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Today & Tomorrow
5e
Cecie Starr | Christine A. Evers | Lisa Starr
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Biology Today and Tomorrow, Fifth Edition Cecie Starr, Christine A. Evers, Lisa Starr
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1 Invitation to Biology
Unit 1 How Cells work
2 Molecules of Life
3 Cell Structure
4 Energy and Metabolism
5 Capturing and Releasing Energy
Unit 2 GenetiCs
6 DNA Structure and Function
7 Gene Expression and Control
8 How Cells Reproduce
9 Patterns of Inheritance
10 Biotechnology
Unit 3 evolUtion and diversity
11 Evidence of Evolution
12 Processes of Evolution
13 Early Life Forms and the Viruses
14 Plants and Fungi
15 Animal Evolution
Unit 4 eColoGy
16 Population Ecology
17 Communities and Ecosystems
18 The Biosphere and Human Effects
Unit 5 How animals work
19 Animal Tissues and Organs
20 How Animals Move
21 Circulation and Respiration
22 Immunity
23 Digestion and Excretion
24 Neural Control and the Senses
25 Endocrine Control
26 Reproduction and Development
Unit 6 How Plants work
27 Plant Form and Function
28 Plant Reproduction and Development
B r
IE F
C o
N T
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T S
BC
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1 Invitation to Biology 1.1 the secret life of earth 4
1.2 life is more than the sum of its Parts 4
1.3 How living things are alike 6
Organisms Require Energy and Nutrients 6
Organisms Sense and Respond to Change 6
Organisms Grow and Reproduce 6
1.4 How living things differ 8
What Is a Species? 8
A Rose by Any Other Name 10
1.5 the science of nature 11
Thinking About Thinking 12
How Science Works 12
Examples of Experiments in Biology 13
1.6 the nature of science 16
Bias in Interpreting Experimental Results 16
Sampling Error 17
Scientific Theories 18
The Scope of Science 19
UNIT 1 HOw CELLS wORk
2 Molecules of Life 2.1 Fear of Frying 24
2.2 start with atoms 25
Why Electrons Matter 26
2.3 From atoms to molecules 28
Ionic Bonds 28
Covalent Bonds 28
2.4 Hydrogen Bonds and water 29
Water Is an Excellent Solvent 30
Water Has Cohesion 31
Water Stabilizes Temperature 31
2.5 acids and Bases 32
2.6 organic molecules 33
What Cells Do to Organic Compounds 33
2.7 Carbohydrates 34
2.8 lipids 36
Fats 36
Phospholipids 36
Waxes 37
Steroids 37
2.9 Proteins 38
The Importance of Protein Structure 39
2.10 nucleic acids 41
3 Cell Structure 3.1 Food for thought 46
3.2 what, exactly, is a Cell? 46
The Cell Theory 46
Components of All Cells 47
Constraints on Cell Size 47
How Do We See Cells? 48
3.3 Cell membrane structure 50
Membrane Proteins 51
3.4 introducing Prokaryotic Cells 52
Biofilms 53
3.5 introducing eukaryotic Cells 54
The Nucleus 54
The Endomembrane System 54
Mitochondria 55
Chloroplasts 56
The Cytoskeleton 56
Extracellular Matrix 58
Cell Junctions 58
3.6 the nature of life 59
C C
o N
T E
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S
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4 Energy and Metabolism 4.1 a toast to alcohol dehydrogenase 64
4.2 life runs on energy 65
4.3 energy in the molecules of life 66
Why Earth Does Not Go Up in Flames 67
Energy In, Energy Out 68
4.4 How enzymes work 68
The Need for Speed 68
Factors That Influence Enzyme Activity 69
Cofactors 70
Metabolic Pathways 71
Controlling Metabolism 71
Electron Transfers 72
4.5 diffusion and membranes 73
Semipermeable Membranes 73
4.6 membrane transport mechanisms 75
Passive Transport 75
Active Transport 76
Membrane Trafficking 76
5 Capturing and Releasing Energy
5.1 a Burning Concern 82
5.2 to Catch a rainbow 83
Storing Energy in Sugars 84
5.3 light-dependent reactions 85
5.4 light-independent reactions 87
Alternative Carbon-Fixing Pathways 87
5.5 a Global Connection 89
Aerobic Respiration in Mitochondria 89
5.6 Fermentation 92
5.7 Food as a source of energy 94
Complex Carbohydrates 94
Fats 94
Proteins 95
UNIT 2 GENETICS
6 DNA Structure and Function 6.1 Cloning 100
6.2 Fame, Glory, and dna structure 102
Discovery of DNA’s Function 102
Discovery of DNA’s Structure 104
DNA Sequence 105
6.3 dna in Chromosomes 106
6.4 dna replication and repair 108
How Mutations Arise 108
7 Gene Expression and Control 7.1 ricin, riP 114
7.2 Gene expression 115
7.3 transcription: dna to rna 116
RNA Modifications 117
7.4 the Genetic Code 118
7.5 translation: rna to Protein 119
7.6 Products of mutated Genes 122
7.7 Control of Gene expression 124
Master Genes 124
Sex Chromosome Genes 125
Lactose Tolerance 125
DNA Methylation 126
CONTENTS v
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8 How Cells Reproduce 8.1 Henrietta’s immortal Cells 132
8.2 multiplication by division 133
Cytoplasmic Division 136
8.3 mitosis and Cancer 137
Cell Division Gone Wrong 137
Cancer 138
Telomeres 139
8.4 sex and alleles 140
On the Advantages of Sex 140
8.5 meiosis in sexual reproduction 142
How Meiosis Mixes Alleles 144
From Gametes to Offspring 144
9 Patterns of Inheritance 9.1 menacing mucus 150
9.2 tracking traits 151
Mendel’s Experiments 151
Inheritance in Modern Terms 151
9.3 mendelian inheritance Patterns 152
Monohybrid Crosses 153
Dihybrid Crosses 154
9.4 Beyond simple dominance 155
Incomplete Dominance 155
Codominance 155
Pleiotropy and Epistasis 156
9.5 Complex variation in traits 158
Continuous Variation 159
9.6 Human Genetic analysis 160
Types of Genetic Variation 160
9.7 Human Genetic disorders 161
The Autosomal Dominant Pattern 162
The Autosomal Recessive Pattern 163
The X-Linked Recessive Pattern 164
9.8 Chromosome number Changes 165
Autosomal Change and Down Syndrome 166
Change in the Sex Chromosome Number 166
9.9 Genetic screening 168
10 Biotechnology 10.1 Personal Genetic testing 174
10.2 Finding needles in Haystacks 175
Cutting and Pasting DNA 175
DNA Libraries 176
PCR 177
10.3 studying dna 178
Sequencing the Human Genome 178
Genomics 179
DNA Profiling 179
10.4 Genetic engineering 181
Genetically Modified Microorganisms 181
Designer Plants 181
Biotech Barnyards 182
10.5 modifying Humans 184
Gene Therapy 184
Eugenics 185
UNIT 3 EVOLUTION AND DIVERSITy
11 Evidence of Evolution 11.1 reflections of a distant Past 190
11.2 Confusing discoveries 191
11.3 a Flurry of new ideas 192
Squeezing New Evidence Into Old Beliefs 192
Darwin and the HMS Beagle 193
A Key Insight—Variation in Traits 194
Great Minds Think Alike 195
11.4 Fossil evidence 196
The Fossil Record 196
vi CONTENTS
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CONTENTS vii
Radiometric Dating 197
Missing Links 199
11.5 drifting Continents 200
Putting Time Into Perspective 201
11.6 evidence in Form 204
Morphological Divergence 204
Morphological Convergence 205
11.7 evidence in Function 206
Patterns in Animal Development 207
12 Processes of Evolution 12.1 superbug Farms 212
12.2 alleles in Populations 213
An Evolutionary View of Mutations 213
Allele Frequency 214
12.3 modes of natural selection 215
Directional Selection 215
Stabilizing Selection 217
Disruptive Selection 217
12.4 natural selection and diversity 218
Survival of the Sexiest 218
Maintaining Multiple Alleles 219
12.5 Genetic drift and Gene Flow 220
Bottlenecks and the Founder Effect 220
Gene Flow 221
12.6 speciation 222
Reproductive Isolation 222
Allopatric Speciation 224
Sympatric Speciation 224
12.7 macroevolution 226
Evolutionary Theory 228
12.8 Phylogeny 229
Applications of Phylogeny 230
13 Early Life Forms and the Viruses 13.1 the Human micobiome 236
13.2 on the road to life 237
Conditions on the Early Earth 237
Origin of the Building Blocks of Life 237
Origin of Metabolism 238
Origin of Genetic Material 238
Origin of Cell Membranes 239
13.3 origin of the three domains 240
Reign of the Prokaryotes 240
Origin of Eukaryotes 241
13.4 viruses 242
Viral Structure and Replication 242
Bacteriophages 242
Plant Viruses 243
Viruses and Human Health 243
HIV—The AIDS Virus 244
Ebola 245
New Flus 245
13.5 Bacteria and archaea 246
Structure and Function 246
Reproduction and Gene Transfers 246
Metabolic Diversity 247
Domain Archaea 248
Domain Bacteria 248
13.6 Protists 250
Flagellated Protozoans 250
Foraminifera 251
Ciliates 251
Dinoflagellates 252
Apicomplexans 252
Water Molds, Diatoms, and Brown Algae 254
Red Algae 255
Green Algae 255
Amoebas and Slime Molds 256
Choanoflagellates 257
14 Plants and Fungi 14.1 Fungal threats to Crops 262
14.2 Plant traits and evolution 263
Life Cycle 263
Structural Adaptations to Life on Land 264
Reproduction and Dispersal 264
14.3 nonvascular Plants 265
Mosses 265
Liverworts and Hornworts 266
14.4 seedless vascular Plants 266
Ferns 266
Horsetails and Club Mosses 267
14.5 rise of the seed Plants 269
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14.6 Gymnosperms 270
Conifers 270
Cycads and Ginkgos 271
Gnetophytes 271
14.7 angiosperms—Flowering Plants 272
Floral Structure and Function 272
A Flowering Plant Life Cycle 273
Keys to Angiosperm Diversity 273
Major Groups 273
Ecology and Human Uses of Angiosperms 274
14.8 Fungal traits and diversity 274
Yeasts, Molds, Mildews, and Mushrooms 274
Lineages and Life Cycles 275
14.9 ecological roles of Fungi 277
Decomposers 277
Parasites 277
Fungal Partnerships 278
Human Uses of Fungi 279
15 Animal Evolution 15.1 medicines From the sea 284
15.2 origins and diversification 285
Animal Origins 285
Evidence of Early Animals 285
Major Groups and Evolutionary Trends 286
15.3 invertebrate diversity 288
Sponges 288
Cnidarians 288
Flatworms 289
Annelids 290
Mollusks 290
Roundworms 291
Arthropods 292
Echinoderms 296
15.4 introducing the Chordates 297
Chordate Traits 297
Invertebrate Chordates 297
Vertebrate Traits and Trends 298
15.5 Fishes and amphibians 299
Jawless Fishes 299
Jawed Fishes 299
Early Tetrapods 300
Modern Amphibians 301
15.6 escape From water—amniotes 302
Amniote Innovations 302
Nonbird Reptiles 302
Birds 303
Mammals 303
15.7 Human evolution 305
Primate Traits 305
Primate Origins and Diversification 305
Australopiths 306
Early Humans 307
Homo Sapiens 308
Neanderthals and Denisovans 308
UNIT 4 ECOLOGy
16 Population Ecology 16.1 a Honkin’ mess 314
16.2 Characteristics of Populations 315
Demographic Traits 315
Collecting Demographic Data 316
16.3 Population Growth 317
Exponential Growth 317
Carrying Capacity and Logistic Growth 318
Density-Independent Factors 319
16.4 life History Patterns 320
Biotic Potential 320
Describing Life Histories 320
Evolution of Life Histories 321
Predation and Life History Evolution 322
viii CONTENTS
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CONTENTS ix
16.5 Human Populations 323
Population Size and Growth Rate 323
Fertility Rates and Future Growth 324
Effects of Industrial Development 325
17 Communities and Ecosystems 17.1 Fighting Foreign Fire ants 330
17.2 Community structure 331
Nonbiological Factors 331
Biological Factors 331
17.3 direct species interactions 332
Commensalism and Mutualism 332
Interspecific Competition 333
Predator–Prey Interactions 334
Plants and Herbivores 335
Parasites and Parasitoids 335
17.4 How Communities Change 337
Ecological Succession 337
Adapted to Disturbance 338
Species Losses or Additions 338
17.5 the nature of ecosystems 339
Overview of the Participants 339
Food Chains and Webs 339
Primary Production and Inefficient Energy Transfers 341
17.6 Biogeochemical Cycles 342
The Water Cycle 342
The Phosphorus Cycle 342
The Nitrogen Cycle 344
The Carbon Cycle 345
The Greenhouse Effect and Global Climate Change 346
18 The Biosphere and Human Effects 18.1 Going with the Flow 352
18.2 Factors that affect Climate 353
Air Circulation Patterns 353
Ocean Circulation 354
18.3 the major Biomes 355
Forest Biomes 355
Grasslands and Chaparral 356
Deserts 356
Tundra 356
18.4 aquatic ecosystems 358
Freshwater Ecosystems 358
Marine Ecosystems 358
18.5 Human impact on the Biosphere 360
Increased Species Extinctions 360
Deforestation and Desertification 362
Acid Rain 362
Biological Accumulation and Magnification 363
The Trouble With Trash 363
Destruction of the Ozone Layer 364
Global Climate Change 364
18.6 maintaining Biodiversity 366
The Value of Biodiversity 366
Conservation Biology 366
Ecological Restoration 367
Reducing Human Impacts 368
UNIT 5 HOw ANIMALS wORk
19 Animal Tissues and Organs 19.1 Growing replacement Parts 374
19.2 animal structure and Function 375
Organization and Integration 375
Evolution of Structure and Function 376
19.3 types of animal tissues 376
Epithelial Tissues 376
Connective Tissues 378
Muscle Tissues 379
Nervous Tissue 380
19.4 organs and organ systems 380
Organ Systems 382
19.5 regulating Body temperature 384
