UXL Complete Life Science Resource Edition 1

COMPLETE LIFE SCIENCE RESOURCE COMPLETE LIFE SCIENCE RESOURCE volume ONE: A–E LEONARD C. BRUNO JULIE CARNAGIE, EDI...

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COMPLETE

LIFE SCIENCE RESOURCE

COMPLETE

LIFE SCIENCE RESOURCE volume ONE: A–E

LEONARD C. BRUNO JULIE CARNAGIE, EDITOR

UXL Complete Life Science Resource LEONARD C. BRUNO Staff Julie L. Carnagie, UXL Senior Editor Carol DeKane Nagel, UXL Managing Editor Meggin Condino, Senior Market Analyst Margaret Chamberlain, Permissions Specialist Randy Bassett, Image Database Supervisor Robert Duncan, Imaging Specialist Pamela A. Reed, Image Coordinator Robyn V. Young, Senior Image Editor Michelle DiMercurio, Art Director Evi Seoud, Assistant Manager, Composition Purchasing and Electronic Prepress Mary Beth Trimper, Manager, Composition and Electronic Prepress Rita Wimberley, Senior Buyer Dorothy Maki, Manufacturing Manager GGS Information Services, Inc., Typesetting Bruno, Leonard C. UXL complete life science resource / Leonard C. Bruno; Julie L. Carnagie, editor. p. cm. Includes bibliographical references. Contents: v. 1. A-E v. 2. F-N v. 3. O-Z. ISBN 0-7876-4851-5 (set) ISBN 0-7876-4852-3 (vol. 1) ISBN 0-7876-4854-X (vol. 2) 1. Life sciences Juvenile literature. [1. Life sciences Encyclopedias.] I. Carnagie, Julie. II. Title. QH309.2.B78 2001 00-56376

This publication is a creative work fully protected by all applicable copyright laws, as well as by misappropriation, trade secret, unfair competition, and other applicable laws. The editors of this work have added value to the underlying factual material herein through one or more of the following: unique and original selection, coordination, expression, arrangement, and classification of the information. All rights to this publication will be vigorously defended. Copyright ©2001 UXL, an Imprint of the Gale Group 27500 Drake Rd. Farmington Hills, MI 48331-3535 All rights reserved, including the right of reproduction in whole or in part in any form. Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

Table of Contents Reader’s Guide • v i i Introduction • i x Timeline of Significant Discoveries in the Life Sciences • x i Words to Know • x v i i Research and Activity Ideas •

xxxiii volume ONE: A–E Abiotic/Biotic Environment • Acid and Base • 2 Acid Rain • 4 Adaptation • 7 Aerobic/Anaerobic • 8 Aging • 1 1 Agriculture • 1 3 AIDS • 1 7 Algae • 2 1 Amino Acids • 2 4 Amoeba • 2 5 Amphibian • 2 7 Anatomy • 3 0 Animals • 3 3 Antibiotic • 3 5 Antibody and Antigen • 3 7 Arachnid • 3 9

1

Arthropod • 4 1 Bacteria • 4 5 Biodiversity • 4 9 Biological Community • Biology • 5 4 Biome • 5 5 Biosphere • 5 9 Birds • 6 1 Blood • 6 6 Blood Types • 6 8 Botany • 7 1 Brain • 7 5 Bryophytes • 7 8 Buds and Budding • 7 9 Calorie • 8 3 Carbohydrates • 8 4 Carbon Cycle • 8 7 Carbon Dioxide • 9 0 Carbon Family • 9 2 Carbon Monoxide • 9 5 Carnivore • 9 7 Cell • 1 0 0 Cell Division • 1 0 5 Cell Theory • 1 0 8 Cell Wall • 1 1 1 Centriole • 1 1 2

U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

52

iii

Contents

Cetacean • 1 1 4 Chaparral • 1 1 6 Chloroplast • 1 1 8 Chromatin • 1 2 0 Chromosome • 1 2 1 Cilia • 1 2 5 Circulatory System • 1 2 6 Class • 1 3 1 Classification • 1 3 2 Cloning • 1 3 6 Cnidarian • 1 4 0 Community • 1 4 1 Competition • 1 4 3 Crustacean • 1 4 5 Cytoplasm • 1 4 7 Decomposition • 1 4 9 Desert • 1 5 0 Diffusion • 1 5 3 Digestive System • 1 5 5 Dinosaur • 1 6 1 DNA (Deoxyribonucleic Acid) •

164 Dominant and Recessive Traits •

168 Double Helix • 1 6 9 Echinoderm • 1 7 3 Ecology • 1 7 5 Ecosystem • 1 8 0 Egg • 1 8 2 Embryo • 1 8 5 Endangered Species • 1 8 7 Endocrine System • 1 9 0 Endoplasmic Reticulum • 1 9 4 Entomology • 1 9 5 Enzyme • 1 9 9 Eutrophication • 2 0 2 Evolution • 2 0 5 Evolution, Evidence of • 2 0 8 Evolutionary Theory • 2 1 1 Excretory System • 2 1 5 iv

Extinction • 2 1 8 For Further Information • Index • x l v

xxxix

volume TWO: F–N Family • 2 2 3 Fermentation • 2 2 4 Fertilization • 2 2 6 Fish • 2 2 9 Flower • 2 3 1 Food Chains and Webs • 2 3 4 Forests • 2 3 6 Fossil • 2 3 8 Fruit • 2 4 0 Fungi • 2 4 2 Gaia Hypothesis • 2 4 5 Gene • 2 4 7 Gene Theory • 2 4 9 Gene Therapy • 2 5 1 Genetic Code • 2 5 3 Genetic Disorders • 2 5 5 Genetic Engineering • 2 5 7 Genetics • 2 6 0 Genus • 2 6 3 Geologic Record • 2 6 4 Germination • 2 6 8 Golgi Body • 2 6 9 Grasslands • 2 7 0 Greenhouse Effect • 2 7 2 Habitat • 2 7 7 Hearing • 2 8 0 Heart • 2 8 3 Herbivore • 2 8 6 Herpetology • 2 8 8 Hibernation • 2 8 9 Homeostasis • 2 9 1 Hominid • 2 9 4 Homo sapiens neanderthalensis •

298 Hormones •

299


Horticulture • 3 0 3 Human Evolution • 3 0 4 Human Genome Project • 3 0 6 Human Reproduction • 3 1 0 Hybrid • 3 1 3 Ichthyology • 3 1 5 Immune System • 3 1 6 Immunization • 3 2 0 Inbreeding • 3 2 2 Inherited Traits • 3 2 3 Insects • 3 2 7 Instinct • 3 3 0 Integumentary System • 3 3 2 Invertebrates • 3 3 5 Karyotype • 3 4 1 Kingdom • 3 4 2 Lactic Acid • 3 4 5 Larva • 3 4 6 Leaf • 3 4 9 Life Cycle • 3 5 1 Light • 3 5 5 Lipids • 3 5 6 Lymphatic System • 3 5 8 Lysosomes • 3 5 9 Malnutrition • 3 6 1 Mammalogy • 3 6 3 Mammals • 3 6 5 Meiosis • 3 6 8 Membrane • 3 7 0 Mendelian Laws of Inheritance •

372 Metabolism • 3 7 4 Metamorphosis • 3 7 7 Microorganism • 3 8 0 Microscope • 3 8 2 Migration • 3 8 6 Mitochondria • 3 8 8 Mollusk • 3 8 9 Monerans • 3 9 2 Muscular System • 3 9 4

Mutation • 3 9 6 Natural Selection • 3 9 9 Nervous System • 4 0 3 Niche • 4 0 9 Nitrogen Cycle • 4 1 0 Nonvascular Plants • 4 1 2 Nuclear Membrane • 4 1 3 Nucleic Acids • 4 1 5 Nucleolus • 4 1 7 Nucleus • 4 1 7 Nutrition • 4 1 9 For Further Information • x x x i x Index • x l v

Contents

volume THREE: O–Z Ocean • 4 2 3 Omnivore • 4 2 5 Order • 4 2 7 Organ • 4 2 8 Organelle • 4 2 9 Organic Compounds • 4 3 0 Organism • 4 3 2 Ornithology • 4 3 4 Osmosis • 4 3 6 Ozone • 4 3 8 Paleontology • 4 4 1 Parasite • 4 4 3 pH • 4 4 5 Pheromone • 4 4 6 Photosynthesis • 4 4 9 Phototropism • 4 5 2 Phylum • 4 5 4 Physiology • 4 5 4 Piltdown Man • 4 5 8 Plant Anatomy • 4 6 0 Plant Hormones • 4 6 4 Plant Pathology • 4 6 6 Plant Reproduction • 4 6 9 Plants • 4 7 1 Plasma Membrane • 4 7 5


v

Contents

vi

Pollution • 4 7 6 Polymer • 4 7 9 Population • 4 8 0 Population Genetics • 4 8 2 Population Growth and Control (Human) • 4 8 4 Predation • 4 8 7 Primate • 4 8 9 Protein • 4 9 1 Protists • 4 9 3 Protozoa • 4 9 5 Punnett Square • 4 9 9 Radioactive Dating • 5 0 1 Rain Forest • 5 0 3 Reproduction, Asexual • 5 0 6 Reproduction, Sexual • 5 0 8 Reproductive System • 5 1 0 Reptile • 5 1 2 Respiration • 5 1 4 Respiratory System • 5 1 5 Rh factor • 5 1 9 RNA (Ribonucleic Acid) • 5 2 0 Root System • 5 2 2 Seed • 5 2 5 Sense Organ • 5 2 8 Sex Chromosomes • 5 2 9 Sex Hormones • 5 3 1 Sex-linked Traits • 5 3 3 Sight • 5 3 4 Skeletal System • 5 3 8

Smell • 5 4 2 Species • 5 4 5 Sperm • 5 4 6 Sponge • 5 4 7 Spore • 5 4 9 Stimulus • 5 5 1 Stress • 5 5 2 Survival of the Fittest • 5 5 4 Symbiosis • 5 5 5 Taiga • 5 6 1 Taste • 5 6 3 Taxonomy • 5 6 5 Territory • 5 6 7 Tissue • 5 7 0 Touch • 5 7 2 Toxins and Poisons • 5 7 4 Tree • 5 7 7 Tundra • 5 7 8 Vacuole • 5 8 1 Vascular Plants • 5 8 2 Vertebrates • 5 8 3 Virus • 5 8 8 Vitamins • 5 9 1 Water • 5 9 5 Wetlands • 5 9 7 Worms • 6 0 0 Zoology • 6 0 5 Zygote • 6 0 6 For Further Information x x x i x Index x l v


Reader’s Guide UXL Complete Life Science Resource explores the fascinating world of the life sciences by providing readers with comprehensive and easyto-use information. The three-volume set features 240 alphabetically arranged entries, which explain the theories, concepts, discoveries, and developments frequently studied by today’s students, including: cells and simple organisms, diversity and adaptation, human body systems and life cycles, the human genome, plants, animals, and classification, populations and ecosystems, and reproduction and heredity. The three-volume set includes a timeline of scientific discoveries, a “Further Information” section, and research and activity section. It also contains 180 black-and-white illustrations that help to bring the text to life, sidebars containing short biographies of scientists, a “Words to Know” section, and a cumulative index providing easy access to the subjects, theories, and people discussed throughout UXL Complete Life Science Resource.

Acknowledgments Special thanks are due for the invaluable comments and suggestions provided by the UXL Complete Life Science Resource advisors: •

Don Curry, Science Teacher, Silverado High School, Las Vegas, Nevada

•

Barbara Ibach, Librarian, Northville High School, Northville, Michigan

•

Joel Jones, Branch Manager, Kansas City Public Library, Kansas City, Missouri

•

Nina Levine, Media Specialist, Blue Mountain Middle School, Peekskill, New York


vii

Reader’s Guide

Comments and Suggestions We welcome your comments on this work as well as your suggestions for topics to be featured in future editions of UXL Complete Life Science Resource. Please write: Editors, UXL Complete Life Science Resource, UXL, 27500 Drake Rd., Farmington Hills, MI 48331-3535; call toll-free: 1-800-877-4253; fax: 248-699-8097; or send e-mail via www.galegroup.com.

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Introduction U X L Complete Life Science Resource is organized and written in a manner to emphasize clarity and usefulness. Produced with grades seven through twelve in mind, it therefore reflects topics that are currently found in most textbooks on the life sciences. Most of these alphabetically arranged topics could be described as important concepts and theories in the life sciences. Other topics are more specific, but still important, subcategories or segments of a larger concept. Life science is another, perhaps broader, term for biology. Both simply mean the scientific study of life. All of the essays included in UXL Complete Life Science Resource can be considered as variations on the simple theme that because something is alive it is very different from something that is not. In some way all of these essays explore and describe the many different aspects of what are considered to be the major characteristics or signs of life. Living things use energy and are organized in a certain way; they react, respond, grow, and develop; they change and adapt; they reproduce and they die. Despite this impressive list, the phenomenon that is called life is so complex, awe-inspiring, and even incomprehensible that our knowledge of it is really only just beginning. This work is an attempt to provide students with simple explanations of what are obviously very complex ideas. The essays are intended to provide basic, introductory information. The chosen topics broadly cover all aspects of the life sciences. The biographical sidebars touch upon most of the major achievers and contributors in the life sciences and all relate in some way to a particular essay. Finally, the citations listed in the “For Further Information” section include not only materials that were used by the author as sources, but other books that the ambitious and curious student of the life sciences might wish to consult. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

ix

Timeline of Significant Developments in the Life Sciences c. 50,000

B.C.

Homo sapiens sapiens emerges as a conscious observer of nature.

c. 10,000

B.C.

Humans begin the transition from hunting and gathering to settled agriculture, beginning the Neolithic Revolution.

c. 1800 c. 350

A.D.

1615

B.C.

B.C.

1543

Process of fermentation is first understood and controlled by the Egyptians. Greek philosopher Aristotle (384–322 B.C.) first attempts to classify animals, considers nature of reproduction and inheritance, and basically founds the science of biology. Flemish anatomist Andreas Vesalius (1514–1564) publishes Seven Books on the Construction of the Human Body which corrects many misconceptions regarding the human body and founds modern anatomy. The modern study of animal metabolism is founded by Italian physician, Santorio Santorio (1561–1636), who publishes De Statica Medicina in which he is the first to apply measurement and physics to the study of processes within the human body.

12,000 B.C. The dog is domesticated from the wolf

15,000 B.C.

3,000 B.C. The world’s population reaches 100,000

7,500 B.C.


A.D.

552 Buddhism reaches Japan

1

1,000 xi

Timeline

1628

The first accurate description of human blood circulation is offered by English physician William Harvey (1578–1657), who also founds modern physiology.

1665

English physicist Robert Hooke (1635–1703) coins the word “cell” and develops the first drawing of a cell after observing a sliver of cork under a microscope.

1669

Entomology, or the study of insects, is founded by Dutch naturalist Jan Swammerdam (1637–1680), who begins the first major study of insect microanatomy and classification.

1677

Dutch biologist and microscopist Anton van Leeuwenhoek (1632–1723) is the first to observe and describe spermatozoa (sperm). He later goes on to describe different types of bacteria and protozoa.

1727

English botanist Stephen Hales (1677–1761) studies plant nutrition and measures water absorbed by roots and released by leaves. He states that the plants convert something in the air into food, and that light is a necessary part of this process, which later becomes known as photosynthesis.

1735

Considered the father of modern taxonomy, Swedish botanist Carl Linnaeus (1707–1778) creates the first scientific system for classifying animals and plants. His system of binomial nomenclature establishes generic and specific names.

1779

Dutch physician Jan Ingenhousz (1739–1799) shows that carbon dioxide is taken in and oxygen is given off by plants during photosynthesis. He also states that sunlight is necessary for this process.

1802

The word “biology” is coined by French naturalist JeanBaptiste Lamarck (1744–1829) to describe the new science of living things. He later proposes the first scientific, but flawed, theory of evolution.

1710 The first copyright law is established in Britain

1650 England’s first coffee house opens

1620 xii

1680

1770 The Boston Massacre occurs

1740

1800


1809

Modern invertebrate zoology is founded by French naturalist Jean-Baptiste Lamarck (1744–1829) who also introduces the term “invertebrate.”

1827

A mammalian egg is discovered by Estonian biologist Karl Ernst von Baer (1792–1876). He states that the human egg is not fundamentally different from that of other animals.

1831

English naturalist Charles Robert Darwin (1809–1882), begins his historic voyage on the H.M.S. Beagle (1831–36).

1839

German physiologist Theodore Schwann (1810–1882) states that all living things are made up of cells, each of which contains certain essential components. Schwann’s theory is applied to both animals and plants and becomes known as the cell theory.

1858

Modern biology begins as German pathologist Rudolph Virchow (1821–1902) founds cellular pathology with his historic statement that “Every cell comes from a cell.”

1859

The landmark book, On the Origin of Species, is published by Charles Darwin. This revolutionary work proposes a theory of evolution based on variation and survival of the fittest.

1864

Pasteurization is invented by French chemist Louis Pasteur (1822–1895). Earlier he recognized the relation between microorganisms and disease as well as microorganisms and fermentation.

1866

The laws of inheritance, or genetics, are first stated by Austrian botanist Gregor Johann Mendel (1822–1884). He also states that both male and female contribute equal factors (genes) to the offspring and that these factors do not blend but remain distinct.

1820 The Spanish Inquisition ends

1810

1860 The internal combustion engine is patented

1840 The brass saxophone is invented

1830


Timeline

1850

1870 xiii

Timeline

1873

Italian histologist Camillo Golgi (1843–1926) devises a way to stain tissue samples with inorganic dye and applies this new method to nerve tissues.

1882

German bacteriologist Robert Koch (1843–1910) establishes the classic method of preserving, documenting, and studying bacteria.

1882

German anatomist Walther Flemming (1843–1905) becomes the first to observe and describe mitosis or splitting of chromosomes, the structure in the cell that carries the cell’s genetic material.

1900

Different types of human blood are discovered by Austrian American physician Karl Landsteiner (1868–1943), who names them A, B, AB, and O.

1901

Spanish histologist Santiago Ramon y Cajal (1852–1911) demonstrates that the neuron is the basis of the nervous system.

1902

Hormones are first named and understood by English physiologists Ernest H. Starling (1866–1927) and William H. Bayliss (1860–1924), who describe them as chemicals that stimulate an organ from a distance.

1905

English biochemist Frederick Gowland Hopkins (1861–1947) provides proof that “essential amino acids” cannot be manufactured by the body and must be obtained from food.

1907

Russian physiologist Ivan Pavlov (1849–1936) conducts pioneering studies on inborn reflexes and the conditioning of animals.

1910

American geneticist Thomas Hunt Morgan (1866–1945) works with the fruit fly Drosophila and establishes the chromosome theory of inheritance. This theory states that chromosomes are composed of discrete entities called genes that are the actual carriers of specific traits.

1880 Thomas Edison receives patent for the light bulb

1875 xiv

1900 Sigmund Freud pioneers psychoanalysis

1890

1905

1920 Suffrage for American women becomes effective

1920


1912

English biochemist Frederick Gowland Hopkins (1861–1947) proves that “accessory substances,” later called vitamins, are essential for health and growth.

1932

German biochemist Hans Krebs (1900–1981) discovers that glucose (sugar) is broken down in a chain of reactions that comes to be called the Krebs cycle.

1953

The double helix structure of deoxyribonucleic acid (DNA) is discovered by American biochemist James Dewey Watson (1928– ) and English biochemist Francis Harry Compton Crick (1916– ). Their model explains how DNA transmits hereditary traits in living organisms, and forms the basis for all genetic discoveries that follow. This is considered one of the greatest of all scientific discoveries.

1961

Messenger ribonucleic acid (mRNA), which transfers genetic information to the ribosomes where proteins are made, is discovered by French biologists Jacques Lucien Monod (1910–1976) and Francois Jacob (1920– ).

1978

The first “test tube” baby is born in England. Physicians remove an egg from the mother’s ovary, fertilize it with the father’s sperm in a petri dish, and reimplant it in the mother’s uterus.

1982

A gene from one mammal (a rat growth hormone gene) functions for the first time in another mammal (a mouse). As a result, the mouse grows to twice its normal size.

1983

American biologist Lynn Margulis (1938– ) discovers that cells with nuclei can be formed by the synthesis of non-nucleated cells (those without a nucleus, like bacteria).

1987

Genetically engineered plants are first developed.

1955 British Prime Minister Winston Churchill resigns

1935 Adolf Hitler creates the Lüftwaffe

1925

1945


Timeline

1975 Microsoft is founded

1965

1985 xv

Timeline

1990

The Human Genome Project is established in Washington, D.C., as an international team of scientists announces a plan to compile a “map” of human genes.

1991

The gender of a mouse is changed at the embryo stage.

1992

The United Nations Conference on Environment and Development is held in Brazil and is attended by delegates from 178 countries, most of whom agree to combat global warming and to preserve biodiversity.

1995

The first complete sequencing of an organism’s genetic make up is achieved by the Institute for Genomic Research in the United States. The institute uses an unconventional technique to sequence all 1,800,000 base pairs that make up the chromosome of a certain bacterium.

1997

The first successful cloning of an adult mammal is achieved by Scottish embryologist Ian Wilmut (1944– ), who clones a lamb named Dolly from a cell taken from the mammary gland of a sheep.

1998

The first completed genome of an animal, a roundworm, is achieved by a British and American team. The genetic map shows the 97,000,000 genetic letters in correct sequence, taken from the worm’s 19,900 genes.

1999

Danish researchers find what they believe is evidence of the oldest life on Earth—fossilized plankton from 3,700,000,000 years ago.

2000

Gene therapy succeeds unequivocally for the first time as doctors in France add working genes to three infants who could not develop their own complete immune systems.

1992 Bill Clinton becomes president of the United States

1990 xvi

1993

1995 The Million Man March takes place

1996

1999 The first nonstop around-the-world balloon trip is made

1999


Words to Know A Abiotic: The nonliving part of the environment. Absorption: The process by which dissolved substances pass through a cell’s membrane. Acid: A solution that produces a burning sensation on the skin and has a sour taste. Acid rain: Rain that has been made strongly acidic by pollutants in the atmosphere. Acquired characteristics: Traits that are developed by an organism during its lifetime; they cannot be inherited by offspring. Active transport: In cells, the transfer of a substance across a membrane from a region of low concentration to an area of high concentration; requires the use of energy. Adaptation: Any change that makes a species or an individual better suited to its environment or way of life. Adrenalin: A hormone released by the body as a result of fear, anger, or intense emotion that prepares the body for action. Aerobic respiration: A process that requires oxygen in which food is broken down to release energy. AIDS: A disease caused by a virus that disables the immune system. Algae: A group of plantlike organisms that make their own food and live wherever there is water, light, and a supply of minerals. Allele: An alternate version of the same gene. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Words to Know

Alternation of generations: The life cycle of a plant in which asexual stages alternate with sexual stages. Amino acids: The building blocks of proteins. Amoeba: A single-celled organism that has no fixed shape. Amphibians: A group of vertebrates that spend part of their life on land and part in water; includes frogs, toads, and salamanders. Anaerobic respiration: A stage in the breaking down of food to release energy that takes place in the absence of oxygen. Anaphase: The stage during mitosis when chromatids separate and move to the cell poles. Angiosperms: Flowering plants that produce seeds inside of their fruit. Anther: The male part of a flower that contains pollen; a saclike container at the tip of the stamen. Antibiotics: A naturally occurring chemical that kills or inhibits the growth of bacteria. Antibody: A protein made by the body that locks on, or marks, a particular type of antigen so that it can be destroyed by other cells. Antigen: Any foreign substance in the body that stimulates the immune system to action. Arachnid: An invertebrate that has four pairs of jointed walking legs. Arthropod: An invertebrate that has jointed legs and a segmented body. Atom: The smallest particle of an element. Autotroph: An organism, such as a green plant, that can make its own food from inorganic materials. Auxins: A group of plant hormones that control the plant’s growth and development. Axon: A long, threadlike part of a neuron that conducts nerve impulses away from the cell.

B Bacteria: A group of one-celled organisms so small they can only be seen with a microscope. Binomial nomenclature: The system in which organisms are identified by a two-part Latin name; the first name is capitalized and identifies the genus; the second name identifies the species of that genus. xviii


Biological community: A collection of all the different living things found in the same geographic area.

Words to Know

Biological diversity: A broad term that includes all forms of life and the ecological systems in which they live. Biomass: The total amount of living matter in a given area. Biome: A large geographical area characterized by distinct climate and soil and particular kinds of plants and animals. Biosphere: All parts of Earth, extending both below and above its surface, in which organisms can survive. Biotechnology: The alteration of cells or biological molecules for a specific purpose. Bipedalism: Walking on two feet; a human characteristic. Binary fission: A type of asexual reproduction that occurs by splitting into two more or less equal parts; bacteria usually reproduce by splitting in two. Blood: A complex liquid that circulates throughout an animal’s body and keeps the body’s cells alive. Blood type: A certain class or group of blood that has particular properties. Brain: The control center of an organism’s nervous system. Breeding: The crossing of plants and animals to change the characteristics of an existing variety or to produce a new one. Bud: A swelling or undeveloped shoot on a plant stem that is protected by scales.

C Calorie: A unit of measure of the energy that can be obtained from a food; one calorie will raise the temperature of one kilogram of water by one degree Celsius. Camouflage: Color or shape of an animal that allows it to blend in with its surroundings. Carbohydrates: A group of naturally occurring compounds that are essential sources of energy for all living things. Carbon cycle: The process in which carbon atoms are recycled over and over again on Earth. Carbon dioxide: A major atmospheric gas. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

xix

Words to Know

Carbon monoxide: An odorless, tasteless, colorless, and poisonous gas. Carnivores: A certain family of mammals that have specially shaped teeth and live by hunting. Carpel: The female organ of a flower that contains its stigma, style, and ovary. Cartilage: Smooth, flexible connective tissue found in the ear, the nose, and the joints. Catalyst: A substance that increases the speed at which a chemical reaction occurs. Cell: The building block of all living things Cell theory: States that the cell is the basic building block of all lifeforms and that all living things, whether plants or animals, consist of one or more cells. Cellulose: A carbohydrate that plants use to form the walls of their cells. Central nervous system: The brain and spinal cord of a vertebrate; it interprets messages and makes decisions involving action. Centriole: A tiny structure found near the nucleus of most animal cells that plays an important role during cell division. Cerebellum: The part of the brain that coordinates muscular coordination and balance; the second largest part of the human brain. Cerebrum: The part of the brain that controls thinking, speech, memory, and voluntary actions; the largest part of the human brain. Cetacean: A mammal that lives entirely in water and breathes air through lungs. Chlorophyll: The green pigment or coloring matter in plant cells; it works by transferring the Sun’s energy in photosynthesis. Chloroplast: The energy-converting structures found in the cells of plants. Chromatin: Ropelike fibers containing deoxyribonucleic acid (DNA) and proteins that are found in the cell nucleus and which contract into a chromosome just before cell division. Chromosome: A coiled structure in the nucleus of a cell that carries the cell’s deoxyribonucleic acid (DNA). Cilia: Short, hairlike projections that can beat or wave back and forth; singular, cilium. Classification: A method of organizing plants and animals into categories based on their appearance and the natural relationships between them.

xx


Cleavage: Early cell division in an embryo; each cleavage approximately doubles the number of cells.

Words to Know

Cloning: A group of genetically identical cells descended from a single common ancestor. Cnidarian: A simple invertebrate that lives in the water and has a digestive cavity with only one opening. Cochlea: A coiled tube filled with fluid in the inner ear whose nerve endings transmit sound vibrations. Community: All of the populations of different species living in a specific environment. Conditioned reflex: A type of learned behavior in which the natural stimulus for a reflex act is substituted with a new stimulus. Consumers: Animals that eat plants who are then eaten by other animals. Cornea: The transparent front of the eyeball that is curved and partly focuses the light entering the eye. Cranium: The dome-shaped, bony part of the skull that protects the brain; it consists of eight plates linked together by joints. Crustacean: An invertebrate with several pairs of jointed legs and two pairs of antennae. Cytoplasm: The contents of a cell, excluding its nucleus.

D Daughter cells: The two new, identical cells that form after mitosis when a cell divides. Decomposer: An organism, like bacteria and fungi, that feed upon dead organic matter and return inorganic materials back to the environment to be used again. Dendrite: Any branching extension of a neuron that receives incoming signals. Deoxyribonucleic acid (DNA): The genetic material that carries the code for all living things. Differentiation: The specialized changes that occur in a cell as an embryo starts to develop. Diffusion: The movement or spreading out of a substance from an area of high concentration to the area of lowest concentration. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Words to Know

Dominant trait: An inherited trait that masks or hides a recessive trait. Double helix: The “spiral staircase” shape or structure of the deoxyribonucleic acid (DNA) molecule.

E Ecosystem: A living community and its nonliving environment. Ectoderm: In a developing embryo, the outermost layer of cells that eventually become part of the nerves and skin. Ectotherm: A cold-blooded animal, like a fish or reptile, whose temperature changes with its surroundings. Element: A pure substance that contains only one type of atom. Endangered species: Any species of plant or animal that is threatened with extinction. Endoderm: In a developing embryo, the innermost layer of cells that eventually become the organs and linings of the digestive, respiratory, and urinary systems. Endoplasmic reticulum: A network of membranes or tubes in a cell through which materials move. Endotherm: A warm-blooded animal, like a mammal or bird, whose metabolism keeps its body at a constant temperature. Energy: The ability to do work. Enzyme: A protein that acts as a catalyst and speeds up chemical reactions in living things. Epidermis: The outer layer of an animal’s skin; also the outer layer of cells on a leaf. Eukaryote: An organism whose cells contain a well-defined nucleus that is bound by a membrane. Eutrophication: A natural process that occurs in an aging lake or pond as it gradually builds up its concentration of plant nutrients. Evolution: A scientific theory stating that species undergo genetic change over time and that all living things originated from simple organisms. Exoskeleton: A tough exterior or outside skeleton that surrounds an animal’s body. Extinction: The dying out and permanent disappearance of a species. xxii


F

Words to Know

Fermentation: A chemical process that breaks down carbohydrates and other organic materials and produces energy without using oxygen. Fertilization: The union of male and female sex cells. Fetus: A developing embryo in the human uterus that is at least two months old. Flagella: Hairlike projections possessed by some cells that whip from side to side and help the cell move about; singular, flagellum. Food chain: A sequence of relationships in which the flow of energy passes. Food web: A network of relationships in which the flow of energy branches out in many directions. Fossil: The preserved remains of a once-living organism. Fruit: The mature or ripened ovary that contains a flower’s seeds. Fungi: A group of many-celled organisms that live by absorbing food and are neither plant nor animal.

G Gaia hypothesis: The idea that Earth is a living organism and can regulate its own environment. Gamete: Sex cells used in reproduction; the ovum or egg cell is the female gamete and the sperm cell is the male gamete. Gastric juice: The digestive juice produced by the stomach; it contains weak hydrochloric acid and pepsin (which breaks down proteins). Gene: The basic unit of heredity. Genetic code: The information that tells a cell how to interpret the chemical information stored inside deoxyribonucleic acid (DNA). Genetic disorder: Conditions that have some origin in a person’s genetic makeup. Genetic engineering: The deliberate alteration of a living thing’s genetic material to change its characteristics. Genetic theory: The idea that genes are the basic units in which characteristics are passed from one generation to the next. Genetic therapy: The process of manipulating genetic material either to treat a disease or to change a physical characteristic. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Words to Know

Genotype: The genetic makeup of a cell or an individual organism; the sum total of all its genes. Geolotic record: The history of Earth as recorded in the rocks that make up its crust. Germination: The earliest stages of growth when a seed begins to transform itself into a living plant that has roots, stems, and leaves. Gland: A group of cells that produce and secrete enzymes, hormones, and other chemicals in the body. Golgi body: A collection of membranes inside a cell that packages and transports substances made by the cell. Greenhouse effect: The name given to the trapping of heat in the lower atmosphere and the warming of Earth’s surface that results. Gymnosperm: Plants with seeds that are not protected by any type of covering.

H Habitat: The distinct, local environment where a particular species lives. Heart: A muscular pump that transports blood throughout the body. Hemoglobin: A complex protein molecule in the red blood cells of vertebrates that carries oxygen molecules in the bloodstream. Herbivore: Animals that eat only plants. Herpetology: The scientific study of amphibians and reptiles. Heterotroph: An organism, like an animal, that cannot make its own food and must obtain its nutrients be eating plants or other animals. Hibernation: A special type of deep sleep that enables an animal to survive the extreme winter cold. Homeostasis: The maintenance of stable internal conditions in a living thing. Hominid: A family of primates that includes today’s humans and their extinct direct ancestors. Hormones: Chemical messengers found in both animals and plants. Host: The organism on or in which a parasite lives. Hybrid: The offspring of two different species of plant or animal. Hypothesis: A possible answer to a scientific problem; it must be tested and proved by observation and experiment. xxiv


I

Words to Know

Ichthyology: The branch of zoology that deals with fish. Immunization: A method of helping the body’s natural immune system be able to resist a particular disease. Inbreeding: The mating of organisms that are closely related or which share a common ancestry. Instincts: A specific inborn behavior pattern that is inherited by all animal species. Interphase: The stage during mitosis when cell division is complete. Invertebrates: Any animal that lacks a backbone, such as paramecia, insects, and sea urchins. Iris: The colored ring surrounding the pupil of the vertebrate eye; its muscles control the size of the pupil (and therefore the amount of light that enters).

K Karyotype: A diagnostic tool used by physicians to examine the shape, number, and structure of a person’s chromosomes when there is a reason to suspect that a chromosomal abnormality may exist.

L Lactic acid: An organic compound found in the blood and muscles of animals during extreme exercise. Larva: The name of the stage between hatching and adulthood in the life cycle of some invertebrates. Lipids: A group of organic compounds that include fats, oils, and waxes. Lysosome: Small, round bodies containing digestive enzymes that break down large food molecules into smaller ones.

M Malnutrition: The physical state of overall poor health that can result from a lack of enough food to eat or from eating the wrong foods. Mammals: A warm-blooded vertebrate with some hair that feeds milk to its young. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Medulla: The part of the brain just above the spinal cord that controls certain involuntary functions like breathing, heartbeat rate, sneezing, and vomiting; the smallest part of the brain. Meiosis: A specialized form of cell division that takes place only in the reproductive cells. Membrane: A thin barrier that separates a cell from its surroundings. Mendelian laws of inheritance: A theory that states that characteristics are not inherited in a random way but instead follow predictable, mathematical patterns. Mesoderm: In a developing embryo, the middle layer of cells that eventually become bone, muscle, blood, and reproductive organs. Metabolism: All of the chemical processes that take place in an organism when it obtains and uses energy. Metamorphosis: The extreme changes that some organisms go through when they pass from an egg to an adult. Metaphase: The stage during mitosis when the chromosomes line up across the center of the spindle. Microorganism: Any form of life too small to be seen without a microscope, such as bacteria, protozoans, and many algae; also called microbe. Migration: The seasonal movement of an animal to a place that offers more favorable living conditions. Mineral: An inorganic compound that living things need in small amounts, like potassium, sodium, and calcium. Mitochondria: Specialized structures inside a cell that break down food and release energy. Mitosis: The division of a cell nucleus to produce two identical cells. Molars: Chewing teeth that grind or crush food; the back teeth in the jaws of mammals. Molecule: A chemical unit consisting of two or more linked atoms. Mollusk: A soft-bodied invertebrate that is often protected by a hard shell. Molting: The shedding and discarding of the exoskeleton; some insects molt during metamorphosis, and snakes shed their outer skin in order to grow larger. Monerans: A group of one-celled organisms that do not have a nucleus. Mutation: A change in a gene that results in a new inherited trait.

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N

Words to Know

Natural selection: The process of survival and reproduction of organisms that are best suited to their environment. Neuron: An individual nerve cell; the basic unit of the nervous system. Niche: The particular job or function that a living thing plays in the particular place it lives. Nitrogen cycle: The stages in which the important gas nitrogen is converted and circulated from the nonliving world to the living world and back again. Nucleic acid: A group of organic compounds that carry genetic information. Nutrients: Substances a living thing needs to consume that are used for growth and energy; for humans they include fats, sugars, starches, proteins, minerals, and vitamins. Nutrition: The process by which an organism obtains and uses raw materials from its environment in order to stay alive.

O Omnivore: An animal that eats both plants and other animals. Organ: A structural part of a plant or animal that carries out a certain function and is made up of two or more types of tissue. Organelle: A tiny structure inside a cell that performs a particular function. Organic compound: Substances that contain carbon. Organism: Any complete, individual living thing. Ornithology: The branch of zoology that deals with birds. Osmosis: The movement of water from one solution to another through a membrane or barrier that separates the solutions. Oviparous: Term describing an animal that lays or spawns eggs which then develop and hatch outside of the mother’s body. Ovoviparous: Term describing an animal whose young develop inside the mother’s body, but who receive nourishment from a yolk and not from the mother. Oxidation: An energy-releasing chemical reaction that occurs when a substance is combined with oxygen. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Ozone: A form of oxygen found naturally in the stratosphere or upper atmosphere that shields Earth from the Sun’s harmful ultraviolet radiation.

P Paleontology: The scientific study of the animals, plants, and other organisms that lived in prehistoric times. Parasite: An organism that lives in or on another organism and benefits from the relationship. Ph: A number used to measure the degree of acidity of a solution. Phenotype: The outward appearance of an organism; the visible expression of its genotype. Pheromones: Chemicals released by an animal that have some sort of effect on another animal. Photosynthesis: The process by which plants use light energy to make food from simple chemicals. Physiology: The study of how an organism and its body parts work or function normally. Pistil: The female part of a flower made up of organs called carpels; located in the center of the flower, parts of it become fruit after fertilization. Plankton: Tiny, free-floating organisms in a body of water. Pollen: Dustlike grains produced by a flower’s anthers that contain the male sex cells. Pollution: The contamination of the natural environment by harmful substances that are produced by human activity. Population: All the members of the same species that live together in a particular place. Predator: An organism that lives by catching, killing, and eating another organism. Primate: A type of mammal with flexible fingers and toes, forward-pointing eyes, and a well-developed brain. Producer: A living thing, like a green plant, that makes its own food and forms the beginning of a food chain, since it is eaten by other species. Prokaryote: An organism, like bacteria or blue-green algae, whose cells lack both a nucleus and any other membrane-bound organelles. xxviii


Prophase: The stage in mitosis when the chromosomes condense or, coil up, and the sister chromatids become visible.

Words to Know

Protein: The building blocks of all forms of life. Protozoa: A group of single-celled organisms that live by taking in food. Pseudopod: A temporary outgrowth or extension of the cytoplasm of an amoeba that allows it to slowly move.

R Radioactive dating: A method of determining the approximate age of an old object by measuring the amount of a known radioactive element it contains. Recessive trait: An inherited trait that may be present in an organism without showing itself. It is only expressed or seen when partnered by an identical recessive trait. Reptiles: A cold-blooded vertebrate (animal with a backbone) with dry, scaly skin and which lays sealed eggs. Respiration: A series of chemical reactions in which food is broken down to release energy. Retina: The lining at the back of the eyeball that contains nerve endings or rods sensitive to light. Rh factor: A certain blood type marker that each human blood type either has (Rh-positive) or does not have (Rh-negative). Rhizome: A creeping underground plant stem that comes up through the soil and grows new stems. Ribonucleic acid (RNA): An organic substance in living cells that plays an essential role in the construction of proteins and therefore in the transfer of genetic information. Rods: Nerve endings or receptor cells in the retina of the eye that are sensitive to dim light but cannot identify colors.

S Sap: A liquid inside a plant that is made up mainly of water and which transports dissolved substances throughout the plant. Sedimentation: The settling of solid particles at the bottom of a body of water that are eventually squashed together by pressure to form rock. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Words to Know

Smooth muscle: Muscle that appears smooth under a microscope; they are involuntary muscles since they cannot be controlled. Sponge: An invertebrate that lives underwater and survives by taking in water through a system of pores. Spontaneous generation: The incorrect theory that nonliving material can give rise to living organisms. Spore: Usually a single-celled structure with a tough coat that allows an organism, like bacteria or fungi, to reproduce asexually under the proper conditions. Stamen: The male organ of a flower consisting of a filament and an anther in which the pollen grains are produced. Stigma: The tip of a flower’s pistil upon which pollen collects during pollination and fertilization. Stimulus: Anything that causes a receptor or sensory nerve to react and carry a message. Stomata: The pores in leaves that allow gases to enter and leave; singular, stoma. Stress: A physical, psychological, or environmental disturbance of the well-being of an organism. Striated muscle: Muscle that appears striped under a microscope; also called skeletal muscles, they are under the voluntary control of the brain. Symbiosis: A relationship between two different species who benefit by living closely together. Synapse: The space or gap between two neurons across which a nerve impulse or a signal is transmitted.

T Taxonomy: The science of classifying living things. Telophase: The near-final phase of mitosis in which the cytoplasm of the dividing cell separates two sets of chromosomes. Territory: An area that an animal claims as its own and which it will defend against rivals. Tissue: The name for a group of similar cells that have a common structure and function and which work together. Toxins: Chemical substances that destroy life or impair the function of living tissue and organs. xxx


Transpiration: Loss of water by evaporation through the stomata of the leaves of a plant.

Words to Know

Tropism: The growth of a plant in a certain manner or direction as a response to a particular stimulus, such as when a plant grows toward the light source.

V Vacuole: A bubble-like space or cavity inside a cell that serves as a storage area. Variation: The natural differences that occur between the individuals in any group of plants or animals; if inherited, these differences are the raw materials for evolution. Vascular plants: Plants with specialized tissue that act as a pipeline for carrying the food and water they need. Vegetative reproduction: The asexual production of new plants from roots, underground runners, stems, or leaves. Vertebrates: Animals that have a backbone and a skull that surrounds a well-developed brain. Virus: A package of chemicals that infects living cells. Vitamins: Organic compounds found in food that all animals need in small amounts. Viviparous: Term describing an animal whose embryos develop inside the body of the female and who receive their nourishment from her.

Z Zygote: A fertilized egg cell; the product of fertilization formed by the union of an egg and sperm.


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Research and Activity Ideas Activity 1: Studying an Ecosystem Ecosystems are everywhere—your backyard, a nearby park, or even a single, rotting log. To study an ecosystem, you need only choose an individual natural community to observe and study and then begin to keep track of all of the interactions that occur among the living and nonliving parts of the ecosystem. Look carefully and study the entire ecosystem, deciding on what its natural boundaries are. Making a map or a drawing on graph paper of the complete site always helps. Next, you should classify the major biotic (living) and abiotic (nonliving) factors in the ecosystem and begin to observe the organisms that live there. Binoculars sometimes help to observe distant objects or to keep from interfering with the activity. A small magnifying glass is also useful for studying small creatures. You should also search for evidence of creatures that you do not see. A camera is also useful sometimes, especially when comparing the seasonal changes in an ecosystem. It is very important to keep a notebook of your observations, keeping track of any creatures you find and where you find them. You can learn more about your ecosystem by counting the different populations discovered there, as well as classifying them according to their ecosystem roles like producer, consumer, or decomposer. A diagram can then be made of the ecosystem’s food web. You can search for evidence of competition as well as other types of relationships such as predator-prey or parasitism. You can even keep a record of changes such as plant or animal growth, the birth of offspring, or weather fluctuations. Finally, you can try to predict what might happen if some part of U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Research and Activity Ideas

the ecosystem were disturbed or greatly changed. Ecosystems themselves are related to other ecosystems in many ways, and it is important to always realize that all the living and nonliving things on Earth are ultimately connected to one another.

Activity 2: Studying the Greenhouse Effect The greenhouse effect is the name given to the natural trapping of heat in the lower atmosphere and the warming of Earth’s surface that results. This global warming is a natural process that keeps our planet warm and hospitable to life. However, when this normal process is exaggerated or enhanced because of certain human activities, too much heat can be trapped and the increased warming could result in harmful climate changes. The greenhouse effect can be produced by trying the following experiment. Using two trays filled with moist soil and some easy-to-grow seeds like beans, place a flat thermometer on the soil surface of each tray. After inserting tall wooden skewers in the four corners of one tray, cover it completely with plastic wrap and secure it with a large rubber band. Leave the other tray uncovered and place both trays outside where they are sheltered from the rain but exposed to the Sun. Record the temperature of each tray at the same time each day and note all the differences between the plants. The plastic-wrapped tray should be warmer and its seedling plants should grow larger. This is evidence of the beneficial aspects of the greenhouse effect. However, if the plastic wrap is left over the seedlings for too long they will overheat, wither, and die.

Activity 3: Studying Photosynthesis If you have ever picked up a piece of wood that has been sitting on the grass for some time and noticed that the patch underneath has lost its greenness and appears yellow or whitish, you have witnessed the opposite of photosynthesis. Since a green plant cannot exist without sunlight, when it is left totally in the dark, the chlorophyll departs from its leaves and photosynthesis no longer takes place. The key role of sunlight can be easily demonstrated by germinating pea seeds and placing them in pots of soil. After placing some pots in a place where they will receive plenty of direct sunlight, place the other pots in a very dark area. After a week to ten days, compare the seedlings in the sunlight to those left in the dark. The root structure of both is especially interesting. Another way of demonstrating the importance of sunlight to a plant is to pick a shrub, tree, or houseplant that has large individual leaves. Using aluminum foil or pieces of cardboard cut into distinct geometrical xxxiv


shapes that are small enough not to cover the entire leaf but large enough to cover at least half, paperclip each shape to a different leaf. After about a week, remove the shapes from the leaves and compare what you see now to those leaves that were not covered. The importance of sunlight will be dramatically noticeable.


Finally, as a way of demonstrating the exchange of gases (carbon dioxide and oxygen) that occurs during photosynthesis, place a large glass over some potted pea seedlings and place them in sunlight. In time, you will notice that some liquid has condensed on the inside of the glass. This condensation is water vapor that has been given off by the plant when it exchanges oxygen for the carbon dioxide it needs.

Activity 4: Studying Osmosis In the life sciences, osmosis occurs at the cellular level. For example, in mammals it plays a key role in the kidneys, which filter urine from the blood. Plants also get the water they need through osmosis that occurs in their root hairs. Everyday examples of osmosis can be seen when we sprinkle sugar on a grapefruit cut in half. We notice that the surface becomes moist very quickly and a sweet syrup eventually forms on its top surface. Once the crystallized sugar is dissolved by the grapefruit juices and becomes a liquid, the water molecules will automatically move from where they are greater in number to where they are fewer, so the greater liquid in the grapefruit forms a syrup with the dissolved sugar. Placing a limp stalk of celery in water will restore much of its crispness and gives us another example of osmosis. Osmosis occurs in plants and animals at the cellular level because their cell membranes are semipermeable (meaning that they will allow only molecules of a certain size or smaller to pass through them). Osmosis can be studied directly by observing how liquid moves through the membrane of an egg. This requires that you get at an egg’s membrane by submerging a raw egg (still in its shell) completely inside a wide-mouth jar of vinegar. Record the egg’s weight and size (length and diameter) before doing this. The acetic acid in the vinegar will eventually dissolve the shell because the shell is made of calcium carbonate or limestone which reacts with acid to produce carbon dioxide gas. You will observe this gas forming as bubbles on the surface. After about 72 hours, the shell should be dissolved but the egg will remain intact because of its transparent membrane. After carefully removing the egg from the jar of vinegar, weigh and measure the egg again. You will notice that its proportions have increased. The egg has gotten larger because the water in the vinegar moved through the egg’s membrane into the egg itself (because of the higher concentraU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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tion of water in the vinegar than in the egg). The contents of the egg did not pass out of the membrane since the contents is too large. The opposite of this activity can be performed using thick corn syrup instead of water. If the egg has its shell removed in the same manner as above but is then immersed for about 72 hours in a jar of syrup, you will find that the egg will have shrunken noticeably. This is because the water concentration of the syrup outside the egg is much less than that inside the egg, so the membrane allows water to move from the egg to the syrup.

Activity 5: Studying Inherited Traits An inherited trait is a feature or characteristic of an organism that has been passed on to it in its genes. This transmission of the parents’ traits to their offspring always follows certain principles or laws. The study of how these inherited traits are passed on is called genetics. Genetics influences everything about us, including the way we look, act, and feel, and some of our inherited traits are very noticeable. Besides these very obvious traits like hair and skin color, there are certain other traits that are less noticeable but very interesting. One of these is foot size. Another is free or attached earlobes. Still another is called “finger hair.” All of these are traits that are passed from parents to their offspring. You can collect data on any particular inherited characteristic and therefore learn more about how genetics works. You will need to collect data about each trait and develop a chart. Any of the above inherited traits can be analyzed. For example, there are generally two types of earlobes. They may be free, and therefore hang down below where the earlobe bottom joins the head, or they may be attached and have no curved bottom that appears to hang down freely. Foot length is simply the size of your own foot and is measured from the tip of the big toe to the back of the heel. The finger hair trait always appears in one of two forms. It is either there or it isn’t. People who have the finger hair trait have some hair on the middle section of one or more fingers (which is the finger section between the two bendable joints of your finger). In order to study one of these interesting traits like finger hair or type of earlobes, you should construct a table or chart that records data on the trait for as many of your family members as you wish. Although it is best to include a large sampling, such as starting with both sets of grandparents and working through any aunts, uncles, and cousins you can contact, even a small sample with only a few members can be helpful. Once you have determined the type of trait each family member has, you should draw your family’s “pedigree” for that trait. This is simply a diagram of connected individuals that looks like any other genealogical diagram xxxvi


(which starts at the top with two parents and draws a line from them down to their offspring, and so on). You should use some sort of easily identifiable code or color to signify which individual has or does not have a certain trait. The standard coding technique for tracing the occurrence of a trait in a family is to represent males by squares and females by circles. Usually, a solid circle or square means that a person has the trait, while an empty square or circle shows they do not. In more elaborate pedigrees, a half-colored circle or square means that the person is a carrier but does not show the trait. Once you have done your pedigree, you may do the same for a friend’s family and compare his or her family’s distribution of the same trait. By comparing the two families’ pedigrees for the same trait, you may be able to find certain general patterns of inheritance and to answer certain basic questions. For example, in studying the finger hair trait, you may be able to answer the question whether or not both parents must have finger hair for their offspring to also have it. You might also discover whether both parents having finger hair means that every offspring must show the same trait.



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A Abiotic/Biotic Environment The environment or surroundings in which every organism, or living thing, exists can be divided into two categories: its abiotic and its biotic environment. Abiotic refers to the nonliving part of the environment, while biotic refers to its living part. Typical examples of abiotic factors that make up an organism’s environment are sun, wind, rain, soil, and water. Examples of biotic factors are any of the organisms, such as bacteria, plants, and animals that live in the environment. Abiotic forces or factors affect and influence life in an environment. Dramatic examples of abiotic factors are acid rain or a severe storm, while more ordinary examples are water temperature or the amount of oxygen dissolved in a stream. Plants are most affected by abiotic factors including quantity of minerals in the soil, the amount of sunlight received by the plant, and the effects of wind, water, and overall temperature of climate. Animals are less affected by abiotic factors, although temperature and the availability of water can impact an animal’s ability to survive. While an abiotic environment consists of nonliving things, biotic factors include all of the living things, plant and animal, that might be found in any given environment. While green plants get much of the energy they need from the abiotic factors in their environment (sunlight, water, minerals), most other living things (like animals) depend more on biotic elements (since these animals must eat plants and/or other living things to survive). U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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The organisms that make up the biotic part of an environment can interact in several different ways. Organisms with a neutral interaction have little or no contact with or impact on each other. Organisms may compete for the same resource. They can have a predatory relationship (in which one species benefits by killing another); they can have a parasitic relationship (in which one benefits by living off another); or they can have a mutually beneficial relationship that is favorable to both. Similarly, abiotic factors can influence and shape one another, just as the wind shapes a dune or sunlight heats a pond. Any environment that contains and supports living organisms is necessarily complex, with living and nonliving elements influencing and modifying each other. Virtually no major environment, or ecosystem, is completely biotic or abiotic, and nearly every living and nonliving element in an environment is related in some way to another. For example, animals and plants (biotic elements) can be affected by climate or weather (abiotic elements); and major changes in the abiotic environment (such as increased rainfall and flooding or a severe and prolonged drought) can alter the conditions and threaten the existence of certain organisms. A drought (abiotic factor) can eliminate many types of green plants, resulting in a decrease in animal population (due to scarcity of food, cover, and shelter). Conversely, too many animals in one area (biotic factor) can destroy important plant life; causing depletion of plant life, erosion, and the possibility of a changed local environment that will not support any life. In a properly balanced environment, abiotic and biotic factors work together and make up a healthy, functional system. In a typical environment like a small pond, algae, plants, and animals make up the biotic forces, while the pond’s water, minerals, and soil (as well as the amount of light it receives) make up some of the more obvious abiotic factors. Altogether, these interacting parts of an environment that are living and nonliving make up what is called an ecological system or ecosystem. [See also Biological Community; Ecosystem]

Acid and Base Acid and base are terms used by chemists to describe the very different actions and opposing properties of certain chemicals when they are dissolved in water. A solution of acid produces a burning sensation on the skin and has a sour taste. A base solution feels slippery and tastes bitter. Both occur naturally and some acids are essential to life. 2


Acids and bases are both biologically important compounds. As compounds, however, they are complete opposites because of what happens when they are put into a solution, or liquid mixture. At the subatomic level of protons and electrons, any substance that releases a hydrogen ion (a positively charged hydrogen atom) when in solution is called an acid. Conversely, any substance that combines with or gains a hydrogen ion in solution is called a base. While this may sound complicated, many acids and bases are actually quite familiar to us. The vinegar used in a salad dressing or in ketchup and pickles gets its flavor from acetic acid. We have only to place an aspirin on our tongues for a moment to recognize its distinctly acidic, sour taste. We also know immediately that lemons and other citrus fruit contain some form of natural acid. On the other hand, when we wash with soap, we can feel the slipperiness of a base substance, and when we take an antacid tablet for an upset stomach we can experience its neutralizing effect on acids.

Acid and Base

People have been using both acids and bases ever since they first started making food and drink. When wine turns sour it changes to vinegar, a diluted or weak form of acetic acid. Spilling some vinegar, or even lemon juice, on a cut lets us feel immediately that it is a mildly burning acid. A common base material that is found in nature is limestone, which people eventually learned to roast and obtain lime. Today we sprinkle this white powder on some soils that are too “acidic” for certain plants to thrive. Acids come in a variety of strengths, from the fiercely strong hydrochloric acid found in the human stomach, to the mild strength of tomatoes, and the very mild strength found in our own saliva. Strong acids however, are poisonous and can cause severe burns. Acid and base strength are measured on a pH scale ranging from 1 (strongest acid) to 14 (strongest base). Since the strength of a particular substance depends on its concentration of hydrogen ions and whether it releases or attracts them in solution, the pH scale was devised to measure this concentration of hydrogen ions. On a scale of 1 to 14, a solution with a pH of 7 is considered neutral. Very potent acids like sulfuric acid and hydrochloric acid have a pH of 1; lemon juice has a pH of 2; vinegar has a pH of 3; tomatoes are 4; black coffee is 5; and urine is 6. Pure water (not rainwater which is slightly acidic) has a neutral pH of 7. Pure water is between the highest acid (1) and the highest base (14), and is actually neither. Continuing on the pH scale to the base side, seawater has a pH of 8; baking soda has a pH of 9; milk of magnesia is 10; household ammonia is 11; lime is 12; hair remover is 13; and lye is 14. On this scale, a change U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Acid Rain

of only 1 point means a change of ten times the concentration. Thus, lemon juice is ten times more acidic than vinegar, and stomach acid is ten times more acidic than lemon juice. An easy way of distinguishing acids from bases is to use litmus paper. It was discovered that certain organic extracts, such as litmus—which is obtained from lichen, a plantfungus organism—turns red when dipped in acid and blue when dipped in a base. Acids play a major biological role at the most fundamental level, since the organic acids called amino acids are necessary for life. Amino acids are known to be the building blocks of proteins. The pH level is also important to life, since every cell is sensitive to it and will not tolerate too great a change from its proper pH level. Because of this, most living systems have several mechanisms to make sure that their internal pH remains fairly constant. An organism’s habitat or external environment must also have a range of pH that is suited to it or it will suffer or even die. Acid rain, which results from air pollution, has damaged certain vulnerable forests. It has also made some lakes too acidic (as much as a pH of 5) for trout and other fish. Finally, both acids and bases have been put to a number of practical uses in industry and in the production of consumer goods. We have acid in our batteries and bases in our soaps.

Acid Rain Acid rain is rain that has been made strongly acidic by pollutants in the atmosphere. It is caused by the burning of fossil fuels and results in declining fish populations and damaged and dead trees. It also damages buildings and statues and aggravates respiratory conditions. Precipitation or rainwater has always been slightly acidic since water naturally dissolves atmospheric carbon dioxide, but the precipitation described as acid rain often shows an alarmingly high concentration of dangerous acids like sulfuric acid and nitric acid. These powerful acids are in the atmosphere because of human activities, despite the fact that no one deliberately or even accidentally put them there. Rather, acid rain is the indirect result of human technological and industrial progress. Automobiles, factories, and power plants usually burn fossil fuels such as oil, gas, natural gas, or coal. Ever since the Industrial Revolution began in the eighteenth century, scientists have noticed a connection between air pollution, acid rain, and downwind damage to animals and plants. The term acid rain was first used in England around 1870. 4


HOW ACID RAIN IS FORMED

Acid Rain

Acid rain is formed in Earth’s atmosphere by a very simple and natural chemical process. First the atmosphere receives a steady dose of gases, such as sulfur and nitrogen oxides, that result from burning huge quantities of fossil fuels. Gas-burning cars and coal-burning electric power plants are two examples of items that give off sulfur dioxide and nitrogen oxides. Sulfur dioxide is an especially smelly toxic gas given off when coal is burned. These gases rise into the sky where they usually react with water, oxygen, and sunlight. It is mainly the moisture in the clouds, after reacting with these gases, that creates nitric acid and sulfuric acid. Sulfuric acid is an especially strong acid and is responsible for well over half of the extra acidity found in acid rain. Nitric acid makes up most of the rest of acid rain, and it too is a very caustic acid.

HOW ACIDIC IS ACID RAIN? Normal rain has a slightly acidic pH of 5.6. This is on a typical pH acid/base scale of 0 to 14 where 7 indicates neutrality. Numbers less than 7 indicate increasing acidity and numbers greater than 7 represent increasing alkalinity. Acid rain has been found to have a pH ranging between 3.4 and 4.5, which is very acidic. This acid-bearing moisture reaches Earth’s surface when it rains, snows, or when fog covers the ground. It is known to be particularly harmful to organisms that live in water. Acid rain can also fall to Earth as particles or gases in a process called dry deposition.

EFFECTS OF ACID RAIN Some regions are more sensitive to acid rain than others, and an area that has highly alkaline soil tends to neutralize much of the acidity, making it less harmful. However, areas that have thin soil on top of granite do not have the same advantages. What happens to the aquatic organisms in these areas is that the ponds, lakes, and streams they live in become acidic to the point where water conditions actually start to kill the inhabitants. The young and small fish die first and are followed by the amphibians (frogs and salamanders). This alters the ecological balance and usually results in a boom in the mosquito population since their natural predators are gone. If the situation continues, the lake will eventually die from a lack of oxygen after becoming full of dead organisms. For plants, too much acid in the soil eventually reduces soil fertility. More important however, is the effect that acid soil has on a plant’s leaves. Since the acid destroys a leaf’s waxy outer coating, it takes away both its protection and its ability to photosynthesize (the process by which the U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Acid Rain

plant makes its own food using sunlight). It is easy to notice evergreen forests damaged by acid rain since the trees usually die from the top down. High altitude trees suffer the worst effects since they are exposed to the highest concentration of acid rain. Plants and trees not killed directly by acid rain are put under stress, making them susceptible to other diseases and enemies. Besides acidifying water bodies and damaging forests, acid rain is linked to corrosion of statues, monuments, and some buildings. Such wellknown monuments like the Lincoln Memorial and the Roman Colosseum were built out of limestone and marble and have shown signs of corroding. Both stone types are composed of calcium carbonate which reacts with the acids and slowly “melts” away the surface detail.

Spruce and fir trees in the Great Smoky Mountains killed by acid rain. (Reproduced by permission of JLM Visuals.)

Today, acid rain is recognized as an international problem since airborne pollutants do not stop at national boundaries. For example, much of the acid rain that harms the forests of Norway, Sweden, and Finland is blown there by prevailing winds from western and eastern Europe where the rain originated. Beginning with the passage of the 1970 Clean Air Act in the United States, national legislation and international agreements have been passed to reduce the amount of acid rain that is produced in developed countries. New technologies like “scrubbers,” whose purpose it is to control the release of sulfur, are now being used in factories and power plants. Automotive emissions also have been controlled somewhat by the use of catalytic converters. Corrective actions like these have improved the situation in certain parts of the world, yet not all nations are making their best efforts, especially when pollution reduction is so expensive. However, it is important that all countries attempt to control their pollution since the future of the world’s ecology depends on it. [See also Acid and base; Forests; Pollution]

6


Adaptation

Adaptation

Adaptation refers to any change that makes a species or an individual better suited to its environment or way of life. All living creatures must be able to adapt to changes in their environment if they are to survive and reproduce. The process of adaptation does not always result in an obvious physical change, but may affect an individual’s behavior or even its internal processes. Adaptation could be described as the theory of evolution in action. This theory was first offered in its best and most complete form in 1859 by the English naturalist, Charles Darwin (1809–1882). Darwin’s theory of evolution suggested that all living things are subject to a gradual process of change over a long period of time. Evolution is therefore the process that results in living things changing through successive generations. Darwin also described a mechanism called “natural selection,” which is the means through which these hereditary changes are passed on from one generation to another. Darwin’s theory of evolution by natural selection—which is highly important to understanding the life sciences—states that individual organisms possessing certain traits or characteristics that are most suited to a particular environment have a better chance of surviving and therefore of passing these traits on to their offspring. In other words, organisms in possession of favorable traits allows these traits to be “selected” by nature (natural selection) so that these organisms survive and produce young that have the same favorable traits. Darwin’s years of travel, study, and thought led him to make three important observations or conclusions. First, he stated that all living things vary, or that each individual varies slightly from the others of its species. This could be seen in any given species in which differences could be found among any group of the same organisms (such as a flower with an extra petal or a deer with larger-than-average antlers). Second, Darwin suggested that individuals were able to pass certain characteristics on to their young, who inherited them. Modern genetics has proven this to be true. Third, Darwin noted that all life is involved in a struggle for survival. To Darwin, these three observations explained why nature allowed most organisms to produce far more offspring than could ever survive. Offspring that did survive and were able to reproduce, according to Darwin, were usually the ones who possessed certain traits better suited to their environment. These individuals had a better chance of surviving and of producing more individuals like themselves. Over many generations, U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Aerobic/Anaerobic

it was simply nature’s “selection” of the individuals with the fittest (or best adapted) characteristics that explained Darwin’s theory of evolution. This accounts for the well-known phrase, “survival of the fittest.” “Survival of the fittest” could also be described as “survival of the best adapted.” Among any group of individuals or organisms of the same species, there will always be variations or differences (in color, shape, behavior, and even chemical makeup). An adaptation then, is considered to be any variation that makes an organism better suited to its environment. Camouflage is one way nature has of providing protection for an individual, and an organism whose color or shape allows it to blend into its environment is more likely to survive and reproduce than one whose coloration makes it easier to be noticed. Woodpeckers are highly specialized birds and are a good example of the process of adaptation. Their main job is to find and eat insects that live in and beneath the bark of trees. Consequently, those woodpeckers with the most powerful and chisel-like beaks, strong neck muscles for hammering, sturdy skulls, grasping feet, stiff-supporting tail feathers, and long tongues proved best able to survive and pass on these traits. Darwin first described his theory of evolution by natural selection in 1859 in his classic book, On the Origin of Species by Means of Natural Selection. The idea of adaptation is key to Darwin’s theory. Darwin distinguishes between genotypic adaptation and phenotypic adaptation. The type of adaptation discussed above—in which an individual possesses a “favorable” gene for a certain characteristic which it passes on to its offspring—is called genotypic or evolutionary adaptation. This type of adaptation is genetic, permanent, and is very different from phenotypic adaptations. In contrast to genotypic adaptations, phenotypic adaptations are traits that are developed during an individual’s lifetime. An example of phenotypic, or nongenetic, adaptation might be a certain type of behavior that is learned or developed by an individual. The macaque monkeys in Japan, for example, have learned to wash their food in water, and newborns soon copy this behavior. Although it is not known exactly why the first macaque washed its food, this behavior in not instinctive with them. Rather it is a case of learned behavior. [See also Evolution; Evolution, Evidence of; Evolutionary Theory]

Aerobic/Anaerobic Aerobic and anaerobic are terms used to describe the presence or absence of oxygen. (Anaerobic means “without air.”) All living things require energy, and when oxygen is used to metabolize (convert or break down) 8


HANS ADOLF KREBS

Aerobic/Anaerobic

German-British biochemist Hans Krebs (1900–1981) was the first to explain how cells release energy during respiration (the chemical process by which food is broken down to release energy). His discovery of a very complicated chain of reactions, which came to be called the Krebs cycle, explained how cells break down glucose (a common sugar) and obtain needed energy. For this discovery he received the 1953 Nobel Prize in Physiology and Medicine and was knighted by Queen Elizabeth in 1958. Hans Krebs was born in Hildesheim, Germany and received his medical degree from the University of Hamburg in 1925. His father was a doctor, and Krebs took up his father’s specialty as an ear, nose, and throat specialist. It was not long, however, before he realized that he preferred doing research to working with patients, and in 1926 he became an assistant to the noted biochemist, Otto Heinrich Warburg (1883–1970) in Berlin. Warburg studied respiration and would himself win a Nobel Prize in 1931 for his work on that subject. By 1932, Krebs was making a name for himself with his own work on amino acids (the building blocks of protein), but in 1933 the German dictator, Adolf Hitler (1889–1945), was appointed chancellor. This political change in Germany meant that all people of Jewish origin, including Krebs, would be persecuted and eventually sent to concentration camps to be worked to death. That year however, Krebs was able to leave Germany and move to England. There he had the good fortune to work with another Nobel Prize winner, the English biochemist, Frederick Gowland Hopkins (1861–1947), at Sheffield University. It was there that Krebs would discover the process for which he is best known, the citric acid cycle, which is also called the Krebs cycle. The Krebs cycle is an important step in the process used by cells when they convert food, such as carbohydrates and fats, into usable energy. During this energy-producing process called respiration, one molecule of glucose combines with six molecules of oxygen to produce six molecules of carbon dioxide, six molecules of water, and a considerable amount of energy. Krebs discovered that this does not happen all at once, but that a complicated chain of reactions occurs during which a little of the original energy is released each time. His work revealed that this series of reactions was actually a chain, or a cycle, of events. His explanation of this highly complex cycle proved to be a major breakthrough in biochemistry and in understanding how an animal’s metabolism really works. Metabolism is all of the chemical processes (all the building up and breaking down) that takes place in an organism to stay alive and grow. The Krebs cycle focuses specifically on the breaking-down aspects of an animal’s metabolism by which energy is released. It explains how, through a series of six chemical reactions that take place inside an animal cell in a recurring loop, food is combined with oxygen to produce the energy needed for life.


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Aerobic/Anaerobic

organic materials to release energy, it is described as being an aerobic process. When no oxygen is needed to metabolize materials, the process is referred to as anaerobic. The terms aerobic and anaerobic are used mostly to describe types of respiration and types of bacteria.

AEROBIC RESPIRATION All living cells need a constant supply of energy to power the chemical activities they conduct to support life. The living cells in both plants and animals use glucose, the most common form of sugar, as their energy source or fuel. However, glucose must be broken down during a process called respiration before it will release usable energy. Finally, energy is produced by the cell in one of two ways: aerobic respiration or anaerobic respiration. Aerobic cellular respiration is a process that necessarily involves the use of oxygen. During this process, glucose and oxygen are chemically combined, which is known as oxidation, in the cell’s mitochondria (part of the cell that produces energy) to yield energy and to release carbon dioxide and water as waste. During aerobic respiration, one molecule of glucose is combined with six molecules of oxygen to produce six molecules of carbon dioxide and six molecules of water. Aerobic respiration also releases a large amount of energy in the form of energy-carrying molecules called adenosine diphosphate (ADP) and adenosine triphosphate (ATP). The series of events or reactions that occur during aerobic respiration are known as the Krebs cycle. This cycle is named after the German biochemist Hans Krebs (1900–1981), who discovered that glucose is broken down in a chain of reactions. Aerobic respiration, therefore, results in the release of a large amount of energy, but only if oxygen is present. It is in this way in which animals and plants obtain energy.

ANAEROBIC RESPIRATION Anaerobic respiration is the opposite of aerobic, since it involves a type of respiration that does not involve oxygen. Also called glycolysis (which literally means the splitting of carbohydrates), this process takes place in the cell and is very slow compared the to oxygen-rich process of aerobic respiration. Anaerobic respiration does not result in the production of a great deal of energy since glucose is only partly broken down. Instead, most of the glucose forms new organic compounds such as acid, methane gas, and alcohol. The most common anaerobic reactions take place during the process known as alcoholic fermentation. During this process, microorganisms like bacteria, molds, or yeast change sugar into carbon dioxide and alcohol. In the best known examples of making bread, beer and wine, yeast is used to bring about this conversion with10


out the use of oxygen. Although the anaerobic process produces little immediate energy, its by-products like methane and alcohol contain a significant amount of potential energy and can be used as fuels.

Aging

AEROBIC AND ANAEROBIC BACTERIA Bacteria can also be described as being aerobic or anaerobic. These single-celled microorganisms are among the most abundant living things on Earth and live and feed in many different ways. Bacteria that need oxygen in order to grow are called aerobic bacteria. An example are the bacteria that cause the lung disease tuberculosis. The moist, warm lungs have a steady supply of oxygen and provide an ideal breeding ground for this potentially fatal bacterium. However, many bacteria can only grow in the absence of oxygen and are called anaerobic bacteria. The bacteria that live in soil and those that inhabit the intestinal tracts of mammals carry out anaerobic respiration.

ANAEROBIC RESPIRATION IN THE MUSCLES Another type of anaerobic respiration occurs in the stressed muscles of an animal when is uses oxygen faster than the blood can supply it. The exhausted muscle quickly switches from aerobic respiration to anaerobic respiration and begins to break down glucose without oxygen. This type of anaerobic respiration results in little energy and produces muscles that ache and eventually shut down. Anaerobic respiration in the muscles produces an acid by-product known as lactic acid. It is the buildup of lactic acid that makes an athlete’s muscles “burn” and then stop working, which is why some runners collapse and can barely use their muscles immediately after a race.

German-British biochemist Hans Krebs discovered the reactions of aerobic respiration, which was later named the Krebs cycle. (Courtesy of The Library Congress.)

[See also Bacteria; Respiration]

Aging Aging is the gradual loss of function in both cells and the overall organism. The natural process of aging, or senescence, results in bodily changes that make an organism less efficient and eventually contribute to its death. Aging is almost certainly affected by genes, and members of the same species have similar life expectancies. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Aging

After living for three weeks as a larva, a mayfly may spend only one day as an adult before it dies. A bird may live up to four years, a frog sometimes up to twenty, a human being can reach one hundred, and a Sierra redwood tree can live as many as four thousand years. Obviously, different species have radically different life spans. (Life span is the maximum time that an individual may live under ideal circumstances.) It is different from life expectancy, which is calculated from the average years lived by individuals of a certain generation. However, it appears that no matter how long or short the life span of an individual organism may be, it generally undergoes a process of getting older that is marked by gradual deterioration of its systems and abilities. In humans, this process becomes very obvious in what is called middle age. By forty or fifty, a person’s body begins to act and appear different. The skin becomes less elastic or smooth and permanent wrinkles appear. These people lose muscle tissue and bone hardness, and their vision and hearing gets less sharp. Even their taste buds start to deteriorate. Eventually all organisms die, and aging can be considered the process through which animals and plants go on their way through their individual life span. However, science has not yet been able to explain exactly why aging occurs. Gerontology, which is the study of all aspects of aging, has no single theory on how or why people age. One theory says that an individual’s life span is programmed by his or her genetic inheritance. Some call this the “time-bomb” theory, claiming that each of us has our own genetic clock or clocks that slow down and eventually cause certain cells to die out. The other major theory of aging is that of wearand-tear. This argument says that cells eventually break down under the constant assaults of heavy use and environmental insults like chemicals and radiation.

EFFECTS OF AGING Although scientists are unsure of the exact cause or causes, they know very well the effects of aging on the human body. These include wrinkly skin, muscle loss, bone thinness, a less efficient heart, weakened lungs, poorer vision and hearing, decrease in mental quickness, reduced kidney function, and diminished resistance to infection, among many others. These effects appear to be the result of our cells becoming less efficient in their jobs. As we age, our cells do not do as good a job in functions like removing wastes, destroying poisons, repairing genes, and making proteins. As the cells get weaker and weaker, they do their jobs less well, which means that the entire body becomes less and less efficient or healthy. Although old age can have its share of diseases, such as hard12


ening of the arteries, stroke, cancer, and the brain condition known as Alzheimer’s disease, it is important to realize that these diseases are not a natural result of the aging process.

Agriculture

Aging occurs in plants as well as animals, and is usually connected to plant growth cycles. Plants that have what is called determinate growth have a built-in time when they stop growing, after which they slowly breakdown and die. Plants that we call annuals and biennials have a programmed time during which they grow, reach a certain size or age, and then wither and die. Plants that continue for a longer period have indeterminate growth. Some perennials can live for years, despite the fact that their above-ground systems die every winter. The below-ground plant stays alive and recovers in the spring. Others, like the common juniper tree, can live for two thousand years. Although science has yet to pinpoint the exact reason that aging occurs in any organism, it is safe to say that genetics probably plays the largest role in determining the life span of an individual. The next largest role is probably that of our external environment. A toxic environment no doubt puts an enormous strain on all body systems, which inevitably deteriorate. While good living habits like a balanced diet and regular exercise can minimize some of the effects of the aging process, the reality of growing old and less efficient is, so far, an inevitable fact of life.

Agriculture Agriculture is the art and science of cultivating the soil, growing and harvesting crops, and raising livestock (animals) for human use. As the world’s oldest and most important industry, agriculture provides the basic substances necessary to sustain human life. Without agriculture, the development of civilization could have never occurred. Long before humans knew how to grow their own supply of food, people practiced hunting, fishing, and the simple gathering of edible plants that happened to be growing locally. This type of existence did not encourage people to stay in one place, since they were often forced to move somewhere else when their supply of game diminished or local plants like fruits, nuts, and roots had a poor growing season. As a result of this hunting and gathering, people were more or less nomads who were often forced to leave and search for food. A constant preoccupation with where the next meal was coming from left people little time to develop any other skills besides those related to finding food. Early humans eventually learned how to domesticate, or tame, dogs and used U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Agriculture

them to help herd the groups of sheep and goats they had captured (as a supply of fresh food). However, as long as humans had to hunt and gather most of their food, they existed on a day-to-day basis, meaning that they could not meaningfully change their lives in any way. The development of agriculture would eventually change nearly every aspect of human society. The word agriculture comes from two Latin words, ager meaning “field” and cultura meaning “cultivation.” Sometime around 11,000 B.C., certain tribes discovered that plants could be grown from seeds, and the first crops were probably raised from the seeds of grasses that could be used for food, like wild barley and wild wheat. This was the beginning of farming, and these first farmers usually settled where the soil was fertile and easy to till and where there was water close by. These conditions were usually found in river valleys. In many ways, agriculture is ultimately responsible for both group living (in villages or cities), and for the actual location of the cities themselves. Since farming seemed to arise in different parts of the world at about the same time, many scientists believe that a favorable change in Earth’s climate may have been responsible for the development of agriculture.

EFFECTS OF AGRICULTURE Whatever the reason, growing crops and raising livestock would unavoidably change the way human beings lived. First, it provided them with a steady supply of food that increased their chances of survival. Second, it allowed them to establish permanent settlements since they not only had a close and reliable source of food, but they had to constantly take care of and guard it. Also, since far fewer people were required to steadily search for and gather food, people were free to develop arts and skills like pottery, weaving, and leatherwork. Their technology also improved as they developed new agricultural tools like plows pulled by oxen. Altogether, a steady food supply based on farming ultimately led to the development of human civilization, including culture, art, laws, customs, religion, and government. It is not an overstatement to say that agriculture is the foundation upon which all of human society is based. A single example of how agriculture influenced science and technology is the annual flooding of the Nile River in Egypt. Every year the Nile would overflow its banks and lay down a rich new layer of soil for that year’s crops. This was beneficial but it also erased everyone’s boundaries. Since it was important to know when the flood would occur as well as whose land was whose, the Egyptians applied their knowledge of astron14


omy (the science that deals with the study of the Sun, Moon, stars and other celestial bodies) and invented an accurate calendar based on the phases of the moon. They also made advances in surveying (the measurement and description of a region, part, or feature on the Earth’s surface) and mathematics, discovering the practical principle behind the Pythagorean theorem (that in any right triangle, the square of the hypotenuse of the triangle is equal to the sum of the squares of the other two sides) long before the Greek mathematician, Pythagoras of Samos (c.580–c.500 B.C.), ever lived. The civilizations of ancient Egypt, Greece, and Rome were all based on and supported by a base of agriculture. The Romans were a civilization of wealthy city-dwellers whose specialized agriculture allowed them to sell and trade. They used many sound agricultural techniques, like resting the land (not planting for a season) and plowing under crops to enrich the soil. They also practiced selective breeding techniques and produced the first specialized breed of dairy cattle. The Middle Ages (500–1450) was a time of minor agricultural improvements, but it was marked by terrible happenings like the Black Death (an extremely deadly form of bubonic plague that was widespread throughout Europe and Asia in the fourteenth century) that killed millions of people and created chaos on farms as well as in cities. However, the voyages of discovery that


Agriculture

This corn field is the result of advances in agricultural technology that led to the mass production of crops. (Reproduced by permission of Photo Researchers, Inc.)

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Agriculture

marked the beginning of the 1400s contributed greatly to a new agricultural diversity as New World (North and South America) crops and methods were brought back to Europe. Many Native American crops were eventually accepted and grown by Europeans, including corn, peanuts, peppers, tomatoes, potatoes, and squash. The Europeans also brought their own seeds, farming methods, and tools to the New World. It was not until after 1700 that the practice of agriculture began to benefit from a series of scientific discoveries and technological inventions, setting the stage for what has been called the agricultural revolution of the 1700s. One of the most important agricultural changes was the adoption of the new four-crop rotation in England. This new method led to greater food supplies since farmers replaced the old system of three crops a year for three years (with no planting the fourth) with the new system. They began to grow wheat, barley, turnips, and clover in succession without ever resting the soil. This was possible because the clover put nitrogen back into the soil and also provided grazing for cattle and sheep, which fertilized the soil with their waste. Farmers also began to suspect that their livestock could be improved by repeatedly breeding animals with desirable traits. By the nineteenth century, agricultural tools had improved greatly. The discovery was also made that chemicals known as phosphates were needed by growing plants, and artificial fertilizers were developed by treating rocks that were rich in phosphate with sulfuric acid. By the twentieth century, science and technology had not only provided farmers new power sources for their tools, but gave farmers ways to further improve their livestock while also increasing their harvest using new agricultural chemicals. These modern chemicals include fertilizers (phosphorous, potassium, and nitrogen), insecticides like DDT dichlorodiphenyltrichloroethane, and herbicides for weed control, as well as other chemicals to fight plant diseases. By the beginning of the twenty-first century, however, both science and farmers have learned that these chemicals can have disastrous environmental side effects, as seen in the DDT experience of the 1950s. This insecticide proved toxic to birds that had eaten insects contaminated with the chemical. As a result, the birds were unable to reproduce, and the U.S. government eventually banned most uses of DDT. Today, although almost half of the world’s labor force is employed in agriculture, in highly developed nations like the United States less than four percent of the population is actively engaged in agriculture. Although the human population is steadily increasing and farmland is shrinking in America, the United States produces an agricultural sur-

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plus almost every year. This is not the case in all countries, however. For example, some countries in Africa and Central America are not able to produced enough food for their growing populations. As a result, thousands of people in these countries die every year because of lack of food.

AIDS

AIDS AIDS, or Acquired Immune Deficiency Syndrome, is a disease caused by a virus that disables the immune system. The virus enters the body through the bloodstream, duplicates itself rapidly, and eventually destroys the body’s immune system. This leaves the victim susceptible to other infectious diseases that usually prove fatal. AIDS is caused by the Human Immunodeficiency Virus (HIV) that was first isolated in 1983. Before 1981, AIDS was unknown, and many health professionals believed that infectious diseases were a thing of the past in developed countries. However, following the discovery in late 1980 of several young homosexual men who had developed rare forms of pneumonia and cancer, health officials in the United States realized they had discovered a new infectious disease that enters the bloodstream and destroys the immune system. From these beginnings, the worldwide estimates of the number of people infected with AIDS has reached 30,000,000. More than 12,000,000 people have already died from AIDS since the beginnings of this worldwide epidemic, and although the number of infected individuals has stabilized in developed countries, the number of AIDS cases has exploded in many African countries.

HOW AIDS SPREADS AIDS is obviously a contagious disease for which there is no cure. It is spread by a virus and transmitted by entering the bloodstream. This means that AIDS cannot be spread by the type of casual contact that usually takes place between family members and friends. HIV must somehow enter the bloodstream to infect a person, and the most common way for this to happen is through some form of intimate sexual contact that allows bodily fluids from one person to enter that of another. This is what occurs during any type of sexual intercourse or sexual penetration of a person’s body. Another way is for an intravenous drug user to share a needle with another person. HIV has also been transmitted to an unborn child by its infected mother, and until programs for blood screening were created, HIV had also been transmitted by blood transfusions. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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AIDS

ROBERT CHARLES GALLO American virologist (a person specializing in the study of viruses) Robert Gallo (1937– ) is credited as the codiscoverer of the human immunodeficiency virus (HIV). Gallo also established that HIV causes acquired immunodeficiency syndrome, or AIDS (a disease caused by a virus that disables the immune system), and developed the blood test for detecting HIV, which is still the main tool in diagnosing the disease. Moreover, Gallo’s blood test for HIV has made the blood supply safe. Robert Gallo was born in Waterbury, Connecticut, and grew up in the same house that his immigrant grandparents bought after they came to the United States from Italy. His father had a welding business, but the dominant theme of young Gallo’s family life was the illness and death of his only sibling, his sister Judy. His sister died of childhood leukemia, and this disease brought Gallo into regular contact with the nonfamily member who most influenced his life, Doctor Marcus Cox. During his senior year in high school, an injury forced Gallo off the basketball team and got him thinking about his future. He began to spend time with Cox, and by the time Gallo was ready for college, he knew he wanted a career in biomedical research. Gallo majored in biology at Providence College in Rhode Island, and went on to earn a medical degree from Jefferson Medical College in Philadelphia in 1963. In 1965 Gallo joined the National Institutes of Health (NIH) to do cancer research, and in 1971 he was appointed head of a new Tumor Cell Biology lab-

AIDS TAKES OVER THE IMMUNE SYSTEM AIDS is an especially difficult disease because, unlike other forms of infections, it attacks the very system that we use to defend ourselves against outside invasions. The HIV virus is a ribonucleic acid (RNA) virus like many other viruses, such as the flu, the common cold, and measles. However, HIV is also a retrovirus, which makes it quite different and deadly. Viruses contain their own forms of deoxyribonucleic acid (DNA) and RNA (their own genetic code), but a retrovirus contains a special enzyme that enables it to put its own DNA into the DNA of the cell it invades. A retrovirus can then use the infected cell’s machinery to continuously reproduce itself and make more and more copies of the retrovirus. Once inside the body and reproducing, HIV goes to work by attacking the very type of cells that the body automatically uses to fight invaders like viruses. It mainly attacks what are called the T-4 white blood cells and eventually causes their number to dramatically decrease. As these 18


oratory at NIH. There he studied retroviruses, which are types of viruses that possess the ability to penetrate other cells and to splice their own genetic material into the genes of their hosts, eventually taking over all of the invaded cell’s reproductive functions. This research led him to discover the first human cancer virus.

AIDS

When AIDS began to be recognized in the United States in early 1981 as a new and terrible disease, it was Gallo’s pioneering work in the field of human retrovirology that led him to be one of the first scientists to hypothesize that the disease was caused by a virus. In 1982, the National Cancer Institute formed an AIDS task force with Gallo as its head. By 1984, Gallo’s team was able to establish that the Human Immunodeficiency Virus caused AIDS, and it developed a blood test for detecting the virus. It was then that a controversy developed which would involve Gallo for the next decade. The controversy began when a French colleague who had earlier and independently discovered the same virus sent Gallo a sample. The result involved legal disputes and hearings, accusations, findings, and reversals of opinions on which scientists had actually discovered the virus first. Overall, it is now thought that the virus sent to Gallo from France may have contaminated the blood samples that he held, making for a mistake but not misconduct. Today, Gallo has survived and overcome the allegations and is the director of the Institute of Human Virology in Baltimore, Maryland. There, Gallo continues his pioneering work in the field of human retroviruses. Gallo has received many honors, including the distinction of being the most referenced scientist in the world between 1980 and 1990.

important disease-fighting cells are killed off, the body’s ability to resist infection is severely impaired, and AIDS patients become more susceptible to what are called “opportunistic” infections such as pneumonia, tuberculosis, and rare forms of cancer.

PHASES OF AIDS The disease typically goes through three major phases. During the first stage, the individual experiences general flu-like symptoms and remains relatively healthy while the immune system keeps fighting back. However, once the immune system begins to weaken (which may take several years), the next stage includes symptoms like swollen glands, severe fatigue, cough, diarrhea, and night sweats, as well as persistent infections like thrush and herpes. Only in the final stage, which is what is technically called AIDS, does the patient develop the serious infections or tumors that will eventually prove fatal. AIDS is a terribly wasting disU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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ease, and its victims are usually reduced to human skeletons as their lungs and brains are also destroyed.

AIDS

Continued and heavily funded research has not yet found a cure for AIDS, although researchers have discovered drug combinations that effectively prolong the period before full-blown AIDS begins. As with any virus however, HIV has already mutated and developed strains that are resistant to some of these drugs. Research on the origins of AIDS suggests that the virus may be a mutant of a strain that is known to infect the African green monkey, an animal that often comes in contact with humans in West Africa. An illustration showing the parts of the body that AIDS attacks and some of the infections that the disease causes. (Illustration by Electronic Illustrators Group.)

To date, information and educational campaigns throughout the world have been effective in making people aware that prevention is, for now, the only sure weapon against AIDS. However, sexual behavior and drugrelated activities are not always conducted with common sense in mind. It is especially important to always practice safe sex by using a condom

CENTRAL NERVOUS SYSTEM LYMPHOPROLIFERATIVE DISEASE

MUCOCUTANEOUS

PNEUMONIA

SKIN DIARRHEA

OPPORTUNISTIC INFECTIONS CAUSED BY AIDS

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(a rubber sheath that is worn over the penis to prevent the exchange of bodily fluids) and not to share intravenous drug needles. Even with widespread awareness, an estimated 6,000,000 new infections occur every year.

Algae

[See also Immune System; Virus]

Algae Algae are a group of plantlike organisms that make their own food and live wherever there is water, light, and a supply of minerals. Like plants, algae contain chlorophyll, which means that they produce their own food by photosynthesis (using sunlight). There are thousands of different species of algae, ranging from microscopic diatoms to huge ribbons of seaweed. Since algae use the Sun’s energy to make their own food, they eventually become food for other kinds of life, just as plants do. Classifying algae is difficult, and experts are constantly revising their ideas. Although algae resemble plants in many ways (they have cell walls and make their own food), some algae can move about and even absorb organic food like animals. However, most algae belong to the kingdom Protista and are considered to be the simplest form of plants (despite the fact that they have no roots, stems, or leaves). Most algae are aquatic, meaning that they need water to survive, grow, and reproduce. Algae can be found almost anywhere there is water. One of the most common is often seen as the green on the surface of ponds or streams. Some algae merely float near the water’s surface while others are capable of moving about on their own. Seaweed is a common form of algae that can be found in the ocean or on the seashore. Even the green, powdery film sometimes seen on trees or on old wooden fences is caused by an algae named Pleurococcus, which is unusual because it is one of the few algae that can survive away from water. Since all algae require light to make food, most are found near the water’s surface, although photosynthesis can occur in extremely clear water at a depth of 100 meters (328.1 feet). Marine algae are very important to life on Earth since they produce about 90 percent of the oxygen that is created by the process of photosynthesis. Like green plants, algae take in the carbon dioxide that humans and animals exhale and release the oxygen that humans and animals need to breathe. Single-celled algae make up a large part of the phytoplankton of the oceans. Phytoplankton are found at the beginning of the food chain and form the basis of all nutrition in the sea. Even some whales feed on phytoplankton. Since all marine life is ultimately dependent on this first link in the food chain, there would be no fish to catch and eat without algae. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Algae

TYPES OF ALGAE The different species of algae are grouped into phyla (related groups) according to their pigment (color) and the form in which they store food. Some are better known and easier to identify than others. The six main phyla of algae are diatoms, dinoflagellates, euglenas, green algae, red algae, and brown algae.

Diatoms. Diatoms make up the largest and most important part of phytoplankton and are also the easiest algae to identify (besides seaweed). Each one-celled diatom is protected by a tiny, two-part case that it makes out of silicon dioxide. When a single marine diatom dies, its hard case or shell drifts to the ocean bottom where over time, thick layers of these cases accumulate and are compressed to form a rock called diatomite. This valuable powdery rock is almost pure silica and is used commercially as an abrasive, filtering, or insulating material.

Dinoflagellates. Dinoflagellates almost always live in salt water and form the second most important part of phytoplankton after diatoms. These onecelled algae move about using whiplike tails called “flagella.” As algae, dinoflagellates possess chlorophyll but they have red pigment rather than green. When the dinoflagellate population sometimes explodes for unknown reasons, it causes what is known as a “red tide.” Red tides sometimes contain a nerve poison that can kill fish and people who eat infected fish.

Euglenas. Euglenas live in fresh water and are able to move with a long, whipping tail. Euglenas combine both plant and animal characteristics. Like plants they are able to produce their own food through photosynthesis. However, like animals, euglenas are also able to capture and eat food. Although they do not have a cell wall, they have a flexible layer inside their membrane as well as an “eyespot” that responds to light.

Green Algae. Green algae make up the phylum Chlorophyta and are distinguished by the presence of chlorophyll. They can be one-celled or many-celled and usually live in water, although they can survive in other environments (like the damp side of a tree trunk). Many species of green algae form colonies (a permanent group of related organisms) or grow in long chains, although some form a ball-shaped colony. Sea lettuce that grows in salt water is a good example of green algae.

Red Algae. Red algae are multicelled and get their name from their distinctive coloring. Since their unique red pigment allows them to absorb even the smallest amount of light, they are able to live far below the ocean surface and still make their own food by photosynthesis. Their food is a type of carbohydrate or starch called carrageenan, and it is used commercially to give toothpaste and even pudding its smooth creaminess. 22


Brown Algae. Brown algae are multicelled and most species live in salt water. Brown algae, also called kelp, grows into fields that are sometimes 100 yards (91.4 meters) long. Kelp or brown algae play an important role in the ocean as they provide both food for many fish and invertebrates (animals without a backbone) and a place to live and hide for many small fish. People in many parts of the world eat brown algae, and it is used commercially in ice cream, marshmallows, and fertilizer. Despite the fact that some kelp can grow as long as 100 feet (30.48 meters), they lack the complex structure of plants and are still considered algae. ESSENTIAL TO LIFE ON EARTH Algae play a key role in sustaining life on Earth since they give off oxygen and absorb much of the carbon dioxide that is produced by not only be humans and animals, but also the burning of fossil fuels. They also form the basis for most food chains in fresh water and ocean habi-


Algae

Green algae magnified fifty times its original size. Green algae receives its color from the presence of chlorophyll. (Reproduced by permission of Custom Medical Stock Photo, Inc. Photography by Alex Rakoey.)

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Amino Acids

tats and have many valuable commercial purposes. For example, brown algae provide a natural source for the manufacture of chemicals called alginates that are used as thickening agents and stabilizers in the industrial preparation of foods and pharmaceutical drugs. [See also Food Web/Food Chain]

Amino Acids A computer-generated model of glycine, and amino acid. (©Scott Camazine, National Audubon Society Collection/ Photo Researchers, Inc. Reproduced by permission.)

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Amino acids are the building blocks of proteins. Living things are able to produce vast types of proteins out of only twenty different amino acids. Nearly half the number of amino acids needed to make proteins cannot be made by the body and must be obtained from food. Proteins are essential to all living things, and for animals especially they perform many critical tasks. Without proteins, there would be no life,


and without amino acids, there would be no proteins. While there are many types of amino acids (some two hundred and fifty have been found in plants and fungi), for humans there are only twenty that are found in proteins. Out of these twenty amino acids, the body is able to manufacture the thousands of different proteins that it needs for growth and repair. The body uses various combinations of the twenty acids to make proteins in much the same way the twenty-five letters of the alphabet are used in various combinations to make different words. Proteins are therefore made of amino acids linked or bonded together in different chainlike combinations. The properties of an individual protein are determined by the sequence or order in which the amino acids are linked together. Therefore, different proteins have different sequences of amino acids.

Amoeba

While most bacteria and plants can make all the amino acids they need, the cells of humans and most other animals are not able to manufacture all of the biologically important amino acids. It is estimated that of the twenty amino acids needed by humans, only ten can be made by the human body. The other ten must be obtained from the food the organism eats. In fact, the human body really gets all of its essential amino acids from the food it consumes since it is only able to make some amino acids by converting the others that it gets from outside its food. Therefore, unless the body obtains certain “essential” amino acids from its protein food source, it does not have the building blocks to make any other amino acids. In such a case, the body’s protein-making ability would break down. Nutritionists have determined that all of these essential amino acids can be obtained from meats, eggs, milk, cheese and other foods derived from animals. [See also Nutrition; Protein]

Amoeba An amoeba is a single-celled organism that has no fixed shape. As a protozoan and a member of the kingdom Protista that has animal-like qualities, an amoeba has to find and eat its food (since it is unable to make its own food as plantlike Protists do). Of the many species of amoeba, some live in water and soil, while others are parasites and live inside plants or animals. Many biologists believe that the first protozoans were similar to today’s amoeba. As members of the phylum (or primary division of the animal kingdom) Sarcodina, amoebas are distinct among protozoa in that they have no definite shape. Although they consist of a single cell surU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Amoeba

Amoebas are formless organisms that use pseudopods, or “false feet” to move about. The false feet are visible on the amoeba in this photo. (Reproduced by permission of Photo Researchers, Inc. Photograph by M.I. Walker.)

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rounded by a membrane, they move by changing the shape of that membrane and therefore do not have one particular shape that makes them immediately recognizable. Some would say that they are recognizable because of this formlessness, while others say they look like a tiny bag of jelly. An amoeba actually moves in a very strange manner using pseudopods or “false feet.” When it wants to move, an amoeba turns its jelly-like body solid in a certain spot to form a temporary “foot” which it stretches in the direction it wants to move. The rest of the amoeba then flows into the pseudopod and basically changes its position to where its pseudopod had reached. This type of movement is called “streaming.” Every time an amoeba does this it changes its shape. Since this is not the fastest way to move about, an amoeba only can move at a top speed of about 1 inch (2.54 centimeters) an hour. An amoeba eats its food in much the same manner as it moves. It flows around and surrounds the food and then the amoeba totally engulfs it. The food is then held in a food vacuole, which is a specialized structure that digests the food. The amoeba mainly eats bacteria, algae, and other protozoans. Any waste that remains after the food is digested is released from a contractile vacuole (one that can open and close). Water


also flows into the amoeba by the process of osmosis (in which water flows through a membrane until the solutions on either side of it are at equal strength). When the amoeba needs to expel some water, it squeezes its contractile vacuole and squirts some out.

Amphibian

There are four types of protozoans that are called amoeboid protozoans. All use pseudopods to move about and to capture their food, and three of them are considered shelled amoeba since they are at least partially covered by a shell made up of the minerals they secrete. The first of the amoeba is the familiar “naked” amoeba called Amoeba proteus. While most of this type of amoeba live in water, some are parasitic and live in the human gut. The infected human then contracts the disease called amoebic dysentery. One of the shelled amoeba that lives in a tiny chambered sea shell and uses threadlike pseudopods to move about is called a foraminiferan. This amoeba can barely be seen with the naked eye. A second shelled amoeba is the freshwater heliozoan or “sun animal.” It has thin pseudopods that look like needles that radiate from its body like rays from the Sun. The third and most intricate of the shelled amoebas is the radiolarian. These sea creatures have skeletons of silica, a mineral that does not dissolve in deep water, so that the deepest seabeds are covered with a thick layer of what is called radiolarian ooze. Amoebas usually reproduce asexually by a process known as binary fission. In this form of reproduction, an amoeba splits in two after pinching in half and forms two smaller but identical cells. This occurs after the cell’s nucleus duplicates its hereditary material and divides in two. [See also Cell; Protozoa; Reproduction, Asexual]

Amphibian An amphibian is a cold-blooded vertebrate (an animal with a backbone) animal that spends part of its life in the water and part on land. After hatching from an egg, an amphibian usually lives in water and breathes through gills. As it grows, it undergoes a metamorphosis, growing legs and developing air-breathing lungs. At home in both water and land, the amphibian lives in damp places where its thin skin will not dry out.

THE LIFE CYCLE OF AN AMPHIBIAN The name amphibian comes from Greek words meaning “having two lives,” and it is this unique life cycle that most characterizes an amphibian. Frogs, toads, newts, and salamanders are the best-known members U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Amphibian

of this smallest class of vertebrates. They all share several other characteristics. Most have thin, moist skin that is smooth and soft. They do not have scales or claws, although the skin of a toad is dry and covered with bumps. They have a three-chambered heart (a fish has a two-chambered heart and birds and mammals have four-chambers) and are ectothermic or cold-blooded. This does not mean that they are always cold, but rather that their body temperature matches that of their surroundings. Because of this, they become sluggish or inactive in the cold and usually hibernate (an inactive state resembling deep sleep) during extreme cold. Most adult amphibians have four limbs and are carnivores (meat-eaters) who will eat almost anything they can catch. Although adult amphibians can live on land, they must return to water to reproduce. There they lay their eggs, which are fertilized by the male outside of the female’s body. The jelly-coated eggs remain in water until they hatch into larvae (the early stage of an organism’s development, which changes structurally as it becomes an adult).

Metamorphosis. Upon hatching, a process called metamorphosis (a series of distinct changes in form through which an organism passes as it develops from and egg to an adult) begins that is unique among vertebrates. After the eggs hatch in the spring, legless tadpoles emerge. These animals look like tiny fish with long tails, and they breathe as fish do, using their external gills. Soon, however, the tadpoles begin to change gradually as their tails shrink and the beginnings of legs start to form. Covers over their gills start to grow as their lungs begin to take shape. It may take as long as two years for a frog larva or tadpole to change completely into an adult, but when it does, it looks entirely different from when it was hatched. As an adult, the frog has a tail-less, squat, compact body with four legs, the back two of which have powerful muscles for jumping. Upon maturity, the frog will mate and the cycle will begin all over again with its offspring. HIBERNATION During the winter months, frogs and toads become inactive since their body temperature decreases. It is at this time that frogs bury themselves in the mud at the bottom of lakes or ponds and hibernate. Toads do the same only in soft, moist soil. During hibernation, their body processes slow down considerably, and they are able to exchange gases (breathe) through their damp skin. Besides frogs and toads, the other major group of amphibians is made up of the tailed amphibians like the salamander and the newt. Both have long bodies, four short, thin legs, and a tail. Salamanders spend most of their adult lives on land, while 28


newts live in the water. As amphibians, they both begin life with gills living underwater, and soon develop lungs as they mature and change into adults.

Amphibian

AMPHIBIANS OFFER GREAT VARIATION Even in a small class like the amphibians, there is amazing variation. For example, some frogs and toads are poisonous. Tree frogs have toes with sticky pads for gripping, while other frogs are able to jump and then glide using their webbed feet as a parachute. The pink salamander is able to reproduce before it even matures, and the Caecilian is an amphibian without any legs at all, having a wormlike body and scales. All, however, have the double life of an amphibian. Recently, biologists have noticed the disappearance of some species and an overall decrease in the amphibian population. Some believe that the health of amphibians may serve as an early warning system for the overall health of the environment. Since amphibians metabolize toxic substances in much the same way that hu-

A flow chart of the life cycle of a frog beginning with the fertilization of the eggs and ending with a sexually mature frog. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

Eye Spiracle External gills Olfactory organ

Tadpole begins feeding on algae at 7 days; a skin fold grows over external gill leaving a pore or spiracle on the left side for exit of water (11 days)

Hatches at 6 days as tadpole with external gills which, clings to submerged vegetation with its sucker

Tail bud At 4 days the embryo has a tail bud and early muscular Sucker movement and subsists on the yolk packed in its gut Each egg undergoes first cleavage in 3-12 hours, depending on temperature; successive cleavages occur more rapidly Each egg has three jelly coats which swell with water to enclose it

Hind limbs appear first, then forelimbs emerge; internal gills replaced by lungs (75+ days)

Tail shortens, metamorphosis nearly complete at 90+ days; functional lungs; juvenile frog for one to two years Sexually mature frog at three years

Clasping by the male stimulates the female to lay 500 to 5,000 eggs, which the male fertilizes as they are shed, over the course of about 10 minutes


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Anatomy

mans do, whenever any serious changes are noticed, such as a sharp drop in population or mutant frogs being born, it suggests that their environment might be equally hazardous to humans. [See also Animals; Metamorphosis; Vertebrates]

Anatomy Anatomy is the study of the biological structure of living things. Although many think of anatomy as being concerned only with the human body, the word actually applies to the structure of plants, animals, and other organisms.

ARTISTOTLE BECOMES THE FIRST TO STUDY ANATOMY As one of the oldest branches of biology (the science of life processes and living organisms), anatomy comes from the Greek word anatome, which means “cutting up.” Until modern times, dissecting or “cutting up” was in fact the only way to learn how living things were actually put together. A knowledge of anatomy was important even in ancient times, since it was recognized early on that it was impossible to understand how the parts of a living thing worked until one knew how they were shaped and how they all fit together. The Greek philosopher and scientist, Aristotle (384–322 B.C.), is considered the first to study anatomy, and he is credited with the idea that each organ has its own function that could be discovered by observing its structure. Because of Aristotle’s work, the structure and function of a body’s organs and parts have been linked from very early times. The Greek scholar, Herophilus of Chalcedon (335–280 B.C.) is credited with founding the first school of anatomy and is believed to have conducted some six hundred dissections. By the first century A.D., dissection of human corpses was becoming discouraged, so the prominent Greek physician, Galen (129–200) dissected apes, dogs, and pigs in order to study anatomy. Galen is considered to be the founder of experimental medicine. Many of his anatomical teachings about the human body were incorrect (as they were based on the anatomy of other animals), but his work was considered to be the final authority for centuries.

VESALIUS BEGINS THE MODERN ERA OF ANATOMY The modern era of anatomy began with the Flemish physician, Andreas Vesalius (1514–1564), who published his classic On the Structure of the Human Body in 1543. His highly accurate drawings were based on his extensive dissection of the bodies of executed criminals. Despite be30


ing condemned in his lifetime because his writings contradicted the old teachings, Vesalius is now considered to be the founder of modern anatomy. By the time the great English physician William Harvey (1578– 1657) correctly described the circulation of the blood in 1628, the study of anatomy had been reestablished in schools and human dissection was once again permissible. Anatomical study was broadened with the seventeenth-century invention of the microscope, since it opened up an entirely new world that was too small for the naked eye to see. Regular microscopic discoveries eventually led to the nineteenth-century finding that all living matter is made up of cells. In that century, the second great revolution in anatomy took place when the English naturalist, Charles Darwin (1809–1882), introduced his theory of evolution in 1859. His argument that all living species are descended from other species led to the branch of anatomy called comparative anatomy. Comparative anatomy is used to study the anatomical differences and similarities between animals, and eventually provided evidence for Darwin’s theory. Today, the study of anatomy has shifted from the invasive techniques, which usually opened the body in some manner in order to observe, to noninvasive techniques, such as x rays, computerized tomography (CT scan), ultrasound, and magnetic resonance imaging (MRI).

Anatomy

The modern age of anatomy began with Andreas Vesalius and the publication of his highly accurate drawings based on the human body. (Courtesy of The Library of Congress.)

ANATOMY OF ANIMALS AND PLANTS The anatomy of higher animals is made up of eleven body systems: the integumentary system (external features that are related to its skin); the skeletal system (external or internal); the muscular system (including three different types); the nervous system (including the brain, sense organs, and nerves); the digestive, circulatory, and respiratory systems (that work together to nourish the body); the excretory system (that rids the body of waste); the reproductive system (that allows new life to be created); the endocrine system (that produces hormones that regulate bodily functions); and the immune system (that protects the body from infection). The anatomy of plants is much simpler than that of animals, having only two main systems— the root system and the shoot system. The root system anchors the plant in the ground and alU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Anatomy

ANDREAS VESALIUS Flemish anatomist (a person who studies the structure of human and animal bodies) Andreas Vesalius (1514–1564) began the modern era of anatomy with the publication of the first accurate book on the human body. Often called the founder of modern anatomy, Vesalius performed many autopsies (examinations of dead bodies to determine the cause of death) and discovered that much of what was taught about the human body was wrong. The illustrations in his book, On the Structure of the Human Body, are at the highest level of both art and science, and it is considered one of the greatest biology books ever written. Andreas Vesalius was born in Brussels, Belgium, and came from a long line of physicians. Although his mother was English, his father was court pharmacist for Emperor Charles V, and Andreas studied medicine in Belgium and France. At that time, medical schools were very conservative in that their teachings were based on very old texts written around A.D. 175 by the Greek physician, Galen (A.D. c.130–c.200). Galen wrote about anatomy, which is the study of the physical structure of living things, and specialized in human anatomy. However, when Galen did his work, it was unlawful to dissect (to cut open and examine) the bodies of dead people, so Galen did most of his anatomical research on dead animals like monkeys, pigs, dogs, and goats. While he did advance the study of anatomy with this work, not all of it was directly applicable to the human body. Nonetheless, some 1500 years later, Galen’s teachings were still being used in many of the more conservative medical schools. Vesalius had an inquiring mind, and when he thought Galen was wrong he told his teachers. This only served to get him in trouble, so he moved to Padua, Italy, and earned his medical degree from that city in 1537. Although dissecting humans was still discouraged, things were much freer in Italy,

lows it to get water and nutrients from the soil. The shoot system is made up of all aboveground stems, branches, leaves, and flowers. In the life sciences, the study of anatomy is essential to our understanding of the overall structure of living things and of how those individual parts relate to one another, influence one another, and work together. Understanding the anatomical similarities of different organisms provides important evidence of how all living things are linked together through the process of evolution. [See also Circulatory System; Digestive System; Endocrine System; Muscular System; Nervous System; Reproductive System; Respiratory System; Skeletal System] 32


and when Vesalius began to teach anatomy himself, he did something that was truly revolutionary. It was the practice for teachers to lecture during dissections and only supervise the actual cutting, which was done by assistants who usually knew little about the human body. Vesalius, however, decided to perform these dissections himself as he taught, and his lectures became popular with the best students. Finally, he realized just how wrong much of Galen’s teachings were, and he decided to produce an anatomical textbook that, once and for all, would actually show the way the body was really constructed. He commissioned talented artists to draw the anatomical features of the human body the way he actually saw them when he dissected. One story tells how, until Vesalius, it was taught that men had one fewer rib than women because of the creation story in the Bible. Vesalius put nothing in his book that he had not observed himself, and after three years of hard work, he published his De humani corporis fabrica (On the Structure of the Human Body), whose highly accurate and artistically beautiful woodcuts raised anatomy to a new level. The publication of this great work instantly marked the beginning of modern anatomy and introduced a new standard for anatomical textbooks. Today it is considered to be one of the greatest medical works ever produced. Yet in its time, it was actually ridiculed by the medical establishment.

Animals

Although this work would eventually revolutionize biology, it would bring Vesalius as much trouble as it did fame. His enemies accused him of snatching bodies to dissect, and he was even accused of religious heresy (having an opinion in opposition to religious beliefs). At one point, Vesalius became so disgusted that he gave up his work altogether. Eventually, he was given a good position at the royal court, but was ordered to make a pilgrimage, or journey, to the Holy Land (the Middle East) to make up for his heresies. It was during his return trip that he died off the coast of Greece when his ship was wrecked in a storm.

Animals Animals are a group of multicelled, living organisms that take in food. Most animals reproduce sexually (with sperm fertilizing an egg), can move about, and are able to respond to their surroundings. Of the separate divisions of living organisms, the animal kingdom forms the largest in terms of the number of species. Animals range in size from barely visible one-celled animals to the 100-foot (3.48 meter) blue whale. Different creatures may slither, burrow, climb, run, swim, and fly—yet they are all considered part of the Animalia kingdom. The Animalia kingdom is one of the five major divisions U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Animals

of all living organisms. The other kingdoms are Monera (bacteria with no nucleus); Protista (one-celled organisms with a nucleus); Fungi (multicelled organisms that take in food); and Plantae (multicelled plants that make their own food).

CHARACTERISTICS OF ANIMALS There are six major characteristics of all animals, whether they are worms or whales. First, animals cannot produce their own food (as plants do) and must therefore rely on eating other living things. Second, animals cannot use protein, fats, and carbohydrates directly and so must first digest or break them down into smaller molecules. Third, because of their need to find food and a mate, as well as escape from an enemy, animals have developed a way to move from place to place. Fourth, animals are multicellular, having many cells that are highly specialized. Fifth, animal cells are eukaryotic, meaning that each has a nucleus surrounded by a membrane. Sixth, animals are able to respond quickly and in the correct manner to changes in their environment.

VERTEBRATES AND INVERTEBRATES To classify the different types of animals or to group them by their similarities, biologists have divided animals into vertebrates (those with a backbone) and invertebrates (those without backbones). Although vertebrates, such human beings, whales, elephants, and dolphins, are the biggest and brainiest of the animals, about 97 percent of the entire animal kingdom is made up of invertebrates like worms, sponges, clams, and insects. The next thing a person who classifies animals considers is the physical arrangement of an animal’s body parts. Some animals (like humans) have what is called “bilateral symmetry.” This means that if an imaginary line were drawn from top to bottom of an animal, each half of its body would be a mirror image of the other. Those with “radial symmetry,” like sea anemones, have their body parts arranged in a circle around a central point. Others like a sponge, have no definite shape at all and are called “asymmetrical.”

IDENTIFYING BY FEATURES Among the major divisions of better-known animals, it is possible to group several according to some of their easily identifiable features. One of these groups includes animals with exoskeletons or strong and light skeletons on the outside of their bodies. Examples of animals with exoskeletons include horseshoe crabs, oysters, and snails as well as insects like spiders and ticks. Animals with a backbone include fishes with gills, 34


amphibians who live on land but need to breed in water, and cold-blooded (animals whose body temperature changes with the environment) reptiles like lizards, snakes, and crocodiles. Birds evolved from reptiles as their scales changed into feathers. They lay eggs and can fly. Mammals are warm-blooded and give birth to live young. The strangest group of animals may be the echinoderms or “spiky skin” animals like starfish. Their structure and shape are completely different from other animals, including five identical parts and a skeleton of plates inside their bodies. Although echinoderms have no head, brain, or blood, they are still members of the animal kingdom. The animal kingdom, of which human beings are one part, includes a wide variety of different but related life forms.

Antibiotic

[See also Arthropods; Echinoderms; Mollusks;]

Antibiotic An antibiotic is a naturally occurring chemical that kills or inhibits the growth of bacteria. Today, antibiotics are used to treat infections and to fight a wide range of bacteria. However, the overuse of antibiotics has caused bacteria that are resistant to antibiotics to become more widespread, and in many cases, the antibiotics have become ineffective. When a person gets an infection, microscopic bacteria have entered the body through an opening or a wound. After quickly finding an abundant supply of food inside, these bacteria reproduce in great numbers and release toxins or poisons as they grow. These toxins can interfere with cell functions or even destroy human cells.

THE HISTORY OF ANTIBIOTICS Antibiotic drugs have been developed to fight and kill bacteria. They are derived from other organisms, like molds, that are naturally harmful to bacteria. Certain molds produce their own toxins that destroy bacterial cells. This may be the means by which a mold would defend itself against bacterial invasion. As early as 1871, the English surgeon Joseph Lister (1827–1912) noted that certain organic compounds seemed to act against bacteria. However, it was not until 1928 that the Scottish doctor Alexander Fleming (1881–1955) made the important discovery that would eventually lead to the development of penicillin (synthetically produced antibiotics derived from molds and used to treat a wide variety of diseases). While he was growing cultures of bacteria in petri dishes for experiments, Fleming accidentally left several dishes uncovered for a few days. He then noticed that a green mold had gotten into one dish (having U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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traveled through the air as a mold spore) and had destroyed or dissolved the bacteria. Examining the situation with his trained eye, Fleming realized that he had come upon a natural substance that could kill bacteria. His later experiments with mice showed that his new “penicillin” killed only the bacteria and did not harm the animals’ cells. Since he was unable to purify and concentrate more penicillin, he published a paper that received little attention. It was not until 1940 that penicillin was taken up experimentally by others who, by 1942, were beginning to make it in large amounts. Fleming’s discovery would eventually lead to the steady production of several different lifesaving antibiotics.

Antibiotic

HOW ANTIBIOTICS WORK

A labeled illustration showing how antibiotics attack and destroy a bacterial cell. (Illustration by Electronic Illustrators Group.)

The key to why an antibiotic works is that it is selectively toxic or poisonous. That is, it works against certain life forms and not others. It does this by interfering with the cell wall of each new bacterial cell, and this eventually kills the cell. Since animal and human cells do not have cell walls, it is not harmful to these types of cells. However, when an antibiotic encounters a bacterial cell, it joins with its cell wall, leaving a gap in the cell wall so that it no longer can protect its contents, which then spill out. Other antibiotics bind to the ribosomes (particles that act in protein synthesis) in a bacterial cell and stop them from making proteins (which a cell needs to stay alive).

Antibiotic

Water enters

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Cell is destroyed

Cell deteriorates


BACTERIOCIDAL AND BACTERIOSTATIC ANTIBIOTICS There are two types of antibiotics: some antibiotics are bacteriocidal while others are bacteriostatic. Bacteriocidal agents kill bacteria, while bacteriostatic agents slow them down so that the host’s immune system has a better chance to defeat them. Today’s many antibiotics can be broadspectrum agents or narrow-spectrum agents. As it sounds, one is effective against a broad range of bacteria while the other works against only a few. There also are many different “families” of antibiotics, some of which are synthetic or man-made. However, no matter how different they are from one another, when a patient is given an antibiotic injection or takes an antibiotic pill, the antibiotic prevents bacterial cells from growing and dividing normally.

Antibody and Antigen

ANTIBIOTIC RESISTANT BACTERIA Bacteria may seem easy to kill with modern medicine. Yet these invisible agents of disease can reproduce every twenty minutes and have proven capable of becoming resistant to antibiotics. They do this by mutations, or changes. that occur in a cell’s genetic material. More and more, antibiotic resistant bacteria are becoming increasingly common due largely, it has been shown, to their routine use by farmers who give antibiotics to their livestock and chickens to prevent them from getting sick. Unfortunately, because people then consume the meat of these animals, humans are ingesting some of the antibiotics given to the animals. It has been shown that overuse of antibiotics in both humans and in animals speeds up the development of antibiotic resistant bacteria. Without antibiotics, humans still would be subject to the terrible diseases that killed millions of people in the past. Before 1950, bacterial diseases like diphtheria, tuberculosis, pneumonia, blood poisoning, food poisoning, bacterial meningitis, and scarlet fever were sure killers. This victory over bacteria may be coming to an end since humans are now faced with the real possibility that entire populations of bacteria are mutating to the point where they will be resistant to any antibiotic available.

Antibody and Antigen An antigen is any foreign substance in the body that stimulates the immune system to action. An antibody is a protein made by the body that locks on, or marks, a particular type of antigen so that it can be destroyed by other cells. Antibodies are an essential part of the immune system of U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Antibody and Antigen

vertebrates (animals with a backbone) and enable the body to resist disease-causing organisms. All vertebrates have an immune system that produces antibodies. The immune system is able to distinguish “self” from “nonself” and recognizes when an antigen (foreign cell) has invaded the body. The immune system then produces special chemicals called antibodies to fight and help kill the invader. The immune system is also able to “remember” these specific invaders, and if they ever return, it is able to respond even faster using specific antibodies whose job is to lock on to that particular type of antigen. After locking on, or binding with it, the antibodies get help from other cells and proteins that destroy the antigen or at least neutralize it. Antibodies really work after the fact. When an antigen, such as a virus, invades the body, two things can happen. However, if this invader is new to the body and has never entered it before, the body has no antibodies to combat it. In such a case, it is the body’s large white blood cells known as macrophages that will attack and try to destroy the antigen. If this virus has entered the host before, then specific antibodies already exist. These antibodies will immediately recognize the virus as “nonself” and bind to it like a key in a lock. Once they lock on to the antigen, they have marked the invader as a target for the body’s killer cells. An antibody will only recognize and help destroy one kind of organism or antigen. If a new and different organism enters the body, a new type of antibody must be produced. Although scientists were aware of antibodies in the 1890s, it was not until the late 1930s that scientists came to discover what they really were. In 1938 antibodies were identified as proteins of the gamma globulin portion of the plasma (the liquid portion of the blood). Later it was found that antibodies are produced by special white blood cells called Blymphocytes. Immunization, sometimes called vaccination, uses the ability of the immune system to remember a previous invader. For example, a child is immunized against certain diseases, like measles, mumps, rubella, diphtheria, whooping cough, tetanus, polio, and chicken pox, through a vaccine. Vaccines contain dead or weakened disease-causing organisms that stimulate the body’s immune system without actually causing the disease. Before vaccination, these diseases were common among children and responsible for many deaths. Now, routine vaccination of children has virtually eliminated these diseases. Vaccination works because once a certain antibody is produced in the body, it usually remains for many years.

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The case of immunization is an excellent example of how an understanding of the body’s systems and operations allows scientists to better use the body’s own natural defense mechanisms to the advantage of the individual.

Arachnid

[See also Blood; Blood Types; Immune System; Immunization; Rh Factor]

Arachnid An arachnid is an invertebrate (an animal without a backbone) that has four pairs of jointed walking legs. Most arachnids, like spiders, ticks, scorpions, and mites, live on land and have two main body parts. Many arachnids prey on other invertebrates, while some are parasites. Unlike insects, however, arachnids have no jaws, antennae, wings, or compound eyes

Light chains

Antigen binding site


Heavy chains



Light chain Disulfide bonds


Heavy chain

Two illustrations of the molecular structure of antigens. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

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Arachnid

(many tiny, individual eyes that altogether make a single image that resembles a mosaic). Arachnids are probably the most unpopular type of arthropod (an invertebrate with jointed appendages and a segmented body). An arachnid is a member of the phylum (or primary division of the animal kingdom) Arthropoda since it has a jointed body case, an exoskeleton (a hard, outer support structure), and no backbone. The class name Arachnida comes from a story in Greek mythology in which a young girl named Arachne spun a silken cloth more perfectly than did the goddess Athena, who became so angry that she turned the girl into a spider. Arachnids are sometimes thought to be insects, but what defines an arachnid and prevents it from being considered an insect is the number of legs it has. An arachnid has eight legs. An insect has six legs. A crustacean has ten legs. Thus a spider and a tick are arachnids, while a centipede and an ant are not. Among other characteristics of arachnids is their highly developed sense of sight. They also have a rigid yet versatile exoskeleton that, combined with muscles and jointed limbs, allows them to have great flexibility and mobility. Its skeleton armor also keeps them waterproof and prevents them from losing body fluids by evaporation. The spider is a good representative of the arachnid class. Besides their eight walking legs, they have a modified pair they use for handling food and another modified pair of appendages they use as poison fangs or claws.

A spider spinning a web. Spiders are probably the best example of the arachnid class. (Reproduced by permission of JLM Visuals.)

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Most spiders breathe by means of “book lungs.” These are special organs located on the abdomen; they include chambers filled with hollow plates connected to tubes that extend to the outside of the skeleton. Air enters via these tubes and passes over plates that are richly supplied with blood. The oxygen diffuses into the blood and carbon dioxide is released. Arachnids, like spiders, are usually hunters, so they have sense detectors in their front end as well as six or eight pairs of eyes. Web-weaving in order to trap prey is probably the most distinctive characteristic of a spider, although not all spiders spin sticky webs. Some literally jump on top of their prey. Weaving a web is instinctive behavior, and webs are made of liquid silk (really a protein) produced from glands in the abdomen and shot out through “spinnerets” at the rear of spiders’ bodies. Sticky threads are added later to catch and hold any prey that gets tangled. Spiders wait in hiding and stay connected to their web by a single thread that acts like a fishing bobber and signals that something is caught. The spider’s feet are coated with an oily film so that it will not be caught in its own web. The spider bites its prey, immobilizing or killing it, and wraps it in silk for a later meal. Other arachnids, like scorpions, simply paralyze their prey with a sting. Similar to a spider, scorpions consume only the prey’s bodily fluids.

Arthropod

Unlike spiders and scorpions, ticks are parasites. They burrow their heads into the skin of a mammal and feed on its blood. A tick is able to take in 200 times its own weight in blood at a single feeding. Although they do not always kill their victims, ticks may sometimes carry infectious diseases, such as lyme disease, which may be fatal if left untreated. Arachnids reproduce sexually (through the union of male sperm and female eggs) and therefore must mate to produce offspring. For spiders however, mating can be a dangerous act. Since spiders not only eat other spiders but also those of their own species, it is not unusual for the female to kill and eat the smaller male immediately after mating with him. To avoid being eaten, some male spiders lock fangs with the female while they mate. Others tie the female in silk before mating, and still others present her with a meal already wrapped and ready to be eaten. Females lay their eggs in silken cocoons and sometimes keep watch over them until they hatch. [See also Arthropods]

Arthropod An arthropod is an invertebrate (an animal without a backbone) that has jointed legs and a segmented body. Arthropods are the world’s oldest U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Arthropod

creatures and the most successful invertebrate group. The phylum Arthropoda contains nearly 1,000,000 species and is therefore the largest in the kingdom Animalia. It is estimated that arthropods account for 75 percent of all the animals on Earth. They range from dust mites to huge crabs.

EXTERNAL STRUCTURE OF ARTHROPODS The name arthropod is translated as “jointed foot” but it is really a jointed or segmented body that is most characteristic of an arthropod. Since all arthropods have an exoskeleton, which is a hard covering that surrounds the outside of an animal’s body, their outer skeleton cannot consist of a single solid piece or they would never be able to move about with any flexibility or speed. A tough covering that is composed of overlapping plates (linked together by tough but flexible hinge joints) allows the arthropod to bend, twist, and move about with great freedom. The same applies to any appendages (legs, arms, tails, pincers) an arthropod may have; these too are made up of tough but flexible joints that allow them to maneuver easily over most surfaces. Whether a lobster or a beetle, most of the arthropod body is covered by an exoskeleton called a cuticle. This protects its soft tissues from predators and disease and supports its entire body. The cuticle is made of a protein called anthropodin, and a carbohydrate called chitin that together produce a tough and flexible covering. This exoskeleton can vary immensely. It may consist of the delicate and flexible wing of an insect or to the heavy and thick shell of a lobster. The major drawback of an exoskeleton is that it makes growth or physical expansion difficult. Since chitin is not living tissue, it cannot expand, and must instead be shed and regrown when an animal’s body gets larger. This periodic shedding is called molting and occurs when an arthropod splits its exoskeleton and walks out of it to later form another. During this stage an arthropod is especially vulnerable to attack. The softshelled crabs that people eat and enjoy are actually hard-shelled crabs that are caught while molting.

INTERNAL STRUCTURE OF ARTHROPODS Inside their suit of armor, all arthropods are basically the same. All have a nervous system made up of a brain, simple eyes, and nerves that connect to a long nerve cord running the length of their body. This allows them to perceive and react to their environment. Arthropods that live on land breathe through a tracheal system rather than through lungs. This system consists of narrow, air-filled tubes in their outer skeletons called tracheae that branch into smaller tubes inside the body and directly supply each cell with the oxygen it needs. Water-living arthropods breath 42


with gills, located sometimes on their bodies and other times on their legs. Their circulatory system is an open one, meaning that blood flows freely throughout their body, pumped by a simple heart. Digestion always begins with a mouth and continues into a single long gut that runs the length of their body. Excretion of waste takes place from a separate opening. Most arthropods reproduce sexually (through the union of male sperm and female eggs), although a few species have both male and female organs. Most females lay their eggs in a protected place where they eventually develop into larvae.

Arthropod

TYPES OF ARTHROPODS Crustaceans. Arthropods are classified by biologists according to the number of legs, antennae, and body regions they have. There are therefore five main groupings: crustaceans, arachnids, insects, centipedes, and millipedes. A crustacean has compound eyes, several pairs of legs (four or more), and two pairs of antennae, with a body divided into two main parts. There are about 32,000 species of crustaceans (lobsters, crayfish, crabs) who get their name from the hard case or “crust” they wear. Crustaceans eat other invertebrates, almost always live in water, and vary greatly in size. A few crustaceans, like the wood louse (also called pill bug), live on land.

Arachnids. The arachnids, which include spiders, mites, ticks, scorpions, and horseshoe crabs, have only four pairs of walking legs and no compound eyes. Their bodies have two main regions. Most live on land and feed on insects and other small animals. Most are harmless although some species are poisonous. Insects. Insects are the most successful invertebrates on land and make up the largest class of arthropods. Well-known examples are bees, ants, grasshoppers, butterflies, moths, and the housefly. Insects live in nearly every habitat on Earth and total at least 800,000 species. All insects have a segmented body that is divided into three regions (head, thorax, and abdomen). The head contains the mouthparts and sense organs (often compound eyes); the thorax (the part of an arthropod’s body where the legs are attached) has three pairs of legs or one or two pairs of wings.

Centipedes and Millipedes. The last two arthropod groups, centipedes and millipedes, both have cylindrical segmented bodies with many joined legs and antennae. The main difference between the two is that centipedes are poisonous and have one pair of legs attached to each segment. Millipedes have two pairs on each body segment. Both live in dark, damp places, but centipedes capture and eat other invertebrates, while millipedes feed mainly on decaying plant material. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Arthropod

BENEFICIAL AND HARMFUL Arthropods are an extremely diverse group of invertebrates. They crawl, swim, run, and fly. They produce honey, silk, and other valuable products and provide the main meal for many of the fish that humans eat. They pollinate flowers and crops and recycle soil nutrients. Although beneficial, they can also be harmful. They can cause illness and even death to humans with their poison, and they can destroy our crops. Overall, however, arthropods are an integral part of many ecosystems (an area in which living things interact with each other and the environment), most of which would collapse without them. [See also Arachnids; Crustaceans; Insects]

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B Bacteria Bacteria are a group of one-celled organisms so small they can only be seen with a microscope. As the most abundant form of life on Earth, they live in every habitat—in the air, soil, and water, as well as within the bodies of living things. As the simplest of all organisms, they reproduce asexually by splitting in two. Although bacteria are often associated with disease, they perform an enormously useful function in the natural world since they are responsible for much of the decay of organic material. Bacteria were first discovered in 1683 by the Dutch microscopist, Anton van Leeuwenhoek (1632–1723), who called them “little animalcules.” However, it was not until the middle of the nineteenth century that biologists began to understand bacteria better. Although they were considered to be animals and then plants, bacteria eventually came to be placed in the Monera kingdom since they do not have a distinct nucleus (a cell’s control center). Members of the four other kingdoms (Protista, Fungi, Plantae, and Animalia) all are eukaryotic, meaning their cells have nuclei kept within a membrane. Most bacteria are single-celled and can be grouped according to three definite shapes. Rod-shaped bacteria are called “bacilli.” One species of bacillus causes the cattle disease anthrax. Spherical or round bacteria are known as “cocci.” Certain “cocci” can cause staph or strep infections. Bacteria that have a helical or coiled shape resembling corkscrews are known as “spirilla.” Spirilla are often carried by rats and can cause a form of rat-bite fever. Bacteria can also sometimes be in the shape of a fat comma, and these are called “vibroids.” A particularly nasty form of this bacteria causes cholera. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Bacteria

ROBERT KOCH German bacteriologist (one who specializes in the study of bacteria) Robert Koch (1843–1910) was the first to prove that a specific bacteria (a group of one-celled organisms so small they can only be seen with a microscope) can cause a given disease, thus helping to establish the germ theory of disease. As a pioneer in bacteria and cell culture, he also established the rules, called Koch’s postulates, upon which modern bacteriology is built. He won the 1905 Nobel Prize for Physiology and Medicine for his work on tuberculosis . Robert Koch was born near Hanover, Germany, and was one of thirteen children. Although his father wanted him to be a shoemaker, Koch was able to go to school and eventually received a medical degree in 1866. After serving his country as a surgeon in the Franco-Prussian War, he settled down as a country doctor in Wollstein in 1872. It was there that he began to study anthrax, a cattle disease, since an epidemic had struck the area. No one knew what caused this disease, and Koch set himself the task of finding out. What he would do however, would be not only to teach the world about anthrax, but help bacteriology to develop as a science as well. Koch first obtained the anthrax bacterium from the spleen of infected cattle and gave the disease to a mouse. He then transferred the infection from that mouse to another, and then to another, until he was sure that he could identify the particular rod-shaped bacterium (called a bacillus) that caused anthrax with absolute certainty. In order to do this, Koch had to essentially invent the techniques of studying microorganisms (organisms that can only be seen with a microscope). For example, he developed a technique in which he spread a liquid gelatin on glass slide plates, which enabled him to examine a pure culture and even to photograph it. What Koch also learned to do was how to cultivate bacteria, or allow it to grow and multiply, outside the living body. Koch used blood serum at body temperature

PASTEURIZATION KILLS BACTERIA Most scientists believe that bacteria were the earliest forms of life on Earth. Today, there are more than 10,000 species that have been identified. Bacteria can live in many different environments—even under extreme conditions such as lack of oxygen. Some live in or on other organisms, and many that grow inside the bodies of animals cause disease and are known commonly as “germs.” Examples of some of the more common infectious diseases caused by bacteria are strep throat, tuberculosis, typhoid fever, and tooth decay. Certain bacteria can also cause milk and wine to go bad. In the nineteenth century, the French chemist Louis Pasteur (1822–1895) discov46


to do this, and was therefore also able to trace the entire life cycle of the anthrax bacterium. The specific techniques that Koch pioneered would serve as a model for others to follow.

Bacteria

Before Koch, many others had put forth the germ theory of disease, arguing that certain diseases are caused and transmitted by specific microorganisms. However, until Koch, no one had been able to prove this theory. With his identification of the anthrax bacillus, Koch not only demonstrated that this theory was true, but he also established the rules for properly identifying the cause of a disease. Called “Koch’s postulates,” these rules guided many a researcher in the right direction, and they still hold true today. First, Koch determined that the suspected microorganism must be found in the infected animal. Second, after being cultured, or grown, it must be able to reinfect a healthy animal with the same disease. Third, the exact same microorganism must be found in the second animal that infected the first. Using his own techniques and rules, Koch isolated the cause of cholera epidemics in Egypt and India and discovered the tubercle bacillus that causes the lung disease, tuberculosis. Until Koch isolated this microorganism, science was baffled by tuberculosis, not knowing how or whether it actually spread or whether it was simply hereditary. Once he identified the bacillus, however, he was able to show that tuberculosis was caused by a germ that could be carried in the air and passed from one person to another. Until Koch had proven the germ theory of disease once and for all, the science of medicine could not really progress much beyond what it was centuries before when people believed in spontaneous generation (the idea that living things can come from nonliving matter, such as maggots from rotting meat). After Koch, the science of microbiology, which is the study of things that can only be seen with a microscope, could really begin to make progress.

ered that heat would kill bacteria. When he examined good wine, he noticed that it contained yeast cells that caused the process of fermentation and therefore produced alcohol. However, when he looked at sour wine under a microscope, he saw bacteria as well. His successful heat remedy to kill unwanted bacteria has come to be known as pasteurization. It was Pasteur who also discovered the bacterial origin of certain diseases like anthrax.

THE BENEFITS OF BACTERIA Many bacteria are harmless and essential to the well-being of certain ecosystems (an area in which living things interact with each other U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Bacteria

An illustration of the anatomy of a typical bacterium. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

Capsule

on the environment). Bacteria have always played a vital role in breaking down organic material after they are dead, and because of bacteria, nature is able to recycle that material’s basic chemicals back into the environment. It is bacteria that immediately break down an animal’s waste products. Without bacteria, the leaves that fall to the ground or the grass that is cut would never rot and get reused by nature. Just as important, the environment would be cluttered and smelly without bacteria decomposing organic matter. Bacteria even help animals digest the food they eat. Humans have mastered killing unwanted, disease-causing bacteria (usually with antibiotics) and have learned to use bacteria in many helpful ways. Some bacteria produce desirable chemicals such as ethyl alcohol and acetic acid, while others are used in the production of food products such as cheese, butter, coffee, wine, and cocoa. Bacteria are used in the manufacture of silk, cotton, and rubber, and help produce useful medical substances like insulin, antibiotics, and interferon. Certain bacteria help clean up oil spills in the ocean by breaking up the oil into its harmless components. Bacteria have little trouble reproducing, since each bacterium can do so on its own. A bacterium reproduces by a process Granules called “binary fission,” meaning that Cell it splits in two, making a pair of membrane identical cells. Under proper condiCytoplasm tions, bacteria do this about every Nuclear twenty minutes. material

Pili

Mesosome

Ribosomes Cell wall Flagellum

While certain bacteria are responsible for human misery and even death, others are absolutely essential to the mechanisms of the natural world. Therefore, much of the proper functioning of nature is attributable to vastly abundant but virtually invisible organisms called bacteria. [See also Antibiotics; Fermentation]

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Biodiversity

Biodiversity

Biodiversity, or biological diversity, is a broad term that includes all forms of life and the ecological systems in which they live. Biodiversity also refers to the degree of variety found in life and ecosystems (an area in which living things interact with each other and the environment). It suggests a level of healthy or balanced diversity of habitat, as well as animal and plant life. Biodiversity has also been described as the total richness of biological variety in a given place and can serve as a measure of the number of different types of organisms that live there. The opposite of biodiversity is the extinction of a species.

EDWARD WILSON COINS THE TERM BIODIVERSITY As first put forth by the American biologist, Edward O. Wilson (1929–), biodiversity has become a standard against which scientists can measure the negative impact or damage that modern human societies have inflicted upon Earth and its creatures. The growing threat to biodiversity posed by human activities makes it one of the most important aspects of the present global environmental crisis that some scientists say we are experiencing. Humans have already caused permanent losses of biodiversity through extinction of certain species. Often this is caused indirectly by destroying a particularly distinctive natural environment in which these species lived. Some ecologists predict that unless there are major changes in the ways that humans affect ecosystems, there will be much larger losses of biodiversity in the near future. In the past, humans have hunted some creatures like the dodo bird into extinction, and sometimes simply crowded others out of their natural habitats. It is known that some 200 species of plants native to the United States already have become extinct, and that as much as 20 percent of all bird species around the world have gone extinct. Wilson estimates that at least 27,000 species of plants and animals are becoming extinct every year by cutting down the rain forests in Brazil’s Amazon region. Since extinction is the permanent death of a species, the world will lose an enormous amount of biodiversity as the years pass. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

Edward O. Wilson brought the world’s attention to biodiversity and how human actions are causing its losses through extinction. (Reproduced by permission of Edward O. Wilson.)

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Biodiversity

EDWARD OSBORNE WILSON Founder of the modern field of sociobiology (the study of what biologically determines social behavior) and the only person to receive both this nation’s highest scientific and literary awards, American entomologist Edward O. Wilson (1929– )is one of the great naturalists in American history. His work has influenced the field of animal taxonomy (the science of classifying living things), and the discovery of pheromones (chemicals that are released by an animal and which affect another animal). He also has become a powerful voice condemning the steady loss of biodiversity (a broad term that includes all forms of life and the ecological systems in which they live). Edward O. Wilson was born in Birmingham, Alabama, and is a descendant of farmers and shipowners. Always drawn to the outdoors, he decided at the age of seven to become a naturalist explorer. His plans changed, however, when he suffered an injury to his right eye while fishing, which made him change his plans. By the age of ten, after reading a National Geographic article titled, “Stalking Ants, Savage and Civilized,” Wilson decided to become an entomologist. (Entomology is a branch of zoology that deals with insects.) After studying biology at the University of Alabama, he received his bachelor’s degree in 1949. By the age of twenty-six, he had earned a Ph.D. in biology from Harvard University, where he continues to work. His early work led taxonomists (classifiers of organisms) to revise their procedures, and by the late 1950s, Wilson had begun to study how social insects, like ants, communicate. His work with the venom glands of fire ants led to his discovery that animal chemicals called pheromones are used to communicate complex instructions. He later wrote that pheromones were “not just a guidepost, but the entire message.” In the 1960s, he offered the theory of species equilibrium, which demonstrates that the number of species on a small island would always remain constant.

To biologists, biodiversity is not just a blanket term for the natural biological wealth found on Earth or a description of a healthy and richly varied ecosystem, but it is something that is good biologically. In other words, biodiversity is something to value, to maintain, and to try to protect. Both the term “biodiversity” and the concept were born in 1986 during the National Forum on Biodiversity, held in Washington, D.C. and sponsored by the National Academy of Sciences and the Smithsonian Institution. Most attribute both the word and the concept to Wilson. In his extensive writings, Wilson argues that the notion of biodiversity has a value of its own whether or not it is related to humans in any way. This argument brings up the ethical question of whether mankind has the “right” to do whatever it wants with Earth and its creatures—even to the 50


In the 1970s, Wilson’s work on the biological basis of social behavior caused a major controversy. In his 1975 book, Sociobiology: The New Synthesis, Wilson suggested that the way people act or behave in certain situations can sometimes be accounted for by their genetic makeup. This daring idea caused a great stir among many colleagues who argued that similar ideas had led to notions that people were “biologically determined” to act a certain way and were therefore not responsible. Wilson’s critics also said that such an idea could lead to some of the policies followed by Nazi Germany during World War II (1939–45) when it claimed that certain races were genetically “inferior” to others. In 1978, Wilson defended his theory with his book On Human Nature in which he more fully explained his views. In this book and others, Wilson made a good case for not ignoring the role of biology in trying to understand human behavior.

Biodiversity

In the 1990s, Wilson also became closely identified with the notion of biodiversity. Technically, this word means the variety of life or the biological diversity of species in an area. However, biodiversity also has come to mean the richness and balance of an ecological system. It does not necessarily mean huge numbers of life forms, but rather suggests equal numbers of individuals of different species. Biodiversity not only has commercial value to people but, argues Wilson, its continued loss is a signal of the worsening health of the environment. Wilson is very concerned with trying to minimize what he sees as mass extinctions that are happening in the modern world. As huge areas of rain forest are destroyed each year, species are going extinct in record numbers as they lose the habitat that support them. As people continue to drive plant and animal species to extinction, they also are killing the spirit and the beauty of the natural world. Today, Wilson is doing his best to prevent this from happening.

point of destroying them. On a more practical level, Wilson says that preserving biodiversity has very specific benefits to humans, as illustrated by the fact that in the United States, one-fourth of our prescription drugs have active ingredients obtained from plants. Wilson gives two dramatic examples. The first is the rosy periwinkle plant that became the source of two of the most effective anticancer agents ever discovered. The other is cyclosporin, an obscure fungus that lives only in Norway. This fungus became a powerful immunosuppressant drug and is entirely responsible for our ability to do organ transplants. More generally, a biologically diverse planet is simply more liveable and functional since nature, if properly balanced, keeps water and air cleaner, recycles nutrients, removes waste, and even controls erosion. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Biological Community

Biodiversity is a very broad term. Overall, there are three general kinds of biodiversity: ecosystem or habitat diversity, species diversity, and genetic diversity. The diversity of an ecosystem refers to the variety of actual ecosystems or places where organisms live in a certain region. Thus a region with forests, streams, ponds, grasslands, and farmland is more biologically diverse than one that has only open agricultural fields. It goes without saying that when an ecosystem or habitat disappears, a great number of species disappear as well. Species diversity refers to the ability of many different types of organisms to exist at a certain time. Finally, genetic diversity refers to the level of variability that is found among individual members of a single species. Variability, or variation, refers to the natural differences between living things, and it is a very important phenomenon since it allows a species to adapt to environmental changes over time through evolution. The threat to our planet’s biodiversity is different from most other ecological problems in that it is irreversible. Once a certain species has gone extinct it has disappeared forever and its entire heritage that took millions of years to evolve is lost. Since scientists admit that the vast majority of species have not yet even been identified, humans may be destroying species about which nothing in known, and therefore never will. Finally, by trying to solve some of the problems associated with threats to biodiversity, people gain new knowledge about managing the Earth and using its natural products for the benefit of all.

