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Biology The Unity and Diversity of Life
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Biology The Unity and Diversity of Life Starr Taggart Evers Starr Twelfth Edition
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Biology: The Unity and Diversity of Life, Twelfth Edition Cecie Starr, Ralph Taggart, Christine Evers, Lisa Starr Publisher: Yolanda Cossio Managing Development Editor: Peggy Williams Assistant Editor: Elizabeth Momb Editorial Assistant: Samantha Arvin Technology Project Manager: Kristina Razmara Marketing Manager: Amanda Jellerichs Marketing Assistant: Katherine Malatesta Marketing Communications Manager: Linda Yip Project Manager, Editorial Production: Andy Marinkovich Creative Director: Rob Hugel
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CONTENTS IN BRIEF
INTRODUCTION 1
Invitation to Biology
UNIT I
PRINCIPLES OF CELLULAR LIFE
2
Life’s Chemical Basis
3
Molecules of Life
4
Cell Structure and Function
5
A Closer Look at Cell Membranes
6
Ground Rules of Metabolism
7
Where It Starts—Photosynthesis
8
How Cells Release Chemical Energy
UNIT VI UNIT II 9 10
PRINCIPLES OF INHERITANCE
32
HOW ANIMALS WORK
Animal Tissues and Organ Systems
How Cells Reproduce
33
Neural Control
Meiosis and Sexual Reproduction
34
Sensory Perception Endocrine Control
11
Observing Patterns in Inherited Traits
35
12
Chromosomes and Human Inheritance
36
Structural Support and Movement
13
DNA Structure and Function
37
Circulation
14
From DNA to Protein
38
Immunity
15
Controls Over Genes
39
Respiration
16
Studying and Manipulating Genomes
40
Digestion and Human Nutrition
41
Maintaining the Internal Environment
42
Animal Reproductive Systems
43
Animal Development
UNIT III
PRINCIPLES OF EVOLUTION
17
Evidence of Evolution
18
Processes of Evolution
19
Organizing Information About Species
UNIT VII
20
Life’s Origin and Early Evolution
44
Animal Behavior
45
Population Ecology
46
Community Structure and Biodiversity
UNIT IV
EVOLUTION AND BIODIVERSITY
PRINCIPLES OF ECOLOGY
21
Viruses and Prokaryotes
47
Ecosystems
22
Protists—The Simplest Eukaryotes
48
The Biosphere
23
The Land Plants
49
Human Impacts on the Biosphere
24
Fungi
25
Animal Evolution—The Invertebrates
26
Animal Evolution—The Chordates
27
Plants and Animals—Common Challenges
UNIT V
HOW PLANTS WORK
28
Plant Tissues
29
Plant Nutrition and Transport
30
Plant Reproduction
31
Plant Development
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DETAILED CONTENTS INTRODUCTION
1
Invitation to Biology
IMPACTS, ISSUES Lost Worlds and Other Wonders 2
1.1
2.2
Putting Radioisotopes to
Use 23
2.3
Why Electrons Matter 24
Life’s Levels of Organization 4
Electrons and Energy Levels 24
Making Sense of the World 4
Why Atoms Interact 24 Shells and Electrons 24
A Pattern in Life’s Organization 4
1.2
FOCUS ON RESEARCH
Atoms and Ions 25
Overview of Life’s Unity 6
From Atoms to Molecules 25
Energy and Life’s Organization 6 Organisms Sense and Respond to Change 6
2.4
What Happens When Atoms Interact? 26 Ionic Bonding 26
Organisms Grow and Reproduce 7
Covalent Bonding 26
1.3
Overview of Life’s Diversity 8
1.4
An Evolutionary View of Diversity 10
1.5
Critical Thinking and Science 11
Polarity of the Water Molecule 28
Thinking About Thinking 11
Water’s Solvent Properties 28
The Scope and Limits of Science 11
Water’s Temperature-Stabilizing Effects 29
How Science Works 12
Water’s Cohesion 29
1.6
Observations, Hypotheses, and Tests 12
1.7
Hydrogen Bonding 27
2.5
2.6
Water’s Life-Giving Properties 28
Acids and Bases 30
About the Word “Theory” 12
The pH Scale 30
Some Terms Used in Experiments 13
How Do Acids and Bases Differ? 30
The Power of Experimental Tests 14
Salts and Water 31
Potato Chips and Stomach Aches 14
Buffers Against Shifts in pH 31
Butterflies and Birds 14 Asking Useful Questions 15
1.8
FOCUS ON SCIENCE
Sampling Error in
Experiments 16
3
IMPACTS, ISSUES Fear of Frying 34
3.1
UNIT I
2
PRINCIPLES OF CELLULAR LIFE
Molecules of Life
Organic Molecules 36 Carbon—The Stuff of Life 36 Representing Structures of Organic Molecules 36
Life’s Chemical Basis
IMPACTS, ISSUES What Are You Worth? 20
2.1
Start With Atoms 22 Characteristics of Atoms 22 The Periodic Table 22
vii
3.2
3.3
3.4
3.5
3.6
From Structure to Function 38
The Nuclear Envelope 64
Functional Groups 38
The Nucleolus 65
What Cells Do to Organic Compounds 39
The Chromosomes 65
4.9
Carbohydrates 40
The Endomembrane System 66
Simple Sugars 40
The Endoplasmic Reticulum 66
Short-Chain Carbohydrates 40
Vesicles 67
Complex Carbohydrates 40
Golgi Bodies 67
Greasy, Oily—Must Be Lipids 42
4.10
FOCUS ON HEALTH
Fats 42
4.11
Other Organelles 68
Lysosome Malfunction 68
Phospholipids 43
Mitochondria 68
Waxes 43
Plastids 69
Cholesterol and Other Steroids 43
The Central Vacuole 69
Proteins—Diversity in Structure and Function 44
4.12 Cell Surface Specializations 70
Proteins and Amino Acids 44
Eukaryotic Cell Walls 70
Levels of Protein Structure 44
Matrixes Between Cells 70
Why Is Protein Structure So Important? 46
Cell Junctions 71
Just One Wrong Amino Acid . . . 46
4.13 The Dynamic Cytoskeleton 72
Proteins Undone—Denaturation 46
3.7
Nucleic Acids 48
4
Cell Structure and Function
Cilia, Flagella, and False Feet 73
5
A Closer Look at Cell Membranes
IMPACTS, ISSUES Food for Thought 52
IMPACTS, ISSUES One Bad Transporter and Cystic Fibrosis 76
4.1
5.1
4.2
The Cell Theory 54 Measuring Cells 54
Revisiting the Lipid Bilayer 78
Animalcules and Beasties 54
The Fluid Mosaic Model 78
The Cell Theory Emerges 55
Variations 78 Differences in Membrane Composition 78
What Is a Cell? 56
Differences in Fluidity 78
The Basics of Cell Structure 56 Preview of Cell Membranes 57
4.3
Organization of Cell Membranes 78
FOCUS ON RESEARCH
How Do We See Cells? 58
5.2
Membrane Proteins 80
5.3
Diffusion, Membranes, and Metabolism 82 Membrane Permeability 82
Modern Microscopes 58
4.4
Introducing Prokaryotic Cells 60
4.5
FOCUS ON THE ENVIRONMENT
4.6
Introducing Eukaryotic Cells 62
4.7
Visual Summary of Eukaryotic Cell Components 63
Concentration Gradients 82 The Rate of Diffusion 83
4.8
viii
The Nucleus 64
Microbial Mobs 61
How Substances Cross Membranes 83
5.4
Passive and Active Transport 84 Passive Transport 84 Active Transport 84
5.5
Membrane Trafficking 86
5.6
Endocytosis and Exocytosis 86
Properties of Light 108
Membrane Cycling 87
The Rainbow Catchers 108
Which Way Will Water Move? 88
7.2
FOCUS ON RESEARCH
Osmosis 88
7.3
Overview of Photosynthesis 111
7.4
Light-Dependent Reactions 112
Tonicity 88 Effects of Fluid Pressure 88
Exploring the Rainbow 110
Capturing Energy for Photosynthesis 112 Replacing Lost Electrons 112
6
IMPACTS, ISSUES A Toast to Alcohol Dehydrogenase 92
6.1
6.3
Accepting Electrons 113
7.5
Energy Flow in Photosynthesis 114
7.6
Light-Independent Reactions: The Sugar Factory 115
7.7
The One-Way Flow of Energy 95
Adaptations: Different Carbon-Fixing Pathways 116
Energy and the World of Life 94 Energy Disperses 94
6.2
Harvesting Electron Energy 112
Ground Rules of Metabolism
Energy in the Molecules of Life 96
Rascally Rubisco 116
Energy In, Energy Out 96
C4 Plants 116
Why the World Does Not Go Up in Flames 96
CAM Plants 117
ATP—The Cell’s Energy Currency 97
7.8
Photosynthesis and the Atmosphere 118
How Enzymes Make Substances React 98
7.9
FOCUS ON THE ENVIRONMENT
A Burning
Concern 119
How Enzymes Work 98 Helping Substrates Get Together 98 Orienting Substrates in Positions That Favor Reaction 98 Inducing a Fit Between Enzyme and Substrate 98 Shutting Out Water Molecules 98 Effects of Temperature, pH, and Salinity 99
8
IMPACTS, ISSUES When Mitochondria Spin Their Wheels 122
8.1
Help From Cofactors 99
6.4
Controls Over Metabolism 100 Redox Reactions 101
6.5
Overview of Carbohydrate Breakdown Pathways 124 Comparison of the Main Pathways 124
Metabolism—Organized, EnzymeMediated Reactions 100 Types of Metabolic Pathways 100
How Cells Release Chemical Energy
Overview of Aerobic Respiration 125
8.2 8.3
Glycolysis—Glucose Breakdown Starts 126 Second Stage of Aerobic Respiration 128 Acetyl–CoA Formation 128
FOCUS ON RESEARCH
Night Lights 102
The Krebs Cycle 128
Enzymes of Bioluminescence 102 A Research Connection 102
8.4
Aerobic Respiration’s Big Energy Payoff 130 Electron Transfer Phosphorylation 130 Summing Up: The Energy Harvest 130
7
Where It Starts—Photosynthesis
IMPACTS, ISSUES Biofuels 106
7.1
Sunlight as an Energy Source 108
8.5
Anaerobic Energy-Releasing Pathways 132 Fermentation Pathways 132 Alcoholic Fermentation 132 Lactate Fermentation 133
ix
8.6
FOCUS ON HEALTH
8.7
Alternative Energy Sources in the Body 134
Segregation of Chromosomes into Gametes 161
10.5 From Gametes to Offspring 162
The Fate of Glucose at Mealtime and Between Meals 134
Gamete Formation in Plants 162
Energy From Fats 134
More Shufflings at Fertilization 162
Energy From Proteins 134
8.8
Crossing Over in Prophase I 160
The Twitchers 133
Reflections on Life’s Unity 136
UNIT II
PRINCIPLES OF INHERITANCE
Gamete Formation in Animals 162
10.6 Mitosis and Meiosis—An Ancestral Connection? 164
11 Observing Patterns in Inherited Traits IMPACTS, ISSUES The Color of Skin 168
9
How Cells Reproduce
11.1
IMPACTS, ISSUES Henrietta’s Immortal Cells 140
9.1
Mendel, Pea Plants, and Inheritance Patterns 170 Mendel’s Experimental Approach 170 Terms Used in Modern Genetics 171
Overview of Cell Division Mechanisms 142
11.2
Mendel’s Law of Segregation 172
Mitosis, Meiosis, and the Prokaryotes 142
11.3
Mendel’s Law of Independent Assortment 174
Key Points About Chromosome Structure 142
11.4 9.2
Introducing the Cell Cycle 144 The Life of a Cell 144
Incomplete Dominance 176
Mitosis and the Chromosome Number 144
9.3 9.4
Beyond Simple Dominance 176 Codominance in ABO Blood Types 176 Epistasis 177
A Closer Look at Mitosis 146
Single Genes With a Wide Reach 177
Cytoplasmic Division Mechanisms 148
11.5
Linkage Groups 178
Division of Animal Cells 148
11.6
Genes and the Environment 179
11.7
Complex Variations in Traits 180
Division of Plant Cells 149 Appreciate the Process! 149
9.5
FOCUS ON HEALTH
Continuous Variation 180
When Control Is Lost 150
Regarding the Unexpected Phenotype 181
The Cell Cycle Revisited 150 Checkpoint Failure and Tumors 150 Characteristics of Cancer 151
12 Chromosomes and Human Inheritance IMPACTS, ISSUES Strange Genes, Tortured Minds 184
10 Meiosis and Sexual Reproduction
12.1
Human Chromosomes 186
IMPACTS, ISSUES Why Sex? 154
Sex Determination 186
10.1
Karyotyping 187
Introducing Alleles 156
10.2 What Meiosis Does 156 Two Divisions, Not One 157
12.2 Examples of Autosomal Inheritance Patterns 188 Autosomal Dominant Inheritance 188
10.3 Visual Tour of Meiosis 158 10.4 How Meiosis Introduces Variations in Traits 160
x
Autosomal Recessive Inheritance 188 What About Neurobiological Disorders? 189
12.3
FOCUS ON HEALTH
Too Young to be Old 189
12.4 Examples of X-Linked Inheritance Patterns 190
13.4 Using DNA to Duplicate Existing Mammals 210 13.5
FOCUS ON RESEARCH
Fame and Glory 211
Hemophilia A 190 Red–Green Color Blindness 191 Duchenne Muscular Dystrophy 191
12.5 Heritable Changes in Chromosome Structure 192
14 From DNA to Protein IMPACTS, ISSUES Ricin and Your Ribosomes 214
14.1
Duplication 192
DNA, RNA, and Gene Expression 216 The Nature of Genetic Information 216
Deletion 192
Converting a Gene to an RNA 216
Inversion 192
Converting mRNA to Protein 216
Translocation 192 Does Chromosome Structure Evolve? 193
14.2 Transcription: DNA to RNA 218 DNA Replication and Transcription Compared 218
12.6 Heritable Changes in the Chromosome Number 194 Autosomal Change and Down Syndrome 194
The Process of Transcription 218
14.3 RNA and the Genetic Code 220 Post-Transcriptional Modifications 220
Change in the Sex Chromosome Number 195 Female Sex Chromosome Abnormalities 195
mRNA—The Messenger 220
Male Sex Chromosome Abnormalities 195
rRNA and tRNA—The Translators 221
12.7 Human Genetic Analysis 196
14.4 Translation: RNA to Protein 222
12.8
14.5 Mutated Genes and Their Protein Products 224
FOCUS ON HEALTH
Prospects in Human
Genetics 198
Common Mutations 224
Genetic Counseling 198
What Causes Mutations? 224
Prenatal Diagnosis 198
The Proof Is in the Protein 225
Preimplantation Diagnosis 198 Regarding Abortion 199 Phenotypic Treatments 199 Genetic Screening 199
15 Controls Over Genes IMPACTS, ISSUES Between You and Eternity 228
15.1
Gene Expression in Eukaryotic Cells 230 Which Genes Get Tapped? 230
13 DNA Structure and Function
Control of Transcription 230
IMPACTS, ISSUES Here, Kitty, Kitty, Kitty, Kitty, Kitty 202
mRNA Processing 231
13.1
Translational Control 231
FOCUS ON RESEARCH
mRNA Transport 231
The Hunt for DNA 204
Post-Translational Modification 231
Early and Puzzling Clues 204 Confirmation of DNA’s Function 205
13.2 The Discovery of DNA’s Structure 206
15.2 A Few Outcomes of Eukaryotic Gene Controls 232 X Chromosome Inactivation 232
DNA’s Building Blocks 206 Patterns of Base Pairing 207
13.3 DNA Replication and Repair 208 Checking for Mistakes 208
Flower Formation 233
15.3
FOCUS ON SCIENCE
There’s a Fly in My
Research 234 Discovery of Homeotic Genes 234
xi
Knockout Experiments 234 Filling In Details of Body Plans 235
17.1
Early Beliefs, Confounding Discoveries 260
17.2
A Flurry of New Theories 262
15.4 Prokaryotic Gene Control 236
Squeezing New Evidence into Old Beliefs 262
The Lactose Operon 236 Lactose Intolerance 236
Voyage of the Beagle 262
17.3 Darwin, Wallace, and Natural Selection 264 Old Bones and Armadillos 264 A Key Insight—Variation in Traits 264
16 Studying and Manipulating Genomes
Natural Selection 265
IMPACTS, ISSUES Golden Rice or Frankenfood? 240
17.4 Great Minds Think Alike 266
16.1
17.5 About Fossils 266
Cloning DNA 242 Cut and Paste 242
How Do Fossils Form? 266
cDNA Cloning 243
The Fossil Record 266
16.2 From Haystacks to Needles 244 Big-Time Amplification: PCR 244
17.6 Dating Pieces of the Puzzle 268 17.7
FOCUS ON RESEARCH
A Whale of a Story 269
16.3 DNA Sequencing 246
17.8 Putting Time Into Perspective 270
16.4
17.9
FOCUS ON SCIENCE
DNA Fingerprinting 247
FOCUS ON RESEARCH
Drifting Continents,
Changing Seas 272
16.5 Studying Genomes 248 The Human Genome Project 248 Genomics 249
18 Processes of Evolution
DNA Chips 249
16.6 Genetic Engineering 250
IMPACTS, ISSUES Rise of the Super Rats 276
16.7 Designer Plants 250
18.1
Variation in Populations 278
Genetically Engineered Plants 250
The Gene Pool 278
16.8 Biotech Barnyards 252
Mutation Revisited 279
Of Mice and Men 252
Stability and Change in Allele Frequencies 279
Knockout Cells and Organ Factories 252
18.2
16.9 Safety Issues 253 16.10
FOCUS ON BIOETHICS
Individuals Don’t Evolve, Populations Do 278
FOCUS ON RESEARCH
A Closer Look at Genetic
Equilibrium 280 Modified Humans? 254
Getting Better 254 Getting Worse 254 Getting Perfect 254 Getting There 254
The Hardy–Weinberg Formula 280 Applying the Rule 281
18.3 Natural Selection Revisited 281 18.4 Directional Selection 282 Effects of Predation 282 The Peppered Moth 282 Pocket Mice 282
UNIT III
PRINCIPLES OF EVOLUTION
17 Evidence of Evolution IMPACTS, ISSUES Measuring Time 258
xii
Resistance to Antibiotics 283
18.5 Selection Against or in Favor of Extreme Phenotypes 284 Stabilizing Selection 284 Disruptive Selection 285
18.6 Maintaining Variation 286
19.3 Comparing Patterns of Development 306
Sexual Selection 286
Similar Genes in Plants 306
Balanced Polymorphism 287
Developmental Comparisons in Animals 306 How Many Legs? 306
18.7 Genetic Drift—The Chance Changes 288
Forever Young 306
Bottlenecks and the Founder Effect 288
18.8 Gene Flow 289
19.4 Comparing DNA and Proteins 308 Molecular Comparisons 308
18.9 Reproductive Isolation 290 Prezygotic Isolating Mechanisms 290 Temporal Isolation 290
19.5
FOCUS ON RESEARCH
Making Data Into Trees 310
19.6 Preview of Life’s Evolutionary History 312
Mechanical Isolation 290 Behavioral Isolation 291 Ecological Isolation 291 Gamete Incompatibility 291 Postzygotic Isolating Mechanisms 291 Reduced Hybrid Viability 291 Reduced Hybrid Fertility 291 Hybrid Breakdown 291
20 Life’s Origin and Early Evolution IMPACTS, ISSUES Looking for Life in All the Odd Places 316
20.1 In the Beginning . . . 318 Origin of the Universe and Our Solar System 318
18.10 Allopatric Speciation 292
Conditions on the Early Earth 318
The Inviting Archipelagos 292
18.11 Other Speciation Models 294
Origin of the Building Blocks of Life 319
20.2 How Did Cells Emerge? 320
Sympatric Speciation 294
Origin of Proteins and Metabolism 320
Polyploidy 294
Origin of the Plasma Membrane 320
Other Examples 294
Origin of Genetic Material 321
Isolation at Hybrid Zones 295
18.12 Macroevolution 296
20.3 Life’s Early Evolution 322 The Golden Age of Prokaryotes 322
Patterns of Macroevolution 296
The Rise of Eukaryotes 323
Coevolution 296 Stasis 296
20.4 Where Did Organelles Come From? 324
Exaptation 296
Origin of the Nucleus, ER, and Golgi Body 324
Adaptive Radiation 296
Evolution of Mitochondria and Chloroplasts 324
Extinction 297
Evidence of Endosymbiosis 325
Evolutionary Theory 297
20.5 Time Line for Life’s Origin and Evolution 326 20.6
FOCUS ON SCIENCE
About Astrobiology 328
19 Organizing Information About Species IMPACTS, ISSUES Bye Bye Birdie 300
19.1
Taxonomy and Cladistics 302
UNIT IV
EVOLUTION AND BIODIVERSITY
A Rose by Any Other Name . . . 302 Ranking Versus Grouping 302
21 Viruses and Prokaryotes
19.2 Comparing Body Form and Function 304 Morphological Divergence 304 Morphological Convergence 305
IMPACTS, ISSUES The Effects of AIDS 332
21.1
Viral Characteristics and Diversity 334
xiii
Viral Discovery and Traits 334
Protist Organization and Nutrition 352
Examples of Viruses 334
Protist Life Cycles 353
Impacts of Viruses 335
22.2 Flagellated Protozoans 354
Viral Origins and Evolution 335
The Anaerobic Flagellates 354
21.2 Viral Replication 336
Trypanosomes and Other Kinetoplastids 355
Steps in Replication 336 Bacteriophage Replication 336
The Euglenoids 355
22.3 Foraminiferans and Radiolarians 356
Replication of Herpes, an Enveloped DNA Virus 336
The Chalky-Shelled Foraminiferans 356 The Glassy-Shelled Radiolarians 356
Replication of HIV, a Retrovirus 336
21.3
FOCUS ON RESEARCH
Viroids and Prions 338
22.4 The Ciliates 357
The Smallest Pathogens 338
22.5 Dinoflagellates 358
Fatal Misfoldings 338
22.6
21.4 Prokaryotes—Enduring, Abundant, and Diverse 339
22.7 The Stramenopiles 360
Evolutionary History and Classification 339
The Diatoms 360
Abundance and Metabolic Diversity 339
The Multicelled Brown Algae 360
21.5 Prokaryotic Structure and Function 340 Cell Structure and Size 340
The Heterotrophic Water Molds 361
22.8
Reproduction and Gene Transfers 340
21.6 The Bacteria 342
The Cell-Dwelling Apicomplexans 359
FOCUS ON HEALTH
FOCUS ON THE ENVIRONMENT
The Plant
Destroyers 361
22.9 Green Algae 362
The Heat Lovers 342
The Chlorophytes 362
The Cyanobacteria 342
Chlorophyte Algae 363
The Metabolically Diverse Proteobacteria 342 The Gram-Positive Heterotrophs 342 Spirochetes and Chlamydias 343
21.7 The Archaeans 344
22.10 Red Algae Do It Deeper 364 22.11
Amoeboid Cells at the Crossroads 365
FOCUS ON RESEARCH
The Third Domain 344 Here, There, Everywhere 344
21.8
FOCUS ON HEALTH
Evolution and Infectious
Disease 346
23 The Land Plants IMPACTS, ISSUES Beginnings and Endings 368
The Nature of Disease 346 An Evolutionary Perspective 346
23.1 Evolution on a Changing World Stage 370
Emerging Diseases 347
23.2 Evolutionary Trends Among Plants 372
The Threat of Drug-Resistance 347
From Haploid to Diploid Dominance 372
It’s a Small World 347
Roots, Stems, and Leaves 372 Pollen and Seeds 373
22 Protists—The Simplest Eukaryotes
23.3 The Bryophytes 374 Liverworts 374
IMPACTS, ISSUES The Malaria Menace 350
Hornworts 374
22.1 The Many Protist Lineages 352
Mosses 374
Classification and Phylogeny 352
xiv
23.4 Seedless Vascular Plants 376
Lycophytes 376
Fungal Endophytes 398
Whisk Ferns and Horsetails 376
Mycorrhizae—The Fungus–Roots 399
Ferns—No Seeds, But Much Diversity 377
23.5
FOCUS ON THE ENVIRONMENT
24.7
FOCUS ON HEALTH
An Unloved Few 399
Ancient
Carbon Treasures 378
23.6 Seed-Bearing Plants 379
25 Animal Evolution—The Invertebrates
Rise of the Seed Plants 379 Human Uses of Seed Plants 379
23.7 Gymnosperms—Plants With Naked Seeds 380
IMPACTS, ISSUES Old Genes, New Drugs 402
25.1 Animal Traits and Body Plans 404 What Is an Animal? 404
Conifers 380
Variation in Animal Body Plans 404
Lesser Known Gymnosperms 380
Organization 404
A Representative Life Cycle 381
Body Symmetry 404 Gut and Body Cavity 404
23.8 Angiosperms—The Flowering Plants 382 Keys to Angiosperm Success 382
Circulation 405
Flowering Plant Diversity 382
Segmentation 405
23.9 Focus on a Flowering Plant Life Cycle 384
25.2 Animal Origins and Adaptive Radiation 406 Becoming Multicellular 406
23.10 FOCUS ON SCIENCE The World’s Most Nutritious Plant 385
A Great Adaptive Radiation 406 Relationships and Classification 407
25.3
FOCUS ON RESEARCH
The Simplest Living
Animal 408
24 Fungi
25.4 The Sponges 408 IMPACTS, ISSUES High-Flying Fungi 388
Characteristics and Ecology 408
24.1 Fungal Traits and Classification 390
Sponge Reproduction and Dispersal 409 Sponge Self-Recognition 409
Characteristics and Ecology 390 Overview of Fungal Life Cycles 390
25.5 Cnidarians—True Tissues 410
Phylogeny and Classification 390
24.2
General Features 410 Diversity and Life Cycles 410
FOCUS ON THE ENVIRONMENT
The Flagellated Fungi 391
25.6 Flatworms—Simple Organ Systems 412
24.3 Zygote Fungi and Relatives 392
Structure of a Free-Living Flatworm 412
Typical Zygote Fungi 392 Microsporidians—Intracellular Parasites 393 Glomeromycetes—Plant Symbionts 393
Flukes and Tapeworms—The Parasites 412
25.7 Annelids—Segmented Worms 414 The Marine Polychaetes 414
24.4 Sac Fungi—Ascomycetes 394
Leeches—Bloodsuckers and Others 414
Sexual Reproduction 394 Asexual Reproduction 395 Human Uses of Sac Fungi 395
The Earthworm—An Oligochaete 414
25.8 Mollusks—Animals With a Mantle 416 General Characteristics 416
24.5 Club Fungi—Basidiomycetes 396 24.6 The Fungal Symbionts 398 Lichens 398
Mollusk Diversity 416
25.9
FOCUS ON EVOLUTION
Cephalopods—Fast and
Brainy 418
xv
Cartilaginous Fishes 439
25.10 Rotifers and Tardigrades—Tiny and Tough 419 25.11 Roundworms—Unsegmented Worms That Molt 420
Bony Fishes 439
26.5 Amphibians—First Tetrapods on Land 440 Adapting to Life on Land 440
25.12 Arthropods—Animals With Jointed Legs 421
Modern Amphibians 440
Key Arthropod Adaptations 421 Hardened Exoskeleton 421 Jointed Appendages 421 Highly Modified Segments 421 Sensory Specializations 421 Specialized Developmental Stages 421
26.6
FOCUS ON THE ENVIRONMENT
26.7 The Rise of Amniotes 442 26.8
FOCUS ON EVOLUTION
So Long, Dinosaurs 443
26.9 Diversity of Modern Reptiles 444
25.13 Chelicerates—Spiders and Their Relatives 422
General Characteristics 444
25.14 The Mostly Marine Crustaceans 423
Major Groups 444
25.15 Myriapods—Lots of Legs 424 25.16 The Insects 424
Turtles 444 Lizards 444 Tuataras 444
Insect Characteristics 424
Snakes 444
Insect Origins 425
Crocodilians 445
25.17 Insect Diversity and Importance 426
26.10 Birds—The Feathered Ones 446
A Sampling of Insect Diversity 426
From Dinosaurs to Birds 446
Ecological Services 426
General Characteristics 446
Competitors for Crops 426
Vanishing Acts 441
Bird Diversity and Behavior 447
Vectors for Disease 426
26.11 The Rise of Mammals 448 25.18 The Spiny-Skinned Echinoderms 428 The Protostome-Deuterostome Split 428
Mammalian Traits 448 Mammalian Evolution 448
Echinoderm Characteristics and Body Plan 428 Echinoderm Diversity 429
26.12 Modern Mammalian Diversity 450 Egg-Laying Monotremes 450 Pouched Marsupials 450
26 Animal Evolution—The Chordates
Placental Mammals 450
26.13 From Early Primates to Hominids 452 IMPACTS, ISSUES Transitions Written in Stone 432
Overview of Key Trends 452
26.1 The Chordate Heritage 434
Origins and Early Divergences 453
Chordate Characteristics 434
26.14 Emergence of Early Humans 454
The Invertebrate Chordates 434 A Braincase but No Backbone 435
26.2 Vertebrate Traits and Trends 436 An Internal Skeleton and a Big Brain 436 Circulatory and Respiratory Systems 437 Other Organ Systems 437
Early Hominids 454 Early Humans 455
26.15 Emergence of Modern Humans 456 Branchings of the Human Lineage 456 Where Did Modern Humans Originate? 456 Multiregional Model 456
26.3 The Jawless Lampreys 438 26.4 The Jawed Fishes 438
xvi
Replacement Model 456 Leaving Home 457
Vascular Tissues 478
27 Plants and Animals—Common
Dermal Tissues 479
Challenges IMPACTS, ISSUES A Cautionary Tale 460
28.3 Primary Structure of Shoots 480 Behind the Apical Meristem 480
27.1
Levels of Structural Organization 462 From Cells to Multicelled Organisms 462
Inside the Stem 480
28.4 A Closer Look at Leaves 482
Growth Versus Development 462
Leaf Epidermis 482
Evolution of Form and Function 462
Mesophyll—Photosynthetic Ground Tissue 482
The Internal Environment 463
Veins—The Leaf’s Vascular Bundles 483
A Body’s Tasks 463
27.2 Common Challenges 464 Gas Exchange 464
28.6 Secondary Growth 486
Internal Transport 464
28.7
Maintaining the Water–Solute Balance 464 Cell-to-Cell Communication 464 On Variations in Resources and Threats 465
27.3 Homeostasis in Animals 466
27.4
28.5 Primary Structure of Roots 484
FOCUS ON RESEARCH
28.8 Modified Stems 489 Stolons 489 Rhizomes 489
Detecting and Responding to Changes 466
Bulbs 489
Negative Feedback 466
Corms 489
Positive Feedback 467
Tubers 489
FOCUS ON HEALTH
Heat-Related Illness 467
Three Rings and Old
Secrets 488
Cladodes 489
27.5 Does Homeostasis Occur in Plants? 468 Walling Off Threats 468
29 Plant Nutrition and Transport
Sand, Wind, and the Yellow Bush Lupine 468 Rhythmic Leaf Folding 469
27.6 How Cells Receive and Respond to Signals 470
IMPACTS, ISSUES Leafy Cleanup Crews 492
29.1 Plant Nutrients and Availability in Soil 494 The Required Nutrients 494 Properties of Soil 494 Soils and Plant Growth 494
UNIT V
HOW PLANTS WORK
How Soils Develop 494 Leaching and Erosion 495
28 Plant Tissues IMPACTS, ISSUES Droughts Versus Civilization 474
28.1 The Plant Body 476 The Basic Body Plan 476 Eudicots and Monocots—Same Tissues, Different Features 476 Introducing Meristems 476
28.2 Plant Tissues 478
29.2 How Do Roots Absorb Water and Nutrients? 496 Root Hairs 496 Mycorrhizae 496 Root Nodules 496 How Roots Control Water Uptake 497
29.3 How Does Water Move Through Plants? 498 Cohesion–Tension Theory 498
29.4 How Do Stems and Leaves Conserve Water? 500
Simple Tissues 478
The Water-Conserving Cuticle 500
Complex Tissues 478
Controlling Water Loss at Stomata 500
xvii
29.5 How Do Organic Compounds Move Through Plants? 502 Pressure Flow Theory 502
Cytokinins 527 Ethylene 527 Other Signaling Molecules 527
31.3 Examples of Plant Hormone Effects 528 Gibberellin and Germination 528
30 Plant Reproduction
Auxin Augmentation 528
IMPACTS, ISSUES Plight of the Honeybee 506
Jeopardy and Jasmonates 529
30.1 Reproductive Structures of Flowering Plants 508
31.4 Adjusting the Direction and Rates of Growth 530 Gravitropism 530
Anatomy of a Flower 508
Phototropism 531
Diversity of Flower Structure 509
Thigmotropism 531
30.2 Flowers and Their Pollinators 510 Getting By With a Little Help From Their Friends 510
30.3 A New Generation Begins 512 Microspore and Megaspore Formation 512 Pollination and Fertilization 512
30.4 Flower Sex 514
31.5 Sensing Recurring Environmental Changes 532 Biological Clocks 532 Setting the Clock 532 When to Flower? 532
31.6 Senescence and Dormancy 534 Abscission and Senescence 534 Dormancy 534
30.5 Seed Formation 515 The Embryo Sporophyte Forms 515
UNIT VI
HOW ANIMALS WORK
Seeds as Food 515
30.6 Fruits 516 30.7 Asexual Reproduction of Flowering Plants 518 Plant Clones 518 Agricultural Applications 518 Cuttings and Grafting 518 Tissue Culture 519 Seedless Fruits 519
32 Animal Tissues and Organ Systems IMPACTS, ISSUES Open or Close the Stem Cell Factories? 538
32.1 Organization of Animal Bodies 540 From Tissue to Organs to Organ Systems 540 Cell Junctions 540
32.2 Epithelial Tissue 541 General Characteristics 541
31 Plant Development IMPACTS, ISSUES Foolish Seedlings, Gorgeous Grapes 522
Glandular Epithelium 541
32.3 Connective Tissues 542 Soft Connective Tissues 542 Specialized Connective Tissues 542
31.1
Patterns of Development in Plants 524
31.2 Plant Hormones and Other Signaling Molecules 526 Plant Hormones 526 Gibberellins 526 Auxins 527 Abscisic Acid 527
xviii
32.4 Muscle Tissues 544 Skeletal Muscle Tissue 544 Cardiac Muscle Tissue 544 Smooth Muscle Tissue 545
32.5 Nervous Tissue 545 32.6 Overview of Major Organ Systems 546
Development of Tissues and Organs 546
An Information Highway 566
Vertebrate Organ Systems 546
Reflex Pathways 566
32.7 Vertebrate Skin—Example of an Organ System 548
32.8
33.10 The Vertebrate Brain 568 The Hindbrain and Midbrain 568
Structure and Function of Skin 548
The Forebrain 568
Sunlight and Human Skin 549
Protection at the Blood–Brain Barrier 568
FOCUS ON HEALTH
The Human Brain 569
Farming Skin 549
33.11 The Human Cerebrum 570
33 Neural Control
Functions of the Cerebral Cortex 570
IMPACTS, ISSUES In Pursuit of Ecstasy 552
Making Memories 571
Connections With the Limbic System 571
33.1 Evolution of Nervous Systems 554 The Cnidarian Nerve Net 554
33.12
FOCUS ON RESEARCH
The Split Brain 572
33.13 Neuroglia—The Neurons’ Support Staff 573
Bilateral, Cephalized Nervous Systems 554
Types of Neuroglia 573
The Vertebrate Nervous System 555
About Brain Tumors 573
33.2 Neurons—The Great Communicators 556 33.3 Membrane Potentials 557 Resting Potential 557 Action Potentials 557
33.4 A Closer Look at Action Potentials 558
34 Sensory Perception IMPACTS, ISSUES A Whale of a Dilemma 576
34.1 Overview of Sensory Pathways 578
Approaching Threshold 558
Sensory Receptor Diversity 578
An All-or-Nothing Spike 558
From Sensing to Sensation 579
Direction of Propagation 559
34.2 Somatic and Visceral Sensations 580
33.5 How Neurons Send Messages to Other Cells 560
Receptors Near the Body Surface 580
Synaptic Integration 561
Muscle Sense 580
Cleaning the Cleft 561
The Sense of Pain 580
33.6 A Smorgasbord of Signals 562
33.7
The Somatosensory Cortex 580
Chemical Synapses 560
34.3 Sampling the Chemical World 582
Neurotransmitter Discovery and Diversity 562
Sense of Smell 582
The Neuropeptides 562
Sense of Taste 582
FOCUS ON HEALTH
Drugs Disrupt Signaling 563
Stimulants 563 Depressants 563
34.4 Sense of Balance 583 34.5 Sense of Hearing 584 Properties of Sound 584
Analgesics 563
The Vertebrate Ear 584
Hallucinogens 563
33.8 The Peripheral Nervous System 564 Axons Bundled as Nerves 564 Functional Subdivisions 564 Somatic and Autonomic Systems 564 Sympathetic and Parasympathetic Divisions 564
33.9 The Spinal Cord 566
34.6
FOCUS ON THE ENVIRONMENT
Noise Pollution 586
34.7 Sense of Vision 586 Requirements for Vision 586
34.8 A Closer Look at the Human Eye 588 Anatomy of the Eye 588 Focusing Mechanisms 589
xix
Type 2 Diabetes 609
34.9 From the Retina to the Visual Cortex 590
Hypoglycemia 609
Structure of the Retina 590 How Photoreceptors Work 591
35.10 The Adrenal Glands 610
Visual Processing 591
34.10 FOCUS
ON HEALTH
Hormonal Control of the Adrenal Cortex 610 Nervous Control of the Adrenal Medulla 610
Visual Disorders 592
Color Blindness 592
35.11 Too Much or Too Little Cortisol 611
Lack of Focus 592
Chronic Stress and Elevated Cortisol 611
Macular Degeneration 592
Low Cortisol Level 611
Glaucoma 592
35.12 Other Endocrine Glands 612
Cataracts 592
The Gonads 612
Nutritional Blindness 592
The Pineal Gland 612
Infectious Agents 592
The Thymus 612
35.13 A Comparative Look at a Few Invertebrates 613
35 Endocrine Control
Evolution of Hormone Diversity 613 Hormones and Molting 613
IMPACTS, ISSUES Hormones in the Balance 596
35.1 Introducing the Vertebrate Endocrine System 598
36 Structural Support and Movement
Intercellular Signaling in Animals 598 Overview of the Endocrine System 598
IMPACTS, ISSUES Pumping Up Muscles 616
Nervous–Endocrine Interactions 598
36.1 Invertebrate Skeletons 618 Hydrostatic Skeletons 618
35.2 The Nature of Hormone Action 600
Exoskeletons 618
From Signal Reception to Response 600
Endoskeletons 619
Intracellular Receptors 600 Receptors at the Plasma Membrane 600
36.2 The Vertebrate Endoskeleton 620
Receptor Function and Diveristy 600
Features of the Vertebrate Skeleton 620 The Human Skeleton 620
35.3 The Hypothalamus and Pituitary Gland 602 Posterior Pituitary Function 602
36.3 Bone Structure and Function 622
Anterior Pituitary Function 603
Bone Anatomy 622
Feedback Controls of Hormone Secretion 603
Bone Formation and Remodeling 622 About Osteoporosis 623
35.4 Growth Hormone Function and Disorders 604 35.5 Sources and Effects of Other Vertebrate Hormones 605 35.6 Thyroid and Parathyroid Glands 606 The Thyroid Gland 606
FOCUS ON THE ENVIRONMENT
36.5
FOCUS ON HEALTH
Those Aching Joints 625
Common Injuries 625 Arthritis and Bursitis 625
The Parathyroid Glands 607
35.7
36.4 Skeletal Joints—Where Bones Meet 624
36.6 Skeletal–Muscular Systems 626 Twisted
Tadpoles 607
36.7 How Does Skeletal Muscle Contract? 628 Fine Structure of Skeletal Muscle 628
35.8 Pancreatic Hormones 608 35.9
FOCUS ON HEALTH
Blood Sugar Disorders 609
Type 1 Diabetes 609
xx
The Sliding-Filament Model 629
36.8 From Signal to Response: A Closer Look at Contraction 630
Nervous Control of Contraction 630 The Roles of Troponin and Tropomyosin 630
37.8 Diffusion at Capillaries, Then Back to the Heart 650 Capillary Function 650
36.9 Energy for Contraction 631
Venous Pressure 651
36.10 Properties of Whole Muscles 632
36.11
37.9
FOCUS ON HEALTH
Blood and Cardiovascular
Motor Units and Muscle Tension 632
Disorders 652
Fatigue, Exercise, and Aging 632
Red Blood Cell Disorders 652
FOCUS ON HEALTH
Disruption of Muscle
White Blood Cell Disorders 652
Contraction 633
Clotting Disorders 652
Muscular Dystrophies 633
Atherosclerosis 652
Motor Neuron Disorders 633
Hypertension—A Silent Killer 653
Botulism and Tetanus 633
Rhythms and Arrhythmias 653 Risk Factors 653
37.10 Interactions With the Lymphatic System 654
37 Circulation
Lymph Vascular System 654 Lymphoid Organs and Tissues 655
IMPACTS, ISSUES And Then My Heart Stood Still 636
37.1
The Nature of Blood Circulation 638 From Structure to Function 638 Evolution of Circulation in Vertebrates 638
37.2 Characteristics of Blood 640 Functions of Blood 640
38 Immunity IMPACTS, ISSUES Frankie’s Last Wish 658
38.1 Integrated Responses to Threats 660 Evolution of the Body’s Defenses 660
Blood Volume and Composition 640 Plasma 640
Three Lines of Defense 660
Red Blood Cells 640
The Defenders 661
White Blood Cells 641 Platelets 641
37.3 Hemostasis 642 37.4 Blood Typing 642
38.2 Surface Barriers 662 38.3
FOCUS ON HEALTH
Remember to Floss 663
38.4 Innate Immune Responses 664 Phagocytes and Complement 664
ABO Blood Typing 642
Inflammation 665
Rh Blood Typing 643
Fever 665
37.5 Human Cardiovascular System 644 37.6 The Human Heart 646
38.5 Overview of Adaptive Immunity 666 Tailoring Responses to Specific Threats 666
Heart Structure and Function 646
First Step—The Antigen Alert 666
How Does Cardiac Muscle Contract? 646
Two Arms of Adaptive Immunity 667
Cardiac Muscle Revisited 646 How the Heart Beats 647
37.7 Pressure, Transport, and Flow Distribution 648 Rapid Transport in Arteries 648 Flow Distribution at Arterioles 648 Controlling Blood Pressure 649
Intercepting and Clearing Out Antigen 667
38.6 Antibodies and Other Antigen Receptors 668 Antibody Structure and Function 668 The Making of Antigen Receptors 669
38.7 The Antibody-Mediated Immune Response 670 An Antibody-Mediated Response 670
xxi
38.8 The Cell-Mediated Response 672 38.9
FOCUS ON HEALTH
39.7 Gas Exchange and Transport 692 The Respiratory Membrane 692
Allergies 673
Oxygen Transport 692
38.10 Vaccines 674
Carbon Dioxide Transport 692
38.11 Immunity Gone Wrong 675
The Carbon Monoxide Threat 693
Autoimmune Disorders 675
39.8
Immunodeficiency 675
FOCUS ON HEALTH
Respiratory Diseases and
Disorders 694
38.12 AIDS Revisited—Immunity Lost 676
Interrupted Breathing 694
HIV Revisited 676
Potentially Deadly Infections 694
A Titanic Struggle 676
Chronic Bronchitis and Emphysema 694
Transmission 677
Smoking’s Impact 695
Testing 677
39.9 High Climbers and Deep Divers 696
Drugs and Vaccines 677
Respiration at High Altitudes 696 Deep-Sea Divers 696
39 Respiration IMPACTS, ISSUES Up in Smoke 680
40 Digestion and Human Nutrition
39.1 The Nature of Respiration 682 The Basis of Gas Exchange 682 Factors Affecting Diffusion Rates 682
Incomplete and Complete Systems 702
Ventilation 683
Dietary Adaptations 703
FOCUS ON THE ENVIRONMENT
40.2 Overview of the Human Digestive System 704 Gasping for
Oxygen 683
39.3 Invertebrate Respiration 684 Integumentary Exchange 684
40.3 Food in the Mouth 705 40.4 Food Breakdown in the Stomach and Small Intestine 706
Invertebrate Gills 684
Digestion in the Stomach 706
Snails with Lungs 684
Digestion in the Small Intestine 707
Tracheal Tubes and Book Lungs 684
Controls Over Digestion 707
39.4 Vertebrate Respiration 686
40.5 Absorption From the Small Intestine 708
The Gills of Fishes 686
From Structure to Function 708
Evolution of Paired Lungs 686
How Are Materials Absorbed? 708
39.5 Human Respiratory System 688 The System’s Many Functions 688 From Airways to Alveoli 689 The Respiratory Passageways 689 The Paired Lungs 689 Muscles and Respiration 689
39.6 Cyclic Reversals in Air Pressure Gradients 690 The Respiratory Cycle 690 Respiratory Volumes 690 Control of Breathing 691
xxii
40.1 The Nature of Digestive Systems 702
Surface-to-Volume Ratio 683 Respiratory Proteins 683
39.2
IMPACTS, ISSUES Hormones and Hunger 700
Water and Solute Absorption 708 Fat Absorption 709
40.6 The Large Intestine 710 Structure and Function of the Large Intestine 710 Disorders of the Large Intestine 710
40.7 Metabolism of Absorbed Organic Compounds 711 40.8 Human Nutritional Requirements 712 USDA Dietary Recommendations 712
Energy-Rich Carbohydrates 712
41.9 Heat Gains and Losses 733
Good Fat, Bad Fat 712
How the Core Temperature Can Change 733
Body-Building Proteins 713
Endotherms, Ectotherms, and Heterotherms 733
About Low-Carb/High-Protein Diets 713
41.10 Temperature Regulation in Mammals 734
40.9 Vitamins, Minerals, and Phytochemicals 714
Responses to Heat Stress 734
40.10 FOCUS ON HEALTH Weighty Questions, Tantalizing Answers 716
Responses to Cold Stress 734
Weight and Health 716 What Is the “Right” Body Weight? 716 Genes, Hormones, and Obesity 717
42 Animal Reproductive Systems IMPACTS, ISSUES Male or Female? Body or Genes? 738
41 Maintaining the Internal Environment
42.1 Modes of Animal Reproduction 740 Asexual Reproduction in Animals 740
IMPACTS, ISSUES Truth in a Test Tube 720
41.1
Costs and Benefits of Sexual Reproduction 740
Maintenance of Extracellular Fluid 722
41.2 How Do Invertebrates Maintain Fluid Balance? 722
Variations on Sexual Reproduction 740
42.2 Reproductive System of Human Males 742 The Male Gonads 742
41.3 Fluid Regulation in Vertebrates 724
Reproductive Ducts and Accessory Glands 743
Fluid Balance in Fishes and Amphibians 724
Prostate and Testicular Problems 743
Fluid Balance in Reptiles, Birds, and Mammals 724
41.4 The Human Urinary System 726
42.3 Sperm Formation 744 From Germ Cells to Mature Sperm 744
Components of the Urinary System 726
Hormonal Control of Sperm Formation 745
Nephrons—The Functional Units of the Kidney 727 Overview of Nephron Structure 727
42.4 Reproductive System of Human Females 746 Components of the System 746
Blood Vessels Around Nephrons 727
Overview of the Menstrual Cycle 747
41.5 How Urine Forms 728 Glomerular Filtration 728
42.5
Female Troubles 747
PMS 747
Tubular Secretion 729
Menstrual Pain 747
Concentrating the Urine 729
Hot Flashes, Night Sweats 747
41.6 Regulation of Water Intake and Urine Formation 730
42.6 Preparations for Pregnancy 748 The Ovarian Cycle 748
Regulating Thirst 730
Correlating Events in the Ovary and Uterus 749
Effects of Antidiuretic Hormone 730 Effects of Aldosterone 730
42.7
Hormonal Disorders and Fluid Balance 731
42.8 When Gametes Meet 750
41.7 Acid–Base Balance 731 41.8
FOCUS ON HEALTH
Tubular Reabsorption 728
FOCUS ON HEALTH
When Kidneys Fail 732
Causes of Kidney Failure 732 Kidney Dialysis 732 Kidney Transplants 732
FOCUS ON HEALTH
FSH and Twins 750
Sexual Intercourse 750 Physiology of Sex 750 Regarding Viagra 750 Fertilization 751
42.9 Preventing or Seeking Pregnancy 752
xxiii
Birth Control Options 752
43.11
FOCUS ON HEALTH
Mother as Provider and
About Abortion 753
Protector 774
Assisted Reproductive Technology 753
Nutritional Considerations 774
42.10 FOCUS ON HEALTH Sexually Transmitted Diseases 754
About Morning Sickness 774 Infectious Agents 775
Consequences of Infection 754
Alcohol and Caffeine 775
Major Agents of Sexually Transmitted Disease 754
Smoking 775
HPV 754 Trichomoniasis 754
Prescription Drugs 775
43.12 Birth and Lactation 776
Chlamydia 754
Giving Birth 776
Genital Herpes 754
Nourishing the Newborn 776
Gonorrhea 754 Syphilis 755 AIDS 755
43 Animal Development IMPACTS, ISSUES Mind-Boggling Births 758
43.1 Stages of Reproduction and Development 760
UNIT VII
PRINCIPLES OF ECOLOGY
44 Animal Behavior IMPACTS, ISSUES My Pheromones Made Me Do It 780
44.1 Behavioral Genetics 782 How Genes Affect Behavior 782
43.2 Early Marching Orders 762
Studying Variation Within a Species 782
Information in the Cytoplasm 762
Comparisons Among Species 783
Cleavage Divides Up the Maternal Cytoplasm 762
Knockouts and Other Mutations 783
Variations in Cleavage Patterns 763 Structure of the Blastula 763
44.2 Instinct and Learning 784 Instinctive Behavior 784
43.3 From Blastula to Gastrula 764
Time-Sensitive Learning 784
43.4 Specialized Tissues and Organs Form 765
Conditioned Responses 785
Cell Differentiation 765 Morphogenesis and Pattern Formation 765
43.5 An Evolutionary View of Development 766 A General Model for Animal Development 766 Developmental Constraints and Modifications 766
43.6 Overview of Human Development 767 43.7 Early Human Development 768 Cleavage and Implantation 768
44.3 Adaptive Behavior 786 44.4 Communication Signals 786 44.5 Mates, Offspring, and Reproductive Success 788 Sexual Selection and Mating Behavior 788 Parental Care 789
44.6 Living in Groups 790
Extraembryonic Membranes 768
Defense Against Predators 790
Early Hormone Production 769
Improved Feeding Opportunities 790
43.8 Emergence of the Vertebrate Body Plan 770 43.9 The Function of the Placenta 771 43.10 Emergence of Distinctly Human Features 772
xxiv
Other Types of Learned Behavior 785
Dominance Hierarchies 791 Regarding the Costs of Group Living 791
44.7 Why Sacrifice Yourself? 792 Social Insects 792
Social Mole-Rats 792
46 Community Structure and Biodiversity
Evolution of Altruism 792
44.8
FOCUS ON SCIENCE
Human Behavior 793
Hormones and Pheromones 793
IMPACTS, ISSUES Fire Ants in the Pants 816
46.1 Which Factors Shape Community Structure? 818
Morality and Behavior 793
The Niche 818 Categories of Species Interactions 818
45 Population Ecology
46.2 Mutualism 819
IMPACTS, ISSUES The Numbers Game 796
46.3 Competitive Interactions 820 Effects of Competition 820
45.1 Population Demographics 798
Resource Partitioning 821
45.2
FOCUS ON SCIENCE
Elusive Heads to Count 799
46.4 Predator–Prey Interactions 822 45.3 Population Size and Exponential Growth 800
Models for Predator–Prey Interactions 822
Gains and Losses in Population Size 800
The Canadian Lynx and Snowshoe Hare 822
From Zero to Exponential Growth 800
Coevolution of Predators and Prey 823
What Is the Biotic Potential? 801
45.4 Limits on Population Growth 802
46.5
FOCUS ON EVOLUTION
An Evolutionary
Arms Race 824
Environmental Limits on Growth 802 Prey Defenses 824
Carrying Capacity and Logistic Growth 802
Adaptive Responses of Predators 825
Two Categories of Limiting Factors 803
45.5 Life History Patterns 804 Life Tables 804
Parasites and Parasitoids 826
Survivorship Curves 804
Biological Control Agents 827
Reproductive Strategies 805
45.6
46.6 Parasite–Host Interactions 826
FOCUS ON SCIENCE
Natural Selection and
Life Histories 806
46.7
FOCUS ON EVOLUTION
Strangers in the Nest 827
46.8 Ecological Succession 828
Predation on Guppies in Trinidad 806
Successional Change 828
Overfishing and the Atlantic Cod 807
Factors Affecting Succession 828
45.7 Human Population Growth 808 The Human Population Today 808 Extraordinary Foundations for Growth 808 Geographic Expansion 808 Increased Carrying Capacity 808 Sidestepped Limiting Factors 808
45.8 Fertility Rates and Age Structure 810
46.9 Species Interactions and Community Instability 830 The Role of Keystone Species 830 Species Introductions Can Tip the Balance 831
46.10 FOCUS
ON THE ENVIRONMENT
Exotic Invaders 832
Battling Algae 832 The Plants That Overran Georgia 832
Some Projections 810
The Rabbits That Ate Australia 833
Shifting Fertility Rates 810
Gray Squirrels Versus Red Squirrels 833
45.9 Population Growth and Economic Effects 812 Demographic Transitions 812 Resource Consumption 812
45.10 Rise of the Seniors 813
46.11 Biogeographic Patterns in Community Structure 834 Mainland and Marine Patterns 834 Island Patterns 834
xxv
Winds and Acid Rain 865
47 Ecosystems
Windborne Particles and Health 865 IMPACTS, ISSUES Bye-Bye, Blue Bayou 838
47.1
48.3 The Ocean, Landforms, and Climates 866
The Nature of Ecosystems 840
Ocean Currents and Their Effects 866
Overview of the Participants 840
Rain Shadows and Monsoons 866
Trophic Structure of Ecosystems 840
47.2 The Nature of Food Webs 842
48.4 Biogeographic Realms and Biomes 868 48.5 Soils of Major Biomes 870
Interconnecting Food Chains 842
48.6 Deserts 871
How Many Transfers? 843
47.3 Energy Flow Through Ecosystems 844 Capturing and Storing Energy 844
Grasslands 872
Ecological Pyramids 844
Dry Shrublands and Woodlands 873
Ecological Efficiency 845
47.4
48.8 More Rain, Broadleaf Forests 874
Magnification 846
Semi-Evergreen and Deciduous Broadleaf Forests 874
DDT and Silent Spring 846
Tropical Rain Forests 874
FOCUS ON THE ENVIRONMENT
Biological
48.9
The Mercury Menace 846
FOCUS ON BIOETHICS
You and the Tropical
Forests 875
47.5 Biogeochemical Cycles 847
48.10 Coniferous Forests 876
47.6 The Water Cycle 848 How and Where Water Moves 848
48.11 Tundra 877
A Global Water Crisis 848
48.12 Freshwater Ecosystems 878 Lakes 878
47.7 Carbon Cycle 850 47.8
48.7 Grasslands, Shrublands, and Woodlands 872
FOCUS ON THE ENVIRONMENT
Greenhouse Gases
Nutrient Content and Succession 878 Seasonal Changes 878
and Climate Change 852
Streams and Rivers 879
47.9 Nitrogen Cycle 854
48.13 FOCUS
Inputs Into Ecosystems 854 Natural Losses From Ecosystems 855 Disruptions by Human Activities 855
ON HEALTH
“Fresh” Water? 880
48.14 Coastal Zones 880 Wetlands and the Intertidal Zone 880 Rocky and Sandy Coastlines 881
47.10 The Phosphorus Cycle 856
48.15 FOCUS ON THE ENVIRONMENT The Once and Future Reefs 882 48.16 The Open Ocean 884
48 The Biosphere
Oceanic Zones and Habitats 884 IMPACTS, ISSUES Surfers, Seals, and the Sea 860
48.1 Global Air Circulation Patterns 862
Upwelling—A Nutrient Delivery System 885
48.17 Climate, Copepods, and Cholera 886
Air Circulation and Regional Climates 862 Harnessing the Sun and Wind 863
48.2
FOCUS ON THE ENVIRONMENT
Something in the
49 Human Impacts on the Biosphere
Air 864 Swirling Polar Winds and Ozone Thinning 864 No Wind, Lots of Pollutants, and Smog 864
xxvi
IMPACTS, ISSUES A Long Reach 890
49.1 The Extinction Crisis 892
Mass Extinctions and Slow Recoveries 892 The Sixth Great Mass Extinction 893
49.2 Current Threats to Species 894
Appendix I
Classification System
Appendix II
Annotations to A Journal Article
Appendix III
Answers to Self-Quizzes and Genetics Problems
Appendix IV
Periodic Table of the Elements
Appendix V
Molecular Models
Appendix VI
Closer Look at Some Major Metabolic Pathways
Appendix VII
A Plain English Map of the Human Chromosomes
Appendix VIII
Restless Earth—Life’s Changing Geological Stage
Appendix IX
Units of Measure
Appendix X
A Comparative View of Mitosis in Plant and Animal Cells
Habitat Loss, Fragmentation, and Degradation 894 Overharvesting and Poaching 894 Species Introductions 895 Interacting Effects 895
49.3
FOCUS ON RESEARCH
The Unknown Losses 896
49.4 Assessing Biodiversity 896 Conservation Biology 896 Monitoring Indicator Species 896 Identifying Regions at Risk 896
49.5 Effects of Development and Consumption 898 Effects of Urban and Suburban Development 898 Effects of Resource Consumption 898
49.6 The Threat of Desertification 900 49.7 The Trouble With Trash 901 49.8 Maintaining Biodiversity and Human Populations 902 Bioeconomic Considerations 902 Sustainable Use of Biological Wealth 902 Using Genetic Diversity 902 Discovering Useful Chemicals 902 Ecotourism 902 Sustainable Logging 903 Responsible Ranching 903
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Preface In preparation for this revision, we invited instructors who teach introductory biology for non-majors students to meet with with us and discuss the goals of their courses. The main goal of almost every instructor was something like this: “To provide students with the tools to make informed choices as consumers and as voters by familiarizing them with the way science works.” Most students who use this book will not become biologists, and many will never take another science course. Yet for the rest of their lives they will have to make decisions that require a basic understanding of biology and the process of science. Our book provides these future decision makers with an accessible introduction to biology. Current research, along with photos and videos of the scientists who do it, underscore the concept that science is an ongoing endeavor carried out by a diverse community of people. The research topics include not only what the researchers discovered, but also how the discoveries were made, how our understanding has changed over time, and what remains undiscovered. The role of evolution is a unifying theme, as it is in all aspects of biology. As authors, we feel that understanding stems mainly from making connections, so we are constantly trying to achieve the perfect balance between accessibility and level of detail. A narrative with too much detail is inaccessible to the introductory student; one with too little detail comes across as a series of facts that beg to be memorized. Thus, we revised every page to make the text in this edition as clear and straightforward as possible, keeping in mind that English is a second language for many students. We also simplified many figures and added tables that summarize key points.
CHANGES IN THIS EDITION
Impacts, Issues To make the Impacts, Issues essays more appealing, we shortened and updated them, and improved their integration throughout the chapters. Many new essays were added to this edition. Key Concepts Introductory summaries of the Key Concepts covered in the chapter are now enlivened with eye-catching graphics taken from relevant sections. The links to earlier concepts now include descriptions of the linked concepts in addition to the section numbers.
Take Home Message Each section now concludes with a Take Home Message box. Here we pose a question that reflects the critical content of the section, and we also provide answers to the question in bulleted list format. Figure It Out Figure It Out Questions with answers allow students to check their understanding of a figure as they read through the chapter.
Data Analysis Exercise To further strengthen a student’s analytical skills and provide insight into contemporary research, each chapter includes a Data Analysis Exercise. The exercise includes a short text passage—
usually about a published scientific experiment—and a table, chart, or other graphic that presents experimental data. The student must use information in the text and graphic to answer a series of questions.
Chapter-Specific Changes Every chapter was extensively revised for clarity; this edition has more than 250 new photos and over 300 new or updated figures. A page-by-page guide to content and figures is available upon request, but we summarize the highlights here. • Chapter 1, Invitation to Biology New essay about the discovery of new species. Greatly expanded coverage of critical thinking and the process of science; new section on sampling error. • Chapter 2, Life’s Chemical Basis Sections on subatomic particles, bonding, and pH simplified; new pH art. • Chapter 3, Molecules of Life New essay about trans fats. Structural representations simplified and standardized. • Chapter 4, Cell Structure and Function New essay about foodborne E. coli; microscopy section updated; new section on cell theory and history of microscopy; two new focus essays on biofilms and lysosome malfunction. • Chapter 5, A Closer Look at Cell Membranes Membrane art reorganized; new figure illustrating cotransport. • Chapter 6, Ground Rules of Metabolism Energy and metabolism sections reorganized and rewritten; much new art, including molecular model of active site. • Chapter 7, Where It Starts—Photosynthesis New essay about biofuels. Sections on light-dependent reactions and carbon fixing adaptations simplified; new focus essay on atmospheric CO2 and global warming. • Chapter 8, How Cells Release Chemical Energy All art showing metabolic pathways revised and simplified. • Chapter 9, How Cells Reproduce Updated micrographs of mitosis in plant and animal cells. • Chapter 10, Meiosis and Sexual Reproduction Crossing over, segregation, and life cycle art revised. • Chapter 11, Observing Patterns in Inherited Traits New essay about inheritance of skin color; mono- and dihybrid cross figures revised; new Punnett square for coat color in dogs; environmental effects on Daphnia phenotype added. • Chapter 12, Chromosomes and Human Inheritance Chapter reorganized; expanded discussion and new figure on the evolution of chromosome structure. • Chapter 13, DNA Structure and Function New opener essay on pet cloning; adult cloning section updated. • Chapter 14, From DNA to Protein New art comparing DNA and RNA, other art simplified throughout; new micrographs of transcription Christmas tree, polysomes. • Chapter 15, Controls Over Genes Chapter reorganized; eukaryotic gene control section rewritten; updated X chromosome inactivation photos; new lac operon art. • Chapter 16, Studying and Manipulating Genomes Text extensively rewritten and updated; new photos of bt corn, DNA fingerprinting; sequencing art revised. • Chapter 17, Evidence of Evolution Extensively revised, reorganized. Revised essay on evidence/inference; new
xxix
focus essay on whale evolution; updated geologic time scale correlated with grand canyon strata. • Chapter 18, Processes of Evolution Extensively revised, reorganized. New photos showing sexual selection in stalk-eyed flies, mechanical isolation in sage. • Chapter 19, Organizing Information About Species Extensively revised, reorganized. New comparative embryology photo series; updated tree of life. • Chapter 20, Life’s Origin and Early Evolution Information about origin of agents of metabolism updated. New discussion of ribozymes as evidence for RNA world. • Chapter 21, Viruses and Prokaryotes Opening essay about HIV moved here, along with discussion of HIV replication. New art of viral structure. New section describes the discovery of viroids and prions. • Chapter 22, Protists—The Simplest Eukaryotes New opening essay about malaria. New figures show protist traits, how protists relate to other groups. • Chapter 23, The Land Plants Evolutionary trends revised. More coverage of liverworts and hornworts. • Chapter 24, Fungi New opening essay about airborne spores. More information on fungal uses and pathogens. • Chapter 25, Animal Evolution—The Invertebrates New summary table for animal traits. Coverage of relationships among invertebrates updated. • Chapter 26, Animal Evolution—The Chordates New section on lampreys. Human evolution updated. • Material previously covered in the Biodiversity in Prespective chapter now integrated into other chapters. • Chapter 27, Plants and Animals—Common Challenges New section about heat-related illness. • Chapter 28, Plant Tissues Secondary structure section simplified; new essay on dendroclimatology. • Chapter 29, Plant Nutrition and Transport Root function section rewritten and expanded; new translocation art. • Chapter 30, Plant Reproduction Extensively revised. New essay on colony collapse disorder; new table showing flower specializations for specific pollinators; new section on flower sex; many new photos added. • Chapter 31, Plant Development Sections on plant development and hormone mechanisms rewritten. • Chapter 32, Animal Tissues and Organ Systems Essay on stem cells updated. New section on lab-grown skin. • Chapter 33, Neural Control Reflexes integrated with coverage of spinal cord. Section on brain heavily revised. • Chapter 34, Sensory Perception New art of vestibular apparatus, image formation in eyes, and accommodation. Improved coverage of eye disorders and disease. • Chapter 35, Endocrine Control New section about pituitary disorders. Tables summarizing hormone sources now in appropriate sections, rather than at end. • Chapter 36, Structural Support and Movement Improved coverage of joints and joint disorders. • Chapter 37, Circulation Updated opening essay. New section about hemostasis. Blood cell diagram simplified. Blood typing section revised for clarity.
xxx
• Chapter 38, Immunity New essay on HPV vaccine; new focus essays on periodontal-cardiovascular disease and allergies; vaccines and AIDS sections updated. • Chapter 39, Respiration Better coverage of invertebrate respiration and of Heimlich maneuver. • Chapter 40, Digestion and Human Nutrition Nutritional information and obesity research sections updated. • Chapter 41, Maintaining the Internal Environment New figure of fluid distribution in the human body. Improved coverage of kidney disorders and dialysis. • Chapter 42, Animal Reproductive Systems New essay on intersex conditions. Coverage of reproductive anatomy, gamete production, intercourse, and fertilization. • Chapter 43, Animal Development Information about principles of animal development streamlined. • Chapter 44, Animal Behavior More on types of learning. • Chapter 45, Population Ecology Exponential and logistic growth clarified. Human population material updated. • Chapter 46, Community Structure and Biodiversity New table of species interactions. Competition section heavily revised. • Chapter 47, Ecosystems New figures for food chain and food webs. Updated greenhouse gas coverage. • Chapter 48, The Biosphere Improved coverage of lake turnover, ocean life, coral reefs, and threats to them. • Chapter 49, Human Impacts on the Biosphere Covers extinction crisis, conservation biology, ecosystem degradation, and sustainable use of biological wealth. Appendix V, Molecular Models New art and text explain why we use different types of molecular models. Appendix VI, Closer Look at Some Major Metabolic Pathways New art shows details of electron transport chains in thylakoid membranes. ACKNOWLEDGMENTS
No list can convey our thanks to the team of dedicated people who made this book happen. The professionals who are listed on the following page helped shape our thinking. Marty Zahn and Wenda Ribeiro deserve special recognition for their incisive comments on every chapter, as does Michael Plotkin for voluminous and excellent feedback. Grace Davidson calmly and tirelessly organized our efforts, filled in our gaps, and put all of the pieces of this book together. Paul Forkner’s tenacious photo research helped us achieve our creative vision. At Cengage Learning, Yolanda Cossio and Peggy Williams unwaveringly supported us and our ideals. Andy Marinkovich made sure we had what we needed, Amanda Jellerichs arranged for us to meet with hundreds of professors, Kristina Razmara continues to refine our amazing technology package, Samantha Arvin helped us stay organized, and Elizabeth Momb managed all of the print ancillaries. cecie starr, christine evers, and lisa starr June 2008
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xxxi
Introduction
Current configurations of the Earth’s oceans and land masses—the geologic stage upon which life’s drama continues to unfold. This composite satellite image reveals global energy use at night by the human population. Just as biological science does, it invites you to think more deeply about the world of life—and about our impact upon it.
1
Invitation to Biology IMPACTS, ISSUES
Lost Worlds and Other Wonders
In this era of satellites, submarines, and global positioning
animals in this particular rain forest—mammals and birds
systems, could there possibly be any more places on Earth
especially—seem too big to have gone unnoticed. Had peo-
that we have not explored? Well, yes. In 2005, for instance,
ple just missed them? Perhaps not. No trails or other human
helicopters dropped a team of biologists into a swamp in the
disturbances cut through that part of the forest. The animals
middle of a vast and otherwise inaccessible tropical forest in
had never learned to be afraid of humans, so the team mem-
New Guinea. Later, team member Bruce Beehler remarked,
bers could simply walk over and pick them up (Figure 1.1).
“Everywhere we looked, we saw amazing things we had
Many other animals have been discovered in the past few
never seen before. I was shouting. This trip was a once-in-
years, including lemurs in Madagascar, monkeys in India and
a-lifetime series of shouting experiences.”
Tanzania, cave-dwelling animals in two of California’s national
The team discovered dozens of animals and plants that
parks, carnivorous sponges near Antarctica, and whales and
had been unknown to science, including a rhododendron
giant jellylike animals in the seas. Most came to light during
with plate-sized flowers. They found animals that are on the
survey trips similar to the New Guinea expedition—when
brink of extinction in other parts of the world, and a bird that
biologists simply were attempting to find out what lives where.
was supposedly extinct. The expedition fired the imagination of people all over the
Exploring and making sense of nature is nothing new. We humans and our immediate ancestors have been at it for
world. It is not that finding new kinds of organisms is such
thousands of years. We observe, come up with explanations
a rare event. Almost every week, biologists discover many
about what the observations mean, and then test the explana-
kinds of insects and other small organisms. However, the
tions. Ironically, the more we learn about nature, the more we realize how much we have yet to learn. You might choose to let others tell you what to think about the world around you. Or you might choose to develop your own understanding of it. Perhaps, like the New Guinea explorers, you are interested in animals and where they live. Maybe you are interested in aspects that affect your health, the food you eat, or your home and family. Whatever your focus may be, the scientific study of life—biology—can deepen your perspective on the world. Throughout this book, you will find examples of how organisms are constructed, where they live, and what they do. These examples support concepts that, when taken together, convey what “life” is. This chapter gives you an overview of basic concepts. It sets the stage for upcoming descriptions of scientific observations and applications that can help you refine your understanding of life.
See the video! Figure 1.1 Biologist Kris Helgen and a rare golden-mantled tree kangaroo in a tropical rain forest in the Foja Mountains of New Guinea. There, in 2005, explorers discovered forty previously unknown species.
Links to Earlier Concepts
Key Concepts Levels of organization We study the world of life at different levels of organization, which extend from atoms and molecules to the biosphere. The quality of “life” emerges at the level of cells. Section 1.1
Life’s underlying unity All organisms consist of one or more cells, which stay alive through ongoing inputs of energy and raw materials. All sense and respond to change; all inherited DNA, a type of molecule that encodes information necessary for growth, development, and reproduction. Section 1.2
Life’s diversity Many millions of kinds of organisms, or species, have appeared and disappeared over time. Each kind is unique in some aspects of its body form or behavior. Section 1.3
Explaining unity in diversity Theories of evolution, especially a theory of evolution by natural selection, help explain why life shows both unity and diversity. Evolutionary theories guide research in all fields of biology. Section 1.4
This book parallels nature’s levels of organization, from atoms to the biosphere. Learning about the structure and function of atoms and molecules primes you to understand the structure of living cells. Learning about processes that keep a single cell alive can help you understand how multicelled organisms survive, because their many living cells all use the same processes. Knowing what it takes for organisms to survive can help you see why and how they interact with one another and with their environments. At the start of each chapter, we will use this space to remind you of such connections. Within chapters, cross-references will link you to relevant sections in earlier chapters.
How we know Biologists make systematic observations, predictions, and tests in the laboratory and in the field. They report their results so others may repeat their work and check their reasoning. Sections 1.5–1.8
How would you vote? The discoverer of a new species usually is the one who gives it a scientific name. In 2005, a Canadian casino bought the right to name a monkey species. Should naming rights be sold? See CengageNOW for details, then vote online.
3
1.1
Life’s Levels of Organization We understand life by thinking about nature at different levels of organization. Nature’s organization begins at the level of atoms, and extends through the biosphere. The quality of life emerges at the level of the cell.
Making Sense of the World Most of us intuitively understand what nature means, but could you define it? Nature is everything in the universe except what humans have manufactured. It encompasses every substance, event, force, and energy —sunlight, flowers, animals, bacteria, rocks, thunder, humans, and so on. It excludes everything artificial. Researchers, clerics, farmers, astronauts, children— anyone who is of the mind to do so attempts to make sense of nature. Interpretations differ, for no one can be expert in everything learned so far or have foreknowledge of all that remains hidden. If you are reading this book, you are starting to explore how a subset of scientists, the biologists, think about things, what they found out, and what they are up to now.
B
molecule
C
Two or more atoms joined in chemical bonds. In nature, only living cells make the molecules of life: complex carbohydrates and lipids, proteins, and nucleic acids.
cell
Smallest unit that can live and reproduce on its own or as part of a multicelled organism. A cell has DNA, an outermost membrane, and other components.
D
A Pattern in Life’s Organization Biologists look at all aspects of life, past and present. Their focus takes them all the way down to atoms, and all the way up to global relationships among organisms and the environment. Through their work, we glimpse a great pattern of organization in nature. The pattern starts at the level of atoms. Atoms are fundamental building blocks of all substances, living and nonliving (Figure 1.2a). At the next level of organization, atoms join with other atoms, forming molecules (Figure 1.2b). Among the molecules are complex carbohydrates and lipids, proteins, and nucleic acids. Today, only living cells make these “molecules of life” in nature. The pattern crosses the threshold to life when many molecules are organized as cells (Figure 1.2c). A cell is the smallest unit of life that can survive and reproduce on its own, given information in DNA, energy inputs, raw materials, and suitable environmental conditions. An organism is an individual that consists of one or more cells. In larger multicelled organisms, trillions
tissue
Organized array of cells and substances that are interacting in some task. Bone tissue consists of secretions (brown) from cells such as this (white).
E
organ
Structural unit of two or more tissues that interact in one or more tasks. This parrotfish eye is a sensory organ used in vision.
F
organ system
Organs that interact in one or more tasks. The skin of this parrotfish is an organ system that consists of tissue layers, organs such as glands, and other parts.
single-celled organisms can form populations A
atom
Atoms are fundamental units of all substances. This image shows a model of a single hydrogen atom.
4 INTRODUCTION
Figure 1.2 Animated Levels of organization in nature.
of cells organize into tissues, organs, and organ systems, all interacting in tasks that keep the whole body alive. Figure 1.2d–g defines these body parts. Populations are at a greater level of organization. Each population is a group of individuals of the same kind of organism, or species, living in a specified area (Figure 1.2h). Examples are all heavybeak parrotfish living on Shark Reef in the Red Sea or all California poppies in California’s Antelope Valley Poppy Reserve. Communities are at the next level. A community consists of all populations of all species in a specified area. As an example, Figure 1.2i shows a sampling of the Shark Reef’s species. This underwater community includes many kinds of seaweeds, fishes, corals, sea anemones, shrimps, and other living organisms that make their home in or on the reef. Communities may be large or small, depending on the area defined. The next level of organization is the ecosystem: a community interacting with its physical and chemical environment. The most inclusive level, the biosphere, encompasses all regions of Earth’s crust, waters, and atmosphere in which organisms live.
Bear in mind, life is more than the sum of its individual parts. In other words, some emergent property occurs at each successive level of life’s organization. An emergent property is a characteristic of a system that does not appear in any of its component parts. For example, the molecules of life are themselves not alive. Considering them separately, no one would be able to predict that a particular quantity and arrangement of molecules will form a living cell. Life—an emergent property—appears first at the level of the cell but not at any lower level of organization in nature.
Take-Home Message How does “life” differ from “nonlife”? The building blocks—atoms—that make up all living things are the same ones that make up all nonliving things. Atoms join as molecules. The unique properties of life emerge as certain kinds of molecules become organized into cells.
Higher levels of organization include multicelled organisms, populations, communities, ecosystems, and the biosphere.
Red Sea
G
multicelled organism
Individual composed of different types of cells. Cells of most multicelled organisms, such as this parrotfish, form tissues, organs, and organ systems.
H
population
Group of single-celled or multicelled individuals of a species in a given area. This is a population of one fish species in the Red Sea.
I
community
J
All populations of all species in a specified area. These populations belong to a coral reef community in a gulf of the Red Sea.
K
ecosystem
A community that is interacting with its physical environment through inputs and outputs of energy and materials. Reef ecosystems flourish in warm, clear seawater throughout the Middle East.
biosphere
All regions of Earth’s waters, crust, and atmosphere that hold organisms. Earth is a rare planet. Life as we know it would be impossible without Earth’s abundance of free-flowing water.
CHAPTER 1
INVITATION TO BIOLOGY 5
1.2
Overview of Life’s Unity Continual inputs of energy and the cycling of materials maintain life’s complex organization. Organisms sense and respond to change. DNA inherited from parents is the basis of growth and reproduction in all organisms.
Energy and Life’s Organization Eating supplies your body with energy and nutrients that keep it organized and functioning. Energy is the capacity to do work. A nutrient is a type of atom or molecule that has an essential role in growth and survival and that an organism cannot make for itself. All organisms spend a lot of time acquiring energy and nutrients, although different kinds get such inputs from different sources. These differences allow us to classify organisms into one of two broad categories: producers or consumers. Producers acquire energy and simple raw materials from environmental sources and make their own food. Plants are producers. By the process of photosynthesis,
they use sunlight energy to make sugars from carbon dioxide and water. Those sugars function as packets of immediately available energy or as building blocks for larger molecules. Consumers cannot make their own food; they get energy and nutrients indirectly—by eating producers and other organisms. Animals fall within the consumer category. So do decomposers, which feed on wastes or remains of organisms. We find leftovers of their meals in the environment. Producers take up the leftovers as sources of nutrients. Said another way, nutrients cycle between producers and consumers. Energy, however, is not cycled. It flows through the world of life in one direction—from the environment, through producers, then through consumers. This flow maintains the organization of individual organisms, and it is the basis of life’s organization within the biosphere (Figure 1.3). It is a one-way flow, because with each transfer, some energy escapes as heat. Cells do not use heat to do work. Thus, energy that enters the world of life ultimately leaves it—permanently.
Organisms Sense and Respond to Change
energy input, mainly from sunlight
A Energy inputs from the environment flow through producers, then consumers.
Organisms sense and respond to changes both inside and outside the body by way of receptors. A receptor is a molecule or cellular structure that responds to a specific form of stimulation, such as the energy of light or the mechanical energy of a bite (Figure 1.4). Stimulated receptors trigger changes in activities of organisms. For example, after you eat, the sugars from
PRODUCERS plants and other self-feeding organisms
nutrient cycling
B Nutrients become incorporated into the cells of producers and consumers. Some nutrients released by decomposition cycle back to producers.
CONSUMERS animals, most fungi, many protists, bacteria
energy output, mainly heat
C All energy that enters an ecosystem eventually flows out of it, mainly as heat.
Figure 1.3 Animated The one-way flow of energy and cycling of materials through an ecosystem.
6 INTRODUCTION
Figure 1.4 A roaring response to signals from pain receptors, activated by a lion cub flirting with disaster.
Figure 1.5 Development of the atlas moth. Instructions in DNA guide the development of this insect through a series of stages, from a fertilized egg (a), to a larval stage called a caterpillar (b), to a pupal stage (c), to the winged adult form (d,e). a
b
c
d
e
your meal enter your bloodstream, and then your blood sugar level rises. The added sugars bind to receptors on cells of the pancreas (an organ). Binding sets in motion a series of events that causes cells throughout the body to take up sugar faster, so the sugar level in your blood returns to normal. In multicelled organisms, the internal environment is all fluid inside of the body but outside of cells. Unless the composition of the internal environment is kept within certain ranges, body cells will die. By sensing and adjusting to change, organisms keep conditions in their internal environment within a range that favors cell survival. This process is called homeostasis, and it is a defining feature of life. All organisms, whether single-celled or multicelled, undergo homeostasis.
Organisms Grow and Reproduce DNA, a nucleic acid, is the signature molecule of life. No chunk of rock has it. Why is DNA so important? It is the basis of growth, survival, and reproduction in all organisms. It is also the source of each individual’s distinct features, or traits. In nature, an organism inherits DNA—the basis of its traits—from parents. Inheritance is the transmission of DNA from parents to offspring. Moths look like moths and not like chickens because they inherited moth DNA, which differs from chicken DNA. Reproduction refers to actual mechanisms by which
Figure 1.6 Animated Three examples of objects assembled in different ways from the same materials.
parents transmit DNA to offspring. For all multicelled individuals, DNA has information that guides growth and development—the orderly transformation of the first cell of a new individual into an adult (Figure 1.5). DNA contains instructions. Cells use some of those instructions to make proteins, which are long chains of amino acids. There are only 20 kinds of amino acids, but cells string them together in different sequences to make a tremendous variety of proteins. By analogy, just a few different kinds of tiles can be organized into many different patterns (Figure 1.6). Different proteins have structural or functional roles. For instance, certain proteins are enzymes—functional molecules that make cell activities occur much faster than they would on their own. Without enzymes, such activities would not happen fast enough for a cell to survive. There would be no more cells—and no life.
Take-Home Message How are all living things alike? A one-way flow of energy and a cycling of nutrients through organisms and the environment sustain life, and life’s organization.
Organisms maintain homeostasis by sensing and responding to change. They make adjustments that keep conditions in their internal environment within a range that favors cell survival. Organisms grow, develop, and reproduce based on information encoded in their DNA, which they inherit from their parents.
CHAPTER 1
INVITATION TO BIOLOGY 7
1.3
Overview of Life’s Diversity Of an estimated 100 billion kinds of organisms that have ever lived on Earth, as many as 100 million are with us today.
Each time we discover a new species, or kind of organism, we assign it a two-part name. The first part of the name specifies the genus (plural, genera), which is a group of species that share a unique set of features. When combined with the second part, the name designates one species. Individuals of a species share one or more heritable traits, and they can interbreed successfully if the species is a sexually reproducing one. Genus and species names are always italicized. For example, Scarus is a genus of parrotfish. The heavybeak parrotfish in Figure 1.2g is called Scarus gibbus. A different species in the same genus, the midnight parrotfish, is S. coelestinus. Note that the genus name may be abbreviated after it has been spelled out one time. We use various classification systems to organize and retrieve information about species. Most systems group species together on the basis of their observable characteristics, or traits. Table 1.1 and Figure 1.7 show a common system in which more inclusive groupings above the level of genus are phylum (plural, phyla), kingdom, and domain. Here, all species are grouped into domains Bacteria, Archaea, and Eukarya. Protists, plants, fungi, and animals make up domain Eukarya. All bacteria (singular, bacterium) and archaeans are single-celled organisms. All of them are prokaryotic, which means they do not have a nucleus. In other organisms, this membrane-enclosed sac holds and protects a cell’s DNA. As a group, prokaryotes have the most diverse ways of procuring energy and nutrients. They are producers and consumers in nearly all of the biosphere, including extreme environments such as frozen desert rocks, boiling sulfur-clogged lakes, and nuclear reactor waste. The first cells on Earth may have faced similarly hostile challenges to survival. Cells of eukaryotes start out life with a nucleus. Structurally, protists are the simplest kind of eukaryote. Different protist species are producers or consumers. Many are single cells that are larger and more complex than prokaryotes. Some of them are tree-sized, multi-
Table 1.1
Comparison of Life’s Three Domains
Bacteria
Single cells, prokaryotic (no nucleus). Most ancient lineage.
Archaea
Single cells, prokaryotic. Evolutionarily closer to eukaryotes.
Eukarya
Eukaryotic cells (with a nucleus). Single-celled and multicelled species categorized as protists, plants, fungi, and animals.
8 INTRODUCTION
A Bacteria These prokaryotes tap more diverse sources of energy and nutrients than all other organisms. Clockwise from upper left, a magnetotactic bacterium has a row of iron crystals that acts like a tiny compass; bacteria that live on skin; spiral cyanobacteria; and Lactobacillus cells in yogurt.
B Archaea Although they often appear similar to bacteria, these prokaryotes are evolutionarily closer to eukaryotes. Left, a colony of methane-producing cells. Right, two species from a hydrothermal vent on the seafloor.
A
Bacteria
B
Archaea
C
Eukarya
Figure 1.7 Animated Representatives of diversity from the three most inclusive branchings of the tree of life.
celled seaweeds. Protists are so diverse that they are now being reclassified into a number of separate major lineages based on emerging biochemical evidence. Cells of fungi, plants, and animals are eukaryotic. Most fungi, such as the types that form mushrooms, are multicelled. Many are decomposers, and all secrete enzymes that digest food outside the body. Their cells then absorb the released nutrients.
C
Eukarya
Protists are single-celled and multicelled eukaryotic species that range from the microscopic to giant seaweeds. Many biologists are now viewing the “protists” as many major lineages.
Plants are multicelled eukaryotes, most of which are photosynthetic. Nearly
Fungi are eukaryotes. Most are multicelled. Different kinds are
Animals are multicelled eukaryotes that ingest tissues or juices of other
parasites, pathogens, or decomposers. Without decomposers such as fungi, communities would become buried in their own wastes.
organisms. Like this basilisk lizard, they actively move about during at least part of their life.
Plants are multicelled species. Most of them live on land or in freshwater environments. Nearly all plants are photosynthetic: They harness the energy of sunlight to drive the production of sugars from carbon dioxide and water. Besides feeding themselves, photosynthesizers also feed much of the biosphere. The animals are multicelled consumers that ingest tissues or juices of other organisms. Herbivores graze, carnivores eat meat, scavengers eat remains of other organisms, and parasites withdraw nutrients from the tissues of a host. Animals grow and develop through a
all have roots, stems, and leaves. Plants are the primary producers in land ecosystems. Redwood trees and flowering plants are examples.
series of stages that lead to the adult form. Most kinds actively move about during at least part of their lives. From this quick overview, can you get a sense of the tremendous range of life’s variety—its diversity? Take-Home Message How do living things differ from one another? Organisms differ in their details; they show tremendous variation in observable characteristics, or traits.
Various classification systems group species on the basis of shared traits.
CHAPTER 1
INVITATION TO BIOLOGY 9
1.4
An Evolutionary View of Diversity A theory of evolution by natural selection is an explanation of life’s diversity.
Individuals of a population are alike in certain aspects of their body form, function, and behavior, but the details of such traits differ from one individual to the next. For instance, humans (Homo sapiens) characteristically have two eyes, but those eyes come in a range of color among individuals. Most traits are the outcome of information encoded in DNA, so they can be passed to offspring. Variations in traits arise through mutations, which are smallscale changes in DNA. Most mutations have neutral or negative effects, but some cause a trait to change in a way that makes an individual better suited to its environment. The bearer of such an adaptive trait has a better chance of surviving and passing its DNA to offspring than other individuals of the population. The naturalist Charles Darwin expressed the concept of “survival of the fittest” like this: First, a natural population tends to increase in size. As it does, the individuals of the population compete more for food, shelter, and other limited resources. Second, individuals of a population differ from one another in the details of shared traits. Such traits have a heritable basis. Third, adaptive forms of traits make their bearers more competitive, so those forms tend to become more common over generations. The differential survival and reproduction of individuals in a population that differ in the details of their heritable traits is called natural selection. Think of how pigeons differ in feather color and other traits (Figure 1.8a). Imagine that a pigeon breeder
prefers black, curly-tipped feathers. She selects birds with the darkest, curliest-tipped feathers, and allows only those birds to mate. Over time, more and more pigeons in the breeder’s captive population will have black, curly-tipped feathers. Pigeon breeding is a case of artificial selection. One form of a trait is favored over others under contrived, manipulated conditions—in an artificial environment. Darwin saw that breeding practices could be an easily understood model for natural selection, a favoring of some forms of a given trait over others in nature. Just as breeders are “selective agents” that promote reproduction of certain pigeons, agents of selection act on the range of variation in the wild. Among them are pigeon-eating peregrine falcons (Figure 1.8b). Swifter or better camouflaged pigeons are more likely to avoid falcons and live long enough to reproduce, compared with not-so-swift or too-flashy pigeons. When different forms of a trait are becoming more or less common over successive generations, evolution is under way. In biology, evolution simply means change in a line of descent. Take-Home Message How did life become so diverse? Individuals of a population show variation in their shared, heritable traits. Such variation arises through mutations in DNA. Adaptive traits improve an individual’s chances of surviving and reproducing, so they tend to become more common in a population over successive generations. Natural selection is the differential survival and reproduction among individuals of a population that differ in the details of their shared, heritable traits. It and other evolutionary processes underlie the diversity of life.
wild rock pigeon
a
Figure 1.8 (a) Outcome of artificial selection: a few of the hundreds of varieties of domesticated pigeons descended from captive populations of wild rock pigeons (Columba livia). (b) A peregrine falcon (left ) preying on a pigeon (right ) is acting as an agent of natural selection in the wild.
10 INTRODUCTION
b
1.5
Critical Thinking and Science
Critical thinking means judging the quality of information. Science is limited to that which is observable.
Table 1.2
What message am I being asked to accept?
Thinking About Thinking Most of us assume that we do our own thinking—but do we, really? You might be surprised to find out just how often we let others think for us. For instance, a school’s job, which is to impart as much information as possible to students, meshes with a student’s job, which is to acquire as much knowledge as possible. In this rapid-fire exchange of information, it is easy to forget about the quality of what is being exchanged. If you accept information without question, you allow someone else to think for you. Critical thinking means judging information before accepting it. “Critical” comes from the Greek kriticos (discerning judgment). When you think this way, you move beyond the content of information. You look for underlying assumptions, evaluate the supporting statements, and think of possible alternatives (Table 1.2). How does the busy student manage this? Be aware of what you intend to learn from new information. Be conscious of bias or underlying agendas in books, lectures, or online. Consider your own biases—what you want to believe—and realize those biases influence your learning. Respectfully question authority figures. Decide whether ideas are based on opinion or evidence. Such practices will help you decide whether to accept or reject information.
The Scope and Limits of Science Because each of us is unique, there are as many ways to think about the natural world as there are people. Science, the systematic study of nature, is one way. It helps us be objective about our observations of nature, in part because of its limitations. We limit science to a subset of the world—only that which is observable. Science does not address some questions, such as “Why do I exist?” Most answers to such questions are subjective; they come from within as an integration of the personal experiences and mental connections that shape our consciousness. This is not to say subjective answers have no value: No human society functions for very long unless its individuals share standards for making judgments, even if they are subjective. Moral, aesthetic, and philosophical standards vary from one society to the next, but all help people decide what is important and good. All give meaning to what we do. Also, science does not address the supernatural, or anything that is “beyond nature.” Science does not
A Guide to Critical Thinking
What evidence supports the message? Is the evidence valid? Is there another way to interpret the evidence? What other evidence would help me evaluate the alternatives? Is the message the most reasonable one based on the evidence?
assume or deny that supernatural phenomena occur, but scientists may still cause controversy when they discover a natural explanation for something that was thought to be unexplainable. Such controversy often arises when a society’s moral standards have become interwoven with traditional interpretations of nature. For example, Nicolaus Copernicus studied the planets centuries ago in Europe, and concluded that Earth orbits the sun. Today this conclusion seems obvious, but at the time it was heresy. The prevailing belief was that the Creator made Earth—and, by extension, humans—as the center of the universe. Galileo Galilei, another scholar, found evidence for the Copernican model of the solar system and published his findings. He was forced to publicly recant his publication, and to put Earth back at the center of the universe. Exploring a traditional view of the natural world from a scientific perspective might be misinterpreted as questioning morality even though the two are not the same. As a group, scientists are no less moral, less lawful, or less compassionate than anyone else. As you will see in the next section, however, their work follows a particular standard: Explanations must be testable in the natural world in ways that others can repeat. Science helps us communicate experiences without bias; it may be as close as we can get to a universal language. We are fairly sure, for example, that laws of gravity apply everywhere in the universe. Intelligent beings on a distant planet would likely understand the concept of gravity. We might well use such concepts to communicate with them—or anyone—anywhere. The point of science, however, is not to communicate with aliens. It is to find common ground here on Earth.
Take-Home Message What is science?
Science is the study of the observable—those objects or events for which valid evidence can be gathered. It does not address the supernatural.
CHAPTER 1
INVITATION TO BIOLOGY 11
1.6
How Science Works Scientists make and test potentially falsifiable predictions about how the natural world works.
Observations, Hypotheses, and Tests To get a sense of how science works, consider Table 1.3 and this list of common research practices: 1. Observe some aspect of nature. 2. Frame a question that relates to your observation. 3. Read about what others have discovered concerning the subject, then propose a hypothesis, a testable answer to your question. 4. Using the hypothesis as a guide, make a prediction: a statement of some condition that should exist if the hypothesis is not wrong. Making predictions is called the if–then process: “if” is the hypothesis, and “then” is the prediction. 5. Devise ways to test the accuracy of the prediction by conducting experiments or gathering information. Experiments may be performed on a model, or analogous system, if experimenting directly with an object or event is not possible. 6. Assess the results of the tests. Results that confirm the prediction are evidence—data—in support of the hypothesis. Results that disprove the prediction are evidence that the hypothesis may be flawed. 7. Report all the steps of your work, along with any conclusions you drew, to the scientific community.
Table 1.3
Example of a Scientific Approach
1. Observation
People get cancer.
2. Question
Why do people get cancer?
3. Hypothesis
Smoking cigarettes may cause cancer.
4. Prediction
If smoking causes cancer, then individuals who smoke will get cancer more often than those who do not.
5. Gather information
Conduct a survey of individuals who smoke and individuals who do not smoke. Determine which group has the highest incidence of cancers.
Laboratory experiment
Establish identical groups of laboratory rats (the model system). Expose one group to cigarette smoke. Compare the incidence of new cancers in each of the two groups.
6. Assess results Compile test results and draw conclusions from them. 7. Report
Submit the results and the conclusions to the scientific community for review and publication.
12 INTRODUCTION
You might hear someone refer to these practices as “the scientific method,” as if all scientists march to the drumbeat of a fixed procedure. They do not. There are different ways to do research, particularly in biology (Figure 1.9). Some biologists do surveys; they observe without making hypotheses. Others make hypotheses and leave tests to others. Some stumble onto valuable information they are not even looking for. Of course, it is not only a matter of luck. Chance favors a mind that is already prepared, by education and experience, to recognize what the new information might mean. Regardless of the variation, one thing is constant: Scientists do not accept information simply because someone says it is true. They evaluate the supporting evidence and find alternative explanations. Does this sound familiar? It should—it is critical thinking.
About the Word “Theory” Most scientists avoid the word “truth” when discussing science. Instead, they tend to talk about evidence that supports or does not support a hypothesis. Suppose a hypothesis has not been disproven even after years of tests. It is consistent with all of the evidence gathered to date, and it has helped us to make successful predictions about other phenomena. When any hypothesis meets these criteria, it is considered to be a scientific theory. To give an example, observations for all of recorded history have supported the hypothesis that gravity pulls objects toward Earth. Scientists no longer spend time testing the hypothesis for the simple reason that, after many thousands of years of observation, no one has seen otherwise. This hypothesis is now a scientific theory, but it is not an “absolute truth.” Why not? An infinite number of tests would be necessary to confirm that it holds under every possible circumstance. A single observation or result that is not consistent with a theory opens that theory to revision. For example, if gravity pulls objects toward Earth, it would be logical to predict that an apple will fall down when dropped. However, a scientist might well see such a test as an opportunity for the prediction to fail. Think about it. If even one apple falls up instead of down, the theory of gravity would come under scrutiny. Like every other theory, this one remains open to revision. A well-tested theory is as close to the “truth” as scientists will venture. Table 1.4 lists a few scientific theories. One of them, the theory of natural selection, holds after more than a century of testing. Like all other scientific theories, we cannot be sure that it will hold under all possible conditions, but we can say it
a
b
c
Figure 1.9 Scientists doing research in the laboratory and in the field. (a) Analyzing data with computers. (b) At the Centers for Disease Control and Prevention, Mary Ari testing a sample for the presence of dangerous bacteria. (c) Making field observations in an old-growth forest.
has a very high probability of not being wrong. If any evidence turns up that is inconsistent with the theory of natural selection, then biologists will revise it. Such a willingness to modify or discard even an entrenched theory is one of the strengths of science. You may hear people apply the word “theory” to a speculative idea, as in the phrase “It’s just a theory.” Speculation is opinion or belief, a personal conviction that is not necessarily supported by evidence. A scientific theory is not an opinion: By definition, it must be supported by a large body of evidence. Unlike theories, many beliefs and opinions cannot be tested. Without being able to test something, there is no way to disprove it. Even though personal conviction has tremendous value in our lives, it should not be confused with scientific theory.
Table 1.4
Examples of Scientific Theories
Some Terms Used in Experiments Careful observations are one way to test predictions that flow from a hypothesis. So are experiments. You will find examples of experiments in the next section. For now, just get acquainted with some of the important terms that researchers use: 1. Experiments are tests that can support or falsify a prediction. 2. Experiments are usually designed to test the effects of a single variable. A variable is a characteristic that differs among individuals or events. 3. Biological systems are an integration of so many interacting variables that it can be difficult to study one variable separately from the rest. Experimenters often test two groups of individuals, side by side. An experimental group is a set of individuals that have a certain characteristic or receive a certain treatment. This group is tested side by side with a control group, which is identical to the experimental group except for one variable—the characteristic or the treatment being tested. Ideally, the two groups have the same set of variables, except for the one being tested. Thus, any differences in experimental results between the two groups should be an effect of changing the variable.
Atomic theory
All substances are composed of atoms.
Gravitation
Objects attract one another with a force that depends on their mass and how close together they are.
Cell theory
All organisms consist of one or more cells, the cell is the basic unit of life, and all cells arise from existing cells.
Germ theory
Microorganisms cause many diseases.
Take-Home Message
Plate tectonics
Earth’s crust is cracked into pieces that move in relation to one another.
Evolution
Change occurs in lines of descent.
How does science work? Scientific inquiry involves asking questions about some aspect of nature, formulating hypotheses, making and testing predictions, and reporting the results.
Natural selection
Variation in heritable traits influences differential survival and reproduction of individuals of a population.
Researchers design experiments to test the effects of one variable at a time. A scientific theory is a long-standing, well-tested concept of cause and effect that is consistent with all evidence, and is used to make predictions about other phenomena.
CHAPTER 1
INVITATION TO BIOLOGY 13
1.7
The Power of Experimental Tests Researchers unravel cause and effect in complex natural processes by changing one variable at a time.
Potato Chips and Stomach Aches In 1996 the FDA approved Olestra®, a type of synthetic fat replacement made from sugar and vegetable oil, as a food additive. Potato chips were the first Olestralaced food product on the market in the United States. Controversy soon raged. Some people complained of intestinal cramps after eating the chips and concluded that Olestra caused them. Two years later, four researchers at Johns Hopkins University School of Medicine designed an experiment to test the hypothesis that this food additive causes cramps. They predicted that if Olestra causes cramps, then people who eat Olestra will be more likely to get cramps than people who do not.
To test the prediction, they used a Chicago theater as the “laboratory.” They asked more than 1,100 people between ages thirteen and thirty-eight to watch a movie and eat their fill of potato chips. Each person got an unmarked bag that contained 13 ounces of chips. The individuals who got a bag of Olestra-laced potato chips were the experimental group. Individuals who got a bag of regular chips were the control group. Afterward, researchers contacted all of the people and tabulated the reports of gastrointestinal cramps. Of 563 people making up the experimental group, 89 (15.8 percent) complained about problems. However, so did 93 of the 529 people (17.6 percent) making up the control group—who had munched on the regular chips! This simple experiment disproved the prediction that eating Olestra-laced potato chips at a single sitting can cause gastrointestinal cramps (Figure 1.10).
Butterflies and Birds
A
Hypothesis Olestra® causes intestinal cramps.
B
Prediction People who eat potato chips made with Olestra will be more likely to get intestinal cramps than those who eat potato chips made without Olestra.
C Experiment
D Results
Control Group
Experimental Group
Eats regular potato chips
Eats Olestra potato chips
93 of 529 people get cramps later (17.6%)
89 of 563 people get cramps later (15.8%)
E Conclusion Percentages are about equal. People who eat potato chips made with Olestra are just as likely to get intestinal cramps as those who eat potato chips made without Olestra. These results do not support the hypothesis.
Figure 1.10 Animated The steps in a scientific experiment to determine if Olestra causes cramps. A report of this study was published in the Journal of the American Medical Association in January of 1998.
14 INTRODUCTION
Consider the peacock butterfly, a winged insect that was named for the large, colorful spots on its wings. In 2005, researchers published a report on their tests to identify factors that help peacock butterflies defend themselves against insect-eating birds. The researchers made two observations. First, when a peacock butterfly rests, it folds its ragged-edged wings, so only the dark underside shows (Figure 1.11a). Second, when a butterfly sees a predator approaching, it repeatedly flicks its paired forewings and hindwings open and closed. At the same time, each forewing slides over the hindwing, which produces a hissing sound and a series of clicks. The researchers asked this question, “Why does the peacock butterfly flick its wings?” After they reviewed earlier studies, they formulated three hypotheses that might explain the wing-flicking behavior: 1. When folded, the butterfly wings resemble a dead leaf. They may camouflage the butterfly, or help it hide from predators in its forest habitat. 2. Although the wing-flicking probably attracts predatory birds, it also exposes brilliant spots that resemble owl eyes (Figure 1.11b). Anything that looks like owl eyes is known to startle small, butterfly-eating birds, so exposing the wing spots might scare off predators. 3. The hissing and clicking sounds produced when the peacock butterfly rubs the sections of its wings together may deter predatory birds. The researchers decided to test hypotheses 2 and 3. They made the following predictions:
b
Table 1.5
c
Figure 1.11 Peacock butterfly defenses against predatory birds. (a) With wings folded, a resting peacock butterfly looks like a dead leaf. (b) When a bird approaches, the butterfly repeatedly flicks its wings open and closed. This defensive behavior exposes brilliant spots. It also produces hissing and clicking sounds.
Results of Peacock Butterfly Experiment*
Wing Spots
Wing Sound
Spots
Sound
No spots
Sound
Spots No spots
Total Number of Butterflies
Number Eaten
Number Survived
9
0
9 (100%)
10
5
5 (50%)
No sound
8
0
8 (100%)
No sound
10
8
2 (20%)
Researchers tested whether the behavior deters blue tits (c). They painted over the spots of some butterflies, cut the sound-making part of the wings on other butterflies, and did both to a third group; then the biologists exposed each butterfly to a hungry bird. The results are listed in Table 1.5. Figure It Out: Which defense, wing spots or sounds, more effectively deterred the tits? Answer: wing spots
a
* Proceedings of the Royal Society of London, Series B (2005) 272: 1203–1207.
1. If brilliant wing spots of peacock butterflies deter predatory birds, then individuals with no wing spots will be more likely to get eaten by predatory birds than individuals with wing spots. 2. If the sounds that peacock butterflies produce deter predatory birds, then individuals that do not make the sounds will be more likely to be eaten by predatory birds than individuals that make the sounds. The next step was the experiment. The researchers painted the wing spots of some butterflies black, cut off the sound-making part of the hindwings of others, and did both to a third group. They put each butterfly into a large cage with a hungry blue tit (Figure 1.11c) and then watched the pair for thirty minutes. Table 1.5 lists the results of the experiment. All of the butterflies with unmodified wing spots survived, regardless of whether they made sounds. By contrast, only half of the butterflies that had spots painted out but could make sounds survived. Most of the butterflies with neither spots nor sound structures were eaten quickly. The test results confirmed both predictions, so they support the hypotheses. Birds are deterred by peacock butterfly sounds, and even more so by wing spots.
Asking Useful Questions Researchers try to design single-variable experiments that will yield quantitative results, which are counts or some other data that can be measured or gathered objectively. Even so, they risk designing experiments and interpreting results in terms of what they want to find out. Particularly when studying humans, isolating a single variable is not often possible. For example, by thinking critically we may realize that the people who participated in the Olestra experiment were chosen randomly. That means the study was not controlled for gender, age, weight, medications taken, and so on. Such variables may well have influenced the results. Scientists expect one another to put aside bias. If one individual does not, others will, because science works best when it is both cooperative and competitive.
Take-Home Message Why do biologists do experiments? Natural processes are often influenced by many interacting variables.
Experiments help researchers unravel causes of such natural processes by focusing on the effects of changing a single variable.
CHAPTER 1
INVITATION TO BIOLOGY 15
FOCUS ON SCIENCE
1.8
Sampling Error in Experiments Biology researchers experiment on subsets of a group. Results from such an experiment may differ from results of the same experiment performed on the whole group.
A Natalie, blindfolded, randomly plucks a jelly bean from a jar. There are 120 green and 280 black jelly beans in that jar, so 30 percent of the jelly beans in the jar are green, and 70 percent are black. B The jar is hidden from Natalie’s view before she removes her blindfold. She sees only one green jelly bean in her hand and assumes that the jar must hold only green jelly beans.
C Blindfolded again, Natalie picks out 50 jelly beans from the jar and ends up with 10 green and 40 black jelly beans.
D The larger sample leads Natalie to assume that one-fifth of the jar’s jelly beans are green (20 percent) and four-fifths are black (80 percent). The sample more closely approximates the jar’s actual green-to-black ratio of 30 percent to 70 percent. The more times Natalie repeats the sampling, the greater the chance she will come close to knowing the actual ratio.
Rarely can researchers observe all individuals of a group. For example, remember the explorers you read about in the chapter introduction? They did not survey the entire rain forest, which cloaks more than 2 million acres of New Guinea’s Foja Mountains. Even if it were possible, doing so would take unrealistic amounts of time and effort. Besides, tromping about even in a small area can damage delicate forest ecosystems. Given such constraints, researchers tend to experiment on subsets of a population, event, or some other aspect of nature that they select to represent the whole. They test the subsets, and then use the results to make generalizations about the whole population. Suppose researchers design an experiment to identify variables that influence the population growth of goldenmantled tree kangaroos. They might focus only on the population living in one acre of the Foja Mountains. If they identify only 5 golden-mantled tree kangaroos in the specified acre, then they might extrapolate that there are 50 in every ten acres, 100 in every twenty acres, and so forth. However, generalizing from a subset is risky because the subset may not be representative of the whole. If the only population of golden-mantled tree kangaroos in the forest just happens to be living in the surveyed acre, then the researchers’ assumptions about the number of kangaroos in the rest of the forest will be wrong. Sampling error is a difference between results from a subset and results from the whole. It happens most often when sample sizes are small. Starting with a large sample or repeating the experiment many times helps minimize sampling error (Figure 1.12). To understand why, imagine flipping a coin. There are two possible results: The coin lands heads up, or it lands tails up. You might predict that the coin will land heads up as often as it lands tails up. When you actually flip the coin, though, often it will land heads up, or tails up, several times in a row. If you flip the coin only a few times, the results may differ greatly from your prediction. Flip it many times, and you probably will come closer to having equal numbers of heads and tails. Sampling error is an important consideration in the design of most experiments. The possibility that it occurred should be part of the critical thinking process as you read about experiments. Remember to ask: If the experimenters used a subset of the whole, did they select a large enough sample? Did they repeat the experiment many times? Thinking about these possibilities will help you evaluate the results and conclusions reached.
Figure 1.12 Animated Demonstration of sampling error.
16 INTRODUCTION
IMPACTS, ISSUES REVISITED
Lost Worlds and Other Wonders
Almost every week, another new species is discovered and we are again reminded that we do not yet know all of the organisms on our own planet. We don’t even know how many to look for. The vast information about the 1.8 million species we do know about changes so quickly that collating it has been impossible—until now. A new web site, titled the Encyclopedia of Life, is intended to be an online reference source and database of species information maintained by collaborative effort. See it at www.eol.org.
Summary Section 1.1 There are emergent properties at each level of organization in nature. All matter consists of atoms, which combine as molecules. Organisms are one or more cells, the smallest units of life. A population is a group of individuals of a species in a given area; a community is all populations of all species in a given area. An ecosystem is a community interacting with its environment. The biosphere includes all regions of Earth that hold life.
Explore levels of biological organization with the interaction on CengageNOW.
Section 1.2 All living things have similar characteristics (Table 1.6). All organisms require inputs of energy and nutrients to sustain themselves. Producers make their own food by processes such as photosynthesis; consumers eat producers or other consumers. By homeostasis, organisms use molecules and structures such as receptors to help keep the conditions in their internal environment within ranges that their cells tolerate. Organisms grow, develop, and reproduce using information in their DNA, a nucleic acid inherited from parents. Information encoded in DNA is the source of an individual’s traits.
Use instructions with the animation on CengageNOW to see how different objects are assembled from the same materials. Also view energy flow and materials cycling.
Section 1.3 Each type of organism is given a name that includes genus and species names. Classification systems group species by their shared, heritable traits. All organisms can be classified as bacteria, archaea, or eukaryotes. Plants, protists, fungi, and animals are eukaryotes.
Use the interaction on CengageNOW to explore characteristics of the three domains of life.
Section 1.4 Information encoded in DNA is the basis of traits that an organism shares with others of its species. Mutations are the original source of variation in traits. Some forms of traits are more adaptive than others, so their bearers are more likely to survive and reproduce. Over generations, such adaptive traits tend to become more common in a population; less adaptive forms of traits tend to become less common or are lost. Thus, the traits that characterize a species can change over generations in evolving populations. Evolution is change in a line of descent. The differential survival and
How would you vote? Discovered in Madagascar in 2005, this tiny mouse lemur was named Microcebus lehilahytsara in honor of primatologist Steve Goodman (lehilahytsara is a combination of the Malagasy words for “good” and “man”). Should naming rights be sold? See CengageNOW for details, then vote online.
reproduction among individuals that vary in the details of their shared, heritable traits is an evolutionary process called natural selection. Section 1.5 Critical thinking is judging the quality of information as one learns. Science is one way of looking at the natural world. It helps us minimize bias in our judgments by focusing on only testable ideas about observable aspects of nature. Section 1.6 Researchers generally make observations, form hypotheses (testable assumptions) about it, then make predictions about what might occur if the hypothesis is correct. They test predictions with experiments, using models, variables, experimental groups, and control groups. A hypothesis that is not consistent with results of scientific tests (evidence) is modified or discarded. A scientific theory is a long-standing hypothesis that is used to make useful predictions. Section 1.7 Scientific experiments simplify interpretations of complex biological systems by focusing on the effect of one variable at a time. Section 1.8 Small sample size increases the likelihood of sampling error in experiments. In such cases, a subset may be tested that is not representative of the whole.
Table 1.6
Summary of Life’s Characteristics
Shared characteristics that underlie life’s unity Organisms grow, develop, and reproduce based on information encoded in DNA, which is inherited from parents. Ongoing inputs of energy and nutrients sustain all organisms, as well as nature’s overall organization. Organisms maintain homeostasis by sensing and responding to changes inside and outside of the body.
Basis of life’s diversity Mutations (heritable changes in DNA) give rise to variation in details of body form, the functioning of body parts, and behavior. Diversity is the sum total of variations that have accumulated, since the time of life’s origin, in different lines of descent. It is an outcome of natural selection and other processes of evolution.
CHAPTER 1
INVITATION TO BIOLOGY 17
Data Analysis Exercise The photographs to the right represent the experimental and control groups used in the peacock butterfly experiment from Section 1.7.
.
move around for at least part of their life.
3.
4. Organisms require and themselves, grow, and reproduce.
to maintain
6. Bacteria, Archaea, and Eukarya are three
8.
c. is transmitted from parents to offspring d. all of the above
is the process by which an organism produces
10. Science only addresses that which is 11.
.
is the transmission of DNA to offspring. a. Reproduction c. Homeostasis b. Development d. Inheritance
9. offspring.
.
are the original source of variation in traits.
12. A trait is if it improves an organism’s chances to survive and reproduce in its environment. 13. A control group is . a. a set of individuals that have a certain characteristic or receive a certain treatment b. the standard against which experimental groups can be compared c. the experiment that gives conclusive results 14. Match the terms with the most suitable description. emergent a. statement of what a hypothesis property leads you to expect to see natural b. type of organism selection c. occurs at a higher organizational scientific level in nature, not at levels below it theory d. time-tested hypothesis hypothesis e. differential survival and reproduction prediction among individuals of a population species that vary in details of shared traits f. testable explanation
Visit CengageNOW for additional questions.
18 INTRODUCTION
but spots visible
f Wings painted but spots visible; wings cut but not silenced
Critical Thinking 1. Why would you think twice about ordering from a cafe menu that lists only the second part of the species name (not the genus) of its offerings? Hint: Look up Ursus americanus, Ceanothus americanus, Bufo americanus, Homarus americanus, Lepus americanus, and Nicrophorus americanus. 2. How do prokaryotes and eukaryotes differ?
5. is a process that maintains conditions in the internal environment within ranges that cells can tolerate. 7. DNA . a. contains instructions for building proteins b. undergoes mutation
e Wings cut but not silenced
painted out; wings silenced
Answers in Appendix III
2. The smallest unit of life is the
b Wing spots
c Wing spots
are fundamental building blocks of all matter.
1.
d Wings painted
visible; wings silenced
See if you can identify each experimental group, and match it with the relevant control group(s). Hint: Identify which variable is being tested in each group (each variable has a control).
Self-Quiz
a Wing spots painted out
3. Explain the relationship between DNA and natural selection. 4. Procter & Gamble makes Olestra and financed the study described in Section 1.7. The main researcher was a consultant to Procter & Gamble during the study. What do you think about scientific information that comes from tests financed by companies with a vested interest in the outcome? 5. Once there was a highly intelligent turkey that had nothing to do but reflect on the world’s regularities. Morning always started out with the sky turning light, followed by the master’s footsteps, which was always followed by the appearance of food. Other things varied, but food always followed footsteps. The sequence of events was so predictable that it eventually became the basis of the turkey’s theory about the goodness of the world. One morning, after more than 100 confirmations of the goodness theory, the turkey listened for the master’s footsteps, heard them, and had its head chopped off. Any scientific theory is modified or discarded when contradictory evidence becomes available. The absence of absolute certainty has led some people to conclude that “facts are irrelevant—facts change.” If that is so, should we stop doing scientific research? Why or why not? 6. In 2005 a South Korean scientist, Woo-suk Hwang, reported that he made immortal stem cells from eleven human patients. His research was hailed as a breakthrough for people affected by currently incurable degenerative diseases, because such stem cells might be used to repair a person’s own damaged tissues. Hwang published his results in a respected scientific journal. In 2006, the journal retracted his paper after other scientists discovered that Hwang and his colleagues had faked their results. Does the incident show that the results of scientific studies cannot be trusted? Or does it confirm the usefulness of a scientific approach, because other scientists quickly discovered and exposed the fraud?
I
PRINCIPLES OF CELLULAR LIFE
Staying alive means securing energy and raw materials from the environment. Shown here, a living cell of the genus Stentor. This protist has hairlike projections around an opening to a cavity in its body, which is about 2 millimeters long. Its “hairs” of fused-together cilia beat the surrounding water. They create a current that wafts food into the cavity.
19
2
Life’s Chemical Basis IMPACTS, ISSUES
What Are You Worth?
Hollywood thinks actor Keanu Reaves is worth $30 million
Elements in a Human Body
plus per movie, the Yankees think shortstop Alex Rodriguez is worth $252 million per decade, and the United States thinks
Number of Atoms (x 1015)
Retail Cost
Hydrogen Oxygen Carbon Nitrogen Phosphorus Calcium Sulfur Sodium Potassium Chlorine Magnesium Fluorine Iron Silicon Zinc Rubidium Strontium Bromine Boron Copper Lithium Lead Cadmium Titanium Cerium Chromium Nickel Manganese Selenium Tin Iodine Arsenic Germanium Molybdenum Cobalt Cesium Mercury Silver Antimony Niobium Barium Gallium Yttrium Lanthanum Tellurium Scandium Beryllium Indium Thallium Bismuth Vanadium Tantalum Zirconium Gold Samarium Tungsten Thorium Uranium
41,808,044,129,611 16,179,356,725,877 8,019,515,931,628 773,627,553,592 151,599,284,310 150,207,096,162 26,283,290,713 26,185,559,925 21,555,924,426 16,301,156,188 4,706,027,566 823,858,713 452,753,156 214,345,481 211,744,915 47,896,401 21,985,848 19,588,506 10,023,125 6,820,886 6,071,171 3,486,486 2,677,674 2,515,303 1,718,576 1,620,894 1,538,503 1,314,936 1,143,617 1,014,236 948,745 562,455 414,543 313,738 306,449 271,772 180,069 111,618 98,883 97,195 96,441 60,439 40,627 34,671 33,025 26,782 24,047 20,972 14,727 14,403 12,999 6,654 6,599 6,113 2,002 655 3 3
$ 0.028315 0.021739 6.400000 9.706929 68.198594 15.500000 0.011623 2.287748 4.098737 1.409496 0.444909 7.917263 0.054600 0.370000 0.088090 1.087153 0.177237 0.012858 0.002172 0.012961 0.024233 0.003960 0.010136 0.010920 0.043120 0.003402 0.031320 0.001526 0.037949 0.005387 0.094184 0.023576 0.130435 0.001260 0.001509 0.000016 0.004718 0.013600 0.000243 0.000624 0.028776 0.003367 0.005232 0.000566 0.000722 0.058160 0.000218 0.000600 0.000894 0.000119 0.000322 0.001631 0.000830 0.001975 0.000118 0.000007 0.004948 0.000103
Total
67,179,218,505,055 x 1015
Element
the average public school teacher is worth $46,597 per year. How much is one human body really worth? You can buy the entire collection of ingredients that make up an average 70-kilogram (150-pound) body for about $118.63 (Figure 2.1). Of course, all you have to do is watch Keanu, Alex, or any teacher to know that a human body is far more than a collection of those ingredients. What makes us worth more than the sum of our parts? The fifty-eight pure substances listed in Figure 2.1 are called elements. You will find the same elements that make up the human body in, say, dirt or seawater. However, the proportions of those elements differ between living and nonliving things. For example, a human body contains far more carbon. Seawater and most rocks have no more than a trace of it. We are only starting to understand the processes by which a collection of elements becomes assembled as a living body. We do know that life’s unique organization starts with the properties of atoms that make up certain elements. This is your chemistry. It makes you far more than the sum of your body’s ingredients—a handful of lifeless chemicals.
See the video! Figure 2.1 Composition of an average-sized adult human body, by weight and retail cost. Manufacturers commonly add fluoride to toothpaste. Fluoride is a form of fluorine, one of several elements with vital functions— but only in trace amounts. Too much can be toxic.
$118.63
Links to Earlier Concepts
Key Concepts Atoms and elements
With this chapter, we turn to the first of life’s levels of organization—atoms and energy—so take a moment to review Section 1.1.
Life’s organization requires continuous inputs of energy (1.2). Organisms store that energy in chemical bonds between atoms.
You will come across a simple example of how the body’s built-in mechanisms maintain homeostasis (1.2).
Atoms are particles that are the building blocks of all matter. They can differ in their numbers of component protons, electrons, and neutrons. Elements are pure substances, each consisting entirely of atoms that have the same number of protons. Sections 2.1, 2.2
Why electrons matter Whether one atom will bond with others depends on the element, and the number and arrangement of its electrons. Section 2.3
Atoms bond Atoms of many elements interact by acquiring, sharing, and giving up electrons. Ionic, covalent, and hydrogen bonds are the main interactions between atoms in biological molecules. Section 2.4
Water of life Life originated in water and is adapted to its properties. Water has temperature-stabilizing effects, cohesion, and a capacity to act as a solvent for many other substances. These properties make life possible on Earth. Section 2.5
The power of hydrogen Life is responsive to changes in the amounts of hydrogen ions and other substances dissolved in water. Section 2.6
How would you vote? Fluoride helps prevent tooth decay, but too much wrecks bones and teeth, and causes birth defects. A lot can kill you. Many communities in the United States add fluoride to drinking water. Do you want it in yours? See CengageNOW for details, then vote online. 21
2.1
Start With Atoms The behavior of elements, which make up all living things, starts with the structure of individual atoms.
proton neutron electron
Characteristics of Atoms Atoms are particles that are the building blocks of all substances. Even though they are about one billion times smaller than basketballs, atoms consist of even smaller subatomic particles called protons (p+), which carry a positive charge; neutrons, which carry no charge; and electrons (e– ), which carry a negative charge. Charge is an electrical property that attracts or repels other subatomic particles. Protons and neutrons cluster in an atom’s central core, or nucleus. Electrons move around the nucleus (Figure 2.2). Atoms differ in the number of subatomic particles. The number of protons, which is the atomic number, determines the element. Elements are pure substances, each consisting only of atoms with the same number of protons. For example, a chunk of carbon contains only carbon atoms, all of which have six protons in their nucleus. The atomic number of carbon is 6. All atoms with six protons in their nucleus are carbon atoms, no matter how many electrons or neutrons they have. Each element has a symbol that is an abbreviation of its Latin name. Carbon’s symbol, C, is from carbo, the Latin word for coal—which is mostly carbon.
1
2
H
He
3
4
5
6
7
8
9
10
Li
Be
B
C
N
O
F
Ne
11
12
13
14
15
16
17
18
Na
Mg
Al
Si
P
S
Cl
Ar
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
55
56
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
Cs
Ba
Lu
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
111
112
113
114
115
116
87
88
103
104
105
106
107
108
109
110
Fr
Ra
Lr
Rf
Db
Sg
Bh
Hs
Mt
Ds
Rg Uub Uut Uuq Uup Uuh
57
58
59
60
61
62
63
64
65
66
67
68
69
70
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
95
96
89
90
91
92
93
94
Ac
Th
Pa
U
Np
Pu
Am Cm
97
98
99
100
101
102
Bk
Cf
Es
Fm
Md
No
118
Uuo
Figure 2.2 Atoms. Electrons move about a nucleus of protons and neutrons. Models such as this do not show what an atom really looks like. A more accurate rendering would show electrons occupying fuzzy, three-dimensional shapes about 10,000 times larger than the nucleus.
All elements occur in different forms called isotopes. Atoms of isotopes have the same number of protons, but different numbers of neutrons. We refer to isotopes by mass number, which is the total number of protons and neutrons in their nucleus. The mass number of an isotope is shown as a superscript to the left of an element’s symbol. For instance, the most common isotope of carbon is 12 C (six protons, six neutrons). Another is 13 C (six protons, seven neutrons).
The Periodic Table Today, we know that the numbers of electrons, protons, and neutrons determine how an element behaves, but scientists were classifying elements by chemical behavior long before they knew about subatomic particles. In 1869, the chemist Dmitry Mendeleev arranged all of the elements known at the time into a table based on their chemical properties. He had constructed the first periodic table of the elements. Elements are ordered in the periodic table by their atomic number (Figure 2.3). Those in each vertical column behave in similar ways. For instance, all of the elements in the far right column of the table are inert gases; their atoms do not interact with other atoms. In nature, such elements occur only as solitary atoms. We find the first ninety-four elements in nature. The others are so unstable that they are extremely rare. We know they exist because they can be made, one atom at a time, for a fraction of a second. It takes a nuclear physicist to do this, because an atom’s nucleus cannot be altered by heat or other ordinary means. Take-Home Message
Figure 2.3 Periodic table of the elements and its creator, Dmitry Mendeleev. Until he came up with the table, Mendeleev was known mainly for his extravagant hair; he cut it only once per year. Atomic numbers are shown above the element symbols. Some of the symbols are abbreviations for their Latin names. For instance, Pb (lead) is short for plumbum; the word “plumbing” is related—ancient Romans made their water pipes with lead. Appendix IV has a more detailed table.
22 UNIT I
PRINCIPLES OF CELLULAR LIFE
What are the basic building blocks of all matter?
Atoms are tiny particles, the building blocks of all substances. Atoms consist of electrons moving around a nucleus of protons and (except for hydrogen) neutrons.
An element is a pure substance. Each kind consists only of atoms with the same number of protons.
FOCUS ON RESEARCH
2.2
Putting Radioisotopes to Use
Some radioactive isotopes—radioisotopes—are used in research and in medical applications.
In 1896, Henri Becquerel made a chance discovery. He left some crystals of a uranium salt in a desk drawer, on top of a metal screen. Under the screen was an exposed film wrapped tightly in black paper. Becquerel developed the film a few days later and was surprised to see a negative image of the screen. He realized that “invisible radiations” coming from the uranium salts had passed through the paper and exposed the film around the screen. Becquerel’s images were evidence that uranium has radioisotopes, or radioactive isotopes. So do many other elements. The atoms of radioisotopes spontaneously emit subatomic particles or energy when their nucleus breaks down. This process, radioactive decay, can transform one element into another. For example, 14C is a radioisotope of carbon. It decays when one of its neutrons spontaneously splits into a proton and an electron. Its nucleus emits the electron, and so an atom of 14C (with eight neutrons and six protons) becomes an atom of 14N (nitrogen 14, with seven neutrons and seven protons). Radioactive decay occurs independently of external factors such as temperature, pressure, or whether the atoms are part of molecules. A radioisotope decays at a constant rate into predictable products. For example, after 5,730 years, we can predict that about half of the atoms in any sample of 14C will be 14N atoms. This predictability can be used to estimate the age of rocks and fossils by their radioisotope content. We return to this topic in Section 17.6.
Researchers and clinicians also introduce radioisotopes into living organisms. Remember, isotopes are atoms of the same element. All isotopes of an element generally have the same chemical properties regardless of the number of neutrons in their atoms. This consistent chemical behavior means that organisms use atoms of one isotope (such as 14C) the same way that they use atoms of another (such as 12C). Thus, radioisotopes can be used in tracers. A tracer is any molecule with a detectable substance attached. Typically, a radioactive tracer is a molecule in which radioisotopes have been swapped for one or more atoms. Researchers deliver radioactive tracers into a biological system such as a cell or a multicelled body. Instruments that can detect radioactivity let researchers follow the tracer as it moves through the system. For example, Melvin Calvin and his colleagues used a radioactive tracer to identify specific reaction steps of photosynthesis. The researchers made carbon dioxide with 14C, then let green algae (simple aquatic organisms) take up the radioactive gas. Using instruments that detected the radioactive decay of 14C, they tracked carbon through steps by which the algae—and all plants—make sugars. Radioisotopes have medical applications as well. PET (short for Positron-Emission Tomography) helps us “see” cell activity. By this procedure, a radioactive sugar or other tracer is injected into a patient, who is then moved into a PET scanner (Figure 2.4a). Inside the patient’s body, cells with differing rates of activity take up the tracer at different rates. The scanner detects radioactive decay wherever the tracer is, then translates that data into an image. Such images can reveal abnormal cell activity (Figure 2.4b).
A A patient is injected with a radioactive tracer and moved into a scanner like this one. Detectors that intercept radioactive decay of the tracer surround the body part of interest.
tumors
B Radioactive decay detected by the scanner is converted into digital images of the body’s interior. Two tumors (blue) in and near the bowel of a cancer patient are visible in this PET scan.
Figure 2.4 Animated PET scanning.
CHAPTER 2
LIFE’S CHEMICAL BASIS 23
2.3
Why Electrons Matter Atoms acquire, share, and donate electrons. Whether an atom will interact with other atoms depends on how many electrons it has.
Electrons and Energy Levels Electrons are really, really small: If they were as big as apples, you would be 3.5 times taller than our solar system is wide. Simple physics explains the motion of, say, an apple falling from a tree. Electrons are so tiny that everyday physics does not explain their behavior, but that behavior underlies interactions among atoms. A typical atom has about as many electrons as protons, so a lot of electrons may be vacancy zipping around one nucleus. Those electrons never collide, despite moving at nearly the speed of light (300,000 kilometers per second, or 670 million miles per hour). Why not? They travel in different orbitals, which are defined volumes of space around the nucleus. Imagine that an atom is a multilevel apartment building, with rooms available for rent by electrons. The nucleus is in the basement, and each “room” is an orbital. No more than two electrons can share a room at the same no vacancy time. An orbital with only one electron has a vacancy, and another electron can move in.
Each floor in the apartment building corresponds to one energy level. There is only one room on the first floor: one orbital at the lowest energy level, closest to the nucleus. It fills up first. In hydrogen, the simplest atom, a single electron occupies that room. Helium has two electrons, so it has no vacancies at the lowest energy level. In larger atoms, more electrons rent the second-floor rooms. When the second floor fills, more electrons rent third-floor rooms, and so on. Electrons fill orbitals at successively higher energy levels. The farther an electron is from the basement (the nucleus), the greater its energy. An electron in a firstfloor room cannot move to the second or third floor, let alone the penthouse, unless an input of energy gives it a boost. Suppose an electron absorbs enough energy from sunlight to get excited about moving up. Move it does. If nothing fills that lower room, though, the electron immediately moves back down, emitting its extra energy as it does. In later chapters, you will see how some types of cells harvest that released energy.
Why Atoms Interact Shells and Electrons We use a shell model to help us check an atom for vacancies (Figure 2.5). With this model, nested “shells” correspond to successive energy levels. Each shell includes all rooms on one floor of the
electron
C Third shell This shell corresponds to the third energy level. It has four orbitals with room for eight electrons. Sodium has one electron in the third shell; chlorine has seven. Both have vacancies, so both form chemical bonds. Argon, with no vacancies, does not.
B Second shell This shell, which corresponds to the second energy level, has four orbitals—room for a total of eight electrons. Carbon has six electrons: two in the first shell and four in the second. It has four vacancies. Oxygen has two vacancies. Both carbon and oxygen form chemical bonds. Neon, with no vacancies, does not. A First shell A single shell corresponds to the first energy level, which has a single orbital that can hold two electrons. Hydrogen has only one electron in this shell, so it has one vacancy. A helium atom has two electrons (no vacancies), so it does not form bonds.
sodium
chlorine
argon
11p+, 11e–
17p+, 17e–
18p+, 18e
carbon
oxygen
neon
6p+, 6e–
8p+, 8e–
10p+, 10e–
hydrogen
helium
1p+, 1e–
2p+, 2e–
Figure 2.5 Animated Shell models, which help us check for vacancies in atoms. Each circle, or shell, represents all orbitals at one energy level. Atoms with vacancies in the outermost shell tend to form bonds. Remember, atoms do not look anything like these flat diagrams.
24 UNIT I
PRINCIPLES OF CELLULAR LIFE
atomic apartment building. We draw an atom’s shells by filling them with electrons (represented as dots or balls) from the innermost shell out, until there are as many electrons as the atom has protons. If an atom’s outermost shell is full of electrons, it has no vacancies. Atoms of such elements are chemically inactive; they are most stable as single atoms. Helium, neon, and the other inert gases in the righthand column of the periodic table are like this. If an atom’s outermost shell has room for an extra electron, it has a vacancy. Atoms with vacancies tend to interact with other atoms; they give up, acquire, or share electrons until they have no vacancies in their outermost shell. Any atom is in its most stable state when it has no vacancies. Atoms and Ions The negative charge of an electron
cancels the positive charge of a proton, so an atom is uncharged only when it has as many electrons as protons. An atom with different numbers of electrons and protons is called an ion. An ion carries a charge; either it acquired a positive charge by losing an electron, or it acquired a negative charge by pulling an electron away from another atom. Electronegativity is a measure of an atom’s ability to pull electrons from other atoms. Whether the pull is strong or weak depends on the atom’s size and how many vacancies it has; it is not a measure of charge. As an example, when a chlorine atom is uncharged, it has 17 protons and 17 electrons. Seven electrons are in its outer (third) shell, which can hold eight (Figure 2.6). It has one vacancy. An uncharged chlorine atom is highly electronegative—it can pull an electron away from another atom and fill its third shell. When that happens, the atom becomes a chloride ion (Cl–) with 17 protons, 18 electrons, and a net negative charge. As another example, an uncharged sodium atom has 11 protons and 11 electrons. This atom has one electron in its outer (third) shell, which can hold eight. It has seven vacancies. An uncharged sodium atom is weakly electronegative, so it cannot pull seven electrons from other atoms to fill its third shell. Instead, it tends to lose the single electron in its third shell. When that happens, two full shells—and no vacancies—remain. The atom has now become a sodium ion (Na+), with 11 protons, 10 electrons, and a net positive charge. From Atoms to Molecules Atoms do not like to have
vacancies, and try to get rid of them by interacting with other atoms. A chemical bond is an attractive force that arises between two atoms when their electrons interact. A molecule forms when two or more atoms
Sodium atom
Chlorine atom
11p+ 11e–
17p+ 17e–
no net charge
no net charge electron loss
electron gain
Sodium ion
Chloride ion
11p+ 10e–
17p+ 18e–
net positive charge
net negative charge
A A sodium atom becomes a positively charged sodium ion (Na+ ) when it loses the electron in its third shell. The atom’s full second shell is now the outermost, and the atom has no vacancies.
B A chlorine atom becomes a negatively charged chloride ion (Cl–) when it gains an electron and fills the vacancy in its third, outermost shell.
Figure 2.6 Animated Ion formation. H
of the same or different elements join in chemical bonds. The next section explains the main types of bonds in biological molecules. Compounds are molecules that consist of two or more different elements in proportions that do not vary. Water is an example. All water molecules have one oxygen atom bonded to two hydrogen atoms. Whether water is in the seas, a waterfall, a Siberian lake, or anywhere else, its molecules have twice as many hydrogen as oxygen atoms. By contrast, in a mixture, two or more substances intermingle, and their proportions can vary because the substances do not bond with each other. For example, you can make a mixture by swirling sugar into water. The sugar dissolves, but no chemical bonds form.
O H
Always two H for every O
Take-Home Message Why do atoms interact? An atom’s electrons are the basis of its chemical behavior. Shells represent all electron orbitals at one energy level in an atom. When the outermost shell is not full of electrons, the atom has a vacancy. Atoms tend to get rid of vacancies by gaining or losing electrons (thereby becoming ions), or by sharing electrons with other atoms. Atoms with vacancies can form chemical bonds. Chemical bonds connect atoms into molecules.
CHAPTER 2
LIFE’S CHEMICAL BASIS 25
2.4
What Happens When Atoms Interact? The characteristics of a bond arise from the properties of the atoms that take part in it.
Cl– Na+ Na+
The same atomic building blocks, arranged in different ways, make different molecules. For example, carbon atoms bonded one way form layered sheets of a soft, slippery mineral known as graphite. The same carbon atoms bonded a different way form the rigid crystal lattice of diamond—the hardest mineral. Bond oxygen and hydrogen atoms to carbon and you get sugar. Although bonding applies to a range of interactions among atoms, we can categorize most bonds into distinct types based on their different properties. Three types—ionic, covalent, and hydrogen bonds—are most common in biological molecules. Which type forms depends on the vacancies and electronegativity of the atoms that take part in it. Table 2.1 compares different ways to represent molecules and their bonds.
Cl– Na+ Na+
Remember from Figure 2.6, a strongly electronegative atom tends to gain electrons until its outermost shell is full. Then it is a negatively charged ion. A weakly electronegative atom tends to lose electrons until its outermost shell is full. Then it is a positively charged ion. Two atoms with a large difference in electronegativity may stay together in an ionic bond, which is a strong mutual attraction of two oppositely charged ions. Such bonds do not usually form by the direct transfer of an
Cl–
Na+
Na+ Cl– Cl– Na+
Na+ Cl–
A A crystal of table salt is a cubic lattice of many sodium and chloride ions.
Chloride ion 17p+, 18e–
Ionic Bonding
Cl–
Cl–
Sodium ion 11p+, 10e–
B The mutual attraction of opposite charges holds the two kinds of ions together in a lattice.
Figure 2.7 Animated Ionic bonds.
electron from one atom to another; rather, atoms that have already become ions stay close together because of their opposite charges. Figure 2.7 shows crystals of table salt (sodium chloride, or NaCl). Ionic bonds in such solids hold sodium and chloride ions in an orderly, cubic arrangement.
Covalent Bonding Table 2.1
Different Ways To Represent the Same Molecule
Common name
Water
Familiar term.
Chemical name
Hydrogen oxide
Systematically describes elemental composition.
Chemical formula
H2O
Indicates unvarying proportions of elements. Subscripts show number of atoms of an element per molecule. The absence of a subscript means one atom.
Structural formula
HsOsH
Represents each covalent bond as a single line between atoms. The bond angles may also be represented.
H
O
H
Structural model
Shows the positions and relative sizes of atoms.
Shell model
Shows how pairs of electrons are shared in covalent bonds.
26 UNIT I
PRINCIPLES OF CELLULAR LIFE
In a covalent bond, two atoms share a pair of electrons. Such bonds typically form between atoms with similar electronegativity and unpaired electrons. By sharing their electrons, each atom’s vacancy becomes partially filled (Figure 2.8). Covalent bonds can be stronger than ionic bonds, but they are not always so. Take a look at the structural formula in Table 2.1. Such formulas show how bonds connect the atoms. A line between two atoms represents a single covalent bond, in which two atoms share one pair of electrons. A simple example is molecular hydrogen (H2), with one covalent bond between hydrogen atoms (HsH). Two lines between atoms represent a double covalent bond, in which two atoms share two pairs of electrons. Molecular oxygen (OtO) has a double covalent bond linking two oxygen atoms. Three lines indicate a triple covalent bond, in which two atoms share three pairs of electrons. A triple covalent bond links two nitrogen atoms in molecular nitrogen (N⬅N).
hydrogen bond
Molecular hydrogen (HsH) Two hydrogen atoms, each with one proton, share two electrons in a nonpolar covalent bond.
A A hydrogen (H) bond is an attraction between an electronegative atom and a hydrogen atom taking part in a separate polar covalent bond.
Molecular oxygen (OtO) Two oxygen atoms, each with eight protons, share four electrons in a double covalent bond.
water molecule
water molecule
B Hydrogen bonds are individually weak, but many of them form. Collectively, they are strong enough to stabilize the structures of large biological molecules such as DNA, shown here.
Water molecule (HsOsH) Two hydrogen atoms share electrons with an oxygen atom in two polar covalent bonds. The oxygen exerts a greater pull on the shared electrons, so it has a slight negative charge. Each hydrogen has a slight positive charge.
Figure 2.8 Animated Covalent bonds, in which atoms with unpaired electrons in their outermost shell become more stable by sharing electrons. Two electrons are shared in each covalent bond. When sharing is equal, the bond is nonpolar. When one atom exerts a greater pull on the electrons, the bond is polar.
Figure 2.9 Animated Hydrogen bonds. Hydrogen bonds form at a hydrogen atom taking part in a polar covalent bond. The hydrogen atom’s slight positive charge weakly attracts an electronegative atom. As shown here, hydrogen (H) bonds can form between molecules or between different parts of the same molecule.
Some covalent bonds are nonpolar, meaning that the atoms participating in the bond are sharing electrons equally. There is no difference in charge between the two ends of such bonds. Nonpolar covalent bonds form between atoms with identical electronegativity. The molecular hydrogen (H2), oxygen (O2), and nitrogen (N2) mentioned earlier are examples. These molecules are some of the gases that make up air. Atoms participating in polar covalent bonds do not share electrons equally. Such bonds can form between atoms with a small difference in electronegativity. The atom that is more electronegative pulls the electrons a little more toward its “end” of the bond, so that atom bears a slightly negative charge. The atom at the other end of the bond bears a slightly positive charge. For example, the water molecule shown in Table 2.1 has two polar covalent bonds (HsOsH). The oxygen atom carries a slight negative charge, but each of the hydrogen atoms carries a slight positive charge. Any such separation of charge into distinct positive and negative regions is called polarity. As you will see in the next section, the polarity of the water molecule is very important for the world of life.
ecule. A hydrogen bond is a weak attraction between a highly electronegative atom and a hydrogen atom taking part in a separate polar covalent bond. Like ionic bonds, hydrogen bonds form by mutual attraction of opposite charges: The hydrogen atom has a slight positive charge and the other atom has a slight negative charge. However, unlike ionic bonds, hydrogen bonds do not make molecules out of atoms, so they are not chemical bonds. Hydrogen bonds are weak. They form and break much more easily than covalent or ionic bonds. Even so, many of them form between molecules, or between different parts of a large one. Collectively, they are strong enough to stabilize the characteristic structures of large biological molecules (Figure 2.9).
Hydrogen Bonding Hydrogen bonds form between polar regions of two molecules, or between two regions of the same mol-
Take-Home Message How do atoms interact? A chemical bond forms when the electrons of two atoms interact. Depending on the atoms, the bond may be ionic or covalent.
An ionic bond is a strong mutual attraction between ions of opposite charge. Atoms share a pair of electrons in a covalent bond. When the atoms share electrons equally, the bond is nonpolar; when they share unequally, it is polar.
A hydrogen bond is an attraction between a highly electronegative atom and a hydrogen atom taking part in a different polar covalent bond. Hydrogen bonds are individually weak, but are collectively strong when many of them form.
CHAPTER 2
LIFE’S CHEMICAL BASIS 27
2.5
Water’s Life-Giving Properties Water is essential to life because of its unique properties. The unique properties of water are a result of the extensive hydrogen bonding among water molecules.
Life evolved in water. All living organisms are mostly water, many of them still live in it, and all of the chemical reactions of life are carried out in water. What is so special about water?
Polarity of the Water Molecule slight negative charge on the oxygen atom H
O
H
– –– O
A The polarity of a water molecule arises because of the distribution of its electrons. The hydrogen atoms bear a slight positive charge, and the oxygen atom bears a slight negative charge.
+ ++
H
H
+ ++
slight positive charge on the hydrogen atoms
The special properties of water begin with the polarity of individual water molecules. In each molecule of water, polar covalent bonds join one oxygen atom with two hydrogen atoms. Overall, the molecule has no charge, but the oxygen pulls the shared electrons a bit more than the hydrogen atoms do. Thus, each of the atoms in a water molecule carries a slight charge: The oxygen atom is slightly negative, and the hydrogen atoms are slightly positive (Figure 2.10a). This separation of charge means a water molecule is polar. The polarity of each water molecule attracts other water molecules, and hydrogen bonds form between them in tremendous numbers (Figure 2.10b). Extensive hydrogen bonding between water molecules imparts unique properties to water that make life possible.
Water’s Solvent Properties
B Many hydrogen bonds (dashed lines) that form and break rapidly keep water molecules clustered together in liquid water.
C Below 0°C (32°F), the hydrogen bonds hold water molecules rigidly in the three-dimensional lattice of ice. The molecules are less densely packed in ice than in liquid water, so ice floats on water. The Arctic ice cap is melting because of global warming. It will probably be gone in fifty years, and so will polar bears. Polar bears must now swim farther between shrinking ice sheets, and they are drowning in alarming numbers.
Figure 2.10 Animated Water, a substance that is essential for life.
28 UNIT I
PRINCIPLES OF CELLULAR LIFE
A solvent is a substance, usually a liquid, that can dissolve other substances. Dissolved substances are solutes. Solvent molecules cluster around ions or molecules of a solute, thereby dispersing them and keeping them separated, or dissolved. Water is a solvent. Clusters of water molecules form around the solutes in cellular fluids, tree sap, blood, the fluid in your gut, and most other fluids associated with life. When you pour table salt (NaCl) into a cup of water, the crystals of this ionically bonded solid separate into sodium ions (Na+) and chloride ions (Cl–). Salt dissolves in water because the negatively charged oxygen atoms of many water molecules pull on each Na+, and the positively charged hydrogen atoms of many others pull on each Cl– (Figure 2.11). The collective strength of many hydrogen bonds pulls the ions apart and keeps them dissolved. Hydrogen bonds also form between water molecules and polar molecules such as sugars, so water easily dissolves polar molecules. Thus, polar molecules are hydrophilic (water-loving) substances. Hydrogen bonds do not form between water molecules and nonpolar molecules, such as oils, which are hydrophobic (water-dreading) substances. Shake a bottle filled with water and salad oil, then set it on a table. The water gathers together, and the oil clusters at the water’s surface as new hydrogen bonds replace the ones broken
a
Figure 2.11 Animated Water molecules that surround an ionic solid pull its atoms apart, thereby dissolving them.
b
c
Figure 2.12 Cohesion of water. (a) After a pebble hits liquid water, individual molecules do not fly apart. Countless hydrogen bonds keep them together. (b) Cohesion keeps fishing spiders from sinking. (c) Water rises to the tops of plants because evaporation from leaves pulls cohesive columns of water molecules upward from the roots.
by shaking. The same interactions occur at the thin, oily membrane that separates water inside cells from water outside them. The organization of membranes— and life itself—starts with such interactions. You will read more about membranes in Chapter 5.
Water’s Temperature-Stabilizing Effects All molecules vibrate nonstop, and they move faster as they absorb heat. Temperature is a way to measure the energy of this molecular motion. The extensive hydrogen bonding in liquid water restricts the jiggling of water molecules. Thus, compared with other liquids, water absorbs more heat before it becomes measurably hotter. This property means that the temperature of water (and the air around it) stays relatively stable. When the temperature of water is below its boiling point, hydrogen bonds form as fast as they break. As the water gets hotter, the molecules move faster, and individual molecules at the water’s surface begin to escape into the air. By this process—evaporation— heat energy converts liquid water to a gas. The energy increase overcomes the attraction between water molecules, which break free. It takes heat to convert liquid water to a gas, so the surface temperature of water decreases during evaporation. Evaporative water loss can help you and some other mammals cool off when you sweat in hot, dry weather. Sweat, which is about 99 percent water, cools the skin as it evaporates. Below 0ºC (32ºF), water molecules do not jiggle enough to break hydrogen bonds, and become locked in the rigid, latticelike bonding pattern of ice (Figure 2.10c). Individual water molecules pack less densely in ice than they do in water, so ice floats on water. During cold winters, ice sheets may form near the sur-
face of ponds, lakes, and streams. Such ice “blankets” insulate liquid water under them, so they help keep fish and other aquatic organisms from freezing.
Water’s Cohesion Another life-sustaining property of water is cohesion. Cohesion means that molecules resist separating from one another. You see its effect as surface tension when you toss a pebble into a pond (Figure 2.12 a). Although the water ripples and sprays, individual molecules do not fly apart. Its hydrogen bonds collectively exert a continuous pull on the individual water molecules. This pull is so strong that the molecules stay together rather than spreading out in a thin film as other liquids do. Many organisms take special advantage of this unique property (Figure 2.12b). Cohesion works inside organisms, too. For instance, plants continually absorb water as they grow. Water molecules evaporate from leaves, and replacements are pulled upward from roots (Figure 2.12c). Cohesion makes it possible for columns of liquid water to rise from roots to leaves inside narrow pipelines of vascular tissues. Section 29.3 returns to this topic.
Take-Home Message Why is water essential to life? Extensive hydrogen bonding among water molecules imparts unique properties to water that make life possible. Water molecules hydrogen-bond with polar (hydrophilic) substances, dissolving them easily. They do not bond with nonpolar (hydrophobic) substances.
Ice is less dense than liquid water, so it floats. Ice insulates water beneath it. The temperature of water is more stable than other liquids. Water also stabilizes the temperature of the air near it.
Cohesion keeps individual molecules of liquid water together.
CHAPTER 2
LIFE’S CHEMICAL BASIS 29
2.6
Acids and Bases Hydrogen ions have far-reaching effects because they are chemically active, and because there are so many of them.
Link to Homeostasis 1.2
100
0—
10–1
more acidic
1—
2—
10–2
3—
10–3
10–4
4—
battery acid
gastric fluid
acid rain lemon juice cola vinegar orange juice tomatoes, wine bananas beer bread black coffee urine, tea, typical rain
5—
10–5
6—
10–6
corn butter milk
7—
10–7
pure water
8—
10–8
blood, tears egg white seawater
The pH Scale At any given instant in liquid water, some of the water molecules are separated into hydrogen ions (H+) and hydroxide ions (OH–): H2O
H+
water
hydrogen ions
OH–
hydroxyl ions
In chemical equations such as this, arrows indicate the direction of the reaction. pH is a measure of the number of hydrogen ions in a solution. When the number of H+ ions is the same as the number of OH– ions, the pH of the solution is 7, or neutral. The pH of pure water (not rainwater or tap water) is like this. The more hydrogen ions, the lower the pH. A one-unit decrease in pH corresponds to a tenfold increase in the amount of H+ ions, and a one-unit increase corresponds to a tenfold decrease in the amount of H+ ions. One way to get a sense of the difference is to taste dissolved baking soda (pH 9), distilled water (pH 7), and lemon juice (pH 2). A pH scale that ranges from 0 to 14 is shown in Figure 2.13. Nearly all of life’s chemistry occurs near pH 7. Most of your body’s internal environment (tissue fluids and blood) is between pH 7.3 and 7.5.
more basic
How Do Acids and Bases Differ? 9—
10–9
baking soda phosphate detergents Tums
10 —
10–10
toothpaste hand soap milk of magnesia
11—
10–11 household ammonia
12 —
10–12
13 —
10–13
hair remover bleach oven cleaner
10–14
14 —
drain cleaner
Figure 2.13 Animated A pH scale. Here, red dots signify hydrogen ions (H+) and blue dots signify hydroxyl ions (OH–). Also shown are approximate pH values for some common solutions. This pH scale ranges from 0 (most acidic) to 14 (most basic). A change of one unit on the scale corresponds to a tenfold change in the amount of H+ ions (blue numbers). Figure It Out: What is the approximate pH of cola?
Answer: 2.5 30 UNIT I
PRINCIPLES OF CELLULAR LIFE
Substances called acids donate hydrogen ions as they dissolve in water. Bases accept hydrogen ions. Acidic solutions, such as lemon juice and coffee, contain more H+ than OH–, so their pH is below 7. Basic solutions, such as seawater and hand soap, contain more OH– than H+. Basic, or alkaline, solutions have a pH greater than 7. Acids and bases can be weak or strong. Weak acids, such as carbonic acid (H2CO3), are stingy H+ donors. Strong acids give up more H+ ions. One example is hydrochloric acid (HCl), which separates into H+ and Cl– very easily in water: HCl
H+
hydrochloric acid
hydrogen ions
Cl– chloride ions
Inside your stomach, the H+ from HCl makes gastric fluid acidic (pH 1–2). The acidity activates enzymes that digest proteins in your food. Acids or bases that accumulate in ecosystems can kill organisms. For instance, fossil fuel emissions and nitrogen-containing fertilizers release strong acids into
Figure 2.14 Emissions of sulfur dioxide from a coal-burning power plant. Airborne pollutants such as sulfur dioxide dissolve in water vapor and form acidic solutions. They are a component of acid rain. The far-right photograph shows how acid rain can corrode stone sculptures.
the atmosphere. The acids lower the pH of rain (Figure 2.14). Some ecosystems are being damaged by such acid rain, which changes the composition of water and soil. Organisms in these regions are being harmed by the changes. We return to this topic in Section 48.2.
Carbon dioxide, a gas that forms in many reactions, takes part in an important buffer system. It becomes carbonic acid when it dissolves in the water component of human blood: H2O CO2 carbon dioxide
Salts and Water A salt is a compound that dissolves easily in water and releases ions other than H+ and OH–. For example, when dissolved in water, the salt sodium chloride separates into sodium ions and chloride ions: NaCl
Na+
sodium chloride
sodium ions
Cl– chloride ions
Many ions are important components of cellular processes. For example, sodium, potassium, and calcium ions are critical for nerve and muscle cell function. As another example, potassium ions help plants minimize water loss on hot, dry days.
Buffers Against Shifts in pH Cells must respond quickly to even slight shifts in pH because most enzymes and other biological molecules can function properly only within a narrow pH range. Even a slight deviation from that range can halt cellular processes completely. Body fluids stay at a consistent pH because they are buffered. A buffer system is a set of chemicals, often a weak acid or base and its salt, that can keep the pH of a solution stable. It works because the two chemicals donate and accept ions that contribute to pH. For example, when a base is added to an unbuffered fluid, the number of OH– ions increases, so the pH rises. However, when a base is added to a buffered fluid, the acid component of the buffer releases H+ ions. These combine with the extra OH– ions, forming water, which does not affect pH. So, the buffered fluid’s pH stays the same even when base is added.
H2CO3 carbonic acid
The carbonic acid can separate into hydrogen ions and bicarbonate ions: H+
H2CO3 carbonic acid
HCO3– bicarbonate
This easily-reversed reaction constitutes the buffer system. Any excess OH– combines with the H+ to form water, which does not contribute to pH. Any excess H+ combines with the bicarbonate; thus bonded, the hydrogen does not affect pH: H+
HCO3–
H2CO3
bicarbonate
carbonic acid
Together, these reactions keep the blood pH between 7.3 and 7.5—but only up to a point. A buffer system can neutralize only so many ions. Even slightly more than that limit causes the pH to swing widely. A buffer system failure in a biological system can be catastrophic. In acute respiratory acidosis, carbon dioxide accumulates, and excess carbonic acid forms in blood. The resulting decline in blood pH may cause an individual to enter a coma, a level of unconsciousness that is dangerous. Alkalosis, a potentially lethal rise in blood pH, can also bring on coma. Even an increase to 7.8 can result in tetany, or prolonged muscle spasm.
Take-Home Message Why are hydrogen ions important in biology? Hydrogen ions contribute to pH. Acids release hydrogen ions in water; bases accept them. Salts release ions other than H+ and OH–. Buffer systems keep the pH of body fluids stable. They are part of homeostasis.
CHAPTER 2
LIFE’S CHEMICAL BASIS 31
IMPACTS, ISSUES REVISITED
What Are You Worth?
Contaminant or nutrient? An average human body contains highly toxic elements such as lead, arsenic, mercury, selenium, nickel, and even a few uranium atoms. The presence of these elements in the body is usually assumed to be the aftermath of environmental pollutants, but occasionally we discover that one of them has a vital function. For example, recently we found that having too little selenium can cause heart problems and thyroid disorders, so it may be part of some biological system we haven’t yet unraveled. The average body contains a substantial amount of fluorine, but as yet we know of no natural metabolic role for this element. Fluorine can substitute for other elements in biological molecules,
How would you vote? When fluorine replaces calcium in teeth and bones, it changes the structural properties of these body parts. One effect is fewer cavities. Many communities add fluoride to drinking water. Do you want it in yours? See CengageNOW for details, then vote online. but the substitution tends to make the molecules toxic. Several kinds of predator-deterring plant toxins are simple biological molecules with fluorine substituted for other elements.
Summary Section 2.1 Most atoms have electrons, which have a negative charge. Electrons move around a nucleus of positively charged protons and, except in the case of hydrogen, uncharged neutrons. Atoms of an element have the same number of protons—the atomic number (Table 2.2). A periodic table lists all of the elements. We refer to isotopes of an element by their mass number.
Table 2.2
Summary of Players in the Chemistry of Life
Section 2.2 Researchers make tracers with detectable substances such as radioisotopes, which emit particles and energy as they decay spontaneously.
Use the animation on CengageNOW to learn how radioisotopes are used in making PET scans.
Section 2.3 We use shell models to view an atom’s electron structure. Atoms with different numbers of electrons and protons are ions. Atoms with vacancies tend to interact with other atoms by donating, accepting, or sharing electrons. They form different chemical bonds depending on their electronegativity. A compound is a molecule of different elements. Mixtures are intermingled substances.
Atom
Particles that are basic building blocks of all matter; the smallest unit that retains an element’s properties
Element
Pure substance that consists entirely of atoms with the same, characteristic number of protons
Proton (p+)
Positively charged particle of an atom’s nucleus
Electron (e–)
Negatively charged particle that can occupy a volume of space (orbital) around an atom’s nucleus
Neutron
Uncharged particle of an atom’s nucleus
Section 2.4 An ionic bond is a very strong association between ions of opposite charge. Two atoms share a pair of electrons in a covalent bond, which may be nonpolar or polar (polarity is a separation of charge). Hydrogen bonds are weaker than either ionic or covalent bonds.
Isotope
One of two or more forms of an element, the atoms of which differ in the number of neutrons
Radioisotope Tracer
Unstable isotope that emits particles and energy when its nucleus disintegrates Molecule that has a detectable substance (such as a radioisotope) attached
Ion
Atom that carries a charge after it has gained or lost one or more electrons
Molecule
Two or more atoms joined in a chemical bond
Compound
Molecule of two or more different elements in unvarying proportions (for example, water)
Mixture
Intermingling of two or more elements or compounds in proportions that can vary
Solute
Molecule or ion dissolved in a solvent
Acid
Substance that releases H+ when dissolved in water
Base
Substance that accepts H+ when dissolved in water
Salt
Substance that releases ions other than H+ or OH– when dissolved in water
32 UNIT I
PRINCIPLES OF CELLULAR LIFE
Use the animation and interaction on CengageNOW to study electron distribution and the shell model.
Use the animation on CengageNOW to compare the types of chemical bonds in biological molecules.
Section 2.5 Evaporation helps liquid water stabilize temperature. Hydrophilic substances dissolve easily in water; hydrophobic substances do not. Solutes are substances dissolved in water or another solvent. Cohesion keeps water molecules together.
Use the animation on CengageNOW to view the structure of the water molecule and properties of liquid water.
Section 2.6 pH reflects the number of hydrogen ions (H+) in a solution. Typical pH scales range from 0 (most acidic) to 14 (most basic or alkaline). At neutral pH (7), the amounts of H+ and OH– ions are the same. Salts are compounds that release ions other than H+ and OH– in water. Acids release H+; bases accept H+. A buffer system keeps a solution within a consistent range of pH. Most biological processes are buffered; they work only within a narrow pH range, usually near pH 7.
Use the interaction on CengageNOW to investigate the pH of common solutions.
Data Analysis Exercise Living and nonliving things have the same kinds of atoms joined together as molecules, but those molecules differ in their proportions of elements and in how the atoms of those elements are arranged. The three charts in Figure 2.15 compare the proportions of some elements in the human body, Earth’s crust, and seawater. 1. Which is the most abundant element in dirt? In the human body? In seawater? 2. What percentage of seawater is oxygen? Hydrogen? How many atoms of hydrogen are there for each atom of oxygen in seawater? In which molecule are hydrogen and oxygen found in that exact proportion?
Human Hydrogen Oxygen Carbon Nitrogen Phosphorus Calcium Sodium Potassium Chlorine
62.0% 24.0 12.0 1.2 0.2 0.2 < 0.1 < 0.1 < 0.1
Earth Hydrogen Oxygen Carbon Nitrogen Phosphorus Calcium Sodium Potassium Chlorine
Seawater
3.1% 60.0 0.3 < 0.1 < 0.1 2.6 < 0.1 0.8 < 0.1
Hydrogen Oxygen Carbon Nitrogen Phosphorus Calcium Sodium Potassium Chlorine
66.0% 33.0 < 0.1 < 0.1 < 0.1 < 0.1 0.3 < 0.1 0.3
3. How many atoms of chlorine are there for every atom of sodium in seawater? What common molecule has one atom of chlorine for every atom of sodium?
Figure 2.15 Comparison of the abundance of some elements in a human, Earth’s crust, and typical seawater. Each number is the percent of the total number of atoms in each source. For instance, 120 of every 1,000 atoms in a human body are carbon, compared with only 3 carbon atoms in every 1,000 atoms of dirt.
Self-Quiz
Critical Thinking
Answers in Appendix III
1. A(n) is a molecule into which a radioisotope has been incorporated. 2. An ion is an atom that has . a. the same number of electrons and protons b. a different number of electrons and protons c. a and b are correct 3. A(n) forms when atoms of two or more elements bond covalently. 4. The measure of an atom’s ability to pull electrons away from another atom is called . 5. Atoms share electrons unequally in a(n)
bond.
6. Symbols for the elements are arranged according to in the periodic table of the elements. 7. Liquid water has a. tracers b. a profusion of hydrogen bonds c. cohesion
. d. resistance to increases in temperature e. b through d f. all of the above
8. A(n)
substance repels water. 9. Hydrogen ions (H+) are . a. indicated by pH c. dissolved in blood b. protons d. all of the above 10. A(n)
is dissolved in a solvent.
11. When dissolved in water, a(n)
donates H+.
12. A salt releases ions other than
in water.
13. A(n) is a chemical partnership between a weak acid or base and its salt. 14. Match the terms with their most suitable description. hydrophilic a. measure of molecular motion atomic number b. number of protons in nucleus mass number c. polar; readily dissolves temperature in water d. number of protons and neutrons in nucleus
Visit CengageNOW for additional questions.
1. Alchemists were medieval scholars and philosophers who were the forerunners of modern-day chemists. Many spent their lives trying to transform lead (atomic number 82) into gold (atomic number 79). Explain why they never did succeed in that endeavor. 2. Meats are often “cured,” or salted, dried, smoked, pickled, or treated with chemicals that can delay spoilage. Ever since the mid-1800s, sodium nitrite (NaNO2) has been used in processed meat products such as hot dogs, bologna, sausages, jerky, bacon, and ham. Nitrites prevent growth of Clostridium botulinum. If ingested, this bacterium can cause a form of food poisoning called botulism. In water, sodium nitrite separates into sodium ions (Na+) and nitrite ions (NO2–), which are called nitrites. Nitrites are rapidly converted to nitric oxide (NO), the compound that gives nitrites their preservative qualities. Eating preserved meats increases the risk of cancer, but nitrites may not be at fault. It turns out that nitric oxide has several important functions, including blood vessel dilation (for example, inside a penis during an erection), cell-to-cell signaling, and antimicrobial activities of the immune system. Draw a shell model for nitric oxide and then use it to explain why the molecule is so reactive. 3. Ozone is a chemically active form of oxygen gas. High in Earth’s atmosphere, it forms a layer that absorbs about 98 percent of the sun’s harmful rays. Oxygen gas consists of two covalently bonded oxygen atoms: OtO. Ozone has three covalently bonded oxygen atoms: OtO s O. Ozone reacts easily with many substances, and gives up an oxygen atom and releases gaseous oxygen (OtO). From what you know about chemistry, why do you suppose ozone is so reactive? 4. David, an inquisitive three-year-old, poked his fingers into warm water in a metal pan on the stove and did not sense anything hot. Then he touched the pan itself and got a nasty burn. Explain why water in a metal pan heats up far more slowly than the pan itself. 5. Some undiluted acids are more corrosive when diluted with water. That is why lab workers are told to wipe off splashes with a towel before washing. Explain. CHAPTER 2
LIFE’S CHEMICAL BASIS 33
3
Molecules of Life IMPACTS, ISSUES
Fear of Frying
The human body requires about one tablespoon of fat each
Hydrogenation, a manufacturing process that adds
day to remain healthy, but most of us eat far more than that.
hydrogen atoms to carbons, changes liquid vegetable oils
The average American consumes the equivalent of one stick
into solid fats. Procter & Gamble Co. developed partially
of butter per day—100 pounds of fat per year—which may be
hydrogenated vegetable oil in 1908 as a substitute for the
part of the reason why the average American is overweight.
more expensive solid animal fats they were using to make
Being overweight increases one’s risk for many diseases
candles. However, the demand for candles began to wane
and health conditions. However, which type of fat we eat may
as more households in the United States became wired for
be more important than how much fat we eat. Fats are more
electricity, and P & G began to look for another way to sell its
than just inert molecules that accumulate in strategic areas
proprietary fat. Partially hydrogenated vegetable oil looks a
of our bodies if we eat too much of them. They are major con-
lot like lard, so in 1911 the company began marketing it as a
stituents of cell membranes, and as such they have powerful
revolutionary new food—a solid cooking fat with a long shelf
effects on cell function.
life, mild flavor, and lower cost than lard or butter.
The typical fat molecule has three tails—long carbon
By the mid-1950s, hydrogenated vegetable oil had become
chains called fatty acids. Different fats have different fatty
a major part of the American diet. It was (and still is) found in
acid components. Those with a certain type of double bond
a tremendous buffet of manufactured and fast foods: butter
in one or more of their fatty acids are called trans fats (Figure
substitutes, cookies, crackers, cakes and pancakes, peanut
3.1). Small amounts of trans fats occur naturally in red meat
butter, pies, doughnuts, muffins, chips, granola bars, break-
and dairy products, but most of the trans fats humans con-
fast bars, chocolate, microwave popcorn, pizzas, burritos,
sume come from partially hydrogenated vegetable oil, an
french fries, chicken nuggets, fish sticks, and so on. For decades, hydrogenated vegetable oil was considered
artificial food product.
to be a more healthy alternative to animal fats. We now know that trans fats in hydrogenated vegetable oils raise the level of cholesterol in our blood more than any other fat, and they directly alter the function of our arteries and veins. O
OH
The effects of such changes are serious. Eating as little as 2 grams per day of hydrogenated vegetable oils increases
C H—C—H
a person’s risk of atherosclerosis (hardening of the arteries),
H—C—H
heart attack, and diabetes. A single serving of french fries
H—C—H
made with hydrogenated vegetable oil contains about
H—C—H
5 grams of trans fats.
H—C
With this chapter, we introduce you to the chemistry of life.
C—H
Although every living thing consists of the same basic kinds
H—C—H
of molecules—carbohydrates, lipids, proteins, and nucleic
H—C—H
acids—small differences in the way those molecules are put
H—C—H
together often have big results.
H—C—H H—C—H H—C—H H—C—H H—C—H H—C—H H—C—H C H
H
trans fatty acid
See the video! Figure 3.1 Trans fats. The arrangement of hydrogen atoms around the carbon–carbon double bond in the middle of a trans fatty acid makes it a very unhealthy food. Consider skipping the french fries.
Links to Earlier Concepts
Key Concepts Structure dictates function
Having learned about atoms, you are about to enter the next level of organization in nature: the molecules of life. Keep the big picture in mind by reviewing Section 1.1.
You will be building on your understanding of how electrons are arranged in atoms (2.3) as well as the nature of covalent bonding and hydrogen bonding (2.4).
Here again, you will consider one of the consequences of mutation in DNA (1.4), this time with sickle-cell anemia as the example.
We define cells partly by their capacity to build complex carbohydrates and lipids, proteins, and nucleic acids. All of these organic compounds have functional groups attached to a backbone of carbon atoms. Sections 3.1, 3.2
Carbohydrates Carbohydrates are the most abundant biological molecules. They function as energy reservoirs and structural materials. Different types of complex carbohydrates are built from the same subunits of simple sugars, bonded in different patterns. Section 3.3
Lipids Lipids function as energy reservoirs and as waterproofing or lubricating substances. Some are remodeled into other molecules. Lipids are the main structural component of all cell membranes. Section 3.4
Proteins Structurally and functionally, proteins are the most diverse molecules of life. They include enzymes, structural materials, signaling molecules, and transporters. A protein’s function arises directly from its structure. Sections 3.5, 3.6
Nucleotides and nucleic acids Nucleotides have major metabolic roles and are building blocks of nucleic acids. Two kinds of nucleic acids, DNA and RNA, interact as the cell’s system of storing, retrieving, and translating information about building proteins. Section 3.7
How would you vote? All packaged foods in the United States now list trans fat content, but may be marked “zero grams of trans fats” even if a serving contains up to half a gram of it. Should hydrogenated vegetable oils be banned from all food? See CengageNOW for details, then vote online.
35
3.1
Organic Molecules All of the molecules of life are built with carbon atoms. We can use different models to highlight different aspects of the same molecule.
Representing Structures of Organic Molecules
Links to Elements 2.1, Covalent bonds 2.4
Carbon—The Stuff of Life Living things are mainly oxygen, hydrogen, and carbon. Most of the oxygen and hydrogen are in the form of water. Put water aside, and carbon makes up more than half of what is left. The carbon in living organisms is part of the molecules of life—complex carbohydrates, lipids, proteins, and nucleic acids. These molecules consist primarily of hydrogen and carbon atoms, so they are organic. The term is a holdover from a time when such molecules were thought to be made only by living things, as opposed to the “inorganic” molecules that formed by nonliving processes. The term persists, even though we now know that organic compounds were present on Earth long before organisms were, and we can also make them in laboratories. Carbon’s importance to life starts with its versatile bonding behavior. Each carbon atom can form covalent bonds with one, two, three, or four other atoms. Depending on the other elements in the resulting molecule, such bonds may be polar or nonpolar. Many organic compounds have a backbone—a chain of carbon atoms—to which other atoms attach. The ends of a backbone may join so that the carbon chain forms one or more ring structures (Figure 3.2). Such versatility means that carbon atoms can be assembled and remodeled into a variety of organic compounds.
Any molecule’s structure can be depicted using different kinds of molecular models. Such models allow us to see different characteristics of the same molecule. For example, structural modH H els such as the one at right show C O H H how all the atoms in a molecule C O H H connect to one another. In such C C models, each line indicates one H O C C O H H H covalent bond. A double line (t) O O indicates a double bond; a triple H H line (⬅) indicates a triple bond. Some of the atoms or bonds in glucose a molecule may be implied but not shown. Hydrogen atoms bonded to a carbon backbone may also be omitted, and other atoms as well. Carbon ring structures such as the ones that occur in glucose and other sugars are often represented as polygons. If no atom is shown at a corner or at the end of a bond, a carbon atom is implied there: CH2OH HO
HO
a
C C
b
Figure 3.2 Carbon rings. (a) Carbon’s versatile bonding behavior allows it to form a variety of structures, including rings. (b) Carbon rings form the framework of many sugars, starches, and fats, such as those found in doughnuts.
36 UNIT I
OH
glucose
glucose
Ball-and-stick models such as the one at right show the positions of the atoms in three dimensions. Single, double, and triple covalent bonds are all shown as one stick connecting two balls, which represent atoms. Ball size reflects relative size of an atom. Elements are usually coded by color:
hydrogen
oxygen
nitrogen
glucose
phosphorus
C
C C
O
OH
carbon C
O
PRINCIPLES OF CELLULAR LIFE
Space-filling models such as the one at right show how atoms that are sharing electrons overlap. The elements in space-filling models are coded using the same color scheme as the ones in balland-stick models.
glucose
red blood cell
Figure 3.3 shows three different ways to represent the same molecule, hemoglobin, a protein that colors your blood red. Hemoglobin transports oxygen to tissues throughout the body of all vertebrates (animals that have a backbone). A ball-and-stick or space-filling model of such a large molecule can appear very complicated if all of the atoms are included. The spacefilling model in Figure 3.3a is an example. To reduce visual complexity, other types of models omit individual atoms. Surface models of large molecules can reveal large-scale features, such as folds or pockets, that can be difficult to see when individual atoms are shown. For example, in the surface model of hemoglobin in Figure 3.3b, you can see folds of the molecule that cradle two hemes. Hemes are complex carbon ring structures that often have an iron atom at their center. They are part of many important proteins that you will encounter in this book. Very large molecules such as hemoglobin are often shown as ribbon models. Such models highlight different features of the structure, such as coils or sheets. In a ribbon model of hemoglobin (Figure 3.3c), you can see that the protein consists of four coiled components, each of which folds around a heme. Such structural details are clues to how a molecule functions. For example, hemoglobin, which is the main oxygen-carrier in vertebrate blood, has four hemes. Oxygen binds at the hemes, so each hemoglobin molecule can carry up to four molecules of oxygen.
A A space-filling model of hemoglobin shows the complexity of the molecule.
B A surface model of the same molecule reveals crevices and folds that are important for its function. Heme groups, in red, are cradled in pockets of the molecule.
Take-Home Message How are all of the molecules of life alike? Carbohydrates, lipids, proteins, and nucleic acids are organic molecules, which consist mainly of carbon and hydrogen atoms. The structure of an organic molecule starts with its carbon backbone, a chain of carbon atoms that may form a ring.
We use different models to represent different characteristics of a molecule’s structure. Considering a molecule’s structural features gives us insight into how it functions.
C A ribbon model of hemoglobin shows all four heme groups, also in red, held in place by the molecule’s coils.
Figure 3.3 Visualizing the structure of hemoglobin, the oxygentransporting molecule in red blood cells (top left). Models that show individual atoms usually depict them color-coded by element. Other models may be shown in various colors, depending on which features are highlighted.
CHAPTER 3
MOLECULES OF LIFE 37
3.2
From Structure to Function Functional Groups
The function of organic molecules in biological systems begins with their structure.
Links to Ions 2.3, Polarity 2.4, Acids and bases 2.6
All biological systems are based on the same organic molecules—a legacy of life’s common origin—but the details of those molecules can differ among organisms. Remember, depending on the way carbon atoms bond together, they can form diamond, the hardest mineral, or graphite, one of the softest (Section 2.4). Similarly, the building blocks of carbohydrates, lipids, proteins, and nucleic acids bond together in different arrangements to form different molecules.
Group
Character
hydroxyl
polar
amino acids; sugars and other alcohols
methyl
nonpolar
fatty acids, some amino acids
Location
Structure
—OH
—
H
—
— C —H H
sugars, amino acids, nucleotides
— C —H
—C— — —
polar, reactive
— —
carbonyl
O
O
An organic molecule that consists only of hydrogen and carbon atoms is called a hydrocarbon. Methane, the simplest hydrocarbon, is one carbon atom bonded to four hydrogen atoms. Most of the molecules of life have at least one functional group: a cluster of atoms covalently bonded to a carbon atom of an organic molecule. Functional groups impart specific chemical properties to a molecule, such as polarity or acidity. Figure 3.4 lists a few functional groups that are common in carbohydrates, lipids, proteins, and nucleic acids. For example, alcohols are a class of organic compounds that have hydroxyl groups (sOH). These polar functional groups can form hydrogen bonds, so alcohols (at least the small ones) dissolve quickly in water. Larger alcohols do not dissolve as easily, because their long, nonpolar hydrocarbon chains repel water. Fatty acids also are like this, which is why lipids that have fatty acid tails do not dissolve easily in water. Methyl groups impart nonpolar character. Reactive carbonyl groups (sCtO) are part of fats and carbohydrates. Carboxyl groups (sCOOH) make amino acids and fatty acids acidic. Amine groups are basic. ATP releases chemical energy when it donates a phosphate group (PO4) to another molecule. DNA and RNA also contain phosphate groups. Bonds between sulfhydryl
(aldehyde) (ketone) amino acids, fatty acids, carbohydrates
— C — OH
— C — O–
OH
O
— —
acidic
— —
carboxyl
O
O
(ionized) H
—
amino acids, some nucleotide bases
— N—H
— N H+ —
basic
—
amine
H
H
O
HO one of the estrogens
testosterone
female wood duck
male wood duck
(ionized)
sulfhydryl
forms disulfide bridges
cysteine (an amino acid)
O–
—
nucleotides (e.g., ATP); DNA and RNA; many proteins; phospholipids
— O — P — O– — —
high energy, polar
O
—SH
Figure 3.4 Animated Common functional groups in biological molecules, with examples of where they occur. Because such groups impart specific chemical characteristics to organic compounds, they are an important part of why the molecules of life function as they do.
38 UNIT I
PRINCIPLES OF CELLULAR LIFE
— P
icon
—S—S—
(disulfide bridge)
Figure 3.5 Estrogen and testosterone, sex hormones that cause differences in traits between males and females of many species such as wood ducks (Aix sponsa). Figure It Out: Which functional groups differ between these hormones? Answer: The hydroxyl and carbonyl groups differ in position, and testosterone has an extra methyl group.
phosphate
Table 3.1
What Cells Do to Organic Compounds
Type of Reaction
What Happens
Condensation
Two molecules covalently bond into a larger one.
Cleavage
A molecule splits into two smaller ones. Hydrolysis is an example.
O + HsOsH
OH + HO
Functional group transfer
A functional group is transferred from one molecule to another.
Electron transfer
Electrons are transferred from one molecule to another.
Rearrangement
Juggling of covalent bonds converts one organic compound into another.
groups (sSH) stabilize the structure of many proteins. Heat and some kinds of chemicals can temporarily break sulfhydryl bonds in human hair, which is why we can curl straight hair and straighten curly hair. How much can one functional group do? Consider a seemingly minor difference in the functional groups of two structurally similar sex hormones (Figure 3.5). Early on, an embryo of a wood duck, human, or any other vertebrate is neither male nor female. If it starts making the hormone testosterone, a set of tubes and ducts will become male sex organs and male traits will develop. Without testosterone, those ducts and tubes become female sex organs, and hormones called estrogens will guide the development of female traits.
What Cells Do to Organic Compounds Metabolism refers to activities by which cells acquire and use energy as they construct, rearrange, and split organic compounds. These activities help each cell stay alive, grow, and reproduce. They require enzymes— proteins that make reactions proceed faster than they would on their own. Some of the most common metabolic reactions are listed in Table 3.1. We will revisit these reactions in Chapter 6. For now, just start thinking about two of them. With condensation, two molecules covalently bond into a larger one. Water usually forms as a product of condensation when enzymes remove an sOH group
O + HsOsH
OH + HO
A Condensation. An sOH group from one molecule combines with an H atom from another. Water forms as the two molecules bond covalently.
B Hydrolysis. A molecule splits, then an sOH group and an H atom from a water molecule become attached to sites exposed by the reaction.
Figure 3.6 Animated Two examples of what happens to the organic molecules in cells. (a) In condensation, two molecules are covalently bonded into a larger one. (b) In hydrolysis, a water-requiring cleavage reaction splits a larger molecule into two smaller molecules.
from one of the molecules and a hydrogen atom from the other (Figure 3.6a). Some large molecules such as starch form by repeated condensation reactions. Cleavage reactions split large molecules into smaller ones. One type of cleavage reaction, hydrolysis, is the reverse of condensation (Figure 3.6b). Enzymes break a bond by attaching a hydroxyl group to one atom and a hydrogen to the other. The sOH and sH are derived from a water molecule. Cells maintain pools of small organic molecules— simple sugars, fatty acids, amino acids, and nucleotides. Some of these molecules are sources of energy. Others are used as subunits, or monomers, to build larger molecules that are the structural and functional parts of cells. These larger molecules, or polymers, are chains of monomers. When cells break down a polymer, the released monomers may be used for energy, or they may reenter cellular pools.
Take-Home Message How do organic molecules work in living systems? An organic molecule’s structure dictates its function in biological systems. Functional groups impart certain chemical characteristics to organic molecules. Such groups contribute to the function of biological molecules. By reactions such as condensation, cells assemble large molecules from smaller subunits of simple sugars, fatty acids, amino acids, and nucleotides. By reactions such as hydrolysis, cells split large organic molecules into smaller ones, and convert one type of molecule to another.
CHAPTER 3
MOLECULES OF LIFE 39
3.3
Carbohydrates they remodel into other molecules. For example, vitamin C is derived from glucose.
Carbohydrates are the most plentiful biological molecules in the biosphere. Cells use some carbohydrates as structural materials; they use others for stored or instant energy.
Short-Chain Carbohydrates
Link to Hydrogen bonds 2.4
Long-chain hydrocarbons such as gasoline are an excellent source of energy, but cells (which are mostly water) cannot use hydrophobic molecules. Instead, cells use organic molecules that have polar functional groups— molecules that are easily assembled and broken apart inside a cell’s watery interior. Carbohydrates are organic compounds that consist of carbon, hydrogen, and oxygen in a 1:2:1 ratio. Cells use different kinds as structural materials and as sources of instant energy. The three main types of carbohydrates in living systems are monosaccharides, oligosaccharides, and polysaccharides.
Simple Sugars “Saccharide” is from a Greek word that means sugar. Monosaccharides (one sugar unit) are the simplest of the carbohydrates. Common monosaccharides have a backbone of five or six carbon atoms, one ketone or aldehyde group, and two or more hydroxyl groups. Most monosaccharides are water soluble, so they are easily transported throughout the inter6 CH2OH nal environments of all organisms. 5 Sugars that are part of DNA and RNA HO O 4 are monosaccharides with five carbon 1 atoms. Glucose (at left) has six carbons. 3 HO OH 2 Cells use glucose as an energy source or OH as a structural material. They also use it glucose as a precursor, or parent molecule, that
CH2OH HO
Complex Carbohydrates The “complex” carbohydrates, or polysaccharides, are straight or branched chains of many sugar monomers —often hundreds or thousands. There may be one type or many types of monomers in a polysaccharide. The most common polysaccharides are cellulose, glycogen, and starch. All consist of glucose monomers, but they differ in their chemical properties. Why? The answer begins with differences in patterns of covalent bonding that link their glucose units (Figure 3.8). For example, the covalent bonding pattern of starch makes the molecule coil like a spiral staircase (Figure 3.8b). Starch does not dissolve easily in water, so it resists hydrolysis. This stability is a reason why starch is used to store chemical energy in the watery, enzymefilled interior of plant cells. Most plants make much more glucose than they can use. The excess is stored as starch, in roots, stems, and leaves. However, because it is insoluble, starch
CH2OH
OH HO
HO
O
HO
An oligosaccharide is a short chain of covalently bonded monosaccharides (oligo– means a few). As examples, disaccharides consist of two sugar monomers. The lactose in milk is a disaccharide, with one glucose and one galactose unit. Sucrose, the most plentiful sugar in nature, has a glucose and a fructose unit (Figure 3.7). Sucrose extracted from sugarcane or sugar beets is our table sugar. Oligosaccharides with three or more sugar units are often attached to lipids or proteins that have important functions in immunity.
O
CH2OH OH
HO
glucose
O CH2OH
OH +
HO
PRINCIPLES OF CELLULAR LIFE
CH2OH + H2O O
O OH
fructose
Figure 3.7 Animated The synthesis of a sucrose molecule is an example of a condensation reaction. You are already familiar with sucrose—it is common table sugar.
40 UNIT I
OH
HO
CH2OH sucrose
+
water
OH O
OH O
O O
O
O
OH O
O O
O
O
O
OH O
O
HO
HO
HO
HO
OH
OH
OH
O
O
HO
O
O
O
O HO
O
O
O
HO OH
O
O
O
HO
O
O
HO
HO
OH O
O
HO
O
O
HO
OH O
O
O
O
O
HO
OH O
O
OH O
O
O
O
HO
HO
OH O
O
O
O
HO
OH O
O
O
O
O
HO
OH O
O
O
O
O
O
O
O
OH
O
O
OH O
O
OH O
O
O
O
O HO
a Cellulose, a structural component of plants. Chains of glucose units stretch side by side and hydrogen bond at many sOH groups. The hydrogen bonds stabilize the chains in tight bundles that form long fibers. Very few types of organisms can digest this tough, insoluble material.
b In amylose, one type of starch, a series of glucose units form a chain that coils. Starch is the main energy reserve in plants, which store it in their roots, stems, leaves, fruits, and seeds (such as coconuts).
OH
OH
OH
O
O
O
O
O
c Glycogen. In animals, this polysaccharide functions as an energy reservoir. It is especially abundant in the liver and muscles of active animals, including people.
OH O O
O
Figure 3.8 Structure of (a) cellulose, (b) starch, and (c) glycogen, and their typical locations in a few organisms. All three carbohydrates consist only of glucose units, but the different bonding patterns that link the subunits result in substances with very different properties.
OH
cannot be transported out of cells and distributed to other parts of the plant. When sugars are in short supply, hydrolysis enzymes nibble at the bonds between starch’s sugar monomers. Cells make the disaccharide sucrose from the released glucose molecules. Sucrose is soluble and easily transported. Cellulose, the major structural material of plants, may be the most abundant organic molecule in the biosphere. Glucose chains stretch side by side (Figure 3.8a). Hydrogen bonding between the chains stabilizes them in tight, sturdy bundles. Plant cell walls contain long cellulose fibers. Like steel rods inside reinforced concrete pillars, the tough fibers help tall stems resist wind and other forms of mechanical stress. In animals, glycogen is the sugar-storage equivalent of starch in plants (Figure 3.8c). Muscle and liver cells store it. When the sugar level in blood falls, the liver cells break down glycogen, and the released glucose subunits enter the blood.
OH
HNCOCH3 O
O O HNCOCH3
O
O OH
HNCOCH3
O HNCOCH3
O
Figure 3.9 Chitin. This polysaccharide strengthens the hard parts of many small animals, such as crabs.
OH
Chitin is a polysaccharide with nitrogen-containing groups on its many glucose monomers (Figure 3.9). Chitin strengthens hard parts of many animals, including the outer cuticle of crabs, beetles, and ticks. It also reinforces the cell wall of many fungi.
Take-Home Message What are carbohydrates? Subunits of simple carbohydrates (sugars), arranged in different ways, form various types of complex carbohydrates.
Cells use carbohydrates for energy, storage, or as structural materials.
CHAPTER 3
MOLECULES OF LIFE 41
3.4
Greasy, Oily—Must Be Lipids The fatty acid tails of saturated fats have only single covalent bonds. Animal fats tend to remain solid at room temperature because their saturated fatty acid tails pack tightly. Fatty acid tails of unsaturated fats have one or more double covalent bonds. Such rigid bonds usually form kinks that prevent unsaturated fats from packing tightly (Figure 3.12a). Most vegetable oils are unsaturated, so they tend to remain liquid at room temperature. Partially hydrogenated vegetable oils are an exception. The double bond in these trans fatty acids keeps them straight. Trans fats pack tightly, so they are solid at room temperature (Figure 3.12b).
Lipids function as the body’s major energy reservoir, and as the structural foundation of cell membranes.
Lipids are fatty, oily, or waxy organic compounds that are insoluble in water. Many lipids incorporate fatty acids: simple organic compounds that have a carboxyl group joined to a backbone of four to thirty-six carbon atoms (Figure 3.10).
Fats Fats are lipids with one, two, or three fatty acids that dangle like tails from a small alcohol called glycerol. Most neutral fats, such as butter and vegetable oils, are triglycerides. Triglycerides are fats with three fatty acid tails linked to the glycerol (Figure 3.11). Triglycerides are the most abundant energy source in vertebrate bodies, and the richest. Gram for gram, triglycerides hold more than twice the energy of glycogen. Triglycerides are concentrated in adipose tissue that insulates and cushions parts of the body.
C
Figure 3.10 Examples of fatty acids. (a) The backbone of stearic acid is fully saturated with hydrogen atoms. (b) Oleic acid, with a double bond in its backbone, is unsaturated. (c) Linolenic acid, also unsaturated, has three double bonds. The first double bond occurs at the third carbon from the end of the tail, so oleic acid is called an omega-3 fatty acid. Omega-3 and omega-6 fatty acids are “essential fatty acids.” Your body does not make them, so they must come from food.
C O
O
C
O
H
C H
H
C
H
H
C
H
H
C H
H
C
H
H
C
H
H
C H
H
C
H
H
C
H
H
C H
H
C
H
H
C
H
H
C H
H
C
H
H
C
H
H
C H
H
C
H
H
C
H
H
C H
H
C
H
H
C
H
H
C H
C
H
H
C H
H
C H
H
C
C
H
H
C
H
C H
H
C
H
H
C H
H
C
H
H
C H
H
C
H
H
C H
H
C
H
H
C H
H
C
H
H
C H
H
C
H
H
C H
H
C
H
H
a
H
H C
H
C
H
C
H
H C
H
C
H
C
H
b
carboxyl group
HO
HO
HO
H C
H
H
C
H
H
C
H
H
c
glycerol H
H
H
H C
C
C H
OH
OH
OH
HO
HO
C O H H H H H H H
Figure 3.11 Animated Triglyceride formation by the condensation of three fatty acids with one glycerol molecule. The photograph shows triglyceride-insulated emperor penguins during an Antarctic blizzard.
42 UNIT I
PRINCIPLES OF CELLULAR LIFE
C C C C C C C
H H H H H H H
HO
C O H H H H H H H
C C C C C C C
H H H H H H H
C C C C C C C
H H H H H H H
H C H H C H H C H
H C H H C H H C H
C H C H H C H
H C H H C H
H C H H C H
H C H H C H
H H H H
H H H H
H H H H
C C C C
H H H H
C C C C
H H H H
C C C C
H H H H
H C H
H C H
H C H
H
H
H
three fatty acid tails
H C
C H
O
O
O
C O
C O
C O
H H H H H H H
C O H H H H H H H
H H C
C C C C C C C
H H H H H H H
H H H H H H H
C C C C C C C
H
H H H H H H H
H H H H H H H
C C C C C C C
+ 3H2O
H H H H H H H
H C H H C H H C H
H C H H C H H C H
C H C H H C H
H C H H C H
H C H H C H
H C H H C H
H H H H
C C C C
H H H H
H H H H
H
C H H
H H H H
C C C C
H H H H
C C C C
H H H H
H C H
H C H
H
H
triglyceride, a neutral fat
CH3 CH2
+
CH2
N
CH3
CH3
O P
O
O–
hydrophilic head
O CH2
CH
O C
cis double bond
trans double bond
a oleic acid
b elaidic acid
O O
C CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH3
CH2
a
b
CH
CH2
CH2
Figure 3.12 The only difference between (a) oleic acid (a cis fatty acid) and (b) elaidic acid (a trans fatty acid) is the arrangement of hydrogens around one double bond. Trans fatty acids form during chemical hydrogenation processes.
two hydrophobic tails
O
CH2
CH2
CH2
CH3
OH
c Cell membrane section
Figure 3.13 Phospholipid, (a) structure and (b) icon. Phospholipids are the main structural component of all cell membranes (c).
Figure 3.14 Right, cholesterol. Notice the rigid backbone of four carbon rings.
Phospholipids Phospholipids have a polar head with a phosphate in it, and two nonpolar fatty acid tails. They are the most abundant lipids in cell membranes, which have two phospholipid layers (Figure 3.13a–c). The heads of one layer are dissolved in the cell’s watery interior, and the heads of the other layer are dissolved in the cell’s fluid surroundings. All of the hydrophilic tails are sandwiched between the heads. You will read about membrane structure and function in Chapters 4 and 5.
Waxes Waxes are complex, varying mixtures of lipids with long fatty acid tails bonded to long-chain alcohols or carbon rings. The molecules pack tightly, so the resulting substance is firm and water-repellent. Waxes in the cuticle that covers the exposed surfaces of plants help restrict water loss and keep out parasites and other pests. Other types of waxes protect, lubricate, and soften skin and hair. Waxes, together with fats and fatty acids, make feathers waterproof. Bees store honey and raise new generations of bees inside honeycomb, which they make from beeswax.
Cholesterol and Other Steroids Steroids are lipids with a rigid backbone of four carbon rings and no fatty acid tails. They differ in the type, number, and position of functional groups. All eukaryotic cell membranes contain steroids. In animal tissues, cholesterol is the most common steroid (Figure 3.14). Cholesterol is remodeled into many molecules, such as bile salts (which help digest fats) and vitamin D (required to keep teeth and bones strong). Steroid hormones are derived from cholesterol. Estrogens and testosterone, hormones that govern reproduction and secondary sexual traits, are examples (Figure 3.5).
