHAPTER so

An Introduction to Ecology and the Biosphere

1105

Behavioral Ecology A Figure 51.1 A courting pair of East Asian red-crowned cranes (Grus japonicus).

51.1 Behavioral ecologists distinguish between proximate and ultimate causes of behavior 51.2 Many behaviors have a strong genetic component 5 1 3 Environment, interacting with an animal's genetic makeup, influences the development of behaviors 5 1 A Behavioral traits can evolve by natural selection . 1 , 5 Natural selection favors behaviors that increase survival and reproductive success 51.6 The concept of inclusive fitness can account for most altruistic social behavior

Studying Behavior

W

e humans have probably studied animal behavior lor as long as we have lived on Earth. As hunters-— and sometimes the hunted—-knowledge of animal behavior was essential to human survival. But other animals are also a source of fascination beyond the need for practical information. For instance, cranes are animals that have long captivated people's interest, perhaps because cranes are large and their behavior is easily observed (Figure 51.1). Male and female cranes engage in elaborate courtship rituals involving graceful dance-like movements and synchronized vocalizations. Observing these rituals, many people have viewed the birds as symbols of fidelity and devotion. Some of the most conspicuous crane behaviors are associated with their annual migrations. Each spring, thousands of cranes fly from wintering grounds in southern Eurasia, North Africa, and North America to northern nesting grounds. Various species of cranes fly 1106

hundreds or thousands of kilometers, stopping periodically to rest and feed. Because migrating cranes fly at high altitudes am call as they fly, some cultures have traditionally viewed them a messengers between Earth and the heavens. The modern scientific discipline of behavioral ecology ex tends such observations of animal behavior by studying how such behavior is controlled and how it develops, evolves, and contributes to survival and reproductive success. For example, a behavioral ecologist might ask how similarities or differences in courtship displays may be related to genetic similarities ox differences among crane species and how learning contributes to the development of courtship displays. Questions aboiit migration might include why migrating cranes call, what environmental cues trigger migration, or how migration contributes to the reproductive success of cranes. Behavioral ecology is essential to solving critically important problems ranging from the conservation of endangered species to the control of emerging infectious diseases. This chapter focuses on sucn questions and others in a quest to understand how behavior js related to genetics, environment, and evolution.

Concept

Behavioral ecologists distinguish between proximate and ultimate causes of behavior The questions that can be posed about any behavior can be divided generally into two classes: those that focus on the immediate stimulus and mechanism for the behavior and those that explore how the behavior contributes to survival and reproduction. First, however, let's consider an even more fundamental question: what the term behavior encompasses.

>Vhat Is Behavior? ihavioral traits are as much a part of an animal's phenotype as the length of its appendages or the color of its fur. Most of what we call behavior is the visible result of an animal's muscular activity, as when a predator chases its prey or a fish raises its fins as part of a territorial display (Figure 51.2). In some behaviors, muscular activity is involved but less obvious, as when a bird uses muscles to force air from its lungs and shape the sounds in its throat, producing a song. Some nonmuscular activities are also considered behaviors, as when an animal secretes a hormone that attracts members of the opposite sex. Furthermore, we can consider learning to be a behavioral process. For examtile, a juvenile bird may learn to reproduce a song that it hears an adult of its species singing, even though the muscular activity based on this memory may not occur until months later, when the young bird itself begins to sing the song. Thus, in addition to studying observable behaviors, mainly in the form of muscle-powered activities, behavioral ecologists also study the mechanisms underlying those behaviors, which may not involve muscles at all. Put more simply, we can think of behavior as everything an animal does and how it does it.

froximate and Ultimate Questions When we observe a certain behavior, we may ask both I wximate and ultimate questions. Proximate questions about behavior focus on the environmental stimuli, if any, that trigger a behavior, as well as the genetic, physiological, and anatomi(al mechanisms underlying a behavioral act. Proximate questjions are often referred to as "how" questions. For example, the red-crowned cranes in Figure 51.1, like many animals, lpreed in spring and early summer. A proximate question about the timing of breeding by this species might be, How does day length influence breeding by red-crowned cranes? A reasonable hypothesis for the proximate cause of this behavior ils that breeding is triggered by the effect of increased day

length on an animals production of and responses to particular hormones. Indeed, experiments with various animals demonstrate that lengthening daily exposure to light produces neural and hormonal changes that induce behavior associated with reproduction, such as singing and nest building in birds. In contrast to proximate questions, ultimate questions address the evolutionary significance of a behavior. Ultimate questions take such forms as, Why did natural selection favor this behavior and not a different one? Hypotheses addressing "why" questions propose that the behavior increases fitness in some particular way A reasonable hypothesis for why the redcrowned crane reproduces in spring and early summer is that breeding is most productive at that time of year. For instance, at that time, parent birds can find ample food for rapidly growing offspring, providing an advantage in reproductive success compared to birds that breed in other seasons. Although proximate causation is distinct from ultimate causation, the two concepts are nevertheless connected. Proximate mechanisms produce behaviors that have evolved because they reflect fitness in some particular way. For instance, increased day length itself has little adaptive significance for red-crowned cranes, but since it corresponds to seasonal conditions that increase reproductive success, such as the availability of food for feeding young birds, breeding when days are longer is a proximate mechanism that has evolved in cranes.

Ethology In the mid-20th century, a number of pioneering behavioral biologists developed the discipline of ethology, the scientific study of how animals behave, particularly in their natural environments. Ethologists such as Niko Tinbergen, of the Netherlands, and Karl von Frisch and Konrad Lorenz, of Austria, who shared a Nobel Prize in 1973, established the conceptual foundations on which modern behavioral ecology is based. In a 1963 paper, Tinbergen suggested four questions that must be answered to fully understand any behavior. His questions, which remain at the core of behavioral ecology today can be summarized as follows: 1. What is the mechanistic basis of the behavior, including chemical, anatomical, and physiological mechanisms? 2. How does development of the animal, from zygote to mature individual, influence the behavior? 3. What is the evolutionary history of the behavior? 4. How does the behavior contribute to survival and reproduction (fitness)?

i. Figure 51.2 A male African cichlid {Neolamprologus tetracephalus) with erect fins. Muscular contraction that raises the fins is a behavioral response to a threat to the fish's territory.

Tinbergen's list includes both proximate and ultimate questions. The first two, which concern mechanism and development of the behavior, are proximate questions, while the second two are ultimate, or evolutionary, questions. The complementary nature of proximate and ultimate perspectives can be demonstrated with behaviors frequently studied by the classical ethologists, such as fixed action patterns and imprinting. CHAPTER 51

Behavioral Ecology

1107

Fixed Action Patterns

BEHAVIOR: A male stickleback fish attacks other male sticklebacks that invade its nesting territory.

A type of behavior studied extensively by the ethologists is the fixed action pattern (PAP), a sequence of unlearned behavioral acts that is essentially unchangeable and, once initiated, is usually carried to completion. A FAP is triggered by an external sensory stimulus known as a sign stimulus. Tinbergen studied what has become a classic example of sign stimuli and FAPs in the male three-spined stickleback fish, which attacks other males that invade its nesting territory. The stimulus for the attack behavior is the red underside of the intruder; the stickleback will not attack an intruding fish lacking a red belly (note that female sticklebacks never have red bellies), but will readily attack unrealistic models as long as some red is present (Figure 51.3). In fact, Tinbergen was inspired to look into the matter by his casual observation that his fish responded aggressively when a red truck passed their tank. As a consequence of his research, which was first reported in 1937, Tinbergen discovered that the color red is a key component of the sign stimulus releasing aggression in male sticklebacks.

4 Figure 51.4 Proximate and ultimate perspectives on aggressive behavior by male sticklebacks.

V Figure 51.3 Sign stimuli in a classic fixed action pattern.

Figure 51.4 offers both proximate and ultimate explanation for this particular FAP among male stickleback fish.

PROXIMATE CAUSE: The red belly of the intruding male acts as a sign stimulus that releases aggression in a male stickleback. ULTIMATE CAUSE: By chasing away other male sticklebacks, a male decreases the chance that eggs laid in his nesting territory will be fertilized by another male.

Imprinting

(a) A male three-spined stickleback fish shows its red underside.

(b) The realistic model at the top, without a red underside, produces no aggressive response in a male three-spined stickleback fish. The other models, with red undersides, produce strong responses.

1108

UNIT EIGHT

Ecology

Another phenomenon studied by the classical ethologists i imprinting, a type of behavior that includes both learning and innate components and is generally irreversible. Imprinting is distinguished from other types of learning by having A sensitive period, a limited phase in an animal's development that is the only time when certain behaviors can be learned. An example of imprinting is young geese following their mother. In species that provide parental care, parent-offspring bonding is a critical part of the life cycle. During the period of bonding, the young imprint on their parent and learn the basic behaviors of their species, while the parent learns to recognize its offspring. Among gulls, for instance, the sensitive period for parental bonding on young lasts one to two days. I bonding does not occur, the parent will not initiate care of the infant, leading to certain death for the offspring and a decrease in reproductive success for the parent. But how do the young know on whom—or what—t print? How do young geese know that they should follow the mother goose? The tendency to respond is innate in the birds; the outside world provides the imprinting stimulus, something to which the response will be directed. Experiments with many species of waterfowl indicate that they have no innate recognition of "mother." They respond to and identify with the first object they encounter that has certain key characteristics. In classic experiments done in the 1930s, Konrad Lorenz showed that the most important imprinting stimulus in graylag geese is movement of an object away from the young. When incubator-hatched goslings spent their first few

hours with Lorenz rather than with a goose, they imprinted on him, and from then on, they steadfastly followed him and showed no recognition of their biological mother or other adults of their own species. Again, there are both proximate and ultimate explanations, as outlined in Figure 51.5. Cranes also imprint as hatchlings, creating both problems and opportunities in captive rearing programs designed to save endangered crane species. For instance, a group of 77 endangered whooping cranes hatched and raised by sandhill cranes imprinted on the sandhill foster parents; none of these whooping cranes ever formed a mating pair-bond with another Vs hooping crane. As a consequence, captive breeding programs now isolate young cranes and expose them to the sights and sounds of members of their own species. But imprinting can also be used to aid crane conservation (Figure 51.6). Young

BEHAVIOR: Young geese follow and imprint on their mother.

r

whooping cranes imprinted on humans in "crane suits" have been taught to follow these "parents" flying ultralight aircraft along new migration routes. And importantly, such cranes have formed mating pair-bonds with other whooping cranes. While research on imprinting and fixed action patterns is much less active than it once was, the early study of these behaviors helped to make the distinction between proximate and ultimate causes of behavior. These studies also helped establish a strong tradition of experimental approaches in behavioral ecology. Concept Check i 1. A ground squirrel that sees a predator may utter a brief, loud call. List four questions about this behavior, one for each of Tinbergen's four questions. State whether each is a proximate or an ultimate question. 2. If an egg rolls out of the nest, a mother graylag goose will retrieve it by nudging with her beak and head. If researchers remove the egg or substitute a ball during this process, the goose will not alter her response. What type of behavior is this? Suggest a proximate and an ultimate explanation. For suggested answers, see Appendix A.

PROXIMATE CAUSE: During an early, critical developmental stage, the young geese observe their mother moving away from them and calling. ULTIMATE CAUSE: On average, geese that follow and imprint on their mother receive more care and learn necessary skills, and thus have a greater chance of surviving than those that do not follow their mother.

Figure 51.5 Proximate and ultimate perspectives on imprinting in graylag geese.

i Figure 51.6 Imprinting for conservation. Conservation biologists have taken advantage of imprinting by young whooping cranes as a means to teach the birds a migration route. A pilot wearing a crane suit in an ultralight plane acts as a surrogate parent,

Concept

Many behaviors have a strong genetic component Extensive research shows that behavioral traits, like anatomical and physiological aspects of a phenotype, are the result of complex interactions between genetic and environmental factors. This conclusion contrasts sharply with the popular conception that behavior is due either to genes (nature) or to environment (nurture). In biology; nature versus nurture is not a debate. Rather, biologists study how both genes and the environment influence the development of phenotypes, including behavioral phenotypes. Though we first discuss genetic influences on behavior and defer the discussion of environmental influences, you should bear in mind that all behaviors are affected by both genes and environment. One approach to studying the influence of different factors on a particular behavior is to view the behavior in terms of the norm of reaction (see Figure 14.13). For instance, we might measure the behavioral phenotypes for a particular genotype that develop in a range of environments. In some cases, the behavior is variable, depending on environmental experience. In other cases, nearly all individuals in a population exhibit virtually the same behavior, despite internal and external CHAPTER si

Behavioral Ecology

1109

environmental differences during development and throughout life. Behavior that is developmentally fixed in this way is called innate behavior. Innate behaviors are under strong genetic Influence, as we will discuss in the following eXarnpleS.

Directed Movements Many animal movements, ranging from simple ones that take place within a few millimeters to complex movements that span hundreds or thousands of kilometers, are under substantial genetic influence. Because of the clear role of genes in the control of these animal movements, we may refer to them as directed movements.

Kinesis A kinesis is a simple change in activity or turning rate in response to a stimulus. For example, sow bugs (also called wood lice), terrestrial crustaceans that survive best in moist environments, exhibit a kinesis in response to variation in humidity (Figure 51.7a). The sow bugs become more active in dry areas and less active in humid areas. Though sow bugs do not move toward or away from specific conditions, their increased movement under dry conditions increases the chance that they will leave a dry area and encounter a moist area. And since they slow down in a moist area, they tend to stay there once they encounter it. Taxis In contrast to a kinesis, a taxis is a more or less automatic, oriented movement toward (a positive taxis) or away from (a • Figure 51.7 A kinesis and a taxis.

negative taxis) some stimulus. For example, many stream fish, such as trout, exhibit positive rheotaxis (from the Greek rheos, I current); they automatically swim or orient themselves in aik

upstream direction (toward the current). This taxis keeps the fish from being swept away and keeps them facing the direction from which food will come {Figure 51.7b). Migration It is easy to assume that a simple behavior, such as kinesis shown by sow bugs or positive rheotaxis by trout, is under strong genetic control. However, genetic influence can be substantial eveh. for more complex behaviors. For example, ornithologists have found that many features of migratory behavior in birds are genetically programmed (Figure 51.8). One of the most thoroughly studied migratory birds is th t blackcap (Sylvia atricapilla), a small warbler that ranges from the Cape Verde Islands off the coast of West Africa to northern Europe. The migratory behavior of blackcaps differs greatly among populations; for instance, while all blackcaps in the northern portion of the range migrate, usually at night, thosfe in the Cape Verde Islands do not migrate at all. During the normal season for migration, captive migratory blackcaps spend their nights hopping restlessly about their cages or rap idly flapping their wings while sitting on a perch. Peter Berthold and his colleagues at the Max Planck search Center for Ornithology in Radolfzell, Germany, studie the genetic basis of this behavior, known as "migratory restjlessness," in several populations of blackcaps. In one study, the research team crossed (mated) migratory blackcaps from southern Germany with nonmigratory blackcaps from Cape Verde and subjected their offspring to environments simulating one location or the other. Approximately 40% of the offspring raised in both conditions showed migratory restlessness,

Dry open

(a) Kinesis increases the chance that a sow bug will encounter and stay in a moist environment.

& Figure 51.8 Bird migration, a behavior that is largely

(b) Positive rheotaxis keep from which most food

1110

UNIT EIGHT

Ecolog"

u n d e r g e n e t i c c o n t r o l . Each spring, migrating western sandpiper: {Cdlidris mauri), such as those shown here, migrate from their wintering grounds, which may be as far south as Peru, to their breeding grounds in Alaska. In the autumn, they return to the wintering grounds.

leading Berthold to conclude that migratbry restlessness is under genetic control and follows a polygenic inheritance pattern (see Chapter 14). Other breeding experiments in Berthold's laboratory have demonstrated genetic influences on many Components of blackcap migration, as we Mil see later in this chapter.

Animal Signals and Communication (b) Within seconds of the alarm substance being .uch of the social interaction between (a) Minnows are widely dispersed in an introduced, minnows aggregate near the aquarium before an alarm substance is inimals involves transmitting information bottom of the aquarium and reduce their introduced. movement. through specialized behaviors called sigls (also known as displays). In behavral ecology, a signal is a behavior that A Figure 51.9 Minnows responding to the presence of an alarm substance. (auses a change in another animal's behavior. The transmission of, reception of, and response to sigattract mates from several kilometers away. Once the moths are nals constitute animal communication, an essential element together, pheromones also trigger specific courtship behaviors. i if interactions between individuals. Though the environment The context of a chemical signal can be as important as the makes significant contributions to all communication syschemical itself. In a honeybee colony, pheromones produced ems, some of their features are under strong genetic control. by the queen and her daughters, the workers, maintain the Many signals are very efficient in energy costs. For inhive's very complex social order. When male honeybees tance, a common signal between territorial fish is to erect (drones) are outside the hive, where they can mate with a their fins, which gives them a larger profile and is generally queen, they are attracted to her pheromone; when drones are Enough to drive off an intruding individual (see Figure 51.2), inside the hive, they are unaffected by the queen's pheromone. t takes less energy to erect fins than it does to push an inPheromones also function in nonreproductive behavior. For ruding male from a territory. example, when a minnow or catfish is injured, an alarm substance that is stored in glands in the fish's skin disperses in the Animals communicate using visual, auditory, chemical (olwater, inducing a fright response among other fish in the area. actory), tactile, and electrical signals. The type of signal used These nearby fish become more vigilant and group together in o transmit information is closely related to an animal's tightly packed schools, often near the bottom, where they are ifestyle and environment. For example, most terrestrial mamsafer from attack (Figure 51.9). Pheromones can be very effective nals are nocturnal, which makes visual displays relatively inat low concentrations. For instance, just 1 cm of skin from a fatffective. But olfactory and auditory signals work as well in head minnow contains sufficient alarm substance to induce an he dark as in the light, and most mammalian species signal alarm reaction even when diluted by 58,000 L of water. iy these means. Birds, by contrast, are mostly diurnal and lave a relatively poor olfactory sense; they communicate prinarily by visual and auditory signals. Unlike most mammals, Auditory Communication .umans are diurnal and, like birds, use mainly visual and audiThe songs of most bird species are at least partly learned. By ory communication. Therefore, we can detect the songs and contrast, in many species of insects, mating rituals include aright colors that birds use to communicate with each other. If characteristic songs that are generally under direct genetic lumans had the well-developed olfactory abilities of most control. In Drosophila species, males produce a song by vi-nammals and could detect the rich world of chemical cues, we brating their wings, and females are able to recognize the would have a very different perspective of nature. songs of males of their species through such details as the intervals between pulses of wing vibrations, the rhythm of the Chemical Communication song, and the length of song pulses. A variety of evidence indicates that song structure in Drosophila males is controlled Many animals that communicate through odors emit chemical genetically and is under strong selective pressure. For insubstances called pheromones. In most cases, both the prostance, males reared in isolation produce a characteristic song duction of pheromones and animal responses to them are confor their species despite having no exposure to other singing trolled genetically. Pheromones are especially common among males. In addition, the male song shows very little variation mammals and insects and often relate to reproductive behavamong individuals within a Drosophila species. ior. Many moths, for example, emit pheromones that can

t

CHAPTER 51

Behavioral Ecolog'

1111

While Drosophila species differ morphologically as well as in their courtship songs, some other insect species can be identified only through their courtship songs or behavior. For example, morphologically identical green lacewings found throughout central to northern Eurasia and North America were once thought to belong to a single species, Chrysoperla carnea. However, studies of their courtship songs by Charles Henry and his colleagues and students at the University of Connecticut revealed the presence of at least 15 different species, each singing a different courtship song (Figure 51.10). Several of these morphologically identical species may occur together in the same habitat (an example of behavioral isolation of species; see Figure 24.4). Figure 51.10

tqyiry Are the different songs of closely related green lacewing species under genetic control? larles Henry, Lucia Martinez, and Kent Holsinger crossed males and females of Chrysoperla plorabunda and Chrysoperla johnsoni, t w o morphologically identical species of lacewings that sing different courtship songs. SONOGRAMS Chrysoperla plorabunda parent -Volley period

V Standard repeating unit

Vibration volleys crossed with

Chrysoperla johnsoni parent Volley period

Standard repeating unit The researchers recorded and compared the songs of the male and female parents with those of the hybrid offspring that had been raised in isolation from other lacewings. The FT hybrid offspring sing a song in which the length of the standard repeating unit is similar to that sung by the Chrysoperla plorabunda parent, but the volley period, that is, the interval between vibration volleys, is more similar to that of the Chrysoperla johnsoni parent. F, hybrids, typical phenotype -Volley period

W Standard repeating unit The results of this experiment indicate that the songs sung by Chrysoperla plorabunda and Chrysoperla johnsoni are under genetic control.

1112

UNIT

EIGHT

Ecology

Though they never observed hybrids of the different green lacewing species in the wild during more than 20 years o fieldwork, Henry and his colleagues have been able to produo hybrids between different species under laboratory conditions While all individuals in each species sing the same song laboratory-raised hybrid offspring sing songs that contain ele ments of the songs of both parental species. These data led the researchers to conclude that the various songs sung by the dif ferent green lacewing species are genetically controlled.

Genetic Influences on Mating and Parental Behavior Behavioral research has uncovered a variety of mammalian be haviors that are under relatively strong genetic control, as wel as the physiological mechanisms for these behaviors. One o the most striking lines of research concerns mating and parental behavior by male prairie voles (Microtus ochrogaster) Prairie voles and a few other vole species are monogamous a social trait found among only about 3% of mammalian species. Male prairie voles also help their mates care for young another relatively uncommon trait among male mammals. Ir contrast with most other vole species, a male prairie vole forms a strong pair-bond with a single female after they mate, associating closely with his mate and engaging in grooming and huddling behaviors (Figure 51.11). Further, though an unmated male prairie vole shows little aggression toward other prairie voles, whether male or female, a mated male becomes intensel) aggressive toward any strange male or female prairie voles while remaining nonaggressive toward his mate, A few days following the birth of pups, a male prairie vole will spend a great deal of time hovering over them, licking them, and carrying them around, while remaining vigilant for intruders. The genetic and physiological controls on the complex social and parental behaviors of prairie voles have been revealec over the past decade by the research of Thomas R. Insel and his colleagues at Emory University Early research suggestec

nm

i

•4 Figure 51.11 A pair of prairie voles {Microtus ochrogaster) huddling. North American prairie voles are monogamous, with males associating closely with their mates, as shown here, and contributing substantially to the care of young.

that arginine-vasopressin (AW), a nine-amino-acid neurotiansmitter released during mating, might mediate both pairmd formation and aggression by male prairie voles. In the mtral nervous system, AVP binds with a receptor called the Vjla receptor. The Emory researchers found significant differences between the distribution of V la receptors in the brains ojf monogamous prairie voles and their distribution in promiscuous montane voles (which live in the mountainous regions I western North America). To test whether the distribution of VIa receptors is a key facr controlling the mating and parental behavior of prairie voles, Iiisel and his colleagues inserted the prairie vole V la receptor gene into laboratory mice. Not only did the transgenic mice develop brains in which the distribution of V la receptors was re.arkably similar to the distribution found in prairie voles, but Ley also showed many of the same mating behaviors as monogmous male prairie voles. In contrast, male wild-type mice did ot have the same Vla receptor distribution and did not show :h behavior. Thus, though many genes influence the mating \ ehavior of these voles, it appears that a single gene may niedi.e a considerable amount of the complex mating behavior (and erhaps parental behavior) that occurs in prairie voles. It relains to be seen whether other complex behaviors are under milarly simple genetic influences. In addition, although genes ifluence behaviors in a multitude of ways, environment also has r lajor effects on behaviors, as we shall see in the next section.

\ Concept Check B 1. Which has greater influence on the development of behaviors, "nature" or "nurture"? Explain. 2. Use an example to explain how researchers study whether a particular behavior has a strong genetic component.

Dietary Influence on Mate Choice Behavior One example of environmental influence on behavior is the role of diet in mate selection by Dwsophila mojavensis, which mates and lays its eggs on the rotting tissues of cactus. D. mojavensis populations in Baja California, Mexico, breed almost entirely on agna cactus, whereas most populations in Sonora, Mexico, and in Arizona use organ pipe cactus. These cacti serve as food for the developing larvae. When D. mojavensis from Baja and Sonora were raised on an artificial banana-based medium in the laboratory, researchers noticed that females from the Sonoran population tended to avoid mating with Baja males. Following up on these initial observations, William Etges and Mitchell Ahrens, of the University of Arkansas, showed that the food eaten by D. mojavensis larvae strongly influences later mate selection by females, especially those from Sonoran populations (Figure 51.12).

Figure 51.12

How does dietary environment affect mate choice by female Drosophila mojavensisl William Etges raised a D. mojavensis population from Baja California and a D. mojavensis population from Sonora on three different culture media: artificial medium, agria cactus (the Baja host plant), and organ pipe cactus (the Sonoran host plant). From each culture medium, Etges collected 15 male and female Baja D. mojavensis pairs and 15 Sonoran pairs and observed the numbers of matings between males and females from the two populations. When D. mojavensis had been raised on artificial medium, females from the Sonoran population showed a strong preference for Sonoran males (a). When D. mojavensis had been raised on cactus medium, the Sonoran females mated with Baja and Sonoran males in approximately equal frequency (b).

For suggested answers, see Appendix A.

Concept

"Environment, interacting with in animal's genetic makeup, nfluences the development rf behaviors Artificial

Vhile experimental demonstrations of genetic influences on ehaviors accumulate, research also reveals that environmental onditions modify many of the same behaviors. Environmental actors, such as the quality of the diet, the nature of social ineractions, and opportunities for learning can influence the levelopment of behaviors in every group of animals.

Organ pipe cactus

Agria cactus

Ciiiiyre medium The difference in mate selection shown by females that developed on different diets indicates that mate choice by females of Sonoran populations of D. mojavensis is strongly influenced by the dietary environment in which larvae develop.

Behavioral Ecology

1113

< Figure 51.13 Therese Markow (right) and a colleague collecting Drosophila mojavensis from a cactus.

But why do Sonoran females avoid Baja males that develop on some diets but not those that develop on other diets? In other words, what is the proximate cause of the behavior? Therese Markow, of Arizona State University, and Eric Toolson, of the University of New Mexico, proposed that the physiological basis for the observed mate preferences was differences in hydrocarbons in the exoskeletons of the flies (Figure 51.13). This was a reasonable hypothesis, since in addition to using courtship songs and other sensory information, Drosophila use their sense of taste to assess the hydrocarbons in the exoskeleton of possible mates. To test the influence of exoskeleton hydrocarbons, Etges and Ahrens raised Baja males on artificial medium and then perfumed them with hydrocarbons extracted from the exoskeletons of males from the Sonoran population. Instead of rejecting these Baja males, Sonoran females accepted them as frequently as they accepted Sonoran males. Etges and Ahrens's study showed how the effects of the nutritional environment can modify a critical behavior. Next we will see how social environment has been found to modify the behavior of at least one species.

Social Environment and Aggressive Behavior The discoveries of genetic influence on mating behaviors in male prairie voles are complemented by parallel studies of another monogamous rodent, the California mouse (Peromyscus californicus). Like prairie

voles, male California mice are highly aggressive toward other mice and provide extensive parental care. In contrast to prairie voles, however, even male California mice that have not mated are already aggressive. Janet Bester-Meredith and Catherine Marler, of the University of Wisconsin at Madison, studied what influence the social environment might have on the behavior 1114

UNIT EIGHT

Ecology

of male California mice. To manipulate the early social environ ment, they placed newborn California mice in the nests o white-footed mice (Peromyscus leucopus), a species in whicr males are not monogamous and engage in little parental care They also placed newborn white-footed mice in the nests of Cal ifornia mice. This "cross-fostering," in which the young of eacl species were placed in the nests of the other species, altered tb behavior of both species (Table 51.1). For instance, when mal( California mice raised by white-footed mice crawled away front the nest, they were not retrieved as frequently as those raised by their own species. When these cross-fostered California mice became parents, they, too, spent less time retrieving their pups. Th(| cross-fostered California mice were also less aggressive toward intruders. In contrast, male white-footed mice raised by Califor} nia mice were more aggressive than those raised by white-footed mice. In one study by Bester-Meredith and Marler, the brains o California mice raised by white-footed mice were found to con tain reduced levels of the same neurotransmitter (AVP) associatec with elevated aggression and parental care in male prairie voles. The fact that cross-fostered California and white-footec mice adopted some of the behaviors of their foster parent; suggests that experience during development can lead tc changes in parental and aggressive behaviors in these rodent: that can be passed from one generation to the next.

Learning One of the most powerful ways that environmental condition can influence behavior is through learning, the modificatior of behavior based on specific experiences. Learned behavior: range from the very simple, such as imprinting, which result: in a young bird following a particular individual it has learnec to recognize as its parent, to the highly complex. Before wt consider some complex forms of learning, let's look briefly a another very simple one: habituation.

Table 51.1 Influence of Cross-Fostering on Male Mice* Aggression Toward an Intruder

Aggression in Neutral Situation

Paternal Behavior

ArginineVasopressin (AVP) Content in Brain

California mice fostered by whitefooted mice

Reduced

No differena

Reduced

Reduced

White-footed mice fostered by California mice

No difference

Increased

No difference

No difference

Species

* Comparisons are wuh mice raised ny yja rents of tlie Data from J. K. fester-Meredith and C. A. Marler, Va= :. Behavioral Neurt

iiission of paternal faehavi s 117(2003):455-463.

I labituation

Figure 51.14

Habituation is a loss of responsiveness to stimuli that convey little or no information. "Examples are widespread. A hydra Contracts when disturbed by a slight touch; if it is repeatedly disturbed by such a stimulus without any further consequences, it stops responding. Many mammals and birds recognize alarm calls of members of their species, but they eventually stop responding if these calls are not followed by n actual attack (the "cry-wolf effect)- In terms oi ultimate ausation, habituation may increase fitness by allowing an nimal's nervous system to focus on stimuli that signal the presence of food, mates, or real danger instead of wasting me or energy on a vast number of other stimuli that are irelevant to the animal's survival and reproduction. Next, let's shift our attention to some more complex forms of learning.

Does a digger wasp use landmarks to find her nest? : emale digger wasp excavates and cares for four or five separate underground nests, flying to each nest daily with food for the single larva in the nest. To test his hypothesis that the wasp uses visual landmarks to locate the nests, Niko Tinbergen marked one nest with a ring of pinecones.

initial Learning I ivery natural environment shows some degree of spatial vantion. For instance, sites suitable for nesting may be more ( ommon in some places than in others. As a consequence, the iitness of an organism may be enhanced by the capacity for patial learning, which is the modification of behavior based >n experience with the spatial structure of the environment, ncluding the locations ot nest sites, hazards, food, and prospective mates. In a classic experiment done in 1932, Niko Tinbergen tudied how digger wasps find their nest entrances. Tinbergen noved a circle of pinecones that had previously surrounded a lest entrance and observed that the wasp landed in the center • if the pinecones, even though the nest entrance was no longer there (Figure 51.14). The wasp was using the pinecones as a andmark, or location indicator. The use of landmarks is a nore complex cognitive mechanism than a taxis or kinesis, ;ince it involves learning. The wasp flies toward a stimulus (in his case, the center of the pinecones), as in a taxis, but the •timulus is an arbitrary landmark the animal must learn rather han a constant stimulus such as light. One nest entrance may lave pinecones around it, while another may be next to a pile :>f stones. Each wasp has to learn the unique landmarks ol :ach individual nest site. Tinbergen's experiment reveals that for spatial learning to a reliable way to navigate through the environment, the andmarks used must be stable (within the time frame of a ^articular activity). For example, the pinecones that indi;ate the presence of the wasp's nest should have a low probability of moving. Using unreliable information for learning may result in a significant cost to the animal. If a nesting A'asp learns to locate its nest using objects that might blow away, for instance, the cost may be an inability to locate and provision the nest and, consequently, reduced reproductive ;uccess for the wasp.

After the mother visited the nest and flew away, Tinbergen moved the pinecones a few feet to one side of the nest. When the wasp returned, she flew to the center of the pinecone circle instead of to the nearby nest. Repeating the experiment with many wasps, Tinbergen obtained the same results.

diggei

The experiment supported the hypothesis that ndmarks to keep track of their nests.

Because some environments are more stable than others, animals may use different kinds of information for spatial learning in different environments. Lucy Odling-Smee and Victoria Braithwaite, of the University of Edinburgh, hypothesized that three-spined stickleback fish from stable pond environments would rely more on landmarks than would sticklebacks from more changeable river environments. Odling-Smee and Braithwaite trained 20 sticklebacks from rivers and 20 from ponds to navigate a T-shaped maze to reach a reward (a combination of food and other fish the sticklebacks could school with). In the first phase of the experiment, the researchers placed the reward at one end of the maze and marked the correct direction to turn for the reward with landmarks in the form of two plastic plants. Behavioral Ecology

1115

Fish that learned to find the reward were given a second set of trials., in which Odling-Smee and Braithwaite put rewards at both ends of the maze and switched the location of the landmarks. The results showed that the sticklebacks from stable pond environments located the rewards using a combination of learning which direction to turn and landmarks, while most river fish learned simply to turn in a particular direction. These results suggest that the degree of environmental variability influences the spatial learning strategies of animals. Cognitive Maps An animal can move around its environment in a flexible and efficient manner using landmark orientation alone. Honeybees, for instance, might learn ten or so landmarks and locate their hive and flowers in relation to those landmarks. A more powerful mechanism is a cognitive map, an internal representation, or code, of the spatial relationships between objects in an animal's surroundings. It is very difficult to distinguish experimentally between an animal that is using landmarks and one that is using a true cognitive map. Researchers have collected evidence that specifically supports the use of cognitive maps by corvids, a family of birds that includes ravens, crows, jays, and nutcrackers. Many corvids store food in caches, from which the birds can retrieve the food later. For instance, pinyon jays {Gymnorhinus cyanocephalus) and Clark's nutcrackers (Nucifraga coiivnbiana) store nuts in as many as thousands of caches that may be widely dispersed. The birds not only relocate each cache, but also keep track of food quality, bypassing caches in which the food was relatively perishable and would have decayed since it was stored. Research by Alan Kami], of the University of Nebraska, suggests that pinyon jays and Clark's nutcrackers use cognitive maps to memorize the locations of their food caches. In one experiment with Clark's nutcrackers, Kamil varied the distance between landmarks and discovered that the birds could learn to find the halfway point between them. Such behavior suggests that the birds are capable of using a general abstract geometric rule for locating a seed cache. In Kamil's experiment, the rule was something like, "Seed caches are found halfway between particular landmarks." Such rules form the basis of cognitive maps, which benefit the organism by reducing the amount of detail that must be remembered to relocate an object. Associative Learning As we have seen, in learning, an animal modifies its behavior based on information from the environment. For instance, an inexperienced white-footed mouse may readily attack a brightly colored, slow-moving caterpillar, such as the larva of a monarch butterfly, only to find its mouth full of a distasteful poisonous fluid. Following this experience, the white-footed mouse may avoid attacking insects with similar coloration and behavior The ability of many animals to associate one feature 1116

UNIT

EIGHT

Ecology

of the environment (a stimulus, such as color) with another (bad taste) is called associative learning. Associative learning and its genetic and neurological bases have been extensively studied in the fruit fly Drosophila melanogasLer. William Quinn, William Harris, and Seymoui Benzer, of the California Institute of Technology, published the first demonstration of associative learning by Drosophila over 30 years ago. The research team trained Dwsophila flies to avoid air carrying a particular scent by coupling exposure to the odor with an electric shock. This is an example oi a type of associative learning called classical conditioning, ir which an arbitrary stimulus, in this case an odor, is associated with a reward or punishment, the electric shock. Drosophilq individuals trained in this way would avoid the particular! odor for as long as 24 hours. Further research over the past! three decades has revealed a surprising capacity for learning by Dro^nphilo., which has become a model for the study oi thi: component of behavior. The laboratory study of associative learning has been extended to natural environments. The role that associative learning may play in helping animals avoid predators has been particularly well studied in fishes and aquatic insects.i Recall our earlier discussion about the alarm substance in the skins of minnows and related fishes that induces an automatic fright response in nearby fish. Nichole Korpi and Briar Wisenden, of Minnesota State University at Moorhead, conducted a study to determine whether this alarm substance might be involved in a process of associative learning through which fish leam to avoid predators. Korpi and Wisenden studied the zebrafish (Danio rerio), a common aquarium fish. For the predator they chose a pike. Zebrafish, a minnow from southern Asia, and pike, a fish of northern lakes, do not occur together in nature. Would the zebrafish leam to associate the scent of this unfamiliar predatorl with their alarm substance if there were a delay between expo -I sure to the alarm substance and exposure to the odor of the predator? Korpi and Wisenden proposed that a delay of 5 minutes would realistically model a possible delay between the occurrence of an alarm and encountering an actual predator. The researchers exposed zebrafish in an experimental group to an influx of 20 mL of water containing an alarm substance and then, 5 minutes later, to 20 mL of water with pike odor Zebrafish in a control group were exposed to an influx of water alone (without alarm substance) and then to water containing pike odor. On day 1 of the experiment, the zebrafish in the control group did not change their activity in response to the influx of either water alone or pike odor (Figure 51.15). This lack of response by the control group is critical to the interpretation of the results, because it indicates that the zebrafish had no innate negative reaction to pike odor. As expected, the zebrafish in the experimental group reduced their activity markedly in response to the alarm substance. When pike odor was added 5 minutes later, their activity actually increased

Before stimulus

I Influx of alarm substance

Influx of water alone

| Influx of pike odor

Day1

Day 3

A Figure 51.16 Operant conditioning. Having received a face full of quills, a young coyote has probably learned to avoid porcupines. Control group

Experiments group

Control group

Experimental group

k Figure 51.15 Associative learning in zebrafish. Changes in chctivity indicated that the experimental group of zebrafish had learned TO associate the odor of pike with the presence of alarm substance.

somewhat, a difference from the typical alarm response. Three days later, the zebrafish in the control group still showed no ref.ponse to the influx of either water alone or pike odor (see "-igure 51.15). The experimental fish also showed no significant response to an influx of water alone. However, they did : educe their activity somewhat in response to pike odor. Korpi and Wisenden concluded that the zebrafish in the experimeni al group had learned to associate pike odor with the alarm substance, even though their exposure to pike odor on the first • lay of the study was delayed for 5 minutes. In terms of classical conditioning, the zebrafish had been conditioned to associate pike odor with the alarm substance. The experimental esults show that this association was retained for three days.

The study of animal cognition, called cognitive ethology, examines die connection between an animals nervous system and its behavior. One area of research in cognitive ethology investigates how an animal's brain represents objects in the environment. For example, researchers have discovered that many animals, including insects, are capable of categorizing objects according to concepts such as "same" and :i different."' Martin Giurfa, of the Centre of Animal Cognition Research in Toulouse. France, and his team trained honeybees to match a sample color to the same color marking one of two arms in a Y-maze. Later, the trained bees were able to match black-and-white patterns in the same orientation on the maze, indicating that they recognized the "sameness" of the problem.

Cognition and Problem Solving

Watching an animal solve a problem makes us aware that its nervous system has a substantial ability to process information. For example, if a chimpanzee is placed in a room with a banana hung high out of reach and several boxes on the floor, the chimp can "size up1' the situation and stack the boxes, enabling it to reach the food. Such novel problemsolving behavior is highly developed in some mammals, especially primates and dolphins, and notable examples have also been observed in some bird species, especially crows, ravens, and jays. Behavioral ecologist Bernd Heinrich, of the University of Vermont, performed an experiment in which ravens had to obtain food hanging from a string. Several ravens solved the problem by using one foot to pull up the string in increments and the other foot to secure the string so the food did not drop down again; other ravens showed great individual variation in their problem-solving attempts.

The study of cognition connects behavior with nervous system unction. The term cognition is variously defined. In a narrow ^ense, it is synonymous with consciousness, or awareness. In a 3road sense—the way we use the term in this book—-cognition Ls the ability of an animals nervous system to perceive, store, process, and use information gathered by sensory receptors.

One powerful source of information used by many animals to solve problems is the behavior of other individuals. Long-term research by Tetsuro Matsuzawa, of the Primate Research Institute of Kyoto University, Japan, has revealed that chimpanzees leam to solve problems by copying the behavior of other chimpanzees. Matsuzawa and his research team have shown that

Another type of associative learning is operant conditioning, . ilso called trial-and-error learning. Here, an animal learns to associate one of its own behaviors with a reward or punishment ind then tends to repeat or avoid that behavior (Figure 51.16). \s in the example of a mouse eating a distasteful caterpillar, or instance, predators quickly learn to associate certain kinds >f potential prey with painful experiences and modify their behavior accordingly.

CHAPTER si

Ben;-\ loral Ecolog1

1117

There are important exceptions to the song-learning scenario seen in white-crowned sparrows. Canaries, for example do not have a single sensitive period for song learning. $ young canary begins with a subsong, but the full song it devel ops is not crystallized in the same way as it is in white-crownec sparrows. Between breeding seasons, the song becomes flexi ble again, and an adult male may learn new song "syllables each year, adding to the song it already sings. Each year, a nev plastic song stage allows the learning of new syllables.

A Figure 51.17 Young chimpanzees learning to crack oil palm nuts by observing older chimpanzees. young wild chimpanzees at Bossou, Guinea, leam how to use two stones as a hammer and anvil to crack oil palm nuts by observing and copying experienced chimpanzees (Figure 51.17).

1. How can cognitive maps increase an animal's capacity to learn spatial relationships? 2. Describe three ways in which an animal's environment can influence the development of its behavior. 3. How might associative learning explain why unrelated distasteful or stinging insects have similar colors? For suggested answers, see Appendix A.

Genetic and Environmental Interaction in Learning Considerable research on the development of bird songs has revealed varying degrees of genetic and environmental influence on the learning ol complex behavior. In some species, learning appears to play only a small part in song development. For instance, New World flycatchers that are reared away from adults of their species will develop the song characteristic of their own species without ever having heard it: In other words, their songs are innate. In contrast, among the songbirds—a group that includes the sparrows, robins, and canaries—learning plays a key role in song development. Techniques for learning songs vary from species to species. Some have a sensitive period for developing their songs. For example, if a white-crowned sparrow is isolated for the first 50 days of life and unable to hear either real sparrows or recordings of sparrow songs, it fails to develop the adult song of its species. Although the young bird does not sing during the sensitive period, it memorizes the song of its species by listening to other white-crowned sparrows sing- During the sensitive period, fledglings seem to be stimulated more by the songs of their own species than by songs of other species, chirping more in response. Thus, young white-crowned sparrows learn the songs they will sing as adults, but the learning appears to be bounded by genetically controlled preferences. The sensitive period when a white-crowned sparrow memorizes its species' song is followed by a second learning phase, when the juvenile bird sings some tentative notes that researchers call a subsong. The juvenile bird hears its own singing and compares it with the song that it memorized during the sensitive period. Once a sparrows own song matches the one it memorized, it "crystallizes" as the final song, and the bird sings only this adult song for the rest of its life. 1118

UNIT

EIGHT

Ecology

Behavioral traits can evolve by natural selection Because of the influence of genes on behavior, natural selectior can result in the evolution of behavioral traits in populations One ol the primary sources ot evidence for this evolution is behavioral variation between and within species.

Behavioral Variation in Natural Populations Behavioral differences between closely related species are common. As you have already seen, the males of different species o. Drosophila sing different courtship songs. Another example i< found in voles: In some species, both males and females care foi young, while in other species, the young receive only materna care. Though often less obvious, significant differences ir behavior can also be found within animal species. When behavioral variation within a species corresponds to variation in environmental conditions, it may be evidence of past evolution. Variation in Prey Selection One of the best-known examples of genetically based variation m behavior within a species is prey selection by the gartei snake Thamnophis elegans (Figure 51.18a). Stevan Arnold, of the University of Chicago, discovered that the natural diet oi this species differs widely across its range in California. Coastal populations feed on salamanders, frogs, and toads but predominantly on slugs. Inland populations feed on frogs.

a) A garter snake (Thamnophis elegans)

\ Figure 51.18 Predator and potential prey.

(b) A banana slug (Ariolimus californicus); not to scale

eeches, and fish, but not on slugs. In fact, slugs are rare or ab;ent in the inland habitats of T. elegans. The contrast in prey availability led Arnold to compare he responses of coastal and inland populations of T. elegans A'hen offered bits of banana slug (Figure 51.18b), which is ound throughout the coastal habitats of T. elegans but is abent from the inland habitats Arnold studied. In a first test, \rnold offered banana slugs to T. elegans from both wild populations. Whereas most of the coastal snakes readily ate the lugs, the inland snakes tended to refuse them. Because the contrast in feeding behavior may have been conditioned by experience in their natural habitat, Arnold ested the responses of young snakes born in the laboratory, -[e found that 73% of the young snakes from coastal mothers ittacked the slugs they were offered, while only 35% of the 'oung snakes from inland mothers attacked the slugs. Arnold proposed that when inland snakes colonized coastal labitats more than 10,000 years ago, a fraction of the populaion had the ability to recognize slugs by chemoreception. because these snakes took advantage of the abundant food iource, they had higher fitness than snakes in the population hat ignored slugs; thus, the capacity to recognize slugs as prey ncreased in frequency in the population over hundreds or housands of generations. The variation in behavior today beween the two populations is evidence oi this evolution.

meets a riparian (riverside) woodland. The obvious contrast in vegetation is only one of many differences, which extend to the behavioral ecology of species inhabiting riparian zones and the surrounding arid environments. Agelcnopsis aperta is a species of funnel web spider that lives in both riparian zones and the surrounding arid environments in the western United States. The web of A. aperta consists of a silken sheet ending in a funnel hidden in some sheltering feature of the habitat. While foraging (behavior associated with recognizing, searching for, capturing, and consuming food), the spider sits in the mouth of the funnel. When prey strikes the web, the spider runs out to make its capture. Susan RiecherL, of the University of Tennessee at Knoxville, and several colleagues found a striking contrast in the behavior of A. aperta spiders inhabiting riparian forests and those in arid and semi-arid habitats in Arizona and New Mexico. In arid, food-poor habitats, A. aperta is more aggressive toward potential prey and toward other spiders, and it returns to foraging more quickly following disturbance. Ann Hedrick and Riechert tested whether the higher aggressiveness of arid-habitat A. aperta spiders reflects a genetic difference between the two populations or a learned response to living in food-poor environments (Figure 51.19). The researchers compared each spider's time to attack, the time that elapsed between a prey's first contact with the web and the point at which the spider first touched it. They found that in their natural environments, the spiders from desert grassland attacked all 15 prey types more quickly than did the riparian spiders. Hedrick and Riechert then brought female spiders from the two habitats into the laboratory, where the spiders laid eggs. Again using attack time as a measure, the researchers compared the aggressiveness of laboratory-raised Desert grassland population Riparian population

Field

Lab-raised generation 1

Variation in Aggressive Behavior

Lab-raised " generation 2

Population

A Figure 51.19 Aggressiveness of funnel web spiders (Agelenopsis aperta) living in two environments. Fieldcollected and lab-raised funnel web spiders from desert grasslands delay less before attacking potential prey compared to A. aperta from riparian environments.

CHAPTER si

Behavioral Ecology

1119

spiders from desert grassland and riparian habitats and repeated the experiment with the laboratory-raised offspring of each population. The experiments revealed a strikingly consistent difference between desert grassland and riparian A. aperLa in their time to attack prey. The highly productive riparian sites are rich in prey for the spiders, but the density of potential predators, particularly birds, is high. Riechert had proposed that, this higher risk of predation was the cause of the more timid behavior of A. aperta in riparian habitats. Based on their results, Hedrick and Riechert concluded that the difference in aggressiveness between desert grassland and riparian A. aperta spiders is genetically based and is the product of natural selection lor differences in foraging and territorial behavior—a genetically determined rather than a learned response. Their findings were reinforced by later experiments with A. aperta transplanted from one environment to the other.

Experimental Evidence for Behavioral Evolution Looking for more direct ways to demonstrate the evolution of behavior, researchers are increasingly turning to laboratory and field experiments using organisms with short lifespans, enabling the observation of change over many generations. Laboratory Studies of Drosophila Foraging Behavior In a few cases, researchers have been able to link behaviors to specific genes. For instance, Maria Sokolowski, of the University of Toronto, studied a polymorphism in a gene for foraging m Drosophila melanogaster called [or. One allefe, jof, results in a behavioral phenotype called "sitter," in which the fly larva moves less than average. A different allele, jorR, results in a "rover" behavioral phenotype, in which the fly larva moves about more than average. Sokolowski found that the allele frequencies in a natural population of Drosophila melanogaster are 70% forR and 30%/or". She noted, however, that while the mechanistic basis for the differences in rover and sitter behaviors are well known, little is known about the evolutionary and ecological significance of different frequencies of these alleles in natural populations. Sokolowski teamed with Sofia Pereira and Kimberly Hughes, of York University Ontario, to conduct a laboratory study of how population density might affect the frequency of the/or* and/or' alleles in Drosophila populations. Sokolowski and her colleagues raised Drosophila at both high and low densities, starting with equal frequencies of/or^ and for5 in both lineages. After 74 generations, larvae from the low-density and high-density lineages showed a clear divergence in behavior, as measured by differences in average foraging path length (Figure 51.20). The researchers concluded that the/or5 allele had increased in frequency in the low-density populations, in which short-distance foraging would yield sufficient 1120

UNIT

EIGHT

Ecology

HI

H2

H3

H4

H5 •

A Figure 51.20 Evolution of foraging behavior by laboratory populations of Drosophila melanogaster. After 74 generations of living at low population density, D, meldnogdster followed a foraging path significantly shorter than that of D. mebnogaster populations that had lived at high density.

food, while long-distance foraging would result in unnecessary energy expenditure. Meanwhile, the jorR allele had increased in frequency in the high-density lineage, where long-distance foraging could enable larvae to move beyond areas of food depletion. In other words, they had observed an evolutionary change in behavior in their laboratory populations. Migratory Patterns in Blackcaps An example of evolutionary change in the behavior of a wild population comes from studies on migratory behavior in a population of blackcaps, which we introduced in our earlier discussion of migration. The migratory patterns of blackcaps are well known in western Europe because of a great interest in bird study and a long history of bird banding. Blackcaps that breed in northwestern Europe generally migrate to the western Mediterranean region for the winter. As a consequence, blackcaps in Germany have historically migrated in a southwest direction, while those in Britain have migrated south. In the 1950s, a few blackcaps began to spend their winters in Britain. and over time, the population of blackcaps wintering in Britain! has grown to many thousands. Surprisingly, these wintering. birds are not birds that bred in Britain and then stayed for the winter. Leg bands on some of the birds show that they had come from continental Europe and migrated westward tc Britain instead of south to the Mediterranean. Peter Berthold and his colleagues captured 20 male anc 20 female blackcaps wintering in Britain and transported them back to their laboratory in southwestern Germany. They compared the migratory orientation of these birds and their F-, offspring to that of young birds originally from southwestern Germany Behavioral ecologists have long known that the migratory restlessness that Berthold had studied previously is not

random activity but rather is directed. That is, caged birds showmigratory restlessness orient their activity in the direction .hat they would migrate if they were not caged. In the autumn when the birds were showing migratory restlessness, Bertholds :eam placed the blackcaps from their three study groups in large

f Figure 51.21 Evidence of a genetic basis for migratory mentation.

a) Blackcaps placed in a funnel cage left marks indicating the direction in which they were trying to migrate.

glass-covered funnel cages lined with carbon-coated paper for 1.5-2 hours. As the birds moved around the funnels, the marks they made on the paper indicated the direction in which they were trying to "migrate" (Figure 51.21a). The results showed that the migratory orientation of wintering adult birds captured in Britain was very similar to the orientation of their laboratory-raised offspring (Figure 51.21b). That similarity contrasted sharply with the migratory orientation of the young birds originally from Germany This study indicates a genetic basis for migratory orientation, since the young of the British blackcaps and the young birds from Germany were raised under similar conditions but showed very different migratory orientations. But has the behavior actually evolved over time? Berthold's study also indicates that the change in migratory behavior in western European blackcaps is both recent and rapid, having taken place over the past 50 years. Before I960, there were no known westward-migrating blackcaps in Germany, but by the 1990s, westward migrants made up 7-11% of the blackcap populations of Germany and parts of Austria. Berthold suggests that once westward migration began, it persisted and increased in frequency as a result of several factors, including milder winter climate in Britain and improved winter food partly due to the widespread use of winter bird feeders in Britain. These hypotheses touch on the subject of the next section, which concerns how behavior can influence survival and reproduction and thus lead to evolution.

Concept Check . 1. Explain why geographic variation in garter snake foraging behavior might demonstrate that the behavior evolved by natural selection. 2. Why did Hedrick and Riechert examine behavioral variation in laboratory-raised spiders rather than wild ones? 3. What conclusions did Berthold and his colleagues draw from their studies of blackcap migrator)' patterns?

J2*»

For suggested answers, see Appendix A.

? Mediterranean _ ^ •v/

S e a

1\ V\

d y \

Concept

k•

\

^/

LH b) Wintering blackcaps captured in Britain and their laboratory-raised offspring had a migratory orientation toward the west, while young birds from Germany were oriented toward the southwest.

1

Natural selection favors behaviors that increase survival and reproductive success The genetic components of behavior, like all aspects of phenotype, evolve through natural selection for traits that enhance CHAPTER

51

Behavioral Ecology

1121

survival and reproductive success in a population. Two of the most direct ways a behavior can affect fitness are through its influences on foraging and mate choice behavior.