Biological Community A biological community is a collection of all the different living things found in the same geographic area. The community may be small or large, but it always consists of different types of living things interacting with one another in the same particular area or habitat. A biological community is held together by the relationships among its members. Biologists can study the natural world from many different ecological perspectives. That is, they can study life or living things according to the size and type of certain groupings in which life is found. At the species level, biologists study the same type of organisms that are capable of mating and producing offspring together. For example, one species of bird is a cardinal, another species is a blue jay. They do not breed with each other so they are different species. One level up from species is a population. A population is made up of all the members of a species who live in a given area at the same time. All the different populations living and 52


interacting within a particular geographic area make up a biological (or biotic) community. To keep going up the scale, the living organisms in a community, together with their nonliving (or abiotic) environment, make up an ecosystem. At this point, in theory, an ecosystem (and the biological community that makes up its living component) can be as small as a few mosquito larvae living in a rain puddle or as large as a prairie stretching hundreds of miles. A very large ecosystem is called a biome.

Biological Community

A biological community is made up of all the populations living in an area. As a community, these populations live together and interact with one another in many ways. In fact, relationships are what biological communities are all about. Some populations eat food that is produced by another. Others get their homes from other populations. Competition is also a way of interacting, and populations may compete with one another for food and shelter. Biological communities are thus tied together by a food chain or food web (consisting of who eats what). When studying a biological community of any size, biologists use certain categories to describe these communities. Communities may have populations that interact in antagonistic ways, meaning that the relationship is detrimental or harmful to one or both species. They may have commensal relationships, meaning one species benefits while the other is unaffected. Or they may have mutualistic or cooperative relationships in which both species benefit. Not surprisingly, in the real world of biological communities, these relationships sometimes shift. Biologists also take note of productivity and trophic levels in a biological community. Productivity describes the amount of biomass (or living matter) produced by green plants as they capture sunlight and create new organic compounds. A tropical rain forest will have a very high rate of productivity compared to that of a desert. Trophic levels describe the level or position of a given species on the food chain. Since a hawk eats mice, which eats plants, the hawk is at a higher trophic level than the mouse. Interestingly, the same species can occupy different trophic levels in different food chains found in different biological communities. Therefore in a food chain containing a killer whale, a leopard seal, and an emperor penguin, the leopard seal is a tertiary consumer, or at the third trophic level, since it eats the penguin, which eats fish, which eats krill. However, in a chain consisting of a killer whale, a leopard seal, and an adelie penguin, the seal is at the second level since it eats the adelie penguin, which is a primary consumer because it only feeds on krill. The technical term “abundance” refers to the total number of organisms in a biological community, while the expression “diversity” is a measure of the number of different species in a given community. “ComU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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plexity” is the term used to describe the variety or number of different ecological niches in the community. A niche is the role a living thing plays in its environment. Niche also refers to the place an organism can fit into, or the way it makes its living. Overall, the concept of a biological community is a practical and very useful scientific way to organize, categorize, and learn more about the dynamic goings-on in a particular habitat. [See also Abiotic/Biotic Environment; Food Web/Food Chain; Niche]

Biology Biology is the study of life or living things. Biology includes a huge range of subjects, all of which are based on studying the way living things work and interact with everything around them. Since biology is concerned solely with living things, it is important to know what the characteristics of being alive are. All living things show four main characteristics. First, they have metabolic processes, which means they conduct some sort of chemical reactions to take in nutrition, process it, and eliminate waste. Second, they have generative processes, which means they are able to grow and to reproduce. Third, they have responsive processes, which means they react to stimuli and can adapt to changing conditions. Fourth, they have control processes, which means they can coordinate their metabolic processes in the right order and can regulate them as well. If something demonstrates every one of these characteristics, we can say that it is alive or that it is an organism. An organism is, therefore, any single living thing that demonstrates the characteristics of life. Organisms or living things can be studied from many different levels or aspects. At the molecular level (a molecule being a chemical unit made up of two or more atoms linked together), biologists study the complex of chemicals that work together in living things. A molecular biologist would study such molecules as proteins and nucleic acids and try to discover exactly how they work in a living thing. In terms of how living things are constructed and function, all living things are made up of cells. Since the cell is the basic unit or building block of all living things, cell biology is the next level of biological study. Some organisms are made up of a single cell, while others are composed of trillions. The next structural and functional level is that of tissues and organs, and beyond that is the complete organism itself. At this point, biologists often focus on one particular group of living thing, such as plants (botany), animals (zool54


ogy), or a certain type of animal, like insects (entomology). Just as all living things have the same characteristics listed above, so all are governed by the same, few biological principles. One of these is the notion of homeostasis. The word “homeostasis” means “staying the same,” and all living things need to stay the same or maintain a constant internal environment. This idea was first suggested by the French physiologist, Claude Bernard (1813–1878), who showed that an organism has control systems that enable it to keep its metabolism (internal chemistry) within certain limits, especially when things around it are always changing. Another biological principle is that all living things are made of the same materials, share the same functions, and have a common origin. Moreover, all follow the laws of heredity and possess genes that are the basic unit of inheritance. This leads to the principle of evolution by natural selection, which explains how life evolves and becomes different over time. The fact that life is very different despite its common origins leads to another principle—diversity. Altogether, biology or the study of life can examine its subject from a highly focused and specialized point of view or from the larger viewpoint of what all living things have in common. At the same time, it can be highly theoretical (such as plant taxonomy, which is the science of classifying or naming plants), or it can be very practical (such as plant breeding or wildlife management). In the future, biology will have to cope with and try to solve some of the more important twenty-first-century issues. These involve problems related to increasing human populations and the need for increased food production. Biology is also the foundation of all medical advances, and this century will most likely focus on the genetic aspects of diseases. Finally, biology will have to face the pressing ecological problems that a growing, highly mechanized world creates. As a result of these issues, many scientists feel that the twenty-first century will necessarily be the biological century.

Biome

[See also Botany; Ecology; Evolution; Genetics; Physiology]

Biome A biome is a large geographical area characterized by a distinct climate and soil as well as particular kinds of plants and animals. Biologists have divided the globe into six main biomes—which they also call major life zones—and have named them after the dominant type of vegetation that grows there. These six terrestrial (or land-based) biomes are the rain forest, tundra, taiga, temperate deciduous forest, desert, and grasslands. There are also three other biomes for the aquatic environment: freshwater, saltwater, and estuaries. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Biome

Biomes can be described as environments that have a lot of things in common, such as climate, vegetation, and animal life. Grouping the world’s environments into separate biomes helps biologists to better understand them and to organize what has been discovered about them. Each biome is home to certain groups or communities of plants and animals that have adapted to its particular temperature and precipitation (amount of rainfall). Since a biome is necessarily a large geographic area, it usually supports a wide variety of life within the same major zone. A biome can be made up of different habitats or places where organisms live; altogether, the biomes of Earth make up the biosphere, which is the region of Earth that support life.

RAIN FOREST BIOMES Among the six major land biomes, the biome known as the rain forest supports more species than any other, containing about half of the world’s plant and animal species. Rain forests are commonly found in tropical areas close to the equator in areas of very heavy rainfall and constantly warm temperatures. They are considered one of the most biologically diverse ecosystems (an area in which living things interact with each other and the environment) on Earth. In a typical rain forest, almost every plant has another plant growing on it and the tallest trees create a canopy or rooflike effect. Actually, there are four other separate layers or levels or vegetation beneath this canopy (including lianas) that can be as thin as a rope or as thick as a tree trunk. Other plants called epiphytes (like ferns and bromeliads) grow in clumps on tree branches, and at ground level there is little vegetation since little sunlight can penetrate to the forest floor. High temperatures, constant moisture, and high humidity cause anything dead to rot very quickly. There are few ground-dwelling animals other than ants and termites; and most birds, mammals, reptiles, and amphibians live somewhere in the multilevel canopy. Fifty-seven percent of all tropical rain forests are located in South America where they are steadily being destroyed. Increasing population pressure has resulted in vast expanses of these forests being cleared for farming, only to prove unsuitable for agriculture. Constant rains quickly wash away the soil’s nutrients (which were contained in a thin layer of humus), and heavy fertilizers must be added regularly. When the habitat is destroyed in this fashion, the variety of life that used to thrive in these areas disappears as well.

TUNDRA BIOMES No environment on Earth is more different from a rain forest than the biome known as tundra. It is the coldest of all terrestrial biomes. Char56


acterized by extremely long, severe winters and short, cool summers, tundra has been described as a polar desert. What makes it unique is the existence of “permafrost”—a condition in which the subsoil, or Earth below the top few feet of soil, remains permanently frozen. The small amount of precipitation this area does receive is trapped at the surface by the permafrost below and forms bogs in the summer. Since the subsoil never thaws, no trees or shrubs can survive, and the vegetation that manages to live there is dominated by mosses, lichens, and other plants that do not grow very large and which can reproduce very quickly during the short growing season. Few animals live on the tundra, although the Snowy owl and Arctic hare are exceptions. There are, however, vast swarms of flies and mosquitoes that appear when the few herbs and grasses flower and attract migratory birds. Although the tundra is a relatively simple environment, it is easily damaged and very slow to recover. The northern parts of Canada are composed primarily of tundra.

Biome

TAIGA BIOMES Taiga, also called the coniferous or evergreen forest biome, supports a diverse and complex community of organisms. Although its winters can be very cold, its summers are longer than those of the tundra, allowing its soil to thaw completely. The taiga is primarily an environment where evergreen trees flourish since they are specially adapted to survive the long, cold winters. The evergreens’ needle-shaped leaves prevent water loss during the harsh winter months and their flexible branches allow heavy snow to slide off without breaking. The ground below them has little vegetation and is home to mice, squirrels, porcupines, grouse, warbler birds, wolves, and moose. This biome is particularly common in southern Canada and is particularly susceptible to the effects of acid rain.

TEMPERATE DECIDUOUS FOREST BIOMES The temperate deciduous forest biome has what we consider to be normal, well-defined seasons and is named after its most common feature—deciduous trees or trees that drop their leaves in the autumn (like oak, maple, hickory, and ash). Most of the eastern part of the United States is typical of this biome, and it is characterized by a moderate climate and fairly high rainfall. Although it is a single biome, temperate deciduous forests can be quite different according to their specific location. All support a wide variety of animal life. Insects are often abundant and some birds live there year-round, despite its winters. In this biome, leaves that fall from deciduous trees decay and result in a deep, rich humus layer that provides nutrients and conserves water. While many different mammals U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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were once present in great numbers in this biome, today they have been reduced substantially by humans. Still, there are many raccoons, deer, moose, bobcats, a large variety of birds, and invertebrates.

DESERT BIOMES In those parts of the world where rainfall is very low and irregular, a desert biome can be found. A desert is not only very dry, with a total rainfall between 1 to 10 inches (2.54 to 25.38 centimeters) a year, but it is also usually very hot. While most would guess that the average temperature of a desert is extremely high, it actually fluctuates or changes dramatically. At night the land cools rapidly because there is seldom any cloud cover to insulate and keep the desert heat from radiating off into space. In a desert like the Kalahari in Africa, this fluctuation can go from well over 100 °F (37.78 °C) to just above freezing. That is why a desert biome is characterized primarily by its low rainfall and not its temperature. Plants that grow in a desert are highly specialized and able to gather and store water in many different ways. For example, the leaves on a cactus became spines, while other cactus have extremely deep and spreading root systems. Despite this harsh environment, many other plants and animals live in deserts, although the desert reptiles, small mammals, and spiders and scorpions usually spend the day in burrows or shaded areas and become active at night (meaning they are nocturnal). Some animals, like the camel, have also adapted to the lack of moisture in interesting ways. Contrary to what most people believe however, the camel does not store water in its hump. Rather, it stores energy in the form of fat, and by concentrating all of its fat in one part of its body, it is allows its body heat to escape more easily from the rest of its body. Camels also can allow their body temperature to fluctuate more when water is scarce.

GRASSLAND BIOMES Certain parts of the world are too rainy for a desert but not wet enough to support a real forest. These areas are called a grassland biome. Grasslands (also called savannahs) have hot summers and cold winters and receive between 15 and 30 inches (38.1 and 76.2 centimeters) of rain a year. Abundant grasses replace trees (that seldom grow in these fertile places). If the soil is deep and rich, tallgrass grows, while shortgrass grows in thinner soil. In the United States, grasslands are called prairies; in South America they are the pampas; in Asia they are called steppes; and in South Africa they are known as veldt. Whatever the actual name, these flatlands are covered by fast-growing grasses that are enormously efficient at converting sunlight into plant tissue. Animals common to grasslands are the 58


prairie dog, coyote, grasshopper, rattlesnake, pronghorn antelope, and meadowlark. Grasslands often catch fire during dry times, but the roots usually remain alive allowing grass to regrow there. Grasslands are amazingly fertile and can support regular farming if treated properly, but they also turn into dustbowls with wrong management.

Biosphere

FRESHWATER, SALTWATER, AND ESTUARY BIOMES Earth’s water systems have their own types of biomes divided simply between freshwater, saltwater, and estuary biomes. Freshwater biomes either move (streams and rivers) or stand (lakes and ponds), and the types of fish and other organisms that live in them are determined by temperature and other variables like oxygen and minerals. The marine, or saltwater, biome is the largest of all the world’s biomes, covering nearly two-thirds of the planet’s surface. With a salt content of between 3.0 and 3.7 percent, saltwater biomes support fish and other organisms that have adapted to this specialized environment. An estuary is a partially closedoff part of the sea that is fed by fresh water. These include bays and tidal marsh inlets. Since the salt content varies greatly, relatively few species can tolerate estuaries but those that do are extremely productive. Because estuaries are fairly shallow, sunlight can penetrate them completely making them ideal habitats for algae, grasses, oysters, and barnacles. Human influence has altered the world’s biomes. Some biomes no longer have the diversity of life that they once contained. The quantity and diversity of mammals found in North America today can in no way compare to those that existed there some five hundred years ago. Nearly all of the original forest trees are also gone. Human activity is thought to result in a global temperature rise, known as the greenhouse effect. This rise in temperature could also alter the nature of each biome causing the extinction of plants and animals. [See also Chaparral; Desert; Ecosystem; Forests; Grasslands; Ocean; Rain Forest; Taiga; Tundra; Wetlands]

Biosphere The biosphere is that part of Earth that contains life. This worldwide ecosystem ( a area in which living things interact with each other and the environment) is made up of the land, water, and atmosphere that support life, and includes every part of Earth where life exists. Its many parts are linked together by nutrient cycles. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Biosphere

The biosphere can be described as all of the world’s ecosystems, or the worldwide ecosystem. As such, it consists of Earth’s lithosphere, atmosphere, and hydrosphere. The lithosphere includes all of Earth’s surface layers of solid substances like soil and rocks. The atmosphere is the envelope of gases or air that completely surround the planet. The hydrosphere includes all of the lakes, rivers, and oceans on its surface. All of the elements and forces in these three spheres are constantly part of the many chemical, biological, and physical processes that make the entire “bio-sphere” or “life-sphere” what it is. To date, no other planet besides Earth has been discovered that contains a biosphere or living world.

EDUARD SUESS COINS THE TERM BIOSPHERE The term biosphere was first used by the Austrian geologist Eduard Suess (1831–1914) in 1875 to describe that part of the Earth that contains life. This concept had little impact on the scientific community until it was discussed by the Russian mineralogist, Vladimir I. Vernadsky (1863–1945) in his 1926 book. Vernadsky argued that in order to study the biosphere, scientists from many fields like geology, chemistry, and biology had to work together. Earth’s biosphere can be considered to be thousands of feet thick— from the bottom of the oceans to about 30,000 feet (9.144 kilometers) above sea level. However, given the size of the entire Earth, this is a fairly thin layer. In fact, most of the life that does exist within the biosphere can be found in the narrow band just below sea level to about 20,000 feet (6.096 kilometers) up. That height is about the limit for animals and most plants to live. Scientists consider Earth’s biosphere a closed system in which the only thing ever added is sunlight. Living things in the biosphere need energy and nutrients and an appropriate environment in order to live and reproduce. The Sun is the ultimate energy source for the biosphere, and its light energy is captured by green plants that use the process of photosynthesis to change light energy into chemical energy. This energy then passes through the biosphere from plants to other organisms and then to others again. It is eventually either lost in the form of heat to the environment or stored for a long time in organic molecules, such as carbon atoms are locked in coal underground. The biosphere can, therefore, be seen as a group of tightly interconnected recycling systems and subsystems that affect and influence each other.

THE BIOSPHERE CONSIDERED AS A GLOBAL CONCEPT The global concept of the biosphere has become increasingly useful and important as technology allows the world to be considered in a truly 60


global way. Spacecraft can not only provide images of the entire globe, but also can monitor the many systems that make up Earth’s total environment. Today, therefore, the term biosphere is used most often when discussing the health of Earth. Current research focuses on the effects that human activities have on the health of the global environment (the biosphere). The pressures and demands of increasing populations have resulted in two major threats to the biosphere’s well-being: the loss of natural resources and the effects of pollution. As more and more of the natural environment is destroyed, such as the continuing destruction of tropical rain forests, many scientists fear that the balance of nature may be so upset that it may permanently harm or change the biosphere. Many also fear that the pollution by-products of everyday modern life could become too great for the biosphere to bear and may permanently damage its systems. Activities such as fuel consumption (burning coal, oil, and gas) and the use of fertilizers could increase the levels of carbon dioxide, nitrogen, and phosphorous in the atmosphere and seriously alter the natural balance of the life-sustaining biosphere.

Birds

The concept of the biosphere is also at the center of a scientific debate as to whether Earth and its biosphere should be considered a living organism with self-regulating mechanisms. Called the “Gaia hypothesis,” this idea was first put forth in the late 1970s by the British scientist, James Lovelock (1919– ), who argued that the biosphere is able to create and maintain the environment that most favors its own stability—as long as it is left alone. Lovelock argued that by tampering with and sometimes altering Earth’s environmental balancing systems, humans are placing themselves and their planet at risk. He and his followers argue that such phenomena as global warming, or the greenhouse effect (caused by too much carbon dioxide in the atmosphere), and ozone depletion from pollution (the resulting decrease in protection from harmful ultraviolet radiation from the Sun, are evidence of a growing risk. Whether the Gaia hypothesis is right or wrong, it has been very useful in generating concern and research about people’s influence on the worldwide ecosystem known as the biosphere. [See also Photosynthesis; Respiration]

Birds A bird is a warm-blooded vertebrate (an animal with a backbone) that has feathers, a beak, and two wings. Its most unique feature is the ability to fly, although not every bird is able to fly. All birds hatch from eggs and U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Birds

JOHN JAMES AUDUBON American ornithologist (a person specializing in the study of birds) John James Audubon (1785–1851) was the most famous artist and naturalist in nineteenth-century America. He not only kept finely detailed studies of birds, but produced the first modern atlas of ornithology. This atlas is also considered to be one of the most beautiful natural history books ever made. John James Audubon was born in what is now Haiti, the son of a French sea captain who owned a plantation on that island. After spending his first few years there, he moved to Paris, France, and studied painting for a short time. Audubon was eventually sent to America when he was eighteen years old in order to avoid being inducted into the army of Napoleon. Throughout his boyhood, he continuously collected and sketched birds, plants, and insects. He also developed the habit of keeping detailed notes of whatever he observed. At his father’s estate at Mill Grove, near Philadelphia, Pennsylvania, he conducted what appear to be the first known bird-banding experiments in North America. In order to learn more about the movements and habits of a bird called the Eastern Phoebe, he caught them, tied bits of colored string on their legs, and was able to prove that they returned to the same nesting sites the following year. After Audubon married Lucy Blakewell in 1808, he tried to become a storekeeper, but found himself unable to stay indoors long enough to run a store. As a natural-born outdoorsman, he was unable to make himself stay away from studying the things he loved. After his business failed, and he went bankrupt (and even spent some time in jail), he decided to pursue what he really loved and dedicated himself to what he now called his “great idea.” He would travel throughout America and draw, in life-size scale, every bird on the continent. Thus, at the age of thirty-five, Audubon set off, with his wife’s blessing, to pursue his ornithological dream. Audubon’s wife worked as a governess and teacher to help support him, although he would sometimes paint portraits and street signs when they needed money. For five years Audubon traveled the American wilderness painting its bird life. Where most bird painters before him had worked from long-dead, stuffed

their bodies have evolved a wide range of adaptations that enable them to fly. Birds are found in nearly all parts of the world.

CHARACTERISTICS OF BIRDS From the soaring eagle to the flightless penguin, from the clumsy loon to the graceful swan, birds come in the widest variety imaginable. All have several characteristic features that distinguish them from other ani62


specimens and drew them in a not very true-to-life way, Audubon pioneered the use of fresh models. He would shoot a bird and wire into a life-like pose in order to try and capture it accurately. It must be remembered that in the early nineteenth century, little thought was ever given to the conservation of birds or any other animals, and it was common for Audubon to shoot several of the same species until he found one that he considered perfect for painting. From the beginning, Audubon’s “great idea” ambitiously included the goal of painting every bird in its real, life-size dimensions. He was able to do this for most birds by having a single book page measure more than 3.5 feet (1.07 meters) by 2.5 feet (0.76 meters). This was larger than any book published to that time. For the very large birds like the whooping crane, he would paint them life-size but have their heads bending toward the ground, so that the drawing would fit on a page.

Birds

By 1825, Audubon had compiled a spectacular set of bird paintings, but when he could interest no American publisher in his work, he went to England. There his work was recognized, and he began to enlist “subscribers,” either individuals or institutions, who agreed to buy the book when published. The actual production of his great work took many years since the plates (a full-page book illustration) were all hand-colored. The first volume of Birds of America appeared in 1827, and the fourth and last volume was published in 1838. Altogether, the books contain 435 hand-colored pictures. Later, Audubon published a five-volume work of bird descriptions to accompany the illustrated work. The production of Birds of America was extremely expensive, and a subscriber paid approximately $1,000 for the set, an extremely high price in the early nineteenth century. Despite the fact that Audubon probably killed more birds than most anyone of his time, he had dedicated his life to capturing their beauty and the essence of their liveliness, and he truly must have also loved birds more than anyone of his time. Today, his name is linked to that of a modern conservation organization, the National Audubon Society, and there are many local Audubon societies throughout America dedicated to learning about and conserving birds.

mals. Only birds have feathers. While other types of animals may have hair or scales or a shell covering their bodies, feathers are unique to birds. As with just about everything in the way they have evolved, a bird’s overlapping feathers give it the ability to fly. The most important factor in flight, whether it is a bird or an airplane, is weight. The heavier something weighs, the more energy is required to get it off the ground and keep it in the air. Feathers are therefore, by design, a lightweight covering for a bird. Like human hair or a bull’s horn, feathers are made of a U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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protein called keratin. Although feathers vary in size, shape, and texture, they share a basic frame or structure: a base like a tube that goes up into a central shaft that itself branches into vanes. A bird actually has several different types of feathers. Short, fluffy, down feathers that are closest to the skin serve as insulation. Powder feathers produce a type of powder that makes a bird waterproof. Contour feathers give the bird a streamlined shape. Flight feathers are strong and provide lift much like an airplane wing. Tail feathers are stiff and provide steering, balance, and control. Altogether, a bird’s feathers are the perfect covering for its wings. Birds are able to fly because when air passes over their wings, they receive an upward push or lift. Feathers also act as a good insulator and keep a bird warm by helping it retain its body heat. All birds have two feet and one beak, although there can be some very dramatic differences. The shape and structure of both beaks and feet give a good indication of what a bird eats and what type of habitat it lives in. The long, tubelike beak of a hummingbird is adapted for sucking nectar from a flower, while a bird of prey like an eagle has a sharp, hooked beak for holding and tearing its meal. Finches have short beaks that are thick and strong for cracking open seeds, and a typical marsh bird has an upward-curved, spoonlike bill for sifting water. Ducks have webbed feet for swimming atop water, while hunters and meat-eaters like hawks have sharp talons that grip and kill their prey. Other birds simply have toes that enable them to perch on branches. Most birds have four toes—three in front and one in back. Finally, all birds are endothermic or warm-blooded and hatch from eggs. Since birds are warm-blooded, meaning that they maintain a constant internal body temperature despite the temperature of their environment, they are able to live in a wide variety of climates and environments. Unlike cold-blooded animals, they do not have to slow down their activity when the temperature drops. Compared to almost any other animal, birds use up an enormous amount of energy and have a very high rate of metabolism (which could be described as the rate at which their internal motor is running). This is because flying is such a high-energy activity. Since they have a high-energy lifestyle, birds have a high-energy diet (such as seeds, worms, fruit, meat, fish) and do not eat many low-energy foods like grasses or leaves. Birds reproduce sexually and lay eggs (one at a time) from which their young hatch. Birds don’t give birth to live young since weight is so critical, and flying while carrying a developing embryo inside would probably be impossible for a female. Bird eggs contain a yolk that feeds the embryo. They are covered by a hard shell, and one or both parents must

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keep them warm with their own bodies for them to develop and hatch. At birth, many birds are helpless and must be fed and cared for. While they may learn certain things from the behavior of their parents, important activities like flying and migrating are instinctive.

Birds

BIRDS BODIES ADAPTED TO FLIGHT As a bird’s feathers are perfectly adapted to flight, so too is the rest of its body built to give it the two things it needs to fly: low weight and high power. Compared to almost any other animal, a bird’s skeleton makes up only a small percentage of its total weight. This is because its bones are hollow, or in fact, filled with tiny air spaces. Despite this, the bird has a very strong frame since many bones that are separate in other animals are fused in birds. Other systems conserve weight as well. A bird has no separate bladder (which would fill and add weight), but rather passes its nitrogenous waste along with its intestinal waste in a single pastelike form called bird droppings. Birds get their flying power from powerful breast muscles, and are able to feed these important muscles all the oxygen they need because of their highly efficient respiratory systems (which can also store air). Birds, like mammals, also have a fourchambered heart which keeps oxygen-rich blood from the lungs separate from the blood that is returning from the rest of its body. This guarantees that the muscles get as much oxygen as they need. Birds also have an above-average nervous system since they must perform highly coordinated movements in order to fly. They also have very sharp eyesight and good hearing. As with insects, flight provides birds with a competitive advantage. They are able to escape quickly from an earthbound predator who cannot pursue them. Birds are also able to move to another habitat if food becomes scarce. Many birds do this annually as a regular part of their life cycle in a process called migration. Some birds make this an entire way of life, migrating as much as 25,000 miles (40,225 kilometers) in one year.

THE IMPORTANCE OF BIRDS Birds play an important role in the ecosystem (an area in which living things interact with each other and the environment). They not only provide people with food (like chicken), but they consume an enormous quantity of insects and play a major role in pollination (the transfer of male pollen to the female part of a flower). Finally, birds make sounds that people find pleasant. Bird sounds are both calls and song, with the former used to communicate with others, and the latter used mainly to attract a mate. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Blood

Blood Blood is a complex liquid that circulates throughout an animal’s body and keeps the body’s cells alive. Blood transports oxygen and nutrients, carries away waste, and helps fight germs that invade the body. Each different function of the blood is carried out by a different type of blood cell. The importance of blood to life was recognized by primitive humans, but they had no understanding of why it was so. They did not know that the cells in an animal’s body are very specialized and that these cells cannot get their own food or dispose of their waste. Primitive humans also did not know that it is the blood that carries oxygen and nutrients to the cells, while also carrying away waste products, and that blood plays a major role in fighting disease by defending the body from invasions of microorganisms and parasites. These ancient humans believed that blood had mystical properties, and in many ways they were right since blood has the ability to sustain life.

SCIENCE LEARNS TO UNDERSTAND BLOOD The first real scientific contribution that concerned blood was made by the English physician, William Harvey (1578–1657). Harvey made the major discovery that blood circulates constantly throughout the body in a one-way direction. Harvey said the heart was a pump which pushed the blood through a one-way, closed-loop circulation, and he was able to demonstrate this by tying off arteries and veins. He proved that blood did not wash back and forth like the ocean tides, but instead kept constantly moving in the same direction. Once this mechanical aspect of blood circulation was understood, science moved toward an understanding of the composition and actual function of blood. After the seventeenth-century invention of the microscope, the Dutch naturalist Jan Swammerdam (1637–1680) discovered the red blood cell in frog’s blood. In 1673 and 1674, his countryman, Anton van Leeuwenhoek (1632–1723), was the first to see and describe red blood cells in human blood. In the next century, white blood cells (known as leukocytes) were first seen, and finally platelets (tiny fragments that cause clotting) were first observed in 1842.

PARTS OF BLOOD By the start of the twentieth century, the composition of blood was known. It was found to be composed of red blood cells, white blood cells, 66


platelets, and plasma. Besides knowing what blood was made of, scientists also were beginning to understand each parts real function.

Blood

Red Blood Cells. Red blood cells are called erythrocytes, from the Greek erythros, meaning red. These cells collect oxygen from the lungs and carry it to all the cells in the body. Under a microscope they look like flattened disks, and they do not contain a nucleus (the control center for most of a cell’s activities). Erythrocytes get their distinctive red color from a pigment called hemoglobin. Hemoglobin is an iron-containing protein that does the actual oxygen-carrying for the blood. People who have a diet low in iron may not have enough hemoglobin, and they would tend to tire easily since their blood does not carry enough oxygen. Hemoglobin also carries carbon dioxide away from the cells to be disposed by the lungs. Red blood cells are made in the bone marrow (a soft, fatty tissues that fills most of the bone cavities and is the source of red blood cells and many white blood cells) and live for about four months.

White Blood Cells. White blood cells are called leukocytes and form part of the body’s defense against invasion. They are larger than red blood cells, and as part of the body’s immune system, their job is to fight infection. They do this by engulfing and killing the invading organism.

Platelets. Blood also contains tiny fragments called platelets or thrombocytes. These small fragments go into action when the body is injured. They help cause blood to clot or form a plug to stop bleeding. They also help repair damaged blood vessels. Clotting or coagulation of the blood prevents foreign organisms from getting in and minimizes blood loss.

Magnified red blood cells. Anton van Leeuwenhoek was the first to see and describe these cells in the human blood. (Reproduced by permission of Phototake. Photograph by Dennis Kunkel.)

Plasms. All of these specialized cells and cell fragments (red cells, white cells, and platelets) are suspended in a liquid called plasma. Plasma is the liquid portion of the blood. It has often been compared to seawater in the amount of sodium chloride (common salt) it contains. BLOOD TYPES AND DISORDERS There are slight but important differences that people have in their blood, and this accounts for what are known as blood types or blood groups. There are four major human blood groups U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Blood Types

(A, B, AB, and O), and people can only receive blood from someone else of the same blood group. These groups are determined by certain proteins in the blood. Blood disorders take the form of problems with red blood cells, the worst of which are hereditary diseases like sickle cell anemia. Leukemia is a type of white blood cell disorder in which these cells reproduce uncontrollably. They should only make up about one percent of the blood. Platelet disorders can result in bleeding problems or dangerous clots. Future breakthroughs in blood research may soon result in cures for these conditions, such as artificial blood or some acceptable blood substitute that will be able to carry oxygen to the cells. [See also Blood Types; Circulatory System; Heart; Respiratory System]

Blood Types A blood type is a certain class or group of blood that has particular properties. There are four major human blood types, which are inherited, and each of which has a characteristic protein on the surface of its red blood cells. Individuals who share the same type of proteins belong to the same blood group or type. It is essential to know a person’s blood type before a blood transfusion can be given. Since the Middle Ages (A.D. 500–1450), doctors thought that if there were only some way to replace the blood a person lost due to injury, they could possibly save lives. Once precision instruments were developed in the seventeenth century that could be used to inject one person’s blood into another, blood transfusions were attempted. In far too many of these experiments, however, the results were just the opposite of what was expected. Many patients died and those that did not often became even more ill. Since no one had any idea why blood could not simply be transfused from one person to another, blood transfusions were eventually banned in most of western Europe after the late seventeenth century.

KARL LANDSTEINER DISCOVERS BLOOD GROUPS Research on blood did continue, and in the late 1800s, several researchers noted that when blood cells from one animal or person were mixed with cells from another, they stuck together in clumps. This was 68


called agglutination. While studying this clumping phenomenon, the Austrian physician Karl Landsteiner (1868–1943) discovered that not all blood always clumped with other blood. For example, one sample would clump with red cells from person A but not with person B. Another sample might clump red cells from person B but not from person A. Still another sample might clump them both, while yet another might clump neither. Eventually, Landsteiner was able to clearly identify four main blood groups that he named A, B, AB, and O.

Blood Types

Further research showed that these four blood types differed because of the type of protein that was located on the surface of the red blood cells and in the blood’s plasma (the fluid part of the blood). These proteins on red blood cells came to be called antigens (a kind of chemical identification tag), while those in plasma were called antibodies (proteins that destroy foreign substances). In what came to be known as the ABO system, there are two antigens (A and B) and four blood groups (A, B, AB, and O). People with type A blood have the A antigen; people with type B blood have the B antigen; type AB people have both; and type O people have neither. This led Landsteiner to formulate a simple pattern of who can receive what from whom. The first rule is that people in the same blood group can accept blood from each other with no ill effects. Next, blood types A and B are incompatible and cannot receive blood from each other, but they can receive blood from O (since it has no antigens). Blood type AB can accept blood from A or B (since they have both A and B antigens), as well as from O (which has no antigens). The AB blood type is therefore called the “universal recipient.” Type O can give blood to all other groups, but can only receive blood from its own type. It is therefore called the “universal donor.”

LANDSTEINER ALSO DISCOVERS THE RHESUS FACTOR The Rh (rhesus) factor system is another blood group that was discovered by Landsteiner and his associates in 1940. First discovered in rhesus monkeys, it was found that about 85 percent of the human population was Rh positive, meaning that their blood cells carried the D antigen or rhesus antigen. Those that did not were Rh negative. A person’s Rh factor becomes important during pregnancy. A fetus (unborn child) that is carried by an Rh-negative woman who developed Rh antibodies by previously carrying an Rh-positive baby can have its red blood cells attacked by these antibodies, resulting in death. The most common blood type in the United States is O+ (O Rh positive). It is found in 38 percent of the population. The type A+ (A Rh positive) is U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Blood Types

KARL LANDSTEINER Austrian-American immunologist (a person specializing in the study of the immune system) Karl Landsteiner (1868–1943) discovered the main types of human blood. His blood-typing system made blood transfusions possible and saved countless lives. Awarded the Nobel Prize for Physiology and Medicine in 1930, he also discovered the Rhesus (Rh) blood factor and that polio is caused by a virus (a disease-causing agent). Since at least the Middle Ages (A.D. 500–1450), doctors had been intrigued by the idea that severe blood loss might be treated simply by injecting the blood of one person into another. Once instruments precise enough to be able to do this were produced in the seventeenth century, blood transfusions were attempted. Sometimes they would work and save a patient, but much more often the transfusion itself would kill the person who was receiving someone else’s blood. This happened so often that blood transfusions eventually were banned in most European countries. Until the problem was taken up by Karl Landsteiner, no one knew the reason that one person’s blood could not be transferred to anyone else. All anyone knew was summed up by folk wisdom which simply said that everyone’s blood was different. Karl Landsteiner was born in Vienna, Austria, and entered medical school at the age of seventeen. By the time he was twenty-three, he had received his doctorate in medicine and went to work in the field of organic chemistry, studying with some of the best chemists in Europe. By around 1896, he became interested in the nature of antibodies, which are special proteins that circulate in the blood and lock on and disable any foreign substance that enters the body. By 1900, he was studying how blood agglutinates, or clumps, together when it is brought into contact with the blood of another person. No one could properly explain why this happened, but Landsteiner believed that it was due to something unique in the blood of each individual. He then began a series of experiments that showed that there were often very different things going on when blood clumped together. For example, the blood of one person might clump the blood from

found in 34 percent of the population. Knowing ahead of time a person’s Rh factor makes it possible to avoid incompatible transfusions and to correct any incompatibility by a blood transfusion either in the womb or directly after birth. Landsteiner’s discovery of blood types has made blood transfusions routine and safe, and has saved the lives of millions of people. [See also Blood; Rh Factor] 70


person A but not from person B, while another sample might clump blood from person B but not from person A. Another might clump both, and yet another might clump neither. Instead of giving up in the face of what seemed chaos, Landsteiner kept at his experiments and data-gathering and eventually saw that a real pattern existed in all of this. From his observations he came up with the idea of mutually incompatible blood groups, which he finally was able to sort out into four groups he called A, B, O, and AB. Landsteiner explained that blood contained certain antibodies that triggered a clumping reaction when one group, or type of blood, was mixed with another. He then showed that blood transfusions were possible if blood was “typed” and if the right type of blood was given to the right patient. Guided by Landsteiner’s work, the first successful blood transfusions were achieved at Mt. Sinai Hospital in New York in 1907. Thereafter, Landsteiner’s achievement saved many lives on the battlefields of World War I (1914–18), where transfusions of “compatible” blood were first performed on a large scale.

Botany

Although Landsteiner continued to work on antibodies, he turned his attention to studying the disease called polio (a viral disease that attacks the nervous system) and was able to show that it was not caused by a bacteria, but was instead traceable to a virus. In the 1920s, Landsteiner joined the Rockefeller Institute for Medical Research in New York and became an American citizen. Although officially retired by 1939, he kept working and, in 1940, discovered yet another blood factor that came to be called the Rh factor (named after the Rhesus monkeys in which it was first discovered). The Rh factor was shown to be responsible for a disease that occurred when mother and fetus have incompatible blood types and the fetus is injured or killed by the mother’s antibodies. Landsteiner’s brilliant work on blood groups has had a major impact on medicine and health, making life-saving blood transfusions possible. His work on blood typing also is regularly applied in legal and criminal cases in which blood is used as evidence. Landsteiner never really stopped working and died after suffering a heart attack at his laboratory bench.

Botany Botany is the scientific study of plants. While early humans were very knowledgeable about identifying harmful and beneficial plants, the ancient Greeks were the first to study plants scientifically (or for the sake of gaining knowledge rather than any practical purpose). The study of plants greatly expanded with the seventeenth-century invention of the miU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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croscope and today, modern botany uses a whole range of tools that investigate plants at their genetic level. Plants are multicelled organisms that live by making their own food using the process of photosynthesis to harness the energy of sunlight and convert it into food. Plants are essential to all living things since they provide food, oxygen, energy, and even wood. Since early humans were hunter-gatherers before they learned how to farm, it was especially important to know which plants were good to eat and which were not. Some plants were sometimes found to have medical uses. Until the Greeks studied plants in the fourth century, knowledge of plants consisted primarily of the following types of practical information—which plants were safe, which were harmful, and which were good to cure illness.

THEOPHRASTUS CONSIDERED THE FOUNDER OF BOTANY The Greek scholar Theophrastus (c.372–c.287 B.C.) began to study plants as life-forms worthy of study by themselves rather than sources of food or drugs. His work, titled Enquiry into Plants, survives today and has earned him the title “founder of botany.” In this work he studied a wide range of plants and discussed seeds, budding, and the effects of dis-

The magnification of a plant cell showing the nucleus, chloroplasts, mitochondria, cytoplasm, vacuoles, and cell wall. (©Photographer, The National Audubon Society Collection/ Photo Researchers.)

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ease and weather. He also attempted a classification of plants and described their different parts. Following Theophrastus, other Greeks and Romans were more interested in the practical aspects of plants and most books written were “herbals” or works that contained mainly medicinal (and sometimes mythical) information on plants. The tradition of herbals, which stressed plants that were useful to people, continued throughout medieval times and the Renaissance.

Botany

OTHERS EXPAND ON THEOPHRASTUS’S WORK By the middle of the sixteenth century, the scientific tradition begun by Theophrastus was revived in Germany as several naturalists began to produce botanical books that were based on facts rather than on the elaborate and sometimes fantastic claims of the herbals. These naturalists also began to investigate plant anatomy (the structure or parts of plants) and plant physiology (the internal life processes that take place). This scientific tradition was reinforced in the next century by the invention of the microscope, which allowed a better view of a plant’s minute parts. By the end of the seventeenth century, plant anatomy was being studied seriously and many correct scientific discoveries were being made. In 1682, the English physician Nehemiah Grew (1641–1712) published The Anatomy of Plants in which he displayed eighty-three full-page plates of microscopic sections of plant stems and roots. Grew was the first to state that flowers contained a plant’s sexual reproductive organs. His work and that of others led to the landmark work of the Swedish clergyman and naturalist, Carolus Linnaeus (1707–1778). After traveling through much of Europe studying plants, Linnaeus published in 1735 his System of Nature in which he created the modern form of systematic classification known as the binomial (two name) system. The first name he used was the genus, or the type of group, to which they belonged. This was followed by the species, or the particular name. His system soon became useful in classifying all living things. Following Linnaeus, botanists became increasingly specialized and nineteenth-century botany became best known for its discovery of photosynthesis and of the cellular structure of plants. By 1900, the earlier work of the Austrian monk Gregor Mendel (1822–1884), in which he worked out the actual laws of inheritance based on his work breeding pea plants, became well known and modern botany truly began. Today, the study of botany has many interconnected branches. The major areas of investigation are plant anatomy or the study of the internal arrangement of plant parts; plant physiology or the processes (like photosynthesis) that take place inside a plant; plant morphology or the U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Botany

THEOPHRASTUS Greek botanist Theophrastus (327 B.C.–287 B.C.) is considered the father of botany, the scientific study of plants. He was the first to study plants solely for their own sake and not just to learn how they might be put to some practical use. Although few of his writings remain, what did survive became the principle source of botanical information for centuries. Theophrastus was born on the Greek island of Lesbos and was lucky to study as a very young man with the great Greek philosopher, Plato (c.427 B.C.–c.347 B.C.). After Plato’s death, Theophrastus met and became a lifelong friend of the second great philosopher of the ancient world, Aristotle (384 B.C.–322 B.C.). In fact, it was Aristotle who gave him his nickname “Theophrastus,” meaning “divine speech.” Aristotle had founded a school called the Lyceum which Theophrastus took over after Aristotle’s retirement. Under Theophrastus’s leadership, the school reached its highest point. There, he carried on Artistotle’s teachings in biology, although he concentrated on the study of plants (botany), where Aristotle had specialized in the study of animals (zoology). Although Theophrastus is believed to have written a great deal on many different subjects, only a small portion of his botanical work survived. In these works, he covered every major aspect of plants—their description, classification, and distribution as well as how plants propagate (reproduce). Significantly, he described the formation of the plant in the seed as being like the fetus of an animal, something produced by it but not a part of it. He identified and grouped more than five hundred species and varieties of plants from those he knew, as well as those from neighboring lands. He classified plants into trees, shrubs, undershrubs, and herbs and

external or visible arrangement of a plant’s parts; plant taxonomy or the identification and classification of plants; plant cytology or the study of plant cells; plant genetics or the study of plant inheritance; plant ecology or the study of a plant’s relationship to its surroundings; plant paleobotany or the study of fossil plants; dendochronology or tree-ring dating; and ethnobotany or the relations between humans and plants (especially the identification of plants with medical properties). [See also Plant Anatomy; Plant Hormones; Plant Pathology; Plant Reproduction; Plants]

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developed a way of naming plants based on their external and internal parts, which he called organs and tissues. He also described sexual reproduction in flowering plants, as well as seed germination (when a seed starts to grow and puts out a root) and development. Although the real function of pollen (dustlike grains that contain the plant’s male sex cells) was not understood, he wrote detailed descriptions of how to pollinate certain fruit-bearing trees. His knowledge of plants was such that he knew that some flowers bear petals while others do not, and that there were major differences in the seed structure of flowering plants (called angiosperms) and cone-bearing trees (called gymnosperms). In fact, he is credited with inventing the term “gymnosperm” which in Greek means “naked seed.” Finally, he described how Greek farmers used certain bean crops to enrich the soil. Today, farmers know that important nitrates (salts from nitric acid) are formed by bacteria that live on the roots of these bean plants, and that they add important nitrogen (a nonmetalic element) to the soil.

Brain

Theophrastus is rightly considered to be the founder of botany. Unlike many who followed him, his plant study was focused on learning about plants not for their practical uses (which are many and important), but from a purely scientific aspect, simply in order to learn more about them. His one surviving botanical work contains all the essentials of what today is considered scientific botany. He observed, collected, and systematized his botanical information, and wrote in a clear and accurate manner. Although missing from his work are all of the fabulous folk tales that surrounded plant lore, he brought a scientific mentality to the study of plants. In many ways, Theophrastus was more modern than anyone who followed him for the next two thousand years.

Brain The brain is the control center of an organism’s nervous system. Composed of specialized cells called neurons, the brain receives information from the body’s sensory systems. It processes and analyzes the information, and responds by sending out messages that control the rest of the body. All vertebrate (an animal with a backbone) brains are organized and divided into three parts. Not all organisms have a brain. Simpler forms of life, such as invertebrates (animals without a backbone) like the sponge, jellyfish, and sea anemone have what are called “nerve nets.” Animals like earthworms and grasshoppers have a larger collection of cells called “cerebral ganglia.” U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Brain

These collections of nerve cells function as a primitive brain and perform the basic operations of receiving sensory information and acting upon it. More complex forms of life, and certainly all vertebrates, have a distinct and separate organ known as the brain. Described as the control center of the body, the brain’s functions are usually described by words like organizing, coordinating, supervising, and governing.

HOW THE BRAIN WORKS The above words are appropriate in referring to what the brain does, since unlike any other organ, the brain performs what might be called higher functions. For example, no other organ in the body is responsible for the entire organism. It is the brain that receives input, or information, from numerous sensory systems and analyzes and combines that information in order to issue commands to other body systems. The brain regulates and controls other organs and makes sure that the body’s “automatic” operations are running automatically. It is the brain that controls

Suprapharyngeal Lateral nerve ganglia (brain)

Corpus callosum

Mouth Subpharyngeal ganglion

Hypothalamus Diencephalon Thalamus

A. Earthworm brain Stomatogastric Deutocerebrum system Protocerebrum

Cerebrum Infundibulum Pituitary gland

Ventral nerve cord Tritocerebrum B. Insect brain

An illustrated comparison of the brains of an earthworm, an insect, a bird, and a human. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

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Cerebellum Midbrain Brain Pons Medulla stem oblongata

Thoracic ganglia Subesophageal ganglion

Cerebral hemisphere

Spinal cord D. Human brain

Cerebellum Optic lobe C. Bird brain


all of the body’s movements. In humans, it is the brain that is responsible for our thought, language, and awareness or consciousness.

Brain

Each brain is able to accomplish these complex tasks because it is composed of neurons, or nerve cells. Neurons are the building blocks of the animal brain. Like any cell, neurons have a nucleus (a cell’s control center), cytoplasm (a jelly-like substance in a cell), and a membrane. Yet they also have structures not found in any other cells. These structures are long, thin fibers, or threads, called axons and dendrites that extend out from the cell’s body. It is these fibers that allow a neuron to receive and send electrical impulses or signals. Axons do the sending and dendrites do the receiving.

PARTS OF THE BRAIN Although the brain is astoundingly complex, the vertebrate brain can be described in simple terms. All brains can be divided into three main parts: the hindbrain, the midbrain, and the forebrain. The hindbrain, or brain stem, is the most primitive part of the brain and consists of the medulla, the pons, and the cerebellum. Located in the rear of the brain, these regulate several of the autonomic bodily functions, like heartbeat and breathing, and coordinate our movements. The midbrain does a great deal of relaying of information and is responsible for much of the sensory data obtained by the eyes and ears. The forebrain, in the front, contains what are referred to as the higher brain centers. The cerebrum, the site for our thinking, reasoning, and language, is located in the forebrain. In humans, the cerebrum is divided into left and right hemispheres or sides. Each side is divided again into four lobes. The occipital lobe receives and analyzes visual information. The temporal lobe deals with memory, hearing, and some language functions. The frontal lobe regulates movement and handles language production. The parietal lobe deals with sensations. These two halves communicate by means of a bundle of axons called the corpus callosum, and each side of the brain controls the opposite side of the body. In humans, the entire brain is protected by the very hard and thick bones of the skull. Between the brain and the skull are three layers of protective tissue called meninges, and a clear, colorless fluid called cerebrospinal fluid that acts as a shock absorber. The brain also contains a group of connected structures that make up what is called the limbic system. Sometimes called the “emotional brain,” this system is made up of the olfactory bulb and the amygdala, which senses smells; the hypothalamus, which controls basic drives like hunger, thirst, and sex; the pituitary U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Bryophytes

gland, that secretes growth and other hormones; the hippocampus, which works for memory; and the thalamus, which coordinates sensory information. Overall, the limbic system is responsible for the basic drives, emotions, and involuntary behavior that are critical for an animal’s survival. The human brain weighs about 3 pounds (48 ounces) and contains about a trillion neurons. It is one of the largest organs in the body and its cells use about 20 percent of the body’s oxygen. The human cerebrum (the large rounded structure of the brain occupying most of the cranial cavity) is bigger than all of the other parts of the brain put together. The brain does most of its growing after birth, which is part of the reason that human childhood is so long. This growth continues until about the age of 18, but learning takes place until death, and the brain is constantly “remodeling” itself. In other words, the brain creates and preserves new connections in response to new experiences.

THE BRAIN AND THE FUNCTION OF MEMORY The phenomenon of memory is still not well understood, but scientists know that the brain has short-term and long-term memory. In the short-term memory, the brain holds on to certain information for only a certain amount of time. In the long-term memory, it stores selected information for many years. The brain also regularly enters an altered state of consciousness called sleep. While there is still much to learn about sleep, it is known that sleep allows the body to rest and repair the daily wear and tear it receives. It somehow also lets the brain “recharge” its batteries. Brain disorders can be serious, since the brain is such an important organ. Organic diseases like Parkinson’s and Alzheimer’s disease are a leading cause of death. Mental disorders like dementia and schizophrenia often render individuals incapable of having a normal life. Strokes, in which the flow of blood to the brain is temporarily cut off usually by a clot, disable 500,000 Americans each year. According to the part of the brain impacted by the blockage, the individual can suffer some localized paralysis or disability. If any part of the brain is without blood for even a few minutes, its cells die. [See also Circulatory System; Nervous System]

Bryophytes Bryophytes are nonflowering plants that make up one of the major divisions of the plant kingdom. They are composed of moss, liverwort, and hornwort, all of which are small, simple land plants that live in damp places. 78


The most widely accepted method of classifying the many types of plants in the kingdom Plantae divides it into ten divisions, one of which is called Bryophyta. Plants in this division are unlike plants in any of the other nine since they make up the nonvascular plants. Unlike typical plants, nonvascular plants lack a transport system made up of tubelike vessels or internal pipelines to move their water and food about. Because of this, these plants live very close to the ground and do not have stems, leaves, or roots.

Buds and Budding

Bryophytes live in almost any part of the world, and are found in the Arctic as well as the tropics. They may have been among the first land plants, since fossils of bryophytes have been found that date to about 400,000,000 years ago. Since bryophytes do not have a specialized method of moving its food and water from one part of the plant to another, all parts of the plant can absorb water and nutrients directly from the environment. Bryophytes also reproduce differently from most plants by using spores rather than seeds. A spore is similar to a seed in that it has an outer coat that protects its inner reproductive cells. Spores are usually released into the air by fungi (such as mushrooms) and are light enough to travel great distances. If a spore lands in a suitable place, it will germinate (begin to grow or sprout) and produce a new fungus. Bryophytes cannot reproduce without water, which is why most are found in moist places. Mosses, part of the bryophyte group, are soft and leafy, usually never more than 2 inches (5.08 centimeters) tall. They grow by anchoring themselves by rootlike growths called rhizoids. There are nearly 10,000 species of moss. Liverwort and hornwort also are both bryophytes and have a ribbonlike or flat, leafy shape that grows low to the ground and is anchored like the mosses. Bryophytes do not produce flowers since they use spores to reproduce. [See also Plants]

Buds and Budding A bud is a swelling or undeveloped shoot on a plant stem that is protected by scales. Within the scales is a cluster of overlapping, immature or undeveloped leaves or flower petals that will eventually open and develop into new leaves, flowers, or stems. Budding describes the developmental process by which immature plant tissue inside expands and grows into mature structures like leaves or flowers. Botanists (people who specialize in the study of plants) also call “budding” the type of asexual reproduction that occurs when a plant makes a genetically idenU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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tical copy of itself (as strawberries do when they send out aboveground runners that form new individual plants). A plant bud is a complex structure that is basically a tiny shoot packed into a small space. It could also be described as a plant’s growing point, for it is from a plant’s buds that new growth happens. After a bud is formed, if the growing season is ending, the bud will remain dormant or inactive until conditions are right. With proper conditions (usually the warmer and longer days of spring), the protective scales fold back, the bud bursts, and the shoot begins to grow. If a bud is found at the tip of a stem, it is called a terminal bud. Buds that form along the sides of a stem are called lateral buds.

A freshwater hydra in the process of budding. (Reproduced by permission of the National Audubon Society Collection/ Photo Researchers, Inc. Photograph by James Bell.)

Dissecting a bud reveals its complexity. Starting at the outside, it is protected by bud scales that protect the delicate tissues inside and help it conserve water. The dormant buds of many woody plants are protected by several tough, overlapping scales. The smallest and least-developed part of the bud is found at its center, and is surrounded by slightly older, larger, and more developed parts that overlap and curve around one another. Within the bud, each bit of tissue is already programmed for differentiation after it starts to grow. This means that each part knows that it will eventually become a certain, specific type of plant tissue—perhaps a flower, a leaf, or a stem. Buds do not grow constantly, having what is called episodic growth, meaning that they grow only when conditions are favorable. In climates where the seasons vary greatly, either because of cold winters or extended droughts, buds remain dormant for long periods of time. They “break” or are stimulated to grow when the temperature warms in the springtime or after rains occur. These condition changes are recognized by the plant, which releases certain hormones known as auxins. The release of certain amounts of these chemicals breaks the dormancy and the buds swell and burst open and budding occurs. Buds can also refer to the individual flower before it blooms. Besides describing the developmental process of expansion, growth, and differentiation, budding also can refer to the form of asexual reproduction known as vegetative reproduction. In such cases, certain plants have the ability to duplicate themselves by developing aboveground

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runners, like strawberry plants do, or belowground roots that send up plantlets, like how grass in a lawn spreads. Other plants like poplar trees put out rhizomes or underground stems that grow outward from the parent. In every case, the new individual plant is genetically identical to the parent. This is unlike sexual reproduction, which produces a genetically unique individual by the mixing of genes. Yeast (which are single-celled fungi) also reproduce by budding as they produce new cells from the parent cells. Some animals reproduce asexually by budding including the many species of hydra, corals, and sea anemones.


Buds and Budding

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C Calorie A calorie is a unit of measurement of the heat given off by a substance when it is completely consumed. Besides this description, it is also a measurement of the amount of energy in food. The number of Calories in a serving of food tells how much heat energy is available in the food for the body to use. The word calorie always relates to the notion of energy, although it has two separate uses. The first and older of the two came about when scientists realized the need for accurate measurement, especially the measurement of energy. Once it was discovered that matter is simply energy in another form, scientists realized they would be able to measure how much energy a piece of matter contained if they consumed that matter completely (usually by burning) and somehow measured the energy that was freed. Thus calorimetry, or the measurement of the heat given off by a substance when it is completely consumed in a chemical reaction, was invented. In scientific terms, it was decided that a calorie would be equal to the amount of heat needed to raise the temperature of 0.035 ounces (1 gram) of water by 1 degree Centigrade at standard atmospheric conditions (since things sometimes act differently at high altitudes). This is a very accurate method of measuring potential energy, and it has proven to be extremely useful to many scientific disciplines. Calorimetry should be distinguished from the other notion of calorie which is used as a measure of the amount of energy in food. This notion of calorie has become so popular and actually so important to good health, that today the word calorie has been replaced by the word “kilocalorie” U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Carbohydrates

(also sometimes called a kilojoule). A kilocalorie is exactly what is sounds like: 1,000 calories, since the prefix “kilo” means one thousand times. One kilocalorie is defined as the amount of heat energy needed to raise the temperature of 0.028 ounces (1 kilogram) of water by 1 degree Centigrade. Finally, when calories are written as “Calories” with a capital “C,” it means a kilocalorie. In the United States, when someone refers to Calories in foods, we are always talking about kilocalories since the original calorie (1/1,000th of a Calorie) is too small a unit of measurement to be practical when labeling foods. It has long been known that people need to take in a certain amount of Calories everyday to maintain good health. This number changes quite a bit according to the age and level of activity of an individual. For example, an expectant mother would need to take in more Calories since her system is supporting a developing fetus. In the same way, a person who spends a great deal of time outdoors in a frigid environment needs many more Calories than a person sitting warmly at home, since the former’s body must burn more Calories just to maintain steady body heat. The average recommended daily requirement for men is considered to be 2500 Cal and 2000 Cal for women, but these totals should decrease as a person gets older. Although it is much easier to say than to carry out, if a person wants to maintain his or her weight, he or she should balance intake of Calories with output of energy. Many adults are able to do this with little or no effort, and therefore maintain a consistent body weight over very long periods. Losing weight requires either a reduction in caloric intake or increase in energy output (or both). Many of today’s packaged food products provide the buyer with nutrition information, including the amount of Calories in an average serving. It is therefore possible to obtain a fairly good estimate of the amount of Calories consumed during a twenty-fourhour period. [See also Nutrition]

Carbohydrates Carbohydrates are a group of naturally occurring compounds that are essential sources of energy for all living things. Carbohydrates are manufactured by green plants, which use them for energy and to build their cell walls. Animals are able to both use and store the energy that carbohydrates contain. Carbohydrates include such organic compounds as sugars, starches, and cellulose. 84


Chemically, carbohydrates are natural compounds of carbon, hydrogen, and oxygen and were described as “carbon hydrates” by the chemists who first encountered them (since they mistakenly thought that carbohydrates were simply a compound of carbon and water). We now know that a carbohydrate molecule is made up of equal numbers of carbon and oxygen atoms (the smallest units of elements), with twice as many hydrogen atoms. Almost all the energy consumed by living things comes from carbohydrates that are manufactured by plants.

Carbohydrates

CARBOHYDRATES PRODUCED BY PHOTOSYNTHESIS Plants manufacture carbohydrates using the process known as photosynthesis. Because plant cells contain chloroplasts inside their cells, plants are able to use the sunlight absorbed by their leaves that they combine with the water from the soil and the carbon dioxide from the air to make a simple form of sugar called glucose, which is a carbohydrate. Glucose is packed with energy and plants use it to make cellulose (their building

A computer graphic representation of fructose, a common carbohydrate found in many fruits and vegetables. (©Chemical Design Ltd./Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.) U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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material). What they do not use, they store as starch in the form of seeds, roots, or fruits. Animals that eat plants take in their carbohydrates and reverse a plant’s metabolic process. This occurs by the process of digestion in which animals break down complex carbohydrates into the original glucose that animals are then able to absorb into their bloodstream. The animals’ bloodstream then carries the glucose to every cell in the body and cells turn the glucose into energy by a process known as oxidation, in which glucose combines with oxygen and releases energy. In a sort of reverse process, an animal turns a plant into the energy that the plant “captured” from the sun.

TYPES OF CARBOHYDRATES There are three major types of carbohydrates: monosaccharides, disaccharides, and polysaccharides. As the names imply, each is more complex than the other. Monosaccharides are the simplest of the carbohydrates since they consist of a single carbohydrate unit that cannot be broken down into anything simpler. The three most common sugars in this group are glucose, fructose, and galactose. Glucose is found in fruits and vegetables; fructose is found in honey and some fruits and vegetables; and galactose is found in milk. Disaccharides are more complex since they consist of two joined monosaccharides. The three most nutritionally important disaccharides are sucrose (ordinary table sugar), maltose (found in sprouts), and lactose (found in the milk of mammals). Polysaccharides are made up of ten or more monosaccharides and are often highly complex chains of long molecules. Carbohydrates are stored in the form of polysaccharides (as starch in plants and as glycogen in animals).

THE IMPORTANCE OF CARBOHYDRATES Much has been learned about the role carbohydrates play in our diet and health. For instance, it was once believed that avoiding carbohydrates like bread, potatoes, and pasta would keep a person slim. Scientists now know that this is not the case since not all carbohydrates are created equal and that the type of carbohydrate eaten is more important than the total amount consumed. Low-fiber carbohydrates are digested very quickly and behave like sugar, giving a quick peak in energy. On the other hand, highfiber carbohydrates like whole grains are digested more slowly, providing a steady and gradual source of energy. Athletes have learned that continuous exercise, such as running a marathon, can burn up all of the body’s supply of glycogen in about two or three hours. After that the body must start converting its stored fat into glucose. Since this is a fairly slow 86


process, the athlete operating on stored fat becomes fatigued. To overcome this, some athletes whose competition lasts longer than ninety minutes began the practice of “carbohydrate loading” before competition. Beginning a week before competition, athletes consume an increasing amount of carbohydrate-rich foods daily.

Carbon Cycle

The importance of carbohydrates to all living things cannot be overemphasized since it is the primary source of energy for plants and animals. We have learned that “starchy foods,” such as potatoes, which were once thought to be inadequate for humans in large amounts, are in fact a necessary part of every meal. The U. S. Food and Drug Administration now recommends that people’s carbohydrate consumption should be increased and fat consumption should be reduced. [See also Fruits; Photosynthesis]

Carbon Cycle The carbon cycle is the process in which carbon atoms are recycled over and over again on Earth. Carbon recycling takes place within Earth’s biosphere (the region of the Earth that supports life) and between living things and the nonliving environment. Since a continual supply of carbon is essential for all living organisms, the carbon cycle is the name given to the different processes that move carbon from one organism to another. The complete cycle is made up of “sources” that put carbon back into the environment and “sinks” that absorb and store carbon. Earth’s biosphere can be thought of as a sealed container into which nothing new is ever added except the energy from the sun. Since new matter can never be created, it is essential that living things be able to reuse the existing matter again and again. For the world to work as it does, everything has to be constantly recycled. The carbon cycle is just one of several recycling processes, but it may be the most important process since carbon is known to be a basic building block of life. Carbon is the basis of carbohydrates, proteins, lipids, and nucleic acids—all of which form the basis of life on Earth. Since all living things contain the element carbon, it is one of the most abundant elements on Earth. The total amount of carbon, whether we are able to measure it accurately or not, always remains the same, although carbon regularly changes its form. A particular carbon atom located in someone’s eyelash may have at one time been part of some nowextinct species, like a dinosaur. Since the dinosaur died and decomposed U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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millions of years ago, its carbon atoms have seen many forms before ending up as part of a human being. It may have been part of several plants and trees, been free-floating in the air as carbon dioxide, been locked away in the shell of a sea creature and then buried at the ocean bottom, or may have been part of a volcanic eruption. Carbon is found in great quantities in Earth’s crust, its surface waters, the atmosphere, and the mass of green plants. It also is found in many different chemical combinations, including carbon dioxide (CO2), calcium carbonate (CaCO3), as well as in a huge variety of organic compounds such as hydrocarbons (like coal, petroleum, and natural gas).

CARBON CYCLE PROCESSES If a diagram were drawn showing the different processes that move carbon from one form to another, its main processes would be photosynthesis, respiration, decomposition, combustion of fossil fuels, and the natural weathering of rocks.

Carbon dioxide in atmosphere

Photosynthesis

Combustion, and the manufacturing of cement

Carbon released from volcanoes

Stored in sediments Aerobic respiration and decay

An illustrated diagram showing the processes found in the carbon cycle. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

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Fires from use of fuel wood and conversion of forest to agriculture

Aerobic respiration and decay (plants, animals, and decomposers)

Fossil fuels


PHOTOSYNTHESIS

Carbon Cycle

Carbon exists in the atmosphere as the compound carbon dioxide. It first enters the ecological food web (the connected network of producers and consumers) when photosynthetic organisms, such as plants and certain algae, absorb carbon dioxide through tiny pores in their leaves. The plants then “fix” or capture the carbon dioxide and are able to convert it into simple sugars like glucose through the biochemical process known as photosynthesis. Plants store and use this sugar to grow and reproduce. When plants are eaten by animals, their carbon is passed on to those animals. Since animals cannot make their own food, they must get their carbon by eating plants or by eating animals that have eaten plants.

RESPIRATION Respiration is the next step in the cycle, and unlike photosynthesis, it occurs in plants, animals, and decomposers. Although we usually think of breathing oxygen when we hear the word “respiration,” it has a broader meaning that involves oxygen. To a biologist, respiration is the process in which oxygen is used to break down organic compounds into carbon dioxide (CO2) and water (H2O). For an animal, respiration includes taking in oxygen (and releasing carbon dioxide) and oxidizing its food (or burning it with oxygen) in order to release the energy the food contains. In both cases, carbon is returned to the atmosphere as carbon dioxide. Carbon atoms that started out as components of carbon dioxide molecules have passed through the body of living organisms and been returned to the atmosphere, ready to be recycled again.

DECOMPOSITION Decomposition is the largest source through which carbon is returned to the atmosphere as carbon dioxide. Decomposers are microorganisms that live mostly in the soil but also in water, and which feed on the rotting remains of plants and animals. It is their job to consume both waste products and dead matter, during which they return carbon dioxide to the atmosphere by respiration. Decomposers not only play a key role in the carbon cycle, but break down, remove, and recycle what might be called nature’s garbage.

HUMANS INCREASE CARBON DIOXIDE IN THE ATMOSPHERE In recent history, humans have added to the carbon cycle by burning fossil fuels. Ever since the rapid growth of the Industrial Revolution when people first harnessed steam to power their engines, human beings have U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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been burning carbon-containing fuels like coal and oil for artificial power. This constant burning produces massive amounts of carbon dioxide, which are released into Earth’s atmosphere. Fossil fuel consumption could be an example of a human activity that affects and possibly alters the natural processes (photosynthesis, respiration, decomposition) that nature had previously kept in balance. Many scientists believe that carbon dioxide is a “greenhouse gas.” This means that it traps heat and prevents it from escaping from Earth. As a result, this trapped gas leads to a global temperature rise, which can have disastrous effects on Earth’s environment. Not all carbon atoms are always moving somewhere in the carbon cycle. Often, many become trapped in limerock, a type of stone formed on the ocean floor by the shells of marine plankton. Sometimes after millions of years, the waters recede and the limerock is eventually exposed to the elements. When limerock is exposed to the natural process of weathering, it slowly releases the carbon atoms it contains, and once again they become an active part of the carbon cycle. [See also Biosphere; Carbon Dioxide; Decomposition; Photosynthesis; Respiration]

Carbon Dioxide Carbon dioxide (CO2) is, along with oxygen and nitrogen, one of the major atmospheric gases. Although it only makes up .03 percent of the atmosphere, it is vitally important for all living things. A colorless and odorless gas, carbon dioxide is made up of a central carbon atom joined to two oxygen atoms. Early scientists were able to observe the effects of carbon dioxide long before they knew exactly what it was. At the start of the seventeenth century, the common air humans and animals breathe was thought to be a single substance or element. However, around 1630, a Flemish physician, Jan Baptista van Helmont (1577–1644), became the first to discover that there were other “vapors” that were different from ordinary air. He coined the word “gas” to describe the vapors that were given off when wood was burned (actually carbon dioxide). He also recognized that this same gas was produced by the process of wine fermentation. In 1756, the Scottish chemist, Joseph Black (1728–1799), proved that carbon dioxide is present in the atmosphere and that it combines with other chemicals to form compounds. Black also learned that the gas he called “fixed air” was present in exhaled breath and that it was heavier than ordinary air. It was also known by then that a candle flame would eventually go out when enclosed in a jar with a 90


limited supply of air and that a small bird would die in the same tight jar. Near the end of the eighteenth century, chemists finally began to realize that gases were important and should be weighed and accounted for whenever chemists analyzed chemical compounds, as was done with solids and liquids. This more scientific method and approach would eventually lead to a real understanding of the nature and role of carbon dioxide.

Carbon Dioxide

Following the 1771 discovery by English chemist Joseph Priestley (1733–1804) that plants give off oxygen (which he called “dephlogisticated air”), the Dutch physician Jan Ingenhousz (1730–1799) demonstrated in 1779 that during photosynthesis (the plant process that uses sunlight to make food), green plants take in carbon dioxide (as well as give off oxygen). He therefore established the key fact that plants, in the presence of light, consume the carbon dioxide produced by humans and animals and give off the oxygen which is in turn consumed by both humans and animals. In the bodies of humans and animals that breathe air, carbon dioxide is generated by the cells as a waste product (similar to the release of carbon dioxide when wood burns). The lungs remove it from the body, which received it from the bloodstream. The amount of carbon dioxide in the blood affects how humans and animals breathe; a rise stimulates the rate of breathing and a drop lowers it. When air consists of 3 percent of carbon dioxide, it is difficult to breathe. If the amount goes above 10 percent, a loss of consciousness occurs. At about 18 percent humans and animals suffocate. Although this rarely happens, in 1986 a huge cloud of carbon dioxide suddenly exploded from Lake Nyos in northwestern Cameroon (Africa) and suffocated more than 1,700 people and 8,000 animals. It was later learned that atmospheric conditions probably caused the colder water at the bottom of the lake to turn over and quickly release a large amount of carbon dioxide at the surface. Increasingly, we are becoming aware of the critical role carbon dioxide plays in the health of the global environment. The amount of carbon dioxide in the atmosphere affects our climate since it acts to trap the heat escaping from Earth’s surface and reflect it back down. Many scientists argue that when there is too much carbon dioxide in the atmosphere, a “greenhouse effect” will occur and cause the planet to overheat. There is no doubt that atmospheric levels of carbon dioxide have increased over the years as a result of massive amounts of forests that would consume carbon dioxide being destroyed. Also, the amount of carbon dioxide in the atmosphere has also increased because of the steady burning of fossil fuels (which releases carbon dioxide as a by-product). Fermentation and the breakdown of organic matter also play a role toward increasing U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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the amount of carbon dioxide in the atmosphere. Many argue that Earth’s temperature could be raised by these influences to a level at which all life is threatened. Despite these dire predictions, recent scientific calculations of the amount of carbon dioxide in the atmosphere suggest that there is significantly less than should be expected. So far, scientists have not been able to account for the “missing” carbon dioxide. Many think that we may be underestimating the amount or rate (or both) of photosynthesis that takes place on a global scale. If that is the case, Earth is in better shape than first thought. [See also Carbon Cycle; Carbon Family; Greenhouse Effect; Photosynthesis; Pollution]

Carbon Family Carbon is a chemical element that is essential to all life on Earth. Since it can link up with the atoms of other elements to form an immense variety of stable compounds, carbon is able to form the chemical structures basic to all life. More than 1,000,000 compounds have been identified in the family of carbon compounds. Carbon has been called the “backbone” element of life. This is because carbon is such an unusual and versatile element that it forms more compounds than any other element. An element is a pure substance that contains only one kind of atom. There are more than ninety different chemical elements on Earth, and less than one third of them seem to be essential to life. Of these, only oxygen, hydrogen, nitrogen, and carbon are needed in any real quantity to sustain life. Of these four, only carbon has the ability to combine so easily with other elements that it forms a part of every compound important to life. A single carbon atom can make four bonds, or link up with four other atoms. These can be other carbon atoms, atoms of other elements, or a combination of the two. The huge variety of compounds it forms is explained by the fact that one carbon atom can easily join with another and form long, stable chains. These can be extended indefinitely, so that if only a few other elements are added, the number of compounds it is possible to make becomes almost limitless. In its pure form, carbon exists only as diamonds and graphite. Both are a form of crystal, although in the diamond (the hardest known mineral), each carbon atom is linked to four others, therefore forming a solid structure. However, in graphite (a soft, almost greasy black solid) the 92


carbon atoms are linked in such a way that they form layers that slide over each other like plates. Aside from diamonds and graphite, most carbon occurs in combination with other elements, and in nearly all of its forms it is linked with, or was once part of, a living organism. In fact, of the nine characteristics of all living things, the presence of carbon is usually listed as the first. In human bodies, carbon, oxygen, and hydrogen are the three most abundant elements, as they represent 93 percent of its weight. Structurally however, carbon is the most important element in the body.

Carbon Family

The carbon family is really a family of compounds. All animals and plants consist of carbon compounds, some of which are groups of small organic molecules, while others are large organic molecules called macromolecules. By definition, an organic compound is a compound that contains carbon. In living things, there are four main types of organic compounds—carbohydrates, lipids, proteins, and nucleic acids. An example of small molecules is table sugar, which is a carbohydrate and therefore an example of a carbon compound. Nucleic acids are examples of macromolecules and are the most complex carbon-containing substances in living things. As a basic building block of life and one of the most abundant elements on Earth, carbon is recycled again and again. This is necessary

FRIEDRICH KEKULE German chemist Friedrich Kekule (1829–1896) was the first to discover the nature of the carbon atom (a unit of matter) and to explain how carbon atoms arrange themselves. This allowed him to understand the structure of organic compounds and to lay the groundwork for the modern structural theory in organic chemistry. Friedrich Kekule was born in Darmstadt, Germany, and entered the University of Giessen intending to become an architect (a person who designs buildings). There, however, he sat in on the lectures of the great German chemist, Justus von Liebig (1803–1873), and switched immediately to chemistry. He eventually obtained his doctorate in chemistry and began to teach and do research. His early interest in architecture may have influenced the chemicals problems he studied, since from the beginning, he was always interested in the actual structure of chemical compounds. Until the mid1850s, chemists would give the atomic makeup of a compound such as water (H2O) by listing the numbers of each element in a certain order. Therefore, H2O meant that water contained two hydrogen atoms and one oxygen atom. They did not, however, state exactly what the arrangement or struc-


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ture of these atoms were. In other words, which atom was bonded or linked to which, and in what order? Many chemists doubted the value of even knowing this, while others believed that their structure might not even be knowable. In 1858, Kekule decided that structure was important and that it was knowable. Kekule felt that such fixed combinations of elements, like H2O, had a basic structure all their own and that they could be represented as a pattern of atoms linked in a certain order. Kekule then focused on carbon, an element found in all living things, and argued that any one carbon atom would always form four bonds, no more and no less, in a compound. He then also suggested that carbon atoms had the ability to bond with each other in endless chains. Therefore, a single carbon atom might bond to four other carbon atoms. When Kekule adopted the suggestion by the Scottish chemist Archibald Scott Couper (1831–1892) that a dash or dotted line be used to represent the chemical bond, Kekule was then able to illustrate the structure of water (H2O) as H-O-H. Kekule’s structural formulas allowed chemists to make sense out of organic compounds (compounds that contain carbon), since he showed that it was the structural arrangement of the atoms that made a compound what it was. For example, the compound known as ethyl alcohol has six hydrogen atoms, two carbon atoms, and one oxygen atom. However, the very different compound known as dimethyl ether has the exact same number of elements (six hydrogen, two carbon, and one oxygen atom), yet a slightly different arrangement of the oxygen atom gives the compound an entirely different nature. Kekule’s structures proved to be the key to understanding organic compounds, and actually laid the foundation for the structural theory of organic chemistry. Kekule’s second great achievement was his discovery of the structure of the benzene (a clear, colorless, flammable liquid used to make insecticides) molecule. Chemists knew that its formula was C6H6, but no one could arrange these atoms in the proper way. The story goes that in 1865, Kekule fell into a half-sleep while still thinking about the benzene problem, and he dreamt that he saw atoms dancing wildly. Suddenly he saw the tail end of one chain attach itself to the front end, and awoke knowing that he had solved the benzene problem. He then immediately drew a structure that had six carbon atoms arranged in a circle, joined by alternative single and double bonds, with a single hydrogen atom attached to each carbon atom. This is a favorite story of many science historians, showing how Kekule’s dreamlike state, in which he saw something like a snake devouring its tail, influenced him to make a real scientific breakthrough. Kekule’s work on structure is valid today, and still helps chemists to both show what an organic compound looks like and to predict its reactions.

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since the total amount of carbon on Earth always remains the same. However, its form changes according to what part of the carbon cycle it is in. For example, the same carbon atom may be absorbed from the atmosphere by a plant that takes in carbon dioxide. It may then become part of an animal who eats the plant, then part of the atmosphere when the animal breathes, or part of the soil when the animal dies, and so on.

Carbon Monoxide

[See also Carbon Dioxide; Carbon Monoxide]

Carbon Monoxide Carbon monoxide is an odorless, tasteless, colorless, and poisonous gas. Although most of the carbon monoxide in the atmosphere comes from natural sources, a great deal is also added by the burning of fossil fuels by automobiles and industry. It is an extremely poisonous gas if inhaled, since it kills by preventing oxygen from reaching the cells. Carbon monoxide (CO) is a compound consisting of one carbon atom (the smallest unit of an element) and one oxygen atom. It is normally in the air people breathe, although in extremely small amounts. It is produced naturally when a substance that contains carbon decays or breaks down in the absence of oxygen. This happens in swamps where there is little oxygen. Carbon monoxide is also produced in greater quantity when carbon-containing substances are burned without enough oxygen being present. This often happens when gas furnaces, wood stoves, and space heaters malfunction or are not properly vented. Automobile engines and other gasoline or diesel-powered motors also generate carbon monoxide. The exhaust from these motors can be deadly if they are operated in enclosed areas or attached garages. Carbon monoxide can kill a person who breathes it. It does this by preventing the blood from being able to carry oxygen. Without oxygen, people and animals soon die. Once inhaled, carbon monoxide combines with the hemoglobin (the oxygen-carrying substance in the blood) to the exclusion of oxygen. In fact, carbon monoxide combines with hemoglobin two hundred times faster than oxygen does. Additionally, the hemoglobin does not release the carbon monoxide as it does the oxygen. Thus, as more and more red blood cells pick up carbon monoxide, the total number available to deliver oxygen to the cells keeps decreasing, and soon the person slowly falls into a sleeplike state. Deprived of oxygen, the brain begins to slow its functions. Eventually, bodily functions stop, and the person dies. This oxygen deprivation does not always kill immediately. Low-level exposure can cause flu-like symptoms including shortness of breath, mild U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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headaches, fatigue, and nausea. Higher levels may cause dizziness, severe headaches, mental confusion, and fainting. Prolonged exposure can cause death. According to the U. S. Consumer Product Safety Commission, more than 2,500 people will die and 100,000 will be seriously injured by carbon monoxide poisoning over the next 10 years. People who smoke also run the risk of harming themselves with this toxic gas. The carbon monoxide present in cigarette smoke not only excludes oxygen from binding to hemoglobin, but it also prevents it from picking up carbon dioxide (which is the waste product of breathing). This means that the person’s heart has to pump harder to try and rid the body of carbon dioxide wastes. Large cities had an especially bad problem in the past as unhealthy amounts of carbon monoxide would build up during rush hour because of automobile exhaust. Newer cars are now equipped with catalytic converters that chemically change carbon monoxide into harmless carbon dioxide. Also, in the last decade or so, carbon monoxide detectors for the

Victims of carbon monoxide poisoning breathe oxygen at a hospital in Philadelphia. (Reproduced by permission of AP/Wide World Photos.)

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home have become practical, and many cities are now requiring that at least one carbon monoxide detector be installed in every home, apartment, and hotel. Since carbon monoxide is odorless, tasteless, and colorless, these monitors are invaluable in alerting people to possible exposure.

Carnivore

[See also Carbon Family]

Carnivore The term carnivore is thought by many to refer to any meat-eating organism, but in the life sciences it is applied to a certain family of mammals (Carnivora) that have specially shaped teeth and live by hunting. Carnivores are animals that obtain most of their nutrition from eating other animals, and their name comes from a combination of Latin words that literally mean “flesh devourers.” Carnivores are always at the top levels of every food chain.

CHARACTERISTICS OF CARNIVORES There are eight families of terrestrial (land-dwelling) carnivores and three families of aquatic (water-based) carnivores. A family is a classification term that includes one or more genera (singular, genus), and a genus contains one or more species. What best defines a carnivore are its teeth, although most have powerful jaws and a keen sense of smell as well. Since it lives primarily by hunting, catching, killing, and eating its prey, a carnivore has teeth that are specially shaped for all of those demanding tasks, especially for gripping and cutting. These specialized teeth come in sets that have particular functions. The canine teeth are the longest. It is with these curved weapons that a carnivore both grabs and punctures its prey. The canine teeth are best used for holding prey since they can pierce it deeply. The chisel-shaped incisors are next to the canines and are at the center of the jaw. With these, the carnivore bites into food and slices it up. Carnivores also have a unique set of teeth called carnassials that form the first set of molars. These sharp teeth have a pointed edge, and the top slides along the bottom like scissors. Carnassials are used to slice through tough flesh and gristle (cartilage especially present in meat) and to crack open bones. Since they are near the hinge of the jaw, they can close with great force. After a carnivore has caught, killed, and eaten its prey, its specialized digestive system takes over. Since its main diet of meat is easier to digest than the tough material of plants (plant cells are surrounded by a tough wall of cellulose), a carnivore’s intestines are much shorter than those of a herbivore (plant-eating organisms) like a horse or cow. A carnivore also a much simpler stomach. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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As a predatory animal or one that lives by killing and eating other animals, a carnivore has other distinguishing characteristics that enable it to capture and subdue its prey. One of these is its brain, which is often fairly large and complex. Such a brain means that a carnivore’s behavior can be somewhat flexible and that it can rear its young. Some carnivores are particularly social creatures and hunt in packs. This allows them to overwhelm creatures that an individual could not catch on its own. Most carnivores are either speedy runners or very quick and nimble. For example, the cheetah is the fastest land animal on Earth, able to reach 75 miles-per-hour (120.68 kilometers-per-hour) in short bursts. Others have endurance like wild African dogs that can run for 3 miles (4.83 kilometers) at a speed of 35 miles-per-hour (56.32 kilometers-per-hour). Because carnivores must find and catch their food, they are often very active and aggressive animals. However, since they do not have to eat continuously the way herbivores do, they are able to spend more time relaxing in between meals.

LAND-DWELLING AND SEA-DWELLING CARNIVORES There are about 240 species of carnivore belonging to two suborders or major groups: Fissipedia, or land-dwelling carnivores, and Pinnipedia, or sea-dwelling carnivores. Some biologists, however, consider Pinnipedia to be a separate order like Carnivora. There are three families that make up Pinnipedia and eight families in Fissipedia, each of which has its own specialties and characteristics.

Mustelidae Family. One of the largest members of the Fissipedia group is the Mustelidae family consisting of small carnivores like skunks, badgers, weasels, and ferrets. These sleek animals can burrow into hard-toreach places and catch their prey. Some of these carnivores depend on foul-smelling spray for defense. They are aggressive and often take on animals that are larger than they are. Procyonidae Family. The Procyonidae family includes raccoons and the tropical coati, which are less carnivorous and more omnivorous creatures (eating both plants and animals). Tropical coati have teeth that reflect their diet since their carnassials are more like grinding teeth. They are usually slow runners but excellent climbers and often live in groups. Canidae Family. The Canidae family is especially diverse, consisting of wolves, foxes, and dogs. Very adaptable animals with a keen sense of smell, they are good at running and often hunt in packs. The dog was probably the first animal to be domesticated (tamed) by humans. They live in groups, take care of their young, and are very territorial (defending where they live). 98


Felidae Family. The Felidae family is made up of some of the most efficient carnivores: the cats. Divided into two main groups called simply big cats and small cats, they can range in size from a tiny 2-pound (.91kilograms) animal to one more than 750 pounds (340.50 kilograms). The big cats include lions, tigers, jaguars, leopards, and cheetahs. Among the small cats are bobcats, lynxes, pumas, and domestic cats. An interesting difference between the two groups is that big cats can roar but not purr, while small cats can purr but not roar. Despite this oddity, all cats are especially good hunters, possessing retractable claws that are kept razorsharp and allow them to pad silently while stalking prey. They have large, pointed canine teeth and forepaws used to swipe at and claw their prey. Finally, most cats are solitary hunters. Hyaenidae Family. One of the more unusual groups are the members of the Hyaenidae family. Hyenas are skilled hunters who work together in packs and are one of the few carnivores that will regularly eat carrion (an already-dead animal). Hyenas are fiercely wild-looking, with heavy bodies and front legs that are longer than the rear ones, giving them a crouching or lurching-forward look. Their skulls are strong and their powerful teeth can easily crush bone, their favorite food.

Viverridae Family. The Viverridae family is represented by civets and genets which live in tropical areas. They resemble weasels and have long


Carnivore

A pack of spotted hyenas in Tanzania eat a zebra. Hyenas are one of the more unusual groups of carnivores. (Reproduced by permission of the National Audubon Society Collection/ Photo Researchers, Inc. Photography by Robert Caputo.)

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noses and short legs. Besides meat, they will also eat insects as well as fruit and eggs, and possess scent glands that produce musk, long-sought for its use in perfumes and lotions.

Herpestidae Family. The Herpestidae family is made of many species of mongoose and is an offshoot from the Viverridae family. Some species are solitary while others are very social and live in groups. Viverridae eat insects as well as meat.

Ursidae Family. Probably the most awe-inspiring and terrifying of the carnivores are the members of the Ursidae family, better known as bears. Including the polar bear, black bear, brown bear, or grizzly bear, all have large bodies and short, powerful limbs. While their sense of smell is much better than their hearing or sight, bears can walk or run upright on the soles of their feet. They are fiercely aggressive when provoked and have few natural enemies. Despite their appearance, bears are mostly omnivorous, and spend more time foraging for insects and berries than catching and eating prey. The exception are polar bears, which are mainly flesh-eaters and are such strong swimmers they have been known to pursue they favorite food (seals) as far as 40 miles (64.36 kilometers) from land.

Phocidae, Otariidae, and Odobenidae Families. The three families of the aquatic Pinnipedia suborder are made up of the Phocidae family (sea lions, fur seals, and earless seals), Otariidae family (eared seals), and the Odobenidae family (walrus). Called pinnipeds, all of these marine animals reproduce on land despite the fact that they spend most of their time in the water. All hunt underwater and have adapted to this environment, having nostrils that can close, limbs that are modified fins, and an insulating layer of fur or blubber to reduce the loss of body heat in cold water. Their hearing is especially keen and they can dive to great depths. All meat eaters do not belong to the Carnivora order, as there are birds (eagles and hawks), reptiles (snakes and alligators), fish (sharks and barracuda), and even plants (Venus’s flytrap) that regularly live on a diet of meat. Although many humans regularly eat meat, they are instead classified as omnivores because of their varied diet.

Cell The cell is the building block of all living things—basic to their makeup and basic to their functions. All cells also have the same processes: they “breathe” and take in food, get rid of wastes, grow, reproduce, and eventu100


ally die. It is a self-contained, living unit that takes in and expends energy. A living organism can be made up of a single cell or trillions of cells. Life scientists have divided the cells of all living things into two types based on whether they have a distinct nucleus (the control center of the cell) or not. They have named these categories prokaryotes (no nucleus) and eukaryotes (nucleus). Prokaryotic cells are found among the simplest of all living things, like bacteria and algae. Not only do these single-celled organisms lack a distinct nucleus but they are also missing many of the other sophisticated structures that perform specialized functions in the cells of more complicated organisms. Eukaryotic cells describe the cells of nearly every other life forms that have more than one cell. Animal cells and plant cells are therefore both eukaryotic. As the basic unit of life, cells make up every living thing—from tigers and people to cockroaches and flowers. However, although all living things are made up of cells, their size and shape will vary according to their function. In plants and animals, cells have specialized jobs to do, and in an animal, skin cells are different from muscle cells, which are different from nerve cells. These individual specialized cells group together to form tissues which, in turn, form organs. Altogether, the complete package of specialized cells will form a certain type of organism—whether a giraffe, oak tree, or a human being.

Cell

CELL STRUCTURE Despite their specialization, all cells have the same basic structure, and their protoplasm (the living substance that makes up the cell) consists of two parts: the cytoplasm and the nucleus. The cytoplasm is a jellylike fluid that contains many tiny, specialized structures, called organelles, each of which performs a particular job. Separate from the cytoplasm— but within the cell body—is the nucleus. This important part directs a cell’s activities and contains its genetic “program” that is written and stored in its deoxyribonucleic acid or DNA (the genetic material that carries the code for all living things). It is this program that makes living things different from each other. Since the cells of both plants and animals share a large number of organelles, there are some similarities. As mentioned, both plant and animal cells have cytoplasm. A jelly-like substance made up mostly of water, the cytoplasm is responsible for keeping the cell alive. Its does this mainly with a division of labor among its mitochondria, endoplasmic reticulum, and ribosomes. Mitochondria “breathe” for the cell, and because they break down food and release energy, they are often described as the “powerhouse” of the cell. The endoplasmic reticulum is a complicated U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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A photograph of animal cells showing some of its common features. All cells have the same basic structure. (©Don W. Fawcett, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.)

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network of tube-shaped membranes that makes and stores a range of substances (like proteins) for the cell. Ribosomes are tiny clusters of granules that actually synthesize or assemble the cell’s proteins. Within the cytoplasm of both plant and animal cells are found the common features of Golgi bodies: a vacuole and a cell membrane. The Golgi complex is a collection of membranes that operate as the cell’s transport system and store and release various products from the cell. These organelles were named after their discoverer, the Italian scientist, Camillio Golgi (1843– 1926). Vacuoles in plants are a large space filled with cell sap that keeps a plant crisp and prevents wilting. In animal cells, vacuoles are smaller and are used for storage and transport. Finally, both plant and animal cells have thin, strong plasma membranes that separate the cell from its surroundings. Both plant and animal cells also have a nucleus separate from the cytoplasm. The nucleus is the most noticeable feature of a cell since it is large and is surrounded by its own double membrane called the nuclear


envelope. Inside the nucleus are the chromosomes and the nucleolus, which function as the cell’s genetic program or chemical instructions; these function mainly when the cell is dividing.

Cell

DIFFERENCES BETWEEN ANIMAL AND PLANT CELLS While plant and animal cells have a great deal in common, each have distinguishing features. Animal cells have lysosomes and centrioles while plant cells do not, yet only plant cells have a cell wall and chloroplast. Lysosomes are small, round bodies that are actually powerful enzymes able to digest or break down the living matter (food) a cell takes in. Centrioles resemble bundles of rods and are important during cell reproduction. Only a plant cell has a cell wall made of a tough carbohydrate called cellulose. Light but strong, it gives the cell its shape and, by fastening together with its neighboring cells, is able to keep a plant growing upright. What most distinguishes a plant cell from that of an animal cell are the existence of chloroplasts and their unique role in the process of photosynthesis. This is the process by which a plant makes its own food in the presence of sunlight. Chloroplasts are disk-shaped sacs that appear green since they contain chlorophyll, which trap the energy in sunlight and produce sugars. Chloroplasts have their own membrane that separates them from the rest of the cell.

SHAPED ACCORDING TO FUNCTION Cells are shaped differently according to what function they perform in the animal or plant. Some look like snowflakes, others like rods, and some look like round balls. Most are able to move about using hairlike projections called cilia when they are numerous, or flagella when they each have a single tail. Sperm cells use flagella that they whip from side to side to move about. Most cells reproduce by a process called mitosis, in which a cell splits in two and makes an identical copy of itself. Specialized sex cells, which are all genetically different from each other, mix their different genes and produce an entirely different individual cell that has half the genetic content from each of the original two cells.

A SINGLE CELL BEGINS LIFE The concept of the cell is one of the most important ideas in the life sciences since every living thing begins life as a single cell. Described as a tiny chemical processing plant, it is also the simplest structure able to exist as an individual unit of life. Cells vary in size, shape, and specialization, with the smallest cells belonging to bacteria. The largest cells are the egg cells of mammals and birds. Knowledge of the structure and funcU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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RUDOLF LUDWIG KARL VIRCHOW German cell biologist Rudolf Virchow (1821–1902) helped establish cell theory and laid the foundations for modern pathology (the study of diseased tissue). Having demonstrated that the cell theory extended to diseased as well as to healthy tissue, Virchow is considered to be the founder of cellular pathology. Rudolf Virchow was born in a part of Germany that is now in Poland. After receiving his medical degree from the University of Berlin in 1843, he began working at a Berlin hospital while also teaching at the university. It was during these early years of his career that he first showed his very strong social conscience. While investigating an outbreak of typhus (several forms of infectious diseases caused by bacteria) in Silesia in 1848, he blamed the epidemic on the terrible conditions in which the people lived, thus indirectly blaming the government. Since the ruling establishment was being threatened by real revolutionaries (people promoting political or social change) at this time, Virchow was labeled a radical and lost his jobs. This was not entirely bad for him, since it allowed him time to pursue his scientific research. Thus, by the time he returned to Berlin in 1856 to join a new institute, he had formulated his ideas concerning the importance of cells. By the 1850s, the cell theory—the idea that all forms of life are made up of cells—had been established for some time, although no one had applied it to pathology. Beginning with his famous statement in Latin, Omnis cellula e cellula, meaning that every cell arises from a previously existing cell, Virchow set out to bring the study of disease down past the tissue level and to the cellular level. In 1858, Virchow wrote a book, titled Cellular Pathology, in which he demonstrated that the cell theory applied to diseased tis-

tion of cells enables scientists to not only understand better the living organisms they make up, but to treat or prevent things that go wrong at the cellular level. Since the cell is the basic unit of our growth and heredity, a disorder at the cellular level could result in genetic disorders or diseases like cancer in which cells divide in a wild manner, eventually forming tumors. Certain viruses also invade living cells, taking over their machinery to reproduce more viruses. Polio, AIDS, and hepatitis are examples of viruses harmful to humans. [See also Cell Division; Cell Theory; Cell Wall; Centriole; Chromosome; Cytoplasm; DNA; Gene; Golgi Body; Karyotype; Meiosis; Mitosis; Mitochondria; Nucleus; Nucleic Acid; RNA] 104


sue as well. In this work, he was able to show that diseased cells were in fact descended from healthy cells of normal tissue. Although this major work founded cellular pathology and laid the groundwork for later, more fundamental studies of the molecules within the cell, it nonetheless managed to go too far at times. It did this by stating that all diseases happened because of some imbalance or mistake in the cell. In other words, he refused to agree with the germ theory of disease put forth by the French chemist, Louis Pasteur (1822–1895). At times in his career, Virchow’s colleagues called him “the pope of medicine,” or “the pope of pathology,” and it was this stubbornness that led him to refuse to acknowledge Pasteur’s research on germs. Virchow’s idea of disease has been described as a civil war between cells rather than an invasion of the enemy from the outside. In fact, it is now known that both Pasteur and Virchow were correct depending on the circumstances, since disease can result from an outside invasion as well as from a breakdown of order within the cell.

Cell Division

Perhaps because Virchow was unwilling to compromise on Pasteur’s germ theory, he eventually put biology aside and took up anthropology (the study of the origin, distribution, and classification of humans) and archaeology (the study of the material remains of past humans). In fact. because of his major contributions to physical anthropology (the study of the origin and evolution of biological man), he is generally regarded as the founder of that subject as well. Later in his life, Virchow was elected to the Reichstag (the German parliament) and decided to work for social reform and the improvement of public health. He exercised his position’s power by designing new sewer systems and hospitals, and worked for better hygiene and food inspection. Still, it is his work in biology that distinguishes Virchow as a major figure in the life sciences. His discoveries can be said to have modernized the entire medical field as well as biology itself.

Cell Division Cell division is the process by which one cell divides to make two. It is the mechanism that enables an organism to grow, repair damaged tissue, and replace dead cells. There are two different forms of cell division: mitosis and meiosis. Mitosis (my-TOH-sis) is the division of a cell nucleus (a cell’s control center) to produce two identical cells. Meiosis (may-OHsis) is a form of cell division that produces differing sex cells. Mitosis is used to grow, replace, and repair with exact copies. Meiosis is used to produce an entirely new individual. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Without cell division, an organism could no longer grow, reproduce, or repair itself. Every day, the human body makes billions of new cells. Yet each human being began life as a single cell that was formed by the union of a sperm and an egg. Once that single cell began dividing (first into two, then into four, then eight, and so on), the process continued until a complete individual was formed. In organisms that are still growing, like seedlings or children, cells divide very rapidly, but as an organism grows older, many of its cells lose their ability to divide. Thus when cells are dying faster than they can be replaced, the organism begins to feel and show the effects of the aging process, and it looks and acts older. Older people develop wrinkles because their skin and muscle tone is lost as fewer cells are replaced. Older people also cannot heal themselves as quickly as they did when young. All cells have a basic cycle of life that they go through, according to their specialty. A healthy young person’s skin cells complete one cycle every twenty-four hours, but a person’s brain cells go through only so many cycles and then stop forever.

PHASES OF MITOSIS Despite the duration of an individual cell’s life cycle, each cell goes through the same process when it divides. Most cells are produced by mitosis. In mitosis, a single cell goes through a process in which it eventually produces an identical cell called a “daughter cell.” Each daughter cell then grows and soon becomes capable of dividing and producing yet another daughter cell. Mitosis takes place in four stages—prophase, metaphase, anaphase, and telophase—during which each chromosome copies itself, the nucleus divides in two, and the whole cell splits into two identical daughter cells. Each new cell receives a set of chromosomes identical to those of the original cell. During the first phase of mitosis, called the prophase, the cell’s chromosomes become shorter and thicker and duplicate themselves, appearing as double-stranded structures. These joined copies are called chromatids. The membrane around the nucleus also begins to disintegrate. During the metaphase, each pair of joined chromosomes line up across the center of the cell and attach themselves to tiny tubes called spindle fibers. In anaphase, the third stage of mitosis, the chromatids (joined chromosome pairs) are pulled apart by the spindle fibers and move toward opposite ends of the cell as it begins to divide. Actual division occurs in the telophase, when an envelope surrounds each set of chromatids to form a new nucleus in each. Finally, the cell splits in two as the cyotplasm (the cell’s jelly-like fluid) turns inward and pinches together, resulting in the 106


production of two new, identical cells. Despite minor differences, mitosis is basically the same for plant and animal cells.

Cell Division

PHASES OF MEIOSIS Where mitosis makes two identical cells, meiosis produces differing cells. Meiosis takes place whenever reproductive cells such as sperm, pollen, or egg cells are produced. The goal of meiosis is to reduce by half the number of chromosomes, so that when two different reproductive cells join together to form a new organism, it will have the exact same number of chromosomes as its parent. If this halving of chromosomes did not happen, the new cell produced would have twice the number of chromosomes that it should have. For example, in order to be human, an individual must have forty-six chromosomes. Without meiosis, that number would be ninety-two chromosomes after fertilization. Because of meiosis, fertilization will result in an offspring with the exact same number of chromosomes as the parents, getting one-half from each. Unlike mitosis, meiosis has only two major stages that result in the creation of four reproductive cells. Besides halving the number of chromosomes, meiosis also performs another major function. It allows genetic material to be “shuffled” since chromosomes cross over each other and swap genes before the cell divides. This is a random exchange of genetic

An illustration showing the phases of cell division or mitosis, in which one cell becomes two identical cells. (Illustration courtesy of Gale Research.)

PHASES OF MITOSIS

Interphase

Prophase

Anaphase


Metaphase

Telophase

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material that assures that an entirely new individual will be produced after fertilization. Because of this shuffle of genetic instructions, each reproductive cell is given its own unique set of instructions (making sure, for example, that no two egg cells will have the exact same combination of genes). This partly explains why brothers or sisters of the same parents have different characteristics. Eventually, when two of these unique cells are joined sexually (sperm and egg) to form a new individual (which further mixes the genetic instructions), a unique organism is created. The exception to this is, of course, the case of identical twins (two complete individuals with the exact same genetic makeup). Identical twins occur after fertilization when a human embryo spontaneously splits during the first cleavage (division) and forms two separate cells. [See also Cell; Chromosome; DNA; Meiosis; Mitosis]

Cell Theory Cell theory states that the cell is the basic building block of all life forms and that all living things, whether plants or animals, consist of one or more cells. It further states that new cells can only be made from existing ones, and that organisms can grow and reproduce because their cells are able to divide. Although the cell is the basic unit of life, it is also the smallest part of a living organism, and it is therefore too tiny to be seen by the naked eye. Because of this, the notion of a cell did not exist until the seventeenth-century invention of the microscope. Before then, it was commonly believed that the basic units of life were things like fibers and vessels, and that new living things came about by a process simply known as “spontaneous generation” in which life developed suddenly from dead or decomposing matter. By the middle of the seventeenth century, however, some individuals were beginning to investigate the new, subvisible world that the recently invented microscope could bring them. One of these pioneers was the Dutch naturalist Jan Swammerdam (1637–1680), who examined blood and in 1658 became the first person to observe a red blood cell. Also in Holland at the same time, the Dutch naturalist Anton van Leeuwenhoek (1632–1723) developed a powerful, simple (single-lens) microscope and saw, among other things, sperm cells and one-celled animals known as protozoa. In England at this time, the most famous microscopist was the physicist Robert Hooke (1635–1703), who published his book, Micrographia, in 1665. It was in this book that Hooke first used the word “cell” to de108


scribe the tiny structures he observed when examining a thin slice of cork under a microscope. He called the structures cells because they resembled a small room (as in “jail cell”). What he was really seeing were the dead remnants of structures that were filled with fluid when the cork was part of a living tree. Although Hooke did not discover living cells, he did coin the word “cell,” which was eventually adopted by biologists. Over the next 150 years, improvements in the microscope allowed observers to better study living tissue, and in 1831, the Scottish botanist Robert Brown (1773–1858) discovered that every plant cell had what he described as a “little nut” or nucleus in it. This discovery paved the way for the cell theory of German botanist Matthias Schleiden (1804–1881) and German physiologist Theodor Schwann (1810–1882). Although others had observed cells and even recognized that animal tissues contained cells, no one had as yet made the connection between cells and life. In 1838 however, Schleiden announced his findings that all vegetable matter is made up of cells and are the fundamental unit of plant life. The following year, Schwann took Schleiden’s basic idea and expanded upon it by stating that the cell is the basic unit of all living matter, plants and animals. Schwann’s clearly stated and well-summarized ideas were more elaborate than Schleiden’s and Schwann usually gets most of the credit for establishing cell theory, which he also named. Schwann also suggested that eggs were actually cells and that all life starts as a single cell.

Cell Theory

Although not the first to discover cells, Theodor Schwann was the first to clearly state that the cell is the basic unit of all living matter. (Photograph courtesy of The Library of Congress.)

By the middle of the nineteenth century, the final major point was added to cell theory by the German pathologist Rudolph Virchow (1821–1902), who summed up his research with the Latin phrase, Omnis cellula e cellula, translated as “all cells arise from cells.” Virchow correctly proposed that all cells originate from other cells (putting an end to notions of spontaneous generation), and further demonstrated that even diseased tissue comes from normal cells through the process of division. With this he founded cellular pathology or the study of diseased cells. Cell theory was essentially complete with the 1861 contribution of the Swiss anatomist and physiologist Rudolf Albert von Kolliker (1817–1905), who was the first scientist to study the developing embryo (a living organism in its early stages before birth) in terms of cell theory. Kolliker showed that eggs and sperm should be considered cells and argued that the cell nuU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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MATTHIAS JAKOB SCHLEIDEN AND THEODOR SCHWANN German botanist Matthias Jakob Schleiden (1804–1881) and German physiologist Theodor Schwann (1810–1882) are credited with establishing cell theory as a basic, unifying theme of all biology. The cell theory states that all forms of life are made up of cells and that all living things grow and reproduce because these cells can divide. While it was Schleiden who first formulated the theory in regard to plants, it was Schwann who applied it to both animals and plants and concluded that biology was a single science. Although they were not collaborators, they discussed and compared each other’s work, and together offered biology one of its most important concepts. Matthias Schleiden was born in Hamburg, Germany and began his professional life as a lawyer. He found this field unsatisfying and returned to school to study medicine, later specializing in botany (the study of plants). Using his microscope to study plant tissue, Schleiden eventually concluded that what he was seeing was the most essential unit, or the basic physical unit, of the living plant. In 1838, therefore, he first offered the thenunknown idea that the cell is the fundamental unit of a living plant. As the first to recognize the importance of cells, Schleiden announced that all the various parts of a plant consisted of cells and that they were all created in the same manner. Schleiden was incorrect, however, about a few things, incorrectly stating that all cells developed from the nucleus (a cell’s control center), which then disappeared after the cell was fully formed. Yet this does not take away from the overall importance of his work to the world of biology. First, his cell theory of plants focused attention on the basic unit of that organism and second, it laid the foundation for Schwann’s broader, more comprehensive work on cell theory. Theodore Schwann was a very different man than Schleiden. Six years younger, he was as gentle and quiet as Schleiden was impulsive and testy. Where Schleiden would publicly denounce his critics, Schwann would go

cleus was the key to the transmittal of hereditary factors. The only significant modern exceptions added to the original cell theory are that viruses (disease-causing agents) are not composed of cells, although they contain some genetic (hereditary) material and can reproduce in a host cell. Also, mitochondria (produces energy in cells) and chloroplasts (contains chlorophyll to capture the sun’s energy) are considered to be parts of cells but contain genetic material and can also reproduce in a cell. As one of the major theories in the life sciences, cell theory serves as the basis for all of today’s breakthroughs in the important field of genetics. 110


out of his way to avoid controversy. Before his work on animal cells, Schwann had done solid work on digestion and was actually the first person to isolate an enzyme (a protein catalyst that speeds up chemical reactions in living things) from animal tissue. An enzyme is a protein catalyst that speeds up chemical reactions in living things. Schwann was able to isolate the enzyme he called pepsin from the lining of the stomach. It was also Schwann who coined the word “metabolism” to describe all of the chemical changes that take place within living tissue.

Cell Wall

In 1839, Schwann was working on disproving, yet again, the ancient idea of spontaneous generation (that living things can be generated out of nonliving matter) when he arrived at his own cell theory. Schleiden of course had formulated his cell theory of plants the year before, and in October 1838, the two men got together for dinner and discussed each other’s work. Schwann listened to Schleiden’s description of what he had seen under his microscope, and said it sounded similar to what he was viewing under his. After dinner, the two went to Schwann’s lab and discovered that the cell structure in the spinal cord of a fish was almost the same as that of plants. It was left to Schwann, therefore, to conclude that a cell structure was common to all living things. Having extended Schleiden’s theory to animals, Schwann then formulated it in its best and clearest way. The cell theory would prove to unite animal and vegetable biology and show that it was fundamentally one science of biology. Both men were not entirely correct about everything they proposed, and while their theories did have specific flaws, their work was responsible for formulating one of the most fundamental concepts in biology. The establishment of the cell theory was a landmark achievement in the history of biology. Besides its unifying importance, it also would lead to further research by the best minds who extended the theory and who began to investigate what went on inside the cell.

[See also Cell; Cell Division; Mitosis]

Cell Wall A cell wall is a tough, semirigid case that surrounds a cell. Both plants and some single-celled organisms have cell walls. Cell walls are outside the cell membrane and are not part of the living cell. They protect the cell and provide it with support. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Centriole

Cell walls are found only in some single-celled organisms like fungi and bacteria, but they are found in all plants. They are one of the characteristics that separate plant cells from animal cells (which do not have cell walls). A cell wall is different from a cell membrane, since all cells have plasma membranes that are a part of the living cell. Membranes are also semipermeable and only allow substances of a certain size to pass in and out of the cell. A cell wall in a plant is a structure that is just outside the membrane and provides a plant with protection and rigidity. In plants, it is made up of a complex carbohydrate called cellulose that, although it is very tough, also allows water and solutions to reach the plasma membrane. Since cellulose is both light and strong, it provides the ideal material for a cell wall, acting as a kind of external skeleton that gives the cell (and therefore the plant) its shape and strength. The stem of a plant is able to hold itself up despite gravity by having thousands of cells lined up next to and on top of each other. As the cells take in water, they expand like a balloon and exert pressure against their own walls and against the stem walls. It is their pressure that holds the stem up. When a plant droops, it is because its cells lack water to push against the walls, and the cells begin to shrink. The cell walls of a green plant are made of cellulose, making it the most abundant organic compound on Earth. The cellulose in a plant’s cell walls is formed by fibers that are very strong because they are linked in a criss-cross mesh pattern. Herbivores or animals who eat nothing but green plants must have special digestive systems since the tough cell walls of a plant make it very difficult to digest. This is why herbivores have a much longer and more elaborate digestive tract than do carnivores (meateaters) who consume mostly easy-to-digest proteins. Herbivores must also consume enormous amounts of plant material since each mouthful of vegetation contains a relatively small amount of energy (compared to a protein diet). A plant’s cell wall helps protect the important membrane and gives the plant cell and the plant its shape and support. Fungi and bacteria also have cell walls, but they are not made of cellulose. Most fungi have a cell wall made of chitin, while yeast (a type of bacteria) cell walls are made of a different complex of carbohydrates. [See also Botany; Cell; Plants]

Centriole A centriole is a tiny structure found near the nucleus (a cell’s control center) of most animal cells that plays an important role during cell division. 112


Shaped like long, hollow tubes, centrioles help the X-shaped chromatids (duplicated chromosomes) to split apart when a cell divides in two.

Centriole

All animal cells must divide in order to repair themselves and to grow. Just before a cell divides and produces an identical cell (a process called mitosis), it duplicates or copies its chromosomes so that the new cell will have the same deoxyribonucleic acid (DNA) code as the old cell. Chromosomes contain DNA, which is the chemical that holds the code for all of an individual’s inherited traits. Most of the time, chromosomes are long and thin and appear as a tangled mass of thin threads in the cell nucleus. However, after the chromosomes make an exact copy of themselves and just before cell division is about to take place, the chromosomes begin to shorten and thicken and continuously fold in upon themselves. As they get shorter and thicker, the copy is attached to the original, and together they form a X-shaped structure. Each separate strand of this X-shaped structure is called a chromatid. While this is going on inside the nucleus of the cell, outside the nucleus small cylindrical tubes called centrioles are preparing to go to work. The centrioles soon move to opposite ends of the nucleus and fibers begin to form between the centrioles as they move away from each other. These fibers make up a structure called a spindle. As division continues to progress, the two connected chromatids are pulled apart by the spin-

A transmission electron micrograph of cellular centrioles. These tiny structures play an important role in cell division. (©Photographer, Science Source/Photo Researchers. Reproduced by permission.) U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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dle. Each spindle pulls a chromatid toward it, separating them and splitting each chromosome away from its duplicate. The two sets of chromosomes continue to move away from each other and toward a centriole. As division nears its end, a membrane forms between the two groups of chromosomes and there are now two new identical cells. Although centrioles are still somewhat mysterious to biologists, it is known that they play an important role during cell division since they act as organizing centers for the spindles that actually separate chromosomes. [See also Cell; Cell Division; Chromatin; Mitosis]

Cetacean A cetacean is a mammal that lives entirely in water and breathes air through lungs. Whales, dolphins, and porpoises are cetaceans. Cetaceans are found in all of the oceans of the world, and some live in fresh water. Whales, dolphins, and porpoises are all members of the order Cetacea. They are mammals that have so completely adapted to life in the sea that they have lost almost all of their hair as well as their hind limbs. Their front limbs have become flat flippers. Their bodies are fishlike and streamlined, with a thick layer of fat beneath the skin that keeps them warm. Despite living in water every day of their lives, they breathe air with typical mammal lungs and therefore must come to the surface to breathe through blowholes at the top of their heads. All cetaceans possess a certain combination of characteristics. All are endothermic, meaning they are warm-blooded. Unlike cold-blooded animals whose body temperature changes with their surroundings, cetaceans generate their own internal heat and are therefore able to function in most environments. This ability plus their fat layer enables them to exist in near-freezing water. As mammals, cetaceans bear live young, and their offspring are nourished by milk produced in the female’s mammary glands. As wateradapted mammals, cetaceans have entirely lost their rear limbs, and their front ones have changed into fins or flat flippers. They have smooth skin which allows them to move easily through water, and are virtually hairless except when born. Their body shapes are streamlined, which allows them to move through water with speed and efficiency. They take in oxygen and expel carbon dioxide through a blowhole at the top of their head. This is named after the manner in which they breathe. When a whale breaks the surface 114


after a long dive, it breathes out with such force that the spray can be seen for miles. After inhaling and going under again, the blowhole closes tightly and prevents water from entering the lungs. Cetaceans’ respiratory systems have adapted to surviving long periods between surfacings, and they are able to use oxygen in a highly efficient manner. There are two main types of cetaceans, characterized by what and how they eat. Those that are filter-feeders are called Mysticeti, and those that eat fish and squid are called Odontoceti. Size has nothing to do with what a cetacean eats, since some of the largest whales consume a diet of crustaceans that are sometimes as small as one-tenth of an inch (0.254 centimeters). Filter feeders are also called baleen whales since they have a curtain of fringed plates in the roofs of their mouths (instead of teeth) called a baleen. Using the baleen, these filter feeders strain out tiny shrimplike water animals called krill. Although baleen whales eat only krill, some, like the blue whale, grow to enormous proportions since they can


Cetacean

Two Atlantic bottlenosed dolphins. Whales, porpoises, and dolphins are all mammals and members of the order Cetacea. (Reproduced by permission of JLM Visuals.)

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eat tons of krill at a time. The great size of the blue whale is possible only because its great weight is mostly supported or held up by water. Toothed cetaceans are smaller creatures but more numerous. There are eleven species of baleen whales and sixty-seven species of toothed whales, dolphins, and porpoises. Toothed whales eat fish and squid, while dolphins and porpoises eat fish and other sea animals. The main difference between dolphins and porpoises is that dolphins have a beaklike snout and cone-shaped teeth. Porpoises have a rounded snout and flat or spade-shaped teeth. Cetaceans are considered to be intelligent mammals, having large brains and showing an ability to learn. They are also social animals and usually live in groups. Females care for their young and whales have been known to try to support an ill member of the group and keep it from drowning. All cetaceans have highly developed hearing, since sight is not an important sense for them. Many species use a form of echolocation by making a sound like a click. Using these sounds, they are able to tell how far away something is by how long it takes for their sound to bounce back to them. Some species communicate with one another through clicks, while others, like the humpback whale, use song. Cetaceans reproduce by internal fertilization and usually only a single offspring is produced. Whales have been hunted commercially since the seventeenth century and today many species like the sperm whale are in danger of extinction. Dolphins and porpoises also are killed accidentally by tuna fisherman whose nets trap them below the surface and drown them. [See also Mammals]

Chaparral A chaparral is a geographical region characterized by mild, cool winters and hot, dry summers. Chaparral is sometimes called “scrubland” because the periods of drought and regular fires allow only a certain type of thorny scrub or thicket to thrive there. A chaparral has what is called a Mediterranean climate because of its mild, moist winters and very dry summers. Besides the Mediterranean however, there are other parts of the world that have a similar climate, such as southwestern Australia, central Chile, the Cape region of South Africa, and southwestern California and northern Baja California, Mexico. The lands in these different areas usually have their own particular names (“mallee” in Australia, “mattoral” in Chile, and “fynbos” in South Africa), and the name chaparral most often refers only to a certain part 116


of California and Mexico. The word chaparral comes from the Spanish word chapparo which originally described a thicket of evergreen shrubs.

Chaparral

PLANT LIFE IN A CHAPARRAL It is the vegetation that grows in this particular climate that most characterizes a chaparral. Typically, a chaparral is composed mainly of woody evergreen shrubs that have adapted to summer drought and fire. There are few tall trees. The leaves of most of these shrubs are small and usually have a waxy, or leathery, covering. Their small size minimizes moisture loss, as does the waxy outer covering. Since the shrubs are evergreen and are not deciduous (they do not lose their leaves all at once), they are ready to absorb rainwater whenever it falls. Dormancy (in which a plant slows down all its processes) is another way that these plants conserve water loss during a drought. Most chaparral plants have two sets of roots that make them ready to take in any avail-


A chaparral plant community in Diablo State Park in California. This plant life is composed mainly of woody evergreen shrubs that have adapted to summer drought and fire. (©Photograper, The National Audubon Society Collection/ Photo Researchers. Reproduced by permission.)

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Chloroplast

able water. One of these is a very long taproot that can obtain water deep underground. Another is a lateral root system that grows barely under the surface and can absorb water before it soaks down deeper into the soil. Fire is another constant in a chaparral, and all of the shrubs that grow there have adapted in some way to surviving fire and sometimes even using fire to their benefit. Chaparral fires happen naturally every ten to forty years, and they are beneficial in the long run. They remove dead plants that built up over the years and release their ash and minerals into the soil to be reused. They also open up the ground, letting more light in and allowing new plants to grow. Most chaparral plants can sprout from their burned base following a fire, and some even need a fire to open their seed coats.

ANIMALS IN A CHAPARRAL Just like the plants that live in a chaparral, the animals that make their home there have adapted to its extremes. Common chaparral animals in the United States are the mule deer and coyotes. Rodents, reptiles, and rabbits use shrubs to hide from the red-tailed hawk and barn owl. Rattlesnakes and deer mice are also usually abundant. Many of these animals have adapted to their environment since they are able to go without water for long periods, and most avoid activity in the intense midday heat. Humans are a different story and they are now threatening the natural balance of chaparral. As the chaparral in California becomes an increasingly popular place to build a home, people are having a major effect. Fires that once were natural and needed in a chaparral are suppressed since they would threaten homes. Thus the chaparral becomes thicker and denser, so that when a fire does break out and cannot be contained immediately, the fire that results is ferociously hot and fast-moving, destroying not only homes but even plants that had been fire-adapted. These super-hot fires destroy both shrubs and seeds entirely. Since the seeds or shrubs cannot resprout as before, mud slides often result since there is no vegetation to hold the soil. As a result, these mudslides may cause harm to people, animals, and plants and result in further damage to the chaparral itself. [See also Biome]

Chloroplast Chloroplasts are the energy-converting structures found in the cells of plants. As one of the many tiny organelles (structures inside a plant that 118


have a particular function) in a plant cell, it is the site where photosynthesis (the process by which plants convert the sun’s energy into food) occurs. Chloroplasts are not found in animal cells and are the most distinguishing feature of a plant cell. Chloroplasts allow a plant to capture light energy from the sun and turn it into chemical energy. Chloroplasts accomplish this conversion because they contain chlorophyll, a bright green pigment that absorbs light energy and carries out a chain of chemical reactions. These chemical reactions result in the production of glucose, which the plant uses as food, either storing it or making cellulose to build its cell walls. Chlorophyll is stored in disk-shaped sacs or membranes called thylakoids. It is here that the light energy absorbed by the chlorophyll is directed and changed into chemical energy. This energy allows the plant to take in carbon dioxide, give off oxygen, and eventually produce the plant’s food.


Chloroplast

A transmission electron micrograph of a chloroplast from a tobacco leaf. Chloroplasts are the organelles the allow a plant to capture light energy from the sun and turn it into chemical energy. (©Dr. Jeremy Burgess/ Science Photo Library, National Audubon Society Collection/ Photo Researchers, Inc. Reproduced by permission.)

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Some plant cells contain only one large chloroplast, but other plant cells may have hundreds of smaller ones. Those areas containing concentrations of chlorophyll are called the grana of the chloroplast, and the spaces between the grana are called the stroma. Inside a plant cell, the chloroplasts are separated from the rest of the cell by a membrane and are usually located around the edges of the cell. Although food production takes place at the cellular level, altogether those cells form an individual leaf on a plant. So it is within the cells of the leaf that photosynthesis occurs, providing the entire plant with food and energy. The energy that the plant has stored is converted back into usable food when the plant is placed in the dark and cannot photosynthesize. Organisms that eat green plants are able to obtain the light energy originally captured by photosynthesis. [See also Cell; Organelle; Photosynthesis; Plants]

Chromatin Chromatin are ropelike fibers containing deoxyribonucleic acid (DNA) and proteins that are found in the cell nucleus and that contract into a chromosome just before cell division. In its unraveled state, chromatin look like beads on a string. In its condensed state, they fold into tight loops that coil up and form x-shaped chromosomes. When scientists first became able to examine the cell under a microscope using stains to distinguish among its many parts, they noticed that a particular granular material inside the nucleus became more brightly colored by the stain than did other structures. These colored granular structures were named chromatin as derived from the Greek word khroma meaning “color.” At much higher magnification however, it was discovered that chromatin was not granular but was much more threadlike, with proteins attached to it like beads on a chain. It was soon discovered that in every cell that is not about to actually divide, the cell’s genetic material floats about the nucleus as unwound, extremely fine threads or strings called chromatin. In human cells, there are forty-six strands of chromatin forming a tangled mass that has been described as “a bowl of microscopic spaghetti.” When a single “noodle” or strand of this mass is examined more closely, it is seen as a coil made up of another compactly folded strand of material which itself is made up a series of loops that are coiled around protein molecules called “histones.” It is within these loops that the “twisted ladder” or double helix structure of DNA is found. 120


Just before a cell is about to divide, this apparently tangled mass of forty-six strands of chromatin begins to condense or gather together to form forty-six easily recognizable, x-shaped packages of genetic information called chromosomes. One of the main purposes of a chromosome is to package the DNA into tight coils so that it all fits into the nucleus. Each chromosome then makes a copy of itself and splits apart, dividing into two identical new cells. In the new cell, the condensed chromosome unravels into its earlier state, containing all the instructions needed to make the cell work and ready to pass on genetic material to the next generation.

Chromosome

[See also Cell; Cell Division; Chromosome; DNA; Nucleus; Mitosis; Protein]

Chromosome A chromosome is a coiled structure in the nucleus of a cell that carries the cell’s deoxyribonucleic acid (DNA). DNA is the genetic blueprint that contains the genes that both direct the cell’s activities and determine the characteristics of the organism. Chromosomes are found in nearly every cell of the body, and different species have different numbers of chromosomes. They are probably the most important part of a living cell since they contain all the necessary information to make a cell work. Chromosomes were given their name, which means “color body,” because they easily take up the dye stain that biologists use to study cell structures under a microscope. Despite this, chromosomes are only visible in the nucleus (a cell’s control center) when the cell is dividing (although they are always present). Just before division occurs in a cell, chromosomes are easily seen because they condense or bunch together forming tightly coiled, rodlike shapes (many look like little “X’s”). Until this happens, chromosomes exist in the nucleus as unwound, extremely fine threads or strings of protein and DNA that are called chromatin. These loose, strung-out forms of chromatin contain DNA which, in turn, consists of genes. In humans, there are forty-six of these ropelike fibers in the nucleus, which condense or contract into a chromosome just before cell division. Humans have forty-six chromosomes. Chromosomes are bunched together strings of DNA and proteins, and it is these DNA molecules that contain the cellular instructions or coded information that we call genes. The gene is considered the basic unit of heredity. In summary, chromosomes are found in nearly every cell of our bodies. Chromosomes are made of DNA, and DNA stores genes. It is genes that carry the vital codes and information that not only tell a cell what to do, but which get passed on to the next generation by sexual reproduction. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Chromosome

THOMAS HUNT MORGAN American geneticist (a person specializing in the study of genes) Thomas Hunt Morgan’s work with the fruit fly established the chromosome (a coiled structure in the nucleus of a cell that carries the cell’s deoxyribonucleic acid, or DNA) theory of inheritance. He discovered that chromosomes are composed of discrete units called genes which are the actual carriers of specific traits. He also showed that when genes mutate, or change, the traits they control also change. Thomas Hunt Morgan (1866–1945) was born in Lexington, Kentucky and grew up surrounded by nature and wildlife. As a youngster, he had an intense interest in biology and later majored in zoology in college. After obtaining his Ph.D. from Johns Hopkins University in 1890, he taught for awhile, and in 1904 became professor of experimental zoology at Columbia University where he would remain until 1928. Morgan had long been interested in heredity and believed that it was an important phenomenon barely understood. When Morgan began his research in 1904, the world had just learned of Austrian monk Gregor Mendel’s laws of heredity. One of these laws stated that mixing traits did not result in a blend of traits, but instead these traits sorted themselves out according to a fixed ratio. Most scientists realized that the behavior of newly discovered chromosomes during cell division seemed to match Mendel’s laws. However, everyone knew that there were only slightly more than two dozen pairs of chromosomes in the human cell, and it did not seem possible that they alone could account for the huge range of inherited characteristics exhibited by people. The explanation might be that each chromosome contained large numbers of different “factors,” or “genes.” In 1907, Morgan decided to attack this problem using a new tool to science called the Drosophila melanogaster, or the common fruit fly. Fruit flies are the very tiny flies attracted by the smell of fruit. Leave a bowl of fruit out in the summer, and chances are these flies will somehow get in the house

Genes have been compared to instructions or to a recipe for making proteins. Proteins can be found in virtually every part of the body, and they help cells do all the complicated things they have to do. There are somewhere between 50,000 and 80,000 genes in the human body, and each contains instructions on making a protein that has a specific purpose. There are genes for proteins that make our eyes, our organs, our hair, and our skin. There are genes that influence how tall we will be or what our skin color is. One of the main functions of chromosomes is to package the DNA that contains these genes that tell our cells what type 122


and swarm onto the peaches and plums, attracted by the odor. For Morgan, however, these flies proved perfect for genetic research since they were inexpensive, very easily bred in large numbers, multiplied rapidly, and best of all, their cells possessed only four chromosomes. Morgan would therefore tackle the problem of inheritance by closely following the generations of flies. It was in his famous “Fly room” with his undergraduate students Calvin Blackman Bridges (1889–1938), Alfred Henry Sturtevant (1891–1970), and Hermann Joseph Muller (1890–1967), all of whom went on to do major work in genetics, that Morgan discovered many instances of mutations. He was able to trace these in later generations and to prove that genes were linked in a series on chromosomes (or inherited together) and were responsible for identifiable, hereditary traits. Morgan was also the first to explain sex-linked inheritance when he located the mutant white-eye gene on the male sex chromosome (fruit flies should all have red eyes). He further explained the “mistake” phenomenon called crossing-over in which traits found on the same chromosome are not always inherited together. In this, Morgan showed that one chromosome actually exchanged material with (or crossed over to) another chromosome. This mistake proved to be an important source for genetic diversity since it can possibly add unpredictable variety.

Chromosome

By 1911, Morgan and his “Fly room” team had created the first chromosome maps for fruit flies. In 1915, Morgan and his students published a summary of their work, The Mechanism of Mendelian Heredity, which would lay the groundwork for all future genetically based research. Morgan’s rigorous work with the humble fruit fly enabled him to discover how genes are transmitted through the action of chromosomes, thus confirming Mendel’s laws of heredity and laying the foundation for modern experimental genetics. In 1933, Morgan was awarded the Nobel Prize in Physiology or Medicine for his work on heredity. His student, Hermann Muller, also experimented with fruit flies and proved that x rays can damage genetic material. For this, Muller received the 1946 Nobel Prize.

of proteins to make. This protein-making is a nearly constant activity in the cell and can be considered a kind of biological housekeeping.

MITOSIS Every day our bodies make billions of new cells that are identical to the ones they will replace. This is because every cell in our bodies has its own life cycle, and some, like skin cells, complete their full cycle in only twenty-four hours. In this steady production of identical or “sister” U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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cells, called mitosis, each chromosome makes a copy of itself, moves to the opposite ends of the cell membrane, and splits into two identical cells after a membrane forms across the cell’s middle. This process assures that each new cell gets the correct genetic material.

MEIOSIS During sexual reproduction, however, an entirely different process called meiosis takes place that involves chromosomes. When a sperm fertilizes an egg, each sex cell (sperm and egg) starts with only twenty-three individual chromosomes, unlike all other cells in the body that have a full set of forty-six (or twenty-three pairs). When the sperm and egg join together, the first new cell created gets twenty-three chromosomes from the mother (egg cell) and twenty-three chromosomes from the father (sperm cell), to form a full complement of forty-six chromosomes. Meiosis also adds a final shuffling of genes that happens before division takes place. During this shuffle, chromosomes cross over each other and actually swap genes, thus further assuring that each sex cell has its own unique combination of genetic instructions. Unlike mitosis, the new cell (and eventually new organism) created is not identical to the cells that formed it but is rather a mixture of the chromosomes of two organisms. This is why in

A scanning electron micrograph of a human X chromosome. Chromosomes are probably the most important part of a living cell since they contain all the necessary information to make a cell work. (©Biophoto Associates, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.)

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organisms that reproduce sexually, an offspring does not look exactly like either parent since it inherited genes from both.

Cilia

At the chromosome level, the difference between a male and a female is only one gene on one chromosome. Chromosomes are always in pairs, and those that determine the sex of an individual make up two of our forty-six chromosomes. These two are known as sex chromosomes. The other forty-four chromosomes are not involved in determining sex and are called autosomes. Females have a pair of sex chromosomes called XX, while males have a pair called XY. Thus the difference is only one (Y) chromosome. If a human embryo is given two X chromosomes, a certain area of cells becomes the egg-making part (the ovaries) and the embryo will develop into a female. If it has an X and a Y chromosome, the Y signals the cells to start producing sperm-making parts (testes), and the offspring develops into a male. Nature, however, can and does make mistakes (usually during meiosis), and when they occur at the chromosome level, they can be disastrous. The most common mistake in humans is called aneuploidy. This occurs when an offspring has an extra or a missing chromosome. Most cases of aneuploidy result in the mother aborting her fetus spontaneously (called a miscarriage). This can be considered nature’s way of putting an end to a mistake. One instance in which fetuses do develop and are born is that of Down’s syndrome in which the offspring has an extra chromosome. However, these people suffer from mental retardation and some physical deformities. [See also Cell; DNA; Genetic Disorders; Genetic Engineering; Genetics; Mendelian Laws of Inheritance; Mutation; Nucleic Acid; Protein]

Cilia Cilia are microscopic, hairlike structures that project from the edges of certain types of cells (the building blocks of all living things) and allow them to move themselves or things that are close by. Not all cells have cilia, and those that do are usually animal cells rather than plant cells. In higher animals, such as humans, cilia also refer to the hairlike lining of the nose, ear, and trachea (the air passage to the lungs) that keep those passages clean from dust, pollen, bacteria, and mucus. Animal cells must often move about, and cilia are the primary means by which they achieve movement. Cilia are composed of microtubules or U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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A computer graphic of cilia found on the surface of the human windpipe, or trachea. (©Photographer, Science Source/Photo Researchers. Reproduced by permission.)

extremely tiny tubes whose action or movement can be controlled by the cell. By coordinating the wavelike action of its cilia, the cell can either send itself through its environment or help to move the environment past itself. If one-celled organisms like the protozoan Paramecium are observed under a microscope, the rhythmic, wavelike motion of its cilia can be easily seen beating against its liquid environment as if it were rowing in a coordinated way. Certain one-celled organisms also use their cilia to capture food and move it into their gullet. Certain cells, like gametes or sex cells, only have a single projection that they use to move about. When a cell has this sort of singular, long, hairlike projection that resembles a tail, it is called a flagellum instead of a cilium. Sperm cells are a good example of cells that have flagella. Besides the epithelial (skin) tissues in most higher animals contain a carpet of cilia whose purpose is to move tiny particles across and away from sensitive surfaces. The wavy motion of the cilia in the uterine tubes in women help move and guide the fertilized ovum (human egg) down to the uterus where it can attach itself and grow into a fetus. Even a clam uses its cilia to fan water containing oxygen into its gills. Cilia are so tiny that just the trachea may contain as many as 1,000,000,000 cilia per square centimeter.

Circulatory System The circulatory system is a network that carries blood throughout an animal’s body. Described as an internal transport system or a distribution system, the circulatory system maintains a constant flow of blood throughout the body, carrying nutrients and oxygen to the body’s tissues and taking away its waste products. It also helps regulate the body’s temperature; carries substances, like an126


tibodies and white cells that protect the body from disease; and transports chemicals, such as hormones, that help the body regulate its activities.

Circulatory System

All animal cells live in a liquid environment, taking in oxygen and nutrients and creating waste products. All, therefore, require a way to gain access to what they need to grow, to perform their specialized task, to reproduce, and to dispose of their waste products. For any animal larger than a single-celled organism—which meets its needs by passing substances through holes in its membrane (called diffusion)—a complex circulatory system is required in order to make sure that every single cell gets what it needs.

OPEN CIRCULATORY SYSTEM There are two main types of circulatory systems—an open system and a closed system. Most invertebrates (animals without a backbone) have an open circulatory system. It consists of a fairly simple network of tubes and hollow spaces. In this system, a heart (or a series of hearts) pumps blood out of the vessels (a duct for circulating blood) and into the sinuses or open spaces of an animal’s body. Mollusks, like clams, and arthropods, such as crayfish and grasshoppers, have an open circulatory system. In such a system blood flows slowly and under low pressure into the open spaces in these organisms’ body cavities. This blood also bathes their cells, allowing them to obtain food and oxygen while eliminating waste. Open circulatory systems pump a bloodlike fluid called “hemolymph” that closely resembles seawater. However, one of the disadvantages of this type of system is that it cannot respond quickly to change, nor can it supply large amounts of oxygen.

CLOSED CIRCULATORY SYSTEM All vertebrates (animals with a backbone) and some invertebrates, like earthworms, have a closed circulatory system. This means that the blood never leaves their vessels. For vertebrates, blood is pumped by the heart throughout the body via a network of closed vessels that become finer and finer as they get farther away from the heart. Closed systems are more efficient than open ones, since it is easier for them to respond to sudden changes and to alter the distribution of blood. Although there are invertebrates with closed systems, it is by far the predominant characteristic of vertebrates—whether fish, amphibian, reptile, bird, or mammal. The basic components of the vertebrate circulatory system are the heart, arteries, capillaries, veins, and the blood itself. In the human body, the heart is a hollow, muscular pump that forces blood to move throughout the body. The human heart consists of two pumps U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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that lie side by side. The left and stronger pump receives oxygen-fresh blood from the lungs and pumps it under great pressure to the cells throughout the body. The weaker right side receives “used” blood from the cells and sends it to the lungs to have carbon dioxide removed and to be freshened with oxygen. The heart muscle beats at its own automatic rhythm and pumps in a certain correct sequence. Arteries are blood vessels that carry blood away from the heart. They are specially designed, with thick, strong walls, since the blood they transport is under high pressure. Puncturing an artery can cause blood to spurt and travel through the air. Arteries also serve to smooth out the flow of blood by absorbing much of the rhythmic shock of the pumping heart. Arteries get smaller farther from the heart and are called arterioles. As oxygen-rich blood continues the one-way trip from the heart and to the cells, the capillaries receive blood from the arterioles and pass it directly to the surrounding tissues and cells. Capillaries are finer than human hair (they are one cell thick) and have very thin walls where the critical exchange of nutrients, oxygen, and waste takes place. Capillaries merge to form venules or tiny veins, which in turn, merge to create large veins that will carry the blood back to the heart. Large veins are thinner than arteries (since the blood is now under much lower pressure) and are the primary blood vessels for the one-way, return trip of blood to the heart. Large veins also have one-way valves that prevent deoxygenated blood (depleted of oxygen and full of carbon dioxide) from flowing backwards. These veins empty into the parts of the heart called the vena cava, which in turn, empty directly into the heart’s right atrium and back on to the lungs to start the cycle all over.

Opposite: An image of the main components of the human circulatory system. The heart (placed between the lungs) delivers blood to the lungs, where it picks up oxygen and circulates it throughout the body by means of blood vessels. (Drawing courtesy of Gale Research.)

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The route that blood travels in humans (as well as in birds and other mammals) is called “double circulation,” which is in contrast to animals whose blood circulates in a single loop from the heart, around the body, and back again. In a double circulation system blood flows alternately through the lungs and throughout the body in a figure-eight pattern. After completing each loop, blood returns to a different side of the heart. In the human body, an entire double-loop cycle takes less than one minute. While the actual path that blood follows will vary among different groups of vertebrates, all are based on a similar system whether single or double loop. For example, fish have single loop circulation and a four-chambered heart, while amphibians and most reptiles have a three-chambered heart.

WILLIAM HARVEY DISCOVERS BLOOD CIRCULATION The English physician William Harvey (1578–1657) discovered blood circulation in 1628. He demonstrated, contrary to widely-held beliefs that U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

carotid artery jugular vein subclavian vein subclavian artery superior vena cava pulmonary artery

axillary vein axillary artery

pulmonary vein heart

inferior vena cava

cephalic vein brachial vein brachial artery

renal vein renal artery

ulnar vein ulnar artery radial vein radial artery

femoral vein femoral artery Arteries carry blood away from the heart.

great saphenous vein

Veins return blood to the heart. aorta

small saphenous vein

superior vena cava pulmonary artery

anterior tibial artery posterior tibial artery pulmonary valve aortic valve

left artium

right artium tricuspid valve

right ventricle

inferior vena cava

pulmonary vein

mitral valve

left ventricle

The Human Heart


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WILLIAM HARVEY English physician and biologist William Harvey (1578–1657) was the first to describe the circulation of the blood through the heart and the blood vessels. As a great experimenter, Harvey was able to prove that blood does not ebb and flow like waves in the body as people believed, but instead travels in a closed, or one-way circle. Many consider Harvey to be the founder of modern physiology (which studies how the different processes in living things work). William Harvey was born in Folkestone, England, the son of well-to-do parents. After earning his degree from Cambridge in 1597, he did what anyone who wanted the best medical education available did: he went to Padua, Italy. At that time, Padua was able to boast that it had many great scholars, including the young astronomer, Galileo Galilei (1564–1642), on its faculty. Harvey stayed in Padua until he obtained his degree in 1602, and then he moved back to England. Given his father’s connections, he soon had important patients, and later became physician to the king. However, Harvey was more interested in experimentation than in his day-to-day medical practice, as evidenced by his claim to have dissected eighty different species of animals by 1616. Harvey always dissected with a purpose, and most often, he focused on learning more about the heart and its vessels. Because of his extensive hands-on experience, Harvey already knew that heart muscle was basically a pump that acted by contracting, which pushed the blood out. He also learned that the valves that separated the heart’s two upper and two lower chambers were one-way valves, and that veins (a branching system of vessels through which blood returns to the heart) even had one-way valves. All of this was leading Harvey far away from what most medical men of his time believed. Traditional medicine was based on the writings of Galen (A.D. 129–c.199) who lived some 1,400 years before Harvey and whose human physiology was based on his dissections of animals. Galen taught that the liver was the im-

blood flowed from the heart in a continuous, one-way cycle. The current understanding of the circulatory system was born of Harvey’s work. It is now known that the circulatory system performs many vital functions in respiration (delivering oxygen to the cells and removing carbon dioxide); in nutrition (carrying nutrients to the cells and liver); and in waste removal (transporting poisons like salts and ammonia to the liver for disposal). The circulatory system also helps the body fight disease by acting as a means of transport for the lymphatic system, which is part of the body’s immune system. This system filters harmful substances out of the 130


portant organ for the blood and that the blood that formed there ebbed and flowed throughout the body like the back-and-forth of the oceans’ tides. Before Harvey decided to refute Galen, who he knew to be wrong, he performed even more experiments on animal and human hearts. He monitored the beating heart and made rough calculations of the volume of blood leaving the heart per beat and then compared that to total blood volume. It seemed impossible to him that blood could be broken down and reformed, as Galen said, fast enough to account for the amount of blood in the human body. The only explanation was that it must be the same blood moving in circles throughout the body. Finally, in 1628, Harvey published his slim, seventy-two-page book in Holland, in which he argued that blood was in motion all the time and that it circulated. The translated title of his landmark work is On the Motions of the Heart and Blood.

Class

Since Harvey’s book was a direct assault on Galen and all those who supported his ancient ideas, it is not surprising that Harvey found himself being ridiculed and condemned. His medical practice suffered for a time, but Harvey refused to be drawn into a defensive debate and instead, let the facts speak for themselves. Throughout this, he retained the king’s favor. By his old age, the theory of the circulation of the blood was accepted and he had become a highly respected and even revered man of science. Interestingly, the one part of Harvey’s theory that he could not prove was the crucial statement that blood moved from the arteries (a branching system of blood vessels that carry blood away from the heart) to the veins. He was unable to point out any visible connections between the two. However, he knew from his dissections that both vessels divided down to smaller and finer vessels, again and again, and so he simply assumed that they were too small to see with the naked eye. He was, of course, correct and was proven so about twenty-five years later when his countryman, Marcello Malpighi (1628–1694), used his new microscope and discovered extra-fine blood vessels called capillaries that connected the smallest arteries to the smallest veins.

bloodstream and carries white blood cells that destroy harmful bacteria, viruses, and other invaders. [See also Blood; Heart; Lymphatic System; Respiratory System]

Class The term class refers to one of the seven major classification groups that biologists use to identify and categorize living things. These seven groups U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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are hierarchical and range in order of size. Class is the third largest group and is located between phylum and order. The classification scheme for all living things is: kingdom, phylum, class, order, family, genus, and species. Members of a class have more characteristics in common than do members of a phylum. The following example compares animals that are included in phylum with those in class. In the former are included mammals, reptiles, and birds, yet each of these are placed in a different class. Humans and mice belong to the class Mammalia because they are characterized by having hair or fur on their bodies and feeding milk to their young. Lizards and snakes belong to the class Reptilia since they are covered with scales and do not feed milk to their young. Sparrows and eagles belong to the class Aves because they have feathers on their bodies and do not feed milk to their young. All, however, belong to the same phylum Chordata since they all have a vertebrae or backbone. In addition to these obvious differences between classes, such as body covering and the way the young are nurtured, there are more subtle ways of determining an organism’s correct class. One of these is called comparative biochemistry. This process compares the deoxyribonucleic acid (DNA) of different organisms to determine if their genetic information is the same. Since the grouping by class is near the midpoint of the seven groupings, the organisms that will remain and be lumped in the next group, order, will have even more in common with each other. [See also Classification; Family; Genus; Kingdom; Order; Phylum; Species]

Classification Classification is a method of organizing plants and animals into categories based on their appearance and the natural relationships between them. Also called scientific classification, it is science’s way of identifying and grouping living things. The classification of organisms is a science called taxonomy, or systematics. The first person to attempt any type of systematic grouping of organisms was the Greek philosopher and scientist, Aristotle (384–322 B.C). Until his work, most people simply divided all plants and animals into two basic categories: useful and harmful. As a careful observer of the natural world, Aristotle began arranging organisms according to their physical similarities. Since there were only about one thousand organisms known in his time, he classified animals according to those with red blood 132


(vertebrates or having a backbone) and those with no red blood (invertebrates or no backbone). He also classified plants by size and by whether they were herbs, shrubs, or trees. Despite many mistakes and an oversimplified idea, Aristotle’s impulse to classify and to categorize organisms was a necessary attempt to make sense of the diversity of life in order to study it better.