Take-Home Message What are lipids? Lipids are fatty, waxy, or oily organic compounds. They resist dissolving in water. The main classes of lipids are triglycerides, phospholipids, waxes, and steroids. Triglycerides function as energy reservoirs in vertebrate animals.
Phospholipids are the main component of cell membranes. Waxes are components of water-repelling and lubricating secretions. Steroids are components of cell membranes, and precursors of many other molecules.
CHAPTER 3
MOLECULES OF LIFE 43
3.5
Proteins—Diversity in Structure and Function Proteins are the most diverse biological molecule. Cells build thousands of different proteins by stringing together amino acids in different orders.
Link to Covalent bonding 2.4
Proteins and Amino Acids A protein is an organic compound composed of one or more chains of amino acids. An amino acid is a small organic compound with an amine group, a carboxyl group (the acid), and one or more atoms called an “R group.” Typically, these groups are all attached to the same carbon atom (Figure 3.15). In water, the functional groups ionize: The amine group occurs as sNH3+, and the carboxyl group occurs as sCOO–. Of all biological molecules, proteins are the most diverse. Structural proteins make up spiderwebs and feathers, hooves, hair, and many other body parts. Nutritious types abound in foods such as seeds and
amine group
carboxyl group
O–
+H
CH3 O–
+H
CH3 valine
Figure 3.15 Generalized structure of amino acids, and an example. Green boxes highlight R groups. Appendix V has models of all twenty of the common amino acids.
+H
O–
+
+H
O–
+H
H2O
A DNA encodes the order of amino acids in a new polypeptide chain. Methionine (met) is typically the first amino acid.
Levels of Protein Structure Each type of protein has a unique sequence of amino acids. This sequence is known as the protein’s primary structure (Figure 3.17a). Secondary structure emerges as the chain twists, bends, loops, and folds. Hydrogen bonding between amino acids makes stretches of the polypeptide chain form a sheet, or coil into a helix a bit like a spiral staircase (Figure 3.17b). The primary structure of each type of protein is unique, but similar patterns of coils and sheets occur in most proteins. Much as an overly twisted rubber band coils back on itself, the coils, sheets, and loops of a protein fold up even more into compact domains. A “domain” is a part of a protein that is organized as a structurally stable unit. Such units are a protein’s tertiary structure, its third level of organization. Tertiary structure
O–
+
+H
O–
H2O
B In a condensation reaction, a peptide bond forms between the methionine and the next amino acid, alanine (ala) in this example. Leucine (leu) will be next. Think about polarity, charge, and other properties of functional groups that become neighbors in the growing chain.
Figure 3.16 Animated Examples of peptide bond formation. Chapter 14 returns to protein synthesis.
44 UNIT I
eggs. Most enzymes are proteins. Proteins move substances, help cells communicate, and defend the body. Amazingly, cells can synthesize thousands of different proteins from only twenty kinds of amino acids. The complete structures of those twenty amino acids are shown in Appendix V. Protein synthesis involves bonding amino acids into chains called polypeptides. For each type of protein, instructions coded in DNA specify the order in which any of the twenty kinds of amino acids will occur at every place in the chain. A condensation reaction joins the amine group of an amino acid with the carboxyl group of the next in a peptide bond (Figure 3.16).
PRINCIPLES OF CELLULAR LIFE
makes a protein a working molecule. For instance, the barrel-shaped domains of some proteins function as tunnels through membranes (Figure 3.17c). Many proteins also have a fourth level of organization, or quaternary structure: They consist of two or more polypeptide chains bonded together or in close association (Figure 3.17d). Most enzymes and many other proteins are globular, with several polypeptide chains folded into shapes that are roughly spherical. Hemoglobin, described shortly, is an example. Enzymes often attach linear or branched oligosaccharides to polypeptide chains, forming glycoproteins such as those that impart unique molecular identity to a tissue or to a body. Some proteins aggregate by many thousands into much larger structures, with their polypeptide chains organized into strands or sheets. Some of these fibrous proteins contribute to the structure and organization of cells and tissues. The keratin in your fingernails is an example. Other fibrous proteins, such as the actin and myosin filaments in muscle cells, are part of the mechanisms that help cells and cell parts move.
a Protein primary structure: Amino acids bonded as a polypeptide chain.
H
H
R
H
R
H
R
H
R
H
R
H
R
H
R
R
b Protein secondary structure: A coiled (helical) or sheetlike array held in place by hydrogen bonds (dotted lines) between different parts of the polypeptide chain.
helix (coil)
sheet
c Protein tertiary structure: A chain’s coils, sheets, or both fold and twist into stable, functional domains such as barrels or pockets.
barrel
Take-Home Message What are proteins? Proteins consist of chains of amino acids. The order of amino acids in a polypeptide chain dictates the type of protein. Polypeptide chains twist and fold into coils, sheets, and loops, which fold and pack further into functional domains.
d Protein quaternary structure: two or more polypeptide chains associated as one molecule.
Figure 3.17
+H
O–
+
+H
Four levels of a protein’s structural organization.
O–
+H
O–
H2O
C A peptide bond forms between the alanine and leucine. Tryptophan (trp) will be next. The chain is starting to twist and fold as atoms swivel around some bonds and attract or repel their neighbors.
D The sequence of amino acid subunits in this newly forming peptide chain is now met–ala–leu–trp. The process may continue until there are hundreds or thousands of amino acids in the chain.
CHAPTER 3
MOLECULES OF LIFE 45
3.6
Why Is Protein Structure So Important?
When a protein’s structure goes awry, so does its function.
Links to Inheritance 1.2, Acids and bases 2.6
Just One Wrong Amino Acid . . . Sometimes a protein’s amino acid sequence changes, with drastic consequences. Let’s use hemoglobin as an example. As blood moves through lungs, the hemoglobin inside red blood cells binds oxygen, then gives it up in regions of the body where oxygen levels are low. After giving up oxygen to tissues, the blood circulates back to the lungs, where the hemoglobin inside red blood cells binds more oxygen.
alpha globin
heme A Globin. The secondary structure of this protein includes several helices. The coils fold up to form a pocket that cradles heme, a functional group with an iron atom at its center.
alpha globin
alpha globin
Hemoglobin’s oxygen-binding properties depend on its structure. Each of the four globin chains in the protein forms a pocket that holds an iron-containing heme group (Figure 3.3 and 3.18). One oxygen molecule can bind to each heme in a hemoglobin protein. Globin occurs in two slightly different forms, alpha globin and beta globin. In adult humans, two of each form fold up into a hemoglobin molecule. Negatively charged glutamic acid is normally the sixth amino acid in the beta globin chain, but a DNA mutation sometimes puts a different amino acid—valine—in the sixth position (Figure 3.19a,b). Valine is uncharged. As a result of that one substitution, a tiny patch of the protein changes from polar to nonpolar—which in turn causes the protein’s behavior to change slightly. Hemoglobin altered this way is called HbS. Under some conditions, molecules of HbS form large, stable, rod-shaped clumps. Red blood cells containing these clumps become distorted into a sickled shape (Figure 3.19c). Sickled cells tend to clog tiny blood vessels and disrupt blood circulation. A human has two genes for beta globin, one inherited from each of two parents. (Genes are units of DNA that can encode proteins.) Cells use both genes to make beta globin. If one of a person’s genes is normal and the other has the valine mutation, he or she makes enough normal hemoglobin to survive, but not enough to be completely healthy. Someone with two mutated globin genes can make only HbS hemoglobin. The outcome is sickle-cell anemia, a severe genetic disorder (Figure 3.19d).
Proteins Undone—Denaturation
beta globin
beta globin
B Hemoglobin is one of the proteins with quaternary structure. It consists of four globin molecules held together by hydrogen bonds. To help you distinguish among them, the two alpha globin chains are shown here in green, and the two beta globin chains are in brown.
Figure 3.18 Animated Globin and hemoglobin. (a) Globin, a coiled polypeptide chain that cradles heme, a functional group with an iron atom. (b) Hemoglobin, an oxygen-transport protein in red blood cells.
46 UNIT I
PRINCIPLES OF CELLULAR LIFE
The shape of a protein defines its biological activity: Globin cradles heme, an enzyme speeds a reaction, a receptor responds to some signal. These—and all other proteins—function as long as they stay coiled, folded, and packed in their correct three-dimensional shapes. Heat, shifts in pH, salts, and detergents can disrupt the hydrogen bonds that maintain a protein’s shape. Without the bonds that hold them in their three-dimensional shape, proteins and other large biological molecules denature—their shape unravels and they no longer function. Consider albumin, a protein in the white of an egg. When you cook eggs, the heat does not disrupt the covalent bonds of albumin’s primary structure. But it destroys albumin’s weaker hydrogen bonds, and so the protein unfolds. When the translucent egg white turns opaque, we know albumin has been altered. For a few proteins, denaturation might be reversed if and
H3N+
H
O
C
C
N
CH H3C CH3
valine
H
O
C
C
N
H
O
C
C
CH2
CH2
C
CH
HN
CH
HC
NH+
histidine
H
O
N
C
C
H
C
N
H
O
C
C
N
OH CH2 CH2
threonine
proline
C
N
H
O
C
C
CH2
CH2
CH2
C O O–
C O O–
glutamic acid
glutamic acid
H3C CH3
leucine
O
CH2
CH2
CH3
H
C
Clumping of cells in bloodstream Circulatory problems, damage to brain, lungs, heart, skeletal muscles, gut, and kidneys Heart failure, paralysis, pneumonia, rheumatism, gut pain, kidney failure
A Normal amino acid sequence at the start of the hemoglobin beta chain.
H3N+
H
O
C
C
N
O
C
C
CH2
CH H3C CH3
valine
H
C HN
CH
HC
NH+
histidine
N
H
O
C
C
CH2 CH
H
O
N
C
C
H
C
N
H
O
C
C
OH CH2 CH2
CH3
CH2
N
H
O
C
C
O
C
C
CH
CH2
H3C CH3
CH2
H3C CH3
leucine
N
H
Spleen concentrates sickle cells
Spleen enlargement
Immune system compromised
C O O–
threonine
proline
valine
glutamic acid
Rapid destruction of sickle cells
B One amino acid substitution results in the abnormal beta chain of HbS molecules. The sixth amino acid in such chains is valine, not glutamic acid. C Glutamic acid carries a negative charge; valine carries no charge. This difference changes the protein so it behaves differently. At low oxygen levels, HbS molecules stick together and form rod-shaped clumps that distort normally rounded red blood cells into sickle shapes. (A sickle is a farm tool that has a crescentshaped blade.)
Anemia, causing weakness, fatigue, impaired development, heart chamber dilation
sickled cell Impaired brain function, heart failure
D Melba Moore is a celebrity spokesperson for sickle-cell anemia organizations. Right, range of symptoms for a person with two mutated genes for hemoglobin’s beta chain. normal cell
Figure 3.19 Animated Sickle-cell anemia’s molecular basis and symptoms. Section 18.6 explores evolutionary and ecological pressures that maintain this genetic disorder in human populations.
when normal conditions return, but albumin is not one of them. There is no way to uncook an egg. A protein’s structure dictates its function. Enzymes, hormones, transporters, hemoglobin—such proteins are critical for our survival. Their coiled, twisted and folded polypeptide chains form anchors, membranespanning barrels, or jaws that attack foreign proteins in the body. Mutations can alter the chains enough to block or enhance an anchoring, transport, or defense function. Sometimes the consequences are awful. Yet such changes also give rise to variation in traits, which
is the raw material of evolution. Learn about protein structure and you are on your way to understanding life’s richly normal and abnormal expressions.
Take-Home Message Why is protein structure important? A protein’s function depends on its structure.
Mutations that alter a protein’s structure may also alter its function.
Protein shape unravels if hydrogen bonds are disrupted.
CHAPTER 3
MOLECULES OF LIFE 47
3.7
Nucleic Acids Nucleotides are subunits of DNA and RNA. Some have roles in metabolism.
NH2
base (adenine)
N
Links to Inheritance 1.2, Diversity 1.4, Hydrogen bonds 2.4
C N
C
HC
Nucleotides are small organic molecules, various kinds of which function as energy carriers, enzyme helpers, chemical messengers, and subunits of DNA and RNA. Each nucleotide consists of a sugar with a five-carbon ring, bonded to a nitrogen-containing base and one or more phosphate groups. The nucleotide ATP (adenosine triphosphate) has a row of three phosphate groups attached to its sugar (Figure 3.20). ATP transfers its outermost phosphate group to other molecules and so primes them to react. You will read about such phosphate-group transfers and their important metabolic role in Chapter 5. Nucleic acids are polymers—chains of nucleotides in which the sugar of one nucleotide is joined to the phosphate group of the next. An example is RNA, or ribonucleic acid, named after the ribose sugar of its component nucleotides. RNA consists of four kinds of nucleotide monomers, one of which is ATP. RNA molecules are important in protein synthesis, which we will discuss in Chapter 14.
O– O–
N 5'
CH2
C N
C
CH
Figure 3.20
2'
C NH
HC
5'
CH2
C N
OH
OH
The structure of ATP.
5
3
5
cytosine (C) base with a single ring structure
C HC
N
HC
C N
CH2
3
A
3
G
5
O
O
4'
1'
5 3'
a
OH
2'
H
3'
OH
2'
H
Figure 3.21 Animated (a) Nucleotides of DNA. The four kinds of nucleotides in DNA differ only in their component base, for which they are named. The carbon atoms of the sugar rings in nucleotides are numbered as shown. This numbering convention allows us to keep track of the orientation of a chain of nucleotides, as shown in (b).
48 UNIT I
PRINCIPLES OF CELLULAR LIFE
C
2'
H NH2
5'
A
3
5
NH2
1'
thymine (T)
1'
base with a double ring structure
O
4'
2'
NH
guanine (G)
O
C
O 1'
O
OH
N
CH2
4'
base with a single ring C O structure N
C
3'
C
5'
CH
DNA, or deoxyribonucleic acid, is another type of nucleic acid named after the deoxyribose sugar of its component nucleotides (Figure 3.21). A DNA molecule consists of two nucleotide chains twisted together as a double helix. Hydrogen bonds between the four kinds of nucleotide hold the two strands of DNA together (Figure 3.22). Each cell starts out life with DNA inherited from a parent cell. That DNA contains all of the information necessary to build a new cell and, in the case of
4'
H
N
O
O
N
sugar (ribose)
HC
CH2
1'
OH
P
3'
CH3
5'
3'
sugar (deoxyribose)
O
C
base with a double ring structure
O
4'
O
C
O
C N
P
O
N
O–
3 phosphate groups
adenine (A)
HC
3 phosphate groups
P O
NH2 N
O–
b
C 3
Summary Section 3.1 Under present-day conditions in nature, only living cells make the molecules of life: complex carbohydrates and lipids, proteins, and nucleic acids. The molecules of life differ, but all of them are organic compounds that consist mainly of carbon and hydrogen atoms. Carbon atoms can bond covalently with as many as four other atoms. Long carbon chains or rings form the backbone of the molecules of life. Section 3.2 Functional groups attached to the carbon backbone influence the function of organic compounds. Table 3.2 (next page) summarizes the molecules of life and their functions. By the process of metabolism, cells acquire and use energy as they make, rearrange, and break down organic compounds. Enzymatic reactions that are common in metabolism include condensation, which makes polymers from smaller monomers, and hydrolysis, which cleaves molecules into smaller ones.
Section 3.3 Cells use carbohydrates as energy sources, transportable or storable forms of energy, and structural materials. The oligosaccharides and polysaccharides are polymers of monosaccharide monomers.
⎫
⎫ ⎪ ⎪ ⎪ ⎪ ⎪ ⎬ ⎪ ⎪ ⎪ ⎪ ⎪ ⎭
⎭
covalent bonding in sugar– phosphate backbone
hydrogen bonding between bases
Figure 3.22
Models of the DNA molecule.
multicelled organisms, an entire individual. The cell uses the order of nucleotide bases in its DNA—the DNA sequence—to construct RNA and proteins. Parts of the sequence are identical or nearly so in all organisms. Other parts are unique to a species, or even to an individual. Chapter 13 returns to DNA structure and function.
Take-Home Message What are nucleotides and nucleic acids? Different nucleotides are monomers of the nucleic acids DNA and RNA, coenzymes, energy carriers, and messengers.
DNA’s nucleotide sequence encodes heritable information. Different types of RNA have roles in the processes by which a cell uses the heritable information in its DNA.
Use the animation on CengageNOW to explore functional groups, condensation, and hydrolysis.
Use the animation on CengageNOW to see how sucrose forms by condensation of glucose and fructose.
Section 3.4 Lipids are greasy or oily nonpolar molecules, often with one or more fatty acid tails, and include triglycerides and other fats. Phospholipids are the main structural component of cell membranes. Waxes are part of water-repellent and lubricating secretions; steroids are precursors of other molecules.
Use the animation on CengageNOW to see how a triglyceride forms by condensation.
Section 3.5 Proteins are the most diverse molecules of life. Protein structure begins as a linear sequence of amino acids called a polypeptide chain (primary structure). The chains form sheets and coils (secondary structure), which may pack into functional domains (tertiary structure). Many proteins, including most enzymes, consist of two or more chains (quaternary structure). Fibrous proteins aggregate further into large chains or sheets.
Use the animation on CengageNOW to explore amino acid structure and learn about peptide bond formation.
Read the InfoTrac article “Protein Folding and Misfolding,” David Gossard, American Scientist, September 2002.
Section 3.6 A protein’s structure dictates its function. Sometimes a mutation in DNA results in an amino acid substitution that alters a protein’s structure enough to compromise its function. Genetic diseases such as sicklecell anemia may result. Shifts in pH or temperature, and exposure to detergent or to salts may disrupt the many hydrogen bonds CHAPTER 3
MOLECULES OF LIFE 49
IMPACTS, ISSUES REVISITED
Fear of Frying
Several countries are ahead of the United States in restricting the use of trans fats in food. In 2004, Denmark passed a law that prohibited importation of foods that contain partially hydrogenated vegetable oils. French fries and chicken nuggets the Danish import from the United States contain almost no trans fats; the same foods sold to consumers in the United States contain 5 to 10 grams of trans fats per serving.
and other molecular interactions that hold a protein in its three-dimensional shape. If a protein unfolds so that it loses its three-dimensional shape (or denatures), it also loses its function.
Use the animation on CengageNOW to learn more about hemoglobin structure and sickle-cell mutation.
Section 3.7 Nucleotides are small organic molecules that consist of a sugar bonded to three phosphate groups
Table 3.2
How would you vote? New York was the first U.S. city to ban trans fats from restaurant food. Should the use of trans fats in food be banned entirely? See CengageNOW for details, then vote online.
and a nitrogen-containing base. ATP transfers phosphate groups to many kinds of molecules. Other nucleotides are coenzymes or chemical messengers. DNA and RNA are nucleic acids, each composed of four kinds of nucleotides. DNA encodes heritable information about a cell’s proteins and RNAs. Different RNAs interact with DNA and with one another to carry out protein synthesis.
Use the animation on CengageNOW to explore DNA.
Summary of the Main Organic Molecules in Living Things
Category
Main Subcategories
Some Examples and Their Functions
CARBOHYDRATES
Monosaccharides Simple sugars
Glucose
Energy source
. . . contain an aldehyde or a ketone group, and one or more hydroxyl groups
Oligosaccharides Short-chain carbohydrates
Sucrose
Most common form of sugar
Polysaccharides Complex carbohydrates
Starch, glycogen
Energy storage
Cellulose
Structural roles
Fats (e.g., butter), oils (e.g., corn oil)
Energy storage
Phospholipids Glycerol backbone, phosphate group, another polar group; often two fatty acids
Lecithin
Key component of cell membranes
Waxes Alcohol with long-chain fatty acid tails
Waxes in cutin
Conservation of water in plants
Steroids Four carbon rings; the number, position, and type of functional groups differs
Cholesterol
Component of animal cell membranes; precursor of many steroids, vitamin D
Mostly fibrous proteins Long strands or sheets of polypeptide chains; often strong, waterinsoluble
Keratin
Structural component of hair, nails
Collagen
Component of connective tissue
Myosin, actin
Functional components of muscles
Enzymes
Great increase in rates of reactions
Hemoglobin
Oxygen transport
Insulin
Control of glucose metabolism
Antibodies
Immune defense
ATP
Energy carrier
LIPIDS
Glycerides Glycerol backbone with one, two,
. . . are mainly hydrocarbon; generally do not dissolve in water but do dissolve in nonpolar substances, such as alcohols or other lipids
or three fatty acid tails (e.g., triglycerides)
PROTEINS . . . are one or more polypeptide chains, each with as many as several thousand covalently linked amino acids
NUCLEIC ACIDS AND NUCLEOTIDES . . . are chains of units (or individual units) that each consist of a five-carbon sugar, phosphate, and a nitrogen-containing base
50 UNIT I
Mostly globular proteins One or more polypeptide chains folded into globular shapes; many roles in cell activities
Adenosine phosphates
cAMP
Messenger in hormone regulation
Nucleotide coenzymes
NAD+, NADP+, FAD
Transfer of electrons, protons (H+) from one reaction site to another
Nucleic acids Chains of nucleotides
DNA, RNAs
Storage, transmission, translation of genetic information
PRINCIPLES OF CELLULAR LIFE
Data Analysis Exercise Main Dietary Fats
Cholesterol does not dissolve in blood, so it is carried through the bloodstream by lipid–protein aggregates called lipoproteins. Lipoproteins vary in structure. Low-density lipoprotein (LDL) carries cholesterol to body tissues such as artery walls, where it can form health-endangering deposits. LDL is often called “bad” cholesterol. High-density lipoprotein (HDL) carries cholesterol away from tissues to the liver for disposal; it is often called “good” cholesterol. In 1990, R.P. Mensink and M.B. Katan published a study that tested the effects of different dietary fats on blood lipoprotein levels. Their results are shown in Figure 3.23. 1. In which group was the level of LDL (“bad” cholesterol) highest?
3. An elevated risk of heart disease has been correlated with increasing LDL-to-HDL ratios. In which group was the LDL:HDL ratio highest? Rank the three diets according to their potential effect on cardiovascular health.
Answers in Appendix III
1. Each carbon atom can share pairs of electrons with up to other atom(s). 2. Sugars are a type of 3.
.
is a simple sugar (a monosaccharide). a. Glucose c. Ribose e. both a and b b. Sucrose d. Chitin f. both a and c
5. Is this statement true or false? Unlike saturated fats, all of the unsaturated fats are beneficial to health because their fatty acid tails bend and do not pack together. 6. Steroids are among the lipids with no
.
7. Which of the following is a class of molecules that encompasses all of the other molecules listed? a. triglycerides c. waxes e. lipids b. fatty acids d. steroids f. phospholipids are to proteins as are to nucleic acids. a. Sugars; lipids c. Amino acids; hydrogen bonds b. Sugars; proteins d. Amino acids; nucleotides
9. A denatured protein has lost its . a. hydrogen bonds c. function b. shape d. all of the above 10. a. sugars
consist(s) of nucleotides. b. DNA c. RNA
d. b and c
are the richest energy source in the body. 11. a. Sugars b. Proteins c. Fats d. Nucleic acids 12. Match each molecule with its most suitable description. chain of amino acids a. carbohydrate energy carrier in cells b. phospholipid glycerol, fatty acids, phosphate c. polypeptide two strands of nucleotides d. DNA one or more sugar monomers e. ATP richest source of energy f. triglycerides
optimal level
LDL
103
117
121
40
ratio
1.87
2.43
2.2
1 million
most arthropods also have paired antennae that can detect touch and waterborne or airborne chemicals. The body plan of many arthropods changes during the life cycle. Individuals often undergo metamorphosis: Tissues get remodeled as juveniles become adults. Each stage is specialized for a different task. For instance, wingless, plant-eating caterpillars metamorphose into winged butterflies that disperse and find mates (Figure 25.31d). Having such different bodies also prevents adults and juveniles from competing for the same resources. Specialized Developmental Stages
Take-Home Message What are arthropods? Arthropods are the most diverse animal phylum. A jointed exoskeleton, a segmented body plan with specialized segments, sensory specializations, and a life cycle that often includes metamorphosis contributed to their success. Trilobites are an extinct arthropod group. Modern arthropod groups include horseshoe crabs, spiders, ticks, crabs, lobsters, centipedes, and insects.
CHAPTER 25
ANIMAL EVOLUTION—THE INVERTEBRATES 421
25.13 Chelicerates—Spiders and Their Relatives Chelicerates include the oldest living arthropod lineage (horseshoe crabs) and other arthropods without antennae.
Link to Disease-causing bacteria 21.6
telson
a telson (with stinger)
b
pedipalp
cephalothorax
abdomen
Chelicerates include horseshoe crabs, scorpions, spiders, ticks, and mites (Figure 25.32). The body has a cephalothorax (fused head and thorax) and abdomen. There are four pairs of walking legs. The head has eyes, but no antennae. Near the mouth are paired feeding appendages called chelicerae and pedipalps. Horseshoe crabs live in seas, eat clams and worms, and have a hard shield over the cephalothorax (Figure 25.32a). A spinelike segment (the telson) acts as a rudder when they swim. Horseshoe crab eggs, laid onshore in spring, are essential food for some migratory birds. All land chelicerates, including spiders, scorpions, ticks, and mites, are arachnids. Scorpions and spiders are predators that subdue prey with venom. Scorpions dispense venom through a stinger on the telson (Figure 25.32b). Spiders deliver venom with a bite. Their fanglike chelicerae have poison glands (Figures 25.32c,d and 25.33). Of 38,000 spider species, about 30 produce venom that can harm humans. Most spiders indirectly help us by eating insect pests. The spider abdomen has paired spinners that eject silk for webs and nests. An open circulatory system allows blood to mingle with tissue fluids. Malpighian tubules move excess water and nitrogen-rich wastes from the tissues to the gut for disposal. In many species, gas exchange occurs at leaflike “book lungs.” All ticks suck blood from vertebrates (Figure 25.32e). Some can transmit bacteria that cause Lyme disease or other diseases (Section 21.6). The 45,000 or so species of mites include parasites, predators, and scavengers (Figure 25.32 f ). Most are less than a millimeter long. Take-Home Message
d
c
What are chelicerates? Chelicerates are arthropods that do not have antennae. Horseshoe crabs are a small, marine lineage. The far more diverse arachnids live mostly on land.
digestive gland eye
f
e
Figure 25.32 Chelicerates. (a) A horseshoe crab (Limulus). All horseshoe crabs are marine. They are the closest living relatives of the extinct trilobites. Members of the arachnid subgroup: (b) A scorpion. Some scorpion stings can be fatal to humans. (c) A jumping spider. It does not make a web. It pounces on its prey. (d) A web-weaving black widow spider (Latrodectus) has venom that can be fatal to humans. Only the females bite. They have a red hourglass marking on their abdomen. (e) Tick swollen after a blood meal. (f) A dust mite.
422 UNIT IV
EVOLUTION AND BIODIVERSITY
brain
heart
Malpighian tubules
poison gland
pedipalp chelicera
anus
mouth
book lung
ovary
sperm receptacle
Figure 25.33 Body plan of a spider.
spinners silk gland
25.14 The Mostly Marine Crustaceans Crustaceans are the only arthropod group that is mainly aquatic. Most crustaceans live in the seas.
Crustaceans are a group of mostly marine arthropods with two pairs of antennae. Some live in freshwater. A few such as wood lice (Figure 25.34a) live on land. Small crustaceans reach great numbers in the seas and are an important food source for larger animals. Krill (euphausids) have a shrimplike body a few centimers long and swim in upper ocean waters (Figure 25.34b). Most copepods are also marine zooplankton, but others live in freshwater (Figure 25.34c). Some copepod parasites of fish or whales can be large; a few of these are as long as your forearm. Larval barnacles swim, but adults are enclosed in a calcified shell and live attached to piers, rocks, and even whales (Figure 25.34d). They filter food from the water with feathery legs. As adults, they cannot move about, so you would think that mating might be tricky. But barnacles tend to settle in groups, and most are hermaphrodites. An individual extends a penis, often several times its body length, out to neighbors. Lobsters, crayfish, crabs, and shrimps all belong to the same crustacean subgroup (the decapods). All are bottom feeders with five pairs of walking legs (Figure 25.35). In some lobsters, crayfish, and crabs, the first pair of legs has become modified into a pair of claws. Like all arthropods, crabs molt as they grow (Figure 25.36). Some spider crabs do quite a bit of growing. With legs that can reach more than a meter in length, these crabs are the largest living arthropods. Take-Home Message What are crustaceans? Crustaceans are mostly marine arthropods that have two pairs of antennae. They are ecologically important as a food source and include the largest living arthropods.
abdomen cephalothorax segments (fused segments)
eyes (two)
a
b
d
c
Figure 25.34 Representative crustaceans. (a) A wood louse (also known as a pill bug, sow bug, or roly-poly) is a scavenger on land. (b) Antarctic krill (Euphausia superba) can be up to 6 centimeters long. Populations can reach densities of 10,000 individuals per cubic meter of seawater. (c) A free-living female copepod (Macrocyclops albidus) from the Great Lakes is about one millimeter long. (d) Goose barnacle. Adults cement themselves to one spot head down, and filter food from the water with feathery jointed legs.
antennae (two pairs)
food-handling appendages (three pairs)
juvenile
egg
adult female
larva tail swimmerets fan
first leg walking legs (five pairs)
Figure 25.35 Body plan of a lobster (Homarus americanus).
Figure 25.36 Animated Crab life cycle. Larval and juvenile stages molt repeatedly and grow in size before they are mature adults. Adults continue to molt. A female carries her fertilized eggs under her abdomen until they hatch.
CHAPTER 25
ANIMAL EVOLUTION—THE INVERTEBRATES 423
25.15 Myriapods—Lots of Legs Centipedes and millipedes use their many legs to walk about on land, hunting prey or scavenging.
25.16 The Insects Arthropods are the most successful animal phylum, and insects are the most successful arthropods.
Myriapod means “many feet,” and aptly describes the centipedes and millipedes. Both have a long body with many similar segments (Figure 25.37). The head has a pair of antennae and two simple eyes. Myriapods are ground-dwellers that move about at night and hide under rocks and leaves during the day. Centipedes have a low-slung, flattened body with a single pair of legs per segment, for a total of 30 to 50. They are fast-moving predators. Their first pair of legs has become modified as fangs that inject paralyzing venom. Most centipedes prey on insects, but some big tropical species eat small vertebrates (Figure 25.37a). Millipedes are slower moving animals that feed on decaying vegetation. Their body is rounded and has two pairs of legs on most segments, with 250 or so pairs in total (Figure 25.37b). Take-Home Message What are myriapods? Myriapods are land-dwelling arthropods with two antennae and an abundance of body segments. Centipedes are predators and millipedes are scavengers.
Link to Genomics 16.5
Insect Characteristics With more than a million species, insects are the most diverse arthropod group. They are also breathtakingly abundant. By some estimates, the ants alone make up about 10 percent of the world’s animal biomass (the total weight of all living animals). Insects have a three-part body plan, with a head, thorax, and abdomen (Figure 25.38). The head has one pair of antennae and two compound eyes. Such eyes consist of many individual units, each with a lens. Near the mouth are jawlike mandibles and other feeding appendages. Insects feed in a variety of ways and their mouthparts reflect their speciality (Figure 25.39). Three pairs of legs extend out from the insect thorax. In some groups, the thorax also has one or two pairs of wings. Insects are the only winged invertebrates. A few insects spend some time in the water, but the group is overwhelmingly terrestrial. A respiratory system consisting of tracheal tubes carries air from openings at the body surface to tissues deep inside the body. Like all other arthropods, insects have an open circulatory system.
abdomen
thorax with six legs
a
head with two eyes, and two antennae b
Figure 25.37 (a) A Southeast Asian centipede feeds on its frog prey. The false antennae on the last segment may prevent predators from attacking the centipede’s head. (b) A millipede.
424 UNIT IV
EVOLUTION AND BIODIVERSITY
Figure 25.38 A bed bug (Cimex lectularius) illustrates the basic insect body plan: a head, thorax, and abdomen. The bug is 7 millimeters long and feeds on human blood.
liplike labrum
antenna
a Direct development: Growth in size between molts but no change in body form
compound eye mandible
young
egg
adult
maxilla maxilla palps
a
b
liplike labrum
labium
c
d
b Incomplete metamorphosis: gradual change with each molt until the nymph becomes adult
c Complete metamorphosis: larvae grow, then molt into a pupa, which is remodeled into the adult form
egg
egg
adult
nymphs
larvae
pupa
adult
Figure 25.39 Animated Examples of insect appendages. Head parts of (a) grasshoppers, which chew fibrous plant parts; (b) flies, which sop up nutrients; (c) butterflies, which siphon nectar from flowering plants; and (d) mosquitoes, which pierce hosts and suck up blood.
Figure 25.40 Insect development. (a) Silverfish show direct development. Young simply change in size with each molt. (b) Bugs, including bedbugs, undergo incomplete metamorphosis. Small changes occur with each molt. (c) Fruit flies show complete metamorphosis. A larva develops into a pupa, which is remodeled into an adult.
Insects have a complete digestive system divided into a foregut, a midgut where food is digested, and a hindgut, where water is reabsorbed. As in spiders and other land-dwelling arthropods, Malpighian tubules inside the abdomen function in excretion. Nitrogenrich wastes produced by digestion of proteins diffuse from blood into these tubes. There, enzymes convert the waste to crystals of uric acid, which an insect excretes. Malpighian tubules help insects eliminate toxic metabolic wastes without losing precious water. An insect abdomen also contains sex organs. Sexes are separate. Depending on the group, a fertilized egg either hatches into a small version of the adult, or a juvenile that will later undergo metamorphosis. During metamorphosis, tissues of a juvenile are reorganized (Figure 25.40). Incomplete metamorphosis means that the changes in body form take place a bit at a time. Juveniles called nymphs, change a little with each molt. Complete metamorphosis is more dramatic. In this case, the juvenile, called a larva, grows and molts with no change in body plan. Then it is transformed into a pupa, which undergoes the tissue remodeling that produces the adult.
Insect Origins Until recently, insects were thought to be close relatives of the myriapods. Both groups have a single pair of antennae and unbranched legs. Then—as we have seen so many times—new information made scientists rethink the connections. The current hypothesis holds that the insects are most closely related to crustaceans. Specifically, insects are thought to be descended from freshwater crustaceans, with silverfish (Figure 25.40a) as an early insect lineage. If this hypothesis is correct, then insects are the crustaceans of the land.
Take-Home Message What are insects? Insects are the most diverse and abundant animals. They have a three-part body plan. The head has compound eyes, a pair of antennae, and specialized mouthparts. A thorax has three pairs of legs and, in some lineages, wings.
Insects are adapted to life on land. A system of tracheal tubes delivers air to their tissues. Malpighian tubules in their abdomen allow them to expel waste while minimizing water loss. By the most recent hypothesis, insects evolved from a crustacean lineage.
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25.17 Insect Diversity and Importance It would be hard to overestimate the importance of insects, for either good or ill.
Links to Flagellate protozoans 22.2, Malaria 22.6
A Sampling of Insect Diversity Again, insects show tremendous diversity, with more than a million species. The representatives in Figure 25.41 provide a glimpse of the variety. Of these, only silverfish (Figure 25.41a) undergo direct development. In addition to bugs, insects with incomplete metamorphosis include earwigs, lice, cicadas, damselflies, termites, and grasshoppers. Earwigs are scavengers that have a flattened body (Figure 25.41b). Like silverfish, they sometimes end up in our basements and garages. Lice are wingless and suck blood from warm-blooded animals (Figure 25.41c). Cicadas (Figure 25.41d) and the related leafhoppers and aphids, are winged and suck juices from plants. Damselflies (Figure 25.41e) and the related dragonflies are agile aerial predators of other insects. Termites live in big family groups. They have prokaryotic and protistan symbionts in their gut that allow them to digest wood (Figure 25.41f ). They are unwelcome when they devour buildings or decks, but are important decomposers. Grasshoppers cannot eat wood, but they do chew their way through tough, nonwoody plant parts (Figure 25.41g). The four most successful insect lineages all have wings and undergo complete metamorphosis. There are approximately 150,000 species of flies, or dipterans (Figure 25.41h), and at least as many beetles, or coleopterans (Figure 25.41i,j). The wasp in Figure 25.41k is one of about 130,000 hymenopterans. This group also includes the bees and ants. Lepidopterans—moths and butterflies (Figure 25.41l)—weigh in with about 120,000 species. As a comparison, consider that there are about 4,500 species of mammals.
Ecological Services As you learned in Section 23.8, the flowering plants coevolved with insect pollinators. The vast majority of these plants are pollinated by members of one of the four most successful insect groups. The other groups contain few or no pollinators. By one hypothesis, the close interactions between pollinating insect groups and flowering plants contributed to an increased rate of speciation in both. Today, declines in populations of insect pollinators are a matter of concern. Development of natural areas, use of pesticides, and the spread of newly introduced 426 UNIT IV
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diseases are reducing the populations of insects that pollinate native plants and agricultural crops. We discuss this problem in more detail in Chapter 30. Insects are also important as food for wildlife. Most songbirds nourish their nestlings on a diet consisting largely of insects. Many migratory songbirds travel long distances to nest and raise young in areas where insect abundance is seasonally high. Aquatic larvae of insects such as dragonflies, mayflies, and mosquitoes serve as food for trout and other freshwater fish. Amphibians and reptiles feed mainly on insects. Even humans eat insects. In many cultures, they are considered a tasty source of protein. Insects dispose of wastes and remains. Flies and beetles are quick to discover an animal corpse or a pile of feces. They lay their eggs in or on this organic material, and the larvae that hatch devour it. By their actions, these insects keep organic wastes and remains from piling up, and help distribute nutrients through the ecosystem.
Competitors for Crops Insects are our main competitors for food and other plant products. It is estimated that about a quarter to a third of all crops grown in the United States are lost to insects. Also, in an age of global trade and travel, we have more than just home-grown pests to worry about. Consider the Mediterranean fruit fly (Figure 25.41h). The Med fly, as it is known, lays eggs in citrus and other fruits, as well as many vegetables. Damage done to plants and fruits by larvae of the Med fly can cut crop yield in half. Med flies are not native to the United States and there is an ongoing inspection program for imported produce, but some Med flies still slip in. So far, eradication efforts have been successful, but they have cost hundreds of millions of dollars. Still, this amount is probably a bargain. If the Med fly were to become permanently established, losses would likely run into the billions.
Vectors for Disease What is the deadliest animal? It may be the mosquito. As you learned earlier, certain mosquito species transmit malaria, which kills more than a million people each year (Section 22.6). Mosquitoes are also vectors for viruses and roundworms that cause disease. Other biting insects can spread other pathogens. Biting flies transmit African sleeping sickness; biting bugs spread
c
b
a
f
e
d
j
g
h
k
Chagas disease (Section 22.2). Fleas that bite rats and then bite humans can transmit bubonic plague. Body lice can transmit typhus. So far as we know, bedbugs like the one in Figure 25.38 do not cause disease. However, a heavy bedbug infestation can cause weakness as a result of blood loss, especially in children.
Take-Home Message What effect do insects have? There are many groups of insects. The four most diverse groups all include members that are pollinators of flowering plants. As such, insects help provide us with food crops. Insects also have important ecological roles as food for animals and as agents of waste disposal. A small number of insect species compete with us for crops or carry pathogens.
i
l
Figure 25.41 A sampling of insect diversity. (a) One of the silverfish, the only insect group with direct development. Insects with incomplete metamorphosis: (b) European earwig, a common household pest. Curved pincers at the tail end indicate this is a male. In females, pincers are straight. (c) Duck louse. It eats bits of feathers and skin. (d) A cicada. Male cicadas are among the loudest of all insects. They have specialized sound-producing organs that they use to attract females. (e) A damselfly, one of the insects that has aquatic larvae. (f) Sterile soldier termites with glue-squirting heads ready to defend their colony. (g) A grasshopper. Members of the four most diverse groups. All are winged and undergo complete metamorphosis. (h) Mediterranean fruit fly. Larvae of this insect destroy citrus fruit and other crops. (i) Ladybird beetles with a distinctive red and black spotted wing cover. (j) Staghorn beetle from New Guinea. Males, such as this one, have huge mandibles. Females have smaller mandibles. (k) A bald-faced hornet, a wasp, is a hymenopteran. This is a fertile female, or queen. She lives in a papery nest with her many offspring. (l) A swallowtail butterfly, a lovely lepidopteran, shown acting as pollinator.
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25.18 The Spiny-Skinned Echinoderms Echinoderms begin life as bilateral larvae and develop into spiny-skinned, radial adults.
Link to Patterns of development 19.3
The Protostome–Deuterostome Split In Section 25.1 we introduced the two major lineages of animals, protostomes and deuterostomes. Thus far, all of the animals with a three-layer embryo that we have discussed—from flatworms to arthropods—have been protostomes. This section begins our survey of deuterostome lineages. Echinoderms are the largest group of invertebrate deuterostomes. We will discuss other invertebrate deuterostomes, and the vertebrates (also deuterostomes), in the next chapter.
Echinoderm Characteristics and Body Plan Echinoderms (phylum Echinodermata) include about 6,000 marine invertebrates. Their name means “spinyskinned” and refers to interlocking spines and plates of calcium carbonate embedded in their skin. Adults have a radial body plan, with five parts (or multiples
of five), around a central axis. The larvae, however, are bilateral, which suggests that the ancestor of echinoderms was a bilateral animal. Sea stars (also called starfish) are the most familiar echinoderms, and we will use them as our example of the echinoderm body plan (Figure 25.42). Sea stars do not have a brain, but they do have a decentralized nervous system. Eye spots at the tips of arms detect light and movement. A typical sea star is an active predator that moves about on tiny, fluid-filled tube feet. Tube feet are part of a water–vascular system unique to echinoderms. Figure 25.42a shows the system of fluid-filled canals in each arm of a sea star. Side canals deliver coelomic fluid into muscular ampullae that function like the bulb on a medicine dropper (Figure 25.42b). Contraction of an ampulla forces fluid into the attached tube foot, extending the foot. A sea star glides along smoothly as coordinated contraction and relaxation of the ampullae redistributes fluid among hundreds of tube feet. Sea stars often feed on bivalve mollusks. They can slide their stomach out through their mouth and into the bivalve’s shell. The stomach secretes acid and
upper stomach anus
gonad
spine
lower stomach coelom digestive gland eyespot
ampulla of a tube foot
canal of watervascular system
spine a
ossicle (tiny skeletal structure)
c
b
Figure 25.42 Animated Body plan of a sea star. (a) Major components of the central body and the radial arms, with a close-up of its little tube feet. (b) Organization of the water–vascular system. In combination with many tube feet, it is the basis of locomotion. (c) A sea star’s toothy feeding apparatus.
428 UNIT IV
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tube feet
enzymes that kill the mollusk and begin to digest it. Partially digested food is taken into the stomach and digestion is completed with the aid of digestive glands in the arms. Gas exchange occurs by diffusion across the tube feet and tiny skin projections at the body surface. There are no specialized excretory organs. Sexes are separate. Either male or female gonads are in the arms. Eggs and sperm are released into the water. Fertilization produces an embryo that develops into a ciliated, bilateral larva. The larva swims about and develops into the adult, nonswimming form. Sea stars and other echinoderms have a remarkable ability to regenerate lost body parts. If a sea star is cut into pieces, any portion with some of the central disk can regrow the missing body parts.
Echinoderm Diversity Brittle stars are the most diverse and abundant echinoderms (Figure 25.43a). They are less familiar than sea stars because they generally live in deeper water. They have a central disk and highly flexible arms that move about in a snakelike way. Most brittle stars are scavengers on the sea floor. In sea urchins, calcium carbonate plates form a stiff, rounded cover from which spines protrude (Figure 25.43b). The spines provide protection and are used in movement. Some urchins graze on algae. Others act as scavengers or prey on invertebrates. Sea urchin roe (eggs) are used in some sushi. Overharvesting for markets in Asia threatens species that produce the most highly prized roe. In sea cucumbers, hardened parts have been reduced to microscopic plates embedded in a soft body. Some species such as the one in Figure 25.43c filter food from the seawater. Others have a wormlike body and, like earthworms, they feed by eating their way through sediments and digesting any organic material. Lacking spines or sharp plates, sea cucumbers have an alternative defense. When threatened, they expel a sticky mass of specialized threads and internal organs out through their anus. If this maneuver successfully distracts the predator, the sea cucumber escapes, and its missing parts grow back. Take-Home Message What are echinoderms? Echinoderms are deuterostome invertebrates that have a radial body as adults. They are brainless and have a unique water–vascular system that functions in locomotion.
a
b
c
Figure 25.43 (a) Brittle star. Its slender arms (rays) make fast, snakelike movements. (b) Underwater “forest” of sea urchins, which can move about on spines and a few tube feet. (c) Sea cucumber, with rows of tube feet along its soft body.
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ANIMAL EVOLUTION—THE INVERTEBRATES 429
IMPACTS, ISSUES REVISITED
Old Genes, New Drugs
Marine invertebrates are important components of ecosystems, a source of food, and a treasure trove of molecules with potential for use in industrial applications or as medicines. Various species of cone snails, sponges, corals, crabs, and sea cucumbers make compounds that show promise as drugs. However, even as we begin to explore this potential, marine biodiversity is on the decline as a result of habitat destruction and overharvesting.
Summary Section 25.1 Animals are multicelled heterotrophs with unwalled cells. Some animals have no body symmetry or have radial symmetry, like a wheel. Most have bilateral symmetry and show cephalization, a concentration of nerves and sensory structures at the head end. Most digest food in a gut. The gut may be surrounded by tissues or inside a fluid-filled cavity. The cavity may be a fully lined coelom, or a partially lined pseudocoel. Two major branches of bilateral animals, protostomes and deuterostomes, have a coelom and a complete gut. In protostomes, the first opening on the embryo becomes a mouth. In deuterostomes, an anus forms first.
Use the animation on CengageNOW to familiarize yourself with terms necessary to understand animal body plans.
Sections 25.2, 25.3 Animals most likely evolved from a colony similar to choanoflagellates, a type of protist. Placozoans are the structurally simplest modern animals. The oldest animal fossils, called the Ediacarans, date back about 600 million years. A great adaptive radiation during the Cambrian gave rise to most modern lineages. Relationships among animal groups are still being investigated. For example, recent genetic studies suggest that all invertebrates that molt are closely related. Section 25.4 Sponges are asymmetrical and do not have tissues or organs. They filter food from water and are hermaphrodites: each makes eggs and sperm. Adults stay put, but immature forms, called larvae, swim.
Use the animation on CengageNOW to explore the body plan of a sponge.
Section 25.5 Cnidarians, such as jellyfishes, corals, and sea anemones, are radially symmetrical. They alone make nematocysts, which they use to catch prey and to defend themselves. They have two tissues with a jellylike layer that functions as a hydrostatic skeleton between them. A nerve net controls movements. A gastrovascular cavity functions in both respiration and digestion.
Use the animation on CengageNOW to compare cnidarian body plans and life cycles.
Section 25.6 Flatworms, such as planarians, are bilateral protostomes and the simplest animals to have organ systems. Nerve cords connect to ganglia in the head that serve as a control center. The gut is saclike and a pharynx takes in food and expels waste. Tapeworms are parasitic 430 UNIT IV
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How would you vote? Bottom trawling helps keep seafood prices low, but can destroy invertebrate habitats. Should it be banned? See CengageNOW for details, then vote online.
flatworms with a body made of units called proglottids. Flukes are also parasites.
Use the animation on CengageNOW to learn about flatworm organ systems and life cycles.
Section 25.7 Annelids are segmented worms (such as earthworms and polychaetes) and leeches. Circulatory, digestive, solute-regulating, and nervous systems extend through all coelomic chambers. Nephridia regulate the composition of body fluid.
Use the animation on CengageNOW to investigate the body plan of an earthworm.
Sections 25.8, 25.9 Mollusks have a sheetlike mantle. Most have respiratory gills in the mantle cavity and feed using a food-scraping radula. Examples are chitons; gastropods (such as snails) which undergo torsion; bivalves (such as clams); and cephalopods.
Use the animation on CengageNOW to compare molluscan body plans.
Section 25.10 Rotifers and tardigrades are tiny animals of damp or aquatic habitats. Rotifers have a ciliated head and a pseudocoelom. Tardigrades, or water bears, have a reduced coelom and molt. Both groups can dry out and survive long periods of adverse conditions. Section 25.11 The roundworms (nematodes) have an unsegmented body, a cuticle that is molted, a complete gut, and a false coelom. Some are parasites of humans.
Use the animation on CengageNOW to learn about the roundworm body plan.
Sections 25.12–25.17 Arthropods, the largest phylum of animals, have a jointed exoskeleton, or external skeleton. Most have one or more pairs of sensory antennae. Malpighian tubules expel waste in land-dwelling groups. Chelicerates include the marine horseshoe crabs and the arachnids (spiders, scorpions, ticks, and mites). The mostly marine crustaceans include wood lice, crabs, lobsters, barnacles, krill, and copepods. Myriapods are predatory centipedes and scavenging millipedes. Insects, the most successful arthropods, include the only winged invertebrates. Most insects undergo metamorphosis, a change in body form between larval and adult stages. Insects pollinate plants, dispose of wastes, and serve as food, but some eat crops or transmit disease.
Use the animation on CengageNOW to learn about arthropod life cycles and body plans.
ee e
Data Analysis Exercise Atlantic horseshoe crabs, Limulus polyphemus, have long been ecologically important. For more than a million years, their eggs have fed migratory shorebirds. More recently, people began to harvest horseshoe crabs for use as bait. More recently still, people started using horseshoe crab blood to test injectable drugs for potentially deadly bacterial toxins. To keep horseshoe crab populations stable, blood is extracted from captured animals, which are then returned to the wild. Concerns about the survival of animals after bleeding led researchers to do an experiment. They compared survival of animals captured and maintained in a tank with that of animals captured, bled, and kept in a similar tank. Figure 25.44 shows the results.
Control Animals Trial 1 2 3 4 5 6 7 8 Total
1. In which trial did the most control crabs die? In which did the most bled crabs die? 2. Looking at the overall results, how did the mortality of the two groups differ? 3. Based on these results, would you conclude that bleeding harms horseshoe crabs more than capture alone does?
Number Number of crabs that died 10 10 30 30 30 30 30 30 200
0 0 0 0 1 0 0 0 1
Bled Animals Number of crabs
Number that died
10 10 30 30 30 30 30 30 200
0 3 0 0 6 0 2 5 16
Figure 25.44 Mortality of young male horseshoe crabs kept in tanks during the 2 weeks after their capture. Blood was taken from half the animals on the day of their capture. Control animals were handled, but not bled. This procedure was repeated 8 times with different sets of horseshoe crabs.
Section 25.18 Echinoderms, such as sea stars, are invertebrate members of the deuterostome lineage. They have skin with spines, spicules, or plates of calcium carbonate. A water–vascular system with tube feet helps most glide about. Adults are radial, but bilateral ancestry is evident in their larval stages and other features.
9. A radula is used to a. detect light b. scrape up food
. c. produce silk d. eliminate excess water
10. Barnacles are shelled a. gastropods b. cephalopods
. c. crustaceans d. copepods
11. The include the only winged invertebrates. a. cnidarians c. arthropods b. echinoderms d. placozoans
Use the animation on CengageNOW to examine a sea star body plan and observe tube feet in action.
Self-Quiz
Answers in Appendix III
1. True or false? Animal cells do not have walls. 2. A body cavity fully lined with tissue derived from mesoderm is a . 3. The modern protist group most closely related to animals is the . 4. A filters food from the water and has no tissues or organs. a. sponge c. cnidarian b. roundworm d. flatworm 5. Cnidarians alone have a. nematocysts b. a mantle
. c. a hydrostatic skeleton d. Malpighian tubules
6. Flukes are most closely related to . a. tapeworms c. arthropods b. roundworms d. echinoderms 7. Nephridia have the same functional role as . a. gemmules of sponges c. flame cells of planarians b. mandibles of insects d. tube feet of echinoderms 8. Which invertebrate phylum includes the most species? a. mollusks c. arthropods b. roundworms d. flatworms
12. The
have a coelom and are radial as adults.
13. Match the organisms with their descriptions. choanoflagellates a. complete gut, pseudocoelom placozoan b. sister group to animals sponges c. simplest organ systems cnidarians d. no tissues, filters out food flatworms e. jointed exoskeleton roundworms f. mantle over body mass annelids g. segmented worms arthropods h. tube feet, spiny skin mollusks i. nematocyst producers echinoderms j. simplest known animal
Visit CengageNOW for additional questions.
Critical Thinking 1. Many different species of flatworms, roundworms, and annelids are parasites of mammals. There are no such parasites among sponges, cnidarians, mollusks, and echinoderms. Propose a plausible explanation for this difference. 2. A massive die-off of lobsters in the Long Island Sound was blamed on pesticides sprayed to control the mosquitoes that carry West Nile virus. Why might a chemical designed to kill insects also harm lobsters? CHAPTER 25
ANIMAL EVOLUTION—THE INVERTEBRATES 431
26
Animal Evolution—The Chordates IMPACTS, ISSUES
Transitions Written in Stone
By Charles Darwin’s time, all major groups of organisms had
the supercontinent Pangea. When bodies of organisms fell
been identified. One objection to acceptance of Darwin’s
into this lagoon, fine sediments quickly covered them. Over
theory of evolution by natural selection was the apparent lack
time, the sediments compacted and hardened. They became
of transitional forms between groups. If new species evolve
a stony tomb for more than 600 species, including marine
from older ones, then where were the “missing links,” species
invertebrates, dinosaurs, and Archaeopteryx.
with traits intermediate between two groups? In fact, workmen at a limestone quarry in Germany had
animal diversity. However, fossils are physical evidence of
already unearthed one such link. The pigeon-sized fossil
changes, and radiometric dating assigns the fossils to places
resembled a small dinosaur. It had teeth, three long clawed
in time. The structure, biochemistry, and gene sequences of
fingers on a pair of forelimbs, and a long bony tail. Later, dig-
living organisms provide information about branchings.
gers found another specimen. Later still, someone noticed
The theory of evolution by natural selection provides the
feathers. If they were fossilized birds, then why did they have
best explanation for the observed genetic similarities and
teeth and a bony tail? If dinosaurs, what were they doing with
differences between species and for the transitional forms we
feathers? The specimen was named Archaeopteryx, meaning
observe in the fossil record. Evolutionists often argue over
ancient winged one (Figure 26.1a).
how to interpret data and which of the known mechanisms
So far, a total of eight Archaeopteryx fossils have been
a
No human witnessed the transitions that led to modern
can best explain life’s history. At the same time, they eagerly
excavated, all from German limestone. Radiometric dating
look to new evidence to support or disprove hypotheses. As
(Section 17.6) revealed that Archaeopteryx lived about 150
you will see, fossils and other evidence form the foundation
million years ago, in the late Jurassic. What is now limestone
for this chapter’s account of vertebrate evolution, including
was once sediments in a shallow lagoon near the shore of
the story of our own origins.
b
See the video! Figure 26.1 Placing Archaeopteryx in time. (a) One of the Archaeopteryx fossils from Germany. It clearly shows feathers, a long bony tail, and teeth. No modern bird has a bony tail or teeth. (b) Painting based on fossils of plants and animals that lived in a Jurassic forest. Foreground, two gliding Archaeopteryx. Behind them a huge herbivorous Apatosaurus (a herbivore) is pursued by Saurophaganax (“king of reptile eaters”). In the distant background are Camptosaurus (left) and Stegosaurus (right).
Links to Earlier Concepts
Key Concepts Characteristics of chordates
This chapter continues the story of the deuterostome lineage first described in Section 25.1.
Be sure you understand the processes of gene duplications (12.5), convergent evolution (19.2), adaptation (17.3), allopatric speciation (18.10), and adaptive radiation (18.12). They come up repeatedly. Knowledge of cladistics (19.1, 19.5) will also be important.
You will see how physical factors such as asteroids striking Earth (Chapter 17 introduction) and plate tectonics (17.9) influenced animal evolution and distribution. You may find it useful to refer back to the geologic time scale (17.8).
We will return to the story of amphibian declines (24.2). In considering vertebrate body plans, we contrast the vertebrate endoskeleton with the exoskeleton (25.12) of arthropods.
A unique set of four traits characterizes the chordates: a supporting rod (notochord); a hollow, dorsal nerve cord; a pharynx with gill slits in the wall; and a tail extending past an anus. Certain invertebrates and all vertebrates belong to this group. Section 26.1
Trends among vertebrates In vertebrate lineages, a backbone replaced the notochord. Jaws and fins evolved in water. Fleshy fins with skeletal supports evolved into limbs that allowed some vertebrates to walk onto land. On land, lungs replaced gills and circulation changed in concert. Section 26.2
Transition from water to land Vertebrates evolved in the seas, where cartilaginous and bony fishes still live. Of all vertebrates, modern bony fishes are most diverse. One group gave rise to aquatic tetrapods (four-legged walkers), the descendants of which moved onto dry land. Sections 26.3–26.6
The amniotes Amniotes—reptiles, birds, and mammals—have waterproof skin and eggs, highly efficient kidneys, and other traits that adapt them to a life that is typically lived entirely on land. Reptiles and birds belong to one amniote lineage, and mammals to another. Sections 26.7–26.11
Early humans and their ancestors Changes in climate and available resources were selective forces that shaped the anatomy and behavior of early humans and their primate ancestors. Behavioral and cultural flexibility helped humans disperse from Africa throughout the world. Sections 26.12–26.14
How would you vote? Some private collectors have purchased rare and valuable vertebrate fossils. Private trade raises purchase costs for museums and encourages theft from protected fossil beds. Is the private sale of significant vertebrate fossils unethical? See CengageNOW for details, then vote online.
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26.1
The Chordate Heritage Table 26.1 Chordates are the most diverse lineage of deuterostomes. Some are invertebrates, but most are vertebrates.
Modern Chordate Groups
Link to Animal classification 25.1
Chordate Characteristics The preceding chapter ended with the echinoderms, a phylum of invertebrate deuterostomes. The majority of deuterostomes are chordates (phylum Chordata). Chordate embryos have four defining traits: (1) A rod of stiff but flexible connective tissue, a notochord, extends the length of the body and supports it. (2) A dorsal, hollow nerve cord parallels the notochord. (3) Gill slits open across the wall of the pharynx (throat region). (4) A muscular tail extends beyond the anus. Depending on the chordate group, some, all, or none of these traits persist in the adult. Chordates are bilateral and coelomate (Section 25.1). They show cephalization (sensory structures are concentrated at the head end) and segmentation (paired structures such as muscles are repeated along either side of the long body axis). They have a complete digestive system and a closed circulatory system. Most of the 50,000 or so chordates are vertebrates (subphylum Vertebrata), animals that have a backbone (Table 26.1). The bulk of this chapter describes their traits and evolution. Here we begin our survey with tunicates and lancelets, two groups of marine invertebrate chordates. We also take a brief look at hagfishes, another in-between group.
The Invertebrate Chordates Lancelets (subphylum Cephalochordata) are invertebrate, fish-shaped chordates 3 to 7 centimeters long
a Dorsal, hollow nerve cord
b Notochord
Group
Named Species
Invertebrate chordates: Lancelets Tunicates Craniates: Hagfishes (jawless fishes) Vertebrates: Lampreys (jawless fishes) Jawed fishes: Cartilaginous fishes Bony fishes Amphibians Reptiles Birds Mammals
30 2,150 60 41 1,160 26,000 4,900 8,200 8,600 4,500
For details of chordate classifi cation, see Appendix I.
(Figure 26.2). They retain all characteristic chordate traits as adults. A dorsal nerve cord extends into the head. A single eyespot at the end of the nerve cord detects light, but the head has no brain, braincase, or paired sensory organs like those of fishes. A lancelet wiggles backward into sediments until it is buried up to its mouth, then filters food from the water. Movement of cilia causes water to flow in through the mouth, into the pharynx, then out of the body through gill slits. Cilia also move food particles that get trapped in mucus on the pharynx to the gut. Like vertebrates, lancelets have segmented muscles. Contractile units in muscle cells run parallel with the body’s long axis. The force that muscles direct against the notochord produces a side-to-side motion that allows lancelets to burrow and swim short distances.
c Pharynx with gill slits
d Tail extends beyond anus
eyespot
tentacle-like structures around mouth
segmented muscles (myomeres) epidermis midgut
aorta
gonad
Figure 26.2 Animated Photo and body plan of a lancelet, a small filter-feeder. Like other chordates, it has a dorsal, hollow nerve cord (a), a supporting notochord (b), a pharynx with gill slits (c), and a tail that extends past the anus (d).
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pore of atrial cavity hindgut
anus
As you will see, that is how fish swim and how the first land vertebrates walked. Tunicates (subphylum Urochordata) are invertebrates in which larvae have typical chordate traits, but adults retain only the pharynx with gill slits (Figure 26.3). Larvae swim about briefly, then undergo metamorphosis. The tail breaks down and other parts become rearranged into the adult body form. A secreted carbohydrate-rich covering or “tunic” encloses the adult body and gives the group its common name. Most tunicates are sea squirts that live attached to an undersea surface. When disturbed, they squirt water. Other tunicates, known as salps, drift or swim in the open seas. Both groups filter food from the water. Water flows in an oral opening and past gill slits, where the food sticks to mucus and gets sent to a gut. Water leaves through another body opening. Until recently, lancelets were considered the closest invertebrate relatives of vertebrates. An adult lancelet certainly looks more like fish than an adult tunicate does, but such superficial similarities are sometimes deceiving. New studies of developmental processes and gene sequences indicate that tunicates are the closest living relatives of vertebrates. Keep in mind that neither tunicates nor lancelets are ancestors of vertebrates. These groups share a recent common relative, but each has unique traits that put it onto a separate branch of the animal family tree.
nerve cord
a
notochord
gut pharynx with gill slits
b
pharynx with gill slits c
Figure 26.3 (a,b) Tunicate larva. It swims briefly, then glues its head to a surface and metamorphoses. Tissues of its tail, notochord, and much of the nervous system are remodeled. (c,d) Adult tunicate. Arrows in (c) indicate direction of water flow: in one opening, into the pharynx, through gill slits, then out through another opening.
tentacles
d
gill slits (twelve pairs)
1 cm
mucous glands
A Braincase but No Backbone Fishes, amphibians, reptiles, birds, and mammals are craniates. A cranium—a braincase of cartilage or bone—encloses and protects their brain, and they have paired eyes and other sensory structures on the head. Hagfishes are the only modern chordates that have a cranium, but no backbone (Figure 26.4). Like lancelets, these soft-bodied, jawless fishes have a notochord that supports the body. Like other craniates, a hagfish has paired ears that detect vibrations and a pair of eyes. However, their eye has no lens so their vision is poor. Sensory tentacles near the mouth respond to touch and dissolved chemicals. They help a hagfish find its food—soft invertebrates and dead or dying fish. There are no fins. A hagfish moves with a wriggling motion, similar to that of a lancelet. Hagfishes are sometimes called slime eels because, when threatened, they can secrete a gallon of slimy mucus. Exuding slime is a useful defense for a softbodied animal and it deters most predators. However, it has not kept humans from harvesting hagfish. Most of what is sold as “eelskin” is actually hagfish skin.
Figure 26.4 Hagfish body plan. The two photographs show a hagfish before and after it coated its body with slimy mucous secretions.
Take-Home Message What traits characterize the chordates?
We define chordates based on traits seen in their embryos. Only in one group of invertebrate chordates, lancelets, do these traits persist in adults. Tunicates are the other group of invertebrate chordates.
Hagfishes are the only craniates that are not vertebrates.
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ANIMAL EVOLUTION—THE CHORDATES 435
26.2
Vertebrate Traits and Trends A supportive backbone, a larger brain, and hardened jaws contributed to vertebrate success.
Link to External skeleton 25.12
An Internal Skeleton and a Big Brain Vertebrates have an endoskeleton, an internal skeleton, consisting of cartilage and (in most groups) bone. The endoskeleton encloses and protects internal organs. It also interacts with skeletal muscles to move the body and its parts. Compared with an external skeleton, an internal one provides less protection, but it has other advantages. It consists of living cells, so it grows and does not have to be molted. It allows greater flexibility and speed of movement. It also provides relatively lightweight support that allows animals to grow big. All large land animals are vertebrates. The notochord of a vertebrate embryo develops into a vertebral column, or backbone. This flexible but
lancelets tunicates
hagfishes
lampreys
cartilaginous fishes
ray-finned fishes
sturdy structure is made of many individual skeletal elements called vertebrae. It encloses and protects the spinal cord that develops from the embryonic nerve cord. The anterior end of that nerve cord develops into a brain, which is protected by a cranium. Vertebrate brains are larger and more complex than those of invertebrate chordates. Paired eyes relay information to the brain, as do paired ears. In fishes, paired ears help maintain balance and detect pressure waves in water. When vertebrates moved onto land, ears became modified to detect pressure waves in air. With the exception of fishes called lampreys, all modern vertebrates have jaws (Figure 26.5a). Jaws are hinged skeletal elements used in feeding. The earliest vertebrates were jawless fishes (Figure 26.5b). Jawed fishes called placoderms appeared during the Silurian. They had bony plates on their head and body. Their jaws were expansions of hard parts that structurally supported the gill slits (Figure 26.6).
lobe-finned fishes
lungfishes
amphibians
“reptiles”
birds
mammals
amniotes tetrapods
swim bladder or lungs
jawed vertebrates vertebrates
craniates ancestral chordates a
Origin of the first jawless fishes.
Jawed fishes, including the placoderms and sharks, evolve.
Adaptive radiation of fishes, and the first amphibians move onto land.
Diversification of fishes and amphibians. Armored fishes go extinct.
Ordovician
Silurian
Devonian
Carboniferous
488
443
416
359
Reptiles arise and start to diversify. Early amphibians in decline.
Dinosaurs and marine reptiles evolve.
Birds, mammals, and modern amphibians arise. Dinosaurs dominate.
Dinosaur diversity peaks, then extinction by period’s end.
Permian
Triassic
Jurassic
Cretaceous
299
251
200
146
Adaptive radiation of mammals.
Tertiary 66
b
436 UNIT IV
EVOLUTION AND BIODIVERSITY
Figure It Out: Which tetrapods
are not also amniotes? Answer: Amphibians
Figure 26.5 Animated The chordate family tree. (a) Compare the size of the human with Dunkleosteus, an extinct placoderm. (b) Time line for events in vertebrate evolution. Numbers indicate millions of years ago. Periods are not to scale.
location of spiracle (modified gill slit)
supporting structure for gill slits
jaw, derived from support structure
gill slits A In early jawless fishes, supporting elements reinforced a series of gill slits on both sides of the body.
jaw support jaw
B In early jawed fishes (e.g., placoderms), the first elements were modified and served as jaws. Cartilage reinforced the mouth’s rim.
C Sharks and other modern jawed fishes have strong jaw supports.
Figure 26.6 Animated Comparison of gill-supporting structures.
The evolution of jaws started an arms race between predators and prey. Fishes with a bigger brain that could better plan pursuit or escape had an advantage, as did those that were fast and maneuverable. Fishes evolved fins, body appendages that help them swim. The fins go by these names:
caudal fin
dorsal fin
dorsal fin pectoral fin (pair)
anal fin
pelvic fin (pair)
Vertebrates have a closed circulatory system. Such systems allow faster blood flow than open systems (Section 25.1). Vertebrate circulatory systems evolved in concert with the respiratory system. In fishes, a twochambered heart pumps blood through one circuit: from the heart, to gills, through the body, and back to the heart. In most land vertebrates, the heart is divided into four chambers and pumps blood through two separate circuits. One circuit carries oxygen-poor blood from the heart to the lungs and returns oxygenenriched blood to the heart. The other circuit then pumps this blood to body tissues. Together, lungs and a two-circuit circulatory system enhance the rate of gas exchange and thus sustain a high level of activity.
Other Organ Systems In the Devonian, fishes underwent a great adaptive radiation. Groups with heavy armor died out and a lineage of fishes with bones in the pelvic and pectoral fins arose. This lineage gave rise to amphibians, the first animals with paired limbs, and the vertebrates began to move onto land.
Circulatory and Respiratory Systems In lancelets and tunicates, some gas exchange occurs at gill slits, but most gases just diffuse across the body wall. Paired gills evolved in early vertebrates. Gills are respiratory organs with moist, thin folds that are richly supplied with blood vessels. Gills enhance the exchange of gases and thereby support higher levels of activity than diffusion alone. The force of a beating heart drives blood flow through vessels in fish gills. Gills became more efficient in larger, more active fishes. But gills cannot function out of water. In fishes ancestral to land vertebrates, two small outpouchings on the side of the gut wall evolved into lungs: moist, internal sacs that serve in gas exchange.
Vertebrates have a pair of kidneys, organs that filter blood and adjust the volume and composition of the extracellular fluid. On land, highly efficient kidneys that help conserve water proved advantageous. Vertebrates reproduce sexually and sexes are usually separate. Fishes and amphibians typically release eggs and sperm into water. The reptiles, birds, and mammals have organs that allow fertilization to take place inside a female, and eggs that resist water loss. Vertebrates have a well-developed immune system. Specialized white blood cells allow this system to recognize, remember, and respond fast to pathogens.
Take-Home Message What are vertebrates? Vertebrates are chordates with an internal skeleton that includes a backbone. Most also have jaws. Compared to invertebrate chordates, vertebrates have a larger, more complex brain.
Paired fins in one lineage of fish were the evolutionary predecessors of limbs of land vertebrates. Moving to land also involved modification of circulatory and respiratory systems, a more efficient kidney, and internal fertilization.
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ANIMAL EVOLUTION—THE CHORDATES 437
26.3
26.4
The Jawless Lampreys Lampreys are vertebrates, but they do not have jaws or paired fins as the jawed fishes do.
Jawed fishes come in a great variety of shapes and sizes. Nearly all have paired fins and a body covered with scales.
The 50 or so species of lampreys are an evolutionarily ancient lineage of fishes. Fossils show that their body plan has been basically unchanged since the Devonian. Like hagfishes, lampreys do not have jaws or fins, but lampreys do have a backbone made of cartilage. Lampreys are among the few fishes that undergo metamorphosis. Larval lampreys live in fresh water and, like lancelets, they burrow into sediments and filter food from the water. After several years, body tissues get remodeled into the adult form. About half of the lamprey species remain in fresh water and do not feed as adults. The other half are parasites. Some of these stay in fresh water; others migrate to the sea. Figure 26.7 shows the distinctive mouth of an adult parasitic lamprey. It has an oral disk with horny teeth made of the protein keratin. A parasitic lamprey uses its oral disk to attach to another fish. Once attached, it secretes enzymes and uses a tooth-covered tongue to scrape up bits of the host’s tissues. The host fish often dies from blood loss or a resulting infection. In the early 1800s, sea lampreys invaded the Great Lakes of North America. They probably entered the Hudson River, then made their way through newly built canals. By 1946, lampreys were established in all the Great Lakes. Their arrival caused local extinction of many native fish species. Today, attempts to reduce the lamprey population cost millions of dollars each year and, so far, have had little success.
The Jawed Fishes
Link to Gene duplications 12.5
Most jawed fishes have paired fins and scales: hard, flattened structures that grow from and often cover the skin. Scales and an internal skeleton make a fish body denser than water and thus prone to sinking. Fish that are highly active swimmers have fins with a shape that helps lift them, something like the way that wings help lift up an airplane. Water resists movements through it, so speedy swimmers typically have a streamlined body that reduces friction. There are two groups of jawed fishes: cartilaginous fishes and bony fishes.
a
b
Figure 26.7 Adult parasitic lamprey with eight gill slits on each side of its body and an impressive oral disk. The lamprey latches on to another fish and feeds on its tissues.
c
Take-Home Message What are lampreys? Lampreys are a lineage of jawless fishes that undergo metamorphosis. As adults, about half are ecologically important parasites of other fishes.
438 UNIT IV
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Figure 26.8 Cartilaginous fishes. (a) Manta ray. Two fleshy projections on its head unfurl and funnel plankton to its mouth. (b) Galápagos sharks. Notice the gill slits on both the ray and the shark. (c) The cavernous mouth of a whale shark. The shark is as long as a city bus. Like the manta ray, a whale shark is primarily a plankton feeder.
kidney
swim bladder
nerve cord brain
ovary
a
cloaca
intestine
stomach liver
heart
gills
a
Figure 26.10 An Australian lungfish, a bony fish. In oxygen-poor waters it fills its lungs by rising to the surface and inhaling air.
b
c
d
Figure 26.9 Ray-finned bony fishes. (a) Body plan of a perch. (b) Sea horse. (c) Coral grouper. (d) Long-nose gar, a fast predator.
Cartilaginous Fishes Cartilaginous fishes (Chondrichthyes) include about 850 species of mostly marine sharks and rays. All have a skeleton of cartilage, and five to seven gill slits (Figure 26.8). Their teeth are modified scales hardened with bone and dentin. The teeth grow in rows and are continually shed and replaced. Rays have a flattened body with large pectoral fins. Manta rays filter plankton from water and some are 6 meters (20 feet) wide (Figure 26.8a). Stingrays are bottom feeders. Their barbed tail has a venom gland. Sharks include predators that swim in upper ocean waters (Figure 26.8b), plankton feeders (Figure 26.8c), and bottom feeders that suck up invertebrates and act as scavengers.
Bony Fishes In bony fishes (Osteichthyes) bone replaces cartilage in much of the skeleton. Unlike most cartilaginous fishes, bony fishes have a cover, or operculum, that protects their gills. Bony fishes also typically have a swim bladder: a gas-filled flotation device. By adjusting the volume of gas inside its swim bladder, a bony fish can stay suspended in water at different depths.
Figure 26.11 A coelacanth (Latimeria), a lobe-finned fish. It is a bony fish that has skeletal elements in its pelvic and pectoral fins.
The three bony fish subgroups are ray-finned fishes, lungfishes, and lobe-finned fishes. Ray-finned fishes (Figure 26.9) have thin, flexible fin supports derived from skin. With 21,000 species, they are the most diverse fishes. Teleosts, the largest rayfinned group, includes the fishes in Figure 26.9a–c, as well as most fishes we eat. Long ago, the entire teleost genome was duplicated. Mutations in copied genes may have facilitated diversification of this group. Lungfishes (Figure 26.10) are bony fishes that have gills and lunglike sacs—modified outpouchings of the gut wall. They fill the sacs by surfacing and gulping air, then oxygen diffuses from the sacs into the blood. Coelacanths (Latimeria) are the only modern group of lobe-finned fishes. The two populations we know about may be separate species. Their ventral fins are fleshy extensions of the body wall and have internal skeletal elements (Figure 26.11). Lobe-finned fishes are the fish most closely related to amphibians. Take-Home Message What are the characteristics of jawed fishes? Jawed fishes are cartilaginous fishes and bony fishes. Both groups typically have scales. The ray-finned lineage of bony fishes is the most diverse group of vertebrates. Lobe-finned fishes are the fish closest to the amphibians.
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ANIMAL EVOLUTION—THE CHORDATES 439
26.5
Amphibians—First Tetrapods on Land Amphibians spend part of their life on land, but most still return to water to breed.
Link to Homologous structures 19.2
Adapting to Life on Land Amphibians are land-dwelling vertebrates that need water to breed and have a three-chambered heart. Their lineage branched from that of lobe-finned fishes during the Devonian. Fossils show how the skeleton was modified as fishes adapted to swimming evolved into four-legged walkers, or tetrapods (Figure 26.12). Bones of a fish’s pelvic and pectoral fins are homologous with amphibian limb bones (Section 19.2). The transition to land was not simply a matter of skeletal changes. Division of the heart into three chambers allowed flow in two circuits, one to the body and one to the increasingly important lungs. Changes to the inner ear improved detection of airborne sounds. Eyes became protected from drying out by eyelids. What was the selective advantage to living on land? An ability to survive out of water would have been favored in seasonally dry places. Also, on land, individuals escaped aquatic predators and had new food— insects, which also evolved during the Devonian.
a
b
Figure 26.13 (a) Red-spotted salamander, with equal-sized forelimbs and hindlimbs. (b) A legless caecilian.
and sperm into water. Their aquatic larvae have gills. Larvae feed and grow until hormonal changes cause them to metamorphose into adults. Most species lose their gills and develop lungs during this transition. However, a few salamanders retain gills as adults. Others lose gills and exchange gases across their skin. The 535 species of salamanders and related newts live mainly in North America, Europe, and Asia. In body form, they are the modern group most like early
Modern Amphibians The three subgroups of modern amphibians are the salamanders, the caecilians, and the frogs and toads. All are carnivores as adults. Amphibians release eggs
a
b
c
Figure 26.12 Skeleton of a Devonian lobe-finned fish (a), and two early amphibians, Acanthostega (b), and Ichthyostega (c). The painting (d) shows what Acanthostega (foreground), and Ichthyostega (background) may have looked like.
440 UNIT IV
EVOLUTION AND BIODIVERSITY
d
FOCUS ON THE ENVIRONMENT
26.6
Vanishing Acts
Amphibians depend on access to standing water to breed and have a thin skin unprotected by scales. These features make them vulnerable to habitat loss, disease, and pollution.
a
b
Figure 26.14 (a) Adult frog displaying the power of its well developed hindlimbs. (b) A larval frog, or tadpole.
tetrapods. Forelimbs and back limbs are of similar size and there is a long tail (Figure 26.13a). As salamanders walk, their body bends from side to side, like the body of a swimming fish. Their ancestors that first ventured onto land probably moved in a similar way. Caecilians are close relatives of salamanders that have adapted to a burrowing way of life. They include about 165 limbless, blind species (Figure 26.13b). Most caecilians burrow through soil and use their senses of touch and smell to pursue invertebrate prey. Frogs and toads belong to the most diverse amphibian lineage; there are more than 5,000 modern species. Muscular, elongated hindlegs allow the tailless adults to swim, hop, and make leaps that can be spectacular, given their body size (Figure 26.14a). The forelegs are much smaller and help absorb the impact of landings. Larvae of salamanders and caecilians have a body shape more or less like that of an adult, except for the presence of gills. In contrast, the larvae of frogs and toads are markedly different from adults. The larvae have gills and a tail, but no limbs. They are commonly known as tadpoles (Figure 26.14b).
Links to Chytrid fungi 24.2, Flukes 25.6
There is no question that amphibians are in trouble. Of about 5,500 known species, population sizes of at least 200 are plummeting. The alarming declines have been best documented in North America and Europe, but the changes are happening worldwide. At this writing, six frog species, four toad species, and eleven salamander species are considered threatened or endangered in the United States and Puerto Rico. One, the California red-legged frog (Rana aurora), inspired Mark Twain’s well-known short story, “The Celebrated Jumping Frog of Calaveras County.” This species is the largest frog native to the western United States. Researchers correlate many declines with shrinking or deteriorating habitats. Developers and farmers commonly fill in low-lying ground that once collected seasonal rains and formed pools of standing water. Nearly all amphibians need to deposit their eggs and sperm in water, and their larvae must develop in water. Also contributing to declines are introductions of new species in amphibian habitats, long-term shifts in climate, increases in ultraviolet radiation, and the spread of certain pathogens and parasites. Section 24.2 discussed chytrid infections of amphibians and Figure 26.15 provides an example of the effects of a parasitic fluke (a subgroup of flatworms). Chemical pollution of aquatic habitats also harms amphibians. We will consider the negative effects of one agricultural chemical on frogs in Chapter 35.
a
Take-Home Message
a
What are amphibians? Amphibians are vertebrates with a three-chambered heart. They start life in water as gilled larvae, then undergo metamorphosis. Adults typically have lungs and are carnivores on land.
b
Figure 26.15 (a) Example of frog deformities. (b) A parasitic fluke (Ribeiroia). It burrows into limb buds of frog tadpoles and physically or chemically alters individual cells. Infected tadpoles grow extra legs or none at all. Where Ribeiroia populations are most dense, the number of tadpoles that successfully complete metamorphosis is low. Nutrient enrichment of water by fertilizers and pesticide contamination make frogs more easily infected.
CHAPTER 26
ANIMAL EVOLUTION—THE CHORDATES 441
26.7
The Rise of Amniotes Amniotes took waterproofing to a new level with their skin and eggs, making them well adapted to dry habitats.
Links to Geologic time scale 17.8, Cladistics 19.5
In the late Carboniferous, one amphibian lineage gave rise to the “stem” reptiles, the first amniotes. Amniotes make eggs having four unique membranes that allow embryos to develop away from water (Figures 26.16a,b and 26.21). Amniotes have waterproof skin and a pair of efficient kidneys. Nearly all fertilize eggs in the female’s body. These traits adapt them to life on land.
a
One early branching of the amniote lineage led to synapsids: mammals and extinct mammal-like species (Figure 26.16c). A now extinct synapsid subgroup, the therapsids, included ancestors of mammals, as well as Lystrosaurus, a tusked herbivore shown in Figure 17.17. Three other branches of the amniote lineage have survived. One branch led to turtles, one to lizards and snakes, and the third to crocodilians and birds. As you can see, the traditional division of birds and “reptiles” into separate classes does not reflect phylogeny; the reptiles are not a clade (Section 19.5). Nevertheless, the term reptile persists as a way to refer to amniotes that lack the defining traits of birds or mammals. That is how we use it in this book. The earliest reptiles had a lizardlike body. With well-muscled jaws and sharp teeth, they could seize and kill their prey with more force than amphibians. Waterproof scales rich in the protein keratin covered the body and suited reptiles to drier habitats, but such scales also prevented gas exchange across the skin. Compared to amphibians, the early reptiles had larger, more efficient lungs. They also had larger brains that allowed more complex behavior.
b
snakes lizards
“stem” reptiles
tuataras
Figure 26.16 Amniote eggs and phylogeny.
ichthyosaurs
(a) A painting of a nest of a duck-billed dinosaur (Maiasaura) that lived about 80 million years ago in what is now Montana. Like the modern crocodilians and birds, this dinosaur protected its eggs in a nest and may have cared for the hatchlings. (b) Two eastern hognose snakes emerging from amniote eggs. (c) Family tree for amniotes. Snakes, lizards, tuataras, birds, crocodilians, turtles, and mammals are modern amniote c groups.
442 UNIT IV
plesiosaurs birds therapod dinosaurs
other dinosaurs pterosaurs archosaurs crocodilians turtles anapsids therapsids mammals
synapsids CARBONIFEROUS
PALEOZOIC ERA
EVOLUTION AND BIODIVERSITY
PERMIAN
TRIASSIC
JURASSIC
MESOZOIC ERA
CRETACEOUS
TERTIARY TO PRESENT
FOCUS ON EVOLUTION
26.8
So Long, Dinosaurs
The effects of an asteroid impact on life on Earth are vividly illustrated by the story of the dinosaurs’ demise.
Figure 26.17 Temnodontosaurus. This ichthyosaur hunted large squids, ammonites, and other prey in the warm, shallow seaways of the early Jurassic. Fossils that measure 30 feet (9 meters) long have been found in England and Germany.
Biologists define the reptiles known as dinosaurs by certain skeletal features, such as the configuration of their pelvis and hips. Dinosaurs evolved by the late Triassic. Early species were the size of a turkey and ran on two legs. Adaptive zones opened up for this lineage as the Jurassic began, after fragments from an asteroid or comet hit what are now France, Quebec, Manitoba, and North Dakota. Nearly all the animals that survived these asteroid impacts were small, had high metabolic rates, and could tolerate big changes in temperature. Surviving dinosaur groups, such as those shown in Figure 26.1, became the “ruling reptiles.” For 125 million years they dominated the land even as other groups, including the ichthyosaurs, flourished in the seas (Figure 26.17). Many kinds of dinosaurs were lost in a mass extinction that ended the Jurassic. Others died off during the Cretaceous. As the Cretaceous ended, another asteroid impact wiped out many groups. Feathered dinosaurs ancestral to birds survived, as did ancestors of modern reptiles: crocodilians, turtles, tuataras, snakes, and lizards.
Take-Home Message What are amniotes? Amniotes are animals whose embryos develop inside a waterproof egg. They also have waterproof skin and highly efficient kidneys that reduce water loss. Dinosaurs are extinct amniotes and birds are their descendants. Reptiles and mammals are the other modern amniotes.
Link to Asteroid impacts (Chapter 17 introduction)
Chapter 17 made reference to a mass extinction that defines the Cretaceous–Tertiary (K–T) boundary. After methodically analyzing the elemental composition of soils, maps of gravitational fields, and other evidence from around the world, Walter Alvarez and Luis Alvarez developed a hypothesis: A direct hit by an enormous asteroid caused the K–T extinction event. This came to be known as the K–T asteroid impact hypothesis. Later, researchers discovered an enormous impact crater on the seafloor in the Gulf of Mexico. Known as the Chicxulub Crater, it is 9.6 kilometers deep and about 300 kilometers across. By one estimate, to make a crater that big, the impact would have blasted least 200,000 cubic kilometers of dense gases and debris into the sky. Did this impact cause the K–T extinction event? Many researchers think so. However, Gerta Keller and others hold that the Chicxulub Crater was formed 300,000 years before the K–T extinction. They hypothesize that a series of asteroid impacts occurred and that the crater formed by the impact at the K–T boundary is yet to be discovered. Researchers also debate the mechanism by which an asteroid impact could have caused the known extinctions. Some argue that atmospheric debris must have blocked sunlight for months, causing a deep, dark freeze that killed plants and starved animals. They are trying to explain the fossil record, which shows that land plant and animal species died off. Others think that the volume of debris blasted aloft would not have been great enough to cause the widespread extinctions recorded in the fossil record. An alternative scenario was proposed after the comet Shoemaker–Levy 9 slammed into Jupiter in 1994. Debris blasted into the Jovian atmosphere and triggered intense heating. That event led Jay Melosh and his colleagues to propose that an enormous asteroid impact raised Earth’s atmospheric temperature by thousands of degrees. In one terrible hour, the world erupted in flames. Any animals out in the open—including nearly all dinosaurs—were broiled alive. Not every living thing disappeared. Snakes, lizards, crocodiles, and turtles survived, as did birds and mammals. The proponents of Melosh’s hypothesis argue that smaller species may have escaped the firestorm by burrowing underground. Critics point out that most birds are ill equipped to burrow. Also, many of the invertebrate species that lived on the ocean floor disappeared, too. How could they have “broiled” deep underwater? In short, one or more asteroids are implicated An asteroid impact in the K–T extinctions. Where they hit and exactly ended the golden age of dinosaurs. what happened next remains an open question.
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ANIMAL EVOLUTION—THE CHORDATES 443
26.9
Diversity of Modern Reptiles Reptile have a scale-covered body. Most have four limbs of approximately equal size, but the snakes are limbless.
General Characteristics “Reptile” is derived from the Latin repto, which means to creep. Some reptiles do creep. Others swim or race or lumber about. Modern reptiles include about 8,160 species. Figure 26.18 shows a typical body plan. Like fishes, reptiles have scales. However, reptile scales develop from the outer layer of skin (epidermis), whereas fish scales arise from a deeper layer (dermis). Like amphibians and fishes, reptiles have a cloaca, an opening that expels digestive and urinary waste and functions in reproduction. All male reptiles, except tuataras, have a penis and fertilize eggs in a female’s body. In most groups, females lay eggs which develop on land. In some lizards and snakes, eggs are held in a female’s body and young are born fully developed. Also like amphibians and fishes, all modern reptiles are ectotherms; their body temperature is determined by the temperature of their surroundings. Reptiles in temperate regions spend the cold season inactive in a burrow on land, or in the case of some freshwater turtles, beneath the mud at a lake bottom.
Major Groups turtles and tortoises is a bony, scale-covered shell that connects to the backbone (Figure 26.19a,b). Turtles do not have teeth; a “beak” made of keratin covers their jaws. Some feed on plants and others are predators. Many sea turtles are endangered. Adults travel back to the same tropical beach where they were hatched to mate and lay their eggs. An increasing human presence on these beaches threatens these species.
olfactory lobe (sense of smell)
are the most diverse reptiles. The smallest fits on a dime (left). The largest, the Komodo dragon, can reach 3 meters (10 feet) in length. It is an ambush predator that snags prey with its peglike teeth. Its saliva contains deadly pathogenic bacteria. Chameleons are lizards that catch prey with a sticky tongue that can be longer than their body. Iguanas are herbivorous lizards. Lizards have interesting defenses to avoid becoming prey themselves. Some try to outrun a predator or startle it (Figure 26.19c, d). Many can detach their tail. The detached tail wriggles briefly, which may distract a predator from its fleeing, tailless owner. Tuataras The two species of tuatara that live on small
islands near the coast of New Zealand are all that remains of a lineage that thrived during the Triassic. Tuatara means “peaks on the back” in the language of New Zealand’s native Maori people. This name refers to a spiny crest (Figure 26.19e). Tuataras are reptiles but walk like salamanders and have amphibian-like brain structures. Also, a third eye develops under the skin of the forehead. It becomes covered by scales in adults and its function, if any, is unclear. Snakes During the Cretaceous, snakes evolved from
Turtles The unique feature of the 300 or so species of
Figure 26.18 Animated Body plan of a crocodile. The heart has four chambers, so blood flows through two entirely separate circuits. This keeps oxygen-poor blood returning from the body from mixing with oxygen-rich blood from the lungs.
Lizards With 4,710 species, lizards
short-legged, long-bodied lizards. Some of the 2,995 modern snakes still have bony remnants of hindlimbs, but most are limbless. All are carnivores. Many have flexible jaws that help them swallow prey whole. All snakes have teeth; not all have fangs. Rattlesnakes and other fanged types bite and subdue prey with venom made in modified salivary glands (Figure 26.19f ). On average, only about 2 of the 7,000 snake bites reported annually in the United States are fatal.
hindbrain, midbrain, spinal forebrain cord vertebral column
gonad
kidney (control of water, solute levels in internal environment)
snout
unmatched rows of teeth on upper and lower jaws
444 UNIT IV
esophagus
EVOLUTION AND BIODIVERSITY
lung
heart
liver
stomach
intestine
cloaca
c
a
hard shell
d
vertebral column
b
Figure 26.19 (a) Galápagos tortoise. (b) Turtle shell and skeleton. Most turtles can pull their head into their shell when threatened, however the shell is reduced in some sea turtles.
e venom gland
(c) A lizard fleeing and (d) lizard confronting a threat. (e) Tuatara (Sphenodon). (f) Rattlesnake. (g) Spectacled caiman, a crocodilian, showing its peglike teeth. Upper and lower teeth do not align as they do in mammals.
Almost a dozen species of crocodiles, alligators, and caimans are the closest living relatives of birds. All are predators in or near water. They have powerful jaws, a long snout, and sharp teeth (Figures 26.18 and 26.19g). They clench prey, drag it under water, tear it apart, and then gulp down the chunks. Crocodilians are the only reptiles that have a fourchambered heart, as mammals and birds do. Such a heart prevents the mixing of oxygen-poor blood from tissues with oxygen-rich blood from the lungs. Crocodilians are the closest living relatives of birds, and like birds they display complex parental behavior. For example, they build and guard a nest, then feed and care for the hatchlings.
hollow fang
Crocodilians
f
Take-Home Message What are the modern reptiles like? Reptiles range in size from tiny lizards to giant crocodiles. Some are aquatic, but most live on land. All lay eggs on land. There are herbivores, but the majority are carnivores. g
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ANIMAL EVOLUTION—THE CHORDATES 445
26.10 Birds—The Feathered Ones In one group of dinosaurs, the scales became modified as feathers. Birds are modern descendants of this group.
yolk sac
embryo
amnion
chorion
allantois
Links to Beak morphology 17.3,18.10
From Dinosaurs to Birds Birds are the only living animals that have feathers. Feathers are modified reptilian scales. Sinosauropteryx prima, a small carnivorous dinosaur that lived in the late Jurassic, was covered with fine, downy feathers (Figure 26.20a). Similar feathers give juvenile birds a fluffy appearance (Figure 26.20b). Downy feathers do not allow flight, but they provide insulation. Archaeopteryx, described and shown in the chapter introduction, was like modern birds in having both short, downy feathers and flight feathers. However, this early bird had teeth and a long bony tail. Confuciusornis sanctus is the earliest known bird with a beak like that of modern birds (Figure 26.20c,d). Its tail was short, with long feathers. Still, its dinosaur ancestry remains apparent—it had grasping, clawed digits at the front tips of its wings.
a
c
b
d
Figure 26.20 (a) A feathered dinosaur, Sinosauropteryx prima. It was covered with downy feathers like those of a modern chick (b). The early bird Confuciusornis sanctus (c), lived about the same time as Archaeopteryx. It had a short tail with long feathers, wings with grasping digits and claws, and a toothless beak, similar to that of a modern bird such as this cardinal (d).
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hardened shell
albumin (“egg white”)
Figure 26.21 Animated A bird egg, a type of amniote egg with four membranes outside the embryo. The chorion assists in gas exchange; the amnion secretes fluid that keeps the embryo moist; the allantois stores waste; and the yolk stack holds yolk that nourishes the developing embryo.
General Characteristics Like other amniotes, birds produce eggs with internal membranes (Figure 26.21). In birds, a shell hardened with calcium encloses the egg. Fertilization is internal. Males do not have a penis; sperm is transferred from the male’s cloaca to the female’s. Birds do not have teeth. Instead, jaw bones covered with layers of the protein keratin form a horny beak. Beak shape varies, with different types of beaks suited to different diets (Sections 17.3, 18.10). Birds are endotherms, which means “heated from within.” Physiological mechanisms allow endotherms to maintain their body temperature within a limited range. A bird’s downy feathers slow the loss of metabolic heat. Feathers also act as a water-shedding body covering, play a role in courtship, and allow flight. Flight feathers are just one of the adaptations that helps birds fly. Birds also have a lightweight skeleton, powerful flight muscles, and highly efficient respiratory and circulatory systems. A wing is a modified forelimb, with feathers that extend outward, increasing its surface area (Figure 26.22a,b). The feathers give the wing a shape that helps lift the bird as air passes over it. Air cavities inside bones keep body weight low, and make it easier for a bird to get and remain airborne. The flight muscles connect an enlarged breastbone, or sternum, to bones of the upper limb (Figure 26.22c). Flight requires a lot of energy, which is provided by aerobic respiration (Section 8.1). To ensure adequate oxygen supply, birds have a unique system of air sacs that keeps air flowing continually through their lungs. A four-chamber heart pumps blood through two fully separated circuits.
a
Flight also requires a great deal of coordination. Much of the bird brain controls movement. Birds also have excellent vision, including color vision.
Bird Diversity and Behavior The approximately 9,000 named species of birds vary in size, proportions, coloration, and capacity for flight. A bee hummingbird weighs 1.6 grams (0.6 ounces). The ostrich, a flightless sprinter, weighs 150 kilograms (330 pounds). More than half of all bird species belong to the subgroup of perching birds. Among them are familiar sparrows, jays, starlings, swallows, finches, robins, warblers, orioles, and cardinals (Figure 26.20d). We see one of the most impressive forms of behavior among birds that migrate with the changing seasons. Migration is a recurring movement from one region to another in response to some environmental rhythm. Seasonal change in daylength is one cue for internal timing mechanisms called “biological clocks.” Such a clock triggers physiological and behavioral changes that induce migratory birds to fly between breeding grounds and wintering grounds. Many types of birds migrate long distances. They use the sun, stars, and Earth’s magnetic field as directional cues. Arctic terns make the longest migrations. They spend summers in the arctic and winters in the antarctic.
Take-Home Message What are birds? Birds are the only living animals with feathers. They evolved from dinosaurs and have a body adapted for flight. Bones are lightweight; air sacs increase the efficiency of respiration; and a four-chambered heart keeps blood moving rapidly.
b
skull
radius pectoral girdle
ulna
humerus
internal structure of bird limb bones
pelvic girdle sternum (breastbone) two main flight muscles attached to keel of sternum c
Figure 26.22 Animated Adaptations for flight. (a) Birds fly by flapping their wings. The downstroke provides lift. (b) Some birds, like this Laysan albatross, have wings that let them glide long distances. With wings more than 2 meters (6.5 feet) across, this bird weighs less than 10 kilograms (22 pounds). It is so at home in the air that it sleeps while aloft. (c) A bird’s skeleton is made up of lightweight bones with internal air pockets. The wing is a modified forelimb (see Figure 19.5). Powerful flight muscles attach to a large breastbone, or sternum.
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26.11 The Rise of Mammals Mammals scurried about while dinosaurs dominated the land, then radiated once they were gone. Links to Morphological convergence 19.2, Plate tectonics 17.9, Adaptive radiation 18.12
Mammalian Traits Mammals are animals in which females nourish their offspring with milk they secrete from mammary glands (Figure 26.23a). The group name is derived from the Latin mamma, meaning breast. Milk is a nutrient-rich food source that also contains immune system proteins that help protect offspring from disease. Mammals are the only animals that have hair or fur. Both are modifications of scales. Like birds, mammals are endotherms. A coat of fur or head of hair helps them maintain their core temperature. Most mammals have whiskers, stiffened hairs on the face that serve a sensory purpose. Mammals are the only animals that sweat, although not all mammals can do so.
incisors
b
molars
premolars
canines
Figure 26.23 Distinctly mammalian traits. (a) A human baby, already with a mop of hair, being nourished by milk secreted from the mammary gland in a breast. (b) Four types of teeth and a single lower jaw bone.
a
Mammalian Evolution As noted earlier (Figure 26.16), mammals belong to the synapsid branch of the amniote lineage. The earliest mammals appeared when the dinosaurs were becoming dominant. Monotremes (egg-laying mammals) and marsupials (pouched mammals) both evolved during the Jurassic. Placental mammals evolved a bit later, in the Cretaceous. Placental mammals are named for their placenta, an organ that allows materials to pass between a mother and an embryo developing inside her body. Placental embryos grow faster than those of other mammals. Also the offspring are born more fully formed and thus are less vulnerable to predation. Continental movements affected the evolution and dispersion of mammal groups. Because monotremes and marsupials evolved while Pangea was intact, they dispersed across this supercontinent (Figure 26.24a). Placental mammals evolved after Pangea had begun breaking up (Figure 26.24b). As a result, monotremes
southern land mass
Pangea
A About 150 million years ago, during the Jurassic, the first monotremes and marsupials evolved and migrated through the supercontinent Pangea.
Only mammals have four different kinds of teeth (Figure 26.23b). In other vertebrates, an individual’s teeth may vary a bit in size, but they are all the same shape. Mammals have incisors that can be used to gnaw, canines that tear and rip flesh, and premolars and molars that grind and crush hard foods. Not all mammals have all four tooth types, but most have some combination. Having a variety of different kinds of teeth allows mammals to eat a wider variety of foods than most other vertebrates. Like birds and crocodilians, mammals have a fourchambered heart that pumps their blood through two fully separate circuits. Gas exchange occurs in a pair of well-developed lungs.
B Between 130 and 85 million years ago, during the Cretaceous, placental mammals arose and began to spread. Monotremes and marsupials that lived on the southern land mass evolved in isolation from placental mammals.
C Starting about 65 million ago, mammals expanded in range and diversity. Marsupials and early placental mammals displaced monotremes in South America.
D About 5 million years ago, in the Pliocene, advanced placental mammals invaded South America. They drove most marsupials and the early placental species to extinction.
Figure 26.24 Animated Effects of continental drift on the evolution and distribution of mammalian lineages.
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Figure 26.25 Paleocene mammals in a sequoia forest in what is now Wyoming. With the exception of the marsupial on the tree branch, all are members of now extinct mammalian lineages.
Figure 26.26 Indricotherium, the “giraffe rhinoceros.” At 15 tons and 5.5 meters (18 feet) high at the shoulder, it is the largest land mammal we know about. It lived in Asia during the Oligocene and is a relative of the rhinoceros.
Figure 26.27 Example of convergent evolution. (a) Australia’s spiny anteater, one of only three modern species of monotremes. (b) Africa’s aardvark and (c) South America’s giant anteater. Compare the antsnuffling snouts. a
b
egg-laying mammal
and marsupial mammals on land masses that broke early from Pangea, lived for millions of years in the absence of placental mammals. For example, Australia split off from Pangea early on and so does not have native placental mammals. Australia remains a separate continent, but continental movement reunited other land masses. When placental mammals entered regions where they were previously unknown, the native monotreme and marsupial populations declined. The new arrivals often outcompeted them and, in many cases, drove them to local extinction (Figure 26.24c,d). After the dinosaurs disappeared at the end of the Cretaceous, mammals underwent the great adaptive radiation illustrated earlier in Figure 18.26. Figures
pouched mammal
c
placental mammal
26.25 and 26.26 provide examples of some of the resulting diversity. Members of different mammal lineages adapted to similar habitats on different continents. For example, Australia’s spiny anteater, South America’s giant anteater, and Africa’s aardvark all hunt ants using their long snout (Figure 26.27). The similar snouts are an example of morphological convergence (Section 19.2). Take-Home Message What are mammals?
Mammals are animals that nourish young with milk and have hair or fur. Their four kinds of teeth allow them to eat many different kinds of foods. Mammals originated in the Jurassic, then underwent an adaptive radiation after dinosaurs died out. Continental movements influenced mammal distribution.
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26.12 Modern Mammalian Diversity Mammals successfully established themselves on every continent and in the seas. What are the existing species like?
Figure 26.28 Female platypus, a monotreme, with two young that hatched from eggs with a rubbery shell. She has a beaverlike tail, a ducklike bill, and webbed feet. Sensory receptors in the bill help a platypus find prey under water. Platypuses burrow into riverbanks using claws exposed when they retract the webbing on their feet. Both males and females have spurs on their hind feet. The male’s spurs deliver venom, making them the only venomous mammals.
Egg-Laying Monotremes Three species of monotremes still exist. Two are spiny anteaters, and one of these is shown in Figure 26.27a. The third species is the platypus (Figure 26.28). All female monotremes lay and incubate eggs that have a leathery shell like that of reptiles. Offspring hatch in a relatively undeveloped state—tiny, hairless, and blind. Young cling to the mother or are held in a skin fold on her belly. Milk oozes from openings on the mother’s skin; monotremes do not have nipples.
Pouched Marsupials Most of the 240 modern species of marsupials live in Australia and on nearby islands. Groups include kangaroos, the koala (Figure 26.29a), and the Tasmanian devil (Figure 26.29b). The opossum (Figure 26.29c) is the only marsupial native to North America. Young marsupials develop briefly in their mother’s body, nourished by egg yolk and by nutrients that diffuse from maternal tissues. They are born at an early developmental stage, and crawl along their mother’s body to a permanent pouch on her ventral surface. They attach to a nipple in the pouch, suckle, and grow.
Placental Mammals
b
a
c
Figure 26.29 Marsupials. (a) A koala, Phascolarctos cinereus, from Australia. It eats only eucalyptus trees and is threatened by destruction of native forests for development. (b) A young Tasmanian devil shows its teeth in a defensive display. It is the only carnivorous marsupial surviving in the wild. (c) A female opossum with her four genetically identical offspring. They form when a single embryo splits in early development.
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Compared to other mammals, the placental mammals develop to a far more advanced stage inside their mother’s body. An organ called the placenta allows materials to pass between maternal and embryonic bloodstreams (Figure 26.30a). The placenta transfers nutrients more efficiently than diffusion does, allowing the embryo to grow faster. After birth, young suckle milk from nipples on the mother’s ventral surface. Placental mammals are now the dominant mammals in most land habitats, and the only ones that live in the seas. Figure 26.30b–i shows a few of the more than 4,000 species. Appendix I lists major groups. Nearly half of mammal species are rodents and, of those, about half are rats. The next most diverse group is the bats, with about 375 species. Bats are the only flying mammals. Although some may resemble flying mice, bats are more closely related to carnivores such as wolves and foxes than to rodents. Take-Home Message What are living mammals like? Most mammals living today are placental mammals. Of these, the rodents and bats are the most diverse groups.
placenta
uterus
embryo
a
Figure 26.30 Placental mammals. (a) Location of the placenta in a pregnant human female. (b) Blue whale and (c) the size of its skeleton relative to a human. At 200 tons, an adult is the largest living animal. (d) A Florida manatee eats plants in warm coastal waters and rivers. (e) A camel traverses hot deserts. (f) Harp seals pursue prey in frigid waters and rest on ice. (g) Flying squirrel, really just a glider. The only flying mammals are bats (h); this one is a Kitti’s hog-nosed bat. (i) Red fox hiding in blue spruce. Thick, insulating fur protects it from winter cold. f
e
b
g
c
d
h
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26.13 From Early Primates to Hominids The primates are the mammalian subgroup to which humans and our closest relatives belong.
Link to Adaptive traits 17.3
Primates include 260 species of prosimians, monkeys, apes, and humans (Figure 26.31). Prosimians (“before monkeys”) evolved first. Modern prosimians include tarsiers and lemurs in Africa, Asia, and Madagascar (Page 17 shows a newly discovered lemur species). Anthropoids include monkeys, apes, and humans; all are widely distributed. Hominoids include apes and humans. Our closest living relatives are chimpanzees and bonobos (previously called pygmy chimpanzees). Humans and extinct humanlike species are hominids. Table 26.2 summarizes primate subgroups.
Table 26.2
Primate Classification
Prosimians
Lemurs, tarsiers
Anthropoids
New World monkeys (e.g., spider monkeys) Old World monkeys (e.g., baboons, macaques) Hominoids: Hylobatids (gibbons, siamangs) Pongids (orangutans, gorillas, chimpanzees, bonobos) Hominids (humans, extinct humanlike species)
Overview of Key Trends Five trends that led to uniquely human traits began in early tree-dwelling species. They came about through modifications to the eyes, bones, teeth, and brain. Enhanced daytime vision. Early primates had an eye on each side of a mouse-shaped head. Later on, some had a more upright, flattened face, with eyes up front. The ability to focus both eyes on an object improved depth perception. Also, eyes became more sensitive to variations in light intensity and in color. During this time, the sense of smell declined in importance. Upright walking. Humans are bipedal: their skeleton and muscles are adapted for upright standing and walking. For example, their S-shaped backbone keeps the head and torso centered over the feet, and arms are shorter than legs. By contrast, prosimians and monkeys move about on four legs, all about the same length. Gorillas walk on two legs while leaning on the knuckles of longer arms (Figure 26.31c,e). How can we determine whether a fossil primate was bipedal? The position of the foramen magnum, an opening in the skull, is one clue. This opening allows the brain to connect with the spinal cord. In animals that walk on all fours, the foramen magnum is located at the back of the skull. In upright walkers, it is near the center of the skull’s base (Figure 26.32). Better grips. Early mammals spread their toes apart to support their weight as they walked or ran on four legs. In ancient tree-dwelling primates, hands became
d
a
b
c
Figure 26.31 Primates. (a) Tarsier, a prosimian climber and leaper. (b) Spider monkey, an agile climber. (c) Male gorilla, using its forearms to support its weight as it walks on its knuckles and two legs. Comparisons of the skeletal structure of (d) a monkey, (e) a gorilla, and (f) a human. Skeletons are not to the same scale.
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e
f
a Hole at back of skull; the backbone is habitually parallel with ground or a plant stem
b Hole close to center of base of skull; the backbone is habitually perpendicular to ground
b
Figure 26.32 A hole in the head, the foramen magnum, in (a) a four-legged walker and (b) an upright walker. Position of this hole helps us determine if a fossil species was bipedal.
modified. Fingers could curl around things (prehensile movements), and the thumb could touch all fingertips (opposable movements). In time, hands were freed from load-bearing functions and were modified in ways that allowed powerful or precision gripping: power grip
precision grip
Having a capacity for versatile hand positions gave the ancestors of humans the ability to make and use tools. Refined prehensile and opposable movements led to the development of technologies and culture. Modified jaws and teeth. Modifications to the jaws correlate with a shift from eating insects, to fruits and leaves, to a mixed, or omnivorous, diet. Rectangular jaws and long canine teeth evolved in monkeys and apes. A bow-shaped jaw and smaller, more uniformly sized teeth evolved in the early hominids. Brain, behavior, and culture. The braincase and the brain increased in size and complexity. As brain size increased, so did length of pregnancy and extent of maternal care. Compared to early primates, later groups had fewer offspring and invested more in them. Early primates were solitary. Later, some started to live in small groups. Social interactions and cultural traits started to affect reproductive success. Culture is the sum of all learned behavioral patterns transmitted among members of a group and between generations.
c
a
Figure 26.33 (a) Southeast Asian tree shrew (Tupaia), a close relative of modern primates. Skull comparisons: (b) Plesiadapis, an early, shrewlike primate. (c) Monkey-sized Aegyptopithecus, one of the Oligocene anthropoids. (d) Proconsul africanus. This early hominoid was the size of a four-year-old child.
d
We know from fossils that the prosimians had evolved by the Eocene. Skeletal changes adapted them to life among the treetops. They had a shorter snout and front-facing eyes. Their brain was bigger than that of early primates. Climbers and leapers had to estimate body weight, distance, wind speed, and suitable destinations. Adjustments had to be quick for a body in motion far above the ground. By 36 million years ago, tree-dwelling anthropoids arose (Figure 26.33c). Between 23 and 18 million years ago, in tropical rain forests, they gave rise to the first hominoids: early apes (Figure 26.33d). Hominoids dispersed through Africa, Asia, and Europe as climates were changing due to shifts in land masses. During this time, Africa became cooler and drier. Tropical forests, with their abundance of edible soft fruits and leaves, were replaced by open woodlands and, later, grasslands. Food became drier, harder, and more difficult to find. Hominoids that had evolved in moist forests either moved into new adaptive zones or died out. Most species died out, but not the shared ancestor of apes and humans. By 6 million years ago, hominids had emerged.
Pleistocene Pliocene Miocene
Oligocene
Eocene
Paleocene
Origins and Early Divergences The first primates arose in the tropical forests of East Africa by about 65.5 million years ago (mya). Early species resembled modern tree shrews (Figure 26.33a,b). They foraged at night among low branches for insects and seeds. They had a long snout and eyes located toward the sides of the head.
Take-Home Message What trends shaped the primate lineage ancestral to humans? Early primates were long-snouted animals that clambered about near the ground. Later species were climbers with a skeleton and brain that better adapted them to this new way of life.
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26.14 Emergence of Early Humans We have fossil evidence of many hominids, but do not know exactly how they are related to one another.
Link to Gene duplication 12.5
Early Hominids Genetic comparisons indicate that hominids diverged from apelike ancestors about 6 to 8 million years ago. Fossils that may be hominids are about 6 million years old. Sahelanthropus tchadensis had a hominid-like flat face, prominent brow, and small canines, but its brain was the size of a chimpanzee’s (Figure 26.34a). Orrorin tugenensis and Ardipithecus ramidus also had hominidlike teeth. Some researchers suspect that those species stood upright; others disagree. More fossils will have to be discovered to clarify the picture. An indisputably bipedal hominid, Australopithecus afarensis, was established in Africa by about 3.9 million
a
b
Sahelanthropus tchadensis 6 million years ago
Australopithecus africanus 3.2–2.3 million years ago
years ago. Remarkably complete skeletons reveal that it habitually walked upright (Figure 26.35). About 3.7 million years ago, two A. afarensis individuals walked across a layer of newly deposited volcanic ash. Soon thereafter, a light rain fell, and transformed the powdery ash they had just crossed into stone, preserving their footprints (Figure 26.35c,d). A. afarensis was one of the australopiths, or “southern apes.” This informal group includes Australopithecus and Paranthropus species. Australopithecus species were petite; they had a narrow jaw and small teeth (Figure 26.34b). One or more species probably are ancestral to modern humans. In contrast, Paranthropus species had a stockier build, a wider face, and larger molars. Jaw muscles attached to a pronounced bony crest at the top of their skull (Figure 26.34c). Large molars and strong jaw muscles indicate that fibrous, difficult-tochew plant parts accounted for a large part of the diet. Paranthropus died out about 1.2 million years ago.
c
d
Paranthropus boisei 2.3–1.2 million years ago
e
Homo habilis 1.9–1.6 million years ago
Homo erectus 1.9 million to 53,000 years ago
Figure 26.34 A sampling of fossilized hominid skulls from Africa, all to the same scale.
d
Figure 26.35 (a) Fossilized bones of Lucy, a female australopith (Australopithecus afarensis). This bipedal hominid lived in Africa 3.2 million years ago.
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b
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Chimpanzees and other apes have a splayed-out big toe (b). Early hominids did not. How do we know? (c,d) At Laetoli in Tanzania, Mary Leakey discovered footprints made in soft, damp volcanic ash 3.7 million years ago. The arch, big toe, and heel marks of these footprints are signs of bipedal hominids.
Early Humans What do the fossilized fragments of early hominids tell us about human origins? The record is still too incomplete for us to be sure how all the diverse forms were related, let alone which might have been our ancestors. Besides, exactly which traits should we use to define humans—members of the genus Homo? Well, what about brains? Our brain is the basis of unsurpassed analytical skills, verbal skills, complex social behavior, and technological innovations. How did an early hominid make the evolutionary leap to becoming human? Comparing the brains of modern primates can give us clues. We know that genes for some brain proteins underwent repeated duplication (Section 12.5) as the primate lineage evolved. Further studies of how these proteins function may provide additional insight into how our uniquely human mental traits arose. Until then, we are left to speculate on the evidence of physical traits among diverse fossils. They include a skeleton that permitted bipedalism, a smaller face, larger cranium, and smaller teeth with more enamel. These traits emerged during the late Miocene and can be observed in Homo habilis. The name of this early human means “handy man” (Figure 26.36). Most of the early known forms of Homo are from the East African Rift Valley. Fossil teeth indicate that these early humans ate hard-shelled nuts, dry seeds, soft fruits, leaves, and insects. H. habilis may have enriched its diet by scavenging carcasses left behind by carnivores such as saber-tooth cats, but it did not have teeth adapted to a diet rich in meat. Our close relatives, the chimpanzees and bonobos, use sticks and other natural objects as tools (Section 44.6). They smash nuts open with rocks and use sticks to dig into termite nests and capture insects. Early hominids most likely did the same. By 2.5 million years ago, some hominids had begun modifying rocks in ways that made them better tools. Pieces of volcanic rock chipped to a sharp edge were found with animal bones that show evidence that they were scraped by such tools. The layers of Tanzania’s Olduvai Gorge document refinements in toolmaking abilities (Figure 26.37). The layers that date to about 1.8 million years ago hold crudely chipped pebbles. More recent layers contain more complex tools, such as knifelike cleavers. Olduvai Gorge also holds hominid fossils. At the time of their discovery, these fossils were classified as Homo erectus. This name means “upright man.” Today, some researchers reserve that name for fossils in Asia.
Figure 26.36 Painting of a band of Homo habilis in an East African woodland. Two australopiths are shown in the distance at the left.
Figure 26.37 A sample of stone tools from Olduvai Gorge in Africa. From left to right, crude chopper, more refined chopper, hand ax, and cleaver.
They prefer to call the African fossils H. ergaster. In our discussions, we will adopt a traditional approach using “H. erectus” in reference to African populations and to descendant populations that, over generations, made their way into Europe and Asia. H. erectus adults averaged about 1.5 meters (5 feet) tall, and had a larger brain than H. habilis. Improved hunting skills may have helped H. erectus get the food needed to maintain a large body and brain. Also, H. erectus built fires, so cooking probably broadened their diet by softening previously inedible hard foods.
Take-Home Message What were the now extinct hominids like? Australopiths and certain hominids that preceded them walked upright. Homo habilis, the earliest known human species, also walked upright. Homo erectus had a larger brain and dispersed out of Africa.
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26.15 Emergence of Modern Humans Modern humans first evolved in Africa and relatively recently spread from there throughout the world.
Link to Allopatric speciation 18.10
Branchings of the Human Lineage By 1.7 million years ago, Homo erectus populations had become established in places as far away from Africa as the island of Java and eastern Europe. At the same time, African populations continued to thrive. Over thousands of generations, geographically separated groups adapted to local conditions. Some populations became so different from parental H. erectus that we call them new species: H. neanderthalensis (Neandertals), H. floresiensis, and H. sapiens, or fully modern humans (Figure 26.38). We know from one fossil found in Ethiopia a that Homo sapiens had evolved by 195,000 years ago. Compared to H. erectus, H. sapiens had smaller teeth, facial bones, and jawbones. H. sapiens also had a higher, rounder skull, a larger brain, and a capacity for spoken language. From 200,000 to 30,000 years ago, Neandertals b lived in Africa, the Middle East, Europe, and Asia. They were stocky enough to endure colder climates. A stocky body has a lower ratio of surface area to volume than a thin one, so it loses heat less quickly. c Neandertals had a big brain. Did they have a spoken language? We do not know. They vanFigure 26.38 ished when H. sapiens entered the same regions. Recent Homo species. (a) H. neanThe new arrivals may have driven Neadertals to derthalensis, (b) extinction through warfare or by outcompeting H. sapiens (modthem for resources. Members of the two species ern human), (c) H. may have occasionally mated, but comparisons floresiensis.
H. erectus
H. sapiens
between DNA from modern humans and DNA from Neandertal remains indicate that Neandertals did not contribute to the gene pool of modern Homo sapiens. In 2003, human fossils about 18,000 years old were discovered on the Indonesian island of Flores. Like H. erectus, they had a heavy brow and a relatively small brain for their body size. Adults would have stood a meter tall. Scientists who found the fossils assigned them to a new species, H. floresiensis. Not everyone is convinced. Some think the fossils belong to H. sapiens individuals who had a disease or disorder.
Where Did Modern Humans Originate? Neandertals evolved from H. erectus populations in Europe and western Asia. H. floresiensis evolved from H. erectus in Indonesia. Where did H. sapiens originate? Two major models agree that H. sapiens evolved from H. erectus but differ over where and how fast. Both attempt to explain the distribution of H. erectus and H. sapiens fossils, as well as genetic differences among modern humans who live in different regions. Multiregional Model By the multiregional model, populations of H. erectus in Africa and other regions evolved into populations of H. sapiens gradually, over more than a million years. Gene flow among populations maintained the species through the transition to fully modern humans (Figure 26.39a). By this model, some of the genetic variation now seen among modern Africans, Asians, and Europeans began to accumulate soon after their ancestors branched from an ancestral H. erectus population. The model is based on interpretation of fossils. For example, faces of H. erectus fossils from China are said to look more like modern Asians than those of H. erectus that lived in Africa. The idea is that much variation seen among modern H. sapiens evolved long ago, in H. erectus.
Africa Asia Europe
a H. erectus
H. sapiens Africa Asia
b
Europe Time
Figure 26.39 Two models for the origin of H. sapiens. (a) Multiregional model. H. sapiens slowly evolves from H. erectus in many regions. Arrows represent ongoing gene flow among populations. (b) Replacement model. H. sapiens rapidly evolves from one H. erectus population in Africa, then disperses and replaces H. erectus populations in all regions.
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By the more widely accepted replacement model, H. sapiens arose from a single H. erectus population in sub-Saharan Africa within the past 200,000 years. Later, bands of H. sapiens entered regions already occupied by H. erectus populations, and drove them all to extinction (Figure 26.39b). If this model is correct, then the regional variations observed among modern H. sapiens populations arose relatively recently. This model emphasizes the enormous degree of genetic similarity among living humans. Fossils support the replacement model. H. sapiens fossils date back to 195,000 years ago in East Africa and 100,000 years ago in the Middle East. In Australia,
Replacement Model
the oldest such fossils date to 60,000 years ago and, in Europe, they date to 40,000 years ago. Global comparisons of markers in mitochondrial DNA, and in the X and Y chromosomes, place the modern Africans closest to the root of the family tree. They also reveal that the most recent common ancestor of all humans now alive lived in Africa approximately 60,000 years ago. a
Leaving Home Long-term shifts in the global climate drove human bands away from Africa (Figure 26.40). About 120,000 years ago, Africa’s interior was getting cooler and drier. As patterns and amounts of rainfall changed, so did the distribution of herds of grazing animals and the humans who hunted them. A few hunters may have journeyed north from East Africa into Israel, where fossils 100,000 years old were found inside a cave. These populations apparently died out. Eruption of Mount Toba in Indonesia 73,000 years ago may have killed them, along with other ancient travelers. The enormous eruption released 10,000 times more ash than the 1981 eruption of Mount St. Helens in Washington State. The resulting cloud of debris had a devastating impact on the global climate. Later waves of travelers had better luck, as some individuals left established groups and ventured into new territory. Successive generations continued along the coasts of Africa, then Australia and Eurasia. In the Northern Hemisphere, much of Earth’s water became locked in vast ice sheets, which lowered the sea level by hundreds of meters. Previously submerged land was drained off between some regions. About 15,000 years ago, one small band of humans crossed such a land bridge from Siberia into North America. Deserts and mountains influenced the dispersal routes (Figure 26.40b). Until about 100,000 years ago, enough rain fell in northern Africa to sustain plants and herds of grazing animals. By 45,000 years ago, blazing hot sand stretched for more than 3,200 kilometers (2,000 miles). Humans whose ancestors had passed through this region no longer had the option of moving back to the grasslands of central Africa. The newly enlarged desert blocked their way. With return to Africa no longer an option, groups moved east into central Asia, where the towering Himalayas and other peaks of the Hindu Kush forced some to detour north, into western China, and others south, into India. Descendants of humans that moved
Figure 26.40 (a) Some dispersal routes for small bands of Homo sapiens. This map shows ice sheets and deserts that prevailed about 60,000 years ago (ya). It is based on clues from sedimentary rocks and ice core drillings. Times when modern humans appeared in these regions are based on fossils studies of genetic markers in mitochondrial DNA and Y chromosomes from 10,000 individuals around the world:
Africa
120,000 ya
Africa by 195,000 years ago Israel 100,000 Australia 60,000 China 50,000 Europe 40,000 North America 11,000 (b) Global climate changes caused expansion and contraction of deserts in Africa and the Middle East. Resulting changes in food sources may have encouraged migrations of small groups out of Africa. Locations of ice sheets, deserts, and tall mountain ranges influenced migration routes.
into Asia eventually reached Siberia, then traveled into North America. Colonists from central Asia moved west, across cold grasslands. Some crossed mountains in the Balkans, and continued on into Europe. With each step of their journey, humans faced and overcame extraordinary hardships. During this time, they devised cultural means to survive in inhospitable environments. Unrivaled capacities for modifying the habitat and for language served them well. Cultural evolution is ongoing. Hunters and gatherers persist in a few parts of the world, while others of us live in the age of “high tech.” This coexistence of such diverse groups is a tribute to the great behavioral flexibility of the human species.
60,000 ya
30,000 ya
b
present
Take-Home Message What do fossils and DNA studies tell us about the evolution of modern humans? Fossils and genetic evidence indicate that modern humans, H. sapiens, evolved from a H. erectus population in Africa.
Modern humans dispersed out of Africa during a time when long-term shifts in climate influenced their options.
CHAPTER 26
ANIMAL EVOLUTION—THE CHORDATES 457
IMPACTS, ISSUES REVISITED
Transitions Written in Stone
Sale of vertebrate fossils is big business for rock shops, auction houses, and websites. Most such fossils are not particularly important to scientists, but some are. For example, one of the few Archaeopteryx fossils in existence is privately held. It shows details of the bird’s feet that are not visible in other fossils. Some scientists argue that private ownership of such fossils thwarts research and endangers an irreplaceable legacy.
Summary Section 26.1 Four features help define chordates: a notochord, a dorsal hollow nerve cord, a pharynx with gill slits, and a tail extending past the anus. All features form in embryos and may or may not persist in adults. Invertebrate chordates include tunicates and lancelets, both marine filter-feeders. Craniates are chordates with a braincase of cartilage or bone. Structurally, a jawless fish called the hagfish is the simplest modern craniate. Most craniates are vertebrates.
Use the animation on CengageNOW to examine the body plan and chordate features of a lancelet.
Section 26.2 Vertebrates have an endoskeleton with a vertebral column (backbone) of cartilaginous or bony vertebrae. Jaws and paired fins evolved in early fishes. In lineages that moved onto land, gills were replaced by lungs, kidneys became better at conserving water, and the circulatory system became more efficient.
Use the animation on CengageNOW to explore the chordate family tree and see how jaws evolved.
Sections 26.3, 26.4 Lampreys are jawless fishes with a backbone. Jawed fishes include the cartilaginous fishes and bony fishes. Both have scales on their skin. A swim bladder helps bony fishes regulate their buoyancy. Sections 26.5, 26.6 Tetrapods, or four-legged walkers, evolved from lobe-finned bony fishes. Amphibians are tetrapods that live on land, but typically return to water to reproduce. Many amphibians now face extinction.
Figure 26.41 Estimated dates for the origin and extinction of three hominid genera. Purple lines show one view of how the human species relate to one another. Number of species, which fossils belong to each species, and how species relate remains a matter of debate.
How would you vote? One-of-a-kind vertebrate fossils are in private collections. Is sale of scientifically important fossils unethical? See CengageNOW for details, then vote online.
Sections 26.7, 26.8 Amniotes, the first vertebrates able to complete their life cycle on dry land, have waterconserving skin and kidneys, and amniote eggs. Reptiles (including extinct dinosaurs) and birds are one amniote lineage; modern mammals are another. The K–T asteroid hypothesis proposes that an asteroid impact led to the extinction of dinosaurs. Section 26.9 Reptiles are ectotherms (cold-blooded animals) with scales. Eggs are laid on land and fertilization is usually internal. A cloaca functions in excretion and reproduction. Lizards and snakes are the most diverse groups. Crocodilians are the closest relatives of birds.
Use the animation on CengageNOW to explore the body plan of a crocodile.
Section 26.10 Birds are endotherms (warm-blooded animals) and the only living animals with feathers. The body plan of most has been highly modified for flight.
Use the animation on CengageNOW to see what is inside a bird egg and how birds are adapted for flight.
Sections 26.11, 26.12 Mammals nourish young with milk secreted by mammary glands, have fur or hair, and have more than one kind of tooth. Three lineages are egg-laying mammals (monotremes), pouched mammals (marsupials), and placental mammals, the most diverse group. A placenta is an organ that facilitates exchange of substances between the embryonic and maternal blood.
Use the animation on CengageNOW to see how the current distribution of mammalian groups arose.
Homo floresiensis Homo rudolfensis Australopithecus anamensis
458 UNIT IV
Homo habilis
Homo sapiens Homo erectus
Australopithecus africanus Australopithecus garhi
Australopithecus afarensis
Homo neanderthalensis
Paranthropus aethiopicus
Paranthropus robustus Paranthropus boisei
4
3
2
Time (millions of years ago)
EVOLUTION AND BIODIVERSITY
1
present
human
chimp
gibbon
lemur
As Section 26.13 mentioned, one trend in primate evolution involved changes in life history traits, such as length of infancy and the time it takes to reach adulthood. Figure 26.42 compares five primate lineages, from most ancient to most recent. It graphs life spans and years spent as “infants” when ongoing maternal care is required. It shows time spent as subadults, when individuals are no longer dependent on their mother for care, but have not yet begun to breed. It shows the length of the reproductive years, and the length of time lived after reproductive years have passed.
macaque
Data Analysis Exercise
60 50
30
10
18
3. Which group has the longest expanse of reproductive years?
Self-Quiz
Answers in Appendix III
1. List the four distinguishing chordate traits.
adult
20
2. Which group reaches adulthood most quickly?
Sections 26.13–26.15 Primates include prosimians such as tarsiers and anthropoids such as monkeys, apes, and hominids—humans and extinct humanlike forms. Early primates were shrewlike. Bipedalism, improved daytime vision, refined hand movements, smaller teeth, bigger brains, social complexity, extended parental care, and, later, culture evolved in some lineages. All hominids, including australopiths and humans, originated in Africa. The human lineage (Homo) arose by 2 million years ago (Figure 26.41). By the multiregional model, H. sapiens evolved in many separate regions. The replacement model postulates that H. sapiens evolved in Africa, then dispersed. It is now the favored model.
postreproductive years
40
1. What is the average life span for a lemur? A gibbon?
4. Which groups survive past their reproductive years?
70
24 30 34 38 Time in uterus (weeks)
0
subadult infancy
Figure 26.42 Trend toward longer life spans and greater dependency of offspring on adults for five primate lineages.
8. The closest modern relatives of birds are a. crocodilians b. prosimians c. tuataras
. d. lizards
9. Only birds have . a. a cloaca b. a four-chambered heart
c. feathers d. amniote eggs
10. An australopith is a a. craniate b. vertebrate c. hominoid
. d. amniote e. placental mammal f. all of the above
11. Match the organisms with the appropriate description. lancelets a. pouched mammals fishes b. invertebrate chordates amphibians c. feathered amniotes reptiles d. egg-laying mammals birds e. humans and close relatives monotremes f. cold-blooded amniotes marsupials g. first land tetrapods hominids h. most diverse vertebrates
Visit CengageNOW for additional questions.
2. Which of these traits are retained by an adult lancelet? 3. Vertebrate jaws evolved from . a. gill supports b. ribs c. scales d. teeth 4. Lampreys and sharks both have . a. jaws d. a swim bladder b. a bony skeleton e. a four-chambered heart c. a cranium f. lungs 5. Which group of bony fish gave rise to tetrapods? 6. Reptiles and birds belong to one major lineage of amniotes, and belong to another. a. sharks c. mammals b. frogs and toads d. salamanders 7. Reptiles are adapted to life on land by . a. tough skin d. amniote eggs b. internal fertilization e. both a and c c. efficient kidneys f. all of the above
Critical Thinking 1. In 1798, a stuffed platypus specimen was delivered to the British Museum. Reports that it laid eggs added to the confusion. To modern biologists, a platypus is clearly a mammal. It has fur and the females produce milk. Young animals have typical mammalian teeth that are replaced by hardened pads as the animal matures. Why do you think modern biologists can more easily accept that a mammal can have some seemingly reptilian traits? 2. The cranial volume of early H. sapiens averaged 1,200 cubic centimeters. It now averages 1,400 cubic centimeters. By one hypothesis, females chose the cleverest mates, the advantage being offspring with genes that favorably affect intelligence. What types of data might a researcher gather to test this sexual selection hypothesis? CHAPTER 26
ANIMAL EVOLUTION—THE CHORDATES 459
27
Plants and Animals—Common Challenges IMPACTS, ISSUES
A Cautionary Tale
A cell can only survive within a certain range of conditions.
already been done. Stringer’s blood clotting mechanism shut
As explained in Section 6.3, changes in acidity, salinity, or
down and he started to bleed internally. Then his kidneys
temperature can inactivate the enzymes that catalyze the
faltered. He stopped breathing and was attached to a respi-
many reactions necessary for life. To remain alive, any
rator, but his heart gave out. Less than twenty-four hours after
multicelled organism must keep conditions inside its body
the football practice had started, Stringer was pronounced
within the range its cells can tolerate.
dead. He was twenty-seven years old.
Heat stroke is an example of what can happen when
The human body functions best when internal temperature
internal conditions get out of balance. It can be deadly. For
remains between about 97°F (36°C) and 100°F (38°C). Above
example, Korey Stringer, a football player for the Minnesota
104°F (40°C), blood flow is increasingly diverted from internal
Vikings, collapsed of heat stroke during a practice (Figure
organs to the skin. Heat is transferred from skin to air, as long
27.1). He and his team were working out in full uniform on
as a body is warmer than its surroundings. Sweating helps
a day when temperature and humidity were high.
get rid of heat, but it is less effective on humid days.
Stringer was rushed to the hospital with an internal body
When internal temperature climbs above 105°F (40.6°C),
temperature of 108.8°F (42.7°C), and a blood pressure too
normal cooling processes fail and heat stroke occurs. The
low to measure. Doctors immersed him in a bath of ice water
body stops sweating, and its core temperature begins to
to bring his temperature down, but irreparable damage had
shoot up. The heart beats faster; fainting or confusion follow. Without prompt treatment, brain damage or death can occur. We use this sobering example as our introduction to anatomy and physiology. Anatomy is the study of body form. Physiology is the study of how the body’s parts are put to use. This information can help you understand what is going on inside your own body. More broadly, it can also help you appreciate how all organisms survive. We discuss the anatomy and physiology of plants and animals separately in later chapters. In this chapter, we provide an overview of the processes and structural traits that the two groups share in common.
See the video! Figure 27.1 Left, Korey Stringer, during his last practice with his team. When the body’s temperature rises, profuse sweating increases evaporative cooling. Also, blood is directed to capillaries of the skin (above), which radiate heat into the air. In Stringer’s case, homeostatic control mechanisms were no match for strenuous activity on a hot, humid day.
Links to Earlier Concepts
Key Concepts Many levels of structure and function
With this chapter, we return to the concept of levels of organization introduced in Section 1.1. We also explore some examples of sensing and responding to stimuli (1.2), one of the signature traits of life.
You will learn how constraints imposed by the ratio of surface area to volume (4.2) affect body structures.
Cellular structures such as cell junctions (4.12) and membrane proteins (5.2) also come into play, as do cellular processes such as transport (5.3) and energyreleasing pathways (8.1).
We discuss the ability of plants and animals to fight infectious disease (21.8) and how their bodies are adapted to life on land (23.1, 26.5, 26.7).
Cells of plants and animals are organized in tissues. Tissues make up organs, which work together in organ systems. This organization arises as the plant or animal grows and develops. Interactions among cells and among body parts keep the body alive. Section 27.1
Similarities between animals and plants Animals and plants exchange gases with their environment, transport materials through their body, maintain volume and composition of their internal environment, and coordinate cell activities. They also respond to threats and to variations in available resources. Section 27.2
Homeostasis Homeostasis is the process of keeping conditions in the body’s internal environment stable. The feedback mechanisms that often play a role in homeostasis involve receptors that detect stimuli, an integrating center, and effectors that carry out responses. Sections 27.3–27.5
Cell communication in multicelled bodies Cells of tissues and organs communicate by secreting chemical molecules into extracellular fluid, and by responding to signals secreted by other cells. Section 27.6
How would you vote? The interior of a vehicle heats up fast on even a mild day. Each year children left in vehicles die as a result of heat stroke. Some states have made it a crime to leave a child alone in a parked car. Do you support such laws? See CengageNOW for details, then vote online.
461
27.1
Levels of Structural Organization Earlier chapters covered plant and animal diversity. Here we begin to consider how their bodies are organized. Links to Levels of organization 1.1, Natural selection 17.3, Land plants 23.1, Land animals 26.5 and 26.7
From Cells to Multicelled Organisms The body of any plant or animal consists of hundreds to hundreds of trillions of cells. In all but the simplest bodies, cells become organized as tissues, organs, and organ systems, each capable of specialized functions. Said another way, there is a division of labor among parts of a plant or animal body (Section 23.1). A tissue consists of one or more cell types—and often an extracellular matrix—that collectively perform a specific task or tasks. Each tissue is characterized by the types of cells it includes and their proportions. For
example, nervous tissue has different types and proportions of cells than muscle tissue or bone tissue. An organ consists of two or more tissues that occur in specific proportions and interact in carrying out a specific task or tasks. For example, a leaf is an organ that serves in gas exchange and photosynthesis (Figure 27.2); lungs are organs of gas exchange (Figure 27.3). Organs that interact in one or more tasks form an organ system. Leaves and stems are components of a plant’s gas exchange system. Lungs and airways are organs of the respiratory system of land vertebrates.
Growth Versus Development A plant or animal becomes structurally organized as it grows and develops. For any multicelled species, growth refers to an increase in the number, size, and volume of cells. We describe it in quantitative terms. Development is a series of stages in which specialized tissues, organs, and organ systems form in heritable patterns. We describe it in qualitative terms; usually by describing the stages. For example, both plants and animals have an early stage called the embryo.
Evolution of Form and Function Flower, a reproductive organ
Cross-section of a leaf, an organ of photosynthesis and gas exchange
shoot system (aboveground parts)
root system (belowground parts, mostly)
Cross-section of a stem, an organ of structural support, storage, and distribution of water and food
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All anatomical and physiological traits have a genetic basis and thus have been affected by natural selection. The traits we see in modern species are the outcome of differences in survival and reproduction among many generations of individuals who varied in their traits. Only traits that proved adaptive in the past have been passed along to modern generations. For example, Section 23.1 discussed how plants adapted to life on dry land. As plants radiated out of the aquatic environment onto land, they faced a new challenge—they had to keep from drying out in air. We see solutions to this challenge in the anatomy of roots, stems, and leaves (Figure 27.2). Internal pipes called xylem convey water that roots absorb from soil upward to leaves. The epidermal tissue that covers leaves and stems of vascular plants secretes a waxy cuticle that reduces evaporative water loss. Stomata, small gaps across a leaf’s epidermis, can open to allow gas exchange or close to prevent water loss.
Figure 27.2 Animated Anatomy of a tomato plant. Its vascular tissues (purple) conduct water, dissolved mineral ions, and organic compounds. Another tissue makes up the bulk of the plant body. A third covers all external surfaces. Organs such as flowers, leaves, stems, and roots are each made up of all three tissues.
Figure 27.3 Parts of the human respiratory system. Cells making up the tissues of this system carry out specialized tasks. Airways to paired lungs are lined with epithelial tissue. Ciliated cells in this tissue whisk any bacteria and particles that might cause infections away from the lungs. Lungs are organs of gas exchange. Inside them are air sacs lined with continually moist epithelial tissue. Tiny vessels (capillaries) filled with blood surround the air sacs and interact with them in the task of gas exchange.
Ciliated cells and mucus-secreting cells of a tissue that lines respiratory airways
Lung tissue (tiny air sacs) laced with blood capillaries—one-cell-thick tubular structures that hold blood, which is a fluid connective tissue
Organs (lungs), part of an organ system (the respiratory tract) of a whole organism
Similarly, animals evolved in water and faced new challenges when they moved onto land (Sections 26.5 and 26.7). Gases can only move into and out of an animal’s body by moving across a moist surface. That is not a problem for aquatic organisms, but on land, evaporation can cause moist surfaces to dry out. The evolution of respiratory systems allowed land animals to exchange gases with air across a moist surface deep inside their body. In land vertebrates, the respiratory system typically includes airways and paired lungs (Figure 27.3). The tissue that lines the airways leading to lungs includes ciliated cells that can capture airborne particles and pathogens. Deep inside the lungs gases are exchanged between air and blood across the thin, continually moistened tissue of tiny air sacs.
The Internal Environment A single-celled organism gets necessary nutrients and gases from the fluid around it. Plant and animal cells are also surrounded by fluid. This extracellular fluid (ECF) is like an internal environment in which body cells live. To keep cells alive, a body’s parts work in concert in ways that maintain the volume and composition of the extracellular fluid.
A Body’s Tasks The next two units describe how a plant or an animal carries out the following essential functions: • Maintains favorable conditions for its cells • Acquires and distributes water, nutrients, and other raw materials; disposes of wastes • Defends itself against pathogens • Reproduces • Nourishes and protects gametes and (in many species) embryos Each living cell engages in metabolic activities that keep it alive. At the same time, integrated activities of cells in tissues, organs, and organ systems sustain the body as a whole. Their interactions keep conditions in the internal environment within tolerable limits—a process we call homeostasis. Take-Home Message How are plant and animal bodies organized? Plant and animal bodies typically consist of cells organized as tissues, organs, and organ systems. The ways in which body parts are arranged and function have a genetic basis and have been shaped by natural selection.
Collectively, cells, tissues, and organs maintain conditions inside the body.
CHAPTER 27
PLANTS AND ANIMALS—COMMON CHALLENGES 463
27.2
Common Challenges Although plants and animals differ in many ways, they share some common challenges. Links to Surface area-to-volume ratio 4.2, Diffusion and transport mechanisms 5.3, Energy-releasing pathways 8.1
Gas Exchange To begin thinking about the processes that occur in both plants and animals, consider how the golfer Tiger Woods is like a tulip (Figure 27.4). Cells inside both bodies release energy by carrying out aerobic respiration (Section 8.1). This pathway requires oxygen and produces carbon dioxide. Some tulip cells also carry out photosynthesis, an energy-storing process that requires carbon dioxide and produces oxygen. All multicelled species respond, structurally and functionally, to this common challenge: Quickly move molecules to and from individual cells. By the process of diffusion, ions or molecules of a substance move from a place where they are concentrated to one where they are more scarce (Section 5.3). Plants and animals keep gases diffusing in directions most suitable for metabolism and cell survival. How? That question will lead you to stomata at leaf surfaces (Section 28.4) and to the circulatory and respiratory systems of animals (Chapters 37 and 39).
Internal Transport Diffusion is most effective over small distances. As an object’s diameter increases, its ratio of surface area to volume decreases (Section 4.2). This means that as the
diameter of a body part becomes larger, interior cells get farther and farther from the body surface, and there is less body surface per cell. As a result of this constraint, plants and animals that rely on diffusion alone to move materials through their body tend to be small and flat. Flatworms and liverworts are two examples (Figure 27.5a,b). Both are just a few cell layers thick. Most plants and animals that are not small and flat have vascular tissues—systems of tubes through which substances move to and from cells. A leaf vein in a vascular plant consists of long strands of xylem and phloem, the two types of vascular tissue (Figure 27.5c). Human blood vessels such as veins and capillaries are our vascular tissues (Figure 27.5d). In both plants and animals, vascular tissue carries water, nutrients, and signaling molecules. In animals, this tissue also distributes gases. Gases move into and through a plant by diffusion. Components of animal blood fight infection. Similarly, phloem of vascular plants carries chemicals made in response to injury.
Maintaining the Water–Solute Balance Plants and animals continually gain and lose water and solutes. Even so, to stay alive they must maintain the volume and composition of their extracellular fluid within limited ranges. How do they do this? Plants and animals differ hugely in this respect, yet you can still find common responses by zooming down to the level of molecules. At the surface of a body or an organ, cells in sheets of tissue carry out active and passive transport. Recall that in passive transport, a solute moves down its concentration gradient with the assistance of a transport protein. In active transport, a protein pumps one specific solute from a region of low concentration to one of higher concentration (Section 5.3). Active transport by cells in plant roots helps control which solutes move into the plant. In leaves, active transport puts sugars made by photosynthesis into phloem, which distributes them through the plant. In animals, active transport moves nutrients from food inside the gut into body cells. In vertebrates, active transport allows kidneys to eliminate wastes and excess solutes and water in the urine.
Cell-to-Cell Communication
Figure 27.4 What do Tiger and the tulips have in common?
464 UNIT IV
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Plants and animals have another crucial similarity: Both depend on communication among cells. Many types of specialized cells release signaling molecules
a
b
Figure 27.5 Having a flattened body allows a liverwort (a) and a flatworm (b) to do just fine without vascular tissues. All their cells lie close to the body surface. Evolution of vascular tissues such as (c) leaf veins in a dicot and (d) blood vessels in a human allow these organisms to grow much larger and have thicker body parts. c
d
a
b
that help coordinate and control events in the body as a whole. Signaling mechanisms guide how the plant or animal body grows, develops, and maintains itself, and also reproduces.
On Variations in Resources and Threats A habitat is a place where members of a species typically live. Each habitat has a specific set of resources and poses a unique set of challenges. Each has unique physical characteristics. Water and nutrients may be plentiful or scarce. The habitat may be brightly lit, a bit shady, or dark. It may be whipped by winds or still. Temperature may vary a lot or a little over the course of a day. Similarly, conditions may change with the season or stay more or less constant. Biotic (living) components of the habitat vary as well. Different producers, predators, prey, pathogens, or parasites may be present. Competition for resources and reproductive partners may be minimal or fierce. Variation in these factors promotes diversity in form and function. Even with all the diversity, we may still see similar responses to similar challenges. Sharp cactus spines or porcupine quills deter most animals that might eat a cactus or porcupine (Figure 27.6). Modified epidermal cells give rise to both spines and quills that defend the body against potential predators.
Figure 27.6 Protecting body tissues from predation: (a) Cactus spines. (b) Quills of a porcupine (Erethizon dorsatum).
Take-Home Message How are plant and animal bodies similar? Plants and animals carry out aerobic respiration and exchange gases with the environment.
Most plants and most animals have vascular tissues that function in transport. Plants and animals keep their internal environment stable by regulating which substances enter their body and which are eliminated.
CHAPTER 27
PLANTS AND ANIMALS—COMMON CHALLENGES 465
27.3
Homeostasis in Animals Detecting and responding to changes is a characteristic trait of all living things and the key to homeostasis.
Link to Sensing and responding to change 1.2
Detecting and Responding to Changes In animals, homeostasis involves interactions among receptors, integrators, and effectors (Figure 27.7). A receptor is a cell or cell component that changes in response to specific stimuli. Some receptors such as those in eyes, ears, and skin respond to external stimuli such as light, sound, or touch. Receptors involved in homeostasis function like internal watchmen. They detect changes inside the body. For example, some receptors detect blood pressure changes, others detect
Receptor
Integrator
Effector
such as the brain or the spinal cord
a muscle or a gland
Negative Feedback In a negative feedback mechanism, a change leads to a response that reverses that change. Think of how a furnace with a thermostat operates. A user sets the thermostat to a desired temperature. When the temperature decreases below this preset point, the furnace turns on and emits heat. When the temperature rises to the desired level, the thermostat turns off the heat. Similar feedback mechanisms help keep a human’s internal body temperature near 98.6°F (37°C) despite changes in the temperature of the surroundings.
STIMULUS Sensory input into the system
such as a free nerve ending in the skin
changes in the level of carbon dioxide in the blood, and still others detect changes in internal temperature. Information from sensory receptors throughout the body flows to an integrator: a collection of cells that receives and processes information about stimuli. In vertebrates, this integrator is the brain. In response to the signals it receives, the integrator sends a signal to effectors—muscles, glands, or both— that carry out responses to the stimulation. Sensory receptors, integrators, and effectors often interact in feedback systems. In such systems, some stimulus causes a change from a set point, which then “feeds back” and affects the original stimulus.
Figure 27.7 The three types of components that interact in homeostasis in animal bodies.
STIMULUS Body’s surface temperature skyrockets after exertion on a hot, dry day.
Receptors
Integrator
Effectors
Sensory receptors in skin and elsewhere detect the change in temperature.
Hypothalamus (a brain region) compares input from receptors against a set point for the body.
Pituitary gland and thyroid gland trigger adjustments in activity of many organs.
RESPONSE Body’s surface temperature falls, which causes sensory receptors to initiate shift in effector output.
dead, flattened skin cell
Effectors Different types of effectors carry out specific (not general) responses: Skeletal muscles in chest wall contract more frequently; faster breathing speeds heat transfer from lungs to air.
Blood vessels in skin expand as muscle in their wall relaxes; more metabolic heat gets shunted to skin, where it dissipates into the air.
Sweat gland secretions increase; the evaporation of sweat cools body surfaces.
Adrenal gland secretions drop off; excitement declines.
Effectors collectively call for an overall slowdown in activities, so the body generates less metabolic heat.
Figure 27.8 Animated Major homeostatic controls over a human body’s internal temperature. Solid arrows signify the main control pathways. Dashed arrows signify the feedback loop.
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sweat gland pore
Scanning electron micrograph of a sweat gland pore at the skin surface. Such glands are among the effectors for this control pathway.
FOCUS ON HEALTH
27.4 Consider what happens when you exercise on a hot day. During exercise, muscles increase their metabolic rate. Because metabolic reactions generate heat, body temperature rises. Receptors sense the increase and trigger changes that affect the whole body (Figure 27.8). Blood flow shifts, so more blood from the body’s hot interior flows to the skin. This maximizes the amount of heat that dissipates to the surrounding air. At the same time, glands in the skin increase their secretion of sweat. Sweat is mostly water and as it evaporates, it helps cool the body surface. Breathing rate and the volume of each breath increase, speeding the transfer of heat from the blood flowing through your lungs to the air. Levels of excitatory hormones decline, so you feel more sluggish. As your activity level slows, and your rate of heat loss to the environment rises, your temperature falls. Thus, the stimulus (high body temperature) that triggered these responses is reversed by the responses. For most people, most of the time, this feedback mechanism will prevent overheating. The heat illness that occurs when negative feedback mechanisms fail is the topic of the next section.
Positive Feedback Positive feedback mechanisms also operate in a body, although they are less common than negative feedback ones. These mechanisms spark a chain of events that intensify change from an original condition. In living organisms, intensification eventually leads to a change that ends feedback. For example, when a woman is giving birth, muscles of her uterus contract and force the fetus against the wall of this organ. The resulting pressure on the uterine wall induces secretion of a signaling molecule (oxytocin) that causes stronger contractions. In a positive feedback loop, as contractions get more forceful, pressure on the uterine wall increases, thus causing still stronger contractions. The positive feedback cycle continues until the child is born.
Take-Home Message What types of mechanisms operate in animal homeostasis? Change-detecting receptors, an information-processing brain, and muscles and glands controlled by the brain interact in homeostasis. Negative feedback mechanisms can reverse changes to conditions within the body. Positive feedback is less common than negative feedback. It causes a temporary intensification of a change in the body.
Heat-Related Illness
Heat stroke is a failure of homeostasis that can cause irreversible brain damage or death.
In a typical year, about 175 Americans die as a direct result of heat exposure. To avoid heat-related problems, listen to your body. Most heat-related deaths in young, healthy adults occur when people continue to exert themselves despite clear warnings that something is amiss. Social pressure to continue an activity often plays a role in exertion-induced heat stress. An attempt to impress a coach or peers, or to satisfy a boss, can push a healthy person beyond safe limits. Symptoms of heat exhaustion include dizziness, blurred vision, muscle cramping, weakness, nausea, and vomiting. Korey Stringer vomited repeatedly during his final practice, but did not stop working out. Similarly, a young firefighter recruit in Florida complained of weakness and blurred vision. Yet he ran until he collapsed with a body temperature of 108°F. Immediate treatment by fellow firefighters and quick hospitalization could not save him; he died nine days later. Part of the problem is that heat exhaustion can impair judgment. Profuse sweating causes loss of water and salts, changing the concentration of the extracellular fluid. Blood flow to the gut and liver decreases. Starved of nutrients and oxygen they need, these organs release toxins into the blood. The toxins interfere with function of the nervous system, as well as other organ systems. As a result, a person may be incapable of recognizing and responding to seemingly obvious signs of danger. To stay safe outside on a hot day, drink plenty of water and avoid excessive exercise. If you must exert yourself, take frequent breaks and monitor how you feel. Wear light-colored, lightweight, breathable clothing. Stay in the shade, or if you must be in direct sunlight wear a hat and use a strong sunscreen. Sunburn impairs the skin’s ability to transfer heat to the air. Keep in mind that high humidity adds to the danger. Evaporation slows when there is more water in the air, so sweating is less effective on humid days. A 95°F (35°C) day with 90 percent humidity puts more heat stress on the body than a 100°F (37.8°C) day accompanied by 55 percent humidity. Responses to heat can vary with age and certain medical conditions. Pregnant women, the elderly, and people with heart problems or diabetes are at an elevated risk for heat stroke and should be especially careful. Use of alcohol, blood pressure medications, antidepressants, and other drugs also make heat-related problems more likely. Also, people can become acclimated to a high external temperature; those who are not used to living with heat are at an increased risk for heat-related problems. If you suspect someone is suffering from heat stroke, call for medical help immediately. Give the heat-stroke victim water to drink, then have them lie down with their feet slightly elevated. Spray or sponge them with cool water and, if possible, place ice packs under their armpits.
CHAPTER 27
PLANTS AND ANIMALS—COMMON CHALLENGES 467
27.5
Does Homeostasis Occur in Plants? Plants too must maintain internal conditions within a range that their cells can tolerate.
Link to Infectious disease 21.8
Directly comparing plants and animals is not always possible. For example, as a plant grows, new tissues arise only at particular sites in roots and shoots. In animal embryos, tissues form all through the body. Plants do not have the equivalent of an animal brain. But they do have some decentralized mechanisms that influence the internal environment and keep the body functioning properly. Two simple examples illustrate the point; chapters to follow include more.
Walling Off Threats Unlike people, trees consist mostly of dead and dying cells. Also unlike people, trees cannot run away from attacks. When a pathogen infiltrates their tissues, trees
A
B
Strong
Moderate
cannot unleash infection-fighting white blood cells in response, because they have none. However, plants do have systemic acquired resistance: a defense response to infections and injured tissues. Cells in an affected tissue release signaling molecules. The molecules cause synthesis and release of organic compounds that will protect the plant against attacks for days or months to come. Some protective compounds are so effective that synthetic versions are being used to boost disease resistance in crop plants and ornamental plants. Most trees also have another defense that minimizes effects of pathogens. When wounded, such trees wall off the damaged tissue, release phenols and other toxic compounds, and often secrete resins. A heavy flow of gooey compounds saturates and protects the bark and wood at the wound. It also seeps into the soil around roots. Some of these toxins are so potent that they can kill cells of the tree itself. Compartments form around injured, infected, or poisoned tissues, and new tissues grow right over them. This plant response to wounds is called compartmentalization. Drill holes into a tree species that makes a strong compartmentalization response and the wound gets walled off fast (Figure 27.9). In a species that makes a moderate response, decomposers cause the decay of more wood surrounding the holes. Drill into a weak compartmentalizer, and decomposers cause massive decay deep into the trunk. Even strong compartmentalizers live only so long. If too much tissue gets walled off, flow of water and solutes to living cells slows and the tree begins to die. What about the bristlecone pine, which grows high in mountain regions (Section 23.7)? One tree we know of is almost 5,000 years old. These trees live under harsh conditions in remote places where pathogens are few. The trees spend most of each year dormant beneath a blanket of snow, and grow slowly during a short, dry summer. This slow growth makes a bristlecone pine’s wood so dense that few insects can bore into it.
Sand, Wind, and the Yellow Bush Lupine
C
Weak
Figure 27.9 Animated Results of an experiment in which holes were drilled into living trees to test compartmentalization responses. From top to bottom, decay patterns (green) in trunks of three species of trees that made strong, moderate, and weak compartmentalization responses, respectively.
468 UNIT IV
EVOLUTION AND BIODIVERSITY
If you have ever walked barefoot across beach sand on a sunny summer day you know how hot it can get. Sandy soil also tends to drain quickly, and to be low in nutrients. Few plants are adapted to survive in this habitat, but the yellow bush lupine, Lupinus arboreus, thrives here (Figure 27.10). This shrubby plant is native to coastal dunes of central and southern California. Several factors contribute to the lupine’s success in its challenging coastal environment. It is a legume and, like other members of this plant family, it shelters
Figure 27.10 Yellow bush lupine, Lupinus arboreus, in a sandy shore habitat. On hot, windy days, its leaflets fold up longitudinally along the crease that runs down their center. This helps minimize evaporative water loss.
1 A.M.
6 A.M.
Noon
3 P.M.
10 P.M.
Midnight
Figure 27.11 Animated Observational test of rhythmic leaf movements by a young bean plant (Phaseolus). Physiologist Frank Salisbury kept the plant in darkness for twenty-four hours. Despite the lack of light cues, the leaves kept on folding and unfolding at sunrise (6 A.M.) and sunset (6 P.M.).
nitrogen-fixing bacteria inside its young roots (Section 24.6). The bacteria share some nitrogen with their host plant, thus giving it a competitive edge in nitrogenpoor soil. Another environmental challenge near the beach is the lack of fresh water. Leaves of a yellow bush lupine are structurally adapted for water conservation. Each leaf has a dense array of fine epidermal hairs that project above it, particularly on the leaf’s lower surface. Collectively, these hairs trap moisture that evaporates from the stomata. The dampened hairs keep humidity around the stomata high, which helps minimize water losses to the air. The yellow bush lupine also makes a homeostatic response. It folds its leaves lengthwise when conditions are hot and windy (Figure 27.10). This folding shelters stomata from the wind and further raises the humidity around them. When winds are strong and the potential for water loss is greatest, the leaves fold tightly. The least-folded leaves are close to the plant’s center or on the side most sheltered from the wind. Folding is a response to heat as well as to wind. When air temperature is highest during the day, leaves fold at an angle that helps minimizes the amount of light they intercept, and the amount of heat they absorb.
Rhythmic Leaf Folding Another example of a plant response is rhythmic leaf folding (Figure 27.11). A bean plant holds its leaves horizontally during the day but folds them close to its stem at night. A plant exposed to constant light or darkness for a few days will continue to move its leaves in and out of the “sleep” position at the time of sunrise and sunset. The response might help reduce heat loss at night, when air cools, and so maintain the plant’s internal temperature within tolerable limits. Rhythmic leaf movements are just one example of a circadian rhythm: a biological activity pattern that recurs with an approximately 24-hour cycle. Circadian means “about a day.” Both plants and animals, as well as other organisms, have circadian rhythms.
Take-Home Message How does homeostasis in plants differ from that animals? Control mechanisms that function in homeostasis in plants are not centrally controlled as they are in most animals. Systemic acquired resistance, compartmentalization, and leaf movements in response to environmental changes are examples of these mechanisms.
CHAPTER 27
PLANTS AND ANIMALS—COMMON CHALLENGES 469
27.6
How Cells Receive and Respond to Signals Coordinated action requires communication among body cells. Signaling mechanisms are essential to that integration.
Links to Cell junctions 4.12, Membrane proteins 5.2
Cells in any multicelled body communicate with their neighbors and often with cells farther away. Section 4.12 described how plasmodesmata in plants and gap junctions in animals allow substances to pass quickly between adjoining cells. Communication among more distant cells involves special molecules. Some molecular signals diffuse from one cell to another through the fluid between them. Others travel in blood vessels or in a plant’s vascular tissues. Molecular mechanisms by which cells “talk” to one another evolved early in the history of life. They often have three steps: signal reception, signal transduction, and a cellular response (Figure 27.12a). During signal reception, a specific receptor is activated, as by reversibly binding a signaling molecule. The receptors are often membrane proteins of the sort shown in Section 5.2. Next, the signal is transduced, or converted to a form that acts inside the signal-receiving cell. Some signal receptor proteins are enzymes that undergo a shape change when a signaling molecule binds. Once
Signal Reception Signal binds to a receptor, usually at the cell surface.
a
Signal Transduction Binding brings about changes in cell properties, activities, or both.
Cellular Response Changes alter cell metabolism, gene expression, or rate of division.
activated in this way, the enzyme catalyzes formation of a molecule that then acts as an intracellular signal. Finally, the cell responds to the signal. For example, it may alter its growth or which genes it expresses. Consider one example, a signaling pathway that occurs as an animal develops. As part of development, many cells heed calls to self-destruct at a particular time. Apoptosis is a process of programmed cell death. It often starts when certain molecular signals bind to receptors at the cell surface (Figure 27.12b). A chain of reactions leads to the activation of self-destructive enzymes. Some of these enzymes chop up structural proteins, such as cytoskeleton proteins and histones that organize DNA. Others snip apart nucleic acids. An animal cell undergoing apoptosis shrinks away from its neighbors. Its membrane bubbles inward and outward. The nucleus and then the whole cell break apart. Phagocytic white blood cells that patrol tissues engulf the dying cells and their remnants. Enzymes in the phagocytes digest the engulfed bits. Many cells committed suicide as your hands were developing. Each hand starts as a paddlelike structure. Normally, apoptosis in vertical rows of cells divides the paddle into individual fingers within a few days (Figure 27.13). When the cells do not die on cue, the paddle does not split properly (Figure 27.14). Besides helping to sculpt certain developing body parts, apoptosis also removes aged or damaged cells from a body. For example, keratinocytes are the main cells in your skin. Normally they live for three weeks or so, then undergo apoptosis. Formation of new cells balances out the death of old ones, so your skin stays
Signal to die docks at receptor.
Signal leads to activation of proteindestroying enzymes.
b
Figure 27.12 (a) Signal transduction pathway. A signaling molecule docks at a receptor. The signal activates enzymes or other cytoplasmic components that cause changes inside the cell. (b) An artist’s fanciful depiction of what happens during apoptosis, the process by which a body cell self-destructs. Figure It Out: What are the blue objects with sharp blades? Answer: Protein-destroying enzymes
470 UNIT IV
EVOLUTION AND BIODIVERSITY
IMPACTS, ISSUES REVISITED
A Cautionary Tale
A parked car can heat up quickly even on a mild day. Children’s bodies do not regulate temperature as well as adults’ bodies do. Together, these facts can add up to tragedy. Between 1997 and 2007, 339 children who were left alone in cars died of heat stroke. In some cases, an adult unknowingly left the child behind, but about 20 percent of deaths occurred after an adult deliberately left an infant or child in the car.
How would you vote? Children left alone in cars have died of heat stroke. Should it be illegal to leave a child in a car for even a minute? See CengageNOW for details, then vote online.
Summary
a
b
Figure 27.13 Animated Formation of human fingers. (a) Forty-eight days after fertilization, tissue webs connect embryonic digits. (b) Three days later, after apoptosis by cells making up the tissue webs, the digits are separated.
Section 27.1 Anatomy is the scientific study of body form, and physiology is the study of body functions. Structural and functional organization emerges during the growth and development of an individual. Bodies have levels of organization. Each cell carries out metabolic tasks that keep it alive. At the same time, individual cells interact in tissues, and often, in organs and organ systems. Together cells, tissues, and organs maintain conditions in the extracellular fluid (ECF), the fluid outside of cells. Maintaining the ECF is an aspect of homeostasis: the process of keeping the conditions inside a body within a range that body cells can tolerate.
Figure 27.14 Digits remained attached when embryonic cells did not commit suicide on cue.
uniformly thick. If you spend too much time in the sun, cells enter apoptosis ahead of schedule, so your skin peels. Peeling is bad news for individual cells but it helps protect your body. Cells exposed to excess UV radiation often end up with damaged DNA and are more likely to become cancerous. Some walled plant cells also die on cue. They get emptied of cytoplasm, and the walls of abutting ones act as pipelines for water. Cells that attach leaves to a stem die in response to seasonal change or stress, and leaves are shed. When a plant tissue is wounded or attacked by a pathogen, signals may trigger the death of nearby cells, which form a wall around the threat, as described in the previous section. Take-Home Message How do cells in a multicelled body communicate? Cell communication involves binding of signaling molecules to membrane receptors, transduction of that signal, and the cellular response to it.
Use the animation on CengageNOW to investigate the structural organization of a tomato plant.
Section 27.2 Plants and animals have adapted to some of the same environmental challenges. Small plants and animals rely on diffusion of material through their body. Larger ones have vascular tissues. Active transport and passive transport maintain water and solute concentrations inside both plants and animals. Both groups have mechanisms that allow them to respond to signals from other cells, as well as to environmental changes. Sections 27.3, 27.4 In animal bodies, receptors detect stimuli and send signals to an integrator such as a brain. Signals from the integrator cause effectors (muscles and glands) to respond. With negative feedback mechanisms, receptors detect a change, then effectors respond and reverse the change. Such mechanisms act in homeostasis. With positive feedback mechanisms, detection of a change leads to a response that intensifies the change. Heat stroke is an example of the consequences of a failure of homeostasis.
Use the animation on CengageNOW to observe the effects of negative feedback on temperature control in humans.
Section 27.5 Plants do not have a brain, but they do have decentralized mechanisms of homeostasis, such as systemic acquired resistance to pathogens and an ability to wall off a wound (compartmentalization). Plants respond to changes in their environment when they fold leaves in ways that minimize water loss or help maintain temperature. Rhythmic leaf folding is a type of circadian rhythm, an event repeated on a 24-hour cycle.
Use the animation on CengageNOW to learn about plant defense mechanisms.
CHAPTER 27
PLANTS AND ANIMALS—COMMON CHALLENGES 471
Data Analysis Exercise As part of ongoing efforts to prevent heat-related illness, the National Weather Service has devised a heat index (HI) to alert people to the risks of high temperature coupled with high humidity. The heat index is sometimes referred to as the “apparent temperature.” It tells you what the temperature feels like, given the level of relative humidity. The higher the HI value, the higher the heat disorder risk with prolonged exposure or with exertion. Figure 27.15 shows the heat index chart. The maximum possible value is 137. Gold indicates temperatures near the danger level, orange indicates danger, and pink means extreme danger.
Relative humidity (%) Temp (˚F) 40 45 50 55 60 65 70 75 80 85 90 95 100
1. What is the heat index on a day when the temperature is 96°F and the relative humidity is 45 percent? 2. What is the heat index on a day when the temperature is 96°F and the relative humidity is 75 percent? 3. How does the danger level indicated by these two heat index values compare? 4. What is the lowest temperature that, when coupled with 100% relative humidity, can cause extreme danger?
Section 27.6 Communication between cells involves signal reception, signal transduction, and a response by a target cell. Many signals are transduced by membrane proteins that trigger reactions in the cell. Reactions may alter gene expression or metabolic activities. An example is a signal that unleashes the protein-cleaving enzymes of apoptosis, the programmed self-destruction of a cell.
Use the animation on CengageNOW to see how a human hand forms.
Self-Quiz
Answers in Appendix III
1. Fill in the blank. An increase in the number, size, and volume of plant cells or animal cells is called . 2. A leaf is an example of a. a tissue b. an organ
. c. an organ system d. none of the above
3. A substance moves spontaneously to a region of lower concentration by the process of . a. diffusion c. passive transport b. active transport d. a and c 4. Aerobic respiration occurs in . a. plants c. both plants and animals b. animals d. neither 5. A plant’s xylem and phloem are tissues. a. vascular c. respiratory b. sensory d. digestive 6. An animal’s muscles and glands are a. integrators c. effectors b. receptors d. all are correct
.
7. Fill in the blank: With feedback, a change in conditions triggers a response that intensifies that change. 472 UNIT IV
EVOLUTION AND BIODIVERSITY
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Figure 27.15 Heat index (HI) chart.
8. Systemic acquired resistance . a. helps protect plants from infections b. is an example of a circadian response c. requires white blood cells d. all are correct 9. When a signal is transduced, it is . a. heightened c. converted to a new form b. dampened d. ignored 10. The process of a paddlelike form. a. apoptosis b. transduction
sculpts a developing hand from c. positive feedback d. diffusion
11. Match the terms with their most suitable description. circadian rhythm a. programmed cell death homeostasis b. 24-hour or so cyclic activity apoptosis c. central command center integrator d. stable internal environment effectors e. muscles and glands negative f. an activity changes some feedback condition, then the change triggers its own reversal
Visit CengageNOW for additional questions.
Critical Thinking 1. The Arabian oryx (Oryx leucoryx), an endangered antelope, lives in the harsh deserts of the Middle East. Most of the year there is no free water, and temperatures routinely reach 47°C (117°F). The most common tree in the region is the umbrella thorn tree (Acacia tortilis). List the common challenges faced by the oryx and acacia that are unlike those faced by plants and animals in other environments. 2. Eating a heavy, protein-rich meal on a hot day can increase the risk of heat illness. Why?
V
HOW PLANTS WORK
The sacred lotus, Nelumbo nucifera, busily doing what its ancestors did for well over 100 million years—flowering spectacularly during the reproductive phase of its life cycle.
473
28
Plant Tissues IMPACTS, ISSUES
Droughts Versus Civilization
The more we dig up records of past climates, the more we
A catastrophic drought contributed to the collapse of the
wonder about what is happening now. In any given year,
Mayan civilization centuries ago (Figure 28.1). More recently,
places around the world have severe droughts—far less rain-
Afghanistan was scorched by seven years of drought—the
fall than we expect to see. In themselves, droughts are not
worst in the past century. The vast majority of Afghans are
that unusual, but some have been severe enough to cause
subsistence farmers; the drought wiped out their harvests,
mass starvation, cripple economies, and invite conflicts
dried up their wells, and killed their livestock. Despite relief
over dwindling resources. What is the long-term forecast?
efforts, starvation was rampant. Desperate rural families sold
As global warming changes Earth’s weather patterns, heat
their land, their possessions, and their daughters. As of this
waves are expected to be more intense, and droughts more
writing, extreme drought is affecting southern China and
frequent and more severe.
about one-third of the continental United States; Australia
Humans built the whole of modern civilization on a vast agricultural base. Today we reel from droughts that last two,
is in the middle of the worst drought in 1,000 years. This unit focuses on seed-bearing vascular plants, espe-
five, seven years or so. Imagine one lasting 200 years! It hap-
cially the flowering types that are integral to our lives. You will
pened. About 3,400 years ago, rainfall dried up and brought
be looking at how these plants function and at their patterns
an end to the Akkadian civilization in northern Mesopotamia.
of growth, development, and reproduction. You will consider
We know about the drought from ice cores. Researchers take
how they are adapted to withstand a variety of stressful con-
such samples by drilling a long pipe down through deep ice,
ditions and why prolonged water deprivation kills them.
then pulling it out. The ice core inside the pipe holds dust and
The vulnerability of the agricultural base for societies around
air bubbles trapped in layers of snow that fell year in, year
the world will impact your future. Which nations will stumble
out. The ice in some regions is more than 3,000 meters (9,800
during long-term climate change? Which ones will make it
feet) thick, and has layers that have accumulated over the last
through a severe drought that does not end any time soon?
200,000 years. These layers hold clues to past atmospheric conditions, and they point to recurring climate changes that may have brought an end to many societies around the world.
See the video! Figure 28.1 We depend on adaptations by which plants get and use resources, which include water. Directly or indirectly, plants make the food that sustains nearly all forms of life on Earth. Left, mute reminder of the failed Mayan civilization. Above, from a Guatemalan field, a stunted corncob—a reminder of prolonged drought and widespread crop failures.
Links to Earlier Concepts
Key Concepts Overview of plant tissues
This chapter builds on what you learned in Sections 23.1, 23.8, and 27.1, which introduced plant structure and growth, and correlated them with present and past functions.
You will revisit some structural specializations of plant cells (4.12, 7.7, 23.2), and see how water-conserving adaptations (27.5) function in plant homeostasis (27.1, 27.2). You will also see how secondary growth is part of compartmentalization (27.5).
Seed-bearing vascular plants have a shoot system, which includes stems, leaves, and reproductive parts. Most also have a root system. Such plants have ground, vascular, and dermal tissues. Plants lengthen or thicken only at active meristems. Sections 28.1, 28.2
Organization of primary shoots Ground, vascular, and dermal tissues are organized in characteristic patterns in stems and leaves. The patterns differ between monocots and eudicots. Stem and leaf specializations maximize sunlight interception, water conservation, and gas exchange. Sections 28.3, 28.4
Organization of primary roots Ground, vascular, and dermal tissues are organized in a characteristic pattern in roots. The pattern differs between monocots and eudicots. Roots absorb water and minerals, and anchor the plant. Section 28.5
Secondary growth In many plants, older branches and roots put on secondary growth that thickens them during successive growing seasons. Wood is extensive secondary growth. Sections 28.6, 28.7
Modified stems Certain types of stem modifications are adaptations for storing water or nutrients, or for reproduction. Section 28.8
How would you vote? Large-scale farms and large cities compete for clean, fresh water. Should cities restrict urban growth? Should farming be restricted to areas with sufficient rainfall to sustain agriculture? See CengageNOW for details, then vote online.
475
28.1
The Plant Body The unique organization of tissues in flowering plants is part of the reason why they are the dominant group of the plant kingdom.
Links to Plant evolution 23.1, Angiosperms 23.8, Evolution of plant structure 27.1
shoot tip (terminal bud) lateral (axillary) bud young leaf flower
node internode dermal tissue
node
vascular tissues
leaf
seeds in fruit
withered seed leaf (cotyledon)
ground tissues
stem SHOOTS ROOTS
primary root lateral root root hairs
root tip
The Basic Body Plan Figure 28.2 shows the body plan of a typical flowering plant. It has shoots: aboveground parts such as stems, leaves, and flowers. Stems support upright growth, a bonus for cells that intercept energy from the sun. They also connect the leaves and flowers with roots, which are structures that absorb water and dissolved minerals as they grow down and outward in the soil. Roots often anchor the plant. All root cells store food for their own use, and some types also store it for the rest of the plant body. Shoots and roots consist of three tissue systems. The ground tissue system functions in several tasks, such as photosynthesis, storage, and structural support of other tissues. Pipelines of the vascular tissue system distribute water and mineral ions that the plant takes up from its surroundings. They also carry sugars produced by photosynthetic cells to the rest of the plant. The dermal tissue system covers and protects exposed surfaces of the plant. The ground, vascular, and dermal tissue systems consist of cells that are organized as simple and complex tissues. Simple tissues are constructed primarily of one type of cell; examples include parenchyma, collenchyma, and sclerenchyma. Complex tissues have two or more types of cells. Xylem, phloem, and epidermis are examples. You will learn more about all of these tissues in the next section.
Eudicots and Monocots—Same Tissues, Different Features The same tissues form in all flowering plants, but they do so in different patterns. Consider cotyledons, which are leaflike structures that contain food for a plant embryo. These “seed leaves” wither after the seed germinates and the developing plant begins to make its own food by photosynthesis. Cotyledons consist of the same types of tissues in all plants that have them, but the seeds of eudicots have two cotyledons and those of monocots have only one. Figure 28.3 shows other differences between these two types of flowering plants. Most shrubs and trees, such as rose bushes and maple trees, are eudicots. Lilies, orchids, and corn are typical monocots.
root cap
Introducing Meristems Figure 28.2 Animated Body plan of a tomato plant (Lycopersicon esculentum). Its vascular tissues (purple) conduct water, dissolved minerals, and organic substances. They thread through ground tissues that make up most of the plant. Epidermis, a type of dermal tissue, covers root and shoot surfaces.
476 UNIT V
HOW PLANTS WORK
All plant tissues arise at meristems, each a region of undifferentiated cells that can divide rapidly. Portions of the descendant cells differentiate and mature into
A
Characteristics of Eudicots
In seeds, two cotyledons (seed leaves of embryo)
B
Flower parts in fours or fives (or multiples of four or five)
Leaf veins usually forming a netlike array
Pollen grains with three pores or furrows
Vascular bundles organized in a ring in ground tissue
Leaf veins usually running parallel with one another
Pollen grains with one pore or furrow
Vascular bundles throughout ground tissue
Characteristics of Monocots
In seeds, one cotyledon (seed leaf of embryo)
Flower parts in threes (or multiples of three)
Figure 28.3 Animated Comparison of eudicots and monocots.
Figure 28.4 Right, locations of apical and lateral meristems.
specialized tissues. New, soft plant parts lengthen by activity at apical meristems in the tips of shoots and roots. The seasonal lengthening of young shoots and roots is called primary growth (Figure 28.4a). Some plants also undergo secondary growth—their stems and roots thicken over time. In woody eudicots and in gymnosperms such as pine trees, secondary growth occurs when cells of a thin cylindrical layer called the lateral meristem divide (Figure 28.4b).
shoot apical meristem (new cells forming) cells dividing, differentiating
three tissue systems developing
three tissue systems developing
cells dividing, differentiating root apical meristem (new cells forming)
a–Many cellular descendants of apical meristems are the start of lineages of differentiated cells that grow, divide, and lengthen shoots and roots.
vascular cambium
Take-Home Message
cork cambium
What is the basic structure of flowering plants? Plants typically have aboveground shoots, such as stems, leaves, and flowers. All have ground, vascular, and dermal tissue systems. The patterns in which plant tissues are organized differ between eudicots and monocots.
Plants lengthen, or put on primary growth, at soft shoot and root tips. Many plants put on secondary growth; older stems and roots thicken over successive growing seasons.
thickening
b In woody plants, the activity of two lateral meristems—vascular cambium and cork cambium—result in secondary growth that thickens older stems and roots.
CHAPTER 28
PLANT TISSUES 477
28.2
Plant Tissues
sclerenchyma (fibers)
parenchyma
epidermis
Different plant tissues form just behind shoot and root tips, and on older stem and root parts. Links to Plant cell surface specializations 4.12, Stomata 7.7, Lignin in plant evolution 23.2, Growth 27.1
Table 28.1 summarizes the common plant tissues and their functions. Some of these tissues are visible in the micrograph shown in Figure 28.5. Plant parts are typically cut along standard planes like this cross-section in order to simplify our interpretation of micrographs (Figure 28.6).
phloem
Simple Tissues
Figure 28.5 Some tissues in a buttercup stem (Ranunculus).
Parenchyma tissue makes up most of the soft primary growth of roots, stems, leaves, and flowers, and it also has storage and secretion functions. Parenchyma is a
simple tissue that consists mainly of parenchyma cells, which are typically thin-walled, flexible, and manysided. These cells are alive in mature tissue, and they can continue to divide. Plant wounds are repaired by dividing parenchyma cells. Mesophyll, the only photosynthetic tissue, is a type of parenchyma. Collenchyma is a simple tissue that consists mainly of collenchyma cells, which are elongated and alive in mature tissue. This stretchable tissue supports rapidly growing plant parts, including young stems and leaf stalks (Figure 28.7a). Pectin, a polysaccharide, imparts flexibility to a collenchyma cell’s primary wall, which is thickened where three or more of the cells abut. Cells of sclerenchyma are variably shaped and dead at maturity, but the lignin-rich walls that remain help this tissue resist compression. Remember, lignin is the organic compound that structurally supports upright plants, and helped them evolve on land (Section 23.2). Lignin also deters some fungal attacks. Fibers and sclereids are typical sclerenchyma cells. Fibers are long, tapered cells that structurally support the vascular tissues in some stems and leaves (Figure 28.7b). They flex and twist, but resist stretching. We use certain fibers as materials for cloth, rope, paper, and other commercial products. The far stubbier and often branched sclereids strengthen hard seed coats, such as peach pits, and make pear flesh gritty (Figure 28.7c).
radial:
tangential:
transverse:
Figure 28.6 Terms that identify how tissue specimens are cut from a plant. Longitudinal cuts along a stem or root radius give radial sections. Cuts at right angles to the radius give tangential sections. Cuts perpendicular to the long axis of a stem or root give transverse sections—that is, cross-sections.
Table 28.1
Overview of Flowering Plant Tissues
Tissue Type
Main Components
Main Functions
Parenchyma
Parenchyma cells
Photosynthesis, storage, secretion, tissue repair, other tasks
Collenchyma
Collenchyma cells
Pliable structural support
Sclerenchyma
Fibers or sclereids
Structural support
Tracheids, vessel members; parenchyma cells; sclerenchyma cells
Water-conducting tubes; reinforcing components
Sieve-tube members, parenchyma cells; sclerenchyma cells
Tubes of living cells that distribute organic compounds; supporting cells
Undifferentiated as well as specialized cells (e.g., guard cells)
Secretion of cuticle; protection; control of gas exchange and water loss
Cork cambium; cork cells; parenchyma
Forms protective cover on older stems, roots
Simple Tissues
Complex Tissues Vascular Xylem
Phloem
Dermal Epidermis
Periderm
xylem
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HOW PLANTS WORK
Vascular Tissues Xylem and phloem are vascular tissues that thread through ground tissue. Both consist of elongated conducting tubes that are often sheathed in sclerenchyma fibers and parenchyma. Xylem, which conducts water and mineral ions, consists of two types of cells, tracheids and vessel members, that are dead at maturity (Figure 28.8a,b). The secondary walls of these cells are stiffened and waterproofed with lignin. They
collenchyma
parenchyma
lignified secondary wall
Figure 28.7 Simple tissues. (a) Collenchyma and parenchyma from a supporting strand inside of a celery stem, transverse section.
a
b
interconnect to form conducting tubes, and they also lend structural support to the plant. The perforations in adjoining cell walls align, so fluid moves laterally between the tubes as well as upward through them. Phloem conducts sugars and other organic solutes. Its main cells, sieve-tube members, are alive in mature tissue. They connect end to end at sieve plates, forming sieve tubes that distribute sugars to all parts of the plant (Figure 28.8c). Phloem’s companion cells are parenchyma cells that load sugars into the sieve tubes. Dermal Tissues The first dermal tissue to form on a plant is epidermis, which usually is a single layer of cells. Secretions deposited on the outward-facing cell walls form a cuticle. Plant cuticle is rich in deposits of cutin, a waxy substance. It helps the plant conserve water and repel pathogens (Figures 28.5 and 28.9). The epidermis of leaves and young stems includes specialized cells. For example, a stoma is a small gap across epidermis; it opens when the pair of guard cells around it swells (Section 7.7). Diffusion of water vapor, oxygen, and carbon dioxide gases across the epidermis is controlled at stomata. Periderm, a different tissue, replaces epidermis in woody stems and roots.
Sclerenchyma: (b) Fibers from a strong flax stem, tangential view. (c) Stone cells, a type of sclereid in pears, transverse section.
c
one cell’s wall
sieve plate of sievetube cell
pit in wall
b
a
parenchyma
companion cell
vessel of xylem
c
phloem
fibers of sclerenchyma
Figure 28.8 Simple and complex tissues in a stem. In xylem, (a) part of a column of vessel members, and (b) a tracheid. (c) One of the living cells that interconnect as sieve tubes in phloem.
Take-Home Message What are the main types of plant tissues? Cells of parenchyma have diverse roles, such as secretion, storage, photosynthesis, and tissue repair. Collenchyma and sclerenchyma support and strengthen plant parts. Xylem and phloem are vascular tissues that thread through the ground tissue. In xylem, water and ions flow through tubes of dead tracheid and vessel member cells. In phloem, sieve tubes that consist of living cells distribute sugars.
leaf surface
cuticle
epidermal cell
photosynthetic cell
Figure 28.9 A typical plant cuticle, with many epidermal cells and photosynthetic cells under it.
Epidermis covers all young plant parts exposed to the surroundings. Periderm that forms on older stems and roots replaces epidermis of younger stems.
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28.3
Primary Structure of Shoots Inside the soft, young stems and leaves of both eudicots and monocots, the ground, vascular, and dermal tissue systems are organized in predictable patterns.
Behind the Apical Meristem The structural organization of a new flowering plant has become mapped out by the time it is an embryo sporophyte inside a seed coat. As you will read later, a tiny primary root and shoot have already formed as part of the embryo. Both are poised to resume growth and development as soon as the seed germinates. Terminal buds are a shoot’s main zone of primary growth. Just beneath a terminal bud’s surface, cells of shoot apical meristem divide continually during the growing season. Some of the descendants divide and differentiate into specialized tissues. Each descendant cell lineage divides in particular directions, at different rates, and the cells go on to differentiate in size, shape, and function. Figure 28.10 shows an example.
Buds may be naked or encased in modified leaves called bud scales. Small regions of tissue bulge out near the sides of a bud’s apical meristem; each is the start of a new leaf. As the stem lengthens, the leaves form and mature in orderly tiers, one after the next. A region of stem where one or more leaves form is called a node; the region between two successive nodes is called an internode (Figure 28.2). Lateral buds, or axillary buds, are dormant shoots of mostly meristematic tissue. Each one forms inside a leaf axil, the point at which the leaf is attached to the stem. Different kinds of axillary buds are the start of side branches, leaves, or flowers. A hormone secreted by a terminal bud can keep lateral buds dormant, as Section 31.2 will explain.
Inside the Stem In most flowering plants, cells of primary xylem and phloem are bundled together as long, multistranded
immature leaf immature leaf
youngest immature leaf
shoot apical meristem
apical meristem
Figure 28.10 Stem of Coleus, a eudicot. (a–c) Successive stages of the stem’s primary growth, starting with the shoot apical meristem. (d) The light micrograph shows a longitudinal cut through the stem’s center. The tiers of leaves in the photograph below it formed in this linear pattern of development. Figure It Out:
What is the transparent layer of cells on the outer surface of b and c?
a Sketch of the shoot tip in the micrograph at right, tangential cut. The descendant meristematic cells are color-coded orange.
epidermis forming lateral bud forming vascular tissues forming
b Same tissue region later on, after the shoot tip lengthened above it
cortex
primary phloem
pith
primary xylem pith
Answer: Epidermis c Same tissue region later still, with lineages of cells lengthening and differentiating
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HOW PLANTS WORK
d
vessel in xylem
meristem cell
epidermis cortex vascular bundle pith
sieve tube in phloem
companion cell in phloem
A Stem fine structure for alfalfa (Medicago), a eudicot air collenchyma sheath cell space
vessel in xylem
epidermis vascular bundle pith
sieve tube in phloem
B Stem fine structure for corn (Zea mays), a monocot
companion cell in phloem
Figure 28.11 Animated Zooming in on a eudicot and a monocot stem.
cords in the same cylindrical sheath of cells. The cords are called vascular bundles, and they thread lengthwise through the ground tissue system of all shoots. Vascular bundles form in two distinct patterns. The vascular bundles of most eudicots form in a cylinder that runs parallel with the long axis of the shoot. Figure 28.11a shows how the cylinder divides the parenchyma of ground tissue into cortex (parenchyma between the vascular bundles and the epidermis) and pith (parenchyma inside the cylinder of vascular bundle). Most monocot and some magnoliids have a different arrangement. Vascular bundles in stems of these
plants are distributed all throughout the ground tissue (Figure 28.11b). In the next chapter, you will see how these vascular tissues take up, conduct, and give up water and solutes throughout the plant. Take-Home Message How are plant tissues organized inside stems? Buds are the main zones of primary growth in shoots. Ground, vascular, and dermal tissues form in organized patterns.
The arrangement of vascular bundles, which are multistranded cords of vascular tissue, differs between eudicot and monocot stems.
CHAPTER 28
PLANT TISSUES 481
28.4
A Closer Look at Leaves All leaves are metabolic factories where photosynthetic cells churn out sugars, but they vary in size, shape, surface specializations, and internal structure. Links to Plasmodesmata 4.12, Photosynthesis in leaf cells 7.7, Water conservation adaptations in plants 27.5
petiole
axillary bud
blade
node sheath blade
stem node a
c
elliptic
palmate
b
lobed
pinnatisect
Leaves differ in size and structure. A leaf of duckweed is 1 millimeter (0.04 inch) across; leaves of one palm (Raphia regalis) can be 25 meters (82 feet) long. Leaves are shaped like cups, needles, blades, spikes, tubes, or feathers. They differ in color, odor, and edibility (some make toxins). Leaves of deciduous species wither and drop from their stems seasonally. Leaves of evergreen plants also drop, but not all at the same time. Figure 28.12 shows examples of leaf shapes. A typical leaf has a flat blade and, in eudicots, a petiole, or stalk, attached to the stem. The leaves of most monocots are flat blades, the base of which forms a sheath called a coleoptile around the stem. Grasses are examples. Simple leaves are undivided, but many are lobed. Compound leaves are blades divided into leaflets. Leaf shapes and orientations are adaptations that help a plant intercept sunlight and exchange gases. Most leaves are thin, with a high surface-to-volume ratio; many reorient themselves during the day so that they stay perpendicular to the sun’s rays. Typically, adjacent leaves project from a stem in a pattern that allows sunlight to reach them all. However, the leaves of plants native to arid regions may stay parallel to the sun’s rays, reducing heat absorption and thus conserving water (Section 27.5). The thick or needlelike leaves of some plants also conserve water. Leaf Epidermis Epidermis covers every leaf surface
d acuminate odd pinnate
elliptic odd pinnate
lobed odd bipinnate
Figure 28.12 Common leaf forms of (a) eudicots and (b) monocots, and a few examples of (c) simple leaves and (d) compound leaves.
exposed to the air. This surface tissue may be smooth, sticky, or slimy, with hairs, scales, spikes, hooks, and other specializations (Figure 28.13). A cuticle coating restricts water loss from the sheetlike array of epidermal cells (Figures 28.9 and 28.14). Most leaves have far more stomata on the lower surface. In arid habitats, stomata and epidermal hairs often are positioned in depressions in the leaf surface. Both of these adaptations help conserve water. Mesophyll—Photosynthetic Ground Tissue Each leaf
has mesophyll, a photosynthetic parenchyma with air spaces between cells (Section 7.7 and Figure 28.14). Carbon dioxide reaches the cells by diffusing into the leaf through stomata, and oxygen released by photo-
Figure 28.13 Example of leaf cell surface specialization: hairs on a tomato leaf. The lobed heads are glandular structures that occur on the leaves of many plants; they secrete aromatic chemicals that deter plant-eating insects. Those on marijuana plants secrete the psychoactive chemical tetrahydrocannabinol (THC).
50 µm
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HOW PLANTS WORK
leaf vein (one vascular bundle) xylem
cuticle upper epidermis
phloem
A
palisade mesophyll
Water, dissolved mineral ions from roots and stems move into leaf vein (blue arrow).
Photosynthetic products (pink arrow) enter vein, will be distributed through plant.
B
spongy mesophyll
lower epidermis C epidermal cell Oxygen and water vapor (blue arrow) diffuse out of leaf through stomata.
Carbon dioxide (pink arrow) in outside air diffuses into leaf through stomata.
stoma (small gap across lower epidermis)
D
Figure 28.14 Animated Leaf organization for Phaseolus, a bean plant. (a) Foliage leaves. (b–d) Leaf fine structure.
synthesis diffuses out the same way. Plasmodesmata connect the cytoplasm of adjacent cells. Substances can flow rapidly across the walls of adjoining cells through these cell junctions (Section 4.12). Leaves oriented perpendicular to the sun have two layers of mesophyll. Palisade mesophyll is attached to the upper epidermis. The elongated parenchyma cells of this tissue have more chloroplasts than cells of the spongy mesophyll layer below (Figure 28.14). Blades of grass and other monocot leaves that grow vertically can intercept light from all directions. The mesophyll in such leaves is not divided into two layers. Leaf veins are vascular bundles typically strengthened with fibers. Inside the bundles, continuous strands of xylem rapidly transport water and dissolved ions to mesophyll. Continuous strands of phloem rapidly transport the products of photosynthesis (sugars) away from mesophyll. In most eudicots, large veins branch into a network of minor veins embedded in mesophyll. In most monocots, all veins are similar in length and run parallel with the leaf’s long axis (Figure 28.15).
Veins—The Leaf’s Vascular Bundles
a
b
Figure 28.15 Typical vein patterns in flowering plants. (a) The netlike array in this grape leaf is common among eudicots. A stiffened midrib runs from the petiole to the leaf tip. Ever smaller veins branch from it. (b) The strong parallel orientation of veins in an Agapanthus leaf is typical of monocots. Like umbrella ribs, stiffened veins help maintain leaf shape.
Take-Home Message How does a leaf’s structure contribute to its function? A leaf’s shape, orientation, and structure typically function in sunlight interception, gas exchange, and distribution of water and solutes to and from living cells. Its epidermis encloses mesophyll and veins.
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28.5
Primary Structure of Roots Roots mainly function to provide plants with a large surface area for absorbing water and dissolved mineral ions.