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20 How Animals Move 20.1 Bulking Up muscles 390
20.2 skeletal systems 391
Types of Skeletons 391
The Human Skeleton 392
Bone Structure and Function 392
Where Bones Meet—Skeletal Joints 393
20.3 Functions of skeletal muscles 395
20.4 How muscle Contracts 396
Muscle Components 396
Sliding Filaments 397
20.5 Fueling muscle Contraction 398
20.6 exercise and inactivity 398
21 Circulation and Respiration 21.1 a shocking save 404
21.2 How substances are moved through a Body 405
Open and Closed Circulatory Systems 405
Evolution of Vertebrate Cardiovascular Systems 406
21.3 Human Cardiovascular system 407
21.4 the Human Heart 408
The Cardiac Cycle 409
Setting the Pace of Contractions 409
21.5 Blood and Blood vessels 410
Components and Functions of Blood 410
High-Pressure Flow in Arteries 410
Adjusting Resistance at Arterioles 411
Capillary Exchange and Function of the Lymph Vessels 411
Back to the Heart 412
21.6 Blood and Cardiovascular disorders 412
Blood Disorders 412
Cardiovascular Disorders 413
21.7 animal respiration 414
Two Sites of Gas Exchange 414
Respiratory Systems 414
21.8 Human respiratory Function 416
From Airways to Alveoli 416
How You Breathe 417
Exchanges at Alveoli 418
Transport of Gases 418
Respiratory Disorders 418
22 Immunity 22.1 Frankie’s last wish 424
22.2 responding to threats 425
The Defenders 426
22.3 innate immunity mechanisms 427
Normal Flora 427
Surface Barriers 427
Complement 428
Phagocytosis 428
Inflammation and Fever 429
Examples of Innate Responses 430
22.4 antigen receptors 431
Antigen Processing 432
22.5 adaptive immune responses 434
Example of an Antibody-Mediated Response 434
Example of a Cell-Mediated Response 436
22.6 immunity Gone wrong 438
Overly Vigorous Responses 438
Immune Deficiency and AIDS 439
22.7 vaccines 441
23 Digestion and Excretion 23.1 Causes and effects of obesity 446
23.2 two types of digestive systems 446
23.3 digestive structure and Function 448
In the Mouth 448
Swallowing 448
The Stomach 449
Digestion in the Small Intestine 450
Absorption in the Small Intestine 451
Concentrating and Eliminating Wastes 452
23.4 Human nutrition 453
Carbohydrates 453
Fats 454
Proteins 454
Vitamins and Minerals 454
USDA Dietary Recommendations 455
23.5 Fluid regulation 456
Fluid Homeostasis 456
Fluid Regulation in Invertebrates 456
Vertebrate Urinary System 457
x CONTENTS
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CONTENTS xi
23.6 kidney Function 458
How Urine Forms 458
Feedback Control of Urine Formation 459
Impaired Kidney Function 460
24 Neural Control and the Senses 24.1 impacts of Concussions 466
24.2 animal nervous systems 467
Invertebrate Nervous Systems 467
Vertebrate Nervous Systems 467
24.3 neuron Function 468
Three Types of Neurons 468
Neuroglia—Neuron Helpers 469
Resting Potential 469
The Action Potential 470
The Chemical Synapse 471
Disrupted Synaptic Function 472
Psychoactive Drugs 472
24.4 the Central nervous system 474
Regions of the Human Brain 474
A Closer Look at the Cerebral Cortex 476
The Limbic System—Emotion and Memory 476
The Spinal Cord 477
24.5 the Peripheral nervous system 478
24.6 the senses 480
Sensory Reception and Diversity 480
Sensation to Perception 480
The Chemical Senses—Smell and Taste 481
Detecting Light 482
The Human Eye 482
At the Retina 484
Hearing 484
Sense of Balance 486
The Somatosensory Cortex 487
25 Endocrine Control 25.1 endocrine disrupters 492
25.2 Hormone Function 493
Types of Hormones 494
Hormone Receptors 494
25.3 the Hypothalamus and Pituitary 496
Posterior Pituitary Function 496
Anterior Pituitary Function 496
Growth Disorders 496
25.4 thyroid and Parathyroid Glands 498
Thyroid Hormone 498
Regulation of Calcium 499
25.5 the Pancreas 500
Controlling Blood Glucose 500
Diabetes Mellitus 501
25.6 the adrenal Glands 502
25.7 Hormones and reproductive Function 504
Gonads 504
The Pineal Gland 504
26 Reproduction and Development 26.1 assisted reproduction 510
26.2 modes of reproduction 511
Asexual Reproduction 511
Sexual Reproduction 511
Variations on Sexual Reproduction 511
26.3 stages of animal development 512
26.4 Human reproductive Function 514
Female Reproductive Anatomy 514
Egg Production and Release 515
The Menstrual Cycle 516
Male Reproductive Anatomy 517
How Sperm Form 518
Sexual Intercourse 518
A Sperm’s Journey 519
26.5 reproductive Health 520
Contraception 520
Infertility 521
Sexually Transmitted Diseases 522
26.6 Human development 523
Fertilization 523
From Cleavage to Implantation 524
Embryonic and Fetal Development 525
Functions of the Placenta 528
Maternal Effects on Prenatal Development 528
26.7 Birth and milk Production 529
Childbirth 529
Nourishing the Newborn 529
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UNIT 6 HOw PLANTS wORk
27 Plant Form and Function 27.1 leafy Cleanup Crews 534
27.2 tissues in a Plant Body 535
Eudicots and Monocots 537
27.3 stems, leaves, and roots 538
Stems 538
Leaves 540
Roots 542
27.4 Fluid movement in Plants 544
Water Moves Through Xylem 544
Sugars Flow Through Phloem 545
27.5 Plant Growth 546
28 Plant Reproduction and Development
28.1 Plight of the Honeybee 554
28.2 sexual reproduction 555
A New Generation Begins 558
28.3 seeds and Fruits 560
28.4 early development 562
28.5 asexual reproduction 564
Agricultural Applications 564
28.6 Plant Hormones 565
Auxin 566
Cytokinin 567
Gibberellin 568
Abscisic Acid 568
Ethylene 568
28.7 Growth responses 570
Tropisms 570
Photoperiodic Responses 572
appendix i answers to self-Quizzes
appendix ii Periodic table of the elements
appendix iii a Plain english map of the Human Chromosomes
appendix iv Units of measure
xii CONTENTS
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P
P r
E Fa
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Biology is a huge field, with a wealth of new discover- ies being made every day, and biology-related issues such as climate change, stem cell research, and per- sonal genetics often making headlines. This avalanche of information can be intimidating to non-scientists. This book was designed and written specifically for students who most likely will not become biologists and may never again take another science course. It is an accessible and engaging introduction to biology that provides future decision-makers with an under- standing of basic biology and the process of science.
a wealth of applications This book is packed with everyday applications of biological processes. At every opportunity, we enliven discussions of biologi- cal processes with references to their effects on human health and the environment. This edition also con- tinues to focus on real world applications pertaining to the field of biology, including social issues arising from new research and developments. Descriptions of current research, along with photos of scientists who carry it out, underscore the concept that biology is an ongoing endeavor carried out by a diverse commu- nity of people. Discussions include not only what was discovered, but also how the discoveries were made, how our understanding has changed over time, and what remains to be discovered. These discussions are provided in the context of an accessible introduction to well-established concepts that underpin modern biology. Every topic is examined from an evolutionary perspective, emphasizing the connections between all forms of life.
accessible text Understanding stems from mak- ing connections between concepts and details, so a text with too little detail reads as a series of facts that beg to be memorized. However, excessive detail can overwhelm the introductory student. Thus, we con- stantly strive to strike the perfect balance between level of detail and accessibility. We once again revised the text to eliminate details that do not contribute to a basic understanding of essential concepts. We also know that English is a second language for many introductory students, so we avoid idioms and aim for a clear, straightforward style.
Analogies to familiar objects and phenomena will help students understand abstract concepts. For exam- ple, in the discussion of transpiration in Chapter 27 (Plant Form and Function), we explain that a column of water is drawn upward through xylem as a drinker draws fluid up through a straw.
in-text learning tools To emphasize connections between biological topics, each chapter begins with an application section that explores a current event or controversy directly related to the chapter’s content. For example, a discussion of binge drinking on col- lege campuses introduces the concept of metabolism in Chapter 4. This section presents an overview of the metabolic pathway that breaks down alcohol, linking the function of enzymes in the pathway to hangovers, alcoholism, and cirrhosis. The section is illustrated with a photo of a tailgate party that preceded a recent Notre Dame–Alabama football game, and also a photo of Gary Reinbach just before he died at age 22 of alcoholic liver disease. (In the index, you’ll find health-related applications denoted by red squares and environmental applications by green squares.)
To strengthen a student’s analytical skills and offer insight into contemporary research, each chapter includes an exercise called digging into data that is placed in a section with relevant content. The exer- cise consists of a short text passage—usually about a published scientific experiment—and a table, chart, or other graphic that presents experimental data. A student can use information in the text and graphic to answer a series of questions. For example, the exercise in Chapter 2 asks students to interpret results of a study that examined the effect of dietary fat intake on “good” and “bad” cholesterol levels.
The chapter itself consists of several numbered sections that contain a manageable chunk of informa- tion. Every section ends with a boxed take-home message in which we pose a question that reflects the critical content of the section, and then answer the question in bulleted list format. Every chapter has at least one figure it out question with an answer immediately following. These questions allow students to quickly check their understanding as they read. Mastering scientific vocabulary challenges many stu- dents, so we have included an on-page glossary of key terms introduced in each two-page spread, in addition to a complete glossary at the book’s end. The end-of-chapter material features a visual summary that reinforces each chapter’s key concepts. A self- quiz poses multiple choice and other short answer questions for self-assessment (answers are in Appen- dix I). A set of more challenging critical thinking questions provides thought-provoking exercises for the motivated student. The end matter of several chapters now includes a visual question that rein- forces learning in a nonverbal style.
Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
xiv PREFACE
design and Content revisions Throughout the book, text and art have been revised to help students grasp difficult concepts. The following list highlights some of the revisions to each chapter.
Introduction 1 Invitation to Biology Renewed and updated emphasis on the rel-
evance of new species discovery and the process of science.
Unit 1 How Cells work 2 Molecules of Life New graphic illustrates radioactive decay. 3 Cell Structure Application section updated with current statistics
and ‘pink slime’ story. Micrograph comparisons now feature Para- mecia and include a confocal image. Essay about the nature of life expanded to add Gerald Joyce’s “life is squishy” concept.
4 Energy and Metabolism Application section now illustrated with a real-life example. Diffusion illustrated with a tea bag in hot water.
5 Capturing and Releasing Energy Application section updated with current statistics and illustrated with a current photo of air pollu- tion in China. Yogurt production added to fermentation section.
Unit 2 Genetics 6 DNA Structure and Function Content reorganized: material on clon-
ing folded into Application section for concept connection, and chromosome structure now appears after DNA structure. New art demonstrates how replication errors become mutations.
7 Gene Expression and Control Ricin discussion revised to include medical applications. New material includes hairlessness mutation (in cats), evolution of lactose tolerance, heritability of DNA meth- ylations, telomeres.
8 How Cells Reproduce New material on telomeres, asexual vs. sexual mud snails. New micrograph shows multiple crossovers.