Classification

THE BENEFITS OF CLASSIFICATION In the life sciences, the need to organize is very important and extremely useful. Classification helps biologists keep track of living things and to study their differences and similarities. It also shows biologists how living things are related to one another through evolution (the process by which living things change over generations). Classifying also saves time and effort. There are many possible ways to classify life: appearance, behavior, evolutionary history, or life development from fertilization to adulthood. The modern classification system is considered a natural system since it represents genuine relationships between organisms. In this natural system, the more closely organisms are related to each other, the more features they have in common. This system is also hierarchical, meaning that its categories are grouped according to size in a series of successively larger ranks.

CAROLUS LINNAEUS DEVELOPS BINOMIAL SYSTEM OF NOMENCLATURE The system used today is based on the work of one individual, the Swedish physician and naturalist, Carolus Linnaeus (1707–1778). In his day, it sometimes took as many as ten words to name a particular organism and no standard system existed upon which everyone agreed. Linnaeus traveled throughout Europe compiling lists of the animals and plants he encountered, and in 1735 published a book which tried to make some sense of this great diversity. By 1758 he had completed his huge encyclopedia called System of Nature which described and classified all known organisms by their structure and placed them in one of the seven levels of his hierarchical system. Linnaeus also developed the binomial system of nomenclature, which gave a distinctive two-word name to each species. This system is still followed with the first part being the genus name, and the second part serving as its species name. For example, both the bobcat and the house cat belong to the genus Felis, but the bobcat’s species is rufa (Felis rufa) while that of the domestic cat is Felis domestica. The second name is usually descriptive of the particular animal. Among the rules for this system, the two-part name is always used. The species part U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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CAROLUS LINNAEUS Swedish botanist (a person specializing in the study of plants) Carolus Linnaeus (1707–1778) devised the first orderly system of classifying living things. He also introduced the binomial system of nomenclature (a twopart naming system) that is still in use today. He is called the father of taxonomy (the science of classifying living things) because his system was able to impose a much-needed order on the study of life itself. Being able to identify a plant or animal, to tell how it is different from others, and to know how it fits into the entire natural world is something that people simply take for granted in today’s world. However, there was a time in the history of the life sciences when naturalists (people specializing in the study of plants and animals in their natural surroundings) used as many as ten words to give something a specific, descriptive name, and even with all that effort, there was no guarantee that it would be used by others or that someone who spoke a different language would know what the name meant. This describes what the life sciences were like before the great classifier, Carolus Linnaeus, gave science a practical way of naming organisms that was based on clear and simple standards upon which everyone could agree. Linnaeus was born Carl von Linne in South Rashult, Sweden. (Linnaeus is the Latin version of his name.) His father was a clergyman, and the very young Linnaeus was so interested in gardens and growing things that the locals called him “the little botanist.” When his father sent him to medical school, Linnaeus was able to combine school with botanical exploring trips that only made him more interested in plants. When he became lecturer in botany at Uppsala University at the age of twenty-three, he was able to go on longer, more extensive excursions to Lapland in 1732. After traveling 4,600 miles (7,407 kilometers) throughout northern Scandinavia discovering new plant species and observing animal life, he began to formulate the details of an idea that he had first expressed in a paper some years before. By 1735, Linnaeus had published his System of Nature in which he proposed the idea of classifying plants in the simplest and most clear way that one could. To Linnaeus, that meant a system based on the specimen’s external characteristics that were most obvious to the eye. His system would there-

is never used alone. The generic name always begins with a capital letter but the species name is always lowercase. Both names are written in italics or are underlined. Latin is used to avoid any confusion in translating different languages. Altogether, this system allows everyone in the world to use the same name for the same organism and to immediately understand each other. 134


fore be based on observable characteristics such as structure, or anatomy, or on the details of the way a thing reproduced itself. In his landmark book, he showed how this could be done. First, he created a hierarchical system (in a hierarchy, things are arranged in a certain order) in which the categories above included all of the ones below it. Thus, he created a system in which living things were grouped according to their similarities, with each succeeding level possessing a larger number of shared traits. He named these levels class, order, genus, and species. He also popularized what is called binomial nomenclature, which gave every living thing a Latin name consisting of its genus and species. For example, this would distinguish two very different species, like a lion and a cougar, simply by their Latin names. The lion belongs to Panthera leo and the cougar belongs to Felis concolor. Thus each organism has a generic name, telling which group it belongs to, and a specific name for itself.

Classification

Although classification might not seem to be as important a subject to science as some others, it proved absolutely essential to such a broad and diverse field as the life sciences. In fact, only after Linnaeus’s system was accepted and regularly used did biology and botany begin to make real progress. The advantages of his system are numerous. For example, his use of Latin allows scientists to communicate worldwide about organisms without having to understand different languages. Since each type of organism can fit into his system in a logical and orderly way, it can be expanded indefinitely. It is also a great advantage that the levels of his hierarchical system provide a framework for seeing and understanding the relationships among different organisms or groups of organisms. His system is also flexible and adaptable. Since it was first introduced, the number of levels have grown, with phylum being inserted above class and kingdom being placed at the very top. Finally, although his system was introduced before the theory of evolution (the process by which gradual genetic change occurs over time to a group of living things), it always has been able to accommodate any new discoveries or modifications which that theory has made. Linnaeus was said to have been an almost obsessive classifier, yet he was a person who turned his passion for an idea into a truly great scientific contribution.

CLASSIFICATION GROUPS Seven major groups or categories make up the scientific classification system. The groups or categories themselves are called taxons (from taxonomy, which is the science of naming and classifying organisms). These groups range in order of size, so from the largest or most general to the smallest and most specific, they are: kingdom, phylum, class, orU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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der, family, genus, and species. Each kingdom is divided into smaller and smaller groups until each type of organism is placed in a unique category. One way of remembering this general-to-specific scheme is the rhyme or formula, “King Philip Came Over From Great Spain.” Kingdom is the largest unit and is composed of five separate kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. Beginning with Linnaeus and for a long time afterward, there were only two kingdoms, Plantae and Animalia. But with the improvement of the microscope and the discovery of microorganisms, the number was expanded to five. From kingdom on down to species, organisms are grouped together with increasing similarity. Besides these seven major groups, biologists are able to use various subgroups to deal with minor differences among organisms when those differences are not large enough to form a new group. For example, species may be divided up into subspecies. Classifying a dog and a wolf offers a good example of how two animals would fit into these seven categories. Both are in the kingdom Animalia since they cannot make their own food. Next, they would both be in the phylum Chordata since they have a notochord (like a vertebrae or backbone). Both also belong to the class Mammalia since they have fur and feed milk to their young. Both are members of the order carnivora since they are meat eaters. They also both belong to the family canidae because they cannot retract their claws and they hunt and stalk their prey. However, while both are similar enough to be in the same genus, canis, they are different enough to be in separate species. Therefore, the wolf’s scientific name is Canis lupus and the dog’s is Canis familiaris. A classification system provides a method that best represents genuine relationships between organisms. It is a natural system that is based on overall resemblances and which reflects how each organism is related from an evolutionary standpoint. [See also Class; Family; Genus; Kingdom; Order; Phylum; Species]

Cloning A clone is a group of genetically identical cells descended from a single common ancestor. A clone can describe a group of cells or a multicellular organism. In both cases, the clone or offspring has the exact same genes as the parent organism. 136


A clone or a genetic double is not as rare in the natural world as one might suppose. Besides identical twins (who are the result of a fertilized egg separating completely during its two-cell stage), there are numerous examples in the plant kingdom. Almost all potatoes are clones, as are all banana trees grown from root cuttings. For plants, this form of asexual reproduction (an individual copies its genetic material) is known as vegetative reproduction. This is how grass and other plants like strawberries grow and spread. Grass puts out underground shoots, and strawberries send out aboveground runners, both of which eventually form independent, new plants that are genetically identical to the original or parent plant. Most bacteria are also natural clones since they reproduce by a process called binary fission in which they basically split in two, making a pair of identical cells. Besides these natural types of cloning, a recently developed artificial type of cloning occurs when a segment of deoxyribonucleic acid (DNA) is duplicated outside the body of a plant or animal. Advances with this type of research in which exact copies of DNA segments were made eventually led to scientists being able to clone a complex organism. For example, in 1968, the English biologist John Gurdon cloned a frog by replacing the nucleus of a frog egg cell with the nucleus (a cell’s control center) of a cell from another frog’s embryo. The egg cell matured into an exact identical twin of the tadpole embryo. Following this success, biologists attempted to clone mice and white rats, but most of the clones did not survive. Cloning mammals proved to be even more difficult and inefficient, with most attempts failing because the cell taken from the embryo was too mature. Its cells had already begun to specialize, as some started making cells for different organs and others making skin cells and limb cells. Overall, it proved very difficult to obtain a mammal embryo cell in its earliest stages of development.

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Dolly the sheep was the first mammal to be cloned from the cell of an adult instead of an embryo. Although a scientific breakthrough, the ability to clone an adult mammal has raised many issues. (Reproduced by permission of Archive Photos, Inc.)

This problem was solved on July 5, 1996 when a sheep named Dolly was born in Edinburgh, Scotland. In a dramatic breakthrough, the Scottish embryologist Ian Wilmut was able to clone a mammal from a cell taken not from an embryo but from an adult. His startling success, announced when Dolly was about seven months old, was achieved by Wilmut’s unique method of “starving” a cell’s nucleus which made it revert U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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IAN WILMUT English embryologist (a person specializing in the study of the early development of living things) Ian Wilmut (1944– ) produced the first mammal to be cloned from an adult animal. This biological breakthrough meant that future cloned animals might be used to produce large quantities of proteins needed for making certain drugs. It also suggested that these animals might provide a safer organ transplant source for humans. Ian Wilmut was born in Hampton Lucey, England, the son of a mathematics teacher. He became fascinated with embryology while earning a degree in agricultural science at the University of Nottingham in 1967. Wilmut continued his studies at Darwin College at Cambridge University in England and received a Ph.D. in animal genetic engineering in 1971. He then took a position at the Animal Breeding Research Station in Scotland, now known as the Roslin Institute. While at Darwin College, his dissertation topic was on techniques for freezing boar sperm, and in 1973 he created the first calf ever produced from a frozen embryo. Wilmut continued his research during the 1980s, always with the goal of cloning an animal in mind. A clone is the offspring that results from a form of asexual reproduction. This means that cloning involves only a single parent and does not require the exchange of sex cells from a male and female. In 1990, Wilmut hired English cell biologist Keith Campbell to work in his cloning laboratory, and it was Campbell’s idea that transplanted adult cells had not been working with embryo cells because the two were not “synchronized.” Since cells go through specific cycles, regularly growing and dividing and making an entirely new package of chromosomes each time, Campbell argued that adult mammal cells had to be slowed down to be in synch with embryos. Wilmut and Campbell then pioneered a new technique

back to an earlier stage of development. First, Wilmut took unfertilized eggs from an adult female and removed all of its DNA. This left it an empty egg that could still support growth. He then took the udder cells from an adult sheep and raised them in a way designed to “turn off” their specialized genes. One of these donor cells was then fused electrically with the empty egg cell, and the artificially fertilized egg started to divide into an embryo. It was then transplanted into the womb of a sheep, and Dolly, the genetic twin of the animal who donated the udder cell and its own DNA, was eventually born. The cloning of a mammal produced fear as well as praise among many people, as it raised the possibility of cloning a human being. Biologists tried to ease this fear by pointing out the medical advantages of being 138


of starving adult cells so they would eventually be in the same cycle as the embryos. Once they “turned off” the specialized adult genes (taken from the udder or milk gland of a six-year-old sheep) and made them act like embryo cells, they fused it with an unfertilized egg that had all of its genetic information-containing deoxyribonucleic acid (DNA) removed. After the artificially fertilized egg started to divide into an embryo, it was transplanted into the womb of a surrogate, or substitute, female sheep where it developed and grew, producing an offspring that was genetically identical to the animal that donated the cell.

Cloning

Wilmut and Campbell, therefore, produced the cloned lamb named “Dolly” on July 5, 1996. As the first clone from an adult mammal, this successful experiment marked an achievement that some had thought would (or should) never be realized. It also set off a wave of discussion and debate about the implications and ethics of cloning. Naturally, that debate focused on the potential for cloning human beings. While Wilmut remained passionate about his achievement, he stated clearly that cloning a person is ethically unacceptable, and that the primary purpose of his work is to advance the development of drug therapies to combat certain life-threatening diseases. As an example of a health-related product developed from cloning, he offers the possibility of cloning an animal that produces the blood clotting factors that hemophiliacs are lacking. He also envisions organ transplants becoming plentiful and routine by means of inserting a human protein into a cloned animal that allows the animal organs to be more easily accepted by the human patient’s body. Wilmut is aware of the ethical concerns many people have about cloning, and he stresses that it is very important to prevent any real misuse if humans are to gain any of cloning potential benefits.

able to clone an animal that contains a certain human gene in its cells. They suggest that such animals could produce a particular enzyme needed by people whose bodies will not produce it, such as the blood-clotting enzyme thrombin, which hemophiliacs lack. However, as with all aspects of genetic engineering, cloning raises many issues with far-reaching social, legal, and ethical implications. These complex issues, in turn, raise many difficult questions, such as who decides what traits are desirable? Are biologists “playing God” by tampering with human DNA? And might a genetic mistake result in some sort of disaster in which a genetic monster like an uncontrollable plague is created? [See also DNA; Genetic Engineering; Nucleic Acid; Reproduction, Asexual] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Cnidarian

Cnidarian A cnidarian (ny-DAIR-ee-uhn) is a simple invertebrate (an animal without a backbone) that lives in the water and has a digestive cavity with only one opening. Jellyfish, sea anemones, corals, and hydra are all cnidarians. Cnidarians catch food using armlike, stinging tentacles and their bodies have only two tissue layers, unlike higher animals who have three layers. While a cnidarian may be more complex than a sponge, it is still a very simple invertebrate. Also called a coelenterate, which means “hollow gut,” a cnidarian has been described as having a body that resembles a sack or a hollow bag with a hole in it. This hole is its baglike digestive cavity that has a single mouth or opening at one end. Both food and waste pass through this opening which is usually surrounded by armlike extensions called tentacles. These tentacles are equipped with stinging cells called cnidoblasts that shoot poisonous, spiny threads. Cnidarians use these threads to paralyze and capture small animals that swim into them. The unmoving fish is then pushed by the tentacles through the mouth and into the digestive cavity. Once there, it is broken down and eventually absorbed by cells lining the cavity. There are three main groups of cnidarians: the hydra, the jellyfish, and the corals. The simplest cnidarian is the hydra, and it is one of the few freshwater cnidarians. Although it is especially tiny, resembling a piece of string on a pond, under a microscope the hydra reveals a hollow trunk with a mouth at one end that is surrounded by a ring of tentacles. It captures tiny animals by stinging them and can stretch when reaching for food. A hydra can reproduce asexually by budding (growing new cells that separate from the parent) or by regeneration (growing a complete organism from a piece). The jellyfish belongs to a class whose name translates as “cup animals.” As an adult, it has an umbrella-shaped or bell-shaped body and swims by pumping water into and out of its digestive cavity. Its mouth is in the center of its underside and has long mouth lobes that hang down from it. The nearly colorless jellyfish we often see while swimming is probably the moon jellyfish. All jellyfish reproduce sexually, joining sperm and egg to produce a larva that first swims around and then attaches itself to the bottom and grows as a polyp. It then breaks off and swims away to assume the shape called a medusa. Since a jellyfish’s body is mostly water, when it dies and is washed ashore, it soon dries and leaves only what appears to be a circle of film.

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Some other cnidarians do not have the freedom to swim about and spend their entire lives attached to something at the bottom of the ocean, usually with many other of their kind in what are called colonies. These are the coral and sea anemones. Both have tubelike bodies and resemble what some call flower animals. They are brightly colored and gather in clusters that resemble blossoms. Most corals are very small and protect themselves by building hard cases of calcium carbonate (limestone) around themselves. During the day, corals hide inside their shell, but at night they extend their tentacles and catch tiny animals swimming by. When a coral dies, its limestone covering remains and coral reefs are built, formed by countless covering after covering. One large example of a coral reef that has developed over time is The Great Barrier Reef off the coast of Queensland, Australia, which is 95 miles (152.9 kilometers) long.

Community

Community A community is made up of all of the populations of different species living in a specific environment. A community consists of only the living components (biotic) of the environment. Ecologists study the different roles each species play in their community and also study the different types of communities and how they can change over time. A community has no particular size. An example of a pond community would be all of the algae, plants, fish, frogs, ducks and other organisms that live in and around a particular pond. These many species living within a community interact with each other in many different ways. These interactions are very important in that they play a significant role in shaping the size and structure of the community. There are three major categories of interactions: competition, predation, and symbiosis.

COMPETITION Competition occurs when members of the same or different species compete for or share a limited resource. A dramatic example of a competitive interaction affecting a community is the death of or departure of a species from a community because others were using up a scarce resource (like food) that the species needed. Since competition for food almost always exists in nature, each species eventually finds its own “niche” in the community. A niche (also called an ecological niche) is a specific job or role in a community that relates to feeding. In general, the niche of one species does not overlap with that of another species. However, U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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ecologists do not agree as to why this is so. Some say that one species out-competes another in a certain area and that the other species is forced out of the community, thus surrendering the niche to the winner. Others say that one species occupies a certain niche because it is the one best suited (or best physically equipped) to do so. A good example of two species having their own niche are the woodpecker and the nuthatch. Although both these birds eat grubs (insect larvae) found in the cracks of trees, they are seldom in serious competition because each has its own niche. While feeding from the same tree, the woodpecker will begin at the bottom and work its way up. The nuthatch does the opposite, working from the top down.

PREDATION The second type of interaction between species is predation. Predation occurs when one organism catches and kills another organism. Predation is a good example of how community interactions can result not only in community change but in the actual evolution (gradual genetic change in a group of living things) of the species involved. As a result of one species hunting another, the one doing the hunting may evolve better tools to catch its prey, while the one trying to get away from the predator may evolve better ways of escaping or avoiding being noticed. The numbers of how many predators and prey exist will influence the community structure. If there are many predators in a particular community, the number of prey will probably decrease since many of them are caught and eaten. This could mean that within the prey population there will be less competition among themselves. However, if too many prey disappear, the number of predators is likely to go down, since there will be less and less for predators to eat.

SYMBIOSIS Symbiosis is the third major interaction that has an effect on a community. Symbiosis describes an especially close relationship between two different species within the same community. Although the term can refer to any type of partnership, it sometimes means a type of relationship that is to the benefit of both species. Flowering plants and bees have a symbiotic relationship that benefits each other. Bees get the nectar they need from flowers and, in turn, the flowers are pollinated by the bees. Certain bacteria have the same relationship with rabbits that cannot digest the cellulose (the walls of a plant cell) in the plants they eat. The bacteria live in the rabbit’s gut and benefit from the warm, moist environment; they in turn break down the cellulose for the rabbit to digest. 142


This interdependence between species can result in evolutionary changes. In the short run, however, a community often experiences a noticeable change in one species when its symbiotic “partner” disappears.

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ECOLOGICAL SUCCESSION Changes that occur over time in a community are called ecological succession. These are usually slow, natural changes that take place in any community. Weather patterns change, as do soil minerals and organisms; even populations rise and fall. Ecologists describe two types of succession, primary and secondary. Primary succession occurs when organisms move into a place that formerly had no life, like a newly formed barrier island. Secondary succession happens after an established community suffers some sort of drastic change, like a fire or volcanic eruption. In this type of succession, there is a pattern by which life introduces itself back into the community. For plant life, a meadow will first develop, to be followed by shrubs and later trees. Studying communities is more complicated than studying populations since the number and type of interactions can be so large and complex. Even relatively simple communities with small numbers of species form a complicated, interrelated web of dynamic interactions. [See also Competition; Environment; Population]

Competition Competition is a situation that arises when two or more organisms have to share the same limited resources. Competition is a constant in nature since plants and animals almost always have to share such important resources as food, water, space, shelter, sunlight, minerals, and mates. Many ecologists (a person specializing in the study of the relationships between organisms and their environment) consider competition a powerful force that shapes populations (a group of the same species) and communities (groups of different species) as well as the adaptation and evolution of species.

INTRASPECIFIC AND INTERSPECIFIC COMPETITION There are different types of competition, depending on whether the competing organisms belong to the same species (intraspecific competition) or are from different species (interspecific competition). Intraspecific competition is the most common and often the most fierce since inU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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dividuals of the same species will have nearly identical needs. The intensity of the competition is also closely related to species population— or how many of the same organisms are in need of the same thing. A rise in population will necessarily result in more competition. In fact, many studies have shown that simple overpopulation in certain species often result in the adults of the species not growing to full size. In other species, overcrowding lowers the number of young produced. Intraspecific competition can take many forms. One of the more obvious types of competition, called “interference competition,” occurs when two competitors of the same species directly confront each other over the same thing. The result can be an actual fight or aggressive poses, displays, and threats. When a hawk aggressively swoops down at another hawk, it is probably engaging in a form of interference competition called territorial competition. Many animals often claim a certain area and will defend it against newcomers entering and trying to take its resources. Males of certain species also engage in a form of interference competition when they fight over who will get to mate with a certain female or with a group of females. Interspecific competition may not be as fierce as intraspecific competition since no two species ever occupy the same ecological niche (or the precise role that a species plays in its environment). However, the closer the ecological niche is between species, the more fierce the competition. This natural rule, that two species cannot occupy the same niche, is called the competitive exclusion principle. When niches are similar, a phenomenon known as “resource partitioning” occurs. Thus species may eat the same thing, but their feeding habits may be different enough that they do not interfere with each other. For example, a woodpecker and a nuthatch, who both dig for and eat insect grubs that live under tree bark, can both work the same tree without interfering with each other since the woodpecker eats from the bottom up and the nuthatch from the top down. Competition could also be described as one of the engines driving evolution, since competition is at the core of the concept of natural selection. In the natural world, living things that are not properly “fitted” or suited to their environment are eventually weeded out because they will fail to survive. The saying “survival of he fittest” originated from this phenomenon, since nature allows only those organisms that are best adapted to survive when its resources are limited. Only the best competitors survive and get to reproduce, passing on to their young the characteristics (adaptations) that made them better competitors. Poor competitors seldom survive. Although competition ensures the survival of the

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fittest, conditions can change, and an individual that survived well in a certain environment may be destroyed if the environment changes, or if the individual moves to a different environment.

Crustacean

[See also Community; Population]

Crustacean A crustacean is an invertebrate (an animal without a backbone) with several pairs of jointed legs and two pairs of antennae. It is covered by a tough exoskeleton (a hard outer support structure) with overlapping plates that thin out at the joints to allow maximum movement. Its body is divided into two main regions that are fused together. Most crustaceans live in water. A crustacean is a member of the phylum Arthropoda, the largest and most successful phylum in the kingdom Animalia. It is also a member of the class Crustacea which is one of the three major groupings of arthropods (the other two are Arachnida and Insecta). The name crustacean is derived from the Latin word cursta meaning “crust,” and refers to the hard outer shell that this class of invertebrate wears. There are about 40,000 species of crustaceans, including the better-known animals like shrimps, lobsters, crayfishes, and crabs as well as barnacles, water fleas, and isopods like the wood louse. Some are predators and eat other invertebrates, while others are herbivores and eat only plant material. While there are simple crustaceans, most usually have a large diversity (and a large number) of paired appendages (like legs, arms, or pincers). To be classified as a crustacean, an animal must have two joined body parts— a cephalothorax (a head and middle region) and an abdomen (the lower part of the body). The head has two compound eyes that are located on the ends of retractable and flexible stalks. A compound eye is made up of many separate compartments, each having its own lens. A crustacean must also have two pairs of antennae with which it feels and receives chemical stimuli. It must also have at least four pairs of walking legs, and often has more. Shrimp, lobster, crabs, and crayfish are called decapods because they have ten legs. All have a broad, paddle-like tail used for swimming. As with a representative crustacean species like the crayfish, its first set of legs are adapted as claws. It uses these claws or pincers to obtain food and to defend itself. The other four pairs of smaller legs are used for walking. Behind these walking legs and attached to the lower half of U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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its body called the abdomen are tiny appendages called swimmerets used for swimming and during reproduction. The respiratory system of a crayfish consists of gills over which water passes as the animal moves. The gills are really feathery outgrowths located on both sides of its body. The crayfish has a heart that moves its blood through arteries. Other crustaceans have variations of these systems. For example, a crab has an especially strong claw used for tearing up seaweed and attacking another animal. The barnacle, which attaches itself to a rock by a long stalk, has appendages almost like feathers that are used to comb or sift the water for microscopic food. The smallest crustacean might be the 7,500 species of copepods, some of which are no more than a few millimeters long. These freeswimming herbivores play an important role in the diet of many fish. All crustaceans reproduce sexually and develop through a series of larval stages.

A crayfish at the Fish Point State Wildlife Area in Michigan displaying its claws. These claws are common traits for crustaceans and are used to catch food and defend themselves. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

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Cytoplasm

Cytoplasm

The term cytoplasm refers to the contents of a cell, excluding its nucleus. More specifically, cytoplasm refers to the jelly-like or semisolid fluid that is enclosed by the cell’s plasma membrane. Before scientists had knowledge of what was contained in a eukaryotic cell (one with a nucleus and an outer membrane), the term cytoplasm was a convenient description for the cell’s contents. Now it is known that cytoplasm contains the cell’s organelles—the many tiny structures that each have a particular function to perform. The cytoplasm, along with the nucleus, make up what is called the protoplasm, or living material of a cell. Cytoplasm is the fluid environment in which the cell’s metabolism (the chemical processes that make a cell a living thing) takes place. Cytoplasm is a gel-like fluid rich in proteins, fats, carbohydrates, salts, and other chemicals. Unlike a common gelatin, however, cytoplasm is constantly moving and transporting materials from one place to another. This cytoplasmic movement can best be observed in slime molds, amoeba (uhMEE-buh), and certain species of algae, in which an ordinary light microscope will reveal what appear to be streams of cytoplasm coursing through the interior of a cell. In addition to these proteins and enzymes in fluid form, the cytoplasm contains a wide variety of organelles. Each of these tiny structures carries out a particular task that is important in maintaining the life of the cell. Some break down food while others move waste around and get it ready to be expelled. Others may store important materials. All organelles in the cytoplasm are surrounded by membranes. Some of the more important organelles found in a cell’s cytoplasm are mitochondria (energy generators), ribosomes (assembly units for proteins), endoplasmic reticulum (material transporters), Golgi bodies (storage), and other components like the cell’s coded plans and instructions that are carried in its ribonucleic acid (RNA). Plant cells have chloroplasts in their cytoplasm, enabling them to convert sunlight into food. Although animal cells do not have chloroplasts, they do have lysosomes which enable them to digest the food they take in. [See also Cell]


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D Decomposition Decomposition is the process by which dead organisms and their wastes are broken down into an organic form that is usable by other organisms. In ecological terms, the chemicals out of which living things are made were only borrowed from the earth, and when they die, they are returned by the process of decomposition. These chemicals are then recycled in order to be used by other living things. Decomposition could be called rotting or decay, and it is carried out by a very important group of organisms called decomposers. Decomposers are bacteria and fungi as well as other small animals called detritivores. These decomposers break down waste and dead matter into smaller and smaller pieces until all the chemicals they contain are released into the air, water, and soil. Decomposers may be tiny in size but they are huge in numbers. In addition to bacteria and fungi, which are the most important decomposers, such larger animals as the slug, snail, earthworm, woodlouse, and centipede also play an important role in breaking down organic matter. A common example of a fungus and decomposer is a mushroom growing out of a dead tree trunk and feeding on the tree’s decaying remains. Decomposition is an essential stage in the cycling of nutrients through nature’s food web (the connected network of producers, consumers, and decomposers). It is nature’s way of recycling nutrients so they can be reused. Some of the major nutrients that are recycled include nitrogen, phosphorous, carbon, and oxygen. The decomposers that are a key link in nature’s food web because they allow nutrients to be cycled through it continuously. If it were not for decomposers, the food web could not be U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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self-sustaining and would break down because it would eventually have to obtain more nutrients from outside the ecosystem (an area in which living things interact with each other and the environment). For example, without the carbon dioxide that is released by decomposition, green plants would eventually die since they use it to make their food. Without green plants, there would be much less oxygen, since they give it off as part of the process of making their own food. No plants and no oxygen means that all animals would starve. Finally, without decomposition, the world would be a mineral-deficient land full of waste and corpses. [See also Bacteria; Fungi]

Desert A desert is generally a very hot, barren region on Earth that receives little rainfall. Most sources describe a region as being a desert if it receives less than 10 inches (25.4 centimeters) of rain a year. It has also been described as a place where more water evaporates than falls as precipitation. Despite being an extremely harsh environment, deserts support a diverse community of both plant and animal life. As one of the six terrestrial (land) biomes (particular types of large geographic regions), deserts cover between one fifth and one quarter of Earth’s surface. A desert is a stark, dramatic place whose topography (surface conditions) is almost immediately recognizable. Its miles of sand dunes or endless stretches of flat, featureless sand are not easily forgettable; nor are its strangely adapted plants (like cacti) apt to be confused with vegetation from some other region. It is easy to understand what makes a desert what it is. Any part of Earth that constantly experiences a water “debt” rather than a water “surplus” is so dry that the need to capture, conserve, and store water is not only overwhelming, but affects and determines everything living in that place. Despite the impression that a desert is a lifeless place, it is home to certain plants and animals who have adapted to its harsh conditions and who do very well there.

THE LOCATION OF DESERTS Most of the world’s deserts are located on two desert belts that wrap around Earth’s equator (the circular band around Earth’s middle which divides the Northern and Southern Hemispheres). The belt in the Northern Hemisphere is along the tropic of Cancer and includes the Gobi Desert in China, the Sahara Desert in North Africa, the deserts of southwestern North America, and the Arabian and Iranian deserts in the Middle East. The belt 150


in the Southern Hemisphere is along the tropic of Capricorn and includes the Patagonia Desert in Argentina, the Kalahari Desert of southern Africa, and the Great Victoria and Great Sandy Deserts of Australia. Altogether, there are about twelve major deserts, the largest of which is the Sahara Desert which measures 3.5 million square miles (5.63 million square kilometers). This is an area as big as the entire United States.

Desert

THE CREATION OF DESERTS In a way, deserts are made and not born, meaning that Earth’s weather patterns created a desert in the first place and continue to work to keep it that way. These regular patterns, or moving currents of hot and cold air interact with each other so that descending currents of air pick up moisture and dry out the land. Mountain ranges also influence these currents, as dry air moving off their slopes evaporate even more moisture. The steady lack of moisture in the air above a desert region leads to extreme changes in temperature once the Sun goes down. In normally humid areas, the moisture in the air acts as an insulating barrier, and clouds keep some of the daytime warmth from the Sun trapped, thereby moderating temperatures. However in a desert, which has no moisture in the air above it, there are no clouds to act as a blanket, meaning that although daytime temperatures are extremely hot, they can be near freezing at night. As with any biome, deserts vary considerably throughout the world, and they can be as diverse as the lifeless-looking and appropriately named Death Valley in California and Nevada, and the almost-lush looking Vazcaino Desert in Mexico when it bursts into flower following its annual spring rain. Even in as harsh an environment as Death Valley or the Sahara Desert, life can be found. Sometimes life is a dormant seed buried for years and waiting for a bit of moisture so that the seed can spring into existence as an aboveground plant. Other times desert life is a toad hibernating below ground and rushing to find a mate and lay its eggs as soon as it rains. Life in a desert is a constant challenge, and plant and animal inhabitants do not have the luxury of being wasteful that other organisms in more temperate climates might have.

PLANT LIFE IN THE DESERT Desert plants have evolved many methods to obtain and efficiently use available water. Certain ones compress their entire life cycle into one growing season. The seeds or bulbs of some flowering desert plants can lie dormant in the soil for years until a heavy rain enables them to germinate (sprout), grow, and bloom. Woody plants may develop deep root systems—like the mesquite whose tap root can measure 50 feet (15.24 U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Desert Cacti in the Sonoran Desert in Arizona. The cactus is specially adapted to life in the dry desert. (Reproduced by permission of JLM Visuals.)

meters) down although the aboveground tree is only about 10 feet (3.048 meters) tall. They may also develop a network of shallow, spreading, hairlike roots that can take up water from dew and the occasional rain shower before it seeps below ground. For many plants, the answer to years of absolute drought is to drop leaves, allow the aboveground part of the plant to die, and keep the underground root alive in a state of dormancy (functioning slowly or not at all). Conserving and storing water becomes important for a plant once it has obtained moisture. Since all plants lose water by evaporation from their leaves, many desert species minimize this by having very small or rolled leaves, or by turning their leaves into spines or barbs. These thornlike leaves protect a plant’s water supply from animals. The problem of storing water is solved by the cactus, which is a succulent and can store water in its leaves, stems, and roots. An amazing example of adaptation is the Saguro cactus of the American southwest. The trunk of this large cactus is folded or pleated like an accordion, which can unfold and expand as the plant absorbs water after a heavy rain. A Saguro that is 20 feet tall (6.1 meters) can hold more than one ton (1.102 metric tons) of water.

ANIMAL LIFE IN THE DESERT Desert animals, like desert plants, have also evolved ways to cope with the desert’s arid environment using avoidance and/or adaptation. Besides the highly adapted camel, most desert animals are small and do not have an extensive range. While their size limits their ability to leave, it does make easier their ability to remain in cool underground burrows during the day and emerge only after dark to feed. Animals that do this are called “nocturnal” or “cre152


puscular.” Other small mammals and reptiles survive the most extreme times by a process called estivation, which is similar to the sleeplike state of hibernation. Other animals have adapted specialized body parts to help them cool off. A well-known example is the huge ears of the small Fennec fox of the Sahara Desert and the Kit fox of North America. Both have enormous ears whose dense network of tiny blood vessels run just below their skin and act as radiators, releasing excess heat. Larger desert animals developed broad hooves that allow them to move more easily over soft sand. Some animals can actually slow down their production of body heat by varying their heart rate, while others reabsorb the water in their urine several times before finally excreting a highly concentrated form of urine.

Diffusion

Just as the animal and plant population in deserts is not overly abundant because the desert’s difficult conditions can only support small numbers, deserts cannot support humans in large numbers. People must, like animals and plants, make adjustments in order to survive in the desert’s extremes, and in the past they have lived in mud houses that kept cool in the daytime and provided warmth at night. Long robes were often used in Africa and the Middle East for protection against the scorching sunlight and blowing sand. With today’s technology, however, people can live comfortably in a desert if they have air conditioning and an adequate water supply. A good, steady source of water also allows humans to raise crops in a desert since they are usually naturally fertile regions because there is seldom enough water to leech away important nutrients. Crops can be grown on desert lands with irrigation, but farmers must be prepared to deal with a buildup of salts in the soil as a result of evaporation (which takes away most of the water they put down). Humans can also be responsible for creating deserts or allowing an existing desert to spread. This is usually the result of burning or overgrazing of animals. When a desert encroaches, or spreads, to arable land (land able to be farmed), that process is called “desertification.” [See also Biome]

Diffusion Diffusion is the movement or spreading out of a substance from an area of high concentration to the area of lowest concentration. Diffusion takes place at the cellular level in both living and nonliving things. Simple animals that do not have an internal circulatory system rely on diffusion to exchange gases and obtain nutrients. The root cells of green plants obtain U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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their water from the soil by diffusion. Diffusion requires no output of energy on the part of the cell. Diffusion is a natural phenomenon that happens under certain conditions and occurs at the molecular level. Because molecules are constantly moving, their natural tendency is for different types to mix with one another. This movement of molecules is random and depends on the amount of energy (called kinetic energy) in each molecule. As different types of molecules move about and mix together, the only pattern noticeable is their overall movement from an area of high concentration (where they are all together) to an area of lowest concentration (where there are the fewest of them). While an individual molecule may not do this exactly, the net or overall movement of the group of molecules will always move in that manner. This net movement is called diffusion.

An illustration of diffusion in a red blood cell. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

A good example of everyday diffusion is the way tobacco smoke spreads throughout the still air of a room. Perfume does the same thing in the motionless air of a room. For the life sciences, diffusion takes place at the cellular level of both plants and animals. Since cell membranes are composed of molecules that are always in motion, there are always temporary openings in cell membranes. Living cells are always bathed in liquid. If the liquid concentration of a certain type of molecule is higher on the outside of a cell membrane than on the inside, those molecules will

Cell Membrane

(a) Simple diffusion

Carrier molecule Red blood cell

(b) Carrier-facilitated diffusion Cytoplasm

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Extracellular fluid


diffuse through the membrane into the cell. The opposite can also occur if the liquid concentration is higher on the inside of a cell. The rate of diffusion depends on the unevenness of the distribution and the size of the molecules. When the molecules on either side of the membrane are equally distributed, no diffusion will take place.

Digestive System

The process of diffusion is an important method for cells to be able to exchange substances with their environment. As a result, the cells are able to refill what they have run out of and dispose what they have too much of. However, since the cell has no real control of this process, it is called passive diffusion. Diffusion also takes place in the lungs of animals. When blood enters the lungs to fill up with oxygen, it is carrying carbon dioxide, a waste product, in high concentration. Immediately, the carbon dioxide diffuses from the blood into the lungs and oxygen diffuses from the lungs into the blood. This same exchange takes place in single-celled animals and in many simple aquatic animals as well. Animals such as sponges, flatworms, and hydras rely on diffusion to get oxygen and food from the surrounding water and to remove waste. Such simple animals can do this either because their bodies have many openings (like sponges) or because they have body walls that are only two or three cells thick (like hydras). Diffusion can work in such animals since they have simple body parts and minimal demands. However, creatures with fast metabolisms (all the chemical processes going on in a living thing) need real circulatory systems that can work fast and transport large amounts. Plants with vascular systems (internal pipelines for transporting food and water) also use diffusion to take water and oxygen into their roots. For example, if the cells of a root have only a little oxygen in them and the surrounding soil has a great deal, oxygen molecules will automatically move from the soil to the root. As with animal cells, the plant does not expend any energy to accomplish this. [See also Respiratory System]

Digestive System A digestive system is a system that allows an organism to take in food, break it down, absorb its nutrients, and excrete what is not usable. All organisms that cannot internally make their own food (as plants do) must ingest or eat it, and therefore must have a digestive system in some form. Different types of animals have different digestive systems according to their main diet and the amount they eat. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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All living things need food in order to continue to live, grow, and reproduce. Except for green plants and some algae that can make their own food using the Sun’s energy, all other living things get their energy from eating other living things, such as plants or animals. However, these plants and animals used as food are made up of large molecules that cannot be used by an organism’s cells unless they are changed into smaller molecules that can be absorbed. The entire process by which food is converted into a form the body can use is called digestion. This process of digestion is carried out by the organism’s digestive system. Digestive systems range from the very simple, primitive systems of one-celled organisms to the complex, many-organ systems used by vertebrates (animals with backbones).

INTRACELLULAR DIGESTION Very simple, single-celled organisms practice what is called intracellular digestion in which they engulf or surround a food particle with their outer membrane. During this type of digestion, these organisms literally bring the food particle inside the cell. Strong enzymes (proteins that control the rate of chemical changes) break the food particle down into its usable components, which are then absorbed into the cell’s cytoplasm (the jelly-like fluid inside a cell). Waste products are packaged up and passed back out through the cell membrane.

EXTRACELLULAR DIGESTION Other slightly more complex organisms like a sponge may have a mouth that leads to a large, open body cavity. Organisms that have only one opening through which passes both their food and their waste are said to have an incomplete digestive system. Flatworms and hydras have this type of digestive system, sometimes called a blind gut. Food enters its mouth and is partially digested by chemicals released into its gut. This is called extracellular digestion because it occurs inside the gut cavity and not inside a cell. Once the food has been broken down, the smaller bits can be absorbed by the cells that line the gut. Waste products are passed back out through the mouth. Because of this two-way traffic, the organism’s cavity cannot be subdivided into specialized compartments. Opposite: A labeled diagram showing all of the parts of the human digestive system. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

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More complex organisms have more complicated digestive systems. These are called complete digestive systems. Higher up the evolutionary ladder, the blind gut develops a separate opening for waste removal, called the anus. This is seen in earthworms, clams, crabs, spiders, and starfish, among others, who have the simplest form of a complete digestive system. Food enters the mouth, is broken down, and passes in one U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

The digestive process begins in the mouth, where ingested food is chewed and softened by saliva for about a minute. A swallowable amount of food is separated off from the mass by the tongue and made into a ball called a bolus at the back of the mouth.

In swallowing, the bolus is forced back into the pharynx by the tongue; a flap of tissue called the epiglottis covers the opening to the larynx and the soft palate closes over the opening to the nasal cavity to prevent food from entering these respiratory passageways.

Nasal cavity Soft palate

Bolus

Tongue

Pharynx

Epiglottis

Esophagus

Bolus

Stomach

A process called peristalsis takes about three minutes to move the bolus into the stomach: muscles that run lengthwise on the esophagus contract, shortening the passageway ahead of the bolus, and muscles that circle the esophagus constrict behind it, pushing it ahead.

Gall bladder Rugae Liver Bile duct Duodenum

Large intestine

The inner lining of the stomach is wrinkled with folds called rugae. As more and more food enters the stomach, the rugae smooth out, stretching the capacity of the stomach to more than a quart (liter). Over a two-to-four-hour period, the muscular wall of the stomach churns and mashes its contents, and gastric juices break down connective tissues within ingested meat and kill bacteria, reducing the food to a semisolid mixture called chyme.

Pancreas Small intestine

Although water, salts, and alcohol pass into the bloodstream through the stomach, the major site for nutrient absorpton is the small intestine. The breakdown of food is completed by enzymes secreted by the pancreas, and by bile secreted by the liver and the gall bladder.

Appendix

The inner lining of the small intestine is composed of up to Rectum five million tiny, finger like projections called villi, which increase the rate of nutrient absorption by extending the surface of the small intestine to about five times that of the surface of the skin. The material that makes the one-to-four-hour journey through the small intestine without being digested and absorbed and arrives at the large intestine is mostly waste; in a process taking from 10 hours to several days, water, vitamins, and salts are removed and passed on to the bloodstream, and the rest, consisting of undigested food, bacteria, small amounts of fat, and cells from the walls of the digestive tract, is passed into the rectum, where it is eliminated from the body.


Villus Artery Vein Lymph vessel

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WILLIAM BEAUMONT American surgeon William Beaumont (1785–1853) conducted pioneering studies on how the human stomach works. His highly accurate and welldocumented firsthand observations of the stomach proved that digestion is basically a chemical process. If U.S. Army surgeon William Beaumont had not been in the right place at the right time, he would never have become a pioneer of science. Until the event occurred that changed his life, Beaumont had done nothing remarkable. Born in Lebanon, Connecticut, on a farm, he was able to study medicine at St. Albans in Vermont. By 1812, he became an assistant surgeon in the U.S. Army. Three years later, he left the army and tried private practice in Plattsburgh, New York. However, in 1820 he returned to the army and was eventually transferred to the frontier post of Fort Mackinac in Michigan. While there, a bizarre medical event occurred on June 6, 1822. That day, Beaumont was called upon to assist a patient who had been accidentally shot at close range with a shotgun. What Beaumont found was a young French-Canadian trapper named Alexis St. Martin who had been struck by the blast on his left side. The shotgun had taken a deep chunk out of his side and no one expected the young man to live through the night. Beaumont tended him with care and skill nonetheless, and to everyone’s surprise he remained alive. Beaumont continued his care, changing his bandages every day for a year, and in time, St. Martin was fully recovered. However, despite the fact that he had regained his full strength and seemed normal, St. Martin was not literally whole. His wound never fully closed, and he was left with an inch-wide opening through which Beaumont could insert his finger all the way into St. Martin’s stomach. The trapper had been left with what is called a permanent traumatic fistula, meaning he had a hole in his side that led directly to his stomach. About a year later when

direction through a straight digestive tube where it is absorbed. Waste passes out of the body through an anus at the organism’s other end. Earthworm waste called casts are deposited on the soil’s surface, adding needed nutrients.

HOW THE DIGESTIVE SYSTEM WORKS As organisms became more complex and evolved into what are called the higher animals, their digestive systems developed an alimentary tract or alimentary canal with specialized structures and compartments. Among vertebrates, a basic digestive scheme came about that resulted in structures responsible for: receiving food, conducting and storing food, break158


St. Martin needed to take some medicine from Beaumont, the surgeon decided to try an experiment and administered the medicine directly into the stomach rather than orally as he would have done normally. Beaumont soon found that the medicine worked on St. Martin exactly as it would have if it had been administered the regular way. This led Beaumont to realize that he had a unique research opportunity at hand, and he soon began a series of experiments and observations on his subject. First Beaumont attached small chunks of food to a string and inserted them directly into the St. Martin’s stomach. Beaumont would then withdraw the string and observe the results of digestion hour by hour. Later, by using a hand lens, he began actually looking into his patient’s stomach to see how it behaved. He was also able to extract and analyze samples of gastric juice and stomach contents, establishing that digestion is indeed a chemical process. He could also observe the muscular movements of the stomach. Over the next few years, Beaumont conducted more than two hundred carefully detailed experiments and, in 1833, published his findings in Experiments and Observations on the Gastric Juice and the Physiology of Digestion. As the first, well-documented and accurate observation of the digestive processes of a living human being, Beaumont’s book was a one-of-kind source on the process of digestion. It also suggested to some scientists the possibility of using artificial fistulas on their research animals as a way of learning more about their bodies.

Digestive System

The human side of this story should also be noted. By about 1835, St. Martin had had enough, not only of Beaumont’s experiments, but also of the bullying Beaumont himself. Poor St. Martin refused to cooperate anymore and eventually just ran away from Beaumont for good and returned to his native Canada. In his own way, the wounded trapper who lived to the ripe old age of eighty-two, had made his own contributions toward understanding more about the human body.

ing food down and absorbing its useful nutrients, and absorbing water while eliminating wastes. The alimentary canal in vertebrates is able to move food from the mouth to the anus entirely on its own. It does this by contracting its circular muscles and pushing food further along. This happens in a rhythmic wave called peristalsis. The canal itself is constantly lubricated by mucus, making the food move easily when the circular muscles contract. Automatically, these important muscles are regularly contracting and relaxing, pushing food and waste along as if a fist were closing on a tube, squeezing its contents in a certain direction. In all vertebrates, food encounters four major areas in the alimentary canal where it is received and ingested (swallowed), broken down, abU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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sorbed, and eliminated. The mouth is responsible for receiving food, and it is usually designed with the animal’s diet in mind. The mouth may have teeth which, helped by the tongue, mechanically break food down into smaller pieces. Some species, like snakes, smell with their tongue, while others, such as frogs, capture their food with it. Since mammals chew their food in their mouth, they also have salivary glands that secrete enzymes that moisten their food and begin the chemical breakdown process. The esophagus connects to the stomach and moves the food downward after it is swallowed. Unlike most vertebrates, however, birds have an enlarged esophagus called a crop that is used to store food before it is sent to the stomach. It also can produce a “milky” substance that a parent bird can regurgitate (spit back up) and feed its young. In most vertebrates, the real grinding and digestion of food is carried out in the stomach. This is a muscular pouch that works to mix the food with a highly acidic combination of chemicals that break it down or dissolve it further. Powerful stomach muscles churn up the food while certain cells lining the stomach produce gastric juice (mainly hydrochloric acid) that turn the broken-down food into a milky substance known as chyme. The stomach itself is protected from the strong acid by a thick layer of mucus. As the stomach fills with chyme, it gradually releases small amounts into the duodenum, the top part of the small intestine. The stomach of some vertebrates, like birds, who swallow their food without chewing (like birds), are called gizzards and often contain pebbles that mechanically break up the food for them as their muscle walls contract. Some plant-eating mammals called ruminants, like cows and horses, have special stomach chambers that help break down their difficult-to-digest diet. They also must have certain microorganisms in their gut in order to break down their food since mammals do not produce the digestive enzymes needed to break down cellulose (tough plant walls). Once the now-liquid food passes into the duodenum, it begins its final trip to becoming completely digested and prepared for absorption. At this point, the liver and the pancreas come into play as both have ducts leading into the duodenum. The pancreas secretes enzymes or pancreatic juice that is alkaline and counteracts the strong acid made by the stomach. It also breaks down food molecules in the duodenum. The liver produces a fluid called bile that is essential if the body is to digest fats. Bile emulsifies (breaks down) large fat globules so they can be absorbed. Without bile, which is stored in a sac called the gall bladder, most of the fat would pass through the digestive system undigested. From the duodenum, the digested food flows into the small intestine, called the ileum in humans. The small intestine has densely folded structures called villi that look like tiny

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tongues or fingers. These villi absorb the molecules of nutrients and then rapidly send them into the bloodstream. The parts of food that have not been digested or absorbed continue to pass down into the large intestine (called the colon in humans) in the chyme. The large intestine’s job is to absorb nearly all its water, and what is left arrives at the rectum where it is stored as feces. When a sufficient quantity has accumulated, the feces are expelled through the anus. This is called elimination.

Dinosaur

Although mammals all have the same basic digestive system, there are differences that reflect their diet. Carnivores, or meat-eaters who have special teeth and live by hunting, have fairly short digestive systems since meat is mostly protein and requires very little hard work to digest. Some carnivores swallow their prey whole and thus do not need to have salivary glands. Herbivores (plant eaters) have to consume huge amounts of vegetative matter that is also hard to break down. As a consequence, their digestive systems are much more complicated than that of a carnivore. For example, a cow has four stomachs and an extremely long and coiled intestine. Some animals, like rodents and rabbits, even reingest their waste pellets so that their food passes through their systems twice. Since their digestion is basically incomplete, they must take in their partially digested fiber and have it pass through their digestive system again to benefit from it fully.

Dinosaur A dinosaur is an extinct vertebrate (an animal with a backbone) reptile. The first dinosaurs appeared on Earth around 220,000,000 years ago and after surviving for 140,000,000 years, suddenly disappeared. Certain dinosaur species, like the Brachiosaurus, were the largest animals ever to have lived. All of the knowledge about dinosaurs is the result of studying the fossil remains that have been discovered in all parts of the world. Dinosaurs lived during a time in Earth’s history called the Mesozoic Era, also called the Age of Reptiles. In many ways, dinosaurs were much like the reptiles we know today—the familiar snakes, turtles, lizards, and crocodiles. Like them, dinosaurs may have been ectothermic or coldblooded, meaning that their internal body temperature would change according to the temperature of their environment. However, the great physical bulk of some species suggests that it would have taken them a very long time to reach their full size, since ectothermic animals grow very slowly. Like today’s reptiles, they varied greatly in size, from those the size of a chicken to others that grew to more than 90 feet (27.4 meters) U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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long. Some dinosaurs were carnivores (meat-eaters) and others were herbivores (plant-eaters). As a reptile, they laid eggs that had tough outer shells and that may have contained enough water and food for the dinosaur embryo to grow and finally hatch.

KNOWLEDGE OF DINOSAURS REVEALED THROUGH FOSSILS A duck-billed dinosaur fossil that is 78,000,000 years ago. Fossils have been a tremendous help to scientists in learning what dinosaurs were like before they became extinct. (Reproduced by permission of Photo Researchers, Inc.)

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Although a great deal more is known about dinosaurs today compared to when the first fossil bones were discovered in England around 1822, new facts are learned every year as new finds reveal more about their lives and habits. One thing that is known is how dinosaurs actually reproduced. It is known, however, that many probably laid eggs as all reptiles do. Also, it is not known how long they normally lived. Some species may have lived in herds, while others could have been solitary. Another mystery is whether plant-eaters ate underwater plants or leaves on trees, or if carnivores ate other dinosaurs. Among the many things not known


about dinosaurs, certainly the biggest and most important of all is the actual reason for their sudden and total disappearance. It is known that after entirely dominating the Earth, dinosaurs went extinct about 63,000,000 years ago.

Dinosaur

The fossilized remains of dinosaurs have been found in all parts of the world because at the time they lived, there was a single land mass or continent called Pangaea. When this single continent slowly broke up and moved apart into the several ones evident today, the fossil remains were scattered along with the land masses. This fossil evidence is available today only because of a particular set of circumstances that occurred millions of years ago. If a dinosaur was stuck in soft mud and died there, or if it died and simply fell in and sank, it would sometimes be covered by more and more sediment (that was moved there by wind, water, or ice). Over even more time, these sediments would be compressed by layers of Earth deposited on top of it until everything was transformed by great pressure into solid rock. After nearly 200 years of collecting and studying fossilized dinosaur bones, fossilized eggs, and footprints left in rock, scientists have been able to reconstruct several species with some degree of certainty. They have also been able to classify them as they would any living animal, and have divided dinosaurs into two main groups according to the structure of their hips. This may sound strange, but when people realize that how their hips and those of animals are shaped and function affects how they move about, it starts to make sense.

DINOSAUR GROUPS The first group or order called Saurischia had a hip structure that resembled a lizard. The second order, called Ornithischia, had hips that were built like those of a bird. The first group included the largest (and most ferocious) dinosaurs. Thus the well-known Apatosaurus (formerly known as the Brontosaurus) is among this group, as is the fearsome Tyrannosaurus rex. Many believe that this large group consisted mainly of carnivores (meat-eaters), and it is known that they walked mainly on four feet, lived mostly on land, and had barrel-like bodies and legs that looked like columns, as well as long, heavy tails. The second group with birdlike hips are believed to be mainly plant-eaters. The well-known Stegosaurus was a member of this order. It had the smallest brain compared to body size of any dinosaur. Although there are many things left to learn about dinosaurs to understand their life cycle, habits, and internal functions, what remains the largest gap in the knowledge about these great beasts is the cause of their sudden extinction. Scientists now believe that a mass extinction must have U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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somehow occurred, but they are still not in agreement as to its cause. Scientists have already eliminated theories that say that dinosaurs simply grew too large to hold themselves up. Instead, many now think that dinosaurs were already on the decline when something very big and destructive happened. One theory says that the key event was Earth being struck by a massive piece of debris from space, like an asteroid. This would have caused tons upon tons of dust and soot to clog Earth’s atmosphere, causing either prolonged darkness that cooled the planet or a greenhouse effect that trapped warmth and caused the surface to overheat. Others say that climates may have simply changed too fast for dinosaurs to adapt. Whatever the exact nature of the cause or causes, something did happen with which the dinosaurs were unable to cope, and they all eventually disappeared. Science may never know for sure what killed all the dinosaurs. [See also Evolution; Fossil; Geologic Record; Paleontology]

DNA (Deoxyribonucleic Acid) Deoxyribonucleic acid, or DNA, is the genetic material that carries the code for all living things. This code determines the form, development, and behavior patterns of an organism and is part of the chromosomes that exist within the nucleus of cells. DNA consists of two long chains joined together by chemicals called bases and coiled together into a twisted-ladder shape. DNA is a large molecule found in almost all organisms and contains codes for the making and using of proteins. Since proteins carry out the work of all cells, it is DNA that ultimately controls and directs all the activities of a living cell. Biologists have known about DNA for a very long time. Even before they discovered that genes control heredity, they were aware that the cell’s chromosomes were made up of protein and a special chemical they called deoxyribonucleic acid (DNA). Although DNA was discovered in 1869, more than fifty years passed before biologists believed that genes were composed of DNA. As a nucleic acid, DNA was considered too simple a chemical to contain the huge amount of complex information needed to determine heredity. Since it is made of only four or sometimes five chemical bases, called nucleotides, no one thought that DNA was complex enough. However, as more experimental evidence began to point toward DNA as key in the transmission of hereditary characteristics, more scientists began to turn their attention to DNA. 164


WATSON AND CRICK DISCOVER THE STRUCTURE OF DNA By the early 1950s, an unusual pair of scientists teamed their efforts in the passionate belief that the structure of DNA held the key to understanding how genetic information is stored and transmitted. In 1951, the twenty-four-year-old American geneticist James Watson met the thirtysix-year-old English physicist (and self-trained chemist) Francis Crick and the two decided to try to solve the puzzle of how such a simple material as DNA could store so much complicated information. Both knew they did not have to make any new discoveries, but instead, had to solve what might be called the “molecular architecture” of DNA.


The key to solving that problem lay in a technique known as x-ray crystallography. When x rays are directed at a crystal of some material, such as DNA, they are reflected and refracted by the atoms that make up the crystal. A refraction pattern is produced from which a skilled observer can determine how the atoms of the crystal are arranged. However, this is more difficult than it sounds, and Watson and Crick were trying to solve a very difficult puzzle. It was the chemical composition of DNA itself that led them to the correct model. They knew that DNA was composed of four chemicals—adenine (A), guanine (G), cytosine (C), and thymine (T)—and that A was always paired with T, and C always with G. Knowing this and studying the x rays led them to realize that this alignment could only happen if DNA was made up of two strands that were twisted together to form what is called a “double helix.” (A double helix is the correct name for a corkscrew-like spiral shape.) In March 1953 Watson and Crick built a wire model showing that the DNA molecule could be thought of as a ladder where the nucleotide bases (A, T, G, C) form the “rungs” connecting the two side rails. The sides were then twisted to make the double helix. In Watson and Crick’s Nobel Prize-winning work, the base pairs were the critical part that allowed them to explain how nature stores and uses a genetic code. Each DNA base is like a letter of the alphabet, and a sequence of nucleotide bases can be thought of as forming a message. Put another way, each “rung” (base pair) of the twisted ladder consists of some combination of the four chemicals (A, T, G, C) that form the coded message.

THE CELL’S INSTRUCTION MANUAL DNA has been called the instruction manual for the cell. It has also been called the chemical language in which “gene recipes” are written. This is because genes can be considered recipes for making proteins, and proteins control the characteristics of all organisms. These codes or recipes are written with the four nucleotide building blocks (the bases A, T, G, and C). Each gene has several thousand bases joined together in a precise U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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JAMES DEWEY WATSON AND FRANCIS HARRY COMPTON CRICK The team of Watson and Crick discovered the structure of deoxyribonucleic acid (DNA), one of the most important discoveries of modern science. Their model explained how genetic information is coded and how DNA makes copies of itself. This discovery formed the basis for all the genetic developments that have followed. English molecular biologist Francis Crick (1916– ) was born in Northampton, England. As a boy, he was very interested in chemistry, although he eventually obtained a degree in physics from University College in London. During World War II (1939–45) he worked on the development of radar (an instrument used to determine an object’s position, speed, or other characteristics) and new weapon design. After the war, Crick decided he wanted to concentrate on biology and did so on his own, eventually taking a job at the Cavendish Laboratory where he would meet the younger Watson. American molecular biologist James Watson (1928– ) was a former “Quiz Kid,” which was the name of a popular radio show in the 1940s. Born in Chicago, Illinois, Watson was a child prodigy (an exceptionally smart person) who graduated from the University of Chicago when he was nineteen and who obtained his Ph.D. from the University of Indiana at twenty-two. After receiving a fellowship to study in Copenhagen, Denmark, he joined the Cavendish Laboratory at Cambridge, England, where he met Crick. Although Crick was twelve years older than Watson, both shared what was described as “youthful arrogance” and found that they got along extremely well both personally and professionally. Crick had been studying protein structure, and Watson came to Cambridge highly interested in discovering the basic substance of genes. With these complementary goals, they teamed up to try to unravel the structure of DNA, the carrier of genetic information at the molecular level.

code that is different from the code for any other gene. For example, a sequence such as A-T-T-C-G-C-T... etc. might tell a cell to make one type of protein (for red hair), while another sequence such as G-C-T-C-T-CG ... etc. might code for a different type of protein (for blonde hair). When cells reproduce by division (a process called mitosis), each parent cell must make sure that its daughter cell (its exact duplicate) gets a complete copy of its DNA. This is accomplished by a process called “replication” in which the two strands or rails of the DNA ladder “unzip” themselves down the middle of the bases (or rungs of the ladder). Since A always links to T and G always to C, each separate rail of the ladder becomes a 166


Beginning in 1951, they worked to create a DNA model that would explain how it could copy and pass on its instructions to every new cell in a living thing. After a great deal of experimentation, they recognized the importance of DNA x rays being done by the English biochemist Rosalind Franklin (1920–1958), and soon built an accurate spiral-shaped model, called a double helix, in which they said two parallel chains of alternate sugar and phosphate groups were linked by pairs of organic bases. Their model looked like a twisted, spiral staircase. They then theorized that replication (the process by which DNA molecules copy themselves) occurs by a parting, or unwinding, of the two strands, or bases, of the staircase that then unite with newly created strands to form new DNA molecules (made up of one old strand and one new strand). Watson and Crick then published their findings in the journal Nature, which appeared on April 25, 1953 (with Watson’s name appearing first due to a coin toss). Other researchers soon confirmed their hypothesis and the Watson-Crick model was accepted as correct.


Watson went on to write several highly regarded books, one of which became the first widely used textbook on molecular biology. He taught at Harvard University, became director of Cold Spring Harbor Laboratory of Quantitative Biology in Massachusetts, and served as the first director of the United States Human Genome Project. Crick joined the Salk Institute for Biological Studies in San Diego, California, in 1977. In 1962, Watson and Crick shared the Nobel Prize in Physiology and Medicine with their colleague, English physicist, Maurice H. F. Wilkins, for their discovery of the structure of DNA. Rosalind Franklin worked on Wilkins’s team, and she would have received the award had she lived. The Watson-Crick discovery of DNA structure ushered in the modern era of molecular biology and made possible all that has happened since. There would be no understanding of human genetics without their discovery.

template or model for free-floating bases to link up to. The result is the existence of two identical double helixes where there was just one. The other important processes to understand are called “transcription” and “translation,” which are the two stages of making a protein. In transcription, DNA “unzips” again and another type of nucleic acid called ribonucleic acid (or messenger RNA) uses one strand of DNA as a template to make an exact, single-strand copy. The RNA then leaves the nucleus with its message and becomes a template for the production of protein (by a ribosome in the cell) in what is known as translation. This process is going on all the time in our bodies, since cells are constantly called upon to make protein molecules. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Dominant and Recessive Traits

Breakthroughs in understanding DNA have also led to the use of DNA in forensic science where “DNA fingerprinting” or “DNA profiling” is conducted. This use of DNA is based on the fact that repetitive sequences of DNA vary greatly among individuals, since each person has his or her own unique code. It has also led to the beginnings of treatment for hereditary disease by genetic engineering. In theory, doctors will be able to replace a defective (inherited) gene that causes a certain disease with a normal gene, thus preventing the patient from getting the disease in the first place. [See also Cell; Double Helix; Gene; Nucleic Acid]

Dominant and Recessive Traits Dominant and recessive traits exist when a trait has two different forms at the gene level. The trait that first appears or is visibly expressed in the organism is called the dominant trait. The trait that is present at the gene level but is masked and does not show itself in the organism is called the recessive trait. In order to understand the concept of dominant and recessive traits, it is necessary to know what is meant by the word “allele.” Alleles have to do with genes, and genes are the carriers of information that determine an organism’s traits. Our height, hair color, blood type and overall looks are but a few examples of traits that are the result of the chemical activities directed by our genes. Every human being is produced by sexual reproduction and therefore receives twenty-three gene-containing chromosomes (coiled structures in the nucleus of a cell that carries the cell’s DNA) from each parent, resulting in a full complement of forty-six chromosomes. When the chromosomes pair up to form a new and unique individual (since chromosomes always exist in pairs), they do so in a very particular way so that the same trait is always carried on the same place or position on the chromosome. In other words, since the offspring receives information on each trait from both its parents, there are corresponding pairs (or two genes) that match together for each trait. Sometimes these are the same (when a person inherits a gene for blood type O from both its parents), and sometimes these are different (when the person inherits blood type O from the father and blood type A from the mother). When these forms of the same type of gene are different or alternative versions, they are called “alleles.” Therefore, alleles are different forms of a gene for a particular characteristic. However, more and more the word allele is used interchangeably for gene. 168


Most often when an individual receives two different alleles for a given trait, one allele is expressed and the other is not. For example, a person may receive one allele for a straight hairline and another for a widow’s peak (when the hair comes down to a point in the middle of the forehead). In such a case, the person will have a widow’s peak since that allele is “dominant” or is the one that is able to express or show itself. In the same way, the allele for brown eyes is known to be dominant over the allele for blue eyes. Conversely, the allele that is masked or is not able to show itself (despite being there) is called “recessive.”

Double Helix

Austrian monk Gregor Mendel (1822–1884) made the first detailed investigation of inherited traits in the 1860s. Since Mendel’s groundbreaking work, the rule has been that when two organisms showing different traits are crossed, the trait that shows up in the first generation is considered the dominant trait. A dominant trait could be compared to an athlete who dominates a game or a person who dominates a conversation. Each of these people monopolize things to the point where others have no chance to express their ability or ideas. It is in this way that a dominant allele expresses itself and suppresses or masks the activity of the other allele for that trait. Although the masked allele is not expressed, it is still there and remains part of the person’s inherited package. This means the recessive allele can still be passed on to the next generation. Masked or recessive traits can only express themselves when the individual has a matching recessive allele (totaling two alleles for that trait). Although Mendel did not know exactly what the gene and the allele were, he knew very well that they existed in some form (he called then “factors”), and that they followed certain laws. He was therefore able to formulate what became known as the law of dominance. This law states that when a dominant and a recessive form of a gene come together, the dominant form masks the recessive form. Thus, even though the recessive allele (or member of the gene pair) is still present, it is not visible. [See also Gene; Inherited Traits; Mendelian Laws of Inheritance]

Double Helix The double helix refers to the “spiral staircase” shape or structure of the deoxyribonucleic acid (DNA) molecule. DNA is the genetic material of all living organisms. Also described as a twisting ladder, this double helix model enabled scientists to finally account for both the similarities as well as the immense variety of life. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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The discovery of the double helix molecular structure of DNA in 1953 by American biochemist James Dewey Watson and English biochemist Francis Harry Compton Crick was one of the major scientific events of the twentieth century and some would say in the history of the life sciences. Prior to this discovery, it was not understood how such a relatively simple nucleic acid as DNA could contain such a vast complex of hereditary material, and few, if any, believed that it did. As a nucleic acid, DNA was known to be composed of only four different submolecules called nucleotides—adenine (A), guanine (G), thymine (T), and cytosine (C)—and Watson and Crick believed that if they could determine the structure of DNA, they could explain how DNA actually works. The major way of learning about the structure of a chemical is to crystallize it and x-ray the crystals. When x rays pass through the crystals, they bend or diffract and create a pattern that can be studied. Watson and Crick worked with English physicist Maurice H. F. Wilkins and his associate Rosalind Franklin, whose excellent x rays in

The structure of a DNA molecule. Watson and Crick knew that the structure of DNA held the key to understanding how genetic information is stored and transmitted. (Reproduced by permission of the Federal Aviation Administration. Photograph by Robert L. Wolke.)

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1951 provided important evidence that DNA had a spiral shape. Franklin’s x rays showed that the DNA molecule was a double strand of twisted material which came to be called a double helix. (Helix is taken from the Greek word for spiral.)

Double Helix

Knowing this did not allow Watson and Crick to correctly describe the actual structure of a DNA molecule, and for some time they tried to build a model of what it might look like. Although, until they discovered that the four nucleotides always formed themselves into a definite pattern of pairs (A always pairs with T, and G always pairs with C), they were unable to make further progress. Once they knew that these “base pairs” were complementary (in other words they always paired up the same way), Watson and Crick designed and built a model in which the correctly paired bases were the “rungs” of a ladder that connected the two sides or “rails” of the ladder. These sides were then twisted in the shape of a compact spiral or coil. Watson and Crick also explained that the rungs on the ladder (called bases) were the coded instructions, and that the order of these four nucleotides (A,T,G,C) spelled out the instructions for all of the different characteristics of an organism. In March 1953, the two scientists announced their discovery of the double helix structure of the DNA molecule, offering to science what was basically the explanation of the chemical basis of life itself. Unlike many discoveries in the life sciences, knowledge of the structure or shape (the double helix) of the DNA molecule was essential to explaining how it could carry all the information needed to make a living creature, as well as how it could make exact duplicates of itself. In 1962 Wilkins shared the Nobel Prize in Physiology or Medicine with Watson and Crick for their discovery. Rosalind Franklin would surely have been included as well, but she had died in 1958 and the prize is only given to living scientists. [See also DNA]


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E Echinoderm An echinoderm is a spiny-skinned invertebrate (an animal without a backbone) that lives in the ocean. Most echinoderms, like starfish and sand dollars, have a distinctive five-part body plan, an endoskeleton (an internal skeleton), and many tiny, sucker-tipped tube feet with which they take in seawater in order to move. As one of the more unusual animals, the echinoderm (meaning “spinyskinned”) numbers about 6,000 related species and includes such exotic animals as the five-armed starfish, the pincushion-like sea urchin, and the bottle-shaped sea squirt. With their armor-like external spines that protect them, it might seem that an echinoderm has an exoskeleton (like a lobster or an insect). In fact, the spines are only extensions of an interior or endoskeleton that makes echinoderms unique among invertebrates. This internal skeleton is made of plates under the skin that have spiny projections. Because of this internal skeleton, echinoderms are considered closer to vertebrates that any other invertebrate phylum. However, unlike vertebrates, echinoderms have no head or centralized nervous system. Nor do they have an excretory or respiratory system. Finally, echinoderms have a circulatory system that is unique in the animal kingdom since they use seawater as their circulatory fluid. Echinoderms use their specialized tube feet to suck in water and create a suction effect that allows them to move about and to feed on other animals. The starfish is an excellent example of features that make an echinoderm such a different animal. Also called sea stars, this invertebrate has a radial symmetry, meaning that its body parts are arranged around a central U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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area or hub. Animals with radial symmetry can be divided into mirror-image halves along many lengthwise lines. A human body that has bilateral symmetry only can be divided perfectly down the middle and, therefore, have only two mirror-image halves. However, a starfish with five arms can be divided evenly several different ways. A starfish has a skeleton made up of hard plates beneath its skin. It also has many canals inside its body that help it operate a water pumping system, allowing it to move about and find the food it needs. These canals are connected to tube feet that are located on its underside and are hollow, suction-cup-like structures that suck in water. This forces the tube feet outward. A starfish can walk on the tips of its arms, using the sucker action of the tube feet to cling to a surface. When the suction is released, it moves forward. The starfish also uses its tube feet to wrap its arms around a clam shell and pry its hinged shells apart. A starfish’s mouth is located in the center of its underside. It eats the soft-bodied mollusk by pushing part of its own stomach through its mouth and engulfing its prey, thereby digesting the clam inside the shell.

A starfish on the California coast. The starfish is an excellent example of an echinoderm because of its five-pointed, radially symmetrical arms. (Reproduced by permission of Field Mark Publication. Photograph by Robert J. Huffman.)

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Finally, the starfish and all other echinoderms reproduce sexually by the union of male sperm and female egg. They also have remarkable powers of regeneration. If an echinoderm’s arm breaks off, it will grow back, and if a part of its central core breaks off, it will grow into a complete organism. Many clam and oyster fishermen viewed the starfish as a rival, since clams and oysters are the starfish’s main diet, and would tear these creatures apart whenever they found them. Unfortunately the fishermen were only multiplying their problem.

Ecology

[See also Animals; Invertebrates]

Ecology Ecology is the branch of the life sciences that studies the relationships between living things and their environment. The basic principle behind the idea of ecology is that organisms interact with their surroundings and are influenced by them, while at the same time changing them. The notion of ecology is therefore something like a two-way street. The study of ecology takes as a basic principle something that we all recognize once it is stated: the dynamic, living world is based on complex relationships between organisms and the places in which they live. Therefore, ecology can be described as the attempt to explain how and why living things interact with each other and their environments. The word ecology comes from the Greek words oikos, meaning “house, household, or place to live” and logos, meaning “the study of.” The word was created in 1866 by Ernst Heinrich Haeckel (1834–1919), a German biologist and philosopher, after he came to recognize the importance of studying the environment as a separate scientific field. Haeckel was greatly influenced by the English naturalist Charles Darwin (1809– 1882), whose classic book, On the Origin of Species by Means of Natural Selection, inspired Haeckel to think about the important role the environment played in the story of nature. Until Haeckel, ecological issues were referred to as natural history, and a student of nature was called a “naturalist.” While there was some notion of the interrelatedness of life and its environment before Darwin’s work, it was mostly a notion of nature as being economical and Earth being a place where things were always balanced harmoniously.

ECOLOGY DRAWS ON MANY SCIENCES Today, scientists who study ecology are called ecologists, and they are well aware that although there may be a type of harmony in nature, U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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RACHEL LOUISE CARSON American biologist Rachel Carson’s book, Silent Spring, is one of the most important works in the history of ecology (the study of the interrelationships of organisms and their environment). This very popular book became an instant classic and informed people about the severe side effects of chemical poisons on all forms of life, thus making people aware for the first time that environmental damage can affect life itself. Her work marks the beginning of the environmental movement in the United States. Rachel Carson (1907–1967) was born in Springdale, Pennsylvania. Her interest in nature came from her mother taking her into the woods and fields around her home and exploring the countryside. This love of nature would remain with Carson throughout her life, and after entering the Pennsylvania College for Women as an English major she was so impressed by a biology teacher that she switched her major. In 1929 she graduated with a degree in zoology and went on to earn her Master’s degree in zoology from Johns Hopkins University in 1932. Her appointment to the Department of Zoology at the University of Maryland allowed her to teach and spend summers doing research at the Woods Hole Marine Biological Laboratory in Massachusetts. This turned her interest more to the sea, but family commitments to her widowed mother and orphaned nieces soon forced her to leave teaching and take a position at the United States Bureau of Fisheries. It was while she worked at the bureau that she began to combine her love of nature with her love of writing. In 1941 she produced her first book about the sea, and during World War II (1939–45) she wrote fisheries information bulletins for the government. By 1949 she had become chief biologist and editor for the new United States Fish and Wildlife Service, and began a new book titled The Sea Around Us. This book appeared in 1951 and won the National Book Award, becoming an immediate best-seller. After a second successful book on the sea, Carson produced her classic, Silent Spring, in 1962. This book has been described as the work that gave rise to the environ-

the balance supporting it is both fragile and difficult to repair if broken. If ecologists can be said to be specialists, their specialty is necessarily a very general one. This is the case because ecologists use knowledge from many disciplines, such as chemistry, computer science, mathematics, and physics, besides the many subspecialties of biology (botany, microbiology, morphology, physiology, and zoology). This multidisciplinary approach is required since ecologists’ subject matter is so broad and deals with all major living and nonliving aspects of what can be called a system. Increasingly however, there are more individuals who are becoming specialists within the general field of ecology, resulting in such spe176


mental movement in the United States, and Carson was the first to draw widespread attention to the fact that the haphazard use of chemical pesticides was creating an ecological disaster for animals and people alike. Carson was above all a scientist, and her book was not a one-sided argument full of accusations and no facts. Rather, it was a well-documented, reasoned analysis of the actual and potential hazards of misusing chemicals.

Ecology

Carson’s book focused specifically on the disastrous effects caused by the highly popular but overused pesticide DDT (declare-diphenyl-trichloroethane). Carson showed that birds would eat insects contaminated with DDT, and as a result lay eggs with shells too brittle for the birds inside the egg to survive to birth. She also documented how DDT would remain poisonous for years since it was unaffected by the sun, rain, or even bacteria or acids in the soil. It would also be stored in body tissues after being eaten and therefore would remain in the food chain (the series of stages energy goes through in the form of food), killing more birds, fish, and small animals. Despite extreme criticism and attacks by the chemical industry, Carson’s book served to awaken people that they were poisoning their own environment. It led to the creation of a Presidential Advisory Committee on the use of pesticides and had a great deal to do with the eventual creation of the Environmental Protection Agency. Carson’s book is rightly considered to be a landmark in the environmental movement since it did more than just alert people to the dangers of pesticides. Perhaps even more important, it took a holistic, or overall view, of life on Earth and showed how all living things were connected and how they ultimately affected one another. This led directly to an awareness of the meaning and importance of ecology. Carson was therefore a pioneering environmentalist and ecologist whose work and dedication inspired a new worldwide movement of environmental concern. Her work will remain a hallmark in the increasing awareness that all people have about how humans interact with and affect the environment in which they live and on which they depend.

cialists as population ecologists and restoration ecologists, to name just a couple.

ECOLOGY, THE TWENTIETH-CENTURY SCIENCE Although ecology began with Haeckel in the 1860s, it is really a twentieth-century science, and it is therefore a relatively young one. As a science, ecology is founded on four basic principles upon which most ecologists agree. First, it states that an organism’s life pattern or the way it lives reflects the patterns of its physical environment. This is true for nearly all plants and animals who survive and prosper when certain faU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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vorable conditions are present (unlike humans who can manipulate their surroundings and create a livable place). The second principle states that living creatures in a certain place tend to group themselves into loosely organized units known as “communities.” A community exists wherever a species has its natural home. The third principle of ecology states that an orderly, predictable sequence of development (change) takes place in any area. The slow process of orderly change over time is called “ecological succession,” and is more understandable once we realize how interdependent all life is with other life forms and its environment. The fourth principle states that a community and its environment make up an ecosystem. The notion of an ecosystem is important because by dividing the natural world into ecosystems it becomes easier to study. Since an ecosystem describes any system that has organisms interacting with other living things as well as their environment, an ecosystem can be anything from a fallen tree rotting in the forest to something as big as a river.

POPULATIONS, COMMUNITIES, AND ECOSYSTEMS In studying the relationships between living things and their environment, ecologists regularly use three main organizing ideas that have proved very helpful. These are the concepts of populations, communities, and ecosystems, which allow ecologists to categorize systems in terms of how they are organized. A population is the smallest system and is defined as a group of individuals from the same species who live in the same area at the same time. For example, the bees in a hive represent a population, but so do a particular forest of beech trees or the deer that live in that forest. Studying a particular population over time allows an ecologist to know what natural forces can change the size of a population (like bad weather or hunting by predators), and to detect when human activities are at fault (such as sewage runoff or acid rain). A community is larger than a population and is defined as a group or a collection of different animal and plant populations that live in the same area. It is also sometimes called a “biotic community” since it is made up of living things (biotic means living things). Ecologists also study the roles that different species play in their communities. They call this the “ecological niche” of a species, and it refers to such factors as what it an animal eats or what amount of moisture is necessary for a certain plant to thrive. More broadly, a niche has been described as the exact way in which a living thing fits into its environment. An example of a highly specialized niche would be an animal that is the only species that eats a certain plant to live. The animal’s specialization serves it well since it has no competition for its food. However, this specialization 178


could be dangerous as well, since the animal might starve if the particular plant suddenly dies off. Finally, an ecosystem (or an ecological system) includes a community (biotic) as well as a community’s environment (abiotic).

Ecology

LANDSCAPE, BIOME, AND BIOSPHERE Besides population, community, and ecosystem, ecologists may use larger categories to describe the environment that makes up an ecosystem. Beginning with a particular ecosystem, the next largest unit is a landscape that includes groups of ecosystems and humans. Landscapes are part of a bigger unit called a biome. Biomes are large geographical areas that include certain type of climates, vegetation, and animals. The six major land biomes are rain forest, desert, grasslands, temperate deciduous forest, taiga, and tundra. Larger than biomes are biogeographic regions such as major continents and oceans. The final and largest category of all is the biosphere. This includes every part of Earth that has living things on it, from the sky to the ocean bottom to the mountain tops. The biosphere includes all of Earth’s ecosystems functioning together. The idea of functioning together is especially important, since the notion of every part of the whole being linked together is what the concept of ecology is all about.

DIVERSITY CREATES HEALTH Knowing how living and nonliving things are linked together helps ecologists understand better how living things depend on each other for survival. This knowledge allows certain types of ecologists called “restoration ecologists” to work toward returning a damaged ecosystem back to its natural state. More recently, one of the areas receiving attention from ecologists is the notion of the importance of biodiversity to the environment. Only in the 1980s did scientists begin to realize that the more diverse and complex an ecosystem was the healthier it was. It was also realized that the loss of biodiversity can be permanent, as when a species goes extinct (life forms that have died out). Since ecosystems interact with one another in ways that we sometimes do not understand, human activity could be harming biodiversity and permanently altering many of Earth’s ecosystems. Ecologists are also concerned about many other ways that human beings affect the planet, and many feel that if we continue to pollute our air and water, destroy the rain forests, and steadily increase our population, our harm to Earth may be irreversible. [See also Biome; Biosphere; Community; Ecosystem; Population] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Ecosystem

Ecosystem An ecosystem (ecological system) is a living community and its nonliving environment. It is a term used by life scientists to break up the biosphere (the entire living world) into smaller parts so as to more easily categorize and study those parts. An ecosystem can be any size and has no set boundaries. The term can be applied to an entire forest, a lake, a vacant city lot, a suburban lawn, and even a crack in a sidewalk. An ecosystem is a complex system made up of communities of living organisms that interact with each other and with their nonliving surroundings. Ultimately, the entire Earth with all of its life and its physical environment can be said to make up the largest ecosystem of all—Earth’s biosphere.

BIOTIC AND ABIOTIC COMPONENTS OF AN ECOSYSTEM All ecosystems, no matter their size, have two interacting parts—the biotic (living) component and the abiotic (nonliving) component. The biotic component is made up of autotrophic organisms (self-nourishing) and heterotrophic (other-nourishing) organisms. Green plants are autotrophic since they can make their own food or nourish themselves by their ability to convert sunlight into energy. Animals are examples of heterotrophic organisms since they cannot make their own food and are able to break down other living things (plants or animals) and use their energy.

OTHER PARTS OF AN ECOSYSTEM These biotic and abiotic components are in fact only two parts of a six-part system that ecologists use to categorize what goes on in every ecosystem. These six elements are based on the flow of energy and the cycle of nutrients within an ecosystem. These six elements are (1) the sun; (2) abiotic substances; (3) primary producers; (4) primary consumers; (5) secondary consumers; (6) decomposers. The sun of course is the ultimate source of all energy, and its light is used by green plants (as primary producers) to make food in a process called photosynthesis. Essential to this process are abiotic substances like carbon dioxide, water, and phosphorus, which the plant uses to carry out its food-making series of chemical reactions. After this, a green plant is ready to be consumed or eaten by a primary consumer called a herbivore (any plant-eating animal from a mouse to a cow or an insect) or an omnivore (any animal able to eat both plants and other animals). Primary consumers are followed, sometimes literally, by secondary consumers who are carnivores and therefore 180


eat the primary consumers. Thus carnivores, like snakes and coyotes, generally feed on herbivores. A subset called tertiary consumers are the world’s scavengers, feeding off dead animals (as vultures do) or dead organic matter (as earthworms do). Finally, the decomposers have a major role to play in every ecosystem. Decomposers such as bacteria and fungi break down dead organic matter and allow it to release its minerals and compounds back into the soil. Beginning with the sun and ending with organic material being used by green plants, the series of stages that energy goes through in the form of food is called a “food chain.” The connected network of producers and consumers is called the “food web.”

Ecosystem

ENERGY FLOW AND BIOGEOCHEMICAL CYCLE The physical environment and all of its components serve to make up the abiotic part of an ecosystem. Included in it are basic elements like air (composed mainly of nitrogen, oxygen, and carbon dioxide) and soil (containing nitrogen calcium, phosphorus, zinc, and other minerals). These and other basic elements are constantly being shifted, transferred, and otherwise moved about in an ecosystem. Ecologists identify two major aspects of this movement: the energy flow and the biogeochemical cycle. In the food web, energy flows through an ecosystem in a series of predictable changes. First, it is changed from light energy into chemical energy by green plants that store it in their cells. Next, the primary consumers eat the plants, digest plant cell walls, and change these into their own form of chemical energy (which they too sometimes store). When a secondary consumer kills and eats a primary consumer, the energy enters the secondary consumer’s body. Animals use this energy to keep their vital functions performing and to carry out actions needed for survival (such as running after and catching another animal). When an animal exerts itself like this, it loses much of its energy in the form of heat that escapes from its body and radiates off into the atmosphere. Ecologists have studied this energy flow and arrived at what they call an energy pyramid. In all ecosystems, plants form the base or the largest part of the pyramid, followed by herbivores in the middle, which is topped by the carnivores. This means that more energy passes through plants than U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

A temperate rain forest ecosystem like the one pictured here contains many organisms that depend on one another and their environment for survival. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

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Egg

passes through herbivores, and still less through carnivores. Creating such a pyramid allows ecologists to know roughly how many pounds of vegetation it takes to support a certain number of herbivores, and how many herbivores are necessary to support one carnivore. Like energy flow, the biogeochemical cycle tells ecologists the routes that chemicals follow as they pass between organisms and their environment. The obvious path of water is a good example as it evaporates off the surface only to fall back as rain or snow. In the phosphorous cycle, plants take phosphorus from the soil, animals get the phosphorus from eating the plants, and the soil gets the phosphorus back from the decomposers when the plants or animals die. Knowledge of these abiotic factors is as essential as understanding biotic factors.

ECOLOGY IS A COMPLEX SCIENCE Recognizing all of the interactions within a complex ecosystem requires a great deal of time and actual field study. If ecologists want to understand the subtle day-to-day changes as well as the major ones in an ecosystem, they must become familiar with biology, physics, chemistry, mathematics, climatology, and even geology of ecosystems. Only then can ecologists know which changes are desirable and which may pose a real threat. This knowledge is vital for ecologists if they want to preserve the well-being of all ecosystems. [See also Abiotic/Biotic Environment; Food Web/Food Chain]

Egg An egg is a specialized female reproductive cell that, if fertilized, can develop to produce a new organism. Nearly all female animals produce eggs, or ova, and in most species the eggs develop inside their body. No matter its size, every egg is a single cell before fertilization. An egg cell is like every other cell in that it has a nucleus (the cell’s control center), cytoplasm (the jelly-like fluid within a cell), and all the other cell structures. However, an egg cell is different from body cells because it has the potential, when fertilized by a male sperm cell, to divide, form an embryo, and develop into a new organism. In contrast to sperm that are small, mobile, and produced in great numbers, eggs tend to be immobile, large, and produced in limited numbers. The larger size of eggs is attributed to the fact that eggs must contain all of the material necessary for the beginning of growth and development. A fertilized egg 182


cannot obtain food, and therefore must store enough nutritional supplies to allow it to be able to divide. Nearly all female animals produce eggs, and while some force the fertilized eggs out of their bodies as birds do, others keep them inside their bodies where they will grow and develop.

Egg

TYPES OF EGGS There are three major types of eggs roughly categorized by the amount of yolk each has. Yolk is rich in protein and lipids (fats) and is basically the egg’s stored food. How much yolk there is in an egg relates to how much its mother is involved with its development. Some species, like humans, have only a very small amount of yolk since the developing embryo will soon switch to nutrients obtained from its mother’s blood. Thus the human egg after it is fertilized needs only enough yolk to last until it has implanted itself in the wall of the uterus (about eight to ten days). Other types of animals need a moderate supply of yolk. A frog is a good example of an animal whose egg has just enough yolk to get the developing embryo to the tadpole stage (during which it will survive on food stored in its tail). Bird eggs, like those of a chicken, have the largest yokes since they are expelled from the mother’s body right after fertilization and must carry their entire food supply with them. The eggs of birds, reptiles, and amphibians—all of whom lay their eggs—are approximately 95 percent yolk, while those of mammals contain less than 5 percent yolk. Eggs that develop outside the body of the mother are enclosed in some form of protective covering or shell which has pores despite its hardness and thickness. Inside this outer covering are membranes that are filled with fluid. These prevent the egg from drying out and also act as a shock absorber. In addition to the yolk of the egg is the germ or nucleus that will develop into an embryo after it has been fertilized. Upon fertilization, the egg undergoes drastic changes as cell division begins. As the developing embryo passes through its growing stages it lives off the yolk, and when it has fully developed inside the egg, it will eventually break through the shell and emerge. The largest known egg is that of the ostrich, which can weigh as much as 3 pounds (48 ounces). Unlike birds, fishes, reptiles, and most insects that lay eggs, most mammals keep the developing egg within their bodies and give birth to the fully developed organism. Animals that reproduce by giving birth to live young are called viviparous. Those that lay or spawn eggs that develop and hatch outside of their bodies are called oviparous. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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KARL ERNST VON BAER Estonian biologist Karl von Baer (1792–1876) is considered the founder of modern embryology, which is the study of the early development of living things. As the discoverer of the mammal egg, he developed a theory explaining early development that showed that mammals, including humans, were not fundamentally different from other animals. Karl von Baer was born in Piep, Estonia, which was then part of old Russia. He was descended from Germans, however, and after being tutored at home as most children of nobility were, he was sent to Austria and then Germany for his higher education. After obtaining a medical degree and studying comparative anatomy (the comparison of different animals’ structures), he moved to Konigsberg in Russia and taught zoology and anatomy. It was there that Baer began to search for something that had eluded everyone to that point—the mammal egg. Some were not even sure it existed, while others thought that the follicles in the ovaries might be eggs. In fact, it was this small sac of cells (called follicles) in the ovary of a mammal that actually contained the egg. Baer searched the follicles of a female dog and discovered the mammalian egg by identifying a yellowish spot inside the follicle that he could see only with a microscope. Baer published his discovery in 1827 and with this proof, it finally became a fundamental part of science that the development of mammals, including humans, was not fundamentally different from that of other animals. Baer then turned to what might be called comparative embryology and began to do research on the developing embryos of different vertebrates (animals with a backbone). It was already known that all these embryos were formed of three layers (called germ layers, although they had nothing to do with bacteria, or the germs, known today). Baer argued and was able to demonstrate that it was out of these three layers that all later organs were formed. This came to be called his germ theory. He also put forth some very important “laws,” one of which stated that the younger the embryos of different species are, the more closely they resemble each other. This led him to state what was called the concept of epigenesis, that an embryo develops from the simple to the complex, and that it goes from undifferentiated or ordinary tissue to specialized tissue that has some specific purpose. Although English naturalist Charles Darwin (1809–1882) and others later used Baer’s work to support the theory of evolution (the process by which gradual genetic change occurs over time to a group of living things), Baer refused to ever concede that Darwin might have been right. Baer was a complex man of wide scientific interests. During his long career he served as chief of a new zoological museum and became the librarian at the St. Petersburg Academy of Sciences in Russia. He also led a scientific expedition to Arctic Russia in 1837, and served as inspector of fisheries for the empire for a year. He continued his work until his death at age eighty-four.

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FERTILIZATION OF EGGS

Embryo

Eggs are usually fertilized in one of two ways. In mammals, snakes, birds, turtles, and insects, mating or the coming together of male and female sex cells takes place inside the body of the female. Unlike mammals, most aquatic animals, like fishes, toads, and frogs, engage in external fertilization. For example, the male frog sprays the female’s eggs with his sperm as she deposits them in the water. For many animals, including humans, eggs (that are laid or spawned) are a source of food since they contain a great deal of nourishment. [See also Fertilization; Reproduction, Sexual; Reproduction System]

Embryo An embryo is an early stage in the development of an organism. It can be applied to plants as well as animals and is the most important growing phase of an organism. The embryonic stages of all animals are similar, although they may occur at different times. All complex organisms, from a plant to a human being, begin as a single fertilized cell. This cell is called a zygote and it is the product of a sperm (a male sex cell) and an egg (a female sex cell) coming together. From the moment this fertilized egg or ovum begins to divide until it has formed its organs, it is known as an embryo. In some animals, like birds, this takes place inside an egg that is expelled from the female’s body. In other cases, like most mammals, this takes place inside the body of the female. In humans, the word embryo refers to the first two months of development, after which it is called a fetus. In the development of every organism from a single cell to a complex living thing, a great many stages or phases take place. The embryonic stage is one of these, and it also has phases or stages of its own. This embryonic stage is probably the most important of all the stages because it is during this time that the growing embryo forms all of the tissues and structures that it will need as a mature adult.

FROM EMBRYO TO ORGANISM A developmental chain begins within an embryo that eventually results in the birth of a fully developed organism. The first stage is called the embryonic stage, and it has four of its own stages: cleavage, gastrulation, neurulation, and organogenesis. Beginning with the zygote, the first U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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A human embryo at five to six weeks of development. (Reproduced by permission of Photo Researchers, Inc.)

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stage of cleavage or repeated cell division occurs when the zygote cleaves or splits into two cells. This division continues as two cells become four, four become eight, and so on, until a ball-shaped cluster is produced called a morula. As division continues, the morula becomes a blastula, which also resembles a ball of cells. It is at this point in mammals that attachment to the uterus occurs. Soon the blastula folds inward and gastrulation, or rearrangement, of the cells begins. This gastrulation creates layers of cells, and in vertebrates each layer will produce different organs. This begins the process of specialization known as differentiation. As differentiation continues, the first specialized tissues begin to form, starting the neurulation phase. During neurulation, organs form at the head of the new organism and continue to develop downwards. By the time organogenesis begins, the embryo has also started to change its shape to make its developing organs fit. The body becomes longer, and a head and trunk become visible. Dramatic and rapid changes now occur as the em-


bryo develops rudimentary or primitive versions of the heart, lungs, liver, kidneys, and other vital organs.

Endangered Species

Although all animals follow the same stages, their time schedules can be very different. While a fish or an amphibian may be ready for birth after six weeks, mammals, like humans, must remain in the female’s womb for another twenty or so weeks to develop further. However, by the time the human embryo reaches eight weeks, all its adult organ systems are in place and it is no longer called an embryo, but is a fetus. Plants also develop from embryos. In flowering plants and conifers (like evergreens), the embryo is the part of the seed from which the mature plant develops. In plants like ferns (nonflowering plants that form spores), the embryo is the mass of cells that develops into a new plant. [See also Fertilization; Plant Anatomy; Plant Reproduction Reproduction, Sexual; Zygote]

Endangered Species An endangered species is any species of plant or animal that is threatened with extinction. Although extinction, or the situation in which no living member of a species exists any longer, can have natural causes, it is often being hastened by the activities of humans. International efforts to identify the most threatened species have begun, and various national and international laws have been passed to protect these endangered organisms. Biologists estimate that about 500,000,000 species of plants and animals have existed since life began on Earth. Since there are only some 2,000,000 to 4,000,000 species in existence today, most of the species that ever lived on Earth are now extinct. This demonstrates that extinction is a natural phenomenon that occurs as the normal process in the course of evolution. Evolution has shown that species that cannot adapt to natural changes in their environment or to increased competition from other species will eventually become extinct. Besides climate or environment change and competition, disease and natural catastrophes can destroy an entire species. It is believed that such an occurrence took place more than 65,000,000 years ago when an asteroid struck the Earth and eliminated more than half of the planet’s plant and animal life, including the dinosaurs.

ENDANGERMENT INCREASES DUE TO HUMANS Although extinction may occur naturally, the growing problem in our modern world is that increased human activities have dramatically inU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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creased the natural rate of extinction. While people have always had an effect on their environment, the degree to which they have affected it has increased substantially since the Industrial Revolution of the eighteenth century. During the second half of the twentieth century in particular, technological advances combined with a rapidly expanding human population have changed the natural world as never before. As the twentyfirst century begins, biologists estimate that the rate of extinction has increased to somewhere between 100 to 1,000 times nature’s normal rate. Today, species are threatened and become endangered primarily because of a combination of three human factors: their habitat is either disturbed or eliminated; they are overhunted; or they are being eliminated by other nonnative “introduced” species.

CAUSES FOR ENDANGERMENT Habitat destruction is the main reason for the increasing number of today’s endangered species. The steadily increasing human population has in many ways taken over areas that formerly were habitats or homes for certain organisms. This “takeover” usually shows itself in the form of houses, highways, and industrial buildings in developed countries, and growing farms and farmland in less developed ones. Whenever a new airport or dam is built in a formerly natural landscape, the life forms that lived there must somehow either adapt, move, or die. Often a habitat is totally destroyed and changed into something that is totally unrecognizable from its former self. Other times however, habitat destruction is less obvious, such as when industrial runoff or city sewage degrades the quality of a habitat but leaves it apparently looking the same. As habitats are regularly chipped away and become smaller and smaller, the pressure on the organisms in the habitat becomes greater, and it becomes harder for them to survive. The inhabitants in a given habitat are often also sought after by people for many different reasons. Whether people have killed animals for sport (such as the American bison was in the nineteenth century) or for profit (like the whale and the rhinoceros), overhunting has placed many of today’s species close to extinction. Finally, people have altered habitats dramatically by taking one species from its native habitat and transplanting it into a different habitat somewhere else. Sometimes this is done deliberately, as when rabbits were introduced to Victoria, Australia, by Thomas Austin in 1859. Austin release twenty-four of the animals from England to be used for sport hunting. However, within twenty years these twenty-four rabbits multiplied to millions. Not only did they became serious pests, but they set about actively destroying certain vegetative life. 188


Other times, introducing a new species is done accidentally, as when the gypsy moth was accidentally introduced into the United States from France in 1869. Sometimes newly introduced species cause little or no harm, but many times they upset the delicate balance that was established by the native organisms, and often out-compete these “natives” for scarce resources. Sometimes a new species can actually eliminate a species that was perfectly adapted to its habitat but had no natural defenses to use against the newcomer.

SAVING ENDANGERED SPECIES Fortunately, something is being done for species that are so threatened that they may disappear altogether. On a global level, the International Union for Conservation of Nature and Natural Resources (IUCN) classifies such species as endangered, critically endangered, threatened, or rare. To the IUCN, an endangered species is in the greatest danger and faces immediate extinction, possibly even if action is taken in an effort to prevent it. A critically endangered species is one that will not survive without human help. Threatened species may still be abundant in their own habitat, but overall their population is rapidly declining. Rare species are considered at risk because of low overall population numbers. The Endangered Species Act of 1973, which became law in the United States that year, obligates the government to protect all animal and plant life threatened with extinction. It also provides for the drawing up of lists of such species and promotes the protection of critical habitats or places that are essential to the survival of a species. One of the first species placed on that list was the nation’s own symbol, the bald eagle. In May 1998, the bird was taken off the list, along with several other species that have apparently been saved.

Endangered Species

The pygmy lemur of West Madagascar is the world’s smallest primate and in danger of becoming extinct. Human actions have led to the endangerment of this species. (Reproduced by permission of World Wide Fund For Nature Photolibrary.)

[See also Extinction; Habitat] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Endocrine System

Endocrine System The endocrine system is made up of a number of glands that control bodily functions by secreting hormones. It is found in all vertebrates (animals with a backbone) and works together with the nervous system to regulate many of the body’s functions. The hormones secreted by glands are chemicals that carry instructions from one set of cells in an animal’s body to another. The endocrine system can be described as the body’s chemical coordinating system. Although the endocrine system works closely with the body’s nervous system in helping it respond appropriately to a changing environment, it is different from that system in many ways. Where nerve transmission is rapid and over quickly, the effect generated by a certain hormone can last several days and is usually concerned with large, longterm processes or situations. For example, the nervous system makes a child jerk his hand away from a hot stove almost as soon as he or she has touched it. In a very different situation however, it is the endocrine system that senses that the child has not been fed for some time and makes sure that his or her brain has the steady supply of glucose (sugar) that he or she needs. Therefore, the messages sent by the endocrine system usually have long-term effects and are almost always concerned with the body’s larger processes, unlike the nervous system which generally brings about short-term changes.

THE ENDOCRINE SYSTEM AND HORMONES In vertebrates, the endocrine system uses several different glands, as well as specialized tissue and cells, to secrete hormones directly into the bloodstream. These glands are called ductless because they have no need for ducts or tubelike connections to the circulatory system (a network that carries blood throughout the body). Rather, they secrete hormones in very small amounts that travel throughout the body via the bloodstream to eventually reach their “target organ” (which is sometimes not close to the secreting gland). Although the hormones cause no changes in the tissue they pass through, they do cause major changes in the target organs, which have special receptors or receivers for hormones. Working together with the nervous system, the hormones released by the body’s endocrine system regulate growth, ovulation (the release of an ovum or egg from an ovary), milk production, sexual development, and many other processes. The endocrine system can be considered the body’s chemical coordinating system as it gives orders to start and stop certain bodily changes as needed. Since stopping a process is sometimes as important as starting, 190


the endocrine system has what are called feedback mechanisms that monitor a situation and signal when a gland should stop secreting hormones. Malfunctions in the system can cause an over or undersupply of a hormone, resulting in such conditions as gigantism (unusually large stature), dwarfism (unusually small stature), or a goiter (visible swelling at the front of the neck).

Endocrine System

Hormones also play a large part in the growth cycle of insects. Since insects have an exoskeleton (a hard shell that surrounds its body), they must periodically molt or shed their skeleton if they are to grow. Molting involves the precise coordination of many separate steps, and hormones begin and coordinate the entire, complicated process. A hormone also signals caterpillars to make a cocoon and enter their pupa (resting) stage. It is in this stage that they undergo a metamorphosis and transform into an entirely different creature, like a moth or butterfly.

GLANDS OF THE ENDOCRINE SYSTEM In humans and other vertebrates, the endocrine system regulates a great deal more than one or two bodily functions and processes. In humans, there are at least nine separate glands that make up the endocrine system, and they are scattered throughout the body. These major endocrine glands are: the pituitary, the hypothalamus, the pineal, the thyroid, the parathyroid, the thymus, the adrenal, the pancreas, and the gonads.

The Pituitary. Located at the base of the brain (of which it is actually a part), the pituitary influences so many glands that it has been called the “master gland.” About the size of a pea, this gland works with the hypothalamus as part of a direct link between the endocrine system and the nervous system. Although small, it is really two glands in one and its parts are called the anterior (front) and the posterior (rear) pituitary. The anterior secretes the hormone prolactin, which makes the female body ready to produce milk. The anterior also secretes five other hormones that start other glands working. The posterior pituitary secretes oxytocin, which makes the uterus contract during birth and stimulates the release of milk. Vasopressin which regulates the balance of water and raises blood pressure is also released by the posterior pituitary. The Hypothalamus. The hypothalamus monitors internal organs and emotional states and supervises the release of hormones from the anterior pituitary gland. It also produces substances called releasing factors that control hormonal secretions from other glands. The Pineal. The pineal gland is a light-sensitive organ that evolved from a third eye that vertebrates had on top of their heads until about U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Hypothalamus Pituitary

Pineal Parathyroid

Thyroid Thymus Adrenals

Pancreas

Ovaries

Testes

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240,000,000 years ago. It secretes the hormone melatonin, which acts as the body’s biological clock and helps regulate its rhythms.

Endocrine System

The Thyroid. The thyroid is located around the windpipe at the front of the neck and produces hormones containing iodine that are necessary for growth, development, and proper metabolism (all the body’s processes involved in using energy). Hypothyroidism, or a lack of these hormones, leads to stunted growth and retardation in children. In adults, hypothyroidism produces dry skin, sluggish behavior, and an inability to tolerate cold. Hyperthyroidism, or an overproduction of hormones, results in nervousness and agitation, weight loss, and heavy sweating. The Parathyroid. There are four parathyroid glands next to the thyroid. They regulate the level of calcium in the blood by taking calcium stored in bones and releasing it into the bloodstream. Proper calcium levels are essential to muscle contraction and the clotting of blood.

The Thymus. The thymus is located behind the breastbone and between the lungs. It is often considered to be part of the immune system since it plays a major role in the development of lymphocytes (white blood cells) that fight infection. It is also considered part of the endocrine system because it produces a hormone that stimulates the growth of these white cells. The Adrenals. The two adrenal glands found in humans are located above each kidney, and their outer region, called the cortex, secretes hormones called glucocorticoids. These help maintain the blood’s important glucose level and also minimize inflammation (swelling and tenderness) that is caused by infection or injury. The adrenal medulla is the inner region of the gland and secretes epinephrine and norepinephrine, which prepare the body for stress by narrowing the blood vessels and increasing both the heart rate and the amount of glucose in the blood. During times of major excitement, these dramatic changes can be easily felt and recognized as the body prepares for “fight or flight.” The Pancreas. The pancreas (an organ involved in digestion) is located below the stomach and contains clusters of endocrine cells called pancreatic islets that secrete glucagon and insulin. These hormones are key to controlling the level of glucose in the bloodstream. When not enough insulin is produced or the target cells do not respond to insulin, a disease called diabetes mellitus is diagnosed.

The Gonads. Finally, the gonads are the main organs of reproduction and secrete sex hormones. The testes in males and the ovaries in females usually come in pairs in most mammals. The testes are responsible for producing androgens—the most important of which is testosterone— U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

Opposite: A labeled illustration showing all of the glands of the human endocrine system. All of the glands are same for males and females except for the gonads. The ovaries are found in females and the testes are found in males. (Illustration by Kopp Illustration.)

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Endoplasmic Reticulum

which help develop sperm and are responsible for what are called secondary sexual characteristics, like a deeper voice and facial hair. The ovaries help in the production of ova (eggs cells) and produce progesterone and estrogen. These hormones are responsible for female development (like breasts) and also control pregnancy. Both of these physical changes come about in boys and girls during puberty, that burst of development that occurs during the early teenage years.

THE IMPORTANCE OF THE ENDOCRINE SYSTEM It is thought that vertebrates have as many as fifty different hormones, suggesting that the endocrine system of humans is a highly complex and very intricate system. It might be said that the overall goal or purpose of the endocrine system is to maintain a balance in all the body’s systems. Despite this general goal, the hormones that the system actually releases can have powerful and even dramatic effects. For example, the gonads change a boy into a man and a girl into a woman; the adrenal gland pumps adrenalin and can make a person unnaturally strong or not feel pain; the pituitary can produce nutritious breast milk to nourish a baby; and the thymus can help wage and win a war against infection. [See also Hormone; Immune System; Reproductive System; Sex Hormones]

Endoplasmic Reticulum Endoplasmic reticulum is a network of membranes or tubes in a cell through which materials move. As an organelle in a eukaryotic cell (complex cells having nuclei and other organelles), it is involved with the production of new proteins as well as with the movement of materials throughout the cell. The name endoplasmic reticulum means “an inside formed network,” and these words properly describe this network of connected folds, tubes, and sacs that are found in all eukaryotic cells (those with a distinct nucleus). As an organelle in a cell, the endoplasmic reticulum is a specialized structure and has certain tasks to perform. The endoplasmic reticulum has been described as the pipeline or conveyer belt through which supplies are moved about the factory (or the cell). Viewed through a microscope, the endoplasmic reticulum looks like flat, tubular membranes that are connected to the nucleus and which spread throughout the interior of the cell. There are two types of endoplasmic reticulum: rough endoplasmic reticulum and smooth endoplasmic reticulum. Rough endo194


plasmic reticulum appears to have a pockmarked, or rough outer surface, and this is because it is covered with ribosomes, which are structures in the cell that make proteins. Smooth endoplasmic reticulum has no ribosomes dotting its surface. Smooth endoplasmic reticulum has the job of packaging and distributing materials needed for making proteins and other substances, and also is responsible for “detoxifying” any substance in the cell that might be poisonous. Both types of endoplasmic reticulum are essential to the cell if it is to be able to move materials around to where they are needed. There is always a large amount of endoplasmic reticulum within a cell, and in many cells it takes up nearly half the amount of interior space. Although both are involved with transport, smooth endoplasmic reticulum curves around the cell’s cytoplasm (its jelly-like fluid) like interconnecting pipelines, while rough endoplasmic reticulum looks more like stacked and flattened sacs. Both smooth and rough endoplasmic reticulum are connected directly to many other organelles in the cell, and without the membrane pipeline that the endoplasmic reticulum provides, eukaryotic cells would not be able to carry out many of their functions.

Entomology

[See also Cell; Organelle]

Entomology Entomology is the branch of zoology dealing with insects. It includes the study of the development, anatomy (structure), physiology (functions), behavior, classification, genetics, and ecology of insects. It is a fascinating branch of science because by their sheer numbers, insects are considered to be the dominant group of animals on Earth today. Based on the Greek word entomon for insects, entomology is the scientific study of insects. As with most branches of zoology, entomology was first seriously studied in the work of the Greek philosopher Aristotle (384–322 B.C.). It was he who gave the first good descriptions of insect anatomy and laid the groundwork for entomology by stating that all insects have a body divided into three parts. After Aristotle’s work, there was little attention paid to insects except to add more types to the list of known species. However, in 1602, the Italian naturalist Ulisse Aldrovandi (1522–1605) devoted an entire book to insects called Of Insect Animals, and by the time the microscope was invented in the latter part of that century, two men in different countries made the first accurate studies of insect anatomy. In Italy, the physiologist Marcello Malpighi (1628–1694) turned his microscope on insects and discovered the tiny, branching tubes with which they breathe. He also devoted an entire volume to the interior U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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JAN SWAMMERDAM Dutch naturalist Jan Swammerdam (1637–1680) is considered to be the founder of entomology or the study of insects. He also was the first to observe red blood corpuscles (an unattached body cell) and did a great deal of work in comparative anatomy (the study of the structure of living things). Swammerdam’s life always seemed to be characterized by extremes. Although he led a productive life, it also was extremely difficult and sad. Born in Amsterdam in the Netherlands, the son of an apothecary, or pharmacist, Swammerdam was interested in natural history and particularly insects at a very early age. Although his father sent him to medical school, he intended that his son become a priest. When the young Swammerdam graduated from the University of Leiden, he took up the study of natural history and never practiced medicine. Since he had also refused to become a priest, his father decided not to support him and cut him off from any financial help. Despite a lack of money, Swammerdam continued to pursue his biological studies and did an enormous amount of work under very difficult conditions. Swammerdam studied the life cycles and anatomy of many species of insects, particularly honeybees, mayflies, and dragonflies. Using his microscope, he made several discoveries about what goes on when certain insects undergo complete metamorphosis. This is when it experiences a complete change in its body shape, such as when a caterpillar turns into a butterfly. From these observations of metamorphic development, Swammerdam was able to classify insects into four major groups, three of which are accepted and used today in insect classification. Aside from this, one of his most significant achievements may have been simply to disprove some of the false notions about insects. Until Swammerdam demonstrated that insect bodies have real structure and internal organs, most had considered them to have simply fluid-filled

organs of the silkworm. About the same time in Holland, the Dutch naturalist Jan Swammerdam (1637–1680) was doing such excellent and highly detailed studies on insect anatomy, that he came to be considered the founder of modern entomology. Today’s entomology deals with a group of animals that are by far the largest of all the classes. There are more than 800,000 known species of insects, and it is estimated that there may be an equal number of still-unknown species. Insects belong to the phylum Arthropoda, which is considered the most successful group of animals on Earth. This is because insects have lived on Earth for about 350,000,000 years, while humans 196


cavities with no specialized organs. Since virtually every habitat on land contains insects, and they make up about three-quarters of all the animal species on Earth, Swammerdam was finally giving nature’s most prolific and most successful class of animals their due. For example, his detailed drawings and descriptions of their reproductive organs showed that they reproduced sexually rather than via such ancient myths as spontaneous generation (which said that living things can be generated from nonliving things).

Entomology

Swammerdam also used his microscope to study some of the internal systems of vertebrates (animals with a backbone). In 1658, he documented certain small particles that he observed in the blood of a frog, making this the first time that anyone had ever seen red blood cells. He discovered valves in the lymph system (a network of vessels that carry lymph throughout the body) of mammals and also studied the fertilization of eggs as well as their development into an embryo. Experimentally, he showed that the muscle removed from a frog’s leg can be stimulated and made to contract. He also theorized and made an excellent guess about the role of oxygen in respiration. He even pioneered the practice of injecting dyes into a cadaver (a dead body) in order to better observe certain anatomical details. Unfortunately, Swammerdam never published his work and most of it went unknown during his lifetime. Since he had little or no income, he did without a great deal and actually suffered both physically and mentally. Eventually he became sick and undernourished, and with overwork and worry, he soon became depressed and mentally unstable. In 1673 he came under the influence of a cultlike figure to whom he remained devoted until his death at the age of forty-three. It was not until 1737 that the Dutch physician Hermann Boerhaave (1668-1738), discovered Swammerdam’s work and paid to have it published. Titled the Bible of Nature, this two-volume Latin translation of his Dutch work contains some of the finest illustrations of insects ever produced and served to lay the foundation for entomology.

have been on Earth less than 2,000,000 years. While insects are considered pests by many people, they are in fact highly beneficial to humans in a number of ways. Insects perform many important functions in any given ecosystem (an area in which living things interact with each other and the environment). Many insects are soil-dwellers, wood-borers, and consumers of dead animals, and therefore help in the decomposition process (breaking things down) and in the recycling of nutrients. Many insects are eaten by fish and birds and are thus an important food source. Many insects kill and eat other insects, helping to regulate populations. Many flowering plants are dependent on the pollinating activities of bees, U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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butterflies, and flies in order to reproduce. Insects also provide people with a number of useful products such as silk, wax, and honey. On the other hand, insects can be terribly harmful by causing huge losses of food and acting as transmitters of diseases to humans, plants, and animals. Modern entomology has two aspects: the scientific side that simply wants to learn as much as possible about all insects, and the practical or applied side (often called economic entomology) that searches for methods to better control insects. This latter part of entomology investigates the physiology, development, genetics, diseases, and behavior of pest insects in order to discover new ways of controlling insect populations. For example, research on insect development has led to the use of specific chemicals that disrupt certain hormones important during metamorphosis (as when a caterpillar changes into a flying insect). Other investigations have shown ways to use chemicals to modify insects’ behavior (and perhaps confuse them during their mating cycle). Such methods, when combined with traditional chemical pesticides, come under the term pest management. However, entomologists do not want to exterminate all insects. Rather, they hope to control and limit any bad effects insects may have. This is probably wise, since most people who study insects realize there are many good reasons why insects are beneficial to humans, and why insects are the most abundant and most successful animal group ever.

A mosquito fossilized in amber. Insects are extremely helpful to scientists in learning what Earth was like millions of years ago. (Reproduced by permission of JLM Visuals.)

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Insects can also be enormously interesting. They have a skeleton on the outside of their bodies; they have no lungs; they smell with their antennae; and some taste with their feet. Some hear with special organs in their abdomens or front legs. Some are highly social and live in colonies. Because insects are highly adaptable, many people say that only insects will remain after every other living thing has disappeared from Earth.

Enzyme

[See also Insects]

Enzyme An enzyme is a protein that acts as a catalyst (a substance that speeds up a chemical reaction) and speeds up chemical reactions in living things. The chemistry of life would not be possible without enzymes since they allow reactions in organisms to happen very quickly and therefore support life. Each enzyme is highly specific and will only work in one particular reaction. Like a catalyst, an enzyme is not consumed or used up during a reaction. Since it is not changed or affected in any way by the process it helps create, an enzyme is immediately ready to be used again for the same purpose. Enzymes are essential to living things because without them, most of the chemical reactions that take place inside an organism would happen too slowly to keep it alive. Temperature is a key factor in a chemical reaction, and the temperature of most living cells is too low to allow the necessary reactions to take place quickly enough. With the proper enzyme, temperature no longer is a limiting factor. Since without the right enzyme, the reaction might occur so slowly that the cell would die. Although enzymes are catalysts, they are far from being typical— while a catalyst can be any type of simple substance, an enzyme is a complex protein. Further, it is by far the most efficient catalyst, since it can, at times, increase reaction rates by factors of 1,000,000 or more. Since enzymes are complex proteins, they have their own unique three-dimensional shape. It is this shape that makes a particular enzyme act in a certain way. Their shape is the determining factor in how they will act because of the nature of a chemical reaction. A chemical reaction results in the formation of a new compound from existing ones. The mechanism of a chemical reaction involves either the breaking or the forming of a chemical bond (the link between its atoms). Whether breaking or forming bonds, energy is always necessary, and it is here that enzymes play their part. Enzymes either add energy to make something happen or reduce the energy required for something to happen. Either way, U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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ANSELME PAYEN French chemist Anselme Payen (1795–1871) investigated the chemical reactions carried out by plants and discovered diastase, the first enzyme known to science. He also introduced the filtering properties of activated charcoal and discovered cellulose, a basic constituent of plant cells. Anselme Payen was born in Paris, France, the son of a lawyer who turned to a career in industry and started up several chemical production factories. At the age of twenty, Payen was put in charge of his father’s borax production plant. Borax is a crystalline mineral often found in salt lakes that is refined and used in metallurgy (metal-making) and to make soaps, glass, and pottery glazes, among other things. Payen discovered a method of preparing borax from boric acid which was readily available from Italy. Since his production costs for this method were so low, he was able to undersell his competitors, the Dutch, who had a monopoly on borax. Five years later he turned his attention to his company’s production of sugar from sugar beets. Seeking a way to remove color impurities from beet sugar, he invented a method of using activated charcoal to filter out, or catch, large molecules. As a result of Payen’s process, the filtering properties of charcoal have since been put to many uses, the most notable of which was their use in gas masks during wartime. In 1833, Payen made another major discovery. That year he separated a substance from malt extract (grain that was germinating or starting to sprout) that seemed to be responsible for speeding up the conversion of starch into sugar. Further tests suggested to Payen that this substance acted as yeast did, meaning that it acted as a catalyst. A catalyst is a chemical that

they are able to create a reaction that would not have occurred without the enzyme. Since each enzyme has a unique shape, it will only “fit” or work for one particular reaction. This means the more reactions an organism needs, the greater the number of different enzymes required. For example, in the process called cellular respiration (in which food is broken down to release energy), approximately thirty chemical reactions take place, and each is controlled by its own enzyme. It is estimated that the typical animal cell (roughly one-billionth the size of a drop of water) contains about three thousand different enzymes, all of which are programmed to work in a certain chemical reaction. Enzymes work at the cellular level and can be considered a cell’s chemical regulator or system of control. The needs of a living cell are 200


usually speeds up the rate of a chemical reaction, but is not itself affected, or changed, in any way. Payen named this new substance “diastase” from a Greek word meaning “to separate,” since in many ways it separated the individual building blocks of starch into its individual components of sugar. To his delight, he found that diastase even worked when it was taken out of the original malt extract that produced it. Diastase, therefore, could be called an organic catalyst, or an enzyme, since an enzyme is a substance that acts as a catalyst in biological systems. Thus, diastase was the first enzyme to be produced in concentrated form. Ever since, enzymes that have been discovered have been named with the “-ase” suffix, the pattern started by Payen.

Enzyme

Payen was the first to isolate cellulose, a carbohydrate (a compound consisting of only carbon, hydrogen, and oxygen). While studying different types of wood, he obtained a substance that was definitely not starch but that nonetheless could be broken down into units of sugar as starch could. Because he obtained it from the cell walls of plants, he named it “cellulose.” Once more, Payen established a naming system, and ever since carbohydrates always end with “-ose,” like glucose and sucrose. Much later, cellulose went on to be used as the building block for many other products. Treated with acids and other additives, it was the main ingredient in the manufacture of guncotton (an explosive), celluloid (film), cellophane, and rayon, among others. By 1835 Payen abandoned business altogether and became professor of industrial and agricultural chemistry in Paris. He died in Paris during the Franco-Prussian War after refusing to leave the city as the Prussian army advanced.

constantly changing as it strives to adjust to the ever-changing demands of its environment. Therefore, the cell needs the flexible system of control that enzymes provide. Another advantage of an enzyme is that the reactions that it stimulates are reversible if the cell wants the opposite reaction. The synthesis, or the making, of an enzyme is controlled by a specific gene. Usually it is a hormone that switches on the gene, which, in turn, signals that a certain enzyme should be produced. Since enzyme production can be turned on, it can also be turned off. Both of these on/off mechanisms are natural and happen all the time, but certain forms of artificial “inhibitors” can turn off production permanently. Many poisons and some drugs have this effect, and can lead to death. For example, certain nerve gases as well as the poison cyanide permanently inhibit or stop U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Computer-generated models of pepsinogen, a pre-enzyme found in the stomach, and pepsin, a digestive enzyme. In the presence of increased acidity, pepsinogen transforms into pepsin. (©Ken Edward/Science Source/National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.)

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the important enzyme that allows the body to use oxygen. This results in a quick death. Besides the natural enzymes in the bodies of organisms, enzymes are put to use everyday in the production of beer, wine, cheese, and bread. Without the proper enzymes contained in yeast cells, none of these important food products could be made. [See also Protein]

Eutrophication Eutrophication is a natural process that occurs in an aging lake or pond as it gradually builds up its concentration of plant nutrients. Cultural or artificial eutrophication occurs when human activity introduces increased amounts of these nutrients. These speed up plant growth and eventually choke the lake of all of its animal life.


In nature, eutrophication is part of the normal aging process of many lakes and ponds. Some never experience it because of a lack of warmth and light, but many do. Over time, these bodies of fresh water change in terms of how productive or fertile they are. While this is different for each lake or pond, those that are fed rich nutrients from a stream or river or some other natural source are described as “eutrophic,” meaning they are nutrient-rich and therefore abundant in plant and animal life. Eutrophication is not necessarily harmful or bad, and the word itself is often translated from the Greek as meaning “well nourished” or “good food.” However, eutrophication can be speeded up artificially, and then the lake and its inhabitants eventually suffer from too much of a good thing.

Eutrophication

HUMANS INCREASE RATE OF EUTROPHICATION Artificial or cultural eutrophication has become so common, that the word eutrophication by itself has come to mean this harmful acceleration of nutrients. Human activities almost always result in the creation of waste, and many of these waste products often contain nitrates and phosphates. Nitrates are a compound of nitrogen, and most are produced by bacteria. Phosphates are phosphorous compounds. Both nitrates and phosphates are absorbed by plants and are needed for growth. However, the human use of detergents and chemical fertilizers has greatly increased the amount of nitrates and phosphates that are washed into our lakes and ponds. When this occurs in a sufficient quantity, they act like fertilizer for plants and algae and speed up their rate of growth. Algae are a group of plantlike organisms that live in water and can make their own food by photosynthesis (using sunlight to make food from simple chemicals). As plants begin to grow explosively and algae “blooms,” two harmful things occur, both involving oxygen. First, the fast-growing plants and algae consume more oxygen than usual. Algae consume oxygen even at night. This in itself is not that harmful, but when combined with the second effect, it sometimes has a fatal result on the body of water. All algae eventually die, and when they do, oxygen is required by bacteria in order for them to decompose this dead material and break it down. A cycle then begins in which more bacteria decompose more dead algae (consuming more oxygen). The bacteria then release more phosphates back into the water. These phosphates feed more algae. Eventually, the lake or pond begins to fill in and starts to be choked with plant growth. As the plants die and turn to sediment that sinks, the lake bottom starts to rise. The waters grow shallower and finally the body U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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A labeled illustration of the structure of an eutrophic lake. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

of water is filled completely and disappears. This can also happen to wetlands, which are already shallow. Eventually, there are shrubs growing where a body of water used to be. In the 1960s and 1970s, Lake Erie was the most publicized example of eutrophication. Called a “dead lake,” the smallest and shallowest of the five Great Lakes was swamped for decades with nutrients from heavily developed agricultural and urban lands. As a result, plant and algae growth choked out most other species living in the lake, and left the beaches unusable due to the smell of decaying algae that washed up on the shores. New pollution controls for sewage treatment plants and agricultural methods by the United States and Canada led to drastic reductions in the amount of nutrients entering the lake. Forty years later, while still not totally free of pollutants and nutrients, Lake Erie is again a biologically thriving lake. [See also Algae; Bacteria; Pollution]

Dense shoreline vegetation

Nutrients Thermal pollution Limnetic zone high concentration of nutrients and plankton Profundal zone Benthic zone Sediment contributing to the filling in of the lake bed Silt, sand, and clay bottom

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Evolution

Evolution

Evolution is the process by which gradual genetic change occurs over time to a group of living things. As one of the major theories in the life sciences, it can be applied in a narrow sense to the development of a species that adapts to its environment over many generations. In a broader sense, evolution can mean that all life on Earth has progressed from simple to more complex organisms. Evolution is one of the great explanatory theories of science because it describes how biological change occurs over time. However, since it also applies to the human species, it has philosophical and even religious implications and can be considered controversial.

DARWIN PROPOSES THEORY The theory of evolution was first presented in 1858 by the English naturalist Charles Darwin (1809–1882). Its basic idea is that species undergo genetic change (changes in inherited characteristics) over time and appear different from their long-dead ancestors (although not noticeably different from other living members of their species). Darwin stated that these changes are brought about by the organism’s response to the environment. He believed that organisms that are best adapted to their environment have a greater chance of passing on those “fit” traits to their offspring. Nature therefore “selects” the best adapted (fittest) and “selects out” those organisms whose genes contain traits that are less fit. Darwin’s theory further stated that evolution has given rise to all the different forms of life that have ever existed. This means that all species came from a single form of life or a common ancestor that may have first appeared about 3,500,000,000 years ago. The branching out from this first life form that eventually resulted in the enormous variety of living things is called “speciation.” Darwin also theorized that the process of evolution is extremely slow. This notion of “gradualism” means that many generations must pass before any changes are noticeable. While Darwin’s theory of evolution is not accepted by some who believe in the biblical account of creation, his theory is supported by a vast amount of factual evidence and has withstood all serious attempts to disprove it.

NATURAL SELECTION The main idea behind evolution is “natural selection,” because this concept explains how evolution occurs. Natural selection favors the U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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passing on of beneficial or “fit” genes to the offspring and discourages the transmittal of less valuable genes. As the “engine that drives” evolution, natural selection does not work on the individual unit of life but at the population level. The term population refers to all members of the same species, which are slightly different from one another and live together in a particular place. For example, some deer have larger antlers than others, others are larger and stronger, and others may have a slightly different coloring. Therefore among any population or group of organisms that breed with one another, there will necessarily be a large genetic pool of slightly different traits. Nature, or the population’s local environmental conditions, selects which individuals are best adapted. Those individuals with traits that best fit their habitat will get more food, be better prepared to escape a predator, or better able to resist disease than those not so well adapted. Those with favorable traits get to survive and to reproduce—passing these traits on to their young. Individuals with less favored traits do not reproduce as much, and eventually, an entire population will have the traits (in their genes) of the favored group. Understanding natural selection allows us to understand the basics of evolution. Often the term “survival of the fittest” is used to mean that only the strongest survive or get to live a very long time. Actually, however, the term means that organisms poorly suited or badly “fitted” to their environment are not able to reproduce as much as the “fittest” among them. Consequently, the “fittest” get to survive longer and produce the most offspring. This process assures that it is their genes that get spread the most throughout the population. The fittest organism does not have to be the strongest (or even smartest) individual. What determines fitness is based entirely on the population’s environmental conditions. This means that it is essentially a random process. Nature has no master plan that it uses to make species change. Rather, a variety of traits exist in a given population, and whichever one gives an individual an edge over others in the population is the trait that is favored. In many ways, natural selection appears to be a matter of chance. An example of this is the peppered moth of England. Before the Industrial Revolution of the eighteenth century took place and created the steampowered factories of London, this particular moth was a beige color that enabled it to blend in almost perfectly against local tree bark. Over time, however, as its environment became more and more sooty from the burning of coal, the moth became a much darker color. Its environment change meant that those slightly darker moths had a greater chance of surviving than the lighter ones who were easily seen (and eaten by birds) against a blacker environment.

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GENETIC VARIETY

Evolution

The idea of natural selection is always connected to that of reproduction. It does no good for an organism to live a very long time if it does not reproduce. The fittest not only survive—they reproduce. Of the many factors that influence natural selection, the factor called “genetic variety” is probably the most important. Genetic variety means that the individuals making up a population must have basic genetic differences between them—that is, they have inherited characteristics that make them slightly different from one another. This becomes obvious when we realize that if each individual were identical genetically to all the others, it would not matter which one reproduced. When genetic differences exist, however, who gets to pass on which traits makes a big difference.

Mutations. Genetic variety occurs in two ways: by mutations, which are accidental changes in a gene, and by genetic recombination, which is the mixing of genes that happens to organisms that reproduce sexually. The word mutation often implies something bad, but such is not always the case. Mutations at the genetic level occur during the reshuffling that happens when meiosis takes place. Meiosis (may-OH-sihs) is the type of cell division that makes sex cells. During this division process, genes are sometimes accidentally altered, simply by chance, or by a change in the shape or number of chromosomes, which are the structures that contain the cell’s deoxyribonucleic acid (DNA). When a mutation occurs, it means that a particular trait has been changed. Sometimes the mutation is unfavorable since it may eliminate a characteristic that proved beneficial, but it can also be advantageous since it may give a new and even better trait to an individual. If a mutation is favored by natural selection, it gets passed on from one generation to the next. In many ways, pure chance allows mutations to happen and contributes to evolution. Genetic Recombination. Sexual reproduction is also a source of genetic variety. Sex cells are unlike cells in the rest of the body in that they have only a single set of genes (or only one copy of each gene). It is only after fertilization occurs (when the egg and sperm unite) that the cell gets two copies of each set of genes. During sexual reproduction (the joining of egg and sperm), genes are combined in new ways that result in an individual having a unique combination of genes. This mixing of genes is called genetic recombination, and it accounts for most of the variety needed for natural selection to occur. ADAPTATION The end result of natural selection is a process called adaptation. Through natural selection, which favors organisms that “fit” their enviU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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ronment best and weeds out those badly “fitted,” living things become better suited, or “adapted,” to their habitat or local environment. Darwin called this process of “molding” organisms to their environment over many generations “adaptation.” It has come to mean any trait or characteristic that allows an organism to fit better in its environment. Furthermore, it includes an organism’s behavior as well as its anatomy (structure) and physiology (internal processes).

THE RATE OF EVOLUTION Ever since Darwin, the pace or rate at which evolution occurs has been thought to be a gradual one, taking place over a long period of time. However, in recent times two biologists, Stephen Jay Gould of Harvard University and Niles Eldredge of the American Museum of Natural History, have proposed the idea of “punctuated equilibrium.” This new hypothesis states that a large number of new species can appear in sudden spurts rather than very gradually over extremely long periods. Gould and Eldredge support their argument by referring to the fossil record, which often shows how some species remained unchanged for millions of years. If change was slow but ever-present, it should appear that way in that fossil record. However, Gould and Eldredge point out cases where new species seem to appear out of nowhere, and use these examples to argue for evolution by sudden bursts. While there may still be controversy over the rate at which evolution occurs, there is little argument about its major points: living things are constantly competing with one another for the necessities of life; the characteristics of each vary a great deal and are inherited; some of these variations make certain individuals better adapted to their environment; the process of natural selection decides which are more “fit”; more “fit” individuals are more likely to survive and pass on their genes; and all of today’s life is the result of evolution progressing from the simple to the complex. [See also Adaptation; Evolution, Evidence of; Human Evolution; Evolutionary Theory; Natural Selection]

Evolution, Evidence of As a scientific theory stating that species undergo genetic change over time and that all living things originated from simple organisms, evolution is considered the most important basic concept in the life sciences. The theory has a great deal of explanatory power, and scientists 208


from many different fields have accumulated a large body of evidence to support it.


DARWIN LAYS FOUNDATION FOR EVOLUTION THEORY Much of the credit for the acceptance of the theory of evolution should go to English naturalist Charles Darwin (1808–1882). Darwin spent a lifetime gathering data and documenting examples that would explain his theory in detail. Although today’s scientists may understand how evolution works in a deeper way than their counterparts did in the past, the foundation of their understanding is built upon Darwin’s original observations. Darwin’s evidence for evolution falls into several categories. The first of these is called adaptive radiation and is based on Darwin’s observation that although mammals are found on all the major landmasses of the Earth, the same mammals are not found in the same habitats. Darwin speculated that when continents moved apart, animals became separated and, like Darwin’s finches in the Galapagos Islands, they adapted or went their own way in order to best fit into their environment. Darwin found thirteen species of finches on the Galapagos, and later realized that they all evolved from the same ancestor who came there from South America. He also realized that although the original ancestor probably ate seeds, its descendants adapted their original diet over time to better fit their new environment. Some therefore, became insect eaters while others ate seeds of different sizes. This process is known as adaptive radiation—meaning that many different forms have evolved from the same stock.

DIVERGENT EVOLUTION The fact that widely separated organisms can have common ancestors suggests that, although these organisms have changed quite a bit over time, some of their basic structure must still be the same. It follows that the more structures these organisms have in common, the more closely related they must be. The study of these structural similarities is called comparative anatomy, and when scientists find two different structures that perform entirely different functions but are basically similar, they argue that this provides evidence of what is called divergent evolution. A good example of divergent evolution are the front limbs of three vertebrates (animals with a backbone). When the arm of a human, the wing of a bird, and the flipper of a whale are compared, it becomes apparent that all are made of the same bones and have five digits (finger-like projections) at their ends. However, each limb has adapted to a different way of life in a different environment. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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EVIDENCE OF EVOLUTION Evidence for evolution can be found by studying three different areas. These are embryology, the fossil record, and the case of the peppered moth of England.

Embryology. Embryology studies the way organisms develop before they are born. Often embryology provides evidence of a common ancestor that cannot be found by studying the adult organism. A good example are two invertebrates (organisms without a backbone): the ragworm and the marine snail. As adults, they are not at all alike, yet when studied as developing larvae, they are remarkably similar and in fact, share a common ancestor.

The Fossil Record. Another area that provides some of the strongest evidence for evolution is the fossil record. Paleontology (the study of fossils) clearly shows that there are many species that no longer exist. By using certain techniques such as carbon dating (a method of determining the age of fossils by measuring the amount of carbon in them) and by studying the placement of fossils within the ground, many fossils can be dated or their age closely estimated. If the fossil record is then placed together based on the ages of each, a gradual change in the form of an organism can be followed and traced eventually to species that exist today. One of the best fossil records of this type is the gradual evolution of reptiles into mammals. Although there are many gaps in the overall fossil record, enough data has been discovered to allow paleontologists to piece together a progression from one-celled life, to simple, multicelled descendants, to more and more complex organisms. Most agree that the weight of fossil evidence in favor of evolution is overwhelming. The Case of the Peppered Moths. Probably the most direct evidence for evolution and also best known is the case of the peppered moths of England. Prior to the Industrial Revolution of the eighteenth century and the development of the factory system, this species of moth blended perfectly with the pale color of lichens (organisms made up of both fungi and algae) found on London trees and were able to avoid being spotted by hungry birds. Decades after the factory system began, pollution from the factories had killed the lichen and blackened the tree trunks, giving the few dark moths that were part of the species an advantage, since now they blended better with their environment. Eventually, dark peppered moths came to be dominant. Finally, when the pollution sources were reduced and eventually removed, the lighter-colored moth began to make a comeback. Direct observation of such a clear-cut and rapid case of evolution in a living species is rare, but very convincing. Today, plant and 210


animal breeders use methods similar to evolution’s natural selection to produce new and more desirable varieties. Their ability to bring about dramatic changes in a fairly short time is more than enough evidence for evolution.

Evolutionary Theory

[See also Evolution; Evolutionary Theory; Fossil; Human Evolution]

Evolutionary Theory In 1858, when English naturalist Charles Darwin (1809–1882) proposed his theory of evolution by natural selection, most of the scientific world believed that the account of the creation of the world as written in the biblical book of Genesis was true. Most thought that the world was not more than a few thousand years old and that every species had been created separately and had remained basically unchanged. However, Darwin’s theory would change that thinking. Even though evolutionary processes had been considered before the publication of Darwin’s 1858 book, Darwin’s theory rocked the world like no scientific idea ever had, before or since.

ANCIENT GREEKS HINT AT EVOLUTION Before Darwin conceptualized the idea of evolution, pieces of his theory had been considered and written about by historical thinkers. Some think the idea of evolution began in Greece more than 2500 years ago when the Greek philosopher, Anaximander (610–c.546 B.C.), taught that the life on Earth had begun in the water and eventually moved to dry land. Centuries later, the Greek philosopher Aristotle (384–322 B.C.) taught that the structures of living things were determined by the purpose they would fulfill. Both of these ideas are part of the theory of evolution. For the next 150 years however, no one took these ideas any further, and it was not until the eighteenth century that scientists (then called “natural philosophers”) began to think seriously about the origin and progress of life on Earth.

CLASSIFICATION SYSTEM SUPPORTS EVOLUTION A major step was taken in 1735 when the Swedish botanist, Carolus Linnaeus (1707–1778), first published his System of Nature in which he offered science his system of classifying living things. This great accomplishment gave science a way of ordering or making sense of the immense variety of organisms in the natural world. The system also enabled U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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scientists to begin to make generalizations (which would lead to hypotheses or educated guesses and then to theories). Linnaeus also made another contribution toward evolutionary thinking when he dared to include human beings in his classification. It was Linnaeus who called people Homo sapiens, which means belonging to the genus Homo (man) and the species sapiens (wise). This is in contrast to another member of the genus Homo, the orangutan, which he designated for some reason as Homo troglodytes (man, cave-dwelling).

BUFFON TOUCHES ON THE THEORY OF EVOLUTION By the mid-1700s, the French naturalist Georges Louis Buffon (1707– 1788) was beginning to think seriously if not systematically about the possibilities of life on Earth changing over time. Although he never stated any theory of evolution, his study of the fossil record and his knowledge of comparative anatomy led him to write that life on Earth had undergone some major changes over what appeared to be a very long time. Although he could not explain how or why this had happened, his ideas were definitely based on some parts of evolution.

LAMARCK BECOMES FIRST TO ARGUE FOR EVOLUTION Finally, at the beginning of the nineteenth century, a student of Buffon’s named Jean Baptiste Lamarck (1744–1829) became the first major scientist to put forward a serious, scientific argument for evolution. He also was the first to suggest what type of mechanism would account for evolutionary change. Lamarck’s theory argued that evolution occurred through the inheritance of acquired characteristics. His theory, however, was incorrect since it was based on two mistakes. First, he stated that nature had a definite plan and that it kept trying to perfect or make a better organism each time a change occurred. Second, he thought that the characteristics an organism acquired during its own lifetime could be passed on directly to the next generation. Lamarck used the recently discovered giraffe as an example, saying that their ancestors, in trying to reach high leaves to eat, would have stretched their necks and passed this trait on to their offspring. Although we now know that from a genetic standpoint this is not possible, Lamarck’s brave new ideas brought the idea of evolution to the scientific forefront.

DARWIN PROPOSES THEORY OF EVOLUTION Although Lamarck was the first scientist to present and argument for evolution, it was actually Darwin who introduced the theory of evolution that is accepted today. In 1858, Darwin finally published his theory in his 212


book, On the Origin of Species by Means of Natural Selection. The inspiration for this book and of Darwin’s own ideas on evolution occurred twenty-seven years before when he began a five-year journey (1831–36) on the British survey ship, HMS Beagle. As the ship’s naturalist, Darwin was able to examine thousands of plants and animals during his trip along the coast South America, the Pacific Islands, and Australia. This voyage would eventually change Darwin’s thinking as well as the thinking of the entire world about nature and the life sciences.

Evolutionary Theory

Throughout his long voyage to exotic places, Darwin was constantly amazed at the variety or diversity of life he always found. One thing he kept noting were similar but not identical species on separate islands. The birds he saw on the Galapagos Islands became his famous example. Darwin found finches whose head and beaks were adapted to the particular type of food they could find on their particular geographic regions. He found the same types of adaptations with turtles of the Galapagos. Although all were related, each species had adapted in some different way to the local conditions. Darwin’s great leap to the theory of evolution was made when, many years later, he realized that there was nothing unusual about these differences but that these variations were a basic fact of life in the natural world. Two related ideas had led him to this conclusion. First, his friend the Scottish geologist, Charles Lyell (1797–1875), had recently written a book (Theory of the Earth) saying that Earth had been formed by natural processes over an extremely long period of time. Darwin soon began to think that perhaps life on Earth had also developed gradually. The second idea that influenced his thinking was another book written by the English economist Thomas Malthus (1766–1834) whose Essay on Population argued that populations tend to outgrow their food supply, resulting in a struggle for existence. These two ideas, combined with Darwin’s knowledge of fossils, his knowledge of the structure of different species, and his awareness of how their bodies and behavior compared, eventually led Darwin to state his theory of evolution and to choose natural selection as its key.