Link to Homeostasis in plants 27.1 and 27.2
Unless tree roots start to buckle a sidewalk or clog a sewer line, flowering plant root systems tend not to occupy our thoughts. Yet these are dynamic systems that actively mine soil for water and minerals. Most grow no deeper than 5 meters (16 feet). However, the roots of one hardy mesquite shrub grew 53.4 meters (175 feet) down into the soil near a streambed. Some
A Organization of a primary root, showing the zones where cells divide, lengthen, and differentiate into primary tissues. The oldest cells in a root are farthest from the apical meristem, which is protected by the root cap. Drawing is of a eudicot root; not to scale.
types of cactus have shallow roots that can radiate 15 meters (50 feet) from the plant. Someone measured the roots of a young rye plant that had been growing for four months in 6 liters (1.6 gallons) of soil. If the surface area of that root system were laid out as one sheet, it would occupy over 600 square meters, or close to 6,500 square feet! A root’s structural organization begins in a seed. As the seed germinates, a primary root pokes through the seed coat. In nearly all eudicot seedlings, that young root thickens.
VASCULAR CYLINDER
endodermis pericycle xylem phloem epidermis cortex
The micrograph below shows a radial section of a root tip of Zea mays (corn), a monocot.
root hair
Vessel members are mature; root hairs are about to form. New root cells lengthen, sieve tubes mature, vessel members start forming. endodermis Most cells have stopped dividing.
root cortex
Meristem cells are dividing fast.
pericycle
primary phloem primary xylem
VASCULAR CYLINDER
root tip No cell division is occurring here. root cap
Figure 28.16 Animated Tissue organization of a typical root.
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HOW PLANTS WORK
B Transverse sections of root and vascular cylinder of a buttercup (Ranunculus) plant.
epidermis cortex pith vascular cylinder primary xylem primary phloem
a eudicot root structure
b monocot root structure
c lateral root growing from pericycle
Figure 28.17 Comparison of root structure of (a) a eudicot (buttercup, Ranunculus) and (b) a monocot (corn, Zea mays). In corn and some other monocots, the vascular cylinder divides the ground tissue into cortex and pith. (c) A lateral root forms and branches from the pericycle of Zea mays.
Look at the root tip in Figure 28.16a. Some descendants of root apical meristem give rise to a root cap, a dome-shaped mass of cells that protects the soft, young root as it grows through soil. Other descendants give rise to lineages of cells that lengthen, widen, or flatten when they differentiate as part of the dermal, ground, and vascular tissue systems. Ongoing divisions push cells away from the active root apical meristem. Some of their descendants form epidermis. The root epidermis is the plant’s absorptive interface with soil. Many of its specialized cells send out fine extensions called root hairs, which collectively increase the surface area available for taking up soil water, dissolved oxygen, and mineral ions. Chapter 29 looks at the role of root hairs in plant nutrition. Descendants of meristem cells also form the root’s vascular cylinder, a central column of conductive tissue. The root vascular cylinder of typical eudicots is mainly primary xylem and phloem (Figure 28.17a); that of typical monocots divides the ground tissue into two zones, cortex and pith (Figure 28.17b). The vascular cylinder is sheathed by a pericycle, an array of parenchyma cells one or more layers thick (Figure 28.16b). These cells are differentiated, but they still divide repeatedly in a direction perpendicular to the axis of the root. Masses of cells erupt through the cortex and epidermis as the start of new, lateral roots (Figure 28.17c). As you will see in Chapter 29, water entering a root moves from cell to cell until it reaches the endodermis, a layer of cells that encloses the pericycle. Wherever endodermal cells abut, their walls are waterproofed. Water must pass through the cytoplasm of endodermal cells to reach the vascular cylinder. Transport proteins in the plasma membrane control the uptake of water and dissolved substances.
Root primary growth results in one of two kinds of root systems. The taproot system of eudicots consists of a primary root and its lateral branchings. Carrots, oak trees, and poppies are among the plants that have a taproot system (Figure 28.18a). By comparison, the primary root of most monocots is quickly replaced by adventitious roots that grow outward from the stem. Lateral roots that are similar in diameter and length branch from adventitious roots. Together, the adventitious and lateral roots of such plants form a fibrous root system (Figure 28.18b).
a eudicot
b monocot
Figure 28.18 Different types of root systems. (a) Taproot of the California poppy, a eudicot. (b) Fibrous roots of a grass plant, a monocot.
Take-Home Message What is the function of plant roots? Roots provide a plant with a tremendous surface area for absorbing water and solutes. Inside each is a vascular cylinder, with long strands of primary xylem and phloem.
Taproot systems consist of a primary root and lateral branchings. Fibrous root systems consist of adventitious and lateral roots that replace the primary root.
CHAPTER 28
PLANT TISSUES 485
28.6
Secondary Growth Secondary growth occurs at two types of lateral meristem, vascular cambium and cork cambium.
Link to Compartmentalization 27.5
cork cambium
A Secondary growth (thickening of older stems and roots) occurs at two lateral meristems. Vascular cambium gives rise to secondary vascular tissues; cork cambium gives rise to periderm.
vascular cambium
pith
cortex
stem surface
primary xylem
primary phloem
vascular cambium
B In spring, primary growth resumes at terminal and lateral buds. Secondary growth resumes at vascular cambium. Divisions of meristem cells in the vascular cambium expand the inner core of xylem, which displaces the vascular cambium (orange) toward the surface of the stem or root.
secondary xylem
secondary phloem
Each spring, as primary growth resumes at buds, secondary growth thickens the girth of stems and roots of some plants. Figure 28.19 shows a typical pattern of secondary growth at the vascular cambium. This lateral meristem forms a cylinder, a few cells thick, inside older stems and roots. Divisions of vascular cambium cells produce secondary xylem on the cylinder’s inner surface, and secondary phloem on its outer surface. As the core of xylem thickens, it also displaces the vascular cambium toward the surface of the stem. The displaced cells of the vascular cambium divide in a widening circle, so the tissue’s cylindrical form is maintained. Vascular cambium consists of two types of cells. Long, narrow cells give rise to the secondary tissues that extend lengthwise through a stem or root: tracheids, fibers, and parenchyma in secondary xylem; and sieve tubes, companion cells, and fibers in secondary phloem. Small, rounded cells that divide perpendicularly to the axis of the stem give rise to “rays” of parenchyma, radially oriented like spokes of a bicycle wheel. Secondary xylem and phloem of the rays conduct water and solutes radially through the stems and roots of older plants. A core of secondary xylem, or wood, contributes up to 90 percent of the weight of some plants. Thinwalled, living parenchyma cells and sieve tubes of secondary phloem lie in a narrow zone outside of the vascular cambium. Bands of thick-walled reinforcing fibers are often interspersed through this secondary phloem. The only living sieve tubes are within a centimeter or so of the vascular cambium; the rest are dead, but they help protect the living cells behind them. As seasons pass, the expanding inner core of xylem continues to direct pressure toward the stem or root surface. In time, it ruptures the cortex and the outer
outer surface of stem or root
division Vascular cambium cell as secondary growth starts
division
One of two daughter cells differentiates into a xylem cell (blue); the other stays meristematic.
One of two daughter cells differentiates into a phloem cell (pink); the other stays meristematic.
Figure 28.19 The pattern of cell division and then differentiation into xylem and phloem continues through growing season.
C Overall pattern of growth at vascular cambium.
486 UNIT V
HOW PLANTS WORK
Animated Secondary growth.
bark secondary phloem sapwood (new xylem)
heartwood (old xylem)
vascular cambium
vessel in xylem
direction of growth
periderm (includes cork cambium, cork, some phloem, and new parenchyma)
early late early late
A Structure of a typical woody stem.
early
late
early late
early
B Early and late wood in ash (Fraxinus). Early wood forms during wet springs. Late wood indicates that a tree did not waste energy making large-diameter xylem cells for water uptake during a dry summer or drought.
Figure 28.20 Animated Structure of wood.
secondary phloem. Then, another lateral meristem, the cork cambium, forms and gives rise to periderm. This dermal tissue consists of parenchyma and cork, as well as the cork cambium that produces them. What we call bark is secondary phloem and periderm. Bark consists of all of the living and dead tissues outside of the vascular cambium (Figure 28.20a). The cork component of bark has densely packed rows of dead cells, the walls of which are thickened with a fatty substance called suberin. Cork protects, insulates, and waterproofs the stem or root surface. Cork also forms over wounded tissues. When leaves drop from the plant, cork forms at the places where petioles had attached to stems. Wood’s appearance and function change as a stem or root ages. Metabolic wastes, such as resins, tannins, gums, and oils, clog and fill the oldest xylem so much that it no longer is able to transport water and solutes. These substances often darken and strengthen the wood, which is called heartwood. Sapwood is moist, still-functional xylem between heartwood and vascular cambium. In trees of temperate zones, dissolved sugars travel from roots to buds through sapwood’s secondary xylem in spring. The sugar-rich fluid is sap. Each spring, New Englanders collect maple tree sap to make maple syrup. Vascular cambium is inactive during cool winters or long dry spells. When the weather warms or moisture returns, the vascular cambium gives rise to early wood, with large-diameter, thin-walled cells. Late wood, with small-diameter, thick-walled xylem cells, forms in dry summers. A transverse cut from older trunks reveals
alternating bands of early and late wood (Figure 28.20b). Each band is a growth ring, or “tree ring.” Trees native to regions in which seasonal change is pronounced tend to add one growth ring each year. Those in desert regions may add more than one ring of early wood in response to a single season of plentiful rain. In the tropics, seasonal change is almost nonexistent, so growth rings are not a feature of tropical trees. Oak, hickory, and other eudicot trees that evolved in temperate and tropical zones are hardwoods, with vessels, tracheids, and fibers in xylem. Pines and other conifers are softwoods because they are weaker and less dense than the hardwoods. Their xylem has tracheids and parenchyma rays but no vessels or fibers. Like other organisms, plants compete for resources. Plants with taller stems or broader canopies that defy the pull of gravity also intercept more light energy streaming from the sun. By tapping a greater supply of energy for photosynthesis, they have the metabolic means to produce large root and shoot systems. The larger its root and shoot systems, the more competitive the plant can be in acquiring resources.
Take-Home Message What is secondary growth in plants? Secondary growth thickens the stems and roots of older plants. Wood is mainly accumulated secondary xylem. Secondary growth occurs at two types of lateral meristem: vascular cambium and cork cambium. Secondary vascular tissues form at a cylinder of vascular cambium. A cylinder of cork cambium gives rise to periderm, which is part of a protective covering of bark.
CHAPTER 28
PLANT TISSUES 487
FOCUS ON RESEARCH
28.7
Tree Rings and Old Secrets The number and relative thickness of a tree’s rings hold clues to environmental conditions during its lifetime.
direction of growth
Tree rings can be used to estimate average annual rainfall; to date archaeological ruins; to gather evidence of wildfires, floods, landslides, and glacier movements; and to study the ecology and effects of parasitic insect populations. How? Some tree species, such as redwoods and bristlecone pines, lay down wood over centuries, one ring per year. Count an old tree’s rings, and you have an idea of its age. If you know the year in which the tree was cut, you can find out which ring formed in what year by counting them backwards from the outer edge. Compare the thicknesses of the rings, and you have clues to events in those years (Figure 28.21). For instance, In 1587, about 150 English settlers arrived at Roanoke Island off of the coast of North Carolina. When ships arrived in 1589 to resupply the colony, they discovered that the island had been abandoned. Searches up and down the coast failed to turn up the missing colonists. About twenty years later, the English established a colony at Jamestown, Virginia. Although this colony survived, the initial years were difficult. In the summer of 1610 alone, more than 40 percent of the colonists died, many of them from starvation. Researchers examined wood cores from bald cypress trees (Taxodium distichum) that had been growing at the time the Roanoke and Jamestown colonies were founded. Differences in the thicknesses of the trees’ growth rings revealed that the colonists were in the wrong place at the wrong time (Figure 28.22). The settlers arrived at Roanoke just in time for the worst drought in 800 years. Nearly a decade of severe drought struck Jamestown. We know that the corn crop of the Jamestown colony failed. Drought-related crop failures probably occurred at Roanoke as well. The settlers also had difficulty finding fresh water. Jamestown was established at the head of an estuary; when the river levels dropped, their drinking water supply mixed with ocean water and became salty. Piecing together these bits of evidence gives us an idea of what life must have been like for the early settlers.
Figure 28.22 (a) Location of two of the early American colonies. (b) Rings of a bald cypress tree, transverse section. This tree was living when English colonists first settled in North America. Narrower annual rings mark years of severe drought.
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HOW PLANTS WORK
A Pine is a softwood. It grows fast, so it tends to have wider rings than slower growing species. Note the difference between the appearance of heartwood and sapwood.
B The rings of this oak tree show dramatic differences in yearly growth patterns over its lifetime.
C
An elm made this series between 1911 and 1950.
Figure 28.21 Animated Tree rings. In most species, each ring corresponds to one year, so the number of rings indicate the age of the tree. Relative thickness of the rings can be used to estimate data such as average annual rainfall long before such records were kept. year:
1
2
3
Jamestown Colony
Virginia North Carolina
a
Lost Colony (Roanoke Island)
b
1587–1589
1606 –1612
28.8
Modified Stems
Many plants have modified stem structures that function in storage or reproduction.
The structure of a typical stem is shown in Figure 28.2, but there are many variations on that structure in different types of plants. Most serve special reproductive or storage functions. a
f
Stolons Stolons, often called runners, are stems that
branch from the main stem of the plant, typically on or near the surface of the soil. Stolons may look like roots, but they have nodes; roots do not have nodes. Adventitious roots and leafy shoots that sprout from the nodes develop into new plants (Figure 28.23a). Rhizomes Rhizomes are fleshy, scaly stems that typi-
cally grow under the soil and parallel to its surface. A rhizome is the main stem of the plant, and it also serves as the plant’s primary storage tissue. Branches that sprout from nodes grow aboveground for photosynthesis and flowering. Examples include ginger, irises, many ferns, and some grasses (Figure 28.23b). Bulbs A bulb is a short section of underground stem
encased by overlapping layers of thickened, modified leaves called scales. The scales contain starch and other substances that a plant holds in reserve when conditions in the environment are unfavorable for growth. When favorable conditions return, the plant uses these stored substances to sustain rapid growth. The scales develop from a basal plate, as do roots. A dry, paperlike outermost scale of many bulbs serves as a protective covering. An onion is an example (Figure 28.23c). Corms A corm is a thickened underground stem that stores nutrients. Like a bulb, a corm has a basal plate from which roots grow. Unlike a bulb, a corm is solid rather than layered, and it has nodes from which new plants develop (Figure 28.23d). Tubers Tubers are thickened portions of underground
e
b
c
d
Figure 28.23 Variations on a stem. Counterclockwise from top: (a) plants such as this aquatic eelgrass (Vallisneria) propagate themselves by sending out stolons. New plants develop at nodes in the stolons. (b) The main stems of turmeric plants (Curcuma longa) are undergound rhizomes. (c) Clearly visible scales of an onion (Allium cepa) surround the stem at the center of the bulb.
stolons; they are the plant’s primary storage tissue. Tubers are like corms in that they have nodes from which new shoots and roots sprout, but they do not have a basal plate. Potatoes are tubers; their “eyes” are the nodes (Figure 28.23e).
(d) Taro, also known as arrowroot, is a corm of Colocasia esculenta plants. Corms, unlike bulbs, do not have layers of scales. (e) Potatoes are tubers that grow on stolons of Solanum tuberosum plants. (f) The stems of prickly pear (Opuntia) are spiky cladodes. These paddlelike structures store water, allowing the plant to survive in very dry regions.
Cladodes Cactuses and other succulents have photo-
Take-Home Message
synthetic stems called cladodes: flattened stems that store water. New plants form at the nodes. The cladodes of some plants appear quite leaflike, but most are unmistakably fleshy (Figure 28.23f ).
Are all stems alike?
Many plants have modified stems that function in storage or reproduction. Stolons, rhizomes, bulbs, corms, tubers, and cladodes are examples.
CHAPTER 28
PLANT TISSUES 489
IMPACTS, ISSUES REVISITED
Droughts Versus Civilization
Even a short drought reduces photosynthesis and crop yields. Like other plants, crop plants conserve water by closing stomata, which of course also stops carbon dioxide from moving in. Without a continuous supply of carbon dioxide, the plant’s photosynthetic cells cannot continue to make sugars. Drought-stressed flowering plants make fewer flowers or stunted ones. Even if flowers get pollinated, fruits may fall off the plant before ripening.
How would you vote? Should cities restrict urban growth? Should farming be restricted to areas with sufficient rainfall to sustain agriculture? See CengageNOW for details, then vote online.
Summary Section 28.1 Most flowering plants have aboveground shoots, including stems, photosynthetic leaves, and flowers. Most kinds also have roots. Shoots and roots consist of ground, vascular, and dermal tissue systems. Ground tissues store materials, function in photosynthesis, and structurally support the plant. Tubes in vascular tissues conduct substances to all living cells. Dermal tissues protect plant surfaces. Monocots and eudicots consist of the same tissues organized in different ways. For example, monocots and eudicots differ in how xylem and phloem are distributed through ground tissue, in the number of petals in flowers, and in the number of cotyledons. All plant tissues originate at meristems, which are regions of undifferentiated cells that retain their ability to divide. Primary growth (or lengthening) arises from apical meristems. Secondary growth (or thickening) arises from lateral meristems.
Use the animation on CengageNOW to explore a plant body plan and to compare monocot and eudicot tissues.
Section 28.2 The simple plant tissues are parenchyma, collenchyma, and sclerenchyma. The living, thin-walled cells in parenchyma have diverse roles in ground tissue. Photosynthetic parenchyma is called mesophyll. Living cells in collenchyma have sturdy, flexible walls that support fast-growing plant parts. Cells in sclerenchyma die at maturity, but their lignin-reinforced walls remain and support the plant. Vascular tissues (xylem and phloem) and dermal tissues (epidermis and periderm) are examples of complex plant tissues. Vessel members and tracheids of xylem are dead at maturity; their perforated, interconnected walls conduct water and dissolved minerals. Phloem’s sievetube members remain alive at maturity. These cells interconnect to form tubes that conduct sugars. Companion cells load sugars into the sieve tubes. Epidermis covers and protects the outer surfaces of primary plant parts. Periderm replaces epidermis on woody plants, which have extensive secondary growth. Section 28.3 Stems of most species support upright growth, which favors interception of sunlight. Vascular bundles of xylem and phloem thread through them. New shoots form at terminal buds and lateral buds on stems. In most herbaceous and young woody eudicot stems, a ring of bundles divides the ground tissue into cortex and 490 UNIT V
HOW PLANTS WORK
pith. In woody eudicot stems, the ring becomes bands of different tissues. Monocot stems often have vascular bundles distributed throughout ground tissue.
Use the animation on CengageNOW to look inside stems.
Section 28.4 Leaves are photosynthesis factories that contain mesophyll and vascular bundles (veins) between their upper and lower epidermis. Air spaces around mesophyll cells allow gas exchange. Water vapor and gases cross the cuticle-covered epidermis at stomata.
Use the animation on CengageNOW to explore the structure of a leaf.
Section 28.5 Roots absorb water and mineral ions for the rest of the plant. Inside each is a vascular cylinder with primary xylem and phloem. Root hairs increase the surface area of roots. Most eudicots have a taproot system; many monocots have a fibrous root system.
Use the animation on CengageNOW to learn about root structure and function.
Sections 28.6, 28.7 Activity at vascular cambium and cork cambium, both lateral meristems, thickens the older stems and roots of many plants. Wood is classified by its location and function, as in heartwood or sapwood. Bark is secondary phloem and periderm. The cork in periderm protects and waterproofs woody stems and roots.
Use the animation on CengageNOW to learn about the structure of wood.
Section 28.8 Stem modifications in many types of plants function in storage or reproduction.
Self-Quiz
Answers in Appendix III
1. Which of the following two distribution patterns for vascular tissues is common among eudicots? Which is common among monocots?
Annual precipitation (PDSI)
Data Analysis Exercise Douglas fir trees (Pseudotsuga menziesii) are exceptionally long-lived, and particularly responsive to rainfall levels. Researcher Henri Grissino-Mayer sampled Douglas firs in El Malpais National Monument, in west central New Mexico. Pockets of vegetation in this site have been surrounded by lava fields for about 3,000 years, so they have escaped wildfires, grazing animals, agricultural activity, and logging. Grissino-Mayer compiled tree ring data from old, living trees, and dead trees and logs to generate a 2,129-year annual precipitation record (Figure 28.24).
2 1 0 -1
*
-2 B.C. 137
1. The Mayan civilization began to suffer a massive loss of population around 770 a.d. Do these tree ring data reflect a drought condition at this time? If so, was that condition relatively more or less severe than the “dust bowl” drought?
1 A.D. 200
400
600
800
1000
1200 1400 1600 1800 1992
Year
2. One of the worst population catastrophes ever recorded occurred in Mesoamerica between 1519 and 1600 a.d., when approximately 22 million people native to the region died. According to these data, which period between 137 b.c. and 1992 had the most severe drought? How long did that particular drought last?
2. Roots and shoots lengthen through activity at a. apical meristems c. vascular cambium b. lateral meristems d. cork cambium
3
Figure 28.24 A 2,129-year annual precipitation record complied from tree rings in El Malpais National Monument, New Mexico. Data was averaged over 10-year intervals; graph correlates with other indicators of rainfall collected in all parts of North America. PDSI: Palmer Drought Severity Index: 0, normal rainfall; increasing numbers mean increasing excess of rainfall; decreasing numbers mean increasing severity of drought. * A severe drought contributed to a series of catastrophic dust storms that turned the midwestern United States into a “dust bowl” between 1933 and 1939.
.
3. In many plant species, older roots and stems thicken by activity at . a. apical meristems c. vascular cambium b. cork cambium d. both b and c 4. Bark is mainly a. periderm and cork b. cork and wood
. c. periderm and phloem d. cork cambium and phloem
5. conducts water and minerals throughout a plant, and conducts sugars. a. Phloem; xylem c. Xylem; phloem b. Cambium; phloem d. Xylem; cambium
Critical Thinking
6. Mesophyll consists of a. waxes and cutin b. lignified cell walls
.
2. Oscar and Lucinda meet in a tropical rain forest and fall in love, and he carves their initials into the bark of a tiny tree. They never do get together, though. Ten years later, still heartbroken, Oscar searches for the tree. Given what you know about primary and secondary growth, will he find the carved initials higher relative to ground level? If he goes berserk and chops down the tree, what kinds of growth rings will he see?
walls.
3. Are the structures shown below left stolons, rhizomes, bulbs, corms, or tubers? (Hint: Notice where the shoots are growing from.) What about the structures shown below right?
. c. photosynthetic cells d. cork but not bark
7. In phloem, organic compounds flow through a. collenchyma cells c. vessels b. sieve tubes d. tracheids 8. Xylem and phloem are a. ground b. vascular
tissues. c. dermal d. both b and c
9. In early wood, cells have diameters, a. small; thick c. large; thick b. small; thin d. large; thin
1. Is the plant with the yellow flower above a eudicot or a monocot? What about the plant with the purple flower?
10. Match each plant part with a suitable description. apical meristem a. massive secondary growth lateral meristem b. source of primary growth xylem c. distribution of sugars phloem d. source of secondary growth vascular cylinder e. distribution of water wood f. central column in roots
Visit CengageNOW for additional questions.
CHAPTER 28
PLANT TISSUES 491
29
Plant Nutrition and Transport IMPACTS, ISSUES
Leafy Cleanup Crews
From World War I until the 1970s, the United States Army
In other types of phytoremediation, groundwater contami-
tested and disposed of weapons at J-Field, Aberdeen
nants accumulate in tissues of the plants, which are then har-
Proving Ground in Maryland (Figure 29.1a). Obsolete
vested for safer disposal elsewhere.
chemical weapons and explosives were burned in open
The best plants for phytoremediation take up many con-
pits, together with plastics, solvents, and other wastes.
taminants, grow fast, and grow big. Not very many species
Lead, arsenic, mercury, and other metals heavily contami-
can tolerate toxic substances, but genetically engineered
nated the soil and groundwater. So did highly toxic organic
ones may increase our number of choices for this purpose.
compounds, including trichloroethylene (TCE). TCE dam-
For example, alpine pennycress (Thlaspi caerulescens)
ages the nervous system, lungs, and liver, and can cause
absorbs zinc, cadmium, and other potentially toxic minerals
coma and death. Today, the toxic groundwater is seeping
dissolved in soil water. Unlike typical cells, the cells of penny-
toward nearby marshes and the Chesapeake Bay.
cress plants store zinc and cadmium inside a central vacuole.
There was too much contaminated soil at J-Field to
Isolated inside these organelles, the toxic elements are kept
remove, so the Army and the Environmental Protection
safely away from the rest of the cells’ activities. Pennycress is
Agency turned to phytoremediation: the use of plants to
a small, creeping plant, so its usefulness for phytoremedia-
take up and concentrate or degrade environmental contami-
tion is limited. Researchers are working to transfer a gene
nants. They planted hybrid poplar trees (Populus trichocarpa
that confers its toxin-storing capacity to larger plants.
deltoides) that cleanse groundwater by taking up TCE and other organic compounds from it (Figure 29.1b). How? The roots of the hybrid poplars take up water from
a
Many adaptations that help the toxin-busters cleanse contaminated areas are the same ones that absorb and distribute water and solutes through the plant body. When considering
the soil. Along with the water come dissolved nutrients and
these adaptations, remember that many details of plant phys-
chemical contaminants, including TCE. The trees break down
iology are adaptations to limited environmental resources. In
some of the TCE, and release some of it into the atmosphere.
nature, plants rarely have unlimited supplies of the resources
Airborne TCE is the lesser of two evils: TCE persists for a long
they require to nourish themselves, and nowhere except in
time in groundwater, but it breaks down quickly in air that is
overfertilized gardens does soil water contain lavish amounts
polluted with other chemicals.
of dissolved minerals.
b
See the video! Figure 29.1 Phytoremediation in action. (a) J-Field, once a weapons testing and disposal site. (b) Today, hybrid poplars are helping to remove substances that contaminate the field’s soil and groundwater.
Links to Earlier Concepts
Key Concepts Plant nutrients and soil
In this chapter, you will be taking a closer look at how fluids move through plants. This movement depends on hydrogen bonding in water (Section 2.4), membrane transporters (5.2–5.4), and osmosis and turgor (5.6).
It will help to review what you learned about nutrients (1.2), ions (2.3), water (2.5), and carbohydrates (3.3), as well as photosynthesis (7.3, 7.6) and aerobic respiration (8.4).
You will use your knowledge of vascular tissues (28.2), leaves (28.4), and roots (28.5). You will also see more examples of plant symbionts (24.6).
We will revisit some adaptations of land plants (23.2), including the cuticle (4.12) and stomata (7.7). You will see an example of how cell signaling (27.6) is part of homeostasis in plants.
Many plant structures are adaptations to limited amounts of water and essential nutrients. The amount of water and nutrients available for plants to take up depends on the composition of soil. Soil is vulnerable to leaching and erosion. Section 29.1
Water uptake and movement through plants Certain specializations help roots of vascular plants take up water and nutrients. Xylem distributes absorbed water and solutes from roots to leaves. Sections 29.2, 29.3
Water loss versus gas exchange A cuticle and stomata help plants conserve water, a limited resource in most land habitats. Closed stomata stop water loss but also stop gas exchange. Some plant adaptations are trade-offs between water conservation and gas exchange. Section 29.4
Sugar distribution through plants Phloem distributes sucrose and other organic compounds from photosynthetic cells in leaves to living cells throughout the plant. Organic compounds are actively loaded into conducting cells, then unloaded in growing tissues or storage tissues. Section 29.5
How would you vote? Transgenic plants may be more efficient at cleaning up contaminated sites than unmodified plants. Do you support using genetically engineered plants for phytoremediation? See CengageNOW for details, then vote online.
493
29.1
Plant Nutrients and Availability in Soil Plants require elemental nutrients from soil, water, and air. Different types of soil affect the growth of different plants.
Properties of Soil
Links to Nutrients 1.2, Ions 2.3
Soil consists of mineral particles mixed with variable amounts of decomposing organic material, or humus. The particles form by the weathering of hard rocks. Humus forms from dead organisms and organic litter: fallen leaves, feces, and so on. Water and air occupy spaces between the particles and organic bits. Soils differ in their proportions of mineral particles and how compacted they are. The particles, which differ in size, are primarily sand, silt, and clay. The biggest sand grains are 0.05 to 2 millimeters in diameter. You can see individual grains by sifting beach sand through your fingers. Individual particles of silt are too small to see; they are only 0.002 to 0.05 millimeters in diameter. Particles of clay are even smaller. Each clay particle consists of thin, stacked layers of negatively charged crystals. Sheets of water molecules alternate between the layers. Because of its negative charge, clay can temporarily bind positively charged mineral ions dissolved in the soil water. Clay latches onto dissolved nutrients that would otherwise trickle past roots too quickly to be absorbed. Even though they do not bind mineral ions as well as clay, sand and silt are necessary for growing plants. Without enough sand and silt to intervene between the tiny particles of clay, the soil packs so tightly that air is excluded. Without air spaces in the soil, root cells cannot secure enough oxygen for aerobic respiration.
The Required Nutrients A nutrient is an element or molecule with an essential role in an organism’s growth and survival. Plants require sixteen nutrients, all elements available in water and air, or as minerals that have dissolved as ions in the water. Examples include calcium and potassium. Nine of the elements are macronutrients, which means that they are required in amounts greater than 0.5 percent of the plant’s dry weight (its weight after all of the water has been removed). Seven other elements are micronutrients, which make up traces (typically a few parts per million) of the plant’s dry weight. A deficiency in any one of these nutrients may affect plant growth (Table 29.1).
Table 29.1
Plant Nutrients and Deficiency Symptoms
Type of Nutrient
Deficiency Symptoms
MACRONUTRIENT
Carbon, oxygen, hydrogen
None; all are available in abundance from water and carbon dioxide
Nitrogen
Stunted growth; chlorosis (leaves turn yellow and die because of insufficient chlorophyll)
Potassium
Reduced growth; curled, mottled, or spotted older leaves, leaf edges brown; weakened plant
Calcium
Terminal buds wither; deformed leaves; stunted roots
Magnesium
Chlorosis; drooped leaves
Phosphorus
Purplish veins; stunted growth; fewer seeds, fruits
Sulfur
Light-green or yellowed leaves; reduced growth
MICRONUTRIENT
Chlorine
Wilting; chlorosis; some leaves die
Iron
Chlorosis; yellow, green striping in leaves of grasses
Boron
Buds die; leaves thicken, curl, become brittle
Manganese
Dark veins, but leaves whiten and fall off
Zinc
Chlorosis; mottled or bronzed leaves; abnormal roots
Copper
Chlorosis; dead spots in leaves; stunted growth
Molybdenum
Pale green, rolled or cupped leaves
494 UNIT V
HOW PLANTS WORK
Soils and Plant Growth Soils with the best oxygen and water penetration are loams, which have roughly equal proportions of sand, silt, and clay. Most plants grow best in loams. Humus also affects plant growth because it releases nutrients, and its negatively charged organic acids can trap the positively charged mineral ions in soil water. Humus swells and shrinks as it absorbs and releases water, and these changes in size aerate soil by opening spaces for air to penetrate. Most plants grow well in soils that contain between 10 and 20 percent humus. Soil with less than 10 percent humus may be nutrient-poor. Soil with more than 90 percent humus stays so saturated with water that air (and the oxygen in it) is excluded. The soil in swamps and bogs contains so much organic matter that very few kinds of plants can grow in them. How Soils Develop Soils develop over thousands of
years. They are in different stages of development in different regions. Most form in layers, or horizons, that are distinct in color and other properties (Figure 29.2).
O HORIZON
Fallen leaves and other organic material littering the surface of mineral soil A HORIZON
Topsoil, with decomposed organic material; variably deep [only a few centimeters in deserts, elsewhere extending as far as 30 centimeters (1 foot) below the soil surface] B HORIZON
Compared with A horizon, larger soil particles, not much organic material, more minerals; extends 30 to 60 centimeters (1 to 2 feet) below soil surface C HORIZON
No organic material, but partially weathered fragments and grains of rock from which soil forms; extends to underlying bedrock BEDROCK
Figure 29.2 From a habitat in Africa, an example of soil horizons.
Figure 29.3 Right: Runaway erosion in Providence Canyon, Georgia, is the result of poor farming practices combined with soft soil. Settlers that arrived in the area around 1800 plowed the land straight up and down the hills. The furrows made excellent conduits for rainwater, which proceeded to carve out deep crevices that made even better rainwater conduits. The area became useless for farming by 1850. It now consists of about 445 hectares (1,100 acres) of deep canyons that continue to expand at the rate of about 2 meters (6 feet) per year.
The layers help us characterize soil in a given place, and compare it with soils in other places. For instance, the A horizon is topsoil. This layer typically contains the greatest amount of organic matter, so the roots of most plants grow most densely in it. Topsoil is deeper in some places than in others. Section 48.5 shows soil profiles for some major classes of ecosystems on land.
est losses (Figure 29.3). For example, each year, about 25 billion metric tons of topsoil erode from croplands in the midwestern United States. The topsoil enters the Mississippi River, which then dumps it into the Gulf of Mexico. Nutrient losses because of this erosion affect not only plants that grow in the region, but also the other organisms that depend on them for survival.
Leaching and Erosion Minerals, salts, and other molecules dissolve in water as it filters through soil. Leaching is the process by which water removes soil nutrients and carries them away. Leaching is fastest in sandy soils, which do not bind nutrients as well as clay soils. During heavy rains, more leaching occurs in forests than in grasslands. Why? Grass plants absorb water more quickly than trees. Soil erosion is a loss of soil under the force of wind and water. Strong winds, fast-moving water, sparse vegetation, and poor farming practices cause the great-
Take-Home Message From where do plants get the nutrients they require? Plants require nine macronutrients and seven micronutrients, all elements. All are available from water, air, and soil. Soil consists mainly of mineral particles: sand, silt, and clay. Clay attracts and reversibly binds dissolved mineral ions.
Soil contains humus, a reservoir of organic material rich in organic acids. Most plants grow best in loams (soils with equal proportions of sand, silt, and clay) and between 10 and 20 percent humus. Leaching and erosion remove nutrients from soil.
CHAPTER 29
PLANT NUTRITION AND TRANSPORT 495
29.2
How Do Roots Absorb Water and Nutrients? Root specializations such as hairs, mycorrhizae, and nodules help the plant absorb water and nutrients.
Links to Plasmodesmata 4.12, Aquaporins 5.2, Transport proteins 5.3, Osmosis 5.6, Nitrogen fixation 21.6, Fungal symbionts 24.6, Root structure 28.5
root hair
In actively growing plants, new roots infiltrate different patches of soil as they replace old roots. The new roots are not “exploring” the soil. Rather, their growth is simply greater in areas where the water and nutrient concentrations best match the requirements of the particular plant. Certain specializations help plants take up water and nutrients from both soil and air. In roots, mycorrhizae and root hairs help plants absorb water and ions from soil, and root nodules help certain plants absorb additional nitrogen from the air. Root Hairs As most plants put on primary growth,
a
their root tips sprout many root hairs (Figure 29.4a). Collectively, these thin extensions of root epidermal cells enormously increase the surface area available for absorbing water and dissolved mineral ions. Root hairs are fragile structures no more than a few millimeters long. They do not develop into new roots, and live only a few days. New ones constantly form just behind the root tip (Section 28.5).
b
Mycorrhizae As Section 24.6 explains, a mycorrhiza
(plural, mycorrhizae) is a form of mutualism between a young root and a fungus. Both species benefit from the association. The fungal hyphae grow as a velvety covering around the root or penetrate its cells (Figure 29.4b). Collectively, hyphae have a far greater surface area than the root itself, so they can absorb scarce minerals from a larger volume of soil. The root’s cells give some sugars and nitrogen-rich compounds to the fungus, and the fungus gives some of the minerals it mines to the plant.
d
root nodule
c
e
Figure 29.4 Examples of root specializations. (a) The hairs on this root of a white clover plant (Trifolium repens) are about 0.2 mm long. (b) Mycorrhizae (white hairs) extending from the tip of these roots (tan) greatly enhance their surface area for absorbing scarce minerals from the soil. (c) Root nodules on this soybean plant fix nitrogen from the air, and share it with the plant. (d) A nodule forms where bacteria infect the root. (e) Soybean plants growing in nitrogen-poor soil show the effect of root nodules on growth. Only the plants in the rows at right were inoculated with Rhizobium bacteria and formed nodules. Figure It Out: Are Rhizobium bacteria parasites or mutualists? Answer: Mutualists
496 UNIT V
HOW PLANTS WORK
Root Nodules Certain types of bacteria in soil are mutualists with clover, peas, and other legumes. Like all other plants, legumes require nitrogen for growth. Nitrogen gas (N⬅N, or N2) is abundant in the air, but plants do not have enzymes that can break it apart. The bacteria do. Their enzymes convert nitrogen gas to ammonia (NH3). The metabolic conversion of nitrogen gas to ammonia is an energy-intensive process called nitrogen fixation (Section 21.6). Other types of soil bacteria convert ammonia to nitrate (NO3–), the form of nitrogen that plants can use most easily. You will read more about nitrogen fixation in Section 47.9. Root nodules are swollen masses of bacteria-infected root cells (Figure 29.4c). The bacteria (Rhizobium and Bradyrhizobium, both anaerobic) fix nitrogen and share it with the plant. In return, the plant provides the bacteria with an oxygen-free environment, and shares its photosynthetically produced sugars with them.
vascular cylinder
Figure 29.5 Animated In most flowering plants, transport proteins in the plasma membranes of root cells control the plant’s uptake of water and dissolved mineral ions from the soil.
epidermis endodermis
How Roots Control Water Uptake Osmosis drives the movement of soil water into a root, then into the walls of parenchyma cells that make up the root cortex. Some of the nutrient-laden water stays in the cell walls; it permeates the cortex by diffusing around the cells’ plasma membranes. Water molecules enter the cells’ cytoplasm by diffusing across plasma membranes directly or through aquaporins (Section 5.2). Active transporters in the membranes pump dissolved mineral ions into the cells. After moving into cytoplasm, the water and ions diffuse from cell to cell through plasmodesmata (Section 4.12). A vascular cylinder is separated from the root cortex by endodermis, a tissue composed of a single layer of parenchyma cells (Figure 29.5a). These cells secrete a waxy substance into their walls wherever they abut. The substance forms a Casparian strip, a waterproof band between the plasma membranes of endodermal cells (Figure 29.5b). The Casparian strip prevents the water that is seeping around the cells in the root’s cortex from passing through endodermal cell walls into the vascular cylinder. Water and ions enter a root’s vascular cylinder by moving through plasmodesmata, or by crossing endodermal cell plasma membranes. Either way, they have to cross at least one plasma membrane. Thus, plasma membrane transport proteins can control the amount of water, and the amount and types of ions, that move from the root cortex into the vascular cylinder (Figure 29.5c). The selectivity of these proteins also offers protection against toxins that may be in soil water. The roots of many plants also have an exodermis, a layer of cells just beneath their surface. Exodermal cells often deposit their own Casparian strip that functions like the one next to the vascular cylinder.
Take-Home Message How do roots take up water and nutrients?
Root hairs, mycorrhizae, and root nodules greatly enhance a root’s ability to take up water and nutrients.
Transport proteins in root cell plasma membranes control the uptake of water and ions into the vascular cylinder.
primary phloem primary xylem
cortex vascular cylinder
tracheids and vessels in xylem
A In roots, the vascular cylinder’s outer layer is a sheet of endodermis, one cell thick.
B Parenchyma cells that make up the layer secrete a waxy substance into their walls wherever they touch. The secretions form a Casparian strip, which prevents water from seeping around the cells into the vascular cylinder.
sieve tubes in phloem endodermal cell
Casparian strip
C Water and ions can only enter the vascular cylinder by moving through cells of the endodermis. They enter the cells via plasmodesmata or via transport proteins in the cells’ plasma membranes. Water and ions must cross at least one lipid bilayer before entering a vascular cylinder. Thus, plasma membrane transport proteins control the movement of these substances into the rest of the plant.
CHAPTER 29
Vascular cylinder
Casparian strip
water and nutrients Cortex
PLANT NUTRITION AND TRANSPORT 497
29.3
How Does Water Move Through Plants? Evaporation from leaves and stems drives the upward movement of water through pipelines of xylem inside a plant. Water’s cohesion allows it to be pulled from roots into all other parts of the plant. Links to Hydrogen bonding 2.4, Properties of water 2.5, Xylem 28.2, Root structure 28.5
Soil water moves into roots and then into the plant’s aboveground parts. How does water move all the way from roots to leaves that may be more than 100 meters (330 feet) above the soil? The movement does not occur by active pumping, but rather is driven by two features of water that you learned about in Section 2.5: evaporation and cohesion.
Cohesion–Tension Theory In vascular plants, water moves inside xylem. Section 28.2 introduced the tracheids and vessel members that make up its water-conducting tubes. These cells are dead at maturity; only their lignin-impregnated walls are left behind (Figure 29.6). Obviously, being dead, the cells are not expending any energy to pump water against gravity.
The botanist Henry Dixon explained how water is transported in plants. By his cohesion–tension theory, water inside xylem is pulled upward by air’s drying power, which creates a continuous negative pressure called tension. The tension extends continuously from leaves to roots. Figure 29.7 illustrates the theory. First, air’s drying power causes transpiration: the evaporation of water from aboveground plant parts. Most of the water a plant takes up is lost by evaporation, typically from stomata on the plant’s leaves and stems. Transpiration creates negative pressure inside the conducting tubes of xylem. In other words, the evaporation of water from leaves and stems pulls on the water that remains in the xylem. Second, the continuous columns of fluid inside the narrow conductive tubes of xylem resist breaking into droplets. Remember from Section 2.5 that the collective strength of many hydrogen bonds among water molecules imparts cohesion to liquid water. Because water molecules are all connected to one another by hydrogen bonds, a pull on one also pulls on the others. Thus, the negative pressure created by transpiration exerts tension on the entire column of water that fills a xylem tube. That tension extends from leaves
perforation plate
vessel member perforation in the side wall of tracheid
a Tracheids have tapered, unperforated end walls. Perforations in the side walls of adjoining tracheids match up.
b Three adjoining vessel members. The thick, finely perforated end walls of dead cells connect to make long tubes that conduct water through xylem.
c Perforation plate at the end wall of one type of vessel member. The perforated ends allow water to flow freely through the tube.
Figure 29.6 Tracheids and vessel members from xylem. Interconnected, perforated walls of dead cells form these water-conducting tubes. The pectin-coated perforations may help control water distribution to specific regions. When hydrated, the pectins swell and stop the flow. During dry periods, they shrink, and water moves freely through open perforations toward leaves.
498 UNIT V
HOW PLANTS WORK
mesophyll (photosynthetic cells) vein
upper epidermis
A The driving force of transpiration Evaporation of water molecules from aboveground plant parts puts water in xylem into a state of tension that extends from roots to leaves. For clarity, tissues inside the vein are not shown.
stoma
xylem
vascular cambium
phloem
B Cohesion of water inside xylem tubes Even though long columns of water that fill narrow xylem tubes are under continuous tension, they resist breaking apart. The collective strength of many hydrogen bonds keeps individual water molecules together.
vascular cylinder
endodermis
cortex
water molecule
root hair cell
C Ongoing water uptake at roots Water molecules lost from the plant are being continually replaced by water molecules taken up from soil. Tissues in the vein not shown.
Figure 29.7 Animated Key points of the cohesion–tension theory of water transport in vascular plants.
that may be hundreds of feet in the air, down through stems, and on into young roots where water is being absorbed from the soil. The movement of water through plants is driven mainly by transpiration. However, evaporation is only one of many other processes in plants that involve the loss of water molecules. Such processes all contribute to the negative pressure that results in water movement. Photosynthesis is an example.
Take-Home Message What makes water move inside plants? Transpiration is the evaporation of water from leaves, stems, and other plant parts.
By a cohesion–tension theory, transpiration puts water in xylem into a continuous state of tension from leaves to roots. Tension pulls columns of water in xylem upward through the plant. The collective strength of many hydrogen bonds (cohesion) keeps the water from breaking into droplets as it rises.
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29.4
How Do Stems and Leaves Conserve Water? Water is an essential resource for all land plants. Thus, water-conserving structures and processes are key to the survival of these plants. Links to Plant cuticle 4.12, Osmosis 5.6, Gases in photosynthesis 7.3, Stomata 7.7, Gases in aerobic respiration 8.4, Land plant adaptations 23.2, Cell signaling 27.6, Leaf structure 28.4
stoma
A Cuticle (gold ) and stoma on a leaf. Each stoma is formed by two guard cells, which are specialized epidermal cells.
B This stoma is open. When the guard cells swell with water, they bend so that a gap opens between them. The gap allows the plant to exchange gases with air. The exchange is necessary to keep metabolic reactions running. 20 µm
guard cells C This stoma is closed. The guard cells, which are not plump with water, are collapsed against each other so there is no gap between them. A closed stoma limits water loss, but it also limits gas exchange, so photosynthesis and respiration reactions slow. solutes water D How do stoma open and close? When a stoma is open, the guard cells are maintaining a relatively high concentration of solutes by pumping solutes into their cytoplasm. Water diffuses into the hypertonic cytoplasm and keeps the cells plump.
ABA signal g
In land plants, at least 90 percent of the water transported from roots to a leaf evaporates right out. Only about 2 percent is used in metabolism, but that amount must be maintained or photosynthesis, growth, membrane functions, and other processes will shut down. If a plant is running low on water, it cannot move around to seek out more, as most animals can. A cuticle and stomata (Sections 4.12 and 23.2) help the plant conserve the water it already holds in its tissues. Both of these structures restrict the amount of water vapor that diffuses out of the plant’s surfaces. However, the cuticle and stomata also restrict gas exchanges between the plant and the air. Why is that important? The concentrations of carbon dioxide and oxygen gases in air spaces inside the plant affect the rate of critical metabolic pathways (such as photosynthesis and aerobic respiration) in the plant’s cells. If a plant were entirely impermeable to water vapor and gases, it could not take in enough carbon dioxide to run photosynthesis. Neither could it sustain aerobic respiration for very long, because too much oxygen would build up in its tissues. Thus, water-conserving structures and mechanisms must balance the plant’s needs for water with its needs for gas exchanges.
The Water-Conserving Cuticle Even mildly water-stressed plants would wilt and die without a cuticle. This water-impermeable layer coats the walls of all plant cells exposed to air (Figure 29.8a). It consists of epidermal cell secretions: a mixture of waxes, pectin, and cellulose fibers embedded in cutin, an insoluble lipid polymer. The cuticle is translucent, so it does not prevent light from reaching photosynthetic tissues.
Controlling Water Loss at Stomata A pair of specialized epidermal cells defines each stoma. When these two guard cells swell with water, they bend
solutes olutes water E When water is scarce, a hormone (ABA) activates a pathway that lowers the concentrations of solutes in guard cell cytoplasm. Water follows its gradient and diffuses out of the cells, and the stoma closes.
Figure 29.8 Water-conserving structures in plants. (a) Cuticle and stoma in a cross-section of basswood (Tilia) leaf. (b–e) Stomata in action. Whether a stoma is open or closed depends on how much water is plumping up these guard cells. The amount of water in guard cell cytoplasm is influenced by hormonal signals. The round structures inside the cells are chloroplasts. Guard cells are the only type of epidermal cell with these organelles.
500 UNIT V
HOW PLANTS WORK
Figure 29.9 Stomata at the leaf surface of a holly plant growing in a smoggy, industrialized region. Airborne pollutants not only block sunlight from photosynthetic cells, they also clog stomata, and can damage them so much that they close permanently.
slightly so a gap forms between them. The gap is the stoma. When the cells lose water, they collapse against each other, so the gap closes (Figure 29.8b,c). Environmental cues such as water availability, the level of carbon dioxide inside the leaf, and light intensity affect whether stomata open or close. These cues trigger osmotic pressure changes in the cytoplasm of guard cells. For example, when the sun comes up, the light causes guard cells to begin pumping solutes (in this case, potassium ions) into their cytoplasm. The resulting buildup of potassium ions causes water to enter the cells by osmosis. The guard cells plump up, so the gap between them opens. Carbon dioxide from the air diffuses into the plant’s tissues, and photosynthesis begins. As another example, root cells release the hormone abscisic acid (ABA) when soil water becomes scarce. ABA travels through the plant’s vascular system to leaves and stems, where it binds to receptors on guard cells. The binding causes solutes to exit these cells. Water follows by osmosis, the guard cells lose plumpness and collapse against each other, and the stomata close (Figure 29.8e).
Most stomata close at night, in most plants. Water is conserved, and carbon dioxide builds up in leaves as cells make ATP by aerobic respiration. The stomata of CAM plants, including most cactuses, open at night, when the plant takes in and fixes carbon from carbon dioxide. During the day, they close, and the plant uses the carbon that it fixed during the night for photosynthesis (Section 7.7). Stomata also close in response to some of the chemicals in polluted air. The closure protects the plant from chemical damage, but it also prevents the uptake of carbon dioxide for photosynthesis, and so inhibits growth. Think about it on a smoggy day (Figure 29.9).
Take-Home Message How do land plants conserve water?
A waxy cuticle covers all epidermal surfaces of the plant exposed to air. It restricts water loss from plant surfaces. Plants conserve water by closing their many stomata. Closed stomata also prevent gas exchanges necessary for photosynthesis and aerobic respiration.
A stoma stays opens when the guard cells that define it are plump with water. It closes when the cells lose water and collapse against each other.
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29.5
How Do Organic Compounds Move Through Plants? Xylem distributes water and minerals through plants, and phloem distributes the organic products of photosynthesis. Links to Carbohydrates 3.3, Active transport 5.4, Osmosis and turgor 5.6, Photosynthetic products 7.6, Plant vascular tissues 28.2
Phloem is a vascular tissue with organized arrays of conducting tubes, fibers, and strands of parenchyma cells. Unlike conducting tubes of xylem, sieve tubes in phloem consist of living cells. Sieve-tube cells are positioned side by side and end to end, and their abutting end walls (sieve plates) are porous. Dissolved organic compounds flow through the tubes (Figure 29.10a,b).
one of a series of living cells that abut, end to end, and form a sieve tube
companion cell (in the background, pressed tightly against sieve tube)
Companion cells that are pressed against the sieve tubes actively transport the organic products of photosynthesis into them. Some of the molecules are used in the cells that make them, but the rest travel through the sieve tubes to the other parts of the plant: roots, stems, buds, flowers, and fruits. Plants store their carbohydrates mainly as starch, but starch molecules are too big and too insoluble to transport across plasma membranes. Cells break down starch molecules to sucrose and other small molecules that are easily transported through the plant. Some experiments with plant-sucking insects demonstrated that sucrose is the main carbohydrate transported in phloem. Aphids feeding on the juices in the conducting tubes of phloem were anesthetized with high levels of carbon dioxide (Figure 29.11). Then their bodies were detached from their mouthparts, which remained attached to the plant. Researchers collected and analyzed fluid exuded from the aphids’ mouthparts. For most of the plants studied, sucrose was the most abundant carbohydrate in the fluid.
Pressure Flow Theory Translocation is the formal name for the process that moves sucrose and other organic compounds through phloem of vascular plants. Phloem translocates photosynthetic products along declining pressure and solute concentration gradients. The source of the flow is any region of the plant where organic compounds are being loaded into sieve tubes. A common source is photosynthetic mesophyll in leaves. The flow ends at a sink, which is any plant region where the products are being used or stored. For instance, while flowers and fruits are forming on the plant, they are sinks.
perforated end plate of sieve-tube cell, of the sort shown in (b)
a
Figure 29.10 (a) Part of a sieve tube inside phloem. Arrows point to perforated ends of individual tube members. (b) Scanning electron micrograph of the sieve plates on the ends of two side-by-side sieve-tube members.
b
502 UNIT V
HOW PLANTS WORK
Figure 29.11 Honeydew exuding from an aphid after this insect’s mouthparts penetrated a sieve tube. High pressure in phloem forced this droplet of sugary fluid out through the terminal opening of the aphid gut.
Translocation
upper leaf epidermis interconnected sieve tubes
photosynthetic cell
SOURCE (e.g., mature leaf cells) sieve tube in leaf vein
A Solutes move into a sieve tube against their concentration gradients by active transport.
C The pressure difference pushes the fluid from the source to the sink. Water moves into and out of the sieve tube along the way.
E Solutes are unloaded into sink cells, which then become hypertonic with respect to the sieve tube. Water moves from the sieve tube into sink cells.
WATER
flow
B As a result of increased solute concentration, the fluid in the sieve tube becomes hypertonic. Water moves in from surrounding xylem, increasing phloem turgor.
companion cell next to sieve tube
lower leaf epidermis
Typical source region
D Both pressure and solute concentrations gradually decrease as the fluid moves from source to sink.
Photosynthetic tissue in a leaf
sieve tube
SINK (e.g., developing root cells)
Typical sink region
Actively growing cells in a young root
Figure 29.12 Animated Translocation of organic compounds. Review Section 7.6 to get an idea of how translocation relates to photosynthesis in vascular plants.
Why do organic compounds in phloem flow from source to sink? High fluid pressure drives the movement of fluid in phloem (Section 5.6). According to the pressure flow theory, internal pressure builds up in sieve tubes at a source. The pressure can be five times higher than the air pressure inside an automobile tire. A pressure gradient pushes solute-rich fluid to a sink, where the solutes are removed from the phloem. Use Figure 29.12 to track what happens to sugars and other organic solutes as they move from the photosynthetic cells into small leaf veins. Companion cells in veins actively transport the solutes into sieve-tube members. When the solute concentration increases in the tubes, water also moves into them by osmosis. The increase in fluid volume exerts extra pressure (turgor) on the walls of the sieve tubes.
Phloem in a sink region has a lower internal pressure than that of a source region. Sucrose is unloaded at a sink, and water is diffusing out of phloem there by osmosis. The difference in fluid pressure between sources and sinks moves the sugar-laden fluid inside phloem through the plant.
Take-Home Message How do organic molecules move through plants?
Plants store carbohydrates as starch, and distribute them as sucrose and other small, water-soluble molecules.
Concentration and pressure gradients in the sieve-tube system of phloem force organic compounds to flow to different parts of the plant. The gradients are set up by companion cells moving organic molecules into sieve tubes at sources, and the unloading of the molecules at sinks.
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IMPACTS, ISSUES REVISITED
Leafy Cleanup Crews
With elemental pollutants such as lead or mercury, the best phytoremediation strategies use plants that absorb and then store these toxins in aboveground tissues, which can be harvested for safe disposal. Researchers have genetically modified such plants to enhance their absorptive and storage capacity. Dr. Kuang-Yu Chen, pictured at right, is analyzing zinc and cadmium levels in plants that can tolerate these elements. In the case of organic toxins such as TCE, the best phytoremediation strategies use plants with biochemical pathways that break down the compounds to less-toxic molecules. Phytoremediation researchers are beefing up these pathways in many plants. Some
Summary Section 29.1 Plant growth requires steady sources of water and nutrients obtainable from carbon dioxide and soil (Figure 29.13). The availability of water and nutrients in soil is largely determined by its proportions of sand, silt, and clay; and its humus content. Loams have roughly equal proportions of sand, silt, and clay. Leaching and soil erosion deplete nutrients in soil, particularly topsoils. Section 29.2 Root hairs greatly increase roots’ surface area for absorption. Fungi are symbionts with young roots in mycorrhizae, which enhance a plant’s ability to absorb mineral ions from soil. Nitrogen fixation by bacteria in root nodules gives a plant extra nitrogen. In both cases, the symbionts receive some of the plant’s sugars. Roots control the movement of water and dissolved mineral ions into the vascular cylinder. Endodermal cells that form a layer around the cylinder deposit a waterproof band, a Casparian strip, in their abutting walls. The strip keeps water from diffusing around the cells. Water and nutrients enter a root vascular cylinder only by moving through the plasma membrane of parenchyma cells. The uptake is controlled by active transport proteins embedded in the membranes. Some plants also have an exodermis, an additional layer of cells that deposit a second Casparian strip just inside the root surface.
Use the animation on CengageNOW to see how vascular plant roots control nutrient uptake.
Section 29.3 Water and dissolved mineral ions flow through conducting tubes of xylem. The interconnected, perforated walls of tracheids and vessel members (cells that are dead at maturity) form the tubes. ATP formation by roots
respiration of sucrose by roots
Figure 29.13
504 UNIT V
absorption of minerals and water by roots
transport of sucrose to roots
transport of minerals and water to leaves
photosynthesis
Summary of processes that sustain plant growth.
HOW PLANTS WORK
How would you vote? Do you support the use of transgenic plants with an enhanced capacity to take up or detoxify pollutants for phytoremediation? See CengageNOW for details, then vote online.
are transferring genes from bacteria or animals into plants; others are enhancing expression of genes that encode molecular participants in the plants’ own detoxification pathways.
Transpiration is the evaporation of water from plant parts, mainly at stomata, into air. By a cohesion–tension theory, transpiration pulls water upward by creating a continuous negative pressure (or tension) inside xylem from leaves to roots. Hydrogen bonds among water molecules keep the columns of fluid continuous inside the narrow vessels.
Use the animation on CengageNOW to learn about water transport in vascular plants.
Section 29.4 A cuticle and stomata balance a plant’s loss of water with its needs for gas exchange. Stomata are gaps across the cuticle-covered epidermis of leaves and other plant parts. Each is defined by a pair of guard cells. Closed stomata limit the loss of water, but also prevent the gas exchange required for photosynthesis and aerobic respiration. Environmental signals, including pollution, can cause stomata to open or close. Hormonal signals trigger guard cells to pump ions into or out of their cytoplasm; water follows the ions (by osmosis). Water moving into guard cells plumps them, which opens the gap between them. Water diffusing out of the cells causes them to collapse against each other, so the gap closes. Section 29.5 Organic compounds become distributed through a plant by translocation. Companion cells actively transport sugars and other organic products of photosynthesis into sieve tubes of phloem at source regions. The molecules are unloaded from the tubes at sink regions. By the pressure flow theory, the movement of fluid through phloem is driven by pressure and solute gradients.
Use the animation on CengageNOW to observe how vascular plants distribute organic compounds.
Self-Quiz
Answers in Appendix III
1. Carbon, hydrogen, and oxygen are plant a. macronutrients d. essential elements b. micronutrients e. both a and d c. trace elements
.
2. A(n) strip between abutting endodermal cell walls forces water and solutes to move through these cells rather than around them.
Data Analysis Exercise
1. How many transgenic plants did the researchers test?
3. On day 6, what was the difference between the TCE content of air around transgenic plants and that around vector control plants? 4. Assuming no other experiments were done, what two explanations are there for the results of this experiment? What other control might the researchers have used?
.
4. The nutrition of some plants depends on a root–fungus association known as a . a. root nodule c. root hair b. mycorrhiza d. root hypha 5. Water evaporation from plant parts is called a. translocation c. transpiration b. expiration d. tension
.
6. Water transport from roots to leaves occurs mainly because of . a. pressure flow b. differences in source and sink solute concentrations c. the pumping force of xylem vessels d. transpiration and cohesion of water molecules 7. Stomata open in response to light when . a. guard cells pump ions into their cytoplasm b. guard cells pump ions out of their cytoplasm 8. Tracheids are part of a. cortex b. mesophyll
. c. phloem d. xylem
9. Sieve tubes are part of a. cortex b. mesophyll
. c. phloem d. xylem
10. When soil is dry, initiates closure of stomata. a. air temperature b. humidity
acts on guard cells and c. abscisic acid d. oxygen
20,000
15,000
10,000
5,000
0 0
2. In which group did the researchers see the slowest rate of TCE uptake? The fastest?
3. A vascular cylinder consists of cells of the a. exodermis d. xylem and phloem b. endodermis e. b and d c. root cortex f. all of the above
Planted vector control Unplanted transgenic Planted transgenic
25,000
TCE concentration (µg/m3)
Plants used for phytoremediation take up organic pollutants from the soil or air, then transport the chemicals to plant tissues, where they are stored or broken down. Researchers are now designing transgenic plants with enhanced ability to take up or break down toxins. In 2007, Sharon Doty and her colleagues published the results of their efforts to design plants useful for phytoremediation of soil and air containing organic solvents. The researchers used Agrobacterium tumefaciens (Section 16.7) to deliver a mammalian gene into poplar plants. The gene encodes cytochrome P450, a type of heme-containing enzyme involved in the metabolism of a range of organic molecules, including solvents such as TCE. The results of one of the researchers’ tests on these transgenic plants are shown in Figure 29.14.
1
2
3
4
5
6
7
Time (days)
Figure 29.14 Results of tests on transgenic poplar trees. Planted trees were incubated in sealed containers with an initial 15,000 micrograms of TCE (trichloroethylene) per cubic meter of air. Samples of the air in the containers were taken daily and measured for TCE content. Controls included a tree transgenic for a Ti plasmid with no cytochrome P450 in it (vector control), and a bare-root transgenic tree (one that was not planted in soil).
11. Match the concepts of plant nutrition and transport. stomata a. evaporation from plant parts plant nutrient b. harvests soil water and nutrients sink c. balance water loss with gas root system exchange hydrogen d. cohesion in water transport bonds e. sugars unloaded from sieve tubes transpiration f. organic compounds distributed translocation through the plant body g. essential element
Visit CengageNOW for additional questions.
Critical Thinking 1. Successful home gardeners, like farmers, make sure that their plants get enough nitrogen from either nitrogen-fixing bacteria or fertilizer. Which biological molecules incorporate nitrogen? Nitrogen deficiency stunts plant growth; leaves yellow and then die. How would nitrogen deficiency cause these symptoms? 2. When moving a plant from one location to another, the plant is more likely to survive if some native soil around the roots is transferred along with the plant. Formulate a hypothesis that explains that observation. 3. If a plant’s stomata are made to stay open at all times, or closed at all times, it will die. Why? 4. Allen is studying the rate at which tomato plants take up water from soil. He notices that several environmental factors, including wind and relative humidity, affect the rate. Explain how they might do so. CHAPTER 29
PLANT NUTRITION AND TRANSPORT 505
30
Plant Reproduction IMPACTS, ISSUES
Plight of the Honeybee
In the fall of 2006, commercial beekeepers in Europe, India,
develop into a fruit unless it receives pollen from another
and North America began to notice something was amiss
flower. Even plants with flowers that can self-pollinate tend
in their honeybee hives. The bees were dying off in unusu-
to make bigger fruits and more of them when they are cross-
ally high numbers. Many colonies did not survive through
pollinated (Figure 30.1).
the winter that followed. By spring, the phenomenon had
Many types of insects pollinate plants, but honeybees are
a name: colony collapse disorder. Farmers and biologists
especially efficient pollinators of a variety of plant species.
began to worry about what would happen if the honeybee
They are also the only ones that tolerate living in man-made
populations continued to decline. Honey production would
hives that can be loaded onto trucks and carted wherever
suffer, but many commercial crops would fail too.
crops require pollination. Loss of their portable pollination
Nearly all of our crops are flowering plants. As Chapter 23 explained, these plants make pollen grains that consist
service is a huge threat to our agricultural economy. We do not know what causes colony collapse disorder.
of a few cells, one of which produces sperm. Honeybees
Honeybees can be infected by a variety of pests and dis-
are pollinators; they carry pollen from one plant to another,
eases that may be part of the problem. For example, Israeli
pollinating flowers as they do. Typically, a flower will not
acute paralysis virus has been detected in many affected hives. Pesticides may also be taking a toll. In the past few years, neonicotinoids have become the most widely used insecticides in the United States. These chemicals are systemic insecticides, which means they are taken up by all plant tissues, including the nectar and pollen that honeybees collect. Neonicotinoids are highly toxic to honeybees. Colony collapse disorder is currently in the spotlight because it affects our food supply. However, other pollinator populations are also dwindling. Habitat loss is probably the main factor, but pesticides that harm honeybees also harm other pollinators. Flowering plants rose to dominance in part because they coevolved with animal pollinators. Most flowers are specialized to attract and be pollinated by a specific species or type of pollinator. Those adaptations put the plants at risk of extinction if coevolved pollinator populations decline. Wild animal species that depend on the plants for fruits and seeds will also be affected. Recognizing the prevalence and importance of these interactions is our first step toward finding workable ways to protect them.
a
b
See the video! Figure 30.1 Importance of insect pollinators. (a) Honeybees are efficient pollinators of a variety of flowers, including berries. (b) Raspberry flowers can pollinate themselves, but the fruit that forms from a self-fertilized flower is of lower quality than that of a cross-pollinated flower. The two berries on the left formed from self-pollinated flowers. The one on the right formed from an insect-pollinated flower.
Links to Earlier Concepts
Key Concepts Structure and function of flowers
A review of what you know about plant tissue organization (Sections 28.2, 28.3, 28.8) and plant life cycles (10.5, 23.2) will be helpful as we examine in detail some of the reproductive adaptations that contributed to the evolutionary success of flowering plants (23.8, 23.9).
This chapter revisits some of the evolutionary processes (18.11, 18.12) that resulted in the current spectrum of structural diversity in flowering plants.
You will draw upon your understanding of membrane proteins (5.2) as you learn more about cell signaling (27.6) and development (15.2) in plant reproduction.
We also revisit meiosis (10.3), Mendelian inheritance (11.1), cloning (13.4), radiometric dating (17.6), aneuploidy (12.6), and polyploidy in plants (18.11) within the context of plant asexual reproduction (10.1).
Flowers are shoots that are specialized for reproduction. Modified leaves form their parts. Gamete-producing cells develop in their reproductive structures; other parts such as petals are adapted to attract and reward pollinators. Sections 30.1, 30.2
Gamete formation and fertilization Male and female gametophytes develop inside the reproductive parts of flowers. In flowering plants, pollination is followed by double fertilization. As in animals, signals are key to sex. Sections 30.3, 30.4
Seeds and fruits After fertilization, ovules mature into seeds, each an embryo sporophyte together with tissues that nourish and protect it. As seeds develop, tissues of the ovary and often other parts of the flower mature into fruits, which function in seed dispersal. Sections 30.5, 30.6
Asexual reproduction in plants Many species of plants reproduce asexually by vegetative reproduction. Humans take advantage of this natural tendency by propagating plants asexually for agriculture and research. Section 30.7
How would you vote? Systemic insecticides get into the nectar and pollen of flowering plants and thus can poison honeybees and other insect pollinators. To protect pollinators, should the use of these chemicals on flowering plants be restricted? See CengageNOW for details, then vote online.
507
30.1
Reproductive Structures of Flowering Plants Specialized reproductive shoots called flowers consist of whorls of modified leaves. Links to Plant life cycles 10.5 and 23.2, ABC model of flowering 15.2, Lateral buds 28.3
The sporophyte dominates the life cycle of flowering plants. A sporophyte is a diploid spore-producing plant body that grows by mitotic cell divisions of a fertilized egg (Sections 10.5 and 23.2). Flowers are the specialized reproductive shoots of angiosperm sporophytes. Spores that form by meiosis inside flowers develop into haploid gametophytes, or structures in which haploid gametes form by mitosis.
Anatomy of a Flower A flower forms when a lateral bud along the stem of a sporophyte develops into a short, modified branch called a receptacle. Master genes that become active in the apical meristem of the branch direct the formation of a flower (Section 15.2). The petals and other parts of a typical flower are modified leaves that form in four spirals or four rings
(whorls) at the end of the floral shoot. The outermost whorl develops into a calyx, which is a ring of leaflike sepals (Figure 30.2a). The sepals of most flowers are photosynthetic and inconspicuous; they serve to protect the flower’s reproductive parts. Just inside the calyx, petals form in a whorl called the corolla (from the Latin corona, or crown). Petals are usually the largest and most brightly colored parts of a flower. They function mainly to attract pollinators. A whorl of stamens forms inside the ring of petals. Stamens are the male parts of a flower. In most flowers, they consist of a thin filament with an anther at the tip. Inside a typical anther are two pairs of elongated pouches called pollen sacs. Meiosis of diploid cells in each sac produces haploid, walled spores. The spores differentiate into pollen grains, which are immature male gametophytes. The durable coat of a pollen grain is a bit like a suitcase that carries and protects the cells inside on their journey to meet an egg. The innermost whorl of modified leaves are folded and fused into carpels, the female parts of a flower. Carpels are sometimes called pistils. Many flowers have one carpel; others have several carpels, or several
stamen
carpel
(male reproductive part)
(female reproductive part)
filament
anther
stigma
style
ovary carpel structure varies
petal (all petals combined are the flower’s corolla) sepal (all sepals combined are flower’s calyx)
ovule (forms within ovary)
receptacle
A Like many flowers, a cherry blossom (Prunus) has several stamens and one carpel. The male reproductive parts are stamens, which consist of pollen-bearing anthers atop slender filaments. The female reproductive part is the carpel, which consists of stigma, style, and ovary.
Figure 30.2 Animated Structure of flowers.
508 UNIT V
HOW PLANTS WORK
ovary position varies
ovule position varies within ovaries
B Flower structure varies among different plant species.
groups of carpels, that may be fused (Figure 30.2b). The upper region of a carpel, a sticky or hairy stigma, is specialized to trap pollen grains. Often, the stigma sits on top of a slender stalk called a style. The lower, swollen region of a carpel is the ovary, which contains one or more ovules. An ovule is a tiny bulge of tissue inside the ovary. A cell in the ovule undergoes meiosis and develops into the haploid female gametophyte. At fertilization, a diploid zygote forms when male and female gametes meet inside an ovary. The ovule then matures into a seed. The life cycle of the plant is completed when the seed germinates, and a new sporophyte forms and matures (Figure 30.3). We return to fertilization and seed development in later sections.
mature sporophyte (2n)
germination zygote in seed (2n)
meiosis in anther
DIPLOID
fertilization
meiosis in ovary
HAPLOID
eggs (n)
microspores (n)
sperm (n)
megaspores (n)
male gametophyte (n) female gametophyte (n)
Diversity of Flower Structure Remember that mutations in some master genes give rise to dramatic variations in flower structure (Section 15.2). We see many such variations in the range of diversity of flowering plants. Regular flowers are symmetric around their center axis: If the flower were cut like a pie, the pieces would be roughly identical (Figure 30.4a). Irregular flowers are not radially symmetric (Figure 30.4b). Flowers may form as single blossoms, or in clusters called inflorescences. Some species, like sunflowers (Helianthus), have inflorescences that are actually composites of many flowers grouped into a single head. Other types of inflorescence include umbrella-like forms (Figure 30.4c) or elongated spikes (Figure 30.4d). A cherry blossom (Figure 30.2) has all four sets of modified leaves (sepals, petals, stamens, and carpels), so it is called a complete flower. Incomplete flowers lack one or more of these structures (Figure 30.4e). Cherry blossoms are also called perfect flowers, because they have both stamens and carpels. Perfect flowers may be fertilized by pollen from other plants, or they can self-pollinate. Self-pollination can be adaptive in situations where plants are widely spaced, such as in newly colonized areas. However, in general, offspring of self-pollinated flowers or plants tend to be less vigorous than those of cross-pollinated plants. Accordingly, adaptations of many plant species encourage or even require cross-pollination. For example, pollen may be released from a flower’s anthers only after its stigma is no longer receptive to being fertilized by pollen. As another example, the imperfect flowers of some species have either stamens or carpels, but not both. Depending on the species, the separate male and female flowers form on different plants, or on the same plant.
Figure 30.3 Animated Typical flowering plant life cycle.
a
b
c
d
e
Figure 30.4 Examples of structural variation in flowers. (a) Arctic rose (Rosa acicularis), a regular flower; (b) white sage (Salvia apiana), an irregular flower; (c) carrot (Daucus carota), an umbrella-like inflorescence; (d) yucca (Yucca sp.), an elongated inflorescence, and (e) meadow-rue (Thalictrum pubescens), an incomplete flower that has stamens but no petals.
Take-Home Message What are flowers? Flowers are short reproductive branches of sporophytes. The different parts of a flower (sepals, petals, stamens, and carpels) are modified leaves. The male parts of flowers are stamens, which typically consist of a filament with an anther at the tip. Pollen forms inside anthers. The female parts of flowers are carpels, which typically consist of stigma, style, and ovary. Haploid, egg-producing female gametophytes form in an ovule inside the ovary. Flowers vary in structure. Many of the variations are adaptations that maximize the plant’s chance of cross-pollination.
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30.2
Flowers and Their Pollinators Flowering plants coevolved with pollination vectors that help them reproduce sexually.
Figure 30.6 Opposite, flowers of a giant saguaro cactus (Carnegia gigantea). Birds and insects sip nectar from these large, white flowers by day, and bats sip by night. The flowers offer a sweet nectar.
Links to Coevolution 18.12, Coevolution of flowers and pollinators 23.8
Getting By With a Little Help From Their Friends Sexual reproduction in plants involves the transfer of pollen, typically from one plant to another. Unlike animals, plants cannot move about to find a mate, so they depend on factors in the environment that can move pollen around for them (Section 23.8). The diversity of flower form in part reflects that dependence. A pollination vector is an agent that delivers pollen from an anther to a compatible stigma. Many plants are pollinated by wind, which is entirely nonspecific in
a
b
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c
HOW PLANTS WORK
where it dumps pollen. Such plants often release pollen grains by the billions, insurance in numbers that some of their pollen will reach a receptive stigma. Other plants enlist the help of pollinators—living pollination vectors—to transfer pollen among individuals of the same species. An insect, bird, or other animal that is attracted to a particular flower often picks up pollen on a visit, then inadvertently transfers it to the flower of a different plant on a later visit. The more specific the attraction, the more efficient the transfer of pollen among plants of the same species. Given the selective advantage for flower traits that attract specific pollinators, it is not surprising that about 90 percent of flowering plants have coevolved animal pollinators. A flower’s shape, pattern, color, and fragrance are adaptations that attract specific animals (Table 30.1). For example, the petals of flowers pollinated by bees usually are bright white, yellow, or blue, typically with pigments that reflect ultraviolet light. Such UV-reflecting pigments are often distributed in patterns that bees can recognize as visual guides to nectar (Figure 30.5). We see these patterns only with special camera filters; our eyes do not have receptors that respond to UV light. Pollinators such as bats and moths have an excellent sense of smell, and can follow concentration gradients of airborne chemicals to a flower that is emitting them (Figure 30.6). Not all flowers smell sweet; odors like dung or rotting flesh beckon beetles and flies. An animal’s reward for a visit to the flower may be nectar (a sweet fluid exuded by flowers), oils, nutritious pollen, or even the illusion of having sex (Figure 30.7). Nectar is the only food for most adult butterflies, and it is the food of choice for hummingbirds. Honeybees collect nectar and convert it to honey, which helps feed the bees through the winter. Pollen is an even richer food, with more vitamins and minerals than nectar. Many flowers have specializations that exclude nonpollinators. For example, nectar at the bottom of a long floral tube or spur is often accessible only to a certain
Figure 30.5 Bees as pollinators. (a) The blueberry bee (Osmia ribifloris) is an efficient pollinator of a variety of plants, including this barberry (Berberis). (b) How we see a gold-petaled marsh marigold. (c) Bee-attracting pattern of the same flower. We can see this UV-reflecting pattern only with special camera filters.
Table 30.1
Common Traits of Flowers Pollinated by Specific Animal Vectors Vector
Floral Trait
Bats
Bees
Beetles
Birds
Butterflies
Flies
Moths
Color:
Dull white, green, purple
Bright white, yellow, blue, UV
Dull white or green
Scarlet, orange, red, white
Bright, such as red, purple
Pale, dull, dark brown or purple
Pale/dull red, pink, purple, white
Odor:
Strong, musty, emitted at night
Fresh, mild, pleasant
None to strong
None
Faint, fresh
Putrid
Strong, sweet, emitted at night
Nectar :
Abundant, hidden
Usually
Sometimes, not hidden
Ample, deeply hidden
Ample, deeply hidden
Usually absent
Ample, deeply hidden
Pollen:
Ample
Limited, often sticky, scented
Ample
Modest
Limited
Modest
Limited
Shape:
Regular, bowlshaped, closed during the day
Shallow with landing pad; tubular
Large, bowlshaped
Large funnelshaped cups, strong perch
Narrow tube with spur; wide landing pad
Shallow, funnelshaped or traplike and complex
Regular; tubeshaped with no lip
Banana, agave
Larkspur, violet
Magnolia, dogwood
Fuschia, hibiscus
Phlox
Skunk cabbage, philodendron
Tobacco, lily, some cactuses
Examples:
pollinator that has a matching feeding device (Figure 18.25). Often, stamens adapted to brush against a pollinator’s body or lob pollen onto it will function only when triggered by that pollinator. Such relationships are to both species’ mutual advantage: A flower that captivates the attention of an animal has a pollinator that spends its time seeking out (and pollinating) only those flowers; the animal receives an exclusive supply of the reward offered by the plant. a
b
Take-Home Message What is the purpose of the nonreproductive traits of flowers? The shape, pattern, color, and fragrance of flowers attract coevolved pollinators. Pollinators are often rewarded for visiting a flower by obtaining nutritious pollen or sweet nectar.
Figure 30.7 Intimate connections. (a) Female burnet moths (Zygaena filipendulae) perch on purple flowers—preferably those of field scabious (Knautia arvensis)—when they are ready to mate. The visual combination attracts males. (b) A zebra orchid (Caladenia cairnsiana) mimics the scent of a female wasp. Male wasps follow the scent to the flower, then try to copulate with and lift the dark red mass of tissue on the lip. The wasp’s movements trigger the lip to tilt upward, which brushes the wasp’s back against the flower’s stigma and pollen.
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A New Generation Begins In flowering plants, fertilization has two outcomes: It results in a zygote, and it is the start of endosperm, which is a nutritious tissue that nourishes the embryo sporophyte.
pollen sac
Links to Evolution of seed-bearing plants 23.8, Life cycle of flowering plants 23.9, Cell signaling 27.6
anther (cutaway view)
Microspore and Megaspore Formation Figure 30.8 zooms in on a flowering plant life cycle. On the male side, masses of diploid, spore-producing cells form by mitosis in the anthers. Typically, walls develop around the cell masses to form four pollen sacs (Figure 30.8a). Each cell inside the sacs undergoes meiosis, forming four haploid microspores (Figure 30.8b). Mitosis and differentiation of microspores produce pollen grains. Each pollen grain consists of a durable coat that surrounds two cells, one inside the cytoplasm of the other (Figure 30.8c). After a period of dormancy, the pollen sacs split open, and pollen is released from the anther (Figure 30.8d). On the female side, a mass of tissue—the ovule— starts growing on the inner wall of an ovary (Figure 30.8e). One cell in the middle of the mass undergoes meiosis and cytoplasmic division, forming four haploid megaspores (Figure 30.8f ). Three of the four megaspores typically disintegrate. The remaining megaspore undergoes three rounds of mitosis without cytoplasmic division. The outcome is a single cell with eight haploid nuclei (Figure 30.8g). The cytoplasm of this cell divides unevenly, and the result is a seven-celled embryo sac that constitutes the female gametophyte (Figure 30.8h). The gametophyte is enclosed and protected by cell layers, called integuments, that developed from ovule tissue. One of the cells in the gametophyte, the endosperm mother cell, has two nuclei (n + n). Another cell is the egg.
Pollination and Fertilization Pollination refers to the arrival of a pollen grain on a receptive stigma. Interactions between the two structures stimulate the pollen grain to resume metabolic activity (germinate). One of the two cells in the pollen grain then develops into a tubular outgrowth called a pollen tube. The other cell undergoes mitosis and cytoplasmic division, producing two sperm cells (the male gametes) within the pollen tube. A pollen tube together with its contents of male gametes constitutes the mature male gametophyte (Figure 30.8d). The pollen tube grows from its tip down through the carpel and ovary toward the ovule, carrying with it the two sperm cells. Chemical signals secreted by 512 UNIT V
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filament
forerunner of one of the microspores
A Pollen sacs form in the mature sporophyte.
Diploid Stage
meiosis
Haploid Stage
B Four haploid (n) microspores form by meiosis and cytoplasmic division of a cell in the pollen sac.
C In this plant, mitosis of a microspore (with no cytoplasmic division) followed by differentiation results in a two-celled, haploid pollen grain. D A pollen grain released from the anther lands on a stigma and germinates. One cell in the grain develops into a pollen tube; the other gives rise to two sperm cells, which are carried by the pollen tube into the tissues of the carpel.
stigma Mature Male Gametophyte
pollen tube sperm cells (male gametes)
carpel
Figure 30.8 Animated Life cycle of cherry (Prunus), a eudicot. Figure It Out: What structure gives rise to a pollen grain by mitosis? Answer: A microspore
30.3
the female gametophyte guide the tube’s growth to the embryo sac within the ovule. Many pollen tubes may grow down into a carpel, but only one typically penetrates an embryo sac. The sperm cells are then released into the sac (Figure 30.8i). Flowering plants undergo double fertilization: One of the sperm cells from the
an ovule
cell inside ovule tissue
ovary wall Sporophyte seedling (2n) ⎫ ⎪ ⎪ ⎪ embryo (2n) ⎬ seed endosperm (3n)⎪⎪ ⎪ ⎭
seed coat
E In a flower of a mature sporophyte, an ovule forms inside an ovary. One of the cells in the ovule enlarges.
ovary (cutaway view)
Diploid Stage
double fertilization
meiosis
Haploid Stage
F Four haploid (n) megaspores form by meiosis and cytoplasmic division of the enlarged cell. Three megaspores disintegrate.
pollen tube
G In the remaining megaspore, three rounds of mitosis without cytoplasmic division produce a single cell that contains eight haploid nuclei.
Female Gametophyte
endosperm mother cell (n + n) egg (n)
I The pollen tube grows down through stigma, style, and ovary tissues, then penetrates the ovule and releases two sperm nuclei. One nucleus fertilizes the egg. The other nucleus fuses with the endosperm mother cell.
H Uneven cytoplasmic divisions result in a seven-celled embryo sac with eight nuclei—the female gametophyte.
pollen tube fuses with (fertilizes) the egg and forms a diploid zygote. The other fuses with the endosperm mother cell, forming a triploid (3n) cell. This cell will give rise to triploid endosperm, a nutritious tissue that forms only in seeds of flowering plants. Right after a seed germinates, endosperm will sustain the rapid growth of the sporophyte seedling until true leaves form and photosynthesis begins.
Take-Home Message How does fertilization occur in flowering plants? In flowering plants, male gametophytes form in pollen grains; female gametes form in ovules. Pollination occurs when pollen arrives on a receptive stigma.
A pollen grain germinates on a receptive stigma as a pollen tube containing male gametes. The pollen tube grows into the carpel and enters an ovule. Double fertilization occurs when one of the male gametes fuses with the egg, the other with the endosperm mother cell.
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30.4 Flower Sex Interactions between pollen grain and stigma govern pollen germination and pollen tube growth. Links to Recognition and adhesion proteins 5.2, Cell signaling 27.6, Plant epidermis 28.2
a
100 µm
The main function of a pollen grain’s coat is to protect the two cells inside of it on what may be a long, turbulent ride to a stigma. Pollen grains make terrific fossils because the outer layer of the coat consists primarily of sporopollenin, an extremely hard, durable mixture of long-chain fatty acids and other organic molecules. In fact, sporopollenin is so resistant to degradation by enzymes and harsh chemicals that we still don’t know exactly what it is. Given the coat’s toughness, how does a pollen grain “know” when to germinate? How does a microscopic pollen tube that grows through centimeters of tissue find its way to a single cell deep inside of the carpel? The answers to such questions involve cell signaling (Section 27.6). Sex in plants, like sex in animals, involves an interplay of signals. It begins when recognition proteins on epidermal cells of a stigma bind to molecules in the coat of a pollen grain. Within minutes, lipids and proteins in the pollen grain’s coat begin to diffuse onto the stigma, and the pollen grain becomes tightly bound via adhesion proteins in stigma cell membranes. The specificity of recognition proteins means that a stigma can preferentially bind pollen of its own species. Pollen is very dry, and the cells inside are dormant. These adaptations make the grains light and portable. After a pollen grain attaches to a stigma, nutrient-rich fluid begins to diffuse from the stigma into the grain. The fluid stimulates the cells inside to resume metabolism, and a pollen tube that contains the male gametes grows out of one of the furrows or pores in the pollen’s coat (Figure 30.9). Gradients of nutrients (and perhaps other molecules) direct the growth of the pollen tube down through the style. Cells of the female gametophyte secrete chemical signals that guide the growth of the pollen tube from the bottom of the style to the egg. These signals are species-specific; pollen tubes of different species do not recognize them, and will not reach the ovule. In some species, the signals are also part of mechanisms that can keep a flower’s pollen from fertilizing its own stigma. Only pollen from another flower (or another plant) can give rise to a pollen tube that recognizes the female gametophyte’s chemical guidance.
b
Figure 30.9 Pollen. (a) Pollen grains from several species. Elaborately sculpted pollen coats are adapted to cling to insect bodies; smooth coats are adapted for wind dispersal. (b) Pollen tubes grow from pollen grains (orange) that germinated on stigmas (yellow) of prairie gentian (Gentiana). Molecular cues guide a pollen tube’s growth through carpel tissues to the egg.
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Take-Home Message What constitutes sex in plants? Species-specific molecular signals stimulate pollen germination and guide pollen tube growth to the egg. In some species, the specificity of the signaling also limits self-pollination.
30.5
Seed Formation many ovules inside ovary wall
After fertilization, mitotic cell divisions transform a zygote into an embryo sporophyte encased in a seed.
The Embryo Sporophyte Forms In flowering plants, double fertilization produces a zygote and a triploid (3n) cell. Both begin mitotic cell divisions; the zygote develops into an embryo sporophyte, and the triploid cell develops into endosperm (Figure 30.10a–c). When the embryo approaches maturity, the integuments of the ovule separate from the ovary wall and become layers of the protective seed coat. The embryo sporophyte, its reserves of food, and the seed coat have now become a mature ovule, a selfcontained package called a seed (Figure 30.10d). The seed may enter a period of dormancy until it receives signals that conditions in the environment are appropriate for germination.
Seeds as Food As an embryo is developing, the parent plant transfers nutrients to the ovule. These nutrients accumulate in endosperm mainly as starch with some lipids, proteins, or other molecules. Eudicot embryos transfer nutrients in endosperm to their two cotyledons before germination occurs. The embryos of monocots tap endosperm only after seeds germinate. The nutrients in endosperm and cotyledons nourish seedling sporophytes. They also nourish humans and other animals. Rice (Oryza sativa), wheat (Triticum), rye (Secale cereale), oats (Avena sativa), and barley (Hordeum vulgare) are among the grasses commonly cultivated for their nutritious seeds, or grains. The embryo (the germ) of a grain contains most of the seed’s protein and vitamins, and the seed coat (the bran) contains most of the minerals and fiber. Milling removes bran and germ, leaving only the starch-packed endosperm. Maize, or corn (Zea mays), is the most widely grown grain crop. Popcorn pops because the moist endosperm steams when heated; pressure builds inside the seed until it bursts. Cotyledons of bean and pea seeds are valued for their starch and protein; those of coffee (Coffea) and cacao (Theobroma cacao), for their stimulants.
embryo
endosperm
integuments
A After fertilization, a Capsella flower’s ovary develops into a fruit. Surrounded by integuments, an embryo forms inside each of the ovary’s many ovules.
embryo
endosperm
B The embryo is heart-shaped when cotyledons start forming. Endosperm tissue expands as the parent plant transfers nutrients into it.
embryo
root apical meristem endosperm
shoot tip
cotyledons
C The developing embryo is torpedo-shaped when the enlarging cotyledons bend inside the ovule.
embryo
seed coat
cotyledons
Take-Home Message What is a seed? After fertilization, the zygote develops into an embryo, the endosperm becomes enriched with nutrients, and the ovule’s integuments develop into a seed coat. A seed is a mature ovule. It contains an embryo sporophyte.
D A layered seed coat that formed from the layers of integuments surrounds the mature embryo sporophyte. In eudicots like Capsella, nutrients have been transferred from endosperm into two cotyledons.
Figure 30.10 Animated Embryonic development of shepherd’s purse (Capsella), a eudicot.
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30.6
Fruits tissue derived from ovary wall
As embryos develop inside the ovules of flowering plants, tissues around them form fruits. Water, wind, and animals disperse seeds in fruits.
a
b
c
seed enlarged receptacle
Figure 30.11 Parts of a fruit develop from parts of a flower. Left, the tissues of an orange (Citrus) develop from the ovary wall. Right, the flesh of an apple is an enlarged receptacle. Figure It Out: How many carpels were there in the flower that
Answer: Eight
Only flowering plants form seeds in ovaries, and only they make fruits. A fruit is a seed-containing mature ovary, often with fleshy tissues that develop from the ovary wall (Figure 30.11). In some plants, fruit tissues develop from parts of the flower other than the ovary wall (such as petals, sepals, stamens, or receptacles). Apples, oranges, and grapes are familiar fruits, but so are many “vegetables” such as beans, peas, tomatoes, grains, eggplant, and squash. An embryo or seedling can use the nutrients stored in endosperm or cotyledons, but not in fruit. The function of fruit is to protect and disperse seeds. Dispersal increases reproductive success by minimizing competition for resources among parent and offspring, and by expanding the area colonized by the species. Just as flower structure is adapted to certain pollination vectors, so are fruits adapted to certain dispersal vectors: environmental factors such as water or wind, or mobile organisms such as birds or insects. Water-dispersed fruits have water-repellent outer layers. The fruits of sedges (Carex) native to American
carpel wall
gave rise to this orange?
marshlands have seeds encased in a bladderlike envelope that floats (Figure 30.12a). Buoyant fruits of the coconut palm (Cocos nucifera) have thick, tough husks that can float for thousands of miles in seawater. Many plant species use wind as a dispersal agent. Part of a maple fruit (Acer) is a dry outgrowth of the ovary wall that extends like a pair of thin, lightweight wings (Figure 30.12b). The fruit breaks in half when it drops from the tree; as the halves drop to the ground, wind currents that catch the wings spin the attached seeds away. Tufted fruits of thistle, cattail, dandelion,
d
Figure 30.12 Examples of adaptations that aid fruit dispersal. (a) Air-filled bladders that encase the seeds of certain sedges (Carex) allow the fruits to float in their marshy habitats. (b) Wind lifts the “wings” of maple (Acer) fruits, which spin the seeds away from the parent tree. (c) Wind that catches the hairy modified sepals of a dandelion fruit (Taraxacum) lifts the seed away from the parent plant. (d) Curved spines make cocklebur (Xanthium) fruits stick to the fur of animals (and clothing of humans) that brush past it. (e) The fruits of the California poppy (Eschscholzia californica) are long, dry pods that split open suddenly. The movement jettisons the seeds. e
f
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(f) The red, fleshy fruit of crabapples attracts cedar waxwings.
Table 30.2
Three Ways To Classify Fruits
How did the fruit originate?
a
b
c
Simple fruit
One flower, single or fused carpels
Aggregate fruit
One flower, several unfused carpels; becomes cluster of several fruits
Multiple fruit
Individually pollinated flowers grow and fuse
What is the fruit’s tissue composition?
Figure 30.13 Aggregate fruits. (a) A strawberry (Fragaria) is not a berry. The flower’s carpels turn inside out as the fruits form. The red, juicy flesh is an expanded receptacle; the hard “seeds” on the surface are individual dry fruits (b). (c) Boysenberries and other Rubus species are not berries, either. Each is an aggregate fruit of many small drupes.
True fruit
Only ovarian wall and its contents
Accessory fruit
Ovary and other floral parts, such as receptacle
Is the fruit dry or fleshy? Dry
and milkweed may be blown as far as 10 kilometers (6 miles) from the parent plant (Figure 30.12c). The fruits of cocklebur, bur clover, and many other plants have hooks or spines that stick to the feathers, feet, fur, or clothing of more mobile species (Figure 30.12d). The dry, podlike fruit of plants such as California poppy (Eschscholzia californica) propel their seeds through the air when they pop open explosively (Figure 30.12e). Colorful, fleshy, fragrant fruits attract insects, birds, and mammals that disperse seeds (Figure 30.12f ). The animal may eat the fruit and discard the seeds, or eat the seeds along with the fruit. Abrasion of the seed coat by digestive enzymes in an animal’s gut can facilitate germination after the seed departs in feces. Botanists categorize fruits by how they originate, their tissues, and appearance (Table 30.2). Simple fruits, such as pea pods, acorns, and Capsella, are derived from one ovary. Strawberries and other aggregate fruits form from separate ovaries of one flower; they mature as a cluster of fruits. Multiple fruits form from fused ovaries of separate flowers. The pineapple is a multiple fruit that forms from fused ovary tissues of many flowers. Fruits also may be categorized in terms of which tissues they incorporate. True fruits such as cherries consist only of the ovary wall and its contents. Other floral parts, such as the receptacle, expand along with the ovary in accessory fruits. Most of the flesh of an apple, an accessory fruit, is an enlarged receptacle. To categorize a fruit based on appearance, the first step is to describe it as dry or juicy (fleshy). Dry fruits are dehiscent or indehiscent. If dehiscent, the fruit wall splits along definite seams to release the seeds inside. California poppy fruits and pea pods are examples. A dry fruit is indehiscent if the wall does not split open; seeds are dispersed inside intact fruits. Acorns and grains (such as corn) are dry indehiscent fruits, as are the fruits of sunflowers, maples, and strawberries. Strawberries are not berries and their fruits are not
Dehiscent Indehiscent Fleshy Drupe Berry
Pome
Dry fruit wall splits on seam to release seeds Seeds dispersed inside intact, dry fruit wall
Fleshy fruit around hard pit surrounding seed Fleshy fruit, often many seeds, no pit Pepo: Hard rind on ovary wall Hesperidium: Leathery rind on ovary wall Fleshy accessory tissues, seeds in core tissue
juicy. A strawberry’s red flesh is an accessory to the dry indehiscent fruits on its surface (Figure 30.13a,b). Drupes, berries, and pomes are three types of fleshy fruits. Drupes have a pit, a hard jacket around the seed. Cherries, apricots, almonds, and olives are drupes, as are the individual fruits of boysenberries and other Rubus species (Figure 30.13c). A berry forms from a compound ovary. It has one to many seeds, no pit, and fleshy fruit. Grapes and tomatoes are berries. Lemons, oranges, and other citrus fruits (Citrus) are a type of berry called a hesperidium: An oily, leathery peel encloses juicy pulp. Each “section” of the pulp started out as an ovary of a partially fused carpel. Pumpkins, watermelons, and cucumbers are pepos, berries in which a hard rind of accessory tissues forms over the somewhat slippery true fruit. A pome has seeds in a core derived from the ovary; fleshy tissues derived from the receptacle enclose the core. Two familiar pomes are apples and pears. Take-Home Message What is a fruit?
A mature ovary, with or without accessory tissues that develop from other parts of a flower, is a fruit.
We can categorize a fruit in terms of how it originated, its composition, and whether it is dry or fleshy.
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30.7
Asexual Reproduction of Flowering Plants Many plants also reproduce asexually, which permits rapid production of genetically identical offspring. Links to Asexual versus sexual reproduction 10.1, Meiosis 10.3, Mendelian inheritance 11.1, Aneuploidy 12.6, Cloning 13.4, Radiometric dating 17.6, Speciation by polyploidy in plants 18.11, Modified stems 28.8
Plant Clones Unlike most animals, most flowering plants can reproduce asexually. By vegetative reproduction, new roots and shoots grow from extensions or fragments of a parent plant. Each new plant is a clone, a genetic replica of its parent. You already know that new roots and shoots sprout from nodes on modified stems (Section 28.8). This is one example of vegetative reproduction. As another example, “forests” of quaking aspen (Populus tremuloides) are actually stands of clones that grew from root suckers, which are shoots that sprout from the aspens’ shallow, cordlike lateral roots. Suckers sprout after aboveground parts of the aspens are damaged or removed. One stand in Utah consists of about 47,000 shoots and stretches for 107 acres (Figure 30.14). No one knows how old those aspen clones are. As long as conditions in the environment favor growth, such clones are as close as any organism gets to being immortal. The oldest known plant is a clone: the one and only population of King’s holly (Lomatia tasmanica), which consists of several hundred stems growing along 1.2 kilometers (0.7 miles) of a river gully in
Tasmania. Radiometric dating of the plant’s fossilized leaf litter show that the clone is at least 43,600 years old—predating the last ice age! The ancient species of Lomatia is triploid. With three sets of chromosomes, it is sterile—it can only reproduce asexually. Why? During meiosis, an odd number of chromosome sets cannot be divided equally between the two spindle poles. If meiosis does not fail entirely, unequal segregation of chromosomes during meiosis results in aneuploid offspring, which are rarely viable.
Agricultural Applications For thousands of years, we humans have been taking advantage of the natural capacity of plants to reproduce asexually. Almost all houseplants, woody ornamentals, and orchard trees are clones that have been grown from stem fragments (cuttings) of a parent plant. Propagating some plants from cuttings may be as simple as jamming a broken stem into the soil. This method uses the plant’s natural ability to form roots and new shoots from stem nodes. Other plants must be grafted. Grafting means inducing a cutting to fuse with the tissues of another plant. Often, the stem of a desired plant is spliced onto the roots of hardier one. Propagating a plant from cuttings ensures that offspring will have the same desirable traits as the parent plant. For example, domestic apple trees (Malus) are typically grafted because they do not breed true for fruit color, flavor, size, or texture. Even trees grown Cuttings and Grafting
Figure 30.14 Quaking aspen (Populus tremuloides). A single plant gave rise to this stand of shoots by asexual reproduction. Such clones are connected by underground lateral roots, so water can travel from roots near a lake or river to those in drier soil some distance away.
518 UNIT V
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c
b
Figure 30.15 Apples (Malus). (a) Commercial growers must plant grafted apple trees in order to reap consistent crops. (b) Fruit of 21 wild apple trees. (c) Gennaro Fazio (left) and Phil Forsline (right) are part of an effort to maintain the genetic diversity of apple trees in the United States. Cross-breeding is yielding new apples with the palatability of commercial varieties, and the disease resistance of wild trees.
a
from seeds of the same fruit produce fruits that vary, sometimes dramatically so. The genus is native to central Asia, where apple trees grow wild in forests. Each tree in the forests is different from the next, and very few of the fruits are palatable (Figure 30.15). In the early 1800s, the eccentric humanitarian John Chapman (known as Johnny Appleseed) planted millions of apple seeds in the midwestern United States. He sold the trees to homesteading settlers, who would plant orchards and make hard cider from the apples. About one of every hundred trees produced fruits that could be eaten out of hand. Its lucky owner would graft the tree and patent it. Most of the apple varieties sold in American grocery stores are clones of these trees, and they are still propagated by grafting. Grafting is also used to increase the hardiness of a desirable plant. In 1862, the plant louse Phylloxera was accidentally introduced into France via imported American grapevines. European grapevines had little resistance to this tiny insect, which attacks and kills the root systems of the vines. By 1900, Phylloxera had destroyed two-thirds of the vineyards in Europe, and devastated the wine-making industry. Today, French vintners routinely graft their prized grapevines onto the roots of Phylloxera-resistant American vines. Tissue Culture An entire plant may be cloned from a
single cell with tissue culture propagation, by which a somatic cell is induced to divide and form an embryo (Section 13.4). The method can yield millions of genetically identical plants from a single specimen. The technique is being used in research intended to improve
food crops. It is also used to propagate rare or hybrid ornamental plants such as orchids. Seedless Fruits In some plants such as figs, blackberries, and dandelions, fruits may form even in the absence of fertilization. In other species, fruit may continue to form after ovules or embryos abort. Seedless grapes and navel oranges are the result of mutations that result in arrested seed development. These plants are sterile, so they are propagated by grafting. Seedless bananas are triploid (3n). In general, plants tolerate polyploidy better than animals do. Triploid banana plants are robust, but sterile: They are propagated by adventitious shoots that sprout from corms. Despite their ubiquity in nature (Section 12.6), polyploid plants rarely arise spontaneously. Plant breeders often use the microtubule poison colchicine to artificially increase the frequency of polyploidy in plants (Section 18.11). Tetraploid (4n) offspring of colchicinetreated plants are then backcrossed with diploid parent plants. The resulting triploid offspring are sterile: They make seedless fruit after pollination (but not fertilization) by a diploid plant, or on their own. Seedless watermelons are produced this way.
Take-Home Message How do plants reproduce asexually? Many plants propagate asexually when new shoots grow from a parent plant or pieces of it. Offspring of such vegetative reproduction are clones.
Humans propagate plants asexually for agricultural or research purposes by grafting, tissue culture, or other methods.
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IMPACTS, ISSUES REVISITED
Plight of the Honeybee
Theobroma cacao (right) is a species of flowering plant that is native to the deep tropical rainforests of middle and south America. The bumpy, football-sized fruits of T. cacao contain 40 or so black, bitter seeds. We make chocolate by processing those seeds, but the tree has proven difficult to cultivate outside of rainforests. Why? T. cacao trees do not produce very many seeds when they are grown in typically sun-drenched cultivated plantations. As plantation owners found out, T. cacao has a preferred pollinator: midges. These tiny, flying insects live and breed only in
How would you vote? Systemic pesticides get into plant nectar and pollen eaten by honeybees and other pollinators. To protect pollinators, should the use of these pesticides on flowering plants be restricted? See CengageNOW for details, then vote online.
damp, rotting leaf litter of tropical rain forest floors. The flowers of T. cacao trees form low to the ground, directly on the woody trunk. This is an adaptation that encourages pollination by—not surprisingly—insects that live in the damp, rotting leaf litter of rain forest floors. Thus, no forests, no midges. No midges, no chocolate.
Summary Section 30.1 Flowers consist of modified leaves (sepals, petals, stamens, and carpels) at the ends of specialized branches of angiosperm sporophytes. An ovule develops from a mass of ovary wall tissue inside carpels. Spores produced by meiosis in ovules develop into female gametophytes; those produced in anthers develop into immature male gametophytes (pollen grains). Adaptations of many flowers restrict self-pollination.
Use the animation on CengageNOW to investigate a flowering plant life cycle and floral structure.
Section 30.2 A flower’s shape, pattern, color, and fragrance typically reflect an evolutionary relationship with a particular pollination vector, often a coevolved animal. Coevolved pollinators receive nectar, pollen, or another reward for visiting a flower. Sections 30.3, 30.4 Meiosis of diploid cells inside pollen sacs of anthers produces haploid microspores. Each microspore develops into a pollen grain. Mitosis and cytoplasmic division of a cell in an ovule produces four megaspores, one of which gives rise to the female gametophyte. One of the seven cells of the gametophyte is the egg; another is the endosperm mother cell. Pollination is the arrival of pollen grains on a receptive stigma. A pollen grain germinates and forms a pollen tube that contains two sperm cells. Species-specific molecular signals guide the tube’s growth down through carpel tissues to the egg. In double fertilization, one of the sperm cells in the pollen tube fertilizes the egg, forming a zygote; the other fuses with the endosperm mother cell and gives rise to endosperm.
Use the animation on CengageNOW to take a closer look at the life cycle of a eudicot.
Section 30.5 As a zygote develops into an embryo, the endosperm collects nutrients from the parent plant, and the ovule’s protective layers develop into a seed coat. A seed is a mature ovule: an embryo sporophyte and endosperm enclosed within a seed coat. 520 UNIT V
HOW PLANTS WORK
Eudicot embryos transfer nutrients from endosperm to their two cotyledons. Carbohydrates, lipids, and proteins stored in endosperm or cotyledons make seeds a nutritious food source for humans and other animals. Section 30.6 As an embryo sporophyte develops, the ovary wall and sometimes other tissues mature into a fruit that encloses the seeds. Fruit functions in the protection and dispersal of seeds.
Use the animation on CengageNOW to see how an embryo sporophyte develops in a eudicot seed.
Section 30.7 Many species of flowering plants reproduce asexually by vegetative reproduction. The offspring produced by asexual reproduction are clones of the parent. Many agriculturally valuable plants are produced by grafting or tissue culture propagation.
Self-Quiz
Answers in Appendix III
1. The of a flower contains one or more ovaries in which eggs develop, fertilization occurs, and seeds mature. a. pollen sac c. receptacle b. carpel d. sepal 2. Seeds are mature a. ovaries; ovules b. ovules; stamens
; fruits are mature c. ovules; ovaries d. stamens; ovaries
3. Meiosis of cells in pollen sacs forms haploid a. megaspores c. stamens b. microspores d. sporophytes 4. After meiosis in an ovule, a. two b. four c. six
megaspores form. d. eight
5. The seed coat forms from the . a. ovule wall c. endosperm b. ovary d. residues of sepals 6. Cotyledons develop as part of . a. carpels c. embryo sporophytes b. accessory fruits d. petioles
.
.
Data Analysis Exercise Figure 30.17 The dull, petal-less, ground-level flowers of Massonia depressa are accessible to rodents, who push their heads through the stamens to reach the nectar. Note the pollen on the gerbil’s snout.
Massonia depressa is a low-growing succulent plant native to the desert of South Africa. The dull-colored flowers of this monocot develop at ground level, have tiny petals, emit a yeasty aroma, and produce a thick, jelly-like nectar. These features led researchers to suspect that desert rodents such as gerbils pollinate this plant (Figure 30.17). To test their hypothesis, the researchers trapped rodents in areas where M. depressa grows and checked them for pollen. They also put some plants in wire cages that excluded mammals, but not insects, to see whether fruits and seeds would form in the absence of rodents. The results are shown in Figure 30.18.
10 mm
Type of rodent
1. How many of the 13 captured rodents showed some evidence of pollen from M. depressa?
Number caught
# with pollen on snout
# with pollen in feces
4 3 4 1 1
3 2 2 0 0
2 2 4 1 0
Namaqua rock rat Cape spiny mouse Hairy-footed gerbil Cape short-eared gerbil African pygmy mouse
2. Would this evidence alone be sufficient to conclude that rodents are the main pollinators for this plant? 3. How did the average number of seeds produced by caged plants compare with that of control plants?
40 mm
a
4. Do these data support the hypothesis that rodents are required for pollination of M. depressa? Why or why not? Figure 30.18 Right, results of experiments testing rodent pollination of M. depressa. (a) Evidence of visits to M. depressa by rodents. (b) Fruit and seed production of M. depressa with and without visits by mammals. Mammals were excluded from plants by wire cages with openings large enough for insects to pass through. 23 plants were tested in each group.
Mammals allowed access to plants Percentage of plants that set fruit Average number of fruits per plant Average number of seeds per plant
Mammals excluded from plants
30.4 1.39 20.0
4.3 0.47 1.95
b
7. Name one reward that a pollinator may receive in return for a visit to a flower of its coevolved plant partner. 8. By , a new plant forms from a tissue or structure that drops or is separated from the parent plant. a. parthenogenesis c. vegetative reproduction b. exocytosis d. nodal growth 9. Wanting to impress friends with her sophisticated knowledge of botany, Dixie Bee prepares a plate of tropical fruits for a party and cuts open a papaya (Carica papaya). Soft skin and soft fleshy tissue enclose many seeds in a slimy tissue (Figure 30.16a). Knowing her friends will ask her how to categorize this fruit, she panics, runs to her biology book, and opens it to Section 30.6. What does she find out? 10. Having succeeded in spectacularly impressing her friends, Dixie Bee prepares a platter of peaches (Figure 30.16b) for her next party. How will she categorize this fruit? 11. Match the terms with the most suitable description. ovule a. pollen tube together with receptacle its contents double b. embryo sac of seven cells, fertilization one with two nuclei anther c. starts out as cell mass in carpel ovary; may become a seed mature female d. female reproductive part gametophyte e. pollen sacs inside mature male f. base of floral shoot gametophyte g. formation of zygote and first cell of endosperm
Visit CengageNOW for additional questions.
a
b
Figure 30.16 Tangential sections reveal seeds of two mature fruits: (a) papaya (Carica papaya) and (b) peach (Prunus).
Critical Thinking 1. Would you expect winds, bees, birds, bats, butterflies, or moths to pollinate the flower pictured to the left? Explain your choice. 2. All but one species of largebilled birds native to New Zealand’s tropical forests are now extinct. Numbers of the surviving species, the kereru, are declining rapidly due to the habitat loss, poaching, predation, and interspecies competition that wiped out the other native birds. The kereru remains the sole dispersing agent for several native trees that produce big seeds and fruits. One tree, the puriri (Vitex lucens), is New Zealand’s most valued hardwood. Explain, in terms of natural selection, why we might expect to see no new puriri trees in New Zealand. CHAPTER 30
PLANT REPRODUCTION 521
31
Plant Development IMPACTS, ISSUES
Foolish Seedlings, Gorgeous Grapes
In 1926, researcher Ewiti Kurosawa was studying what
gibberellins also help dormant seeds and buds resume
Japanese call bakane, the “foolish seedling” effect. The
growth in spring.
stems of rice seedlings infected with a fungus, Gibberella
Applications of synthetic gibberellins make celery stalks
fujikuroi, grew twice the length of uninfected seedlings. The
longer and crispier. They prevent the rind of navel oranges in
abnormally elongated stems were weak and spindly, and
orchard groves from ripening before pickers can get to them.
eventually toppled. Kurosawa discovered that he could cause
Walk past plump seedless grapes in produce bins of grocery
the lengthening experimentally by applying extracts of the
stores and marvel at how fleshy fruits of the grape plant (Vitis)
fungus to seedlings. Many years later, other researchers puri-
grow in dense clusters along stems. Seedless grapes tend
fied the substance from fungal extracts that brought about
to be smaller than seeded varieties, because their undevel-
the lengthening. They named it gibberellin, in reference to the
oped seeds do not produce normal amounts of gibberellin.
name of the fungus.
Farmers spray their seedless grape plants with synthetic gib-
Gibberellins, as we now know, are a major class of plant
berellin, which increases the size of the resulting fruit (Figure
hormones. Hormones are secreted signaling molecules
31.1). Gibberellin also makes the stems elongate between
that stimulate some response in target cells. Cells that bear
nodes, which opens up space between individual grapes.
molecular receptors for a hormone may be in the same tissue
Improved air circulation between the fruit reduces infections
as the hormone-secreting cell, or in a distant tissue.
by fruit-damaging fungi.
Researchers have isolated more than eighty different forms
Gibberellin and other plant hormones control the growth
of gibberellin from seeds of flowering plants and fungi. These
and development of plants. Plant cells secrete hormones in
signaling molecules cause young cells in stems to elongate,
response to environmental cues, as when warm spring rains
and the collective elongation lengthens plant parts. In nature,
arrive after a cold winter, and the hours of daylight increase. With this chapter, we complete our survey of plant structure and function. So far, you read about the tissue organization of primary and secondary growth in flowering plants. You considered the tissue systems by which plants acquire and distribute water and solutes that sustain their growth. You learned how flowering plants reproduce, from gamete formation and pollination on through the formation of a mature embryo sporophyte inside a protective seed coat. At some point after its dispersal from a parent plant, remember, a seed germinates and growth resumes. In time, the mature sporophyte typically forms flowers, then seeds of its own. Depending on the species, it may drop old leaves throughout the year or all at once, in autumn. Continue now with the internal mechanisms that govern plant development, and the environmental cues that turn the mechanisms on or off at different times.
Figure 31.1 Seedless grapes radiate market appeal. The hormone gibberellin causes grape stems to lengthen, which improves air circulation around individual grapes and gives them more room to grow. The fruit also enlarges, which makes growers happy (grapes are sold by weight).
Links to Earlier Concepts
Key Concepts Patterns of plant development
This chapter revisits hormones (Section 27.2), homeostasis (27.5), and signaling pathways (27.6) in the context of plant physiology. In plants, development depends on cell-to-cell communication, just as animal development does (15.3).
Plant hormones are involved in gene expression and control (15.1), and the function of structures such as meristems (28.3) and stomata (29.4).
As you learn about plant responses to environmental stimuli, you will be drawing upon your understanding of carbohydrates (3.2, 3.3); how turgor (5.6) pushes on plant cell walls (4.12); light (7.1); and photosynthesis (7.4, 7.6). You will also revisit cell components, including plastids (4.11), the cytoskeleton (4.13), and membrane transport proteins (5.2).
Plant development includes seed germination and all events of the life cycle, such as root and shoot development, flowering, fruit formation, and dormancy. These activities have a genetic basis, but are also influenced by environmental factors. Section 31.1
Mechanisms of hormone action Cell-to-cell communication is essential to development and survival of all multicelled organisms. In plants, such communication occurs by hormones. Sections 31.2, 31.3
Responses to environmental cues Plants respond to environmental cues, including gravity, sunlight, and seasonal shifts in night length and temperatures, by altering patterns of growth. Cyclic patterns of growth are responses to changing seasons and other recurring environmental patterns. Sections 31.4–31.6
How would you vote? 1-Methylcyclopropene, or MCP, is a gas that keeps ethylene from binding to cells in plant tissues. It is used to prolong the shelf life of cut flowers and the storage time for fruits. Should produce treated with MCP be labeled to alert consumers? See CengageNOW for details, then vote online.
523
31.1
Patterns of Development in Plants Patterns of development in plants have a genetic basis, and they are also influenced by the environment.
seed coat fused with ovary wall
Links to Carbohydrates 3.3, Plant cell walls 4.12, Gene control 15.1, Hormones 27.2, Meristems 28.3
In Chapter 30, we left the embryo sporophyte after its dispersal from the parent plant. What happens next? An embryonic plant complete with shoot and root apical meristems formed as part of the embryo (Figure 31.2). However, the seed dried out as it matured, and the desiccation caused the embryo’s cells to stop dividing. The embryo entered a period of temporarily suspended development called dormancy. An embryo may idle in its protective seed coat for years before it resumes metabolic activity. Germination is the process by which a mature embryo sporophyte resumes growth. The process begins with water seeping into a seed. The water activates enzymes that start to hydrolyze stored starches into sugar monomers. It also swells tissues inside the seed, so the coat splits open and oxygen enters. Meristem cells in the embryo begin to use the sugars and the oxygen for aerobic respiration as they start dividing rapidly. The embryonic plant begins to grow from the meristems. Germination ends when the first part of the embryo—the embryonic root, or radicle—breaks out of the seed coat. Seed dormancy is a climate-specific adaptation that allows germination to occur when conditions in the environment are most likely to support the growth of a seedling. For example, the weather in regions near the equator does not vary by season, so seeds of most plants native to such regions do not enter dormancy; they can germinate as soon as they are mature. By contrast, the seeds of many annual plants native to colder regions are dispersed in autumn. If they germinated immediately, the seedlings would not survive the cold winter. Instead, the seeds stay dormant until spring, when milder temperatures and longer daylength are more suitable for tender seedlings. How does a dormant embryo sporophyte “know” when to germinate? The triggers, other than the presence of water, differ by species, and all have a genetic basis. For example, some seed coats are so dense that they must be abraded or broken (by being chewed, for example) before water can even enter the seed. Seeds of some species of lettuce (Lactuca) must be exposed to bright light. The germination of wild California poppy seeds (Eschscholzia californica) is inhibited by light and enhanced by smoke. The seeds of some species of pine (Pinus) will not germinate unless they have been previously burned. The seeds of many cool-climate plants require exposure to freezing temperatures. 524 UNIT V
HOW PLANTS WORK
endosperm cells
cotyledon coleoptile plumule (embryonic shoot)
embryo
hypocotyl
radicle (embryonic root)
Figure 31.2 Anatomy of a corn seed (Zea mays). During germination, cell divisions resume mainly at apical meristems of the plumule (the embryonic shoot) and radicle (the embryonic root). A plumule consists of an apical meristem and two tiny leaves. In grasses such as corn, the growth of this delicate structure through soil is protected by a sheathlike coleoptile.
Germination is just one of many patterns of development in plants. As a sporophyte grows and matures, its tissues and parts develop in other patterns characteristic of its species (Figures 31.3 and 31.4). Leaves form in predictable shapes and sizes, stems lengthen and thicken in particular directions, flowering occurs at a certain time of year, and so on. As in germination, these patterns have a genetic basis, but they also have an environmental component. Development includes growth, which is an increase in cell number and size. Plant cells are interconnected by shared walls, so they cannot move about within the organism. Thus, plant growth occurs primarily in the direction of cell division—and cell division occurs primarily at meristems. Behind meristems, cells differentiate and form specialized tissues. However, unlike animal cell differentiation, plant cell differentiation is often reversible, as when new shoots form on mature roots, or when new roots sprout from a mature stem. Take-Home Message What is plant development? In plants, growth and differentiation results in the formation of tissues and parts in predictable patterns. Germination and other patterns of plant development are an outcome of gene expression and environmental influences.
coleoptile
primary leaf branch root
coleoptile
adventitious (prop) root
primary root coleoptile
branch root
hypocotyl
primary root
radicle
A After a corn grain (seed) germinates, its radicle and coleoptile emerge. The radicle develops into the primary root. The coleoptile grows upward and opens a channel through the soil to the surface, where it stops growing.
B The plumule develops into the seedling’s primary shoot, which pushes through the coleoptile and begins photosynthesis. In corn plants, adventitious roots that develop from the stem afford additional support for the rapidly growing plant.
Figure 31.3 Animated Early growth of corn (Zea mays), a monocot.
seed coat
primary leaf
radicle
primary leaf withered cotyledon
cotyledons (two) hypocotyl
branch root primary root
A After a bean seed germinates, its radicle emerges and bends in the shape of a hook. Sunlight causes the hypocotyl to straighten, which pulls the cotyledons up through the soil.
primary root
branch root nodule roots
B Photosynthetic cells in the cotyledons make food for several days, then the seedling’s leaves take over the task. The cotyledons wither and fall off.
Figure 31.4 Animated Early growth of the common bean plant (Phaseolus vulgaris), a eudicot.
CHAPTER 31
PLANT DEVELOPMENT 525
31.2
Plant Hormones and Other Signaling Molecules Plant development depends on cell-to-cell communication, which is mediated by plant hormones. Links to Transcription factors 15.1, Cell communication in animal development 15.3, Function of stomata 29.4
Plant Hormones You may be surprised to learn that plant development depends on extensive coordination among individual cells, just as it does in animals (Section 15.3). A plant is an organism, not just a collection of cells, and as such it develops as a unit. Cells in different parts of a plant coordinate their activities by communicating with one another. Such communication means, for example, that root and shoot growth occur at the same time. Plant cells use hormones to communicate with one another. Plant hormones are signaling molecules that can stimulate or inhibit plant development, including growth. Environmental cues such as the availability of water, length of night, temperature, and gravity influence plants by triggering the production and dispersal of hormones. When a plant hormone binds to a target cell, it may modify gene expression, solute concentrations, enzyme activity, or activate another molecule in the cytoplasm. Later sections give examples. Five types of plant hormones—gibberellins, auxins, abscisic acid, cytokinins, and ethylene—all interact to orchestrate plant development (Table 31.1).
Table 31.1
Primary Source
Gibberellins
Auxins
Abscisic acid
Cytokinins
Effect
Site of Effect
Stem tip, young leaves
Stimulates cell division, elongation
Stem internode
Embryo
Stimulates germination
Seed
Embryo (grass)
Stimulates starch hydrolysis
Endosperm
Stem tip, young leaves
Stimulates cell elongation
Growing tissues
Initiates formation of lateral roots
Roots
Inhibits growth (apical dominance)
Axillary buds
Developing embryos
526 UNIT V
Figure 31.5 Foolish cabbages! The three tall cabbage plants were treated with gibberellins. The two short plants in front of the ladder were not treated.
Major Plant Hormones and Some of Their Effects
Hormone
Ethylene
Gibberellins Growth and other processes of development in all flowering plants, gymnosperms, mosses, ferns, and some fungi are regulated in part by gibberellins. These hormones induce cell division and elongation in stem tissue; thus, they cause stems to lengthen between the nodes. As mentioned in the chapter introduction, this effect can be demonstrated by application of gibberellin to the leaves of young plants (Figure 31.5). The short stems of Mendel’s dwarf pea plants (Section 11.3) are the result of a mutation that reduces the rate of gibberellin synthesis in these plants. Gibberellins are also involved in breaking dormancy of seeds, seed germination, and the induction of flowering in biennials and some other plants.
Leaves
Stimulates differentiation of xylem
Cambium
Inhibits abscission
Leaves, fruits
Stimulates fruit development
Ovary
Closes stomata
Guard cells
Stimulates formation of dormant buds
Stem tip
Ovule
Inhibits germination
Seed coat
Root tip
Stimulates cell division
Stem tip, axillary buds
Inhibits senescence (aging)
Leaves
Inhibits cell elongation
Stem
Stimulates senescence (aging)
Leaves
Stimulates ripening
Fruits
Damaged or aged tissue
HOW PLANTS WORK
Table 31.2
Some Commercial Uses of Plant Hormones
Gibberellins Increase fruit size; delay citrus fruit ripening; synthetic forms can make some dwarf mutants grow tall
Synthetic auxins Promote root formation in cuttings; induce seedless fruit production before pollination; keep mature fruit on trees until harvest time; widely used as herbicides against broadleaf weeds in agriculture
ABA Induces nursery stock to enter dormancy before shipment to minimize damage during handling
Cytokinins Tissue culture propagation; prolong shelf life of cut flowers Ethylene Allows shipping of green, still-hard fruit (minimizes bruises and rotting). Carbon dioxide application stops ripening of fruit in transit to market, then ethylene is applied to ripen distributed fruit quickly
Figure 31.6 Effect of rooting powders that contain auxin. Cuttings of winter honeysuckle (Lonicera fragrantissima) that were treated with a lot of auxin (right), some auxin (middle), and no auxin (left).
Auxins Auxins are plant hormones that promote or
inhibit cell division and elongation, depending on the target tissue. Auxins that are produced in apical meristems result in elongation of shoots. They also induce cell division and differentiation in vascular cambium, fruit development in ovaries, and lateral root formation in roots (Figure 31.6). Auxins also have inhibitory effects. For example, auxin produced in a shoot tip prevents the growth of lateral buds along a lengthening stem, an effect called apical dominance. Gardeners routinely pinch off shoot tips to make a plant bushier. Pinching the tips ends the supply of auxin in a main stem, so lateral buds give rise to branches. Auxins also inhibit abscission, which is the dropping of leaves, flowers, and fruits from the plant. Abscisic Acid Abscisic acid (ABA) is a hormone that
was misnamed; it inhibits growth, and has little to do with abscission. ABA is part of a stress response that causes stomata to close (Section 29.4). It also diverts photosynthetic products from leaves to seeds, an effect that overrides growth-stimulating effects of other hormones as the growing season ends. ABA inhibits seed germination in some species, such as apple (Malus). Such seeds do not germinate before most of the ABA they contain has been broken down, for example by a long period of cold, wet conditions. Cytokinins Plant cytokinins form in roots and travel
via xylem to shoots, where they induce cell divisions in the apical meristems. These hormones also release lateral buds from apical dominance, and inhibit the normal aging process in leaves. Cytokinins signal to shoots that roots are healthy and active. When roots stop growing, they stop producing cytokinins, so shoot growth slows and leaves begin to deteriorate.
Ethylene The only gaseous hormone, ethylene, is produced by damaged cells. It is also produced in autumn in deciduous plants, or near the end of the life cycle as part of a plant’s normal process of aging. Ethylene inhibits cell division in stems and roots. It also induces fruit and leaves to mature and drop. Ethylene is widely used to artificially ripen fruit that has been harvested while still green (Table 31.2).
Other Signaling Molecules As we now know, other signaling molecules have roles in various aspects of plant development. For example, brassinosteroids stimulate cell division and elongation; stems remain short in their absence. FT protein is part of a signaling pathway in flower formation. Salicylic acid, a molecule similar to aspirin, interacts with nitric oxide in regulating transcription of gene products that help plants resist attacks by pathogens. Systemin is a polypeptide that forms as insects feed on plant tissues; it enhances transcription of genes that encode insect toxins. Jasmonates, derived from fatty acids, interact with other hormones in control of germination, root growth, and tissue defense. You will see an example of how jasmonates help defend plant tissues in the next section.
Take-Home Message What regulates growth and development in plants?
Plant hormones are signaling molecules that influence plant development.
The five main classes of plant hormones are gibberellins, auxins, cytokinins, abscisic acid, and ethylene. Interactions among hormones and other kinds of signaling molecules stimulate or inhibit cell division, elongation, differentiation, and other events.
CHAPTER 31
PLANT DEVELOPMENT 527
31.3
Examples of Plant Hormone Effects
Links to Carbohydrates 3.2 and 3.3, Membrane proteins 5.2, Turgor 5.6, Plant cell walls 4.12, Rubisco 7.6, Gene expression 15.1, Signal transduction 27.6
lase is released into the endosperm’s starchy interior, where it proceeds to break down stored starch molecules into sugars. The embryo takes up the sugars and uses them for aerobic respiration, which fuels rapid cell divisions at the embryo’s meristems.
Gibberellin and Germination
Auxin Augmentation
During germination, water absorbed by a barley seed causes cells of the embryo to release gibberellin (Figure 31.7). The hormone diffuses into the aleurone, a protein-rich layer of cells surrounding the endosperm. In the aleurone, gibberellin induces transcription of the gene for amylase, an enzyme that hydrolyzes starch into sugar monomers (Sections 3.2 and 3.3). The amy-
There are a few naturally occurring auxins, but the one with the majority of effects is indole-3-acetic acid (IAA). This molecule plays a critical role in all aspects of plant development, starting with the first division of the zygote. It is involved in polarity and tissue patterning in the embryo, formation of plant parts (primary leaves, shoot tips, stems, and roots), differentiation of vascular tissues, formation of lateral roots (and adventitious roots in some species), and, as you will see in the next sections, responses to environmental stimuli. How can one molecule have so many roles? Part of the answer is that IAA has multiple effects on plant cells. For example, it causes cells to expand by increasing the activity of proton pumps, which are membrane transporter proteins that pump hydrogen ions from the cytoplasm into the cell wall. The resulting increase in acidity causes the wall to become less rigid. Turgor pushing on the softened wall from the inside stretches the cell irreversibly. IAA also affects gene expression by binding to certain regulatory molecules. The binding results in the degradation of repressor proteins that block transcription of specific genes (Section 15.1). IAA can exert different effects at different concentrations. Although present in almost all plant tissues, IAA is unevenly distributed through them. In a sporophyte, IAA is made mainly in shoot tips and young leaves, and its concentration is highest there. It forms gradients in plant tissues by moving away from these developing parts, but the movement is more complicated than diffusion alone can explain. IAA is transported in phloem over long distances, such as from shoots to roots. Over shorter distances, it moves by a cell-to-cell transport system that involves active transport. IAA diffuses into a cell, but it also is actively transported through membrane proteins located on the top of the cell. It moves out of the cell only through efflux carriers, which are active transport proteins present on the bottom of the cell. In other words, IAA moves into a cell on the top, and out of it on the bottom. Thus, it tends to be transported in a polar fashion through local tissues, from the tip toward the base of a stem (Figure 31.8). A different mechanism moves auxin molecules upward from the root tip to the shoot–root junction.
Plant hormones are involved in signal perception, transduction, and response.
aleurone
endosperm
embryo
gibberellin
A Absorbed water causes cells of a barley embryo to release gibberellin, which diffuses through the seed into the aleurone layer of the endosperm.
amylase
B Gibberellin triggers cells of the aleurone layer to express the gene for amylase. This enzyme diffuses into the starch-packed middle of the endosperm.
sugars
C The amylase hydrolyzes starch into sugar monomers, which diffuse into the embryo and are used in aerobic respiration. Energy released by the reactions of aerobic respiration fuels meristem cell divisions in the embryo.
Figure 31.7 Action of gibberellin in barley seed germination.
528 UNIT V
HOW PLANTS WORK
auxin
time
time
auxin
A A coleoptile stops growing if its tip is removed. A block of agar will absorb auxin from the cut tip.
B Growth of a de-tipped coleoptile will resume when the agar block with absorbed auxin is placed on top of it.
C If the agar block is placed to one side of the shaft, the coleoptile will bend as it grows.
Figure 31.8 Animated A coleoptile lengthens in response to auxin produced in its tip. Auxin moves down from the tip by passing through cells of the coleoptile. The directional movement is driven by different types of active transporters positioned at the top and bottom of the cells’ plasma membranes (right).
Jeopardy and Jasmonates Many plants protect themselves with thorns or nastytasting chemicals that deter herbivores (plant-eating animals). Some get help from wasps. Damage to a leaf, such as occurs when an herbivore chews on it, triggers a stress response in the plant. The wounding results in the cleavage of certain peptides (such as systemin) in mesophyll cells. Thus activated, the peptides stimulate synthesis of jasmonates, which turn on transcription of a variety of genes. Some of the resulting gene products break down molecules used in normal activities, such as rubisco (Section 7.6), so growth temporarily slows. Other gene products produce chemicals that the plant releases into the air. The chemicals are detected by wasps that parasitize herbivores (Figure 31.9). The signaling is quite specific: A leaf releases a different set of chemicals depending on which herbivore is chewing on it. Certain wasp species recognize these chemicals as a signal leading to preferred prey. They follow airborne concentration gradients of the chemicals back to the plant, where they attack the herbivores. Take-Home Message What are some examples of plant hormone effects? Gibberellin affects expression of genes for nutrient utilization in germination; auxin causes cell lengthening; and jasmonates are involved in plant defensive signaling.
b
c
d
a
Figure 31.9 Jasmonates in plant defenses. (a) Consuelo De Moraes studies chemical signaling in plants. (b) A caterpillar chewing on a tobacco leaf (Nicotiana) triggers a chemical response from the leaf’s cells. The cells release certain chemicals into the air. (c,d) A parasitoid wasp follows the chemicals back to the stressed leaves, then attacks a caterpillar and deposits an egg inside it. When the egg hatches, it will release a caterpillar-munching larva. De Moraes discovered that such interactions are highly specific: Leaf cells release different chemicals in response to different caterpillar species. Each chemical attracts only the wasps that parasitize the particular caterpillar that triggered the chemical’s release.
CHAPTER 31
PLANT DEVELOPMENT 529
31.4
Adjusting the Direction and Rates of Growth Plants alter growth in response to environmental stimuli. Hormones are typically part of this effect.
Links to Plastids 4.11, Cytoskeleton 4.13, Pigments 7.1
Plants respond to environmental stimuli by adjusting the growth of roots and shoots. These responses are called tropisms, and they are mediated by hormones. For example, a root or shoot “bends” because of differences in auxin concentration. Auxin that accumulates in cells on one side of a shoot causes the cells to elongate more than the cells on the other side. The result is that the shoot bends away from the side with more auxin. Auxin has the opposite effect in roots: It inhibits elongation of root cells. Thus, a root will bend toward the side with more auxin. Gravitropism No matter how a seed is positioned in
the soil when it germinates, the radicle always grows down, and the primary shoot always grows up. Even if a seedling is turned upside down just after germina-
tion, the primary root and shoot will curve so the root grows down and the shoot grows up (Figure 31.10). A growth response to gravity is called gravitropism. How does a plant “know” which direction is up? Gravity-sensing mechanisms of many organisms are based on statoliths. In plants, statoliths are starch-grainstuffed amyloplasts (Section 4.11) that occur in root cap cells, and also in specialized cells at the periphery of vascular tissues in the stem. Starch grains are heavier than cytoplasm, so statoliths tend to sink to the lowest region of the cell, wherever that is (Figure 31.11). When statoliths move, they put tension on actin microfilaments of the cell’s cytoskeleton. The filaments are connected to the cell’s membranes, and the change in tension is thought to stimulate certain ion channels in the membranes. The result is that the cell’s auxin efflux carriers move to the new “bottom” of the cell within minutes of a change in orientation. Thus, auxin is always transported to the down-facing side of roots and shoots.
statoliths
A Gravitropism of a corn seedling. No matter what the orientation of a seed in the soil, a seedling’s primary root grows down, and its primary shoot grows up.
A Heavy, starch-packed statoliths are settled on the bottom of gravity-sensing cells in a corn root cap. B These seedlings were rotated 90° counterclockwise after they germinated. The plant adjusts to the change by redistributing auxin, and the direction of growth shifts as a result.
C In the presence of auxin transport inhibitors, seedlings do not adjust their direction of growth after a 90° counterclockwise rotation. Mutations in genes that encode auxin transport proteins have the same effect.
Figure 31.10 Gravitropism.
B Ten minutes after the root was rotated, the statoliths settled to the new “bottom” of the cells. The redistribution causes auxin redistribution, and the root tip curves down.
Figure 31.11 Animated Gravity, statoliths, and auxin. Figure It Out: In which direction was this root rotated? Answer: 90° counterclockwise
530 UNIT V
HOW PLANTS WORK
light
A Sunlight strikes only one side of a coleoptile.
B Auxin is transported to the shaded side, where it causes cells to lengthen.
Phototropism Light streaming in from one direction
causes a stem to curve toward its source. This response, phototropism, orients certain parts of the plant in the direction that will maximize the amount of light intercepted by its photosynthetic cells. Phototropism in plants occurs in response to blue light. Nonphotosynthetic pigments called phototropins absorb blue light, and translate its energy into a cascade of intracellular signals. The ultimate effect of this cascade is that auxin is redistributed to the shaded side of a shoot or coleoptile. As a result, cells on the shaded side elongate faster than cells on the illuminated side. Differences in growth rates between cells on opposite sides of a shoot or coleoptile causes the entire structure to bend toward the light (Figure 31.12).
Figure 31.12 Animated Phototropism. (a,b) Auxin-mediated differences in cell elongation between two sides of a coleoptile induce bending toward light. The photo shows shamrock (Oxalis) responding to a directional light source.
Thigmotropism A plant’s contact with a solid object
may result in a change in the direction of its growth, a response called thigmotropism. The mechanism that gives rise to this response is not well understood, but it involves the products of calcium ions and at least five genes called TOUCH. We see thigmotropism when a vine’s tendril touches an object. The cells near the area of contact stop elongating, and the cells on the opposite side of the shoot keep elongating. The unequal growth rates of cells on opposite sides of the shoot cause it to curl around the object (Figure 31.13). A similar mechanism causes roots to grow away from contact, which allows them to “feel” their way around rocks and other impassable objects in the soil. Mechanical stress, as inflicted by wind or grazing animals, inhibits stem lengthening in a touch response related to thigmotropism (Figure 31.14). Take-Home Message How do plants respond to environmental stimuli? Plants adjust the direction and rate of growth in response to environmental stimuli that include gravity, light, contact, and mechanical stress.
Figure 31.13 Passion flower (Passiflora) tendril twisting thigmotropically around a wire support.
a
b
c
Figure 31.14 Effect of mechanical stress on tomato plants. (a) This plant, the control, was not shaken. (b) This plant was mechanically shaken for thirty seconds each day, for twenty-eight days. (c) This one had two shakings each day. All plants were the same age.
CHAPTER 31
PLANT DEVELOPMENT 531
31.5
Sensing Recurring Environmental Changes Seasonal shifts in night length, temperature, and light trigger seasonal shifts in plant development.
JANUARY
dormancy
Links to Photosynthesis 7.4 and 7.6, Master genes in flowering 15.2, Homeostasis in plants 27.5
MARCH APRIL
seed germination or renewed growth; short-day plant flowering
Biological Clocks
FEBRUARY
MAY JUNE
Most organisms have a biological clock—an internal mechanism that governs the timing of rhythmic cycles of activity. Section 27.5 showed a bean plant changing the light-intercepting position of its leaves over twenty-four hours even when it was kept in the dark. A cycle of activity that starts anew every twenty-four hours or so is called a circadian rhythm (Latin circa, about; dies, day). In the circadian response called solar tracking, a leaf or flower changes position in response to the changing angle of the sun throughout the day. For example, a buttercup stem swivels so the flower on top of it always faces the sun. Unlike a phototropic response, solar tracking does not involve redistribution of auxin and differential growth. Instead, the absorption of blue light by photoreceptor proteins increases fluid pressure in cells on the sunlit side of a stem or petiole. The cells change shape, which bends the stem. Similar mechanisms cause flowers of some plants to open only at certain times of day. For example, the flowers of many bat-pollinated plants unfurl, secrete nectar, and release fragrance only at night. Closing flowers periodically protects the delicate reproductive parts when the likelihood of pollination is low.
Setting the Clock Like a mechanical clock, a biological one can be reset. Sunlight resets biological clocks in plants by activating
long-day plant flowering JULY
short-day plant flowering
AUGUST SEPTEMBER
onset of dormancy OCTOBER
dormancy
NOVEMBER DECEMBER
14 12 10 8 Length of night (hours of darkness)
Figure 31.16 Plant growth and development correlated with seasonal climate changes in northern temperate zones.
and inactivating photoreceptors called phytochromes. These blue-green pigments are sensitive to red light (660 nanometers) and far-red light (730 nanometers). The relative amounts of these wavelengths in sunlight that reaches a given environment vary during the day and with the season. Red light causes phytochromes to change from an inactive form to an active form. Far-red light causes them to change back to their inactive form (Figure 31.15). Active phytochromes bring about transcription of many genes, including some that encode components of rubisco, photosystem II, ATP synthase, and other proteins used in photosynthesis; phototropin for phototropic responses; and molecules involved in flowering, gravitropism, and germination.
When to Flower? red 660 nm
far-red 730 nm
red light Pr
inactive
far-red light
Pfr
response
activated
Pfr influences gene expression
Pfr reverts to Pr in darkness
Figure 31.15 Animated Phytochromes. Red light changes the structure of a phytochrome from inactive to active form; far-red light changes it back to the inactive form. Activated phytochromes control important processes such as germination and flowering.
532 UNIT V
HOW PLANTS WORK
Photoperiodism is an organism’s response to changes in the length of night relative to the length of day. Except at the equator, night length varies with the season. Nights are longer in winter than in summer, and the difference increases with latitude (Figure 31.16). You have probably noticed that different species of plants flower at different times of the year. In these plants, flowering is photoperiodic. Long-day plants such as irises flower only when the hours of darkness fall below a critical value (Figure 31.17a). Chrysanthemums and other short-day plants flower only when the hours of darkness are greater than some critical value (Figure 31.17b). Sunflowers and other day-neutral plants flower when they mature, regardless of night length.
critical night length night
will flower
will not flower 0
day
will not flower
night day 4 8 12 16 20 Time being measured (hours)
A Long-day plants flower only when hours of darkness are less than the critical value for the species. Irises will flower only when night length is less than 12 hours.
will flower 24
B Short-day plants flower only when hours of darkness are greater than the critical value for the species. Chrysanthemums will flower only when night length exceeds 12 hours.
Figure 31.17 Animated Different plant species flower in response to different night lengths. Each horizontal bar represents 24 hours.
Long-Day Plant:
Short-Day Plant:
critical night length
a
Figure 31.18 shows two experiments that demonstrated how phytochromes play a role in photoperiodism. In the first experiment, a long-day and a short-day plant were exposed to long “nights,” interrupted by a brief pulse of red light (which activates phytochrome). Both plants responded in their typical way to a season of short nights. In the second experiment, the pulse of red light (which activates phytochrome) was followed by a pulse of far-red light (which deactivates phytochrome). Both plants responded in their typical way to a season of long nights. Leaves detect night length and produce signals that travel through the plant. In one experiment, a single leaf was left on a cocklebur, a short-day plant. The leaf was shielded from light for 8–1/2 hours every day, which is the threshold amount of darkness required for flowering. The plant flowered. Later, the leaf was grafted onto another cocklebur plant that had not been exposed to long hours of darkness. After grafting, the recipient plant flowered, too. How does a compound produced by leaves cause flowering? In response to night length and other cues, leaf cells transcribe more or less of a flowering gene. The transcribed mRNA migrates from leaves to shoot tips, where it is translated. Its protein product helps activate the master genes that control the formation of flowers (Section 15.2). The length of night is not the only cue for flowering. Some biennials and perennials flower only after exposure to cold winter temperatures (Figure 31.19). This process is called vernalization (from Latin vernalis, which means “to make springlike”).
did not flower
flowered
b
did not flower 0
4 8 12 16 20 Time being measured (hours)
24 flowered
Figure 31.18 Phytochrome plays a role in flowering. (a) An flash of red light interrupting a long night causes plants to respond as if the night were short: Long-day plants flower. (b) A pulse of far-red light, which inactivates phytochrome, cancels the effect of the red flash: Short-day plants flower.
Figure 31.19 Local effect of cold on dormant buds of lilac (Syringa). For this experiment, a single branch was positioned to protrude from a greenhouse through a cold winter. The rest of the plant was kept inside and exposed only to warm temperatures. Only buds exposed to the low outside temperatures resumed growth and flowered in springtime.
Take-Home Message Do plants have biological clocks?
Flowering plants respond to recurring cues from the environment with recurring cycles of development.
The main environmental cue for flowering is the length of night relative to the length of day, which varies by the season in most places. Low winter temperatures stimulate the flowering of many plant species in spring.
CHAPTER 31
PLANT DEVELOPMENT 533
31.6
Senescence and Dormancy Dropping of plant parts and dormancy are triggered by seasonal changes in environmental conditions.
Link to Plant extracellular matrix 4.12
Abscission and Senescence Senescence is the phase of a plant life cycle between full maturity and the death of plant parts or the whole plant. In many species of flowering plants, recurring cycles of growth and inactivity are responses to conditions that vary seasonally. Such plants are typically native to regions that are too dry or too cold for optimal growth during part of the year. Plants may drop leaves during such unfavorable intervals. The process by which plant parts are shed is abscission. It occurs in deciduous plants in response to shortening daylight hours, and year-round in evergreen plants. Abscission may also be induced by injury, water or nutrient deficiencies, or high temperatures. Let’s use deciduous plants as an example. As leaves and fruits grow in early summer, their cells produce auxin. The auxin moves into the stems, where it helps maintain growth. By midsummer, the nights are getting longer. Plants begin to divert nutrients away from their leaves, stems, and roots, and into flowers, fruits, and seeds. As the growing season comes to a close, nutrients are routed to twigs, stems, and roots, and auxin production declines in leaves and fruits. The auxin-deprived structures release ethylene that diffuses into nearby abscission zones—twigs, petioles, and fruit stalks. The ethylene is a signal for cells in the
control (pods not removed)
experimental plant (pods removed)
Figure 31.21 Experiment in which seed pods removed from a soybean plant as soon as they formed delayed senescence.
zone to produce enzymes that digest their own walls and the middle lamella (Section 4.12). The cells bulge as their walls soften, and separate from one another as their middle lamella—the layer that cements them together—dissolves. Tissue in the zone weakens, and the structure above it drops (Figure 31.20). If the seasonal diversion of nutrients into flowers, seeds, and fruits is interrupted, leaves and stems stay on a deciduous plant longer (Figure 31.21).
Dormancy For many species, growth stops in autumn as a plant enters dormancy, a period of arrested growth that is triggered by (and later ended by) environmental cues. Long nights, cold temperatures, and dry, nitrogen-poor soil are strong cues for dormancy in many plants. Dormancy-breaking cues usually operate between fall and spring. Dormant plants do not resume growth until certain conditions in the environment occur. A few species require exposure of the dormant plant to many hours of cold temperature. More typical cues include the return of milder temperatures and plentiful water and nutrients. With the return of favorable conditions, life cycles begin to turn once more as seeds germinate and buds resume growth.
Take-Home Message Figure 31.20 Horse chestnut (Aesculus hippocastanum) leaves changing color in autumn. The horseshoe-shaped leaf scar at right is all that remains of an abscission zone that formed before a leaf detached from the stem.
534 UNIT V
HOW PLANTS WORK
What triggers dropping of plant parts and dormancy? Abscission and dormancy are triggered by environmental cues such as seasonal changes in temperature or daylength.
IMPACTS, ISSUES REVISITED
Foolish Seedlings, Gorgeous Grapes
Fruit ripening is a type of senescence. Like wounded tissues, senescing tissues (including ripening fruit) release ethylene gas. This plant hormone stimulates the production of enzymes such as amylase. These enzymes convert stored starches ethylene and acids to sugars, and soften the cell walls of fleshy fruits—sweetening and softening effects that we associate with ripening. Ethylene emitted by one fruit can stimulate the ripening—and over-ripening—of nearby fruits. Fruit that is harvested at the peak of ripeness can be stored for months or even years after treatment with MCP. MCP binds per-
How would you vote? MCP prevents ethylene from binding to receptors on cells in plant tissues. Fruit is often treated with MCP to retard ethylene’s ripening effect. Should such fruit be labeled to alert consumers? See CengageNOW for details, then vote online.
manently to ethylene receptors on fruit, but unlike ethylene, does not stimulate them. Thus, ripe fruit treated with MCP becomes insensitive to ethylene, so it will not over-ripen. MCP treatment is marketed as SmartFresh technology.
Summary Section 31.1 Gene expression and cues from the environment coordinate plant development, which is the formation and growth of tissues and parts in predictable patterns (Figure 31.22). Germination is one pattern of development in plants.
Use the animation on CengageNOW to compare monocot and eudicot growth and development.
Sections 31.2, 31.3 Like animal hormones, plant hormones secreted by one cell alter the activity of a different cell. Plant hormones can promote or arrest growth of a plant by stimulating or inhibiting cell division, differentiation, elongation, and reproduction. Gibberellins lengthen stems, break dormancy in seeds and buds, and stimulate flowering. Auxins lengthen coleoptiles, shoots, and roots by promoting cell enlargement. Cytokinins stimulate cell division, release lateral buds from apical dominance, and inhibit senescence. Ethylene promotes senescence and abscission. It also inhibits growth of roots and stems. Abscisic acid promotes bud and seed dormancy, and it limits water loss by causing stomata to close.
Use the animation on CengageNOW to observe the effect of auxin on plant growth.
Section 31.4 In tropisms, plants adjust the direction and rate of growth in response to environmental cues. In gravitropism, roots grow down and stems grow up in response to gravity. Statoliths are part of this response. In phototropism, stems and leaves bend toward or away from light. Blue light is the trigger for such phototropic responses. In some plants, the direction of growth changes in response to contact (thigmotropism). Growth may also be affected by mechanical stress.
Use the animation on CengageNOW to investigate plant tropisms.
Sections 31.5, 31.6 Internal timing mechanisms such as biological clocks (including circadian rhythms) are set by daily and seasonal variations in environmental con-
ditions. Solar tracking is one type of circadian rhythm. Another, photoperiodism, is a response to changes in length of night relative to length of day. Light-detection in plants involves nonphotosynthetic pigments called phytochromes (in photoperiodism) and phototropins (in phototropism). Short-day plants flower in spring or fall, when nights are long. Long-day plants flower in summer, when nights are short. Day-neutral plants flower whenever they are mature enough to do so. Some plants require exposure to cold before they can flower, a process called vernalization. Dormancy is a period of arrested growth that does not end until specific environmental cues occur. Dormancy is typically preceded by abscission. Senescence is the part of the plant life cycle between maturity and death of the plant or plant parts.
Use the animation on CengageNOW to learn how plants respond to night length.
mature sporophyte (2n)
germination zygote in seed (2n)
meiosis in anther
DIPLOID
fertilization
meiosis in ovary
HAPLOID
eggs (n)
microspores (n)
sperm (n)
megaspores (n)
male gametophyte (n) female gametophyte (n)
Figure 31.22 Summary of development in the life cycle of a typical eudicot.
CHAPTER 31
PLANT DEVELOPMENT 535
Data Analysis Exercise In 2007, researchers Casey Delphia, Mark Mescher, and Consuelo De Moraes (pictured in Figure 31.9a) published a study on the production of different volatile chemicals by tobacco plants (Nicotiana tabacum) in response to predation by two types of insects: western flower thrips (Frankliniella occidentalis) and tobacco budworms (Heliothis virescens). Their results are shown in Figure 31.23. 1. Which treatment elicited the greatest production of volatiles? 2. Which volatile chemical was produced in the greatest amount? What was the stimulus? 3. Which one of the chemicals tested is most likely produced by tobacco plants in a nonspecific response to predation? 4. Are there any chemicals produced in response to predation by budworms, but not in response to predation by thrips?
Self-Quiz
Volatile Compound Produced
C
T
W
WT
HV
HVT
Myrcene β-Ocimene Linalool Indole Nicotine β-Elemene β-Caryophyllene α-Humulene Sesquiterpene α-Farnesene Caryophyllene oxide Total
0 0 0 0 0 0 0 0 0 0 0 0
0 433 0 0 0 0 100 0 7 15 0 555
0 15 0 0 233 0 40 0 0 0 0 288
0 121 0 0 160 0 124 0 0 0 0 405
17 4,299 125 74 390 90 3,704 123 219 293 89 9,423
22 5,315 178 142 538 102 6,166 209 268 457 166 13,563
Figure 31.23 Volatile compounds produced by tobacco plants (Nicotiana tabacum) in response to predation by different insects. Groups of plants were untreated (C), attacked by thrips (T), mechanically wounded (W), mechanically wounded and attacked by thrips (WT), attacked by budworms (HV), or attacked by budworms and thrips (HVT). Values are indicated in nanograms/day.
Answers in Appendix III
1. Which of the following statements is false? a. Auxins and gibberellins promote stem elongation. b. Cytokinins promote cell division, retard leaf aging. c. Abscisic acid promotes water loss and dormancy. d. Ethylene promotes fruit ripening and abscission. 2. Plant hormones . a. may have multiple effects b. are influenced by environmental cues c. are active in plant embryos within seeds d. are active in adult plants e. all of the above 3.
Treatment
is the strongest stimulus for phototropism. a. Red light c. Green light b. Far-red light d. Blue light
6. In some plants, flowering is a response. a. phototropic c. photoperiodic b. gravitropic d. thigmotropic 7. Match the observation with the hormone most likely to be its cause. ethylene a. Your cabbage plants bolt (they cytokinin form elongated flowering stalks). auxin b. The philodendron in your room gibberellin is leaning toward the window. abscisic acid c. The last of your apples is getting really mushy. d. The seeds of your roommate’s marijuana plant do not germinate no matter what he does to them. e. Lateral buds on your Ficus plant are sprouting branch shoots.
4. light makes phytochrome switch from inactive to active form; light has the opposite effect. a. Red; far-red c. Far-red; red b. Red; blue d. Far-red; blue
5. The following oat coleoptiles have been modified: either cut or placed in a light-blocking tube. Which ones will still bend toward a light source?
Critical Thinking
Visit CengageNOW for additional questions.
1. Reflect on Chapter 28. Would you expect hormones to influence primary growth only? What about secondary growth in, say, a hundred-year-old oak tree? 2. Photosynthesis sustains plant growth, and inputs of sunlight sustain photosynthesis. Why, then, do seedlings that germinated in a fully darkened room grow taller than different seedlings of the same species that germinated in full sun?
a
b
c
d
3. Belgian scientists discovered that certain mutations in common wall cress (Arabidopsis thaliana) cause excess auxin production. Predict the impact on the plant’s phenotype. 4. Beef cattle typically are given somatotropin, an animal hormone that makes them grow bigger (the added weight means greater profits). There is concern that such hormones may have unforeseen effects on beef-eating humans. Do you think plant hormones can affect humans? Why or why not?
536 UNIT V
HOW PLANTS WORK
VI
HOW ANIMALS WORK
How many and what kinds of body parts does it take to function as a lizard in a tropical forest? Make a list of what comes to mind as you start reading Unit VI, then see how resplendent the list can become at the unit’s end.
537
32
Animal Tissues and Organ Systems IMPACTS, ISSUES
Open or Close the Stem Cell Factories?
Imagine being able to grow new body parts to replace lost
In theory, embryonic stem cell treatments could provide
or diseased ones. This dream motivates researchers who
new nerve cells for paralyzed people. Treatments might also
study stem cells. Stems cells are self-renewing; they divide
help treat other nerve and muscle disorders such as heart
and produce more stem cells. In addition, some descendants
disease, muscular dystrophy, and Parkinson’s disease.
of stem cells differentiate into the specialized cells that make
Despite the promise of embryonic stem cell research,
up specific body parts. In short, all cells in your body “stem”
some people oppose it. They are troubled by the original
from stem cells.
source of the cells—early human embryos. The embryos
Cell types that your body continually replaces, such as blood and skin, arise from adult stem cells. Such stem cells are specialists that normally differentiate into a limited variety
typically come from fertility clinics that would otherwise have destroyed them and are donated by their parents. So far, scientists have not found any adult stem cells that
of cells. For example, stem cells in adult bone marrow can
have the same potential of embryonic stem cells. However,
become blood cells, but not muscle cells or brain cells.
they may be able to genetically engineer such cells. For
Embryos have stem cells that are more versatile. After all,
example, James Thompson and Junying Yu (Figure 32.1)
these cells are the source of all tissue types in the new body.
used viruses to insert genes from embryonic cells into skin
Embryonic stem cells are formed soon after fertilization when
cells of a newborn boy. The result was easy-to-grow cells
cell division produces a pinhead-sized ball of cells. By birth,
that showed the same features as embryonic stem cells in
embryonic stem cells have disappeared.
culture. A research team in Japan achieved similar results
Stem cells that can become nerve cells or muscle cells are rare in adults. Thus, unlike skin and blood cells, nerves
by using viruses to insert genes into adult skin cells. Does that mean using embryonic stem cells will become
and muscles are not replaced if they get damaged or die.
unnecessary? Possibly, but there are still obstacles. First,
This is why an injury to the nerves of the spinal cord can
the retroviruses used to insert the genes can cause cancer.
cause permanent paralysis.
Thus cells created by this method cannot safely be placed in a human body. Second, while the engineered cells seem to behave like embryonic stem cells in the lab, they might behave differently once implanted in a person. Further research will be necessary to see whether stem cells can be engineered in a safer way, and if they actually have the same potential as embryonic stem cells in a clinical context. Stem cells, the source of all tissues and organs, are a fitting introduction to this unit. The unit deals with animal anatomy (how a body is put together) and physiology (how a body works). In this unit, you will return repeatedly to a concept outlined in Chapter 27. Cells, tissues, and organs interact smoothly when the body’s internal environment is maintained within a range that individual cells can tolerate. In most kinds of animals, blood and interstitial fluid are the internal environment. The processes involved in maintaining this environment are collectively called homeostasis. Regardless of the species, the body parts must interact and perform the following tasks : 1. Coordinate and control activities of its individual parts. 2. Acquire and distribute raw materials to individual cells and dispose of wastes.
See the video! Figure 32.1 Junying Yu at the University of WisconsinMadison is part of a research team that developed a method of turning a newborn’s skin cells into cells that behave like embryonic stem cells.
3. Protect tissues against injury or attack. 4. Reproduce and, in many species, nourish and protect offspring through early growth and development.
Links to Earlier Concepts
Key Concepts Animal organization
With this chapter, we begin to consider the tissue and organ system levels of organization in animals (Section 1.1). You will also learn more about the cells involved in sensing and responding to stimuli (1.2).
This chapter expands on the nature of animal body plans (25.1) and trends in vertebrate evolution (26.2).
You will think again about the importance of diffusion across cell membranes (5.3), aerobic respiration (8.1), and the structure and metabolism of lipids (3.4, 8.7). The protein hemoglobin (3.6) comes up as we discuss blood.
Cancer (9.5) and the effects of UV radiation (14.5) are revisited in the context of skin and sunlight exposure.
All animals are multicelled, with cells joined by cell junctions. Typically, cells are organized in four tissues: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. Organs, which consist of a combination of tissues, interact in organ systems. Section 32.1
Types of animal tissues Epithelial tissue covers the body’s surface and lines its internal tubes. Connective tissue provides support and connects body parts. Muscle tissue moves the body and its parts. Nervous tissue detects internal and external stimuli and coordinates responses. Sections 32.2–32.5
Organ systems Vertebrate organ systems compartmentalize the tasks of survival and reproduction for the body as a whole. Different systems arise from ectoderm, mesoderm, and endoderm, the primary tissue layers that form in the early embryo. Section 32.6
A closer look at skin Skin is an example of an organ system. It includes epithelial layers, connective tissue, adipose tissue, glands, blood vessels, and sensory receptors. It helps protect the body, conserve water, control temperature, excrete wastes, and detect external stimuli. Sections 32.7, 32.8
How would you vote? Human embryonic stem cells have potential medical benefits, but some people object to their use. Should scientists be allowed to destroy embryos created in fertility clinics and donated by their parents as a source of cells for research? See CengageNOW for details, then vote online.
539
32.1
Organization of Animal Bodies Cells of animal bodies are united by cell junctions, and typically organized as tissues, organs, and organ systems.
Link to Levels of organization 1.1
From Tissue to Organs to Organ Systems All animals are multicelled, and nearly all have cells organized as tissues. A tissue consists of interacting cells and extracellular substances that carry out one or more specialized tasks. Four types of tissue occur in all vertebrate bodies. Epithelial tissues cover body surfaces and line internal cavities. Connective tissues hold body parts together and provide structural support. Muscle tissues move the body and its parts. Nervous tissues detect stimuli
and relay information. We will consider each type of tissue in detail in the sections that follow. Typically, animal tissues are organized into organs. An organ is a structural unit of two or more tissues organized in a specific way and capable of carrying out specific tasks. Your heart is an organ that consists of all four types of tissues in certain proportions and arrangements. In organ systems, two or more organs and other components interact physically, chemically, or both in a common task, as when the force generated by a beating heart moves blood through the body. A body’s cells, tissues, and organs interact smoothly when the internal environment stays within a range that the cells can tolerate. In most animals, blood and interstitial fluid (fluid between cells) are the internal environment. Homeostasis is the process of maintaining the internal environment (Section 27.1).
Cell Junctions
B
A
C
Tight junctions
Adhering junction
Gap junction
Rows of proteins that run parallel with the free surface of a tissue; stop leaks between adjoining cells
A mass of interconnected proteins that welds two cells together; anchored under the plasma membrane by intermediate filaments of cytoskeleton
Cylindrical arrays of proteins spanning the plasma membrane of adjoining cells, paired as open channels
Figure 32.2 Animated Examples of cell junctions in animal tissues.
540 UNIT VI
HOW ANIMALS WORK
Cells in most animal tissues connect to their neighbors by way of one or more types of cell junctions. In epithelial tissues, rows of proteins that form tight junctions between plasma membranes of adjacent cells prevent fluid from seeping between these cells. To cross an epithelium, a fluid must pass through the epithelial cells. Transport proteins in cell membranes control which ions and molecules cross the epithelium (Section 5.2). An abundance of tight junctions in the lining of the stomach normally keeps acidic fluid from leaking out. If a bacterial infection damages this lining, acid and enzymes can erode the underlying connective tissue and muscle layers. The result is a painful peptic ulcer. Adhering junctions hold cells together at distinct spots, like buttons hold a shirt closed (Figure 32.2b). Skin and other tissues that are subject to abrasion or stretching are rich in adhering junctions. Gap junctions permit ions and small molecules to pass from the cytoplasm of one cell to another (Figure 32.2c). Heart muscle and other tissues in which the cells perform some coordinated action have many of these communication channels.
Take-Home Message How is an animal body organized? Nearly all animals have cells united by cell junctions and organized into tissues, organs, and organ systems. All body parts work together in homeostasis, the process of keeping internal conditions within the range cells can tolerate.
32.2
Epithelial Tissue
Sheets of epithelial tissue cover the body’s outer surface and line its internal ducts and cavities.
Link to Diffusion 5.3
Simple squamous epithelium • Lines blood vessels, the heart, and air sacs of lungs • Allows substances to cross by diffusion
General Characteristics An epithelium (plural, epithelia), or epithelial tissue, is a sheet of cells that covers an outer body surface or lines an internal cavity. One surface of the epithelium faces the outside environment or a body fluid. A secreted extracellular matrix, known as the basement membrane, attaches the epithelium’s opposite surface to an underlying tissue (Figure 32.3). Epithelial tissues are described in terms of the shape of the constituent cells and the number of cell layers. A simple epithelium is one cell thick; a stratified epithelium has multiple cell layers. Squamous epithelium cells are flattened or platelike. Cells of cuboidal epithelium are short cylinders that look like cubes when viewed in cross-section. Cells in columnar epithelium are taller than they are wide. Figure 32.4 shows these shapes in the three types of simple epithelium. Different kinds of epithelia are suited to different tasks. Simple squamous epithelium is the thinnest type. It lines blood vessels and the tiny air sacs inside lungs. Because it is thin, gases and nutrients diffuse across it easily. In contrast, thicker stratified squamous epithelium has a protective function. The outer layer of your skin consists of this tissue. Cells of cuboidal and columnar epithelium act in absorption and secretion. In some tissues, such as the lining of the kidneys and small intestine, fingerlike projections called microvilli extend from the free surface of epithelial cells. These projections increase the surface area across which substances are absorbed. In other tissues, such as the upper airways and oviducts, the free surface is ciliated. Action of the cilia helps move the mucus secreted by the epithelium.
Simple cuboidal epithelium • Lines kidney tubules, ducts of some glands, oviducts • Functions in absorption and secretion, movement of materials
Simple columnar epithelium
mucus-secreting gland cell
• Lines some airways, parts of the gut • Functions in absorption and secretion, protection
Figure 32.4 Micrographs and drawings of three types of simple epithelia in vertebrates, with examples of their functions and locations.
Only epithelial tissue contains gland cells. These cells produce and secrete substances that function outside
the cell. In most animals, secretory cells are clustered inside glands, organs that release substances onto the skin, or into a body cavity or the interstitial fluid. Exocrine glands have ducts or tubes that deliver their secretions onto an internal or external surface. Exocrine secretions include mucus, saliva, tears, milk, digestive enzymes, and earwax. Endocrine glands have no ducts. They secrete their products, hormones, directly into the interstitial fluid between cells. Hormone molecules diffuse into blood, which carries them to target cells.
free surface of a simple epithelium
Take-Home Message
Glandular Epithelium
What are epithelial tissues?
basement membrane (material secreted by epithelial cells)
Epithelial tissues are sheetlike layers of cells attached by a basement layer to an underlying tissue. They cover body surfaces and line cavities and ducts.
underlying connective tissue
Some epithelial cells are ciliated or have microvilli that aid absorption. Secretory epithelium forms endocrine and exocrine glands.
Figure 32.3 Generalized structure of a simple epithelium.
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32.3
Connective Tissues Connective tissues connect body parts and provide structural and functional support to other body tissues. Links to Lipids 3.4, Hemoglobin 3.6, Storage of excess sugars as fats 8.7
Connective tissues consist of cells in an extracellular matrix of their own secretions. Connective tissues are classified by the cell types that they include and the composition of their extracellular matrix. There are two kinds of soft connective tissues: loose and dense. In both, fibroblasts are the main type of cell. Fibroblasts secrete a matrix of complex carbohydrates with long fibers of the structural proteins collagen and elastin. Cartilage, bone tissue, adipose tissue, and blood are specialized connective tissues.
Soft Connective Tissues Loose and dense connective tissues are made up of the same components but in different proportions. Loose connective tissue has fibroblasts and fibers dispersed widely through its matrix. Figure 32.5a is an example. This tissue, the most common type in the vertebrate body, helps hold organs and epithelia in place. In dense, irregular connective tissue, the matrix is packed full of fibroblasts and collagen fibers that are oriented every which way, as in Figure 32.5b. Dense,
collagen fiber
irregular connective tissue makes up deep skin layers. It supports intestinal muscles and also forms capsules around organs that do not stretch, such as kidneys. Dense, regular connective tissue has fibroblasts in orderly rows between parallel, tightly packed bundles of fibers (Figure 32.5c). This organization helps keep the tissue from being torn apart when placed under mechanical stress. Tendons and ligaments are mainly dense, regular connective tissue. The tendons connect skeletal muscle to bones. Ligaments attach one bone to another and are stretchier than tendons. Elastic fibers in their matrix facilitate movements around joints.
Specialized Connective Tissues All vertebrate skeletons include cartilage, which has a matrix of collagen fibers and rubbery glycoproteins. Cartilage cells (chondrocytes) secrete the matrix, which eventually imprisons them (Figure 32.5d). When you were an embryo, cartilage formed a model for your developing skeleton; then bone replaced most of it. Cartilage still supports the outer ears, nose, and throat. It cushions joints and acts as a shock absorber between vertebrae. Blood vessels do not extend through cartilage, so nutrients and oxygen must diffuse from vessels in nearby tissues. Also, unlike cells of other connective tissues, cartilage cells do not divide often in adults.
glycoprotein-rich matrix with fine collagen fibers
collagen fibers
fibroblast collagen fibers
elastic fiber
a Loose connective tissue • Underlies most epithelia • Provides elastic support and serves as a fluid reservoir
b Dense, irregular connective tissue
c Dense, regular connective tissue
• In deep skin layers, around intestine, and in kidney capsule
• In tendons connecting muscle to bone and ligaments that attach bone to bone
• Binds parts together, provides support and protection
Figure 32.5 Micrographs and drawings of connective tissues.
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fibroblast
HOW ANIMALS WORK
• Provides stretchable attachment between body parts
cartilage cell (chondrocyte) d Cartilage • Internal framework of nose, ears, airways; covers the ends of bones • Supports soft tissues, cushions bone ends at joints, provides a lowfriction surface for joint movements
Adipose tissue is the body’s main energy reservoir. Most cells can convert excess sugars and lipids into fats (Section 8.7). However, only the cells of adipose tissue bulge with so much stored fat that the nucleus gets pushed to one side and flattened (Figure 32.5e). Adipose cells have little matrix between them. Small blood vessels run through the tissue and carry fats to and from cells. In addition to its energy-storage role, adipose tissue cushions and protects body parts, and a layer under the skin functions as insulation. Bone tissue is a connective tissue in which living cells (osteocytes) are imprisoned in a calcium-hardened matrix that they secreted (Figure 32.5f ). Bone tissue is the main component of bones, organs that interact with muscles to move a body. Bones also support and protect internal organs. Figure 32.6 shows a femur, a leg bone that is structurally adapted to bear weight. Blood cells form in the spongy interior of some bones. Blood is considered a connective tissue because its cells and platelets are descended from stem cells in bone (Figure 32.7). Red blood cells filled with hemoglobin transport oxygen (Section 3.6). White blood cells help defend the body against dangerous pathogens. Platelets are cell fragments that function in clot formation. Cells and platelets drift in plasma, a fluid extracellular matrix consisting mostly of water, with dissolved nutrients and other substances.
cartilage at the end of long bone
compact bone tissue
spongy bone tissue
Figure 32.6 Locations of cartilage and bone tissue. Spongy bone tissue has hard parts with spaces between. Compact bone tissue is more dense. The bone shown here is the femur, the largest and strongest bone in the human body.
Figure 32.7 Cellular components of human blood. Cells and cell fragments (platelets) drift along in plasma, the fluid portion of the blood. Plasma consists of water with dissolved proteins, salts, and nutrients.
white blood cell red blood cell platelet
compact bone tissue nucleus cell bulging with fat droplet
blood vessel
Take-Home Message
bone cell (osteocyte)
What are connective tissues? Connective tissues consist of cells in a secreted extracellular matrix.
e Adipose Tissue
f Bone Tissue
• Underlies skin and occurs around heart and kidneys
• Makes up the bulk of most vertebrate skeletons
• Serves in energy storage, provides insulation, cushions and protects some body parts
• Provides rigid support, attachment site for muscles, protects internal organs, stores minerals, produces blood cells
Various soft connective tissues underlie epithelia, form capsules around organs, and connect muscle to bones or bones to one another. Cartilage is a specialized connective tissue with a rubbery extracellular matrix.
Adipose tissue is a specialized connective tissue with fat-filled cells. Bone is a specialized connective tissue with a calcium-hardened matrix.
Blood is considered a connective tissue because blood cells form in bone. The cells are carried along by plasma, the fluid portion of the blood.
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32.4
Muscle Tissues
Muscle tissue is made up of cells that can contract.
Links to Cytoskeletal proteins 4.13, Aerobic respiration 8.1
Cells of muscle tissues contract—or forcefully shorten —in response to signals from nervous tissue. Muscle tissues consist of many cells arranged in parallel with one another, in tight or loose arrays. Coordinated contractions of layers or rings of muscles move the whole body or its parts. Muscle tissue occurs in most animals, but we focus here on the kinds found in vertebrates.
Skeletal Muscle Tissue Skeletal muscle tissue, the functional partner of bone (or cartilage), helps move and maintain the positions of the body and its parts. Skeletal muscle tissue
has parallel arrays of long, cylindrical muscle fibers (Figure 32.8a). The fibers form during development, when embryonic cells fuse together, so each fiber has multiple nuclei. A fiber is filled with myofibrils—long strands with row after row of contractile units called sarcomeres. The rows of sarcomeres are so regular that skeletal muscle has a striated, or striped, appearance. Each sarcomere consists of parallel arrays of the proteins actin and myosin (Section 4.13). ATP-powered interactions between the actin and myosin filaments shorten sarcomeres and brings about muscle contraction. We describe this process in detail in Section 36.7. Skeletal muscle tissue makes up 40 percent or so of the weight of an average human. Reflexes activate it, but we also can cause it to contract when we want to move a body part. That is why skeletal muscles are commonly called “voluntary” muscles.
Cardiac Muscle Tissue Cardiac muscle tissue occurs only in the heart wall (Figure 32.8b). Like skeletal muscle tissue, it contains sarcomeres and looks striated. Unlike skeletal muscle tissue, it consists of branching cells. Cardiac muscle cells are attached at their ends by adhering junctions that prevent them from being ripped apart during forceful contractions. Signals to contract pass swiftly from cell to cell at gap junctions that connect cells along their length. Rapid flow of signals ensures that all cells in cardiac muscle tissue contract as a unit.
nucleus adjoining ends of abutting cells
nucleus
a Skeletal muscle
b Cardiac muscle
c Smooth muscle
• Long, multinucleated, cylindrical cells with conspicuous striping (striations)
• Striated cells attached end to end, each with a single nucleus
• Cells with a single nucleus, tapered ends, and no striations
• Interacts with bone to bring about movement, maintain posture
• Occurs only in the heart wall
• Found in the walls of arteries, the digestive tract, the reproductive tract, the bladder, and other organs
• Reflex activated, but also under voluntary control
• Contraction is not under voluntary control
Figure 32.8 Micrographs of muscle tissues, and a photo of skeletal muscles in action.
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• Contraction is not under voluntary control
32.5 Compared to other muscle tissues, cardiac muscle has far more mitochondria, which provide the beating heart with a dependable supply of ATP from aerobic respiration. Unlike skeletal muscle, cardiac muscle has little stored glycogen. If blood flow to cardiac cells is interrupted, cells run out of glucose and oxygen fast, so aerobic respiration slows. A heart attack interrupts blood flow, and cardiac muscle dies as a result. Cardiac muscle and smooth muscle tissue occur in “involuntary” muscle, so named because most people cannot make it contract just by thinking about it.
Nervous Tissue
Nervous tissue detects internal and external stimuli, and coordinates responses to these stimuli.
Link to Sensing and responding 1.2
We find layers of smooth muscle tissue in the wall of many soft internal organs, such as the stomach, uterus, and bladder. This tissue’s unbranched cells contain a nucleus at their center and are tapered at both ends (Figure 32.8c). Contractile units are not arranged in an orderly repeating fashion, as they are in skeletal and cardiac muscle tissue, so smooth muscle tissue is not striated. Even so, cells of this tissue contain actin and myosin filaments, which are anchored to the plasma membrane by intermediate filaments. Smooth muscle tissue contracts more slowly than skeletal muscle, but its contractions can be sustained longer. Smooth muscle contractions propel material through the gut, shrink the diameter of blood vessels, and close sphincters. A sphincter is a ring of muscle in a tubular organ.
Nervous tissue consists of specialized signaling cells called neurons, and the cells that support them. A neuron has a cell body that holds its nucleus and other organelles (Figure 32.9). Projecting from the cell body are long cytoplasmic extensions that allow the cell to receive and send electrochemical signals. When a neuron receives sufficient stimulation, an electrical signal travels along its plasma membrane to the ends of certain of its cytoplasmic extensions. Here, the electrical signal causes release of chemical signaling molecules. These molecules diffuse across a small gap to an adjacent neuron, muscle fiber, or gland cell, and alter that cell’s behavior. Your nervous system has more than 100 billion neurons. There are three types. Sensory neurons are excited by specific stimuli, such as light or pressure. Interneurons receive and integrate sensory information. They store information and coordinate responses to stimuli. In vertebrates, interneurons occur mainly in the brain and spinal cord. Motor neurons relay commands from the brain and spinal cord to glands and to muscle cells, as in Figure 32.10. Neuroglial cells keep neurons positioned where they should be and provide metabolic support. They constitute a significant portion of the nervous tissue. More than half of your brain volume is neuroglia.
Take-Home Message
Take-Home Message
What is muscle tissue?
What is nervous tissue?
Smooth Muscle Tissue
Skeletal muscle, cardiac muscle, and smooth muscle consist of cells that contract when stimulated. Contraction requires ATP. Skeletal muscle, which interacts with bone, is the only muscle tissue that can be voluntarily controlled.
Figure 32.9 Micrograph of a motor neuron. It has a cell body with a nucleus (visible as a dark spot), and long cytoplasmic extensions.
Nervous tissue consists of excitable cells called neurons and supporting cells called neuroglia. Neurons make up the body’s internal communication lines. Messages travel along neuron membranes and are relayed to muscle and gland cells.
Figure 32.10 One example of the coordinated interaction between skeletal muscle tissue and nervous tissue. Interneurons in the brain of this lizard, a chameleon, calculate the distance and the direction of a tasty fly. In response to this stimulus, signals from the interneurons flow along certain motor neurons and reach muscle fibers inside the lizard’s long, coiled-up tongue. The tongue uncoils swiftly and precisely to reach the very spot where the fly is perched.
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32.6
Overview of Major Organ Systems
Interacting tissues form organs and organ systems. cranial cavity
Links to Animal body plans 25.1, Trends in vertebrate evolution 26.2
spinal cavity
Development of Tissues and Organs How do tissues of a vertebrate body develop? After fertilization, mitotic cell divisions form a ball of cells that arrange themselves as three germ layers, or primary tissue layers (Figure 25.2). Growth and differentiation of these germ layers yields all adult tissues. Ectoderm, the outermost germ layer, becomes the nervous tissue and the epithelium of skin. Mesoderm, the middle germ layer, gives rise to muscle, connective tissue, and the lining of body cavities derived from the coelom. The innermost germ layer, endoderm, forms epithelium of the gut and also organs—such as lungs—that evolved from outpocketings of the gut. As noted in the chapter introduction, stem cells are self-renewing cells; some of their descendants are stem cells, while others differentiate to form specific tissues. An embryonic stem cell that develops before the germ layers form can give rise to any adult tissue. Stem cells of later embryos or after birth are more specialized; each gives rise to only specific tissue types.
Vertebrate Organ Systems Like other vertebrates, humans are bilateral and have a lined body cavity known as a coelom (Section 25.1). A sheet of smooth muscle, the diaphragm, divides the coelom into an upper thoracic cavity and a cavity that has abdominal and pelvic regions (Figure 32.11a). The heart and lungs are in the thoracic cavity. The stomach, intestines, and liver lie inside the abdominal cavity. The bladder and reproductive organs are in the pelvic cavity. A cranial cavity in the head and spinal cavity in the back are not derived from the coelom. Figure 32.12 introduces organ systems that divide up the necessary tasks that ensure survival and reproduction of a vertebrate body. Structure and function of these systems is the topic of the remaining chapters in this unit. Figure 32.11b,c introduces some anatomical terms we will use in these discussions. Take-Home Message How do vertebrate organ systems arise and function? In vertebrates, organs arise from three embryonic germ layers: ectoderm, mesoderm, and endoderm. All vertebrates have a set of organ systems that compartmentalize the many specialized tasks required for survival and reproduction of a body.
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thoracic cavity diaphragm abdominal cavity
pelvic cavity
Dorsal Surface
A transverse
midsagittal
ANTERIOR
POSTERIOR
frontal
B
Ventral Surface SUPERIOR (of two body parts, the one closer to head)
frontal plane (aqua)
midsagittal plane (green)
ANTERIOR (at or near front of body)
C
distal (farthest from trunk or from origin of a body part) proximal (closest to trunk or to point of origin of a body part)
POSTERIOR (at or near back of body)
INFERIOR (of two body parts, the one farthest from head)
transverse plane (yellow)
Figure 32.11 Animated (a) Main body cavities in humans. (b,c) Directional terms and planes of symmetry for the body. For vertebrates that keep their main body axis parallel with Earth’s surface, dorsal refers to the upper surface (back) and ventral to the lower surface. For upright walkers, anterior (the front) corresponds to ventral and posterior (the back) to dorsal.
Figure 32.12 Animated Facing page, human organ systems and their functions.
Integumentary System Protects body from injury, dehydration, and some pathogens; controls its temperature; excretes certain wastes; receives some external stimuli.
Nervous System Detects external and internal stimuli; controls and coordinates responses to stimuli; integrates all organ system activities.
Lymphatic System Collects and returns some tissue fluid to the bloodstream; defends the body against infection and tissue damage.
Respiratory System Rapidly delivers oxygen to the tissue fluid that bathes all living cells; removes carbon dioxide wastes of cells; helps regulate pH.
Muscular System Moves body and its internal parts; maintains posture; generates heat by increases in metabolic activity.
Skeletal System Supports and protects body parts; provides muscle attachment sites; produces red blood cells; stores calcium, phosphorus.
Digestive System Ingests food and water; mechanically, chemically breaks down food and absorbs small molecules into internal environment; eliminates food residues.
Circulatory System Rapidly transports many materials to and from interstitial fluid and cells; helps stabilize internal pH and temperature.
Urinary System Maintains the volume and composition of internal environment; excretes excess fluid and bloodborne wastes.
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Endocrine System Hormonally controls body functioning; with nervous system integrates short- and long-term activities. (Male testes added.)
Reproductive System Female: Produces eggs; after fertilization, affords a protected, nutritive environment for the development of new individuals. Male: Produces and transfers sperm to the female. Hormones of both systems also influence other organ systems.
ANIMAL TISSUES AND ORGAN SYSTEMS 547
32.7
Vertebrate Skin—Example of an Organ System
Skin is the body’s interface with the environment.
Links to Cancer 9.5, UV radiation and mutations 14.5
Structure and Function of Skin The integumentary system, or skin, is the vertebrate organ system with the largest surface area. It includes sensory receptors that detect changes in external conditions. Skin forms a barrier that helps defend a body against pathogens. It helps control internal temperature and, in land vertebrates, it helps conserve water. In humans, it helps make vitamin D. Skin consists of two layers, an outer epidermis and a deeper dermis (Figure 32.13). Underlying the dermis is a layer of connective tissue called the hypodermis. The dermis consists of dense connective tissue with stretch-resistant collagen fibers. Blood vessels, lymph vessels, and sensory neurons run through the dermis. Nutrients delivered to the dermis by blood vessels diffuse up to cells in the epidermis. There are no blood vessels in this upper layer. The epidermis is stratified squamous epithelium. Its structure varies among vertebrate groups. Evolution of a thick layer of keratinocytes—cells that make the waterproof protein keratin—accompanied the move onto land. Ongoing mitotic divisions in the deepest epidermal layers push newly formed keratinocytes toward the skin’s surface. As cells move toward the
surface, they become flattened, lose their nucleus, and die. Dead cells at the skin surface form an abrasionresistant layer that helps prevent water loss. Surface cells are continually abraded or flake off. The epidermis is the body’s first line of defense against pathogens. Phagocytic dendritic cells prowl through it. These white blood cells engulf pathogens and alert the immune system to these threats. As vertebrate lineages evolved, some keratinocytes became specialized and keratin-rich structures such as claws, nails, and beaks evolved. Hair and fur of mammals consist of dead keratinocytes. Hair follicles lie in the dermis, but are of epidermal origin. An average human scalp has about 100,000 hairs. Genes, nutrition, and hormones all affect hair growth. Epidermally derived gland cells also lie in the dermis. In humans, these include about 2.5 million sweat glands. Sweat glands help humans and many other mammals dissipate heat. Sweat is mostly water, with dissolved salts. Most regions of the mammalian dermis also have oil glands (sebaceous glands). The oily secretions lubricate and soften hair and skin, and deter bacterial growth. Amphibians do not have sweat glands, but most have mucous glands that help keep their surface moist. Many also have glands that secrete distasteful substances or poisons. Pigmented cells in the dermis give some highly poisonous frogs a distinctive coloration that predators learn to avoid (Figure 32.14).
hair’s cuticle
hair outer flattened epidermal cells
epidermis
one hair cell dermis cells being flattened
keratin polypeptide chain
dividing cells
hypodermis
oil gland hair follicle blood vessels
sensory neuron sweat gland smooth muscle
dermis B
Figure 32.13 Animated (a) Skin structure. (b) Section through human skin. (c) Structure of a hair. It arises from a hair follicle derived from epidermal cells that have sunk into the dermis.
548 UNIT VI
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C
Figure It Out: How many polypeptide chains
are in a keratin macrofibril?
Answer: Three
A
keratin macrofibril
FOCUS ON HEALTH
32.8
Farming Skin
Commercially grown skin substitutes are already in use for treatment of chronic wounds. Skin may be a source of stem cells that could be used to grow other organs.
mucous gland
poison gland pigmented cell
Figure 32.14 Skin of a frog (Dendrobates azureus). The dermis contains epidermally derived glands that secrete mucus and poison. Pigment cells in the dermis give the frog its distinctive color and warn predators that it is poisonous.
Sunlight and Human Skin As the Chapter 11 introduction explained, skin color variation has a genetic basis. Color variations arise from differences in the distribution and activity of melanocytes. These cells make the brownish pigment called melanin and donate it to keratinocytes. In pale skin, little melanin is formed. Such skin appears pink because the red color of the iron in hemoglobin shows through thin-walled blood vessels and the epidermis. Melanin has a protective function. It absorbs ultraviolet (UV) radiation that might otherwise damage underlying skin layers. Exposure to sunlight causes increased production of melanin, producing a “tan.” A bit of UV radiation is a good thing; it stimulates melanocytes to make a molecule that the body later converts to vitamin D. We need this vitamin to absorb calcium ions from food. However, excessive UV exposure damages collagen and causes elastin fibers to clump. Chronically tanned skin gets less resilient and becomes leathery. UV also damages DNA, increasing the risk of skin cancer (Section 9.5). As we age, epidermal cells divide less often. Skin thins and becomes less elastic as collagen and elastin fibers become sparse. Glandular secretions that kept it soft and moist dwindle. Wrinkles deepen. Many people needlessly accelerate the aging process by tanning or smoking, which shrinks the skin’s blood supply. Take-Home Message
Adults make few new muscle cells or nerve cells, but they do constantly renew their skin cells. Each day you lose skin cells, and new ones move up to replace them. The whole epidermis is renewed every month, and an adult sheds about 0.7 kilogram (1.5 pounds) of skin each year. Skin cells are already being cultured for medical uses (Figure 32.15). Commercially available cultured skin substitutes are made using infant foreskins that were removed during routine circumcisions. The foreskins (a tissue that covers the tip of the penis) provide a rich source of keratinocytes and fibroblasts. These cells are grown in culture with other biological materials, and the resulting products are used to close chronic wounds, help burns heal, and cover sores on patients with epidermolysis bullosa. Epidermolysis bullosa (EB) is a rare inherited disorder caused by mutations in the structural proteins of skin, such as keratin, collagen, or laminin. The protein defect causes skin layers to separate easily, so upper layers blister and slough off. Affected people are covered with open sores and must avoid touch. Even the friction of clothing on their skin can open a wound. Use of cultured skin substitutes cannot cure EB, but it does help wounds heal faster, thus reducing pain and the risk of life-threatening infections. Unlike real skin, cultured skin substitutes do not include melanocytes, sweat glands, oil glands, and other differentiated structures. Use of adult epidermal stem cells may one day allow production of cultured skin as complex as real skin. Stem cells, recall, divide and produce more stem cells, as well as specialized cells that make up specific tissues. As noted in the chapter introduction, researchers also have more ambitious hopes for epidermal cells. If these cells could be genetically engineered, and their differentiation controlled, they might provide starting material to replace other types of tissues, without the controversy raised by use of embryonic stem cells.
b
a
What are the properties of vertebrate skin? Vertebrate skin consists of all four tissue types arranged in two layers, an outer epidermis and a deeper dermis. Skin’s keratinized, melanin-containing cells provide a waterproof barrier that protects internal body cells.
Figure 32.15 (a) A commercially available cultured skin substitute called Apligraf. It has a two-layered structure, with living keratinocytes on top, and fibroblasts below. (b) When placed over a wound, as shown here, the cultured skin cells can help prevent infection while encouraging faster healing.
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IMPACTS, ISSUES REVISITED
Open or Close the Stem Cell Factories?
In vitro fertilization—uniting egg and sperm outside the body— is a common practice in fertility clinics. It produces a cell cluster smaller than a grain of sand. The cluster is implanted in a woman’s uterus or frozen for later use. An estimated 500,000 such “embryos” are now frozen and many will never be implanted in their mother. They are a potential source of stem cells, or a potential child—if a woman is willing to carry them to term.
How would you vote? Should embryos unwanted by parents and stored in fertility clinics be used as a source of stem cells for research? See CengageNOW for details, then vote online.
Summary Section 32.1 Animal cells are organized as tissues, aggregations of cells and intercellular substances that interact in specific tasks. Animal tissues have a variety of cell junctions. Tight junctions stop fluid from leaking across an epithelium. Adhering junctions hold neighboring cells together. Gap junctions are open channels that connect the cytoplasm of abutting cells and permit rapid transfer of ions and small molecules between them. Tissues are organized into organs, which interact as components of organ systems. Together, all body parts maintain homeostasis—they keep conditions in the internal environment stable and suitable for life.
Use the animation on CengageNOW to compare the structure and function of the main animal cell junctions.
Section 32.2 Epithelial tissues cover the body surface and line its internal spaces. They have one free surface exposed to a body fluid or the environment. A secreted basement membrane connects the epithelium to underlying tissue. Microvilli increase the free surface area of epithelia that absorb substances. Epithelia may also be ciliated or secretory. Gland cells and secretory glands are derived from epithelium. Endocrine glands secrete hormones into blood. Exocrine glands secrete products such as sweat or digestive enzymes through ducts. Section 32.3 Connective tissues “connect” tissues to one another, both functionally and structurally. Different types bind, organize, support, strengthen, protect, and insulate other tissues. All contain cells scattered in a secreted matrix. Soft connective tissue underlies skin, holds internal organs in place, and connects muscle to bone, or bones to one another. The different types of soft connective tissues all have the same components (fibroblasts and a matrix with elastin and collagen fibers) but in different proportions. Rubbery cartilage, calciumhardened bone tissue, lipid-storing adipose tissue, and blood are specialized connective tissues. Section 32.4 Muscle tissues contract and move a body or its parts. Muscle contraction is a response to signals from the nervous system and it requires ATP energy. The three types of muscle are skeletal muscle, cardiac muscle, and smooth muscle tissue. Only skeletal muscle and cardiac muscle tissues appear striated. Only skeletal muscle is under voluntary control. 550 UNIT VI
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Skeletal muscle is the functional partner of bones and consists of long cells with many nuclei. Cardiac muscle occurs only in the heart wall. Its cells are joined together end to end. Smooth muscle occurs in walls of hollow and tubular organs such as blood vessels and the bladder. Section 32.5 Nervous tissue makes up the communication lines that extend through the body. Neurons are cells that can become excited and relay messages along their plasma membrane. Sensory neurons detect stimuli. Interneurons integrate information and call for responses. Motor neurons deliver commands to muscles and glands that carry out responses. Nervous tissue also contains a diverse collection of neuroglial cells. Neuroglia protect and support the neurons. Section 32.6 An organ system consists of two or more organs that interact chemically, physically, or both in tasks that help keep individual cells as well as the whole body functioning. Most vertebrate organ systems contribute to homeostasis; they help maintain conditions in the internal environment within tolerable limits and so benefit individual cells and the body as a whole. All tissues and organs of an adult animal arise from three primary tissue layers, or germ layers, that form in early embryos: ectoderm, mesoderm, and endoderm. Cells in all tissues are derived from stem cells. Stem cells in early embryos—before germ layers form—can become any tissue. Stem cells in later stages are more specialized and produce only a limited number of tissues.
Use the animation on CengageNOW to investigate the function of vertebrate organ systems and learn terms that describe their locations.
Sections 32.7, 32.8 The skin is an organ system that functions in protection, temperature control, detection of shifts in external conditions, vitamin production, and defense. It has two-layers, the outer epidermis and the deeper dermis. Hair, fur, and nails are rich in keratin and derived from epidermal cells. A brownish pigment called melanin protects the skin from ultraviolet radiation that can damage DNA. Skin is continually renewed. Some kinds of skin cells are already being cultured for medical uses.
Use the animation on CengageNOW to explore the structure of human skin.
Data Analysis Exercise Diabetes is a disorder in which the blood sugar level is not properly controlled. Among other effects, this disorder reduces blood flow to the lower legs and feet. As a result, about 3 million diabetes patients have ulcers, or open wounds that do not heal, on their feet. Each year, about 80,000 require amputations. Several companies provide cultured cell products designed to promote the healing of diabetic foot ulcers. Figure 32.16 shows the results of a clinical experiment that tested the effect of the cultured skin product shown in Figure 32.15 versus standard treatment for diabetic foot wounds. Patients were randomly assigned to either the experimental treatment group or the control group and their progress was monitored for 12 weeks.
Percent of wounds healed
60
1. What percentage of wounds had healed at 8 weeks when treated the standard way? When treated with cultured skin? 2. What percentage of wounds had healed at 12 weeks when treated the standard way? When treated with cultured skin? 3. How early was the healing difference between the control and treatment groups obvious?
Self-Quiz 1. 2.
Answers in Appendix III
tissues are sheetlike with one free surface. function in cell-to-cell communication. a. Tight junctions c. Gap junctions b. Adhering junctions d. all of the above
3. In most animals, glands are formed of a. epithelial c. muscle b. connective d. nervous 4. A sweat gland is an a. endocrine
tissue.
gland. b. exocrine
5. Most have many collagen and elastin fibers. a. epithelial tissues c. muscle tissues b. connective tissues d. nervous tissues
50 40
standard treatment
cultured skin treatment
30 20 10
4 weeks
8 weeks
12 weeks
Figure 32.16 Results of a multicenter study of the effects of standard treatment versus use of a cultured cell product for diabetic foot ulcers. Bars show the percentage of foot ulcers that had completely healed.