9 Patterns of Inheritance Epistasis is now illustrated with human skin color. New material about environmentally-triggered hemoglobin production in Daphnia; continuous variation in dog face length arising from short tandem repeats foreshadows DNA fingerprint- ing in chapter 10.
10 Biotechnology Updated coverage of personal genetic testing includes social impact of Angelina Jolie’s response to her test. New photos illustrate genetically modified animals. New “who’s the daddy” critical thinking question offers students an opportu- nity to analyze a paternity test based on SNPs.
Unit 3 Evolution and Diversity 11 Evidence of Evolution Photos of 19th century naturalists added
to emphasize the process of science that led to natural selection theory. How banded iron formations provide evidence of the evo- lution of photosynthesis added to fossil section. Plate tectonics art updated to reflect new evidence of lava lamp mantle movements.
12 Processes of Evolution New opening essay on resistance to anti- biotics as an outcome of agricultural overuse (warfarin material now exemplifies directional selection). New art illustrates founder effect, and hypothetical example in text replaced with reduced
diversity of ABO alleles in Native Americans. New art illustrates stasis in coelacanths.
13 Early Life Forms and the Viruses New introductory essay about study of the human microbiome, new coverage of Ebola, and new figure depicting mechanisms of gene exchange in prokaryotes.
14 Plants and Fungi Additional coverage of fungal ecology, including information about white-nose syndrome in bats.
15 Animal Evolution New introductory essay about invertebrates as a source of medicines. Updated information about Neanderthals and added coverage of the newly discovered Dennisovans.
Unit 4 Ecology 16 Population Ecology Updated coverage of human demographics. 17 Communities and Ecosystems New photos illustrate species interac-
tions; updated coverage of the increases in greenhouse gases. 18 The Biosphere and Human Effects New essay about dispersion of the
radioactive material released at Fukushima and new Digging Into Data about bioaccumulation of this material in tuna.
Unit 5 How Animals work 19 Animal Tissues and Organs Updated information about stem cell
research and tissue regeneration in animals. Improved figures depict epithelial and connective tissues.
20 How Animals Move New information about how different muscle fiber types relate to animal locomotion.
21 Circulation and Respiration Improved coverage of insect respiration, including a new photo.
22 Immunity New photos show skin as a surface barrier, a cytotoxic T cell killing a cancer cell, and victims of HIV. Immune response and lymphatic system illustrations updated.
23 Digestion and Excretion Revised essay about obesity and new com- parative information about the ruminant digestive system.
24 Neural Control and the Senses New opening essay about the effects of concussions. Discussion of the human nervous system has been reorganized. New information about echolocation.
25 Endocrine Control Opening essay now focuses on phthalates as endocrine disruptors. New Digging Into Data about BPA’s effect on insulin secretion.
26 Reproduction and Development Updated coverage of assisted repro- ductive technologies. Discussion of human reproductive structure and function has been reorganized.
Unit 6 How Plants work 27 Plant Form and Function Reorganization consolidates growth into a
separate section. Many new photos illustrate stem, leaf, and root structure(s). Material on fire scars added to dendroclimatology.
28 Plant Reproduction and Development Updates reflect current research on colony collapse and ongoing major breakthroughs in the field of plant hormone function. New photos illustrate fruit classification, asexual reproduction, early growth, ABA inhibition of seed germination, and tropisms.
Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
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We owe a special debt to the members of our advisory board, listed below. They helped us shape the book’s design and to choose appro priate content. We appreciate their guidance.
Andrew Baldwin, Mesa Community College Charlotte Borgeson, University of Nevada, Reno Gregory A. Dahlem, Northern Kentucky University Gregory Forbes, Grand Rapids Community College Hinrich Kaiser, Victor Valley Community College Lyn Koller, Pierce College Terry Richardson, University of North Alabama
We also wish to thank the reviewers listed below.
Idris Abdi, Lane College Meghan Andrikanich, Lorain County Community College Lena Ballard, Rock Valley College Barbara D. Boss, Keiser University, Sarasota Susan L. Bower, Pasadena City College James R. Bray Jr., Blackburn College Mimi Bres, Prince George’s Community College Randy Brewton, University of Tennessee Evelyn K. Bruce, University of North Alabama Steven G. Brumbaugh, Green River Community College Chantae M. Calhoun, Lawson State Community College Thomas F. Chubb, Villanova University Julie A. Clements, Keiser University, Melbourne Francisco Delgado, Pima Community College Elizabeth A. Desy, Southwest Minnesota State University Brian Dingmann, University of Minnesota, Crookston Josh Dobkins, Keiser University, online Hartmut Doebel, The George Washington University Pamela K. Elf, University of Minnesota, Crookston Johnny El-Rady, University of South Florida Patrick James Enderle, East Carolina University Jean Engohang-Ndong, BYU Hawaii Ted W. Fleming, Bradley University Edison R. Fowlks, Hampton University Martin Jose Garcia Ramos, Los Angeles City College J. Phil Gibson, University of Oklahoma Judith A. Guinan, Radford University Carla Guthridge, Cameron University Laura A. Houston, Northeast Lakeview–Alamo College Robert H. Inan, Inver Hills Community College Dianne Jennings, Virginia Commonwealth University Ross S. Johnson, Chicago State University Susannah B. Johnson Fulton, Shasta College Paul Kaseloo, Virginia State University Ronald R. Keiper, Valencia Community College West Dawn G. Keller, Hawkeye Community College Ruhul H. Kuddus, Utah Valley State College Dr. Kim Lackey, University of Alabama Vic Landrum, Washburn University Lisa Maranto, Prince George’s Community College Catarina Mata, Borough of Manhattan Community College Kevin C. McGarry, Keiser University, Melbourne Timothy Metz, Campbell University Ann J. Murkowski, North Seattle Community College Alexander E. Olvido, John Tyler Community College Joshua M. Parke, Community College of Southern Nevada Elena Pravosudova, Sierra College Nathan S. Reyna, Howard Payne University Carol Rhodes, Cañada College Todd A. Rimkus, Marymount University Laura H. Ritt, Burlington County College Lynette Rushton, South Puget Sound Community College Erik P. Scully, Towson University
Marilyn Shopper, Johnson County Community College Jennifer J. Skillen, Community College of Southern Nevada Jim Stegge, Rochester Community and Technical College Lisa M. Strain, Northeast Lakeview College Jo Ann Wilson, Florida Gulf Coast University
We were also fortunate to have conversations with the following workshop attendees. The insights they shared proved invaluable.
Robert Bailey, Central Michigan University Brian J. Baumgartner, Trinity Valley Community College Michael Bell, Richland College Lois Borek, Georgia State University Heidi Borgeas, University of Tampa Charlotte Borgenson, University of Nevada Denise Chung, Long Island University Sehoya Cotner, University of Minnesota Heather Collins, Greenville Technical College Joe Conner, Pasadena Community College Gregory A. Dahlem, Northern Kentucky University Juville Dario-Becker, Central Virginia Community College Jean DeSaix, University of North Carolina Carolyn Dodson, Chattanooga State Technical Community College Kathleen Duncan, Foothill College, California Dave Eakin, Eastern Kentucky University Lee Edwards, Greenville Technical College Linda Fergusson-Kolmes, Portland Community College Kathy Ferrell, Greenville Technical College April Ann Fong, Portland Community College Kendra Hill, South Dakota State University Adam W. Hrincevich, Louisiana State University David Huffman, Texas State University, San Marcos Peter Ingmire, San Francisco State Ross S. Johnson, Chicago State University Rose Jones, NW-Shoals Community College Thomas Justice, McLennan Community College Jerome Krueger, South Dakota State University Dean Kruse, Portland Community College Dale Lambert, Tarrant County College Debabrata Majumdar, Norfolk State University Vicki Martin, Appalachian State University Mary Mayhew, Gainesville State College Roy Mason, Mt. San Jacinto College Alexie McNerthney, Portland Community College Brenda Moore, Truman State University Alex Olvido, John Tyler Community College Molly Perry, Keiser University Michael Plotkin, Mt. San Jacinto College Amanda Poffinbarger, Eastern Illinois University Johanna Porter-Kelley, Winston-Salem State University Sarah Pugh, Shelton State Community College Larry A. Reichard, Metropolitan Community College Darryl Ritter, Okaloosa-Walton College Sharon Rogers, University of Las Vegas Lori Rose, Sam Houston State University Matthew Rowe, Sam Houston State University Cara Shillington, Eastern Michigan University Denise Signorelli, Community College of Southern Nevada Jennifer Skillen, Community College of Southern Nevada Jim Stegge, Rochester Community and Technical College Andrew Swanson, Manatee Community College Megan Thomas, University of Las Vegas Kip Thompson, Ozarks Technical Community College Steve White, Ozarks Technical Community College Virginia White, Riverside Community College Lawrence Williams, University of Houston Michael L. Womack, Macon State College
Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Today & Tomorrow
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acknowledgments
Writing, revising, and illustrating a biology text- book is a major undertaking for two full-time au- thors, but our efforts constitute only a small part of what is required to produce and distribute this one. We are truly fortunate to be part of a huge team of very talented people who are as commit- ted as we are to creating and disseminating an exceptional science education product.
Biology is not dogma; paradigm shifts are a common outcome of the fantastic amount of research in the field. Ideas about what material should be taught and how best to present that material to students changes from one year to the next. It is only with the ongoing input of our many academic reviewers and advisors (previous page) that we can continue to tailor this book to the needs of instructors and students while inte- grating new information and models. We con- tinue to learn from and be inspired by these dedicated educators.
On the production side of our team, the indis- pensable Grace Davidson orchestrated a continu- ous flow of files, photos, and illustrations while managing schedules, budgets, and whatever else happened to be on fire at the time. Grace, thank you as always for your patience and dedication. Thank you also to Cheryl DuBois, John Saranta- kis, and Christine Myaskovsky for your help with photoresearch. Copyeditor Anita Hueftle and proofreader Diane Miller, your valuable sugges- tions kept our text clear and concise.
Yolanda Cossio, thank you for continuing to support us and for encouraging our efforts to innovate and improve. Thanks also to Cengage Production Manager Hal Humphrey, Marketing Manager Tom Ziolkowski, and to Lauren Oliveira, who creates our exciting technology package, Associate Content Developers Casey Lozier and Kellie Petruzzelli, and Product Assistant Victor Luu.
Lisa Starr and Christine Evers, November 2014
Cengage learning testing Powered by Cognero is a flexible, online system that allows you to: • author, edit, and manage test bank content from
multiple Cengage Learning solutions • create multiple test versions in an instant • deliver tests from your LMS, your classroom, or
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instructor Companion site Everything you need for your course in one place! This collection of book- specific lecture and class tools is available online via www.cengage.com/login. Access and download Power- Point presentations, images, instructor’s manual, videos, and more.
Cooperative learning Cooperative Learning: Making Connections in General Biology, 2nd Edi- tion, authored by Mimi Bres and Arnold Weisshaar, is a collection of separate, ready-to-use, short coop- erative activities that have broad application for first year biology courses. They fit perfectly with any style of instruction, whether in large lecture halls or flipped classrooms. The activities are designed to address a range of learning objectives such as reinforcing basic concepts, making connections between various chapters and top- ics, data analysis and graphing, developing problem solving skills, and mastering terminology. Since each activity is designed to stand alone, this collection can be used in a variety of courses and with any text.
mindtap A personalized, fully online digital learn- ing platform of authoritative content, assignments, and services that engages students with interactivity while also offering instructors their choice in the configuration of coursework and enhancement of the curriculum via web-apps known as MindApps. MindApps range from ReadSpeaker (which reads the text out loud to students) to Kaltura (which allows you to insert inline video and audio into your curriculum). MindTap is well beyond an eBook, a homework solution or digital supplement, a resource center website, a course delivery platform, or a Learning Management System. It is the first in a new category—the Personal Learning Experience.
New for this edition! MindTap has an integrated Study Guide, expanded quizzing and application activi- ties, and an integrated Test Bank.
aplia for Biology The Aplia system helps students learn key concepts via Aplia’s focused assignments and active learning opportunities that include randomized, automatically graded questions, exceptional text/art inte- gration, and immediate feedback. Aplia has a full course management system that can be used independently or in conjunction with other course management systems such as MindTap, D2L, or Blackboard.
Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Today & Tomorrow
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Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
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1.1 The Secret Life of Earth 4
1.2 Life Is More Than the Sum of Its Parts 4
1.3 How Living Things Are Alike 6
1.4 How Living Things Differ 8
1.5 The Science of Nature 11
1.6 The Nature of Science 16
Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
4 INTroDucTIoN
1.1 The Secret Life of Earth In this era of detailed satellite imagery and cell phone global positioning systems, could there possibly be any places left on Earth that humans have not yet explored? Actually, there are plenty of them. In 2005, for example, helicopters dropped a team of scientists into the middle of a vast and otherwise inaccessible cloud forest atop New Guinea’s Foja Mountains. Within a few minutes, the explorers realized that their landing site, a dripping, moss-covered swamp, had been untouched by humans. Team member Bruce Beehler remarked, “Everywhere we looked, we saw amazing things we had never seen before. I was shouting. This trip was a once-in-a- lifetime series of shouting experiences.”