DARWIN SHARES CREDIT Ever since Buffon, the idea of evolution had been in the air, and it is not entirely surprising that in 1858, Darwin’s contemporary, the English naturalist Alfred Russel Wallace (1823–1913) wrote and sent to Darwin a paper that essentially outlined Darwin’s not-yet-published ideas on evolution. The two therefore decided to share credit and they jointly published their theory in a scientific journal. Since then, however, Darwin has rightly received most of the credit for the full and complete theory, U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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as a result of his book, On the Origin of Species, in which he offered his lifetime of research and documentation. In 1871, Darwin also published The Descent of Man, which discussed evidence that suggested that human beings had descended from subhuman forms. This argument understandably created a great deal of controversy in both scientific and nonscientific circles. Furthermore, even Darwin’s supporters realized that despite the soundness of his theory and wealth of evidence, he had never provided an adequate explanation of exactly how biological variations were produced or passed on.

MENDEL’S WORK SUPPORTS DARWIN’S THEORY This problem was solved in 1901 when the unrecognized work of the Austrian botanist Gregor Johann Mendel (1822–1884) was discovered and published by the Dutch botanist, Hugo Marie De Vries (1848–1935). De Vries discovered that as far back as 1857, Mendel had been experimenting with varieties of the common garden pea and had published in 1867 what were essentially the laws of heredity. By carefully selecting plantbreeding combinations, Mendel discovered that an organism’s characteristics are inherited in pairs during sexual reproduction. As a result of this finding, Mendel was able to state several laws of genetics that described exactly how characteristics are passed to the next generation and how they are expressed in an individual. After Mendel’s discoveries, it was realized that traits did not blend but remained distinct. Mendel’s mathematical laws of inheritance removed the final weakness in Darwin’s theory of evolution.

EVOLUTIONARY THEORY STILL STRONG Darwin’s theory is still considered the most important basic concept in the life sciences, and today’s biologists continue to study the patterns, pace, and mechanisms of evolution. Some have even attempted to modify the original theory. One of these focused on Darwin’s emphasis on the notion of gradualism, which assumes slow, steady rates of change. A newer model, called punctuated equilibrium, suggested by American paleontologist Stephen Jay Gould and American biologist Niles Eldredge, proposes that change may occur in relatively quick bursts, followed by longer periods when nothing happens. Such theory changes do no harm to the very strong and vital theory of evolution. [See also Classification; Evolution; Evolution, Evidence of; Human Evolution; Mendelian Laws of Inheritance]

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Excretory System

An animal’s excretory system is made up of organs that remove waste products from the body in order to maintain a stable internal environment. The system also regulates the acid/alkaline balance of the body’s fluids. The method an animal uses to dispose of its waste depends on its body structure and where it lives. All animals, no matter how simple or complex, must get rid of the waste they generate through metabolism. This means that all of the processes that keep animals alive and functioning, such as the conversion of food into energy, produce some form of useless by-product called waste that must be eliminated from their systems. If waste is not removed, it will eventually poison and kill the animal. Therefore, waste removal is key to an organism maintaining a healthy or balanced internal environment. This is called homeostasis and involves keeping an organism’s internal systems stable or within certain limits. In order to maintain homeostasis, an excretory system has to be capable of three main functions: it must be able to get rid of the organism’s waste products; it must be able to keep both the fluid and the salt content of the organism within normal limits; and it must keep the concentration of other substances in body fluids at normal levels. Although the body eliminates some waste via its lungs (respiratory system) and skin (sweating and evaporation), the removal of nitrogen-based substances and water is the job of the excretory system (also called the urinary system in some animals).

SIMPLE SYSTEMS The organs that make up the excretory system often differ in shape, size, and location depending on the animal. The structure of an organism’s excretory system depends on the complexity of the organism. Among all animals, from the simple planarium to a human being, there are four main types of organs that do the work of the excretory system: a contractile vacuole (an opening or pore through which water is squeezed out); nephridia (a bunch of tubelike vessels); Malpighian tubules (a filtering system of tubes); and kidneys (that filter the blood and form urine). The simplest one-celled organism does not have any need for an actual system that removes its waste. Instead, it eliminates waste by the process of simple diffusion (the movement of a substance from an area of high concentration to one of low concentration). Other slightly more complex water-based organisms like a freshwater protozoan, have a special method of expelling waste and getting rid of excess water. They use a contractile vacuole, a baglike structure inside a cell that fills up with waste and waU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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ter and then suddenly contracts and squeezes the contents out of the cell. Other invertebrates (animals without a backbone), like the flatworm, have a simple excretory system using structures called nephridial organs or nephridia. These are simple tubes that lead waste to pores or openings in its body from which the waste is expelled. Insects have a similar system of organs known as Malpighian tubules. These tubes have grown out from the insect’s digestive tract and lead into the gut rather than outside the insect’s body. This allows insects to be able to successfully conserve water since they can reabsorb it instead of dumping it outside their bodies.

MORE COMPLEX SYSTEMS As organisms become more complex, so do their excretory systems, and vertebrates (animals with a backbone) have adapted their excretory systems so that they can live in almost any type of habitat. Most vertebrates have a pair of specialized organs called kidneys that perform three main functions: filtration, reabsorption, and secretion. Thus, these more complex excretory systems are able to collect, store, and dispose of their liquid waste. The waste generated by vertebrates mainly consists of a nitrogenous waste product that is eliminated in the form of ammonia, urea, or uric acid. In fish, the ammonia is released into the water by their gills, and their kidneys remove the salts. Birds produce uric acid which is a more solid type of nitrogenous waste. Their kidneys remove it from their blood and it is eliminated by being mixed in with their solid waste. Mammals produce nitrogen-containing waste called urea which is excreted in a concentrated solution called urine.

THE HUMAN EXCRETORY SYSTEM

Opposite: An illustration showing the main components of the excretory system. This system is responsible for eliminating water, urea, and other waste products from the body in the form of urine. (Illustration by Kopp Illustration, Inc.)

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The human excretory system consists of two kidneys that filter the blood, the ureters (a tube that leads from each kidney), the bladder (a stretchable, balloon-like bag that stores urine), and the urethra, which is a tube from the bladder through which urine is able to flow out of the body. The human kidneys are about the size of a fist and lie at the back of the body cavity below the ribs and above the hipbones. They are highly complex organs made up of filtering units called nephrons. Each kidney contains about 1,000,000 nephrons that remove nitrogen products, excess salts, and water from the blood in the form of urine. The nephrons are able to replenish the body’s salt and water content after receiving a hormonal message from the endocrine system. The nephrons then release salt and water into the bloodstream where it can be distributed throughout the body. As the nephrons form urine, it passes through a duct into the ureter that empties into the bladder. This collects and stores urine until it beU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e


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comes full (its adult capacity is about two cans of soda), after which it signals to be emptied and urine is expelled through the tube called the urethra. When kidneys do not work properly, it is difficult to maintain homeostasis and severe disruptions of the body’s chemistry can result. Fluid may be retained throughout the body and swelling may occur. Actual kidney failure is a serious, life-threatening condition that can be caused by infection, poisoning, tumors, shock, circulatory disorders, and immune disorders. Treatment for chronic kidney failure includes dialysis, which circulates a patient’s blood through a mechanical filtration system outside its body. Permanent kidney failure can only be cured by a transplant.

Extinction Extinction is the permanent disappearance of an entire species. (A species is a group of closely related, physically similar living organisms that can breed with one another.) A species goes extinct when every single one of its kind has died. Extinctions have been happening since life first began on Earth. However, human activity has greatly accelerated the pace of extinction, and many believe that a great extinction may be taking place today. While some use the word extinction to describe what might better be described as “local or regional extinction,” to most it means the complete and total elimination of a certain species from the face of Earth. Others describe extinction as the loss of a species and its replacement by an evolved version. However, although a particular species may no longer exist, the fact that it adapted or changed so much that it became a distinctly different species means that it never died out completely. Scientists know from fossils that the earliest ancestor of today’s horse stood less than 2 feet (0.61 meters) high. Although that species of horse no longer exists, in a way it does, because it slowly and gradually evolved into the very large horse that is common today. If the smaller horse had died out completely and never had a chance to adapt and change and to pass on those changes to its descendants, then it would have gone extinct.

A NATURAL PART OF EARTH’S LIFE CYCLE Extinction is an ongoing feature of the Earth’s ever-changing ecosystem. It is usually caused by major environmental or climate changes. Humans also indirectly cause extinction by the effects their activities have on the environment. Hunting, habitat destruction, and pollution are some 218


of the ways that humans have driven species to extinction. Also, a phenomenon known as “coextinction” can take place when the disappearance of one species of plant or animal dooms the existence of others that are entirely dependent upon it. It is believed that the saber-toothed tiger of prehistoric times died out completely when its sole prey, the mastodon, went extinct.

Extinction

Throughout the enormously long time that life has existed on Earth, millions of species have simply come and gone. It is estimated that more than 90 percent of all the species that ever existed during Earth’s history are now extinct. This is known from the fossil record, which reveals that a number of mass extinctions took place; huge events that resulted in the destruction of large numbers of different species. (A fossil is the remains of a living thing that has tuned into rock and therefore has been preserved.) The different layers, or strata, of Earth’s crust represent different times in Earth’s history. By this record, we know that the earliest mass extinction was about 650,000,000 years ago during a time called the Precambrian period when life on Earth consisted mostly of algae (plants or plantlike organisms that contain chlorophyll and other pigments that trap light from the Sun) floating on oceans. Scientists think that about 70 percent of the algae species were killed by a major ice age (a period where glaciers covered much of the Earth). Another mass extinction, probably caused by a climate change and considered the largest ever, occurred about 250,000,000 years ago, killing about 90 percent of all land and sea creatures. However, the best-known extinction killed the dinosaurs and many other species during the Cretaceous period about 65,000,000 years ago. Scientists are now fairly certain that this event was caused by a meteorite that crashed into the Earth, sending a vast cloud of dust into the atmosphere and blotting out all sunlight. Most think that the food chain was disrupted since plants could not grow without light, resulting in the death of herbivores (plant-eaters) and the eventual death of carnivores (flesheaters). Scientists do not agree as to whether the dense cloud would have cooled the atmosphere or warmed it, but either scenario would have changed the environment so quickly that plant and animal life would have had no time to adapt. Finally, the Ice Age that occurred during the Pleistocene era (between 1,600,000 and 10,000 years ago) caused the destruction of many plants and animals. This was a time of extremely diverse animal life, and mammals rather than reptiles were the dominant species.

HUMANS INCREASE EXTINCTION RATE Since the evolution (the process by which gradual genetic change occurs) of humans about 10,000 years ago, there have been dramatic changes U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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LUIS WALTER ALVAREZ American physicist (a person specializing in the study of physics) Luis Alvarez (1911–1988) was a physicist who had wide-ranging interests and abilities that led him to suggest that the extinction of dinosaurs was caused indirectly by an asteroid that struck Earth. During World War II (1939–45), he was involved with radar and the atomic bomb, and in 1968 he won the Nobel Prize for Physics. Luis Walter Alvarez was born in San Francisco, California, the son of a medical researcher and physician. His paternal grandfather was originally from Spain, but had run away to Cuba before making his fortune in real estate in California. Young Alvarez attended school in San Francisco, but when his father accepted a position at the Mayo Clinic in Rochester, Minnesota, he attended high school there. After enrolling in the University of Chicago in 1928 to study chemistry, he soon came to love physics and switched his major. Alvarez stayed at Chicago through his bachelor’s, master’s, and the doctorate degree he received in 1936. He then joined the faculty at the University of California at Berkeley, where he remained until his retirement in 1978. At Berkeley, Alvarez soon was given the title “prize wild idea man” by his colleagues because of his involvement in such a wide variety of research activities. His earliest research was in the area of nuclear physics and cosmic rays, and when World War II broke out in Europe he worked at Massachusetts Institute of Technology’s radiation laboratory on radar (a method of

in Earth’s biosphere (all the parts of Earth that make up the living world). With the invention of agriculture (farming) and the domestication (taming) of some animals, humans spread throughout the world and began to use its resources. Some ecologists argue that early humans hunted certain species into extinction, but probably more have been simply crowded out by countless human populations who have taken over the species’ habitat. For example, it is known that at least 200 plant species that were native to North America have become extinct only in the past few thousand years. Worldwide, it is estimated that as many as 20 percent of all bird species have disappeared since the evolution of humans. While the threat of extinction has always existed in nature, the actual time it takes to happen has rapidly speeded up in modern times. The difference today is the overwhelming pressure placed upon the environment by human activity. Habitat destruction, pollution, and over-harvesting are some of the reasons why today, some ecologists argue that we are driving species to extinction at a rate between 1,000 and 10,000 times faster 220


detecting distinct objects) and helped develop a narrow beam radar system that allowed airplanes to land in bad weather. He was also involved in the Manhattan Project to develop the world’s first nuclear weapons. Since he helped develop the bomb’s detonating device, Alvarez flew aboard a B-29 airplane that followed the Enola Gay when it dropped the first atomic bomb on Hiroshima, Japan. Returning to Berkeley after the war, Alvarez built a huge bubble chamber that could track extremely short-lived particles. It was for this invention that he won the Nobel Prize for Physics in 1968.

Extinction

In 1980, he and his son, Walter, who was a professor of geology at Berkeley, accidentally discovered a band of sedimentary rock in Italy that contained an unusually high level of the rare metal iridium. Dating techniques set the age of this layer at about 65,000,000 years old. Similar iridium-rich layers were later found in other parts of the world. This led the father-son team to propose a theory regarding the extinction of dinosaurs that occurred 65,000,000 years ago. They then proposed that the iridium had come from an asteroid that struck Earth and sent huge volumes of smoke and dust (including the iridium) into Earth’s atmosphere. They suggested that the cloud produced by the asteroid’s impact covered Earth for so long that it blocked out sunlight, causing widespread death of plant life. This loss of plant life in turn brought about the extinction of dinosaurs who fed on the plants. While this overall theory has found favor among many scientists and has been confirmed to some extent by additional findings, it is still the subject of much debate, although most now agree with the first half of the hypothesis (the asteroid impact).

than has ever happened before. The American biologist Edward O. Wilson (1929– ) currently estimates that the world may have already lost onefifth of its present species. Today’s human interference in nature is probably the most destructive since much of it is the result of chemical pollutants that easily enter an organism’s reproductive system and cause a wide range of birth defects. Habitat destruction also accounts for a great amount of loss as the rain forests of the world, which contains a vast variety of plant and animals species, are steadily being cut down.

COUNTRIES TRY TO CURB EXTINCTION RATES Many countries are trying to control the effects of human activity, which can be most devastating but is also most manageable. Laws have been passed in many countries that protect certain habitats from destruction and individual species from being hunted. Zoos are changing from places that exhibit animals to places that keep an endangered species alive. One such success story is the resurgence of the American bison. Where U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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it once dominated the plains from Alaska and western Canada to the United States and northern Mexico, the American bison was so ruthlessly hunted that by 1899 there were fewer than 1,000 left. When guarded preserves were established and breeding programs begun, the numbers of plains bison rose to more than 50,000 and the species is no longer threatened by extinction. The bison resurgence however, required an enormous and expensive effort on a national scale, and would be impossible to duplicate for every valued species that is threatened. Instead, it is easier and more efficient to regulate the activities that threaten the diversity of life on Earth in the first place. While this may sound simple however, it often involves very sensitive issues of economics, which many times takes priority over the protection of plant and animal species. [See also Endangered Species; Habitat; Pollution]

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Abbot, David, ed. Biologists. New York: Peter Bedrick Books, 1983.

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Agosta, William. Bombardier Beetles and Fever Trees. Reading, Mass.: Addison-Wesley Publishing Co., 1996. Alexander, Peter and others. Silver, Burdett & Ginn Life Science. Morristown, N.J.: Silver, Burdett & Ginn, 1987. Alexander, R. McNeill, ed. The Encyclopedia of Animal Biology. New York: Facts on File, 1987. Allen, Garland E. Life Science in the Twentieth Century. New York: Cambridge University Press, 1979. Attenborough, David. The Life of Birds. Princeton, N.J.: Princeton University Press, 1998. Attenborough, David. The Private Life of Plants. Princeton, N.J.: Princeton University Press, 1995. Bailey, Jill. Animal Life: Form and Function in the Animal Kingdom. New York: Oxford University Press, 1994.

Burton, Maurice, and Robert Burton, eds. Marshall Cavendish International Wildlife Encyclopedia. New York: Marshall Cavendish, 1989. Coleman, William. Biology in the Nineteenth Century. New York: Cambridge University Press, 1977. Conniff, Richard. Spineless Wonders. New York: Henry Holt & Co., 1996. Corrick, James A. Recent Revolutions in Biology. New York: Franklin Watts, 1987. Curry-Lindahl, Kai. Wildlife of the Prairies and Plains. New York: H. N. Abrams, 1981. Darwin, Charles. The Origin of Species. New York: W.W. Norton & Company, Inc., 1975. Davis, Joel. Mapping the Code. New York: John Wiley & Sons, 1990. Diagram Group Staff. Life Sciences on File. New York: Facts on File, 1999.

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Drlica, Karl. Understanding DNA and Gene Cloning. New York: John Wiley & Sons, 1997.


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Edwards, Gabrielle I. Biology the Easy Way. New York: Barron’s, 1990.

Kordon, Claude. The Language of the Cell. New York: McGraw-Hill, 1993.

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Farrington, Benjamin. What Darwin Really Said. New York: Schocken Books, 1982. Finlayson, Max, and Michael Moser, eds. Wetlands. New York: Facts on File, 1991. Goodwin, Brian C. How the Leopard Changed Its Spots: The Evolution of Complexity. New York: Simon & Schuster, 1996. Gould, Stephen Jay, ed. The Book of Life. New York: W.W. Norton & Company, Inc., 1993. Greulach, Victor A., and Vincent J. Chiapetta. Biology: The Science of Life. Morristown, N.J.: General Learning Press, 1977. Grolier World Encyclopedia of Endangered Species. 10 vols. Danbury, Conn.: Grolier Educational Corp., 1993. Gutnik, Martin J. The Science of Classification: Finding Order Among Living and Nonliving Objects. New York: Franklin Watts, 1980. Hall, David O., and K.K. Rao. Photosynthesis. New York: Cambridge University Press, 1999. Hare, Tony. Animal Fact-File: Head-to-Tail Profiles of More than 100 Mammals. New York: Facts on File, 1999.

Leakey, Richard, and Roger Lewin. Origins Reconsidered. New York: Doubleday, 1992. Leonard, William H. Biology: A Community Context. Cincinnati, Ohio: South-Western Educational Pub., 1998. Levine, Joseph S., and David Suzuki. The Secret of Life: Redesigning the Living World. Boston, Mass.: WGBH Boston, 1993. Little, Charles E. The Dying of the Trees. New York: Viking, 1995. Lovelock, James. Healing Gaia. New York: Harmony Books, 1991. McGavin, George. Bugs of the World. New York: Facts on File, 1993. McGowan, Chris. Diatoms to Dinosaurs. Washington, D.C.: Island Press/Shearwater Books, 1994. McGowan, Chris. The Raptor and the Lamb. New York: Henry Holt & Co., 1997. McGrath, Kimberley A. World of Biology. Detroit, Mich.: The Gale Group, 1999. Magner, Lois N. A History of the Life Sciences. New York: Marcel Dekker, Inc., 1979.

Hare, Tony, ed. Habitats. Upper Saddle River, N.J.: Prentice Hall, 1994.

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Huxley, Anthony Julian. Green Inheritance. New York: Four Walls Eight Windows, 1992. Jacob, François. Of Flies, Mice, and Men. Cambridge, Mass.: Harvard University Press, 1998. Jacobs, Marius. The Tropical Rain Forest. New York: Springer-Verlag, 1990. Johanson, Donald, and Blake Edgar. From Lucy to Language. New York: Simon & Schuster, 1996. Jones, Steve. The Language of Genes. New York: Doubleday, 1994. Kapp, Ronald O. How to Know Pollen and Spores. Dubuque, Iowa: W. C. Brown, 1969.

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Leakey, Richard. The Origin of Humankind. New York: Basic Books, 1994.

Margulis, Lynn, and Karlene V. Schwartz. Five Kingdoms. New York: W.H. Freeman, 1998. Margulis, Lynn, and Dorian Sagan. The Garden of Microbial Delights. Dubuque, Iowa: Kendall Hunt Publishing Co., 1993. Marshall, Elizabeth L. The Human Genome Project. New York: Franklin Watts, 1996. Mauseth, James D. Plant Anatomy. Menlo Park, Calf.: Benjamin/Cummings Publishing Co., 1988. Mearns, Barbara. Audubon to X’antus. San Diego, Calf.: Academic Press, 1992. Moore, David M. Green Planet: The Story of Plant Life on Earth. New York: Cambridge University Press, 1982.


Morris, Desmond. Animal Days. New York: Morrow, 1979. Morton, Alan G. History of the Biological Sciences: An Account of the Development of Botany from Ancient Times to the Present Day. New York: Academic Press, 1981. Nebel, Bernard J., and Richard T. Wright. Environmental Science: The Way the World Works. Upper Saddle River, N.J.: Prentice Hall, 1998. Nies, Kevin A. From Priestess to Physician: Biographies of Women Life Scientists. Los Angeles, Calf.: California Video Institute, 1996. Norell, Mark, A., Eugene S. Gaffney, and Lowell Dingus. Discovering Dinosaurs in the American Museum of Natural History. New York: Knopf, 1995.

Singleton, Paul. Bacteria in Biology, Biotechnology and Medicine. New York: John Wiley & Sons, 1999. Snedden, Robert. The History of Genetics. New York: Thomson Learning, 1995. Stefoff, Rebecca. Extinction. New York: Chelsea House, 1992. Stephenson, Robert, and Roger Browne. Exploring Variety of Life. Austin, Tex.: Raintree Steck-Vaughn, 1993. Sturtevant, Alfred H. History of Genetics. New York: Harper & Row, 1965. Tesar, Jenny E. Patterns in Nature: An Overview of the Living World. Woodbridge, Conn.: Blackbirch Press, 1994. Tocci, Salvatore. Biology Projects for Young Scientists. New York: Franklin Watts, 1999.

O’Daly, Anne, ed. Encyclopedia of Life Sciences. 11 vols. Tarrytown, N.Y.: Marshall Cavendish Corp., 1996.

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Watson, James D. The Double Helix: A Personal Account of the Discover of the Structure of DNA. New York: Scribner, 1998.

Ross-McDonald, Malcom, and Robert Prescott-Allen. Man and Nature: Every Living Thing. Garden City, N.Y.: Doubleday, 1976.

Wilson, Edward O. The Diversity of Life. Cambridge, Mass.: Belknap Press of Harvard University Press, 1992.

Sayre, Anne. Rosalind Franklin and DNA. New York: W.W. Norton & Co., 1975. Shearer, Benjamin F., and Barbara Smith Shearer. Notable Women in the Life Sciences: A Biographical Dictionary. Westport, Conn.: Greenwood Press, 1996. Shreeve, Tim. Discovering Ecology. New York: American Museum of Natural History, 1982. Singer, Charles Joseph. A History of Biology to about the Year 1900. Ames, Iowa: Iowa State University Press, 1989.


Videocassettes Attenborough, David. Life on Earth. 13 episodes. BBC in association with Warner Brothers & Reiner Moritz Productions. Distributor, Films Inc. Chicago, Ill.: 1978. Videocassette. Attenborough, David. The Living Planet. 12 episodes. BBC/Time-Life Films. Distributor, Ambrose Video Publishing, Inc., N.Y. Videocassette.


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Web Sites ALA (American Library Association): Science and Technology: Sites for Children: Biology. http://www.ala.org/parentspage/greatsites/ science.html#c (Accessed August 9, 2000). Anatomy and Science for Kids. http://kidscience.about.com/kids/kidscience/ msub53.htm (Accessed August 9, 2000). ARS (Agricultural Research Service): Sci4Kids. http://www.ars.usda.gov/is/kids/ (Accessed August 9, 2000).

Cornell University: Cornell Theory Center Math and Science Gateway. http://www.tc.cornell.edu/Edu/ MathSciGateway/ (Accessed August 9, 2000). Defenders of Wildlife: Kids’ Planet. http://www.kidsplanet.org/ (Accessed August 9, 2000). DLC-ME (Digital Learning Center for Microbiology Ecology). http://commtechlab.msu.edu/sites/dlc-me/ index.html (Accessed August 9, 2000). The Electronic Zoo. http://netvet.wustl.edu/e-zoo.htm (Accessed August 9, 2000). Explorer: Natural Science. http://explorer.scrtec.org/explorer/ explorer-db/browse/static/NaturalScience/ index.html (Accessed August 9, 2000).

Fish Biology Just for Kids: Florida Museum of Natural History. http://www.flmnh.ufl.edu/fish/Kids/ kids.htm (Accessed August 9, 2000).

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GO Network: Biology for Kids. http://www.go.com/WebDir/Family/Kids/ At_school/Science_and_technology/ Biology_for_kids (Accessed August 9, 2000). Howard Hughes Medical Institute: Cool Science for Curious Kids. http://www.hhmi.org/coolscience/ (Accessed August 9, 2000). Internet Public Library: Science Fair Project Resource Guide. http://www.ipl.org/youth/projectguide/

Best Science Links for Kids (Georgia State University). http://www.gsu.edu/chevkk/kids.html (Accessed August 9, 2000).

Federal Resources for Educational Excellence: Science. http://www.ed.gov/free/ s-scienc.html (Accessed August 9, 2000).

Franklin Institute Online: Science Fairs. http://www.fi.edu/qanda/spotlight1/ spotlight1.html (Accessed August 9, 2000).

Internet School Library Media Center: Life Science for K-12. http://falcon.jmu.edu/ramseyil/ lifescience.htm (Accessed August 9, 2000). K-12 Education Links for Teachers and Students (Pollock School). http://www.ttl.dsu.edu/hansonwa/k12.htm (Accessed August 9, 2000). Kapili.com: Biology4Kids! Your Biology Web Site!. http://www.kapili.com/biology4kids/ index.html (Accessed August 9, 2000). Lawrence Livermore National Laboratory: Fun Science for Kids. http://www.llnl.gov/llnl/03education/ science-list.html (Accessed August 9, 2000). LearningVista: Kids Vista: Sciences. http://www.kidsvista.com/Sciences/ index.html (Accessed August 9, 2000). Life Science Lesson Plans: Discovery Channel School. http://school.discovery.com/lessonplans/ subjects/lifescience.html (Accessed August 9, 2000). Life Sciences: Exploratorium’s 10 Cool Sites. http://www.exploratorium.edu/ learning_studio/cool/life.html (Accessed August 9, 2000). Lightspan StudyWeb: Science. http://www.studyweb.com/Science/ (Accessed August 9, 2000).


Lycos Zone Kids’ Almanac. http://infoplease.kids.lycos.com/ science.html (Accessed August 9, 2000).

Ranger Rick’s Kid’s Zone: National Wildlife Federation. http://www.nwf.org/nwf/kids/index.html (Accessed August 9, 2000).

Mr. Biology’s High School Bio Web Site. http://www.sc2000).net/czaremba/ (Accessed August 9, 2000).

The Science Spot. http://www.theramp.net/sciencespot/ index.html (Accessed August 9, 2000).

Mr. Warner’s Cool Science: Life Links. http://www3.mwis.net/science/life.htm (Accessed August 9, 2000). Naturespace Science Place. http://www.naturespace.com/ (Accessed August 9, 2000). NBII (National Biological Information Infrastructure): Education. http://www.nbii.gov/education/index.html (Accessed August 9, 2000). PBS Kids: Kratts’ Creatures. http://www.pbs.org/kratts/ (Accessed August 9, 2000). Perry Public Schools: Educational Web Sites: Science Related Sites. http://scnc.perry.k12.mi.us/ edlinks.html#Science (Accessed August 9, 2000). QUIA! (Quintessential Instructional Archive) Create Your Own Learning Activities. http://www.quia.com/ (Accessed August 9, 2000).


South Carolina Statewide Systemic Initiative (SC SSI): Internet Resources: Math Science Resources. http://scssi.scetv.org/mims/ssrch2.htm (Accessed August 9, 2000). ThinkQuest: BodyQuest. http://library.thinkquest.org/10348/ (Accessed August 9, 2000). United States Department of the Interior Home Page: Kids on the Web. http://www.doi.gov/kids/ (Accessed August 9, 2000). USGS (United States Geological Service) Learning Web: Biological Resources. http://www.nbs.gov/features/education.html (Accessed August 9, 2000). Washington University School of Medicine Young Scientist Program. http://medinfo.wustl.edu/ysp/ (Accessed August 9, 2000).


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F Family The term family is one of the seven major classification groups that biologists use to identify and categorize living things. These seven groups are hierarchical or range in order of size. Family is located between the groups order and genus. The classification scheme for all living things is: kingdom, phylum, class, order, family, genus, and species. Since the grouping family includes organisms that are even more alike than those in the group order, it is not always obvious which should or should not be included. One example that is easy to distinguish would be that of cat and dog. Both belong to the kingdom Animalia, the phylum Chordata, the class Mammalia, and the order Carnivora, but each is placed in a different family. Since the dog has nonretractable claws and hunts its prey by chasing it, it belongs to the family Canidae. The cat has retractable claws and hunts by stalking and surprise, and therefore belongs to the family Felidae. For plants, all family names end in -aceae, while for animals, the family names end in -idae. The grouping family is certainly important to biologists, but in most cases, they often find that stating a genus and species is more than sufficient for consistent understanding and identification of an organism under consideration. [See also Class; Classification; Genus; Kingdom; Order; Phylum; Species] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Fermentation

Fermentation Fermentation is a chemical process that breaks down carbohydrates and other organic materials and produces energy without using oxygen. This process is carried out by microorganisms such as bacteria, molds, and fungi. Alcohol fermentation is a well-known type of fermentation where sugar is broken down into alcohol and carbon dioxide. Fermented products have been used by people for thousands of years, primarily to make the alcohol in beer and wine and to make bread dough

LOUIS PASTEUR One of the most extraordinary scientists in history, French chemist and microbiologist (a person specializing in the study of microorganisms) Louis Pasteur (1822–1895) is considered the founder of microbiology. He also contributed to our understanding of fermentation (a chemical process that breaks down carbohydrates and other organic materials and produces energy without using oxygen), developed the germ theory of disease, improved immunization, and proved that heating kills microorganisms (an organism of microscopic size). This process of using heat was named pasteurization after the famed scientist. Louis Pasteur was born in Dole, France, and his family moved to Arbois when he was very young. He attended school there and appeared to be a mediocre student in every way. Still, he stayed in school despite near poverty, and after attending a lecture in chemistry and being inspired by it, he decided to study this new and fascinating subject. Studying chemistry, he suddenly became an excellent student, and by the age of twentysix earned his Ph.D. and made a major discovery concerning crystals for which he won a national award. By 1854, the “mediocre” student had become dean of the Faculty of Sciences at the University of Lille, and was asked by the French wine industry to help them with their spoilage problem. Very often, wine and beer spoiled, or went sour, as they aged, ruining tons of good beverage without anyone knowing why or what to do. Pasteur took up the problem and discovered almost immediately with his microscope that the yeast (various single-celled fungi capable of fermenting carbohydrates) in sour wine had an elongated shape, while the yeast in good wine was spherical, or round. When the good (round) yeast ferments, it produces alcohol. When the bad (elongated) yeast ferments, it produces lactic acid (a syrupy liquid). He suggested heating the wine or beer gently at about 120°F (48.9°C) after it had been properly made. Pasteur stated that the heat would kill any yeast left,

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rise. Although they did not understand what made it happen, the ancient Egyptians knew that if they allowed bread dough to stand for several hours, it became lighter and better tasting than if baked immediately. What they did not know was that the dough was lightened by the carbon dioxide gas produced by the fermentation of sugar. This happened not because the Egyptians knew enough to add yeast (a single-celled fungus) to the dough, but because leaving the dough uncovered allowed microscopic organisms like yeast and bacteria to float in on the breeze and break down the dough’s sugars into alcohol and carbon dioxide. The carbon dioxide gas then became trapped in the dough and made it rise, while the alcohol

Fermentation

especially the bad yeast, and if the wine were properly corked, it would not go sour. Heating wine seemed barbaric to the French, but they tried the experiment and it worked. Ever since, a gentle heating that kills unwanted microorganisms has been called pasteurization. Besides beer and wine, milk also is now pasteurized. Saving the French wine industry made Pasteur a hero, so it is not surprising that its silk industry asked him to do the same thing for them. He did by showing them how to get rid of a killer parasite (an often harmful organism that lives on or in a different organism) that was killing the silk worms. This led Pasteur to work with communicable (contagious) diseases. He had long felt that disease was something that was caused by unseeable organisms and then was spread person-to-person. By now, Pasteur had considerable experience using his microscope to identify different kinds of microorganisms such as bacteria and fungi. So when he decided to work on what is now called the germ theory of disease, he was following one of his favorite sayings, “Chance favors the prepared mind.” Pasteur then developed techniques for culturing (growing) and examining several disease-causing bacteria. He identified both Staphylococcus and Streptococcus, which cause serious, sometimes fatal infections, and also cultured the bacteria that cause cholera. It was in working with these infectious bacteria that Pasteur realized that weakening them allows them to be used as a vaccine. From this discovery, he developed a vaccine for the disease anthrax, as well as one for rabies, a deadly disease contracted from the bite of an infected, rabid animal. If any individual had achieved one or two of these accomplishments, he or she would be considered among the pioneers in the life sciences. Yet Pasteur did all this and more. His germ theory of disease is considered by many scientists as the single most important medical discovery of all time because it not only showed doctors how to fight and prevent disease, but it supplied the all-important correct theory that would guide future research. Pasteur is truly one of the giants of biology.


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Fertilization

would evaporate during baking. The Egyptians also discovered that by allowing certain grains like barley to begin to spoil, they could obtain a drink with a pleasing side effect (alcohol). The same effect could be achieved by allowing grapes to spoil since grapes contain yeast that grow naturally on their skins. Throughout history, the process of fermentation was shrouded in mystery and superstition. During the seventeenth century, the English chemist Robert Boyle (1627–1691) correctly predicted that an understanding of the fermentation process would lead to the discovery of the causes of other phenomena like disease. Boyle’s prediction came true when the French chemist, Louis Pasteur (1822–1895), proved that yeast caused fermentation in beer and wine. After this discovery, Pasteur turned his research toward the spread of diseases caused by other microorganisms. Pasteur’s work saved France’s wine industry, which could not understand why its burgundy wine was spoiling. Pasteur discovered that wine normally contained yeast cells that produced alcohol. However, he also realized that wine containing bacteria and other microorganisms produced lactic acid when they fermented, and thus spoiled the wine. Pasteur showed that fermentation caused by living organisms is too small to be seen without a microscope, and that the end product of the fermentation process depends on both what is being fermented and what microbes are the catalyst (something that starts a chemical reaction). Pasteur taught France’s wine industry how to kill unwanted bacteria by the gentle heating of the wine at about 120°F (48.9°C). This process is called pasteurization after the great scientist. Today, fermentation is well understood and can be controlled. Fermentation is a large part of today’s food industry, with some form of fermentation taking place in the production of many food products like yogurt, buttermilk, cheese, soy sauce, cured meats, pickled vegetables, and chocolate, as well as in alcoholic beverages and bread. In some cases, antibiotics and other medications can be produced by fermentation, as can ethyl alcohol that is added to gasoline to produce gasohol. Fermentation is also critical to today’s disposal of solid waste by converting it to carbon dioxide, water, and mineral salts. [See also Bacteria; Carbohydrates; Carbon Dioxide; Fungi]

Fertilization Fertilization is the union of male and female sex cells. Also called conception in humans, fertilization occurs during sexual reproduction that 226


necessarily involves two parents. If fertilization is successful, a pregnancy occurs that results in the creation of a unique, new individual.

Fertilization

Fertilization is the key moment of sexual reproduction. It is the moment that begins the combining of the genes (hereditary characteristics) of two separate individuals to produce a unique, new offspring. For fertilization to occur, the male sperm and the female egg, or ovum, must be brought together or come together physically under the proper conditions. One they meet and actually touch, the process of fertilization can begin. This process has two stages. In the first stage, the ovum is “activated” by the contacting sperm, setting in motion a series of chemical reactions. The second stage is the actual fusion or uniting of the male gamete (sex cell) with the female gamete.

FERTILIZATION FOR AQUATIC ANIMALS Before either stage can happen in an animal’s reproductive system, other important things must take place. Sexual reproduction in animals requires that male sperm swim to the egg. For animals that already live in a watery environment (like fish and frogs), this is no problem, and they usually engage in external fertilization. In this process, the female lays her eggs or releases them in the water while the male simultaneously deposits his sperm in the same area. Fertilization is left to chance as the female eggs release chemicals that attract the sperm.

FERTILIZATION FOR LAND ANIMALS Among animals that live on land however, fertilization is usually done internally, meaning that the male deposits his sperm inside the body of the female. The egg is large and does not move, while the sperm are small and very mobile. Since sperm must swim to the egg, the male produces a fluid that transports them, while the female also provides additional internal fluid. Most male animals use a specialized organ called a penis to deposit their sperm safely inside the female. Once the sperm reaches the egg and comes into physical contact with it, a chemical reaction begins. First the head of the individual sperm bursts open and releases a chemical that bores a hole into the outer covering of the egg. As this is happening, tiny membranes called “microvilli” emerge from both the sperm head and the egg, and actually join together or fuse. At this point, fertilization occurs and the sperm is engulfed in egg’s cytoplasm (the jelly-like fluid in a cell). As the sperm continues deeper into the egg, the nuclei (the control center of a cell) of both eventually meet and form a new nucleus. Basically, the genetic contents of the sperm are joined to those of the egg. The now-fertilized egg is called U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Fertilization

a zygote. It is from this zygote that the entire, complete new individual will develop. This process begins with division: first into two cells, then four, then eight, and so on. As soon as the division begins, the zygote becomes an embryo. It is from this single fertilized cell that a multicellular individual with tissues, organs, and organ systems comes to be. When an embryo has formed organs, it is then called a fetus. Also, as soon as penetration by one sperm occurs, the egg takes action to prevent penetration by any other sperm. It changes its electrical charge from negative to positive and creates a hard protective layer that cannot be penetrated.

ARTIFICIAL FERTILIZATION For a long time fertilization was not understood. Now it is not only understood but it can be achieved artificially. Fertilization by artificial insemination is achieved in animals by introducing sperm-containing se-

Sperm penetrating a hamster ovum. When the genetic material of the sperm and egg fuses, fertilization is complete. (Reproduced by permission of Photo Researchers, Inc. Photograph by David Phillips.)

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men into the vagina or uterus of the female without any sexual contact. This process was first developed for breeding cattle and horses, and is sometimes used in humans as long as the male partner is able to produce sperm. Another form of artificial insemination is called “in vitro” fertilization. This involves mixing the sperm and ovum in a laboratory dish and transplanting the fertilized egg into the woman’s uterus where it will develop normally. Children born through this form of artificial insemination were often called “test-tube babies.” The first such test-tube baby was Mary Louise Brown, born in England in 1978. The procedure was used successfully in the United States for the first time in 1981, and since then more than 50,000 American babies have been born as a result of this technique.

Fish

[See also Egg; Human Reproduction; Reproduction, Sexual; Sperm]

Fish A fish is a vertebrate (an animal with a backbone) that lives in the water and breathes through gills. It has a streamlined body and is usually protected by a hard coat of scales. Fish species vary greatly in size and shape, but all are cold-blooded and most have fins. Fish live in watery habitats as diverse as stagnant ponds and subzero polar water. As a vertebrate that lives its entire life underwater, a fish is ectothermic or cold-blooded. This means that it is not necessarily cold but that its own, internal temperature rises or lowers to meet that of its environment. If anything defines a fish as a fish, it is that it breathes through gills. These respiratory organs that lie behind and to the side of the mouth are able to absorb oxygen that is dissolved in water and to give off carbon dioxide as the water passes over the gills’ filaments. Oxygen-rich blood is then pumped by a heart to the rest of its body. Most fish have a streamlined body over which water flows easily as it moves through the water. Fish are able to swim forward by contracting the muscles on each side of their body in turn so that their tail whips from side to side and pushes them forward. Fins allow them to maneuver and have control and balance, while an inflatable swim bladder keeps them from sinking when they are not swimming. As a fish descends to deeper waters, the increase in pressure compresses and deflates the bladder, allowing the fish to swim deeper. As the fish rises again and the pressure decreases, the bladder begins to inflate with gas. The majority of fish have scales that are overlapping plates that protect its body. A fish does U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Fish

not shed these scales, because they grow as its body grows. Mucus usually covers these scales, as it helps the fish to glide more easily through the water. Most fish reproduce sexually through the union of male sperm and female eggs, but it takes place outside the female’s body by what is called spawning. Spawning is the release of eggs by the female into the water. The male then releases his sperm over the eggs and some of the eggs are fertilized.

A close-up photo of a long nose gar showing its overlapping scales used to protect its body. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

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Biologists have grouped fish into three classes: jawless fish, cartilage fish, and bony fish. A jawless fish is a primitive, wormlike fish without a hinged jaw. This means that it usually has a simple, sucker-like mouth instead. A lamprey eel is a good example of this ancient type of fish. The lamprey is a parasite and sucks the blood and juices from live fish. The only other jawless fish is the scavenger fish called the hagfish. It has a round mouth and attaches itself to the bodies of dead or dying fish, feeding on the contents of the victim’s body.


Cartilage fish have an endoskeleton (internal skeleton) made entirely of strong, flexible cartilage instead of bone. These fish are almost always hunters such as sharks, skates, and rays who live in the ocean. They all have jaws, scales, and paired fins. Their skin is covered with tiny scales that feel like sandpaper. The shark is an especially ferocious predator that must swim all the time because it has no swimbladder. Sharks are powerful swimmers and have teeth that grow in rows that move forward to replace lost teeth.

Flower

Bony fish make up the third and largest category of fish. Also called “true fish,” these have an endoskeleton made up mostly of bone. The familiar trout, salmon, cod, and sardine are all bony fish. Bony fish all have a gill cover or flap and like all fish, a keen sense of smell. Bony fish usually have highly maneuverable fins that allow them to make rapid and complex movements. Although bony fish are, as their name implies, all bone, their skeleton is in fact very light and thin because they use the natural buoyancy of water to support their bodies. Fish are an important food source for people. [See also Ichthyology]

Flower A flower is a plant structure that contains the organs needed for sexual reproduction. The function of a flower is to make seeds. Flowers and the seeds they produce are also a major source of nutrients for almost all animals.

THE STRUCTURE OF FLOWERS Most plants have flowers, and any plant that produces some sort of flower, even a small, colorless one, is a flowering plant. Grasses and oak trees are flowering plants just like roses and cherry trees. As many as 200,000 types of flowers have been classified, from tiny pondweed flowers to the bathtub-size flower of the tropical Giant Rafflesia. Despite the enormous variety in size, shape, color, and fragrance, all flowers have similar structure. Every flower has four basic organs or parts: the sepals, the petals, the stamen, and the pistil.

The Sepals and Petals. The sepals are the outermost part of the flower (where the flower emerges from the stem), and resemble green flaps that protect the flower when it is still a bud. The petals are usually the flower’s most distinctive part and signal to animal pollinators U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Flower

A close-up photograph of an open bleeding heart flower showing its petals, stamen, and pistil. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

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(animals who transfer pollen, containing male sex cells to the pistil containing female sex cells) with bright colors and strong scents. Sometimes a flower’s petals resemble a female insect, attracting males who pollinate as they land on their “mate.” However, flowers that use the wind as a pollinator rather than animals usually have small petals or none at all.

The Stamen. The stamen is the male reproductive organ of a flower and lies inside the petals. Each is a slender stalk or a ribbon-like thread called a filament with an enlarged tip or head called an anther. The anther is like a small sac and contains the pollen, which are dustlike particles that contain the plant’s male sex cells. The Pistil. The pistil is the female reproductive organ of a flower and is usually located in the center of the flower. It is here that the seed is actually produced. Each pistil has three parts: the stigma, the style, and the


ovary. The stigma is the sticky top of the style, which is a slender stalk or tube that connects with the ovary below. The ovary contains the ovules that store the female sex cells. In terms of functions, the stigma catches or collects the pollen, the style is the tube down which the pollen travels to the ovary, and the ovary is where the pollen fertilizes the ovules. It is the ovule that develops into the seed after fertilization, and the ovary that becomes the plant’s fruit.

Flower

There are several other names for a flower’s different subparts. For example, the protective petals around the stamens are collectively called the corolla. The sepals are also collectively called the calyx. In flowers that have two or more pistils (called compound pistils), the individual pistil is called a carpel. Flowers that have all these parts are termed perfect or complete, while those missing one or more parts are called incomplete or imperfect flowers.

REPRODUCTION IS THE KEY TO CLASSIFICATION The flowering plants of the world have been classified into roughly three hundred families according to these flower parts. Thus, even though certain plants may grow in very different climates and soils and have varying shapes and colors, they may still be part of the same family because of how their reproductive organs look and function. Since reproduction is the key in classifying flowers, the lily family includes not only the tulip and hyacinth but also the onion, garlic, aloe, and yucca plant. All are pollinated by insects, contain three sepals and petals that closely resemble each other, six stamens, one pistil, long sheathlike leaves with parallel veins, and fruit that contains many seeds within one capsule. Further, the pea family includes not only beans, lentils, and peanuts but trees like the locust, vines like the wisteria, and herbs like licorice. Flowers are critically important as the key to a flowering plant’s ability to reproduce more of its own kind. Exactly when a flower blooms is also very important, since plants need to flower and form fruits and seeds before the cold season resumes. The length of daylight and darkness, as well as consistent temperature change, are mechanisms that signal a plant it should begin to blossom. Besides acting as a reproductive agent for plants, flowers also beautify the world for people. Since ancient times, humans have prized flowers for their shapes, colors, and fragrances. They also have used flowers as medicines and given them symbolic meanings. [See also Botany; Plant Anatomy; Plant Reproduction] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Food Chains and Webs

An ecological or energy pyramid. The lowest level consists of producers, the next higher level of firstorder consumers, the next higher level of second-order consumers, and so on. Note that the total number organisms found in any one level decreases as one goes up the pyramid. (Illustration courtesy of Gale Research.)

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Food Chains and Webs Food chains and food webs show how energy is transferred in an ecosystem (an area in which living things interact with each other and their environment) from one organism to another in the form of food. Since all organisms in an ecosystem need nutrients in the form of food, the terms food chain and food web describe the feeding relationship between an ecosystem’s different populations. Among the many processes that occur in every ecosystem, none is more important than the transfer of energy from one living thing to another. Without this energy exchange, no organisms would be able to survive. Ecologists use the term food chain to describe the typical path or route that energy (food) takes as it moves from one group of living things to another. A simple food chain would be: green plant to mouse to snake to eagle. In this example, the green plant is the first link in the chain, producing chemical energy from sunlight through a process of photosynthesis. The plant is eaten by the mouse, which absorbs the plant’s energy. In turn, the mouse is eaten by the snake, who absorbs the mouse’s energy.


Finally, the eagle consumes the snake and obtains the snake’s energy. This chain becomes circular when the eagle dies and its organic matter is reduced to nutrients, which are returned to the soil by organisms called decomposers.

Food Chains and Webs

Ecologists now prefer the term “food web” as a more realistic description of what really happens in this process. Since they feel that the word “chain” implies an orderly linking of equal parts, they use instead “food web” which can be described as a collection of food chains. In the natural world, food chains are extraordinarily complex, since there is no exact order stating which creature has to eat which. The notion of a web is more complicated than that of a chain and suggests a network of connections rather than a direct, one-to-one linking. The first link in a food chain (or first stage of a food web) are the organisms known as primary producers. These are also called “autotrophs” since they can make their own food. Green plants and some forms of bacteria are primary producers since they begin the chain by performing photosynthesis. By capturing sunlight and using its energy to carry out a series of chemical reactions, green plants make glucose which is packed with energy. In the sea, primary producers are floating microorganisms known as plankton. On land, green plants are eaten by herbivores (planteating animals) who are considered to be the primary consumers. In turn, these primary consumers become food for other animals called secondary consumers. Each step up the ladder consists of fewer flesh-eating animals. Every food chain then begins with an autotroph and ends with a carnivore (flesh-eating animal) that itself is not eaten by a larger animal. The bear is a good example of a carnivore that is at the top of its food chain. When top carnivores die, their bodies are usually eaten by scavengers, like vultures. What remains is broken down by decomposers (bacteria and fungi) and returned to Earth as nutrients to be used by autotrophs (green plants). Ecologists also use a model called an “energy pyramid” to depict the actual energy-transfer relationships in a food web. They have found that each species occupies a certain place or level on that pyramid that they call its “trophic level.” Trophic refers to nourishment or nutrition, and a certain trophic level is the stage which a given organism occupies on the pyramid. In an energy pyramid, energy flows from one trophic level to the next, but only in one direction (bottom to top). At the base or bottom of the pyramid are the producers or plants, the most plentiful resource. Primary consumers are at the next level and are just below the secondary consumers. The pyramid is not just another way of showing a pattern, but demonstrates that the same amount of energy is never transferred up the U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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pyramid. Studies have shown that the amount of energy passed on to the next higher pyramid level is only about 10 percent of the energy that the organism at the lower level received. This is because a great deal of energy is lost into the environment as heat, which cannot be reused. As energy passes up to the higher levels of the pyramid, less and less of it is left to reach the top. As a result, the total number of organisms that can participate in a food web is severely limited. It also shows how important the producers at the bottom of the pyramid are, since the less energy they produce the less energy is passed on up the pyramid. Ecologists use models like energy pyramids to evaluate how much energy an ecosystem can produce, and therefore how much life it can sustain. They usually try to measure the total productivity of the producers and turn that into an amount of heat energy per unit of area. Another way of measuring productivity is to measure the “biomass” (total amount of organic matter produced) of an ecosystem. Certain ecosystems are naturally more productive than others. For example, a forest is more productive than a desert. Also, human activity can have a disruptive effect on a food web if it causes a major change at one of its levels. [See also Ecosystem]

Forests A temperate, or deciduous, forest is a geographic area usually dominated by deciduous trees (trees that lose their leaves before winter). These forests experience four separate seasons and are home to a wide variety of insects, birds, and animals. Only a small fraction of the original forests that once covered the world’s temperate areas remains today. Dense deciduous forests once covered most of the world’s temperate zones. Found on the eastern side of North America, most of Europe (except for its mountains), eastern Asia, and parts of southwestern South America, they are named after their most characteristic feature, which is large trees that lose their leaves before winter. Today, in whatever natural forest remains, we can only get a hint of what this original deciduous forest must have looked like. Some deciduous forests are referred to as “climax” forests. A climax community is a balanced, stable habitat that has reached its highest or most developed form. It is usually dominated by one type of plant species. For example, the few giant sequoia trees remaining today on the West Coast of North America once lived, and were dominant in, a climax community. 236


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Now, as in the past, forests not only support a wide range of other plant and animal life, but they play a vital role in many ecosystems (areas in which living things interact with each other and the environment). Forests maintain the structure and fertility of forest soil and prevent soil erosion and floods. Forests also absorb carbon dioxide and release oxygen through photosynthesis (the process by which plants use light energy to make their food). In this manner, forests help control the level of greenhouse gases, which, in turn, helps prevent global warming. Forest trees also provide a huge number of products upon which human society depends. We still build most of our homes out of wood and make most of our paper from wood pulp. Trees also still provide fuel and fibers that are used not only in construction but in making foods and medicines.

PLANT LIFE IN FORESTS The dominant plant life in a temperate forest is the broad-leaved tree, so-called because its green leaves have a flat, broad shape. These deciduous trees drop their colorful leaves as winter begins and go dormant for the colder months. Trees cannot draw water from the soil if the temperature of the air is less than about 40°F (5°C). The leaves from deciduous trees fall to the forest floor and begin the process of recycling. Worms, insects, fungi, and other microscopic organisms slowly convert this fallen debris into humus, which builds up the minerals in the soil. Like a tropical rain forest, a temperate forest also has several horizontal layers of activity (like floors in an apartment house). The tallest trees form a canopy, or umbrella, although nowhere near as dense as that of a rain forest. However, it still shades the next layer of shorter trees, which are often oddly shaped as they reach in any direction they can to get more sunlight. Both types of trees are home to climbing animals and birds. Below these shorter trees are woody shrubs and bushes, many of which have berries. These provide cover and food for birds as well as large mammals like bears that thrive on late summer berries. Under the woody bushes and shrubs are grasses, ferns, flowers, and herbs. A great amount of animal activity occurs at this level. At the very bottom is the forest floor where everything dead usually falls and is slowly decomposed, to be recycled and reused in another form.

ANIMAL LIFE IN THE FORESTS Today, many of the larger carnivores (animals that eat other animals) have left the remaining temperate forests and gone north to the coniferous (evergreen) forests. These evergreen forests are also called boreal U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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forests, or the taiga, and have long, cold, dry winters and short summers. However, black bears, deer, raccoons, and squirrels are still commonly seen in temperate forests. The richest or most productive of all the North American forests are those in the southern Appalachians where maple, beech, hickory, horse chestnut, and birch trees shelter rhododendron, wild cherry, and magnolia shrubs. [See also Biome; Rain Forest; Taiga]

Fossil Fossils are the preserved remains of a once-living organism. They form in many different ways and they can provide us with information on the climate, geology, and geography of ancient Earth. Fossils also provide strong evidence for evolution (the process by which living things change over generations). Fossils are usually thought of as the remains of once-living things that have been mineralized or turned into rock. While this describes a fossil, the notion of fossilization includes several other methods of preserving the evidence of ancient life. The use of fossils to study ancient life and its development, or evolution, is called paleontology. The word fossil comes from the Latin word fossilis, meaning “something dug up.” This is actually how most fossils are discovered. Most are found below the surface of Earth in a preserved form, since they had been covered up at one time or another. A fossil can be the partial or complete bodily remains of a plant or animal, or it can be the more common “trace fossil” which is more like some evidence of the organism’s life or activities. Trace fossils are things like trails, coprolites (fossilized animal dung), or footprints.

THE FORMATION OF FOSSILS Fossils can be formed in many ways, but they all have one process in common. They all replace the relatively fragile organic structure of a living thing with something that is harder and which lasts longer. This can only happen when certain conditions apply. Under normal conditions, if an animal falls dead on the ground, it will eventually begin to rot, or decompose and will be reduced to its basic organic compounds, which are then recycled. In the end, it completely and totally disappears. However, if the animal falls dead and slides into a tar pool, asphalt lake, or peat bog, it may be completely preserved because of the lack of oxygen or the absence of bacteria, which are both needed in order for decomposition to occur. In an extremely hot and dry climate, an animal may undergo mum238


mification, or be rapidly dehydrated, under the right conditions; therefore, it will be preserved. In other extreme climates in the north, completely preserved specimens of woolly mammoths and even humans have been recovered after being frozen under glacial conditions for thousands of years. A final way that nature can capture and preserve a complete specimen is in amber (the sticky resin of a pine tree). Insects were usually fossilized this way when they became trapped inside the amber. When the resin itself became fossilized, the insect was forever frozen in a drop of what looks like yellow glass.

Fossil The fossil remains of a baby dinosaur. This is considered a true fossil by scientists since it has been preserved in stone. (Reproduced by permission of AP/Wide World Photos.)

Although these are examples are more like preservation than true fossilization, they are all considered to be fossils. True fossils are the remains of once-living organisms that are preserved in stone. When geology (the study of the Earth) became more of a real science in the late eighteenth and early nineteenth centuries, geologists began exploring below Earth’s surface. These scientists went deeper into successively older and older layers of what is called sedimentary rock (formed by the gradual settling of sediments). It was in these buried layers that they found entire communities of fossils in each layer. Often, the deeper layers of rock contained animals that were very different from any known animals, since the deeper the geologists dug, the older the fossil. These unknown animal bodies that were preserved were those that had been covered by sediments very quickly before they could start to decay. However, they needed to be more than just covered to become fossils. They also needed to be deprived of oxygen. This often happened when a dead animal sunk to the bottom of a body of water and was deeply covered by mud. As more sediments piled up and pressure inU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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creased, the organic material that made up the animal was slowly replaced by minerals and the body eventually became a fossil made of stone. These type of fossils might be described as a replica of the real living thing. In the rare process known as petrifaction, both the external shape and the internal structure of a plant or animal are perfectly reproduced. The Petrified Forest of Arizona contains trees that were buried by volcanic eruptions and underwent petrifaction. In this rare process, a molecule-bymolecule replacement occurred, with the end result being the replacement of natural wood fibers by silica. This replacement is usually so accurate that even the cell structure of the tree can be determined. Paleontology, or the study of fossils, informs scientists about the organisms that lived on Earth long before human life evolved. It also tells us a great deal about Earth’s climate in those ancient times. The oldest known fossils are 3,500,000-year-old bacteria. They represent not only the oldest known form of life on Earth but help scientists to learn more about the origins of life itself. Fossils also tell scientists about extinction that has taken place. Finally, most of what is known about evolution is based on the fossil record. It is fossils that provide the most reliable evidence for evolution since for some organisms, paleontologists are able to compare their fossils from different layers of rock (which are from different ages) and actually trace how they evolved physically. [See also Geologic Record]

Fruit Fruit refers to anything that contains seeds. From a botanical standpoint, a fruit is the mature or ripened ovary that contains a flower’s seeds. Therefore, fruit may be dry and hard as well as soft and juicy, and many of the foods commonly thought of as vegetables are in fact fruit. Flowering plants produce fruit either to protect their seeds or to help the seed be better dispersed or scattered over a wide area. As the seed-bearing part of a flowering plant, fruit develops and grows from a flower’s ovaries. After a flower has been pollinated and its ovules (female sex cells) have been fertilized by a pollen grain (male sex cells), the flower begins to change. Since the flower no longer needs them, its stamens (male reproductive organs) and pistils (female reproductive organs) wither and the petals fall off. The ovary, a hollow structure located near the base of the flower that contains the ovules or female sex cells, starts to grow into fruit. The ovules become the seeds. The plant may produce a fruit containing one or more seeds, depending on the species. 240


Horticulturalists, or people who grow crops for commercial purposes, have a different way of defining fruit than do botanists (people specializing in the study of plants) since both plants and fruits contain seeds. To fruit growers, neither nuts nor cucumbers are considered to be a fruit, but they are definitely fruit to a botanist since they both contain seeds. Unlike the sometimes complicated rules used by horticulturalists, botanists classify a fruit by its structure, designating fruit as either simple or compound. A simple fruit is formed from a single ripened ovary. A compound fruit is the product of two or more ovaries. By far, the majority of fruit are simple, such as peaches, nuts, and berries. There are two types of simple fruit: fleshy or dry, depending upon their texture. Fleshy fruits are exactly what they sound like and come in three types: berries, drupes, and pomes. To botanists, berries include bananas and tomatoes since they have a completely fleshy ovary wall; drupes have a single pit or stone for a seed, like olives or peaches; and pomes have an inedible core, like an apple or a pear. Simple dry fruit include grains like corn and rice, as well as the more obvious pods of beans and peas. Compound fruits, which have developed from several ovaries, are fewer in number than simple fruit. There are two types of compound fruit: aggregate and multiple fruit. In an aggregate fruit, like an orange or a raspberry, the fruit developed from a single flower that had several ovaries. Multiple fruits are less common and have ovaries from several flowers. The fig and the pineapple are examples of multiple fruits that develop from a cluster of flowers on a single stem.

Fruit

Although many people think of tomatoes as a vegetable, they are really classified as a fruit since they contain seeds. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

Fruit can also be described by the manner in which they disperse their seeds into the environment. Fleshy fruit ripen, fall to the ground, and decay, leaving their seed to possibly germinate (begin to grow or sprout) and start a new plant. Plants have evolved systems to avoid the overcrowding that would result if new plants grew only near the parent plant. Therefore many fleshy U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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fruits are edible and very tasty to animals that either spit out the seeds as they eat or consume the seeds with the fruit and eventually pass the undigested seeds in their feces, depositing them somewhere else. Nuts and other dry fruits are carried off by animals and buried for later eating. Those fruits left buried may germinate in the spring. Coconuts have a waterproof covering around their single seed, allowing them to be dispersed by the tides. Other dry fruits have burrs or thistles around their seeds that catch onto an animal’s fur and are transported elsewhere for germination. Fruit is very important to the human diet. Many of the fleshy fruits contain a high content of sugar and important vitamins needed by humans and other animals. Fruit also provides humans and other animals a means of obtaining the energy that plants have harnessed from the Sun. Without fruits, the human body lacks the nutrients and vitamins to fight off certain diseases.

Fungi Fungi are a group of many-celled organisms that live by absorbing food and are neither plant nor animal. They are so different that biologists have given them their own separate kingdom among the five kingdoms or forms of life (monerans, protists, fungi, plants, and animals). Fungi play a key role as decomposers and recyclers, but they can also cause disease in plants and animals. The kingdom Fungi is made up of yeasts, molds, and mushrooms. Most are many-celled organisms with a complex cell structure. Although some fungi resemble plants and have roots, they lack leaves and chlorophyll and cannot make their own food. Instead, they absorb their food directly from their surroundings, including living or dead matter. Fungi also digest their food outside their bodies. They do this by releasing enzymes onto their food, which breaks down the food for absorption. Fungi are different in the way they reproduce. Nearly all species of fungi reproduce asexually by forming special reproductive cells called spores. They do not produce embryos as plants and animals do. Instead, every fungus produces powdery spore cells that are so light they can be dispersed by the wind. These microscopic spores can resist harsh conditions and remain ready to germinate when conditions are right. Fungi play an essential role in the cycles of nature because they break down organic matter like dead plants or animals and allow their basic nutrients to be recycled. Without fungi in the soil acting as nature’s de242


composers, we would be living in a sea of waste. When fungi eat, they break down or decompose organic matter into simple substances like carbon, nitrogen, and hydrogen, which become available for other living things to use. Fungi are usually too small to see, but when they form fruiting bodies (such as mushrooms growing on a rotting log), they become obvious. Those mushrooms we notice on a log are hard at work breaking down the dead wood. Although beneficial, fungi can also be harmful to certain forms of life. This kind of fungi are parasitic and are known as biotrophs. Biotrophs are organisms that live by absorbing organic compounds from living matter. These fungi can destroy crops by attacking a plant’s major systems, or they can contribute to plant diseases. Certain fungi, like molds, can cause foods to spoil, sneakers to smell, and toes to itch from athlete’s foot. Other fungi, like yeast, are put to good use and are essential to making bread, wine, beer, and certain cheeses, since they cause the all-important fermentation process (the breaking down of carbohydrates into alcohol and carbon dioxide). In the twentieth century, scientists discovered that important drugs could be derived from fungi. Antibiotics—like penicillin as well as the wonder drug cyclosporin, which makes organ transplants possible—are good examples. Finally, mushrooms are cultivated like any crop and are sold commercially to be eaten as a food.


Fungi

Magnified yeast cells. Yeast is an important fungi since it is essential in making products such as bread, cheese, beer, and wine. (Reproduced by permission of Photo Researchers, Inc.)

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The main groups of fungi are chyrids, water molds, sporangium fungi, sac fungi, and club fungi. Chyrids live in muddy or aquatic habitats and feed on decaying plants, although some live as parasites. Water molds are important decomposers in watery environments. Sporangium fungi are commonly known as bread mold and are also found in soil and manure. Sac fungi, which comprise more than 30,000 species, include the yeast used to leaven bread and to make alcoholic beverages. Club fungi include the familiar mushroom as well as stinkhorns and puffballs. All fungi can reproduce asexually, and making spores is the most common method of reproduction. Spores are similar to seeds but much smaller and simpler since they usually contain only one or two cells. Fungi are able to respond to changes in their environment and can produce a large amount of spores in a short time if necessary (as in drought conditions). Most fungi release their spores into the air; the wind carries the ultralight spores into the atmosphere where they can travel great distances. If a spore lands on organic matter and conditions are good, it will germinate or sprout and produce a new fungus. With a typical mushroom, the part we notice aboveground is the spore-producing part of the organism. When a fungus is mature enough, a good rainfall will cause this spore-bearing part to swell and push aboveground, making mushrooms appear where there were none the day before. A world with no fungi would be a world full of dead plants and animals that would neither rot and nor disappear naturally. It would be a world in constant need of basic substances since it could not break down organic matter and therefore could not recycle. It would also be a world without mushrooms. Mushroom farming is a large industry since people in all parts of the world eat mushrooms both raw and cooked. Wild mushrooms should never be eaten, however, since it takes an expert to know the difference between an edible mushroom and a poisonous one. [See also Antibiotics; Decomposition]

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G Gaia Hypothesis The Gaia (pronounced guy-ah) hypothesis is the idea that Earth is a living organism and can regulate its own environment. This idea argues that Earth is able to maintain conditions that are favorable for life to survive on it, and that it is the living things on Earth that give the planet this ability. The idea that Earth and its atmosphere are some sort of “superorganism” was actually first proposed by the Scottish geologist (a person specializing in the study of the Earth) James Hutton (1726–1797), although this was not one of his more accepted and popular ideas. As a result, no one really pursued this notion until some two hundred years later, when the English chemist, James Lovelock (1919– ), put forward a similar idea in his 1979 book, Gaia: A New Look at Life on Earth. Gaia is the name of the Greek goddess of the Earth, and in modern times has come to symbolize “Earth Mother” or “Living Earth.” In this book, Lovelock proposed that Earth’s biosphere (all the parts of Earth that make up the living world) acts as a single living system that if left alone, can regulate itself. Lovelock arrived at this hypothesis (theory) by studying Earth’s neighboring planets, Mars and Venus. Suggesting that chemistry and physics seemed to argue that these barren and hostile planets should have an atmosphere just like that of Earth, Lovelock stated that Earth’s atmosphere is different because it has life on it. Both Mars and Venus have an atmosphere with about 95 percent carbon dioxide (gas), while Earth’s is about 79 percent nitrogen (gas) and 21 percent oxygen. He explained this dramatic difference by saying that Earth’s atmosphere was probably U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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very much like that of its neighbors at first, and that it was a world with hardly any life on it. The only form that did exist was what many consider to be the first forms of life—anaerobic bacteria that lived in the ocean. This type of bacteria cannot live in an oxygen environment, and its only job is to convert nitrates to nitrogen gas. This accounts for the beginnings of a nitrogen buildup in Earth’s atmosphere. The oxygen essential to life as we know it did not start to accumulate in the atmosphere until organisms that were capable of photosynthesis evolved. Photosynthesis is the process that some algae and all plants use to chemically convert the Sun’s light into food. This process uses carbon dioxide and water to make energy-packed glucose (sugar), and it gives off oxygen as a by-product. These very first photosynthesizers were a blue-green algae called cyanobacteria that live in water. Eventually, these organisms produced so much oxygen that they put the older anaerobic bacteria out of business. As a result, the only place that anaerobic bacteria could survive was on the deep-sea floor (as well as in heavily waterlogged soil and in our own intestines). Lovelock’s basic point was that the existence of life (bacteria) eventually made the Earth a very different place by giving it an atmosphere. Lovelock eventually went beyond the notion that life can change the environment, and proposed the controversial Gaia hypothesis. He said that Gaia is the “living Earth” and that Earth itself should be viewed as being alive. Like any living thing, Earth always strives to maintain homeostasis (constant, or stable, conditions) for itself. In the Gaia hypothesis, it is the presence and activities of life that keeps Earth in homeostasis and allows it to regulate its systems and maintain steady-state conditions. Lovelock claims that it is the living things on Earth that provide it with the feedback so necessary to regulating something. (A feedback mechanism is something that can detect and reverse any unwanted changes.) Lovelock offers several examples of cycles in the environment that work to keep things on an even keel. He also warns that since Earth has the capacity to keep things in a stable range, human tampering with Earth’s environmental balancing mechanisms places everyone at great risk. While environmentalists insist that human activity is upsetting Earth’s ability to regulate itself, others argue that Earth can continue to survive very well no matter what humans do exactly because of its builtin adaptability. An important aspect of the Gaia hypothesis is that it offers scientists a new model to consider. It places great emphasis on what promises to be the planet’s greatest future problem—the quality of Earth’s environment.

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Gene

The gene is the basic unit of heredity. All living organisms have genes that are the part of the cell that determines the traits offspring inherit from their parents. Genes are composed of deoxyribonucleic acid (DNA) and form part of the cell’s chromosome. Genes produce their individual effect chemically by issuing the necessary instructions that tell a cell to make certain proteins. Genes have been described as recipes for making proteins. Proteins build and control the activities of cells, so by making different proteins at different times, genes act as switches that control and change the way cells work. There are approximately 80,000 genes in each molecule of DNA in the human body, and a single gene can be thought of as a section of a single strand of DNA. Thus, if each gene is a recipe, the DNA is the chemical language in which the recipes are written. Genes may be


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considered a message and DNA as the letters that make up the words of the message. Since DNA itself is made up of only four different chemicals called nucleotides—adenine (A), thymine (T), cytosine (C), and guanine (G)—all gene recipes or messages are also made up of some combination of these four “bases.” It was the Austrian monk, Gregor Johann Mendel (1822–1884), who introduced the world to the notion that there were certain factors in cells that determined all the specific hereditary traits. Even though Mendel never saw them or gave them their name, he knew by his experiments with cross-breeding peas that something like genes—tiny, independent units that passed on certain traits—existed. In 1910 the American geneticist Thomas Hunt Morgan (1866–1945) discovered that genes are located on chromosomes. Chromosomes were soon seen as threadlike structures that were made up of genes. Chromosomes were the chain, and genes were the links in the chain. Since each gene carries a different piece of information, each chromosome carries many different pieces of information. In the cells of the body, chromosomes exist in pairs, The same trait (such as height) is carried on both chromosomes of a certain pair, and it is always found in the same position or location on each chromosome. The two chromosomes of the pair carry the genes for either the same form (tall/tall) or the different form (tall/short) of a certain trait (height). These genes are passed from parents to their offspring when sex cells (sperm and egg) join during fertilization. The new cell formed by this union contains twenty-three chromosomes from the male and twentythree from the female. Thus a new organism gets half of its genes (and therefore its traits) from each parent. After the American biochemist, James Dewey Watson, and his colleague, the English biochemist Francis Harry Compton Crick, discovered the molecular structure of DNA in 1953, it was soon learned how DNA duplicated itself and how it was able to form a specific protein. DNA was then proven to be the letters that formed the genetic code or message (which was the genes). Understanding genes and the role they play in life has transformed research in the life sciences, and has led not only to attempts to cure inherited diseases (known as gene therapy), but to a continuing push to understand the biochemical mechanisms in the body, especially the key proteins that govern all living processes. [See also Chromosome; DNA; Gene Therapy; Gene Theory; Genetic Code; Genetic Disorders; Genetic Engineering; Genetics; Inherited Traits; Mendelian Laws of Inheritance; Nucleic Acid]

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Gene Theory

Gene theory is the idea that genes are the basic units in which characteristics are passed from one generation to the next. Genes themselves are the basic units of heredity. The gene theory provides the basis for understanding how genes enable parents to transmit traits to their offspring. It is also a key element in the study of genetics. Genes are the central objects studied by the science of genetics. The theory of genes (or gene theory) enables the science of genetics to be able to explain how information that is needed to make a new organism is passed from one generation to the next. Today we know that genes are made up of deoxyribonucleic acid (DNA), and we are able to state clearly what are known as the rules or laws of inheritance. However, less than 150 years ago scientists knew nothing about what went on at the cellular level that affected heredity. Since then, the science of heredity, or genetics (taken from the Greek word genes meaning “born”) has been making regular and spectacular advances, so that at the beginning of the twentyfirst century, scientists are close to learning the entire set of genetic instructions that go to form a single human being. Today scientists know that the 80,000 or so genes that make up what might be called the human blueprint are so individual that no two people (in a world of billions) are exactly alike—except for identical twins. Gene theory shows us how this extreme individuality can actually occur.

GREGOR MENDEL DISCOVERS DOMINANT AND RECESSIVE TRAITS The existence of something like genes was recognized by the Austrian monk Gregor Johann Mendel (1822–1884), whose experiments with breeding different types of pea plants led him to describe what he called “hereditary factors,” or genes. The first thing that Mendel discovered was that in crossing plants with different pure traits, such as all-tall plants with all-short ones, only a single trait was expressed. He therefore considered this expressed trait “dominant.” Traits were therefore not blended, resulting in a medium-height plant, but were “expressed” as individual traits. He also found that a regular ratio of 3 to 1 existed for the number of dominant (tall) versus recessive (short) traits. This led him to decide that plants must contain what he called “factors” and “particles of inheritance,” or what is now called genes. Mendel’s other contribution was his correct assumption that both male and female parent contributed one “factor” per trait to an offspring. By 1900 it was realized that Mendel had given biU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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ology the basis for a new science of heredity, and the search began for the single “factor,” or key, in all living things that contained the crucial information that dictated every detail of what an organism would be.

WATSON AND CRICK DISCOVER DNA STRUCTURE By 1900, the existence of chromosomes was also known, and three years later, the American geneticist Thomas Hunt Morgan (1866–1945) announced the findings of his fruit-fly experiments, stating that chromosomes (the coiled structure in a cell that carries the cell’s DNA) were made up of other, smaller things—later called genes. It was not until 1953 that the American biochemist, James Dewey Watson (1928– ), and his colleague, the English biochemist, Francis Harry Compton Crick (1916– ), were able to explain the molecular structure of DNA. With this new understanding, life scientists could formulate a fuller and more satisfying gene theory. Put simply, chromosomes are found in nearly every cell of our bodies. Chromosomes are made of DNA, and DNA stores genes. It is genes that carry the vital codes and information that not only tell a cell what to do, but which get passed on to the next generation by sexual reproduction. The final part of gene theory explains how traits are passed on, and how no two individuals are exactly alike. During sexual reproduction, when a single human sperm fertilizes a single human egg, each contains only half the full set of forty-six chromosomes. Unlike other cells in the human body that have a complete set of forty-six chromosomes, sex cells contain only twenty-three. Consequently, when egg and sperm unite, the first new cell created gets twenty-three chromosomes from the mother and twenty-three chromosomes from the father to form a complete set of forty-six. This process, along with other “shuffling” of genes that occurs, guarantees that the new organism created is a unique individual. Gene theory is the key to the genetics of the twenty-first century. Understanding how genes work and the knowledge that genes can change, or mutate, will lead to the prevention and cure of genetic diseases, as well as to the use of genetic engineering (the deliberate alteration of a living thing’s genetic material to change its characteristics) to improve certain animal and plant species. [See also Chromosome; DNA; Gene Therapy; Genetic Code; Genetic Disorders; Genetic Engineering; Genetics; Inherited Traits; Nucleic Acid] 250


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Gene Therapy

Gene therapy is the process of manipulating genetic material either to treat a disease or to change a physical characteristic. It is accomplished by introducing a normal gene into the cell to make up for a defective or missing gene. Although some successes have been achieved, this technique is still in the research stage. Once the scientific knowledge about genes and genetics had reached a certain level of sophistication by the early 1970s, scientists started thinking seriously about going right to the source of certain conditions and diseases and replacing bad genes with good genes. That is how gene therapy might be described in a simple way. Instead of introducing a therapeutic product like a drug into the body, scientists would try to deliver a gene to cure the problem once and for all. In theory, the notion of gene therapy seems very attractive. It would allow new genetic material to be inserted into the cells in a patient’s body to correct or improve a particular cell function; to make diseased cells even more vulnerable to being destroyed; or to block the functioning of diseased cells altogether. Again in theory, the way this is accomplished sounds feasible. If an individual has a disease that is the result of inher-


A scientist using computerized equipment to perform a DNA microinjection, a form of gene therapy. (Reproduced by permission of Photo Researchers, Inc.)

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iting two copies of a defective gene, he or she would be a candidate for gene therapy. First, a new and properly functioning gene would be isolated from normal cells. Called the “transfer gene,” this gene would be removed by cutting deoxyribonucleic acid (DNA) at specific locations using a technique called “gene splicing.” Then a genetically altered virus would be used as the vehicle (called a “vector”) to deliver the new gene to the cells. In fact, this has already been accomplished, and the first successful gene therapy experiment was performed in 1990. In that case, doctors replaced the defective genes of a four-year-old girl whose immune system did not work. Her body would not produce the necessary enzyme named adenosine deaminase (ADA). Doctors inserted a normal ADA gene into immune cells taken from her body and then returned the treated cells to her with a blood transfusion. The new gene gave the cell instructions to produce the enzyme and her immune system began to recover.

STILL NEEDS TO BE PERFECTED Despite this success, researchers have discovered that the major limitation of gene therapy is the delivery system. They must always use a safe, efficient vector, or vehicle, to carry the new DNA to targeted cells. The early use of viruses as vectors was a logical strategy since viruses have been easily invading people’s cells and causing trouble for thousands of years. Once there, viruses inject their own genes into the human cells, and the viral cells instruct them to make more viruses. Biologists have tried to turn the tables on viruses, and they carefully treat viral DNA in the laboratory so that it cannot reproduce itself once it gets into the body. Then human genes are inserted into the treated viral DNA, and this modified viral DNA is then introduced into human cells. Because it is carried by a virus, the modified DNA quickly finds its way into the DNA of the cells and becomes active. In practice, however, viruses infect cells in an unpredictable manner and can sometimes cause unwanted results. For example, inserting viral DNA into a cell might accidentally trigger the cell’s oncogenes—specialized genes that can activate an uncontrolled cellular growth of cancer. In 1993 gene therapy for cystic fibrosis had to be stopped when the patient developed lung inflammation. In 1999 a young male patient actually died from a gene therapy treatment that went wrong. In the fall of that year, eighteen-year-old Jesse Gelsinger of Arizona died after undergoing experimental genetic therapy a the University of Pennsylvania. The young man suffered from an inherited liver disorder and was given new genes as an attempt to cure him. While it is not yet known if mistakes were made in his treatment and therapy, his tragic loss underscores the dangers or conducting experiments on humans. 252


One alternative to the use of viruses in gene therapy is the technique of packaging genes into fatty globules. This has been used to fight tumors that often grow when the immune system fails to recognize the tumor cells as foreign and does not kill them. By wrapping the new gene in droplets of fat, called liposomes, which bind easily to cell surfaces, scientists have been able to experimentally insert a gene that marks the tumor cells in a way that forces the immune system to attack them. Researchers are working steadily at trying to make their methods of transferring genes into human cells more effective, efficient, and safe.

Genetic Code

Although gene therapy is still in its early stages, researchers are confident that it will become a standard, accepted strategy for fighting diseases like cancer in the future. Contrary to what many people believe, however, gene therapy is different from genetic engineering since it does not target sex cells that pass along genetic traits. Rather, gene therapy attempts to fix existing problems in ordinary cells and does not tamper with genes in eggs or sperm. Thus, a person who has a single-gene disorder corrected by gene therapy can still pass on a faulty copy of a gene to his or her children. Unlike gene therapy, the more ambitious and fundamentally different goal of genetic engineering is preventative. Genetic engineering seeks to keep diseases from happening in the first place. This process is more controversial since it involves changing genes in the sex cells and gives rise to many ethical and legal issues. [See also Chromosome; DNA; Gene; Gene Theory; Genetic Code; Genetic Disorders; Genetic Engineering; Genetics; Inherited Traits; Nucleic Acid]

Genetic Code The genetic code tells a cell how to interpret the chemical information stored inside deoxyribonucleic acid (DNA). This information is in the form of a sequence of chemicals that tell a cell which proteins to make. Without the genetic code, the cell would be unable to interpret the DNA sequence, and therefore could not make the proteins that build cells and make them work. By the early 1950s, scientists knew that genes were made of DNA, and that specific proteins were made by specific genes. DNA is found in the chromosomes in the nucleus of cells, and it controls the characteristics of living things by means of a chemical code of instructions. The structure of the DNA molecule was found to resemble a twisted ladder called a double helix. The rungs on this ladder are called “bases” and are U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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the coded instructions. These instructions are written with only four chemicals—adenine (A), thymine (T), guanine (G), and cytosine (C)—that make up what might be considered a four-letter alphabet. These bases must pair up a certain way (A only with T, and G only with C). Each pair of bases is called a nucleotide. It is the order of these nucleotides along the DNA that spells out the instructions for making proteins, which control the characteristics of organisms. Proteins are chains made of twenty different units called amino acids, and it is the order of the amino acids that determines what type of protein will be produced. After the 1950s discovery of the molecular structure of DNA, the question that drove geneticists during the 1960s was: “How is a gene, whose information is contained in the sequence of only a four-letter alphabet (A, T, G, C) able to code enough messages for twenty different amino acids?” If a single base coded for one amino acid, only four amino acids could be made. If two bases coded for one amino acid, then a maximum of sixteen arrangements was possible. However, if the four bases somehow combined in groups of three to form one amino acid, sixty-four combinations were possible. After a great deal of difficult research, this triplet code called a “codon,” proved to be the answer. The explanation somewhat resembles that of the Morse code, which is able to code all twenty-six letters of the alphabet using only two symbols—a dot and a dash. It does this by using different combinations of dots or dashes to code for each letter of the alphabet. With DNA, the answer lies in the codons or triplet code. Each codon is three bases long and has an exact meaning. In other words, a group of three bases in a certain order forms the codon for a specific amino acid. Therefore, the sequence GAG would specify the amino acid glutamic acid. Once the idea of a triplet code was discovered, years of work resulted in what might be called a working dictionary of codes. It was found that of the sixty-four possible combinations, sixty-one of the codons are actually used to form the twenty amino acids. This means that some amino acids can be specified by more than one type of codon. It also means that the remaining three codons do not code for any amino acid but instead act as punctuation in a long message. Thus, these three codons can signal the end of a genetic “sentence” and therefore the completion of a code. It makes sense that, just as a paragraph of words has punctuation guiding the reader, a continuous sequence of hundreds of thousands of bases needs punctuation to make it a meaningful set of precise instructions. These three codons therefore not only end a code, but are thought to also signal something like, “I am not a gene,” or “I am not a gene but one is coming soon.” Interestingly, however, no commas or internal punctuation

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are found within the code. Punctuation is limited to stop or start signals at the beginning or end of a continuous run of triplets. The code was also found to be linear, meaning that just like a sentence, its sequence of bases are supposed to be read from a fixed starting point (the beginning). Once this genetic code was broken, it was found that the code is universal. That is, the very same three-letter codons specify the exact same proteins for all living things—from humans to bacteria. All life is therefore guided or directed by a common language that is the genetic code written in all DNA.

Genetic Disorders

[See also DNA; Gene; Gene Theory; Genetics]

Genetic Disorders Genetic disorders are conditions that have some origin in a person’s genetic makeup. Genetic disorders are more severe than simple abnormalities and usually result in some type of medical problem. A genetic disorder is not the same thing as a disease. Genetic disorders are generally one of two types: those that are inherited (and are governed by the same rules that determine all our traits), and those that are the result of some type of mutation or change that took place while the embryo was developing. Sometimes this mutation is caused by environmental factors. Within this first set of inherited disorders, a disorder that is transmitted by genes inherited from only one parent is called an “autosomal dominant disorder.” The term “autosomal” refers to any of the twenty-two sets of chromosomes (forty-four individual chromosomes) common to both males and females that determine all of the traits except a person’s gender. (The single pair of chromosomes that determine sex is called the sex chromosome and is different between men and women.) In an autosomal dominant disorder (ADD), an individual inherits a dominant gene from one parent that causes the disorder. Since it is dominant, the gene has a fifty percent chance of being expressed. Sometimes an individual will inherit a gene for a genetic disorder that is not a dominant gene. In that case, it is a recessive gene that is “covered” by the normal gene (since genes are always inherited in pairs), and the person will not have the disorder. There are approximately 2,000 ADDs, ranging from the inconvenient (like an extra toe) to eventual death (as in Huntington’s disorder). Although certain disorders reduce a person’s chance of surviving (and therefore of passing the gene on to offspring), many ADDs do not affect reproduction or are diagnosed fairly late in life. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Autosomal recessive disorders, also called recessive genetic disorders (RGD), are the result of both parents supplying a recessive gene to their offspring. In this case, the offspring has no chance to avoid the disorder since it does not have any normal gene to possibly “cover” for the recessive one. Instead, the individual has two recessive genes for the same trait. There are about 1,000 RGDs, and the odds for getting two of the same recessive genes are as high as 25 percent. Some of the better-known recessive genetic disorders are sickle-cell anemia, cystic fibrosis, and TaySachs disease. Galactosemia is an example of what is called a metabolic RGD. In this case, a person with this disorder lacks a certain enzyme needed to break down the sugar found in milk. Another disorder, adenosine deaminase deficiency, is, like galactosemia, one of the few treatable genetic diseases. This immune disease is sometimes cured by a bone marrow transplant. Certain other traits are not received from the set of twenty-two pairs of autosomal chromosomes but are instead found on a single pair of sex

Down syndrome is a genetic disorder resulting from chromosome pair twenty-one receiving three chromosomes instead of just two. (Reproduced by permission of Custom Medical Stock Photo, Inc.)

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chromosomes. Since females have two X chromosomes and males have one X and one Y, males are more susceptible to X-related gene defects. Since females have two X chromosomes, any defect on one is no more likely to be expressed than a gene defect on any other chromosome (unless they inherited two X’s with defects). However, since males have only a single X chromosome, any defective gene on that chromosome will be expressed. The bleeding condition Hemophilia A is a sex-linked or Xlinked recessive disorder, as is color blindness.

Genetic Engineering

Finally, there are genetic disorders that are caused not by gene mutations but by things going wrong right from the beginning of fertilization and cell splitting. Sometimes an abnormal number of chromosomes are formed (either too few or too many), resulting in such conditions as Down syndrome in which children are born with some degree of mental retardation and sometimes heart defects. Although genetic disorders are not common, it is estimated that of all newborns, as many as one percent will have some form of chromosome-related condition. [See also Chromosome; Embryo; Gene; Gene Therapy; Gene Theory; Genetic Code; Genetic Engineering; Genetics; Mendelian Laws of Inheritance; Mutation; Nucleic Acid]

Genetic Engineering Genetic engineering is the deliberate alteration of a living thing’s genetic material to change its characteristics. It is also a general term that describes a range of techniques that allow geneticists to transfer genes from one organism to another. The applications of genetic engineering are vast, and since it is technically possible to produce new gene combinations that could never occur in nature, the implications of this new technology are controversial. Genetic engineering has actually been practiced under another name for thousands of years. Probably the oldest version of it was conducted under the name of agriculture when farmers deliberately crossed plants with certain desirable characteristics and did not breed those without them. They did the same thing with farm animals and called it selective breeding. For example, animal offspring that showed certain desired characteristics were bred with their like, while those not showing desired characteristics were not allowed to reproduce. It was such a technique that gave us the many different types of dogs we have today. However, selective breeding is a slow, trial-and-error process since it must allow animals the time to grow and mature sexually. By the 1970s, once science U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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had developed ways to isolate individual genes and to reintroduce them into cells, they were able to directly and quickly alter the deoxyribonucleic acid (DNA) of an organism and accomplish overnight what would have taken generations.

RECOMBINANT DNA TECHNOLOGY

DNA being injected into a mouse embryo. The discovery of DNA in 1953 led to the new field of genetic engineering. (Reproduced by permission of Archive Photos, Inc. Photograph by Jon Gordon.)

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Genetic engineering is also called “recombinant DNA technology” because the DNA of existing organisms is actually recombined into new organisms. Genetic engineering is a form of gene manipulation that results in a new arrangement of genes. This is achieved by removing a certain part of DNA and attaching it to another piece of DNA, sometimes from a different organism or even a different species. According to what it was intended to do, this transfer might give its new host a new trait, or it might enable it to produce substances that it never before could. Certain crops have been genetically altered or engineered to withstand the


effects of a herbicide that kills weeds. Other plants have had their ability to resist certain diseases transferred genetically to plants without that natural ability. In medicine, genetic engineers have manipulated bacteria and made them cheap, efficient producers of substances needed by humans.

Genetic Engineering

Most of today’s genetic engineering is accomplished by inserting genes into bacteria. This recombining is achieved by a relatively simple process described as “cutting and pasting.” First a gene is identified, isolated, and spliced or cut out of its DNA strand. In gene splicing, special proteins called “restriction enzymes” are used as scissors. These enzymes react chemically with a certain part of the DNA and break it off, leaving each piece with what are described as “sticky ends.” With the help of another enzyme called ligase, these ends will easily attach to any other piece of DNA, even if it comes from a different organism. It is because of this ability that DNA is so easily transferrable between organisms, even ones as different as humans and bacteria. After the gene-containing DNA has been cut out and spliced to another DNA fragment, a hybrid (having mixed composition) molecule called recombinant DNA is formed and inserted back into the cell. When the cell divides, the number of recombinant DNA molecules also increases.

GENETIC ENGINEERING HAS MANY USES Today genetic engineering has found many uses in agriculture, industry, and medicine. Plants have been engineered to withstand herbicides that kill weeds, as well as to resist insects and even grow in poor soil. In one of the more amazing experimental uses of genetic engineering, genes from fish that thrive in arctic waters have been spliced into plants in an attempt to make them tolerate freezing temperatures. A genetically altered tomato, the “Flavr Savr,” can remain on the vine longer and thus does not have to be picked and shipped while still green. In industry, genetic engineering is creating microorganisms that can more efficiently clean up oil spills by naturally breaking down the oil. This also has potential for neutralizing toxic substances and other waste products. Genetic engineering has received most of its public notice in the medical field. Already, bacteria have been genetically engineered to produce human insulin in large quantities, enabling diabetes treatment to be easier and less expensive. The same has been done for the production of human hormones. Gene therapy is a form of genetic engineering, which introduces a normal gene to make up for a missing or defective gene. Finally, for those who might pass a genetic disease on to their offspring, genetic counseling is now available that allows people to be more informed before they decide to have children. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Genetics

The recently acquired ability to alter organisms at the level of their genes is more than just another powerful tool of modern science. If we can insert a normal gene in a human to replace a defective one, who is to say we cannot change or modify an existing trait in order to have one that is more “desirable?” The ability to manipulate human beings genetically is an awesome responsibility that some argue should never be attempted. Others say it should be done only under strict guidelines, while others are simply confused by its far-reaching social and ethical implications. There is always the fear that an accident may occur and an uncontrollable bacteria or some other engineered life form might prove to be environmentally disastrous. However, genetic engineering or recombinant DNA technology has such enormous potential for useful and beneficial applications in so many areas that it can never be simply ignored. Fortunately, ethical and legal committees from many disciplines are in place to oversee this crucial work and to consider its future. [See also Chromosome; DNA; Gene; Gene Therapy; Gene Theory; Genetic Code; Genetic Disorders; Genetics; Inherited Traits; Nucleic Acid]

Genetics Genetics is the branch of biology that is concerned with the study of heredity or the passing on of characteristics from one generation to the next. It is also concerned with variation or what makes one living thing different from another. Geneticists are people who study genes in an attempt to understand how the inherited information genes contain is stored and passed on. Genetics is a fairly young science and was started by the landmark work of the Austrian monk and botanist Gregor Johann Mendel (1822– 1884), who first put forward his theory of heredity in 1865. Until Mendel, most everyone knew that noticeable traits were usually passed on from one generation to the next, but no one knew where to begin to find out what controlled and influenced the passing on of these traits. It was Mendel who carried out the first scientific study of how traits pass from one generation to the next by conducting a series of experiments with pea plants. The experiments took him eight years to complete. Mendel decided to use ordinary green peas, like the ones commonly eaten today, since they are easy to breed for what are called “pure traits.” This means that a purebred plant that produces yellow pods will always produce yellow pods. Mendel selected pea varieties that differed in a single trait (such 260


as height or color or wrinkled seeds) and then crossed them with plants of a different trait (short with tall, yellow with green). For example, he would cross a pure tall with a pure short and then accurately record the number of each type he harvested for each generation. Luckily for Mendel, he had concentrated on only a few traits, and each of these happened to be governed by a single gene (which he did not yet even know existed). The first thing that Mendel discovered was that crossing a tall plant with a short one did not result in a blend of medium-height plants, but rather in plants that were all tall. After allowing these offspring plants to pollinate (the transfer of pollen containing male sex cells to the pistil containing female sex cells) themselves, he found that the next generation produced plants three-quarters of which were tall (which he then called a dominant factor) and one-quarter of which were short (which he called a recessive factor). After crossing hundreds of plants and keeping careful records, Mendel discovered that a regular 3 to 1 ratio or pattern existed for the number of dominant versus recessive traits. This led him to decide that plants must contain what he called “factors” and “particles of inheritance,” or what we now call genes. This also led him to believe that there must be laws or rules that determined how these “particles” were passed on. After much study, Mendel formulated what are now called the “Mendelian laws of inheritance.” He stated correctly that traits did not blend but remained distinct; that they combined and sorted themselves out according to fixed rules; and that both male and female contributed equally. Mendel was also correct when he said that each parent contributed one “factor” for a particular trait to its offspring. Although Mendel’s laws had laid the foundation of the new science of genetics, his work was unknown until 1900 when it was separately discovered by three different botanists (in three different countries) who realized that Mendel had discovered the laws of genetics long before they did. Although each published his own version of these laws, each cited Mendel as having been the real discoverer, and all said that their work was merely a confirmation of what Mendel had accomplished thirty-five years before.

Genetics

MORGAN INTRODUCES CHROMOSOME THEORY OF INHERITANCE By 1900, when biologists realized that Mendel had given them the basis of a new science of heredity, they began to search for the key part in all living things that contained the crucial information that determined every detail of what an organism would look like. This search soon led to an understanding of chromosomes and then to the discovery of a single molecule called deoxyribonucleic acid or DNA. By 1900, science alU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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ready knew that chromosomes (the coiled structure in a cell that carries the cell’s DNA) existed in the nucleus of every cell, and by 1903, some biologists were making a connection between chromosomes and the process of heredity. One scientist, the American geneticist Thomas Hunt Morgan (1866–1945), conducted experiments on the rapidly bred fruit fly and arrived at the idea that chromosomes are themselves made up of other, smaller things (later named “genes”) that were linked together and arranged in a long line. In 1911 he put forth this chromosome theory of inheritance based on genes. The next question was “how did the genes actually work?” As early as the late nineteenth century, it was known that a unique type of acid named deoxyribonucleic acid (DNA) existed in every cell nucleus. It had been long ignored, however, and was considered unimportant to heredity. By 1944, though, better research and improved technology enabled biologists to realize that DNA was actually the key chemical at the center of heredity. Research continued and by 1952, biologists were able to demonstrate that DNA was indeed the genetic material for which they had been searching. However, since its structure was still unknown, it was not possible to describe how such an apparently simple molecule of DNA could contain the vast and complicated information or code that was needed to develop a human being.

WATSON AND CRICK COMPLETE THE PUZZLE The final breakthrough to this puzzle was achieved in 1953 by the American biochemist James Watson and his colleague, the English biochemist Francis Harry Compton Crick. That year they were able to successfully construct a “double helix” model of a DNA molecule that solved the puzzle. DNA, they explained, is made up of two long strands connected by “base pairs” (like rungs on a ladder), and that the entire model looks like a curving, twisting ladder or a spiral staircase. Watson and Crick also found that the bases always paired up in a specific order, so that if they knew the sequence of one strand, they could accurately tell the sequence of the other. Finally, they discovered that the order of the chemical bases represented a code that was translated by the cell and used as a guide to make proteins. Since their achievement, scientists have learned that DNA is the blueprint for all life on Earth. It is now known that almost every cell in our bodies has a set of chromosomes that store this DNA. It also is known that each DNA base is like a letter in the alphabet, and that a sequence of bases forms a message. These messages are called genes, and each gene instructs the cell that contains it on how to make a specific protein. 262


We also know that while all living things have their own unique code of DNA, the molecule that forms that code is basically the same for all.

Genus

Finally, we know that it is through chromosomes that genetic information is passed from one generation to another. Today’s genetics has gone beyond all of these discoveries and has begun to put them to good use. Now that scientists actually are able to look at the genes themselves, they can know what sequences produce what effect. This sometimes allows scientists to move genes around, fix mistakes, and even transfer them between species. These many years of research culminated in 1997, when scientists in Scotland produced a lamb that was cloned from a cell nucleus taken from the udder of a sheep. As the twenty-first century begins, science is preparing to cope with what promises to be a difficult and probably very controversial period as it becomes able to artificially alter genes through a process called genetic engineering. This powerful new tool can produce great benefits, such as curing a genetic disease like Alzheimer’s, but it also has the potential for serious misuse. Genetics may be a young science, but it promises to put its stamp on the twenty-first century. [See also Chromosome; DNA; Gene Therapy; Gene Theory; Genetic Code; Genetic Disorders; Genetic Engineering; Genetics; Inherited Traits; Nucleic Acid]

Genus The term genus is one of the seven major classification groups that biologists use to identify and categorize living things. These seven groups are hierarchical or range in order of size, and genus is one of the smaller, important, and more frequently used groups. The classification scheme for all living things is: kingdom, phylum, class, order, family, genus, and species. Coming as it does between the larger group, family, and the smaller group, species, members of the same genus have more in common than those in the same family and less than those in the same species. Although members of the same genus are very similar (like a wolf and a coyote), members of different groups usually cannot breed with one another. Members of the same genus, however, are known to be very closely related in terms of their evolutionary history, and it is obvious that they share the same basic shape and structure as well as similar biochemistry (the chemistry of biological processes) and even behavior. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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The group genus is almost always used with the more particular grouping, species. All organisms are referred to scientifically by a two-word, Latin name called a binomial. Humans therefore are Homo sapiens. This example of Homo is unusual, for only one living species occurs in that genus. Most genera are “polyspecific” and contain more than one species. This is especially the case for plants known as sedges (Carex) and insects known as fruit flies (Drosophila), each which has hundreds of species in the genus. [See also Class; Classification; Family; Genus; Kingdom; Order; Phylum; Species]

Geologic Record The geologic record is the history of Earth as recorded in the rocks that make up its crust. Rocks have been forming and wearing away since Earth first started to form, creating sediment that accumulates in layers of rock called strata. The way these strata are arranged and what fossils are in them give scientists clues about what Earth was like billions of years ago. The concept of what is called geologic time is somewhat difficult to fully grasp because it deals in such enormous blocks of time. When people first began to seriously study Earth around the seventeenth century, their first estimate of Earth’s age was in the thousands of years. One famous example is that of the Irish clergyman James Ussher (1581–1656), who used the Bible to calculate that Earth was created in 4004 B.C. A century later, estimates by others had only raised that number to about 75,000 years, and it was not until the Scottish geologist (a person specializing in the study of Earth) James Hutton (1726–1797) made his famous statement that the Earth contains “no vestige of a beginning—no prospect of an end,” did the notion of millions and perhaps billions of years begin to be considered. Today, with advanced tools, scientists are able to say with some certainty that Earth is about 4,600,000,000 years old.

READING THE GEOLOGIC RECORD By examining the progress of geology, humans touch upon the major breakthroughs that allowed scientists to be able to “read” Earth’s geologic record. This geologic history of the planet’s evolution (changes occurring over time) and developmental changes is recorded in its rocks. One of the earliest breakthroughs was the eighteenth-century realization that there was something to be learned by the obvious relationships of

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one type of rock to another. Called the law of superposition, this idea stated that in an undisturbed section of sedimentary rocks (formed in layers or “strata” by weathering and erosion), each layer is older than the one above it and younger than the one below it. Although this seems fairly simple and obvious today, it was a major breakthrough in being able to start to date the age of Earth.

Geologic Record

A related idea that also proved very helpful was the principle of faunal succession. This states that the fossils found in rocks also succeed one another in a definite order, and that a time period can be recognized by the type of fossils contained in its rocks. This principle applies throughout the world, so that geologists can identify rocks of the same age even if they are found in widely separated locations. The importance of fossils cannot be overemphasized, since without them, science would lose its primary tool for subdividing geologic time periods into smaller and smaller chunks.

ERAS, PERIODS, AND EPOCHS Geologists have divided the geologic record into periods that can be organized or charted onto a timescale. The major divisions of geologic time are known as eras which are described by some as “chapters” in Earth’s history. Each era is naturally different from another, especially in terms of the nature of life it contained. The eras are then divided into periods. These have nothing to do with the passage of a certain amount of time, meaning that they are not of equal length. Instead, they are based upon the nature of the rocks and fossils found there. Some may be longer than others. The main subdivisions of periods are called epochs. These eras, periods, and epochs usually were named after places on Earth (mostly in Western Europe) where the rocks of those times were first discovered. In terms of the geologic record, life on Earth is first seen about 3,500,000,000 years ago. This means that for about 1,000,000,000 years from the time Earth first formed, there was no life on the planet. The oldest primitive fossils found were simple prokaryotic organisms (bacteria whose cells did not have a nucleus or any other structures). The first eukaryotic organisms (whose cells contain a nucleus that is surrounded by a membrane) appeared about 1,800,000,000 years ago in what is called Precambrian era. The first multicellular organisms did not appear on Earth until somewhere between 700,000,000 and 1,000,000,000 years ago. Then, during the Cambrian period of the Paleozoic era, an explosion of multicellular U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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CHARLES LYELL Called the father of modern geology (the study of the Earth), Scottish geologist Charles Lyell (1797–1875) offered proof that Earth’s surface was the result of natural forces operating very slowly over millions of years. His work laid the foundations not only for modern geology but for the study of evolutionary biology as well. Charles Lyell was born to a well-to-do family in Kinnordy in eastern Scotland; his family moved to Southampton, England, when he was two years old. His father was an amateur naturalist who kept a well-stocked library that the young Lyell often used. As a youngster, Lyell was more interested in observing nature and collecting butterflies and insects than in school. He entered Oxford University at nineteen and his interest in geology increased, although he prepared for a law career. At twenty-two he graduated and moved to London to study law, although he spent every free moment on geological trips and excursions. By the time he finished law school and was admitted to the bar, he had conducted several geological tours in England and the continent and met with some of the best minds in geology. By the age of twenty-eight, Lyell still had his father’s financial support and was doing much more geology than law. By now, he was doing serious geological research and writing and beginning to formulate his own ideas. After making a long and difficult geological trip through France and Italy, Lyell returned to begin his Principles of Geology, which would become one of the most influential textbooks ever written. The state of geology at this time was such that it was still dominated by individuals who believed that Earth was not very old, perhaps only several thousand years, and that the obvious changes that had taken place were the result of sudden, catastrophic occurrences. Lyell’s readings, and mostly his field trips, had convinced him of just the opposite. He agreed with the earlier ideas of his coun-

life in the sea took place. Continuing explosions occurred, going from marine invertebrates (animals without a backbone) to the beginnings of actual fishes. Around 435,000,000 years ago, the first land plants evolved, followed by great swamp trees, amphibians, and primitive reptiles. The dinosaurs came on the scene during the Triassic period (about 225,000,000 years ago), and by about 180,000,000 years ago, the Jurassic period saw the first birds and mammals. Sometime during the Paleocene epoch of the Cenozoic era (about 65,000,000 years ago), the Age of Mammals began. Around 2,000,000 years ago, Homo habilis appeared and was the first human species to be given the genus name Homo, meaning “man.” 266


tryman, the Scottish physician James Hutton (1726–1797), who had long before put forth his idea of “uniformitarianism.” This theory (written before Lyell was born) said that natural geological forces, like earthquakes, volcanoes, and erosion acted upon the Earth over an extremely long period of time, and that geological change was very slow-acting and sometimes barely noticeable. Lyell’s Principles of Geology was published in three volumes from 1830 to 1833. It was so well-written and so well-documented, that despite the fact that it contained few original ideas, it communicated and explained the basic principles of the new geology so well and so effectively that it became a steady best-seller. It went through twelve editions in Lyell’s lifetime.

Geologic Record

One of those who closely read Lyell’s books and who was deeply influenced by them was the English naturalist Charles Darwin (1809–1882). Darwin and Lyell had become good friends, and when Darwin left to take his famous trip in 1831 on the H.M.S. Beagle, he had the first volume of Lyell’s book with him. Lyell’s ideas of timelessness and gradual change proved to be highly influential when Darwin began to formulate his own ideas about biological evolution (physical changes that occur over generations). In fact, Darwin is said to have once stated, “I always feel as if my books came half out of Lyell’s brain.” Darwin admitted that he drew heavily on Lyell’s book, both for its excellent writing style and its real content. As a result, both men would shape the thinking of their disciplines with their ideas that Earth and life itself was much more ancient than thought, and that existing species appeared to have evolved from previous ones now extinct. Another important geological concept championed by Lyell concerned what is called the geological record. That is, that older rocks are generally buried beneath younger ones, and that careful excavation and study of the geological layers and fossils found there contain the evolutionary history of Earth itself.

Finally, because of the geologic record, scientists also know that a major phenomenon like continental drift (the movement of the plates on Earth’s crust) is responsible for the current position of the continents. Scientists also know that the record contains evidence of several extinctions. None of these are more dramatic and puzzling than the disappearance of the dinosaurs about 100,000,000 years ago during the Cretaceous period. Scientists have described the geologic record as a history book to be read to learn about Earth’s past. However, the geologic record is only useful if one knows how to read its signs and interpret them. [See also Fossils; Radioactive Dating] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Germination Germination is the earliest stages of growth when a seed begins to transform itself into a living plant that has roots, stems, and leaves. Although conditions vary according to species, all seeds require a certain amount of moisture and oxygen as well as a suitable temperature before they will germinate.

A kidney bean sprouting into a seedling. This is the beginning of germination, a process that cannot be reversed. (Reproduced by permission of Photo Researchers, Inc.)

Some seeds are ready to germinate almost as soon as they are ripe and will sprout open wherever and whenever they land in a suitable environment. However, the seeds of most plants need to lie dormant (inactive or resting) for a period of time before they will germinate. This enforced dormancy can be caused by many factors within the seed itself. First, the seed coat may be so hard that it will not allow water or oxygen to penetrate until it has begun to soften or break down over time. Second, seeds may contain chemicals that prevent germination, and sprouting will not occur until these antigermination hormones have been washed away by rainwater. Other seeds need to be exposed to prolonged periods of cold, while others must pass through the gut of an animal or even be exposed to fire before germination will occur. During this dormancy period, the seed is inactive and no growth occurs. Seeds have remarkable properties, and some can remain dormant for extremely long periods of time. In fact, some seeds have been known to germinate after remaining dormant for centuries. A mature or ripe seed is surrounded by a hard coat called a testa. Inside this coat is the beginning of a plant called an embryo. The embryo has one or more seed leaves called cotyledons. Also inside is all the food the embryo will need to fuel its early growth. Germination usually happens in the spring when the soil warms, and the seed breaks its dormancy. At the beginning of germi-

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nation, the seed takes in water very quickly. This process is called imbibition, and as the dry seed takes in more water, it swells and finally bursts the seed coat. Once dormancy ends and the seed coat bursts, germination becomes irreversible. With its coat now open, the seed can take in even more water and oxygen, so that it soon doubles in size. The next stage of germination begins when the food stored in the endosperm is converted into useable forms and sent to the seed’s growing points. These points consist of the seeds beginning root system called the radicle and its early stem and leaf stage called the plumule. The radicle or young root is the first to emerge from the seed and begins to grow downward into the soil. Soon after, the young shoot or plumule appears and starts to grow upward. The plumule breaks through the soil with its tip bent over, protecting the young, tender tip and allowing the older, stronger part of the shoot to bear the brunt of pushing upwards. In some plants like garden beans, the cotyledon (the first leaf to appear from a sprouting seed) is also raised out of the ground, while others, like peas, the cotyledon stays buried. After the radicle becomes a root system and the plumule straightens out, the cotyledon begins to open and the first true leaves start to grow. By now, the seedling is well established and the life cycle of another generation has begun.

Golgi Body

A high resolution scanning electron micrograph of the Golgi body of an olfactory bulb cell. The Golgi body collects proteins and lipids made by the endoplasmic reticulum. (©Photographer, Science Source/Photo Researchers, Inc.)

[See also Botany; Embryo; Fertilization; Seed]

Golgi Body A Golgi body is a collection of membranes inside a cell that packages and transports substances made by the cell. All eukaryotic cells have one or more Golgi bodies that work closely with the endoplasmic reticulum (a network of membranes, or tubes, in a cell through which materials move). A eukaryotic cell is one with a distinct nucleus (such as a plant or animal cell). U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Golgi bodies were named after Italian anatomist (a person specializing in the structure of animals) Camillo Golgi (1843–1926), who first saw them in brain cells late in the nineteenth century. Golgi was able to see these tiny organelles (or cell structures that have certain functions) because of his discovery of a particular cell stain. Golgi found that by staining nerve cells with silver nitrate, he was able to see details that were not otherwise visible. He was also able to make out all of the very fine extensions that branched off the Golgi bodies. Looking like a stack of flattened bags, these piles of membranes are usually found close by the nucleus and work closely with the cell’s endoplasmic reticulum (which make proteins). Their function is to package the proteins made by the cell, and they literally wrap these proteins in membranes. The proteins then travel either to another part of the cell or to the cell membrane to be transferred outside the cell. Golgi bodies also work on any “unfinished” proteins that the endoplasmic reticulum has created. Sometimes these proteins need a bit more tinkering or finishing before storage or use, and it is the Golgi bodies that perform whatever chemical modifications are necessary. Golgi bodies are also called Golgi apparatus. They have also been referred to as the Golgi complex. [See also Cell; Endoplasmic Reticulum; Membrane]

Grasslands Grasslands are particular geographic regions in which grasses are the dominant vegetation. Grasslands typically have only two seasons: a long, dry season and a shorter, rainy season. In their natural state, grasslands can support a variety of large grazing animals. Most grasslands are called temperate grasslands since they spread across geographic areas where conditions are more moderate than extreme. In temperate grasslands, summers are hot and winters are cold. Grasslands receive less rain than forested areas but more rain than deserts. Grasslands are found in both the Northern and the Southern Hemisphere and are called by a variety of names. In North America, they are the “plains,” in Argentina and Chile they are the “pampas,” in the Andes of South America they are called “paramo,” in Eurasia the “steppes,” the “veld” in South Africa, and the “outback” in Australia. Besides sharing similar climates, all of these regions have few trees, except near rivers and streams. The land is either flat or has rolling hills. Different types of grasses grow in different regions, although all have adapted to moist winters and dry summers. Some grass roots may go as deep as 9 feet (2.74 270


meters) in search of water, although more often they rely on a huge, spreading system of roots and subsurface stems called rhizomes.

Grasslands

In the United States, what are called shortgrass prairies existed east of the Rocky Mountains. These were home to some 60,000,000 bison at one time. Shortgrass prairies, such as those found in South Dakota, experience strong winds, extreme light, and infrequent rainfall. Most of the original North American Great Plains have been plowed under to grow wheat. This contributed to the Great Dust Bowl of the 1930s when poor farming practices, drought, and strong winds transformed the prairie into a wasteland. Tallgrass prairies also once extended west across North America from the temperate deciduous (trees that lose their leaves before winter) forests. It is said that they grew as high as a person on horseback. These grasslands were covered with herds of grazing animals, especially bison. The main predators were wolves, although coyotes predominated in the drier


Stands of trees in a grassland during the summer. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

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west. Smaller animals included prairie dogs and their predators (foxes, ferrets, hawks, and eagles). In the American Midwest and the Russian steppe, most wild herbivores (animals that eat mainly plants) have all but disappeared. In the grasslands of Africa, buffalo, antelope, and rhinoceros all live together. In African tallgrass, a particular type of “grazing succession” takes place where each herbivore has a niche or proper role to play. As the zebras eat the tall grass, they expose the lower grasses. These grasses are grazed on by the wildebeest whose teeth cannot handle the tall grass. Wildebeest grazing simulates the growth of new grasses, which is then consumed by Thomson’s gazelle. Many of these animals eat grass seeds that are then passed through their systems and deposited elsewhere in their dung (waste). Grasslands have proven well suited to farming, since they have few trees, rich soil, mostly flat, that little of any original grasslands now exist. The temperate grasslands have become what is called the “breadbasket of the world.” Corn and wheat have mostly replaced grasses. However, ecologists are concerned that this switch from grasses to crops has increased the amount of carbon dioxide in the atmosphere, possibly contributing to global warming. [See also Biome]

Greenhouse Effect The greenhouse effect is the name given to the trapping of heat in the lower atmosphere and the warming of Earth’s surface that results. Although a natural phenomenon, this warming effect tends to increase as more human-produced gases are released into the atmosphere. This increased warming could result in climate changes that affect crops, as well as melting glaciers and coastal flooding. Only in the past few decades has the term “greenhouse effect” implied something bad for the environment. In fact, the phenomenon that it describes is absolutely essential to life on Earth. Without it, Earth would be a cold and lifeless planet. What this phenomenon does is hold on to some of the heat given off by the Sun. Specifically, Earth’s atmosphere is the mixture of gases and water vapor that surrounds the entire Earth. Composed mainly of oxygen and nitrogen as well as small amounts of other trace gases, the atmosphere is essential to photosynthesis (the process a plant uses light to make food). It also protects organisms from the Sun’s infrared rays because it absorbs much of these. Since the atmosphere is located physically between the surface of Earth and the Sun, 272


the molecules of gas that make it up act somewhat like a pane of glass in a greenhouse, which is where the name “greenhouse effect” originates. The glass on a greenhouse lets light in and out but holds in its heat. What the atmosphere does is allow wavelengths of visible light from the Sun to reach Earth’s surface. While doing that, however, the atmosphere blocks the escape into space of the longer infrared wavelengths. That is, they trap the light’s heat by reabsorbing these wavelengths, much of which get sent back down to Earth again. Overall, this makes Earth a warm place that is hospitable to life. Human activities have begun to alter this process of capturing heat, however. The result of these activities may be the experiencing of too much of a good thing. Too much of a greenhouse effect means that things are heating up too fast. In the past few decades, human activities have begun to change, if not harm, the atmosphere, so that it is trapping more heat than it should. Specifically, our steady burning of fossil fuels (coal, oil, natural gas) has increased the amount of carbon dioxide in the atmosphere. Carbon dioxide is the most important gas in the greenhouse effect, since it is the molecules of carbon dioxide that do the actual absorbing of longwave infrared radiation. The more carbon dioxide in the atmosphere, the more heat it keeps in. Besides all of the carbon dioxide being pumped into the air by car exhausts and factories, there is the added problem that hu-


Greenhouse Effect

An atmosphere with natural levels of greenhouse gases (left) compared with an atmosphere of increased greenhouse effect (right). (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

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Greenhouse Effect

SVANT AUGUST ARRHENIUS Swedish chemist Svant Arrhenius (1859–1927) is considered to be the founder of physical chemistry, a fairly new field which blends chemistry and physics. He not only contributed to the founding of a new branch of science, but added a new concept, now called the “greenhouse effect,” to the study of the life sciences. Svant Arrhenius was a child prodigy (exceptionally smart) who taught himself to read at three years of age. Born at Vik, Sweden, the youngster is said to have become interested in mathematics while watching his father, a surveyor (a person who determines boundaries), add columns of figures. Naturally brilliant in school, he earned a bachelor’s degree from the University of Uppsala at the age of nineteen while studying physics, mathematics, and chemistry. For his doctoral dissertation in 1884, he offered a theory that explained what occurred when electricity passes through a solution at the atomic level. His ideas were so revolutionary, however, that his committee gave him the lowest passing grade possible. Nineteen years later, Arrhenius would receive the Nobel Prize in Chemistry for the same work that had barely earned him a degree. All his life, Arrhenius was regularly pushing the limits of science, and in 1908 he published a book titled Worlds in the Making that marked him as one of the forerunners of molecular biology. Molecular biology is the study

mans are vastly reducing the world’s forests, whose trees and plants take in carbon dioxide and give off oxygen as part of photosynthesis. Since humans are artificially warming Earth this way, many scientists feel that Earth is already beginning to experience the negative consequences. Climatologists (scientists who study Earth’s climate or weather) now believe that Earth’s long-term climate patterns are changing. Some project as much as a 4 or 5 degree increase by the middle of the twentyfirst century. This could have disastrous effects, since such global warming could cause the polar ice caps and the mountain glaciers to melt. This could result in mass flooding, making many islands disappear, and putting coastal cities completely under water. Since it is climate that mostly determines what will grow where, crops also could be seriously affected. A global temperature rise could produce entirely new patterns and extremes of rainfall or drought in certain areas. Steps are being taken to prevent possible disastrous effects from happening. Efforts to reduce the pumping of greenhouse gases (carbon dioxide, methane, and nitrous oxide) into the atmosphere have begun in cer274


of the complex chemicals, like proteins and nucleic acids, that make up living things. In this work, Arrhenius offered what he called the universality of life, meaning that life was not to be found only on Earth. It was his belief that life on Earth had sprung from “spores” that had been driven though outer space by radiation pressure from other planets. This meant to him that life had been spread throughout the universe and took hold wherever conditions were favorable. There are now many physical reasons why his spore theory is not correct, and it also offers no explanation as to where or how life originated in the first place.

Greenhouse Effect

It was in this book, however, that Arrhenius also suggested what we now call the greenhouse effect. There he speculated that carbon dioxide gas in the atmosphere heats Earth by first allowing sunlight to reach its surface, and then trapping much of the heat radiation or preventing the heat from escaping back into space. He argued that because of this phenomenon, any rise in the amount of carbon dioxide in the atmosphere would raise Earth’s temperature since it would act like a greenhouse and trap even more of the heat. He also argued that the reverse could happen and that a major decrease in carbon dioxide could result in a cooling effect that might even cause another Ice Age. Today, many scientists think that global warming is in fact occurring, and that it is caused by the carbon dioxide released when fossil fuels, like coal, are burned in factories and power plants. Arrhenius was certainly a man ahead of his time.

tain countries. This involves making automobile engines more efficient and clean, or better still, encouraging the use of public transportation. Electric cars soon may become practical. The use of nitrogen-based fertilizers can be reduced, and the destruction of entire forests can be stopped. International agreements have already been made to reduce the production of such gases by industry. Many scientists point to the nearby planet Venus as an example of what could happen if a “runaway” greenhouse effect ever occurs. The atmosphere around Venus keeps its surface temperatures as high as 932°F (504°C). To date, there is no evidence of life on Venus. [See also Carbon Dioxide; Forests; Ozone; Pollution; Rain Forests]


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H Habitat A habitat designates the distinct, local environment where a particular species lives. Habitat has also been described simply as a place where a certain organism usually can be found. A shark’s habitat is the ocean, and a trout’s is a freshwater stream. Habitat requirements of the world’s organisms vary widely and can be extremely different even for similar species. Habitat changes can affect the survival of a species. The habitat where an organism lives is one in which that organism’s particular requirements are met. Habitat has been described as the place where an organism can best do its job, or its biological “street address.” A favorable habitat for any animal or plant means that this organism is able to get all the things is needs to live, and that it has a slight advantage over others. Habitats are usually described in terms of an outstanding geographical feature such as being mountainous, being a desert, or a being a grassland. However, the concept of habitat is dominated by the idea of a particular type of place. The place where an organism thrives can be anywhere, from the human gut where the bacterium Escherichia coli lives, to the soil an earthworm digs, to the rocky crevices a mountain goat leaps across. Habitats can be nearly any size, although when a habitat becomes especially large it is called a biome. The word “environment” is sometimes used in place of habitat, but environment usually suggests a larger area, while habitat suggests a more local area. Within its habitat, every species eventually finds its niche or the particular role that fits it best. An organism’s ecological niche is defined by many factors such as the food it lives on, who are its predators, what temperatures it tolerates, or how much water it needs to consume. While two U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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THOMAS EUGENE LOVEJOY A leader in the emerging discipline of conservation biology, Thomas Lovejoy (1941– ) conducted a unique twenty-year experiment to determine the best-sized habitat for conserving biological diversity (a broad term that includes all forms of life and the ecological systems in which they live). The goal of international conservation also has been fostered by his idea of a “debt-for-nature” swap in which a country’s financial debt is partially forgiven if it carries out certain conservation measures. Thomas Lovejoy was born in New York City into a life of wealth and privilege. While attending the Millbrook School in New York, Lovejoy was first made aware of the wonders of the natural world by that school’s founder, Frank Trevor. The young Lovejoy was inspired to study field biology, especially birds, and became fascinated with biology and the natural world. After entering Yale University, he was able to study under the eminent English ecologist, George Evelyn Hutchinson. Lovejoy received his bachelor’s degree in 1964. While in pursuit of his doctorate at Yale, Lovejoy was able to spend two years in Brazil as part of a Smithsonian Institution project. There he introduced the technique of bird-banding (tying bits of colored string on birds’ legs in order to observe their behavior) to that country, and his doctoral thesis proved to be the first major long-term study of birds in the Amazon region. He was also able to study birds in Africa. In 1971 he received his Ph.D. from Yale University. After serving the World Wildlife Fund for fourteen years, eventually becoming its vice president, Lovejoy joined the Smithsonian Institution in 1987. There he has been able to continue a long-term experiment that he had begun at the World Wildlife Fund. In 1978, Lovejoy initiated a twentyyear experiment (which has since been extended) to try and determine the best strategy for conserving biological diversity. Called the Minimum Critical Size of Ecosystems (MCSE) Project, its goal is to discover whether bi-

species may live in the same habitat, they can never share the same niche. A niche is a smaller concept than habitat, since a niche is part of the larger habitat. That does not mean, however, that a niche is any less complex. An organism’s niche is a highly complex set of activities, impacts, relationships, and roles that can be difficult to understand fully. Plant and animal habitats are often negatively impacted by humans. It is difficult for human populations to live near a species’ habitat and not have an impact on it. Human impact can include climate change, acidification (from acid rain), reduction of wooded areas, lowering of water ta278


ological diversity is better favored by conserving a single large piece of land or by preserving a large number of smaller habitats. Although this was a serious issue that needed to be resolved scientifically, it seemed to be impossible to carry out until Lovejoy came up with an idea. Aware that Brazil had begun a program of developing the Amazon basin, Lovejoy also knew that that country allowed developers to clear only half of the rain forest and leave the remaining half untouched. Seeing that he had the raw materials for a large-scale experiment at hand, Lovejoy was able to get Brazil’s support for his new idea. That idea consisted of twenty-four separate reserves, or habitats, that varied in size from 2.5 to 25,000 acres. After two decades of study, the project has yielded valuable results. One of the most interesting is called the “edge effect.” This phenomenon occurs when the trees at the edge of the preserved habitat die off and the butterfly populations decline. This suggests that a reserve should always be made larger than is required to support its biological diversity, since there will always be an inevitable shrinkage of life-supporting land around the edges of the preserved habitat. Recent results further suggest that there may also be a loss of diversity deeper into the habitat than first thought.

Habitat

Lovejoy is one of the first of a new wave of biologists who must combine their science with real-world concerns, such as politics, in order to advance their goals. If a scientist is studying biological diversity and seeks to preserve it, the realities of today’s world are such that he or she can no longer remain in the laboratory or jungle and conduct research. Instead, if scientists want to preserve the subject of their study, like the world’s essential biological diversity, they are being forced to become actively involved in the politics and economics of the twenty-first century. Lovejoy is a good example of a scientist who has become involved in politics and economics. He has done this through his “debt-for-nature” idea. Today, Lovejoy also is recognized as one of the most effective spokespersons on such issues as global warming and the loss of biological diversity.

bles, and desertification (or the spread of a desert area). Such activities degrade, break up, and eventually destroy entire habitats. These human activities threaten the existence of other organisms. This can be particularly true for species that are highly specialized. An example is the giant panda that feeds only on a certain species of bamboo. Given the rate at which its habitat containing the bamboos species is being destroyed by human development, ecologists do not expect the giant panda to survive in the wild. As a result, giant pandas will eventually only be able to be found in zoos. [See also Niche; Species] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Hearing

Hearing Hearing is the sense that enables an organism to detect sound waves. It serves mainly to allow an animal to detect danger, to locate its prey, to communicate, and even to express emotion. As one of the five human senses, the ability to hear is an especially important sense because of its connection to human speech and language. To understand what hearing is about, it is important to understand the nature of sound. Sound needs air to be heard. This means that in a vacuum there is no sound because no air is present. This was proven in the seventeenth century when the English chemist Robert Boyle (1627– 1691) placed a bell inside an airtight jar and gradually withdrew all the air as he rang the bell. When all the air was gone, the ringing bell made no sound at all. This is because sound is created by waves of pressure or vibrations that happen when something disturbs the air. It is much like how a pebble thrown into still water creates an increasing ring of ripples. When a noise is made, it disturbs the air and a sound wave begins as the air vibrates from the original noise. Without air to travel through, sound does not exist. A drawing of a sound wave would therefore look like a wavy line. Only vertebrates (animals with a backbone) and some insects have the ability to hear. This means that only these types of animals have special organs to receive and then interpret these vibrations of the air. Just as smell and taste use chemoreceptors (a nerve cell that responds to chemical stimuli) to detect dissolved or airborne chemicals, and touch and sight use tactile (touch) and visual receptors, hearing employs auditory receptors. Each sense has a specialized type of receptor that is geared to respond to a certain type of stimulus. Most vertebrates have a system that enables them to detect sound waves and then to convert them into nerve impulses that the brain identifies.

Opposite: A diagram of the hearing process. The out ear collects external sounds and funnels them through the auditory system to the eardrum (far right of illustration). (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

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HEARING IN MAMMALS Hearing reached its highest level of development in mammals, a class that includes humans. The human ear is a complicated organ that serves as a good model of how animals hear. Humans and most other mammals have an outer, middle, and inner ear.

The Outer Ear. The outer, or external ear, is called the pinna and is the part that is outside of the head. Its shape is designed to catch and direct sound inwards. Mammals have two ears so they can better locate the direction of a sound. The brain actually calculates the location by comparU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e


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ing the speed at which the sound reaches each ear, since it will reach the ear closer to the sound first. As the outer ear collects the sound, it funnels it into a passageway called the auditory canal and then on through to the tympanic membrane or eardrum. This delicate, tight piece of tissue is like the tight skin on a drum. When the sound, made up of certain vibrations, reaches the eardrum, it causes it to vibrate.

The Middle Ear. Beyond the eardrum is the middle ear, and here the vibrations are transferred to three bones inside of it called the hammer, the anvil, and the stirrup. The eardrum makes the hammer vibrate. Like a chain reaction, the hammer makes the anvil vibrate. This makes the stirrup do the same, each time increasing the intensity of the sound. The Inner Ear. The stirrup then vibrates against the inner ear’s oval window, which covers a snail-shaped structure called the cochlea. The cochlea is filled with fluid, which helps the body keep its balance, and is covered with tiny hair cells. It is here that the vibrations somehow get changed into nerve impulses. Once the mechanical vibrations are converted into electrical impulses, they travel through the auditory nerve to the brain’s cerebral cortex. There they are interpreted as sounds. The type of sound sensed by the brain depends on which hair cells are triggered. The Eustachian Tube. It is important for the pressure on both sides of the eardrum to be equal, so the ear has mechanism to do this. Called the Eustachian tube, this tube connects the middle ear with the throat. It is not a permanently open tube but works like a valve that opens and closes as necessary. We experience this pressure imbalance when our ears feel uncomfortable on an airplane and our hearing seems to fade. We also experience the “pop” as we yawn or swallow and suddenly equalize the pressure. The Cochlea. The ear’s cochlea also functions to help people keep their balance as we move about. The fluid inside the cochlea moves and shifts and tells the brain about the body’s position. Spinning about makes someone dizzy afterwards because this fluid keeps sloshing about and does not stop all at once. The brain thus receives confusing and chaotic signals until the fluid slows and stops sloshing.

HEARING IN INVERTEBRATES Most invertebrates (animals without a backbone) do not have specific receptors for detecting these air vibrations that produce sound, and most have no hearing. They do feel the vibrations of the air, water, or soil in which they live. Insects are an exception to the above, since crickets, grasshoppers, katydids, cicadas, butterflies, moths, and flies are capable of 282


hearing. Crickets and katydids have receptors called tympanic membranes, which are similar to ears, on their legs, and grasshoppers and cicadas have them on their abdomen. The tympanic membrane of grasshoppers and crickets are known to function much the way the human eardrum does.

Heart

ECHOLOCATION Some mammals can move their outer ears to better focus on a sound. Certain deep-diving marine mammals like seals and dolphins can close off their ear canals when submerged. Bats and dolphins use echolocation as a way of sensing their surroundings. They send out a sound and then listen for the echo as it bounces off an object. Both can determine not only how far they are from the object, but their brains can actually analyze the echo pattern and form an image of it.

HEARING LOSS Hearing in humans can be impaired or lost altogether. The most common reason for hearing loss is a stiffening and eventual death of the important hair cells in the inner ear. Loud noises make this happen. It is estimated that the average person has lost more than 40 percent of his or her hair cells by the age of sixty-five. Loudness is measured in decibels, and ears can be permanently damaged by two to three hours of exposure to 90 decibels. Since the music at some rock concerts is often as high as 130 decibels, it is not surprising that some rock musicians and their fans have diminished hearing. Since communication is such an important part of being human, the loss of hearing is an especially isolating disability for many people. Helen Keller (1880–1968), who became blind and deaf during infancy, said that her lack of sight was nothing compared to how her deafness isolated her from others.

Heart The heart is a muscular pump that transports blood throughout the body. As an essential part of an organism’s respiratory system, the heart circulates blood through the lungs. It exchanges carbon dioxide (gas) for oxygen which it distributes to the rest of the body. The heart is made of tough cardiac muscle that never needs to rest. All animals that have blood and a circulatory system also need a version of a heart to move that blood throughout the body. The design and U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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complexity of a heart is generally related to the complexity of an organism and to the type of lifestyle it leads. For example, an earthworm’s simple system needs nothing more than two pulsating tubes running up and down its body. In vertebrates (animals with a backbone), there is also a degree of complexity depending on the structure of the vertebrate’s body. For example, a fish’s heart has two separate chambers. One is called an “atrium” that receives blood after it has circulated through the fish’s body. The other is called a “ventricle” that forces the blood received from the atrium over its gills. Here the exchange of carbon dioxide and oxygen takes place. Still higher up the vertebrate ladder, the amphibian, like the frog, heart has three chambers, while birds and mammals have four chambers.

THE HUMAN HEART High-energy animals, like birds and mammals that move about quickly and maintain a constant body temperature, need a very efficient heart. Such a heart can keep oxygen-rich blood entirely separate from blood that has surrendered all its oxygen (deoxygenated blood). The human heart is a good example of how high efficiency is achieved. The human heart is actually two hearts or pumps. One serves the lungs and the other serves the body. Located at the center of the chest cavity behind the breastbone, the heart is about the size of a clenched fist. The surface of the heart is covered with a number of small arteries, known as coronary arteries. These supply the heart’s muscle fibers with oxygen-rich blood. The inside of the heart is divided into four chambers. The two at the top, called the atria (singular, atrium), are the receiving chambers. The two at the lower half are called the ventricles and are the pumping chambers. Each atrium is separated from a ventricle by a valve that allows blood to travel only in one direction and prevents any backup. The right and left sides of the heart are separated by a thick wall of muscle called the septum. Blood flows from the right atrium into the right ventricle. The right ventricle pumps the blood to the lungs where it leaves its waste (carbon dioxide) and receives oxygen. From the lungs, blood flows into the left atrium and then to the powerful left ventricle which pumps it throughout the rest of the body.

Opposite: A cutaway view of the anatomy of the human heart. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

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The heart is made of tireless cardiac muscle that beats, or contracts and relaxes, an average of seventy-two times a minute. This beating is known as the pulse rate. The heart beats in a regular sequence due to a complex electrical network. This network stimulates the muscle fibers and make the chambers contract or pump in the proper sequence. The heart also regulates blood pressure, or the pressure exerted by flowing blood against the inside of the walls of the veins and arteries through which it is flowing. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

Left common carotid artery

Left subclavian artery

Brachiocephalic trunk Superior vena acva

Aorta Right pulmonary artery

Trunk of left pulmonary artery

Right pulmonary veins

Left pulmonary veins Left atrium

Semilunar valve Semilunar valve Atrioventricular valve Left ventricle

Apex of heart Atrioventricular valve

Septum

Right ventricle

Inferior vena cava Oxygen-rich blood from the lungs Oxygen-poor blood from the upper part of the body

oxygen-poor blood from the lower part of the body


ventricles relaxed

Oxygen-rich blood to the body Oxygen-poor blood to the lungs

ventricles contracted

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The heart is always able to adjust to physical and chemical changes in the body. Its beating strength, rate, and blood pressure are affected by the needs and demands of the body. The heart also responds to chemical changes in the body, such as the increased demands placed upon it during strenuous exercise. In this example, waste products must be eliminated at a much faster rate than when resting. Hormones, the body’s chemical messengers, affect the rate at which the heart beats, and the heart is able to react appropriately to all types of stress, whether physical or emotional.

HEART DISEASES AND DISORDERS Finally, a variety of diseases and conditions can affect the heart. An infection can inflame the sac around the heart (pericarditis), the lining of the heart (endocarditis), or the cardiac muscle itself (pericarditis). The electrical impulses that make the heart beat regularly can be upset. This is called arrhythmia. Also, the arteries that supply the heart muscle can become clogged. This can lead to a heart attack in which a part of the heart muscle dies due to a lack of a proper blood supply. High blood pressure and clogged arteries can also make the heart have to work harder, causing it to enlarge. This condition is known as congestive heart failure, and if not treated, is fatal. [See also Circulatory System]

Herbivore Herbivores are animals that eat only plants. Since animals cannot make their own food, they must obtain their energy either by being herbivorous (eating plants), carnivorous (eating other animals), or omnivorous (eating both plants and animals). Herbivores are called primary consumers on the food chain since they are the first organisms to consume the energy stored by primary producers (plants). As plant eaters, herbivores have certain physical characteristics that make them different from carnivores and omnivores. Since green plants are the only organisms capable of producing their own food, they are at the beginning of the world’s food chain. This food chain or food path connects species in terms of how food and energy is passed from one species to another. Food chains or webs are divided into organisms that produce energy and those that consume it. Producers, who use energy and make their own food, are called “autotrophs.” Plants are autotrophs since they make their own food by converting the Sun’s light energy into chemical energy via photosynthesis. Animals cannot do this 286


and must eat other living organisms in order to survive. They are called “heterotrophs” since they are forced to obtain their energy and nutrients from the food they ingest or eat. Animals are considered to be primary, secondary, or tertiary consumers according to the order of who eats whom. A caterpillar is a primary consumer because it eats only plants. A pigeon would be a secondary consumer when it eats a caterpillar. A fox would be a tertiary consumer when it eats a pigeon.

Herbivore

Herbivores have developed many traits that are specialized only for plant eaters. Cows and sheep are examples of herbivores that possess several physical characteristics because of their diet. An examination of their skulls shows that they have no canine teeth, or the two long, pointy teeth in the front of a carnivore’s mouth that it uses for ripping and holding its prey. Since these herbivores are grazing animals and have no need to hunt and catch their food, they have instead chisel-like front teeth called incisors to break off blades of grass. Behind the incisors and the back molars is a gap or a space called a diastema, providing the necessary “give” in a jaw that moves side-to-side. These herbivores’ molars are flat teeth with ridges on the surface that serve as powerful grinding tools. Sheep and cows also have loose joints in their jaws so they can chew side-toside, which improves the grinding action of their teeth. After herbivores chew the leaves or grass into a pulp with their molars, it passes into a highly specialized digestive system that is very different from that of a carnivore. The best example is that of a cow, since it has four chambers in its stomach. Grass is very difficult to digest because of its tough cell walls. Sometime after the pulp enters a cow’s first chamber or rumen, it is regurgitated or coughed up into its mouth to be chewed again. This helps break the grass down even more and is what we describe as a cow “chewing its cud.” The grass later passes on through the remaining chambers of a cow’s stomach where it is digested even more. Since herbivores are able to get only a small amount of energy from each mouthful of their vegetarian diet, they must eat enormous amounts of food. This is why a cow spends nearly all its waking hours grazing. Besides cows and sheep, other herbivores include caterpillars, fishes, birds, and many other mammals. Many eat seeds and fruit instead of grass or leaves, and because of this specialized diet, have specialized tools, like a certain type of beak or bill. Certain finches have beaks that allow them to eat a certain type of seed. Other birds like the toucan have long, sharp bills to pluck berries or chop larger fruit into bite-size pieces. Herbivores are critically important to carnivores, since without them, a meat-eater would have no way of obtaining the life-giving energy first captured by green plants. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Herpetology

Herpetology Herpetology is the scientific study of amphibians and reptiles. As one of the many subfields of vertebrate (animals with a backbone) biology, it focuses on the anatomy (structure), physiology (processes), behavior, genetics, and ecology of amphibians and reptiles. Although amphibians are very different from reptiles, they are grouped together in the discipline called herpetology.

English naturalist John Ray wrote the first book that systematically arranged animals. In this work, however, Ray incorrectly grouped amphibians and reptiles together. (Photograph courtesy of The Library of Congress.)

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Amphibians live both on land and in the water, and need to keep both their skin and their eggs moist. Frogs, toads, newts, and salamanders are amphibians. Reptiles are best suited to life on land, and both their skin and their eggs have tough outer coverings. Turtles, snakes, lizards, and crocodiles are reptiles. Both amphibians and reptiles are ectothermic (cold-blooded), meaning that their temperature matches their environment. The grouping together of these different types of vertebrate animals into one field is believed to have come from the very old tradition of lumping together all creeping or crawling organisms. In Greek, herpeton means a crawling thing and logos means reasoning or knowledge. The first book to systematically arrange animals in some sort of order was written by the English naturalist, John Ray (1628–1705), in 1693, and in that work he grouped amphibians and reptiles together. A herpetologist, or one who practices herpetology, must be familiar with a wide variety of animals. Amphibians (whose name in Greek means “having two lives”) undergo a unique process called metamorphosis. This is a dramatic but natural change in body shape that transforms an organism (like a tadpole that lives totally underwater) into an entirely different organism (like a frog that breathes air). Reptiles are often confused with amphibians from which they evolved, but they do not undergo metamorphosis and breathe air through lungs their entire lives. Amphibians must return to water to reproduce, and fertilization occurs externally (outside of their bodies). Reptiles produce a sealed egg that hatches on land, and the eggs are fertilized internally, or within the body of the female. All amphibians and reptiles belong to the phylum Chordata and the subphylum Vertebrata. Both also belong to a superclass called Tetrapoda. A suU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

perclass is a name devised by taxonomists (biologists who specialize in classifying things) when an extra classification group is needed. It is similar to the categories called superfamily and subphylum, which are also used when the realities of life do not completely fit into the standard seven-part classification scheme.

Hibernation

The study of herpetology has a significance beyond the animals themselves and extends to humans as well. Since amphibians and reptiles are easily kept in captivity and have systems that manage and respond to foreign and toxic substances in much the same way that human’s do, they are ideal subjects for studies that benefit people. In fact, because of this similarity, many scientists believe that the health of both (especially amphibians) may serve as an early warning system for the overall health of our environment. Thus, if frogs become unexplainably sick, it may be a warning that there is something harmful in the environment, which may eventually cause health problems in humans. [See also Amphibians; Reptiles]

Hibernation Hibernation is a special type of deep sleep that enables an animal to survive the extreme winter cold. Hibernation lowers an animal’s energy needs and allows it to live off stored fat and not have to search for scarce food. Hibernation is a form of cyclic behavior and is triggered by different cues in the animal’s environment. All animals have different survival tactics that allow them to live through difficult or life-threatening situations. The steady, severe cold that comes with winter poses a problem to animals who do not escape it (by migrating or leaving) nor adapt to it (such as by growing an extra-thick coat of fur or a layer of fat). Winter is difficult for all warm-blooded animals (those that maintain a constant internal temperature despite their environment), since they must spend most of their energy just keeping warm. When the temperature falls below freezing, these animals must eat even more than they usually do simply to produce enough internal heat to stay alive. Nature makes things even more difficult in winter, since at a time when warm-blooded animals need to increase their intake of food, it has suddenly become very scarce. For certain animals, hibernation is a simple way to solve the particular problems posed by winter cold, since they basically sleep through winter and wake when the weather has become mild. However, hibernation is a fairly complicated physiological event. When an animal hiberU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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nates, its body processes such as breathing and heartbeat slow down, sometimes to the point that the animal appears dead. While hibernation could be described simply as a very deep sleep, it is anything but simple. True hibernators are usually small mammals like woodchucks, mice, or ground squirrels. When the time comes to hibernate, the animal responds to one of several environmental “cues,” such as a certain low temperature or a reduction in the hours of daylight. These cues trigger the release of a hormone (a chemical messenger) called hibernation induction trigger (HIT) that causes major changes in the animal’s body. Its heartbeat becomes slow and weak and its body temperature drops many degrees. It takes a few breaths every minute and it makes hardly any waste (urine). As a true hibernator, the animal falls into such a deep sleep that it looks dead and sometimes cannot even be awakened if picked up. All of these reactions triggered by the hormone allow the animal to maintain its necessary body processes while using far less energy than if it were awake. Before they fall asleep for the season, hibernators usually develop huge appetites that allow them to store as much fat as possible to be burned later while sleeping. They also usually prepare the den or burrow where they sleep to comfortably insulate themselves from the cold. As spring approaches, different cues in their environment, like warmer temperatures or lengthening daylight, awaken them and they soon resume their normal level of activity. This does not happen immediately, however. As their heart rate increases, along with their blood pressure and respiration, hibernators usually begin to shiver, which slowly raises their body temperature. After readjusting to this now-high rate of metabolism (the chemical processes that take place in an organism), they are ready for normal activity. Like their warm-blooded counterparts, cold-blooded animals (whose temperature changes with the surroundings) also hibernate. Animals like frogs, turtles, and snakes bury themselves in the mud where their sloweddown systems find just enough trapped oxygen to stay alive. Some insects like butterflies also hibernate in the open, and their systems produce chemicals that act as antifreeze. Other animals like bears, raccoons, and skunks are not true hibernators, although they do very much the same thing. Rather, they are considered “light sleepers” since their bodily functions do not fall to such low rates as those of true hibernators. These “light sleepers” also sometimes wake up and eat something that they may have stored or even go outside to eliminate waste. Brown bears can awaken very quickly, and often give birth during a long, cold winter. Even true hibernators have built-in mechanisms to awaken them under unusual conditions. For ex-

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ample, if temperatures fall to such a low that even the hibernating animal is in danger of freezing to death, its body will automatically switch to actively producing heat.