14. Match the terms with the most suitable description. exocrine gland a. strong, pliable; like rubber endocrine gland b. secretion through duct endoderm c. outermost primary tissue ectoderm d. contracts, not striated cartilage e. innermost primary tissue smooth muscle f. muscle of the heart wall cardiac muscle g. cements cells together blood h. fluid connective tissue adhering i. ductless secretion junction
Visit CengageNOW for additional questions.
Critical Thinking
6. What is the fluid portion of the blood called? 7. Your body converts excess carbohydrates and proteins to fats. specializes in storing the fats. a. Epithelial tissue c. Adipose tissue b. Dense connective tissue d. both b and c 8. Only cells of a. epithelial tissue b. connective tissue
can shorten (contract). c. muscle tissue d. nervous tissue
9. detects and integrates information about changes and controls responses to those changes. a. Epithelial tissue c. Muscle tissue b. Connective tissue d. Nervous tissue 10. Which type of muscle can be voluntarily controlled? 11. Which type of neuron delivers signals to muscles? 12. Exposure to sunlight causes increased production of , which shields against harmful UV radiation. a. melanin c. keratin b. hemoglobin d. collagen 13. The main cell type in the epidermis is a. neuroglia c. keratinocytes b. motor neurons d. osteocytes
.
1. Many people oppose the use of animals for testing the safety of cosmetics. They say alternative test methods are available, such as the use of lab-grown tissues in some cases. Given what you learned in this chapter, speculate on the advantages and disadvantages of tests that use specific lab-grown tissues as opposed to living animals. 2. Porphyria is a name for a set of rare genetic disorders. Affected people lack one of the enzymes in the metabolic pathway that forms heme, the iron-containing group of hemoglobin. As a result, intermediates of heme synthesis (porphyrins) accumulate. When porphyrins are exposed to sunlight, they absorb energy and release energized electrons. Electrons careening around the cell can break bonds and cause damaging free radicals to form. In the most extreme cases, gums and lips can recede, which makes some front teeth—the canines—look more fanglike. Affected individuals must avoid sunlight, and garlic can exacerbate their symptoms. By one hypothesis, people who were affected by the most extreme forms of porphyria may have been the source for vampire stories. Would you consider this hypothesis plausible? What other kinds of historical data might support or disprove it? CHAPTER 32
ANIMAL TISSUES AND ORGAN SYSTEMS 551
33
Neural Control IMPACTS, ISSUES
In Pursuit of Ecstasy
Ecstasy, an illegal drug, can make you feel socially accepted,
and a racing heart. Blood pressure soars, and the body’s
less anxious, and more aware of your surroundings and of
internal temperature can rise out of control. Spinks became
sensory stimuli. It also can leave you dying in a hospital,
dizzy, flushed, and incoherent after taking just two Ecstasy
foaming at the mouth and bleeding from all orifices as your
tablets. She died because her increased temperature caused
temperature skyrockets. It can send your family and friends
her organ systems to shut down.
spiraling into horror and disbelief as they watch you stop
Few Ecstasy overdoses end in death. Panic attacks and
breathing. Lorna Spinks ended life that way when she was
fleeting psychosis are more common short-term effects. We
nineteen years old (Figure 33.1).
do not know much about the drug’s long-term effects; users
Her anguished parents released these photographs because they wanted others to know what their daughter did not: Ecstasy can kill. Ecstasy is a psychoactive drug; it alters brain function. The
are unwitting guinea pigs for unscripted experiments. We know that Ecstasy use depletes the brain’s store of serotonin and that this shortage can last for some time. In animals, multiple doses of MDMA alter the structure and number
active ingredient, MDMA (3,4-methylenedioxymethamphet-
of serotonin-secreting neurons. This is a matter of concern
amine), is a type of amphetamine, or “speed.” As one effect,
because, low serotonin levels in humans are associated with
it makes neurons release an excess of the signaling molecule
inability to concentrate, memory loss, and depression.
serotonin. The serotonin saturates receptors on target cells
Human MDMA users do have memory loss, and the more
and cannot be cleared away, so cells cannot be released
often a person uses the drug, the worse their memory gets.
from overstimulation.
Fortunately, at least over the short term, capacity for memory
The abundance of serotonin promotes feelings of energy, empathy, and euphoria. But the unrelenting stimulation calls for rapid breathing, dilated eyes, restricted urine formation,
seems to be restored when Ecstasy use stops. However, undoing the neural imbalances often takes many months. Think about it. The nervous system evolved as a way to sense and respond fast to changing conditions inside and outside the body. Vision and taste, hunger and passion, fear and rage—awareness of stimulation starts with a flow of information along communication lines of the nervous system. Even before you were born, excitable cells called neurons started organizing in newly forming tissues and chattering among themselves. All through your life, in moments of danger or reflection, excitement or sleep, their chattering has continued and will continue for as long as you do. Each of us possesses a complex nervous system, a legacy of millions of years of evolution. Its architecture and its functions give us an unparalleled capacity for learning and sharing experiences with others. Perhaps the saddest consequence of drug abuse is the implicit denial of this legacy— the denial of self when we choose not to assess how drugs can harm our brain, or cease to care.
See the video! Figure 33.1 Photos of Lorna Spinks alive (left), and minutes after her death (right). She died after taking two Ecstasy tablets. If you suspect someone is having a bad reaction to Ecstasy or any other drug, get medical help fast and be honest about the cause of the problem. Immediate, informed medical action may save a life.
Links to Earlier Concepts
Key Concepts How animal nervous tissue is organized
In this chapter, you will find many examples of the cell processes covered in Unit One. Nervous signals involve receptor proteins (5.2) and transport mechanisms (5.3, 5.4, 5.5). They depend on ion gradients, a type of potential energy (6.1).
You will reconsider trends in animal evolution (25.1, 26.2) and chordate traits (26.1) with emphasis on nervous systems.
You will also revisit some health applications such as cancer (9.5), alcohol abuse (Chapter 6 introduction), and stem cell research (Chapter 32 introduction).
You will see examples of PET scans, a technique that uses radioisotopes, as explained in Section 2.2.
In radially symmetrical animals, excitable neurons interconnect as a nerve net. Most animals are bilaterally symmetrical with a nervous system that has a concentration of neurons at the anterior end and one or more nerve cords running the length of the body. Section 33.1
How neurons work Messages flow along a neuron’s plasma membrane, from input to output zones. Chemicals released at a neuron’s output zone may stimulate or inhibit activity in an adjacent cell. Psychoactive drugs interfere with the information flow between cells. Sections 33.2–33.7
Vertebrate nervous system The central nervous system consists of the brain and spinal cord. The peripheral nervous system includes many pairs of nerves that connect the brain and spinal cord to the rest of the body. The spinal cord and peripheral nerves interact in spinal reflexes. Sections 33.8, 33.9
About the brain The brain develops from the anterior part of the embryonic nerve cord. A human brain includes evolutionarily ancient tissues and newer regions that provide the capacity for analytical thought and language. Neuroglia make up the bulk of the brain. Sections 33.10–33.13
How would you vote? Should people caught using illegal drugs enter mandatory drug rehabilitation programs as an alternative to jail? Or does the threat of jail make some think twice before experimenting with possibly dangerous drugs? See CengageNOW for details, then vote online.
553
33.1
Evolution of Nervous Systems Interacting neurons give animals a capacity to respond to stimuli in the environment and inside their body.
Link to Trends in animal evolution 25.1
Of all multicelled organisms, animals respond fastest to external stimuli. Activities of neurons are the key to these quick responses. A neuron is a cell that can relay electrical signals along its plasma membrane and can communicate with other cells by way of specific chemical messages. Cells called neuroglia functionally and structurally support neurons in most animals. A typical animal has three types of neurons. Sensory neurons detect internal or external stimuli and signal interneurons or motor neurons. Interneurons process information received from sensory neurons or other interneurons, then send signals along to interneurons or motor neurons. Motor neurons signal and control muscles and glands.
The Cnidarian Nerve Net Cnidarians such as the hydras and jellyfishes, are the simplest animals with neurons. These radial, aquatic animals have a nerve net that allows them to respond to food or threats that arrive from all directions (Figure 33.2a). A nerve net is a mesh of interconnected neurons.
pair of ganglia
Information can flow in any direction among cells of the nerve net, and there is no centralized, controlling organ that functions like a brain. By causing cells in the body wall to contract, the nerve net can alter the size of the animal’s mouth, change the body shape, or shift the position of tentacles.
Bilateral, Cephalized Nervous Systems Most animals have a bilaterally symmetrical body (Section 25.1). Evolution of bilateral body plans was accompanied by cephalization, the concentration of neurons that detect and process information at the body’s anterior, or head, end. Planarians and the other flatworms are the simplest animals with a bilateral, cephalized nervous system. A planarian’s head end has a pair of ganglia (Figure 33.2b). A ganglion (plural, ganglia), is a cluster of neuron cell bodies that functions as an integrating center. A planarian’s ganglia receive signals from eye spots and chemical-detecting cells on its head. The ganglia also connect to a pair of nerve cords that run the length of the body. The cords have no ganglia. Nerves cross the body between the cords, giving the nervous system a ladderlike appearance. The cross connections help coordinate activities of the two sides of the body.
optic lobe (one pair, for visual stimuli)
pair of nerve cords crossconnected by lateral nerves
a nerve net (highlighted in purple) controls the contractile cells in the epithelium
HOW ANIMALS WORK
paired ventral nerve cords ganglion
rudimentary brain
ganglion
554 UNIT VI
branching nerves
b Planarian, a flatworm
ventral nerve cord
a Hydra, a cnidarian
brain
brain
c Earthworm, an annelid
d Crayfish, a crustacean (a type of arthropod)
e Grasshopper, an insect (a type of arthropod)
Figure 33.2 (a) Hydras and other cnidarians have a nerve net. (b) A planarian has a ladderlike nervous system with two nerve cords and a pair of ganglia in the head. (c,d,e) Annelids and arthropods have paired ventral nerve cords with ganglia in each segment. The nerve cords connect to a simple brain.
Annelids and arthropods have paired ventral nerve cords that connect to a simple brain (Figure 33.2c–e). In addition, a pair of ganglia in each body segment provides local control over that segment’s muscles. Chordates have a single, dorsal nerve cord (Section 26.1). In vertebrates, the anterior region of this cord evolved into a brain. Bigger brains gave some animals a competitive edge in finding resources and reacting to danger. Also, among vertebrates that moved onto land, certain brain centers became modified and expanded in ways that helped animals better move about and respond to stimuli in their new environment.
The Vertebrate Nervous System The nervous system of vertebrates has two functional divisions (Figure 33.3). Most interneurons are located in the central nervous system—the brain and spinal cord. Nerves that extend through the rest of the body make up the peripheral nervous system. These nerves are further classified as autonomic or somatic, based on which organs they are associated with. Figure 33.4 shows the location of the human brain, spinal cord, and some peripheral nerves. As you will learn, each nerve contains long extensions, or axons, of sensory neurons, motor neurons, or both. Afferent axons carry sensory signals into the central nervous system; efferent axons relay commands for response out of it. For instance, you have a sciatic nerve in each of your legs. These nerves swiftly relay signals from sensory receptors in leg muscles, joints, and skin in toward the spinal cord. At the same time, they relay signals from the spinal cord to leg muscles. In sections to follow, you will consider the kinds of messages that flow along these communication lines.
Central Nervous System Brain
Spinal Cord
Peripheral Nervous System (cranial and spinal nerves)
Autonomic Nerves
Somatic Nerves
Nerves that carry signals to and from smooth muscle, cardiac muscle, and glands
Nerves that carry signals to and from skeletal muscle, tendons, and the skin
Sympathetic Parasympathetic Division Division Two sets of nerves that often signal the same effectors and have opposing effects
Figure 33.3 Functional divisions of vertebrate nervous systems. The spinal cord and brain are its central portion. The peripheral nervous system includes spinal nerves, cranial nerves, and their branches, which extend through the rest of the body. Peripheral nerves carry signals to and from the central nervous system. Section 33.8 explains the functional divisions of the peripheral system.
Brain cranial nerves (twelve pairs)
cervical nerves (eight pairs)
Spinal Cord thoracic nerves (twelve pairs)
Take-Home Message
ulnar nerve (one in each arm)
lumbar nerves (five pairs)
What are the features of animal nervous systems? Most animals have three types of interacting neurons— sensory neurons, interneurons, and motor neurons. The simplest animals that have neurons are cnidarians. Their neurons are arranged as a nerve net. Most animals are bilaterally symmetrical and have a nervous system with a concentration of nerve cells at their head end. Bilateral invertebrates usually have a pair of ventral nerve cords. In contrast, the chordates have a dorsal nerve cord.
Cnidarians do not have a central information-processing organ. Flatworms have a pair of ganglia that serve this function. Other invertebrates have larger and more complex brains.
sciatic nerve (one in each leg)
sacral nerves (five pairs) coccygeal nerves (one pair)
The vertebrate nervous system includes a well-developed brain, a spinal cord, and peripheral nerves.
Figure 33.4 Some of the major nerves of the human nervous system.
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33.2
Neurons—The Great Communicators Neurons have cytoplasmic extensions specialized for receiving and sending signals.
Like other body cells, each neuron has a nucleus and organelles; both are inside its cell body. Unlike other cells, a neuron also has special cytoplasmic extensions that allow it to receive and send messages (Figure 33.5). Dendrites are short, cytoplasmic branches that receive information from other cells and convey it to the cell body. A neuron usually has several dendrites. A neuron also has an axon, a longer extension that can send chemical signals to other cells. The cell body and dendrites function as signal input zones, where arriving signals alter ion concentration gradients across the plasma membrane. The resulting ion disturbance spreads into a trigger zone, which connects with the axon. From here, the disturbance is con-
ducted along the axon to axon terminals. When it reaches these output zones, the disturbance causes release of signaling molecules. Information usually flows from sensory neurons, to interneurons, to motor neurons (Figure 33.6). The three types of neurons differ somewhat in the type and arrangement of their cytoplasmic extensions. A sensory neuron typically has no dendrites. One end of its axon has receptor endings that can detect a specific stimulus (Figure 33.6a). Axon terminals at the other end send chemical signals, and the cell body lies in between. An interneuron has many signal-receiving dendrites and one axon (Figure 33.6b). In vertebrates, nearly all interneurons reside in the central nervous system and some have many thousands of dendrites. A motor neuron also has multiple dendrites and one axon (Figure 33.6c).
Take-Home Message dendrites
cell body
How do different parts of the three types of neuron function in communication?
⎭ ⎪ ⎬ ⎪ input zone ⎫
Sensory neurons have an axon with one end that responds to a specific stimuli and another that sends signals to other cells. Interneurons and motor neurons have many signal-receiving dendrites and one signal-sending axon.
trigger zone
output zone
conducting zone
axon
Figure 33.5 Animated Scanning electron micrograph and sketch of a motor neuron. Dendrites receive information and relay it to the cell body. Signals that spread to the trigger zone may be conducted along the axon to its endings. From here, signals flow to another cell—in the case of a motor neuron, a muscle cell.
10 µm
receptor endings
peripheral axon
cell body
axon
axon terminals
axon terminal
cell body
axon
cell body
axon terminals
axon
dendrites dendrites a sensory neuron
b interneuron
c motor neuron
Figure 33.6 The three types of neurons. Arrows indicate direction of information flow. (a) Sensory neurons detect stimuli and signal other cells. (b) Interneurons relay signals between neurons. (c) Motor neurons signal effectors—muscle or gland cells.
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33.3
Membrane Potentials
Properties of the neuron membrane affect ion movement.
Links to Transport mechanisms 5.3, 5.4, Potential energy 6.1
interstitial fluid
Resting Potential All cells have an electric gradient across their plasma membrane. The cytoplasmic fluid near this membrane has more negatively charged ions and proteins than the interstitial fluid outside the cell does. As in a battery, these separated charges have potential energy. We call the voltage difference across a cell membrane a membrane potential and measure it in thousandths of a volt, or millivolts (mV). An unstimulated neuron has a resting membrane potential of about –70 mV. Distributions of three kinds of ions are important in generating the resting potential. First, the cytoplasm of a neuron includes many negatively charged proteins that are not present in the interstitial fluid. Being large and charged, these proteins cannot diffuse across the lipid bilayer of the cell membrane. The other two important ions are positively charged potassium ions (K+) and positively charged sodium ions (Na+). These ions move in and out of the neuron with the assistance of transport proteins (Section 5.3). Sodium–potassium pumps (Figure 33.7a and Section 5.4) use energy from a molecule of ATP to transport two potassium ions into the cell and three sodium ions out. Since the pump moves more positive charges out of the cell than in, its action increases the charge gradient across the neuron membrane. Action of the pump also contributes to concentration gradients for sodium and potassium across this membrane. Nearly all sodium pumped out of the neuron stays out—as long as the cell is at rest. In contrast, some potassium ions flow down their concentration gradient (out of the cell) through channel proteins (Figure 33.7b). Leaking of potassium (K+) outward increases the number of unbalanced negative ions in the cell. In summary, the cytoplasm of a resting neuron has negatively charged proteins that the interstitial fluid lacks. It also has fewer sodium ions (Na+) and more potassium ions (K+). We can show the relative concentrations of the relevant ions this way, with the green ball representing negatively charged proteins:
150 Na+
interstitial fluid
5 K+
plasma membrane 15 Na+ 150 K+
65
neuron’s cytoplasm
neuron cytoplasm A Sodium–potassium pumps actively transport 3 Na+ out of a neuron for every 2 K+ they pump in.
B Passive transporters allow K+ ions to leak across the plasma membrane, down their concentration gradient.
C In a resting neuron, gates of voltage-sensitive channels are shut (left). During action potentials, the gates open (right), allowing Na+ or K+ to flow through them.
Figure 33.7 Animated Icons for protein channels and pumps that span a neuron’s plasma membrane. (a) Sodium–potassium pumps (Na+/ K+ pumps) and (b) open potassium (K+) channels contribute to the resting potential. (c) Voltage-gated channels are required for action potentials.
Action Potentials Neurons and muscle cells are said to be “excitable” because, when properly stimulated, they undergo an action potential—an abrupt reversal in the electric gradient across the plasma membrane. Channels with gates that open at a particular voltage, or membrane potential, are essential to action potentials. Neurons have such voltage-gated channels in the membrane of their trigger zone and conducting zone (Figure 33.7c). Some of these voltage-gated channels let potassium ions diffuse across the membrane through their interior. Others let sodium ions move across. The voltage-gated channels are shut in a neuron at rest, but they swing open during an action potential. With this bit of background on membrane proteins and ion gradients, you are ready to look at how an action potential arises at a neuron’s trigger zone and propagates itself, undiminished, to an output zone.
Take-Home Message How do gradients across a neuron membrane contribute to neuron function? The interior of a resting neuron is more negative than the fluid outside the cell. The presence of negatively charged proteins and activity of transport proteins contribute to this charge difference, or resting membrane potential. A resting neuron also has concentration gradients for sodium and potassium across its membrane, with more sodium outside and more potassium inside. When properly stimulated, a neuron undergoes an action potential. Voltagegated channels open and the membrane potential briefly reverses.
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33.4
A Closer Look at Action Potentials makes the neuron cytoplasm more positive, so more sodium channels open. Now the stimulus that brought the neuron to threshold becomes unimportant. Sodium rushing into the neuron—not diffusion of ions from the input zone—drives the feedback cycle:
Movement of sodium and potassium ions through gated channels causes a brief reversal of the membrane potential.
Link to Transport mechanisms 5.3, 5.4
Approaching Threshold A small alteration in the ion concentration gradients across the plasma membrane of a neuron can shift the membrane potential. We call the resulting change a local, graded potential. “Local” means it only spreads out for a millimeter or so. “Graded” means that the change in potential can vary in size. A local potential occurs when ions enter a region of neuron cytoplasm and change the membrane potential in that region. For example, a little sodium entering may shift membrane potential in a region from –70 millivolts to –66 mV. Stimulation of a neuron’s input zone can cause a local, graded potential. If the stimulus is sufficiently intense or long-lasting, ions diffuse from the input zone into the adjacent trigger zone. The membrane here includes sodium channels with voltage-sensitive gates (Figure 33.8a). When the difference in charge across the membrane increases to a specific level, the threshold potential, the gated sodium channels in the trigger zone open and start an action potential. Opening of these voltage-gated channels allows sodium to flow down its electrical and concentration gradients into the neuron (Figure 33.8b). In an example of positive feedback (Section 27.3), gated sodium channels open in an accelerating way after threshold is reached. As sodium starts to flow in, it
interstitial fluid with high Na+, low K+
more Na+ flows into the neuron more gated channels for Na+ open
neuron becomes more positive inside
An All-or-Nothing Spike Researchers can study changes in membrane potentials by inserting one electrode into an axon and another into the fluid just outside of it (Figure 33.9). They connect these electrodes to a device that shows membrane potential. Figure 33.10 shows what a recording looks like before, during, and after an action potential. Once threshold level is reached, membrane potential always rises to the same level as an action potential peak. Thus, an action potential is an all-or-nothing event. The reversal of charge during an action potential lasts only milliseconds. Above a certain voltage, gates on sodium channels swing shut. About the same time, gates on potassium (K+) channels open (Figure 33.8c). The resulting outflow of positively charged potassium makes cytoplasm once again more negative than the interstitial fluid. Diffusion of ions quickly restores the Na+ and K+ ion gradients to match those set up by action of sodium–potassium pumps (Figure 33.8d).
Na+ Na+ Na+
Na+K+ pump
voltage-gated ion channels
cytoplasm with low Na+, high K+
A Close-up of the trigger zone of a neuron. One sodium–potassium pump and some of the voltage-gated ion channels are shown. At this point, the membrane is at rest and the voltage-gated channels are closed. The cytoplasm’s charge is negative relative to interstitial fluid.
Na+
B Arrival of a sufficiently large signal in the trigger zone raises the membrane potential to threshold level. Gated sodium channels open and sodium (Na+) flows down its concentration gradient into the cytoplasm. Sodium inflow reverses the voltage across the membrane.
Figure 33.8 Animated Propagation of an action potential along part of a motor neuron’s axon.
558 UNIT VI
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Na+ Na+
A Resting membrane potential is 70 mV.
electrode outside
++++ ++++++++ ––––––––––––
B Stimulation causes an influx of positive ions and a rise in the membrane potential.
unstimulated axon
Figure 33.9 How membrane potentials can be investigated. Electrodes placed inside and outside an axon allow researchers to measure membrane potential. Figure 33.10 shows the record this method produces when a neuron is stimulated enough to produce an action potential.
C Once potential exceeds threshold (60 mV), the sodium (Na+) gates begin to open, and Na+ rushes in. This causes more gates to open, and so on. Voltage shoots up rapidly as a result.
D Every action potential peaks at +33 mV; no more, no less. At this point, Na+ gates have closed and potassium (K+) gates have opened.
D
+30
Membrane potential (millivolts)
electrode inside
action potential
C threshold level
E
E Flow of K+ out of the neuron causes the potential to fall.
–60
B resting level
F So much K+ exits that potential declines below resting potential.
G
–70
A F 1
0
2
3
4
5
6
G Na+–K+ pump action restores resting potential.
Time (milliseconds)
Figure 33.10 Animated How membrane potential changes during an action potential. Figure It Out: How long does the increase in potential last?
Answer: About 2 milliseconds
Direction of Propagation
Each action potential is self-propagating. Some of the sodium that enters one region of an axon diffuses into an adjoining region, driving that region to threshold and opening sodium gates. As these gates swing open in one region after the next, the action potential moves toward the axon terminals without weakening. Once sodium gates close, another action potential cannot occur right away. The brief refractory period limits the maximum speed of signals and causes them to move one way, toward axon terminals. Diffusion of ions from a region undergoing an action potential can only open gated channels that did not already open.
Take-Home Message What happens during an action potential? An action potential begins in the neuron’s trigger zone. A strong stimulus decreases the voltage difference across the membrane. This causes gated sodium channels to open, and the voltage difference reverses. An action potential travels along an axon as consecutive patches of membrane undergo reversals in membrane potential. At each patch of membrane, an action potential ends when potassium ions flow out of the neuron, and voltage difference across the membrane is restored. Action potentials move in one direction, toward axon terminals, because gated sodium channels are briefly inactivated after an action potential.
Na+ K+ pump
K+
K+ K+
K+
K+ K+
Na+
Na+ Na+
Na+ Na+
Na+ K+
C The charge reversal makes gated Na+ channels shut and gated K+ channels open. The K+ outflow restores the voltage difference across the membrane. The action potential is propagated along the axon as positive charges spreading from one region push the next region to threshold.
D After an action potential, gated Na+ channels are briefly inactivated, so the action potential moves one way only, toward axon terminals. Na+ and K+ gradients disrupted by action potentials are restored by diffusion of ions that were put into place by activity of sodium–potassium pumps.
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33.5
How Neurons Send Messages to Other Cells Action potentials do not pass directly from a neuron to another cell; chemicals carry the signals between cells.
Links to Receptor proteins 5.2, Exocytosis 5.5
Chemical Synapses An action potential travels along a neuron’s axon to axon terminals at its tips. The region where an axon terminal sends chemical signals to a neuron, a muscle fiber, or a gland cell is called a synapse. At a synapse, the signal-sending neuron is called the presynaptic cell. A fluid-filled space about 20 nanometers wide separates it from the input zone of a postsynaptic cell
Neuromuscular junctions A An action potential propagates along a motor neuron.
that receives the signal. Figure 33.11 shows a synapse between a motor neuron and a skeletal muscle fiber. Such a synapse is called a neuromuscular junction. Action potentials arrive at a neuromuscular junction by traveling along the axon of a motor neuron to axon terminals (Figure 33.11a,b). Inside the axon terminals are vesicles with molecules of neurotransmitter, a type of signaling molecule that relays messages between presynaptic and postsynaptic cells. Release of the neurotransmitter requires an influx of calcium ions (Ca++). The plasma membrane of an axon terminal has gated channels for these ions. In a resting neuron, these gates are closed and calcium
Close-up of a neuromuscular junction (a type of synapse)
B The action potential reaches axon terminals that lie close to muscle fibers.
axon of a motor neuron
C Arrival of the action potential causes calcium ions (Ca++) to enter an axon terminal. D Ca++ causes vesicles with signaling molecule (neurotransmitter) to move to the plasma membrane and release their contents by exocytosis.
one axon terminal of the presynaptic cell (motor neuron) plasma membrane of the postsynaptic cell (muscle cell) synaptic vesicle receptor protein in membrane of postsynaptic cell
muscle fiber synaptic cleft (gap between pre- and postsynaptic cells)
axon terminal
Close-up of neurotransmitter receptor proteins in the plasma membrane of the postsynaptic cell binding site for neurotransmitter is vacant
muscle fiber
channel through interior is closed E When neurotransmitter is not present, the channel through the receptor protein is shut, and ions cannot flow through it.
Figure 33.11 Animated How information is transmitted at a neuromuscular junction, a synapse between a motor neuron and a skeletal muscle fiber. The micrograph shows several such junctions.
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neurotransmitter in binding site ion crossing plasma membrane through the nowopen channel
F Neurotransmitter diffuses across the synaptic cleft and binds to the receptor protein. The ion channel opens, and ions flow passively into the postsynaptic cell.
Synaptic Integration Typically, a neuron or effector cell gets messages from many neurons at the same time. Certain interneurons in the brain are on the receiving end of synapses with 10,000 neurons! An incoming signal may be excitatory and push the membrane potential closer to threshold. Or it may be inhibitory and nudge the potential away from threshold. How does a postsynaptic cell respond to all of this information? Through synaptic integration, a neuron sums all inhibitory and excitatory signals arriving at its input zone. Incoming synaptic signals can amplify, dampen, or cancel one another’s effects. Figure 33.12 illustrates how an excitatory signal and an inhibitory signal of differing sizes that arrive at a synapse at the same time are integrated. Competing signals cause the membrane potential at the postsynaptic cell’s input zone to rise and fall. When the excitatory signals outweigh inhibitory ones, ions diffuse from the input zone into the trigger zone and drive the postsynaptic cell to threshold. Gated sodium channels swing open, and an action potential occurs as described in the preceding section.
Membrane potential (millivolts)
pumps actively transport calcium out of the cell. As a result, there are fewer calcium ions in the neuron cytoplasm than in the interstitial fluid. Arrival of an action potential opens gated calcium channels, and calcium flows into the axon terminal. The resulting increase in calcium concentration causes exocytosis; vesicles filled with neurotransmitter move to the plasma membrane and fuse with it. This releases neurotransmitter into the synaptic cleft (Figure 31.11c,d). At a neuromuscular junction, the neurotransmitter released by the motor neuron is acetylcholine (ACh). The plasma membrane of a postsynaptic cell has receptors that bind neurotransmitter (Figure 31.11e). When ACh binds to receptors in the membrane of a skeletal muscle fiber, channels for sodium ions open (Figure 33.11f ). Sodium ions stream passively through these channels into the muscle cell. Like a neuron, a muscle fiber is excitable; it can undergo an action potential. The rise in sodium caused by the binding of ACh drives the fiber’s membrane toward threshold. Once threshold is reached, action potentials stimulate muscle contraction by a process described in detail in Section 36.8. Some neurotransmitters bind to more than one type of postsynaptic cell, causing a different result in each. For example, ACh stimulates contraction in skeletal muscle but it slows contraction in cardiac muscle.
what action potential spiking would look like
threshold
–60
excitatory signal
integrated potential resting membrane potential
–70
inhibitory signal –75
Figure 33.12 Synaptic integration. Excitatory and inhibitory signals arrive at a postsynaptic neuron’s input zone at the same time. The graph lines show a postsynaptic cell’s response to an excitatory signal (yellow), to an inhibitory signal (purple) and to both at once (red). In this example, summation of the two signals did not lead to an action potential (white waveform).
Neurons also integrate signals that arrive in quick succession from a single presynaptic cell. An ongoing stimulus can trigger a series of action potentials in a presynaptic cell, which will bombard a postsynaptic cell with waves of neurotransmitter.
Cleaning the Cleft After signaling molecules do their work, they must be removed from synaptic clefts to make way for new signals. Some diffuse away. Membrane pumps move others back into presynaptic cells or neuroglial cells. Secreted enzymes break down specific kinds, as when the enzyme acetylcholinesterase breaks down ACh. When neurotransmitter accumulates in a synaptic cleft, it disrupts the signaling pathways. That is how nerve gases such as sarin exert their deadly effects. After being inhaled, they bind to acetylcholinesterase and thus inhibit ACh breakdown. ACh accumulates, causing skeletal muscle paralysis, confusion, headaches, and, when the dosage is high enough, death.
Take-Home Message How does information pass between cells at a synapse? Action potentials travel to a neuron’s output zone. There they stimulate release of neurotransmitters—chemical signals that affect another cell. Neurotransmitters are signaling molecules secreted into a synaptic cleft from a neuron’s output zone. They may have excitatory or inhibitory effects on a postsynaptic cell. Synaptic integration is the summation of all excitatory and inhibitory signals arriving at a postsynaptic cell’s input zone at the same time. For a synapse to function properly, neurotransmitter must be cleared from the synaptic cleft after the chemical signal has served its purpose.
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33.6
A Smorgasbord of Signals
Different types of neurons release different neurotransitters.
Link to PET scans 2.2
Neurotransmitter Discovery and Diversity In the early 1920s, Austrian scientist Otto Loewi was working to find out what controls the heart’s beating. He surgically removed a frog heart—with the nerve that adjusts its rate still attached—and put it in saline solution. The heart continued to beat and, when Loewi stimulated the nerve, the heartbeat slowed a bit. Loewi suspected stimulation of the nerve caused release of a chemical signal. To test this hypothesis, he put two frog hearts into a saline-filled chamber and stimulated the nerve connected to one of them. Both
Table 33.1
Major Neurotransmitters and Their Effects
Neurotransmitter
Examples of Effects
Acetylcholine (ACh)
Induces skeletal muscle contraction, slows cardiac muscle contraction rate, affects mood and memory
Epinephrine and norepinephrine
Speed heart rate; dilate the pupils and airways to lungs; slow gut contractions; increase anxiety
Dopamine
Dampens excitatory effects of other neurotransmitters; has roles in memory, learning, fine motor control
Serotonin
Elevates mood; role in memory
GABA
Inhibits release of other neurotransmitters
hearts started to beat more slowly. As expected, the nerve had released a chemical that not only affected the attached heart, but also diffused through the liquid and slowed the beating of the second heart. Loewi had discovered one of the responses to ACh, the neurotransmitter you read about in the preceding section. ACh acts on skeletal muscle, smooth muscle, the heart, many glands, and the brain. In myasthenia gravis, an autoimmune disease, the body mistakenly attacks its skeletal muscle receptors for ACh. Eyelids droop first, then other muscles weaken. Interneurons in the brain also use ACh as a signaling molecule. A low ACh level in the brain contributes to memory loss in Alzheimer’s disease. Affected people often can recall long-known facts, such as a childhood address, but have trouble remembering recent events. There are many other neurotransmitters (Table 33.1). Norepinephrine and epinephrine (commonly known as adrenaline) prepare the body to respond to stress or to excitement. They are made from the amino acid tyrosine. So is dopamine, a neurotransmitter that influences reward-based learning and fine motor control. Parkinson’s disease involves impairment or death of dopamine-secreting neurons in a brain region that governs motor control (Figure 33.13). Hand tremors are often the earliest symptom. Later, sense of balance may be affected, and any movement can be difficult. The neurotransmitter serotonin affects memory and mood. The drug fluoxetine (Prozac) lifts depression by raising serotonin levels. GABA (gamma-aminobutyric acid) inhibits release of neurotransmitters by other neurons. Diazapam (Valium) and alprazolam (Xanax) are drugs that lower anxiety by boosting GABA’s effects.
The Neuropeptides
b
a
Some neurons also make neuropeptides that serve as neuromodulators, molecules that influence the effects of neurotransmitters. One neuromodulator, substance P, enhances pain perception. Neuromodulators called enkephalins and endorphins are natural painkillers. They are secreted in response to strenuous activity or injuries and inhibit release of substance P. Endorphins also are released when people laugh, reach orgasm, or get a comforting hug or a relaxing massage.
c
Take-Home Message Figure 33.13 Battling Parkinson’s disease. (a) This neurological disorder affects former heavyweight champion Muhammad Ali, actor Michael J. Fox, and about half a million other people in the United States. (b) A normal PET scan and (c) one from an affected person. Red and yellow indicate high metabolic activity in dopamine-secreting neurons. Section 2.2 explains PET scans.
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What kinds of signaling molecules do neurons make? Neurons make neurotransmitters that signal other neurons or effector cells. Some neurons also make neuromodulators that can influence a neurotransmitter’s effects on other cells.
FOCUS ON HEALTH
33.7
Drugs Disrupt Signaling
Psychoactive drugs exert their effects by interfering with the action of neurotransmitters.
Link to Alcohol’s effects Chapter 6 introduction
People take psychoactive drugs, both legal and illegal, to alleviate pain, relieve stress, or feel pleasure. Many drugs are habit-forming, and users often develop tolerance; it takes larger or more frequent doses of the drug to obtain the desired effect. Habituation and tolerance can lead to drug addiction, by which a drug takes on a vital biochemical role. Table 33.2 lists the main warning signs of addiction. Three or more signs may be cause for concern. All major addictive drugs stimulate release of dopamine, a neurotransmitter with a role in reward-based learning. In just about all animals with a nervous system, dopamine release provides pleasurable feedback when an animal engages in behavior that enhances survival or reproduction. This response is adaptive; it helps animals learn to repeat the behaviors that benefit them. When drugs cause dopamine release, they tap into this ancient learning pathway. Drug users inadvertently teach themselves that the drug is essential to their well-being.
Stimulants Stimulants make users feel alert but also anxious, and they can interfere with fine motor control. Nicotine is a stimulant that blocks brain receptors for ACh. The caffeine in coffee, tea, and many soft drinks is also a stimulant. It blocks receptors for adenosine, which acts as a signaling molecule to suppress brain cell activity. Cocaine, a powerful stimulant, is inhaled or smoked. Users feel elated and aroused, then become depressed and exhausted. Cocaine stops the uptake of dopamine, serotonin, and norepinephrine, from synaptic clefts. When norepinephrine is not cleared away, blood pressure soars. Overdoses may cause strokes or heart attacks that can end in death. Cocaine is highly addictive. Heavy cocaine use remodels the brain so that only cocaine can bring about a sense of pleasure (Figure 33.14). Amphetamines reduce appetite and energize users by increasing secretion of serotonin, norepinephrine, and dopamine in the brain. Various types of amphetamine are ingested, smoked, or injected. The chapter introduction focused on the synthetic amphetamine found in Ecstasy. Crystal meth is another widely abused amphetamine. As with cocaine, users require more and more to get high or just to feel okay. Long-term use shrinks the brain areas involved in memory and emotions. Depressants Depressants such as alcohol (ethyl alcohol) and barbiturates slow motor responses by inhibiting ACh output. Alcohol stimulates the release of endorphins and GABA, so users typically experience a brief euphoria followed by depression. Combining alcohol with barbiturates can be deadly. As the introduction to Chapter 6 explains,
alcohol abuse damages the brain, liver, and other organs. Alcoholics deprived of the drug undergo tremors, seizures, nausea, and hallucinations.
Analgesics Analgesics mimic a body’s natural painkillers—endorphins and enkephalins. The narcotic analgesics, such as morphine, codeine, heroin, fentanyl, and oxycodone, suppress pain. They cause a rush of euphoria and are highly addictive. Ketamine and PCP (phencyclidine) belong to a different class of analgesics. They give users an out-of-body experience and numb the extremities, by slowing the clearing of synapses. Use of either drug can lead to seizures, kidney failure, and fatal heat stroke. PCP can induce a violent, agitated psychosis that sometimes lasts more than a week.
a
Hallucinogens Hallucinogens distort sensory perception and bring on a dreamlike state. LSD (lysergic acid diethylamide) resembles serotonin and binds to receptors for it. Tolerance b develops, but LSD is not addictive. However, users can get hurt, and even die, because they Figure 33.14 PET do not perceive and respond to hazards, such scans revealing (a) as oncoming cars. Flashbacks, or brief distornormal brain activity tions of perceptions, may occur years after the and (b) cocaine’s longlast intake of LSD. Two related drugs, mescaline term effect. Red shows and psilocybin, have weaker effects. areas of most activity, Marijuana consists of parts of Cannabis and yellow, green, and blue show successively plants. Smoking a lot of marijuana can cause reduced activity. hallucinations. More often, users become relaxed and sleepy as well as uncoordinated and inattentive. The active ingredient, THC (delta-9-tetrahydrocannabinol), alters levels of dopamine, serotonin, norepinephrine, and GABA. Chronic use can impair short-term memory and decision-making ability.
Table 33.2
Warning Signs of Drug Addiction
1. Tolerance; takes increasing amounts of the drug to get the same effect. 2. Habituation; takes continued drug use over time to maintain the self-perception of functioning normally. 3. Inability to stop or curtail drug use, even if desire to do so persists. 4. Concealment; not wanting others to know of the drug use. 5. Extreme or dangerous actions to get and use a drug, as by stealing, by asking more than one doctor for prescriptions, or by jeopardizing employment by using drugs at work. 6. Deterioration of professional and personal relationships. 7. Anger and defensiveness if someone suggests there may be a problem. 8. Drug use preferred over previous favored activities.
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33.8
The Peripheral Nervous System voltage difference reverses abruptly. By jumping from node to node in long axons, a signal can move as fast as 120 meters per second. In unmyelinated axons, the maximum speed is about 10 meters per second.
Peripheral nerves run through your body and carry information to and from the central nervous system.
Axons Bundled as Nerves In humans, the peripheral nervous system includes 31 pairs of spinal nerves that connect to the spinal cord and 12 pairs of cranial nerves that connect directly to the brain. Each peripheral nerve consists of axons of many neurons bundled together inside a connective tissue sheath (Figure 33.15a). All spinal nerves include axons from both sensory and motor neurons. Cranial nerves may include axons of motor neurons, axons of sensory neurons, or axons of both sensory and motor neurons. Interneurons, remember, are not part of the peripheral nervous system. The neuroglial cells called Schwann cells wrap like jelly rolls around the axons of most peripheral nerves (Figure 33.15b). The Schwann cells collectively form an insulating myelin sheath that makes action potentials flow faster. Ions cannot cross a sheathed neural membrane. As a result, ion disturbances associated with an action potential spread through an axon’s cytoplasm until they reach a node, a small gap between Schwann cells. At each node, the membrane contains numerous gated sodium channels. When these gates open, the
Functional Subdivisions We subdivide the peripheral system into the somatic nervous system and the autonomic nervous system. Somatic and Autonomic Systems The sensory part
of the somatic nervous system conducts information about external conditions from sensory neurons to the central nervous system. The motor part of the somatic system relays commands from the brain and spinal cord to the skeletal muscles. It is the only part of the nervous system normally under voluntary control. The autonomic nervous system is concerned with signals to and from internal organs and glands. The nerves of the autonomic system are in two categories: sympathetic and parasympathetic. Both service most organs and work antagonistically, meaning the signals from one type oppose signals from the other (Figure
Sympathetic and Parasympathetic Divisions
unsheathed node
myelin sheath
a
b “Jellyrolled” Schwann cells of an axon’s myelin sheath
axon
blood vessels nerve fascicle (a number of axons bundled inside connective tissue) the nerve’s outer wrapping
Figure 33.15 Animated (a) Structure of one type of nerve. (b–d) In axons with a myelin sheath, ions flow across the neural membrane at nodes, or small gaps between the cells that make up the sheath. Many gated channels for sodium ions are exposed to extracellular fluid at the nodes. When excitation caused by an action potential reaches a node, the gates open and sodium rushes in, starting a new action potential. Excitation spreads rapidly to the next node, where it triggers a new action potential, and so on down the axon to the output zone.
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axon
Na+
c
----
++++
++++
++++ ++++
-------
-------
----
++++
++++
action potential
resting potential
resting potential
K+
d
HOW ANIMALS WORK
Na +
++++
----
++++
-------
++++ ++++
-------
++++
----
++++
resting potential restored
action potential
resting potential
eyes
Figure 33.16
optic nerve
medulla oblongata
salivary glands
Animated (a) Sympathetic and (b) parasympathetic nerves of the autonomic system. Each half of the body has nerves of the same type.
heart
vagus nerve
larynx bronchi lungs
Ganglia containing the cell bodies of sympathetic neurons lie near the spinal cord. Ganglia of the autonomic neurons lie in or near the organ they control.
midbrain
cervical nerves (8 pairs)
stomach liver spleen pancreas
Figure It Out: Which para-
thoracic nerves (12 pairs)
kidneys adrenal glands
sympathetic nerve has branches that send signals to the heart, stomach, and kidneys?
small intestine upper colon
Answer: Vagus nerve
lower colon rectum
(most ganglia near spinal cord)
(all ganglia in walls of organs)
bladder uterus
pelvic nerve
lumbar nerves (5 pairs) sacral nerves (5 pairs)
genitals
A
Sympathetic outflow from spinal cord
B Parasympathetic outflow from spinal cord and brain
Some responses to sympathetic outflow: • Heart rate increases. • Pupils of eyes dilate (widen, let in more light). • Glandular secretions decrease in airways to lungs. • Salivary gland secretions thicken. • Stomach and intestinal movements slow down. • Sphincters contract.
Some responses to parasympathetic outflow: • Heart rate decreases. • Pupils of eyes constrict (keep more light out). • Glandular secretions increase in airways to lungs. • Salivary gland secretions become more watery. • Stomach and intestinal movements increase. • Sphincters relax.
33.16). Sympathetic neurons are most active in times of stress, excitement, and danger. Their axon terminals release norepinephrine. Parasympathetic neurons are most active in times of relaxation. Release of ACh by their axon terminals promotes housekeeping tasks, such as digestion and urine formation. What happens when something startles or scares you? Parasympathetic input falls. Sympathetic signals increase. When unopposed, sympathetic signals raise your heart rate and blood pressure, make you sweat more and breathe faster, and induce adrenal glands to secrete epinephrine. The signals put you in a state of intense arousal, so you are primed to fight or make a fast getaway. Hence the term fight–flight response. Opposing sympathetic and parasympathetic signals govern most organs. For instance, both act on smooth muscle cells in the gut wall. As sympathetic neurons are releasing norepinephrine at synapses with these
cells, parasympathetic neurons are releasing ACh at other synapses with the same muscle cells. One signal tells the gut to slow down contractions; the other calls for increased activity. The outcome is finely adjusted through synaptic integration. Take-Home Message What is the peripheral nervous system? The peripheral nervous system includes nerves that connect the body with the central nervous system. A nerve consists of the bundled axons of many neurons. Typically each axon is wrapped in a myelin sheath that increases the speed of action potential transmission. Neurons of the somatic part of the peripheral system control skeletal muscle and convey information about the external environment to the central nervous system. The autonomic system carries information to and from smooth muscle, cardiac muscle, and glands. Signals from its two divisions—sympathetic and parasympathetic—have opposing effects on effectors.
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33.9
The Spinal Cord The spinal cord serves as an information highway for traffic to and from the brain, and also as a reflex center. Spinal reflexes do not involve the brain.
An Information Highway Your spinal cord is about a thick as your thumb. It runs through the vertebral column and connects peripheral nerves with the brain (Figure 33.17). The brain and spinal cord together are the central nervous system (CNS). Three membranes, called meninges, cover and protect these organs. The central canal of the spinal cord and spaces between the meninges are filled with cerebrospinal fluid. The fluid cushions blows and thus protects central nervous tissue. The outermost portion of the spinal cord is white matter: bundles of myelin-sheathed axons. In the CNS, such bundles are called tracts, rather than nerves. The tracts carry information from one part of the central nervous system to another. Gray matter makes up the bulk of the CNS. It consists of cell bodies, dendrites, and many neuroglial cells. In cross-section, the spinal cord’s gray matter has a butterfly-like shape. Spinal nerves of the peripheral nervous system connect to the spinal cord at dorsal and ventral “roots.” Remember, all spinal nerves have sensory and motor components. Sensory information travels to the spinal cord through a dorsal root. Cell bodies of sensory neurons are found in dorsal root ganglia. Motor signals
ventral
travel away from the spinal cord through a ventral root. Cell bodies of motor neurons are in the spinal cord’s gray matter. An injury that disrupts the signal flow through the spinal cord can cause a loss of sensation and paralysis. Symptoms depend on what portion of the cord is damaged. Nerves carrying signals to and from the upper body lie higher in the cord than nerves that govern the lower body. An injury to the lumbar region of the cord often paralyzes the legs. An injury to higher cord regions can paralyze all limbs, as well as muscles used in breathing. More that 250,000 Americans now live with a spinal cord injury.
Reflex Pathways Reflexes are the simplest and most ancient paths of information flow. A reflex is an automatic response to a stimulus, a movement or other action that does not require thought. Basic reflexes do not require any learning. With such reflexes, sensory signals flow to the spinal cord or the brain stem, which then calls for a response by way of motor neurons. For example, the stretch reflex is one of the spinal reflexes. It causes a muscle to contract after gravity or some other force stretches it. Suppose you hold a bowl as someone drops fruit into it. The increased load makes your hand drop a bit, which stretches the biceps muscle in your arm. Stretching of the muscle
dorsal
dorsal horn (gray matter, including interneurons that receive input from sensory neurons)
spinal cord meninges (protective coverings) spinal nerve vertebra
dorsal root ganglion (cell bodies of sensory neurons)
white matter (myelinated axons)
ventral horn (gray matter, including the cell bodies of motor neurons) location of intervertebral disk
Figure 33.17 Animated Location and organization of the spinal cord.
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dorsal root (axons of sensory neurons that relay signals from peripheral regions)
ventral root (axons of motor neurons that relay signals toward peripheral regions)
STIMULUS Biceps stretches.
A Fruit being loaded into a bowl puts weight on an arm muscle and stretches it. Will the bowl drop? NO! Muscle spindles in the muscle’s sheath also are stretched.
B Stretching stimulates sensory receptor endings in this muscle spindle. Action potentials are propagated toward spinal cord.
E Axon terminals of the motor neuron synapse with muscle fibers in the stretched muscle.
C In the spinal cord, axon terminals of the sensory neuron release a neurotransmitter that diffuses across a synaptic cleft and stimulates a motor neuron. D The stimulation is strong enough to generate action potentials that self-propagate along the motor neuron’s axon.
RESPONSE
F ACh released from the motor neuron’s axon terminals stimulates muscle fibers.
Biceps contracts.
G Stimulation makes the stretched muscle contract. Ongoing stimulations and contractions hold the bowl steady.
muscle spindle
neuromuscular junction
Figure 33.18 Animated Stretch reflex, a spinal reflex. Muscle spindles in skeletal muscle are stretch-sensitive receptors of sensory neurons. Stretching generates action potentials, which form a synapse with a motor neuron in the spinal cord. Signals for contraction flow along the motor neuron’s axon, from spinal cord back to the stretched muscle. The muscle contracts, steadying the arm.
causes muscle spindles between the muscle fibers to stretch. Muscle spindles are sensory organs that house receptor endings of sensory neurons (Figure 33.18). The more the biceps muscle stretches, the greater the frequency of action potentials along axons of the muscle spindle neurons. Inside the spinal cord, these axons synapse with motor neurons that control the stretched muscle. Signals from the sensory neurons cause action potentials in the motor neurons, which release ACh at the neuromuscular junction. In response to this signal, the biceps contracts and steadies the arm against the added load. The knee-jerk reflex is another stretch reflex. A tap just below the knee stretches the thigh muscle. The stretch is detected by muscle spindles in this muscle. The muscle spindles send signals to the spinal cord, where motor neurons become excited. As a result, signals flow from the spinal cord back to the leg, and the leg jerks in response.
Another spinal reflex, the withdrawal reflex, allows quick action when you touch something hot. Touch a hot surface and signals flow to the spinal cord. Unlike the stretch reflex, the withdrawal response involves an interneuron of the spinal cord. A heat-detecting sensory neuron sends signals to the spinal interneuron, which then relays the signal to motor neurons. Before you know it, your biceps has contracted, pulling your hand away from the potentially damaging heat. Take-Home Message What are the functions of the spinal cord? Tracts of the spinal cord relay information between peripheral nerves and the brain. The axons involved in these pathways make up the bulk of the cord’s white matter. Cell bodies, dendrites, and neuroglia make up gray matter. The spinal cord also has a role in some simple reflexes, automatic responses that occur without conscious thought or learning. Signals from sensory neurons enter the cord through the dorsal root of spinal nerves. Commands for responses go out along the ventral root of these nerves.
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33.10 The Vertebrate Brain The brain is part of the central nervous system and is the body’s main information integrating organ.
Link to Trends in vertebrate evolution 26.2
In all vertebrates, the embryonic neural tube develops into a spinal cord and brain. During development the brain becomes organized as three functional regions: the forebrain, midbrain, and hindbrain (Figure 33.19).
The Hindbrain and Midbrain The hindbrain sits atop the spinal cord. The portion just above the cord, the medulla oblongata, influences
forebrain
midbrain
The Forebrain
hindbrain
a
HINDBRAIN
MIDBRAIN N
FOREBRAIN
b
c
Cerebrum
Localizes, processes sensory inputs; initiates, controls skeletal muscle activity; governs memory, emotions, abstract thought in the most complex vertebrates
Olfactory lobe
Relays sensory input from nose to olfactory areas of cerebrum
Thalamus
Relays sensory signals to and from cerebral cortex; has a role in memory
Hypothalamus
With pituitary gland, functions in homeostatic control. Adjusts volume, composition, temperature of internal environment; governs organ-related behaviors (e.g., sex, thirst, hunger), and expression of emotions
Limbic system
Governs emotions; has roles in memory
Pituitary gland (Chapter 35)
With hypothalamus, provides endocrine control of metabolism, growth, development
Pineal gland (Chapter 35)
Helps control some circadian rhythms; also has role in mammalian reproduction
Roof of midbrain (tectum)
In fishes fishes and amphibians amphibians, it coordinates sensory input (as from optic lobes) with motor responses. In mammals, it is reduced and mainly relays sensory input to the forebrain
Pons
Tracts bridge cerebrum and cerebellum cerebellum, also connect spinal cord with forebrain. With the medulla oblongata, controls rate and depth of respiration
Cerebellum
Coordinates motor activity for moving limbs and maintaining posture, and for spatial orientation
Medulla oblongata
Tracts relay signals between spinal cord and pons; functions in reflexes that affect heart rate, blood vessel diameter, and respiratory rate. Also involved in vomiting, coughing, other vital functions
568 UNIT VI
the strength of heartbeats and the rhythm of breathing. It also controls reflexes such as swallowing, vomiting, and sneezing. Above the medulla oblongata lies the pons, which assists in regulation of breathing. Pons means “bridge,” and tracts extend through the pons to the midbrain. The cerebellum, the largest hindbrain region, lies at the back of the brain and serves mainly to coordinate voluntary movements. Fishes and amphibians have the most pronounced midbrain (Figure 33.20). It sorts out sensory input and initiates motor responses. In primates, the midbrain is the smallest of the three brain regions and plays an important role in reward-based learning. The pons, medulla, and midbrain are collectively referred to as the brain stem.
HOW ANIMALS WORK
Early vertebrates relied heavily on their forebrain’s olfactory lobes; odors provided essential information about the environment. Paired outgrowths from the brain stem integrated olfactory input and responses to it. Especially among land vertebrates, these outgrowths expanded into the two halves of the cerebrum, the two cerebral hemispheres. Most sensory signals destined for the cerebrum pass through the adjacent thalamus. The hypothalamus (“under the thalamus”) is the center for homeostatic control of the internal environment. It regulates behaviors related to internal organ activities, such as thirst, sex, and hunger and governs temperature. The hypothalamus is also an endocrine gland. It interacts with the adjacent pituitary gland to control hormone secretions. Another endocrine gland, the pineal gland, lies deep in the forebrain. We discuss endocrine function in detail in Chapter 35. Also in the forebrain is a group of structures that we refer to collectively as the limbic system. We discuss the role of the human system in the next section.
Protection at the Blood–Brain Barrier The neural tube’s lumen—the space inside it—persists in adult vertebrates as a system of cavities and canals filled with cerebrospinal fluid. This clear fluid forms
Figure 33.19 Neural tube to brain. The human neural tube at (a) 7 weeks of embryonic development. The brain at (b) 9 weeks, and (c) at birth. The chart lists and describes major components in the three regions of the adult vertebrate brain.
Figure 33.20 Animated (a) Major brain regions of five vertebrates, dorsal views. The sketches are not to the same scale. (b) Right half of a human brain in sagittal section, showing the locations of the major structures and regions. Meninges around the brain were removed for this photograph.
olfactory lobe forebrain midbrain hindbrain
a
FISH
AMPHIBIAN
REPTILE
BIRD
MAMMAL
shark
frog
alligator
goose
human
when water and small molecules are filtered out of the blood into brain cavities called ventricles. The fluid then seeps out and bathes the brain and spinal cord. It returns to the bloodstream by entering veins. A blood–brain barrier protects the spinal cord and brain from harmful substances. The barrier is formed by the walls of blood capillaries that service the brain. In most parts of the brain, tight junctions form a seal between adjoining cells of the capillary wall, so water-soluble substances must pass through the cells to reach the brain. Transport proteins in the plasma membrane of these cells allow essential nutrients to cross. Oxygen and carbon dioxide diffuse across the barrier, but most waste urea cannot breach it. No other portion of extracellular fluid has solute concentrations maintained within such narrow limits. Even changes brought on by eating and exertion are limited. Why? Hormones and other chemicals in blood affect neural function. Also, changes in ion concentrations can alter the threshold for action potentials. The blood–brain barrier is not perfect; some toxins such as nicotine, alcohol, caffeine, and mercury slip across. Also, inflammation or a traumatic blow to the head can damage it and compromise neural function.
The Human Brain The average human brain weighs 1,330 grams, or 3 pounds. It contains about 100 billion interneurons, and neuroglia makes up more than half of its volume. The human midbrain is relatively smaller than that of other vertebrates. A human cerebellum is the size of a fist and has more interneurons than all other brain regions combined. As in other vertebrates, the cerebellum plays a role in the sense of balance, but it took on added functions as humans evolved. It affects learning of motor and some mental skills, such as language. A deep fissure divides the forebrain’s cerebrum into two halves, the cerebral hemispheres (Figure 33.20).
corpus callosum part of optic nerve
hypothalamus
thalamus
pineal gland location
midbrain cerebellum pons medulla oblongata
b
Each half deals mainly with input from the opposite side of the body. For instance, signals about pressure on the right arm reach the left hemisphere. Activity of the hemispheres is coordinated by signals that flow both ways across the corpus callosum, a thick band of nerve tracts. The next section focuses on the cerebral cortex, the thin outer layers of the cerebrum. Take-Home Message What are the structural and functional divisions of the vertebrate brain? We recognize three regions, the forebrain, midbrain, and hindbrain, based on the embryonic tissue from which they develop. The brain stem, which includes parts of the hindbrain and the midbrain, is the most evolutionarily ancient region of brain tissue. It is involved in reflex behaviors. The forebrain includes the cerebrum, which evolved as an expansion of the olfactory lobe and is now the main processing center in humans. It also includes the hypothalamus, which has important roles in thirst, temperature regulation, and other responses related to homeostasis.
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The Human Cerebrum Our capacity for language and conscious thought arises from the activity of the cerebral cortex. The cortex interacts with other brain regions in shaping our emotional responses and memories.
Functions of the Cerebral Cortex Each half of the cerebrum, or cerebral hemisphere, is divided into frontal, temporal, occipital, and parietal lobes (Figure 33.21). The cerebral cortex, the outermost gray matter on each lobe, contains distinct areas that receive and process diverse signals. The cerebral hemispheres overlap in function, but there are differences. Most often, mathematical skills and language arise mainly from activity in the left hemisphere. The right hemisphere interprets music, judges spatial relations, and assesses visual inputs. The body is spatially mapped out in the primary motor cortex of each frontal lobe, which controls and
thu ex ne mb c bro k w eye lid a nd face eye b lips
vocalization
jaw
i at
a m
liv sa
on
st
gue
ica
ing
ow
all
sw
tion
a
Figure 33.22 (a) Slice of the primary motor cortex through the region identified in (b). Sizes of body parts draped over the artful slice are distorted to indicate which ones get the most precise control.
b
Figure 33.23 Three PET scans that identify which brain areas were active when a person performed three kinds of tasks. Yellow and orange indicate high activity.
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ee kn ankle
toes
all
ton
trunk
elbow
shoulder
d
han
wrist
le litt g e rin ddl i m
ind
hip
33.11
HOW ANIMALS WORK
frontal lobe (planning of movements, aspects of memory, inhibition of unsuitable behaviors)
primary motor cortex
primary somatosensory cortex
parietal lobe (visceral sensations)
Wernicke’s area
Broca’s area
temporal lobe (hearing, advanced visual processing)
occipital lobe (vision)
Figure 33.21 Animated Lobes of the brain, with primary receiving and integrating centers of the human cerebral cortex.
coordinates the movements of skeletal muscles on the opposite side of the body. Much of the motor cortex is devoted to finger, thumb, and tongue muscles, which can make fine movements. Figure 33.22 depicts the proportions of the motor cortex that are devoted to controlling different body parts. The premotor cortex of each frontal lobe regulates complex movements and learned motor skills. Swing a golf club, play the piano, or type on a keyboard, and the premotor cortex coordinates the activity of many different muscle groups. Broca’s area in the frontal lobe helps us translate thoughts into speech. It controls the tongue, throat, and lip muscles and gives humans our capacity to speak complex sentences. In most people, Broca’s area is in the left hemisphere. Damage to Broca’s area often prevents normal speech, although an affected individual can still understand language. The primary somatosensory cortex is at the front of the parietal lobe. Like the motor cortex, it is organized as a map that corresponds to body parts. It receives sensory input from the skin and joints, and one part has a role in taste perception (Section 34.3). The perceptions of sound and odor arise in sensory areas of each temporal lobe. Wernicke’s area, in this lobe, functions in the comprehension of spoken and
Motor cortex activity when speaking
Prefrontal cortex activity when generating words
Visual cortex activity when seeing written words
(olfactory tract)
Sensory stimuli, as from the nose, eyes, and ears
cingulate gyrus thalamus hypothalamus
Temporary storage in the cerebral cortex
Input forgotten
SHORT-TERM MEMORY
amygdala hippocampus
Figure 33.24 Limbic system components.
written language, including Braille, a written language for the blind. A primary visual cortex at the back of each occipital lobe receives sensory input from both eyes. Association areas are scattered through the cortex, but not in the primary motor and sensory areas. Each integrates diverse inputs (Figure 33.23). For instance, one visual association area around the primary visual cortex compares what we see with visual memories.
Connections With the Limbic System The limbic system encircles the upper brain stem. It governs emotions, assists in memory, and correlates organ activities with self-gratifying behavior such as eating and sex. That is why the limbic system is known as our emotional-visceral brain. “Gut reactions” called up by the limbic system can often be overridden by the cerebral cortex. The hypothalamus, hippocampus, amygdala, and cingulate gyrus are part of the limbic system (Figure 33.24). The hypothalamus is the major control center for homeostatic responses and it correlates emotions with visceral activities. The hippocampus helps store memories and access memories of earlier threats. The almond-shaped amygdala helps interpret social cues, and contributes to the sense of self. It is highly active during episodes of fear and anxiety, and often it is overactive in people afflicted with panic disorders. The cingulate gyrus has a role in attention and in emotion. It is often smaller and less active than normal in people with schizophrenia. Evolutionarily, the limbic system is related to the olfactory lobes. Olfactory input causes signals to flow to the hippocampus, amygdala, and hypothalamus as well as to the olfactory cortex. That is one reason why specific odors can call up emotionally significant memories. Information about taste also travels to the limbic system and can trigger emotional responses.
Recall of stored input
Emotional state, having time to repeat (or rehearse) input, and associating the input with stored categories of memory influence transfer to long-term storage
LONG-TERM MEMORY
Input irretrievable
Figure 33.25 Stages in memory processing.
Making Memories The cerebral cortex receives information continually, but only a fraction of it becomes memories. Memory forms in stages. Short-term memory lasts seconds to hours. This stage holds a few bits of information, a set of numbers, words of a sentence, and so forth. In long-term memory, larger chunks of information get stored more or less permanently (Figure 33.25). Different types of memories are stored and brought to mind by different mechanisms. Repetition of motor tasks can create skill memories, which are highly persistent. Once you learn to ride a bicycle, drive a car, dribble a basketball, or play an accordion, you seldom forget how. Skill memories involve the cerebellum, which controls motor activity. Declarative memory stores facts and impressions of events, as when it helps you remember how a lemon smells or that a quarter is worth more than a dime. It starts when the sensory cortex signals the amygdala, a gatekeeper to the hippocampus. A memory will be retained only if signals loop repeatedly in the sensory cortex, hippocampus, and thalamus. Emotions influence memory retention. For instance, epinephrine released during times of stress helps place short-term memories into long-term storage. Take-Home Message What are the functions of the cerebral cortex? The cerebral cortex controls voluntary activity, sensory perception, abstract thought, and language and speech. It receives information and processes some of it into memories. It also oversees the limbic system, the brain’s center of emotional responses.
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33.12 The Split Brain Investigations by Roger Sperry into the importance of information flow between the cerebral hemisphere showed that the two halves of the brains have a division of labor.
As mentioned in the preceding section, the two cerebral hemispheres look alike but differ a bit in their functions. The differences first became apparent in the mid-1800s, through studies of people who had brain injuries. For instance, damage to Broca’s area in the left frontal cortex interfered with the ability to vocalize words. Injury to Wernicke’s area in the left temporal lobe did not interfere with the capacity to say words, but the affected person could not put words into sentences. Fast-forward to the 1960s. Ever more evidence of the importance of the left hemisphere continued to flow in. Researchers began wondering what role, if any, the right hemisphere plays in the advanced functions of typical right-handed people. Roger Sperry and his coworkers decided to find out. Sperry became interested in “split-brain” patients. These people had undergone surgery to sever their corpus callosum, a thick band of nerves that connects the two cerebral hemispheres. At the time, this was an experimental way to treat severe epilepsy. Epileptic seizures are like electrical storms in the brain. Surgeons severed a patient’s corpus callosum to prevent flow of disturbed electrical signals from one hemisphere to the other. After a brief recovery, patients were able to lead what seemed to be normal lives, with fewer seizures.
Left Half of Visual Field
Right Half of Visual Field
COWBOY
But were those patients really normal? The surgery had stopped the flow of information across 200 million or so axons in the corpus callosum. Surely something had to be different. Something was. Sperry designed elegant experiments to examine the split-brain experience. He devised a mechanism of presenting the two halves of affected patients with two different parts of a visual stimulus. At the time, researchers already knew that the visual connections to and from one hemisphere are mainly concerned with the opposite half of the visual field, as in Figure 33.26. Sperry projected a word—say, COWBOY—onto a screen so that COW fell in the left half of the visual field, and BOY fell in the right (Figure 33.27). The subjects of this experiment reported seeing the word BOY. The left hemisphere, which controls language, recognized the word. However, when asked to write the word with the left hand—which was hidden from view— the subject wrote COW. The right hemisphere “knew” the other half of the word (COW) and had directed the left hand’s motor response. But it could not tell the left hemisphere what was going on because of the severed corpus callosum. The subject knew a word was being written but could not say what it was! “The surgery,” Sperry reported, “left these people with two separate minds, two spheres of consciousness.” Sperry concluded that both hemispheres contribute to normal perception by sharing information that shapes the experience we call consciousness.
COW BOY
pupil COWBOY
optic nerves
retina optic chiasm
corpus callosum right visual cortex
left visual cortex
A Pathway by which sensory input about visual stimuli reaches the visual cortex of the human brain.
B Each eye gathers visual information at the retina, a thin layer of densely packed photoreceptors at the back of the eyeball (Section 34.7). Light from the left half of the visual field strikes receptors on the right side of both retinas. Parts of two optic nerves carry signals from the receptors to the right cerebral hemisphere. Light from the right half of the visual field strikes receptors on the left side of both retinas. Parts of the optic nerves carry signals from them to the left hemisphere.
Figure 33 33.26 26 Animated Vi Visuall information i f ti and d th the b brain. i
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COWBOY
Figure 33.27 One example of the response of a split-brain patient to visual stimuli. As described in the text, this type of experiment demonstrated the importance of the corpus callosum in coordinating activities between the two cerebral hemispheres.
33.13 Neuroglia—The Neurons’ Support Staff Although we focus on the neurons, neuroglial cells make up the bulk of the brain and have important roles too.
Links to Cell Cycle 9.2, Cancer 9.5
Types of Neuroglia Neuroglial cells, or neuroglia, outnumber neurons in a human brain by about 10 to 1. Neuroglia act as a framework that holds neurons in place; glia means glue in Latin. While a nervous system is developing, new neurons migrate along highways of neuroglia to reach their final destination. An adult brain has four main types of neuroglial cells: oligodendrocytes, microglia, astrocytes, and ependymal cells. The oligodendrocytes make myelin sheaths that insulate axons in the central nervous system. As mentioned earlier, Schwann cells are neuroglia that perform this same function for peripheral nerves. Multiple sclerosis (MS) is an autoimmune disorder in which white blood cells wrongly attack and destroy the myelin sheaths of oligodendrocytes. The myelin is replaced by scar tissue and the conduction ability of the affected axons declines. Certain genes increase the likelihood of MS, but a viral infection might set it in motion. Once it begins, information flow is disrupted. Dizziness, numbness, muscle weakness, fatigue, visual problems, and other symptoms commonly follow. MS affects at least 300,000 people in the United States. Microglia are, as the name implies, the smallest of the neuroglial cells. They continually survey the brain. If brain tissue is injured or infected, microglia become active, motile cells that engulf dead or dying cells and debris. They also produce chemical signals that alert the immune system to the threat. Star-shaped astrocytes are the most abundant cells in the brain (Figure 33.28). They have diverse roles. They wrap around blood vessels that supply the brain and stimulate formation of the blood-brain barrier, take up neurotransmitters released by neurons, assist in immune defense, make lactate that fuels activities of neurons, and synthesize nerve growth factor. A growth factor is a molecule that is secreted by one cell and causes division or differentiation of another cell. Neurons do not divide; they are stopped in G1 of the cell cycle (Section 9.2). But nerve growth factor causes a neuron to form new synapses with its neighbors. Ependymal cells are neuroglia that line the brain’s fluid-filled cavities (ventricles) and the spinal cord’s central canal. Some ependymal cells are ciliated and the action of their cilia keeps the cerebrospinal fluid flowing in a consistent direction through the system of cavities and canals.
Figure 33.28 Astrocytes (orange) and a neuron (yellow) in brain tissue. The cells in this light micrograph were made visible by immunofluorescence. This procedure attaches fluorescent dye molecules to antibodies that then bind to specific molecules on a cell.
About Brain Tumors Neurons do not divide, so they do not give rise to tumors. However, sometimes neuroglial cells divide uncontrollably, and the result is a glioma. This is the most common kind of primary brain tumor—a tumor that arises from cells in the brain. Brain tumors also arise from uncontrolled division of cells in meninges, or as a result of metastasis—arrival of cancerous cells from elsewhere in the body (Section 9.5). Men are more prone to brain tumors than women. Exposure to ionizing radiation, such as x-rays, or to chemical carcinogens increases risk. What about the radio waves from cell phones? No study has shown that use of a cell phone causes brain cancer. However, cell phones are a relatively recent invention and brain tumors can take years to develop. To be cautious, some doctors recommend use of a headset, which keeps the wave-emitting part of the phone away from the brain.
Take-Home Message What are the functions of neuroglia? Neuroglial cells make up the bulk of the brain. They provide a framework for neurons, insulate neuron axons, assist neurons metabolically, and protect the brain from injury and disease. Unlike neurons, neuroglia continue to divide in adults. Thus, neuroglia can be a source of brain tumors.
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In Pursuit of Ecstasy
Now that you know a bit more about how a brain functions, take a moment to reconsider effects of MDMA, the active ingredient in Ecstasy. MDMA harms and possibly kills brain interneurons that produce the neurotransmitter serotonin. Remember, neurons do not divide, so damaged ones are not replaced. MDMA also impairs the blood–brain barrier, so it allows larger than normal molecules to pass into the brain for as long as 10 weeks after use.
How would you vote? Should people who are caught using illegal drugs be offered addiction treatment as an alternative to jail time? See CengageNOW for details, then vote online.
Summary Section 33.1 Neurons are electrically excitable cells that signal other cells by means of chemical messages. Sensory neurons detect stimuli. Interneurons relay signals between neurons. Motor neurons signals effectors (muscles and glands). Neuroglia support the neurons. Radially symmetrical animals have a nerve net. Most animals have a bilateral nervous system with cephalization; they have paired ganglia (clusters of neuron cell bodies) or a brain at the head end. The vertebrate central nervous system is a brain and spinal cord. The peripheral nervous system includes all nerves that run through the body. Sections 33.2–33.4 A neuron’s dendrites receive signals and its axon transmits signals. Neurons maintain a resting membrane potential, a slight voltage difference across their plasma membrane. An action potential is a brief reversal of the membrane potential. It occurs only if membrane potential increases to the threshold potential. An action potential occurs when opening of voltagegated sodium channels allows sodium to flow down its concentration gradient into the neuron. Then, opening of voltage-gated potassium channels allows potassium ions to flow out of the neuron. All action potentials are the same size and travel in one direction only, away from the cell body and toward the axon terminals.
Use the animation on CengageNOW to learn about a neuron’s structure and its membrane properties and to view an action potential step by step.
Sections 33.5–33.7 Neurons send chemical signals to cells at synapses. A synapse between a motor neuron and a muscle fiber is a neuromuscular junction. Arrival of an action potential at a presynaptic cell’s axon terminals triggers the release of neurotransmitter, a type of chemical signal. Neurotransmitter diffuses to receptors on a postsynaptic cell and binds to them. A postsynaptic cell’s response is determined by synaptic integration of all messages arriving at the same time. Neuromodulators are chemicals secreted by neurons that can alter neurotransmitter effects. Psychoactive drugs disrupt neurotransmitter-based signaling. Some cause drug addiction, a dependence on the drug that interferes with normal functioning.
Use the animation on CengageNOW and learn about a synapse between a motor neuron and a muscle cell.
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Section 33.8 Nerves are bundles of axons that carry signals through the body. Myelin sheaths enclose most axons and increase signal conduction rates. The peripheral nervous system is functionally divided into the somatic nervous system, which controls skeletal muscles, and the autonomic nervous system, which controls internal organs and glands. Signals from sympathetic neurons of the autonomic system increase in times of stress or danger. The signals cause a fight–flight response. During less stressful times, signals from parasympathetic neurons dominate. Organs receive signals from both types of neurons.
Use the animation on CengageNOW to explore the structure of a nerve and to compare the effects of sympathetic and parasympathetic stimulation.
Section 33.9 Like the brain, the spinal cord consists of white matter (with myelinated axons) and gray matter (with cell bodies, dendrites, and neuroglia). The spinal cord and brain are enclosed by membranous meninges and cushioned by cerebrospinal fluid. Spinal reflexes involve peripheral nerves and the spinal cord. A reflex is an automatic response to stimulation; it does not require conscious thought.
Use the animation on CengageNOW to explore the spinal cord and see what happens during a stretch reflex.
Sections 33.10–33.12 The neural tube of a vertebrate embryo develops into the spinal cord and brain. The brain stem is the evolutionarily oldest brain tissue. It includes the pons and medulla oblongata, which control reflexes involved in breathing and other essential tasks. The cerebellum acts in motor control. The thalamus and hypothalamus function in homeostasis. A blood–brain barrier protects the brain from many harmful chemicals. The cerebral cortex, the most recently evolved brain region, governs complex functions. It has specific areas that receive different types of sensory input or control voluntary movements. The cerebral cortex interacts with the limbic system in emotions and memory. Activity of the two halves of the cerebrum is coordinated by means of the corpus callosum that connects them.
Use the animation on CengageNOW to learn about the structure and function of the human brain.
Section 33.13 Neuroglial cells make up the bulk of the brain. Unlike neurons, they continue to divide in adults.
Data Analysis Exercise Animal studies are often used to assess effects of prenatal exposure to illicit drugs. For example, Jack Lipton used rats to study the behavioral effect of prenatal exposure to MDMA, the active ingredient in Ecstasy. He injected female rats with either MDMA or saline solution when they were 14 to 20 days pregnant. This is the period when their offsprings’ brains were forming. When those offspring were 21 days old, Lipton tested their ability to adjust to a new environment. He placed each young rat in a new cage and used a photobeam system to record how much each rat moved around before settling down. Figure 33.29 shows his results.
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1. Which rats moved around most (caused the most photobeam breaks) during the first 5 minutes in a new cage, those prenatally exposed to MDMA or the controls?
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2. How many photobeam breaks did the MDMA-exposed rats make during their second 5 minutes in the new cage? 3. Which rats moved around the most during the last 5 minutes of the study? 4. Does this study support the hypothesis that MDMA affects a developing rat’s brain?
Self-Quiz
Answers in Appendix III
1. relay messages from the brain and spinal cord to muscles and glands. a. Motor neurons b. Interneurons c. Sensory neurons 2. When a neuron is at rest, . a. it is at threshold potential b. gated sodium channels are open c. the sodium–potassium pump is operating d. both a and c 3. Action potentials occur when . a. a neuron receives adequate stimulation b. more and more sodium gates open c. sodium–potassium pumps kick into action d. both a and b 4. True or false? Action potentials vary in their size. 5. Neurotransmitters are released by . a. axon terminals c. dendrites b. the cell body d. the myelin sheath 6. What chemical is released by axon terminals of a motor neuron at a neuromuscular junction? a. ACh b. serotonin c. dopamine d. epinephrine 7. Which neurotransmitter is important in reward-based learning and drug addiction? a. ACh b. serotonin c. dopamine d. epinephrine
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Figure 33.29 Effect of prenatal exposure to MDMA on activity levels of 21-day-old rats placed in a new cage. Movements were detected when the rat interrupted a photobeam. Rats were monitored at 5-minute intervals for a total of 20 minutes. Blue bars are results for rats whose mothers received saline, red bars are rats whose mothers received MDMA.
11. Which of the following are not in the brain? a. Schwann cells b. astrocytes c. microglia 12. True or false? Neurons do not divide in adults. 13. Match each item with its description. muscle spindle a. start of brain, spinal cord neurotransmitter b. connects the hemispheres limbic system c. protects brain and spinal corpus callosum cord from some toxins cerebral cortex d. type of signaling molecule neural tube e. support team for neurons neuroglia f. stretch-sensitive receptor white matter g. roles in emotion, memory blood–brain h. most complex integration barrier i. myelinated axons of neurons
Visit CengageNOW for additional questions.
Critical Thinking 1. In humans, the axons of some motor neurons extend more than a meter, from the base of the spinal cord to the big toe. What are some of the functional challenges involved in the development and maintenance of such impressive cellular extensions?
9. When you sit quietly on the couch and read, output from neurons prevails. a. sympathetic b. parasympathetic
2. Some survivors of disastrous events develop posttraumatic stress disorder (PTSD). Symptoms include nightmares about the experience and suddenly feeling as if the event is recurring. Brain-imaging studies of people with PTSD showed that their hippocampus was shrunken and their amygdala unusually active. Given these changes, what other brain functions might be disrupted in PTSD?
10. Cell bodies of the sensory neurons that deliver signals to the spinal cord are in the . a. white matter b. gray matter c. dorsal root ganglia
3. In human newborns, especially premature ones, the blood–brain barrier is not yet fully developed. Why is this one reason to pay careful attention to the diet of infants?
8. Skeletal muscles are controlled by . a. sympathetic signals c. somatic nerves b. parasympathetic signals d. both a and b
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5-Minute Intervals
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Sensory Perception IMPACTS, ISSUES
A Whale of a Dilemma
Imagine yourself in the sensory world of a whale, 200 meters
humpback whale, make very low-pitched sounds that can
(650 feet) beneath the ocean surface. Almost no sunlight
travel across an entire ocean basin. Their ears are adapted
penetrates this deep, so the whale sees little as its moves
to detect those sounds.
through water. Many fishes detect motion with a lateral line
The ocean is becoming a lot noisier, and the superb
system, which responds to differences in water pressure.
acoustical adaptations of whales now put them at risk. For
Fishes also use dissolved chemicals as navigational cues.
example, in 2001 some whales beached themselves near
However, a whale has no lateral line, and it has a very poor
an area where the United States Navy was testing a sonar
sense of smell. How does it know where it is going?
system (Figure 34.1). This system emits loud low-frequency
All whales use sounds—acoustical cues. Water is an ideal medium for transmitting sound waves, which move five times faster in water than in air. Unlike humans, whales do not have
sounds and uses their echoes to locate submarines. Humans cannot hear the sonar sounds. Whales can. As autopsies later revealed, the beached whales had
a pair of ear flaps that collect sound waves. Some whales do
blood in their ears and in acoustic fat. Apparently the intense
not even have a canal leading to ear components inside their
sounds emitted by the sonar made them race to the surface
head. Others have ear canals packed with wax. How, then, do
in fear. Rapid change in pressure damaged internal tissues.
whales hear? Their jaws pick up vibrations traveling through
Sonar testing continues because the threat of stealth
water. The vibrations are transmitted from the jaws, through a
submarine attacks against the United States is real. Also,
layer of fat, to a pair of pressure-sensitive middle ears.
noise from commercial shipping may be a worse problem for
Whales use sound to communicate, locate food, and find
whales. Massive tankers generate low-frequency sounds that
their way around underwater. Killer whales and some other
frighten whales or drown out acoustical cues. Realistically,
species of toothed whales use echolocation. The whale emits
global shipping of oil and other resources that industrial
high-pitched sounds and then listens as the echoes bounce
nations require is not going to stop. If research shows that
off objects, including prey. Its ears are especially sensitive
whales are at risk, will those same nations be willing to design
to sounds of high frequencies. Baleen whales, including the
and deploy newer, more expensive tankers that are quieter? In this chapter, we turn to sensory systems. Using these organ systems, animals detect stimuli inside and outside their body and become aware of touches, sounds, sights, odors, and other sensations. As you will learn, animals differ in their type and number of sensory receptors that sample the environment, and thus also differ in their perception of that environment.
See the video! Figure 34.1 A few children drawn to one of the whales that stranded itself during military testing of a new sonar system. Of sixteen stranded whales, six died on the beach. Volunteers pushed the others out to sea. Their fate is unknown.
Links to Earlier Concepts
Key Concepts How sensory pathways work
This chapter builds heavily on the previous one. You will see examples of action potentials (Section 33.3), and learn more about neuromodulators (33.6), the stretch reflex (33.9), and the limbic system and cerebral cortex (33.11).
Our discussions of evolution of sensory organs will refer to earlier sections about morphological convergence (19.2), vertebrate evolution (26.2), and primate evolution (26.13) in particular.
In discussing vision, we return to the topic of pigments (7.1), and to the effects of Vitamin A deficiency (Chapter 16 introduction).
You will also learn about how pathogenic amoebas (22.11) and roundworms (25.11) can harm vision.
Sensory receptors detect specific stimuli. Different animals have receptors for different stimuli. Information from sensory receptors becomes encoded in the number and frequency of action potentials sent to the brain along particular nerve pathways. Section 34.1
Somatic and visceral senses Somatic sensations such as touch are easily localized and arise from receptors in the skin, muscles, or near joints. Visceral sensations, such as a feeling of fullness in your stomach, are less easily pinpointed. They arise from receptors in the walls of internal organs. Section 34.2
Chemical senses The senses of smell and taste require chemoreceptors, which bind molecules of specific substances dissolved in the fluid bathing them. Section 34.3
Balance and hearing Organs in the ear function in balance and in hearing. The inner ear’s vestibular apparatus detects body position and motion. The outer and middle ear collect and amplify sound waves. Mechanoreceptors in the inner ear send signals about sound to the brain. Sections 34.4–34.6
Vision Most organisms have light-sensitive pigments, but vision requires eyes. Vertebrates have an eye that operates like a film camera. Their retina, which has photoreceptors, is analogous to the film. A sensory pathway starts at the retina and ends in the visual cortex. Sections 34.7–34.10
How would you vote? Maritime activities such as shipping cause an underwater ruckus. Would you support a ban on activities that generate excessive noise levels from territorial waters of the United States and other nations? See CengageNOW for details, then vote online.
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34.1
Overview of Sensory Pathways An animal’s sensory receptors determine what features of the environment it can detect and respond to.
Links to Action potentials 33.3, Stretch reflex 33.9
As the previous chapter explained, an animal’s sensory neurons detect specific stimuli, or forms of energy, in the internal or external environment. Stimulation of the receptor endings of a sensory neuron causes action potentials that travel along the plasma membrane.
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Sensory Receptor Diversity All animals that have neurons have sensory neurons. However, the types of stimuli these neurons detect vary among animal groups. We can classify sensory neurons based on the kinds of stimuli to which they respond. Mechanoreceptors are sensory endings that respond to mechanical energy. Some detect a body’s position or acceleration. For example, a jellyfish can tell which way is up because it has cells with statoliths. A statolith is a dense object that shifts position when a cell’s orientation changes. Shifts trigger action potentials. Other mechanoreceptors fire off action potentials in response to touch or to stretching of a body part. The muscle spindles involved in the human stretch reflex (Section 33.9) are a type of mechanoreceptor. Still other mechanoreceptors respond to vibrations caused by pressure waves. Hearing involves this type of receptor. As the chapter introduction noted, different animals detect sound waves of different frequencies. Whales detect ultra-low frequencies that humans cannot hear. Bats emit and respond to sounds too high for humans to perceive (Figure 34.2a). Pain receptors, also called nociceptors, detect tissue damage. They have a protective function and are often involved in reflexes that minimize further harm. Some thermoreceptors respond to a specific temperature; others fire in response to a temperature change. Pythons and some other snakes have thermoreceptors concentrated in pits on their head (Figure 34.2b). These receptors help a snake detect warm-blooded prey. Chemoreceptors detect specific solutes dissolved in a fluid. Nearly all animals have chemoreceptors that help them locate chemical nutrients and avoid taking in poisons. Chemoreceptors also function in smell. Osmoreceptors detect a change in the concentration of solutes in a body fluid, such as blood.
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Figure 34.2 Examples of sensory receptors. (a) Mechanoreceptors inside a bat’s inner ear allow the animal to detect high-pitched, or ultrasonic, pressure waves. (b) Thermoreceptors in pits above and below a python’s mouth allow it to detect body heat, or infrared energy, of nearby prey.
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b
Figure 34.3 A marsh marigold looks yellow to humans (a), but photographing it with UV-sensitive film reveals a dark area around the reproductive parts (b). This pattern is caused by UV-absorbing pigment and is visible to insect pollinators.
Photoreceptors detect light energy. Humans detect only visible light, but insects and some other animals, including rodents, also respond to ultraviolet light. Flowers often have UV-absorbing pigments arranged in patterns that are invisible to us, but obvious to their insect pollinators (Figure 34.3).
From Sensing to Sensation
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In animals that have a brain, the processing of sensory signals gives rise to sensation: awareness of a stimulus. Sensation is different than perception, which refers to a conscious understanding of what a sensation means. Sensory receptors in skin, skeletal muscles, or near the joints give rise to somatic sensations. Sensations of touch and warmth are examples. Visceral sensations, such as the feeling that your bladder or stomach is full, arise from receptors in internal organs. Sensory receptors restricted to specific sensory organs, such as eyes or ears, function in special senses—vision, smell, balance, hearing, and taste. For example, stretch receptors in a gymnast’s arm and leg muscles keep the brain informed of changes in muscle length (Figure 34.4a). The gymnast’s brain integrates this sensory input with signals from eyes and the organs of balance in the inner ear, then issues commands that cause muscles to adjust their length and help maintain balance and posture. Stimulation of a sensory receptor produces action potentials which, remember, are always the same size (Section 33.3). The brain gets additional information about stimuli by noting which nerve pathways carry the action potentials, the frequency of action potentials traveling on each axon in the pathway, and the number of axons recruited by the stimulus. First, an animal’s brain is prewired, or genetically programmed, to interpret action potentials in certain ways. That is why you may “see stars” after an eye gets poked, even in a dark room. Photoreceptors in the eye that are mechanically disturbed send signals along one of two optic nerves to the brain. The brain interprets all signals from an optic nerve as “light.” Second, a strong signal makes receptors fire action potentials more often and longer than a weak signal. The same receptors are stimulated by a whisper and a whoop. Your brain interprets the difference by variations in frequency of signals (Figure 34.4b). Third, a stronger stimulus recruits more sensory receptors, compared to a weak stimulus. A gentle tap on the arm activates fewer receptors than a slap. Stimulus duration also affects response. In sensory adaptation, sensory neurons cease firing in spite of
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Figure 34.4 Animated (a) A young gymnast benefiting from information flowing from his muscle spindles and other sensory receptors to his brain. (b) Recordings of action potentials from a pressure receptor with endings in a human hand. The graphs chart the variations in stimulus strength. A thin rod was pressed against skin with varying amounts of pressure. Vertical bars above each thick horizontal line record individual action potentials. Frequency of action potentials rises with each increase in stimulus strength.
continued stimulation. Put on a sock and you briefly feel it against your skin, but you quickly lose your awareness of it. Mechanoreceptors in the skin adapt to this stimulus, allowing you to focus on other things.
Take-Home Message How do animals detect and process sensory stimuli?
Sensory neurons undergo action potentials in response to specific stimuli. Different kinds of sensory receptors respond to different types of stimuli.
In animals with a brain, input from sensory neurons can give rise to sensation.
Action potentials are all the same size, but which axons are responding, how many are responding, and the frequency of action potentials provides the brain with information about stimulus location and strength.
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34.2
Somatic and Visceral Sensations Signals from receptors in the skin, joints, muscles, and internal organs flow through the spinal cord to the brain.
Links to Neuromodulators 33.6, Cerebral cortex 33.11
Sensory neurons responsible for somatic sensations are located in skin, muscle, tendons, and joints. Somatic sensations are easily localized to a specific part of the body. In contrast, visceral sensations, which arise from neurons in the walls of soft internal organs, are often difficult to pinpoint. It is easy to determine exactly where someone is touching you, but less easy to say exactly where you feel a stomachache.
The Somatosensory Cortex
trunk neck head er should arm w elbo arm fore t is wr d n ha
lit rin tle g d ind dle thu ex mb eye nos e face mi
hip le g
Signals from the sensory neurons involved in somatic sensation travel along axons to the spinal cord, then along tracts in the spinal cord to the brain. The signals end up in the somatosensory cortex, a part of the cerebral cortex. Like the motor cortex (Section 33.11), the somatosensory cortex has neurons arrayed like a map of the body (Figure 34.5). Body parts shown as disproportionately large in the “body” mapped onto this brain correspond to body regions with the most sensory receptors, such as the fingertips, face, and lips. Body parts, such as legs, that have relatively fewer sensory neurons appear disproportionately small.
foot toes
genitalia
upper lip
lips
Receptors Near the Body Surface As an example of the types of receptors that report to the somatosensory cortex, consider those in the human skin (Figure 34.6). Free nerve endings that coil around the roots of hairs in the dermis detect even the slightest pressure. Other free nerve endings detect temperature changes or tissue damage. Free nerve endings also occur in skeletal muscles, tendons, joints, and walls of internal organs. Here, they give rise to sensations that range from itching, to a dull ache, to sharp pain. Other skin receptors are surrounded by a capsule and are named for the scientists who first described them. Meissner’s corpuscles and Pacinian corpuscles are the main receptors that detect touch and pressure in hairless skin regions such as fingertips, palms, and the soles of feet. Small Meissner’s corpuscles lie in the upper dermis and detect light touches. Pacinian are larger and respond to stronger pressure. They lie deeper in the dermis and also occur near joints and in the wall of some organs. Concentric layers of connective tissue wrap around their sensory endings. Either pressure or a specific temperature can cause other encapsulated receptors to respond. Ruffini endings adapt more slowly than Meissner’s and Pacinian corpuscles. If you hold a stone in your hand, Ruffini endings inform your brain that the stone is still there even after other receptors have adapted and stopped responding. Ruffini endings also fire when temperature exceeds 45°C (113°F). The bulb of Krause, also an encapsulated receptor, responds to touch and cold. It is found in skin and certain mucous membranes.
Muscle Sense Remember those stretch receptors in muscle spindle fibers (Section 33.9)? The more a muscle stretches, the more frequently stretch receptors fire. In concert with receptors in tendons and near movable joints, they inform the brain about positions of the body’s limbs.
lower lip s, and jaw teeth, gum e u tong nx ary l ph - ina ra n i t dom b a
Figure 34.5 A map showing where the different body regions are represented in the human primary somatosensory cortex. This brain region is a narrow strip of the cerebral cortex that runs from the top of the head to just above each ear. Compare Figure 33.21.
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The Sense of Pain Pain is the perception of a tissue injury. Somatic pain is a response to signals from pain receptors in skin, skeletal muscles, joints, and tendons. Visceral pain is associated with organs inside body cavities. It occurs as a response to a smooth muscle spasm, inadequate blood flow to an internal organ, over-stretching of a hollow organ, and other abnormal conditions. Injured or distressed body cells release chemicals that stimulate nearby pain receptors. Signals from the
hair shaft inside follicle
epidermis
lungs, diaphragm heart stomach liver, gallbladder
dermis
pancreas small intestine ovaries colon appendix urinary bladder kidney ureter
free nerve endings
Pacinian corpuscle
Ruffini endings
bulb of Meissner’s Krause corpuscle
Figure 34.6 Animated Sensory receptors in human skin.
pain receptors then travel along the axons of sensory neurons to the spinal cord. Here, the sensory axons synapse with the spinal interneurons that relay signals about pain to the brain. The signals proceed through the brain to the cerebral cortex, where they are assessed and the appropriate responses are set in motion. Numerous substances affect signal transmission at the synapse between a pain-detecting sensory neuron and a spinal interneuron. For example, substance P (a neuromodulator) makes the interneurons more likely to send signals to the sensory cortex. In contrast, the natural opiates—endorphins and enkephalins (Section 33.6)—impair flow of signals along the pain pathway. Pain relievers, or analgesics, interfere with steps in the pain pathway. For example, aspirin reduces pain by slowing production of prostaglandins. These local signaling molecules, which are released by damaged tissues, increase the sensitivity of pain receptors to stimulation. As another example, synthetic opioids such as morphine mimic the activity of endorphins. The drug ziconotide is a chemical first discovered in the venom of a cone snail (Chapter 24 introduction). When injected into the spinal cord, ziconotide blocks calcium channels in axon terminals of pain receptor neurons. Because calcium ion inflow is necessary for neurotransmitter release (Section 33.5), preventing it keeps signals from reaching spinal interneurons that normally convey pain signals to the brain.
Figure 34.7 Animated Sites of referred pain. Colored regions indicate the area that the brain interprets as affected when specific internal organs are actually distressed.
Sometimes, the brain mistakenly interprets signals about a visceral problem as if the signals were coming from the skin or joints. The result is called referred pain. The classic example is a pain that radiates from chest across the shoulder and down the left arm during a heart attack (Figure 34.7). Tissue in the heart, not the arm, is affected so why does the arm hurt? The answer lies in the construction of the nervous system. Each level of the spinal cord receives sensory input from the skin as well as from some of the organs. The skin encounters more painful stimuli than the organs do, so its signals more often flow along the pathway to the brain. The brain sometimes attributes signals that arrive along a pathway to their most common source—skin—even if they originate elsewhere.
Take-Home Message How do somatic and visceral sensations arise? Somatic sensations are signals from sensory receptors in skin, skeletal muscle, and joints. They travel along sensory neuron axons to the spinal cord, then to the somatosensory cortex. Visceral sensations begin with the stimulation of sensory neurons in the walls of organs inside the body. These signals are relayed to the spinal cord, and then the brain. Pain is the sensation associated with tissue damage. Because pain signals originate most often with somatic sources, the brain sometimes misinterprets visceral pain as if it were caused by a problem in the skin or a joint.
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34.3
Sampling the Chemical World
Both smell and taste begin with chemoreceptors.
Link to Limbic system 33.11
Sense of Smell Olfaction, a sense of smell, starts with chemoreceptors that bind specific substances. A stimulus can trigger action potentials that olfactory nerves transmit to the cerebral cortex. Messages also travel to the limbic system, which integrates them with emotional state and stored memories (Section 33.11).
olfactory tract from receptors to the brain
olfactory bulb bony plate
Olfactory receptors detect water-soluble or volatile (easily vaporized) chemicals. A human nose has about 5 million olfactory receptors; a bloodhound nose has 200 million. Receptor axons send action potentials to two olfactory bulbs. These small brain structures sort out components of a scent, then signal the cerebrum for further processing (Figure 34.8). Many animals use olfactory cues to find their way, locate food, and communicate socially. A pheromone is a type of signaling molecule that is secreted by one individual and affects the behavior of other members of its species. For example, female silk moths secrete a sex pheromone. Male silk moths have antennae with olfactory receptors that help them detect a pheromonesecreting female more than a kilometer upwind. Reptiles and most mammals, have a vomeronasal organ, a collection of sensory neurons in the nasal cavity that is sensitive to pheromones. Humans and our closest primate relatives have a reduced version of this organ. Whether humans make and respond to pheromones remains a matter of debate. We discuss the role of pheromones in more detail in Chapter 44.
Sense of Taste
ciliated endings of olfactory receptor that project into mucus inside nose
Figure 34.8 Pathway from the sensory endings of olfactory receptors in the human nose to the cerebral cortex and limbic system. Axons of these sensory receptors pass through holes in a bony plate between the lining of the nasal cavities and the brain.
taste bud
hairlike ending of taste receptor
Taste receptors are also chemoreceptors that detect chemicals dissolved in fluid, but they have a different structure and location than olfactory receptors. Taste receptors help animals locate food and avoid poisons. An octopus “tastes” with receptors in suckers on its tentacles; a fly “tastes” using receptors in its antennae and feet. In humans, many taste buds are embedded in the upper surface of the tongue (Figure 34.9). These sensory organs are located in specialized epithelial structures, or papillae, that look like raised bumps or red dots on the tongue surface. You perceive many tastes, but all are a combination of five main sensations: sweet (elicited by glucose and the other simple sugars), sour (acids), salty (sodium chloride or other salts), bitter (plant toxins, including alkaloids), and umami (elicited by amino acids such as glutamate which, as in aged cheese and aged meat, has a savory taste). The food additive MSG (monosodium glutamate) can enhance flavor by stimulating the taste receptors that contribute to the sensation of umami.
sensory nerve section through circular papilla
Figure 34.9 Taste receptors in the human tongue. Taste buds are clusters of receptor cells and supporting cells inside special epithelial papillae. One type, a circular papilla, is shown in section here. The tongue has about 5,000 taste buds, each enclosing as many as 150 taste receptor cells.
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Take-Home Message How are the senses of smell and taste similar? Smell and taste begin with the stimulation of chemoreceptors by the binding of specific dissolved molecules.
34.4
Sense of Balance semicircular canals
Organs inside your inner ear are essential to maintaining posture and a sense of balance. Somatic sensory receptors also contribute to balance.
Organs of equilibrium are parts of sensory systems that monitor the body’s positions and motions. Each vertebrate ear includes such organs inside a fluid-filled sensory structure called the vestibular apparatus. The organs are located in three semicircular canals and in two sacs, the saccule and utricle (Figure 34.10a). Organs of the vestibular apparatus have hair cells, a type of mechanoreceptor with modified cilia at one end. Fluid pressure inside the canals and sacs makes the cilia bend. The mechanical energy of this bending deforms the hair cell plasma membrane just enough to let ions slip across and stimulate an action potential. A vestibular nerve carries the sensory input to the brain. As you will see, other hair cells function in hearing. The three semicircular canals are oriented at right angles to one another, so rotation of the head in any combination of directions—front/back, up/down, or left/right—moves the fluid inside them. An organ of equilibrium rests on the bulging base of each canal. The cilia of its hair cells are embedded in a jellylike mass (Figure 34.10b). When fluid moves in the canal, it pushes against the mass and generates the pressure required for initiating action potentials. The brain receives signals from semicircular canals on both sides of the head. By comparing the number and frequency of action potentials coming from each side of the head, the brain senses dynamic equilibrium: the angular movement and rotation of the head. Among other things, this sense allows you to keep your eyes locked on an object even when you swivel your head or nod. Organs in the saccule and utricle act in the sense of static equilibrium. These organs help the brain keep track of the head’s position and how fast it is moving in a straight line. They also help keep the head upright and maintain posture. Inside the saccule and utricle, is a jellylike mass weighted with calcite statoliths. This mass lies on top of mechanoreceptors (hair cells). When you tilt your head, or start or stop moving, the weighted mass shifts, bending hair cells and altering their rate of action potentials.
vestibular nerve A Vestibular apparatus inside a human inner ear. The organs of equilibrium in its fluid-filled sacs and canals contribute to a sense of balance.
saccule
utricle B Components of one of the organs inside a semicircular canal. Shifts in the position of the head bend hair cells and alter their frequency of action potentials.
gelatinous membrane in a semicircular canal hair cells with their cilia embedded in membrane sensory neurons
Figure 34.10 Animated Organs of equilibrium in the inner ear.
The brain also takes into account information from the eyes, and from receptors in the skin, muscles, and joints. Integration of the signals provides awareness of the body’s position and motion in space, as shown by figure skater Sarah Hughes at left. A stroke, an inner ear infection, or loose particles in the semicircular canals can cause vertigo, a sensation that the world is moving or spinning around. Vertigo also arises from conflicting sensory inputs, as when you stand at a height and look down. The vestibular apparatus reports that you are motionless, but your eyes report that your body is floating in space. Mismatched signals can cause motion sickness. On a curvy road, passengers in a car experience changes in acceleration and direction that scream “motion” to their vestibular apparatus. At the same time, signals from their eyes about objects inside the car tell their brain that the body is at rest. Driving can minimize motion sickness because the driver focuses on sights outside the car such as scenery rushing past, so the visual signals are consistent with vestibular signals.
Take-Home Message What gives us our sense of balance? Mechanoreceptors in the fluid-filled vestibular apparatus of the inner ear detect the body’s position in space, and when we start or stop moving.
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34.5
Sense of Hearing INNER EAR
Your ears collect, amplify, and sort out sound waves, which are pressure waves traveling through the air.
vestibular apparatus, cochlea
Link to Vertebrate evolution 26.2
Properties of Sound Hearing is the perception of sound, which is a form of mechanical energy. A sound arises when vibration of an object causes pressure variations in air, water, or some other medium. We can represent the pressure variations as waveforms. The amplitude of a sound— the magnitude of its pressure waves—determines its intensity or loudness. The frequency of a sound—the number of wave cycles per second—determines pitch (Figure 34.11). The more wave cycles per second, the higher the frequency. Sounds also differ in their timbre or quality. Differences in timbre can help you recognize people by their voices, or discern the difference between the sounds of a flute and a trumpet, even when both play the same note at the same volume.
OUTER EAR
pinna, auditory canal
MIDDLE EAR
eardrum, ear bones
A The outer ear’s flap and canal collect sound waves.
oval window (behind stirrup)
MIDDLE EAR BONES:
stirrup
auditory nerve
anvil hammer
The Vertebrate Ear
Amplitude
Water readily transfers vibrations to body tissues, so fishes do not require elaborate ears to detect sounds. When vertebrates left water for land, their capacity to collect and amplify vibrations evolved in response to a new environmental challenge: transfer of sound waves from the air to body tissues is inefficient. The structure of human ears helps maximize the efficiency of transfer. As Figure 34.12a indicates, the outer ear of humans and most other mammals is adapted to gathering sounds from the air. one cycle The pinna, a skin-covered flap of cartilage projecting from the side of the head, collects sound waves and directs them into the auditory canal. The canal conveys sounds to the middle ear. Frequency per The middle ear amplifies and transmits unit time air waves to the inner ear. An eardrum, or tympanic membrane, first evolved in early Soft reptiles as a shallow depression on each Loud side of the head. Pressure waves cause this thin membrane to vibrate. Behind the earSame frequency, different amplitude drum is an air-filled cavity and three small bones known as the hammer, anvil, and stirLow rup (Figure 34.12b). These bones transmit note the force of sound waves from the eardrum High note
Same amplitude, different frequency
Figure 34.11 Animated Wavelike properties of sound.
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auditory canal
EARDRUM
round window
COCHLEA
B The eardrum and middle ear bones amplify sound.
Figure 34.12 Animated How humans hear.
to the smaller surface of the oval widow. This flexible membrane is the boundary between the middle ear and the inner ear. The inner ear, remember, has a vestibular apparatus that functions in the sense of balance (Section 34.4). It also has a cochlea, which in humans is a pea-sized, fluid-filled structure that resembles a coiled snail shell (the Greek word koklias means snail). If you could straighten out a cochlea and look inside it, you would notice two fluid-filled compartments (Figure 34.12c). One compartment bends in a U-shape. Its two arms are known as the vestibular duct and tympanic duct. The other compartment, the cochlear duct, lies between the arms of the “U.” When sound waves make the three tiny bones of the middle ear vibrate, the stirrup pushes against the oval window. The oval window bows inward, creating a fluid pressure wave. The wave travels through fluid of the vestibular and tympanic ducts, until it reaches the round window, which bows outward in response.
the cochlea, “uncoiled” for clarity waves of air pressure
oval window
vestibular duct waves of fluid pressure
eardrum
cochlear duct
tympanic duct
round window C Pressure waves are transferred to fluid inside the ducts of the cochlea (shown here uncoiled). hair cells of organ of Corti vestibular duct
cochlear duct organ of Corti tectorial membrane sensory neurons (to the auditory nerve)
tympanic duct E Movement of the basilar membrane (the floor of the cochlear duct) bends hair cells against the organ of Corti’s tectorial membrane. This bending causes hair cells to fire. The action potentials travel along the auditory nerve to the brain.
D Pressure waves are detected by the organ of Corti in the cochlear duct.
As fluid shifts back and forth between the round window and oval window, pressure waves cause the lower wall of the cochlear duct to begin vibrating up and down. This lower wall is the basilar membrane (Figure 34.12d,e). Sitting on top of the membrane is the organ of Corti, an acoustical organ with arrays of hair cells. A hair cell is a mechanoreceptor with a tuft of modified cilia at one end. The cilia project into a tectorial membrane that drapes over them. Movement of the basilar membrane pushes cilia against the tectorial membrane. When the cilia bend, the hair cells undergo action potentials, which then travel along an auditory nerve to the brain. The number of hairs cells that fire and the frequency of their signals inform the brain how loud a sound is. The louder a sound, the more action potentials flow along the auditory nerve to the brain. The brain can determine the pitch of a sound by assessing which part of the basilar membrane is vibrating most. The basilar membrane is not uniform along its length. It is stiff and narrow near the oval
basilar membrane
window, and broader and more flexible deeper into the coil. High-pitched sounds make the stiff, narrow, closer-in part of the basilar membrane vibrate most. Low-pitched sounds cause vibrations mainly in the wide flexible part close to the membrane’s tip. More vibrations make more hair cells in that region fire. Hearing loss or deafness can occur because sound waves do not reach the inner ear, as when an eardrum is ruptured or ear bones do not move properly. It can also occur because of auditory nerve damage or hair cell loss. Some antibiotic drugs can kill hair cells. So can loud noise, a topic we consider in the next section.
Take-Home Message How do vertebrates hear? Human ears collect pressure waves from the surroundings and convert them to pressure waves in fluid inside the inner ear. Pressure waves in this fluid stimulate hair cells, which are auditory receptors that send action potentials along auditory nerves to the brain.
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FOCUS ON THE ENVIRONMENT
34.6
34.7
Noise Pollution Excessive noise caused by human activity is a threat to humans and animals.
Many organisms are sensitive to light, but only those with a camera eye see an image as you do.
As detailed in the chapter introduction, human activities have made the world’s oceans a noisy place. This noise alters the sensory world through which marine animals move, alters their behavior, and endangers their health. Things are not much quieter on land. We measure the intensity of a sound in decibels. An increase of 10 on this scale means an increase of ten fold in loudness. A normal conversation is about 60 decibels, a food blender operating at high speed is about 90 decibels, and a chain saw is about 100 decibels. Music at a rock concert is about 120 decibels. So is the sound heard through the earbuds of an iPod or similar device cranked up to its maximum volume. Noise louder than 90 decibels damages hair cells in the cochlea (Figure 34.13). Humans have about 30,000 such cells at birth, and the number declines with age. Exposure to loud noise accelerates loss of hair cells and of hearing. In humans, a high level of environmental noise also impairs concentration and interferes with sleep patterns. It raises anxiety and increases the risk of high blood pressure and other cardiovascular problems. Land animals are also affected by the increasing din. Loud sounds can frighten animals away from food or young. It can also distract them, making them vulnerable to predators. In birds that rely heavily on auditory signals during courtship, man-made noise can interfere with the ability to find and secure a mate. Canadian researchers recently reported the effects of noisy compressors used to extract oil and gas on ovenbirds, a type of song bird. Birds that share their habitat with the noisy machinery have 15 percent fewer offspring than those in quiet forest habitat.
Sense of Vision
Links to Morphological convergence 19.2, Primates 26.13
Requirements for Vision Vision is detection of light in a way that provides a mental image of objects in the environment. It requires eyes and a brain with the capacity to interpret visual stimuli. Image perception arises when the brain integrates signals regarding shapes, brightness, positions, and movement of visual stimuli. Eyes are sensory organs that hold photoreceptors. Pigment molecules inside the photoreceptors absorb light energy. That energy is converted to the excitation energy in action potentials that are sent to the brain. Certain invertebrates, such as earthworms, do not have eyes, but they do have photoreceptors dispersed under the epidermis or clustered in parts of it. They use light as a cue to orient the body, detect shadows, or adjust biological clocks, but they do not have a true sense of vision. Detecting visual detail requires many photoreceptors, and many invertebrate eyes do not have many such receptors. The quality of the image formed by an eye improves with a lens, a transparent body that bends light rays
lens crystalline cone pigmented cells photoreceptor cells sensory neuron
ommatidium
Figure 34.13 Results of an experiment on the effect of intense sound on the inner ear. Left, from a guinea pig ear, two rows of hair cells that normally project into the tectorial membrane in the organ of Corti. Right, hair cells inside the same organ after twenty-four hours of exposure to noise levels comparable to extremely loud music.
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Figure 34.14 The compound eye of a deerfly, with many densely packed, identical units called ommatidium. Each unit has a lens that focuses light on photoreceptor cells. Although the mosaic image produced by such an eye is fuzzy, the eye is very good at detecting movement.
retina
lens
optic tract
Figure 34.15 An octopus has a camera eye, with a single lens that focuses light on a retina. The retina is a layer of densely packed photoreceptor cells. Axons of these sensory neurons combine to form an optic tract that relays information to the brain.
from any point in the visual field so that rays converge on photoreceptors. Light rays bend at boundaries between substances of different densities. Insects have compound eyes with many lenses, each in a separate unit known as an ommatidium (Figure 34.14). The brain constructs images based on the light intensities detected by the different units. Compound eyes do not provide the clearest vision, but they are highly sensitive to movement. Cephalopod mollusks such as squids and octopuses have the most complex eyes of any invertebrate (Figure 34.15). Their camera eyes have an adjustable opening that allows light to enter a dark chamber. Each eye’s single lens focuses incoming light onto a retina, a tissue densely packed with photoreceptors. The retina of a camera eye is analogous to the lightsensitive film used in a traditional film camera. Signals from the photoreceptors in each eye travel along one of the two optic tracts to the brain. Compared to compound eyes, camera eyes yield a more sharply defined and detailed image. Vertebrates also have camera eyes, and because they are distant relatives of cephalopod mollusks, camera eyes are presumed to have evolved independently in the two lineages. This is an example of morphological convergence (Section 19.2).
Figure 34.16 In owls, eyes face forward and photoreceptors are concentrated near the top of the inner eyeball. Such birds mainly look down for prey. When on the ground, they must turn their heads almost upside down to see something above their head.
Many animals have eyes placed on either side of the head, which maximizes the visible area. Predators, including owls, tend to have two eyes that face forward (Figure 34.16). Having two eyes that both survey the same area supplies the brain with overlapping information that enhances depth perception. The brain can compare information from the eyes to determine how far apart objects are. Primates have good depth perception. As Section 26.13 explained, primates evolved from a shrewlike ancestor that had eyes on either side of its head. The enhanced depth perception from forward-facing eyes may have provided an advantage when early primates began living in and moving through the treetops.
Take-Home Message How do animal visual systems differ? Some animals such as earthworms have photoreceptors that detect light, but do not form any sort of image.
Other animals, including insects, have compound eyes. A compound eye has many individual units, each with its own lens. It produces a mosaic image that is fuzzy, but highly sensitive to movement. A camera eye with an adjustable opening and a lens that focuses light on a photoreceptor-rich retina provides a richly detailed image. Camera eyes evolved independently in cephalopod mollusks and vertebrates.
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34.8
A Closer Look at the Human Eye The human eye is a multilayered structure with a lightbending cornea, a focusing lens, and a photoreceptor-rich retina. The eye is surrounded by protective structures.
Anatomy of the Eye Each human eyeball sits inside a protective, cuplike, bony cavity called the orbit. Skeletal muscles that run from the rear of the eye to the bones of the orbit move the eyeball up and down or side to side. Eyelids, eyelashes, and tears all help protect delicate eye tissues. Periodic blinking is a reflex that spreads a film of tears over the eyeball’s exposed surface. Tears are secreted by glands in the eyelids and consist of water, lipids, salts, and proteins. Among the proteins are enzymes that break down bacterial cell walls and thus help prevent eye infections. A protective mucous membrane, the conjunctiva, lines the inner surface of the eyelids and folds back to cover most of the eye’s outer surface. Conjunctivitis, commonly called pinkeye, is an inflammation of this membrane. A viral or bacterial infection can cause it. The eyeball is spherical, and has a three-layered structure (Figure 34.17). The front portion of each eye is covered by a cornea made of transparent crystalline proteins. A dense, white, fibrous sclera covers the rest of the eye’s outer surface. The eye’s middle layer includes the choroid, iris, and the ciliary body. The blood vessel–rich choroid
is darkened by the brownish pigment melanin. This dark layer prevents light reflection within the eyeball. Attached to the choroid, and suspended behind the cornea, is a muscular, doughnut-shaped iris. It too has melanin. Whether your eyes are blue, brown, or green depends on the amount of melanin in your iris. Light enters the eye’s interior through the pupil, an opening at the center of the iris. Muscles of the iris can adjust pupil diameter in response to light conditions. Bright light causes the iris muscle encircling the pupil to contract, so the pupil contracts (shrinks). In low light, the spoke-like radial muscle contracts and the pupil dilates (widens). A ciliary body of muscle, fibers, and secretory cells, attaches to the choroid. The ciliary body holds the lens in its proper place, just behind the pupil. The stretchable, transparent lens is about 1 centimeter (1/2 inch) in diameter and bulges outward on both sides. The eye has two internal chambers. The ciliary body produces the fluid that fills the anterior chamber. Called aqueous humor, this fluid bathes the iris and lens. A jellylike vitreous body fills the larger chamber behind the lens. The innermost layer of the eye, the retina, is at the back of this chamber. The retina contains the light-detecting photoreceptors. The cornea and lens both bend incoming light so that rays converge at the back of the eye, on the retina. The image formed on the retina is upside down and the mirror image of the real world (Figure 34.18). The
Wall of eyeball (three layers) Outer layer
Sclera. Protects eyeball Cornea. Focuses light
sclera
Middle layer
Pupil. Serves as entrance for light
choroid
Iris. Adjusts diameter of pupil Ciliary body. Its muscles control the lens shape; its fine fibers hold lens in place Choroid. Its blood vessels nutritionally support wall cells; its pigments stop light scattering Start of optic nerve. Carries signals to brain Inner layer
Retina. Absorbs, transduces light energy
Lens Aqueous humor Vitreous body
fovea iris lens pupil cornea aqueous humor
Interior of eyeball Focuses light on photoreceptors Transmits light, maintains fluid pressure Transmits light, supports lens and eyeball
ciliary body vitreous body
Figure 34.17 Animated Components and structure of the human eye.
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retina
optic disk (blind spot) part of optic nerve
Figure 34.18 Animated Pattern of retinal stimulation in the human eye. The curved, transparent cornea changes the trajectory of light rays that enter the eye. As a result, light rays that fall on the retina produce a pattern that is upside down and inverted left to right.
brain makes the necessary adjustments so you perceive the correct orientation when you view an object.
relaxed ciliary muscle
contracted ciliary muscle
fibers taut
fibers slack
distance vision
close vision
A Relaxed ciliary muscle pulls fibers taut; the lens is stretched into a flatter shape that focuses light from a distant object on the retina.
B Contracted ciliary muscle allows fibers to slacken; the lens rounds up and focuses light from a close object on the retina.
Focusing Mechanisms
Figure 34.19 Animated How the eye varies its focus. The lens is encircled by ciliary muscle. Elastic fibers attach the muscle to the lens. The shape of the lens is adjusted by contracting or relaxing the ciliary muscle, increasing or decreasing the tension on the fibers, and thus changing the shape of the lens. Figure It Out: The thicker a lens, the more it bends light. Does the lens bend light more with distance vision or close vision? Answer: Close vision
With visual accommodation, the shape or position of a lens adjusts so that incoming light rays fall on the retina, not in front of it or behind it. Without these adjustments, only objects at a fixed distance would stimulate retinal photoreceptors in a focused pattern. Objects closer or farther away would appear fuzzy. Fishes and reptiles have eyes with a lens that can be shifted forward or back, but lens shape is constant. Extending or decreasing the distance between the lens and retina keeps light focused on the retina. In birds and mammals, the lens is elastic; pulling on the lens changes its shape. A ring-shaped ciliary muscle (part of the ciliary body) encircles the lens and attaches to it by short fibers. Contraction of this muscle adjusts the shape of the lens. When the ciliary muscle is relaxed, fibers are taut, the lens is under tension, and it flattens (Figure 34.19a). When the ciliary muscle contracts, fibers attached to the lens slacken allowing the lens to become more round (Figure 34.19b). The curvature of the lens determines the extent to which light rays will bend, and thus where they will fall in the eye. A flat lens will focus light from a distant object onto the retina. However, the lens must be rounder to focus light from nearby objects. When you read a book, ciliary muscle contracts and fibers that connect this muscle to the lens slacken. The decreased tension on the lens allows it to round up enough to focus light from the page onto your retina. Gaze into the distance and ciliary muscle around the lens relaxes, allowing the lens to flatten. Continual viewing of a close object, such as a computer screen or book, keeps ciliary muscle contracted. To reduce eyestrain, take breaks and focus on more distant objects.
Take-Home Message How is the structure of the human eye related to its function?
The eye consists of delicate tissues that are surrounded by a bony orbit and constantly bathed in infection-fighting tears. The cornea at the front of the eye bends light rays, which then enter the eye’s interior through the pupil. The diameter of the pupil can be regulated depending on the amount of available light.
Behind the pupil, the lens focuses light on the retina, the eye’s innermost photoreceptor-containing layer. Muscle contractions can alter the shape of the lens to focus light from near or distant objects.
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34.9
From the Retina to the Visual Cortex Processing of visual signals begins in the retina and continues along the pathway to the brain.
Link to Pigments 7.1
Structure of the Retina As explained in the previous section, the cornea and lens bend light rays so they fall on the retina. Figure 34.20 shows what a physician sees when she uses a lighted magnifying instrument to examine the retina inside the eyeball. The fovea, the area of the retina that is richest in photoreceptors, appears as a reddish spot in an area relatively free of blood vessels. With normal vision, most light rays are focused on the fovea. Also visible in this photo is the start of the optic nerve. The retina consists of multiple cell layers. Nearest the source of light are several layers of interneurons such as amacrine cells, horizontal cells, and bipolar cells (Figure 34.21). These cells are involved in processing of visual signals. The two types of photoreceptors, rod cells and cone cells, lie in the deepest retinal layer, the one closest to the choroid. Rod cells are photoreceptors that detect dim light. They are the basis for coarse perception of movement and for peripheral vision. They are the most abundant outside the fovea. Cone cells detect bright light and are the basis for sharp vision and for color perception. The fovea has the greatest density of cone cells.
horizontal cell bipolar cell
a
start of an optic nerve
fovea
b
Figure 34.20 (a) Examining the retina. (b) View of the retina, showing the fovea and start of the optic nerve.
cone cell rod cell
amacrine cell rod cells incoming rays of light
cone cell
cone cell stacked, pigmented membrane ganglion cell (axon is part of one of two optic nerves)
Figure 34.21 Animated Organization of the retina. The light-sensitive rods and cones lie beneath and send signals to interneurons involved in visual processing.
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rod cell
Figure 34.22 Scanning electron micrograph and diagrams of rod cells and cone cells. There are three types of cones. Each responds to a different wavelength of light.
Figure 34.23 Animated An experiment into the response of cells of the visual cortex. David Hubel and Torsten Wiesel implanted an electrode in a cat’s brain. They placed the cat in front of a screen upon which different patterns of light were projected, here, a hard-edged bar. Light or shadow falling on part of the screen excited or inhibited signals sent to a single neuron in the visual cortex. Tilting the bar at different angles, as shown in the tan box, produced changes in the neuron’s activity, shown in the purple box. A vertical bar image produced the strongest signal (numbered 5 in the sketch). When the bar image tilted slightly, signals were less frequent. When the bar was tilted past a certain angle, signals stopped.
signals picked up by oscilloscope 1
electrode in cat’s brain
2 3 4 5 6 screen
7 projector
8 0 1 2 3 time (seconds)
How Photoreceptors Work Stacks of membranous disks fill much of the interior of a rod cell (Figure 34.22). Each membranous disk holds molecules of rhodopsin. Rhodopsin consists of a protein (opsin) and retinal, a light-absorbing pigment synthesized from vitamin A. As long as rod cells are in the dark, they undergo action potentials and release an inhibitory neurotransmitter at their synapses with bipolar cells. Exposure to blue-green light causes rhodopsin to change shape, and halts release of the inhibitory neurotransmitter. With this inhibition lifted, the bipolar cells are free to signal other interneurons in the retina. Eventually, this signaling causes action potentials that travel along the optic nerve to the brain. Humans have three types of cone cells—red, green, and blue—each with a slightly different kind of opsin. Differences in opsins affect which wavelength of light a cone absorbs. As in rods, photon absorption by cones leads indirectly to action potentials in other cells.
Visual Processing Interneurons that connect to photoreceptors receive, process, and begin to integrate visual signals. Input from hundreds of rods and cones converges on each bipolar cell. Information also flows laterally among the amacrine cells and horizontal cells of the retina. Eventually, all of the signals converge on about one million ganglion cells. These are the output neurons; their axons are the start of an optic nerve. The region where the optic nerve exits the eye is known as the blind spot because it does not have photoreceptors. You do not normally notice your blind spots because the visual fields of your eyes overlap. The portion of the visual field that is missed because of the blind spot in one eye is seen by the other eye.
Figure 34.24 Flow of information from the retina to processing centers in the brain. Signals from both eyes reach both of the brain’s two hemispheres. The signals from the left half of the visual field end up in the brain’s right visual cortex. Signals from the right half of the visual field end up in the left cortex.
left half of visual field
right half of visual field pupil
optic nerves
corpus callosum left visual cortex
optic chiasm (cross) lateral geniculate nucleus right visual cortex
Different neurons inside the brain’s visual cortex respond to different visual patterns. Figure 34.23 shows an experiment that demonstrated this mechanism. Signals from the right visual field of each eye travel to the left hemisphere. Signals from the left visual field go to the right hemisphere (Figure 34.24). Each optic nerve ends in a brain region (lateral geniculate nucleus) that processes signals. From here, the signals travel to the visual cortex where the final integration process produces visual sensations.
Take-Home Message How does the retina function? The retina’s deepest layer, closest to the choroid, contains photoreceptors: rod cells that work in dim light and cone cells that allow sharp color vision. Interneurons that overlie the photoreceptors receive signals from them. Signal processing begins in the brain and is completed in the visual cortex.
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34.10 Visual Disorders Genetic conditions, age-related changes, nutritional deficits, and infectious agents can impair vision. Links to X-linked inheritance 12.4, Vitamin A deficiency Chapter 16 introduction, Amoebas 22.11, Roundworms 25.11
Color Blindness Color blindness arises when one or more types of cones fail to develop or do not work properly. With the most common type, an affected person has trouble distinguishing reds from greens. This X-linked recessive trait affects about 7 percent of men in the United States. As is the case for other X-linked traits, it shows up predominantly in males (Section 12.4). Only 0.4 percent of women are affected. Lack of Focus About 150 million Americans have disorders in which light rays do not converge as they should. Astigmatism results from an unevenly curved cornea, which cannot properly focus incoming light on the lens. Nearsightedness occurs when the distance from the front to the back of the eye is longer than normal or when ciliary muscles react too strongly. With either disorder, images of distant objects get focused in front of the retina instead of on it (Figure 34.25a). In farsightedness, the distance from front to back of the eye is unusually short or ciliary muscles are too weak. Either way, light rays from nearby objects get focused behind the retina (Figure 34.25b). Also, the lens loses its flexibility as a person ages. That is why most people who are over age forty have relatively impaired close vision. Glasses, contact lenses, or surgery can correct some focusing problems. About 1.5 million Americans undergo laser surgery (LASIK) annually. Typically, LASIK eliminates the need for glasses during most activities, although older adults usually continue to require reading glasses. Macular Degeneration In the United States, an estimated 13 million people have age-related macular degeneration (AMD). The macula is the cone-rich region that surrounds and includes the fovea. Destruction of photoreceptors in the macula clouds the center of the visual field more than the periphery (Figure 34.26b).
distant object
close object
a
b
Figure 34.25 Focusing problems. (a) In nearsightedness, light rays from distant objects converge in front of the retina. (b) In farsightedness, light rays from close objects have not yet converged when they arrive at the retina.
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Mutations in certain genes can increase the risk of AMD. So do smoking, obesity, and high blood pressure. A diet rich in vegetables seems to protect against it. Damage caused by AMD usually cannot be reversed, but drug treatments and laser therapy can slow its progression.
Glaucoma With glaucoma, too much aqueous humor builds up inside the eyeball. The increased fluid pressure damages blood vessels and ganglion cells. It also can interfere with peripheral vision and visual processing. Although we often associate chronic glaucoma with old age, the conditions that give rise to the disorder start to develop long before symptoms show up. When doctors detect the increased fluid pressure before the damage becomes severe, they can manage the disorder with medication, surgery, or both. Cataracts A cataract is a clouding of the lens. It typically develops slowly. The cloudy lens reduces the amount and focusing of light that reaches the retina. Early symptoms are poor night vision and blurred vision (Figure 34.26c). Vision ends after the lens becomes fully opaque. Excessive exposure to ultraviolet radiation, use of steroids, and some diseases such as diabetes can promote the onset and development of cataracts. An artificial implant can replace a badly clouded lens. Millions of people in developed countries undergo cataract surgery each year. Worldwide, about 16 million are currently blind as a result of cataracts. Nutritional Blindness Each year, as many as half a million children worldwide go blind because they do not have enough vitamin A in their diet. Among other things, the body needs vitamin A to make retinal, the pigment in both rods and cones. The Chapter 16 introduction described efforts to genetically engineer rice to contain vitamin A, as a partial solution to vitamin A deficiency. This vitamin can be obtained as part of a balanced diet that includes meat, eggs, and yellow and orange vegetables. Infectious Agents The bacterium Chlamydia trachomatis causes the disease trachoma. The bacteria infect the conjunctiva, the membrane that lines the eyelids and covers the sclera (the white part of the eye). Repeated infections cause corneal scarring and lead to blindness. About 6 million people have been blinded by trachoma in Africa, Asia, the Middle East, Latin America, and the Pacific Islands. It is the leading cause of infectious blindness. Roundworms (Section 25.11) cause onchocerciasis, the second most common type of infectious blindness. It is also called “river blindness” because the biting flies that transmit it are most common around African rivers. Other bacterial diseases and viral diseases, including syphilis, can also cause blindness. So can infection by certain kind of amoebas (Section 22.11). These amoebas have turned up in batches of certain contact lens solutions, as have eye-damaging fungi.
FOCUS ON HEALTH
Summary
a Normal vision
Section 34.1 The types of sensory receptors that an animal has determine the types of stimuli it detects and can respond to. Stimulation of a sensory receptor causes action potentials. Mechanoreceptors respond to mechanical energy such as touch. Pain receptors respond to tissue damage. Thermoreceptors are sensitive to temperature. Chemoreceptors fire in response to dissolved chemicals. Osmoreceptors sense and respond to water concentration. Photoreceptors respond to light. The brain evaluates action potentials from sensory receptors based on which of the body’s nerves delivers them, their frequency, and the number of axons firing in any given interval. Continued stimulation of a receptor may lead to a diminished response (sensory adaptation). The somatic sensations arise from sensory receptors located in skin, or near muscles or joints. Visceral sensations arise from receptors near organs in body cavities. The receptors for special senses—taste, smell, hearing, balance, and vision—are in specific sensory organs.
b Vision with macular degeneration
Use the animation on CengageNOW to see how stimulus intensity affects action potential frequency.
Section 34.2 Signals from free nerve endings, encapsulated receptors, and stretch receptors in the skin, skeletal muscles, and joints reach the somatosensory cortex. Interneurons in this part of the cerebral cortex are laid out like a map of the body surface. Pain is the perception of tissue damage. In vertebrates, a variety of neuromodulators enhance or lessen signals about pain. With referred pain, the brain mistakenly attributes signals that come from an internal organ to the skin or muscles.
Use the animation on CengageNOW to learn about sensory receptors in human skin and referred pain.
Section 34.3 The senses of taste and smell (olfaction) involve chemoreceptors and pathways to the cerebral cortex and limbic system. In humans, taste receptors are concentrated in taste buds on the tongue and walls of the mouth. Olfactory receptors line human nasal passages. Pheromones are chemical signals that act as social cues among many animals. A vomeronasal organ functions in detection of pheromones in many vertebrates.
c Vision with cataracts
Figure 34.26 Photos that simulate how normal vision (a) compares with vision of a person with age-related macular degeneration (b) or cataracts (c). Macular degeneration obscures the center of the visual field. Cataracts lessen the amount of the light that reaches the retina and scatter that light so the resulting image appears fuzzy.
Section 34.4 Organs of equilibrium detect gravity, acceleration, and other forces that affect body positions and motions. The vestibular apparatus is a system of fluid-filled sacs and canals in the inner ear. The sense of dynamic equilibrium arises when body movements cause shifts in the fluid, which causes cilia of hair cells to bend. Static equilibrium depends on signals from hair cells that lie beneath a weighted, jellylike mass. A shift in head position or a sudden stop or start shifts the mass, bends the hair cells, and makes these cells fire.
Use the animation on CengageNOW to explore static and dynamic equilibrium.
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IMPACTS, ISSUES REVISITED
A Whale of a Dilemma
Animal sensory systems evolved over countless generations in a world without human activity. Now, we have dramatically altered the sensory landscape for many animals. The world has become noisier and more brightly lit. Our communication systems fill the air with radio waves. How do these changes affect the species with which we share the planet? How much harm do the changes do? We do not know the answers to these questions.
Sections 34.5, 34.6 Hearing is the perception of sound, which is a form of mechanical energy. Sound waves are pressure waves. We perceive variations in the amplitude of the waves as differences in loudness. We perceive variations in wave frequency as differences in pitch. Human ears have three functional regions. The skincovered flap of the outer ear collects sound waves. The middle ear contains the eardrum and a set of tiny bones that amplify sound waves and transmit them to the inner ear. The inner ear is where pressure waves elicit action potentials inside a cochlea. This coiled structure with fluid-filled ducts holds the mechanoreceptors responsible for hearing in its organ of Corti. Pressure waves traveling through the fluid inside the cochlea bend hair cells of the organ of Corti. The brain gauges the loudness of a sound by the number of signals the sound elicits. It determines a sound’s pitch by which part of the cochlea’s coil the signals arrive from. Hearing loss may be caused by nerve problems, damaged hair cells, or failure of signals to reach the inner ear. Exposure to loud noise can damage hair cells. Noise also disrupts human health and animal behavior.
Use the animation on CengageNOW to learn about the properties of sound and the human sense of hearing.
Section 34.7 Most organisms can respond to light, but vision requires eyes and brain centers capable of processing the visual information. An eye is a sensory organ that contains a dense array of photoreceptors. Insects have a compound eye, with many individual units. Each unit has a lens, a structure that bends light rays so they fall on the photoreceptors. Like squids and octopuses, humans have camera eyes, with an adjustable opening that lets in light, and a single lens that focuses the light on a photoreceptor-rich retina. In animals with eyes that face forward, the brain gets overlapping information about the viewed area. This allows more accurate depth perception. Sections 34.8–34.10 A human eye is protected by eyelids lined by the conjunctiva. This membrane also covers the sclera, or white of the eye. The clear, curved cornea at the front of the eye bends incoming light. Light enters the eye’s interior through the pupil, an adjustable opening in the center of the muscular, doughnut-shaped iris. Light that enters the eye falls on the retina. The retina sits on a pigmented choroid that absorbs light so it is not reflected inside the eye. 594 UNIT VI
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How would you vote? Excessive noise can harm marine organisms. Should we regulate the maximum allowable noise level underwater? See CengageNOW for details, then vote online.
With visual accommodation, the ciliary muscle adjusts the shape of the lens so that light from a near or distant object falls on the retina’s photoreceptors. Humans have two types of photoreceptors. Rod cells detect dim light and are important in coarse vision and peripheral vision. Cone cells detect bright light and colors; they provide a sharp image. The greatest concentration of cones is in the portion of the retina called the fovea. The rods and cones interact with other cells in the retina that start processing visual information before sending it to the brain. Visual signals travel to the cerebral cortex along two optic nerves. There are no photoreceptors in the eye’s blind spot, the area where the optic nerve begins. Abnormalities in eye shape, in the lens, and in cells of the retina can impair vision.
Use the animation on CengageNOW to investigate the structure, function, and organization of the eye and retina.
Self-Quiz
Answers in Appendix III
1. A stimulus is a specific form of energy in the outside environment that is detected by . a. a sensory neuron c. a motor neuron b. an interneuron d. all of the above 2. is defined as a decrease in the response to an ongoing stimulus. a. Perception c. Sensory adaptation b. Visual accommodation d. Somatic sensation 3. Which is a somatic sensation? a. taste c. touch e. a through c b. smell d. hearing f. all of the above 4. Chemoreceptors play a role in the sense of a. taste c. touch e. both a and b b. smell d. hearing f. all of the above
.
5. In the , interneurons are arranged like maps that correspond to different parts of the body surface. a. somatosensory cortex c. basilar membrane b. retina d. all of the above 6. Mechanoreceptors in the send signals to the brain about the body’s position relative to gravity. a. eyes b. ears c. tongue d. nose 7. The middle ear functions in . a. detecting shifts in body position b. amplifying and transmitting sound waves c. sorting sound waves out by frequency 8. The organ of Corti responds to a. sound b. light c. heat
. d. pheromones
Frequent exposure to noise of a particular pitch can cause loss of hair cells in the part of the cochlea’s coil that responds to that pitch. Many workers are at risk for such frequencyspecific hearing loss because they work with or around noisy machinery. Taking precautions such as using ear plugs to reduce sound exposure is important. Noise-induced hearing loss can be prevented, but once it occurs it is irreversible. Dead or damaged hair cells are not replaced. Figure 34.27 shows the threshold decibel levels at which sounds of different frequencies can be detected by an average 25-year-old carpenter, a 50-year-old carpenter, and a 50-year-old who has not been exposed to on-the-job noise. Sound frequencies are given in hertz (cycles per second). The more cycles per second, the higher the pitch. 1. Which sound frequency was most easily detected by all three people?
3. Which of the three people had the best hearing in the range of 4,000 to 6,000 hertz? Which had the worst? 4. Based on this data, would you conclude that the hearing decline in the 50-year-old carpenter was caused by age or by job-related noise exposure?
12. Label the parts of the human eye in this diagram:
25-year-old carpenter
20
50-year-old with no onthe-job noise exposure
30 40 50
50-year-old carpenter
60 70 500
1,000
2,000
3,000
4,000
6,000
Figure 34.27 Effects of age and occupational noise exposure. The graph shows the threshold hearing capacities (in decibels) for sounds of different frequencies (given in hertz) in a 25-year-old carpenter (blue), a 50-year-old carpenter (red), and a 50-year-old who did not have any on-the-job noise exposure (brown).
Critical Thinking
9. Color vision begins with signals from . a. rods b. cones c. hair cells d. the blind spot
11. Bright light causes the to shrink. a. lens b. pupil c. fovea d. blind spot
10
Frequency (Hertz)
2. How loud did a 1,000-hertz sound have to be for the 50-year-old carpenter to detect it?
10. When you view a close object, your lens gets a. more rounded c. more flattened b. cloudier d. more transparent
Hearing Level (Decibels)
Data Analysis Exercise
.
1. Laura loves to eat broccoli and brussels sprouts. Lionel cannot stand them. Everyone has the same five kinds of taste receptors, so what is going on? Is Lionel just being difficult? Perhaps not. The number and distribution of receptors that respond to bitter substances vary among individuals of a population—and studies now indicate that some of this variation is heritable. People who have the greatest number of receptors for bitter substances find many fruits and vegetables highly unpalatable. These supertasters make up about 25 percent of the general population. They tend to be slimmer than average but are more likely to develop colon polyps and colon cancer. How might Lionel’s highly sensitive taste buds put him at increased risk for colon cancer? 2. Are organs of dynamic equilibrium, static equilibrium, or both activated during a roller-coaster ride?
13. Match each structure with its description. fovea a. sensitive to vibrations cochlea b. functions in balance lens c. type of photoreceptor cell hair cell d. has most cone cells rod cell e. contains chemoreceptors taste bud f. focuses rays of light vestibular g. sorts out sound waves apparatus h. helps brain assess heat, free nerve ending pressure, pain
Visit CengageNOW for additional questions.
3. The strength of Earth’s magnetic field and its angle relative to the surface vary with latitude. Diverse species sense these differences and use them as cues for assessing their location and direction of movement. Behavioral experiments have shown that sea turtles, salamanders, and spiny lobsters use information from Earth’s magnetic field during their migrations. Whales and some burrowing rodents also seem to have a magnetic sense. Evidence about humans is contradictory. Suggest an experiment to test whether humans can detect a magnetic field. 4. After a leg injury, pain makes a person avoid putting too much weight on the affected leg. An injured insect shows no such shielding response and does not make natural pain-relieving chemicals. Is this sufficient evidence to conclude that insects do not have a sense of pain? CHAPTER 34
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35
Endocrine Control IMPACTS, ISSUES
Hormones in the Balance
Atrazine has been used widely as an herbicide for more than
agency regulates chemical applications in agriculture. It
forty years. Each year in the United States, about 76 million
called for further study of atrazine’s effects on amphibians
pounds are sprayed, mostly to kill weeds in cornfields. From
and is encouraging farmers to minimize atrazine-laden runoff
there, atrazine gets into soil and water. Atrazine molecules
from their fields.
break down within a year but they still turn up in ponds, wells,
Numerous hormone disruptors infiltrate aquatic habitats.
groundwater, and rain. Do they have bad effects? Tyrone
For instance, the estrogens in birth control pills are excreted
Hayes, a University of California biologist, thinks so. His data
in urine and cannot be removed by standard wastewater
suggest that atrazine is an endocrine disruptor: a synthetic
treatments. In streams or rivers, estrogen-tainted water
compound that alters the action of natural hormones and
causes male fish to develop female traits.
adversely affects health and development (Figure 35.1). Hayes studied atrazine’s effects on African clawed frogs
An excess of estrogen-like chemicals may lower sperm counts. Estrogen is a sex hormone. Both men and women
(Xenopus laevis) and leopard frogs (Rana pipiens). He found
produce it and have receptors for it, although females make
that exposing male tadpoles to atrazine in the laboratory
much more. In males, estrogen docks at receptors on target
caused some to develop both female and male reproductive
cells in reproductive organs and helps sperm to mature.
organs. This effect occurred even at atrazine levels far below
Other synthetic chemicals, including kepone and DDT, bind
those allowed in drinking water.
to estrogen receptors, thus blocking estrogen’s actions,
Does atrazine have similar effects in the wild? To find out, Hayes collected leopard frogs from ponds and ditches
including its role in sperm maturation. Both chemicals are now banned in the United States.
across the Midwest. Male frogs from every contaminated
This chapter focuses on the hormones—their sources,
pond had abnormal sex organs. In the pond with the most
targets, effects, and interactions. All vertebrates have similar
atrazine, 92 percent of males had ovary tissue.
hormone-secreting glands and systems. Keep this point in
Other scientists have also reported that atrazine causes or
mind when you think about the endocrine disruptors. What
contributes to frog deformities. The Environmental Protection
you learn in this chapter will help you evaluate the costs and
Agency found the data intriguing. Among other tasks, this
benefits of synthetic chemicals that affect hormone action.
See the video! Figure 35.1 Benefits and costs of herbicide applications. Left, atrazine can keep cornfields nearly weed-free; no need for constant tilling that causes soil erosion. Tyrone Hayes (right) suspects that the chemical scrambles amphibian hormonal signals.
Links to Earlier Concepts
Key Concepts Signaling mechanisms
This chapter continues the story of cell signaling that began in Section 27.6. You will see many examples of feedback mechanisms (27.3). We will also revisit gap junctions (32.1) and glandular epithelium (32.2).
Knowing the properties of steroids (3.4), proteins (3.5), and the function of the plasma membrane (5.4) will help you understand how different types of hormones interact with cells.
The nervous and endocrine systems work together. You will hear again about action potentials (33.3), synapses (33.5), sympathetic neurons (33.8), the anatomy of the brain (33.10), and visual processing (34.9).
You will see how hormones affect metabolism of glucose (8.7), gamete formation (10.5), and molting (25.11).
Genetics concepts relevant to this chapter include gene duplications (12.5), gene expression (14.1), the role of promoters (14.2), introns (14.3), and techniques of genetic engineering (16.6).
Hormones and other signaling molecules function in communication among body cells. A hormone travels through the blood and acts on any cell that has receptors for it. The receptor may be at a target cell’s surface or inside the cell. Sections 35.1, 35.2
A master integrating center In vertebrates, the hypothalamus and pituitary gland are connected structurally and functionally. Together, they coordinate activities of many other glands. Pituitary hormones affect growth, reproductive functions, and composition of extracellular fluid. Sections 35.3, 35.4
Other hormone sources Negative feedback loops to the hypothalamus and pituitary control secretions from many glands. Signals from the nervous system and internal solute concentrations also influence hormone secretion. Sections 35.5–35.12
Invertebrate hormones Hormones control molting and other events in invertebrate life cycles. Vertebrate hormones and receptors for them first evolved in ancestral lineages of invertebrates. Section 35.13
How would you vote? Some widely used agricultural chemicals may disrupt hormone action in untargeted species. Should potentially harmful chemicals be kept on the market while researchers investigate them? See CengageNOW for details, then vote online.
597
35.1
Introducing the Vertebrate Endocrine System Animal cells communicate with one another by way of a variety of short-range and long-range chemical signals.
Links to Gap junctions 32.1, Glandular epithelium 32.2, Synapses 33.5
Intercellular Signaling in Animals In all animals, cells constantly signal one another in response to changes in the internal and external environments. Receiving such signals can influence a cell’s metabolic activity, division, or gene expression. Gap junctions allow signals to move directly from the cytoplasm of one cell to that of an adjacent cell (Section 32.1). Other cell–cell communication involves signaling molecules that are secreted into interstitial fluid (the fluid between cells). These molecules exert effects only when they bind to a receptor on or inside another cell. We refer to a cell that has receptors that bind and respond to a specific signaling molecule as a “target” of that molecule. Some secreted signaling molecules diffuse a short distance through interstitial fluid and bind to nearby cells. For example, neurons secrete signaling molecules called neurotransmitters into the synaptic cleft that separates them from a target cell. Neurotransmitter diffuses the short distance across the cleft and binds to the target (Section 33.5). Only neurons release neurotransmitters, but many cells secrete local signaling molecules that affect their neighbors. Prostaglandins are one type of local signal. When released by injured cells, they activate pain receptors and increase local blood flow. The enhanced blood flow delivers more infection-fighting proteins and white blood cells to the injured region. Animal hormones are longer-range communication molecules. After being secreted into interstitial fluid, they enter capillaries and are distributed throughout the body. Compared to neurotransmitters or local signaling molecules, hormones last longer, travel farther, and exert their effects on a greater number of cells. Some animals produce intercellular communication signals called pheromones that diffuse through water or air and bind to target cells in other individuals. Pheromones help integrate social behavior. We discuss them in Chapter 44, in the context of social behavior. For the rest of this chapter, our focus is hormones.
Overview of the Endocrine System The word “hormone” dates back to the early 1900s. Physiologists W. Bayliss and E. Starling were trying to determine what triggers the secretion of pancreatic 598 UNIT VI
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juices when food travels through a dog’s gut. As they knew, acids mix with food in the stomach. Arrival of the acidic mixture inside the small intestine triggers pancreatic secretions that reduce the acidity. Was the nervous system stimulating this pancreatic response, or was some other signaling mechanism at work? To find an answer, Bayliss and Starling blocked the nerves—but not blood vessels—to the small intestine of a laboratory animal. The pancreas still responded when acidic food from the stomach entered the small intestine. The pancreas even responded to extracts of cells from the intestinal lining, which is a glandular epithelium (Section 32.2). Apparently, some substance produced by glandular cells signaled the pancreas to start its secretions. That substance is now called secretin. Identifying its mode of action supported a hypothesis that dated back centuries: The blood carries internal secretions that influence the activities of the body’s organs. Starling coined the term “hormone” for glandular secretions (the Greek word hormon means to set in motion). Later on, researchers identified many other hormones and their sources. Glands and other hormone sources are collectively referred to as an animal’s endocrine system. Figure 35.2 surveys major sources of hormones in the human endocrine system.
Nervous–Endocrine Interactions The endocrine system and nervous system are closely linked. Both neurons and endocrine cells are derived from an embryo’s ectodermal layer. Both respond to the hypothalamus, a command center in the forebrain (Section 33.10). Most organs receive and respond to both nervous signals and hormones. Hormones influence the development of the brain, both before and after birth. Hormones can also affect nervous processes such as sleep/wake cycles, emotion, mood, and memory. Conversely, the nervous system affects hormone secretion. For example, in a stressful situation, nervous signals call for increased secretion of some hormones and decreased secretion of others.
Take-Home Message How do cells of an animal body communicate with one another? Animals cells communicate through gap junctions and by release of molecules that bind to receptors in or on other cells. Neurotransmitters and local signaling molecules disperse by diffusion and affect only nearby cells. Hormones enter the blood and are distributed throughout the body, so they have wider reaching effects.
hypothalamus
closer view of the hypothalamus and pituitary gland
Hypothalamus Makes and secretes releasers and inhibitors, hormones that act in the anterior lobe of the pituitary. Also makes antidiuretic hormone and oxytocin, which are stored in and released from the posterior lobe of the pituitary.
pituitary gland
Pineal gland Makes and secretes melatonin (affects sleep/wake cycles, onset of puberty).
Pituitary gland Anterior lobe makes and secretes ACTH, TSH, LH, FSH (stimulate secretion by other endocrine glands), prolactin (acts on mammary glands) and growth hormone (affects overall growth).
Thyroid gland Makes and secretes thyroid hormone (metabolic and developmental effects) and calcitonin (lowers blood calcium).
Posterior lobe secretes antidiuretic hormone (acts on kidneys) and oxytocin (acts on uterus and mammary glands). Both are made in hypothalamus.
Parathyroid glands (four) Make and secrete parathyroid hormone (raises blood calcium level).
Thymus gland Makes and secretes thymosins (act in maturation of T cells, a type of white blood cell).
Adrenal glands (one pair) Adrenal cortex makes and secretes cortisol (affects metabolism, immune response), aldosterone (acts in kidneys), small amount of sex hormones. Adrenal medulla makes and secretes norepinephrine and epinephrine, which prepare body for exciting or dangerous situations.
Pancreas Makes and secretes insulin (lowers blood glucose level) and glucagon (raises blood glucose level).
Ovaries (one pair of female gonads) Make and secrete progesterone and estrogens (affect primary sex organs and influence secondary sexual traits).
Figure 35.2 Animated Main components of the human endocrine system and the effects of their secretions. Hormone-secreting cells are also present in the glandular epithelia of the stomach, small intestine, liver, heart, kidneys, adipose tissue, skin, placenta, and other organs.
Testes (one pair of male gonads) Make and secrete testosterone and other androgens (affect primary sex organs and influence secondary sexual traits).
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35.2
The Nature of Hormone Action For a hormone to have an effect, it must bind to receptors on or inside a target cell. Links to Steroids 3.4, Proteins 3.5, Cell membranes 5.4, Sex determination 12.1, Promoters 14.2, Cell signaling 27.6
From Signal Reception to Response Cell communication involves three steps (Section 27.6). A signal activates a target cell receptor, the signal is transduced (changed into form that affects target cell behavior), and the cell makes a response: Signal Reception
Signal Transduction
Cellular Response
Enzymes make hormones from a variety of sources. Steroid hormones are derived from cholesterol. Amine hormones are modified amino acids. Peptide hormones are short chains of amino acids; protein hormones are longer chains. Table 35.1 lists a few examples of each. Hormones initiate responses in different ways. In all cases, binding to a receptor is reversible and the effect of the hormone declines over time. The decline occurs as the body breaks the hormones down so they no longer bind to receptors and elicit a response. Intracellular Receptors Steroid hormones are made from cholesterol and, like other lipids, they easily diffuse across a plasma membrane. Once inside a cell, steroid hormones form a hormone–receptor complex by binding to a receptor in the cytoplasm or nucleus. Most often, this hormone–receptor complex binds to and activates a promoter (Section 14.2). Activation of the promoter allows binding of RNA polymerase, which then transcribes an adjacent gene or genes. Transcription and translation produce a protein product, such as an enzyme, that carries out the target cell’s response to the signal. Figure 35.3a is a simple illustration of this type of steroid hormone action.
Table 35.1 Steroids
Categories and Examples of Hormones Testosterone and other androgens, estrogens, progesterone, aldosterone, cortisol
Amines
Melatonin, epinephrine, thyroid hormone
Peptides
Glucagon, oxytocin, antidiuretic hormone, calcitonin, parathyroid hormone
Proteins
Growth hormone, insulin, prolactin, follicle-stimulating hormone, luteinizing hormone
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Receptors at the Plasma Membrane Most amine hor-
mones, and all peptide or protein hormones, are too big and polar to diffuse across a membrane. They bind to receptors that span a target cell’s plasma membrane. Often, this binding activates an enzyme that converts ATP to cAMP (cyclic adenosine monophosphate). The cyclic AMP then functions as a second messenger: a molecule that forms inside a cell in response to an external signal and affects that cell’s activity. For example, when there is too little glucose in the blood, certain cells in the pancreas secrete the peptide hormone glucagon. When glucagon binds to receptors in the plasma membrane of target cells, it causes formation of cAMP inside them (Figure 35.3b). The cAMP activates an enzyme that activates a different enzyme, setting into motion a cascade of reactions. The last enzyme activated catalyzes breakdown of glycogen into glucose and thus raises the blood glucose level. Some cells have receptors for steroid hormones at their plasma membrane. Binding of a steroid hormone to such a receptor does not influence gene expression. Instead, it triggers a faster response by way of a second messenger or by affecting the membrane. For example, when the steroid hormone aldosterone binds to receptors at the surface of kidney cells, the membrane of these cells becomes more permeable to sodium ions.
Receptor Function and Diversity A cell can only respond to a hormone for which it has appropriate and functional receptors. All hormone receptors are proteins and gene mutations can make them less efficient or even nonfunctional. In this case, even though the hormone that targets the mutated receptor is present in normal amounts, the hormone will have a lesser or no effect. For example, typical male genitals will not form in an XY embryo without testosterone, one of the steroid hormones (Section 12.1). XY individuals who have androgen insensitivity syndrome secrete testosterone, but a mutation alters their receptors for it. Without functional receptors, it is as if testosterone is not present. As a result, the embryo forms testes, but they do not descend into the scrotum, and the genitals appear female. Such individuals are often raised as females, as discussed in more detail in Chapter 42. Variations in receptor structure also affect responses to hormones. Different tissues often have receptor proteins that respond in different ways to binding of the same hormone. For example, in Chapter 41, you will learn how ADH (antidiuretic hormone) from the posterior lobe of the pituitary acts on kidney cells and
Step 1 A peptide hormone molecule, glucagon, diffuses from blood into interstitial fluid bathing the plasma membrane of a liver cell.
Step 1 A steroid hormone molecule is moved from blood into interstitial fluid bathing a target cell.
Step 2 Being lipid soluble, the hormone easily diffuses across the cell’s plasma membrane.
unoccupied glucagon receptor at target cell’s plasma membrane
Step 3 The hormone diffuses through the cytoplasm and nuclear envelope. It binds with its receptor in the nucleus.
cyclic AMP
+ Pi
ATP Step 2 Glucagon binds with a receptor. Binding activates an enzyme that catalyzes the formation of cyclic AMP from ATP inside the cell.
Step 3 Cyclic AMP activates another enzyme in the cell.
receptor
gene product
hormone– receptor complex
Step 4 The hormone– receptor complex triggers transcription of a specific gene.
Step 4 The enzyme activated by cyclic AMP activates another enzyme, which in turn activates another kind that catalyzes the breakdown of glycogen to its glucose monomers.
B
A
Figure 35.3 Animated (a) Typical steroid hormone action inside a target cell. (b) Typical peptide hormone action at the plasma membrane. Cyclic AMP, which serves as the second messenger, relays a signal from a plasma membrane receptor into the cell.
helps maintain solute concentrations in the internal environment. ADH is sometimes referred to as vasopressin, because it also binds to receptors in the wall of blood vessels and causes these vessels to narrow. In many mammals, ADH helps maintain blood pressure. ADH also binds to brain cells and influences sexual and social behavior, as we will discuss in Section 44.1. This diversity of responses to a single hormone is an outcome of variations in ADH receptors. In each kind of cell, a different kind of receptor summons up a different cellular response.
Step 5 The enzyme activated by cyclic AMP also inhibits glycogen synthesis.
Figure It Out: Where does the second messenger form after
glucagon binds to a cell?
Answer: In the cytoplasm
Step 5 The resulting mRNA moves into the cytoplasm and is transcribed into a protein.
Take-Home Message How do hormones exert their effects on target cells?
Hormones exert their effects by binding to protein receptors, either inside a cell or at the plasma membrane.
Most steroid hormones bind to a promoter inside the nucleus and alter the expression of specific genes.
Peptide and protein hormones usually bind to a receptor at the plasma membrane. They trigger formation of a second messenger, a molecule that relays a signal into the cell. Variations in receptor structure affect how a cell responds to a hormone.
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35.3
The Hypothalamus and Pituitary Gland The hypothalamus and pituitary gland deep inside the brain interact as a central command center. Links to Feedback controls 27.3, Action potentials 33.3, Human brain 33.10, Exocrine glands 32.2
The hypothalamus is the main center for control of the internal environment. It lies deep inside the forebrain and connects, structurally and functionally, with the pituitary gland (Figure 35.4). In humans, this gland is no bigger than a pea. Its posterior lobe secretes hormones made in the hypothalamus. Its anterior lobe synthesizes its own hormones. Table 35.2 summarizes the hormones released from the pituitary gland.
hypothalamus
posterior lobe of pituitary
anterior lobe of pituitary
Figure 35.4 Location of the hypothalamus and pituitary gland. The two lobes of the pituitary (anterior and posterior) release different hormones.
Table 35.2
The hypothalamus signals the pituitary by way of secretory neurons that make hormones, rather than neurotransmitters. These neurons have their cell body in the hypothalamus. Axons of some of these neurons extend into the pituitary’s posterior lobe. Axons from others end in the stalk just above the pituitary.
Posterior Pituitary Function Antidiuretic hormone and oxytocin are hormones produced in the cell bodies of secretory neurons of the hypothalamus (Figure 35.5a). These hormones move through axons to axon terminals inside the posterior pituitary (Figure 35.5b). Arrival of an action potential (Section 33.3) at the axon terminals causes these terminals to release hormone. The hormone diffuses into capillaries (small blood vessels) inside the posterior pituitary (Figure 35.5c). From here, blood distributes the hormone throughout the body, where it exerts its effect on target cells (Figure 35.5d). Antidiuretic hormone (ADH) affects certain kidney cells. The hormone causes these cells to reabsorb more water, thus making the urine more concentrated. Oxytocin (OT) triggers muscle contractions during childbirth. It also makes milk move into the ducts of mammary glands when a female is nursing her young, and it affects social behavior in some species.
Primary Actions of Hormones Released From the Human Pituitary Gland
Pituitary Lobe
Posterior Nervous tissue (extension of hypothalamus)
Anterior Glandular tissue, mostly
602 UNIT VI
Secretions
Designation
Main Targets
Primary Actions
Antidiuretic hormone (vasopressin)
ADH
Kidneys
Induces water conservation as required to maintain extracellular fluid volume and solute concentrations
Oxytocin
OT
Mammary glands Uterus
Induces milk movement into secretory ducts Induces uterine contractions during childbirth
Adrenocorticotropic hormone
ACTH
Adrenal glands
Stimulates release of cortisol, an adrenal steroid hormone
Thyroid-stimulating hormone
TSH
Thyroid gland
Stimulates release of thyroid hormones
Follicle-stimulating hormone
FSH
Ovaries, testes
In females, stimulates estrogen secretion, egg maturation; in males, helps stimulate sperm formation
Luteinizing hormone
LH
Ovaries, testes
In females, stimulates progesterone secretion, ovulation, corpus luteum formation; in males, stimulates testosterone secretion, sperm release
Prolactin
PRL
Mammary glands
Stimulates and sustains milk production
Growth hormone (somatotropin)
GH
Most cells
Promotes growth in young; induces protein synthesis, cell division; roles in glucose, protein metabolism in adults
HOW ANIMALS WORK
A Cell bodies of secretory neurons in hypothalamus synthesize inhibitors or releasers that are secreted into the stalk that connects to the pituitary.
A Cell bodies of secretory neurons in hypothalamus synthesize ADH or oxytocin. B The ADH or oxytocin moves downward inside the axons of the secretory neurons and accumulates in the axon terminals. C Action potentials trigger the release of these hormones, which enter blood capillaries in the posterior lobe of the pituitary.
D Blood vessels carry hormones to the general circulation.
B The inhibitors or releasers picked up by capillaries in the stalk get carried in blood to the anterior pituitary.
D When encouraged by a releaser, anterior pituitary cells secrete hormone that enters blood vessels that lead into the general circulation.
C The inhibitors or releasers diffuse out of capillaries in the anterior pituitary and bind to their target cells.
Figure 35.5 Animated Interactions between the pituitary gland’s posterior lobe and the hypothalamus.
Figure 35.6 Animated Interactions between the pituitary gland’s anterior lobe and the hypothalamus.
Anterior Pituitary Function
Growth hormone (GH) has targets in most tissues. It triggers secretions of signals that promote growth of bone and soft tissues in the young. It also influences metabolism in adults.
The anterior pituitary produces hormones of its own, but hormones from the hypothalamus control their secretion. Most hypothalamic hormones that act on the anterior pituitary are releasers; they encourage secretion of hormones by target cells. Hypothalamic inhibitors call for a reduction in target cell secretions. Hypothalamic releasers and inhibitors are secreted into the stalk that connects the hypothalamus to the pituitary (Figure 35.6a). They diffuse into blood and are carried to the anterior lobe of the pituitary (Figure 35.6b). Here, they diffuse out of capillaries and bind to target cells (Figure 35.6c). When stimulated by a releaser, the target cell releases an anterior pituitary hormone into the blood (Figure 35.6d). The target cells of some anterior pituitary hormones are inside other glands: Adrenocorticotropic hormone (ACTH) stimulates the release of hormones by adrenal glands. Thyroid-stimulating hormone (TSH) regulates the secretion of thyroid hormone by the thyroid gland. Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) affect sex hormone secretion and production of gametes by gonads—a male’s testes or a female’s ovaries. Prolactin (PRL) targets the mammary glands, which are exocrine glands (Section 32.2). It stimulates and sustains milk production after childbirth.
Feedback Controls of Hormone Secretion The hypothalamus and pituitary are involved in many feedback controls. With positive feedback mechanisms, a stimulus causes a response, such as hormone secretion, that increases the intensity of the stimulus. For example, Section 27.3 described how the stretching of muscles during childbirth causes oxytocin secretion, which causes more stretching, and so on. Negative feedback mechanisms are more common. In this case, a stimulus elicits a response that decreases the stimulus. Several examples of negative feedback mechanisms that involve the hypothalamus and pituitary gland are described later in this chapter.
Take-Home Message How do the hypothalamus and pituitary gland interact?
Some secretory neurons of the hypothalamus make hormones (ADH, OT) that move through axons into the posterior pituitary, which releases them.
Other hypothalamic neurons produce releasers and inhibitors that are carried by the blood into the anterior pituitary. These hormones regulate the secretion of anterior pituitary hormones (ACTH, TSH, LH, FSH, PRL, and GH).
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35.4
Growth Hormone Function and Disorders Disturbances of growth hormone production or function can cause excessive or reduced growth.
Link to Genetic engineering 16.6
Growth hormone (GH) secreted by the anterior pituitary affects target cells throughout the body. Among other effects, GH calls for production of cartilage and bone and increases muscle mass. Normally, GH production surges during teenage years, causing a growth spurt. Level of the hormone then declines with age. Excessive secretion of GH during childhood causes gigantism. Affected people have a normally proportioned body, but are unusually large (Figure 35.7a). Excess production of GH during adulthood causes acromegaly. Bones can no longer lengthen, and instead become thicker. The hands, feet, and facial bones are most often visibly affected (Figure 35.7b). The Greek word acro means extremities, and megas means large. Both gigantism and acromegaly usually arise as the result of a benign (noncancerous) pituitary tumor. Pituitary dwarfism occurs when the body produces too little GH or receptors do not respond to it properly during childhood. Affected individuals are short but normally proportioned (Figure 35.7c). Pituitary dwarfism can be inherited, or it can result from a pituitary tumor or injury.
Figure 35.7 Examples of effects of disrupted growth hormone function.
Human growth hormone can now be made through genetic engineering (Section 16.6). Injections of recombinant human growth hormone (rhGH) increase the growth rate of children who have a naturally low GH level. However, such treatment is expensive ($10,000 to $20,000 a year) and controversial. Some people object to the idea of treating short stature as a defect to be cured. Injections of rhGH are also used to treat adults who have a low GH level because of pituitary or hypothalamic tumors or injury. Injections that restore normal GH level can help affected individuals maintain bone and muscle mass, while reducing body fat. Injections of rhGH have also been touted by some as a way to slow normal aging or to boost athletic performance. However, such uses are not approved by regulatory agencies, have not been shown effective in clinical trials, and can have negative side effects, including increased risk of high blood pressure and diabetes. Take-Home Message What are the effects of too much or too little growth hormone? Excessive growth hormone causes faster than normal bone growth. When the excess occurs during childhood, the result is gigantism. In adults, the result is acromegaly.
A deficiency of GH during childhood can cause dwarfism.
age 16
(a) Standing 6 feet 5 inches tall, this 12-year-old boy with pituitary gigantism towers over his mother. (b) A woman before and after she became affected by acromegaly. Notice how her chin elongated. (c) Dr. Hiralal Maheshwari, right, with two men from a village in Pakistan where a heritable form of dwarfism is common. The men of the village average 130 centimeters (a little over 4 feet) tall. Dr. Maheshwari found that these men make less than the typical amount of GH because their pituitary gland does not respond to the hypothalamic releaser that normally stimulates GH secretion.
604 UNIT VI
age 52
a
HOW ANIMALS WORK
b
c
35.5
Sources and Effects of Other Vertebrate Hormones
A cell in a vertebrate body is a target for a diverse array of hormones from endocrine glands and secretory cells.
The next few sections of this chapter describe effects of the main vertebrate hormones that are released by endocrine glands other than the pituitary. Table 35.3 provides an overview of this information. In addition to major endocrine glands, vertebrates have hormone-secreting cells in some organs. As noted earlier, cells of the small intestine make secretin, which acts on the pancreas. Parts of the gut also secrete other hormones that affect appetite and digestion. In addition, adipose (fat) tissue makes leptin, a hormone that acts in the brain and suppresses appetite. When oxygen level in blood falls, kidneys secrete erythropoietin, a hormone that stimulates maturation and production of oxygen-transporting red blood cells. Even the heart makes a hormone: atrial natriuretic peptide. It stimulates water and salt excretion by kidneys.
Table 35.3
As you learn about the effects of specific hormones, keep in mind that cells in most tissues have receptors for more than one hormone. The response called up by one hormone may oppose or reinforce that of another. For example, every skeletal muscle fiber has receptors for glucagon, insulin, cortisol, epinephrine, estrogen, testosterone, growth hormone, somatostatin, and thyroid hormone, as well as others. Thus, blood levels of all of these hormones affect the muscles.
Take-Home Message What are the sources and effects of vertebrate hormones? In addition to the pituitary gland and hypothalamus, endocrine glands and endocrine cells secrete hormones. The gut, kidneys, and heart are among the organs that are not considered glands, but do include hormonesecreting cells.
Most cells have receptors for multiple hormones, and the effect of one hormone can be enhanced or opposed by that of another.
Sources and Actions of Vertebrate Hormones Discussed in Sections 35.6 to 35.12
Source
Examples of Secretion(s)
Thyroid
Thyroid hormone
Parathyroids Pancreatic islets
Adrenal cortex
Adrenal medulla
Main Target(s)
Primary Actions
Most cells
Regulates metabolism; has roles in growth, development
Calcitonin
Bone
Lowers calcium level in blood
Parathyroid hormone
Bone, kidney
Elevates calcium level in blood
Insulin
Liver, muscle, adipose tissue
Promotes cell uptake of glucose; thus lowers glucose level in blood
Glucagon
Liver
Promotes glycogen breakdown; raises glucose level in blood
Somatostatin
Insulin-secreting cells
Inhibits digestion of nutrients, hence their absorption from gut
Glucocorticoids (including cortisol) Mineralocorticoids (including aldosterone)
Most cells
Promotes breakdown of glycogen, fats, and proteins as energy sources; thus help raise blood level of glucose Promotes sodium reabsorption (sodium conservation); help control the body’s salt–water balance
Kidney
Epinephrine (adrenaline)
Liver, muscle, adipose tissue
Raises blood level of sugar, fatty acids; increases heart rate and force of contraction
Norepinephrine
Smooth muscle of blood vessels
Promotes constriction or dilation of certain blood vessels; thus affects distribution of blood volume to different body regions
Testes (in males)
Androgens (including testosterone)
General
Required in sperm formation; development of genitals; maintenance of sexual traits; growth, development
Ovaries (in females)
Estrogens
General
Required for egg maturation and release; preparation of uterine lining for pregnancy and its maintenance in pregnancy; genital development; maintenance of sexual traits; growth, development
Progesterone
Uterus, breasts
Prepares, maintains uterine lining for pregnancy; stimulates development of breast tissues
Pineal gland
Melatonin
Brain
Influences daily biorhythms, seasonal sexual activity
Thymus
Thymosins
T lymphocytes
Poorly understood regulatory effect on T lymphocytes
Gonads
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35.6
Thyroid and Parathyroid Glands The thyroid regulates metabolic rate, and the adjacent parathyroids regulate calcium levels.
Link to Feedback mechanisms 27.3
The Thyroid Gland The human thyroid gland is at the base of the neck, and attaches to the trachea (Figure 35.8). The gland secretes two iodine-containing molecules (triiodothyronine and thyroxine) that we refer to collectively as thyroid hormone. Thyroid hormone increases the metabolic activity of tissues throughout the body. The thyroid gland also secretes calcitonin, a hormone that causes deposition of calcium in the bones of growing children. Normal adults produce little calcitonin. The anterior pituitary gland and hypothalamus regulate thyroid hormone secretion by way of a negative
epiglottis thyroid cartilage (Adam’s apple)
pharynx
Thyroid Gland Parathyroid Glands
trachea (windpipe) anterior
posterior
Figure 35.8 Location of human thyroid and parathyroid glands.
STIMULUS
Blood level of thyroid hormone falls below a set point.
+
feedback loop. Figure 35.9 shows what happens when the level of thyroid hormone in the blood declines. In response to this decline, the hypothalamus secretes a releasing hormone (TRH) that acts in the anterior lobe of the pituitary. The releaser causes the pituitary to secrete thyroid-stimulating hormone (TSH). TSH in turn induces the thyroid gland to release thyroid hormone. As a result, the blood level of thyroid hormone rises back to its set point. Once that point is reached, the secretion of TRH and TSH slows. Thyroid hormone includes iodine, a nutrient that humans obtain from their food. Thus, too little iodine in the diet is one cause of hypothyroidism—a low level of thyroid hormone. A goiter, or enlarged thyroid, is often a symptom (Figure 35.10a). The thyroid enlarges because the feedback loop illustrated in Figure 35.9 is disrupted and the gland receives constant stimulation to increase its output. Use of iodized salt is an easy, inexpensive way to ensure adequate iodine intake, but such salt is not available everywhere. Hypothyroidism can cause developmental problems. If a mother lacks iodine during her pregnancy, or a child has a genetic defect that interferes with thyroid hormone production, the child’s nervous system may not form properly. A low level of thyroid hormone during infancy or early childhood also stunts growth and impairs mental ability. Hypothyroidism sometimes arises in adults as the result of an injury or an immune disorder that affects the thyroid or pituitary. Regardless of the cause, symptoms of insufficient thyroid hormone often include
RESPONSE
Hypothalamus
TRH
Anterior Pituitary
TSH
Rise of thyroid hormone level in blood inhibits the secretion of TRH and TSH.
Thyroid Gland
a
b
Thyroid hormone is secreted.
Figure 35.9 Negative feedback loop to the hypothalamus and the pituitary’s anterior lobe that governs thyroid hormone secretion.
606 UNIT VI
HOW ANIMALS WORK
Figure 35.10 (a) A goiter caused by a diet that includes too little iodine. (b) A child with rickets caused by a lack of vitamin D has characteristic bowed legs.
FOCUS ON THE ENVIRONMENT
35.7 weight gain, sluggishness, forgetfulness, depression, joint pain, weakness, and increased sensitivity to cold. The use of synthetic thyroid hormone can eliminate symptoms, but treatment must be continued for life. A goiter can also be a symptom of Graves’ disease. In this case, an immune malfunction causes the thyroid to produce an excess of thyroid hormone. The resulting hyperthyroidism, causes anxiety, insomnia, heat intolerance, protruding eyes, weight loss, and tremors. Drugs, surgery, or radiation can be used to reduce thyroid hormone level in the blood.
The Parathyroid Glands Four parathyroid glands, each about the size of a grain of rice, are located on the thyroid’s posterior surface (Figure 35.8). The glands release parathyroid hormone (PTH) in response to a decline in the level of calcium in blood. Calcium ions have roles in neuron signaling, blood clotting, muscle contraction, and other essential physiological processes. PTH targets bone cells and kidney cells. In bones, it induces specialized cells called osteoclasts to secrete bone-digesting enzymes. Calcium and other minerals released from the bone enter the blood. In the kidneys, PTH stimulates tubule cells to reabsorb more calcium. It also stimulates secretion of enzymes that activate vitamin D, transforming it to calcitriol. Calcitriol is a steroid hormone that encourages cells in the intestinal lining to absorb more calcium from food. A nutritional disorder known as rickets occurs in children who do not get enough vitamin D. Without adequate vitamin D, the child does not absorb much calcium, so formation of new bone slows. At the same time, low calcium in the blood triggers PTH secretion. As PTH rises, the child’s body breaks down existing bones. Bowed legs and deformities in pelvic bones are common symptoms of rickets (Figure 35.10b). Tumors and other conditions that cause excessive PTH secretion also weaken bone, and they increase risk of kidney stones, because calcium released from bone ends up in the kidney. Disorders that reduce PTH output lower blood calcium. The resulting seizures and unrelenting muscle contractions can be deadly. Take-Home Message What are the functions of the thyroid and parathyroid glands? The thyroid gland has roles in regulation of metabolism and in development. Iodine is required to make thyroid hormone. The parathyroid glands are the main regulators of blood calcium level.
Twisted Tadpoles
Impaired thyroid function in frogs is another indication of hormone disruptors in the environment.
A tadpole is an aquatic larva of a frog. It undergoes a major remodeling in body form—a metamorphosis—when it makes the transition to an adult. For instance, it sprouts legs, lungs replace its gills, and its tail disappears. A surge in thyroid hormone triggers these changes. A tadpole keeps growing if its thyroid tissue is removed, but it will never undergo metamorphosis or take on adult form. Some water pollutants may be the chemical equivalent of thyroid removal. For one study, investigators exposed embryos of African clawed frogs (X. laevis) to water drawn from lakes in Minnesota and Vermont. Half of the water samples came from lakes where deformity rates were low. The other half came from “hot spots,” places where the water has as many as twenty kinds of dissolved pesticides and where deformity rates are high. The embryos that were raised in hot-spot water often developed into tadpoles that had a bent spine and other abnormalities, as in Figure 35.11. Some tadpoles never did undergo metamorphosis and change into adult form. Control embryos raised in water from other lakes developed normally. To find out if something in the water was interfering with thyroid hormone, the researchers added thyroid hormone to hot-spot water. Embryos raised in this mix developed into tadpoles that had fewer deformities or none at all. This result suggested that something in the water impaired normal thyroid hormone action. Frogs are highly sensitive to disturbances in thyroid function, and thyroid disruptions are easy to detect. That is why toxicologists use laboratory frogs to test whether chemicals are thyroid disruptors. These scientists also use frogs to determine exactly how disruptive chemicals exert their effects. Among the chemicals under study are perchlorates, which are widely used in explosives, propellants, and batteries. Perchlorates can interfere with the metabolism of iodine. As little as 5 parts per billion in water may stop a frog’s forelimbs from developing.
Figure 35.11 Evidence that pollutants affect frog development. The uppermost Xenopus laevis tadpole in this photographic series was raised in water from a lake with few deformed frogs. Tadpoles below it developed in water taken from three “hot-spot” lakes with increasingly higher concentrations of dissolved chemical compounds. As later tests showed, supplemental thyroid hormone can lessen or eliminate hot-spot deformities.
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35.8
Pancreatic Hormones As you can see, glucagon and insulin have opposing effects on blood glucose level. Together, their actions keep blood glucose within the range that body cells can tolerate. When blood glucose level rises above a set point, alpha cells secrete less glucagon and beta cells secrete more insulin (Figure 35.12a–c). As glucose is taken up and stored inside cells, blood glucose declines (Figure 35.12d,e). In contrast, any decline in blood glucose below the set point turns up glucagon secretion and slows insulin secretion (Figure 35.12f–h). The resulting release of glucose from the liver causes blood glucose to rise (Figure 35.12i,j).
Two pancreatic hormones with opposing effects work together to regulate the level of sugar in the blood.
Link to Endocrine and exocrine glands 32.2
The pancreas is an organ that lies in the abdominal cavity, behind the stomach (Figure 35.12) and has both endocrine and exocrine functions. Its exocrine cells secrete digestive enzymes into the small intestine. Its endocrine cells are in clusters called pancreatic islets. Alpha cells of the pancreatic islets secrete the hormone glucagon. Glucagon targets cells in the liver and causes the activation of enzymes that break glycogen into glucose subunits. By its action, glucagon raises the level of glucose in blood. Beta cells of the islets secrete the hormone insulin. This hormone’s main targets are liver, fat, and skeletal muscle cells. Insulin stimulates muscle and fat cells to take up glucose. In all target cells, insulin activates enzymes that function in protein and fat synthesis, and it inhibits the enzymes that catalyze protein and fat breakdown. As a result of its actions, insulin lowers the level of glucose in the blood.
Take-Home Message How do the actions of pancreatic hormones help maintain the level of blood glucose within a range body cells can tolerate? Insulin is secreted in response to high blood glucose and it increases glucose uptake and storage by cells. Glucagon is secreted in response to low blood glucose and it increases breakdown of glycogen to glucose.
A Stimulus
F Stimulus
Increase in blood glucose
Decrease in blood glucose
stomach pancreas small intestine
PANCREAS
PANCREAS
Figure 35.12 Animated Above, location of the pancreas. Right, how cells that secrete insulin and glucagon react to shifts in the blood level of glucose. Insulin and glucagon work antagonistically to regulate glucose level, an example of homeostasis. (a) After a meal, glucose enters blood faster than cells can take it up. Its level in blood increases. (b,c) In the pancreas, the increase stops alpha cells from secreting glucagon and stimulates beta cells to secrete insulin. (d) In response to insulin, muscle and adipose cells take up and store glucose, and liver cells synthesize more glycogen. (e) The outcome? Insulin lowers the glucose blood level. (f) Between meals, the glucose level in blood declines. (g,h) This stimulates alpha cells to secrete glucagon and stops beta cells from secreting insulin. (i) In the liver, glucagon causes cells to break glycogen down into glucose, which enters the blood. (j) The outcome? Glucagon raises the amount of glucose in blood.
608 UNIT VI
HOW ANIMALS WORK
B alpha cells
C beta cells
X–
+
glucagon
insulin
LIVER
MUSCLE
G alpha cells
+ glucagon
FAT CELLS
D Body cells, especially those muscle and adipose tissue, take up and use more glucose.
H beta cells
X– insulin
LIVER
I
Cells in liver break down glycogen faster. The released glucose monomers enter blood.
Cells in skeletal muscle and liver store glucose in the form of glycogen.
E Response Decrease in blood glucose
J Response Increase in blood glucose
FOCUS ON HEALTH
35.9
Blood Sugar Disorders
Glucose is the main energy source for brain cells and the only one for red blood cells. Having too much or too little glucose in blood causes problems throughout the body.
products of fat breakdown, but when too many build up, the result is ketoacidosis. The altered acidity and solute levels can interfere with brain function. Extreme cases may lead to coma or death.
Diabetes mellitus is a metabolic disorder in which cells do not take up glucose as they should. As a result, sugar accumulates in blood and in urine. Complications develop throughout the body (Table 35.4). Excess sugar in the urine encourages growth of pathogenic bacteria, and it damages small blood vessels in the kidneys. Diabetes is the most common cause of permanent kidney failure. Uncontrolled diabetes also damages blood vessels and nerves elsewhere, especially in the arms, hands, legs, and feet. Diabetics account for more than 60 percent of lower limb amputations.
Type 2 Diabetes Type 2 diabetes is by far the most common form of the disorder. Insulin levels are normal or even high. However, target cells do not respond to the hormone as they should, and blood sugar levels remain elevated. Symptoms typically start to develop in middle age, when insulin production declines. Genetics also is a factor, but obesity increases the risk. Diet, exercise, and oral medications can control most cases of type 2 diabetes. However, if glucose levels are not lowered by these means, pancreatic beta cells receive continual stimulation. Eventually they falter, and insulin production declines. When that happens, a type 2 diabetic may require insulin injections. Worldwide, rates of type 2 diabetes are soaring. By one estimate, more than 150 million people are now affected. Western diets and sedentary life-styles are contributing factors. The prevention of diabetes and its complications is acknowledged to be among the most pressing public heath priorities around the world.
Type 1 Diabetes There are two main types of diabetes mellitus. Type 1 develops after the body has mounted an autoimmune response against its insulin-secreting beta cells. Certain white blood cells wrongly identify the cells as foreign (nonself) and destroy them. Environmental factors add to a genetic predisposition to the disorder. Symptoms usually start to appear during childhood and adolescence, which is why this metabolic disorder is also known as juvenile-onset diabetes. Individuals with type 1 diabetes require injections of insulin, and must monitor their blood sugar level carefully (Figure 35.13). Type 1 diabetes accounts for only 5 to 10 percent of all reported cases, but it is the most dangerous in the short term. Insulin discourages metabolism of fats and proteins, so too little insulin causes excessive fat and protein breakdown. Two outcomes are weight loss and accumulation of ketones in blood and urine. Ketones are normal acidic
Table 35.4
Hypoglycemia In hypoglycemia, the level of blood glucose falls low enough to disrupt normal body functions. Rare insulin-secreting tumors can cause it, but most cases occur after an insulin-dependent diabetic miscalculates and injects a bit too much insulin to balance food intake. The result is insulin shock. The brain stalls as its fuel source dwindles. Common symptoms are dizziness, confusion, and difficulty speaking. Insulin shock can be lethal, but an injection of glucagon quickly reverses the condition.
Some Complications of Diabetes
Eyes
Changes in lens shape and vision; damage to blood vessels in retina; blindness
Skin
Increased susceptibility to bacterial and fungal infections; patches of discoloration; thickening of skin on the back of hands
Digestive system
Gum disease; delayed stomach emptying that causes heartburn, nausea, vomiting
Kidneys
Increased risk of kidney disease and failure
Heart and blood vessels
Increased risk of heart attack, stroke, high blood pressure, and atherosclerosis
Hands and feet
Impaired sensations of pain; formation of calluses, foot ulcers; poor circulation in feet especially sometimes leads to tissue death that can only be treated by amputation
Figure 35.13 A diabetic checks his blood glucose by placing a blood sample into a glucometer. Compared with Caucasians, Hispanics and African Americans are about 1.5 times more likely to be diabetic. Native Americans and Asians are at even greater risk. Proper diet helps control blood sugar, even in type 1 diabetics.
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35.10 The Adrenal Glands Atop each kidney is an adrenal gland with two parts. Each part produces and releases different hormones. Links to Alternative energy sources 8.7, Sympathetic neurons 33.8
There are two adrenal glands; one perches above each kidney. (In Latin ad– means near, and renal refers to the kidney.) Each adrenal gland is about the size of a big grape. Its outer layer is the adrenal cortex and its inner portion is the adrenal medulla. The two parts of the gland are controlled by different mechanisms, and they secrete different hormones.
Hormonal Control of the Adrenal Cortex The adrenal cortex secretes three steroid hormones. One of these, aldosterone, controls sodium and water reabsorption in the kidneys. Chapter 41 explains its function in great detail. The adrenal cortex also produces and secretes small amounts of both male and female sex hormones, which we discuss in Section 35.12 and Chapter 42. For now, we focus on cortisol, an adrenal hormone that has wide-reaching effects on metabolism and immunity.
A negative feedback loop governs the cortisol level in blood (Figure 35.14). A decrease in cortisol triggers secretion of CRH (corticotropin-releasing hormone) by the hypothalamus. CRH then stimulates secretion of ACTH (adrenocorticotropic hormone). This anterior pituitary hormone causes the adrenal cortex to release cortisol. The blood level of cortisol keeps increasing until it reaches a set point. Then the hypothalamus and anterior pituitary slow their release of CRH and ACTH, and cortisol secretion also winds down. Cortisol has many effects. It induces liver cells to break down their store of glycogen, and it suppresses uptake of glucose by other cells. Cortisol also prompts adipose cells to degrade fats, and skeletal muscles to degrade proteins. Breakdown products of fats and proteins function as alternative energy sources (Section 8.7). Cortisol also suppresses immune responses. With injury, illness, or anxiety, the nervous system overrides the feedback loop, and cortisol in blood can soar. In the short term, this response helps get enough glucose to the brain when food supplies are likely to be low. Cortisol also suppresses inflammatory responses. As the next section explains, long-term stress and elevation of cortisol level can cause health problems.
Nervous Control of the Adrenal Medulla STIMULUS
+
A Blood level
RESPONSE
Hypothalamus
of cortisol falls below a set point.
B CRH
Anterior Pituitary adrenal cortex
ACTH adrenal medulla
Adrenal Cortex
D Hypothalamus and pituitary detect rise in blood level of cortisol and slow its secretion.
C Cortisol is secreted and has the following effects:
Cellular uptake of glucose from blood slows in many tissues, especially muscles (but not in the brain). Protein breakdown accelerates, especially in muscles. Some of the amino acids freed by this process get converted to glucose.
kidney
Fats in adipose tissue are degraded to fatty acids and enter blood as an alternative energy source, indirectly conserving glucose for the brain.
Figure 35.14 Animated Structure of the human adrenal gland. An adrenal gland rests on top of each kidney. The diagram shows a negative feedback loop that governs cortisol secretion.
610 UNIT VI
HOW ANIMALS WORK
The adrenal medulla contains specialized neurons of the sympathetic division (Section 33.8). Like other sympathetic neurons, those in the adrenal medulla release norepinephrine and epinephrine. However, in this case, the norepinephrine and epinephrine enter the blood and function as hormones, rather than acting as neurotransmitters at a synapse. Epinephrine and norepinephrine released into the blood have the same effect on a target organ as direct stimulation by a sympathetic nerve. Remember that sympathetic stimulation plays a role in the fight–flight response. Epinephrine and norepinephrine dilate the pupils, increase breathing, and make the heart beat faster. They prepare the body to deal with an exciting or dangerous situation.
Take-Home Message What is the function of the adrenal glands? The adrenal cortex secretes aldosterone, cortisol, and small amounts of sex hormones. Aldosterone affects urine concentration and cortisol affects metabolism and the stress response. The adrenal medulla releases epinephrine and norepinephrine, which prepare the body for excitement or danger.
35.11
Too Much or Too LIttle Cortisol
Short-term responses to stress help us function in hard times, but chronic stress is unhealthy.
Link to Memory 33.11
Chronic Stress and Elevated Cortisol Each summer, a troop of olive baboons (Papio anubis) on East Africa’s Serengeti plains has visitors. For more than twenty years, neurobiologist Robert Sapolsky and his Kenyan colleagues have been studying how these baboons interact and how a baboon’s social position influences its hormone levels and health. Remember, when the body is stressed, commands from the nervous system trigger secretion of cortisol, epinephrine, and norepinephrine. As these secretions find their targets, they help the body deal with the immediate threat by diverting resources from longerterm tasks. This stress response is highly adaptive for short bursts of activity, as when it diverts blood flow to muscles of an animal fleeing from a predator. Sometimes stress does not end. The baboons live in big troops with a clearly defined dominance hierarchy. Those on top of the hierarchy get first access to food, grooming, and sexual partners. Those at the bottom must relinquish resources to a higher ranking baboon or face attack (Figure 35.15). Not surprisingly, the lowranking baboons tend to have elevated cortisol levels. Physiological responses to chronic stress interfere with growth, the immune system, sexual function, and cardiovascular function. Chronically high cortisol levels also harm cells in the hippocampus, a brain region central to memory and learning (Section 33.11). We also see the impact of long-term elevated cortisol levels in humans affected by Cushing’s syndrome, or hypercortisolism. This rare metabolic disorder might be triggered by an adrenal gland tumor, oversecretion of ACTH by the anterior pituitary, or ongoing use of the drug cortisone. Doctors often prescribe cortisone to relieve chronic pain, inflammation, or other health problems. The body converts it to cortisol. The symptoms of hypercortisolism include a puffy, rounded “moon face” and increased fat deposition around the torso. Blood pressure and blood glucose become unusually high. White blood cell counts are low, so affected people are more prone to infections. Thin skin, decreased bone density, and muscle loss are common. Wounds may be slow to heal. Women’s menstrual cycles are erratic or nonexistent. Men may be impotent. Often, the hippocampus shrinks. Patients with the highest cortisol level also have the greatest reduction in the volume of the hippocampus, and the most impaired memory.
Figure 35.15 A dominant baboon (right) raising the stress level—and cortisol level—of a less dominant member of its troop.
Can status-related social stress affect human health? People who are low in a socioeconomic hierarchy do tend to have more health problems—obesity, hypertension, and diabetes—than those who are better off. These differences persist even after researchers factor out the obvious causes, such as variations in diet and access to health care. By one hypothesis, a heightened cortisol level caused by low social status may be one of the links between poverty and poor health.
Low Cortisol Level Tuberculosis and other infectious diseases can damage the adrenal glands, and slow or halt cortisol secretion. The result is Addison’s disease, or hypocortisolism. In developed countries, this hormonal disorder more often arises after autoimmune attacks on the adrenal glands. President John F. Kennedy had this form of the disorder. Symptoms often include fatigue, weakness, depression, weight loss, and darkening of the skin. If cortisol levels get too low, blood sugar and blood pressure can fall to life-threatening levels. Addison’s disease is treated with a synthetic form of cortisone.
Take-Home Message What are the effects of abnormal cortisol levels? High cortisol levels, produced by chronic stress or an endocrine disorder, impair growth, healing, sexual function, and memory. Blood pressure and blood sugar are higher than normal. With low cortisol levels, blood pressure and blood sugar fall. If they decline too far, the result can be life threatening.
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35.12 Other Endocrine Glands Outputs from the gonads, pineal gland, and thymus all change as an individual enters puberty.
Links to Gamete formation 10.5, Visual signals 34.9
The Gonads The gonads, or primary reproductive organs, produce gametes (eggs or sperm) as well as sex hormones. The gonads of male vertebrates are testes (singular, testis) and the main hormone they secrete is testosterone, the male sex hormone. The female gonads are the ovaries. They secrete mainly estrogens and progesterone, the female sex hormones. Figure 35.16 shows the location of the human gonads. Puberty is a post-embryonic stage of development when the reproductive organs and structures mature. At puberty, a female mammal’s ovaries increase their estrogen production, which causes breasts and other female secondary sexual traits to develop. Estrogens and progesterone control egg formation and ready the uterus for pregnancy. In males, a rise in testosterone output triggers the onset of sperm formation and the development of secondary sexual traits. The hypothalamus and anterior pituitary control the secretion of sex hormones (Figure 35.17). In both males and females, the hypothalamus produces GnRH (gonadotropin-releasing hormone). This releaser causes the anterior pituitary to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH cause the gonads to secrete sex hormones. Testes secrete mostly testosterone, but they also make a little bit of estrogen and progesterone. The estrogen is necessary for sperm formation. Similarly, a female’s ovaries make mostly estrogen and progesterone, but also a little testosterone. Hypothalamus The presence of testosterone contributes to libido—the desire for sex. GnRH We discuss the role of sex hormones in gamete formation, the menstrual cycle, Anterior Pituitary and development in detail in Chapter 42.
testis (where sperm originate)
ovary (where eggs develop)
Figure 35.16 Location of human gonads, which produce gametes and secrete sex hormones.
Melatonin may affect human gonads. A decline in the production of this hormone starts at puberty and may help trigger it. Some pineal gland disorders are known to accelerate or delay puberty. Melatonin also targets neurons that can lower body temperature and make us drowsy in dim light. The blood level of melatonin peaks in the middle of the night. Exposure to bright light sets a biological clock that controls sleeping versus arousal. Travelers who cross many time zones are advised to spend time in the sun after reaching a destination. Doing so helps them reset their biological clock and minimize jet lag. In winter, seasonal affective disorder, also called “winter blues,” causes some people to be depressed, to binge on carbohydrates, and to crave sleep. Bright artificial light in the morning decreases pineal gland activity and can improve mood.
The Thymus The thymus lies beneath the breastbone. It secretes thymosins, hormones that help the infection-fighting white blood cells called T cells mature. The thymus grows until puberty, when it is about the size of an orange. Then, the surge in sex hormones causes it to shrink, and its secretions decline. However, the thymus enhances immune function even in adults.
FSH, LH
The Pineal Gland Gonads
Sex hormones
Figure 35.17 Generalized diagram showing control of sex hormone secretion.
612 UNIT VI
Deep in the vertebrate brain is the pineal gland. This small, pine cone–shaped gland secretes melatonin, a hormone that serves as part of an internal timing mechanism, or biological clock. Melatonin secretion declines when the retina detects light and sends signals along the optic nerve to the brain (Section 34.9). HOW ANIMALS WORK
Take-Home Message What are the roles of the gonads, pineal gland, and thymus? A female’s ovaries or a male’s testes are gonads that make sex hormones as well as gametes. The pineal gland is inside the brain and produces melatonin, which influences sleep-wake cycles and onset of puberty. The thymus is in the chest and it secretes thymosins that are necessary for the maturation of white blood cells called T cells.