How did the explorers know they had landed in uncharted territory? For one thing, the forest was filled with plants and animals previously unknown even to native peoples that have long inhabited other parts of the region. During the next month, the team members discovered many new species, including a rhododendron plant with flowers the size of a plate and a frog the size of a pea. They also came across hundreds of species that are on the brink of extinction in other parts of the world, and some that supposedly had been extinct for decades. The animals had never learned to be afraid of humans, so they could easily be approached. A few were discovered as they casually wandered through campsites (Figure 1.1A).
New species are discovered all the time, often in places much more mundane than Indonesian cloud forests (Figure 1.1B). How do we know what species a par- ticular organism belongs to? What is a species, anyway, and why should discovering a new one matter to anyone other than a scientist? You will find the answers to such questions in this book. They are part of the scientific study of life, biology, which is one of many ways we humans try to make sense of the world around us.
Trying to understand the immense scope of life on Earth gives us some per- spective on where we fit into it. For example, hundreds of new species are discov- ered every year, but about 20 species become extinct every minute in rain forests alone—and those are only the ones we know about. The current rate of extinctions is about 1,000 times faster than normal, and human activities are responsible for the acceleration. At this rate, we will never know about most of the species that are alive on Earth today. Does that matter? Biologists think so. Whether or not we are aware of it, humans are intimately connected with the world around us. Our activities are profoundly changing the entire fabric of life on Earth. These changes are, in turn, affecting us in ways we are only beginning to understand.
Ironically, the more we learn about the natural world, the more we realize we have yet to learn. But don’t take our word for it. Find out what biologists know, and what they do not, and you will have a solid foundation upon which to base your own opinions about how humans fit into this world. By reading this book, you are choos- ing to learn about the human connection—your connection—with all life on Earth.
1.2 Life Is More Than the Sum of Its Parts What, exactly, is the property we call “life”? We may never actually come up with a good definition, because living things are too diverse, and they consist of the same basic components as nonliving things. When we try to define life, we end up with a long list of properties that differentiate living from nonliving things. These
Figure 1.1 Newly discovered species. Each of the thousands of species discovered every year is a reminder that we do not yet know all of the organ- isms living on our own planet. We don’t even know how many to look for. Information about the 1.8 million species we do know about is being collected in The Encyclopedia of Life, an online database maintained by collaborative effort (www.eol.org). (A) Tim Laman/National Geographic Stock; (B) Courtesy East Carolina University.
Application
A. Paul oliver discovered this tree frog perched on a sack of rice during a rainy campsite lunch in New Guinea’s Foja Mountains. The explorers dubbed the new species “Pinocchio frog” after the Disney character because the male frog’s long nose inflates and points upward during times of excitement.
B. Dr. Jason Bond holds a new species of trapdoor spider he discovered in sand dunes of california beaches in 2008. Bond named the spider Aptostichus stephencol- berti, after TV personality Stephen colbert.
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Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.
INVITATIoN To BIoLoGy ChApter 1 5
properties often emerge from the interactions of basic components. To understand how that works, take a look at these groups of squares:
A property called “roundness” emerges when the squares are organized one way, but not other ways. The idea that different structures can be assembled from the same basic building blocks is a recurring theme in our world, and also in biology.
Life has successive levels of organization, with new properties emerging at each level (Figure 1.2). This organization begins with interactions between atoms, which are fundamental units of matter—the building blocks of all substances
1
. Atoms bond together to form molecules
2
. There are no atoms unique to living things, but there are unique molecules. In today’s natural world, only living things make the “molecules of life,” which are lipids, proteins, DNA, RNA, and complex carbohydrates. The emergent property of “life” appears at the next level, when many molecules of life become organized as a cell
3
. A cell is the smallest unit of life. Cells survive and reproduce themselves using energy, raw materials, and information in their DNA.
Some cells live and reproduce independently; others do so as part of a mul- ticelled organism
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. An organism is an individual that consists of one or more cells. In most multicelled organisms, cells are organized as tissues, organs, and organ systems that interact to keep the body working properly.
A population is a group of interbreeding individuals of the same type, or spe- cies, living in a given area
5
. At the next level, a community consists of all popula- tions living in a given area
6
. Communities may be large or small, depending on the area defined.
The next level of organization is the ecosystem, which is a community inter- acting with its physical and chemical environment
7
. The most inclusive level, the biosphere, encompasses all regions of Earth’s crust, waters, and atmosphere in which organisms live
8
.
Figure 1.2 Levels of organization in nature.
1
Atoms are fundamental units of matter.
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Molecules consist of atoms.
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cells consist of molecules.
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organisms consist of cells.
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Populations consist of organisms.
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communities consist of populations.
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Ecosystems consist of communities interacting with their environment.
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The biosphere consists of all ecosystems on Earth.
Take-Home Message 1.2 how do living things differ from nonliving things?
• All things, living or not, consist of the same building blocks: atoms. Atoms bond together to form molecules.
• In today’s natural world, only living things make lipids, proteins, DNA, rNA, and com- plex carbohydrates. The unique properties of life emerge as these molecules become organized into cells.
• Higher levels of life’s organization include multicelled organisms, populations, com- munities, ecosystems, and the biosphere.
atom Fundamental building block of all matter.
biology The scientific study of life.
biosphere All regions of Earth where organisms live.
cell Smallest unit of life.
community All populations of all species in a given area.
ecosystem A community interacting with its environment.
molecule Two or more atoms bonded together.
organism Individual that consists of one or more cells.
population Group of interbreeding individuals of the same species that live in a given area.
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6 INTroDucTIoN
1.3 How Living Things Are Alike Even though we cannot precisely define “life,” we can intuitively understand what it means because all living things share a particular set of key features. All require ongoing inputs of energy and raw materials; all sense and respond to change; and all pass DNA to offspring.
Organisms Require Energy and Nutrients Not all living things eat, but all require energy and nutrients on an ongoing basis. Inputs of both are essential to maintain the functioning of individual organisms and the organization of life in general. A nutrient is a substance that an organism needs for growth and survival but cannot make for itself.
Organisms spend a lot of time acquiring energy and nutrients (Figure 1.3). However, the source of energy and the type of nutrients acquired differ among organisms. These differences allow us to classify living things into two catego- ries: producers and consumers. A producer makes its own food using energy and simple raw materials it obtains from nonbiological sources. Plants are producers; by a process called photosynthesis, they use the energy of sunlight to make sugars from water and carbon dioxide (a gas in air). Consumers, by contrast, cannot make their own food. A consumer obtains energy and nutrients by feeding on other organisms. Animals are consumers. So are decomposers, which feed on the wastes or remains of other organisms. The leftovers from consumers’ meals end up in the environment, where they serve as nutrients for producers. Said another way, nutri- ents cycle between producers and consumers.
Unlike nutrients, energy is not cycled. It flows through the world of life in one direction: from the environment, through organisms, and back to the environ- ment. This flow maintains the organization of every living cell and body, and it also influences how individuals interact with one another and their environment. The energy flow is one-way, because with each transfer, some energy escapes as heat, and cells cannot use heat as an energy source. Thus, energy that enters the world of life eventually leaves it (we return to this topic in Chapter 5).
Organisms Sense and Respond to Change An organism cannot survive for very long in a changing environment unless it adapts to the changes. Thus, every living thing has the ability to sense and respond to change both inside and outside of itself (Figure 1.4). Consider how, after you eat, the sugars from your meal enter your bloodstream. The added sugars set in motion a series of events that causes cells throughout the body to take up sugar faster, so the sugar level in your blood quickly falls. This response keeps your blood sugar level within a certain range, which in turn helps keep your cells alive and your body functioning properly.
All of the fluids outside of cells make up a body’s internal environment. That environment must be kept within certain ranges of temperature and other con- ditions, or the cells that make up the body will die. By sensing and adjusting to change, organisms keep conditions in the internal environment within a range that favors survival. Homeostasis is the name for this process, and it is one of the defin- ing features of life.
Organisms Grow and Reproduce With little variation, the same types of mol- ecules perform the same basic functions in every organism. For example, informa- tion in an organism’s DNA (deoxyribonucleic acid) guides ongoing functions that sustain the individual through its lifetime. Such functions include development:
Figure 1.3 the one-way flow of energy and the cycling of materials in the world of life. Top, © Victoria Pinder, www.flickr.com/photos/vixstarplus.
P R O D U C E R S plants and other self-feeding organisms
E N E R G Y I N S U N L I G H T
C O N S U M E R S animals, most fungi, many protists, bacteria
Producers harvest energy from the environment. Some of that energy flows from producers to consumers.
Nutrients that get incorporated into the cells
of producers and consumers are eventually released back into the environment (by decomposi-
tion, for example). Producers then take up some of the
released nutrients.
All energy that enters the world of life eventually flows out of it, mainly as heat released back to the environment.
consumer acquiring energy and nutrients by eating a producer
producer acquiring energy and nutrients from its environment
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INVITATIoN To BIoLoGy ChApter 1 7
DNA
Figure 1.4 Organisms sense and respond to stimulation. This baby orangutan is laughing in response to being tickled. Apes and humans make different sounds when being tickled, but the airflow patterns are so similar that we can say apes really do laugh. © Dr. Marina Davila Ross, University of Portsmouth.
consumer organism that gets energy and nutrients by feeding on the tissues, wastes, or remains of other organisms.
development Multistep process by which the first cell of a new multicelled organism gives rise to an adult.
DNA Deoxyribonucleic acid; carries hereditary infor- mation that guides development and other activities.
growth In multicelled species, an increase in the number, size, and volume of cells.
homeostasis Process in which an organism keeps its internal conditions within tolerable ranges by sensing and responding to change.
inheritance Transmission of DNA to offspring.
nutrient Substance that an organism needs for growth and survival but cannot make for itself.
photosynthesis Process by which a producer uses light energy to make sugars from carbon dioxide and water.
producer organism that makes its own food using energy and nonbiological raw materials from the environment.
reproduction Process by which parents produce offspring.
the process by which the first cell of a new individual becomes a multicelled adult; growth: increases in cell number, size, and volume; and reproduction: processes by which individuals produce offspring.
Individuals of every natural population are alike in certain aspects of their body form and behavior because their DNA is very similar: Orangutans look like orangutans and not like caterpillars because they inherited orangutan DNA, which differs from caterpillar DNA in the information it carries. Inheritance refers to the transmission of DNA to offspring. All organisms receive their DNA from one or more parents.
DNA is the basis of similarities in form and function among organisms. How- ever, the details of DNA molecules differ, and herein lies the source of life’s diversity. Small variations in the details of DNA’s structure give rise to differences among indi- viduals, and also among types of organisms. As you will see in later chapters, these differences are the raw material of evolutionary processes.
Take-Home Message 1.3 how are all living things alike?
• A one-way flow of energy and a cycling of nutrients sustain life’s organization. • organisms sense and respond to conditions inside and outside themselves. They
make adjustments that keep conditions in their internal environment within a range that favors cell survival, a process called homeostasis.
• All organisms use information in the DNA they inherited from their parent or parents to develop, grow, and reproduce. DNA is the basis of similarities and differences in form and function among organisms.
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Figure 1.5 A few representative prokaryotes. (A) top left, Dr. Richard Frankel; top right, Science Source; bottom left, www.zahnarzt-stuttgart .com; bottom right, © Susan Barnes; (B) left, Dr. Terry Beveridge, Visuals Unlimited/Corbis; right, © Dr. Harald Huber, Dr. Michael Hohn, Prof. Dr. K.O. Stetter, University of Regensburg, Germany.
A. Bacteria are the most numerous organisms on Earth. clockwise from upper left, a bacterium with a row of iron crystals that acts like a tiny compass; a common resident of cat and dog stomachs; spiral cyanobacteria; types found in dental plaque.
B. Archaea may resemble bacteria, but they are more closely related to eukaryotes. These are two types of archaea from a hydrothermal vent on the seafloor.
1.4 How Living Things Differ Living things differ tremendously in their observable characteristics. Various clas- sification schemes help us organize what we understand about the scope of this variation, which we call Earth’s biodiversity.
For example, organisms can be grouped on the basis of whether they have a nucleus, which is a saclike structure containing a cell’s DNA. Bacteria (singular, bacterium) and archaea (singular, archaeon) are organisms whose DNA is not contained within a nucleus. All bacteria and archaea are single-celled, which means each organism consists of one cell (Figure 1.5). Collectively, these organisms are the most diverse representatives of life. Different kinds are producers or consumers in nearly all regions of Earth. Some inhabit such extreme environments as frozen des- ert rocks, boiling sulfurous lakes, and nuclear reactor waste. The first cells on Earth may have faced similarly hostile conditions.