125 %

1

o

100

Foraging Behavior Because adequate nutrition is essential to an animals survival and reproductive success, we should expect natural selection to refine behaviors that enhance the efficiency of feeding. Food-obtaining behavior, or foraging, includes not only eating, but also any mechanisms an animal uses to recognize, search for, and capture food items. Optimal foraging theory views foraging behavior as a compromise between the benefits of nutrition and the costs of obtaining food, such as the energy expenditure or the risk of being eaten by a predator while foraging. According to this theory, natural selection should favor foraging behavior that minimizes the costs of foraging and maximizes the benefits. Some behavioral ecologists are applying cost-benefit analysts to study the proximate and ultimate causes of diverse foraging strategies. Energy Costs and Benefits Reto Zach, of the University of British Columbia, conducted a cost-benefit analysis of feeding behavior in crows of the Pacific Northwest. Northwestern crows (Corvus caurinus), like other crow species, are opportunistic feeders that eat a variety of food items. On Mandarte Island, off British Columbia, crows search the rocky tide pools for gastropod molluscs called whelks. A bird grasps a whelk in its beak, then flies upward and drops it onto the rocks to break the shell. If the drop is successful, the crow can dine on the mollusc's soft parts. If the shell doesn't break, the crow flies up and drops the whelk again and again until the shell breaks. The higher the bird flies before dropping a whelk, the fewer drops are required to break the shell. But there is an energy cost correlated with the height of the crows ascent. Zach predicted that crows would, on average, fly to a height that would provide the most food relative to the amount of total energy required to break the whelk shells. To determine the optimal height, Zach erected a 15-m pole and then dropped shells of relatively uniform size onto rocks from different heights along the pole. He recorded the number of drops required to break a shell from each height and multiplied it by the drop height to arrive at the total flight height. He then calculated the average total height required to break shells as the average number of drops from a particular height times the drop height. Zach's experiments indicated that a drop height of about 5 m is optimal, breaking the shells with the least total flight height—in other words, with the least amount of work. When Zach measured the actual average flight height for crows in their whelk-eating behavior, it was 5.23 m, very close to the prediction based on an optimal trade-off between energy gained (food) versus energy expended (Figure 51.22). 1122

UNIT

EIGHT

Ecology

g -B Average number of drops

75

Total flight height

*

-50

1 1

_•_

-25

3

5

7

15

Height of drop (m)

A Figure 51.22 Energy costs and benefits in foraging behavior. Experimental results indicate that dropping shells from a height of 5 m results in breakage with the least amount of work. The actual drop height preferred by crows corresponds almost exactly to the height that minimizes total flight height.

The feeding behavior of the bluegill sunfish provides further support for the optimal foraging theory. These animals feed on small crustaceans called Daphnia, generally selecting larger individuals, which supply the most energy per unit time. However, bluegill sunflsh select smaller prey if larger prey are too far away The optimal foraging theory predicts that the proportion of small prey to large prey eaten will also vary with the overall density of prey. When prey density is high, it is efficient to concentrate on larger crustaceans, whereas at very low prey densities, bluegill sunfish should exhibit little size selectivity, because all available prey are needed to meet energy requirements. In actual experiments, young bluegill sunfish did become more selective at higher prey densities, though not to the extent that would theoretically maximize their efficiency (Figure 51.23). Young bluegill sunfish forage fairly efficiently but not as close Lo the optimum as older individuals. Tt may be that younger fish judge size and distance less accurately because their vision is not yet completely developed; learning may also improve the foraging efficiency of bluegill sunfish as they age. Though crows and bluegill sunfish provide good examples of how energy costs and benefits affect an animals choice of food items, the following example shows that energy is not the only cost of foraging behavior.

Risk ofPredation One of the most significant potential costs to a forager is risk of predation. Clearly, an animal that feeds in a way that maximizes energy benefits and minimizes energy costs without regard to its

behavior reflects variation in predation risk more than variation in food availability. This result, like the results ol the bluegill sunfish study, underscores the fact that behavior often reflects a compromise among competing selective pressures.

Large prey at 3 far distance ~~~~~^

Mating Behavior and Mate Choice

High prey density

Low prey density

Mating behavior, which includes seeking or attracting mates, choosing among potential mates, and competing for mates, is the product of a form of natural selection called sexual selection (see Chapter 23). How mating behavior enhances reproductive success varies, depending on the species' mating system.

Percentage available 33% Medium prey . Large prey

33% Predicted percentage in diet I 2%

Mating Systems and Parental Care

Obser1' eo porcer.iage n jiei k > Figure 51.23 Feeding by bluegill sunfish. In feeding on Daphnia (water fleas), the fish c o not feed randomly but select prey based on both size and distance, tending to pursue prey that I )ok5 largest. Small prey (low energy yield) at the middle distance may be ignored. But small prey i. t close distances may be taken with a relatively small energy expenditure. In the experiments described here, when prey was at low density, young bluegill sunfish foraged nonselectively, s predicted by optimal foraging theory.

isk of becoming a meal for some predator has not optimized its araging behavior. Researchers have investigated the influence of sedation risk on foraging behavior for a number of species, inluding the mule deer (pdocoileus hemionus). A major predator f mule deer throughout their range in the mountainous egions of western North America is the mountain lion (Puma oncolor). A team of researchers from several institutions tudied populations of mule deer in Idaho to determine if the nule deer forage in a way that reduces their risk of falling prey 0 mountain lions. The team investigated how food availabilty and risk of predation varied across the study landscape, vhich consisted of both patches of forest and open, nonorested areas. The researchers found that the food available or mule deer was fairly uniform across the potential foraging reas, though somewhat lower in open areas. In contrast, risk f predation differed greatly; mountain lions killed most mule eer at forest edges and only a small number in open areas nd forest interiors (Figure 51.24). How does mule deer feeding behavior respond to the dramatic differences in predation risk in different foraging areas? The researchers found that the mule deer in their study area eed predominantly in open areas, avoiding both forest edges nd forest interiors (see Figure 51.24). In addition, when deer re at Forest edges, they spend significantly more time scanning leir surroundings than when they are in either open areas or orest interiors. The evidence indicates that mule deer foraging

The mating relationship between males and females varies a great deal from species to species. In many species, mating is promiscuous, with no strong pairbonds or lasting relationships. In species in which the mates remain together for a longer period, the relationship may be monogamous (one male mating with one female) or polygamous (an individual of one sex mating with several of the other). Polygamous relationships most often involve a single male and many females,

—^™ Predation risk

• Relative deer use 70-

g 601 .5040c

:°

1 Q_

3020100-

I

l•

15

s

>

I/I Open

:-S...

\

/

20

-1.0 \

•• 1Im

Forest edge Habitat

Forest interior

1 J

|

-5

-O.

A Figure 51.24 Risk of predation and use of foraging areas by mule deer. Optimal foraging theory predicts that prey will forage in a way that minimizes the risk of predation. The risk of predation by mountain lions is lowest in open areas and forest interiors and highest at forest edges. Mule deer feed preferentially in open areas, avoiding both forest edges and interiors. CHAPTER 51

Behavioral Ecoiogy

1123

a system called polygyny, though some species exhibit polyandry, in which a single female mates with several males. Among monogamous species, males and females are often so much alike morphologically that they may be difficult or impossible to distinguish based on external characteristics (Figure 51.25a). Potygynous species are generally dimorphic, with males being more showy and often larger than females (Figure 51.25b). Polyandrous species are also dimorphic, but in this case, females are generally ornamented and larger than males (Figure 51.25c) The needs of the young are an important factor constraining the evolution of mating systems. Most newly hatched birds, for instance, cannot care for themselves and require a large, continuous food supply that a single parent may not be able to provide. In such cases, a male may ultimately leave more viable offspring by helping a single mate than by going off to seek more mates. This may explain why most birds are monogamous. In contrast, birds with young that can feed and care for themselves almost immediately after hatching, such as pheasants and quail, have less need for their parents to stay together. Males of such species can maximize their reproductive success by seeking other mates, and polygyny is relatively common in such birds. In the case of mammals, the lactating female is often the only food source for the young; males usually play no role in raising the young. In mammalian species where males protect the females and young, such as lions, a male or small group of males typically takes care of many females at once in a harem. Another factor influencing mating behavior and parental care is certainty of paternity Young born or eggs laid definitely contain the mother's genes. But even within a normally monogamous relationship, these young may have been fathered by a male other than the females usual mate. The certainty of paternity is relatively low in most species with internal fertilization because the acts of mating and birth (or mating and egg laying) are separated over time. This could explain why exclusively male parental care occurs in very few species of birds and mammals. However, the males of many species with internal fertilization engage in behaviors that appear to increase their certainty of paternity, including guarding females, removing any sperm from the female reproductive tract before copulation, and introducing large quantities oi fpevm to displace [he .sperm of other mates. Certainty of paternity is much higher when egg laying and mating occur together, as in external fertilization. This may explain why parental care in aquatic invertebrates, fishes, and amphibians, when it occurs at all, is at least as likely to be by males as by females (Figure 51.26). Male parental care occurs in only 7% of fish and amphibian families with internal fertilization, but in 69% of families with external fertilization. In fishes, even when parental care is provided exclusively by males, the mating system is often polygynous, with several females laying eggs in a nest tended by one male. 1124

UNIT EIGHT

Ecology

(a) Since monogamous species, such as these trumpeter swans, are often monomorphic, males and females are difficult to distinguish using external characteristics only.

(b) Among polygynous species, such as elk, the male (left) is often highly ornamented.

(c) In polyandrous species, such as these Wilson's phalaropes, females (top) are generally more ornamented than males.

& Figure 51.25 Relationship between mating system and male and female forms.

I Figure 51.26 Paternal care by a male jawfish. Male wfish, which live in tropical marine environments, hold the eggs they hjave fertilized in their mouths, keeping them aerated and protecting lem from egg predators until the young hatch.

A Figure 51.27 Zebra finches, native to Australia. The male zebra finch (left) is more strikingly patterned and more colorful than the female zebra finch.

It is important to point out that when behavioral ecologists se the expression certainty ojpaternity, they clo not mean that nimals are aware of those factors when they behave a certain r ay. Parental behavior correlated with certainty of paternity xists because it has been reinforced over generations by natral selection. The relationship between certainty of paternity nd male parental care remains an area of active research, enIvened by controversy.

8 days old, approximately 2 days before they opened their eyes. (The artificial ornaments represent novel anatomical features that might appear naturally in populations as a consequence of genetic mutations.) A control group of zebra finches were raised by unadorned parents. When the chicks from the experiments matured, Witte and Sawka tested their mate preferences by giving them a choice between prospective mates that were either ornamented with a red feather or nonornamented. Males showed no preference for either ornamented or nonornamented females. Females raised by nonornamented parents or by parents in which only the female was ornamented also showed no preference for either ornamented or nonornamented males (Figure 51.28, on the next page). However, females raised by parents that were both ornamented or by a pair in which the male was ornamented preferred ornamented males as their own mates. These experiments suggest that the females imprint on their fathers rather than on their mothers and that mate choice by female zebra finches has played a key role in the evolution of ornamentation in male zebra finches. As another example of how female choice affects the evolution of males, consider the courtship behavior of stalk-eyed flies. The eyes of these insects are at the tips of stalks, which are longer in males than in females (Figure 51.29, on the next page). During courtship, a male presents himself to a female, front end on. Researchers have documented that females are more likely to mate with those males that have relatively long eyestalks; thus, female choice has been a strong selection factor in the evolution of long eyestalks in males. But why would the females favor this seemingly arbitrary trait? Behavioral ecologists have correlated certain genetic disorders in the male flies with an inability lo develop long eyestalks. Such studies support the hypothesis that females are basing their mate choices on characteristics that correlate with male quality. As

Sexual Selection and Mate Choice \s you read in Chapter 23, the degree of sexual dimorphism vithin a species results from sexual selection, a form of natual selection in which differences in reproductive success mong individuals are a consequence of differences in mating uccess. Recall from that chapter that sexual selection can take ae form of intersexud selection, in which members of one sex hoose mates on the basis of particular characteristics of the ther sex, such as courtship songs, or intrasexual selection, A'hich involves competition among members of one sex for nates. Lets look next at some experimental evidence for sex,al selection. Mate Choice by Females. Mate preferences by females may lay a central role in the evolution of male behavior and natomy through intersexual selection. Klaudia Witte and ^adia Sawka, of the University of Bielefeld, Germany, used experiments to test whether imprinting by young zebra finches n their parents may influence their choice of mates when ley mature. Male zebra finches are more ornate than females, lough neither have crests on their heads (Figure 51.27). A'7itte and Sawka taped a 2.5-cm-long red feather to the foreead feathers of both male and female parents, to the male arent only, or to the female parent only when the chicks were

Behavioral Ecology

1125

Experimental Groups

Control Group

we discussed in Chapter 23, in genera. ornaments such as long eye stalks in thess flies or brightly colored feathers in rnal

birds correlate with the males' health ant vitality. A lemale that chooses a health male increases the probability of produ< ing healthy offspring. Male Competition for Mates. The pie vious examples show how female choic can select for one best type of male in given situation, resulting in low variatio1 among males. Male competition fo mates is a source of intrasexual selectioi that also can reduce variation amon males. Such competition may involve Females reared by Females reared by agonistic behavior, an often ritualizec ornamented mothers or ornamented parents nonornamenied parents cr ornamented fathers contest that determines which competi showed no preference preferred ornamented tor gains access to a resource, such a; for either ornamented or males K nates food or mates (Figure 51.30). The out' come of such contests may be determined by strength, size, or the form o horns, teeth, and so forth, but the victories may be psychological rather thar physical. Despite the potential for such competition to select for reduced variation, male behavioral and morphologica variation is extremely high in some vertebrate species ranging from fish to deei Males reared by si- experiments! groi preference for either ornamented or and in a wide variety of invertebrates. Ir female mates. some species, more than one mating behavior can result in successful reproA Figure 51.28 Sexual selection influenced by imprinting. Experiments demonstrated duction; in these cases, intrasexua" that female zebra finch chicks that had imprinted on artificially ornamented fathers preferred selection has led to the evolution of alornamented males as adults. ternative male mating behavior anc morphology. One invertebrate in which alternative male mating behaviors and morphology have been well documented is the marine intertidal isopod Paracerceis sculpta, which lives within sponges in the Gulf of California. This species includes three genetically distinct male types (Figure 51.31). Stephen M. Shuster, of the University of New Mexico, discovered that large a males defend harems of females within intertidal1 sponges, mainly against other a males. Meanwhile, (3 males mimic female morphology and behavior and do not elicit a defensive response in a males; thus, they are able to gain access to guarded harems. Tiny 7 males invade and live within large harems, which can be inhabited by all three types of males at the same time. Shuster reported that the mating success of each type of Paracerceis male depends on the relative densities of males and • Figure 51.29 females within sponges. The a males father the majority of Male stalk-eyed flies. young when defending a single female. If more than one 1126

UNIT

EIGHT

Ecology

male is present, 3 males sire approximately 60% of the spring. Finally, the reproductive rate of 7 males increases li learly with harem size. Using this information and the patrns of distribution of Paracerceis females and males in their natural environment, Shuster and Michael Wade, of the University of Chicago, concluded that a, p, and 7 males have

approximately equal mating success, indicating that a diversity of mating behavior and anatomy can result from intrasexual selection due to male competition for mates, Variation among males in this species is sustained, since none of the three male types is most successful in all circumstances; each type of male has high mating success at some female densities but reduced mating success at other female densities (an example of balanced polymorphism, which we discussed in Chapter 23).

Applying Game Theory

A Figure 51.30 Agonistic behavior. These two male polar bears (Ursus mdritimus) are engaging in "play-fighting," ritualized contests that usually do not injure the bears. By contrast, during the spring breeding season, male polar bears fight fiercely as they compete for access to females in estrus. Large Paracerceis a males defend harems of females within intertidal sponges

The three types of Paracerceis males discovered by Shuster can coexist because they have equal mating success, which likely gives them equal evolutionary fitness. However, in some other species, the fitness of different types of males appears to be unequal. How can alternative reproductive strategies exist in such a situation? Why doesn't natural selection favor one strategy to the exclusion of the others? In studying this question, behavioral ecologists use a range of tools, including game theory Originally developed by John Nash and other mathematicians to model human economic behavior, game theory evaluates alternative strategies in situations where the outcome depends not only on each individual's strategy but also on the strategies of other individuals. Currently applied to a wide variety of problems in behavioral ecology, game theory is a way of thinking about evolution in situations where the fitness of a particular behavioral phenotype is influenced by other behavioral phenotypes in the population. Barry Sinervo and Curt Lively, of Indiana University, used game theory to account for the coexistence of three different male phenotypes in populations of the side-blotched lizard (Uta slansburiana) in the inner Coast Ranges of California (Figure 51.32). Sinervo and Lively identified males with three different types of genetically controlled coloration: orange throats, blue throats, and yellow throats. Orange-throat males are the most aggressive and defend large territories that contain many females. Blue-throat males are also territorial but defend smaller territories and fewer females. Yellow-throats

Tiny y males are able to invade and live within large harems.

p males mimic female morphology and behavior and do not elicit a defensive response in a males and so are able to gain access to guarded harems. A Figure 51.31 Male polymorphism in the marine intertidal isopod Paracerceis sculpta. Different morphology affects the mating behavior of three genetically different types.

A Figure 51.32 Male polymorphism in the side-blotched lizard (Uta stansburiana). An orange-throat male, left; a bluethroat male, center; a yellow-throat male, right.

CHAPTER 51

Behavioral "Ecology

1127

are nonterritorial males thai mimic females and use "sneaky" tactics to obLain matings. Sinervo and Lively found thaL the three types go through cycles of relative abundance; over a period of several years, their study population went from high frequency of blue-throats to high frequency of orange-throats to high frequency of yellow-throats and finally back to high frequency of blue-throats. Sinervo and Lively connected their study population to game theory when they realized that the relative mating success of each male type is not fixed but changes with the relative abundance of the other male types in the populations. Sinervo and Lively suggested that the cycling of male abundance in side-blotched lizards is like the children's game of paperrock-scissors, in which paper defeats rock, rock defeats scissors, and scissors defeats paper. When blue-throats are abundant, they can successfully defend the few females in their territories from the advances of the sneaky yellow-throat males; in terms of the game, blue-throats "defeat" yellowthroats. However, blue-throat males cannot defend their territories against the hyperaggressive orange-throat males. As a consequence, orange-throat males take over blue-throat territories and increase in abundance; orange-throats "defeat" blue throats. But the numerical dominance by orange-throat males is temporary, since they cannot defend all the females in their large territories from the sneaky yellow-throat males. So eventually orange-throat males, the most aggressive males, are replaced as the most numerous by yellow-throat males, the least aggressive males; yellow-throats "defeat" orange-throats. And then the cycle begins again; Since the blue-throat tactics of guarding small territories and few females can restrict yellowthroat access to females, blue-throat males soon replace yellow-throats as the most abundant males. Game theory gives behavioral ecologists a way to think about complex evolutionary problems in which relative performance, not absolute performance, is the key to understanding the evolution of behavior. This makes game theory an important tool, since the relative performance of one phenotype compared to others is how evolutionary7 biologists assess Darwinian fitness.

Concept Check 1. Why is male parental care more likely to evolve among species with external fertilization than among species with internal fertilization? 2. How does optimal foraging theory explain why mule deer spend more time foraging in open areas than near or in forests? 3. How is a female bird's fitness associated with her ability to choose a mate by discerning among displays and adornments that "advertise" the health of the male? For suggested answers, see Appendix A.

1128

UNIT

EIGHT

Ecolo£\

The concept of inclusive fitness can account for most altruistic social behavior Many social behaviors are selfish, meaning that they benefi the individual at the expense of others, especially competitors Even in species in which individuals do not engage in agonistic behavior, most adaptations that benefit one individual will indirectly harm others. For example, superior foraging ability by one individual may leave less food for others. It is easy to understand the pervasive nature of selfishness if natural selection shapes behavior. Behavior that maximizes an individual':; survival and reproductive success is favored by selection, regardless of how much damage such behavior does to anothe' individual, a local population, or even an entire species. How, then, can we explain observed examples of what appears to b "unselfish" behavior?

Altruism On occasion, some animals do behave in ways that reductf their individual fitness but increase the fitness of other individuals in the population; this is our functional definition of altruism, or selflessness. Consider the Belding's ground squir-j rel, which lives in some mountainous regions of the westerr United States and is vulnerable to predators such as coyotes and hawks. If a squirrel sees a predator approach, the squirre often gives a high-pitched alarm call, which alerts unawart individuals, who then retreat to their burrows. Field observa-l tions have confirmed that the conspicuous alarm behavior in-i creases the risk of being killed, because it brings attention tc the caller's location. Another example of altruistic behavior occurs in bee societies, in which the workers are sterile. The workers themselves never reproduce, but they labor on behalf of a single fertile queen. Furthermore, the workers sting intruders, a behavior that helps defend the hive but results in the death of those workers. Still another example of altruism is seen in naked mole rats (Heterocephalus glaber), highly social rodents that live in underground chambers and tunnels in southern and northeast-, ern Africa (Figure 51.33). The naked mole rat, which is almost hairless and nearly blind, lives in colonies of 75 to 250 or more individuals. Each colony has only one reproducing female, the queen, who mates with one to three males, called kings. The rest of the colony consists of nonreproductive females and males who forage for underground roots and tubers and care for the queen, the kings, and new offspring still dependent on the queen. The nonreproductive individuals may sacrifice their own lives in trying to protect the queen or kings from snakes or other predators that invade the colony.

i. Figure 51.33 Naked mole rats, a species of colonial mammal that exhibits altruistic behavior. Pictured here is a cueen nursing offspring while surrounded by other members of the colony.

L

iclusive Fitness

How can a naked mole rat, a worker bee, or a Belding's ground squirrel enhance its fitness by aiding other members of the population, which may be its closest competitors? How can altruistic behavior be maintained by evolution if it does not enhance—and in fact may even reduce—the survival and reproductive success of the self-sacrificing individuals? it is easiest to see how such behavior might be selected for when It applies to parents sacrificing for offspring. When parents sacrifice their own personal well-being to produce and aid offspring, this actually increases the fitness of the parents, because it maximizes their genetic representation in the population. But individuals sometimes help others who are not their offspring. Like parents and offspring, siblings have half their genes in common. Therefore, selection might also favor helpng siblings or helping one's parents produce more siblings. Evolutionary biologist William Hamilton was the first to realze that selection could result in an animal's increasing its genetic representation in the next generation by ''altruistically" lelping close relatives other than its own offspring. This realzation led to the concept of inclusive fitness, the total effect an individual has on proliferating its genes by producing its own offspring and by providing aid that enables other close relatives, who share many of those genes, to produce offspring.

produces. Suppose, for example, that members of a human population average two children each. Now consider two brothers who are close in age. reproductively mature, equally fertile, but not yet fathers. One of these young men is close to drowning in heavy surf, and his brother risks his own life to swim out and pull his sibling to safety. The benefit to the almostdrowned brother, the recipient of this altruistic act, is two offspring. Had he drowned, his reproductive output would have been zero; but now, if we use the average, he can father two children: B = 2.0. The cost to the heroic brother depends on the risk to his own life in saving his sibling. Let's say that in this kind of surf, an average swimmer has a 5% chance of drowning. Thus, the cost of the altruistic act is 5% of the number of offspring we would expect if the altruist had not taken the risky plunge. The cost is 0.05 X 2 = 0.1. We now know that B = 2.0 and C — 0.1 for this hypothetical act of altruism, but what about r? The coefficient of relatedness equals the probability that if two individuals share a common parent or ancestor, a particular gene present in one individual will also be present in the second individual. With two siblings, such as our imaginary brothers at the beach, there is a 50% chance that any gene in one brother will also be present in the other brother. Thus, for siblings, r = 0.5. One way to see this is in terms of the segregation of homologous chromosomes that occurs during meiosis of gametes (Figure 51.34; also see Chapter 13). We can now use values of B, C, and r to evaluate whether natural selection would favor the altruistic act in our imaginary scenario. Natural selection favors altruism w-hen the

Hamilton's Rule and Kin Selection Hamilton proposed a quantitative measure for predicting when natural selection would favor altruistic acts among related individuals. The three key variables in an act of altruism are the benefit to the recipient (B), the cost to the altruist (C), and the coefficient of relatedness (r). The benefit and cost measure the change in the average number of offspring produced by the recipient and altruist, respectively, resulting from the altruistic act. Thus, B, the benefit, is the average number of extra offspring that the beneficiary of an altruistic act produces; and C, the cost, is how many fewer offspring the altruist

A Figure 51.34 The coefficient of relatedness between siblings. The red band indicates a particular gene on a chromosome of a homologous chromosome pair in one parent. Sibling 1 has inherited the gene from parent A. There is a probability of V; that sibling 2 will also inherit this gene from parent A. The coefficient of relatedness between the t w o siblings is thus V?, or 0.5.

CHAPTER 51

Behavioral Ecology

1129

benefit to the recipient multiplied by the coefficient of relatedness exceeds the cost to the altruist—in other words, when rB > C, This inequality is called Hamilton's rule. For our swimming brothers, rB = 0.5 X 2 = 1 and C = 0.1. This satisfies Hamilton's rule; thus, natural selection would favor this altruistic act of one brother saving another. Any particular gene in the altruist will, on average, be passed on to more offspring if that brother risks the rescue than if he does not. (And among those genes may be some that actually contribute to altruistic behavior, so these genes, too, are propagated.) The natural selection that favors this kind of altruistic behavior by enhancing reproductive success of relatives is called kin selection. Kin selection weakens with hereditary distance. Siblings have an r of 0.5, but between an aunt and her niece, r = 0.25 QA), and between first cousins, r — 0.125 (Vs). Notice that as the degree of relatedness decreases, the rB term in the Hamilton inequality also decreases. Would natural selection favor our strong swimmer rescuing his cousin? For this altruistic act, rB = 0.125 X 2 = 0.25, which, luckily for the drowning cousin, is still much greater than C " 0.1, the cost to the altruist. Of course, the degree of risk the altruist takes comes into play, too. II the potential rescuer is a poor swimmer, he may have a 50% chance of drowning instead of the 5% chance for a strong swimmer. In this case, the cost to the altruist would be 0.5 X 2 = 1. That's greater than the rB of 0.25 we calculated for the drowning cousin, who'd better hope a lifeguard is near. The British geneticist J. B. S. Haldane anticipated the concepts of inclusive fitness and kin selection by jokingly saying that he would lay down his life lor two brothers or eight cousins. In todays terms, we would say that he would do this because either two brothers or eight cousins would result in as much representation of Haldane's genes as would two of his own offspring. If kin selection explains altruism, then the examples of unselfish behavior we observe among diverse animal species should involve close relatives. This is in fact the case, but often in complex ways. Like most mammals, female Beldings ground squirrels settle close to their site of birth, whereas males settle at distant sites. Since nearly all alarm calls are given by females (Figure 51.35), they are most likely aiding close relatives. If all of a iemale's close relatives are dead, she rarely gives alarm calls. In the case of worker bees, who are all sterile, anything they do to help the entire hive benefits the only permanent member who is reproductively active—the queen, who is their mother. In the case of naked mole rats, DNA analyses have shown that all the individuals in a colony are closely related. Genetically, the queen appears to be a sibling, daughter, or mother of the kings, and the nonreproductive rats are the queen's direct descendants or her siblings. Hence, when a nonreproductive individual enhances a queens or king's chances of reproducing, it increases the chances that some genes identical to its own will be passed to the next generation. 1130

UNIT EIGHT

Ecology

A Figure 51.35 Kin selection and altruism in Belding's g r o u n d squirrels. This graph helps explain the male-female difference in altruistic behavior of ground squirrels. After they are weaned, females are more likely than males to live near close relations. Alarm calls that warn these relatives increase the inclusive fitness of the femaie altruist.

Reciprocal Altruism Some animals occasionally behave altruistically toward others who are not relatives. A baboon may help an unrelated companion in a fight, or a wolf may offer food to another wolf even though they share no kinship. Such behavior can be adaptive if the aided individual returns the favor in the future. This sort of exchange of aid, called reciprocal altruism, is commonly invoked to explain altruism between unrelated humans. Reciprocal altruism is rare in other animals; it is limited largely to species (such as chimpanzees) with social groups stable enough that individuals have many chances to exchange aid. It is generally thought to be most likely where individuals are likely to meet again and where there would be negative consequences associated with not returning favors to individuals who had been helpful in the past, a pattern of behavior that behavioral ecologists refer to as "cheating." It is likely that all behavior that seems altruistic actually has at least the potential to increase fitness in some way However, because cheating may provide a large benefit to the cheater, behavioral ecologists have questioned how reciprocal altruism could evolve. To find answers to this question, many behavioral ecologists have turned to game theory. In 1981, Robert Axelrod and William Hamilton, then at the University of Michigan, proposed that reciprocal altruism can evolve and persist in a population where individuals adopt a behavioral strategy they called til [or tat. In the tit-for-tat strategy, an individual treats another in the same way it was treated the last time they met. Individuals adopting this behavior are always altruistic, or cooperative, on the first encounter with another individual and will remain so as long as their altruism is reciprocated. When their cooperation is not reciprocated,

however individuals employing til for tat will retaliate immediately hut return to cooperative behavior as soon as the other individual becomes cooperative. The tit-for-tat strategy has been used to explain the few apparently reciprocal altruistic interactions observed in animals—ranging from blood sharing between nonrelated vampire bats to social grooming in primates.

Social Learning When we discussed learning earlier in this chapter, we focused largely on the genetic and environmental influences that lead animals LO acquire new behaviors. Learning can also have a Significant social component, as seen in social learning, which is learning through observing others (see Figure 51.17). Social learning forms the roots of culture, which can be defined as a system of information transfer through social learning or teaching that influences the behavior of individuals in a population. Cultural transfer of information has the potential to alter behavioral phenotypes and, in turn, to influence the fitness of individuals. Culturally based changes in the phenotype occur on a much shorter time scale than changes resulting from natural selection. Because we recognize social learning most easily in humans, we may take the process for granted or assume that social learning occurs only in humans. However, social learning can be seen among lineages of animals that diverged from ours very long ago. some of which we describe next. Mate Choice Copying We have seen how female mate choice can lead to mtersexual selection for elaborate male ornamentation (see Figure 51.28). In many species, mate choice is strongly influenced by social learning. Mate choice copying, a behavior in which individuals in a population copy the mate choice of others, has been extensively studied in the guppy Poecilia retiadata. Female wild guppies generally prefer to mate with males showing a high percentage of orange coloration. However, they are also known to copy the mate choices of other females. In other words, female guppies appear to mate with males that have been successful in attracting other females. With this background in mind, Lee Dugatkin, of the University of Louisville, designed an ingenious experiment to compare the influences of male phenotype and social learning on mate choice by female guppies. He gave female guppies a choice of mating with males with varying degrees of orange coloration. In the control samples, a female chose between males with no other females present. In the experimental treatments, males with the same range of orange coloration as the control males were present, but the experimental female also observed a model of a female engaging in courtship with a male with relatively little orange. In the control samples, females overwhelmingly chose males with higher percentages of orange

Controi Sample

Female qiopif-S :xe!e'" males with more orange coloration.

Experimentai Sample

ale guppies prefer less igfe mates that are associated i another female.

A Figure 51.36 Mate choice copying by female guppies (Poecilia reticulata). Female guppies generally choose the males with more orange coloration. However, when males were matched for orange or differed in amount of orange by 1 2 % or 2 4 % , experimental females chose the less orange male that had been presented with a model female. Females ignored the apparent choice of the model female only where the alternative male had 4 0 % more orange coloration.

coloration (Figure 51.36). However, experimental females in most cases chose the less orange male that had been presented in association with a model female, choosing a male not associated with a model female only when he had a much higher percentage of orange coloration. Dugatkin concluded that below a certain threshold of difference in male color, mate choice copying by female guppies can mask genetically controlled female preference for orange males. A female that mates with males that are attractive to other females may increase the probability that her male offspring will also be attractive and have high reproductive success. Mate choice copying, one form of social learning, has also been observed in several other fish and bird species. Social Learning of Alarm Calls In their studies of vervet monkeys (Cercopithecus aethiops) in Amboseli National Park, Kenya, Dorothy Cheney and Richard Seyfarth, of the University of Pennsylvania, determined that performance ot a behavior by the vervets could improve through learning. Vervet monkeys, which are about the srze of a domestic cat, produce a complex set of alarm calls. The CHAPTER 51

Behavioral Ecofog-

1131

Amboseli vervets give distinct alarm calls when they see leopards, eagles, or snakes, all of which prey on vervets. When a verveL sees a leopard, it gives a loud barking sound; when it sees

M Figure 51.38 Both genes and culture build human nature.

an eagle, it gives a short double-syllabled cough; and the snak

Teaching ot a

alarm call is a "chatter." Upon hearing a particular alarm call, other vervets in the group behave in an appropriate way: They run up a tree on hearing the alarm for a leopard (vervets are nimbler than leopards in the trees); look up on hearing the alarm for an eagle; and look down on hearing the alarm for a snake (Figure 51.37) Infant vervet monkeys give alarm calls, but in a relatively undiscriminating way. For example, they give the "eagle" alarm on seeing any bird, including harmless birds such as bee-eaters. With age, the monkeys improve their accuracy. In fact, adult vervet monkeys give the eagle alarm only on seeing an eagle belonging to either of the two species that eat vervets. Infants probably learn how to give the right call by observing other members of the group and receiving social confirmation. For instance, if the infant gives the call on the right occasion—for instance, an eagle alarm when there is an eagle overhead—another member ol the group will also give the eagle call. But if the infant gives the call when a bee-eater flies by, the adults in the group are silent. Thus, vervet monkeys have an initial, unlearned tendency to give calls on seeing potentially threatening objects in the environment. Learning fine-tunes the call so that by adulthood, vervets give calls only in response to genuine danger and are prepared to fine-tune the alarm calls of the next generation. However, neither vervets nor any other species comes close to matching the social learning and cultural transmission that occurs among humans (Figure 51.38).

younger generation by an older generation is one of the basic ways in which all cultures are transmitted.

A Figure 51.37 Vervet monkeys learning correct use of alarm calls. On seeing a python (foreground), vervet monkeys give a distinct "snake" alarm call (inset), and the members of the group stand upright and look down. 1132

UNIT EIGHT

Ecology

Evolution and Human Culture Human culture is related to evolutionary theory in the discipline of sociobiology, whose main premise is that certain behavioral characteristics exist because they are expressions of genes that have been perpetuated by natural selection. In hii seminal 1975 book Sociobiology: The New Synthesis, E. O. Wilson speculated about the evolutionary basis of certain kinds of social behavior mainly in nonhuman animals, but also including human culture, sparking a debate that remains heated today. The spectrum of possible human social behaviors may be influenced by our genetic makeup, but this is very different from saying that genes are rigid determinants of behavior. This distinction is at the core of the debate about evolutionary perspectives on human behavior. Skeptics fear that evolutionary interpretations of human behavior could be used to justify the status quo in human society, thus rationalizing current social injustices. Evolutionary biologists argue that this is a gross oversimplification and misunderstanding of what the data tell us about human biology. Evolutionary explanations of human behavior do not reduce us to robots stamped out of rigid genetic molds. Just as individuals vary extensively in anatomical features, we should expect inherent variations in behavior as well. Environment intervenes in the pathway from genotype to phenotype for physical traits and even more so for behavioral traits. And because of our capacity for learning and versatility, human behavior is probably more plastic than that of any other animal. Over our recent evolutionary history, we have built up a diversity of structured societies with governments, laws, cultural values, and religions that define what is acceptable behavior and what is not, even when unacceptable behavior might enhance an individual's Darwinian fitness.

And yet we read in ilie media about newly discovered genes for complex human behavioral traits, such as depression, violence, or alcoholism. Doesn't this reinforce the idea that our behavior is. in fact, "hardwired"? According to Robert Plomin, director of the Center for Developmental and Health Genetics at Pennsylvania State University, research into the heritability of behavior is the best demonstration of the importance of environment. As Plomin puts it, genes and nongenetic, environmental factors "build on each other/' For instance, it might seem that the human ability to speak is completely genetic. However, the ability to learn a specific language, such as English or Spanish, is a function of a complex brain that develops in a particular environmental context under the guidance of a human genome and with the aid of social learning. If the behavior of humans, like that of other species, is the result of interactions between genes and environment, what is unique about our species? Perhaps it is oar

Chapter Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY CONCEPTS Behavioral ecologists distinguish between proximate and ultimate causes of behavior K What Is Behavior? (p. 1107) Behavior, which includes muscular as well as nonmuscular activity, is everything that an animal does and how it does it. Learning is also generally considered a behavioral process. I* Proximate and Ultimate Questions (p. 1107) Proximate, or "how," questions focus on the environmental stimuli, if any, that trigger a behavior, as well as the genetic, physiological, and anatomical mechanisms underlying a behavioral act. Ultimate, or "why," questions address the evolutionary significance of a behavior. I*- Ethology (pp. 1107-1109) Ethology is the scientific study of animal behavior, particularly in natural environments. The mid-2 Oth-century ethologists developed a conceptual framework defined by a set of questions that highlight the complementary nature of proximate and "ultimate perspectives. Investigation How Can Pill Bug Responses to Environments Be Tested?

Many behaviors have a strong genetic component • Biologists study the ways both genes and the environment influence the development of behavioral phenotypes (pp. 1109-1110). Directed Movements (pp. 1110-1111) Kinesis, a behavior involving a change in activity or turning raie, and taxis, an

^

social and cultural institutions that make us truly unique and that provide the only feature in which there is no continuum between humans and other animals.

Concept Check 1. What is the ultimate cause for altruistic behavior among kin? 2. What hypothesis could explain cooperative behavior among nonrelated animals? Explain. 3. Could the changes in behavior produced by crossfostering of white-footed mice and California mice (described in Concept 51.3) extend beyond one generation? Explain. For suggested answers, see Appendix A.

Review oriented movement toward or away from some stimulus, are examples of innate behaviors that are under strong genetic influence. Experiments have demonstrated that bird migration is one complex behavior that is at least partly under genetic control. • Animal Signals and Communication (pp. 1111-1112) The transmission of, reception of, and response to signals constitute animal communication. Animals communicate using visual, auditory, chemical (olfactory), tactile, and electrical signals. Activity Honeybee Waggle Dance Video • Genetic Influences on Mating and Parental Behavior (pp. 1112-1113) A variety of mammalian behaviors are under relatively strong genetic control. For instance, research has revealed the genetic and neural basis for the mating and parental behavior of male prairie voles.

Environment, interacting with an animal's genetic makeup, influences the development of behaviors • Dietary Influence on Mate Choice Behavior (pp. 11131114) Laboratory experiments have demonstrated that the type of food eaten during larval development strongly influences later mate selection by Drosophila mojavensis females. • Social Environment and Aggressive Behavior (p. 1114) Cross-fostering studies of California mice and white-footed mice have uncovered an influence of social environment on the aggressive and parental behaviors of these mice. • Learning (pp. 1114-1118) Learning is the modification of behavior based on specific experiences. Types of learning include habituation, spatial learning, use of cognitive maps, and associative learning. Studies of cognition and problem solving are revealing levels of mental sophistication in animals ranging from insects and birds to primates. Environment and genetics can interact in influencing the learning process.

CHAPTER si

Behavioral Ecology

1133

TESTING YOUR KNOWLEDGE Behavioral traits can evolve by natural selection • Behavioral Variation in Natural Populations (pp. 1118-1120) When behavioral variation within a species corresponds to variation in environmental conditions, it may be evidence of past evolution. • Experimental Evidence for Behavioral Evolution (pp. 1120-1121) Laboratory studies of Drosophila populations raised in high- and low-density conditions show a clear divergence in behavior linked to specific genes. Field and laboratory studies have documented a change in migratory behavior in western European blackcaps over a period of 40 years.

Evolution Connection In human affairs, we often explain our behavior in terms of subjective feelings, motives, or reasons; but evolutionary explanations for behavior are based on reproductive fitness. What is the relationship between the two kinds of explanation? For instance, is a human explanation for behavior, such as "falling in love," incompatible with an evolutionary explanation? Does falling in love become more meaningful or less meaningful (or neither) if it has an evolutionary basis?

Natural selection favors behaviors that increase survival and reproductive success • Foraging Behavior (pp. 1122-1123) Several studies provide support for optimal foraging theory, which posits that natural selection should favor foraging behavior that minimizes the costs of foraging and maximizes the benefits. • Mating Behavior and Mate Choice (pp. 1123-1127) How mate choice enhances reproductive success varies, depending on the species' mating system. The mating relationship between males and females, which includes monogamous, polygynous, and polyandrous mating systems, varies a great deal from species to species. Certainty of paternity has a significant influence on mating behavior and parental care. Mate preferences by females may play a central role m the evolution of male behavior and anatomy through intersexual selection. Male competition for mates is a source of intrasexual selection that also can reduce variation among males. Intrasexual selection can lead to the evolution of alternative male mating behavior and morphology • Applying Game Theory (pp. 1127-1128) Applied to problems in behavioral ecology, game theory provides a way of thinking about evolution in situations where the fitness of a particular behavioral phenotype is influenced by other behavioral phenotypes in the population.

The concept of inclusive fitness can account for most altruistic social behavior • Altruism (pp. 1128-1129) On occasion, animals behave in altruistic ways that reduce their individual fitness but increase the fitness of the recipient of the behavior. • Inclusive Fitness (pp. 1128-1131) Altruistic behavior can be explained by the concept of inclusive fitness, the total effect an individual has on proliferating its genes by producing its own offspring and by providing aid that enables other close relatives to produce offspring. Kin selection favors altruistic behavior by enhancing the reproductive success of relatives. Altruistic behavior toward unrelated individuals can be adaptive if the aided individual returns the favor in the future, an exchange of aid called reciprocal altruism. • Social Learning (pp. 1131-1132) Social learning forms the roots of culture, which can be defined as a system of information transfer through observation or teaching that influences the behavior of individuals in a population. • Evolution and Human Culture (pp. 1132-1133) Human behavior, like that of other species, is the result of interactions between genes and environment. However, our social and cultural institutions may provide the only feature in which there is no continuum between humans and other animals.

1134

UNIT

EIGHT

Ecology

Scientific Inquiry Scientists studying scrub jays found that it is common for "helpers" to assist mated pairs of birds in raising their young. The helpers lack territories and mates of their own. Instead, they help the territory owner gather food for their offspring. Propose a hypothesis to explain what advantage there might be for the helpers to engage in this behavior instead of seeking their own territories and mates. How would you test your hypothesis? If your hypothesis is correct, what kind of results would you expect your tests to yield? Investigation How Can Pill Bug Responses to Environments Be Tested? Biological Inquiry: A Workbook of Investigative Cases Explore the behavior oj a large gull population in a marina, and human attempts to control the population, in the case "Back to the Buy."

Science, Technology, and Society Researchers are very interested in studying identical twins who were separated at birth and raised apart. So far, the data suggest that such twins are much more alike than researchers would have predicted; they frequently have similar personalities, mannerisms, habits, and interests. What general question do you think researchers hope to answer by studying twins that have been raised apart? Why do identical twins make good subjects for this kind of research? What are the potential pitfalls of this research? What abuses might occur if the studies are not evaluated critically and if the results are carelessly cited in support of a particular social agenda?

A Figure 52.1 Population of northern fur seals (Callorhinus ursinus) on St. Paul Island, off the coast of Alaska.

Key Concepts 52-1 Dynamic biological processes influence population density, dispersion, and demography 52.2 Life history traits are products of natural selection 52.3 The exponential model describes population growth in an idealized, unlimited environment 52.4 The logistic growth model includes the concept of carrying capacity 52.5 Populations are regulated by a complex interaction of biotic and abiotic influences 52.6 Human population growth has slowed after centuries of exponential increase

Earth's Fluctuating Populations

T

he size of the human population and its impact are now among Earth's most significant problems. With a population of more than 6 billion individuals', our species requires vast amounts of materials and space, including places to live, land to grow our food, and places to dump our waste. By rapidly expanding our presence on Earth, we have devastated the environment for many other species and now threaten to make it unfit for ourselves. To understand human population growth, we must consider the general principles of" population ecology. No population, including the human population, can continue to grow indefinitely. Though many species exhibit population explosions, their numbers inevitably decline. In contrast to such

1136

radical booms and busts, other populations are relatively stable over time, with only minor changes in size. Our earlier study of populations in Chapter 23 emphasized the relationship between population genetics—the structure and dynamics of gene pools—and evolution. Evolution remains our central theme as we now view populations in the context of ecology Population ecology, the subject of this chapter, is the study of populations in relation to the environment, including environmental influences on population density and distribution, age structure, and variations in population size. The fur seal population of St. Paul Island (Figure 52.1), off the coast of Alaska, is one population that has experienced dramatic fluctuations in size. Reduced to low numbers by hunting early in the 20th century, the seal population grew rapidly once it was protected. In this case, the population had been reduced by human predation, but there are many reasons why populations grow or decline. Later in this chapter, we will apply the basic population concepts we develop to the human population. Let's begin by exploring some of the structural and dynamic aspects of populations that apply to all species.

Concept

Dynamic biological processes influence population density, dispersion, and demography A population is a group of individuals of a single species living in the same general area. Members of a population rely on the same resources, are influenced by similar environmental factors, and have a high likelihood of interacting with and

breeding with one another. Populations can evolve through natural selection acting on heritable variations among individuals and changing the frequencies of various traits over time (see Chapter 23).

Density and Dispersion At any given moment, every population has specific boundaries and a specific size (the number of individuals living within those boundaries). Ecologists usually begin investigating a population by defining boundaries appropriate to the organisms under study and to the questions being asked. A population's boundaries may be natural ones, as in the case of terns nesting on a particular island in Lake Superior, or they may be arbitrarily defined by an investigator, as in the case of oak trees within a specific county in Minnesota. Once defined, the population can be described in terms of its density and its dispersion. Density is the number of individuals per unit area or volume—the number of oak trees per square kilometer in the Minnesota county, for example, or the number of Escherichia coli bacteria per milliliter in a test tube. Dispersion is the pattern of spacing among individuals within the boundaries of the population.

total number of individuals in the population. Note that the mark-recapture method assumes that each marked individual has the same probability of being trapped as each unmarked individual. This is not always a safe assumption, because an animal that has been trapped once may be wary of traps in the future. Density is not a static property of a population, but is rather [he result of a dynamic interplay between processes that add individuals to a population and those that remove individuals from it (Figure 52.2). Additions to populations occur through birth (which we will define here to include all forms of reproduction) and immigration, the influx of new individuals from other areas. The factors that remove individuals from a population are death (mortality) and emigration, the movement of individuals out of a population. While birth and death rates have obvious influences on the density of all populations, immigration and emigration can

Births and immigration add individuals to a population.

Density: A Dynamic Perspective li rare cases, it is possible to determine population size and density by actually counting all individuals within the boundaries of the population. We could count all the sea stars in a tide pool, for example. Herds of large mammals, such as buffalo or elephants, can sometimes be counted accurately from airplanes. In most cases, however, it is impractical or impossible to count ail individuals in a population. Instead, ecologists use a variety of sampling techniques to estimate densities and total population sizes. For example, they might count the number of oak trees in several randomly located 10 X 100 m plots (samples), calculate the average density in the samples, and then extrapolate to estimate the population size in the entire area. Such estimates are most accurate when there are many sample plots and when the habitat is homogeneous. In some cases, instead of counting individual organisms, population ecologists estimate density from some index of population size, such as the number of nests, burrows, tracks, or fecal droppings. Another sampling technique commonly used to estimate wildlife populations is the mark-recapture method. The researchers place traps within the boundaries of the population under study. Captured animals are marked with tags, collars, bands, or spots of dye, and then released. After a few days or weeks-—enough time for the marked individuals to mix with unmarked members of the population—traps are set again. This second capture yields both marked and unmarked individuals. From these data, researchers can estimate the

Deaths and emigration remove individuals from a population.

A Figure 52.2 Population dynamics.

CHAPTER 52

Population Ecology

1137

also make important contributions to the density of local populations. For instance, long-term studies of Beldings ground squirrels (Spermophilus beldingi) in the vicinity of Tioga Pass, in the Sierra Nevada mountains of California, show that some of the squirrels move nearly 2 km from where they are born, making them immigrants to other populations. Paul Sherman and Martin Morton, at that time at Cornell University and Occidental College, respectively, estimated that immigrants made up 1-8% of the males and 0.7-6% of the females in the study population. While these may seem like small percentages, over time such rates represent biologically significant exchanges between populations.

Random dispersion (unpredictable spacing) occurs in the a > sence of strong attractions or repulsions among individuals of a population or where key physical or chemical factors are relatively homogeneous across the study area; the position of each individual is independent ol other individuals. For example,

Patterns of Dispersion Within a population's geographic range, local densities may vary substantially Variations in local density are among the most important characteristics that a population ecologist might study, since they provide insight into the environmental associations and social interactions of individuals in the population. Environmental differences—even at a local level—contribute to variation in population density; some habitat patches are simply more suitable for a species than are others. Social interactions between members of the population, which may maintain patterns of spacing between individuals, can also contribute to variation m population density. The most common pattern of dispersion is dumped, with the individuals aggregated in patches. Plants or lungi are often clumped where soil conditions and other environmental factors favor germination and growth. For example, mushrooms may be clumped on a rotting log. Many animals spend much of their time in a particular microenvironment that satisfies their requirements. Forest insects and salamanders, for instance, are frequently clumped under logs, where the humidity tends to be higher than in more exposed areas. Clumping of animals may also be associated with mating behavior. For example, mayflies often swarm in great numbers, a behavior that increases mating chances for these insects, wThich survive only a day or two as reproductive adults. Group living may also increase the effectiveness of certain predators; for example, a wolf pack is more likely than a single wolf to subdue a large prey animal, such as a moose (Figure 52.3a). A uniform, or evenly spaced, pattern of dispersion may result from direct interactions between individuals in the population. For example, some plants secrete chemicals that inhibit the germination and growth of nearby individuals that could compete for resources. Animals often exhibit uniform dispersion as a result of antagonistic social interactions, such as territoriality-—the detense of a bounded physical space against encroachment by other individuals (Figure 52.3b). Uniform patterns are not as common in populations as clumped patterns.

1138

UNIT EIGHT

Ecology

(a) Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory.

(b) Uniform. Birds nesting on small islands, such as these king penguins on South Georgia island in the South Atlantic Ocean, often exhibit uniform spacing, maintaine by aggressive interactions between neighbors.

(c) Random. Dandelions grow from windblown seeds that land at random and later germinate.

A Figure 52.3 Patterns of dispersion within a population's geographic range.

plants established by windblown seeds, such as dandelions, are sometimes randomly distributed in a fairly consistent habitat (Figure 52.3c). Random patterns are not as common in nature as one might expect; most populations show at least a tendency toward a clumped distribution.

Demography The factors that influence population density and dispersion patterns—ecoLogical needs of a species, structure of the environment, and interactions between individuals within the population—also influence other characteristics of populations. Demography is the study of the vital statistics of populations and how they change over time. Of particular interest to demographers are birth rates and how they vary among individuals (specifically among females) and death rates. A useful way of summarizing some of the vital statistics of a population is with a life table.

Life Tables About a century ago, when Life insurance first became available, insurance companies needed to estimate how long, on average, individuals of a given age could be expected to live. To do this, demographers developed life tables, age-specific summaries of the survival pattern of a population. Population ecologists adapted this approach to the study of nonliuman populations and developed quantitative demography as a branch of population ecology

The best way to construct a life table is to follow the fate of a cohort, a group of individuals of the same age, from birth until all are dead. To build the life table, we need to determine the number of individuals that die in each age group and calculate the proportion of the cohort surviving from one age to the next. Cohort life tables are difficult to construct for wild animals and plants and are available for only a limited number of species. Sherman and Morton's studies of the Tioga Pass Belding's ground squirrels resulted in the life table in Table 52.1. The table reveals many things about the population. For instance, the third and eighth columns list, respectively, the proportions of females and males in the cohort that are still alive at each. age. A comparison of the fifth and tenth columns reveals that males have higher death rates than females.

Survivorship Curves A graphic way of representing the data in a life table is a survivorship curve, a plot of the proportion or numbers m a cohort still alive at each age. Let's use the data for Belding's ground squirrels in Table 52.1 to construct a survivorship curve for this population. Generally, a survivorship curve is constructed beginning with a cohort of 1,000 individuals from a population. We can do this for the Belding's ground squirrel population by multiplying the proportion alive at the start of each year (third and eighth columns of Table 52.1) by 1,000 (the hypothetical beginning cohort). The result is the

liable 52.1 Life Table for Belding's Ground Squirrels (Spermophilus beldingi) at Tioga Pass, in the Sierra Nevada Mountains of California* FEMALES

Age (years) 0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9

9-10

MALES

Number Alive at Start of Year

Proportion Alive at Start of Year

Number of Deaths During Year

Death Rate*

Average Additional Life Expectancy (years)

337

1.000 0.386 0.197 0.106 0.054 0.029 0.014 0.008 0.006 0.002

207 125 60 32 16 10 4 1 3 1

0.61 0.50 0.47 0.48 0.46 0.53 0.44 0.20 0.75 1.00

1.33 1.56 1.60 1.59 1.59 1.50 1.61 1.50 0.75 0.50

252" 127 67 35 19 y 5 4 J

Number Alive at Start of Year

Proportion Alive at Start of Year

Number of Deaths During Year

Death Rate*

Average Additional Life Expectancy (years)

349

1.000 0.350 0.152 0.048 0.015 0.003

227 140 74 23 9 0

0.65 0.56 0.69 0.68 0.82 1.00

1.07 1.12 0.93 0.89 0.68 0.50

248' f 108 34 11 2 0

'Males and Females have different morality ^heciules. =,0 trie) a;e is.lkvl separately. 'The death rale is the proportion of individuals dying in the specific time interval. 1+ lncludes 122 females and 126 males first c&p Lured as one-year-olds and therefore not included m the covjii of squirrels age 0-1 Source: Data from V. W Sherman ami M. L. Morton, " Demography of Beldings Ground Squirrel," FcoJngy 65(1984): 1617-1628.

CHAPTER 52

Population Ecology

1139

number alive at the start of each year. Plotting these numbers versus age for female and male Belding's ground squirrels yields Figure 52.4. The relatively straight lines of the plots indicate relatively constant rates of death; however, males have a lower survival rate overall than females. Figure 52.4 represents just one of many patterns of survivorship exhibited by natural populations. Though diverse, survivorship curves can be classified into three general types (Figure 52.5). A Type I curve is flat at the start, reflecting low death rates during early and middle life, then drops steeply as death rates increase among older age groups. Humans and many other large mammals that produce few offspring but provide them with good care often exhibit this kind of curve. In contrast, a Type III curve drops sharply at the start, reflecting very high death rates for the young, but then flattens out as death rates decline for those few individuals that have survived to a certain critical age. This type of curve is usually associated

with organisms that produce very large numbers of offspring but provide little or no care, such as long-lived plants, many fishes, and marine invertebrates. An oyster, for example, may release millions of eggs, but most offspring die as larvae from predation or other causes. Those few that survive long enough to attach to a suitable substrate and begin growing a hard shell will probably survive for a relatively long time. Type II curves are intermediate, with a constant death rate over the organisms life span. This kind of survivorship occurs in Belding's ground squirrels (see Figure 52.4) and some other rodents, various invertebrates, some lizards, and some annual plants. Many species fall somewhere between these basic types of survivorship or show more complex patterns, In birds, for example, mortality is often high among the youngest individuals (as in a Type III curve) but fairly constant among adults (as in a Type II curve). Some invertebrates, such as crabs, mayshow a "stair-stepped" curve, with brief periods of increased mortality during molts, followed by periods of lower mortality when the exoskeleton is hard. In populations without immigration or emigration, survivorship is one of the two key factors determining changes in population size. In such populations, the other key factor determining population trends is reproductive rate. Reproductive Rates

A Figure 52.4 Survivorship curves for male and female Belding's g r o u n d squirrels. The logarithmic scale on they-axis makes changes in number of survivors visible across the entire range (2 to 1,000 individuals) on the graph.