35.13 A Comparative Look at a Few Invertebrates Genes that encode hormone receptors and enzymes involved in hormone synthesis have evolved over time.
Links to Gene duplication 12.5, Introns 14.3, Molting 25.11
Evolution of Hormone Diversity We can trace the evolutionary roots of some vertebrate hormones and receptors back to signaling molecules in invertebrates. For example, receptors for the hormones FSH, LH, and TSH all have a similar structure. The genes that encode these receptors have a similar sequence and have introns (noncoding DNA) in the same places. The slightly different forms of receptor most likely evolved when a gene was duplicated, then copies mutated over time (Section 12.5). When did the ancestral gene arise? Sea anemones do not have an endocrine system, but they do have a receptor protein gene like that for FSH. This suggests that the ancestral receptor gene existed long ago in a common ancestor of sea anemones and vertebrates. Estrogen receptors may also have a long history. Sea slugs (Figure 35.18), a kind of mollusk, have receptors that are similar to vertebrate estrogen receptors.
Figure 35.18 The sea hare (Aplysia), a type of mollusk. Some receptors in its plasma membrane are similar to vertebrate receptors that bind the steroid hormone estrogen.
a
Hormones and Molting Other hormones are unique to invertebrates. For example, arthropods, which include crabs and insects, have a hardened external cuticle that they periodically shed as they grow (Section 25.11). Shedding of the old cuticle is called molting. A soft new cuticle forms beneath an old one before the animal molts. Although details vary among arthropod groups, molting is generally under the control of ecdysone, a steroid hormone. The arthropod molting gland produces and stores ecdysone, then releases it for distribution throughout the body when conditions favor molting. Hormonesecreting neurons inside the brain control ecdysone’s release. The neurons respond to internal signals and environmental cues, including light and temperature. Figure 35.19 is an example of the control steps in crabs and other crustaceans. In response to seasonal cues, secretion of a molt-inhibiting hormone declines and ecdysone secretion rises. Ecdysone causes changes in the animal’s structure and physiology. The existing cuticle separates from the epidermis and the muscles. Inner layers of the old cuticle break down. At the same time, cells of the epidermis secrete the new cuticle. The steps in molting differ a bit in insects, which do not have a molt-inhibiting hormone. Rather, stimulation of the insect brain sets in motion a cascade of signals that trigger the production of molt-inducing
Absence of suitable stimuli
Presence of suitable stimuli
X organ releases molt-inhibiting hormone (MIH)
Signals from brain inhibit release of MIH
MIH prevents Y organ from making ecdysone
Y organ makes and releases ecdysone
No molting
Molting b
c
Figure 35.19 Hormonal control of molting in crustaceans such as crabs. Two hormone-secreting organs play a role. The X organ is in the eye stalk. The Y organ is at the base of the crab’s antennae. (a) In the absence of environmental cues for molting, secretions from the X organ prevent molting. (b) When stimulated by proper environmental cues, the brain sends nervous signals that inhibit X organ activity. With the X organ suppressed, the Y organ releases the ecdysone that stimulates molting. (c) A newly molted blue crab with its old shell. The new shell remains soft for a about 12 hours, making it a “soft-shelled crab.” In this state, the crab is highly vulnerable to predators, including human seafood lovers.
ecdysone. Chemicals that mimic ecdysone or interfere with its function are used as insecticides. When such insecticides run off from fields and get into water, they can affect ecdysone-related responses in other arthropods, such as crayfish, crabs, or shrimps.
Take-Home Message What types of hormone systems do we see in invertebrates? We can trace the evolutionary roots of the vertebrate endocrine system in invertebrates. Cnidarians such as sea anemones, and mollusks such as sea slugs, have receptors that resemble those that bind vertebrate hormones.
Invertebrates also have hormones with no vertebrate counterparts. Hormones that control molting in arthropods are an example.
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IMPACTS, ISSUES REVISITED
Hormones in the Balance
Testosterone and estrogens have a very similar structure and enzymes can convert one to the other. The enzyme aromatase converts testosterone to estrogens. When human cells growing in culture are exposed to the herbicide atrazine, their aromatase activity rises, so more testosterone gets converted to estrogen. Atrazine may have the same effect in frogs, which would explain the altered sex organs first reported by Tyrone Hayes.
Summary Section 35.1 Hormones, neurotransmitters, local signaling molecules, and pheromones are chemicals that are secreted by one cell type and that adjust the behavior of other, target cells. Any cell is a target if it has receptors for a signaling molecule. All vertebrates have an endocrine system of secretory glands and cells. In most cases, the hormonal secretions travel through the bloodstream to nonadjacent targets.
Use the animation on CengageNOW to learn about the main sources of hormones in the human body.
Section 35.2 Some steroid hormones enter a target cell and bind to receptors inside it. Others bind to the cell’s plasma membrane and alter the membrane properties. The peptide and protein hormones bind to plasma membrane receptors. Binding may lead to formation of a second messenger, which relays a signal into the cell.
Use the animation on CengageNOW to compare the mechanisms of steroid and protein hormone action.
Sections 35.3, 35.4 The hypothalamus, a forebrain region, is structurally and functionally linked with the pituitary gland as a major center for homeostatic control. The posterior pituitary releases two hormones made by neurons of the hypothalamus. Antidiuretic hormone acts in kidneys to concentrate urine. Oxytocin acts on the uterus and milk ducts. Other hypothalamic neurons secrete releasers and inhibitors that encourage or slow the secretion of anterior pituitary hormones. The anterior pituitary produces several hormones that regulate other glands. Adrenocorticotropic hormone acts on the adrenal glands. Follicle-stimulating hormone and luteinizing hormone regulate the gonads. The thyroid is stimulated by thyroid-stimulating hormone. Mammary glands are stimulated by prolactin. The anterior pituitary also makes growth hormone, which affects cells throughout the body and stimulates bone growth. Gigantism, dwarfism, and acromegaly result from mutations that affect growth hormone function.
Use the animation on CengageNOW to study how the hypothalamus and pituitary interact.
Section 35.5 In addition to major endocrine glands, there are hormone-secreting cells in tissues and organs throughout the body. Most cells have receptors for, and are influenced by, many different hormones. 614 UNIT VI
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Sections 35.6, 35.7 A feedback loop to the anterior pituitary and hypothalamus governs the thyroid gland in the base of the neck. The thyroid affects metabolic rate and development. Iodine is required for thyroid function. Four parathyroid glands make a hormone that acts on bone and kidney cells and raises blood calcium level. Sections 35.8, 35.9 The pancreas in the abdominal cavity has exocrine and endocrine functions. Beta cells secrete insulin when blood glucose level is high. Insulin stimulates uptake of glucose by muscle and liver cells. When blood glucose is low, alpha cells secrete glucagon, which calls for glycogen breakdown and glucose release by the liver. The two hormones work in opposition to keep blood glucose levels within the optimal range. Diabetes occurs when the body does not make insulin or its cells do not respond to it.
Use the animation on CengageNOW to see how the actions of insulin and glucagon regulate blood sugar.
Sections 35.10, 35.11 There is an adrenal gland on each kidney. The adrenal cortex secretes aldosterone which targets the kidney, and cortisol, the stress hormone. Cortisol secretion is governed by a negative feedback loop to the anterior pituitary gland and hypothalamus. In times of stress, the nervous system overrides feedback controls. Norepinephrine and epinephrine released by neurons of the adrenal medulla influence organs as sympathetic stimulation does; they cause a fight–flight response.
Watch the animation on CengageNOW to see how cortisol levels are maintained by negative feedback.
Section 35.12 The gonads (ovaries or testes) secrete sex hormones. Ovaries secrete mostly estrogens and progesterone. Testes secrete mostly testosterone. Sex hormones control gamete formation and, in puberty, regulate the development of secondary sexual traits. Light suppresses secretion of melatonin by the pineal gland in the brain. Melatonin affects biological clocks— internal timing mechanisms. The thymus in the chest produces hormones that help some white blood cells (T cells) mature. Section 35.13 Some vertebrate hormone receptor proteins resemble similar receptor proteins in invertebrates. This suggests the receptors evolved in a common ancestor of both groups. The steroid hormone ecdysone affects molting in arthropods and has no vertebrate counterpart.
Data Analysis Exercise Contamination of water by agricultural chemicals affects the reproductive function of some animals. Are there effects on humans? Epidemiologist Shanna Swann and her colleagues studied sperm collected from men in four cities in the United States (Figure 35. 20). The men were partners of women who had become pregnant and were visiting a prenatal clinic, so all were fertile. Of the four cities, Columbia, Missouri, is located in the county with the most farmlands. New York City in New York is in an area with no agriculture. 1. In which cities did researchers record the highest and lowest sperm counts? 2. In which cities did samples show the highest and lowest sperm motility (ability to move)? 3. Aging, smoking, and sexually transmitted diseases adversely affect sperm. Could differences in any of these variables explain the regional differences in sperm count? 4. Do these data support the hypothesis that living near farmlands can adversely affect male reproductive function?
Self-Quiz
Answers in Appendix III
1. are signaling molecules that travel through the blood and affect distant cells in the same individual. a. Hormones d. Local signaling molecules b. Neurotransmitters e. both a and b c. Pheromones f. a through d 2. A is synthesized from cholesterol and can diffuse across the plasma membrane. a. steroid hormone c. peptide hormone b. pheromone d. all of the above 3. Match each pituitary hormone with its target. antidiuretic hormone a. gonads (ovaries, testes) oxytocin b. mammary glands, uterus luteinizing hormone c. kidneys growth hormone d. most body cells 4. Releasers secreted by the hypothalamus cause the secretion of hormones by the pituitary lobe. a. anterior b. posterior 5. In adults, too much a. growth hormone b. cortisol
can cause acromegaly. c. insulin d. melatonin
6. A diet lacking in iodine can cause a. rickets c. diabetes b. a goiter d. gigantism
.
7. Low blood calcium triggers secretion by a. adrenal glands c. ovaries b. parathyroid glands d. the thyroid gland 8.
.
lowers blood sugar levels; raises it. a. Glucagon; insulin b. Insulin; glucagon
9. The has endocrine and exocrine functions. a. hypothalamus c. pineal gland b. pancreas d. parathyroid gland 10. Secretion of suppresses immune responses. a. melatonin c. thyroid hormone b. antidiuretic hormone d. cortisol
Location of clinic Columbia, Missouri
Los Angeles, Minneapolis, New York, California Minnesota New York
Average age
30.7
29.8
32.2
36.1
Percent nonsmokers
79.5
70.5
85.8
81.6
Percent with history of STD
11.4
12.9
13.6
15.8
Sperm count (million/ml)
58.7
80.8
98.6
102.9
Percent motile sperm
48.2
54.5
52.1
56.4
Figure 35.20 Data from a study of sperm collected from men who were partners of pregnant women that visited prenatal health clinics in one of four cities. STD stands for sexually transmitted disease.
11. Exposure to bright light lowers blood levels. a. glucagon c. thyroid hormone b. melatonin d. parathyroid hormone 12. True or false? Some heart cells and kidney cells secrete hormones. 13. True or false? Only women make follicle-stimulating hormone (FSH); only men make luteinizing hormone (LH). 14. True or false? All hormones secreted by arthropods such as crabs and insects are also secreted by vertebrates. 15. Match the term listed at left with the most suitable description at right. adrenal medulla a. affected by day length thyroid gland b. a local signaling molecule posterior pituitary c. secretes hormones made gland in the hypothalamus pancreatic islets d. source of epinephrine pineal gland e. secrete insulin, glucagon prostaglandin f. hormones require iodine
Visit CengageNOW for additional questions.
Critical Thinking 1. A large study of nurses suggests that night shift work may raise the risk of breast cancer. Changes in melatonin level may contribute to the increased risk. There is evidence that this hormone can slow the rate of cancer cell division. Nurses who work night shifts tend to have lower melatonin levels than those working days. Why is secretion of this hormone especially likely to be reduced by night work? 2. Sex hormone secretion is governed by a negative feedback loop to the hypothalamus and pituitary, similar to that for thyroid hormone or cortisol. Because of this, a veterinarian can tell whether or not a female dog has been neutered with a blood test. Dogs that still have their ovaries have a lower blood level of luteinizing hormone (LH) than dogs that have been neutered. Explain why removing a dog’s ovaries would result in an elevated level of LH. CHAPTER 35
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36
Structural Support and Movement IMPACTS, ISSUES
Pumping Up Muscles
The male sex hormone testosterone has anabolic effects;
Administration announced that, in light of these side effects,
it encourages protein synthesis and thus increases muscle
it was banning the sale of andro. Even with all the negative
mass. That’s one reason why men, who naturally make a lot
publicity, some athletes continued to use anabolic steroids,
of testosterone, tend to be more muscular than women, who
risking both their health and reputation.
make far less (Figure 36.1). It is also why some body builders
Athletes also use approved nutritional supplements such
and athletes turn to anabolic steroids (synthetic derivatives
as creatine, which is a short chain of amino acids. The body
of testosterone), or to supplements that claim to raise natural
makes some creatine and obtains more from food. When
testosterone levels.
muscles must contract hard and fast, they normally turn first
For example, in the late 1990s, androstenedione, or “andro,” soared in popularity after a baseball player, Mark
to phosphorylated creatine as an instant energy source. Does creatine work? In some controlled studies, creatine
McGwire, said he had used it during his successful attempt
improved performance during brief, high-intensity exercise.
to break Major League Baseball’s single-season home-run
Nevertheless, excessive creatine intake puts a strain on the
record. Andro forms naturally in the body as an intermediate
kidneys, and it is too soon to know whether creatine supple-
in the synthesis of the sex hormone testosterone.
ments have any long-term side effects. Also, no regulatory
Does taking andro as a dietary supplement improve athletic performance? Results from the few controlled studies are mixed. Moreover, andro, like all anabolic steroids, has
agency checks to see how much creatine is actually present in any commercial product. With this chapter, we turn to the skeletal and muscular
side effects. It increases a man’s level of the female hormone
systems. What you learn here can help you evaluate how far
estrogen, which can also be formed from andro. Estrogen
both systems can and should be pushed in the pursuit of
has feminizing effects on males, including shrunken testicles,
enhanced performance.
formation of female-like breasts, and hair loss. Also, like all anabolic steroids, andro increases risk of liver damage and cardiovascular attack. In 2004, the U.S. Food and Drug
See the video! Figure 36.1 Left, a male with an abundance of skeletal muscle tissue, which has parallel rows of muscle fibers (above).
Links to Earlier Concepts
Key Concepts Invertebrate skeletons
This chapter elaborates on some of the animal traits and evolutionary trends you learned about in Chapters 25 and 26.
You will also build on your knowledge of connective (32.3) and muscle (32.4) tissues.
You will learn more about the X-linked disorder muscular dystrophy (12.4), and how bacterial endospores (21.6) can affect muscles.
You will see examples of active transport (5.4) and revisit the filaments involved in cell movement (4.13).
Nervous control of muscle (33.5) and the effects of some hormones (35.6) are also discussed again.
Contractile force exerted against a skeleton moves animal bodies. In many invertebrates a fluid-filled body cavity is a hydrostatic skeleton. Others have an exoskeleton of hard structures at the body surface. Still others have a hard internal skeleton, or endoskeleton. Section 36.1
Vertebrate skeletons Vertebrates have an endoskeleton of cartilage, bone, or both. Bones interact with muscles to move the body. They also protect and support organs, and store minerals. Blood cells form in some bones. A joint is a place where bones meet; there are several kinds. Sections 36.2–36.5
The muscle–bone partnership Skeletal muscles are bundles of muscle fibers that interact with bones and with one another. Some cause movements by working as pairs or groups. Others oppose or reverse the action of a partner muscle. Tendons attach skeletal muscles to bones. Section 36.6
Skeletal muscle function Muscle fibers contract in response to signals from a motor neuron. A muscle fiber contains many myofibrils, each divided crosswise into sarcomeres. ATP-driven interactions between protein filaments shorten sarcomeres, causing muscle contraction. Sections 36.7–36.11
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617
36.1
Invertebrate Skeletons
A skeleton can be internal or external.
Links to Cnidarians 25.5, Annelids 25.7, Arthropods 25.12, Echinoderms 25.18
When you think of a skeleton, you probably picture an internal framework of bones, but this is just one type of skeleton. In other animals, a skeleton consists of a fluid or of external hard parts. Animal body parts move when muscles interact with the skeleton.
Hydrostatic Skeletons Cnidarians and annelids are among the animals with a hydrostatic skeleton: a fluid-filled closed chamber or chambers that muscles act against. For example, a sea anemone’s body is inflated by water that flows in through its mouth and fills its gastrovascular cavity (Figure 36.2). Beating of cilia causes the inward flow of water. Contraction of a ring of muscle around the mouth traps the water inside the body. Contractions
of other muscles can redistribute the water and alter body shape. By analogy, think about how squeezing or pulling on a water-filled balloon changes its shape. An anemone has circular muscles that ring its body and longitudinal ones that run from its top to bottom. Contracting circular muscles and relaxing longitudinal ones makes an anemone taller and thinner. When circular muscles relax and longitudinal ones contract, the anemone gets shorter and fatter. The animal can also open its mouth, contract both sets of muscles, and draw in its tentacles. This action forces most fluid from the gastrovascular cavity out of the body, and the body shrinks into a protective resting position (Figure 36.2b). In earthworms, a coelom divided into many fluidfilled segments is the hydrostatic skeleton (Section 25.7). Longitudinal and circular muscles put pressure on the coelomic fluid in each segment, causing it to become long and narrow or short and wide. Waves of contraction that run the length of the body move the worm through the soil (Figure 36.3).
Exoskeletons An exoskeleton is a stiff body covering to which the muscles attach. For example, bivalve mollusks such as clams and scallops have a hinged two-part shell.
mouth
gastrovascular cavity; the mouth can close and trap fluid inside this cavity
a
b
Figure 36.2 Animated Hydrostatic skeleton of a sea anemone. (a) Water is drawn into the gastrovascular cavity through the mouth. When the cavity is filled and mouth is closed, muscles can act on the trapped fluid and alter body shape. There are two sets of muscles: circular muscles ring the body; longitudinal ones run the length of the body. (b) One anemone inflated with water (left) and another that has expelled water from the gastrovascular cavity and pulled in its tentacles (right).
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thorax
longitudinal muscle contracts
Figure 36.3 How an earthworm moves through the soil. Muscles act on coelomic fluid in individual body segments, causing the segments to change shape. A segment narrows when circular muscle ringing it contracts and longitudinal muscle running its length relaxes. The segment widens when circular muscle relaxes and longitudinal muscle contracts.
A powerful muscle attached to the two halves of the shell can pull them together, shutting the shell. Some scallops can swim through the water by opening and closing their shell. Each time the shell is pulled shut, forcing water out, the scallop scoots backwards a bit. Crabs, spiders, insects, and other arthropods have a hinged exoskeleton with attachment sites for sets of muscles that pull on the hardened parts. For example, a fly’s wings flap when muscles attached to its thorax alternately contract and relax (Figure 36.4). Redistribution of body fluid also has a role in some arthropod movements. In spiders, muscles attached to the exoskeleton contract and pull the legs inward, but there are no opposing muscles to pull legs out again. Instead, a large muscle of the thorax contracts, which causes blood to surge into the hind legs (Figure 36.5). Similarly, redistribution of fluid extends the proboscis of a moth or butterfly, allowing the insect to sip nectar.
vertical muscle relaxes A Wings pivot down as the relaxation of vertical muscle and the contraction of longitudinal muscle pulls in sides of thorax.
longitudinal muscle relaxes
vertical muscle contracts B Wings pivot up when the contraction of vertical muscle and relaxation of longitudinal muscle flattens the thorax.
Figure 36.4 Animated Fly wing movement. Wings attach to the thorax at pivot points. When muscles inside the thorax contract and relax, the thorax changes shape and the wings pivot up and down at their attachment point.
Figure 36.5 Side view of a jumping spider making a leap. When a large muscle in the thorax contracts, volume of the thoracic cavity decreases, forcing blood into the hind legs. The resulting surge of high fluid pressure extends the legs. Some jumping spiders can leap a distance 25 times the length of their body.
Endoskeletons An endoskeleton is an internal framework of hardened elements to which the muscles attach. Echinoderms and vertebrates have an endoskeleton. The skeleton of echinoderms such as sea stars (Figure 36.6) and sea urchins consists of calcium-carbonate plates embedded in the body wall.
Take-Home Message What kinds of skeletons do invertebrates have? Soft-bodied animals such as sea anemones and earthworms have a hydrostatic skeleton, which is an enclosed fluid that contracting muscles act upon. Some mollusks and all arthropods have a hardened external skeleton, or exoskeleton. Echinoderms have an endoskeleton, or internal skeleton.
element of endoskeleton
Figure 36.6 A sea star. The sketch shows a cross-section through one arm. Hard plates embedded in the body wall form an endoskeleton.
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36.2
The Vertebrate Endoskeleton All vertebrates have an endoskeleton. In most groups, the endoskeleton consists primarily of bones. Links to Vertebrate evolution 26.2, Transition to life on land 26.5, Human bipedalism 26.13, Connective tissues 32.3
Features of the Vertebrate Skeleton All vertebrates (the fishes, reptiles, amphibians, birds, and mammals) have an endoskeleton (Figures 36.7 and 36.8). The skeleton of sharks and other cartilaginous fishes consists of cartilage, a rubbery connective tissue. Other vertebrate skeletons include some cartilage, but consist mostly of bone tissue (Section 32.3). The term “vertebrate” refers to the vertebral column, or backbone, a feature common to all members of this group. The backbone supports the body, serves as an attachment point for muscles, and protects the spinal cord, which runs through a canal inside it. Bony segments called vertebrae (singular, vertebra) make up the backbone. Intervertebral disks of cartilage between vertebrae act as shock absorbers and flex points. The vertebral column, along with the bones of the head and rib cage, constitute the axial skeleton. The appendicular skeleton consists of the pectoral (shoulder) girdle, the pelvic (hip) girdle, and limbs (or bony fins) attached to them. You learned earlier how vertebrate skeletons have evolved over time. For example, jaws are derived from the gill supports of ancient jawless fishes (Section 26.2). As another example, bones in the limbs of land vertebrates are homologous to those in fins of lobefinned fishes (Section 26.5).
The Human Skeleton For a closer look at vertebrate skeletal features, think about a human skeleton. The human skull’s flattened
cranial bones fit together to form the braincase that surrounds and protects the brain (Figure 36.8a). The brain and spinal cord connect through an opening called the foramen magnum. In upright walkers such as humans, this opening lies at the base of the skull (Section 26.13). Facial bones include cheekbones and other bones around the eyes, the bone that forms the bridge of the nose, and bones of the jaw. Both males and females have twelve pairs of ribs (Figure 36.8b). Ribs and the breastbone, or sternum, form a protective cage around the heart and lungs. The vertebral column extends from the base of the skull to the pelvic girdle (Figure 36.8c). In humans, natural selection favored an ability to walk upright and led to modification of the backbone. Viewed from the side, our backbone has an S shape that keeps our head and torso centered over our feet (Section 26.13). Maintaining an upright posture requires that vertebrae and intervertebral disks stack one on top of the other, rather than being parallel to the ground, as in four-legged walkers. The stacking puts additional pressure on disks and, as people age, their disks often slip out of place or rupture, causing back pain. The scapula (shoulder blade), and clavicle (collarbone) are bones of the human pectoral girdle (Figure 36.8d). The thin clavicle transfers force from the arms to the axial skeleton. When a person falls on an outstretched arm, the excessive force transferred to the clavicle frequently causes it to fracture or break. The upper arm has one bone, the humerus. The forearm has two bones, the radius and ulna. Carpals are bones of the wrist, metacarpals are bones of the palm, and phalanges (singular, phalanx) are finger bones. The pelvic girdle consists of two sets of fused bones, one set on each side of the body. It protects organs inside the pelvic cavity and supports the weight of the upper body when you stand upright (Figure 36.8e).
vertebral column pectoral girdle
rib cage
vertebral column
skull bones
a pelvic girdle pelvic girdle
Figure 36.7 Skeletal elements typical of (a) a cartilaginous fish and (b) an early reptile. Compare Figures 26.12, 26.22, and 26.31.
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pectoral girdle b
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A Skull bones CRANIAL BONES Enclose, protect brain and sensory organs
D Pectoral girdle and upper limb bones
FACIAL BONES Framework for facial area, support for teeth
Bones with extensive muscle attachments, arranged for great freedom of movement:
B Rib cage These bones and some vertebrae enclose, protect heart, lungs; assist breathing:
CLAVICLE (collarbone) SCAPULA (shoulder blade)
STERNUM (breastbone)
HUMERUS (upper arm bone)
RIBS (twelve pairs) RADIUS (forearm bone) ULNA (forearm bone) CARPALS (wrist bones)
C Vertebral column, or backbone VERTEBRAE (twenty-six bones) Enclose, protect spinal cord; support skull, upper extremities; attachment sites for muscles
1 2 3 5
INTERVERTEBRAL DISKS Fibrous, cartilaginous structures between vertebrae; absorb movement-induced stresses; impart flexibility to backbone
4
METACARPALS (palm bones) PHALANGES (thumb, finger bones) E Pelvic girdle and lower limb bones PELVIC GIRDLE (six fused bones) Supports weight of backbone; helps protect soft pelvic organs FEMUR (thighbone) Body’s strongest weight-bearing bone; works with large muscles in locomotion and in maintaining upright posture PATELLA (kneebone) Protects knee joint; aids leverage
ligament bridging a knee joint, side view, midsection
TIBIA (lower leg bone) Major load-bearing role FIBULA (lower leg bone) Muscle attachment sites; no load-bearing role TARSALS (ankle bones) METATARSALS (sole bones) PHALANGES (toe bones)
Figure 36.8 Animated Bone (tan) and cartilage (light blue) elements of the human skeleton. Left, labels for the axial portion, and (right) for the appendicular portion.
Bones of the leg include the femur (thighbone), the patella (kneecap), and the tibia and fibula (bones of the lower leg). Tarsals are ankle bones, and metatarsals are bones of the sole of the foot. Like the bones of the fingers, those of the toes are called phalanges.
Take-Home Message What type of skeleton is present in humans and other vertebrates? The endoskeleton of vertebrates usually consists mainly of bone. Its axial portion includes the skull, vertebral column, and ribs. Its appendicular part includes a pectoral girdle, a pelvic girdle, and the limbs. Some features of the human skeleton such as an S-shaped backbone are adaptations to upright posture and walking.
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36.3
Bone Structure and Function Bones consist of living cells in a secreted extracellular matrix. Proper diet and exercise will help keep them healthy.
Links to Extracellular matrix 4.12, Parathyroid glands 35.6
Bone Anatomy The 206 bones of an adult human’s skeleton range in size from middle ear bones as small as a grain of rice to the massive thighbone, or femur, which weighs about a kilogram (2 pounds). The femur and other bones of the arms and legs are long bones. Other bones, such as the ribs, the sternum, and most bones of the skull are flat bones. Still other bones, such as the carpals in the wrists, are short and roughly squarish in shape. Table 36.1 summarizes the functions of bones. Each bone is wrapped in a dense connective tissue sheath that has nerves and blood vessels running through it. Bone tissue consists of bone cells in an extracellular matrix (Section 4.12). The matrix is mainly collagen (a protein) with calcium and phosphorus salts.
space occupied by living bone cell
blood vessel
There are three main types of bone cells. Osteoblasts are the bone builders; they secrete components of the matrix. In adult bones, osteoblasts lie beneath a sheath of connective tissue. Osteocytes are former osteoblasts that are now surrounded by the hardened matrix they secreted. These are the most abundant bone cells in adults. Osteoclasts are cells that can break down the matrix by secreting enzymes and acids. A long bone such as a femur includes two types of bone tissue, compact bone and spongy bone (Figure 36.9). Compact bone forms the outer layer and shaft of the femur. It is made up of many functional units called osteons, each having concentric rings of bone tissue, with bone cells in spaces between the rings. Nerves and blood vessels run through a canal in the osteon’s center. Spongy bone fills the shaft and knobby ends of long bones. It is strong yet lightweight; open spaces riddle its hardened matrix. The cavities inside a bone contain bone marrow. Red marrow fills the spaces in spongy bone and is the major site of blood cell formation. Yellow marrow fills the central cavity of an adult femur and most other mature long bones. It consists mainly of fat.
Bone Formation and Remodeling
nutrient canal location of yellow marrow compact bone tissue spongy bone tissue
a 55 µm
The first skeleton that forms in a vertebrate embryo consists of cartilage. It remains cartilage in sharks and other cartilaginous fishes. In other vertebrates, early cartilage serves as a model for an adult skeleton that is largely bone (Figure 36.10). Most bones in these animals form when osteoblasts move into and replace cartilage models. A few bones in the head and part of the clavicle do not start as cartilage; they form when osteoblasts colonize membranes of connective tissue. Many bones continue to grow in size until early adulthood. Even in adults, bone remains a dynamic tissue that the body continually remodels. Microscopic
Table 36.1
Functions of Bone
1. Movement. Bones interact with skeletal muscle and change or maintain positions of the body and its parts. 2. Support. Bones support and anchor muscles.
spongy bone tissue compact bone tissue
b
blood vessel
outer layer of dense connective tissue
Figure 36.9 Animated (a) Structure of a human femur, or thighbone, and (b) a section through its spongy and compact bone tissues.
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3. Protection. Many bones form hardened chambers or canals that enclose and protect soft internal organs. 4. Mineral storage. Bones are a reservoir for calcium and phosphorus ions. Deposits and withdrawals of these ions help maintain their concentrations in body fluids. 5. Blood cell formation. Only certain bones contain the tissue where blood cells form.
Embryo: cartilage model of bone forms Fetus: blood vessel invades model; osteoblasts start producing bone tissue; marrow cavity forms Newborn: remodeling and growth continue; secondary boneforming centers appear at knobby ends of bone Adult: mature bone
a
b
Figure 36.11 (a) Normal bone tissue. (b) Bone weakened by osteoporosis. The term osteoporosis means “porous bones.”
and stronger. Later in life, as osteoblasts become less active, bone mass gradually declines.
About Osteoporosis Figure 36.10 Long bone formation, starting with osteoblast activity in a cartilage model formed earlier in the embryo. The bone-forming cells are active first in the shaft region, then at the knobby ends. In time, cartilage is left only at the ends.
fractures that result from normal body movements are repaired. In response to hormonal signals, osteoclasts dissolve portions of the matrix, releasing stored mineral ions into the blood. Osteoblasts secrete new matrix, which replaces that broken down by osteoclasts. Bones and teeth contain most of the body’s calcium. Hormones regulate calcium concentration in blood by affecting calcium uptake from the gut and calcium release from bone. When the blood calcium level is too high, the thyroid gland secretes calcitonin. This hormone slows the release of calcium into blood by inhibiting osteoclast action. When blood has too little calcium, the parathyroid glands release parathyroid hormone, or PTH (Section 35.6). This hormone stimulates osteoclast activity. It also decreases calcium loss in urine and helps activate vitamin D. The vitamin stimulates cells in the gut lining to absorb calcium. Other hormones also affect bone turnover. The sex hormones estrogen and testosterone encourage bone deposition. Cortisol, the stress hormone, slows it. Until an individual is about twenty-four years old, osteoblasts secrete more matrix than osteoclasts break down, so bone mass increases. Bones become denser
Osteoporosis is a disorder in which bone loss outpaces bone formation. As a result, the bones become weaker and more likely to break (Figure 36.11). Osteoporosis is most common in postmenopausal woman because they no longer produce the sex hormones that encourage bone deposition. However, about 20 percent of osteoporosis cases occur in men. To reduce your risk of osteoporosis, ensure that your diet provides adequate levels of vitamin D and calcium. A premenopausal woman requires 1,000 milligrams of calcium daily; a post-menopausal woman requires 1,500 milligrams a day. Avoid smoking and excessive alcohol intake, which slow bone deposition. Get regular exercise to encourages bone renewal and avoid an excessive intake of cola soft drinks. Several studies have shown that women who drink more than two such soft drinks a day have a slightly lower than normal bone density.
Take-Home Message What are the structural and functional features of bones?
Bones have a variety of shapes and sizes. A sheath of connective tissues encloses the bone, and the bone’s inner cavity contains marrow. Red marrow produces blood cells. All bones consist of bone cells in a secreted extracellular matrix. A bone is continually remodeled; osteoclasts break down the matrix of old bone and ostoeoblasts lay down new bone. Hormones regulate this process.
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36.4
Skeletal Joints—Where Bones Meet Bones interact with one another at joints. Depending on the type, they allow no, little, or much range of motion.
A joint is an area of contact or near contact between bones. There are three types of joints: fibrous joints, cartilaginous joints, and synovial joints (Figure 36.12a). At fibrous joints, bones are held securely in place by dense, fibrous connective tissue. Fibrous joints hold teeth in their sockets in the jaw. Pads or disks of cartilage connect bones at cartilaginous joints. The flexible connection allows just a bit of movement. Cartilaginous joints connect vertebrae to one another and connect some ribs to the sternum. Synovial joints are the most common kind of joints. They include joints of knees, hips, shoulders, wrists,
fibrous joint attaches tooth to jawbone
and ankles. At these joints, bones are separated by a small cavity and smooth cartilage covers their ends, reducing friction. Cords of dense connective tissue called ligaments hold bones in place at a synovial joint. Some ligaments form a capsule that encloses the joint. The capsule’s lining secretes a lubricating synovial fluid. Synovial means “egglike” in Latin, and describes the thick consistency of the fluid. Different synovial joints allow different kinds of movements. For example, joints at the shoulders and hips are ball-and-socket joints that allow a wide range of rotational motion. At other joints, including some in the wrists and ankles, bones glide past one another. Joints at the elbows and knees function like a hinged door; they allow the bones to move back and forth in one plane only. Figure 36.12b shows some of the ligaments that hold the fibula and tibia together at the knee joint. The knee also is stabilized by wedges of cartilage called menisci (singular, meniscus).
Take-Home Message What are joints?
Joints are areas where bones meet and interact.
synovial joint (ball and socket) between humerus and scapula
In the most common type, synovial joints, the bones are separated by a small fluid-filled space and are held together by ligaments of fibrous connective tissue.
cartilaginous joint between rib and sternum
femur
cartilaginous joint between adjacent vertebrae
patella
cartilage ligaments
synovial joint (hinge type) between humerus and radius
menisci tibia synovial joint (ball and socket) between pelvic girdle and femur
fibula b
Figure 36.12 (a) Examples of the three types of joints. (b) Simplified diagram of the structure of the left knee with muscles stripped away. Several ligaments attach the femur to the tibia and chunks of cartilage called menisci help keep the bones properly aligned. Compare the photo in Figure 36.8.
a
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FOCUS ON HEALTH
36.5
Those Aching Joints
We ask a lot of our joints when we engage in sports, carry out repetitive tasks, or slip on a pair of high heels.
Common Injuries A sprained ankle is the most common joint injury. It occurs when one or more of the ligaments that hold bones together at the ankle joint overstretches or tears. A sprained ankle is usually treated immediately with rest, application of ice, compression with an elastic bandage, and elevation of the affected area. After the ankle heals, exercises may help strengthen muscles that stabilize the joint and prevent future sprains. A tear of the cruciate ligaments in the knee joint may require surgery. Cruciate means cross, and these short ligaments cross one another in the center of the joint. They are visible in Figure 36.12b. The cruciate ligaments stabilize the knee and when they are torn completely, bones may shift so the knee gives out when a person tries to stand. A blow to the lower leg, as often occurs in football, can injure
a cruciate ligament, but so can a fall or misstep. Female athletes are at a higher risk for cruciate ligament tears than men who play the equivalent sport. For example, female soccer players tear these ligaments four times as often as male soccer players do. Another common knee injury is a torn meniscus. A meniscus is a C-shaped wedge of cartilage that reduces friction between the bones, cushions them, and helps keep them in place. Each knee has two menisci. A minor tear at the edge of the meniscus may heal on its own, but cartilage repairs itself only very slowly. If a chunk of meniscus cartilage gets torn off, it can drift about in the synovial fluid of the joint and end up jammed into a spot where it interferes with normal function. A dislocation means that bones of a joint are out of place. It is usually highly painful and requires immediate treatment. The bones must be placed back into proper position and immobilized for a time to allow healing.
Figure 36.13 High heels now, may lead to aching knees later. A study by researchers at Tufts University showed that shoes with heels 2.7 inches high increased pressure on the knee joint by 20 to 25 percent over barefoot walking. Wide heels increased pressure on knees more than narrow ones, perhaps because women walked more confidently in them.
Arthritis and Bursitis Arthritis means inflammation of a joint. As you will learn in Chapter 38, inflammation is the body’s normal response to injury. However, with arthritis, inflammation—and the associated pain and swelling— become chronic. The most common type of arthritis is osteoarthritis. It usually appears in old age, after cartilage wears down at a frequently used joint. It affects different joints in different people. For example, women who habitually wear highheeled shoes increase their risk of osteoarthritis of the knees (Figure 36.13). Such shoes put added pressure on the cartilage that cushions the knee joint, increasing the chances that it will wear down and fail. Rheumatoid arthritis is an autoimmune disorder; the immune system mistakenly attacks the fluid-secreting lining of synovial joints. It can occur at any age and women are two to three times more likely than men to be affected. Gout is another form of arthritis. It occurs when crystals of uric acid accumulate in certain joints, most notably those of the big toes. The resulting pain can be chronic and excruciating. Uric acid is a natural product of protein breakdown, but certain genes, excess alcohol intake, or obesity can cause blood levels to rise. Arthritis can be treated with drugs that relieve pain and minimize inflammation. Joints affected by osteoarthritis can also be replaced with artificial, or prosthetic, joints. Knee and hip replacements are now common and allow a person to resume normal activities. With bursitis, a bursa becomes inflamed. A bursa (as shown in Figure 36.16b) is a fluid filled sac that functions as a cushion between parts in many joints. Repeating a movement that puts pressure on a particular bursa usually causes the inflammation. For example, swinging a tennis racket or golf club can lead to inflammation of a bursa in the shoulder or elbow. Continually leaning on an elbow, kneeling to work on something on the floor, or even sitting or standing a certain way can also cause bursitis.
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36.6
Skeletal–Muscular Systems
Only skeletal muscles attach to and pull on bones.
Link to Types of muscle 32.4
Skeletal muscles consist of bundles of muscle fibers sheathed in dense connective tissue. A muscle fiber is a long, cylindrical cell with multiple nuclei that holds contractile filaments. It has several nuclei because it is descended from a group of cells that fused together in the developing embryo. Most muscles and bones interact as a lever system, in which a rod is attached to a fixed point and moves about it. The bone is a rigid rod near a joint (the fixed point). Muscle contraction transmits force to the bone and makes it move, as in Figure 36.14. Fully extend your right arm, place your left hand over the upper arm, and slowly bend your elbow, as in Figure 36.15a. Can you feel the muscle contracting? By causing this muscle to shorten a bit, you caused the bone attached to the muscle to move a large distance. Besides acting on bone, skeletal muscles also can interact with one another. Some work in pairs
C The first muscle group in the upper hindlimb contracts again and draws it back toward body.
or groups to bring about a movement. Muscles can only pull on bones; they cannot push. Often two muscles work in opposition; action of one resists or reverses action of another. For example, the biceps in the upper arm opposes the triceps. Such pairings are the case for most muscles in the limbs (Figures 36.14 and 36.15). Bear in mind, only skeletal muscle moves bones. As you read in Section 32.4, smooth muscle is mainly a component of soft internal organs, such as the stomach. Cardiac muscle is found only in the heart wall. Later chapters consider the structure and function of smooth muscle and cardiac muscle. The human body has close to 700 skeletal muscles, some near the surface, others in the body wall (Figure 36.16). A straplike tendon of connective tissue attaches skeletal muscles to bone. As an example, the Achilles tendon attaches calf muscles to the heel bone and is the largest tendon in the body (Figure 36.16a). Later chapters explain the roles skeletal muscles play in respiration and in blood circulation. We turn now to mechanisms that bring about muscle contraction.
Take-Home Message How do muscles interact with bones? Tendons attach skeletal muscles to bone. When a muscle contracts, it pulls on the attached bone. Often, two muscles attached to a bone have opposing actions.
B An opposing muscle group attached to the limb forcefully contracts and pulls it back. The contractile force, applied against the rock, now propels the frog forward.
Triceps relaxes. Biceps contracts at the same time, and pulls forelimb up.
Triceps contracts, pulls the forelimb down.
At the same time, biceps relaxes.
A A muscle attached to each upper hindlimb contracts and pulls it slightly forward relative to main body axis.
A When the triceps relaxes and its opposing partner (biceps) contracts, the elbow joint flexes and the forearm is pulled upward.
Figure 36.14 A frog on a rock demonstrating how small contractions and the action of opposing muscles can cause big movements.
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B When the triceps contracts and the biceps relaxes, the forearm is extended downward.
Figure 36.15 Animated Two opposing muscle groups in human arms.
HOW ANIMALS WORK
TRICEPS BRACHII Straightens the forearm at elbow
BICEPS BRACHII Bends the forearm at the elbow
PECTORALIS MAJOR Draws the arm forward and in toward the body
DELTOID Raises the arm
SERRATUS ANTERIOR Draws shoulder blade forward, helps raise arm, assists in pushes
TRAPEZIUS Lifts the shoulder blade, braces the shoulder, draws the head back
EXTERNAL OBLIQUE Compresses the abdomen, assists in lateral rotation of the torso
LATISSIMUS DORSI Rotates and draws the arm backward and toward the body
RECTUS ABDOMINIS Depresses the thoracic (chest) cavity, compresses the abdomen, bends the backbone
GLUTEUS MAXIMUS Extends and rotates the thigh outward when walking, running, and climbing
ADDUCTOR LONGUS Flexes, laterally rotates, and draws the thighs toward the body
BICEPS FEMORIS (Hamstring muscle) Draws thigh backward, bends the knee
SARTORIUS Bends the thigh at the hip, bends lower leg at the knee, rotates the thigh in an outward direction QUADRICEPS FEMORIS Set of four muscles that flex the thigh at the hip, extend the leg at knee
GASTROCNEMIUS Bends the lower leg at the knee when walking, extends the foot when jumping
TIBIALIS ANTERIOR Flexes the foot toward the shin
Achilles tendon
a
muscle
Figure 36.16 Animated (a) Muscles of the human musculoskeletal system. These are the skeletal muscles that gym enthusiasts are familiar with; many more are not shown. Also labeled is the Achilles tendon, the largest tendon in the body and the most frequently injured. It attaches muscles in the calf to the heel bone. (b) Tendons at a synovial joint. Bursae form between tendons and bones or some other structure. These fluid-filled sacs help reduce friction between adjacent tissues.
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tendon bursae
b
synovial cavity
STRUCTURAL SUPPORT AND MOVEMENT 627
36.7
How Does Skeletal Muscle Contract? ATP-fueled movements of protein filaments inside a muscle fiber result in muscle contraction.
Link to Cytoskeleton 4.13
Fine Structure of Skeletal Muscle A skeletal muscle’s function arises from its internal organization. Long muscle fibers run parallel with the muscle’s long axis. The muscle fibers are packed with myofibrils, each a bundle of contractile filaments that run the length of the fiber (Figure 36.17a). Light-todark crossbands show up along the entire length of
myofibrils stained for microscopy, as in Figure 36.17b. The bands give the muscle fiber a striated, or striped, appearance. These bands define the units of muscle contraction called sarcomeres. A mesh of cytoskeletal elements called Z bands anchors adjacent sarcomeres to one another (Figure 36.17c). The sarcomere has parallel arrays of thin and thick filaments (Figure 36.18a). Thin filaments attached to Z bands extend inward, toward the sarcomere center. A thin filament consists mainly of two chains of actin, a globular protein (Figure 36.17d). Two other proteins associate with the actin, but we can ignore their role for now. Thick filaments are centered in a sarcomere.
one bundle of many muscle fibers in parallel inside the sheath
outer sheath of one skeletal muscle
one myofibril inside fiber
one myofibril in one fiber
a b Skeletal muscle fiber, longitudinal section. All bands of its myofibrils line up in rows and give the fiber a striped appearance.
sarcomere Z band c Sarcomeres. Many thick and thin filaments overlap in an A band. Only thick filaments extend across the H zone. Only thin filaments extend across I bands to the Z bands. Different proteins organize and stabilize the array.
one actin molecule
sarcomere Z band
H zone
Z band
I band
A band
I band
part of a thin filament
d Arrangement of actin molecules in the thin filaments
Figure 36.17 Animated From the Dance Theatre of Harlem, an example of exquisite control of skeletal muscle movements. (a–e) Zooming down through skeletal muscle from a biceps to molecules having contractile properties.
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part of a myosin molecule
part of a thick filament
e Arrangement of myosin molecules in the thick filaments
Each consists of myosin, a motor protein with a clublike head (Figure 36.17e). The head is positioned just a few nanometers away from a thin filament. Muscle fibers, myofibrils, thin filaments, and thick filaments all run parallel with a muscle’s long axis. As a result, all sarcomeres in all fibers of a muscle work together and pull in the same direction.
A Relative positions of actin and myosin filaments inside a sarcomere between contractions actin
myosin
Z band
actin
Z band
The Sliding-Filament Model The sliding-filament model explains how interactions between thick and thin filaments bring about muscle contraction. According to this model, filaments do not change length and myosin filaments do not change position. Instead, myosin heads bind to actin filaments and slide them toward the center of a sarcomere. As actin filaments are pulled inward, Z bands attached to them are drawn closer together, and the sarcomere shortens (Figure 36.18a,b). Part of the myosin head can bind ATP and break it into ADP and phosphate. This reaction readies myosin for action (Figure 36.18c). Muscle contraction occurs when signals from the nervous system cause calcium levels around filaments to rise, a process we consider in the next section. For now, it is enough to know that a rise in calcium allows myosin heads to bind actin, forming a cross-bridge between the actin and myosin filaments (Figure 36.18d). After binding actin, each myosin head tilts toward the sarcomere center, and the ADP and phosphate are released (Figure 36.18e). Movement of the myosin head slides the attached actin filament toward the center of the sarcomere. The collective sliding of many myosin heads pulls the Z bands toward one another. Binding of a new ATP frees the myosin head from actin, and the head goes back to its original position (Figure 36.18f ). The head attaches to another binding site on the actin, tilts in another stroke, and so on as long as calcium and ATP are available. Hundreds of myosin heads perform a series of repeated strokes all along the length of the actin filaments.
Take-Home Message What is the sliding-filament model for muscle contraction? The sliding-filament model explains how interactions among protein filaments within a muscle fiber’s individual contractile units (its sarcomeres) bring about muscle contractions. By this model, a sarcomere shortens when actin filaments are pulled toward the center of the sarcomere by ATP-fueled interactions with myosin filaments.
B Relative positions of actin and myosin filaments in the same sarcomere, contracted
myosin head
one of many myosin-binding sites on actin
C Myosin in a muscle at rest. Earlier, all myosin heads were energized by binding ATP, which they hydrolyzed to ADP and inorganic phosphate.
cross-bridge
cross-bridge
D A rise in the local concentration of calcium exposes binding sites for myosin on actin filaments, so cross-bridges form.
E Binding makes each myosin head tilt toward the sarcomere’s center and slide the bound actin along with it. ADP and phosphate are released as the myosin heads drag the actin filaments inward, which pulls the Z bands closer.
F New ATP binds to myosin heads, which detach from actin. ATP is hydrolyzed, which returns myosin heads to their original positions.
Figure 36.18 Animated A sliding-filament model for the contraction of a sarcomere in skeletal muscle. (a,b) Organized, overlapping arrays of actin and myosin filaments interact and reduce each sarcomere’s width. (c–f) For clarity, we show the action of two myosin heads only. Each head binds repeatedly to an actin filament and slides it toward the center of the sarcomere. Collective action of many myosin heads makes the sarcomere shorten (contract).
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36.8
From Signal to Response: A Closer Look at Contraction Like neurons, muscle cells are excitable. Action potentials in muscle trigger calcium release that allows contraction.
Links to Active transport 5.4, Neuromuscular junctions 33.5
Nervous Control of Contraction A neuromuscular junction is a synapse between a motor neuron and a muscle fiber (Section 33.5 and Figure 36.19a,b). For a skeletal muscle to contract, an action potential must first travel to a neuromuscular junction and cause the release of acetylcholine (ACh) from a motor neuron’s axon terminals. Like a neuron, a muscle fiber is excitable, and the binding of ACh to
motor neuron
A A signal travels along the axon of a motor neuron, from the spinal cord to a skeletal muscle.
The Roles of Troponin and Tropomyosin
section from spinal cord B The signal is transferred from the motor neuron to the muscle at neuromuscular junctions. Here, ACh released by the neuron’s axon terminals diffuses into the muscle fiber and causes action potentials.
receptors at its plasma membrane causes an action potential. The action potential travels along the muscle plasma membrane, then down T tubules that extend from this membrane. The T tubules deliver the action potential to the sarcoplasmic reticulum, a special type of smooth endoplasmic reticulum that wraps around myofibrils and stores calcium ions (Figure 36.19c). The arrival of action potentials opens voltage-gated channels in the sarcoplasmic reticulum, allowing calcium ions to flow out, down their concentration gradient. This raises the calcium concentration around the actin and myosin filaments, allowing them to interact, and muscle contraction occurs. When contraction ends, calcium pumps of the type described and illustrated in Section 5.4 transport the calcium ions back into the sarcoplasmic reticulum. The muscle fiber is ready for another signal.
neuromuscular junction
section from skeletal muscle
T sarcoplasmic tubule reticulum C Action potentials propagate along a muscle fiber’s plasma membrane down to T tubules, then to the sarcoplasmic reticulum, which releases calcium ions. The ions promote interactions of myosin and actin that result in contraction.
one myofibril in muscle fiber
muscle fiber’s plasma membrane
Figure 36.19 Animated Pathway by which the nervous system controls skeletal muscle contraction. A muscle fiber’s plasma membrane encloses many individual myofibrils. Tubelike extensions of the membrane connect with part of the sarcoplasmic reticulum, which wraps lacily around the myofibrils.
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How does the release of calcium from the sarcoplasmic reticulum allow actin and myosin to interact? Calcium affects troponin and tropomyosin, two proteins that regulate binding of myosin to actin filaments. Figure 36.20a,b shows a single thin filament in a muscle fiber at rest. Under these circumstances, there is little calcium in the fluid around the thin filament. Tropomyosin, a fibrous protein, wraps around actin and covers myosin-binding sites, preventing myosin from binding. Troponin, a globular protein attached to the tropomyosin, has a site that can reversibly bind calcium ions. When an action potential causes release of calcium from the sarcoplasmic reticulum, some of the calcium binds to troponin (Figure 36.20c). As a result, troponin changes shape and pulls tropomyosin—to which it is attached—away from the myosin-binding site on actin (Figure 36.20d). With this binding site cleared, myosin can bind to actin, and the sliding action described in the previous section takes place (Figure 36.20e,f ). So, to summarize events of muscle contraction, a signal (ACh) from a motor neuron causes an action potential in a muscle fiber, which opens calcium gates in the sarcoplasmic reticulum. Some released calcium ions bind to troponin, which pulls tropomyosin away from the myosin-binding site on actin. Cross-bridges form, sarcomeres shorten, and the muscle contracts. Afterwards, calcium pumps transport calcium ions back into the sarcoplasmic reticulum. As the calcium level in the muscle fiber declines, troponin resumes its resting shape, tropomyosin settles back into place over the myosin-binding site, and the muscle relaxes.
36.9
Energy for Contraction
Multiple metabolic pathways can supply the ATP required for muscle contraction.
A Actin (tan) with troponin (teal) and tropomyosin (green) in a thin filament of muscle at rest. myosin-binding site blocked by tropomyosin
B View of a section through the filament shown above.
C Some calcium ions (orange) released by the sarcoplasmic reticulum bind to troponin.
D Troponin changes shape and pulls tropomyosin away from the myosin-binding site.
myosin head
Links to Energy-releasing pathways 8.1, Fermentation 8.5
The availability of ATP affects whether and how long a muscle can contract. ATP is the first energy source a muscle uses, but cells store little ATP. Once that ATP gets used up, the muscle turns to creatine phosphate. Phosphate transfers from creatine phosphate to ADP can produce more ATP (Figure 36.21), and thus keep a muscle going until ATP output from other pathways increases. This is why taking creatine supplements, as described in the chapter introduction, may enhance athletic feats that require short bursts of activity. Most of the ATP used during prolonged, moderate activity is produced by aerobic respiration. Glucose derived from stored glycogen fuels five to ten minutes of activity. Next, glucose and fatty acids that the blood delivers to muscle fibers are broken down. Fatty acids fuel activities that last more than half an hour. Not all fuel is broken down aerobically. Even in resting muscle, some pyruvate is converted to lactate by fermentation. Lactate production rises with exercise. This pathway does not yield much ATP, but it can operate even when oxygen is low.
E The myosin head binds to the now-exposed binding site.
Take-Home Message What is the source of ATP that powers muscle contraction? Muscles first use any stored ATP, then transfer phosphate from creatine phosphate to ADP to form ATP. With ongoing exercise, aerobic respiration and lactate fermentation yield the ATP that supplies the energy for muscle contraction.
F A cross-bridge forms between actin and myosin.
Figure 36.20 Animated The interactions among actin, tropomyosin, and troponin in a skeletal muscle cell.
Take-Home Message What initiates muscle contraction? What role does calcium play in muscle contraction?
A skeletal muscle contracts in response to a signal from a motor neuron. Release of ACh at a neuromuscular junction causes an action potential in the muscle cell. An action potential results in release of calcium ions, which affect proteins attached to actin. Resulting changes in the shape and location of these proteins open the myosin-binding site on actin, allowing cross-bridge formation.
pathway 1 dephosphorylation of creatine phosphate
creatine
ADP + P i
ATP
pathway 2
pathway 3
aerobic respiration
lactate fermentation
oxygen
glucose from bloodstream and from glycogen breakdown in cells
Figure 36.21 Animated Three metabolic pathways by which muscles obtain the ATP molecules that fuel their contraction.
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STRUCTURAL SUPPORT AND MOVEMENT 631
36.10 Properties of Whole Muscles So far, we have been concentrating on individual muscle fibers, but in bodies many fibers respond as a unit.
Motor Units and Muscle Tension
A A single, brief stimulus causes a twitch, a rapid contraction followed by immediate relaxation.
Force
B Repeated stimuli over a short time have an additive effect; they increase the force of contraction.
Force
A motor neuron has many axon terminals that synapse on different fibers in a muscle. One motor neuron and all of the muscle fibers it synapses with constitute one motor unit. Briefly stimulate a motor neuron, and the fibers of its motor unit contract for a few milliseconds. That contraction is a muscle twitch (Figure 36.22a).
relaxation starts
stimulus
contraction
Fatigue, Exercise, and Aging
Force
six stimulations per second
C Sustained stimulation causes tetanus, a sustained contraction with several times the force of a twitch.
twitch
tetanic contraction
repeated stimulation Time
Figure 36.22 Animated Recordings of twitches in a muscle fiber when the motor neuron controlling is artificially stimulated. Figure It Out: Which graph allows you to compare the force generated by a twitch and tetanus?
Answer: C
contracted muscle can shorten
a
A new stimulus that occurs before a response ends makes the fibers twitch again. Repeatedly stimulating a motor unit during a short interval makes all of the twitches run together in a sustained contraction called tetanus (Figure 36.22c). Force generated by tetanus is three or four times the force of a single twitch. Muscle tension is the mechanical force exerted by a muscle. The more motor units stimulated, the greater the muscle tension. Opposing muscle tension is a load, either the weight of an object or gravity’s pull on the muscle. Only when muscle tension exceeds opposing forces does a stimulated muscle shorten. Isotonically contracting muscles shorten and move some load, as when you lift an object (Figure 36.23a). Isometrically contracting muscles tense but do not shorten, as when you try to lift an object but fail because it is too heavy (Figure 36.23b).
contracted muscle can’t shorten
b
When unrelenting stimulation keeps a skeletal muscle in tetanus, muscle fatigue follows. Muscle fatigue is a decrease in a muscle’s capacity to generate force; muscle tension declines despite ongoing stimulation. After a few minutes of rest, the fatigued muscle will contract again in response to stimulation. In humans, all muscle fibers form before birth and exercise does not stimulate the addition of new ones. Aerobic exercise—low intensity, but long duration— makes muscles more resistant to fatigue. It increases their blood supply and the number of mitochondria, the organelles that produce the bulk of ATP during aerobic respiration. Brief, intense exercise such as weight lifting results in synthesis of actin and myosin. This helps a muscle exert more tension but does not improve endurance. As people age, the number and size of their muscle fibers decline. The tendons that attach muscle to bone stiffen and are more likely to tear. Older people may exercise intensely for long periods, but their muscle mass can no longer increase as much. Even so, aerobic exercise does improve blood circulation, and modest strength training can slow the loss of muscle tissue.
Take-Home Message
Figure 36.23 (a) Isotonic contraction. The load is less than a muscle’s peak capacity to contract. The muscle can contract, shorten, and lift the load. (b) Isometric contraction. The load exceeds a muscle’s peak capacity, so the muscle contracts but cannot shorten.
632 UNIT VI
HOW ANIMALS WORK
How do whole muscles respond to stimulation and exercise? Brief stimulation of a muscle causes a twitch; ongoing stimulation results in a more forceful contraction called tetanus. Exercise cannot add muscle fibers, but it can increase the number of protein filaments and mitochondria in existing ones.
FOCUS ON HEALTH
36.11 Disruption of Muscle Contraction Some genetic disorders, diseases, or toxins can cause muscles to contract too little or too much.
Links to X-linked inheritance 12.4, Endospores 21.6
Muscular Dystrophies Muscular dystrophies are a class of genetic disorders in which skeletal muscles progressively weaken. With Duchenne muscular dystrophy, symptoms begin to appear in childhood. Myotonic muscular dystrophy is the most common kind in adults. A mutation of a gene on the X chromosome causes Duchenne muscular dystrophy. This affected gene encodes dystrophin, a protein found in the plasma membrane of muscle fibers. A mutant form of dystrophin allows foreign material to enter a muscle fiber, which causes the fiber to break down (Figure 36.24). Muscular dystrophy arises in about 1 in 3,500 males. Like other X-linked disorders, it rarely causes symptoms in females, who nearly always have a normal version of the gene on their other X chromosome. Affected boys usually begin to show signs of weakness by the time they are three years old, and require a wheelchair in their teens. Most die in their twenties of the respiratory failure that occurs when the skeletal muscles involved in breathing stop functioning. Motor Neuron Disorders When motor neurons cannot signal muscles to contract, or signaling is impaired, skeletal muscles weaken or become paralyzed. For example, poliovirus can infect and kill motor neurons. Children are most frequently infected; those who survive an infection may be paralyzed or have a weakened voluntary muscle response as a result. Polio vaccines have been available since the 1950s, so the disease is on the decline. No new cases have been reported in the United States since 1979. However, infections continue to occur in less-developed countries. Also, some people who had polio as children develop post-polio syndrome as adults. Fatigue and progressive muscle weakness are the main symptoms. There are at least 250,000 polio survivors in the United States, and may be as many as a million. Amyotrophic lateral sclerosis (ALS) also kills motor neurons. It is sometimes called Lou Gehrig’s disease, after a famous baseball player whose career was cut short by the disease in the late 1930s. ALS usually causes death by respiratory failure within three to five years of diagnosis, but some people survive much longer. For example, the astrophysicist Stephen Hawking was diagnosed with ALS in 1963. Though now confined to a wheelchair and unable to speak, he continues to write and to lecture with the assistance of a voice synthesizer. Botulism and Tetanus Bacteria of the genus Clostridium produce toxins that disrupt the flow of signals from nerves to muscles. Resting spores (endospores) of C. botulinum sometimes are in canned food. When spores germinate, the bacteria that grow make botulinum, an odorless toxin. When a person eats tainted food, botulinum enters motor
a
b
Figure 36.24 Electron micrographs of (a) normal skeletal muscle tissue and (b) muscle tissue of a person affected by muscular dystrophy.
Figure 36.25 An 1809 painting showing a casualty of a battle wound as he lay dying of tetanus in a military hospital.
neurons and keeps them from releasing acetylcholine (ACh). Muscles cannot contract without this neurotransmitter. Affected people can die if skeletal muscles with roles in breathing become paralyzed. A related bacterium, C. tetani, lives in the gut of cattle, horses, and other grazing animals, and even some people. Its endospores can last for years in soil. C. tetani spores sometimes get into a deep wound and germinate there. The bacteria grow and produce a toxin that blood or nerves deliver to the spinal cord and brain. In the spinal cord, the bacterial toxin blocks release of neurotransmitters such as GABA (Section 33.6) that exert inhibitory control over motor neurons. Without these controls, nothing dampens signals to contract, so symptoms of the disease known as tetanus begin. Overstimulated muscles stiffen and cannot be released from contraction. Fists and the jaw may stay clenched; lockjaw is a common name for the disease. The backbone may become locked in an abnormally arching curve (Figure 36.25). Death occurs when respiratory and cardiac muscles become locked in contraction. Vaccines have eradicated tetanus in the United States. Worldwide, the annual death toll is over 200,000. Most are newborns infected during an unsanitary delivery.
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STRUCTURAL SUPPORT AND MOVEMENT 633
IMPACTS, ISSUES REVISITED
Pumping Up Muscles
In a 2004 article, scientists in Germany reported on their study of an unusual child. Even at birth the boy had bulging biceps and thigh muscles. Investigation showed that he has a mutation in the gene for myostatin, a regulatory protein that slows muscle growth. He apparently makes little or no myostatin. Gene mutations that decrease myostatin levels may give some athletes a natural edge in putting on muscle mass. The boy’s mother is a sprinter.
Summary Sections 36.1, 36.2 Nearly all animals move by applying the force of muscle contraction to their skeletal elements. A hydrostatic skeleton is a confined fluid upon which muscle contractions act. An exoskeleton consists of hardened parts at the body surface. An endoskeleton consists of hardened parts inside the body. Humans, like other vertebrates, have an endoskeleton. The axial skeleton consists of skull bones, a vertebral column (backbone), and a rib cage. The appendicular skeleton is the pelvic girdle, pectoral girdle, and paired limbs. The vertebral column consists of individual segments called vertebrae, with intervertebral disks between them. The spinal cord runs through the vertebral column and connects with the brain through the foramen magnum, a hole in the base of the skull. Placement of this hole, and other features of the human skeleton, are adaptations for upright walking in humans.
Use the animation on CengageNOW to learn about the skeletal systems of invertebrates and humans.
Sections 36.3–36.5 Bones are organs rich in collagen, calcium, and phosphorus. In addition to having a role in movement, they store minerals and protect organs. Some have red marrow that makes blood cells; most have yellow marrow. In a human embryo, bones develop from a cartilage model. Even in adults, bones are continually remodeled. Osteoblasts are cells that synthesize bone, whereas osteoclasts break bone down. Osteocytes are former osteoblasts enclosed in a matrix of their secretions. A joint is an area of close contact between bones. One or more ligaments hold bones together at most joints. Bits of cartilage and fluid-filled bursae cushion joints.
Use the animation on CengageNOW to study the structure of a human femur.
Section 36.6 A muscle fiber is a long, cylindrical cell with multiple nuclei. In a skeletal muscle, muscle fibers are bundled inside a dense connective tissue sheath that extends beyond the fibers. Tendons are extensions of this sheath. They attach most skeletal muscles to bones. When skeletal muscles contract, they transmit force to bones and move them. Some muscles work together, and others work as opposing pairs.
Use the animation on CengageNOW to review the location and function of human skeletal muscles.
634 UNIT VI
HOW ANIMALS WORK
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Section 36.7 The internal organization of a skeletal muscle promotes a strong, directional contraction. Many myofibrils make up a skeletal muscle fiber. A myofibril consists of sarcomeres, units of muscle contraction, lined up along its length. Each sarcomere has parallel arrays of actin and myosin filaments. The sliding-filament model describes how ATP-driven sliding of actin filaments past myosin filaments shortens the sarcomere. Shortening of all sarcomeres in all myofibrils of all muscle fibers of a muscle bring about the muscle’s contraction.
Use the animation on CengageNOW to explore muscle structure and observe muscle contraction.
Sections 36.8, 36.9 Signals from motor neurons result in action potentials in muscle fibers, which in turn cause the sarcoplasmic reticulum to release stored calcium. The flow of this calcium into the cytoplasm makes accessory proteins associated with the thin filaments shift in such a way that actin and myosin heads can interact and bring about a muscle contraction. Muscle fibers produce the ATP needed for contraction by way of three pathways: dephosphorylation of creatine phosphate, aerobic respiration, and lactate fermentation.
Use the animation on CengageNOW to observe how the nervous system controls muscle contraction, and how a muscle gets the energy for contraction.
Sections 36.10, 36.11 A motor neuron and all the muscle fibers it controls are a motor unit. Brief stimulation of a motor unit causes a twitch. Repeated stimulation causes a tetanus, or sustained contraction. Muscle tension is the force exerted by a contracting muscle. Muscle fatigue is a decline in muscle tension despite ongoing stimulation. Genetic disorders that affect muscle structure impair muscle function. So do some diseases and toxins that affect motor neurons.
Use the animation on CengageNOW to observe how a muscle fiber responds to stimulation of a motor neuron.
Self-Quiz
Answers in Appendix III
1. A hydrostatic skeleton consists of . a. a fluid in an enclosed space b. hardened plates at the surface of a body c. internal hard parts d. none of the above
Data Analysis Exercise Tiffany, shown in Figure 36.26, was born with multiple fractures in her arms and legs. By age six, she had undergone surgery to correct more than 200 bone fractures. Her fragile, easily broken bones are symptoms of osteogenesis imperfecta (OI), a genetic disorder caused by a mutation in a gene for collagen. As bones develop, collagen forms a scaffold for deposition of mineralized bone tissue. The scaffold forms improperly in children with OI. Figure 36.26 also shows the results of an experimental test of a new drug. Treated children, all less than two years old, were compared to similarly affected children of the same age who were not treated with the drug.
Vertebral area in cm2 Fractures Treated child (Initial) (Final) per year 1 2 3 4 5 6 7 8 9 Mean
1. An increase in vertebral area during the 12-month period of the study indicates bone growth. How many of the treated children showed such an increase? 2. How many of the untreated children showed an increase in vertebral area? 3. How did the rate of fractures in the two groups compare? 4. Do the results shown support the hypothesis that giving young children who have OI this drug, which slows bone breakdown, can increase bone growth and reduce fractures?
2. Bones are . a. mineral reservoirs b. skeletal muscle’s partners
c. sites where blood cells form (some bones only) d. all of the above
3. Bones move when a. cardiac b. skeletal
muscles contract. c. smooth d. all of the above
4. A ligament connects a. bones at a joint b. a muscle to a bone
. c. a muscle to a tendon d. a tendon to bone
7. The is the basic unit of contraction. a. osteoblast c. twitch b. sarcomere d. myosin filament
18.2 16.5 16.4 13.5 16.2 18.9 16.6
13.7 12.9 11.3 7.7 16.1 17.0 13.1
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Critical Thinking
9. A sarcomere shortens when . a. thick filaments shorten b. thin filaments shorten c. both thick and thin filaments shorten d. none of the above
c. muscular dystrophy
1 2 3 4 5 6 Mean
Figure 36.26 Results of a a clinical trial of a drug treatment for osteogenesis imperfecta (OI), which affects the child shown at right. Nine children with OI received the drug. Six others were untreated controls. Surface area of certain vertebrae was measured before and after treatment. Fractures occurring during the 12 months of the trial were also recorded.
8. In sarcomeres, phosphate-group transfers from ATP activate . a. actin b. myosin c. both d. neither
11. A virus causes . a. polio b. botulism
1 1 6 0 6 1 0 4 4 2.6
13. Match the words with their defining feature. osteoblast a. stores and releases calcium muscle twitch b. all in the hands muscle tension c. blood cell production joint d. decline in tension myosin e. bone-forming cell red marrow f. motor unit response metacarpals g. force exerted by cross-bridges myofibrils h. area of contact between bones muscle fatigue i. muscle fiber’s threadlike parts foramen magnum j. actin’s partner sarcoplasmic k. hole in the head reticulum
attaches to the pelvic girdle. c. femur d. tibia
10. ATP for muscle contraction can be formed by a. aerobic respiration b. lactate fermentation c. creatine phosphate breakdown d. all of the above
16.7 16.9 16.5 11.8 14.6 15.6 15.9 13.0 13.4 14.9
12. A motor unit is . a. a muscle and the bone it moves b. two muscles that work in opposition c. the amount a muscle shortens during contraction d. a motor neuron and the muscle fibers it controls
5. Parathyroid hormone stimulates . a. osteoclast activity c. red blood cell formation b. bone deposition d. all of the above 6. The a. radius b. sternum
14.7 15.5 6.7 7.3 13.6 9.3 15.3 9.9 10.5 11.4
Vertebral area in cm2 Control Fractures child (Initial) (Final) per year
1. Compared to most people, long-distance runners have far more mitochondria in skeletal muscles. In sprinters, skeletal muscle fibers have more of the enzymes required for glycolysis but not as many mitochondria. Suggest why. .
2. Zachary’s younger brother Noah had Duchenne muscular dystrophy and died at the age of 16. Zachary is now 26 years old, healthy, and planning to start a family of his own. However, he worries that his sons might be at high risk for muscular dystrophy. His wife’s family has no history of this genetic disorder. Review Sections 12.4 and 36.11 and decide whether Zachary’s concerns are well founded. CHAPTER 36
STRUCTURAL SUPPORT AND MOVEMENT 635
4 7 8 5 8 6 6.3
37
Circulation IMPACTS, ISSUES
And Then My Heart Stood Still
The heart is the body’s most durable muscle. It starts to beat
arrest while playing in a high school football game. Nader’s
during the first month of human development, and keeps on
parents, who were watching the game, rushed from their
going for a lifetime. Each heartbeat is set in motion by an
seats and begin CPR on their son. At the same time, some-
electrical signal generated by a natural pacemaker in the
one ran to get the school’s automated external defibrillator
heart wall. In some people, this pacemaker malfunctions.
(AED). This device is about the size of a laptop computer
Electrical signaling gets disrupted, the heart stops beating,
(Figure 37.1b). It provides simple voice commands about
and blood flow halts. This is called sudden cardiac arrest.
how to attach electrodes to a person in distress, then checks
In the United States, it strikes more than 300,000 people per
for a heartbeat and, if required, shocks the heart. The AED restarted Nader’s heart, and he went on to testify
year. An inborn heart defect causes most cardiac arrests in people under age 35. In older people, heart disease usually
before the Texas Legislature about his experience. Thanks
causes the heart to stop functioning.
in part to his efforts, Texas passed a law requiring all high schools to have AEDs at athletic events and practices.
The chance of surviving sudden cardiac arrest rises by 50
Because most cardiac arrests do not occur in a hospital,
percent when cardiopulmonary resuscitation (CPR) is started within four to six minutes of the arrest. With this technique,
the presence of a bystander willing to carry out CPR and use
a person alternates mouth-to-mouth respiration with chest
an AED often means the difference between life and death.
compressions that keep the victim’s blood moving.
Yet studies show only about 15 percent of sudden cardiac arrest victims get CPR before trained personnel arrive. The
CPR cannot restart the heart. That requires a defibrillator, a device that delivers an electric shock to the chest and resets
problem is most people do not know how to administer CPR
the natural pacemaker. You have probably seen this proce-
or use an AED. A half-day course given by the American Red
dure depicted in hospital dramas.
Cross or another community health organization can teach you both skills. Taking the time to learn these skills is some-
Matt Nader (Figure 37.1a) learned about the importance
thing we can all do for one another.
of CPR and defibillation when he went into sudden cardiac
b
See the video! Figure 37.1 Surviving sudden cardiac arrest. (a) Matt Nader, a talented high school football player, discovered he had a heart defect when his heart stopped during a game. CPR and quick defibrillation saved his life.
a
(b) One type of automated external defibrillator. Such devices are designed to be simple enough to be used by a trained layperson. AEDs are increasingly available in public places, but they only make a difference if someone uses them.
Links to Earlier Concepts
Key Concepts Overview of circulatory systems
In this chapter, you will see examples of the role that diffusion (Section 5.3) plays in exchange of substances. You will revisit metabolism of alcohol (Chapter 6 introduction), and how glucose gets stored as glycogen (8.7).
You will learn more about blood as a connective tissue (32.3) and how the muscle of the heart contracts (36.7, 36.8). You will also see how cell junctions (32.1) play a role in this contraction.
ABO blood typing (11.4) and membrane proteins (5.2) are discussed again, as are hemoglobin and sickle-cell anemia (3.6, 18.6), hemophilia (12.4), and thalassemia (14.5).
You will be reminded again of how diabetes affects the circulatory system (35.9), the role of the thymus gland (35.12), the effects of autonomic stimulation (33.8), and the overriding importance of homeostasis (27.1, 27.3).
Evolutionary changes to the circulatory system (25.1, 26.2) also receive additional attention here.
Many animals have either an open or a closed circulatory system that transports substances to and from all body tissues. All vertebrates have a closed circulatory system, in which blood is always contained within the heart or blood vessels. Section 37.1
Blood composition and function Vertebrate blood is a fluid connective tissue. It consists of red blood cells, white blood cells, platelets, and plasma (the transport medium). Red blood cells function in gas exchange; white blood cells defend tissues, and platelets function in clotting. Sections 37.2–37.4
The human heart and two flow circuits The four-chambered human heart pumps blood through two separate circuits of blood vessels. One circuit extends through all body regions, the other through lung tissue only. Both circuits loop back to the heart. Sections 37.5, 37.6
Blood vessel structure and function The heart pumps blood rhythmically, on its own. Adjustments at arterioles regulate how blood volume is distributed among tissues. Exchange of gases, wastes, and nutrients between the blood and tissues takes place at capillaries. Sections 37.7, 37.8
When the system breaks down Cardiovascular problems include clogged blood vessels or abnormal heart rhythms. Some problems have a genetic basis; most are related to age or life-style. Section 37.9
Links with the lymphatic system A lymph vascular system delivers excess fluid that collects in tissues to the blood. Lymphoid organs cleanse blood of infectious agents and cellular debris. Section 37.10
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637
37.1
The Nature of Blood Circulation In a closed circulatory system, the blood volume moves continually through large and small vessels. Blood moves fastest where it is confined in a few big vessels and it slows in capillaries, the vessels with the smallest diameter. The slowdown in capillaries gives the blood and interstitial fluid time to exchange substances by diffusion (Section 5.3). Blood slows in capillaries not because these vessels are small, but because of their huge numbers. Your body has billions, and their collective cross-sectional area is much greater than that of the far fewer, larger vessels that deliver blood to them. When blood enters capillaries, its speed declines, as if a narrow river (the few larger vessels) were delivering water to a wide lake (the many capillaries). Figure 37.3 d illustrates the concept. Velocity picks up again in the larger, but far fewer, vessels that return blood to the heart. Similarly, water picks up speed when it flows from a wide lake into a narrow river.
A circulatory system distributes materials throughout the body of some invertebrates and all vertebrates. Links to Animal evolution 25.1, Vertebrate evolution 26.2, Diffusion 5.3
From Structure to Function A circulatory system moves substances into and out of cellular neighborhoods. Blood, its transport medium, typically flows inside tubular vessels under pressure generated by a heart, a muscular pump. Blood makes exchanges with interstitial fluid—fluid that fills spaces between cells. Interstitial fluid in turn exchanges substances with cells. Blood and interstitial fluid serve as the body’s internal environment. Interactions among organ systems keep the composition and volume of this environment within ranges that cells can tolerate (Section 25.1). Structurally, there are two main kinds of circulatory systems. Arthropods and most mollusks have an open circulatory system. Their blood moves through hearts and large vessels but also mixes with interstitial fluid (Figure 37.2a). Annelids and vertebrates have a closed circulatory system. Their blood remains inside a heart or blood vessel at all times (Figures 37.2b and 37.3).
aorta
Evolution of Circulation in Vertebrates All vertebrates have a closed circulatory system, but fishes, amphibians, birds, and mammals differ in their pumps and plumbing. These differences evolved over
pump
heart
spaces or cavities in body tissues A In a grasshopper’s open system, a heart (not like yours) pumps blood through a vessel, a type of aorta. From there, blood moves into tissue spaces, mingles with interstitial fluid, then reenters the heart at openings in the heart wall.
dorsal blood vessel
pump
large-diameter blood vessels two of five ventral blood hearts vessels
gut cavity
capillary bed (many small vessels that serve as a diffusion zone)
B The closed system of an earthworm confines blood inside pairs of muscular hearts near the head end and inside many blood vessels.
Figure 37.2 Animated Comparison of open and closed circulatory systems.
638 UNIT VI
HOW ANIMALS WORK
large-diameter blood vessels
hundreds of millions of years after some vertebrates left the water for land. The earliest vertebrates, recall, had gills. Like other respiratory structures, gills have a thin, moist surface, across which oxygen and carbon dioxide diffuse. In time, internally moistened sacs called lungs evolved and supported the move to dry land. Other modifications helped blood flow faster in a loop between the heart and lungs (Section 26.2). In most fishes, blood flows in one circuit (Figure 37.3a). The contractile force of a two-chambered heart drives it through a capillary bed inside each gill. From there, blood flows into a large vessel, then through capillary beds in body tissues and organs, and back to the heart. The blood is not under much fluid pressure when it leaves the gill capillaries, so it moves slowly through the single circuit back to the heart. In amphibians, the heart is partitioned into three chambers, with two atria emptying into one ventricle. Oxygenated blood flows from the lungs to the heart in one circuit, then a forceful contraction pumps it through the rest of the body in a second circuit. Still, the oxygenated blood and oxygen-poor blood mix a bit in the ventricle (Figure 37.3b). In birds and mammals, the heart has fully separate right and left halves, each with two chambers, and it pumps blood in two separate circuits (Figure 37.3c). In the pulmonary circuit, oxygen-poor, carbon dioxide– rich blood flows from the right half of the heart to the lungs. There, blood picks up oxygen, gives up carbon dioxide, and flows into the left half of the heart. In the longer systemic circuit, the heart’s left half pumps oxygenated blood to tissues where oxygen is used and carbon dioxide forms in aerobic respiration. Blood gives up oxygen and picks up carbon dioxide at tissues, then flows to the heart’s right half. With two fully separate circuits, blood pressure can be regulated independently in each circuit. Strong contraction of the heart’s left ventricle provides sufficient force to keep blood moving fast through the long systemic circuit. Less forceful contraction of the right ventricle protects delicate lung capillaries that would be blown apart by high pressure.
A In fishes, the heart has two chambers: one atrium and one ventricle. Blood flows through one circuit. It picks up oxygen in the capillary beds of the gills, and delivers it to capillary beds in all body tissues. Oxygen-poor blood then returns to the heart.
capillary beds of gills
heart
rest of body
lungs
right atrium
left atrium ventricle
rest of body
lungs
left atrium
right atrium right ventricle
left ventricle
rest of body
lake river in
river out
1 2 3
1 2 3
Take-Home Message What are the two types of animal circulatory systems?
123
Some animals, including insects, have an open circulatory system in which blood leaves vessels and mingles with the interstitial fluid. Other animals, including annelids and all vertebrates, have a closed circulatory system, in which materials are exchanged across the walls of small blood vessels.
B In amphibians, the heart has three chambers: two atria and one ventricle. Blood flows along two partially separated circuits. The force of one contraction pumps blood from the heart to the lungs and back. The force of a second contraction pumps blood from the heart to all body tissues and back to the heart.
C In birds and mammals, the heart has four chambers: two atria and two ventricles. The blood flows through two fully separated circuits. In one circuit, blood flows from the heart to the lungs and back. In the second circuit, blood flows from the heart to all body tissues and back.
D Why flow slows in capillaries. Picture a volume of water in two fast rivers flowing into and out of a lake. The flow rate is constant, with an identical volume moving from points 1 to 3 in the same interval. However, flow velocity decreases in the lake. Why? The volume spreads out through a larger cross-sectional area and flows forward a shorter distance during the specified interval.
Figure 37.3 Animated (a–c) Comparison of flow circuits in the closed circulatory systems of fishes, amphibians, birds, and mammals. Red indicates oxygenated blood; blue, oxygen-poor blood. (d) Analogy illustrating why blood flow slows in capillaries.
CHAPTER 37
CIRCULATION 639
37.2
Characteristics of Blood cell is an unspecialized cell that retains a capacity for mitotic cell division. Some portion of its daughter cells divide and differentiate into specialized cell types.
Tumbling along in the fluid plasma of a vertebrate bloodstream are cells that distribute oxygen through the body and defend the body from pathogens.
Link to Connective tissues 32.3, Hemoglobin 3.6
Plasma About 50 to 60 percent of the blood’s total
Functions of Blood Blood is the fluid connective tissue that carries oxygen, nutrients, and other solutes to cells and picks up their metabolic wastes and secretions, including hormones. Blood helps stabilize internal pH. It is a highway for cells and proteins that protect and repair tissues. In birds and mammals, it helps keep body temperature within tolerable limits by moving excess heat to skin, which can give up heat to the surroundings.
Blood Volume and Composition Body size and the concentrations of water and solutes dictate the blood volume. Average-sized humans hold about 5 liters (a bit more than 10 pints), which is 6 to 8 percent of the total body weight. In vertebrates, blood is a viscous fluid that is thicker than water, and slower flowing. Blood’s fluid portion is plasma. Its cellular portion consists of blood cells and platelets that arise from stem cells in bone marrow (Section 32.3). A stem
Components
volume is plasma (Figure 37.4). Plasma is 90 percent water. Besides being the transport medium for blood cells and platelets, it acts as a solvent for hundreds of different plasma proteins. Some proteins transport lipids and fat-soluble vitamins; others have a role in blood clotting or immunity. Plasma also holds sugars, lipids, amino acids, vitamins, and hormones, as well as the gases oxygen, carbon dioxide, and nitrogen. Red Blood Cells Erythrocytes, or red blood cells, trans-
port oxygen from lungs to aerobically respiring cells and help carry carbon dioxide wastes from them. In all mammals, red blood cells lose their nucleus, mitochondria, and other organelles as they mature. Mature red blood cells are flexible disks with a depression at their center. They slip easily through narrow blood vessels and the flattened shape facilitates gas exchange. Most oxygen that diffuses into your blood binds to hemoglobin in red blood cells. You learned about this protein in Section 3.6. Stored hemoglobin fills about 98 percent of the interior of human red blood cells. It
Main Functions
Amounts
Plasma Portion (50–60% of total blood volume) 1. Water
91–92% of total plasma volume
Solvent
2. Plasma proteins (albumins, globulins, fibrinogen, etc.)
7–8%
Defense, clotting, lipid transport, extracellular fluid volume controls
3. Ions, sugars, lipids, amino acids, hormones, vitamins, dissolved gases, etc.
1–2%
Nutrition, defense, respiration, extracellular fluid volume controls, cell communication, etc.
Cellular Portion (40–50% of total blood volume; numbers per microliter) 1. Red blood cells
4,600,000–5,400,000
Oxygen, carbon dioxide transport to and from lungs
2. White blood cells: Neutrophils Lymphocytes Monocytes (macrophages) Eosinophils Basophils
3,000–6,750 1,000–2,700 150–720 100–380 25–90
Fast-acting phagocytosis Immune responses Phagocytosis Killing parasitic worms Anti-inflammatory secretions
3. Platelets
250,000–300,000
Roles in blood clotting
Figure 37.4 Typical components of human blood. Numbers for cellular components are all per microliter. The sketch of a test tube shows what happens when you prevent a blood sample from clotting. The sample separates into straw-colored plasma, which floats on a reddish cellular portion. The scanning electron micrograph shows these components.
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red blood cell
white blood cell
platelet
stem cell in bone marrrow
myeloid stem cell
lymphoid stem cell
red blood cell
granulocyte
monocyte
precursor
precursor
precursor
megakaryocytes
platelets
red blood cells (erythrocytes)
neutrophils
eosinophils
basophils
monocytes (immature phagocytes)
B lymphocytes T lymphocytes (mature in (mature in thymus) bone marrow)
Figure 37.5 Main cellular components of mammalian blood and how they originate.
makes the cells and oxygenated blood appear bright red. Oxygen-poor blood is dark red, but it looks blue through blood vessel walls near the body surface. In addition to hemoglobin, a mature red blood cell has enough stored glucose and enzymes to last about 120 days. In a healthy person, ongoing replacements keep red blood cell numbers at a fairly stable level. A cell count measures the quantity of cells of one type per microliter of blood. Men typically have a higher red blood cell count than women of reproductive age, who lose blood during menstruation. White Blood Cells Leukocytes, or white blood cells,
carry out ongoing housekeeping tasks and function in defense. The cells differ in their size, nuclear shape, and staining traits (Figure 37.5), as well as function. Neutrophils, basophils, and eosinophils all develop from one type of precursor cell. They are sometimes collectively referred to as the granulocytes, because their cytoplasm contains granules that can be stained by specific dyes. Neutrophils are the most abundant white cells; they are phagocytes that engulf bacteria and debris. Eosinophils attack larger parasites, such as worms, and have a role in allergies. Basophils secrete chemicals that have a role in inflammation.
Monocytes circulate in the blood for a few days, then move into the tissues, where they develop into phagocytic cells known as macrophages. As you will see in the next chapter, macrophages interact with lymphocytes to bring about immune responses. There are two types of lymphocytes, B cells and T cells. B cells mature in bone, whereas T cells mature in the thymus. Both protect the body against specific threats. Platelets Megakaryocytes are ten to fifteen times big-
ger than other blood cells that form in bone marrow. They break up into membrane-wrapped fragments of cytoplasm called platelets. After a platelet forms, it will last five to nine days. When activated, it releases substances needed for blood clotting.
Take-Home Message What are the components of human blood and what are their functions?
Blood consists mainly of plasma, a protein-rich fluid that carries wastes, gases, and nutrients.
Blood cells and platelets form in bone marrow and are transported in plasma. Red blood cells contain hemoglobin that carries oxygen from lungs to tissues. White cells help defend the body from pathogens. Platelets are cell fragments that have a role in clotting.
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37.3
37.4
Hemostasis
Plasma proteins and platelets interact in clotting.
Link to Hemophilia 12.4
Genetically determined differences in molecules on the surface of red blood cells are the basis of blood typing.
The blood vessels are vulnerable to ruptures, cuts, and similar injuries. Hemostasis is a three-phase process that stops blood loss and constructs a framework for repairs. In the initial vascular phase, smooth muscle in the damaged vessel wall contracts in an automatic spasm. In the second phase, platelets stick together at the injured site. They release substances that prolong the spasm and attract more platelets. In the final coagulation phase, plasma proteins convert blood to a gel and form a clot. During clot formation, fibrinogen, a soluble plasma protein, is converted to insoluble threads of fibrin. Fibrin forms a mesh that traps cells and platelets (Figure 37.6). Clot formation involves a cascade of enzyme reactions. Fibrinogen is converted to fibrin by the enzyme thrombin, which circulates in blood as the inactive precursor prothrombin. Prothrombin is activated by an enzyme (factor X) that is activated by another enzyme, and so on. What starts the cascade of reactions? The exposure of collagen in the damaged vessel wall. If a mutation affects any one of the enzymes that acts in the cascade of clotting reactions, the blood may not clot properly. Such mutations cause the genetic disorder hemophilia (Section 12.4). Stimulus A blood vessel is damaged. Phase 1 response A vascular spasm constricts the vessel. Phase 2 response Platelets stick together plugging the site. Phase 3 response Clot formation starts: 1. Enzyme cascade results in activation of Factor X. 2. Factor X converts prothrombin in plasma to thrombin. 3. Thrombin converts fibrinogen, a plasma protein, to fibrin threads. 4. Fibrin forms a net that entangles cells and platelets, forming a clot.
Figure 37.6 The three-phase process of hemostasis. The micrograph shows the result of the final clotting phase—blood cells and platelets in a fibrin net.
Take-Home Message How does the body respond to blood vessel damage and halt bleeding? The vessel constricts, platelets accumulate, and cascading enzyme reactions involving protein components of plasma cause clot formation.
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Blood Typing
Links to Membrane proteins 5.2, ABO genetics 11.4
The plasma membrane of any cell includes many molecules that vary among individuals. An individual’s body ignores versions of these molecules that occur on its own cells, but unfamiliar cell surface molecules elicit defensive responses by the immune system. Agglutination is a normal response in which plasma proteins called antibodies bind foreign cells, such as bacteria, and form clumps that attract phagocytes. Agglutination can also occur when red blood cells with unfamiliar surface molecules are transfused into a person’s body. The result is a transfusion reaction, in which the recipient’s immune system attacks the donated cells, causing them to clump together (Figure 37.7). The clumps of cells clog small blood vessels and damage tissues. A transfusion reaction can be fatal. Blood typing—analysis of specific surface molecules on red blood cells—can help prevent mixing of blood from incompatible donors and recipients. It can also put physicians on the alert for blood-related problems that can arise during some pregnancies.
ABO Blood Typing ABO blood typing analyzes variations in one type of glycolipid on the surface of red blood cells. Section 11.4 describes the genetics of these variations. People who have one form of the molecule have type A blood. Those with a different form have type B blood. People with both forms of the molecule have type AB blood. Those who have neither form are type O. See below. ABO Type
Glycolipid(s) on Red Cells
Antibodies Present
A B AB O
A B Both A and B Neither A nor B
Anti-B Anti-A None Anti-A, Anti-B
If you are blood type O, your immune system treats both type A and type B cells as foreign. You can accept blood only from people who are type O (Figure 37.8). However, you can donate blood to anyone. If you are blood type A, your body will recognize type B cells as foreign. If you are type B, your blood will react against type A cells. If you are blood type AB, your immune system treats both type A and type B as “self,” so you can receive blood from anyone.
Rh+
a
Rh–
Rh + markers on the red blood cells of a fetus
b
Figure 37.7 Light micrographs showing (a) an absence of agglutination in a mixture of two different yet compatible blood types and (b) agglutination in a mixture of incompatible types. fetus
O
Blood Type of Donor A B
AB
A An Rh+ man and an Rh– woman carrying his Rh+ child. This is the mother’s first Rh+ pregnancy, so she has no anti-Rh+ antibodies. But during birth, some of the child’s Rh+ cells get into her blood.
Blood Type of Recipient
O
anti-Rh + antibody molecules
A
B
any subsequent Rh + fetus
AB
Figure 37.8 Animated Results of mixing blood of the same or differing ABO blood types. Figure It Out: How many incompatible combinations are shown? Answer: Seven
Rh Blood Typing Rh blood typing is based on the presence or absence of the Rh protein (first identified in blood of Rhesus monkeys). If you are type Rh+, your blood cells bear this protein. If you are type Rh–, they do not. Normally, Rh– individuals do not have antibodies against the Rh protein. However, they will produce such antibodies if they are exposed to Rh+ blood. This can happen during some pregnancies. If an Rh+ man impregnates an Rh– woman, the resulting fetus may be Rh+. The first time that an Rh– woman carries an Rh+ fetus, she will not have antibodies against the Rh protein (Figure 37.9a). However, fetal red blood cells may get into her blood during childbirth, causing her to form anti-Rh+ antibodies. If the woman gets pregnant again, these antibodies cross the placenta and get into fetal blood. If a fetus is Rh+, the antibodies attack
B The foreign marker stimulates antibody formation. If this woman gets pregnant again and if her second fetus (or any other) carries the Rh+ protein, her anti-Rh+ antibodies may attack the fetal red blood cells.
Figure 37.9 Animated How Rh differences can complicate pregnancy.
its red blood cells and can kill the fetus (Figure 37.9b). To prevent any problems, an Rh– mother who has just given birth to an Rh+ child should be injected with a drug that blocks production of antibodies that could cause problems during future pregnancies. ABO blood types do not cause a similar condition because maternal antibodies for A and B molecules do not cross the placenta and attack the fetal cells. Take-Home Message What is a blood type? Blood type refers to the kind of surface molecules on red blood cells. Genes determine which form of these molecules is present in a particular individual. When blood of incompatible types mixes, the immune system attacks the unfamiliar molecules, with results that can be fatal.
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37.5
Human Cardiovascular System The term “cardiovascular” comes from the Greek kardia (for heart) and Latin vasculum (vessel). Links to Alcohol metabolism Chapter 6 introduction, Glycogen storage 8.7, Homeostasis 27.1 and 27.3
left pulmonary artery
right pulmonary artery
capillary bed of left lung
capillary bed of right lung pulmonary trunk
to systemic circuit
from systemic circuit
A
Pulmonary Circuit for Blood Flow
(pulmonary vessels to and from thoracic cavity)
pulmonary veins
heart
capillary beds of head, upper extremities to pulmonary circuit
aorta from pulmonary circuit
heart
(diaphragm, the muscular partition between thoracic and abdominal cavities)
capillary beds of other organs in thoracic cavity
capillary bed of liver
In humans, as in all mammals, the heart is a double pump that propels blood through two cardiovascular circuits. Each circuit extends from the heart, through arteries, arterioles, capillaries, venules, and veins, and reconnects with the heart (Figures 37.10 and 37.11). A short loop, the pulmonary circuit, oxygenates blood (Figure 37.10a). It leads from the heart’s right half to capillary beds in the lungs. Blood is oxygenated in the lungs, then flows to the heart’s left half. The systemic circuit is a longer loop (Figure 37.10b). The heart’s left half pumps oxygenated blood into the main artery in the body: the aorta. That blood gives up oxygen in all tissues, then the oxygen-poor blood flows back to the heart’s right half. In the systemic circuit, most blood flows through one capillary bed, then returns to the heart. However, blood that passes through the capillaries in the small intestine then flows through the hepatic portal vein to a capillary bed in the liver. This arrangement allows the blood to pick up glucose and other substances absorbed from the gut, and deliver them to the liver. The liver stores some of the absorbed glucose as glycogen (Section 8.7). It also breaks down some absorbed toxins, including alcohol (Chapter 6 introduction). As Figure 37.12 shows, the cardiovascular system distributes nutrients, gases, and other substances that enter the body by way of the digestive system and respiratory system. It moves carbon dioxide and other metabolic wastes to the respiratory and urinary systems for disposal. These are the main systems that keep operating conditions of the internal environment within tolerable ranges, a process we call homeostasis (Sections 27.1 and 27.3).
Take-Home Message What are the two circuits of the human circulatory system? In the pulmonary circuit, oxygen-poor blood flows from the heart, through a pair of lungs, then back to the heart. It takes up oxygen and gives up carbon dioxide in the lungs. In the systemic circuit, oxygenated blood flows from the heart to capillary beds of all tissues. There it gives up oxygen and takes up carbon dioxide, then flows back to the heart.
capillary beds of intestines
B
Systemic Circuit for Blood Flow
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HOW ANIMALS WORK
Figure 37.10 Animated (a,b) Pulmonary and systemic circuits of the human cardiovascular system. Blood vessels carrying oxygenated blood are shown in red. Those that hold oxygen-poor blood are color-coded blue.
Jugular Veins
Carotid Arteries
Receive blood from brain and from tissues of head
Deliver blood to neck, head, brain
Ascending Aorta Superior Vena Cava
Carries oxygenated blood away from heart; the largest artery
Receives blood from veins of upper body
Pulmonary Arteries
Pulmonary Veins
Deliver oxygen-poor blood from the heart to the lungs
Deliver oxygenated blood from the lungs to the heart
Coronary Arteries Service the incessantly active cardiac muscle cells of heart
Hepatic Vein Carries blood that has passed through small intestine and then liver
Brachial Artery
Renal Vein
Renal Artery
Carries processed blood away from kidneys
Delivers blood to kidneys, where its volume, composition are adjusted
Delivers blood to upper extremities; blood pressure measured here
Inferior Vena Cava
Abdominal Aorta
Receives blood from all veins below diaphragm
Delivers blood to arteries leading to the digestive tract, kidneys, pelvic organs, lower extremities
Iliac Veins
Iliac Arteries
Carry blood away from the pelvic organs and lower abdominal wall
Deliver blood to pelvic organs and lower abdominal wall
Femoral Artery Femoral Vein
Delivers blood to the thigh and inner knee
Carries blood away from the thigh and inner knee
Figure 37.11 Animated Major blood vessels of the human cardiovascular system. This art is simplified for clarity. For example, the arteries or veins labeled for one arm occur in both arms.
food, water intake
oxygen intake
Digestive System nutrients, water, salts
Respiratory System oxygen
elimination of carbon dioxide
carbon dioxide
Circulatory System
Urinary System water, solutes
Figure 37.12 Functional links between the circulatory system and other organ systems with major roles in maintaining the internal environment.
elimination of food residues
rapid transport to and from all living cells
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CIRCULATION 645
elimination of excess water, salts, wastes
37.6
The Human Heart The heart is a durable, spontaneously beating, muscular pump. It contracts with a wringing motion.
Links to Cell junctions 32.1, Muscle contraction 36.7, 36.8
Heart Structure and Function The human heart’s durability arises from its structure. Outermost is the pericardium, a tough, two-layered sac of connective tissue (Figure 37.13). Fluid between the layers lubricates the heart during its wringing motions. The inner pericardial layer attaches to the heart wall, or myocardium, of cardiac muscle. Each half of the heart has an atrium (plural, atria), an entrance chamber for blood, and a ventricle that pumps blood out. Endothelium, a kind of epithelium, lines the heart chambers and all blood vessels. To get from an atrium into a ventricle, blood must travel through an atrioventricular (AV) valve. To flow from a ventricle into an artery, it has to pass through a semilunar valve. Heart valves are like one-way doors. High fluid pressure forces the valve open. When fluid pressure declines, the valve shuts and prevents blood from flowing backwards.
In the cardiac cycle, heart muscle alternates through diastole (relaxation) and systole (contraction). First, the relaxed atria expand with blood (Figure 37.14a). Fluid pressure forces the AV valves open. This allows blood to flow into the relaxed ventricles, which expand as the atria contract (Figure 37.14b). Once the ventricles have filled, they contract. As they do, the fluid pressure in them rises so sharply above the pressure in the great arteries that both semilunar valves open, and blood flows out (Figure 37.14c). Now emptied, the ventricles relax while the atria fill again (Figure 37.14d). Contraction of the thick-walled ventricles provides the driving force for blood circulation. Contractions of thinner-walled atria serve only to fill the ventricles.
How Does Cardiac Muscle Contract? Sections 36.7 and 36.8 describe skeletal muscle contraction. Cardiac muscle, found only in the heart, contracts by the same type of ATP-driven sliding-filament mechanism. Compared to skeletal muscle and smooth muscle, cardiac muscle has more mitochondria. Cardiac Muscle Revisited
arch of aorta
superior vena cava (flow from head, arms)
trunk of pulmonary arteries (to lungs)
right semilunar valve (shown closed) to pulmonary trunk
left semilunar valve (closed) to aorta
right pulmonary veins (from lungs)
right lung
left lung
B The heart is located between the lungs in the thoracic cavity.
1 2
ribs 1–8 3 4 5
left pulmonary veins (from lungs)
6 7
right atrium
left atrium
8
pericardium right AV valve (opened)
left AV valve (opened)
right ventricle
left ventricle
(muscles that prevent valve from everting)
endothelium and underlying connective tissue
inferior vena cava (from trunk, legs)
myocardium
septum (partition between heart’s two halves)
inner layer of pericardium
A
heart’s apex
A cutaway view shows the heart’s internal organization.
Figure 37.13 Animated The human heart.
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diaphragm
C Outer appearance. Pads of fat on the heart’s surface are normal.
intercalated disk A Atria fill. Fluid pressure opens the AV valves, blood flows into the ventricles.
B Next, atria contract. As fluid pressure rises in the ventricles, AV valves close.
a branching cardiac muscle cell (part of one cardiac muscle fiber)
a D Ventricles relax. Semilunar valves close as atria begin filling for the next cardiac cycle.
C Ventricles contract. Semilunar valves open. Blood flows into aorta and pulmonary artery.
Figure 37.14 Animated Cardiac cycle. You can hear the cycle through a stethoscope as a “lub-dup” near the chest wall. At each “lub,” the heart’s AV valves are closing as its ventricles are contracting. At each “dup,” the heart’s semilunar valves are closing as its ventricles are relaxing.
Sarcomeres arranged along the length of each cell give cardiac muscle a striated appearance. The cells attach end to end at intercalated disks, regions with many adhering junctions (Figure 37.15a). Neighboring cells communicate through gap junctions. These gap junctions allow waves of excitation to wash swiftly over the entire heart (Section 32.1 and Figure 37.15b).
b Part of a gap junction across the plasma membrane of a cardiac muscle cell. The junctions connect cytoplasm of adjoining cells and allow electrical signals that stimulate contraction to spread swiftly between them.
Figure 37.15 (a) Cardiac muscle cells. Compare Figure 32.8b. Many adhering junctions in intercalated disks at the ends of cells hold adjacent cells together, despite the mechanical stress caused by the heart’s wringing motions. (b) The sides of cardiac muscle cells are subject to less mechanical stress than the ends. The sides have a profusion of gap junctions across the plasma membrane.
SA node (cardiac pacemaker) AV node (the only point of electrical contact between atria and ventricles)
junctional fibers
branchings of junctional fibers (carry electrical signals through the ventricles)
How the Heart Beats In cardiac muscle, some special-
ized cells do not contract. Instead, they are part of the cardiac conduction system, which initiates and distributes signals that tell other cardiac muscle cells to contract. As Figure 37.16 shows, the system consists of a sinoatrial (SA) node and an atrioventricular (AV) node, functionally linked by junctional fibers. These fibers are bundles of long, thin cardiac muscle cells. The SA node, a clump of noncontracting cells in the right atrium’s wall, is the cardiac pacemaker. Its cells have specialized membrane channels that allow them to fire action potentials about seventy times per minute. The brain does not have to direct the SA node to fire; this natural pacemaker has spontaneous action potentials. Nervous signals from the brain only adjust the rate and strength of contractions. Even if a heart is removed from the body, it will keep beating for a short time. A signal from the SA node starts the cardiac cycle. The signal spreads through the atria, causing them to contract. Simultaneously, the signal excites junctional fibers, which conduct it to the AV node. This clump of cells is the only electric bridge to the ventricles. The
Figure 37.16 Animated The cardiac conduction system.
time it takes for a signal to cross this bridge is enough to keep ventricles from contracting before they fill. From the AV node, a signal travels along a bundle of fibers. These junctional fibers branch in the septum, between the heart’s left and right ventricles. Branching fibers extend down to the heart’s lowest point and up the ventricle walls. Ventricles contract from the bottom up, with a twisting motion.
Take-Home Message How is the human heart structured and how does it function? The four-chambered heart is partitioned into two halves, each with an atrium and a ventricle. Contraction of ventricles drives blood circulation. The SA node is the cardiac pacemaker. Its spontaneous, rhythmically repeated signals make cardiac muscle fibers of the heart wall contract in a coordinated fashion.
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CIRCULATION 647
37.7
Pressure, Transport, and Flow Distribution Contracting ventricles put pressure on the blood, forcing it through a series of vessels.
Link to Autonomic nervous system 33.8
Figure 37.17 compares the structure of blood vessels. Arteries are rapid-transport vessels for blood pumped out of the heart’s ventricles. They deliver blood to the arterioles: smaller vessels where controls over distribution of blood flow operate. Arterioles branch into capillaries, small, thin-walled vessels that substances diffuse into and out of easily. Venules are small vessels located between capillaries and veins. Veins are large vessels that deliver blood back to the heart and serve as blood volume reservoirs.
Blood pressure is the pressure exerted by the blood on the walls of the vessels that enclose it. Ventricular contractions put blood under pressure, and because the right ventricle contracts less forcefully than the left ventricle, blood entering the pulmonary circuit is under less pressure than blood entering the systemic circuit. In either circuit, blood pressure is highest in the arteries and declines as blood flows through the circuit (Figure 37.18). The rate of flow between two points in a circuit depends on the pressure difference between those points, and the resistance to flow. The wider and smoother a vessel is, the less resistance there is, and the faster fluid can move through it.
Rapid Transport in Arteries
outer coat
smooth muscle
basement membrane
endothelium
Artery
elastic tissue
With their large diameter and low resistance to flow, arteries are efficient rapid transporters of oxygenated blood. They also are pressure reservoirs that smooth out pressure differences during every cardiac cycle. Their thick, muscular, elastic wall bulges whenever a heartbeat forces a large volume of blood into them. Between contractions, the wall recoils.
Flow Distribution at Arterioles
elastic tissue
a
outer coat
smooth muscle rings basement over elastic tissue membrane
endothelium
Arteriole b basement membrane
endothelium
No matter how active you are, all blood from the right half of your heart flows to your lungs, and all blood from the left half is distributed to other tissues along the systemic circuit. The brain gets a constant supply of blood, but flow to other organs varies with activity. When you are resting, the blood flow is distributed as shown in Figure 37.19. When you exercise, less blood flows to the kidneys and gut, and more flows to skeletal muscles in your legs. Like traffic cops, your arterioles guide the flow
Capillary arteries
outer coat
smooth muscle, elastic fibers
basement membrane
endothelium
Vein valve
Blood pressure (mm Hg)
(venules have a similar structure)
c
capillaries
veins
(systolic)
120
80
(diastolic)
40
0
arterioles
d
Figure 37.17 Structural comparison of human blood vessels. The drawings are not to the same scale.
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venules
Figure 37.18 Plot of fluid pressure for a volume of blood as it flows through the systemic circuit. Systolic pressure occurs when ventricles contract, diastolic when ventricles are relaxed.
100%
20%
Figure 37.19 Distribution of the heart’s output in a resting person. How much blood flows through a given tissue can be adjusted by selectively narrowing and widening arterioles all along the systemic circuit.
15%
Figure It Out: What
13%
percentage of the brain’s blood supply arrives from the heart’s right half?
lungs
heart’s right half liver digestive tract kidneys skeletal muscle brain skin bone cardiac muscle all other regions
heart’s left half
6% 21%
9%
Answer: None
based on orders from the autonomic nervous system (Section 33.8) and endocrine system. Signals from both act on rings of smooth muscle cells in arteriole walls (Figure 37.17b). Some signals cause dilation, or widening, of a blood vessel by causing the smooth muscle cells in its wall to relax. Other signals decrease blood vessel diameter by causing the smooth muscle in its wall to contract. When arterioles that supply a particular organ widen, more blood flows to that organ. Arterioles also respond to shifts in concentrations of substances in a tissue. As an example, when you exercise, your skeletal muscle cells use up oxygen, and carbon dioxide concentration around them rises. Arterioles in the muscle widen in response to these localized changes. As a result, more oxygenated blood flows through the tissue, and more metabolic waste products are carried away. When skeletal muscles relax, they require less oxygen. The concentration of oxygen rises locally, and the arterioles narrow.
5% 3% 8%
Controlling Blood Pressure We generally measure blood pressure at the brachial artery in an upper arm (Figure 37.20). In each cardiac cycle, systolic (peak) pressure occurs when contracting ventricles force blood into arteries. Diastolic pressure, the lowest pressure, occurs when ventricles are most relaxed. Blood pressure is measured in millimeters of mercury (mm Hg) and recorded as “systolic pressure over diastolic pressure,” as in 120/80 mm Hg. Blood pressure depends on the total blood volume, how much blood the ventricles pump out (the cardiac output), and whether the arterioles are constricted or dilated. Receptors in the aorta and in the carotid arteries of the neck send signals to a control center in the medulla (a portion of the brain stem) when blood pressure increases or decreases. In response, this brain region calls for changes in cardiac output and arteriole diameter. This reflex response is a short-term control over blood pressure. Over the longer term, kidneys influence blood pressure by adjusting fluid loss and thus altering the total blood volume. The greater the blood volume, the higher the blood pressure.
Take-Home Message What determines blood pressure and distribution? The rate and strength of heartbeats and resistance to flow through blood vessels dictates blood pressure. Pressure is greatest in contracting ventricles and at the start of arteries.
How much blood flows to specific tissues varies over time and is altered by adjustments to the diameter of arterioles.
Figure 37.20 Animated Measuring blood pressure. Left, a hollow inflatable cuff attached to a pressure gauge is wrapped around the upper arm. A stethoscope is placed over the brachial artery, just below the cuff. The cuff is inflated with air to a pressure above the highest pressure of the cardiac cycle, when ventricles contract. Above this pressure, you will not hear sounds through the stethoscope, because no blood is flowing through the vessel. Air in the cuff is slowly released until the stethoscope picks up soft tapping sounds. Blood flowing into the artery under the pressure of the contracting ventricles—the systolic pressure—causes the sounds. When these sounds start, a gauge typically reads about 120 mm Hg. That amount of pressure will force mercury (Hg) to move up 120 millimeters in a glass column of a standard diameter. More air is released from the cuff. Eventually the sounds stop. Blood is now flowing continuously, even when the ventricles are the most relaxed. The pressure when the sounds stop is the lowest during a cardiac cycle. This diastolic pressure is usually about 80 mm Hg. Right, compact monitors are now available that automatically record the systolic/diastolic blood pressure.
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37.8
Diffusion at Capillaries, Then Back to the Heart A capillary bed is a diffusion zone, where blood exchanges substances with the interstitial fluid bathing cells before veins carry it back to the heart.
Links to Epithelium 32.2, Diffusion 5.3, Endocytosis 5.5
Capillary Function A capillary is a cylinder of endothelial cells, one cell thick, wrapped in basement membrane (Section 32.2). Figure 37.21 shows a few of the 10 billion to 40 billion capillaries that service a human body. Collectively, they offer a huge surface area for exchange of substances with interstitial fluid. In nearly all tissues, cells are very close to one or more capillaries. Proximity is essential. Diffusion distributes molecules and ions so slowly that it is effective only over small distances. Red blood cells, which are about 8 micrometers in diameter, have to squeeze in single file through the capillaries. The squeeze puts oxygen-transporting red
blood cells and solutes in the plasma in direct or near contact with the exchange surface—the capillary wall. To move between the blood and interstitial fluid, a substance must cross a capillary wall. Oxygen, carbon dioxide, and small lipid-soluble molecules can diffuse across endothelial cells of a capillary. Proteins are too big to diffuse across plasma membranes, but some do enter endothelial cells by endocytosis, diffuse through the cell, then escape by exocytosis on the opposite side. Also, fluid with small solutes and ions leaks out of capillaries through spaces between adjacent cells. Compared to other capillaries in the body, those in the brain are much less leaky. Brain endothelial cells adhere so tightly to one another that plasma does not leak between them. This property of brain capillaries creates the blood–brain barrier (Section 33.10). As blood flows through a typical capillary bed, it is subject to two opposing forces. Hydrostatic pressure, an outward-directed force, results from contraction of
blood to venule
high pressure causes outward flow
inward-directed osmotic movement
10 µm
cells of tissue
blood from arteriole
A
Arteriole end of capillary bed Figure 37.21 Fluid movement at a capillary bed. Fluid crosses a capillary wall by way of ultrafiltration and reabsorption. (a) At the capillary’s arteriole end, a difference between blood pressure and interstitial fluid pressure forces out plasma, but few plasma proteins, through clefts between endothelial cells of the capillary wall. Ultrafiltration is the outward flow of fluid across the capillary wall as a result of hydrostatic pressure. (b) Reabsorption is the osmotic movement of some interstitial fluid into the capillary. It happens when the water concentration between interstitial fluid and the plasma differs. Plasma, with its dissolved proteins, has a greater solute concentration and therefore a lower water concentration. Reabsorption near the end of a capillary bed tends to balance ultrafiltration at the start of it. Normally, there is only a small net filtration of fluid, which vessels of the lymphatic system return to blood (Section 37.10).
650 UNIT VI
HOW ANIMALS WORK
B
Outward-Directed Pressure: Hydrostatic pressure of blood in capillary: Osmosis due to interstitial proteins:
Venule end of capillary bed Outward-Directed Pressure:
35 mm Hg
Hydrostatic pressure of blood in capillary:
15 mm Hg
28 mm Hg
Osmosis due to interstitial proteins:
28 mm Hg
Inward-Directed Pressure: Hydrostatic pressure of interstitial fluid: Osmosis due to plasma proteins:
Inward-Directed Pressure: 0
Hydrostatic pressure of interstitial fluid:
0
3 mm Hg
Osmosis due to plasma proteins:
3 mm Hg
Net Ultrafiltration Pressure: (35 – 0) – (28 – 3) = 10 mm Hg
Ultrafiltration favored
Net Reabsorption Pressure: (15 – 0) – (28 – 3) = –10 mm Hg
Reabsorption favored
blood flow to heart
Figure 37.22 Venous valve action. (a) Valves in medium-sized veins prevent the backflow of blood.
valve open
valve closed
valve closed
valve closed
venous valve a
b
the ventricles. Osmotic pressure, an inward-directed force, results from differences in solute concentration between blood and interstitial fluid. At the arterial end of a capillary bed, hydrostatic pressure is high. It forces fluid out between cells of the capillary wall, into interstitial fluid (Figure 37.21a). This process is ultrafiltration. The fluid forced out has high levels of oxygen, ions, and nutrients such as glucose. Ultrafiltration moves large quantities of essential substances from blood into interstitial fluid. As the blood continues on to the venous end of the capillary bed, hydrostatic pressure drops and osmotic pressure predominates (Figure 37.21b). Water is drawn by osmosis from interstitial fluid into the protein-rich plasma. This process is capillary reabsorption. Normally, there is a small net outward flow of fluid from capillaries, which lymph vessels return to blood. If high blood pressure causes too much fluid to flow out or something interferes with fluid return, interstitial fluid collects in tissues. The resulting swelling is called edema. Roundworm infections that damage lymph vessels also cause severe edema (Section 25.11).
c
Adjacent skeletal muscles helps raise fluid pressure inside a vein. (b) These muscles bulge into a vein as they contract. Pressure inside the vein rises and helps keeps blood flowing forward. (c) When muscles relax, the pressure that they exerted on the vein is lifted. Venous valves shut and cut off backflow.
Sometimes venous valves lose their elasticity. Then veins become enlarged and bulge near the surface of skin. This elasticity loss commonly occurs in veins of the legs; they become varicose veins. Failure of valves in veins around the anus causes hemorrhoids. The vein wall can bulge quite a bit under pressure, much more so than an arterial wall. Thus, veins act as reservoirs for great volumes of blood. When you rest, they hold about 60 percent of the total blood volume. During exercise, fluid pressure in veins increases, and less blood collects inside them. Veins have a bit of smooth muscle inside their wall, and exercise-induced signals from the nervous system make it contract. The contraction causes veins to stiffen so they cannot hold as much blood, and the pressure inside them rises. At the same time, skeletal muscles that move limbs bulge and press against veins, squeezing blood toward the heart (Figure 37.22b,c). Exercise-induced deep breathing also raises venous pressure. As the chest expands, organs get squeezed and press against adjacent veins. The pressure assists in moving blood toward the heart.
Venous Pressure Blood from several capillaries flows into each venule. These thin-walled vessels join together to form veins, the large-diameter, low-resistance transport tubes that carry blood to the heart. Many veins, especially in the legs, have flaplike valves that help prevent backflow (Figure 37.22). These valves automatically shut when blood in the vein starts to reverse direction.
Take-Home Message How do capillaries and the venous system function? Capillary beds are diffusion zones where blood exchanges substances with interstitial fluid. Outward flow of fluid through capillary walls also contributes to the fluid balance between blood and interstitial fluid. Venules deliver blood from capillaries to veins. Veins are blood volume reservoirs. The amount of blood in the veins varies with activity level.
CHAPTER 37
CIRCULATION 651
37.9
Blood and Cardiovascular Disorders High blood pressure and atherosclerosis increase the risk of both heart attack and stroke. Links to Cholesterol 3.4, Sickle-cell anemia 3.6, 18.6, Hemophilia 12.4, Thalassemia 14.5, Diabetes 35.9
Red Blood Cell Disorders In anemias, red blood cells are few or compromised. As a result, oxygen delivery and metabolism falter. Shortness of breath, fatigue, and chills follow. Hemorrhagic anemias result from sudden blood loss, as from a wound; chronic anemias result from low red blood cell production or a slight but persistent blood loss. Bacteria and protozoans that replicate in red blood cells cause some hemolytic anemias. The pathogens get into red cells, divide inside them, and then cause the cell to break apart and die. A diet with too little iron causes iron-deficiency anemia, in which red blood cells cannot make enough iron-containing heme. Sickle-cell anemia arises from a mutation that alters hemoglobin and allows cells to change shape (Section 3.6). Beta-thalassemias occur when mutations disrupt or stop synthesis of a globin chains of hemoglobin (Section 14.5). Few red blood cells form and those that do are thin and fragile. Polycythemia is an excess of red blood cells. This increases oxygen delivery, but also makes blood more viscous and elevates blood pressure. White Blood Cell Disorders Epstein–Barr virus can cause infectious mononucleosis. The virus infects B lymphocytes and the body produces large numbers of monocytes in response. Symptoms typically last weeks and include a sore throat, fatigue, muscle aches, and low-grade fever. Leukemias are cancers that originate in cells of the bone marrow. They cause overproduction of abnormally formed white blood cells that do not function properly. Lymphomas are cancers that originate from B or T lymphocytes. Division of the cancerous lymphocytes produces tumors in lymph nodes and other parts of the lymphatic system. Clotting Disorders Too much or too little clotting can cause health problems. Hemophilia is a genetic disorder
in which clotting is impaired (Section 12.4). Other disorders cause clots to form spontaneously inside a vessel. A clot that forms inside a vessel and stays put is called a thrombus. A clot that breaks loose and travels in blood is an embolus. Both clot types can block vessels and cause problems. For example, a stroke occurs when a vessel in the brain ruptures or gets blocked by an embolus. Either way, blood flow to brain cells is disrupted. A person who survives a stroke often has impairments caused by death of blood-starved brain cells.
Atherosclerosis In atherosclerosis, buildup of lipids in the arterial wall narrows the lumen, or space inside the vessel. As you may know, cholesterol plays a role in this “hardening of the arteries.” The human body requires cholesterol to make cell membranes, myelin sheaths, bile salts, and steroid hormones (Section 3.4). The liver makes enough cholesterol to meet these needs, but more is absorbed from food in the gut. Genetics affects how different people’s bodies deal with an excess of dietary cholesterol. Most of the cholesterol dissolved in blood is bound to protein carriers. The complexes are known as low density lipoproteins, or LDLs, and most cells can take them up. A lesser amount is bound up in high density lipoproteins, or HDLs. Cells in the liver metabolize HDLs, using them in the formation of bile, which the liver secretes into the gut. Eventually, the bile leaves the body in the feces. When the LDL level in blood rises, so does the risk of atherosclerosis. The first sign of trouble is a buildup of lipids in an artery’s endothelium (Figure 37.23). Fibrous connective tissue forms over the entire mass. The mass, an atherosclerotic plaque, bulges into the vessel’s interior, narrowing its diameter and slowing blood flow. A hardened plaque can rupture an artery wall, thereby triggering clot formation. A heart attack occurs when a cardiac artery is completely blocked, most commonly by a clot. If the blockage is not removed fast, cardiac muscle cells die. Clot-dissolving drugs can restore blood flow if they are given within an hour of the onset of an attack, so a suspected heart attack should receive prompt attention. In coronary bypass surgery, doctors open a person’s chest and use a blood vessel from elsewhere in the body
atherosclerotic plaque
wall of artery, cross-section
Figure 37.23 Sections from (a) a normal artery and (b) an artery with a lumen narrowed by an atherosclerotic plaque. A clot clogged this one.
blood clot sticking to plaque
unobstructed lumen of a normal artery
652 UNIT VI
narrowed lumen a
HOW ANIMALS WORK
b
FOCUS ON HEALTH
(usually a leg vein) to divert blood around the clogged coronary artery. (Figure 37.24). In laser angioplasty, laser beams vaporize plaques. In balloon angioplasty, doctors inflate a small balloon in a blocked artery to flatten the plaques. A wire mesh tube called a stent is then inserted to keep the vessel open.
coronary artery
Hypertension—A Silent Killer Hypertension refers to chronically high blood pressure (above 140/90). Often the cause is unknown. Heredity is a factor, and African Americans have an elevated risk. Diet also plays a role; in some people high salt intake causes water retention that raises blood pressure. Hypertension is sometimes described as a silent killer, because people often are unaware they have it. Hypertension makes the heart work harder than normal. This can cause the heart to enlarge and to function less efficiently. High blood pressure also increases risk of atherosclerosis. An estimated 180,000 Americans die each year as a result of hypertension. Rhythms and Arrhythmias As you read in Section 37.6, the SA node controls the rhythmic beating of the heart. Electrocardiograms, or ECGs, record the electrical activity during the cardiac cycle (Figure 37.25a). ECGs can reveal arrhythmias, which are abnormal heart rhythms (Figure 37.25b–d). Arrhythmias are not always dangerous. For example, endurance athletes commonly experience bradycardia, a below-average resting heart rate. Ongoing exercise has made their heart more efficient, and the nervous system has adjusted the firing rate of the cardiac pacemaker downward. Tachycardia, a faster than normal heart rate, can be caused by exercise, stress, or some underlying heart problem. In atrial fibrillation, the atria do not contract normally. They quiver, which increases the risk of blood clots and stroke. Ventricular fibrillation is the most dangerous type of arrhythmia. It causes the ventricles to flutter, and their pumping action falters or stops. Blood flow halts, leading to loss of consciousness and death. A shock administered by a defibrillator such as the new AEDs mentioned in the chapter introduction can restore a heart’s normal rhythm. It does so by resetting the heart’s natural pacemaker, the SA node. Risk Factors Cardiovascular disorders are the leading cause of death in the United States. Each year, they affect about 40 million people, and about 1 million die. Tobacco smoking tops the list of risk factors. Other factors include a family history of such disorders, hypertension, a high cholesterol level, diabetes mellitus, and obesity (Section 35.9). Age also is a factor. The older you get, the greater the risk of cardiovascular disorders. Physical inactivity, too, increases the risk. Regular exercise helps lower the risk of cardiovascular disorders even when the exercise is not particularly strenuous. Gender is another factor; until about age fifty, males are at greater risk.
aorta
coronary artery blockage
location of a shunt made of a section taken from one of the patient’s other blood vessels
Figure 37.24 The photo shows coronary arteries and other blood vessels that service the heart. Resins were injected into them. Then the cardiac tissues were dissolved to make an accurate, three-dimensional corrosion cast. The sketch shows two coronary bypasses (color-coded green), which extend from the aorta past two clogged parts of the coronary arteries.
one normal heartbeat
0
Figure 37.25 (a) ECG of one normal beat of the human heart. (b–d) Recordings that identified three types of arrhythmias.
0.2 0.4 0.6 0.8
a time (seconds) bradycardia (here, 46 beats per minute)
b tachycardia (here, 136 beats per minute)
c ventricular fibrillation
d
CHAPTER 37
CIRCULATION 653
37.10 Interactions With the Lymphatic System Vessels and organs of the lymphatic system interact closely with the circulatory system.
Link to Thymus gland 35.12
Tonsils Defense against bacteria and other foreign agents Right Lymphatic Duct Drains right upper portion of the body
Thymus Gland Site where certain white blood cells acquire means to chemically recognize specific foreign invaders
Lymph Vascular System A portion of the lymphatic system, called the lymph vascular system, consists of vessels that collect water and solutes from interstitial fluid, then deliver them to the circulatory system. The lymph vascular system includes lymph capillaries and vessels (Figure 37.26). Fluid that moves through these vessels is the lymph. The lymph vascular system serves three functions. First, its vessels are drainage channels for water and plasma proteins that leaked out of capillaries and that must be returned to the circulatory system. Second, it delivers fats absorbed from food in the small intestine to the blood. Third, it transports cellular debris, pathogens, and foreign cells to lymph nodes, which serve as disposal sites. The lymph vascular system extends to capillary beds. There, excess fluid enters lymph capillaries. These capillaries have no obvious entrance;
lymph capillary
Thoracic Duct Drains most of the body
interstitial fluid flaplike “valve” made of overlapping cells at tip of lymph capillary
Spleen Major site of antibody production; disposal site for old red blood cells and foreign debris; site of red blood cell formation in the embryo
b
capillary bed
lymph trickles past organized arrays of lymphocytes
Some Lymph Vessels Return excess interstitial fluid and reclaimable solutes to the blood
Some Lymph Nodes Filter bacteria and many other agents of disease from lymph
c Bone Marrow Marrow in some bones is production site for infectionfighting blood cells (as well as red blood cells and platelets)
Figure 37.26 Animated (a) Components of the human lymphatic system and their functions. Not shown are patches of lymphoid tissue in the small intestine and in the appendix. (b) Diagram of lymph capillaries at the start of a drainage network, the lymph vascular system. (c) Cutaway view of a lymph node. Its inner compartments are packed with organized arrays of infection-fighting white blood cells.
a
654 UNIT VI
valve (prevents backflow)
HOW ANIMALS WORK
water and solutes move into clefts between cells. As you can see from Figure 37.26b, endothelial cells overlap, forming flaplike valves. Lymph capillaries merge into larger diameter lymph vessels, which have smooth muscle in their wall and valves that prevent backflow. Finally, lymph vessels converge onto collecting ducts, which drain into veins in the lower neck.
Lymphoid Organs and Tissues The other portion of the lymphatic system has roles in the body’s defense responses to injury and attack. It includes the lymph nodes, tonsils, adenoids, spleen, and thymus, as well as some patches of tissue in the wall of the small intestine and appendix. Lymph nodes are located at intervals along lymph vessels (Figure 37.26c). Lymph filters through at least one node before it enters the blood. Large numbers of lymphocytes (B and T cells) that formed in the bone marrow take up stations inside the nodes. When they identify pathogens in the lymph, they sound the alarm that summons up an immune response, as described in detail in the next chapter. Tonsils are two patches of lymphoid tissue at the back of the throat. Adenoids are similar tissue clumps at the rear of the nasal cavity. Tonsils and adenoids help the body respond fast to inhaled pathogens. The spleen is the largest lymphoid organ, about the size of a fist in an average adult. In embryos only, it functions as a site of red blood cell formation. After birth, the spleen filters pathogens, worn-out red blood cells and platelets from the many blood vessels that branch through it. The spleen has white blood cells that engulf and digest pathogens and altered body cells. It also holds antibody-producing B cells. People can survive removal of their spleen, but they become more vulnerable to infections. In the thymus gland, T lymphocytes differentiate and become capable of recognizing and responding to particular pathogens. The thymus gland also makes the hormones that influence these actions. It is central to immunity, the focus of the next chapter.
Summary Section 37.1 A circulatory system moves substances to and from interstitial fluid faster than diffusion alone could move them. Interstitial fluid fills spaces between cells. It exchanges substances with cells and with blood, a fluid transport medium. Some invertebrates have an open circulatory system, in which blood spends part of the time mingling with tissue fluids. In vertebrates, a closed circulatory system confines blood inside a heart, a type of muscular pump, and blood vessels, the smallest of which are capillaries. As lungs took on added importance in vertebrates on land, the circulatory system also evolved, making gas exchange more efficient. In birds and mammals, the heart has four chambers, so blood travels in two fully separated circuits. The systemic circuit carries blood from the heart to body tissues, then returns it to the heart. Blood in the pulmonary circuit moves from the heart to the lungs, then back to the heart.
Sections 37.2, 37.3 Blood is a fluid connective tissue that consists of plasma, blood cells, and platelets. Plasma is mostly water in which diverse ions and molecules are dissolved. Red blood cells, or erythrocytes, contain the hemoglobin that functions in rapid transport of oxygen and, to a lesser extent, carbon dioxide. They do not have a nucleus when mature. A variety of white blood cells, or leukocytes, have roles in day-to-day tissue maintenance and repair and in defenses against pathogens. Cell fragments called platelets interact with blood cells and plasma proteins in hemostasis after a vessel is damaged. Platelets and all blood cells arise from stem cells in bone marrow. A cell count is the number of blood cells of a specific type in a given volume. Section 37.4 Among the molecules on the surface of red blood cells are glycolipids and proteins that can be used to type an individual’s blood. The body mounts an attack against any cells that bear unfamiliar molecules, causing agglutination, or a clumping of cells. ABO blood typing helps match the blood of donors and recipients to avoid blood transfusion problems. Rh blood typing and the appropriate treatment prevent problems that can arise when maternal and fetal Rh blood types differ.
Take-Home Message What are the functions of the lymphatic system? The lymph vascular system consists of tubes that collect and deliver excess water and solutes from interstitial fluid to blood. It also carries absorbed fats to the blood, and delivers disease agents to lymph nodes.
The system’s lymphoid organs, including lymph nodes, have specific roles in body defenses.
Use the animation on CengageNOW to compare animal circulatory systems.
Use the animation on CengageNOW to learn about blood types and blood transfusions.
Section 37.5 The human heart is a four-chambered muscular pump, the contraction of which forces blood through two separate circuits. In the pulmonary circuit, oxygen-poor blood from the heart’s right half flows to the lungs, picks up oxygen, then flows to the heart’s left half. In the systemic circuit, the oxygen-rich blood flows from the heart’s left half, out the aorta, and to body tissues. Oxygen-poor blood returns to the heart’s right half. CHAPTER 37
CIRCULATION 655
IMPACTS, ISSUES REVISITED
And Then My Heart Stood Still
Traditional CPR alternates blowing into a person’s mouth to inflate their lungs with chest compressions. The requirement for mouthto-mouth contact makes many people reluctant to use this method on strangers. A new method called CCR (cardiocerebral resuscitation) relies on chest compressions alone. This method may be as good as or even better than traditional CPR as treatment for most people who have sudden cardiac arrest or a heart attack.
Most blood flows through only one capillary system, but blood in intestinal capillaries will later flow through liver capillaries. The liver metabolizes or stores nutrients and neutralizes some bloodborne toxins.
Use the animation on CengageNOW to explore the human cardiovascular system.
Section 37.6 A human heart is a double pump that consists mainly of cardiac muscle. It is partitioned into two halves, each with two chambers: an atrium that receives blood and a ventricle that expels it. During one cardiac cycle, all heart chambers undergo rhythmic relaxation (diastole) and contraction (systole). When a cycle starts, each atrium expands as blood fills it. Both ventricles already are filling as the atria contract. When ventricles contract, they force blood into the aorta and pulmonary arteries. Ventricular contraction provides the force that powers movement of blood through blood vessels. Atrial contraction simply fills the ventricles. A cardiac conduction system produces and distributes electrical signal that cause the heart’s beating. It consists of an SA node in the right atrium that is functionally linked by conducting fibers to an AV node. The SA node, the cardiac pacemaker, spontaneously generates the action potentials that set the pace for cardiac contractions. The nervous system does not initiate heartbeats; it only adjusts their rate and strength. Waves of excitation wash over the heart’s atria, down fibers in its septum, then up the walls of the ventricles.
Use the animation on CengageNOW to learn about the structure and function of the human heart.
Section 37.7 Blood pressure varies in the circulatory system. It is highest in contracting ventricles. It declines as blood travels through arteries, arterioles, capillaries, venules, and veins of the systemic or pulmonary circuit. It is lowest in relaxed atria. The speed of flow depends on heartbeat strength and rate, and on resistance to flow in the blood vessels. Adjusting the diameter of arterioles that supply different parts of the body redistributes the blood volume as necessary. In any interval, when a tissue needs more blood, the arterioles that supply it widen, allowing increased blood flow.
Use the animation on CengageNOW to see how blood pressure is measured.
Section 37.8 Substances move between the blood and interstitial fluid at capillary beds. Ultrafiltration pushes 656 UNIT VI
HOW ANIMALS WORK
How would you vote? Knowledge of CPR can save lives. Should high schools require students to learn CPR? See CengageNOW for details, then vote online.
a small amount of fluid out of capillaries. Fluid moves back in by capillary reabsorption. Normally, inward and outward directed forces are nearly balanced, but there is a small net outward flow from a capillary bed. Several capillaries drain into each venule. Veins are transport vessels that serve as a blood volume reservoir where the flow volume back to the heart is adjusted. Section 37.9 In a blood disorder, an individual has too many, too few, or abnormal red or white blood cells. Formation of blood clots inside vessels can cause health problems. Common circulatory disorders include atherosclerosis, hypertension (chronic high blood pressure), heart attacks, strokes, and certain arrhythmias. Regular exercise, maintaining normal body weight, and not smoking lower risk for these disorders. Section 37.10 Some fluid that leaves capillaries enters the lymph vascular system. The fluid, now called lymph, is filtered by lymph nodes. White blood cells in the nodes attack any pathogens. The spleen and thymus are organs of the lymphatic system. The spleen filters the blood and removes any old red blood cells. The thymus gland produces hormones and is the site where T lymphocytes (a kind of white blood cell) mature.
Learn about the human lymphatic system with the animation on CengageNOW.
Self-Quiz 1. The velocity of blood flow capillaries. a. increases b. decreases
Answers in Appendix III when blood enters c. stays the same
2. All vertebrates have . a. an open circulatory system b. a closed circulatory system c. a four-chambered heart d. both b and c 3. Which are not found in the blood? a. plasma b. blood cells and platelets c. gases and dissolved substances d. All of the above are found in blood. 4. A person who has type O blood . a. can receive a transfusion of blood of any type b. can donate blood to a person of any blood type c. can donate blood only to a person of type O d. cannot be a blood donor
Data Analysis Exercise Risk of death by stroke is not distributed evenly across the United States. Epidemiologists refer to a swath of states in the Southeast as the “stroke belt” because of the increased incidence of stroke deaths there. By one hypothesis, the high rate of deaths from stroke in this region results largely from a relative lack of access to immediate medical care. Compared to other parts of the country, more stroke-belt residents live in rural settings with few medical services. Figure 37.27 compares the rate of stroke deaths in strokebelt states (Alabama, Arkansas, Georgia, Mississippi, North Carolina, South Carolina, and Tennessee) with that of New York State. It also breaks down the death risk in each region by ethnic group and by sex.
Stroke-Belt States Total
63
American Indian Asian
3. Which group has the higher rate of stroke deaths, blacks living in New York, or whites living in the stroke belt? 4. Do these data support the hypothesis that poor access to care causes the high rate of death by stroke in the stroke belt?
5. In the blood, most oxygen is transported . a. in red blood cells c. bound to hemoglobin b. in white blood cells d. both a and c 6. Which has a more muscular wall? a. right atrium b. left ventricle
.
7. Blood flows directly from the left atrium to . a. the aorta c. the right atrium b. the left ventricle d. the pulmonary arteries 8. All blood cells descend from stem cells in a. the spleen c. the right atrium b. the left ventricle d. bone marrow
.
White
Female
.
12. At the start of a capillary bed (closest to arterioles), ultrafiltration moves . a. proteins into the capillary b. interstitial fluid into the capillary c. proteins into the interstitial fluid d. water, ions, and small solutes into interstitial fluid 13. Which is not a function of the lymphatic system? a. filters out pathogens b. returns fluid to the circulatory system c. helps certain white blood cells mature d. distributes oxygen to the tissues
Male 100
80
60 40 20 0 0 Age-adjusted death rate per 100,000 people
20
40
Figure 37.27 Comparison of the age-adjusted rate of deaths by stroke in Southeastern “stroke-belt” states and in New York State. Source: National Vital Statistics System—Mortality (NVSS-M), NCHS, CDC.
14. Match the components with their functions. capillary bed a. filters out pathogens lymph node b. cardiac pacemaker blood c. main blood volume ventricle reservoir SA node d. largest artery veins e. fluid connective tissue aorta f. zone of diffusion g. contractions drive blood circulation Visit CengageNOW for additional questions.
Critical Thinking
10. Blood pressure is highest in the and lowest in the . a. arteries; veins c. veins; arteries b. arterioles; venules d. capillaries; arterioles 11. At rest, the largest volume of blood is in a. arteries c. veins b. capillaries d. arterioles
Too few to compare
Black
9. Contraction of drives the flow of blood through the aorta and pulmonary arteries. a. atria c. ventricles b. arterioles d. skeletal muscle
33
Hispanic
1. How does the rate of stroke deaths among blacks living in the stroke-belt compare with whites in the same region? 2. How does the rate of stroke deaths among blacks living in New York compare with whites in the same region?
New York State
1. The highly publicized deaths of a few airline travelers led to warnings about economy-class syndrome. The idea is that sitting motionless for long periods on flights allows blood to pool and clots to form in legs. More recent studies suggest that long-distance flights cause problems in about 1 percent of air travelers, and that the risk is the same regardless of whether a person is in a first-class seat or an economy seat. Physicians suggest that air travelers drink plenty of fluids and periodically get up and walk around the cabin. Given what you know about blood flow in the veins, explain why these precautions can lower the risk of clot formation. 2. Mitochondria occupy about 40 percent of the volume of human cardiac muscle but only about 12 percent of the volume of skeletal muscle. Explain this difference. 3. In some people the valve between an atrium and ventricle does not close properly. This condition can be diagnosed by listening carefully to the heart. The listener will hear a whooshing sound called a murmur when the ventricle of the affected chamber contracts. What causes this sound? CHAPTER 37
CIRCULATION 657
38
Immunity IMPACTS, ISSUES
Frankie’s Last Wish
In October of 2000, Frankie McCullough had known for a few
times cause genital warts, but usually there are no symptoms
months that something was not quite right. She hadn’t had
of infection. Genital HPV is spread very easily by sexual con-
an annual checkup in many years; after all, she was only 31
tact. At least 80% of women have been infected by age 50.
and had been healthy her whole life. It never occurred to her
A genital HPV infection usually goes away on its own, but
to doubt her own invincibility until the moment she saw the
not always. A persistent infection with one of about 10 strains
doctor’s face change as he examined her cervix. Frankie
is the main risk factor for cervical cancer. Types 16 and 18
had cervical cancer.
are particularly dangerous: One of the two is found in more
The cervix is the lowest part of the uterus, or womb.
than 70% of all cervical cancers. In 2006, the FDA approved
Epithelial or endocrine cells of the cervix can become
Gardasil, a vaccine against four types of genital HPV, includ-
cancerous, but the process is usually slow. The cells pass
ing 16 and 18. The vaccine prevents cervical cancer caused
through several precancerous stages that are detectable by
by these HPV strains. It is most effective in girls who have not
routine Pap tests (Figure 38.1). Precancerous and even early-
yet become sexually active, because they are least likely to
stage cancerous cells can be removed from the cervix before
have become infected with any of those four strains of HPV.
they spread to other parts of the body. However, plenty of
The HPV vaccine came too late for Frankie McCullough.
women like Frankie do not take advantage of regular exams.
Despite radiation treatments and chemotherapy, her cervi-
Those who end up at the gynecologist’s office with pain or
cal cancer spread quickly. She died on September 16, 2001,
bleeding may be experiencing symptoms of advanced cer-
leaving a wish for other young women: awareness. “If there
vical cancer, the treatment of which offers only about a 9%
is one thing I could tell a young woman to convince them to
chance of survival. About 3,600 women die of cervical cancer
have a yearly exam, it would be not to assume that your youth
each year in the United States; many more than that die in
will protect you. Cancer does not discriminate; it will attack
places where routine gynecological testing is not common.
at random, and early detection is the answer.” She was right;
What causes cancer? At least in the case of cervical can-
almost all of the women with newly diagnosed invasive cervi-
cer, we know the answer to that question: Healthy cervical
cal cancer have not had a Pap test in at least five years, and
cells are transformed into cancerous ones by infection with
many of them have never had one.
human papillomavirus (HPV). HPV is a DNA virus that infects
Pap tests, HPV vaccines, and all other medical tests and
skin and mucous membranes. There are about 100 different
treatments are direct benefits of our increasing understand-
types of HPV; a few cause warts on the hands or feet, or in
ing of the interplay of the human body with its pathogens, an
the mouth. About 30 others that infect the genital area some-
interaction that we call immunity.
See the video! Figure 38.1 HPV and cervical cancer. Left, Frankie McCullough (waving) died of cervical cancer in 2001. Above, a Pap test reveals cancer cells (with enlarged, irregularly shaped nuclei) among normal squamous epithelial cells of the cervix. Cells with multiple nuclei are indicative of HPV infection. The orange ball is a model of an HPV16 virus.
Links to Earlier Concepts
Key Concepts Overview of body defenses
In this chapter you will be integrating what you have learned about diseasecausing agents and their hosts (Section 21.8). You will apply your knowledge of prokaryotic cells and viruses (4.4, 4.5, 16.1, 21.1, 21.2) as you learn about their interactions with eukaryotic cells.
You will revisit what you know about protein structure (3.5), the endomembrane system (4.9), membrane proteins (5.2), endocytosis and phagocytosis (5.5), osmosis (5.6), fever (6.3), alternative splicing (14.3), cell junctions (32.1), and apoptosis (27.6) to understand the immune defenses of vertebrates.
This chapter has several examples of what happens when pathogens invade internal environments (27.1), including the human nervous system (33.13), joints (36.5), and the cardiovascular system (37.9).
Earlier sections on cell signaling (27.3, 33.6, 35.1) gave you the background to understand immune signaling mechanisms. You will see how body systems, including exocrine glands (32.2), skin (32.7), the circulatory system (37.2, 37.8), and the lymphatic system (37.10) work together to fight infection.
The vertebrate body has three lines of immune defenses. Surface barriers prevent invasion by ever-present pathogens; general innate responses rid the body of most pathogens; adaptive responses specifically target pathogens and cancer cells. Section 38.1
Surface barriers Skin, mucous membranes, and secretions at the body’s surfaces function as barriers that exclude most microbes. Sections 38.2, 38.3
Innate immunity Innate immune responses involve a set of general, immediate defenses against invading pathogens. Innate immunity includes phagocytic white blood cells, plasma proteins, inflammation, and fever. Section 38.4
Adaptive immunity In an adaptive immune response, white blood cells destroy specific pathogens or altered cells. Some make antibodies in an antibodymediated immune response; others destroy ailing body cells in a cell-mediated response. Sections 38.5–38.8
Immunity in our lives Vaccines are an important part of any health program. Failed or faulty immune mechanisms can result in allergies, immune deficiencies, or autoimmune disorders. The immune system itself is a target of human immunodeficiency virus (HIV). Sections 38.9–38.12
How would you vote? Clinical trials of some vaccines take place in underdeveloped countries that have fewer regulations governing human testing than the United States. Should clinical trials be held to the same ethical standards no matter where they take place? See CengageNOW for details, then vote online.
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38.1
Integrated Responses to Threats In vertebrates, the innate and adaptive immune systems work together to combat infection and injury.
Links to Phagocytosis 5.5, Coevolution of pathogens and hosts 21.8, Neuropeptides 33.6, White blood cells 37.2
Evolution of the Body’s Defenses Humans continually cross paths with a tremendous array of viruses, bacteria, fungi, parasitic worms, and other pathogens, but you need not lose sleep over this. Humans coevolved with these pathogens, so you have defenses that protect your body from them. Immunity, an organism’s capacity to resist and combat infection, began well before multicelled eukaryotes evolved from free-living cells. Mutations in membrane protein genes introduced new patterns in the proteins, patterns that were unique in cells of a given type. As multicellularity evolved, so did mechanisms of identifying the patterns as self, or belonging to one’s own body. By 1 billion years ago, nonself recognition had also evolved. Cells of all modern multicelled eukaryotes
Table 38.1
Innate and Adaptive Immunity Compared Innate Immunity
Adaptive Immunity
Response time
Immediate
About a week
How antigen is detected
Fixed set of receptors for molecular patterns found on pathogens
Random recombinations of gene sequences generates billions of receptors
Specificity of response
None
Specific antigens targeted
Persistence
None
Long-term
Table 38.2
Some Chemical Weapons in Immunity
Substance
Functions
Complement
Direct cell lysis; enhancement of lymphocyte responses
Cytokines
Cell-to-cell and cell–tissue communication:
Interleukins
Inflammation, T cell and B cell proliferation and differentiation, bone marrow stem cell stimulation, neutrophil chemotaxis, NK cell activation, fever
Interferons
Resistance to virus infection, NK cell activation
TNFs
Inflammation; tumor cell destruction
bear a set of receptors that collectively can recognize around 1,000 different nonself cues, which are called pathogen-associated molecular patterns (PAMPs). As their name suggests, PAMPs occur mainly on or in pathogens. They include some components of prokaryotic cell walls, bacterial flagellum and pilus proteins, double-stranded RNA unique to some viruses, and so on. When a cell’s receptors bind to a PAMP, they trigger a set of immediate, general defense responses. In mammals, for example, binding triggers activation of complement. Complement is a set of proteins that circulate in inactive form throughout the body. Activated complement can destroy microorganisms or flag them for phagocytosis (Section 5.5). Pattern receptors and the responses they initiate are part of innate immunity, a set of fast, general defenses against infection. All multicelled organisms start out life with these defenses, which do not change within the individual’s lifetime. Vertebrates have another set of defenses carried out by interacting cells, tissues, and proteins. This adaptive immunity tailors immune defenses to a vast array of specific pathogens that an individual may encounter during its lifetime. It is triggered by antigen: a PAMP or any other molecule or particle recognized by the body as nonself. Most antigens are polysaccharides, lipids, and proteins typically present on viruses, bacteria or other foreign cells, tumor cells, toxins, and allergens.
Three Lines of Defense
Other chemicals (enzymes, peptides, clotting factors, toxins, hormones, protease inhibitors)
Figure 38.2 A physical barrier to infection: mucus and the mechanical action of cilia keep pathogens from getting a foothold in the airways to the lungs. Bacteria and other particles get stuck in mucus secreted by goblet cells (gold ). Cilia (pink ) on other cells sweep the mucus toward the throat for disposal.
Antimicrobial activities, cell lysis, complement activation and binding, coagulation, signaling, other diverse functions
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The mechanisms of adaptive immunity evolved within the context of innate immunity. The two systems were once thought to operate independently of each other, but we now know they function together.
We describe both systems together in terms of three lines of defense. The first line comprises the physical, chemical, and mechanical barriers that keep pathogens on the outside of the body (Figure 38.2). Innate immunity, the second line of defense, begins after tissue is damaged, or after a PAMP is detected inside the body. Its general response mechanisms rid the body of many different kinds of invaders before populations of them become established in the internal environment. Activation of innate immunity triggers the third line of defense, adaptive immunity. White blood cells form huge populations that target a specific antigen and destroy anything bearing it. Some of the cells persist after infection ends. If the same antigen returns, these memory cells mount a secondary response. Adaptive immunity can specifically target billions of antigens. Table 38.1 compares innate and adaptive immunity.
The Defenders White blood cells (Figure 38.3) carry out all immune responses. Many kinds circulate through the body in blood and lymph; others populate the lymph nodes, spleen, and other tissues. Some white blood cells are phagocytic; all are secretory. Their secretions include cell-to-cell signaling molecules called cytokines. These peptides and proteins coordinate all aspects of immunity. Vertebrate cytokines include interleukins, interferons, and tumor necrosis factors (Table 38.2). Different types of white blood cells are specialized for specific tasks, such as phagocytosis. Neutrophils are the most abundant of the circulating phagocytes. Macrophages that patrol tissue fluids are mature monocytes, which patrol the blood. Dendritic cells alert the adaptive immune system to the presence of antigen. Some white blood cells contain secretory vesicles: granules that hold cytokines, enzymes, or pathogenbusting toxins. Eosinophils target parasites too big for phagocytosis. Basophils circulating in blood and mast cells anchored in tissues secrete substances contained by their granules in response to injury or antigen. Often associated with nerves, mast cells also respond to neuropeptides (Section 33.6), so they link the nervous and immune systems. Lymphocytes are a special category of white blood cells that are central to adaptive immunity. B and T lymphocytes (B and T cells) have the capacity to collectively recognize billions of specific antigens. There are several kinds of T cells, including some that target infected or cancerous body cells. Natural killer cells (NK cells) can destroy infected or cancerous body cells that are undetectable by cytotoxic T cells.
Macrophage
Phagocyte; presents antigen to helper T cells; secretes cytokines. Circulates in blood in immature form; matures only after it enters damaged tissue.
Neutrophil
Fast-acting and most abundant phagocyte. Circulates in blood; migrates into damaged tissues.
Eosinophil
Granules contain enzymes that target parasitic worms. Circulates in blood; migrates into damaged tissues.
Basophil
Granules contain histamine and other substances that cause inflammation. Circulates in blood.
Mast cell
Anchored in tissues. Granules contain histamine, other substances that cause inflammation; contributes to allergies.
Dendritic cell
Lymphocytes:
Phagocyte that presents antigen to naive T cells. Circulates in blood in immature form; takes up residence in tissues when mature.
Act in most immune responses. After antigen recognition, clonal populations of effector and memory cells form and circulate in blood and tissue fluid.
B cell
Recognizes antigens via membranebound antibodies. It is the only type of cell that produces antibodies.
T cell
Helper T cells coordinate all immune responses, and activate naive B cells and T cells. Cytotoxic T cells recognize antigen–MHC complexes, and touch-kill infected, cancerous, or foreign cells.
Natural killer (NK) cell
Cytotoxic; kills stressed body cells that lack MHC markers; also kills antibodytagged cells.
Figure 38.3 White blood cells (leukocytes). Staining shows details such as cytoplasmic granules that contain enzymes, toxins, and signaling molecules.
Take-Home Message What is immunity? The innate immune system is a set of general defenses against a fixed number of antigens. It acts immediately to prevent infection. Vertebrate adaptive immunity is a system of defenses that can specifically target billions of different antigens. White blood cells are central to both systems; signaling molecules such as cytokines integrate their activities.
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38.2
Surface Barriers Table 38.3 A pathogen can cause infection only if it enters the internal environment by penetrating skin or other protective barriers at the body’s surfaces.
Vertebrate Surface Barriers
Links to Bacterial cell walls 4.4, Internal environment 27.1, Hair follicles and skin 32.7
Your skin is in constant contact with the external environment, so it picks up many microorganisms. It normally teems with about 200 different kinds of yeast, protozoa, and bacteria (Figure 38.4a). If you showered today, there are probably thousands of them on every square inch of your external surfaces. If you did not, there may be billions. They tend to flourish in warmer, moister parts, such as between the toes. Huge populations inhabit cavities and tubes that open out on the body’s surface, including the eyes, nose, mouth, and anal and genital openings. Microorganisms that typically live on human surfaces, including the interior tubes and cavities of the digestive and respiratory tracts, are called normal flora. Our surfaces provide them with a stable environment and nutrients. In return, their populations deter more aggressive species from colonizing (and penetrating) body surfaces; help us digest food; and make nutrients that we depend on, including a cobalt-containing vitamin (B12) made only by bacteria. Normal flora are helpful only on the outside of body tissues. Consider a type of rod-shaped bacteria that is a major constituent of normal flora, Propionibacterium acnes (Figure 38.4b). It feeds on sebum, a greasy mixture of fats, waxes and glycerides that lubricates hair and skin. Sebaceous glands secrete sebum into hair follicles (Section 32.7). During puberty, higher levels
a
2 µm
b
Physical
Intact skin and epithelia that line tubes and cavities such as the gut and eye sockets; established populations of normal flora
Mechanical
Mucus; broomlike action of cilia; flushing action of tears, saliva, urination, diarrhea
Chemical
Secretions (sebum, other waxy coatings); low pH of urine, gastric juices, urinary and vaginal tracts; lysozyme
of steroid hormones trigger sebaceous glands to make more sebum than before. Excess sebum combines with dead, shed skin cells and so blocks the openings of hair follicles. P. acnes can survive on the surface of the skin, but far prefer anaerobic habitats such as the interior of blocked hair follicles. There, they multiply to tremendous numbers. Secretions of the flourishing P. acnes populations leak into internal tissues, attracting neutrophils that initiate inflammation in the tissue around the follicles. The resulting pustules are called acne. Normal flora can cause serious illness if they invade tissues. The bacterial agent of tetanus, Clostridium tetani, passes through our intestines so often that we consider it a normal inhabitant. The bacteria responsible for diphtheria, Corynebacterium diphtheriae, was normal skin flora before widespread use of the vaccine eradicated the disease. Staphylococcus aureus, a resident of human skin, nasal membranes, and intestines, is also a leading cause of human bacterial disease (Figure 38.4c). Normal flora cause or worsen pneumonia; ulcers; colitis; whooping cough; meningitis; abscesses of the lung and brain; and colon, stomach, and intestinal cancers.
1 µm
c
Figure 38.4 Some microbial inhabitants of human surfaces. (a) Staphylococcus epidermidis, the most common colonizer of human skin. (b) Propionibacterium acnes, the bacterial cause of acne. (c) Staphylococcus aureus cells (yellow) adhering to mucus-coated cilia of human nasal epithelial cells. S. aureus is a common inhabitant of human skin and linings of the mouth, nose, throat, and intestines. It is also the leading cause of bacterial disease in humans. Antibiotic-resistant strains of S. aureus are now widespread. A particularly dangerous kind (MRSA) that is resistant to all penicillins is now endemic in most hospitals around the world. MRSA is called a “superbug.”
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FOCUS ON HEALTH
38.3
skin surface epithelial cells die and become filled with keratin as they are pushed toward skin surface
Remember to Floss
Nine of every ten cardiovascular disease patients have serious periodontal disease. There is a connection. Links to Biofilms 4.5, Cell junctions 32.1, Cardiovascular disease 37.9
epidermis
dividing epithelial cells
Figure 38.5 One surface barrier to infection: epidermis of human skin. 0.1 mm
In contrast to body surfaces, the blood and tissue fluids of healthy people are typically microorganismfree. Physical, chemical, and mechanical barriers keep microorganisms on the outside of body tissues (Table 38.3). For example, healthy, intact skin is an effective physical barrier. Vertebrate skin has a tough outer layer (Figure 38.5). Microorganisms flourish on this waterproof, oily surface, but rarely penetrate it. Sticky mucus that coats the surfaces of many epithelial linings can trap microorganisms. Broomlike cilia on cells of the linings sweep the trapped microorganisms toward the outside of the body (Figure 38.4c). Mucus also contains lysozyme, an enzyme that chops up the polysaccharides in bacterial cell walls and so unravels their structure. Lysozyme ensures that bacteria stuck in the mucus do not survive long enough to breach the walls of the sinuses and lower respiratory tract. Normal flora in the mouth resist lysozyme in saliva. Most microorganisms that enter the stomach are killed by gastric fluid, a potent brew of protein-digesting enzymes and acid. Most of those that survive to reach the small intestine are killed by bile salts. The hardy ones that make it to the large intestine must compete with about 500 resident species. Any that displace normal flora there are typically flushed out by diarrhea. Lactic acid produced by Lactobacillus helps keep the vaginal pH outside the range of tolerance of most fungi and other bacteria. Urination’s flushing action usually stops pathogens from colonizing the urinary tract.
Your mouth is a particularly inviting habitat for microorganisms, offering plenty of nutrients, warmth, moisture, and surfaces for colonization. Accordingly, it harbors huge populations of various species of Streptococcus, Lactobacillus, Staphylococcus, and other bacteria. A few of the 400 or so species of microorganisms that normally live in the mouth cause dental plaque, a thick biofilm of various bacteria and occasional archaea, their extracellular products, and saliva glycoproteins. Plaque sticks tenaciously to teeth (Figure 38.6). Some bacteria that live in it are fermenters. They break down bits of carbohydrate that stick to teeth and then secrete organic acids, which etch away tooth enamel and make cavities. In young, healthy people, tight junctions (Section 32.1) between the gum epithelium and teeth form a barrier that keeps oral microorganisms out of the internal environment. As we age, the connective tissue beneath gum epithelium thins, and the barrier becomes vulnerable. Deep pockets form between the teeth and gums, and a very nasty gang of anaerobic bacteria and archaea accumulates in these pockets. Their noxious secretions, including destructive enzymes and acids, cause inflammation of surrounding gum tissues—a condition called periodontitis. Porphyromonas gingivalis is one of those anaerobic species. Along with every other species of oral bacteria associated with periodontitis, P. gingivalis also occurs in atherosclerotic plaque (Section 37.9). Periodontal wounds are an open door to the circulatory system and its arteries. Atherosclerosis is now known to be a disease of inflammation. Macrophages and T cells are attracted to lipid deposits in the vessel walls. Their secretions initiate inflammation that further attracts lipids, and the lesion grows as the immune cells die and become part of the deposits. What role the oral microorganisms play in this scenario is not yet clear, but one thing is certain—they contribute to the inflammation that fuels coronary artery disease.
Take-Home Message What prevents ever-present microorganisms from entering the body’s internal environment?
Surface barriers keep microorganisms that contact or inhabit vertebrate surfaces from invading the internal environment.
Figure 38.6 Plaque. Left, micrograph of toothbrush bristles scrubbing plaque on a tooth surface. Right, the main cause of plaque, Streptococcus mutans.
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38.4
Innate Immune Responses Innate immune mechanisms nonspecifically protect animals from pathogens that invade internal tissues. Links to Osmosis 5.6, Fever 6.3, Lysis 21.2, Effectors 27.3, Prostaglandins 35.1, Blood 37.2, Capillary function 37.8
What happens if a pathogen slips by surface defenses and enters the body’s internal environment? All animals are normally born with a set of fast-acting, offthe-shelf immune defenses that can keep an invading pathogen from establishing a population in the body’s internal environment. These innate immune defenses include phagocyte and complement action, inflammation, and fever—all general mechanisms that normally do not change much over an individual’s lifetime. Phagocytes and Complement Macrophages are large
phagocytes that engulf and digest essentially everything except undamaged body cells. They patrol the interstitial fluid, so they are often the first white blood cells to encounter an invading pathogen. When receptors on a macrophage bind to antigen, the cell begins to secrete cytokines. These signaling molecules attract more macrophages, neutrophils, and dendritic cells to the site of invasion.
activated complement
Antigen also triggers complement activation (Figure 38.7a,b). In vertebrates, about 30 different types of complement protein circulate in inactive form throughout the blood and interstitial fluid. Some become activated when they encounter antigen, or an antibody bound to antigen (we will return to antibodies in Section 38.6). The activated complement proteins are enzymes that cut other inactive complement proteins, which thereby become activated and cut other inactive complement proteins, and so on. These cascading reactions quickly produce tremendous concentrations of activated complement localized at the site of invasion. Activated complement attracts phagocytic cells. Like snuffling bloodhounds, these cells can follow complement gradients back to an affected tissue. Some complement proteins attach directly to pathogens. Phagocytes have complement receptors, so a pathogen coated with complement is recognized and engulfed faster than an uncoated pathogen. Other activated complement proteins self-assemble into complexes that puncture bacterial cell walls or plasma membranes (Figure 38.7c–e). Activated complement proteins also work in adaptive immunity, by guiding the maturation of immune cells and mediating some interactions among them.
attack complex that causes a pore to form through the lipid bilayer of the bacterium
activated complement
antibody molecule A In some responses, complement proteins become activated when antibodies (the Y-shaped molecules) bind to antigen—in this case, antigen on the surface of a bacterium.
activated complement
bacterial cell
B Complement also becomes activated when it binds directly to antigen.
C By cascading reactions, huge numbers of different complement molecules form and assemble into structures called attack complexes.
D The attack complexes become inserted into the target cell’s lipid envelope or plasma membrane. Each complex makes a large pore form across it.
Figure 38.7 Animated One effect of complement protein activation. Activation causes lysis-inducing pore complexes to form. The micrograph shows holes in a pathogen’s surface that were made by membrane attack complexes.
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E The pores bring about lysis of the cell, which dies because of the severe structural disruption.
A Bacteria invade a tissue and release toxins or metabolic products that damage tissue.
B Mast cells in tissue release histamine, which widens arterioles (causing redness and warmth) and increases capillary permeability.
C Fluid and plasma proteins leak out of capillaries; localized edema (tissue swelling) and pain result.
D Complement proteins attack bacteria. Clotting factors also wall off inflamed area.
E Neutrophils and macrophages engulf invaders and debris. Macrophage secretions kill bacteria, attract more lymphocytes, and initiate fever.
Figure 38.8 Animated Inflammation in response to bacterial infection. Above, in this example, white blood cells and plasma proteins enter a damaged tissue. Right, the micrograph shows a phagocyte squeezing through a blood vessel wall.
Activated complement and cytokines trigger inflammation, a local response to tissue damage. The outward symptoms include redness, warmth, swelling, and pain. Inflammation begins when pattern receptors on basophils, mast cells, or neutrophils bind to antigen, or when mast cells directly bind to activated complement. In response to the binding, the cells release prostaglandins, histamines, and other substances into the affected tissue (Section 35.1). These substances have two effects. First, they cause nearby arterioles to widen. As a result, blood flow to the area increases, reddening and warming the tissue. The increased flow speeds the arrival of more phagocytes, which are attracted to the cytokines. Second, the signaling molecules cause spaces between cells in capillary walls to widen, so they make capillaries in an affected tissue “leakier.” Phagocytes and plasma proteins squeeze between the cells, out of the blood vessel and into interstitial fluid (Figure 38.8). The transfer changes the osmotic balance across the capillary wall, so more water diffuses from the blood into tissue. The tissue swells with fluid, putting pressure on free nerve endings and thus giving rise to sensations of pain.
Inflammation
temperature set point. As long as the temperature of the body is below the new set point, the hypothalamus signals effectors (Section 27.3) to give rise to a sensation of cold, to constrict blood vessels in the skin, and to trigger shivering, or “chills.” All of these responses help raise the internal temperature of the body. Fever enhances immune defenses by increasing the rate of enzyme activity, thus speeding up metabolism, tissue repair, and formation and activity of phagocytes. Some pathogens multiply more slowly at the higher temperature, so white blood cells can get a head start in the proliferation race against them. A fever is a sign that the body is fighting something, so it should never be ignored. However, a fever of 40.6°C (105°F) or less does not necessarily require treatment in an otherwise healthy adult. Body temperature usually will not rise above that value, but if it does, immediate hospitalization is recommended because a fever of 42°C (107.6°F) can result in brain damage or death.
Take-Home Message
Fever is a temporary rise in body temperature above the normal 37°C (98.6°F) that often occurs in response to infection. Some cytokines stimulate brain cells to make and release prostaglandins, which act on the hypothalamus to raise the body’s internal
Fever
What is innate immunity? Innate immunity is the body’s built-in set of general immune defenses. Complement, phagocytes, inflammation, and fever quickly eliminate most invaders from the body before their populations become established.
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38.5
Overview of Adaptive Immunity Vertebrate adaptive immunity is defined by self/nonself recognition, specificity, diversity, and memory.
of different antigen receptors, so an individual has the potential to counter billions of different threats.
Links to Lysosomes 4.9, Recognition proteins 5.2, Phagocytosis 5.5, Lymphatic system 37.10
Memory refers to the capacity of the adaptive immune
If innate immune mechanisms do not quickly rid the body of an invading pathogen, populations of pathogenic cells may become established in body tissues. By that time, long-lasting adaptive immune mechanisms have begun to target the invaders specifically.
system to “remember” an antigen. It take a few days for B and T cells to respond in force the first time they encounter an antigen. If the same antigen shows up again, they make a faster, stronger response. That is why we do not get as sick the second time around.
First Step—The Antigen Alert Tailoring Responses to Specific Threats Life is so diverse that the number of different antigens is essentially unlimited. No system can recognize all of them, but vertebrate adaptive immunity comes close. Unlike innate immunity, the adaptive immune system changes: It “adapts” to different antigens an individual encounters during its lifetime. Lymphocytes and phagocytes interact to effect the four defining characteristics of adaptive immunity: self/nonself recognition, specificity, diversity, and memory. Self versus nonself recognition starts with the molec-
MHC marker
ular patterns that give each kind of cell or virus a unique identity. The plasma membrane of your cells bears MHC markers (left), which are self-recognition proteins named after the genes that encode them. Your T cells also bear antigen receptors called T cell receptors, or TCRs. Part of a TCR recognizes MHC markers as self; part also recognizes an antigen as nonself. Specificity means that defenses are tailored to target
specific antigens. Diversity refers to the antigen receptors on a body’s
collection of B and T cells. There are potentially billions
cell engulfs an antigen-bearing particle
Figure 38.9 Antigen processing. (a) A macrophage ingests a foreign cell. (b) From encounter to display, what happens when a B cell, macrophage, or dendritic cell engulfs an antigenic particle—in this case, a bacterium. These cells engulf, process, and then display antigen bound to MHC markers. The displayed antigen is presented to T cells.
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b
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HOW ANIMALS WORK
Recognition of a specific antigen is the first step of the adaptive immune response. A new B or T cell is naive, which means that no antigen has bound to its receptors yet. Once it binds to an antigen, it begins to divide by mitosis, and tremendous populations form. T cell receptors do not recognize antigen unless it is presented by an antigen-presenting cell. Macrophages, B cells, and dendritic cells do the presenting. First, they engulf something antigenic (Figure 38.9a). Vesicles that contain the antigenic particle form in the cells’ cytoplasm and fuse with lysosomes. Lysosomal enzymes digest the particle into bits (Sections 4.9 and 5.5). The lysosomes also contain MHC markers that bind to some of the antigen bits. The resulting antigen–MHC complexes become displayed at the cell’s surface when the vesicles fuse with (and become part of) the plasma membrane (Figure 38.9b). The display of MHC markers paired with antigen fragments is a call to arms. Any T cell that bears a receptor for this antigen will bind the antigen–MHC complex. The T cell then starts secreting cytokines, which signal all other B or T cells with the same antigen receptor to divide again and again. Huge populations of B and T cells form after a few days; all of the cells recognize the same antigen. Most are effector cells, differentiated lymphocytes that
endocytic vesicle forms
particle is digested into bits
antigen–MHC complexes become displayed on cell surface MHC markers bind fragments of particle
Antibody-Mediated Immune Response
Cell-Mediated Immune Response antigen-presenting cells
naive B cells
+
antigen
+ complement
naive helper T cells
activated B cells
effector helper T cells
naive cytotoxic T cells
effector B cells
memory helper T cells
effector cytotoxic T cells
lymph node, midsection
(thymus gland)
+
+
memory B cells
spleen
+
memory cytotoxic cells
Figure 38.10 Overview of key interactions between antibody-mediated and cell-mediated responses—the two arms of adaptive immunity. A “naive” cell is one that has not made contact with its specific antigen.
act at once. Some are memory cells, long-lived B and T cells reserved for future encounters with the antigen.
Two Arms of Adaptive Immunity Like a boxer’s one-two punch, adaptive immunity has two separate arms: the antibody-mediated and the cellmediated immune responses (Figure 38.10). These two responses work together to eliminate diverse threats. Not all threats present themselves in the same way. For example, bacteria, fungi, or toxins can circulate in blood or interstitial fluid. These cells are intercepted quickly by B cells and other phagocytes that interact in the antibody-mediated immune response. In this response, B cells produce antibodies, which are proteins that can bind to specific antigen-bearing particles. We return to antibodies in the next section. Some kinds of threats are not targeted by B cells. For example, B cells cannot detect body cells altered by cancer. As another example, some viruses, bacteria, fungi, and protists can hide and reproduce inside body cells; B cells can detect them only briefly, when they slip out of one cell to infect others. Such intracellular pathogens are targeted primarily by the cellmediated immune response, which does not involve antibodies. In this response, cytotoxic T cells and NK cells detect and destroy altered or infected body cells.
Intercepting and Clearing Out Antigen After engulfing an antigen-bearing particle, a dendritic cell or macrophage migrates to a lymph node (Section 37.10), where it will present antigen to many T cells
Figure 38.11 Battlegrounds of adaptive immunity. Lymph nodes along lymph vascular highways hold macrophages, dendritic cells, B cells, and T cells. The spleen filters antigenic particles from blood.
that filter through the node (Figure 38.11). Every day, about 25 billion T cells pass through each node. T cells that recognize and bind to antigen presented by a phagocyte initiate an adaptive response. Antigen-bearing particles in interstitial fluid flow through lymph vessels to a lymph node, where they meet up with arrays of resident B cells, dendritic cells, and macrophages. These phagocytes engulf, process, and present antigen to T cells that are passing through the node. Any antigenic particle that escapes a lymph node to enter blood is taken up by the spleen. During an infection, the lymph nodes swell because T cells accumulate inside of them. When you are ill, you may notice your swollen lymph nodes as tender lumps under the jaw or elsewhere. The tide of battle turns when the effector cells and their secretions destroy most antigen-bearing agents. With less antigen present, fewer immune fighters are recruited. Complement proteins assist in the cleanup by binding antibody–antigen complexes, forming large clumps that can be quickly cleared from the blood by the liver and spleen. Immune responses subside after the antigenic particles are cleared from the body.
Take-Home Message What is the adaptive immune system? Phagocytes and lymphocytes interact to bring about vertebrate adaptive immunity, which has four defining characteristics: self/nonself recognition, specificity, diversity, and memory. The two arms of adaptive immunity work together. Antibody-mediated responses target antigen in blood or interstitial fluid; cell-mediated responses target altered body cells.
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38.6
Antibodies and Other Antigen Receptors Antigen receptors give lymphocytes the potential to recognize billions of different antigens. Links to Protein structure 3.5, Membrane proteins 5.2, Alternative splicing 14.3, Exocrine glands 32.2
Antibody Structure and Function If we liken B cells to assassins, then each one has a genetic assignment to liquidate one particular target— an antigen-bearing extracellular pathogen or toxin. Antibodies are their molecular bullets. Antibodies are proteins, Y-shaped antigen receptors made only by B cells. Each can bind to the antigen that prompted its synthesis. Many antibodies circulate in blood and enter interstitial fluid during inflammation, but they do not kill pathogens directly. Instead, they activate complement, facilitate phagocytosis, prevent pathogens from attaching to body cells, and neutralize toxins. An antibody molecule consists of four polypeptides: two identical “light” chains and two identical “heavy”
chains (Figure 38.12). Each chain has a variable and a constant region. When the chains fold up together as an intact antibody, the variable regions form two antigen binding sites that have a specific distribution of bumps, grooves, and charge. These binding sites are the antigen receptor part of an antibody: they can bind only to antigen with a complementary distribution of bumps, grooves, and charge. In addition to the antigen-binding sites, each antibody also has a constant region that determines its structural identity, or class. There are five antibody classes: IgG, IgA, IgE, IgM, and IgD (Ig stands for immunoglobulin, which is another name for antibody). The different classes serve different functions (Table 38.4). Most of the antibodies circulating in the bloodstream and tissue fluids are IgG, which binds pathogens, neutralizes toxins, and activates complement. IgG is the only antibody that can cross the placenta to protect a fetus before its own immune system is active. IgA is the main antibody in mucus and other exocrine
Table 38.4
Structural Classes of Antibodies
Secreted antibodies IgG
Main antibody in blood; activates complement, neutralizes toxins; protects fetus and is secreted in early milk.
IgA
Abundant in exocrine gland secretions (e.g., tears, saliva, milk, mucus), where it occurs in dimeric form (shown). Interferes with binding of bacteria and viruses to body cells.
a
binding site for antigen
variable region (dark green) of heavy chain
binding site for antigen
Membrane-bound antibodies IgE
Anchored to surface of basophils, mast cells, eosinophils, and some dendritic cells. IgE binding to antigen induces anchoring cell to release histamines and cytokines. Factor in allergies and asthma.
constant region of light chain
IgD
B cell receptor.
constant region (bright green) of heavy chain, including a hinged region
IgM
variable region of light chain
b
Figure 38.12 Antibody structure. (a) An antibody molecule has four polypeptide chains joined in a Y-shaped configuration. In this ribbon model, the two heavy chains are shown in green, and the two light chains are teal. (b) Each chain has a variable and a constant region.
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B cell receptor, as a monomer. Also is secreted as pentamer (group of five, shown).
V1
V2
V3
Vn
J1
J2
J3
V3
J2
C
V3
J2
C
V3
J2
C
C Processing yields a mature mRNA (introns excised, exons spliced together).
V3
J2
C
D mRNA is translated into one of the polypeptide chains of an antibody molecule.
Figure 38.13 Animated! How antigen receptor diversity arises, with an antibody light chain as the example. Antibodies are proteins. Genes encode instructions for synthesizing them. Instructions for an antibody molecule’s variable regions are not one continuous stretch along one chromosome; they are divided up in different segments along its length. Here we show different kinds of V, J, and C segments of a light chain on a chromosome. In this region, a recombination event occurs as each B cell is maturing. Any one of the V segments may be joined to any one of the J segments. The joined sequence is attached to a constant region segment. The combined gene will be present in all of the B cell’s descendants.
gland secretions (Section 32.2). Bound to antigen, it interacts with mast cells, basophils, macrophages, and NK cells to initiate inflammation. IgA is secreted as a dimer (two antibodies bound together), which makes it stable enough to patrol harsh environments such as the interior of the digestive tract. There, IgA encounters pathogens before they contact body cells. IgE is incorporated into the plasma membrane of mast cells, basophils, and some types of dendritic cells. Binding of antigen to IgE triggers the anchoring cell to release histamines and cytokines. A new B cell bristles with B cell receptors, which are membrane-bound IgM or IgD antibodies. Secreted IgM pentamers (polymers of five) efficiently bind antigen and activate complement.
The Making of Antigen Receptors Most humans can make about 2.5 billion unique antigen receptors. This diversity arises because the genes that encode the receptors do not occur in a continuous stretch on one chromosome; instead, they occur in several segments on different chromosomes, and there are several different versions of each segment. The segments are spliced together during B and T cell differentiation, but which version of each segment gets spliced into the antigen receptor gene of a particular cell is random (Section 14.3 and Figure 38.13). As a B or T cell differentiates, it ends up with one out of about 2.5 billion different combinations of gene segments. Before a new B cell leaves bone marrow, it already is synthesizing its unique antigen receptors. The constant
Jn
C
A As a B cell matures, different segments of antibody-coding genes recombine at random into a final gene sequence.
B The final sequence is transcribed into mRNA.
region of each receptor is embedded in the lipid bilayer of the cell’s plasma membrane, and the two arms project above the membrane. In time the B cell bristles with more than 100,000 antigen receptors. It is now a “naive” B cell, meaning it has not yet met its antigen. T cells also form in bone marrow, but they mature only after they take a tour in the thymus gland (Section 37.10). There, they encounter hormones that stimulate them to make MHC receptors and T cell receptors. Because of the random splicing of antigen receptor gene segments, the TCRs of some new T cells bind body proteins instead of antigen, and most do not recognize MHC markers. So how does an individual end up with a working set of T cells that does not attack its own body? Thymus cells have a built-in quality control that weeds out “bad” TCRs. They snip small peptides from a variety of body proteins and attach them to MHC markers. T cells that bind to a peptide–MHC complex have TCRs that recognize a self protein; those that do not bind any complex do not recognize MHC markers. Both types of cells die. Thus, any T cell that leaves the thymus to begin its journey through the circulatory system bristles with functional TCRs. Take-Home Message What are antigen receptors? The adaptive immune system has the potential to recognize about 2.5 billion different antigens via receptors on B cells and T cells. Antibodies are secreted or membrane-bound antigen receptors. They are made only by B cells.
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38.7
The Antibody-Mediated Immune Response In an antibody-mediated immune response, B cells are stimulated to produce antibodies targeting a specific antigen.
Link to Receptor-mediated endocytosis 5.5
An Antibody-Mediated Response Suppose that you accidentally nick your finger. Being opportunists, some Staphylococcus aureus cells on your skin invade your internal environment. Complement in interstitial fluid quickly attaches to carbohydrates in the bacterial cell walls, and cascading complement activation reactions begin. Within an hour, complementcoated bacteria tumbling along in lymph vessels reach a lymph node in your elbow. There they filter past an army of naive B cells. As it happens, one of the naive B cells in that lymph node makes antigen receptors that recognize a polysaccharide in S. aureus cell walls. This and every other B cell has receptors that recognize a complement coating on bacteria. Binding to antigen and complement together stimulates the B cell to engulf one of the bacteria by receptor-mediated endocytosis (Section 5.5). The B cell is now activated (Figure 38.14a).
Meanwhile, more S. aureus cells have been secreting metabolic products into interstitial fluid around your cut. The secretions attract phagocytes. A dendritic cell engulfs several bacteria, then migrates to the lymph node in your elbow. By the time it gets there, it has digested the bacteria and is displaying their fragments bound to MHC markers on its surface (Figure 38.14b). Each hour, about 500 different naive T cells travel through the lymph node, inspecting resident dendritic cells. In this case, one of those T cells has TCRs that bind the S. aureus antigen–MHC complexes displayed by the dendritic cell. For the next 24 hours, the T cell and the dendritic cell interact. When they disengage, the T cell returns to the circulatory system and begins to divide (Figure 38.14c). A huge population of genetically identical T cells forms; each cell has receptors that can bind the S. aureus antigen. These clones differentiate into helper T cells and memory T cells. By the theory of clonal selection, the T cell was “selected” because its receptors bind to the S. aureus antigen. T cells with receptors that do not bind the antigen do not divide to form huge clonal populations.
A The B cell receptors on a naive B cell bind to a specific antigen on the surface of a bacterium. The bacterium’s complement coating triggers the B cell to engulf it. Fragments of the bacterium bind MHC markers, and the complexes become displayed at the surface of the now-activated B cell.
bacterium
naive B cell
B A dendritic cell engulfs the same kind of bacterium that the B cell encountered. Digested fragments of the bacterium bind to MHC markers, and the complexes become displayed at the dendritic cell’s surface. The dendritic cell is now an antigen-presenting cell. C The antigen–MHC complexes on the antigen-presenting cell are recognized by antigen receptors on a naive T cell. Binding causes the T cell to divide and differentiate into effector and memory helper T cells.
naive T cell D cytokines
D Antigen receptors of one of the effector helper T cells bind antigen–MHC complexes on the B cell. Binding makes the T cell secrete cytokines.
memory helper T cell
E The cytokines induce the B cell to divide, giving rise to many identical B cells. The cells differentiate into effector B cells and memory B cells. E memory B cell
effector B cell
Figure 38.14 Animated Example of an antibody-mediated immune response.
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antigenpresenting dendritic cell
C
effector helper T cell
B cell
F The effector B cells begin making and secreting huge numbers of IgA, IgG, or IgE, all of which recognize the same antigen as the original B cell receptor. The new antibodies circulate throughout the body and bind to any remaining bacteria.
B
complement
A
dendritic cell
F
B cell with bound antigen antigen mitosis Antigen binds only to a matching B cell receptor. primary immune response effector cells
memory cells mitosis
mitosis clonal population of effector B cells
secondary immune response effector cells
Many effector B cells secrete many antibodies.
A Clonal selection of one B cell. Only B cells with receptors that bind to antigen divide and differentiate.
memory cells
B A first exposure to antigen generates a primary immune response in which effector cells fight the infection. Memory cells also form in a primary response but are set aside, sometimes for decades. If the antigen returns, the memory cells initiate a secondary response.
Let’s go back to that B cell in the lymph node. By now, it has digested the bacterium, and it is displaying bits of S. aureus bound to MHC molecules on its plasma membrane. One of the new helper T cells recognizes the antigen–MHC complexes displayed by the B cell. Like long-lost friends, the B cell and the helper T cell stay together for a while and communicate. One of the messages that is communicated consists of cytokines secreted by the helper T cell. The cytokines stimulate the B cell to begin mitosis after the two cells disengage (Figure 38.14d). The B cell divides again and again to form a huge population of genetically identical cells, all with receptors that can bind to the S. aureus antigen (Figure 38.15a). These clones differentiate into effector and memory B cells (Figure 38.14e). The effector cells start working immediately. They switch antibody classes, which means they begin to produce and secrete IgG, IgA, or IgE instead of making membrane-bound B cell receptors. The new antibody molecules recognize the same S. aureus antigen as the original B cell receptor. Antibodies now circulate throughout the body and attach themselves to any remaining bacterial cells. An antibody coating prevents bacteria from attaching to body cells, and flags them for phagocytosis and disposal (Figure 38.14f ). Memory B and T cells also form, but these do not act right away. They persist long after the initial infection
Antibody concentration
Figure 38.15 Animated B cell maturation.
first exposure
0
1
second exposure
2
3
4
5
6
7
8
9
10
Weeks
Figure 38.16 Antibody levels in a primary and secondary immune response. A secondary immune response is faster and stronger than the primary response that preceded it.
ends. If the same antigen enters the body at a later time, these memory cells will initiate a secondary response (Figures 38.15b and 38.16). In the secondary response, larger populations of effector cell clones form much more quickly than they did in the primary response, so more antibodies can be produced in a shorter time. Take-Home Message What happens during an antibody-mediated immune response? Antigen-presenting cells, T cells, and B cells interact in an antibodymediated immune response targeting a specific antigen. Populations of B cells form; these make and secrete antibodies that recognize and bind the antigen.
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38.8
The Cell-Mediated Response In a cell-mediated immune response, cytotoxic T cells and NK cells are stimulated to kill infected or altered body cells.
Link to Apoptosis 27.6
If B cells are like assassins, cytotoxic T cells are specialists in cell-to-cell combat. Antibody-mediated immune responses efficiently target pathogens that circulate in blood and interstitial fluid, but they are not as effective against pathogens hidden inside cells. As part of a cell-mediated immune response, cytotoxic T cells kill ailing body cells that may be missed by an antibodymediated response. Such cells typically display antigen: Cancer cells display altered body proteins, and body cells infected with intracellular pathogens display polypeptides of the infecting agent. Both types of cell are detected and killed by cytotoxic T cells.
dendritic cell
A A dendritic cell engulfs a virus-infected cell. Digested fragments of the virus bind to MHC markers, and the complexes are displayed at the dendritic cell’s surface. The dendritic cell, now an antigen-presenting cell, migrates to a lymph node.
A
naive cytotoxic T cell
antigenpresenting dendritic cell
A typical cell-mediated response begins in interstitial fluid during inflammation when a dendritic cell recognizes, engulfs, and digests a sick body cell or the remains of one (Figure 38.17a). The dendritic cell begins to display antigen that was part of the sick cell, and migrates to the spleen or a lymph node. There, the dendritic cell presents its antigen–MHC complexes to huge populations of naive helper T cells and naive cytotoxic T cells. Some of the naive cells have TCRs that recognize the complexes on the dendritic cell. Those helper T cells and cytotoxic T cells that bind the antigen–MHC complexes on the dendritic cell become activated. The activated helper T cells divide and differentiate into populations of effector and memory helper T cells (Figure 38.17b). The effector cells immediately begin to secrete cytokines. Activated cytotoxic T cells
B Receptors on a naive helper T cell bind to antigen– MHC complexes on the dendritic cell. The interaction activates the helper T cell, which then begins to divide. A large population of descendant cells forms. Each cell bears T cell receptors that recognize the same antigen. The cells differentiate into effector and memory cells.
naive helper T cell
C
C Receptors on a naive cytotoxic T cell bind to the antigen–MHC complexes on the surface of the dendritic cell. The interaction activates the cytotoxic T cell.
B
D activated cytotoxic T cell
memory cytotoxic T cell
cytokines
effector helper T cell
effector cytotoxic T cell
memory helper T cell
E The new cytotoxic T cells circulate through the body. They recognize and touch-kill any body cell that displays the viral antigen–MHC complexes on its surface.
E
Figure 38.17 Animated Example of a primary cell-mediated immune response. Figure It Out: What do the large red spots represent? Answer: Viruses
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D The activated cytotoxic T cell recognizes cytokines secreted by the effector helper T cells as signals to divide. A large population of descendant cells forms. Each cell bears T cell receptors that recognize the same antigen. The cells differentiate into effector and memory cells.
FOCUS ON HEALTH
38.9 cytotoxic T cell cancer cell b
a
Figure 38.18 T cell receptor function. (a) A TCR (green) on a T cell binds to an MHC marker (tan) on an antigen-presenting cell. An antigen (red) is bound to the MHC marker. (b) A cytotoxic T cell caught in the act of touch-killing a cancer cell.
recognize the cytokines as signals to divide and differentiate, and tremendous populations of effector and memory cytotoxic T cells form (Figure 38.17c,d). All of them recognize and bind the same antigen—the one displayed by that first ailing cell. As in an antibodymediated response, the memory cells that form in a primary cell-mediated response will mount a secondary response if the antigen returns at a later time. The effector cytotoxic T cells start working immediately. They circulate throughout blood and interstitial fluid, and bind to any other body cell displaying the original antigen together with MHC markers (Figure 38.18a). After it is bound to an ailing cell, a cytotoxic T cell releases perforin and proteases. These toxins poke holes in the sick cell and induce it to die by apoptosis (Figures 38.17e and 38.18b). Cytotoxic T cells also recognize the MHC markers of foreign body cells (cytotoxic T cells are responsible for rejection of transplanted organs). They must recognize MHC molecules on the surface of a body cell in order to kill it. However, some infections or cancer can alter a cell so that it is missing part or all of its MHC markers. NK (“natural killer”) cells are crucial for fighting such cells. Unlike cytotoxic T cells, NK cells can kill body cells that lack MHC markers. Cytokines secreted by helper T cells (Figure 38.17d) also stimulate NK cell division. The resulting populations of effector NK cells attack body cells tagged for destruction by antibodies. They also recognize certain proteins displayed by body cells under stress. Stressed body cells with normal MHC markers are not killed; only those with altered or missing MHC markers are destroyed.
Allergies
An immune response to a typically harmless substance is an allergy. Allergies can be annoying or life-threatening.
In millions of people, exposure to harmless substances stimulates an immune response. Any substance that is ordinarily harmless yet provokes such responses is an allergen. Sensitivity to an allergen is called an allergy. Drugs, foods, pollen, dust mites, fungal spores, poison ivy, and venom from bees, wasps, and other insects are among the most common allergens. Some people are genetically predisposed to having allergies. Infections, emotional stress, and changes in air temperature can trigger reactions. A first exposure to an allergen stimulates the immune system to make IgE, which becomes anchored to mast cells and basophils. With later exposures, antigen binds to the IgE. Binding triggers the anchoring cell to secrete histamine and cytokines that initiate inflammation. If this reaction occurs at the lining of the respiratory tract, a copious amount of mucus is secreted and the airways constrict; sneezing, stuffed-up sinuses, and a drippy nose result (Figure 38.19a). Contact with an allergen that penetrates the skin’s outer layers causes the skin to redden, swell, and become itchy. Antihistamines relieve allergy symptoms by dampening the effects of histamines. These drugs act on histamine receptors, and also inhibit the release of cytokines and histamines from basophils and mast cells. Some people are hypersensitive to drugs, insect stings, foods, or vaccines. A second exposure to the allergen can result in anaphylactic shock, a severe, whole-body allergic reaction. Huge amounts of cytokines and histamines released in all parts of the body provoke an immediate, systemic reaction. Fluid leaking from blood into tissues causes the blood pressure to drop too much (shock), and tissues to swell. Swelling tissue constricts airways and may block them. Anaphylactic shock is rare but life-threatening and requires immediate treatment (Figure 38.19c). It may occur at any time, upon exposure to even a tiny amount of allergen. Risks include a prior allergic reaction of any kind.
Take-Home Message What happens during a cell-mediated immune response? Antigen-presenting cells, T cells, and NK cells interact in a cell-mediated immune response targeting body cells that have been altered by cancer or infected.
a
b
c
Figure 38.19 Allergies. (a) A mild allergy may cause upper respiratory symptoms. (b) Ragweed pollen, a common allergen. (c) Anaphylactic shock is a severe allergic reaction that requires immediate treatment.
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38.10 Vaccines
Vaccines are designed to elicit immunity to a disease.
Immunization refers to processes designed to induce immunity. In active immunization, a preparation that contains antigen—a vaccine—is administered orally or injected. The first immunization elicits a primary immune response, just as an infection would. A second immunization, or booster, elicits a secondary immune response for enhanced immunity. In passive immunization, a person receives antibodies purified from the blood of another individual. The treatment offers immediate benefit for someone who has been exposed to a potentially lethal agent, such as tetanus or rabies, Ebola virus, or a venom or toxin. Because the antibodies were not made by the recipient’s lymphocytes, memory cells do not form, so benefits last only as long as the injected antibodies do. The first vaccine was the result of desperate attempts to survive smallpox epidemics that swept repeatedly through cities all over the world. Smallpox is a severe disease that kills up to one-third of the people it infects (Figure 38.20). Before 1880, no one knew what caused infectious diseases or how to protect anyone from getting them, but there were clues. In the case of smallpox, survivors seldom contracted the disease a second time. They were immune, or protected from infection.
Table 38.5
Recommended Immunization Schedule
Vaccine
Age of Vaccination
Hepatitis B Hepatitis B boosters Rotavirus DTP: diphtheria, tetanus, and pertussis (whooping cough) DTP boosters
Birth to 2 months 1–4 months and 6–18 months 2, 4, and 6 months 2, 4, and 6 months
HiB (Haemophilus influenzae) HiB booster Pneumococcal Pneumococcal booster Inactivated poliovirus Inactivated poliovirus boosters
15–18 months, 4–6 years, and 11–12 years 2, 4, and 6 months 12–15 months 2, 4, and 6 months 12–15 months 2 and 4 months 6–18 months and 4–6 years
Influenza MMR (measles, mumps, rubella) MMR booster Varicella (chicken pox) Varicella booster Hepatitis A series HPV series Meningococcal
Yearly, 6 months–18 years 12–15 months 4–6 years 12–15 months 4–6 years 1–2 years 11–12 years 11–12 years
Source: Centers for Disease Control and Prevention (CDC), 2008
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Figure 38.20 Young survivor and the cause of her disease, smallpox viruses. Worldwide use of the vaccine eradicated naturally occurring cases of smallpox; vaccinations for it ended in 1972.
The idea of acquiring immunity to smallpox was so appealing that people had been risking their lives on it for two thousand years. For example, many people poked into their skin bits of scabs or threads soaked in pus. Some survived the crude practices and became immune to smallpox, but many others did not. By the late 1700s, it was widely known that dairymaids did not get smallpox if they had already recovered from cowpox (a mild disease that affects cattle as well as humans). In 1796, Edward Jenner, an English physician, injected liquid from a cowpox sore into the arm of a healthy boy. Six weeks later, Jenner injected the boy with liquid from a smallpox sore. Luckily, the boy did not get smallpox. Jenner’s experiment showed directly that the agent of cowpox elicits immunity to smallpox. Jenner named his procedure “vaccination,” after the Latin word for cowpox (vaccinia). The use of Jenner’s vaccine spread quickly through Europe, then to the rest of the world. The last known case of naturally occurring smallpox was in 1977, in Somalia. The vaccine had eradicated the disease. We now know that the cowpox virus is an effective vaccine for smallpox because the antibodies it elicits also recognize smallpox virus antigens. Our knowledge of how the immune system works has allowed us to develop many other vaccines that save millions of lives every year. These vaccines are an important part of worldwide public health programs (Table 38.5).