Traditionally, organisms without a nucleus have been called prokaryotes, but the designation is now used only informally. This is because, despite the similar appearance of bacteria and archaea, the two types of cells are less related to one another than we once thought. Archaea turned out to be more closely related to eukaryotes, which are organisms whose DNA is contained within a nucleus. Some eukaryotes live as individual cells; others are multicelled (Figure 1.6). Eukaryotic cells are typically larger and more complex than bacteria or archaea.
Protists are the simplest eukaryotes, but as a group they vary dramatically, from single-celled consumers to giant, multicelled producers.
Fungi (singular, fungus) are eukaryotic consumers that secrete substances to break down food externally, then absorb nutrients released by this process. Many fungi are decomposers. Most fungi, including those that form mushrooms, are mul- ticellular. Fungi that live as single cells are called yeasts.
Plants are multicelled eukaryotes, and the vast majority of them are photosyn- thetic producers that live on land. Besides feeding themselves, plants also serve as food for most other land-based organisms.
Animals are multicelled eukaryotic consumers that ingest tissues or juices of other organisms. Unlike fungi, animals break down food inside their body. They also develop through a series of stages that lead to the adult form. All animals actively move about during at least part of their lives.
What Is a Species? Each time we discover a new species, or unique kind of organism, we name it. Taxonomy, the practice of naming and classifying spe- cies, began thousands of years ago, but naming species in a consistent way did not become a priority until the eighteenth century. At the time, European explor- ers who were just discovering the scope of life’s diversity started having more and more trouble communicating with one another because species often had multiple names. For example, the dog rose (a plant native to Europe, Africa, and Asia) was alternately known as briar rose, witch’s briar, herb patience, sweet briar, wild briar, dog briar, dog berry, briar hip, eglantine gall, hep tree, hip fruit, hip rose, hip tree, hop fruit, and hogseed—and those are only the English names! Species often had multiple scientific names too, in Latin that was descriptive but often cumbersome. The scientific name of the dog rose was Rosa sylvestris inodora seu canina (odorless woodland dog rose), and also Rosa sylvestris alba cum rubore, folio glabro (pinkish white woodland rose with smooth leaves).
An eighteenth-century naturalist, Carolus Linnaeus, standardized a two-part naming system that we still use. By the Linnaean system, every species is given a
animal Multicelled consumer that develops through a series of stages and moves about during part or all of its life.
archaea Group of single-celled organisms that lack a nucleus but are more closely related to eukaryotes than to bacteria.
bacteria The most diverse and well-known group of single-celled organisms that lack a nucleus.
biodiversity Scope of variation among living organisms.
eukaryote organism whose cells characteristically have a nucleus.
fungus Single-celled or multicelled eukaryotic con- sumer that breaks down material outside itself, then absorbs nutrients released from the breakdown.
plant A multicelled, typically photosynthetic producer.
prokaryote Single-celled organism with no nucleus.
protists A group of diverse, simple eukaryotes.
species unique type of organism.
taxonomy Practice of naming and classifying species.
8
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INVITATIoN To BIoLoGy ChApter 1 9
Animals are multicelled con- sumers that ingest tissues or juices of other organisms. All actively move about during at least part of their life.
Fungi are eukaryotic consumers that secrete substances to break down food outside their body. Most are multicelled (left), but some are single-celled (above).
plants are multicelled eukaryotes. Almost all plants are photosynthetic producers, and most of them have roots, stems, and leaves.
protists are a group of extremely diverse eukary- otes that range from giant multicelled seaweeds to microscopic single cells.
Figure 1.6 A few representative eukaryotes. Protists: from left, © worldswildlifewonders/Shutterstock.com; top middle, Courtesy of Allen W. H. Bé and David A. Caron; bottom middle, © Emiliania Huxleyi photograph, Vita Pariente, scanning electron micrograph taken on a Jeol T330A instrument at Texas A&M University Electron Microscopy Center; top right, M I Walker/Science Source; middle right, © Carolina Biological Supply Company; bottom right, Oliver Meckes/Science Source; Plants: left, © Jag.ca.Shutterstock.com; right, © Martin Ruegner/Radius Images/Getty Images; Fungi, left, Edward S. Ross; right, London Scientific Films/Oxford Scientific/Getty Images; Animals: left, Shironina/Shutterstock.com; middle, © Martin Zimmerman, Science, 1961, 133:73–79, © AAAS; right, © Pixtal/SuperStock.
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10
unique two-part scientific name. The first part of a scientific name is the genus (plural, genera), a group of species that share a unique set of features. The second part is the specific epithet. Together, the genus name and the specific epithet desig- nate one species. Thus, the dog rose now has one official name, Rosa canina, that is recognized worldwide.
Genus and species names are always italicized. For example, Panthera is a genus of big cats. Lions belong to the species Panthera leo. Tigers belong to a different species in the same genus (Panthera tigris), and so do leopards (P. pardus). Note how the genus name may be abbreviated after it has been spelled out once.
A Rose by Any Other Name The individuals of a species share a unique set of inherited traits. For example, giraffes normally have very long necks, brown spots on white coats, and so on. These are morphological (structural) traits. Individuals of a species also share biochemical traits (they make and use the same molecules) and behavioral traits (they respond the same way to certain stimuli, as when hungry giraffes feed on tree leaves). We can rank species into ever more inclusive catego- ries based on some subset of traits it shares with other species. Each rank, or taxon (plural, taxa), is a group of organisms that share a unique set of traits. Each category above species—genus, family, order, class, phylum (plural, phyla), kingdom, and domain—consists of a group of the next lower taxon (Figure 1.7). Using this system, we can sort all life into a few categories (Figure 1.8).
It is easy to tell that orangutans and caterpillars are different species because they appear very different. Distinguishing between species that are more closely related may be much more challenging (Figure 1.9). In addition, traits shared by members of a species often vary a bit among individuals, as eye color does among
Figure 1.7 taxonomic classification of five species that are related at different levels. Each species has been assigned to ever more inclusive groups, or taxa: in this case, from genus to domain. From the left, Joaquim Gaspar; © kymkemp.com; Sylvie Bouchard/Shutterstock.com; Courtesy of Melissa S. Green, www.flickr.com/photos/henkimaa; © Grodana Sarkotic.
Answer: Marijuana, apple, prickly rose, and dog roseFigure It Out: Which of the plants shown here are in the same order?
domain kingdom phylum
class order
family genus
species
A “species” is a convenient but artificial construct of the human mind.
genus A group of species that share a unique set of traits.
taxon Group of organisms that share a unique set of traits.
Eukarya Plantae Magnoliophyta Magnoliopsida Apiales Apiaceae Daucus carota
wild carrot Eukarya Plantae Magnoliophyta Magnoliopsida rosales rosaceae Malus domestica
apple Eukarya Plantae Magnoliophyta Magnoliopsida rosales rosaceae Rosa acicularis
prickly rose Eukarya Plantae Magnoliophyta Magnoliopsida rosales rosaceae Rosa canina
dog rose Eukarya Plantae Magnoliophyta Magnoliopsida rosales cannabaceae Cannabis sativa
marijuana
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INVITATIoN To BIoLoGy ChApter 1 11
Bacteria Archaea Eukarya
B. Three-domain classification system. The Eukarya domain includes protists, plants, fungi, and animals.
Figure 1.9 Four butterflies, two species: Which are which? The top row shows two forms of the species Heliconius melpomene; the bottom row, two forms of H. erato.
H. melpomene and H. erato never cross-breed. Their alternate but similar patterns of coloration evolved as a shared warning signal to predatory birds that these but- terflies taste terrible. © 2006 Axel Meyer, “Repeating Patterns of Mimicry.” PLoS Biology Vol. 4, No. 10, e341 doi:10.1371/journal.pbio.0040341. Used with Permission.
Figure 1.8 two little ways to see the big picture of life. Lines in such diagrams indicate evolutionary connections.
people. How do we decide whether similar-looking organisms belong to the same species? The short answer to that question is that we rely on whatever information we have. Early naturalists studied anatomy and distribution—essentially the only methods available at the time—so species were named and classified according to what they looked like and where they lived. Today’s biologists are able to compare traits that the early naturalists did not even know about, including biochemical ones.
The discovery of new information sometimes changes the way we distinguish a particular species or how we group it with others. For example, Linnaeus grouped plants by the number and arrangement of reproductive parts, a scheme that resulted in odd pairings such as castor-oil plants with pine trees. Having more information today, we place these plants in separate phyla.
Evolutionary biologist Ernst Mayr defined a species as one or more groups of individuals that potentially can interbreed, produce fertile offspring, and do not interbreed with other groups. This “biological species concept” is useful in many cases, but it is not universally applicable. For example, we may never know whether two widely separated populations could interbreed if they got together. As another example, populations often continue to interbreed even as they diverge, so the exact moment at which two populations become two species is often impossible to pinpoint. We return to speciation and how it occurs in Chapter 12, but for now it is important to remember that a “species” is a convenient but artificial construct of the human mind.
1.5 The Science of Nature Most of us assume that we do our own thinking, but do we, really? You might be surprised to find out how often we let others think for us. Consider how a school’s job (which is to impart as much information to students as quickly as possible)
Take-Home Message 1.4 how do organisms differ from one another?
• organisms differ in their details; they show tremendous variation in observable characteristics.
• We divide Earth’s biodiversity into broad groups based on traits such as having a nucleus or being multicellular.
• Each species is given a unique, two-part scientific name. • classification systems group species on the basis of shared traits.
Bacteria Archaea FungiPlants AnimalsProtists
A. Six-kingdom classification system. The protist kingdom includes the most ancient multicelled and all single-celled eukaryotes.
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12
How do my own biases affect what I’m learning?
© JupiterImages Corporation.
meshes perfectly with a student’s job (which is to acquire as much knowledge as quickly as possible). In this rapid-fire exchange of information, it can be very easy to forget about the quality of what is being exchanged. Any time you accept informa- tion without questioning it, you let someone else think for you.
Thinking About Thinking Critical thinking is the deliberate process of judg- ing the quality of information before accepting it. “Critical” comes from the Greek kriticos (discerning judgment). When you use critical thinking, you move beyond the content of new information to consider supporting evidence, bias, and alterna- tive interpretations. How does the busy student manage this? Critical thinking does not necessarily require extra time, just a bit of extra awareness. There are many ways to do it. For example, you might ask yourself some of the following questions while you are learning something new:
What message am I being asked to accept? Is the message based on facts or opinion? Is there a different way to interpret the facts? What biases might the presenter have? How do my own biases affect what I’m learning?
Such questions are a way of being conscious about learning. They can help you decide whether to allow new information to guide your beliefs and actions.
How Science Works Critical thinking is a big part of science, the systematic study of the observable world and how it works. A scientific line of inquiry usually begins with curiosity about something observable, such as (for example) a decrease in the number of birds in a particular area. Typically, a scientist will read about what others have discovered before making a hypothesis, a testable explanation for a natural phenomenon. An example of a hypothesis would be, “The number of birds is decreasing because the number of cats is increasing.”
A prediction, or statement of some condition that should exist if the hypoth- esis is correct, comes next. Making predictions is often called the if–then process, in which the “if ” part is the hypothesis, and the “then” part is the prediction: If the number of birds is decreasing because the number of cats is increasing, then reduc- ing the number of cats should stop the decline.
Next, a researcher will test the prediction. Tests may be performed on a model, or analogous system, if working with an object or event directly is not possible. For
A. Studying the ecological benefits of weedy buffer zones on farms.
B. Measuring how much wood is produced by extremely old trees.
control group Group of individuals identical to an experimental group except for the independent vari- able under investigation.
critical thinking Evaluating information before accepting it.
data Experimental results.
experiment A test designed to support or falsify a prediction.
experimental group In an experiment, a group of individuals who have a certain characteristic or receive a certain treatment.
hypothesis Testable explanation of a natural phenomenon.
model Analogous system used for testing hypotheses.
prediction Statement, based on a hypothesis, about a condition that should exist if the hypothesis is correct.
science Systematic study of the observable world.
scientific method Making, testing, and evaluating hypotheses.
variable In an experiment, a characteristic or event that differs among individuals or over time.
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INVITATIoN To BIoLoGy ChApter 1 13
example, animal diseases are often used as models of similar human diseases. Care- ful observations are one way to test predictions that flow from a hypothesis. So are experiments: tests designed to support or falsify a prediction. A typical experiment explores a cause-and-effect relationship using variables, which are characteristics or events that can differ among individuals or over time.