1,000

A Figure 52.5 Idealized survivorship curves: Types I, II, a n d III. The y-axis is logarithmic and the x-axis is on a relative scale, so species with widely varyinq life spans can be compared on the same graph.

1140

UNIT E I G H T

Ecology

Demographers who study sexually reproducing species generally ignore males and concentrate on females in a population because only females produce offspring. Demographers view populations in terms of females giving rise to new females; males are important only as distributors of genes. The simplesL way to describe the reproductive pattern of a population is to ask how reproductive output varies with the ages of females. A reproductive table, or fertility schedule, is an agespecific summary of the reproductive rates in a population. The best way to construct a fertility schedule is to measure the reproductive output of a cohort from birth until death. For sexual species, the reproductive table tallies the number of female offspring produced by each age group. Table 52.2 illustrates a reproductive table for Belding's ground squirrels. Reproductive output for sexual species such as birds and mammals is the product of the proportion of females of a given age that are breeding and the number of female offspring of those breeding females. Multiplying these numbers gives the average number of female offspring for each female in a given age class (the last column in Table 52.2). For Belding's ground squirrels, which begin to reproduce at age 1 year, reproductive output rises to a peak at 4 years and then falls off in older females. Reproductive tables vary greatly, depending on the species. Squirrels have a litter of two to six young once a year for less than a decade, whereas oak trees drop thousands of acorns each year for tens or hundreds of years. Mussels and other invertebrates may release hundreds of thousands of eggs in a

able 52.2 Reproductive Table for Belding's Ground Squirrels at Tioga Pass Proportion of Females Weaning a Litter

Mean Size of Litters (Males + Females)

Mean Number of Females in a Litter

Average Number of Female Offspring*

0-1

0.00

0.00

0.00

0.00

1-2

0.65

3.30

1.65

1.07

2-3

0.92

4.05

2.03

1.87

3-4

0.90

4.90

2.45

2.21

4-5

0.95

5.45

2.73

2.59

5-6

1.00

4.1.5

2.08

2.08

6-7

1.00

3.40

1.70

1.70

7-8

1.00

3.85

1.93

1.93

8-9

1.00

3.85

1.93

1.93

9-10

1.00

3.15

1.58

1.58

Age (years)

'The average number ol it'iii.sie o.'i;yr.nz i~ ihe proportion weaning a tin multiplied by the mean number of females in a litter. Data from P. W. Sherman and M. L. Morton, "Demography of Belding's Ground Squirrel; frologv 65 (1984): 1617-1628.

spawning cycle. Why a particular type of reproductive pattern evolves in a particular population is one of the many questions at the interface of population ecology and evolutionary biology. This is the subject of life history studies.

1 Concept Check t 1. One species of forest bird is highly territorial, while a second lives in flocks. What is each species' likely pattern of dispersion? Explain. 2. Each female of a particular fish species produces millions of eggs per year. What is its likely survivorship pattern? Explain, 3. What is the average proportion of females born into the population of Belding's ground squirrels described by Table 52.2? For suggested answers, see Appendix A.

1 Concept

ss ^i$si I

Life history traits are products of natural selection .Natural selection favors traits that improve an organism's chances of survival and reproductive success. In every species,

there are trade-offs between survival and traits such as frequency of reproduction, the number of offspring produced (the number ol seeds produced by plants and litter or clutch size for animals), and investment in parental care. The traits that affect an organisms schedule of reproduction and survival (from birth through reproduction to death) make up its life history. Life histories entail three basic variables: when reproduction begins (the age at first reproduction or age at maturity), how often the organism reproduces, and how many offspring are produced during each reproductive episode. Keep in mind that, with the important exception of humans (which we will consider later in the chapter), organisms do not choose consciously when to reproduce or how many offspring to have. Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism.

Life History Diversity Life histories are very diverse. Pacific salmon, for example, hatch in the headwaters of a stream, then migrate to open ocean, where they require one to four years to mature. The salmon eventually return to freshwater streams to spawn, producing thousands of small eggs in a single reproductive opportunity, and then they die. This "one-shot" pattern of big-bang reproduction, or semelparity (from the Latin semei, once, and parito, to beget), also occurs in some plants, such as the agave (Figure 52.6). Agaves, or century plants, generally grow in arid climates with sparse and unpredictable rainfall. Agaves grow for several years, then send up a large flowering stalk, produce seeds, and die. The shallow roots of agaves catch water after rain showers but are dry during droughts. This irregular water supply may prevent seed production or seedling establishment for several years at a time. By growing and storing nutrients until an unusually wet year and then putting all its resources into reproduction, the agave's life history is an adaptation to erratic climate. In contrast to semelparity is iteroparity (from the Latin item, to repeat), or repeated reproduction. For example, some lizards produce a few large eggs during their second year of life, then repeat the reproductive act annually for several years. What factors contribute to the evolution of semelparity A Figure 52.6 An agave, or versus iteroparity? In other cen tury plant, an example words, bow much does an of big-bang reproduction. CHAPTER 52 Population Ecology

1141

individual gain in reproductive success through one pattern versus the other? The critical factor is survival rate of the offspring. Where the survival rate of offspring is low, as in highly variable or unpredictable environments, big-bang reproduction (semelparity) is favored. Production of large numbers of offspring in such environments increases the probability that at least some will survive. Repeated reproduction (iteroparity) will be favored in more dependable environments where competition for resources may be intense. In such environments, a few relatively large, well-provisioned offspring will have a better chance of surviving to reproductive age.

"Trade-offs" and Life Histories We might imagine an organism that could produce as many offspring as a semelparous species, provision them well like an iteroparous species, and do so repeatedly, but such organisms do not exist. Time, energy, and nutrients that are used for one thing cannot be used for something else. As a consequence, natural selection cannot maximize all these reproductive variables simultaneously Organisms have finite resources, and limited resources mean trade-offs, in the broadest sense, there is a trade-off between reproduction and survival, as

demonstrated by studies of a number of organisms. For example, a study of red deer in Scotland showed that females that reproduced in one summer suffered higher mortality over the next winter than females that did not reproduce. And a study of European kestrels demonstrated the survival cost to parents of caring for young (Figure 52.7). Selective pressures also influence the trade-off between the number and size of offspring. Plants and animals whose young are subject to high mortality rates often produce large numbers of relatively small offspring. Plants that colonize disturbed environments, for example, usually produce many small seeds, only a few of which may reach a suitable habitat. Small size may also increase the chance of seedling establishment by enabling the seeds to be carried longer distances to a broader range of habitats (Figure 52.8a). Animals that suffer high predation rates, such as quail sardines, and mice, also tend to produce large numbers of offspring. In other organisms, extra investment on the part of the parent greatly increases the offspring's chances of survival. Oak,

Figure 52,7

How does caring for offspring affect parental survival in kestrels? EXPERIMENT

iesearchers in the Netherlands studied the effects of parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.) (a) Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least some will grow into plants and eventually produce seeds themselves.

The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents.

(b) Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring.

A Figure 52.8 Variation in seed crop size in plants.

1142

UNIT

EIGHT

walnut, and coconut trees all have large seeds with a large store of energy and nutrients that help the seedlings become established (Figure 52.8b). In animals, parental investment in offspring does not always end after incubation or gestation. For instance, primates generally bear only one or two offspring at a time- Parental care and an extended period of learning in the first several years of life are very important to offspring fitness in these species.

(Concept Check » 1. Consider two rivers: One is spring fed and is constant m water volume and temperature year-round; the other drains a desert landscape and floods and dries out at unpredictabe intervals. Which is more likely to support many species of iteroparous animals? Why? For suggested answers, see Appendix A.

I Concept

The exponential model describes population growth in an idealized, unlimited environment The concepts of life history provide a biological toundation for a quantitative understanding of population growth. To appreciate the potential for population increase, consider a single bacterium that can reproduce by fission every 20 minutes under ideal laboratory conditions. There would be 2 bacteria after 20 minutes, 4 alter 40 minutes, 8 after 60 minutes, and so i m. If reproduction continued at this rate, with no mortality, for only a day and a half—a mere 36 hours—there would be enough bacteria to form a layer a foot deep over the entire globe. At the other life history extreme, an elephant may produce only 6 offspring in a 100-year life span. Still, Charles Darwin once estimated that the descendants of a single pair of mating elephants would number 19 million within only 750 years. Though Darwin's estimate may not have been precisely correct, such analyses led him to recognize the tremendous capacity for growth in all populations. However, unlimited population increase does not occur indefinitely for any species, either in the laboratory or in nature. A population that begins at a small size in a favorable environment may increase rapidly for a while, but eventually, as a result of limited resources and other factors, its numbers must stop growing. Nevertheless, it is useful to study population growth in an idealized., unlimited environment because such studies reveal the capacity of species for increase and the conditions in which that capacity may be expressed.

Per Capita Rate of Increase Imagine a hypothetical population consisting of a few individuals living in an ideal, unlimited environment. Under these conditions, there are no restrictions on the abilities of individuals to harvest energy, grow, and reproduce, aside from the inherent biological limitations of their life history traits. The population will increase in size with every birth and with the immigration of individuals from other populations, and it will decrease in size with every death and with the emigration of individuals out of the population. For simplicity here, we will ignore the effects of immigration and emigration (a more complex formulation would certainly include these factors). We can thus define a change in population size during a fixed time interval with the following verbal equation: Change in population size during time interval

Births during time interval

Deaths during time interval

We can use mathematical notation to express this relationship more concisely If N represents population size and t represents time, then AN is the change in population size and At is the time interval (appropriate to the life span or generation time of the species) over which we are evaluating population growth. (The Greek letter delta, A, indicates change, such as change in time.) We can now rewrite the verbal equation as

where B is the number of births in the population during the time interval and D is the number of deaths. Next, we can convert this simple model into one in which births and deaths are expressed as the average number of births and deaths per individual during the specified time interval The per capita birth rate is the number of offspring produced per unit time by an average member of the population. If, for example, there are 34 births per year in a population of 1,000 individuals, the annual per capita birth rate is 34Aooo, or 0.034. If we know the annual per capita birth rate (expressed as b), we can use the formula B = bN to calculate the expected number of births per year in a population of any size. For example, if the annual per capita birth rate is 0.034 and the population size is 500, B = hH B » 0.034 X 500 B — 17 per year Similarly, the per capita death rate (symbolized as m. for mortality) allows us to calculate the expected number of deaths per unit time in a population of any size. If m = 0.016 per year, we would expect 16 deaths per year in a population of 1,000 individuals. For natural populations or those in the laboratory, the per capita birth and death rates can be calculated from estimates of population siae and data in life tables and reproductive tables (for example, Tables 52.1 and 52.2). C H A P T E R 52

Population Ecology

1143

Now we can revise the population growth equation again, this time using per capita birth and death rates rather than the numbers of births and deaths: AM - — - bN - mN At

One final simplification is in order. Population ecologists are most interested in the difference between the per capita birth rate and per capita death rate. This difference is the per capita rate, ojincrease, or r:

The value of r indicates whether a given population is growing (r > 0) or declining (r < 0). Zero population growth (ZPG) occurs when the per capita birth and death rates are equal (r = 0). Births and deaths still occur m such a population, of course, but they balance each other exactly. Using the per capita rate of increase, we now rewrite the equation for change in population size as

rmax, and larger populations experience more births (and deaths) than small ones growing at the same per capita rate. It is also clear from Figure 52.9 that a population with a higher intrinsic rate of increase (dN/dt = 1.0N) will grow faster than one with a lower rate of increase (dN/dt = 0.5N), TheJ-shaped curve of exponential growth is characteristic of some populations that are introduced into a new or unfilled environment or whose numbers have been drastically reduced by a catastrophic event and are rebounding. For example, Figure 52.10 illustrates the exponential population growth that occurred in the population of elephants in Kruger National Park, South Africa, after they were protected from hunting. After approximately 60 years of exponential growth, the large number of elephants had caused enough damage to the park vegetation

Most ecologists use differential calculus to express population growth as growth rate at a particular instant in time;

If you have not yet studied calculus, don't be intimidated by the form of the last equation; it is essentially the same as the previous one, except that the time intervals At are very short and are expressed in the equation as dt.

Exponential Growth Earlier we described a population whose members all have access to abundant food and are free to reproduce at their physiological capacity. Population increase under these ideal conditions is called exponential population growth, also known as geometric population growth. Under these conditions, the per capita rate of increase may assume the maximum rate for the species, called the intrinsic rate oj increase and denoted as rmax. The equation for exponential population growth is

A Figure 52.9 Population growth predicted by the e x p o n e n t i a l m o d e l . This graph compares growth in two populations with different values of rmax. Increasing the value from 0.5 to 1.0 increases the rate of rise in population size with time, as reflected by the relative slopes of the curves.

Tt = r-N The size of a population that is growing exponentially increases at a constant rate, resulting eventually in a j-shaped growth curve when population size is plotted over time (Figure 52.9). Although the intrinsic rate of increase is constant, the population accumulates more new individuals per unit of time when it is large than when it is small; thus, the curves in Figure 52.9 get progressively steeper over time. This occurs because population growth depends on N as well as 1144

UNIT

EIGHT

Ecology

A Figure 52.10 Exponential growth in the African elephant population of Kruger National Park, South Africa.

that a collapse in the elephant food supply was likely, leading to an end to population growth through starvation. To protect other species and the park ecosystem before that happened, park managers began limiting the elephant population by using birth control and exporting elephants to other countries.

Concept Check 1. Explain why a constant rate of increase (rmax) for a population produces a growth graph that is j-shaped rather than a straight line. 2. Where is exponential growth by a plant population more likely—on a newly formed volcanic island or in a mature, undisturbed rain forest? Why? For suggested answers, see Appendix A.

To construct the logistic model, we start with the exponential population growth model and add an expression that reduces the per capita rate of increase as N increases (Figure 52.11). If the maximum sustainable population size (carrying capacity) is K, then K - N is the number of additional individuals the environment can accommodate, and (K — N)/K is the fraction of K that is still available for population growth. By multiplying the exponential rate of increase rmaxN by (K — N)/K, we modify the growth rate of the population as N increases:

Table 52.3 shows calculations of population growth rate at various population sizes for a hypothetical population growing

Maximum

Concept

The logistic growth model includes the concept of carrying capacity The exponential growth model assumes resources are unlimited; never the case in the real world. As population density increases, each individual has access to fewer resources. Ultimately, there is a limit to the number of individuals that can occupy a habitat. Ecologists define carrying capacity, symbolized as K, as the maximum population size that a particular environment can support. Carrying capacity is not fixed, but varies over space and time with the abundance of limiting resources. Energy, shelters, refuges from predators, soil nutrients, water, and suitable nesting sites can all be limiting factors. For example, the carrying capacity for bats may be high in a habitat with abundant flying insects and roosting sites but lower where there is abundant food but fewer suitable shelters. Crowding and resource limitation can have a profound effect on population growth rate. If individuals cannot obtain sufficient resources to reproduce, the per capita birth rate (b) will decline. If they cannot find and consume enough energy to maintain themselves, the per capita death rate (m) may increase. A decrease in b or an increase in m results in a lower per capita rate of increase, r.

The Logistic Growth Model

Population size (N) A Figure 52.11 Influence of population size (Af) on per capita rate of increase (r). The logistic model assumes that the per capita rate of increase decreases as N increases. If N is greater than K, then the population growth rate is negative, and population size decreases. An equilibrium is reached at the white line when N = K.

Table 52.3 A Hypothetical Example of Logistic Population Growth, Where K = 1,000 and rm,x = 0.05 per Individual per Year Population Size: N 20

Per Capita Growth Rate:

Intrinsic Rate of Increase:

0.05

\ k - N\ j """[ K j 0.98

Population Growth Rate:*

max

\ K 1

0.049

+1 +5

100

0.05

0.90

0.045

250

0.05

0.75

0.038

+9

500

0.05

0.50

0.025

+ 13

750

0.05

0.25

0.013

+9

1,000

0.05

0.00

0.000

0

We can modify our mathematical model to incorporate changes in growth rate as the population size nears the carrying capacity In the logistic population growth model, the per capita rate of increase declines as carrying capacity is reached. CHAPTER 52 Population Ecology

1145

according to the logistic model. When N is small compared to K, the term (K — N)/K is large, and the per capita rate of increase rmax(K — N)/K, is close to the intrinsic (maximum) rate of increase. But when N is large and resources are limiting, then (K — N)/K is small, and so is the per capita rate of increase. When N equals K, the population stops growing. Notice in Table 52.3 that the overall population growth rate is highest, +13, over a year when the population size is 500, or half the carrying capacity. Why is population growth rate greatest at 500 and not at smaller population sizes? This is due to the balance between per capita rate of increase and population size. At a population size of 500, the per capita rate of increase remains relatively high (one-half the maximum rate), and there are many more reproducing individuals in the population than at lower population sizes. The logistic model of population growth produces a sigmoid (S-shaped) growth curve when N is plotted over time (the red line in Figure 52.12). New individuals are added to the population most rapidly at intermediate population sizes, when there is not only a breeding population of substantial size, but also lots of available space and other resources in the environment. The population growth rate slows dramatically as N approaches K. Notice that we haven't said anything about why the population growth rate slows as N approaches K For a population's growth rate to decrease, either the birth rate b must decrease, the death rate m must increase, or both. Later in the chapter, we will consider some of the factors affecting these rates.

5

10

Number of generations

A Figure 52.12 Population growth predicted by the logistic m o d e l . The rate of population growth slows as population size (N) approaches the carrying capacity (K) of the environment. The red line shows logistic growth in a population where rmax = 1.0 and K = 1,500 individuals. For comparison, the blue line illustrates a population continuing to grow exponentially with the same rmax.

1146

U N I T EIGHT

Ecolog)

The Logistic Model and Real Populations The growth of laboratory populations of some small animals,

such as beetles and crustaceans, and of some microorganisms, such as paramecia, yeasts, and bacteria, fits an S-shaped curve fairly well (Figure 52.13a). These populations are grown in a constant environment lacking predators and other species that may compete for resources, conditions that rarely occur in nature. Some of the basic assumptions built into the logistic model clearly do not apply to all populations. For example, the logistic model assumes that populations adjust instantaneously to growth and approach carrying capacity smoothly In most natural populations, however there is a lag time before the negative effects of an increasing population are realized. If food becomes limiting for a population, for instance, reproduction will be reduced. However, the birth rate may not decline immediately, since females may use their energy reserves to continue reproducing for a short time. This may cause the population to overshoot its carrying capacity before settling down to a relatively stable density Figure 52.13b illustrates this overshoot for a laborator}' population of water fleas (Daphma). If the population then drops below carrying capacity, there will be a delay in population growth until the increasing number of offspring are actually born. And still other populations fluctuate greatly, making it difficult even to define carrying capacity For example, Figure 52.13c shows marked population changes in the song sparrow on a small island in southern British Columbia. The population increases rapidly but suffers periodic catastrophes in winter; consequently, there is no stable population size. We will examine some possible reasons for these fluctuations later in the chapter. The logistic model also incorporates the idea that regardless of population density, each individual added to a population has the same negative effect on population growth rate. However, some populations show an Alkc effect (named after W. C. Allee, of the University of Chicago, who 6rst described it), in which individuals may have a more difficult time surviving or reproducing if the population size is too small. For example, a single plant may be damaged by excessive wind if it is standing alone, but it would be protected in a clump of individuals. Conservation biologists fear that if populations of certain solitary animals, such as rhinoceroses, drop below a critical size, individuals will not be able to locate mates in the breeding season. Although the logistic model fits few, if any, real populations closely, it is a useful starting point Tor thinking about how populations grow and for constructing more complex models. The model is also useful in conservation biology lor estimating how rapidly a particular population might increase in numbers after it has been reduced to a small size, or for estimating sustainable harvest rates for fish and wildlife populations. And like any good starting hypothesis, the logistic model has stimulated research leading to a better understanding of the factors affecting population growth.

1985 T990 Time (years) (b) A Daphnia population in the lab. The growth (a) A Paramecium population of a population of Daphnia in a small laboratory in the lab. The growth of culture (black dots) does not correspond well to Paramecium aurelis in small the logistic model (red curve). This population cultures (black dots) closely overshoots the carrying capacity of its artificial approximates logistic growth environment and then settles down to an (red curve) if the experimenter approximately stable population size. maintains a constant environment.

1995 2000

(c) A song sparrow population in its natural habitat. The population of female song sparrows nesting on Mandarte Island, British Columbia, is periodically reduced by severe winter weather, and population growth is not well described by the logistic model.

. Figure 52.13 How well do these populations fit the logistic growth model?

The Logistic Model and Life Histories The logistic model predicts different per capita growth rates for populations of low or high density relative to the carrying capacity of the environment. At high densities, each individual has few resources available, and the population grows slowly, if at all. At low densities, the opposite is true: Per capita resources are relatively abundant, and the population can grow rapidly. Different life history features are favored under each condition. At high population density, selection favors adaptations that enable organisms to survive and reproduce with few resources. Thus, competitive ability and efficient use of resources should be favored in populations that are at or near their carrying capacity. These are the traits we associated earlier with iteroparity. At low population density, on the other hand, even in the same species, adaptations that promote rapid reproduction, such as the production of numerous, small offspring, should be favored. Ecologists have attempted to connect these differences in favored traits at different population densities with the logistic growth model. Selection for life history traits that are sensitive to population density is known as K-selection, or densitydependent selection. In contrast, selection for life history traits that maximize reproductive success in uncrowded environments (low densities) is called r-selection, or densityindependent selection. These names follow from the variables of the logistic equation. K-selection tends to maximize population size and operates in populations living at a density near the limit imposed by their resources (the carrying capacity, K). By contrast, r-selection tends to maximize r, the rate of increase, and occurs in environments in which population densities fluctuate well below carrying capacity or individuals are likely to face little competition.

Laboratory experiments have shown that different populations of the same species may show a different balance of K-selected and r-selected traits, depending on conditions. For example, cultures of the fruit fly Drosophila melanogaster raised for 200 generations under crowded conditions with minima] food are more productive at high density than other populations raised for many generations in uncrowded conditions with abundant food, Apparently, when given equal access to food, larvae from cultures selected for living in crowded conditions feed faster than larvae selected for living in uncrowded cultures. The fruit fly genotypes that are most fit at low density do not have high fitness at high density, as predicted by r- and K-selection theory. The concepts of r- and K-selection have been criticized as an oversimplification of the variation seen in the natural histories of species. The characteristics of most species place them somewhere in between the extremes represented by r- and K-selection. The critical assessment of r- and K-selection has led ecologists to propose alternative theories of life history evolution. These alternative theories, in turn, have stimulated more thorough study of how factors such as disturbance, stress, and the frequency of opportunities for successful reproduction affect the evolution of life histories.

Concept Check i 1. Explain why a population that fits the logistic growth model increases more rapidly at intermediate size than at relatively small or large sizes. For suggested answers, see Appendix A.

CHAPTER 52

Population Ecology

1147

1-VIP lnai^tir mnrlpl. When M i T ^rricrif

study of population regulation, Andrew Watkinson and John Harper, plant population ecologists from the University of Wales, found that the mortality of dune fescue grass is mainly due to physical factors that kill similar proportions of a local population, regardless of its density. In contrast, a death rate that rises as population density rises is said to be density dependent, as is a birth rate that falls with rising density. Watkinson and Harper found that reproduction by dune fescue declines as population density increases. Thus, in this grass population, the key factors regulating birth rate were density dependent, while death rate was largely regulated by density-independent factors. Figure 52.14 models how a population may stop increasing and reach equilibrium due to various combinations of density-dependent and densityindependent regulation.

Density-Dependent Population Regulation Density-dependent birth and death rates are an example ol negative feedback, a type of regulation described in Chapter 1. Without some type of negative feedback between population density and the vital rates of birth and death, a population would not stop growing. However, at increased densities, birth rates decline and/or death rates increase, providing the negative feedback that halts continued population growth. Once we know how birth and death rates change with population density, we need to determine the mechanisms causing these changes, which may involve many factors. Competition for Resources In crowded populations, increasing population density intensifies intraspecific competition for declining nutrients and other resources, resulting in a lower birth rate. For example, crowding

(b) Birth rate changes with population density while death rate is constant.

A Figure 52.14 Determining equilibrium for population density. This simple model considers only birth and death rates (immigration and emigration rates are assumed to be either zero or equal).

1148

UNIT EIGHT

Ecoloe;

(c) Death rate changes with population density while birth rate is constant.

can reduce seed production by plants (Figure 52.15a). And available food supplies often limit the reproductive output of songbirds; as bird population density increases in a particular habitat, each female lays fewer eggs, a density-dependent response (Figure 52.15b). In an experiment in which female song sparrows living in high-density conditions were given extra food, they did not suffer a reduction in clutch size,

Territoriality

^

4.0-

10,000-

-g 3.6i 3.4en

"* 3.0-

•

""*"•"

2.8. 0

10

100

3

10

20 30

Seeds planted per (a) Plantain. The number of seeds produced by plantain (Piantago major) decreases as density increases.

In. many vertebrates and some invertebrates, territoriality may limit density. In this case, territory space becomes the resource for which individuals compete. Cheetahs, for example, are highly territorial, using chemical communication to warn other cheetahs of their territoriaf boundaries (Figure 52.16). Oceanic birds, such as gannets, often nest on rocky islands, where they are relatively safe from predators (Figure 52.17). Up to a certain population density, most birds can find a suitable nest site, but beyond that threshold, few birds breed successfully. Thus, the limiting resource that determines breeding population density for gannets is safe nesting territory. Birds that cannot obtain a nesting spot do not reproduce. The presence of surplus, or nonbreeding, individuals is a good indication that territoriality is restricting population growth, as it does in many bird populations.

40

50 60

70

80

Density of females (b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.

.4 Figure 52.15 Decreased reproduction at high p o p u l a t i o n densities. Log scales are used on both x- and y-axes.

Health Population density can also influence the health and thus the survival of organisms. If the transmission rate of a disease depends on a certain level of crowding in a population, the disease's impact may be density dependent. Field experiments conducted by Charles Mitchell, David Tilman, and James Groth, of the University of Minnesota, demonstrated that the severity of infection of plants by fungal pathogens is greater where the density of the host plant population is higher. Animals, too, can experience increased infection by pathogens at high population densities. Steven Kohler and Wade Hoiland of the Illinois Natural History Survey showed that in casebuilding caddis flies (stream-dwelling insects) peaks in diseaserelated mortality followed years of high insect abundance. Their study indicated that such disease-related mortality was largely responsible for cyclic fluctuations in the density of the insect population. Human pathogens can also show densitydependent infection rates. For example, tuberculosis, which is caused by bacteria that spread through the air when an infected person sneezes or coughs, strikes a greater percentage of people living in high-density cities than those in rural areas.

A Figure 52.16 Cheetah staking out a territory with a chemical marker.

& Figure 52.17 Territories. Gannets nest virtually a peck apart and defend their territories by calling and pecking at one another.

CHAPTER 52

PopulationEcolog'

1149

study of population regulation, Andrew Watkinson and John

Predation Predation may be an important cause of density-dependent mortality for some prey populations if a predator encounters and captures more food as the population density of the prey increases. As a prey population builds up, predators may feed preferentially on that species, consuming a higher percentage of individuals. For example, trout may concentrate for a few days on a particular species of insect that is emerging from its aquatic larval stage, then switch prey as another insect species becomes more abundant. Toxic Wastes The accumulation of toxic wastes can contribute to densitydependent regulation ot population size. In laboratory cultures of small microorganisms, for example, metabolic by-products accumulate as the populations grow, poisoning the organisms within this limited, artificial environment. For example, ethanol accumulates as a by-product of yeast fermentation. The alcohol content of wine is usually less than 13% because that is the maximum concentration of ethanol that most wineproducing yeast cells can tolerate.

can determine an average population size for many species, the average is often of less interest than the variation in numbers trom year to year or place to place. The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size. Stability and Fluctuation Populations of large mammals, such as deer and moose, were once thought to remain relatively stable over time. But some long-term studies have challenged that hypothesis. The moose population on Isle Royale, an island in Lake Superior, is a striking example. Moose from the mainland colonized the island around 1900 by walking across the frozen lake. However, the lake has not frozen over in recent years, so the moose population has been isolated from immigration and emigration. Nevertheless, as Figure 52.18 shows, the population has been anything but stable, with two major increases and collapses over the last 40 years. In general, the seventy of winter loss in large grazers and browsers inhabiting temperate and polar regions is proportional to the harshness of the winter. Colder temperatures increase energy requirements (and therefore the: need for food), while deeper snow makes it harder to find food.

Intrinsic Factors For some animal species, intrinsic (physiological) factors, rather than the extrinsic (environmental) factors we've just discussed, appear to regulate population size. White-footed mice in a small field enclosure will multiply from a few to a colony of 30 to 40 individuals, but eventually reproduction will decline until the population ceases to grow. This drop in reproduction is associated with aggressive interactions that increase with population density, and it occurs even when food and shelter are provided in abundance. Although the exact mechanisms by which aggressive behavior affects reproductive rate are not yet understood, researchers have learned that high population densities in mice induce a stress syndrome in which hormonal changes can delay sexual maturation, cause reproductive organs to shrink, and depress the immune system. In this case, high densities cause an increase in mortality and a decrease in birth rates. Similar effects of crowding occur in some other wild rodent populations. These various examples of population regulation by negative feedback show how increased densities cause population growth rates to decline by affecting reproduction, growth, and survivorship. This helps answer our first question about populations: What causes a population to stop increasing? Now let's turn to our second question: Why do some populations fluctuate dramatically over time while others remain more stable?

Figure 52.1S

ImfMiry How stable is the Isle Royale moose population? Researchers regularly sun/eyed the population of moose on Isle Royale, Michigan, from 1960 to 2003. During that time, the lake never froze over, and so the moose population was isolated from the effects of immigration and emigration. Over 43 years, this population experienced t w o significant increases and collapses, as well as several less severe fluctuations in size.

winter weather and food shortage, leading to starvation of more than 7 5 % of the population 1990

1970 Year

Population Dynamics While some populations appear to be more stable in size than others, over the long term, all populations for which we have data show some fluctuation in numbers. Although ecologists 1150

UNIT EIGHT

Ecology

The pattern of population dynamics observed n this isolated population indicates that various biotic and abiotic actors can result in dramatic fluctuations over time in a moose population.

Although large mammal populations are more dynamic than once thought, they show much more stability than other populations. The Dungeness crab is a classic example of an erratically fluctuating population. As shown in Figure 52.19, the crab population at Fort Bragg on the northern California coast varied between 10,000 and hundreds of thousands over a 40-year period. By contrast, over this same period, the moose population of Isle Royale varied only between 500 and 2.500 (see Figure 52.18). One key factor involved in the large fluctuations in the Dungeness crab population is cannibalism. Females release up to 2 million eggs each fall, and large numbers of juvenile ciabs are eaten by older juveniles and by adult crabs. In addition, successful settlement of larval crabs occurs only in shallow waters and depends on ocean currents and water temperature. If winds and currents move larval crabs too far offshore, they cannot reach the ocean bottom and settle successfully. Small changes in environmental variables seem to be magnified by density-dependent cannibalism, and together these factors explain the marked fluctuations in Dungeness crab populations. These results support the hypothesis that the dynamics of many populations result from a complex interaction of biotic and abiotic factors. Metapopulations and Immigration To this point in our discussion of population dynamics, we have focused mainly on the contributions of births and deaths. However, as you have read, immigration and emigration can also influence populations. This is particularly true when a group of populations is linked, forming a metapopulation. For example, immigration and emigration link the Belding's ground squirrel population we discussed earlier to other populations of the species, forming a metapopulation.

100,000

A Figure 52.19 Extreme population fluctuations. This graph shows the commercial catch of male Dungeness crabs (Cancer magister) at Fort Bragg, California. Note the log scale of the y-axis.

How might immigration influence population dynamics? A metapopulation of song sparrows on Mandarte Island and a cluster of smaller islands off the coast of British Columbia provides an example. On relatively isolated Mandarte Island, differences between births and deaths are the principal influences maintaining the highly dynamic population of song sparrows (see Figure 52.13c). In 1988-1991, only ] 1% of the breeding song sparrows were immigrants. By contrast, during the same period, immigrants made up 57% of the breeding song sparrow population on a cluster of small islands near Vancouver Island. This higher level of song sparrow immigration combined with a higher survival rate on small islands resulted in greater stability in song sparrow populations on the small islands than on Mandarte Island (Figure 52.20). The metapopulation concept underscores the significance of immigration and emigration in the contrasting song sparrow populations, and is important for understanding populations in patchy habitats.

Population Cycles While many populations fluctuate at unpredictable intervals, others undergo regular boom-and-bust cycles, fluctuating in density with a remarkable regularity. For example, some small herbivorous mammals, such as voles and lemmings, tend to have 3- to 4-year cycles, and some birds, such as ruffed grouse and ptarmigans, have 9- to 11-year cycles. One striking example of population cycles is the 10-year cycles of snowshoe hares and lynx in the far northern forests

E

20-

A Figure 52.20 Song sparrow populations and immigration. Song sparrow populations on a cluster of small islands make up a metapopulation. Immigration keeps the linked populations more stable than the isolated population on larger Mandarte Island. Population Ecology

1151

of Canada and Alaska. Lynx are specialist predators of snowshoe hares, so it is not surprising that lynx numbers rise and fall with the numbers of hares (Figure 52.21). But why do hare numbers rise and fall in 10-year cycles? Three main hypotheses have been proposed. First, the cycles may be caused by food shortage during winter. Hares eat the terminal twigs of small shrubs such as willow and birch in winter and may suffer from malnutrition due to overgrazing. Second, the cycles may be due to predator-prey interactions. Many predators other than lynx eat hares, and they may overexploit their prey Third, the cycles may be affected by a combination of food resource limitation and excessive predation. If hare cycles are due to winter food shortage, then they should stop if extra food is added to a field population. Researchers have conducted such experiments in the Yukon for 20 years—over two hare cycles—and report two results. First, hare populations in the areas with extra food increased about threefold in density. The carrying capacity of a habitat for hares can clearly be increased by adding food. Second, the hares with extra food continued to cycle in the same way as the unfed control populations. In particular, cyclic collapses occurred in both experimental and control areas, and the decline in numbers could not be stopped by adding food. Thus, food supplies alone are not the cause of the hare cycle shown in Figure 52.21, so we can discard the first hypothesis.

By putting radio collars on hares, field ecologists can find individual hares as soon as they die and determine the immediate cause of death. Almost 90% of the hares that died were killed by predators; none appeared to have died of starvation. These data support either the second or third hypothesis. Ecologists tested these hypotheses by excluding predators from one area with electric fences and by both excluding predators and adding food to another area. The results supported the hypothesis that the hare cycle is driven largely by excessive predation but that food availability also plays an important role, particularly in the winter. Perhaps better-fed hares are more likely to escape from predators. Many different predators contribute to these losses; the cycle is not simply a hare-lynx cycle. For the lynx, great-horned owls, weasels, and other predators that depend heavily on a single prey species, the availability of prey is the major factor influencing their population changes. When prey become scarce, predators often turn on one another. Coyotes kill foxes and lynx, and great-horned owls kill smaller birds of prey as well as weasels, accelerating the collapse of the predator populations. Long-term experimental studies will help to unravel the complex causes of such population cycles.

Concept Check i 1. Identify three density-dependent factors that limit population size, and explain how each exerts negative feedback. For suggested answers, see Appendix A.

Concept

Human population growth has slowed after centuries of exponential increase As you have read, no population can grow indefinitely, anc humans are no exception. In this last section of the chapter, we'll apply the concepts of population dynamics to the specific case of the human population.

The Global Human Population A Figure 52.21 Population cycles in the snowshoe hare a n d lynx. Population counts are based on the number of pelts sold by trappers to the Hudson Bay Company.

1152

U N I T EIGHT

Ecolo

The exponential growth model in Figure 52.9 approximates the population explosion of humans since 1650. Ours is a singular case; it is unlikely that any other population of large animals has ever sustained so much growth for so long. The human

population increased relatively slowly until about 1650, at which time approximately 500 million people inhabited Earth (Figure 52.22). The population doubled to 1 billion within the next two centuries, doubled again to 2 billion between 1850 and 1930, and doubled still again by 1975 to more than 4 billion. The global population now numbers over 6 billion people and is increasing by about 73 million each year. The population grows by approximately 201,000 people each day, the equivalent of adding a city the size of Amarillo, Texas, or Madison,

Wisconsin. Every week the population increases by the size of San Antonio, Milwaukee, or Indianapolis. It takes only four years for world population growth to add the equivalent of another United States. Population ecologists predict a population of 7.3-8.4 billion people on Earth by the year 2025. Though the global population is still growing, the rate of growth began to slow during the 1960s. Figure 52.23 presents the percentage increase in the global population from 1950 to 2003 and projected to the year 2050. The rate of increase in the global population peaked at 2.19% in 1962; by 2003 it had declined to 1.16%. Current models project a decline in the overall growth rate to just over 0.4% by 2050. This reduction in growth rate shows that the human population has departed from true exponential growth, which assumes a constant rate. These declines are the result of fundamental changes in population dynamics due to diseases, such as AIDS, and voluntary population control. Regional Patterns of Population Change So far we have described changes in the global population. But population dynamics vary widely from region to region. To maintain population stability, a regional human population can exist in one of two configurations:

1 Figure 52.22 H u m a n p o p u l a t i o n g r o w t h ( d a t a as of 2 0 0 3 ) . The global human population has grown almost continuously throughout history, but it skyrocketed after the Industrial Revolution. Though it is not apparent at this scale, the rate of population growth has slowed in recent decades, mainly as a result of decreased birth rates throughout the world.

Zero population growth = High birth rates — High death rates Zero population growth = Low birth rates — Low death rates The movement from the first toward the second state is called the demographic transition. Figure 52.24 compares the demographic transition in one of the most economically developed countries, Sweden, and in a developing country, Mexico. The demographic transition in Sweden took about 150 years,

•2003

10- Sweden

Mexico

— Birth rate — Death rate

— Birth rate — Death rate

04

1750

A Figure 52.23 Percent increase in t h e g l o b a l h u m a n p o p u l a t i o n ( d a t a as of 2 0 0 3 ) . The dashed portion of the curve indicates projected data. The sharp dip in the 1960s is due mainly to a famine in China in which about 60 million people died.

1800

1850

1900 Year

A Figure 52.24 D e m o g r a p h i c t r a n s i t i o n in S w e d e n and Mexico, 1750-2050 (data as of 2003).

CHAPTER 52

Population Ecology

1153

of the current global population growth (1.4% per year) is concentrated in developing countries, where about 80% of the world's people now live.

from 1810 to 1960; in Mexico, the changes are projected to continue until sometime after 2050, almost the same length of time. Demographic transition is associated with an increase in

the quality of health care and sanitation as well as improved

A unique feature of human population growth is the ability

access to education, especially for women. After 1950, mortality rates declined rapidly in most developing countries, but birth rates have declined in a more variable manner. Birth rate decline has been most dramatic in China. In 1970, the Chinese birth rate predicted an average of 5.9 children per woman per lifetime (total fertility rate); by 2004, largely because of the government's strict one-child policy, the expected total fertility rate was 1.7 children. In India, birth rates have fallen more slowly. In some countries of Africa, the transition to lower birth rates has been dramatic, though birth rates remain high in most of sub-Saharan Africa. How do such variable birth rates affect the growth of the world's population? In the developed nations, populations are near equilibrium (growth rate about 0.1% per year), with reproductive rates near the replacement level (total fertility rate = 2.1 children per female). In many developed countries, including Canada, Germany, Japan, Italy, and the United Kingdom, total reproductive rates are in fact below replacement. These populations will eventually decline if there is no immigration and if the birth rate does not change. In fact, the population is already declining in many eastern and central European countries. Most

to control it with family planning and voluntary contraception. Reduced family size is the key to the demographic transition. Social change and the rising educational and career aspirations of women in many cultures encourage women to delay marriage and postpone reproduction. Delayed reproduction helps to decrease population growth rates and to move a society toward zero population growth under conditions of low birth rates and low death rates. However, there is a great deal of disagreement among world leaders as to how much support should be provided for global family planning efforts. Age Structure One important demographic variable in present and future growth trends is a country's age structure, the relative number of individuals of each age. Age structure is commonly represented in "pyramids," such as those shown in Figure 52,21, For Italy, the pyramid has a small base, indicating that individuals younger than reproductive age are relatively underrepresented in the population. This situation contributes to the projection of continuing population decrease in Italy. In contrast, Afghanistan has an age structure that is bottom-

Rapid growth

Slow growth

Afghanistan

United States

Male

4

2

Female

0

2

4

Percent of population

Male

4

2

Decrease Italy

Female

0

2

4

Percent of population

A Figure 52.25 Age-structure pyramids for the human population of three countries ( d a t a f r o m 2 0 0 0 ) . As of 2004, Afghanistan was growing at 2.6% per year, the United States was growing at 0.6% per year, and Italy was declining at - 0 . 1 % per year.

1154

UNIT

EIGHT

Ecology

Male

4

2

Female

0

2

4

Percent of population

Age-structure diagrams not only predict a population's growth trends, but they can also illuminate social conditions. Based on the diagrams in Figure 52.25, we can predict, for instance, that employment and education opportunities will continue to be a significant problem for Afghanistan in the loreseeable future. The large number of young entering the Afghan population could also be a source of continuing social and political unrest, particularly if their needs and aspirations are not met. In Italy and the United States, a decreasing proportion of younger working-age people will soon be supporting an increasing population of retired "boomers." In the United States, this demographic feature has made the future of Social Security and Medicare a major political issue. Understanding age structures can help us plan for the future.

Infant Mortality and Life Expectancy [nfant mortality, the number of infant deaths per 1,000 live births, and life expectancy at birth, the predicted average length of life at birth, vary widely among different human populations. These differences reflect the quality of life faced by children at birth. Figure 52.26 contrasts average infant mortality and average life expectancy in the developed and developing countries of the world in 2000. While these averages are markedly different, they do not capture the broad range of the human condition. In 2003, for example, Afghanistan had an infant mortality rate of 143 (14.3%). By contrast, in Japan only 3 children out of every 1,000 born died in infancy. Moreover, life expectancy at birth in Afghanistan was 47 years, compared to 81 years in Japan. While global life expectancy has been increasing since about 1950, more recently it has dropped in a number of regions, including countries of the former Soviet Union and in subSaharan Africa. In these regions, the combination of social upheaval, decaying infrastructure, and infectious diseases such as AIDS and tuberculosis is reducing life expectancy In the East African country of Rwanda, for instance, life expectancy in 2003 was approximately 39 years, about half that in Japan, Sweden, Italy, and Spain.

1 ifant mor tality (death: > per 1,ooo bi

heavy, skewed toward young individuals who will grow up and may sustain the explosive growth with their own reproduction. The age structure for the United States is relatively even (until the older, postreproductive ages) except for a hulge that corresponds to the "baby boom" that lasted for about two decades after the end of World War II. Even though couples bom during those years have had an average of fewer than two children, the nation's overall birth rate still exceeds the death rate because there are still so many "boomers" and their offspring of reproductive age. Moreover, although the current total reproductive rate in the United States is 2.1 children per woman—approximately replacement rate—the population is projected to grow slowly through 2050 as a result of immigration.

605040-

3020100-

Developed Developing countries countries

Developed Developing countries countries

A Figure 52.26 Infant mortality and life expectancy at birth in developed and developing countries. (Data as of 2003.)

Global Carrying Capacity No ecological question is more important than the future size of the human population. The projected worldwide population size depends on assumptions about future changes in birth and death rates. For 2050, the United Nations projects a global population of approximately 7.5 to 10.3 billion people. In other words, without some catastrophe, an estimated 1.2-4.0 billion people will be added to the population in the next four decades because of the momentum of population growth. But just how many humans can the biosphere support? Will the world be overpopulated in 2050? Is it already overpopulated? Estimates of Carrying Capacity For more than three centuries, scientists have asked, What is the carrying capacity of Earth for humans? The first known estimate, 13.4 billion, was made by Anton van Leeuwenhoek in 1679. Since then, estimates have varied from less than 1 billion to over ] ,000 billion (1 trillion) people, with an average of 10-] 5 billion. Carrying capacity is difficult to estimate, and the scientists who provide these estimates use different methods to get their answers. Some researchers use curves like that produced by the logistic equation (see Figure 52.12) to predict the future maximum of the human population. Others generalize from existing "maximum" population density and multiply this by the area of habitable land- Still other estimates are based on a single limiting factor, such as food, and require many assumptions (such as the amount of available farmland, the average yield of crops, the prevalent diet—vegetarian or meat-based—and the number of calories needed per person per day). Ecological Footprint A more comprehensive approach to estimating [he carrying capacity of Earth is to recognize that humans have multiple CHAPTER 52 Population Ecology

1155

constraints: We need food, water, fuel, building materials, and other requisites, such as clothing and transportation. The ecological footprint concept summarizes the aggregate land

and water area appropriated by each nation to produce all the resources it consumes and to absorb all the waste it generates. Six types of ecologically productive areas are distinguished in calculating the ecological footprint: arable land (land suitable for crops), pasture, forest, ocean, built-up land, and fossil energy land. (Fossil energy land is calculated on the basis of the land required for vegetation to absorb the CO2 produced by burning fossil fuels.) All measures are converted to land area as hectares (ha) per person (1 ha = 2.47 acres). Adding up all the ecologically productive land on the planet yields about 2 ha per person. Reserving some land for parks and conservation means reducing this allotment to 1.7 ha per person—the benchmark for comparing actual ecological footprints. Figure 52.27 graphs the ecological footprints for 13 countries and for the whole world as of 1997. We can draw two key conclusions from the graph. First, countries vary greatly in their individual footprint size and in their available ecological capacity (the actual resource base of each country). The United States has an ecological footprint of 8.4 ha per person but has only 6.2 ha per person of available ecological capacity. In other words, the U.S. population is already above carrying capacity. By contrast, New Zealand has a larger ecological footprint of 9.8 ha per person but an available capacity of 14.3 ha per person, so it is below its carrying capacity The second conclusion is that, in general, the world was already in ecological deficit when the study was conducted. The overall analysis suggests that the world is now at or slightly above its carrying capacity.

We can only speculate about Earth's ultimate carrying capacity for the human population or about what factors will eventually limit our growth. Perhaps food will be the main factor. Malnutrition and famines are common in some regions, but they result mainly from unequal distribution, rather than inadequate production, of food. So far, technological improvements in agriculture have allowed food supplies to keep up with global population growth. However, the principles of energy flow through ecosystems (explained in Chapter 54) tell us that environments can support a larger number of herbivores than carnivores. If everyone ate as much meat as the wealthiest people in the world, less than half of the present world population could be fed on current food harvests. Perhaps we will eventually be limited by suitable space, like the gannets on ocean islands. Certainly, as our population grows, the conflict over how space will be utilized will intensify, and agricultural land will be developed for housing. There seem to be few limits, however, on how closely humans can be crowded together. Humans could also run out of nonrenewable resources, such as certain metals and fossil fuels. The demands of many populations have already far exceeded the local and even regional supplies of one renewable resource-—water. More than 1 billion people do not have access to sufficient water to meet their basic sanitation needs. It is also possible that the human population will eventually be limited by the capacity of the environment to absorb its wastes. In such cases, Earths current human occupants could lower the planets long-term carrying capacity for future generations. Some optimists have suggested that because of our ability to develop technology, human population growth has no practical limits. Technology has undoubtedly increased Earth's carrying capacity for humans, but as we have emphasized, no population can continue to grow indefinitely. Exactly what the world's human carrying capacity is and under what circumstances we will approach it are topics of great concern and debate. Unlike other organisms, we can decide whether zero population growth will be attained through social changes based on human choices or through increased mortality due to resource limitation, plagues, war, and environmental degradation.

Concept Check

A Figure 52.27 Ecological footprint in relation to available ecological capacity. Countries indicated by red dots were in an ecological deficit in 1997 when the study was conducted. Countries indicated by black dots still had resource surpluses relative to the demands of their populations.

1156

UNIT EIGHT

Ecology

1. How does a population's age structure affect its growth rate? 2. What are the relationships among carrying capacity, ecological capacity, and ecological footprint for a country's population? Define each term in your answer. For suggested answers, see Appendix A.

Chapter 52 Review Go to the Campbell Biology website (www.campbellbiology.com) or CDROM to explore Activities, Investigations, and other interactive study aids.

SUMMARY OF KEY

I Dynamic biological processes influence population density, dispersion, and demography I*- Density and Dispersion (pp. 1137-1139) Population density-—the number of individuals per unit area or volume— results from the interplay of births, deaths, immigration, and emigration. Environmental and social factors influence the spacing of individuals into clumped, •uniform, or random dispersion patterns. Activity Techniques for Estimating Population Density and Size • Demography (pp. 1139-1141) Populations increase from births and immigration and decrease from deaths and emigration. Life tables, survivorship curves, and reproductive tables summarize specific demographic trends. Activity Investigating Survivorship Curves

Life history traits are products of natural selection • Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism. (p. 1141).

• The Logistic Growth Model (pp. 1145-1146) According to the logistic equation dN/dt = rmaxN(K - N)/K, growth levels off as population size approaches the carrying capacity • The Logistic Model and Real Populations (p. 1146) The logistic model fits few real populations, but it is useful for estimating possible growth. • The Logistic Model and Life Histories (p. 1147) Two hypothetical, but controversial, life history patterns are K-selection, or density-dependent selection, and r-selection, or density-independent selection.

Populations are regulated by a complex interaction of biotic and abiotic influences • Population Change and Population Density (p. 1148) In density-dependent populations, death rates rise and birth rates fall with increasing density. In density-independent populations, birth and death rates do not change with increasing density. • Density-Dependent Population Regulation (pp. 1148-1150) Density-dependent changes in birth and death rates curb population increase through negative feedback and can eventually stabilize a population near its carrying capacity. Density-dependent limiting factors include intraspecific competition for limited food or space, increased predation, disease, stress due to crowding, and buildup of toxins.

• Life History Diversity (pp. 1141-1142) Big-bang, or semelparous, organisms reproduce a single time and die. In contrast, iteroparous organisms produce offspring repeatedly.

• Population Dynamics (pp. 1150-1151) Because changing environmental conditions periodically disrupt them, all populations exhibit some size fluctuations. Metapopulations are groups of populations linked by immigration and emigration.

• "Trade-offs" and Life Histories (pp. 1142-1143) Life history traits such as brood size, age at maturity, and parental caregiving represent trade-offs between conflicting demands for limited time, energy, and nutrients.

• Population Cycles (pp. 1151-1152) Many populations undergo regular boom-and-bust cycles that are influenced by complex interactions between biotic and abiotic factors. Biology Labs On-Line Population EcohgyLab

The exponential model describes population growth in an idealised, unlimited environment • Per Capita Rate of Increase (pp. 1143-1144) If immigration and emigration are ignored, a population's growth rate (the per capita rate of increase) equals birth rate minus death rate. • Exponential Growth (pp. 1144-1145) The exponential growth equation dN/dt — TmaxN represents a population's potential growth in an unlimited environment, where rmax is the maximum per capita, or intrinsic, rate of increase and N is the number of individuals in the population. This model predicts that the larger a population becomes, the faster it grows. Plotting population size over time in such a population yields a J-shaped graph.

The logistic growth model includes the concept of carrying capacity

Human population growth has slowed after centuries of exponential increase • The Global Human Population (pp. 1152-1155) Since about 1650, the global human population has grown exponentially, but within the last 40 years, the rate of growth has fallen by nearly 50%. Differences in age structure show that while some nations are growing rapidly, others are stable or declining in size. Infant mortality rates and life expectancy at birth differ markedly between developed and developing countries. Activity Human Population Growth Activity Analyzing Age-Structure Pyramids Biology Labs On-line DemographyLab • Global Carrying Capacity (pp. 1155-1156) The carrying capacity of Earth for humans is uncertain. Ecological footprint, the aggregate land and water area needed to sustain the people of a nation, is one measure of how close we are to the carrying capacity of Earth, At more than 6 billion people, the world is already in ecological deficit.

• Exponential growth cannot be sustained for long in any population. A more realistic population model limits growth by incorporating carrying capacity (K), the maximum population size the environment can support (p. 1145). CHAPTER 52 Population Ecology

1157

Science, Technology; and Society

Evolution Connection Write a paragraph contrasting the conditions that favor the evolution of semelparous (one-time) versus iteroparous (repeated) reproduction.

Scientific Inquiry You are testing the hypothesis that population density of a particular plant species influences the rate at which a pathogenic fungus infects the plant. Because the fungus causes visible scars on the leaves, you can easily determine whether a plant is infected. Design an experiment to test your hypothesis. Include your experimental treatments and control, the data you will collect, and the results expected if your hypothesis is correct. Biology Labs On-Line Population EcologyLab Biology Labs On-Line DemographyLab

1158

U N I T EIGHT

H co logy

Many people regard the rapid population growth of developing countries as our most serious environmental problem. Others think that the population growth in developed countries, though smaller, is actually a greater environmental threat. What problems result from population growth in (a) developing countries and (b) the industrialized world? Which do you think is a greater threat, and why?

Community Ecology A Figure 53.1 A savanna community in Chobe National Park, Botswana.

Key Concepts 53.1 A community's interactions include competition, predation, herbivory, symbiosis, and disease 53.2 Dominant and keystone species exert strong controls on community structure 53.3 Disturbance influences species diversity and composition 53.4 Biogeographic factors affect community biodiversity 53.5 Contrasting views of community structure are the subject of continuing debate

community of trees and shrubs in Shenandoah National Park. The various animals as well as the grass and trees surrounding the watering hole in Figure 53.1 are all members of a savanna community in southern Africa. In this chapter, we will examine the factors that are most significant in structuring a community—in determining how many species there are overall, which particular species are present, and the relative abundance of these species. We begin writh a fundamental factor influencing community structure: the interactions between the organisms in a community.

Concept

What Is a Community?