Take-Home Message How does immunization work?
Immunization is the administration of an antigen-bearing vaccine designed to elicit immunity to a specific disease.
38.11
Immunity Gone Wrong
The immune system of some people does not function properly. The outcome is often severe or lethal.
Links to Multiple sclerosis 33.13, Arthritis 36.5
Despite the redundancies of immune system functions and built-in quality controls, immunity does not always work as well as it should. Its sheer complexity is part of the problem, because there are so many points at which it could go wrong. Autoimmune disorders occur when an immune response is misdirected against the person’s own body cells. In immunodeficiency, the immune response is insufficient to protect a person from disease.
Autoimmune Disorders Sometimes lymphocytes and antibody molecules fail to discriminate between self and nonself. When that happens, they mount an autoimmune response, or an immune response that targets one’s own tissues. For example, autoimmunity occurs in rheumatoid arthritis, a disease in which self antibodies form and bind to the soft tissue in joints. The resulting inflammation leads to eventual disintegration of bone and cartilage in the joints (Section 36.5). Antibodies to self proteins may bind to hormone receptors, as in Graves’ disease. Self antibodies that bind stimulatory receptors on the thyroid gland cause it to produce excess thyroid hormone, which quickens the body’s overall metabolic rate. Antibodies are not part of the feedback loops that normally regulate thyroid hormone production. So, antibody binding continues unchecked, the thyroid continues to release too much hormone, and the metabolic rate spins out of control. Symptoms of Graves’ disease include uncontrollable weight loss; rapid, irregular heartbeat; sleeplessness; pronounced mood swings; and bulging eyes. A neurological disorder, multiple sclerosis, occurs when self-reactive T cells attack the myelin sheaths of axons in the central nervous system (Section 33.13). Symptoms range from weakness and loss of balance to paralysis and blindness. Specific MHC gene alleles increase susceptibility, but a bacterial or viral infection may trigger the disorder. Immune responses tend to be stronger in women than in men, and autoimmunity is far more frequent in women. We know that estrogen receptors are part of gene expression controls throughout the body. T cells have receptors for estrogens, so these hormones may enhance T cell activation in autoimmune diseases. Women’s bodies have more estrogen, so interactions between their B cells and T cells may be amplified.
Figure 38.21 A case of severe combined immunodeficiency (SCID). Cindy Cutshwall was born with a deficient immune system. She carries a mutated gene for adenosine deaminase (ADA). Without this enzyme, her cells cannot break down adenosine completely, so a reaction product that is toxic to white blood cells accumulated in her body. High fevers, severe ear and lung infections, diarrhea, and an inability to gain weight were outcomes. In 1991, when Cindy was nine years old, she and her parents consented to one of the first human gene therapies. Genetic engineers spliced the normal ADA gene into the genetic material of a harmless virus. The modified virus delivered copies of the normal gene into her bone marrow cells. Some cells incorporated the gene in their DNA and started making the missing enzyme. Now in her twenties, Cindy is doing well. She still requires weekly injections to supplement her ADA production. Other than that, she is able to live a normal life. She is a strong advocate of gene therapy.
Immunodeficiency Impaired immune function is dangerous and sometimes lethal. Immune deficiencies render individuals vulnerable to infections by opportunistic agents that are typically harmless to those in good health. Primary immune deficiencies, which are present at birth, are the outcome of mutations. Severe combined immunodeficiencies (SCIDs) are examples. A genetic disorder called adenosine deaminase (ADA) deficiency is a type of SCID (Figure 38.21). Secondary immune deficiency is the loss of immune function after exposure to an outside agent, such as a virus. AIDS (acquired immunodeficiency syndrome, described in the next section) is the most common secondary immune deficiency.
Take-Home Message What happens when the immune system does not function as it should? Misdirected or compromised immunity, which sometimes occurs as a result of mutation or environmental factors, can have severe or lethal outcomes.
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38.12 AIDS Revisited—Immunity Lost Table 38.6
Global HIV and AIDS Cases
AIDS is an outcome of interactions between the HIV virus and the human immune system.
Links to cDNA 16.1, Viruses 21.1, HIV replication 21.2
Region Sub-Saharan Africa
Acquired immune deficiency syndrome, or AIDS, is a constellation of disorders that occur as a consequence of infection with HIV, the human immunodeficiency virus (Figure 38.22a). This virus cripples the immune system, so it makes the body very susceptible to infections and rare forms of cancer. Worldwide, approximately 39.5 million individuals currently have AIDS (Table 38.6 and Figure 38.22b). There is no way to rid the body of the HIV virus, no cure for those already infected. At first, an infected person appears to be in good health, perhaps fighting “a bout of flu.” But symptoms eventually emerge that foreshadow AIDS: fever, many enlarged lymph nodes, chronic fatigue and weight loss, and drenching night sweats. Then, infections caused by normally harmless microorganisms strike. Yeast infections of the mouth, esophagus, and vagina often occur, as well as a form of pneumonia caused by the fungus Pneumocystis carinii. Colored lesions erupt. These lesions are evidence of Kaposi’s sarcoma, a type of cancer that is common among AIDS patients (Figure 38.22c). HIV is a retrovirus that has a lipid envelope. Remember, this type of envelope is a small piece of plasma membrane a virus particle acquires as it buds from a cell (Section 21.2). Proteins jut out from the enve-
HIV Revisited
a
b
c
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AIDS Cases
New HIV Cases
22,500,000
1,700,000
South/Southeast Asia
4,000,000
340,000
Central Asia/East Europe
1,600,000
150,000
Latin America
1,600,000
100,000
North America
1,300,000
46,000
East Asia
800,000
92,000
Western/Central Europe
760,000
31,000
Middle East/North Africa
380,000
35,000
Caribbean Islands
230,000
17,000
75,000
14,000
33,200,000
2,500,000
Australia/New Zealand Worldwide total
Source: Joint United Nations Programme HIV/AIDS, 2007 data
lope, span it, and line its inner surface. Just beneath the envelope, more viral proteins enclose two RNA strands and copies of reverse transcriptase. When a virus particle infects a cell, the reverse transcriptase copies the viral RNA into DNA, which becomes integrated into the host cell’s DNA. A Titanic Struggle HIV mainly infects macrophages,
dendritic cells, and helper T cells. When virus particles enter the body, dendritic cells engulf them. The dendritic cells then migrate to lymph nodes, where they present processed HIV antigen to naive T cells. An army of HIV-neutralizing IgG antibodies and HIVspecific cytotoxic T cells forms. We have just described a typical adaptive immune response. It rids the body of most—but not all—of the virus. In this first response, HIV infects a few helper T cells in a few lymph nodes. For years or even decades, the IgG antibodies keep the level of HIV in the blood low, and the cytotoxic T cells kill HIV-infected cells. Patients are contagious during this stage, although they might show no symptoms of AIDS. HIV viruses persist in a few of their helper T cells, in a few lymph nodes. Eventually, the level of virus-neutralizing IgG in the blood plummets, and T cell production slows. Why IgG decreases is still a major topic of research, but its effect is certain: The adaptive immune system
Figure 38.22 AIDS. (a) A human T cell (blue), infected with HIV (red ). (b) This Romanian baby contracted AIDS from his mother’s breast milk. He did not live long enough to develop lesions of Kaposi’s sarcoma (c), a cancer that is a common symptom of HIV infection in older AIDS patients.
becomes less and less effective at fighting the virus. The number of virus particles rises; up to 1 billion HIV viruses are built each day. Up to 2 billion helper T cells become infected. Half of the viruses are destroyed and half of the helper T cells are replaced every two days. Lymph nodes begin to swell with infected T cells. Eventually, the battle tilts as the body makes fewer replacement helper T cells and the body’s capacity for adaptive immunity is destroyed. Other types of viruses make more particles in a day, but the immune system eventually wins. HIV demolishes the immune system. Secondary infections and tumors kill the patient. Transmission HIV is transmitted most frequently by
having unprotected sex with an infected partner. The virus occurs in semen and vaginal secretions, and can enter a partner through epithelial linings of the penis, vagina, rectum, and the mouth. The risk of transmission increases by the type of sexual act; for example, anal sex carries 50 times the risk of oral sex. Infected mothers can transmit HIV to a child during pregnancy, labor, delivery, or breast-feeding. HIV also travels in tiny amounts of infected blood in the syringes shared by intravenous drug abusers, or by patients in hospitals of poor countries. HIV is not transmitted by casual contact. Testing Most AIDS tests check blood, saliva, or urine
for antibodies that bind to HIV antigens. These antibodies are detectable in 99 percent of infected people within three months of exposure to the virus. One test can detect viral RNA at about eleven days after exposure. Currently, the only reliable tests are performed in clinical laboratories; home test kits may result in false negatives, which may cause an infected person to unknowingly transmit the virus. Drugs cannot cure AIDS, but they can slow its progress. Of the twenty or so FDAapproved AIDS drugs, most target processes unique to retroviral replication. For example, RNA nucleotide analogs such as AZT are called reverse transcriptase inhibitors. They interrupt HIV replication when they substitute for normal nucleotides in the viral RNA-toDNA synthesis process (Sections 16.1 and 21.2). Other drugs such as protease inhibitors affect different parts of the viral replication cycle. A three-drug “cocktail” of one protease inhibitor plus two reverse transcriptase inhibitors is currently the most successful AIDS therapy, and has changed the course of the disease from a short-term death sentence to a long-term, often manageable illness.
Drugs and Vaccines
Figure 38.23 At the Global AIDS Program’s International Laboratory Branch of the National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention, researcher Amanda McNulty examines a DNA electrophoresis gel. She is investigating HIV drug resistance in people from Africa, Vietnam, and Haiti.
Researchers are using several strategies to develop an HIV vaccine. At this writing, organizations around the world are testing 42 different HIV vaccines. Most of them consist of isolated HIV proteins or peptides, and many deliver the antigens in viral vectors. Live, weakened HIV virus is an effective vaccine in chimpanzees, but the risk of HIV infection from the vaccines themselves far outweighs their potential benefits in humans. Other types of HIV vaccines are notoriously ineffective. IgG antibody exerts selective pressure on the virus, which has a very high mutation rate because it replicates so fast. The human immune system just cannot produce antibodies fast enough to keep up with the mutations (Figure 38.23). At present, our best option for halting the spread of HIV is prevention, by teaching people how to avoid being infected. The best protection against AIDS is to avoid unsafe behaviors. In most circumstances, HIV infection is the consequence of a choice: either to have unprotected sex, or to use a shared needle for intravenous drugs. Educational programs around the world are having an effect on the spread of the virus: In many (but not all) countries, the incidence of new cases of HIV each year is beginning to slow. Overall, however, our global battle against AIDS is not being won.
Take-Home Message What is AIDS? AIDS occurs as a result of infection by HIV, a virus that infects lymphocytes and so cripples the human immune system.
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Frankie’s Last Wish
The Gardasil HPV vaccine consists of viral capsid proteins that self-assemble into virus-like particles (VLPs). These proteins are produced by a recombinant yeast, Saccharomyces cerevisiae. The yeast carries genes for one capsid protein from each of four strains of HPV, so the VLPs carry no viral DNA. Thus, the VLPs are not infectious, but the antigenic proteins they consist of elicit an immune response at least as strong as infection with HPV virus.
Summary Section 38.1 Three lines of immune defense protect vertebrates from infection. An antigen-bearing pathogen that breaches surface barriers triggers innate immunity, a set of general defenses that usually prevents populations of pathogens from becoming established in the internal environment. Adaptive immunity, which can specifically target billions of different antigens, follows. Complement and signaling molecules such as cytokines coordinate the activities of white blood cells (dendritic cells, macrophages, neutrophils, basophils, mast cells, eosinophils, B and T lymphocytes, and NK cells) in immunity. Sections 38.2, 38.3 Vertebrates can fend off pathogens such as those that cause dental plaque at body surfaces with physical, mechanical, and chemical barriers (including lysozyme). Most normal flora do not cause disease unless they penetrate inner tissues. Section 38.4 An innate immune response includes fast, general responses that can eliminate invaders before an infection can become established. Complement attracts phagocytes, and punctures some invaders. Inflammation begins when mast cells in tissue release histamine, which increases blood flow and also makes capillaries leaky to phagocytes and plasma proteins. Fever fights infection by increasing the metabolic rate.
Use the animation on CengageNOW to investigate inflammation and the action of complement.
Section 38.5 Adaptive immunity is characterized by self/nonself recognition, target specificity, diversity (the capacity to intercept billions of different pathogens), and memory. B and T cells carry out adaptive responses. The antibody-mediated immune response and the cellmediated immune response work together to rid the body of a specific pathogen. Macrophages, dendritic cells, and B cells engulf and digest viruses or bacteria into bits. The phagocytes then present the antigenic bits on their surfaces bound to MHC markers (self markers). T cells that recognize the complexes via T cell receptors (TCRs) initiate the formation of many effector cells that target other antigenbearing particles. Memory cells that are reserved for later encounters with the same antigen also form. Sections 38.6, 38.7 B cells, assisted by T cells and signaling molecules, carry out antibody-mediated immune responses. B cells make antibodies that bind to specific 678 UNIT VI
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antigens. Antigen receptors—T cell receptors and B cell receptors (a type of antibody)—recognize specific antigens. These receptors are the basis of the immune system’s capacity to recognize billions of different antigens.
Use the animations on CengageNOW to see an antibodymediated immune response, how antigen receptor diversity is generated, and clonal selection of B cells.
Section 38.8 Antigen-presenting cells, T cells, and NK cells interact in cell-mediated responses. They target and kill body cells altered by infection or cancer.
Use the animation on CengageNOW to observe a cellmediated immune response.
Sections 38.9–38.11 Allergens are normally harmless substances that induce an immune response; sensitivity to an allergen is called an allergy. Immunization with vaccines designed to elicit immunity to specific diseases saves millions of lives each year. In an autoimmune response, a body’s own cells are inappropriately recognized as foreign and attacked. Immune deficiency is a reduced capacity to mount an immune response. Section 38.12 AIDS is caused by HIV, a virus that destroys the immune system mainly by infecting helper T cells. At present, AIDS cannot be cured.
Self-Quiz 1.
is/are the first line of defense against threats. a. Skin, mucous membranes d. Resident bacteria b. Tears, saliva, gastric fluid e. a through c c. Urine flow f. all of the above
2. Complement proteins a. form pore complexes b. promote inflammation 3.
Answers in Appendix III
. c. neutralize toxins d. a and b
trigger immune responses. a. Cytokines d. Antigens b. Lysozymes e. Histamines c. Immunoglobulins f. all of the above
4. Name one defining characteristic of innate immunity. 5. Name one defining characteristic of adaptive immunity. 6. Antibodies are . a. antigen receptors b. made only by B cells 7. a. IgA
c. proteins d. all of the above
binding antigen triggers allergic responses. b. IgE c. IgG d. IgM e. IgD
Data Analysis Exercise
1. At 110 months into the study, what percentage of women who were not infected with any type of cancer-causing HPV had cervical cancer? What percentage of women who were infected with HPV16 also had cervical cancer? 2. In which group would women infected with both HPV16 and HPV18 fall? 3. Is it possible to estimate from this graph the overall risk of cervical cancer that is associated with infection of cancercausing HPV of any type?
20
Cumulative incidence rate (%)
In 2003, Michelle Khan and her coworkers published their findings on a 10-year study in which they followed cervical cancer incidence and HPV status in 20,514 women. All women who participated in the study were free of cervical cancer when the test began. Pap tests were taken at regular intervals, and the researchers used a DNA probe hybridization test to detect the presence of specific types of HPV in the women’s cervical cells. The results are shown in Figure 38.24 as a graph of the incidence rate of cervical cancer by HPV type. Women who are HPV positive are often infected by more than one type, so the data were sorted into groups based on the women’s HPV status ranked by type: either positive for HPV16; or negative for HPV16 and positive for HPV18; or negative for HPV16 and 18 and positive for any other cancer-causing HPV; or negative for all cancer-causing HPV.
15
10
5
0 4.5
15.0
27.0
39.0
51.0
63.0
75.0
87.0
99.0
Follow-up time (months)
Figure 38.24 Cumulative incidence rate of cervical cancer correlated with HPV status in 20,514 women aged 16 years and older. The data were grouped as follows: HPV16 positive (closed circles), or else HPV18 positive (open circles), or else all other cancer-causing HPV types combined (closed triangles). Open triangles: no cancer-causing HPV type was detected.
4. Do these data support the conclusion that being infected with HPV16 or HPV18 raises the risk of cervical cancer?
8. Antibody-mediated responses work against a. intracellular pathogens d. both a and b b. extracellular pathogens e. both b and c c. cancerous cells f. a, b, and c 9. Cell-mediated responses work against a. intracellular pathogens d. both a and b b. extracellular pathogens e. both a and c c. cancerous cells f. a, b, and c 10.
.
12. Match the immunity concepts. anaphylactic shock a. neutrophil antibody secretion b. effector B cell phagocyte c. general defense immune memory d. immune response autoimmunity against own body antigen receptor e. secondary response inflammation f. B cell receptor g. hypersensitivity to an allergen
2. Elena developed chicken pox when she was in first grade. Later in life, when her children developed chicken pox, she remained healthy even though she was exposed to countless virus particles daily. Explain why.
are targets of cytotoxic T cells. a. Extracellular virus particles in blood b. Virus-infected body cells or tumor cells c. Parasitic flukes in the liver d. Bacterial cells in pus e. Pollen grains in nasal mucus
11. Allergies occur when the body responds to a. pathogens c. toxins b. normally harmless d. all of the above substances
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Critical Thinking 1. As described in Section 38.10, Edward Jenner was lucky. He performed a potentially harmful experiment on a boy who managed to survive the procedure. What would happen if a would-be Jenner tried to do the same thing today in the United States?
.
3. Before each flu season, you get a flu shot, an influenza vaccination. This year, you get “the flu” anyway. What happened? There are at least three explanations. .
110.0 119.5
4. Monoclonal antibodies are made by immunizing a mouse with a particular antigen, then removing its spleen. Individual B cells producing mouse antibodies specific for the antigen are isolated from the mouse’s spleen and fused with cancerous B cells from a myeloma cell line. The resulting hybrid myeloma cells—hybridoma cells—are cloned, or grown in tissue culture as separate cell lines. Each line produces and secretes antibodies that recognize the antigen to which the mouse was immunized. These monoclonal antibodies can be purified and used for research or other purposes. Monoclonal antibodies are sometimes used in passive immunization. They tend to be effective, but only in the immediate term. IgG produced by one’s own immune system can last up to about six months in the bloodstream, but monoclonals delivered in passive immune therapy typically last for less than a week. Why the difference? CHAPTER 38
IMMUNITY 679
39
Respiration IMPACTS, ISSUES
Up in Smoke
Each day, 3,000 or so teenagers join the ranks of habitual
The highly addictive stimulant nicotine constricts blood
smokers in the United States. Most are not even fifteen years
vessels, which increases blood pressure. The heart has to
old. When they first light up, they cough and choke on the
work harder to pump blood through the narrowed tubes.
irritants in the smoke. Most become dizzy and nauseated,
Nicotine also triggers a rise in “bad” cholesterol (LDL) and
and develop headaches. Sound like fun? Hardly. Why, then,
a decline in the “good” kind (HDL) in blood. It makes blood
do they ignore signals about threats to the body and work so
stickier, encouraging clots that can block blood vessels.
hard to be a smoker? Mainly to fit in. To many adolescents,
Tobacco smoke has more than forty known carcinogens
a misguided perception of social benefits overwhelms the
and 80 percent of lung cancers occur in smokers. Women
seemingly remote threats to health (Figure 39.1).
who smoke are more susceptible to cancers than men. On
Despite teenage perceptions, changes that can make
average, women develop cancers earlier, and with lower
the threat a reality start right away. Ciliated cells keep many
exposure to tobacco. Fewer than 15 percent of women
pathogens and pollutants that enter airways from reaching
diagnosed with lung cancer survive five years. Smoking also
the lungs. These cells can be immobilized for hours by the
increases breast cancer risk; females who start to smoke
smoke from a single cigarette. Smoke also kills white blood
as teenagers are about 70 percent more likely to get breast
cells that patrol and defend respiratory tissues. Pathogens
cancer than those women who never smoked. Therefore, the
multiply in the undefended airways. The result is more colds,
trend of increased smoking among women in less-developed
more asthma attacks, and more bronchitis.
countries especially troubling. Families, coworkers, and friends get unfiltered doses of the carcinogens in tobacco smoke. Each year in the United States, lung cancers arising from secondhand smoke kill about 3,000. Children exposed to secondhand smoke also are more likely to develop chronic middle ear infections, asthma, and other respiratory problems later in life. This chapter samples a few respiratory systems. All exchange gases with the outside environment. They also contribute to homeostasis—maintaining the body’s internal operating conditions within ranges that cells can tolerate. If you or someone you know smokes, you might use the chapter as a guide to smoking’s impact on health. For a more graphic preview, find out what goes on every day with smokers in hospital emergency rooms or intensive care units. There’s no glamor there. It is not cool, and it is not pretty.
See the video! Figure 39.1 Learning to smoke is easy, compared with trying to quit. In one survey, two-thirds of female smokers who were sixteen to twenty-four wanted to give up smoking entirely. Of those who tried to quit, only about 3 percent remained nonsmokers for an entire year.
Links to Earlier Concepts
Key Concepts Principles of gas exchange
Understanding diffusion (Section 5.3) and aerobic respiration (8.1) will help you understand the need for gas exchange and the process by which it occurs. You will also revisit the role of red blood cells (37.2) and the hemoglobin they hold (3.6).
You will learn about the role of the brain stem (33.10), autonomic nervous system (33.8), and chemoreceptors (34.1) in the regulation of breathing. You will also be reminded of the role of the respiratory system in temperature regulation (27.3).
You will see how adaptations of animal body plans (25.1, 17.1) and the evolutionary changes that accompanied the move of vertebrates onto land (26.5) allow respiration in specific environments.
Respiratory effects of algal blooms (22.5), tuberculosis (21.8), and marijuana use (33.7) are also discussed.
Respiration is the sum of processes that move oxygen from air or water in the environment to all metabolically active tissues and move carbon dioxide from those tissues to the outside. Oxygen levels are more stable in air than in water. Sections 39.1, 39.2
Gas exchange in invertebrates Gas exchange occurs across the body surface or gills of aquatic invertebrates. In large invertebrates on land, it occurs across a moist, internal respiratory surface or at fluid-filled tips of branching tubes that extend from the surface to internal tissues. Section 39.3
Gas exchange in vertebrates Gills or paired lungs are gas exchange organs in most vertebrates. The efficiency of gas exchange is improved by mechanisms that cause blood and water to flow in opposite directions at gills, and by muscle contractions that move air into and out of lungs. Sections 39.4–39.7
Respiratory problems Respiration can be disrupted by damage to respiratory centers in the brain, physical obstructions, infectious disease, and inhalation of pollutants, including cigarette smoke. Section 39.8
Gas exchange in extreme environments At high altitudes, the human body makes short-term and long-term adjustments to the thinner air. Built-in respiratory mechanisms and specialized behaviors allow sea turtles and diving marine mammals to stay under water, at great depths, for long periods. Section 39.9
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681
39.1
The Nature of Respiration All animals must supply their cells with oxygen and rid their body of carbon dioxide.
Figure 39.3 How a mercury barometer measures atmospheric pressure. That pressure makes mercury (Hg), a viscous liquid, rise or fall in a narrow tube. At sea level, it rises 760 millimeters (29.91 inches) from the tube’s base. Atmospheric pressure varies with altitude. On the top of Mount Everest, atmospheric pressure is only about one-third the pressure at sea level.
Links to Diffusion 5.3, Aerobic respiration 8.1
All animals move their body or body parts during at least some part of their life cycle. This movement requires energy, which is usually supplied by ATP. The most efficient way to make ATP is aerobic respiration, a pathway that requires oxygen and releases carbon dioxide as a by-product (Section 8.1). How does an animal supply its cells with the oxygen necessary for aerobic respiration and rid itself of carbon dioxide waste? In animals that have organ systems, a respiratory system carries out these tasks. In humans and other vertebrates, the respiratory system interacts with other organ systems as shown in Figure 39.2.
The Basis of Gas Exchange Respiration is the physiological process by which an animal exchanges oxygen and carbon dioxide with its environment. Respiration depends upon the tendency of gaseous oxygen (O2) and carbon dioxide (CO2) to diffuse down their concentration gradients—or, as we say for gases, their pressure gradients—between the external and internal environments. Aquatic animals live in an environment where the availability of O2 can vary widely from place to place and change over time. Air is a more reliable source of oxygen. Earth’s atmosphere is 78 percent nitrogen,
760 mm Hg
21 percent oxygen, 0.04 percent carbon dioxide, and 0.06 percent other gases. Total atmospheric pressure as measured by a mercury barometer is 760 mm at sea level (Figure 39.3). Oxygen’s contribution to the total, its partial pressure, is 21 percent of 760, or 160 mm Hg. “Hg” is the symbol for mercury. Gases enter and leave the internal environment by crossing a respiratory surface: a moist layer thin enough for gases to diffuse across. The surface has to be moist because gases can only diffuse quickly across a membrane if they first dissolve in fluid.
Factors Affecting Diffusion Rates Several factors affect how much gas diffuses across a respiratory surface. For example, the steeper the partial pressure gradient, the faster the rate of diffusion.
food, water intake
oxygen intake
Digestive System nutrients, water, salts
Respiratory System oxygen
elimination of carbon dioxide
carbon dioxide
Circulatory System
Urinary System water, solutes
elimination of food residues
rapid transport to and from all living cells
elimination of excess water, salts, wastes
Figure 39.2 A dog’s breathing helps meet its cells’ need for oxygen. In dogs and other vertebrates, the respiratory system interacts with other organ systems that contribute to homeostasis.
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FOCUS ON THE ENVIRONMENT
39.2 The greater the area of a respiratory surface, the more molecules can cross it in any given interval. Remember, as an animal grows, its volume increases faster than its surface area does (Section 4.2). If an animal does not have specialized respiratory organs, it usually has a small, flattened body. In such animals, diffusion alone delivers enough oxygen to cells, because no cell is more than a few millimeters away from gases outside the body.
Surface-to-Volume Ratio
Ventilation Moving air or water past a respiratory surface keeps the pressure gradient across the surface high and thus increases the rate of gas exchange. For example, frogs and humans breathe in and out, which ventilates their lungs. Breathing forces stale air with waste CO2 away from the respiratory surface in the lungs, and draws in fresh air with more O2. Fish and other animals that live in water have mechanisms that keep water moving across their respiratory surface. Respiratory Proteins Respiratory proteins contain one or more metal ions that reversibly bind oxygen atoms. Oxygen atoms bind to these proteins when the partial pressure of oxygen is high, and are released when the partial pressure of oxygen declines. By reversibly binding oxygen, respiratory proteins help maintain a steep partial pressure gradient for oxygen between cells and the blood. The gradient is steepened because any oxygen that is bound to a molecule in solution does not contribute to the partial pressure of O2 in that solution. Hemoglobin, an iron-containing respiratory protein, fills vertebrate red blood cells (Sections 3.6 and 37.2). It also circulates in the blood of annelids, mollusks, and crustaceans, which do not have red blood cells. The respiratory proteins hemerythrin (with iron) and hemocyanin (with copper) also aid oxygen transport in some invertebrates. Myoglobin a heme-containing respiratory protein, is found in muscle of vertebrates and some invertebrates. It helps stabilize the oxygen level inside muscle cells.
Take-Home Message What is respiration and what factors influence it? Respiration supplies cells with oxygen for aerobic respiration and removes carbon dioxide wastes. Gases are exchanged by diffusion across a respiratory surface: a thin, moist membrane. The area of a respiratory surface and the partial pressure gradients across it influence the rate of exchange. Ventilation and respiratory proteins help keep partial pressure gradients steep and thus enhance gas exchange.
Gasping for Oxygen
Rising water temperatures, slowing streams, and organic pollutants reduce the oxygen available for aquatic species.
Link to Algal bloom 22.5
Any animal can tolerate only a limited range of environmental conditions. For aquatic animals, dissolved oxygen content of water (DO) is one of the most important factors affecting their survival. More oxygen dissolves in cooler, fast-flowing water than in warmer, still water. When water temperature increases or water becomes stagnant, aquatic species that have high oxygen needs suffocate (Figure 39.4). As oxygen levels in water fall, so does biodiversity. Pollution can cause DO to decline. A lake enriched with runoff that contains manure or sewage offers a nutrition boost to aerobic bacteria living on the lake bottom. The bacteria are decomposers. As their populations soar, they use up lots of oxygen, so the amount available to other species plummets. The same thing can happen after phosphate-rich or nitrogen-rich fertilizers cause an algal bloom—a population explosion of protists such as dinoflagellates (Section 22.5). The protists multiply rapidly, then die. Their decomposition depletes the water of oxygen. In freshwater lakes and streams, aquatic larvae of mayflies and stoneflies are the first invertebrates to disappear when oxygen levels fall. These insect larvae are active predators that demand considerable oxygen. Gilled snails disappear, too. Such invertebrate declines have cascading effects on fishes that feed on them. Some fish are more directly affected. Trout and salmon are especially intolerant of low oxygen. Carp (including koi and goldfish) are among the most tolerant of oxygen declines; they survive even in warm algae-rich ponds or tiny goldfish bowls. When oxygen levels fall below 4 parts per million, no fishes can survive. Leeches thrive as most competing invertebrates disappear. In waters with the lowest oxygen concentration, annelids called sludge worms (Tubifex) often are the only animals. They are colored red by large amounts of hemoglobin. Compared to the hemoglobin in most organisms, the Tubifex hemoglobin is better at binding oxygen when oxygen levels are low. A high affinity for oxygen allows these worms to exploit low-oxygen habitats such as sediments in deep lakes, where food is plentiful and competitors and predators are scarce.
Figure 39.4 A fish kill. When the oxygen level in water declines, fishes and other aquatic organisms can suffocate.
CHAPTER 39
RESPIRATION 683
39.3
Invertebrate Respiration Most aquatic mollusks draw water into their mantle cavity, where it flows over a gill (as shown earlier in Figures 25.23, 25.25, and 25.26). In some sea slugs, gills are visible on the body surface (Figure 39.5c). Many aquatic arthropods such as lobsters and crabs have feathery gills inside their exoskeleton, where the delicate tissues are protected from damage. The gills evolved from walking legs.
Invertebrates arose in water but some groups evolved respiratory organs that allow them to breathe air.
Link to Animal body plans 25.1
Integumentary Exchange Some invertebrates do not have any respiratory organs (Figure 39.5a,b). Sponges, cnidarians, flatworms, and earthworms are examples. Such animals live in aquatic or continually damp land environments and rely on integumentary exchange: the diffusion of gases across their outer body surface, or integument. Animals that depend on this method of gas exchange usually are small and flat, or when larger, have cells arranged in thin layers. Integumentary exchange also supplements the effects of respiratory organs in many invertebrates that have gills, and even some vertebrates.
Snails With Lungs Snails and slugs that spend some time on land have a lung instead of, or in addition to, their gill. A lung is a saclike respiratory organ. Inside it, branching tubes deliver air to a respiratory surface serviced by many blood vessels. In snails and slugs, a pore at the side of the body can be opened to allow air into the lung, and it can be shut to conserve water (Figure 39.6).
Invertebrate Gills Tracheal Tubes and Book Lungs
Gills are filamentous respiratory organs that increase the surface area available for gas exchange in many aquatic animals. Blood vessels in gill filaments pick up oxygen and distribute it throughout the body.
The most successful air-breathing land invertebrates are insects and arachnids, such as spiders. They have a hard integument that helps conserve water but also
gill
siphon
mantle
c
a
Figure 39.5 Respiration in water. (a) A jellyfish and (b) a marine flatworm do not have respiratory organs. All of the cells in these animals lie close to the body surface, and gas exchange takes place by diffusion across that surface. (c) Gill of the marine sea slug Aplysia, a mollusk. Having a gill increases the surface area for gas exchange. Blood vessels that run through the gill carry gases to and from body tissues.
b
684 UNIT VI
HOW ANIMALS WORK
trachea (tube nside body) rachea tube inside( body)
Figure 39.6 A land snail (Helix aspersa) with the opening that leads to its lung visible at the left. Compare Figure 25.24.
blocks gas exchange. Insects and some spiders have a tracheal system that consists of repeatedly branching, air-filled tubes reinforced with chitin. Tracheal tubes start at spiracles—small openings across the integument (Figure 39.7). There is usually a pair of spiracles per segment: one on each side of the body. They can be opened or closed to regulate the amount of oxygen that enters the body. Substances that clog spiracles are used as insecticides. For example, horticultural oils sprayed on fruit trees kill scale bugs, aphids, and mites by clogging their spiracles. At the tips of the finest tracheal branches is a bit of fluid in which gases dissolve. The tips of insect tracheal tubes are adjacent to body cells, and oxygen and carbon dioxide diffuse between these tubes and the tissues. Because tracheal tubes end next to cells, insects have no need for a respiratory protein such as hemoglobin to carry gases. Some insects can force air into and out of tracheal tubes. For example, when a grasshopper’s abdominal muscles contract, organs press on the pliable tracheal tubes and force air out of them. When these muscles relax, pressure on tracheal tubes decreases, the tubes widen, and air rushes in. Some spiders have one or two book lungs in addition to or instead of tracheal tubes. In a book lung, air and blood exchange gases across thin sheets of tissue (Figure 39.8). Hemocyanin in a spider’s blood picks up oxygen and turns blue-green as it passes through a book lung. It gives up oxygen and becomes colorless in body tissues.
(opening tospiracle body surface) to body (opening surface) spiracle
Figure 39.7 Insect tracheal system. Chitin rings reinforce branching, air-filled tubes in such respiratory systems.
air-filled space blood-filled space
book lung
Figure 39.8 Above, a spider’s book lung. The lung contains many thin sheets of tissue, somewhat like the pages of a book. As blood moves through spaces between the “pages,” it exchanges gases with air in adjacent spaces. Left, horseshoe crab blood. Like spider blood, it contains the respiratory pigment hemocyanin, which turns blue-green when carrying oxygen.
Take-Home Message How do invertebrates exchanges gases with their environment?
Some invertebrates do not have respiratory organs and exchange gases across the body wall. This process also supplements the action of gills in many invertebrates.
Gills are filamentous organs that increase the surface area for gas exchange in aquatic habitats. Blood vessels run through gill filaments.
Some land snails have a lung in their mantle cavity. Land arthropods have tracheal tubes or book lungs, respiratory organs that bring air deep inside their body.
CHAPTER 39
RESPIRATION 685
39.4
Vertebrate Respiration Fishes use gills to extract oxygen from water; land vertebrates obtain it from air that enters their lungs.
Link to Move to land 26.5
The Gills of Fishes
a gill cover
All fishes have gill slits that open across the pharynx (their throat region). In jawless fishes and cartilaginous fishes, the gill slits are visible from the outside, but bony fishes have a gill cover that hides them (Figure 39.9a). In all fishes, respiration occurs when water flows into the mouth, enters the pharynx, then moves out of the body through the gill slits. Some sharks swim constantly with their mouth open, so water flows passively over their gills. However, most fish actively draw water over their gills. A bony fish sucks water inward by opening its mouth, closing its gill covers, and contracting muscles that enlarge the oral cavity
mouth open
mouth closed
gill cover open
gill cover closed
b
c
Figure 39.9 (a) Location of the gill cover of a bony fish. (b) Water is sucked into the mouth and over the gills when a fish closes its gill covers, opens its mouth, and expands its oral cavity. (c) The water moves out when the fish closes its mouth, opens its gill covers, and squeezes the water past its gills.
gill filaments one gill arch
(Figure 39.9b). Water is forced out when the fish closes its mouth, opens the gill cover, and contracts muscles that make the oral cavity smaller (Figure 39.9c). If you could remove the gill cover of a bony fish, you would see that the gills themselves consist of bony gill arches, each with many gill filaments attached (Figure 39.10a,b). Each gill filament holds many capillary beds where gases are exchanged with blood. Blood in a gill capillary and water flowing past gill filaments move in opposite directions (Figure 39.10c). The result is a countercurrent exchange, in which two fluids exchange substances while flowing in opposite directions. Oxygen-poor blood enters a capillary and travels past water with an increasing oxygen content. Because these fluids flow in opposite directions, their oxygen content can never equalize, as it would if they flowed in the same direction. As a result, oxygen diffuses from water into the blood all along the capillary.
Evolution of Paired Lungs The first vertebrate lungs evolved from outpouchings of the gut wall in some bony fishes. Such lungs may have helped these fishes survive short trips on land. Gills would have been useless in air: Without water to buoy them up and keep them moist, gills would collapse under their own weight and dry out. Lungs became increasingly important as aquatic tetrapods spent more time on land (Section 26.5). Amphibian larvae have external gills. Most often, as the animal develops, these gills disappear and are replaced by paired lungs. Amphibians also exchange some gases across their thin-skinned body surface. In all amphibians, most carbon dioxide that forms during aerobic respiration leaves the body across the skin.
gill arch
respiratory surface
gill filament
fold with a capillary bed inside
Water exits through gill slits
water is sucked into mouth
A A bony fish with its gill cover removed. Water flows in through the mouth, flows over the gills, then exits through gill slits. Each gill has bony gill arches to which the gill filaments attach.
water flow
oxygen-poor blood oxygenated blood from deep in body back toward body B
Two gill arches with filaments
Figure 39.10 Animated Structure and function of the gills of a bony fish.
686 UNIT VI
HOW ANIMALS WORK
direction of blood flow
C
Countercurrent flow of water and blood
A Lowering the floor of the mouth draws air inward through nostrils.
B Closing nostrils and raising the floor of the mouth pushes air into lungs.
C Rhythmically raising and lowering the floor of the mouth assists gas exchange.
D Contracting chest muscles and raising the floor of the mouth forces air out of lungs, and the frog exhales.
Figure 39.11 Animated How a frog breathes.
Frogs have paired lungs. They inhale by lowering the floor of the mouth, which draws air in through their nostrils. Then they close their nostrils and lift the floor of the mouth and throat, pushing air into the lungs (Figure 39.11). Reptiles, birds, and mammals—the amniotes—have waterproof skin and no gills as adults. Gas exchange occurs in their two well-developed lungs. Contraction of chest muscles draws air through airways and into the lungs. In reptiles and mammals, gas exchange occurs in sacs at the ends of the smallest airways. In birds, there are no such “dead ends” inside the lung. Birds have small, inelastic lungs that do not expand and contract when the bird breathes. Instead, air sacs attached to the lungs inflate and deflate. It takes two breaths to move air through this system (Figure 39.12). Oxygenrich air flows through tiny tubes in the lung during both inhalations and exhalations. The lining of these tubes is the respiratory surface. Continual movement of air past this surface greatly increases the efficiency of gas exchange. We turn next to the human respiratory system. Its operating principles apply to most vertebrates, even though lungs evolved differently among them.
Take-Home Message What kind of respiratory systems do vertebrates have?
Most fish exchange gases with water that flows over their gills. The direction of blood flow in gill capillaries is opposite that of water flow. This countercurrent flow aids gas exchange. Amphibians exchange gases across their skin and (usually) at the respiratory surface of paired lungs.
Reptiles, birds, and mammals do not exchange any gases across the skin. They rely on paired lungs. Birds have the most efficient vertebrate lungs. A system of air sacs ensures that air moves constantly through a bird’s lung.
A Inhalation 1 Muscles expand chest cavity, drawing air in through nostrils. Some of the air flowing in through the trachea goes to lungs and some goes to posterior air sacs.
B Exhalation 1 Anterior air sacs empty. Air from posterior air sacs moves into lungs.
trachea
anterior air sacs lung
posterior air sacs
C Inhalation 2 Air in lungs moves to anterior air sacs and is replaced by newly inhaled air.
D Exhalation 2 Air in anterior air sacs moves out of the body and air from posterior sacs flows into the lungs.
Figure 39.12 Animated Respiratory system of a bird. Large, stretchy air sacs attach to two small, inelastic lungs. Contraction and expansion of chest muscles cause air to flow into and then out of this system. Air flows in through many air tubes inside the lung, and into posterior air sacs. The lining of the tiniest air tubes, sometimes called air capillaries, is the site of gas exchange—the respiratory surface. It takes more than one breath for air to flow through the system, but air flows continuously through the lungs and over the respiratory surface. This unique ventilating system supports the high metabolic rates that birds require for flight and other energy-demanding activities. Right, this scanning electron micrograph of lung tissue shows the tubes through which air flows to and from air sacs. Gas exchange takes place across the lining of these tubes.
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RESPIRATION 687
39.5
Human Respiratory System The human respiratory system functions in gas exchange, but also in speech, in the sense of smell, and in homeostasis.
Link to Temperature regulation 27.3
The System’s Many Functions Figure 39.13 shows the human respiratory system and lists the functions of its parts. It also shows skeletal muscles that assist in respiration. Rhythmic contraction and relaxation of these muscles cause air to move into and out of the lungs.
The respiratory system functions in gas exchange, but it has a wealth of additional roles. We can speak, sing, or shout by controlling vibrations as air moves past our vocal cords. We have a sense of smell because airborne molecules stimulate olfactory receptors in the nose. Cells lining nasal passages and other airways of the system help defend the body; they intercept and neutralize airborne pathogens. The respiratory system contributes to the body’s acid–base balance by getting rid of carbon dioxide wastes. Controls over breathing even help maintain body temperature, because water evaporating from airways has a cooling effect.
Nasal Cavity Chamber in which air is moistened, warmed, and filtered, and in which sounds resonate
Oral Cavity (Mouth) Supplemental airway when breathing is labored
Pharynx (Throat) Airway connecting nasal cavity and mouth with larynx; enhances sounds; also connects with esophagus
Epiglottis Closes off larynx during swallowing
Larynx (Voice Box) Airway where sound is produced; closed off during swallowing
Pleural Membrane Double-layer membrane with a fluid-filled space between layers; keeps lungs airtight and helps them stick to chest wall during breathing
Trachea (Windpipe) Airway connecting larynx with two bronchi that lead into the lungs
Lung (One of a Pair) Lobed, elastic organ of breathing; enhances gas exchange between internal environment and outside air
Intercostal Muscles At rib cage, skeletal muscles with roles in breathing. There are two sets of intercostal muscles (external and internal)
Bronchial Tree Increasingly branched airways starting with two bronchi and ending at air sacs (alveoli) of lung tissue
Diaphragm Muscle sheet between the chest cavity and abdominal cavity with roles in breathing A
bronchiole
Figure 39.13 Animated (a) Components of the human respiratory system and their functions. The diaphragm and other muscles, as well as certain bones of the axial skeleton, have secondary roles in respiration. (b,c) Location of alveoli relative to the bronchioles and the lung (pulmonary) capillaries.
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alveolar sac (sectioned)
alveolar sac
alveolar duct
alveoli B
HOW ANIMALS WORK
pulmonary capillary C
From Airways to Alveoli Take a deep breath. Now look at Figure 39.13 to get an idea of where the air traveled in your respiratory system. If you are healthy and sitting quietly, air probably entered through your nose, rather than your mouth. As air moves through your nostrils, tiny hairs filter out any large particles. Mucus secreted by cells of the nasal lining captures most fine particles and airborne chemicals. Ciliated cells in the nasal lining also help remove any inhaled contaminants. Air from the nostrils enters the nasal cavity, where it gets warmed and moistened. It flows next into the pharynx, or throat. It continues to the larynx, a short airway commonly known as the voice box because a pair of vocal cords projects into it (Figure 39.14). Each vocal cord is skeletal muscle with a cover of mucussecreting epithelium. Contraction of the vocal cords changes the size of the glottis, the gap between them. When the glottis is wide open, air flows through it silently. When muscle contraction narrows the glottis, flow of air outward through the tighter gap makes vocal cords vibrate and produces sounds. The tension on the cords and the position of the larynx determine the sound’s pitch. To get a feel for how this works, place one finger on your “Adam’s apple,” the laryngeal cartilage that sticks out most at the front of your neck. Hum a low note, then a high one. You will feel the vibration of your vocal cords and how laryngeal muscles shift the position of your larynx. In laryngitis, overuse or infection has inflamed the vocal cords. The swollen cords cannot vibrate as they should, which makes speaking difficult. At the entrance to the larynx is an epiglottis. When this tissue flap points up, air moves into the trachea, or windpipe. When you swallow, the epiglottis flops over, points down, and covers the larynx entrance, so food and fluids enter the esophagus. The esophagus connects the pharynx to the stomach. The trachea branches into two airways, one to each lung. Each airway is a bronchus (plural, bronchi). Its epithelial lining has many ciliated and mucus-secreting cells that fend off respiratory tract infections. Bacteria and airborne particles stick to the mucus. Cilia sweep the mucus toward the throat for expulsion. The Respiratory Passageways
Human lungs are cone-shaped organs in the thoracic cavity, one on each side of the heart. The rib cage encloses and protects the lungs. A two-layer-thick pleural membrane covers each lung’s outer surface and lines the inner thoracic cavity wall.
The Paired Lungs
glottis open
glottis closed vocal cords
Figure 39.14 Human vocal cords, inside the larynx. Contraction of skeletal muscle in these cords changes the width of the glottis, the gap between them. The glottis closes tightly when you swallow. It is open during quiet breathing. It narrows when you speak, so that air flow causes the cords to vibrate.
glottis (closed) epiglottis tongue’s base
Once inside a lung, air moves through finer and finer branchings of a “bronchial tree.” The branches are called bronchioles. At the tips of the finest bronchioles are respiratory alveoli (singular, alveolus), little air sacs where gases are exchanged (Figure 39.13b,c). Each alveolus has a wall that is only one cell thick. Collectively, the many alveoli provide an extensive surface for gas exchange. If all 6 million alveoli in your lungs could be stretched out in a single layer, they would cover half of a tennis court! Air in alveoli exchanges gases with blood flowing through pulmonary capillaries (Latin pulmo, lung). At this point, a different organ system gets involved. The circulatory system transports oxygen to body tissues and carries carbon dioxide away from them. A broad sheet of smooth muscle beneath the lungs, the diaphragm, partitions the coelom into a thoracic cavity and an abdominal cavity. Of all smooth muscle, it alone can be controlled voluntarily. You can make it contract by deliberately inhaling. The diaphragm and intercostal muscles, the skeletal muscles between the ribs, interact to change the volume of the thoracic cavity during breathing.
Muscles and Respiration
Take-Home Message What roles do the components of the human respiratory system play? In addition to gas exchange, the human respiratory system acts in the sense of smell, voice production, body defenses, acid–base balance, and temperature regulation. Air enters through the nose or mouth. It flows through the pharynx (throat) and larynx (voice box) to a trachea that branches into two bronchi, one to each lung. Inside each lung, additional branching airways deliver air to alveoli, where gases are exchanged with pulmonary capillaries.
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39.6
Cyclic Reversals in Air Pressure Gradients Rhythmic signals from the brain cause muscle contractions that cause air to flow into the lungs.
6
Links to Autonomic signals 33.8, Brain stem 33.10, Chemoreceptors 34.1
5
The Respiratory Cycle A respiratory cycle is one breath in (inhalation) and one breath out (exhalation). Inhalation is always active; muscle contractions drive it. Changes in the volume of the lungs and thoracic cavity during a respiratory cycle alter pressure gradients between air inside and outside the respiratory tract (Figures 39.15 and 39.16). When you inhale, the diaphragm flattens, moving downward. External intercostal muscles contract and lift the rib cage up and outward (Figure 39.15a). As the thoracic cavity expands, so do the lungs. Pressure in the alveoli falls below atmospheric pressure, and air flows down the pressure gradient, into the airways.
Inward flow of air
A Inhalation. Diaphragm contracts, moves down. External intercostal muscles contract, lift rib cage upward and outward. Lung volume expands.
Outward flow of air
B Exhalation. Diaphragm, external intercostal muscles return to resting positions. Rib cage moves down. Lungs recoil passively.
Figure 39.15 Animated Changes in the size of the thoracic cavity during a single respiratory cycle. The x-ray images reveal how inhalation and expiration change the lung volume.
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Lung volume (liters)
4
forced inhalation volume
tidal volume
3 2 1
vital capacity
total lung capacity
forced exhalation volume residual volume
0 Time
Figure 39.16 Animated Respiratory volumes. In normal breathing, lungs hold 2.7 liters at the end of inhalation and 2.2 liters at the end of exhalation; the tidal volume of air entering and leaving is 0.5 liter. Lungs never deflate completely. When air flows out and lung volume is low, the wall of the smallest airways collapses and prevents further air loss.
Exhalation is usually passive. When muscles that caused inhalation relax, the lungs passively recoil and lung volume decreases. This compresses alveolar sacs, raising air pressure inside them. Air moves down the pressure gradient, out of the lungs (Figure 39.15b). Exhalation is active only when you exercise vigorously or consciously attempt to expel more air. During active exhalation, internal intercostal muscles contract, pulling the thoracic wall inward and downward. At the same time, muscles of the abdominal wall contract. Abdominal pressure increases and exerts an upwarddirected force on the diaphragm. The volume of the thoracic cavity decreases more than normal, and a bit more air is forced out. Upward-directed force on the diaphragm is also the reason that the Heimlich maneuver works (Figure 39.17). Performing this procedure can save the life of a person who is choking. A choking person has food lodged in their trachea. By making upward thrusts into the choker’s upper abdomen, a rescuer raises the intra-abdominal pressure, which forces the choker’s diaphragm upward. The force of air rushing out of the lungs into the trachea can dislodge the food, allowing the victim to resume breathing.
Respiratory Volumes The maximum volume of air that the lungs can hold, total lung volume, averages 5.7 liters in men and 4.2 liters in women. Usually lungs are less than half full. Vital capacity, the maximum volume that can move
in and out in one cycle, is one measure of lung health. Tidal volume—the volume that moves in and out in a normal respiratory cycle—is about 0.5 liter (Figure 39.16). Your lungs never fully deflate; thus air inside them always is a mix of freshly inhaled air and “stale air” that was left behind during the previous exhalation. Even so, there is plenty of oxygen for exchange.
Control of Breathing Neurons in the medulla oblongata of the brain stem serve as a control center for respiration. When you rest, these neurons fire spontaneous action potentials 10 to 14 times per minute. Nerves carry these signals to the diaphragm and intercostal muscles, causing the contractions that result in inhalation. Between action potentials, the muscles relax and you exhale. Breathing patterns change with activity level. When you are more active, muscle cells increase their rate of aerobic respiration and produce more CO2. This CO2 enters blood, where it combines with water and forms carbonic acid (Section 39.7). The acid dissociates and H+ levels rise in the blood and in cerebrospinal fluid. Chemoreceptors inside the medulla oblongata and in carotid artery and aorta walls detect the change. These receptors signal the respiratory center, which calls for changes in the breathing pattern (Figure 39.18). Chemoreceptors in the carotid arteries also signal the medulla oblongata when the O2 partial pressure in arterial blood falls below a life-threatening 60 mm Hg. Ordinarily, the O2 partial pressure does not fall that low. This control mechanism has survival value only at high altitudes and during severe lung diseases. Reflexes such as swallowing or coughing can briefly halt breathing. Breathing patterns can also be deliberately altered, as when you hold your breath to dive, or break normal breathing rhythm to talk. In addition, commands from sympathetic nerves make you breathe faster when you are frightened (Section 33.8). Take-Home Message What happens when we breathe? Inhalation is always an active process. Contraction of the diaphragm and external intercostal muscles increase the volume of the thoracic cavity. This reduces air pressure in alveoli below atmospheric pressure, so air moves inward. Exhalation is usually passive. As muscles relax, the thoracic cavity shrinks back down, air pressure in alveoli rises above atmospheric pressure, and air moves out. Only some of the air in the lungs is replaced with each breath. The lungs are never fully emptied of air. The brain controls the rate and depth of breathing.
A
B
Figure 39.17 Animated How to perform the Heimlich maneuver on an adult who is choking. 1. Determine that the person is actually choking; a person who has an object lodged in their trachea cannot cough or speak. 2. Stand behind the person and place one fist below his or her rib cage, just above the navel, with your thumb facing inward as in (a). 3. Cover the fist with your other hand and thrust inward and upward with both fists as in (b). Repeat until the object is expelled.
STIMULUS CO2 concentration and acidity rise in the blood and cerebrospinal fluid.
RESPONSE Chemoreceptors in wall of carotid arteries and aorta
Respiratory center in brain stem
Diaphragm, Intercostal muscles
CO2 concentration and acidity decline in the blood and cerebrospinal fluid.
Tidal volume and rate of breathing change.
Figure 39.18 Respiratory response to increased activity levels. An increase in activity raises the CO2 output. It also makes the blood and cerebrospinal fluid more acidic. Chemoreceptors in blood vessels and the medulla sense the changes and signal the brain’s respiratory center, also in the brain stem. In response, the respiratory center signals the diaphragm and intercostal muscles. The signals call for alterations in the rate and depth of breathing. Excess CO2 is expelled, which causes the level of this gas and acidity to decline. Chemoreceptors sense the decline and signal the respiratory center, so breathing is adjusted accordingly.
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39.7
Gas Exchange and Transport Gases are exchanged by diffusion in alveoli. Red blood cells play a role in transport of both oxygen and carbon dioxide.
Links to Hemoglobin 3.6, Red blood cells 37.2
The Respiratory Membrane Gases diffuse between an alveolus and a pulmonary capillary at the lung’s respiratory membrane. This thin membrane consists of alveolar epithelium, capillary endothelium, and the fused basement membranes of the alveolus and capillary (Figure 39.19). Secretions keep the alveolar side of the respiratory membrane moist so that gases can diffuse quickly across it. O2 and CO2 diffuse passively across the respiratory membrane. Therefore, the net direction of movement of these gases depends upon their partial pressure gradients across the membrane. Air flow into and out of the lungs and blood flow through pulmonary capillaries keep O2 and CO2 partial pressure gradients steep.
Oxygen Transport The inhaled air that reaches alveoli contains a great deal of O2 compared to the blood in pulmonary capillaries. As a result, O2 in the lungs tends to diffuse into blood plasma inside the pulmonary capillaries, and then into red blood cells. As many as 30 trillion red blood cells circulate in your blood. Each holds many millions of hemoglobin molecules. Again, the hemoglobin molecule consists of four polypeptide chains, each associated with one heme group (Figure 39.20a). Each heme group includes one iron atom that reversibly binds O2. Hemoglobin with oxygen bonded to it is oxyhemoglobin, or HbO2.
About 98.5 percent of the oxygen you inhale gets bound to heme groups of hemoglobin. The amount of HbO2 that forms in a given interval depends on the partial pressure of O2. The higher the partial pressure of O2, the more HbO2 will form. Heme binds O2 only weakly. It releases O2 in places where the partial pressure of O2 is much lower than that in the alveoli. This is true in metabolically active tissues, as the boxes color-coded pink in Figure 39.21 show. Other factors that encourage release of O2 from heme, including high temperature, low pH, and high CO2 partial pressure, also are typical of these tissues. Myoglobin, also an iron-containing respiratory protein, helps cardiac muscle and some skeletal muscles store oxygen. Structurally, myoglobin resembles the globin in hemoglobin, but it holds more tightly onto oxygen (Figure 39.20b). The O2 that hemoglobin gives up near a cardiac muscle cell diffuses into the cell and binds to myoglobin inside it. When blood flow cannot keep up with a cell’s increased O2 needs, as during periods of intense exercise, the myoglobin releases O2, which allows mitochondria to keep on making ATP.
Carbon Dioxide Transport Carbon dioxide diffuses into blood capillaries in any tissue where its partial pressure is higher than it is in blood. This is the case in metabolically active tissues, as the boxes color-coded blue in Figure 39.21 show. Carbon dioxide is transported to the lungs in three forms. About 10 percent remains dissolved in plasma. Another 30 percent reversibly binds with hemoglobin and forms carbaminohemoglobin (HbCO2). However, most CO2 that diffuses into the plasma—60 percent— is transported as bicarbonate (HCO3–).
red blood cell inside pulmonary capillary
pore for air flow between adjoining alveoli
air space inside alveolus
a Surface view of capillaries associated with alveoli
b Cutaway view of one of the alveoli and adjacent pulmonary capillaries
Figure 39.19 Zooming in on the respiratory membrane in human lungs.
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alveolar epithelium capillary endothelium fused basement membranes of both epithelial tissues c Three components of the respiratory membrane
alpha globin
Figure 39.20 (a) Structure of hemoglobin, the oxygen-transporting protein of red blood cells. It consists of four globin chains, each associated with an iron-containing heme group, color-coded red.
alpha globin
(b) Myoglobin, an oxygen-storing protein in muscle cells. Its single chain associates with a heme group. Compared to hemoglobin, myoglobin has a higher affinity for oxygen, so it helps speed the transfer of oxygen from blood to muscle cells.
a
beta globin
beta globin
b
heme
How does bicarbonate form? Carbon dioxide first combines with water, forming carbonic acid (H2CO3). This compound separates into bicarbonate and H+: CO2 H2O
H2CO3 carbonic acid
DRY INHALED AIR
MOIST EXHALED AIR
160 0.03
HCO3– H+
120
27
bicarbonate
Red blood cells have carbonic anhydrase, an enzyme that catalyzes the above reaction. The bicarbonate that forms in red blood cells diffuses into plasma, whereas most of the H+ binds to hemoglobin. When red blood cells reach the alveolar capillaries— where the CO2 partial pressure is relatively low—the reactions reverse, forming water and CO2. The CO2 diffuses into the air in an alveolus and is exhaled.
alveolar sacs pulmonary arteries 40
104
pulmonary veins
40
45
100
40
The Carbon Monoxide Threat Carbon monoxide (CO) is a colorless, odorless gas. It is present in the smoke from cigarettes and fossil fuel combustion. Hemoglobin has a higher affinity for CO than for O2. When CO builds up in the air, it fills O2 binding sites on hemoglobin, preventing transport of O2 and causing carbon monoxide poisoning. Nausea, headache, confusion, dizziness, and weakness set in as tissues are starved of oxygen. In the United States, accidental CO poisoning kills about 500 people each year. To minimize your risk, be sure that fuel-burning appliances have been properly vented to the outside, and install a carbon monoxide detector.
start of systemic veins
start of systemic capillaries
40
100
45
40
Take-Home Message
Most carbon dioxide is transported in blood in the form of bicarbonate, nearly all of which forms by enzyme action inside red blood cells.
cells of body tissues less than 40
more than 45
Figure 39.21 Animated Partial pressures (in mm Hg) for oxygen (pink boxes) and carbon dioxide (blue boxes) in the atmosphere, blood, and tissues. Figure It Out: What is the partial pressure of oxygen in arteries that carry blood to systemic capillary beds? Answer: 100 mm Hg
How are gases transported in blood? Most oxygen in blood is bound to hemoglobin, which binds oxygen in alveoli where oxygen partial pressure is high, and releases it in tissues where oxygen partial pressure is lower.
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RESPIRATION 693
39.8
Respiratory Diseases and Disorders Genetic disorders, infectious disease, and lifestyle choices can increase the risk of respiratory problems.
Links to Tuberculosis 21.8, Marijuana’s effects 33.7
Interrupted Breathing A tumor or other damage to the brain stem’s medulla oblongata can affect respiratory controls. It can cause apnea, a disorder in which breathing repeatedly stops and restarts spontaneously, especially during sleep. More often, sleep apnea occurs when the tongue, tonsils, or another soft tissue obstructs the upper airways. Breathing stops for up to several seconds many times each night. Interrupted sleep patterns and daytime fatigue follow. The risk for heart attacks and strokes rises, because each time breathing stops, blood pressure soars. Changes in sleeping positions or using a mouthpiece or other kinds of devices can help mild sleep apnea. Severe cases require surgical removal of the soft tissues that block the airways. Sudden infant death syndrome (SIDS) occurs when an infant does not awaken from an apneic episode. Infants who sleep on their back are less vulnerable to SIDS than stomach sleepers. They are more at risk if their mother smoked or was exposed to smoke during pregnancy. Hannah Kinney of Harvard Medical School reported that an underlying weakness in the respiratory control center may be fatal when combined with environmental
a
free surface of a mucussecreting cell
free surface of a cluster of ciliated cells b
Figure 39.22 (a) Cigarette smoke about to enter bronchi that lead to the lungs. Smoke irritates ciliated and mucus-secreting cells that line the airways (b) and can exacerbate bronchitis.
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stresses. She compared brains of infants who died of SIDS with those of infants who died of other causes. The SIDS babies had fewer receptors for serotonin in their medulla oblongata. This neurotransmitter carries signals between neurons (Section 33.6). Weak signaling may impair the responses to potentially deadly respiratory stress.
Potentially Deadly Infections About one-third of the human population is infected by Mycobacterium tuberculosis, the cause of tuberculosis (Section 21.8). These bacteria colonize the lungs, but infection does not always result in disease. Carriers can be identified by a TB skin test. If untreated, about 10 percent of them eventually will develop the disease. They start to cough and may have chest pain. They may have trouble breathing and cough up bloody mucus. Antibiotics cure TB, but only if they are taken diligently for at least six months. An active, untreated infection can be fatal. Lungs also get infected by bacteria, viruses, and—less commonly—fungi that cause pneumonia. Pneumonia is not one disease; it is a general term for lung inflammation caused by an infectious organism. Coughing, an aching chest, shortness of breath, and fever are usual symptoms. An x-ray can reveal infected tissues filled with fluid and immune cells instead of air. The treatment and outcome depend on the type of pathogen. Chronic Bronchitis and Emphysema Facing the lumen of your bronchioles is a ciliated, mucus-producing epithelium (Figure 39.22). It is one of many defenses that protect you from respiratory infections. Chronic irritation of the lining may lead to bronchitis. With this respiratory disease, epithelial cells become irritated and secrete too much mucus. Excessive mucus triggers coughing, and provides a moist, nutrient-rich place for pathogens to grow. Early attacks of bronchitis are treatable. When the aggravation continues, bronchioles become chronically inflamed as bacteria, chemical agents, or both attack the lining of these airways. The lining’s ciliated cells die, and mucus-secreting cells multiply. Fibrous scar tissue forms. Over time, scarring narrows or obstructs the airways. Breathing becomes labored and difficult. Chronic bronchitis can lead to emphysema. With this condition, tissue-destroying bacterial enzymes digest the thin, stretchable alveolar wall. As walls deteriorate, inelastic fibrous tissue builds up around them. Alveoli enlarge, and gas exchange becomes less efficient. In time, the lungs become distended and inelastic, so the balance between air flow and blood flow is compromised. It becomes hard even to catch a breath. About 2 million people in the United States currently have emphysema, and it causes or contributes to about 100,000 deaths every year. A number of individuals are genetically predisposed to develop emphysema. They do not have a workable gene for antitrypsin, an enzyme that inhibits bacterial attacks
FOCUS ON HEALTH
Reduction in Risks by Quitting
Risks Associated With Smoking Shortened life expectancy Nonsmokers live about 8.3 years longer than those who smoke two packs a day from their midtwenties on.
Cumulative risk reduction; after 10–15 years, the life expectancy of ex-smokers approaches that of nonsmokers.
Chronic bronchitis, emphysema Smokers have 4–25 times higher risk of dying from these diseases than do nonsmokers.
Greater chance of improving lung function and slowing down rate of deterioration.
Cancer of lungs Cigarette smoking is the major cause.
After 10–15 years, risk approaches that of nonsmokers.
Cancer of mouth 3–10 times greater risk among smokers.
After 10–15 years, risk is reduced to that of nonsmokers.
Cancer of larynx 2.9–17.7 times more frequent among smokers.
After 10 years, risk is reduced to that of nonsmokers.
Cancer of esophagus 2–9 times greater risk of dying from this.
Risk proportional to amount smoked; quitting should reduce it.
Cancer of pancreas 2–5 times greater risk of dying from this.
Risk proportional to amount smoked; quitting should reduce it.
Cancer of bladder 7–10 times greater risk for smokers.
Risk decreases gradually over 7 years to that of nonsmokers.
Cardiovascular disease Cigarette smoking a major contributing factor in heart attacks, strokes, and atherosclerosis.
Risk for heart attack declines rapidly, for stroke declines more gradually, and for atherosclerosis it levels off.
Impact on offspring Women who smoke during pregnancy have more stillbirths, and the weight of liveborns is lower than the average (which makes babies more vulnerable to disease and death).
When smoking stops before fourth month of pregnancy, risk of stillbirth and lower birth weight eliminated.
Impaired immunity More allergic responses, destruction of white blood cells (macrophages) in respiratory tract.
Avoidable by not smoking.
Bone healing Surgically cut or broken bones may take 30 percent longer to heal in smokers, perhaps because smoking depletes the body of vitamin C and reduces the amount of oxygen delivered to tissues. Reduced vitamin C and reduced oxygen interfere with formation of collagen fibers in bone (and many other tissues).
Avoidable by not smoking.
a
on alveoli. Poor diet and persistent or recurring colds and other respiratory infections also invite emphysema later in life. Air pollution and chemicals in the workplace may contribute to the problem. However, tobacco smoking is by far the main risk factor for emphysema. Most of those affected are over age 50. Twenty or thirty years of smoke exposure leave lungs looking like those in Figure 39.23c.
Smoking’s Impact Globally, cigarette smoking kills 4 million people each year. By 2030, the number may rise to 10 million, with about 70 percent of the deaths occurring in developing countries. In the United States, the direct medical costs of treating smoke-induced disorders drains $22 billion a year from the economy. As G. H. Brundtland—a medical doctor and the former director of the World Health Organization—points out, tobacco is the only legal consumer product that kills half of its regular users. If you are a smoker, you may wish to reflect on the information in Figure 39.23a. Cigarettes also do more than sicken and kill smokers. Nonsmokers die of cancers and disease brought on by breathing secondhand smoke. Children who breathe cigarette smoke at home have a heightened risk for developing lung problems. Smoking while pregnant increases risk of miscarriage and low birth weight.
b
c
Figure 39.23 (a) From the American Cancer Society, a list of major risks incurred by smoking and the benefits of quitting. (b) Appearance of normal lung tissue in humans. (c) Appearance of lung tissues from someone who was affected by emphysema.
Smoking marijuana (Cannabis) also poses significant respiratory risks. Although marijuana contains fewer toxic particles, or “tar,” than tobacco, marijuana is usually smoked without a filter. Also, people smoking marijuana tend to inhale more deeply than tobacco smokers, to hold hot smoke in their lungs for longer periods, and to smoke their cigarettes down to stubs, where tar accumulates. As a result, long-term marijuana smokers have an increased risk of respiratory problems, and they tend to show lung damage earlier than cigarette smokers. On the other hand, unlike tobacco, marijuana has not been shown to increase the risk of lung cancer.
CHAPTER 39
RESPIRATION 695
39.9
High Climbers and Deep Divers Specialized features of some respiratory systems adapt organisms to high altitude or deep dives.
Links to Evolutionary adaptation 17.1, Hypertension 37.9
Respiration at High Altitudes Atmospheric pressure decreases with altitude. Above 5,500 meters, or about 18,000 feet, it is 380 mm Hg— half of what it is at sea level. Oxygen still is 21 percent of the total pressure, so there is about half as much oxygen as there is at sea level. Llamas are animals that live at high altitudes in the Andes (Figure 39.24). Their hemoglobin helps them survive in the “thin air,” with its lower oxygen level. Compared to the hemoglobin of humans and most other mammals, llama hemoglobin binds oxygen more efficiently. Also, the lungs and the heart of a llama are unusually large relative to the animal’s body size. Most people live at lower altitudes where there is plenty of oxygen. When they ascend too fast to high altitudes, the transport of oxygen to cells plummets. Hypoxia, or cellular oxygen deficiency, is the result. In an acute compensatory response to hypoxia, the brain commands the heart and respiratory muscles to work harder. People breathe faster and more deeply than usual; they hyperventilate. As a result CO2 is exhaled faster than it forms and ion balances in the cerebrospinal fluid get skewed. Shortness of breath, a
pounding heart, dizziness, nausea, and vomiting are symptoms of the resulting altitude sickness. Compared to people at low elevations, people who grew up at high altitudes have more alveoli and blood vessels in their lungs. Their heart has larger ventricles and pumps greater volumes of blood. A healthy person who is unaccustomed to life at a high altitude can become physiologically adjusted to such an environment. Through acclimatization, the body makes long-term adjustments in cardiac output, and the rate and magnitude of breathing. Hypoxia also stimulates kidney cells to secrete more erythropoietin. This hormone induces stem cells in the bone marrow to divide repeatedly, and induces descendant cells to develop as red blood cells. Under typical conditions, the body produces 2 million to 3 million red blood cells per second to replace those that die off. Under extreme oxygen deprivation, increased erythropoietin secretion can result in a six-fold rise in red blood cell formation. Increased numbers of circulating red blood cells improve the oxygen-delivery capacity of blood. However, an altitude-induced increase in red blood cell count can put a strain on the heart. Having more blood cells thickens blood, so the heart has to work harder to pump blood through the circulatory system. Stronger contractions increase blood pressure, putting a person at risk for the health problems associated with chronic hypertension (Section 37.9).
Deep-Sea Divers
Heme binding site saturation (%)
100
llama hemoglobin
80 60 typical range for hemoglobin in most mammals
40 20
human hemoglobin 0
80 60 0 40 20 O2 partial pressure (mm Hg)
Figure 39.24 Saturation curve for hemoglobin of humans, llamas, and other mammals. Figure It Out: At what partial pressure of oxygen do half the heme groups in human blood have oxygen bound? Answer: 30 mm Hg
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Water pressure increases with depth. Human divers using tanks of compressed air risk nitrogen narcosis, sometimes called “raptures of the deep.” The deeper a diver goes, the more gaseous nitrogen (N2) dissolves in interstitial fluid. N2 affects the lipid bilayer of cell membranes. In neurons, this dissolved nitrogen can disrupt signaling, causing a diver to feel euphoric and drowsy. The deeper divers descend, the more weakened and clumsy they become. Returning to the surface from a deep dive also has risks. As a diver ascends, pressure falls and N2 moves from interstitial fluid into blood and is exhaled. If a diver rises too fast, N2 bubbles form inside the body. The resulting decompression sickness, also known as “the bends,” usually begins with joint pain. Bubbles of N2 can slow the flow of blood to organs. If bubbles form in the brain or lungs, the result can be fatal. Humans who train to dive without oxygen tanks can remain submerged for about three minutes. So far, the human free diving record is 210 meters. Compare that with the impressive depth records for species
Species
Maximum Depth
Sperm whale (Physeter macrocephalus)
2,200 meters
Leatherback turtle (Dermochelys coriacea)
1,200 meters
Southern elephant seal (Mirounga leonina)
1,620 meters
Weddell seal (Leptonychotes weddelli ) Bottlenose dolphin (Tursiops truncatus) Emperor penguin (Aptenodytes forsteri )
741 meters >600 meters 565 meters
listed in Figure 39.25. What types of adaptations make deep dives possible? Leatherback sea turtles leave water only to lay eggs (Figure 39.25a). They spend the a b rest of their time in open oceans diving for jellyfishes, their main prey. As a turtle or Figure 39.25 (a) Two Atlantic leatherback sea turtles returning to the sea after laying other air-breathing animal dives deeper eggs. The leathery shell is adapted for deep diving; it bends rather than breaks under and deeper, the weight of more and more extreme pressure. (b) Bottlenose dolphins. The chart at left lists a few diving records. water presses down onto the body. Lungs filled with air would collapse inward, but most diving animals move air out of the muscle in a dog has. A muscle in a sperm whale has lungs and into cartilage-reinforced airways before they 7 times as much as the dog muscle. dive too deep. Also, the pressure at great depths could Third, more oxygen gets distributed to the heart, crack a typical turtle’s hard shell, but the leatherback’s brain, and other organs that require an uninterrupted soft shell bends and flexes under such pressure. supply of ATP for a deep dive. The blood volume and Making a deep dive means spending long intervals dissolved gases are stored and distributed efficiently without access to air. The longest dive recorded for a with the assistance of valves and plexuses–—meshes of leatherback turtle lasted for a little more than an hour. blood vessels in local tissues. Metabolic rate and heart Sperm whales can stay submerged for two hours. rate also decrease. So do oxygen uptake and carbon If a diving animal’s lungs are emptied of air and if dioxide formation. it has no access to the surface, then how does it meet Fourth, whenever possible, a diving animal makes its oxygen requirements? It does so in four ways. the most of its oxygen stores by sinking and gliding First, before it dives, it breathes deeply. A sperm instead of actively swimming. It conserves energy by whale blows out about 80–90 percent of the air in its avoiding unnecessary movements. lungs with each exhalation; you exhale only about 15 percent. The deep breaths keep oxygen pressure inside alveoli high, so more oxygen diffuses into the blood. Second, diving animals can store great amounts of Take-Home Message oxygen inside their blood and muscles. They tend to What are some adaptations that aid respiration in extreme environments? have a large blood volume relative to their body size, Hemoglobin with a high affinity for oxygen adapts some animals to life at a high red blood cell count, and considerable amounts high altitudes where oxygen partial pressure is low. of myoglobin in their muscles. A skeletal muscle of a A high red blood cell count, large amount of myoglobin, and other traits bottlenose dolphin (Figure 39.25b) has about 3.5 times allow some animals to hold their breath for long, deep dives. the amount of myoglobin that a comparable skeletal CHAPTER 39
RESPIRATION 697
IMPACTS, ISSUES REVISITED
Up in Smoke
In the United States, tobacco use is declining. Smoking is banned from airline cabins and airports. Many states and cities ban it in theaters, restaurants, and other enclosed spaces. Cigarette sales to minors are prohibited, as is cigarette advertising on television or near schools. However, in most developing countries smoking is largely unrestricted and the proportion of smokers continues to rise, especially among women.
Summary Section 39.1 Respiration is the physiological process by which O2 enters the internal environment and CO2 leaves by diffusing across a respiratory surface. Each gas moves down its own partial pressure gradient into or out of animal bodies. Constraints imposed by the surface-tovolume ratio shape respiratory structures and ventilation mechanisms. Respiratory proteins such as hemoglobin in red blood cells and myoglobin in muscle bind oxygen and help maintain gradients that favor gas exchange. Section 39.2 Oxygen content of water can vary and affects the survival of aquatic species. Section 39.3 Some invertebrates do not have special respiratory organs and rely on integumentary exchange, diffusion of gases across the body surface. Gills enhance respiration in other aquatic invertebrates. On land, lungs, book lungs, and tracheal systems aid gas exchange. Section 39.4 Water flowing over fish gills exchanges gases with blood flowing in the opposite direction inside gill capillaries. This countercurrent exchange is highly efficient. Most amphibians have lungs, and also exchange gases across the skin. Reptiles, birds, and mammals rely on lungs for gas exchange. In birds, air sacs connected to lungs keep air flowing continually through them.
Use the animation on CengageNOW to compare various vertebrate respiratory systems.
Section 39.5 In humans, air flows through two nasal cavities and a mouth into the pharynx (throat), then the larynx (voice box). A flap of tissue called the epiglottis directs air through the glottis, the opening to the trachea (windpipe). The trachea branches into two bronchi that enter the lungs. In the lungs, bronchi lead to finely branching bronchioles that have alveoli at their tips. Gases are exchanged at these thin-walled air sacs. Contractions of the dome-shaped diaphragm and the intercostal muscles between the ribs alter the volume of the thoracic cavity during breathing.
Use the animation on CengageNOW to explore the human respiratory system.
Section 39.6 Each respiratory cycle consists of one inhalation and one exhalation. Inhalation is always an active process. As muscle contractions expand the chest cavity, pressure in lungs decreases below atmospheric 698 UNIT VI
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pressure, and air flows into the lungs. These events are reversed during exhalation, which normally is passive. If a person is choking, the Heimlich maneuver can be used to expel food from their trachea. Tidal volume is normally far less than vital capacity. The medulla oblongata in the brain stem adjusts the rate and magnitude of breathing.
Use the animation on CengageNOW to learn about the respiratory cycle and the Heimlich maneuver.
Section 39.7 In human lungs, the alveolar wall, the wall of a pulmonary capillary, and their fused basement membranes form a thin respiratory membrane between air inside an alveolus and the internal environment. O2 following its partial pressure gradient diffuses across the respiratory membrane, into the plasma of the blood, and finally into red blood cells. Red blood cells are filled with hemoglobin that binds O2 where its partial pressure is high, forming oxyhemoglobin. In metabolically active tissue, O2 released from hemoglobin diffuses out of capillaries, through interstitial fluid, and into cells. CO2 diffuses from cells to blood. Most CO2 reacts with water inside red blood cells, to form bicarbonate. The enzyme carbonic anhydrase catalyzes this reaction, which is reversed in the lungs. There, CO2 and water vapor form and are expelled in exhalations. Carbon monoxide (CO) is a dangerous gaseous pollutant that binds to hemoglobin more strongly than oxygen.
Use the animation on CengageNOW to compare partial pressures of gases in different body regions.
Section 39.8 Respiratory disorders include apnea and sudden infant death syndrome (SIDS). Respiratory diseases include tuberculosis, pneumonia, bronchitis, and emphysema. Smoking worsens or increases risk of many respiratory problems. Worldwide, smoking remains a leading cause of debilitating diseases and deaths. Section 39.9 Air’s oxygen concentration declines with altitude. Short-term physiological changes that occur in response to high altitude are called acclimatization. They include altered breathing patterns and an increase in erythropoietin, a hormone that stimulates red blood cell formation. Specialized mechanisms and behaviors allow some turtles and marine mammals to dive deeply for long intervals.
Data Analysis Exercise Radon is a colorless, odorless gas emitted by many rocks and soils. It is formed by the radioactive decay of uranium and is itself radioactive (Section 2.2). There is some radon in the air almost everywhere, but routinely inhaling a lot of it raises the risk of lung cancer. Radon also seems to increase cancer risk far more in smokers than in nonsmokers. Figure 39.26 is an estimate of how radon in homes affects risk of lung cancer mortality. Note that this data shows only the death risk for radon-induced cancers. Smokers are also at risk from lung cancers that are caused by tobacco.
Risk of Cancer Death From Lifetime Radon Exposure
1. If 1,000 smokers were exposed to a radon level of 1.3 pCi/L over a lifetime (the average indoor radon level) how many would die of a radon-induced lung cancer? 2. How high would the radon level have to be to cause approximately the same number of cancers among 1,000 nonsmokers? 3. The risk of dying in a car crash is about 7 out of 1,000. Is a smoker in a home with an average radon level (1.3 pCi/L), more likely to die in a car crash or of radon-induced cancer?
Self-Quiz
Answers in Appendix III
1. The most abundant gas in the atmosphere is a. nitrogen c. oxygen b. carbon dioxide d. hydrogen
.
2. Respiratory proteins such as hemoglobin a. contain metal ions b. occur only in vertebrates c. increase the efficiency of oxygen transport d. both a and c 3. In insects, most gas exchange occurs at a. the tips of tracheal tubes c. gills b. the body surface d. paired lungs
.
Never Smoked
Current Smokers
20
36 out of 1,000
260 out of 1,000
10
18 out of 1,000
150 out of 1,000
8
15 out of 1,000
120 out of 1,000
4
7 out of 1,000
62 out of 1,000
2
4 out of 1,000
32 out of 1,000
1.3
2 out of 1,000
20 out of 1,000
0.4
>1 out of 1,000
6 out of 1,000
Figure 39.26 Estimated risk of lung cancer death as a result of lifetime radon exposure. Radon levels are measured in picocuries per liter (pCi/L). The Enviromental Protection Agency considers a radon level above 4 pCi/Liter to be unsafe. For information about testing your home for radon and what to do if the radon level is high, visit the EPA’s Radon Information Site at www.epa.gov/radon.
10. At high altitudes, . a. nitrogen bubbles out of the blood b. hemoglobin has fewer oxygen-binding sites c. atmospheric pressure is lower than at sea level d. both b and c 11. Myoglobin helps muscles to a. synthesize hemoglobin b. store oxygen c. form bicarbonate d. both b and c
.
.
12. True or false? Hemoglobin has a higher affinity for carbon dioxide than for oxygen.
4. Countercurrent flow of water and blood increases the efficiency of gas exchange in . a. fishes c. birds b. amphibians d. all of the above 5. In human lungs, gas exchange occurs at the a. two bronchi c. alveolar sacs b. pleural sacs d. both b and c
Radon Level (pCi/L)
.
6. When you breathe quietly, inhalation is and exhalation is . a. passive; passive c. passive; active b. active; active d. active; passive 7. During inhalation, . a. the thoracic cavity expands b. the diaphragm relaxes c. atmospheric pressure declines d. both a and c
13. Match the words with their descriptions. trachea a. muscle of respiration pharynx b. gap between vocal cords alveolus c. between bronchi and alveoli hemoglobin d. windpipe bronchus e. respiratory protein bronchiole f. site of gas exchange glottis g. airway leading to lung diaphragm h. throat
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Critical Thinking
8. True or false? Human lungs hold some air, even after a forced exhalation.
1. The red blood cell enzyme carbonic anhydrase contains the metal zinc. Humans obtain zinc from their diet, especially from red meat and some seafoods. A zinc deficiency does not reduce the number of red blood cells, but it does impair respiratory function by reducing carbon dioxide output. Explain why a zinc deficiency has this effect.
9. Most oxygen being transported in blood . a. is bound to hemoglobin b. combines with carbon to form carbon dioxide c. is in the form of bicarbonate d. is dissolved in the plasma
2. Look again at Figure 39.21. Notice that the oxygen and carbon dioxide content of blood in pulmonary veins is the same as at the start of the systemic capillaries. Notice also that systemic veins and pulmonary arteries have equal partial pressures. Explain the reason for these similarities. CHAPTER 39
RESPIRATION 699
40
Digestion and Human Nutrition IMPACTS, ISSUES
Hormones and Hunger
Like other mammals, humans have adipose tissue with
Ghrelin, another hormone, increases appetite. Some cells
cells that store fat. This energy warehouse served our early
in the stomach lining and the brain secrete ghrelin when the
hominid ancestors well. As foragers, they could seldom be
stomach is empty. Secretions slow after a big meal. In one
certain of where their next meal was coming from. Filling their
study of ghrelin’s effects, a group of obese volunteers stayed
adipose cells with fat when food was abundant helped them
on a low-fat, low-calorie diet for six months. They lost weight,
survive when food became scarce.
but the concentration of ghrelin in their bloodstream climbed
Lean pickings are not a problem for most Americans. With
dramatically—they were hungrier than ever!
60 percent of adults overweight or obese, they are among the
Some extremely obese people undergo gastric bypass
fattest people in the world. “Obesity” means there is too much
surgery, which effectively reduces the size of the stomach
fat in adipose tissue. It increases the risk of heart disease,
and small intestine. The surgery makes people feel full faster.
diabetes, and some cancers. Many people try to lose weight,
It also reduces the amount of nutrients that they absorb from
but extra pounds are tough to shed. Why? For one thing,
food. The results can be dramatic (Figure 40.1b). However,
hormones are involved.
the surgery raises risk for vitamin and mineral deficiencies.
When you take in more calories than you burn, your fat-
Gastric bypass is more effective than standard weight loss
storing cells plump up and increase their secretion of leptin.
methods. Post-bypass patients are far less likely to regain
This hormone acts on a brain region that affects appetite.
pounds. One reason may be that these patients secrete less
Mutant mice that cannot make leptin eat and eat, until they
ghrelin after the bypass surgery, so they feel less hungry.
look like inflated balloons (Figure 40.1a). Inject an obese mutant mouse with leptin, and it eats less and slims down. However, lack of leptin or leptin receptors is extremely rare
Discussion of food intake and body weight lead us into the world of nutrition. The word encompasses all the processes by which an animal ingests and digests food, then absorbs
in humans. Having more fat, obese people make more leptin
the released nutrients as energy sources and building blocks
than slim ones, but for unknown reasons an obese person’s
for cells. When all works well, inputs balance the outputs, and
body does not heed leptin’s call to stop eating.
weight remains within a healthy range.
a
b
See the video! Figure 40.1 Examples of hormonal effects on appetite. (a) Two normal mice (left) weigh less than one mutant mouse (right) that cannot synthesize leptin. This hormone that acts in the brain to suppress appetite. Compared with normal mice, a leptin-deficient mutant eats and weighs a lot more. (b) A young woman before (left) and after gastric bypass surgery (right). This surgery reduces the amount of food a person can take in and the amount of ghrelin she secretes.
Links to Earlier Concepts
Key Concepts Overview of digestive systems
You already know about the structure of carbohydrates (Section 3.3), lipids (3.4), and proteins (3.5). In this chapter you will learn about how your body digests these molecules. You will also learn how the body obtains vitamins and minerals required to make coenzymes (6.3), electron transfer chain components (6.4), hemoglobin (37.2), and certain hormones.
You will discover how low pH (2.6) and the action of enzymes (6.3) help break down food, and how the products of digestion (3.3– 3.5) cross cell membranes.
Characteristics of epithelium (32.2) and smooth muscle (32.4), as well as the sense of taste (34.3) and the action of the autonomic nervous system (33.8), are discussed again, as is the anatomy of the throat (39.5).
You will be reminded of the variety of animal body plans (25.1), and the way in which natural selection affects traits related to feeding (17.3, 18.5, and 18.10).
Some animal digestive systems are saclike, but most are a tube with two openings. In complex animals, a digestive system interacts with other organ systems in the distribution of nutrients and water, disposal of residues and wastes, and homeostasis. Section 40.1
Human digestive system Human digestion starts in the mouth, continues in the stomach, and is completed in the small intestine. Secretions of the salivary glands, liver, and pancreas aid digestion. Most nutrients are absorbed in the small intestine. The large intestine concentrates wastes. Sections 40.2–40.6
Organic metabolism and nutrition Nutrients absorbed from the gut are raw materials used in synthesis of the body’s complex carbohydrates, lipids, proteins, and nucleic acids. A healthy diet normally provides all nutrients, vitamins, and minerals necessary to support metabolism. Sections 40.7–40.9
Balancing caloric inputs and outputs Maintaining body weight requires balancing calories taken in with calories burned in metabolism and physical activity. Section 40.10
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701
40.1
The Nature of Digestive Systems All animals are heterotrophs that ingest food, break it down, and absorb its nutrient subunits into their body.
Incomplete and Complete Systems
Links to Animal body plans 25.1, Bill shape and natural selection 17.3, 18.5, and 18.10
Remember from Section 25.1 that some invertebrates have an incomplete digestive system. Food enters their saclike gut through an opening at the body surface. Wastes leave through the same opening. In flatworms, a saclike, branching gut cavity opens at the start of the pharynx, a muscular tube (Figure 40.3a). Food that enters the sac is digested, its nutrients are absorbed, then wastes are expelled. This two-way movement of material does not favor specialization of gut regions for specific tasks. Most groups of invertebrates and all vertebrates have a complete digestive system: a tubular gut with an opening at both ends. Along the tube’s length are regions that specialize in processing food, absorbing nutrients, or concentrating wastes. Figure 40.3b shows the complete digestive system of a frog. The tubular portion consists of the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus. A liver, gallbladder, and pancreas are accessory organs that assist digestion by secreting enzymes and other products into the small intestine. A complete digestive system carries out five tasks:
An animal’s digestive system is a body cavity or a tube that mechanically and chemically breaks food down to small particles, then to molecules that can be absorbed into the internal environment. The digestive system also expels any unabsorbed residues. Together with other organ systems, it plays an essential role in homeostasis (Figure 40.2).
food, water intake
oxygen intake
Digestive System nutrients, water, salts
elimination of carbon dioxide
Respiratory System carbon dioxide
oxygen
Circulatory System
Urinary System water, solutes
elimination of food residues
elimination of excess water, salts, wastes
rapid transport to and from all living cells
Figure 40.2 Organ systems with key roles in the uptake, processing, and distribution of nutrients and water in complex animals.
branching saclike gut
1. Mechanical processing and motility: movements that break up, mix, and directionally propel food material 2. Secretion: release of substances, especially digestive enzymes, into the lumen (the space inside the tube) 3. Digestion: breakdown of food into particles, then into nutrient molecules small enough to be absorbed
bill mouth only opening to gut
esophagus
pharynx
A Flatworm (planarian)
crop pharynx
stomach
flip-out tongue in mouth liver gallbladder pancreas
gizzard
small intestine
intestines large intestine cloaca (terminal opening; serves in excretion and reproduction)
B Amphibian (frog)
C Bird (pigeon)
Figure 40.3 Animated (a) Incomplete digestive system. (b,c) Two complete digestive systems.
702 UNIT VI
glandular part of stomach
HOW ANIMALS WORK
ingestion, regurgitation, reswallowing of food through esophagus
stomach chamber 1
gumline crown
stomach chamber 2 stomach chamber 3
root
stomach chamber 4
antelope molar
to small intestine
b
crown
Figure 40.4 Animated (a) Human and pronghorn antelope molars. (b) An antelope’s multiple stomach chambers. In the first two, food is mixed with fluid and exposed to microbes (prokaryotes, protists, and fungi) that engage in fermentation. Some of the microbes degrade cellulose; others synthesize organic compounds, fatty acids, and vitamins. Partly digested food is regurgitated into the mouth, chewed, then swallowed. It enters the third chamber and is digested again before entering the last chamber.
root
human molar a
4. Absorption: uptake of digested nutrients and water across the gut wall, into extracellular fluid 5. Elimination: expulsion of undigested or unabsorbed solid residues
Dietary Adaptations In birds, the size and shape of the bill are diet-related traits shaped by natural selection (Sections 17.3, 18.5, and 18.10). So are other traits. For example, a pigeon (Figure 40.3c) uses its bill to pick up small seeds from the ground. Like other seed-eating birds, a pigeon has a large crop, a saclike food-storing region above the stomach. The bird quickly fills its crop with seeds, then flies off and digests them later. This eat-and-run strategy reduces the amount of time that the bird is on the ground, where it is most vulnerable to predators. Birds do not have teeth. They grind up food inside a gizzard: a stomach chamber lined with hard protein particles. Compared to hawks and other meat-eating birds, seed eaters have larger gizzards relative to their body size. Also, seed eaters have a relatively longer intestinal tract, because seeds require more processing time than easier-to-digest meat. In all birds, undigested residues collect in a cloaca before being expelled. Mammalian teeth are adaptations to specific diets. For example, pronghorn antelope browse on grass and nibble on shrubby plants. Antelope molars (cheek teeth) have a flattened crown that serves as a grinding platform (Section 26.11). The crown on your molars is proportionately much smaller (Figure 40.4a). Why? You do not brush your mouth against dirt as you eat,
but an antelope does. Abrasive soil particles mix with the animal’s food, so the crown of an antelope molar gets a lot of wear. An enlarged crown is an adaptation that keeps the molars from wearing down to nubs. The antelope’s gut also shows specializations for a diet of plant material. Like cattle, goats, and sheep, antelopes are ruminants, hoofed mammals that have multiple stomach chambers (Figure 40.4b). Microbes that live inside the first two stomach chambers carry out fermentation reactions that break down cellulose in plant cell walls. Solids accumulate in the second chamber, forming “cud” that is regurgitated—moved back into the mouth for a second round of chewing. Nutrient-rich fluid moves from the second chamber to the third and fourth chambers, and finally to the intestine. This system allows ruminants to maximize the amount of nutrients they extract from plant foods rich in cellulose. Cellulose is so tough and insoluble that most animals cannot digest it.
Take-Home Message What are digestive systems and how do they vary among animal groups?
Digestive systems mechanically and chemically degrade food into small molecules that can be absorbed, along with water, into the internal environment. These systems also expel the undigested residues from the body. Digestive systems may be incomplete or complete. Incomplete digestive systems are a saclike cavity with one opening. A complete digestive system is a tube with two openings and regional specializations in between. Some digestive traits, such as the shape of teeth or the length of different portions of the digestive tract, are adaptations that allow an animal to exploit a particular type or types of foods.
CHAPTER 40
DIGESTION AND HUMAN NUTRITION 703
40.2
Overview of the Human Digestive System If the tubular gut of an adult human were fully stretched out, it would extend up to 9 meters (30 feet). Accessory organs along the length of the gut secrete enzymes and other substances that aid the breakdown of food into its component molecules.
Links to Epithelium 32.2, Taste 34.3, Trachea 39.5
Humans have a complete digestive system, a tubular gut with two openings (Figure 40.5). Mucus-covered epithelium (Section 32.2) lines the tube, and different parts of the tube specialize in digesting food, absorbing released nutrients, or concentrating and storing the unabsorbed waste. The salivary glands, the pancreas,
Accessory Organs
Major Organs Mouth Oral cavity. Its teeth break food into smaller bits. Tongue mixes food with saliva. Pharynx (throat) Entrance to the gut and respiratory system. Action of the epiglottis keeps food from entering the trachea.
Salivary Glands Produce and secrete saliva, which moistens food and begins the process of carbohydrate digestion.
Esophagus Muscular tube through which food moves to the stomach. Stomach J-shaped muscular sac that receives food and mixes it with gastric fluid secreted by cells in its lining.
Liver Produces bile, which aids digestion and absorption of fats.
Small Intestine Longest tube of the gut. Its first part receives secretions from the liver, gallbladder, and pancreas. These secretions help complete the process of digestion. Most water and products of digestion are absorbed across the highly folded wall of this organ. Large Intestine (colon) Wider than the small intestine, but shorter. It absorbs most remaining water, thus concentrating any undigested waste and forming the feces. Rectum Expandable sac that stores feces. Anus Opening through which feces are expelled from the body.
Figure 40.5 Animated Overview of the components of the human digestive system, together with a brief description of their primary functions in digestion.
704 UNIT VI
HOW ANIMALS WORK
Gallbladder Stores and concentrates bile, then secretes it into the small intestine. Pancreas Secretes enzymes and bicarbonate (a buffer) into the small intestine.
40.3 Food in the Mouth the liver, and the gallbladder are accessory organs involved in secreting substances into the tube. Food enters the mouth, and travels through the pharynx and esophagus to the gut. A human gut, or gastrointestinal tract, starts at the stomach and extends through the intestines to the tube’s terminal opening. Food is partially processed inside the mouth, or oral cavity. The tongue is a bundle of membrane-covered skeletal muscle attached to the floor of the mouth. The tongue positions food so it can be swallowed, and the many chemoreceptors in taste buds at the tongue’s surface contribute to our sense of taste (Section 34.3). Swallowing forces food into the pharynx. A human pharynx, or throat, is the entrance to the digestive and respiratory tracts (Section 39.5). The presence of food at the back of the throat triggers a swallowing reflex. When you swallow, the flaplike epiglottis flops down and the vocal cords constrict, so the route between the pharynx and larynx is blocked. This reflex keeps food from getting stuck in an airway and choking you. A muscular tube called the esophagus connects the pharynx with the stomach. The esophagus undergoes peristalsis, rhythmic muscle contractions that propel food or liquid through a tubular digestive organ. The stomach is a stretchable sac that stores food, secretes acid and enzymes, and mixes them all together. Between the esophagus and stomach is a sphincter. Like all sphincters, this ring of smooth muscle blocks the flow of substances past it when it has contracted. In people who have gastroesophageal reflux disease (GERD), this sphincter does not shut properly. As a result, acidic stomach fluids splash back and irritate esophageal tissues causing burning pain (heartburn). The stomach leads to the small intestine, the part of the gut where most carbohydrates, lipids, and proteins are digested and where most of the released nutrients and water are absorbed. Secretions from the liver and pancreas assist the small intestine in these tasks. The large intestine absorbs most remaining water and ions, thus compacting wastes. Wastes are briefly stored in a stretchable tube, the rectum, before being expelled from the gut’s terminal opening, or anus.
Chewing your food begins the process of digestion.
Mechanical digestion begins when teeth rip and crush food. Each tooth is embedded in the jaw at a fibrous joint and consists mostly of bonelike dentin (Figure 40.6a). Dentin-secreting cells reside in a central pulp cavity. These cells are serviced by nerves and blood vessels that extend through the tooth’s root. Enamel— the hardest material in the body—covers the tooth’s exposed crown and reduces wear. Human adults have thirty-two teeth of four types (Figure 40.6b). Chisel-shaped incisors shear off bits of food. Cone-shaped canines tear up meats. Premolars and molars have broad bumpy crowns that serve as platforms for grinding and crushing food. Chemical digestion begins when food mixes with saliva from salivary glands. Saliva is mostly water, with bicarbonate, enzymes, and mucins. Bicarbonate, a buffer, keeps the pH in the mouth from becoming too acidic. The enzyme salivary amylase hydrolyzes starch, breaking it into disaccharides. Mucin proteins combine with water and form mucus that makes food pieces stick together in easy-to-swallow clumps. Take-Home Message How does the mouth function in digestion?
Digestion begins when teeth mechanically break food into smaller bits and salivary amylase chemically breaks starch into disaccharides.
enamel dentin pulp cavity (contains nerves and blood vessels)
crown gingiva (gum)
ligaments root canal
root
periodontal membrane bone
a
Figure 40.6 Human teeth. (a) Cross-section of a human tooth. The crown is the portion extending above the gum; the root is embedded in the jaw. Tiny ligaments attach the tooth to the jawbone. (b) The four types of teeth in adult. Molars and premolars grind up food. Incisors and canines rip and tear off bits.
molars (12)
Take-Home Message What type of digestive system do humans have? Humans have a complete digestive system with a muscular, mucosa-lined gut.
premolars (8) canines (4) incisors (8)
Accessory organs positioned adjacent to the gut secrete substances into its interior. These substances aid digestion or absorption of food.
b
lower jaw
CHAPTER 40
upper jaw
DIGESTION AND HUMAN NUTRITION 705
40.4 Food Breakdown in the Stomach and Small Intestine In the stomach and small intestine, smooth muscle contractions mix food with digestive enzymes.
Digestion in the Stomach
Links to pH 2.6, Enzymes 6.3, Smooth muscle 32.4, Autonomic nervous system 33.8
The stomach is a muscular, stretchable sac with three functions. First, the stomach stores food and controls the rate of passage to the small intestine. Second, it pummels and mechanically breaks down food. Third, it secretes substances that aid in chemical digestion. A mucus-secreting epithelium—the mucosa—lines the inner gut wall. In the stomach, cells of the mucosa secrete about two liters of gastric fluid each day. This fluid includes mucus, hydrochloric acid, and enzymes such as pepsinogens. Acid lowers the pH to about 2. When food enters the stomach, endocrine cells in the stomach lining secrete the hormone gastrin into blood. Gastrin binds to secretory cells of the mucosa, causing them to step up secretion of acid and pepsinogens. Rhythmic contraction of smooth muscle in the stomach wall mixes gastric fluid and food into a semiliquid mass called chyme. Eventually, the contractions propel chyme through the pyloric sphincter that connects the stomach with the small intestine (Figure 40.7). The acidity of chyme makes proteins unfold, exposing their peptide bonds. Acid also causes pepsinogens to become pepsins, enzymes that break peptide bonds. The strong acidity kills most bacteria, but acid-tolerant Helicobacter pylori sometimes infects the lining of the stomach and the upper intestine. A chronic H. pylori infection can harm the lining and expose the underlying tissues to acid, causing a painful ulcer. Antibiotics are now routinely used to treat such ulcers.
Carbohydrate breakdown, again, starts in the mouth. Protein breakdown begins in the stomach. Digestion of both is completed in the small intestine. Lipids are also digested in the small intestine. Digestion occurs as contractions of smooth muscle in the gut wall mix food with enzymes (Figure 40.7 and Table 40.1).
esophagus
serosa longitudinal muscle circular muscle
pyloric sphincter
oblique muscle
submucosa duodenum
mucosa
Figure 40.7 Structure of the stomach wall. The outermost layer, the serosa, is connective tissue covered by epithelium. Beneath the serosa, three layers of smooth muscle differ in their orientation and direction of contraction. Their coordinated action mixes stomach contents with gastric fluid secreted by the mucosa that lines the stomach’s interior.
Table 40.1
Summary of Chemical Digestion
Location
Enzymes Present
Enzyme Source
Enzyme Substrate
Main Breakdown Products
Salivary amylase Pancreatic amylase Disaccharidases
Salivary glands Pancreas Intestinal lining
Polysaccharides Polysaccharides Disaccharides
Disaccharides Disaccharides
Pepsins Trypsin, chymotrypsin Carboxypeptidase Aminopeptidase
Stomach lining Pancreas Pancreas Intestinal lining
Proteins Proteins Protein fragments
Amino acids*
Lipase
Pancreas
Triglycerides
Free fatty acids, monoglycerides*
Pancreatic nucleases Intestinal nucleases
Pancreas Intestinal lining
DNA, RNA Nucleotides
Nucleotide bases, monosaccharides*
Carbohydrate Digestion Mouth, stomach Small intestine
Monosaccharides* (such as glucose)
Protein Digestion Stomach Small intestine
Protein fragments Protein fragments
Amino acids*
Lipid Digestion Small intestine Nucleic Acid Digestion Small intestine
* Breakdown products small enough to be absorbed into the internal environment. 706 UNIT VI
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Nucleotides
submucosa
serosa
blood vessels time
gut lumen
a A section of highly folded mucosa
circular muscle
longitudinal muscle
autonomic nerves
b
Figure 40.8 (a) Structure of the small intestine. Its wall has a highly folded inner lining, the mucosa. (b) Rings of circular muscle inside the wall contract and relax in a pattern. Back-and-forth movement propels, mixes, and forces chyme up against the wall, enhancing digestion and absorption.
Digestion in the Small Intestine
Controls Over Digestion
Chyme from the stomach and various secretions from the pancreas enter the duodenum, the first part of the small intestine. Pancreatic enzymes break down large organic compounds in chyme into monosaccharides, monoglycerides, fatty acids, amino acids, nucleotides, and nucleotide bases (Table 40.1). Bicarbonate from the pancreas buffers acids, thus protecting the intestinal lining and ensuring intestinal enzymes work properly. In addition to enzymes, fat digestion requires bile. Bile is a mixture of salts, pigments, cholesterol, and lipids. It is made in the liver, and is concentrated and stored in the gallbladder. A fatty meal stimulates the gallbladder to contract, forcing bile out through a duct that leads to the small intestine. Bile salts enhance fat digestion by emulsification, a process which disperses any droplets of fat in a fluid. Water-insoluble triglycerides from food tend to clump together and form fat globules. Movement of the small intestine counteracts this tendency. Rings of smooth muscle in the intestinal wall contract in an oscillating pattern (Figure 40.8b). These contractions mix the chyme and break up fat globules into small droplets that become coated with bile salts. This coating of bile salts keeps the droplets separated. The smaller drops present a greater surface area to enzymes that break up fats into fatty acids and monoglycerides. Gallstones, hard pellets of cholesterol and bile salts, can form in the gallbladder. Most are harmless. If they block the bile duct or otherwise interfere with function of the gallbladder, they can be removed surgically. The breakdown products of digestion are absorbed across the epithelial lining of the small intestine, into the internal environment. How each kind gets across is the focus of the next section.
The nervous system, endocrine system, and nerves in the gut wall control digestion. Arrival of food in the stomach causes signals to flow along reflex pathways to gut muscles and glands. Other pathways alert the brain. In response, gut muscles contract and glands secrete hormones into the blood (Table 40.2). A large meal stimulates more forceful contractions than a small one. Composition of a meal also has an effect. Stomach emptying takes longer after a high-fat meal than after a meal lower in fat. With stress or exercise, sympathetic neurons signal gut muscles to contract more slowly (Section 33.8). This is why chronic stress or exercising immediately after a meal can cause digestive problems.
Table 40.2
Main Hormonal Controls of Digestion
Hormone
Source
Effects on Digestive System
Gastrin
Stomach
Stimulates stomach acid secretion
Cholecystokinin (CCK)
Small intestine
Stimulates pancreatic enzyme secretion and gallbladder contraction
Secretin
Small intestine
Stimulates pancreas to secrete bicarbonate and slows contractions of small intestine
Take-Home Message Where and how does digestion occur?
Digestion begins in the mouth and continues in the stomach, but the bulk of it occurs in the small intestine.
Enzyme activity, acidity, and mechanical processes break food into small organic molecules that can be absorbed.
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40.5 Absorption From the Small Intestine The small intestine is the main site of absorption for the products of digestion.
Links to Organic monomers 3.3–3.5, Lysozyme 38.2
A One of many permanent folds on the inner wall of the small intestine. Each fold is covered with villi.
villi (fingerlike projections of mucosa covered by epithelium blood capillaries
connective tissue vesicle epithelium
artery vein
lymph vessel
B At the free surface of each mucosal fold are many fingerlike absorptive structures called villi.
C A villus is covered with specialized epithelial cells. It also contains blood capillaries and lymph vessels.
D Epithelial cells in the intestinal mucosa. The four types shown below are color-coded enlargements of cells on the surface of the villus shown in (c). Absorptive brush border cells are the most abundant cells on a villus. Their crown of microvilli extends into the intestinal lumen. The small-intestinal enzymes discussed in the previous section are built into brush border cell plasma membranes. Other cells of the mucosa secrete mucus, hormones, or lysozyme (an enzyme that digests bacterial cell walls).
From Structure to Function The small intestine is “small” only in its diameter— about 2.5 cm (1 inch). It is the longest segment of the gut. Uncoiled, it would extend for about 5 to 7 meters (16 to 23 feet). Water and nutrients cross the lining of this long tube to reach the internal environment. Three features of the small intestine lining enhance absorption. First, this lining is folded (Figure 40.9a). Second, millions of multicelled, fingerlike absorptive structures called villi (singular, villus) extend out from each of the folds (Figure 40.9b). Each villus houses a lymph vessel and blood vessels (Figure 40.9c). Third, most cells on the villus surface are brush border cells (Figure 40.9d). These specialized cells have membrane extensions called microvilli (singular, microvillus) that project into the lumen. Collectively, all of the folds and projections make the surface area of intestinal mucosa about the size of half a tennis court! The brush border cells function in both digestion and absorption. Digestive enzymes at the surface of the microvilli break down sugars, protein fragments, and nucleotides as listed in Table 40.1. Also at the microvillus surface are many transport proteins that act in absorption, as explained below. In addition to brush border cells, the lining of the small intestine includes secretory cells (Figure 40.9d). These cells secrete hormones, mucus, and bacteriakilling chemicals, such as lysozyme (Section 38.2).
How Are Materials Absorbed? Water and Solute Absorption Each day, eating and
drinking puts 1 to 2 liters of fluid into the small intestine. Secretions from the stomach, accessory glands, and intestinal lining contribute another 6 to 7 liters. About 80 percent of the water in that fluid is absorbed across the small intestinal lining and into the internal
lumen secretes lysozyme
secretes hormones
secretes mucus
absorbs nutrients
microvilli at free surface of a brush border cell
cytoplasm
brush border cell
Figure 40.9 Animated The lining of the small intestine.
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Lumen of Small Intestine
carbohydrates A proteins
monosaccharides
fat globules (triglycerides) + bile salts
amino acids
free fatty acids, monoglycerides C emulsification droplets
+ bile salts
D micelles
E triglycerides + proteins Brush Border Cell
B
F lipoproteins
Internal Environment (interstitial fluid inside a villus)
A Enzymes secreted by the pancreas and cells of the intestinal mucosa complete the digestion of carbohydrates to monosaccharides, and proteins to amino acids.
B Monosaccharides and amino acids are actively transported across the plasma membrane of brush border cells in the intestinal lining, then out of the same cells and into the internal environment.
C Movements of the intestinal wall break up fat globules into small droplets. Bile salts coat the droplets, so that globules cannot form again. Pancreatic enzymes digest the droplets to fatty acids and monoglycerides.
D Micelles form when bile salts combine with products of fat digestion: monoglycerides and fatty acids. These products slip into and out of micelles.
E Concentrating monoglycerides and fatty acids in micelles enhances diffusion of these substances into brush border cells. These lipids diffuse across the plasma membrane’s lipid bilayer, into the cells.
F In a brush border cell, the products of fat digestion form triglycerides, which associate with proteins. The resulting lipoproteins are then expelled by exocytosis into the interstitial fluid inside the villus.
Figure 40.10 Animated Summary of digestion and absorption in the small intestine. Figure It Out: What do the purple dots in the micelles represent? Answer: Bile salts
environment by osmosis (Section 5.6). Transport proteins in the plasma membrane of brush border cells move salts, sugars, and amino acids from the intestinal lumen into these cells. Other transport proteins then move these solutes from the brush border cells into interstitial fluid inside a villus (Figure 40.10b). This movement of solutes creates an osmotic gradient, so water moves in the same direction. From the interstitial fluid, water, salts, sugars, and amino acids enter the blood capillary inside the villus. The blood then distributes them throughout the body. Fat Absorption Being lipid soluble, the fatty acids and monoglycerides released by fat digestion enter a villus by diffusing across the lipid bilayer of brush border cells. Remember, bile salts aid fat digestion by coating fatty droplets (Section 40.4 and Figure 40.10c). Bile salts also combine with the products of fat digestion (fatty acids and monoglycerides) to form tiny droplets called micelles (Figure 40.10d). When a micelle contacts a brush border cell, the micelle’s fatty
acids and monglycerides diffuse into that cell (Figure 40.10e). The bile salts that were in the micelle remain in the intestinal lumen, where they will become part of new micelles. Inside the brush border cells, monoglycerides and fatty acids form triglycerides that join with proteins. The resulting lipoproteins move by exocytosis into the interstitial fluid inside a villus (Figure 40.10f ). From the interstitial fluid, triglycerides enter lymph vessels. Lymph—and triglycerides—eventually drain into the bloodstream (Section 37.10).
Take-Home Message How are substances absorbed from the small intestine? With a folded mucosa, villi, and microvilli, the small intestine has a vast surface area for absorbing water and nutrients.
Substances are absorbed through the brush border cells that line the free surface of each villus. Passive and active transport mechanisms help water and solutes cross; micelle formation helps lipid-soluble products cross.
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40.6 The Large Intestine The large intestine is wider than the small intestine, but also much shorter—only about 1.5 meters (5 feet) long.
Structure and Function of the Large Intestine Not everything that enters the small intestine can be or should be absorbed. Muscular contractions propel indigestible material, dead bacteria and mucosal cells, inorganic substances, and some water from the small intestine into the large intestine. As the wastes travel through the large intestine, they become compacted as feces. Compaction occurs as the large intestine actively pumps sodium ions out of the lumen, into the internal environment. Water follows by osmosis. A cup-shaped cecum, is the first part of the large intestine (Figure 40.11a). A wormlike pouch, called the appendix extends from it. From the cecum, material
enters the ascending colon, which extends up along the wall of the abdominal cavity. The transverse colon extends across this cavity, and the descending colon connects to the rectum (Figures 40.5 and 40.11). Contraction of the smooth muscle in the colon wall mixes its contents and propels them along its length. Compared with other gut regions, wastes move more slowly through the colon, which also has a moderate pH. These conditions favor growth of bacteria such as Escherichia coli. The bacteria make vitamins K and B12, which are absorbed across the colon lining. After a meal, gastrin and signals from autonomic nerves cause much of the colon to contract forcefully and propel feces to the rectum. The rectum stretches, which activates a defecation reflex to expel feces. The nervous system can override the reflex by calling for contraction of a sphincter at the anus.
Disorders of the Large Intestine
ascending colon
cecum
appendix
last portion of small intestine
a
transverse colon
colon polyp
Healthy adults typically defecate about once a day, on average. Emotional stress, a diet low in fiber, minimal exercise, dehydration, and some medications can lead to constipation, in which defecation occurs fewer than three times a week, is difficult, and yields small, hardened, dry feces. Occasional constipation usually goes away on its own. A chronic problem should be discussed with a doctor. Diarrhea—frequent passing of watery feces—can result from bacterial infection or problems with nervous controls. If prolonged, it can cause dehydration and disrupt blood solute levels. Appendicitis—an inflammation of the appendix— requires prompt treatment. Removing the inflamed appendix prevents it from bursting and releasing large numbers of bacteria into the abdominal cavity. Such a rupture could cause a life-threatening infection. Some people are genetically predisposed to develop colon polyps, small growths on the colon wall (Figure 40.11b). Most polyps are benign, but some can become cancerous. If detected in time, colon cancer is highly curable. Blood in feces and dramatic changes in bowel habits may be symptoms of colon cancer and should be reported to a doctor. Also, anyone over the age of 50 should have a colonoscopy, in which clinicians use a camera to examine the colon for polyps or cancer.
descending colon
Take-Home Message
b
What is the function of the large intestine?
Figure 40.11 (a) Location of cecum and appendix of the large intestine. (b) Sketch and photo of polyps in the transverse colon.
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The large intestine completes the process of absorption, then concentrates, stores, and eliminates wastes.
40.7
Metabolism of Absorbed Organic Compounds Most of the body’s fat-soluble vitamins such as vitamins A and D are stored in the liver. The liver also stores glucose. After a meal, liver and muscle cells take up glucose and convert it to glycogen (Section 3.3). Excess carbohydrates and proteins are also converted to fats, which are stored mainly in adipose tissue. In between meals, the brain takes up much of the glucose circulating in the blood. The brain cannot use fats or proteins as an energy source. Other body cells dip into their stores of glycogen and fat. Adipose cells degrade fats to glycerol and fatty acids, which enter blood. Liver cells break down glycogen and release glucose, which also enters blood. Body cells take up the released fatty acids and glucose and use them to fuel ATP production.
Most absorbed organic compounds are broken down for energy, stored, or used to build larger organic compounds. Links to Glycogen 3.3, Alcohol metabolism Chapter 6 introduction, Systemic circulation 37.5
Figure 40.12a shows the main routes by which organic molecules from food are shuffled and reshuffled in the body. Living cells constantly recycle some carbohydrates, lipids, and proteins by breaking them apart. They use breakdown products as energy sources and building blocks. The nervous and endocrine systems regulate this turnover. The liver is a large organ that functions in digestion, metabolism, and homeostasis (Figure 40.12b). All blood from the capillaries in the small intestine enters the hepatic portal vein, which delivers it to the liver. The blood flows through capillaries in the liver before returning to the heart (Section 37.5). The liver helps protect the body against dangerous substances that were ingested or formed as a result of digestion. For example, Chapter 6 explained the role of the liver in detoxifying alcohol, and how alcohol abuse can damage this essential organ. As another example, ammonia (NH3) is a toxic product of amino acid breakdown. The liver converts ammonia to urea, a much less toxic compound. Urea is carried by the blood to kidneys and is excreted in the urine.
Take-Home Message What happens to compounds absorbed from the gut? Absorbed compounds are carried by the blood to the liver. The liver detoxifies dangerous substances and stores vitamins and glucose. The glucose is stored as glycogen. Adipose tissue takes up absorbed carbohydrates and proteins and converts them to fats.
In between meals, the liver breaks down stored glycogen, and releases its glucose subunits into the blood. This ensures that the brain, which can only use carbohydrates as fuel, always has an adequate supply of energy.
Liver Functions FOOD INTAKE
dietary carbohydrates, lipids
Forms bile (assists fat digestion), rids body of excess cholester ol and blood’s respiratory pigments
dietary proteins, amino acids
Cytoplasmic Pool of Carbohydrates, Fats
Controls amino acid levels in the blood; converts potentially toxic ammonia to urea
Cytoplasmic Pool of Amino Acids
(interconvertible forms)
Controls glucose level in blood; major reservoir for glycogen
ammonia
storage forms (e.g., glycogen)
building blocks for cell structures
specialized derivatives (e.g., steroids, acetylcholine)
instant energy sources for cells
urea
excreted in urine
Removes hormones that served their functions from blood Removes ingested toxins, such as alcohol, from blood
building nitrogencontaining blocks for structural derivatives (e.g., hormones, proteins, nucleotides) enzymes
Breaks down worn-out and dead red blood cells, and stores iron Stores some vitamins
b
a
Figure 40.12 (a) Summary of major pathways of organic metabolism. Cells continually synthesize and tear down carbohydrates, fats, and proteins. Most urea forms in the liver, an organ that is at the crossroads of organic metabolism (b).
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40.8 Human Nutritional Requirements Eating provides your cells with a source of energy and a supply of essential building materials. Links to Trans fats Chapter 3 introduction, Carbohydrates 3.3, Lipids 3.4, Proteins 3.5, Quinoa 23.10
USDA Dietary Recommendations Scientists at the Department of Agriculture and other United States government agencies research diets that may help prevent diabetes, cancers, and other health problems. They periodically update their nutritional guidelines. In 2005, they replaced their traditional onesize-fits-all food pyramid with a new Internet-based program that generates recommendations specific for
USDA Nutrition Guidelines Food Group
Vegetables
Amount Recommended 2.5 cups/day
Dark green vegetables
3 cups/week
Orange vegetables
2 cups/week
Legumes
3 cups/week
Starchy vegetables
3 cups/week
Other vegetables
6.5 cups/week
Fruits
2 cups/day
Milk Products
3 cups/day
Grains
6 ounces/day
Whole grains
3 ounces/day
Other grains
3 ounces/day
Fish, poultry, lean meat
5.5 ounces/day
Oils
24 grams/day
Figure 40.13 Example of nutritional guidelines from the United States Department of Agriculture (USDA). These recommendations are for females between ages ten and thirty who get less than 30 minutes of vigorous exercise daily. Portions add up to a 2,000-kilocalorie daily intake.
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a person’s age, sex, height, weight, and activity level (Figure 40.13). To generate your own healthy eating plan, visit the USDA web site: www.mypyramid.gov. In stark contrast to the diet of a typical American, the new guidelines recommend lowering the intake of refined grains, saturated fats, trans-fatty acids, added sugar or caloric sweeteners, and salt (no more than a teaspoon per day). They also recommend eating more vegetables and fruits with a high potassium and fiber content, fat-free or low-fat milk products, and whole grains. About 55 percent of daily caloric intake should come from carbohydrates.
Energy-Rich Carbohydrates Fresh fruits, whole grains, and vegetables—especially legumes such as peas and beans—provide abundant complex carbohydrates (Section 3.3). The body breaks the starch in these foods into glucose, your primary source of energy. These foods also provide essential vitamins and fiber. Eating foods high in soluble fiber helps lower one’s cholesterol level and may reduce the risk of heart disease. A diet high in insoluble fiber helps prevent constipation. Foods that contain a lot of processed carbohydrates such as white flour, refined sugar, and corn syrup are sometimes said to be full of “empty calories.” This is a way of saying that these foods provide little in the way of vitamins or fiber.
Good Fat, Bad Fat You cannot stay alive without lipids. Cell membranes incorporate phospholipids and cholesterol, one of the sterols. Fats serve as energy reserves, insulation, and cushioning. They also help store fat-soluble vitamins. Linoleic acid and alpha-linolenic acid are essential fatty acids. The human body cannot synthesize them, so you must get them from your diet. Both are polyunsaturated fats; their long carbon tails include two or more double bonds (Table 40.3). Unsaturated fats are liquid at room temperature (Section 3.4). We divide the polyunsaturated fatty acids into two categories: omega-3 fatty acids and omega-6 fatty acids. Omega-3 fatty acids, the main fat in oily fish such as sardines, seem to have special health benefits. Studies suggest that a diet high in omega-3 fatty acids can reduce the risk of cardiovascular disease, lessen the inflammation associated with rheumatoid arthritis, and help diabetics control their blood glucose. Oleic acid, the main fat in olive oil, may also have health benefits. It is monounsaturated, which means
its carbon tails have only one double bond. A diet in which olive oil is substituted for saturated fats helps prevent heart disease. Dairy products and meats are rich in saturated fats and cholesterol. Overindulging in these foods increases risk for heart disease, stroke, and some cancers. Trans fatty acids, or trans fats, are manufactured from vegetable oils. However, they have a molecular structure that makes them even worse for the heart than saturated fats (Chapter 3 introduction).
Body-Building Proteins Amino acids are building blocks of proteins (Section 3.5). Your cells can make some amino acids but you must get eight essential amino acids from food. The eight essential amino acids are methionine (or cysteine, its metabolic equivalent), isoleucine, leucine, lysine, phenylalanine, threonine, tryptophan, and valine. Most proteins in meat are “complete”; their amino acid ratios match a human’s nutritional needs. Nearly all plant proteins are incomplete, in that they lack one or more amino acids essential for humans. Proteins of quinoa (Chenopodium quinoa) are a notable exception (Section 23.10). To get required amino acids from a vegetarian diet, one must combine plant foods so that the amino acids missing from one component are present in some others. As an example, rice and beans together provide all necessary amino acids, but rice alone or beans alone do not. You do not have to eat the two complementary foods at the same meal, but both should be consumed within a 24-hour period.
About Low-Carb/High-Protein Diets Many people turn to diets lower in carbohydrates and higher in proteins and fats to promote rapid weight loss. The long-term effectiveness and health effect of these diets is controversial. We know that increased
Table 40.3
Main Types of Dietary Lipids
Polyunsaturated Fatty Acids: Liquid at room temperature; essential for health. Omega-3 fatty acids Alpha-linoleinc acid and its derivatives Sources: Nut oils, vegetable oils, oily fish Omega-6 fatty acids Linoleic acid and its derivatives Sources: Nut oils, vegetable oils, meat Monounsaturated Fatty Acids: Liquid at room temperature. Main dietary source is olive oil. Beneficial in moderation. Saturated Fatty Acids: Solid at room temperature. Main sources are meat and dairy products, palm and coconut oils. Excessive intake may raise risk of heart disease. Trans Fatty Acids (Hydrogenated Fats): Solid at room temperature. Manufactured from vegetable oils and used in many processed foods. Excessive intake may raise risk of heart disease.
protein intake increases ammonia production (Section 40.7). Enzymes in the liver convert ammonia to urea, which kidneys filter from blood and excrete in urine. Also, when a body uses fats rather than carbohydrates as its main source of energy, large amounts of acidic metabolic wastes called ketones form. Ketones must be filtered from blood and excreted. Thus, high-fat, high-protein diets make kidneys work harder, raising the risk of kidney problems. Anyone with impaired kidney function should avoid such a diet. Take-Home Message What are the main types of nutrients that humans require? Carbohydrates are broken down to glucose, the body’s main energy source. Foods rich in complex carbohydrates also supply fiber and vitamins. Fats are burned for energy and used as building materials. Polyunsaturated and monounsaturated fats should provide most of your fat calories. Excessive consumption of saturated fats and trans fats raises risk of heart disease. Proteins are the source of amino acids used to build your body’s own proteins. Meat provides all essential amino acids. Most plant foods lack one or more amino acids, but when combined correctly these foods can meet all human amino acid needs.
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40.9 Vitamins, Minerals, and Phytochemicals In addition to major nutrients, the body requires certain organic and inorganic substances to function properly. Links to Electron transfer chains 6.4, Coenzymes 6.3, Thyroid hormones 35.6, Blindness 34.10, Hemoglobin 37.2
Vitamins are organic substances that are essential in very small amounts; no other substance can carry out their metabolic functions. At a minimum, human cells require the thirteen vitamins listed in Table 40.4. Each
Table 40.4 Vitamin
has specific roles. For instance, the B vitamin niacin is modified to make NAD, a coenzyme (Section 6.3). Minerals are inorganic substances that are essential for growth and survival because no other substance can serve their metabolic functions (Table 40.5). As an example, all of your cells use iron as a component of electron transfer chains (Section 6.4). Red blood cells require iron to make oxygen-transporting hemoglobin (Section 37.2). Iodine is essential for development of a
Major Vitamins: Sources, Functions, and Effects of Deficiencies or Excesses* Common Sources
Main Functions
Effects of Chronic Deficiency
Effects of Extreme Excess
Fat-Soluble Vitamins A
Its precursor comes from beta-carotene in yellow fruits, yellow or green leafy vegetables; also in fortified milk, egg yolk, fish, liver
Used in synthesis of visual pigments, bone, teeth; maintains epithelia
Dry, scaly skin; lowered resistance to infections; night blindness; permanent blindness
Malformed fetuses; hair loss; changes in skin; liver and bone damage; bone pain
D
Inactive form made in skin, activated in liver, kidneys; in fatty fish, egg yolk, fortified milk products
Promotes bone growth and mineralization; enhances calcium absorption
Bone deformities (rickets) in children; bone softening in adults
Retarded growth; kidney damage; calcium deposits in soft tissues
E
Whole grains, dark green vegetables, vegetable oils
Counters effects of free radicals; helps maintain cell membranes; blocks breakdown of vitamins A and C in gut
Lysis of red blood cells; nerve damage
Muscle weakness; fatigue; headaches; nausea
K
Enterobacteria form most of it; also in green leafy vegetables, cabbage
Blood clotting; ATP formation via electron transport
Abnormal blood clotting; severe bleeding (hemorrhaging)
Anemia; liver damage and jaundice
Water-Soluble Vitamins B1 (thiamin)
Whole grains, green leafy vegetables, legumes, lean meats, eggs
Connective tissue formation; folate utilization; coenzyme action
Water retention in tissues; tingling sensations; heart changes; poor coordination
None reported from food; possible shock reaction from repeated injections
B2 (riboflavin)
Whole grains, poultry, fish, egg white, milk
Coenzyme action (FAD)
Skin lesions
None reported
B3 (niacin)
Green leafy vegetables, potatoes, peanuts, poultry, fish, pork, beef
Coenzyme action (NAD+)
Contributes to pellagra (damage to skin, gut, nervous system, etc.)
Skin flushing; possible liver damage
B6
Spinach, tomatoes, potatoes, meats
Coenzyme in amino acid metabolism
Skin, muscle, and nerve damage; anemia
Impaired coordination; numbness in feet
Pantothenic acid
In many foods (meats, yeast, egg yolk especially)
Coenzyme in glucose metabolism, fatty acid and steroid synthesis
Fatigue; tingling in hands; headaches; nausea
None reported; may cause diarrhea occasionally
Folate (folic acid)
Dark green vegetables, whole grains, yeast, lean meats; enterobacteria produce some folate
Coenzyme in nucleic acid and amino acid metabolism
A type of anemia; inflamed tongue; diarrhea; impaired growth; mental disorders
Masks vitamin B12 deficiency
B12
Poultry, fish, red meat, dairy foods (not butter)
Coenzyme in nucleic acid metabolism
A type of anemia; impaired nerve function
None reported
Biotin
Legumes, egg yolk; colon bacteria produce some
Coenzyme in fat, glycogen formation and in amino acid metabolism
Scaly skin (dermatitis); sore tongue; depression; anemia
None reported
C (ascorbic acid)
Fruits and vegetables, especially citrus, berries, cantaloupe, cabbage, broccoli, green pepper
Collagen synthesis; possibly inhibits effects of free radicals; structural role in bone, cartilage, and teeth; used in carbohydrate metabolism
Scurvy; poor wound healing; impaired immunity
Diarrhea, other digestive upsets; may alter results of some diagnostic tests
* Guidelines for appropriate daily intakes are being worked out by the Food and Drug Administration.
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healthy nervous system and to make thyroid hormone (Section 35.6). Healthy people can get all the vitamins and minerals they need from a well-balanced diet. In most cases, vitamin and mineral supplements are necessary only for strict vegetarians, the elderly, and people who are chronically ill or taking a medicine that interferes with nutrient absorption. In addition to vitamins and minerals, a healthy diet should include a variety of phytochemicals, also known as phytonutrients. These organic molecules are found in plant foods and while not essential, they may reduce the risk of certain disorders. For example, eating leafy green vegetables ensures adequate intake of the plant pigments lutein and zeaxanthin. A diet low in these phytochemicals raises the risk of macular
Table 40.5
degeneration, a major cause of blindness (Section 34.10). As another example, isoflavones in soy products can help lower cholesterol level in the blood and protect against heart diseases. Keep this in mind: The more colors you see among the vegetables on your plate, the greater the variety of beneficial phytochemicals in your food.
Take-Home Message What roles do vitamins, minerals, and phytonutrients play?
Vitamins are organic molecules with an essential role in metabolism. Minerals are inorganic substances with an essential role. Phytochemicals are plant molecules that are not essential but may reduce the risk of certain disorders.
Major Minerals: Sources, Functions, and Effects of Deficiencies or Excesses* Effects of Chronic Deficiency
Effects of Extreme Excess
Bone, tooth formation; blood clotting; neural and muscle action
Stunted growth; fragile bones; nerve impairment; muscle spasms
Impaired absorption of other minerals; kidney stones in susceptible people
Table salt (usually too much in diet)
HCl formation in stomach; contributes to body’s acid–base balance; neural action
Muscle cramps; impaired growth; poor appetite
Contributes to high blood pressure in certain people
Copper
Nuts, legumes, seafood, drinking water
Used in synthesis of melanin, hemoglobin, and some transport chain components
Anemia; changes in bone and blood vessels
Nausea; liver damage
Fluorine
Fluoridated water, tea, seafood
Bone, tooth maintenance
Tooth decay
Digestive upsets; mottled teeth and deformed skeleton in chronic cases
Iodine
Marine fish, shellfish, iodized salt, dairy products
Thyroid hormone formation
Enlarged thyroid (goiter) with metabolic disorders
Toxic goiter
Iron
Whole grains, green leafy vegetables, legumes, nuts, eggs, lean meat, molasses, dried fruit, shellfish
Formation of hemoglobin and cytochrome (transport chain component)
Iron-deficiency anemia; impaired immune function
Liver damage; shock; heart failure
Magnesium
Whole grains, legumes, nuts, dairy products
Coenzyme role in ATP–ADP cycle; roles in muscle, nerve function
Weak, sore muscles; impaired neural function
Impaired neural function
Phosphorus
Whole grains, poultry, red meat
Component of bone, teeth, nucleic acids, ATP, phospholipids
Muscular weakness; loss of minerals from bone
Impaired absorption of minerals into bone
Potassium
Diet alone provides ample amounts
Muscle and neural function; roles in protein synthesis and body’s acid–base balance
Muscular weakness
Muscular weakness; paralysis; heart failure
Sodium
Table salt; diet provides ample to excessive amounts
Key role in body’s salt–water balance; roles in muscle and neural function
Muscle cramps
High blood pressure in susceptible people
Sulfur
Proteins in diet
Component of body proteins
None reported
None likely
Zinc
Whole grains, legumes, nuts, meats, seafood
Component of digestive enzymes; roles in normal growth, wound healing, sperm formation, and taste and smell
Impaired growth; scaly skin; impaired immune function
Nausea, vomiting, diarrhea; impaired immune function and anemia
Mineral
Common Sources
Main Functions
Calcium
Dairy products, dark green vegetables, dried legumes
Chloride
* Guidelines for appropriate daily intakes are being worked out by the Food and Drug Administration.
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DIGESTION AND HUMAN NUTRITION 715
40.10 Weighty Questions, Tantalizing Answers Fat cells do not increase in number after birth. Putting on weight simply fills existing fat cells with more fat. Links to Fat tissue 32.3, Limbic system 33.11, Insulin 35.8, Diabetes 35.9, Inflammation 38.4
Weight and Health Being overweight has a negative effect on health. Among other things, it increases the risk of type 2 diabetes, high blood pressure, heart disease, breast and colon cancer, arthritis, and gallstones. Why does excess weight have ill effects? As Section 8.7 explained, triglycerides in fat cells are the body’s main form of energy storage. Fat cells of people who are at a healthy weight hold a moderate amount of triglycerides and function normally. In obese people, an excess of these molecules distends fat cells and impairs their function. Like cells damaged in other ways, the overstuffed fat cells respond by sending out signals that summon up an inflammatory response (Section 38.4). The resulting chronic inflammation harms organs throughout the body and increases risk of cancer. Overstuffed fat cells also increase secretion of signals that interfere with the action of insulin. Remember that this hormone encourages cells to take up sugar from the blood (Section 35.8). When insulin becomes ineffective, the result is type 2 diabetes (Section 35.9). Armed with an understanding of how weight impairs health, researchers are looking for ways to dampen or offset harmful signals secreted by fat cells. One day, it may be possible to keep fat cells from causing inflammation or interfering with insulin function. But for now, the only way to prevent these effects is by losing the excess weight.
Figure 40.14 How to estimate “ideal” weights for adults. Values shown are consistent with a long-term Harvard study into the link between excess weight and risk of cardiovascular disorders. The “ideal” varies. It is influenced by specific factors such as having a small, medium, or large skeletal frame; bones are heavy.
716 UNIT VI
What Is the “Right” Body Weight? Figure 40.14 shows one of the widely accepted weight guidelines for women and men. The body mass index (BMI) is another guideline. It is a measurement designed to help assess increased health risk associated with weight gains. You can calculate your body mass index with this formula: BMI =
weight (pounds) × 703 height (inches)2
Generally, individuals with a BMI of 25 to 29.9 are said to be overweight. A score of 30 or more indicates obesity: an overabundance of fat in adipose tissue that may lead to severe health problems. How body fat gets distributed also helps predict the risks. Fat deposits just above the belt, as in a “beer belly,” are associated with the greatest risk of heart problems. Fat deposits just below the skin of arms and legs, commonly referred to as “cellulite,” have less of an effect on the heart. If your BMI is too high, dieting alone will probably not lower it to a healthy level. When you simply eat less than normal, your body slows its metabolic rate to conserve energy. So how do you lose weight? You must decrease your caloric intake and increase your energy output. For most people, this means eating reasonable portions of low-calorie, nutritious foods and exercising regularly. Energy stored in food is expressed as kilocalories or Calories (with a capital C). One kilocalorie equals 1,000 calories, which are units of heat energy. Here is a way to calculate how many kilocalories you should take in daily to maintain a preferred weight. First, multiply the weight (in pounds) by 10 if you are not active physically, by 15 if you are moderately active, and by 20 if
Weight Guidelines for Women
Weight Guidelines for Men
Starting with an ideal weight of 100 pounds for a woman who is 5 feet tall, add five additional pounds for each additional inch of height. Examples:
Starting with an ideal weight of 106 pounds for a man who is 5 feet tall, add six additional pounds for each additional inch of height. Examples:
Height (feet) 5 2 5 3 5 4 5 5 5 6 5 7 5 8 5 9 5 10 5 11 6
Height (feet) 5 2 5 3 5 4 5 5 5 6 5 7 5 8 5 9 5 10 5 11 6
Weight (pounds) 110 115 120 125 130 135 140 145 150 155 160
HOW ANIMALS WORK
Weight (pounds) 118 124 130 136 142 148 154 160 166 172 178
FOCUS ON HEALTH
you are highly active. Next, subtract one of the following amounts from the multiplication result: Age: 25–34 35–44 45–54 55–64 Over 65
Subtract: 0 100 200 300 400
×
For example, if you are 25 years old, are highly active, and weigh 120 pounds, you will require 120 × 20 = 2,400 kilocalories daily to maintain weight. If you want to gain weight you will require more; to lose, you will require less. The amount is only a rough estimate. Other factors, such as height, must be considered. A person who is 5 feet, 2 inches tall and is active does not require as much energy as an active 6-footer whose body weight is the same.
Genes, Hormones, and Obesity Numerous studies have explored the role that genetics plays in obesity. As one example, Claude Bouchard studied experimental overeating by twelve pairs of male twins. All were lean young men in their early twenties. For 100 days they did not exercise, and they adhered to a diet that provided 6,000 more kilocalories a week than usual. They all gained weight, but some gained three times as much as others. Members of each set of twins tended to gain a similar amount, which suggests that genes affect the response to overfeeding. For another test, Bouchard put sets of obese twins on a low-calorie diet. Once again, each set of twins lost a similar amount. As the chapter introduction indicated, we are learning more about how genes that encode hormones contribute to obesity. Figure 40.15 details how researchers uncovered the role of the appetite-suppressing hormone leptin in mice. Researchers have now also identified the leptin gene in humans; it is on chromosome 7 (Appendix VII). Leptin deficiency of the sort seen in mice is extremely rare in humans. However, three cousins in a Turkish family have been found to be entirely leptin deficient. All three were greatly obese. When UCLA researchers gave them daily leptin injections, the leptin-deficient men lost an average of 50 percent of their body weight without even trying to diet. The injections apparently caused changes in their brains. Scans showed increases in the gray matter of the cingulate gyrus, a portion of the limbic system known from other research to affect cravings (Section 33.11).
a 1950. Researchers at the Jackson Laboratories in Maine notice that one of their laboratory mice is extremely obese, with an uncontrollable appetite. Through cross-breeding of this apparent mutant individual with a normal mouse, they produce a strain of obese mice.
b Late 1960s. Douglas Coleman of the Jackson Laboratories surgically joins the bloodstreams of an obese mouse and a normal one. The obese mouse now loses weight. Coleman hypothesizes that a factor circulating in blood may be influencing its appetite, but he is not able to isolate it.
ob gene
protein product (leptin)
c 1994. Late in the year, Jeffrey Friedman of Rockefeller University discovers a mutated form of what is now called the ob gene in obese mice. Through DNA cloning and gene sequencing, he defines the protein that the mutated gene encodes. The protein, now called leptin, is a hormone that influences the brain’s commands to suppress appetite and increase metabolic rates. d 1995. Three different research teams develop and use genetically engineered bacteria to produce leptin, which, when injected in obese and normal mice, triggers significant weight loss, apparently without harmful side effects.
Figure 40.15 Chronology of research developments that identified leptin as a heritable factor that affects body weight.
CHAPTER 40
DIGESTION AND HUMAN NUTRITION 717
IMPACTS, ISSUES REVISITED
Hormones and Hunger
Americans are eating fewer and fewer meals at home. An everincreasing array of fast-food outlets benefits from this trend. However, frequent fast-food meals increase the risk of obesity and diabetes. One part of the problem is over-sized portions. Another is that people simply do not make healthy choices. Many fast-food restaurants now offer salads or veggie burgers, but most diners prefer higher fat, higher calorie options.
Summary Section 40.1 A digestive system breaks food down into molecules that are small enough to be absorbed into the internal environment. It also stores and eliminates any unabsorbed materials, and promotes homeostasis by its interactions with other organ systems. Some invertebrates have an incomplete digestive system: a saclike gut with a single opening. Most animals, and all vertebrates, have a complete digestive system: a tube with two openings (mouth and anus) and specialized areas between them. Features of the digestive system may adapt an animal to a particular diet. For example, the multiple stomach chambers of cattle and other ruminants allow them to maximize the nutrients they get from plant food.
Use the animation on CengageNOW to compare vertebrate digestive systems.
Section 40.2 The human pharynx is the entrance to the digestive and respiratory systems. Peristalsis moves food down the esophagus and through a sphincter (a ring of muscle that can close off an opening) into the stomach, the start of the gastrointestinal tract. From the stomach, material moves to the small intestine. Most digestion occurs and most nutrients and water are absorbed here. The large intestine concentrates undigested wastes, which are stored in the rectum until expelled through the anus.
Use the animation on CengageNOW to explore the components of the human digestive system.
Section 40.3 Teeth are mostly bonelike dentin, with a covering of hard enamel. They break food into bits that become coated with saliva from salivary glands. Saliva contains the enzyme salivary amylase, which begins the process of starch digestion. Section 40.4 Protein digestion starts in the stomach, where cells in its lining (the mucosa) release gastric fluid. This fluid contains protein-digesting enzymes and acid. It mixes with food and forms the semiliquid chyme. Most digestion is completed in the small intestine, which receives a variety of digestive enzymes from the pancreas. Bile, which assists in fat digestion, is made in the liver and stored in the gallbladder. Delivery of bile into the small intestine, causes the emulsification of fats, breaking them into smaller, more easily digested droplets. The nervous and endocrine systems respond to the volume and composition of food in the gut. They cause 718 UNIT VI
HOW ANIMALS WORK
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changes in muscle activity and in the rate of secretion of hormones and enzymes. Section 40.5 The lining of the small intestine is highly folded. Multicelled, absorptive structures called villi are on each fold. Most cells at the surface of each villus are brush border cells that have microvilli on their surface. Brush border cells function in digestion and absorption. Their many membrane proteins transport salts, simple sugars, and amino acids from the intestinal lumen into the villus interior. A blood vessel inside each villus takes up absorbed sugars and amino acids. Monoglycerides and fatty acids diffuse into a brush border cell, where they combine with proteins. The result is lipoproteins, which move by exocytosis into interstitial fluid, then enter lymph vessels that deliver them to blood.
Use the animation on CengageNOW to learn about the small intestine’s structure and how it absorbs nutrients.
Section 40.6 The large intestine absorbs water and ions, thus compacting undigested solid wastes as feces. The appendix is a thin extension of the first part of the large intestine. Section 40.7 The small organic compounds absorbed from the gut are stored, used in biosynthesis or as energy sources, or excreted by other organ systems. Blood that flows through the small intestine travels next to the liver, whic eliminates ingested toxins and stores some excess glucose as glycogen. Sections 40.8, 40.9 Food must provide both energy and raw materials, including essential amino acids and essential fatty acids. It must also include two additional types of compounds needed for metabolism: vitamins, which are organic, and minerals, which are inorganic. Phytochemicals are plant molecules that are not essential, but may improve health or prevent certain disorders. Section 40.10 An unhealthy overabundance of fat, or obesity, stresses fat cells and increases the risk of many disorders. To maintain your body weight, energy (caloric) intake must balance with energy output. Genetic factors influence how difficult it is for a person to reach and maintain a healthy weight. Hormones can influence both appetite and metabolic rate.
Use the interaction on CengageNOW to calculate your body mass index.
Data Analysis Exercise 1.0 Cumulative proportion of individuals
The human AMY-1 gene encodes salivary amylase, an enzyme that breaks down starch. The number of copies of this gene varies, and people who have more copies generally make more enzyme. In addition, the average number of AMY-1 copies differs among cultural groups. George Perry and his colleagues hypothesized that duplications of the AMY-1 gene would confer a selective advantage in cultures in which starch is a large part of the diet. To test this hypothesis, the scientists compared the number of copies of the AMY-1 gene among members of seven cultural groups that differed in their traditional diets. Figure 40.16 shows their results. 1. Starchy tubers are a mainstay of Hadza hunter–gatherers in Africa, whereas fishing sustains Siberia’s Yakut. Almost 60 percent of Yakut had fewer than 5 copies of the AMY1 gene. What percent of the Hadza had fewer than 5 copies? 2. None of the Mbuti (rain-forest hunter–gathers) had more than 10 copies of AMY-1. Did any European Americans? 3. Do these data support the hypothesis that a starchy diet favors duplications of the AMY-1 gene?
Self-Quiz
Answers in Appendix III
1. A digestive system functions in . a. secreting enzymes c. eliminating wastes b. absorbing compounds d. all of the above
digestion and absorption. c. protein d. amino acid
5. Monosaccharides and amino acids absorbed from the small intestine enter . a. blood vessels c. fat droplets b. lymph vessels d. the large intestine
7. The pH is lowest in the a. stomach b. small intestine
European American
0.4
Low starch Biaka Mbuti
0.2
Datog Yakut 0.0 2
3
4 5 6 7 8 AMY-1 diploid gene copy number
.
. c. large intestine d. esophagus
8. Most water that enters the gut is absorbed across the lining of the . a. stomach c. large intestine b. small intestine d. esophagus 9. are inorganic substances with essential metabolic roles that no other substance can fulfill. a. Phytonutrients c. Vitamins b. Minerals d. both a and c 10. True or false? Glucose-rich blood flows from the small intestine to the liver, which stores glucose as glycogen.
9
10
Figure 40.16 Number of copies of the AMY-1 gene among members of cultures with traditional high-starch or low-starch diets. The Hadza, Biaka, Mbuti, and Datog are tribes in Africa. The Yakut live in Siberia.
11. Ammonia is a toxic product of the digestion of . a. fats b. proteins c. carbohydrates d. vitamins
13. The essential fatty acids are . a. trans fats c. polyunsaturated fats b. saturated fats d. lysine and methionine
3. Most nutrients are absorbed in the . a. mouth c. small intestine b. stomach d. colon
6. The largest number of bacteria thrive in the a. stomach c. large intestine b. small intestine d. esophagus
High starch Japanese Hadza
0.6
12. Ammonia is converted to less toxic urea by the . a. liver b. stomach c. gallbladder d. rectum
2. Protein digestion begins in the . a. mouth c. small intestine b. stomach d. colon
4. Bile has roles in a. carbohydrate b. fat
0.8
14. Match each organ with a digestive function. gallbladder a. makes bile large b. compacts undigested residues intestine c. secretes most digestive enzymes liver d. absorbs most nutrients small e. secretes gastric fluid intestine f. stores, secretes bile stomach pancreas
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Critical Thinking 1. Anorexia nervosa is an eating disorder in which people, most often women, starve themselves. Although the name means “nervous loss of appetite,” most affected people are obsessed with food and continually hungry. Anorexia nervosa has complex causes, including some recently discovered genetic factors. Reported incidence of anorexia has soared during the past 20 years. Is it likely that a rise in the frequency of alleles that put people at risk for anorexia has caused this rise in reported cases? 2. Starch and sugar have the same number of calories per gram. However, not all vegetables are equally calorie dense. For example, a serving of boiled sweet potato provides about 1.2 calories per gram, while a serving of kale yields only 0.3 calories per gram. What could account for the difference in the calories your body obtains from these two foods? CHAPTER 40
DIGESTION AND HUMAN NUTRITION 719
41
Maintaining the Internal Environment IMPACTS, ISSUES
Truth in a Test Tube
Light or dark? Clear or cloudy? A lot or a little? Asking about
Do-it-yourself urine tests are popular. If a woman is hoping
and examining urine is an ancient art (Figure 41.1). About
to become pregnant, she can use one test to keep track of
3,000 years ago in India, the pioneering healer Susruta
the amount of luteinizing hormone, or LH, in her urine. About
reported that some patients formed an excess of sweet-
midway through a menstrual cycle, LH triggers ovulation, the
tasting urine that attracted insects. In time, the disorder was
release of an egg from an ovary. Another over-the-counter
named diabetes mellitus, which loosely translates as “pass-
urine test can reveal whether she has become pregnant. Still
ing honey-sweet water.” Doctors still diagnose it by testing
other tests help older women check for declining hormone
the sugar level in urine, although they have replaced the taste
levels in urine, a sign that they are entering menopause.
test with chemical analysis. Today, physicians routinely check the pH and solute con-
Not everyone is in a hurry to have their urine tested. Olympic athletes can be stripped of their medals when man-
centrations of urine to monitor their patients’ health. Acidic
datory urine tests reveal they use prohibited drugs. Major
urine suggests metabolic problems. Alkaline urine can indi-
League Baseball players agreed to urine tests only after
cate an infection. Damaged kidneys will produce urine high
repeated allegations that certain star players took prohibited
in proteins. An abundance of some salts can result from
steroids. The National Collegiate Athletic Association (NCAA)
dehydration or trouble with the hormones that control kidney
tests urine samples from about 3,300 student athletes per
function. Special urine tests detect chemicals produced by
year for any performance-enhancing substances as well as
cancers of the kidney, bladder, and prostate gland.
for “street drugs.” If you use marijuana, cocaine, Ecstasy, or other kinds of psychoactive drugs, urine tells the tale. After the active ingredient of marijuana enters blood, the liver converts it to another compound. As kidneys filter blood, they add the compound to newly forming urine. It can take as long as ten days for all molecules of the compound to become fully metabolized and removed from the body. Until that happens, urine tests can detect it. It is a tribute to the urinary system that urine is such a remarkable indicator of health, hormonal status, and drug use. Each day, a pair of fist-sized kidneys filter all of the blood in an adult human body, and they do so more than forty times. When all goes well, kidneys rid the body of excess water and excess or harmful solutes, including a variety of metabolites, toxins, hormones, and drugs. So far in this unit, you have considered several organ systems that work to keep cells supplied with oxygen, nutrients, water, and other substances. Turn now to the kinds that maintain the composition, volume, and even the temperature of the internal environment.
See the video! Figure 41.1 This page, a seventeenthcentury physician and a nurse examining a urine specimen. Urine’s consistency, color, odor, and—at least in the past— taste provide clues to health conditions. Urine forms inside kidneys, and it provides clues to abnormal changes in the volume and composition of blood and interstitial fluid. Facing page, testing for the presence of drugs in urine samples.
Links to Earlier Concepts
Key Concepts Maintaining the extracellular fluid
In this chapter, you will see how osmosis (Section 5.6) affects water gain and loss in animal bodies, and learn about an animal group that has contractile vacuoles (22.2). You will also learn more about the efficient kidneys of amniotes (26.7). You will be reminded that aerobic respiration (8.1) produces water, and protein metabolism (40.7) yields ammonia, which is why a high-protein diet (40.8) can stress kidneys.
Your knowledge of pH and buffer systems (2.6) will help you understand acid–base balance in the body.
You will learn the roles of osmoreceptors (34.1), the hypothalamus (33.10), the pituitary gland (35.3), the adrenal glands (35.10) and the autonomic nervous system (33.8), in regulating body fluids. You will also learn about another spinal reflex (33.9).
The discussion of body temperature will refer back to properties of water (2.5), forms of energy (6.1), feedback controls (27.3), heat illness (27.4), sweat glands (32.7), and fever (38.4).
Animals continually produce metabolic wastes. They continually gain and lose water and solutes. Yet the overall composition and volume of extracellular fluid must be kept within a narrow range. Most animals have organs that accomplish this task. Sections 41.1–41.3
The human urinary system The human urinary system consists of two kidneys, two ureters, a bladder, and a urethra. Inside a kidney, millions of nephrons filter water and solutes from the blood. Most of this filtrate is returned to the blood. Water and solutes not returned leave the body as urine. Section 41.4
What kidneys do Urine forms by filtration, reabsorption, and secretion. Its content is adjusted continually by hormonal and behavioral responses to shifts in the internal environment. Hormones, as well as a thirst mechanism, influence whether urine is concentrated or dilute. Sections 41.5–41.8
Adjusting the core temperature Heat losses to the environment and heat gains from the environment and from metabolic activity determine an animal’s body temperature. Adaptations in body form and behavior help maintain core temperature within a tolerable range. Sections 41.9, 41.10
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721
41.1
41.2
Maintenance of Extracellular Fluid All animals constantly acquire and lose water and solutes, yet they must keep the volume and the composition of their internal environment—the extracellular fluid—stable.
Links to Osmosis 5.6, Aerobic respiration 8.1, Ammonia 40.7
By weight, all organisms consist mostly of water, with dissolved salts and other solutes. Fluid outside cells— the extracellular fluid (ECF)—functions as the body’s internal environment. In humans and other vertebrates, extracellular fluid consists mostly of interstitial fluid, which fills the spaces between cells, and plasma, the fluid portion of the blood (Figure 41.2). Keeping the solute composition and volume of the extracellular fluid within the range that living cells can tolerate is a major aspect of homeostasis. Water and solute gains need to be balanced by water and solute losses. An animal can lose water and solutes in feces and urine, in exhalations, and in secretions. The animal gains water by eating and drinking. In aquatic animals, water also moves into or out of the body by osmosis across the body surface (Section 5.6). In all animals, metabolic reactions put water and solutes into the ECF. The most abundant molecules of metabolic waste are carbon dioxide and ammonia. Aerobic respiration produces carbon dioxide and water (Section 8.1). Ammonia forms when amino acids or nucleic acids are broken down (Section 40.7). Carbon dioxide diffuses out across the body surface or leaves with the help of respiratory organs. In most animals, excretory organs rid the body of ammonia and other unwanted solutes, as well as excess water.
How Do Invertebrates Maintain Fluid Balance?
Most invertebrates regulate the volume and composition of their body fluid through the action of excretory organs.
Link to Contractile vacuole 22.2
Sponges are among the simplest invertebrates; they do not have tissues or organs (Section 25.4). A sponge excretes metabolic wastes at the cellular level. All of a sponge’s cells are located close to the body surface, so metabolic wastes can simply diffuse from that surface into the surrounding water. Freshwater sponges face a challenge common to all freshwater animals. Their body fluid contains a higher concentration of solutes than the surrounding water. As a result, water constantly moves into the body by osmosis. In freshwater sponges, this inward flow is countered by action of contractile vacuoles similar to those of freshwater protists (Section 22.2). Fluid accumulates inside this organelle, which then contracts and expels the fluid to the outside through a pore. In planarians, a group of freshwater flatworms, a pair of branching, tubular, excretory organs run the length of the body (Figure 41.3). Along the tubes are flame cells, so called because the cells contain a tuft of cilia that looks like a flickering flame when viewed with a microscope. Movement of the cilia draws interstitial fluid into the tubes, propels it along, and forces it out of the body through pores at the body surface. An earthworm is a segmented annelid worm with a fluid-filled body cavity (a coelom) and a closed circulatory system (Section 25.7). Most body segments have a
plasma
nucleus
lymph, cerebrospinal fluid, mucus, and other fluids
interstitial fluid
Intracellular Fluid
Extracellular Fluid (ECF)
(28 liters)
(15 liters)
Human Body Fluids
cilia
pair of highly branched tubules that adjust water and solute levels in body
fluid filters through membrane folds flame cell
(43 liters)
Figure 41.2 Fluid distribution in the human body. opening at body surface
Take-Home Message What is the function of excretory organs? Excretory organs help maintain the volume and composition of the extracellular fluid by getting rid of water and certain solutes.
722 UNIT VI
HOW ANIMALS WORK
Figure 41.3 Planarian excretory organs. Action of cilia in flame cells drives flow of interstitial fluid into branching tubes, then out of the body through pores at the body surface.
body wall
storage bladder
loops where blood vessels take up solutes
funnel where coelomic fluid enters the nephridium (coded green)
pore where ammoniarich fluid leaves the body one body segment of an earthworm
Figure 41.4 Excretory system of an earthworm. Most body segments have a pair of nephridia. One nephridium is shown in the diagram in green. Coelomic fluid enters a nephridium through a ciliated funnel in the segment just anterior to it. As fluid travels through the nephridium, essential solutes leave this tube and enter into adjacent blood vessels (shown in red). This process yields an ammonia-rich fluid that exits the body through a pore.
pair of tubular excretory organs called nephridia. The anterior end of each nephridium is a ciliated funnel that collects coelomic fluid from the adjacent segment (Figure 41.4). As the fluid flows through the tubular portion of the nephridium, essential solutes and some water leave the tubes and are reabsorbed by adjacent blood vessels, but wastes remain in the tubule. The ammonia-rich fluid that forms through this process is stored in a bladderlike organ before leaving the body through a pore. Land-dwelling arthropods such as insects, spiders, and centipedes do not excrete ammonia. Instead, some enzymes in their blood convert ammonia to uric acid. Uric acid and other solutes are actively transported into Malpighian tubules. These tubules are long, thin excretory organs that connect to and empty into a region of the gut (Figure 41.5). Solutes are pumped from the blood into Malpighian tubules, then water follows by osmosis. Both the water and solutes move through the tubules and enter the gut. Unlike ammonia, uric acid need not be dissolved in a large amount of water in order to be excreted from the body. Thus, nearly all of the water taken up by Malpighian tubules can be reabsorbed into the blood across the wall of the rectum. The uric acid is then excreted from the rectum in the form of crystals mixed with just a tiny bit of water to produce a thick paste.
Malpighian tubule
Portion of the gut
Figure 41.5 Scanning, colorized electron micrograph of Malpighian tubules (gold) in a honeybee. The tubules are outpouchings of the gut (pink). They are bathed by the bee’s blood, and take up substances from it.
Take-Home Message How do invertebrates regulate the volume and composition of their body fluid? Sponges are simple animals with no excretory organs. Wastes diffuse out across the body wall and excess water is expelled by contractile vacuoles.
Flatworms and earthworms have tubular excretory organs that deliver fluid with dissolved ammonia to a pore at the body surface.
Insects convert ammonia to uric acid, which Malpighian tubules deliver to the gut. Excreting uric acid rather than ammonia reduces water loss.
CHAPTER 41
MAINTAINING THE INTERNAL ENVIRONMENT 723
41.3
Fluid Regulation in Vertebrates All vertebrates have paired kidneys—excretory organs that filter metabolic wastes and toxins out of the blood and adjust the level of solutes.
water loss by osmosis gulps water
Links to Osmosis 5.6, Amniote traits 26.7
Vertebrates have a urinary system that filters water and solutes from their blood, then reclaims or excretes water and certain solutes as needed to maintain the volume and composition of the extracellular fluid. A pair of organs called kidneys filter blood. The urinary system interacts with other vertebrate organ systems as illustrated in Figure 41.6.
Fluid Balance in Fishes and Amphibians Most marine invertebrates have body fluids that have the same concentration of solutes as seawater. As a result, there is no net movement of water into or out of their body as a result of osmosis. Body fluids of sharks and other cartilaginous fishes are also isotonic with seawater, although the fluids have different types of solutes. The fishes maintain a high internal solute concentration by retaining large amounts of urea, a solute that is scarce in seawater. Bony fishes have body fluids that are less salty than seawater, but saltier than fresh water. Thus, wherever they live, they face an osmotic challenge. A marine bony fish loses water by osmosis across its body surfaces, especially its gills. To replace this lost water, the fish gulps seawater, then pumps the unwanted salts out through its gills (Figure 41.7a). It produces a small amount of urine that contains some salts.
food, water intake
oxygen intake
Digestive System
Respiratory System
nutrients, water, salts
oxygen
elimination of carbon dioxide
Marine bony bonyfifish; bodyflfluids areless lesssalty salty than aa Marine sh: Body uids are than thethe surrounding water; water;they theyare arehypotonic. hypotonic. surrounding water gain by osmosis
does not drink water
cells in gills pump solutes in
water loss in large volume of dilute urine
Freshwater bony bony fifish; bodyflfluids areless saltier b Freshwater sh: Body uids are saltythan thanthe surrounding water; they areare hypertonic. the surrounding water; they hypertonic.
Figure 41.7 Fluid and solute balance in bony fishes.
In contrast, a freshwater bony fish continually gains water. It does not drink and still produces a large volume of dilute urine. Solutes lost in the urine are offset by solutes absorbed from the gut, and by sodium ions pumped in across the gills. When in water, amphibians face the same challenge as freshwater bony fishes. Water moves inward across their skin. Most keep their body fluid from becoming too dilute by pumping ions in across the skin. On land, amphibians tend to lose water when it evaporates across their skin. Most amphibians excrete either ammonia or urea as adults, but some that spend much of their time in dry habitats excrete uric acid. Converting urea to uric acid takes energy, but this cost is offset by the benefit of reducing the amount of water required for excretion.
Fluid Balance in Reptiles, Birds, and Mammals Urinary System water, solutes
rapid transport to and from all living cells
elimination of excess water, salts, wastes
Figure 41.6 Functional links between the urinary, digestive, respiratory, and circulatory systems. Guided by the nervous and endocrine systems, these systems help maintain homeostasis.
724 UNIT VI
water loss in very small volume of concentrated urine
carbon dioxide
Circulatory System
elimination of food residues
cells in gills pump solutes out
HOW ANIMALS WORK
Waterproof skin and a pair of highly efficient kidneys are among the features that adapt amniotes—reptiles, birds, and mammals—to life on land (Section 26.7). Reptiles and birds convert ammonia to uric acid, while mammals convert it to urea. It takes twenty to thirty times more water to excrete 1 gram of urea than to excrete 1 gram of uric acid. Thus, a typical mammal requires more water than a bird or reptile of similar body size. Even so, some mammals have adaptations
Kangaroo Rat Human Daily Water Gain (milliliters): By ingesting solids By ingesting liquids By metabolism Daily Water Loss (milliliters): In urine In feces By evaporation
Figure 41.8 Water gains and losses in two mammals, a human and a kangaroo rat. In both, water intake must balance water losses.
2600 ml
10% 0% 90%
33% 54% 13%
60 ml 23% 4% 73%
2600 ml 58% 8% 34%
Figure It Out: Which species loses a greater percentage of
its water to evaporation?
Answer: Kangaroo rat
that allow them to get along with very little water. For example, the kangaroo rat (Dipodomys deserti) is a small mammal that lives in the New Mexico desert where standing water is scarce, except during a brief rainy season. The rat conserves water by sheltering in its burrow during the heat, then foraging at night for dry seeds and bits of plants. The kangaroo rat hops rapidly and far as it searches for seeds and flees from predators. All that activity requires ATP energy. Aerobic respiration (Section 8.1) of compounds in food provides energy and produces carbon dioxide and water. Each day, the “metabolic water” derived from this and other reactions makes up 90 percent of a kangaroo rat’s water intake. In contrast, metabolic water accounts for about 13 percent of a human’s daily water gain (Figure 41.8). A kangaroo rat conserves and recycles water when it rests in its cool burrow. It moistens and warms the air that it inhales. When it exhales, water condenses in its cooler nose, and some diffuses back into the body. Seeds emptied from a kangaroo rat’s cheek pouches soak up water dripping from the nose. The kangaroo rat reclaims water when it eats dripped-on seeds. A kangaroo rat has no sweat glands and its feces contain only half the water that human feces do. Like a human, the kangaroo rat must eliminate metabolic wastes in urine, but the rat’s highly efficient kidneys minimize urinary water loss. A kangaroo rat produces urine that can be as much as three to five times more concentrated than human urine.
60 ml
As another example of how mammalian kidneys can help an animal adapt to an unusual habitat, think about whales and dolphins. These marine mammals had land-dwelling ancestors, so solute concentrations in their blood are like those of other land mammals. Yet whales and dolphins eat highly salty food and do not drink fresh water. How do they rid their body of ingested salts and obtain the water needed to maintain proper solute concentration in their body fluid? The kidneys of marine mammals tend to be larger than those of land mammals of a similar size, and marine mammal kidneys are divided into many small lobes that increase their surface area. Having large, highly efficient kidneys allows whales and dolphins to make and excrete urine that is saltier than seawater. As for meeting their water needs, like kangaroo rats, whales and dolphins conserve nearly all the water released by digestion and metabolism of their food.
Take-Home Message How do vertebrates regulate volume and composition of their body fluid?
All vertebrates have a urinary system with two kidneys that filter the blood and adjust its solute concentration. Fish and amphibians also adjust their internal solute concentration by pumping solutes across their gills or skin. Reptiles and birds excrete uric acid but mammals excrete urea, which requires more water to excrete.
Some mammals have highly efficient kidneys and other adaptations that allow them to live in habitats where fresh water is scarce.
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41.4
The Human Urinary System The human urinary system forms urine, stores it, and then expels it from the body.
Link to Reflexes 33.9
Components of the Urinary System As in all other vertebrates, the human urinary system includes two kidneys, two ureters, a urinary bladder, and a urethra (Figure 41.9a). Kidneys filter blood and form urine. The other organs collect and store urine, and channel it to the body surface for excretion. Each human kidney is a bean-shaped organ about the size of an adult fist. The kidneys are located at the rear of the abdominal cavity, with one on each side of the backbone (Figure 41.9a,b). Kidneys lie beneath the peritoneum, the tissue that lines the abdominal cavity. The outermost layer of a kidney is a renal capsule that consists of fibrous connective tissue (Figure 41.9c). The Latin renal means “relating to the kidneys.” The bulk of tissue inside the renal capsule is divided into two zones: the outer renal cortex and the inner renal medulla. A renal artery carries blood to each kidney and a renal vein transports blood away from it.
Kidney (one of a pair)
Urine collects in the renal pelvis, a central cavity inside each kidney. A tubular ureter conveys the fluid from a kidney into the urinary bladder. This muscular organ stores urine until a sphincter at its lower end opens and urine flows into the urethra. As the bladder fills with urine, it stretches and a reflex action occurs. Stretch receptors in the bladder wall signal neurons in the spinal cord. These neurons then command the smooth muscle in the bladder wall to contract. As the bladder contracts, sphincters that encircle the urethra relax, so urine can flow out of the body. After age two or three, the brain overrides this spinal reflex and prevents urine from flowing through the urethra at inconvenient moments. In males, the urethra runs the length of the penis. Urine and semen flow through it, but a sphincter cuts off urine flow during erections. In females, the urethra opens onto the body surface between the vagina and the clitoris. A female’s urethra is a relatively short tube (about 4 centimeters, or 1.5 inches long), so pathogens move more easily through it to the urinary bladder. That is one reason why women get bladder infections more often than men do.
renal cortex
heart
Blood-filtering organ; filters water, all solutes except proteins from blood; reclaims only amounts body requires, excretes rest as urine
renal medulla
diaphragm (back of body) adrenal gland
right backbone kidney
left kidney
Ureter (one of a pair)
renal artery
Channel for urine flow from one kidney to urinary bladder
abdominal aorta
Urinary Bladder
inferior vena cava
Stretchable urine storage container
peritoneum
abdominal cavity
renal vein
(front of body)
Urethra Urine flow channel between urinary bladder and body surface
A The human urinary system, like that of other vertebrates, includes paired kidneys that filter blood and form urine. Other organs of this system convey urine to the body surface for excretion.
Figure 41.9 Animated Human urinary system.
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B The paired kidneys are located between the peritoneum, which lines the abdominal cavity, and the abdominal wall.
renal capsule
C
renal pelvis
ureter
Structure of a human kidney
Figure 41.10 Animated (a) The structure of a nephron. Nephrons are functional units of a kidney. They interact with neighboring blood vessels to form the urine. (b) The arterioles and blood capillaries associated with each nephron. Large gaps between cells in the walls of glomerular capillaries make the capillaries about a hundred times more permeable than any others in the body. Only a thin basement membrane separates each capillary wall from cells of the innermost layer of Bowman’s capsule. Cells of this inner layer have long extensions that interdigitate with one another, like interlaced fingers. Fluid flows through the narrow slits between these extensions.
Bowman’s capsule (red)
proximal tubule (orange)
distal tubule (brown) efferent arteriole afferent arteriole
Renal Cortex Renal Medulla
peritubular capillaries threading around tubular nephron regions
collecting duct (tan)
loop of Henle (yellow)
A Bowman’s capsule and tubular regions of one nephron, cutaway view
glomerular capillaries inside Bowman’s capsule
B Arterioles and the two sets of blood capillaries associated with the nephron
Nephrons—The Functional Units of the Kidney In the section to follow, you will be taking a look at three processes that rid the body of excess water and solutes in the form of urine. Tracking the steps of the processes will be simpler if you first acquaint yourself with the structures that carry out these functions. Overview of Nephron Structure A kidney has more
than 1 million nephrons—microscopically small tubes with a wall only one cell thick. Each nephron begins in the renal cortex, where its wall balloons outward and folds back on itself, to form a cup-shaped Bowman’s capsule (Figure 41.10a). Past the capsule, the nephron twists a bit and straightens out as a proximal tubule (the part nearest the beginning of the nephron). After extending down into the renal medulla, the nephron makes a hairpin turn, the loop of Henle. It reenters the cortex and it twists again, as the distal tubule (the farthest from the start of the nephron), which drains into a collecting duct. Up to eight nephrons drain into each duct. Many collecting ducts extend through the kidney medulla and open onto the renal pelvis. Blood Vessels Around Nephrons Inside each kidney,
the renal artery branches into many afferent arterioles. Each arteriole branches into a glomerulus, a capillary bed inside Bowman’s capsule (Figure 41.10b). As the next section explains, these capillaries interact with Bowman’s capsule as a blood-filtering unit.
As blood passes through the glomerulus, a portion of it is filtered into Bowman’s capsule. The rest enters an efferent arteriole. This arteriole quickly branches to become peritubular capillaries, which thread lacily around the nephron (peri–, around). Blood inside these capillaries continues into venules, and then through a vein leading out of the kidney. Urine forms by three physiological processes that involve all the nephrons, glomerular capillaries, and peritubular capillaries. The processes are glomerular filtration, tubular reabsorption, and tubular secretion. They are the topic of the next section. Each minute, nephrons of both kidneys collectively filter about 125 milliliters (1/2 cup) of fluid from the blood flowing past, which amounts to 180 liters (about 47.5 gallons) per day. At this rate of flow, the kidneys filter the entire volume of blood about 40 times a day!
Take-Home Message How do the components of the human urinary system function? Kidneys filter water and solutes from blood. The body reclaims most of the filtered fluid, or filtrate. The rest flows as urine through ureters into a bladder that stores it. Urine flows out of the body through the urethra.
The functional unit of the human kidneys is the nephron, a microscopic tube that interacts with two systems of capillaries to filter blood and form urine.
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41.5
How Urine Forms Urine consists of water and solutes that were filtered from the blood and not returned to it, along with unwanted solutes secreted from the blood into the nephron’s tubular regions.
Link to Autonomic nervous system 33.8
Urine formation begins when blood pressure drives water and small solutes from the blood into a nephron. Variations in permeability along a nephron’s tubular parts affect whether components of the filtrate return to blood or leave in urine. Figure 41.11 and Table 41.1 provide overviews of the steps in this process.
The pressure forces about 20 percent of the fluid that enters the glomerulus out across its wall and into the first portion of a nephron. Collectively, a glomerular capillary’s walls and the inner wall of the Bowman’s capsule function like a filter. Plasma proteins, platelets, and blood cells are too large to go through this filter. They leave the glomerulus via the efferent arteriole, along with the 80 percent of the fluid that did not get filtered out. The protein-free plasma that does enter the nephron becomes the filtrate: glomerulus inside Bowman’s capsule
outer wall of Bowman’s capsule
Glomerular Filtration Blood pressure generated by the beating heart drives glomerular filtration, the first step of urine formation.
efferent arteriole (to peritubular capillaries) filtrate (to proximal tubule)
afferent arteriole (from renal artery)
A Glomerular filtration Occurs at glomerular capillaries in Bowman’s capsule. Glomerular filtration nonselectively moves water, ions, and solutes from blood into Bowman’s capsule.
B Tubular reabsorption Occurs all along a nephron’s tubular parts. Most of the filtrate leaks or is transported out of the nephron’s tubular parts into interstitial fluid, then is selectively reabsorbed into blood.
A Glomerular filtration Driven by pressure from a beating heart, water and solutes are forced across the wall of glomerular capillaries and into Bowman’s capsule.
Tubular Reabsorption proximal tubule
distal tubule
glomerular capillaries
C Tubular secretion Starts at proximal tubule and continues all along a nephron’s tubular parts. Secretion moves other solutes from blood into interstitial fluid, then into tubular parts of the nephron.
Cortex Medulla
increasing solute concentration
peritubular capillaries
Only a small fraction of the filtrate will be excreted. Most water and solutes are reclaimed during tubular reabsorption. By this process transport proteins move sodium ions (Na+), chloride ions (Cl–), bicarbonate, glucose, and other substances across the tubule wall and into peritubular capillaries. Movement of these solutes causes water to follow by osmosis: lumen of tubule Na+ glucose
wall of tubule
interstitial fluid
peritubular capillary
Na+, glucose
Cl–
Cl–
H2O
H2O
loop of Henle
D Solutes pumped out of the ascending loop of Henle and the collecting duct set up a solute concentration gradient in the medulla that allows urine to become concentrated as it flows through collecting ducts.
urine outflow from collecting duct into renal pelvis
Figure 41.11 Animated How urine forms and becomes concentrated. The lettered in-text art in this section provides a closer look at each of the processes designated by a letter in this diagram.
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B Tubular reabsorption As the filtrate flows through the proximal tubule, ions and nutrients are actively and passively transported from the filrate into interstitial fluid. Water follows by osmosis. Cells of peritubular capillaries transport ions and nutrients into blood. Water again follows by osmosis.
Tubular reabsorption returns close to 99 percent of the water that enters a nephron to the blood. It also returns all the glucose and amino acids, most Na+ and bicarbonate, and about half of the urea.
Tubular Secretion A build-up of excess hydrogen (H+) ions, potassium (K+) ions, or wastes such as urea can harm the body. By tubular secretion, transport proteins in the walls of peritubular capillaries actively transport urea and excess ions into the interstitial fluid. Then active transport proteins in a nephron’s wall pump urea and ions into the filtrate, so they may be excreted in the urine: lumen of tubule
wall of tubule
interstitial fluid
peritubular capillary
H+
H+
K+
K+
urea
urea
C Tubular secretion Transport proteins actively transport H+, K+, and urea out of peritubular capillaries and into filtrate.
As Section 41.7 explains, secretion of H+ is essential to maintenance of the body’s acid–base balance.
Filtrate becomes concentrated as it flows through the descending part of the loop of Henle and loses water by osmosis. It then becomes less concentrated when salt is actively transported out in the ascending part of the loop. As a result, filtrate entering the distal tubule is less concentrated than normal body fluid. The distal tubule delivers filtrate to the collecting duct, which—like the descending loop of Henle— extends down into the medulla. In the deepest part of the medulla, urea is pumped out of the collecting duct, making the interstitial fluid nearby even saltier. As urine passes through the collecting duct, the increasing saltiness of the interstitial fluid around it favors flow of water out of the duct by osmosis. The body can adjust how much water is reabsorbed at distal tubules and collecting ducts. When it needs to conserve water, distal tubules and collecting ducts become more permeable to water, so less leaves in urine. When the body needs to rid itself of excess water, the distal tubule and collecting ducts become less permeable to water and the urine remains dilute. As the next section explains, hormones adjust the permeability of the distal tubule and collecting duct.
Concentrating the Urine Table 41.1
Sip soda all day and your urine will be dilute; sleep eight hours and it will be concentrated. Urine often has far more solutes than either plasma or most interstitial fluid. What concentrates the urine? Urine gets concentrated when water moves out of a nephron by osmosis. For urine to become concentrated, interstitial fluid surrounding the nephron must be saltier than the filtrate inside it. Only in the renal medulla does an outward-directed solute concentration gradient form, with the interstitial fluid saltiest deep in the medulla. This concentration gradient is established as filtrate flows though the loop of Henle which extends into the medulla. The loop’s two arms are close together and differ in permeability:
Na+ Cl– H2O
renal medulla saltiest near turn
D The ascending limb of the loop of Henle actively pumps out salt, but is not permeable to water. Pumping salt outward creates a concentration gradient, with the saltiest interstitial fluid in the deepest part of the medulla. The descending part of the loop is permeable to water, but not to salt. As filtrate flows through the loop, it first loses water by osmosis, then loses salt by active transport.
Processes of Urine Formation
Process
Characteristics
Glomerular filtration
Pressure generated by heartbeats drives water and small solutes (not proteins) out of leaky glomerular capillaries and into Bowman’s capsule, the entrance to the nephron.
Tubular reabsorption
Most water and solutes in the filtrate move from a nephron’s tubular portions, into interstitial fluid around the nephron, then into blood inside the peritubular capillaries.
Tubular secretion
Urea, H+, and some other solutes move out of peritubular capillaries, into interstitial fluid, then into the filtrate inside the nephron for excretion in urine.
Take-Home Message How does urine form and become concentrated? The force of the beating heart drives protein-free plasma out of glomerular capillaries and into the nephron’s tubular portion as filtrate. Nearly all of the water and solutes that leave the blood as filtrate later leave the tubule and return to the blood in peritubular capillaries. Water and solutes that remain in the tubule, and solutes secreted into the tubule along its length, become the urine. Concentration of urine as it flows down through the loop of Henle sets up a solute concentration gradient in the surrounding interstitial fluid of the renal medulla. The existence of this gradient allows urine to become concentrated as it flows through the collecting duct to the renal pelvis.
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41.6
Regulation of Water Intake and Urine Formation
When you do not drink enough fluid to make up for normal fluid losses, the concentration of sodium and other solutes in your blood rises. You make less saliva and your dry mouth stimulates nerve endings that signal the thirst center, a region of the hypothalamus. At the same time, the thirst center receives input from osmoreceptors that detect the level of solutes inside the brain (Section 34.1). The thirst center responds by notifying the cerebral cortex, which in turn compels you to search for and drink fluid. While thirst mechanisms call for uptake of water, hormonal controls act to conserve water already inside the body. The hormones exert their effects mainly at distal tubules and collecting ducts.
permeable to water. As a result, water moves out of the filtrate more freely, peritubular capillaries reabsorb more of it, and less departs in the urine (Figure 41.12). In time, solute levels decline because the volume of extracellular fluid rises, and ADH secretion slows. Other factors also stimulate ADH secretion. With heavy blood loss, receptors in the atria sense a decline in blood pressure and call for increased ADH. Stress, heavy exercise, or vomiting also cause internal changes that trigger a rise in ADH output. ADH increases water reabsorption by stimulating the insertion of proteins called aquaporins into the plasma membrane of the distal tubules and collecting ducts. An aquaporin is a porelike passive transport protein that selectively allows water to cross the membrane. When ADH binds to cells of distal tubules and collecting ducts, vesicles that hold aquaporin subunits move toward the cells’ plasma membrane. As these vesicles fuse with the plasma membrane, the subunits assemble themselves into functional aquaporins. Once in place, aquaporins facilitate the rapid flow of water out of the filtrate, back into the interstitial fluid.
Effects of Antidiuretic Hormone
Effects of Aldosterone
When internal sodium levels rise, the hypothalamus stimulates the pituitary gland to secrete antidiuretic hormone (ADH). ADH binds to cells of distal tubules and collecting ducts, and causes them to become more
Any decrease in the volume of extracellular fluid also activates some cells in the arterioles that deliver blood to the nephrons. These cells release renin, an enzyme that sets in motion a complex chain of reactions.
Urine consists of water and solutes that were filtered from the blood and not returned to it, along with solutes secreted from the blood into the nephron. Links to Hypothalamus 33.10, Osmoreceptors 34.1, Pituitary hormones 35.3, Adrenal glands 35.10
Regulating Thirst
hypothalamus
ADH alert!
Stimulus
Response
a Water loss lowers the blood volume Sensory receptors in the hypothalamus detect a big deviation from the set point.
f Sensory receptors in the hypothalamus detect the increase in blood volume. Signals calling for ADH secretion slow down.
b The hypothalamus stimulates the pituitary gland to step up its secretion of ADH. pituitary gland
c ADH circulates in blood, reaches nephrons in the kidneys. By acting on cells of distal tubules and collecting ducts, it makes the tube walls more permeable to water.
Figure 41.12 Feedback control of ADH secretion, one of the negative feedback loops from kidneys to the brain that helps adjust the volume of extracellular fluid. Nephrons in the kidneys reabsorb more water when we do not take in enough water or lose too much, as by profuse sweating.
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e The blood volume rises. d More water is reabsorbed by peritubular capillaries around the nephrons, so less water is lost in urine.
41.7 Renin coverts angiotensinogen, a protein secreted by the liver into the blood, into angiotensin I. Another enzyme converts angiotensin I to angiotensin II, which acts on the adrenal cortex. The cortex is the outer part of the adrenal gland that sits atop the kidney. The adrenal cortex responds to angiotensin II by secreting the hormone aldosterone into the blood. Aldosterone acts on the kidney’s collecting ducts, increasing the activity of sodium–potassium pumps so that more sodium gets reabsorbed. Water follows the sodium by osmosis, and the urine becomes more concentrated. Thus, both ADH and aldosterone cause urine to become more concentrated, although they do so by different mechanisms. Atrial natriuretic peptide (ANP) is a hormone that makes urine more dilute. Muscle cells in the heart’s atria release ANP when high blood volume causes the atrial walls to stretch. ANP directly inhibits secretion of aldosterone by acting on the adrenal cortex. It also acts indirectly by inhibiting renin release. In addition, ANP increases the glomerular filtration rate, so more fluid enters kidney tubules.
Hormonal Disorders and Fluid Balance
Acid–Base Balance
The kidneys help maintain the pH of body fluids. They are the only organs that can selectively rid the body of H+ ions.
Link to pH and buffer systems 2.6
Metabolic reactions such as protein breakdown and lactate fermentation add H+ to the extracellular fluid. Despite these continual additions, a healthy body can maintain its H+ concentration within a tight range; a state known as acid–base balance. Buffer systems, and adjustments to the activity of respiratory and urinary systems are essential to this balance. A buffer system involves substances that reversibly bind and release H+ or OH–. Such a system minimizes pH changes as acidic or basic molecules enter or leave a solution (Section 2.6). The pH of human extracellular fluid usually stays between 7.35 and 7.45. In the absence of any buffer, adding acids to ECF could make its pH decrease. But excess hydrogen ions react with buffers, including the bicarbonate–carbonic acid buffer system: _ H+ + HCO3 bicarbonate
H2CO3
CO2 + H2O
carbonic acid
The metabolic disorder diabetes insipidus arises if the pituitary gland secretes too little ADH, ADH receptors do not respond to ADH, or aquaporins are impaired or missing. A large volume of highly dilute urine, and unquenchable thirst are symptoms of the disorder. Some cancers, infections, and medications such as antidepressants, stimulate ADH oversecretion. With an excess of ADH, the kidneys retain too much water. Solute concentrations in the interstitial fluid decrease, which is bad news, especially for brain cells; they are highly sensitive to solute concentrations. If untreated, ADH oversecretion can be fatal. An adrenal gland tumor may cause oversecretion of aldosterone, or hyperaldosteronism. The excess of aldosterone causes fluid retention, which can increase the blood pressure to dangerous levels.
Adjustments in the rate and depth of breaths help offset changes in pH. When the blood pH decreases, breathing quickens and deepens, so CO2 is expelled faster than it forms. As you can tell from the equation above, less CO2 means less carbonic acid can form, so the pH rises. Slower, shallower breathing allows CO2 to accumulate, so more carbonic acid can form. Control of bicarbonate reabsorption and secretion of H+ can adjust the pH inside kidneys. Reabsorbed bicarbonate moves into peritubular capillaries, where it buffers excess acid. H+ secreted into tubule cells combines with phosphate or ammonia ions and forms compounds that are excreted in the urine. When the kidney’s secretion of H+ falters, or excess + H is formed by metabolic reactions, or not enough bicarbonate is reabsorbed, the pH of body fluids can fall below 7.1, a condition called acidosis.
Take-Home Message
Take-Home Message
How do hormones affect urine concentration?
What mechanisms maintain the pH of the extracellular fluid?
Antidiuretic hormone released by the pituitary causes an increase in water reabsorption. It concentrates the urine. Aldosterone released by the adrenal cortex increases salt reabsorption, and water follows. It concentrates the urine. Atrial natriuretic peptide released by the heart discourages the secretion of aldosterone, increases the rate of glomerular filtration, and thus makes urine more dilute.
The kidneys, buffering systems, and the respiratory system work together to tightly control the acid–base balance of extracellular fluid. By reversible reactions, a bicarbonate–carbonic acid buffer system neutralizes excess H+. Shifts in the rate and depth of breathing affect this buffer system, and thus can alter the pH of blood.
The kidneys also can shift the pH of blood when they adjust bicarbonate reabsorption and H+ secretion.
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FOCUS ON HEALTH
41.8
When Kidneys Fail Kidney failure can be treated with dialysis, but only a kidney transplant can fully restore function.
Link to High-protein diets 40.8
Causes of Kidney Failure The vast majority of kidney problems occur as complications of diabetes mellitus or high blood pressure. These disorders damage small blood vessels, including capillaries that interact with nephrons. Some people are genetically predisposed to infections or conditions that damage kidneys. Kidneys also fail after filtering lead, arsenic, pesticides, or other toxins from the blood. Occasionally, repeated high doses of aspirin or other drugs irreversibly damage the kidneys. High-protein diets force the kidneys to work overtime to dispose of nitrogen-rich breakdown products (Section 40.8). Such diets also increase the risk for kidney stones. These hardened deposits form when uric acid, calcium, and other wastes settle out of urine and collect in the renal pelvis. Most kidney stones are washed away in urine, but sometimes one becomes lodged in a ureter or the urethra and causes severe pain. Any stone that blocks urine flow raises risk of infections and permanent kidney damage.
filter where blood flows through semipermeable tubes and exchanges substances with dialysis solution abdominal cavity, lined with peritoneum (green)
dialysis solution flowing into abdominal cavity
dialysis solution with unwanted wastes and solutes draining out
patient’s blood inside tubing
A Hemodialysis Tubes carry blood from a patient’s body through a filter with dialysis solution that contains the proper concentrations of salts. Wastes diffuse from the blood into the solution and cleansed, solute-balanced blood returns to the body.
B Peritoneal dialysis Dialysis solution is pumped into a patient’s abdominal cavity. Wastes diffuse across the lining of the cavity into the solution, which is then drained out.
Figure 41.13
Two types of kidney dialysis.
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We usually measure kidney function in terms of the rate of filtration through glomerular capillaries. Kidney failure occurs when the filtration rate falls by half, regardless of whether it is caused by low blood flow to the kidneys or by damaged tubules or blood vessels. Kidney failure can be fatal. Wastes build up in the blood and interstitial fluid. The pH rises and changes in the concentrations of other ions, most notably Na+ and K+, interfere with metabolism.
Kidney Dialysis Kidney dialysis is used to restore proper solute balances in a person with kidney failure. “Dialysis” refers to exchanges of solutes across a semipermeable membrane between two solutions. With hemodialysis, a dialysis machine is connected to a patient’s blood vessel (Figure 41.13a). The machine pumps a patient’s blood through semipermeable tubes submerged in a warm solution of salts, glucose, and other substances. As the blood flows through the tubes, wastes dissolved in the blood diffuse out and solute concentrations return to normal levels. Cleansed, solute-balanced blood is returned to the patient’s body. Typically a person has hemodialysis three times a week at an outpatient dialysis center. Each treatment takes several hours. Peritoneal dialysis can be done at home. Each night, dialysis solution is pumped into a patient’s abdominal cavity (Figure 41.13b). Wastes diffuse across the peritoneal lining into the fluid, which is drained out the next morning. Thus this body lining serves as the dialysis membrane. Kidney dialysis can keep a person alive through an episode of temporary kidney failure. When kidney damage is permanent, dialysis must be continued for the rest of a person’s life, or until a donor kidney becomes available for transplant surgery. Kidney Transplants Each year in the United States, about 12,000 people are recipients of kidney transplants. More than 40,000 others remain on a waiting list because there is a shortage of donated kidneys. The National Kidney Foundation estimates that every day, 17 people die of kidney failure while waiting for a transplant. Most kidneys used as transplants come from people who had arranged to be organ donors after their death. However, an increasing number of kidneys are removed from a living donor, most often a relative. A kidney transplant from a living donor has a better chance of success than one from a deceased person. One kidney is adequate to maintain good health, so the risks to a living donor are mainly related to the surgery—unless a donor’s remaining kidney fails. The benefits of organs from living donors, a lack of donated organs, and high dialysis costs have led some to suggest that people should be allowed to sell a kidney. Critics argue that it is unethical to tempt people to risk their health for money. Section 16.8 describes another potential alternative. Some day, genetically modified pigs may become organ factories.
41.9
Heat Gains and Losses
Maintaining the body’s core temperature is another aspect of homeostasis. Some animals expend more energy than others to keep their body warm.
Links to Properties of water 2.5, Forms of energy 6.1
How the Core Temperature Can Change Metabolic reactions release heat, so the heat generated by metabolism affects the temperature of an animal’s internal core. Animals also gain heat from, and lose heat to, their surroundings. An animal’s internal core temperature is stable only when the metabolic heat produced and the heat gained from the environment balance any heat losses to the environment: change in body heat
=
heat + heat – produced gained
heat lost
Heat is gained or lost at body surfaces by radiation, conduction, convection, and evaporation. Thermal radiation is emission of heat from a warm object into the space around it. Just as the sun radiates heat energy into space, an animal radiates metabolically produced heat. A typical human at rest gives off about as much heat as a 100-watt light bulb. In conduction, heat is transferred within an object or among objects that contact one another. An animal loses heat when it contacts a cooler object, and gains heat when it contacts a warmer one. In convection, heat is transferred by the movement of heated air or water away from the source of heat. As air or water heats up, it rises and moves away from the object, such as a body, that warmed it. In evaporation, heat energy converts a liquid into a gas, a process that cools any remaining liquid (Section 2.5). When that liquid is water at a body surface, this cooling helps decrease body temperature. Evaporative cooling is most effective with dry air and a breeze; high humidity and still air slow it.
Endotherms, Ectotherms, and Heterotherms Fishes, amphibians, and reptiles are ectotherms, which means that they are “heated from the outside”; their body temperature fluctuates with the temperature of their environment. Ectotherms typically have a low metabolic rate and little insulation; they lack fur, hair, or feathers. They regulate their internal temperature by altering their position, rather than their metabolism. A rattlesnake (Figure 41.14a) is an example. When its body is cold, the snake basks in the sun. When the snake becomes too hot, it moves into shade.
a
b
Figure 41.14 (a) Sidewinder, an ectotherm. (b) Pine grosbeak, an endotherm, using fluffed feathers as insulation against winter cold.
Most birds and mammals are endotherms, which means “heated from within.” Compared to ectotherms, endotherms have relatively high metabolic rates. For example, a mouse uses thirty times more energy than a lizard of the same body weight. An ability to produce a large amount of metabolic heat helps endotherms remain active in a wider range of temperatures than ectotherms. Fur, hair, or feathers insulate endotherms and minimize heat transfers (Figure 41.14b). Some birds and mammals are heterotherms. They can maintain a fairly constant core temperature some of the time, but let it shift at other times. For example, hummingbirds have a very high metabolic rate when foraging for nectar during the day. At night, metabolic activity decreases so much that the bird’s body may become almost as cool as the surroundings. Warm climates favor ectotherms, which do not have to spend as much energy as endotherms do to maintain core temperature. Thus, in tropical regions, reptiles exceed mammals in numbers and diversity. In all cool or cold regions, however, most vertebrates tend to be endotherms. About 130 species of mammals and 280 species of birds occur in the arctic, but fewer than 5 species of reptiles are native to this region.
Take-Home Message How do animals regulate their body temperature? Animals can gain heat from the environment, or lose heat to it. They can also generate heat by metabolic reactions.
Fishes, amphibians, and reptiles are ectotherms that warm themselves mostly by heat gained from the environment. Birds and mammals are endotherms that maintain body temperature with their own metabolic heat.