Homeostasis

Another form of hibernation occurs in the summer. This is called “estivation,” and is a process similar to hibernation. Certain desert animals estivate underground when they are threatened by prolonged, extreme heat or drought. Estivation is what enables many desert dwellers to survive during the very hot summer months. [See also Metabolism]

Homeostasis Homeostasis is the maintenance of stable internal conditions in a living thing. Organisms use a variety of systems and processes that help regulate and maintain a constant environment within their bodies. All organisms use a self-adjusting balance to make sure that what is going on inside their bodies is kept within certain boundaries. One of the main characteristics of living things, or organisms, is that they have the ability to adjust to their environment. In the highly competitive struggle for survival, an organism would be at a great disadvantage if it could not adjust and be ready to cope with a changed situation. Such change can occur inside or outside an organism. Since living things are extremely complex organisms with constant energy demands, there are countless cellular reactions going on all the time. For example, chemicals are combining and breaking apart, fluids are passing in and out of membranes, and substances are being converted from one form to another. All of this constant activity means that the environment inside an organism’s body is dynamic, and that it is always in a state of movement and change.

CLAUDE BERNARD DEVELOPS THE CONCEPT OF HOMEOSTASIS The concept of homeostasis was developed by the French physiologist (a person specializing in the study of life processes, activities, and functions) Claude Bernard (1813–1878). Bernard investigated how the body keeps itself in a stable, or steady state. It was Bernard who first recognized the idea behind homeostasis—that an organism is designed and operates on the principle that it will always attempt to maintain a balance in its systems. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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After Bernard, science eventually discovered that living things use two simple self-adjusting elements, input and output, as regulators. Although these elements are uncomplicated, an organism has many mechanisms and structures that it uses to maintain homeostasis. Some of these mechanisms work automatically and are under the control of the autonomic nervous system. It is this system that regulates the body’s involuntary processes like internal body temperature, blood pressure, and food digestion, among other function. Certain changes in the external environment may automatically trigger an organ to take a certain action. Under certain conditions, our bodies will perspire whether we want them to or not. Other mechanisms are controlled by the body’s endocrine system. This system uses chemical messages, known as hormones, inside the body to regulate functions. A hormone is secreted by the organ that produces it when something happens to the organism that warrants regulation. Hormones then travel through the bloodstream to their target cells, which take the appropriate action.

THE FEEDBACK SYSTEM For this system to really work, however, it must have some way of getting updated information as to what is going on inside and outside the organism. This is achieved by a feedback system that operates something like the thermostat in a house. Every thermostat has a sensor that, when set at a certain temperature, automatically turns the furnace on when the temperature gets lower than its setting, and turns it off when it reaches it. In this way, a built-in feedback “loop” regularly monitors its environment and is able to maintain a constant temperature by turning the furnace on or off. The feedback system in our bodies works mostly with what is called negative feedback. Negative feedback operates by detecting an unwanted change and countering it in order to balance it. A typical example would be the body’s use of feedback mechanisms to raise or lower its internal temperature during extremely cold or hot weather.

LEVELS OF HOMEOSTASIS A diagram showing a negative feedback loop regulating blood pressure. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

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In the human body, homeostasis takes place at many different levels. These include the molecular level (in which atoms are linked together by chemical bonds), the cellular level, the organism level, and the population level. An example of homeostasis at the molecular level would be the body limiting how much of something is produced by a certain chemical reaction. At the cellular level, an example would be how certain cells U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

Control center (brain)

Resistance HR

Effector (heart)

Normal range of blood pressure

BP

Receptor (aorta in heart)

BP

Receptor (aorta in heart) Time Resistance

HR Control center (brain) Effector (heart)


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will stop dividing if they become so numerous that they touch each other. Sensations of hunger and thirst are good examples of a homeostasis mechanism at the organism level. Finally, at the population level, an increase in the number of prey animals usually results in an increase in the number of predators that eat the prey. In this way, the population size of both animals is kept in balance. [See also Endocrine System; Hormones]

Hominid A hominid is a family of primates that includes today’s humans and their extinct direct ancestors. Before humans evolved into what they are today, several human-like species existed, some of which went extinct and some of which evolved into today’s species—the only living species of hominids. Fossil discoveries suggest that the complete story of human evolution is still not fully known, although many of the major hominids species are documented. The word hominid, which includes only human beings and their direct or immediate ancestors, should not be confused with the similar word hominoid. Hominoid includes both humans and apes, and therefore refers to a much larger and more diverse group of primates. All hominids are hominoids, but not all hominoids are hominids. The first hominids are thought to have appeared on Earth about 3,000,000 to 4,000,000 years ago. The earliest known fossils have been discovered in southern and eastern Africa, and all appear to have three features in common: bipedalism or upright walking; an omnivorous diet (plant and animal); and an expansion of the brain. Eventually, over very long periods of time, these and other biological changes occurred and hominids became less and less apelike and more like today’s modern humans. Many think that the first or earliest hominids came down from living in the trees and moved into the open fields or plains. Some think that a major climate shift brought this about since it is known that when hominids first appeared, the savannas were starting to replace forests. This is believed to have forced hominids to make a transition from being forest and tree dwellers to living in a more mixed habitat with woodland and open grasslands. It is important to take note of the actual physical changes that would make hominids different from other primates. One obvious difference were different facial features. Hominids would lose much of their muzzle or protruding jaw, and the overall size of their faces would be reduced, 294


especially their teeth and jaw. Their teeth would also develop thicker enamel and become less specialized. This is an indication that their diet was also less specialized and was probably omnivorous (both plant and meat eaters). Also reduced would be the bony ridges over their eyes, and the back of the skull would lose its crest or raised edge. The brain would also become larger in comparison to the rest of the body. At some point in their transition from trees to plains, hominids became bipedal, meaning that they could walk upright on two feet. This not only made them taller and able to see farther, but left their arms and hands free to carry things, to use tools, or otherwise do things that would further promote their survival. Bipedal walking resulted in significant changes to a hominid’s lower spine, leg bones, and pelvis. The oldest known hominid was found in South Africa and is called Australopithecus ramidus. Dated at about 4,400,000 years ago, this species walked on two legs but had a fairly small brain. There probably were even earlier types of hominids, but no one has yet found fossil remains. Paleoanthropologists (scientists who study the fossil remains of hominids) are not sure if Australopithecus is our direct ancestor or not. The first hominid to be considered human and therefore given the genus name Homo appeared probably about 2,000,000 years ago. Called Homo habilis meaning “handy man,” it had a much larger brain than Australopithecus and is known to have used stone tools. Its skull and teeth were also different, and its face was smaller and more in proportion with the rest of its body. Between 1,500,000 and 500,000 years ago, Homo habilis was either replaced by or evolved into Homo erectus or “upright man.” This is believed to be the first hominid to venture out of Africa and move into Asia and Europe. Significantly, its brain was even larger and it was able to use fire and make hand axes. About 300,000 years ago, the first Homo sapiens neanderthalensis, or Neanderthal man, appeared. Although it had a brain as large as humans are today, its head was still different, as its eye ridges were heavy, probably making it look fierce. Neanderthal man also made tools but unlike Homo habilis buried its dead in special graves.

Hominid

British paleoanthropologist Louis Leakey holding skull fragments of an early hominid he discovered in Africa. (Photograph courtesy of The Library of Congress.)

About 40,000 years ago, humans similar to today’s species first appeared. Called Homo sapiens sapiens (“wise man”), they may have interU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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LOUIS SEYMOUR BAZETT LEAKEY British paleoanthropologist Louis Leakey (1903–1972) was a pioneer in the field of paleoanthropology, which is the study of the fossils of early humans and prehumans. He discovered the earliest known hominid (a family of primates that includes humans and the immediate ancestors of humans) and showed that humans were not only older than previously believed but that they may have first evolved in Africa. Louis Leakey’s parents were missionaries who were trying to convert African natives to Christianity. Leakey was, therefore, born in Kabete, Kenya, which was then part of the British Empire. He was raised among the Kikuyu tribe, a group of Africans who lived in the area where the mission was located. The young Leakey was able to speak the Kikuyu language as well as his own English, and although he had a governess who instructed him, he spent most of his time with other Kikuyu children exploring the countryside. This would remain with him all his life, and it is said that Leakey always thought of himself as an African instead of an Englishman. When Leakey was finally sent to England at age sixteen to begin his formal education, he found he could not get along with the typical English schoolboy with whom he had nothing in common. Although he got along better at Cambridge University, he was forced to take a year out of school when he suffered a head injury when kicked twice in a rugby match. This absence from school enabled him to join a fossil-hunting expedition to Tanganyika (now Tanzania), an experience that showed him what he really wanted to do in life. After Leakey obtained his degree from Cambridge in 1926, he decided to devote his career to studying the origins of humanity, which he believed would be found in Africa. At this time, most scientists believed that Asia and not Africa was the original center of human evolution (the process by which humans changed over generations). Leakey began his work at two African fossil sites, one at Lake Victoria and the other at Olduvai Gorge, now in Tanzania. Olduvai was a 350-mile (217.36 kilometers) ravine that contained a great deal of evidence, like primitive stone tools, that some forms of humans had lived there very long ago.

bred with Neanderthals, or Neanderthals may have simply died out. This newest species began using its brain in ways not seen before, made better tools, began cultivating crops, and created sculptures and cave paintings. They developed language, music, built cities, and eventually created civilizations. All of these and other activities are suggested when we say that humans developed culture. Since today’s particular species burst on 296


During the mid-1930s, Leakey divorced his wife and married one of his students, Mary Douglas Nicol (1913–1996). Together, they would spend more than thirty years at Olduvai searching for the fossil remains of the creatures who had made and used those tools. The Leakeys were very determined scientists and put up with a great deal of hardships at Olduvai. They seldom had enough financial support and the remoteness of the site made their supplies and equipment scarce and difficult to haul. Finally, in 1959 while Leakey himself was in his tent sick with malaria, Mary discovered the fossil they had been looking for. She located the skull fragments of a hominid with a small brain and near-human teeth that they named Zinjanthropus boisei and later renamed Australopithecus boisei. This was the first more or less complete skull of its kind, and it was also the first to be accurately dated. Potassium-argon testing showed that it was about 1,800,000 years old. Although Leakey argued it was probably an evolutionary deadend and not a direct ancestor of modern humans, it nonetheless added considerably to the knowledge of human origins and showed that humans are older than previously thought.

Hominid

The following year, Leakey’s son, Jonathan, discovered the fossil remains of the larger-brained Homo habilis, or “handy man,” which Leakey claimed was the direct ancestor of modern Homo. For this claim, Leakey received a great deal of criticism, and it must be said that he often would overstate his claims and overpublicize himself and his work. Leakey was an ambitious man who recognized the value of publicity in terms of obtaining financial support for his work. Despite his sometimes overblown claims, his significance resides in the fact that he did change the views concerning human development and pushed back the date when humans first appeared to a time much earlier than scientists had originally thought. He also showed that human evolution began in Africa rather than in Asia, which was also an early belief. As recently as 1977, five years after his death, his wife Mary discovered a set of footprints that were dated to about 4,000,000 years ago. After Mary died in 1996, the Leakey’s son, Richard, continued their work.

the scene some 40,000 years ago, too little time has passed for us to notice any real biological changes, and any evolution that humans have made since then has been primarily cultural rather than biological. [See also Fossils; Homo sapiens neanderthalensis; Human Evolution] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Homo sapiens neanderthalensis

An illustration of an early Neanderthal man. Scientists still are not sure whether Neanderthals are ancestors to modern humans or not. (Reproduced by Corbis Corporation (New York).)

Homo sapiens neanderthalensis Homo sapiens neanderthalensis, commonly referred to as Neanderthal man, is a species of the hominid (human) family Homo sapiens that disappeared about 30,000 years ago. As an early member of this species, Neanderthals were shorter and stockier than today’s humans and had differently shaped heads with heavy ridges over the eyes, although their brains were as large as that of modern humans. Neanderthals existed about the same time as modern man emerged, and it is not known whether they were assimilated into the new group by interbreeding or were somehow made extinct by violence or disease. The first fossil finds of Neanderthals were made in Germany in 1856, and simply by studying the heavy-ridged brows of the skulls, it was realized that if these bones were human, they were those of a distant ancestor. Eventually these and other bones were dated to between 70,000 to 35,000 years ago, and their rugged bodies indicated that they had adapted to an existence in a cold climate. For some time, Neanderthals were considered to be a form of prehuman brutes, but the size of their brains was shown to be as big or bigger than modern man’s. When Neanderthal stone tools and weapons were later found that were more advanced than their predecessors, Homo erectus, it was realized that Neanderthals were not as primitive as believed. Neanderthals also demonstrated the beginnings of certain cultural activities that would become a human trademark. One of these was the simple fact that they buried their dead in special graves, suggesting that they had some awareness and sensitivity to the permanent loss of an individual. Neanderthal fossils have also been found in Asia dating from as far back as 125,000 years ago to as recent as 35,000 years, and aside from anatomical differences in their pelvis, shoulder blades, and skull, they did not look that much different from modern humans. Their skulls, however, did show heavy eyebrow ridges and facial bones. This would have made their facial features much less delicate or refined than those of today’s humans. The contents of their skulls, however, were similar, and their brain size ranged between 1,300 and 1,750 cubic centimeters (512.2 to 689.5 cubic inches), much like

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modern man’s. Neanderthals had sophisticated tools and they probably lived in caves and rock shelters. Today, there is disagreement as to whether Neanderthals are part of the gene pool that gave rise to modern humans. A controversial comparison of Neanderthal DNA obtained from fossil bones has not proved conclusively whether Neanderthals were our ancestors or whether they were a dead-end branch on the human tree.

Hormones

Researchers had previously thought that human speech began about 40,000 years ago when Homo sapiens sapiens emerged. However, a recent study argues that for thousands of years prior to this, Neanderthals had the ability to speak. This claim is based on the diameter of a nerve canal that connects the brain and the tongue in Neanderthal skulls. This “hypoglossal” canal is roughly the same size as that in a modern human skull, and implies that Neanderthals may have had the necessary physical equipment for speech. Whether they were modern humans’ direct ancestors, an evolutionary deadend, or a species that succumbed to disease or slaughter, researchers continue to study Neanderthal fossils in order to educate themselves about human evolution. [See also Fossil; Hominid; Human Evolution]

Hormones Hormones are chemical messengers found in both animals and plants. In animals, hormones are produced by glands and travel through the blood to certain target tissues. There they act as chemical regulators. Hormones influence reproduction, growth, and overall bodily balance, among other things. Hormones are important to both plants and animals, but especially to animals. Hormones regulate key bodily functions like body growth, sexual maturity, reproduction, and the maintenance of a stable, or balanced, internal environment. Some hormones have a temporary effect, such as those that regulate the body’s blood sugar level. Others cause permanent changes, such as those that make a person grow tall and mature sexually. Still others are present only in certain situations, such as those that prepare a body for stressful situations. Whatever their effect, hormones help an organism to adapt to its environment in the best manner possible. The word hormone comes from the Greek hormaein meaning to excite or to set into motion, and this describes what hormones do—they have a stimulating effect. In vertebrate animals (animals with a backbone), hormones are produced by certain glands, tissues, or organs. They travel through the cirU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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culatory system (a network that carries blood throughout an animal’s body) to target cells. Hormones do not produce an effect until they reach these specifically receptive cells. The target cells are programmed to react when stimulated by a certain hormone. Only the target cells in the target organ are able to produce the desired effect, since they have receptors that recognize and bind to the hormone.

THE ENDOCRINE SYSTEM It is estimated that vertebrates have at least fifty different hormones, and many are produced by what is called the endocrine system. Some of the major glands in the human endocrine system are the pituitary gland and the pineal gland at the base of the brain; the thyroid and parathyroid in the throat; and the adrenal, gonads, thymus, and pancreas in the trunk or lower half of the body. Each of these endocrine glands releases its own particular hormone into the bloodstream and each produces the desired effect when it reaches the appropriate target cells. Thus, some hormones

A beef insulin hormone. Most hormones fall into two main categories called peptides and lipids. (Reproduced by permission of Phototake.)

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stimulate the growth of muscle and bone, others begin the secretion of milk, and still others influence blood pressure. All are usually produced in tiny quantities, yet all have profound and major effects on the body.

Hormones

HORMONES IN HUMANS For example, female sex hormones, like the group called estrogens, are produced by the pituitary gland as well as the ovaries. These hormones produce the dramatic physical changes that take place when a young girl starts to become a young woman. The estrogens trigger the development of what are called secondary sexual characteristics, like breasts. Later, if a woman becomes pregnant, these and other hormones prepare her body to carry and nourish a developing fetus. The male sex hormone, testosterone, is made by the testes and produces the typical male secondary sex characteristics during puberty. Another well-known hormone is adrenaline. This produces what is known as the “fight-or-flight” response. When an individual senses danger, the body automatically enters a state of readiness to either fight for survival or to take measures to avoid a conflict. This powerful hormone works with great suddenness and a person can actually feel its effects. The heart rate and blood pressure quicken, the skin goes pale, the body’s blood sugar level rises, and a person’s strength increases. All of these and other physiological reactions take place immediately and without the person’s conscious will. These reactions give people a better chance to act immediately and possibly survive a threat. In humans and animals, hormones are made of either proteins or steroids (which are a type of lipid or organic compound that includes fats, oils, and waxes). The adrenal gland and the gonads (male testes and female ovaries) produce steroid hormones, while the rest of the endocrine system makes hormones that are protein-based and, therefore, are made out of amino acids (the building blocks of proteins). Abnormally functioning endocrine glands can result in too much or too little hormones. For example, a lack of human growth hormone from the pituitary gland can result in dwarfism, while too much can produce giants who suffer from a condition known as acromegaly.

HORMONES IN OTHER ORGANISMS While other living things also have hormones, their hormones are nowhere near as dominating an influence as they are for animals. Other organisms do not have as elaborate a system of transport and reception as do vertebrates. However, hormones are still very important to invertebrates (animals without a backbone). Invertebrates use hormones mainly U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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ERNEST HENRY STARLING English physiologist Ernest Starling (1866–1927) helped create endocrinology, a major branch of medicine and physiology that studies the glands of the body. Starling not only discovered the digestive hormone secretin, but he also suggested the name “hormone” to describe the body’s chemical messengers. Ernest Starling was born in London, England. His father worked in Bombay, India, for the British government and was able to come home only once every three years. The young Starling was educated at King’s College School in London and then enrolled at Guy’s Hospital Medical College in London. While there, he took advantage of the opportunity to do research in Heidelberg, Germany, and study with the eminent German physiologist, Wilhelm Kuhne (1837–1900). After obtaining his medical degree in 1889, he was appointed demonstrator of physiology at Guy’s, and he eventually became head of the department of physiology there. By the time he left there in 1899 to become Professor of Physiology at University College, he had made a name for himself studying the conditions that cause fluids to leave blood vessels and enter the tissues. In fact, in 1896 he demonstrated a phenomenon that came to be known as “Starling equilibrium.” Starling is best known, however, for his work with the English physiologist William Maddock Bayliss (1860–1924), who became his brother-in-law in 1893 when he married Starling’s sister. Together they began a study of the secretion of digestive juices by the pancreas (a gland). The normal pancreas

in their growth and development. For example, insects that molt, or periodically shed, their skin produce a hormone that allows this to happen at the right time. Metamorphosis (the complete bodily change that takes place in an insect, such as when a caterpillar changes into a butterfly) is controlled by hormones. When an octopus changes its color during stress, it is a hormone that causes this dramatic reaction. For plants, hormones allow them to react to the changing conditions of their environment. Some hormones promote cell division, others stimulate or slow growth, and others cause a plant’s fruit to ripen. Plants do not have specialized structures for hormones as animals do. In fact, a plant can even be affected by the hormone of its neighbor. This sometimes occurs when a plant releases the ripening hormone called ethylene into the atmosphere. Thus, a fruit like an apple continues to produce this ripening hormone even after it is picked, and will therefore speed up the ripening of any other fruit nearby. 302


releases several different juices into the duodenum (the top part of the small intestine) to assist digestion. After they had cut all the nerves to the pancreas, they found that the organ continued to release its juices. This proved that it was not under nervous control (that is, not controlled directly by the brain), and so they concluded in 1902 that it must have received a chemical rather than an electrical message. This meant that the message must have been sent to the pancreas through the blood when food entered the duodenum.

Horticulture

They soon found that the small intestine secretes a substance, or a chemical messenger, into the blood that they named “secretin.” Further research showed that the secretin was released under the influence of stomach acid. This was the first time that a certain chemical had been proven to act as a stimulus for an organ that was located somewhere else in the body. Starling and Bayliss eventually came to call any chemical that transmits a message from one part of the body to another part a “hormone.” This word was taken from the Greek root meaning “to excite.” Although hormones had actually been known before the discovery of secretin in 1902, it was Starling who first clearly defined the concept in 1905 and who detailed the role that such substances play in the body. It was thought that Starling and Bayliss were strong candidates for a Nobel Prize, but World War I (1914–18) intervened, and no awards were given for those years. As for recognition from his own country, Starling had been such an outspoken critic of the way his country had managed the war effort that he was given no awards in his lifetime.

[See also Endocrine System; Reproduction System]

Horticulture Horticulture is a branch of agriculture that deals with fruits, vegetables, and ornamental plants. It includes the production of fruits and vegetables for food, and the use of plants in landscaping and decorations. The word horticulture comes from the Latin words hortus meaning “garden” and colere meaning to cultivate, and was first used in England in 1678. The word hortus or garden is an important part of the idea of horticulture, since the concept of the garden as being different from the open field dates back to the Middle Ages (500–1450). During this era, there were three types of areas where things grew. First were the large, open fields where farmers raised mostly grain and fiber crops. Next came U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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the garden or hortus which meant a much smaller space that was intensively cultivated with plants used mainly in the kitchen. Finally there was the forest where timber and wild game were found. Today, horticulture includes the art and science of gardening, and is closest to the second of these categories. However, modern horticulture has gone beyond the tiny kitchen garden and has become an entire industry. It is from this industry that people obtain the fresh fruits and vegetables that they eat, the flowers they use to beautify our environment, and the trees and shrubs they use to decorate the outside of their buildings. Although the notion of intensive gardening in a fairly small space distinguishes horticulture from agriculture (which is large-scale), the boundary between the two becomes less clear with an activity like commercial vegetable production. Modern horticulture is usually divided into two large categories: food crops (olericulture and pomology) and ornamentals (floriculture and ornamental horticulture). Olericulture deals with vegetables grown for food, and pomology deals with fruit and nut crops. Floriculture is concerned with the production of flowers and potted plants, while ornamental horticulture deals with the use of trees, bushes, shrubs, and grass in outdoor landscaping. However, no matter which aspect of horticulture is being practiced, the gardener or grower must be familiar with all the factors that may increase or decrease a plant’s growth and development. While the growers need not be botanists (people who specialize in the study of plants), a great deal of serious horticultural research goes on at colleges and universities, agricultural experiment stations, and botanical gardens from which growers benefit.

Human Evolution Human evolution is the process by which the modern species of humans was formed and developed. Although the major stages of human evolution are known, there are large gaps in our knowledge. It is now widely accepted that apes and humans evolved from the same ancestor. Many scientists believe that the earliest modern humans evolved first in Africa and then spread throughout the world. Today’s human beings, or Homo sapiens sapiens, belong to the hominid family tree. Hominid means “human types,” and describes early creatures that split off from the apes and took to walking upright, or on their hind legs. In the overall history of life on Earth, the human species is a very recent product of evolution (the process by which living things change over generations). Since there are no human-like fossils older than 304


4,000,000 years, this makes us only one-thousandth the age of life on Earth. Today, most scientists agree that humans and apes evolved from the same ancestor. Where scientists cannot agree is how long ago this happened, where it happened, and how it happened. However, it has been demonstrated that the difference between the human genetic (heredity) code and that of a chimpanzee is only 0.7 percent.

Human Evolution

It is not difficult to pinpoint what separates today’s humans from the apes (gorillas, chimpanzees, orangutans, and gibbons). Some of these distinctions are more significant than mere physical difference. Humans have bigger brains and are intellectually superior. They have used this brainpower to develop tools, which have helped humans adapt to nearly any environment in which we choose to live. Humans are bipedal, meaning that they are able to walk on two feet, leaving their arms and hands free to make tools and do things. Finally, humans have developed language, a very sophisticated form of communication. But how did they separate from the apes and begin our own distinct evolutionary path toward what humans are today?

FOSSIL EVIDENCE It is known humans evolved from the same ancestor as the apes. However, genetic evidence points to apes and humans going in different paths on the African continent between 10,000,000 and 6,000,000 years ago. Scientists have been able to assemble an impressive amount of fossil evidence that documents some of the branches on the hominid family tree.

Australopithecus afarensis. The oldest hominid scientists know about is called Australopithecus afarensis. It lived about 3,000,000 to 4,000,000 years ago and stood about 3.5 to 4 feet (1.07 to 1.22 meters) tall. It had a brain the size of a chimpanzee but it was clearly built for upright walking.

Homo habilis. Many scientists believe that the next known stage, Homo habilis who appeared about 2,000,000 years ago, came from Australopithecus. As the first hominid to be given the genus Homo, or “man,” it was taller and used tools made of stone. Its name therefore means “capable man” or “handy man.”

Homo erectus. By 1,500,000 years ago, it is known that a new, taller human species appeared and possessed a brain that was about half the size of humans today. Called Homo erectus, or “upright man,” it was the first hominid to use fire and hand axes and to substantially move about. In fact, it is thought that this species left Africa and gradually migrated into Asia and parts of Europe. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Homo sapiens. Homo sapiens, or “wise man,” are thought to have appeared about 400,000 years ago. With a still-larger brain and more human features like smaller teeth and a clearly defined chin, it used more sophisticated tools and buried its dead. Neanderthals and Cro-Magnons. One form of Homo sapiens, known as Neanderthals, eventually died out. Some argue that Neanderthals evolved into the Cro-Magnons. Others say that the Cro-Magnons were a separate species and simply out-competed or killed the Neanderthals. In 1997, genetic studies of a Neanderthal bone revealed no evidence of a genetic connection to modern man, suggesting that they may have been a separate and distinct species that went extinct. Nonetheless, it was the Cro-Magnons who survived and continued to reproduce. They were humans who mostly looked like us, although with possibly broader faces, and they had moved into Europe. Their population increased rapidly and they began to develop a culture of some kind. Homo sapiens sapiens. Finally, the emergence of fully modern humans, now called Homo sapiens sapiens, or doubly “wise” man, came about some 40,000 to 15,000 years ago. This creature was physically identical to today’s humans and had real language. Humans moved from being strictly hunters and gatherers to domesticating animals and plants and creating fine artwork. Soon, their settlements had turned into real cities, and a civilization was created based on agriculture (farming). Recent studies of fossil fragments found in a cave in Israel suggest that modern humans (Homo sapiens sapiens) may have existed as far back as 92,000 years ago. Despite more information regularly coming to light, the human race still has not solved the complete mystery of human evolution at the beginning of the twenty-first century. [See also Evolution; Fossil; Genetics; Hominid; Homo sapiens neanderthalensis; Primates]

Human Genome Project The Human Genome Project is the scientific research effort to construct a complete map of all of the genes carried in human chromosomes. The finished blueprint of human genetic information will serve as a basic reference for research in human biology and will provide insights into the genetic basis of human disease. The human “genome” is the word used to describe the complete collection of genes found in a single set of human chromosomes. It was in 306


the early 1980s that medical and technical advances first suggested to biologists that a project was possible that would locate, identify, and find out what each of the 100,000 or so genes that make up the human body actually do. After investigations by two United States government agencies—the Department of Energy and the National Institutes of Health— the U. S. Congress voted to support a fifteen-year project, and on October 1, 1990, the Human Genome Project officially began. It was to be coordinated with the existing related programs in several other countries. The project’s official goals are to identify each of the more than 100,000 genes in human deoxyribonucleic acid (DNA) and to determine the sequences of the 3,000,000,000 base pairs that make up human DNA. The project will also store this information in databases, develop tools for data analysis, and address any ethical, legal, and social issues that may arise. In order to understand how mammoth an undertaking this ambitious project is, it is necessary to know how genetic instructions are carried on the human chromosome. Humans have forty-six chromosomes, which are coiled structures in the nucleus of a cell that carry DNA. DNA is the genetic material that contains the code for all living things, and it consists of two long chains or strands joined together by chemicals called bases, or nucleotides, all of which is coiled together into a twisted-ladder shape called a double helix. The bases are considered to be the “rungs” of the twisted ladder. These rungs are made up of only four different types of submolecules called nucleotides—adenine (A), thymine (T), guanine (G), and cytosine (C)—and are critical to understanding how nature stores and uses a genetic code. The four bases always form a “rung” in pairs, and they always pair up the same way. Scientists know that A always pairs with T, and G always pairs with C. Therefore, each DNA base is like a letter of the alphabet, and a sequence, or chain of, bases can be thought of as forming a certain message.


The Human Genome Project has made great strides under the leadership of Francis Collins. In June 2000, researchers announced that they had completed a rough draft of the human genome. (Reproduced by permission of AP/Wide World Photos, Inc.)

The human genome, which is the entire collection of genes found in a single set of chromosomes (or all the DNA in an organism), consists of 3,000,000,000 nucleotide pairs or bases. To get some idea about how much information is packed into a very tiny space, a single large gene may consist of tens of thousands of nucleotides or bases, and a single chromosome may contain as U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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FRANCIS SELLERS COLLINS American geneticist Francis Collins (1950– ) became the director of the National Human Genome Research Institute in 1993. The goal of this international scientific research effort is to construct a complete map of all of the genes (the units of heredity) carried on human chromosomes (a coiled structure in the nucleus of a cell that carries the cell’s deoxyribonucleic acid). This finished blueprint of human genetic information will then serve as a basic reference for research in human biology and will provide insights into the genetic basis of human disease. Collins’s own research is in the identification and understanding of genes that cause disease. Francis Collins was raised on a small farm in Staunton, Virginia, where he was home-schooled until the sixth grade. After obtaining his bachelor’s degree from the University of Virginia in 1970, he went on to obtain a doctorate in physical chemistry from Yale University in 1974. At this point, he became so intrigued by what he saw as the beginnings of a revolution in molecular biology (the study of the complex chemical molecules in all living things), that he switched fields and went back to school. After enrolling in medical school at the University of North Carolina, he began to study medical genetics and knew he was finally doing what he really wanted. After obtaining his medical degree in 1977, he returned to Yale for a fellowship in human genetics. It was there that he began working on the problem of trying to identify genes in human deoxyribonucleic acid (DNA) that cause disease. He continued studying this problem after joining the fac-

many as 100,000,000 nucleotide base pairs and 4,000 genes. What is most important about these pairs of bases is the particular order of the A’s, T’s, G’s, and C’s. Their order dictates whether an organism is a human being, a bumblebee, or an apple. Another way of looking at the size of the human genome present in each of our cells is to consider the following phone book analogy. If the DNA sequence of the human genome were compiled in books, 200 volumes the size of the Manhattan telephone book (1,000 pages) would be needed to hold it all. This would take 9.5 years to read aloud without stopping. In actuality, since the human genome is 3,000,000,000 base pairs long, it will take 3 gigabytes of computer data storage to hold it all. In light of the project’s main goal—to map the location of all the genes on every chromosome and to determine the exact sequence of nucleotides of the entire genome—two types of maps are being made. One of these is a physical map that measures the distance between two genes in terms of nucleotides. A very detailed physical map is needed before 308


ulty at the University of Michigan in 1984 where he developed an approach that is now called positional cloning.


Collins’s approach was a major breakthrough in human genetic research as it allowed him to identify “disease genes” for almost any condition without knowing ahead of time what the particular abnormality might be. This approach has been adopted as a standard method of modern molecular genetics, and Collins demonstrated its usefulness in 1989 when his research team identified the gene for cystic fibrosis. The following year it did the same for the disease known as neurofibromatosis. This inherited disease causes tumors to form and destroys bones. In 1993 the gene for Huntington’s disease was also located. That same year, Collins succeeded American biochemist James Dewey Watson (1928– ), and became the second director of the National Center for Human Genome Research at the National Institutes of Health (NIH), heading the American part of an international project that involves at least eighteen countries. Collins also founded a new research program in genome research at NIH, and his own laboratory continued to focus on the identification and understanding of the genes that cause disease. Scheduled to be completed in 2003, the Human Genome Project is regarded by many as the most important scientific undertaking of our time, and Collins is bringing it to conclusion ahead of schedule and under budget. By the year 2000, the project had already compiled what might be called a rough draft of the human genome, having put together a sequence of about 90 percent of the total.

real sequencing can be done. Sequencing is the precise order of the nucleotides. The other map type is called a genetic linkage map and it measures the distance between two genes in terms of how frequently the genes are inherited together. This is important since the closer genes are to each other on a chromosome, the more likely they are to be inherited together. As an international project involving at least eighteen countries, the Human Genome Project was able to make unexpected progress in its early years, and it revised its schedule in 1993 and again in 1998. Completion is expected in 2003, coinciding with the fiftieth anniversary of Watson and Crick’s description of the molecular structure of DNA. During December 1999, an international team announced it had achieved a scientific milestone by compiling the entire code of an complete human chromosome for the first time. Another achievement was made in June 2000, when researchers announced that they had completed what might be called a rough draft of the human genome, having put together a sequence of about 90 percent of the total. Researchers chose chromosome twenty-two U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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because of its relatively small size and its link to many major diseases. The sequence they compiled is over 23,000,000 letters in length and is the longest continuous stretch of DNA ever deciphered and assembled. What was described as the “text” of one chapter of the twenty-three-volume human genetic instruction book has been completed. Francis Collins, Director of the National Human Genome Research Institute of the National Institutes of Health said, “To see the entire sequence of a human chromosome for the first time is like seeing an ocean liner emerge out of the fog, when all you’ve ever seen before were rowboats.” Another major advance on the project was made in June 2000 when researchers announced that they had completed a rough draft of the fully mapped human genome. With this fully mapped genome, biologists can for the first time stand back and look at each chromosome as well as the entire human blueprint. They will start to understand how a chromosome is organized, where the genes are located, how they express themselves, how they are duplicated and inherited, and how disease-causing mutations occur. This could lead to the development of new therapies for diseases thought to be incurable as well as to new ways of manipulating DNA. It also could lead to testing people for “undesirable” genes. However, such a statement leads to all sorts of potential dangers involving ethical and legal matters. Fortunately, such issues have been considered from the beginning, and part of the project’s goal is to address these difficult issues of privacy and responsibility, and to use the completely mapped and fully sequenced genome to everyone’s benefit. [See also Chromosomes; Genes]

Human Reproduction Human reproduction is essential for the continuance of the human species. Humans reproduce sexually by the uniting of the female and male sex cells. Although the reproductive systems of the male and female are different, they are structured to function together to achieve internal fertilization. It is a characteristic of all living things on Earth that they reproduce or produce offspring, and humans are no different. If humans are examined as large, complex land mammals, then we can say from a strictly biological point of view that the male and female have the same role as other mammals. The male’s job is to produce sperm cells and deliver them into the female reproductive tract. The female’s job is to produce ova 310


(eggs), receive the sperm, and nourish the embryo that grows inside her. She must also give birth and produce milk for the offspring during its early years.

Human Reproduction

Unlike other land mammals, humans are not simply physical creatures who must follow the course nature has planned for them. People have intelligence and the ability to do what they want. People have invented culture and civilization, and have made up rules and values. A good example of how different humans are from all other living things is reproduction itself. Humans are the only ones who can choose not to reproduce, for whatever reason. So, despite the millions of years of evolution and adaptation that thousands of generations went through to produce a certain human being, that person can decide not to reproduce with another human and therefore not to pass on its genetic inheritance. Although humans are greatly influenced by their biological makeup, they are not compelled by it they way other animals are.

MALE REPRODUCTIVE SYSTEM Among humans who do choose to reproduce (and this is by far the greater part of the human species) reproduction is basically a biological process. As mammals, humans practice internal fertilization, which means that the sperm and egg come together inside the female’s body. In order for this to happen, both male and female need a set of organs and systems that work together. The male reproductive system produces, stores, and releases its gametes, or sex cells, known as spermatozoa. Sperm are produced in the testes, two oval-shaped organs contained in the scrotum, which is a like a pouch of skin. Located outside the body, it keeps sperm a few degrees lower than 98.6°F (37°C), since normal body temperature would kill most sperm. The testes are made of tightly coiled tubes in which sperm cells are formed. They are stored internally in a liquid called seminal fluid that keeps them nourished. The penis is the external part of the male reproductive system and contains a central channel called the urethra. Sperm flow out of the urethra at the proper time. As the specialized organ through which sperm is introduced into the female reproductive tract, the penis is made of spongy tissue that lengthens and stiffens when excited. At this point, it is ready and able to be inserted into the female’s vagina.

FEMALE REPRODUCTIVE SYSTEM The female gametes, or sex cells, are known as eggs, or ova, and are produced and stored in the ovaries. An egg is 75,000 times larger than an individual sperm cell. Females are born with about 40,000 immature eggs U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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and do not produce any more during their lifetime. In most females, only about 400 of these eggs actually mature. From the onset of puberty until sometime in their forties, females release one mature ovum approximately every month. This monthly release of an egg that is ready to be fertilized is part of the female’s menstrual cycle. The term menstrual comes from the Latin word mensis, which means “month.” Therefore, every twenty-eight days an egg matures and is positioned to meet with a sperm cell in the Fallopian tubes. These two, 3-inch (7.6-centimeter) tubes connect the ovaries with the uterus. It is in the Fallopian tubes that fertilization takes place.

FERTILIZATION An ultrasound of a nine-month-old human fetus. A woman’s reproductive system is designed to be able to carry and protect an unborn child. (Reproduced by permission of Photo Researchers, Inc. Photograph by Matt Meadows.)

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The uterus is a muscular structure that houses the developing egg if it is fertilized. The vagina is the muscular tube leading from the uterus to the outside of the body and it is the entrance or canal through which the male deposits his sperm. The act of human sexual reproduction is called sexual intercourse or coitus, and for it to work properly, both partners must usually be excited or experience what is called sexual arousal. This sexual stimulation results in body changes, such as the male’s erect penis and the female’s lubricated vagina. Nature has arranged it so that sexual reproduction is pleasurable or feels good to most organisms, and for humans, the inward and outward movement of the penis in the vagina


causes friction that leads the male to an orgasm. In the male, orgasm causes very strong, involuntary contractions of muscles at the base of the penis. These contractions forcefully expel semen, which contains sperm, from the penis. This release of sperm is called ejaculation. The female may or may not experience a similarly intense feeling, but even if she does not, she can still become pregnant.

Hybrid

Hundreds of millions of sperm cells are released during an ejaculation, and they swim through the uterus and into the Fallopian tubes. Many sperm attach themselves to the egg but only one actually enters it. Once a sperm enters the egg, the egg prevents any others from doing so. If fertilization occurs—and for many reasons it often does not—the zygote (fertilized egg) begins to divide and grow. It will then implant itself in the uterus where it will be nourished and grow into a baby. If the egg was not fertilized, it is eventually discharged out of the vagina with other uterine tissues and blood. This is sometimes called a woman’s “period.” A fertilized egg that successfully attaches to the uterus will take about 270 days to grow into a fully developed fetus or baby. When it is ready to be born, the baby’s adrenal glands secrete a hormone that signals the mother’s pituitary gland to secrete a hormone called oxytocin. This causes the uterine muscles to contract rhythmically, and eventually the baby is born, or expelled, from the uterus.

Hybrid A hybrid is the offspring produced by organisms of two different varieties or species. Hybridization occurs often in nature between different varieties of the same species, but much less often between related or different species. The product of such a cross is usually unable to reproduce itself. A hybrid is basically the product of two different organisms. In the plant kingdom, hybrids occur all the time both by natural pollination and when humans deliberately cross different types. Wheat is a hybrid that came about naturally, although the types grown now are hybridized even further by farmers so they will resist certain diseases and produce higher yields. Probably the most famous hybridizer was the Austrian monk and botanist (a person specializing in the study of plants) Gregor Johann Mendel (1822–1884), whose famous experiments with garden peas led to his discovery of the laws of heredity. Mendel spent years deliberately crossing different varieties within a species to produce other new varieties. Mendel started by crossing plants that bred different true traits (such as all tall or all dwarf), and produced hybrid plants whose varied offspring U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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eventually led him to discover the phenomena he called dominant and recessive traits. Today, different types of plants are crossbred by farmers to produce a particular combination of desirable features. In the animal world, a hybrid is more of an exception. Nature seems to work against members of different species trying to mate and reproduce, and if fertilization does somehow occur, the result is usually not able to survive much past birth. Very often, the fertilized egg does not develop properly and it dies. In instances where the two animals are members of different but closely related species, the offspring is born but it is usually sterile or unable to reproduce. The best-known animal hybrid is probably the mule. In this case, two closely related species, the horse and the donkey, are able to mate and produce an offspring. However, because the horse has sixty-two chromosomes and the donkey has sixty-four, the hybrid mule is born with sixty-three. It is therefore unable to successfully fertilize another animal because its odd-numbered chromosomes are unable to pair up correctly during meiosis (the special form of cell division that produces sex cells). A mule is a cross between a female horse and a male donkey. A “hinny” (also sterile) is the result of a male horse mating with a female donkey. While both mules and hinnies can be healthy, vigorous animals, they are nonetheless unable to reproduce because of what is called hybrid infertility. As a result, self-sustaining mule or hinny population could never develop since both must necessarily be produced by repeating the original crossbreeding. [See also Fertilization; Genetics]

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I Ichthyology Ichthyology is the branch of zoology (the study of animals) that deals with fish. It includes the study of the development, anatomy (structure), physiology (function), behavior, classification, genetics, and ecology of fish, among other things. Since fish are a major food source for people, the study of ichthyology also has economic importance. Taken from the Greek word ichthys for fish, ichthyology had its beginning with the Greek philosopher Aristotle (384–32 B.C.), since the ancient world was more interested in and more knowledgeable about fish than they were about many other animals. This may have been because fish were both a relatively easy-to-obtain source of food as well as an animal group that was readily accessible, since fishing is one of humankind’s oldest occupations. Until the end of the nineteenth century, however, more attention was paid to describing and classifying fish than any other aspect. By then, ichthyology was well on its way to becoming a separate field of zoology. By the end of the first half of the twentieth century, the emergence of oceanography (the science of the ocean) and the newfound ability to conduct underwater observations, allowed scientists to be able to study fish in their natural environment for the first time. The development of improved techniques for keeping fish in tanks for study also spurred further advances. There are more than 22,000 known species of fish in the world, and they live in nearly every imaginable body of water, from stagnant ponds to the deepest oceans. They live in water all of the time and breathe through gills. Together with mammals, birds, reptiles, and amphibians, fish are one of the major groups of vertebrates (animals with a backbone). U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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They are considered the most successful vertebrate group, outnumbering birds two to one and mammals seven to one. An ichthyologist, therefore, must contend with a great variety of subjects, from the bony, snakelike eel and the shark with all cartilage and no bone, to the bioluminescent deep-sea fish that can make its own light. Today, fish are not studied just for their own sake or to simply learn more about them. Since fish are a major food source and fishing is an important industry, a great deal of fishery research is conducted in government laboratories as well as institutional aquariums. It is not surprising that much of this work is aimed at learning more about diseases in fish as well as understanding the effect that pollution has on them. Fishes are as vulnerable to infections by viruses as are higher vertebrates, and often they are the first to show signs of disease. They are also susceptible to tumors, and sick fish are a signal that they live in an environmentally degraded body of water. Increasingly, ichthyologists must know as much about the environment of a certain type of fish as they do about the fish itself in order to note any irregularities in the environment—and thus in the fish. [See also Fish]

Immune system The immune system is the body’s biological defense mechanism and protects it against foreign invaders, such as bacteria and viruses. The system is a collection of cells and tissues in the body that protect it against disease-causing organisms. It works by using a simple system in which it distinguishes self (acceptable) from nonself (nonacceptable), and then it attacks and attempts to destroy anything nonself. Nearly all animals, simple and complex, have an immune system based on this self/nonself mechanism.

NATURAL IMMUNITY The immune system has two different types of defense. The first is called natural immunity and is composed of the basic physical and chemical barriers that every body has at its disposal to fight a foreign invasion. The body’s skin is its first line of natural defense since healthy, unbroken skin acts as a physical barrier against microorganisms. If, however, these tiny organisms try to get into the body through normal openings, like the nose and eyes, the body is prepared. These passages are lined with sticky mucus that catches the microorganisms, and with hairlike cilia that sweep them back out of the body. The body also uses secretions, like 316


tears and saliva to protect itself. These secretions contain an enzyme called lysozyme that breaks down the walls of invading bacteria. If after these three defenses, microorganisms still manage to get into the body, the blood contains certain types of white cells called phagocytes that literally swallow up and destroy foreign cells or substances. The body then activates its complement system, which releases proteins that cause an inflammatory response. With this response, the body releases a fluid called histamine that helps fight the invader and results in local swelling.

Immune System

ACQUIRED IMMUNITY Sometimes the phagocytes, which make a general attack and attempt to destroy anything detected as foreign, cannot cope with the invader. When this happens, the body’s acquired immunity goes into action. If natural immunity is the body’s “nonspecific defense,” meaning it will attack anything detected as foreign, then acquired immunity is its “specific defense.” Acquired immunity allows the body to “remember” and link past infections to a particular bacteria or virus. The body is then able to respond more quickly the next time it encounters the invader (now called an antigen). Only vertebrates (animals with a backbone) have acquired immunity. Acquired immunity enables the immune system to produce certain types of white cells called antibodies to fight a particular type of pathogen (disease-producing organism). It also enables the immune system to “remember” that pathogen and to respond more quickly the next time it appears. The body produces three types of white blood cells, macrophages, T lymphocytes, and B lymphocytes, that work together and carry out a complex series of events known as the immune response. (Macrophages alert the immune system that specific foreign agents are present.) The primary immune response involves the B-cell lymphocytes producing antibodies that capture and kill the invading antigen. However, when a virus invades a cell, the virus is safe from antibodies, so the T-cell lymphocytes begin what is called cell-mediated immunity. The T-cell is able to recognize any infected cell, and when it does, it kills the cell. Lymphocytes are transported throughout the body via the lymphatic system, a type of secondary circulatory system that acts as a bridge to the immune system.

EDWARD JENNER DEVELOPS IMMUNIZATION The natural ability of the immune system to be able to recognize a particular antigen is the basis for immunization. Since ancient times, medical observers had noticed that the body seemed to have powers to protect itself and resist disease. In particular, people who had survived a certain infectious disease did not suffer from that disease again during their U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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PAUL EHRLICH German bacteriologist (a person specializing in the study of bacteria) Paul Ehrlich (1854–1915) is the founder of chemotherapy, which is the use of a chemical substance to treat a disease. He also identified substances that could be used as drugs to destroy bacteria in the body, and made important contributions to the understanding of immunity (the body’s natural resistance to a foreign substance). Paul Ehrlich was born in Strehlin, Silesia (then part of Germany, now part of Poland). His family was educated and well-off, and although young Ehrlich did not do well in school at first, he came to be very interested in both chemistry and biology. He attended German universities and received his medical degree in 1878. Throughout his medical education, Ehrlich was always interested in its chemical aspects, and he became especially interested in the new dyes that were being introduced. Ehrlich was particularly fascinated by the staining (dyeing) of cells and tissues and their reactions to dyes. For his graduate thesis he discovered several practical stains for bacteria and even wrote his thesis on that subject. After working with the famous German bacteriologist Robert Koch (1843–1910) studying tuberculosis, he was appointed a professor at the University of Berlin in 1890. There he began work with others on the study of immunity, or the body’s own defense against disease. The group he joined was trying to find a cure for diphtheria, a childhood respiratory disease that killed many. Ehrlich was searching for a substance that would give immunity against diphthe-

lifetime. In 1796, the English physician Edward Jenner (1749–1823) discovered that it was possible to make people immune to a disease they never had. First, he gave a person an injection of a dead or weakened microorganism (called a vaccine) that caused a certain disease (like cowpox). The vaccine was not strong enough to give the person cowpox, but still the patient’s body would react by producing antibodies against the disease. Jenner found that immunization protected his patients from the dreaded smallpox disease. Eventually, successful methods of immunization were developed against such diseases as diphtheria, whooping cough, mumps, measles, rubella, polio, rabies, anthrax, typhoid fever, typhus, yellow fever, cholera, and the plague.

HIV AND AIDS The last few decades of the twentieth century witnessed not a new disease to fight, but the emergence of a disorder of the human immune 318


ria by using antitoxins. Antitoxins are antibodies produced by the body’s immune system to fight poisons invading the body. An antibody is a special protein in the blood that locks on to a specific foreign substance and kills it. By 1892, Ehrlich had worked out an antitoxin for diphtheria that could be used medically. He obtained the right antitoxin from large animals that had been immunized against diphtheria. He then concentrated and purified it and administered it to 220 children with success. For his work on immunity, Ehrlich later won the 1908 Nobel Prize for Physiology and Medicine. After this achievement, Ehrlich returned to studying dyes and stains, and decided to pursue a fascinating idea. He knew that stains were useful because they colored some cells but not others, thereby making the stained ones stand out. He also knew that a stain would not color a bacterium (plural, bacteria) unless it combined with something in the bacterium. Knowing also that when this happened the bacterium usually died, he theorized that if he could find a dye that stained bacteria but not ordinary cells, then maybe it was also a chemical that killed bacteria without harming the host (the human being). He described such a chemical as a “magic bullet,” saying that it would seek and destroy only the invader. Eventually, he did discover one dye, called trypan red, that worked against such diseases as sleeping sickness. Much later, he discovered a dye he named Salvarsan that would kill the microorganism that caused syphilis, a sexually transmitted disease. These two chemicals marked the beginning of modern chemotherapy. Ehrlich proved to be a pioneer not only in the field of immunology, but in the newer field of chemotherapy as well.

Immune System

system itself called AIDS (Acquired Immune Deficiency Syndrome). Infection by this new Human Immunodeficiency Virus (HIV) caused the immune system to collapse, leaving the body defenseless. Specifically, the HIV virus attacked certain T-cells and made them unable to do their job helping B-cells make antibodies. The result was that once a person’s natural immune system shut down, they became host to a number of devastating infectious organisms. AIDS is not a single disease but a syndrome of symptoms that are caused by infectious invaders taking advantage of an immune system that cannot function. HIV can remain dormant in the body for some time without producing any signs. There is still no cure for AIDS, although great progress has been made in coping and managing this disease and prolonging the lives of its victims. Also in the last few decades, biologists have discovered that the immune system can be affected by a person’s psychological health or state of mind. Apparently there exists a complex network of nerves, hormones, and brain chemicals that link the immune system to a person’s mental U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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state, and it has been demonstrated that extreme psychological stress can suppress the immune system and accelerate certain diseases. Recent immunological research indicates that the mind/body connection is more significant than previously thought. [See also AIDS; Antibody and Antigen; Immunization; Lymphatic System; Vaccine]

Immunization Immunization is a method of helping the body’s natural immune system be able to resist a particular disease. It is usually carried out by giving someone a mild version of the disease. This allows the body to make antibodies that will resist the disease in the future. Active immunization, or vaccination, has proven to be a highly successful method of disease prevention. Long before modern science discovered the causes of disease, it was folk practice in some parts of the world to give a powder made from the scabs of recovering smallpox patients to healthy children in the belief that it would somehow protect them in the future. If this risky custom, which originated in China, did not kill the child, it often did grant him or her immunity against a full-blown case of smallpox (a highly infectious viral disease). This same idea was at work when the English physician Edward Jenner (1749–1823) decided to try a dangerous experiment. He based his experiment on the fact that people who had suffered a case of the less serious cowpox (a contagious skin disease found in cattle) often did not catch the deadly smallpox. In 1796 Jenner prepared what he called a vaccine (because the cowpox virus name was “vaccinia”) and gave it to a young boy. Months later, he injected real smallpox into the boy. Fortunately, the boy did not get the disease. This marked the modern beginnings of immunization. Jenner, however, made no claims that he understood why immunization worked. It was not until a century later that the French chemist and microbiologist (a person specializing in the study of microorganisms) Louis Pasteur (1822–1895) proved experimentally that disease-causing microorganisms (organisms that can only be seen through a microscope) that were “attenuated,” or weakened, would create an immune response in a person without actually causing the disease itself. On the basis of this breakthrough, active immunization began. It is now known that immunization uses the mechanisms of the body’s natural immune system to protect the body against future diseases. 320


IMMUNIZATION AND ANTIBODIES In the twentieth century, science learned that the body produces substances called antibodies. These fight and kill what the body recognizes as foreign invaders (disease-producing microorganisms). These antibodies are specific to that particular disease and will remain in the body’s “memory cells” for a long time, ever ready to fight should the body recognize the disease in the future. Today, many viral vaccines are made from live, weakened viruses, including those for yellow fever, measles, mumps, rubella, and polio. Using a live form means that the body will react with a very strong immune response to the particular virus, thereby protecting the individual against future infection. Other vaccines, like rabies, flu, and intravenous polio, use dead viruses and do not confer as strong a protection.

PASSIVE IMMUNIZATION All of the above are considered to be forms of active immunization, but there is also another method called passive immunization. This method is used when a quick response to a disease is required. Passive immunization consists of injecting specific antibodies into a person to fight a specific disease. For example, a person who is bitten by a snake or who has been exposed to hepatitis cannot wait for his own system to build up antibodies against them. Instead, the person is given a direct and immediate dose of the antibodies in order to neutralize the venom or to kill the microorganism. While passive immunization usually works, it is not long-lasting like active immunization and usually will not protect the person in the future.

Immunization

Doctor Jonas Salk, a pioneer in immunization, immunizing a child against polio. Immunization has proved to be the best way to protect people against contagious diseases. (Reproduced by permission of AP/Wide World Photo, Inc.)

IMMUNIZATION WORKS Immunization has proven to be the safest, least expensive, and most effective means of protecting people U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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against contagious diseases. In many ways it is ideal, since it prevents the disease rather than trying to cure it once it has taken hold. Today, immunization in the developed world has nearly eliminated the threat of typical childhood diseases like measles, mumps, rubella, whooping cough, and polio. In 1980 the World Health Organization declared that smallpox was the first disease to be totally eradicated (destroyed) worldwide. [See also Antibody and Antigen; Immune System; Vaccine]

Inbreeding Inbreeding is the mating of organisms that are closely related or that share a common ancestry. It is used deliberately by people to try to retain desirable traits and eliminate undesirable ones in animals. However, inbreeding can result in harmful recessive genes that had been masked in parents but later appear in the offspring. Inbreeding of animals has been conducted by people ever since they first began to keep animals for food, clothing, and transport. It probably was done initially when a particularly useful characteristic was displayed in an animal, and the animal was then encouraged to mate with another of its kind that had the same, desirable trait. Today, inbreeding is used in animal husbandry, which is the scientific control and management of animals. It is performed for the same original purpose of encouraging the development of certain desirable traits in offspring. A good example of inbreeding is done with dogs and cats who are bred primarily for their appearance. There are also some types of inbreeding that occur in nature. Self-fertilization is one example that occurs in bisexual flowering plants. This is probably the most “inbred” an organism can be since its offspring are the result of the fusion of the male and female sex cells of the same individual. This particular form of inbreeding is sometimes necessary since it allows an isolated individual plant to create a local population. One disadvantage of such an inbred population, however, is that its ability to adapt to environmental changes is limited since its members all share the same pool of genes. This limitation of inbreeding applies to animals as well, and all farmers know that they can only mate animal siblings (brothers and sisters) for a few generations before they start to show signs of being less healthy and less fertile. The reason for this is because of the same lack of variability (genetic differences) that the isolated flower population suffered. Inbreeding can cause harmful genes that are recessive in both parents to become expressed in their offspring. A recessive gene is not expressed if 322


there is a dominant one to offset it or mask it. However, it is still part of the individual’s genetic makeup. The recessive gene will be expressed if the other parent also has a recessive gene for the same trait.

Inhertited Traits

Continued inbreeding can result in an accumulation of recessive genes, which can cause what is called “inbreeding depression.” This is not a state of mind but rather a physical state that results in low fertility, poor general health, and particularly negative characteristics like stunted growth or feebleness. One example of a species that is threatened by its lack of genetic diversity is the cheetah. Because today’s existing cheetahs are all so closely related, most have weakened immune systems and are very susceptible to disease. To avoid this, agricultural breeders sometimes practice a form of inbreeding called “linebreeding.” This is accomplished by mating a female animal with its grandfather or uncle (rather than with a sibling or parent). This reduces the probability of undesirable genes in the offspring. The opposite of inbreeding is outbreeding, which is defined as mating individuals that are not related at all. While this can make it more difficult to regularly achieve a certain desirable quality or trait, it does produce more vigorous and healthy offspring. The most extreme example of outbreeding is called crossbreeding in which individuals of different but closely related species are mated. The mule is an example of the crossbreeding of a horse and a donkey, and although it is a very strong and useful animal, the mule is nonetheless sterile or cannot reproduce. This is the case with all crossbred animals. [See also Genetic Disorders]

Inherited Traits An inherited trait is a feature or characteristic of an organism that has been passed on to it in its genes. This transmission of parental traits to their offspring always follows certain principles or laws. The study of how inherited traits are passed on is called genetics. The study of genetics or heredity began in the early 1800s when scientists first began trying to explain the existence of different species and variations within the same species. The French naturalist, Jean Baptiste de Lamarck (1744–1829), was one of the first to seriously consider the idea that present-day life forms descended from common ancestors. This is the principal idea of biological evolution. However, Lamarck also put forth some incorrect notions about biological development and heredity. One of these was that new organs and capabilities could be developed out of need and would also grow and improve when routinely used over time. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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GREGOR JOHANN MENDEL Austrian botanist (a person specializing in the study of plants) Gregor Mendel (1822–1884) is considered to be the father of genetics. After years of breeding peas and studying their characteristics, he discovered the basic laws of heredity that apply to all plants and animals. His work not only explained English naturalist Charles Darwin’s theory of evolution (the process by which living things change over generations) by natural selection, but laid the foundation of modern genetics. Gregor Mendel was named Johann when he was born in Heinzendorf, Austria (now part of the Czech Republic). The son of a peasant who took care of the fruit trees on a rich man’s estate, the young Mendel took the name Gregor when he became a priest. Although very poor, he had been helped by the church to obtain a basic education and eventually received some higher training in mathematics and science. However, when his financial situation got very bad, he entered a monastery in 1843, mainly as a means of trying to continue his education. Although he had not intended on becoming a priest when he entered the monastery, four years later he decided to become a priest and was ordained that year. Eventually he was sent to the University of Vienna to study zoology, botany, chemistry, and physics. After becoming a science teacher, he repeatedly failed the examination that would have enabled him to teach at a higher level, so he finally just gave up and decided to pursue his own interests while remaining a priest at the monastery. Since he was particularly interested in both mathematics and botany, he decided to combine his two loves and to see if it were possible to predict the kinds of fruits and flowers a plant would produce. Until Mendel, no one had ever done any real statistical analysis of breeding experiments. So starting in 1868, Mendel began a long-term project— almost a hobby—to see if he could conduct a range of breeding experiments and keep accurate records of his results. Mendel wanted to see if he could begin to understand how traits pass from one generation to another, so after taking over the monastery’s garden plot, he chose to breed pea plants (today’s “sweet peas”). Peas were an especially good experimental plant be-

In other words, Lamarck said that a characteristic could be acquired (the long neck of a giraffe was acquired by continuously stretching its neck to reach for leaves) and then could be passed on to its offspring. He also said the opposite, that those characteristics that were not used would eventually disappear. Lamarck also argued that when an organism acquired a new skill it passed on that ability to its offspring. Of course, we know today that if one person learns a foreign language, there is no genetic way he or she can pass that talent on to its children. However, Lamarck was 324


cause they had characteristics, or traits, that could be easily observed (tall or short, wrinkled seeds or smooth seeds, yellow or green seeds). Mendel was very careful in all of his experiments, transferring pollen by hand from the male to the female part of the flowers to produce seeds. He even wrapped his plants so they would not be accidentally pollinated by insects. He would save the seeds from each self-pollinated plant, plant them separately, and study the new generation. He also crossbred plants with different characteristics.

Inhertited Traits

After eight years of work and careful recording, Mendel found that, indeed, he was able to predict what a certain plant would produce as long as he knew which plants were the parents. In fact, he was so certain of what he found that his conclusions are now called Mendel’s “laws” of inheritance. What he discovered after years of breeding more than 30,000 plants was that there are powerful traits, called dominant, and weaker traits, called recessive. He also found that when mixed together they do not blend. For example, although he at first expected to breed a medium-size plant when he crossed a tall plant (dominant trait) with a dwarf plant (recessive trait), what he found was that he eventually wound up with a mixture of tall and short plants according to a given ratio. He concluded, therefore, that in every instance, mixing traits did not result in a blend but instead sorted themselves out according to a fixed ratio. Mendel also concluded that each parent plant contributed a factor, later found to be a gene, that determines what a certain trait will be. Unfortunately for Mendel, he published his results in a journal not read by many in Europe. When he wrote directly to a prominent botanist of his time, the heavily statistical arguments he offered confused a man who was unused to seeing mathematical data in a botany paper. Discouraged, Mendel later put away his work and died totally unnoticed. It was not until 1900 that his work was discovered and made public. Upon close consideration, a new generation of life scientists realized that Mendel’s laws of inheritance supported and even explained Darwin’s theory of evolution by natural selection. Thus, the quiet priest who worked with the humble garden pea is now recognized as the giant who laid the groundwork for the modern science of genetics.

on the right track since he did suggest that traits can be inherited from generation to generation and that species do undergo long-term evolutionary changes. In 1859, the English naturalist Charles Robert Darwin (1809–1882) published his landmark work, On the Origin of Species, in which he outlined his theory of evolution through natural selection. Darwin argued that members of a particular species always have slightly different traits or U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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characteristics, and that in the competition for food, space, and shelter, some of these differences would make one member more suited to survive and produce offspring than others of its species. He continued this line of reasoning and said that those traits that were an advantage would be passed on to later generations, while those that were not would eventually disappear as their carriers died out. This meant that after centuries upon centuries of competition, or natural selection, recent members of a species could be quite different from their ancestors. Despite its eventual acceptance by the scientific community, Darwin’s theory lacked an explanation for the mechanism or manner in which these random variations were inherited. It was not until 1900 that the means of transmission of inherited traits was understood. Some fortyfive years earlier, the Austrian monk, Gregor Johann Mendel (1822–1884), had begun experimenting with pea plants at about the same time that Darwin set forth his ideas on natural selection. Through his careful experiments, Mendel demonstrated that what he called “hereditary factors,” now called genes, are transmitted to offspring. He also discovered that traits are inherited in pairs and that usually only one trait in each pair is actually expressed in the offspring. Although Mendel had established the laws of heredity by 1865, his work remained unnoticed until it was independently rediscovered by three scientists in 1900. With their discovery, it became apparent that Mendel had formulated the fixed rules of inheritance that applied to the entire plant and animal kingdoms. After 1900, science began its search for the key part in all living things that contained the actual information that determined every de-

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tail of what an organism was. This search soon led to an understanding of chromosomes (a coiled structure in the nucleus of a cell that carries the cell’s heredity information), and then to the realization that they in turn were made up of other, smaller things later named “genes.” By the early 1950s, science knew that the chemical deoxyribonucleic acid (DNA) was somehow at the center of heredity, and in 1953 the American biochemist, James Dewey Watson, and the English biochemist, Francis Harry Compton Crick, explained exactly how. That year they discovered the “double helix” structure of the DNA molecule, demonstrating exactly how DNA carries the genetic code for all living things.

Insects

By the end of the twentieth century, our knowledge of the mechanism of inherited traits had nearly reached the point where the actual location of every human gene on every chromosome was identified and every letter in the 3,000,000,000-base code deciphered. Finally, on June 26, 2000, scientists announced that they had completed a rough draft of the human genome—the complete set of chromosomes that determines humans inherited traits. When completed, this human genome will lead to an understanding of each gene’s precise chemical structure and its function in health and disease. This information will be invaluable since it may lead to cures, or possibly preventions, of certain genetic disorders. [See also Dominant and Recessive Traits; Genes; Genetics; Mendelian Laws of Inheritance]

Insects An insect is an invertebrate animal with six legs and a body that is clearly divided into three main segments. The heads of most insects have a pair of antennae, compound eyes, and large jaws. Insects inhabit nearly every part of Earth and make up the most numerous class of living animal. They are considered to be the most successful group of living creatures ever to have lived on Earth. Insects belong to the phylum Arthropoda and make up its largest class. Well-known examples include bees, ants, butterflies, grasshoppers, beetles, moths, mosquitos, and the house fly. Insects are so diverse that there is probably no typical insect, although there is a basic insect anatomy or structure. An adult insect has three distinct body segments—the head, thorax, and abdomen. The head contains the sense organs like antennae and eyes, as well as three pairs of mouthparts that are adapted for either biting, chewing, puncturing, or sucking, depending on the species. The thorax has three segments, each of which has a pair of legs that are used for U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

Opposite: A nineteenth-century diagram illustrating Gregor Mendel’s discovery of the patterns of inheritance as shown by sweet peas. This diagram shows the original crossing, the first generation, and the next when recessive traits appear in the proportions discovered by Mendel. (Reproduced by permission of The Granger Collection Ltd.)

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A labeled diagram showing the external and internal features of a generalized insect. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

walking and clinging. If an insect has wings, they are attached at the thorax. Also located at the thorax are small tubelike openings called trachea that insects use to take in oxygen and expel carbon dioxide. Some of these openings are also found on the forward part of the abdomen. Since insect tissue gets oxygen directly through these tracheae, the circulatory system is fairly simple. The insect abdomen is used primarily for reproductive purposes. Digestion occurs in a three-part gut and wastes are excreted out of very specialized organs called Malpighian tubules. Named after the Italian anatomist (a person specializing in the structure of animals) who discovered them, these function like kidneys and remove waste from the insect’s system. Insects also possess an exoskeleton, a hard outer support structure that protects their soft internal organs and provides some protection against predators. Insects have been described as the dominant form of life on Earth. One writer even argues that if visitors from other planets came to Earth and studied its life forms, they might want to communicate first with an

Brain

Head

Front wing Respiratory system

Thorax

Main nerve cord Hind wing Coxa Trochanter Femur Leg

Tibia

Claws Tarsus

Spiracles Trachea Abdomen

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insect because of their staggering diversity and overwhelming numbers that the insect represents. One reason for this steady success is the small size of insects. A small insect does not need as much food as a large animal does, so it is easier for an insect to find food. Insects also have highly specialized mouthparts and digestive systems that allow them to consume almost any plant in existence. If necessary, however, they will eat anything they can find. An insect’s small size also allows it to hide more easily from its enemies, thus possibly enabling the insect to avoid predators and live longer.

Insects

Another reason for their success could be the distinct advantage having wings provide insects. Being able to fly means insects can easily escape earthbound predators, but it also confers the advantage of being able to leave a certain habitat if it proves difficult or dangerous to live there. Flying also allows insects to populate other habitats that most animals would have a hard time reaching. Altogether, their small size and flying ability gives insects a competitive advantage in the struggle for survival. Another major reason that certain insects prosper is because they are social insects. Although some live independently of others, some insects live together in what are called colonies. A colony is a group of animals that live together and share work and food. Honeybees, termites, ants, and wasps are social insects. In a colony, an individual insect does not have to provide itself with food and shelter, since all the tasks needed for living are carried out by different members of the colony. Each member has a specific job to do, and it performs only that role and nothing else. For example, in an ant colony, a queen ant lays eggs and contributes to the growth of the colony; soldier ants guard the colony and do the fighting during an attack; nurse ants tend only to the queen, her eggs, and the larvae; and worker ants locate food outside the colony and bring it back for all. All of the roles are performed totally by instinct. The system works because each insect in a colony spends every day of its life doing the job it must do and nothing else. Besides being social, insects also have a staggering reproductive capacity. For example, if all the eggs of a single fly were to survive and reproduce through only six more generations, there would be more than 5,000,000,000,000 flies. In many ways, insects are mankind’s most aggressive competitor. They will eat crops as well as stored food. They can swarm and consume every green thing in sight. They also can destroy paper, wood, and cloth. They bite humans and other animals and transmit diseases. On the other hand, insects are important and necessary pollinators of flowers and crops, and many beneficial insects (like the ladybug and praying mantis) attack or destroy many insects harmful to humans or crops. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Bees provide honey as well as pollination. Overall, human beings have now learned that we cannot defeat or even diminish the range, extent, and diversity of the insect population. However, humans can control insects’ negative effects by learning more about their habits, needs, and life cycles. [See also Invertebrate]

Instinct Instinct is a specific inborn behavior pattern that is inherited by all animal species. Instinctive behavior exists at birth and does not have to be learned. Most instinctive behavior is related to an animal’s survival. The word instinct could be used to describe the set of wired instructions that are built into an animal’s nervous system. These instructions are passed genetically from one generation to the next. From observing animal behavior, it is known that particular species will do certain things automatically almost from the moment of birth. For example, newborn chicks “instinctively” open their mouths when an adult bird returns to the nest. A baby kangaroo rat instantly performs an escape jump maneuver when it hears the sound of a striking rattlesnake, even if it has never seen a snake before. Nest-building and web-spinning also are examples of instinctive behavior that can be seen and observed. All of these and other instinctive behavior patterns have things in common. Given the proper condition, situation, or stimulus, instinctive behavior patterns are automatic and are performed in a fixed, regular way by each member of the species. Each cocoon-spinning spider builds its silk cocoon in a certain sequence. The spider first spins a base plate, then the walls, lays its eggs, and adds a lid. The spider can only build the cocoon this way. If interrupted and moved elsewhere after having built the base plate, it will not start over but will continue as if the plate were already there. This means that the eggs will fall out the bottom when laid. Another feature common to all instinctive behavior is that it requires no learning and is carried out fully and completely the first time it is performed.

SCIENTISTS TROUBLED BY INSTINCT Although the term instinct is commonly understood by most people, for scientists it presents a problem. The word does little to explain the real mechanisms that are at work. If instinct is used to describe behavior that is performed as if guided by some mysterious and unknown force, 330


then “instinct” is not a real scientific word, since science seeks explanations. Many scientists do not use the word “instinct” anymore, since this is too general a word. Instead, they refer to what is called a fixed action pattern (FAP). A fixed action pattern of behavior describes an activity like that of the spinning spider. Once a stimulus has started her spinning, she will continue automatically in a step-by-step process no matter what happens. In other words, a fixed action pattern ignores feedback from the senses and makes the animal continue. For example, after a goose lays her eggs, she uses her neck to pull them together into a clump. If an egg is quickly removed, she will continue to use her neck to pull at the nonexistent egg until completion. In other words, she is oblivious to her senses, which would tell her that there is no egg to pull. Scientists have found that fixed action patterns are begun by what they call a sign stimulus or releaser. This is a type of cue in the animal’s environment that triggers what might be described as genetically pro-


Instinct

These baby birds instinctively open their mouths in anticipation of food when an adult bird returns to the nest. Because of this instinct, the baby bird is able to compete for limited resources, increasing chances for survival. (Reproduced by permission of The Stock Market. Photograph by Roy Morsch.)

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grammed behavior. Although this is still not fully understood, it is known that certain “releasers” trigger a reaction in the animal’s central nervous system called the innate releasing mechanism (IRM). However, this scientific explanation cannot go beyond saying only that the IRM is genetically encoded. Despite people’s inability to fully understand it, they know that instinct serves primarily to help an animal survive, and that it is controlled by its genes (meaning that instinct was inherited and will also be passed on to later generations). Instinctive behavior patterns can be modified, or changed, and can even be used to trick the animal. However, for the most part, natural selection (the process of survival and reproduction of organisms best suited to their environment) has found that an automatic response in certain important situations is best suited to assure the survival of the species. Thus a certain environmental cue, or releaser, always causes an appropriate biological response, or trigger. This permits the animal to perform the “right” action immediately. [See also Genetics; Inherited Traits]

Integumentary System The integumentary system of an organism is its protective outer covering. All organisms have an integument or covering that separates the organism from its environment and serves several other important functions. The integument of vertebrates (animals with a backbone) is called skin. Skin can vary widely, from the impenetrable shell of an armadillo to the amazingly smooth skin of a porpoise. Every organism has some sort of covering that holds together its body organs and fluids and makes it separate from its environment. This outer covering protects it from foreign bodies and matter and sometimes allows it to communicate with the world outside itself. In both one-celled organisms and plants, the integument is the same as its cell membrane and any secretion or coating that it produces. More complex invertebrates (animals without a backbone) have an integument that consists of a single layer of cells called an ectoderm. Only in vertebrates is the integument a many-layered, complex organ system that serves many functions.

THE INTEGUMENTARY SYSTEM OF PLANTS The integument, or outer covering, of plants does the same thing that skin does for animals—it protects plants from injury and prevents under332


lying tissue from drying out. Higher plants, or those that have seeds and a vascular system (an internal system of tubing that carries fluid), have a living epidermis usually one cell thick. It may be thin, like the covering on lettuce leaves, or thick and tough like that on pine needles. The epidermis has openings called stomates that regulate temperature and water loss. It can have coatings like the wax on an apple or sensitive hairs like those on a Venus’s flytrap. Overall, most plant integuments are fairly fragile compared to animal coverings.


THE INTEGUMENTARY SYSTEM OF INVERTEBRATES The integument of more complex invertebrates is usually a single layer of cells that secrete some type of cuticle. The cuticle oozes out of the epidermis and hardens, affording it some type of protection. In crustaceans like crabs and lobsters, this functions as an external skeleton and is very hard and tough. Insects have an outer covering made up of chitin fibers that is secreted by the epidermis. Chitin forms a type of flexible, natural plastic and acts as an outer skeleton to which muscles are attached. In insects, the cuticle is a living structure and can produce sensitive hairs as well as bristles, scales, claws, or wings, depending on the species. Insects grow by shedding the cuticle and growing a new, larger one. Mollusks, like clams and snails, also form a hard, external shell.

INTEGUMENTARY SYSTEMS IN VERTEBRATES Only in vertebrates, however, is the integumentary system considered to be a vital organ. That is, it not only provides protection for the delicate tissue underneath, but it gathers and conveys information to the organism itself about the outside environment. The skin of all vertebrates consists of two layers: the relatively thin (outer) epidermis, and the tough, inner dermis. The epidermis is several cells thick, and its outermost layer of cells is made up of dead cells composed of keratin, the protein found in hair, nails, claws, beaks, feathers, scales, and quills, among other things. Vertebrates replace this outer layer of dead cells every twenty-eight days, with the new layer identical to the old. In mammals, the inner dermis is highly developed, and is richly supplied with blood vessels, glands, and nerve endings. Besides acting as a barrier against infection and retaining the body’s fluids, the dermis also has a regulatory function of letting the organism know whether to raise or lower body temperature or to move to where it is cooler or warmer. In endothermic (warm-blooded) animals, the skin plays an important role in regulating the body’s temperature. In humans, the epidermis also contains a dark pigment called melanin that protects the skin from the Sun’s ultraviolet radiation. It is the amount of U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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melanin an individual’s body produces that accounts for what we describe as the many different colorings of human “races.”


Mammals. In mammals, the presence of hair is a distinguishing characteristic. As an outgrowth from a mammal’s skin, hair grows from a pit in the dermis called a hair follicle. This pit also has a small gland that secretes an oily substance that keeps the hair oiled. For most mammals, the hair’s main function is to act as insulation against the cold. Hair also serves as a sensory organ (like long whiskers) for certain night-prowling animals. Eyelashes in humans serve to make the eyes reflexively shut if they are hit by a speck of dust. There are also other glands in the dermis that keep the skin oiled and waterproof. Humans also have sweat glands in the dermis that act as a temperature control by means of evaporation, and most mammals also have dermal glands that produce odors that are thought to be a form of sexual communication. A cross section of the skin, part of the integumentary system. Structures used for sensing are labeled on the right. (Illustration of Hans & Cassidy. Courtesy of Gale Research.)

In humans, the skin is considered the largest organ of the body. It changes considerably over time, and as it ages it becomes less elastic and more wrinkled. As with all vertebrates, human skin provides both protection from and communication with its environment. The dermis is rich with nerve fibers that can respond rapidly to changing environmental conditions, reporting its findings to the brain, which makes the necessary ad-

Hair shaft Merkel's discs (touch, pressure) Stratum corneum

Naked nerve endings (pain)

Epidermis Stratum basale Arrector pili muscle

End-bulb of Krause (cold) Dermis

Sebaceous gland Collagen fibers

Adipose tissue

Meissner's corpuscle (touch, pressure) Ruffini's end organ (heat) Pacinian corpuscle (deep pressure)

Subcutaneous tissue Hair follicle Adipose tissue Vein Artery Sweat gland

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justments. Besides its practical functions as barrier and regulator, the human skin also possesses the exquisite and indescribable sense of touch. Its surface is usually home to many bacteria, mostly harmless, and the skin can be subject to many diseases or injuries.

Invertebrates

Invertebrates An invertebrate is a multicelled animal that does not have a backbone. Within this seemingly simple grouping there is an amazing variety of complex life forms: from sponges and starfish to earthworms, clams, spiders, and butterflies. Of the roughly 1,500,000 different species of animals in the world, more than 95 percent are invertebrates. They inhabit nearly every type of environment on Earth and vary greatly in the way they live and reproduce. An invertebrate may be as soft as a jellyfish or as hard as a lobster but they have one distinction in common—they have no bony vertebral column or backbone. In nature there is no actual dividing line that separates animals with backbones from those without one, but grouping the members of the kingdom Animalia in this manner allows biologists to sort them into very broad groupings. The animal kingdom is divided into major groups called “phyla,” (singular, phylum), and of all the animal phyla identified (some say there are as many as thirty-eight), only one includes vertebrates. The rest are invertebrates. This gives some sense of how successful these “lower” animals have been in the race for survival. Invertebrates not only live almost everywhere on Earth, but range in size from an organism too small to be seen without a microscope to a giant squid measuring 60-feet (18.29 meters). Invertebrates are often considered to be pests, yet despite our best efforts to exterminate them, they seem to adapt and thrive.

SPONGES The sponge is the simplest of all invertebrates (and therefore the simplest of all animals) and lives at the bottom of the sea. Early naturalists considered sponges to be plants since they looked like a plant and did not move. Later, as they were studied more, sponges came to be considered “zoophytes” or plant-animals. Today they are considered to be the simplest of animals and are placed in the phylum Porifera. The light brown, oddly shaped sponge we sometimes use to bathe ourselves or wash the car began its life attached to the bottom of one of Earth’s seas, where it used its many holes or pores to draw in water and filter it for food. Sponges have bodies resembling a sack with an opening at the top. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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They have no organs or nervous system, and usually reproduce sexually (with sperm fertilizing an egg). However, some sponges can also reproduce asexually. For example, when a sponge piece breaks off, floats away, and happens to settle in a proper place it begins to grow. Sponge “farmers” cut up living sponges and place their pieces on a rock underwater where a full-sized sponge will grow in a few years. The sponge we use in the bath is the dried (but very absorbent) skeleton of an invertebrate that was once alive. Sponges are so different from all other animals that some biologists believe that they should have their own animal subphylum.

CNIDARIANS One step up the invertebrate ladder of complexity from sponges are the members of the phylum Cnidarian (the “C” is silent). Also called coelenterates (Latin for “hollow gut”), cnidarians include jellyfish, sea anemones, corals, hydra, and their relatives, all who live in the water. Although they are not related to sponges, cnidarians are usually listed after sponges up the invertebrate ladder because they started with the sponge’s simple multicelled form and added many features not found in sponges. One of the things cnidarians added was a “hollow gut” or specialized digestive cavity, which is attached to a type of mouth (that also serves as an opening to expel waste). A cnidarian has only one opening that serves two purposes: to take in food and expel waste. All cnidarians have armlike projections called tentacles that hang down around their mouth. When a small fish bumps into them, the tentacles react and sting or grab the fish, reeling it into its mouth. Since cnidarians are designed to be organized around a single food-gathering mouth, their body form is described as having “radial symmetry.” This means that the cnidarian body has no definite right or left side but resembles spokes radiating from the hub of a wheel. Some cnidarians, like coral, are filter feeders and stay in one place, while others, like the jellyfishes, can swim around. Cnidarians reproduce sexually, but they can also duplicate themselves asexually by budding or producing new cells that separate from the parent and become independent organisms. Cnidarians are named because of specialized nerve cells called “cnidoblasts,” which makes their stingers work. Stinging tentacles are used to get food and for defense. These tentacles are not linked by a central nervous system but operate independently and almost automatically in a stimulus/response manner. This is why a dead jellyfish can continue to inflict a bad sting to someone touching its tentacle. 336


WORMS

Invertebrates

Worms are more complex than sponges or cnidarians, and although the word worm refers to any animal that has a long, soft body without legs, a worm is far from a simple animal. The most important difference between worms and the other two invertebrate types is not obvious, however. Instead of having two layers of cells in their bodies, like sponges and cnidarians, worms and other “higher” animals have three layers of cells. Because of this middle layer between its external and internal layers, the worm has specialized tissues and organs that sponges and cnidarians do not have. The many different kinds of worms are gathered into three groups: flatworms, roundworms, and segmented worms. All members of the last two groups show bilateral symmetry, meaning that if they were cut down the middle, there would be two matching halves.

Flatworms. Flatworms, which belong to the phylum Platyhelminthes, move about for their food and therefore have a definite front and back end. This means that they have developed a head or at least a forward part in which nerves and senses are concentrated. Along with a nervous system, flatworms also have a separate excretory system and a reproductive system. The simplest flatworm is flat and always found in water. The most common is the planarian, which has only one opening to take in and

A millipede is a good representative of invertebrates. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.) U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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expel waste. A tapeworm is a parasitic flatworm that lives in the digestive system of its host where it attaches to an intestine wall and absorbs already-digested food. Flatworms reproduce sexually and asexually, and if cut in half, both parts will grow into a complete animal.

Roundworms. Roundworms have a cylindrical body, a tough outer cuticle, and are found in both water and soil. Also called Nematoda, many are parasitic. Pinworms are a well-known roundworm, as are the trichina that causes trichinosis (a disease caused by eating infected pork or meat). Roundworms reproduce sexually and have a separate digestive and circulatory system. Segmented worms. Segmented worms or true worms belong to the phylum, Annelida which means “little rings.” An annelid’s body is therefore made up of segments or little rings attached together. The earthworm is a good example of a typical annelid in that its body is more complicated than that of other types of worms. Their digestive system contains organs with special jobs, and their nervous system has a distinct brain in the front end or head. Since each body segment has a set of muscles, annelids can slowly move about by changing their segment shapes. A very important annelid advance is the development of a “coelum,” a lined body cavity that not only provides support but allows organs to be suspended inside the body. The coelum is found in all the more complex animals, including humans. Most annelids live in the soil, and some, like leeches, are parasitic. Reproduction in annelids is usually sexual. Although an earthworm is both male and female, it must mate with another earthworm before each can lay eggs. MOLLUSKS Although the next invertebrate phylum Mollusca means “soft,” most of these invertebrate have a hard shell. Clams, oysters, and scallops are all mollusks as are squid, octopus, and snails. Despite the shell of some, they all are soft-bodied with some form of covering or mantle. In some, it is hard tissue and in others it is a very hard shell. Most mollusks have some sort of foot or appendage for feeding and moving about. Besides a digestive and circulatory system, they also have a well-developed nervous system, and some even have eyes. Finally, mollusks that live under water have specialized gills that take oxygen out of the water and put it into their blood. Mollusks also reproduce sexually.

ECHINODERMS The phylum Echinoderm means “spiny skinned” and is made up of invertebrates like starfish and sand dollars that have hard outer plates. 338


These plates usually cover a body that has five separate parts like the spokes of a wheel. Echinoderms live in the sea and have a specialized system of canals in their bodies that connect to their many tube feet. These feet suck in water and allow the echinoderm to attach itself to something solid (like a clam shell which it can then pry open). The echinoderm has a complete digestive system although it has no excretory or respiratory system. All echinoderms reproduce sexually. They also are able to grow back any lost parts through regeneration.

Invertebrates

ARTHROPODS The phylum Arthropoda is considered the largest and most successful phylum in the kingdom Animalia. Arthropods live nearly everywhere on Earth. All have at least three pairs of jointed legs and a body divided into jointed segments that is covered by a hard, outer case called an exoskeleton. Arthropods have internal body systems that break down food, take in oxygen, circulate blood, and carry away wastes. Most reproduce sexually. Arthropods are so varied that there are five major types: crustaceans (lobsters, crabs); arachnids (spiders, ticks, mites); insects (bees, ants, beetles); centipedes; and millipedes. As the largest animal group, invertebrates are an essential part of every ecosystem. Humans could not function without them since they are responsible for the decomposition of organic waste, which allows the recycling of essential chemicals. Invertebrates are also involved with the pollination of plants and are a crucial link in the food chain where herbivores (plant-eating animals) convert the energy in plants into a form useful to meat-eating animals. [See also Arachnids; Arthropods; Crustaceans; Insects; Mollusks; Protozoa]


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K Karyotype Karyotyping is a diagnostic tool (a way of identifying a disease or condition) used by physicians to examine the shape, number, and structure of a person’s chromosomes (a coiled structure in a cell’s nucleus that carries the cell’s DNA) when there is a reason to suspect that a chromosomal abnormality may exist. A karyotype is made by arranging pictures of the chromosomes in matching pairs (called homologous pairs) according to their size, shape, and length. This particular technique was developed because chromosomes are very difficult to observe in a cell unless the cell is about to undergo division. When division is about to happen, the chromosomes that could be seen as only long, tangled threads suddenly begin to shorten, thicken, and condense. Chromosomes are easily stained when they are in this form. Staining them with a dye not only makes them easier to see but also makes their distinctive bands show up well. The technique used by biochemists to make a karyotype is now fairly routine. Skin cells are often used since they divide frequently. Mitosis or cell division can be started by adding the proper stimulating chemical to the cells that are in a liquid. Once the cells begin to divide, another substance is added to fix, or freeze, the division, and the fixed cells are placed on microscope slides and dyed so that the light and dark bands on the chromosomes show up. These bands indicate the position of deoxyribonucleic acid (DNA) along the chromosomes. A photograph of the chromosomes is then taken and enlarged. Individual chromosomes are cut out and paired up or arranged by size, shape, and the length of their “arms.” Biochemists use the pattern of stained bands as well as chromosome length and the position of the centromere U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Short region of DNA double helix

"Beads-ona-string" form of chromatin

30-nm chromatin fiber of packed nucleosomes

Section of chromosome in an extended form

Condensed section of chromosome

(the strand linking two chromosomes) to identify pairs of matching chromosomes. All the chromosomes are then numbered and arranged in order, from the largest to the smallest. Today, the use of fluorescent stains, microscopes, and computers make karyotyping an easy, standard way of searching for and identifying genetic disorders. In the hands of a trained professional, a karyotype can provide a great deal of genetic information. Some problems can be spotted with little analysis, such as Down’s syndrome. This condition, which causes mental handicaps and certain facial and body characteristics, is easily seen on a karyotype. It shows up as three copies of chromosome number 2 nm twenty-one instead of the normal two chromosomes. Karyotyping identiNucleosome fies extra or missing chromosomes as well as banding irregularities. 11 nm

Histone

30 nm

300 nm

700 nm

Doctors often recommend making a karyotype of a fetus when there is a high risk of genetic disorders. Cells are obtained from the fetus in the mother’s womb using a technique called amniocentesis. In this procedure, a long thin needle is inserted through a mother’s abdomen and into the fluid-filled membrane surrounding a developing fetus. Karyotyping can also be done after a baby is born to determine if a certain physical disability that it has was caused by a problem in its chromosomes. [See also Chromosomes; DNA; Genetics; Genetic Engineering; Gene Therapy]

Kingdom Entire duplicated chromosome

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1400 nm

The term kingdom is one of the seven major classification groups that biologists use to identify and categorize living things. These seven groups are U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

hierarchical or range in order of size. The kingdom group is the first and largest group. The classification scheme for all living things is: kingdom, phylum, class, order, family, genus, and species.

Kingdom

As the broadest of all classification groups, kingdom is made up of phyla (singular, phylum). From the time of Greek philosopher and scientist Aristotle (384–322 B.C.) to the mid-twentieth century, only two kingdoms were recognized—Animalia (animal) and Plantae (plant). However, with increasingly modern and sophisticated techniques, biologists eventually came to recognize a five kingdom approach. These additional kingdoms were necessary in order to include other forms of life that did not belong in either the plant or animal kingdom. Today, the five kingdoms are monerans, protists, fungi, plants, and animals. Monera, the smallest kingdom (with only about 4,000 species), includes the prokaryotic bacteria (single cells that do not have a nucleus) and certain types of algae. Bacteria play an important role as decomposers, and some monerans can make their own food through photosynthesis. Organisms in the kingdom Protista are eukaryotic (their cells contain a nucleus) but are both plantlike and animal-like. Some algae and protozoans are protists. The kingdom Fungi consists of molds, yeasts, and mushrooms. These are all multicelled organisms that live by absorbing food. Although these organisms look like plants, they do not make their own food. Members of the kingdom Plantae make their own food and often grow flowers and form seeds. The kingdom Animalia includes multicelled organisms that live by taking in food. Animalia is made up of vertebrates (animals with a backbone) and invertebrates (animals without a backbone). This is the largest kingdom, containing more than 2,000,000 species. [See also Class; Classification; Family; Genus; Phylum; Species]


Opposite: A figure of a human karyotype. Karyotyping is often a good way of detecting possible genetic disorders in unborn children. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

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L Lactic Acid Lactic acid is an organic compound found in the blood and muscles of animals during extreme exercise. It also is produced in some plants as a result of the fermentation (the process of splitting complex organic compounds into simple substances) of certain carbohydrates. A buildup of lactic acid in the body is toxic and causes muscle fatigue, pain, and cramps. Athletes who push themselves or exert their muscles and bodies beyond what is a comfortable level of exercise often experience a “burn” in their muscles. If they do not stop exercising, they sometimes will feel muscle cramps, pain, and overall fatigue or exhaustion. This is the direct result of lactate, or lactic acid, building up in the blood and muscles. This occurs naturally when a person’s muscles are not given a rest and are made to keep contracting (which is how a muscle works). During this or any kind of exercise, the body uses up energy that it gets from a process called respiration. This is not the respiration we refer to as breathing, but rather it is the chemical process of breaking food down to release the energy it contains. After we eat and our body’s enzymes break down the food we have consumed into glucose (sugar), our bodies store the glucose if we do not need to use it immediately. This depends on our level of activity. A certain amount of glucose will always be needed just to maintain all of our systems and keep everything “running.” When we start to increase our body’s energy demands by doing something strenuous like running or exercising, the process of aerobic respiration starts up. Our cells break down the glucose the body has stored by U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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An illustration of the molecular structure of lactic acid, an organic compound found in the blood and muscles of animals during extreme exercise. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

combining it with oxygen. Aerobic respiration releases a large amount of energy. Sometimes we exercise so vigorously or steadily that our muscles start to consume or use up the much-needed oxygen faster than we can take it in, which is why we breathe heavier and faster as we exert ourselves. When this “oxygen debt” occurs and we still do not stop exercising and consuming energy, our cells get the message that they should begin the alternative process called anaerobic respiration. Anaerobic respiration involves the release of energy without needing to consume any oxygen (anaerobic means no oxygen). This is basically the same process that occurs in fermentation, since fermentation is the breaking down of organic materials without the consumption of oxygen. Although anaerobic respiration provides the muscles with much-needed energy, it also has a toxic or poisonous by-product called lactate or lactic acid. If lactic acid is allowed to buildup in muscles, it causes cramps and pain, makes them work less efficiently, and eventually will simply shut them down. Lactic acid will only begin to be broken down and removed from the system once the body is able to begin normal, aerobic respiration (with oxygen). Once the body’s “oxygen debt” is replenished by enough heavy breathing and an activity slowdown, the unpleasant side effects caused by lactic acid will disappear. Fitness training is thought to increase an athlete’s tolerance to lactic acid buildup. Lactic acid is also found outside the body. Not surprisingly, it is an important acidic component of fermented food products such as yogurt, buttermilk, sauerkraut, green olives, and pickles. The formation of lactic acid in these food products is the result of the activity of lactic acid bacteria. Lactic acid also has industrial uses, as it is used in a variety of processes including tanning (converting animal hide into leather) and wool dyeing. [See also Blood; Carbohydrates; Fermentation]

Larva A larva is the name of the stage between hatching and adulthood in the life cycle of some invertebrates (animals without a backbone). A sexually immature organism that lives on its own, a 346


larva seldom resembles its final adult form and usually has entirely different life habits.

Larva

A larva, like a caterpillar, is sometimes thought to be a complete, separate, sexually mature organism that has a life of its own and produces more caterpillars. On the contrary, a caterpillar is only one stage (the larval stage) between the hatched egg of a butterfly and the adult butterfly itself. This is typical of one of the major characteristics of larvae—they seldom resemble their final adult stage. There is no better example than that of the fat, slow-moving, hairy caterpillar that spends all its time eating, and the graceful, often beautiful butterfly that flits and darts from flower to flower. Because a butterfly is an invertebrate that undergoes metamorphosis (a total change in its body shape) as part of its development, it must pass through a larval stage (caterpillar) before it can become a sexually mature butterfly. Like the butterfly, moths also live as caterpillars before they reach their adult flying stage. Among several other invertebrates that pass through larval stages are bees, wasps, and beetles (as grubs), flies (as maggots), mosquitoes (as wrigglers), and frogs and toads (as tadpoles). All these types of larvae are eating machines, since their main goal is to grow and develop as much as possible. For example, caterpillars


A moth larva on a leaf, which it uses as food. Larva are eating machines, since their main goal is to grow and develop as much as possible. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

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have powerful jaws for chewing leaves. They also do not need to move quickly (nor can they), since they often simply attach themselves to their food source. While this makes them vulnerable to predators, they use different strategies to avoid being eaten. Some use camouflage and blend in perfectly with the leaf color of their favorite food. Others arm themselves with prickly hairs that irritate or with sharp spines. Some are poisonous or taste bad. Larvae also seldom eat the same thing or live in the same habitat that they will as an adult. This means that the immature organism does not compete with the adult organism. At some point in its short life as a larva, the invertebrate will receive a hormonal signal that will trigger the beginning of its metamorphosis into an adult. According to the type of invertebrate, the larva will go through either complete or incomplete metamorphosis. Incomplete metamorphosis has three stages (egg, larva, adult). Complete metamorphosis has four stages (egg, larva, pupa, and adult). Insects like grasshoppers, dragonflies, and termites go through the shorter, threestage or incomplete metamorphosis. For these insects, the larva are often called nymphs since they actually resemble miniature adults in their larval stage. The nymph or larva changes into an adult by molting or shedding its outer skin several time as its internal systems develop and enlarge. Complete metamorphosis is much more dramatic since the adult that finally emerges is so drastically different from the organism it used to be. During complete metamorphosis, the larva goes through a resting stage called the pupa, during which all of these changes take place. At the beginning of the pupal stage, the larva attaches itself to something solid, sheds its skin, and forms a tough outer case around itself called a chrysalis. Usually this is a hard shell, but it sometimes can be a silken covering called a cocoon. The changes that go on during pupation consist mostly of breaking down cells and developing new cells. Eventually the adult insect is formed inside the pupa and it escapes from its casing by breaking it open. What emerges is the former larva now transformed into an adult. About 90 percent of all insects undergo complete metamorphosis. Certain other invertebrates, like sponges, have larval stages as a way of dispersing their offspring. For example, after sexual fertilization (union of male and female sex cells), many sponges develop thousands of tiny, freeswimming larvae that are released to be carried away by currents and to finally settle and attach themselves to the ocean bottom. [See also Life Cycle; Metamorphosis]

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Leaf

Leaf

A leaf is the main energy-capturing and food-producing organ of most plants. Nearly every plant on Earth owes its continued existence to its leaves, which collect energy from sunlight and convert it into food through a process known as photosynthesis. Leaves may vary widely in size and shape, but all are designed primarily to capture as much light as possible without drying out.

PARTS OF A LEAF Leaves are attached to and supported by the plant stem, which provides the leaves with water and inorganic nutrients from the soil. Most leaves have two main parts: the blade and the petiole. The blade, or lamina, is the broad, flattened surface of the leaf that absorbs radiant energy from the sun. The blade is attached to the stem by a stalk called a petiole that also supports it. The blade is made up of two layers of cells—a tight, outer layer of cells called the epidermis, and a thicker, inner layer of mesophyll cells. The epidermis is covered by a waxy coating called a cuticle that helps cut down water loss from the leaf. It is in the inner mesophyll cells where photosynthesis is carried out. The petiole not only joins the leaf to the stem, but contains tiny tubes that connect with veins inside the blade. Besides strengthening the blade, the veins’ main purpose is to act as pipelines and transport water and food to and from its cells. A large vein called the midrib usually runs along the center of the leaf and smaller branching veins run out to its edges. Edges of leaves can differ greatly, and while narrow leaves like grass have smooth edges, many broadleaf blades have jagged points called teeth at their edges. In some plants, these teeth act as valves and release excess water, while in others they function as tiny glands producing a liquid that repels insects. Leaves also contain a stoma (plural, stomata), which is critically important to the leaves’ operations. Because the wax coating of its blade is not porous, leaves have developed special openings that allow gases (carbon dioxide and oxygen) to be exchanged and water to be released. A stoma is similar to a tiny slit that opens or closes by the action of two guard cells on either side. These cells can change shape and make the stoma open or close. This is essential during photosynthesis when the plant must take in carbon dioxide (and give off oxygen as a by-product). The stomata also enables a plant to regulate how much water it loses. When the stomata are open, they allow water to escape into the atmosphere. To minimize water loss, stomata tend to close at night when phoU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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tosynthesis is not occurring, and open during the day when rapid gas exchange is necessary. During unusually dry conditions the stomata may close to prevent wilting, and photosynthesis is reduced.

LEAVES ARE ESSENTIAL TO PLANTS

A close-up photograph of a tulip tree leaf showing the veins, which transport water and food to and from the leaf’s cells. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

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Leaves are like a food factory for a plant since they begin with raw materials and process them internally to produce glucose, which the plant uses for growth, development, and reproduction. The plant itself can also become food for primary consumers that eat parts of the plant and obtain the energy the plant has stored. Within the leaf’s internal structures called chloroplasts are the plant’s main light-absorbing compound called chlorophyll. It is this pigment that gives plants their typically green color. Photosynthesis begins as the leaf lets carbon dioxide in through its stomata and obtains water from its roots through its veins. When sunlight strikes the chlorophyll in the chloroplasts, light energy splits the water into hy-


drogen and oxygen. Hydrogen combines with carbon dioxide to make the simple sugar glucose, and oxygen is released through the stomata as a byproduct. All of this occurs within the leaf at the cellular level.

Life Cycle

HOW LEAVES DEVELOP At the very beginning of their existence, leaves are contained in embryo form within the seed and are called a cotyledon. Once the seed germinates, or sprouts, the cotyledon emerges and eventually becomes the first true leaf. As the plant matures, more leaves are formed from buds that formed on the stem. Once the bud begins to unfold and open, the leaf begins its growth period and reaches full size anywhere from one to several weeks. The mature leaf turns a deep green and begins to make food for itself and the rest of the plant. Although a leaf contains other colors, they are masked by the chlorophyll (a green pigment). As autumn approaches, however, the plant releases a hormone and the chlorophyll starts to break down and eventually disappears from the leaves, allowing the remaining colors of yellow, orange, or red to finally be seen. Once the chlorophyll breaks down, the leaf no longer makes food, its veins become plugged, and it soon withers and dies. A layer of cells grow across the base of its petiole, shutting it off from the stem, and the leaf soon dries, twists in the wind, and breaks off. When a tree’s dead leaves fall to the ground, they take away some of the waste products the tree produced. They also eventually become food for bacteria and decay on the ground, adding essential humus or organic matter to the soil and offering new nourishment for other plants to use. Leaves are vital to life on Earth. Since they are the actual site where photosynthesis occurs, they are the first link in the food chain (the series of stages energy goes through in the form of food), providing food to animals. They are not only the factories of the primary producers (plants), but they help make the air breathable for animals. Without the oxygen that plants give off during photosynthesis, Earth’s supply of breathable oxygen might be eventually used up. People also use leaves for many products, from tea and herbs, to lettuce and spinach, to drugs like digitalis and tobacco. [See also Photosynthesis; Plant Anatomy; Plants]

Life Cycle The term life cycle describes the series of predictable changes that an organism goes through until it is mature enough to reproduce. Knowledge U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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of the major stages or changes that all species undergo during their lives is essential to the study of the life sciences. Studying an organism from birth to sexual maturity is an ideal way to learn what is most important and essential to its life and continuance. For some species, a complete life cycle is only fifteen days, while for others it can be decades. However, during the normal life cycle of every organism, growth and reproduction always take place. Between birth and sexual maturity, some species go through a long sequence of basic changes over time while others appear to make a direct trip. For example, although mammals are relatively complex animals, their life cycle is fairly straightforward. Mammals begin to develop from a fertilized egg and once born, they simply continue to develop or grow. There is certainly much variation between mammals, since a human baby takes about eighteen months to learn to walk, while a horse will stand up almost immediately at birth and romp in a day. Childhood for mammals also varies in length. Humans enter puberty (the stage at which they begin to mature sexually) in their early teens, while a dog may be ready to have a puppy before it is a year old. Despite these differences, the life cycle basics are nearly the same for all of the higher animals (birth, growth and sexual maturation, fertilization, birth).

INCOMPLETE METAMORPHOSIS

Opposite: A labeled diagram of a butterfly’s life cycle, which is a good example of complete metamorphosis. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

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While some lower organisms have simpler life cycles, there are many animals and plants with life cycles that are not so straightforward. Some animals go through complex life cycles in which they physically become an entirely different type of individual. In other cases, a period of asexual reproduction (without the union of sperm and egg) is followed by a period of sexual reproduction. For example, a grasshopper has a threestage life cycle called incomplete metamorphosis. After an adult female grasshopper lays an egg and buries it, the egg develops and eventually hatches. What emerges from the shell is called a nymph. At this stage in its life, the nymph may look like a miniature adult but it has no wings and no working reproductive organs. As the nymph grows, it periodically sheds its outer skin or molts, and with every molt it becomes more of an adult. When it sheds its skin for the fifth and final time, it has become an adult grasshopper and is ready to mate and reproduce.

COMPLETE METAMORPHOSIS Other insects, like a moth or butterfly, go through a much more complicated process called complete metamorphosis. After an adult female moth lays its eggs and they develop and mature, what hatches looks like U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

The Adult (Imago)

Wing vein Forewing

Cell

The Egg

Hind wing Thorax Head Antenna

Labial palp Compound eye Proboscis

Spiracle Abdomen Hind leg

The Caterpillar

Foreleg Middle leg Thorax

Simple eye Head Mandible Walking leg

The Chrysalis Abdominal segment

Cremaster

Proleg Spiracle

Abdomen Anal proleg

Wing Metathorax Antenna

Mesothorax Prothorax


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a worm and is called a larva. A larva is the caterpillar stage in a moth’s life cycle. As a caterpillar, the larva is nothing more than an eating machine, and its body is built to help it consume as much food as possible. It has a long body with three pairs of true legs. It also has a large head with strong jaws that allow it to feed on plants. Many caterpillars have some form of camouflage or coloring that allows them to blend in with the plants they eat. Others may have bright warning colors and irritating hairs that keep predators away. After a series of molts, or outer skin shedding, the caterpillar produces an outer covering around itself called a cocoon and attaches itself and the cocoon to a branch. Inside these coverings, most of the larva’s cells are broken down and begin to reform as a pupa. As the pupa develops inside, it is reformed and transformed into an adult insect, and a moth or butterfly emerges. As an adult, the insect is soon ready to reproduce and its life cycle is complete.

ALTERNATION OF GENERATIONS Discovering the details of an organism’s life cycle can sometimes be essential to understanding its true nature. For centuries, no one was able to discover how ferns reproduced. It was long thought that since a fern was a green plant, it had to produce seeds (and therefore reproduce sexually with male and female sex cells). Yet finding a fern’s seeds proved impossible. Botanists (people who specialize in the study of plants) were only able to solve this problem by closely studying a fern’s life cycle. It was finally discovered that ferns, as well as other plants like mosses, reproduce by spores and not seeds. Also, it was found that a fern has a sexual stage that alternates with an asexual stage that produces spores. This process of going through two different plant forms in one life cycle is called the alternation of generations. A fern’s life cycle begins when a mature fern plant produces spores inside little cases, which are attached to the underside of the fronds (leaves). Called sori (singular, sorus), these dark brown dots are sometimes mistaken for bugs or disease spots. When the spores mature, their cases split open and the tiny, light spores are sometimes carried great distances from the parent plant by the wind. When it lands in an inviting place, the spore develops into a tiny green plant called a gametophyte and produces sperm and egg. This is the sexual stage of the fern. When sperm and egg unite during rains or with dew, a fertilized egg forms and the asexual stage of its life begins. The egg develops into a new individual spore-producing fern plant, which will begin the cycle all over again. [See also Larva; Metamorphosis] 354


Light

Light

Light is energy from the Sun that we can see. Light is essential to all life on Earth, as it is the source of food, air, and warmth. Visible light is actually made up of a spectrum of colors. Although all life on Earth depends on light, it is easy to take it for granted. However, a world without light is almost impossible to imagine. Without light from the Sun, people’s eyes would not work. Also, plants would not make their own food, which feeds other animals. Nor would plants give off any oxygen as a by-product of making food, and there would be no breathable air. Without light, there would be no warmth, and Earth would be as cold as the deepest part of outer space. On Earth, therefore, light means livable conditions and life itself. All light is really energy that travels through space from the Sun. Sunlight is a form of energy called electromagnetic energy, or electromagnetic radiation. Physicists (a person specializing in the study of matter and energy and the interactions between the two) have long known that there are many kinds of this radiant energy that streams from the Sun in waves. The visible light from the Sun is only one type of radiant energy. The other types of radiant energy are known as gamma rays, x rays, ultraviolet, infrared, microwaves, and radio waves. The entire range of this energy, including visible light, is called the electromagnetic spectrum. Each of these forms of radiant energy travels in waves and each has its own wavelength. A wavelength is the distance from one wave peak to the next. Gamma rays have the shortest length, while at the opposite end of the spectrum are radio waves, which have the longest length. Visible light is somewhere in between these two and is the part of the electromagnetic spectrum to which the human eye is sensitive. Although this light appears white to the average person (some may say it is clear), it is really made up of another spectrum, a spectrum of colors. Thus the visible spectrum actually includes all the colors of the rainbow. Finally, when this light streaming from the Sun encounters matter, the light is either reflected, absorbed, or transmitted to someplace else. What determines this is the color, or pigment, of the matter the light meets. Different pigments absorb different parts of visible light. The color of a pigment is determined by the type of light that it reflects or transmits. Thus, green pigment looks green to human eyes because it transmits and reflects green light. It also absorbs red and blue light, which we do not see. A black pigment absorbs all visible light, while a white pigment reflects all colors of visible light. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Lipids

One of the great discoveries of science is the first law of thermodynamics. It is also called the law of conservation of energy. It states that energy can be neither created nor destroyed, although it can be changed from one form to another. This changing of the form of energy is what enables light to play such an essential role in the maintenance of life on Earth. Light energy from the Sun can be transformed into heat energy when it is absorbed by Earth. Even more important is the change from light energy to chemical energy. This occurs during photosynthesis. Remarkably, plants absorb less than 1 percent of the sunlight that reaches Earth. This is enough, however, to allow every plant on Earth to grow and make food through the process of photosynthesis. This chemical process begins with sunlight. It carries out a chain of chemical reactions that produces not only food for the plant but oxygen for the atmosphere. When humans breathe the air and eat their food (animal or vegetable), they are incorporating the energy from the Sun (light) into their own beings. Light is therefore truly the source of all life. [See also Photosynthesis]

Lipids Lipids are a group of organic compounds that include fats, oils, and waxes. Lipids are important because they are a concentrated source of energy. They also serve as an important building material for cells and have many industrial and commercial applications. Lipids are organic or natural substances that are produced by animals and plants. Common lipids include butter, vegetable oil, and beeswax. Lipids are not soluble in water, meaning that they cannot be dissolved in it. In fact, lipids repel water. Fats and oils are both lipids, yet they are different. Fats are usually solid or semisolid at room temperature (like butter), while oils are liquid at room temperature. Lipids are classified as saturated or unsaturated depending on their chemical structure (the type of bonds between the atoms). It is these bonds that make fats solid and oils liquid at room temperature. Animals and plants store fats in their cells to use as an energy reserve. Plants usually store lipids in their seeds, while animals store them in cells of their skin. When needed, both can convert them back into fatty acids (which are made up of carbon, hydrogen and oxygen and are therefore a basic energy source). Many mammals use this layer of fatty deposits below their skin to keep warm in cold weather. Body fat is an insulator against low temperatures and internal heat loss. It is also an 356


excellent shock absorber. Animal fats are rich in saturated fatty acids, while plant oils are rich in unsaturated fatty acids.

Lipids

Lipids are an important part of a healthy human diet and are needed for normal growth, blood clotting, and healthy skin. They also are essential to the proper hormonal functioning of many animals. People also have found many practical uses for lipids, and use them in the production of many industrial products such as cosmetics, cleaners, and lubricants. Much of the soap, detergents, and cosmetics we use are made from purified animal and plant sources. Besides fats and oils, lipids also include waxes. Waxes are soft, slippery substances that are similar in their chemical structure to fats and oils. Waxes usually resist attack by other chemicals and are produced by plants and animals. Plants use waxy lipids to coat their leaves and fruit and to prevent moisture loss. Animal skin is covered with a waxy lipid, and lamb’s wool is protected by a very soft wax called lanolin. The wax made


A transmission electron micrograph of lipid droplets in a rat fat cell. (©Photographer, Science Source/Photo Researchers.)

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Lymphatic System

by bees is also considered to be a lipid. While lipids are generally considered a type of biological fuel since they can be converted into energy, wax can be digested by very few animals. Although some portion of a healthy diet should include lipids, diets high in animal fats are known to cause serious health problems, such as arteriosclerosis (the abnormal thickening and hardening of the arterial walls), heart disease, and cancer. Generally, animal foods are rich in both saturated fats and cholesterol, while plant foods contain unsaturated fats and no cholesterol. Cholesterol is a kind of lipid called a steroid, and although it is essential to the body, too much cholesterol can accumulate in the arteries and cause heart disease. For this reason, it is important for humans to maintain a diet low in saturated fat and cholesterol. [See also Metabolism; Nutrition]

Lymphatic System The lymphatic system is a network of vessels and channels that branch throughout the body and bathe cells in a fluid called lymph while also filtering out foreign material from the blood. As the body’s second circulatory system, the lymphatic system is the body’s transport mechanism for necessary fluid (lymph) and also makes up part of the body’s defense system by carrying lymphocytes (specialized white blood cells) that fight infection. As blood circulates throughout the human body, a tiny fraction of its plasma (the fluid part of the blood) leaks out and collects around body cells and tissues. The lymphatic system exists partly to prevent this fluid, called lymph, from building up in the tissues of the body. It does this by a system of tubes with thin walls that absorb the excess fluid and move it slowly through the body. The lymphatic system does not have a pump like the heart to move its fluid, but instead uses the normal movements of the body to slowly push the fluid in a one-way direction. Lymph vessels do more than just transport excess fluid however, for they also pick up fat from the digestive tract and transport it to the blood where it will be used for energy. It also plays an important role in the movement of white blood cells in the body’s immune response. The complete lymphatic system includes lymph nodes and lymph ducts as well as the thymus and the spleen. The system branches to nearly all parts of the body, excluding the brain or spinal cord, draining and filtering lymph and returning it back into the bloodstream. Lymph is a col358


orless fluid that, although it is derived from blood, contains no red blood cells. As it seeps out of the capillaries (hairlike blood vessels) and circulates through lymph vessels, lymph is filtered as it passes through lymph nodes that are clustered throughout the body. These lymph nodes are round structures that are about the size of a pea. They act as filters, catching bacteria, toxins, dead cells and other particles, and destroying them. The main groups of lymph nodes are located in the neck, armpit, chest, abdomen, and groin. These nodes often become swollen and painful as they produce and supply extra white blood cells to fight an infection. When this happens, we experience what are called “swollen glands.”

Lysosomes

The thymus, located behind the breastbone and between the lungs, is also part of the lymphatic system since it secretes a hormone that tells the body’s bone marrow (tissue that fills the bone cavities) to produce specialized white blood cells called lymphocytes. It is these cells that attack and destroy invading microorganisms as well as create antibodies (a specific protein targeted to kill a specific invader). The lymphatic system is the main highway for the patrolling lymphocytes that are always on the lookout for foreign bodies. Another part of the lymphatic system, the spleen, filters lymph and removes waste and other materials (like old red blood cells). In humans, the spleen is located below and behind the left side of the stomach. Altogether, the lymphatic system can be thought of as the body’s fluid drainage network that filters and destroys foreign particles through the transportation of infection-fighting cells. [See also Antibody and Antigen; Immune System]

Lysosomes Lysosomes are small, round bodies containing digestive enzymes that break down large food molecules into smaller ones. They are found in the cytoplasm, or jelly-like fluid, of all eukaryotic cells (cells with a distinct nucleus). Lysosomes are the main site where digestion takes place inside a cell. As organelles or specialized, membrane-bound structures inside a cell that have a certain job to do, lysosomes contain very powerful enzymes called “hydrolases” that are capable of breaking down many different types of substances. These enzymes work on food molecules such as proteins, carbohydrates, and fats and quickly break them down into smaller particles that can be easily used by the cell. The powerful enzymes in lysosomes are also sometimes put to another use by the cell when it needs to rid itself of a damaged or defective organelle. In such a case, the lysoU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Lysosomes

somes attack an organelle and quickly break it down and destroy it. At other times, a cell may use lysosomes to actually destroy itself. This process is known as “autolysis” (auto means “self” and lysis means “destruction”). This usually happens for a very good reason, as in metamorphosis when an animal has to entirely reshape its tissues (as when a caterpillar changes into a butterfly). While biologists know that lysosomes are used by the cell to digest the food it takes in, they do not yet fully understand how the lysosome membrane itself avoids being broken down by the enzymes it carries. This is especially puzzling since the membrane is made of the same compounds that the enzymes easily destroy. [See also Cells; Enzymes]

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M Malnutrition Malnutrition is the physical state of overall poor health. It can result from a lack of enough food to eat or from eating the wrong foods. Malnutrition is most common in developing countries where people do not get enough to eat or are able to eat only one type of food. If nutrition means eating enough of the right kinds of food to stay alive and healthy, then malnutrition is literally “bad” nutrition. Malnutrition affects a large part of the world’s population. Even in developed countries, many children suffer from some forms of diet deficiency similar to malnutrition. Since animals cannot photosynthesize, or make their own food, as plants do, animals must get all their nutrients (the substances necessary for life) from their food. A balanced diet would contain all the basic types of nutrients that an animal needs. These include proteins, fats, carbohydrates, vitamins, and minerals. Malnutrition can be the result of a lack of many of these vital nutrients or of one particular nutrient. Few people, if any, choose to be malnourished. This unhealthy condition can have one or more causes. External circumstances, such as war or a crop failure, can make it impossible to get enough food to eat. Very often, essential foods can become scarce or completely unavailable. Poor eating habits can also lead to a person eating only one kind of food and excluding many others. Finally, a physical condition can prevent or impair the proper digestion and absorption of food. For example, people in developing countries who drink contaminated water often get prolonged cases of diarrhea. They then lose essential nutrients from their bowels. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Malnutrition

Two malnourished Sudanese children. Their huge bellies are the result of a fluid imbalance due to protein deficiency. (Reproduced by permission Reuters/CorbisBettmann.)

PROTEIN-ENERGY MALNUTRITION Whatever the external reason, probably the most serious form of malnutrition is called protein-energy malnutrition. The terribly sad pictures of very young children with large, swollen bellies are examples of protein deficiency. Their huge bellies, made so large by a fluid imbalance, seem to deny the fact that they are in fact starving to death. Proteins, which are made of amino acids, are essential to human life since they are needed for cell growth and repair. Of the twenty amino acids needed by the body, ten can only be obtained through diet. Without a certain amount of these essential amino acids, a person’s body systems will stop working normally, resulting in sickness and even death. In many very poor countries, people often eat very little protein and live mainly on a diet very high in carbohydrates. This is understandable since carbohydrates, like corn and other grains, are fairly cheap to grow and process. The protein that is found in meat, fish, poultry, and milk is naturally more expensive and in shorter supply. Lacking proteins, the malnourished person’s body also cannot make the enzymes (proteins that control the rate of chemical changes) necessary for all of the chemical reactions required during digestion. When a person first begins to be severely deprived of food, called starvation, the body starts a process of using up its stores. First to be used are the carbohydrates, since the body stores very little of these. After two days or so, the carbohydrates are gone and the body turns to any stored fat it may have. Not until it has turned all of the fat it has into energy, and only after it has no more, will the body begin to turn to its own protein sources. Some describe fat as a protein protector. However, with all fat gone, the body must begin to use protein as a source of energy. Proteins are essential for enzymes and hormones (chemical mes-

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sengers) and serve as the body’s building blocks. Using up this precious source without replacing it can only result in the body’s cells not doing their jobs. The body, in effect, starts to consume itself. In children, this results in slowed mental and physical growth. Since their immune systems (a collection of cells and tissues that protect the body against disease-causing organisms) have shut down, these children cannot fight infections and easily contract diseases. Besides their swollen bellies, their hair often falls out and their muscles waste away. Needless to say, they are listless and have little energy before they finally die.

Mammalogy

Adults who are malnourished show the same mental dullness as children, and they also easily contract diseases. Mothers give birth to severely underweight babies and cannot produce enough breast milk to feed them. The lack of a certain vitamin or mineral can also result in a particular deficiency disease, but this is more easily corrected than is a severe case of general malnutrition. As the world population continues to grow at an alarming rate, it appears that malnutrition may continue be a constant problem for developing countries. [See also Blood; Nutrition]

Mammalogy Mammalogy is the branch of zoology that deals with mammals. Major subject areas in mammalogy include anatomy (structure), physiology (function), behavior, ecology, evolution, and classification. Humans are mammals and belong to the class Mammalia which is one of the most diverse groupings of animals. Mammals are among the smartest, fastest, and largest animals on Earth. The cheetah is the fastest mammal; the blue whale is the largest; and human beings are the most intelligent. There are 18 orders of mammals, containing about 4,000 living species. Aside from a couple of exceptions, all mammals have certain things in common. They are warmblooded (they maintain a constant internal temperature despite their environment), have hair on their bodies, give birth to live young, and feed their newborns with milk from their mammary glands. It is because of these milk glands that mammals got their name. Beginning with the Greek philosopher Aristotle (384–322 B.C.), the Greeks were the first to systematically study, categorize, and write about mammals. In fact, it was Aristotle who recognized that both whales and dolphins were really mammals and had more in common with land-based animals than they did with fish. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Mammalogy

Mammalogy allows us to understand how and where a mammal lives, what are its habits and behavior, and how it reproduces. Aside from the common mammal features already mentioned, there are other characteristics of mammals that deserve mention. All mammals have a basic structure. They all have skulls that house a brain, and they all have seven vertebrae (bony segments) in their necks, whether their neck is as long as a giraffe’s or as short as a dormouse. Most have four limbs that end in five digits (finger-like projections). Their teeth are adapted to their feeding habits, and most carry their unborn inside their bodies until birth. Mammals have highly developed senses, although not all are so sharp in every mammal. Some rely more on keen vision, while others depend most on their sense of smell. Most mammals are herbivorous (feeding on plants), while fewer are carnivorous (feeding on meat or other animals). Mammals are found in almost all habitats, and many actually build some sort of shelter or dwelling for themselves. For example, beavers build underwater lodges, gorillas make beds of palms, and prairie dogs have underground tunnels. Some mammals hibernate, or enter a sleeplike state, during winter, while others migrate or travel some distance to avoid winter weather. While many mammals live alone as adults, many also live in groups of different sizes. For example, humans live in families typically consisting of a male, female, and their offspring while beavers live in family groups. Monkeys live in larger groupings called bands, while sheep live in larger groups called herds. The largest grouping of mammals are called colonies, and this is how bats live. As social animals, mammals need to communicate, and they do this in many different ways. Some communicate by a scent and give off a certain smell when they are in heat (and are ready to mate). Many use visual signals. For instance, while a gorilla may make certain facial gestures, a wolf will assume a certain body stance. Most mammals, however, use sounds to communicate, from a coyote’s howl to a beaver’s tail-slapping. At the beginning of the twenty-first century, it is safe to say that nearly all of the world’s mammal species are known to science. Yet the same cannot be said about the biology of every species. Twenty-first century techniques and technology will give mammalogists (people who study mammals) the ability to study free-living (animals in the wild) animals by the use of data obtained from tiny radio transmitters placed on the animals. These transmitters will also allow scientists to learn more about the genetics of mammals. [See also Mammals]

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Mammals

Mammals

A mammal is a warm-blooded vertebrate (an animal with a backbone) animal with some hair that feeds milk to its young. Mammals are the most diverse as well as the most successful vertebrate, and can be found living in nearly every habitat on Earth. Mammals are also the most advanced or intelligent animal and have become the dominant form of life on Earth. Humans are mammals. The animals that make up the amazingly diverse class known as Mammalia range from the largest animal that ever lived, the blue whale that can reach 100 feet (30.48 meters) and 150 tons (136.05 metric tons) to the hog-nosed bat of Thailand that is the size of a bumblebee. Mammalia includes human beings, whose intelligence shaped the world as it is today, and the grotesque and slow-moving sloth that hangs upside-down from a tree most of its life. Rats are mammals, and so are dolphins, monkeys, and giraffes.

COMMON TRAITS OF MAMMALS Despite the extreme differences among species of mammals, all have several traits in common.

Although they act more like fish, whales are considered mammals. The blue whale pictured here is one of the largest animals to ever live. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Mammals

Mammary Glands. First, mammals get their name from the characteristic “mammary” glands that females use to feed their young. After a pregnant female gives birth, her mammary glands secrete milk that the young drink by sucking. This liquid substance provides all the nutrients that a young mammal requires to grow and develop. Some mammal species suckle their young for only a few days, while others, like the elephant and humans, may nurse them for more than a year. A mammal also usually gives birth to live young. Except for only three species out of approximately 4,300, the young of every mammal develop inside the body of the mother who delivers or gives birth to live young instead of laying eggs. This makes mammals placental animals (named after the organ called a placenta which nourishes the developing embryo while it is inside its mother’s uterus). Another characteristic related to their offspring is that mammals usually care for their young, even after the young cease nursing on mother’s milk. Mammal parents both protect the young from enemies and teach them necessary survival skills. Hair and Teeth. Interestingly, body hair is a mammalian trait, and all mammals have at least some hair or fur on their bodies at some stage in their development. Even smooth-skinned whales and dolphins have hair at birth. Mammal hair and fur is made of long thin strands of protein called keratin. Many mammals have long, stiff hairs around their head or mouth that act as feelers and allow them to get around in the dark. The keratin in some mammals has adapted into a protective device, such as the quills of a porcupine. Hair insulates mammals in much the same way that feathers keep birds warm. This hair insulates and helps keep body heat from escaping. The fact that mammals can generate their own heat (as long as they have enough food to eat) means that they are endothermic or warm-blooded. This is a distinct advantage because it means that mammals can maintain their own internal body heat despite living in a cold climate. Unlike a cold-blooded animal whose body temperature rises and falls with that of its environment (and as a result, becomes lethargic in cold weather), mammals are always ready to spring into action. This is a necessary trait if one is either the hunter or the hunted. Most mammals are also able to cool their bodies in hot weather because they have sweat glands on their skin that produce moisture. This moisture then evaporates and cools the mammals’ bodies. Mammal teeth are also specially adapted to their feeding habits. Mammal carnivores (meat-eaters) have distinctive canine teeth that allow them to catch, hold, kill, and eat their prey. Herbivores (plant-eaters), however, have flat, grinding teeth for breaking down the tough cell walls of plants. Beavers and squirrels have teeth that continue to grow, and

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an elephant’s tusks are really its incisor teeth (what humans call their front teeth).

Mammals

Senses. Mammals also have highly developed senses, although few species have acute capabilities in all five senses (sight, smell, taste, touch, hearing). Most have an excellent sense of smell since this sense is not only necessary in locating food that may be out of sight, but it is crucially important in alerting an animal to danger. Mammals also usually have an excellent sense of hearing and usually, the larger a mammal’s ears, the more it relies on that sense for its survival. For example, a rabbit’s primary defense is its quickness and, therefore, its large, long ears serve as an excellent early-warning system. Skeletal System. As vertebrate animals, all mammals also share a basic skeletal structure. They all have a bony skull that houses a brain and key sense organs (such as eyes, ears, and nose). All have a backbone or vertebral column consisting of individual vertebrae. Except for sloths and manatees, all mammals have exactly seven vertebrae in their necks. Thus a giraffe has seven very large ones, and a mouse also has seven very small ones. Most mammals have four limbs, each of which usually ends with five digits (fingers and toes in humans). Mammals also have a distinctive, four-chambered heart (as birds do). This type of heart keeps oxygen-rich blood separate from the oxygen-deficient blood by having each flow in and out of the heart in its own system. Because of this system, the mammal heart can quickly deliver large amounts of high-energy oxygen on demand.

Behavior. Unlike invertebrates (animals without a backbone), mammals exhibit what is usually called complex behavior. For example, mammals act on instinct, a pattern of behavior that is inborn rather than learned. When a newborn human immediately seeks its mother’s breast to suck or when a bear hibernates for the winter, these mammals are following instincts (or natural drive to do something). Mammals have larger brains than other animals and can be said to learn in some ways. If a mammal changes its behavior because of repeated experiences, then mammals can learn. When a mammal avoids a situation that was dangerous in the past and seeks another alternative that was previously beneficial, the mammal also exhibits a type of learning. Such behavior leads to the statement that mammals, and not just humans, are the smartest animals on Earth.

The Brain. Mammals have a large brain with a well-developed cerebral cortex. The cerebral cortex is the part of the brain involved in memory, sensory perception, and learning. Mammals also often live in social groups of different sizes. The smallest social group includes a male, a female, and young. Others live in larger groups called bands, and others in stillU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Meiosis

larger groups called colonies. The largest group of mammals would be composed of thousands and is called a herd. In bands of mammals, like monkeys, there is always a ranking of individuals with the most dominant member acting as a sort of boss or leader.

Communication. Mammals also establish bonds between one another. Mammals interact with one another by communicating in many different ways. Many use smells to tell friend from stranger or to mark their territory. Visual signals are also used, as when a rabbit flashes a patch of white hair under its tail (danger), or when a dog bares its teeth and lowers its tail. Sound is the most obvious way to communicate, and cries or whistles of danger are different noises than those of mating calls. Altogether, the behavior of mammals can be very complex.

Development of Young. Among the 4,300 species of mammals, all can be placed in one of three groups based on how their young develop: monotremes, marsupials, and placentals. There are only two species of monotremes, which are by far the strangest type of mammals, since they lay eggs. The duck-billed platypus and the spiny anteater lay leathery eggs instead of giving birth to live young. However, they have mammary glands and nurse their young. A marsupial is a mammal whose young complete their development inside the mother’s pouch. Kangaroos and koalas, as well as wallabies and opossums, carry their young inside their bodies for a short period, after which they give birth to a tiny, barely developed marsupial that crawls into its mother’s pouch where it nurses and grows. All other mammals are placental, meaning that they are nourished inside the female’s body until birth. Some mammals, like the horse, are able to walk within minutes of birth, while others, such as a human infant, are helpless and require years of care. The diversity of mammals can be amazing. For example, bats are the only mammals that can fly, and rodents are the largest group of mammals. Whales, porpoises, and dolphins are aquatic mammals (their fins are limbs) that breathe air, and elephants are the largest land mammals. This being the case, mammals are an interesting group of animals to study and learn from. [See also Mammalogy]

Meiosis Meiosis (may-OH-sis) is a specialized form of cell division that takes place only in the reproductive cells. The goal of meiosis is to produce sex 368


cells (sperm and egg) that have only one set of twenty-three chromosomes. When a sex cell unites with another sex cell, the zygote (fertilized egg) will have the proper total of forty-six chromosomes.

Meiosis

It is important to distinguish meiosis from mitosis (my-TOH-sis). Although both are a form of cell division, mitosis produces two identical cells, while meiosis produces four different cells. Mitosis makes new, identical cells so that an organism will be able to replace damaged and dead cells and be able to grow. Nearly all the cell division in an organism can be described as mitosis. Meiosis only occurs in an organism’s sex cells and is structured so that it deliberately produces different rather than identical cells. Without meiosis producing differing cells, there would be no variation in offspring, who also would have twice the number of chromosomes than they should have. If mitosis occurred in reproductive cells the way it does in all other cells of the body, the new cell produced would have twice the number of chromosomes that it should have. For example, the exact amount of chromosomes needed to be human is forty-six. Without meiosis cutting the number of chromosomes in half, that number would be ninetytwo chromosomes after fertilization has taken place. Meiosis splits in half the number of chromosomes in sperm and egg cells, so when the cells unite, the zygote will get half the number of chromosomes from each parent. Besides halving the number of chromosomes, meiosis also performs another very important function. It allows genetic material to be “shuffled” as the chromosomes cross over each other and swap genes before the cell divides. This is a random exchange of genetic material that guarantees that an entirely new individual will be produced after fertilization. Because of this shuffle of genetic instructions, each reproductive cell is given its own unique set of instructions. This assures that no two sperm or egg cells have the same exact combination of genes. This also partly explains why brothers and sisters (except identical twins) of the same parents have different characteristics. Eventually, when two of these unique sex cells are joined as sperm and egg and form a new individual (thus further mixing the genetic instructions), an entirely unique organism is created unlike any other existing organism. The variations or differences caused by meiosis are very important to evolution, since the process of natural selection (the process of survival and reproduction of organisms that are best suited to their environment) needs genetic variety from which to “select.” If there were no differences, there would be no evolution. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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In humans, meiosis occurs in the gametes or sex cells (sperm and egg). In males, the process of gamete production is known as spermatogenesis. During this process, each dividing cell in the testes produces four functional sperm cells, all basically the same size. In contrast, the female process of producing eggs, called oogenesis, makes four eggs, only one of which survives. This is because nature gives all the necessary cytoplasm (living material) and organelles (structures with particular functions) to only one egg, thereby increasing its chances of survival should it become fertilized. [See also Cell Division; Fertilization; Reproduction, Sexual]

Membrane A cell membrane or plasma membrane is a thin barrier that separates a cell from its surroundings. It also keeps the cell’s cytoplasm and organelles on the inside. Membranes are selectively permeable, meaning that some things can pass through the membrane and some cannot. All cells have a cell membrane that is a thin but double layer of molecules that surrounds it. This membrane acts as a barrier and helps protect the cell while controlling the movement of substances in and out of

A freeze fracture image across the cell wall and membrane of a blue-green alga. (Reproduced by permission of Phototake NYC. Photograph by Dr. Dennis Kunkel.)

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the cell. The membrane also allows a cell to maintain a constant internal environment, despite changes in its external environment. It is able to do this because of the semipermeable nature of its layers that regulate the passage of all substances going through it. The membrane is able to keep things out that it does not want or need while allowing in what it must have. A membrane surrounds not only the entire cell, but each organelle or specialized structure inside the cell also has a membrane around it. For both the cell and its organelles, the membrane is a place of constant activity. Although this membrane is extremely thin, it is very strong and can heal itself if broken. Examined very closely, a membrane is like a mesh bag whose little, square holes are small and strong enough to hold a dozen oranges or five pounds of potatoes, but which will also let water flow out or in completely. The bag is semipermeable, since it keeps certain-size things in (oranges) but let things of another size (water molecules) pass through. Most cell membranes are permeable to oxygen and water but not to large organic molecules like proteins.

Membrane

One way that substances move through a membrane is by a process called passive transport. Passive transport involves no use of energy on the part of the cell, since certain substances are able to move freely in or out of it. Diffusion and osmosis are forms of passive transport since the cell does not need to use any energy to move substances. In diffusion, molecules of a substance spread themselves out more evenly from an area of high concentration. Substances like carbon dioxide, salts, and oxygen move in and out of cells this way. In osmosis, water molecules move from an area where they are crowded and cross a membrane to where they are less crowded. This process stops of its own accord when the solutions on either side of the membrane are at equal strength. Active transport is a different way that a cell moves molecules in or out through its membrane, and it involves the use of energy. Active transport occurs when a cell wants to bring in more of a substance than will enter via passive transport. This happens when a plant’s nearly-full root cells want to store even more minerals, and must move them from an area where they are less crowded to one where they are more crowded (inside the cell). In order to pack in more molecules, the cell uses carrier molecules that literally carry the desired molecule to a membrane slot into which it fits and then forces it into the cell. This process requires that the cell uses its own energy. Cell membranes are much more than walls or barriers that hold a cell together and keep it separate from its environment, since membranes control the movement of substances into and out of a cell. [See also Cell; Diffusion; Osmosis] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Mendelian Laws of Inheritance

Mendelian Laws of Inheritance The Mendelian laws of inheritance laid down the basic principles of genetics. They state that characteristics are not inherited in a random way, but instead follow predictable, mathematical patterns. Mendelian laws were formulated by Austrian monk and botanist (a person specializing in the study of plants) Gregor Johann Mendel (1822–1887) in 1865, but went unnoticed for nearly a half century. Before Mendel, many scientists had realized that certain traits, or characteristics, were passed on from one generation to the next, but in the middle of the nineteenth century, no one had any idea about where to begin to discover what controlled them or how or why these traits were passed on. In 1857 Mendel was able to combine his interest in both botany and mathematics by undertaking a long-term study breeding garden peas. For the next eight years, Mendel was able to conduct a thorough scientific study of how traits pass from one generation to the next. By using ordinary garden peas—like those we eat today and call sweet peas—Mendel was able to easily breed for what are called “pure traits.” This means that a self-pollinated (plants that contain both male and female reproductive organs and are able to transfer pollen between these parts) plant with pure traits will always produce offspring like itself. For example, a purebred plant that produces yellow pods will always produce yellow pods. Mendel then selected pea varieties that differed in single traits (such as height or pod color), and then he crossed them with plants that had a different trait (crossing tall plants with short, or yellow pods with green). After crossing a pure tall with a pure short, he would record the number of each type harvested and save the seeds produced by each plant for later planting, recording, and study. While Mendel was conducting these careful experiments, neither he nor anyone else had any idea that such things as chromosomes (coiled structures in a cell’s nucleus that carries the cell’s heredity information) and genes (basic units of heredity) existed, although he would eventually decide that plants contained something he called “factors” and “particles of inheritance.” He came to this conclusion because of the pattern of results he eventually saw. The very first thing that Mendel discovered after crossing a pure tall plant with a pure short one was that it did not result in the production of medium-size offspring. Instead, in the first generation, all the plants were tall. However, after allowing these plants to self-pollinate, he saw that the next generation produced plants that were a mix of tall and short. In fact, three-quarters were tall (which he called

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a dominant factor), and one-quarter were short (which he called a recessive factor). Mendel continued crossing hundreds of plants and kept careful records. Eventually he was able to state that a regular 3 to 1 ratio or pattern existed for the number of dominant versus recessive traits. This led him to realize that there must be laws or rules that make this mathematical ratio happen. Continued work and study eventually allowed him to formulate what are now called the Mendelian laws of inheritance. He stated correctly that the characteristics of an organism are passed on from on generation to another by definite particles (which he called factors and we call genes). These genes exist in pairs, which are really different versions of the same genetic instructions. In this pair, one of the two factors comes from the male parent and the other comes from the female parent (each contributes equally). Finally, traits do not blend but remain distinct, and they combine and sort themselves out according to fixed rules. Mendel also stated


Mendelian Laws of Inheritance

Two labeled diagrams showing Mendel’s first law of inheritance, the law of segregation. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

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that dominant genes always have an effect on an individual, but that two recessive factors have to be present before they are expressed. Recessive factors can therefore be present in an individual but not have any effect on its characteristics. Mendel published his findings in an obscure journal, and although his laws had laid the foundation for the new science of genetics, his work remained unknown for nearly two decades after his death. In 1900 Mendel’s work was separately discovered by three different botanists (in three different countries) who realized that Mendel had discovered the laws of genetics long before they had. Although each published his own version of these laws, each man cited Mendel as the real discoverer. All three honorably stated that their work was merely a confirmation of what Mendel had accomplished in 1865. [See also Chromosomes; Gene; Genetics; Inherited Traits]

Metabolism Metabolism refers to all of the chemical processes that take place in an organism when it obtains and uses energy. Metabolism can be divided into two major phases in which substances are broken down and other substances are made. All organisms conduct both phases constantly. If the body of a living thing is thought of as a machine, then its metabolism is similar to a running motor. In an organism, however, the motor is not only never turned off, but it is able to monitor itself and make adjustments according to internal and external changes that are always taking place. The job of the entire organism, just like the job of every living cell, is to conduct metabolic reactions continuously. These reactions all center around the processing and use of energy. Such reactions are needed by cells and the entire organism to constantly fuel itself, repair itself, and grow. The entire range of these chemical reactions make up an organism’s metabolism.

TYPES OF METABOLISM Metabolism can be divided into two phases or categories: catabolic metabolism (catabolism) and anabolic metabolism (anabolism). Catabolism is also known as destructive metabolism. It involves the breaking down of the molecules in nutrients taken in and the release of the energy they contain. Through catabolic processes, complex compounds like fats, carbohydrates, and proteins, are degraded (broken down) into simple molecules so 374


their energy can be released. Catabolism takes place in the body when food is digested. However, if the system needs energy and no food is available, catabolism can also break down the body’s stored fat and protein.

Metabolism

Anabolism is also called biosynthesis or constructive metabolism and can be considered the reverse of catabolism. By its different names, it is apparent that these types of chemical reactions involve the synthesis or the making of essential, complex molecules from simpler components. Anabolism is the body’s building-up phase in which it uses the complex substances it has just formed for growth and overall body maintenance. For instance, by combining amino acids (the building blocks of protein molecules), the body’s cells can form structural proteins and use them to repair and replace worn-out tissues. It can also form functional proteins such as enzymes to speed up chemical reactions, antibodies to fight disease, and hormones to regulate body processes. The remarkable thing about an organism’s metabolism is that its two phases of chemical reactions are going on constantly, stopping only when the organism dies. This constant and highly complex level of chemical activity has built-in control mechanisms that regularly monitor and adjust to all sorts of changing conditions. Hormones control metabolism, and thyroxine, which is secreted by the thyroid gland, determines the rate of metabolism (meaning the rate at which the body uses up energy to perform a certain metabolic function). The pancreas determines whether anabolism or catabolism is being performed, and then releases either insulin or glucagon. Since eating causes the level of glucose in the blood to rise, the pancreas responds by releasing insulin which, in turn, starts the process of anabolism. If the glucose level is low, the pancreas releases the hormone glucagon, which triggers the catabolic processes.