O

n your next walk through a field or a woodland or even across campus or a park, try to observe some of the interactions between the species present. You may see birds using trees as nesting sites, bees pollinating flowers, spiders trapping insects in their webs, ferns growing in shade provided by trees—a tiny sample of the many interactions between species that exist in any ecological theater. In addition to the physical and chemical factors discussed in Chapter 50, an organism's environment includes biotic factors: other individuals of the same species as well as individuals of other species. Such an assemblage of populations of various species living close enough for potential interaction is called a biological community. Ecologists define the boundaries of a particular community to fit their research questions. They might study, for example, the community of decomposers and other organisms living on a rotting log, the benthic community in Lake Superior, or the

A community's interactions include competition, predation, herbivory, symbiosis, and disease Some key relationships in the life of an organism, are its interactions with other species in the community. Ecologists refer to these relationships as interspecific interactions. We will start with the simplest situation: interactions between populations of just two species. The possible interactions that can link species include competition, predation, herbivory, symbiosis (parasitism, mutualism, and commensalism), and disease. Here we will use the symbols + and — to indicate how each interspecific interaction affects survival and reproduction of the two species engaged in the interaction. For example, in mutualism, the survival and reproduction of each species is increased in the presence of the other; therefore, this is a +/+ interaction. Predation is an example of a +/— interaction, with a positive effect on the survival and reproduction of the predator population and a negative 1159

effect on that of the prey population. A 0 indicates that a population is not affected by the interaction in any known way. Historically, most ecological research has focused on interactions with a negative effect on at least one species, such as competition and predation. However, positive interactions are ubiquitous, and their contributions to community structure are the subject of much study.

Competition Interspecific competition occurs when species compete for a particular resource that is in short supply For instance, the weeds growing in a garden compete with garden plants for soil nutrients and water. Grasshoppers and bison in the Great Plains compete for the grass they both eat. Lynx and foxes in the northern forests of Alaska and Canada compete for prey such as snowshoe hares. In contrast, some resources, such as oxygen, are rarely in short supply; thus, although almost all species use this resource, they do not compete for it. When two species do compete for a resource, the result is detrimental to one or both species (—/—). Strong competition can lead to the local elimination of one of the two competing species, a process called competitive exclusion. The Competitive Exclusion Principle In 1934, the Russian ecologist G. F. Gause studied the effects of interspecific competition in laboratory experiments with two closely related species of protists, Paramecium aurelia and Paramecium caudatum. He cultured the protists under stable conditions with a constant amount of food added every day

When Gause grew the two species in separate cultures, each population grew rapidly and then leveled off at what was apparently the carrying capacity of the culture. But when Gause

cultured the two species together, P caudatum was driven to extinction in the culture. Gause inferred that P. aurelia had a competitive edge in obtaining food, and he concluded that two species competing for the same limiting resources cannot coexist in the same place. One species will use the resources more efficiently and thus reproduce more rapidly than the other. Even a slight reproductive advantage will eventually lead to local elimination of the inferior competitor. Ecologists call Gauses concept the competitive exclusion principle. Ecological Niches The sum total of a species' use of the biotic and abiotic resources in its environment is called the species' ecological niche. One way to grasp the concept is through an analogy made by ecologist Eugene Odum: If an organisms habitat is its "address," the niche is the organisms "profession." Put another way, an organism's niche is its ecological role—how it "fits into" an ecosystem. For example, the niche of a tropical tree lizard consists of, among many components, the temperature range it tolerates, the size of branches on which it perches, the time of day when it is active, and the sizes and kinds of insects it eats. We can use the niche concept to restate the competitive exclusion principle: Two species cannot coexist in a community if their niches are identical. However, ecologically similar species can coexist in a community if there are one or more significant differences in their niches (Figure 53.2). As a result of competition, a species' fundamental niche, which is the niche potentially

Can a species' niche be influenced by interspecific competition? EXPERIMENT

Ecologist Joseph Connell studied two barnacle species—Balanus balanoides and Chthamalus stellatus—that have a stratified distribution on rocks along the coast of Scotland.

When Connell removed Balanus from the lower strata, the Chthamalus population spread into that area.

High tide

'amalus & Balanus

In nature, Balanus fails to survive high on the rocks because it is unable to resist desiccation (drying out) during low tides. Its realized niche is therefore similar to its fundamental niche. In contrast, Chthamalus is usually concentrated on the upper strata of rocks. To determine the fundamental niche of Chthamalus, Connell removed Balanus from the lower strata.

1160

UNIT EIGHT

Ecology

:«.]««mn[.»

The spread of Chthamalus when Balanus was removed indicates that competitive exclusion makes the realized niche of Chthamalus much smaller than its fundamental niche.

occupied by that species, may be different from its realized niche, the niche it actually occupies in a particular environment.

G. fuliginosa

Resource Partitioning When competition between species having identical niches does not lead to local extinction of either species, it is generally because one species' niche becomes modified. In other words, evolution by natural selection can result in one of the species using a different set of resources. The differentiation of niches that enables similar species to coexist in a community is called resource partitioning (Figure 53.3). You can think of resource partitioning in a community as "the ghost of competition past"—indirect evidence of earlier interspecific competition resolved by the evolution of niche differentiation. Character Displacement

i Daphne

A related line of indirect evidence of the effects of competition comes from comparisons of closely related species whose populations are sometimes allopatric (geographically separate; see Chapter 24) and sometimes sympatric (geographically overlapping). In some cases, the allopatric populations of such species are morphologically similar and use similar resources. By contrast, sympatric populations, which would potentially compete for resources, show differences in body structures and in the resources they use. This tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species is called character displacement. An example of character displacement is the variation in beak size between different popula-

2 40-

G. fortis, aiiopatric

§208

10

12

14

Beak depth (mm)

A Figure 53.4 Character displacement: indirect evidence of past c o m p e t i t i o n . Allopatric populations of Geospiza fuliginosa and Geospiza fortis on Los Hermanos and Daphne Islands have similar beak morphologies {bottom t w o graphs) and presumably eat similarly sized seeds. However, where the two species are sympatric on Santa Maria and San Cristobal, G. fuliginosa has a shallower, smaller beak and 6. fortis a deeper, larger one (top graph), adaptations that favor eating different sizes of seeds.

tions of the Galapagos finches Geospiza fuliginosa and Geospiza fortis (Figure 53.4).

Predation

Figure 53.3 Resource partitioning among Dominican Republic lizards. Seven species of Anoiis lizards live in close proximity, and all feed on insects and other small arthropods. However, competition for food is reduced because each lizard species has a different perch, thus occupying a distinct niche.

Predation refers to a +/— interaction between species in which one species, the predator, kills and eats the other, the prey. Though the term predation generally elicits such images as a lion attacking and eating an antelope, it applies to a wide range of interactions defined by the killing of prey. For instance, animals such as certain weevils that chew up or digest plant seeds, thereby killing them, are called seed predators. Because eating and avoiding being eaten are prerequisite to reproductive success, the adaptations of both predators and prey tend to be refined through natural selection. Many important feeding adaptations of predators are both obvious and familiar. Most predators have acute senses that enable them to locate and identify potential prey. In addition, many predators have adaptations such as claws, teeth, fangs, stingers, or poison that help catch and subdue the organisms on which they feed. Rattlesnakes and other pit vipers, for example, locate their prey with heat-sensing CHAPTER 53

Community Ecology

1161

organs located between each eye and nostril, and they kill small birds and mammals by injecting them with toxins through their fangs. Predators that pursue their prey are generally fast and agile, whereas those that lie in ambush are often disguised in their environments, Just as predators possess adaptations for securing prey, prey animals have adaptations that help them avoid being eaten. Behavioral defenses include hiding, fleeing, and selfdefense. Active self-defense is not common, though some large grazing mammals vigorously defend their young from predators such as lions. Other behavioral defenses include alarm calls that summon many individuals of the prey species, which then mob the predator. Animals also display a variety of morphological and physiological defensive adaptations. For example, cryptic coloration, or camouflage, makes prey difficult to spot (Figure 53.5). Other animals have mechanical or chemical defenses. For example, most predators are strongly discouraged by the familiar de-

• Figure 53.5 Cryptic coloration: canyon tree frog.

(a) Hawkmoth larva

UNIT EIGHT

toxins from the plants they eat. Animals with effective chemical

defenses often exhibit bright warning coloration, or aposematic coloration, like that of the poison arrow frog (Figure 53.6). Aposematic coloration seems to be adaptive; there is evidence that predators are particularly cautious in dealing with potential prey having bright color patterns (see Chapter 1). Sometimes, one prey species may gain significant protection by mimicking the appearance of another. In Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful model. For example, the larva of the hawkmoth puffs up its head and thorax when disturbed, looking like the head of a small poisonous snake (Figure 53.7). In this case, the mimicry even involves behavior; the larva weaves its head back and forth and hisses like a snake, in Mullerian mimicry, two or more unpalatable species, such as the cuckoo bee and yellow jacket, resemble eacn other (Figure 53.8). Presumably, each species gains an additional advantage because the greater the number of unpalatable prey the more quickly and intensely predators adapt, avoiding any prey with thai particular appearance. The mimicry thus acts as a kind of aposematic coloration. In an example of convergent evolution, unpalatable animals in several different taxa have similar patterns of coloration: Black and yellow or red stripes characterize unpalatable animals as diverse as yellow jackets and coral snakes A Figure 53.6 Aposematic (see Figure 1.27). coloration: poison arrow frog. Predators also use mimicry in a variety of ways. For example, some snapping turtles have tongues that resemble a wriggling worm, thus luring small fish; any fish that tries to eat the "bait" is itself quickly consumed as the turtle's strong jaws snap closed.

(b) Yellow jacket

A Figure 53.7 Batesian mimicry: A harmless species mimics a harmful one. 1162

fenses of porcupines and skunks. Some animals, such as the poison arrow frog of Costa Rica, can synthesize toxins, while others passively acquire a chemical defense by accumulating

Ecology

A Figure 53.8 Mullerian mimicry: Two unpalatable species mimic each other.

Eerbivory Geologists use the term herbivory to refer to a +/— interaction in which an herbivore eats parts of a plant or alga. While the large mammalian herbivores such as cattle, sheep, and water buffalo may be most familiar, most herbivores are small invertebrates, such as grasshoppers and beetles. In the ocean, herbivores include snails, sea urchins, and some tropical fishes, such as the surgeon hsh. Like predators, herbivores have specialized adaptations. Because plants are so chemically diverse, it is advantageous for herbivores to be able to distinguish between toxic and nontoxic plants as well as between more nutritious and less nutritious plants. Many herbivorous insects have chemical sensors on their feet that recognize appropriate food plants. Some mammalian herbivores, such as goats, use their sense of smell to examine plants, rejecting some and eating others. In some cases, they eat just a specific part of a plant, such as the flowers. Many herbivores also have specialized teeth or digestive systems adapted for processing vegetation (see Chapter 41). Unlike prey animals, of course, plants cannot run away to avoid being eaten. Instead, a plant's main arsenal against herbivores includes chemical toxins, often in combination with spines and thorns. Among the plant compounds that serve as chemical weapons are the poison strychnine, produced by the tropical vine Strychnos toxijera; nicotine, from the tobacco plant; and tannins, from a variety of plant species. Compounds that are not toxic to humans but may be distasteful to many herbivores are responsible for the familiar flavors of cinnamon, cloves, and peppermint. Certain plants produce chemicals that cause abnormal development in some insects :hal eat them.

Parasitism Parasitism is a +/— symbiotic interaction in which one organism, the parasite, derives its nourishment from another organism, its host, which is harmed in the process. (This text adopts the most general definition of symbiosis as an interaction in which two organisms of different species live together in direct contact, including parasitism, mutualism, and commensalism. However, some biologists use symbiosis more specifically as a synonym for mutualism.) Parasites that live within the body of their host, such as tapeworms and malarial parasites, are called endoparasites; parasites that feed on the external surface of a host, such as ticks and lice, are called ectoparasites. In a particular type of parasitism called parasitoidism, insects—usually small wasps—lay eggs on or in living hosts. The larvae then feed on the body of the host, eventually killing it. Many parasites have a complex life cycle involving a number of hosts. For instance, the life cycle of the blood fluke, which currently infects approximately 200 million people

around the world, involves two hosts: humans and freshwater snails (see Figure 33.11)- Some parasites change the behavior of their hosts in a way that increases the probability of the parasite being transferred from one host to another. For instance, the presence of parasitic acanthocephalan (spinyheaded) worms leads their crustacean hosts to engage in a variety of atypical behaviors, including leaving the protection of cover and moving into the open. As a result of their modified behavior, the crustaceans have a greater chance of being eaten by the birds that are the second host in the parasitic worm's life cycle. Parasites can have a significant effect on the survival, reproduction, and density of their host population, either directly or indirectly. For example, ticks that live as ectoparasites on moose weaken their hosts by withdrawing blood and causing hair breakage and loss, increasing the chance that the moose will die from cold stress or predation by wolves. Some of the declines of the moose population on Isle Royale, Michigan, have been attributed to tick outbreaks (see Figure 52.18).

Disease In terms of their effect on host organisms, pathogens, or disease-causing agents, are similar to parasites (+/—). Pathogens are typically bacteria, viruses, or protists, but fungi and prions (protein bodies; see Chapter 18) may also be pathogenic. In contrast to many parasites, which are relatively large, multicellular organisms, most pathogens are microscopic. Further, while most parasites cause nonlethal damage to their hosts—by pilfering nutrients, for example—many pathogens inflict lethal harm. Despite the potential of pathogenic organisms to limit populations, pathogens have been the subject of relatively few ecological studies. This imbalance is now being addressed as dramatic events highlight the ecological importance of disease. One such event was the appearance of sudden oak death, a tree disease caused by a fungus-like protistan pathogen called Phytophthora ramorum. By 2004, sudden oak death had spread 650 km from where it was first discovered in Marin County, California, in 1994. In one decade, the disease killed tens of thousands of oaks of several species from the central California coast to southern Oregon and in the process changed the structure of the affected forest communities. Animal populations also tend to decline in the face of disease epidemics. From 1999 to 2004, the mosquito-transmitted West Nile virus spread from New York to 46 other states, killing hundreds of thousands of birds as it moved across the United States. Crows appear to be particularly susceptible to this disease. Meanwhile, the number of human cases in the United States rose from 62 in 1999 to 9,862 in 2003, resulting in 7 and 264 deaths, respectively. These statistics indicate a fraction of the total exposure, since most people exposed to the virus show no symptoms. CHAPTER S3

Community Ecology

1163

A Figure 53.9 Mutuafism between acacia trees and ants. Certain species of acacia trees have hollow thorns that house stinging ants of the genus Pseudomyrmex. The ants feed on sugar produced by nectaries on the tree and on protein-rich swellings (orange in the photograph) at the tips of leaflets. The acacia benefits because the pugnacious ants, which attack anything that touches the tree, remove fungal spores and other debris and clip vegetation that grows close to the acacia.

Mutualism Mutualistic symbiosis, or mutualism, is an interspecific interaction that benefits both species (+/+). Figure 53.9 illustrates mutualism between ants and acacia trees in Central and South America. We have described many other examples of mutualism in previous chapters: nitrogen fixation by bacteria in the root nodules of legumes; the digestion of cellulose by microorganisms in the digestive systems of termites and ruminant mammals; photosynthesis by unicellular algae in the tissues of corals; and the exchange of nutrients in mycorrhizae, the association of fungi and the roots of plants. Mutualistic relationships sometimes involve the evolution of related adaptations in both species, with changes in either species likely to affect the survival and reproduction of the other. For example, most flowering plants have adaptations such as fruit or nectar that attract animals that function in pollination or seed dispersal. The hypothesis is that natural selection might favor evolution of such adaptations because by sacrificing organic materials such as nectar rather than pollen or seeds to a consumer, the plant loses fewer gametes and thereby increases its relative reproductive success. In turn, many animals have adaptations that help them find and consume nectar.

Commensalism Commensalism is defined as an interaction between species that benefits one of the species but neither harms nor helps the other (+/0). Commensal interactions have been difficult to document in nature because any close association between species likely affects both species, if only slightly. For instance,

1164

UNIT EIGHT

Ecology

A. Figure 53.10 A possible example of commensalism between cattle egrets and water buffalo.

"hitchhiking" species, such as algae that grow on the shells or aquatic turtles or barnacles that attach to whales, are sometimes considered commensal. The hitchhikers gain access to a substrate while seeming to have little effect on their ride. However, the hitchhikers may in fact slightly decrease the reproductive success of their hosts by reducing the hosts' efficiency of movement in searching for food or escaping from predators. Conversely, the hitchhikers may provide a benefit in the form of camouflage. Some putative commensal associations involve one species obtaining food that is inadvertently exposed by another. For instance, cowbirds and cattle egrets feed on insects flushed out of the grass by grazing bison, cattle, horses, and other herbivores (Figure 53.10). Because the birds increase their feeding rates when following the herbivores, they clearly benefit from the association. Much of the time, the herbivores may be unaffected by the relationship. However, they, too, may sometimes derive some benefit; the birds tend to be opportunistic feeders that occasionally remove and eat ticks and other ectoparasites from the herbivores. They may also give warning to the herbivores of a predator's approach.

Interspecific Interactions and Adaptation Earlier in this book, we discussed the concept of coevolution, reciprocal evolutionary adaptations of two interacting species. A change in one species acts as a selective force on another species, whose adaptation in turn acts as a selective force on the first species. This linkage of adaptations requires that genetic change in one of the interacting populations of the two species be tied to genetic change in the other population. An example of such dual adaptation that probably qualifies as

coevolution is the gene-for-gene recognition between a plant species and a species of avirulent pathogen (see Figure 39.31). In contrast, the aposematic coloration of various tree frogs and the corresponding aversion reactions of various predators do not qualify as coevolution because these are adaptations to multiple species in the communiLy rather than coupled genetic changes in just two interacting species. In fact, the term coevolution may often be used too loosely in describing the adaptations of certain organisms to the presence of other organisms in a community There is little evidence for true coevolution in most cases of interspecific interactions. Nevertheless, the more generalized adaptation of organisms to other organisms in their environment is a fundamental characteristic of life. At present, much evidence seems to indicate that competition and predation are the key processes driving community dynamics. But this conclusion is based mainly on research in temperate communities. Far fewer data exist for interspecific interactions in tropical communities. In addition, the hypothesis that competition and predation control community structure is being challenged by ecologists exploring the influences of parasitism, disease, mutualism, and commensalism on communities.

Species Diversity The species diversity of a community—the variety of different kinds of organisms that make up the community—has two components. One is species richness, the total number of different species in the community The other is the relative abundance of the different species, the proportion each species represents of the total individuals in the community. For example, imagine two small forest communities, each with 100 individuals distributed among four different tree species (A, B, C, and D) as follows: Community 1: 25A, 25B, 25C, 25D Community 2: 80A, 5B, 5C, 10D The species richness is the same for both communities because they both contain four species, but the relative abundance is very different (Figure 53.11). You would easily notice the four different types of trees in community 1, but without looking carefully, you might see only the abundant species A in the second forest. Most observers would intuitively describe community 1 as the more diverse of the two communities. Indeed, for an ecologist, the species diversity of the community is dependent on both species richness and relative abundance.

Concept Check 1. Explain how interspecific competition, predation, and mutualism differ in their effects on the interacting populations of two species. 2. According to the competitive exclusion principle, what outcome is expected when two species compete for a resource? Why? 3. Is the evolution of Batesian mimicry an example of coevolution? Explain your answer. For suggested answers, sec Appendix A.

Community 1 A: 25% B: 25% C: 25% D: 25%

Concept

(Mr

Dominant and keystone species exert strong controls on community structure In general, a small number of the species in a community exert strong control on that community's structure, particularly on the composition, relative abundance, and diversity of its species. Before examining the effects of these particularly influential species, we Erst need to consider two fundamental features of community structure: species diversity and feeding relationships.

Community 2 A: 80% B: 5% C: 5% D: 10% A Figure 53.11 Which forest is more diverse? Ecologists would say that community I has greater species diversity, a measure that includes both species richness and relative abundance.

CHAPTER 53

Community Ecology

1165

Determining the number of species in a community is easier said than done. Various sampling techniques can be employed, but since most species in a community are relatively rare, it may be hard to come up with a sample size large enough to be representative. Ii LS particularly difficult to accurately census the highly motile or less visible members of communities, such as mites and nematodes. Nevertheless, measuring species diversity is essential not only for understanding community structure, but for conserving biodiversity, as you will read in Chapter 55.

Trophic Structure The structure and dynamics of a community depend to a large extent on the feeding relationships between organisms—the trophic structure of the community. The transfer of food energy up the trophic levels from its source in plants and other photosynthetic organisms (primary producers) through herbivores (primary consumers) to carnivores (secondary and tertiary consumers) and eventually to decomposers is referred to as a food chain (Figure 53.12).

Food Webs In the 1920s, Oxford University biologist Charles Elton recognized that food chains are not isolated units but are linked together in food webs. An ecologist can summarize the trophic relationships of a community by diagramming a food web with arrows linking species according to who eats whom. For example, Figure 53.13 is a simplified food web for an antarctic pelagic community. The primary producers m the community are phytoplankton, which serve as food for the dominant grazing zooplankton, especially euphaustds (krill) and copepods, both of which are crustaceans. These zooplankton species are in turn eaten by various carnivores, including other plankton, penguins, seals, fishes, and baleen whales. Squids, which are carnivores that feed on fishes as well as zooplankton, are another important link in these food webs, as they are in turn eaten by seals and toothed whales. During the time

Quaternary consumers

Tertiary . consumers

Primary consumer; Zooplankton

Primary producers Plant

Phytoplankton

A terrestrial food chain

A marine food chain

A Figure 53.12 Examples of terrestrial and marine food chains. The arrows trace energy and nutrients that pass through the trophic levels of a community when organisms feed on one another. Decomposers, which "feed" on organisms from all trophic levels, are not shown here. 1166

UNIT EIGHT

Ecology

A Figure 53.13 An antarctic marine food web. Arrows follow the transfer of food from the producers (phytoplankton) up through the trophic levels. For simplicity, this diagram omits decomposers.

Juvenile striped bass

Fish eggs

Zoopiankton

A Figure 53.14 Partial food web for the Chesapeake Bay estuary on the U.S. Atlantic coast. The sea nettle (Chrysaora quinquecirrha) and juvenile striped bass (Morone saxatilis) are the main predators of fish larvae (bay anchovy and several other species). Note that sea nettles are secondary consumers (black arrows) when they eat zoopiankton and tertiary consumers (red arrows) when they eat fish larvae, which are themselves secondary consumers of zoopiankton.

when whales were commonly hunted for food, humans became the top predator in this food web. Having hunted many whale species to low numbers, humans are now harvesting at lower trophic levels, catching krill as well as fishes. How are food chains linked into food webs? First, a given species may weave into the web at more than one trophic level. For example, in the antarctic food web, euphausids feed on phytoplankton as well as on other grazing zoopiankton, such as copepods. Such "nonexclusive" consumers are also found in terrestrial communities. For example, foxes are omnivores with diets that include berries and other plant materials, herbivores such as mice, and other predators, such as weasels. Humans are among the most versatile of omnivores. Food webs can be very complicated, but we can simplify them for easier study in two ways. First, we can group species with similar trophic relationships in a given community into broad functional groups. For example, in Figure 53.13, more than 100 phytoplankton species are grouped as the primary producers in the food web. A second way to simplify a food web for closer study is to isolate a portion of the web that interacts very little with the rest of the community. Figure 53.14 illustrates a partial food web for sea nettles (jellies) and juvenile striped bass in Chesapeake Bay. Limits on Food Chain Length Each food chain within a food web is usually only a few links long. In the antarctic web of Figure 53.13, there are rarely more than seven links from the producers to any top-level

predator, and there are even fewer links in most chains. In fact, most food webs studied to date consist of five or fewer links, as Charles Elton first pointed out in the 1920s. Why are food chains relatively short? There are two main hypotheses. One, the energetic hypothesis, suggests that the length of a food chain is limited by the inefficiency of energy transfer along the chain. As you will read in Chapter 54, only about 10% of the energy stored in the organic matter of each trophic level is converted to organic matter at the next trophic level. Thus, a producer level consisting of 100 kg of plant material can support about 10 kg of herbivore biomass and 1 kg of carnivore biomass. The energetic hypothesis predicts that food chains should be relatively longer in habitats of higher photo-synthetic productivity, since the starting amount of energy is greater. A second hypothesis, the dynamic stability hypothesis, proposes that long food chains are less stable than short chains. Population fluctuations at lower trophic levels are magnified at higher levels, potentially causing the local extinction of top predators. In a variable environment, top predators must be able to recover from environmental shocks (such as extreme winters) that can reduce the food supply all the way up the food chain. The longer the food chain, the slower the recovery from environmental setbacks for top predators. This hypothesis predicts that food chains should be shorter in unpredictable environments. Most of the data available support the energetic hypothesis. For example, ecologists have used tree-hole communities in tropical forests as experimental models to test the energetic hypothesis. Many trees have small branch scars that rot, forming small holes in the tree trunk. The tree holes hold water and provide a habitat for tiny communities consisting of decomposer microorganisms and insects that feed on leaf litter, as well as predatory insects. Figure 53.15 shows the results of

65

g-

-6 •1 No. of species ^ N o . o f trophic - 5 links •4

|^H

| 3 -

•3

Q

-2

= 21-

LHH_

0High {control)

Medium

• 1

Productivity k Figure 53.15 Test of the energetic hypothesis for the restriction of food chain length. Researchers manipulated the productivity of tree-hole communities in Queensland, Australia, by providing leaf litter input at three levels: high litter input = natural (control) rate of litter fall; medium = Vie natural rate; and low = Vioo natural rate. Reducing energy input reduced food chain length, a result consistent with the energetic hypothesis.

CHAPTER 53

Community Ecology

1167

a set of experiments in which researchers manipulated productivity (leaf litter falling into the tree holes). As predicted by the energetic hypothesis, holes with the most leaf litter, and hence the greatest total food supply at the producer level, supported the longest food chains. Another factor that may limit food chain length is that animals in a food chain tend to be larger at successive trophic levels (except for parasites). The size of an animal and its feeding mechanism put some upper limit on the size of food it can take into its mouth. And except in a few cases, large carnivores cannot live on very small food items because they cannot procure enough food in a given time to meet their metabolic needs. Among the exceptions are baleen whales, huge suspension feeders with adaptations that enable them to consume enormous quantities of krill and other small organisms (see Figure 41.2).

Species with a Large Impact Certain species have an especially large impact on the structure of entire communities either because they are highly abundant or because they play a pivotal role in community dynamics. The impact of these species can occur either through their trophic interactions or through their influences on the physical environment.

killed all the chestnut trees in eastern North America. The once-dominant tree of many of these forests was eliminated. In this case, removing the dominant species apparently had

a relatively small impact on most of the other species, Species of oak, hickory, beech, and red maple that were already present in the forest increased in abundance and replaced the chestnut. No mammals or birds seemed to be seriously affected by the loss of this dominant species. Some insect species, however, were significantly affected. Fifty-six species of moths and butterflies fed on the American chestnut. Of these, 7 species became extinct, though the other 49 species, which did not feed exclusively on the chestnut, still survive. This is only one example of a community response to the loss of a dominant species. More research is needed before we can generalize about the overall effects of such losses. Keystone Species In contrast to dominant species, keystone species are not necessarily abundant in a community (Figure 53.16a). They (a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates.

Dominant Species

1168

UNIT EIGHT

Ecology

.8

,With Pisaster (control)

20-

C

1510-

Num

Dominant species are those species in a community that are the most abundant or that collectively have the highest biomass (the total mass of all individuals in a population). As a result, dominant species exert a powerful control over the occurrence and distribution of other species. For example, the abundance of sugar maples, the dominant plant species in many eastern North American forest communities, has a major impact on abiotic factors such as shading and soil, which in turn affect which other species live there. There is no single explanation for why a species becomes dominant in a community. One hypothesis suggests that dominant species are most competitive in exploiting limited resources such as water or nutrients. Another explanation is that dominant species are most successful at avoiding predation or the impact of disease. This latter idea could explain the high biomass that invasive species (species, generally introduced by humans, that take hold outside their native range) can attain in environments lacking their natural predators and pathogens. One way to discover the impact of a dominant species is to remove it from the community. Humans have carried out this type of experiment many times by accident. For example, the American chestnut was a dominant tree in eastern North American deciduous forests before 1910, making up more than 40% of the canopy (top-story) trees. Then humans accidentally introduced the fungal disease called chestnut blight to New York City via nursery' stock imported from Asia. Between 1910 and 1950, this fungus, which attacks only chestnuts,

5

/ Without Pisaster (experimental)

\ < \

0

1963 '64 '65 '66 '67 '68 '69 70 71 72 73

(b) When Pisaster was removed from an intertidal zone, mussels eventually took over the rock face and eliminated most other invertebrates and algae. In a control area from which Pisasterwas not removed, there was little change in species diversity.

A Figure 53.16 Testing a keystone predator hypothesis.

exert strong control on community structure not by numerical might but by their pivotal ecological roles, or niches. A good way to identify keystone species is by removal experiments, which is how ecologist Robert Paine of the University of Washington first developed the concept of keystone species. The sea star Pisaster ochraceous is a predator of mussels such as Mytilus calijornianus in rocky intertidal communities of western North America. M. calijornianus is a dominant species, a superior competitor for space. Predation by Pisaster offsets this competitive edge and allows other species to use the space vacated by Mytilus. After Paine removed Pisaster from rocky intertidal areas, the abundant mussels were able to monopolize space and exclude other invertebrates and algae from attachment sites {Figure 53.16b). When sea stars were present, 15 to 20 species of invertebrates and algae occurred in the intertidal zone. But after experimental removal of Pisaster, species diversity quickly declined to fewer than 5 species. Pisaster thus acts as a keystone species, exerting an influence on the whoie community chat is not reflected in its abundance.

972

Food chain before killer whale involvement in chain

1985

(c) Total kelp density

Food chain after killer whales started preying on otters

The sea otter, Enhydra tutris, a key- * Figure 53.17 Sea otters as keystone predators in the North Pacific. The graphs stone predator in the North Pacific, offers correlate changes over time in sea otter abundance (a) with changes in sea urchin biomass (b) another example Sea otters feed on sea a n d chan 9es in kelp density (c) in kelp forests at Adak Island (part of the Aleutian Island chain).

" ' The food chains alongside the graphs symbolically indicate the relative abundance of species urchins, and sea urchins feed mainly on before and after killer whales entered the chain. kelp. In areas where sea otters are abundant, sea urchins are rare and kelp forests are well developed. Where sea otters are rare, sea urchins are common and kelp is almost absent. Over the last 20 years, killer whales have been preying on sea otters as the whales1 usual prey has declined. As a result, sea otter populations have declined precipitously in large areas off the coast of western Alaska (Figure 53.17), sometimes at rates as high as 25% per year. The loss of this keystone species has allowed sea urchin populations to increase, resulting in the destruction of kelp forests. Ecosystem "Engineers" (Foundation Species) Some organisms exert their influence not through their trophic interactions but by causing physical changes in the environment that affect the structure of the community. Such organisms may alter the environment through their behavior or by virtue of their large collective biomass. An example of a species that dramatically alters its physical environment is the beaver (Figure 53.18), which, through tree

A Figure 53.18 Beavers as ecosystem "engineers" in temperate and boreal forests. By felling trees, building dams, and creating ponds, beavers can transform large areas of forest into flooded wetlands. CHAPTER 53

Community Ecology

1169

versa. In this situation, herbivores are limited by vegetation, but vegetation is not limited by herbivory. In contrast, V*— H means that an increase in herbivore biomass will have an impact on vegetation (decreasing it), but not vice versa. A double-headed arrow indicates that feedback flows in both directions, with each trophic level sensitive to changes in the biomass of the other.

With Salt marsh with Juncus (foreground)

Without

Junqus Juncus Conditions

A Figure 53.19 Facilitation by black rush {Juncus gerardi) in New England salt marshes. Black rush facilitates the occupation of the middle upper zone of the marsh, which increases local plant species richness.

felling and dam building, can transform landscapes on a very large scale. Species with such effects are called ecosystem "engineers" or, to avoid implications of conscious intent, "loundation species." By altering the structure or dynamics of the environment, foundation species act as facilitators thai have positive effects on the survival and reproduction of some of the other species in the community. For example, by modifying soils, the black rush Juncus gerardi increases the species richness m some zones of New England salt marshes. Juncus helps prevent salt buildup in the soil by shading the soil surface, which reduces evaporation. Juncus also prevents the salt marsh soils from becoming anoxic as it transports oxygen to its below-ground tissues. Sally Hacker and Mark Bertness, of Brown University, uncovered some of Junais's facilitation effects through removal experiments (Figure 53.19). When they removed Juncus from study plots in a New England salt marsh, three other plant species died out. Hacker and Bertness suggest that without Juncus, the upper middle intertidal zone would support approximately 50% fewer plant species.

Bottom-Up and Top-Down Controls Simplified models based on relationships between adjacent trophic levels are useful for discussing how biological communities might be organized. For example, lets consider the three possible relationships between plants (V for vegetation) and herbivores (H):

The arrows indicate that a change in the biomass (total mass) of one trophic level causes a change in the other trophic level. V —* H means that an increase in vegetation will impact (increase) the numbers or biomass of herbivores, but not vice 1170

UNIT

EIGHT

Based on the possible interactions in our example, we can define two models of community organization: the bottom-up model and the top-down model, The V—» H linkage suggests a bottom-up model, which postulates a unidirectional influence from lower to higher trophic levels. In this case, the presence or absence of mineral nutrients (N) controls plant (V) numbers, which control herbivore (H) numbers, which in turn control predator (P) numbers. The simplified bottom-up model is thus N -^ V —* H —> P. To change the community structure of a bottom-up community, you need to alter biomass at the lower trophic levels. For example, if you add mineral nutrients to stimulate growth of vegetation, then the higher trophic levels should also increase in biomass. If you add predators to or remove predators from a bottom-up community, however, the effect will not extend down to the lower trophic levels. In contrast, the top-down model postulates that the influence moves in the opposite direction: It is mainly predation thai controls community organization because predators limit herbivores, which in turn limit plants, which in turn limit nutrient levels through their uptake of nutrients during growth and reproduction. The simplified top-down model is thus TV -e— V *— H Nitrogen fixation by Dryas and alder increases the soil nitrogen content. A Figure 53.24 Changes in plant community structure and soil nitrogen during succession at Glacier Bay, Alaska. Plant species composition changes dramatically during succession at Glacier Bay.

by the alder. By altering soil properties, pioneer plant species permit new plant species to grow, and the new plants in turn alter the environment in different ways, contributing to succession.

1. Why do high and low levels of disturbance usually reduce species diversity? Why does an intermediate level of disturbance promote species diversity? 2. How do primary and secondary succession differ? 3. During succession, how might the early species facilitate the arrival of other species? For suggested answers, see Appendix A.

Concept

Biogeographic factors affect community biodiversity Communities vary greatly in the number of species they contain. Two key factors correlated with a community's species diversity are its geographic location and its size. In the 1850s, both Charles Darwin and Alfred Wallace pointed out that plant and animal life was generally more abundant and varied in the tropics than in other parts of the globe. Darwin and Wallace also noted that small or remote islands have fewer species than large islands or those nearer continents. Such observations imply that biogeographic patterns in biodiversity conform to a set of basic principles rather than being CHAPTER 53 Community Ecoloe1

1175

historical coincidences, One way to understand such largescale patterns in the diversity of life is to examine variation along environmental gradients.

.• •• ^«

Equatorial-Polar Gradients As Darwtn and Wallace noted, tropical habitats support many more species than do temperate and polar regions. For example, one study found that a 6.6-hectare [1 hectare (ha) = 10,000 m ] area in Sarawak (part of Malaysia) contained 711 tree species, while a 2-ha plot ol deciduous forest in Michigan contained just 10 to 15 tree species. And the whole of Western Europe north of the Alps, an area of more than 2 million km2, has only 50 tree species. Similarly, there are more than 200 species of ants in Brazil, 73 in Iowa, and only 7 in Alaska. The two key factors in equatorial-polar gradients of species richness are probably evolutionary history and climate. Over the course of evolutionary time, species diversity may increase in a community as more speciation events occur. And tropical communities are generally older than temperate or polar communities. This age difference stems partly from the fact that the growing season in tropical forests is about five times longer than the growing season in the tundra communities of high latitudes- In effect, biological time, and hence time for speciation, runs about five times faster in the tropics than near the poles. And many polar and temperate communities have repeatedly "started over'' as a result of major disturbances in the form of glaciations. Climate is likely the primary cause of the latitudinal gradient in biodiversity The two main climatic factors correlated with biodiversity are solar energy input and water availability. These factors can be considered together by measuring a community's rate of evapotranspiration, the evaporation of water from soil plus the transpiration of water from plants. Actual evapotranspiration, which is determined by the amount of solar radiation, temperature, and water availability, is much higher in hot areas with abundant rainfall than in areas with low temperatures or low precipitation. Potential evapotranspiration, a measure of energy availability but not water availability, is determined by the amount of solar radiation and temperature and is highest in regions of high solar radiation and temperature. The species richness of plants and animals correlates with measures of evapotranspiration (Figure 53.25).

Area Effects In 1807, Alexander von Humboldt described one of the first patterns of biodiversity to be recognized, the species-area curve, which quantifies what probably seems obvious: All other factors being equal, the larger the geographic area of a community, the greater the number of species. The likely explanation for this pattern is that larger areas offer a greater diversity of habitats and microhabitats than smaller areas. In conservation biology, developing species-area curves for the 1176

UNIT EIGHT

Ecology

1 12CH

1 1OO j

40 H

'

t

[/•

•

•• V* JY• • -..• • ••

100

300 500 700 900 Actual evapotran:jpiration (mrn/yr)

t,K

(a) Trees

500

1,000

1,500

2,000.

Potential evapotranspiration (mm/yr) (fa) Vertebrates

A Figure 53.25 Energy, water, and species richness. (a) Species richness of North American trees increases most predictably with actual evapotranspiration, while (b) vertebrate species richness in North America increases most predictably with potential evapotranspiration. Evapotranspiration values are expressed as rainfall equivalents in millimeters per year.

key taxa in a community helps ecologists to predict how the potential loss of a certain area of habitat is likely to affect the community's biodiversity. Figure 53.26 is a species-area curve for North American breeding birds (birds with breeding populations in the mapped area, as opposed to migrant populations). The slope indicates the extent to which species richness increases with community area. The slopes of different species-area curves vary, depending on the taxon being sampled and the type of

X 1 "

TO

100

10 3

10 4

10 5

10 6

10 7

10 8

1G9 10 1 0

Area (acres)

A Figure 53.26 Species-area curve for North American b r e e d i n g birds. Both area and number of species are plotted on a logarithmic scale. The data points range from a 0.5-acre plot with 3 species in Pennsylvania to the whole United States and Canada (4,6 billion acres) with 625 species.

community But the basic concept of diversity increasing with increasing area applies in a variety of situations, from surveys of ant diversity in New Guinea to the number of plant species on islands of different sizes. In fact, island biogeography provides some of the best examples of speciesarea curves, as we see next.

Island Equilibrium Model Because of their isolation and limited size, islands provide excellent opportunities for studying the biogeographic fao

tors that affect the species diversity of communities. By "islands," we mean not only oceanic islands, but also habitat islands on land, such as lakes, mountain peaks separated by lowlands, or natural woodland fragments surrounded by areas disturbed by humans—in other words, any patch surrounded by an environment not suitable for the "island" species. In the 1960s, American ecologists Robert MacArthur and E. O. Wilson developed a general model of island biogeography identifying the key determinants of species diversity on an island with a given set of physical characteristics (Figure 53.27) Consider a newly formed oceanic island that receives colonizing species from a distant mainland. Two factors determine the number of species that will eventually inhabit the island: the rate at which new species immigrate to the island and the rate at which species become extinct on the island. And two physical features of the island affect immigration and extinction rates: its size and its distance from the mainland. Small islands generally have lower immigration rates because potential colonizers are less likely to reach a small island. For instance, birds blown out to sea by a storm are more likely to land by chance on a larger island than on a small one. Small islands also have higher extinction rates, as they generally contain fewer resources and less diverse habitats for colonizing species to partition. Distance from the mainland is also important; for two islands of equal size, a closer island generally has a higher immigration rate than one farther away. And because of their higher immigration rates, closer islands also have lower extinction rates, as arriving colonists help sustain the presence of a species on a near island and prevent its extinction.

Equilibrium n u r n b e r - A Number of species on island

Far i s l a n d - m *~

(a) i m m i g r a t i o n a n d e x t i n c t i o n rates. The equilibrium number of species on an island represents a balance between the immigration of new species and the extinction of species already there.

Number of s (b) Effect of isfand size. ultimately have a iargei ber of species than sm; immigration rates tend extinction rates lower c

Jkr-Hear island

Number of species on isiand — — — > • (c) Effect of distance f r o m m a i n l a n d . Near islands tend to have larger equiiibnim numbers of species than' fa: islands because ^rinvg^ation rates to , vi si C

i 5 is oxidized and NAD"1" is reduced. Concept Check 9,2 1. NAD' acts as ihe oxidizing agent in step 6, accepting electrons from glyceraldehyde-3phosphate, which thus acts as the reducing agent. Concept Check 9,3 L NADH and FADH2; they will donate electrons to the electron transport chain. 2. CO2 is removed from pyruvate, which is produced by glycolysis, and CO, is produced by the citric acid cycle. Concept Check 9.4 1. Oxidative phosphorylation would stop entirely, resulting in no ATP production. Without oxygen to "pull"' electrons down the electron transport chain, H+ would not be pumped into the mitochondrion's intermembrane space and chemiosmosis would not occur. 2. Because addition of H+ (decreasing the pH) would establish a proton gradient even without the function of the electron transport chain, we would expect ATP synthase to function and synthesize ATP (In fact, it was experiments like this one that provided support for chemiosmosis as an energy-coupling mechanism.) Concept Check 9.5 1. A derivative of pyruvate—either acetaldehyde during alcohol fermentation or pyruvate itself during lactic acid fermentation; oxygen 2. The cell would need to consume glucose at a rate about 19 times the consumption rate in the aerobic environment (2 ATP are generated by fermentation versus up to 38 ATP by cellular respiration).

Concept Check 9.6 1. The fat is much more reduced: it has many —CH2— units. The electrons present in a carbohydrate molecule are already somewhat oxidized, as some of them are bound to oxygen. 2. When we consume more food than necessar" lor metabolic processes, our bodies synthesize fat as a way of storing energy [or later. 3. AMP will accumulate, stimulating phosphofructokinase, which increases the rate of glycolysis Since oxygen is not present, ihe cell will convert pyruvate to lactate in lactic acid fermentation, providing a supply of ATP. CHAPTER 10 Concept Check 10.1 1. CO2 enters leaves via stomaia, and water enters via roots and is carried to leaves through veins, 2. Using a heavy isotope of oxygen as a label, O, van Niel was able to show that ihe oxygen produced during photosynthesis originates in water, not in carbon dioxide. 3. The Calvin cycle depends on the NADPH and ATP that the light reactions generate, and the light reactions depend on the NADP+ and ADP and©i that die Calvin cycle generates. Concept Check 10.2 1. Green, because green light is mostly transmitted and reflected—not absorbed-—by photosynthetic pigments 2. In chloroplasts, light-excited electrons are trapped by a pri mary electron acceptor, which prevents them from dropping back to the ground state. In isolated chlorophyll, there is no electron acceptor, so the phoioexcited electrons immediately drop back down to the ground state, with the emission of light and heat, 3. Water (H2O) is the electron donor; NADPf accepts electrons at the end of the electron transport chain, becoming reduced to NADPH. Concept Check 10.3 1. 6, 18, 12 2. The more potential energy that a molecule stores, the more energy and reducing power required for the formation of that molecule. Glucose is a valuable energy source because it is highly reduced, storing lots of potential energy in its electrons. To reduce CO2 to glucose, much energy and reducing power are required in the form of a high number of ATP and NADPH molecules, respectively. 3. The light reactions require ADP and NADP~, which would not be formed from ATP and NADPH if the Calvin cycle stopped. Concept Check 10.4 1. Photorespiration decreases photosynthetic output by adding oxygen, instead of carbon dioxide, to the Calvin cycle. As a result, no sugar is generated (no carbon is fixed), and O2 is used rather than generated. 2. C4 and CAM species would replace many of the C-, species.

CHAPTER 11 Concept Check 11.1 1. The secretion of neurotransmitter molecules at a synapse is an example oflocal signaling. The electrical signal that travels along a very long nerve cell and is passed to the next nerve cell can be considered an example of long-distance signaling. (Note, however, that local signaling at the synapse between iwo cells is necessary for the signal to pass from one cell to the next.) 2. No glucose-1-phosphate is generated, because the activation of the enzyme requires an intact cell membrane with an intact receptor in the membrane. The enzyme cannot be activated directly by interaction with the signal molecule in the test tube. Concept Check 11,2 1. The NGF receptor is in the plasma membrane. The water-soluble NGF molecule cannot pass through the lipid membrane to reach intracellular receptors, as steroid hormones can. Concept Check 11.3 1. A protein kinase is an enzyme that transfers a phosphate group from ATP to a protein, usually activating that protein (often a second type of protein kinase). Many signal transduction pathways include a series of such interactions, in which each phosphorylated protein kinase, in turn phosphorylates the next protein kinase in the series. Such phosphorylation cascades carry a signal irom outside the cell to the cellular protein(s) that will carry out the response. 2. Protein phosphatases reverse the effects of the kinases. 3. The IPj-gated channel opens, allowing calcium ions to flow out of the ER, which raises the cytosolic Ca2~ concentration. Concept Check 11.4 1. By a cascade of sequential activations, at each step of which one molecule may activate numerous molecules functioning in the next step 2. Scaffolding proteins hold molecular components of signaling pathways in a complex with each other. Different scaffolding proteins would assemble different collections of proteins, leading to different cellular responses in the two cells. CHAPTER 12 Concept Check 12.1 1. 32 cells 2. 2 3. 39; 39; 78; 39 Concept Check 12.2 1. From the end of S phase of interphase through the end of metaphase of mitosis 2. 4; 8

3. Cytokinesis results in two genetically identical daughter cells in both plant cells and animal cells, but the mechanism of dividing the cytoplasm is different between animals and plants. In an animal cell, cytokinesis occurs by cleavage, which divides the parent cell in two with a contractile ring of actin filaments. In a plant cell, a cell plate forms in the middle of the cell and grows until its membrane fuses with the plasma membrane of the parent cell. A new cell wall is also produced from the cell plate, resulting in two daughter cells. 4. They elongate the cell during anaphase. 5. Sample answer: Fach type of chromosome consists of a single molecule of DNA with attached proteins. If stretched out, the molecules oi DNA would be many times longer than the cells in which they reside. During cell division, the two copies of each type of chromosome actively move apart, and one copy ends up in each of the two daughter cells. Concept Check 12.3 1. G, 2. The nucleus on the right was originally in the G] phase; therefore, it had not yet duplicated its chromosome. The nucleus on the left was in ihe M phase, so it had already duplicated its chromosome. 3. A sufficient amount of MPF has to build up lor a cell to pass the G2 checkpoint. 4. The cells might divide even in the absence of PDGF, in which case they would not stop when the surface was covered; they would continue to divide, piling on top of one another. 5. Most body cells are not in the cell cycle, but rather are in a nondividing state called Go. 6. Both types of tumors consist oi abnormal cells. A benign tumor stays at the original site and can usualh Iv ^uigically r^movei... Cancer cells from a malignant tumor spread from the original site b\ metastasis and may impair the functions of one or more organs. CHAPTER 13 Concept Check 13.1 1. Parents pass genes to their offspring that program cells to make specific enzymes and other proteins whose cumulative action produces an individual's inherited traits. 2. Such organisms reproduce by mitosis, which generates offspring whose genomes are virtually exact copies of die parents genome. 3. Offspring resemble their parents but are not genetically identical to them or their siblings because sexual reproduction generates different combinations of genetic information. Concept Check 13.2 1. A female has two X chromosomes; a male has an X and Y. 2. In meiosis, the chromosome count is reduced from diploid to haploid; the union ol two haploid gametes in fertilization restores the diploid chromosome count.

3. Haploid number (n) is 39; diploid number (2n) is 78. 4. Meiosis is involved in the production of gametes in animals. Mitosis is involved in the production of gametes in plants and most fungi (see Figure 13.6). Concept Check 13.3 1. In mitosis, a single replication of the chromosomes is followed by one division of the cell, so the number of chromosome sets in daughter cells is the same as in the parent cell. In meiosis, a single replication of the chromo- fi somes is followed by two cell divisions that WJ reduce the number of chromosome sets from i two (diploid) to one (haploid). 2. The chromosomes are similar in that each is composed of two sister chromatids, and the individual chromosomes are positioned similarly on the metaphase plate. The chromosomes differ in that in a mitotically dividing cell, sister chromatids of each chromosome are genetically identical, but in a meiotically dividing cell, sister chromatids are genetically distinct because of crossing over in meiosis I. Moreover, the chromosomes in meiaphase of mitosis can be composed ol a diploid set or a haploid set, but the chromosomes in metaphase of meiosis II always consist of a haploid set. Concept Check 13.4 1. Even in the absence of crossing over, independent assortment of chromosomes during meiosis I theoretically can generate 2" possible haploid gametes, and random fertilization can produce 2n x 2" possible diploid zygotes. Since the haploid number (n) of honeybees is 16 and that of Iruit flies is 4, two honeybees would be expected to produce a greater variety of zygotes than would two fruit flies. 2. If the segments of the maternal and paternal chromatids that undergo crossing over are genetically identical, then the recombinant chromosomes will be genetically equivalent to the parental chromosomes. Crossing over contributes to genetic variation only when it involves rearrangement of different versions of genes. CHAPTER 14 Concept Check 14.1 1. First, all the F, plants had flowers with the same color (purple) as the Qowers of one of the parental varieties, rather than an intermediate color as predicted by the "blending" hypothesis. Second, the reappearance of white flowers in ihe F2 generation indicates that the allele controlling the white-flower trait was not lost in the Fx generation; rather its phenotypic effect was masked by the effect of the dominant purple-flower allele. 2. According to the law of independent assortment, 25 plants (Vie of the offspring) are predicted to be aatt, or recessive for both characters. The actual result is likely to differ slightly from this value.

Appendix A

Concept Check 14.2 1. V2 dominant homozygous CCC), 0 recessive homozygous (cc), and V2 heterozygous (Cc) 2. 'A BBDD; LA BbDD; 'A BBDd; LA BbDd 3. 0; since only one of the parents has a recessive allele for each character, there is no chance of producing homozygous recessi\e offspring that would display the recessive trails. Concept Check 14.3 1. The black and white alleles are incompletely dominant, with heterozygot.es being gray in color. A cross between a gray rooster and a black hen should yield approximately equal numbers of gray and black offspring. 2. Height is at least partially hereditary and appears to exhibit polygenic inheritance with a wide norm of reaction, indicating that environmental factors have a strong influence on phenotype. Concept Check 14.4 1. % (Since cystic fibrosis is caused by a recessive allele, Beth and Tom's siblings who have CF must be homozygous recessive. Therefore, each parent must be a carrier of the recessive allele. Since neither Beth nor Tom has CF, this means they each have a 2/J chance of being a carrier. If they are both carriers, there is a Yt chance that they will have a child with CF 2/3 X % X 'A = Ye); 0 (Both Beth and Tom would have to be carriers to produce a child with the disease.) 2. Joan's genotype is Dd. Because the allele for polydactyly (D) is dominant to the allele for five digits per appendage (d), the trail is expressed in people with either the DD or Dd genotype. If Joan's mother were homozygous dominant (DD), then all of her children would have polydactyly. But since some of Joan's siblings do not have this condition, her mother must be heterozygous (Dd). All the children born to her mother (Dd) and father (dd) have either the dd genotype (normal phenotype) or the Dd genotype (polydactyly phenotype). Genetics Problems 1. Parental cross is AARR X aarr. Genotype ofFi is AaRr, phenotype is all axial-pink. Genotypes of ¥2 are 4 AaRr : 2 AaRR : 2 AARr : 2 aaRr : 2 Aarr : 1 AAP& : 1 aaRR : 1 AArr : 1 aarr. Phenotypes of F2 are 6 axialpink : 3 axial-red : 3 axia!-white : 2 terminalpink : 1 terminal-white : 1 terminal-red. 2. a. V54 b. VM C. Va d. Y32 3. Albino (b) is a recessive trait; black (B) is dominant. First cross: parents BB X bb; gametes B and b; offspring all Bb (black coat). Second cross: parents Bb X bb; gametes V2 B and V2 b (heterozyous parent) and b; offspring V2 Bb and lh bb. 4. a. PPLl X PPL1, PPLI X PpLl, or PPLI X ppLl b. ppLl X ppLl. c. PPLL X any of the 9 possi-

Appendix A

ble genotypes or PP11 X ppLL d. PpLl X Ppli. e. PpLl X PpLl. 5. Man A; woman IBi; child H. Other genotypes for children are 'A r V , 'A IAi, 'A l"i. 6. a. 3A X 3A X 3A = 27 /M c 'A X 'A d . 1 - 1/64 7. a. Y256 b. 8. a. 1 b.