Biological systems are typically complex, with many interdependent variables. It can be difficult to study one variable separately from the rest. Thus, biology researchers often test two groups of individuals simultaneously. An experimental group is a set of individuals that have a certain characteristic or receive a certain treatment. An experimental group is tested side by side with a control group, which is identical to the experimental group except for one independent variable: the characteristic or the treatment being tested. Any differences in experimental results between the two groups is likely to be an effect of changing the variable. Test results—data—that are consistent with the prediction are evidence in support of the hypothesis. Data inconsistent with the prediction are evidence that the hypothesis is flawed and should be revised.
A necessary part of science is reporting one’s results and conclusions in a stan- dard way, such as in a peer-reviewed journal article. The communication gives other scientists an opportunity to evaluate the information for themselves, both by check- ing the conclusions drawn and by repeating the experiments. Forming a hypothesis based on observation, and then systematically testing and evaluating the hypothesis, are collectively called the scientific method (Table 1.1).
Examples of Experiments in Biology There are many different ways to do research, particularly in biology (Figure 1.10). Some biologists survey, simply observing without making or testing hypotheses. Others make hypotheses based on observations, and leave the testing to others. However, despite a broad range of approaches, scientific experiments are typically designed in a consistent way, so the effects of changing one variable at a time can be measured. To give you a sense of how biology experiments work, we summarize two published studies here.
In 1996 the U.S. Food and Drug Administration (FDA) approved Olestra®, a fat replacement manufactured from sugar and vegetable oil, as a food additive. Potato chips were the first Olestra-containing food product to be sold in the United States. Controversy about the chip additive soon raged. Many people complained of intes- tinal problems after eating the chips, and thought that the Olestra was at fault. Two
table 1.1 the Scientific Method
observe some aspect of nature.
Think of an explanation for your observation (in other words, form a hypothesis).
Test the hypothesis. a. Make a prediction based on the hypothesis. b. Test the prediction using experiments or
surveys. c. Analyze the results of the tests (data).
Decide whether the results of the tests support your hypothesis or not (form a conclusion).
report your results to the scientific community.
Figure 1.10 A few examples of scientific research in the field of biology. (A) Photo by Scott Bauer, USDA/ARS; (B) MICHAEL NICHOLS/National Geographic Creative; (C) © Roger W. Winstead, NC State University; (D) National Cancer Institute; (E) Courtesy of Susanna López-Legentil.
e. Discovering medically active natural products made by marine animals.
C. Improving efficiency of biofuel production from agricultural waste.
D. Devising a vaccine that helps prevent cancer.
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14 INTroDucTIoN
years later, researchers at the Johns Hopkins University School of Medicine designed an experiment to test whether Olestra causes cramps. The researchers made the fol- lowing prediction: if Olestra causes cramps, then people who eat Olestra should be more likely to get cramps than people who do not eat it. To test the prediction, they used a Chicago theater as a “laboratory.” They asked 1,100 people between the ages of thirteen and thirty-eight to watch a movie and eat their fill of potato chips. Each person received an unmarked bag containing 13 ounces of chips. In this experiment, the individuals who received Olestra-laden potato chips were the experimental group, and the individuals who received regular chips were the control group.
A few days after the movie, the researchers contacted all of the people who participated in the experiment and collected any reports of post-movie gastrointes- tinal problems. Of 563 people making up the experimental group, 89 (15.8 percent) reported having cramps. However, so did 93 of the 529 people (17.6 percent) mak- ing up the control group—who had eaten the regular chips. People were about as likely to get cramps whether or not they ate chips made with Olestra. These results did not support the prediction, so the researchers concluded that eating Olestra does not cause cramps (Figure 1.11).
A different experiment that took place in 2005 investigated whether certain behaviors of peacock butterflies help the insects avoid predation by birds. The researchers performing this experiment began with two observations. First, when a peacock butterfly rests, it folds its wings, so only the dark underside shows (Fig- ure 1.12A). Second, when a butterfly sees a predator approaching, it repeatedly flicks its wings open, while also moving them in a way that produces a hissing sound and a series of clicks (Figure 1.12B).
The researchers were curious about why the peacock butterfly flicks its wings. After they reviewed earlier studies, they came up with two hypotheses that might explain the wing-flicking behavior.
Figure 1.11 the steps in a scientific experiment to determine whether Olestra causes intestinal cramps. A report of this study was published in the Journal of the American Medical Association in January 1998. Left, © Bob Jacobson/Corbis; background right, © SuperStock.
Eats regular potato chips
Eats Olestra potato chips
Olestra® causes intestinal cramps.
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.
89 of 563 people get cramps later (15.8%)
93 of 529 people get cramps later (17.6%)
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.
Control Group Experimental Group
Hypothesis
Prediction
Experiment
Results
Conclusion
A
B
C
D
E
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INVITATIoN To BIoLoGy ChApter 1 15
1. Wing-flicking probably attracts predatory birds, but it also exposes brilliant spots that resemble owl eyes. Anything that looks like owl eyes is known to startle small, butterfly-eating birds, so exposing the wing spots might scare off predators.
2. The hissing and clicking sounds produced when the peacock butterfly moves its wings may be an additional defense that deters predatory birds.
The researchers then used their hypotheses to make the following predictions:
1. If exposing brilliant wing spots startles butterfly-eating birds, then peacock but- terflies missing their spots will be more likely to get eaten.
2. If hissing and clicking sounds deter birds butterfly-eating birds, then peacock butterflies unable to make these sounds will be more likely to get eaten.
The next step was the experiment. The researchers used a black marker to cover up the wing spots of some butterflies, and scissors to cut off the sound-making part of the wings of others. A third group had both treatments, their wings painted and also cut. The researchers then put each butterfly into a large cage with a hungry blue tit (Figure 1.12C) and watched the pair for thirty minutes.
Figure 1.12D lists the results of the experiment. All 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
B. When a predatory bird approaches, a butterfly flicks its wings open and closed, reveal- ing brilliant spots and producing hissing and clicking sounds.
A. With wings folded, a resting peacock butterfly resembles a dead leaf, so it is appropriately camou- flaged from predatory birds.
C. researchers tested whether the wing-flicking behavior of peacock but- terflies affected predation by blue tits.
D. The researchers painted out the spots of some butterflies, cut the sound-making part of the wings on others, and did both to a third group; then exposed each butterfly to a hungry blue tit for 30 minutes. results are listed on the right.
experimental treatment
Number of Butterflies eaten (of total)
Spots painted out 5 of 10
Wings cut 0 of 8
Spots painted, wings cut 8 of 10
None 0 of 9
Figure 1.12 testing peacock butterfly defenses. (A) © Matt Rowlings, www.eurobutterflies.com; (B) © Adrian Vallin; (C) © Antje Schulte; (D) Proceedings of the Royal Society of London, Series B (2005) 272: 1203–1207.
Answer: 20 percent
Figure It Out: What percentage of butterflies with spots painted and wings cut survived the test?
Digging Into Data peacock Butterfly predator Defenses The photographs below represent the experimental and control groups used in the peacock butterfly experiment. Identify the experimental groups, and match them up with the relevant control group(s). Hint: Identify which variable is being tested in each group (each variable has a control). Adrian Vallin, Sven Jakobsson, Johan Lind and Christer Wiklund, Proc. R. Soc. B (2005: 272, 1203, 1207). Used with permission of The Royal Society and the author.
A. Wing spots painted out
B. Wing spots vis- ible; wings silenced
C. Wing spots painted out; wings silenced
D. Wings painted but spots visible
e. Wings cut but not silenced
F. Wings painted, spots visible; wings cut, not silenced
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16 INTroDucTIoN
of the silenced butterflies with painted-out spots were eaten quickly. The test results confirmed both predictions, so they support the hypotheses. Predatory birds are indeed deterred by peacock butterfly wing-flicking behavior.
1.6 The Nature of Science Bias in Interpreting Experimental Results Experimenting with a single vari- able apart from all others is not often possible, particularly when studying humans. For example, remember that the people who participated in the Olestra experiment were chosen randomly, which means the study was not controlled for gender, age, weight, medications taken, and so on. These variables may well have influenced the experiment’s results.
Humans are by nature subjective, and scientists are no exception. Research- ers risk interpreting their results in terms of what they want to find out. That is
Take-Home Message 1.5 how does science work?
• The scientific method consists of making, testing, and evaluating hypotheses. It is one way of critical thinking—systematically judging the quality of information before allowing it to guide one’s beliefs and actions.
• Natural processes are often very complex and influenced by many interacting variables.
• Experiments help researchers unravel causes of complex natural processes by focus- ing on the effects of changing a single variable.
Figure 1.13 example of how generalizing from a subset can lead to a conclusion that is incorrect. (A) Tim Laman/ National Geographic Stock; (B) © Bruce Beehler/ Conservation International.
B. In science, discov- ery of an error is not always bad news. Kris Helgen holds a golden- mantled tree kangaroo he found during the 2005 Foja Mountains survey. This kangaroo species is extremely rare in other areas, so it was thought to be criti- cally endangered prior to the expedition.
A. The cloud forest that covers about 2 million acres of New Guinea’s Foja Mountains is extremely remote and difficult to access, even for natives of the region. The first major survey of this forest occurred in 2005.
The scientific community consists of critically thinking people trying to poke holes in one another’s ideas.
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INVITATIoN To BIoLoGy ChApter 1 17
why they typically design experiments that will yield quantitative results, which are counts or some other data that can be measured or gathered objectively. Quantita- tive results minimize the potential for bias, and also give other scientists an oppor- tunity to repeat the experiments and check the conclusions drawn from them. This last point gets us back to the role of critical thinking in science. Scientists expect one another to recognize and put aside bias in order to test hypotheses in ways that may prove them wrong. If a scientist does not, then others will, because exposing errors is just as useful as applauding insights. The scientific community consists of critically thinking people trying to poke holes in one another’s ideas. Ideally, their collective efforts make science a self-correcting endeavor.
Sampling Error Researchers cannot always observe all individuals of a group. For example, the explorers you read about in Section 1.1 did not—and could not— survey every uninhabited part of the Foja Mountains. The cloud forest alone cloaks more than 2 million acres (Figure 1.13A), so surveying all of it would take unrealis- tic amounts of time and effort.
When researchers cannot directly observe all individuals of a population, all instances of an event, or some other aspect of nature, they may test or survey a subset. Results from the subset are then used to make generalizations about the whole. However, generalizing from a subset is risky because subsets are not neces- sarily representative of the whole. Consider the golden-mantled tree kangaroo, an animal first discovered in 1993 on a single forested mountaintop in New Guinea. For more than a decade, the species was never seen outside of that habitat, which is getting smaller every year because of human activities. Thus, the golden-mantled tree kangaroo was considered to be one of the most endangered animals on the planet. Then, in 2005, the New Guinea explorers discovered that this kangaroo spe- cies is fairly common in the Foja Mountain cloud forest (Figure 1.13B). As a result, biologists now believe its future is secure, at least for the moment.
Sampling error is a difference between results obtained from a subset, and results from the whole (Figure 1.14A). Sampling error may be unavoidable, but knowing how it can occur helps researchers design their experiments to minimize it. For example, sampling error can be a substantial problem with a small subset, so experimenters try to start with a relatively large sample, and they repeat their experiments (Figure 1.14B). To understand why these practices reduce the risk of sampling error, think about flipping a coin. There are two possible outcomes of each flip: The coin lands heads up, or it lands tails up. Thus, the chance that the coin will land heads up is one in two (1/2), or 50 percent. However, when you flip a coin repeatedly, it often lands heads up, or tails up, several times in a row. With just 3 flips, the proportion of times that the coin actually lands heads up may not even be close to 50 percent. With 1,000 flips, however, the overall proportion of times the coin lands heads up is much more likely to approach 50 percent.
Probability is the measure, expressed as a percentage, of the chance that a particular outcome will occur. That chance depends on the total number of pos- sible outcomes. For instance, if 10 million people enter a drawing, each has the same probability of winning: 1 in 10 million, or (an extremely improbable) 0.00001 per- cent. Analysis of experimental data often includes probability calculations. If there is a very low probability that a result has occurred by chance alone, the result is said to be statistically significant. In this context, the word “significant” does not refer to the result’s importance. Rather, it means that a rigorous statistical analysis has shown a very low probability (usually 5 percent or less) of the result being incorrect because of sampling error.
A. Natalie chooses a random jelly bean from a jar. She is blindfolded, so she does not know that the jar contains 120 green and 280 black jelly beans.
The jar is hidden from Natalie’s view before she removes her blindfold. She sees one green jelly bean in her hand and assumes that the jar must hold only green jelly beans. This assumption is incorrect: 30 percent of the jelly beans in the jar are green, and 70 percent are black. The small sample size has resulted in sampling error.
B. Still blindfolded, Natalie randomly chooses 50 jelly beans from the jar. She ends up choosing 10 green and 40 black beans.
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 larger 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 her chance of guessing the actual ratio.