THE BASAL METABOLISM RATE (BMR) A person’s metabolic rate, or the rate at which it releases energy, is influenced by a number of factors, such as an individual’s age, sex, level of activity, general health, and hormone levels. Because an individual’s metabolic rate can provide a doctor with a great deal of information, it often needs to be measured. Since almost all of the energy used by the body is eventually converted to heat, the metabolic rate is usually calculated by measuring the amount of heat loss an individual displays during basal (resting) conditions. A person’s Basal Metabolism Rate (BMR), therefore, measures the amount of energy the body consumes in performing its maintenance operations, like normal breathing, heart beating, and minor movements while it is at rest (but not asleep). A person’s BMR is then judged to be normal or abnormal by comparing it to standardized U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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SANTORIO SANTORIO Italian physiologist Santorio Santorio (1561–1636) founded the modern study of animal metabolism (all of the chemical processes that take place in an organism when it obtains and uses energy). He introduced the idea of quantification (number or quantity of ) and measurement into the study of human physiology (the study of how different processes in living things work) and used mathematics and experimentation as his tools. He was an original thinker who was far ahead of his time. As part of a tradition that was not unusual for Italy, Santorio was given the same first name as his last. Even when he became well-known and his name was Latinized as Sanctorius Sanctorius, it was still the same double name. He was born in Capodistria (what is now Croatia), and his father was an official in the Republic of Venice. Santorio was soon sent to Venice where he was educated by tutors. He began the study of philosophy and medicine at the age of fourteen, and obtained his medical degree from the University of Padua in 1582. For the next decade or so, it is unclear whether he spent time in Poland working as a physician for the king. Biographers disagree, but it is known that he was often in Venice during this period. In 1611, however, he was appointed professor of theoretical medicine at the University of Padua on the strength of a medical book he had written in 1602. At Padua he was a very popular lecturer, and students came from all over Europe to attend. It was in this 1602 medical book that he first began writing about how important it was to measure things exactly when doing physiology. He also stressed that close and careful observation was equally important. Ten years later he published another medical book based on his more extensive

rates or rates that reflect the average BMR of healthy individuals of various ages. Typically, the BMR of males is higher than that of females, and both sexes have a lower BMR as they age. A recent discovery relates metabolism to the aging process, since animal studies suggest that there is some connection between substantially reducing calorie intake and living longer. It has yet to be determined what role metabolism plays in this phenomenon. Finally, since enzymes and hormones play such a key role in every metabolic process, a glandular problem or a genetic fault that affects the production of an enzyme can result in major metabolism problems. Such conditions as diabetes and Addison’s disease throw a person’s metabo376


research, and in it he described his use of a type of thermometer, probably invented by Italian physicist and astronomer Galileo Galilei (1564–1642), which he adapted to measure the warmth of the body. He was thus the first physician to use a thermometer on a person and to write about it. This could be said to be the first clinical thermometer. In yet another book written in 1614, Santorio described his experiment weighing a human being every day to determine the influence of everything that went into and came out of a body, including perspiration. He even designed an apparatus that had a chair built onto a large scale and was able to prove with this that people lost weight by the evaporation of their perspiration. His ability to measure all things related to the body was further improved when he invented a device to measure a person’s pulse rate. For more than twenty-five years, Santorio performed experiments on more than 10,000 subjects, using scales and similar measuring instruments. This led modern biologists to call him “the father of the science of metabolism.” If metabolism can be described as all of the chemical processes that take place in a living thing, then metabolism is indeed what Santorio was pioneering.

Metamorphosis

However, his work did not have any great impact on the science of his era, and many scientists believe it is because he was simply too far ahead of his time. Nonetheless, as science progressed, his modern ideas came to be more understood and appreciated. Santorio always stressed the importance of measurement, facts, and solid information, and regularly argued against such unscientific practices as astrology (the use of stars and planets to predict their influence on human affairs). He was in favor of applying all the new tools and instruments that science had in its possession to the study of the human body. Although he did not know the word for what he was doing, he was studying what is now called human metabolism.

lism off severely, while other people simply cannot tolerate or process certain foods, like milk.

Metamorphosis Metamorphosis comes from a Greek word meaning “transformation,” and is the term biologists use to describe the extreme changes that some organisms go through when they pass from an egg to a adult. A caterpillar turning into a butterfly is an example of complete metamorphosis. Metamorphosis often gives an organism some type of competitive advantage and usually occurs in organisms with short life spans like amphibians, U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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some fish, and various invertebrates (animals without a backbone), especially insects. Metamorphosis means much more than a physical change, however. In the process called complete metamorphosis, major differences occur not only in outward appearance but in an organism’s internal organs and processes as well. Sometimes, an adult or mature organism has an entirely different set of cells and organs compared to what it had in an earlier stage of its development. Many organisms look different as adults compared to what they looked like when just born or very young. For example, a crowing adult rooster does not resemble the chirping, yellow ball of fluff that it was as a chick. This is not an example of metamorphosis, but simply a difference in size and an elaboration of certain characteristics. For metamorphosis to occur, an organism must go through at least three stages of development during which it changes radically both inside and out. Metamorphosis does not happen in the life cycles of what are called “higher animals,” such as dogs, cats, or human beings. Rather, metamorphosis occurs only in certain “lower animals” like ants, butterflies, sea urchins, and frogs. During the most dramatic example of complete metamorphosis, an organism passes through four distinct stages of development and ends up being a completely different type of organism.

Opposite: Beginning with the larva in the lower left-hand corner, this illustration shows the transformations a gypsy moth goes through during metamorphosis. (Illustration courtesy of The Library of Congress.)

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The butterfly is probably the best-known example of an organism whose life cycle undergoes complete metamorphosis. In its embryonic or egg stage, it is deposited as an egg on a green plant that will serve as its food when it hatches. Its second stage is its feeding time and is called its larva stage. A larva is an insect that is in its wormlike stage. This stage begins when the egg hatches. In the case of a butterfly, what emerges from the egg is a caterpillar. It may be hairy or smooth and have distinct markings or little color according to what species of butterfly its parent was. At this point in its life, the larva or caterpillar has legs, and chewing mouthparts but no wings, and is best described as an eating machine. It looks more like a worm than an insect that will fly. The caterpillar eats and continues to grow and go through several molts (the shedding of its outer skin). As it grows, its body parts continue to be rearranged and modified and it finally matures into an adult caterpillar. At this point, it is ready to enter the third stage of complete metamorphosis called its pupa stage or cocoon stage. When ready, the larva attaches itself to a branch or a twig and forms a protective covering around itself. Butterflies form a hard, shiny shell called a chrysalis that hangs suspended from a twig, while moths spin their coverings out of silk and wrap themselves almost flat against the twig. It is during this pupal or resting stage that the larva U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

changes into the adult butterfly it will eventually become. Inside the chrysalis or cocoon, most of the larva’s cells are broken down and new tissues and organs begin to develop. Eventually, an entirely new and different organism is created using the raw materials left by the old one. When this transformation is finished, the chrysalis or cocoon breaks open and the adult butterfly emerges. After its shrunken wings stretch and fill with blood, they soon are strong enough for the butterfly to fly away. This is its final stage—called its adult stage. As an adult, the insect will eventually reproduce and lay its eggs, beginning the cycle and metamorphosis once more.

Metamorphosis

The often startling aspect about metamorphosis is how suddenly and dramatically one form of life is changed into another that seems completely different. A butterfly in its larva stage crawls about and eats leaves, while as an adult it flies from flower to flower and sips nectar. The same transformation occurs for some amphibians. An example is frog eggs that develop into swimming tadpoles and breathe with gills like a fish. After they change into an adult, nearly every organ has changed. The tadpoles’ tails are absorbed into the legs of a frog and they breathe with lungs. For all organisms that go through metamorphosis (beetles, flies, ants, bees, and wasps as well as butterflies and frogs), their hormones begin the process and keep it going. Unlike butterflies and frogs, some insects go through only a threestage process (egg, nymph or larva, and adult) called incomplete metamorphosis. In this abbreviated version, the pupa stage is omitted and the changes are more gradual. Grasshoppers, crickets, cockroaches, and termites go through incomplete U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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metamorphosis. Even by undergoing only incomplete metamorphosis, it is believed that species that live entirely different lives as a young organism and as an adult have an advantage over those who do not. For example, a species that is slow-moving and limited in range in one stage of its life (such as a caterpillar) can suddenly move around quickly as an adult and can therefore lay its eggs far away in a more advantageous habitat. Another advantage is that overall competition is reduced, since at one stage an organism is seeking one certain type of food, and in a later stage it eats something entirely different. [See also Amphibians; Insects; Larva; Life Cycle]

Microorganism A microorganism is any form of life that is too small to be seen without a microscope. Also called microbes, these tiny organisms include bacteria, protozoa, single-celled algae, and fungi as well as viruses. Microorganisms are nearly everywhere and are essential to the production of certain medicines, foods, and drinks. They play a key role in nature’s oxygen, carbon, and nitrogen cycles, but can also be harmful to humans. The study of microorganisms is called microbiology. It was founded in the seventeenth century after the microscope was invented. Not until the Dutch naturalist Anton van Leeuwenhoek (1632–1723) saw what he called “little animalcules” (or little animals) with his own microscope in 1673, did science know of the existence of a subvisible world that was teeming with life. Despite this major discovery, no one knew where these microscopic forms of life came from. Most believed that the life forms simply sprang out of rotten wheat or from the soil. This incorrect notion of spontaneous generation was held until 1861 when the French chemist Louis Pasteur (1822–1895) was able to prove that spontaneous generation does not occur and that the air itself is full of microorganisms. Pasteur also discovered that fermentation was caused by microorganisms. (Fermentation is a process in which cells break down sugar or starch into carbon dioxide, ether, alcohol or lactic acid.) Pasteur went on to make many other contributions and is considered the founder of the science of microbiology. He discovered that microorganisms are present in nonliving matter as well as in the air, and that some of these tiny organisms caused disease. He also showed how microorganisms could be killed by heat and how they could be manipulated for use in vaccines. In 1876, the German physician Robert Koch (1843–1910) was able to demonstrate that a particular bacterium could cause a partic380


ular disease. He isolated the bacteria that produced the cattle disease anthrax, and he discovered the bacteria that caused tuberculosis and cholera. With the discovery of viruses just before the turn of the century, it was realized that although viruses are not living organisms (since they cannot grow and reproduce on their own), they are even smaller than bacteria and are physically microorganisms.

Microorganism

There are several types of microorganisms. Protists are a group of single-celled plantlike or animal-like organisms that have complex or eukaryotic cells. This means that their genetic material is contained in a nucleus that is bound by a membrane. Protists (kingdom Protista) are far more diverse than plants or animals and obtain their food by simply absorbing it from their environment since most live in water. They also have different means of getting about, such as flagella (tails) and cilia (hairs). The well-known euglena is a protist, as are all single-celled algae and the many differently shaped diatoms that form glasslike shells. Bacteria are another group of microorganisms (too small to be seen with the naked eye), that belong to the kingdom Monera instead of Protista. Among the most abundant life forms on Earth, bacteria are single-celled organisms that have a cell wall but no nucleus (meaning that they are prokaryotic and not eukaryotic). Some bacteria feed on dead matter and play an important role in recycling nutrients, while others cause disease. Fungi are microorganisms that make up their own kingdom (Fungi). The smallest fungi can be microscopic and single-celled. Like protists, fungi also absorb their food from their environment. Finally, viruses are microorganisms by virtue of their size. They are not considered living organisms because they are parasites and can reproduce only by taking over their host’s cellular machinery. They are the tiniest of all the microorganisms and can only be seen with an electron microscope. Microorganisms play a major role in the environment. As recyclers, they break down many substances into usable forms for plants and animals. Without microorganisms the world would be full of waste. Microorganisms are also crucial to several key industries. For example, the production of antibiotics, vaccines, beer, cheese, wine, and bread would be impossible without microorganisms. Although sometimes beneficial, microorganisms also can cause diseases. Some of these diseases are less dangerous than in the past because scientists have been able to develop cures. However, other diseases continue to be harmful as microorganisms adapt and mutate in response to treatment. Knowledge of microorganisms has allowed biologists to use them as experimental models to study the chemical processes of more complex organisms. Microorganisms have been crucial to our knowledge of deU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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oxyribonucleic acid (DNA) and have proven to be an essential tool of genetic engineering in which biologists experiment with the genetic code of living organisms. Altogether, microorganisms are not only essential to life on Earth, but also help scientists solve problems in medicine, agriculture, industry, and the environment.

Microscope A microscope is a scientific instrument that magnifies objects that are too small to be seen by the naked eye. As one of the most important scientific tools ever invented, it is especially significant to the life sciences, since it made possible the discovery of an entirely unseen world of microorganisms. Today’s increasingly powerful and highly specialized microscopes can achieve magnifications of a million times or more. The earliest types of magnifiers were probably globes of water-filled glass or chips of transparent rock crystal used by the Romans. The first microscope could not be invented, however, until the first lenses were devised for use in eyeglasses sometime around the year 1300. These first eyeglasses or spectacles were made with convex lenses (curved inward) that helped farsightedness (the inability to see objects up close). By 1500, it is known that concave lenses (curving outward) were crafted to help with myopia, or nearsightedness (the inability to see object far away). Lenscrafters had learned that by grinding any clear glass or crystal into a certain shape, usually with the edges thinner than the center, a magnifying effect was achieved. The first real microscope was therefore a single, handheld lens, and it is called a simple microscope. Today, we would call it a magnifying glass. The individual most identified with the improvement and use of the simple microscope is the Dutch naturalist Anton van Leeuwenhoek (1632–1723), whose secret grinding, polishing, and mounting techniques allowed him to achieve possibly as much as 270 times magnification. Beginning in the 1670s, he examined mainly biological specimens and was the first to observe spermatozoa (male sex cells), red blood cells, and bacteria. The typical microscope used today is a tubelike instrument with a lens at its top and bottom. It is called a compound microscope because it has more than one lens. This device is believed to have come about in Holland near the end of the sixteenth century when the telescope was invented there at the same time. Apparently, it was soon realized that a telescope could be used as a microscope when reversed. The first compound microscopes were, therefore, two lenses housed in a long tube (in which 382


the enlarged image produced by the first lens is further magnified by the second one). The first scientist to improve the compound microscope and to put it to real scientific use was the English physicist, Robert Hooke (1635–1703). In 1665, Hooke published Micrographia, which contained excellent drawings of what he had observed with his improved microscope. Hooke was the first to use a microscope to observe the structure of plants (actually thin slices of cork), finding that they consisted of tiny walled chambers that he called “cells.” After Hooke, there were minor improvements in microscopy until the mid-1800s, when the German physicist Ernst Abbe (1840–1905) collaborated with the German optician Carl Zeiss (1816–1888), and produced high-quality lenses with no blurring or distortions. Later developments resulted in the basic microscope with built-in illumination (lighting) that is used today in schools and small laboratories. These generally have a magnification of up to 400 power (times).


Microscope

A labeled diagram showing the components of a modern compound light microscope. The invention of the microscope was truly revolutionary since it allowed scientists to see objects unable to be seen with the naked eye. (Reproduced by permission of Carolina Biological Supply Company/Phototake NYC.)

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ROBERT HOOKE English physicist (a person specializing in the study of energy and matter and their interactions) Robert Hooke (1635–1703) was one of the earliest and greatest of the microscope pioneers. His microscopic studies of insects, feathers, and fish scales are both beautiful and accurate, and he published the first book dedicated to microscopy. He is responsible for first using the word “cell,” which later would become the cornerstone of microbiology (the study of microorganisms). Born on the Isle of Wight in England, Robert Hooke was born sickly and with a backbone that did not grow straight. As a child prodigy (exceptionally smart) Hooke impressed everyone with his mechanical gifts. He built elaborately complicated toys as a child, and as a young man he attended Westminster School and later Oxford University. At Oxford, his abilities caught the eye of the great English physicist and chemist, Robert Boyle (1627–1691), and Hooke quickly was made Boyle’s assistant. It is known that it was Hooke who designed and built an improved air pump that Boyle used to establish his gas laws. When Hooke was made a member of the Royal Society in 1663, he also became its “curator of experiments,” which allowed him access to the society’s facilities. He remained in this position for the rest of his life, and was able to pursue whatever interested him scientifically. As a man of wide talents, Hooke’s scientific interests were even broader, and he went on to make contributions in physics, astronomy, architecture, microscopy, and biology, among other fields. Although Hooke was not the first to experiment using a microscope, he was the first to dedicate an en-

Today’s school and lab microscopes are called compound light microscopes because they let light pass through the object being studied and then through two or more lenses. The lenses enlarge the image and bend the light toward the eye. Such a microscope has two lenses: an objective lens and an ocular lens. The ocular is also called the eyepiece and is what you look through. The objective lens, sometimes only called the objective, magnifies the object just below it on a slide. If the objective lens has a power of 50X (magnifying an object 50 times), and the ocular has a power of 10X, then together they have a total magnification of 500X (10X times 50X). Such a magnifying power would allow a cell to be easily observed. While such a microscope is adequate for schools and modest labs, greater magnification is often needed for more advanced research. In 1931 the electron microscope was invented. Using the knowledge that beams of electrons (particles that make up a single atom) could be focused us384


tire book to microscopy. In 1665, he published his Micrographia, which was written in English despite its Latin title. This work contains descriptions and illustrations of the structures of insects, fossils, and plants in neverbefore-seen detail. His drawings of tiny insects, parts of bird feathers, and even fish scales are both artistically beautiful and scientifically accurate. The biological discovery for which he is best remembered is the porous (having pores or holes) structure of cork. When he took a thin slice of cork and put it under the compound microscope that he had built himself, he noticed that it was made up of tiny rectangular holes that he called “cells.” It was an appropriate name for these little boxes, or empty rectangular structures, since the word usually meant a small room (like a jail cell). In fact, what he was seeing were the now-dead remnants of once-living structures that had been filled with fluid. That is, he actually was viewing what had been cells. Hooke’s word “cell” came to be adopted by biologists once they were able to observe living structures under a microscope. Ever since, the word and the concept it stands for has become one of the cornerstones of biology. Hooke’s Micrographia is recognized today as containing some of the best microscopic views of nature. In addition to his microscopic studies of insects and plants, he studied fossils a great deal, which led him to offer some early ideas about what is now realized to be evolution (the process by which living things change over generations). Hooke was described by some as quarrelsome, miserly, and a hypochondriac (someone who believes that are always sick), and he engaged some of the best minds of his time in some terrible feuds. Yet his contributions to biology are, in the full range of his life, but a small part of what he accomplished and contributed to other fields of science.

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ing a magnetic field the same way a glass lens focuses light, scientists built an electron microscope a thousand times more powerful than a regular compound microscope. Today there are two types of electron microscopes: the transmission electron microscope (TEM) and the scanning electron microscope (SEM). In a TEM, electrons pass through the object and cannot be used to magnify living things. In a SEM, electrons are bounced off the surface of the object, meaning that it is possible to examine things that are alive. Although the SEM cannot magnify things as greatly as the TEM, it produces a more three-dimensional image. Both microscopes display their magnified images on a video screen. Besides the TEM and SEM, other more advanced type of microscopes include the phase contrast and dark-field microscopes. The phase microscope contrasts light waves that pass through a specimen with those that do not, making for a sharper contrast. The dark field microscope makes U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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an object appear bright against a dark background. Finally, a scanning optical microscope uses a laser to obtain pinpoint illumination, while the atomic-force microscope uses a diamond-tipped probe that moves across the surface of the specimen and is able to “see” individual living cells without damaging them. A microscope extends the sense of sight to an incredible degree, and while it has been an important instrument to all of science, it is an essential tool for the life sciences.

Migration Migration is the seasonal movement of an animal to a place that offers more favorable living conditions. Migrations can be long or short distances and are usually annual, involving a round trip. Although there are many risks involved in migrating, they are usually outweighed by the benefits. Although some people consider the one-way, permanent relocation of an animal to be a migration (such as when a species is driven from its natural habitat by a sudden change in living conditions), most scientists consider migration be a periodic, regular occurrence in which certain animals make a regular, round-trip journey to the same place. A good example is the Arctic tern, the animal that makes the longest known migration. This remarkable bird leaves its arctic home as winter approaches and flies south to Antarctica to take advantage of its “summer.” When winter approaches in the south, the tern returns north to take advantage of the arctic “summer” there. Altogether, it flies about 25,000 miles in a single year. However, migrations do not necessarily have to be as long as the Arctic tern’s to be considered real migrations. Some species, like the grizzly bears that frequent the Rocky Mountains, make “vertical” migrations in which they migrate to high-altitude tundra meadows in early summer where they feed. Later in the season they come down to feed on plants growing in lower-altitude valleys and meadows. However, most animals usually make longer migrations. The North American bison once migrated in enormous herds from their summer range in the northern prairie to their winter habitats further south. Certain fish can also be long-distance migrants, such as the Atlantic salmon. These salmon hatch in large rivers and migrate as juveniles to the open ocean. There they feed for several years until they are sexually mature, after which they migrate back upriver to the stream where they were born in order to breed. Most migrations take place on a yearly basis. The effort that an animal has to make in order to do this is usually considerable, making it an 386


“expensive” activity. This means that it costs an animal great amounts of energy to move itself from one place to another. Migrating also can involve substantial risk, since it becomes a time when the animal is exposed to stress and predators as well as to the possibility of getting lost. Despite this, migration is an adaptive behavior that is carried out because it confers some sort of benefit on the animal that does it. These “benefits” usually outweigh the “costs.”

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REASONS FOR MIGRATION Among the benefits of migration are a greater abundance of food and more favorable weather conditions. Some animals actually follow their food. For example, the dramatic migration of millions of monarch butterflies from eastern Canada and the United States to Mexico is believed to be related to the availability of milkweed plants on which these butterflies feed and lay their eggs. Certain animals migrate to specific areas to mate, lay eggs, or give birth, while others migrate in order to raise their young under favorable conditions. For example, many birds migrate where there are favorable nesting places and plentiful insects to feed their ravenous young. Humpback whales migrate from their polar waters and give birth in warm tropical waters. In these waters the young can easily thrive and can build up layers of insulating blubber to fight the extreme cold of their future feeding grounds. Whatever the reason, it appears that migration in certain animals is instinctive or inherited behavior. This means that the migrating animal reacts to certain “cues” and acts out a pattern of behavior that all the generations before it have done. Studies have shown that different animals respond to different cues. For some, it is the declining hours of daylight. Others respond to temperature or weather changes, and some react specifically to changes in rainfall amounts. It is believed that these changes cause an animal to respond because they trigger the release of certain hormones that are the real stimulants to migration. There is mystery in exactly how these different species manage to find their way, sometimes over great distances. It is known that some, like salmon, are guided by their sense of smell, while others, like whales, use echolocation or bounce sounds off objects to orient themselves. Some, like the caribou, simply follow well-trodden trails or use landmarks. Many birds use the positions of the Sun and stars to navigate or are believed to be able to sense Earth’s magnetic fields. Overall, migration is an animal’s response to seasonal changes in the availability of the things it values, such as food, shelter, and mates. If their existing habitat cannot U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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provide those things, then these animals will physically relocate in order to find them.

Mitochondria Mitochondria are specialized structures inside a cell that break down food and release energy. If a cell is like a tiny chemical processing plant, then the mitochondria are the power plants of the cell. Without mitochondria, a cell with a nucleus cannot use oxygen and cannot live. Mitochondria are found only in eukaryotic cells (those with a nucleus) and are not found in prokaryotic cells (those without a nucleus). Mitochondria (singular, mitochondrian) are described as organelles. An organelle is a tiny structure inside a cell that is controlled by the nucleus and has a specific function to play in maintaining the life of the cell. For a cell, mitochondria play a most important role in carrying out aerobic respiration. In other words, mitochondria break down the food a cell takes in and release the energy it contains. Examined under a microscope, mitochondria appear as oval- or sausage-shaped structures that have a double membrane. The outer membrane is smooth and permeable to certain enzyme molecules, meaning that molecules of a certain size

A colored transmission electron micrograph of a mitochondrian, and rough endoplasmic reticulum. The mitochondria are considered the “powerhouse” of the cell since they break down food and release energy. (©1996 by SPL/SecchiLecaque/RousselUCLAF. Reproduced by permission of Custom Medical Stock Photo, Inc.)

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can flow through its walls. The inner membrane consists of many folds that allow it to have an increased surface area and make it able to pack in many more enzymes than would be possible without the folds. A eukaryotic cell typically has anywhere from a dozen to a thousand mitochondria, and animal cells usually have more mitochondria than plant cells. Mitochondria are especially abundant in cells whose functions have high energy demands.

Mollusk

As the “powerhouse” of the cell, mitochondria carry out respiration. Respiration is the chemical process that breaks down food to release energy. Two key ingredients are necessary for respiration—sugar and oxygen. The sugar that cells use is usually in the form of glucose, which contains a great deal of stored energy. Oxygen is used to get the energy out of the glucose. This process takes place in the folds of the mitochondria where a substance known as adenosine triphosphate or ATP is created. ATP is the molecular storage form of energy that mitochondria produce. When a cell needs energy, it draws on the stored ATP, which releases the energy. Here is a simplified example of how the human body uses mitochondria to release energy from food. For an organism like the human body, most of its energy comes from mitochondria, which are the body’s power plant that burns the fuel to produce the energy to run it. The food a person eats is the fuel that is “burned” in the body’s furnaces (mitochondria). The ATP produced by this process is the energy or “electricity” produced by the power plant (which powers the cells in the body). When a person breathes, he or she is taking in oxygen for the mitochondria to use to release the energy the body needs. This process is similar in all organisms, since all organisms must break down food to get energy. As a result, all life forms have cells that contain mitochondria. [See also Cells; Organelle]

Mollusk A mollusk is a soft-bodied invertebrate (an animal without a backbone) that is often protected by a hard shell. Most mollusks live in water and make up the second largest group of invertebrates (next to arthropods). Mollusks are represented by such diverse invertebrates as clams, slugs, snails, and octopuses. There are about 100,000 species of mollusks, most of which live in the water. All are soft-bodied, nonsegmented, and usually enclosed in some sort of covering made of calcium carbonate. Whether a land snail U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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This tree snail, hiding in its shell, is part of a group of mollusks called gastropods because it has a sucker-like foot. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

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or an underwater clam, all mollusks share certain traits. They all have a “mantle,” or a covering of tissue that protects their internal organs. Many species secrete a substance that forms a hard shell, protecting them from predators. Their digestive systems are made up of a mouth, throat, gullet, stomach, intestine, and anus. Mollusks that live underwater use gills to get their oxygen, while land-dwelling mollusks have lungs. All have a two-chambered heart that pumps blood through vessels that branch throughout their bodies. The mollusk nervous system consists of two pairs of nerve cords. Some mollusks have eyes and other sense organs. Most mollusks also have a muscular “foot” that is used for slow crawling or digging. In an octopus, this foot is divided into arms or tentacles lined with suction cups. There are at least eight species of mollusks known, but most fit into three main groups—gastropods, bivalves, and cephalopods—that are grouped according to the shape of their muscular foot. The name gastropod means “stomach-foot” and describes those mollusks, like snails, that


have a sucker-like foot. This is the largest group of mollusks and is best represented by the common garden snail. Most gastropods, excluding slugs, have a single shell often shaped like a spiral, or coil. They also have a large, flat foot that they use to slide along on top of the mucus they secrete. This is why they leave a visible, slimy trail behind them. A gastropod has a distinct head with tentacles that can move up and down like a periscope and act as eyes or sense organs. A snail also has rows of tiny teeth. Not just confined to land, there are many ocean-dwelling gastropods like periwinkles, abalones, and sea slugs.

Mollusk

A bivalve is a mollusk that has a two-part shell joined by a hinge. Bivalves are water animals and are best represented by clams, oysters, and scallops. All bivalves are filter feeders. They filter water through large gills that catch bits of food and absorb oxygen. To do this, however, they must keep their shells open, and as a result, expose their vulnerable soft body. A mollusk, like a clam, moves on the ocean’s bottom by thrusting its large, hatchet-shaped, muscular foot between the open shells, thus pushing it along slowly. Unlike gastropods, bivalves have no head, and their sense organs are not well-developed. Bivalves do have a powerful adductor muscle that they use to clamp their shells tightly together. Members of the cephalopod group of mollusks, like the octopus and squid, are free-swimming and have no shell. Unlike typical mollusks, they are fast-moving hunters and prey on other animals. An octopus has a sharp beak, like a parrot’s, that it uses to rip its food and break open shells. In squid and octopuses, the mollusk foot has evolved into long arms, or tentacles, around the head. They use these tentacles to capture fish. In some ways, cephalopods resemble vertebrates because they have a solid internal support structure, like a skeleton, and a large head with eyes, as well as a good-sized brain. Cephalopods move by a form of jet propulsion in which they forcefully squirt water. They also can eject a cloud of darkcolored chemicals that covers their escape as they zoom away. A giant squid may extend its tentacles as far as 60 feet (18.29 meters) and has the largest eyes in the animal kingdom—up to 1 foot (0.3 meters) wide. Some squid and octopuses can change color like a chameleon. The only cephalopod with a shell on the outside of its body is the nautilus whose shell is divided into chambers. It lives in the largest chamber and adds another when it grows. It fills the empty chambers with a gas that makes it easier to swim. All three groups of mollusks reproduce sexually through the union of male sperm and female eggs. Unlike gastropods and cephalopods, however, bivalves are not necessarily either male or female. Instead they may contain both male and female organs. [See also Invertebrate] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Monerans

Monerans Monerans are a group of one-celled organisms that do not have a nucleus. Along with Protists, Fungi, Plants, and Animals, Monerans make up the five kingdoms of living things. As one of the first life forms to evolve, they are today the most abundant living organisms on Earth. Monerans are found throughout the world and can live in freezing as well as extremely hot conditions. Monerans belong to the kingdom Monera, which, unlike other kingdoms, is made up of only one member—bacteria. Monerans are usually microscopic life forms, and although some are smaller than viruses, others can be seen by the naked eye. They live not only on Earth, from hot springs to frozen wastelands, but inside other organisms as well. Nearly all multicelled plants and animals act as hosts to Monerans. Monerans are so abundant that it is estimated that the number of bacteria found in the human mouth would outnumber all of the people who have ever lived. They have left no fossil record that scientists can learn from, yet are believed to be among the oldest type of organisms still thriving. Unlike other living cells, Monerans are prokaryotic, meaning that they have no nucleus or any organelles (tiny structures inside a cell that have certain functions) inside their walls. Instead, the material that is usually found in the nucleus is scattered throughout the cell. Monerans can also reproduce asexually by binary fission, meaning that a single cell can divide itself into two identical “daughter” cells. Monerans can reach maturity in a phenomenally short time (about fifteen minutes), so that they are able to rapidly mutate or adapt to a changing environment.

TYPES OF BACTERIA For some time, Monerans were not considered to be a separate kingdom, but advances in molecular biology now suggest that this kingdom is made up of three different types of bacteria: the archaebacteria, the eubacteria, and the cyanobacteria. Some scientists argue that cyanobacteria (formerly called blue-green algae) are part of the eubacteria, and that archaebacteria should form their own kingdom.

Archaebacteria. Archaebacteria are unique, since some species live in such harsh places as boiling mud, hot springs, and extremely salty water, while others live in the intestinal tracts of some mammals. The archaebacteria called thermoacidophiles thrive in the recently discovered deepsea volcanic vents where it is extremely hot. 392


Eubacteria. Eubacteria are considered to be “true” bacteria and are composed of types with which scientists are most familiar. Some make their own food as plants do, while others get their energy by fermentation (a process by which cells break down sugar and starch into energy). Others are considered dangerous parasites. The bacteria in the soil that are able to “fix” or capture nitrogen from the air are eubacteria, as are those that live in ticks and cause Lyme disease and Rocky Mountain spotted fever. These bacteria are also responsible for such transmissible diseases as syphilis and gonorrhea, as well as botulism and diarrhea. Although sometimes harmful, eubacteria also can be beneficial. For example, the bacteria Escherichia coli that normally lives in the gut of mammals produces enzymes that help with the digestion of fats. Eubacteria also are necessary for the production of cheese, yogurt, and other fermented milk products. Cyanobacteria. Cyanobacteria are a group that make their own food by photosynthesis. Also called blue-green algae, they use the process that plants employ of capturing the energy of the Sun and changing it into simple food substances. Most live in or on the water’s surface, either by themselves or in large clusters called colonies.

Monerans

IDENTIFYING BACTERIA Although all bacteria are prokaryotic, it is the cell wall that gives different types of bacteria their different shapes and allows biologists to identify them. The most commonly occurring shapes are round (cocci), rodshaped (bacillus), and coiled (spirillum). The bacteria with a round shape called cocci (singular, coccus) often form chains and are usually found in the human body. An example is Streptococcus mutans, which inhabits the mouth and causes tooth decay. A single rod-shaped bacterium is called a bacillus (plural, bacilli). The species Bacillus anthrax causes the cattle disease anthrax. Finally, the corkscrew-shaped bacteria are like twisted spirals and are called Spirillum (plural, Spirilla) or Spirochaete. Despite our familiarity with the many disease-causing bacteria, most bacteria are not only harmless to human beings, but some have become indispensable. Besides the bacteria that allow us produce cheese and wine, and naturally break down and recycle our waste and sewage, bacteria have been used in medicine to produce such modern miracle drugs as antibiotics (like erythromycin and streptomycin). More and more, bacteria are used in the biotechnology industry, which is genetically altering certain bacteria so they will produce important compounds like insulin that are difficult to make artificially. As a result, the Monerans or bacteria are an important and essential part of the life cycle on Earth. [See also Kingdom] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Muscular System

Muscular System The muscular system is composed of all those body tissues capable of contracting and relaxing and, therefore, of producing movements in its body parts. Muscles are able to produce movement by converting chemical energy into mechanical energy. This conversion occurs due to the ability of muscles to contract and relax. Animals use muscles not only to move about but to operate their many necessary internal processes such as blood circulation. Since animals cannot make their own food as plants do, they must be able to move about to locate things to eat. Movement is therefore a trait shared by all animals, from a one-celled amoeba to a killer whale. Animals are able to move because they have a support framework (a jointed skeleton) that is moved by muscles, which push bones together and pull them apart. Muscles are made up of long fibers of contractile tissue, meaning they have the ability to contract or grow shorter by means of a chemical change, as well as relax or return to their normal length. Muscles are rich in blood vessels that bring them food and oxygen and take away their wastes. The harder the muscles work, the more blood is transported to them.

TYPES OF MUSCLES

Opposite: Some of the estimated 800 muscles found in the human body. No exact figure is available because scientists disagree about which ones are separate muscles and which ones are part of a large muscles. (Illustration by Kopp Illustration, Inc.)

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There are three types of muscle found in the muscular system: skeletal muscle, smooth muscle, and cardiac muscle. Although all three contract in basically the same manner, they look very different when examined under a microscope. Skeletal Muscle. Skeletal muscle is attached to bones and can be said to make up the flesh of an animal. It is also called striated muscle since it is made up of long, striped muscle fibers. Skeletal muscle is a voluntary muscle because it can be consciously controlled. A person can, therefore decide when and how to move his or her arms or legs to walk or run, or to move facial muscles to smile or frown. Most skeletal muscles are attached to bones and are connected at both ends by a tough, connecting tissue called a tendon. Tendons can be felt in the forearms near the wrist, in back of the leg near the knee, and in the back of the ankle. Skeletal muscles move bone by acting in pairs. One muscle, called the flexor, contracts and pulls the bones connected by a joint together. Another muscle, called the extensor, pulls, or moves the bones apart. Muscles, such as the flexor and extensor that work together but in opposing ways, are called an antagonistic pair. Smooth Muscles. Smooth muscle is a second type of muscle, which actually appears smooth under a microscope. It makes up the walls of many U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e


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organs inside the body, including the digestive tract, reproductive organs, bladder, arteries, and veins. Since it does not respond to a person’s will or command—that is, it cannot deliberately be controlled—it is called involuntary muscle. Blood is pushed through the veins, and the lungs expand whether a person wants them to or not. Smooth muscles contract like any muscle, and although they do so more slowly than skeletal muscle, they can maintain these contractions for a longer period of time.

Cardiac Muscle. A third type of muscle is cardiac muscle. As its name implies, it is found only in the heart and it is responsible for the strong, regular contractions known as the heartbeat. Like smooth muscle, cardiac muscle is involuntary, but unlike any other type of muscle, cardiac muscle contracts independently of any nerve supply. This means that cardiac muscle contracts and relaxes automatically according to its own, built-in rhythm. Cardiac muscle is amazingly strong and resilient, given that it has to beat continuously over an individual’s lifetime. Unlike skeletal muscle, which requires regular periods of rest, cardiac muscle neither requires—nor is allowed—to take a rest. WHY MUSCLES CONTRACT The actual contraction or shortening of any muscle occurs because a muscle contains two proteins, actin and myosin, that form long threads called filaments. These filaments are arranged in parallel stacks, like overlapping decks of cards. When a muscle gets a nerve signal to contract or shorten, the actin filaments physically slide over the myosin filaments and overlap them. The more they overlap, the more the muscle shortens. When the filaments slide back again, the muscle relaxes. The human body has more than 600 muscles, and in the adult male, muscles make up about 40 percent of the total body mass. Although skeletal muscle fiber cannot replace itself by cell division after an organism is born, it can increase in size through exercise, which is important for healthy, strong muscles. Regular physical activity creates an increase in the number of blood vessels, which means the muscle receives more nutrients and oxygen. Muscles that are not used for a long period of time undergo a wasting process called atrophy. [See also Heart]

Mutation A mutation is any change in the genetic structure of a living cell. When that cell divides, the mutation is transmitted to the new cell and in the 396


case of an organism, may result in offspring that look or behave different than its parents. Mutations are random events that can have a variety of causes. The word mutation is credited to the Dutch botanist Hugo Marie De Vries (1848–1935), who observed random changes that suddenly appeared in the flowers he studied. De Vries named the changes mutations after the Latin word mutare, which means to change. De Vries had independently discovered Mendel’s laws of inheritance, and when he added his theory of mutations (which meant there could be sudden “jumps” in evolution) to Mendel’s laws, he was able to provide Darwin’s theory of evolution with its missing mechanisms of change. We now know that mutations occur constantly in nature and that they are essential to life. Mutations are the source of variation in all living things, and without them populations would never evolve. Mutations occur at the cellular level. Cells carry their codes for inherited characteristics in threadlike structures called chromosomes which,


Mutation

A mutation caused this frog to be born with six legs instead of just four. (Reproduced by permission of JLM Visuals.)

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in turn, carry genes that consist of a substance called deoxyribonucleic acid, or DNA. The coded information contained in the DNA determines the characteristics of living things. DNA looks like a spiral staircase or ladder, with the stair steps or ladder rungs determining the code for a particular organism. Any change or break in this DNA ladder alters the genetic code and results in a change in the next generation of the organism, whether it is a bacterium or a human being. [See also Chromosome; Genetic Disorders; Genetics]

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N Natural Selection Natural selection is the process of survival and reproduction of organisms that are best suited to their environment. It is a unifying idea for all of biology, and it also explains how the theory of evolution (the process by which living things change over generations) actually works. The theory of evolution was first suggested by the English naturalist Charles Robert Darwin (1809–1882) in 1858. As a major theory, it attempted to account for the amazing diversity in the living world and to explain how present-day organisms came to be. It stated that all life progressed from simple to more complex organisms, and that gradual genetic changes occurred over a long period of time. Darwin’s idea of natural selection was the key to explaining how this is accomplished. Natural selection has been described as the mechanism of evolution or even as the cause of evolution. The end result of natural selection is that organisms are able to adapt to their environment and change over time. Natural selection has been described as the “survival of the fittest” because it is an unforgiving natural process that weeds out those traits that are less fit. This was explained by Darwin himself who said that natural selection rested on a few obvious facts. The first of these is that in the natural world, reproduction has the potential to produce more individual organisms than can survive. For example, a single leopard frog has the capability to produce 3,000 frogs a year. Since every species’ living space or habitat has limited resources, population increases result in competition between individuals. Darwin said that at any one time, each organism is competing with others as well as members of its own species for such things as food, shelter, and members of the opposite sex with U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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CHARLES ROBERT DARWIN English naturalist Charles Darwin (1809–1882) is without doubt one of the greatest and most influential life scientists who ever lived. His theory of evolution became the dominant concept for all of biology. Darwin’s explanation that individual species can change or adapt over time, that humans have evolved from earlier “less human” forms, and that ultimately all life on Earth is connected and passed from simple to more complex organisms, is the “big idea” of biology. Born in Shrewsbury, England, to a well-to-do family, the young Darwin had a physician father, and his two grandfathers were both wealthy and influential individuals. Expected to become a doctor like his father, Darwin could not tolerate watching surgery while in medical school, so his father suggested a career in the church. When the young man proved unwilling to pursue that profession, his father lost hope and declared he would grow up as a disgrace to the family. Throughout all this, however, the young man was starting to cultivate an interest in the natural world, and he eventually made natural history his hobby. His first real exposure to science was a field trip he took that was led by the English geologist (a person specializing in the origin, history, and structure of Earth) Adam Sedgwick (1785–1873). It was Sedgwick who recognized that there was something special about Darwin. This also enabled him to meet an English botany (the study of plants) professor, John Stevens Henslow (1796–1861), who helped Darwin obtain a position on a scientific expedition that would be making a five-year voyage to South America and the South Pacific Islands. Darwin’s father eventually allowed him to go, and the twenty-two-year-old Darwin became an unpaid naturalist on the government survey ship, the H.M.S. Beagle, as it set sail in December 27, 1831. This grand voyage by sea not only transformed Darwin into a real naturalist (what is now called a biologist), but it proved to be one of the most important scientific voyages ever undertaken. Darwin’s job on the trip was to make geological and biological observations, keep records, and collect specimens. With each day, he learned a little more about the incredible variety in the natural world, and after awhile, he began to question why he found species that were closely related but still had noticeably different characteristics. Four years into his journey, Darwin landed on the Galapagos Islands far off the coast of Ecuador. There he noticed that there were about fourteen different types of finches on these different islands, with each bird apparently having adapted perfectly to its particular island environment. He also found that the natives could tell just by looking which island a giant tortoise had come from because of its distinctive features. All the while, Darwin searched for a pattern of

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meaning in this, and after some thought, he began to realize that it might be that species could actually change. It would make very good sense if one type of land finch colonized these islands and then each adapted, or changed, slightly to better fit its particular island. However, Darwin could not explain how this might occur, and he eventually returned home with no real answers.

Natural Selection

After publishing his very popular book, A Naturalist’s Voyage on the Beagle, Darwin began to seriously work out an explanation for the ideas he was considering. Those ideas were influenced not only by the writings of his good friend, the Scottish geologist Charles Lyell (1797–1875), but by those of the English economist, Thomas Robert Malthus (1766–1834). Malthus had written that a population always grows faster than does its food supply, and when Darwin applied this notion to the natural world, he realized that it might explain how species change. Since each individual is slightly different from every other, those that possess a certain trait that gives them some sort of advantage in competing for food would have a better chance of surviving and passing that trait on. A new species, therefore, is developed that is better able to survive in its environment. In 1844, Darwin started a book that would explain his theory, but by 1858 he still did not have it completed. So when the English naturalist Alfred Russel Wallace (1823–1913) sent Darwin a draft of his own paper on this very same subject, the two decided to issue their papers together. Darwin, however, went on to elaborate much more fully in his 1859 book, On the Origin of Species by Means of Natural Selection. Although his book was immediately controversial, most of the scientific community was persuaded, and every copy sold out on its first day. With the post-1900 discovery of Austrian botanist Gregor Mendel’s laws of inheritance (characteristics are not inherited in a random way but instead follow predictable, mathematical patterns), and the later discovery that genes are the basic units of heredity, Darwin’s theory of evolution at last had a mechanism that explained exactly how it could take place. Although Darwin lived a fairly long life, he was always troubled by a variety of physical problems, and while it was thought by many that he was probably a hypochondriac (someone who believes that they are always ill), many now think that he ruined his health during the Beagle trip by contracting a tropical disease. No book has been as important or as controversial as Darwin’s, given that it goes against the Biblical view of creation. Although some today still disagree with its conclusions and implications, most life scientists agree that his theory remains the only viable scientific explanation for the amazing variety and diversity of life on Earth.


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whom to mate. The organism’s habitat or environment is the key to natural selection since it is the standard against which an individual’s “fitness” is measured. Simply, if one organism possesses traits that make it more fit for its particular environment than does another (whether it is a longer neck, different color, larger antlers, or aggressive temperament), it is more likely that the “fitter” one will have a better chance to survive, to reproduce, and to pass on those advantageous traits. Since the environment is the measure, natural selection becomes an essentially random process. This means that nature has no master plan to favor one trait over another. Rather, a variety of traits usually exist in a given population, and whichever one happens to give its owner an edge over others becomes the trait that the environment favors.

GENETIC VARIETY English naturalist Charles Darwin is considered the father of the theory of evolution by natural selection. (Photograph courtesy of The Library of Congress.)

This leads to the second fact stated by Darwin: that close examination of any population of the same organisms reveals that not all individuals are exactly alike. In every group of like animals, there is always variety in both form and function. Simply, no two animals of the same species are identical because of the factor Darwin called “genetic variety.” Genetic variety states that the individuals making up a population have inherited characteristics that make them slightly different from one another. This becomes obvious when we realize that if each individual were identical genetically, then it would make no difference which one survived to reproduce. But when genetic differences do exist, who gets to pass on what trait makes a very big difference. Genetic variety comes about in two ways. The first is the physical result of sexual reproduction in which a unique individual is created who possesses a mixture of genes from both parents. This is called genetic recombination. The other way that genetic variety occurs is by mutations or accidental changes in a gene. A mutation is not necessarily something bad, and sometimes a chance change in a gene can result in a trait that gives an individual an advantage over others. Finally, natural selection is always tied to reproduction, since it does no good for an organism to live a very long time if it does not reproduce. The way natural selection works is that

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those best suited to their environment (the fittest) survive better and get to produce more offspring, thus passing on their genes to future generations. The end result of natural selection is a process called “adaptation.” Through natural selection, which favors organisms that fit their environment best and which weeds out those badly fitted, living things become better suited, fit, or “adapted” to their local environment or habitat. As this process continues over millions of years, new species evolve which are better adapted to their habitat or ecological niche (a specific job, or role, in a community that relates to feeding).

Nervous System

Although the theory of natural selection is popularly identified with Darwin, his contemporary, the English naturalist Alfred Russel Wallace (1823–1913), came up with the same theory in the same year. Darwin was at first amazed when he received Wallace’s not-yet-published ideas in 1858, but the two men became allies and published their ideas together in a scientific journal that year. Later, Darwin went on to produce a fuller and more complete theory with his book, On the Origin of Species, and as a result received much more recognition than Wallace. [See also Evolution; Evolution, Evidence of; Evolutionary Theory; Human Evolution]

Nervous System The nervous system is a network of nerve cells that allows an animal to collect, process, and respond to information. As an internal communications system, the nervous system enables an animal to react and adjust to changes in its environment. Almost all animals have some type of nervous system, but the human nervous system allows us to speak, solve problems, and have creative ideas—activities that make humans different from all other animals. Animals have a nervous system but plants do not. Plants react and respond to changes in their environment by slowly altering their growth patterns using different hormones. When a plant inclines itself toward a light source or drops its leaves, it is doing so on command from a naturally occurring chemical called a hormone that it produces as a response to something outside itself. Such a slow system of internal communication could only be practical for an organism that can make its own food and does not move. For animals, however, their very existence and reproductive ability often rests on being able to react immediately to something in their environment. Often they are either trying to catch something to eat or they are trying to escape being caught. Movement is, therefore, essential to anU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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imals, and their nervous systems must always allow them to act appropriately, efficiently, and most important of all, rapidly.

EVOLUTIONARY DEVELOPMENT OF THE NERVOUS SYSTEM Even the simplest multicellular organism has to constantly gather and analyze information about its environment if it is to maintain its inner balance (known as homeostasis) and survive in a constantly changing habitat. The single-celled amoeba does not have a real nervous system, but it still responds appropriately to stimuli like light or food. However, more complex organisms need to do more than simply react to stimuli and therefore need a more complex communications system. Probably the simplest nervous system is the one used by the class of animals called Scyphozoa (phylum Cnidaria), better known as jellyfish, hydra, and sea anemones. These animals use a system described as a nerve net that directly connects the receiving cell to the cell that does the responding. Flatworms are more complex than jellyfish and concentrate both their receiving and sending sensors in their forward end (like a head). This means that their front part is the first to meet the stimuli, and this permits them to react more rapidly. Flatworms also have bilateral symmetry (a body that is basically the same on both halves). Bilateral symmetry includes a typical vertebrate system with pairing of nerves down either side of a central column.

Opposite: The brain and the spinal cord are the two major components of the central nervous system, which is considered the command center of the body. (Illustration of Kopp Illustration, Inc.)

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Continuing up the ladder past the jellyfish and the flatworm, the beginnings of the vertebrate (an animal with a backbone) model can be seen inside an earthworm or a grasshopper. Besides having bilateral symmetry, they also demonstrate what is called segmentation. They have identical segments (each of which has a pair of nerves) linked together and connected to a central organ that could be called a primitive brain. Once this model was established and seemed to work well, it eventually evolved into the sophisticated nervous system that is basic to humans and all other animals with backbones. The backbone or the vertebral column evolved into its present form because it proved, first of all, to be an ideal up-and-down framework to support the body and make it both strong and flexible. Its hollow center was eventually expanded and modified so that it could hold a spinal cord connected to a brain. Finally, the skull that held the brain then slowly developed a range of sensory equipment (ears, eyes, nose) that gathered information about the outside world and fed it into the brain.

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SANTIAGO RAMON Y CAJAL Spanish histologist (a person specializing in the study of tissues and organs) Santiago Ramon y Cajal (1852–1934) laid the foundations of modern neurology, which is the scientific study of the nervous system. He established that the nervous system is made up of independent units of nerve cells called neurons, and also made important discoveries relating to the transmission of nerve impulses and the cellular structures of the brain. Santiago Ramon y Cajal was born in the remote village of Petilla de Aragon, Spain. His father was a self-trained country doctor who wanted his son to have a real medical education. Therefore, the family moved to the university city of Zaragoza, and young Santiago studied medicine there. Santiago was a rebellious youth who was more interested in drawing than studying, and his father finally forced him to work for a barber and a shoemaker as a way of disciplining him and making him appreciate school. This apparently worked, and he earned his medical degree from the University of Zaragoza in 1873. He then joined Spain’s army medical service and served as army surgeon in Cuba for a year. However, while in Cuba he caught malaria and returned to Spain to recuperate. Going back to school, he earned his doctorate in medicine in 1879 and began a teaching career. Preferring to do research and to teach rather than practice medicine, he turned to the field of anatomy, which had always been his favorite subject. Anatomy studies the structure of living things and figures out how the different parts of an organism are shaped and how they fit together. His favorite branch of anatomy was histology. Working with an old, abandoned microscope he found at the University of Zaragoza, Ramon y Cajal eventually turned toward the study of the most

tem. The central nervous system consists of the brain and spinal cord and acts as network central, or the “main switchboard” for the entire system. The cord itself lies within and is protected by a central canal that is made up of stacks of bony vertebrae. Like an electrical cable, it has pathways that lead to and from the brain, and its nerve cells are lined up in a columnar way. Inside the backbone, the spinal column is bathed in cerebrospinal fluid that circulates around both it and the brain and acts as a liquid shock absorber. At the top of the column sits the brain—the system’s real control center. It is actually a continuation of the spinal cord, which extends upwards and expands into segments called ventricles and aqueducts. The brain itself has three parts: the hindbrain, the midbrain, and the forebrain. The hindbrain occupies the rear 406


complex tissues in the body, which are those of the nervous system. These can only be studied under the microscope if they are stained (dyed), and Ramon y Cajal was able to improve upon some of the stains in use at this time. When he began his research on the nervous system, little or nothing was known about what might be called the path that a nervous impulse takes. Most scientists thought it traveled over something like a connected grid, or network of wires, but no one really knew for sure since they had never been able to examine nervous tissue closely. Using his own improved stain techniques on brain tissue, Ramon y Cajal was able, by 1889, to demonstrate that the nervous system was by far much more complex than anyone had imagined. He then went on to show that the neuron, or nerve cell, was the basic unit of the nervous system, and that it was very different from the other ordinary cells of the body. Ramon y Cajal then offered a controversial “neuron theory” that few accepted. He stated that the nervous system consists of a network of separate nerve fibers whose ends never actually “touch” or are not actually connected with the surrounding nerve cells. Today it is known that neurons are indeed not “hard-wired” together but instead have a synapse or space between them, across which the electrical charge or nervous impulse “jumps.” He also stated that nerve impulses travel in only one direction, and that the brain’s neurons had different structural patterns in different areas, suggesting that one part might have a different job than another. We now call this “localization,” meaning that a certain part of the brain controls memory and another intelligence. Ramon y Cajal also conducted important research on the tissues of the inner ear and eye. Overall, modern neurology really began with his work, since he established the correct role played by the neuron and the nervous impulse.

Nervous System

of the skull and connects with the cord. It contains the medulla oblongata and the cerebellum. The medulla controls the body’s involuntary processes like breathing and the heartbeat. The cerebellum coordinates the body’s many muscles and allows it to move properly. The midbrain is sort of a coordinating center for information collected by sight and hearing, and it also relays information to higher centers of the brain. The forebrain contains the higher brain centers such as the cerebrum. The cerebrum is the part of the brain involved in voluntary actions and with functions related to memory, learning, and sensing things. In humans, the cerebrum is larger than the hindbrain and midbrain put together, and is the place that governs thinking, reasoning, and the use of complex language. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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THE PERIPHERAL NERVOUS SYSTEM The peripheral nervous system can be described as a branching network of nerves that carry signals to and from the central nervous system. This system runs throughout the body of a vertebrate and has two different types of nerves: afferent nerves (or sensory neurons) bring input into the central nervous system, while efferent nerves (or motor neurons) take output away and communicate it to the muscles and glands. Some nerves in the peripheral nervous system can be controlled voluntarily, such as those that allow us to walk or run, but those that are not under our control (like the production of saliva or our beating heart) are considered a division of the peripheral nervous system called the autonomic nervous system. This system regulates functions like digestion that we cannot control. These nerves keep the body running smoothly by automatically adjusting its many systems.

NEURONS The entire system works as it does because it is based on the neuron or nerve cell, the fundamental unit of communication in all nervous systems. Neurons never act alone. Rather, they transmit impulses to one another in the form of electrical signals and link together the entire nervous system. In many ways, neurons are the body’s electrical highway. The neuron is a specialized cell and consists of three parts: the soma, the dendrites, and the axons. The soma is the cell body with its cytoplasm and nucleus. Dendrites and axons are hairlike arms or branches that extend from the body and channel information in one direction. Dendrites form the input part of the system and carry information toward the cell body. Axons are for output and therefore carry information away from the cell body. A typical neuron has several dendrites and one axon. Finally, neurons pass impulses to one another in a one-way direction across a space called a synapse. When a nerve impulse reaches the end of an axon and arrives at the synapse, a transmitter substance is released from the axon into the synapse. This chemical neurotransmitter goes across and binds to a receptor in the adjoining dendrite and triggers an impulse in it. The brain of the average adult contains about 1,000,000,000 neurons. Since the functioning of the entire system is dependent on the precise and proper functioning of the different types of neurons, anything that interrupts or disturbs that synaptic function can cause a problem with the organism. Today, scientists know of many genetic and infectious diseases that can severely interfere with this function. Many drugs have also been developed that can positively or negatively affect it. [See also Brain; Muscular System] 408


Niche

Niche

The term niche refers to the particular job, or function, that a living thing plays in the particular place it lives. Also called an ecological niche, this concept refers to the precise way in which an organism fits into its environment. A niche includes all the factors that are important to the organism’s existence. No two species can occupy the same niche. In order to study a niche, life scientists must understand all of the factors that are important to a living thing’s existence. These factors include: an organism’s: diet; energy, light, and moisture requirements, ideal temperature; ideal habitat; and ideal reproduction conditions. Therefore, the word function is key to understanding the idea of an ecological niche. The idea of niche as a function has also been described as the job a living thing has—and as with any job—it can be very specialized or very general. Some organisms have very broad niches, meaning that they are fairly flexible in terms of the type of things they eat and temperature they tolerate. In other words, their overall living needs can be met in a less specialized environment. The opossum and the raccoon are examples of animals with very broad niches since they eat a wide variety of plants and animals and adjust well to different climates. However, many species play a narrow or specialized role and therefore have a narrow niche. Two examples are the giant panda of central China and the koala of Australia. The panda lives in the bamboo forests and eats only one type of bamboo, while the koala can only live where certain species of eucalyptus trees grow and survives by eating the leaves. Other factors influence a niche, and in the case of the koala, its existence becomes more fragile since it must live in a warm climate and it does not produce its young in great numbers. Not surprisingly, animals with the broader niche survive fairly easily and thrive in certain areas, while those animals with the more specialized niche, like the koala and the giant panda, are more likely to become endangered species as their habitats are destroyed. Some life scientists say that a niche can be understood primarily in terms of competition, while others say it has more to do with one species being best fitted for a certain role. As early as the 1930s, life scientists developed the principle called competitive exclusion. One of the pioneers arguing for the primary importance of competition was the Russian microbiologist Georgil F. Gauze (1910– ), who conducted tests on different species of protozoa (one-celled organisms). Gauze successfully raised different species of protozoa, each in its own environment. He then put them together and discovered that one died out completely while the other thrived. In another test, he found that although the two species survived, U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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neither did so in great numbers, and each occupied a different territory (or a separate part of the test tube). From this work came Gauze’s nowaccepted principle that although two species may occupy the same habitat, they never share the same niche. A common example of such a phenomenon is that of the woodpecker and nuthatch. Both are birds that eat grubs (insects) that they find under tree bark. However, despite the fact that they are both after the same meal, neither occupies the niche of the other since woodpeckers start at the bottom of a tree and work their way up, while nuthatches begin at the top and work down. Beyond an organism’s feeding habits, there are scores of other factors that go into describing an organism’s niche. For example, the simple earthworm plays a key role in its habitat as a consumer of dead organic matter and as food for other animals (such as birds). It is a host for certain parasites, and its burrowing has a beneficial plowing effect on the soil, loosening it up and allowing air to circulate. As the example of the earthworm shows, understanding an organism’s niche also allows scientists to better understand both the organism and the environment it lives in.

Nitrogen Cycle The nitrogen cycle describes the stages in which the important gas nitrogen is converted and circulated from the nonliving world to the living world and back again. There are five main steps in the nitrogen cycle, four of which are carried out by bacteria. Since nitrogen cannot be used by the cells of living things until it has been converted by bacteria into a useful form, these nitrogen-fixing bacteria not only play a key role in the nitrogen cycle but are in fact essential to all life on Earth. All plants and animals need a certain amount of nitrogen in their systems in order to live. Nitrogen is an important component of amino acids, which are the building blocks of life, and nucleic acids, which make up genetic material. Although nitrogen is a free gas (it remains a gas under normal atmospheric conditions) that makes up nearly 80 percent of Earth’s atmosphere, only bacteria can use it in this form. This means that for every plant and animal on Earth, the abundant nitrogen in the atmosphere is entirely useless unless it is somehow combined with hydrogen or oxygen. Bacteria that produce compounds of nitrogen, called nitrates, work to form this necessary combination. Most nitrates are produced by bacteria in the soil and are therefore absorbed by plants. Animals obtain the nitrates either by eating plants or other animals that eat plants. When plants and animals die, the nitrates in their systems are returned to the atmosphere and back to the beginning of the nitrogen cycle. 410


THE FIVE STEPS OF THE NITROGEN CYCLE

Nitrogen Cycle

The nitrogen cycle has five main steps: fixation, nitrification, assimilation, ammonification, and denitrification. All except the third step (assimilation) are carried out by bacteria.

Nitrogen Fixation. The first step, nitrogen fixation, is probably the most important since it frees what was previously unusable nitrogen. Only a few types of bacteria are able to break the chemical bonds that hold the paired atoms in nitrogen gas together. Nitrogen-fixing bacteria that live in the soil have an enzyme called nitrogenase that they use to break that bond or “fix” and capture nitrogen gas from the atmosphere and convert it to useable ammonia. Blue-green algae, called cyanobacteria, is one example of bacteria that can change atmospheric nitrogen to ammonia.

Nitrification. The second step in the cycle, called nitrification, occurs when the converted nitrogen is combined with oyxgen and hydrogen. This combination results in the production of nitrogen compounds, like nitrates,

A flowchart showing all five steps of the nitrogen cycle beginning with atmospheric fixation. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

Atmospheric nitrogen (N2) Ammonia (NH3)

Atmospheric fixation

Lightning Spreading of ammonia, ammonium and nitrate fertilizers

Industrial fixation Protein

) Denitrification Plant and animal wastes, dead organisms

Uptake by plants

Ammonia (NH3 Nitrites

Nitrogen-fixing bacteria in soil and root nodules

Nitrous oxide Leaching of ground water

Nitrates


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that other organisms can use. During the third stage, assimilation, plants take up these nitrates with their roots and use the nitrates to build proteins and nucleic acids. In other words, the plants use the nitrates to grow. Animals, like plants, also need proteins to function and grow, and proteins are composed of amino acids that contain nitrogen atoms.

Ammonification. The fourth stage in the nitrogen cycle is called ammonification. This is the process in which these nitrogen compounds are returned to the soil and reconverted to ammonia by other bacteria. Ammonification occurs in two ways. The first is when a plant or animal dies and their organic matter is acted upon by decomposer bacteria. These bacteria play an important role because the nitrogen trapped in dead animals and plants would be useless and wasted were it not converted. A second way for ammonification to occur is when animals leave their urine and feces on the ground. Almost immediately by the same decomposer bacteria work to release the nitrogen found in these waste products.

Denitrification. The last step in the nitrogen cycle is called denitrification and involves the reduction of ammonia to nitrogen gas. Again, this process is carried out by a certain type of bacteria whose actions eventually release nitrogen gas back into the atmosphere. The complete nitrogen cycle is, therefore, a process in which nitrogen from the atmosphere is captured and converted, passes through a number of organisms (including bacteria), and is finally returned to the atmosphere to enter the cycle again. There is a more dramatic way that nitrogen is converted to a usable form. The atmosphere of Earth is roughly four parts nitrogen to one part oxygen, and these two gases never normally react with one another. However, during a thunderstorm when lightning strikes, the temperature and the pressure of the air is such that the two gases react chemically and form nitrous oxide gases. In the rainwater that accompanies the storm, nitrous oxide is dissolved and falls to Earth as nitric acid. This enters the nitrogen cycle when it soaks into the soil and forms nitrites and nitrates. However, far more nitrates are formed by bacteria in the soil than by atmospheric lightning flashes.

Nonvascular Plants Nonvascular plants are plants that do not have any special internal pipelines or channels to carry water and nutrients. Instead, nonvascular plants absorb water and minerals directly through their leaflike scales. Nonvascular plants are usually found growing close to the ground in damp, moist places. 412


Nonvascular plants are made up of mosses, liverworts, and hornworts—all of which belong to the subdivision of plants called Bryophyta or bryophytes. They are described as nonvascular because they do not have an internal transport or circulatory system as do other plants. Bryophytes are also nonflowering plants, meaning that they reproduce without growing flowers. Lacking a system to move food and minerals, these plants are unable to grow very tall since they depend on direct contact with moisture. Bryophytes lack true leaves and do not have roots, using rhizoids instead. Rhizoids are slender, rootlike hairs that both anchor and absorb like roots.

Nuclear Membrane

After rhizoids perform this initial absorption, movement throughout the plant takes place by the processes of diffusion and active transport. Diffusion is the distribution of a substance away from an area where it is highly concentrated. In this way, water and nutrients move from cells that are full to cells that are empty. Diffusion uses no energy. Active transport, however, does require a plant to use energy. It is used when a plant needs to achieve the opposite of diffusion and must try to concentrate a substance in one place. A plant carries out active transport by using carrier protein molecules. These molecules actually carry the needed substance from one side of a cell membrane to the other. Because of this essentially primitive means of obtaining and moving water and nutrients, a humid, moist environment is essential to nonvascular plants. [See also Plant Anatomy; Plants]

Nuclear Membrane The nuclear membrane is the outermost part of a cell’s nucleus that separates it from the cytoplasm. Also called the nuclear envelope, this double-membrane structure acts as a boundary for the nucleus, allowing it to keep its shape. It also allows controlled exchanges through its pores. The nucleus is by far the most important structure in any cell, plant or animal, since it functions as the control center directing all of the cell’s activities. The nucleus contains the chemical instructions deoxyribonucleic acid (DNA) needed to make a cell work properly. The nucleus is usually the largest separate structure in a cell and it typically has a round or oval shape. It keeps this shape because it has a double-layer membrane that keeps it separate from the rest of the cell’s cytoplasm. Cytoplasm is the jelly-like contents of a cell that contains all of its other structures. Besides acting as a boundary that keeps the nucleus together, the nuclear membrane also controls what passes between the nucleus and U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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the cytoplasm. It carries out this regulating function by using nuclear pores that dot its surface. These pores are like a sieve (a strainer with certain size holes) and they allow small molecules in and out of the nucleus. They also selectively permit large molecules to pass through their openings. The nuclear membrane also plays a key role during mitosis (myTOH-sis), which occurs when a cell makes a copy of itself. During the later stages of mitosis, the nuclear membrane begins to break down, allowing the already-duplicated chromosomes to split into two groups. After each complete set of chromosomes moves to opposite ends of the cell, a nuclear membrane reforms around each group. Soon each new cell has a separate nucleus surrounded by its own nuclear membrane. This reforming of the nuclear membrane begins the completion phase of mitosis. [See also Cell; Membrane; Mitosis; Nucleus]

A high resolution scanning electron micrograph of the nuclear membrane. The nuclear membrane is the double layer with the granular appearance surrounding the chromosomecontaining cell nucleus. (©Photographer, Science Source/Photo Researchers.)

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Nucleic Acids

Nucleic Acids

Nucleic acids are a group of organic compounds that carry genetic information. Nucleic acids are essential to life since they contain not only a cell’s genetic information, but also instructions for carrying out cellular processes. Deoxyribonucleic acid (DNA) is the particular type of nucleic acid out of which genes are made, and genes are the bearers of hereditary traits from parents to offspring. Nucleic acid was discovered in 1869 by the Swiss biochemist Johann Friedrich Miescher (1844–1895), who first found a sticky, clear chemical in the nucleus of cells. He named it nuclein, and although it later became known as nucleic acid, no one had any idea that it was in some way connected to heredity. The name nucleic acid itself indicates that these clear molecules were first found in the cell nucleus and that they have a mildly acidic character. By 1929, scientists had discovered that there were two types of nucleic acids. One of these contained the sugar ribose (and became known as ribonucleic acid or RNA) and the other contained the sugar deoxyribose (and was called deoxyribonucleic acid or DNA). By the 1930s, most geneticists agreed that the gene was crucial to heredity and was made of some sort of complex chemical, but no one thought it was made of nucleic acid because the acid did not seem to have a complicated enough structure to carry genetic information. By 1950, however, nucleic acid had been established as the key factor in inheritance, yet in the fall of that year when the young American biochemist James Dewey Watson (1928– ), traveled to Europe to study the chemistry of nucleic acids, no one knew how this chain of fairly simple molecules could contain all the information necessary to form a complex organism. In 1951 when Watson met the English biochemist Francis Harry Compton Crick (1916– ) at the Cavendish Laboratory at Cambridge, England, the two began a collaboration with the goal of determining the structure of DNA (which they believed would then explain how DNA actually works). In March 1953, the team of Watson and Crick announced that they had discovered the “double helix structure” of the DNA molecule and offered to science what was essentially an explanation of the chemical basis of life itself. Their theory was that the nucleic acid DNA was made up of two twisted strands that are held together by base pairs that make up the actual coded instructions. Each DNA base is, therefore, like a letter in the alphabet, and a sequence of bases can be thought of as forming U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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a message. Nucleic acids were found not only to contain genetic information (DNA), but were also able to carry that information from genes in the nucleus to other structures in the cell. Thus, the building of proteins was found to be controlled by the group of nucleic acids known as ribonucleic acid (RNA). Geneticists eventually came to describe RNA according to its function in the cell, messenger RNA (mRNA) and transfer RNA (tRNA). Both types of RNA are essential for a DNA molecule to make a copy of itself (which in turn is how proteins are made). mRNA passes out of the nucleus and carries the message for making a protein. tRNA reads this message and transfers the right amino acid to where they form proteins. Watson and Crick’s landmark discovery of the nature and function of nucleic acids provided the foundation for understanding the chemical basis of life. [See also Chromosome; DNA; Enzyme; Genetic Engineering; Genetics; RNA]

A high resolution scanning electron micrograph of the nucleolus of an eightweek-old human embryo. (©Photographer, Science Source/Photo Researchers.)

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Nucleolus

Nucleus

The nucleolus is the part of the nucleus (the cell’s control center) of a cell that helps produce ribosomes. Ribosomes are those parts of a cell that help make proteins. The nucleolus is easily recognized as a dark, dense area near the center of the nucleus. Most cells have only one nucleolus, although some have two or more. When a cell nucleus is stained in order to observe it better under a microscope, the nucleolus is always seen as a dark-stained body. Its shape is usually irregular, probably because it is not walled off from the rest of the nucleus by any type of membrane. It looks like a mass rather than something that is sharply defined. The nucleolus has an important job, however, in that it assembles ribosomes. After the nucleolus assembles them, the ribosomes leave the nucleus through its membrane’s pores and enter the cell’s cytoplasm (the jelly-like contents of a cell). Here they go to work making proteins. Since nucleoli are indirectly involved in making proteins, they perform a key function in the cell. Rapidly growing cells require a great deal of protein, which means that they must also have a lot of ribosomes. The name nucleolus means “little nucleus,” and the nucleolus does resemble a small nucleus within a large nucleus. [See also Cell; Nucleus]

Nucleus The nucleus is the control center that directs the activities of the cell. Most important is its control of cell reproduction and the construction of materials like proteins. The nucleus also functions as the cell’s main repository of genetic information in the form of deoxyribonucleic acid (DNA). A eukaryotic cell is a cell that contains a separate nucleus. Plants, animals, fungi, and some forms of single-celled life are eukaryotic or eukaryotes. Those living things, like bacteria, whose cells do not have a distinct nucleus are called prokaryotes. A eukaryotic cell contains many structures called organelles, each of which has a separate function. The most important organelle in a cell is its nucleus (named after a Latin word meaning “kernel” because it looks like a seed in the center of a fruit). If a cell can be described as a miniature factory in which conditions are carefully regulated, then the nucleus in the cell is the factory’s main office or control center. The nucleus is a very busy place as it simultaneously conU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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trols many cellular activities, responds to changes in its environment, and helps make ribonucleic acid (RNA). As the heart of every cell, the nucleus is easily seen when a cell is observed with a microscope. Because of its large size, the nucleus is by far the outstanding feature of every cell. It usually appears as a rounded structure near the center of the cell. Besides its size, what makes the nucleus so distinct is that it is surrounded by a double membrane called the nuclear envelope. This keeps the nucleus separate from the rest of the cell’s living material called the cytoplasm. This envelope has many tiny openings known as pores that allow certain substances to pass in and out of the nucleus to the rest of the cell. Inside the nucleus are two important structures: threadlike structures called chromosomes and small, round structures called nucleoli. The chromosomes contain the cell’s genetic material called DNA, which contains all the instructions needed to make a cell work. DNA also contains a cell’s genes, which are the basic units of heredity. The number of chromosomes a nucleus contains will change from species to species (humans have fortysix chromosomes). Also found in the nucleus are one or more small, round nucleoli (singular, nucleolus) which help the cell make ribosomes. Ribosomes are organelles that play an important role in the manufacture of proteins. The nucleolus sends RNA to the ribosomes, which use amino acids to make proteins. Ribosomes are outside the nucleus. The nucleus of a cell

The nucleus and perinuclear area in a rat’s liver cell. The nucleus is the dark area in the middle of the cell. (Reproduced by permission of Phototake. Photograph by Dr. Dennis Kunkel.)

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is therefore the control center of the cell because it directs the cell’s activities by controlling the synthesis of production of proteins. It is with these chemicals called proteins that the nucleus actually runs the cell. Proteins are also a key ingredient in the material out of which cells, are made.

Nutrition

[See also Cell; Cell Division; Chromosome; DNA; Protein]

Nutrition Nutrition is the process by which an organism obtains and uses raw materials from its environment in order to stay alive. Autotrophs, like plants, are able to make their own nutrients, while heterotrophs, like animals, must ingest or eat them. Nutritional needs of plants and animals are different. These needs may vary considerably for animals according to their age, sex, level of activity, and reproductive status. Nutrients are any substances that are taken in by a living thing in order to survive or to sustain its life. All living things must take in nutrients from their environment in order to grow and repair themselves and to provide the energy they need. This process of taking in nutrients and using them is called nutrition.

AUTOTROPHS Plants and animals not only have different nutritional requirements, but they obtain their nutrients in different ways. Plants are called autotrophs because they produce many of their own nutrition requirements by photosynthesis. Photosynthesis is the chemical conversion of the Sun’s light into simple food chemicals. A plant makes its own nutrients from inorganic materials like nitrogen (gas), carbon dioxide (gas), and sunlight. The basic elements that a plant needs are mostly in the soil in which they grow. Elements required in large amounts are called macronutrients. These macronutrients are carbon, oxygen, hydrogen, nitrogen, phosphorous, potassium, calcium, and magnesium. Plants also require smaller amounts of micronutrients such as chlorine, iron, boron, manganese, zinc, copper, molybdenum, and nickel. Plants, or autotrophs, are considered the “primary producers” in the food chain (the series of stages energy goes through in the form of food) because all other life depends on them directly or indirectly.

HETEROTROPHS Heterotrophs include humans and other animals. Heterotrophs cannot make organic material the way plants can. Instead, they must obtain their U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Nutrition

nutrients from the food they eat. Heterotrophs are divided into herbivores that eat plants, carnivores that eat other animals, and omnivores who eat both plants and animals. Humans are omnivores and must obtain their nutrients from plants and animals. Like plants, the nutrients humans take in are also divided into macronutrients and micronutrients. Macronutrients are substances humans need in large amounts, like carbohydrates, proteins, and fats. All three of these essential nutrients are large, complex molecules that must be digested, or broken down, into smaller units so they can be absorbed or taken into the body’s cells. Micronutrients are substances like vitamins and minerals that people need in much smaller amounts. However, just because micronutrients are not needed in great quantity does not mean that they are not essential to the proper functioning of the body. Vitamins are especially important because they help enzymes (proteins that control the rate of chemical changes) regulate chemical reactions and are not generally stored by the body. This means that vitamins must be obtained by one’s diet.

The food pyramid developed by the U.S. Department of Agriculture. The bottom level consists of cereal foods, the second of fruits and vegetables, the third of proteins, and the fourth of fats and oils. For a healthy, nutritious diet, one should eat more of the foods represented in the lower levels. (Illustration courtesy of Gale Research.)

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For all living things, food is a key to life since it contains the raw materials, or nutrients, needed to stay alive. Food performs three functions for animals. It acts as fuel for all of the various reactions that take place in the human body, and it provides energy to move about. Food also provides the organic raw materials for the body to make new structures and to repair others. Finally, food contains the nutrients that animals cannot make for themselves. A good example are the much-needed ten “essential amino acids” that the body cannot make on its own, but must be obtained through food.

Nutrition

MALNUTRITION The condition known as malnutrition literally means “bad nutrition,” and an organism is described as malnourished when it is not getting enough of one or more nutrients. The lack of enough carbohydrates and fats means that eventually, the body will start to consume its own proteins. This only happens when a person is actually starving, and it leads to a wasting away that eventually proves fatal.

VARYING NUTRITIONAL NEEDS The absence of an important vitamin usually leads to a specific deficiency disease, and the lack of a needed mineral also can lead to serious conditions. For humans, a diet that supplies the proper amount of nutrients is said to be “balanced,” but a person’s nutritional needs vary greatly according to their stage in life. During infancy, a person’s total energy requirements are the highest. Later in childhood, growth is rapid, and protein intake should be fairly high to develop new tissue. During adolescence, growth spurts also occur, which make impressive nutritional demands. In adulthood, growth has stopped and nutritional requirements are less, although they are still very important to maintaining good health. A person’s level of activity also affects their nutritional needs, and a pregnant woman or an athlete both have different nutritional needs compared to an inactive senior citizen. Although the nutrient amounts taken in depends on these and other variables, a general rule of thumb would be that carbohydrates should provide half of the total energy intake; fats should make up about one third; and protein should comprise the remainder. [See also Amino Acid; Carbohydrate; Lipids; Malnutrition; Protein; Vitamin]


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O Oceans Oceans are huge bodies of salt water that are extremely deep and which cover about 71 percent of Earth’s surface. As the largest home for life on Earth, oceans contain a diverse set of environments that support a tremendous variety of animals and plants. All of the world’s oceans are linked together, although they do not makeup one uniform environment. Nearly three-quarters of Earth’s surface is covered by oceans or seawater. To a scientist, an ocean is different from a sea. To qualify as a sea, a body of water must be large enough to have an effect on its environment, and usually be landlocked. An area is only considered an ocean if it is at least 6.562 feet (2,000 meters) deep. According to this, there are, therefore only six oceans: the Arctic, the North Atlantic, the South Atlantic, the North Pacific, the South Pacific, and the Indian Ocean. All of the Earth’s oceans are connected to one another, meaning that a person could actually go completely around the world and never touch land. All oceans are salty because seawater is made up of dissolved inorganic substances, of which sodium and chloride are the two largest, forming about 85 percent. Together, sodium and chloride form salt. All oceans also have tides (the regular rise and fall of their water level). These are caused mostly by the gravitational pull of the moon that moves around Earth. All oceans also have currents or a stream of water moving in one direction both at their surface and deeply below it. The currents usually are caused by the wind and the differences in water temperature. Oceans lose water by evaporation but regain it by precipitation (rain) and from rivers that drain into them. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Oceans

The coast of the Pacific Ocean in California. The oceans are the world’s largest biomes and are home to the most fascinating plant and animals species. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

PARTS OF AN OCEAN The marine or ocean environment is divided into two major parts. The pelagic environment makes up all of the ocean water, and the benthic environment makes up the ocean floor. The pelagic environment is further subdivided into what are called provinces. The neritic province is from the shoreline out to open ocean of about 656.2 feet (200 meters) deep. The oceanic province is the ocean at a depth greater than 200 meters. No ocean is uniform in any of its features, and there are enormous differences between its several different zones. For example, in coastal areas close to shore, the primary producer, or the first level on the food chain (the transfer of energy in an ecosystem), is the phytoplankton (singlecelled algae that live by photosynthesis). On the surface of the open ocean, however, the primary producers are the macroscopic algae, also known as seaweeds. Differences in the amount of light, as well as pressure and temperature, affect what will grow or thrive in which zone.

The Littoral Zone. Starting at the shoreline, or the littoral zone, where the land and ocean come together, high levels of light and nutrients make this a productive area. Since the water is so shallow, most organisms lie on or in the seabed, which may be rocky, sandy, or muddy. Rocky shores allow seaweeds to anchor themselves firmly and also provide protection for tiny animals.

The Continental Shelf. Extending outward from the littoral zone is the continental shelf, which is really just an underwater extension of the land. This area is by far the most productive part of any ocean. The ocean above the continental shelf, which extends from shore anywhere from 40 to 200 miles (64.4 to 321.8 kilometers), is fairly shallow compared to the rest of the ocean. It is usually in this shallower part of the shelf that tuna, porpoises, and sharks are found because that is where most of the fish they eat are found. The richness of 424


the shelf is due to the light it receives as well as to the nutrients that rivers often pour into it. As the shelf gradually slopes away from shore, it eventually falls off rather sharply. This eventually leads to the abyssal zone that can reach an incredible depth of 7 miles (11.3 kilometers) in some places. Since light cannot penetrate below a depth of about 650 feet (198.1 meters), this depth is where the aphotic (no light) zone begins.

Omnivore

The Aphotic Zone. In such a deep ocean habitat, there is little life because there is no sunlight, the pressure is enormous, and the temperatures are constantly cold (barely above freezing). The few fish that do inhabit this dark zone have adaptations like huge jaws that allow them to swallow another fish whole and are often able to generate some light of their own. It was once believed that nothing could live below 1,800 feet (548.6 meters), but the fairly recent discovery of activity around the hydrothermal vents showed this not to be true. During the 1970s, new underwater technology allowed scientists to penetrate to great depths and they came upon “vent communities,” or areas around underwater volcanoes that were teeming with life. This heated water was loaded with minerals that kept special bacteria as well as clams, crabs, and fish alive. As with many other natural habitats, oceans also are threatened by humankind. Pollution of the oceans is usually a direct result of human activities, as oceans have long been a dumping ground for waste and land runoff. Recently, contamination from oil spills has become the most severe threat to their health. Overfishing also is becoming more of a threat. Human technology has created floating fish factories that in the long run may seriously endanger entire species. If something is not done to protect the oceans, people may soon lose the largest and most fascinating environment on Earth. [See also Biome; Water]

Omnivore An omnivore is an animal that eats both plants and other animals. Because of the wide variety in their diet, omnivores are adaptable to many different environments. They also can be found at several different levels on a food web (the connected network of producers, consumers, and decomposers). Unlike carnivores who eat a diet almost completely of meat, and herbivores who eat only plants, omnivores are generalists when it comes to what they can and will eat. One of nature’s principles is that animals’ bodies are adapted to what they eat. Their systems are designed with their U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Omnivore

Besides humans, bears are probably the most common omnivore. These grizzly bears will eat both plants and animals. (©U.S. Fish & Wildlife Service. Photograph by Chris Servhenn.)

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diet in mind. Therefore, carnivores, who must hunt and kill other animals in order to eat their flesh, are designed for those purposes. Their senses are sharp, they have deadly claws and teeth, and their digestive systems are prepared to process high-protein meat. On the other hand, herbivores do not have to track, catch, and kill their food. Since they eat only green plants, herbivores need only find and eat these plants. Their teeth are not sharp and pointed but are designed for grinding tough plant material. Herbivores’ digestive systems are also specially built to break down cell walls made of cellulose (the main component of plant tissues) by containing microorganisms just for that purpose. The name omnivore is taken from the Latin omnis meaning all, and vorare to eat or devour. According to this broad definition, humans are probably the most omnivorous of all animals, since they can eat almost anything. Besides classifying animals as omnivore, carnivore, or herbivore, biologists categorize the larger group of all living things as autotrophs or heterotrophs.


Autotrophs, like all plants and some bacteria, can make their own food. Heterotrophs (like animals) cannot make their own food and must eat plants or other animals. Autotrophs are considered to be producers since they are at the first level of the food chain (the series of stages energy goes through in the form of food). This is because the food they make supports all other life in the chain. Heterotrophs are considered to be consumers since they cannot make their own food and must eat others (plants or animals) to survive. Omnivores are also heterotrophs.

Order

Unlike herbivores, who always are one level above the plants they eat, different omnivores can be found at different levels on the food chain at different times. Omnivores can be first-level consumers because they eat plants, but they can also be higher up since they also eat animals. For example, a bear will eat berries as well as a fish. Many kinds of mammals are omnivores, such as humans, pigs, bears, apes, raccoons, and hedgehogs. Because of this varied diet, their digestive systems are not as specialized as others who eat only one type of food. Carnivores have a short digestive tract since their systems must break down easy-to-process protein. Herbivores, however, have elaborate and multichambered stomachs to process and reprocess the hard-to-breakdown cell walls of the plants they eat. Omnivores have intermediate digestive systems that can handle both meat and plants. It would seem that the varied food supply of omnivores gives them an advantage over other more specialized eaters. [See also Carnivore; Herbivore]

Order The term order is one of the seven major classification groups that biologists use to identify and categorize living things. These seven groups are hierarchical or range in order of size. Order is at the exact middle of the seven groups, located between class and family. The classification scheme for all living things is: kingdom, phylum, class, order, family, genus, and species. Organisms in the same order are much closer to each other, genetically and on the evolutionary scale, than are those in the larger group called class. For example, although all animals in the class Mammalia produce milk for their young, those in the order Carnivora eat meat, while others in the order Insectivora eat insects. A house pet like a dog (Carnivora) is distinguished by its eating habits and preferences from a mole (Insectivora), although both are in the same class. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Organ

The order names of plant groups generally use the suffix -ales (e.g., Rosales), while the order names of animals usually end with an -a (e.g., Carnivora). In practical terms and in most scientific discussions, the order of an organism is seldom considered. Rather, the more specific terms of family, genus, and species are used. [See also Class; Classification; Family; Genus; Kingdom; Phylum; Species]

Organ An organ is a structural part of a plant or animal that carries out a certain function and is made up of two or more types of tissue. Each organ plays a key role in keeping the organism alive. A group of organs that work together to do a certain job is called an organ system. Very often, a person’s liver, heart, brain, and kidneys (among others) are referred to as internal organs, or vital organs. This is because they are usually inside the body (although some, like eyes, are not), and they are vital, or essential, to keeping us alive. The word vital can also mean living or alive. Plants also have many kinds of vital organs, such as leaves, stems, roots, and the various parts of a flower. However, whether in plants or animals, an organ is usually composed of two or more types of tissue that work together to do a particular job. For example, the heart is an organ whose job it is to pump blood throughout the body. It is made up of muscle tissue that contracts, nerve tissue that transmits impulses, connective tissue that binds it together, and epithelial tissue that lines its surfaces. Tissue is considered to be a group of similar cells that all do the same job. Some organs, like the heart, have only one job to do, while others, like the liver, may have many tasks to perform. When a group of organs are linked together to perform a certain function, it is called an organ system. In large, complex animals, there are ten major organ systems that work together to make up the organism. The main organ systems of complex animals include the skeletal system, nervous system, circulatory system, respiratory system, muscular system, digestive system, excretory system, endocrine system, reproductive system, and integumentary system. The skeletal system consists of bones and cartilage and helps support and protect the body. The nervous system consists of the brain, spinal cord, sense organs, and nerves. It collects, processes, and distributes information. The circulatory system includes the heart and blood vessels. It transports essential materials throughout the body. The respiratory sys428


tem is made up of the lungs and air passageways. It supplies necessary oxygen while removing carbon dioxide (a major atmospheric gas). The muscular system consists of large, skeletal muscles that contract, as well as cardiac muscle and the smooth muscle. The digestive system includes the mouth, stomach, intestines and other organs that break down and absorb food. The excretory system consists of the kidneys, bladder, and other ducts, or tubes, that remove waste from the blood. The endocrine, or glandular, system consists of the thyroid, pituitary, mammary, and other glands. These glands release hormones (chemical messengers) into the circulatory system to regulate metabolic activities (all the body’s chemical processes). The reproductive system is made up of the testes, ovaries, penis, vagina, and uterus. It passes on genes to its offspring. The integumentary system consists of the skin, hair, and nails that serve to protect the body and to regulate its temperature, as well as to receive stimuli.

Organelle

The organs of a plant are its leaves, stems, roots, and the various parts of its flowers. A flower is actually an example of an organ system in a plant, since it is usually composed of stamens, pistils, petals, bracts, and receptacle. The stamens are its male reproductive organ, and are composed of anthers and filaments. The pistil is the female reproductive organ and consists of a stigma, style, and ovary. [See also Brain; Circulatory System; Digestive System; Endocrine System; Excretory System; Heart; Integumentary System; Muscular System; Nervous System; Reproductive System; Respiratory System; Sense Organ; Skeletal System]

Organelle An organelle is a tiny structure inside a cell that performs a particular function. Organelles are only found in eukaryotic cells (those with a distinct nucleus), and are not found in prokaryotic cells (those without a distinct nucleus). Both animal and plant contain many types of organelles (or “little organs”). Just as any organ has a specialized, particular function to perform as part of a larger system, so these “little organs” within a cell have certain tasks they perform. Organelles are bounded by a membrane and are run by the cell’s control center, the nucleus (which itself is an organelle). Each organelle has a job to do that is crucial to maintaining the life of the cell and in most eukaryotic cells, organelles can be grouped into three categories according to their general function. The organelle that directs a cell’s activities and holds the cell’s genetic information is the nucleus. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Organic Compounds

Both plant and animal cells have a nucleus. It is the largest structure in animal cells and is separated from the rest of the cell’s cytoplasm (jellylike fluid) by a double membrane called a nuclear envelope. Organelles also function in transport, synthesis (making things), storage, and recycling. In plant and animal cells, the organelles responsible for these activities are called the endoplasmic reticulum, ribosomes, Golgi bodies, and lysosomes. Plant cells also have organelles called vacuoles. The endoplasmic reticulum is a complex network of folded membranes that form tubes and transport, or move, materials to all parts of the cell. They are something like a pipeline. Ribosomes play an important role in the synthesis, or making of, proteins. Golgi bodies look like a stack of flattened pancakes. They put the finishing touches on proteins, and then sort the proteins and pack them for transport. Lysosomes are bags of enzymes that help a cell digest the food it takes in. Only plant cells have a large sac called a vacuole that they use for storage. Other organelles function to produce energy. Called mitochondria and chloroplasts, these organelles are responsible for changing energy from one form to another. Mitochondria are found in both plant and animal cells and are called the powerhouses of the cell because these organelles break down a cell’s food and release energy. Although chloroplasts also produce energy for cell, they are a little different than mitochondria for two reasons. First, chloroplasts are not found in animal cells, they are only found in plant cells. Second, unlike animals that must take food into their bodies, plants can make their own food from which they obtain their energy. Plants create this food using small, green organelles called chloroplasts that capture the energy in sunlight. Chloroplasts use this trapped radiant energy of the Sun and turn it into chemical energy for use or storage. When the plant is ready to use the energy, the mitochondria take over and release the stored energy. [See also Chloroplasts; Endoplasmic Reticulum; Golgi Bodies; Lysosomes; Mitochondria; Nucleus; Ribosomes]

Organic Compounds Organic compounds are substances that contain carbon (a nonmetallic element that occurs in all plants and animals). All living things have an essential dependence on organic compounds, since carbon occurs in almost every chemical compound found in living things. There are four main types of organic compounds in living things: carbohydrates, proteins, lipids, and nucleic acids. 430


An organic compound is a combination of carbon and almost any other element. Because of its unique atomic structure (the way a single atom of carbon is built), a carbon atom is able to link up with as many as four other atoms of another element. Since it can also link up with other carbon atoms and form long, stable chains, the variety of combinations carbon can form with other elements is almost limitless. Scientists have already identified more than 1,000,000 organic compounds.

Organic Compounds

Until the nineteenth century, it was commonly believed that organic compounds only could be produced by something that was living. In those days, it was thought that some sort of “vital force” existed only in living things, and that it was this force that made living things uniquely capable of producing organic compounds. Two hundred years ago, organic meant “vital,” or “living.” Therefore, in the past, an organic compound was the tissue or the remains of a living thing, while an inorganic compound was something lifeless like a rock or the waters of the earth. In 1828 the German chemist Friedrich Wohler (1800–1882) changed all of this thinking. That year, he quite unintentionally produced urea, an organic substance formed naturally in the bodies of mammals, in his laboratory using strictly inorganic substances. Starting with this laboratory breakthrough, science eventually came to recognize that no “vital force” was necessary for a substance to be considered an organic compound. Eventually, it was learned that what was important was molecular structure, or the way the atoms arranged themselves into molecules. This led to the modern definition of what became the study of organic chemistry— the chemistry of carbon compounds. Today, it is known that all living things are not only organic compounds, but that they also are critically dependent on organic compounds. Specifically, foods are all organic compounds since they are made up of carbohydrates, fats, and proteins. Materials such as the cotton and wool in clothing, the petroleum for cars and factories, and all synthetic (manmade) drugs and plastics are organic compounds. Finally, the very chemistry that carries our genetic information—nucleic acids—are complex organic compounds made up of small molecules called nucleotides. Interestingly, the word organic has been taking on more of its much older (and less precise) meaning, as people now speak positively of the benefits of “organic gardening,” “organic food,” and “organic vitamins.” This use of the term organic suggests that some sort of mysterious “vital” force is at work in these compounds that gives them special qualities that synthetic products do not have. While an organic tomato harvested from a small farm may taste much better than one grown U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Organism

commercially and picked green, from a chemical standpoint, however, they are identical.

Organism An organism is any complete, individual living thing. As a living thing, an organism necessarily has certain attributes or displays certain characteristics that make it different from a nonliving thing. The things that organisms must do to maintain life are called life processes. Although both living and nonliving things are made up of many of the same types of atoms (the building blocks of an element that come together to form a molecule), there are drastic differences between the two in terms of how energy is used and how materials are organized. A bacterium living in a cow’s gut is an organism, as is a worm or a tree—but a rock or a fire is not. Despite how dissimilar organisms can often appear, they exhibit certain features that are common to all. Knowing these life characteristics allows us to determine whether something is alive (and therefore is an organism) or not.

METABOLIC ACTIVITY All organisms share the characteristics of taking in materials and releasing waste. Plants take in carbon dioxide and water and use sunlight to make food. Animals eat plants or other animals to take in nutrients or food. Plants give off oxygen and animals give off carbon dioxide and other waste materials. After taking in materials, all organisms show some form of metabolic activity. This means that organisms are able to break down materials and release the energy these materials contain. They can store this energy or use it to fuel their life processes. An organism can also build more of itself and grow or increase its size. It can repair itself and grow new and larger cells. Organisms pass through their own life cycles. These cycles include a series of changes called development. All organisms also use part of their energy to produce more of their own kind. Reproduction is another characteristic of life. Every organism is itself the product of reproduction. This means that every organism is the offspring of one or more parent organisms. An individual organism can also be terminated or its life processes stopped permanently. If this happens because of an outside cause, then something has killed the organism. Whether or not it is killed, every organism still goes out of existence because of death. Death is therefore a characteristic of life. 432


INTERNAL ORGANIZATION AND HOMEOSTASIS

Organism

There are other characteristics of life important to organisms. All organisms show a capacity for internal organization. They are not a random jumble of cells, but are instead very organized internally and externally. Each organism has its own individual form of organization, so that a worm has a characteristic pattern that is quite different from a shrub. Another characteristic of organisms is their reactions, or responses to changes in their environment or surroundings. Animals often respond by movement, but nonmoving plants respond as well. A typical example of a plant responding to its environment is a potato plant forming an overwintering tuber (an underground bulb) as the days get shorter, or an onion plant forming bulbs during the long, hot days of summer. Organisms also react to their environment by making internal adjustments called homeostasis. Through homeostasis, organisms adjust their internal environment so that they maintain balanced or stable conditions. Organisms instinctively work at keeping things in control and constant. In terms of their behavior, all organisms have some degree of adaptive potential, which means that they can adjust to environmental changes over both the short term and the long term. This long-term adjustment occurs through the process of evolution (the changes an organism goes through over generations). A final characteristic of organisms is that their cells all contain deoxyribonucleic acid (DNA), which carries the genetic information specific to each organism. This DNA is carried in almost every cell within an organism and contains the instructions for reproducing traits that are to be inherited, or passed on, from parent to offspring. An organism displays all of these traits or life processes. A nonliving thing may show some. For example, a fire can move, consume or take in materials, and give off a gas, or an automobile engine takes in materials and gives off a waste product. However, no nonliving thing can show all of them as an organisms does.

BASIC NEEDS Organisms also have certain basic needs that must be met if they are to continue to live. Given the great variety of living things, those essential needs can be generalized by the following four requirements: energy, water, appropriate gases, and proper temperature. Organisms also share one common trait that is perhaps the most basic. All organisms are made up of one or more cells. Therefore, each organism shares the cell as the basic unit of life. Thus, a single-celled algae is as much an organism as a human being made up of trillion of cells. [See also Homeostasis; Metabolism] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Ornithology

Ornithology Ornithology is the branch of zoology that deals with birds. It includes the study of the development, anatomy (structure), physiology (function), behavior, classification, genetics, and ecology of birds, among other things. Scientists believe that they know more about birds than any other group of organisms. Named after the Greek word for birds, ornithology is the scientific study of birds. The Greek philosopher, Aristotle (384–322 B.C.), was very interested in all aspects of biology and wrote a great deal on birds. He listed about 40 species and described their habits, migration (movement from one place to another), and relationship to the environment. Many consider the thirteenth-century emperor, Frederick II (1194–1250), as the first real ornithologist because of his book, On the Art of Hunting with Birds. Although Frederick II was a falconer and wrote his book about using falcons to catch prey, his work was also a serious, scientific account of the habits and the structure of many types of birds. Most books written on birds during his time focused on the practical aspects of birds, such as falconry or game-bird management. However, the fifteenth-century discovery of the Americas eventually gave students of the natural world a huge number of new and sometimes wildly different and exotic birds to describe and understand. By the 1750s, birds were included in all classification systems, and by the end of the next century, their place in the world’s evolutionary history was becoming understood. However, until about 1900, most bird studies still were concerned only with classifying and describing them. Soon after the beginning of the twentieth century, universities began to grant degrees in ornithology, and professional ornithologists came on the scene. As a result, the twentieth century saw an emphasis on further understanding bird anatomy and behavior as well as their role in the overall balance of life (ecology). There are nearly 9,000 species of birds, and ornithologists have decided that what most distinguishes an organism as a bird is not its ability to fly, but rather its feathers. Birds are vertebrates (animals with a backbone) that have feathers covering their body, and have forelimbs that are modified into wings. They are endothermic (warm-blooded; maintain an internal temperature despite their environment) and many parts of their bodies have adapted to their ability to fly. For example, they have large, powerful breast muscles, a streamlined shape, and hollow bones (since weight is a critical factor in flying). Birds lay eggs, have an extremely active life cycle, and use song and sounds for many different reasons, such as a warning, to mark territory, and to show dominance or attract a

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mate. Birds are very diverse. There are 27 orders of birds in the class Aves, divided into about 166 families that contain about 9,000 species. Some migrate extremely long distances, while others stay in the same habitat. They can vary in size from the 300-pound (136.2-kilogram) ostrich to the 0.08-ounce (2.27-gram) hummingbird. Some, like the Arctic tern, stay in the air for weeks at a time, while others, like the penguin, are entirely flightless. Although traditionally a great deal of information about birds has been obtained by simple field observations, ornithology has put available technology to greater use with such techniques as banding. This technique of putting a ring or band on a bird’s leg has been practiced for a long time, but now it has become a major way of obtaining information about bird movements since it is used with sensitive radar equipment. Ornithologists are also able to study bird calls in natural environment since they now have high quality, portable sound equipment. One thing that makes ornithology different from almost every other branch of the life sciences is the fact that amateurs have regularly made major contributions to the field. Many a nonscientist who has only a passion for birds and plenty of time has been able to learn a great deal about birds and actually discovered things unknown to science. The best


Ornithology

The banding of an ovenbird. This banding technique is a major way of obtaining information about bird movements. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

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known amateur may be the American artist John James Audubon (1785– 1851), whose four-volume The Birds of America (1827–38) is a monument to art and science. More recently, the best work on the life history of the song sparrow was done by a housewife and mother of five, Margaret M. Nice. Today, ornithologists are necessarily concerned with environmental conditions that affect birds since birds seem especially sensitive to pollution and climate changes. Because of this, the more ornithologists learn about birds, the more knowledge they gain about our own environment. It is significant that the environmentalist Rachel Carson chose to focus on the lack of birdsong (Silent Spring) as a tangible example of the disappearance of certain bird species due to pollution. She showed how, in many ways, birds are a sentinel species for humans, meaning that they are the first to suffer when the environment becomes degraded. [See also Birds]

Osmosis Osmosis is the movement of water from one solution to another through a membrane or barrier that separates the solutions. Osmosis is a natural process that takes place whenever the proper conditions arise. It occurs when solutions of different strengths are separated by a barrier whose pores will only let molecules of a certain size pass through. Water moves from the weaker solution to the stronger solution in order to make both solutions of equal strength. Osmosis occurs in both animals and plants. One of the characteristics of nature is that it always tries to equalize situations, to bring things into balance, or to make extremes more similar. This characteristic leads to a phenomenon known as diffusion in which a substance will always spread from an area where it is highly concentrated to one of lesser concentration. Osmosis is a variation of diffusion and might be described as diffusion involving a solution and a barrier. Osmosis necessarily involves liquids called solutions. A solution is a liquid with something dissolved in it. If a teaspoon of sugar is stirred into a glass of pure water until it dissolves, a solution is made. If that container of sugar water is somehow arranged so that only a permeable barrier (one with holes the size of water molecules) comes between it and another container of pure water, after a few hours the pure water will have moved 436


through the barrier and into the sugar solution. The water moved into the sugar solution because water molecules are smaller than sugar molecules. Therefore, only the water molecules are able to move and equalize the solutions. This passage or movement stops when the osmotic pressure is reached, or the equalization of pressure on both sides of the barrier. Equal pressure means that the solution concentrations on both sides of the barrier are no longer high and low but the same. In the life sciences, osmosis occurs at the cellular level. Since animals have watery body fluids that contain a variety of dissolved salts, osmosis is necessary to keep their salt and water levels constant. Osmosis plays a key role in the kidneys of mammals since these organs filter urine from the blood, reabsorb water and nutrients, and secrete wastes. Plants obtain the water they need through osmosis that occurs at their root hairs. Root hairs contain more dissolved substances (such as sugar and salts) than there are in the soil. Because of this difference in the solution concentration between the outside and the inside of the plant, water is able to pass through the root hair cells and into the roots by osmosis. Osmosis also helps a plant stay upright and stiff. Osmosis is an ideal method of moving small amounts of water slowly, whether in a plant or an animal, since it does not require the use of any energy or tubelike transport systems.

Osmosis

Two illustrations showing how osmosis equalizes the concentration of a liquid. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

Solvent flow

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Ozone

Ozone Ozone gas is a form of oxygen found naturally in the stratosphere or upper atmosphere that shields Earth from the Sun’s harmful ultraviolet radiation. Ozone is also found in the lower atmosphere as a man-made pollutant best known as smog. In recent years, it was discovered that certain synthetic gases are destroying Earth’s stratospheric ozone shield. Ozone is a natural component of Earth’s upper atmosphere and is essential to the continuance of life on this planet. It is located between 6 and 28 miles (9.7 and 45.1 kilometers) above Earth. As a high-energy form of oxygen, it is formed naturally in the air both by the electrical charges given off during lightning and by high intensity, short wavelength light. Without this ozone layer absorbing the Sun’s ultraviolet radiation, living things on Earth’s surface could not survive. Many scientists believe that life on Earth could not have evolved without this protective ozone shield because direct penetration of the atmosphere by solar radiation would make Earth unlivable. Besides suffering life-threatening radiation burns and cancer, all aboveground organisms would eventually experience severe genetic damage. Although beneficial in the atmosphere, at ground level ozone is a bad-smelling, colorless gas that is poisonous and usually formed by pollutants people put into the air. Ozone at this level does not shield life from harmful radiation and is instead a major component of urban smog and a threat to all living things. Ground-level ozone, therefore, has only bad effects. Because smog can cause shortness of breath, chest pain, coughing nausea, and respiratory congestion in some people, many large cities have established “Ozone Action Day” campaigns to educate citizens how to reduce ground-level ozone and to warn them when to avoid strenuous outdoor activities.

OZONE DEPLETION What might be considered good ozone is the gas found naturally high up in the atmosphere around Earth. It was this layer of ozone that was discovered in the late 1970s to be getting thinner. Further research revealed a severely thin spot, which some called a hole, in the ozone layer over Antarctica. By the mid-1980s, monitoring by satellites and high-altitude planes revealed that ozone levels over Antarctica had declined about 50 percent. Scientists argue that such a dramatic decrease could be related to increases in human skin cancers as well as cataracts and a weakening of the body’s immune system. The decline in the ozone layer might also contribute to crop failures and a reduction in phytoplankton, which

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forms the basis of a major food web (the connected network of producers, consumers, and decomposers).

Ozone

Many scientists argue that there is extensive evidence that a group of synthetic chemicals called chlorofluorocarbons (CFCs) are the primary cause of ozone reduction. While certain natural events like volcanoes and surges in solar activity certainly affect the ozone layer, the argument against CFCs as the primary human-influenced cause is very strong. CFCs are compounds of chlorine, fluorine, and carbon. They are odorless, invisible, and otherwise harmless gases that were put to so many uses that they were thought to be miracle compounds. CFCs were widely used as propellants in aerosol spray cans, although they soon came to be used regularly as coolants in refrigerators and air conditioners. Finally, CFCs proved useful in making Styrofoam cups and packaging materials. With all this use of CFCs, molecules of the gases were released, spreading upwards into the stratosphere where they took part in an unusual and ultimately dangerous set of chemical reactions.

A series of NASA photos depicting ozone depletion over the South Pole from October 1979 to October 1990. (Reproduced by permission of the U.S. National Aeronautics and Space Administrations (NASA).)


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Ozone

When a molecule of CFC absorbs ultraviolet radiation, it releases a chlorine atom. This atom then reacts with the ozone (O3) to form an oxygen molecule (O2) and a chlorine monoxide molecule. At this stage, a molecule of ozone has already been converted into oxygen. The cycle then continues, as the chlorine monoxide reacts with a free oxygen atom and releases yet another atom, which in turn attacks another molecule of ozone. It has been estimated that each chlorine atom released from a CFC reaction can convert as many as 10,000 molecules of ozone to oxygen. Since ozone reduction is greatest when the atmosphere is at its coldest, the extreme atmospheric conditions of the Antarctic region provide excellent conditions for these reactions to occur, thus accounting for the “ozone hole” found there. During the 1990s, ozone depletion was shown to be happening seasonally over populated areas. This led to a decision by the United States government and several European governments to begin phasing out the production and use of CFCs by the year 2000. There is some evidence that the buildup of CFCs is declining since then, but unfortunately, the amount of highly stable CFCs already pumped into the atmosphere will remain for a century doing their destructive work. However, some comfort can be taken in the knowledge that not only are humans no longer contributing to the problem, but that an amazingly small amount of ozone in the stratosphere still can screen out more than 95 percent of the Sun’s ultraviolet radiation. [See also Pollution]

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P Paleontology Paleontology is the scientific study of the animals, plants, and other organisms that lived in prehistoric times. It largely involves the study of fossil remains. The fossil record enables paleontologists to reconstruct what type of life existed at various periods in Earth’s long history. Paleontology has often been described simply as the scientific study of fossils. Fossils are the remains of once-living plants or animals that were preserved in rock or other material. Fossils are found in sedimentary rock, or rock that started out as wet sand or mud. Fossilization occurs if an animal or plant dies and is quickly covered with a sediment like mud. Although its soft body parts usually decay, the harder parts (like teeth and bone) are sometimes preserved by petrifaction. This means that its organic material is replaced over millions of years by minerals that turn it into rock. By studying this evidence of the ancient past, paleontologists can reconstruct an account of what kind of life existed in various periods of Earth’s history. This, in turn, allows them to go further and try to establish a record of how all the animals and plants that make up today’s biosphere (part of Earth that contains life) evolved from their earliest beginnings. In this way, paleontology has often been able to provide actual evidence to support the theory of evolution (the process by which living things change over generations). Paleontology also has its practical aspects and sometimes helps in locating deposits of oil and natural gas. This is not surprising since these “fossil fuels” are often found in the same location as the fossils themselves. Paleontology combines the skills of geology with that of biology. Geology is essential, since a paleontologist would be lost if he or she had U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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no knowledge of the type of rock where fossils are typically found. The paleontologist also needs to be able to know the ages of different rocks (which indicate the age of the fossils found in those rocks). Biology helps paleontologists figure out how the ancient plants and animals that they discover may have actually lived. Although the word “paleontology” does not have an especially scientific meaning—literally, it means “a discourse on ancient beings”—in practice it has been able to scientifically establish the evolutionary development of life. Paleontology tells us when minor or major extinctions occurred, and sometimes why they happened. Paleontology helps recreate these ancient environments and explains why the plants and animals that lived then were built the way they were. Like geology (the study of the history of Earth), paleontology is a fairly young science. Before it was established, geologists had no way of determining the relative ages of rocks. By comparing fossils of organisms that lived for a short time, geologists were able to put together a chronology or time line of Earth’s history. Serious paleontological research dates back to the early 1800s, and is based on the work of two men born the same year. In England, the geologist William Smith (1769–1839) established the principle of stratigraphy—that the deeper layers of the earth were deposited earlier and were thus older than the layers closer to the surface. This would hold true for the fossils contained in those layers. Based on his theory, Smith was able to arrange fossils in terms of their age. The French anatomist, Georges Cuvier (1769–1832), was so skilled in animal anatomy (their body structure), that he was able to reconstruct a complete animal from an incomplete collection of bones. It was Cuvier who brought the new science of classification (the scientific way of identifying and grouping living things) to fossils. By grouping animals according to their skeletons or internal structure, instead of their outer form or how they looked, Cuvier eventually laid the basis for evolution despite his disagreement with that theory. Modern scientists were eventually able to show that the oldest fossils were in fact ones that differed from the more recent fossils and even more so from living animals. Modern paleontology uses several advanced dating techniques and has become, like many other branches in the life sciences, more specialized. The study of fossil plants is called paleobotany; that of fossil communities is paleoecology; that of extremely small fossils is micropaleontology; and that branch concerned with the study of fossil spores and pollen is called palynology. Altogether, these branches of paleontology seek to learn the history of life on Earth. [See also Dinosaurs; Fossil; Geologic Record; Radioactive Dating]

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Parasite

Parasite

A parasite is an organism that lives in or on another organism and benefits from the relationship. Most parasites harm their host organisms in some way and often cause disease in plants and animals. There are many different forms of parasitism.

SYMBIOSIS True parasites do not simply benefit at the expense of another organism, since that is also what a predator does (it benefits by killing and eating another organism). Instead, a parasite relies completely on the organism (called the host) it lives on or in for its nutrients. If the host dies, the parasite dies. Parasitism is a form of symbiosis, which is described as some type of relationship between two different species. There are three types of symbiosis: commensalism, mutualism, and parasitism.

Commensalism. In commensalism, a relationship exists in which one of the participants benefits but the other is neither helped nor harmed. The Spanish moss is an example of a commensalism parasite. It seems to live off certain host trees, but is not a parasite since it harmlessly attaches itself to branches while getting its nutrients not from the tree but from the decaying leaves at the base of the tree.

Mutualism. In mutualism, both members in the relationship benefit. Sometimes, neither can live without the other. A good example are the microorganisms that live in the gut of herbivores (plant-eaters) like cows. The cow could not digest its plant diet without these tiny organisms in its digestive tract, and the microorganisms themselves could not get the food they need from anywhere but the cow’s gut.

Parasitism. In parasitism, however, the relationship not only benefits one partner much more than the other, but it benefits that partner at the expense of the other. Nearly every animal and plant is a host or home to some form of parasite, and many parasites have evolved ways to minimize the damage they do to their host since if they quickly kill their host, they will die also.

TYPES OF PARASITES A parasite can be a single-celled organism, like a protozoan, or a complex arthropod, like a tick. Protozoans usually cause diseases directly, while arthropods usually function as “vectors” and cause disease indirectly. For example, the mosquito that carries the parasite that causes yellow fever is described as the vector since the mosquito transfers the parU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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An orange infested with parasites. This type parasite is not beneficial since it causes crop damage leading to a possible shortage of food and financial hardships for farmers. (Reproduced by permission of Dr. Edward Ross.)

asite to the host when the mosquito bites it. The mosquito’s action allows the parasite to enter the host’s system. Parasites also can be divided into those that live inside the host (endoparasite), and those that live outside or on the surface of the host (ectoparasite). For instance, the endoparasite that causes malaria lives in the liver and red blood cells of a human being, whereas a flea is an ectoparasite because it lives off its host’s blood while remaining only on the host’s outer surface or skin. A flea is actually a temporary ectoparasite, since it does not remain permanently attached to its host, but leaves after feeding and will later jump off to bite another host. Many parasites are host-specific, which means that they can only live on or in a certain host. The adult beef tapeworm is such a parasite since it can only live in the large intestine of a human being. Other parasites can make hosts of any type of animal. Certain parasites only can live off plants. A good example is the well-known Dutch elm disease that is caused by a parasitic fungus that feeds off living elm wood and destroys its internal transport systems that carry the nutrients the wood needs to survive.

RESPONSES TO PARASITES When a host is invaded by a parasite, it often attempts to fight back. The human immune system reacts defensively to try to rid itself of invading parasites. The fever, chills, and sweating associated with malaria are the body’s attempts to destroy the parasite. Since the death of the host means that the parasite also dies, some parasites have evolved ways to live off their hosts without killing them too quickly. However, if the parasite has not developed these ways, and the host is unable to adapt mechanisms of its own to fight back, an entire species may be killed off. This happened when the American chestnut tree was suddenly invaded by a parasitic fungus from Europe. The tree is almost extinct now. 444


Parasites are not some low form of life that dumbly live off a higher form. Rather, they are highly adapted and adaptable organisms that are genetically programmed to know when, how, and what to attack (and when not to). They may communicate with one another and even compete or avoid competing. While some parasites can only live a parasitic existence (and are called obligate parasites), others are more flexible and can provide for some of their own needs. It is important to remember, however, that some parasites are beneficial and vital to both plant and animal life.

pH

[See also Arthropod; Protozoa; Symbiosis]

pH pH is a number used to measure the degree of acidity of a solution. It is used on a pH scale that ranges from 0 to 14, with the difference between each number being a factor of 10. In the life sciences, as well as in chemistry, many chemical reactions depend on the pH of a solution. pH is also used to analyze body secretions, to test soil suitability, and for industrial purposes. pH refers to the amount of acid in a substance. The letters are said to have come from the French for “hydrogen power,” meaning how many

A litmus paper experiment demonstrating the pH of a bar of soap and an orange. Litmus paper is a common pH indicator. (Reproduced by permission of Phototake.) U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Pheromone

hydrogen atoms are concentrated in a solution. The lowercase p means its “power,” or its logarithmic value. This means that each time a number is raised to another power (from a 2 to a 3), it increases by a factor of 10. Another explanation for pH is that it stands for “potential of hydrogen.” Either way, it is known that the pH symbol was first used by the Danish chemist, Soren Sorenson (1868–1939), in 1909. He used the pH symbol on what he called a Sorenson scale. Today, however, it is called a pH scale, and it is a 0 to 14 scale that tells us exactly how acidic a substance is. This scale uses as a reference point the number 7 which is the midpoint between the scale’s two extremes of 0 and 14. A pH of 7 is considered to be neutral—or neither acid nor its opposite, base. Acids and bases are two types or classes of biological compounds. They affect every living cell as well as the habitats of organisms. The pH of a solution can be measured with an electronic pH meter or by various paper or liquid indicators. These change color depending on the pH of the mixture. A pH meter will give a digital readout, or number, indicating the pH of a solution. A treated paper indicator turns darker pink for more acid and darker blue for more base. The paper color is checked against a standard chart that indicates the pH number. The scale itself tells the exact degree of acid in a solution. Starting with the lowest number, the strongest acid, a pH of 0, would be concentrated nitric acid. Following that, in approximate values, stomach acid has a pH of 1, lemon juice 2, vinegar 3, fresh tomatoes 4, black coffee 5, and peas 6. Distilled water is neutral and has a pH of 7. After this, the base part of the scale begins. Baking soda has a pH of 8, borax 9, ammonia 10, lime 12, oven cleaner 13, and lye 14. Since these values are logarithmic, the difference between each one is a factor of 10. Thus a solution of pH 5 is 10 times more acidic than a solution of pH 6. In the living world, almost all biological processes take place in a pH environment between 6 and 8. There are, however, a few exceptions such as digestive acids that are extremely powerful (with a pH of 1). Many organisms have built-in regulators that act as buffers and either soak up or join with small amounts of excess acid or base. [See also Acid and Base]

Pheromone Pheromones are chemicals released by an animal that have some sort of effect on another animal. They are used to communicate or pass a signal to another animal. They will provoke either an immediate response or a 446


more generalized and longer-lasting one. Both types are intended to affect or to modify the behavior of another animal.

Pheromone

Unlike hormones, which are described as chemical messengers inside the body of an animal, pheromones are part of the exocrine system. This system releases chemical signals outside the body. Hormones are used internally and are one way an organism’s body systems communicate and cooperate. Pheromones are exactly the opposite. Although they are chemical signals, they are used only outside of the body and are intended to communicate something to another animal. While hormones have as their object certain “target cells” inside the body that they seek out for a response, pheromones have other organisms as their target. In terms of the amount of energy an organism uses by communicating this way, a chemical communication system is highly efficient. The animal usually uses substances that it already produces as waste or debris. Like hormones, pheromones are powerful and are highly effective in small amounts. Another reason that animals use pheromones or chemical signals is that these convey stable and simple signals. The message they contain is easily understood and often remains around for quite some time. Chemicals can also carry information in the dark. Pheromones are usually intended only for other animals of the same species. They are often released by animals into their environment in the form of urine or sweat. They may also be passed from one animal to another by glands in the skin. Pheromones are classified in two major categories: primers or signalers (also called releasers).

PRIMING PHEROMONES Priming pheromones are not used often. These cause a long-term response in the body of another animal that later influences how it behaves. An example of this is a female moth who releases an airborne chemical mixture. The mixture causes male moths of her species who are downwind to fly toward her (sometimes from as far away as 4 miles [6.44 meters]). As a result, the male moths change their behavior and keep flying upwind until they reach the female.

SIGNALING PHEROMONES The more common signaling pheromones result in an immediate response, such as fear or aggression. An animal leaving its scent markers on its territory is an example of the use of signaling pheromones. Aside from these broad categories of pheromones, pheromones can also be grouped according to the type of behavior they provoke and by the role they play for a certain species. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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BEHAVIORAL PHEROMONES Pheromones can stimulate or deter (stop or prevent) a certain behavioral response such as egg-laying. Pheromones can also attract an animal to seek out the source of the chemical as some males do when they are get the scent of a female who is ready to mate. Other pheromones can repel or warn animals away from the source (as a territorial urine marker would do). Pheromones can serve as sexual or courtship chemicals to attract mates. Aggregation pheromones serve to attract other members of the species to the same spot, as when ants lay down a chemical trail for other ants to guide them to a food source.

An experiment where pheromones are placed in a pouch to attract insects. Pheromones are usually used to communicate with another animal of the same species. (Reproduced by permission of Photo Researchers, Inc.)

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Animals that are social and live in groups may release pheromones for defensive purposes in order to alert the band to some danger. Territorial, or marking, pheromones serve to communicate the boundaries of an animal’s territory to others and to warn others to keep out. Finally, pheromones are an important means of regulating the behavior of individuals in a group. Studies have shown that when the odor of a male mouse is introduced among a group of female mice, the female’s reproductive cycles become synchronized. It also is thought that humans release pheromones, but that they are mostly unaware of them. [See also Excretory System]


Photosynthesis

Photosynthesis

Photosynthesis is the process by which plants use light energy to make food from simple chemicals. Photosynthesis is vital to all life on Earth since all food comes from this process, either directly or indirectly. People not only eat green plants and their fruit and grains, but they also eat the animals that feed on the green plants. The word photosynthesis means “putting together with light,” and perfectly describes a process by which a plant converts carbon dioxide and water into food by using light. The miracle of photosynthesis is that it captures light energy and converts it into chemical energy that can be used by organisms. Photosynthesis occurs inside the leaf of a plant at the cellular level. Plant cells contain chloroplasts. These chloroplasts contain a green pigment called chlorophyll. The flat leaf, acting as a solar collector, allows the light to strike the chlorophyll, which is stimulated to absorb it. In the chloroplasts, the light reacts with carbon dioxide (that the plant breathes in through microscopic holes in its leaves called stomata) and with water (that the plant takes in through its roots). During a series of complicated reactions, the water molecules are broken down into hydrogen and oxygen, and the hydrogen combines with carbon dioxide to produce glucose, a simple sugar that is used as a building block for starch and other complex carbohydrates. The excess oxygen is later released through the stomata into the atmosphere. The plants use the glucose as food. What they do not use they convert to starch for storage and to build cells walls.

A chart showing the stages of photosynthesis and how each stage is related. (Reproduced by permission of McGraw-Hill, Inc.)

If all life on Earth depends on photosynthesis, then all life really begins with what makes photosynthesis work—light. Sunlight is the energy that travels from the Sun. It arrives on Earth in waves of different lengths, and those lengths give it U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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JAN INGENHOUSZ Dutch plant physiologist (a person who studies how an organism and its body parts work or function normally) Jan Ingenhousz (1730–1799) discovered photosynthesis and plant respiration. He demonstrated that plants use sunlight and carbon dioxide (atmospheric gas) to make their own food, and that they give off oxygen as a by-product. He was the first to indicate the close connection between animals and plants and to show how much animals depended on green plants. Jan Ingenhousz was born in Breda, The Netherlands, where he received his basic education. He then attended the University of Louvain and later the University of Leyden. After receiving his medical degree, he worked at a private practice in Breda, but left for England when his father died in 1765. While at work in a hospital there, he became an expert in the new technique of smallpox inoculation. This was a hazardous occupation since he administered a live virus instead of today’s weakened vaccine. However, his treatments were so effective that he was called to Vienna, Austria, to inoculate the royal family. He was then appointed court physician and given a lifetime income. This allowed him to pursue his research, and in 1779, after seven years in Vienna, he returned to England where he would remain for the remainder of his life. It was also that year that he began experiments that led to his discovery of photosynthesis, the process by which plants convert sunlight into food. Ingenhousz found that green plants take in carbon dioxide and give off

its different colors. As a combination of different wavelengths, sunlight is really a mixture of violet, blue, green, yellow, orange, and red light. These wavelengths can be observed by passing sunlight through a prism. Chlorophyll is extremely efficient and absorbs red, orange, and blue light, allowing only green light to pass through. This is what makes a leaf look green. When chlorophyll absorbs the Sun’s light, the first part of photosynthesis begins as light energy splits up water molecules (hydrogen and oxygen). This process is called photolysis and produces energy-carrying molecules called adenosine triphosphate (ATP). In the second part of photosynthesis, energy from ATP and other energy carriers remove oxygen from carbon dioxide, allowing the carbon and hydrogen to combine and form glucose. It has taken scientists hundreds of years to understand what happens during photosynthesis. Beginning in the early seventeenth century, the work of Flemish physician Jan Baptist van Helmont (1577–1644), and 450


oxygen, but they do this only in the presence of sunlight. In the dark, he found that the opposite happens, and like animals, they absorb oxygen and give off carbon dioxide. This is called respiration. This was the first recognition that sunlight played a key role in the life of plants. Ingenhousz also proved that only the actual, visible light and not the Sun’s heat, was necessary for photosynthesis to work. Others had been experimenting with air at this time, and the work of English chemist, Joseph Priestley (1733–1804), showed that a candle flame burning in a closed container eventually would go out. He also found that small animals placed in a similar space eventually died since all the oxygen was consumed and only carbon dioxide was left. Ingenhousz realized that since plants give off oxygen, which is essential to animal life, and then took in the carbon dioxide that animals breathed out as a waste product, there was a fundamental connection between plants and animals that no one before had realized. To Ingenhousz, photosynthesis meant that animals and plants are totally dependent on one another. We now know that photosynthesis is the key to all life on Earth since it provides food, either directly (for plants) or indirectly (for animals that eat the plants or eat other animals that have eaten plants) for virtually every living thing. For Ingenhousz, plants “purified” the air and “revitalized” it. His 1779 book detailed his plant discoveries and laid the foundation for the continued study of photosynthesis. Ingenhousz also broke new ground in physics and chemistry. For example, he improved phosphorous matches, invented a hydrogen-fueled lighter, and mixed an explosive propellant for firing pistols.

Photosynthesis

later the English botanist (a person specializing in the study of plants) Stephen Hales (1677–1761), demonstrated that plants needed air and water to grow. In the eighteenth century, chemists began to identify individual gases, and in 1779, the Dutch physician, Jan Ingenhousz (1730– 1799), showed that plants take in carbon dioxide and release oxygen when light shines on them. By the 1880s, the German physiologist (a person specializing the study of the processes of living things), Theodor Wilhelm Engelmann (1843–1909), showed that the light reactions that capture solar energy and convert it into chemical energy occur in chloroplasts. It was not until the twentieth century, however, that scientists began to fully understand the complex biochemistry of photosynthesis. Photosynthesis is a key part of a cycle that not only maintains life on Earth but keeps Earth’s levels of carbon dioxide and oxygen in balance. Plants convert carbon dioxide into food and oxygen, which animals “burn” in a process called respiration (combining food with oxygen to release U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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energy). Respiration is therefore the opposite or reverse of photosynthesis. In respiration, oxygen is used up and carbon dioxide and water are given off (which plants use to start photosynthesis again). [See also Carbon Dioxide; Carbohydrates; Chloroplast; Light; Plant Anatomy; Plant Hormones; Plants]

Phototropism Phototropism is plants’ response to the direction and amount of light they receive. The seedling at the left received light on only one side, while the plant in the center received no light, and the plant on the right was grown in normal, all-around light. (Reproduced by permission of Photo Researchers, Inc. Photograph by Nigel Cattlin.)

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Phototropism is the term used to describe a plant’s response to light. When we notice that a potted plant on a windowsill has turned its leaves toward the light, we are witnessing phototropism. This is but one form of tropism or plant “behavior.” A tropism is a phenomenon in which a plant grows in response to some outside stimulus. Although plants cannot move in the manner of other organisms, plants are living things and, therefore, are sensitive to external stimuli. Their reactions to the many different outside forces they meet sometimes give the impression that they have indeed moved. Although plants usually appear motionless unless they are moved by the wind, they are in fact growing


much of the time and responding to a variety of environmental stimuli. When a plant’s reactions to a stimulus result directly in any type of plant growth, botanists call this phenomenon a tropism. Tropisms can be positive or negative. A positive tropism means that the plant begins to grow toward the outside stimulus. Negative tropism means that it grows away from the source.

Phototropism

When a plant responds to a light source by growing in the direction of that source, it is called positive phototropism. (Photo means light in Latin.) There are many other forms of tropisms, but all must involve plant growth as a response. Chemotropism is a plant’s response to chemicals; thigmotropism is its response to being touched; geotropism is its response to the force of gravity. Since all tropisms can be positive or negative, the growth of a seed’s roots downward into the soil is positive geotropism (in the direction of the source) but also negative phototropism (away from the light). The upward growth of the new shoot is the reverse (positive phototropism and negative geotropism). Plants have evolved tropism in order to maximize a particular function and therefore be better able to compete, survive, and reproduce. When a plant grows toward the light (positive phototropism), it can grow more rapidly and if necessary, out-compete its neighbor for scarce resources. An obvious example of tropism is hydrotropism in which a plant during a drought will make its root system grow away from its natural, gravity-pulled downward course and off in a direction containing lifesustaining water. Tropisms are one means that plants have to battle for their survival. The actual mechanism by which a tropism (stimulus/growth) occurs could be described as uneven growth. Specifically, when a stem or root moves toward or away from an outside stimulus, it must grow in a curve. It achieves this “curved” growth by having the outside of the curve grow faster than the inside. This is caused by the plant hormone called auxin. According to what type of specialized tissue is receiving the stimulus (such as a root or a stem), a larger amount of auxin moves from the growing tip and down one side than moves down the other. Since auxins come from the tip, when a plant wants to move toward the light it sends an unequal amount of auxin down its sides. More auxin goes to the shaded side and less to the sunny side, meaning that the shaded side grows more than the sunny side and the plant therefore grows in a curve toward the light source. [See also Light; Plants] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Phylum The term phylum is one of the seven major classification groups that biologists use to identify and categorize living things. These seven groups are hierarchical or range in order of size. Phylum is the second largest and is located between kingdom and class. The classification scheme for all living things is: kingdom, phylum, class, order, family, genus, and species. The category phylum is fairly broad, and members of the same phylum can be very different and have only basic similarities. Organisms in the same phylum, however, are presumed to have a common evolutionary ancestry. To determine an organism’s place in a particular phylum, biologists study it to find similarities and differences between it and other organisms within the kingdom. An example within the animal kingdom is Chordata, which contains all vertebrates (animals with a backbone). Fish, amphibians, reptiles, birds, and mammals all belong to the phylum Chordata. However, invertebrates (no backbone) of the animal kingdom, like snails, clams, and octopus, belong to the phylum Mollusca. Others, like insects and crabs, belong to the phylum Arthropoda. The animal kingdom can be divided into twenty or more phyla. Within the plant kingdom, the term “division” is used instead of phylum. Examples of some divisions include Coniferophyta (cone-bearing plants such as pine trees) and Anthophyta (plants that have flowers able to develop into seeds). Altogether, there are ten divisions (or phyla) in the plant kingdom. [See also Class; Classification; Family; Genus; Kingdom; Order; Species]

Physiology Physiology is the study of how an organism and its body parts work or function normally. It is closely related to anatomy, which studies an organism’s structure. There are different types of physiology, such as human physiology, plant physiology, and comparative physiology. As a branch of biology that studies exactly how the different processes in living things work, physiology seeks to answer a more difficult question than its counterpart anatomy. Where anatomy wants to know how things are built, shaped, and how they fit together, physiology wants to know how something functions. For example, how does human lung tissue work? How can a seal survive underwater for ten minutes without 454


taking a breath? How do camels survive so long without water? These and many other questions have been asked by people ever since they started to wonder about the natural world. In order to answer them, physiology must examine an organism’s functions at several different structural levels.

Physiology

At the simplest or chemical level, physiology studies atoms and molecules. The cellular level is next and focuses on cells, which are the smallest units of a living thing. In complex organisms like humans, groups of similar cells form tissues, resulting in the tissue level. An organ is composed of two or more tissue types that perform a certain function, and it is at the organ level that really complex functions take place. Organs that work together to accomplish a common purpose form an organ system. Together, all the organ systems make up the living body or the complete organism. This is the highest structural level, and each of the levels has its own physiology (functions). It is difficult if not impossible to study the body’s functions without some knowledge of how it is built and organized (anatomy). Studying physiology without an understanding of anatomy would be like trying to understand how a car’s engine works without having any idea what an engine looks like.

HISTORY OF PHYSIOLOGY Physiology is thought to have first developed in the Greek Hippocratic school of medicine (before 350 B.C.). Around the same time, the Greek philosopher Aristotle (384–322 B.C.) stated that every part of the body is made for a purpose, and, therefore, that the function (physiology) of something can be figured out by learning its structure (anatomy). The publication that marks the beginning of modern physiology, On the Movement of the Heart and Blood in Animals, was published by the English physician, William Harvey (1578–1657), in 1628. For nearly the next three centuries, physiology was closely associated with anatomy, and it was not until the nineteenth century that it became recognized as a separate discipline. By the middle of that century, discoveries suggesting the unity of all life (as well as the functions of all living things) led to the development of general physiology. This branch seeks to learn the physiological functions that are common to all living things. Comparative physiology is similar and tries to find the evolutionary connections between living things. One of the fundamental concepts of both is the notion of “homeostasis.” This is the ability of an organism to function under very different conditions. Put another way, it is the ability of an organism to maintain an internal environment that compensates or corrects for changes that take place in its external environment. It is U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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ARISTOTLE Greek philosopher (a person who studies the source and nature of human knowledge) and naturalist Aristotle (384 B.C.–322 B.C.) is considered not only the greatest philosopher who ever lived but also the father of biology. His great interest in the natural world led him to closely observe living things and to classify them according to a system. He taught the world that true knowledge could be obtained by observation and experience. Aristotle was born in Stagira, Macedonia, then a Greek colony north of Greece. His father was physician to the Macedonian king, Amyntas II. It is said that Aristotle lost both parents at an early age and was brought up by a family friend. At age seventeen, he traveled to Athens and joined Plato’s Academy. Plato (c. 427 B.C.–c. 347 B.C.) was one of the greatest philosophers of all time, and Aristotle became his best pupil. When Plato died, Aristotle began a journey to various parts of the Greek world and was able to pursue his long-held interest in the natural world. The study of animals was his first love, and he began to systematically learn as much as could about them. However, in 342 B.C. he was called to Macedon by the new king, Philip II, the son of Amyntas II, to become tutor to his own young son named Alexander II (later called Alexander the Great). Aristotle schooled the young man for about six years until Alexander became king when Philip II was assassinated. When the new king began his own conquering campaigns, he no longer needed a tutor, and Aristotle returned to Athens. There he founded his own school he called the Lyceum, where he would do his research and give lectures. When Alexander II died in 323 B.C., however, Aristotle moved to his mother’s hometown called Chalcis, where he died the next year.

important to the survival of any organism that it be able to adapt quickly to its environment while at the same time keeping its internal functions and systems in a steady or balanced state. Control is another concept that is very important to physiologists. It is related to homeostasis and might be described as the way in which an organism’s different systems communicate with and influence each other.

HUMAN PHYSIOLOGY Human physiology is the branch that studies how the human body and its parts work or function. Among the major functional systems studied are the integumentary system (the skin), the skeletal system (the framework), the muscular system (causing movement), the nervous system (control), the endocrine system (glandular control), the circulatory system 456


Aristotle wrote on many subjects, from logic (the principles of reasoning) and ethics (the general nature of morality), to politics and biology. However, of all of his scientific writings, most consider his work in biology to be the most successful. To begin with, he was one of the first to look at animals in a scientific way. Always a careful observer, he eventually began to place all of the known animals in to some sort of order. This is called classification. After studying, observing, and even dissecting (cutting apart for anatomical study) them, Aristotle arranged more than 500 animals according to their physical similarities and tried to understand the relationships between them. Always an excellent observer, he noticed, for example, that dolphins give birth to live young, so he put them in with all of the land beasts (now known as mammals) despite the fact that they were considered fishes. He also dissected a great many animals and wrote about the complex stomach of cattle. His work on the developing embryo of the chick showed that this early phase of life was equally worthy of study. In his writings on biology, he first taught that the form of natural objects is determined by their purpose. This, he said, leads to the conclusion that the structures of organisms are determined by the functions they are supposed to serve. He also hinted at an idea of evolution (the process by which living things change over generations) because he arranged living things in a lower-to-higher order that suggested a sort of chain of progress.

Physiology

While known today for his ethical and political writings, it was Aristotle who showed the earliest “natural philosophers,” or scientists, that experience, observation, and experiment are necessary when one investigates the natural world. He regularly stressed the importance of theory being based on facts. For this and his regular observations, not to mention his love of the natural world, he is rightly called the father of biology.

(transport and delivery), the respiratory system (oxygen supply), the digestive system (breakdown and absorption of food), the urinary system (eliminating waste), and the reproductive system (to produce offspring). These highly organized and complex systems work together and contribute to what are called the necessary life functions. These are: movement (actively getting around), responsiveness (sensing and reacting to stimuli), digestion (breaking down food so it can be absorbed), metabolism (all the chemical reactions in cells), excretion (removing waste), reproduction (producing new life), and growth (increasing in size). Taken together, these are the internal processes and functions that maintain life, the study of which is physiology. While human physiology is of great value to medicine, plant physiology is most useful to agriculture and forestry. However, whether the U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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type of physiology studied is that of plants or animals, it is important to know how a living organism works normally or is supposed to function, since only then can one determine if something is wrong and what might be done to correct the problem. Without the knowledge of anatomy, physiology would not be as useful as it now. [See also Brain; Circulatory System; Digestive System; Endocrine System; Excretory System; Heart; Integumentary System; Muscular System; Nervous System; Reproductive System; Respiratory System; Skeletal System]

Piltdown Man Piltdown man is the name given to the “fossil” bones found in England that turned out to be the greatest hoax in the history of science. When discovered in 1912, these remains were claimed to provide evidence of the missing link between apes and humans. It was not until the 1950s, however, that scientists were able to prove that Piltdown man was a complete fake. Around 1900, science knew that Neanderthal man was an extinct form of Homo sapiens who was similar to modern humans. Many scientists then believed that, according to evolutionary theory (the belief that all living things change over generations), since man evolved from apes, there must be some link or in-between stage that came between this Neanderthal and the apes themselves. Scientists, therefore, assumed that the next great discovery would be this “missing link.” Although most paleoanthropologists (scientists who study fossils to try to discover how humans evolved) thought that if this link were found it would be in Africa or Asia, in 1912 it was suddenly found at a dig on Piltdown Common in Sussex, England. An amateur archaeologist (one who studies the material remains of past cultures) named Charles Dawson supposedly stumbled upon nine fossilized pieces of a skull, as well as a jawbone and molars. When he put them together, it appeared that he had discovered actual evidence of the “missing link” between apes and humans. What Dawson’s discovery showed was a complete skull that was literally half man and half ape. Its upper skull was definitely human, since it had the high brow typical of intelligent humans. Its lower part was surely that of an ape since it had both a protruding jaw (jutting out) and a receding chin. Besides the bones themselves, Dawson found crude flint (a substance used to make fire) and bone tools along with the bones of other, long-extinct animals. Piltdown man was soon hailed as evidence 458


of evolution’s missing link, and this new fossil, dated at about 200,000 years old, was given the scientific name Eoanthropus dawsoni, meaning “dawn man of Dawson.” During the next thirty-five years, many hominid (human-like species) fossils were found in other parts of the world, but none ever came close to matching the features that Piltdown man displayed. This gave many a scientist a reason for doubting the find, and in 1948, testing began on Piltdown man that used new dating techniques. When preliminary results suggested that the bones were of very recent origin, they were tested again each time a new dating method was invented. By the time the new and highly reliable carbon-14 method was used in 1959 to confirm those conclusions, it was apparent to all that Piltdown man was a deliberate forgery. The jaw belonged to an orangutan that probably was killed in the Middle Ages (500–1450), and the cranium was human, but only slightly older than the jaw. Someone also had deliberately filed the molar teeth to make them look old and used, and someone had purposely stained the fragments. Eventually, no one could dispute the fact that the entire discovery had been planted, and that one or more persons had decided to make their own “missing link.” Since then, the strangest and most unexplainable piece of paleontology has been resolved, and Piltdown man is now regarded only as a hoax that fooled people for forty years. Conclusive proof


Piltdown Man

Four busts of prehistoric man’s evolution (from left to right): Pithecanthropus erectus, “Piltdown” man (which was a hoax put over on the scientific community; its fossils turned out to be modern-day human skull combined with an orangutan jawbone), Neanderthal man, Cro-Magnon man, and modern man. (Reproduced by Corbis-Bettmann.)

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of who planned and carried out the hoax was never obtained, and despite several books about Piltdown man, no one has ever been able to absolutely link one or more persons to this deception. The Piltdown man was beneficial to science in two ways, however. First was the fact that new methods of dating were demonstrated and proven in the field. Second, it forced scientists to become more rigorous and demanding when confronted with sudden, new discoveries. [See also Evolution; Evolution, Evidence of; Evolutionary Theory; Human Evolution]

Plant Anatomy Plant anatomy is the study of the shape, structure, and size of plants. As a part of botany (the study of plants), plant anatomy focuses on the structural or body parts and systems that make up a plant. A typical plant body consists of three major vegetative organs: the root, the stem, and the leaf, as well as a set of reproductive parts that include flowers, fruits, and seeds. As a living thing, all of a plant’s parts are made up of cells. Although plant cells have a flexible membrane like animal cells, a plant cell also has a strong wall made of cellulose that gives it a rigid shape. Unlike animal cells, plant cells also have chloroplasts that capture the Sun’s light energy and convert it into food for itself. Like any complex living thing, a plant organizes a group of specialized cells into what are called tissues that perform a specific function. For example, plants therefore have epidermal tissue that forms a protective layer on its surface. They also have parenchyma tissue usually used to store energy. The “veins” or pipeline of a plant are made up of vascular tissue that distribute water, minerals, and nutrients throughout the plant. Combined tissues form organs that play an even more complex role.

THE ROOTS A plant’s roots, like the foundation of a skyscraper, help it to stay upright. They also absorb water and dissolved minerals from the ground and give the plant what it needs to make its own food. Most roots grow underground and move downward because of the influence of gravity, although the roots of some water plants float. Other root systems, like that of the English ivy, actually attach themselves to a vertical surface and allow the plant to climb. There are two main types of root systems: taproot 460


and fibrous. Plants that have taproots grow a single, long root that penetrates straight down and firmly anchors the plant. Trees and dandelions have taproots that serve this function. Fibrous roots are shorter and more shallow and form a branching network. Grass has a fibrous root system that grows at a shallow level and in all directions. Inside a root are pipelines or veins that carry water and minerals to the rest of the plant. These pipes are concentrated in the center of the root, like the lead in the center of a pencil. At the end of each root is a cap that protects it as it pushes farther into the soil. Extending from the sides of the root, but further back from the root cap are root hairs. These hairs are the main water and oxygen absorbing parts of a plant. Materials enter and leave roots by two main processes: diffusion and osmosis. When molecules are distributed unequally, nature always seeks a balance and molecules will move from an area of high concentration to one of low concentration. When the cells of a root hair have little oxygen and the soil around the root hair has a lot, oxygen will move from the soil to the root automatically without the plant having to expend any energy. Osmosis is a similar situation (from high to low concentration), but it occurs when molecules, like those of water, move across a membrane that will not allow other materials to pass. Like diffusion, osmosis does not require the plant to use any energy.

Plant Anatomy

THE STEMS Plant stems perform two functions. They support the parts of the plant aboveground (usually the buds, leaves, and flowers), and they carry water and food from place to place within the plant itself. A stem is made up of an outer layer, the epidermis; an inner layer, the cortex; and a central zone called the pith. The stem of a green plant holds itself up by having thousands of cells lined up next to and on top of each other. As the cells take in water, they expand like a full balloon, and since their walls are elastic, they stretch very tight against each other and against the stem wall. It is their pressure that holds the stem up. A plant droops when its cells lack water and have begun to shrink. Woody plants, like trees, also contain a material called lignin that strengthen cell walls and make them more rigid. A plant’s stem also functions as its circulatory system and uses what is called vascular tissue to form long tubes through which materials move from the roots to the leaves and from the leaves to the roots.