X 'A = Vo4 = 6%4 Yie c. Y256 Y32 c. Ys

d. Ye4 e. Y128 d. Y2

9. V) 10. Matings of the original mutant cat with true-breeding noncurl cats will produce both curl and noncurl Fj offspring if the curl allele is dominant, but only noncurl offspring if the curl allele is recessive. You would obtain some true-breeding offspring homozygous for the curl allele from matings between the Fx cats resulting from the original curl X noncurl crosses whether the curl trait is dominant or recessive. You know that cats are true-breeding when curl X curl matings produce only curl offspring. As it turns out, the allele that causes curled ears is dominant. 11. Yis 12. 25% will be cross-eyed; all of the cross-eyed offspring will also be white. 13. The dominant allele I is epistatic to the P/p locus, and thus the genotypic ratio for the Fi generation will be 9 I_P__ (colorless) : 3 Ijpp (colorless) : 3 UP__ (purple) : 1 l\pp (red). Overall, the phenotypic ratio is 12 colorless : 3 purple ; 1 red. 14. Recessive. All affected individuals (Arlene, Tom. Wilma, and Carla) are homozygous recessive aa. George is Aa, since some of his children with Arlene are affected. Sam, Ann, Daniel, and Alan are each Aa, since they are all unaffected children with one affected parent. Michael also is Aa, since he has an affected child (Carla) with his heterozygous wife Ann. Sandra, Tina, and Christopher can each have the AA or Aa genotype. 15. Y2 16. Ye 17. 9 B_A_ (agouti) : 3 B_aa (black) : 3 bbA_ (white) : 1 bbaa (white). Overall, 9 agouti : 3 black : 4 white. CHAPTER 15 Concept Check 15.1 1. The law of segregation relates to the inheritance of alleles for a single character; the physical basis is the separation of homologues in anaphase I. The law of independent assortment of alleles relates to the inheritance ol alleles for two characters; the physical basis is the alternative arrangements of homologous chromosome pairs in metaphase I. 2. About 3A of the F2 offspring would have red eyes, and about Y4 would have white eyes. About half of the white-eyed flies would be female and half would be male; about half of

the red-eyed flies would be female and half would be male. Concept Check 15.2 1. Crossing over during meiosis I in the heterozygous parent produces some gametes with recombinant genotypes for the two genes. Offspring with a recombinant phenolype arise from fertilization of die recombinant gametes by homozygous recessivt gametes from the double-mutant parent. 2. In each case, the alleles contributed by the female parent determine the phenotype of the offspring because the male only contributes recessive alleles in this cross. 3. No, the order could be A-C-B or C-A-B. To determine which possibility is correct, you need to know the recombination frequency between B and C. Concept Check 15.3 1. Because the gene for this eye-color character is located on the X chromosome, all female offspring will be red-eyed and heterozygous QC X^); all male offspring will be white-eyed (XWY). 2.. V4; Y2 chance that the child will inherit a Y chromosome from the father and be male X Y2 chance that he will inherit the X carrying the disease allele from his mother. Concept Check 15.4 1. At some point during development, one of the embryo's cells may have failed (o carry out mitosis alter duplicating its chromosomes. Subsequent normal cell cycles would produce genetic copies of this tetraploid cell. 2. In meiosis, a combined 14-21 chromosome will behave as one chromosome. If a gamete receives the combined 14-21 chromosome and a normal copy of chromosome 21, trisomy 21 will result when this gamete combines with a normal gameie during fertilization. 3. An aneuploid male cat with more than one X chromosome could have a tortoiseshell phenotype if its X chromosomes have different alleles of the fur-color gene. Concept Check 15.5 1. Inactivation ol an X chromosome in females and genomic imprinting. Because of Xinactivation, ihe effective dose of genes on the X chromosome is the same in males and females. As a result of genomic imprinting, only one allele of certain genes is phenotypically expressed. 2. The genes for leaf coloration are located in plastids within the cytoplasm. Normally, only the maternal parent transmits plastid genes to offspring. Since variegated offspring are produced only when the female parent is of the B varieiy, we can conclude that variety B contains both the wild-type and mutant alleles of pigment genes, producing variegated leaves. 3. Each cell contains numerous mitochondria, and in affected individuals, most cells contain a variable mixture of normal and abnormal lT.iLOchondria.

Genetics Problems 1. 0; Vi, Vi6 2. Recessive; if the disorder were dominant, it would affect at least one parent of a child born with the disorder. The disorder's inheritance is sex-linked because it is seen only in boys. For a girl to have the disorder, she would have to inherit recessive alleles from both parents. This would be very rare, since males with the recessive allele on their X chromosome die in their early teens. [ 3. A for each daughter (V2 chance that child will be female X V2 chance of a homozygous recessive genotype); lh for first son 4. 17% 5. 6%. Wild type (heterozygous for normal wings and red eyes) X recessive homozygote with vestigial wings and purple eyes 6- The disorder would always be inherited from the moiher. 7. The inactivation of two X chromosomes in XXX women would leave them with one genetically active X, as in women with the normal number of chromosomes. Microscopy should reveal two Barr bodies in XXX women. 8. D-A-B-C 9. Fifty percent of the offspring would show phenotypes that resulted From crossovers. These results would be the same as those from a cross where A and 8 were not linked. Further crosses involving other genes on the same chromosome would reveal the linkage and map distances. 10. Between I and A. 12%; between A and 5, 5% 11. Between TandS, 18%; sequence of genes is T-A-5 12. No. The child can be either IAlAi or JAIL An ovum with the genotype ii could result from nondisjunction in the mother, while a sperm of genotype 1AIA could result from nondisj unction in the father. 13. 450 each of blue-oval and white-round (parentals) and 50 each of blue-round and white-oval (recombinants) 14. About one-third of the distance from the vestigial-wing locus to the brown-eye locus CHAPTER 16 Concept Check 16.1 1. DNA from the dead pathogenic 5 cells was somehow taken up by the living, nonpathogenic R cells. The DNA from the S cells enabled the R cells to make a capsule, which protected them from the mouse's defenses. In this way, the R cells were transformed into pathogenic S cells. 2. When the proteins were radioactively labeled (batch 1), the radioactivity would have been found in the pellet of bacterial cells. 3. Chargaffs rules state that in DNA, the percentages of A and T and of C and C are essentially the same, and the fly data are consistent with those rules. (Slight variations are most likely due to limitations of analytical technique.)

4. Each A hydrogen-bonds to a T, so in a DNA double helix, their numbers are equal; the same is true for G and C. Concept Check 16.2 1. Complementary base pairing ensures that the two daughter molecules are exact copies of the parent molecule. When the two strands of the parent molecule separate, each serves as a template on which nucleotides are arranged, by the base-pairing rules, into new complementary strands. 2. DNA pol 111 covalently adds nucleotides to new DNA strands and proofreads each added nucleotide for correct base pairing. 3. The leading strand is initiated by an RNA primer, which must be removed and replaced with DNA, a task performed by DNA pol I, Tn Figure 16.16 just to the left of the origin of replication, DNA pol I would replace the primer of the leading strand with DNA nucleotides. 4. The ends of eukaryotic chromosomes become shorter with each round of DNA replication, and telomeres at the ends of DNA molecules ensure that genes are not lost after numerous rounds of replication. CHAPTER 17 Concept Check 17.1 1. The nontemplate strand would read 5'TGGTTTGGCTCA-3'. The 5' -» 3' direction is the same as that for the mRNA; the base sequence is the same except for the presence of U in the mRNA where there is T in the nontemplate strand of DNA. 2. A polypeptide made up of 10 Gly (glycine) amino acids Concept Check 17.2 1. Both assemble nucleic acid chains from monomer nucleotides using complementary base pairing to a template strand. Both synthesize in the 5'—*3' direction, antiparallel to the template. DNA polymerase requires a primer, but RNA polymerase can start a nucleotide chain from scratch. DNA polymerase uses nucleotides with the sugar deoxyribose and the base T, whereas RNA polymerase uses nucleotides with the sugar ribose and the base U. 2. Upstream end 3. In a prokaryote, RNA polymerase recognizes the gene's promoter and binds to it. In a eukaryote, transcription factors mediate the binding of RNA polymerase 10 the promoter. 4. A prokaryotic primary transcript is immediately usable as mRNA, but a eukaryotic primary transcript must be modified before it can be used as mRNA. Concept Check 17.3 1. The 5' cap and poly-A tail facilitate rnRNA export from the nucleus, prevent the mRNA from being degraded by hydrolytic enzymes, and lacilila'.e nbosoiv.L- a.tacnment.

2. snRNPs join with other proteins and form spliceosomes that cut introns out of a prernRNA molecule and join its exons together. 3. Alternative RNA splicing produces different mRNA molecules from a pre-mRNA molecule depending on which exons are included in the mRNA and which are not. By yielding more than one version of mRNA, a single gene can code for more than one polypeptide. Concept Check 17.4 1. First, each aminoacyl-tRNA synthetase specifically recognizes a single amino acid and will only attach it to an appropriate tRNA. Second, a tRNA charged with its specific amino acid has an anticodon that will only bind to an mRNA codon for that amino acid. 2. Polyribosomes enable the cell to produce multiple copies of a polypeptide in a short amount of time. 3. A signal peptide on ihe leading end of the polypeptide being synthesized is recognized by a signal-recognition particle that brings the ribosome to the ER membrane. There the nbosome attaches and continues to synthesize the polypeptide, depositing it in the ER lumen. Concept Check 17.5 1. RNA can form hydrogen bonds with either DNA or RNA, take on a specific three-dimensional shape, and catalyze chemical reactions. These abilities enable RNA to interact functionally with all the major types of molecules in a cell. Concept Check 17.6 1. The RNA polymerase farthest to the right is first, since it has traveled the farthest along the DNA (and its mRNA is longest). The first ribosome is at the top of each mRNA, because it has traveled the farthest along the mRNA starting from the 5' end; the second is immediately below it, and so on. 2. No, the processes of transcription and translation are separated in space and time in a eukaryotic cell, a result of the eukaryotic cell's compartrnental organization. Concept Check 17.7 1. In the mRNA, the reading frame downstream from the deletion is shifted, leading to a long string of incorrect amino acids in the polypeptide and, in most cases, premature termination. The polypeptide will most likely be nonfunctional. 2. The amino acid sequence of the wild-type protein is Met-Asn-Arg-Leu. The amino acid sequence of ihe mutant protein sequence would be the same, because the mRNA codons 5'-CUA-3' and 5'-UUA-3' both code for Leu.

Appendix A

CHAPTER 18 Concept Check 18.1 1. Lytic phages can only carry out lysis of the host cell, whereas lysogenie p h t p s may either lyse the host cell or integrate into the host chromosome. In the latter case, the viral DNA (prophage) is simply replicated along with the host chromosome. Under certain conditions, a prophage may exit the host chromosome and initiate a lytic cycle. 2. The genetic material of these viruses is RNA, which is replicated inside the infected cell by special enzymes encoded by the virus. The viral genome (or a complementary copy of it) serves as mRNA for the synthesis o( viral proteins. 3. Because it synthesizes DNA from its RNA genome. This is the reverse ("retro") of the usual DNA —* RNA information flow. Concept Check 18.2 1. Mutations can lead to a new strain of a virus that can no longer be recognized by the immune system, even if an animal has been exposed to the original strain; a virus can jump from one species to a new host; and a rare virus can spread if a population becomes less isolated. 2. In horizontal transmission, a plant is infected from an external source of virus, which could enter through a break in the plant's epidermis due to damage by insects or other animals. In vertical transmission, a plant inherits viruses lrom its parent either via infected seeds (sexual reproduction) or via an infected cutting (asexual reproduction). 3. A source of infection, such as prion-infected cattle, may show no symptoms for many years. Beef prepared from such animals before symptoms appear would not be recognized as hazardous and could transmit infection to people who eat the meat. Concept Check 18,3 1. In transformation, naked, foreign DNA from the environment is taken up by a bacterial cell. In transduction, phages carry bacterial genes from one bacterial cell to another. In conjugation, a bacterial eel! directly transfers plasmid or chromosomal DNA to another cell via a mating bridge that temporarily connects the two cells. 2. Both are episomes—that is, they can exist as part of the bacterial chromosome or independently However, phage DNA can leave the cell in a protein coal (as a complete phage), whereas a plasmid cannot. Also, plasmids are generally beneficial to the cell, while phage DNA can direct the production ol complete phages that may harm or kill the cell. 3. In an F X F mating, only plasmid genes are transferred, but in an Hfr X F mating, bacterial genes may be transferred, because the F factor is integrated into the donor cells chromosome. In the latter case, the transferred genes may then recombine with the recipient F~ cell's chromosome.

Appendix A

Concept Check 18.4 1. The cell would continuously produce j3galactosidase and the two other enzymes for lactose utilization, even in the absence of lac-

tose, thus wasting cell resources. 2. Binding by the trp corepressor (tryptophan^ activates the tvp repressor, shutting off transcription of the trp operon; binding by the lac inducer (allolactose) inactivates the lac repressor, leading to transcription of the lac operon. CHAPTER 19 Concept Check 19.1 1. A nucleosome is made up of eight histone proteins, two each of four different types, around which DNA is wound. Linker DNA runs from one nucleosome to the next one. 2. Histones contain many basic (positively charged) amino acids, such as lysine and arginine, which can form weak bonds with the negatively charged phosphate groups on the sugar-phosphate backbone of the DNA molecule. 3. RNA polymerase and other proteins required for transcription do not have access to the DNA in tightly packed regions of a chromosome. Concept Check 19.2 1. Histone acetylation is generally associated with gene expression, while DNA methylation is generally associated wLli lack oi expression. 2. General transcription factors function in assembling the transcription initiation complex at the promoters for all genes. Specific transcription factors bind to control elements associated with a particular gene and once bound either increase (activators! or decrease (repressers) transcription of that gene. 3. The three genes should have some similar or identical sequences in the control elements of their enhancers. Because of this similarity, the same specific transcription factors could bind to the enhancers of all three genes and stimulate their expression coordinaiely. 4. Degradation of the mRNA, regulation of translation, activation oi the protein (by chemical modification, for example), and protein degradation Concept Check 19.3 1. The protein product of a proto-oncogene is usually involved in a pathway that stimulates cell division. The protein product of a tumorbuppressor gene is usually involved in a pathway that inhibits cell division. 2. A cancer-causing mutation in a protooncogene usually makes the gene product overactive, whereas a cancer-causing mutation in a tumor-suppressor gene usually makes the gene product nonfunctional. 3. When an individual has inherited an oncogene or a mutant allele of a tumor-suppressor gene.

Concept Check 19.4 1. The number of genes is 5-15 times higher in mammals, and the amount of noncoding DNA is about 10,000 times greater. The pres-

ence of tons m mammalian genes makes them about 27 times longer, on average, than prokaryotic genes. 2. Introns are interspersed within the coding sequences of genes. Many copies of each transposable element are scattered throughout the genome. Simple sequence DNA is concentrated at the centromeres and tefomeres. 3. In the rRNA gene family, identical transcription units encoding three different RNA products are present in long, tandemly repeated arrays. The large number of copies of the rRNA genes enable organisms to produce the rRNA for enough ribosomes to carry out active protein synthesis. Each globin gene family consists of a relatively small number of nonidentical genes clustered near each other. The differences in the globin proteins encoded by these genes result in production of hemoglobin molecules adapted to particular developmental stages of the organism. Concept Check 19.5 1. II cytokinesis is lanky, two copies of the entire genome can end up in a single cell. Errors in crossing over during meiosis can lead to one segment being duoliciued while another is deleted. During DNA replication, slippage backward along the template strand can result in a duplication. 2. Gene duplication and divergence by mutation. Movement of genes to different chromosomes also occurred. 3. For either gene, a mistake in crossing over during meiosis could have occurred between the two copies of that gene, such that one ended up with a duplicated exon. This could have happened several times, resulting in the multiple copies of a particular exon in each gene. 4. Homologous transposable elements scattered throughout the genome provide sites where recombination can occur between different chromosomes. Movement of these elements into coding or regulatory sequences may change expression oi genes. Transposable elements also can carry genes with them, leading io dispersion of genes and in some cases diflerent patterns ol expression. Or transport of an exon during transposition and its insertion into a gene may add a new functional domain to die originally encoded protein, a type of exon shuffling. CHAPTER 20 Concept Check 20.1 1. White (no functional kuZ gene is present1: 2. A cDNA library, made using mRNA from developing red blood cells, which would be expected to contain many copies of fj-globin mRNAs 3. Some human genes are too large to be incorporated into bacterial plasmids. Bacterial cells

lack [he means to process RNA transcripts, and even ii the need for RNA processing is avoided by using cDNA, bacteria lack enzymes to catalyze the post-translational processing that many human proteins undergo. Concept Check 20.2 1. Any restriction enzyme will cut genorm'c DNA in many places, generating such a large number of fragments that they would appear as a smear rather than distinct bands when the gel is stained after elecirophoresis. 2. RFLPs are inherited in a Mendelian fashion, and variations in RFLPs among individuals can be detected by Southern blotting. Concept Check 20.3 1. In a genetic linkage map, genes and other markers are ordered wiih respect to each other, but only the relative distances between them are known. In a physical map, the aciual distances between markers, expressed in base pairs, are known. 2. The three-stage approach employed in the Human Genome Project involves genetic mapping, physical mapping, and then sequencing of short, overlapping fragments lhat previously have been ordered relative to each other (see Figure 20.11). The shotgun approach eliminates the genetic mapping and physical mapping stages; instead, short fragments generated by multiple restriction enzymes, are sequenced and then subsequently ordered by computer programs that identify overlapping regions (see Figure 20.13). Concept Check 20.4 1. Alternative splicing of RNA transcripts from a gene and post-translational processing of polypeptides 2. It allows expression of thousands of genes to be examined simultaneously, thus providing a genorne-wide view of which genes are expressed in different lissues, under particular conditions, or at different slaves of development. 3. Because the human species arose more recently than many other species, there has been less time for genetic variations in coding and noncoding DNA to accumulate. Concept Check 20.5 1. Stem cells continue to reproduce themselves. 2. Herbicide resistance, pest resistance, disease resistance, delayed ripening, and improved nutritional value CHAPTER 21 Concept Check 21.1 1. Cells undergo differentiation during embryonic development, becoming different from each other; in the adult organism, there are many highly specialized cell types. 2. During animal development, movement of cells and tissues is a major mechanism, which is not the case in plants. In plants, growth and morphogenesis continue throughout the

life of the plant. This is true of only a few types of animal cells. Concept Check 21.2 1. Information deposited by the mother in the egg (cytoplasmic determinants) is required for embryonic development. 2- No, primarily because of subtle (and perhaps not so subtle) differences in their environments 3. By binding to a receptor on the receiving cells >ur!ace and triggering a signal iransduction pathway that affects gene expression Concept Check 21.3 1. Because their products, made by the mother, determine the head and tail ends, as well as the back and belly, of the egg (and eventually the adult fly) 2. The prospective vulval cells require an inductive signal from the anchor cell before they can differentiate into vulval cells. 3. A shoot is a differentiated structure, yet some of the cells that make it up are able to dedilferenuate and redifferentiate, forming all of the organs of an entire new plant. Concept Check 2 1 A 1. Homeotic genes differ in their mmhomeobox sequences, which determine their interactions with other transcription factors and hence which genes are regulated by the homeotic genes. These differ in the tw^o organisms, as do the expression patterns of the homeobox genes. CHAPTER 22 Concept Check 22.1 1. Aristotle, Linnaeus, and Cuvier viewed species as fixed (though Cuvier noted that the species present in a particular location could change over time). I-amarck, Erasmus Darwin, and Charles Darwin thought species could change. 2. Lamarck observed evidence of changes in species over time and noted that evolution could result in organisms" adaptations to iheir environments, though his theory was based on an incorrect mechanism for evolution: that modifications an organism acquires during its lifetime can be passed to its offspring. Concept Cheek 22.2 1. Species have the potential to produce more offspring than survive (overproduction), leading to a struggle for resources, which are limited. Populations exhibit a range of heritable variations, some of which confer advantages to their bearers that make them more likely to leave more offspring than less well-suited individuals. Over time this natural selection can result in a greater proportion of favorable traits in a population (evolutionary adaptation). 2. Though an individual may become modified during its lifetime through interactions with its environment, this does not represent evolution. Evolution can only be measured as a change in proportions of heritable variations from generation to generation.

Concept Check 22.3 1. An environmental factor such as a drug does not create new traits such as drug resistance, but rather selects for traits among those that are already present in the population. 2. Despite their different functions, the forelimbs of different mammals are structurally similar because they all represent modifications ol a structure found in the common ancestor. The similarities between the sugar glider and flying squirrel indicate that similar environments selected for similar adaptations despite different ancestry 3. If molecular biology or btogeography indicates a particular branching pattern of descent from a single group of ancestral organisms, representatives ol (he ancestral group should appear earlier in the fossil record than representatives of the later organisms. Likewise, the many transitional forms that link ancieni organisms to present-day species are evidence of descent with modification. CHAPTER 23 Concept Check 23.1 1. Mendel showed that inheritance is paniculate, and subsequently it was shown that this type of inheritance can preserve the variation on which natural selection acts. 2. 750. Half the loci (250) are fixed, meaning only one alkie exists for each locus: 250 X 1 = 250. There are two alleles each for the other loci: 250 X 2 - 500. 250 + 500 750. 3. 2pq + q\ 2pq represents heieroz) gores with one PKU allele and tf represents homozygotes with two PKU alleles Concept Check 23-2 1. Most mutations occur insomalic cells that do not produce gametes and so are lost when the organism dies. Ol mutations that do occur in cell lines lhat produce gametes, many do not have a phenotypic effect on which natural selection can act. Others have a harmful effect and are thus unlikely to spread in a population from generation to generation because they decrease the reproductive success of their bearers. 2. A population contains a vast number ot possible mating combinations, and fertilization brings together the gameies of individuals with different genetic backgrounds. Sexual reproduction reshuffles alleles into fresh combinations every generation. Concept Check 23.3 1. Natural selection is more "predictable" in that it tends to increase or decrease the frequency of alleles that correspond to variations that increase or decrease an organisms fitness in its environment. Alleles subject to genetic drift all have the same likelihood of increasing or decreasing.

Appendix A

2. Genetic drift results from chance fluctuations of allele frequencies from generation LO generation; ii tends to decrease variation over time. Gene flow is the exchange of alleJ.es between populations; it tends to increase variation within a population but decrease allele frequency differences between populations. Concept Check 23.4 1. No: many nucleotides are in noncoding porLions of DNA or in pseudogenes that have been inactivated by mutations. A change in a nucleotide may not even change the amino acid encoded because of the redundancy of the genetic code. 2. Zero, because fitness includes reproductive contribution to the next generation, and a sterile mule cannot produce offspring. 3. In sexual selection, organisms may compete for mates through behaviors or displays of secondary sexual characteristics; only the competing sex is selected for these characteristics. 4. Only half of the members (the females) of a sexual population actually produce offspring, while all the members of an asexual population can produce offspring. CHAPTER 24 Concept Check 24.1 1. Since the birds are known to breed successfully in captivity the reproductive barrier in nature must be prezygotic. Given the species differences in habitat preference, the reproductive barrier is most likely to be habitat isolation. 2. a. All species concepts except the biological species concept can be applied to both asexual and sexual species because they define species on the basis of characteristics other than ability to reproduce, b. The biological species concept can only be applied to extant sexual species, c. The easiest species concept to apply on a field trip would be the morphological species concept because it is based only on the appearance of the organism. Additional information about its ecological habits, evolutionary history, and reproduction are not required. Concept Check 24.2 1. Continued gene flow between mainland populations and those on a nearby island reduces the chance thai enough genetic divergence will take place for allopatric speciation to occur. 2. The diploid and tetraploid watermelons are separate species. Their hybrids are triploid and as a result are sterile because of problems carrying out meiosis. 3. According to the model of punctuated equilibrium, in most cases the time during which speciation (that is, the distinguishing evolutionary changes) occurs is relatively short compared with the overall duration of the species' existence. Thus, on the vast geologic time scale of the fossil record, the transition of one species to another seems abrupt, and instances of gradual change in the fossil

Appendix A

record are rare. Furthermore, some ol the changes that transitional species underwent may not be apparent in fossils.

Concept Check 24.3 1. Such complex structures do not evolve all at once, but in increments, with natural selection selecting for adaptive variants of the earlier versions. 2. Although an exaptation is co-opted for new or additional functions in a new environment, it existed in the first place because it worked as an adaptation to the original environment. 3. The timing of different developmental pathways in organisms can change in different ways (heterochrony). This can result in differentia! growth patterns, such as those producing different patterns of webbing in salamander feet.

2. Orthologous genes are homologous genes that have ended up in different gene pools, whereas paralogous genes are found in multiple copies in a singie genome because they

ate the result of gene duplication. Concept Check 25.5 1. The molecular clock is a method of estimating the actual time of evolutionary events based on numbers of base changes in orthologous genes. It is based on the assumption that the regions of genomes being compared evolve at constant rates. 2. There are many portions of the genome that do not code for genes, in which, many base changes could accumulate through drift without affecting an organism's fitness. Even in coding regions of the genome, some mutations may not have a critical effect on genes or proteins.

CHAPTER 25 CHAPTER 26 Concept Check 25.1 1. (a) Analogy, since porcupines and cacti are not closely related and aiso most other animals and plants do not have similar structures; (b) homology, since cats and humans are both mammals and have homologous forelimbs, of which the hand and paw are the lower part; (c) analogy, since owls and hornets are not closely related, and also the structure of their wings is very different. 2. The latter is more likely, since just small genetic changes can produce divergent physical appearances, but if genes have diverged greatly, that implies the lineages have been separate for some time. Concept Check 25.2 1. We are classified the same down to the class level; both the leopard and human are mammals. Leopards belong to order Carnivora, whereas humans do not. 2. The branching pattern of the tree indicates that the skunk and the wolf share a common ancestor that is more recent than the ancestor these two animals share with the leopard. Concept Check 25.3 1. No; hair is a shared primitive character common to all mammals and thus cannot be helpful in distinguishing different mammalian subgroups. 2. The principle of maximum parsimony states that the theory about nature we investigate first should be the simplest explanation found to be consistent with the facts. But nature does not always take the simplest course; thus, the most parsimonious tree (reflecting the fewest evolutionary changes) may not reflect reality. Concept Check 25.4 1. Proteins are gene products. Their amino acid sequences are determined by the nucleotide sequences of the DNA that codes for them. Thus, differences between comparable proteins in two species reflect underlying genetic differences.

Concept Check 26.1 1. The hypothesis that conditions on the early Earth could have permitted the synthesis of organic molecules from inorganic ingredients 2. In contrast to random mingling of molecules in an open solution, segregation of molecular systems by membranes could concentrate organic molecules, and electrical charge gradients across the membrane could assist biochemical reactions. 3. An RNA molecule that functions as a catalyst Concept Check 26.2 1. 22,920 years (four half-life reductions) 2. About 2,000 million years, or 2 billion years Concept Check 26.3 1. Prokaryotes must have existed at least 3.5 billion years ago, when the oldest fossilized stromatolites were formed. 2, Free oxygen attacks chemical bonds and can inhibit enzymes and damage cells. Some organisms were able to adapt, however. Concept Check 26.4 1. All eukaryotes have mitochondria or genetic remnants of these organelles, but not all eukaryotes have plastids. 2. The chimera of Greek mythology contained parts from different animals. Similarly, a eukaryotic cell contains parts from various prokaryotes: mitochondria from one type of bacterium, plastids from another type, and a nuclear genome from parts of the genomes of these endosymbionts and at least one other prokaryote. Concept Check 26.5 1. A single-celled organism must carry out all of the functions required to stay alive. Most multicellular organisms have many types of specialized cells, and life functions are divided among specific cell types. 2. Fossils of most major animal phyla appear suddenly in the first 20 million years of the

Cambrian period. Molecular clocks suggest that many animal phyla originated much earlier. Concept Check 26.6 1. Protista, Plantae, Fungi, and Animalia 2. Monera included both bacteria and archaea, but archaea are more closely related to eukaryotes than to bacteria. CHAPTER 27 Concept Check 27.1 1. Adaptations include the capsule (shields prokaryotes from host's immune system), plasmids (confer "contingency" functions such as antibiotic resistance), and the formation of endospores (enable cells LO survive harsh conditions and to revive when the environment becomes favorable). 2. Prokaryotic cells generally lack the internal compartmentalization of eukaryotic cells. Prokaryoiic genomes have much less DNA than eukaryotic genomes, and most of this DNA is contained in a single ring-shaped chromosome located in a nucleoid region rather than within a true membranebounded nucleus. In addition, many prokaryotes also have plasmids, small ringshaped DNA molecules containing a few genes. 3. Rapid reproduction enables a favorable mutation to spread quickly through a prokaryotic population by natural selection. Concept Check 27.2 1. Chemoheierotrophy; the bacterium must rely on chemical sources of energy, since il is not exposed to light, and it must be a heterotroph if it requires an organic source of carbon rather than CO2. 2. Anabaena is a photoautotroph that obtains its carbon from CO2. As a nitrogen-fixing prokaryote, Anabaena obtains its nitrogen from N2. Concept Check 27.3 1. Before molecular systematics, taxonomists classified prokaryotes according to phenotypic characters that did not clarify evolutionary relationships. Molecular comparisons indicate key divergences in prokaryotic lineages. 2. Both diseases are caused by spirochetes. 3. The ability of various archaea to use hydrogen, sulfur, and other chemicals as energy sources and to survive or even thrive without oxygen enables them to live in environments where more commonly needed resources are not present. Concept Check 27.4 1. Although prokaryotes are small, mostly unicellular organisms, they play key roles in ecosystems by decomposing wastes, recycling chemicals, and providing nutrients to other organisms.

2. Bactcradis ihet.aiotcicwikror]. which lives in-

side the human intestine, benefits by obtaining nutrients From the digestive system and by receiving protection from competing bacteria from host-produced antimicrobial compounds to which it is not sensitive. The human host benefits because the bacterium manufactures carbohydraies, vitamins, and other nutrients. Concept Check 27.5 1. Exotoxins are proteins secreted by prokaryotes; endotoxins are lipopolysaccharides released from the outer membrane of gramnegative bacteria that have died. 2. Their quick reproduction can make it difficult to combat them with antibiotics, particularly as they may evolve resistance to the drugs. Some also have the ability to form endospores and withstand harsh environments, surviving until conditions become more favorable. 3. Sample answers: eating Lermented foods such as yogurt, sourdough bread, or cheese; receiving clean water from sewage treatment; taking prokaryote-produced medicines CHAPTER 28 Concept Check 28.1 1. Sample response: Protists include unicellular, colonial, and multicellular organisms; photoautotrophs, heterotrophs, and mixotrophs; and species that reproduce asexually sexually, or both ways; and species that live in marine, freshwater, and moist terrestrial habitats. 2. Four: the inner and outer membranes of the bacterium, and the lood vacuole membrane and plasma membrane of the eukaryotic cell Concept Check 28.2 1. Their mitochondria do not have DNA, an electron transport chain, or citric-acid cycle enzymes. 2. Its flagella and undulating membrane enable it to move along the mucus-coated lining of these tracts inside its host. Concept Check 28.3 1. The proteins have slightly different structures, but only one protein at a time is expressed. Frequent changes in expression prevent the host from developing immunity 2. Euglena could be considered an alga because it is a photosynthetic autotroph; however, it could also be considered a fungus-like protist because it can absorb organic nutrients from its environment. Concept Check 28.4 1. Membrane-bounded sacs under the plasma membrane 2. A red tide is a bloom of dinoflagellates, some of which produce deadly toxins that accumulate in molluscs and can affect people who eat the molluscs.

3. During conjugation, two ciliates exchange micronuclei, but no new individuals are produced. Concept Check 28.5 1. A pair of flagella, one hairy and one smooth 2. Oomycetes acquire nutrition mainly as decomposers or parasites; golden algae are photosynthetic, but some also absorb dissolved organic compounds or ingest food particles and bacteria by phagocytosis. 3. The holdfast anchors the alga to the rocks, while the wide, flat blades provide photosynthetic surfaces. The cellulose and algin in the algas cell walls cushion the thallus from waves and protect it from drying out. Concept Check 28.6 1. Because foram tests are hardened with cal- I cium carbonate, they form long-lasting fossils in marine sediments and sedimentary rocks. 2. Forams feed by extending their pseudopodia through pores in their tests. "Radiolarians ingest smaller microorganisms by phagocytosis using their pseudopodia; cytoplasmic streaming carries the engulfed prey to the main part of the cell. Concept Check 28.7 1. Amoebozoans have lobe-shaped pseudopodia, whereas forams have threadlike pseudopodia. 2. Slime molds are fungus-like in that they produce fruiting bodies that aid in the dispersal of spores, and they are animal-like in that they are motile and ingest food. However, slime molds are more closely related to gymnamoebas and entamoebas than to fungi or animals. 3. In the life cycle of a cell ular slime mold, individual amoebas may congregate in response to a chemical signal, forming a sluglike aggregate form that can move. Then some of the cells form a stalk that supports an asexual fruiting body. Concept Check 28.8 1. Many algae contain an accessory pigment called phycoerythrin, which gives them a reddish color and allows them to carry out photosynthesis in relatively deep coastal water. Also unlike brown algae, red algae have no flagellated stages in their life cycle and must depend on water currents to bring gametes together for fertilization. 2. l/Iva's thallus contains many cells and is differentiated into leaflike blades and a rootlike holdfast. Caulerpa's thallus is composed of multinucleate filaments without cross-walls, so it is essentially one large cell. CHAPTER 29 Concept Check 29.1 1. Land plants share some key traits only with charophyceans: rosette cellulose-producing complexes, presence of peroxisome enzymes, similarity in sperm structure, and similarity

Appendix A

in cell division (the formation of a phragmoplast). Comparisons of nuclear and chloroplast genes also point to a common ancestry. Concept Check 29.2 1. Spore walls toughened by sporopollenin; muhicellular, dependent embryos; cuticle 2. a. diploid, b. haploid; c. haploid; d. diploid; e. haploid; f. haploid Concept Check 29.3 1. Bryophytes are described as nonvascular plants because they do not have an extensive transport system. Another difference is that their life cycle is dominated by gametophytes rather than sporophytes. 2. Answers may include the following: large surface area of protonema enhances absorption of water and minerals; the vase-shaped archegonia protect eggs during fertilization and transport nutrients to the embryos via placental transfer cells; the stalklike seta conducts nutrients from ihe gametophyte to the capsule where sperm are produced; the peristome enabies gradual spore discharge; stomata enable CO2/O2 exchange while minimizing water loss, Lightweight spores are winddispersed; mosses can lose water without drying and rehydrate when moisture is available. Concept Check 29.4 1, Some characteristics that distinguish seedless vascular plants from bryophytes are a sporophyte-dominant life cycle, the presence of xylem and phloem, and the evolution of true roots and leaves. A key similarity is flage.iated sperm requiring moisture for fertilization. 2. Most lycophytes have microphylls, whereas ferns and most fern relatives have megaphvlls. CHAPTER 30 Concept Check 30.1 1. To have any chance of reaching the eggs, ihe flagellated sperm of seedless vascular plants must rely on swimming through a film of water, usually limited to a range of less than a few centimeters. In contrast, the sperm of seed plants are produced within durable pollen grains that can be carried long distances by wind or by animal pollinators. Although flagellated in some species, the sperm of most seed plants do not require water because pollen lubes convey them directly to the eggs. 2. The reduced gametophytes of seed plants are nurtured by sporophytes and protected from stress, such as drought conditions and UV radiation. Pollen grains have tough protective coats and can be carried long distances, facilitating widespread sperm transfer without reliance on water. Seeds are more resilient than spores, enabling better resistance to environmental stresses and wider distribution.

Appendix A

Concept Check 30.2 1. Although gymnosperms are similar in not having their seeds enclosed in ovaries and fruits, their seed-bearing structures vary greatly For instance, cycads have large cones, whereas some gymnosperms. such as Ginkfit and Gne.tu.rn, have small cones that look somewhat like berries, even though they are not fruits. Leaf shape also varies greatly, from the needles of many conifers to the palmlike leaves of cycads to Gnetum leaves thai look like those of flowering plants. 2. The life cycle illustrates heterospory, as ovulate cones produce megaspores and pollen cones produce microspores. The reduced gamelophytes are evident in the form of the microscopic pollen grains and the microscopic female gametophyie within the megaspore. The egg is shown developing within an ovule, and the pollen tube is shown conveying the sperm. The figure also shows the protective and nutritive features of a seed. Concept Check 30.3 1. In the oaks life cycle, the tree (the sporophyte) produces flowers, which contain gametophytes in pollen grains and ovules; the eggs in ovules are fertilized; the mature ovaries develop into dry fruits called acorns; and the acorn seeds germinate, resulting in embryos giving rise to seedlings and finally to mature trees, which produce flowers and ihen acorns. 2. Pine cones and flowers both have sporophylls, modified leaves that produce spores. Pine trees have separate pollen cones (with pollen grains) and ovulale cones (with ovules inside cone scales). In flowers, pollen grains are produced by the anthers of stamens, and ovules are within the ovaries of carpels. Unlike pine cones, many flowers produce both pollen and ovules. 3. Traditionally, angiosperms have been classiiied as either monocots or dicots, based on certain traits, such as the number of cotyledons. However, receni molecular evidence reveals that while monocots are a clade, dicots are not. Based on phylogenetic relationships, most dicots form a clade, now known as eudicots. Concept Check 30.4 1. Because extinction is irreversible, it decreases the total diversity of plants, many of which may have brought important benefits to humans.

2. The extensive network of hyphae puts a large surface area in contact with the food source, and rapid growth of the mycelium extends hyphae into new territory. Concept Check 31.2 1. The majority of the fungal life cycle consists of haploid stages, whereas the majority of the human life cycle consists of diploid stages. 2. The two mushrooms might be reproductive structures of the same mycelium (the same organism). Or they might be parts of two separate organisms that have arisen from a single parent organism through asexual reproduction and thus carry the same genetic information. Concept Check 31.3 1. The fungal lineage thought to be the most primitive, the chytrids, have posterior flagella, as do most other opisthokonts. This suggests that other fungal lineages lost their flagella after diverging from the chytrid lineage. 2. This indicates that fungi had already established symbiotic relationships with plants by the date ihose fossiles formed. Concept Check 31.4 1. Flagellated spores 2. Most plants form arbuscular mycorrhizae with glomeromycetes; without the fungi, the plants would be poorly nourished. 3. Possible answers include the following: In zygomycetes, the sturdy, thick-walled zygosporangium can withstand harsh conditions and then undergo karyogamy and meiosis when the environment is favorable for reproduction. In ascomycetes, the asexual spores (conidia; are produced in chains or clusters at the tips of conidiophores, where they are easily dispersed by wind. The often cup-shaped ascocarps house the sexual spore-forming asci. In basidiomycetes, the basidiocarp supports and protects a large suriace area ofbasidia, from which spores are dispersed. Concept Check 31.5 1. A suitable environment for growth, retention ol water and minerals, protection irom sunlight, and protection from being eaten 2. A hardy spore stage enables dispersal to host organisms through a variety of mechanisms: their ability to grow rapidly in a favorable new environment enables them to capitalize on the host's resources. CHAPTER 32

CHAPTER 31 Concept Check 31.1 1. Both a fungus and a human are lieterotrophs. A fungus digests its food externally by secreting enzymes into the food and then absorbing the small molecules that result from digestion. In contrast, humans (and other animals) ingest relatively large pieces of food and digest the food within their bodies.

Concept Check 32.1 1. Plants are autotrophs; animals are heterotrophs. Plants have cell walls that provide structural support; animals lack strong cell walls (their bodies are held together by structural proteins, including collagen). Animals have unique cell types and tissues (muscle and nerve) and unique patterns of development, including the multicellular blastula stage.

2. These basic patterns of early development arose early in animal evolution and have been conserved across the diverse phyla within this clade today. Concept Check 32.2 1. c b . a . d 2. This diversification may have resulted from such external factors as changing ecological relationships (for instance, predator-prey interactions) and environmental conditions (for instance, increased oxygen levels) and from such internal factors as the evolution of the Hox complex. Concept Check 32.3 1. Grade-level characteristics are those that multiple lineages share regardless of evolutionary history Some grade-level characteristics may have evolved multiple times independently. Features that unite clades are those that were possessed by a common ancestor and were passed on to the various descendants. 2. Snail has spiral and determinate cleavage pattern; human has radial, indeterminate cleavage. Snail has schizocoelous development (characterized by a coelomic cavity formed by splitting of mesoderm masses); human has enterocoelous development (coelom forms from folds of archenteron). In a snail, the mouth forms from the blastopore; in a human, the anus develops from the blastopore, Concept Check 32.4 1. Cnidarians possess true tissues, while sponges do not. Also unlike sponges, cnidarians exhibit body symmetry, though it is radial and not bilateral as in other animal phyla. 2. The morphology-based tree divides Briaiena into two clades: Deuterostomia and Protostomia. The molecule-based tree recognizes three clades: Deuterostomia, Ecdysozoa, and Lophotrochozoa, as well as the phylum Rotifera. 3. Each type of data contributes to scientists' ability to test hypotheses about relationships; the more lines of data that support a particular hypothesis, the more likely it is to be valid. CHAPTER 33 Concept Check 33.1 1. The flagella of choanocytes draw water through their collars, which trap food particles. The particles are engulfed by phagocytosis and digested, either by choanocytes or by amoebocytes. 2. Sponges release their sperm into the surrounding water; changes in current direction will affect the odds that the sperm will be drawn inio neighboring individuals. Concept Check 33.2 1. Both the polyp and the medusa are composed of an outer epidermis and an inner gastrodermis separated by a gelatinous layer.

the mesoglea. The polyp is a cylindrical form that adheres to the substrate by its aboral end; the medusa is a flattened, mouth-down form that moves freely in the water. 2. Cnidarian stinging cells (cnidocytes) function in defense and prey capture. They contain capsule-like organelles (cnidae), which in turn contain inverted threads. The threads either inject poison or stick to and entangle small prey. Concept Check 33.3 1. Tapeworms can absorb food from their environment and release ammonia into their environment through their body surface because their body is very flat. 2. No. Rotifers are microscopic but have an alimentary canal, whereas tapeworms can be very large but lack any digestive system. 3. Ectoprocts and corals are both sessile animals that collect suspended food with their tentacles and build reefs with their exoskeletons. Concept Check 33.4 1. The function of the foot reflects the locomotion required in each class. Gastropods use their foot as a holdfast or to move slowly on the substrate. In cephalopods, the foot functions as a siphon and tentacles. 2. The shell has become divided in two halves connected by a hinge, and the mantle cavity has enlarged gills that function in feeding as well as gas exchange. The radula has been lost, as bivalves have become adapted to suspension feeding. Concept Check 33.5 1. The inner tube is the alimentary canal, which runs the length of the body. The outer tube is the body wall. The two tubes are separated by the coelom. 2. Each segment is surrounded by longitudinal and circular muscles. These muscles work against the fluid-filled coelom, which acts as a hydrostatic skeleton. Coordinated contraction of the muscles produces movement. Concept Check 33.6 1. Incomplete cooking doesni kill nematodes and other parasites that might be present in the meat. 2. Nematodes lack body segments and a true coelom; annelids have both. Concept Check 33.7 1. Arthropod mouthparts are modified appendages, which are bilaterally paired. 2. Two-thirds of all known animal species are arthropods, which are found in nearly all habitats of the biosphere. 3. The arthropod exoskeleton, which had already evolved in the ocean, allowed terrestrial species to retain water and support their bodies on land. Wings allowed them to disperse quickly to new habitats and to find food and mates.

Concept Check 33.8 1. Both echinoderms and cnidarians have radial symmetry However, the ancestors of echinoderms had bilateral symmetry, and adult echinoderms develop from bilaterally symmetrical larvae. Therefore, the radial symmetry of echinoderms and cnidarians is analogous (resulting from convergent evolution), not homologous. 2. Each tube foot consists of an ampulla and a podium. When the ampulla squeezes, it forces water into the podium and makes the podium expand. When the muscles in the wall of the podium contract, they force water back into the ampulla, making the podium shorten and bend.

y; 4J 3' direction. leaf The main photosynthetic organ of vascular plants. leaf primordia Fingerlike projections along the flanks of a shoot apical meristem, from which leaves arise. leaf trace A small vascular bundle that extends from the vascular tissue of the stem through the petiole and into a leaf,

learning A behavioral change resulting from experience. lens The structure in an eye that focuses light rays onto the retina. lenticels Small raised areas in the bark of stems and roots that enable gas exchange between living cells and the outside air. lepidosaur Member of the reptilian group that includes lizards, snakes, and two species of New Zealand animals called tuataras. leukocyte (lu'-ko-sit) A white blood cell; typically functions in immunity, such as phagocytosis or antibody production. Leydig cell A cell that produces testosterone and other androgens and is located between the seminiferous tubules of the testes. lichen (li'-ken) The symbiotic collective formed by the mutualistic association between a fungus and a photosynthetic alga or cyanobacterium. life cycle The generation-to-generation sequence of stages in the reproductive history of an organism. life expectancy at birth The predicted average length of life at birth. life history The series of events from birth through reproduction and death. life table A table of data summarizing mortality in a population. ligament A type of fibrous connective tissue that joins bones together at joints. ligand (lig'-und) A molecule that binds specifically to a receptor site of another molecule. ligand-gated ion channel A protein pore in the plasma membrane that opens or closes in response to a chemical signal, allowing or blocking the flow of specific ions. light chain One of the two types of polypeptide chains that make up an antibody molecule and B cell receptor; consists of a variable region, which contributes to the antigenbinding site, and a constant region. light-harvesting complex Complex of proteins associated with pigment molecules (including chlorophyll a, chlorophyll b, and carotenoids) that captures light energy and transfers it to reaction-center pigments in a photosystem. light microscope (LM) An optical instrument with lenses that refract (bend) visible light to magnify images of specimens. light reactions The steps in photosynthesis that occur on the thylakoid membranes of the chloroplast and that convert solar energy to the chemical energy of ATP and NADPH, evolving oxygen in the process. lignin (lig'-nin) A hard material embedded in the cellulose matrix of vascular plant cell wails that functions as an important adaptation for support in terrestrial species. limbic system (lim'-bik) A group of nuclei (clusters of nerve cell bodies) in the lower part of the mammalian forebrain that interact with the cerebral cortex in determining

emotions; includes the hippocampus and the amygdala, limiting nutrient An element that must be added for production to increase in a particular area. limnetic zone In a lake, the well-lit, open surface waters farther from shore. linkage map A genetic map based on the frequencies of recombination between markers during crossing over of homologous chromosomes. linked genes Genes located close enough together on a chromosome to be usually inherited together. lipid (lip ""-id) One of a family of compounds, including fats, phospholipids, and steroids, that are insoluble in water. littoral zone In a lake, the shallow, well-lit waters close to shore, liver The largest organ in the vertebrate body. The liver performs diverse functions, such as producing bile, preparing nitrogenous wastes for disposal, and detoxifying poisonous chemicals in the blood. liverwort A small, herbaceous nonvascular plant that is a member of the phylum Hepatophyta. loam The most fertile of all soils, made up of roughly equal amounts of sand, silt, and clay. lobe-fin Member of the vertebrate subgroup Sarcopterygii, osteichthyans with rodshaped muscular Ens, including coelacanths and lungfishes, as well as the lineage that gave rise to tetrapods. local regulator A chemical messenger that influences cells in the vicinity. locomotion Active movement from place to place. locus (lo'-kus) (plural, loci) A specific place along the length of a chromosome where a given gene is located. logistic population growth A model describing population growth that levels off as population size approaches carrying capacity. long-day plant A plane that flowers (usually in late spring or early summer) only when the light period is longer than a critical length. long-term memory The ability to hold, associate, and recall information over one's life, long-term potentiation (LTP) An enhanced responsiveness to an action potential (nerve signal) by a receiving neuron, loop of Henle The long hairpin turn, with a descending and ascending limb, of the renal tubule in the vertebrate kidney; functions in water and salt reabsorption. loose connective tissue The most widespread connective tissue in the vertebrate body It binds epithelia to underlying tissues and functions as packing material, holding organs in place. lophophore (lof'-uh-for) A horseshoe-shaped or circular fold of the body wall bearing ciliated tentacles that surround the mouth.

lophotrochozoan Member of a group of animal phyla with prolostome development that some systematists hypothesize form a clade,

characterized by lophophorts or trochophore larvae, low-density lipoprotein (LDL) A cholesterolcarrying particle in the blood, made up of cholesterol and other lipids surrounded by a single layer of phospholipids in which proteins are embedded. An LDL particle carries more cholesterol than a related lipoprotein, HDL, and high LDL levels in the blood correlate with a tendency to develop blocked blood vessels and heart disease. lung An invaginated respiratory surface of terrestrial vertebrates, land snails, and spiders that connects to the atmosphere by narrow tubes. luteal phase That portion of the ovarian cycle during which endocrine cells of the corpus luteum secrete female hormones. luteinizing hormone (LH) (lu'-te-uh-m'-zing) A tropic hormone produced and secreted by the anterior pituitary that stimulates ovulation in females and androgen production in males, lycophyte An informal name for any member of the phylum Lycophyta, which includes club mosses, spike mosses, and quillworts. lymph The colorless fluid, derived from interstitial fluid, in the lymphatic system of vertebrate animals. lymph node Organ located along a lymph vessel. Lymph nodes filter lymph and help attack viruses and bacteria. lymphatic system A system of vessels and lymph nodes, separate from the circulatory system, that returns fluid, proteins, and cells to the blood. lymphocyte A type of white blood cell that mediates acquired immunity Lymphocytes that complete their development in the bone marrow are called B cells, and those that mature in the thymus are called T cells. lysogenic cycle (li'-so-jen'-ik) A phage replication cycle in which the viral genome becomes incorporated into the bacterial host chromosome as a prophage and does not kill the host. lysosome (li'-so-som) A membrane-enclosed sac of hydrolytic enzymes found in the cytoplasm of eukaryotic cells. lysozyme (ll'-so-zim) An enzyme in sweat, tears, and saliva that attacks bacterial cell walls. lytic cycle (lit'-ik) A type of viral (phage) replication cycle resulting in the release of new phages by lysis (and death) of the host cell, M phase See mitotic (M) phase. macrochmate Large-scale variations in climate; the climate of an entire region. macroevolution Evolutionary change above the species level, including the appearance of major evolutionary developments, such as flight, that we use to define higher taxa. rnacromolecule A giant molecule formed by the joining of smaller molecules, usually by a condensation reaction. Polysaccharides,

proteins, and nucleic acids are macromolecules. macronutrient A chemical substance that an organism must obtain in relatively large amounts. See also micronutrient. macrophage (mak'-ro-faj) A phagocytic cell present in many tissues that functions in innate immunity by destroying microbes and in acquired immunity as an antigenpresenting cell. magnetic reversal A reversal of the polarity of Earths magnetic field. magnoliids A flowering plant clade that evolved later than basal angiosperms but before monocots and eudicots. Extant examples are magnolias, laurels, and black pepper plants. major depression Depressive mental illness characterized by experiencing a low mood most of the time. major histocompatibility complex (MHC) A family of genes that encode a large set of cell surface proteins called MHC molecules. Class I and class II MHC molecules function in antigen presentation to T cells. Foreign MHC molecules on transplanted tissue can trigger T cell responses that may lead to rejection of the transplant. malignant tumor A cancerous tumor that is invasive enough to impair the functions of one or more organs. malleus The first of the three middle ear bones. malnourished "Referring to an animal whose diet is missing one or more essential nutrients. Malpighian tubule (mal-pig'-e'-un) A unique excretory organ of insects that empties into the digestive tract, removes nitrogenous wastes from the hemolymph, and functions in osmoregulation, mammal Member of the class Mammalia, amniotes with mammary glands that produce milk. mammary glands Exocrine glands that secrete milk to nourish the young. These glands are characteristic of mammals. mandible One of a pair of jaw-like feeding appendages found in myriapods, hexapods, and crustaceans. mantle A fold of tissue in molluscs that drapes over the visceral mass and may secrete a shell. mantle cavity A water-filled chamber that houses the gills, anus, and excretory pores of a mollusc, map unit A unit of measurement of the distance between genes. One map unit is equivalent to a 1% recombination frequency. marine benthic zone The ocean floor, mark-recapture method A sampling technique used to estimate wildlife populations. marsupial (mar-su'-pe-ul) A mammal, such as a koala, kangaroo, or opossum, whose young complete their embryonic development inside a maternal pouch called the marsupium.

mass number The sum of the number of protons and neutrons in an atom's nucleus. mast cell A vertebrate body cell that produces histamine and other molecules that trigger the inflammatory response. mate choice copying Behavior in which individuals in a population copy the mate choice of others, apparently as a result of social learning. maternal effect gene A gene that, when mutant in the mother, results in a mutant phenotype in the offspring, regardless of the genotype. matter Anything that takes up space and has mass. maximum likelihood A principle that states that when considering multiple phylogenetic hypotheses, one should take into account the one that reflects the most likely sequence of evolutionary events, given certain rules about how DNA changes over time. maximum parsimony A principle that states that when considering multiple explanations for an observation, one should first investigate the simplest explanation that is consistent with the facts. mechanoreceptor A sensor)" receptor thai detects physical deformations in the body's environment associated with pressure. touch, stretch, motion, and sound. medulla oblongata The lowest part of the vertebrate brain, commonly called the medulla; a swelling of the hindbrain dorsal to the anterior spinal cord that: controls autonomic, homeosuuic functions, including breathing, heart and blood vessel activity, swallowing, digestion, and vomiting. medusa (muh-du'-suh) The floating, flattened, mouth-down version of the cnidarian body plan The alternate form is the polyp. megapascal (MPa) (rneg'-uh-pas-kaF) A unit of pressure equivalent to 10 atmospheres of pressure. megaphyll A leaf with a highly branched vascular system, characteristic ol the vast majority of vascular plants. megaspore A spore from, a heterosporous plant species that develops into a female gametophyte. meiosis (mi-o'-sis) A two-stage type of cell division in .sexually reproducing organisms that results in cells with half the chromosome number ol" the original cell meiosis I The first division of a iwo-stage process of cell division in sexually reproducing organisms that results in cells with half the chromosome number of ihe original cell. meiosis II The second division of a two-stage process oi cell division in sexually reproducing organisms that results in cells with half the chromosome number of the original cell. melanocyte-stimulating hormone (MSH) A hormone produced and secreted by the anterior pituitary that regulates the activity of pigment-containing cells in the skin of some vertebrates.

melatonin A hormone secreted by the pineal gland that regulates body functions related to seasonal day length. membrane attack complex (MAC) A molecular complex consisting of a set of complement proteins that forms a pore in the membrane of bacterial and transplanted cells, causing the cells to die by lysis. membrane potential The charge difference between a cells cytoplasm and the extracellular fluid, due to the differential distribution of ions. Membrane potential affects the activity of excitable cells and the transmembrane movement of all charged substances. memory cell One of a clone of long-lived lymphocytes, formed during the primary immune response, that remains in a lymphoid organ until activated by exposure to the same antigen that i ng«ered us formation. Activated memory cells mount the secondary immune response. menopause The cessation of ovulation and menstruation. menstrual cycle (men'-stru-ul) A type of reproductive cycle in higher female primates, in which the nonpregnam endometrium is shed as a bloody discharge through [he cervix into the vagina. menstrual flow phase That portion of the uterine (menstrua]) cycle when menstrual bleeding occurs. menstruation The shedding ol portions of the endomelrium during a uterine (.menstrual! cycle. meristem (mar'-uh-stem) Plant tissue that remains embryonic as long as the plant lives, allowing for indeterminate growth. meristem identity gene A plant gene ihat promotes the switch Iron", vegetative growth to (lowering. meroblastic cleavage (mar'-o-bias -tik} A type of cleavage in which there is incomplete division of yolk-rich egg, characteristic of" avian development. mesentery (mez'-en-tar-e*) A membrane that suspends many ol the organs ol vertebrates inside fluid-filled body cavities. mesoderm (mez-o-derm) The middle primary germ layer of an early embryo that develops into the uotochord, the lining of the coelom, muscles, skeleton, gun ads. kidneys, and most of die circulatory system. mesohyl (mes'-uh-hil) A gelatinous region between the two layers of cells of a sponge. mesophyll (mez'-o-fil) The ground tissue of a leal, sandwiched between the upper and lower epidermis and specialized for photosynthesis. mesophyll cell A loosely arranged photosyntheiic cell located between the bundle sheath and [he leaf surface. messenger RNA (mRNA) A type of RNA, synthesized from DNA, that attaches to ribosomes in the cytoplasm and specifies the primary structure of a protein.