Figure 1.14 how sample size affects sampling error. © Gary Head.
probability The chance that a particular outcome of an event will occur; depends on the total number of outcomes possible.
sampling error Difference between results derived from testing an entire group of events or individuals, and results derived from testing a subset of the group.
statistically significant refers to a result that is sta- tistically unlikely to have occurred by chance alone.
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18 INTroDucTIoN
0
4
8
12
16
20
24
W in
g �i
ck s
p er
m in
ut e
– spots + sound
– spots – sound
+ spots – sound
Variation in data is often shown as error bars on a graph (Figure 1.15). Depend- ing on the graph, error bars may indicate variation around an average for one sample set, or the difference between two sample sets.
Scientific Theories Suppose a hypothesis stands even after years of tests. It is consistent with all data ever gathered, and it has helped us make successful predic- tions about other phenomena. When a hypothesis meets these criteria, it is consid- ered to be a scientific theory (Table 1.2). To give an example, all observations to date have been consistent with the hypothesis that matter consists of atoms. Scien- tists no longer spend time testing this hypothesis for the compelling reason that, since we started looking 200 years ago, no one has discovered matter that consists of anything else. Thus, scientists use the hypothesis, now called atomic theory, to make other hypotheses about matter.
Scientific theories are our best objective descriptions of the natural world. How- ever, they can never be proven absolutely, because to do so would necessitate testing under every possible circumstance. For example, in order to prove atomic theory, the atomic composition of all matter in the universe would have to be checked—an impossible task even if someone wanted to try.
Like all hypotheses, a scientific theory can be disproven by a single observa- tion or result that is inconsistent with it. For example, if someone discovers a form of matter that does not consist of atoms, atomic theory would have to be revised. The potentially falsifiable nature of scientific theories means that science has a built-in system of checks and balances. A theory is revised until no one can prove it to be incorrect. The theory of evolution, which states that change occurs in a line of descent over time, still holds after a century of observations and testing. As with all other scientific theories, no one can be absolutely sure that it will hold under all possible conditions, but it has a very high probability of not being wrong. Few other theories have withstood as much scrutiny.
You may hear people apply the word “theory” to a speculative idea, as in the phrase “It’s just a theory.” This everyday usage of the word differs from the way it is used in science. Speculation is an opinion, belief, or personal conviction that is not necessarily supported by evidence. A scientific theory is different. By definition, a scientific theory is supported by a large body of evidence, and it is consistent with all known data.
A scientific theory also differs from a law of nature, which describes a phe- nomenon that has been observed to occur in every circumstance without fail, but for which we do not have a complete scientific explanation. The laws of
Science helps us communicate our experiences without bias.
Figure 1.15 example of error bars in a graph. This graph was adapted from the peacock butterfly research described in Section 1.5.
The researchers recorded the number of times each butterfly flicked its wings in response to an attack by a bird.
The squares represent average frequency of wing flicking for each sample set of butterflies. The error bars that extend above and below the dots indicate the range of values—the sampling error.
Answer: 22 times per minute
Figure It Out: What was the fastest rate at which a butterfly with no spots or sound flicked its wings?
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INVITATIoN To BIoLoGy ChApter 1 19
thermo dynamics, which describe energy, are examples. We understand how energy behaves, but not exactly why it behaves the way it does.
The Scope of Science Science helps us be objective about our observations in part because of its limitations. For example, science does not address many questions, such as “Why do I exist?” Answers to such questions can only 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, because no human society can function for long unless its individuals share standards for making judgments, even if they are subjective. Moral, aesthetic, and philosophi- cal standards vary from one society to the next, but all help people decide what is important and good. All give meaning to our lives.
Neither does science address the supernatural, or anything that is “beyond nature.” Science neither assumes nor denies that supernatural phenomena occur, but scientists often cause controversy when they discover a natural explanation for something that was thought to have none. Such controversy arises when a society’s moral standards are interwoven with its understanding of nature. Nicolaus Copernicus proposed in 1540 that Earth orbits the sun. Today that idea is generally accepted, but the prevailing belief system had Earth as the immovable center of the universe. In 1610, Galileo Galilei published evidence for the Copernican model of the solar system, an act that resulted in his imprisonment. He was publicly forced to recant his work, spent the rest of his life under house arrest, and was never allowed to publish again.
As Galileo’s story illustrates, exploring a traditional view of the natural world from a scientific perspective is often misinterpreted as a violation of morality. As a group, scientists are no less moral than anyone else, but they follow a particular set of rules that do not necessarily apply to others: Their work concerns only the natu- ral world, and their ideas must be testable by other scientists.
Science helps us communicate our experiences of the natural world without bias. As such, it may be as close as we can get to a universal language. We are fairly sure, for example, that the laws of gravity apply everywhere in the universe. Intelli- gent beings on a distant planet would likely understand the concept of gravity. Thus, we might well use gravity or another scientific concept 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 1.6 Why does science work?
• researchers minimize sampling error by using large sample sizes and by repeating their experiments. Probability calculations can show whether a result is unlikely to have occurred by chance alone.
• Science is concerned only with testable ideas about observable aspects of nature. • Ideally, science is a self-correcting process because it is carried out by a community
of people who systematically check one another’s work and conclusions. • Because a scientific theory is thoroughly tested and revised until no one can prove it
wrong, it is our best way of objectively describing the natural world.
law of nature Generalization that describes a consistent natural phenomenon for which there is incomplete scientific explanation.
scientific theory Hypothesis that has not been disproven after many years of rigorous testing.
theory Main premises
Atomic theory All matter consists of atoms.
Big bang The universe originated with an explosion and continues to expand.
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.
Evolution change occurs in the inher- ited traits of a population over generations.
Global warming Human activities are causing Earth’s average temperature to increase.
Plate tectonics Earth’s crust is cracked into pieces that move in relation to one another.
table 1.2 examples of Scientific theories
© Raymond Gehman/Corbis.
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20
Summary Section 1.1 Biology is the scientific study of life. We know about only a fraction of the organisms that live on Earth, in part because we have explored only a fraction of its inhabited regions.
Section 1.2 Biologists think about life at different levels of organization, with new properties emerging at successively higher levels. all matter consists of atoms, which bond together to form molecules. Organisms are individuals that consist of one or more cells, the organizational level at which life emerges. a population is a group of interbreeding 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.
Section 1.3 life has underlying unity in that all living things have similar characteristics: (1) all organisms require energy and nutrients to sustain themselves. Producers harvest energy from the environment to make their own food by processes such as
photosynthesis; consumers ingest other organisms, or their wastes or remains. (2) organisms keep the conditions in their internal environment within ranges that their cells tolerate—a process called homeostasis. (3) DNA contains information that guides an organism’s growth, development, and reproduction. the passage of Dna from parents to offspring is inheritance.
Section 1.4 the many types of organisms that currently exist on Earth differ greatly in details of body form and function. Biodiversity is the sum of differences among living things. Bacteria and archaea are prokaryotes, single-celled organisms whose Dna
is not contained within a nucleus. the Dna of single-celled or multicelled eukaryotes (protists, plants, fungi, and animals) is contained within a nucleus.
Each species has a two-part name. the first part is the genus name. When combined with the specific epithet, it designates the particular species. With taxonomy, species are ranked into ever more inclusive taxa on the basis of shared traits.
Section 1.5 Critical thinking, the self-directed act of judging the quality of information as one learns, is an important part of science. generally, a researcher observes something in nature, forms a hypothesis (testable explanation) for it, then makes
a prediction about what might occur if the hypothesis is correct. Predictions are tested with observations, experiments, or both.
Experiments typically are performed on an experimental group as compared with a control group, and sometimes on model systems. Conclusions are drawn from data. a hypothesis that is not consistent with data is modified or discarded. the scientific method consists of making, testing, and evaluating hypotheses, and sharing results with the scientific community.
Biological systems are usually influenced by many interacting variables. Research approaches differ, but experiments are designed in a consistent way, in order to study a single cause-and- effect relationship in a complex natural system.
Section 1.6 Small sample size increases the potential for sampling error in experimental results. In such cases, a subset may be tested that is not representative of the whole. Researchers design experiments carefully to minimize sampling error and
bias, and they use probability calculations to check the statistical significance of their results.
Science helps us be objective about our observations because it is concerned only with testable ideas about observable aspects of nature. opinion and belief have value in human culture, but they are not addressed by science. a scientific theory is a long- standing hypothesis that is useful for making predictions about other phenomena. It is our best way of objectively describing nature. a law of nature is a phenomenon that occurs without fail, but has an incomplete scientific explanation.
Answers in Appendix I
1. are fundamental building blocks of all matter. a. Cells c. organisms b. atoms d. Molecules
2. the smallest unit of life is the . a. atom c. cell b. molecule d. organism
3. is the transmission of Dna to offspring. a. Reproduction c. Homeostasis b. Development d. Inheritance
4. a process by which an organism produces offspring is called . a. reproduction c. homeostasis b. development d. inheritance
Self-Quiz
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INVITATIoN To BIoLoGy ChApter 1 21
15. In one survey, fifteen randomly selected students were found to be taller than 6 feet. this data led to the conclusion that the average height of a student is greater than 6 feet. this is an example of . a. experimental error c. a subjective opinion b. sampling error d. experimental bias
1. a person is declared to be dead upon the irreversible ceasing of spontaneous body functions: brain activity, or blood circulation and respiration. However, only about 1% of a person’s cells have to die in order for all of these things to happen. How can some- one be dead when 99% of his or her cells are still alive?
2. Explain the difference between a one-celled organism and a single cell of a multicelled organism.
3. Why would you think twice about ordering from a restaurant 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 america- nus, Lepus americanus, and Nicrophorus americanus.
4. 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 were 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 this theory, the turkey listened for the master’s footsteps, heard them, and had its head chopped off.
any scientific theory is modified or discarded upon discovery of contradictory evidence. the absence of absolute certainty has led some people to conclude that “theories are irrelevant because they can change.” If that is so, should we stop doing scientific research? Why or why not?
5. In 2005, researcher Woo-suk Hwang reported that he had made immortal stem cells from human patients. His research was hailed as a breakthrough for people affected by degenerative diseases, because stem cells may be used to repair a person’s own damaged tissues. Hwang published his results in a peer- reviewed journal. In 2006, the journal retracted his paper after other scientists discovered that Hwang’s group had faked their data. Does the incident show that results of scientific studies cannot be trusted? or does it confirm the usefulness of a scien- tific approach, because other scientists discovered and exposed the fraud?
5. organisms require and to maintain themselves, grow, and reproduce. a. Dna; energy c. nutrients; energy b. food; sunlight d. Dna; cells
6. move around for at least part of their life.
7. By sensing and responding to change, an organism keeps conditions in its internal environment within ranges that its cells can tolerate. this process is called . a. sampling error c. homeostasis b. development d. critical thinking
8. Dna . a. guides form c. is transmitted from
and function parents to offspring b. is the basis of traits d. all of the above
9. a butterfly is a(n) (choose all that apply). a. organism e. consumer b. domain f. producer c. species g. prokaryote d. eukaryote h. trait
10. a bacterium is (choose all that apply). a. an organism c. an animal b. single-celled d. a eukaryote
11. Bacteria, archaea, and Eukarya are three .
12. a control group is . a. a set of individuals that have a characteristic under study or
receive an experimental treatment b. the standard against which an experimental group is
compared c. the experiment that gives conclusive results
13. Science addresses only that which is . a. alive c. variable b. observable d. indisputable
14. Match the terms with the most suitable description. life a. if–then statement probability b. unique type of organism species c. emerges with cells scientific theory d. testable explanation hypothesis e. measure of chance prediction f. makes its own food producer g. time-tested hypothesis
Critical thinking
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22
2.1 Fear of Frying 24
2.2 Start With Atoms 25
2.3 From Atoms to Molecules 28
2.4 Hydrogen Bonds and Water 29
2.5 Acids and Bases 32
2.6 Organic Molecules 33
2.7 Carbohydrates 34
2.8 Lipids 36
2.9 Proteins 38
2.10 Nucleic Acids 41
M o
le c
u le
s o
f l
if e
2
22
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24 Unit 1 HOW CeLLS WOrk
2.1 Fear of Frying The human body requires only about a tablespoon of fat each day to stay healthy, but most people in developed countries eat far more than that. The average Ameri- can eats about 70 pounds of fat per year, which may be part of the reason why the average American is overweight. Being overweight increases one’s risk for many chronic illnesses. However, the total quantity of fat in the diet may have less impact on health than the types of fats. Fats are more than inert molecules that accumulate in strategic areas of our bodies. They are the main constituents of cell membranes, and as such they have powerful effects on cell function.