THE LEAVES The leaf of a green plant manufactures food for plant growth and repair. A leaf is a highly specialized part of a plant since it is the place U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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STEPHEN HALES English botanist (a person who studies plants) and physiologist (a person who studies how the many different processes going on inside a living thing actually work) Stephen Hales (1677–1761) is considered the founder of plant physiology. A pioneer in the study of blood circulation and blood pressure measurement, Hales applied the physics of his time to the problems of biology. In all of his experiments on plants and animals, he regularly emphasized the need for careful measurement of data. Hales was born in Kent, England, and little is known of his life before he entered Cambridge University in 1696. There he studied science and religion, and in 1703 he was ordained in the church as a deacon (a clergyman just below a priest). In 1709 he became a clergyman at Teddington where he would remain for the rest of his life. At this time it was not unusual for a clergyman to also be a man of science, and Hales was able to do both well. It was at Teddington that Hales began to use some of the broad scientific education he had received and, in the spirit of English physicist and mathematician Isaac Newton (1642–1727), he tried to take what he knew of physics (the study of matter and energy) and apply it to biology. Thus, in 1719, Hales began his first experiments on plants. Before this, he had done quite a bit of experimenting on animals and had achieved the first blood pressure measurements using a glass tube device of his own design. He also investigated the reflex actions in a frog whose head he had cut off, but after a while, Hales became in his own words, “discouraged by the disagreeableness of anatomical dissections.” He therefore switched to plants

where photosynthesis takes place. In photosynthesis, the chlorophyll (green pigment) in the leaf absorbs energy from the Sun, combines it with water and minerals from the soil and carbon dioxide from the air, and produces the plant’s food. Everything about a leaf is designed to intercept or capture sunlight. For example, a leaf is a flat structure with a large surface area and consists of a thin, flat blade called the lamina. The lamina is attached to a stalk called the petiole. The petiole is the leaf’s main supporting rib and often branches into a network of veins. Leaves with only one blade are called simple, and those with two or more blades are called compound. Compound leaves often look like several small leaves attached to the same stalk. Leaves also grow in patterns to assure that they do not shade each other, and some plants have alternate leaves while others have leaves opposite each other. Leaves can control the amount of water they lose by opening or closing tiny slits called stomata (singular, stoma). 462


and carried over his blood-related experiments on animals to the study of the movement of sap in plants. Soon he was able to measure the force of a plant’s sap flow just as he had measured blood pressure in animals. In his book, Vegetable Staticks, published in 1727, Hales described many of his discoveries concerning plant physiology. Hales detailed what he had learned concerning plant anatomy and what a plant does in order to survive and grow. He stated that plants take in part of the air and use it for food, that they need light for growth, and that they lose water mainly through their leaves. He showed that sap is under considerable pressure and that water flows in a plant in one direction only. He even calculated the actual velocity (its speed) of the sap and discovered that it differs according to the type of plant. As he did in his animal experiments, he investigated the role of water and air in an organism and explored all aspects of its growth.

Plant Anatomy

Hales also had a very practical and even humanitarian side, and he was a pioneer in the field of public health. He used his knowledge of air and respiration to devise ventilators to remove “spent,” or bad air (probably carbon dioxide), from closed spaces in hospitals, prisons, and merchant ships. He worked on ways to distill fresh water from seawater, and worked at water purification and food preservation. He even adapted a gauge from his plant experiments to measure the ocean depths. Besides all of the specific botanical knowledge and understanding he offered in his book on plant physiology, Hales’ application of physics to biology and his emphasis on quantitative (measurable) experimentation provided an important model for those who were to follow.

FLOWERS AND SEEDS The reproductive part of a seed-producing plant is called the flower. Flowers have male and female cells that produce a seed when they unite. The stamen is the male reproductive organ in a flower and contains the male cells (pollen) in its anther that grows at the tip of its long, narrow stalk. The pistil is the female reproductive organ and looks like a longnecked bottle. It has a round base containing the ovary, a slender tube or long neck called the style, and a flattened, sticky top called the stigma. Once a flower opens, its petals (which are a type of leaf) protect the sex organs and serve to help pollination (the transfer of pollen to the female parts) by attracting animals like bees and birds. When this happens, fertilization occurs and the ovaries become seeds. Seeds have three main parts: the coat, the embryo, and the food storage tissue. The coat protects the embryo, which is the beginning of a plant U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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and grows by using food stored in the seed. Most seeds are enclosed in fruit that can be dry like a ripe bean pod, or fleshy like an apple or a peach. Other plants, like fir trees, have naked or uncovered seeds that form on the upper side of the scales that make up a pine cone. All are designed to be scattered as far as possible from the parent plant to ensure the further survival of the species. [See also Botany; Photosyntheis; Plant Hormones; Plant Pathology; Plant Reproduction; Plants]

Plant Hormones Plant hormones are naturally occurring chemicals that influence plant development and growth. Often called plant growth regulators to distinguish them from animal hormones, they are similar to animal hormones in that they function as chemical messengers. There are three major groups of plant hormones, as well as two other hormones that do not fit in any group. Plant hormones were developed by plants as one way of assuring their survival. Since a plant cannot move away from a threatening situation, it is important that it have an internal messenger system that ensures that the entire plant is able to react in a proper way to its environment. This role is filled by the messenger plant hormones.

HORMONE GROUPS Besides influencing growth rate, plant hormones also control a plant’s response to its environment. Like their animal counterparts, plant hormones are effective in small amounts and tend to be made in one place within the plant and transported somewhere else where they do their work. Unlike animal hormones, however, they are not produced by special glands nor do they only work on specialized target cells. Instead, plant hormones can have an effect on any part of the plant that produces them. Plant hormones were discovered in 1926 by the Dutch botanist (a person specializing in the study of plants), Frits W. Went (1903–1990), who isolated the first plant hormone, which he called “auxin.” He chose this name from the Latin word meaning “to increase” since that word describes its result. Now known to be indoleacetic acid (IAA), this hormone is transported to the roots of young plants where it stimulates growth. Besides IAA, other growth-stimulating auxins besides IAA have been identified, and auxins are now considered to be one of the three major hormone groups. 464


In addition to auxins, there are two other major hormone groups: cytokinins and gibberellins. Cytokinins are an important group of plant hormones since they stimulate cell division and delay aging in older tissues. Cytokinins are thought to be produced in the tips of roots from where they travel upwards through the plant and stimulate branching rather than the lengthy growth promoted by auxins. Gibberellins are a chemically complex family of plant hormones that stimulate the growth of shoots (that part of a beginning plant that first pops out of a seed and reaches for the light). Gibberelins are important for plant embryos and seedlings, and stimulate the beginnings of root growth.

Plant Hormones

ABSCISIC ACID AND ETHYLENE The other plant hormones that do not fall under any of the major three groups are abscisic acid and ethylene. While most plant hormones usually involve stimulating growth in one part or another, the hormone abscisic acid is actually an inhibitor since it turns off growth or development when conditions are not right for it. Sometimes certain environmental conditions, such as a drought, make water conservation a necessity. For a plant to survive in these conditions, it must slow down or stop its growth with the release of abscisic acid. Abscisic acid got its name because it was believed to play a key role in “abscission” or the seasonal loss of leaves. It is now know, however, that other hormones are more important in causing a tree to drop its leaves in autumn. Even though abscisic acid is not solely responsible for trees losing their leaves, it is extremely important given that the total lack of this hormone results in the inability of a plant’s embryos to stop growing inside their seeds. Without abscisic acid premature eruption of a shoot through the seed coat may occur at a time and place where the seedling may not be able to grow. The final and possibly best-known plant hormone, ethylene, plays a major role in the ripening of fruit. Ethylene is an unusual plant hormone in that it is released outside the plant and into the atmosphere. One plant is therefore able to influence its neighbors. It is ethylene gas that explains the old saying, “One bad apple spoils the whole bunch.” Since apples continue to release ethylene after they are picked, any wound in its skin will stimulate extra production of ethylene gas, which in turn speeds up the ripening or aging of any apples nearby.

COMMERCIAL USES FOR PLANT HORMONES Greater understanding of plant hormones has led to their increased commercial use. Farmers and gardeners are now able to use certain hormones regularly to achieve desired effects. For example, a well-known U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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use of ethylene is by farmers who use ethylene gas to ripen tomatoes that are mistakenly harvested while they are still green. Plant hormones have also been used for military purposes. An example is IAA, which if used in very high doses, can have the opposite effect of not only slowing down growth, but it also may prove poisonous to plants. This was the case when the United States used one of these auxin-related compounds during the Vietnam War (1954–75). Called “Agent Orange,” this chemical spray caused a plant’s leaves to dry and fall off, thus supposedly denying the military enemy any hiding places in the jungle. Unfortunately, a chemical by-product of Agent Orange is dioxin, a cancer-causing agent. [See also Botany; Plant Anatomy; Plant Pathology; Plant Reproduction; Plants]

Plant Pathology Plant pathology is the study of diseases, injuries, or other factors that affect the welfare of plants. It is mostly an applied science, meaning that it is studied with a specific, practical purpose in mind, usually of a commercial or economic nature. Also called phytopathology, it is studied by plant pathologists who try to understand and control the many factors that may affect a plant’s health and productivity. All species of plants are subject to disease that may be caused by infectious agents, poor environmental conditions, or the effects of parasites or predators. Unlike animals, there is sometimes no clear distinction between a healthy and an unhealthy plant, and plant pathologists generally describe a plant as diseased when it is regularly disturbed or badly affected by something outside itself. The actual cause might be a living, disease-carrying organism (called a pathogen) or unfavorable environmental conditions. If either factor results in a plant’s biochemical or physiological (functions) systems being disturbed, the plant is considered diseased. As with animals, a plant’s disease often shows itself in what are called symptoms, and it is therefore important to know how a certain plant looks and behaves in its normal, healthy state in order to recognize any symptoms of disease. Thus, plant pathologists must have knowledge of a plant’s normal growth habits as well as the range of variability in the species—or what differences are normal.

CAUSES OF PLANT DISEASE Plant pathologists divide plant diseases into categories. Infectious diseases include those caused by transmissible (capable of being spread) bi466


ological agents or pathogens such as bacteria, fungi, or viruses. Noninfectious disease (usually physiological or functional disorders in which something goes wrong with the way its major systems work) can be caused by environmental factors like nutrient deficiency, excess of minerals, a lack or an excess of moisture, soil temperature that is too high or low, a lack or an excess of light, a lack of oxygen, extreme soil acidity or alkalinity, and air pollution. Finally, noninfectious biological agents that harm plants by either eating them or living off them, called parasites, include such organisms as arthropods (like a mite, spider, or centipede), nematodes (roundworms), and other parasitic plants (like mistletoe that can only live off another plant). The most common causes of disease among plants grown commercially are pathogens (like bacteria), pests (like mites), and bad weather.

Plant Pathology

In treating plant diseases, the first thing a plant pathologist must do is to determine whether the diseases are caused by a pathogen or by a noninfectious factor in its environment. In many cases, a diseased plant shows obvious symptoms such as an unusual color change in its leaves or a loss of flowers, that immediately indicate its cause. A good example of disease symptoms are the bands of white on the foliage of pine trees caused by too much ozone in the air. Other times, identical symptoms can have very different causes.

A diseased phlox leaf covered by a powdery mildew. (Reproduced by Field Mark Publications. Photograph by Robert J. Huffman.) U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Viruses. Among diseases classified as infectious diseases are those caused by a virus. A virus is an extremely small organism that cannot live or reproduce outside its host. A virus usually spreads easily and can severely damage or kill a plant. Viruses can be transmitted by insects feeding off a diseased plant or by a gardener’s shears. Viral infections in plants are often called mosaics because of the blotchy appearance they cause. Viruses are extremely difficult to remove and it is often best to try to prevent infection from the start. Bacteria. Bacteria (a group of one-celled organisms so small they can only be seen with a microscope) are essential to the life cycle on Earth and most are beneficial, but some attack and even destroy plants. Bacteria enter a plant through wounds or natural openings and can be spread by the wind. Like viral infections, bacterial diseases are often difficult to control. Bacteria cause a plant to wilt and even rot. For example, fire blight is a type of bacterial disease that makes an apple tree appear scorched. Fungi. The greatest numbers of infectious plant diseases are caused by fungi (a group of many-celled organisms that live by absorbing food and are neither plant nor animal) that enter a plant through any openings, as do bacteria. Dutch elm disease is one of the best known fungal diseases. It killed millions of elms in the United States. Fungi cause diseases that are described by words like rust, smut, and mildew. Parasites. Parasites also can be harmful to plants. For example, nematodes are parasitic worms too small to be seen with the naked eye that suck vital juices from a plant, causing major losses of fruit and vegetables. Other plants can be parasites as well. The mistletoe is a good example of a plant that invades the tissues of another and steals its nutrients. Other plants like strangleweed and witchweed stunt or kill their hosts by robbing them of water and food.

CONTROLLING PLANT DISEASES Control of plant disease is usually a combination of several strategies, but it always begins by using healthy seeds. To combat actual pests, pathologists might use chemicals called toxicants that kill bacteria, fungi, or parasites. Seeds and soil can be treated chemically and leaves can also be sprayed. Without the widespread use of these chemicals, the successful commercial production of fruits and vegetables would probably not be possible. However, other less dangerous methods can be used, since strong chemicals can harm people as well as pests. Some of these strategies involve simply keeping anything infected away from other plants, 468


properly rotating crops (not growing the same thing in the same location every season), pruning and burning diseased tissue, and ensuring that a plant’s environment is the best it can be. Ideal disease control is best achieved by breeding plant varieties that can resist diseases on their own.

Plant Reproduction

[See also Botany; Plant Anatomy; Plant Reproduction; Plants; Pollution]

Plant Reproduction Plants reproduce either sexually or asexually. In sexual reproduction, two parents produce a genetically different individual. In asexual reproduction, a plant propagates (reproduces) itself and produces a genetically identical individual. Some plants reproduce both ways. Sexual reproduction in plants requires separate male and female parts whose separate sex cells come together and fuse or unite. This unification produces a seed that if cultivated under the proper conditions, will grow into a unique offspring. In flowering seed plants or angiosperms, the reproductive parts are in the flowers. For humans, flowers may be simply a source of pleasure since they add color and fragrance to the world, but for plants they are complex and hardworking organs. Flowers vary in size, shape, and color, yet all have the same common structures. A flower’s male reproductive parts, called stamens, consist of a filament or stalk and a pollen head called an anther. Its female parts, called carpels or pistils, consist of an ovary (where the seed eventually will develop), and a stalk or style at the end of which is a sticky flat top called a stigma, which will receive the pollen. The entire male part looks like an antennae with pads on their ends. The female parts resemble a long-necked, round-bottom bottle with a flat top.

POLLINATION Before a flower can produce seeds, pollination must occur. This means that pollen (which contains the plant’s male sex cells) has to somehow travel from its anthers to its stigma. While many plants have both male and female organs and can therefore easily pollinate themselves (self-pollination), this does not result in much genetic variety. Therefore, flowering plants have evolved several different methods and strategies to make sure that pollen is transferred from one plant to another (cross-pollination). Since plants cannot move on their own and accomplish this task, they require agents to do their work for them. The most common method is animal pollination in which a colorful, perfumed flower attracts an inU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Plant Reproduction An electron micrograph of the reproductive system of a flower. The bulbous carpel with the shortstalked female sigmas emerging from it is at lower center. The tip of the third stigma at the right is covered with pollen. The droplets in the bottom right and left corners are nectar to attract pollinating insects. (©Photographer, Science Source/Photo Researchers, Inc.)

sect, bird, or bat by producing a sugary liquid called nectar. In extracting the nectar from the flower, the animal picks up sticky pollen grains, which it then accidentally rubs onto another flower’s stigma as it gets more nectar. This transfer of pollen is called pollination. Besides animals, the wind is another useful way of transferring pollen from one plant to another. After a grain of pollen lands on the stigma of a suitable flower, the pollen begins to grow a thin tube that penetrates down the long neck or style of the female part and enters the embryo sac containing the ovary (female sex cells). The male cells travel down the tube and fertilize the egg or ovum. Soon a seed is formed, containing an embryo (the beginning of a new individual plant that has its own unique collection of genes), some stored food (the endosperm), and a protective coat (the testa). After a seed has developed, a plant’s next job is to distribute the seed some distance from the parent plant, mainly to prevent its own habitat from becoming overcrowded. Plants that use animals to disperse their seeds often produce fruit that contain seeds. Fruit can be soft and juicy like a peach or hard and dry like a burr. Some fruit are eaten by animals that discard the seed somewhere else or have it pass through their system undigested. Other fruit have hooks or barbs that easily catch on to the fur of passing animals. Some seeds are mechanically dispersed by fruit that explode (like impatiens), while other seeds can float and use water to move about. When a seed falls into an environment where conditions are right for it, the seed takes in water and begins to grow. This beginning growth is called germination.

VEGETATIVE PROPAGATION Plants can also produce more of their kind through asexual reproduction. Many perennial plants (plants that grow for several years) reproduce asexually by producing new parts that become entirely new plants. This form of asexual reproduction is called vegetative propaga470


tion. This method is favored by plants that live in especially harsh or severe conditions, such as those near a mountaintop, where the opportunity to attract animal pollinators is minimal or unreliable. The grass that grows on our lawns reproduces asexually by sending up new above-ground plants from its underground root system. The strawberry plant sometimes found as a weed in our lawns sends out runners along the ground that take root and produce a new plant while still attached to the parent plant. Strawberry plants can also reproduce by making seeds. Other plants can regenerate (regrow) parts from any plant parts that remain. Thus if we pull up a dandelion from our lawn but do not get the entire taproot and leave a piece underground, the plant will regrow new roots, stems, and leaves. Many plants with belowground tubers (stems), rhizomes (stems like roots), or bulbs (like tulips) can do the same and produce an entirely new plant. However, all of the plants produced by vegetative propagation are genetically identical to the parent plant. Farmers and gardeners use this technique to produce large numbers of desirable, identical plantlets. Nature, however, favors sexual over asexual reproduction since it prefers to have plants that vary genetically. Plants that are genetically identical can be susceptible to disease or a sudden climate change, and they can all be destroyed. If a species has genetic variety, though, some may be more resistant to disease or others may tolerate climate change and survive.

Plants

[See also Botany; Plant Anatomy; Plant Hormones; Plants; Reproduction, Asexual; Reproduction, Sexual]

Plants A plant is a multicelled organism that makes its own food by photosynthesis. Although plants show a variety of form, function, and activity, all belong to the kingdom Plantae and generally are characterized by being immobile, or anchored, in soil, having strong woody tissues for support, and by being green and carrying on photosynthesis. Plants are essential to life on earth, especially human life, since they are at the beginning of the food chain and take in carbon dioxide (an atmospheric gas) and give off oxygen. Plants are also a source of medicine and useful materials. Botanists (people who study plants) have identified about 500,000 species of plants, although there are many undiscovered species yet to be classified. The plant kingdom is one of the five main groupings of organisms; the four others being the monerans, protists, fungi, and animals. Although algae were long considered to be part of the plant kingdom, they are now U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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regarded as being part of either the Moneran or Protista kingdom. Plants are found in virtually all land and water habitats and can range in size from tiny mosses to giant sequoia trees more than 300 feet (10.94 kilometers) tall. Whatever their size or habitat, all plants have the following characteristics: they are multicellular at some point in their life; they are eukaryotic (their cells have nuclei); they reproduce sexually (through the union of sperm and egg); they have chloroplasts (are the energy-converting structures) for photosynthesis; they have cell walls; they develop organs; and they have life cycles.

TYPES OF PLANTS Although plants have all these things in common, scientists distinguish among the many different types of plants and classify plants as they do every other living thing. Therefore, in the kingdom Plantae, there are ten phyla or divisions of plants, each of which represents a number of classes, or more specific types. Most of these ten divisions can be grouped into five major types: seed plants, ferns, lycophytes, horsetails, and bryophytes.

Seed Plants. Seed plants are exactly what they sound like—plants that use seeds to reproduce. Plants that produce seeds that are enclosed in a protective case are called angiosperms. These include most of well-known plants like trees, wildflowers, and fruits and vegetables. Plants that produce seeds without any covering are called gymnosperms. Most gymnosperms produce their seeds in cones. Evergreens like firs and pine trees are a good example of gymnosperms. Ferns. Ferns often vary greatly in size, but almost all grow in moist areas. Only their leaves, called fronds, grow above ground. The rest of the plant spreads out in stems that grow horizontally underground.

Lycophytes, Horsetails, and Bryophytes. Lycophytes are mostly mosses and have a single, central stem. Horsetails have tiny leaves and hollow, jointed stems that are scratchy. Bryophytes grow in shady areas and are a type of moss, but they do not have any vascular tissue or tubing that carries water throughout the plant. Sphagnum or peat moss is a good example of bryophytes. PLANT ANATOMY Despite the many types and divisions of plants, most of the common plants reproduce in one of two ways, have the same basic parts, and make their food the same way. Since the seed plants make up the largest single group of plants (around 250,000 species) and is the one most famil472


iar, they will serve as a good example for plant anatomy. Nearly all these plants have three major body parts: roots, stems, and leaves.

Plants

Roots. A plant’s roots anchor it in the soil and provide the plant with what it needs to grow by absorbing water and minerals. This underground root system also places a major restriction on plants since they are unable to move about and must cope with changing conditions instead of moving away as an animal would. Some plants use their roots to store food for the aboveground part of the plant to use, such as radishes and carrots. Others, like potatoes, are examples of plants with tubers, or a swollen underground stem, in which food is stored. As a plant grows in size, its root system must expand not only to feed it more, but also to simply hold it upright. There are two main types of root systems: taproots and fibrous systems. A taproot is a large main root that grows straight down and has smaller, lateral roots growing off it. Carrots and dandelions have taproots, as do trees which sometimes send down an anchor as far as 15 feet below ground. Fibrous roots are all the same size and spread out horizontally not very far beneath the surface. Grass, wheat, and corn have fibrous roots. As roots grow, the tip of each slender branch is protected by a root cap as it pushed through the soil. Behind the cap are threadlike tubes, or root hairs, that spread out and increase a plant’s ability to absorb what it needs from the soil. Some water plants have roots that float, while other plants like orchids have roots that attach themselves to the branches of another plant or tree.

Stems. The stem is that part of a plant that supports the plant’s buds, leaves, and flowers. Although stems vary greatly in size and type, they all connect the roots to the leaves by a network of pipelines, and they also hold up the part of the plant that needs to reach for sunlight. Some stems are short and green, like those of lettuce that appear to have no stem at all, while others are woody and large like the trunk of a tree. Almost all plants grow by putting out buds from different parts of their stems. Terminal buds grow near the end or apex of a stem, while lateral buds grow where a leaf joins a stem. Buds are specialized and may grow into new branches, leaves, or flowers. Stems are made of vascular tissue that serve as the plant’s pipeline, or tubing system, that performs two functions: one system (made of vascular tissue called xylem) is used mainly for transporting material from the roots to the leaves, and the other (made of tissue called phloem) is used for moving material from the leaves to the roots or other parts of the plant. The stringy strands of celery that get caught between our teeth are a good examples of a plant’s vascular tissue. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Leaves. The leaves of a plant are where the really important and amazing work is done—making food. This food-making process in which green leaves change inorganic raw materials into organic nutrients is called photosynthesis. Photosynthesis involves a leaf’s ability to trap light energy and convert it into chemical energy. As a solar collector, a leaf consists of a thin, flat blade that is attached to a stalk called a petiole. The blade is where photosynthesis takes place. The blade, also called the lamina, is made up of two layers of cells—the epidermis on the outer surface and the mesophyll on the inside—and is strengthened by a network of veins that also transport materials to and from the blade. The underside of a leaf has microscopic openings or pores called stomata (singular, stoma) that can open and close and allow oxygen to flow out and carbon dioxide to flow in. These pores also regulate the amount of water that a plant will lose. Leaves are usually green because their cellular structures called chloroplasts contain the green pigment chlorophyll. It is chlorophyll that traps the Sun’s energy and begins a four-step biochemical process of using carbon dioxide and water to produce a sugar called glucose and to release oxygen. The plant uses some of this food as fuel for itself, some to grow and repair, and some it stores. When a primary consumer eats a plant, it obtains the original light energy that was captured by the plant. Leaves vary greatly in size and usually are arranged in definite patterns to make sure that each receives the most sunlight it can and does not shade its neighbor. Leaves also die during a process known as abscission (separation). As the days grow shorter in autumn, a layer of cells grows across the base of the petiole and stops the flow of food to it. This trapping of sugar in the leaf produces a bright red pigment called anthocyanin or a yellow pigment called carotene. In deciduous plants, all the leaves fall off at the same time, but in evergreen plants they are shed and replaced regularly, so the plant is never without leaves.

SEXUAL REPRODUCTION IN PLANTS Plants reproduce either by sexual reproduction, in which a male sperm cell unites with a female egg cell to produce a unique individual plant, or by asexual reproduction, in which the plant divides itself up to produce an identical replica. A flower is the reproductive part of many plants, and it may contain the male or female reproductive structures or even both.

Flowers. Flowers have four main parts: the calyx, the corolla, the stamens, and the pistils. The calyx protects the petals (corolla), inside which are the stamens (the male reproductive part) and pistils (the female reproductive part). The purpose of a flower is to bring about pollination, 474


which is the transfer of pollen (male sex cells) to the female parts. Insects, birds, bees, and even the wind play a role in pollination. When a pollen grain lands on a receptive pistil, fertilization takes place and an entirely new cell is formed that is the start of a seed. A seed containing an embryo of the new plant is often enclosed in something called a fruit.

Plasma Membrane

Fruit. Whether fruit are hard and dry (like a walnut) or soft and juicy (like a raspberry), it is the plant’s way of scattering its seeds as far as possible. Some fruit have burrs that cling to an animal’s fur, while some fruit are capable of floating on water, and others can resist being digested as they pass through an animal’s gut to be deposited somewhere else. By producing fruit that animals want to eat, plants are using animals to distribute their seeds and to make sure that they end up in a place where they may germinate or begin to grow. When conditions are right, the seed uses the water it receives and the food it has stored to send a root, or radicle, through its seed coat and into the soil, while a shoot (containing the beginnings of a stem, buds, leaves, and flowers) grows aboveground and toward the light. This begins a new plant. ASEXUAL REPRODUCTION IN PLANTS Plants can also spread asexually, usually by sending up new aboveground plants from an existing root system. Grass and strawberries grow this way, as do tulips. Many plants with tubers, like a dandelion or potato, will regenerate into a new plant if only a piece is left in the ground.

PLANTS ARE ESSENTIAL TO LIFE Plants are basic to life on Earth. A world without plants would be a world humans could not recognize. Besides missing the beauty and pleasure that plants give, the world would be without any of the food the people and animals know and need. People would also be lacking the many medicines, shelter, and useful products that are based on plants. By capturing the energy of the Sun, plants make all other life on Earth possible. [See also Botany; Photosynthesis; Plant Anatomy; Plant Hormones; Plant Pathology; Plant Reproduction; Reproduction, Asexual; Reproduction, Sexual]

Plasma Membrane The plasma membrane is a thin, continuous sheet that separates all living cells from their environment. This membrane maintains conditions within U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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the cell and allows certain things to pass in and out of the cell. It is strong and can repair itself if damaged. Every living cell in every living thing, plant or animal, is surrounded by a plasma membrane. If cells did not have some sort of barrier to separate them from their fluid environment, they could not perform any functions. Their organelles (tiny structures that each have a job to do) would be floating around all over the place. More important, they would not be cells, since cells are separate units of living matter. It is the plasma membrane that makes cells a separate entity, or a self-contained unit of living matter. The membrane is so thin that it was not proven to exist until the late twentieth-century invention of the super powerful electron microscope. Although incredibly thin, this delicate structure is very strong and even stretchable. It is made up of a double layer of molecules of a lipid (fats, oils, and waxes) that each have a head and a tail. Their tails point in toward the center of the cell and repel water. Their larger heads face outward and attract water. Plasma membranes are selectively permeable. This means that they allow certain molecules to enter the cell through its pores while blocking the path of others. The plasma membrane is sometimes called the living gatekeeper. Although strong and flexible, it can be damaged. However, the plasma membrane has the ability to repair any breaks it may suffer. Although the plasma membrane is one of the few structures common to every living cell, the plasma membrane in a plant cell is not identical to what is called its cell wall. Animal cells do not have cell walls. They are found only in plant cells and are located outside the plasma membrane. Unlike the plasma membrane, the cell wall is made of tough cellulose (the main component of plant tissue). Fungi and bacteria also have cell walls, but they are not made of cellulose. Cells walls give plant cells a rigid shape, while the plasma membrane of animal cells give them the appearance of a tiny, jelly-filled bag. [See also Cell]

Pollution Pollution is the contamination of the natural environment by harmful substances that are produced by human activity. Pollution of Earth’s air and water have become so widespread that the problem is now global and could threaten the biosphere (a life-supporting zone extending from Earth’s crust into the atmosphere). Pollution by chemicals and nuclear material may have especially severe and long-lasting effects. 476


The term pollution can have several different meanings. Derived from the Latin pollutus, it suggests something that is made unclean or dirty. Today, the word pollution is almost always applied to the environment and usually has one of two meanings. The wider of the two suggests that pollution includes any unpleasant or unwanted environmental change, whatever its cause. According to this meaning, a natural event like a volcano can certainly cause pollution.

Pollution

TYPES OF POLLUTION In a narrower sense, pollution can be interpreted as any harmful environmental changes caused specifically by human activity. Humancaused pollution will be discussed here, since it not only poses the bigger threat today but is really the only type that humans can prevent. Pollution caused by human activities is extensive, since people produce an enormous variety of pollutants that eventually end up contaminating the air and the water that is so important to human life.

Air Pollution. Air pollution is sometimes the most obvious since at its worst, we can literally see the pollution in the sky. Air is polluted by the release of harmful gases and particles into the atmosphere. The gases given off by running an automobile are the major causes of today’s air pollution. Although the United States has drastically improved its performance on auto emissions, the fact remains that although it has only 5 percent of the world’s population, it accounts for 70 percent of the carbon monoxide (an odorless, tasteless, colorless, and poisonous gas) and 45 percent of the nitrous oxides (a poisonous gas) that are pumped into the atmosphere. Heavy industry and power plants around the world also contribute to a steady stream of sulfur dioxide (toxic gas) released into the air. All of these gases are noxious or have harmful effects. Carbon monoxide increases the demands on the heart and reduces the blood’s supply of oxygen to the tissues. Nitrogen oxides dissolve in water and form nitric acid, one of the components of acid rain. Sulfur oxides also combine with water to produce sulfuric acid. All of these air pollutants work to undermine humans’ health, erode buildings, and kill green plants. More alarmingly, what we are putting into the atmosphere may be changing the climate of the entire Earth. It has been argued that the accumulation of what are called “greenhouse gases,” like carbon dioxide, methane, and chlorofluorocarbons (CFCs), have the effect of trapping the Sun’s heat close to Earth. This can create a greenhouse effect that will lead to global warming, the results of which may be the melting of the polar ice cap (causing widespread flooding). Also, agricultural regions U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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may become too hot and dry to grow crops. The protective ozone layer is also becoming thinner. This upper atmospheric layer shields Earth and its inhabitants from the harmful ultraviolet radiation contained in ordinary sunlight. When CFCs are released from our refrigerators, air conditioners, and spray cans, they react with the ultraviolet radiation and release chlorine atoms. These in turn destroy the oxygen molecules that make up the protective ozone layer.

Water Pollution. As important as air pollution is, the issue of water

This group of dead fish was killed by water pollution. Clean water is essential to life on Earth. (©U.S. Fish and Wildlife Service. Photograph by W. French.)

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pollution is equally so. Clean water is as essential to life on Earth as is clean air, yet human activity has routinely polluted its sources of fresh water by allowing it to be contaminated by organic and industrial waste. Organic waste is mainly sewage, and even the best sewage treatments plants in the biggest cities cannot keep drinking water supplies from being contaminated. In fact, many larger cities get their drinking water from filtered and treated sewage water. Industrial pollution of water adds many dangerous inorganic (man-made) chemicals to our water supply. These chemicals may cause cancer and interfere with reproduction.

Pesticides and Nuclear Waste. In addition to the vast problems the world faces with air and water pollution, the pollution problems relating to pesticides (chemicals that kill insects) and nuclear power are also critically important. Ever since World War II (1939–45), increasingly pow-


erful chemicals have been used to kill or control insects that are considered harmful to people or products. The first major success was the synthetic chemical DDT. Although able to kill any insect, DDT also was later found to infect fish and kill birds. It has since been found in the tissues of living things from the poles to the most remote forests. It sometimes can even be found in mother’s milk. As a result of these harmful effects, the United States banned the use of DDT in 1972 and many other countries followed suit.

Polymer

While the use of DDT has been banned in the United States and other countries, many other chemical pesticides (as well as herbicides and even fertilizers) used by commercial agriculture have been found to severely interfere with normal human and animal life processes, causing illness, mutations, and even death. However, as bad as these chemicals can be for living things, they have nowhere near the permanence found in radioactive materials. While nuclear power plants generate no air pollution, they result in the steady production of nuclear or radioactive waste. There is no known way of safely disposing of this near-permanent material which will remain radioactive for up to half a million years. The radioactive waste can result in cancers, birth defects, and death.

POLLUTION CONTROL Beginning in the 1960s, a growing public and governmental awareness of the importance of the environment led to campaigns for laws and international agreements to curb pollution. In the United States alone, water quality has been improved (with dramatic reversals of “dead” lakes like Lake Erie), and air pollution has been reduced substantially. Recycling has also become a standard way of conserving resources. However, the world’s pollution problems cannot be solved in only one country. All countries share the growing realization that the biosphere, or all the parts of Earth that make up the living world, includes everyone on Earth. For this reason, international cooperation is essential if Earth is to be restored to its natural and livable state. [See also Carbon Dioxide; Carbon Monoxide; Ozone]

Polymer A polymer is a chemical compound formed by the linking of many smaller molecules (particles) into a long chain. Polymers can be both natural and synthetic, and most living things are composed of natural polymers. Polymers can store energy and information and have structural uses as well. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Population

In Greek, a polymer has many (polys) parts (meros). It is an organic compound or a natural substance containing carbon that is made up of many smaller repeating parts, or units, called monomers. Polymers play an important role in the chemistry of life, and without chemicals and chemical reactions, there would be no living things. Since they are compounds or are made up of many smaller units, polymers are also called macromolecules (meaning they are giant molecules). A polymer is also described as a “chain” molecule since the smaller units that make it up are linked together like a long chain. Polymers are so useful because although this chain has only a certain length, it can, like a piece of string, be tied and twisted and turned into all sorts of different shapes. Polymers combine flexibility with strength since their chains can be stretched several times their normal length without breaking. It is no surprise therefore that examples of substances composed of natural polymers would be starch, wool, and rubber. Without polymers there would be no plants or trees since cellulose is a polymer. Plants use cellulose to build their cell walls, and trees use it to make their woody parts. Green plants also store their food as starch, which is a polymer. Proteins are another type of polymer, and wool is a variety of protein, as are the life-coding molecules of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). It could be said that life on Earth is based to a great degree on the existence, properties, and reactions of two classes of polymers—proteins and nucleic acids. Aside from these organic forms of polymers, there are inorganic polymers and synthetic polymers. Since an organic compound is a chemical compound that contains carbon, an inorganic compound is one that does not have any carbon in it. Most of the inorganic polymers that are found in nature are typically very hard and strong. Many minerals, like diamond, quartz, and silica, are inorganic polymers. Synthetic polymers or polymers that are made by people can be organic (as with rubber that occurs both naturally and as a human-made product), or inorganic (like plastics and adhesives). Once chemists understood the process of polymerization or synthetic creation of polymers, they were able to produce everything from vinyl, polyurethanes, and silicones and to vary their properties in ways that exceeded even what nature could produce.

Population The term population refers to all the members of the same species that live together in a particular place. This concept has proved very useful to 480


the life sciences since it has only two basic requirements. Its individual members must belong to the same species (and be able to mate and produce young), and they must also live in the same area at the same time. Ecologists refer to all of the animal and plant populations that live together and interact in a given environment as a community. This is a larger category than population.

Population

Every ecosystem (living organisms and their environment) is composed of populations or groups of the same type of organisms. In any particular natural place, such as a forest or a lake, there are populations of certain animals (deer, earthworms, trout) along with certain plant populations (maple trees, spruce trees, honeysuckle) that thrive in the same place. Ecologists have decided to categorize these same-species groups, such as all of trout or all of the maple trees, that live in the same place as populations. Population size is influenced by the interaction of two major factors: the rate that a population grows under ideal conditions and the rate at which certain external or environmental factors limit population growth. An example of population growth under ideal conditions would be placing a pair of fertile animals in a “new” habitat with no predators, no diseases, more than enough food, and a perfect climate. Since there are no external factors hindering the animals in any way, their population growth would be limited only by “intrinsic” or internal factors as how quickly they can produce young, how quickly their young become fertile, and how long they naturally live. Under these conditions, the maximum rate of population growth is limited only by the biology of the species. Ecologists know that any population growing at this unnatural rate would eventually reach what is called “carrying capacity,” when external or environmental factors come into play. Carrying capacity is described as the largest size of a particular population that can be supported by a particular environment. Once a population has reached a certain critical size (which is technically past its carrying capacity), it begins to feel the limitations of its habitat in terms of space and resources. When this limit is surpassed, the rate of population growth can only decline since individuals now have to compete with one another for living space and resources like food and water. Intense competition by itself can result in stress, which results in lower birth rates and increases mortality. In the real world, populations of animals and plants change in size all the time. Some factors that put limits on a population include: low food supply, predators, bad weather, disease, competition, and human interference. Ecologists have developed mathematical models, called populaU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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tion dynamics, which are used to study and predict the condition of a population in an ecosystem. Ecologists base a model on the four major environmental factors that affect populations: birth rate (BR), immigration rate (IR), death rate (DR), and emigration rate (ER). Such a model states that P = BR - DR + IR - ER. Therefore, the change in population size (P) is equal to the birth rate less the death rate, plus the immigration rate (how many new members join the habitat) minus the emigration rate (how many old members leave the habitat). This statistical model is true of all populations, including humans. Population studies are important if we are to understand how human activities affect the natural world. Being able not only to measure or count a single group of the same species in a particular ecosystem, but to be able to understand the dynamics of what forces make its numbers grow or decline, is especially valuable. Understanding a population also gives ecologists insight into the larger and more complex concept of communities. [See also Population Genetics; Population Growth and Control (Human)]

Population Genetics Population genetics is the statistical study of the natural differences found within a group of the same organisms. Instead of examining the genes of individuals, it looks at the dominant (the trait that first appears or is visibly expressed in the organism) and recessive (the trait that is present at the gene level but is masked and does not show itself in the organism) genes found within an entire population. Population genetics seeks to understand the factors that control which genes are expressed. It also creates mathematical models to try to predict which differences will be expressed and with what frequency. In the life sciences, a population consists of all the individuals of the same species (all of the same kinds of organisms, like all the tigers) that live in a particular habitat at the same time. Scientists know that in any population, whether it be tigers or people, the individuals that make it up are all different. They may all be tigers, but each has individual and very recognizable traits. Although some might think that all animals of the same species look exactly alike, it is known that once someone becomes familiar with a certain group of the same species, he or she can usually tell one from another. At first all black labrador retrievers look alike. After a closer look, 482


it can be seen that there are very obvious and easily recognizable differences among them. It is known that it is mostly the individual’s genetic inheritance that accounts for these minor differences. This means that the unique combination of dominant and recessive genes that the individual has inherited is responsible for all of its individual traits (color, size, abilities, and tendencies, to name only a few).

Population Genetics

WHAT IS POPULATION GENETICS? Population genetics is a tool used to study the genetic basis of evolution (the process by which gradual genetic change occurs over time to a group of living things), and it is helpful in allowing scientists to understand the relative importance of the many factors that influence evolution. It studies a given population’s gene pool (which is the total of all of the genes available to a generation). Knowing what the gene pool consists of enables scientists to establish a sort of genetic base out of which future offspring will be composed. This assumes that over time, the population is made up of individuals that breed only with others of their species that live in the same habitat.

HARDY-WEINBERG LAW Once the gene pool is established, scientists are able to use Mendel’s laws of inheritance (concerning patterns of dominant and recessive genes) and predict what differences there will be among individuals in that population. Scientists are able to establish what are called gene frequencies, or percentages at which certain genes will be expressed. Scientists also have been able to establish a law that actually measures what changes will take place. Called the Hardy-Weinberg law, since it was proposed independently in 1906 by the English mathematician Godfrey H. Hardy (1877–1947), and the German physician Wilhelm Weinberg (1862– 1937), this is a mathematical formula that has become the basis of population genetics. Using this formula (which only works perfectly when certain ideal conditions are met), scientists are able to describe a steady state called genetic equilibrium. In this state, gene frequencies stay the same and nothing changes unless some outside force intervenes. Naturally, the real world, especially that involving human beings, is not perfect and there are many factors always at work that make conditions less than ideal. Chance events happen all the time. Reproduction does not always work and individuals leave populations while others may wander in. These are only a few potential variables. However, the HardyWeinberg formula is still useful and helps in being able to arrive at some relative frequencies, so it is still applies in some way to the real world. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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WHY STUDY POPLUATION GENETICS? By studying what makes individuals in the same population different, science is able to learn more about evolutionary change. Population genetics can also draw very useful conclusions. For example, when populations of interbreeding individuals are very small, they are highly susceptible to extinction by any number of chance events. This is because their interbreeding has not given them much genetic variation (differences). When something in their habitat changes, they may be unable to adapt quickly enough. Population genetics, therefore, is a valuable, if not always statistically perfect, tool for life scientists. [See also Dominant and Recessive Traits; Evolution; Genetics; Population]

Population Growth and Control (Human) The human population, or the number of people, on Earth has increased enormously during the past two centuries. Upon examining the reasons for this growth and the factors that could influence its continuance, it is not known whether Earth can sustain the great numbers of people predicted for the near future. On October 12, 1999, the United Nations issued a population estimate that said the 6,000,000,000 mark had been reached. That figure is twice the population of 1960, a mere 40 years ago. Since the development of agriculture and the beginnings of settled human communities some 10,000 years ago, it took thousands of years for the human population to reach 1,000,000,000 around the year 1800. It took another 130 years to reach 2,000,000,000, but only 30 years to reach 3,000,000,000, 15 years to reach 4,000,000,000, 12 years to reach 5,000,000,000, and another 12 years to reach 6,000,000,000. Today, the human population is growing at about 1.5 percent a year, or the equivalent of an additional 89,000,000 people a year. Every second five people are born and two people die, for a net gain of three people. The latest United Nations forecast for the year 2050 contains a projected low of 7,800,000,000 and a projected high of 12,500,000,000. However, if the world population continues to grow at its present rate, the high projection will be easily reached. 484


POPULATION HISTORY The biological history of Homo sapiens (humans) extends back for more than 1,000,000 years. For almost all that time it consisted of a very small population that was barely staying alive by hunting and gathering. However, about 10,000 years ago, things began to change. People discovered primitive agriculture and began to domesticate a few plants and animals. With the discovery of the properties of metals and other technologies, the growth of the human population went from about 300,000,000 in the year A.D. 1 to about 500,000,000 in 1650. Around that time, the rate of population growth began to noticeably increase. High birth rates were accompanied by decreasing death rates due to better technologies for sanitation and medicine.

REASONS FOR POPULATION GROWTH In 1800, the population reached 1,000,000,000. Yet in the past two centuries, its has skyrocketed to today’s 6,000,000,000. There are three reasons why such unprecedented population growth has occurred. The first is the ability of the human species to adapt to new habitats. Early humans possessed the ability to learn and to remember, as well as the ability of being able to communicate what they knew to others. Humans have been able to use their technologies not only to disperse themselves virtually all over the globe, but to make their environment perfectly suitable for them if it is not naturally so. Today, people live in climates that might be considered extreme. They can live cooly in Phoenix, Arizona, in July; and they can play a professional football game indoors in Minnesota during a December blizzard. While most animals are “designed” to fit a certain habitat, humans have almost always made wherever they chose to live their habitat.


English economist Thomas R. Malthus was the first to draw attention to the fact that the human population could not keep growing indefinitely. (Reproduced by permission of Archive Photos, Inc.)

Humans have also been able to bypass what might have been natural limits on Earth’s “carrying capacity.” Carrying capacity is the maximum population that a habitat can support over a long period of time. However, humans have been able to increase that natural capacity by improvements in agriculture, as well as human use and management of natural resources. The use of U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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fertilizers, pesticides, and irrigation is only some examples of how Earth can be exploited more efficiently to support greater numbers. The third and probably most significant reason why our population has exploded in the past 200 years is that many of the natural or built-in limits on growth were eventually limited themselves or even done away with. While death is still inevitable for every human born, the death rate has gone down dramatically. This means that more people are living longer than they used to. Until fairly recently, people died early and regularly from many conditions that were really preventable. Childhood was an especially dangerous time, as youngsters often died from malnutrition (the physical state of overall poor health), the flu, or diarrhea. Contagious diseases would spread rapidly and kill great numbers in epidemics, especially in crowded cities. Today, however, simple improvements in hygiene and sanitation have prevented millions of early deaths. Furthermore, medical victories over diseases like cholera, measles, polio, whooping cough, and tuberculosis were based on the development of vaccines, antibiotics, and other new drugs. As a result, these diseases are no longer a threat.

EFFECTS OF CONTINUED POPULATION GROWTH In 1798 the English economist, Thomas R. Malthus (1766–1834), was the first to draw attention to the fact that the growth of the human population could not keep on indefinitely. He argued that the population would eventually outgrow its food supply and start to fall back because of famine, disease, or war. It may once have been argued that human ingenuity has been able to overcome any of Earth’s natural limits to population growth, and that Malthus’s theories no longer applied. However, many argue that if the growth of the human population does not stabilize at some point, we may unhappily discover that there is an actual limit to the carrying capacity of the planet. No one knows how many humans Earth can support. Some scientists argue that we have already reached it, given the rate at which developed countries (as opposed to less developed or third world countries) consume available natural resources. A goal of all population planners is what is called zero population growth. This desirable situation represents stability in that it happens when the birth rate roughly equals the death rate. Presently, the people of Earth are nowhere near such a stable figure. [See also Population]

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Predation

Predation is the act of one animal hunting, killing, and eating another animal. A predator is an animal that survives by killing and eating other animals. Predation can be important in regulating the size of a prey species (the hunted animal). It is also a mechanism of evolution since it weeds out animals that are poorly adapted to being hunted, or to their environment, thus promoting adaptation by natural selection. Predation can be coldly described as nature’s “kill or be killed” approach to who survives in the wild and who does not. While there are some species of animals that have no natural enemies or are simply too difficult to catch and eat (such as a mature, healthy elephant), virtually all animals are at some point in their lives either predator or prey. All predators are heterotrophs, meaning that they cannot make their own food as plants (autotrophs) do, so they must consume another organism and digest it to obtain its energy. Many animals are herbivores (planteaters) and do not kill other animals to eat. They eat living plants that do not have to be hunted, caught, and killed before they can be consumed. Animals that exclusively eat plants are not considered preda-

Spiders, like this redkneed tarantula, are successful predators because they are able to catch prey in the webs they spin. (Reproduced by permission of the Corbis Corporation.)


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tors (although from a living plant’s point of view, it could be said that the plant is being preyed upon). Carnivorous animals as well as omnivores are predators, since carnivores exclusively eat other animals, and omnivores eat both plants and animals, usually according to what is available. Successful predation requires that a catch be made, and a catch can occur in a great variety of ways. Spiders spin webs to trap their prey; cats lie in wait for the proper moment to spring; hyenas stalk in groups and exhaust their prey; and humans use their technology to kill from a distance. Specialized predators usually go after a single species, while generalized predators will feed on a variety of other species. Predation is one of the ways in which nature selects who has done the best job of adapting to its environment and who will have the best chance to survive. In this way, predation can be said to be a mechanism or a means that nature employs to continue the important role of evolution. Predators almost always select a meal that will give them more energy than they will use up to catch it. When faced with an opportunity to select an individual animal from a group to eat, predators will usually select the easiest to catch, such as the weakest, the slowest, or the youngest or oldest. Those animals that are poorly adapted will probably not survive and therefore not be able to pass on their poorly adapted traits to another generation. In the dynamic relationship between predator and prey, there is a continuing type of improvement that goes on in which the prey that survive pass on traits that make their offspring slightly better at avoiding being caught. On the other hand, predators that survive because they are excellent hunters will also pass on the good traits that made them better at catching prey. So in the long run, the seemingly cruel standards of the natural world use predation to improve the species.

Opposite: A female pygmy chimpanzee with her young. Primates, like this chimpanzee, are the most highly developed group of animals. (Reproduced by permission of the World Wide Fund for Nature Photolibrary.)

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In terms of entire populations in a certain habitat, predator and prey relationships often move along the same lines. Thus, when conditions favor a prey population—such as when field mice thrive during a good growing season and their population increases—the predator population will also be well fed and grow larger as its members grow strong and live longer. The opposite happens when populations drop. Finally, when the only predator of a certain species disappears (sometimes human intervention causes this to happen), prey populations will take off. The systematic killing of timber wolves and gray wolves in the American West has led to an increase in the number of rabbits and rodents that regularly served as their prey. [See also Evolution; Natural Selection; Survival of the Fittest] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

Primate

Primate

A primate is a type of mammal with flexible fingers and toes, forwardpointing eyes, and a well-developed brain. Humans are primates, as are lemurs, monkeys, and apes. Except for humans, primates, are found in mostly tropical habitats. Primates are the most highly developed group in the animal kingdom. Most primates either live in trees or have evolved from tree-dwelling ancestors. All are placental mammals meaning that before birth, their young are nourished by a structure in the mother’s body called a placenta. There are about 180 species of primates, all which make up the order called Primata. This name comes from the Latin word primus meaning first, and the case can be made that primates are in may ways first among animals.

CHARACTERISTICS OF PRIMATES Primates have many physical attributes that account for and contribute to their being considered in this manner. First, they have binocular vision, meaning that they use both eyes together. Because both eyes are located in the front of their faces and point forward rather than from the sides, they have sharp three-dimensional vision and good depth perception. Primates also have specially adapted hands for grasping things. Most have five flexible fingers on each hand and five flexible toes on each foot (with flattened nails on their ends instead of claws), as well as an opposable thumb. With a thumb that can be opposed to or that is able U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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to touch the other fingers, primates can fully encircle or grasp something like a tree branch or a tool with their hand. They also can reach out and bring food to their mouth. Some primates are equally agile with their feet as well. All primates have large, well-developed and highly complex brains. Convolutions are folds in the brain that increase surface area and allow for a greater number of nerve cells. This brain enables chimpanzees to make and use tools and humans to reason and solve problems. Primates also have all four different kinds of teeth, meaning that they can eat all types of food. Primate teeth are less specialized for tearing and ripping and more useful for grinding and chewing a plant-based diet. All primates also have a clavicle or collarbone that gives them a flexible shoulder whose joint allows free movement of the arm in all directions. Wrist and elbow joints are also highly mobile. Primates usually have only two mammae (or breasts), and give birth to one or sometimes two offspring per pregnancy. Compared to other mammals, their young are dependent for quite some time and take a long time to mature. Primates are very social and exhibit the most complex behavior of all the mammals. They bond together in pairs of two, in larger family groups, or in even larger bands. In the larger groups, there is usually a leader or dominant male and a hierarchy after him. Communication is very important among primate species, and they use many visual signals as well as sounds to interact with one another. Primates regularly send each other messages, whether they are warnings of danger or calls for mating.

PROSIMIANS AND ANTHROPOIDS Most biologists divide primates into two groups: prosimians and anthropoids. Prosimians are tree-dwelling, squirrel-like insect eaters including lemurs, lorises, and tarsiers. They were the first primates to evolve and are considered primitive compared with anthropoids. Most primates belong to the second group, the anthropoids. Monkeys, apes, and humans are all anthropoids. There are two types of monkeys who were probably split apart when the continents of South America and Africa drifted away from one another. New World monkeys are found in the tropical forests of Central and South America and include howler monkeys, capuchins, marmosets, and tamarins. Many of these monkeys are small and highly vocal. Old World monkeys are found in the tropics of Asia and Africa and include baboons, colobuses, and macaques. They are usually larger than their New World counterparts but have much shorter tails. Another way to tell them apart is by their noses. New World monkeys have flattened noses with widely set nostrils, while Old World monkeys have more definite noses and closely set nostrils. 490


HOMINOIDS

Protein

A separate and useful “superfamily” or extra grouping of primates, called hominoids, is often used by life scientists when they are discussing both apes and hominids. Hominids are humans and their direct ancestors of humans (meaning that humans are the only living species of hominid). Unlike other primates, hominoids have large heads and no tails. They also can move about on their back legs (although many apes cannot do this for a long distance). The ape family is made up of gibbons, who swing from tree to tree with their extra long arms, as well as orangutans, gorillas, and chimpanzees. These are humans’ closest relatives in the animal kingdom, and they are able to see in color, move their lips, and even make a great number of facial expressions, all human-like actions. As for hominids, there is only one species living today, and that is Homo sapiens or human beings. While humans are the most intelligent species of primate (Homo sapiens means “wise human”) gorillas are the largest and most powerful primate. Deoxyribonucleic acid (DNA) comparisons have shown that chimpanzees are the closest living relatives of human beings. [See also Homo sapiens; Human Evolution; Mammals]

Protein Proteins are the building blocks of all forms of life. As an organic compound (meaning that they are based on the element carbon) made up of amino acids (the building blocks of proteins), they are key to the major functions of growth and repair as well as to other important specialized functions. The human body makes proteins by following the coded instructions in its genes. All living things are made up of cells, and the major ingredient of all cells is protein. As organic compounds, proteins play a major role in many of the functions that take place in living things. Proteins called structural proteins make up all of the parts, or building material, of a plant’s or animal’s body. They make up the walls of an organism’s cells. In a human being, hair and fingernails are made of a structural protein called keratin, and tendons (linking muscle to bone) are made of a different structural protein called collagen. The enzymes, hormones, and antibodies in our bodies are all made of protein. Enzymes are essential to all of the chemical reactions that take place inside us; hormones are the body’s chemical messengers; and antibodies help fight foreign substances or infection. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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A computer graphic of a protein molecule found in fruit. This protein functions similar to human digestive enzymes that break down proteins in food. (©Photographer, Science Source/Photo Researchers, Inc.)

Proteins are made up of amino acids. Humans and most other animals require twenty separate types of amino acids in order to be able to make the many different types of proteins they need. Proteins are amino acids that are linked, or bonded, together like chains. Therefore, it is the order of the individual amino acids in the chain that make proteins different from one another. The twenty different amino acids form different proteins in a way similar to how the twenty-five different letters of the alphabet are used in different combinations to form different words. Different proteins have different sequences of amino acids. The shape that these long protein chains eventually form has a great deal to do with the properties of a protein. Chains that form themselves into a springlike shape build a material that is flexible and can be stretched, while those that form a sheetlike structure are more rigid. An example often used to illustrate the former is the protein keratin that forms wool. Because keratin has a springlike structure, a length of wool can be stretched to nearly twice its original length. On the other hand, the natural fiber silk is composed of the protein called fibroin whose shape is not coiled. As a result, silk cannot be stretched to anywhere near the length that wool can. Proteins have important structural uses. They are also a necessary part of an organism’s diet. Of the twenty different amino acids commonly found in proteins, only ten can be made in cells. The other remaining ten amino acids (called “essential amino acids”) must be obtained from food. Unless all these amino acids are obtained by eating various protein-based foods, the body will not have the necessary building blocks to form new protein molecules. The growth and repair of body cells could be harmed or even halted. Nutritionists have determined that these essential proteins can be obtained from meat, eggs, milk, cheese, and other foods usually derived from animals.

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Finally, most proteins are species-specific, meaning that each species has slightly different proteins. Thus, the type of proteins found in the cells of a domestic house cat are different from those of a tiger. The farther apart two species are on the evolutionary ladder, the greater difference there will be in the proteins found in their cells.

Protists

[See also Antibodies and Antigens; Amino Acids; Enzymes; Hormones; RNA]

Protists Protists are a group of single-celled organisms that make up the kingdom Protista. Although some protists behave like animals and others like plants, they are all organisms with a complex eukaryotic cell, or a cell that contains a nucleus and organelles (structures inside a cell that have a particular function). Protista is the most diverse of all the eukaryotic kingdoms, and its members range from free-floating plankton to deadly parasites that live in mammals. There are so many different types of organisms that are classified as protists that biologists are not certain if they all share a common ances-

This diatom plankton is a good representative of the Protista kingdom. (Reproduced by permission of the Corbis Corporation.) U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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tor with higher life forms, or even if all known protists have evolved from the same ancestor. Recent studies suggest that even the name is not large enough, and that protist refers only to the microscopic members of this kingdom. Many leading scientists suggest that a more correct and inclusive name is Protoctist, since it includes every member of this kingdom from slime mold to huge brown kelp. However, one thing all scientists agree on is that protists are eukaryotes since they have at least one nucleus and many other little structures or cell organs called organelles. Further, all protists live in some form of a watery environment, even if that water is within the tissues of another organism. Protists do all the things that other living things can do—they eat, grow, excrete waste, and reproduce. Some are able to make their own food, as plants do. Some reproduce sexually (with male and female sex cells), while others reproduce asexually and simply divide into new identical parts.

TYPES OF PROTISTS Recent studies indicate that there may be as many as 250,000 different species of protists. Despite these large numbers, all can be divided into three main types: animal-like, plantlike, and fungus-like cells. Many are familiar to us, such as slime molds, amoebas, red tide, pond scum, and green seaweeds, but many more are hardly known. For example, the protist known as Trychonympha lives inside the gut of termites and helps them digest and use the nutrients found in the wood they eat. Similarly, the stomachs of grazing animals contain countless protists without which they could not break down tough plant cells. Although they are singlecelled organisms, protists are far from simple. Unlike multicelled organisms that have evolved specialized organs, some protists have evolved specialized structures, like hairs or a tail that allows them to move about.

Animal-like Protists. Many of the animal-like protists, called protozoans, cause disease in animals. For example, among the animal-like group known as flagellates (because they have a tail or flagellum that they use to propel themselves), the genus Trypanosoma causes African sleeping sickness in humans and their livestock. Other animal-like protists, like amoeba, live as parasites (organisms that live in or on another organism and benefit from the relationship) in animals and cause dysentery.

Fungus-like Protists. Some protists, such as the slime molds, have distinct fungus-like characteristics. Like a fungus, they disperse spores, but unlike fungus, these protists can also move about like an animal. One especially strange slime mold is actually a group of amoebas that get together when food is scarce and form a new being. Called a plasmodium, this sluglike creature oozes over surfaces leaving a visible track, and when 494


it runs out of food, it grows upward into a “spore tower,” dries out, and disperses its spores with the wind. Later, under proper conditions, the spores come to life as individual amoebas.

Protozoa

Plantlike Protists. The plantlike protists known as Euglena are usually recognized by anyone who has taken a biology class or seen an algae “bloom” on a lake. Some individual euglenoids have chloroplasts (energy-converting structures found in plant cells) and perform photosynthesis (the process by which plant use light energy to make food from simple compounds) the way plants do, yet also are able to eat food as animals do. Other plantlike protists called dinoflagellates have a pair of long, whiplike tails that they use to move about. Certain dinoflagellates live in warm seas where they give the surface a bright, bluish light, while others produce powerful toxins that can cause illness or death in animals that eat them. Protists are among the strangest organisms on Earth, yet they play an important role. Many provide food for oysters, clams, snails and other ocean organisms that are important to humans. Many protists, called plankton, simply float on the water and provide food for shrimp and other aquatic animals. It is possible that without protists there would be no life on Earth at all since it is believed Earth’s life-giving atmosphere is the result of billions of years of protists conducting photosynthesis and therefore producing oxygen. [See also Protozoa]

Protozoa Protozoa are a group of single-celled organisms that live by taking in food. As a major group in the kingdom Protista, protozoa are described as having animal-like—rather than plantlike—qualities, since they move about to find and eat their food. Since they are protists, protozoa are eukaryotes (they contain a nucleus).

GROUPS OF PROTOZOA The word protozoa literally means “first animal,” indicating that they are considered to be the early ancestor cells from which more complex, multicelled animals evolved. As animal-like protists, protozoa are divided into four phyla (one of the seven major classification groups that biologists use to identify and categorize living things) based mainly on how they move about. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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CHARLES LOUIS ALPHONSE LAVERAN French biologist Charles Laveran (1845–1922) discovered the parasite (an organism that lives in or on another organism and benefits from the relationship) that causes human malaria (a disease characterized by cycles of chills, fever, and sweating). This was the first time that a disease was shown to be caused by living animal cells called protozoa (a group of single-celled organisms that live by taking in food). His creative work not only led to understanding the disease and its transmission by certain species of mosquito but also directed many other researchers into this new field. Charles Laveran was born in Paris, France, the son of a military doctor. When he was five years old, the family was transferred to Algeria. Returning to Paris at the age of eleven, Laveran eventually entered the same military medical school his father had attended and graduated in 1867. Continuing in his father’s footsteps, he joined the military medical service and saw active duty during the Franco-Prussian War (1870–71). It was then that he saw how disease can ravage an army worse than any enemy. In 1878 he was sent to Algeria as his father had been, and it was there that he began a careful study of the disease malaria. Malaria was common in many parts of Algeria, and had affected humans for centuries, with no one able to do anything to prevent it. For a long time it was believed to be caused by mal aria, the Italian words for “bad air.” During Laveran’s time, it was thought that perhaps it had a bacterial cause since French chemist and microbiologist Louis Pasteur (1822–1895) was discovering more and more bacterial diseases. Laveran went about his research in a careful, methodical way, and although he was limited by a primitive, low-powered microscope, he spent a great

Flagellates. The first group is the flagellates called Mastigophora, which move about by the use of one or more flagella (a whiplike tail). This phylum, also known as Zoomastigina, lives in a watery environment. Flagellates are very diverse since some live as parasites (organisms that live in or on other organisms and benefit from the relationship) and others as free-living organisms.

Sarcodina. The second group of protozoa are the members of the phylum Sarcodina, commonly known as amoeba. An amoeba is recognized because it has no particular, fixed shape. It has been described as looking formless like a bag of jelly. The reason for this is that the amoeba’s single cell is surrounded by a plasma membrane and is constantly changing shape. It is this constant shape-changing that allows the amoeba to move. Amoeba can be found in all types of water and even in the soil. 496


deal of time examining blood samples from malaria patients both living and dead. Finally, in November 1880, he first observed under the microscope tiny circular and cylindrical bodies that had moving flagella or hairlike filaments. This, he knew, was no bacteria. Instead, he knew it was a living animal cell, a minute, single-celled creature called a protozoon (plural, protozoa). Once inside the human body, protozoa act like parasites. A parasite is a species that lives in or on another species at the expense of that species. In other words, the parasite thrives and the host gets sick or even dies. The particular protozoan parasite that Laveran discovered was later named plasmodium.

Protozoa

Laveran had a great deal of trouble, however, convincing a skeptical medical community of his protozoan discovery, and eventually it took the great Pasteur to agree with him before everyone was convinced. Laveran was able to study the cycle of malaria in the red blood cell and discovered exactly what goes on there. He found that the protozoa increased in size inside the red blood cell until they almost filled it, at which time they then divided and formed spores. When these spores were freed from the destroyed blood cell, they invaded healthy blood cells and continued to do the same thing. Laveran had a strong feeling that the malaria protozoa were nurtured and transmitted to humans by certain types of mosquito, but it remained for the English physician, Ronald Ross (1857–1932), to finally prove this in 1897. Laveran went on to study other protozoan diseases and eventually joined the Pasteur Institute devoting the rest of his life to the study of tropical diseases. For his discovery of the protozoa plasmodium, Laveran was awarded the 1907 Nobel Prize in Physiology and Medicine.

Some are parasites of humans and can cause fever, abdominal cramps, and diarrhea. There is also a species of shelled amoeba in which the organism’s cytoplasm (jelly-like cell contents) is protected by a shell it has created out of its own mineral secretions. With names like foraminiferams, heliozoans, and radiolarians, the shells of these tiny water creatures represent some of the most intricate and beautiful designs of nature.

Sporozoa. The third group of protozoa belong to the phylum Sporozoa and are parasitic spore-formers. In their adult stage, they cannot move, but during another stage, they live in a host who transfers them to yet another organism who they infect. This is how the species Plasmodium vivax goes from a mosquito to a human and infects the latter with malaria. This particular protozoan kills between 2,000,000 and 4,000,000 people every year. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Ciliophora. Finally, the phylum Ciliophora makes up the fourth group of protozoa. These organisms are characterized by short, flexible, hairlike strands or filaments called cilia that cover their bodies. These move in a rhythmic, coordinated manner and are able to propel the protozoan through its liquid environment. Some species, like the Paramecium, have as many as 15,000 cilia per cell and can move very quickly. Ciliates are the most structurally complex of all the single-celled organisms on Earth, and although most ciliates reproduce asexually, some species conduct a complicated form of sexual reproduction called conjugation. Conjugation is similar to mating since one of the ciliates (the donor or “male”) produces a small protein tube through which genes are transferred to the recipient or “female” when the cells are joined together. Although conjugation achieves gene mixing, it does not result in the production of an entirely new individual. Most ciliates are found in fresh and salt water and are fierce predators, eating bacteria and other small organisms. Many protozoans serve as an essential food source for a wide range of animals. Therefore, they are very important to the ecological food web (the transfer of energy in an ecosystem) of higher organisms. Some are also used for medical purposes while others serve such practical uses as purification of sewage beds. [See also Protists]

Protozoa are considered to be the early ancestors of more complex, multicelled animals. (Reproduced by permission of Photo Researchers, Inc.)

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Punnett Square

Punnett Square

A Punnett square is a diagram used to calculate inheritance patterns. Resembling a checkerboard, this device makes it possible to figure out the exact gene combinations that an offspring will inherit from its two parents. Understanding the Punnett square, which is a handy tool for predicting results in genetics, first involves having knowledge of alleles (pronounced uh-LEELZ). Alleles have to do with genes, which are the carriers of the information that determines all of the traits or characteristics of an individual organism. Every human being receives 23 gene-carrying chromosomes (coiled structures in a cell’s nucleus that carries genetic material) from each parent, resulting in a full set of 46 chromosomes (and some 100,000 genes). When these chromosomes pair up at fertilization to form a new and unique individual, they do so in a way that related chromosomes always pair off (since the same trait is also located in the same place on each chromosome). Since the new individual receives information from both parents concerning a single trait, it always has two sets of directions for that trait. This pair is called “alleles.” When the two sets of instructions are the same (such as both coding for brown hair), they are called “homozygous.” When they are different (such as one coding brown hair and the other coding red hair), they are called “heterozygous.” An allele is therefore a single member that makes up a gene pair. Two alleles are a kind of partnership, and in some cases, one partner is stronger than the other. When this is the case, the stronger one is called the “dominant allele.” The other one in this relationship is called the “recessive allele.” Ever since Austrian monk and botanist (a person who studies plants) Gregor Mendel (1822–1884) began experimenting with pea plants and their traits in the 1860s, the rule concerning dominance has been that when two organisms showing different traits are crossed (like a tall and a short pea plant), the trait that shows up or is expressed in the first generation is considered the dominant trait. Just as an athlete may dominate a game to the point where the opponent has no chance to do anything, so a dominant allele expresses itself and suppresses, or masks, the activity of the recessive allele for that trait. However, the recessive allele does not go away just because it is masked. It is still part of the organism’s inherited package called its “genotype.” The word “phenotype” is the opposite of genotype and describes only the visible characteristics. Since Mendel stated what became known as the laws of inheritance, biologists have been able to use these principles to predict what will hapU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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pen when organisms with specific traits are crossed. The easiest way to do this is with the diagram named for the English geneticist, Reginald Crundall Punnett (1875–1967). Punnett devised a square that he divided into four equal parts. Across the outside top of the square are the symbols for the alleles from the male parent. The allele symbols from the female parent are written outside the left-hand side of the square. A capital letter is used for the dominant allele and a small letter stands for a recessive allele. The inside four squares show all the ways in which these alleles can combine. The answers are achieved much like a multiplication problem with the answer in each box being the product (Ll) of one top allele (L) being multiplied by one side allele (l). A Punnett square is most useful when the results of one gene are considered. Calculating for two genes can be a difficult and complicated process. A Punnett square does not tell how many offspring will be produced, but it does predict what the genotype (genetic makeup including masked traits) and the phenotype (visible traits) will be. [See also Gene; Genetics; Inherited Traits; Mendelian Laws of Inheritance]

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R Radioactive Dating Radioactive dating is a method of determining the approximate age of an old object by measuring the amount of a known radioactive element it contains. Rocks as well as fossil plants and animals can be dated by this process. It has given paleontologists (a person specializing in the study of fossils) as well as geologists (a person specializing in the study of the origin, history, and structure of Earth) a powerful way of dating ancient objects. Until the discovery of radioactive dating, scientists had no way of approximating how old any part of Earth was. Once the principle behind this method was discovered, however, it became possible to gather reliable information about the age of Earth and its rocks and fossils. Radioactive dating was not possible until 1896, when the radioactive properties of uranium (a radioactive metallic element) were discovered by French physicist (a person specializing in the study of energy and matter), Antoine Henri Becquerel (1852–1908). When a substance is described as radioactive, it means that at the subatomic (relating to parts of an atom) level, some parts of it are unstable. When a substance is described as unstable, it means that it has a tendency to break down or decay. During this decay, one substance actually changes into another and radiation is released. As long ago as 1907, the American chemist Bertram B. Boltwood (1870–1927) suggested that knowledge of radioactivity might be used to determine the age of Earth’s crust. He suggested this because he knew that the end product of the decay of uranium was a form of lead. Since each radioactive element decays at a known rate, it can be thought of as a ticking clock. Boltwood explained that by studying a rock containing U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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uranium, its age could be determined by measuring its amounts of uranium and lead. The more lead the rock contained, the older it was. Although this was a major breakthrough, Boltwood’s dating method made it possible to date only the oldest rocks. This is because uranium decayed or changed into lead at such a slow rate that it was not reliable for measuring the age of rocks that were younger than 10,000,000 years old. Another drawback was that uranium is not found in every rock. A later method that used rubidium (which changes into strontium) proved more useful because it is found in nearly all rocks, although it still was not useful for younger specimens. Perhaps the best method for rock dating is the potassium-argon method. This method proved useful to date rocks as young as 50,000 years old. In 1947 another dating breakthrough occurred. The American chemist Willard F. Libby (1908–1980) discovered the radiocarbon method for determining the age of organic materials. Called the carbon-14 dating technique, this ingenious method used the simple knowledge that all living plants and animals contain carbon (a nonmetallic element that occurs in all plants and animals). Libby also knew that while most of this carbon is a common, stable form called carbon-12, a very small amount of the total carbon is radioactive carbon-14. All plants absorb carbon during photosynthesis (the process in which plants use light energy to create food), and animals absorb this carbon by eating plants or eating other animals that ate plants. Libby also found that as long as an organism remains alive, its supply of carbon-14 remains the same. However, once the organism dies, the supply stops and the carbon-14 in its body begins to decrease according to its own rate of decay. Libby realized that this could be a practical dating tool. He eventually designed a device that used Geiger counters (which measure radiation) to accurately measure the amount of carbon-14 left in an organic substance. Libby won the 1960 Nobel Prize in chemistry for his discovery. The discovery allowed him to correctly date a piece of wood from an Egyptian tomb that was known to be about 4,600 years old. In the last 40 years, radiocarbon dating has been used on more than 100,000 samples in 80 different laboratories. Besides dating plant and animal life, this method has been used to verify the age of such different artifacts as the Dead Sea Scrolls (2,100 years), a charcoal sample from an ancient South Dakota campsite (7,000 years), and a pair of sandals from an Oregon cave (9,300 years). Improvements have raised its accuracy to nearly 70,000 years, with an uncertainty of plus-or-minus 10 percent. [See also Fossil; Paleontology]

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Rain Forest

Tropical rain forests are large areas that are warm and wet throughout the year, and whose tall evergreen trees are so dense they form a canopy. As the richest ecosystem (an area in which living things interact with each other and the environment) on Earth, rain forests support such a diversity of life that at least half of all the world’s species of plants and animals live there. Rain forests also play a role in the world’s climate, and are among the most fragile ecosystems. Most rain forests are located in the central region of Earth, near the equator, where temperatures typically range between 73° and 87°F (22.78°C and 30.56°C). Most receive some rain almost every day, averaging more than 100 inches (254 centimeters) a year, and sometimes twice that much. This rain is soaked up from the soil by the lush plants and enormous trees that then return it to the air via transpiration. Transpiration occurs in all plants as they naturally lose water vapor through their leaves. At least half of this water released by plants falls back down again onto the rain forest as rain. Since all tropical rain forests lie near the equator, daylight lasts 12 hours throughout the year, and the steady warmth of

The Amazonian rain forest, like all rain forests, is rich in plant and animal life. (Reproduced by permission of Conservation International. Photograph by R.A. Mettermeier.) U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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the land heats the air above it, causing it to rise and release its moisture (rain). Everything growing below is always green and flowers regularly.

LAYERS OF A RAIN FOREST The largest rain forest is the Amazon in South America, and it is typical of what is called a tropical rain forest, as opposed to a temperate rain forest which is located in a cooler climate. A tropical rain forest like the Amazon is a complex system that can be divided into different horizontal layers (like floors in an apartment building). As with an apartment building, there are different things going on at different levels. Forming the topmost layer are the tallest of the rain forest’s trees. Able to sometimes grow as high as a football field is long, these giants are scattered throughout the forest. The next level or layer is called the canopy, because it shades everything below. This green roof is created by a dense thicket of trees that stand between 60 and 150 feet (18.29 and 45.72 meters) high and whose branches and leaves are so close together that they form an umbrella over the rest of the forest below. These trees grow so tightly that rainfall reaches the ground only by running down the tree trunks or the stems of other plants. The canopy is alive with animal life as well, such as iguanas, tree frogs, monkeys, and bats. The next section is called the understory and includes smaller trees, ferns, vines and palms, and smaller bushes. Since the canopy traps and holds much of the heat and moisture, the understory is extremely hot and humid. It also does not have many flowering plants because of the lack of direct sunlight. The bottom level, or the forest floor, is covered with shade-loving mosses, herbs, and fungi, as well as dead plants and animals. The forest floor receives only 2 percent of the sunlight needed for photosynthesis (the process by which plants use light energy to make food). Dead material on the forest floor decomposes very quickly, thus providing nutrients to the soil and everything that grows in it. Masses of insects and the animals that feed on them, like the anteater, also live on the forest floor. Many of these animals, like bats, mice and rats, and porcupines, are nocturnal or active only at night. Some, like monkeys and apes, are busy during the day, while still others, like large cats and wild dogs, are most active at dusk and dawn.

PLANT LIFE IN THE RAIN FOREST No other biome (a particular type of large geographic region) on Earth can match the tropical rain forest for its diversity of plant and animal species. All of these species, however, have the same thing in common. As rain forest dwellers, they all have adapted in many ways to life in this 504


special ecosystem. One example of plant adaptation is a tree’s buttresses or “prop roots” that spread out above ground and help support the tree in the shallow, unstable soil of the forest floor. Other trees often grow stilt roots or “air roots” which grow out from its trunk as high up as 10 to 12 feet (3.0 to 3.66 meters) and help spread the weight of the tree over a wider area.

Rain Forest

Leaf structure is also highly adapted. Some leaves have what are called “drip tips”—a special shape that helps rainwater and condensation run off easily. This allows the leaf to breathe better. Other plants called epiphytes grow on or in trees and never actually touch the soil.

ANIMAL LIFE IN THE RAIN FOREST Animal life in the rain forest is equally adapted and strange and fascinating. The vast majority of all species are insects, although there are twice as many mammals and birds in its tropical environment as there are in temperate zones. South America is especially rich in birds, bats, and fishes. Rain forest species often have spectacular shapes, colors, or sizes. These include the huge-billed toucan, the brilliantly colored American butterfly with a wingspan of nearly 8 inches (20.32 centimeters), the huge, scary-looking fruit bat, the goliath frog that can weigh as much as 33 pounds (14.98 kilograms), and the bizarre-looking anteater. These are but a few of the thousands of exotic life forms that crawl, slither, hop, fly, and teem about in the tropical rain forest.

CONSEQUENCES OF RAIN FOREST DESTRUCTION Despite this great diversity and obvious abundance of life, tropical rain forests can be easily and severely damaged, and they take an extremely long time to recover. Their soil is usually ancient and poor in minerals that are mostly bound up in the incredible amounts of vegetation it has to support. Although rain forests are highly productive ecosystems, this productivity is based on the constant sunlight and steady rains, and not on rich, thick soil. When trees are cut down and burned in a typical practice called “slash and burn,” the landscape suffers severely. This is usually done to clear the land for farming and roadbuilding. Logging also clears large areas. When this occurs, the thin soil is exposed and the steady rains wash it away, leaving behind “wet deserts” and causing devastating floods. An area twice the size of Maine is cleared this way every year, and with it often go the unique plants and animals it supported. Rain forest destruction means not only plant and animal extinction because of habitat loss but possibly an increase in the “greenhouse effect” as fewer trees exist to absorb the increasing amounts of carbon dioxide U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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people produce. This causes Earth’s atmosphere to trap too much heat and could lead to global warming. [See also Biodivesity; Biome; Forest]

Reproduction, Asexual Asexual reproduction occurs when a new organism is produced from just one parent. It is a form of reproduction that does not involve the union of gametes (male and female sex cells), and therefore results in an offspring with the same genetic blueprint as the parent. As a result, this offspring is actually a clone of the parent.

BINARY FISSION Reproduction is the process by which new organisms are produced from existing ones. Asexual reproduction means reproduction without sex, or without male and female sex cells uniting. The simplest form of asexual reproduction is when a single-celled organism like a bacterium splits into two. Called binary fission, this process occurs when a one-celled organism duplicates its DNA and divides into two to form two new identical organisms.

BUDDING Other organisms like yeast, which is a microscopic, single-celled fungus, reproduce by budding or growing new cells that eventually separate from the parent. Some very simple animals, like hydras and corals, also reproduce this way. Like yeast, a new bud grows directly on the body of the parent. It eventually breaks off and establishes itself as a new, separate organism.

REGENERATION Reproduction by fragmentation or regeneration is related to budding. It occurs when a part of the parent’s body breaks off and grows into a complete, new organism. For example, if a starfish loses an arm, it can grow the arm back. Sometimes the severed arm itself can grow into an entire starfish. A flatworm can also be cut into two and each part can grow into a complete flatworm. Many plants also use a form of asexual reproduction called vegetative reproduction to duplicate themselves. This common method occurs when a plant splits in two, and each segment develops into a separate new plant. 506


RHIZOMES Other plants like strawberries and certain types of grass form runners, or rhizomes, that spread out from the parent plant to form plantlets at their ends. The plantlets become independent plants when they develop roots, and the connecting runners disintegrate. Spider plants are excellent examples of plants that use vegetative reproduction.

Reproduction, Asexual

A CLONE NAMED “DOLLY” Whether it is a plant or an animal, or whether reproduction occurs by binary fission, budding, or vegetative reproduction, all reproduction that is asexual involves one parent passing on a duplicate of all of its genes to its offspring. This means that the offspring produced by asexual reproduction are genetically identical copies, or clones, of the parent. Until recently, only certain animal cells could be regularly cloned or reproduced asexually. However, in 1996 a sheep named “Dolly” was cloned from a cell taken from an adult sheep. The achievement of such a difficult and complex feat (that is, asexually reproducing a mammal from a single cell taken from an adult rather than an embryo) raises the possibilities of asexual human reproduction. However, such possibilities raise many legal, moral, and ethical questions.

THE ADVANTAGES AND DISADVANTAGES OF ASEXUAL REPRODUCTION Humans aside, asexual reproduction is a fairly common occurrence in nature and it has its advantages and disadvantages. First of all, organisms that reproduce without sex do not have to expend any energy or resources toward the production of gametes (sex cells). Neither do they have to maintain an elaborate reproductive system or spend time and energy finding and fertilizing (or being fertilized by) a mate. Also, under certain conditions, a great number of individuals can be rapidly produced by asexual reproduction. For example, bacteria can divide every twenty seconds. Another great advantage is that even if there remains only a single individual, organisms capable of asexual reproduction can continue their species without a mate. Finally, when an organism is perfectly adapted to its habitat or particular environment, it will never change and always remain perfectly suited. However, habitats or environments often change. If an organism does not have the capacity to change, what had been an advantage may become a disadvantage. With asexual reproduction, there is no opportunity for genetic variety. Since offspring are clones of the original parU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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ent, there is no way for any new (and sometimes advantageous) traits to be introduced into the population. When all the individuals are identical, that means if one cannot adapt then none can, and the entire species may be endangered. [See also Buds and Budding; Cell Division; Mitosis]

Reproduction, Sexual Sexual reproduction is the creation of new individuals by the joining of the separate sex cells of two parents. Each offspring produced by sexual reproduction has a unique collection of genes and are well-equipped to adapt to change. Most animals reproduce sexually. Sexual reproduction describes the process in which the gametes (sex cells) of the two parents come together and form a fertilized egg cell called a zygote. The zygote then develops into a new individual. Since each parent contributes genetic information to the new and unique individual created, sexual reproduction is a far more complex phenomenon than asexual reproduction. (Asexual reproduction is when just one parent copies its genetic material creating a new, but identical, individual.) The gametes of each parent must have specific characteristics in order for sexual reproduction to work properly. Specifically, each set of gametes must have only half the total number of chromosomes that nonsex cells of that species contain. This ensures that when the nuclei (singular, nucleus; the cell’s control center) of the male sperm and female egg join together, instead of having twice the normal number of chromosomes, the fertilized egg will have exactly the right number. In the case of humans, that number is forty-six chromosomes (with twenty-three obtained from each parent). Sexual reproduction also applies to many plants—pollen is the equivalent of male sperm and it fertilizes the female sex cell of the flower, called an ovum. However, sexual reproduction in the animal world is the focus here. Organisms that reproduce sexually are usually fairly complex, and the individuals involved are usually either male or female, with the sex of each being separate. Males and females have different types of gonads or sex organs in which the gametes develop. The males produce sperm in gonads called testes, and the females produce egg, or ova, in gonads called the ovaries. Sperm are generally smaller than the egg and must swim through a liquid to get to the egg in order to fertilize it. 508


EXTERNAL AND INTERNAL FERTILIZATION

Reprodution, Sexual

Although there are some instances where one individual produces both sperm and eggs, these are the exception. Fertilization or the union of a male sex cell and a female sex cell can occur inside the female (internal fertilization) or outside (external fertilization). Both methods have advantages and disadvantages. Aquatic organisms (those that live in water) more commonly use external fertilization. The sperm and eggs are released into the water where they meet, and fertilization occurs. (Actually, the sex cells are scattered into the environment and the rest is left to chance.) To increase the odds for fertilization, gametes are released in large numbers. For example, oysters release millions of sperm and eggs into the water. Although there may be great waste in this process, there is little cost to the parents who do not have to care for the offspring produced. Animals that use external fertilization, like fish and frogs, have generally short lives and do not reproduce many times. Internal fertilization is practiced by most land animals, including humans. Since the sperm and egg are united inside the body of the female, both are protected, and there is a better chance the gametes will meet. Although there are still large numbers of sperm, fewer eggs are needed. The female’s body also provides the proper environmental conditions for the survival of the gametes (unlike the harsh conditions of external fertilization in which many eggs are often devoured by predators). For most animals, internal fertilization must take place within a certain time period. For example, in humans, there is only one twenty-fourhour period every twenty-eight days that the egg is able to be fertilized. Internal fertilization also requires special adaptations such as gamete delivery. This means that specialized sex organs (such as the male penis and female vagina) are used to place the sperm as close as possible to the egg inside the female’s body. After internal fertilization occurs, the zygote (fertilized egg cell) is either released from the body within some sort of protective covering, like a shell, or it remains within the body where it develops fully. Development inside a shell or body means that the zygote goes through a series of cell divisions. These begin to specialize and form individual body parts.

ADVANTAGES OF SEXUAL REPRODUCTION Probably the biggest advantage of sexual reproduction over asexual reproduction is that there is great genetic diversity since the new individual inherits genetic material from two different individuals. Because of this, populations (or the many individuals that make up a single species) U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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are better able to survive any environmental changes that may occur. While those less-adapted might die, other better-adapted individuals would survive and therefore reproduce passing on their better-adapted genes. [See also Egg; Embryo; Fertilization; Human Reproduction; Reproductive System; Sperm; Sex Chromosome; Sex Hormones; Sex-linked Traits; Zygote]

Reproductive System The reproductive system is made up of the organs and processes that enable an organism to produce offspring. Although reproduction is one of the characteristics of living things, and each member of the five kingdoms of life (Monerans, Protists, Fungi, Plants, and Animals) is able to reproduce, only the more complex organisms have actual “systems” for reproducing. Since every living thing has a limited life span, reproduction is the means by which organisms are able to continue their species. Among the simpler life forms, like bacteria, protozoa, and fungi, reproduction often takes place by processes like binary fission in which a cell simply divides in two. For plants and animals, which are more complicated forms of life, reproduction usually involves certain systems and processes. Whether they reproduce with a partner (sexually) or alone (asexually), plants and animals need a reproductive system that has certain essential parts and functions.

ASEXUAL REPRODUCTION Asexual reproduction involves a single parent and is less complicated than sexual reproduction (which involves two organisms). Many plants reproduce asexually by a process known as vegetative reproduction. This involves a plant putting out stems or roots that develop into entirely new plants. Grass is an example of a plant that grows and spreads this way. Although each plant is a separate individual from the parent, each is an exact duplicate or clone of the parent plant. Some animals can also reproduce asexually. For example, a cnidarian such as a hydra can achieve reproduction by budding off small parts of itself. Compared to sexual reproduction, reproduction that involves only one organism is not very complex.

SEXUAL REPRODUCTION IN PLANTS Sexual reproduction usually involves two parents. Even when it involves a single organism, it still requires two different types of gametes, 510


or sex cells. In sexual reproduction the male and female gametes fuse or unite to produce a zygote (a fertilized egg cell), which develops into a separate, new individual.

Reproductive System

Pollination. Many flowering plants that produce seeds are able to pollinate and fertilize themselves. Pollination is the process that transfers the flower’s male sex cells to its female parts. Fertilization is the actual uniting of the male and female sex cells. A single flower contains both male (called a stamen) and female (called a pistil) reproductive organs. Not all flowers have both parts, and those that do are called perfect flowers. Other flowering plants depend on animals, like bees and birds, to pollinate them. These animals often carry the male pollen to the female ovary of a plant. Animal-pollinated flowers are usually very showy or have a strong scent in order to attract these pollinators. Other flowers produce sweet nectar as a reward for animal pollinators. For plants, the end product of this process is the production of a seed, which is the beginning of another plant.

SEXUAL REPRODUCTION IN ANIMALS Sexual reproduction among animals is a more complex process since animals are more complicated organisms. Unlike plants, animals must first find a mate. They must then deliver the spermatozoa, or sperm (the male sex cells), to the ovum, or egg (the female sex cells). Sperm delivery is accomplished by internal or external fertilization.

External Fertilization. Many animals that live in a watery environment, like fish and frogs, rely on external fertilization. This process takes place outside of the animals’ bodies, as the female releases her eggs directly into the water while the male sprays them with his sperm. Animals that practice external fertilization usually have fairly short life spans and do not reproduce very often. They also produce large numbers of offspring who receive little or no care from the parents. Internal Fertilization. Other animals practice internal fertilization. In this case, the male gametes unite with the female gametes inside the female’s body. To accomplish this, both sexes need specialized sex organs as well as what are called accessory glands. The reproduction systems of mammals are probably the most complicated of animals that practice internal fertilization. Since the female carries the developing fetus inside her body, her reproductive system must accommodate itself to nourishing and eventually giving birth to a new individual. Animals that practice internal fertilization usually live a long time and reproduce more often, although they produce relatively few young. Their young usually need U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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some degree of parental care after birth. The reproductive systems of mammals, as well as of all living things, are the products of evolution, meaning that they are best-suited and best-adapted to the reproductive needs of a certain species. [See also Egg; Embryo; Fertilization; Human Reproduction; Reproduction, Sexual; Sperm; Sex Chromosome; Sex Hormones; Sex-linked Traits; Zygote]

Reptile

Fox snakes, like this one, are good representatives of reptiles. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

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A reptile is a cold-blooded vertebrate (animal with a backbone) with dry, scaly skin that lays sealed eggs. Dinosaurs once dominated this class, which is now represented by turtles, snakes, lizards, crocodiles, and alligators. Reptiles have endoskeletons (internal skeletons) and are mainly carnivorous (meat-eaters). They do not move especially fast and their name “reptile” means “creeper.” All reptiles have a tough skin made of waterproof scales that resists drying out. In this and many other ways, they are entirely different from amphibians from whom they evolved and with whom they are often con-


fused. Reptiles do not go through any metamorphosis (a series of distinct changes in which an organism passes as it develops from an egg to an adult) and breathe air through lungs their entire lives. They do, however, lay eggs like amphibians—but even these eggs are substantially different. Reptiles always live on land, although they are ectothermic or coldblooded like amphibians. This does not mean that they are cold, but rather that their body temperature reflects that of their environment. Unlike warm-blooded vertebrates that use food to keep their internal body temperature constant despite the outside temperature, reptiles have a body temperature that fluctuates with their environment. This can be an advantage since reptiles require less energy and therefore less food, and as a result are able to survive in a habitat where there is little food. As vertebrates, reptiles have an internal skeleton. Turtles are not an exception to this, but they may appear to be. Although they have hard, bony shells above and below their bodies and into which they can withdraw their head and legs for protection, they still have an endoskeleton.

Reptile

All reptiles reproduce sexually through the joining of male sperm and female eggs. It was the ability of reptiles to produce a sealed egg, or one with a strong membrane, that allowed them to evolve beyond amphibians and remain on land, freeing them from needing water to reproduce or hatch in. The reptilian egg is fertilized internally or within the body of the female, and is called an amniotic egg because it contains the necessary water and food for the embryo to develop and grow before hatching. (This is different than amphibian eggs, which are fertilized than released by the female into the water where they will hatch.) These leathery eggs are usually buried in sand or soil where they hatch, releasing live, miniature versions of their parents. Although reptiles range in size from a tiny lizard, like the gecko, to a 20-foot (6.1-meter) alligator, they all have skin that is dry to the touch. Reptile skin is made of scales created by a waterproof substance called keratin. The outer layers of this skin are regularly shed by most reptiles as they grow in size. Reptiles can vary from the slow-moving turtle whose body is encased in a shell and has a jaw like a bird’s beak, to the fiercely aggressive crocodile. The tiny gecko lizard can walk on ceilings, where as the basilisk lizard can run on two legs using its tail for balance and can even run across water. A chameleon can change its color to blend in perfectly with its surroundings and catch its prey with its tongue. Snakes can unhinge their jaws and swallow whole a prey that is larger than their own head. They also sniff the air with their tongue, which carries the chemical particles it catches in the air to a specialized detector in the roof of their U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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mouth. Some snakes have hollow teeth to inject venom or poison into the prey they bite, while others coil around their prey and squeeze until it suffocates. [See also Dinosaur; Vertebrates]

Respiration Respiration, or cellular respiration, is a series of chemical reactions in which food is broken down to release energy. In order to live, all living things must be able to extract needed energy from the organic compounds they take in. The chemical process of respiration may be either aerobic or anaerobic. The living cells of every organism need a constant supply of energy, or fuel, which they use to stay alive. Therefore, all living things obtain their energy from the food they make themselves (as plants are able to use light and chemicals to make their own food) or which they capture from their environment (by eating plants or other animals). In an animal, once this food is broken down into simpler forms by its digestive system, it is absorbed into the bloodstream. There the circulatory system (a network that carries blood throughout an animal’s body) transports these small bundles of nutrients to each cell. It is at this level that the process known as cellular respiration takes place. During this process, cells convert the stored energy into usable energy. To do this, these bundles of food, or nutrients, must have their bonds broken so that the chemicals necessary to sustain life are released. There are two types of cellular respiration. One involves oxygen and is called aerobic respiration, and the other does not need oxygen and is known as anaerobic respiration. Most cells use aerobic respiration. It is by far the most common and efficient method of obtaining energy. During aerobic respiration, nutrients reach the cells in the form of glucose (a form of sugar). The cell breaks down the glucose by combining it with oxygen, thereby releasing usable energy. This occurs in four separate stages and is called oxidation. The process of oxidation (the combining of glucose and oxygen) releases a large amount of energy to the cell. Oxidation also gives off carbon dioxide (a major atmospheric gases) and water as waste products. Anaerobic respiration, which does not use oxygen to break down glucose, is less common and does not result in the release of as much energy as aerobic respiration. It is performed mainly by bacteria and some fungi, some of which use other inorganic compounds like nitrates or sulfates in 514


place of oxygen. Fermentation is a type of anaerobic respiration since it breaks down glucose without using oxygen. The term respiration is also used to describe the process of breathing, in which oxygen is taken into an animal’s body. This type of respiration is called external respiration.

Respiratory System

[See also Bacteria; Blood; Diffusion; Fermentation; Metabolism; Respiratory System]

Respiratory System The respiratory system is composed of those organs and processes that allow an animal to take oxygen into its body and expel carbon dioxide. All living things require oxygen to survive, and although respiratory systems may vary in size and the way they function, they share many basic features and operate on the same principles. All animals need oxygen to survive, because oxygen is the fuel they use to convert their food into energy. As with any fuel that is consumed, there is always a byproduct given off, and in the case of animals, this waste product is carbon dioxide. Since all animals need to constantly take in oxygen and eliminate carbon dioxide, they need some system to perform this gas exchange, which is called respiration. Although a variety of systems and methods accomplish this, all operate on the same basic principle called diffusion.

DIFFUSION VITAL TO RESPIRATION Diffusion can be described as the movement or spreading out of a substance from an area in which it is highly concentrated to an area of its lowest concentration. Diffusion takes place at the level of molecules, and since molecules are constantly in motion, they have a natural tendency to mix randomly with one another. Although it is not known exactly why molecules behave this way, they have a natural tendency to move from where they are all together to where there are the fewest of them, in an effort to spread themselves out more evenly. Although any given single molecule may not always behave this way, the net or overall movement of a group of molecules will always do so. This net movement is called diffusion. An everyday example of diffusion is the way in which tobacco smoke or strong perfume will spread throughout the still air of a closed room. The respiratory system of an animal always carries out diffusion across a respiratory surface. There is always some sort of membrane across which the gases (oxygen and carbon dioxide) pass in and out of a body. When there is more oxygen in the environment outside a membrane than U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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there is inside, oxygen will automatically drift through the membrane to equalize the pressure on both sides. Although this may sound like osmosis (which could be described as diffusion across a membrane), osmosis only involves the passage of a substance dissolved in a liquid solution. In respiration, however, gases are exchanged.

INTEGUMENTARY EXCHANGE In the simplest animal respiratory system, gases are exchanged directly through the body tissues themselves. This is called integumentary exchange, since an animal’s integument is its protective outer covering or skin. The one-celled organisms that practice this form of respiration are too simple to have any skin, and instead have a membrane that allows oxygen and carbon dioxide to diffuse right through it and reach every part of its “body.” More complicated animals, like earthworms, also perform this type of “skin breathing,” but they also use a simple type of circulatory system to move the gases from the inside of their body to their skin. Studying an earthworm also reveals a major characteristic of all respiratory systems—they must always be wet. The slimy, mucus-covered skin of an earthworm demonstrates that diffusion will only take place if the respiratory surface or membrane is always kept moist. If an earthworm dries out, it will suffocate.

TRACHEAL SYSTEM Skin breathing may be fine for very small organisms, but when the outer area of an animal cannot provide a large enough surface area or is too thick or hardened to allow a good gas exchange, then the animal needs some type of specialized respiratory organs. One simple form of such specialized organs is the tracheal system found in insects. In this unique system, oxygen and carbon dioxide are exchanged through diffusion by means of a network of small, stiff tubes called tracheae that extend into the insect’s body and are small enough to supply individual cells. Insects have no need of lungs or any connection to a circulatory system.

GILLS

Opposite: A labeled diagram of the human respiratory system. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

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The gills that fish use to breathe are the next most sophisticated system, and they are designed specifically to work in water. A gill is a collection of thin flaps (called lamellae) that are able to obtain oxygen dissolved in water. As water moves over the lamellae’s thin membrane, dissolved oxygen diffuses into the fish’s blood, while carbon dioxide diffuses out through the gills and into the water. Since water contains only five percent of the oxygen that air does, gills must be especially efficient. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

Upper Respiratory System

Sinuses Tongue

Air entering the nostrils is filtered of dust, germs, and foreign particles by fine hairs and mucus within the nose. The air is warmed by heat from the network of blood vessels that lines the interior of the nasal cavity.

Pharynx Epiglottis Larynx Trachea Esophagus Bronchioles Primary bronchus Space occupied by heart Secondary bronchus Right lung Left lung

Alveolus

Smooth muscle

Bronchiole Vein Artery

Bronchiole Alveolus

Capillary Red blood cells Capillaries (in cross section)

Alveoli

The bronchioles lead to clusters of tiny air sacs called alveoli. The wall of an alveolus consists of a single layer of cells and elastic fibers that allow it to expand and contract during breathing.

Epithelial cell of the adjacent alveolus Each alveolus is covered with capillaries that bring deoxygenated blood from the rest of the body via the right side of the heart.


Epithelial cell of the wall of the alveolus

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They only will work, however, if water keeps moving over their surface, and a fish needs to keep its mouth open as its swims to keep the water moving over the gills. When a fish is not moving, it closes its gills, takes a mouthful of water, and forces it out its gills. Fish cannot breathe out of water because the filaments that contain their lamellae collapse when they are not supported by water. The gills of most fish are located behind their head.

THE HUMAN RESPIRATORY SYSTEM Lungs are the respiratory organs used by mammals, birds, and reptiles, as well as some amphibians. Lungs exchange gases, since it is the job of the circulatory system to collect and distribute gases throughout the organism. The human lungs are located in the chest cavity, although the human respiratory system actually begins at the nose and mouth where air is inhaled and exhaled. The mouth and nose are connected to a common tube at the back of the throat called a pharynx. Air then passes through the larynx, also called the voice box, into a branching system that resembles an upside-down tree. The larynx flows into the trachea (the tree trunk), which divides into two large limbs called the right and left bronchi. Each of these branch off into multiple smaller bronchi, which continue dwindling down into smaller and smaller tubes that finally end in terminal bronchioles. At the end of these bronchioles, like the leaves of tree, are clusters of moist air sacs called alveoli where the actual exchange of gases takes place. Tiny, thin-walled blood vessels called capillaries create vast networks surrounding the alveoli. It is across these thin walls that oxygen is passed into the blood and carbon dioxide out of the blood by diffusion. Humans have a total of 300,000,000 alveoli in both lungs. Air is actually forced into the lungs by a large muscle beneath the lungs called a diaphragm. When its muscles contract and pull down, the ribs above are lifted upward and outward, and air rushes into the elastic lungs. When air is automatically sucked into the lungs as they expand (inspiration), it is called negative pressure breathing. Expiration occurs when the diaphragm muscles relax, allowing the lungs to go back to their retracted state. Air containing carbon dioxide is then forced out of the lungs. Breathing is controlled by a brain command that sends a signal every few seconds. The average adult human takes between twelve and fifteen breaths a minute. Because the human respiratory system brings in substances from the outside environment, there are safeguards to fight infectious agents that may enter with the oxygen. The normal human lung is sterile, meaning that there are no bacteria or viruses present. The first line of defense includes hair in the nostrils that filters large particles. The epiglottis between the pharynx and larynx acts as a trap door and prevents food and 518


other swallowed substances from entering the larynx and then the trachea. If it does, we cough involuntarily and say that “something went down the wrong pipe.” The acts of sneezing and coughing are usually both started by irritants in the respiratory system, and work to forcibly expel them. Finally, mucus exists throughout the system and serves to trap dust and infectious organisms. Cells called macrophages line the respiratory tract and engulf and kill anything they consider an invader.

Rh Factor

Plants can be said to “breathe” since they exchange gases during photosynthesis (which is actually the reverse of respiration since it takes in carbon dioxide and gives off oxygen in the process of making food). However, they do not have any respiratory system approximating that of an animal, since plants exchange gases through simple openings or pores in their leaves called stomata. [See also Blood; Respiration]

Rh Factor Rh factor describes a certain blood-type marker that each human blood type either has (Rh-positive) or does not have (Rh-negative). As a further refinement of determining exactly what blood type a person is, the Rh factor is important not only for proper blood transfusions but also during pregnancy. In 1909 the Austrian physician, Karl Landsteiner (1868–1943), explained why blood cells agglutinate, or clump, together when blood from different people were mixed. He showed that all human blood can be divided into four main groups, some of which are compatible and many which are not. He named these four groups blood type A, type B, type AB, and type O according to the particular type of antigen they had on their red blood cells. (An antigen is a protein that works as a type of chemical identification tag.) People from the same blood type could exchange their blood, but those with different blood types had to follow certain exact rules if they were to successfully receive a blood transfusion. Overall, Landsteiner’s discovery made blood transfusions fairly safe and saved many a life. However, despite his breakthrough in understanding blood types, there continued to be enough unexplained accidents during blood transfusions to send Landsteiner searching for the possibility that some other factor might exist that was yet unknown. By 1940, Landsteiner and his associates discovered a different factor in the blood of rhesus monkeys, and he later found that this factor also existed in huU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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RNA (Ribonucleic Acid)

man blood. As with the four major human blood types, each of which had (or did not have) a certain, specific type of antigen, certain human blood also was found to have an antigen that he named antigen D. If blood had this antigen, it was therefore Rh-positive. Those without the antigen were considered Rh-negative. The symbol Rh was used as an abbreviation for Rhesus. Studies have since shown that about 85 percent of the population are Rh-positive and 15 percent are Rh-negative. Blood type, like a person’s Rh factor, is inherited. The Rh factor only becomes important when a person receives a blood transfusion or when a woman is pregnant. If a person who is Rh-negative receives blood from an Rh-positive person, then the Rh-negative blood will produce antibodies against the Rh-positive factor. Antibodies are proteins in the plasma (the liquid part of the blood) that destroy foreign substances. Therefore, the next time that person’s system encounters Rh-positive blood, it will cause what is called a hemolytic reaction and make the blood agglutinate together. This can cause serious injury or death to the patient. The Rh factor is also critical during pregnancy. If an Rh-negative woman gives birth to an Rh-positive child whose father was RH-positive, there is a risk that her system will become sensitized to her baby’s Rh factor and will begin to produce Rh-positive antibodies. While her first baby usually will not be affected, her next child may be in danger since the mother’s system may send antibodies into the child’s bloodstream while she is still carrying the fetus. This could threaten the child’s life. This condition is known as erythroblastosis fetalis, or hemolytic disease of the newborn (HDN). It can cause severe anemia (lack of hemoglobin or red blood cells), brain damage, or even death. Before the Rh factor became known, it was a major cause of stillbirth pregnancies (when the baby is born dead). Today, this situation can be prevented. At twenty-eight weeks into her pregnancy and immediately after birth, the mother can be given an injection that destroys any Rh-positive antibodies she has in her system. Also, the baby can be given a blood transfusion just before birth while it is still in the womb, or immediately after it is born. A check of blood type and Rh factor are now routine in all blood transfusions and pregnancies. [See also Antibody and Antigen; Blood; Blood Types]

RNA (Ribonucleic Acid) RNA, or ribonucleic acid, is an organic substance in living cells that plays an essential role in the construction of proteins and, therefore, in the trans520


fer of genetic information. If deoxyribonucleic acid (DNA) is the storehouse of information for cells, RNA is the delivery system of that information. RNA copies the coded DNA instructions and carries them to the part of the cell that forms proteins. Like DNA, RNA is a nucleic acid (so called because it is found in the cell nucleus). Because RNA is involved with making proteins, and because living tissue is made up of proteins, RNA is a critically important substance. Since proteins cannot reproduce themselves, DNA and RNA act together and tell the cells how and which protein to make. Each protein is responsible for a particular characteristic, and some proteins help nourish the cell itself while others determine how tall a person will be. All of this and much more information is encoded in genes in the DNA. However, since DNA is found in the nucleus of the cell and protein is made outside the nucleus in the cytoplasm (the jelly-like fluid that circulates inside the cell and surrounds its nucleus), information about a certain protein must be transferred from the nucleus where it is stored to


RNA (Ribonucleic Acid)

A computer-generated model of RNA. RNA is essential for the production of proteins and the transfer of genetic material. (Reproduced by the National Audubon Society Collection/Photo Researchers, Inc. Photograph by Ken Eward.)

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outside of the cell where it is manufactured. It is RNA that performs this important function. There are actually two types of RNA that handle the information necessary to make proteins. The first is called messenger RNA (mRNA) because it copies the DNA code and carries it out of the nucleus to the ribosomes in the cell (the “factory” where proteins are actually made). This process has been compared to the photocopying of a cookbook. Just as we do not tear out the page of a booklet that contains a recipe, but instead photocopy it for use elsewhere, so the genetic information contained in the DNA is copied by the RNA. This occurs when the DNA double helix or “spiral staircase” splits down the middle, or unzips itself, allowing a single strand of it to act as a template or pattern for RNA to copy. When this has happened, in a process called “transcription,” the single RNA strand leaves the nucleus and enters the cytoplasm. The two DNA strands then reunite. After the mRNA has delivered the instructions to the cell’s ribosomes (its protein factories), its code is read by another type of RNA called transfer RNA (tRNA). Transfer RNA also carries amino acids (the building blocks of protein molecules) to the ribosomes, which need them to make proteins. In this process, called “translation,” the tRNA molecules read the instructions on the mRNA, and whenever the mRNA needs a particular amino acid, the corresponding tRNA molecule drags the correct amino acid into the protein factory. This process is going on constantly in our bodies, since every second of every minute, our cells are using what might be called gene recipes to make the specific proteins they need. [See also Amino Acid; DNA; Gene; Nucleic Acid; Protein]

Root System The root system of a plant is one of its three main organs. (The others are its stems and leaves.) Roots grow underground and perform several functions: they anchor the plant firmly in the soil, they absorb and transport water and nutrients throughout the plant, and they may also store food. All seed-producing plants and most spore-producing plants have roots. The exceptions are bryophytes (a type of nonvascular plant, such as mosses, liverworts, and hornworts that grow only in moist environments), since they do not have true roots. Roots are essential to a plant for many reasons. Without a root system that penetrates the soil and spreads below ground, a plant would never be able to grow toward the life-giving Sun. 522


The beginnings of a plant root are already contained in the embryo found in a seed. When conditions are right and the seed begins to take in water and burst its seed coat (germination), a root or radicle breaks through the coat and begins to grow downward into the soil. Root tissue already has a specialized function, and since it is sensitive to the pull of gravity, it responds appropriately and grows downward. In this way it demonstrates positive geotropism (by growing toward the external stimulus of gravity). As the radicle becomes a mature primary root, it eventually develops secondary roots which, in turn, branch out even more in all directions. These are called fibrous roots. Grass is an example of a plant with a fibrous root system. Some plants also produce a taproot that is Protoxylem pole much larger than secondary roots and penetrates deeper into the soil. A tapProtophloem root is a primary food storage root pole and may be fleshy like a carrot or Root hair woody like a tree root. If the taproot of certain plants is broken, they will die. Other roots that form slightly Cortex above ground level and go into the soil are called adventitious roots. Pericycle Roots that emerge from the base of a corn stalk are adventitious roots or Endodermis Mature xylem element prop roots and serve to support the Immature xylem element plant. Plants that have aerial roots, Epidermis like certain orchids and mistletoe, are Mature part of sieve tube Immature part of sieve tube not in contact with the ground at all but absorb water from aboveground sources (like other plants). Other Protoderm parts of a plant may grow underGround ground, like tubers (potatoes), bulbs meristem (onions), and corms (gladioli), but Procambium these are not really roots but rather only modified stems. Apical meristem A root is made up of three types of tissue: the epidermis or surface Root cap layer; the cortex or root wall where water and food are stored; and the vascular core in its center, which carries food and water into the stem. The U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

Root System

A labeled diagram of a root. The root system is vital to a plant’s health. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

Region of maturation

Region of elongation

Region of cell division

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epidermis is a type of outer layer or protective skin. Roots also have hairs that are modified epidermis cells. These long, tubelike hairs give the root a larger surface area for absorbing substances from the soil. It is the root hairs that perform most of the absorption of water and minerals. The core (also called the stele) is made up of vessels and tubes that transport food and water up and down the roots and stem. The tip or apex of the root is its growing point, and it is protected by a thimble-shaped cap as it penetrates into the ground. Roots help plants adapt to yearly cold seasons. In perennial plants (plants that live for many years), the entire plant survives the winter by using food stored in the roots and stems. In biennial plants, such as carrots, the leaves die off the first winter and a new plant develops from the root the following spring. After the second year, the plant flowers, forms seeds, and dies. In annual plants, the entire plant, including the root system, dies after flowering and producing seeds. The roots of many plants are edible. For example, important root crops include beets, carrots, cassava, parsnips, sweet potatoes, and turnips. Plant roots also benefit the environment by preventing soil erosion by wind and water. [See also Plant Anatomy; Plants; Seed; Tissue]

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S Seed A seed contains the embryo plant from which a new plant will develop. It also contains food storage tissue and a protective coat. The creation of a seed depends on pollination (the transfer of pollen containing male sex cells to the pistil containing female sex cells) and fertilization (the process in which an egg cell and a sperm cell unite to form one cell). Seeds are produced by more than 250,000 different types of plants. Seeds also vary greatly in size and the methods by which they are dispersed. The first seeds are thought to have developed about 360,000,000 years ago, marking a major step in the evolution of land plants. These early seeds were gymnosperms or “naked seeds” since they were totally exposed. Over time, however, seeds came to develop a coating (called angiosperms) that protected the seed. This coating proved so successful that angiosperms are now the dominant type of plant. A seed is formed sexually when a grain of pollen containing male sperm fertilizes an ovule containing the female egg. This fused egg cell begins to divide rapidly and grows a structure called the embryo. This part of the seed will become the new plant. It contains, in miniature, the radicle or embryonic root that will grow into a primary root after germination (sprouting). It also has the beginnings of a shoot and early leaves, called cotyledon. After sprouting, the seed lives off its endosperm or stored food until it develops roots and leaves to make its own food. Seed dispersal, or the methods plants have developed to scatter their seeds away from the parent plant, are extremely varied and interesting. In the simplest cases, seeds ripen on the plant, fall to the ground, germinate, and grow into a new plant. While this may seem like a good U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Seed

method, it soon creates an overcrowded situation where seedlings are competing for the same resources as the parent plant. Plants, therefore, have developed strategies and methods to disperse their seeds some distance from the parent plant. One method is the creation of fruit that is the ripened ovary of the plant and that surrounds the seed itself. Fruit can be hard and dry or fleshy and soft. Many plants enclose their seeds in tasty, attractive fruit to encourage an animal to eat it. Once eaten, the seeds pass through the animal’s digestive tract and are deposited elsewhere in the animal’s waste—often far away from the parent plant. The fruit of berry seeds, for example, are typically small, thin-skinned, and brightly colored with no smell. It is the fruit’s bright color that attracts birds that then eat the fruit. Fruits that appeal to mammals are less colorful but often have a pleasant smell. Others have their nutritious seed enclosed in a hard shell (like a nut) that a rodent will chew through and eat. Sometimes rodents bury nuts to eat later. The ones that are not dug up will germinate and grow into a new plant. Some fruits are meant to be eaten, but contain toxins that make an animal regurgitate (vomit) them elsewhere allowing the fruit’s seeds to be dispersed in a new location. Animals also disperse seeds by carrying both fruit and seeds on their bodies. Certain plants produce seeds with sticky surfaces, while others have barbs or hooks that become attached to an animal’s fur or feathers and are then taken away to fall elsewhere to the ground. Some plant species develop extremely light seeds that are easily blown great distances by the wind. The fluffy coverings of dandelion or cottonwood seeds are good examples of wind dispersal. Other plants use the wind more mechanically and have developed seeds with wings (like the sycamore tree) or appendages that act as parachutes. Tumbleweed is an example of an entire plant becoming loose and blowing away with the wind. Other plants develop seeds that can float on water and even have a waterproof coat, such as the coconut. Other mechanical means of dispersal are used by plants that produce capsules that explode when ripe, resulting in the scattering of seeds. Many such plants, like the popular impatiens, grow little curled capsules that build up tension as they develop. When they finally dry out in early fall, they split and abruptly uncurl, flinging their seeds as far as possible from the parent plant. Many seeds require a period of dormancy or inactivity before they are able to germinate. The plant usually assures that such a seed will not sprout as soon as it ripens by giving it a hard coat or by including a growth-inhibiting hormone in the seed. Some seeds even require a period of freezing temperatures before they will germinate. Seeds are usually

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very dry and therefore need a great deal of water before they can sprout. By making sure that a seed stays dormant for a certain time, the plant ensures that it will only germinate under the best possible conditions, and not prematurely on a warm fall day. Some seeds can remain dormant for weeks, others months, and still others for years. Seeds are a major source of food for both animals and human beings. Many species, such as cereal grains (corn, nuts, oats, rice), are a staple for people throughout the world. Many oils are obtained from seeds (sunflower, corn, peanut, and linseed), and are also used to make other products. Two of the most popular seeds consumed today—the cocoa bean and the coffee bean—give us chocolate and coffee. [See also Plant Anatomy; Plant Reproduction; Plants; Reproduction, Sexual]


Seed

A close-up of grass seed on the tips of growing grass. (Reproduced by permission of The Stock Market. Photograph by Roy Morsch.)

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Sense Organ

Sense Organ A sense organ is any collection of cells in an organism that responds to information about certain changes in the organism’s internal or external environment. Each sense organ reacts to a particular type of stimulus. The sense organ converts the stimulus into a nerve impulse that is sent to the organism’s brain to be processed and identified. Human sense organs are the eyes, ears, nose, mouth, and skin—each having its own particular type of receptors. Sense organs are essential if an animal is to obtain any information about its surroundings. Without them, the individual organism would probably not survive long. Therefore, sense organs are essential to survival since they are the key link to the external, or outside, world. The type of sense organs an animal has greatly determines how it will perceive its environment. Humans see a world rich in colors. Animals who hunt at night see things only as shades of gray. Dogs recognize others mainly by their smell. Insects see an ultraviolet world all their own.

SENSORY RECEPTORS Although there is great variety in how different animals experience the world through their senses, they all have sensing systems that operate by using sensory receptors. Thus, all sense organs, no matter what they detect, have and need sensory receptors. A receptor is a group, or cluster, of nerve cells that react to a particular stimulus and receive information. Sensory receptors make up the most familiar sense organs, such as the ears and eyes. These receptors can be classified according to the type of energy or stimulus to which they respond. Chemoreceptors (for taste and smell) respond to certain chemical compounds. Mechanoreceptors (for touch) respond to mechanical energy. Auditory receptors (for hearing) respond to sound wave vibrations, although some consider them to be a form of mechanoreceptor since the pressure waves of sound are a real, physical force. Photoreceptors (for sight) respond to light energy. These four types of receptors correspond to the five human senses (taste and smell both use chemoreceptors) and, therefore, to our five major sense organs. However, there are other types of receptors that are used by organisms that are not necessarily linked to a particular sense organ. For example, certain insects such as ticks, and animals such as snakes, possess thermoreceptors with which they respond to temperature. A tick is aware of your presence and jumps on you by sensing your body heat. 528


Certain fish also have electroreceptors in their skin that allows them to detect electrical energy given off by other objects. Another important type of receptor is known as the proprioceptors. These are sensory receptors located inside an animal’s muscles and joints. Proprioceptors allow the animal to be aware of its entire body position, as well as to keep its balance. It is because of this sense that a person is able to dress in complete darkness.

Sex Chromosomes

The human body has two senses that employ chemoreceptors—taste (gustation) and smell (olfaction). The first uses taste buds (mainly in the tongue) as the receptors for dissolved chemicals. The second uses olfactory epithelium (“smelling skin”) in the nasal cavities to detect airborne chemicals. The sense of touch is located in the skin. Here, mechanoreceptors are activated when they change shape by being pushed or pulled. Auditory, or hearing, receptors are located in the cochlea, deep within the ear. They detect or respond to pressure waves (since sound is actually a vibration of the air). Finally, the rods and cones in the retina of the eyes are the photoreceptors that enable people to see. Humans would have none of their five senses if they did not also have a vast, branching network of nerves. These nerves take all of the coded messages to the brain, which translates or interprets them. The brain tells a person exactly what he or she is sensing. As a result, people do not really see with the sense organs called the eyes, for it is the brain that interprets the signals the eyes are sending. The brain tells a person that he or she is seeing a beautiful rainbow and not a dangerous fire. The role of the sense organs then is only to gather the information. Understanding that information is left to the brain. [See also Brain; Hearing; Integumentary System; Organ; Sight; Smell; Taste]

Sex Chromosomes Sex chromosomes are the chromosomes within the nucleus of a cell that determine the sex of an organism. Chromosomes pass on genetic material from one generation to another, and different species have different numbers of chromosomes. Humans have forty-six chromosomes arranged in twenty-three pairs, but only a single pair decides what sex (male or female) an individual will be. These are the sex chromosomes. In all animals and some plants, each cell contains sets of chromosomes that are arranged in matched pairs and are thus called “homoloU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Sex Chromosomes

A scanning electron micrograph of human sex chromosomes. The XY represents the male chromosome and the XX represent the female chromosome. (Reproduced by permission of Photo Researchers, Inc.)

gous” chromosomes. Half of these chromosomes came from the organism’s male parent and half came from the female parent. This explains why their offspring, or the unique organism produced by them, is similar to but not exactly like either parent. In humans, the sperm of the male contributes twenty-three chromosomes and the egg of the female also contributes twenty-three—totalling forty-six chromosomes for their offspring. When these two half-sets of chromosomes combine, every chromosome seeks and joins with its matching chromosome, each of which contains different versions (traits) of the same genes. For example, the gene for height that is contributed by the father will match up next to the mother’s height gene, and so on. Of these twenty-three pairs of chromosomes that together form a unique individual (with a complete set of forty-six chromosomes), twenty-two pairs determine or affect every characteristic of that individual except its sex. These twenty-two pairs are called “autosomes.” It is the remaining single pair of chromosomes that determines the sex of the individual, and these are called sex chromosomes. In females, the sex chromosomes are designated as XX, and in males the sex chromosomes are XY. In human beings, the forty-six chromosomes in each body cell are arranged in pairs. When the chromosome pairs of a female are examined under a powerful microscope, they are seen to all have the same general appearance, resembling a slightly crooked X-shape. In males however, all but one pair of chromosomes have this same X shape, with only the twenty-third pair being noticeably different. In both men and women, this twenty-third pair is the sex chromosomes. However where for women, the twenty-third chromosome pair appears like all others as a crooked X, for a man, only one chromosome in the pair looks like an X. The other is considerably smaller and rather stumpy-looking and is called a Y chromosome. This is known as the XY chromosome pair and determines the sex of a child. At the mo-

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ment when a single sperm fertilizes an egg, the sex of a child is determined. This is because each individual sperm carries only twenty-three chromosomes (since it is ready to link up with the egg’s other twentythree chromosomes), one of which is either an X or a Y (having split its forty-six in half). Since the sex chromosomes of the egg are always X (having two X chromosomes), if the egg is fertilized by an X-containing male sperm, the resulting egg will develop into a female (XX). If it is fertilized by a Y-containing sperm, then a male will be born (XY). Although the new child will inherit traits from both its parents, it is the father who determines its sex.

Sex Hormones

[See also Chromosome; Nucleus; Human Reproduction; Reproduction, Sexual]

Sex Hormones Sex hormones are certain types of chemical substances that prepare an animal’s body for reproduction. They are secreted by a gland or an organ. Sex hormones determine both male and female sexual characteristics and can also influence a person’s behavior. Hormones are chemical messengers in both animals and plants. In animals, they are produced by glands. Hormones travel through the blood to target tissues where the hormones act as chemical regulators. Hormones influence reproduction, growth, and overall bodily balance, among other things. Sex hormones are important to all animals, but are especially so to vertebrates (animals with a backbone). In humans, the sex hormones— androgens, estrogens, and progestins—are essential for those body processes that are related to reproduction. Although scientists have only begun to understand sex hormones in the last fifty years, their effects have long been known and recognized. Farmers have known for millennia that castration (or removing a male animal’s testes; the sperm-making organs) not only makes these animals more manageable but improves the quality of their meat. In Renaissance times (fourteenth to sixteenth century), boys in the church choir were sometimes castrated to keep their beautiful high voices from changing.

THE PITUITARY GLAND In humans, the glands that produce hormones involved with sex and reproduction are the pituitary gland, the gonads, and the adrenal glands. The pituitary gland is a small organ below or at the base of the brain. It U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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is about the size of a pea, yet it has a powerful effect on several other glands in the body. The pituitary gland releases eight different hormones. These stimulate other parts or organs in the body, including the gonads. For example, the pituitary gland’s follicle-stimulating hormone (FSH) promotes the production of sex cells in males (sperm) and the maturing of sex cells in women (ovaries). The prolactin produced by the pituitary gland gets the female body ready to produce milk when needed.

THE GONADS The gonads consist of the testes in males and the ovaries in females. These usually come in pairs. The ovaries produce egg cells or ova as well as the hormones known as the estrogens and the progestins. These hormones are usually produced in regular cycles and control female sexual development, thus triggering female secondary sexual characteristics like breasts. The hormones also prepare the body once pregnancy occurs and help ensure that it can sustain the developing fetus (the human embryo).

THE ADRENAL The main estrogens are estradiol, estrone, and estriol. The main progestin is called progesterone. There are also several kinds of androgens (or male sex hormones) produced by the gonads, but the primary one is testosterone. This hormone stimulates hair growth and the lowering of the voices when a male goes through puberty. The adrenal glands also come in pairs in humans, and their name comes from being located so close to the kidneys. Only the outer part, called the cortex, secretes sex hormones, since it is made up of tissue that is similar to that found in the ovaries and testes.

THE IMPORTANCE OF SEX HORMONES Sex hormones are important throughout life. They come into play at times other than sexual maturity. These hormones are working from the time of early development to influence the developing fetus. For example, testosterone begins to work before a baby is born by stimulating the growth of male genitals. Like all sex hormones, it also influences the development of other organs that are not directly related to reproduction. Later in life, the production of female and male sex hormones is gradually reduced with aging. Major changes can occur as a woman no longer produces eggs that can be fertilized. Sex hormones also influence behavior. This is most noticeable in animals. Since sexual reproduction is mostly a joint, or cooperative, effort in which each partner supplies half of the needed ingredients (egg and 532


sperm), many animals go through cycles during which they are receptive to mating. With the exception of humans and a few other mammals, most animals go through only one or two estrous cycles (called “heat”) during which their hormones motivate them to reproduce.

Sex-linked Traits

Sex hormones also influence animals to do all the necessary things to care for their young. These include nest-building, nursing, and feeding. In males especially, sex hormones account for much of the aggressive behavior displayed during the mating season. Studies of rhesus monkeys have shown that those with the highest levels of testosterone in their blood are the ones doing all of the threatening, chasing, and fighting. These monkeys are dominant, or usually near the top of their social order. If the goal of all living things is to reproduce, then sex hormones are the key to all life. These hormones guide the reproductive process, and affect bodily changes as well as behavior. [See also Endocrine System; Hormone; Human Reproduction; Reproduction, Sexual; Reproductive System]

Sex-linked Traits Sex-linked traits are characteristics other than sex that are carried by the sex chromosomes (coiled structures in a cell’s nucleus that carries the cell’s genetic information). Sex chromosomes in humans do more than determine whether a person is male or female, and they can carry such traits as the condition color blindness and the disease muscular dystrophy. Most sex-linked traits occur only on the X chromosome since it is larger than the Y chromosome. In humans and many other species, females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY). At fertilization (the union of sperm and egg), the new embryo that is created receives half (twenty-three) its chromosomes from the female parent and half (twenty-three) from the male parent. The twenty-three chromosomes in one cell join together as matched pairs with the twenty-three chromosomes in the other cell, so that genes for the same trait (such as height) are situated together on the same chromosome. Once this occurs, whichever gene is dominant (such as tallness) usually gets expressed in the offspring. However, since the male and female sex chromosomes are so different from the other twenty-two sets of chromosomes, which are called autosomes, different rules apply to them. This is because the X chromosome is much larger than the Y chromosome. Since it is larger, it naturally has room for more genes, which carry specific traits. Therefore, U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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when the sex chromosomes pair off and an X chromosome matches up with a Y chromosome (to make a male), many of the genes on the X chromosome do not have matching partners on the Y chromosome. Since the X chromosome is larger and has space for genes that cannot fit on the smaller Y chromosome, males carry slightly less genetic information than females. The fact that a male’s sex chromosomes (XY) are different from a woman’s (XX) and are able to carry fewer genes has certain implications that are sometimes very important. It definitely leads to different inheritance patterns between the sexes. Since females have two X chromosomes, if one of these contains a recessive mutation (some type of change in the genetic code), it is likely to be overridden or offset by a dominant, normal gene on the other X chromosome. However, when a male inherits an X chromosome with a recessive mutation, it will appear in him if his Y chromosome does not have a matching gene on it (which it usually does not). When a male has only one gene for a certain trait, it will always be expressed or appear in him. A well-known sex-linked trait is color blindness. The typical form of this condition is an inability to tell the color green apart from the color red. This condition is caused by a recessive gene carried on the X chromosome, but the trait only shows up when there is no dominant gene to offset it. That is why color blindness shows up most often in males. If a female has an X chromosome with the defective gene carrying color blindness, chances are her other X chromosome has a gene with normal vision, and she will not be color blind. Males have no “other” X chromosome, and so whatever recessive gene they inherit on their one X chromosome is expressed. That is why females are considered “carriers” of certain conditions. One of the more serious conditions for which a female can act as a carrier is for the disease hemophilia, which prevents the blood from clotting. Another is muscular dystrophy, a disease in which the muscles waste away. For a woman to get either hemophilia or muscular dystrophy, both her X chromosomes would have to have the defective recessive gene. [See also Chromosome; Fertilization; Inherited Traits; Human Reproduction; Reproduction, Sexual; Sex Chromosome]

Sight Sight, or vision, is the sense that enables an organism to detect light. It serves mainly to allow an organism to see and move about its environ534


ment. As one of the five senses, the ability to see is critically important to the independence of the individual. Sight affects all aspects of an organism’s survival.

Sight

Not all organisms that have receptors for light can actually see. Some of the simplest light-sensitive organisms (like certain cnidarians such as the jellyfish, or flatworms) have eyespots that detect light but cannot see objects. The best their light-sensitive cells can do is determine the direction and intensity of a light. Being able to form an effective image requires a much more complex organ. This organ is called an eye, usually with a lens. Besides having a lens, which concentrates light for the photoreceptors, a complete system of sight needs a brain that can interpret the light images it receives. In the animal world, there are basically two types of eyes. These include the “camera” eye of vertebrates (animals with a backbone), and the compound eye of arthropods (a phylum that includes insects).

SIGHT IN HUMANS As vertebrates, humans naturally have the “camera” eye common to higher animals. It is described as a camera because it has an adjustable lens. The human visual system consists of two eyes located in the front of the head, thus allowing both to be focused on the same object. Called “binocular vision,” this creates an overlap of information that allows people to judge distance and depth accurately. Sight begins when rays of light hit an object and bounce back, entering the eye through the cornea. This front part of the eye is a thin, transparent membrane. This membrane refracts, or bends, the incoming light through a watery fluid called “aqueous humor” and then through the lens. The elliptical, or egg-shaped, lens has muscles on either side of it that allow it to adjust its shape by expanding or contracting (according to whether the object is close or distant). The lens serves to focus this incoming light onto the retina at the back of the eye. Once light reaches the retina, it is absorbed by its more than 100,000,000 photoreceptor cells. These are called cones and rods. Rods are very sensitive to light and enable people to see even in dim light. Cones need more light to work, but they allow people to see colors. These specialized light receptor cells generate electrical impulses that stimulate the optic nerve. The optic nerve carries the impulses to the visual cortex at the back of the brain. At this point, the brain decodes and integrates all this information and produces an image, or a picture. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Vertical section of the right eye, shown from the nasal side

Superior lateral rectus muscle

Choroid (mainly blood vessels) Sclera

Retina Ora serrata

Ciliary body

Sclera Vitreous chamber (vitreous body)

Cilary muscle Cilary process

Scleral venous sinus (canal of Schlemm) Anterior cavity (aqueous humor)

Light

Central fovea of macula lutea

Posterior chamber Anterior chamber

Retinal arteries and veins

Visual axis

Central retinal vein Central retinal artery

Pupil Cornea Lens Iris Bulbar conjunctiva Suspensory ligament of lens

Optic nerve Optic disc (blind spot)

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SIGHT IN ANIMALS

Sight

The vertebrate eye is made so it will form sharp images. However, not all vertebrates see the same thing when they look at an object. Each animal species sees in a way that best helps it to survive. For example, animals that hunt at night, like cats or owls, have many more rods than cones, making them capable of night vision. They cannot, however, distinguish color. Such night hunters also have a type of pigment in their eyes that reflects any available light into their photoreceptor cells, explaining why a cat’s eyes shine in the dark. Birds and primates who hunt by day need sharp vision. Birds that soar high above their prey have phenomenally sharp sight. Unlike vertebrates whose eye has a single lens, the compound eye of arthropods (crustaceans and insects) is made up of thousands of separate, little lenses. Each lens is covered by its own cornea. A dragonfly has about 28,000. Each one of these receives light from a narrow field of view. The animal’s brain puts these all together to form a single image. This type of vision is not geared to giving a clear, sharp image. Instead, it detects the slightest movement in a very wide field of view. Some insects can see an area as wide as 180 degrees without moving their eyes or their head.

THE BRAIN IS ESSENTIAL TO SIGHT Like the other four senses that necessarily involve the brain, vision would not work if the brain was unable to tell a person what his or her eyes are seeing. If the part of the brain that processes visual information is damaged, a person may not be able to recognize a visual object despite having a pair of healthy eyes. Real blindness can result from an injury or disease of the eyeball, the optic nerve, or the nerve connections to the brain.

EYE DISORDERS A common eye condition that is correctable is nearsightedness, or myopia. In this case, the lens bends the light too much so that it focuses before it reaches the retina. Another condition is farsightedness, which is also correctable, and happens when the lens does not bend the light enough. As a result, the light reaches the retina before it is focused. Astigmatism results in blurred vision caused by a misshapen cornea. Glasses, contact lenses, and laser surgery can help these problems. Cataracts, or a clouding of the lens, common among elderly people, and can be corrected by surgery. Glaucoma, which is a type of high blood U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

Opposite: A cutaway anatomy of the human eye. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

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pressure in the eye, can cause blindness, although it is controllable. It is not true that one’s vision can be damaged by reading in poor light or by sitting too close to a television, but these habits can tire the eyes and make them sore. [See also Organ; Sense Organ]

Skeletal System The skeletal system is the structural framework that supports an animal’s body. It also provides protection for an animal’s soft tissues and internal organs and serves as an attachment for the body’s muscles that push against it and apply force, resulting in movement. The skeletal system of invertebrate animals (animals without a backbone) is on the outside and is called an exoskeleton. Most vertebrates (animals with a backbone) have a skeleton on the inside of the body, called an endoskeleton. Without some type of strong, rigid frame for support, an animal’s body would be a soft mass of tissue without any real shape. Only animals have skeletons, and because of them, their muscles have an anchor or something to push against. Muscles and bones are really inseparable, since they were designed to always work together to allow an animal to move. The skeletal system of vertebrates has been described mechanically as bones that act as levers that, in turn, apply a force that muscles have generated.

HYDROSKELETON The simplest and most primitive of all skeletal systems, the hydrostatic skeleton, does not resemble any type of rigid framework. Rather, it is what a soft-bodied animal, like an earthworm or a jellyfish, uses to keep its shape. Since these and other animals, like the hydra, have no rigid parts and are composed of fluid and soft tissue, they maintain their shape and get their support from the internal pressure of fluid pushing against the outer walls of their body cavity. Like air inside a balloon, the fluid that fills the animal’s body cavity pushes against its sides and keeps it “inflated.” These animals often live in water and move about by contracting their bodies and squeezing water out. Thus, when a jellyfish propels water from one opening, it darts quickly in the opposite direction. Earthworms make their internal fluid move inside their many segments in a particular, coordinated way and are able to lengthen and shorten their bodies and, as a result, crawl. 538


EXOSKELETON

Skeletal System

Evolution transformed the primitive hydrostatic skeleton into an exoskeleton by simply hardening its outer wall. An exoskeleton covers all or part of an animal’s body like a suit of armor, and gives it support from the outside. All arthropods (such as crabs, lobsters, spiders, and insects) have exoskeletons, and their muscles are attached to its inside and pull against it to create movement. The typical arthropod body is divided into several jointed regions all composed of a hard shell of dead tissue called chitin. This shell covers every part of the arthropod, and is especially thick and hard around its vital organs. However, it is thin and flexible at the joints so that the animal can move. Although a hard outer shell provides excellent protection, it also limits the size that an animal can reach since the larger it gets, the heavier its shell becomes. It also makes growing difficult and complicated, since the exoskeleton is made up of dead tissue and itself cannot grow. Arthropods achieve growth by periodic molts or splitting and shedding of an outer case. Molting is always a time of danger for arthropods, since they are vulnerable to attack and cannot move well. Mollusks (like clams and snails) have solved this problem since the space inside their extremely hard, outer shell gets bigger as the animal grows.

ENDOSKELETON Unlike an exoskeleton, an endoskeleton cannot be seen since it is found inside the soft flesh of an animal. It is also very different from most exoskeletons because it grows in step with the rest of the body. Since endoskeletons are lighter than those carried on the outside, they can grow much larger in size. It is no surprise therefore that the largest animals on Earth have endoskeletons. An interior skeleton may be light, but it also provides little protection. While some vital organs may be surrounded by bones (like the ribs around the heart and lungs), other organs (like those below the ribs) are totally vulnerable to injury by force. Surprisingly, there are some animals that, despite being invertebrates, nonetheless have an endoskeleton. Sponges do not have backbones, but they do have a form of endoskeleton. Other invertebrates, such as echinoderms like sea urchins and starfish, seem to have an exoskeleton, but their spines are really only extensions of their endoskeleton that lies just below their skin. Both squid and octopi are also invertebrates with an endoskeleton. There are also animals that are considered to be vertebrates but who do not have a single, real bone in their body. This is because their supporting framework is usually made up of cartilage. Cartilage is a tough, slippery substance that is both strong and flexible. The shark, U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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ray, and lamprey are examples of vertebrates with an endoskeleton of cartilage, and they swim in a noticeably different way than true fish do.

PARTS OF AN ENDOSKELETON All other vertebrates have an endoskeleton made of bone. As a living substance, bone is made up of cells surrounded by layers of hard mineral salts, such as calcium phosphate. Most bones have three structural parts: the periosteum or compact bone that is its dense outer layer and gives it strength; spongy bone, which is the light, softer inside layer; and marrow that fills its inside core and makes red and white blood cells. Before vertebrates are born, their skeleton exists in a cartilage form, which “ossifies” or hardens and turns into real bone at birth. After birth, some of the bones fuse together. For example, a human baby has 270 bones at birth. However, by the time the baby reaches adulthood many of the bones will have fused together, leaving only 206 separate bones in the human skeletal system. For a skeleton to really work, however, it cannot be all bone. Bones must be connected to one another to make it all work together, and ligaments are the connective material that links bone to bone at the joints. A ligament is made up of many bands of tissue that, if torn, can heal; but if severed, must be surgically sewn back together. Muscles are connected to bones by connective tissue called tendons. Although tendons act like a very tough cord, they cannot be stretched. If severed, they must be surgically repaired or the muscle will not work the bone (since the connection is broken). Cartilage is also a form of connective tissue and serves to fill the spaces between bones and prevents them from scraping painfully against one another. Cartilage is slippery (called “gristle” in animals) and it allows bones to slide over each other at the joints.

Opposite: A diagram showing some of the major bones that make-up the human skeletal system. (Illustration by Kopp Illustration, Inc.)

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Because vertebrate skeletons are jointed, they allow movement. There are six types of moving joints in vertebrate skeletons: ball-and-socket joints (like the hip and shoulder joint); gliding joints (wrist and ankle); pivotal joints (allow two kinds of movement—side-to-side and up-anddown); saddle joints (thumb joint); hinge joint (elbows, fingers, and knees); ellipsoid joints or condyloid joints (movement in two axes such as the tiny bones in people’s fingers, toes, and jaw). Joints move easily because of their slippery cartilage but also because they are lubricated by a special fluid called synovial fluid. There are also some body joints that do not move, such as the bones of the skull and hip. In these joints, bones come together in joints that are really more like seams. The basic form of all vertebrate skeletons is essentially the same for many different types of animals and is made up of an axial skeleton and U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e


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an appendicular skeleton. The standard axial model (for a typical mammal) has a vertebral column made up of individual small bones (vertebrae) that provide up-and-down or lengthwise support while also being flexible. The vertebrae (singular, vertebra) also connect the skull to the rest of the body. Ribs, also part of the axial skeleton, provide support and protection for the chest area. The appendicular skeleton is made up of the forelimbs (arms and hands), the shoulders, and the pelvic bones to which the hindlimbs are attached. Finally, ligaments keep the bones securely attached at the joints. There are naturally many variations to this model. For instance, a snake is a vertebrate but it has no limbs at all. A manatee and a whale have only forelimbs (flippers). Humans have arms and birds have wings. The long, flexible human thumb allows us to perform many precise movements, whereas the chimpanzee, our close relative, has a shorter, much less useful thumb. While the number, size, and shape of skeletal elements may vary greatly within species during their development, and sometimes even between sexes, the basic vertebrate model is always the same.

Smell Smell is the sense that enables an organism to detect airborne chemicals. It serves as a way of identifying, sorting out, and warning an organism about its environment. As one of the five senses, the sense of smell plays a key role in human development and is also linked to human emotions and memory. Smell is technically called “olfaction,” and it is one of the senses an animal uses to orient itself to its surroundings. Olfaction is sometimes described as a “sensitivity to substances in a gaseous phase.” This means that smell allows an odor (which is an airborne chemical or a gas) to be detected. Being able to smell is one sense that the earliest life forms probably used to find food and avoid being eaten themselves. Smell can be very important to the survival of both humans and other animals. It affects people’s quality of life, or how much they enjoy life. Like the sense of taste, smell involves a complex process called chemoreception. Described simply, things can be smelled if they give off molecules (small particles) into the air. Organisms that have the sense of smell have specialized receptors, or cells, that receive these molecules. Substances that give off molecules are called “volatiles.” 542


THE SENSE OF SMELL IN HUMANS

Smell

For humans, smell begins with the nose. The nose breathes in air carrying these molecules. This air enters through the nasal cavities, or nostrils (and sometimes the mouth). It passes over the body’s “smelling skin,” called its olfactory epithelium. This is a small patch of moist, specialized cells located in the upper part of the very rear of the nasal cavities just above the bridge of the nose. Here the body’s chemoreceptors are stimulated by the gas molecules. The chemoreceptors are covered with extremely tiny hairs, called cilia, and a fluid. When the gas molecules dissolve in the fluid and touch the cilia, the cell reacts. Scientists still do not know if there is a special receptor for every different type of possible scent, or if there are certain receptors that are triggered in a different sequence by a particular odor. Most think that the latter is probably true. When a chemoreceptor cell reacts, a nerve impulse is produced. The impulse travels to the brain’s olfactory cortex (a layer of gray matter that

A diagram showing the process by which olfactory information is transmitted to the brain. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

Olfactory bulb

Mucus producing gland

Olfactory nerve Brain

Olfactory bulb

Bone A chemical reaction within the olfactory nerve cell causes it to send a signal to the neurons in the olfactory bulb. Supporting cell

Olfactory epithelium

Olfactory nerve cell Dendrite Olfactory hair (cilium) Mucus layer Substance being smelled Frontal sinus

Nasal cavity Odor

Nasal conchae

Some neurons within the olfactory bulb lead directly to the limbic system of the brain, the area responsible for emotions, sexuality and drive. Because of this connection, the sense of smell has a more direct route to this area than hearing or vision.

Tongue

Nasopharynx Odor molecules are transmitted to the olfactory epithelium on air currents as a person breathes or are forced upward from the throat as a person chews or swallows.