CHAPTER 41
MAINTAINING THE INTERNAL ENVIRONMENT 733
41.10 Temperature Regulation in Mammals A variety of mechanisms help mammals keep their core temperature from fluctuating with that of the environment. Links to Feedback control of temperature 27.3, Heat illness 27.4, Sweat glands 32.7, Fever 38.4
The hypothalamus is the main regulatory center for the control of mammalian body temperature. This brain region receives signals from thermoreceptors (Section 34.1) in the skin, as well as from others located deep inside the body. When the temperature deviates from a set point, the hypothalamus integrates the responses of skeletal muscles, smooth muscle of arterioles in the skin, and sweat glands. Negative feedback loops back to the hypothalamus inhibit the responses when core temperature returns to the set point (Section 27.3). Most mammals maintain body temperature within a few degrees. Dromedary camels are an exception; they can adjust their hypothalamic set point (Figure 41.15). Over the course of a day, their body temperature can vary from 34°C to 41.7°C (93°F to 107°F).
Responses to Heat Stress When any mammal becomes too hot, the temperature control centers in the hypothalamus issue commands that widen the diameter of blood vessels in the skin. Increased blood flow to the skin delivers more metabolic heat to the body surface, where it can be given up by radiation to the surroundings (Table 41.2). Another response to heat stress, evaporative heat loss, occurs at moist respiratory surfaces and across skin. Animals that sweat lose some water this way. For instance, humans and some other mammals have sweat glands that release water and solutes through pores at the skin’s surface (Section 32.7). An average human adult has more than 2 million sweat glands.
For each liter of sweat produced, the body loses about 600 kilocalories of heat energy through evaporative cooling. During strenuous exercise, sweating helps the body rid itself of the extra heat produced by increased metabolic activity of skeletal muscles. Sweat dripping from skin dissipates little heat. The body cools greatly when sweat evaporates. On humid days, the air’s high water content slows evaporation, so sweating is less effective at cooling the body. Not all mammals sweat. Many drool, lick their fur, or pant to speed cooling. “Panting” refers to shallow, rapid breathing. It assists evaporative water loss from the respiratory tract, nasal cavity, mouth, and tongue. Sometimes peripheral blood flow and evaporative heat loss cannot counter heat stress, so the body’s core temperature increases above normal, a condition called hyperthermia. In humans, an increase in core temperature above 40.6°C (105°F) is dangerous (Section 27.4). A fever is an increase in body temperature that most often occurs as a response to infection (Section 38.4). Chemicals released by an infectious agent or by white blood cells that detect the infection influence the hypothalamus. In response to these chemicals, the hypothalamus allows the core temperature to rise a bit above normal set point. Increased temperature makes the body less hospitable for pathogens and encourages immune responses. Generally, the hypothalamus does not let the core temperature rise above 41.5°C (105°F). When a fever exceeds that point or lasts more than a few days, the condition causing it is life threatening and medical evaluation is essential.
Responses to Cold Stress A mammal responds to the cold by redistributing its blood flow, fluffing its hair or fur, and shivering.
Table 41.2 Figure 41.15 Shortterm adaptation to desert heat stress. Dromedary camels let their core temperature rise during the hottest hours of the day. A hypothalamic mechanism adjusts their internal thermostat, so to speak. By allowing their temperature set point to rise, camels minimize their sweat production and thus can conserve water.
734 UNIT VI
Stimulus Heat stress
Cold stress
HOW ANIMALS WORK
Heat and Cold Stress Compared Main Responses
Outcome
Widening of blood vessels in skin; behavioral adjustments; in some species, sweating, panting
Dissipation of heat from body
Decreased muscle action
Heat production decreases
Narrowing of blood vessels in skin; behavioral adjustments (e.g., minimizing surface parts exposed)
Conservation of body heat
Increased muscle action; shivering; nonshivering heat production
Heat production increases
Figure 41.16 Two responses to cold stress. (a) Polar bears (Ursus maritimus, “bear of the sea”). A polar bear is active even during severe arctic winters. It does not get too chilled after swimming because the coarse, hollow guard hairs of its coat shed water quickly. Thick, soft underhair traps heat. An insulating layer of brown adipose tissue about 11.5 centimeters (4–1/2 inches) thick helps generate metabolic heat. (b) In 1912, the Titanic collided with an immense iceberg on her maiden voyage. It took about 2–1/2 hours for the Titanic to sink and rescue ships arrived in less than two hours. Even so, 1,517 people died. Many died in lifeboats or while afloat in life vests. Hypothermia killed them.
Thermoreceptors in the skin signal the hypothalamus when conditions get chilly. The hypothalamus then causes smooth muscle in arterioles that deliver blood to the skin to constrict. For example, when your fingers or toes are chilled, all but 1 percent of the blood that would usually flow to skin is diverted to other body regions. Constriction of arterioles that supply the skin lessens movement of metabolic heat to the body surface, where it would be lost to the surroundings. As another response to cold, reflex contractions of smooth muscle in the skin cause fur (or hair) to “stand up.” This response creates a layer of still air next to skin, thus reducing heat lost by convection and thermal radiation. Minimizing exposed body surfaces can also prevent heat loss, as when polar bear cubs curl up and cuddle against their mother (Figure 41.16a). With prolonged cold exposure, the hypothalamus commands skeletal muscles to contract ten to twenty times each second. Although this shivering response increases heat production, it has a high energy cost. Long-term or severe cold exposure also leads to an increase in thyroid activity that raises the rate of metabolism. Thyroid hormone binds to cells of brown adipose tissue, causing nonshivering heat production. By this process, mitchondria in cells of brown adipose tissue carry out reactions that release energy as heat, rather than storing it in ATP. Brown adipose tissue occurs in mammals that live in cold regions and in the young of many species. In human infants, this tissue makes up about 5 percent of body weight. Unless cold exposure is ongoing, the tissue disappears after childhood ends. Failure to protect against cold causes hypothermia, a condition in which the core temperature falls. In humans, a decline to 35°C (95°F) alters brain functions. “Stumbles, mumbles, and fumbles” are said to be the symptoms of early hypothermia. Severe hypothermia causes loss of consciousness, disrupts heart rhythm, and can be fatal (Figure 41.16b and Table 41.3).
a
b
Table 41.3
Impact of Increases in Cold Stress
Core Temperature
Physiological Responses
36°–34°C (about 95°F)
Shivering response; faster breathing, metabolic heat output. Peripheral vasoconstriction, more blood deeper in body. Dizziness, nausea.
33°–32°C (about 91°F)
Shivering response ends. Metabolic heat output declines.
31°–30°C (about 86°F)
Capacity for voluntary motion is lost. Eye and tendon reflexes inhibited. Consciousness lost. Cardiac muscle action becomes irregular.
26°–24°C (about 77°F)
Ventricular fibrillation sets in (Section 37.9). Death follows.
Take-Home Message How do mammals maintain their body temperature? Temperature shifts are detected by thermoreceptors that send signals to an integrating center in the hypothalamus. This center serves as the body’s thermostat and calls for adjustments that maintain core temperature.
Mammals respond to cold with reduced blood flow to skin, fluffing up of fur or hair, increased muscle activity, shivering, and nonshivering heat production. Mammals counter heat stress by increasing blood flow to the skin, sweating and panting, and by reducing their activity level.
CHAPTER 41
MAINTAINING THE INTERNAL ENVIRONMENT 735
IMPACTS, ISSUES REVISITED
Truth in a Test Tube
Solutes and nutrients the body needs are reabsorbed from the filtrate that enters kidney tubules. Water-soluble drugs and toxins are generally not reabsorbed, so they end up in the urine. How quickly the kidneys clear a substance from the blood depends in part on the efficiency of the kidneys, which can vary with age and health. A healthy 35-year-old eliminates drugs from the body about twice as fast as a healthy 85-year-old.
How would you vote? Should employers be allowed to require potential employees to pass a urine test as a condition of employment? See CengageNOW for details, then vote online.
Summary Section 41.1 Plasma and interstitial fluid are the main components of extracellular fluid. Maintaining the volume and composition of extracellular fluid is an essential aspect of homeostasis. All organisms balance solute and fluid gains with solute and fluid losses, and all eliminate metabolic wastes. Most have excretory organs that rid the body of ammonia and other unwanted solutes. Section 41.2 Sponges are simple animals in which excretion occurs at a cellular level. In freshwater sponges and other freshwater animals, water flows into the body by osmosis. Like some protists, sponge cells eliminate excess water using organelles called contractile vacuoles. In flatworms, the action of ciliated flame cells draws interstitial fluid into a system of tubes that delivers it to the body surface. Earthworms have excretory organs called nephridia that take up coelomic fluid, and deliver wastes to a pore at the body surface. In insects and spiders, Malpighian tubules take up fluid, uric acid, and solutes from the blood and deliver them to the gut. Uric acid is formed from ammonia, but requires less water to be excreted. Section 41.3 Vertebrates have a urinary system that interacts with other organ systems in homeostasis. A pair of kidneys filter water and solutes from their blood. Cartilaginous fishes retain urea in their body, so they do not lose or gain water by osmosis. Marine bony fish constantly gain water by osmosis, while those that live in fresh water lose water by osmosis. On land, the main challenge is avoiding dehydration. Birds and reptiles save water by eliminating nitrogenrich wastes as uric acid crystals. Mammals excrete urea, which must be dissolved in a lot of water. Section 41.4 The human urinary system consists of two kidneys, a pair of ureters, a urinary bladder, and the urethra. Kidney nephrons are small, tubular structures that interact with nearby capillaries to form urine. Each nephron starts as Bowman’s capsule in a kidney’s outer region, or renal cortex. The nephron continues as a proximal tubule, a loop of Henle that descends into and ascends from the renal medulla, and a distal tubule that drains into a collecting duct. Bowman’s capsule and capillaries of the glomerulus that it cups around serve as a blood-filtering unit. Most filtrate that enters Bowman’s capsule is reabsorbed into 736 UNIT VI
HOW ANIMALS WORK
peritubular capillaries around the nephron. The portion of the filtrate not returned to blood is excreted as urine.
Use the animation on CengageNOW to explore the anatomy of the human urinary system.
Sections 41.5, 41.6 Blood pressure drives glomerular filtration, which puts protein-free plasma into the kidney tubules. Most water and solutes return from these tubules to the blood by tubular reabsorption. Substances move from the blood into the tubules by tubular secretion. A part of the hypothalamus serves as a thirst center. The hypothalamus signals the pituitary gland to release antidiuretic hormone, which increases the reabsorption of water. Aldosterone, a hormone secreted by the adrenal cortex, increases sodium reabsorption. Both antidiuretic hormone and aldosterone concentrate the urine. Atrial natriuretic peptide, a hormone made by the heart, slows secretion of aldosterone and makes urine more dilute.
Use the animation on CengageNOW to learn about the three processes of urine formation.
Section 41.7 The urinary system helps regulate acid– base balance by eliminating H+ in urine and reabsorbing bicarbonate, which has a role in the main buffer system. Section 41.8 When kidneys fail, frequent dialysis or a kidney transplant is required to sustain life. Section 41.9 Animals produce metabolic heat. They also gain or lose heat by thermal radiation, conduction, and convection; and lose it by evaporation. Ectotherms such as reptiles control core temperature mainly by behavior. Endotherms (most mammals and birds) regulate temperature mainly by controlling production and loss of metabolic heat. Heterotherms control core temperature only part of the time. Section 41.10 In mammals, the hypothalamus is the main center for temperature control. A fever is elevation of body temperature as a defensive response to infection. Widening of blood vessels in the skin, sweating, and panting are responses to heat. Mammals alone can sweat, but not all mammals have this ability. Exposure to cold causes constriction of blood vessels in skin, makes hair (or fur) stand upright, and elicits a shivering response. Long-term exposure to cold can alter metabolism and encourage nonshivering heat production, in which brown adipose tissue produces heat.
Data Analysis Exercise Products labeled as “organic” fill an increasing amount of space on supermarket shelves. What does this label mean? A food that carries the USDA’s organic label must be produced without pesticides such as malathion and chlorpyrifos, which conventional farmers typically use on fruits, vegetables, and many grains. Does eating organic food significantly affect the level of pesticide residues in a child’s body? Chensheng Lu of Emory University used urine testing to find out (Figure 41.17). For fifteen days, the urine of twenty-three children (aged 3 to 11) was monitored for breakdown products of pesticides. During the first five days, children ate their standard, nonorganic diet. For next five days, they ate organic versions of the same types of foods and drinks. Then, for the final five days, the children returned to their nonorganic diet. 1. During which phase of the experiment did the children’s urine contain the lowest level of the malathion metabolite? 2. During which phase of the experiment was the maximum level of the chlorpyrifos metabolite detected? 3. Did switching to an organic diet lower the amount of pesticide residues excreted by the children?
Self-Quiz
Malathion Metabolite Study Phase 1. Nonorganic
No. of Samples
Mean (µg/liter)
Chlorpyrifos Metabolite
Maximum (µg/liter)
Mean Maximum (µg/liter) (µg/liter)
87
2.9
96.5
7.2
31.1
2. Organic
116
0.3
7.4
1.7
17.1
3. Nonorganic
156
4.4
263.1
5.8
25.3
Figure 41.17 Above, levels of metabolites (breakdown products) of malathion and chlorpyrifos detected in the urine of children taking part in a study of the effects of an organic diet. The difference in the mean level of metabolites in the organic and inorganic phases of the study was statistically significant. Right, the USDA organic food label.
4. Even in the nonorganic phases of this experiment, the highest pesticide metabolite levels detected were far below those known to be harmful. Given this data, would you spend more to buy organic foods?
2. Body fluids of a marine bony fish have a solute concentration that is its surroundings. a. higher than b. lower than c. equal to
10. Match each structure with a function. ureter a. start of nephron Bowman’s capsule b. delivers urine to urethra body surface collecting duct c. carries urine from pituitary kidney to bladder gland d. secretes ADH e. target of aldosterone
3. Bowman’s capsule, the start of the tubular part of a nephron, is located in the . a. renal cortex c. renal pelvis b. renal medulla d. renal artery
11. The main control center for maintaining the temperature of the mammalian body is in the a. anterior pituitary c. adrenal gland b. renal cortex d. hypothalamus
4. Fluid that enters Bowman’s capsule flows directly into the . a. renal artery c. distal tubule b. proximal tubule d. loop of Henle
12. An animal with a low metabolism that maintains its temperature mainly by adjusting its behavior is . a. an endotherm b. an ectotherm
1. An insect’s a. nephridia b. nephrons
Answers in Appendix III deliver nitrogen wastes to its gut. c. Malpighian tubules d. contractile vacuoles
5. Blood pressure forces water and small solutes into Bowman’s capsule during . a. glomerular filtration c. tubular secretion b. tubular reabsorption d. both a and c 6. Kidneys return most of the water and small solutes back to blood by way of . a. glomerular filtration c. tubular secretion b. tubular reabsorption d. both a and b 7. ADH binds to receptors on distal tubules and collecting ducts, making them permeable to . a. more; water c. more; sodium b. less; water d. less; sodium 8. Increased sodium reabsorption a. will make urine more concentrated b. will make urine more dilute c. is stimulated by aldosterone d. both a and c
.
9. True or false? Increased secretion of H+ into kidney tubules helps lower the pH of the blood.
.
13. True or false? Exposure to cold increases blood flow to your skin, thus warming the skin.
Visit CengageNOW for additional questions.
Critical Thinking 1. The kangaroo rat kidney efficiently excretes a very small volume of urine (Section 41.3). Compared to a human, its nephrons have a loop of Henle that is proportionally much longer. Explain how a long loop helps the rat conserve water. 2. In cold habitats, ectotherms are few and endotherms often show morphological adaptations to cold. Compared to closely related species that live in warmer areas, cold dwellers tend to have smaller appendages. Also, animals adapted to cool climates tend to be larger than relatives in warmer places. The largest bear is the polar bear and the largest penguin is Antarctica’s emperor penguin. Think about heat transfers between animals and their habitat, then explain why smaller appendages and larger overall body size are advantageous in very cold climates. CHAPTER 41
MAINTAINING THE INTERNAL ENVIRONMENT 737
42
Animal Reproductive Systems IMPACTS, ISSUES
Male or Female? Body or Genes?
Athlete Santhi Soundarajan was born in a rural area in India
selves as women, but had a Y chromosome. In the 1996
in 1981. She overcame poverty and malnutrition to become a
Summer Olympics, 8 of 3,387 women athletes tested positive
competitive runner, and in 2006 she represented her country
for an SRY gene. Further tests revealed that each of them
in the Pan Asian Games (Figure 42.1). She won a silver
had some sort of genetic abnormality. Because these genetic
medal, but her triumph was short-lived. A few days after the
conditions prevented testosterone from exerting muscle-
close of the games, the Olympic Council of Asia announced
building effects, the women were not considered to have any
that Soundarajan had been stripped of her medal. Although
unfair advantage and they were allowed to compete.
she had been raised as a female, she has a Y chromosome, rather than a typical woman’s two X chromosomes. The International Olympics Committee (IOC) began a
The IOC and most other groups that govern competitive athletic events have now banned gender testing. They did so in response to geneticists and physicians who spoke
program of gender testing in 1968. At first, they required that
out against the practice. These professionals argued that
athletes “prove” their femaleness by undergoing a physical
disqualifying athletes on the basis of such tests is a form of
exam. In the early 1970s, the committee switched to a less
discrimination that can cause great hardship to athletes with
intrusive method. Experts examined a few of an athlete’s cells
genetic abnormalities.
under a microscope for evidence of two X chromosomes. In
Generally, when a child is born, a quick look at their
1992 the committee upgraded its methods again, this time
genitals (external sex organs) reveals their sex. Males have
to a test that detects the SRY gene. SRY is the gene on the Y
a penis; females, a vagina. Chromosomal sex (XX or XY)
chromosome that normally causes development of testes in a
determines which gonads (ovaries or testes) form. Hormones
human XY embryo (Section 12.1).
secreted by the gonads then shape the genitals and other
The Olympic testing program did not turn up any men
phenotypic aspects of sex. However, mutations can result in
deliberately pretending to be women. It did detect athletes
ambiguous genitals. A boy may be born with a tiny penis and
who had been brought up as females and thought of them-
with testes deep within his abdomen. Or, a girl may have a large clitoris and no opening to her vagina. In other cases, a child who has typical female genitals is actually a genetic male whose body either does not make or does not respond to the male sex hormone testosterone. Such a female lacks ovaries and a uterus, so she will not menstruate, but in terms of her body shape and strength, she is typically female. Such intersex conditions challenge our thinking about what it means to be male or female. In the United States, children who have unusual genitals have traditionally been operated on within their first year to make them appear as normal as possible. Sometimes the best cosmetic outcome is obtained by assigning a child to the opposite genetic sex. Some doctors and some intersex individuals who underwent genital surgery as infants now argue against early surgery. They advocate accepting a child’s unusual appearance and putting off any surgery until after puberty. Postponing surgery until this time allows affected individuals to make their own decision about what type of surgery, if any, they want to have. In this chapter and the next, we consider the structure of reproductive systems and their normal function. Unlike the other organ systems, a reproductive system is not necessary
Figure 42.1 Indian athlete Santhi Soundarajan rests on the track after the 800-meter race for which she won a silver medal at the Pan Asian Games in 2006. She lost the medal after testing indicated that she has a Y chromosome.
for an individual’s survival. It is, however, the key to passing on genes and thus to ensuring the survival of one’s lineage. In humans, it is also an important component of our self-identity.
Links to Earlier Concepts
Key Concepts Modes of animal reproduction
Section 10.1 introduced the concepts of sexual and asexual reproduction, which we expand upon here. Gamete formation (10.5) is also explained in more detail.
This chapter draws upon your knowledge of human sex determination (12.1), and revisits the subject of prenatal diagnosis (12.8).
You will learn more about how the hypothalamus and pituitary (35.3) affect sexual organs, and about the sex hormones (35.12). You will also see how the autonomic nervous system (33.8) affects intercourse.
In considering reproductive health, we revisit tumors (9.5) and effects of prostaglandins (35.1). We end the chapter with a look at the infectious diseases (21.8) that are transmitted sexually, including AIDS (Chapter 21 introduction, 21.2).
Some animals reproduce asexually, but sexual reproduction predominates in most animals. Some sexual reproducers make both eggs and sperm, but most are either male or female. Living on land favored fertilization of eggs inside the female body. Section 42.1
Male reproductive function A human male has a pair of testes that make sperm and secrete the sex hormone testosterone. Sperm mixes with secretions from other glands and leaves the body through ducts. Sections 42.2, 42.3
Female reproductive function A human female has a pair of ovaries that produce eggs and sex hormones. An approximately monthly hormonal cycle causes release of eggs. Ducts carry eggs toward the uterus, where offspring develop. The vagina receives sperm and is the birth canal. Sections 42.4–42.7
Intercourse and fertilization Sexual intercourse requires coordinating nervous and hormonal signals. It can lead to pregnancy, which humans use a variety of methods to prevent, promote, or terminate. Sections 42.8, 42.9
Sexually-transmitted diseases A variety of pathogens make their home in the human reproductive tract. They are passed between partners by sexual interactions and may be transmitted to offspring during childbirth. Effects of sexually transmitted diseases range from discomfort to death. Section 42.10
How would you vote? Children born with intersex disorders have traditionally had surgery early in life. Some people think such surgery should be delayed until after puberty so a child can choose or reject it. Would you delay surgery if your child was thus affected? See CengageNOW for details, then vote online.
739
42.1
Modes of Animal Reproduction Sexual reproduction dominates the life cycle of most animals, including many that can also reproduce asexually.
Link to Sexual and asexual reproduction 10.1
Asexual Reproduction in Animals With asexual reproduction, a single individual makes offspring that are genetically identical to it, so a parent has all its genes represented in each offspring. Asexual reproduction can be advantageous in a stable environment. Gene combinations that make the parent successful can be expected to do the same for offspring. Many invertebrates reproduce asexually. Some can reproduce by fragmentation—a piece breaks off and grows into a new individual. New hydras bud from existing ones (Figure 42.2a). Some insects and rotifers produce offspring from unfertilized eggs, a process called parthenogenesis. Most animals that reproduce asexually can also switch to sexual reproduction. Among the vertebrates, some fishes, amphibians, and lizards can form offspring from unfertilized eggs. However, no mammals reproduce asexually.
Costs and Benefits of Sexual Reproduction With sexual reproduction, two parents make gametes that combine at fertilization to produce offspring with gene combinations unlike either parent (Section 10.4).
b
a
Sexual reproducers incur higher genetic and energetic costs than asexual reproducers. On average, only half of a sexually reproducing parent’s genes end up in each offspring. Producing gametes, and finding and courting an appropriate mate also has costs. What benefits offset these costs? Most animals live where resources and threats change over time. In such environments, production of offspring that differ from both parents and from one another can be advantageous. By reproducing sexually, a parent increases the likelihood that some of its offspring will have a gene combination that suits them to their new environment.
Variations on Sexual Reproduction Some animals that reproduce sexually produce both eggs and sperm; they are hermaphrodites. Tapeworms and some roundworms are simultaneous hermaphrodites. They produce eggs and sperm at the same time, and can fertilize themselves. Earthworms and slugs are simultaneous hermaphrodites too, but they require a partner. So do hamlets, a type of marine fish (Figure 42.2b). During a bout of mating, hamlet partners take turns in the “male” and “female” roles. Other fishes are sequential hermaphrodites. They switch from one sex to another during the course of a lifetime. More typically, vertebrates have separate sexes that remain fixed for life; an individual is either male or female.
c
Figure 42.2 Examples of animal reproduction. (a) A hydra reproducing asexually by budding. (b) Barred hamlets mating. The fish are hermaphrodites that fertilize eggs externally. During mating, each fish alternates between laying eggs and fertilizing its partner’s eggs. (c) A male elephant inserting his penis into his female partner. The eggs will be fertilized and the offspring will develop inside the mother’s body, nourished by nutrients delivered by her bloodstream.
740 UNIT VI
HOW ANIMALS WORK
a
b
c
d
Figure 42.3 A look at where some invertebrate and vertebrate embryos develop, how they are nourished, and how (if at all) parents protect them. (a) Most snails lay eggs and abandon them. (b) Spider eggs develop in a silk egg sac. Females often die soon after they make the sac, but some species guard the sac, then cart spiderlings about for a few days while they feed them. (c) Ruby-throated hummingbirds and all other birds lay fertilized eggs with big yolk reserves. The eggs develop and hatch outside the mother. One or both parent birds typically expend energy feeding and caring for the young. (d) Embryos of some sharks, lizards, and snakes develop in their mother, receive nourishment continuously from yolk reserves, and are born in a well-developed state. Shown here, live birth of a lemon shark. Embryos of most mammals draw nutrients from maternal tissues and are born live. (e) In kangaroos and other marsupials, embryos are born “unfinished.” They complete embryonic development inside a pouch on the mother’s ventral surface. (f) Juveniles (joeys) continue to be nourished with milk secreted by mammary glands inside the pouch.
e
A human female (g) retains a fertilized egg inside her uterus. Her own tissues nourish the developing individual until birth.
Most aquatic invertebrates, fishes, and amphibians release gametes into the water, where they combine during external fertilization. Most land animals have internal fertilization; sperm and egg meet inside the female’s body. A specialized organ is typically used to deliver sperm. In mammals, a male’s penis serves this purpose (Figure 42.2c). Internally fertilized eggs may be laid outside the body and abandoned (Figure 42.3a), or a parent may lay and protect the eggs and later the young (Figure 42.3b,c). In other animals, offspring develop from eggs held inside the mother’s body (Figure 42.3d–g). Most female animals make some investment in yolk, a thick fluid rich in proteins and lipids that nourish the developing individual. The amount of yolk varies among species. Sea urchins release tiny eggs that hold little yolk. Not much yolk is needed because a fertilized sea urchin egg becomes a self-feeding, swimming larva in less than a day. In contrast, birds make eggs with a large quantity of yolk. Yolk is the embryo’s only nourishment during its time in an eggshell. Kiwi birds
f
g
have the longest incubation time, about 11 weeks, and their eggs have an unusually large amount of yolk. A typical bird’s egg is about one-third yolk; whereas the kiwi’s egg is two-thirds yolk. A human mother nourishes her offspring through nine months of development from a nearly yolkless, fertilized egg. Nutrients in the mother’s blood diffuse into an offspring’s blood and support its development (Figure 42.3g). You will learn more about how humans develop and nourish their young in Chapter 43.
Take-Home Message How do animal reproductive systems vary? Many invertebrates and some vertebrates can reproduce asexually. Most of these species can also reproduce sexually. Animals that reproduce sexually have genetically variable offspring. Sexual reproducers may produce eggs and sperm at the same time, produce both at different times in their life, or always produce only one or the other. Fertilization may occur in the mother’s body, or outside it. Internally fertilized eggs may be laid in the environment or develop in a mother’s body.
CHAPTER 42
ANIMAL REPRODUCTIVE SYSTEMS 741
42.2
Reproductive System of Human Males A human male’s reproductive system produces hormones and sperm, which it delivers to a female’s reproductive tract.
Table 42.1
Link to Human sex determination 12.1
Human Male Reproductive System
Reproductive organs Testes (2)
Sperm, sex hormone production
Epididymides (2)
Site of sperm maturation and subsequent storage
Vasa deferentia (2)
Rapid transport of sperm
Ejaculatory ducts (2)
Conduction of sperm to penis
Penis
Organ of sexual intercourse
Accessory glands Seminal vesicles (2)
Secrete most fluid in semen
Prostate gland
Secretes some fluid in semen
Bulbourethral glands (2)
Secrete a lubricating mucus
The Male Gonads Human gametes form in primary reproductive organs, or gonads. Males have a pair of gonads called testes (singular, testis) that produce sperm. Testes also make and secrete the sex hormone testosterone. In addition to gonads, the male reproductive system includes a system of ducts and accessory glands (Table 42.1 and Figure 42.4). Earlier, Figure 12.2 showed how two testes form on the wall of an XY embryo’s abdominal cavity. Before birth, testes descend into the scrotum, a pouch of loose skin suspended below the pelvic girdle. Inside this pouch, smooth muscle encloses the testes. Contraction and relaxation of this muscle in response to threats or temperature adjusts the position of the testes. When a man feels cold or afraid, reflexive muscle contractions draw his testes closer to his body. When he feels warm, relaxation of muscle in the scrotum allows his testes to hang lower, so the sperm-making cells do not overheat. These cells function best when they are just a bit below normal body temperature.
Prostate Gland
Ejaculatory Duct One pair of ducts that carry sperm to the penis
An exocrine gland that contributes some fluid to the semen
Seminal Vesicle One of a pair of exocrine glands that contributes fructose-rich fluid to semen
urinary bladder
Urethra Duct with dual functions; channel for ejaculation of sperm during sexual arousal and for excretion of urine at other times
Bulbourethral gland One of a pair of exocrine glands that secrete mucus anus Vas deferens One of a pair of ducts that carry sperm to the penis
scrotum
cylinders urethra of spongy tissue that swell with blood during an erection
Penis
Testis
Male organ of sexual intercourse
One of a pair of gonads, packed with small, spermproducing tubes (seminiferous tubules) and cells that secrete testosterone and other sex hormones
Figure 42.4 Animated Components of the human male reproductive system and their functions.
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HOW ANIMALS WORK
Epydidymis One of a pair of ducts in which sperm mature and are stored
A male enters puberty—the stage of development when reproductive organs mature—sometime between the ages of 11 and 16 years. Testes enlarge and sperm production begins. Secretion of testosterone increases and leads to development of secondary sexual traits: thickened vocal cords that deepen the voice; increased growth of hair on the face, chest, armpits, and pubic region; and an altered distribution of fat and muscle.
contributor to semen volume. Its secretions help raise the pH of the female reproductive tract, so sperm can swim more efficiently. Both seminal vesicles and the prostate gland also secrete prostaglandins, which are local signaling molecules. The two pea-sized bulbourethral glands secrete a lubricating mucus into the urethra. This mucus helps clear the urethra of urine prior to ejaculation.
Reproductive Ducts and Accessory Glands
Prostate and Testicular Problems
Sperm form by meiosis in the testes, a process we discuss in the next section. Here we consider the path sperm travel to the body surface. The journey begins when the immature, nonmotile sperm are pushed by cilia action from the testis into the epididymis (plural, epididymides), a coiled duct perched on a testis. The Greek epi– means upon and didymos means twins. In this context, the “twins” refers to the two testes. Secretions from the epididymis wall nourish the sperm and help them mature. The last region of each epididymis stores mature sperm and is continuous with the first portion of a vas deferens (plural, vasa deferentia). In Latin, vas means vessel, and deferens, to carry away. A vas deferens is a duct that carries sperm away from an epididymis, and to a short ejaculatory duct. Ejaculatory ducts deliver sperm to the urethra, the duct that extends through a male’s penis to open at the body surface. The penis is the male organ of intercourse and also has a role in urination. Beneath its outer layer of skin, connective tissue encloses three elongated cylinders of spongy tissue. When a male becomes sexually excited, nervous signals cause blood to flow into the spongy tissue faster than it flows out. As fluid pressure rises, the normally limp penis becomes erect. Sperm stored in the epididymides and first part of the vasa deferentia typically continue their journey toward the body surface only when a male reaches the peak of sexual excitement and ejaculates. During ejaculation, smooth muscle in the walls of the epididymides and vasa deferentia undergoes rhythmic contractions that propel sperm and accessory gland secretions out of the body as a thick, white fluid called semen. Semen is a complex mix of sperm, proteins, nutrients, ions, and signaling molecules. Sperm constitute less than 5 percent of semen’s volume; the bulk of it is secretions from accessory glands. Seminal vesicles, exocrine glands near the base of the bladder, secrete fructose-rich fluid into the vasa deferentia. Sperm use fructose (a sugar) as their energy source. The prostate gland, which encircles the urethra, is the other major
In a young, healthy man, the prostate gland is about the size of a walnut. However, inflammation or age can cause this gland to enlarge. Because the urethra runs through the prostate gland, even benign prostate enlargement can narrow this duct and cause difficulty urinating. Medication, laser treatments, and surgery are used to relieve symptoms. Prostate enlargement can be a symptom of prostate cancer. This cancer is a leading cause of death for men, surpassed only by lung cancers. In the United States, more than 200,000 men are diagnosed with prostate cancer each year, and about 35,000 die. Many prostate cancers grow relatively slowly, but some grow fast and spread easily to other parts of the body. Risk factors for prostate cancer include advancing age, a diet rich in animal fats, smoking, and a couch-potato life-style. Genes also play a role. If a man has an affected father or brother, his own risk of prostate cancer doubles. Doctors can diagnose prostate cancer by blood tests that detect increases in prostate-specific antigen (PSA) and by physical examination. Surgery and radiation therapy can cure cancers that are detected early. Testicular cancer is relatively rare, with 7,000 cases a year in the United States. Even so, it is the most common cancer among men aged 15 to 34. Once a month, after a warm shower or bath, a male should examine his testes for lumps, enlargement, or hardening. The treatment of testicular cancer is usually successful if the cancer is detected early, before it has spread.
Take-Home Message What are the functions of the male reproductive organs? A pair of testes, the primary reproductive organs in human males, produce sperm. They also make and secrete the sex hormone testosterone. Sperm and secretions from accessory glands form semen. During sexual arousal, semen is propelled through a series of ducts and leaves the body through an opening in the penis.
One accessory gland, the prostate, frequently becomes enlarged with age. It is also a common site for male cancers.
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ANIMAL REPRODUCTIVE SYSTEMS 743
42.3
Sperm Formation In his reproductive years, a male continually produces new germ cells, which undergo meiosis to produce sperm. Sperm formation is controlled by hormones.
Links to Gamete formation 10.5, Sex hormones 35.12
vas deferens
seminal vesicle prostate gland bulbourethral gland
urethra penis epididymis
seminiferous tubule testis a
mitosis
b
From Germ Cells to Mature Sperm Although smaller than a golf ball, a testis holds coiled seminiferous tubules that would extend for 125 meters (longer than a football field) if stretched out (Figure 42.5a). Leydig cells that cluster between these tubules secrete the hormone testosterone (Figure 42.5b). Male germ cells, or spermatogonia (singular, spermatogonium) line the inner wall of each semiferous tubule. In a sexually mature male, these diploid cells undergo mitosis again and again. With each division, the youngest descendants force older ones farther into the interior of the tubule. The displaced older cells are primary spermatocytes. Sertoli cells, another type of cell inside the tubules, provide metabolic support to the spermatocytes. Primary spermatocytes enter meiosis while they are being displaced—but their cytoplasm does not quite divide. Thin cytoplasmic bridges keep them connected to one another during the nuclear divisions. Signaling molecules cross the bridges freely and induce them to mature at the same rate. The completion of meiosis I yields two secondary spermatocytes (Figure 42.5c). These are haploid cells with duplicated chromosomes (Section 10.5). The sister chromatids of each chromosome will move apart during meiosis II, which produces immature sperm, or spermatids. As spermatids mature into sperm, the bridges of cytoplasm between them break down.
meiosis I
meiosis II
lumen
c
wall of seminiferous tubule
spermatogonium (diploid)
Leydig cells between tubules
primary spermatocyte Sertoli cell
secondary spermatocyte
Figure 42.5 Animated Where and how sperm form. (a) Male reproductive tract, posterior view. (b) Light micrograph of cells in three adjacent seminiferous tubules, cross-section. Testosteronesecreting Leydig cells, occupy spaces between the tubules. (c) Diploid germ cells (spermatogonia) line a seminiferous tubule. These cells undergo mitosis to form primary spermatocytes, which undergo meiosis to form spermatids. Spermatids mature into sperm.
744 UNIT VI
HOW ANIMALS WORK
early spermatids
immature sperm (haploid) late spermatid
a Level of testosterone in blood decreases; the hypothalamus secretes GnRH, a releasing hormone. (+ )
head, with DNA and a cap of enzymes
Figure 42.6 Structure of a mature sperm, a male gamete.
(– )
f Elevated level of testosterone in blood inhibits secretion of GnRH.
midpiece with mitochondria
tail, with its core of microtubules
A spermatozoan, or mature sperm, is a haploid, flagellated cell (Figure 42.6). A sperm uses its flagellum, or “tail,” to swim toward an egg. Mitochondria in the adjacent midpiece supply the energy required for flagellar movement. A sperm’s “head” is packed full of DNA and tipped by an enzyme-containing cap. The enzymes can help a sperm penetrate an oocyte by partly digesting away its outer layer. Sperm formation takes about 100 days, from start to finish. An adult male makes sperm on an ongoing basis, so that many millions of cells are in different stages of development on any given day.
Hypothalamus
(– )
(– )
g High sperm count induces Sertoli cells to secrete inhibin, which inhibits secretion of GnRH and LH.
Anterior Pituitary b GnRH stimulates secretion of LH, FSH from anterior lobe of pituitary. (+ )
Testes c LH prompts Leydig cells d Sertoli cells bind FSH and in testes to produce and testosterone, and function in release testosterone. spermatogenesis at puberty.
e Testosterone and secretions from Sertoli cells encourage sperm production.
Figure 42.7 Signaling pathways in sperm formation. Negative feedback loops control hormonal secretions of the hypothalamus, the anterior lobe of the pituitary gland, and the testes.
Hormonal Control of Sperm Formation Four hormones—GnRH, LH, FSH, and testosterone— are part of the signaling pathways that control sperm formation (Figure 42.7). Gonadotropin-releasing hormone (GnRH) is one of the hypothalamic hormones that targets the pituitary gland (Figure 42.7a and Section 35.3). GnRH stimulates anterior pituitary cells to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH) (Figure 42.7b). As you will learn, these two hormones have a role in both male and female reproductive function. In males, both LH and FSH target cells inside the testes. LH binds to the Leydig cells that lie in between the seminiferous tubules, stimulating them to secrete testosterone (Figure 42.7c). FSH targets Sertoli cells, inside seminiferous tubules. FSH, in combination with testosterone, prompts Sertoli cells to produce growth factors and other molecular signals (Figure 42.7d). These substances bathe neighboring male germ cells and encourage the development and maturation of sperm (Figure 42.7e).
A negative feedback loop regulates testosterone secretion and sperm formation. A high concentration of testosterone in the blood slows secretion of GnRH by the hypothalamus (Figure 42.7f ). The decrease in GnRH then lowers the output of LH and FSH by the testes. In addition, a high sperm count encourages the Sertoli cells to release the hormone inhibin (Figure 42.7g). Like testosterone, inhibin calls for a slowdown in GnRH and FSH secretion.
Take-Home Message How do sperm form and what role do hormones play?
Meiosis in germ cells in seminiferous tubules of the testes produces sperm— the haploid male gametes. Hormonal control of the process begins with GnRH from the hypothalamus. GnRH causes secretion of the hormones FSH and LH by the pituitary gland.
FSH and LH act on the testes, where they stimulate release of testosterone and other factors needed for formation and development of sperm.
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ANIMAL REPRODUCTIVE SYSTEMS 745
42.4
Reproductive System of Human Females The reproductive system of human females functions in the production of gametes and sex hormones. The system receives sperm, and has a chamber in which developing offspring are protected and nourished until birth.
Components of the System Figures 42.8 and 42.9 show the reproductive organs of a human female, and Table 42.2 lists their functions. The gonads are a pair of ovaries that produce oocytes (immature eggs) and secrete sex hormones on a cyclic
pelvic girdle
uterus
urinary bladder
ovary
vagina
Figure 42.8 Location of the human female reproductive system relative to the pelvic girdle and the urinary bladder.
basis. Upon its release, an oocyte enters into one of the pair of oviducts, or Fallopian tubes. Fertilization most often occurs in the oviduct. The fertilized egg tumbles into the uterus, a hollow, pearshaped organ above the urinary bladder. An embryo forms and development is completed in the uterus. A thick layer of smooth muscle, the myometrium, makes up most of the uterine wall. Endometrium lines the uterus and consists of glandular epithelium, connective tissues, and blood vessels. The narrowed-down, lowest portion of the uterus is the cervix, which opens into the vagina. The vagina, a muscular, mucosa-lined tube, extends from the cervix to the body’s surface. It is lubricated by its own mucus secretions, and it functions as the female organ of intercourse. The vagina also functions as the birth canal in childbirth. Two pairs of skin folds enclose the surface openings of the vagina and urethra. Adipose tissue fills the pair of outer folds (the labia majora). Thin inner folds (the labia minora) have a rich blood supply and swell during sexual arousal. The tip of the clitoris, a highly sensitive sex organ, is positioned between the two inner folds, just in front of the urethra. The clitoris and penis develop from the
Ovary
Oviduct
Uterus
One of a pair of gonads that makes oocytes and sex hormones; during the course of a monthly cycle, releases hormones that stimulate maturation of an oocyte and prepares the lining of the uterus for a potential pregnancy
One of a pair of ciliated channels through which oocytes are propelled from an ovary to the uterus; usual site of fertilization
Chamber in which embryo develops; its narrowed portion, the cervix, secretes mucus that helps sperm travel into the uterus and defends the embryo against many bacteria
Myometrium Thick muscle layers of uterus; stretch greatly during pregnancy
Urinary bladder
Endometrium opening of cervix
Urethra
Clitoris Small organ responsive to sexual stimulation
Labium Minora One of a pair of innermost thin, skin folds; part of the genitals anus
Labium Majora Vagina
One of a pair of outermost, fat-padded skin folds; part of the genitals
Organ of sexual intercourse; also the birth canal
Figure 42.9 Animated Components of the human female reproductive system and their functions.
746 UNIT VI
HOW ANIMALS WORK
Inner lining of the uterus into which a blastocyst implants itself; gets thicker and has increased blood supply during pregnancy; gives rise to maternal portion of placenta, an organ that metabolically supports embryonic and fetal development
FOCUS ON HEALTH
42.5 Table 42.2 Ovaries (2)
Female Reproductive Organs Oocyte production and maturation, sex hormone production
Oviducts (2) Tubes between the ovaries and the uterus; fertilization normally takes place here Uterus
Chamber in which new individual develops
Cervix
Entrance to the uterus; secretes mucus that enhances sperm travel into uterus and reduces embryo’s risk of infection
Vagina
Organ of sexual intercourse; birth canal
same embryonic tissue. Both have an abundance of highly sensitive touch receptors, and both swell with blood and become erect during sexual arousal.
Overview of the Menstrual Cycle Females of most mammalian species follow an estrous cycle, meaning they are fertile and “in heat” (sexually receptive to males) only at certain times. Females of humans and some other primates follow a menstrual cycle. Their fertile periods are cyclic, intermittent, and not tied to sexual receptivity. In other words, they can get pregnant only during certain times in their cycle but may be receptive to sex at any time. Human females typically begin to menstruate at about age twelve. Section 42.6 describes the menstrual cycle in detail, but here is an overview: Every twentyeight days or so, an oocyte matures in an ovary, and is released. During a two-week interval, the uterus gets primed for pregnancy. If the oocyte does not get fertilized, blood and bits of endometrium flow out through the vagina. This menstrual flow indicates the start of a new cycle. A woman goes through such monthly cycles until she reaches her late forties or early fifties, when her sex hormone output dwindles. The decline in hormone secretions correlates with the onset of menopause, the twilight of a female’s fertility.
Take-Home Message What are the main female reproductive organs? Ovaries are female gonads; they produce eggs and secrete sex hormones. Eggs travel through oviducts to the uterus, the chamber where offspring develop. The vagina receives sperm and serves as the birth canal.
Female Troubles
Hormonal changes cause premenstrual symptoms, menstrual pain, and hot flashes.
Links to Prostaglandins 35.1, Benign tumors 9.5
PMS Many women regularly experience discomfort a week or so before they menstruate. Body tissues swell because premenstrual changes influence aldosterone secretion. This adrenal gland hormone stimulates reabsorption of sodium and, indirectly, water (Section 41.6). Breasts may become tender because hormones cause their milk ducts to enlarge. Cycle-induced changes also cause depression, irritability, or anxiety. Headaches and sleeping problems are common. The regular recurrence of these symptoms is known as premenstrual syndrome (PMS). A balanced diet and regular exercise make PMS less likely and less severe. Taking oral contraceptives minimizes hormone swings and therefore PMS. In some cases, drugs that completely suppress the secretion of sex hormones can help. Menstrual Pain Prostaglandins secreted during menstruation stimulate contractions of smooth muscle in the uterine wall. Many women do not notice the contractions, but others experience a dull ache or sharp pain. Women who secrete high levels of prostaglandins are more likely to feel uncomfortable while menstruating. Endometriosis, the growth of endometrial tissue in the wrong regions of the pelvis, affects about 15 percent of women and can cause pain during menstruation. Hormones cause misplaced tissue to bleed, then heal, forming scars that can be painful. Hormone suppression methods help, but only surgery can provide a cure. More than one-third of women over age thirty have benign uterine tumors called fibroids. Most fibroids cause no symptoms, but some result in pain, long menstrual periods, and excessive bleeding. A woman who needs to change pads or tampons on an hourly basis should discuss this condition with her doctor. Surgical removal of fibroids halts the excessive bleeding and the pain. Hot Flashes, Night Sweats Three-fourths of the women entering menopause have hot flashes. They get abruptly and uncomfortably hot, flushed, and sweaty as blood surges to their skin. When episodes occur at night, they disrupt sleep. Hormone replacement therapy can relieve these symptoms, but the therapy raises risk of breast cancer and stroke, especially if continued for more than a few years. Exercising, avoiding alcohol, and eating soy-based products can also help reduce symptoms.
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ANIMAL REPRODUCTIVE SYSTEMS 747
42.6
Preparations for Pregnancy A fertile woman undergoes hormonal changes and releases eggs in an approximately monthly cycle.
Links to Gamete formation 10.5, Sex hormones 35.12
The Ovarian Cycle At birth, a girl has about 2 million primary oocytes, immature eggs that have entered meiosis but stopped short in prophase I. Beginning with her first menstrual cycle, these oocytes mature, typically one at a time, in approximately a 28-day cycle. Figure 42.10 depicts the events of this cycle in an ovary. A primary oocyte and the cells that surround it constitute an ovarian follicle (Figure 42.10a). In the first part of the ovarian cycle—the follicular phase—cells around the oocyte divide repeatedly, while the oocyte enlarges and secretes glycoproteins. These secreted glycoproteins form a noncellular layer known as the zona pellucida (Figure 42.10b). As the follicle matures, a fluid-filled cavity opens in the cell layer around the
A A primary oocyte, not yet released from meiosis I. A layer of cells is forming around it. A mature follicle consists of this cell layer and the oocyte inside it.
B The zona pellucida, a transparent, slightly elastic layer, starts to form around primary oocyte.
oocyte (Figure 42.10c). More than one follicle often starts to mature during the follicular phase, but typically only one goes on to become fully mature. Follicle maturation requires about 14 days and is under hormonal control. As the follicular phase starts, the hypothalamus secretes GnRH. This hormone stimulates cells in the anterior pituitary to increase their secretion of FSH and LH (Figure 42.11a). Rising levels of FSH and LH in the blood allow follicle maturation and stimulate follicle cells to secrete estrogens, a type of sex hormone (Figure 42.11b,c). The pituitary detects the rising level of estrogens in blood and responds with an outpouring of LH. The LH surge encourages the primary oocyte to complete meiosis I and undergo cytoplasmic division. One of the resulting haploid cells, the secondary oocyte, gets most of the cytoplasm. The other haploid cell is the first polar body, a cell that will eventually degenerate (Figure 42.10d). The LH surge also causes the follicle to swell and eventually to burst. The secondary oocyte,
C A fluid-filled cavity starts to form in the follicle’s cell layer.
ovary
first polar body primordial follicle
secondary oocyte
D Mature follicle. Meiosis I is over. A secondary oocyte and the first polar body have formed.
G If no pregnancy occurs, the corpus luteum breaks down.
F A corpus luteum forms from remains of ruptured follicle.
E Ovulation. Mature follicle ruptures, releasing the secondary oocyte and first polar body.
Figure 42.10 Animated Cyclic events in a human ovary, cross-section. The follicle does not “move around” as in this diagram, which simply shows the sequence of events. All of these structures form in the same place during one menstrual cycle. In the cycle’s first phase, a follicle grows and matures. At ovulation, the second phase, the mature follicle ruptures and releases a secondary oocyte. In the third phase, a corpus luteum forms from the follicle’s remnants.
748 UNIT VI
HOW ANIMALS WORK
A
still surrounded by the zona pellucida and some follicle cells, is released into an oviduct. Thus, the midcycle surge of LH is the trigger for ovulation, the release of a secondary oocyte from an ovary (Figure 42.10e). Ovulation is followed by the luteal phase of the ovarian cycle. During this phase, the ruptured follicle becomes a yellowish glandular structure known as the corpus luteum (Figure 42.10f ). In Latin, corpus means body, and luteum means yellow. The corpus luteum secretes a large amount of the sex hormone progesterone, and a lesser amount of estrogens. The high progesterone level feeds back to the brain and reduces secretion of LH and FSH, so a new follicle does not develop. If pregnancy does not occur, the corpus luteum lasts no more than 12 days. In the final days of the luteal phase, a decline in LH causes it to break down (Figure 42.10g). Then a new follicular phase begins.
FSH and LH levels in blood
FSH LH LH surge triggers ovulation
FSH and LH stimulate follicle maturation B
Follicle changes in an ovary follicle matures
ovulation
corpus luteum forms
corpus luteum secretes estrogens, progesterone
follicle secretes estrogens C
corpus luteum breaks down
Estrogen and progesterone levels in blood Progesterone
Correlating Events in the Ovary and Uterus Menstruation, the flow of blood and endometrial tissue out of the uterus and through the vagina, coincides with the beginning of the follicular phase in the ovary (Figure 42.11c,d). Menstruation usually lasts for 1 to 5 days. Then, as the follicular phase goes on, estrogens secreted by a maturing follicle encourage the uterine lining to repair itself and thicken. After ovulation, in the luteal phase, estrogens and progesterone secreted by the corpus luteum act on the endometrium. These hormones encourage the growth of blood vessels and of glands that secrete glycogen. The uterus is now ready to sustain a pregnancy. If no pregnancy occurs, the corpus luteum breaks down and progesterone and estrogen levels plummet. Blood vessels supplying the endometrium wither and the endometrium starts to break down. As the bloody tissue is shed, a new follicular phase begins.
Estrogen
estrogens, progesterone, cause uterine lining to thicken
low estrogen D
Changes in uterine lining
menstrual flow
0
2
4
6
8
10
Follicular phase
12 14 16 Days of cycle
18
20
22
24
26
28
Luteal phase
Take-Home Message What cyclic changes occur in the ovary and uterus? Every 28 days or so FSH and LH stimulate maturation of an ovarian follicle. A midcycle surge of LH triggers ovulation—the release of a secondary oocyte into an oviduct. Estrogen secreted by a maturing follicle causes the endometrium to thicken. After ovulation, progesterone secreted by the corpus luteum encourages secretion by endometrial glands. If pregnancy does not occur, the corpus luteum breaks down, hormone levels drop, the endometrial lining is shed, and the cycle begins again.
Figure 42.11 Animated Changes in a human ovary and uterus correlated with changing hormone levels. We start with the onset of menstrual flow on day one of a twenty-eight-day menstrual cycle. (a,b) Prompted by GnRH from the hypothalamus, the anterior pituitary secretes FSH and LH, which stimulate a follicle to grow and an oocyte to mature in an ovary. A midcycle surge of LH triggers ovulation and the formation of a corpus luteum. A decline in FSH after ovulation stops more follicles from maturing. (c,d) Early on, estrogen from a maturing follicle calls for repair and rebuilding of the endometrium. After ovulation, the corpus luteum secretes some estrogen and more progesterone that primes the uterus for pregnancy. If pregnancy occurs, the corpus luteum will persist, and its secretions will stimulate the maintenance of the uterine lining.
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FOCUS ON HEALTH
42.7
42.8
FSH and Twins Typically, only a single egg matures and gets released during each menstrual cycle. Abundant FSH can cause two eggs to mature and possibly lead to fraternal twins.
When a female and a male engage in sexual intercourse, the excitement of the moment may obscure what can happen if a secondary oocyte is in an oviduct.
Sometimes two oocytes mature at the same time and are released during one menstrual cycle. If both become fertilized, the outcome will be two genetically different zygotes that develop into fraternal twins. Fraternal twins are no more alike than any other siblings. They may be the same sex, or different sexes. A high level of FSH, the hormone that stimulates egg maturation, increases the likelihood of fraternal twins. FSH level and the prevalence of fraternal twinning varies among families and among ethnic groups. A woman who is herself a fraternal twin has double the average chances of giving birth to fraternal twins. If she does so once, her odds triple for a second set. Fraternal twins are most common among women of African descent, less common among Caucasians, and rare among Asians. The Yoruba people of Africa have the highest incidence of twin or triplet births—about one in every twenty-two pregnancies (Figure 42.12). They also have unusually high levels of FSH. Age also has an effect. A woman’s FSH levels rise from puberty through her midthirties, causing her likelihood of having fraternal twins to rise. Thus, a trend toward later childbearing is contributing to a rise in fraternal twinning. FSH level does not influence formation of identical twins. Such twins arise when a zygote or early embryo splits, and two genetically identical individuals develop. A split is a chance event; a tendency to produce identical twins does not run in families and such twins are equally likely among women of all ethnic groups and ages.
Figure 42.12 Yoruba mother. The rate of twin births among the Yoruba is the world’s highest, but the mortality rate also is high; half of the twins die shortly after birth. Grieving mothers use a carving (Ere Ibeji) as a ritual point of contact with lost infants. Commercially produced plastic dolls are now being substituted for traditional wood carvings.
750 UNIT VI
When Gametes Meet
Links to Autonomic signals 33.8, Pituitary hormones 35.3
Internal fertilization involves coordinated changes in the physiology of two individuals and then additional interactions between their gametes. It all begins with sexual intercourse, or coitus.
Sexual Intercourse For males, intercourse requires an erection. Long cylinders of spongy tissue make up the bulk of the penis (Figure 42.4). When a male is not sexually aroused, his penis remains limp, because the large blood vessels that transport blood to the spongy tissue are constricted. When a male becomes aroused, parasympathetic signals induce the vessels that supply the spongy tissue to widen. Inward flow of blood now exceeds outward flow, and the increase in fluid pressure expands the internal chambers. As a result, the penis enlarges and stiffens, so it can be inserted into a female’s vagina. During intercourse, pelvic thrusting stimulates the mechanoreceptors in the male’s penis and the female’s clitoris. The female’s vaginal wall, labia, and clitoris swell with blood. In both partners, the heart rate and breathing rate rise. The posterior pituitary steps up its secretion of oxytocin, which inhibits signals from the brain center that controls fear and anxiety (the amygdala). When stimulation continues, oxytocin surges at orgasm. At orgasm, oxytocin causes rhythmic contractions of smooth muscle of the reproductive tract. At the same time, endorphin release in the brain evokes feelings of pleasure. In a male, orgasm is usually accompanied by ejaculation, in which contracting muscles force the semen out of the penis. You may have been told that a female will not become pregnant as long as she does not reach orgasm. Do not believe it.
Physiology of Sex
Regarding Viagra The ability to get and sustain an
erection peaks during the late teens. As a male grows older, he may have episodes of erectile dysfunction. With this disorder, the penis cannot stiffen enough for intercourse. Men who have circulatory problems are most often affected. Smoking also increases risk. The National Institutes of Health estimates that 30 million men are affected in the United States alone. Viagra and similar drugs prescribed for erectile dysfunction cause blood vessels that carry blood into the penis to HOW ANIMALS WORK
oviduct
Fertilization zona pellucida
ovary Ovulation
oocyte nucleus
zona follicle pellucida cell
uterus haploid egg and sperm nuclei
opening of cervix vagina
A Fertilization most often occurs in the oviduct. Many human sperm travel swiftly through the vaginal canal into oviducts (blue arrows). Inside an oviduct, the sperm surround a secondary oocyte that was released by ovulation.
C The oocyte nucleus completes meiosis II, forming a nucleus with a haploid maternal genome. The sperm’s tail and other organelles degenerate. Its DNA is enclosed by a membrane, forming a haploid nucleus with paternal genes.
B Enzymes released from the cap of each sperm clear a path through the zona pellucida. Penetration of the secondary oocyte by a sperm causes the oocyte to releases substances that harden the zona pellucida and prevent other sperm from binding.
Later, the two nuclear membranes will break down and paternal and maternal chromosomes will become arranged on a bipolar spindle in preparation for the first mitotic division.
Figure 42.13 Animated Events in human fertilization. The light micrograph shows a fertilized human oocyte.
Figure It Out: In the micrograph, what are the small cells on the
widen and deliver more blood. Such drugs can cause headaches and (rarely) sudden hearing loss. They can also interact with other drugs, and should never be taken without a prescription.
and divide (Figure 42.13c). This division produces a mature egg, or ovum, and a polar body. This polar body, along with the one formed earlier by meiosis I, will degenerate. In most animals, egg and sperm nuclei fuse to form a diploid nucleus in the zygote, the first cell of a new individual. In humans and other mammals, nuclei do not fuse. Instead, the nuclear membranes of the egg and the sperm break down. The maternal and paternal chromosomes then become oriented on a mitotic spindle for the first cell division. This division is the first step in development, a process explained in detail in the next chapter.
On average, an ejaculation can put 150 million to 350 million sperm in the vagina. Less than thirty minutes later, hundreds of them reach the oviducts. The sperm swim toward the ovaries. As the sperm travel, they undergo changes that prepare them to bind to and penetrate an oocyte. Fertilization most often occurs in an oviduct (Figure 42.13a). Sperm bind to an oocyte’s zona pellucida, and this binding triggers the release of acrosomal enzymes from the cap on a sperm’s head. The enzymes digest the zona pellucida, clearing a passage to the oocyte’s plasma membrane (Figure 42.13b). Usually only one sperm enters the secondary oocyte. The sperm’s tail and other organelles break down. Penetration of an oocyte by a sperm has two major effects. First, penetration causes changes in the oocyte that prevent other sperm from entering it. Second, the penetration causes the oocyte to complete meiosis II
Answer: The polar bodies
Fertilization
right, just beneath the zona pellucida?
Take-Home Message What happens during intercourse and fertilization? Sexual arousal involves nervous and hormonal signals.
Ejaculation releases millions of sperm into the vagina. Sperm travel through the uterus toward the oviducts, the site where fertilization most often occurs. Penetration of a secondary oocyte by a single sperm causes the oocyte to complete meiosis II, and prevents additional sperm from penetrating it.
Sperm organelles disintegrate. The DNA of the sperm, along with that of the oocyte, become the genetic material of the zygote.
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ANIMAL REPRODUCTIVE SYSTEMS 751
42.9
Preventing or Seeking Pregnancy There are many options for people who wish to put off reproducing or improve their chances of becoming a parent.
Link to Prenatal diagnosis 12.8
Birth Control Options Emotional and economic factors often lead people to seek ways to control their fertility. Table 42.3 and Figure 42.14 list common fertility control options and compare their effectiveness. Most effective is abstinence—no sex—which may take great self-discipline. Rhythm methods are forms of abstinence; a woman simply avoids sex in her fertile period. She calculates when she is fertile by recording how long menstrual
Most Effective Total abstinence
100%
Tubal ligation or vasectomy
99.6%
Hormonal implant
99%
Highly Effective IUD + slow-release hormones
98%
IUD + spermicide
98%
Depo-Provera injection
96%
IUD alone High-quality latex condom + spermicide with nonoxynol–9
95%
“The Pill” or birth control patch
94%
95%
Effective Cervical cap
89%
Latex condom alone
86%
Diaphragm + spermicide
84%
Billings or Sympto-Thermal Rhythm Method
84%
Vaginal sponge + spermicide
83%
Foam spermicide
82%
Moderately Effective Spermicide cream, jelly, suppository
75%
Rhythm method (daily temperature)
74%
Withdrawal
74%
Condom (cheap brand)
70%
Unreliable Douching
40%
Chance (no method)
10%
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HOW ANIMALS WORK
cycles last, checking her temperature each morning, monitoring the thickness of her cervical mucus, or some combination of these methods. Miscalculations are frequent. Sperm deposited in the vagina just before ovulation may live long enough to meet an egg. Withdrawal, or removing the penis from the vagina before ejaculation, requires great willpower and may fail. Pre-ejaculation fluids from the penis hold sperm. Douching, or rinsing out the vagina immediately after intercourse, is unreliable. Some sperm can travel through the cervix within seconds of ejaculation. Surgical methods are highly effective, but are meant to make a person permanently sterile. Men may opt for a vasectomy. A doctor makes a small incision into the scrotum, then cuts and ties off each vas deferens. A tubal ligation blocks or cuts a woman’s oviducts. Other fertility control methods use physical and chemical barriers to stop sperm from reaching an egg. Spermicidal foam and spermicidal jelly poison sperm. They are not always reliable, but their use with a condom or diaphragm reduces the risk of pregnancy. A diaphragm is a flexible, dome-shaped device that is positioned inside the vagina so it covers the cervix. A diaphragm is relatively effective if it is first fitted by a doctor and used correctly with a spermicide. A cervical cap is a similar but smaller device. Condoms are thin, tight-fitting sheaths worn over the penis during intercourse. Good brands may be as much as 95 percent effective when used correctly with a spermicide. Only condoms made of latex offer protection against sexually transmitted diseases (STDs). However, even the best ones can tear or leak. An intrauterine device, or IUD, is inserted into the uterus by a physician. Some IUDs make cervical mucus thicken so sperm cannot swim through it. Others shed copper, which interferes with implantation. The birth control pill is the most common fertility control method in developed countries. “The Pill” is a mixture of synthetic estrogens and progesterone-like hormones that prevents both maturation of oocytes and ovulation. When used correctly, the pill is at least 94 percent effective. It can reduce menstrual cramps, but sometimes causes nausea, headaches, and weight gain. Its use lowers risk of ovarian and uterine cancer but raises risk of breast, cervical, and liver cancer.
Figure 42.14 Comparison of the effectiveness of some methods of contraception. These percentages also indicate the number of unplanned pregnancies per 100 couples who use only that method of birth control for a year. For example, “94% effectiveness” for oral contraceptives means that 6 of every 100 females will still become pregnant, on average.
A birth control patch is a small, flat adhesive patch applied to skin. The patch delivers the same mixture of hormones as an oral contraceptive, and it blocks ovulation the same way. Like birth control pills, it is not for everyone. Some women, especially those who smoke, can develop dangerous blood clots and other serious cardiovascular disorders. Hormone injections or implants prevent ovulation. Injections act for several months, whereas the implant Implanon lasts for three years. Both methods are quite effective, but may cause sporadic, heavy bleeding. Some women turn to emergency contraception after a condom tears, or after unprotected consensual sex or rape. Such “morning-after pills” are now available without a prescription to women over age 18. They prevent ovulation and work best if taken immediately after intercourse. However, they can be effective up to five days later. The pills are not meant to be used on a regular basis. Nausea, vomiting, abdominal pain, headache, and dizziness are side effects.
Figure 42.15 In vitro fertilization. A fertility doctor uses a micromanipulator to insert a human sperm into an oocyte. The video screen shows the view through the microscope.
may turn to technology for help. With in vitro fertilization, a doctor combines eggs and sperm outside the body (Figure 42.15). The resulting zygotes are allowed to divide, then one or more small clusters of cells are transferred to a woman’s uterus to undergo development. Assisted reproduction attempts are costly and most fail. In 30-year-old women, about one-third of in vitro attempts result in a birth. In 40-year-olds, only one in six attempts succeeds.
About Abortion About 10 percent of detected pregnancies end in a spontaneous abortion, or miscarriage. Many other pregnancies end without ever having been detected. By some estimates, as many as 50 percent of all pregnancies are cut short by some genetic problem. Risk of miscarriage increases with maternal age. Induced abortion is the deliberate dislodging and removal of an embryo or fetus from the uterus. In the United States, about half of all unplanned pregnancies end in induced abortion. Parents who find out through genetic tests that an embryo has a genetic abnormality may decide to terminate the pregnancy. About 80 percent of embryos diagnosed with Down syndrome are aborted (Section 12.8). From a clinical standpoint, abortion usually is a brief, low-risk procedure, especially during the first trimester of pregnancy. Mifepristone (RU-486) and similar drugs can induce abortion during the first nine weeks. They interfere with how the body sustains the uterine lining for the pregnancy. Use of a suction device terminates pregnancies as late as fourteen weeks. Later abortions require more difficult surgical procedures.
Take-Home Message What methods do humans use to control their fertility? Physical barriers and hormonal treatments can prevent pregnancy.
Spontaneous or induced abortion ends an existing pregnancy. Assisted reproductive technology helps some couples who are having trouble conceiving.
Table 42.3
Common Methods of Contraception
Method Abstinence
Avoid intercourse entirely
Rhythm method
Avoid intercourse in female’s fertile period
Withdrawal
End intercourse before male ejaculates
Douche
Wash semen from vagina after intercourse
Vasectomy
Cut or close off male’s vasa deferentia
Tubal ligation
Cut or close off female’s oviducts
Condom
Enclose penis, block sperm entry to vagina
Diaphragm, cervical cap
Cover cervix, block sperm entry to uterus
Spermicides
Kill sperm
Intrauterine device
Prevent sperm entry to uterus or prevent implantation of embryo
Assisted Reproductive Technology About 15 percent of couples in the United States are infertile; either the woman does not become pregnant or repeatedly miscarries. When a couple make normal sperm and oocytes but cannot conceive naturally, they
Description
Oral contraceptives
Prevent ovulation
Hormone patches, implants, or injections
Prevent ovulation
Emergency contraception pill Prevent ovulation
CHAPTER 42
ANIMAL REPRODUCTIVE SYSTEMS 753
42.10 Sexually Transmitted Diseases Sex acts transfer body fluids in which some human pathogens travel from one host to another.
Links to Infectious disease 21.8, HIV 21.2
Consequences of Infection Each year, the pathogens that cause sexually transmitted diseases, or STDs, infect about 15 million people in the United States (Table 42.4). Two-thirds of those infected are under age twenty-five, one-fourth are teenagers. Over 65 million Americans now live with an incurable STD. Treating these diseases and their complications costs about $8.4 billion in an average year. The social consequences of STDs are sobering. Women become infected more easily than men, and develop more complications. Each year, about 1 million American women develop pelvic inflammatory disease (PID), a complication of some bacterial STDs. PID scars the reproductive tract, can cause chronic pain and infertility, and increases the risk of a tubal pregnancy (Figure 42.16a). A mother can transmits an STD to her child. Herpes simplex type 2 virus kills about 50 percent of the embryos it infects and causes neural defects in many survivors. Exposure to Chlamydia during childbirth can lead to an infection of the newborn’s throat or eyes (Figure 42.16b).
Table 42.4 STD
New STD Cases Annually U.S. Cases
HPV infection Trichomoniasis Chlamydia Genital herpes Gonorrhea Syphilis AIDS
Global Cases
6,200,000 5,000,000 3,000,000 1,000,000 650,000 70,000 40,000
400,000,000 174,000,000 92,000,000 20,000,000 * 62,000,000 12,000,000 4,900,000
* Global data on genital herpes last compiled in 1997.
a
b
c
Figure 42.16 A few downsides of unsafe sex. (a) Increased risk of tubal pregnancy. Scarring caused by STDs can cause an embryo to implant itself in an oviduct, rather than the uterus. Untreated tubal pregnancies can rupture an oviduct and cause bleeding, infection, and death. (b) An infant with chlamydiainflamed eyes. The child’s mother passed on the bacterial pathogen during the birth process. (c) Chancres (open sores) caused by syphilis.
754 UNIT VI
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Major Agents of Sexually Transmitted Disease HPV Human papillomavirus (HPV) infection is the most widespread and fastest growing STD in the United States. At least 20 million are already infected. Some of the 100 or so HPV strains can cause genital warts: bumpy growths on external genitals and the area around the anus. A few HPV strains are the main cause of cervical cancer. Sexually active females should have an annual Pap smear to check for cervical changes. A vaccine can prevent HPV infection if given before viral exposure (Chapter 38 introduction). Trichomoniasis The flagellated protist Trichomonas vaginalis causes the disease trichomoniasis (Section 22.2). In women, symptoms typically include vaginal soreness, itching, and a yellowish discharge. Infected males often show no symptoms. Untreated infections damage the urinary tract, cause infertility, and invite HIV infection. A single dose of an antiprotozoal drug quickly cures an infection. Both sexual partners should be treated. Chlamydia Chlamydial infection is primarily a young person’s disease. Forty percent of those infected are between ages fifteen and nineteen; 1 in 10 sexually active teenage girls is infected. Chlamydia trachomatis causes the disease (Figure 42.17a). Antibiotics can quickly kill this bacterium. Most infected females remain undiagnosed; they have no symptoms. Between 10 and 40 percent of those who are untreated will develop pelvic inflammatory disease. Half of infected males have symptoms, such as abnormal discharges from the penis and painful urination. Untreated males risk an inflamed reproductive tract and infertility. Genital Herpes About 45 million Americans have genital herpes, caused by Herpes simplex type 2 virus. Transmission to new hosts requires direct contact with active herpesviruses or with sores that contain them. Mucous membranes of the mouth and genitals are most vulnerable. Early symptoms are often mild or absent. Small, painful blisters may form on the genitals. Within three weeks, the virus enters latency. Blisters crust over and heal, but viral particles remain hidden in the body. Sporadic reactivation of herpesvirus typically causes painful blisters at or near the original infection site. Sexual intercourse, menstruation, emotional stress, or other types of infection trigger flare-ups. An antiviral drug decreases healing time and pain, but genital herpes is incurable. Gonorrhea The STD gonorrhea is caused by Neisseria gonorrhoeae (Figure 42.17b). This bacterium can cross the mucous membranes of the urethra, cervix, or anal canal during sexual intercourse. An infected female may notice a vaginal discharge or burning sensation while urinating. If the bacterium enters her oviducts, it may cause cramps, fever, vomiting, and scarring that can lead to sterility. Less than one week after a male is infected, yellow pus oozes
FOCUS ON HEALTH
Figure 42.17 Light micrographs of bacteria that cause (a) chlamydia, (b) gonorrhea, and (c) syphilis. All can be killed with antibiotic drugs. a
from his penis. Urination becomes more frequent, and it also may be painful. Prompt treatment with antibiotics quickly cures this disease, which is rampant. Many ignore early symptoms or wrongly believe infection confers immunity. A person can contract gonorrhea repeatedly, probably because there are now at least sixteen strains of N. gonorrhoeae.
Syphilis The spirochete bacterium Treponema pallidum causes syphilis, a dangerous STD (Figure 42.17c). During sex with an infected partner, these bacteria get onto the genitals or into the cervix, vagina, or oral cavity. They then slip into the body through tiny cuts. One to eight weeks later, many T. pallidum cells are twisting about inside a flattened, painless chancre, a localized ulcer. The chancre is a sign of the primary stage of syphilis. It usually heals, but treponemes multiply inside the spinal cord, brain, eyes, bones, joints, and mucous membranes. In the infectious secondary stage, a skin rash develops and more chancres form (Figure 42.16c). In about half of the cases, immune responses succeed and symptoms subside or disappear. In the remainder of cases, lesions and scars appear in the skin and liver, bones, and other organs. Few treponemes form during this tertiary stage, but the host’s immune system is hypersensitive to them. Chronic immune reactions may damage the brain and spinal cord, and cause paralysis. Possibly because the symptoms are so alarming, more people seek early treatment for syphilis than they do for gonorrhea. Later stages require prolonged treatment. AIDS Infection by HIV, human immunodeficiency virus, can lead to AIDS—acquired immune deficiency syndrome (Chapter 21 introduction). At first, a person may not know that he or she is infected. Over time, the virus begins to destroy the immune system, and the set of chronic disorders that characterize AIDS develop. Some normally harmless bacteria already living in and on the body are the first to take advantage of the lowered resistance. This infection can open the door to other, more dangerous pathogens. Over time, these agents can overwhelm the compromised immune system and cause death. Most often, HIV spreads by way of anal, vaginal, and oral intercourse, and through intravenous drug use. Virus particles in blood, semen, urine, or vaginal secretions enter a new host through cuts and abrasions of the penis, vagina, rectum, or mouth. Oral sex is least likely to cause
b
c
infection. Anal sex is 5 times more dangerous than vaginal sex and 50 times more dangerous than oral sex. Most health care workers advocate safe sex, although there is confusion over what “safe” means. The use of high-quality latex condoms together with a nonoxynol-9 spermicide helps prevent viral transmission. However, as mentioned earlier, this practice still carries a slight risk. Open-mouth kissing with an HIV-positive individual is risky. Caressing is not, as long there are no lesions or cuts where HIV-laden body fluids can enter the body. Skin lesions caused by any other sexually transmitted disease can serve as points of entry for the virus. Confidential testing for HIV exposure is now widely available, and early diagnosis saves lives. It keeps a person from unknowingly infecting others and allows treatment to be started when it is most effective. An HIV infection cannot be cured, but drug therapies can extend the life of those infected (Figure 42.18). When early diagnosis and treatment are followed by ongoing medical care, an HIVpositive person can have a nearly normal life span. However, once infected, a person can always infect others. Also, the drugs that keep people alive often have unpleasant side effects, including nausea, fatigue, diarrhea, and bone loss. These side effects cause many HIV-positive people to risk their lives by stopping treatment or taking less than the recommended amount of their medication.
Figure 42.18 Basketball legend Magic Johnson, one of the torch bearers of the 2002 Winter Olympics. He was diagnosed as HIV positive in 1991. He contracted the virus through heterosexual sex, and credits his survival to AIDS drugs and informed medical care. Johnson continues to campaign to educate others about the risk of AIDS.
CHAPTER 42
ANIMAL REPRODUCTIVE SYSTEMS 755
IMPACTS, ISSUES REVISITED
Male or Female? Body or Genes?
Parents who have a child with ambiguous genitals face a difficult choice. Surgery can make their child appear more normal, but it can harm nerves and impair sexual function. The best cosmetic result may even require sex reassignment, as when a boy with a micropenis is surgically altered and reared as a female. On the other hand, parents who opt to avoid surgery worry about the psychological trauma that having an unusual body may cause.
How would you vote? Should parents of a child who has unusual genitals wait and allow the child to choose or decline normalizing surgery? See CengageNOW for details, then vote online.
?
Summary Section 42.1 Asexual reproduction produces genetic copies of the parent. Sexual reproduction is energetically more costly, and a parent does not have as many of its genes represented among the offspring. However, sexual reproduction produces variable offspring, which may be advantageous in environments where conditions fluctuate from one generation to the next. Most animals reproduce sexually and have separate sexes, but some are hermaphrodites that produce both eggs and sperm. With external fertilization, gametes are released into water. Most animals on land have internal fertilization; gametes meet in a female’s body. Offspring may develop inside or outside the maternal body. Yolk helps nourish developing young. Sections 42.2, 42.3 The human reproductive system consists of primary reproductive organs, or gonads, and accessory organs and ducts. Male gonads are testes, which produce sperm and the sex hormone testosterone. Testosterone influences reproduction, as well as development of gender-specific secondary sexual traits that emerge when sexual organs mature at puberty. Gonadotropin-releasing hormone (GnRH) released by the hypothalamus causes the pituitary gland to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones affect gamete formation in both males and females. Sperm form in a series of ducts. Glands that empty into these ducts supply components of the semen. The penis is the male organ of intercourse.
Use the animation on CengageNOW to learn about the reproductive system of human males and how sperm form.
Sections 42.4–42.7 Ovaries, the female gonads, produce eggs and secrete progesterone and estrogens. Eggs are released into oviducts that connect to the uterus, the chamber where offspring develop. The vagina serves as the female organ of intercourse and as the birth canal. A menstrual cycle is an approximately monthly cycle of fertility. Feedback loops from ovaries to the hypothalamus and the anterior pituitary gland control it. In the cycle’s follicular phase, FSH stimulates maturation of a primary oocyte and cells that surround it. Women who have high FSH levels are more likely to release more than one egg at a time and have fraternal twins. FSH and LH also prompt ovaries to secrete estrogens that cause thickening of the lining of the uterus. A midcycle surge 756 UNIT VI
HOW ANIMALS WORK
in LH triggers ovulation, release of a secondary oocyte from an ovary. During the luteal phase, a corpus luteum forms from cells that surrounded the egg. Its hormonal secretions, mainly progesterone, cause the uterine wall to thicken. If fertilization does not occur, the corpus luteum degenerates and menstrual fluid flows out of the vagina as the cycle starts again. Menstrual cycles continue until a woman’s fertility ends at menopause.
Use the animation on CengageNOW to learn about the female reproductive system, cyclic changes in an ovary, and hormonal changes during the menstrual cycle.
Sections 42.8–42.10 Hormones and nerves govern the physiological changes that occur during arousal and intercourse. Millions of sperm are ejaculated, but usually only one penetrates the secondary oocyte. Fertilization forms a zygote, which will develop into a new individual. Humans prevent pregnancy by abstinence, surgery, physical or chemical barriers, and by influencing female sex hormones. Unsafe sex and other behaviors promote the spread of pathogens that cause sexually transmitted diseases, or STDs.
Use the animation on CengageNOW to see what happens during fertilization.
Self-Quiz
Answers in Appendix III
1. Sexual reproduction . a. requires internal fertilization b. produces offspring that vary in their traits c. is more efficient than asexual reproduction d. puts all of a parent’s genes in each offspring 2. Testosterone is secreted by the . a. testes c. prostate gland b. hypothalamus d. all of the above 3. Semen contains secretions from the . a. adrenal gland c. prostate gland b. pituitary gland d. all of the above 4. Male germ cells undergo meiosis in the a. urethra c. prostate gland b. seminiferous tubules d. vasa deferentia
.
5. The female is derived from the same embryonic tissue as the male penis. a. cervix b. clitoris c. vagina d. oviduct
Data Analysis Exercise Adrenal glands normally make a little testosterone, but a mutation in the gene for the enzyme 21-hydroxylase causes excess production of this hormone. A female child who has a 21-hydroxylase deficiency is exposed to abnormally high levels of testosterone during development. This hormone can enlarge her clitoris and cause her labia to fuse, giving her genitals a more male appearance. The drug dexamethasone slows the adrenal glands’ testosterone production. Figure 42.19 shows data from a study in which doctors gave this drug to pregnant women carrying daughters with 21-hydroxylase deficiency. Sixteen of these women had previously given birth to a daughter with 21-hydroxylase deficiency. These daughters (sisters of the treated newborns) serve as a point of comparison. 1. How many daughters produced by dexamethasonetreated pregnancies had normal female genitals? 2. How many phenotypically normal girls had the women’s earlier untreated pregnancies produced? 3. How many women who previously had girls with level 4 or 5 masculinization saw an improvement with treatment? 4. Do the data support the hypothesis that giving the drug dexamethasone to a pregnant woman can reduce the effects of her developing daughter’s 21-hydrolase deficiency?
6. The cervix is the entrance to the a. oviducts b. vagina c. uterus
. d. clitoris
7. During a menstrual cycle, a midcycle surge of triggers ovulation. a. estrogens b. progesterone c. LH d. FSH 8. The corpus luteum develops from and secretes hormones that cause the lining of the uterus to thicken. a. follicle cells c. a primary oocyte b. polar bodies d. a secondary oocyte 9. A male has an erection when . a. muscles running the length of the penis contract b. Leydig cells release a surge of testosterone c. the posterior pituitary releases oxytocin d. spongy tissue inside the penis fills with blood 10. Birth control pills deliver synthetic a. estrogens and progesterone b. LH and FSH c. testosterone d. oxytocin and prostaglandins
.
11. Match each hormone with its source. FSH and LH a. pituitary gland GnRH b. ovaries estrogens c. hypothalamus testosterone d. testes 12. Match each disease with the type of agent that causes it. The choices can be used more than once. chlamydial infection a. bacteria AIDS b. protist syphilis c. virus genital warts gonorrhea genital herpes trichomoniasis
Increasing masculinization
5
Normal female genitals
4
3
2
1
0 1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17
Week drug treatment began
Figure 42.19 Degree of masculinization of 21-hydroxylase deficient females exposed to dexamethasone in the womb (open circles), compared to that of older affected sisters who were untreated during development (dark circles). Graphics along the side depict appearance of the newborn’s genital area.
13. Match each structure with its description. testis a. conveys sperm out of body epididymis b. secretes semen components labia majora c. stores sperm urethra d. produces testosterone vagina e. produces estrogens and ovary progesterone oviduct f. usual site of fertilization prostate gland g. lining of uterus endometrium h. fat-padded skin folds i. birth canal
Visit CengageNOW for additional questions.
Critical Thinking 1. Drugs that inhibit signals of sympathetic neurons may be prescribed for males who have high blood pressure. How might such drugs interfere with sexual performance? 2. In most groups of birds, males do not have a penis. Both males and females have a single opening, called a cloaca, through which wastes leave the body. The male’s sperm also exit through this opening. During mating, a male perches on a female’s back and bends his abdomen under, so his cloaca covers hers. This action is referred to as a “cloacal kiss.” Some birds even carry out this feat in midair. Flightless birds such as ostriches and kiwis do have a penis. Did the common reptile ancestor of all birds have a penis or not? What types of information would help you answer this question? 3. Some sperm mitochondria do get into an egg during fertilization, but they do not persist. As sperm mature, their mitochondria become tagged with a protein (ubiquitin) that signals the egg to destroy them. What organelle would you expect to be involved in this destruction process? CHAPTER 42
ANIMAL REPRODUCTIVE SYSTEMS 757
43
Animal Development IMPACTS, ISSUES
Mind-Boggling Births
In December of 1998, Nkem Chukwu of Houston, Texas,
mothers who are more than forty years old doubled in the
gave birth to six girls and two boys. They were the first set
past decade. Many had turned to reproductive intervention,
of human octuplets to be born alive (Figure 43.1). The births
including fertility drugs and in vitro fertilization.
were premature. In total, all eight newborns weighed a bit
Weigh the rewards against risks. Carrying more than one
more than 4.5 kilograms (10 pounds). Odera, the smallest,
embryo increases the risk of miscarriage, premature delivery,
weighed about 300 grams (less than 1 pound), and six days
or stillbirth. Multiple-birth newborns weigh less than normal
later she died when her heart and lungs gave out. Two others
and are more likely to have birth defects, including cleft lip,
required surgery. All seven survivors had to spend months in
heart malformations, and disorders in which the bladder or
the hospital before going home, but now are in good health.
spinal cord is exposed at the body surface.
Why did octuplets form in the first place? Chukwu had
With this example, we turn to one of life’s most amazing
trouble getting pregnant. Her doctors gave her hormone
dramas—the development of complex animals. How does a
injections, which caused many of her eggs to mature and be
single fertilized egg of a human—or frog or bird or any other
released at the same time. When the doctors realized that
animal—give rise to so many specialized kinds of cells? How
she was carrying a large number of embryos, they suggested
does development yield an adult with all the complex tissues
reducing the number. Chukwu chose instead to try to carry all
and organs discussed throughout this unit?
of them to term. Her first child was thirteen weeks premature. The others were surgically delivered two weeks later. Over the past two decades, the incidence of multiple births has increased by almost 60 percent. There have been four times as many higher order multiple births—triplets or
Answers to these questions will emerge as we consider the developmental processes common to all animals. You will see how experiments helped scientists approach these questions and how these experimental studies led to our current understanding of developmental processes. We will also continue the story of human reproduction and
more. What is going on? A woman’s fertility peaks in her
the human life cycle, which we began in the previous chap-
midtwenties. By thirty-nine, her chance of conceiving natu-
ter. We will see how humans develop from a single cell to an
rally has declined by about half. Yet the number of first-time
adult body with trillions of specialized cells.
See the video! Figure 43.1 Testimony to the potency of fertility drugs—seven survivors of a set of octuplets. Besides manipulating so many other aspects of nature, humans are now manipulating their own reproduction.
Links to Earlier Concepts
Key Concepts Principles of animal embryology
This chapter builds on our discussions of cleavage (Section 9.4), and gamete formation and fertilization (10.5, 42.3, 42.6, 42.8). We revisit RNA localization (15.3), as well as the cell differentiation and master genes that influence it (15.1–15.3, 19.3).
You will learn more about primary tissue layers of embryos (25.1, 32.6), and see more examples of feedback controls (27.3) and cell signaling (27.6).
You will build upon your understanding of the evolution of vertebrate body plans (25.1, 26.1, 26.12) and of the two main animal lineages (25.7).
The effects of thyroid hormone (35.6), and carbon monoxide (38.7) on an embryo are discussed, as is the protective effect of maternal antibodies (38.6).
Animals develop through cleavage, gastrulation, organ formation, and then growth and tissue specialization. Cleavage parcels out material stored in different parts of the egg cytoplasm into different cells, thus starting the process of cell specialization. Sections 43.1–43.5
Human development begins A pregnancy starts with fertilization and implantation of a blastocyst in the uterus. After implantation, a three-layered embryo forms and organ formation begins. All organs have formed by the end of the eighth week. Sections 43.6–43.8
Function of the placenta The placenta allows substances to diffuse between bloodstreams of a mother and her developing child. It also produces hormones that help sustain the pregnancy. Section 43.9
Later human development By the time the fetal period begins, the developing individual appears distinctly human. Harmful substances that get into a mother’s blood can cross the placenta and cause birth defects in the developing embryo or fetus. Sections 43.10, 43.11
Birth and lactation Positive feedback control plays a role in the process of labor, or childbirth. After birth, the newborn is nourished by milk secreted by mammary glands. Section 43.12
How would you vote? Fertility drugs make many eggs mature at the same time and increase the odds of multiple pregnancies. Should the use of such drugs be discouraged to lower the number of highrisk pregnancies? See CengageNOW for details, then vote online.
759
43.1
Stages of Reproduction and Development Animals as different as sea stars and sea otters pass through the same stages in their developmental journey from a single, fertilized egg to a multicelled adult.
Links to Gamete formation 10.5, 42.3, 42.6, Animal body plans 25.1, Germ layers 32.6, Fertilization 42.8
Figure 43.2 shows six sequential processes that occur in reproduction and development of all animals with tissues and organs. This group includes most invertebrates and all vertebrates (Sections 25.1 and 26.1). In the first process, gamete formation, eggs or sperm arise from germ cells in the parental body (Sections 10.5, 42.3, and 42.6). During fertilization (Section 42.8), the first cell of a new individual—the zygote—forms after a sperm penetrates a mature egg. Cleavage carves up the zygote by repeated mitotic cell divisions. The number of cells increases, but the
a Eggs form and mature in female reproductive organs. Sperm form and mature in male reproductive organs.
Gamete Formation
transformation to adult nearly complete
adult, three years old
Sexual reproduction (gamete formation, external fertilization) tadpole
larva (tadpole)
organ formation cleavage
eggs and sperm zygote
A We zoom in on the life cycle as a female releases her eggs into the water and a male releases sperm over the eggs. A frog zygote forms at fertilization. About one hour after fertilization, a surface feature called the gray crescent appears on this type of embryo. It establishes the frog’s head-to-tail axis. Gastrulation will start at the gray crescent.
Figure 43.3 Animated Reproduction and development in the life cycle of the leopard frog, Rana pipiens. b A sperm penetrates an egg. Their nuclei fuse. A zygote has formed.
Fertilization
c Mitotic cell divisions form a ball of cells, a blastula. Each cell gets regionally different par ts of the egg cytoplasm.
Cleavage
d A gastrula, an early embryo that has primary tissue layers, forms by cell divisions, cell migrations, and rearrangements.
Gastrulation
e Details of the body plan fill in as different cell types interact and form tissues and organs in predictable patterns.
Organ Formation
f Organs grow in size, take on mature for m, and gradually assume specialized functions.
Growth, Tissue Specialization
Figure 43.2 Overview of reproductive and developmental processes that occur in animals with tissues and organs. We discussed gamete formation and fertilization in Chapter 42.
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HOW ANIMALS WORK
zygote’s original volume does not. Cells become more numerous but smaller (Figure 43.3b,c). Cells formed during cleavage are called blastomeres. They typically are arranged as a blastula: a ball of cells that enclose a cavity (blastocoel) filled with their own secretions. In the fourth stage, gastrulation, cells self-organize as an early embryo—a gastrula—that has two or three primary tissue layers. The tissues are the germ layers of the new individual. Germ layers, remember, are the forerunners of the adult animal’s tissues and organs (Section 32.6). During organ formation, tissues become arranged into organs. Many organs incorporate tissues derived from more than one germ layer. Growth and tissue specialization is the final process of animal development. The tissues and organs continue to grow, and they slowly take on their final sizes, shapes, proportions, and functions. Growth and tissue specialization will continue into adulthood. Figure 43.3 shows examples of the stages for one vertebrate, the leopard frog (Rana pipiens). A female releases eggs into the water and a male releases sperm onto them. Fertilization is external. The zygote formed by fertilization undergoes cleavage (Figure 43.3b). The
blastocoel
blastula
gray crescent B Here we show the first three divisions of cleavage, a process that carves up the zygote’s cytoplasm. In this species, cleavage results in a blastula, a ball of cells with a fluid-filled cavity.
C Cleavage is over when the blastula forms.
neural tube
ectoderm dorsal lip future gut cavity
yolk plug
neural plate
ectoderm
notochord
mesoderm endoderm
gut cavity
D The blastula becomes a three-layered gastrula—a process called gastrulation. At the dorsal lip, a fold of ectoderm above the first opening that appears in the blastula, cells migrate inward and start rearranging themselves.
E Organs begin to form as a primitive gut cavity opens up. A neural tube, then a notochord and other organs form from the primary tissue layers.
Tadpole, a swimming larva with segmented muscles and a notochord extending into a tail.
Sexually mature, four-legged adult leopard frog.
Limbs grow and the tail is absorbed during metamorphosis to the adult form.
F The frog’s body form changes as it grows and its tissues specialize. The embryo becomes a tadpole, which metamorphoses into an adult.
repeated mitotic divisions form a blastula consisting of several thousand cells (Figure 43.3c). The blastula undergoes gastrulation, which forms the three germ layers (Figure 43.3d). After the three primary tissues have formed, tissue specialization and organ formation begins. A typical vertebrate’s neural tube and notochord form (Figure 43.3e). In frogs, as in some other animals, a larva (in this case as tadpole) undergoes metamorphosis, the remodeling of tissues into the adult form (Figure 43.3f ).
Each stage in the development process builds on the one that precedes it. Take-Home Message What are the stages in reproduction and development in a typical animal? Most animal life cycles start with gamete formation and fertilization. Development involves cleavage, gastrulation, organ formation, and then growth and tissue specialization.
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43.2
Early Marching Orders The location of materials in an egg and distribution of those materials to descendant cells affects early development. Links to Egg formation 10.5, RNA localization 15.3, Protostome and deuterostome lineages 25.7
Information in the Cytoplasm A sperm, recall, consists of paternal DNA and a bit of equipment that helps it swim to and penetrate an egg. An oocyte, or immature egg, has far more cytoplasm (Section 10.5). Its cytoplasm has yolk proteins that will nourish a new embryo, mRNA transcripts for proteins that will be translated in early development, tRNAs and ribosomes to translate the mRNA transcripts, and proteins required to build mitotic spindles. Certain components are not distributed all through the egg cytoplasm; they are localized in one particular region or another. This cytoplasmic localization is a feature of all oocytes (Section 15.3). Cytoplasmic localization gives rise to the polarity that characterizes all animal eggs. In a yolk-rich egg, the vegetal pole has most of the yolk and the animal pole has little. In some amphibian eggs, dark pigment molecules accumulate in the cell cortex, a cytoplasmic region just beneath the plasma membrane. Pigment is
Figure 43.4 Animated Experimental evidence of cytoplasmic localization in an amphibian oocyte.
Cleavage Divides Up the Maternal Cytoplasm Once an oocyte is fertilized, the resulting zygote enters cleavage. By this process, a ring of microfilaments just beneath the plasma membrane contracts and pinches the cell in two (Section 9.4). The zygote’s cytoplasm does not grow in size during cleavage; the repeated cuts divide its volume into ever smaller blastomeres. Simply by virtue of where the cuts are made, different blastomeres receive different portions of the
animal pole pigmented cortex
(a) Many amphibian eggs have dark pigment concentrated in cytoplasm near the animal pole. At fertilization, the cytoplasm shifts, and exposes a gray crescent-shaped region just opposite the sperm’s entry point. With normal first cleavage, each resulting cell gets half of the gray crescent.
yolk-rich cytoplasm vegetal pole
gray crescent of salamander zygote
gray crescent of salamander zygote
First cleavage plane; gray crescent split equally. The blastomeres are separated experimentally.
First cleavage plane; gray crescent missed entirely. The blastomeres are separated experimentally.
sperm penetrating egg
(b) In one experiment, the first two cells formed by normal cleavage were physically separated from each other. Each developed into a normal larva. (c) In another experiment, a zygote was manipulated so the first cleavage plane missed the gray crescent. Only one of the descendant cells received gray crescent material, and only it developed normally.
the most concentrated close to the animal pole. After a sperm penetrates the egg at fertilization, the cortex rotates. Rotation reveals a gray crescent, a region of the cell cortex that is lightly pigmented (Figure 43.4a). Early in the 1900s, experiments by Hans Spemann showed that some substances essential to development are localized in the gray crescent. In one experiment, he separated the first two blastomeres that formed at cleavage. Each blastomere had half of the gray crescent and developed into an embryo (Figure 43.4b). In the next experiment, Spemann altered the cleavage plane (Figure 43.4c). One blastomere received all of the gray crescent, and developed normally. The other, with no gray crescent, formed only a ball of cells.
gray crescent
egg after fertilization
Two normal larvae develop from the two blastomeres.
A ball of Only one undifferentiated normal larva cells forms. develops.
Figure It Out: Is the gray crescent
region required for normal amphibian development?
A
Answer:Yes
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B Experiment 1
C Experiment 2
a Early protostome embryo. Its four cells are undergoing spiral cleavage, oblique to the anterior–posterior axis:
a Sea urchin egg, with little yolk. Cleavage is complete. First cells formed are equally sized.
b Early deuterostome embryo. Its four cells are undergoing radial cleavage, parallel with and perpendicular to the anterior–posterior axis:
b Frog egg, with moderate amount of yolk. Yolk slows cleavage so lower cells are larger.
Figure 43.5 Examples of the two cleavage patterns most common in the two main lineages of bilateral animals.
c Fish egg, with a large amount of yolk. Cleavage is restricted to the layer of cytoplasm on top of the yolk.
maternal cytoplasm. Orientation of the cell divisions is not random and has major implications for future development. The pattern of cleavage determines how much and which portion of the maternal cytoplasm a blastomere will receive. As a result of cytoplasmic localization of material inside the egg, cleavage distributes different kinds and quantities of materials into different blastomeres. For example, cleavage may put a specific maternal mRNA into one blastomere but not others. Thus, cleavage creates cell lineages that differ in the contents of their cytoplasm. Later, possessing different maternal materials will cause different cell lineages to express different genes, forming specialized tissues.
Two cells formed by first cleavage
mass of yolk
Figure 43.6 Comparison of cleavage patterns amoung deuterostomes that have different amounts of yolk in their eggs. Yolk slows division.
yolkless eggs of mammals. Frogs and other amphibians also undergo complete cleavage, but it proceeds more slowly at the yolk-rich vegetal pole than at the yolk-free animal pole. As a result, cells vary a bit in their size (Figure 43.6b). Eggs of reptiles, birds, and most fishes are so yolky that cuts are exceedingly slow or blocked entirely, except in the small, disk-shaped region that has the least yolk (Figure 43.6c).
Variations in Cleavage Patterns
Structure of the Blastula
The details of cleavage vary among species. Differences start with the first division, which determines whether the first two cells will be equal or unequal in size and what part of the egg cytoplasm they receive. There are two major animal lineages, protostomes and deuterostomes (Section 25.1), and they differ in their cleavage pattern. Most bilateral invertebrates are protostomes, which undergo spiral cleavage (Figure 43.5a). Echinoderms and all vertebrates are deuterostomes, and typically undergo radial cleavage (Figure 43.5b). Mammals, however, have a somewhat different pattern called rotational cleavage. The first cleavage divides the zygote along a plane that runs from top to bottom. Next, one cell divides the same way and the other divides in half at the cell equator. The amount of yolk stored inside an egg also affects cleavage patterns. When there is little yolk, cleavage is complete; the first cut divides all the cytoplasm. An abundance of yolk will impede divisions, so cleavage is incomplete. Sea urchin eggs have little yolk, so their cleavage is complete and all blastomeres are similar in size (Figure 43.6a). The same is true for the nearly
Collectively, the cells produced by cleavage constitute the blastula. Tight junctions hold the loose collection of cells together. Structure of the blastula varies with a species’ cleavage pattern. In sea urchins, complete cleavage produces a blastula that is a hollow ball of cells. In animals with highly yolky eggs, such as birds and many fish, a disk-shaped collection of cells, called a blastodisc, forms atop the yolk. There is no large fluid-filled space. A mammal’s blastula is a blastocyst, with outer cells that secrete fluid into the ball’s cavity and other cells clustered in a mass against the cavity wall. The inner cells will develop into the embryo.
Take-Home Message What are the effects of cytoplasmic localization and cleavage? In an unfertilized egg, many enzymes, mRNAs, yolk, and other materials are localized in specific parts of the cytoplasm. This cytoplasmic localization helps guide development. Cleavage divides a fertilized egg into a number of small cells but does not increase its original volume. The cells (blastomeres) inherit different parcels of cytoplasm that will make them behave differently, later in development.
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43.3
From Blastula to Gastrula The first tissues of an animal body form during gastrulation, when cells of the blastula rearrange themselves.
Link to Primary tissues 32.6
Hundreds to thousands of cells form during cleavage, depending on the species. Starting with gastrulation, cells begin to migrate and rearrange themselves. Figure 43.7 provides an example. Mechanisms of gastrulation vary among species. For example, an entire sheet of cells may bend inward, individual cells may migrate, or rows of cells may bend back on themselves. In most animals, gastrulation produces a gastrula with three primary tissue layers: an outermost layer of ectoderm, a middle layer of mesoderm, and an inner layer of endoderm (Section 32.6). What initiates gastrulation? Hilde Mangold, one of Spemann’s students, discovered the answer. She knew that during gastrulation, some cells of a
salamander blastula move inward through an opening on its surface. Cells in the dorsal (upper) lip of the opening are descended from cells in a zygote’s gray crescent. Mangold hypothesized that signals from dorsal lip cells caused gastrulation. She predicted that a transplant of dorsal lip material from one embryo to another would cause gastrulation at the recipient site. Mangold carried out many transplants (Figure 43.8a), and the results supported her prediction. Cells migrated inward at the transplant site, as well as at the usual location (Figure 43.8b). A salamander larva with two joined sets of body parts developed (Figure 43.8c). Apparently, signals from transplanted cells had caused their new neighbors to develop in a novel way. This experiment also explained the results shown in Figure 43.4c. Without any gray crescent cytoplasm, an embryo does not have cells that would normally become the dorsal lip. In the absence of the signals produced by these cells, development stops short. The effect of cells of a salamander gastrula’s dorsal lip region on nearby cells is an example of embryonic induction. By this process, the fate of one group of embryonic cells is affected by its proximity to another group of cells. In this case, the cells of the dorsal lip alter the behavior of their neighbors.
Take-Home Message What is gastrulation and how is it controlled?
Figure 43.7 Gastrulation in a fruit fly (Drosophila). In these insects, cleavage is restricted to the outermost region of cytoplasm; the interior is filled with yolk. The series of photographs, all cross-sections, shows sixteen cells (stained gold ) migrating inward. The opening that the cells move in through will become the fly’s mouth. Descendants of the stained cells will form mesoderm. Movements shown in the photos occur during a period of less than 20 minutes.
A Dorsal lip excised from donor embryo, grafted to novel site in another embryo.
B Graft induces a second site of inward migration.
Gastrulation is the developmental process during which cells rearrange themselves into primary tissue layers. Gastrulation occurs when certain cells of the blastula make and release short-range signals that cause nearby cells to move about, either singly or as a cohesive group. This process is an example of embryonic induction.
C The embryo develops into a “double” larva, with two heads, two tails, and two bodies joined at the belly.
Figure 43.8 Animated Experimental evidence that signals from dorsal lip cells start amphibian gastrulation. A dorsal lip region of a salamander embryo was transplanted to a different site in another embryo. A second set of body parts started to form.
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43.4
Specialized Tissues and Organs Form
Cell differentiation lays the groundwork for formation of specialized tissues and organs.
A Gastrulation produces a sheet of ectodermal cells.
Links to Cell differentiation 15.1, Master genes 15.2, Apoptosis 27.6
B As microtubules constrict or lengthen in different cells, the cells change shape, and the sheet forms a neural groove.
Cell Differentiation From gastrulation onward, selective gene expression occurs: Different cell lineages express different genes. That is the start of cell differentiation, the process by which cell lineages become specialized (Section 15.1). Intercellular signals can encourage differentiation, as during induction. In addition, morphogens, signalling molecules encoded by master genes, diffuse out from their source and form a concentration gradient in the embryo. A morphogen’s effects on target cells depends on its concentration. Cells close to the source of a morphogen are exposed to a high concentration and turn on different genes than distant cells exposed to a lower morphogen concentration.
Morphogenesis and Pattern Formation Cellular signals help bring about morphogenesis, the process by which tissues and organs form. During morphogenesis, some cells migrate to new locations. For example, neurons in the center of the developing brain creep along extensions of glial cells or the axons of other neurons until they reach their final position. Sheets of cells change shape, forming organs such as the neural tube, the forerunner of the vertebrate brain and spinal cord (Figure 43.9). Some cells even die on cue. By the process of apoptosis, signals from cells cause others to self-destruct. Apoptosis sculpts human fingers from a paddlelike body part (Section 27.6).
mesoderm of chick embryo forelimb
AER (region of signalsending ectoderm)
neural groove
C Edges of the groove meet and detach from the main sheet, forming the neural tube.
ectoderm
neural tube
Figure 43.9 Animated Neural tube formation. Microtubule changes alter cell shape, causing the sheet of ectoderm to fold into a tubular form.
Why does a hand form at the end of an arm? Why not a foot? Pattern formation is the process by which body parts form in a specific place. For example, a tissue called AER (apical ectodermal ridge) forms at tips of a chick’s limb buds and induces mesoderm beneath it to develop as a limb (Figure 43.10a). Whether a wing or a leg forms depends on positional information set down earlier in development (Figure 43.10b). Take-Home Message What processes produce specialized cells, tissues, and organs? Selective gene expression is the basis of cell differentiation. Signaling molecules contribute to differentiation. Morphogens diffuse through an embryo and have different effects depending on their concentration in each region.
Organs take shape as cells migrate, fold as sheets, and die on cue.
A Experiment 1: Remove wing bud’s AER
B Experiment 2: Graft a bit of leg mesoderm under the AER of a wing
AER removed
mesoderm from leg
no limb forms
wing
leg forms
Figure 43.10 Animated Control of limb formation in a chick. (a) Cells at the tip of a limb bud tell mesoderm under it to form a limb. Remove these AER cells and no limb forms. (b) Whether a limb becomes a wing or a leg depends on positional signals that the mesoderm received earlier.
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An Evolutionary View of Development Similarities in developmental pathways among animals are evidence of common ancestry.
Link to Homeotic genes 15.3
A General Model for Animal Development Through studies of animals such as roundworms, fruit flies, fish, and mice, researchers have come up with a general model for development. The key point of the model is this: Where and when particular genes are expressed determines how an animal body develops. First, molecules confined to different areas of an unfertilized egg induce localized expression of master genes in the zygote. Products of these master genes diffuse outward, so concentration gradients for these products form along the head-to-tail and dorsal-toventral axes of the developing embryo. Second, depending on where they fall within these concentration gradients, cells in the embryo activate or suppress other master genes. The products of these genes become distributed in gradients, which affect other genes, and so on. Third, this positional information affects expression of homeotic genes, genes that regulate development of specific body parts, as introduced in Section 15.3. All animals have similar homeotic genes. For example, a mouse’s eyeless gene guides development of its eyes. Introduce the mouse version of this gene into a fruit fly, and eyes will form in tissues where the introduced gene is expressed.
a
Developmental Constraints and Modifications The developmental model just described helps explain why we only see certain types of animal body plans. We know that body plans are influenced by physical constraints such as the surface-to-volume ratio. An animal cannot evolve large size unless it has circulatory and respiratory mechanisms to service body cells that reside far from the body surface. There are also architectural constraints. These are constraints imposed by the existing body framework. For example, the first vertebrates on land had a body plan with four limbs. Evolution of wings in birds and bats occurred through modification of existing forelimbs, not by sprouting new limbs. Although it might be advantageous to have both wings and arms, no vertebrate with both has ever been discovered. Finally, there are phyletic constraints on body plans. These constraints are imposed by interactions among genes that regulate development in a lineage. Once master genes evolved, their interactions determined the basic body form. Mutations that dramatically alter effects of these master genes are often lethal. For example, vertebrates have paired bones and skeletal muscles along the body’s head-to-tail axis. This pattern arises early in development, when the mesoderm on either side of the embryo’s neural tube becomes divided into blocks of cells called somites (Figure 43.11). The somites will later develop into bones and skeletal muscles. A complex pathway involving many genes governs somite formation. Any mutation that disrupts this pathway and halts somite formation is lethal during development. Thus, we do not find vertebrates with an unsegmented body plan, although the number of somites does vary among species. In short, mutations that affect development led to a variety of forms among animal lineages. These mutations brought about morphological changes through the modification of existing developmental pathways, rather than by blazing entirely new genetic trails.
Take-Home Message somite
Why are developmental processes and body plans similar among animal groups?
c
b
Figure 43.11 (a) An adult zebrafish. (b) Normal zebrafish embryo with somites that give rise to bone and muscle. (c) Mutant embryo that cannot form somites. It will die in early development.
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In all animals, cytoplasmic localization sets the stage for cell signaling. The signals activate sets of master genes shared by most animal groups. The products of these genes cause embryonic cells to form tissues and organs at certain locations. Once a developmental pathway evolves, drastic changes to genes that govern this pathway are generally not favored.
43.6
Overview of Human Development
Like all animals, humans begin life as a single cell and go through a series of developmental stages.
Links to Placental mammals 26.12, Human fertilization 42.8
Chapter 42 introduced the structure and function of human reproductive organs, and explained how an egg and sperm meet at fertilization to form a zygote (Section 42.8). The remaining sections of this chapter will continue this story, with an in-depth look at human development. In this section, we provide an overview of the process and define the stages that we will discuss. Prenatal (before birth) and postnatal (after birth) stages are listed in Table 43.1. It takes about five trillion mitotic divisions to go from the single cell of a zygote to the ten trillion or so cells of an adult human. The process gets underway during a pregnancy that typically lasts an average of thirty-eight weeks from the time of fertilization. The first cleavage occurs about 12 to 24 hours after fertilization. It takes about one week for a blastocyst to form. Again, a blastocyst is a mammalian blastula. In humans and other placental mammals, a blastocyst embeds itself in its mother’s uterus. As the offspring develops, nutrients diffusing from the maternal bloodstream across the placenta sustain it (Section 26.12). All major organs, including the sex organs, form during the embryonic period, which ends after eight weeks. The bones of the developing skeleton are laid down as cartilage models, which are then invaded by bone cells that convert the cartilage to bone. At the end of the embryonic period, the developing individual is referred to as a fetus. In the fetal period, from the start of the ninth week until birth, organs grow and become specialized. We divide the prenatal period into three trimesters. The first trimester includes months one through three; the second trimester is months four through six; the third trimester is months seven through nine. Births before 37 weeks are considered premature. A fetus born earlier than 22 weeks rarely survives because its lungs are not yet fully mature. About half of births that occur before 26 weeks result in some sort of long-term disability. After birth, the human body continues to grow and its body parts continue to change in proportion. Figure 43.12 shows the proportional changes during development. Postnatal growth is most rapid between 13 and 19 years. Sexual maturation occurs at puberty, and bones stop growing shortly thereafter. The brain is the last organ to become fully mature: Portions of it continue to develop until the individual is about 19 to 22 years old.
8-week embryo
12-week embryo
newborn
2 years
5 years
13 years (puberty)
22 years
Figure 43.12 Observable, proportional changes in prenatal and postnatal periods of human development. Changes in overall physical appearance are slow but noticeable until the teens.
Table 43.1
Stages of Human Development
Prenatal period Zygote
Single cell resulting from fusion of sperm nucleus and egg nucleus at fertilization.
Blastocyst (blastula)
Ball of cells with surface layer, fluid-filled cavity, and inner cell mass.
Embryo
All developmental stages from two weeks after fertilization until end of eighth week.
Fetus
All developmental stages from ninth week to birth (about 38 weeks after fertilization).
Postnatal period Newborn
Individual during the first two weeks after birth.
Infant
Individual from two weeks to fifteen months.
Child
Individual from infancy to about ten or twelve years.
Pubescent
Individual at puberty; secondary sexual traits develop. Girls, between 10 and 15 years; boys, between 11 and 16 years.
Adolescent
Individual from puberty until about 3 or 4 years later; physical, mental, emotional maturation.
Adult
Early adulthood (between 18 and 25 years); bone formation and growth finished. Changes proceed slowly after this.
Old age
Aging processes result in expected tissue deterioration.
Take-Home Message How does human development proceed? Humans are placental mammals, so offspring develop in the mother’s uterus. By the end of the second week, the blastocyst is embedded in the uterus.
By the end of the eighth week, the embryo has all the typical human organs. Most of a pregnancy is taken up with the fetal period, during which organs grow and take on their specialized functions.
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Early Human Development After a human blastocyst forms, it burrows into the wall of its mother’s uterus and a system of membranes forms outside the embryo.
Link to Human fertilization 42.8
Cleavage and Implantation Fertilization of a human egg typically occurs in one of the oviducts. Cleavage gets underway within a day or two of fertilization, as the zygote travels through the oviduct toward the uterus (Figure 43.13a,b). By the time it reaches the uterus, the zygote has become a cluster of sixteen cells called a morula (Figure 43.13c). A blastocyst of a few hundred cells forms by the fifth day. It consists of an outer layer of cells, a cavity filled with their secretions (a blastocoel), and an inner cell mass (Figure 43.13d). The embryo develops from the inner cell mass. The outer cells will help form membranes that surround the developing embryo. About six days after fertilization, the blastocyst is usually in the uterus. It now expands by cell divisions and uptake of fluid. This increase in size ruptures the noncellular zona pellucida, allowing the blastocyst to slip out of this enclosing layer. Implantation begins when the blastocyst attaches to the endometrium and
burrows into it. During implantation, the inner cell mass develops into two flattened layers of cells called the embryonic disk (Figure 43.13e,f ). In an ectopic pregnancy, the blastocyst implants in tissue other than the uterus—most commonly an oviduct. Such a pregnancy cannot be carried to term and must be removed surgically to protect the life of the mother. Use of birth control pills, a history of sexually transmitted disease, and certain inflammatory disorders increase the risk of ectopic pregnancy.
Extraembryonic Membranes Membranes start forming outside the embryo during implantation (Table 43.2). A fluid-filled amniotic cavity opens up between the embryonic disk and part of the blastocyst surface (Figure 43.13f ). Many cells migrate around the wall of the cavity and form the amnion, a membrane that will enclose the embryo. Fluid in the cavity will function as a buoyant cradle in which an embryo can grow, move freely, and be protected from abrupt temperature changes and any potentially jarring impacts. As the amnion forms, other cells migrate around the inner wall of the blastocyst and form a lining that
endometrial epithelium
fertilization in oviduct
implantation in the uterus
cavity inside the uterus
surface layer cells of the blastocyst endometrium
inner cell mass
blastocoel inner cell mass
A DAYS 1–2. The first cleavage furrow extends between the two polar bodies. Later cuts are angled, so cells become asymmetrically arranged. Until the eight-cell stage forms, they are loosely organized, with space between them.
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B DAY 3. After the third cleavage, cells abruptly huddle into a compacted ball, which tight junctions among the outer cells stabilize. Gap junctions formed along the interior cells enhance intercellular communication.
HOW ANIMALS WORK
C DAY 4. By 96 hours there is a ball of sixteen to thirty-two cells shaped like a mulberry. It is a morula (after morum, Latin for mulberry). Cells of the surface layer will function in implantation and will give rise to a membrane, the chorion.
D DAY 5. A blastocoel (fluid-filled cavity) forms in the morula as a result of surface cell secretions. By the thirty-two-cell stage, differentiation is occurring in an inner cell mass that will give rise to the embryo proper. This embryonic stage is the blastocyst.
E DAYS 6–7. Some of the blastocyst’s surface cells attach themselves to the endometrium and start to burrow into it. Implantation has started. actual size
becomes the yolk sac. In reptiles and birds, this sac holds yolk. In humans, cells of the yolk sac give rise to the embryo’s blood cells and to germ cells. Before a blastocyst is fully implanted, spaces that open in maternal tissues become filled with blood seeping in from ruptured capillaries. In the blastocyst, a new cavity opens up around the amnion and yolk sac. The lining of this cavity becomes the chorion, a membrane that is folded into many fingerlike projections that extend into blood-filled maternal tissues. It will become part of the placenta. The placenta is an organ that functions in exchanges of materials between the bloodstreams of a mother and her developing child. After the blastocyst is implanted, an outpouching of the yolk sac will become the fourth extraembryonic membrane—the allantois. It gives rise to the urinary bladder and the placenta’s blood vessels.
Table 43.2 Amnion
Human Extraembryonic Membranes Encloses, protects embryo in a fluid-filled, buoyant cavity
Yolk sac
Becomes site of red blood cell formation; germ cell source
Chorion
Lines amnion and yolk sac, becomes part of placenta
Allantois
Source of urinary bladder and blood vessels for placenta
HCG can be detected in a mother’s urine as early as the third week of pregnancy. At-home pregnancy tests include a treated “dipstick” that changes color when exposed to urine that contains HCG.
Take-Home Message
Early Hormone Production Once implanted, a blastula releases human chorionic gonadotropin (HCG). This hormone causes the corpus luteum to keep secreting progesterone and estrogens. These hormones prevent menstruration and maintain the uterine lining. After about three months, the placenta takes over the secretion of HCG.
start of amniotic cavity
What occurs during the first two weeks of human development? Cleavage produces a morula and then a blastocyst, which slips out of the zona pellucida and implants itself in the endometrium, the lining of the uterus. During implantation, projections from the blastocyst grow into maternal tissues. Connections that will support the developing embryo begin to form. The inner cell mass of the blastocyst will become the embryo. Other portions of the blastocyst give rise to four external membranes. The outermost of these is the amnion, which encloses and protects the embryo in a fluid-filled cavity.
blood-filled spaces
start of embryonic disk
chorion chorionic villi
chorionic cavity
amniotic cavity
connective tissue start of yolk sac
F DAYS 10–11. The yolk sac, embryonic disk, and amniotic cavity have started to form from parts of the blastocyst.
start of chorionic cavity
actual size
yolk sac H DAY 14. A connecting stalk has formed between the embryonic actual disk and chorion. Chorionic villi, size which will be features of a placenta, start to form.
G DAY 12. Blood-filled spaces form in maternal actual tissue. The chorionic size cavity starts to form.
Figure 43.13 Animated From fertilization through implantation. A blastocyst forms, and its inner cell mass becomes an embryonic disk two cells thick. It will later become the embryo. Three extraembryonic membranes (the amnion, chorion, and yolk sac) start forming. A fourth membrane (allantois) forms after implantation is over.
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Emergence of the Vertebrate Body Plan Gastrulation occurs in the third week as the embryo sets off down the pathway of typical vertebrate development.
Link to Coelomic cavity 25.1
By two weeks after fertilization, the inner cell mass of a blastocyst is a two-layered embryonic disk. During gastrulation in the third week, cells migrate inward along a depression, the primitive streak, that forms on the disk’s surface (Figure 43.14a). The resulting three germ layers of the gastrula are the forerunners of all tissues (Table 43.3). The primitive streak’s location establishes the body’s head-to-tail axis. Many master genes are now being expressed and the tissues and organs are beginning to take shape. For example, by the eighteenth day after fertilization,
Table 43.3
Derivatives of Human Germ Layers
Ectoderm (outer layer)
Outer layer (epidermis) of skin; nervous tissue
Mesoderm (middle layer)
Connective tissue of skin; skeletal, cardiac, smooth muscle; bone; cartilage; blood vessels; urinary system; gut organs; peritoneum (coelom lining); reproductive tract
the embryonic disk has two folds that will merge into a neural tube, which will develop into the spinal cord and brain (Figure 43.14b). Mesodermal folding also forms a notochord, which acts as a structural model for the bony segments of the vertebral column. Spina bifida is a birth defect in which the neural tube and one or more vertebrae do not form as they should. As a result, the spinal cord protrudes out of the vertebral column at birth. By the end of the third week, somites form. These paired segments of mesoderm will develop into bones, skeletal muscles of the head and trunk, and overlying dermis of the skin. Pharyngeal arches (Figure 43.14c) that start to form at this time will later contribute to the pharynx, larynx, and the face, neck, mouth, and nose. Small spaces begin to open up in certain parts of the mesoderm; these spaces will eventually interconnect as a coelomic cavity (Section 25.1).
Take-Home Message
Endoderm (inner layer)
Lining of gut and respiratory tract, and organs derived from these linings
paired neural folds
yolk sac embryonic disk amniotic cavity chorionic cavity
What happens during weeks three and four of a pregnancy?
Gastrulation takes place, producing a three-layered embryo.
The neural tube and notochord form.
Somites appear on either side of the neural tube.
future brain
pharyngeal arches
primitive streak
neural groove (below, notochord is forming)
A DAY 15. A faint band appears around a depression along the axis of the embryonic disk. This band is the primitive streak, and it marks the onset of gastrulation in vertebrate embryos.
somites
B DAYS 18–23. Organs start to form through cell divisions, cell migrations, tissue folding, and other events of morphogenesis. Neural folds will merge to form the neural tube. Somites (bumps of mesoderm) appear near the embryo’s dorsal surface. They will give rise to most of the skeleton’s axial portion, skeletal muscles, and much of the dermis.
Figure 43.14 Hallmarks of the embryonic period of humans and other vertebrates. A primitive streak and then a notochord form. Neural folds, somites, and pharyngeal arches form later. (a,b) Dorsal views of the embryo’s back. (c) Side view.
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C DAYS 24–25. By now, some embryonic cells have given rise to pharyngeal arches. These will contribute to the formation of the face, neck, mouth, nasal cavities, larynx, and pharynx.
43.9
The Function of the Placenta
The placenta allows transfer of substances between a mother and her developing child without mixing their blood.
All exchange of materials between an embryo and its mother takes place by way of the placenta, a pancakeshaped, blood-engorged organ that consists of uterine lining and extraembryonic membranes. At full term, the placenta covers about one-fourth of the uterus’s inner surface (Figure 43.15). The placenta begins forming early in pregnancy. By the third week, maternal blood has begun to pool in spaces in the endometrial tissue. Chorionic villi—tiny fingerlike projections from the chorion—extend into the pools of maternal blood. Embryonic blood vessels extend outward through the umbilical cord to the placenta, and then into the chorionic villi. Embryonic blood exchanges substances
with maternal blood, but the two bloodstreams do not mix. If they mixed, some maternal antibodies could attack the embryo. Oxygen and nutrients diffuse from maternal blood and into embryonic blood vessels in the villi. Wastes diffuse the other way, and the mother’s body disposes of them. After the third month, the placenta produces large amounts of HCG, progesterone, and estrogens. These hormones encourage the ongoing maintenance of the uterine lining. Take-Home Message What is the function of the placenta? Vessels of the embryo’s circulatory system extend through the umbilical cord to the placenta, where they run through pools of maternal blood. Maternal and embryonic blood do not mix; substances diffuse between the maternal and embryonic bloodstreams.
appearance of the placenta at full term
umbilical cord
4 weeks
amniotic fluid around fetus
uterine tissue 8 weeks
fetal blood vessels
maternal blood vessels
12 weeks
movement of solutes to and from maternal blood vessels (red and blue arrows)
umbilical cord blood-filled space between villi
chorionic villus
Figure 43.15 Relationship between fetal and maternal blood circulation in a full-term placenta. Blood vessels extend from the fetus, through the umbilical cord, and into chorionic villi. Maternal blood flows into spaces between villi. However, the two bloodstreams do not intermingle. Oxygen, carbon dioxide, and other small solutes diffuse across the placental membrane surface.
fused amniotic and chorionic membranes
tissues of uterus
CHAPTER 43
ANIMAL DEVELOPMENT 771
43.10 Emergence of Distinctly Human Features A human embryo’s tail and pharyngeal arches label it as a chordate. The features disappear during fetal development.
Link to Sex organ formation 12.1
When the fourth week ends, the embryo is 500 times the size of a zygote, but still less than 1 centimeter long. Growth slows as details of organs begin to fill in. Limbs form; paddles are sculpted into fingers and
WEEK 4
toes. The umbilical cord and the circulatory system develop. Growth of the head now surpasses that of all other regions (Figure 43.16). Reproductive organs begin forming, as described in Section 12.1. At the end of the eighth week, all organ systems have formed and we define the individual as a human fetus. In the second trimester, reflexive movements begin as developing nerves and muscles connect. Legs kick, arms wave about, and fingers grasp. The fetus frowns,
WEEKS 5–6 yolk sac connecting stalk embryo
forebrain
head growth exceeds growth of other regions
future lens
retinal pigment future external ear
pharyngeal arches developing heart upper limb bud somites neural tube forming lower limb bud tail
actual length
Figure 43.16 Human embryo at successive stages of development.
772 UNIT VI
HOW ANIMALS WORK
upper limb differentiation (hand plates develop, then digital rays of future fingers; wrist, elbow start forming) umbilical cord formation between weeks 4 and 8 (amnion expands, forms tube that encloses the connecting stalk and a duct for blood vessels) foot plate
actual length
squints, puckers its lips, sucks, and hiccups. When a fetus is five months old, its heartbeat can be heard clearly through a stethoscope positioned on the mother’s abdomen. The mother can sense movements of fetal arms and legs. By now, soft, fetal hair (lanugo) covers the skin; most will be shed before birth. A thick, cheesy coating (vermix) protects the skin from abrasion. In the sixth month, eyelids and eyelashes form. Eyes open during
the seventh month, the start of the final trimester. By this time, all portions of the brain have formed and have begun to function. Take-Home Message What occurs during the late embryonic and the fetal periods? The embryo takes on its human appearance by week eight but remains tiny. During the fetal period, organs begin functioning and growth is rapid.
placenta
WEEK 8
WEEK 16
Length:
final week of embryonic period; embryo looks distinctly human compared to other vertebrate embryos upper and lower limbs well formed; fingers and then toes have separated primordial tissues of all internal, external structures now developed tail has become stubby
Weight:
16 centimeters (6.4 inches) 200 grams (7 ounces)
WEEK 29
Length: 27.5 centimeters (11 inches) Weight: 1,300 grams (46 ounces) WEEK 38 (full term)
Length: 50 centimeters (20 inches) Weight: 3,400 grams (7.5 pounds) During fetal period, length measurement extends from crown to heel (for embryos, it is the longest measurable dimension, as from crown to rump).
actual length
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43.11 Mother as Provider and Protector An embryo depends on its mother to supply nutrients and is subjected to toxins or pathogens to which she is exposed.
development. Her own demands for vitamins and minerals increase, but both are absorbed preferentially across the placenta and taken up by the embryo. Taking B-complex vitamins in early pregnancy reduces the embryo’s risk of neural tube defects. Folate (folic acid) is especially critical in this respect. Dietary deficiencies affect many developing organs. For example, if a mother does not get enough iodine, her newborn may be affected by cretinism, a disorder that affects brain function and motor skills (Section 35.6). A diabetic woman who does not control her blood sugar during pregnancy provides excess sugar to her fetus. This excess can cause birth defects. Also, the fetus converts the extra sugar to fat and becomes unusually large. An oversized fetus can cause problems during delivery.
Links to Thyroid hormone 35.6, Antibodies 38.6, Carbon monoxide 38.7
According to the Centers for Disease Control, about 3 percent of children born in the United States have some sort of birth defect. The defects include visible problems such as a cleft lip or a club foot, as well as internal problems such as heart or intestinal malformations. Some birth defects have a genetic basis, but others result from an environmental factor such as poor nutrition or exposure to a teratogen. A teratogen is a toxin or infectious agent that interferes with development. Figure 43.17 shows the periods when specific organs are the most vulnerable to damage by exposure to teratogens.
About Morning Sickness About two-thirds of pregnant women begin to have episodes of nausea with or without vomiting around the sixth week of pregnancy. Although commonly known as morning sickness, the symptoms can occur at any time of day. They typically end by the twelfth
Nutritional Considerations A pregnant woman who eats a well-balanced diet supplies her future child with all of the proteins, carbohydrates, and lipids it needs for growth and
defects in physiology; physical abnormalities minor
major morphological abnormalities weeks: 1
2
cleavage, implantation
3 future heart future brain
4
future eye
limb buds
5
6
future ear
7
8
9
20 – 36
16
38
palate forming
teeth
external genitals central nervous system heart upper limbs eyes lower limbs teeth palate external genitals
insensitivity to teratogens
ear
774 UNIT VI
HOW ANIMALS WORK
Figure It Out: Is teratogen exposure in the
16th week more likely to affect the heart or the genitals? Answer: Genitals
Figure 43.17 Teratogen sensitivity. Teratogens are drugs, infectious agents, and environmental factors that cause birth defects. Dark blue signifies the highly sensitive period for an organ or body part; light blue signifies periods of less severe sensitivity. For example, the upper limbs are most sensitive to damage during weeks 4 through 6, and somewhat sensitive during weeks 7 and 8.
FOCUS ON HEALTH
week. Morning sickness generally does not cause problems and may have an adaptive function. Morning sickness most often occurs during the period when organs of the child she is carrying are developing and are most vulnerable to teratogens. Women who have morning sickness are less likely to miscarry than women who are not affected, and women who vomited were more likely to carry a child to term than those who only felt nauseous. The foods women who have morning sickness report they are most likely to avoid—fish, poultry, meat, and eggs—are the ones most likely to be tainted by dangerous microorganisms.
Infectious Agents Some antibodies in a pregnant woman’s blood cross the placenta and protect an embryo or fetus from bacterial infections (Section 38.6). But some viral diseases can be dangerous in the early weeks after fertilization. Rubella, or German measles, is an example. A woman may sidestep the risk of passing on the rubella virus by getting vaccinated before she becomes pregnant. A relative of the protist that causes malaria sometimes lurks in garden soil, cat feces, and undercooked meat. It causes toxoplasmosis. The disease often does not cause symptoms, so a pregnant woman may become infected and not realize it. If the parasite crosses the placenta, it can infect her child and lead to developmental problems, a miscarriage, or stillbirth. To minimize the risk, pregnant women should eat well-cooked meat and avoid cat feces. Alcohol and Caffeine Alcohol passes across the placenta, so when a pregnant woman drinks alcohol, her embryo or fetus feels the effects. Alcohol exposure can cause fetal alcohol syndrome, or FAS. A small head and brain, facial abnormalities, slow growth, mental impairment, heart problems, and poor coordination characterize affected infants (Figure 43.18). The damage is permanent. Children affected by FAS never catch up, physically or mentally. Most doctors now advise women who are pregnant or attempting to become pregnant to avoid alcohol entirely. Even before a woman knows she is pregnant, tissues of the embryonic nervous system have begun forming, and alcohol can damage them. Even moderate drinking during pregnancy increases risk of miscarriage and stillbirth. Laboratory studies have shown that caffeine interferes with nervous system development in animals, and physicians have suspected that it may also harm human embryos. A recent study supports this hypothesis. The study showed that women who took in 200 milligrams a day of caffeine (the equivalent of one and a half cups of coffee), had twice as many miscarriages as those who avoided caffeine. The study’s authors advise pregnant women to chose decaffeinated or caffeine-free beverages. Smoking Smoking or exposure to secondhand smoke increases the risk of miscarriage and adversely affects fetal growth and development. Carbon monoxide in the smoke can outcompete oxygen for the binding sites on
Figure 43.18 An child with fetal alcohol syndrome—FAS. Obvious symptoms are low and prominently positioned ears, improperly formed cheekbones, and an abnormally wide, smooth upper lip. Growth-related complications, heart problems, and nervous system abnormalities are also common.
hemoglobin (Section 38.7), so the embryo or fetus of a smoker gets less oxygen than that of a nonsmoker. In addition, levels of the addictive stimulant nicotine in amniotic fluid can be higher than those in the mother’s blood. Effects of maternal smoking persist long after birth. One British study tracked a group of children born in the same week over the course of seven years. More children of smokers died of postdelivery complications, and the survivors were smaller, with twice as many heart defects. When the study ended, the children of smokers were nearly half a year behind the normal reading age.
Prescription Drugs Some medications cause birth defects. For example, thalidomide was routinely prescribed to treat morning sickness during the 1960s in Europe. Infants of some of the women who used it during the first trimester had severely deformed arms and legs or none at all. The FDA never approved use of thalidomide for pregnant women in the United States. Isotretinoin (Accutane) is widely used in the United States and elsewhere. This highly effective treatment for severe acne is often prescribed for young women. If taken early in a pregnancy, it can cause heart problems or facial and cranial deformities in the embryo. Certain antidepressants increase the risk of birth defects. Paroxetine (Paxil) and related drugs inhibit the reuptake of serotonin. Use of these drugs during early pregnancy increases the likelihood of heart malformations. Taking them later in pregnancy increases risk that an infant will have fatal heart and lung disorders.
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ANIMAL DEVELOPMENT 775
43.12 Birth and Lactation As in other placental mammals, human fetuses are born live and are nourished with nutritious milk secreted from the mother’s mammary glands. Shifts in the levels of hormones help control these processes.
milk-producing mammary gland
nipple
Links to Pituitary hormones 35.3, Positive feedback 27.3
Giving Birth A mother’s body changes as her fetus nears full term, at about 38 weeks after fertilization. Until the last few weeks, her firm cervix helped prevent the fetus from slipping out of her uterus prematurely. Now cervical connective tissue becomes thinner, softer, and more flexible. These changes will allow the cervix to stretch enough to permit the fetus to pass out of the body. The birth process is known as labor. Typically, the amnion ruptures right before birth, so amniotic fluid drains out from the vagina. The cervical canal dilates. Strong contractions propel the fetus through it, then out through the vagina (Figure 43.19). A positive feedback mechanism operates during labor. When the fetus nears full term, it typically shifts position so that its head puts pressure on the mother’s cervix. Receptors inside the cervix sense pressure and signal the hypothalamus, which signals the posterior lobe of the pituitary to secrete oxytocin. In a positive feedback loop, oxytocin binds to smooth muscle of the uterus, causing uterine contractions that push the fetus against the cervix. The added pressure triggers more oxytocin secretion, which causes more contractions and more cervical stretching. Forceful uterine contractions continue until the fetus is forced through the cervix and out of the mother’s body. Strong muscle contractions also detach and expel the placenta from the uterus as the “afterbirth.” The umbilical cord that connects the newborn to this mass of expelled tissue is clamped, cut short, and tied. The short stump of cord left in place withers and falls off. The navel marks the former attachment site.
adipose tissue
a
b
milk duct
Figure 43.20 Cutaway views of (a) a breast of a woman who is not pregnant and (b) a breast of a lactating woman.
Nourishing the Newborn Before a pregnancy, a woman’s breast tissue is mostly adipose tissue. Milk ducts and mammary glands are small and inactive (Figure 43.20). During pregnancy, these structures enlarge in preparation for lactation, or milk production. Prolactin, a hormone secreted by the mother’s anterior pituitary, triggers milk synthesis. After birth, a decline in progesterone and estrogens causes milk production to go into high gear. The stimulus of a newborn’s suckling causes the release of oxytocin. The hormone stimulates muscles around the milk glands to contract and force milk into the ducts. Besides being nutrient-rich, human breast milk has antibodies that protect a newborn from some viruses and bacteria. Nursing mothers should remember that drugs, alcohol, and other toxins end up in milk. Take-Home Message What roles do hormones play in birth and lactation? During birth, the hormone oxytocin stimulates muscle contractions that force a fetus out of its mother’s body. Prolactin stimulates milk production and oxytocin causes milk secretion from milk ducts.
placenta
placenta detaching from wall of uterus
wall of uterus umbilical cord
umbilical cord
dilating cervix
A
B
C
Figure 43.19 Animated Expulsion of (a,b) a human fetus and (c) afterbirth during labor. The afterbirth consists of the placenta, tissue fluid, and blood.
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HOW ANIMALS WORK
IMPACTS, ISSUES REVISITED
Mind-Boggling Births
Multiple births that result from use of fertility drugs not only put offspring at risk, they also threaten the health of the mother. Among other things, carrying two or more fetuses requires a greater blood volume, which puts a strain on the mother’s heart and raises her risk of high blood pressure. Also, such pregnancies require more placental area, raising the risk of dangerous blood loss when the placenta detaches after birth.
Summary Section 43.1 Most animal life cycles have six stages of development. Gametes form, then fertilization takes place. Cleavage produces a blastula. Gastrulation results in an early embryo (a gastrula) that has two or three primary tissue layers, or germ layers. Finally, organs form, and tissues and organs become specialized.
Use the animation on CengageNOW to track development of a frog.
Section 43.2, 43.3 Cytoplasmic localization, storage of different substances in different parts of the cytoplasm, is a feature of all oocytes. Cleavage distributes different portions of the egg cytoplasm to different cells. Patterns of cleavage vary among animal lineages. Cleavage ends with formation of a blastula. The mammalian blastula is a blastocyst, which has a fluid-filled cavity and an inner cell mass. The inner cell mass will become the embryo. During gastrulation, cell rearrangement produces layers of tissue. Most often, three tissue layers form: outer ectoderm, inner endoderm, and mesoderm in between ectoderm and endoderm. Gastrulation is controlled by signal-sending cells that cause movement of neighboring cells. This type of signalling interaction is an example of embryonic induction.
Use the animation on CengageNOW to learn about cytoplasmic localization and control of gastrulation.
Sections 43.4, 43.5 Selective gene expression leads to cell differentiation: cells become specialized by activating different subsets of their genome. Morphogens, products of master genes, act as long-range signals that diffuse out from a source and form a concentration gradient. This gradient affects which genes a cell turns on or off. Morphogenesis, the formation of tissues and organs, occurs as cells migrate, change shape, and undergo programmed cell death (apoptosis). Development of organs and limbs in particular places is pattern formation. Cues about position play a role in pattern formation. A general model for animal development is based on comparative studies. By this model, cytoplasmic localization in an oocyte causes localized expression of master genes in the zygote. Diffusion of morphogens—products of these master genes—creates gradients that cause the differential expression of other genes such as homeotic genes, which govern the formation of specific body parts.
How would you vote? Should use of fertility drugs be discouraged to prevent higher risk multiple pregnancies? See CengageNOW for details, then vote online.
Master genes are similar among all major animal groups. Developmental changes are constrained by interactions among master genes, as well as by physical and architectural factors. For example, in all vertebrates, paired blocks of mesoderm called somites form and give rise to paired skeletal muscles and bone.
Use the animation on CengageNOW to learn how the neural tube forms and how a chick wing develops.
Section 43.6 Human prenatal development takes nine months. Organs take shape during the embryonic period, which is over at the end of the eighth week. For the remainder of the pregnancy, the fetus grows larger and organs take on their specialized roles. Growth and development continue after birth (in the postnatal period). Sections 43.7–43.11 Human fertilization usually occurs inside an oviduct. Cleavage produces a morula, then a blastocyst. During implantation, a blastocyst buries itself in the uterine wall. Membranes form outside the blastocyst and support its development. The amnion encloses and protects the embryo in a fluid-filled sac. The chorion and allantois become part of the placenta, the organ that allows exchange of substances between the maternal and fetal bloodstreams. An implanted blastula makes human chorionic gonadotropin, a hormone that prevents menstruation and thus maintains the pregnancy. Gastrulation occurs after implantation. The first organ to form, the neural tube, later becomes the brain and spinal cord. Somites form on either side of the neural tube. By the end of the eighth week, the embryo has lost its tail and pharyngeal arches and has a distinctly human appearance. It continues to grow in size and its organs continue to mature during the fetal period. Nutrients and antibodies move across the placenta from mother to embryo or fetus, as do teratogens, which can cause birth defects.
Use the animation on CengageNOW to observe the events of human development.
Section 43.12 Hormones typically induce labor at about 38 weeks. Positive feedback controls secretion of oxytocin, a hormone that causes contractions that expel the fetus and then the afterbirth. Prolactin regulates the maturation of the mammary glands and then oxytocin causes lactation.
Use the animation on CengageNOW to observe labor.
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ANIMAL DEVELOPMENT 777
Data Analysis Exercise People considering fertility treatments should be aware that such treatments raise the risk of multiple births, and that multiple pregnancies are associated with an increased risk of some birth defects. Figure 43.21 shows the results of Yiwei Tang’s study of birth defects reported in Florida from 1996 to 2000. Tang compared the incidence of various defects among single and multiple births. She calculated the relative risk for each type of defect based on type of birth, and corrected for other differences that might increase risk such as maternal age, income, race, and medical care during pregnancy. A relative risk of less than 1 means that multiple births pose less risk of that defect occurring. A relative risk greater than 1 means multiples are more likely to have a defect. 1. What was the most common type of birth defect in the single-birth group?
Oral defects
2. Was that type of defect more or less common among the multiple-birth newborns than among single births?
Prevalence of Defect Multiples Singles Total birth defects
Relative Risk
358.50
250.54
1.46
Central nervous system defects
40.75
18.89
2.23
Chromosomal defects
15.51
14.20
0.93
Gastrointestinal defects
28.13
23.44
1.27
Genital/urinary defects
72.85
58.16
1.31
189.71
113.89
1.65
Musculoskeletal defects
20.92
25.87
0.92
Fetal alcohol syndrome
4.33
3.63
1.03
19.84
15.48
1.29
Heart defects
4. Does a multiple pregnancy increase the relative risk of chromosomal defects in offspring?
Figure 43.21 Prevalence, per 10,000 live births, of various types of birth defects among multiple and single births. Relative risk for each defect is given after researchers adjusted for the mother’s age, race, previous adverse pregnancy experience, education, Medicaid participation during pregnancy, as well as the infant’s sex and number of siblings.
Self-Quiz
10.
3. Tang found that multiples have more than twice the risk of single newborns for one type of defect. Which type?
Answers in Appendix III
1. The typical end product of cleavage is a a. zygote c. gastrula b. blastula d. gamete
.
2. Is this statement true or false? Materials are randomly distributed in egg cytoplasm, so cleavage parcels out same kinds of cytoplasmic components to all cells. 3. Cells differentiate as a direct result of . a. selective gene expression c. gastrulation b. morphogenesis d. all of the above 4.
help bring about morphogenesis. a. Cell migrations c. Cell suicide b. Changes in cell shape d. all of the above
5. Match each term with the most suitable description. apoptosis a. blastomeres form embryonic induction b. cellular rearrangements cleavage form primary tissues gastrulation c. cells die on cue pattern formation d. cells influence neighbors e. tissues, organs emerge in the correct places 6. A a. zygote
implants in the lining of the human uterus. b. gastrula c. blastocyst d. fetus
7. The , a fluid-filled sac, surrounds and protects an embryo and keeps it from drying out. a. amnion b. allantois c. yolk sac d. chorion 8. At full term, a placenta . a. is composed of extraembryonic membranes alone b. directly connects maternal and fetal blood vessels c. keeps maternal and fetal blood vessels separated 9. During the second trimester of pregnancy, . a. gastrulation ends b. eyes open c. heartbeats start 778 UNIT VI
HOW ANIMALS WORK
stimulates milk synthesis in mammary glands. a. HCG c. Testosterone b. Prolactin d. Oxytocin
11. Number these events in human development in the correct order. gastrulation occurs blastocyst forms morula forms zygote forms neural tube forms pharyngeal arches form 12.
gives rise to skeletal muscle and bone. a. Mesoderm c. Ectoderm b. Endoderm d. all of the above
Visit CengageNOW for additional questions.
Critical Thinking 1. By UNICEF estimates, each year 110,000 people are born with birth defects as a result of prenatal rubella infections. Deafness and blindness only occur if the mother becomes infected during the first trimester of pregnancy. Why? 2. The most common ovarian tumors in young women are ovarian teratomas. The name comes from the Greek word teraton, which means monster. What makes these tumors “monstrous” is the presence of well-differentiated tissues, most commonly bones, teeth, fat, and hair. Early physicians suggested that teratomas arose as a result of nightmares, witchcraft, or intercourse with the devil. Unlike all other tumors, which arise from somatic cells, teratomas arise from germ cells. Explain why a tumor derived from a germ cell is able to produce more differentiated cell types than one derived from a somatic cell.
VII
PRINCIPLES OF ECOLOGY
Lioness and her cub at sunset on the African savanna. What are the consequences of their interactions with each other, with other kinds of organisms, and with their environment? By the end of this last unit, you might find worlds within worlds in such photographs.
779
44
Animal Behavior IMPACTS, ISSUES
My Pheromones Made Me Do It
One spring day as Toha Bergerub was walking down a street
All honeybees defend their hives by stinging. Each can
near her Las Vegas home, she felt a sharp pain above her
sting only once, and all make the same kind of venom. Even
right eye—then another, and another. Within a few seconds,
so, compared with European honeybees, Africanized ones
hundreds of stinging bees covered the upper half of her
get riled up more easily, attack in greater numbers, and stay
body. Firefighters in protective gear rescued her, but not
agitated longer. Some are known to have chased people for
before she was stung more than 500 times. Bergerub, who
more than a quarter of a mile.
was seventy-seven years old at the time, spent a week in the hospital, but recovered fully. Bergerub’s attackers were Africanized honeybees, a hybrid
What makes Africanized bees so testy? Part of the answer is that they have a heightened response to alarm pheromone. A pheromone is a social cue, a type of chemical signal that
between gentle European honeybees and a more aggressive
is emitted by one individual and influences another individual
subspecies native to Africa (Figure 44.1). Bee breeders had
of the same species. For instance, when a honeybee worker
imported African bees to Brazil in the 1950s. The breeders
guarding the entrance to a hive senses an intruder, it releases
thought cross-breeding might yield a mild-tempered but more
alarm pheromone. Pheromone molecules diffuse through the
active pollinator for commercial orchards. However, some
air and excite other bees, which fly out and sting the intruder.
African bees escaped and mated with European honeybees that had become established in Brazil before them. Then, in a grand example of geographic dispersal, some
In one study, researchers tested hundreds of colonies of Africanized honeybees and European honeybees to quantify their responses to alarm pheromone. The researchers
descendants of the hybrids buzzed all the way from Brazil
positioned a seemingly threatening object, such as a scrap
to Mexico and on into the United States. So far, they have
of black cloth, near the entrance of each hive. Then they
settled in Texas, New Mexico, Nevada, Utah, California,
released a small quantity of an artificial alarm pheromone.
Oklahoma, Louisiana, Alabama, and Florida.
The Africanized bees flew out of their hive and zeroed in
Africanized honeybees became known as “killer bees,” although they rarely kill humans. They have been in the United States since 1990, yet no more than fifteen people have died after being attacked.
on the perceived threat much faster. Those bees plunged six to eight times as many stingers into the target. The two strains of honeybees also show other behavioral differences. Africanized bees are less picky about where they establish a colony. They are more likely to abandon their hive after a disturbance. Of greater concern to beekeepers, the Africanized bees are less interested in storing large amounts of honey. Such differences among honeybees lead us into the world of animal behavior—to coordinated responses that animal species make to stimuli. We invite you to reflect first on behavior’s genetic basis, which is the foundation for its instinctive and learned mechanisms. Along the way, you will also come across examples of the adaptive value of behavior.
See the video! Figure 44.1 Two Africanized honeybees stand guard at their hive entrance. If a threat appears, they will release an alarm pheromone that stimulates hivemates to join an attack.
Links to Earlier Concepts
Key Concepts Foundations for behavior
This chapter builds on your knowledge of sensory and endocrine systems (Sections 34.1, 35.3). We will discuss the role of hormones in lactation (43.12) and other behaviors. We will also look in more detail at pheromones 35.1.
You may wish to review the concepts of adaptation (17.3) and sexual selection (18.6). You will see another example of the use of knockout experiments (15.3).
You will be reminded again of the limits of science (1.5), and the rise of cultural traits (26.13).
Behavioral variations within or among species often have a genetic basis. Behavior can also be modified by learning. When behavioral traits have a heritable basis, they can evolve by way of natural selection. Sections 44.1–44.3
Animal communication Interactions between members of a species depend on evolved modes of communication. Communication signals hold clear meaning for both the sender and the receiver of signals. Section 44.4
Mating and parental care Behavioral traits that affect the ability to attract and hold a mate are shaped by sexual selection. Males and females are subject to different selective pressure. Parental care can increase reproductive success, but it has energetic costs. Section 44.5
Costs and benefits of social behavior Life in social groups has reproductive benefits and costs. Selfsacrificing behavior has evolved among a few kinds of animals that live in large family groups. Human behavior is influenced by evolutionary factors, but humans alone make moral choices. Sections 44.6–44.8
How would you vote? Africanized bees are expanding their range in North America. Learning more about them may help us devise ways to protect ourselves. Should research into the genetic basis of their behavior be a high priority? See CengageNOW for details, then vote online.
781
44.1
Behavioral Genetics Variations in behavior within or among species often have their basis in genetic differences. Links to Knockout experiments 15.3, Sensory systems 34.1, Pituitary hormones 35.3, Lactation 43.12
How Genes Affect Behavior Animal behavior requires a capacity to detect stimuli. A stimulus, recall, is some type of information about the environment that a sensory receptor has detected (Section 34.1). Which types of stimuli an animal is able to detect and the types of responses it can make start with the structure of its nervous system. Differences in genes that affect the structure and activity of the nervous system cause many differences in behavior. Keep in mind, however, that mutations that affect metabolism or structural traits also influence behav-
Figure 44.2 (a) Banana slug, the food of choice for adult garter snakes of coastal California. (b) A newborn garter snake from a coastal population, tongueflicking at a cotton swab that had been drenched with fluids from a banana slug.
a
b
Characteristics
Rover
Sitter
Foraging behavior
Switches feeding area frequently
Tends to feed in one area
Genotype
FF or Ff
ff
PKG (enzyme) level
Higher
Lower
Speed of learning olfactory cues
Faster
Slower
Long-term memory for olfactory cues
Shorter
Longer
Figure 44.3 Characteristics of rovers and sitters, two behavioral phenotypes that occur in wild fruit fly populations. The two types differ in foraging behavior, learning, and memory, but not in general activity level. When food is not present, rovers and sitters are equally likely to move about.
782 UNIT VII
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ior. For example, suppose you notice that some birds routinely eat large seeds and others focus on small seeds. Those that eat large seeds might do so because they cannot detect the smaller seeds. Or, they might see but ignore small seeds because the structure of their beaks allows them to easily open larger ones.
Studying Variation Within a Species One way to investigate the genetic basis of behavior is to examine behavioral differences among members of a single species. For example, Stevan Arnold studied feeding behavior in two populations of garter snakes. Some garter snakes live in coastal forests of the Pacific Northwest and their preferred food is banana slugs, which are common on the forest floor (Figure 44.2a). Farther inland, there are no banana slugs and the garter snakes prefer to eat fishes and tadpoles. Were these prey preferences inborn? To find out, Arnold offered newborn garter snakes of both populations a banana slug as their first meal. Most offspring of coastal snakes ate it. Offspring of inland snakes usually ignored it. Newborn coastal snakes also flicked their tongue more often at a cotton swab soaked in slug juices, as in Figure 44.2b. (Tongue-flicking pulls molecules into the mouth.) Arnold hypothesized that inland snakes lack the genetically determined ability to associate the scent of slugs with “food!” He predicted that if coastal garter snakes were crossed with inland snakes, the resulting offspring would make an intermediate response to slug odors. Results from his experimental crosses confirmed this prediction. Hybrid baby snakes tongue-flick at cotton swabs with slug juices more than newborn inland snakes do, but not as often as newborn coastal snakes do. Exactly which gene or genes underlie this difference has not been determined. We do know about one gene that influences feeding behavior in fruit flies (Drosophila melanogaster). Marla Sokolowski showed that in wild fruit fly populations about 70 percent of the flies are “rovers”; they tend to move from place to place when food is present. About 30 percent of flies are “sitters”; they tend to feed in one place. Genotype at the foraging ( for) locus determines whether a fly is rover or a sitter. Flies that have the dominant allele (F) are rovers. Those homozygous for the recessive allele ( f ) are sitters. Sokolowski went on to uncover the molecular basis for the observed differences in behavior. She showed that the for gene encodes a cGMP-dependent protein kinase (PKG). This enzyme activates other molecules by donating a phosphate group to them, and it plays a role in many intercellular signaling pathways. Rovers
make a bit more PKG than sitters. Having more PKG in the brain allows rovers to learn about new odors faster than sitters, but it also makes rovers forget what they learned faster. Figure 44.3 summarizes genotypes and behaviors of the rover and sitter phenotypes. Examples such as this one, in which researchers can point to a single gene as the predominant cause of natural variations in behavior, are extremely rare. More typically, differences in many genes and exposure to different environmental factors cause members of a species to differ in their behavior.
Comparisons Among Species Comparing behavior of related species can sometimes help clarify the genetic basis of a behavior. For instance, all mammals secrete the pituitary hormone oxytocin (OT), which acts in labor and lactation (Section 35.3). In many mammals, OT also influences pair bonding, aggression, territoriality, and other forms of behavior. Among small rodents called prairie voles (Microtus ochrogaster), OT is the hormonal key that unlocks the female’s heart. The female bonds with a male after a night of repeated matings, and she mates for life. In one experimental test of OT’s influence, researchers injected pair-bonded female prairie voles with a drug that blocks OT action. Females that got the injection immediately dumped their partners. Genetic differences in the number and distribution of OT receptors may help explain differences in mating systems among vole species. For example, prairie voles, which are monogamous and mate for life, have more OT receptors than mountain voles (M. montanus), which are highly promiscuous (Figure 44.4). Compared to males of promiscuous vole species, males of monogamous species also have more antidiuretic hormone (ADH) receptors in their forebrain. To test the effect of this difference, scientists isolated the gene for the ADH receptor in prairie voles. They then used a virus to add copies of this gene into the forebrain of some naturally promiscuous male meadow voles (M. pennsylvanicus). Results confirmed the role of ADH receptors in monogamy. Experimentally treated males preferred a female with whom they had mated over a new one. Control males that received the gene in a different brain region or virus with a different gene showed no preference for a familiar partner.
Knockouts and Other Mutations Study of mutations can also help researchers understand behavior. As an example, fruit fly males with
a
Figure 44.4 PET scans of the distribution of oxytocin receptors (red) inside the brain of (a) a mate-for-life prairie vole and (b) a promiscuous mountain vole.
b
a mutation in the fruitless ( fru) gene do not perform normal courtship movements and they court males in addition to females. When researchers compared the brains of male fru mutants to brains of normal males, they found the mutants—like normal females—lacked a certain set of neurons. Apparently development of that set of neurons has an integral role in governing typical male mate preference and courtship behavior. As another example, knockout experiments (Section 15.3) confirmed the importance of oxytocin in mouse maternal behavior. Researchers produced female mice in which the gene for the OT receptor was knocked out. Lacking a functional receptor for OT, these mice could not respond to this hormone. As expected, these females did not lactate; oxytocin is required for contraction of milk ducts (Section 43.12). Knockout females also were less likely than normal mice to retrieve pups that researchers moved out of the nest. Based on these results, researchers concluded that oxytocin is required for normal maternal behavior in mice.
Take-Home Message How do researchers study the effect of genes on animal behavior? Studying variations in behavior within a species or among related species allows researchers to determine whether the variation has a genetic basis. Such differences are rarely caused by variation in a single gene; many genes affect behavior. Researchers sometimes can determine the effect of a gene on a specific behavior by studying individuals in which the gene is nonfunctional.
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44.2
Instinct and Learning Some behaviors are inborn and can be performed without any practice. Most behaviors are modified as a result of experience.
Instinctive Behavior All animals are born with the capacity for instinctive behavior—an innate response to a specific and usually simple stimulus. A newborn coastal garter snake behaves instinctively when it attacks a banana slug. A male fruit fly instinctively waves its wings during courtship of a female. The life cycle of the cuckoo bird provides several examples of instinct at work. This European bird is a social parasite. Females lay eggs in nests of other birds. A newly hatched cuckoo is blind, but contact with an egg laid by its foster parent stimulates an instinctive response. That hatchling maneuvers the egg onto its back, then shoves it out of the nest (Figure 44.5a). This behavior removes any potential competition for the foster parent’s attention. A cuckoo’s egg-dumping response is a fixed action pattern: a series of instinctive movements, triggered by a specific stimulus, that—once started—continues to completion without the need for further cues. Such fixed behavior has survival advantages when it permits a fast response to an important stimulus. However, a fixed response to simple stimuli has limitations. For example, the cuckoo’s foster parents are not equipped to note color and size of offspring. A simple stimulus —a chick’s gaping mouth—induces the fixed action pattern of parental feeding behavior (Figure 44.5b).
Figure 44.6 Nobel laureate Konrad Lorenz with geese that imprinted on him. The smaller photograph shows results of a more typical imprinting episode.
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a
Figure 44.5 Instinctive behavior. (a) A young cuckoo shoves its foster parent’s eggs out of the nest. (b) The foster parent feeds the cuckoo chick in response to one simple cue: a gaping mouth.
b
Time-Sensitive Learning Learned behavior is behavior that is altered by experience. Some instinctive behavior can be modified with learning. A garter snake’s initial strikes at prey are instinctive, but the snake learns to avoid dangerous or unpalatable prey. Learning may occur throughout an animal’s life, or be restricted to a critical period. Imprinting is a form of learning that occurs during a genetically determined time period. For example, baby geese learn to follow the large object that bends over them in response to their first peep (Figure 44.6). With rare exceptions, this object is their mother. When mature, the geese will seek out a sexual partner that is similar to the imprinted object. A genetic capacity to learn, combined with actual experiences in the environment, shapes most forms of behavior. For example, a male songbird has an inborn capacity to recognize his species’ song when he hears older males singing it. The young male uses these overheard songs as a guide to fill in details of his own song. Males reared alone sing a simplified version of their species’ song. So do males exposed only to the songs of other species. Many birds must learn their species-specific song during a limited period early in life. For example, a male white-crowned sparrow will not sing normally if he does not hear a male “tutor” of his own species during his first 50 or so days. Hearing a same-species tutor later in life will not influence his singing. Most birds must also practice their song to perfect it. In one experiment, researchers temporarily paralyzed throat muscles of zebra finches who were beginning to sing. After being temporarily unable to practice, these
birds never mastered their song. In contrast, temporary paralysis of throat muscles in very young birds or adults did not impair later song production. Thus, in this species, there is a critical period for song practice, as well as for song learning.
Conditioned Responses Nearly all animals are lifelong learners. Most learn to associate certain stimuli with rewards and others with negative consequences. With classical conditioning, an animal’s involuntary response to a stimulus becomes associated with another stimulus that is presented at the same time. In the most famous example, Ivan Pavlov rang a bell whenever he fed a dog. Eventually, the dog’s reflexive response to food—increased salivation—was elicited by the sound of the bell alone. With operant conditioning, an animal modifies its voluntary behavior in response to consequences of that behavior. This type of learning was first described for conditions in the lab. For example, a rat that presses a lever in a laboratory cage and is rewarded with a food pellet becomes more likely to press the lever again. A rat that receives a shock when it enters a particular area of a cage will quickly learn to avoid that area.
Figure 44.7 Getting to know one another. Two male lobsters battle at their first meeting. Later, the loser will remember the odor of the winner and avoid him. Without another meeting, memory of the defeat lasts up to two weeks.
Other Types of Learned Behavior With habituation, an animal learns by experience not to respond to a stimulus that has neither positive nor negative effects. For example, pigeons in cities learn not to flee from the large numbers of people who walk past them. Many animals learn about the landmarks in their environment and form a sort of mental map. This map may be put to use when the animal needs to return home. For example, a fiddler crab foraging up to 10 meters (30 feet) away from its burrow is able to scurry straight home when it perceives a threat. Many animals also learn the details of their social landscape; they learn to recognize mates, offspring, or competitors by appearance, calls, odor, or some combination of cues. For example, when two male lobsters meet up for the first time they will fight (Figure 44.7). Later, they will recognize one another by scent and behave accordingly, with the loser actively avoiding the winner. A lobster also recognizes its mate’s scent. With observational learning, an animal imitates the behavior of another individual. For example, Ludwig Huber and Bernhard Voelkel allowed marmoset monkeys to watch another marmoset demonstrate how to
Figure 44.8 Observational learning. A marmoset opens a container using its teeth. After watching one individual successfully perform this maneuver, other marmosets used the same technique. Analysis of videos of their movements showed that the observers closely imitated the behavior they had seen earlier.
open a plastic container and retrieve the treat inside it. Marmosets who had seen the demonstrator open the container with its hands imitated this behavior, using their hands in the same way. In contrast, those who had watched a demonstrator open the box with its teeth attempted to do the same (Figure 44.8). Take-Home Message How do instinct and learning shape behavior? Instinctive behavior can initially be performed without any prior experience, as when a simple cue triggers a fixed action pattern. Even instinctive behavior may be modified by experience.
Certain types of learning can only occur at particular times in the life cycle.
Learning affects both voluntary and involuntary behaviors.
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44.3
44.4 Communication Signals
Adaptive Behavior If a behavior varies and some of that variation has a genetic basis, then it will be subject to natural selection.
Link to Adaptive traits 17.3
Cooperating to mate or in other ways requires individuals to share information about themselves and their environment.
Behavior that increases an individual’s reproductive success is adaptive. For example, Larry Clark and Russell Mason studied the nest decorating behavior of starlings. These birds tuck sprigs of aromatic plants such as wild carrot into their nests. Clark and Mason suspected that the plant bits control parasitic mites that feed on nestlings. To test their hypothesis, the researchers replaced natural starling nests with manmade ones that either had wild carrot sprigs or were sprig-free. They predicted that the decorated nests would have fewer mites than undecorated ones. After the starling chicks left the nests, Clark and Mason recorded the number of mites left behind. The number was greater in sprig-free nests (Figure 44.9). Why? As it turns out, one organic compound in the leaves of wild carrot prevents mites from maturing. Mason and Clark concluded that decorating a nest with sprigs deters bloodsucking mites. They inferred that this nest-decorating behavior is adaptive because it promotes nestling survival, increasing reproductive success for the nest-decorating birds. As you will learn in Section 44.7, some behavior that increases the reproductive success of relatives at the expense of the individual can also be adaptive.
Link to Pheromones 35.1
Communication signals are cues for social behavior between members of a species. Chemical, acoustical, visual, and tactile signals transmit information from signalers to signal receivers. Pheromones are chemical cues. Signal pheromones make a receiver alter its behavior fast. The honeybee alarm pheromone is an example. So are sex attractants that help males and females find each other. Priming pheromones cause longer-term responses, as when a chemical dissolved in the urine of certain male mice triggers ovulation in females of the same species. Many acoustical signals, such as bird song, attract mates or define a territory. Others are alarm signals, such as a prairie dog’s bark that warns of a predator. One visual signal is a male baboon threat display, which communicates readiness to fight a rival (Figure
Take-Home Message What makes a behavior adaptive? Most behavior is adaptive because it increases the reproductive success of the individual performing it. Some is adaptive because it benefits relatives.
a
b
1,000,000
Number of mites
100,000 10,000 1,000 100 10 Without With wild wild carrot carrot
Figure 44.9 Results of an experiment to test the effect of wild carrot sprigs on the number of mites in starling nests. Nests with wild carrot pieces had significantly fewer mites than those with no greenery. There may be a selective advantage to using wild carrot and other aromatic plants as nest materials.
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c
Figure 44.10 Visual signals. (a) A male baboon shows his teeth in a threat display. (b) Penguins engaged in a courtship display. (c) A wolf’s play bow tells another wolf that behavior that follows is play, not aggression.
Figure 44.11 Animated Honeybee dances, an example of a tactile display. (a) Bees that have visited a source of food close to their hive return and perform a round dance on the hive’s vertically oriented honeycomb. The bees that maintain contact with the dancer later fly out and search for food near the hive. (b) A bee that visits a feeding source more than 100 meters (110 yards) from her hive performs a waggle dance. Orientation of an abdomen-waggling dancer in the straight run of her dance informs other bees about the direction of the food. (c) If the food is in line with the sun, the dancer’s waggling run proceeds straight up the honeycomb. (d) If food is in the opposite direction from the sun, the dancer’s waggle run is straight down. (e) If food is 90 degrees to the right of the direction of the sun, the waggle run is offset by 90 degrees to the right of vertical. The speed of the dance and the number of waggles in the straight run provide information about distance to the food. A dance inspired by food that is 200 meters away is much faster and has more waggles per straight run, than a dance inspired by a food source that is 500 meters away. Figure It Out: Do the dances shown in parts c–e indicate different distances from
the hive?
B
Answer: No. The number of waggles in the straight run does not vary.
A
When bee moves straight up comb, recruits fly straight toward the sun.
When bee moves straight down comb, recruits fly to source directly away from the sun.
When bee moves to right of vertical, recruits fly at 90° angle to right of the sun.
C
D
E
44.10a). Visual signals are part of courtship displays that often precede mating in birds (Figure 44.10b). Unambiguous signals work best, so movements often get exaggerated and body form evolves in ways that draw attention to the movements. With tactile displays, information is transmitted by touch. For example, after discovering food, a foraging honeybee worker returns to the hive and performs a complex dance. The bee moves in a defined pattern, jostling a crowd of other bees that surround her. The signals give other bees information about the distance and the direction of the food source (Figure 44.11). The same signal sometimes functions in more than one context. For example, dogs and wolves solicit play behavior with a play bow (Figure 44.10c). A play bow informs an animal’s prospective playmate that signals that follow, which would ordinarily be construed as aggressive or sexual, are friendly play behavior. A communication signal evolves and persists only if it benefits both sender and receiver. If the signal has disadvantages, then natural selection will tend to favor individuals that do not send or respond to it. Other factors can also select against signalers. For example,
male tungara frogs attract females with complex calls, which also make it easier for frog-eating bats to zero in on the caller. When bats are near, male frogs call less, and usually with less flair. The subdued signal is a trade-off between locating a partner for mating and the need for immediate survival. There are illegitimate signalers, too. For example, fireflies attract mates by producing flashes of light in a characteristic pattern. Some female fireflies prey on males of other species. When a predatory female sees the flash from a male of the prey species, she flashes back as if she were a female of his own species. If she lures him close enough, she captures and eats him.
Take-Home Message What are the benefits and costs of communication signals? A communication signal transfers information from one individual to another individual of the same species. Such signals benefit both the signaler and the receiver. Signals have a potential cost. Some individuals of a different species benefit by intercepting signals or by mimicking them.
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44.5
Mates, Offspring, and Reproductive Success In studying behavior, we expect that each sex will evolve in ways that maximize its benefits, and minimize its costs, which can lead to conflicts.
Link to Sexual selection 18.6
Sexual Selection and Mating Behavior Males or females of a species often compete for access to mates, and many are choosy about their partners. Both situations lead to sexual selection. As explained in Section 18.6, this microevolutionary process favors characteristics that provide a competitive advantage in attracting and often holding on to mates. But whose reproductive success is it—the male’s or the female’s? Male animals, remember, produce many small sperm, and females produce far larger but fewer eggs. For the male, success generally depends on how many eggs he can fertilize. For the female, it depends more on how many eggs she produces or how many offspring she can raise. Usually, the most important factor in a female’s sexual preference is the quality of the mate, not the quantity of partners. Female hangingflies (Harpobittacus) will mate only with males that supply food. A male hunts and kills a moth or some other insect. Then he releases a sex pheromone, which attracts females to him and his “nuptial gift” (Figure 44.12a). The female begins to eat the male’s offering and copulation begins. Only after
the female has been eating for five minutes or so does she start to accept sperm from her partner. Even after mating begins, a female can break off from her suitor, if she finishes eating his gift. If she does end the mating, she will seek out a new male and his sperm will replace the first male’s. Thus, the larger the male’s gift, the greater the chance that mostly his sperm will actually end up fertilizing the eggs of his mate. Females of certain species shop around for males who have appealing traits. Consider the fiddler crabs that live along many sandy shores. One of the male’s two claws is enlarged; it often accounts for more than half his total body weight (Figure 44.12b). During their breeding season, hundreds of males excavate mating burrows near one another. Each male stands next to his burrow, waving his oversized claw. Female crabs stroll along, checking out males. If a female likes what she sees, she inspects her suitor’s burrow. Only when a burrow has the right location and dimensions does she mate with its owner and lay eggs in his burrow. Some female birds are similarly choosy. Male sage grouse (Centrocercus urophasianus) converge at a lek, a type of communal display ground, where each stakes out a few square meters. With tail feathers erect, the males emit booming calls by puffing and deflating big neck pouches (Figure 44.12d). As they do, they stamp about on their patch of prairie. Females tend to select and mate with one male sage grouse. Afterward, they
b
c
a
d
Figure 44.12 (a) Male hangingfly dangling a moth as a nuptial gift for a potential mate. Females of some hangingfly species choose sexual partners that offer the largest gift to them. By waving his enlarged claw, a male fiddler crab (b) may attract the eye of a female fiddler crab (c). A male sage grouse (d) showing off as he competes for female attention at a communal display ground.
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go off to nest and raise any offspring by themselves. Often, many females favor the same few males, and most males never have an opportunity to mate. In another behavioral pattern, the sexually receptive females of some species cluster in defendable groups. Where you come across such a group, you are likely to observe males competing for access to the clusters. Competition for ready-made harems has resulted in combative male lions, sheep, elk, elephant seals, and bison, to name a few examples (Figure 44.13).
Parental Care When females fight for males, we can predict that the males provide more than sperm delivery. Some, such as the male midwife toad, help with parenting. The male holds strings of fertilized eggs around his legs until the eggs hatch (Figure 44.14a). Once her eggs are being cared for, a female can mate with other males, if she can find some that are not already caring for eggs. Late in the breeding season, males without strings of eggs are rare, and females fight for access to them. The females even attempt to pry mating pairs apart. Parental behavior uses up time and energy, which parents otherwise might spend on living long enough to reproduce again. However, for some animals, the benefit of increased survival of the young outweighs the cost of parenting. Few reptiles provide care for young. Crocodilians, the reptiles most closely related to birds, are a notable exception. Crocodile parents bury their eggs in a nest. When young are ready to hatch, they call and parents dig them out and care for them for some time. Most birds are monogamous, and both parents often care for the young (Figure 44.14b). In mammals, males typically leave after mating. Females raise the young alone, and males attempt to mate again or conserve energy for the next breeding season (Figure 44.14c). Mammalian species in which males help care for the young tend to be monogamous, at least over the course of a breeding season. Only about 5 percent of mammals are monogamous.
Figure 44.13 Male bison locked in combat during the breeding season.
a
b
Take-Home Message How does natural selection affect mating systems? Males and females each behave in ways that will maximize their own reproductive success.
Most males compete for females and mate with more than one. Monogamy and male parental care are not common.
c
Figure 44.14 (a) Male midwife toad with developing eggs wrapped around his legs. (b) A pair of Caspian terns cooperate in the care of their chick. (c) A female grizzly will care for her cub for as long as two years. The male takes no part in the cub’s upbringing.
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44.6 Living in Groups Survey the animal kingdom and you find evolutionary costs and benefits across a range of social groups.
Link to Culture 26.13
Defense Against Predators In some groups, cooperative responses to predators reduce the net risk to all. Vulnerable individuals can be on the alert for predators, join a counterattack, or engage in more effective defenses (Figure 44.15). Birds, monkeys, meerkats, prairie dogs, and many other animals make alarm calls, as in Figure 44.15a. A prairie dog makes a particular bark when it sights an eagle and a different signal when it sights a coyote. Others dive into burrows to escape an eagle’s attack or stand erect and observe the coyote’s movements. Sawfly caterpillars feed in clumps on branches and benefit by coordinated repulsion of predatory birds. When a potential predator approaches, the caterpillars rear up and vomit partly digested eucalyptus leaves (Figure 44.15b). Birgitta Sillén-Tullberg demonstrated that predatory birds prefer individual caterpillars to a wiggling group. When offered caterpillars one at a time, the birds ate an average of 5.6. Birds offered a cluster of twenty caterpillars ate an average of 4.1. Whenever animals cluster, some individuals shield others from predators. Preference for the center of a group can create a selfish herd, in which individuals hide behind one another. Selfish-herd behavior occurs in bluegill sunfishes. A male sunfish builds a nest by scooping out a depression in mud on the bottom of a
a
b
lake. Females lay eggs in these nests, and snails and fishes prey on eggs. Competition for the safest sites is greatest near the center of a group, with large males taking the innermost locations. Smaller males cluster around them and bear the brunt of the egg predation. Even so, the nests of small males are safer at the edge of the group than they would be alone in the open.
Improved Feeding Opportunities Many mammals, including wolves, lions, wild dogs, and chimpanzees, live in social groups and cooperate in hunts (Figure 44.16). Are cooperative hunters more efficient than solitary ones? Often, no. In one study, researchers observed a solitary lion that caught prey about 15 percent of the time. Two lions cooperatively hunting caught prey twice as often but had to share it, so the amount of food per lion balanced out. When more lions joined a hunt, the success rate per lion fell. Wolves show a similar pattern. Among carnivores that hunt cooperatively, hunting success does not seem to be the major advantage of group living. Individuals hunt together, but they also may fend off scavengers, care for one another’s young, and protect territory. Group living also allows transmission of cultural traits, or behaviors learned by imitation. For example, chimpanzees make and use simple tools by stripping leaves from branches. They use thick sticks to make holes in a termite mound, then insert long, flexible “fishing sticks” into the holes (Figure 44.17). The long stick agitates the termites, which attack and cling to it.
c
Figure 44.15 Group defenses. (a) Black-tailed prairie dogs bark an alarm call that warns others of predators. Does this call put the caller at risk? Not much. Prairie dogs usually act as sentries only after they finish feeding and happen to be standing next to their burrows. (b) Australian sawfly caterpillars form clumps and regurgitate a fluid (the yellow blobs) that predators find unappealing. (c) Musk oxen adults (Ovibos moschatus) form a ring of horns, often around their young.
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Chimps withdraw the stick and lick off termites, as a high-protein snack. Different groups of chimpanzees use slightly different tool-shaping and termite-fishing methods. Youngsters of each group learn by imitating the adults.
Figure 44.16 Members of a wolf pack (Canis lupus). Wolves cooperate in hunting, caring for young, and defending territory. Benefits are not distributed equally. Only the highest ranking individuals, the alpha male and alpha female, breed.
Dominance Hierarchies In many social groups, subordinate individuals do not get an equal share of resources. Most wolf packs, for instance, have one dominant male that breeds with just one dominant female. The others are nonbreeding brothers and sisters, aunts and uncles. All hunt and carry food back to individuals that guard the young in their den. Why would a subordinate give up resources and often breeding privileges? It might get injured or die if it challenges a strong individual. It might not be able to survive on its own. A subordinate might even get a chance to reproduce if it lives long enough or if its dominant peers are taken out by a predator or old age. As one example, some subordinate wolves move up the social ladder when the opportunity arises.
Figure 44.17 Chimpanzees (Pan troglodytes) using sticks as tools for extracting tasty termites from a nest. This behavior is learned by imitation.
Regarding the Costs of Group Living If social behavior is advantageous, then why are there so few social species? In most habitats, costs outweigh benefits. For instance, when individuals are crowded together they compete more for resources. Cormorants and other seabirds form dense breeding colonies, as in Figure 44.18. All compete for space and food. Large social groups also attract more predators. If individuals are crowded together, they are vulnerable to parasites and contagious diseases that jump from host to host. Individuals may also be at risk of being killed or exploited by others. Given the opportunity, a pair of breeding herring gulls will cannibalize the eggs and even the chicks of their neighbors. Take-Home Message What are the benefits and costs of social groups?
Living in a social group can provide benefits, as through cooperative defenses or shielding against predators. Group living has costs: increased competition, increased vulnerability to infections, and exploitation by others.
Figure 44.18 Nearly uniform spacing in a crowded cormorant colony.
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44.7
Why Sacrifice Yourself? Extreme cases of sterility and self-sacrifice have evolved in only a few groups of insects and one group of mammals. How are genes of the nonreproducers passed on?
supplies the female with sperm. Winged reproductive termites of both sexes develop seasonally.
Social Insects
Social Mole-Rats
Animals that are eusocial live together for generations in a group that has a reproductive division of labor. Eusocial insects include the honeybees, termites, and ants. In all of these groups, sterile workers care cooperatively for the offspring produced by just a few breeding individuals. Such workers often are highly specialized in their form and function (Figure 44.19). A queen honeybee is the only fertile female in her hive. She is larger than other females, partly because of her enlarged ovaries (Figure 44.20a). She secretes a pheromone that makes all other female bees sterile. All of the 30,000 to 50,000 worker bees are females that develop from fertilized eggs laid by the queen. They feed the larvae, maintain the hive, and construct honeycomb from wax they secrete. Workers also gather nectar and pollen that feeds the colony. They guard the hive and will sacrifice themselves to repel intruders. In spring and summer, the queen lays unfertilized eggs that develop into drones. These male bees are stingless and subsist on food gathered by their worker sisters. Each day, drones fly in search of a mate. If one is lucky, he will meet a virgin queen on her one flight away from a colony. He dies after mating. A young queen mates with many males, and stores their sperm for use over her lifetime of several years. Like honeybees, termites live in enormous family groups with a queen specialized for producing eggs (Figure 44.20b). Unlike the honeybee hive, a termite mound holds sterile individuals of both sexes. A king
Sterility and extreme self-sacrifice are uncommon in vertebrates. The only eusocial mammals are African mole-rats. The best studied is Heterocephalus glaber, the naked mole-rat. Clans of this nearly hairless rodent build and occupy burrows in dry parts of East Africa. A mole rat clan consists of a reproductive “queen” (Figure 44.20c), the one to three “kings,” with whom she mates, and their nonbreeding worker offspring. Workers care for the queen, the king(s), and the young. Some workers serve as diggers that excavate tunnels and chambers. When a digger finds an edible root, it hauls a bit back to the main chamber and chirps. Its chirps recruit other workers to help carry food back to the chamber. Still other workers function as guards. When a predator appears, they chase and attack it at great risk to themselves.
a
b
Evolution of Altruism A sterile worker in a social insect colony or a naked mole-rat clan shows altruistic behavior: behavior that enhances another individual’s reproductive success at the altruist’s expense. How did this behavior evolve? According to William Hamilton’s theory of inclusive fitness, genes associated with altruism are selected if they lead to behavior that promotes the reproductive success of an altruist’s closest relatives. A sexually reproducing, diploid parent caring for offspring is not helping exact genetic copies of itself.
c
Figure 44.19 Specialized ways of serving and defending the colony. (a) An Australian honeypot ant worker. This sterile female is a living container for her colony’s food reserves. (b) Army ant soldier (Eciton burchelli) with formidable mandibles. (c) Eyeless soldier termite (Nasutitermes). It bombards intruders with a stream of sticky goo from its nozzle-shaped head.
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FOCUS ON SCIENCE
44.8
Human Behavior
Evolutionary forces shaped human behavior—but humans alone can make moral choices about their actions.
a
c
b
Figure 44.20 Three queens. (a) Queen honeybee with her sterile daughters. (b) A termite queen (Macrotermes) dwarfs her offspring and mate. Ovaries fill her enormous abdomen. (c) A naked mole-rat queen.
Each of its gametes, and each of its offspring, inherits one-half of its genes. Other individuals of the social group that have the same ancestors also share genes. Siblings (brothers or sisters) are as genetically similar as a parent and offspring. Nephews and nieces share about one-fourth of their uncle’s genes. Sterile workers promote genes for self-sacrifice by helping close relatives survive and reproduce. In honeybee, termite, and ant colonies, sterile workers assist fertile relatives with whom they share genes. A guard bee will die after she stings, but her sacrifice preserves many copies of her genes in her hivemates. Inbreeding increases the genetic similarity among relatives and may play a role in mole-rat sociality. A clan is highly inbred as a result of many generations of sibling, mother–son, and father–daughter matings. Dry habitats and patchy food sources also may favor cooperation in digging, locating food, and fending off competitors and predators.
Take-Home Message How can altruistic behavior be selectively advantageous?
Altruistic behavior may be favored when individuals pass on genes indirectly, by helping relatives survive and reproduce.
Link to Limits of science 1.5
Hormones and Pheromones Are humans, too, influenced by hormones that contribute to bonding behavior in other mammals? Perhaps. Consider that autism, a developmental disorder in which people have trouble making social contacts, is often associated with low oxytocin levels. Oxytocin is known to affect bonding behavior in other mammals. Pheromones in sweat may also affect human behavior. Women who live together often have synchronized menstrual cycles and experiments have shown that a woman’s menstrual cycle will lengthen or shorten after she has been exposed to sweat from a woman who was in a different phase of the cycle. Other experiments have shown that exposure to male sweat can alter a woman’s cortisol level.
Morality and Behavior If we are comfortable with studying the evolutionary basis of behavior of termites, naked mole-rats, and other animals, why do some people resist the idea of analyzing the evolutionary basis of human behavior? A common fear is that an objectionable behavior will be defined as “natural.” To evolutionary biologists, however, “adaptive” does not mean “morally right.” It simply means a behavior increases reproductive success. Scientific studies do not address moral issues (Section 1.5). For example, infanticide is morally repugnant. Is it unnatural? No. It happens in many animal groups and all human cultures. Male lions often kill the offspring of other males when they take over a pride. Thus deprived of parenting tasks, the lionesses can now breed with the infanticidal male and increase that male’s reproductive success. Biologists would predict that unrelated human males are a threat to infants. Evidence supports the prediction. The absence of a biological father and the presence of an unrelated male increases risk of death for an American child under age two by more than sixty times. What about parents who kill their own offspring? In her book on maternal behavior, primatologist Sarah Blaffer Hrdy cites a study of one village in Papua New Guinea in which parents killed about 40 percent of the newborns. As Hrdy argues, when resources or social support are hard to come by, a mother’s fitness might increase if a newborn who is unlikely to survive is killed. The mother can allocate child-rearing energy to her other offspring or save it for children she may have in the future. Do most of us find such behavior appalling? Yes. Can considering the possible evolutionary advantages of the behavior help us prevent it? Perhaps. An analysis of the conditions under which infanticide occurs tells us this: When mothers lack the resources they need to care for their children, they are more likely to harm them. We as a society can act upon such information.
CHAPTER 44
ANIMAL BEHAVIOR 793
IMPACTS, ISSUES REVISITED
My Pheromones Made Me Do It
When a European queen bee mates with an Africanized drone, her worker offspring are just as aggressive as workers in a pure Africanized colony. In contrast, a cross between an Africanized queen and a European drone yields workers with an intermediate level of aggression. Unfortunately, European queen–Africanized male pairings occur far more frequently than the reciprocal cross. Africanized males outcompete European males for matings.
Summary Section 44.1 Behavior refers to coordinated responses that an animal makes to a stimulus. Genes that affect the nervous system often affect behavior, but other genes may also influence it. Studies of natural behavioral variations within and among species provide information about the genetic basis for behaviors, as does the study of induced or natural mutations. Section 44.2 Instinctive behavior can occur without having been learned by experience. A fixed action pattern is an instinctive series of responses to a simple cue. Learned behavior is altered by experience. Imprinting is one form of learning that happens only during a sensitive period early in life. With classical conditioning, an animal learns to associate an involuntary response to one stimulus with another stimulus. With operant conditioning, an animal modifies a voluntary behavior in response to the behavior’s consequences. With habituation, an animal stops responding to an ongoing stimulus. With observational learning, it imitates another’s actions. Section 44.3 A behavior that has a genetic basis is subject to evolution by natural selection. Adaptive forms of behavior evolved as a result of individual differences in reproductive success in past generations. Section 44.4 Communication signals allow animals of the same species to share information. Such signals evolve and persist only if they benefit both senders and receivers of the signal. Chemical signals such as pheromones have roles in social communication, as do acoustical signals, visual signals that are part of courtship and threat displays, and tactile signals.
Use the animation on CengageNOW to explore the honeybee dance language.
Section 44.5 Sexual selection favors traits that give an individual a competitive edge in attracting and often holding on to mates. The females of many species select males that have traits or engage in behaviors they find attractive. When large numbers of females cluster in a defensible area, males may compete with one another to control the areas. Parental care has reproductive costs in terms of future reproduction and survival. It is adaptive when benefits to a present set of offspring offset the costs. 794 UNIT VII
PRINCIPLES OF ECOLOGY
How would you vote? Africanized honeybees continue to increase their range. Should study of their genetics be a high priority? See CengageNOW for details, then vote online.
Section 44.6 Animals that live in social groups may benefit by cooperating in predator detection, defense, and rearing the young. A selfish herd forms when animals hide behind one another. Benefits of group living are often distributed unequally. Species that live in large groups incur costs, including increased disease and parasitism, and increased competition for resources. Section 44.7 Ants, termites, and some other insects as well as two species of mole-rats are eusocial. They live in colonies with overlapping generations and have a reproductive division of labor. Most colony members do not reproduce; they assist their relatives instead. According to the theory of inclusive fitness, such altruistic behavior is perpetuated because altruistic individuals share genes with their reproducing relatives. Altruistic individuals help perpetuate the genes that led to their altruism by promoting the reproductive success of close relatives that also carry copies of these genes. Section 44.8 Hormones and possibly pheromones influence human behavior. A behavior that is adaptive in the evolutionary sense may still be judged by society to be morally wrong. Science does not address morality.
Self-Quiz
Answers in Appendix III
1. Genes affect the behavior of individuals by . a. influencing the development of nervous systems b. affecting the kinds of hormones in individuals c. determining which stimuli can be detected d. all of the above 2. Stevan Arnold offered slug meat to newborn garter snakes from different populations to test his hypothesis that the snakes’ response to slugs . a. was shaped by indirect selection b. is an instinctive behavior c. is based on pheromones d. is adaptive 3. A behavior is defined as adaptive if it . a. varies among individuals of a population b. occurs without prior learning c. increases an individual’s reproductive success d. is widespread across a species 4. The honeybee dance language transmits information about distance to food by way of signals. a. tactile c. acoustical b. chemical d. visual
Data Analysis Exercise Honeybees disperse by forming new colonies. An old queen leaves the hive along with a group of workers. These bees fly off, find a new nest site, and set up a new hive. Meanwhile, at the old hive, a new queen emerges, mates, and takes over. A new hive can be several kilometers from the old one. Africanized honeybees form new colonies more often than European ones, a trait that contributes to their spread. Africanized bees also spread by taking over existing hives of European bees. In addition, in areas where European and Africanized hives coexist, European queens are more likely to mate with Africanized males, thus introducing Africanized traits into the colony. Figure 44.21 shows the counties in the United States in which Africanized honeybees became established from 1990 through 2006.
CA
AZ
5. A is a chemical that conveys information between individuals of the same species. a. pheromone c. hormone b. neurotransmitter d. all of the above 6. In , males and females typically cooperate in care of the young. a. mammals c. amphibians b. birds d. all of the above 7. Generally, living in a social group costs the individual in terms of . a. competition for food, other resources b. vulnerability to contagious diseases c. competition for mates d. all of the above 8. Social behavior evolves because . a. social animals are more advanced than solitary ones b. under some conditions, the costs of social life to individuals are offset by benefits to the species c. under some conditions, the benefits of social life to an individual offset the costs to that individual d. under most conditions, social life has no costs to an individual 9. Eusocial insects . a. live in extended family groups b. include termites, honeybees, and ants c. show a reproductive division of labor d. a and c e. all of the above 10. Helping other individuals at a reproductive cost to oneself might be adaptive if those helped are . a. members of another species b. competitors for mates c. close relatives d. illegitimate signalers
AK MS
TX 1990 1991 1992 1993 1994 1995
2. In what states did Africanized bees first appear in 2005?
4. Based on this map, would you expect Africanized honeybees to colonize additional states in the next five years?
OK
NM
LA
1. Where in the United States did Africanized bees first become established? 3. Why is it likely that human transport of bees contributed to the spread of Africanized honeybees to Florida?
NV
1996 1997 1998 1999 2000 2001
AL FL
2002 2003 2004 2005 2006
Figure 44.21 The spread of Africanized honeybees in the United States, from 1990 through 2006. The USDA adds a county to this map only when the state officially declares bees in that county Africanized. Bees can be identified as Africanized on the basis of morphological traits or analysis of their DNA.
11. True or false? Some mammals live in colonies and act as sterile workers that serve close relatives. 12. Match the terms with their most suitable description. fixed action a. time-dependent form of pattern learning requiring exposure altruism to key stimulus basis of b. genes plus actual experience instinctive c. series of responses that and learned runs to completion behavior independently of feedback imprinting from environment pheromone d. assisting another individual at one’s own expense e. one communication signal
Visit CengageNOW for additional questions.
Critical Thinking 1. For billions of years, the only bright objects in the night sky were stars or the moon. Night-flying moths used them to navigate in a straight line. Today, the instinct to fly toward bright objects causes moths to exhaust themselves fluttering around streetlights and banging against brightly lit windowpanes. This behavior is not adaptive, so why does it persist? 2. Damaraland mole-rats are relatives of naked mole-rats (Figure 44.19). In their clans, too, nonbreeding individuals of both sexes cooperatively assist one breeding pair. Even so, breeding individuals in wild Damaraland mole-rat colonies usually are unrelated, and few subordinates move up in the hierarchy to breeding status. Researchers suspect that ecological factors, not genetic ones, were the more important selective force in Damaraland mole-rat altruism. Explain why. CHAPTER 44
ANIMAL BEHAVIOR 795
45
Population Ecology IMPACTS, ISSUES
The Numbers Game
In 1722, on Easter morning, a European explorer landed on
Those in power built statues to appeal to the gods. They
a small volcanic island in the South Pacific and discovered
directed others to carve images of unprecedented size and
a few hundred hungry, skittish people living in caves. He
move the new statues to the coast. Wars broke out and by
noticed withered grasses and scorched, shrubby plants—
1550, no one ventured offshore to fish. They could not build
and the absence of trees. He wondered about the hundreds
any more canoes because there were no more trees.
of massive stone statues near the coast and 500 unfinished,
As central authority crumbled, the dwindling numbers of
abandoned ones in inland quarries (Figure 45.1). Some
islanders retreated to caves and launched raids against one
weighed 100 tons and stood 10 meters (33 feet) high.
another. Winners ate the losers and tipped over statues. Even
Easter Island, as it came to be called, is no larger than 165 square kilometers (64 square miles). Archaeologists have determined that voyagers from the Marquesas discovered
if the survivors had wanted to, they had no way to get off the island. The once-flourishing population collapsed. Any natural population has the capacity to increase in
this eastern outpost of Polynesia more than 1,650 years ago.
number, given the right conditions. In North America, white-
The place was a paradise. Its volcanic soil supported dense
tailed deer are behaving like early settlers on Easter Island.
forests and lush grassland. The colonists used long, straight
With plenty of food and few predators, deer numbers are
palms to build canoes that were strengthened with rope
soaring. Deer overpopulation harms forests, damages crops,
made of fibers from hauhau trees. They used wood as fuel to
and increases the incidence of highway accidents.
cook fishes and dolphins. They cleared forests to plant crops. They had many children. By 1440, as many as 15,000 people were living on the
With this chapter, we begin a survey of principles that govern the growth and sustainability of all populations. The principles are the bedrock of ecology—the systematic study
island. Crop yields declined; ongoing harvests and erosion
of how organisms interact with one another and with their
had depleted the soil of nutrients. Fish vanished from the
environment. Those interactions start within and between
waters close to the island, so fishermen had to sail farther
populations and extend to communities, ecosystems, and
and farther out on the open ocean.
the biosphere.
See the video! Figure 45.1 Row of massive statues on Easter Island. Islanders set them up long ago, apparently as a plea for help after their once-large population wreaked havoc on their tropical paradise. Their plea had no effect whatsoever on reversing the loss in biodiversity on the island and in the surrounding sea. The human population did not recover, either.
Links to Earlier Concepts
Key Concepts The vital statistics
Earlier chapters defined and explored the evolutionary history and genetic nature of populations, including those of humans (Sections 18.1 and 26.15). Now you will consider factors that limit population growth, including contraception (42.9).
You will be reminded of the effects of infectious disease (Chapter 21 introduction, 21.8), and the stunning reproductive capacity of prokaryotes (21.5).
Gene flow (18.8) and directional selection (18.4) are discussed in the context of evolving populations. We also consider how sampling error (1.8) affects population studies.
Ecologists explain population growth in terms of population size, density, distribution, and number of individuals in different age categories. Field studies allow ecologists to estimate population size and density. Sections 45.1, 45.2
Exponential rates of growth A population’s size and reproductive base influence its rate of growth. When the population is increasing at a rate proportional to its size, it is undergoing exponential growth. Section 45.3
Limits on increases in number Over time, an exponentially growing population typically overshoots the carrying capacity—the maximum number of individuals of a species that environmental resources can sustain. Some populations stabilize after a big decline. Others never recover. Section 45.4
Patterns of survival and reproduction Resource availability, disease, and predation are major factors that can restrict population growth. These limiting factors differ among species and shape their life history patterns. Sections 45.5, 45.6
The human population Human populations sidestepped limits to growth by way of global expansion into new habitats, cultural interventions, and innovative technology. Even so, no population can continue to expand indefinitely. Sections 45.7–45.10
How would you vote? Soaring numbers of white-tailed deer threaten forest plants and the animals that depend on them. Is encouraging deer hunting in regions where their overabundance is a threat to other species the best solution? See CengageNOW for details, then vote online.
797
45.1
Population Demographics A population’s size, density, distribution, and age structure are shaped by ecological factors, and may shift over time.