metabolic pathway A series of chemical reactions that either builds a complex molecule (anabolic pathway) or breaks down a complex molecule into simpler compounds (catabolic pathway). metabolic rate The total amount of energy an animal uses in a unit of time. metabolism (muh-tab'-uh-lizm) The totality of an organisms chemical reactions, consisting of cacabolic and anabolic pathways. metamorphosis (met'-uh-mor'-fuh-sis) The resurgence of development in an animal larva that transforms it into a sexually mature adult. metanephridium (met'-uh-nuh-frid'-e-um) (plural, metanephridia) In annelid worms, a type of excretory tubule with internal openings called nephrostornes that collect body fluids and external openings called nephridiopores. metaphase The third stage of mitosis, in which the spindle is complete and the chromosomes, attached to micro tubules at their kinetoehores, are all aligned at the metaphase plate. metaphase plate An imaginary plane during metaphase in which the centromeres of all the duplicated chromosomes are located midway between the two poles. metapopulalion A subdivided population of a single species. metastasis (muh-tas'-tuh-sis) The spread of cancer cells to locations distant from their original site. methanogen A microorganism that obtains energy by using carbon dioxide 10 oxidize hydrogen, producing methane :ts a waste product. microclimate Very fine scale variations of climate, such as the specific climatic conditions underneath a log. microevolution Evolutionary change below the species level; change in the genetic makeup of a population from generation lo generation. microfilamen! A solid rod ol aclin protein in ihe cytoplasm of almost all eukaryotic cells, making up part o1 the c\ioskeleton and acting alone or with myosin to cause cell contraction. micronutrient An element that an organism needs in very small amounts and that functions as a component or cofactor of enzymes. See also macronutrient. microphyll In lycophytes, a small leaf with a single unbranched vein. micropyle A pore in the integument(s) of an ovule. micro-RNA (miRNA) A small, single-stranded RNA molecule that binds to a complementary sequence in mRNA molecules and directs associated proteins to degrade or prevent translation of the target mRNA. microspore A spore from a heierosporous plant species that develops into a male gametophyte.

microsporidia Unicellular parasites of animals and protists that molecular comparisons suggest may be most closely related to zygomycete fungi. microtubuk A hollow rod of tubulin protein in the cytoplasm of all eukaryotic cells and in cilia, flagella, and the cytoskeleton. microvillus (.plural, microvilli) One of many fine, fingerlike projections of the epithelial cells in the lumen of the small intestine that increase its surface area. midbrain One of three ancestral and embryonic regions of the vertebrate brain; develops into sensory integrating and relay centers that send sensory information to the cerebrum. middle ear One of three main regions of the vertebrate ear; a chamber containing three small bones (the hammer, anvil, and stirrup) that convey vibrations from the eardrum to the oval window. middle lamella (luh-mel'-uh) A thin layer of achesivt extracellular material, primarily pectins, found between the primary walls of adjacent young plant cells. mineral In nutrition, a chemical element other than h\ drogeti, oxygen, or nitrogen that an organism requires for proper body functioning. mineral nutrient An essential chemical element absorbed from the soil in ihe form of inorganic ions. mineralocorticoid A steroid hormone secreted by ihe adrena! cortex that regulates salt and water homeostasis. minimum viable population (MVP) The smallest population size at which a species is able to sustain its numbers and survive. mismatch repair The cellular process that uses special enzymes to fix incorrectly paired nucleotides. missense mutation The most common type of mutation, a base-pair substiiution in which the new codon makes sense in that it still codes for an amino acid. mitochondrial matrix The compartment of the mitochondrion enclosed by the inner membrane and containing enzymes and substrates for the Krebs cycle. mitochondrion (rm'-to-kon'-dre-on) (plural, mitochondria) An organelle in eukaryotic cells that serves as the site of cellular respiration. mitosis (mi-to'-sis) A process of nuclear division in eukaryotic cells conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Mitosis conserves chromosome number by equally allocating replicated chromosomes to each of the daughter nuclei. mitotic (M) phase Ihe phase ol ihe cell cycle that includes mitosis and cytokinesis. mitotic spindle An assemblage of microtubules and associated proteins that is involved in the movements of chromosomes during mitosis.

mixotroph An organism trial i- cw.xible ol both photosynthesis and heterotrophy. model A representation of a theory or process.

model organism An organism chosen to study broad biological principles. modern synthesis A comprehensive theory of evolution emphasizing popular.ons as uniis of evolution and integrating ideas from many fields, including genetics, statistic?, paleontology, taxonomy, and biogeography molarity A common measure of solute concentration, referring to the number of moles of solute per liter of solution. mold A rapidly growing fungus that reproduces asexually by producing spores. mole (mol) The number of grams of a substance that equals its molecular weight in daltons and contains Avogadro's number of molecules. molecular clock An evolutionan timing method based on the observation that at least some regions of genomes evolve at constant rates. molecular formula A type of molecular notation indicating only the quantity of the constituent atoms. molecular mass The sum of the masses ol all the atoms in a molecule; sometimes called molecular weight. molecular systematic^ The comparison of nucleic acids or other molecules in different species to infer relatedness molecule Two or more atoms he'd together by covalent bonds. molting A process in arthropods in which die exoskeleton is shed at intervals, allow m« growth by the production o! a larger exoskeleton. monoclonal antibody (mon'-o-klon-ul) Any of a preparation oi antibodies thai have been produced by a single clone of cultured cells and tnus are ah specific for the same epitope. monocots A clade consisting of flowering plants that have one embryonic seed leaf, or cotyledon. monocyte A type ol white blood cell that migrates into tissues and develops into a macrophage. monoecious (muh-ne'-shus) A term typically used to describe an angiosperm species in which carpellate and staminate flowers are on the same plant. monogamous A type of relationship in which one male mates with just one female. monohybrid An organism that is heierozvgaus with respect to a single gene o! interest All the o![spring from a cross between parents nomorvgoLis for different alleles are monohybrids. For example, parents of genotypes AA and aa produce a monoiiybud ui cenotype Aa. monomer (mon'-uh-mer) The subunit thai serves as the building block of a polymer. monophyletic (mon'-o-fi-let'-ik) Pertaining to a grouping of species consisting of an ances^ lnil species and nil its descendants: a clade.

monosaccharide (mon'-o-sak'-uh-rld) The simplest carbohydrate, active alone or serving as a monomer for disaccharides and polysaccharides. Also known as simple sugars, i he molecular formulas of monosaccharides are generally some multiple of CH2O. monosomic Referring to a cell that has only one copy of a particular chromosome, instead of the normal two. monoLreme (mon'-uh-trern) An egg-laying mammal, represented by the platypus and echidna. morphogen A substance, such as Bicoid protein, thai provides positional information in the form of a concentration gradient along an embryonic axis. morphogenesis (mor'-fo-jen'-uh-sis) The development of body shape and organization, morphological species concept Defining species by measurable anatomical criteria. morula Cmor'-yuh-luh) A solid ball of blastomeres formed by early cleavage. mosaic evolution The evolution of different Features of organisms at different rates. moss A small, herbaceous nonvascukr plant that is a member of the phylum Bryophyta. motor neuron A nerve cell that transmits signals from the brain or spina! cord to muscles or glands. motor unit A single motor neuron and all the muscle fibers it controls. movement corridor A series of small clumps or a narrow strip of quality habitat disable by organisms) that connects otherwise isolated patches of quality habitat. MPF Maturation-promoting factor (M-phasepromoting factor); a protein complex required for a cell to progress from late interphase to mitosis. The active form consists of cyclin and a protein kinase. mucous membrane (myii'-kus) Smooth moist epithelium that lines the digestive tract ;, biological augmentation project using, 1226 Albumin, 419 regulation of gene expression for synthesis of, 367/ Alcohol(s), 64/ Alcohol fermentation, 175

Aldehydes, 64/ Aldoses, 70/ Aldosterone. 937 Alevria avrantia, 616/ Algae

Alveolates (Alveolata), 555-58 apicomplexans, 555-56 ciliates, 556-58 dinoflagellates, 555

alternation of generations in, 242 blue-green (see Cyanobacteria) brown (phaeophytes), 560-62 diatoms (baciilanophues), 559-60 fungi and (lichens), 621-22 golden (chrysophytes), 560 green (chlorophytes). 567-69 life cycles, 242/, 562/, 569/ photosynthesis in, 560 red algae (rhodophytes), 567 secondary endosymbiosis and diversity of, 551 Alimentary canal, 648, 854/ 855 evolutionary adaptations in, 863/ peristalsis in, 855 Alkaptonuria, 310 Allantois, 688, 1001 Allee,WC,1146 Allee effect, 1146 Alleles, 254. See also Gene(s) as alternate form of gene, 253-54, 255/ for blood groups, 260 codominant, 260 complete dominance of, 260 correlating behavior of chromosome pair with, 276, 277/ dominant, 254, 260, 261-62, 267-68 elimination of, by bottleneck effect, 461 fertilization and segregation of, as chance events, 259/ frequencies of. in populations, 455-62 frequency of dominant, 261-62 genetic disorders and (see Genetic disorders) incomplete dominance of, 260, 261/ law of independent assortment of, 256-58, 274, 275/ law of segregation of, 253-55, 275/ microevolution and change in frequencies of, 454, 455-56, 459-70 multiple, 262 mutations in (see Mutations) natural selection and changes in frequencies of, 460,462-70 recessive. 254, 283-84 relationship between phenotype and dominance of, 261 spectrum of dominance of, 260-62 Allergens. 916 Allergies anaphylactic shock and, 917 histamine release and, 901, 916-17 mast cells, IgE, and, 916, 917/ Alligators, 691/ metabolic rate, 834 Allolactose, 355, 356 Allometric growth, 484/ Allopatric speciation, 476/, 477,480 Allopolypbidy, 478, 479/ 480 Allosteric regulation of enzymes, 156/-57 Almonds, 605 Alpha (a) helix of protein, 8 2 / 85 Alpine pennycress (Thlaspi caerulcsceu^. 763 Alternation of generations, 242, 461, 561, 576 heteromorphic and isomorphic. 562 in plants, 242/, 576/ in protists (algae), 242/ 561-62 Alternative RNA splicing, 319, 368, 399 Altman, Sidney, 515 Altruism, 1128-33 Hamilton's rule and kin selection, 1130 inclusive fitness and, 1129-31 reciprocal, 1130-31 Ah elements, 376, 381 Alvarez, Walter and Luis, 520

Alvin research submarine, 514/ Alzheimer's disease, 290, 1040-41 Amacrine cells, 1061/, 1062 Ambordla trichopoda, 602/ Amebic dysentery; 564 American alligator (Alligator m:si;ss;;)i9 Bound ribosomes, 102, 103/, 325 Boven, Theodor, 274 Bowman's capsule, 931, 932/

Boysen-Jensen, Peter, 792 Brachiopods (lamp shells), 640/, 649 Bracts, 716/ brainstem, 1028/, 1029-30 cerebellum, 1030 cerebral cortex, 1028f, 1029, 1031, 1032-37 cerebrum, 1028/, 1031-32 control of breathing by, 890 development of embryonic, 1028/-29 diencephalon, 1028. 1030-31 evolution of chordate, 675 human (see Human brain) injuries to, 1035, 1037-41 mammalian, 694 neural processing of vision in, 1062/, 1063 primary motor and somatosensory cortices of, 1033/ thalamus and epithalamus of, 1030 ventricles of, 1026/ Brain hormone. 960 Brainstem, 1028/, 1029-30 Braithwaite, Victoria, 1113-14 Branchiostoma, 675/ Brassinosteroids, 791, 798 Brazil, Biological Dynamics of Forest Fragments Project, 1221 BRCA1 and BRCA2 genes and breast cancer, 374 Bread mold (Neurospom crassa), 310, 3 1 1 / 616/ life cycle, 617/ Breast cancer, 400 genetic basis of some, 374 growth and metastasis of malignant, 233/ Breathing. 888-90 in amphibians, 888 in birds, 889/ centers in human brain for automatic control of, 890 in mammals, 888, 889/ negative pressure, 888, 889/ positive pressure, 888 Breathing control centers, 890 Brenner, Sidney, 429 Briggs, Robert, 416 Brightfield microscopes, 96/ Bristlecone pine (Pinus hngaeva), 595/ Brittle stars (Ophiuroidea), 666, 667/ Broad Institute, Eric Under at, 236-37 Broca, Pierre, 1034 Broca's area, brain, 1034, 1035 Bronchi, 887 Bronchioles, 887 Brown algae (phaeophytes), 560-62 alternation of generations in, 561-62/ human uses of, 561 structure. 560, 561/ Brown fat. 838 Brush border, 858, 860 Bryophyta (division), 580 Bryophytes, 578(, 580-83 diversity of, 582/ ecological and economic importance of 583 gametophyte and sporophyie of, 580-81 583 L. Graham on evolution of, 510 life cycle, 581/ Br^ozoans (ectoprocts), 640/ Bt toxin, 784 Budding, asexual reproduction by, 965 in yeast, 611/ Buffers, 55 Bulbourethral glands, 972 Bulbs, 715/ Bulk feeders. 845/" Bulk flow, 743 in circulatory systems, 868, 878 long distance transport in plants and, 743-44 transport of xylem sap as. 748/ 749 Bulk transport across plasma membrane, ] 37, 138/ Bundle-sheath cells, 196/, 724 Burgess Shale, 629/ Burkitt's lymphoma, 370

Butterflies metamorphosis in, 6 6 1 / 943/ nonheritable variation with populations of map, 462/ Buttress roots. 714/ C 3 plants, 195 Q plants, 196 anatomy and pathway of, 196/ CAM plants compared to, 197/ Cacti, 749 Cactus ground finch (Geospiza scandens), 443/ Cadherins, 1002 role in development of blastula, 1003/ Caecilians (Apoda), 685/, 686 Caenorhabdms z'.egars trematode) ceil lineage of, 426/ cell signaling and induction in, 425-26, 427/ genome. 394. 399;. 400 hydrostatic skeleton in, 1063 as model organism, 412/ programmed cell death in, 427-29 as pseudocoelomate, 631/ Calcitonin, 954, 955 calcium homeostasis and, 954/, 955 Calcium cortical reaction in egg fertilization and role of, 989, 990/ deforestation and loss of soil, 1199 hormonal control of homeostasis in, 954/, 955 maintenance of calcium ions in animal cells, 211/ muscle contraction and role of, 1068, 1069/ neural chemical synapses and role of, 1022/ regulation of blood concentrations of, in kidney. 934 as second messenger, 211, 212/, 790 storage of, in smooth ER tissue, 104-5 Callus, 782 Calmodulin, 1072 Caloric imbalance, 846-48 Calorie, 49, 828 Calvin, Melvin, 185 Calvin cycle, photosynthesis, 184-85, 198/ conversion of carbon dioxide to glucose in, 193, 194f, 195 cooperation between light reactions and, 185/ roleofG3Pm, 194 Calyptra, 5 8 1 / Cambrian explosion, 526-27, 629 Camel, role of fur in water conservation by, 926/ Camouflage, 446/ CAM (crassulacean acid metabolism) plants, 196, 197 Q plants compared to. 197/ cAMP_ See Cyclic AMP Campylobacter. 542/ Canavanine, 813 Cancer, 370-74 abnormal cell cycle and, 370, 371-73 breast, 232/ cdl-sigr.aliiig Dathv.ays, interference with, 371, 372f, 373 colon/colorectal, 305, 373/, 374 ' cytotoxic T cell response to, 910, 9 1 1 / environmental factors and, 370 fai.kv gene exmession as cause of, 362 inherited predisposition to, 374 leukemia, 288/. 404, 882 loss of cell cycle controls and, 232-33, 371-73 multisiep model of development of, 3 7 3 / 374 mutations as cause of, 370, 330 obesity and, 847 oncogenes, proto-oncogenes, and. 371 skm, 305/ 306 tumor-suppressor genes, faulty, 371 \ DCS oi genes associated with, 370-71 viruses as cause of, 370, 374 Candida dbicans, 623 Cannibalism, population fluctuations and, 1151 Canopy, forest, 1098 Canyon tree frog, cryptic coloration in, 1162/

CAP (catabolite activator protein), 356 Capases, 428 Capillaries, 869 as base of hypothalamus, 951 blood flow through, 877, 878/ blood flow velocity through, 875 exchange between interstitial fluid and blood at, 875, 879/ structure of, 874, 875/ Capillary bed, 869 blood flow through, 877, 878/ Capsid, viral, 335,336/ Capsomeres, 335, 336/ Capsule bryophyte sporangium, 580, 581/ prokaryotic, 536/ Carbohydrates, 69-74 catabolism of, 161, 176, 177/ cell-cell recognition and membrane, 129 digestion of, 859/ disaccharides, 70, 7 1 / monosaccharid.es (sugars), 70 polysaccharides (sugar polymers), 71-74 role of membrane, in cell-cell recognition, 129 ct Carbon, 78 Carbon, 58-67 as backbone of biological molecules, 58 formation of chemical bonds with, 59-61 functional groups attached to compounds containing, 63-66 organic chemistry and study of, 58-59 valences of, 60/ variations in skeletons of, 61/, 62, 63 versatility of, in molecular formation, 59-63 Carbon cycle in ecosystems, 1196/ Carbon dioxide (CO 2 ) atmospheric, and climate change, 1196/ 1203-5 atmospheric, in Carboniferous period. 588 Calvin cycle and conversion of, to sugar, 193-95 covalent bonding of, 60 gas exchange in animals and disposal of, 750 peat bogs and absorption of, 511 plant guard-cell opening/closing and levels of, 751 respiratory pigments and transport of respiratory, 8 9 3 / 894 Carbon fixation, 185 alternative mechanisms of, 195-98 Calvin cycle and, 185, 193-95 Carbonic acid, 54, 55 Carboniferous period, 596 swamp forests of, 588/ Carbon monoxide (CO) as neurotransmitter. 1025 Carbon skeletons, 61-63 animal diet and provision of, 844, 849-52 Carbony! functional group. 64/ Carboxyl enc, protein, 82/. 323 Carbnxyl times ionai group, 64/ Carboxylic acids, 64/ Cardiac cycle, 8 7 2 , 8 7 3 / Cardiac muscle, 826/, 1072 acetylcholine, G proteins and synapiie transmissions in,1024 Cardiac output, 873 Cardiovascular disease, 76, 847, 882-83 Cardiovascular system, 869-71 biood composition and function, 879—82 blood flow velocity in, 875/ blood pressure in, 876-77 blood vessels of, 874, 875/ diseases of, in humans, 76 heart of, 868, 871-74 lymphatic system and, 878, 879/ metabolic rate and. 870 Caribou, 849/ populations of, 455/ Carnivores, 844 ciigestive tract and dentition in, 862, 863/ plants as, 767, 768f Carotenoids, 188, 189

Carpels, flower, 429/. 430, 598, 772 Carrier proteins t 130 facilitated diffusion and, 133, 134/ Carriers of genetic disorders, 266 testing to identify, 269 Carrots, test-tube cloning of, 782/ Carrying capacity, 1145 estimates of, 1155 human ecological footprint and, 1155-56 Carson, Rachel, 1083/ Cartilage, 825/ vertebrate skeletons of, 678 Casparian strip, 745 Castor bean (Rincinus communis), 778/ Cat Cloning of, 417/ four stages of food processing in, 853/ X chromosome inactivation and, fur color in, 284/ Catabolism and catabolic pathways, 142 cellular respiration as, 142, 161-74 connection of, to odier metabolic pathways, 176-78 fermentation as, 174-76 Catalyst, 77, 150. See also Enzyme(s); Ribozymes Catalytic cycle, 78/ Catastrophism, 440 Catecholamines, 957 Cation, 41 membrane potential and uptake of, 740/ Cation exchange in soil, 760, 761/ Catolite activator protein (CAP), 356/ Cattle egret distribution of, 1084 mutualism between water buffalo and, 1164/ Caulerpa sp., 568/, 569 CD4 cell surface protein, 909 CDS cell surface protein, 910 Cdks (cyclin-dependem kinases}. 229-30 cDNA (complementary DNA), 390, 400 cDNA library, 390 Cech, Thomas, 515 Cecum, 862 cc&-3 gene, 427-28 ced-4 gene, 427-28 ced-9 gene, 428 Celera Genomics, 398 Cell(s), 5 / 6 - 5 , 9 4 - 1 2 3 animal (see Animal cell) ATP and work performed by, 148-50, 828 as biological theme, 27f cell fraciionaiion and study of, 97 cell-to-cell communication (see CeL signaling) differentiation of (see Cell differentiation) division of (see Cell division) eukaryotic, 8, 9/ (see also Eukaryotic cells) heritable information in, 7/ importance of, 94 information flow in (DNA -» RNA -* protein), 86/, 87 membranes of (see Plasma membrane) microscopes and study of, 95-97 morphogenesis and changes in shape of, 1001/ motility of (see Cell motililty) organization of genetic material in, 219 plant (see Plant cell) programmed death of (see Apoptosis) prokaryouc. 8, 9/ 'ysve also Prokaryotic cell) protobionts as precursors to. "313, 515 size of, compared with virus and bacterium, 335/ size range of, 95/, 98 surface-to-volume ratio of, 99/ ihree types ol work performed by. 148 ultrasiructure, 95 water channel proteins of, 93 Cell adhesion molecules (CAMs), 1002 Cell body (neuron), 1014 Cell-cell communication. See Cell signaling Cell-cell recognition membrane carbohydrates, and, 129 membrane proteins and, 128/

Cell crawling during morphogenesis, 1001-2 Cell cycle, 218-35 alternation of micotic phase and interphase in, 221-2S cancer and abnormal, 232-33, 370,371-73 cell division, roles of. in, 218-20 cell division and formation of daughter cells in, 219-20 cell division by binary fission and, 226-27 cytokinesis in, 224-26 defined, 219 phases of, 2 2 1 / regulation of, by cyiop!as:r.ic signals, 228-33 Cell-cycle control system, 229-31 cancer as failure of. 232-33 clock for, 229-30 Gi checkpoint in, 229/ G2 checkpoint in, 230/ mechanical analogy for, 229/ stop and go (internal and external signals) in, 230-31 Cell death, programmed. See Apoptosis Cell differentiation, 362, 413, 987 cell-cell signals and, 420, 4 2 1 / 429-30 cell determination and, 418-20, 1003-8 cytoplasmic determinants affecting, 420, 421f, 987 determination of muscle cells and, 4 1 9 / 420 embryonic development and (See Embryonic development) in plants, 732.733/ Cell division, 218-28 asymmetrical, 730 binary fission as bacterial. 226, 2271" cancer as uncontrolled, 232-33 ce'.luijr organization of genetic material and, 219 '.• :TC!"!L?M•niv 1 o S'.r bv.u.i'". u . j i i n c z i 9 — 2 0

cytokinesis and, 224-26 daughter cells produced by, 219-20 density-dependent inhibition of, 231, 232/ embryonic development and role of, 218/ [unctions of. 218/ key roles of, 218-19 mitosis and, 221, 222-231,224,225/ 226/ 227-28 plane and symmetry of, and plant morphogenesis, 729/ platelet-derived growth factor (PDGF) and, 2 3 1 / preprophase band and plane of, 730/ reproduction and role of, 218/ tissue renewal and role of, 218/ Cell expansion, 730 in plants, 731/ Cell fate in animal development, 1003-8 blastopore dorsal lip, effect on, 1006/ determination of, and pattern formation by inductive signals, 1006-8 determination of, and transcriptional regulation of gene expression, 418-20/ establishing cellular asymmetries, 1004-5 fate mapping. 1004/ Cell fate in plant development, 732-33 Cell fractionation, 97/ Cell lineage. 426 of nematodes, 426/ Cell-mediated immune response, 899/ 908, 909 cytotoxic T cell Function against p^l-o^ens -r. 910,911/ helper T lymphocyte function in, 909, 910/ overview of, 909/ Cell migration in animal morphogenesis, ] 001-2 role of fibronectin in, 1002/ Cellmoiility, 112, 114, 115/ 116f, 117/, 118f in animal morphogenesis, 1001/, 1002-3 Cell plate, 225/, 226 Celi signaling. 201-17 in animal development, 425-29, 1006-8 cell differentiation and, 420, 421/, 429-30 control of coordinate gene expression by, 367 conversion of external signals into cell responses, 201-4

efficiency of, 215 in embryonic development, 420, 421/ endocrine system and, 945-48 evolution of, 201-2

interference with normal, and development of cancer, 371,372/, 373 local and distant, 202/, 203/ nerve cell development and, 1037, 1038/ nervous system, neurotransmitters, and, 1021-25 overview of, 201,204/ pattern formation in animal development, and inductive, 1006-8 in plant development, 429-30 responses to, through regulation or transcription, 204,212-15 root nodule formation in legumes and role of, 766 reception of signals, 204-8, 945-46 signal amplification, 214 signal transduction pathways in, 202, 208, 945-48 specificity of, 214/ termination of, 215 three stages of, 203-4 transduction and relay of signals to target cells, 204.208-12 Cell size, 335/ Cell-surface receptors for water-soluble hormones, 946-47 Ceilular energev.c?. Set Bioenergeties Cellular respiration, 142, 160, 161-80 anabolic pathways and, 177 ATP synthesis in, 170-74 catabolic pathways of, 161-65 citric acid cycle of, 168, 169f, 170 disequilibrium in, 147/ electron transport chain in, 163/, 164 as exergonic reaction, 146, 15.3/ feedback mechanisms lor regulation of, 177, 17Sf fermentation compared to. 175-76 formula, 146 glycolysisin, 165, 166/, 167 mitochondria as site of, 109, 110/ 164-65, 168-74 redox reactions in, 161-64 stages of, overview, 164/, 65 versatility of catabolism and, 176, 177/ Cellular slime molds, 565 life cycle of Dictyosielntm, 5661' Cellulose, 52, 72, 106, 558 arrangement of,in plant cell walls, 7 3 / bacterial digestion of, 74/ cell expansion and orientation of microfibrils of, 72, 73/ structure of, compared with starch, 73/ Cell wall (plant), 118, 119/ transport in plants and, 743 water balance and, 132, 133/ Cell wall (prokaryote), 534-36 water balance and. 132 Celsius scale, 49 Cenozoic era, 5\% 5 2 1 / 629 Center for Conservation Research and Training, Hawaii. 436-37 Centipedes (Chilopoda), 659, 660/ Central canal of central nervous system, 1026 Central nervous system (C.NS), 1012, 1026/ brain, 1028-37 central canal of, 1026 injuries and diseases of, 1035, 1037-41 oligodendrocyit-s of, 1015 organization of vertebrate, 1012-13 Central vacuole, 108/ Cenrrioles, 114/ Centromere, 220, 222/ 991 Centrosome, 1 1 4 / 221, 222/ 223/ Cephalization, 630 CephalochordiKes (Cephalochordata), 674, 675/ Cephalopods (Cephalopoda), 652, 653/ Ceratium tripos, 550/ Cercozoans (Cercozoa). 363 Cerebellum, 1030

Cerebral cortex, 1028/, 1029, 1031, 1032-37 consciousness and, 1036-37 emotions and, 1034-35 information processing in, 1032-33 language, speech, and, 706, 1034/ lateralization of function in, 1033-34 lobes of, 1032/.. 1035 memory, learning, and, 1035-36 primary motor and somatosensory cortices of, 1033/ Cerebral hemispheres, 1031 lateralization of cortical function and, 1033-34 Cerebrospinal fluid, 1026 Cerebrum, 1028/ 1031-32. See also Cerebral cortex hemispheres of, 1031/ Certainty of paternity, 1124-25 Certhidea. olivacea(grten warbler finch), 443/ Cervix, 970 Cestoda Ctapeworms), 646£, 648/ cGMP (cyclic GMP), 211, 790 Chagas' disease, 554 Chambered nautilus, 653/ Channel proteins, 130 facilitated diffusion and, 133. 134/ Chaparral, 1101/ Chaperonin, 85/, 325 Chaperon protein. 85, 325 Character, genetic. 252. See also Trail, genetic discrete. 463 law of independent assortment and, 256-58 law of segregation and, 253-56 multiiaciorial 264 quantitative, 263, 463 Character displacement, 1161 Characters shared derived, and shared primitive, 498 table of, 499/ Charasp.,574/' ChsTgaff, Erwin, experiments on DN'A nucleotides by, 296 Chargaffs rules, 296, 298, 313/ wobble and relaxation of, 322 Charophyceans, 567, 569 evolution of land plants from, 510-11, 573-74 examples of, 574/ Chase, Martha, experiments on genetic material by, 294, 295/ 296 Cheating and noncheating cells on cellular slime mold, 566 Checkpoints in cell-cycle control system, 229/, 230/ Cheetah energy transformations and metabolism in, 143/ territoriality in, 1149/ Chelicerae, 658 Cheliceriforms, 658f, 659/ Chemicalls) as mutagens, 330 protein denaturation caused by, 85 Chemical bonds, 39-43 of carbon, 59-63 chemical reactions and, 44-45 covalent, 39/, 40, 60 hydrogen, 42 ionic, 41f,42 van der Waal? mterae'.Lons and, 42 weak, 42 Chemical cycling in ecosystems, 160/, 1184-86, 1195-98 carbon cycle, 1196/ decomposition and, 1185-86 disruption of, by human population, 1200-1206 general model of, 1195 nitrogen cycle, 1197/ overview, 1185/ phosphorus cycle, 1197/ regulation of, by vegetation, 1198-99 role n: 'irokavvok'^ in. 54-^ trophic relationships as determinants of, 1185 water cycle, 1196/ Chemical elements. Sec Flement(s)

Chemical energy, 142. See also Chemical reactions conversion of light energy into, in photosynthesis, 182-85 Chemical equilibrium, 44 Chemical markers of territory, 1149/ Chemical messengers, cell signaling using, 205. See also Signal transduaion pathways Chemical properties, electron configuration and, 37-38 Chemical reactions effects of enzymes on (see Enzyme(s)} energy levels of electrons and, 36 exergonic and endergonic, 146, 147/ 148, 149/ free energy and, 146 making and breaking chemical bonds in, 44-45 reactants and products of, 44 Chemical signals in animals. See ako Cell signaling animal communication behavior and, 1149/ cell-surface receptors for, 946, 947/ endocrine system and, 205, 943-44, 945/(see also Endocrine system) hormones as, 945-48, 949t, 951-59 intracellular receptors for. 205/, 947 local. 202-3 nervous system and, 943-44, 9 4 5 / 1021-25 {see aha Nervous system) neurosecretory cells as, 944 neurotransmitters as, 947-48, 1024/ 1025 paracrine signaling, 947-48 pheromones as, 437, 610, 945 signal transduction pathways initiated by, 1024 Chemical signals in plants. See Cell signaling; Plant hormones Chemical synapses, 1021-25 diagram of, 1022/ direct synaptic transmission and. 1022-23 indirect synaptic transmission and, 1023-24 neurotransmitters in, 1024-25 synaplic vesicles and, 1021 Chemical work, cellular, 148 driven by ATP, 149/, 828 Chemiosmosis, 171, 740 in chloroplasts, during photosynthesis, 110, 1 1 1 / 173 in chloroplasts versus in mitochondria, 109-11, 173,192-93 in mitochondria, during cellular respiration, 109, 110/, 164-65, 171-73 inprokaryotes, 521-22 transport in plants and, 740 Chemistry, 32-46 atomic structure and properties of elements, 34-39 chemical bonding of atoms and molecule formation and function, 39-43 chemical reactions, 44-45 foundations of biology in, 32 L. Makhubu on medicine and, 30-31 matter, elements, and compounds, 32-34 of metabolism (see Metabolism) molecular shape and biological function, 42-43 Chemoautotrophs, 538, 539( Chemoheterotrophs, 5391 Chemokines. 901 Chemoreceptors, 1048 in insects, 1048, 1049/, 1055/ for taste and olfaction, 1054-56 Che mo: ax is, 537 Chemotrophs, 538-39 Cheny, Dorothy, 1131-32 Chesapeake Bay, partial food web for, 1167/ Chiasma. 247 Chick embryo, homologous structures in human and, 449/ Chicxulub asteroid and crater, Caribbean Sea, 520/ Chief cells, 857/ Childbirth, 981/ positive feedback mechanisms in, 833/ Chile, relationship between rainfall and herbaceous plant cover in desert community in, 1171/ Chilopoda (centipedes). 659. 6(?y){

Chimeras, 430 Chimpanzees, 701/ cognition and problem solving in, 1117, 1118/ skull of, 484/ 494 China birth rate and population growth in, 1154 fossils found in, 676, 677/ Chinese river dolphin, 1211/ Chitin, 74, 609, 1064 in exoskeletons, 7 4 / 1064 in fungal walls, 609 medical uses of, 74/ structure of, 74/ Chitons (Polyplacophora), 651/ nervous system, 1012/ Chlamydia tmchomatis, 543/ Chlamydomonas sp., 567, 568/, 569/ Chlorofluorocarbons (CFCs), atmospheric ozone depletion and, 1205-6 Chlorophyll(s), 182 chlorophyll a, 187, 188, 189, 190 chlorophyll b, 188. 189, 190 light reception by, 187f, 189 photoexcitation of, 188, 189/ structure of, 188/ Chlorophyll :,greer algae), 567-69. 575/ colonial andmulticellular, 568/ life cycle of Qilamyur-niiincu, 5c9f Chloroplasts, 109, 110,111/ capture of light energy by, 110, 111/ chemiosmosis in mitochondria versus in, 109-11, 173,192-93 chlorophylls in, 182 (set1 also Chlorophyll(s)) in green algae, 567 green leaves as interaction of light with, 186/ as site of photosynthesis. 109, 110, 111/, 182, 183/ starch storage in, 71, 72/ structure of chlorophyll in, 188/ Choanocytes, 642 Choanoftagellates, 628/ Cholecystokinm (CCK), 860/ Cholera (Vibrio chokrae), 206/ 211, 542/ 546 Cholesterol, 77 diseases associated with, 882-83 low-density and high-density lipoproteins of, 137, 883 membrane fluidity and role of, 126f structure of, 77/ transport of, 137, 882-83 Chondrichthyans, 680-82 Chondrocytes, 825/ Chordates (Chordata), 6 4 1 / 671 craniates as, 675-77 derived characters of, 673 early evolution of, 674-75 fate mapping for two, 1004/ invertebrate, 667 lancelets as, 674, 675/ nervous system, 1012/ (see aho Nervous system) phytogeny, 671,672/ 673 skeleton of, 1064, 1065/ tunicates as, 673-74 Chorion,1000 Chorionic villus sampling (CVS), 269, 270/ Choroid, 1058, 1059/ Choryjoanne, 798 C.i.Ji;--"i".ois ii'i-]':1 \ ' o i m . 6;'B,'

Chromatids chiasmata of nonsister, 247 crossing over of nonsister. 247 sisier, 219,220/ tetrad of, 247 Chromaun, 102, 219. 359 DNA packing and structure of eukaryotic, 350-60,361/ euchromatin and heterochromatin in, 360 nuclear transplantation and changes in, 416 nucleosomes of, 360, 361/

structural modifications of, as regulation of eukaryotic gene expression, 363-64 Chromatium, 542/ Chromoplasts, 110 Chromosomal basis of inheritance, 274-92 alterations of chromosomes, genetic disorders, and 285-88 behavior of chromosomes, and Medelian inheritance, 274-77 linked genes and, 277-82 locating genes on chromosomes and. 274 select inheritance patterns as exceptions to theory of. 288-90 sex-linked genes and, 282-84 Chromosome(s), 98, 102, 219. See also DNA (deoxyribonucleic acid); Gene(s) alterations in structure of, 286/ 287, 288 bacterial, 226, 227/ 346, 347/ bacterial artificial, 396 during cell division, 218/ 219, 220/ chromatici ri '.s;'e Crrornatin) description and terms related to, 241/ distribution of, to daughter cells during mitosis, 219,220/222-23/225/ eukaryotic, 219/ 359/ extra set of (polyploidy), 285, 378 gene locus on, 239 genetic disorders due to structural alterations of, 288 genetic maps of, 279-81 genetic recombination and, 278-79, 280/ independent assortment of, 247, 248/ 274, 275/ 278 homologous, 240 (see also Homologous chromosomes) karyotype of, 240/ locating genes on, 274 Mendelian inheritance based in behavior of, 274-77 mutations in, leading to genetic variation, 463-64 nondisjunction of, 2S5/-86 number of human (see Chromosome number m humans) Philadelphia, 288 prokaryotic, 537/ reduction of, from diploid to haploid during meiosis, 242. 243-47 sex, 282-83 (see also X chromosome; Y chromosome) translations of, 286/ 287, 288/ 459 "walking" of, during mitosis, 224, 225/ Chromosome number in humans, 102, 240-41 abnormal, 285/-S6 aneuploidy and polyploidy, 285, 378 genetic disorders caused by alteration of, 285-88 reduction of. by meiosis. 241—47 in somatic cells, 219, 240 Chromosome theory of inheritance, 274-77 exceptions to, 288-90 Mendelian inheritance related to, 274, 275/ 276-77 (see also Mendelian inheritance) T.H Morgans experimental evidence supporting, 276-77 sex chromosomes and, 285-88 Chronic myelogenous leukemia (CML), 288/ ChrysophySes ;golden algae), 560 Chylomicrons, 861/ Chytndium, 613/ Chytrids (Chytridiomycota), 613 Cichlid fishes speciationin,479, 480/ territoriality in, 1107/ Cilia, 15/", 114-16 dynein "walking" and movement of, 116/" motion of, 115/ ultrastructure of, 115/ Ciliary body, 1059 Ciiiates, 550/ 556, 557/ 558 Orcadian rhythms, 751, 805 in animals, 1030-31 in plants, 751, 805

Circular DNA, 289 Circulatory system, 867-83 adaptations of, for thermoregulation, 835-37 amphibian, 870/ 871 animal phylogeny reflected in, 867-71 arthropod, 657 blood composition and function, 879-82 blood flow velocity, 875/ blood pressure, 876-77 bloodvessels, 874, 875/ capillary function, 877-78 closed, 868, 869/ diseases of, in humans, 882-83 double, in mammals, 871-74 in fish, 870/871 heart, 871-74 human, 871-83 invertebrate, 868-69 gas exchange and, 885, 891-95 lymphatic system and, 878-79 mammalian, 870/871-83 open, 657,868, 869/ physical principles governing, 874-79 placenta, and fetal, 980/ reptile, 870/ 871 vertebrate, 869-71 Cisternae of golgi apparatus, 105, 106/ Cisternal maturation model. 106 Citric acid cycle, 164, 168-70 connection of, to other metabolic pathways, 176-78 junction between glycolysis and, 168/ overview, 168/ steps of, 169/ Clade(s), 497 grades versus, 630 lizard-bird, 504/ rnammal-bird, 504/ plant kingdom, 575/ Cladistics, 497, 498 Cladogenesis as evolutionary pattern, 472/ Cladogram, 497 construction of, 499/ Clams, 639/, 652 anatomy of, 652/ Clark's nutcrackers (\V:/i j.ijc coiumbiana), 1116 Class (taxonomy), 496 Classical conditioning, 1116-17 Classification of life, 12, 13/ 14-15. See a!so Taxonomy Citivkfns panmrea, 622 Clayton, David, 818 Cleavage, 224-26, 627, 978, 992 in amphibian, 992, 993/ in animal cells, 225/, 627/ 978, 979/ 992-94 in bird embryo, 994/ cleavage of zygote, 627/ 978, 979/ 992-94 cytokinesis and. 224, 225/ 226 in frog embryo, 992, 993/ gray crescent, distribution at first, 1005/ in human zjgote, 978, 979/ 999, 1000/ in mammals, 999, 1000/ 1001 meroblastic, and holohla^.ie 99in plant cells, 225/ in protostomes versus deuterostomes, 631-32 in sand dollar embryo, 992/ Cleavage furrow, 224 Clements, E E., 1178 Climate, 1087-92 as abiotic factor, 1087-92 air circulation, wind and, 1089/ alternative carbon fixation in plants living in arid, 195-98 changes in (global warming), 1204-5 Cretaceous mass extinctions and, 518-20 Earth's latitude and, 1088/ effect ofbodies of water on, 1987, 1090/ effect of mountains on, 1090/ effect of seasonalily on, ]088(. 1090. 109 if global patterns of, 1087, 1088-89/ latitudinal gradient in biodiversity and, 1176

long-term change/warming in, 1091, 1092/ microclimate, 1087, 1091 terrestrial biome distribution and structure determined by, 1098-1104 Climate change, 1091-92 effect of, on forests and trees, 1092/ global warming, greenhouse effect, and, 1204-5 Climotius acanthodian, 680/ Climograph, 1098/ Cline, 464 Clitoris, 970 Cloaca, 682, 968 Clock mutants, 805 Clonal selection, 907 Clone, 239, 385, 415. See also Gene clones animal, 415-17 asexual reproduction and production of, 239 plant, 415, 781-83 Cloning, 415 of animals, 415-17 of DNA and genes, 385-92 of plants, 415,782-83 Cloning vectors, 386 expression vector as, 390 phagesas, 388, 389/ plasmids as, 386-88 Closed circulatory system, 650, 868 open circulatory system versus, 869/ in vertebrates (see Circulatory system) Closed system, 143 equilibrium and work in, 147/ Clostridium botulinum (botulism), 206/ 339, 543/ 1069 Clownfish (Amphiprion ocellaris), 683/ Club fungus (Basidiomycota), 618-19 Club moss, 586, 587/ Clumped pattern of population dispersion, 1138/ Cnidarians (Cnidaria), 633, 639/ 643-46 anthozoans, 645 cubozoans, 645 classes of, 6441 digestion in, 854/ hydra (see Hydra) hydrozoans, 644, 645/ hydrostatic skeleton in, 1063 internal transport m Aureha, 868 life cycle of Obdia, 645/ nervous system, 1012/ origins of, 526 polyp and medusa forms of, 643/ scyphozoans, 644 Cnidocytes (cnidarians), 643 Coal formation, 588 Cocci (spherical-shaped) prokaryotes, 534, 535/ Cocci diotdomycosis, 623 Coccosteus placoderm, 680/ Cochlea, hearing and, 1051/" pitch and, 1052, 1053/" transduction in, 1052/ Coconu!, seed crop size in, 1142/ Codominance of allele, 260 Codons, 312,313 start and stop, 313, 314/324 Coefficient of relatedness, 1129-30 Coelocanch (Latimeria). 683/ 684 Coelom, 631 Coelomates, 631/ deuterostomes {see Deu:erosiomes (Deuterostomia)) protostomes, 369, 370 Coenocytic fungi, 609/ Coensyme, 155 Coevolution, 1164-65 relationship between angiosperms and animals as, 604 Cofactors, enzyme. 155 Cognition, 1117-18 Cognitive ethology, 1117 Cognitive maps, 1116 Cohesion, 48 in ascent of xylem sap, 748/

Cohort, population, 1139 Coitus, 972 Cold, plant response to, 812 Cold virus, 337, 343 Cokochaete sp., 574/ Coleoptile, 778 response of, to light, 792/ Coleorhiza, 778 Collagen, 119, 380 animal cells, 626-27 in extracellular matrix of animal cell, 119 quaternary structure of protein in, 83/ Collagenous fibers. 823 Collecting duct, 931, 932/, 933/, 934 Collenchyma cells, 718 Colloid, 52 Colon, 861-62 cancer of, 305 Colonies, 526 ofeukaryotes, 526 of prokaryotes, 539/ of protists, 568/ Coloration aposematic, 1162/ cryptic, 1162 in planl leaves, 789/, 790/, 791 in skin, 685, 946 warning, in snakes, 21-24 Colorblindness, 283, 1061 Colorectal cancer, model for development of, 373/, 374 Color vision, 1059-60, 1061 Colpidmm, cilia motion in, 115/ Columnar epithelia, 823, 824/ Comb jellies (Ctenophora), 633, 640/ Commensalism, 545,1164 Commercial applications/uses of angiosperms, 605 of fungi, 623 Pharmaceuticals (see Pharmaceutical products) Common juniper (Juniperus communis). 595/ Communication, animal, 1111-12 Community communities), 4/, 1082, 1159 adaptation and interspecific interactions in, 1164-65 biogeographical factors affecting biodiversity in, 1175-78 bottom-up versus top-down models of structure in, 1170-71 commensalism in, 545, 1164 competition in. 1160-61 conservation of {see Conservation biology) conirasting views on structure of, 1178-80 controls in, 1170-71 disease in, 1163 disturbances in, 1171-75 dominant species in, 1168 ecological succession in, 1173-75 foundation species in, 1169-70 herbivory in, ] 163 I ' l l l i i d i ] ! ^ ai- ^.vl' 1 ' : v ••• dlSLl."."" "'IL.IC-C in

1 1 / i

individualistic versus interactive hypotheses on structure of, 1178,1179/ interspecific interactions in, overview, 1159-60 keystone species in. 1168/", 1169/, 1214 mutualism in, 1164 parasitism in, 1163 predationin, 1161-62 recovery time for disasters in, 1226/ rivet and redundancy models on structure of, 1180 species diversity and richness in, 1165-66 stability and change in, 1171-72 trophic structure of, 1166-68 Community ecology, 1082, 1159-82 biogeographic factors affecting biodiversily, 1175-78 debate on structure of communities, 1178-80 defining "community,'" 1159 disturbances aLeciin^ ^or< \e^, chversily Lir.d composition, 1171-75 dominant and keystone species and, 1165-71 interspecific interactions, 1159-65

Companion cells, 719 Competition, 16, 1160-61 character displacement and, 1161/ competitive exclusion principle and, 1160 ecological niches and, 1160-61 i'tsi'Liure pr.rutionx.^ and, 1161/ Competitive enzyme inhibitors, 155/ Competitive exclusion, 1160 Complementary DNA (cDNA), 390, 400 Complement system, 900, 914 Complete digestive tract, 855. See also Alimentary canal Complete dominance of allele, 260 Complete flowers, 773 Complete metamorphosis., 661 Compound(s), 33 elements and 33-34 emergent properties of, 33/ organic, 59-66,513-14 Compound eyes, 1058/ Compound leaf, 716/ Concentration gradient, 131, 135 cotransport as active transport driven by, 136 Concentricycloidea (sea daisies), 666[, 667/ Conception, 978, 979/ Condensation reactions, 68. 69/ Condom, contraceptive, 983 Conduction, 835 Cone cells, eye, 1059, 1061 Confocal microscopy, 96/ Conformers versus regulators, 832 Conidia, 616 Coniferous forest, 1102/ Conifers (Coniferophyta), 593 diversity of, 595/ forest biome of, 1102 life cycle of pine, 596, 597/ Conjugation, 349, 558 in bacteria, 349/, 536, 538 in ciliaie Parumecmm. 557/' plasmids and bacterial, 349-51 Conjunctiva, 1058. 1059/ Connective tissue, 823, 825 types of, 825/ Connell, Joseph, studies of competition and niches of barnacles, 1160/ Conodonts, 678, 679/ Consciousness, human, 1036-37 Conservation biology 1209-32 biodiversity and human welfare in, 1211-12 human threats to biodiversity and, 1212-14 K. Kaneshiro on, 437 at landscape and regional level, 1220-24 levels of biodiversity addressed by, 1210-11 at population and species levels, 1215-20 restoration ecology and, 1224-28 species area curves used in, 1 ] 76 sustainable development and, 1229-30 threat of human activities and need for, 1209-14 Conservation of energy, 143 Conservative model of DNA replication, 300/ Constant (C) regions, lymphocytes, 904f Consumers, 6 Continental drift, 527, 528/ history of, during Phanerozoic, 529/ plate movements and, 528/' species distribution and, 528 Contour tillage, 762/ Contraception, 982-83 mechanisms of, 982/ reproductive technology and, 983-84 Contractile vacuoles, 108 in Paramedum, 132,133/ Control center, animal's internal environment and, 832,944 Control elements, eukaryotic gene, 364-66, 367/ proximal, and distal, 364-65 Controlled experiment, 23-24 Convection, 835 Convergent evolution

of aquatic animals, 8 2 1 / of marsupials and eutherian mammals, 450, 494/, 495, 696/ of oomycetes and fungi, 558 in terrestrial biomes, 1099 Convergent extension, 1002 animal morphogenesis and cell sheet, 1002/ Cooperativity, enzyme activity and, 156/, 157 Cope pods, 664 Coppola, Francis Ford, and family, 238/ CoquereVs sifakas (Pi'iifii/kMas •crcciux- coquerdi), 697/ Coral reef, 1097/ algae and, 567/ Corals, 639/. 645 Coral snake, eastern, case study on mimicry of warning coloration in, 21-24 Corepressor, 354 Cork cambium, 720 production of periderm and, 728 Corn. See Maize {Zea mays) Corn borer, European, 1085 Cornea, 1058, 1059/ Coronavirus, 344 Corpus callosum, 1031/, 1032 Corpus luteum, 969 Correns, Karl, 289 Corridors between ecosystems, 1221-22 Cortex, plant tissue, 717 in roots, 722-23 Cortical granules, 989 Cortical nephrons, 931, 932/ Cortical reaction, 989/, 990/ Corticosteroids, 958 Coruzzi, Gloria, 25/ Costa Rica infant mortality and life expectancy in, 1229/ parks and zoned reserves in, 1223/, 1224 restoration project in tropical dry forest of, 1228/ sustainable development in, 1229-30 Cotransport, 136, 740 solute transport in plants and, 740/ Cotyledons, 600, 780/ Countercurrent exchange, 885 in blood vessels of fish gills, 885, 886/ thermoregulation and, 836/ Countercurrent heat exchanger, 836 Countercurreni "T:i! M I p.'C" svsierns, 936 Courtship behaviors, 1106. See also Klaling in Dmopkh, 436-37 Covalem bonds, 39-40 carbon and, 60 double, 40 formation of, 39/ in four molecules, 40/ nonpolar and polar, 4 0 , 4 1 / Cowbird, brown-headed (Molothrus ater), 1221 Cowpox, 914 Cows (dlulose-digescmg oauena m, 74/ ruminani digestion in, 864/ Coyote, digestive traci of, 863/ Crabs, 664 ghost crabs (Ocypode), 664/, 829/ population fluctuations in Dungeness, 1151/ Cranial nerves, 1026 Craniaies, 675.676 derived characters of, 676 hagfishes as, 676, 677/ neural cresi of, 676/ origin of craniates, 676, 677/ Crassulacean acid metabolism (CAM), 197 Crawling, 1073-74 Crayfish, 664 gills, 885/ stretch receptors in, 1047/ Crenarchaeota, 544 Cretaceous period mass extinctions, 518-21 Cretinism, 953, 954 Creutzfeldl-Jacob disease, 345

Cribrostatin, 643 Crick, Francis, 9 on DNA replication, 299 modeling of DNA structure by J. Watson and, 88-89, 293/ Cri du chat syndrome, 288 Crinoidea (sea lilies, feather star), 666(, 667/ Cristae, 110/ Critical load of nutrients in ecosystems, 1200 Crocodiles, 691/ Crop plants. See also Agriculture angiosperms, 605 genetic engineered, 407 improving protein yield in, 764 polyploidy in, 478 Crop rotation, 766 Cross-fostering, influence of, on male mice, 1114( Crossing over, 247, 278, 348 genetic recombination of linked genes by, 278, 279/, 280/ introns and, 319 production of recombinant chromosomes by, 248, 249/ unequal, and gene duplication, 378/, 379 Cross-pollination, 599 Crows (Corvus caurinus), energy costs/benefits of feeding behavior m, 1122 Crustaceans (lobsters, shrimp, crabs), 641/, 658(, 664J-65 exoskeleton in, 657/ Hox gene expression in, 432/ Cryoprotectants, 840 Cryphonectriaparasitica, 622 Cryptic coloration, 1162 Cryptochromes, 802 Crystallin, 419 regulation of gene expression lor synthesis of, 367/ Ctenophora (comb jellies), 633, 640/ ctr (constitutive triple-response) mutant, 800/ Cuban tree snails, 454/ Cuboidal epithelia, 823, 824/ Cubozoans (box jellies, sea wasps), 644/ 645 Cultural eutrophication of lakes, 1201 Culiure, 1131 Cuticle, 575,655 plant, 575, 717 Cuttings, plant, 781-82 Cuvier, Georges, 440 Cyanobacteria, 537/, 539/, 543/ blooms of, 1171,1190/ lichens and, 621-22 oxygen production by, 522 plastids and, 551/ Cycads (Cycadophyta), 593, 594/ Cycas rwolute, 594/ Cyclic AMP (cAMP), 210, 356 as second messenger, 210/, 211/, 1024, 1056 Cyclic electron flow, photosynthesis, 191/, 192 Cyclin, 229-30 Cyclin-dependent kinases (Cdks), 229-30 Cycliophoran Symbion Pandora, 6 4 1 / Cystic fibrosis, 266, 403 as recessively inherited disorder, 266 Cystinuria, 134 Cytochromes (cyt), 170, 851 cytochrome c, 428 Cytogenetic maps, 281, 396/ Cytokines, 903, 948 Cytokinesis, 220, 223/, 245/ in animal and plant cells, 225/ cleavage and, 224, 225/, 226 Cytokinins, 796-97 anti-aging effects of, 797 control of apical dominance in plants by, 796/, 797 control of plant cell division and differentiation by, 796 Cytolog), microscopes as tool in, 97 Cytoplasm. 98 cell cycle regulated by signals in, 228-33

cytokinesis and division of, 220, 224-26 response of, to cell signaling, 212, 213/ Cytoplasmic determinants, 420, 987 cell differentiation and, 4 2 0 , 4 2 1 / pattern formation and*, 425-25 Cytoplasmic streaming, 117/, 118 Cytosine (C), 88 DNA structure and, 296/, 297/, 298/, 364 Cytoskeleton, 112-18 animal morphogenesis and changes in, 1001/ components of, 113-18 intermediate filaments of, 113(, 118 membrane proteins and, 128/ microfilatnems of, 113t, 116-18 microtubules of, 113t, 114-16 motor molecules in, 112/ role of, in cell support, motility and regulation, 112/-13 structure and function of, 113t Cytosol, 98 ionic gradients across neuron membrane and, 1016/ transport in plant cells and, 743 Cytotoxic T cells, 904, 905/ function of, against imracellular pathogens, 910, 911/ interaction of, with MHC molecules, 905/ Daffodil, radial symmetry in, 773/ DAG(diacylglycerol),212 Daily torpor, 841 DaUon, 34,52 Dalton, John, 34 Dandelion, seed crop size, 1142/ Danielli, James, 125 Darwin, Charles, 15/-19, 438-51, 482, 497, 601, 682, 1081,1143,1175 adaptation in research and theory of, 442, 443/, 444-46 Beagle voyage and field research of, 441-43 descent with modification and theory of", 443-44 evidence supporting theory of, 446-51 genetic variation and theory of, 248-49 historical context of life and ideas of, 439/ modern syruhesi; o: Mendenan .nhentance and evolutionary theory of, 455 natural selection and evolutionary adaptation as theory of, 16, 17/, 438. 441-46 On the Origin ojSpecies by, 15, 438, 439, 443-46, 455 on resistance to idea of evolution and, 439-40 studies on phototropism by, 792 Darwin, Francis, 792 Data, scientific, 19-20 Davson, Hugh, 125 Davson-Danielli membrane model, 125 Day-neutral plants, 806 DCC (deleted in colorectal cancer) tumor-suppressor gene, 373/ Ddel restriction site, 393/ D-dopa, 62, 63/ DDT pesticide," 155 biological magnification of, in iood webs, 1202-3 Death rate in populations, per capita, 1143-44 Decapods, 664 Declining-population approach to species conservation, 1218-19 case study of red-cockaded woodpecker, 1218, 1219/ steps for analysis and intervention, 1218 Decomposers, 544, 1185 fungi as, 620, 1186/ prokaryotes as, 544, 1186 Decomposition in ecosystems, 1185-86 rates of, and nutrient cycling, 1198 Deductive reasoning, 20-21 Deep-diving mammals, respiratory adaptaiions of, 894-95 Deep-sea hydrothermal vents, 514/, 541, 1097/