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covers most of the brain’s surface) where the smell is identified. If the smell is recognized from a previous experience it is easily identified. If it is new, it is stored and remembered for the next time. In humans, the olfactory cortex is located deep within the brain’s limbic system. This is the part of the brain that is the source of people’s emotions. Smell is also linked to the brain’s hippocampus and amygdala in the limbic system, which controls memories. It is thought that this link to the parts of the brain that control emotion and memory is the reason why smells can cause people to feel certain emotions or have strong memories. All people have experienced a particular smell bringing back a flood of childhood feelings or making them remember an event. In this way, smell plays a large role in forming life experiences and influencing moods. Odors associated with a pleasant experience instantly bring back fond memories, while those associated with an unpleasant experience trigger negative emotions. In humans, smell is closely related to taste since they both operate by chemoreception. If food has a foul smell, it becomes unappetizing and someone knows not to eat it. Without a sense of smell, humans’ ability to taste would be severely impaired. People experience this when they are badly stuffed up from a head cold and cannot smell. Newborn babies recognize their mother within three days of birth by her smell. People with no sense of smell can hardly taste cheese and pepper. The sense of smell can be damaged or lost as a result of a head injury, infection, a brain tumor, or exposure to toxic chemicals.

THE SENSE OF SMELL IN ANIMALS While the sense of smell does not play an obvious role in the lives of humans, it is a critically important sense for many animals. Those animals who depend a great deal on their sense of smell usually possess an acute hunting ability, and smelling plays many different roles in their lives. It is known that a dog’s area of “smelling skin” in its nose is roughly fifty times larger than a human’s olfactory epithelium. Such extreme sensitivity to odor by animals is exemplified by a shark, which can smell a few drops of blood mixed with sea water from great distances. While many mammals smell with their noses, some animals and insects smell with their tongues, feet, or antennas. For example, a snake picks up chemical odor molecules from the air by flicking its tongue, while a butterfly senses sweetness with its feet and detects the smell of the opposite sex with its antennas. Ants and bees also smell with their antennas, and salmon are guided upstream by their sense of smell. Other animals, like fish and amphibians, have scent-detecting organs all over their bodies. 544


In the natural world, animals use smell to identify their own kind, find food, and avoid predators. They also use this sense to communicate using pheromones, or “messenger substances.” These chemicals indicate when an individual is receptive, or ready, to mate. They also can guide a predator to its prey or warn the prey that a predator is close by. Many mammals use their urine to mark their territory and signal others to keep out. Animals of the same species can smell a number of things about the “marker” of the territory, such as its sex, readiness to mate, and even its general age.

Species

Mammals have the best sense of smell among all vertebrates (animals with a backbone). Within the group of mammals, carnivores (animals that eat other animals) and rodents have the best sense of smell. Primates, like humans and apes, have the poorest sense of smell. [See also Organ; Sense Organ]

Species A species is a group of organisms of the same type. The term species is one of the seven major classification groups that biologists use to identify and categorize living things. These seven groups are hierarchical or range in order of size. Species is the last and smallest complete group. The classification scheme for all living things is: kingdom, phylum, class, order, family, genus, and species. Species can be considered the basic unit of scientific classification. It is used to describe a group of closely related, physically similar organisms that can breed with one another. Members of the same species often are so close in features that it is difficult to tell them apart. For example, although a wolf and a dog look similar, they do not belong to the same species. The wolf belongs to the species lupus (Canis lupus), while the dog to the species familiaris (Canis familiaris). Some organisms, however, are more difficult to differentiate. This is the case with sunflowers of different species. Some species of organisms may differ very little in their conspicuous features, shape, behavior, and habitat. However, only members of the same species will share a common gene pool. The only correct way to identify one species from another is by the binomial (two-word) scientific name. For example, the correct way to distinguish between two species of seabirds would be to use their binomial names: Sula sula and Sula nebouxii. In these examples, Sula is the genus name, while sula and nebouxii are the species names. (The species name is always the lowercase Latin word that follows the uppercase Latin genus U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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word.) Very often, however, it is the common name for the species that is most used to refer to one particular type. So, for the example above, most people would refer to the two species of seabirds as the red-footed booby and the blue-footed booby instead of using the binomial scientific name. [See also Class; Classification; Family; Genus; Kingdom; Order; Phylum]

Sperm moving over the surface of a uterus. Sexual reproduction involves the union of an egg and sperm to produce a cell that is genetically different than the parent cells. (Reproduced by permission of the National Audubon Society Collection/Photo Researchers, Inc. Photograph by Fawcett/Phillips.)

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Sperm A sperm cell, or spermatozoon, is a sex cell produced by male organisms. In humans and other vertebrates (animals with a backbone), sperm are produced in great numbers by the male gonads or testes (sex organs). The job of a sperm is to swim to the female egg, penetrate its surface, and deliver its package of genetic material. This is called fertilization and usually results in the production of an offspring. Sexual reproduction involves the fusion, or uniting, of male and female gametes (sex cells) to produce a new individual. It also usually involves a male parent that produces sperm and a female that produces an egg, or ovum. The egg is large and does not move, while sperm are usually small and highly mobile. When a sperm contacts and penetrates an egg, fertilization occurs. This fertilization produces a zygote that will develop into a genetically unique offspring. All animals that engage in sexual reproduction manage to unite a sperm with an egg. For some species, the sperm fertilizes the egg externally (as do frogs or fish in water). The method of external fertilization used by aquatic animals would barely work if only a few sperm and eggs were released each season because the eggs and sperm are unprotected and left to unite by chance. That is why each aquatic parent produces very large quantities of sperm or eggs. Land animals, like mammals, are more complex, and their reproductive systems are more specialized. They engage in internal fertilization where the sperm it directly deposited in the female’s body. While the male produces sperm in great numbers, the female makes only a very small number of eggs available for fertilization at any one time. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

Nearly all male animals produce sperm as their reproductive cell. With the exception of certain worms, decapods (like crayfish), diplopods (like millipedes), and mites, all sperm have two main parts. These include a head and some form of a whiplike tail. The tail, or flagellum, whips sideto-side and gives the sperm cell its movement. The head of a sperm is different for each species and is made up mainly of a nucleus (the cell’s control center). It is there that the sperm carries chromosomes, the important genetic material responsible for transmitting certain characteristics to the new individual it helps create.

Sponge

It is the sperm (and not the egg) that also carries the chromosome that determines the sex of the offspring. The head of the sperm is covered with a cap called the acrosome. This cap contains chemicals that eat through the egg’s protective covering and help the sperm burrow into the egg and fertilizes it. When the sperm nucleus and egg nucleus meet, their membranes fuse together and form a new nucleus. This new nucleus receives twenty-three chromosomes from the sperm and twenty-three from the egg to make a complete set of forty-six. Once an egg is fertilized by one sperm, it prevents any others from penetrating its outer layer. Sperm that do not fertilize an egg keep swimming either in water or in the female’s reproductive tract until they die. Most animals release sperm in large numbers, and a healthy human male can release 250,000,000 sperm in a single ejaculation (the ejection of sperm and fluids from the penis). Human males do not start producing sperm until puberty begins (between the ages of twelve and fourteen when they evolve from children into adolescents). The male gonads or testes where sperm are produced are housed outside the body in a bag of skin called the scrotum. Since sperm are unable to survive at normal body temperature, they are housed outside the body where it is at least 5°F (-15°C) cooler. [See also Fertilization; Human Reproduction; Reproduction, Sexual; Reproductive System]

Sponge A sponge is an invertebrate (an animal without a backbone) that lives underwater and survives by taking in water through a system of pores. A sponge has no organs or nervous system and lacks most features that are common to animals. It is the simplest and one of the oldest of all multicellular organisms. There are about 5,000 species of sponges. All of these belong to the phylum Porifera, which means “to have pores.” Most inhabit a saltwater U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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environment and live in one place on the seabed. They always attach themselves to something hard and unmoving. Although biologists consider the sponge to be an evolutionary dead end, it is nonetheless a very successful organism in terms of its ability to survive.

CHARACTERISTICS OF SPONGES

A vase sponge with fishes swimming inside of it. Sponges are one of the simplest and one of the oldest of all multicellular organisms. (Reproduced by Photo Researchers, Inc.)

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All sponges have certain things in common. First, all are filter feeders, meaning that they get their food by trapping whatever their watery environment provides them. They allow water to enter and leave their body cavity through a system of tiny holes or pores. In this way, sponges obtain the water, oxygen, and nutrients they need. They rid themselves of waste like carbon dioxide by using the same system. Sponges expel their waste out of their largest opening called the osculum. As water travels through the sponge, any small food particles in the water are captured and absorbed by its cells.


This points out a unique thing about sponges: each of their cells is in a sense on its own. For example, although each cell responds to stimuli, the sponge as an organism cannot react as a whole. Similarly, digestion and waste disposal is the job of each cell. This is necessary since a sponge has no specialized tissues or organs to perform these functions. A sponge might therefore be described as a loose association of cells.

Spore

While a sponge may be as small as a fingernail or as large as a chair, all have some sort of skeleton or framework that supports them. In fact, sponges are classified according to the type of skeleton that they form by secretion. One class of sponges has a chalky skeleton made of calcium carbonate spikes. Another class has needle-like glass spikes, while a third class has a supporting skeleton made of a fibrous protein called spongin. The “natural” sponge we associate with taking a bath is actually a dried spongin skeleton minus the sponge’s living cells. Most of today’s household sponges are synthetic (man-made) plastic versions of a sponge. As an invertebrate, most sponges are hermaphroditic, meaning that the same individual can produce both sperm and eggs. They can reproduce sexually, although they must obtain the sex cells of another individual. After fertilization (the union of a male sex cell and a female sex cell), a free-swimming larva is released that eventually attaches itself to the bottom of the ocean where it remains anchored for its entire life. Sponges can also reproduce asexually (without another individual’s sex cells). This happens when a small fragment is broken off and settles on an appropriate spot on the ocean floor where it buds or develops into a new sponge. It is estimated that this extremely simple organism, from which few multicellular organisms have evolved, has successfully lived on Earth for more than 5,000,000,000 years.

Spore A spore is an extremely tiny, specialized package of cells used by some organisms during reproduction. Plants use spores as they do seeds, for reproduction; while certain algae, fungi, bacteria, and protozoans use spores to help disperse themselves widely and to protect themselves from unfavorable conditions. Spores vary in size but all are microscopic and usually contain a single cell. In botany (the study of plants), spores are regarded as reproductive cells that are capable of developing into a new individual plant, either directly or after fusion with another spore. Plants that do not flower, like mosses and ferns, do not grow from a seed that was created sexually by U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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A fern leaf with spore clusters. Spores, not seeds, are the reproductive cells of ferns. (©National Audubon Society Collection/Photo Researchers, Inc. Photograph by Hugh Spencer.)

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male pollen combining with a female ovule. Instead, they reproduce by means of spores. The plant that develops from a spore does not resemble the parent plant, since this plant is in the first phase of a life cycle called the “alternation of generations.” This means that in the life cycle of a plant, two generations exist alternately, one after the other. For example, for centuries no one knew how ferns actually reproduced. Most thought that since a fern was a green plant, it had to produce seeds (and therefore reproduce sexually with male and female sex cells). Yet it was impossible to find a fern’s seeds until botanists studied the entire life cycle of a fern. Finally, they discovered that ferns reproduce with spores. They also discovered that a fern has a sexual stage (producing sperm and egg) that alternates with an asexual stage (producing spores). In the life cycle of a fern, a mature fern plant develops little brown spore cases called sporangia that are attached to the underside of their leaves, or fronds. When the spores are ripe, the cases split open in the dry air and the dustlike spores are carried away by the wind. When the spore lands where there are damp conditions, it germinates (begins to sprout) and grows into a very small, heart-shaped plant. This tiny plant begins to mature and develops sex organs which, after fertilization (the process in which an egg cell and a sperm cell unite to form one cell) occurs, grows into a plant we recognize as a fern. Organisms besides plants use spores for other purposes. Certain kinds of algae, bacteria, fungi, and protozoans all form what are described as survival spores. Besides the ability of spores to be easily and widely dispersed, they also have the ability to survive under harsh conditions. For fungi that are not able to move, spores are an ideal way to spread themselves around, and when conditions are right, fungi produce dispersal spores that function like seeds and germinate quickly under proper conditions. They grow, mature, and produce more spores out of which more fungi will grow. But when the environment becomes unfavorable—too hot or cold or too dry—fungi produce survival spores that may live for years before germinating. Certain types of bacteria form spores for protective reasons. Bacterial spores, which are bacterial cells that have gone dormant (resting) and developed a thick, hard wall, can usually survive even in hot, boiling water. Protozoans—most of which are parasites that live in U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

or on other animals—also form protective spores by a type of cell division. The protozoan that causes malaria is injected into a healthy person by a mosquito carrying its spores. Although the spores cannot move, they multiply by cell division in the person’s liver and then enter the blood cells. Soon the blood cells burst and release the spores into the liquid part of the blood. When a mosquito stings this infected person and takes its blood, it also takes the spores and eventually deposits them into yet another healthy person. For plants and other organisms, spores have proven to be a successful means of establishing themselves in new environments as well as a way of surviving unfavorable conditions.

Stimulus

[See also Algae; Botany; Bacteria; Fungi; Plant Reproduction; Plants; Protozoans]

Stimulus A stimulus (plural, stimuli) is any event that triggers a response in an organism. When an organism reacts, or changes, its behavior because of some environmental change, it is responding to a stimulus. An ability to respond to what is going on around them is one of the characteristics of living things. In order to survive in a world full of constant change, activity, and motion, organisms have developed a wide range of receptors that react to particular types of stimuli. If organisms are not immediately aware of changes in their environment, they may not be able to adapt. Therefore, an organism must be able to detect change and to adjust itself. The organism must respond to change in an appropriate way, or it will not live long. Vertebrates (animals with a backbone) have developed elaborate and sensitive nervous systems that enable them to know what is going on about them. This system enables them to react quickly to each new situation. Vertebrates have five main senses. Each have specialized receptors that respond to certain types of stimuli. A person’s eyes have receptors that detect light, while ears detect sound; the skin detects pressure and temperature; the tongue detects dissolved chemicals; and the nose detects airborne chemicals. Although each type of receptor reacts to only one particular type of stimulus, they all work on the same principle. A sense receptor is activated by a certain type of energy change. Therefore, when a certain energy change, or stimulus, occurs in the external environment, the appropriate cell, or nerve ending, reacts and converts this into a nerve impulse. The nerve impulse is carried to the brain by sensory neurons. The brain decodes the signal and tells a person what he or she is sensing. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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It also transmits any necessary signals to the muscles or glands (organs in the body that produce a special substance like hormones or enzymes) to carry out a particular action. Organisms also have rapid or immediate responses to certain stimuli that are built into their nervous systems. These are called reflex actions and are usually geared to matters of well-being and survival. As a result, a person will instantly pull his or her hand away from a hot stove without thinking. The heat sensed by the person’s skin is the stimulus and the hand jerking back is the response. Such an instantaneous reaction is called an unconditioned response because it occurs with no learning or experience involved. The idea of stimulus is closely associated with that of life or living, since all organisms, from the simplest to the most complex, are geared to receive and react to stimuli. [See also Brain; Hearing; Integumentary System; Sight; Smell; Taste]

Stress Stress is a physical, psychological, or environmental disturbance of the well-being of an organism. The body’s nervous and endocrine systems usually respond automatically to stress. Some stress is unavoidable and even natural, but prolonged stress can be harmful. All organisms seek to maintain homeostasis, which can be described as a balanced, constant, or stable internal environment. Some of the major mechanisms that organisms use to maintain homeostasis include the control of temperature, fluid balance, blood pressure, and the production of energy. The body’s many mechanisms and processes are constantly adjusting themselves in order to cope with these highly dynamic internal and external situations. Stress, however, is something that is slightly out of the ordinary. Stress can be considered something that strains or interferes with the functioning of an organism. Stress is a real challenge to homeostasis. Stress can come in many shapes and forms. It can be temporary or longlasting, and it can be mild or severe. Stress can be purely physical, such as a lack of food, an injury to a muscle or bone, or an infection. It can be the result of certain undesirable environmental conditions, such as extreme altitude, humidity, heat, or cold. It can also be psychological, such as a person’s fear of public speaking, or an animal’s fear of being hunted by a predator. 552


All organisms experience stress at one time or another. While each organism may react differently to stress, all of them experience the same general physiological, or bodily, reactions. Many of these reactions happen without the organism knowing it. Most of these responses are difficult, if not impossible, to consciously control. In humans, most responses are controlled by the body’s autonomic nervous system. As it sounds, this is the part of the nervous system that controls involuntary muscles and glands. When a stressful situation is perceived by the nervous system, the brain sends a stimulating message to the adrenal gland. This gland sits above each of the kidneys and secretes adrenalin and other hormones (chemical messengers) when stimulated. These hormones enable a person to be instantly ready for “fight or flight.” This means that the major systems take measures to prepare for the worst and get the body ready to fight harder, run faster, and think quicker than normal. The body’s heart rate skyrockets and the blood moves from the internal organs to the muscles. The body interrupts all nonessential processes and causes the rate of breathing to speed up, taking in as much oxygen as the body needs. As a result, the body becomes ready to either fight if necessary or use its strength to get away as quickly as possible. Besides adrenaline, the adrenal gland also produces cortisol when stimulated. This powerful hormone provides the body with a ready supply of energy and enables it to function at peak efficiency.

Stress

All of these reactions to stress are normal and sometimes necessary for survival. However, prolonged and repeated stress (with no resolution) is damaging to the body. This situation is called chronic stress, and it can be very common given the way people live today. People may encounter situations in everyday life that are not threatening but cause the body to have the same “fight or flight” response, sometimes over and over again. When the body is preparing for a physical extreme, and the situation does not require or permit it, people “stew” in their hormones that are urging them to take action. Chronic stress can be damaging to the body, since it has been found that regularly high levels of cortisol can damage the body’s immune system. Also, steady adrenaline can cause high blood pressure and high cholesterol. Animals often show signs of abnormal behavior when stressed. They may rock or pace when caged too closely together. Plants suffer physical stress when they do not receive enough water or light. Each living thing has developed some ability to cope with stressful situations. For people, it helps to be aware of what makes them feel stressed. If they cannot remove the stressor from their lives, they can attempt to minimize its effects. Relief can come through exercise, meditation, psychotherapy, and U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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biofeedback (in which a person tries to control the body’s hormone level). Overall, stress can be a major factor in people’s health. [See also Endocrine System; Homeostasis; Nervous System]

Survival of the Fittest Survival of the fittest is a simple way of describing how evolution (the process by which gradual genetic change occurs over time to a group of living things) works. It describes the mechanism of natural selection by explaining how the best-adapted individuals are better suited to their environment. As a result, these individuals are more likely to survive and pass on their genes. The theory of natural selection was first offered by English naturalist, Charles R. Darwin (1809–1882), during the 1850s to explain how evolution worked. Darwin suggested that all living things were connected to one another because they had evolved from a few common ancestors. He used the mechanism of natural selection to explain how this could be possible. Natural selection is based on the idea that although the individuals that make up a given species all seem alike, there are in fact many important characteristics that make each slightly different from the other. These differences were inherited from the individual’s parents. Organisms would pass these differences on to their own offspring if they lived long enough to reproduce. Darwin’s idea proposed that since each individual was different from any other, certain ones possessed particular traits, or characteristics, that favored individual over another. For example, a single pair of rabbits can produce up to six litters a year. Darwin realized that if each of these offspring of every species of rabbits survived to reproduce, the entire world be overrun with rabbits, to the point where there would not be enough resources to keep them all alive. Since there was a limited amount of resources, Darwin argued that each individual had to compete with others for what it needed to stay alive, grow, and reproduce. Darwin explained that it is the environment or nature itself that “selects” which individuals are best adapted to it or are best “fit.” For the six litters of rabbits, those whose particular traits give them an advantage in their particular environment are the ones who most likely will survive. These offspring also will grow strong, and pass on these “fit” traits to their offspring. Depending on the environment, it may not always be the fastest rabbit that survives. Instead, it could be the one with a certain coat 554


color that allows it to be easily camouflaged that has the “fit” advantage. This point was recognized by scientists who adopted the term “fitness” to refer to the members of a group whose traits made them the bestadapted. For this reason, natural selection is often called “survival of the fittest.” Darwin did not use these words. Instead, they were first used by the nineteenth-century English philosopher, Herbert Spencer (1820– 1903), to help explain Darwin’s theory of evolution.

Symbiosis

[See also Evolution; Evolutionary Theory; Natural Selection]

Symbiosis Symbiosis describes the relationship, close association, or interaction between two organisms of different species. Although the term is often used to describe a relationship that benefits both species, there are different types of symbiosis. Symbiotic relationships that have occurred over very long periods of time can sometimes result in evolutionary changes in the organisms involved in the relationship. Symbiosis literally means “living together,” and there are many examples in nature of organisms of entirely different species that are in-


A cape buffalo with an oxpecker on its back in Kenya, Africa. The relationship between the oxpecker and the buffalo is a type of symbiosis called mutualism since the oxpecker feeds from the supply of ticks on the buffalo, which in turn benefits from the tick removal. (Reproduced by permission of JLM Visuals.)

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LYNN MARGULIS American biologist Lynn Margulis (1938– ) has suggested some of the more revolutionary ideas in the history of modern biology. Her symbiotic theory of evolution has offered a new approach to both evolution and the origin of cells within the nucleus. She also subscribes to the “Gaia” hypothesis, which states that the Earth acts a superorganism, or single living system, that can regulate itself. Lynn Margulis was born in Chicago, Illinois. Her parents, Morris and Leone Alexander, had three other daughters. An exceptional student, Margulis was fifteen when she completed her second year at Hyde Park High School and was accepted into an early entrant program at the University of Chicago. There she was immediately inspired by her science courses and took to reading the original works of the world’s great scientists. She also became interested in the deeper aspects of heredity and genetics. While at Chicago, she met Carl Sagan (1934–1996), who would become an astronomer (a person who studies the universe beyond Earth) and one of the best-known scientists in the world. Sagan was a graduate student and Margulis was nineteen in the year she both received her bachelor’s degree and married Sagan. She then entered the University of Wisconsin to pursue a joint master’s degree in zoology and genetics, and in 1960 she and Sagan moved to the University of California at Berkeley where she conducted genetic research for her doctoral dissertation. The marriage to Sagan produced two sons but ended before she received her Ph.D. in 1965. After teaching at Brandeis University, she joined Boston University and married crystallographer (a person who studies crystal structure) Thomas N. Margulis, with whom she had two children before they divorced in 1980. Since 1988, Margulis has been a distinguished university professor at the University of Massachusetts at Amherst. Margulis has regularly questioned the commonly accepted theories of genetics yet she has been called the most gifted theoretical biologist of her generation by numerous colleagues. As a graduate student she became in-

volved in some form of close, beneficial relationship or association. In fact, there are some symbiotic relationships that are necessary for the survival of the participating organisms. There are three types of symbiosis—mutualism, commensalism, and parasitism—depending on the nature of the relationship. As with any classification system, there are always exceptions, and sometimes it is difficult to categorize a certain situation. It is also a mistake to make a judgment of one type of symbiosis being better than another, since each is simply an organism’s adaptation to survive. 556


terested in what is called non-Mendelian inheritance, which is when the genetic makeup of a cell’s descendants cannot be traced solely to the genes in the cell’s nucleus (the cell’s control center). This puzzling phenomenon led her to search for genes in the cytoplasm of cells, that is, inside the cell but not inside its nucleus. In the early 1960s, Margulis actually found deoxyribonucleic acid (DNA, which is the carrier of genetic information) in the cytoplasm (jelly-like substance) of plant cells, suggesting that heredity in higher organisms might not be totally determined by genetic information carried only in the cell nucleus. This led her to eventually formulate her most startling idea, called the serial endosymbiotic theory (SET). Margulis stated that prokaryotes (cells that do not have a nucleus, such as very simple life forms like bacteria), which simply carry their genetic information inside the cell’s cytoplasm, were the evolutionary forerunners of the more complex eukaryotes (which are cells that have a separate nucleus). All plants and animals have eukaryotic cells. She argued that eukaryotes evolved from prokaryotes when different types of prokaryotes formed symbiotic systems to increase their chances of survival. Symbiosis means that they had some sort of relationship, usually a type of partnership in which both members benefitted. An example of this, she says, is a cell’s mitochondria (specialized structures inside a cell that break down food and release energy) that process oxygen.

Symbiosis

Most scientists now agree that these cell structures evolved from oxygenusing bacteria, which joined with fermenting bacteria. Margulis is unique in her argument that traditional evolutionary theory cannot explain what she calls the “creative novelty” of life. Margulis also extends her concept of symbiosis to the entire biosphere (that part of Earth that contains life) and therefore accepts the Gaia hypothesis put forth by English chemist James E. Lovelock. This theory states that all life, and Earth itself, including its oceans and the atmosphere, are parts of a single, all-encompassing symbiosis that in turn form a single “organism,” or a single living system. For Margulis, the concept of symbiosis is a powerful explanatory tool.

Mutualism is a type of symbiotic relationship that results in a mutual benefit. Both species realize some type of gain by living together and cooperating within the same habitat. An example of mutualism would be the close relationship between a certain bacteria (Rhizobia) that lives under the soil and is attached to the roots of certain plants like peas, beans, clover, and alfalfa. These bacteria are nitrogen-fixing, meaning they are able to take in nitrogen gas that exists in the atmosphere and change it into nitrates that plants can use. This plant/bacteria relationship is mutualistic because both organisms benefit: the plant gains the necessary niU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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trogen in a usable form, and the bacteria gains access to a source of energy (using the plant’s ready-made glucose). This is also an example of what is called “obligatory mutualism,” since both partners are completely dependent on each other. Another example of this type of mutualism is the lichen, which is really made up of a fungus (plural, fungi), and an alga (plural, algae) living together. An alga can make its own food but can only live in wet places. A fungus cannot make its own food but can store a great deal of water. Together, they can live anywhere since the alga makes food (and lives inside the fungus), while the fungus provides it with its necessary water. The other form of mutualism is called “facultative” and describes a relationship in which both partners benefit, but which each could still survive if the relationship did not exist. The relationship between the oxpecker (also called the tickbird) of Africa and the black rhinoceros is a good example, since these birds spend most of their time clinging to the bodies of large animals like the rhinoceros and eating ticks and maggots that infest the rhinoceros’ hides. The birds also make a hissing sound that alerts the rhinos to possible danger. The rhinoceros benefits by having blood-sucking insects removed from its body, as well as having an early warning system. However, although both animals benefit from their relationship, the bird could obtain insects elsewhere if the rhino were to vanish, and the rhino could survive being infested with ticks. Commensalism is the second type of symbiosis and describes a relationship in which one species benefits while the other experiences basically no effect (it neither benefits nor suffers). A bromeliad (an air plant) growing on the high branch of a rainforest tree is an example of such a relationship since it benefits by being closer to the sunlight while the tree is not harmed in any way (it also does not receive anything beneficial from the bromeliad). Another example is the tiny mollusks or crustaceans called barnacles that attach themselves to the body of a humpback whale enjoy the benefit of being moved through the water so they can filter microscopic food. The whale is neither bothered nor benefitted by the mollusks. Commensalism is usually practiced by one species on another to obtain something it cannot provide for itself such as transportation, protection, or nutrition. Finally, a symbiotic relationship is described as parasitism if it results in the host organism being somehow harmed. In this type of relationship, the organism that benefits is called the parasite, while the organism that the parasite lives in or on is called the host. Disease-producing organisms are probably the best examples of parasitism. Such is the case with a tapeworm that lives inside the digestive organs of mammals. Because the tape-

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worm takes nutrition from the host, the host is left weakened and may also suffer tissue damage. An example of a strange and interesting form of parasitism is that conducted by a brood parasite. In this phenomenon, one species of animal uses the adult or parent of another species to raise its young. The common cuckoo bird does this regularly by laying its eggs in the nest of a species with similar-looking eggs. As soon as the cuckoo hatches, it pushes all the other eggs from the nest and eats all the food provided by its foster parents (which have been tricked into raising the cuckoo as their own).

Symbiosis

[See also Parasite]


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T Taiga The taiga is a geographical region characterized by dense, coniferous (evergreen) forests broken up by bodies of water. With its long, cold winters and low light, the taiga does not support a wide variety of species. However, the taiga covers a larger area than any other type of forest in the world. The name taiga comes from a Russian word used to describe the evergreen forest of Siberia. It is also called a boreal forest and is taken from the word boreas meaning north wind. The taiga is always subject to the north wind because it is located only in the northern latitudes of the Northern Hemisphere, from Alaska, through Canada, across Eurasia, to Siberia where the largest tracts of taiga are located. At a similar latitude in the Southern Hemisphere, the land is too narrow, and therefore influenced by the ocean, to support a taiga-like environment.

PLANT LIFE IN THE TAIGA A typical taiga forest is cold and dark, and broken up by lakes, rivers, and swamps. The dominant tree is the conifer, an evergreen that reproduces by making cones. The conifer is well-adapted to the taiga. It has branches that slope downward to prevent snow from building up, as well as a pointed shape and flexible trunk to cope with high winds. Growing conditions are harsh, since winters are extremely long. The taiga soil is poor because decomposition (breaking down of waste) takes so long. The soil is also highly acidic and low in minerals. Ground-level vegetation consists mostly of ferns, mosses, and shrubs that have adapted to the short growing season. Because of the slow rate at which things decompose, U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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there are usually dense layers of peat (rotted vegetable matter) in the everpresent bog (wet, spongy ground consisting of decaying plant matter).

ANIMAL LIFE IN THE TAIGA

Autumn colors of a taiga in Alaska. Taiga is a Russian word that means “land of little sticks.” (©Photographer, The National Audubon Society Collection/Photo Researchers, Inc.)

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Animal life in the taiga includes woodland caribou, moose, brown bears, beaver, lynx, and wolves. Most animal life is medium to smallsized, including rodents, rabbits, sable, and mink. Most birds migrate (seasonally move) to warmer climates for the winter. Swarms of biting insects during the summer make the taiga a miserable place for humans and other warm-blooded animals (their internal temperature remains constant despite their environment). Most of the taiga is not well-suited for farming, although it does yield great quantities of lumber and support a fur trade in mink, sable, and ermine, among others. [See also Biome]


Taste

Taste

Taste is the sense that enables an organism to detect dissolved chemicals. It serves primarily to tell an animal the difference between something that is good to eat and something that is dangerous. As one of the five senses, the sense of taste is closely associated with the sense of smell. Taste is influenced by habit, learning, and other cultural and psychological factors.

HOW THE SENSE OF TASTE WORKS Humans are born with the ability to taste. The sense of taste begins with the tongue. The skin over this muscular organ located inside the mouth is covered with about 10,000 receptor cells, or chemical-sensing bodies. These are called taste buds. Each of these funnel-shaped clusters has an opening called a taste pore. Molecules (small particles) of dissolved substances, containing chemicals, flow into these holes and trigger, or activate, a receptor cell. The taste buds also respond to other stimuli. When people smell or think of a food they like, their mouth starts to water. This means that people start to produce saliva. In order for humans to actually taste something, it has to be dissolved in saliva. Like smell (called olfaction), taste (also called gustation) operates on the principle of chemoreception. Certain receptors are triggered when chemicals contact them. It was once thought that certain parts of the tongue responded only to one of the four basic categories or sensations of taste (sweet, sour, bitter, salty). Now it is thought that individual receptors are not specifically sensitive to only one sensation. Unless the brain is involved in this process, a person will not be able to actually identify anything he or she has dissolved on the tongue. Science still does not know exactly how this occurs. Somehow, when a dissolved molecule triggers a taste bud, or cell, certain nerves at the root of the cell are also stimulated. These carry impulses to the brain stem, then to the thalamus or the front of the brain stem. The impulses finally end up in the cerebral cortex of the brain, the brain’s taste control center. The brain interprets this signal, or impulse, and tells people what they are tasting. As with each of the senses, all of this happens instantaneously.

THE SENSE OF TASTE IN ANIMALS Some invertebrates (organisms without a backbone) do not have a separate sense of taste or smell. Both are linked in a sense called chemoreception, which means the ability to detect chemicals. Single-celled proU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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tozoans as well as most insects and crustaceans (like crabs, shrimp, and lobster) all use chemoreception. Among vertebrates (animals with a backbone), the organs of taste can be very different. Although birds have their taste receptors on their tongues, adult amphibians, like frogs and toads, and certain fish have chemical receptors in their mouths and over parts of their skin. Most mammals have their taste receptors on their tongue.

THE SENSE OF TASTE IN HUMANS The taste regions of the tongue (left) and taste bud anatomy (right). (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

The sense of taste plays an important evolutionary role for all animals. It allows them to know what is good to eat and what is to be avoided. A fruit that has not yet ripened usually tastes very sour. Humans will probably not eat much of it since they cannot tolerate something that is strongly sour. On the other hand, ripe fruit tastes sweet—a taste people prefer. Interestingly, many plants that are toxic to people also have a sour

Bitter Sour

Circumvallate papillae Filiform papillae Salt Sweet

Fungiform papillae

Trough Taste buds

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or bitter taste. When the body is salt-depleted from overwork, it usually craves something salty to eat. Babies who are Vitamin D deficient seem to prefer the strong fishy taste of cod-liver oil, whereas it seems to revolt almost everyone else.

Taxonomy

In humans, taste is also affected by other sensations such as temperature, texture or feel, and even looks. Certain foods have a particular feel in people’s mouths that affect whether or not they like them. Some cold drinks are not good at room temperature. Finally, human culture, habit, and learning play a major role in what people find appetizing and enjoyable to eat. For many people, these psychological factors are even more important than their basic biological needs. [See also Organ; Sense Organ]

Taxonomy Taxonomy is the science of classifying living things. It follows rules and procedures for classifying organisms according to their differences and similarities. Classifying living things enables scientists to see how an organism is related to others and to trace its evolution (the process by which gradual genetic change occurs over time to a group of living things). Until modern taxonomy was invented by the Swedish naturalist, Carolus Linnaeus (1707–1778), a name for any given organism was usually a long, detailed description rather than an actual name. Since there were no rules, or standards, to follow when naming a newly discovered form of life, most people went about it in an unscientific and rather individual way. Linnaeus realized that some system of order had to be imposed on the great diversity of life if people were to understand one another. As a result, Linnaeus not only devised a system that identified and named each living thing, but was able to place each living thing in one of several related categories.

HIERARCHICAL CLASSIFICATION Most people do this type of organizing in their daily lives without thinking about it. For example, a music store groups all CDs together and keeps them separate from music on tape. Within the CD section, the store makes more detailed categories of music, like rock, jazz, hip-hop, and others. Within one of these categories, it may further break it down into groups or solo artists, and alphabetize within one of those categories. This type of systematic arranging that goes from general groups to more specific groups is called hierarchical classification. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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It was hierarchical thinking that Linnaeus used when he first built his classification system. He divided all forms of life into the first and largest group called kingdoms. In the mid-eighteenth century, science recognized only two kingdoms, plant and animal. With more knowledge, scientists have added three more: fungi, protists, and bacteria. Taxonomy is an always-growing and adaptable science that allows room for additions, changes, and even corrections. As a result, each kingdom was divided into smaller and smaller categories, with each level more general than the one below it. These seven categories are kingdom, phylum, class, order, family, genus, and species. A useful trick to remember this order is the sentence, “King Philip Came Over From Great Spain.” These categories, or taxonomic groupings, are also called taxa (singular, taxon). For example, the taxon Carnivora is an order that includes many different families. Amazingly, the seven taxa, or hierarchical levels, that were finalized by Linnaeus in 1758 are still the ones used today.

BINOMIAL SYSTEM OF NOMENCLATURE Linnaeus made another major contribution to organizing the natural world. He gave a two-part scientific name to every living thing. Since this method was based on certain standards, or rules, and was made up from Greek or Latin words, it gave everyone an agreed-upon name to use for a particular organism. This system avoided the confusion that could arise when different languages or countries used different names for the same animal. It also allowed all scientists to speak the same language. As a result, a Russian researcher knows exactly what organism is discussed in a paper published by a French biologist. The Linnean system came to be called the “binomial system of nomenclature,” and there are several strictly followed rules that govern its use. The system uses two Latin names—the genus and species to which an organism belongs—for its scientific name. Binomial names are always underlined or italicized. The first letter of the genus is always capitalized, while the species is always written in lowercase letters. For instance, Homo sapiens is the scientific name for humans. The genus name may be used alone, but the particular species name is never used without the genus name. There are now taxonomic rules that govern the worldwide classification of organisms. No changes can be made without the approval of the appropriate international body.

NEW METHODS OF CLASSIFICATION Since taxonomy is an evolving science, it is not surprising that it includes more sophisticated ways of grouping and classifying organisms. 566


One new method is to take into account new molecular and biochemical information. Another looks only at the chromosomes (coiled structures in the nucleus of a cell that carries the cell’s genetic information) of an organism to see how it the organism is related to others. In many ways, the aim of modern taxonomy is to arrive at an evolutionary-based classification system that will let scientists see more clearly the real relationships between all living organisms.

Territory

[See also Class; Classification; Family; Genus; Kingdom; Order; Phylum; Species]

Territory A territory is an area that an animal claims as its own and that it will defend against rivals. Competition for territory often comes from within an animal’s own species. Territorial animals usually have a better chance at survival and reproduction than animals with no territory of their own. Many animals that reproduce sexually exhibit a type of behavior that is called territorial. These animals will defend a certain territory or portion of their habitat against other individuals of their own species. Territoriality is often described as adaptive behavior. This means that being territorial somehow works to the advantage of the organism that engages in this behavior. Being territorial means being willing to defend a certain area by using threats or by engaging in actual fighting. Studies have shown that when an animal is successfully territorial (meaning that it is able to exert control over a certain area and keep all rivals out) it obtains many benefits, all of which have survival value. One of these benefits could be protection from other species that want to eat it. Other benefits might be plentiful food and safe breeding areas. A male animal that controls a certain area also demonstrates its dominance to females. These females may then want to mate with the territorial male. An animal may choose a particular area as its place to defend for many reasons. Whatever the reasons, they all are based on the idea that the place contains resources vital to the animal’s survival. Territories are usually smaller segments of a larger habitat, but some predators, like wolves, actually claim and defend several square miles of territory. On the other hand, a sea bird may be territorial only around its actual nesting site. Territoriality can be permanent, as when an eagle returns to and will defend the same general area every nesting season. Prairie dogs remain U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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IVAN PETROVICH PAVLOV Russian physiologist (a person who studies how an organism and its body parts work or function normally) Ivan Pavlov (1849–1936) is best known for his systematic studies of the conditioning of dogs. His work on animal behavior and inborn reflexes revealed a great deal about animals’ true nature. His work has also been applied to understanding human learning behavior. Ivan Pavlov was born in Ryazan, in western Russia. Since his father was a priest, it was expected that he would become one too, and he was sent to study at a theological seminary. There, however, the young man somehow happened to read English naturalist Charles Darwin’s On the Origin of Species, and he turned immediately toward the study of science. By the time he was twenty-one, he had transferred from the seminary and the study of religion to St. Petersburg University and the study of science. There he had the good fortune to study with two of Russia’s best chemists, and in 1879 he received his medical degree from the St. Petersburg Military Medical Academy. Four years later he earned his Ph.D. For the next few years he studied and did more research in Germany, and by 1890 he had returned to St. Petersburg and become a professor of physiology there. By then, Pavlov had already begun his surgical experiments on the physiology of digestion (how digestion actually works). He demonstrated this skill by actually disconnecting a dog’s esophagus (the tube leading to its stomach) so that when the dog ate, the food would drop out and not enter the stomach. Since he found that the stomach produced gastric juice as if there were food being put in it, he suggested correctly that a message must have been sent from the brain by way of nerves to the digestive glands, which then secreted the gastric juices. This helped establish the importance of the autonomic nervous system, which is that part of the nervous

in the same burrows all their lives and will drive away any others of its species. Territoriality can also be temporary, as when a bird abandons a certain shrub on which it has been feeding after the bush stops producing the food it needs. Possessing control means benefits to the territorial animal, but it also means that the animal must expend some resources to enforce that control. The most important of these enforcing actions is called marking. Many animals use their own urine or feces to mark the perimeter (boundary) of their territory. Dogs and cats do this, although they will also “mark” another’s territory temporarily as they are passing through it. Other marking systems includes howls and shrieks by monkeys and sea lions, calls and whistles by birds, and aggressive displays and warnings by certain 568


system that controls an animal’s involuntary actions. It was for this work on the physiology of digestion that Pavlov won the 1904 Nobel Prize in physiology and medicine. He then went on to develop the idea of the conditioned reflex, the discovery for which he is most famous. Pavlov began this work studying an animal’s natural reflexes, which could be described as the simplest form of instinct. Instinct is a behavior pattern that is inherited, or which is built into its nervous system. Instinct is thus handed down from one generation to another. It was Pavlov’s goal to see if he could somehow change, or alter, in some way an animal’s instincts. Noticing that a dog would naturally salivate, or drool, when it thought it was about to be fed, he rang a bell before giving it food. Soon the dog would salivate whenever Pavlov rang the bell, whether food was produced or not. With this, Pavlov had actually changed its natural reflex and developed in the dog what he called a learned, or a conditioned, reflex. This led to his theory that a good part of an animal’s behavior patterns are actually the result of conditioned reflexes.

Territory

The notion of an animal’s territory is a good example of instinctive behavior. In territorial species, or those that will defend a certain part of a habitat against intruders, this behavior is simply part of their genetic makeup. When a male bird selects a territory at the beginning of the breeding season, he does this because of the high concentration of sex hormones in his blood. Despite the fact that territoriality is an inborn trait for many species, Pavlov’s work demonstrated that it, like most other instinctual behavior patterns, can be conditioned or modified. Pavlov’s contribution of the notion of the conditioned reflex showed life scientists that an animal’s reaction is not always purely instinctive but may also have been somehow learned through a sequence of associations.

animals if a rival gets too close. Bears are known to scratch deep gouges on certain trees as markers. Although territoriality may sometime involve conflict or actual fighting, studies have shown that in the long run, it tends to reduce conflict within a species. Once territories are established and understood, most respect the boundaries of others and fighting actually diminishes. This might have something to do with another interesting fact. In most species, the “owner” of a certain territory almost always wins a fight with an intruder, often despite a difference in size. It has been shown that the same animal that acts timid and surrenders in a conflict located on neutral ground becomes an unbeatable fighter when it is defending its “home field.” Scientists also say that individual humans are often territorial U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Tissue

when they display a need for privacy or for personal space in crowded situations.

Tissue Tissue is the name for a group of similar cells that have a common structure and function and which work together. Tissues fit together to form organs in higher animals These animals have four basic types of tissue.

A microscopic view of human fetal tissue. (Reproduced by Custom Medical Stock Photo, Inc.)

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The animal body is made up of many different kinds of cells. These specialize to perform certain functions, or specific tasks. A group of closely associated or similar cells that work together to do one thing is called a tissue. All the cells in a tissue look very similar and all do the same work.


TYPES OF TISSUE

Tissue

There are four types of animal tissue: epithelial, connective, muscle, and nervous tissue. Every cell in each of these tissues is an independent unit, yet all of the cells in a given tissue interact with one another. They must do this in order to perform a certain necessary function for the animal’s body. For example, an animal’s brain is made of nerve tissue that is composed of millions of connected nerve cells. In animals, when two or more tissues are associated and work together, they form an organ such as the stomach or the heart. The same is true for plants, except they have only three types of tissue. Plant tissue, like animal tissue, is also organized into organs, so that epidermal plant tissue makes up the organ known as a leaf. For both plants and animals, a tissue is the “stuff,” or specific type of cellular material, out of which specialized organs are made.

HUMAN TISSUE As animals, humans have four different types of tissues: epithelial, connective, muscle, and nervous tissue.

Epithelial Tissue. The epithelial tissue, whose closely packed cells make it ideal for forming a lining or a covering, is what makes up the skin. This tissue also makes up the lining for the organs and the many passageways. Because its cells are so tightly packed together, it allows the skin to be an excellent barrier and protects the body against injury and invading microorganisms (any form of life that is too small to be seen without a microscope). It also regulates fluid loss.

Nervous Tissue. The nervous tissue is made up of cells called neurons. These are designed to carry impulses. This highly specialized tissue carries electrical signals from one part of the body to another and allows it to function as an organized unit.

Muscle Tissue. Muscle tissue is responsible for producing movement. Its cells are designed to work by contracting and relaxing. They also are built to respond to stimuli transmitted by neurons. There are three types of muscle tissue. Smooth muscle makes up organs that people do not control, such as the intestines. Striated, or skeletal muscle, is under people’s control and makes up the muscles that allow people to move. Cardiac muscle is also called heart muscle and is responsible for the regular and powerful contraction of the heart. Unlike skeletal muscle, it never needs to rest.

Connective Tissue. Finally, connective tissue is the most common tissue in the body, and it is what holds the entire body together. Bone, carU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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tilage, and blood are made of connective tissue. Bone is a type of mineralized connective tissue, cartilage is found between joints, and blood is considered to be a connective tissue in liquid form since it circulates throughout the body.

PLANT TISSUE Plants have three types of tissue. Their epidermal tissue serves a purpose similar to humans, in that it covers the surfaces of leaves, stems, and roots and protects its inner parts. The tissue through which a plant transports materials is called its vascular tissue. Its cells are elongated and form tubelike organs. The rest of a plant is made up of what is called fundamental plant tissue. In plants and animals then, tissues are the intermediary stage between an organism’s individual cells and its specialized organs. Therefore, cells are arranged into groups, called tissues, and tissues are eventually grouped into organs. [See also Cell; Organ]

Touch Touch is the sense that enables an organism to get information about things that are in direct contact with its body. As one of the five senses, touch allows a person to feel heat, cold, pain, and pressure. Touch is a very important sense, since it tells an organism a great deal about its immediate environment. Touch is one of the least-thought about senses, perhaps because it does not involve a complex and highly specialized organ or specific organs that are identified with that sense. The eyes, ears, nose, and mouth are linked immediately and automatically with vision, hearing, smell, and taste. Yet the sense of touch is identified with the largest organ in the human body—the skin.

HOW THE SENSE OF TOUCH WORKS Touch is detected by sensory nerves in the skin called “mechanoreceptors.” These receptors are designed to respond to specific stimuli, because touch actually involves a variety of very different sensations. On the human body, there are well more than 500,000 sensory cells unevenly distributed over the skin’s surface, and not all of them are alike. There are at least a half dozen different types of known touch receptor cells in humans. Some respond to light pressure, others to vibrations, and others to pain or temperature changes. 572


Each of these different types of receptors has a different structure. For example, capsule-like receptors called “Merkel’s discs” are located in parts of the body that are extremely sensitive, such as the fingertips. As one would expect, there are far fewer of these type of cells where there is less need for them, such as on a person’s leg or shoulders. Certain other parts of the body have uncovered nerve endings called “free endings.” The skin around the hair roots has the largest amount of these free nerve endings.

Touch

In order for a person to feel sensation, one of these points on the skin must be stimulated in some way. When this happens, the receptor cells respond by generating a nerve impulse. The impulse is transmitted along pathways via the spinal cord to the brain. The nerves that carry impulses or information from the skin to the spinal cord are called spinal nerves. Each pair of these nerves transmits signals from a particular skin area. The first stop in the brain is the thalamus. The thalamus directs the signals to the appropriate part of the brain’s cerebral cortex (the outer layer of gray matter covering the cerebrum) responsible for processing touch information. The brain then decides whether to pay attention to the signal and if so, how to respond to it. These steps all must happen instantaneously, since any sort of delay could mean serious injury or worse. Little is known about the exact way that the brain interprets what is basically a coded impulse, which it then converts into the actual sensation that we experience or feel. The normal brain correctly lets people feel pain when they burn their finger, and pleasure when a sore muscle is massaged. It also allows people to gently squeeze a fruit and tell if it is ripe, sense an ant crawling up their leg, check a child’s temperature with their hand, or “read” the raised dots of a Braille book. Touch also acts as an early warning system, informing people of serious or dangerous conditions before they encounter them fully.

THE SENSE OF TOUCH IN ANIMALS The sense of touch is no less important for all other organisms. The variety found in the sense organs of animals is based on evolution according to the particular needs of the species. In other words, nature keeps what works best, even if it is a little different. Examples from nature make this point very well. The overly large ears of most rabbits best serve an animal whose first and only defense is a quick getaway. Rabbits’ acute hearing sometimes gives then a slight edge over predators. As for the sense of touch, certain animals rely more on this sense than others. An earthworm that spends its life crawling about and has a minimal amount of specialized organs has special hairs that generate a nerve impulse when U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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they contact something. Some insects have similar hairs on their legs and antennas that “touch” the air and sense changes in currents. This is why it is so difficult to swat a housefly, which knows what is coming by the moving air it senses. Fish also have a sense of touch founded in their lateral line. These are nerves running lengthwise on their bodies that allow them to detect the presence of nearby organisms or objects. The tongues and beaks of many birds have a well-developed sense of touch. The whiskers of a cat or a rat enable them to “feel” things with their long bristles almost in the way humans do with their fingertips. For all organisms, including humans, touch allows us to coordinate bodily movements. Because of touch, people unconsciously sense where their legs, arms, head, eyes, muscles, and internal organs are. Touch disorders can range from the minor but troublesome itching caused by allergies, diseases, infections, or bites. Numbness or pins-and-needles can signal a more serious nerve injury or can result from a blood clot, tumor, heart attack, or stroke. Chronic pain, which has no explanation, is another type of touch disorder.

THE BENEFITS OF TOUCH Science has only recently realized that touching and being touched has important psychological and physical advantages, especially to mammals. It is now known that the stimulation of the skin an infant receives by being held while feeding is as important as the food. Studies have shown that in cases where mammals, like a cow or horse, lick their newborn, licking the skin actually stimulates the nervous system and gets it working. Newborn animals that are not licked usually die, because their intestinal or urinary systems never begin working. Human babies that were never held often display signs of “failure to thrive.” Finally, humans have a psychological need to touch and be touched. For example, studies have shown that seniors who live alone but have an animal to care for and “pet,” are in better health and spirits than those who do not. [See also Organ; Sense Organ]

Toxins and Poisons Toxins and poisons are chemical substances that destroy life or impair the function of living tissue and organs. Although used interchangeably, a toxin is usually considered to be any poison produced by an organism. 574


A poison, however, is more generally any type of substance that harms an organism by its chemical action. As a chemical harmful to living things, a poison can be almost anything. Nearly every prescribed drug can have negative effects if taken in the wrong dosage. In certain situations, whether or not something is harmful depends entirely on the dosage or amount taken. Other substances can be beneficial to one living form (penicillin cures a sick person) but deadly to another (it kills bacteria). Although in common usage, the words toxic and poisonous are often used interchangeably, as are the words toxin and poison, it is safe to say that poison is the broader term. Poisons are so diverse in their origin, chemistry, and toxic action that it is nearly impossible to classify them simply. Many have natural origins, such as those that are produced by living things. Some of the most potent toxins known are produced by simple bacteria. For example, bacteria are responsible for the diseases botulism, diphtheria, and tetanus.


Toxins and Poisons

All of these common household products are toxic and poisonous when absorbed into the systems of humans and animals. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

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Certain mushrooms are highly toxic, while a fungus is responsible for the usually deadly disease called ergotism. More complex plants contain a large variety of poisonous substances that are part of the alkaloid group. Many of the better-known drugs, such as quinine, curare, morphine, and nicotine, come from this family of plants. These drugs can be poisonous depending on the dosage. Many animals also produce toxins, including certain snakes, frogs, and insects. A mammal, like the shrew, can produce a poison in its salivary glands. Poisons are also found in the nonliving part of the natural world. Some organic elements like mercury, arsenic, and lead, all called heavy metals, are poisonous when absorbed into the systems of living things. Many strong acids and bases are damaging to soft tissues. A number of gases, like chlorine and ammonia, are toxic even at low doses. Many alcohols and solvents (a liquid that dissolves other substances) can do extreme damage to people’s organs, while many plastics made from petroleum can be toxic. As a result, today’s industrial society has come to present people with a major source of human-made poisons. The action of these and other poisons is generally described in terms of what part of the body they affect or what biochemical changes they produce. While the actual chemical mechanism may be different for each poison, they can be grouped generally according to their effects. Many poisons block, or inhibit, something that should be happening in the body. Thus, the toxic gas carbon monoxide makes the body’s cells unable to transport necessary oxygen. Other poisons prevent a certain critical enzyme (a protein that acts as a catalyst and speeds up chemical reactions in living things) from being released, while others block a crucial step in a person’s metabolism (which are all of the chemical reactions and sequences that happen in living things). The effects of toxins are classified according to time-based categories. That is, an acute effect happens immediately, as would the distress caused by a venomous snake bite. Acute effects often involve damage to organs. If the effects appear over days, weeks, or months, they are called subacute. For example, a tetanus infection takes some time to develop and show its worst symptoms. Effects that appear very gradually and last for an extended period of time are called chronic effects. A cancer caused by exposure to toxic waste is described as chronic. Often, an individual’s susceptibility to toxins and poisons varies depending on his or her genetic background, age, weight, gender, overall health, and previous exposure to the toxin or poison. [See also Pollution]

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Tree

Tree

A tree is a tall, woody plant with one main trunk and many branches. It is perennial (grows for many years) and distinguished from a shrub by its size (usually 15 feet [4.6 meters] or taller) as well as by its single woody stem or trunk. A shrub is a short, woody plant that usually divides at its base. Trees are the most visible and dominate part of a forest. They also play an important role in the world’s environment and economy. Trees vary greatly in size, with the tallest being the redwood trees of coastal California that reach well over 350 feet (106.7 meters). Trees began their evolution more than 400,000,000 years ago. The oldest known living trees are the bristlecone pine (Pinus aristata) growing in the Rocky Mountains which are believed to be more than 4,500 years old. The widest tree is a giant sequoia (Sequoia gigantea) growing in central California. Nicknamed the “General Sherman,” it has a circumference (measured completely around) of 115 feet (35.1 meters). Trees also make up the largest living organism in the world, with some stands or groups of trembling aspen trees covering as many as 100 acres (404,700 square meters). These trees reproduce by sending out roots that send up new sprouts that in turn become trees. Since the trees are all genetically identical and actually con-

A pine forest. Pine trees are classified as conifers because they reproduce using cones. Pine trees also are classified as softwoods. (Reproduced by permission of JLM Visuals.) U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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nected, they can be said to represent a single organism. Trees are found in every region of the world except deserts, the Arctic, and Antarctica. Woody plants, like trees, have tough stems covered by bark that do not die when the growing season has ended. It is their tough, woody stems that allow them to grow as tall as they do. Trees can be divided by the type of wood they form. Conifers (produced in cones) like pine and spruce are called softwoods, while most others are considered hardwood. A tree is like any other green plant in that it draws in water from its roots and makes food in its leaves through photosynthesis (the process by which plants use light energy to make food from simple chemicals). Deciduous trees, like maples and oaks, lose their leaves for a season and grow new, flat broadleaves each spring. Evergreen trees, like pines, are always covered with leaves (which have evolved into sharp needles), although they are constantly losing them and replacing them in small numbers. As with all plants, trees are either angiosperms (producing flowers and seeds with coverings), or gymnosperms (producing naked seeds). Conifers are gymnosperms since they produce seeds on modified leaf structures called scales that are on their cones. Most other trees produce flowers and are angiosperms. While spring flowers on a fruit tree are obvious and beautiful, the flowers on some trees do not resemble anything like a flower. The age of a tree can be determined by counting its rings after it has been cut down. Each year new wood is formed in a layer that is outside that of the previous year’s wood growth. Very dry years result in thin rings, and years of good rainfall result in thicker rings. Forests of trees have been called the “lungs of the world,” since they provide an enormous amount of oxygen to the environment as a by-product of photosynthesis. Besides providing oxygen, trees and the forests they make up form the habitat for many animals, and virtually every part of a tree has been put to some use by humans—even a tree’s sap and bark. Trees supply wood for fuel as well as for lumber, paper, and plastic products. Humans dependence on trees for products has begun to have a negative impact on forests. Today, deforestation is a growing problem, especially in less developed countries where trees are being cut down faster than they are able to grow. [See also Forest; Plants; Rain Forests]

Tundra The tundra is a geographical region that is a cold, treeless, boggy (wet, spongy ground) plain with a permanently frozen layer of subsoil. Its harsh 578


conditions will not support a wide variety of species. Tundra occupies approximately one-fifth of Earth’s land surface. The tundra can be found only in the extreme northern latitudes where snow melts only in the summertime. It has extremely long, cold winters and short, cool summers, along with regularly low precipitation. Despite receiving barely more rain than a desert does, it is among the wettest landscapes on Earth. This is because it is usually overcast, which minimizes evaporation. All tundra have permafrost, which is a layer of permanently frozen earth that lies just below the surface. This permafrost (permanently frozen subsoil) also prevents water from draining downward. For about fifty days in the summer, the tundra has temperatures warm enough to partially thaw the permafrost. The name tundra comes from the Finnish word tunturi meaning “completely treeless heights.” All tundra are treeless, mainly because tree roots could only spread horizontally along the surface of the ground since they cannot penetrate the permafrost. These roots would therefore not be able to anchor themselves and grow tall.


Tundra

A view of the alpine tundra in Kizlar Sivrisi, Turkey. (Reproduced by permission of JLM Visuals.)

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PLANT LIFE IN THE TUNDRA As with every demanding climate, the plants and animals that live in the tundra have adapted to it in order to live. The soil that lies above the permafrost is a thin layer of cold, moist earth that contains little nutrients for plants. The cold kills off most of the bacteria, so decomposition (breaking down of waste) barely occurs. The common plants found in all parts of the tundra are blankets of fast-growing moss and lichen. Both are able to grow or go dormant at any time and can quickly respond to any sudden weather change. Wildflowers do bloom, however, sometimes through the melting snow so as to take full advantage of the short summer growing season.

ANIMAL LIFE IN THE TUNDRA The animals that survive in the tundra are also well-adapted to its demands. The larger animals, like the musk ox and polar bear, are well insulated by thick fur. Other predators like the Arctic wolf, wolverine, peregrine falcon, snowy owl, and white fox, prey on lemmings and Arctic ground squirrels. Caribou also migrate (seasonally move) from the taiga (coniferous forest) to the tundra in the summertime in search of food. Many birds migrate north to the tundra in summer to nest and feed on heavy swarms of insects. There are no reptiles or amphibians in the tundra. Because of its scarce resources and delicate balances, the tundra is one of nature’s more delicate biomes (particular types of large geographic regions). It has been shown that the tundra is most susceptible to environmental damage, mainly because it takes so long to regenerate after being disturbed. Ecologists know that even small disturbances can have immediate and devastating chain reactions throughout the entire system. [See also Biome]

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V Vacuole A vacuole is a large, fluid-filled storage sac inside a cell. Many types of cells contain vacuoles, but they are most prominent in plant cells. A vacuole can store materials that a cell needs. It also provides the necessary internal pressure for a plant to remain erect and not wilt. A vacuole is an organelle or a specialized structure inside a cell that has a particular function. Its main purpose involves storage and transport of materials inside a cell. Vacuoles in animal cells can store a variety of substances, including lipids (a kind of fat) and carbohydrates. In plants, vacuoles are very prominent, and when seen under a microscope they appear as large, round, clear structures. Vacuoles are filled with a watery fluid called cell sap (made up of water, salts, and sugar). In some plant cells, a vacuole may occupy as much as 90 percent of a cell’s total volume. When a vacuole takes up this much space in a cell, it presses the cytoplasm (jelly-like fluid in a cell) against the cell wall, which eventually remains stretched out under this force. As long as the vacuole remains full, the plant maintains its turgor pressure (like the air pressing against the inside wall of a balloon). When the vacuole loses this fluid and decreases the pressure, it collapses temporarily and the plant wilts. Maintaining this cellular pressure is the prime responsibility of the vacuole in a mature plant. When pressure is maintained, the entire plant is able to keep its crisp shape. When it does not, the whole plant wilts. [See also Cell; Organelle; Plant Anatomy] U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Vascular Plants

Vascular Plants Vascular plants are plants with specialized tissue that act as a pipeline for carrying the food and water they need. All plants, except bryophytes like mosses, liverworts, and hornworts, have developed special, internal systems to transport their requirements from one part to another. Vascular plants have systems that move nutrients and water from the soil to the stems and leaves and transport food to where it is needed. A green plant is about 90 percent water, but it loses much of its water through evaporation from its leaves. Because of this, plants need a steady supply of water to grow and reproduce. Much of this need for water is satisfied by the plant’s root system that develops in a network spreading down and out from the stem. Once the roots reach water, however, they must be able to get the water back up to the branches and leaves. Plants are able to achieve this by having tissues, or cells that are specialized, to perform a particular function. Tissue moves water is called vascular tissue. Vascular comes from the Latin for “little vessel.” There are two types of vascular tissue in a plant: xylem tissue and phloem tissue. The xylem is a ring or a series of hollow tubes that extend from the roots, up through the stem, and out to every branch and leaf. The xylem is located near the surface of the stem and in a tree, it forms a band just below the bark. The xylem is made up of dead, hollow cells arranged end-to-end so that they look like a tube or a straw. The xylem carries water and minerals, like sap, from the roots on a one-way trip to the stem and leaves. The second part of a plant’s circulatory system is the vascular tissue called phloem. Unlike xylem, the cells that make up phloem are living. The phloem carries food (in the form of organic molecules) that the leaves and stems have made by photosynthesis (the process by which plants use light energy to make food from simple chemicals) to parts of the plant that are unable to make their own food (such as the roots and stem tip). In a vascular plant, xylem and phloem are always found together in groups that are called vascular bundles. These bundles may be distributed evenly through the body of a stem, grouped in rings near the outside, or found in a ring in the outer part of the stem according to the type of plant. Water and minerals are moved from the belowground roots to the aboveground stem and leaves by a phenomenon known as water potential. Although plants do not have a pump (as animals have a heart) to push water up from their roots, they nonetheless are able to move water upwards by water potential without expending any energy. Water potential

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works according to the laws of physics. When a wet and dry place are joined by a tube of water, the water always flows toward the dry area. This is similar to the law of physics that says when hot and cold objects come together, the heat always travels toward the cold. In a plant, there is usually a large moisture difference between roots (which are surrounded by water) and leaves (which are constantly losing water through their surface pores). This steady water loss is called transpiration. As the roots absorb water, the pressure builds up in them, causing the water to move up the xylem to the drier parts of the plant (stem and leaves). As a result of water potential and transpiration, water always moves on its own from the roots to the leaves.

Vertebrates

Plants also rely on the water supplied by their vascular systems to keep their shape. Water that is drawn into its cells create what is called “turgor pressure” and, like an inflated balloon, the cell becomes full or rigid. If a plant loses more water through its leaves than it is able to take in from its roots, it will lose some of this turgor and begin to go limp or wilt. If such a situation continues for any length of time, the plant will eventually die. [See also Plant Anatomy]

Vertebrates A vertebrate is an animal with a backbone. They also have internal skeletons and a system of muscles and bones that allows them to move about easily. All vertebrates have bilateral symmetry, well-developed body systems, and a brain that controls many functions. Vertebrates are divided into five groups, all of which vary in structure, life cycle, and behavior. These five groups are further divided according to whether or not they are cold-blooded or warm-blooded.

CHARCTERISTICS OF VERTEBRATES All five major classes of vertebrates—fish, amphibians, reptiles, birds, and mammals—belong to the phylum Chordata. The animals in these five classes all have a true backbone, meaning that it is made of bone and not cartilage. A backbone, or spinal column, is made up of individual vertebra (plural, vertebrae) that first form as cartilage in the embryonic stage and then ossify, or harden, into bone. The vertebrae all lock together and give the entire column rigidity and support as well as flexibility. The vertebrae are separated and cushioned from each other by soft, flexible structures known as discs. Inside the hollow center of each vertebra runs a colU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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ALFRED SHERWOOD ROMER American paleontologist (a person who studies animals, plants, and other organisms that lived in prehistoric times) Alfred Romer (1894–1973) was one of the most influential evolutionary biologists of the twentieth century. Romer’s detailed studies of the evolution of extinct fishes, amphibians, and reptiles enabled him to be able to trace the basic changes in their structure and function that came about as they evolved into more complex land animals. As the ultimate authority on the evolution of vertebrates, Romer can be said to have demonstrated Darwin’s ideas. Alfred Romer was born in White Plains, New York, the son of a newspaper man. Since his family moved many times, he was eventually sent back to White Plains at the age of fifteen to live with his grandmother. At age eighteen, he spent a year doing odd jobs to earn money for college, and entered Amherst College entirely on his own initiative. He was able to pay for his schooling through scholarships, jobs, and loans, and finally graduated in 1917 knowing that he wanted to become a paleontologist (one who studies the life of the past by examining its fossil remains). During World War I (1914–18), the army sent Romer to France, and on his return home in 1919, he entered graduate school at Columbia University and earned his Ph.D. in

umn of soft nervous tissue called the spinal cord that is connected to the brain and is an essential part of the animal’s nervous system. The hard, bony, yet flexible vertebrae protect the delicate cord. Vertebrates also have several other things in common. All have what might be called a “head end” containing a control center and sensory organs. As vertebrates are forward-moving animals, it is important that their sense organs be located in the front-most part of their bodies. Vertebrates also have bilateral symmetry, which means that a line drawn lengthwise (top-to-bottom) through the center of the body divides it into halves that are mirror images of each other. This could only be the case for an animal that has a head end, a tail end, and a right and left side. All vertebrates have an endoskeleton, which is a hard bone and cartilage framework located inside their bodies and which supports and maintains their body shape. An endoskeleton linked to a system of muscles enables vertebrates to move. Vertebrates also have a brain and highly developed nervous systems. Finally, vertebrates have a closed circulatory system and a well-developed heart that pumps blood to all parts of the body. Although there are some 40,000 species of vertebrates in the world

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just two years. His dissertation on myology (the scientific study of muscles) became a classic in that field. In 1923 Romer joined the University of Chicago, and in 1934 moved to Harvard University where he remained until his retirement.

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Romer spent nearly all of his career investigating vertebrate evolution (the process by which all living things change over generations). His comprehensive books, Vertebrate Paleontology and The Vertebrate Body among others shaped much of the thinking of his subject for decades. Romer was able to focus attention on the importance of form and function as it related to an animal’s environment as being the key to how it evolved. His painstaking studies of fossils enabled him to document the slow changes that occurred over long periods of time. Romer’s work would bring English naturalist Charles Darwin’s theory of evolution to life as Romer used evidence taken from comparative anatomy and even embryology (the study of the early development of organisms). His work also contained scores of examples of specific anatomical adaptations that had taken place in organisms as a result of or in response to environmental changes. Romer not only shaped and defined his field, but was able to give tangible evidence of Darwin’s ideas of adaptation through natural selection.

today, vertebrates actually make up only a very small percentage of the total number of animals.

FISH Fish are the most successful group of vertebrates and belong to the class, Osteichthyes which means “bony fishes.” As the name implies, these fish have skeletons made of real bone and cartilage, like eels, sharks, and rays, which are not true fish. A fish is a vertebrate animal that lives in the water and breathes through gills. The bones of a fish are usually thin and light since they can use the natural buoyancy of water to support their bodies. Fish are also ectotherms (“cold-blooded”) which means that their body temperature changes with that of their surroundings. Fish spend their entire lives in water and, therefore, have a respiratory system with gills that uses the oxygen that is dissolved in water. They cannot take oxygen directly from the air. Fish are streamlined in body design and made to move forward efficiently. Fish also have fins that are winglike structures used for balance and control. Most fish have overlapping scales that are covered with mucus to help them glide through the water. They have a U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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heart, but their circulatory system is the simplest of the vertebrates. Fish reproduce sexually through the union of male sperm and female eggs, and they lay these eggs.

AMPHIBIANS Amphibians are the second group of vertebrates and they, like fish, are also cold-blooded. Amphibians are considered to be the first vertebrates to develop legs and move from water to the land. Their name is taken from Greek words meaning “to have two lives.” This is so because the natural life cycle of an amphibian requires that it live partly on land and partly in the water. An amphibian, like a frog, spends part of its life underwater as a tadpole having the characteristics of a fish. The tadpole then undergoes a metamorphosis, or total change in body shape and function, and develops into a land animal. Amphibians, therefore, have “two lives.” Most amphibians, like a salamander, frog, toad, and newt, lack a waterproof layer to the skin and, therefore, must keep their skin moist. They usually have two pairs of limbs as adults. Amphibians reproduce sexually and the female lays her eggs in the water. When the eggs hatch they look like little fish and even breathe with gills. Soon, however, with the release of a certain hormone (chemical messenger), they will begin to grow legs as their tails shrink and the body enlarges, and they will develop air-breathing lungs and lose their gills.

REPTILES The third group of cold-blooded amphibians is made up of reptiles. Although people immediately think of giant dinosaurs that dominated Earth when they think of reptiles, the name now includes snakes, lizards, turtles, and alligators and crocodiles. A reptile is a vertebrate with dry, scaly skin and sealed eggs. Reptiles are not to be confused with amphibians because reptiles have fully adapted to life on land. Although many live in and around water, they always breathe air through lungs. The complete transition to land was made possible when reptiles began laying eggs with a thick, leathery shell that contained water and food. This allowed the egg to be laid on dry land where it would remain safe until it eventually hatched. With the exception of snakes, reptiles usually have four limbs. Snakes have long, thin bodies, no legs, and a jaw that can unhinge and open wide to swallow large prey. Lizards are much like snakes but have two pairs of legs. Turtles have hard protective shells above and below their bodies, but they also have an endoskeleton. Alligators and crocodiles are similar and have very tough armored skin and a powerful tail for swimming. Reptiles reproduce sexually and most lay eggs. 586


BIRDS AND MAMMALS

Vertebrates

Birds and mammals make up the warm-blooded vertebrates. A warmblooded animal is able to maintain a constant body temperature no matter how hot or cold its surroundings are. Unlike cold-blooded animals, which slow down in the cold, warm-blooded animals can remain active because they convert their food into body heat. They also have a covering of feathers or hair that keeps them warm. Birds are the only animals with feathers, and along with bats, are the only vertebrates able to fly. A bird is a warm-blooded animal with wings and feathers. All birds have a beak, two legs, and reproduce by laying eggs. They are found in nearly every environment. The act of flying consumes a high amount of energy, and birds have very efficient respiratory and circulatory systems. They also have powerful breast muscles to push their wings back against the air, and hollow bones that are extremely light. Birds’ beaks and feet are especially adapted to their habitat and diet, and the shape of a bird’s beak and the type of feet it has can usually give clues to what the bird eats. Birds reproduce sexually and lay eggs that have a hard shell. Being able to fly gives a bird a definite competitive advantage since it allows them to escape quickly and easily from danger, as well as leave, or migrate, to another place where food is more plentiful. A mammal is a warm-blooded vertebrate with hair that feeds milk to its young. Mammals are named after the mammary glands of females that secrete milk and provide food for their young. There are about 4,500 species of mammals, which include humans. Rats and elephants are mammals as are bats and horses. All mammals have some hair or fur on at least part of their body that helps to keep them warm. The mammal brain is also larger than other vertebrates and allows for greater learning. Mammals breathe air through lungs and have specialized teeth and a four-chambered heart. Mammals also exhibit complex behavior that is often directed by instinct, or an inborn pattern, of doing things a certain way. In most mammal species, the unborn young remain inside the mother’s body until they are fully developed. Female mammals that mate and are fertilized develop a special organ called a placenta that carries food and oxygen to the embryo and also takes waste away. Most mammals live on land, although whales, dolphins, seals, and manatees live in water. The current geological era is called the Age of Mammals because mammals have successfully spread throughout the world. Mammals are considered by humans (who themselves are mammals) to be among the “higher” vertebrate animals. Although mammals may earn this distinction because they demonstrate evidence of obvious learning and exhibit complex behavior, U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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mammals may also be classed as “higher” vertebrate animals since humans are the ones making up the rules of classification. [See also Amphibians; Birds; Fish; Invertebrates; Mammals; Reptiles]

Virus A virus is a package of chemicals that infects living cells. Composed only of some genetic material inside a protein coat, it is not considered a living thing since it does not reproduce on its own nor does it carry on respiration (the process by which living things obtain energy). A virus is only able to do these things while inside a host cell, which it usually kills in the process of duplicating itself, often causing a disease. Ever since the existence of viruses was first demonstrated just before the twentieth century, viruses have regularly puzzled biologists who sought to classify them. Examining their characteristics seems to place viruses on the borderline between living and nonliving things. Viruses are smaller than the smallest bacteria, and were not able to actually be seen until the postwar invention of the electron microscope. Being able to see them and watch how they behaved did not make it any easier to classify them, however. Viruses at first appear to be a living organism since they are made of some of the same compounds and chemicals that are found in living cells. However, they are not themselves cells nor are they made up of cells (as all living things are). Furthermore, viruses do not have a nucleus or any other cell parts, and they cannot reproduce unless they are inside another living cell. In fact, they do not carry out any of the typical life processes, or metabolic activity, that living cells do. Despite all of this, when they find an appropriate cell, they quickly reproduce themselves, usually to the disadvantage of the host cell and sometimes the entire organism. Anyone who has ever had a case of the chicken pox, measles, or the flu has been infected by a virus. Some viral diseases like polio, AIDS, yellow fever, meningitis, encephalitis, and rabies can be fatal. Others, like herpes or the common cold, only make us sick temporarily. Overall, viruses are responsible for at least 60 percent of the infectious diseases around the world. A virus can also infect bacteria and fungi as well as plant and animal cells. Viruses are incredibly tiny and come in different shapes according to what type of cells they invade. Animal viruses are usually round and look like little puffballs. Plant viruses resemble rods, while bacterial viruses look like tadpoles with little tails. Despite their ap588


pearance, they do not have a true cell structure with a nucleus or other parts of a cell. Instead, viruses consist of a core of nucleic acid covered by a protective coat of protein called the capsid. The nucleic acid (either DNA or RNA) contains directions for making more viruses.

Virus

Unlike a living cell, a virus has one job only—to reproduce or replicate itself. Once inside, it acts like a computer program for making new viruses. When it enters or infects a cell, a virus takes over the cell’s metabolism or its chemical reactions, since it does not have the machinery, the energy, or the raw materials to do anything on its own. Like an invader, it takes charge of the cell and instructs it to produce everything that the virus needs to reproduce itself. In a sense, it “reprograms” the cell it invades so that the infected cell’s systems and energy are available to and controlled by the virus. Biologists have been able to study viruses in action by observing how they infect bacteria. First, viruses do not attack just any type of cell. Viruses do not cross kingdoms, so a virus that attacks a plant cannot infect an animal. Other viruses only attack certain species within a kingdom, while others will only attack certain types of cells in a species. Once a virus finds its proper cell type, it first attaches itself at a certain place on the surface of the cell. It next penetrates the cell by drilling into it, which immediately stops the cell from making its own genetic material.

Three labeled diagrams showing differently shaped viruses. (Illustration by Hans & Cassidy. Courtesy of Gale Research.)

RNA Membranous envelope

Spike Capsid DNA

Capsid

RNA Capsid

Spikes

Rod-like virus

Icosahedral virus


Spherical virus

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With the virus DNA or RNA now inside the cell, the cell begins to go to work copying and making virus DNA. After a half-hour or so when the cell has made many copies (one polio virus can produce 100,000 copies of itself inside a single cell), it makes a certain enzyme called a lysozyme. This enzyme attacks its cell walls, which soon burst open, killing the cell but releasing the many virus copies. These go on to invade and take over more cells, all the while reproducing more and more of the virus. Some viruses do not destroy the cell but simply join their DNA to that of the cell which, when it divides, also reproduces the virus. The organism whose cells are infected by the virus becomes diseased either because the virus is destroying its cells or damaging those that it takes over. Fighting a virus is very difficult. Drugs that work against bacteria cannot work on viruses since viruses have no metabolic activities (the internal processes that make a cell work) of their own to attack. Since viruses use all of the infected cell’s systems, anything that would stop a virus from reproducing would also attack the cell itself. By destroying the virus, the cell is also destroyed. An organism’s natural defenses are the best antivirus weapon, so virus-caused diseases can be prevented by the use of certain vaccines. A vaccine is a substance that contains a weakened version of a virus that, although it can no longer cause a disease, encourages the body to create substances called antibodies that will later specifically attack and kill the virus when it invades. Vaccines have proven successful for many virus-caused diseases like polio, measles, and the many different types of influenza. Viruses have ways of fighting back, including mutating or changing their makeup. Viruses like the flu or HIV (the Human Immunodeficiency Virus that causes AIDS) are able to change frequently enough so that a certain type of vaccine that may have been successful will no longer work on the new version. Other viruses can stay hidden for a long time. This type of virus is called a latent virus, and it does not take control of the cell as soon as its penetrates it. It still duplicates itself but does not harm the cell until at some later time when it becomes active and begins its destruction. Although modern medicine has made great strides in fighting or preventing viral infections, it has proven especially difficult to create new drugs that will limit or stop the growth of a virus inside a cell without doing the same to the cell itself. Ever since their discovery, viruses have proven to be not only a baffling phenomenon but also an ultimate type of parasite that has proven to be a potent and highly resourceful enemy. [See also AIDS; Immune System; Immunization; Nucleic Acid]

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Vitamins

Vitamins

Vitamins are organic compounds (substances that contain carbon) found in food that all animals need in small amounts. Although they usually cannot be made by the body, they are essential to its proper functioning. Vitamins are not a source of energy, but instead serve to help enzymes (proteins that act as catalysts and speed up chemical reactions in living things) make important chemical reactions occur. Vitamins are essential to life. Although all animals need them for proper metabolism (the chemical processes that take place in a living thing), different animals need different types or varying amounts. All animals need some vitamins in their systems, since most vitamins are necessary coenzymes. A coenzyme is something that helps an enzyme speed up a chemical reaction in the body. Specifically, it helps regulate the chemical reactions that take place inside an organism when an organism converts food into energy. Some enzymes cannot work at all by themselves and require a vitamin to do their job.

Some of the different types of vitamin supplements. An overdose of some vitamins can accumulate to harmful levels in the human body. (Reproduced by permission of Archive Photos, Inc.)

Vitamins are named by letters, and there are thirteen vitamins that have been identified as being necessary for human health. They are vitamins A, C, D, E, K and eight B vitamins. Since the B vitamins were originally thought to be one vitamin, they were first given different numbers, like B1, B2, etc. Later, all but B6 and B12 were given actual names. The deficiency, or lack, of one certain vitamin usually shows itself in a very specific condition that is described as a deficiency-related disease.

TYPES OF VITAMINS There are two types of vitamins: water-soluble vitamins and fat-soluble vitamins. Something is soluble if it is capable of being dissolved. Fatsoluble vitamins are stored in the body in the fats and oils that are taken U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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FREDERICK GOWLAND HOPKINS English biochemist Frederick Hopkins (1861–1947) discovered the existence of “accessory food factors”—now known as vitamins—which are needed in animal diets to maintain health and life. He also laid the foundation for the concept of essential amino acids (those that cannot be made by the body and must be supplied in the diet). Frederick Hopkins was born in Sussex, England, and had an especially lonely and unhappy childhood being raised by a widowed mother and an unmarried uncle who ignored him. When he was seventeen, his uncle found him a job in the insurance business. Despite doing this for several years, he managed to take part-time courses at the University of London and eventually earned a degree in chemistry when he was twenty-seven. By then he had received a small inheritance and was able to attend medical school at Guy’s Hospital in London. After receiving his medical degree at the age of thirty-three, he finally began his real career at Cambridge University when he was thirty-seven years old. Although a very late starter, Hopkins made up for lost time, and in 1900 made his first discovery in his area of dietary research. He found that rats fed only gelatin would not live and grow, despite the fact that gelatin was a protein. He then found that the important amino acid called tryptophan was missing in gelatin. Additional research revealed that tryptophan (as well as several other amino acids) could not be manufactured in the body. He discovered that this amino acid had to be supplied in the diet. In doing this, Hopkins thus laid the foundation for the

in. Water-soluble vitamins are not able to be stored, and any excess passes out of the body. Vitamins A, D, E, and K are fat-soluble vitamins. Vitamin A is essential for the sense of sight, and a lack of Vitamin A is a common cause of blindness in developing countries. Vitamin D is important for healthy bones and teeth (a lack causes the disease rickets), and is found mainly in fish and eggs. It is also formed in the skin when it is exposed to sunlight. Vitamin E plays a major role in the reproductive system, and Vitamin K helps the blood to clot. The other nine vitamins are water-soluble vitamins. Vitamin B1 is called thiamin and is found in the outer layer of cereal grains. It helps break down carbohydrates (group of naturally occurring compounds that are essential sources of energy for all living things). B2 is known as riboflavin and is necessary for cellular respiration (the breaking down of food into energy). B3 is called niacin and also works in cellular respiration. B5 is pantothenic acid which helps convert carbohydrates, fats, and proteins into energy. B6 does not have a commonly used name, although 592


concept of the essential amino acid, which was detailed by others a generation later.

Vitamins

Hopkins then went on to study diet and its effect on metabolism (all of the chemical processes that take place in a living thing). At this point in time, nutritional science was not very advanced, and most scientists believed that such diet-related diseases as scurvy, beriberi, or rickets were caused by some sort of toxic substances in food. Hopkins’ research soon led him to have serious doubts about this thinking, and his experiments only confirmed this. He had already noticed that his laboratory rats failed to grow when fed a diet of artificial nutrients. However, those whose artificial nutrients contained a tiny amount of cow’s milk grew rapidly. This led him to suspect that normal food must contain substances missing from the artificial nutrients that contained only pure fats, proteins, and carbohydrates. Hopkins never was able to isolate them or find out what these trace substances were, but in 1906 he wrote a paper in which he described them as “accessory food factors.” He pointed out that whatever they were, they were obviously essential for growth. This paper is considered to be the first explanation of the concept of vitamins. In 1925, the young man who started life as an insurance man and whose determination got him his first teaching job at thirty-seven, was knighted and thereafter was called “Sir.” Four years later he was awarded the Nobel Prize in physiology and medicine for being the first to suggest what became known as the “vitamin concept.”

it is technically called pyridoxine, and it helps break down fatty acids and amino acids (the building blocks of proteins). B12 can be called cobalamin and helps make proteins. Biotin also is a B vitamin and is necessary for the formation of some fatty acids. Folic acid is needed to help make nucleic acids (genetic material). Finally, Vitamin C, also called ascorbic acid, is needed for healthy teeth, gums, and bones as well as for healing. While each of these thirteen vitamins plays a very specific role in the body, each also fills a more general need in promoting overall wellbeing or good health.

HOW TO OBTAIN VITAMINS Most physicians say that the best way to get the right amounts of vitamins into our systems is to eat a regular balanced diet. This means that a variety of foods from each of the basic food groups (fats, carbohydrates, proteins, and minerals) should give people the relatively small amounts of the thirteen vitamins they need. Today, however, many people take viU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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tamin supplements just to be sure they are getting enough. Recent trends in taking what are called megadoses of certain vitamins are more controversial, and some physicians warn that large doses can be dangerous. While some of the water-soluble vitamins people take are simply excreted in their urine, if they take too many fat-soluble vitamins they will not pass the body and can accumulate to harmful levels. Humans’ knowledge about vitamin deficiency diseases is fairly certain. For example, it is known that Vitamin C cures scurvy and Vitamin D can cure rickets. However, we do not yet fully understand all of the biochemical roles played by vitamins, nor are all of the interactions among vitamins and other nutrients known. [See also Malnutrition; Nutrition]

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W Water Water is a liquid that is essential to life and plays many important roles. Water molecules (small particles) are the most common molecules in the body of every organism. Water is the ideal medium for transporting essential nutrients into the body and for transporting waste products out of the body. Water is a chemical compound made up of one oxygen atom bonded to two hydrogen atoms (H2O). Without water, it is safe to say that there would probably be no life on Earth. Virtually all of the chemical reactions that occur in every organism take place in some sort of a watery environment. Because it is an excellent solvent (meaning that it has the ability to dissolve many different substances), water plays a key role in life. Water is the best way to move substances into and out of cells because of its ability to be a “universal solvent.” Since many substances dissolve in water, they are able to cross cell membranes and enter cells. Water also is the major constituent of all living things. If all the molecules in an organism were sorted and counted, the majority would be water molecules. In some living things, this percentage is as low as 60 percent water, but in some ocean animals like the jellyfish, the percentage is closer to 90 percent. The human body is about 65 percent water. Humans know that they cannot live without drinking water. Although some people can go as long as six weeks without any food, without water they would die in a matter of days. Water is the primary component of blood, lymph (a colorless liquid that bathes the tissues of the body), and the fluids in all of the body’s tissues. The body regulates its temperature with water by perspiring, and eliminate wastes with water. U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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Water molecules are the most common molecules in the body of every organism. Without water there would probably be no life on Earth. (Reproduced by permission of the National Park Service.)

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Water is just as essential to plants as it is to all animals. Water is a key raw material that plants use during photosynthesis (when they make their own food). During photosynthesis, plants use the hydrogen atoms from water molecules (H2O) and add them to carbon atoms to make carbohydrates (a group of naturally occurring compounds that are essential sources of energy for all living things) and other forms of food. The oxygen molecules (O2) given off during photosynthesis come from water molecules. Water is a unique substance with special properties. Water is able to absorb a great deal of heat, and on a global scale (that is, on something the size of Earth), it can act as an enormous moderating influence. When Earth’s surface temperatures begin to rise too high, the oceans act like thermal sponges and soak up much of the excessive heat. Water also has a high surface tension, which means that its molecules closely stick together. This explains why it is able to support a broad, flat object, like a boat, and why certain insects can glide across its surface.


Water is also the only common, natural substance that is able to be found in three different physical states. Ice is its solid state, and water assumes a rigid crystalline structure at or below 32°F (0°C). It is found as snow, hail, frost, glaciers, and ice caps. Its liquid state has no particular shape and exists up to a temperature of 148°F (64.4°C). Water in this form covers three-fourths of the Earth’s surface in the form of oceans, lakes, rivers, swamps, as well as rain clouds, dew, and ground water. The gaseous state of water occurs above 148°F and does not have a definite volume. This means it takes on the exact shape and volume of its container. It occurs naturally as fog, steam, and clouds.

Wetlands

Because of water’s unique hydrogen bonding, ice is less dense than water. If this were not so and ice did not float, then all bodies of water would freeze from the bottom up, becoming solid masses of ice and destroying all life in them. Water also is a major force for changing the Earth’s surface, as erosion regularly carves away softer parts and can cause major alterations and changes in the Earth’s natural features. The major use of water by humans is for crop irrigation, although a great deal is used industrially, as well as to keep individuals, homes, and communities clean. Since much of the water used to irrigate crops either evaporates or is given off by plants, humans are a major factor in the hydrologic or water cycle. The water cycle describes the process in which water circulates between the sea, the atmosphere, living things, and the land in a continuous cycle [See also Ocean; Pollution; Wetlands]

Wetlands Wetlands are a diverse group of areas in which land is saturated with water for prolonged periods. They are among the most productive environments on earth and support a wide range of plant and animal life. Wetlands can be freshwater, saltwater, or brackish (part fresh and part salt) and are one of the most delicate and complex ecosystems (areas in which living things interact with each other and the environment). Wetlands are nether totally aquatic (watery) nor terrestrial (dry land), but rather are in the zone between permanently wet and normally dry conditions. Wetlands can be found on every continent, and are often called marshes, swamps, or bogs. Despite their fragility, wetlands perform a variety of important, natural functions. Where the sea meets the land, tidal estuaries or salt marshes form a buffer zone, protecting the mainland from the onrushing sea. Wetlands improve water quality by filtering the water that passes through them. They also remove much of the excess carbon U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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and nitrogen that humans pump into the air. Wetlands contain a great deal of food for sea life and other animals, while providing a safe haven for spawning (egg laying and hatching).

CHARACTERISTICS OF WETLANDS

Wetlands, such as the one pictured here, are a diverse group of areas in which land is saturated with water for prolonged periods. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

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No matter what wetlands are called, they all have three characteristics. First, they all have waterlogged soil. In such soil, water is at or above the soil surface for a long enough time during a year to influence and determine what grows there. Second, a certain type of plant called a “hydrophyte” grows in wetlands. Although there are as many as 5,000 species of such specialized plants, from mangroves and cattails to bald cypress and rushes, all are hydrophytes because they have in some manner developed their own way of getting a steady supply of air to their submerged roots. Third, the soil found in a typical wetland is mostly poor in oxygen and very acidic. This also determines what can grow there.


Wetlands are usually either freshwater wetlands or saltwater wetlands, although there are some in which the two meet and mingle. Saltwater wetlands naturally occur at the coasts where the sea meets the land. Tidal salt marshes with salt-tolerant plant species can be found along the Arctic and Atlantic seaboards of North America, the Gulf of Mexico, and the European coastline. Mangrove forests whose thick trees are concentrated in the Indian Ocean and West Pacific region, are the subtropical equal of tidal salt marshes. Most of the wetlands in the temperate parts of the world are freshwater marshes. In the continental United States, 90 percent of the wetlands are freshwater marshes. Worldwide, wetlands account for about 6.5 percent of the Earth’s total land surface, and the bulk of them are found in the world’s tropical zones.

Wetlands

A river like the Mississippi River, which cuts down across the United States from Minnesota to the Gulf of Mexico, is a good example of how such a large moving body of water with seasonal flooding can build wetlands and keep them alive. Each spring, as the gorged river would search for the shortest route to the Gulf of Mexico, it would overflow its banks, depositing rich sediment. The river also pushed most of the saltwater at the Gulf back into the sea. The combination of sediment buildup and freshwater infiltration allowed wetlands to develop. Many plants that were sensitive to salt were able to take hold and grow, keeping the land in place. Until the eighteenth century, wetlands were considered worthless and places to be avoided, and this annual flooding process was left undisturbed. But with the development of the levee system (a bank of earth along a river to prevent flooding), begun in New Orleans in the early 1700s, the flooding was stopped and the now-dry land became “useable” by people. With the continued development of levees and permanent flood control, the Mississippi River now has no choice but to deposit its wealth of sediment into the Gulf of Mexico. Since then, the wetlands of New Orleans have been disappearing.

PLANT AND ANIMAL LIFE IN WETLANDS The variety of plant and animal life found in and around wetlands is huge and amazing. They are home to all types of birds, snakes, fishes, turtles, raccoons, beavers, alligators, and shellfish. All these and many more animals depend on wetlands for food and shelter. Wetlands also provide a welcome resting and feeding place for migrating (seasonally moving) birds. Many animals need to be in wetlands in order to reproduce. A great deal of today’s commercial fishing industry depends in some way on the fertility of wetlands. Because they are so rich in nutrients and in all manner of interdependent life forms, wetlands are very delicate, intriU X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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cate, and vulnerable ecosystems. Major changes in wetlands can produce a ripple effect.

HUMAN EFFECTS ON WETLANDS Despite national laws and international agreements protecting critical wetlands, the impact of human development has had a profoundly negative effect on wetlands. Wetlands are, therefore, among the most threatened of habitats, and hundreds of thousands of acres are disappearing every year in the United States alone. Without the enforcement of laws put in place to protect wetlands, they may be one of the first ecosystems to vanish completely. [See also Biome; Water]

Worms A worm is a common name given to a diverse group of invertebrate animals that have a long, soft body and no legs. All worms used to be classified together in one phylum called Vermes, but biologists now divide them into three phyla. The bodies of worms mark a higher level of complexity on the invertebrate evolutionary ladder, and they are by no means the simplest of animals. The least complex of the worms is the flatworm, a member of the phylum Platyhelminthes. As their name implies, they are flat like a ribbon and are either free-living or parasitic (organisms that live in or on another organism and benefit from the relationship). Flatworms include planarians, flukes, and tapeworms—the last two are parasitic. Planarian are found living in bodies of water. They are very small and have a simple digestive system and one opening that both receives its food and excretes its waste. Planaria can crawl and swim with hairlike cilia and reproduce sexually and asexually. Flukes are flatworms that live as parasites in the liver or blood of an animal host, and tapeworms are found in the intestines of vertebrates. Members of the phyla Nemotoda (roundworms) and Annelida (true, or segmented worms), have bodies with three cell layers—a trait common to all “higher” animals. This advance means that they have a fluidfilled cavity in which internal, specialized organs can be suspended. Roundworms, or nematodes, are an abundant species and are more complex than flatworms. They have a mouth at one end and an anus at the other through which waste is excreted. They are not really round but have 600


long, tapered bodies than come to a point at each end. They move about by contracting a single, long muscle that pulls their head and tail end closer together. They are covered by a hard, outer layer called a cuticle. Many roundworm are parasites, and some are very harmful to humans. One example is the hookworm that enters the human body by boring through the soles of the feet, entering the bloodstream, and traveling to the lungs. There they bore through the bronchi and the windpipe and enter the throat to be swallowed, passing eventually into the intestines where they finally attach themselves. Hookworms drain a host’s blood and cause severe anemia (lack of blood). Another dangerous worm is the trichina roundworm. Humans can contract trichinosis by eating meat infected by the trichina roundworm. This worm reproduces in the intestines and its larva pass into other body parts and form cysts in muscle tissue, causing pain, fever, weakness, and even death. Few segmented or true worms are parasitic, although the leech is one of them. The leech is a segmented worm with a sucker at each end of its body. It lives by feeding off the blood of other animals. Called annelids, segmented worms have bodies composed of identical ringlike sections or segments. The word annelid means “little ring” in Latin. Annelids have three tissue layers and therefore can support specialized organ systems. They also have a hydrostatic skeleton, which means that they keep their


Worms

Worms are beneficial because their digging action improves the soil. (Reproduced by permission of Field Mark Publications. Photograph by Robert J. Huffman.)

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JEAN-BAPTISTE LAMARCK French naturalist Jean-Baptiste Lamarck (1744–1829) founded modern invertebrate zoology. He was the first to use the words “vertebrate” and “invertebrate” and also popularized the word “biology.” He was a pioneer of evolutionary theory and was the first scientist to describe the adaptability of organisms. Born in Picardy in northern France, Jean-Baptiste Lamarck’s full name was Jean-Baptiste Pierre Antoine de Monet Lamarck and he had the title of “Chevalier.” This was, in fact, the lowest rank of the French nobility, and although his family was part of the aristocratic class, it was still very poor. As one of eleven children, he was supposed to become a priest, but entered the army instead when his father died. Six years of fighting for France left him with medals for bravery but also bad health, so he resigned and tried several different occupations. Having taken an interest in plant life when he was stationed on the Mediterranean coast, he wrote a book about the plants of France, which caused him to be noticed by the well-known French naturalist, Georges Louis Leclerc Buffon (1707–1788). It was through Buffon that Lamarck eventually was appointed botanist (a person who studies plants) to King Louis XVI. After the French Revolution (1789–99), Lamarck became professor of zoology at the Museum of Natural History in Paris. Nearly fifty years old, Lamarck finally started to focus his energies on one thing. The object of his attention was a large group of organisms that, until Lamarck, had been hardly considered at all. In fact, the great Swedish botanist Carolus Linnaeus (1707–1778) himself had grouped these creatures

body shape by the pressure of their internal fluid pushing against the walls of their body. This is similar to the air inside a full balloon. Annelids can move about on the ground by contracting different sets of muscles that move their segments. Earthworms are a type of roundworm that have a complex digestive system. This system includes a mouth and a tubelike esophagus that leads to the storage crop, which connects to a gizzard where food is broken down. From there, food passes into an intestine and eventually waste is expelled through the anus. Earthworms also have a closed circulatory system and several “hearts” that pump blood. They lack a respiratory system since they exchange gases or breathe through their skin (which must always be coated with mucus). Their nervous system consists of a simple brain and a main nerve cord. Earthworms are not only an important food source for many animals, but they improve the soil by their digging 602


into only two general categories: insects and worms. Although Linnaeus had basically not even attempted to classify these organisms, Lamarck set out to give this huge group some order. The first thing he did was to name the entire group “invertebrates” since they did not have a backbone and were therefore different from “vertebrates,” or animals with a backbone. He then set out to classify invertebrates according to their anatomic similarities. So he said that eight-legged arachnids (spiders) were different from six-legged insects, and that echinoderms, like starfish and sea urchins, were different from crustaceans, like crabs and shrimp. He continued this work on invertebrates even as a very old man and finally produced a huge, seven-volume Natural History of Invertebrates that essentially founded modern invertebrate zoology and became his most important contribution to the life sciences.

Worms

While he was creating his classification of invertebrates, Lamarck began to develop an evolutionary theory to explain the differences between living animals and fossils. He soon proposed the idea that species gradually change over time and argued that all living things could be arranged in such a way as to show how some species gradually changed into another. However, it was in his specific explanations as to how this actually came about that he created the incorrect theory now known as “Lamarckism.” This theory wrongly states that characteristics, or traits, acquired by an individual during its lifetime are passed on to its offspring. Despite his errors, Lamarck’s concern with evolutionary theory gave it much-needed attention, and he should be credited as a true pioneer of evolutionary theory as well as the founder of invertebrate zoology.

action (which allows air and water to filter in) and by their waste, which fertilizes it. Most segmented worms have both male and female organs, although they cannot fertilize their own eggs. Fertilization is accomplished when worms exchange sperm with one another. They can also regenerate or grow back a part that has been cut off. [See also Invertebrates]


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Z Zoology Zoology is the scientific study of animals. Its subjects are highly diverse, and it includes a wide range of other disciplines. Some of these study the whole animal in a very broad way, such as animal ecology, while others examine animals on a smaller scale, such as their anatomy (structure) or physiology (function). Zoology is one of the two main branches of biology. The other branch of biology is botany or the study of plants. Zoology includes the study of every type of animal, from the 180-ton blue whale to the one-celled bacteria. Animals make up the largest of the five kingdoms that were created by biologists to organize and describe the living world. Besides animals, the other kingdoms are monerans, protists, fungi, and plants. Described in the simplest way possible, animals are multicelled organisms that move about and live by taking in food. They are classified into two main groups: vertebrates have bony internal skeletons and invertebrates do not. There are eight different groups, or phyla, of invertebrates, all of which differ greatly in body structure, where they live, and how they reproduce. Vertebrates all belong to the same phylum and are classified as cold-blooded (the internal body temperature changes with the environment) or warmblooded (internal body temperature remains the same despite the environment). Finally, animals react to what goes on in their environment. Much of this behavior involves communicating with other animals, and searching for food, mates, and a place to live. As a science, zoology began with the Greek philosopher Aristotle (384–322 B.C.), who developed his own system to classify animals. His accomplishments and reputation were so great that much of what he wrote U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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about animals remained unquestioned for nearly 2000 years. It was not until the middle of the eighteenth century that a modern system of classifying animals was created by the Swedish botanist, Carolus Linnaeus (1707–1778). Another century would also pass before biology, and therefore zoology, would be given a theory of evolution (the process by which living things can change over time) to explain the origins and development of animal life. That great theory, published in 1859 by the English naturalist Charles Robert Darwin (1809–1892), laid a new foundation for zoology. Since the nineteenth century, the field encompassed by zoology has grown so large that it eventually became divided up into many other fields or branches. Some of these are broad subject areas, like embryology, which studies the development of individual animals, and anatomy that studies the structure of an animal’s body. Other branches focus on only one type of animal, such as ichthyology, which studies fish, or entomology, which studies insects. There are also other fields in zoology that focus only on animal behavior. Technical advances have always played a major role in the advancement of science, and they have allowed zoology to progress rapidly as well. Beginning with the seventeenth-century invention of the microscope, zoology benefitted by the new ability to see not only tiny life forms but also smaller details of other animals that had not been known. Further improvements allowed zoologists to examine animals at their cellular level. Recent breakthroughs in gene technology have given zoologists the ability to manipulate an animal’s genetic makeup for a number of scientific and commercial purposes. Zoological research benefits humans in many ways, one example of which is the use of bacterial studies to learn how to protect people from being infected by disease. Since cattle and chicken are key to humans’ food supply, zoologists regularly search for ways to produce healthier animals. Finally, zoologists also work to preserve those animal species that are endangered and could possibly go extinct.

Zygote A zygote in animals is a fertilized egg or ovum. It is the first cell produced when a male gamete (sperm) unites with a female gamete (egg). Since it contains one set of chromosomes from each parent, the zygote will develop into a unique individual. The act of fertilization brings together the sex cells of two different members of the same animal species. It eventually produces a fully de606


veloped offspring, which begins as a one-celled fertilized egg called a zygote. Whether fertilization is external (as when a frog sprays his sperm over the female’s eggs in the water) or internal (when a male mammal deposits them inside the body of the female), the moment an individual sperm makes physical contact with the egg, fertilization begins. The egg is surrounded by a jelly-like film called the “zona pellucida” that the sperm must break through. It does this by actually contacting the film, which causes the tip of the sperm head to rupture. This releases a chemical that opens a hole through the outer layers of the egg.

Zygote

As the sperm head descends through the layers, tiny projections called “microvilli” emerge from both the sperm and the egg, and it is these that first fuse together. Following this, the actual membranes of both sperm and egg fuse and the cytoplasm (jelly-like fluid) of the egg engulfs the sperm. The sperm releases its genetic material into the egg as the nucleus (the cell’s control center) of both merge into one new nucleus. At this point, the sperm and egg have fully merged. The sex of the new offspring

An electron micrograph of a fertilized human zygote. This zygote is seen one day after fertilization and is preparing for its first cell division. (©Photographer, Science Source/Photo Researchers, Inc.) U X L C o m p l e t e L i f e S c i e n c e R e s o u r c e

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and all the instructions needed to produce this new organism are already in place and are beginning to work. Because each parent has donated one set of chromosomes, the genetic information (or genome) of the offspring will represent a unique combination of deoxyribonucleic acid (DNA). The DNA contains the genetic instructions for each cell. The zygote, therefore, is the first cell of this future offspring. As soon as the zygote is formed, a series of rapid cell divisions called cleavage begins. The zygote divides into two cells, then four cells, then eight, and so on. Each new cell has the same genetic makeup as the original zygote. As the cleavage process continues and the developing cell mass grows, it forms a berry-like, compact ball called a “morula.” This ball soon develops different layers, which eventually begin to form their own specialized cells. Some layers become muscle and bone while others become part of the different organs and systems. In humans, five days after fertilization and the creation of a zygote, the zygote has been transformed into the multicelled beginnings of an embryo, which will eventually become a fully developed offspring. [See also Egg; Embryo; Fertilization; Human Reproduction; Reproduction, Sexual; Sperm]

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Index Italic type indicates volume number; boldface indicates main entries and their page numbers; (ill.) indicates photos and illustrations.

A Abbe, Ernst 2: 383 Abdomen 1: 145 Abiotic/Biotic environment 1: 1–2, 180–81 Abscisic acid 3: 465 Abyssal zone 3: 425 Acid rain 1: 4–6 effects of, 1: 6 (ill.) Acids 1: 2–4 Acquired immune deficiency syndrome. See AIDS (acquired immune deficiency syndrome). Acquired immunity 2: 316–17 Acrosome 3: 547 Adaptation 1: 7–8, 207, 403 Adaptive radiation 1: 209 Adenine 1: 165, 170 Adenosine triphosphate (ATP) 2: 389 Adrenal glands 1: 191, 193; 3: 531, 553 Aerobic/anaerobic 1: 8–11 Aerobic respiration 2: 346; 3: 514 Agent Orange 3: 466 Agglutination 1: 69 Aging 1: 11–13

study of 1: 12–13 Agricultural revolution 1: 16 Agriculture 1: 13–17, 15 (ill.) AIDS (acquired immune deficiency syndrome) 1: 17–21, 20 (ill.), 2: 319 Air pollution 3: 477 Aldrovandi, Ulisse 1: 195 Algae 1: 21–24, 23 (ill.) Alimentary canal 1: 158 Alleles 1: 168; 3: 499 Alternation of generations 2: 354; 3: 550 Alvarez, Luis Walter 1: 220 Amber 2: 239 Amino acids 1: 24–25, 24 (ill.); 3: 492 Ammonification 2: 411 Amoebas 1: 25–27, 26 (ill.); 3: 496 Amoeboid protozoans 1: 27 Amphibians 1: 27–30; 2: 288; 3: 586 life cycle of, 1: 29 (ill.) Anabolic metabolism 2: 374 Anabolism 2: 374 Anaerobic. See Aerobic/anaerobic. Anaerobic respiration 2: 346; 3: 514 Anaphase 1: 106 Anatomy 1: 30–33 The Anatomy of Plants 1: 73 Anaximander 1: 211 Androgens 3: 532 Aneuploidy 1: 125 Angiosperms 3: 472, 525 Animacules 2: 380


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Index

Animal cell 1: 102 (ill.) Animals 1: 33–35 embryonic stages 1: 185 migration of 2: 386 pheromones 3: 446 study of 3: 605 Annelida 2: 338; 3: 600 Annelids 3: 601 Anteater, spiny 2: 368 Anthropodin 1: 42 Anthropoids 3: 490 Anthropology, physical 1: 105 Antibiotics 1: 35–37, 36 (ill.) Antibodies 1: 37–39; 2: 317, 321; 3: 491 Antigens 1: 37–39, 39 (ill.), 69; 2: 317 Antitoxins 2: 319 Apes 3: 491 Arachnid 1: 39–41, 40 (ill.), 43 Archaebacteria 2: 392 Aristotle 1: 132, 195; 2: 363; 3: 455–456, 605 anatomy 1: 30 evolution 1: 211 Arrhenius, Svant August 2: 274–75 Arteries 1: 127–28 Arthropods 1: 41–44, 145–46; 2: 339 Artificial insemination 2: 228 Ascorbic Acid 3: 593 Asexual reproduction. See Reproduction, asexual. Assimilation 2: 411 ATP. See Adenosine triphosphate (ATP). Atrium 2: 284 Auditory receptors 3: 528 Audubon, John James 1: 62–63; 3: 436 Australopitecus 2: 295, 297, 305 Autolysis 2: 360 Autonomic nervous system 2: 292; 3: 553 Autosomal disorders 2: 255 Autotrophs 2: 235, 286, 419; 3: 426 Auxins 3: 464

B Bacteria 1: 11, 25, 45–48, 48 (ill.); 2: 392–93; 3: 468 Bacteriocidal agents 1: 37

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Baer, Karl Ernst von 1: 184 Basal Metabolism Rate 2: 375 Bases 1: 2–4 Bayliss, William Maddock 2: 302 Beaumont, William 1: 158–59 Becquerel, Antoine Henri 3: 501 Benthic environment 3: 424 Benzene 1: 94 Bernard, Claude 1: 55; 2: 291 Bible of Nature 1: 197 Bilateral symmetry 1: 34 Bile 1: 160 Binary fission 1: 27; 3: 506 Binocular vision 3: 535 Binomial system of nomenclature 1: 73, 133–34; 3: 566 Biodiversity 1: 49–52; 2: 278 Biogeochemical cycle 1: 181 Biological community 1: 52–54 Biological diversity. See Biodiversity. Biology 1: 54–55. See also Anatomy, Botany, Zoology. Biomass 2: 236 Biomes 1: 55–59, 179 Biosphere 1: 59–61 Biotic environment 1: 1–2 Biotin 3: 593 Biotrophs 2: 243 Birds 1: 61–65; 3: 587 study of 3: 434–36 The Birds of America 1: 63; 3: 436 Bivalves 2: 390–91 Black, Joseph 1: 90 Bladder 1: 216 Blood 1: 66–68, 67 (ill.) Blood types 1: 67, 68–70; 3: 519 Blue–green algae 2: 392 Boerhaave, Hermann 1: 197 Boltwood, Bertram B. 3: 501 Bone 3: 540 Boreal forest. See Taiga. Botany 1: 71–74 Boyle, Robert 2: 226, 280 Brain 1: 75–78, 76 (ill.); 2: 406 Breeding 2: 322 Bridges, Calvin Blackman 1: 123 Brown algae 1: 22–23


Brown, Robert 1: 109 Bryophytes 1: 78–79; 2: 413; 3: 472 Buds and budding 1: 79–81; 80 (ill.), 3: 506 Buffon, Georges Louis 1: 212

C Calorie 1: 83–84 Campbell, Keith 1: 138 Canidae 1: 98 Canine teeth 1: 97 Capillaries 1: 127–28, 131 Carbohydrates 1: 84–87, 85 (ill.) Carbon cycle 1: 87–90, 88 (ill.) Carbon dating. See Radioactive dating. Carbon dioxide 1: 90–92 Carbon family 1: 92–95 Carbon monoxide 1: 95–97 Carbon monoxide poisoning 1: 96 (ill.) Cardiac muscle 2: 284, 394, 396; 3: 571 Carnassials 1: 97 Carnivores 1: 97–100; 3: 425 Carpels 3: 469 Carrying capacity 3: 481 Carson, Rachel Louise 1: 176–77 Cartilage 3: 539 Catabolic metabolism 2: 374 Catabolism 2: 374 Catalysts 1: 199–200 Cell division 1: 105–08, 107 (ill.) Cell membranes 1: 102, 112; 2: 370 Cells 1: 72 (ill.), 100–05, 102 (ill.), 108 Cell sap 3: 581 Cell theory 1: 108–10 Cellular pathology 1: 104 Cellular respiration 3: 514 Cellulose 1: 112, 201; 3: 460 Cell walls 1: 103, 111–12; 3: 476 Centipedes 1: 43 Central nervous system 2: 404 Centrioles 1: 103, 112–14, 113 (ill.) Cephalopods 2: 390–91 Cephalothorax 1: 145 Cerebellum 1: 77 Cerebral ganglia 1: 75 Cerebrospinal fluid 1: 77

Cetacean 1: 114–16, 115 (ill.) echo-location 2: 283 CFCs (chlorofluorocarbons) 3: 439 Chaparral 1: 116–18, 117 (ill.) Chemoreceptors 3: 528, 542, 563 Chemotherapy 2: 318 Chitin 1: 42; 2: 333 Chlorofluorocarbon. See CFCs (chlorofluorocarbons). Chlorophyll 1: 119; 3: 449 Chloroplasts 1: 103, 118–20, 119 (ill.); 3: 430, 449 Chromatids 1: 106 Chromatin 1: 120–21 Chromosomes 1: 103, 121–25, 124 (ill.), 2: 248, 250, 418 diagrams 2: 341 sex 3: 529 Chromosome theory of inheritance 1: 122 Chyme 1: 160 Cilia 1: 103, 125–26, 126 (ill.) Ciliophora 3: 498 Circulatory system 1: 126–31, 129 (ill.); 3: 428 Class 1: 131–32; 3: 427 Classification 1: 132–36 Clean Air Act, United States 1: 6 Cleavage 1: 185 Climax community 2: 236 Cloning 1: 136–39, 137 (ill.), 3: 507 Cnidarian 1: 140–41; 2: 336 Cnidoblasts 2: 336 CO. See Carbon monoxide. CO2. See Carbon dioxide. Cobalamin 3: 593 Cochlea 2: 282 Coelenterates. See Cnidarian. Coelum 2: 338 Coenzymes 3: 591 Coextinction 1: 219 Collagen 3: 491 Collins, Francis Sellers 2: 307 (ill.), 308–10 Colors 2: 355 Commensalism 3: 443, 556, 558 Community 1: 141–43 Comparative anatomy 1: 31 Competition 1: 141, 143–45


Index

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Index

Competitive exclusion 1: 144; 2: 409 Compound eyes 1: 145 Compound fruit 2: 241 Compound light microscopes 2: 384 Conception 2: 226 Connective tissue 3: 571 Continental shelf 3: 424 Continental slope 3: 425 Contractile vacuole 1: 215 Coral 1: 141 Cotyledons 2: 268 Couper, Archibald Scott 1: 94 Crick, Francis Harry Compton 1: 166–67 DNA (deoxyribonucleic acid) 1: 164 double helix 1: 170; 2: 262, 416 Cro–Magnons 2: 306 Crustaceans 1: 145–47, 146 (ill.) Cuticle 1: 42 Cuvier, Georges 3: 442 Cyanobacteria 2: 392 Cytokinins 3: 465 Cytoplasm 1: 100–01, 147; 2: 413 Cytosine 1: 165

D Darwin, Charles Robert 1: 184; 2: 267, 400–01, 402 (ill.) adaptation 1: 208 comparative anatomy 1: 31 ecology 1: 175 evolution, evidence of 1: 209 inherited traits 2: 325–26 natural selection 2: 399–403 survival of the fittest 2: 554–55 theory of evolution 1: 7, 205, 211–14 Dawson, Charles 3: 458 DDT (declare–diphenyl–trichloroethane) 1: 16, 177 Death rate 3: 485 Decomposition 1: 89, 147–48 Denitrification 2: 411 Deoxyribonucleic acid. See DNA (deoxyribonucleic acid). Dermis 2: 333 The Descent of Man 1: 214 Desert 1: 58, 150–53, 152 (ill.)

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polar regions 1: 57 De Vries, Hugo Marie 1: 214; 2: 397 Diamonds 1: 92 Diastase 1: 200 Diatoms 1: 22 Diet 3: 593 Diet imbalance. See Malnutrition. Differentiation 1: 186 Diffusion 1: 153–55, 154 (ill.), 216; 3: 515 Digestive system 1: 155–61, 157 (ill.); 3: 428–29 Dinoflagellates 1: 22 Dinosaurs 1: 161–64, 162 (ill.) extinction 1: 218 Dioxin 3: 466 Disaccharides 1: 86 Divergent evolution 1: 209 DNA (deoxyribonucleic acid) 1: 164–68; 2: 247 (ill.), 418 chromosomes 1: 121 double helix 1: 169–71, 170 (ill.) genetics 2: 262 human genome project 2: 306–10 Dolly 1: 137, 137 (ill.) Dolphins 1: 115 Dominant and recessive traits 1: 168–69 Double circulation 1: 128 Double helix 1: 167, 169–71, 170 (ill.)

E Ear 2: 281 (ill.) Echinoderm 1: 173–75, 174 (ill.); 2: 338 Ecological niche. See Niche. Ecological succession 1: 143, 178 Ecology 1: 175–79 Ecosystem 1: 59, 178, 180–82, 181 (ill.) Ectoparasite 3: 444 Egg 1: 182–85. See also Ovum. Ehrlich, Paul 2: 318–19 Eldredge, Niles 1: 208, 214 Electromagnetic radiation 2: 355 Electromagnetic spectrum 2: 355 Electron microscope 2: 384 Embryo 1: 185–87 human 1: 186 (ill.) Embryology 1: 210


Embryonic stage 1: 185 Endangered species 1: 187–89, 189 (ill.) Endocrine system 1: 190–94, 192 (ill.); 2: 292; 3: 428–29 Endoparasite 3: 444 Endoplasmic reticulum 1: 101, 147, 194–95; 3: 430 Endoskeleton 3: 539 Endosperm 2: 268 Energy flow 1: 181 Energy, law of conservation of 2: 356 Energy, measurement of. See Calorie. Energy pyramid 2: 235 Engelmann, Theodor William 3: 451 Enquiry into Plants 1: 72 Entomology 1: 195–99, 198 (ill.) Enzymes 1: 199–202, 202 (ill.); 3: 491 digestion 2: 359 Epidermal tissue 2: 333, 349; 3: 572 Epigenesis 1: 184 Epithelial tissue 3: 571 Epochs 2: 265 Eras 2: 265 Erythrocytes 1: 67 Estivation 2: 291 Estrogens 2: 301; 3: 532 Estuaries 1: 59 Ethylene 3: 465 Eubacteria 2: 392 Euglena 1: 22; 3: 495 Eukaryotes 1: 100; 2: 269, 417; 3: 429 Eustachian tube 2: 282 Eutrophication 1: 202–04, 204 (ill.) Evergreen forest biome 1: 57 Evolution 1: 205–08; 3: 483. See also Darwin, Charles. human 2: 304 symbiotic theory of evolution 2: 556 Evolutionary theory 1: 211–215 Evolution, evidence of 1: 208–11 Excretory system 1: 215–18; 3: 428–29 human 1: 217 (ill.) Exercise and lactic acid 2: 345 Exoskeleton 1: 42; 2: 328; 3: 539 Experiments and Observations on the Gastric Juice and the Physiology of Digestion 1: 159 External fertilization 3: 509

Extinction 1: 187, 218–22 Extracellular digestion 1: 156 Eye 3: 536 (ill.) compound eye 1: 145

Index

F Facultative mutualism 3: 558 Failure to thrive 3: 574 Family 2: 223 Farming. See Agriculture. Farsightedness 3: 537 Fats. See Lipids. Fat–soluble vitamins 3: 591 Faunal succession 2: 265 Feathers 3: 434 Felidae 1: 99 Fermentation 1: 10; 2: 224–26 Ferns 3: 472 Fertilization 2: 226–29, 228 (ill.), 3: 509, 606 First law of thermodynamics 2: 356 Fish 2: 229–31, 230 (ill.), 3: 585–86 study of 2: 315–16 Fissipedia 1: 98 Fixation 2: 411 Fixed action pattern (FAP). See Instinct. Flagellates 1: 103; 3: 494, 496 Flatworm 3: 600 Fleming, Alexander 1: 35 Floriculture 2: 304 Flowers 2: 231–33, 232 (ill.); 3: 429, 463–64, 474 Folic acid 3: 593 Food chains and webs 2: 234–36, 234 (ill.) Food pyramid 2: 420 (ill.) Forests 2: 236–38 Fossil fuels 1: 89 Fossils 2: 238–40, 239 (ill.) study of 3: 441–42 Francis, Collins 2: 307 (ill.) Franklin, Rosalind 1: 167, 170 Frederick II 3: 434 Freshwater biome 1: 59 Frog, life cycle 1: 29 (ill.) Frontal lobe 1: 77


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Fructose 1: 85 (ill.) Fruit 2: 240–42, 241 (ill.); 3: 526. See also Seeds. study of 2: 303 Fundamental plant tissue 3: 572 Fungi 2: 242–44, 243 (ill.), 343; 3: 468

G Gaia hypothesis 1: 61; 2: 245–46 Galen 1: 30, 130 Gallo, Robert Charles 1: 18–19 Gametes 2: 370 Gastropods 2: 390 Gastrulation 1: 185–86 Gauze, Georgil F. 2: 409 Genes 1: 168; 2: 247–48 Human Genome Project 2: 306–10 Gene theory 2: 249–50 Gene therapy 2: 251–53, 251 (ill.), 259 Genetic code 2: 253–55 Genetic disorders 2: 255–57, 256 (ill.) Genetic engineering 2: 257–60, 258 (ill.) Genetic linkage map 2: 309 Genetic recombination 1: 207 Genetics 2: 260–63 aging 1: 12 Mendel, Gregor Johann 2: 324–25 Mendelian laws of inheritance 2: 372–74 Genetic variety 1: 207 Genome 2: 306 Genotype 3: 499 Genotypic adaptation 1: 8 Genus 2: 263–64 Geologic record 2: 264–67 Geologic time 2: 264 Germination 2: 268–69, 268 (ill.) Germ layers 1: 184 Germs. See Bacteria. Germ theory 1: 184 Germ theory of disease 1: 46 Gerontology 1: 12 Gibberellins 3: 465 Gills 2: 229; 3: 516 Glands 1: 190 Glandular system. See Endocrine system. Global warming. See Greenhouse effect.

l

Glucose 1: 10 Glycolysis 1: 10 Golgi bodies 1: 102, 147; 2: 269–70, 269 (ill.); 3: 430 Golgi, Camillo 1: 102; 2: 270 Gonads 1: 191, 193; 3: 531–32, 547 Gould, Stephen Jay 1: 208, 214 Gradualism 1: 205, 214 Graphite 1: 92 Grasslands 1: 58; 2: 270–72, 271 (ill.) Green algae 1: 22, 23 (ill.) Greenhouse effect 1: 91; 2: 272–75, 273 (ill.) Grew, Nehemiah 1: 73 Growth hormones 2: 302 Guanine 1: 165, 170 Gustation. See Taste. Gymnosperms 3: 472, 525

H H2O. See Water. Habitat 2: 277–79 Haeckel, Ernst Heinrich 1: 175 Hair 2: 334, 366 Hales, Stephen 3: 451, 462–63 Hardy, Godrfrey H. 3: 483 Hardy–Weinberg Law 3: 483 Harvey, William 1: 31, 66, 128, 130–31; 3: 455 Hearing 2: 280–83, 281 (ill.) Heart 1: 127; 2: 283–85, 285 (ill.) Helmont, Jan Baptist van 1: 90; 3: 450 Hemoglobin 1: 67 Hemolymph 1: 127 Henslow, John Stevens 2: 400 Herbivores 2: 286–87 Heredity 2: 260 Herophilus of Chalcedon 1: 30 Herpestidae 1: 100 Herpetology 2: 288–89 Heterotrophs 2: 287, 419; 3: 426, 487 Hibernation 1: 28; 2: 289–91 Histamine 2: 317 HIV (human immunodeficiency virus) 1: 17; 2: 319 Homeostasis 1: 215; 2: 291–94, 293 (ill.), 3: 455, 552


Hominids 2: 294–97, 304; 3: 491 human evolution 2: 304 Hominoids 2: 294; 3: 491 Homo erectus 2: 295, 298, 305 Homo habilis 2: 266, 295, 305 Homo sapiens 2: 295, 304, 306; 3: 491 Homo sapiens neanderthalensis 2: 297–99, 298 (ill.) Hooke, Robert 1: 108; 2: 383, 384–85 Hopkins, Frederick Gowland 3: 592–93 Hormones 1: 190–91; 2: 299–303, 300 (ill.); 3: 491 metabolism 2: 375 plant 3: 464 sex 3: 531 Horticulture 2: 303–04 Human evolution 2: 304–06, 3: 459 (ill.) Human Genome Project 2: 306–10 Human Immunodeficiency Virus. See HIV (human immunodeficiency virus). Human reproduction 2: 310–13, 312 (ill.) Humans, population growth of 3: 484 Hutton, James 2: 245, 264, 267 Hyaenidae 1: 99 Hybrids 2: 313–14 Hydra 1: 140 Hydrogen ion 1: 3 Hydrophytes 3: 599 Hydrostatic skeleton 3: 538 Hyperopia 3: 537 Hypothalamus 1: 191

I Ichthyology 2: 315–16 Immune system 1: 38; 2: 316–20 Immunization 1: 38; 2: 320–22, 321 (ill.) Inbreeding 2: 322–23 Incomplete digestive system 1: 156 Indoleacetic acid 3: 464 Infections, treatment of 1: 35 Ingenhousz, Jan 1: 91; 3: 450–51 Inheritance, non–Mendalian 3: 557 Inheritance patterns 3: 499 Inherited traits 2: 323–27, 327 (ill.) Innate releasing mechanism (IRM) 2: 332

Insects 2: 327–30, 328 (ill.) study of 1: 195–99 Instinct 2: 330–32, 331 (ill.); 3: 569 Integumentary exchange 3: 516 Integumentary system 2: 332–35, 334 (ill.); 3: 428–29 Intercourse 2: 312. See also Reproduction, Sexual. Internal fertilization 3: 509 International Union for Conservation of Nature and Natural Resources (IUCN) 1: 189 Intracellular digestion 1: 156 Invertebrates 2: 335–40 Invertebrate zoology 3: 602

Index

J Jellyfish 1: 140 Jenner, Edward 2: 318, 320 Joints 3: 540

K Karyotype 2: 341–42, 342 (ill.) Kekule, Friedrich 1: 93–94 Keratin 1: 64; 2: 333; 3: 491 Kidneys 1: 216 Kilocalorie 1: 83 Kilojoule 1: 84 Kingdom 1: 136; 2: 342–43 Koch, Robert 1: 46–47; 2: 380 Koch’s postulates 1: 47 Kolliker, Rudolf Albert von 1: 109 Krebs cycle 1: 9–10 Krebs, Hans Adolf 1: 9–10, 11 (ill.)

L Lactic acid 2: 345–46, 346 (ill.) Lamarck, Jean–Baptiste 1: 212; 2: 323; 3: 602–03 Lamina 2: 349 Landsteiner, Karl 1: 69–71; 3: 519 Larva 2: 346–48, 347 (ill.) Laveran, Charles Louis Alphonse 3: 496–97


li

Index

Law of conservation of energy 2: 356 Law of dominance 1: 169 Law of superposition 2: 265 Laws of inheritance 3: 499 Leakey, Louis Seymour Bazett 2: 296–97, 295 (ill.) Leaves 2: 349–51, 350 (ill.); 3: 429, 461, 473–74, 572 Leeches 3: 601 Leeuwenhoek, Anton van 1: 45, 66, 108; 2: 380, 382 Leukocytes 1: 66 Libby, Willard F. 3: 502 Life cycle 2: 351–54, 353 (ill.) Life span 1: 12 Ligament 3: 540 Light 2: 355–56 Limbic system 1: 77 Linnaeus, Carolus 1: 73, 133–35, 211; 2: 343; 3: 565, 606 Lipids 2: 356–58, 357 (ill.) Lister, Joseph 1: 35 Litmus test 1: 4; 3: 445 (ill.) Littoral zone 3: 424 Lovejoy, Thomas Eugene 2: 278–79 Lovelock, James 1: 61; 2: 245 Lungs 3: 518 Lycophytes 3: 472 Lyell, Charles 1: 213; 2: 266–67, 401 Lymph 2: 358 Lymphatic system 2: 358–59 Lysosomes 1: 103; 2: 359–60; 3: 430

M Macronutrients 2: 419 Magnifying glass 2: 382 Malaria 3: 497 Malnutrition 2: 361–63, 362 (ill.), 421 Malpighi, Marcello 1: 131, 195 Malthus, Thomas Robert 1: 213; 2: 401; 3: 485 (ill.), 486 Mammalogy 2: 363–64 Mammals 2: 365–67, 365 (ill.); 3: 587 placental 3: 489 study of 2: 363–64 Mammary glands 2: 366

lii

Margulis, Lynn 3: 556–70 Marine biome 1: 59 Marsupials 2: 368 Mechanoreceptors 3: 528, 572 Medulla 1: 77 Meiosis 1: 105, 124; 2: 368–70 Melanin 2: 333 Membranes 2: 370–72, 370 (ill.) Mendel, Gregor Johann 2: 260, 324–25 genes 2: 248–49 inherited traits 2: 326 laws of inheritance 1: 73, 214; 2: 372–74 Punnett square 3: 499 Mendelian laws of inheritance 2: 261, 372–74, 373 (ill.) Menstrual cycle 2: 312 Mesozoic era 1: 161 Metabolism 1: 111; 2: 374–77 Metamorphosis 2: 347, 352, 377–80, 379 (ill.) Metaphase 1: 106 Microbiology 2: 380 Micrographia 1: 108; 2: 383, 385 Microorganisms 2: 380–82 Microscope 2: 382–86, 383 (ill.) Miescher, Johann Friedrich 2: 415 Migration 2: 386–88 Millipedes 1: 43 Mitochondria 1: 101, 147; 3: 388–89, 388 (ill.), 430 Mitosis 1: 103, 105, 107 (ill.) Mollusk 2: 338, 389–91, 390 (ill.) Molting 1: 42; 3: 539 Monerans 1: 45; 2: 343, 381, 392–93 Monkeys 3: 490 Monosaccharides 1: 86 Monotremes 2: 368 Morgan, Thomas Hunt 1: 122–23; 2: 248, 250, 262 Morula 3: 608 Muller, Hermann Joseph 1: 123 Muscle tissue 3: 571 Muscular system 2: 394–96, 395 (ill.); 3: 428–29 Mustelidae 1: 98 Mutation 1: 207; 2: 396–98, 397 (ill.) Mutualism 3: 443, 556–57


Myopia 3: 537 Mysticeti 1: 115

N Natural history. See Ecology. Natural History of Vertebrates 3: 603 Natural immunity 2: 316 A Naturalist’s Voyage on the Beagle 2: 401 Natural selection 1: 7, 205; 2: 325, 399–403; 3: 554 Neanderthals. See Homo sapiens neanderthalensis. Nearsightedness 3: 537 Nematoda 2: 338; 3: 600 Nerve nets 1: 75 Nervous system 2: 403–08, 405 (ill.); 3: 428 Nervous tissue 3: 571 Neurons 1: 77 Neuron theory 2: 407 Niche 1: 178; 2: 409–10 Nicol, Mary Douglas 2: 297 Nitrates 1: 203 Nitrification 2: 411 Nitrogen cycle 2: 410–12, 411 (ill.) Non–Mendelian inheritance 3: 557 Nonvascular plants 2: 412–13 Nuclear envelope 1: 102 Nuclear membrane 2: 413–14, 414 (ill.) Nuclear waste 3: 478 Nucleic acids 1: 164; 2: 415–16 Nucleolus 2: 417, 417 (ill.) Nucleotides 1: 170 Nucleus 1: 100, 109; 2: 417–19, 418 (ill.) Nutrients 2: 419 Nutrition 2: 361, 419–21

O Obligatory mutualism 3: 558 Occipital lobe 1: 77 Oceans 3: 423–25, 424 (ill.) Odobenidae 1: 100 Odontoceti 1: 115 Oil. See Lipids.

Olduvai Gorge 2: 296 Olfaction 3: 542 Olfactory cortex 3: 543 Omnivores 3: 425–27, 426 (ill.) On Human Nature 1: 51 On the Art of Hunting with Birds 3: 434 On the Motions of the Heart and Blood 1: 131 On the Movement of the Heart and Blood in Animals 3: 455 On the Origin of Species by Means of Natural Selection 1: 8; 2: 401 On the Structure of the Human Body 1: 30 Oogenesis 2: 370 Order 3: 427–28 Organ 3: 428–29 Organelles 1: 101, 118, 147; 2: 388; 3: 429–30 Organic compounds 1: 93; 2: 356; 3: 430–32 Organism 3: 432–33 Organogenesis 1: 185 Organ system 3: 428 Ornamental horticulture 2: 304 Ornithischia 1: 163 Ornithology 3: 434–36, 435 (ill.) Osmosis 3: 436–37, 437 (ill.) Outbreeding 2: 323 Oviparous 1: 183 Ovum 2: 311; 3: 546 Oxygen processes. See Aerobic/Anaerobic. Oxytocin 1: 191 Ozone 3: 438–40 depletion of, 3: 439 (ill.)

Index

P Paleontology 2: 240; 3: 441–42 Pancreas 1: 191, 193 Pangaea 1: 163 Pantothenic Acid 3: 592 Parasites 3: 443–45, 444 (ill.), 468, 556, 558 Parathyroid 1: 191 Parietal lobe 1: 77 Passive transport 2: 371 Pasteurization 1: 47; 2: 224, 226 Pasteur, Louis 1: 46, 105; 2: 224–26, 320, 380


liii

Index

liv

Pathogen 2: 317 Pathology, cellular 1: 104 Pavlov, Ivan Petrovich 3: 568–69 Payen, Anselme 1: 200–01 Pelagic environment 3: 424 Penicillin 1: 35 Penis 2: 311 Pepsin 1: 111 Periods 2: 265 Peripheral nervous system 2: 404, 408 Pesticides 3: 478 Petals 2: 231 Petrifaction 2: 240 pH 1: 3; 3: 445–46 Phenotype 3: 499 Phenotypic adaptation 1: 8 Pheromones 3: 446–48, 448 (ill.) Phloem 3: 582 Phocidae 1: 100 Phosphates 1: 203 Photosynthesis 1: 89, 91; 3: 449–52, 474 carbohydrate production 1: 85 leaves 2: 349–51 stages of 3: 449 (ill.) water 3: 596 Phototropism 3: 452–53, 452 (ill.) Phylum 2: 343; 3: 454 Physical anthropology 1: 105 Physiology 3: 454–58 Phytoplankton 1: 21 Pigments 2: 355 Piltdown Man 3: 458–60, 459 (ill.) Pineal 1: 191 Pinnipedia 1: 98 Pistils 2: 232; 3: 469 Pituitary gland 1: 191; 3: 531 Placental mammals 2: 368; 3: 489 Plankton 3: 495 Plantae 2: 343 Plant anatomy 1: 73; 3: 460–64 Plant cell 1: 72 (ill.) Plant hormones 3: 464–66 Plant pathology 3: 466–69, 467 (ill.) Plant physiology 1: 73 Plant reproduction 3: 469–71, 470 (ill.) Plants 3: 471–75 nonvascular plants 2: 412

response to light 3: 452–53 root system 3: 522 study of 1: 71–74 tissue 3: 572 Plasma membrane 2: 370 Plasmodium 3: 494 Platelets 1: 66 Platyhelminthes 2: 337; 3: 600 Platypus 2: 368 Plumule 2: 269 Poisons 3: 574–76, 575 (ill.) Pollen 3: 469 Pollination 3: 469, 511 Pollution 1: 43; 3: 476–79 effects of, 3: 478 (ill.) Polymers 3: 479–80 Polysaccharides 1: 86 Pomology 2: 304 Pons 1: 77 Population 1: 53; 3: 480–82 statistical study of 3: 482 Population genetics 3: 482–84 Population growth 3: 484–86 Porifera 2: 335; 3: 547 Predation 1: 141–42; 3: 487–89, 487 (ill.) Priestley, Joseph 1: 91 Primary consumers 2: 235 Primary producers 2: 235 Primates 3: 489–91, 489 (ill.) Priming pheromones 3: 447 Principles of Geology 2: 266 Procyonidae 1: 98 Productivity 1: 53 Progesterone 3: 532 Progestin 3: 532 Prokaryotes 1: 100 Prolactin 1: 191 Prophase 1: 106 Prosimians 3: 490 Protein 1: 122; 3: 491–93, 492 (ill.) Protein deficiency 2: 362 Protista 1: 21, 25; 2: 343, 381; 3: 493 Protists 3: 493–95, 493 (ill.) Protoplasm 1: 100 Protozoa 1: 25; 3: 494, 495–98, 498 (ill.) Punctuated equilibrium 1: 208, 214 Punnett, Reginald Crundall 3: 500


Punnett square 3: 499–500 Pyridoxine 3: 593

R Radial symmetry 1: 34 Radicle 2: 269; 3: 523 Radioactive dating 3: 501–02 Radiocarbon dating 3: 502 Rain forest 3: 503–06, 503 (ill.) Ramon y Cajal, Santiago 2: 406–07 Ray, John 2: 288, 288 (ill.) Recessive genetic disorders 2: 256 Recessive traits 1: 168–69 Recombinant DNA technology. See Genetic engineering. Red algae 1: 22 Red blood cells 1: 66 Red tide 1: 22 Regeneration 3: 506 Reproduction, asexual 3: 506–08 Reproduction, plants 3: 469 Reproduction, sexual 3: 508–10 Reproductive system 3: 428–29, 510–12 Reptile 3: 512–14, 512 (ill.), 586 study of 2: 288 Respiration 1: 10, 89; 2: 345, 388; 3: 514–15 Respiratory system 3: 428, 515–19, 517 (ill.) human 3: 517 (ill.) Retrovirus 1: 18 Rh factor 3: 519–20 Rhizomes 3: 507 Riboflavin 3: 592 Ribosomes 1: 101–02, 147; 2: 417; 3: 430 RNA (ribonucleic acid) 1: 147; 2: 416; 3: 520–22, 521 (ill.) Romer, Alfred Sherwood 3: 584–85 Roots 3: 429, 572 Root system 3: 460, 473, 522–24, 523 (ill.) Ross, Ronald 3: 497 Roundworms 3: 600

S Salk, Jonas 2: 321 (ill.) Saltwater biome 1: 59

Santorio, Santorio 2: 376–77 Sarcodina 3: 496 Saurischia 1: 163 Savannahs. See Grasslands. Schleiden, Matthias Jakob 1: 109–11 Schwann, Theodor 1: 109–11, 109 (ill.) Scrotum 3: 547 Scrubland. See Chaparral. Sea 3: 423 Sea anemones 1: 141 The Sea Around Us 1: 176 Secondary consumer 2: 235 Secretin 2: 302 Sedgwick, Adam 2: 400 Seed 3: 463–64, 525–27, 527 (ill.) Seed plants 3: 472 Segmented worms 3: 600 Selective breeding 2: 257 Seminal fluid 2: 311 Sense organ 3: 528–29 Sensory receptors 3: 528 Sepals 2: 231 Serial endosymbiotic theory (SET) 3: 557 Sex chromosomes 3: 529–31, 530 (ill.) Sex hormones 3: 531–33 Sex–linked traits 3: 533–34 Sexual reproduction 3: 508–10 Shortgrass prairies 2: 271 Sight 3: 534–38, 536 (ill.) Signalling pheromones 3: 447 Silent Spring 1: 176 Simple fruit 2: 241 Skeletal muscle 2: 394; 3: 571 Skeletal system 3: 428, 538–42 human 3: 541 (ill.) Skin. See Integumentary system. Smell 3: 542–45, 543 (ill.) Smith, William 3: 442 Smooth muscle 2: 394; 3: 571 Social insects 2: 329 Sociobiology: The New Synthesis 1: 51 Speciation 1: 205 Species 3: 545–46 Spencer, Herbert 3: 555 Sperm 2: 311, 228 (ill.); 3: 546–47, 546 (ill.) Spermatogenesis 2: 370


Index

lv

Index

Spermatozoa 2: 311 Spiders. See Arachnids. Spinal cord 2: 406 Spiny anteater 2: 368 Spleen 2: 358 Sponge 2: 335; 3: 547–49, 548 (ill.) Spore 2: 242; 3: 549–51, 550 (ill.) Sporozoa 3: 497 Stamens 2: 232; 3: 469 Starfish 1: 173, 174 (ill.) Starling, Ernest Henry 2: 302–03 Stems 3: 429, 461, 572 Stimulus 3: 551–52 Strata 2: 264 Stratigraphy 3: 442 Stress 3: 552–54 Striated muscle 3: 571 Sturtevant, Alfred Henry 1: 123 Suess, Eduard 1: 60 Superposition, law of 2: 265 Survival of the fittest 3: 554–55 Survival spores 3: 550 Swammerdam, Jan 1: 66, 108, 196–97 Symbiosis 1: 141–42; 3: 443, 555–59, 555 (ill.) Symbiotic theory of evolution 3: 556 Systematics. See Taxonomy. System of Nature 1: 73, 133–34, 211

T Taiga 1: 57; 3: 561–62, 562 (ill.) Taste 3: 563–65, 564 (ill.) Taxonomy 1: 132; 3: 565–67 Taxons 1: 135 Telescope 2: 382 Telophase 1: 106 Temperate deciduous forest biome 1: 57 Temporal lobe 1: 77 Territory 3: 567–70 Testa 2: 268 Testes 2: 311; 3: 547 Testosterone 2: 301; 3: 532 Theophrastus 1: 72, 74–75 Theory of evolution 1: 7; 2: 399 Theory of natural selection 3: 554 Thiamin 3: 592

lvi

Thrombocytes 1: 67 Thylakoids 1: 119 Thymine 1: 165 Thymus 1: 191, 193; 2: 358 Thyroid 1: 191, 193 Thyroxine 2: 375 Tissue 3: 570–72, 570 (ill.) Tongue 3: 564 (ill.) Touch 3: 572–74 Toxins 3: 574–76, 575 (ill.) Tracheal system 3: 516 Traits, dominant and recessive 1: 168–69 Transcription 1: 167 Transfer gene 2: 252 Translation 1: 167 Transpiration 3: 583 Trees 3: 577–78 Trophic levels 1: 53 Tropical rain forest 1: 56; 3: 503–06 Tundra 1: 56; 3: 578–80, 579 (ill.) Turgor pressure 3: 583

U Uniformitarianism 2: 267 Urea 1: 216 Urethra 1: 216; 2: 311 Ursidae 1: 100 Ussher, James 2: 264 Uterus 2: 312

V Vaccination. See Immunization. Vaccines 3: 590 Vacuoles 1: 102; 3: 430, 581 Vascular plants 3: 582–83 Vascular tissue 3: 572 Vectors 2: 252; 3: 443 Vegetables and fruits, study of. See Horticulture. Vegetative propagation 3: 470 Vegetative reproduction 1: 80; 3: 506 Veins 1: 127 Ventilators 3: 463 Ventricle 2: 284


Vernadsky, Vladimir I. 1: 60 Vertebrae 2: 406 Vertebrate Body 3: 585 Vertebrate Paleontology 3: 585 Vertebrates 3: 583–88, 605 Vesalius, Andreas 1: 30, 31 (ill.), 32 Vibroids 1: 45 Virchow, Rudolf Ludwig Karl 1: 104–05, 109 Virus 1: 17; 3: 468, 588–90, 589 (ill.) Vision 3: 534–38 Vital organs 3: 428 Vitamins 3: 591–94, 591 (ill.) Viverridae 1: 99 Viviparous 1: 183 Von Liebig, Justus 1: 93 Von Linne, Carl. See Linnaeus, Carolus.

White blood cells 1: 66 Wilkins, Maurice H. F. 1: 170 Wilmut, Ian 1: 137–39 Wilson, Edward Osborne 1: 49–51, 49 (ill.), 221 Wohler, Friedrich 3: 431 Worlds in the Making 2: 274 Worms 2: 337; 3: 600–03

Index

X X–ray crystallography 1: 165 Xylem 3: 582

Y Yolk 1: 183

W Wallace, Alfred Russel 1: 213; 2: 403 Water 3: 595–97, 596 (ill.) pollution 3: 478 Water–soluble vitamins 3: 591 Watson, James Dewey 1: 164, 166–67, 170; 2: 250, 262, 309, 327, 416 Waxes. See Lipids. Weinberg, Wilhelm 3: 483 Wetlands 3: 597–600, 598 (ill.)

Z Zeiss, Carl 2: 383 Zoology 3: 605–06. See also Entomology, Ichthyology Mammalogy, and Ornithology. Zoophytes 2: 335 Zygote 1: 185; 3: 509, 606–08, 607 (ill.)


lvii

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