Link to Population genetics 18.1
Ecologists typically use the term “population” to refer to all members of a species within an area defined by the researcher. Studies of population ecology start with demographics: statistics that describe population size, age structure, density, distribution, and other factors. Population size is the number of individuals in the population. Age structure is the number of individuals in each of several age categories. Individuals are often grouped as pre-reproductive, reproductive, or postreproductive. Those in the pre-reproductive category
clumped
random
a
b
nearly uniform
have the capacity to produce offspring when mature. Together with individuals in the reproductive group, they make up the population’s reproductive base. Population density is the number of individuals in a specified portion of a habitat. A habitat, remember, is the type of place where a species lives. We characterize a habitat by its physical and chemical features, and its array of species. Density refers to how many individuals are in an area but not how they are dispersed through it. Even a habitat that looks uniform, such as a sandy shore, has variations in light, moisture, and many other variables. A population may live in only a small part of the habitat, and it may do so all of the time or only some of the time. The pattern in which individuals are dispersed in their habitat is the population distribution. It may be clumped, nearly uniform, or random (Figure 45.2). A clumped distribution is most common, for several reasons. First, the conditions and resources that most species require tend to be patchy. Animals cluster at a water hole, seeds sprout only in moist soil, and so on. Second, most seeds and some animal offspring cannot disperse far from their parents. Third, some animals spend their lives in social groups that offer protection and other advantages. With a nearly uniform distribution, individuals are more evenly spaced than we would expect on the basis of chance alone. Such distribution is relatively rare. It happens when competition for resources or territory is fierce, as in a nesting colony of seabirds. We observe random distribution only when habitat conditions are nearly uniform, resource availability is fairly steady, and individuals of a population or pairs of them neither attract nor avoid one another. Each wolf spider does not hunt far from its burrow, which can be almost anywhere in forest soil (Figure 45.2b). The scale of the study area and timing of a study can influence the observed pattern of distribution. For example, seabirds often are spaced almost uniformly at a nesting site, but nesting sites are clustered along a shoreline. Also, these birds crowd together during the breeding season, but disperse when breeding is over.
Take-Home Message c
Figure 45.2 Three patterns of population distribution: (a) clumped, as in squirrelfish schools; (b) random, as when wolf spiders dig their burrows almost anywhere in forest soil; and (c) more or less uniform, as in a royal penguin nesting colony.
798 UNIT VII
PRINCIPLES OF ECOLOGY
How do we describe a natural population? Each population has characteristic demographics, such as size, density, distribution pattern, and age structure. Environmental conditions and species interactions shape these characteristics, which may change over time.
FOCUS ON SCIENCE
45.2
Elusive Heads to Count
Ecologists carry out field studies to test hypotheses about populations and to monitor the status of populations that are threatened or endangered.
Link to Sampling error 1.8
Many white-tailed deer (Odocoileus virginianus) live in the forests, fields, and suburbs of North America. How could you find out how many deer live in a particular region? A full count would be a careful measure of absolute population density. In the United States, census takers attempt such a count of human populations every ten years, although not everyone answers the door. Ecologists sometimes make counts of large species in small areas, such as fur seals at their breeding grounds, and sea stars in a tidepool. More often, a full count would be impractical, so they sample part of a population and estimate its total density. For instance, you could divide a map of your county into small plots, or quadrats. Quadrats are sampling areas of the same size and shape, such as rectangles, squares, and hexagons. You could count individual deer in several plots and, from that, extrapolate the average number for the county as a whole. Ecologists often make such estimates for plants and other species that stay put (Figure 45.3). Such estimates run the risk of sampling error (Section 1.8), if the number of sampled plots is not large. Ecologists use capture–recapture methods to estimate the population sizes of deer and other animals that do not stay put. First, they trap and mark some individuals. Deer get collars, squirrels get tattoos, salmon get tags, birds
get leg rings, butterflies get wing markers, and so forth (Figure 45.4). Marked animals are released at time 1. At time 2, traps are reset. The proportion of marked animals in the second sample is then taken to be representative of the proportion marked in the whole population: marked individuals in sampling at time 2 total captured in sampling 2
=
marked individuals in sampling at time 1 total population size
Ideally, both marked and unmarked individuals of the population are captured at random, no marked animal is overlooked, and marking does not affect whether animals die or otherwise depart during the study interval. In the real world, recaptured individuals might not be a random sample; they might over- or underrepresent their population. Squirrels marked after being attracted to bait in boxes might now be trap-happy or trap-shy. Instead of mailing tags of marked fish to ecologists, a fisherman may keep them as souvenirs. Birds lose leg rings. Estimates of population size may also vary depending on the time of year they are made. The distribution of a population may change seasonally. Many types of animals move between different parts of their range in response to seasonal changes in resource abundance. As with other population data, the accuracy of size estimates can be increased by repeated samplings. The more data that can be accumulated, the lower the risk of sampling error.
a
b
Figure 45.3 Easy-to-count creosote bushes near the eastern base of the Sierra Nevada. They are an example of a relatively uniform distribution pattern. Individual plants compete for scarce water in this desert, which has extremely hot, dry summers and mild winters.
CHAPTER 45
Figure 45.4 Two individuals marked for population studies. (a) Florida Key deer and (b) Costa Rican owl butterfly (Caligo).
POPULATION ECOLOGY 799
45.3
Population Size and Exponential Growth Populations are dynamic units. They are continually adding and losing individuals. All populations have a capacity to increase in number.
Link to Bacterial reproduction 21.5
Gains and Losses in Population Size Populations continually change size. They increase in size because of births and immigration, the arrival of new residents from other populations. They decrease in size because of deaths and emigration, departure of individuals that then take up permanent residence elsewhere. For example, a freshwater turtle population changes size in the spring when young turtles emigrate from their home pond. The young emigrants typically become immigrants at another pond some distance away. What about the individuals of species that migrate daily or seasonally? A migration is a recurring roundtrip between regions, usually in response to expected shifts or gradients in environmental resources. Some or all members of a population leave an area, spend time in another area, then return. For our purposes, we may ignore these recurring gains and losses, because we can assume that they balance out over time.
From Zero to Exponential Growth Zero population growth is an interval during which the number of births is balanced by an equal number of deaths. Population size remains stable, with no net increase or decrease in the number of individuals. We can measure births and deaths in terms of rates per individual, or per capita. Capita means head, as in a head count. Subtract a population’s per capita death rate (d) from its per capita birth rate (b) and you have the per capita growth rate, or r: r (per capita growth rate)
b
d
= (per capita – (per capita birth rate)
death rate)
As long as r remains constant and greater than zero, exponential growth will continue: Population size will increase by the same proportion in every successive time interval. Imagine a population of 2,000 mice living in a field. If 1,000 mice are born each month, the birth rate is 0.5 per mouse per month (1,000 births/2,000 mice). If 200 mice die each month, the death rate is 200/2,000 0.1 per mouse per month. Given these birth and death rates, r is 0.5 − 0.1 0.4 per mouse per month. In other words, the mouse population grows by 4 percent each
1,200,000
G= r r r r r r r r r r r r r r r r r r r
Figure 45.5 Animated (a) Net monthly increases in a hypothetical population of mice when the per capita rate of growth (r) is 0.4 per mouse per month and the starting population size is 2,000. (b) Graph these numerical data and you end up with a J-shaped growth curve.
800 UNIT VII
× × × × × × × × × × × × × × × × × × ×
2,000 2,800 3,920 5,488 7,683 10,756 15,058 21,081 29,513 41,318 57,845 80,983 113,376 158,726 222,216 311,103 435,544 609,762 853,667
A
PRINCIPLES OF ECOLOGY
Net New Monthly Population Increase Size = 800 2,800 = 1,120 3,920 = 1,568 5,488 = 2,195 7,683 = 3,073 10,756 = 4,302 15,058 = 6,023 21,081 = 8,432 29,513 = 11,805 41,318 = 16,527 57,845 = 23,138 80,983 = 32,393 113,376 = 45,350 158,726 = 63,490 222,216 = 88,887 311,103 =124,441 435,544 =174,218 609,762 =243,905 853,667 =341,467 1,195,134
1,100,000 1,000,000 900,000 Number of individuals (N)
Starting Population Size
800,000 700,000 600,000 500,000 400,000 300,000 200,000 100,000
B
0 2 4 6 8 10 12 14 16 18 20 Time (months)
Number of individuals (× 100,000)
curve 1
Figure 45.6 Effect of deaths on the rate of increase for two hypothetical populations of bacteria. Plot the population growth for bacterial cells that reproduce every half hour and you get growth curve 1. Next, plot the population growth of bacterial cells that divide every half hour, with 25 percent dying between divisions, and you get growth curve 2. Deaths slow the rate of increase, but as long as the birth rate exceeds the death rate and is constant, exponential growth will continue.
curve 2
10 8 6 4 2 0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Time (hours)
month. We can calculate the population growth (G) for each interval based on the per capita growth rate (r) and the number of individuals (N): G (population growth per unit time)
r
N
growth rate)
individuals)
= (per capita × (number of
After one month, 2,800 mice are scurrying about in the field (Figure 45.5a). A net increase of 800 fertile mice has made the reproductive base larger. They all reproduce, so the population size expands, for a net increase of 0.4 2,800 1,120. Population size is now 3,920. At this growth rate, the number of mice would rise from 2,000 to more than 1 million in two years! Plot the increases against time and you end up with a J-shaped curve that is characteristic of exponential growth (Figure 45.5b). With exponential growth, a population grows faster and faster, although the per capita growth rate stays the same. It is like the compounding of interest on a bank account. The annual interest rate remains fixed, yet every year the amount of interest paid increases. Why? The annual interest paid into the account adds to the size of the balance, and the next interest payment will be calculated based on that larger balance. In exponentially growing populations, r is like the interest rate. Although r remains constant, population growth accelerates as the population size increases. When 6,000 individuals reproduce, population growth is three times higher than it was when there were only 2,000 reproducers. As another example, think of a single bacterium in a culture flask. After thirty minutes, the cell divides in two. Those two cells divide, and so on every thirty minutes. If no cells die between divisions, then the population size will double in every interval—from 1 to 2, then 4, 8, 16, 32, and so on. The time it takes for a population to double in size is its doubling time. Consider how doubling time works in our flask of bacteria. After 9–1/2 hours, or nineteen doublings, there are more than 500,000 bacterial cells. After ten hours, or twenty doublings, there are more than one
million. Curve 1 in Figure 45.6 is a plot of this change over time. The size of r affects the speed of exponential growth. Suppose 25 percent of the bacteria in our hypothetical flask die every 30 minutes. Under these conditions, it would take 17 hours, rather than 10, for the population to reach 1 million (curve 2 in Figure 45.6). The higher death rate decreases r, so exponential growth occurs more slowly. However, as long as r is greater than zero and constant, growth plots out as a J-shaped curve.
What Is the Biotic Potential? Now imagine a population living in an ideal habitat, free of all threats such as predators and pathogens. Every individual has plenty of shelter, food, and other vital resources. Under such conditions, a population would reach its biotic potential: the maximum possible per capita rate of increase for its species. All species have a characteristic biotic potential. For many bacteria, it is 100 percent every half hour or so. For humans, it is about 2 to 5 percent per year. The actual growth rate depends on many factors. A population’s age distribution, how often its individuals reproduce, and how many offspring an individual can produce are examples. The human population has not reached its biotic potential, but it is growing exponentially. We will return to the topic of the human population later in the chapter.
Take-Home Message What determines the size of a population and its growth rate? The size of a population is influenced by its rates of births, deaths, immigration, and emigration.
Subtract the per capita death rate from the per capita birth rate to get r, the per capita growth rate of a population. As long as r is constant and greater than zero, a population will grow exponentially. With exponential growth, the number of individuals increases faster and faster over time.
The biotic potential of a species is its maximum possible population growth rate under optimal conditions.
CHAPTER 45
POPULATION ECOLOGY 801
45.4
Limits on Population Growth
Natural populations seldom continue to grow unchecked. Competition and crowding can slow growth.
Environmental Limits on Growth Most of the time, a population cannot fulfill its biotic potential because of environmental limits. That is why sea stars—the females of which could make 2,500,000 eggs each year—do not fill the oceans with sea stars. Any essential resource that is in short supply is a limiting factor on population growth. Food, mineral ions, refuge from predators, and safe nesting sites are examples (Figure 45.7). Many factors can potentially
limit population growth. Which specific factor is the first to be in short supply and thus limit growth varies from one environment to another. To get a sense of the limits on growth, start again with a bacterial cell in a culture flask, where you can control the variables. First, enrich the culture medium with glucose and other nutrients bacteria require for growth. Next, let the cells reproduce. Initially, growth may be exponential. Then it slows, and population size remains relatively stable. After a brief stable period, population size plummets until all the bacterial cells are dead. What happened? The larger population required more nutrients. Over time, nutrient levels declined, and the cells could no longer divide. Even after cell division stopped, existing cells kept taking up and using nutrients. When the nutrient supply was exhausted, the last cells died out. Suppose you continued adding nutrients to the flask. Population growth would still slow and then halt. As before, the bacteria would eventually die. Why? Like other organisms, bacteria generate metabolic wastes. Over time, this waste would accumulate and poison the habitat preventing further growth. No population can grow exponentially forever. Remove one limiting factor and another one becomes limiting.
Carrying Capacity and Logistic Growth
a
b
Figure 45.8 Animated Idealized S-shaped curve characteristic of logistic growth. After a rapid growth phase (time B to C), growth slows and the curve flattens as carrying capacity is reached (time C to D). In the real world, population size often declines when a change in the environment lowers carrying capacity (time D to E). That happened to the human population of Ireland in the mid-1800s. Late blight, a disease caused by a water mold, destroyed the potato crop that was the mainstay of Irish diets (Section 22.8).
802 UNIT VII
PRINCIPLES OF ECOLOGY
Population size (number of individuals)
Figure 45.7 One example of a limiting factor. (a) Wood ducks build nests only inside cavities of specific dimensions. With the clearing of old growth forests, the access to natural cavities of the correct size and position is now a limiting factor on wood duck population size. (b) Artificial nesting boxes are being placed in preserves to help ensure the health of wood duck populations.
Time A
Carrying capacity refers to the maximum number of individuals of a population that a given environment can sustain indefinitely. Ultimately, it means that the sustainable supply of resources determines population size. We can use the pattern of logistic growth, shown in Figure 45.8, to reinforce this point. By this pattern, a small population starts growing slowly in size, then it grows rapidly, then its size levels off as the carrying capacity is reached.
initial carrying capacity
new carrying capacity
B
C
D
Change in growth pattern over time
E
Population size (number of individuals)
6,000
4,500
3,000
1,500
carrying capacity
0 1944
1957 1963 1966
1980
Time (years in which counts were made)
Graphing logistic growth yields an S-shaped curve, as shown in Figure 45.8 (A to C). In equation form, population growth per unit time
=
maximum per capita population growth rate
×
proportion number × of resources of not yet used individuals
An S-shaped curve is simply an approximation of what takes place in nature. Often a population that is growing fast overshoots its carrying capacity. Figure 45.9 shows what happened to a small population of reindeer. As the population size increased, more and more individuals competed for resources such as food and shelter, so each reindeer received a smaller share. More individuals died of starvation and fewer young were born. Deaths began to outnumber births. Finally, the death rate soared and the birth rate plummeted.
Figure 45.9 Graph of changes in a reindeer population that exceeded its habitat’s carrying capacity (blue dashed line) and did not recover. In 1944, during World War II, a United States Coast Guard crew established a station on St. Matthew, an island 320 kilometers (200 miles) west of Alaska in the Bering Sea. They brought in 29 reindeer as a backup food source. Reindeer eat lichens. Thick mats of lichens cloaked the island, which is no more than 51 kilometers long and 6.4 kilometers (32 miles by 4 miles) across. World War II drew to a close before any reindeer were shot. The Coast Guard pulled out, leaving behind seabirds, arctic foxes, voles—and a herd of healthy reindeer with no predators big enough to hunt them. In 1957, biologist David Klein visited St. Matthew. On a hike from one end of the island to the other, he counted 1,350 well-fed reindeer and saw trampled and overgrazed lichens. In 1963, Klein and three other biologists returned to the island. They counted 6,000 reindeer. They could not help but notice the profusion of reindeer tracks and feces, and a lot of trampled, dead lichens. Klein returned to St. Matthew in 1966. Bleached-out reindeer bones littered the island. Forty-two reindeer were still alive. Only one was a male; it had abnormal antlers, which made it unlikely to reproduce. There were no fawns. Klein figured out that thousands of reindeer had starved to death during the unusually harsh winter of 1963–1964. By the 1980s, there were no reindeer on the island at all.
Two Categories of Limiting Factors Density-dependent factors lower reproductive success and appear or worsen with crowding. Competition for limited resources leads to density-dependent effects, as does disease. Pathogens and parasites can spread more easily when hosts are crowded. As one example, human populations in cities support huge numbers of rats that can carry bubonic plague, typhus, and other deadly infectious diseases. Density-dependent factors control population size through negative feedback. High density causes these factors to come into play, then their effects act to lower population density. A logistic growth pattern results from this feedback effect. Density-independent factors decrease reproductive success too, but their likelihood of occurring and their magnitude of effect are unaffected by crowding. Fires, snow storms, earthquakes, and other natural disasters affect crowded and uncrowded populations alike. For
example, in December of 2004, a powerful tsunami (a giant wave caused by an earthquake) hit Indonesia. It killed about 250,000 people. The degree of crowding did not make the tsunami any more or less likely to happen, or to strike any particular island. The logistic growth equation cannot be used to predict effects of density-independent factors.
Take-Home Message How do limiting factors affect population growth? Carrying capacity is the maximum number of individuals of a population that can be sustained indefinitely by the resources in a given environment. With logistic growth, population growth is fastest when density is low, slows as the population approaches carrying capacity, and then levels off. Density-dependent factors such as disease result in a pattern of logistic growth. Density-independent factors such as natural disasters also affect population size.
CHAPTER 45
POPULATION ECOLOGY 803
45.5
Life History Patterns Life span, age at maturity, and the number of offspring produced vary widely among organisms. Natural selection influences these life history traits.
So far, you have looked at populations as if all of their members are identical with regard to age. For most species, however, individuals that make up a group are at many different stages of development. Often, those stages require different resources, as when catTable 45.1
Life Table for an Annual Plant Cohort*
Age Interval (days)
Survivorship (number surviving at start of interval)
Number Dying During Interval
0–63 63–124 124–184 184–215 215–264 264–278 278–292 292–306 306–320 320–334 334–348 348–362 362–
996 668 295 190 176 172 167 159 154 147 105 22 0
328 373 105 14 4 5 8 5 7 42 83 22 0
Death Rate “Birth” Rate (number dying/ During Interval number (number of seeds surviving) from each plant) 0.329 0.558 0.356 0.074 0.023 0.029 0.048 0.031 0.045 0.286 0.790 1.000 0
0 0 0 0 0 0 0 0.33 3.13 5.42 9.26 4.31 0
996 * Phlox drummondii; data from W. J. Leverich and D. A. Levin, 1979.
Table 45.2
Age Interval 0–1 1–5 5–10 10–15 15–20 20–25 25–30 30–35 35–44 44–45 45–50 50–55 55–60 60–65 65–70 70–75 75–80 80–85 85–90 90–95 95–100 100+
Life Table for Humans in the United States (based on 2003 conditions)
Number at Start of Interval
Number Dying During Age Interval
100,000 99,313 99,189 99,116 99,022 98,693 98,219 97,752 97,210 96,444 95,287 93,585 91,185 87,760 82,668 75,535 65,710 52,741 36,988 21,344 8,977 2,363
687 124 73 95 328 474 467 542 767 1,157 1,702 2,441 3,425 5,092 7,133 9,825 12,969 15,753 15,648 12,363 6,614 2,363
804 UNIT VII
Life Expectancy (Years Remaining) Reported at Start of Interval Live Births 77.5 77.0 73.1 68.2 63.2 58.4 53.7 48.9 45.2 39.5 35.0 30.6 26.3 22.2 18.4 14.9 11.8 9.0 6.8 5.0 3.6 2.6
PRINCIPLES OF ECOLOGY
6,781 415,262 1,034,454 1,104,485 965,633 475,606 103,679 5,748 374
erpillars that eat leaves later develop into butterflies that sip nectar. In addition, individuals might be more or less vulnerable to danger at different stages. In short, each species has a life history pattern. It has a set of adaptations that affect when an individual starts reproducing, how many offspring it has at one time, how often it reproduces, and other traits. In this section and the next, we will consider variables that underlie these age-specific patterns.
Life Tables Each species has a characteristic life span, but only a few individuals survive to the maximum age possible. Death is more likely at some ages. Individuals tend to reproduce during an expected age interval and to be most likely to die during another interval. Age-specific patterns in populations are useful to life insurance and health insurance companies as well as ecologists. Such investigators focus on a cohort—a group of individuals born during the same interval— from their time of birth until the last one dies. Ecologists often divide a natural population into age classes and record the age-specific birth rates and mortality. The resulting data is summarized in a life table (Table 45.1). Such tables inform decisions about how changes, such as harvesting a species or altering its environment, might affect the species’ numbers. Birth and death schedules for the northern spotted owl are one case in point. They were cited in federal court rulings that halted mechanized logging in the owl’s habitat—old-growth forests of the Pacific Northwest. Human life tables are usually not based on a real cohort. Instead, information about current conditions is used to predict the births and deaths for a hypothetical group. Table 45.2 is such a life table for humans based on conditions in the United States during 2003.
Survivorship Curves A survivorship curve is a graph line that emerges when you plot a cohort’s age-specific survival in its habitat. Each species has a characteristic survivorship curve. Three types are common in nature. A type I curve indicates survivorship is high until late in life. Populations of large animals that bear one or, at most, a few offspring at a time and give these young extended parental care show this pattern (Figure 45.10a). For example, a female elephant has one calf at a time and cares for it for several years. Type I curves are typical of human populations when individuals have access to good health care.
Number of survivors (logarithmic scale)
A type II curve indicates that death rates do not vary much with age (Figure 45.10b). In lizards, small mammals, and big birds, old individuals are about as likely to die of disease or predation as young ones. A type III curve indicates that the death rate for a population peaks early in life. It is typical of species that produce many small offspring and provide little or no parental care. Figure 45.10c shows how the curve plummets for sea urchins, which release great numbers of eggs. Sea urchin larvae are soft and tiny, so fish, snails, and sea slugs devour most of them before protective hard parts can develop. A type III curve is common for marine invertebrates, insects, fishes, fungi, and for annual plants such as phlox (Table 45.1).
Type I population Age
a Elephants have type I surviorship, with low mortality until advanced age.
Reproductive Strategies Some organisms such as bamboo and Pacific salmon reproduce just once, then die. Others such as oak trees, mice, and humans reproduce repeatedly. A one-shot strategy is favored when an individual is unlikely to have a second chance to reproduce. For Pacific salmon, reproduction requires a life-threatening journey from the sea to a stream. For bamboo, environmental conditions that favor reproduction occur only sporadically. Population density may also influence the optimal reproductive strategy. At low density, there will be little competition for resources, so individuals who turn resources into offspring fast are at an advantage. Such individuals reproduce while still young, produce many small offspring, and invest very little in parental care. Selection that favors traits that maximize number of offspring is called r-selection. When population density nears the carrying capacity, outcompeting others for resources becomes more important. Big individuals that reproduce later in life and produce fewer, higher quality offspring have the advantage in this scenario. Selection for traits that improve offspring quality is K-selection. Some organisms have traits associated mainly with r-selection or with K-selection, but most have a mixture of these traits.
Type II population
b Snowy egrets are type II populations, with a fairly constant death rate.
Type III population
Take-Home Message c Sea urchins are type III populations. Spines protect this adult, but the larvae are tiny, soft-bodied, and vulnerable to predation.
How do researchers study and describe life history patterns? Tracking a cohort (a group of individuals) from their birth until the last one dies reveals patterns of reproduction, death, and migrations.
Survivorship curves reveal differences in age-specific survival among species or among populations of the same species.
Different environments and population densities can favor different reproductive strategies.
Figure 45.10 Three generalized survivorship curves and examples.
CHAPTER 45
POPULATION ECOLOGY 805
45.6
Natural Selection and Life Histories Predation can serve as a selection pressure that shapes life history patterns.
Predation on Guppies in Trinidad Several years ago, two evolutionary biologists drenched with sweat and clutching fishnets were wading through a stream. John Endler and David Reznick were in the mountains of Trinidad, an island in the southern Caribbean Sea. They wanted to capture guppies (Poecilia reticulata), small fishes that live in the shallow freshwater streams (Figure 45.11). The biologists were beginning what would become a long-term study of guppy traits, including life history patterns.
Male guppies are usually smaller and more colorful than female guppies of the same age. A male’s colors serve as visual signals during courtship rituals. The drabber females are less conspicuous to predators and, unlike males they continue to grow after reaching sexual maturity. Reznick and Endler were interested in how predators influence the life history of guppies. For their study sites, they decided on streams with many small waterfalls. These waterfalls are barriers that prevent guppies in one part of a stream from moving easily to another. As a result, each stream holds several populations of guppies, and very little gene flow occurs among those populations (Section 18.8). The waterfalls also keep guppy predators from moving into different parts of the stream. In this habitat, the main
a Right, guppy that shared a stream with killifishes (below).
b Right, guppy that shared a stream with cichlids (below).
Links to Directional selection 18.4, Gene flow 18.8
c
Figure 45.11 (a,b) Guppies and two guppy eaters, a killifish and a cichlid. (c) Biologist David Reznick contemplating interactions among guppies and their predators in a freshwater stream in Trinidad.
806 UNIT VII
PRINCIPLES OF ECOLOGY
a
14
0
b
18
16
14
0
c
26
14
0
guppy predators are killifish and cichlids. These two types of predatory fish differ in size and prey preferences. The killifish is relatively small and preys mostly on immature guppies. It ignores the larger adults. The cichlids are large fish. They tend to pursue mature guppies and ignore the small ones. Some parts of the streams hold one type of predator but not the other, so different guppy populations face different predation pressures. As Reznick and Endler discovered, guppies in streams with cichlids grow faster and are smaller at maturity than those in streams with killifish (Figure 45.12). Also, guppies hunted by cichlids reproduce earlier, have more offspring at a time, and breed more frequently. Were the differences in life history traits genetic, or did environmental differences cause them? To find out, the scientists collected guppies from cichlid-dominated and killifish-dominated streams. They reared these two groups in separate aquariums under identical conditions, with no predators present. Two generations later, the life history traits of these groups still differed, as they had in natural populations. Apparently, the differences in life history traits observed in the wild do have a genetic basis. Reznick and Endler hypothesized that predators serve as selective agents that influence guppy life history traits. The scientists made a prediction: If life history traits are adaptive responses to predation, then these traits will change when a population is exposed to a new predator. To test their prediction, Reznick and Endler found a stream region above a waterfall that had killifish but no guppies or cichlids. They brought in some guppies from a region below the waterfall where there were cichlids but no killifish. At the experimental site, the guppies that had previously lived only with cichlids were now exposed to killifish. The control site was the downstream region below the waterfall, where relatives of the transplanted guppies still coexisted with cichlids. Reznik and Endler revisited the stream over the course of eleven years and thirty-six generations of guppies. They monitored traits of guppies above and below the waterfall. Their data showed that guppies at the upstream experimental site were evolving. Exposure to a novel predator had caused big changes in their rate of growth, age at
Embryo weight (milligrams)
16
reared with cichlids (which eat big fishes)
Brood interval (days)
18
reared with killifish (which eat small fishes) Male size (millimeters)
Female size (millimeters)
FOCUS ON SCIENCE
d
1.3
0.9
0
Figure 45.12 Experimental evidence of natural selection among guppy populations subject to different predation pressures. Compared to the guppies raised with killifish (green bars), guppies raised with cichlids (tan bars) differed in body size and in the length of time between broods.
first reproduction, and other life history traits. By contrast, guppies at the control site showed no such changes. As Reznick and Endler concluded, life history traits in guppies can evolve rapidly in response to the selective pressure exerted by predation.
Overfishing and the Atlantic Cod The evolution of life history traits in response to predation pressure is not merely interesting. It has commercial importance. Just as guppies evolved in response to predators, the North Atlantic codfish (Gadus morhua) evolved in response to fishing pressure. North Atlantic codfish can be big (below). From the mid-1980s to early 1990s, the number of fisherman pursuing codfish rose. Fishermen kept the largest fish, and threw smaller ones back. This human behavior put codfish that became sexually mature when they were still small at an advantage, and such fish became increasingly common. As codfish numbers declined, smaller and smaller fish were kept. Looking back, a rapid decline in age at first reproduction was a sign that the cod population was under great pressure. In 1992, Canada banned cod fishing in some areas. That ban, and later restrictions, came too late to stop the Atlantic cod population from plummeting. The population still has not recovered from this decline. Had biologists recognized the life history changes as a warning sign, they might have been able to save this fishery and protect the livelihood of thousands of workers. Monitoring the life history data for other economically important fishes may help prevent over-fishing of other species in the future.
CHAPTER 45
POPULATION ECOLOGY 807
45.7
Human Population Growth The size of the human population is at its highest level ever and is expected to continue to increase.
Links to Infectious disease 21.8, Human dispersal 26.15
The Human Population Today In 2008, the estimated average rate of increase for the human population was 1.16 percent per year. As long as birth rates continue to exceed death rates, annual additions will drive a larger absolute increase each year into the foreseeable future. Although many people enjoy abundant resources, about a fifth of the human population lives in severe poverty, and more than 800 million are malnourished (Figure 45.13). More than 1 billion people lack access to clean drinking water. More than 2 billion people face a shortage in fuelwood, which they depend on to heat their homes and cook their food. Rising populations will only increase pressure on limited resources.
Extraordinary Foundations for Growth How did we get into this predicament? For most of its history, the human population grew very slowly. The growth rate began to increase about 10,000 years
ago, and during the past two centuries, growth rates soared (Figure 45.14). Three trends promoted the large increases. First, humans were able to migrate into new habitats and expand into new climate zones. Second, humans developed new technologies that increased the carrying capacity of existing habitats. Third, humans sidestepped some limiting factors that tend to restrain the growth of other species. Geographic Expansion Early humans evolved in the
dry woodlands of Africa, then moved into the savannas. We assume they subsisted mainly on plant foods, but they probably also scavenged bits of meat. Bands of hunter–gatherers moved out of Africa about 2 million years ago. By 44,000 years ago, their descendants were established in much of the world (Section 26.15). Few species can expand into such a broad range of habitats, but the early humans had large brains that allowed them to develop the necessary skills. They learned how to start fires, build shelters, make clothing, manufacture tools, and cooperate in hunts. With the advent of language, knowledge of such skills did not die with the individual. Compared to most species, humans displayed a greater capacity to disperse fast over long distances and to become established in physically challenging new environments. Beginning about 11,000 years ago, bands of hunter–gatherers were shifting to agriculture. Instead of counting on the migratory game herds, they were settling in fertile valleys and other regions that favored seasonal harvesting of fruits and grains. They developed a more dependable basis for life. A pivotal factor was the domestication of wild grasses, including species ancestral to modern wheat and rice. Now people harvested, stored, and planted seeds all in one place. They domesticated animals as sources of food and to pull plows. They dug irrigation ditches and diverted water to croplands. Agricultural productivity became a basis for increases in population growth rates. Towns and cities formed. Later, food supplies increased yet again. Farmers started to use chemical fertilizers, herbicides, and pesticides to protect their crops. Transportation and food distribution improved. Even at its simplest, the management of food supplies through agricultural practices increased the carrying capacity for the human population.
Increased Carrying Capacity
Banks of corn silos in Wisconsin
Figure 45.13 Far from well-fed humans in highly developed countries, an Ethiopian child shows the effects of starvation. Ethiopia is one of the poorest developing countries, with an annual per capita income of $ 120. Average caloric intake is more than 25 percent below the minimum necessary to maintain good health. Malnutrition stunts the growth, weakens the body, and impairs the brain development of about half of Ethiopia’s children. Despite ongoing food shortages, Ethiopia’s population has one of the highest annual rates of increase in the world. If growth continues at its current rate, the population of 75 million will double in less than 25 years.
808 UNIT VII
PRINCIPLES OF ECOLOGY
Until about 300 years ago, malnutrition and infectious diseases
Sidestepped Limiting Factors
1999
Projected for 2050
8.9 billion
By 1999
6 billion
By 1987
5 billion
By 1974
4 billion
By 1960
3 billion
By 1927
2 billion
By 1804
1 billion
Estimated size by 10,000 years ago
5 million
6
5
1975
4
3
2 domestication of plants, animals 9000 B.C. (about 11,000 years ago)
beginning of industrial, scientific revolutions
agriculturally based urban societies
1 number of individuals (billions)
B.C. A.D.
14,000 13,000 12,000 11,000 10,000 9000
8000
7000
6000
5000
4000
3000
2000
1000
1000
2000
Figure 45.14 Growth curve (red ) for the world human population. The blue box indicates how long it took for the human population to increase from 5 million to 6 billion. The dip between years 1347 and 1351 marks the time when 60 million people died during a pandemic that may have been a bubonic plague.
kept death rates high enough to more or less balance birth rates. Infectious diseases are density-dependent controls. Plagues swept through crowded cities. In the mid-1300s, one third of Europe’s population was lost to a pandemic known as the Black Death. Waterborne diseases such as cholera that are associated with poor sanitation ran rampant. Then plumbing improved and vaccines and medications began to cut the death toll from disease. Births increasingly outpaced deaths—r became larger and exponential growth accelerated. The industrial revolution took off in the middle of the eighteenth century. People had discovered how to harness the energy of fossil fuels, starting with coal. Within decades, cities of western Europe and North America became industrialized. World War I sparked the development of more technologies. After the war, factories turned to mass production of cars, tractors, and other affordable goods. Advances in agricultural practices meant that fewer farmers were required to support a larger population. In sum, by controlling disease agents and tapping into fossil fuels—a concentrated source of energy—the human population sidestepped many factors that had previously limited its rate of increase.
Where have the far-flung dispersals and ongoing advances in technology and infrastructure gotten us? It took more than 100,000 years for the human population size to reach 1 billion. As Figure 45.14 shows, it took just 123 years to reach 2 billion, 33 more to reach 3 billion, 14 more to reach 4 billion, and then 13 more to get to 5 billion. It took only 12 more years to arrive at 6 billion! No doubt new technology will continue to increase Earth’s human carrying capacity, but growth cannot be sustained indefinitely. Why not? Ongoing increases in population size will cause density-dependent controls to exert their effects. For instance, globe-hopping travelers can carry pathogens to dense urban areas all around the world in a matter of weeks (Section 21.8). Also, limited resources cause economic hardship and civil strife. Take-Home Message Why have human populations grown so much, and what can we expect? Through expansion into new habitats, cultural interventions, and technological innovations, the human population has temporarily skirted environmental resistance to growth. Without technological breakthroughs, density-dependent controls will kick in and slow human population growth.
CHAPTER 45
POPULATION ECOLOGY 809
45.8
Fertility Rates and Age Structure Acknowledgment of the risks posed by rising populations has led to increased family planning in almost every region.
Links to AIDS Chapter 21 introduction, Contraception 42.9
order. China (with 1.3 billion people) and India (with 1.09 billion) dwarf other countries; together, they hold 38 percent of the world population. Next in line is the United States, with 294 million.
Some Projections Most governments recognize that population growth, resource depletion, pollution, and quality of life are interconnected. Many offer family planning programs, and the United Nations Population Division estimates that about 60 percent of the world’s married women now use some sort of contraception. An increase in contraceptive use is contributing to a global decline in birth rate. Death rates are also falling in most regions. Improved diet and health care are lowering the infant mortality rate (the number of infants per 1,000 who die in their first year). On the other hand, AIDS has caused the death rate to soar in some African countries (Chapter 21 introduction). World population is expected to peak at 8.9 billion by 2050, and possibly to decline as the century ends. Think of all the resources that will be required. We will have to boost food production, and find more energy and fresh water to meet even the most basic needs of billions more people. Utilizing natural resources on a larger scale will intensify pollution. We expect to see the most growth in India, China, Pakistan, Nigeria, Bangladesh, and Indonesia, in that
Population in 2006
298 million 188 million 132 million
Population in 2025 (projected)
349 million 211 million 206 million 20%
Population under age 15 Population above age 65
26% 42% 13% 6% 3%
Total fertility rate (TFR)
2.1 1.9
Shifting Fertility Rates The total fertility rate (TFR) is the average number of children born to the women of a population during their reproductive years. In 1950, the worldwide TFR averaged 6.5. Currently it is 2.7, which is still above the replacement level of 2.1—or the average number of children a couple must bear to keep the population at a constant level, given current death rates. TFRs vary among countries. TFRs are at or below replacement levels in many developed countries; the developing countries in western Asia and Africa have the highest. Figure 45.15 has some examples of the disparities in demographic indicators. Comparing age structure diagrams is revealing. In Figure 45.16, focus on the reproductive age category for the next fifteen years. Women generally bear children when they are 15 to 35 years old. We can expect populations that have a broad base to grow faster. The United States population has a relatively narrow base below a wide area that represents the 78 million babyboomers (Figure 45.16c). This cohort began forming in 1946 when American soldiers came home after World War II and started to raise families. Global increases in population seem certain. Even if every couple from this time forward has no more than two children, population growth cannot slow for sixty years. About 1.9 billion are about to enter their reproductive years. More than one-third of the world population is in the broad pre-reproductive base. China has the most wide-reaching family planning program. Its government discourages premarital sex. It urges people to delay marriage and limit families to one or two children. It offers abortions, contraceptives, and sterilization at no cost to married couples, which mobile units and paramedics provide even in remote areas. Couples who follow guidelines get more food, free medical care, better housing, and salary bonuses.
5.5 Infant mortality rate
6 per 1,000 live births 29 per 1,000 live births 97 per 1,000 live births
Life expectancy
78 years 72 years 47 years 43,740 $ $,460 3 560 $
810 UNIT VII
Answer: 31 years
Per capita income
Figure 45.15 Key demographic indicators for three countries, mainly in 2006. The United States (brown bar) is highly developed, Brazil (red bar) is moderately developed, and Nigeria (beige bar) is less developed. Figure It Out: What is the difference in life expectancy between the United States and Nigeria?
PRINCIPLES OF ECOLOGY
male
female
Rapid Growth
85+ 80–84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4
Slow Growth
Zero Growth
1955
1985
Negative Growth
a
2015
United States
India 2035
c
Mexico
Figure 45.16 Animated (a) General age structure diagrams for countries with rapid, slow, zero, and negative rates of population growth. The pre-reproductive years are the green bars; reproductive years, purple; post-reproductive years, light blue. A vertical axis divides each graph into males (left) and females (right). Bar widths correspond to the proportions of individuals in each age group.
China
(b) 1997 age structure diagrams for six nations. Population sizes are measured in millions. b
Canada
Their offspring get free tuition and special treatment when they enter the job market. Parents having more than two children lose benefits and pay more taxes. Since 1972, China’s TFR has fallen sharply, from 5.7 to 1.75. An unintended consequence has been a shift in the country’s sex ratio. Traditional cultural preference for sons, especially in rural areas, led some parents to abort female fetuses or commit infanticide. Worldwide, 1.06 boys are born for every girl. However, among those under age 15 in China, there are 1.134 boys for every girl. More than 100,000 girls are abandoned each year. The government is offering additional cash and tax incentives to the parents of girls. In the meantime,
(c) Sequential age structure diagrams for the United States population. Gold bars track the baby-boomer generation.
Australia
the population time bomb keeps on ticking in China. About 150 million of its young females now make up the pre-reproductive age category.
Take-Home Message How has the human fertility rate changed and what can we expect? The worldwide total fertility rate has been declining but it is still above the replacement level. Even if total fertility rate declines to the replacement level worldwide, the population will continue to increase; more than one-third of the population is in a broad pre-reproductive base.
CHAPTER 45
POPULATION ECOLOGY 811
Population Growth and Economic Effects The most developed countries have the slowest growth rates and use the most resources. As more countries become industrialized, pressure on Earth’s resources will increase.
Demographic Transitions The demographic transition model describes how the population growth rate changes as a country becomes more developed (Figure 45.17). Living conditions are harsh in the preindustrial stage, before technological and medical advances spread. Birth and death rates are both high, so the rate of population growth is low. In the transitional stage, industrialization begins. Food production and health care improve, and the death rate slows. Not surprisingly, in agricultural societies where families are expected to help in the fields, the birth rate is high. The annual growth rates in such societies are between 2.5 to 3 percent. When living conditions improve, the birth rate starts to fall and the population size levels off. In the industrial stage, population growth slows. Cities filled with employment opportunities attract people, and average family size declines. Large numbers of children are no longer required to work a farm, and higher survival means it is not necessary to have many offspring to ensure that a few live. In the postindustrial stage, the population growth rate becomes negative. The birth rate falls below the death rate, and the population size slowly decreases. The United States, Canada, Australia, the bulk of western Europe, Japan, and much of the former Soviet
Stage 1 Preindustrial
Stage 2 Transitional
Union have reached the industrial stage. Developing countries such as Mexico are now in the transitional stage, with people continuing to migrate to cities from agricultural regions. Many currently developing countries are expected to enter the industrial stage in the next few decades. However, there are concerns that the continued rapid population growth in these countries will overwhelm their economic growth, food production, and health care systems. The demographic transition model was developed to describe what happened when western Europe and North America became industrialized. It may not be relevant to today’s less developed countries, which receive aid from existing highly developed countries, and must also compete against these countries in a global market. There are also regional differences in how well the transition to an industrial stage is proceeding. In Asia, rising affluence is bringing higher life expectancy and lowered birth rates, as predicted. However, in subSaharan Africa, the AIDS epidemic is keeping some countries from moving out of the lowest stage of economic development.
Resource Consumption Industrialized nations use the most resources. As an example, the United States accounts for about 4.6 percent of the world’s population, yet it uses about 25
Stage 3 Industrial
Stage 4 Postindustrial
80 relative population size
70 60 50
Change in population size
Births and deaths (number per 1,000 per year)
45.9
births
40 deaths
30 20 10 0 low
increasing
very high
decreasing
low
zero
Growth rate over time
Figure 45.17 Animated Demographic transition model for changes in population growth rates and sizes, correlated with long-term changes in the economy.
812 UNIT VII
PRINCIPLES OF ECOLOGY
negative
45.10 Rise of the Seniors Changes since 1900
While some countries face overpopulation, others have declining birth rates and an increasing average age.
1900
2000 industrial output food other resources
2100 population pollution
Figure 45.18 Computer-based projection of what might happen if human population size continues to skyrocket without dramatic policy changes and technological innovation. The assumptions were that the population has already overshot the carrying capacity and current trends will continue unchanged.
percent of the world’s minerals and energy supplies. Billions of people living in India, China, and other less developed nations dream of owning the same kinds of consumer goods as people in developed countries. Earth does not have enough resources to make that possible. For everyone now alive to have a lifestyle like an average American would require four times the resources present on Earth. What will happen if the human population keeps on increasing as predicted? How will we find the food, energy, water, and other basic resources needed to sustain so many people? Can we provide the necessary education, housing, medical care, and other social services? Some models suggest not (Figure 45.18). Other analysts claim we can adapt to a more crowded world if innovative technologies improve crop yields, if people rely less on meat for protein, and if resources are shared more equitably among regions. We have made great strides in increasing our agricultural output, but have been less successful in getting food to the people who need it.
In some developed countries, the decreasing total fertility rate and increasing life expectancy have resulted in a high proportion of older adults. In Japan, people over 65 currently make up about 20 percent of the population. In the United States, the proportion of people over 65 is projected to reach this level by 2030 (Figure 45.19). In 2050, there could be as many as 31 million Americans over age 85. The aging of a population has social implications. Older individuals have traditionally been supported by a younger workforce. In the United States, most older people receive social security payments and government-subsidized medical care. As a result of inflation and increases in life expectancy, the benefits being distributed to current seniors exceed the contributions these people paid into the program. When baby boomers begin to receive benefits, the deficit will skyrocket. Keeping the system going will require ever greater contributions from the younger, still-working population. Increasing numbers of debilitated seniors will also challenge the health care system. Thus, finding ways to keep people healthy later in life is both a social and an economic priority.
Take-Home Message How does industrialization affect population growth and resource consumption? Differences in population growth and resource consumption among countries can be correlated with levels of economic development. Growth rates are typically greatest during the transition to industrialization.
Global conditions have changed so that the demographic transition model may no longer apply to modern nations. An average person living in a highly developed nation uses far more resources than a person in a less-developed nation.
Figure 45.19 Two of the 37 million Americans over age 65.
Take-Home Message How does slowing population growth affect age distribution?
When population growth slows, the proportion of older individuals rises.
CHAPTER 45
POPULATION ECOLOGY 813
IMPACTS, ISSUES REVISITED
The Numbers Game
Many states are struggling to control rising numbers of white-tailed deer. In Ohio, the number has risen from 17,000 deer in 1970 to more than 700,000. In West Virginia, deer are overbrowsing plants that grow on the forest floor, including wild ginseng, which is an important export crop. Biologist James McGraw argues that controlling deer and saving West Virginia’s forests will require either reintroducing big predators or increasing deer hunting.
Summary Sections 45.1, 45.2 Each population is a group of individuals of the same species. Its growth is affected by its demographics. These include population size and age structure, such as the size of the reproductive base. They also include population density and population distribution. Most populations in nature have a clumped distribution pattern. Counting the number of individuals in quadrats is a way to estimate the density of a population in a specified area. Capture–recapture methods can be used to estimate the population density for mobile animals.
Use the interaction on CengageNOW to learn how to estimate population size.
Section 45.3 Immigration and emigration permanently affect population size, but migration does not. The per capita birth rate minus the per capita death rate gives us r, the population’s per capita growth rate. When births equal deaths we have zero population growth. In cases of exponential growth, a population’s growth is proportional to its size. The population size increases at a fixed rate in any given interval. The time required for a population to double is the doubling time. The maximum possible rate of increase is a species’ biotic potential.
View the animation on CengageNOW to observe a pattern of exponential growth.
Section 45.4 Limiting factors constrain population increases. With logistic growth, a small population starts growing slowly, then grows rapidly, then levels off once carrying capacity is reached. Density-dependent factors are conditions or events that lower reproductive success and have an increasing effect with crowding. Densityindependent factors are conditions or events that can lower reproductive success, but their effect does not vary with crowding.
Watch the animation on CengageNOW to learn about logistic growth.
Sections 45.5, 45.6 The time to maturity, number of reproductive events, number of offspring per event, and life span are aspects of a life history pattern. A cohort is a group of individuals that were born at the same time. Three types of survivorship curves are common: a high death rate late in life, a constant rate at all ages, or a high rate early in life. Life histories have a genetic basis 814 UNIT VII
PRINCIPLES OF ECOLOGY
How would you vote? Without natural predators, deer numbers are soaring. Is encouraging deer hunting the best solution? See CengageNOW for details, then vote online.
and are subject to natural selection. At low population density, r-selection favors quickly producing as many offspring as possible. At a higher population density, K-selection favors investing more time and energy in fewer, higher quality offspring. Most populations have a mixture of both r-selected and K-selected traits. Section 45.7 The human population has surpassed 6.6 billion. Expansion into new habitats and agriculture allowed early increases. Later, medical and technological innovations raised the carrying capacity and sidestepped many limiting factors. Section 45.8 A population’s total fertility rate (TFR) is the average number of children born to women during their reproductive years. The global TFR is declining and most countries have family planning programs of some sort. Even so, the pre-reproductive base of the world population is so large that population size will continue to increase for at least sixty years.
Use the interaction on CengageNOW to compare age structure diagrams.
Section 45.9 The demographic transition model predicts how human population growth rates will change with industrialization. Generally, the death rate and birth rate both fall with rising industrialization, but conditions in countries can vary in ways that affect this trend. Developed nations have a much higher per capita consumption of resources than developing nations. Earth does not have enough resources to support the current population in the style of the developed nations.
Use the interaction on CengageNOW to learn about the demographic transition model.
Section 45.10 Slowing population growth leads to an increase in the proportion of elderly in the population.
Self-Quiz
Answers in Appendix III
1. Most commonly, individuals of a population show a distribution through their habitat. a. clumped c. nearly uniform b. random d. none of the above 2. The rate at which population size grows or declines depends on the rate of . a. births c. immigration e. a and b b. deaths d. emigration f. all of the above
Data Analysis Exercise 180 Number of marked iguanas
In 1989, Martin Wikelski started a long-term study of marine iguana populations in the Galápagos Islands (Section 17.2). He marked the iguanas on two islands— Genovesa and Santa Fe—and collected data on how their body size, survival, and reproductive rates varied over time. The iguanas eat algae and have no predators, so deaths are usually the result of food shortages, disease, or old age. His studies showed that numbers decline during El Niño events, when the surrounding waters heat up. In January 2001, an oil tanker ran aground and leaked a small amount of oil into the waters near Santa Fe—Figure 45.20 shows the number of marked iguanas that Wikelski and his team counted in their census of study populations just before the spill and about a year later.
3. Wikelski concluded that changes on Santa Fe were the result of the oil spill, rather than sea temperature or other climate factors common to both islands. How would the census numbers be different from those he observed if an adverse event had affected both islands?
3. Suppose 200 fish are marked and released in a pond. The following week, 200 fish are caught and 100 of them have marks. There are about fish in this pond. a. 200 b. 300 c. 400 d. 2,000 4. A population of worms is growing exponentially in a compost heap. Thirty days ago there were 400 worms and now there are 800. How many worms will there be thirty days from now, assuming conditions remain constant? a. 1,200 b. 1,600 c. 3,200 d. 6,400 5. For a given species, the maximum rate of increase per individual under ideal conditions is its . a. biotic potential c. environmental resistance b. carrying capacity d. density control 6. is a density-independent factor that influences population growth. a. Resource competition c. Predation b. Infectious disease d. Harsh weather 7. A life history pattern for a population is a set of adaptations that influence the individual’s . a. longevity c. age at reproductive maturity b. fertility d. all of the above 8. The human population is now over 6.6 billion. It was about half that in . a. 2004 b. 1960 c. 1802 d. 1350 9. Compared to the less developed countries, the highly developed ones have a higher . a. death rate c. total fertility rate b. birth rate d. resource consumption rate 10. population growth increases the proportion of older individuals in a population. a. Slowing b. Accelerating
120 90 60 30 0
1. Which island had more marked iguanas at the time of the first census? 2. How much did the population size on each island change between the first and second census?
150
Jan Dec
Jan Dec
Genovesa Island
Santa Fe Island
Figure 45.20 Shifting numbers of marked marine iguanas on two Galápagos islands. An oil spill occurred near Santa Fe just before the January 2001 census (green bars). A second census was carried out in December 2001 (tan bars).
11. Match each term with its most suitable description. carrying a. maximum rate of increase per capacity individual under ideal conditions exponential b. population growth plots out growth as an S-shaped curve biotic c. maximum number of individuals potential sustainable by the resources limiting in a given environment factor d. population growth plots out logistic as a J-shaped curve growth e. essential resource that restricts population growth when scarce
Visit CengageNOW for additional questions.
Critical Thinking 1. Think back to Section 45.6. When researchers moved guppies from populations preyed on by cichlids to a habitat with killifish, the life histories of the transplanted guppies evolved. They came to resemble those of guppy populations preyed on by killifish. Males became gaudier; some scales formed larger, more colorful spots. How might a decrease in predation pressure on sexually mature fish favor this change? 2. The age structure diagrams for two hypothetical populations are shown at right. Describe the growth rate of each population and discuss the current and future social and economic problems that each is likely to face. CHAPTER 45
POPULATION ECOLOGY 815
46
Community Structure and Biodiversity IMPACTS, ISSUES
Fire Ants in the Pants
Step on a nest of red imported fire ants, Solenopsis invicta
foreign ant’s spread. The chemicals might even be facilitating
(Figure 46.1a), and you will be sorry. The ants are quick to
dispersal by preferentially wiping out native ant populations.
defend their nest. Ants stream out from the ground and inflict
Ecologists are enlisting biological controls. Phorid flies
a series of stings. Venom injected by the stinger causes burn-
control S. invicta in its native habitat (Figure 46.1b). The
ing pain and results in the formation of a pus-filled bump that
flies are parasitoids, a type of parasite that kills its host in a
is slow to heal. Multiple stings can cause nausea, dizziness,
rather gruesome way. A female fly pierces the cuticle of an
and—rarely—death.
adult ant, then lays an egg in the ant’s soft tissues. The egg
S. invicta arrived in the United States from South America
hatches into a larva, which grows and eats its way through
in the 1930s, probably as stowaways on a ship. The ants
tissues to the ant’s head. After the larva gets big enough,
spread out from the Southeast and have been found as far
it causes the ant’s head to fall off (Figure 46.1c). The larva
west as California and as far north as Kansas and Delaware.
develops into an adult within the detached head.
Like many introduced species, the ants disrupt natural
Several phorid fly species have now been introduced in
communities. They attack livestock, pets, and wildlife. They
various southern states. The flies are surviving, reproducing,
also outcompete native ants and may be contributing to the
and increasing their range. They probably will never kill off all
decline of other native wildlife. For example, the Texas horned
S. invicta in affected areas, but they are expected to reduce
lizard vanished from most of its home range when S. invicta
the density of colonies.
moved in and displaced the native ants—the lizard’s food of
This example introduces community structure: patterns in
choice. The horned lizard cannot tolerate eating the imported
the number of species and their relative abundances. As you
fire ants.
will see, species interactions and disturbances to the habitat
Invicta means “invincible” in Latin and S. invicta is living up to its species name. Pesticides have not managed to halt the
can shift community structure in small and large ways—some predictable, others unexpected.
b
c a
See the video! Figure 46.1 (a) Red imported fire ant (S. invicta) mounds. (b) A phorid fly that lays its eggs on the ants. (c) An ant that lost its head after the larva of a phorid fly moved into it.
Links to Earlier Concepts
Key Concepts Community characteristics
In this chapter, you will see how natural selection (Section 17.3) and coevolution (18.12) shape traits of species in communities.
You will revisit examples of interspecific interactions such as bacteria that live inside protists (20.4), plant– pollinator interactions (23.8, 30.2), lichens (24.6), and root nodules and mycorrhizae (29.2).
You will consider again the evolution of prey defenses such as ricin (Chapter 14 introduction), nematocysts (25.5), and the way that evolution affects pathogens (21.8).
Knowledge of biogeography (17.1) will help you understand how communities in different regions differ.
A community consists of all species in a habitat. Each species has a niche—the sum of its activities and relationships. A habitat’s history, its biological and physical characteristics, and the interactions among species in the habitat affect community structure. Section 46.1
Types of species interactions Commensalism, mutualism, competition, predation, and parasitism are types of interspecific interactions. They influence the population size of participating species, which in turn influences the community’s structure. Sections 46.2–46.7
Community stability and change Communities have certain elements of stability, as when some species persist in a habitat. Communities also change, as when new species move into the habitat and others disappear. Physical characteristics of the habitat, species interactions, disturbances, and chance events affect how a community changes over time. Sections 46.8–46.10
Global patterns in community structure Biogeographers identify regional patterns in species distribution. They have shown that tropical regions hold the greatest number of species, and also that characteristics of islands can be used to predict how many species an island will hold. Section 46.11
How would you vote? Currently, only a fraction of the crates imported into the United States are inspected for the inadvertent or deliberate presence of exotic species. Would added inspections that better protect native communities be worth the cost? See CengageNOW for details, then vote online.
817
46.1
Which Factors Shape Community Structure? Community structure refers to the number and relative abundances of species in a habitat. It changes over time.
Table 46.1
Direct Two-Species Interactions
Link to Coevolution 18.12 Type of Interaction
Effect on Species 1
Effect on Species 2
The type of place where a species normally lives is its habitat, and all species living in a habitat represent a community. A community has a dynamic structure. It shows shifts in its species diversity—the number and relative abundances of species. Many factors influence community structure. First, climate and topography influence a habitat’s features, including temperature, soil, and moisture. Second, a habitat has only certain kinds and amounts of food and other resources. Third, species themselves have traits that adapt them to certain habitat conditions, as in Figure 46.2. Fourth, the species interact in ways that cause shifts in their numbers and abundances. Finally, the timing and history of disturbances, both natural and human-induced, affect community structure.
the conditions, resources, and interactions necessary for survival and reproduction. Aspects of an animal’s niche include temperatures it can tolerate, the kinds of foods it can eat, and the types of places it can breed or hide. A description of a plant’s niche would include its soil, water, light, and pollinator requirements.
The Niche
Categories of Species Interactions
All species of a community share the same habitat— the same “address”—but each also has a “profession,” or unique ecological role, that sets it apart. This role is the species’ niche, which we describe in terms of
Species in a community interact in a variety of ways (Table 46.1) Commensalism benefits one species and does not affect the other. Most bacteria in your gut are commensal. They benefit by living inside you, but do not help or harm you. Mutualism provides benefits to both species. Interspecific competition hurts both species. Predation and parasitism help one species at another’s expense. Predators are free-living organisms that kill their prey. Parasites live on or in a host and usually do not kill it. Parasitism, commensalism, and mutualism can all be types of symbiosis, which means “living together.” Symbiotic species, or symbionts, spend most or all of their life cycle in close association with each other. An endosymbiont is a species that lives inside its partner. Regardless of whether one species helps or hurts another, two species that interact closely for extended periods may coevolve. With coevolution, each species is a selective agent that shifts the range of variation in the other (Section 18.12).
a
b
c
Figure 46.2 Three of twelve fruit-eating pigeon species in Papua New Guinea’s tropical rain forests: (a) pied imperial pigeon, (b) superb crowned fruit pigeon, and (c) the turkey-sized Victoria crowned pigeon. The forest’s trees differ in the size of fruit and fruit-bearing branches. The big pigeons eat big fruit. Smaller ones, with smaller bills, cannot peck open big, thick-skinned fruit. They eat the small, soft fruit on branches too spindly to hold big pigeons. Trees feed the birds, which help the trees. Seeds in fruit resist digestion in the bird gut. Flying pigeons disperse seed-rich droppings, often some distance from mature trees that would outcompete new seedlings for water, minerals, and sunlight. With dispersal, seedlings have a better chance of surviving.
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Commensalism
Helpful
None
Mutualism
Helpful
Helpful
Interspecific competition
Harmful
Harmful
Predation
Helpful
Harmful
Parasitism
Helpful
Harmful
Take-Home Message What is a biological community? A community consists of all species in a habitat, each with a unique niche, or ecological role. Species in a community interact and may benefit, harm, or have no net effect on one another. Some are symbionts; they associate closely for most or all of their life cycle.
46.2
Mutualism
A mutualistic interaction benefits both partners.
Links to Endosymbiosis and organelles 20.4, Pollination 23.8 and 30.2, Lichens 24.6, Plant mutualisms 29.2
Mutualists are common in nature. For example, birds, insects, bats, and other animals serve as pollinators of flowering plants (Sections 23.8 and 30.2). Pollinators feed on energy-rich nectar and pollen. In return, they transfer pollen between plants, facilitating pollination. Similarly, pigeons take food from rain forest trees but disperse their seeds to new sites (Figure 46.2). In some mutualisms, neither species can complete its life cycle without the other. Yucca plants and the moths that pollinate them show such interdependence (Figure 46.3). In other cases, the mutualism is helpful but not a life-or-death requirement. Most plants, for example, use more than one pollinator. Mutualists help most plants take up mineral ions (Section 29.2). Nitrogen-fixing bacteria living on roots of legumes such as peas provide the plant with extra nitrogen. Mycorrhizal fungi living in or on plant roots enhance the plant’s mineral uptake. Other fungi partner with photosynthetic bacteria or algae, thus forming lichens (Section 24.6). In all mutualisms, there is some conflict between partners. In a lichen, the fungus would do best by obtaining as much sugar as possible from its photosynthetic partner. That partner would do best by keeping as much sugar as possible for its own use. Some mutualists defend one another. For example, most fishes avoid sea anemones, which have stinging cells called nematocysts in their tentacles. However, an anemone fish can nestle among those tentacles (Figure 46.4). A mucus layer shields the anemone fish from stings, and the tentacles keep it safe from predatory fish. The anemone fish repays its partner by chasing off the few fishes that feed on sea anemone tentacles. Finally, reflect on a theory outlined in Section 20.4, whereby certain aerobic bacteria became mutualistic endosymbionts of early eukaryotic cells. The bacteria received nutrients and shelter. In time, they evolved into mitochondria and provided the “host” with ATP. Cyanobacteria living inside eukaryotic cells evolved into chloroplasts by a similar process.
Figure 46.3 Mutualism in the high desert of Colorado. Each species of Yucca plant is pollinated by one species of yucca moth, which cannot complete its life cycle with any other plant. The moth matures when yucca plants flower. A female moth collects yucca pollen and rolls it into a ball. She flies to another flower and pierces the floral ovary, and lays eggs inside. As she crawls out, she pushes a ball of pollen onto the flower’s pollen-receiving platform. After pollen grains germinate, they give rise to pollen tubes, which grow through the ovary tissues and deliver sperm to the plant’s eggs. Seeds develop after fertilization. Meanwhile, moth eggs develop into larvae that eat a few seeds, then gnaw their way out of the ovary. Seeds that larvae do not eat give rise to new yucca plants.
Take-Home Message What is mutualism? Mutualism is a species interaction in which each species benefits by associating with the other.
In some cases the mutualism is necessary for both species; more often it is not essential for one or both partners.
Figure 46.4 The sea anemone Heteractis magnifica, which shelters about a dozen fish species. It has a mutualistic association with the pink anemone fish (Amphiprion perideraion). This tiny but aggressive fish chases away predatory butterfly fishes that would bite off tips of anemone tentacles. The fish cannot survive and reproduce without the protection of an anemone. The anemone does not need a fish to protect it, but it does better with one.
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COMMUNITY STRUCTURE AND BIODIVERSITY 819
46.3
Competitive Interactions Resources are limited and individuals of different species often compete for access to them.
Links to Natural selection 17.3, Limiting factor 45.4
As Charles Darwin understood, intense competition for resources among individuals of the same species leads to evolution by natural selection (Section 17.3). Competitive interactions between different species— interspecific competition—is not usually as intense. Why not? The requirements of two species might be similar, but they can never be as close as they are for individuals of the same species. With interference competition, one species actively prevents another from accessing some resource. As an example, one species of scavenger will often chase
a
b
Figure 46.5 Interspecific competition among scavengers. (a) A golden eagle and a red fox face off over a moose carcass. (b) In a dramatic demonstration of interference competition, the eagle attacks the fox with its talons. After this attack, the fox retreated, leaving the eagle to exploit the carcass.
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PRINCIPLES OF ECOLOGY
another away from a carcass (Figure 46.5). As another example, some plants use chemical weapons against potential competition. Aromatic chemicals that ooze from tissues of sagebrush plants, black walnut trees, and eucalyptus trees seep into the soil around these plants. The chemicals prevent other kinds of plants from germinating or growing. In exploitative competition, species do not interact directly; each reduces the amount of resources available to the other by using that resource. For example, deer and blue jays both eat acorns in oak forests. The more acorns the birds eat, the fewer there are for the deer.
Effects of Competition Deer and blue jays share a fondness for acorns, but each also has other sources of food. Any two species differ in their resource requirements. Species compete most intently when the supply of a shared resource is the main limiting factor for both (Section 45.4). In the 1930s, G. Gause conducted experiments with two species of ciliated protists (Paramecium) that compete for bacterial prey. When cultured separately, the growth curves for these species were about the same. When grown together, growth of one species outpaced the other, and drove it to extinction (Figure 46.6). Experiments by Gause and others are the basis for the concept of competitive exclusion: Whenever two species require the same limited resource to survive or reproduce, the better competitor will drive the less competitive species to extinction in that habitat. Competitors can coexist when their resource needs are not exactly the same, however, competition generally supresses population growth of both species. For instance, Gause also studied two Paramecium species with differing food preferences. When grown together, one fed on bacteria suspended in culture tube liquid. The other ate yeast cells near the bottom of the tube. When grown together, population growth rates fell for both species, but they continued to coexist. Experiments by Nelson Hairston showed the effects of competition between slimy salamanders (Plethodon glutinosus) and Jordan’s salamanders (P. jordani). The salamanders coexist in wooded habitats (Figure 46.7). Hairston removed all slimy salamanders from certain test plots and Jordan’s salamanders from others. He left a final group of plots unaltered as controls. After five years, the numbers and abundances of the two species had not changed in the control plots. In the plots with slimy salamanders alone, population density had soared. Numbers also increased in plots with Jordan’s salamanders alone. Hairston concluded
Relative population density
P. caudatum and P. aurelia
P. aurelia
P. caudatum
0
4
8 12 16 Time (days)
20
24
0
4
8 12 16 Time (days)
20
24
0
A Paramecium caudatum and P. aurelia grown in separate culture flasks established stable populations. The S-shaped graph curves indicate logistic growth and stability.
that whenever these salamanders coexist, competitive interactions suppress the population growth of both.
Resource Partitioning Think back on those fruit-eating pigeon species. They all require fruit, but each eats fruits of a certain size. Their preferences are a case of resource partitioning: a subdividing of an essential resource, which reduces the competition among species that require it. Similarly, three annual plant species live in the same field. They all require minerals and water, but their roots take them up at different depths (Figure 46.8). When species with very similar requirements share a habitat, competition puts selective pressure on them. In each species, individuals who differ most from the competing species are favored. The outcome may be character displacement: Over the generations, a trait of one species diverges in a way that lowers the intensity of competition with the other species. Modification of the trait promotes partitioning of a resource.
8 12 16 Time (days)
20
24
B For this experiment, the two species were grown together. P. aurelia (brown curve) drove P. caudatum toward extinction (green curve).
0 Soil depth (centimeters)
Figure 46.6 Animated Results of competitive exclusion between two related species that compete for the same food. Two species cannot coexist indefinitely in the same habitat when they require identical resources.
4
20 40 60
bristly foxtail roots Indian mallow roots
80
bristly foxtail smartweed roots
100
Figure 46.8 A case of resource partitioning among three annual plant species in a plowed but abandoned field. Roots of each species take up water and mineral ions from a different soil depth. This reduces competition among them and allows them to coexist.
Indian mallow
smartweed
For example, researchers Peter and Rosemary Grant demonstrated a change in beak size in the Galápagos finch Geospiza fortis. It occurred after a larger finch, G. magnirostris, moved onto the island where G. fortis had previously been alone. Arrival of G. magnirostris put big-beaked G. fortis individuals at a disadvantage. They now had to compete with G. magnirostris for big seeds. Small-beaked G. fortis had no such competition, and enjoyed higher reproductive success. As a result, the average beak size of G. fortis declined over time. Take-Home Message What happens when species compete for resources? In some interactions, one species actively blocks another’s access to a resource. In other interactions, one species is simply better than another at exploiting a shared resource.
Figure 46.7 Two species of salamanders, Plethodon glutinosus (top) and P. jordani (bottom), that compete in areas where their habitats overlap.
When two species compete, selection favors individuals whose needs are least like those of the competing species. Indian mallow smartweed
CHAPTER 46
COMMUNITY STRUCTURE AND BIODIVERSITY 821
46.4 Predator–Prey Interactions The relative abundances of predator and prey populations of a community shift over time in response to species interactions and changing environmental conditions.
Link to Coevolution 18.12
Models for Predator–Prey Interactions Predators are consumers that get energy and nutrients from prey, which are living organisms that predators capture, kill, and eat. The quantity and types of prey species affect predator diversity and abundance, and predator types and numbers do the same for prey. The extent to which a predator species affects prey numbers depends in part on how individual predators respond to changes in prey density. Figure 46.9a compares models for the three main predator responses to increases in density. In a type I response, the proportion of prey killed is constant, so the number killed in any given interval depends solely on prey density. Web-spinning spiders and other passive predators tend to show this type of response. As the number of flies in an area increases, more and more become caught in each spider’s web. Filter-feeding predators also show a type I response.
In a type II response, the number of prey killed depends on the capacity of predators to capture, eat, and digest prey. When prey density increases, the rate of kills rises steeply at first because there are more prey to catch. Eventually, the rate of increase slows, because each predator is exposed to more prey than it can handle at one time. Figure 46.9b is an example of this type of response, which is common in nature. A wolf that just killed a caribou will not hunt another until it has eaten and digested the first one. In a type III response, the number of kills increases slowly until prey density exceeds a certain level, then rises rapidly, and finally levels off. This response is common in nature in three situations. In some cases, the predator switches among prey, concentrating its efforts on the species that is most abundant. In other cases, the predators need to learn how to best capture each prey species; they get more lessons when more prey are around. In still other cases, the number of hiding places for prey is limited. Only after prey density rises and some individual prey have no place to hide, does the number of kills increase. Knowing which type of response a predator makes to prey helps ecologists predict long-term effects of predation on a prey population.
The Canadian Lynx and Snowshoe Hare
A
Number of kills per day
Number of prey killed per predator per unit time
In some cases, a time lag in the predator’s response to prey density leads to cyclic changes in abundance of predators and prey. When prey density becomes low, the number of predators declines. As a result, prey are safer and their number increases. This increase allows predators to increase. Then predation causes another prey decline, and the cycle begins again. Consider a ten-year oscillation in populations of a predator, the Canadian lynx, and the snowshoe hare
I II III
0.12 0.08 0.06 0.04 0.02 0
Prey population density
822 UNIT VII
B
PRINCIPLES OF ECOLOGY
0.5 1 1.5 2 2.5 Caribou per square kilometer
Figure 46.9 Animated (a) Three models for responses of predators to prey density. Type I: Prey consumption rises linearly as prey density rises. Type II: Prey consumption is high at first, then levels off as predator bellies stay full. Type III: When prey density is low, it takes longer to hunt prey, so the predator response is low. (b) A type II response in nature. For one winter month in Alaska, B. W. Dale and his coworkers observed four wolf packs (Canis lupus) feeding on caribou (Rangifer tarandus). The interaction fit the type II model for the functional response of predators to the prey density.
160
Number of pelts taken (× 1,000)
140 120
Figure 46.10 Graph of the abundances of Canadian lynx (dashed line) and snowshoe hares (solid line), based on counts of pelts sold by trappers to Hudson’s Bay Company during a ninety-year period. Charles Krebs observed that predation causes heightened alertness among snowshoe hares, which continually look over their shoulders during the declining phase of each cycle. The photograph at right supports the Krebs hypothesis that there is a three-level interaction going on, one that involves plants.
100 80 60
The graph may be a good test of whether you tend to accept someone else’s conclusions without questioning their basis in science. Remember those sections in Chapter 1 that introduced the nature of scientific methods?
40 20 0 1845
1865
1885 1905 Time (years)
1925
What other factors may have had an impact on the cycle? Did the weather vary, with more severe winters imposing greater demand for hares (to keep lynxes warmer) and higher death rates? Did the lynx compete with other predators, such as owls? Did the predators turn to alternative prey during low points of the hare cycle?
that is its main prey (Figure 46.10). To determine the causes of this pattern, Charles Krebs and coworkers tracked hare population densities for ten years in the Yukon River Valley of Alaska. They set up one-squarekilometer control plots and experimental plots. They used fences to keep predatory mammals out of some plots. Extra food or fertilizers that helped plants grow were used in other plots. The researchers captured and put radio collars on more than 1,000 snowshoe hares, lynx, and other animals, and then released them. In predator-free plots, the hare density doubled. In plots with extra food, it tripled. In plots having extra food and fewer predators, it increased elevenfold. The experimental manipulations delayed the cyclic declines in population density but did not stop them. Why not? Owls and other raptors flew over the fences. Only 9 percent of the collared hares starved to death; predators killed some of the others. Krebs concluded that a simple predator–prey or plant–herbivore model did not fully explain his results. Other variables were at work, in a multilevel interaction.
Coevolution of Predators and Prey Interactions among predators and prey can influence characteristic species traits. If a certain genetic trait in a prey species helps it escape predation, that trait will increase in frequency. If some predator characteristic helps overcome a prey defense, it too will be favored. Each defensive improvement selects for a countering improvement in predators, which selects for another defensive improvement, and so on, in a never-ending arms race. The next section describes some outcomes.
Take-Home Message How do predator and prey populations change over time? Predator populations show three general patterns of response to changes in prey density. Population levels of prey may show recurring oscillations. The numbers in predator and prey populations often vary in complex ways that reflect the multiple levels of interaction in a community. Predator and prey populations exert selective pressures on one another.
CHAPTER 46
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46.5
An Evolutionary Arms Race Predators select for better prey defenses, and prey select for more efficient predators.
Links to Ricin Chapter 14 introduction, Coevolution 18.12, Nematocysts 25.5
Prey Defenses Earlier chapters, including Chapter 25, introduced some examples of prey defenses. Many species have hard parts that make them difficult to eat. Spikes in a sponge body, clam and snail shells, lobster and crab exoskeletons, sea urchin spines—all of these traits help deter predators and thereby contribute to evolutionary success. Also, many heritable traits function in camouflage: body shape, color pattern, behavior, or a combination of factors make an individual blend with its surroundings. Predators cannot eat prey they cannot find. Section 18.4 explains how alleles that improved the camouflage of a prey species, the desert pocket mouse, were adaptive in particular habitats. Camouflage is widespread. Marsh birds called bitterns live among tall reeds. When threatened, a bittern points its beak skyward and blends with the reeds (Figure 46.11a). On a breezy day, the bird enhances the effect by swaying slightly. A caterpillar with mottled color patterns appears to be a bird dropping (Figure 46.11b). Desert plants of the genus Lithops usually look like rocks (Figure 46.11c). They flower only during a brief rainy season, when plenty of other plants tempt herbivores. Many prey species contain chemicals that taste bad or sicken predators. Some produce toxins through metabolic processes. Others use chemical or physical weapons that they get from their prey. For instance, after sea slugs dine on a sea anemone or a jellyfish, they can store its stinging nematocysts in their own tissues (Figure 25.24c). Leaves, stems, and seeds of many plants contain bitter, hard-to-digest, or toxic chemicals. Remember the Chapter 14 introduction? It explains how ricin acts to kill or sicken animals. Ricin evolved in castor bean seeds as a defense against herbivores. Caffeine in coffee beans and nicotine in tobacco leaves evolved as defenses against insects. Many prey species advertise their bad-tasting or toxinladen properties by warning coloration. They have flashy patterns and colors that predators learn to recognize and avoid. For instance, a toad might catch a yellow jacket once. But a painful sting from this wasp teaches the toad that black and yellow stripes mean AVOID ME! Mimicry is an evolutionary convergence in body form; species come to resemble one another. In some cases, two or more well-defended organisms end up looking alike.
a
b
Figure 46.11 Prey camouflage. (a) What bird? When a predator approaches its nest, the least bittern stretches its neck (which is colored like the surrounding withered reeds), points its bill upward, and sways like reeds in the wind. (b) An inedible bird dropping? No. This caterpillar’s body coloration and its capacity to hold its body in a rigid position help camouflage it from predatory birds. (c) Find the plants (Lithops) hiding in the open from herbivores with the help of their stonelike form, pattern, and coloration.
c
824 UNIT VII
PRINCIPLES OF ECOLOGY
FOCUS ON EVOLUTION
a A dangerous model
b One of its edible mimics
c Another edible mimic
d And another edible mimic
Figure 46.12 Examples of mimicry. Edible insect species often resemble toxic or unpalatable species that are not at all closely related. (a) A yellow jacket can deliver a painful sting. It might be the model for nonstinging wasps (b), beetles (c), and flies (d) of strikingly similar appearance. In others, a tasty, harmless prey species evolves the same warning coloration as an unpalatable or well-defended one (Figure 46.12). Predators may avoid the mimic after experiencing the disgusting taste, irritating secretion, or painful sting of the species it resembles. When an animal is cornered or under attack, survival may depend on a last-chance trick. Opossums “play dead,” Other animals startle predators. Section 1.7 describes an experiment that tested the peacock butterfly defenses—a show of eye-like spots and hissing. Other species puff up, bare sharp teeth, or flare neck ruffs (Figure 26.19d ). When cornered, many animals, including skunks, some snakes, many toads, and certain insects, secrete or squirt stinky or irritating repellents (Figure 46.13a).
Adaptive Responses of Predators A predator’s evolutionary success hinges on eating prey. Stealth, camouflage, and ways of avoiding repellents are countermeasures to prey defenses. For example, some edible beetles spray noxious chemicals at their attackers.
a
b
A grasshopper mouse grabs the beetle and plunges the sprayer end into the ground, and then chews on the tasty, unprotected head (Figure 46.13b). Some evolved traits in herbivores are responses to plant defenses. The digestive tract of koalas can handle tough, aromatic eucalyptus leaves that would sicken other herbivorous mammals. Also, a speedier predator catches more prey. Consider the cheetah, the world’s fastest animal on land. One was clocked at 114 kilometers (70 miles) per hour. Compared with other big cats, a cheetah has longer legs relative to body size and nonretractable claws that act like cleats to increase traction. Thomson’s gazelle, its main prey, can run longer but not as fast (80 kilometers per hour). Without a head start, the gazelle is likely to be outrun. Camouflaging helps predators as well as prey. Think of white polar bears stalking seals on ice, striped tigers crouched in tall-stalked, golden grasses, and scorpionfish on the sea floor (Figure 46.13c). Camouflage can be quite stunning among predatory insects (Figure 46.13d). Even so, with each new, improved camouflaging trait, predators select for enhanced predator-detecting ability in prey.
c
d
Figure 46.13 Predator responses to prey defenses. (a) Some beetles spray noxious chemicals at attackers, which deters them some of the time. (b) Grasshopper mice plunge the chemicalspraying tail end of their beetle prey into the ground and feast on the head end. (c) This leaf scorpionfish, is a venomous predator with camouflaging fleshy flaps, multiple colors, and many spines. (d) Where do the pink flowers end and the pink praying mantis begin?
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46.6
Parasite–Host Interactions Predators have only a brief interaction with prey, but parasites live on or in their hosts.
Link to Evolution and disease 21.8
Parasites and Parasitoids Parasites spend all or part of their life living in or on other organisms, from which they steal nutrients. Although most parasites are small, they can have a major impact on populations of their hosts. Many parasites are pathogens; they cause disease in their hosts. For example, Myxobolus cerebralis is a parasite of trout, salmon and related fishes. Following infection, a host fish develops deadly whirling disease (Figure 46.14). Even when a parasite does not cause such dramatic symptoms, infection can weaken the host so it is more vulnerable to predation or less attractive to potential mates. Some parasitic infections cause sterility. Others shift the sex ratio of their host species. Parasites affect host numbers by altering birth and death rates. They also indirectly affect species that compete with their host. The decline in trout caused by whirling disease allows competing fish populations to increase.
a
b
Figure 46.14 (a) A young trout with a twisted spine and darkened tail caused by whirling disease, which damages cartilage and nerves. Jaw deformities and whirling movements are other symptoms. (b) Spores of Myxobolus cerebralis, the parasite that causes the disease. The disease now occurs in many lakes and streams in western and northeastern states.
Figure 46.15 Dodder (Cuscuta), also known as strangleweed or devil’s hair. This parasitic flowering plant has almost no chlorophyll. Leafless stems twine around a host plant during growth. Modified roots penetrate the host’s vascular tissues and absorb water and nutrients from them.
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PRINCIPLES OF ECOLOGY
Sometimes the gradual drain of nutrients during a parasitic infection indirectly leads to death. The host is so weak that it cannot fight off secondary infections. A rapid death is rare. Usually death happens only after a parasite attacks a novel host—one with no coevolved defenses—or after the body is overwhelmed by a huge population of parasites. In evolutionary terms, killing the host too quickly is bad for the parasite. Ideally, a host will live long enough to give the parasite time to produce plenty of offspring. The longer the host survives, the more offspring the parasite can produce. That is why we can predict that natural selection will favor parasites with less-than-fatal effects on hosts (Section 21.8). Unit Four describes many parasites. Some spend their entire life in or on a single host species. Others have different hosts during different stages of the life cycle. Insects and other arthropods can act as vectors: organisms that convey a parasite from host to host. Even a few plants are parasitic. Nonphotosynthetic species such as dodders obtain energy and nutrients from a host plant (Figure 46.15). Other species carry out photosynthesis but steal nutrients and water from their host. Most mistletoe are like this; their modified roots tap into the vascular tissues of host trees. Many tapeworms, flukes, and certain roundworms are parasitic invertebrates (Figure 46.16). So are ticks, many insects, and some crustaceans. Parasitoids are insects that lay eggs in other insects. Larvae hatch, develop in the host’s body, eat its tissue, and eventually kill it. The fire ant–killing phorid flies described in this chapter’s introduction do this. As many as 15 percent of all insects may be parasitoids. Social parasites are animals that take advantage of the behavior of a host to complete their life cycle. Cuckoos and North American cowbirds, as explained shortly, are social parasites.
Figure 46.16 Adult roundworms (Ascaris), an endoparasite, inside the small intestine of a host pig. Sections 25.6 and 25.11 show more examples of parasitic worms.
FOCUS ON EVOLUTION
46.7
Strangers in the Nest
The brown-headed cowbird’s genus name (Molothrus) means intruder in Latin. They intrude into other birds’ nests and lay their eggs there.
Figure 46.17 Biological control agent: a commercially raised parasitoid wasp about to deposit an egg in an aphid. After the egg it laid hatches, a wasp larva will devour the aphid from the inside.
Biological Control Agents Some parasites and parasitoids are now raised commercially for use as biological control agents. Use of such agents is promoted as an alternative to pesticides. For example, some parasitoid wasps attack aphids, which are widespread plant pests (Figure 46.17). Effective biological control agents are adapted to a specific host species and to its habitat. They are good at finding the hosts. Their population growth rate is high compared to the host’s. Their offspring are good at dispersing. Also, they make a type III response to changes in prey density (Section 46.4), without much lag time after the prey or host population size shifts. Biological control is not without risks of its own. Releasing multiple species of biological control agent in an area may allow competition among them, and lower their effectiveness against an intended target. Also, introduced parasites sometimes go after nontargeted species in addition to, or instead of, those species they were introduced to control. For example, parasitoids deliberately introduced to the Hawaiian Islands attacked the wrong target. They were brought in to control stinkbugs that are pests of Hawaii’s crops. Instead, the parasitoids decimated the population of koa bugs, Hawaii’s largest native bug. Introduced parasitoids also have been implicated in ongoing declines of many native Hawaiian butterfly and moth populations.
Brown-headed cowbirds (Molothrus ater) evolved in the Great Plains of North America and they were commensal with bison. Great herds of these hefty ungulates stirred up plenty of tasty insects as they migrated through the grasslands, and, being insect-eaters, cowbirds wandered around with them (Figure 46.18a). Cowbirds are social parasites that lay their eggs in the nests constructed by other birds, so young cowbirds are reared by foster parents. Many species became “hosts” to cowbirds; they did not have the capacity to recognize the differences between cowbird eggs and their own eggs. Concurrently, cowbird hatchlings became innately wired for hostile takeovers. They demand to be fed by unwitting, and often smaller, foster parents (Figure 46.18b). For thousands of years, cowbirds have perpetuated their genes at the expense of hosts. When American pioneers moved west, many cleared swaths of woodlands for pastures. Cowbirds now moved in the other direction. They adapted easily to a life with new ungulates—cattle—in the man-made grasslands; hence their name. They started to penetrate adjacent woodlands and exploit novel species. Today, brown-headed cowbirds parasitize at least fifteen kinds of native North American birds. Some of those birds are threatened or endangered. Besides being successful opportunists, cowbirds are big-time reproducers. A female can lay an egg a day for ten days, give her ovaries a rest, do the same again, and then again in one season. As many as thirty eggs in thirty nests—that is a lot of cowbirds.
Take-Home Message What are parasites, parasitoids, and social parasites? Parasitic species feed on another species but generally do not kill their host.
Parasitoids are insects that eat other insects from inside out. Social parasites manipulate the social behavior of another species to their own benefit.
a
b
Figure 46.18 (a) Brown-headed cowbirds (Molothrus ater) originally evolved as commensalists with bison of the North American Great Plains. (b) Cowbirds are social parasites. The large nestling at the left is a cowbird. The smaller foster parent is rearing the cowbird in place of its own offspring.
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46.8
Ecological Succession Which species are present in a community depends on physical factors such as climate, biotic factors such as which species arrived earlier, and the frequency of disturbances.
Links to Mosses 23.3, Lichens 24.6, Nitrogen-fixing bacteria 29.2
b
a
c
d
Successional Change Species composition of a community can change over time. Species often alter the habitat in ways that allow other species to come in and replace them. We call this type of change ecological succession. The process of succession starts with the arrival of pioneer species, which are opportunistic colonizers of new or newly vacated habitats. Pioneers species have high dispersal rates, grow and mature fast, and produce many offspring. Later, other species replace the pioneers. Then replacements are replaced, and so on. Primary succession is a process that begins when pioneer species colonize a barren habitat with no soil, such as a new volcanic island or land exposed by the retreat of a glacier (Figure 46.19). The earliest pioneers to colonize a new habitat are often mosses and lichens (Sections 23.3 and 24.6). They are small, have a brief life cycle, and can tolerate intense sunlight, extreme temperature changes, and little or no soil. Some hardy, annual flowering plants with wind-dispersed seeds are also among the pioneers. Pioneers help build and improve the soil. In doing so, they may set the stage for their own replacement. Many pioneer species partner with nitrogen-fixing bacteria, so they can grow in nitrogen-poor habitats. Seeds of later species find shelter inside mats of the pioneers. Organic wastes and remains accumulate and, by adding volume and nutrients to soil, this material helps other species take hold. Later successional species often shade and eventually displace earlier ones. In secondary succession, a disturbed area within a community recovers. If improved soil is still present, secondary succession can be fast. It commonly occurs in abandoned fields, burned forests, and tracts of land where plants were killed by volcanic eruptions.
Factors Affecting Succession
e
Figure 46.19 One observed pathway of primary succession in Alaska’s Glacier Bay region. (a) As a glacier retreats, meltwater leaches minerals from the rocks and gravel left behind. (b) Pioneer species include lichens, mosses, and some flowering plants such as mountain avens (Dryas), which associate with nitrogen-fixing bacteria. Within 20 years, alder, cottonwood, and willow seedlings take hold. Alders also have nitrogen-fixing symbionts. (c) Within 50 years, alders form dense, mature thickets in which cottonwood, hemlock, and a few evergreen spruce grow. (d) After 80 years, western hemlock and spruce crowd out alders. (e) In areas deglaciated for more than a century, tall Sitka spruce are the predominant species.
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PRINCIPLES OF ECOLOGY
When the concept of ecological succession was first developed in the late 1800s, it was thought to be a predictable and directional process. Physical factors such as climate, altitude, and soil type were considered to be the main determinants of which species appeared in what order during succession. Also by this view, succession culminates in a “climax community,” an array of species that will persist over time and will be reconstituted in the event of a disturbance. Ecologists now realize that the species composition of a community changes frequently, in unpredictable ways. Communities do not journey along a well-worn path to some predetermined climax state.
Figure 46.20 A natural laboratory for succession after the 1980 Mount Saint Helens eruption (a). The community at the base of this Cascade volcano was destroyed. (b) In less than a decade, pioneer species came in. (c) Twelve years later, seedlings of a dominant species, Douglas firs, took hold.
Species richness
Random events can determine the order in which species arrive in a habitat and thus affect the course of succession. Arrival of a certain species may make it easier or more difficult for others to take hold. As an example, surf grass can only grow along a shoreline if algae have already colonized that area. The algae act as an anchoring site for the grass. In contrast, when sagebrush gets established in a dry habitat, chemicals it secretes into the soil keep most other plants out. Ecologists had an opportunity to investigate these factors after the 1980 eruption of Mount Saint Helens leveled about 600 square kilometers (235 square miles) of forest in Washington State (Figure 46.20). Ecologists recorded the natural pattern of colonization. They also carried out experiments in plots inside the blast zone. They added seeds of certain pioneer species to some plots and left other plots seedless. The results showed that some pioneers helped other later arriving plants become established. Different pioneers kept the same late arrivals out. Disturbances also can influence the species composition in communities. According to the intermediate disturbance hypothesis, species richness is greatest in communities where disturbances are moderate in their intensity or frequency. In such habitats, there is enough time for new colonists to arrive and become established but not enough for competitive exclusion to cause extinctions:
Disturbance:
a
b
High
Low Major Frequent Soon after
Minor Infrequent Long after
c
In short, the modern view of succession holds that the species composition of a community is affected by (1) physical factors such as soil and climate, (2) chance events such as the order in which species arrive, and (3) the extent of disturbances in a habitat. Because the second and third factors may vary even between two geographically close regions, it is generally difficult to predict exactly what any given community will look like at any point in the future.
Take-Home Message What is succession? Succession, a process in which one array of species replaces another over time. It can occur in a barren habitat (primary succession), or a region in which a community previously existed (secondary succession).
Chance events make successional changes difficult to predict.
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46.9
Species Interactions and Community Instability The loss or addition of even one species may destabilize the number and abundances of species in a community.
Link to Sudden oak death 22.8
The Role of Keystone Species
Algal species diversity
As you read earlier, short-term physical disturbances can influence the species composition of a community. Long-term changes in climate or some other environmental variable also have an effect. In addition, a shift in species interactions can dramatically alter the community by favoring some species and harming others. The uneasy balance of forces in a community comes into focus when we observe the effects of a keystone species. A keystone species has a disproportionately large effect on a community relative to its abundance. Robert Paine was the first to describe the effect of a keystone species after his experiments on the rocky shores of California’s coast. Species living in the rocky
intertidal zone withstand pounding surf by clinging to rocks. A rock to cling to is a limiting factor. Paine set up control plots with the sea star Pisaster ochraceus and its main prey—chitons, limpets, barnacles, and mussels. In experimental plots he removed all sea stars. Mussels (Mytilus) happen to be the prey of choice for sea stars. In the absence of sea stars, they took over Paine’s experimental plots; they became the strongest competitors and crowded out seven other species of invertebrates. In this intertidal zone, predation by sea stars normally keeps the number of prey species high because it restricts competitive exclusion by mussels. Remove all the sea stars, and the community shrinks from fifteen species to eight. The impact of a keystone species can vary between habitats that differ in their species arrays. Periwinkles (Littorina littorea) are alga-eating snails that live in the intertidal zone. Jane Lubchenco found removing them can increase or decrease the diversity of algal species, depending on the habitat (Figure 46.21).
12 10 8 6 4 2 0
0 100 200 300 Periwinkles per square meter
a
Algal species diversity
d Algal diversity in tidepools 12 10 8 6 4 2 0
0 100 200 300 Periwinkles per square meter
b
c
Figure 46.21 Effect of competition and predation in an intertidal zone. (a) Grazing periwinkles (Littorina littorea) affect the number of algal species in different ways in different marine habitats. (b) Chondrus and (c) Enteromorpha, two kinds of algae in their natural habitats. (d) By grazing on the dominant alga in tidepools (Enteromorpha), the periwinkles promote the survival of less competitive algal species that would otherwise be overgrown. (e) Enteromorpha does not grow on rocks. Here, Chondrus is dominant. Periwinkles find Chondrus tough and dine instead on less competitive algal species. By doing so, periwinkles decrease the algal diversity on the rocks.
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PRINCIPLES OF ECOLOGY
e Algal diversity on rocks that become exposed at high tide
Table 46.2
Outcomes of Some Species Introductions Into the United States
Species Introduced Water hyacinth
Origin South America
Dutch elm disease: Ophiostoma ulmi (fungus) Asia (by way Bark beetle (vector) of Europe)
Mode of Introduction
Outcome
Intentionally introduced (1884)
Clogged waterways; other plants shaded out
Accidental; on infected elm timber (1930) Accidental; on unbarked elm timber (1909)
Millions of mature elms destroyed
Chestnut blight fungus
Asia
Accidental; on nursery plants (1900)
Nearly all eastern American chestnuts killed
Zebra mussel
Russia
Accidental; in ballast water of ship (1985)
Clogged pipes and water intake valves of power plants; displaced native bivalves in Great Lakes
Japanese beetle
Japan
Accidental; on irises or azaleas (1911)
Close to 300 plant species (e.g., citrus) defoliated
Sea lamprey
North Atlantic
Accidental; on ship hulls (1860s)
Trout, other fish species destroyed in Great Lakes
European starling
Europe
Intentional release, New York City (1890)
Outcompetes native cavity-nesting birds; crop damage; swine disease vector
Nutria
South America
Accidental release of captive animals being raised for fur (1930)
Crop damage, destruction of levees, overgrazing of marsh habitat
In tidepools, periwinkles prefer to eat a certain alga (Enteromorpha) which can outgrow other algal species. By keeping that alga in check, periwinkles help other, less competitive algal species survive. On rocks of the lower intertidal zone, Chondrus and other tough, red algae dominate. Here, periwinkles preferentially graze on competitively weaker algae. Periwinkles promote species richness in tidepools but reduce it on rocks. Not all keystone species are predators. For example, beavers can be a keystone species. These large rodents cut down trees by gnawing through their trunks. Some of the felled trees are used to build dams that create a pool where only a shallow stream would otherwise exist. Thus the presence of beavers affects which types of fish and aquatic invertebrates are present.
Species Introductions Can Tip the Balance Instabilities are also set in motion when residents of an established community move out from their home range, then successfully take up residence elsewhere. This type of directional movement, called geographic dispersal, happens in three ways. First, over a number of generations, a population might expand its home range by slowly moving into any outlying regions that prove hospitable. Second, a population might be moved away from a home range by continental drift, at an almost imperceptibly slow pace over long spans of time. Third, some individuals might be rapidly transported across great distances, an event called jump dispersal. Birds that travel long distances facilitate such jumps by carrying seeds of plants. For some time now, humans have been a major cause of jump dispersal. They have introduced species that benefit them, as by bringing crop plants from
the Americas to Europe. They have also unknowingly transported stowaways, as when Asian long-horned beetles were imported along with wood products. When you hear someone speaking enthusiastically about exotic species, you can safely bet the speaker is not an ecologist. An exotic species is a resident of an established community that dispersed from its home range and became established elsewhere. Unlike most imports, which never do take hold outside the home range, an exotic species permanently insinuates itself into a new community. In its new locale, the exotic species is often untroubled by competitors, predators, parasites, and diseases that kept it in check back home. Freed from its usual constraints, the exotic species can often outcompete similar species native to its new habitat. You have already learned how some imports are affecting community structure. The chapter introduction described how red imported fire ants that arrived from South America outcompete North American ant species. Sudden oak death, described in Section 22.8, is caused by a protist from Asia. A parasite from Europe is the cause of whirling disease in trout. The list of detrimental exotic species is depressingly long. Table 46.2 lists some well-known imports, and the next section describes four others in some detail.
Take-Home Message How can a single species affect community structure? A keystone species is one that has a major effect on species richness and relative abundances in a habitat. Removal of a keystone species or introductions of an exotic species can affect the types and abundances of species in a community.
CHAPTER 46
COMMUNITY STRUCTURE AND BIODIVERSITY 831
46.10 Exotic Invaders Nonnative species introduced by human activities are affecting native communities on every continent.
Link to Green algae 22.9
Battling Algae The long, green, feathery branches of Caulerpa taxifolia look great in saltwater aquariums, so researchers at the Stuttgart Aquarium in Germany developed a sterile strain of this green alga and shared it with other marine institutions. Was it from Monaco’s Oceanographic Museum that the hybrid strain escaped into the wild? Some say yes, Monaco says no. In any case, a small patch of the aquarium strain was found growing in the Mediterranean near Monaco in 1984. Boat propellers and fishing nets dispersed the alga, and it now blankets tens of thousands of acres of sea floor in the Mediterranean and Adriatic (Figure 46.22a). Just how bad is C. taxifolia? The aquarium strain can thrive on sandy or rocky shores and in mud. It can live ten days after being discarded in meadows. Unlike its tropical parents, it can also survive in cool water and polluted water. It has the potential to displace endemic algae, overgrow reefs, and destroy marine food webs. Its success is due in part to production of a toxin (Caulerpenyne) that poisons invertebrates and fishes, including algae eaters that keep other algae in check. In 2000, scuba divers discovered C. taxifolia growing near the southern California coast. Someone might have drained water from a home aquarium into a storm drain or into the lagoon itself. The government and private groups
quickly sprang into action. So far, eradication and surveillance programs have worked, but at a cost of more than $3.4 million. Importing C. taxifolia or any closely related species of Caulerpa into the United States is now illegal. To protect native aquatic communities, aquarium water should never be dumped into storm drains or waterways. It should be discarded into a sink or toilet so wastewater treatment can kill any algal spores (Section 22.9).
The Plants That Overran Georgia In 1876, kudzu (Pueraria montana) was introduced to the United States from Japan. In its native habitat, this perennial vine is a well-behaved legume with an extensive root system. It seemed like a good idea to use it for forage and to control erosion on slopes. But kudzu grew faster in the American Southeast. No native herbivores or pathogens were adapted to attack it. Competing plant species posed no serious threat to it. With nothing to stop it, kudzu can grow 60 meters (200 feet) per year. Its vines now blanket streambanks, trees, telephone poles, houses, and almost anything else in their path (Figure 46.22b). Kudzu withstands burning, and grows back from its deep roots. Grazing goats and herbicides help. But goats eat most other plants along with it, and herbicides taint freshwater supplies. Kudzu invasions now stretch from Connecticut down to Florida and are reported in Arkansas. It crossed the Mississippi River into Texas. Thanks to jump dispersal, it is now an invasive species in Oregon.
Figure 46.22 (a) Aquarium strain of Caulerpa taxifolia suffocating yet another richly diverse marine ecosystem.
b
a
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PRINCIPLES OF ECOLOGY
(b) Kudzu (Pueraria montana) taking over part of Lyman, South Carolina. This vine has become invasive in many states from coast to coast. Ruth Duncan of Alabama (above), who makes 200 kudzu vine baskets a year, can’t keep up.
FOCUS ON THE ENVIRONMENT
Figure 46.23 Rabbit-proof fence? Not quite. This photo shows part of a fence built in 1907 to hold back rabbits that were wreaking havoc with the vegetation in Australia. The fence did not solve the rabbit problem, but it did restrict movements of native wildlife such as kangaroos and emus.
On the bright side, Asians use a starch extracted from kudzu in drinks, herbal medicines, and candy. A kudzu processing plant in Alabama may export this starch to Asia, where the demand currently exceeds the supply. Also, kudzu may help save forests; it can be an alternative source for paper and other wood products. Today, about 90 percent of Asian wallpaper is kudzu-based.
The Rabbits That Ate Australia During the 1800s, British settlers in Australia just could not bond with koalas and kangaroos, and so they imported familiar animals from home. In 1859, in what would be the start of a major ecological disaster, a landowner in northern Australia imported and released two dozen European rabbits (Oryctolagus cuniculus). Good food and great sport hunting—that was the idea. An ideal rabbit habitat with no natural predators—that was the reality. Six years later, the landowner had killed 20,000 rabbits and was besieged by 20,000 more. The rabbits displaced livestock and caused the decline of native wildlife. Now 200 million to 300 million are hippity-hopping through the southern half of the country. They graze on grasses in good times and strip bark from shrubs and trees during droughts. Thumping hordes turn shrublands as well as grasslands into eroded deserts. Their burrows undermine the soil and set the stage for widespread erosion. Rabbits have been shot and their warrens fumigated, plowed under, and dynamited. The first assaults killed 70 percent of them, but the rabbits rebounded in less than a year. When a fence 2,000 miles long was built to protect western Australia, rabbits made it from one side to the other before workers could finish the job (Figure 46.23). In 1951, the government introduced a myxoma virus that normally infects South American rabbits. The virus causes myxomatosis. This disease has mild effects on its
coevolved host but nearly always kills O. cuniculus. Fleas and mosquitoes transmit the virus to new hosts. With no coevolved defenses against the import, European rabbits died in droves. But natural selection has since favored a rise in rabbit populations resistant to the imported virus. In 1991, on an uninhabited island in Australia’s Spencer Gulf, researchers released rabbits that were injected with a calicivirus. The rabbits died from blood clots in their lungs, heart, and kidneys. Then, in 1995, the test virus escaped from the island to the mainland, perhaps on insect vectors. The combination of the two imported viruses, along with traditional control methods has brought the rabbit population under control. There still are some rabbits, but vegetation is growing back and native herbivores are increasing in numbers.
Gray Squirrels Versus Red Squirrels The eastern gray squirrel (Sciurus carolinensis) is native to eastern North America, where it is a welcome sight in forests, yards, and parks. It has become similarly common throughout Britain and parts of Italy where it has been introduced. Here, the squirrel is considered an exotic pest that has thrived at the expense of Europe’s native red squirrel (Sciurus vulgaris). In Britain, the imported grays now outnumber the native reds 66 to 1. The gray squirrels are at an advantage over their European cousins because they excel at detecting and stealing nuts that red squirrels stored for the winter. In addition, gray squirrels carry and spread a virus that kills Britain’s red squirrels, but are not themselves affected by the virus. To protect the remaining red squirrels, the British have begun trapping and killing gray squirrels. Efforts are also under way to develop a contraceptive drug that would be effective against grays, but not the native reds.
CHAPTER 46
COMMUNITY STRUCTURE AND BIODIVERSITY 833
46.11 Biogeographic Patterns in Community Structure Mainland and Marine Patterns
The richness and relative abundances of species differ from one habitat or region of the world to another.
Link to Biogeography 17.1
Species richness
Biogeography is the scientific study of how species are distributed in the natural world (Section 17.1). We see patterns that correspond with differences in sunlight, temperature, rainfall, and other factors that vary with latitude, elevation, or water depth. Still other patterns relate to the history of a habitat and the species in it. Each species has its own unique physiology, capacity for dispersal, resource requirements, and interactions with other species.
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Island Patterns
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As you saw in Section 45.4, islands are laboratories for population studies. They have also been laboratories for community studies. For instance, in the mid-1960s volcanic eruptions formed a new island 33 kilome-
90°S
Number of vascular plant species
Figure 46.24 Two patterns of species diversity corresponding to latitude. The number of ant species (a) and breeding birds (b) in the Americas.
Perhaps the most striking pattern of species richness corresponds with distance from the equator. For most major plants and animal groups, the number of species is greatest in the tropics and declines from the equator to the poles. Figure 46.24 illustrates two examples of this pattern. Consider just a few factors that help bring about such a pattern and maintain it. First, for reasons explained in Section 48.1, tropical latitudes intercept more intense sunlight and receive more rainfall, and their growing season is longer. As one outcome, resource availability tends to be greater and more reliable in the tropics than elsewhere. One result is a degree of specialized interrelationships not possible where species are active for shorter periods. Second, tropical communities have been evolving for a long time. Some temperate communities did not start forming until the end of the last ice age. Third, species richness may be self-reinforcing. The number of species of trees in tropical forests is much greater than in comparable forests at higher latitudes. Where more plant species compete and coexist, more species of herbivores also coexist, partly because no single herbivore species can overcome all the chemical defenses of all plants. In addition, more predators and parasites can evolve in response to more kinds of prey and hosts. The same principles apply to tropical reefs.
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834 UNIT VII
PRINCIPLES OF ECOLOGY
Figure 46.25 Surtsey, a volcanic island, at the time of its formation (a) and in 1983 (b). The graph (c) shows the number of vascular plant species found in yearly surveys. Sea gulls began nesting on the island in 1986.
Distance effect: Species richness on islands of a given size declines as distance from a source of colonists rises. Green circles are values for islands less than 300 kilometers from the colonizing source. Orange triangles are values for islands more than 300 kilometers (190 miles) from a source of colonists. Area effect: Among islands the same distance from a source of colonists, larger islands tend to support more species than smaller ones. Figure It Out: Which is likely to have more species, a 100-km2 island more than 300 km from a colonizing source or a 500-km2
Species richness (number of species)
Figure 46.26 Island biodiversity patterns.
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islands less than 300 kilometers from source
500
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islands more than 300 kilometers from source
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island less than 300 km from a colonist source? Answer: The 500-km2 island
ters (21 miles) from the coast of Iceland. The island was named Surtsey (Figure 46.25). Bacteria and fungi were early colonists. The first vascular plant became established on the island in 1965. Mosses appeared two years later and thrived (Figure 46.25b). The first lichens were found five years after that. The rate of arrivals of new vascular plants picked up considerably after a seagull colony became established in 1986 (Figure 46.25c). This example illustrates the important role birds play in introducing species to islands. The number of species on Surtsey will not continue increasing forever. Can we estimate how many species there will be when the number levels off? The equilibrium model of island biogeography addresses this question. According to this model, the number of species living on any island reflects a balance between immigration rates for new species and extinction rates for established ones. The distance between an island and a mainland source of colonists affects immigration rates. An island’s size affects both immigration rates and extinction rates. Consider first the distance effect: Islands far from a source of colonists receive fewer immigrants than those closer to a source. Most species cannot disperse very far, so they will not turn up far from a mainland. Species richness also is shaped by the area effect: Big islands tend to support more species than small ones. More colonists will happen upon a larger island simply by virtue of its size. Also, big islands are more likely to offer a variety of habitats, such as high and low elevations. These options make it more likely that a new arrival will find a suitable habitat. Finally, big islands can support larger populations of species than small islands. The larger a population, the less likely it is to become locally extinct as the result of some random event.
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Figure 46.26 illustrates how interactions between the distance effect and the area effect can influence the number of species on islands. Robert H. MacArthur and Edward O. Wilson first developed the equilibrium model of island biogeography in the late 1960s. Since then it has been modified and its use has been expanded to help scientists think about habitat islands—natural settings surrounded by a “sea” of degraded habitat. Many parks and wildlife preserves fit this description. Island-based models can help estimate the size of an area that must be set aside as a protected reserve to ensure survival of a species. One more note about island communities: An island often differs from its source of colonists in physical aspects, such as rainfall and soil type. It also differs with regard to species array; not all species reach the island. As a result of these differences, a population on an island often faces different selection pressures than its same-species relatives on the mainland and evolves in a different way as a result. In a pattern that is the opposite of character displacement, a species may find itself on an island that lacks a major competitor found on the mainland. In the absence of this competition, traits of the island population may become more like those of the competitor that it left behind.
Take-Home Message What are some biogeographic patterns in species richness? Generally, species richness is highest in the tropics and lowest at the poles. Tropical habitats have conditions that more species can tolerate, and tropical communities have often been evolving for longer than temperate ones. When a new island forms, species richness rises over time and then levels off. The size of an island and its distance from a colonizing source influence its species richness.
CHAPTER 46
COMMUNITY STRUCTURE AND BIODIVERSITY 835
IMPACTS, ISSUES REVISITED
Fire Ants in the Pants
Increased global trade and faster ships are contributing to a rise in the rate of species introductions into North America. Faster ships mean shorter trips, which increases the likelihood that pests will survive a voyage. Wood-eating insects from Asia turn up with alarming frequency in the wood of packing crates and spools for steel wire. Some of these insects, such as the Asian long-horned beetle, now pose a serious threat to North America’s forests.
Summary Section 46.1 Each species occupies a certain habitat characterized by physical and chemical features and by the array of other species living in it. All populations of all species in a habitat are a community. Each species in a community has its own niche, or way of living. Species interactions between members of a community include commensalism, which does not help or harm either species, mutualism, which benefits both species, interspecific competition, which harms both species, and parasitism and predation, in which one species benefits at the expense of another. Commensalism, mutualism, and parasitism may be a symbiosis, in which species live together. Interacting species undergo coevolution. Section 46.2 In a mutualism, two species interact and both benefit. Some mutualists cannot complete their life cycle without the interaction. Section 46.3 By the process of competitive exclusion, one species outcompetes a rival with the same resource needs, driving it to extinction. Character displacement makes competing species less similar, which facilitates resource partitioning.
Use the animation on CengageNOW to learn about competitive interactions.
Sections 46.4, 46.5 Predators are free-living and usually kill their prey. Predator and prey numbers often fluctuate in cycles. Carrying capacity, predator behavior, and availability of other prey affect these cycles. Predators and their prey exert selection pressure on one another. Evolutionary results of such selection include warning coloration, camouflage, and mimicry.
Use the interaction on CengageNOW to learn about three alternative models for predator responses to prey density.
Sections 46.6, 46.7 Parasites live in or on a host and withdraw nutrients from its tissues. Hosts may or may not die as a result. An animal vector often carries the parasite between hosts. Parasitoids lay eggs on a host, then their larvae devour the host. Social parasites manipulate some aspect of a host’s behavior. Section 46.8 Ecological succession is the sequential replacement of one array of species by another over time. Primary succession happens in new habitats. Secondary 836 UNIT VII
PRINCIPLES OF ECOLOGY
How would you vote? Is inspecting more imported goods to detect potentially harmful exotic species worth the added cost? See CengageNOW for details, then vote online.
succession occurs in disturbed ones. The first species of a community are pioneer species. The pioneers may help, hinder, or have no effect on later colonists. The older idea that all communities eventually reach a predictable climax state has been replaced by models that emphasize the role of chance and disturbances. The intermediate disturbance hypothesis holds that disturbances of moderate intensity and frequency maximize species diversity. Sections 46.9, 46.10 Community structure reflects an uneasy balance of forces that operate over time. Major forces are competition and predation. Keystone species are especially important in maintaining the composition of a community. The removal of a keystone species or introduction of an exotic species—one that evolved in a different community—can alter community structure in ways that may be permanent. Section 46.11 Species richness, the number of species in a given area, varies with latitude, elevation, and other factors. Tropical regions tend to have more species than higher latitude regions. The equilibrium model of island biogeography helps ecologists estimate the number of species that will become established on an island. The area effect is the tendency of large islands to have more species than small islands. The distance effect is the tendency of islands near a source of colonists to have more species than distant islands.
Learn about the area effect and distance effect with the interaction on CengageNOW.
Self-Quiz
Answers in Appendix III
1. A habitat . a. has distinguishing physical and chemical features b. is where individuals of a species normally live c. is occupied by various species d. all of the above 2. A species’ niche includes its a. habitat requirements b. food requirements c. reproductive requirements d. all of the above
.
3. Which cannot be a symbiosis? a. mutualism c. commensalism b. parasitism d. interspecific competition
Data Analysis Exercise Ant-decapitating phorid flies are just one of the biological control agents used to battle imported fire ants. Researchers have also enlisted the help of Thelohania solenopsae, another natural enemy of the ants. This microsporidian is a parasite that infects ants and shrinks the ovaries of the colony’s eggproducing female (the queen). As a result, a colony dwindles in numbers and eventually dies out. Are these biological controls useful against imported fire ants? To find out, USDA scientists treated infested areas with either traditional pesticides or pesticides plus biological controls (both flies and the parasite). The scientists left some plots untreated as controls. Figure 46.27 shows the results.
Percent of initial ant numbers
140
1. How did population size in the control plots change during the first four months of the study?
4. How did the two types of treatment (pesticide alone versus pesticide plus biological controls) differ in their longer-term effects?
4. Lizards and songbirds that share a habitat and both eat flies are an example of competition. a. exploitative d. interspecific b. interference e. both a and d c. intraspecific 5. With character displacement, two competing species become . a. more alike c. symbionts b. less alike d. extinct 6. Predator and prey populations . a. always coexist at relatively stable levels b. may undergo cyclic or irregular changes in density c. cannot coexist indefinitely in the same habitat d. both b and c 7. Match the terms with the most suitable descriptions. predation a. one free-living species feeds mutualism on another and usually kills it commensalism b. two species interact and both parasitism benefit by the interaction interspecific c. two species interact and one competition benefits while the other is neither helped nor harmed d. one species feeds on another but usually does not kill it e. two species attempt to utilize the same resource 8. By a currently favored hypothesis, species richness of a community is greatest between physical disturbances of intensity or frequency. a. low c. high b. intermediate d. variable 9. True or false? Parasitoids usually live inside their host without killing it.
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2. How did population size in the two types of treated plots change during this same interval? 3. If this study had ended after the first year, would you conclude that biological controls had a major effect?
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Figure 46.27 Effects of two methods of controlling red imported fire ants. The graph shows the numbers of red imported fire ants over a 28-month period. Orange triangles represent untreated control plots. Green circles are plots treated with pesticides alone. Black squares are plots treated with pesticide and biological control agents (phorid flies and a microsporidian parasite).
10. Match the terms with the most suitable descriptions. geographic a. opportunistic colonizer of dispersal barren or disturbed habitat area effect b. greatly affects other species pioneer c. individuals leave home range, species become established elsewhere climax d. more species on large islands community than small ones at same distance keystone from the source of colonists species e. array of species at the end of exotic successional stages in a habitat species f. allows competitors to coexist resource g. often outcompete, displace native partitioning species of established community
Visit CengageNOW for additional questions.
Critical Thinking 1. With antibiotic resistance rising, researchers are looking for ways to reduce use of these drugs. Some cattle once fed antibiotic-laced food now get probiotic feed that can bolster populations of helpful bacteria in the animal’s gut. The idea is that if a large population of beneficial bacteria is in place, then harmful bacteria cannot become established or thrive. Which ecological principle is guiding this research? 2. Flightless birds that live on islands often have relatives on the mainland that can fly. The island species presumably evolved from fliers that, in the absences of predators, lost their ability to fly. Many flightless birds on islands are now declining because rats and other predators have been introduced to their previously isolated island. Despite the change in selective pressure, no flightless island bird has yet regained the ability to fly. Why is this unlikely to happen? CHAPTER 46
COMMUNITY STRUCTURE AND BIODIVERSITY 837
47
Ecosystems IMPACTS, ISSUES
Bye-Bye, Blue Bayou
Each Labor Day, the coastal Louisiana town of Morgan City
In 2005, the category 5 hurricane Katrina slammed into
celebrates the Louisiana Shrimp and Petroleum Festival.
the Gulf Coast. High winds and flooding ruined countless
The state is the nation’s top shrimp harvester and the third-
buildings, and more than 1,700 people died. Climate change
largest producer of petroleum, which is refined into gasoline
models suggest that if temperatures continue to rise, more
and other fossil fuels. But the petroleum industry’s success
hurricanes are likely to reach category 5 status.
may be contributing indirectly to the decline of the state’s
The models also indicate that warming seas will promote
fisheries. Why? The lower atmosphere is warming up, and
overgrowth of algae, which can kill fish. Warmer water can
fossil fuel burning is one of the causes (Section 7.9). As the
encourage growth of many types of pathogenic bacteria, so
climate heats up, the ocean’s surface waters get warmer and
more people are expected to become sick after swimming in
expand, glaciers melt, and sea level rises.
contaminated water, or eating shellfish harvested from it.
If current trends continue, some coastal lowlands will be
Inland, heat waves are becoming more intense as global
submerged. With more than 40 percent of the nation’s salt-
temperatures rise, and more people are dying of heat stroke.
water wetlands, Louisiana has the most to lose. This state’s
Fueled by rising temperatures and extended dry seasons,
coastal marshes, or bayous, are already in danger. Dams and
wildfires are becoming more frequent and more devastat-
levees keep back sediments that would normally be depos-
ing. Disease-spreading mosquitoes are now spreading into
ited in the marshes. Since the 1940s, Louisiana has lost an
regions that were too cold for them even a few years ago.
area of marshland the size of Rhode Island (Figure 47.1). Louisiana’s marshes are an ecological treasure. Millions of
This chapter is about the flow of energy and nutrients through ecosystems. It will give you the tools to do some of
migratory birds overwinter there. The marshes are also the
your own critical thinking about human impacts on Earth’s
source of more than $ 3.5 billion worth of fish, shrimp, and
environments. We have become major players in the global
shellfish. If the marshes disappear, so will the revenue.
flows of energy and nutrients even before we fully under-
Equally troubling is what will happen to low-lying towns
stand how ecosystems work. Decisions we make today
and cities along the coasts after the marshes are gone. Then,
about global climate change and other environmental issues
there will be nothing to buffer devastating storm surges that
are likely to shape Earth’s environments—and the quality of
threaten the coasts during hurricanes.
human life—far into the future.
See the video! Figure 47.1 Left, Fishing camp in Louisiana. It was built in a once-thriving marsh that has since given way to the open waters of Barataria Bay. Above, a marsh restoration project in Louisiana’s Sabine National Wildlife Refuge. In marshland that has become open water, sediments are barged in and marsh grasses are planted on them.
Links to Earlier Concepts
Key Concepts Organization of ecosystems
This chapter builds on your understanding of the laws of thermodynamics (Section 6.1). We discuss ecological roles of producers such as phytoplankton (22.7), and of decomposers (21.6 and 24.5).
You will be reminded of the importance of water to the world of life (2.5) and how transpiration works (29.3). We also revisit the effects of acid rain (2.6) and the role of water in leaching nutrients (29.1).
You will see how nitrogen fixation (21.6 and 29.2) plays an essential role in nutrient cycles and how excess nitrogen contributes to algal blooms (22.5). You will also learn more about carbon imbalances (7.9), and be reminded that carbon is stored in peat bogs (23.3) and the shells of protists such as foraminiferans (22.3). You will also hear again about attempts to control the protist-caused disease malaria (22.6).
Discussions of nutrient cycles will also draw on your knowledge of tectonic plates (17.9).
An ecosystem consists of a community and its physical environment. A one-way flow of energy and a cycling of raw materials among its interacting participants maintain it. It is an open system, with inputs and outputs of energy and nutrients. Section 47.1
Food webs Food chains are linear sequences of feeding relationships. Food chains cross-connect as food webs. Most of the energy that enters a food web returns to the environment, mainly as metabolic heat. Nutrients are recycled within the food web. Section 47.2
Energy and materials flow Ecosystems differ in how much energy their producers capture and how much is stored in each trophic level. Some toxins that enter an ecosystem can become increasingly concentrated as they pass from one trophic level to another. Sections 47.3, 47.4
Cycling of water and nutrients The availability of water, carbon, nitrogen, phosphorus, and other substances influences primary productivity. These substances move slowly in global cycles, from environmental reservoirs, into food webs, then back to reservoirs. Sections 47.5–47.10
How would you vote? Exhaust from motor vehicles contains greenhouse gases. The better mileage a vehicle gets, the fewer greenhouse gases it emits per mile. Should minimum fuel economy standards for cars and trucks be increased? See CengageNOW for details, then vote online.
839
47.1
The Nature of Ecosystems In an ecosystem, energy and nutrients from the environment flow among a community of species.
Links to Laws of thermodynamics 6.1, Leaching 29.1
Overview of the Participants Diverse natural systems abound on Earth’s surface. In climate, soil type, array of species, and other features, prairies differ from forests, which differ from tundra and deserts. Reefs differ from the open ocean, which differs from streams and lakes. Yet, despite all these differences, all systems are alike in many aspects of their structure and function. We define an ecosystem as an array of organisms and a physical environment, all interacting through a one-way flow of energy and a cycling of nutrients. It is an open system, because it requires ongoing inputs of energy and nutrients to endure (Figure 47.2). All ecosystems run on energy captured by primary producers. These autotrophs, or “self-feeders,” obtain energy from a nonliving source—generally sunlight— and use it to build organic compounds from carbon dioxide and water. Plants and phytoplankton are the main producers. Chapter 7 explains how they capture energy from the sun to assemble sugars from carbon dioxide and water, by the process of photosynthesis. Consumers are heterotrophs that get energy and carbon by feeding on tissues, wastes, and remains of producers and one another. We can describe consumers by their diets. Herbivores eat plants. Carnivores eat the flesh of animals.
Parasites live inside or on a living host and feed on its tissues. Omnivores devour both animal and plant materials. Detritivores, such as earthworms and crabs, dine on small particles of organic matter, or detritus. Decomposers feed on organic wastes and remains and break them down into inorganic building blocks. The main decomposers are bacteria and fungi. Energy flows one way—into an ecosystem, through its many living components, then back to the physical environment (Section 6.1). Light energy captured by producers is converted to bond energy in organic molecules, which is then released by metabolic reactions that give off heat. This is a one-way process because heat energy cannot be recycled; producers cannot convert heat into chemical bond energy. In contrast, many nutrients are cycled within an ecosystem. The cycle begins when producers take up hydrogen, oxygen, and carbon from inorganic sources, such as the air and water. They also take up dissolved nitrogen, phosphorus, and other minerals necessary for biosynthesis. Nutrients move from producers into the consumers who eat them. After an organism dies, decomposition returns nutrients to the environment, from which producers take them up again. Not all nutrients remain in an ecosystem; typically there are gains and losses. Mineral ions are added to an ecosystem when weathering processes break down rocks, and when winds blow in mineral-rich dust from elsewhere. Leaching and soil erosion remove minerals (Section 29.1). Gains and losses of each mineral tend to balance out over time in a healthy ecosystem.
Trophic Structure of Ecosystems energy input, mainly from sunlight
PRODUCERS plants and other self-feeding organisms
A Energy from the environment flows through producers, then consumers. All energy that entered this ecosystem eventually flows out of it, mainly as heat.
All organisms of an ecosystem take part in a hierarchy of feeding relationships called trophic levels (“troph” means nourishment). When one organism eats another, energy is transferred from the eaten to the eater. All organisms at the same trophic level in an ecosystem are the same number of transfers away from the energy input into that system.
nutrient cycling
CONSUMERS animals, most fungi, many protists, bacteria
840 UNIT VII
B Producers and consumers concentrate nutrients in their tissues. Some nutrients released by decomposition get cycled back to producers.
PRINCIPLES OF ECOLOGY
Figure 47.2 Animated Model for ecosystems on land, in which energy flow starts with autotrophs that capture energy from the sun. Energy flows one way, into and out of the ecosystem. Nutrients get cycled among producers and heterotrophs.
hawk
Fourth Trophic Level carnivore (third-level consumer)
sparrow
Third Trophic Level carnivore (second-level consumer)
Figure 47.3 Example of a food chain and corresponding trophic levels in tallgrass prairie, Kansas.
grasshopper
Second Trophic Level
A food chain is a sequence of steps by which some energy captured by primary producers is transferred to organisms at successively higher trophic levels. For example, big bluestem grass and other plants are the major primary producers in a tallgrass prairie (Figure 47.3). They are at this ecosystem’s first trophic level. In one food chain, energy flows from bluestem grass to grasshoppers, to sparrows, and finally to hawks. Grasshoppers are primary consumers; they are at the second trophic level. Sparrows that eat grasshoppers are second-level consumers and at the third trophic level. Hawks are third-level consumers, and they are at the fourth trophic level. At each trophic level, organisms interact with the same sets of predators, prey, or both. Omnivores feed at several levels, so we would partition them among different levels or assign them to a level of their own. Identifying one food chain is a simple way to start thinking about who eats whom in ecosystems. Bear in mind, many different species usually are competing for food in complex ways. Tallgrass prairie producers (mainly flowering plants) feed grazing mammals and herbivorous insects. But many more species interact in the tallgrass prairie and in most other ecosystems, particularly at lower trophic levels. A number of food chains cross-connect with one another—as food webs —and that is the topic of the next section.
herbivore (primary consumer)
big bluestem grass
First Trophic Level autotroph (primary producer)
Take-Home Message What is the trophic structure of an ecosystem? An ecosystem includes a community of organisms that interact with their physical environment by a one-way energy flow and a cycling of materials. Autotrophs tap into an environmental energy source and make their own organic compounds from inorganic raw materials. They are the ecosystem’s primary producers. Autotrophs are at the first trophic level of a food chain, a linear sequence of feeding relationships that proceeds through one or more levels of heterotrophs, or consumers.
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ECOSYSTEMS 841
47.2
The Nature of Food Webs All food webs consist of multiple interconnecting food chains. Ecologists who untangled the chains of many food webs discovered patterns of organization. The patterns reflect environmental constraints and the inefficiency of energy transfers from one trophic level to the next.
human (Inuk)
Interconnecting Food Chains A food web diagram illustrates trophic interactions among species in one particular ecosystem. Figure 47.4 shows a small sampling of the participants in an arctic food web. Nearly all food webs include two types of food chains. In a grazing food chain, the energy stored
arctic fox
arctic wolf
HIGHER TROPHIC LEVELS
A sampling of carnivores that feed on herbivores and one another gyrfalcon
snowy owl
ermine
SECOND TROPHIC LEVEL
mosquito
Major parts of the buffet of primary consumers (herbivores)
flea
Parasitic consumers feed at more than one trophic level.
vole
arctic hare
lemming
FIRST TROPHIC LEVEL
This is just part of the buffet of primary producers.
grasses, sedges
purple saxifrage
arctic willow
Figure 47.4 Animated A very small sampling of organisms in an arctic food web on land.
842 UNIT VII
PRINCIPLES OF ECOLOGY
Detritivores and Decomposers (nematodes, annelids, saprobic insects, protists, fungi, bacteria)
Figure 47.5 Computer model for a food web in East River Valley, Colorado. Balls signify species. Their colors identify trophic levels, with producers (coded red) at the bottom and predators (yellow) at top. The connecting lines thicken, starting from an eaten species to the eater.
in producer tissues flows to herbivores, which tend to be relatively large animals. In a detrital food chain, the energy in producers flows to detritivores, which tend to be smaller animals, and to decomposers. In most land ecosystems, the bulk of the energy that becomes stored in producer tissues moves through detrital food chains. For example, in an arctic ecosystem, grazers such as voles, lemmings, and hares graze on some plant parts. However, far more plant matter becomes detritus. Bits of dead plant material sustain detritivores such as nematodes and soil-dwelling insects, and decomposers such as soil bacteria and fungi. Grazing food chains tend to predominate in aquatic ecosystems. Zooplankton (heterotrophic protists and tiny animals that drift or swim) consume most of the phytoplankton. A smaller amount of phytoplankton ends up on the ocean floor as detritus. Detrital food chains and grazing food chains interconnect to form the overall food web. For example, animals at higher trophic levels often eat both grazers and detritivores. Also, after grazers die, the energy in their tissues flows to detritivores and decomposers.
How Many Transfers? When ecologists looked at food webs for a variety of ecosystems, they discovered some common patterns. For example, the energy captured by producers usually passes through no more than four or five trophic levels. Even in ecosystems with many species, the number of transfers is limited. Remember that energy transfers are not that efficient (Section 6.1). Energy losses limit the length of a food chain.
Field studies and computer simulations of aquatic and land food ecosystems reveal more patterns. Food chains tend to be shortest in habitats where conditions vary widely over time. Chains tend to be longer in stable habitats, such as the ocean depths. The most complex webs tend to have a large variety of herbivores, as in grasslands. By comparison, the food webs with fewer connections tend to have more carnivores. Diagrams of food webs help ecologists predict how ecosystems will respond to change. Neo Martinez and his colleagues constructed the one shown in Figure 47.5. By comparing different food webs, they realized that trophic interactions connect species more closely than people thought. On average, each species in any food web was two links away from all other species. Ninety-five percent of species were within three links of one another, even in large communities with many species. As Martinez concluded in a paper discussing his findings, “Everything is linked to everything else.” He cautioned that extinction of any species in a food web may have an impact on many other species.
Take-Home Message How does energy flow affect food chains and food webs? Tissues of living plants and other producers are the basis for grazing food chains. Remains of producers are the basis for detrital food webs.
Nearly all ecosystems include both grazing food chains and detrital food chains that interconnect as the system’s food web. The cumulative energy losses from energy transfers between trophic levels limits the length of food chains.
Even when an ecosystem has many species, trophic interactions link each species with many others.
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47.3
Energy Flow Through Ecosystems Primary producers capture energy and take up nutrients, which then move to other trophic levels.
top carnivores (gar and bass)
1.5
Link to Phytoplankton 22.7
Capturing and Storing Energy
11
carnivores (smaller fishes, invertebrates)
37
herbivores (plant-eating fishes, invertebrates, turtles)
5
The flow of energy through an ecosystem begins with primary production: the rate at which producers (most often plants or photosynthetic protists) capture and store energy. The amount of energy captured by all producers in the ecosystem is defined as the system’s gross primary production. The portion of energy that producers invest in growth and reproduction (rather than in maintenance) is net primary production.
809
producers (algae and aquatic plants)
detritivores (crayfish) and decomposers (bacteria)
Figure 47.7 Biomass (in grams per square meter) for Silver Springs, a freshwater aquatic ecosystem in Florida. In this system, primary producers make up the bulk of the biomass.
Factors such as temperature and the availability of water and nutrients affect producer growth, and thus influence primary production. As a result, the primary production varies among habitats and may also vary seasonally (Figure 47.6). Per unit area, the net primary production on land tends to be higher than that in the oceans. However, because oceans cover about 70 percent of Earth’s surface, they contribute nearly half of the global net primary productivity.
Ecological Pyramids a
North America
b
Atlantic Ocean in Winter
Africa
North America
c
Atlantic Ocean in Spring
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Africa
Ecologists often represent the trophic structure of an ecosystem in the form of ecological pyramids. In such diagrams, primary producers collectively form a base for successive tiers of consumers above them. A biomass pyramid illustrates the dry weight of all organisms at each trophic level in an ecosystem. Figure 47.7 shows the biomass pyramid for Silver Springs, an aquatic ecosystem in Florida. Most commonly, primary producers make up most of the biomass in a pyramid, and top carnivores make up very little. If you visited Silver Springs, you would see a lot of aquatic plants but very few gars (the main top predator in this ecosystem). Similarly, when you walk through a prairie, you would see more grams of grass than of hawks. However, if producers are small and reproduce rapidly, a biomass pyramid can have its smallest tier at the bottom. For example, producers in the open ocean are
Figure 47.6 Primary productivity. (a) Summary of satellite data on net primary production during 2002. Productivity is coded as red (highest) down through orange, yellow, green, blue, and purple (lowest). (b,c) Satellite data showing seasonal shifts in net primary productivity for the North Atlantic Ocean.
single-celled protists that devote most energy that they harness to rapid reproduction, rather than to building a big body. They get eaten as fast as they reproduce, so a smaller biomass of phytoplankton can support a greater biomass of zooplankton and bottom feeders. An energy pyramid illustrates how the amount of usable energy diminishes as it is transferred through an ecosystem. Sunlight energy is captured at the base (the primary producers) and declines with successive levels to its tip (the top carnivores). Energy pyramids are always “right-side-up,” with their largest tier at the bottom. Such pyramids depict energy flow per unit of water (or land) per unit of time. Figure 47.8 shows the energy pyramid for the Silver Springs ecosystem and the energy flow that this pyramid represents.
top carnivores
Take-Home Message
producers
20,810
A Energy pyramid for the Silver Springs ecosystem. The size of each step in the pyramid represents the amount of energy that enters that trophic level annually, as shown in detail below.
Energy Input 1,700,000 kcal per square meter per year
B Every year 1,700,000 kcal of solar energy fall on each square meter of the Silver Springs ecosystem.
C 98.8 percent of this incoming energy is not captured by producers. 1,679,190 (98.8% )
Energy flow through living components
20,810 (1.2% )
producers Energy in wastes, remains D Producers harness 20,810 kcal of energy, but transfer only 3,368 kcal to herbivores. The rest is lost as heat or ends up in wastes and remains.
Energy flow to the next trophic level
4,245
3,368
Energy lost as heat or to flow downstream
13,197
herbivores
720
383
2,265
carnivores
90
21
272
top carnivores E With each subsequent transfer, only a small fraction of the energy reaches the next trophic level.
5
16 detritivores and decomposers
5,060 ⎫ ⎪ ⎬ ⎪ ⎭
Energy output
20,810 +1,679,190 ⎫ ⎪ ⎪ ⎪ ⎪ ⎪ ⎬ ⎪ ⎪ ⎪ ⎪ ⎪ ⎭
How does energy flow through ecosystems? Primary producers capture energy and convert it into biomass. We measure this process as primary production. A biomass pyramid depicts dry weight of organisms at each trophic level in an ecosystem. Its largest tier is usually producers, but the pyramid for some aquatic systems is inverted.
383 3,368
Ecological Efficiency Anywhere between 5 and 30 percent of the energy in the tissues of organisms at one trophic level ends up in the tissues of those at the next trophic level. Several factors influence the efficiency of transfers. First, not all energy harvested by consumers is used to build biomass. Some is lost as metabolic heat. Second, not all biomass can be digested by most consumers. Few herbivores have the ability to break down the lignin and cellulose that reinforce bodies of most land plants. Similarly, many animals have some biomass tied up in an internal or external skeleton. Hair, feathers, and fur are also part of the biomass that is difficult to digest. The ecological efficiency of energy transfers is usually higher in aquatic ecosystems than on land. Algae lack lignin, and so are more easily digested than land plants. Also, aquatic ecosystems usually have a higher proportion of ectotherms (cold-blooded animals), such as fish, than land ecosystems do. Ectotherms lose less energy as heat than endotherms (warm-blooded animals) so more is transferred to the next level. Higher efficiencies of transfers allow for longer food chains.
detritivores +decomposers =5,060
21
carnivores herbivores
Total annual energy flow
1,700,000 (100% )
Figure 47.8 Animated Annual energy flow in Silver Springs measured in kilocalories (kcal) per square meter per year. Figure It Out: What percent of the energy carnivores received from herbivores was later passed on to top carnivores? Answer: 21/383 100 = 5.5 percent
An energy pyramid depicts the amount of energy that enters each level. Its largest tier is always at the bottom (producers). Efficiency of transfers tends to be greatest in aquatic systems, where primary producers usually lack lignin and consumers tend to be ectotherms.
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FOCUS ON THE ENVIRONMENT
47.4
Biological Magnification Some harmful substances become more and more concentrated as they pass from one trophic level to the next.
Link to Malaria 22.6
DDT and Silent Spring The synthetic pesticide dichlorodiphenyl-trichloroethane, or DDT, was invented in the late 1800s and came into widespread use in the 1940s. Spraying DDT saved many human lives by killing lice that spread typhus, and mosquitoes that carried malaria. Farmers also embraced this new chemical that increased crop yields by killing common agricultural pests. In the 1950s, swelling numbers of suburbanites turned to DDT to keep their shrubbery free of leaf-munching insects.
DDT Residues (In parts per million wet weight of the whole organism) Ring-billed gull fledgling (Larus delawarensis) 75.5 18.5 Herring gull (Larus argentatus) 13.8 Osprey (Pandion haliaetus) 3.57 Green heron (Butorides virescens) 2.07 Atlantic needlefish (Strongylura marina) 1.28 Summer flounder (Paralichthys dentatus) Sheepshead minnow (Cyprinodon variegatus) 0.94 0.47 Hard clam (Mercenaria mercenaria) 0.33 Marsh grass shoots (Spartina patens) 0.30 Flying insects (mostly flies) 0.26 Mud snail (Nassarius obsoletus) 0.16 Shrimps (composite of several samples) 0.083 Green alga (Cladophora gracilis) 0.040 Plankton (mostly zooplankton) 0.00005 Water
Figure 47.9 Biological magnification in an estuary on Long Island, New York, as reported in 1967 by George Woodwell, Charles Wurster, and Peter Isaacson. Effects of DDT vary among species. Ospreys such as the one in the upper photo are highly sensitive. At 4 ppm of DDT, osprey eggs are fragile and unlikely to hatch. Gulls tolerate far higher doses of DDT without eggshell effects.
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PRINCIPLES OF ECOLOGY
Unfortunately, DDT also affected nonpest species. Where DDT was sprayed to control Dutch elm disease, songbirds died. In forests sprayed to kill budworm larvae, DDT got into streams and killed fishes. Rachel Carson, who had worked for the U.S. Fish and Wildlife Service, began compiling information about the harmful effects of pesticide use. She published her findings in 1962 as the book Silent Spring. The public embraced Carson’s ideas but the pesticide industry mounted a campaign to discredit her. At the time, Carson was battling terminal breast cancer. Yet she vigorously defended her position until her death in 1964. After Carson’s death, study of DDT’s impact increased. Researchers showed that DDT, like some other synthetic chemicals, undergoes biological magnification. By this process, a chemical that degrades slowly or not at all becomes increasingly concentrated in tissues of organisms as it moves up a food chain (Figure 47.9). In birds that are top carnivores such as ospreys, brown pelicans, bald eagles, and peregrine falcons, high DDT levels made eggs fragile, causing population sizes to plummet. In recognition of the ecological effects of DDT, the United States has banned its use and export. Predatory bird populations in this country have largely recovered. Some countries still use DDT to fight malaria-carrying mosquitoes, but application is limited to indoor spraying. Even this use is controversial; some people would like to see a worldwide ban on the chemical. In additional to the environmental concerns, they cite studies indicating that maternal exposure to DDT during pregnancy may cause premature births and affects a child’s mental development.
The Mercury Menace Birds bore the brunt of DDT’s effects but fish get the spotlight when it comes to mercury pollution. Coal-burning power plants and some industrial processes put mercury into the air, then rain washes it into aquatic habitats. In some regions, runoff from abandoned or operating mines also contributes to aquatic mercury. Like DDT, mercury accumulates as it moves up through food chains. Mercury adversely affects development of the human nervous system, so children and women who are pregnant or nursing should not eat fish that are top carnivores. Shark, swordfish, king mackerel, and tilefish are riskiest. You should also avoid these high-mercury fish if you are planning on becoming pregnant in the near future. Once mercury settles into your tissues, it can take a year for your body to get rid of it. Everyone should avoid making fish that can have a high mercury content a major part of their diet. You can receive the health benefits of eating fish by choosing other species that are lower in mercury. For example, catfish, salmon, sardines, pollack, and canned light tuna are good choices. If you fish and plan to eat what you catch, check for local advisories about contaminants. The EPA website www.epa.gov/waterscience/fish/states.htm can link you to the appropriate agency.
47.5
Biogeochemical Cycles
Nutrients move from nonliving environmental reservoirs into living organisms, then back into those reservoirs.
Links to Tectonic plates 17.9, Nitrogen fixation 21.6
In a biogeochemical cycle, an essential element moves from one or more nonliving environmental reservoirs, through living organisms, then back to the reservoirs (Figure 47.10). As explained in the Chapter 2 introduction, oxygen, hydrogen, carbon, nitrogen, and phosphorus are some of the elements essential to all forms of life. We refer to these and other required elements as nutrients. Depending on the element, environmental reservoirs may include Earth’s rocks and sediments, waters, and atmosphere. Chemical and geologic processes move elements to and from these reservoirs. For example, elements that had been locked in rocks become part of the atmosphere as a result of volcanic activity. Uplifting elevates rocks where they are exposed to erosive forces of wind and rain. The rocks slowly dissolve; elements in them enter rivers, and eventually seas. Elements enter the living part of an ecosystem by way of primary producers. Photosynthetic organisms take up essential ions dissolved in water. Land plants also take up carbon dioxide from the air. Some bacteria fix nitrogen gas (Section 21.6). Their action makes this nutrient available to producers.
Nutrients move through food webs when organisms eat one another. Fungi and prokaryotes speed nutrient cycling within an ecosystem by decomposing remains and wastes of other organisms, so elements that were tied up in those materials are once again available to primary producers. The next sections describe the four biogeochemical cycles that affect the most abundant elements in living organisms. In the water cycle, oxygen and hydrogen move on a global scale as part of molecules of water. In atmospheric cycles, a gaseous form of a nutrient such as carbon or nitrogen moves through ecosystems. A nutrient that does not often occur as a gas, such as phosphorus, moves in sedimentary cycles. Such nutrients accumulate on the ocean floor, then return to land by slow movements of Earth’s crust (Section 17.9).
Take-Home Message What are biogeochemical cycles? Biogeochemical cycles describe the continual flow of nutrients between nonliving environmental reservoirs and living organisms. Prokaryotes play a pivotal role in transfers between the living and nonliving portions of the cycle. Elements that occur in gases move through atmospheric cycles. Elements that do not normally occur as a gas move in sedimentary cycles.
Atmosphere
Rocks and sediments
Living organisms
Seawater and fresh water
Nonliving environmental reservoirs
Figure 47.10 Generalized biogeochemical cycle. In such cycles, a nutrient moves among nonliving environmental reservoirs and into and out of the living portion of an ecosystem. For all nutrients, the portion tied up in environmental reservoirs far exceeds the amount in living organisms.
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ECOSYSTEMS 847
47.6
The Water Cycle All organisms are mostly water and the cycling of this essential resource has implications for all life.
Table 47.1
Links to Properties of water 2.5, Leaching and erosion 29.1, Transpiration 29.3
How and Where Water Moves The world ocean holds most of Earth’s water (Table 47.1). As Figure 47.11 shows, in the water cycle, water moves among the atmosphere, the oceans, and environmental reservoirs on land. Sunlight energy drives evaporation, the conversion of water from liquid form to a vapor. Transpiration, explained in Section 29.3, is evaporation of water from plant parts. In cool upper layers of the atmosphere, condensation of water vapor into droplets gives rise to clouds. Later, clouds release the water as precipitation—as rain, snow, or hail. A watershed is an area from which all precipitation drains into a specific waterway. It may be as small as a valley that feeds a stream, or as large as the Mississippi River Basin, which covers about 41 percent of the continental United States. Most precipitation falling in a watershed seeps into the ground. Some collects in aquifers, permeable rock layers that hold water. Groundwater is water in soil and aquifers. When soil gets saturated, water becomes runoff; it flows over the ground into streams.
Environmental Water Reservoirs
Main Reservoirs
Volume (103 cubic kilometers)
Ocean Polar ice, glaciers Groundwater Lakes, rivers Soil moisture Atmosphere (water vapor)
1,370,000 29,000 4,000 230 67 14
Flowing water moves dissolved nutrients into and out of a watershed. Experiments in New Hampshire’s Hubbard Brook watershed illustrated that vegetation helps slow nutrient losses. Experimental deforestation caused a spike in loss of mineral ions (Figure 47.12).
A Global Water Crisis Our planet has plenty of water, but most of it is too salty to drink or use for irrigation. If all Earth’s water filled a bathtub, the amount of fresh water that could be used sustainably in a year would fill a teaspoon. Of the fresh water we use, about two-thirds goes to agriculture, but irrigation can harm soil. Piped-in water
atmosphere
wind-driven water vapor 40,000
evaporation from ocean 425,000
precipitation into ocean 385,000
precipitation onto land 111,000
evaporation from land plants (transpiration) 71,000
surface and groundwater flow 40,000
ocean
Figure 47.11 Animated The water cycle. Arrows identify processes that move water. The numbers shown indicate the amounts moved, as measured in cubic kilometers per year.
848 UNIT VII
PRINCIPLES OF ECOLOGY
land
losses from disturbed watershed plot
Concentration (mg/liter)
11
a
b
c
9 7 5
time of deforestation
3 1 0
Jan 1966
Jan 1967
Jan 1968
Figure 47.12 Hubbard Brook experimental watershed. (a) Runoff in this watershed is collected by concrete basins for easy monitoring. (b) This plot of land was stripped of all vegetation as an experiment. (c) After experimental deforestation, calcium levels in runoff increased sixfold (medium blue). A control plot in the same watershed showed no similar increase during this time (light blue).
often has high concentrations of salts. Salinization, the buildup of mineral salts in soil, stunts crop plants and decreases yields. Groundwater supplies drinking water to about half of the United States population. Pollution of this water now poses a threat. Chemicals leaching from landfills, hazardous waste facilities, and underground storage tanks often contaminate it. Unlike flowing rivers and streams, which can recover fast, polluted groundwater is difficult and expensive to clean up. Water overdrafts are also common; water is drawn from aquifers faster than natural processes replenish it. When too much fresh water is withdrawn from an aquifer near the coast, salt water moves in and replaces it. Figure 47.13 highlights regions of aquifer depletion and saltwater intrusion in the United States. Overdrafts have now depleted half of the Ogallala aquifer, which extends from South Dakota into Texas. This aquifer supplies the irrigation water for about 20 percent of the nation’s crops. For the past thirty years, withdrawals have exceeded replenishment by a factor of ten. What will happen when water runs out? Contaminants such as sewage, animal wastes, and agricultural chemicals make water in rivers and lakes unfit to drink. In addition, pollutants disrupt aquatic ecosystems, and in some cases they drive vulnerable species to local extinction. Desalinization, the removal of salt from seawater, may help increase freshwater supplies. However, the process requires a lot of fossil fuel. Desalinization is feasible mainly in Saudi Arabia and other places that have small populations and very large fuel reserves. In addition, the process produces mountains of waste salts that must be disposed of.
Hawaiian Islands
Alaska
Groundwater overdrafts: High Moderate
Significant groundwater contamination
Insignificant
Saltwater intrusion from nearby seas
Figure 47.13 Groundwater problems in the United States.
Take-Home Message What is the water cycle and how do humans affect it? In the water cycle, water moves on a global scale. It moves slowly from the world ocean—the main reservoir—through the atmosphere, onto land, then back to the ocean. Of the fresh water that human populations use, about two-thirds sustains agriculture. Aquifers that supply much of the world’s drinking water are becoming polluted and depleted.
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ECOSYSTEMS 849
47.7
Carbon Cycle Most of the annual carbon movement takes place between the ocean and atmosphere. The ocean holds 38,000–40,000 gigatons of dissolved carbon, primarily in the form of bicarbonate and carbonate ions. The air holds about 766 gigatons of carbon, mainly combined with oxygen in the form of carbon dioxide (CO2). On land, detritus in soil holds 1,500–1,600 gigatons of carbon. Peat bogs and the permafrost, a perpetually frozen layer of soil that underlies arctic regions, are major reservoirs. Another 540–610 gigatons is present in biomass, or tissues of organisms. Ocean currents move carbon from upper ocean waters into deep sea reservoirs. Carbon dioxide enters warm surface waters and is converted to bicarbonate. Then, prevailing winds and regional differences in density drive the flow of bicarbonate-rich seawater in a gigantic loop from the surface of the Pacific and Atlantic oceans down to the Atlantic and Antarctic sea floors. Here, bicarbonate moves into cold, deep storage
Carbon dioxide in air makes the carbon cycle an atmospheric cycle, but most carbon is in sediments and rocks. Links to Carbon fixation 7.6, Foraminiferans 22.3, Peat bogs 23.3
In the carbon cycle, carbon moves through the lower atmosphere and all food webs on its way to and from its largest reservoirs (Figure 47.14). Earth’s crust holds the most carbon—66 million to 100 million gigatons. A gigaton is a billion tons. There are 4,000 gigatons of carbon in the known fossil fuel reserves. Organisms contribute to Earth’s carbon deposits. Single-celled protists such as foraminiferans (Section 22.3) produce shells rich in calcium carbonate. Over hundreds of millions of years, uncountable numbers of these cells died, sank, and were buried in seafloor sediments. The carbon in their remains cycles slowly, as movements of Earth’s crust uplift portions of the sea floor, making it part of a land ecosystem.
Figure 47.14 Animated Right, carbon cycling in (a) marine ecosystems and (b) land ecosystems. Gold boxes highlight the most important carbon reservoirs. The vast majority of carbon atoms are in sediments and rocks, followed by lesser amounts in seawater, soil, the atmosphere, and biomass (in that order). Typical annual fluxes in global distribution of carbon, in gigatons, are:
diffusion between atmosphere and ocean
From atmosphere to plants by carbon fixation 120 From atmosphere to ocean 107 To atmosphere from ocean 105 To atmosphere from plants 60 To atmosphere from soil 60 To atmosphere from fossil fuel burning 5 To atmosphere from net destruction of plants 2 To ocean from runoff 0.4 Burial in ocean sediments 0.1
bicarbonate and carbonate dissolved in ocean water
photosynthesis
combustion of fossil fuels
aerobic respiration
marine food webs producers, consumers, decomposers, detritivores
h y, s alt
incorporation into sediments
r r ent a llow cu
ss es w a r m, l t rren c o l d , s a l t y, d e e p c u
Figure 47.15 Loop that moves carbon dioxide to carbon’s deep ocean reservoir. The loop sinks in the cold, salty North Atlantic. It rises in the warmer Pacific.
850 UNIT VII
PRINCIPLES OF ECOLOGY
death, sedimentation
uplifting over geologic time sedimentation
marine sediments, including formations with fossil fuels
A
reservoirs before water loops back up (Figure 47.15). Storage of carbon in the deep sea helps dampen any short-term effects of increases in atmospheric carbon. Biologists sometimes refer to the global cycling of carbon in the form of carbon dioxide and bicarbonate as a carbon–oxygen cycle. Plants, phytoplankton, and some bacteria fix carbon when they engage in photosynthesis (Section 7.6). Each year, they tie up billions of metric tons of carbon in sugars and other organic compounds. Breakdown of those compounds by aerobic respiration releases carbon dioxide into the air. More carbon dioxide escapes into the air when fossil fuels or forests burn and when volcanoes erupt. The time that an ecosystem holds a given carbon atom varies. Organic material decomposes rapidly in tropical forests, so carbon does not build up at the soil surface. By contrast, bogs and other anaerobic habitats do not favor decomposition, so material accumulates, as in peat bogs (Section 23.3).
Humans are altering the carbon cycle. Each year, we withdraw 4 to 5 gigatons of fossil fuel from environmental reservoirs. Our activities put about 6 gigatons more carbon in the air than can be moved into ocean reservoirs by natural processes. Only about 2 percent of the excess carbon entering the atmosphere becomes dissolved in ocean water. Carbon dioxide in the air traps heat, so increased outputs of it may be a factor in global climate change. The next section looks at this possibility and some environmental implications.
Take-Home Message What is the carbon cycle? In the carbon–oxygen cycle, carbon moves into and out of ecosystems mainly combined with oxygen, as in carbon dioxide, bicarbonate, and carbonate. Earth’s crust is the largest carbon reservoir, followed by the world ocean. Most of the annual cycling of carbon occurs between the ocean and atmosphere.
atmosphere (mainly carbon dioxide)
combustion of fossil fuels
volcanic action
terrestrial rocks
weathering
photosynthesis
combustion of wood (for clearing land; or for fuel)
aerobic respiration
deforestation
land food webs producers, consumers, decomposers, detritivores
soil water (dissolved carbon) death, burial, compaction over geologic time
peat, fossil fuels
leaching, runoff
B
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ECOSYSTEMS 851
47.8
Greenhouse Gases and Climate Change Concentrations of gases in Earth’s atmosphere help determine the temperature near Earth’s surface. Human activities are altering gas concentrations and causing climate change.
Link to Carbon imbalances 7.9
Concentrations of various gaseous molecules profoundly influence the average temperature of the atmosphere near Earth’s surface. That temperature, in turn, has far-reaching effects on global and regional climates. Atmospheric molecules of carbon dioxide, water, nitrous oxide, methane, and chlorofluorocarbons (CFCs) are among the main players in interactions that can shift global temperatures. Collectively, the gases trap heat a bit like a greenhouse does, hence the familiar name “greenhouse gases.” Radiant energy from the sun passes through the atmosphere and is absorbed by Earth’s surface. The energy warms the surface, which means that the surface emits infrared radiation (heat). The infrared energy radiates back toward space, but greenhouse gases in the atmosphere interfere with its progress. How? The gases absorb some of the infrared energy, and then emit a portion of it back toward Earth’s surface (Figure 47.16). Without this process, which is called the greenhouse effect, Earth’s surface would be so cold that very little life would survive. In the 1950s, researchers at a laboratory on Hawaii’s highest volcano began to measure the atmospheric concentrations of greenhouse gases. That remote site is almost free of local airborne contamination. It also is representative of atmospheric conditions for the Northern Hemisphere. What did they find? Briefly, concentrations of CO2 follow annual cycles of primary production. They decline in summer, when the rates of photosynthesis are highest. They rise in winter, when photosynthesis declines but aerobic respiration and fermentation continue.
A Radiant energy from the sun penetrates the lower atmosphere, and it warms Earth’s surface.
The alternating troughs and peaks along the graph line in Figure 47.17a are annual lows and highs of global CO2 concentrations. For the first time, researchers saw the effects of carbon dioxide fluctuations for the entire hemisphere. Notice the midline of the troughs and peaks in the cycle. It shows that carbon dioxide concentration is steadily increasing—as are concentrations of other major greenhouse gases. Atmospheric levels of greenhouse gases are far higher than they were for most of the past. Carbon dioxide may
B The warmed surface radiates heat (infrared radiation) back toward space. Greenhouse gases absorb some of the infrared energy, and then emit a portion of it back toward Earth.
Figure 47.16 Animated The greenhouse effect.
852 UNIT VII
Figure 47.17 Facing page, graphs of recent increases in four categories of atmospheric greenhouse gases. A key factor is the sheer number of gasoline-burning vehicles in large cities. Above, Mexico City on a smoggy morning. With 10 million residents, it is the world’s largest city.
PRINCIPLES OF ECOLOGY
C Increased concentrations of greenhouse gases trap more heat near Earth’s surface. Sea surface temperatures rise, so more water evaporates into the atmosphere. Earth’s surface temperature rises.
a Carbon dioxide (CO2). Of all human activities, the burning of fossil fuels and deforestation contribute the most to rising atmospheric levels.
375 365 355 345 335 1982 1986
1990 1994 1998 2002 2006
600
b CFCs. Until restrictions were in place, CFCs were widely used in plastic foams, refrigerators, air conditioners, and industrial solvents.
500
400
300
1978
Concentration (parts per billion)
1978
Concentration (parts per trillion)
Concentration (parts per million)
385
1982 1986 1990 1994 1998 2002 2006
Figure 47.18 Recorded changes in the global mean temperature over land and sea between 1880 and 2005, given as degrees above or below average temperature during 1960–1990.
Deviation from long-term annual mean temperature (°C)
Concentration (parts per million)
FOCUS ON THE ENVIRONMENT
1.80
c Methane (CH4). Production and distribution of natural gas as fuel adds to methane released by some bacteria that live in swamps, rice fields, landfills, and in the digestive tract of cattle and other ruminants (Section 21.7).
1.75 1.70 1.65 1.60
1.55 1978 1982 1986 1990 1994 1998 2002 2006
322
d Nitrous oxide (N2O). Denitrifying bacteria produce N2O in metabolism. Also fertilizers and animal waste from large-scale feedlots release large amounts.
318 314 310 306 302 298 1978 1982 1986 1990 1994 1998
2002 2006
0.4 0.2 0 –0.2 –0.4 1880
1900
be at its highest level since 470,000 years ago, possibly since 20 million years ago. There is scientific consensus that human activities—mainly the burning of fossil fuels— are contributing significantly to the current increases in greenhouse gases. The big worry is that the increase may have far-reaching environmental consequences. The increase in greenhouse gases may be a factor in global warming, a long-term increase in temperature near Earth’s surface (Figure 47.18). In the past thirty years, the global surface temperature increased at a faster rate, to 1.8°C (3.2°F) per century. Warming is most dramatic at the upper latitudes of the Northern Hemisphere. Data from satellites, weather stations and balloons, research ships, and computer programs suggest that some irreversible climate changes are already under way. Water
1920
1940
1960
1980
2000
expands as it is heated, and heating also melts glaciers and other ice. Together, thermal expansion and addition of meltwater will cause sea level to rise. In the past century, the sea level may have risen as much as 20 centimeters (8 inches) and the rate of rise appears to be accelerating. Scientists expect continued temperature increases to have far-reaching effects on climate. An increased rate of evaporation will alter global rainfall patterns. Intense rains and flooding probably will become more frequent in some regions, while droughts increase in others. Hurricanes probably will become more intense. It bears repeating: As investigations continue, a key research goal is to investigate all of the variables in play. With respect to consequences of climate change, the most crucial variable may be the one we do not know.
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ECOSYSTEMS 853
47.9
Nitrogen Cycle Gaseous nitrogen makes up about 80 percent of the lower atmosphere, but most organisms can’t use this gaseous form.
Links to Acid rain 2.6, Nitrogen fixation 21.6 and 29.2, Algal blooms 22.5, Decomposers 21.6 and 24.5, Leaching 29.1
Inputs Into Ecosystems Nitrogen moves in an atmospheric cycle known as the nitrogen cycle (Figure 47.19). Gaseous nitrogen makes up about 80 percent of the atmosphere. Triple covalent bonds hold its two atoms of nitrogen together as N2, or N⬅N. Plants cannot use gaseous nitrogen, because they do not make the enzyme that can break its triple bond. Volcanic eruptions and lightning can convert some N2 into forms that enter food webs. Far more is
converted through nitrogen fixation. By this process, bacteria break all three bonds in N2, then incorporate the N atoms into ammonia (NH3). Ammonia gets converted into ammonium (NH4+) and nitrate (NO3–). These two nitrogen salts dissolve readily in water and are taken up by plant roots. Many species of bacteria fix nitrogen (Section 21.6). Nitrogen-fixing cyanobacteria live in aquatic habitats, soil, and as components of lichens. Another nitrogenfixing group, Rhizobium, forms nodules on the roots of peas and other legumes. Each year, nitrogen-fixing bacteria collectively take up about 270 million metric tons of nitrogen from the atmosphere. The nitrogen incorporated into plant tissues moves up through trophic levels of ecosystems. It ends up in
gaseous nitrogen in atmosphere
nitrogen fixation
food webs on land
fertilizers
uptake by autotrophs
ammonia, ammonium in soil
excretion, death, decomposition
nitrogen-rich wastes, remains in soil
uptake by autotrophs
loss by denitrification
nitrate in soil
nitrification
ammonification loss by leaching
nitrification
Figure 47.19 Animated Nitrogen cycle in an ecosystem on land. Nitrogen becomes available to plants through the activities of nitrogen-fixing bacteria. Other bacterial species cycle nitrogen to plants. They break down organic wastes to ammonium and nitrates.
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PRINCIPLES OF ECOLOGY
nitrite in soil
loss by leaching
nitrogen-rich wastes and remains, which bacteria and fungi decompose (Sections 21.6 and 24.5). By the process of ammonification, these organisms break apart proteins and other nitrogen-containing molecules and produce ammonium. Some of the ammonium product gets released into the soil, where plants and nitrifying bacteria take it up. Nitrification begins when bacteria convert ammonium to nitrite (NO2–). Other nitrifying bacteria then use the nitrite in reactions that end with the formation of nitrate. Nitrate, like ammonium, can be taken up by plant roots.
Natural Losses From Ecosystems Ecosystems lose nitrogen through denitrification. By this process, denitrifying bacteria convert nitrate or nitrite to gaseous nitrogen or to nitrogen oxide (NO2). Denitrifying bacteria are typically anaerobes that live in waterlogged soils and aquatic sediments. Ammonium, nitrite, and nitrate also are lost from a land ecosystem in runoff and by leaching, the removal of some nutrients as water trickles down through the soil (Section 29.1). Nitrogen-rich runoff enters streams and other aquatic ecosystems.
Disruptions by Human Activities Deforestation and conversion of grassland to farmland also causes nitrogen losses from an ecosystem. With each clearing and harvest of plants, nitrogen stored in plant tissues is removed. Plant removal also makes soil more vulnerable to erosion and leaching. Farmers can counter nitrogen depletion by rotating their crops. For example, they plant corn and soybeans in the same field in alternating years. Nitrogen-fixing bacteria that associate with legumes such as soybeans add nitrogen to the soil (Section 29.2). In developed countries, most farmers also spread synthetic nitrogen-rich fertilizers. High temperature and pressure converts nitrogen and hydrogen gases to ammonia fertilizers. Although the manufactured fertilizers improve crop yields, they also modify soil chemistry. Adding ammonium to the soil increases the concentration of hydrogen ions, as well as nitrogen. High acidity encourages ion exchange: Nutrient ions bound to particles of soil get replaced by hydrogen ions. As a result, calcium and magnesium ions needed for plant growth seep away in soil water. Burning of fossil fuel in power plants and by vehicles releases nitrogen oxides. These gases contribute to global warming and acid rain (Section 2.6). Winds frequently carry gaseous pollutants far from their sources.
Figure 47.20 Dead and dying trees in Great Smoky Mountains National Park. Forests are among the casualties of nitrogen oxides and other forms of air pollution.
By some estimates, pollutants blowing into the Great Smoky Mountains National Park have increased the amount of nitrogen in the soil sixfold (Figure 47.20). Nitrogen in acid rain can have the same effects as use of manufactured fertilizers. Different plant species respond in different ways to increased nitrogen level. Changes in soil nitrogen disrupt the balance among competing species in a community, causing diversity to decline. The impact can be especially pronounced in forests at high elevations or at high latitudes, where soils tend to be naturally nitrogen-poor. Some human activities disrupt aquatic ecosystems through nitrogen enrichment. For instance, about half of the nitrogen in fertilizers applied to fields runs off into rivers, lakes, and estuaries. More nitrogen enters waters in sewage from cities and in animal wastes. As one result, nitrogen inputs promote algal blooms (Section 22.5). Phosphorus in fertilizers has the same negative effects, as explained in the next section.
Take-Home Message What is the nitrogen cycle? The ecosystem phase of the nitrogen cycle starts with nitrogen fixation. Bacteria convert gaseous nitrogen in the air to ammonia and then to ammonium, which is a form that plants easily take up. By ammonification, bacteria and fungi make additional ammonium available to plants when they break down nitrogen-rich organic wastes and remains. By nitrification, bacteria convert nitrites in soil to nitrate, which also is a form that plants easily take up. The ecosystem loses nitrogen when denitrifying bacteria convert nitrite and nitrate back to gaseous nitrogen, and when nitrogen is leached from soil.
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ECOSYSTEMS 855
47.10 The Phosphorus Cycle Unlike carbon and nitrogen, phosphorus seldom occurs as a gas. Like nitrogen, it can be taken up by plants only in ionized form, and it, too, is often a limiting factor on plant growth.
In the phosphorus cycle, phosphorus passes quickly through food webs as it moves from land to ocean sediments, then slowly back to dry land. Earth’s crust is the largest reservoir of phosphorus. Phosphorus in rocks is mainly in the form of phosphate (PO43–). Weathering and erosion put phosphate ions from rocks into streams and rivers, which deliver them to oceans (Figure 47.21). There, the phosphates accumulate as underwater deposits along the edges of continents. After millions of years, movements of Earth’s crust result in uplifting of parts of the sea floor. Once uplifted, the rocky phosphate deposits on land are subject to weathering and erosion, which release phosphates from the rocks and start the phosphorus cycle over again. Phosphates are required building blocks for ATP, phospholipids, nucleic acids, and other compounds. Plants take up dissolved phosphates from soil water. Herbivores get them by eating plants; carnivores get them by eating herbivores. Animals lose phosphate in
urine and in feces. Bacterial and fungal decomposers release phosphate from organic wastes and remains, then plants take them up again. The water cycle helps move phosphorus and other minerals through ecosystems. Water evaporates from the ocean and falls on land. As it flows back to the ocean, it transports silt and dissolved phosphates that the primary producers require for growth. Of all minerals, phosphorus most frequently acts as the limiting factor for plant growth. Only newly weathered, young soil has an abundance of phosphorus. Many tropical and subtropical ecosystems that are already low in phosphorus are likely to be further depleted by human actions. In an undisturbed forest, decomposition releases phosphorus stored in biomass. When forest is converted to farmland, the ecosystem loses phosphorus that had been stored in trees. Crop yields soon decline. Later, after the fields are abandoned, regrowth remains sparse. Spreading finely ground, phosphate-rich rock can help restore fertility, but many developing countries lack this resource. Many developed countries have a different problem. Phosphorous in runoff from heavily fertilized fields pollutes water. Sewage from cities and factory farms also contain phosphorus. Dissolved phosphorus that
mining excretion
fertilizers
guano
agriculture uptake by autotrophs
marine food webs
weathering
dissolved in ocean water
uptake by autotrophs
leaching, runoff
dissolved in soil water, lakes, rivers
death, decomposition
death, decomposition settling out
sedimentation
weathering uplifting over geologic time
marine sediments
Figure 47.21 Animated Phosphorus cycle. In this sedimentary cycle, phosphorus moves mainly in the form of phosphate ions (PO43–) to the ocean. It moves through phytoplankton of marine food webs, then to fishes that eat plankton. Seabirds eat the fishes, and their droppings (guano) accumulate on islands. Humans collect and use guano as a phosphate-rich fertilizer.
856 UNIT VII
land food webs
PRINCIPLES OF ECOLOGY
rocks
Summary gets into aquatic ecosystems can promote destructive algal blooms. Like the plants, algae require nitrogen, phosphorus, and other ions to keep growing. In many freshwater ecosystems, nitrogen-fixing bacteria keep the nitrogen levels high, so phosphorus becomes the limiting factor. When phosphate-rich pollutants pour in, algal populations soar and then crash. As aerobic decomposers break down remains of dead algae, the water becomes depleted of the oxygen that fishes and other organisms require. Eutrophication refers to nutrient enrichment of any ecosystem that is otherwise low in nutrients. It can occur naturally, but human activities often accelerate it, as the experiment shown in Figure 47.22 demonstrated. Eutrophication of a lake is difficult to reverse. It can take years for excess nutrients that encourage algal growth to be depleted.
Take-Home Message What is the phosphorus cycle?
The phosphorus cycle is a sedimentary cycle that moves this element from its main reservoir (Earth’s crust), through soils and sediments, aquatic habitats, and bodies of living organisms.
Section 47.1 An ecosystem consists of an array of organisms along with nonliving components of their environment. There is a one-way flow of energy into and out of an ecosystem, and a cycling of materials among resident species. All ecosystems have inputs and outputs of energy and nutrients. Sunlight supplies energy to most ecosystems. Primary producers convert sunlight energy into chemical bond energy. They also take up the nutrients that they, and all consumers, require. Herbivores, carnivores, omnivores, decomposers, and detritivores are consumers. Energy moves from organisms at one trophic level to organisms at another. Organisms are at the same trophic level if they are an equal number of steps away from the energy input into the ecosystem. A food chain shows one path of energy and nutrient flow among organisms. It depicts who eats whom.
Use the animation on CengageNOW to learn about energy flow and nutrient cycling.
Section 47.2 Food chains interconnect as food webs. The efficiency of energy transfers is always low, so most ecosystems have no more than four or five trophic levels. In a grazing food chain, most energy captured by producers flows to herbivores. In detrital food chains, most energy flows from producers directly to detritivores and decomposers. Both types of food chains interconnect in nearly all ecosystems.
Use the animation on CengageNOW to explore a food web.
Section 47.3 A system’s primary production is the rate at which producers capture and store energy in their tissues. It varies with climate, seasonal changes, nutrient availability, and other factors. Energy pyramids and biomass pyramids depict how energy and organic compounds are distributed among the organisms of an ecosystem. All energy pyramids are largest at their base. If producers get eaten as fast as they reproduce, the biomass of consumers can exceed that of producers, so the biomass pyramid is upside down.
nitrogen, carbon added
nitrogen, carbon, phosphorus added
Figure 47.22 A eutrophication experiment. Researchers put a plastic curtain across a channel between two basins of a natural lake. They added nitrogen, carbon, and phosphorus to the water on one side of the curtain (here, the lower part of the lake) and added nitrogen and carbon to the water on the other side. Within months, the basin with phosphorous was eutrophic, with a dense algal bloom (green) covering its surface.
Use the animation on CengageNOW to see how energy flows through one ecosystem.
Section 47.4 With biological magnification, a chemical substance is passed from organisms at each trophic level to those above and becomes increasingly concentrated in body tissues. Section 47.5 In a biogeochemical cycle, water or some nutrient moves from an environmental reservoir, through organisms, then back to the environment. Section 47.6 In the water cycle, evaporation, condensation, and precipitation move water from its main reservoir —oceans—into the atmosphere, onto land, then back to oceans. Runoff is water that flows over ground into streams. A watershed is an area where all precipitation drains into a specific waterway. Water in aquifers and in the soil is groundwater. Use of irrigation can cause CHAPTER 47
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IMPACTS, ISSUES REVISITED
Bye-Bye, Blue Bayou
In 2006, China overtook the United States as the country that emits the most carbon dioxide. Still, an average American life-style causes about 20 tons of carbon emissions per year. That’s more than four times the emissions of an average person in China. It’s also more than twice that of people in western Europe. Automotive emissions are one factor; fuel efficiency standards in both China and Europe are more stringent than they are in the United States.
salinization—salt buildup—in soil. Desalinization is an energy-intensive method of obtaining fresh water from salt water.
Use the animation on CengageNOW to learn about the water cycle.
Section 47.7 The carbon cycle moves carbon from reservoirs in rocks and seawater, through its gaseous forms (methane and CO2) in the air, and through ecosystems. Deforestation and the burning of wood and fossil fuels are adding more carbon dioxide to the atmosphere than the oceans can absorb.
Use the animation on CengageNOW to observe the flow of carbon through its global cycle.
Section 47.8 The greenhouse effect refers to the ability of certain gases to trap heat in the lower atmosphere. It warms Earth’s surface. Human activities are putting larger than normal amounts of greenhouse gases, including carbon dioxide, into the atmosphere. The rise in these gases correlates with a rise in global temperatures (global warming) and other climate changes.
Use the animation on CengageNOW to explore the greenhouse effect and global warming.
Section 47.9 The nitrogen cycle is an atmospheric cycle. Air is the main reservoir for N2, a gaseous form of nitrogen that plants cannot use. In nitrogen fixation, certain bacteria take up N2 and form ammonia. Ammonification releases ammonia from organic remains. Nitrification involves conversion of ammonium to nitrite and then nitrate, which plants are able to take up. Some nitrogen is lost to the atmosphere by denitrification carried out by bacteria. Human activities add nitrogen to ecosystems; for example, through fossil fuel burning (which releases nitrogen oxides) and application of fertilizers. The added nitrogen can disrupt ecosystem processes.
Use the animation on CengageNOW to learn how nitrogen is cycled in an ecosystem.
Section 47.10 The phosphorus cycle is a sedimentary cycle; Earth’s crust is the largest reservoir and there is no major gaseous form. Phosphorus is often the factor that limits population growth of plant and algal producers. Excessive inputs of phosphorus to an aquatic ecosystem can accelerate eutrophication.
Use the animation on CengageNOW to learn how phosphorus is cycled in an ecosystem.
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PRINCIPLES OF ECOLOGY
How would you vote? Should the United States increase fuel efficiency standards for cars and trucks to lower carbon dioxide output? See CengageNow for details, then vote online.
Self-Quiz
Answers in Appendix III
1. In most ecosystems, the primary producers use energy from to build organic compounds. a. sunlight b. heat c. breakdown of wastes and remains d. breakdown of inorganic substances in the habitat 2. Organisms at the lowest trophic level in a tallgrass prairie are all . a. at the first step away from the original energy input b. autotrophs d. both a and b c. heterotrophs e. both a and c 3. Decomposers are commonly . a. fungi b. plants c. bacteria
d. a and c
4. All organisms at the first trophic level . a. capture energy from a nonliving source b. obtain carbon from a nonliving source c. would be at the bottom of an energy pyramid d. all of the above 5. Primary productivity on land is affected by a. nutrient availability c. temperature b. amount of sunlight d. all of the above
.
6. If biological magnification occurs, the will have the highest levels of toxins in their systems. a. producers c. primary carnivores b. herbivores d. top carnivores 7. Most of Earth’s fresh water is . a. in lakes and streams c. frozen as ice b. in aquifers and soil d. in bodies of organisms 8. Earth’s largest carbon reservoir is . a. the atmosphere c. seawater b. sediments and rocks d. living organisms 9. Carbon is released into the atmosphere by . a. photosynthesis c. burning fossil fuels b. aerobic respiration d. b and c 10. Greenhouse gases . a. slow the escape of heat energy from Earth into space b. are produced by natural and human activities c. are at higher levels than they were 100 years ago d. all of the above 11. The a. water b. carbon
cycle is a sedimentary cycle. c. nitrogen d. phosphorus
12. Earth’s largest phosphorus reservoir is . a. the atmosphere c. sediments and rocks b. guano d. living organisms
Data Analysis Exercise
2. During this period, how many times did carbon dioxide reach a level comparable to that measured in 1980?
13. Plant growth requires a. nitrogen b. carbon c. phosphorus
uptake from the soil. d. both a and c e. all of the above
14. Nitrogen fixation converts a. nitrogen gas; ammonia b. nitrates; nitrites c. ammonia; nitrogen gas
to . d. ammonia; nitrates e. nitrogen gas; nitrogen oxides
15. Match each term with its most suitable description. producers a. steps from energy source herbivores b. feed on small bits of decomposers organic matter detritivores c. degrade organic trophic level wastes and remains to biological inorganic forms magnification d. capture sunlight energy e. feed on plants f. toxins accumulate
300
250
200
150
3. The industrial revolution occurred around 1800. What was the trend in carbon dioxide level in the 800 years prior to this event? What about in the 175 years after it? 4. Was the rise in the carbon dioxide level between 1800 and 1975 larger or smaller than the rise between 1980 and 2007?
350
Industrial Revolution
1. What was the highest carbon dioxide level between 400,000 b.c. and 0 a.d.?
Atmospheric carbon dioxide (ppm)
To assess the impact of human activity on the carbon dioxide level in Earth’s atmosphere, it helps to take a long view. One useful data set comes from deep core samples of Antarctic ice. The oldest ice core that has been fully analyzed dates back a bit more than 400,000 years. Air bubbles trapped in the ice provide information about the gas content in Earth’s atmosphere at the time the ice formed. Combining ice core data with more recent direct measurements of atmospheric carbon dioxide—as in Figure 47.23—can help scientists put current changes in the atmospheric carbon dioxide into historical perspective.
400
400,000 B.C.
0 A.D.
1000 Time interval
1975
1980
Figure 47.23 Changes in atmospheric carbon dioxide levels (in parts per million). Direct measurements began in 1980. Earlier data are based on ice cores.
a
b
Figure 47.24 Antarctica’s Larsen B ice shelf in (a) January and (b) March 2002. About 720 billion tons of ice broke from the shelf, forming thousands of icebergs. Some of the icebergs project 25 meters (82 feet) above the surface of the ocean. About 90 percent of an iceberg’s volume is hidden underwater.
Visit CengageNOW for additional questions.
Critical Thinking 1. Marguerite has a vegetable garden in Maine. Eduardo has one in Florida. What are some of the variables that influence primary production in each place? 2. Where does your water come from? A well, a reservoir? Beyond that, what area is included within your watershed and what are the current flows like? Visit the Science in Your Watershed site at water.usgs.gov/wsc and research these questions. 3. Look around you and name all of the objects, natural or manufactured, that might be contributing to amplification of the greenhouse effect.
2007
4. Polar ice shelves are vast, thickened sheets of ice that float on seawater. In March 2002, 3,200 square kilometers (1,250 square miles) of Antarctica’s largest ice shelf broke free from the continent and shattered into thousands of icebergs (Figure 47.24). Scientists knew the ice shelf was shrinking and breaking up, but this event was the single largest loss ever observed at one time. Why should this concern people who live in more temperate climates? 5. Nitrogen-fixing bacteria live throughout the ocean, from its sunlit upper waters to 200 meters (650 feet) beneath its surface. Recall that nitrogen is a limiting factor in many habitats. What effect would an increase in populations of marine nitrogen-fixers have on primary productivity in the waters? What effect would that change have on carbon uptake in those waters? CHAPTER 47
ECOSYSTEMS 859
48
The Biosphere IMPACTS, ISSUES
Surfers, Seals, and the Sea
Professional surfer Ken Bradshaw has ridden a lot of waves,
The decline in fish populations during an El Niño can have
but one in particular stands out. In January of 1998, he found
devastating effects on marine mammals that normally feed on
himself off the coast of Hawaii riding the biggest wave he had
those fish. During the 1997–1998 El Niño, about half of the sea
ever seen (Figure 48.1). It towered more than 12 meters (39
lions on the Galápagos Islands starved to death. California’s
feet) high and gave him the ride of a lifetime.
population of northern fur seals also suffered a sharp decline.
That wave was one manifestation of a climate event that
The temperature change in Pacific waters during the
happens about every three to seven years. During such an
1997–1998 El Niño was the largest on record, and it affected
event, Pacific waters along the west coast of South America
climates around the world. Giant waves, including the one
and westward become warmer than normal. This change in
that Bradshaw rode, battered eastern Pacific coasts. Heavy
water temperature leads to shifts in marine currents and wind
rains caused massive flooding and landslides in California
patterns, and causes wave-generating winter storms.
and Peru. At the same time, less rain than normal fell in
The rise in water temperature also disrupts currents that
Australia and Indonesia, leading to crop failures and wildfires.
normally carry nutrients from the deep ocean toward western
As you will learn in this chapter, the circulation pattern
coasts of the Americas. The resulting nutrient shortage slows
of water in Earth’s oceans is just one of the physical factors
the growth of marine primary producers, causing cascading
that affect the distribution of species through the biosphere.
effects throughout marine food webs. One effect, which most
We define the biosphere as all the places where we find life
often begins around Christmas, is a shortage of fish in waters
on Earth. It includes the hydrosphere (the ocean, ice caps,
near the coast of Peru. Peruvian fisherman noted this pattern
and other bodies of water, liquid and frozen), the lithosphere
and named the periodic climate effect El Niño, meaning “the
(Earth’s rocks, soils, and sediments), and the lower portions
baby boy,” in reference to the birth of Jesus.
of the atmosphere (gases and particles that envelop Earth).
See the video! Figure 48.1 A powerful El Niño caused this enormous wave in the Pacific. It also affected fish populations, causing sea lion pups (photo at left) and seals to starve.
Links to Earlier Concepts
Key Concepts Air circulation patterns Air circulation patterns start with regional differences in energy inputs from the sun, Earth’s rotation and orbit, and the distribution of land and seas. These factors give rise to the great weather systems and regional climates. Sections 48.1, 48.2
With this chapter, you reach the highest level of organization in nature (Section 1.1).
You will learn more about soils (29.1), distribution of primary productivity (47.3), carbon-fixing pathways (7.7), and the effects of deforestation (Chapter 23 introduction).
Our discussions of aquatic provinces will draw on your knowledge of properties of water (2.5), acid rain (2.6, 47.9), the water cycle (47.6), and eutrophication (47.10). You will learn more about coral reefs (25.5) and life at hydrothermal vents (20.2).
You will be reminded of the effects of fossil fuel use (23.5), including global warming (47.8). You will learn about threats to the ozone layer (20.3).
The chapter ends with an example of a scientific approach to problem solving (1.6, 1.7).
Ocean circulation patterns Interactions among ocean currents, air circulation patterns, and landforms produce regional climates, which affect where different organisms can live. Section 48.3
Land provinces Biogeographic realms are vast regions characterized by species that evolved nowhere else. They are divided into biomes characterized mainly by the dominant vegetation. Sunlight intensity, moisture, soil, and evolutionary history vary among biomes. Sections 48.4–48.11
Water provinces Water provinces cover more than 71 percent of Earth’s surface. All freshwater and marine ecosystems have gradients in light availability, temperature, and dissolved gases that vary daily and seasonally. The variations influence primary productivity. Sections 48.12–48.16
Applying the concepts Understanding interactions among the atmosphere, ocean, and land can lead to discoveries about specific events—in one case, recurring cholera epidemics—that impact human life. Section 48.17
How would you vote? We cannot stop an El Niño from happening, but we might be able to minimize its severity. Would you support the use of taxpayer dollars to fund research into the causes and effects of El Niño? See CengageNOW for details, then vote online.
861
48.1
Global Air Circulation Patterns How much solar energy reaches Earth’s surface varies from place to place and with the season.
Link to Fossil fuels 23.5
Air Circulation and Regional Climates Climate refers to average weather conditions, such as cloud cover, temperature, humidity, and wind speed, over time. Regional climates differ because the factors that influence winds and ocean currents—intensity of sunlight, the distribution of land masses and seas, and elevation—vary from place to place.
D Spring equinox (March) Sun’s direct rays fall on equator; length of day equals that of night.
A Summer solstice (June). Northern hemisphere is most tilted toward sun; has its longest day. 23°
Sun
B Autumn equinox (September) Sun’s direct rays fall on equator; length of day equals that of night.
C Winter solstice (December) Northern hemisphere is most tilted away from sun; has its shortest day.
Figure 48.2 Animated Earth’s tilt and yearly rotation around the sun cause seasonal effects. The 23° tilt of Earth’s axis causes the Northern Hemisphere to receive more intense sunlight and have longer days in summer than in winter.
a
b
Figure 48.3 Variation in intensity of solar radiation with latitude. For simplicity, we depict two equal parcels of incoming radiation on an equinox, a day when incoming rays are perpendicular to Earth’s axis. Rays that fall on high latitudes (a) pass through more atmosphere (blue) than those that fall near the equator (b). Compare the length of the green lines. Atmosphere is not to scale. Also, energy in the rays that fall at the high latitude is spread over a greater area than energy that falls on the equator. Compare the length of the red lines.
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PRINCIPLES OF ECOLOGY
Each year, Earth rotates around the sun in an elliptical path (Figure 48.2). Seasonal changes arise because Earth’s axis is not perpendicular to the plane of this ellipse, but rather is tilted about 23 degrees. In June, when the Northern Hemisphere is angled toward the sun, it receives more intense sunlight and has longer days than the Southern Hemisphere (Figure 48.2a). In December, the opposite occurs (Figure 48.2c). Twice a year—on spring and autumn equinoxes—Earth’s axis is perpendicular to incoming sunlight. On these days, every place on Earth receives 12 hours of daylight and 12 hours of darkness (Figure 48.2b,d). On any particular day, equatorial regions get more sunlight energy than higher latitudes for two reasons (Figure 48.3). First, fine particles of dust, water vapor, and greenhouse gases absorb some solar radiation or reflect it back into space. Because sunlight traveling to high latitudes passes through more atmosphere to reach Earth’s surface than light traveling to the equator, less energy reaches the ground. Second, energy in any incoming parcel of sunlight is spread out over a smaller surface area at the equator than at the higher latitudes. As a result of these factors, Earth’s surface warms more at the equator than at the poles. This regional difference in surface warming is the start of global air circulation patterns (Figure 48.4). Warm air can hold more moisture than cooler air and is less dense, so it rises. Near the equator, air warms, picks up moisture from the oceans, and rises (Figure 48.4a). Air cools when it rises to higher altitudes and flows north and south, releasing moisture as rain that supports lush tropical rain forests. Deserts often form at latitudes of about 30°, where the drier and cooler air descends (Figure 48.4b). Farther north and south, the air picks up moisture again. It rises, and then releases moisture at latitudes of about 60° (Figure 48.4c). In the polar regions cold air that holds little moisture descends (Figure 48.4d). Precipitation is sparse, and polar deserts form. Prevailing winds do not blow directly north and south because Earth’s rotation and curvature influence the air circulation pattern. Air masses are not attached to Earth’s surface, so as an air mass moves north or south this surface rotates beneath it, rotating faster at the equator than the poles. As a result, when viewed from Earth’s surface, air masses that move north or south will seem to be deflected east or west, with the deflection greatest at high latitude (Figure 48.4 e,f ). Regional winds occur where the presence of land masses cause differences in air pressure near Earth’s surface. Because land absorbs and releases heat faster than water does, air rises and falls faster over land
Initial t a Pattern atte o of Air C Circulation cu at o D At the poles, cold air sinks and moves toward lower latitudes.
Prevailing eva g Wind W d Patterns atte s Cooled, dry air descends easterlies (winds from the east)
C Air rises again at 60° north and south, where air flowing poleward meets air coming from the poles.
westerlies (winds from the west)
B As the air flows toward higher latitudes, it cools and loses moisture as rain. At around 30° north and south latitude, the air sinks and flows north and south along Earth’s surface.
northeast tradewinds (doldrums)
A Warmed by energy from the sun, air at the equator picks up moisture and rises. It reaches a high altitude, and spreads north and south.
southeast tradewinds
E Major winds near Earth’s surface do not blow directly north and south because of Earth’s rotation. Winds deflect to the right of their original direction in the Northern Hemisphere and to the left in the Southern Hemisphere.
F For example, air moving from 30° south toward the equator is deflected to the left (west), as the southeast trade winds. The winds are named by the direction from which they blow.
westerlies easterlies
Figure 48.4 Animated Global air circulation patterns and their effects on climate.
Figure It Out: What is the direction of prevailing winds in the
than it does over the ocean. Air pressure is lowest where air rises and greatest where air sinks.
How increased leakage of hydrogen into the air would affect the environment is unknown. We use solar energy indirectly by harnessing winds. Wind energy is only practical where winds blow faster than 8 meters per second (18 miles per hour). Winds seldom blow constantly, but wind energy can charge batteries to supply power even on still days. Energy from winds of North and South Dakota alone could meet 80 percent of the United States’ energy needs. Wind farms have drawbacks. Turbine blades can be noisy and can kill birds and bats. Large facilities may alter local weather patterns. Also, some people see wind farms as a form of “visual pollution” that ruins otherwise scenic views and lowers property values.
The need for energy to support human activities continues to increase. Fossil fuels, including gasoline and coal, are nonrenewable energy sources (Section 23.5). Solar and wind energy are renewable. The amount of solar energy that Earth receives per year is about 10 times the energy of all fossil fuel reserves combined. Solar energy can be harnessed directly to heat air or water that can then be pumped through buildings to heat them. Solar energy can also be captured by photovoltaic cells and used to generate electricity. The electricity can be used directly, stored in a battery, or used to form oxygen and hydrogen gases from water. Proponents of solar–hydrogen energy argue that it could end smog, oil spills, and acid rain without any of the risks of nuclear power. Hydrogen gas can fuel cars and heat buildings. However, hydrogen is a small molecule that leaks easily from pipelines or containers.
Answer: Winds blow from west to east.
Harnessing the Sun and Wind
central United States?
Take-Home Message What causes global air circulation patterns and differences in climate? Longitudinal differences in the amount of solar radiation reaching Earth produce global air circulation patterns.
Earth’s shape and rotation also affect air circulation patterns.
CHAPTER 48
THE BIOSPHERE 863
48.2
Something in the Air Particles and gases act as air pollutants that endanger human health and disrupt ecosystems.
gases were once widely used as propellants in aerosol cans, as coolants, and in solvents and plastic foam. CFCs interact with ice crystals and UV light in the stratosphere. These reactions release chlorine radicals that degrade ozone. A single chlorine radical can break apart thousands of ozone molecules. Ozone thins the most at the poles because swirling winds concentrate CFCs in this region during dark, cold polar winters. In the spring, increasing daylight and the presence of ice clouds allow a surge in the formation of chlorine radicals from the highly concentrated CFCs. In response to the potential threat posed by ozone thinning, developed countries agreed in 1992 to phase out the production of CFCs and other ozone destroyers. As a result of that agreement, the concentrations of CFCs in the atmosphere are now starting to decline (Section 47.8). However, they are expected to stay high enough to significantly affect the ozone layer for the next twenty years.
Links to Acid rain 2.6 and 47.9, Ozone 20.3, CFCs 47.8
A pollutant is a natural or synthetic substance released into soil, air, or water in greater than natural amounts; it disrupts normal processes because organisms evolved in its absence, or are adapted to lower levels of it. Today, air pollution threatens biodiversity and human health.
Altitude (kilometers above sea level)
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Swirling Polar Winds and Ozone Thinning High in Earth’s atmosphere, molecules of ozone (O3) absorb most of the ultraviolet (UV) radiation in incoming sunlight. Between 17 and 27 kilometers above sea level (10.5 and 17 miles), the ozone concentration is so great that scientists refer to this region as the ozone layer (Figure 48.5a). In the mid-1970s, scientists started to notice that the ozone layer was getting thinner. Its thickness had always varied a bit with the season, but now there was steady decline from year to year. By the mid-1980s, the spring ozone thinning over Antarctica was so pronounced that people were calling it an “ozone hole” (Figure 48.5b). Declining ozone quickly became an mesosphere international concern. With a thinner ozone layer, people would be exposed to more UV radiation and get more skin cancers (Section 14.5). Higher UV levels also harm wildlife, which do not have the option of stratosphere rubbing on more sunscreen. Higher UV levels might even harm plants and other producers, slowing rates of photosynthesis ozone layer and release of oxygen into the atmosphere. Chlorofluorocarbons, or CFCs, are the main ozone destroyers. These odorless troposphere
No Wind, Lots of Pollutants, and Smog Often, weather conditions cause a thermal inversion: A layer of cool, dense air becomes trapped under a warm, less dense layer. Trapped air sets the stage for smog, an atmospheric condition in which air pollutants accumulate to high concentration. The accumulation occurs because winds cannot disperse pollutants trapped under a thermal inversion layer (Figure 48.6). Thermal inversions have contributed to some of the highest recorded air pollution levels. Industrial smog forms as a gray haze over cities that burn a lot of coal and other fossil fuels during cold, wet winters. Photochemical smog forms above big cities in warm climate zones. Photochemical smog is most dense over cities in natural topographic basins, such as Los Angeles and Mexico City. Exhaust fumes from vehicles contain nitric oxide, a pollutant that combines with oxygen and forms nitrogen dioxide. Exhaust fumes also contain
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cooler air cool air warm air
South America
a cool air
Antarctica
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864 UNIT VII
Figure 48.5 Animated (a) The atmospheric layers. Ozone concentrated in the stratosphere helps shield life from UV radiation. (b) Seasonal ozone thinning above Antarctica in 2001. Dark blue represents the low ozone concentration, at the ozone hole’s center.
PRINCIPLES OF ECOLOGY
warm inversion layer cool air
b
Figure 48.6 (a) Normal air circulation in smog-forming regions. (b) Air pollutants trapped under a thermal inversion layer.
FOCUS ON THE ENVIRONMENT
hydrocarbons that react with nitrogen dioxide to form ozone and other photochemical oxidants. A high ozone level in the lower atmosphere harms plants and animals.
Winds and Acid Rain Coal-burning power plants, smelters, and factories emit sulfur dioxides. Vehicles, power plants that burn gas and oil, and nitrogen-rich fertilizers emit nitrogen oxides. In dry weather, airborne oxides coat dust particles and fall as dry acid deposition. In moist air, they form nitric acid vapor, sulfuric acid droplets, and sulfate and nitrate salts. Winds typically disperse these pollutants far from their source. They fall to Earth in rain and snow. We call this a wet acid deposition, or acid rain. The pH of typical rainwater is about 5 (Section 2.6). Acid rain can be 10 to 100 times more acidic—as potent as lemon juice! It corrodes metals, marble, rubber, plastics, nylon stockings, and other materials. It alters soil pH and can kill trees (Section 47.9) and other organisms. Rain in much of eastern North America is thirty to forty times more acidic than it was even a few decades ago (Figure 48.7a). The heightened acidity has caused fish populations to vanish from more than 200 lakes in the Adirondack Mountains of New York (Figure 48.7b). It also is contributing to the decline of forests. Windborne Particles and Health Pollen, fungal spores, and other natural particles are carried aloft by winds, along with pollutant particles of many sizes. Inhaling small particles can irritate nasal passages, the throat, and lungs. It triggers asthma attacks and can increase their severity. The smallest particles are most likely to reach the lungs, where they can interfere with respiratory function. Exhaust from vehicles is a major source of particulate pollution. Diesel-fueled engines are the worst offenders because they emit more of the smallest, most dangerous particles than their gasoline-fueled counterparts.
Woods Lake
a pH >5.3 5.2–5.3 5.1–5.2 5.0–5.1 4.9–5.0 4.8–4.9 4.7–4.8 4.6–4.7 4.5–4.6 4.4–4.5 4.3–4.4