The typical fat molecule has three fatty acid tails, each a long chain of carbon atoms that can vary a bit in structure. Fats with a certain arrangement of hydrogen atoms around those carbon chains are called trans fats. Small amounts of trans fats occur naturally in red meat and dairy products, but the main source of these fats in the American diet is an artificial food product called partially hydrogenated vegetable oil. Hydrogenation is a manufacturing process that adds hydrogen atoms to oils in order to change them into solid fats. In 1908, Procter & Gamble Co. developed partially hydrogenated soybean oil as a substitute for the more expensive solid animal fats they had been using to make candles. By 1911, more households in the United States became wired for electricity, so the demand for candles was waning. P & G needed another way to sell its proprietary fat. Partially hydrogenated vegetable oil looks a lot like lard, so the company began aggressively marketing it as a revolutionary new food: a solid cooking fat with a long shelf life, mild flavor, and lower cost than lard or butter.
By the mid-1950s, hydrogenated vegetable oil had become a major part of the American diet. For decades, it was considered to be healthier than animal fats because it was made from plants, but we now know otherwise. Trans fats, which are abun- dant 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 arter- ies and veins. The effects of such changes are quite serious. Eating as little as 2 grams per day (about 0.4 teaspoon) of hydrogenated vegetable oil measurably increases one’s risk of atherosclerosis (hardening of the arteries), heart attack, and diabetes. A small serv- ing of french fries made with hydrogenated vegetable oil contains about 5 grams of trans fat (Figure 2.1). At this writing, hydrogenated oil is still a component of many manufactured and fast foods: french fries, stick margarines, ready-to-use frostings, cookies, crackers, cakes and pancakes, peanut butter, pies, doughnuts, muffins, chips, microwave popcorn, pizzas, burritos, chicken nuggets, fish sticks, and so on.
All organisms consist of the same kinds of molecules, but small differences in the way those molecules are put together can have big effects. With this concept, we introduce you to the chemistry of life. This is your chemistry. It makes you far more than the sum of your body’s molecules.
Figure 2.1 Unhealthy trans fats are abundant in partially hydrogenated oils commonly used to make manufactured and fast foods. © Kentoh/Shutterstock.com.
Application
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MOLeCuLeS OF LiFe Chapter 2 25
2.2 Start With Atoms reMeMBer: Atoms are fundamental units of matter—the building blocks of all substances (Section 1.2).
Even though atoms are about 20 million times smaller than a grain of sand, they consist of even smaller subatomic particles. Positively charged protons (p+) and uncharged neutrons occur in an atom’s core, or nucleus. Negatively charged elec- trons (e–) move around the nucleus (Figure 2.2). Charge is an electrical property: Opposite charges attract, and like charges repel. A typical atom has about the same number of electrons and protons. The negative charge of an electron is the same magnitude as the positive charge of a proton, so the two charges cancel one another. Thus, an atom with the same number of electrons and protons carries no charge.
All atoms have protons. The number of protons in the nucleus is called the atomic number, and it determines the type of atom, or element. Elements are pure substances, each consisting only of atoms with the same number of protons in their nucleus. For example, the element carbon has an atomic number of 6 (Figure 2.3). All atoms with six protons in their nucleus are carbon atoms, no matter how many electrons or neutrons they have. Elemental carbon (the substance) consists only of carbon atoms, and all of those atoms have six protons. Each of the 118 known ele- ments has a symbol that is typically an abbreviation of its Latin or Greek name (see Appendix II). Carbon’s symbol, C, is from carbo, the Latin word for coal. Coal is mostly carbon.
All atoms of an element have the same number of protons, but they can differ in the number of other subatomic particles. Those that differ in the number of neu- trons are called isotopes. The total number of neutrons and protons in the nucleus of an isotope is its mass number. Mass number is written as a superscript to the left of the element’s symbol. For example, the most common isotope of hydrogen has one proton and no neutrons, so it is designated 1H. Other hydrogen isotopes include deuterium (2H, one proton and one neutron) and tritium (3H, one proton and two neutrons).
The most common carbon isotope has six protons and six neutrons (12C). Another naturally occurring carbon isotope, 14C, has six protons and eight neutrons (6 + 8 = 14). Carbon 14 is an example of a radioisotope, or radioactive isotope. Atoms of a radioisotope have an unstable nucleus that breaks up spontaneously. As a nucleus breaks up, it emits radiation (subatomic particles, energy, or both), a process called radioactive decay. The atomic nucleus cannot be altered by ordinary means, so radioactive decay is unaffected by external factors such as temperature, pressure, or whether the atoms are part of molecules.
Each radioisotope decays at a predictable rate into predictable products. For example, when carbon 14 decays, one of its neutrons splits into a proton and an electron. The nucleus emits the electron as radiation. Thus, a carbon atom with eight neutrons and six protons (14C) becomes a nitrogen atom, with seven neutrons and seven protons (14N):
This process is so predictable that we can say with certainty that about half of the atoms in any sample of 14C will be 14N atoms after 5,730 years. The predictability of
atomic number Number of protons in the atomic nucleus; determines the element.
charge electrical property; opposite charges attract, and like charges repel.
electron Negatively charged subatomic particle.
element A pure substance that consists only of atoms with the same number of protons.
isotopes Forms of an element that differ in the num- ber of neutrons their atoms carry.
mass number Of an isotope, the total number of protons and neutrons in the atomic nucleus.
neutron uncharged subatomic particle in the atomic nucleus.
nucleus Core of an atom; occupied by protons and neutrons.
proton Positively charged subatomic particle that occurs in the nucleus of all atoms.
radioactive decay Process by which atoms of a radioisotope emit energy and subatomic particles when their nucleus spontaneously breaks up.
radioisotope isotope with an unstable nucleus.
Figure 2.2 atoms consist of subatomic particles. Models such as this do not show what atoms really look like. electrons move in defined, three-dimensional spaces about 10,000 times bigger than the nucleus. Protons and neutrons occur in the nucleus.
Figure 2.3 example of an element: carbon. Left, Theodore Gray/Visuals Unlimited, Inc.
electron
neutron
proton
an atom
+ +
–
–
–
C 6
12mass number
element symbol
atomic number
carbon
elemental substance
element name
nucleus of 14C, with 6 protons, 8 neutrons
nucleus of 14N, with 7 protons, 7 neutrons
nucleus of 14C, with 6 protons, 8 neutrons
nucleus of 14N, with 7 protons, 7 neutrons
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26 Unit 1 HOW CeLLS WOrk
Figure 2.5 Shell models. each circle (shell) represents one energy level. To make these models, we fill the shells with electrons from the innermost shell out, until there are as many electrons as the atom has protons. The number of protons in each model is indicated.
radioactive decay makes it possible for scientists to estimate the age of a rock or fos- sil by measuring its isotope content (we return to this topic in Section 11.4).
Radioisotopes are often used in tracers, which are substances with a detect- able component. For example, a molecule in which an atom (such as 12C) has been replaced with a radioisotope (such as 14C) can be used as a radioactive tracer. When delivered into a biological system, a radioactive tracer may be followed as it moves through the system with instruments that detect radiation (Figure 2.4).
Why Electrons Matter The more we learn about electrons, the weirder they seem. Consider that an electron has mass but no size, and its position in space is described as more of a smudge than a point. It carries energy, but only in incremen- tal amounts (this concept will be important to remember when you learn how cells harvest and release energy). An electron gains energy only by absorbing the precise amount needed to boost it to the next energy level. Likewise, it loses energy only by emitting the exact difference between two energy levels.
Imagine that an atom is a multilevel apartment building with a nucleus in the basement. Each “floor” of the building corresponds to a certain energy level, and each has a certain number of “rooms” available for rent. Two electrons can occupy each room. Pairs of electrons populate rooms from the ground floor up. The farther an electron is from the nucleus in the basement, the greater its energy. An elec- tron can move to a room on a higher floor if an energy input gives it a boost, but it immediately emits the extra energy and moves back down.
brain
lungs
heart
liver
kidneys
Nonsmoker Smoker
Figure 2.4 pet scans. PeT scans use radioactive tracers to form a digital image of a process in the body’s interior. These two PeT scans reveal the activity of a molecule called MAO-B in the body of a nonsmoker (left) and a smoker (right). The activ- ity is color-coded from red (highest activity) to purple (lowest). Low MAO-B activity is associated with violence, impulsiveness, and other behavioral problems. Brookhaven National Laboratory.
a. the first shell corresponds to the first energy level, and it can hold up to 2 electrons. Hydrogen has one proton, so it has 1 electron and one vacancy. A helium atom has 2 protons, 2 electrons, and no vacancies.
B. the second shell corresponds to the second energy level, and it can hold up to 8 electrons. Carbon has 6 electrons, so its first shell is full. its second shell has 4 electrons and four vacancies. Oxygen has 8 electrons and two vacancies. Neon has 10 electrons and no vacancies.
C. the third shell corresponds to the third energy level, and it can hold up to 8 electrons. A sodium atom has 11 electrons, so its first two shells are full; the third shell has one electron. Thus, sodium has seven vacancies. Chlorine has 17 electrons and one vacancy. Argon has 18 electrons and no vacancies.
answer: Hydrogen, carbon, oxygen, sodium, and chlorineFigure it Out: Which of these models have unpaired electrons in their outer shell?
carbon (C)second shell oxygen (O) neon (Ne)
6 8 10
sodium (Na)third shell chlorine (Cl) argon (Ar)
181711
one electron
one proton
hydrogen (H)first shell
1
helium (He)
2
carbon (C)second shell oxygen (O) neon (Ne)
6 8 10
sodium (Na)third shell chlorine (Cl) argon (Ar)
181711
one electron
one proton
hydrogen (H)first shell
1
helium (He)
2
carbon (C)second shell oxygen (O) neon (Ne)
6 8 10
sodium (Na)third shell chlorine (Cl) argon (Ar)
181711
one electron
one proton
hydrogen (H)first shell
1
helium (He)
2
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MOLeCuLeS OF LiFe Chapter 2 27
A shell model helps us visualize how electrons populate atoms (Figure 2.5). In this model, nested “shells” correspond to successively higher energy levels. Thus, each shell includes all of the rooms on one floor (energy level) of our atomic apart- ment building. We draw a shell model of an atom by filling it with electrons from the innermost shell out, until there are as many electrons as the atom has protons. There is only one room on the first floor, and it fills up first. In hydrogen, the sim- plest atom, a single electron occupies that room (Figure 2.5A). Helium, with two protons, has two electrons that fill the room —and the first shell. In larger atoms, more electrons rent the second-floor rooms (Figure 2.5B). When the second floor fills, more electrons rent third-floor rooms (Figure 2.5C), and so on.
When an atom’s outermost shell is filled with electrons, we say that it has no vacancies, and it is in its most stable state. Helium, neon, and argon are examples
of elements with no vacancies. Atoms of these elements are chemically stable, which means they have very little tendency to interact with other atoms. Thus, these elements occur most frequently in nature as solitary atoms. By contrast, when an atom’s outermost shell has room for another electron, it has a vacancy. Atoms with vacancies tend to get rid of them by interacting with other atoms; in other words, they are chemi- cally active. For example, the sodium atom (Na) in Figure 2.5C has one electron in its outer (third) shell, which can hold eight. With seven vacancies, we can predict that this atom is chemically active. In fact, this particular sodium atom is not just active, it is extremely so. Why? The shell model shows that a sodium atom has an unpaired electron, but in the real world, electrons really like to be in pairs when they populate atoms. Atoms that have
unpaired electrons are called free radicals. With a few exceptions, free radicals are very unstable, easily forcing electrons upon other atoms or ripping electrons away from them. This property makes free radicals dangerous to life. A sodium atom with 11 electrons (a sodium radical) quickly evicts the one unpaired electron, so that its second shell—which is full of electrons—becomes its outermost, and no vacancies remain. This is the atom’s most stable state. The vast majority of sodium atoms on Earth are like this one, with 11 protons and 10 electrons.
Atoms with an unequal number of protons and electrons are ions. An ion car- ries a net (or overall) charge. Sodium ions (Na+) offer an example of how atoms gain a positive charge by losing an electron (Figure 2.6A). Other atoms gain a negative charge by accepting an electron (Figure 2.6B).
free radical Atom with an unpaired electron.
ion Atom or molecule that carries a net charge.
shell model Model of electron distribution in an atom.
tracer A substance that can be traced via its detect- able component.
Figure 2.6 ion formation.
vacancy
no vacancy
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el S
. Y am
as hi
ta / C
or bi
s.
answer: No
Figure it Out: Does a chloride ion have an unpaired electron?
11p+
11e–
charge: 0
Sodium atom
Chlorine atom
charge: 0
17p+
17e–
Chloride ion
charge: –1
17p+
18e–
11p+
10e–
charge: +1
Sodium ion
electron loss
1111
1717
electron gain
a. A sodium atom (Na) becomes a positively charged sodium ion (Na+) when it loses the single electron in its third shell. The atom’s full second shell is now its outer- most, so it has no vacancies.
B. A chlorine atom (Cl) becomes a negatively charged chloride ion (Cl–) when it gains an electron and fills the vacancy in its third, outermost shell.
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