De-etiolation in plants, 789/ as response to cell signaling, 789-91 Defensins, 901 Deforestation, 606

m Hubbattl Brfiok Experimental hffift, 1078-79, 1198, 1199/ of tropical forests, 1209/, 1212, 1221/ Dehydration in animal cells, 136 Dehydration reactions, 68, 69/ synthesis of fats, 75/ synthesis of maltose, and sucrose, 7 1 / Ddesseria sanguinea, 550/ Deletions chromosomal, 286/ of nucleotides, as point mutation, 329, 330/ Demographic transition, 1153-54, 1230 Demography, 1139 age structure and sex ratio as factors in, 1154-55 life tables and survivorship curves in, 1139-40 reproductive rates and, 1140-41 Denaturation of DNA, 388 of proteins, 84, 85/ Dendrites, 1014 stretch receptors and, 1047/ thermoreceptors and, 1048/, 1049 Dendritic cells. 900, 909/, 910/ Dense bodies, 1072 Density-dependent birth/death rates, 1148-50 Density-dependent inhibition of cell division, 231, 232/ Density-independent birth/death rates, 1148 Density of populations, 1137-38 density dependent birth/death rates, 1148-50 density independent birth/death rates, 1148 determining equilibrium for, 1148/ dispersion patterns and, 1137-39 population change and, 1148 Dental plaque, 539/ Dentition, evolutionary adaptions of, 862, 863/ Deoxynbonucleic acid. See DNA (deoxyribonucleic acic) Deoxyribose, 88 Depolarization, membrane potential and, 1018/ Depression, mental, 1025, 1040 Derivatives, 721 Derived traits of plants, 575, 576-77/ Dermal tissue system, 717 Dennis, animal, 835/ Descent with modification, 16, 18/, 443, 444/. See also Natural selection Desert biomes, 1100/ Desmosomes, 120, 121/, 627 Determinant growth, 720 Determinate cleavage, 632 Determination, 419. See also Cell differentiation Detritus, 1093, 1185 Detrivores, 1185, 1186. See also Decomposers Deuterotnycetes (imperfect fungi), 612 Deuterostome, 633-34 Chordata, 641/(see also Chordates) cleavage and, 631-32 Ectiinodermata, 6 4 1 / fate of blastopore, 632-33 protostome development versus, 632/ Deuterostome development mode, 631-33 Development, 411-33. See aha Animal development; Embryonic development; Plant development cell division, cell differentiation, and morphogenesis dunng, 412-14 cell fate during, 1003^3 differential gene expression in, 415-20 evolutionary novelty and, 482-83 evolution of, and morphological diversity, 431-33 evolution of genes controlling, 484-86 hormonal regulation of insect, 960/ model organisms for study of, 412-13/ pattern formation mechanisms in process of, 421-31 as property of life, 3

Developmental potential of cells, 418, 1004 Devonian period, 629 Diabetes mellitus, 956 two forms of, 956 Diacylglycerol(DAG),212 Diaphragm, contraceptive, 983 Diaphragm, respiratory, 888 Diapsids, 689 Diastole, cardiac cycle, 872 Diastolic pressure, 876 Diatoms (bacillariophyta), 559, 560/ Dicots (Dicotyledon), 602 germination and cotyledons of, 780/ Dktyostelium discoideum, life cycle, 566/ Dideoxy chain-termination method, sequencing DNA using, 397/ 398 Diencephalon, 1028 Diet. See also Food; Nutrition dentition and, 862, 863/ disease and, 847, 849/ 851t, 8521 evolutionary adaptations of vertebrate digestive systems and, 862-64 mate selection and, in Drosophila, 1113/ nutritional requirements in animal, 844, 849-52 vegetarian, 850/ Differential gene expression, 362-63 Differential-interference-contrast (Nomarski) microscopy, 96/ Diffusion, 130 facilitated, 133-34 free energy and, 146/ of solutes, 131/ Digestion, 853-55 in alimentary canals, 854/ 855 enzyme function in, 857, 858, 859/ extracellular, 853-55 in gastrovascular cavities, 854 hormonal control of, 860/ intracellular, 853 in small intestine, 858-61 in stomach, 857-58 Digestive system, 855-62 bacteria in, 862 evolutionary adaptations of, 862-64 hormones and, 860/ human, 855/-62 large intestine, 861-62 oral cavity, pharynx, and esophagus. 856 small intestine. 858-61 stomach, 857-58 Digestive tract bird, 854/, 863 diet and evolutionary adaptions in, 862-64 earthworm, 854/ insect, 660/, 854/ mammalian, 855-62 ruminant, 864/ DiggeT wasp nest-locatmg behavior, 1115/ Dihybrid(s),257 Dihybrid cross, 257 Dikaryon, 611 Dimetrodon, jaw and ear bones of, 695/ Dinobryon, 560/ Dinoflagellates (Dinoflagellata). 555/ mitosis in, 228 Dinosaurs, 629, 689 blood pressure and feeding in, 877 parental care among, 690/ F Sereno on searching for fossils of, 15/ Dioecious plants, 773 Diphasiastrum tristachyum club moss, 587/ Diphtheria, 339 Diploblastic animals, 631 Diploid cells, 241 genetic variation preserved in, 466 reduction of chromosomes from, to haploid cells, 242,243-47 Diplomonads, 552, 5531" Diplopodes (millipedes), 659/

Directional selection, 465/ Direct synapiic transmission, 1022-23 Disaccharides, 70, 7 1 / See also Sucrose Discovery science, 19-20 Discrete characters, 463 Diseases and disorders, 1163. See also Genetic disorders; names of specific diseases allergies, 916-17 autoimmune, 917,956 of brain and nervous system, 1039-41 cancer (see Cancer) cardiovascular, 76, 882-83 DNA technology applied to diagnosis of, 403 erectile disfunction, 211, 948 fungal. 622-23 gene therapy for, 403-4 G-proteins and, 206/ 211 growth disorders, 952, 953 hormonal, 952, 953f, 954, 956 hypercholesterolemia, 137 immunodeficiency, 917-19 of muscle fiber excitation by motor neurons, 1069 obesity, 847-48 parasites as cause of (see Parasite(s)) pathogens as cause of (see Pathogens) in plants (see Plant disease) sexual reproduction and resistance to, 469 vaccination against, 914 viral, in animals, 343-44 viroids and prions as cause of, 345, 346/ Disease vectors, insects as, 554/ 555, 556/, 664. See also Pathogens Dispersal of organisms, 1084-85 Dispersion paiterns, population, 1137, 1138-39 Dispersive model of DNA replication, 299, 300/ Disruptive selection, 465/ 466 Dissociation curve of oxygen for hemoglobin, 892f Distal control elements, 365 Distal tubule, 931,932/ regulation of calcium and potassium concentration by, 934 Distribution of species, 1083-92 abiotic factors alSecting, 1086-87 behavior, habitat selection, and, 1085 biotic factors affecting, 1085-86 climate and, 1087-92 dispersal and, 1084-85 species transplants and, 1084-85 Disturbances in communities, 1172-73 community recovery from, 1226/ ecological succession following, 1173-75 fire as, 1172/. 1173/ humans as agents of, 1173 immediate, 1172 Disulfide bridges, 83 Diuresis, 936 DNA (deoxyribonucleic acid), 7/, 86 amplification of, by polymerase chain reaction, 391/ chromatin structure and packing of, 359-60, 3 6 1 / chromosomes formed from (see Chromosome(s)) circular, 289 (see also Plasmids) complementary (cDNA), 390, 400 denaturation of, 388 discovery of, as key genetic material, 293-98 DNA-protein complex (see Chromalin) double helix structure of, If, 8 8 / 89, 2 9 3 / 296-98 duplication and divergence in segments of, 378-80 genes as segments of (see Gene(s)) intron and exon segments of, 318 (see also Exons; Intn is) methylation of, 284, 289, 364, 417 nitrogenous bases of, 8 7 / 88, (see also Nitrogenous bases) physical mapping of fragments of, 396/ plasmids, 537 proteins specified by, 86/(see also Protein synthesis) radioactive isotopes and research on, 35/ recombinant, 385/ 386/ repetitive, 375, 376-77

replication of (see DNA replication) restriction fragment analysis of differences in, 392-94 satellite, 377 sequences of (see DNA sequences) sticky ends, 386 strands of (see DNA strand(s)) telomeres ending molecules of, 306/, 307 transcription of RNA under direction of (see Transcription) DNA cloning, 385-92 of eukaryotic gene in bacterial plasmid, 386, 387/ 388, 389/ gene cloning and applications, 385/ 386 storing cloned genes in DNA libraries, 388, 389/ 390 using restriction enzymes to make recombinant DNA, 386/ DNA fingerprinting, forensic investigations and, 405/ DNA ligase, 303/, 304t, 386 creating recombinant DNA with, 386/ DNA methylation, 284, 417 control of eukaryotic gene expression and effects of, 364 genomic imprinting and. 289 DNA microarray, 384/ 400 DNA microarray assays, 400, 401f DNA nucleotide sequence. See DNA sequences DNApolymerases, 301, 302, 303, 304t, 305, 306 DNA replication, 8 8 / 89 antiparallel arrangement of DNA strands and, 89, 297/ in bacteria, 346, 347/ 350/ base pairing to template strands in, 89, 299/ 300/ models of, 299/ 300/ origins of, in eukaryotes, 302/ proofreading and repair of, 305-6 priming, 303 in prokarytes and eukaryotes, 300-305, 537 proteins facilitating in, 301-3, 304( radioactive tracers used to study, 35/ rolling, 3 5 Of summary of bacterial, 304/ DNA sequences, 88 analysis of, 392-94 homeobox, 431-32, 627 identifying protein-coding genes in, 399 insertion, 351,352/ introns and exons, 318 (see also Exons; Introns) mapping, 396, 397/, 398 molecular homologies in, 494, 495/ mutations in (see Mutations) noncoding, 374-78 phylogeny inferred from, 492, 494-95 principle of maximum likelihood applied to, 501/ promoter. 315/. 316/ repetitive, 375 TATA b o x , 315, 3 1 6 / 3 6 4 terminator, 3 1 5 / 317 types of, in human genome, 375/ DNAstrand(s),7/ antiparallel arrangement and elongation of, 89, 297/, 302-3 complementary, 89 directionality (5' end and 3' end), 88, 89, 302/ elongation of, in replication process, 301-2 incorporation of nucleotide. into, 302/ noniemplate, 313 structure of, 8 8 / 296/ synthesis of leading and lagging strands of, 302/, 303/ telomeres at end of, 306/, 307 template, 313 DNA technology, 2 6 / 384-409 DNA cloning and, 385-92 genomics and, 398-402 Human Genome Project (see Human Genome Project) manipulation of genomes as, 384

mapping genomes at DNA level, 394-98 practical applications of, 402-7 restriction Fragment analysis. 392-94 safety and ethical questions related to, 26, 404, 407-8 Dobzhansky, Theodosius, 455 Dodd, Diane, on reproductive isolation in divergent Drosophila populations, 477f Dodder (Cuscuta), 768/ Dolphin, 821/ Chinese river, 1211/ countercurrent heat exchanger in, 836/ Domains, protein, 319 correspondence between exons and. 319/ Domains of life. 13-14,496. See also Archaea (domain); Bacteria (domain); Eukarya (domain) phylogeny of (see Phylogeny) three-domain system, 14/, 530-31/, 532 Dominant allele, 254 complete dominance of, 260 frequency of, 261-62 genetic disorders due to, 267-68 incomplete dominance, 260, 261/ relationship between phenotype and, 261 Dominant species, 1168 Dominant trait, 253 Donoghue, Philip, 679 Doolittle, Ford, 525 Dopamine, 1025, 1039, 1041 Doppler, Christian, 252 Dormancy, seed, 779-80 abscisic acid and, 798, 799/ Dorsal lip, blastopore, 995 as induction organizer, 1006/ Dorsal side, body 630 Double bond, 40 Double circulation, 871 in mammals, 871-74 Double fertilization, 599, 600/, 776, 777 Double helix, 7/, 88/, 89, 296, 297/, 298. See also DNA (deoxyribonucleic acid) antiparallel structure of, 89, 297/ base pairing in, 89, 298/ complementary strands of, 89 key features of, 297/ Douglas fir (Pseudotsuga menzjesii'), 595/ Down syndrome, 287/ Downy mildews (Oomycota), 558 Dragonfly, 661 fossil, 491/ Drosophila. See Fruit fly (Drosophila sp.) Drought abscisic acid and plant tolerance of, 799 plant response to, 811 Drug(s). See also Pharmaceutical products antibiotics (see Antibiotics) antiviral, 343-44 DNA technology and development of new, 404 enantiomers as, 62, 63/ for HIV and AIDS, 918-19 opiates, as mimics of endorphins, 43, 81, 1025 penicillin, 623 Prozac, 1025, 1040 psychoactive, 1025 smooth endoplasmic reticulum and detoxification of, 104 Viagra, 201,211 Drug resistance in bacteria, 535 in HIV 447, 448/ Dubautia laxa, 4 8 1 / Dubautialinearis, 4 8 1 / Dubautia scabra, 4 8 1 / Dubautia waialealae, 4 8 1 / Duchenne muscular dystrophy, 283-84, 403 Dugatkin, Lee, 1131 Dulse (Palmaria palmate), 567/ Dungeness crab, population fluctuations in, 1151/

Duodenum, 858/-59f Duplication chromosomal, 286/ gene, 459 Dust mites, 659/ Dutch elm disease (Ophiostoma ulmi), 622 Dwarfism, 952 Dynamic stability hypothesis on food chain length, 1167 "Dynein, 114-16 'walking" of, and cilia and flagella movement, 116/ Dyson, Freeman, 516 Ear balance, equilibrium,and, 1052-53 evolution of mammalian jaw and bones of, 695/ hearing in mammalian (human), 1050,1051/, 1052 Earlobes, attached, as recessive trait, 265/ Earth. See also Environment atmosphere of (see Atmosphere) biodiversity hotspots on, 1222/ carrying capacity of, 1145, 1153 chemical conditions of early, and origins of life, 513-16 classifying life on (see Systematics; Taxonomy) clock analog)' for key events in history of, 5 2 1 / continental drift and plate tectonics of, 527-29 continuum of life on, 512 global climate patterns, 1088-89/ global energy budget, 1186-87 moderation of temperatures on, by water, 50 prokaryote evolution and changes to, 521-23 regional annual net primary production for, 1188/ Earthworm (Oligochaeta), 640/, 653-54 anatomy, 654/ closed circulatory system, 869/ defense systems, 903 digestion in, 854/ excretory system (metanephridia) of, 930/ gas exchange in,884 peristalsis and locomotion in, 1064/ reproduction in, 964/ Eastern box turtle (Tenapene Carolina Carolina), 691/ Ebola virus, 344 Ecdysis, 634, 635/, 657 Ecdysone, 960 Ecdysozoans (Ecdysozoa). 634 Echinoidea (sea urchins), 666t, 667/, 988-990 Echinoderms (Echinodermata), 641/, 665-67 anatomy, 665/ brittle stars, 666, 667/ classes of, 666t nervous system, 1012/ sea daisies, 666, 667/ sea lilies, feather stars, 666, 667/ sea stars, 665/, 666, 667/ sea urchins, sand dollars, 666, 667/(see also Sea urchin) sea cucumbers, 666, 667/ E. colt See Escherichia coli (E. coif) Ecological capacity, 1156 Ecological footprint, 1156. See also Carrying capacity in relation to available ecological capacity, 1156/ Ecological niche, 1160 Ecological pyramids, 1192-93 Ecological species concept, 476 Ecological succession. 1173 Ecology, 1080-1105 abiotic andbiotic factors and, 1081 of aquatic biomes, 1092-93 of behavior (see Behavioral ecology) biosphere and, 1083 (see also Biosphere) of communities (see Community ecology) conservation and (see Conservation biology) distribution of species and, 1083-92 ecosystem approach to, 1082 (see also Ecosystem(s)) environmental issues and, 1083 evolutionary biology related to, 1081 landscape, 1082, 1220-24

of organisms, 1082 organisms and environment in study of, 1081 of populations (see Population ecology) prokaryotes and, 544-45 restoration of degraded, 1224-28 scope of, 1080 subfields of, 1082-83 of terrestrial biomes, 1098-1104 Ecosystem(s), 4f, 1082, 1184-1208. See also Biomes aquatic, 1188-90 as biodiversity level, 1211 chemical cycling m, 160/, 1184-86, 1195-99 communities of species in (see Community(ies)) disruption of chemical cycles by humans, 1200-1206 edges between. 1220/, 1221 effects of acid precipitation on, 55-56, 1201-2 energy and matter in, 1184 energy budgets of, 1186-88 energy conversion in. 6 energy flow in, 160/, 1184-86 energy transfer between trophic levels in, 1191-94 "engineers" (foundation species) in, 1169-70 fragmentation of, 1220-21 human impact on, 1200-1206 limitations on primary production in marine and freshwater. 1188-91 producers and consumers in, 6 role of fungi in, 620 role of prokaryotes in, 544-45 services of, promoting human welfare, 1212 terrestrial and wetland, 1190-91 trophic structure of, 1166-68 Ecosystem ecolog)', 1082. See aho Ecosystem(s) Ecosystem services, 1212 Ecotone, 1099 Ectoderm, 631,994 gastrulation and formation of, 994 organs and tissues formed from, 999t Ectomycorrhizae, 610, 766, 767/ Ectoparasites, 1163 Ectoprocts (bryozoans), 640/, 649/ Ectotherms (ectothermic animals), 689, 829, 833 energy budgets in, 830, 831/ thermoregulation in, 833-34 reptiles as, 689 Edges (boundaries) between ecosystems, 1220/. 1221 Ediacaran fauna, 628 Effective population size, 1216-17 Effector cells. 907, 944, 1013 Efferent arteriole, 933 Efferent signal, 944 Egg, 964. See also Ovum activation of, 990 amniotic, 688/ cytoplasmic determinants in, 987 development of human, 973, 974/ fertilization of human, 978, 979/, 991 fertilization of mammalian, 990-91/ fertilization of sea urchin, 988, 989f, 990/ Egg-polarity genes, 424 Ehrlich, Anne, 1180 Ehrlich, Paul, 1180 ein (ethylene-insensitive) mutants, 800/ Ejaculation, 971 Ejaculatory duct, 972 Elastic fibers, 823 Eldredge, Niles, 481 Electrical membrane potential. See Membrane potential; Electrical synapses, 1021 Electrocardiogram (EKG/ECG), 874 Electrochemical gradient, 135 Electrogenicpump, 136/ Electromagnetic receptors, 1048, 1049/ Electromagnetic spectrum, 186/ Electron(s), 34 configuration of,and chemical properties, 37-38 cyclic and noncyclic flow of, in photosynthetic light reactions. 190/", 191/, 192

energy levels of, 36-37/ ionic bonding and transfer of, 4 1 / orbitals of, 38/ 39 transport of, in cellular respiration, 170-3 valence, 38 Electronegativity, 40 Electron microscope (EM), 95-97 freeze-fracture preparation of cells for, 125, 126/ scanning (SEM), and transmission (TEM), 96/ Electron shells, 36 of eighteen elements, 37/ valences for elements of major organic molecules, 60/ Electron transport chain, 163-64 in cellular respiration, versus in uncontrolled reaction, 163/ energy coupling to chemiosmosis, and ATP synthesis, 171,172/ 173 evolution of. 521-22 free energy change during, 171/ pathway of, during oxidative phosphorylation, 170-71 in photosynthetic light-harvesting, 190/ Electroporation, 390-91 Element(s), 33 atomic structure and properties of, 34 cycling of, in ecosystems, 160/ 1184-86, 1195-98 electron shells of eighteen, 37/ essential, required by life, 33-34 in human body, 331 periodic table of, 37 plant nutritional requirements for, 756-59 trace, 34 Elephants evolutionary tree of, 443, 444/ exponential growth in population of African, 1144/ overexploitation of African, 1214 thermoregulation in, 837/ Elephant seals, bottleneck effect in populations of northern, 461/ Elicitors, 814 Elimination as stage in food processing, 853 Elk, mating behaviors in, 1124/ El Nino Southern Oscillation, 1170, 1171/ Elongation factors, polypeptide synthesis, 323, 324/ Elton, Charles, 1166, 1167 Embolus, 883 Embryo cleavage of zygote and formation of human, 978, 979/ development (Embryonic development) ensuring survival of animal, 967-68 epigenesis and, 987 fertilization and formation of animal, 967, 978, 979/988-91 gastrulation and tissue formation in, 994-97 homologous structures in, 448, 449/ maternal effect (egg-polarity) genes, effects of, 424/ organogenesis in, 997/ 998/ pattern formation in (see Pattern formation) plant, 577/ 777/ 778 restriction of cellular potency in, 1005 stem cells of, 418 Embryology, 411. See also Development; Embryonic development Embryonic development, 412-14 of animal, 412-13, 414f, 485-86, 627/ 988-1001 cell division, cell differentiation and, 413 cell fate during, 1003-8 development of brain in, 1028/ 1029 differential gene expression and, 415-20 effects of DNA methylation on, 364 Hox genes and,485-86, 627 human, 978, 979/, 980/ 987/, 1028-29 morphogenesis and, 413, 431-33, 987 pattern formation and, 421-31 plant. 414/, 777/-7S restriction of cellular potency in, 1005 Embryonic lethals, 423

Embryonic stem cells, 418/ 1039 Embryophytes, plant, 575/ 577/ Embryo sac, plant, 599 Encephalitis virus, 337 Emergent properties. 9 of biological systems, 9 as biological theme, 27f cellular functions, 120, 121/ chemistry of life, 89 of compound, 33/ of water, 48-53 Emigration, population density and influence of, 1137 Emotions, human brain and, 1034-35 Emu, 693/ Enantiomers, 62, 63/ amino acids in proteins, 79/ Pharmaceuticals and, 62, 63/ Endangered species, 1210-11 Endangered Species Act, U.S., 1210 Endemic species, 450 Endergonic reactions. 146 coupling of, to exergonic reactions by ATP. 148. 149/ free energy changes in, 146, 147/ Endler, John, research on guppy evolution by 446, 447/ Endocrine glands, 944 adrenal, 956-58 gonads, 958-59 hypothalamus, 948, 950-51 major glands and hormones, 949t, 950t pancreas, 955-56 parathyroid, 954-55 pineal, 959 pituitary, 948-52 thyroid, 953-54 Endocrine pathway, 945/ Endocrine signaling, 203 Endocrine system, 943-63 control pathways and feedback loops in, 944, 945/ defined, 944 hormones as chemical signals of. 945-48 invertebrate, 959-61 major glands and hormones of, 949t, 950/ nervous system overlap with, 943, 944 nonpituitary glands of, 953-59 pituitary gland and hypothalamus of, 948-52 Endocytosis, transport by, 137, 138/ Endoderm, 631,994 gastrulation and formation of, 994 organs and tissues formed from, 999! Endodermis, plant root, 723, 744 water/mineral absorption and, 744, 745f, 746 Endolymph in inner ear, 1053/ Endomembrane system, 104-9 endoplasmic reticulum, 104, 105/ Golgi apparatus, 105, 106/ 107 lysosomes, 107/ 108 relationships of organelles within, 108, 109/ review of, 108 vacuoles, 108/ Endometrium, 970 Endomycorrhizae, 610. 766, 767/ Endoparasites, 1163 Endoplasmic reticulum (ER), 104, 105/ origins of, 525 rough, functions of, 105 smooth, 104, 105/ transitional, 105 Endorphin(s), 952, 1025 opiate drugs as mimics of, 43, 81, 1025 pain reduced by, 43, 81, 952 structure, 43/ Endorphin receptors, 43 Endoskeleton, 1064 human, 1065f vertebrate, 682 Endosperm, 600, 777 development of, 777

Endospores, 537. 538/ Endosymbiont, 523 Endosymbiosis, 550-51 origins of mitochondria and plastids in, 523-25 secondary, 551/ serial, 524 Endothelium, 874 in bloodvessels, 874 Endotherms (endothermic animals), 689. 829, 833 birds as, 689 energy budgets in, 830,831/ 871 [hermoregulation in, 833—34 Endotoxins, 546 Energetic hypothesis on food chain length, 1167 Energy, 36,142. See also Bioenergetics animal communication and cost of, 1111 chemical, 142, 828 free, 145-48 kinetic, 49, 142 forms of, 142-43 laws of thermodynamics applied to, 143-44 life and, as biological theme, 27/ locomotion and requirements for, 1074/ potential, 36, 142 species richness, water, and, 1176/ storage of, in fats. 76 thermal (heat), 142 (see also Heat) transfer of, between ecosystem trophic levels, 1191-94 transformations of, in metabolic processes, 141-44 Energy budget of animals, 830, 8 3 1 / 844-48, 928 of ecosystems, 1186-88 Energy costs of animal locomotion. 1074 of animal signals, 1111 of foraging behavior, 1122 Energy coupling, 148 Energy flow in ecosystems, 6/ 160/ 1184-86 decomposition and, 1185-86 overview of, 1185/ physical laws and, 1185 primary productivity and energy budget, 1186-91 secondary productivity and 1191 trophic efficiency, ecological pyramids, and, 1192-93 trophic relationships as determinants of, 1185 Energy levels of electrons, 36, 37/ Energy processing as property of life, 3/ Energy transformation, laws of, 143-44 Engelmann, Theodor, 187/ 188 Engelmanns experiment, 187/ 188 Enhancers, 364, 365/ function of, in control of eukaryotic gene expression, 365, 366/ Entamoebas, 564 Enteric division of autonomic nervous system, 1028 Enterocoelous development, 632 Enthalpy, change in (AH), 145 Entropy 144 change in (AS), 145 Environment. See also Biosphere; Earth cancer caused by factors in, 370 (see also Cancer) carrying capacity of, 1145, 1155 emerging viruses linked to changes in, 344 genetics combined with influences of, resulting in animal behaviors, 1113-18 impact of, on phenotype, 264 influence of, on animal nitrogenous waste, 928 interactions between organisms and, 27/ 1080-83 (see also Ecology) regulation of animals internal, 831-33 signal transduction pathways in plant responses to, 788-91 water and fitness of, 47-57 Environmental education, 437, 1079 EnvironmentalisiTL 1083 Environmental problems acid precipitation, 55-56, 1201-2

atmospheric carbon dioxide and climate change, 1203-5 decline in amphibian species due to, 686 disrupted chemical cycling, 1200-1206

DNA technology applied to solving, 405-6 ecology and evaluation of. 1083 human population growth as, 1155-56 humans as cause of (see Human environmental impact) ozone depletion, 1205-6 soil depletion, 1200 toxins in ecosystems, 1202-3 Environmental stress, plant responses to, 810-12 Enzymatic hydrolysis, 853 Enzyme(s), 77. 150-55 activation energy barrier and, 150-51 active site of, 152, 153/ allosteric regulation of, 156-57 catalysis in active site of, 152, 153/, 154 as catalysts, 77, 150 catalytic cycle of, 78/, 153/ cofactorsof, 155 conformation of lysozyme, 8 1 / digestive, 857, 858, 859/ DNA proofreading and damage repair by, 305-6 DNA replication and role of, 301-3, 304( effects of temperature and pH on, 154 induced fit between substrate and, 152, 153/ inhibitors of, 155f localization of, within cells, 157 lowering of activation energy barrier by, 152 membrane protein as, 128/ normal binding of, 155/ one gene-one enzyme hypothesis, 310, 311/ repressible, and inducible, 354-56 substrates of, 152 substrate specificity of, 81, 152 Enzyme-substra'.e complex, 152 Eosinophils, 880/, 881, 900 Ephedra, 594/ Ephrussi, Boris, 310 Epiblast, 994, 1000 Epicotyl, 778 Epidermis animal. 835/ plant, 717, 724,813 Epididymis, 971 Epigenesis. 987 Epigenetic inheritance, 364 Epiglottis, 856 Epilepsy, 1032 Epinephrine, 944, 956,1025 cell signaling and glycogen breakdown by, 203-4, 213/, 946 different responses to, 947/ effects of, on cardiovascular and respiratory systems, 874, 957 stress and secretion of, 874, 946, 947/ Epiphytes. 767, 768 Episome, 349 Epistasis, 262, 263/ Epithalamus, 1030 Epithelial tissue, 823 transport, 926-27 Epitope, 903 Epstein-Barr virus, 370, 910 Equatorial-polar gradients in biodiversity 1176 Equilibrium free energy and, 145, 146/ metabolism and, 147-48 organs of, in vertebrates, 1052, 1053f work and, in closed and open systems, 147/ Equilibrium potential (E lon ), 1016 Equisetum arvense field horsetail, 587/ Erectile dysfunction, 211. 948, 1025 Ericksson, Peter, 1038-39 Ergotism, 622 Erythrocytes, 825/, 880-81. See also Red blood cells (erythrocytes)

Erythromycin, 537 Erythropoietin (EPO), 881 Eidiei ichia coli (E. coll) chaperone protein in, 85

chemotaxis in, 537 chromosome and DNA of, 537/ chromosome length of, 226 DNA replication in, 300-305, 347/ genetic recombination in, 460, 346-51 genome and replication of, 346, 347/ genome map, 394 Hershey and Chase experiment on viral DNA infection of, 294, 295/, 296 in human colon, 542/, 862 as pathogen, 460, 546 restriction site, 386/ reproduction rates of, 346, 538 T2 bacteriophage infection of, 294, 295f T4 bacteriophage infection of, 334/ transcription and translation in, 328/ transposition of genetic elements in. 331-52 E site, tRNA binding, 322/ Esophagus, 856 peristalsis in, 856/ Essential ammo acids, 849-50 in vegetarian diet, 850/ Essential element, 757 Essential fatty acids, 850 Essential nutrients, 849 in animals, 849-52 in plants, 757, 758t Estivation, 841 Estradiol, 63/ Estrogen, 958 human pregnancy and childbirth and role of, 981 intracellular receptors for, 947 Estrous cycles, 973 Estrus, 973 Estuary(ies), 1095/ partial food web for Chesapeake Bay, 1167/ Etges, William, 1113 Ethane, molecular shape of, 60/ EthaneihioL 65/ Ethanol, 6 4 / Ethene, molecular shape of, 60/ Ethics DNA technology and, 26, 404, 407-8 human cloning and, 417 Ethology, 1107-9 Ethylene, 799-801 fruit ripening and. 203, 801 leaf abscission and, 800, 801/ programmed cell death and, 800 root apoptosis and, 811 triple response induced by, 799-80 Etiolation in plants, 789 etc (ethylene-overproducing) mutants, 800 Euchromatin, 360 Eudicots, 602, 603/ embryo development, 777/ monocots compared with, 603/ stem tissue in, 724/ Euglenas-p., 529, 554/ Euglenids, 554/ Euglenozoans, 553-54 Eukarya (domain), 1 4 / 5 0 7 / 5 2 1 compared with Archaea and Bacteria, 54U Eukaryotes animals as (see Animal(s)) cells of (see Eukaryotic cell) cilia of, 15/ (see also Cilia) cloning and expression genes of, 390-91 cloning genes of, in bacterial plasmid, 386. 387/, 388, 389/ endosymbiosis in evolution of, 523-25, 550-51 gene expression in (see Gtnt expression) genome (see Eukaryotic genome) fungi as (see Fungi) multicellularity and evolution of, 525-28

origins of, and relationship to prokaryotes, 523, 524/ 525 phylogeny of, 552/ plants as (see Plant(s))

protists as, 519-51 (see fllso Fronsts) Eukaryotic cell, 8, 98-109 animal, 100/ (see also Animal cell) chromosomes of, 219/ cytoskeleton of. 112-18 DNA replication in, 301/, 306-7 endomembrane system of, 104-9 extracellular components of, 118-20, 121/ gene expression in, differential, 362-63 gene expression in prokaryotic cell compared with, 327, 328/ as genetic chimera, 525 infection of, by microspondia, 615/ as living unit greater than sum of parts, 120 mitochondria and chloroplasts of, 109-11 modification of RNA after transcription in, 317-19 nucleus of, 102, 103/ panoramic view of, 99, 100/ 101/ plant, 101/(see also Plant cell) prokaryotic cell compared with, 8/, 98-99, 312/ protein synthesis in, versus m prokaryotes, 327-28 ribosomes of, 102, 103/, 104, 322 RNA polymerases of, 316 RNA processing in, 312/ RNA types in, 327t size of, 98 transcription in, 311, 312/, 315, 316/, 317, 328/, 331/ translation in. 311, 312/, 328/, 331/ Eukaryotic genome, 359-81. See aho Genome cancer resulting from genetic changes affecting cell cycle controls in, 370-74 complexity of, 374-75 chromatin structure based on DNA packing in, 359-61 control of gene expression in (see Gene expression) DNA duplications, rearrangement, and mutations contributing to evolution of, 378-81 gene and mukigene families in, 377-78 gene expression, regulation of, 362-70 human (see Human genome) noncoding DNA sequences in, 374-78 overview, 359 as product of genetic annealing, 525 relationship between organismal complexity and complexity of, 374-75 transposable elements in, 375-76 Eumetazoans, 633, 643 Europa, 514 Europe acid precipitation in, 55/, 1201/ blackcap migration in, 1120, 1121/ European kestrel, parental caregiving by, 1142f Euryarchaeota. 544 Euryhaline animals, 923 Eurypterids, 658 Eustachian tube, 1051 Eutherian (placental) mammals, 697-701 biogeography and evolution of, 528 convergent evolution of marsupials and, 450, 494/ reproduction in, 967-84 Eutrophication in freshwater ecosystems, 1190 Eutrophic lakes, 1094 experimental creation of, 1190 human impact and creation of, 1201 Evaporation, 50, 835 Evaporative cooling, 50 in animals, 837 transpiration in plants and, 749 Evapotranspiration, 1176 actual. 1190 species richness and, 1176/ "Evo-devo" (evolutionary developmental biology) 431 origin of flowers and, 601-2

Evolution, 15-19,438 adaptation and (see Evolutionary adaptation) anagenesis and cladogenesis as patterns of, 472/ ot angiosperms, 601-2 of animals, 628-30 of behavioral traits, 1118-21 of birds, 692-93 chordate, 674-75 coevolution, 1164-65 converger:: Uv Convergent evolution) C. Darwin's field research and his ideas aboat, 441-43 C. Darwin's theory of, 438, 443-53 of development, leading to morphological diversity, 431-33 DNA and proteins as measure of, 89 effects of differential predation on guppy, 446, 447/ of eukaryotes, 523-28 evolutionary novelty 482-83 fossil, homologies, and biogeography as evidence 0 £ 448-51 of genes controlling development, 484-86 of genetic code, 314 gradual is™ theory. 440 of gymnosperms, 596 human culture and, 1132-33 influence of, on animal nitrogenous waste, 528 of jaw and ear bones in mammals, 695/ j. B. Lamarck's theory of, 440-41 E. Lande1.' or! ^en.or/.us and. 237 of leaves, 585. 586/ macroevolution, 472, 482-88 of mammals, 694-95 microevolution (see Microevolution) of mitosis, 2 2 7 / 2 2 8 modern synthesis of Mendelian and Darwinian ideas, 455 mosaic, 703 of muUicellularHy, 525-28 natural selection and. 16/ 17/(see also Natural selection) obesity and, 848 of organic compounds, 59 origin of species (SC.T SDccianon, Species) of plants, from i/harop:i\ res.is i^reen algae). 573-74 of plants, highlights of, 579/ of plants, seed plants, 591-93 of plants, vascular, 584-86 of populations, 454-70 of prokaryotes. 521-23 of reptiles, 689-90 resistance to idea of, 439-40 of roots in plants, 585-85 of tetrapods, 684, 685/ vertebrate digestive systems, 862-64 of viruses, 342-43 Evolutionary adaptation, 16, 438. See ako Natural selection camouflage as example of, 446/ C. Darwins theory of natural selection and, 16, 442-43,444-46 in prokaryotes. 537-40 as property of life, 3/ Evolutionary biology. See also Evolution ecology and, 1081 K. Kaneshiro on, 436-37 modern synthesis of Darwinism and Mendeiism in, 455 Evolutionary fitness, 464-65 Exaptation, 483 ExciUtory postsynaptic potential (EPSP), 1022 Excretion, 922 Excretory systems, 928-39 excretory processes and, 929 homeostasis and, 937/ key functions of, 929/ kidneys, 931-39 Malpighian tubules as, 930/-31 mammal-an (human,1. 931-38

metanephridia as, 930/ nitrogenous waste and, 927-28 protonephridia (flame-bulb) as, 929/-30 Exercise blood pressure during, 876 health benefits of, 883 Exergonic reaction. 146 cellular respiration as. 146. 163/ coupling of, to endergonic reactions by A IT, 148, 149/ energy profile of, 151/ DNA replication, 302/ free energy changes in, 146. 147/ Exit tunnel, ribosome, 323 Exocytosis, transport of large molecules by, 137 Exoenzymes, 609 Exons, 318 correspondence between protein domains and, 319/ duplication of, 380 Exon shuffling, 319, 380-81 evolution of new gene by, 380/ Exoskeleton. 657. 1064 arthropod, 74/, 657/ molluscs, 1064 Exotic (inirodiiced) species, 1213-14 Exotoxins, 546 Expansins, 795 Exponential population growth, 1144-45 global human, 1152,1153/ intrinsic rate of increase (rmax) and, 1144/ Expression vector. 390 Express sequence tags (ESTs), 399 External fertilization, 967 Exteroreceptors, 1046 Extinction of species, 1209-10 mass, 518-21 overexploitation and, 1214 Extinction vortex, 1215 greater prairie chicken case study, 1216 grizzly bear case study, 1217-18 process culminating in, 1215/ Extracellular components, 118-20 Extracellular digestion, 853-55 Extracellular fluid, ionic gradients across neuron plasma membrane and, 1016/ Extracellular matrix (ECM), 119-20 ainma! moii"l:."'i:i_vie':ri:: J"."K. roie cf. 1002—3 membrane proteins and, 128/ structure, 119/ Extraembryonic membranes. 688. 999 in birds/reptiles, 688, 999/ as derived character of arnniotes, 688/ uman emDryo, luuuj Extranuclear genes, 289-90 Extraterrestial sources of life, 514 Extreme halophiles, 541, 544/ Extreme thermophiles. 534/ 541 Extremophiles. 541 Eye compound, 1058 of Dmsophila fruit fly, 276/ 277/ 280, 2 8 1 / 411/ evolution of, 483 invertebrate, 1057/, 1058/ lens cells of. gene expression in. 367) molluscs, 483/ resolving power of, 95 single-lens, 1058 vertebraie, 1058, 1059/. 1060/ Eye cup. 1058 See also Ocellus Eyespot, 1058 See ako Ocellus F t generation, 253 incomplete dominance and, 261/ I s> tionand 253/ 254t, 255/, 257/ E 2 generation, 253 incomplete dominance and, 261/ law of segregation and. 2 5 3 / 254i, 255/, 257/

Facilitated diffusion, 133, 134/ Facilitators in communities, 1170 Faculiaiive anaerobes, 176, 539 FADH2, as source of electrons for electron trans "chain, 170, 171/ Fairy ring, 618/ Family (taxonomy), 496 Famine, 558, 1156 Fasck'.edifjallele, 430 Pass mutants, plant growth and, 730, 731/ Fast block to polyspermy, 989 Fast muscle fibers, 1071, 1072t Fat(s), 75-76 catabolism of, 177/ digestion and absorption of, 859/ 861/ hydrocarbons and, 6 1 / saturated and unsaturated, 7 5 / 76 synthesis and structure of, 75/ Fat, body adipose tissue, 61/, 76 obesity and, 847-48 Fate maps, 1004 or frog and tunicate embryos, 1004/ Fatty acids, 75 digestion of, 8 6 1 / essential dietary, 850 saturated and unsaturated, 75/, 76 Feathers, bird, 472, 692f Feather star (Crinoidea), 666, 667/ Feces: 862 Feedback inhibition of enzyme activity, 157/ in cellular respiration, 177, 178/ reedback mechanisms inbiological systems. 11-12 in cellular respiration, 177. 178/ i.! enc.ocnr.c j"iCi nr:"vo>.-s ^."^i1.""" p...jnv,ij',j, 886,' lungs and, 886-87, 888/ oxygen uptake and carbon dioxide disposal, 867, 892-94 partial pressure gradients and, 891 in protists, 884 respiratory pigments and gas transport in blood, 892-94 respiratory surfaces for, 884-87 tracheal systems in insects and 886. 887,' Gastric gland, 857/ Gastric juice, 857 Gastric ulcers, 858/ Gastropods (Gastropoda), 651-52 torsion in, 651/ Gastrovascular cavity 643, 854 cmdarian, 643, 868/ internal transport and circulation through invertebrate, 868 Gastrula, 627, 994 Gastrula organizer, 1006 Gastrulation, 627, 994-97 in bird embryo, 995-96, 997/ embryonic tissue layers formed by, 994 in frog embryo, 995, 996/ in human embryo, 1000/ of sea urchin embryo, 995/ Gated channels, 133 Gated ion channels, action potentials in nervous system and. 1017-21, 1046 Gause, G. F, competitive exclusion principle and, 1160 Gecko lizard, van der Waals interactions and climbing ability of, 42

Geese, imprinting of graying, 1108, 1109/ Gel electrophoresis, 392 of restriction fragments, 392, 393/ Gene(s), 7, 86, 239. See also Chromosome(s); DNA

genomics and study of interacting group? o: genes, 400, 401/ flow of genetic information and, 309 identifying protein-coding genes in DNA

sequences, 399

(deoxyribonucleic acid) activation of, by growth-factor, 213/ allele as alternate form of, 253-54, 255/ (see also Alleles) avirulence (avr) gene, 813, 814/ BRCA1 and BRCA2, and breast cancer, 374 cloning of (see Gene cloning) coordinately controlled, 367 definition of, 330 determining function of, 400 duplications of, 378/, 380. 459 egg-polarity, 424 evidence for association of specific chromsosome with specific, 276-77 evolution of. controlling development, 484-86 exon duplication and exon shuffling in, 380 expression of (see Gene expression) extending Mendelmn genetics for single, 260-62 extending MenOelian genetics for two or more. 262-63 extranuclear, 289-90 FOXP2, 706 gap, 425 genetic recombination of unlinked and linked, 278-79, 280/ hedgehog, 499/ homeotic, 483 homologous, 505/ horizontal transfer of, in prokaryotes, 538, 541, 546 Hox (see Hox genes) in human genome (see Human genome) immunoglobulin, gene rearrangement in, 906/ jumping, 351-52, 371, 375-76 E. Lander on identifying. 236-37 linked, 277-82 location of, on chromosomes (see Gene locus) maiernal effect, 424 molecular bomologies in, 494, 495/ multigene families, 377-78 mutations in (see Mutations) nod,766 with novel functions, 380 number of, in select genomes, 399t organelle, inheritance of, 289-90 organ-identity, 430 organization of typical eukaryotic, 364, 365/ orthologous, 505/ pair-rule. 425 paralogous, 505/ point mutations in, 328-30, 459 R, 813, 814/ rearrangement of, in lymphocytes, 906-7 relationship of proteins to (see also Protein synthesis'! RNA splicing and split, 3 1 8 / 3 1 9 S-, and self-incompatibility in plants. 776 segmentation, 425 segment polarity, 425 sex-linked, 282-84 transcription factors and controls on, 205, 364-66 transfer of, in bacteria, 348-51 Gene clones, 385 identification of, 388, 389/ libraries of, 388, 389/ Gene cloning, 385 bacterial plasmids and production of, 385, 386, 387/, 388, 389/ overview of, 385/-S.6 recombinant DNA and, 385/, 386/ Gene expression, 309-33 cellular differentiation in plants and, 732, 733/ cloning and, in eukaryotic genes, 390-91 different cell types resulting from differential, 415-20 expression systems, in bacteria, 390 genetic code and, 312-14

point mutations and, 328—30 in prokaryotes (bacteria), and metabolic adjustments, 352-56 in prokaaotes compared witheukaryotes, 327, 328/ regulation of, during development, 418-20 relationship between genes and proteins and, 309-10 roles of RNA in, 327 summary, 331/ translation, 311-12, 320-26 transcription, 311-12. 315-17 transcription in eukaryotic cells, 317-19 transcription in eukaryoi ic cells, regulation of, 362-70 viruses and, 339 Gene flow, 462 lack of. 458 Gene-for-gene recognition, 813-14 Gene locus. 239 mapping of, using recombination data, 279-81 Gene pool. 455 allele frequencies in, 455-56 of nonevolving population, 457-58 mutations and sexual recombination as cause of change in, 459-60 natural selection, genetic drift, and gene flow as cause of change in, 460-62 reproductive barriers and isolation of, 473, 474-75/ Generalized transduction. 348/, 349 Generative cell, 599 Gene regulation in eukaryotic cells, 362-70 negative, 354-56 positive, 356 in prokaryotic cells, 352-56 Gene segments (/), 90S, 907 Gene therapy, 403-4 Genetic anneal ing. 525 Genetically modified (GM) organisms. 407-8 plants as, 784-86 Genetic code, 8, 312-14. 459 deciphering, 313-14 dictionary of, 314/ evolution of, 314 reading frame for, 314 redundancy in, 314 triplet code of, 312, 313/ Genetic disorders. See also names of specific JiwdeLs cancer as, 305 (see also Cancer) carriers of, 266 chromosomal alterations as cause of, 285-88 colorblindness, 283, 1061 cystinuria, 134 doi

gene therapy for, 403-4 genetic drift, founder effect and, 462 genomic imprinting and, 289 genomics and, 401-2 growth disorders, 952 karyotyping for determining, 287/ Mendelian patterns of inheritance and, 266-68 metabolic, 309-10 multifactorial, 268 phenotype, recessive traits, and, 261 population genetics and, 458 relay proteins in cell signaling, and, 21.5 sex-linked, 283-84 testing and counseling for, 268-70 Genetic diversity. See Genetic variation

Genetic drift, 460-62 bottleneck effect and, 4 6 1 / example of, in wildflower population, 461/ founder effect and, 462/

lack of, ^58 Genetic engineering, 384. See also DNA technology in animals, 406, 416 in plants, 783-86 Genetic map, 279 cytogenetic map as, 281, 396/ partial, of Drosophila chromosome, 281/ Genetic markers, RFLPs as. 394, 396/, 403/ Genetic polymorphisms, 463 Genetic prospecting, 541 Genetic recombination, 278. 5ee also Recombinant DNA in bacteria, gene transfer and, 348-51 in bacteria, mutations and, 346-48 in Drosophila fruit fly. 278-81 mutations and. 459-60 (see dso Mutations) produced by crossing over, 278, 279/, 280/ produced by independent assortment of chromosomes, 278 Genetics, 238. See aiso Gene(s); Gene expression; Genome of bacteria (prokaryotes), 334, 346-52 ofbehavior, 1109-13 chromosomes and (see Chromosomal basis of inheritance) of developmcr.. •: SLV Develop™em ' environmental factors combined with, effects on behavior, 1113-18 of eukaryotes (see Eukaryotic genome) genetic variation (see Genetic variation) inheritance of genes and, 239 E. Lander on research in, 236-37 meiosis, sexual life cycle, and, 240-47 Mendelian (see Mendelian inheritance) molecular (see Molecular basis of inheritance) of populations (see Population genetics) sexual and asexual reproduction and. 239 of viruses, 334-46 Genetics problems laws of probability used for solving. 259-60 Punnett square for solving, 254. 255/ Genetic testing carrier recognition, 269 fetal testing, 269, 270/ newborn screening, 269-70 Genetic variation, 238, 247-49, 462-64 benefits of, to human welfare. 1211-12 biodiversity crisis and loss of, 1210 independent asson mem of chromosomes as source of, 247, 2 4 8 / 278 crossing over as source of, 248. 249/ evolutionary significance of, in populations. 248-49, 459-60 measuring, 463 mutation as source of, 459-60 natural selection and role of, 462-64 between populations, 463-64 preservation of, by diploidy and balanced polymorphism, 466-68 random fertilization as source of, 248 sexual recombination as source of, 460 Genome, 8, 219. See also Gene(s) analysis of, 392-94 of Arabidopsis,729( comparing, of different species. 399,-, 400-402 determining gene function in, 400 eukarvoiic (see L'ukaryoiic genome1' evolutionary history of organisms found in, 504-6 evolution of, 378-81, 505-6 future studies in. 402 human (see Human genome) identifying proiein-coding genes in DNA sequences, 399 mapping of, at DNA level, 394-98 prokaryotic (see Prokaryotic genome)

relationship between composition of, and organismal complexity, 374-75 size of, and estimated number of genes in select, 399t studying expression of uueraaing gene groups, 400, 401/ viral, 335 Genome mapping, 279, 394-98 cytogenetic map as, 281, 396/ DNA sequencing as. 396-98 linkage map, 279-81,396 partial, oi Drosopkila chromosome. 281/" physical, 396 whole-genome shotgun approach to, 398/ Genomic equivalence, 415-18 animal stem cells and, 418 nuclear transplantation in animals and, 415-17 totipotency in plants and, 415/ Genomic imprinting, 288-89, 364 of mouse Ig/2 gene, 289/ Genomic library, 388, 389/ Genomics, 398-402 animal pliylogeny based on molecular data and. 635/ comparing genomes, 399t, 400-402 determining gene function, 400 future directions in, 402 genome mapping, 394-98 identifying protein-coding genes in DNA sequences, 399 E. Lander on evolution and, 237 studying expression of gene groups, 400 Genotype, 256 Hardy-Weinberg theorem and calculating frequencies of, 457-58 norm of reaction, 264, 1109 phenotype versus, 256/ Genus (taxonomy). 496 '-_T?og"orr>hic 'ivirners. spec"..Alien vvkh and wunoul, 476-82 Geographic distribution of species, 1083 abiotic factors and, 1086-87 behavior and habitat selection and. 1085 biotic factors and, 1085-86 climate and, 1087-92 dispersal and distribution factors, 1084-85 facwrs limiting, 1084/ Geographic variation, 463-64 in mice populations, 463/ in yarrow plants, 464/ Geologic record, 518. 519( Geometric isomers, 62/ Geospizajortis, 1161/ ' Geospiza iuiiginosa. 11M/ '.jcc^yvz-,: mugi:\. P25f 880f. 881 See alw While blood cells (leukocytes) Lewis, Edward, 423. 425 Leydig cells, 971 Lichens, 621-22 Life, introduction to the study of, 2-29 biology and study of, 2-8 biological diversity of {see Biodiversity) cells as basic units of, 6-8 chemical context of (see Chemistry) classification of, (see ako Systematic^: Taxonomy) continuum of, on Earth, 512 ecosystems and, 6 elements required by, 33-34 evolution of, 15-19 (see also Evolution) exLraierresirial sources of, 514 grouping into species and domains, 12-14 order as "characteristic of, 144/ properties and processes of, 3/ protobionts and ongins of, 515 scientific inquiry into, 19-26 systems concept applied to, 9-12 themes con:i^"1!i]>^ •~i\i• o^ L?.\ >iL.dy of, 26, 2.71 tree of (sec Tree oflile) unity underlying all, 14,15/ Life cycle, 240. See aho Sexual life cycle of angiosperm, 600/", 772/ o: apicomplexan Plasmodium sp.,556/" of ascomycete Neuwspom cmssa, 617/ of basidiomycete, 619/ of cellular slime mold, 566/ of chlorophyte Chlamydomonas, 569/ of DmsophUa fruit fly 421, 422/ 423 of fern, 585/ of fungi, 6 1 1 / 614/ of hydrozoan Obelia, 645/ of moss Poly trichum, 581/ of pine (gymnosperm). 596, 597/ of plants, 576, 581/, 585/ 597/ 600/ ol plasmodial slime moid. 563,' ^' 'T".H :^'L ' X:'Vi?'(iM[?, 5 6 2 /

of trematade blood fluke, 647/ of water mold, 559/ Life expectancy, 1155 in Costa Rica, 1229 Life history; 1141-43 diversity of, 1141-42 logistic population growth model and, 1147 reproduction versus survival trade-offs in, 1142—43 Life history traits, natural selection and production of, 1141-43 Life tables, population, 1139 of Belding's ground squirrel, 11391 Ligaments, 825/ Ligand, 137, 205 Ligand-gated ion channels, 208. 1017 chemical synapse and, 1022f, 1022-25 as membrane receptors, 208/ Light. See aho Sunlight as abiotic factor, 1087 animal brecL.^.i.1 JOC. 1V: animal hibernation and estivation related to, 84Q, 841

characteristics of, 186 circadian rhythms and, 1030, 1031/ conversion of energy in. into chemical energy, 182-85 energy levels of electrons and, 36-37 as limiting factor for ecosystem productivity, 1188 sensory receptors ioi (s."...1 iricmi?rjir.c

extraembryonic, 688, 999 fluidity of, 126/ models of. 125 nuclear envelope as, 102, 103/ structure of, 99 transport across, 126-27 ^

, •:"• I I 11 .1

°i . .

• I

j • I •i :

^ • ,

. ! ,

1 1 ,

i :

i, 1

l

l

'

>

, ' • ! : ! !

904 Membrane attack complex (MAO, 914 Membrane potentials, 135, 739, 1015-17 basis of, 134-35 cation uptake and, in transport in plants, 740/ intracellular recording for measurement or, 1016/ .'."..!•".' r':i: v L i ic e o i .

ox

Meristem identity genes, 734 Meroblastic cleavage. 994 Meselson, Matthew models of DNA replication by ¥. Stahl and, 299-300 research on parthenogenic reproduction in rotifers, 649 Mesencephalon, 1028 Mesenchyme cells. 994-95 Mesenteries, 827 Mesoderm, 631,994 --. vsi i _:. jiion and formation of. 994 organs and tissues formed from, 999! Mesohyl.642 Mesophyll, 182, 724 Mesophyll cells, 196/ loading sucrose inio phloem from, 752/ Mesozoicera, 519(, 520, 5 2 1 / 527-28, 596, 629 Messenger RNA (mRNA), 86/, 87, 102. 311, 327f codons of, 313,314/ degradation of, and regulation of gene expression, 368, 369/ DNA library, 390 transcription of, 311, 315-17, 790-91 translation of polypeptides under direction of, 320-26 Metabolic defects, evidence for gene-directed protein synthesis horn. 309-10 Metabolic pathways, 141-42 Metabolic rate, 828 activity and, 829-30 adjustment of, for thermoregulation, 838 ;-n'i:r.a. nioeiiergelic strategies and. 829 animal body size and, 829 basal, 829 calculating, 829/ cardiovascular system and, 870 human, 834 maximum, over select time spans, 830/ standard, 829 Metabolism, 141-59

i o n 'M.Jiip:1. 1.34—36

resting. 1016-17 functions of, 127, 128/ 129 movement in, 1271" Memory human brain, learning, and, 1035-36 imimmological 903, 908/ short-term and long-term, 1035 Memory cells, 907 Mendel', Gregor Johann, 249, 251/-53, 293. See also Mendelian inheritance Mendelian inheritance, 251-73 basis of, in chromosomal behavior, 274, 275/ 276-77 genetic variation preserved by, 456/ genetic testing and counseling based on, 268-70 genotype and phenoiype relationship, 256/ in humans, 265-70 inheritance "^tinT.^ nv^c complex i i.in ^XD.J.I i".". by, 260-64 integrating, 264 ]aw of independent assortment and, 256-58, 274, 275/ law of segregation, 253-56, 274. 275/ laws of probability governing, 258-60 Mendel's research/approach. 251-53 modem synthesis of Darwinism and, 455 Menopause, 977 Menstrual cycle, 973 Menstrua] flow phase, of menstrual cycle, 977 Menstruation, 973 Mercury, biological magnification of, 1203 Meristems, 720 apical, and plant primary growth, 576/", 720/, 721-25 flower development and, 429/ lateral, and secondary growth, 720/ 725-28 ransiiion of, and flowering. 808

156/ 157 anabolic pathways in. 142 (see also Protein synthesis) ATP in, 148-50, 828 (see also ATP fadenosme tri phosphate)) catabolic pathways in, 1 -2 i^ec also Cellular respiration) energy transformations (laws of thermodynamics), 143-44 enzymes and, 150-55 enzvmes. rc^v.-L-uio.i c-i. ^nd '-O-itro!^ on, 156—57 forms of energy and, 142-43 free energy-and, 145-48 gene expression and adjusi merits of, in bacteria, 352-56 metabolic pathways in, 141-42 organelles and structural order in, 157/ in prokaryoles, 352-56, 539-40 protobionis and simple, 515 regulation of, 352-56 i h e n n o r e ^ L u j l i. • l .!

K, ,'i

IIIU;L-'^

I

H*^

transformation of matter and energy in, 141-44 Metamorphosis, 627 in frogs, 686/ m insects. 6 6 1 / 9 4 3 / Metanephridia, 930/ Meiaphase meiosis. 244/', 245/ mitosis, 221, 2 2 3 / 224/ 226/ Metaphaseplaie, 224/ tetrads on, 247 Metapopulations, 1151 Metastasis, 233 Metencephalon, 1028 Methane combustion of, as redox reaction, 161, 162/ covalent bonding in, 40/" as greenhouse gas, 540

molecular shape of, 4 3 / 60/ production of, by methanogens, 544 Methanogens, 544 Methionine, 313, 323 Methylation, DNA, 284 genomic imprinting and. 289, 364 regulation of eukaryotic gene expression and, 364 Methylation, histone, 363 < i ' < . i * i

•

•

•

i

I ' I . I M

1 1 5 3 /

MHC. See Major histocompatibility complex (MHC) Microclimate, 1087, 1091 Microevolution, 454, 472 gene flow as cause of, 462 genetic drift as cause of, 460-62 mutations as cause of. 459-60 natural selection as cause ol", 460, 462-70 sexual recombination and, 460 Microhbnls in plain cellulose, 72, , 3j Microfilaments, 113;, 116-18 actinand, 116,226,1068/ cell motility and, 117/ cytoplasmic streaming and, 117/, 118 structural role of, 117/ structure and function, 113! Micronulnents. plant, 757, 758( Microphylls, 585, 586/" Micropyle, 599 MicroRNA (miRNA), 327t, 368 regulation of gene expression by, 369/ Microscopy, 94-97 Microsporangia, 774/ 775 Microspores, 586, 774/-7S Microsporidia, 615/ Microtubule-organizing center, 221. 222/ 223/ Microtubules, 113!, 1.14-16 cell motilily and. 112-18 centrosomes and cemrioles, 114/ cilia and flagella and, 114, 115/, 116 kinetochore, 221, 224, 225/ of mitotic spindle, and chromosome movement, 224 9 + 2 pattern of, 114, 115/, 525 plant growth and cytoplasmic, 730, 731/ polar, 224 structure and function, 113! Microvilli, 860 Midbrain, 1028, 1029 Middle ear. ]051 Middle lamella, 118, 119/ Migration bird, 1106,1109/ 1110-11 morphogenesis ;.nd cell 1001-2 Migratory restlessness, 1110-11, 1121 Milk, 694 a-lactalbumin and production of mammalian, 380 passive immunity provided by, 914 prolactin and production of, 952, 981 Miller, Carlos O., 796 Miller, Stanley, 59/ 513/, 514 Millipedes (Diplopoda), 659/ Mimicry 11. Bates hypothesis regarding, 22 case study of, in snake populations, 21-24 Mimaia fJii'J'uii1' sensitive plant), rapid turgor movements in, 810/ Mimulus sp., 474/ 481 Minerals, 851 absorption of, by plant roots, 744-46 availability of soil, to plants, 761-62 deficiencies of, in plants. 758, 759/ as essential nutritional requirement, 851, 8521 as plant nutrients, 756-59 Mineral nutrients, 756 Mineralocorticoids. 958 Minimum viable population size (MVP), 1216 Mining industry, use of prokaryotes in, 405, 546-47 miRNA (microRNA), 327!, 368, 369/ Mismatch repair of DNA, 305 Missense mutations, 328, 329/

Mistletoe (Pharadendron), 768/ Mitchell, Charles, 1149 Mixes, 659/ Mitochondria, 109-11 chemical energy conversion in, 109-10 chemiosmosis in. versus in chloroplasts, 109, 173, 192-93 endosymbiosis and origins of, 523-25 as site'of cellular respiration, 109, 110/, 164-65, 168-74 structure, 110/ Mitochondria! DNA (mtDNA), human genetic disorders and,290 Mitochondrial matrix, 13 0 ATP synthase as molecular mill m, 171/ citric acid cycle enzymes in, 169/ Mitochondrial myopathy, 290 Mitosis, 220 in animal cells, 222-23/ chromosome duplication and distribution during, 219-20 cytokinesis and, 224-26 evolution of, 227/ 228 meiosis compared to, 246/ 247 miiotic spindle in, 221, 224f phases of, 221, 222-23/ in plant cells, 226/ Mitotic CM) phase of cell cycle, 2 2 1 / Mitotic spindle, 221, 222/ 223j Mixotricha paradoxa. symbiotic relationships of, 525 Mixotrophs, 550 Model-building in science, 24, 25/ Model organisms, study of development using, 412-13/ Model system, microbial, 334 Modern synthesis of evolutionary theory, 455 Molanty, 53 Molar mass, 52 Molds, 6 1 1 / black bread moid {Rhizopus uolomfcr). 614/ bread (Neurospora cmssa), 616,r. 617/ as pathogens. 622-23 Mole (mol), 52-53 Mole(s), convergent evolution of analogous burrowing character!MICS in Australian and North American, 494/, 495 Molecular basis of inheritance, 293-308 DNA as genetic material, evidence lor, 293-98 DNA proofreading and repair, 305-6 DNA replication, base pairing to template strands in, 299f, 300/" DNA replication, models of, 299J, 300/ DNA replication, proteins assisting in, 303, 304/ DNA replication, steps in, 300-305 DNA replication, summary of bacterial, 304/ DNA structural model, scientific process of building, 296-98 replicating ends of "DNA molecules, 306-7 Molecular clocks, 506-7 applung, to origin of HIV, 507 difficulties with, 506-7 of multicellular eukaryotes, 526 neutral theory and, 506 of plants, 578 Moleculai formula. 40 Molecular geneaology, 89 Vnit'uil.'.r honolo^ies 4-9 Molecular mass, 52 Molecular systematic^, 491 animal phylogeny based on, 635/ applying parsimony to problem of, 502-3/ prokaryotic phylogeny and lessons of, 540-41 Molecules, 5/, 39-43. See also Macromolecules amphipathic, 124 chemical bonds and formation of. 39-42 denned, 40 shape and function of. 42-43 small, as second messengers, 210-12 Mole rats, naked (Heterocephalus gfobn), altruistic behavior in, 1128, 1129/

Molluscs (Mollusca), 639/, 650-53 basic body plan, 650/ Bivalvia (clams, oysters). 652 Cephalopoda (cephalopods), 652-53/

Motor neurons. 1013/ axon growth direction, 1038/ muscle contraction stimulated by, 1069-71 recruitment of, 1070

exoskeleton of, 650. 1064 Gastropoda (snails, slugs), 651/-52 hormonal regulation in, 960 major classes of, 6511 nervous system of, 1012/ Polyplacophora (chitons). 651/ Molting, 657 nutritional requirements during. 850* Monkey flower (Mimuius), 474/, 481 Monkeys Old World, and New World, 697, 700/ social learning of alarm calk in vervet, 1131-32 Monoclonal antibodies, 913 Monocots (Monocotyledones). 602, 603/ dicots compared to, 603f germination and cotyledons of, 780/ stem tissue in. 724/ Monocytes, 880/, 881, 900 Monoecious plants, 773 Monogamous mating, 1123, 1124/ Monogeneans, 6461, 647 Monohybnd, 256-57 Monohybrid cross, 256 multiplication and addition rules applied to, 258, 259/ Monomers. 68 amino acid. 78-80 nucleotide, 87/, 88 polymers built from, 68, 69/ Monomicleosis, 370 Monophylelic grouping, 498/ Monosaccharides, 70-71. 5eeako Glucose disacchande synthesis from, 70, 7 If structure and classifier.; son a. 70* Monosomic cells, 285 Monotremes, 695, 696/ Montreal Protocol, 1206 Moose, stabilny and fluctuation of population size on Isle Royale, Michigan, 1150/ Moray eel, fine-spotted (Gymnotborax dovii), 683/ MorciicHa t'iau-rnta, 616/ Morgan, Thomas Hunt experimental evidence lor chromosome-gene association from, 276-77, 293 experimental evidence for linked genes ['rum. 277-78, 279/ MtugtimiaicJon, jaw and ear bones of, 695/ Morphogen(s), 424 Vtorphogenesis, development and, 41 3, 987 in animals, 987, 1001-3 in plants, 728-35 Morphological .species concept, 476 Morphology a-r.:-:il phyogeny based on, 634f pb> iogeny and evidence of, 492-93 plant, 712 Morton, Martin, 1138, 1139 Morula, 992 Mosaic evolution, 703 Mosquitos as disease vector. 555,556/, 664, 1085. 1163 as fluid feeders, 845/ habitat and distribution of, 1085 mechanoreceplors n. 1050 Mosses (Bryophyia). 580. 3821. 586 5H7,'" life cycle of Porymdium, 581/ sporophytes of, 577/ Moths bat predation of, 1045/ chemoreceptors in, 1049/ predator-prey relationship with blue jays, 467/ sphinx, 820/ thermoregulation in, 837/, 839/ Motility. See Moverneir. Motor mechanisms, 1063-74

structure of, 1014/ Motor output, nervous system function and, 1013/. See also Locomotion; Movement Motor proteins, 78L 1 1 2 / 114 mechanical work in cells and, 149/ Motor unit of vertebrate muscle, 1070, 1071/ Mountain, effects of, on climate, 1090/ Mountain lion (Puma concolor). 1123 Mouse {Mm musculus) apoptosis arid paw formation in, 428, 429/ effect of social environment on aggressive behavior in California, 11141 energy budget for deer, 830, 831/ genome, 394, 401 genomic imprinting of ]g\2 gene, 289/ geographic variation of chromosomal mutations in, 463/, 464 homeotic genes in, 431J lack of V-., receptor in male wild-type, 1113 a- rr.odd organise. 41?.; obesity in, 848/ Movement See also Locomotion cell crawling during morphogenesis, 1001-2 cell motility, 112-18 in plants, 810/ prokaryotic cell, 536/ 537 inprolists, 117/, US Movement corridor, 1221 MPF (maturation promoting factor). 230/ Mucin, 856 Mucosa tissue layer, of stomach, 827/ Mucous membranes. 82.3 as body defense mechanism, 899/ Mucus. 899 Mule, as hybrid between horse and donkey, 475/ Mule dee: iQdocmlcai kemwnus), foraging and risk of predation, 1123/ Muller, Hermann, 329 Mullerian mimicry, 1162 Multicellularity, origins of, 525-28 Vttskifactorial characters, 264 as genetic disorders, 268 Multigene family, 377-78 Multiple alleles, 262 Multiple fruii, 779 Multiple sclerosis, 917, 1015 Multiplication rule and Mendelian inheritance, 258, 259f Muscle(s) cardiac, 826/, 1024. 1072 contraction b a ' Muscle contraction) effects of anabolic steroids on, 959/ fatigue in, and accumulation of lad ate, 1070-71 interaction between skeleton and, 1066/ ocomotioii a'"ic co r i iv.'_ti^n ol. 1066f neural control of tension in, 1069-71 skeletal, 1066/-72 smooth, 1072 stretch receptors for, 1047/ structure, 1066, 1067/ Muscle cell, 130, 627 determination and cell differentiation of, 419/ 420 micrufi laments and motililty in, 116, 117/ lactic acid fermentation in. 1 75 Muscle contraction myosin and actin interactions and. 116. 11//, 1068; neural controls and, 1069-71 role of calcium and regulatory proteins in, 1068, 1069/ role of sarcoplasmic reticulurn and T tubules in, 1069/ 5liding-filamem model of, 1067/ 1068/ Muscle fiber contraction of (see Muscle contraction) fast, and slow, 1071, 1072r

neural control of tension in, 1069-71 relaxed and contracted, 1067/ types of. 1071, 1072( Muscle spindles, 1048 Muscle tissue, 823 Muscularis tissue layer, of stomach, 827/ Mushrooms, 608, 618-19 Mustard, vegetables artificially selected from wild, 445/ Mutagens, 329-30 Mutant phenotype. 276 fruit fly eye color, 277/" Mutations, 328, 459 in bacteria, 344-48 base-pair substitutions as, 328, 329/ cancer and, 370 as cause of microevolution m populations, 459-60 chromosomal, 285-88, 463-64 deletions, 329, 330/ duplications, 459 as embryonic lethals, 423 frameshifts, 329 gene number and sequence alterations as, 459 genetic variation, in nopuialions caused by, 459-60 genomic evolution and role of, 378-81 insertions, 329, 330/ missense. 328, 329/ nonsense, 328, 329/ point, 328-30 populations lacking, 458 p r o i o - o n c p L ^ ! ."s " :"di "iS'i"iij^"ec ±i~sT• ~• cr.;.o£enes oy.

371/ rates of, 459-60 t r a n s l a t i o n s , 286/, 287, 288/ 459 Mumahsm, 545. 1164 roots and mycorrhizal fungi, 620 Myasihenia gra\is. 1069 Mycelium, fungal, 609 Mycetozoans (slime molds), 564-66 Mycoplasmas, 98, 543/ Mycorrhizae, 610, 744, 766-67 agricultural importance of, 620 arbuscular, 615/ symbiotic relationship of root nodules to, 620 two types of, 766, 767/ water absorption in plants and role of, 744, 745/ Mycosis, 622 Myelencephalon, 1028 Myelin sheath, 1014 conduction of action potentials and, 1020-21 multiple sclerosis and destruction of, 917 Schwann cells and, 1015/ Myoblasts, 419/, 420 MyoD gene, 419/ 420 Vyonbrils, 1066, 1067/ Myofilaments, 1066. 1067/ Myogenic heart, 873 Myoglobin, 895,1071 in oxidative muscle fibers, 1071, 1072/ Myosin, 117 cell motility, actm filaments and, 117/ cytokinesis and, 226 muscle coniraction and, 117/ 1068/ 1069/ Myotonia, 972 Myriapods (Myriapoda), 6581, 659/-60/ NAD+ (nicotinamide adenine dmucleotide), 162 fermentation and, 174-75 as oxidizing agent and electron shuttle during cellular respiration, 162, 163/ 170, 175 NADH citric acid cycle and, 168/ 169/, 170 fermentation and, 174, 175/ glycolysis and, 165/ 166/ as source of electrons for electron transport chain, 170, 171/ NADP+ (nicotinamide adenine dinucleotide nhos'ihate.1. nhotosynthetic light reactions and role of, 184, 185/ 193,1"

NADPH Calvin cycle and conversion of carbon dioxide to sugar using, 193-95 photosynthetic light reactions, and conversion of solar energy to, 185, 190, 191, 193/ Naked mole rat [Heterocephalusglaber). n":n.i-.m m. 1128,1129/ Nash, John, 1127 National parks in Costa Rica, 1223/1224 Grand Canyon, 440/ 477/ Grand Teton, 1222,1223/ Yellowstone, 544, 1172, 1173/ 1217/, 1218, 1220/ 1222, 1223/ Native Americans, genome of. 505 Natural family planning. 982 Natural killer (NK) cells, 902, 910 Natural selection, 16, 17/ 438, 444-46, 460, 462-70 artificial selection versus, 445 behavioral traits evolving from, 1118-28 C. Darwin's theory of, 444-46 directional, disruptive, and stabilizing modes of, 465/-66 effects of differential oredaiion on, 446, 447/ evolutionary fitness and, 464-65 evolution in populations due to, 460, 462-70 evolution of drug-resistant HIV as example of, 447, 448/ evolution of sex and sexual reproduction, 469 in finch populations, 443/ frequency-dependent, 467-68 genetic variation and, 462—64 perfection not created by, 469-70 populations with no, 458 preservation of genetic variation despite effects of, 466-68 RNA, protobionts, and dawn of, 515-16 sexual selection and, 468 summary of, 445-46 survival/reproductive success increased by behaviors favored by 1121-28 Nature reserves, 1222-24 philosophy of, 1222-23 zoned, 1223-24 Nealson, Kenneth, 522/ Neanderthals. 705 Negative feedback, 11/, 832. 944 kidney regulation by, 937/ as mechanism of homeostasis, 832/ Negative pressure breathing, 888, 889/ Nematocysts, 644 Nematodes (roundworms), 640f, 655-56. See also Caerwrhabditis rieguns Sjiemacode) hydr^sLiik >kdeton in, 1063 as modd organism. 41 If as pseudocoelornate. 631/ Tritfiinella spiralis, 656/ Nemerteans (proboscis worms/ribbon worms), 639/ 649/, 650 Neocortex, 1031, 1035 Neon, electron orbitals of, 38/ Neoprolerozoic err?, 628 Nephron, 931,932/ blood \t\ssds associated with, 933 production of urine from blood filtrate in, 933/, 934 regional functions of transport epithelium and, 933/ structure and function of, 931-33 Neritic zone, 1093/ 1097 Nernst equation, 1016-17 Nerve(s), 627, 1012. See also Neuron(s) cranial, 1026/ development of, 1037-38 herpesvirus in, 340 spinal, 1026/ Nerve cord, dorsal and hollow, in chordates, 673 Nerve net, 1012 Nervous system, 1011-44 action potentials in, production and conduction. 1017-21

adrenal gknci 3.nc. - - 7 central nervous system and brain, 1012, 1026f, 1028-37 central nervous system injuries and diseases, 1037-41 as command and control center, 1011-12 endocrine system overlap with, 943-44 human brain (see Human brain) niioz mar !(jT: piv •.'•.' -"•?"1^'; ?*-

' :•'

invertebrate, 1012/ maintenance of resting potential in neurons by ion pumps and ion channels of, 1015-17 membrane ^oter.i'.j!^ m, 1015 neuron communication at synapses in, 1021-25 neuron structure in, 1013, 1014/ nerve signal '.impulses) in, 203 organization of, 1012/ 1013 peripheral nervous system, 1012, 1026/ 1027/ 1028 senses ?,nd {see Sensory reception) supporting cells (glia) of, 1014-15 vertebrate, 1026-32 Nervous tissue in animals, 823. See also Nervous system; Neuron(s) Nest, digger wasps and behaviors for locating, 1115/ Net primary productivity (NPP). 1187 idealized pyramid of, 1192/ Neural crest, 676, 998 as embryonic source of vertebrate characters, 676f Neural stem cells, 1038-39 Neural tube. 998 Neuroendocrine pathway, 945/ Neurofibrillary tangles, 1040/ Neurogenic heart, 873 Neurohormone, 944 Neurohormone pathway, 945/ Neurohypophv'sis 950 Neuromasts, 1054 Neurons, 8 2 6 / 943-44, 1011 action potentials of, 1017-21 development of, 1037-38 gated ion channels of, 1017 mterneurons, 1013/ 1014/ ion pumps/ion channels and. 1015-17 membrane potential of, 1015-16 motor, 1013/. 1014/ uc'f also Motor neurons) production of new, in adults, 1038-39 resting potential of, 1016-17 sensory, 1013, 1014/ siructureof, 1013,1014/ supporting cells for, 1014-15 synapses and, 1021-25 Neuropeptides as neurotransmitters. 1025 Neurosecretoiy cells, 944 bleuivspora cras$a (bread mold) G. Beadle and E. Tatum experiment on geneenzyme relationships in, 310, 31 If life cycle of, 615, 617/ Neurotransmitters, 1014, 1024-25 acetylcholine as, 10241 amino acids and peptides as, 10241, 1025 biogenic amines as, 10241. 1025 as intraceliular messenger at neural synapses, 1022,1024-25 as local regulators, 947-48 major, 10241 migration of vesicles containing, 112/ Neutral theory, 506 Neutral variations in populations, 468 Neutrons, 34 mass number and. 34-35 Neutrophils, 880/ 881. 900 Newborns, screening for genetic disorders, 269-70 New World monkeys, 697, 700/ New Zealand, 690 Nicolson, G., 125 Nicotine, 1024,1163 Night, plant photopenodism and length ol, 806. 807/

9 + 2 structural microtubule pattern in flagella and cilia. 114, 115/, 525 Nirenberg, Marshall, 313 Nitric oxide (NO), 205, 948 effects on male sexual function, 948 as local regulator, 948 asneurotransmiuer,205, 1025 Nitrogen improving protein yield in crops and, 764 as limiting nutrient in ecosystems, 1188-89 plant requirement for, and deficiencies of, 33f, 759/, 761 prokaryotic metabolism and, 539 soil bacteria and fixing of, for plant use, 763/, 764-66 valences of, 60/ Nitrogenase, 764 Nitrogen cycle in ecosystems, 1197/ critical load and, 1200-1201 human disturbance of, 1200-1201 Nitrogen fixation, 539, 764 by bacteria, 539, 763F, 764-66 Nitrogen-fixing bacteria, 764 Nitrogenous bases base-pair insertions and deletions, 329, 330/ base-pair substitutions, 328, 329/ Chargafft rules on pairing of, 296, 298, 313/ genetic code and triplets of, 312-14 nucleic acid structure and, 87/, 88 pairing of, and DNA replication, 87-88 pairing of, and DNA structure, 87/, 296, 297/, 298/ pyrimidine and purine families of, 88, 298 sequences (see DNA sequences) structure of DNA strand and, 296/ wobble in pairing of, 322 Nitrogenous wastes, animal phylogeny correlated with form of, 927-28 Nitrosomonas, 542/" NMDA receptors,' 1036, 1037/ Nociceplors, 1049 Nodes, of stem, 715 Nodes of Ranvier, 1015/, 1021/ \~f);?' seres. 766 Nodules, rooi, 764, 765/ molecular biology of formation of, 766 Noncoding DNA sequences, 374-78 restric';on fragment length polvmonhi.-ms and. 394 Noncompeuuve enzyme inhibitors, 155/ Noncyclic electron flow, photosynthesis, 190/, 191 Nondisjiinction of cniomosomes, 285 genetic disorders associated with, 285/. 286 Nonequilibrium model of communities, 1172 \'"••!".|"i:i] :.".i.r ;\'.i.-- ._[-,"- i' .i vV"""''^. i ic.1-. !>" 'OCT'. .l^-1 .">]"!.-

ment of, 258, 274, 275/ Nonpolar covalent bond, 40 Nonsense mutations, 328, 329/ Nonshivering thermogenesis (NST), 838 Nontemplate DNA strand, 313 Nontropic hormones, 952 Norepmephnne, 956, 1025 stress and secretion of, 957/ Norm of" reaction, 264 behavior and, 1109 North America acid precipitation in, 55-56, 1201/ climograph for terrestrial biomes of, 1098/ plate tectonics of, 527, 528/ species richness of birds in, 1177/ species richness of trees and vertebrates in, 1177/ Northern fur seal (Calhrhinus ursmus), 1136/ Notochord, 498, 673. 997 chordate, 673 organogenesis and formation of, 997/. 998 Nottebohm, Fernando, 818 N-F-K ratio, 761 NTS (nonshivering thermogenesis), 838 Nuclear DNA, 109

Nuclear envelope, 102.103/ Nuclear lamina, 102, 103/ Nuclear magnetic resonance (NMR), 85 Nuclear matrix, 102 Nuclear pore, 102, 103/ Nuclear transplantation in animals, 415-17 Nudeases, 305 Nucleic acid(s), 86-89. See also DNA (deoxyribonudeic acid); RNA (ribonucleic acid) components ot, 87f digestion of, 859/ hereditary information transmitted by, 293 as polymer of nucleotides, 87-88 (see also Nucleotide(s)) roles of, 86-87 structure of, 87f, 88 Nucleic acid hybridization, 388, 389/ Nucleic acid probe, 388, 389/ Nucleoid, 98/, 346 Nucleoid region, 537 Nucleolus, 102, 103/ Nucleomorph. 551 Nuckoside, structure of, 87/ Nucleoside triphosphate, DNA strand elongation and, 301, 302/ Nucleosomes, 360, 361/ Nucleotide(s), 7-8, 87. See also DNA sequences amino acids specified by triplets of, 312-14 base pairing of, in DNA strands, 89, 296 (see also Nitrogenous bases) as component of nucleic acid, 87/, 88 excision repair of DNA damage by, 305/ incorporation of, into 3' end of DNA strand, 302/ mapping sequences o'. 396—98 point mutations in. 328-29 repairing mismatched and dar_-.;wd. 30;/. 306 restriction fragment analysis of, 392-94, 395/ structure, 87/ triplet code of, 312, 313/ Nucleotide excision repair. 305f Nucleus, eukaryotic cell, 98, 102 chcmica! ^er.alrig and responses in, 213/, 214 division of (sec Mitosis) structure of. 102, 103/ transplantation of, 415-17 Nudibranch, 6 5 1 / Numbers, pyramid of, 1193/ Ntisslein-Volhard, Christiane, 423 Nutrient cycling m ecosystems. See also Chemical cycling in ecosystems general model of, 1195/ vegetation and, 1198,1199/ Nutnents absorption of, in small intestine, 859-61 essential animal, 849-52 essential plant, 756-59 as limiting factor in ecosystems, 1188-90 plans root absorption of, 7-4-46 translocation of, in plant phloem tissue. 751-53 Nutrition. Sec also Diet; Food absorptive, in fungi, 608-10 fiber, 74 in plants (see Plant nutrition) in prokaryotes, 538, 5391 Obdia, life cycle, 645/ Obesity, 847-48 evolution and, 848 Obligate aerobes, 539 Obligate anaerobes, 539 Occam's Razor, 501 Ocean benthic zone, 1097/ as biome {sec Marine biomes) coral reefs, 1097/ Ei Niiio Southern Oscillation in, 1170. 1171/ global warming and increased sea level, 1205 pelagic zone, 1096/

primary production and nutrients in, 1188. 11891, 1190 upwellirtg, 1190 zones of, 1093/ Oceanic pelagic biome, 1096 Oceanic zone, 1093/ Ocellus, 1057/-1058 Octopuses, 639/, 653/ Odling-Smee, Lucy, 1115-16 Odum, Eugene, 1160 Oil spills, bioremediation of, 406, 546/ Okazaki fragments of DNA lagging stands, 302/, 303/, 305 Old World monkeys, 697, 700/ Oleander (Nerium okcmcki). 751/ Olfaction, 1055 in humans, 1034, 1035/, 1056, 1057/ Olfactory bulb, 1034, 1035/, 1057/ Olfactory receptors, genes for, 459, 505 Oligochaeta (segmented worms), 654/ See also Earthworms Oligodendrocytes, 1015 Oligosaccharins in plants, 814 Oligotrophic lakes, 1094 Ommatidia, 1058/ Omnivores, 844 dentition in, 863/ Oncogenes, 371 conversion of proto-oncogenes into, 371/ One gene-one enzyme hypothesis, 310, 311/ One gene-one polypeptide hypothesis, 310, 330 One gene-one protein hypothesis, 310 On the Origin of Species (Darwin), 15, 438, 439, 443-46, 455 Onychophorans (velvet worms), 641/ Oogenesis, 973, 974/ Oogonia, 974 Oomycetes, 558, 559/ Oi\u]i'.e-2 mutant, 783 Oparin.A. 1., 513 Open circulatory system, 657, 868. 869/ Open system, 143 equilibrium and work in, 147/ Operant conditioning, 1117/ Operator, 353 Operculum, 682 Operons, 353-54 positive control of lac, 356/ regulated synthesis ol inducible enzymes by lac, 355/ regulated synthesis of repressible enzymes by trp, 354/ repressible versus mducible, 354-56 represser protein and regulatory ^ene control of, 353 Ophiuroidea (brittle star), 666t, 667/ Opiates as mimics of endorphins, 43, 81, 1025 Opisthokonts, 612 Opossums, 695,696/ Opposabie thumb, 697 Opsin, 1060 Opsonization,913 OpLicchiasm, 1063 Optimal foraging theory, 1122 C'ral ia\uy, digestion and 856 Orangutans, 701/ Orbitals, electron, 38/-39 hybridization ol", 4 3 / Orchid bilateral symmetry in flower of, 773/ unity and diversity in family of, 16/ Order evolution of biological, 144 as properties of life. 3/ Orders (taxonomy), 496 mammalian, 699/ Organ, 5/, 712 Organ, animal. 827. See also Org,;n systems, anund digestive, 855-62 reproductive, 969-73

Lissues of, 823 transplants of, and immune response, 916 vestigial, 448 Organ, plant. See Leaf (leaves); Root system; Shoot system Organdies, 5/. 95 of endomembrane system, 104-9 inheritance of genes located in, and genetic disorders, 289-90 isolating, by cell fractionauon, 97 microscopy and views of. 95-97 mitochondria and chtoroplasts, 109-11, 192-93 peroxisomes, 109, 110-11 relationship among endomembrane system, 109/ structural order in metabolism and, 157/ Organic chemistry, 58-59 Organic compounds abiotic synthesis of, 59/ carbon atoms as basis of, 59-61 carbon skeleton variations and diversity of, 61-63 in Earth's early atmosphere, 5 9 / 513-14 extraterrestrial sources of, 514 functional groups of, 63-66 shapes of three simple, 60/ Organic phosphates, 65/ Organ identity gene, 430, 734 ABC hypothesis for flower formation and, 734, 735/ Organisms, 4/. See also Species anatomy and physiology of, 820 binomial nomenclature for naming, 496 classification of (see Systematics; Taxonomy) colonies of, and origin of multicellular, 526/ communities of (see Community(ies)) complexity of, related to genomic composition, 374-75 as conformers and as regulators, 832 development of individual (see Development) evolutionary history of (see Phylogeny) genetically modified, 407-8 homeostasis in (see Homeostasis) interactions between environment and, 27/, 1080-83 dee also Ecology) life cycles of (see Life cycle) life history, 1141-43, 1147 model. Cor research studies, 412-13/ Overproduction of, 445f populations of (see Population(s); Population ecology) thermoregulation in, 833-41 transgemc, 406, 782, 783, 784-86 Organismal ecology. 1082 Organizer (.gastr.iin organizer), 1006 Organ ogenesis, 979, 997 in bird embryo, 998/ in frog embryo, 997/ from three germ layers, 997-98, 999( Organ systems, 827 animal, 827t animal reproductive, 969-73 Orgasm, 972-73 Origins of replication (DNA), 226, 227/ in eukaryotes, 3 0 1 / Qrrorin tugenenus, 703( Orihoiogous genes, 505 Osculum, 642 Osmoconformers, 923 Osmolarity, 923 Osmoregulation, 132, 922-27 kidneys and, 936, 937/, 938 osmosis and, 922-23 role of transport epithelia in, 926-27 iw o szrategies for challenges of, 923-26 water balance and, 131, 132/ 133 Osmoregulators, 923 Osmosis, 132,740 water balance 131, 132/, 133 water conservation, kidneys, and role of, 936 Osmotic potential, 741 Ostcichthyans, 682-84

Osteoblasts, 8 2 5 / Osteons, 825/ Ostia, 868 Ostracoderms, 679 Otoliths, 1052 Outer ear, 1051 Oulgroup, 498 Ova. See Ovum Oval window, 1051 Ovarian cycle, 973 hormonal regulation of, 976/ 977 Ovaries, animal (human), 241, 969 Ovary, plant, 598, 772, 774/, 775 fruit development from, 778, 779/ variation in locations of, 773/ Overexploitation of species, 1214 Overnourishment, 846-48 Overproduction, natural selection and, 16 Oviduct, 969 Oviparous species, 681 Oviraptor, 690/ Ovoviviparous species, 681 Ovulation, 969, 977 Ovule, 592, 772, 774 fertilization and development of seed from, 592, 593/, 777/-78 Ovum, 964. See also Egg development of human, 973, 974/ plant, 5 9 2 , 5 9 3 / 7 7 2 / Oxidation, 161-62 beta. 177 NAD+ as agent of, 162-63 peroxisomes and, 110-11 Oxidr«;"vt' ohi:sphorvla:;on, 164 ATP synthesis during, 164, 165, 170-74 photophosphorylation compared to, 192 Oxidizing agent, 161 Oxygen covalent bonding of, 40/ deep-diving mammals and storage of, 894-95 dissociation curves of, for hemoglobin, 892/ loading/unloading of, by hemoglobin, 892/ myoglobin and storage of, 895 origins of, in Earth's atmosphere, 522-23 photosynthesis and production nl, 183. 184f, 522-23 prokaryotic metabolism and, 539 respiratory pigments and transport of, 892/', 893 valences of, 60/ Oxytocin, 951 human pregnancy and childbirth and role of, 981 secretion of, by posterior pituitary, 950/ 951 Ozone, depletion of atmospheric, 1205-6 p53 gene, 373 cancer development and faulty, 373 P 680 Photosystem II. 190 P 700 Photosystem I, 190 Pace, Norman, 541 Pacemaker, heart, 873 Pacific yew (Taxa brevifolia), 595/ Paedomorphosis, 485, 685 Pain endorphins and reduction of, 43, 81, 952 prostaglandins and sensation of, 948, 1049 sensory receptors for, 1049 Paine, Robert. 1169 Pain receptors (nociceptors), 1049 Pair-rule genes, 425 Paiccanth rooology, 702 Paleontological species concept, 476 Paleontology, 440 Paleozoic era, 519c, 520, 5 2 1 / 527, 596, 629 Palisade mesophyll, 724 Panama, 1080/ Pancreas, 855, 955 digestive function, 858, 859/ glucose homeostasis and role o:. H4r:i '•>=•=• :>6 hormones secretions by, 9491, 955/", 956

Pangaea, 527.3291,696 Papaya, genetically engineered, 784/ Papillae, 1055 Papilloma virus, 370 Parabasalids (trichonior.acs) 553 Parabronchi, 889 Pamcerctis sculpta, mating behavior and male polymorphism in, 1126, 1127/ Paracrine signaling, 203/, 947-48 Paralogous genes, 505 Parametium sp., 556 cilia of, 15/ conjugation and reproduction in, 557/ 558 interspecific competition between species of, 1160 logistic population growth model applied to, 1147/ structure and function of, 537/ water balance and role of contractile vacuole in, 132,133/ Paramyosin, 1072 Paraphyletic grouping, 4 9 8 / Parareptiles, 689 Parasites, 545,1163 blood flukes as, 647/ brown-headed cowbird as, 1221 flowers as, 771 leeches as, 655 nematodes as, 656/ plants as, 767, 768/ tapeworms as, 648/ viruses as (see Viruses} Parasitism, 545, 1163 Parasitoidism, 1163 Parasitoid wasp, 813/ Parasympathetic division of autonomic nervous system, 1027/, 1028 Parathion pesticide, 155 Parathyroid glands, 954 hormones secreted by, 949t, 954-55 Parathyroid hormone (PT11), 954 Parazoan organization, 633 Parenchyma cells, 718, 722/ Parental behavior care of offspring. 690,1, 967. 968f, i 12-. i 1 2 5 / 1142/, 1143 genetic influences on, 1112-13 Parental types, 278 Parietal cells, 857/ Parker, John, 93 Parkinson's disease, 623,1041 E.Jarvison, 819 lackofdopamineand, 1025 L-dopa and treatment of, 62, 63/ Parsimony, principle of maximum. 501 analogy-versus-homology pitfall and, 504/ applied to problem in molecular sysiematics, 502-3/ Parthenogenesis, 648, 965 in rotifers, 648-49 in whiptail lizards, 965, 966/ Partial pressi-re, joad.r.g/unloading of respiratory gases and, 8 9 1 / Parturition, 981 Passive immunity, 914 Passive transport, 130-34. 738 active transport compared to, 135/ defined, 131 diffusion as, 130 facilitated diffusion as, 133-34 osmosis as, 131-33 Patchmess, environmental, 1082, 1172, 1173/ Paternity certainty of, 1124-25 DNA technology and establishment of, 405 Pathogens, 1163 agglutination of, 913 bacteria as, 2 9 5 / 339, 460, 545-46 B and T lymphocytes response to. Q10-14 drug-resistant, 447, 448/

fungi as, 622-23 immunity to (see Immune system) insects as vectors for, 554/, 555, 556/, 664 parasites as (see Parasite(s))

plant (see Plant disease) plant defenses against, 813-15 population density of hosts and, 1149 prokaryoies as, 545-46 promts as. 552, 553,'". 554/". 553. 556/, 564 viruses as, 337, 339', 343-45 Pattern formation, 421-31, 731, 1007 cell fate determination and, by inductive signals, 1006-8 cell signaling and, in C. ckgans, 425-27 cell signaling and transcriptional regulation in plants, 429-30 gene-activation cascade and, in Drosophila, 421-25 homeobox sequences and, 431-32 homeotic genes and, 425 positional information and, 421, 731, 732—33 in plant cell development, 730-32 programmed cell death (apoptosis) in, 427-29 during vertebrate limb development, 1006-8 Pauling, Linus, 296 PCBs (polychlorinated biphenyls), biological magnification of, in food web, 1202/ Peacocks, sexual dimorphism in, 468/ Pea plants, 603/ fruit development in, 779/ gibberellin and stem elongation in, 7L)7 Mendel's studies on inheritance in, 251-58, .309 Rhtzobium bacteria and nitrogen fixation in, 765/ triple response to stress in seedlings of, 799/ Peat, 583 Peal bogs, L. Graham on ecological function of, 511 Pectin, 106 Pediastrum, 526/ Pedigree. Mendeh^n inheritance patterns in humans revealed in analysis of, 265-66 Pelagic zone, 1093/ Penguins, 6 9 3 , 8 2 1 / energy budget for, 830, 831/" nutritional requirements during molting, 850/ population dispersion patterns of, 1138/ Penicillin, 155, 611/, 623/ Poiiciiliumsp., 611/, 623/ Penis, 972 ereciile dysfunction in, 211, 948, 1025 Pentose sugars, 70/ PEP carboxylase, 196 Pepsin, 857 Fepsinogen, 857 Peptides, as neurotransmliters, 1024t, 1025 Peptide bond, 80 Peptidoglycan, 535 Perception, sensory input and, 1046. See also Sensory mechanisms Pereira, Sofia, 1120 Perennials, plant, 720 Pericarp, 598 Pericycle, 723 Periderm,717,814 cork cambia and produciion of, 728 Periodic table of elements, 37 Peripheral nervous system (PNS), 1012, 1026/ enteric division of, 1028 functional hierarchy of, 1027/ homestasis and, 1028 parasympathetic divisions of, 1027/, 1028 role of. in homeosi.asis, 1028 Schwann cells of, 1015/ sympathetic division of, 1027/ Peripheral proteins, 128 Peripheral resistance, 876 Peristalsis, 855, 1064 m digestive tract, 855 esopbageal, 856/ movement by, in annelids, 1064

Peristome, 581 Peritubular capillaries, 933 PeTiviteBine space, 989 Permian period, 596

mass extinctions in, 518/, 520 Peroxisome, 109, 110, 111/ Peroxisome enzymes, 574 Pert, Candace, 1025 Pertussis (whooping liougb), 206j Petals, flower, 429/, 430, 598, 772 Petiole, 715 Pel rels, excessive weight in chicks of, 848/ PET scans, 36/ Pettit, Robert, 643 Pfenning, David and Karin, inquiry- into mimicry of warning coloration in snakes, 22, 23/, 24 Pfiesteria shumwoyse, 555/ P generation, 253 incomplete dominance and, 2 6 1 / law of segregation and, 2 5 3 / 255/, 257/, 275/ pH. 54 acids,bases,and, 53-54 buffers, 55 enzyme activity and effects of, 154/ pH. scale, 53, 54/ 55 Phaeophyta (brown algae), 560-62 Phage(s), 294, 336. Sec also Bacteriophage (phage) Phage X, lytic and lysogenic cycles of. 338, 339;" Phagocytes (phagocytic cells), 899-900 Phagocytosis, 107, 137, 138/, 881, 899, 900/ antibody-mediated disposal of antigens and, 913/ lysosQines and, 107/ lour leukocytes capable of, 900 Pharmaceutical products See aho Drug(s); Medicine biodiversity and development of new, 1211-12 derived from plants, 30-31, 594/ 595/, 605t, 1211, 1212/ DNA technology and development of, 404 enantiomers and, 62, 63/ G-proiein pathways and, 206 "Pharrrf animals, 406/ Pharyngeal clefts, 673, 676 PharyngeaJ slits, 673 Pharyngoleph, 679/ Pharynx, 856 Phase changes, 733 Phase-contrasi microscopy, 96/ Phelloderm, 728 Phenotype, 256 effects of natural selection on, 462-70 genotype versus, 256/ multiple eHecis in (pleiotropy), 262 mutant, 276, 277/ norm of reaction in, 264 relationship of, to allele dominance, 261 wild type, 276/, 277/ Phenotypk: poh rnorphism, 46.3 Phenylalanine, 313,320 Phenylketonuria (PKU), 269-70, 458 Pheromones, 610, 945, 967, 1111 in insects, 437, 1048, 1409/ Philadelphia chromosome, 288 Philippine eagle, "1211/ Phloem, 584, 717, 751-53 loading and unloading of, 752/ pressure flow of sap in, 753/ in roots. 722/ sieve-tube members and companion cells of, 719 transport (translocation) of sap through, 584, 751-53 Phoronids (marine worms), 640/ 649 Phoroms hippocrepia, 649/ Phosphate bonds, high-energy 148-49 Phosphate functional group, 6 5 / Phosphofructokinase, stimulation of, 177/ Phospholipids, 76-77 bilayer structure formed by self-assembly of, in aqueous environments, 77/

membrane models and, 125 steroids as, 77 structure of, 76/ Phospholipid bilayer, 125/

permeability of, 126,130 Phosphorus as plant nutrient, 761 deficiency of, 759/ 761/ Phosphorylation, 149 ATP hydrolysis and. 149-50 cascade, 209f oxiuauve, 164 photophosphorylation, 185 of proteins, as mechanism of signal transductlon, 209-10 substrate-level. 165/ Photic zone, 1093 Photoautotrophs, 181, 182/ prokaryotes as, 538, 539/ protists as, 550 Phoioexcitation of chlorophyll, 188, 189/ Photoheterotrophs, 539( Photomorphogenesis, 802 Photons, light, 186 photoexcitation of chlorophyll and absorption of, 188,189/ Photoperiodisrn, 806 flowering and, 806-8 meristem transition and, 808 Phoiophosphorylation, 185, 189-92 oxidative phosphorylation compared to, 192 Photoprotection, 188 Photopsins, 1061 Phoioreceptors, 1048 blue-light, in plants, 750, 802 brain processing of visual information and, 1061-63 for infrared light, 1049/ in in vertebrates, 1057-58 phototropism in plants and, 792-93 phytochromes as plant, 789-91, 802-5 invertebrate eye, 1057-63 Photorespiraiion, 195-96 Photosynthesis, 181-200 absorption and action spectra for, 187/ 188 alternative mechanisms of, for hot and arid climates, 195-98 Calvin cycle and conversion of CO 2 to sugar in, 184, 185/", 193-95 chemical equation for, 44, 183 chloroplasts as site of, 109, 111/, 182-83 conversion of light energy into chemical energy m, 182-85 importance of, 197. 198/ light reactions in, 184, 185/ 186-93 origins of, 522-23 overview of two stages of, 184, 185/ role of, in biosphere, 181, 182/ splitting of water and redox reactions of, 184 tracking atoms through, 183, 184/ Photosystem I (PS1), 190, 191/ Photosystem II (PSII), 190, 191/ Photosystems, photosynthetic, 189-90 light harvesting by, 189/ Phototrophs, 538-39 Phototropin, 802 Phototropism, 792 early studies of, and discovery of plant hormones. 792-93 light wavelengths and, 803/ Phragmoplasts, 574 pH scale, 53,54/, 55 soil pH, 762 Phyla (taxonomy). 496 Phylogenetic species concept, 476 Phylogenetic trees, 496, 497-504 of amniotes, 687/ of angiosperms, 602/ of animals, 633, 634/ 635/, 638/ of biological diversity, 530-31f

oFchordates, 672/ cladograms, cladistics, and. 497-99 of eukarvotrs. T52T" of fungi, 613/ of Galapagos finches, 18/ as hypoiheses. 501-4 mammalian, 698/ maximum parsimony, maximum likelihood, and, 501,502-3/ of plants, 579/ of prokaryoies, 540/ timing, chronologies, and, 499-500 Phytogeny, 491-95 of amniotes, 697/ animal, 633-36, 638/ chordate, 671, 672f, 67.3 circulatory system as reflection of, 867-71 connection between classification and, 496, 497/ continental drift and biogeographical basis of, 528 of eukaryotes, 552/ fossil record as evidence for, 492 of fungi, 612, 613/ genome and, 504-6 mammals. 698-99/ "-r.orphoioiiica] and molecular bomologies and, 492-95 "^hylogenaic :rees (see Phylogeiifik: trecsi plants, 575-59 of prokaryotes, 540-44 systematics [see Sys?ern;uu~s) taxonomie classification and (see Taxonomy) tree of life and, 491, 507 Phylogram, 499/, 500 PViMip-Lirn poiyccphalum, 564;" Physical map, DNA fragments, 396 Physiology, 820. See also Animal anatomy and physiology Phyioalexins, 814 Phytochemicals, 188 Phytochromes, 802 as molecular switching mechanism, 804/ as photoreceptors, in plants, 789, 790/, 802-5 structure of, 804/ Phytolacca dodecandra, medicinal properties of, 30 I'hy'ophthora injestani, 558 Phytophthom ramorum Csudden oak death disease), 1163 Phytoplankton, 550 nutrients for, and ecosystem productivity, 1189) turnover time for, 1192 Phy t ore mediation. 763 Pigmentation, skin, 952 Pigments carotenoids, 188, 189 chlorophyll (MT Chlorophyll(s)) photosynihetic, 186-88 respirator). 892-94 Pili (pilus), 349, 536 Pihbolus, 614. 615/ Pine, life cycle of, 596, 597/ Pineal gland, hormone produced by, 9491, 959 Pm flower type, 775/ Pinocytosis, 137, 138/ Pinyonjays (Gymnorhnus lyanou phalli. 1116 Phonia, seed dispersal in, 480/ Pistil, 598, 772 Pitch (sound), detection of, 1052 Pitcher plant, 768/ Pith, plani tissue, 717, 722 Pittsburgh Compound-B (P1B), 1041 Pituitary gland. 948. 950 hormones of anterior, 949t, 951/, 952, 976/ hormones of posterior, 9491, 950/. 951 production and release of hormones by, 950/ relationship of, to hypothalamus, 950-51 Pivot joint, I065J PKU (phenylkeionu:ia}. 2.69-70. 458 Placenta, 695, 915, 979 circulation in fetus and. 980/

Placental mammals. See EuuVriar. (placental) mammals Placental transfer cells, 577 Placoderms, 680 Placozoa, 639/ Plagioihila ddiohka liverwort, 582/ Plague, 546 Planarians, 639/ 646 anatomy of, 647/ ocelli and orientation behavior of, 1057/ nervous system, 1012/" protonephndmm (flame-bulb) of, 929/, 930 Plankton, 550 Plants, 573-90 adaptations of, for life on land, 573, 574, 575-79 alternation of generations in, 242, 576/ angiosperms and evolution of fruits and flowers (see Angiosperms) in arid and hot climates, 195-98 artificial selection and selective breeding of, 783-84 biotechnology applied to, 783-86 bryophytes, 580-83 carnivorous, 767, 768/ cells of (see Plant cell) colonization of land by, 527 crop (see Crop plants) defenses of, against predators. 813 derived traits of, 575, 576-77/ development of (see Development; Plant development) diseases in (sire Plant disease) epiphytes, 768/ evolution of. from charophyceans (green algae), 573-74 evolution of, highlights, 579/ extranuclear genes and leaf color in, 289, 290/ genetic engineering in, 406-7, 784-86 L. Graham on evolution of terrestrial, 510-11 greening of (de-etiolation), 789/ 7901, 791 growth in (see Plant growth) gymnosperms (see Gymnosperms) hybridization of, 252-53 kudzu as introduced species of, 1213/ medicines derived from, 594/ 595/ 605f movements in, 805f, 810/ nutrition in (see Plant nutrition) origin and diversification of, 575-79 parasitic, 768/ photosynthesis in (see Photosynthesis) phyla of extant, 5781 polyploidy in, 285 reproduction in (see Plant reproduction) responses of, to signals (see Plant responses) seedless vascular plants, 584-88 seed vascular plants (see Seed 'nhunsi 775-76 sexual life cycle in, 242/ structure (see Plant structure) sympatric speciation and evolution of, 4 7 8 / 479/ temperature moderation in, 50 threats to, 606 totipotency in, 415 transgenic, 406-7, 782, 783, 784-86 transport (see Transport in vascular plants) vascular (see Vascular plants) vascular tissues (see Phloem; Xylem) water transport in, 48/ Plantae (kingdom), 575/ See also Plant(s) Plantain, seed production and population density of, 1149/ Plant anatomy See Plant cell; Plant structure Plant cells, 717,718-19/ abscisic acid effects on, 798-99 auxin and elongation of, 794 ; 795/ brassinosteroid effects on, 798 cell plate of, 2 2 5 / 2 2 6 cell walls of, 118,119/, 743/ central vacuole of, 108/

chloroplasts in (see Chloroplasts) collenchyma cells. 718/ companion cells, 719/ compartments of, and short-distance transport, 743/ cytokinesis and cell plate formation in, 225/ 226 cytokinin and division/differentiation of, 796 cytoplasmic streaming in, 117/, 118 ethylene effects on, 799-801 expansion of, orientation. 730, 731/ flaccid and turgid, 132-33, 750/ gibberellin effects on, 309. 797" location and fate of, 732-33 mitosis in, 226/ orientation of expansion of, 730, 731/ parenchyma, 718/ pattern formation and development of, 730-32 plane and symmetry of division and growth of, 729/ 730/ plasmodesmataof. 120/ positional information and development of, 731, 732-33 protoplast of, 782/ sclerenchyma cells, 718/ sieve-tube members, 719/ storage ol starch in, 71, 72/ structure of generalized, 101/ tracheids, 719/ vacuoles of, 108/ vessel elements, 719/ water balance in, 132, 133/ water relations in, 742/ Plant control systems. See Plant responses Plant development, 728-35. See also Plant growth apical menstems and, 576/ animal development compared to, 433 cell division and cell expansion in, 729/ 730/ 796 cell location and developmental fate of cells in, 732-33 cell signaling, induction and, 420, 4 2 1 / cell signaling, transcriptional regulation, and, 429-30 embryo development, 777/, 778 gene expression and control of cell differentiation in, 732. 733/ mechanisms of, 429 model organism Arabidonw, for study of, 413/, 728,729/ molecular biology and study of, 728-29 organ-identity genes in, 430, 4 3 1 / pattern formation and, 730-32 pattern formation in flowers and, 429/ 430/, 4 3 1 / phase changes and shifts in, 733-34 loiipotency, 415 Plant disease chestnut blight, 1168 fungi as cause of, 622, 797 plant defenses against pathogens, 813-15 protistan,1163 viral, 334, 335/", 345 white rusts and downy mildew, 558 Plant growth, 720-32. See also Plant development apical meristems and primary, 576/ 720/ 721-25 auxin and, 793/, 794. 795/ cell division, cell expansion, and, 729/, 730 determinate, 720 indeterminate, 720 inhibitors of, 793 lateral meristems and secondary, 720/, 721, 725-28 morphogenesis, dilfereniiation, and, 728-35 pattern formation and, 730-32 Plant hormones, 791-802 abscisic acid, 794t, 798-99 auxin, 794t-97 brassinosteroids, 791. 794t, 798 cytokinins, 794(, 796-97 discover)' of, 792-93