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Chapter 1 Introduction to Cells 1

Cells Under the Microscope 1

The Invention of the Light Microscope Led to the Discovery of Cells 2

Cells，Organelles，and Even Molecules Can Be Seen Under the Microscope 3

The Eucaryotic Cell 9

The Nucleus Is the Information Store of the Cell 9

Mitochondria Generate Energy from Food to Power the Cell 10

Chloroplasts Capture Energy from Sunlight 12

Internal Membranes Create Intracellular Compartments with Different Functions 13

The Cytosol Is a Concentrated Aqueous Gel of Large and Small Molecules 15

The Cytoskeleton Is Responsible for Cell Movements 16

Unity and Diversity of Cells 17

Cells Vary Enormously in Appearance and Function 19

Living Cells All Have a Similar Basic Chemistry 21

All Present-Day Cells Have Apparently Evolved from the Same Ancestor 21

Bacteria Are the Smallest and Simplest Cells 22

Molecular Biologists Have Focused on E.coli 25

Giardia May Represent an Intermediate Stage in the Evolution of Eucaryotic Cells 25

Brewer’s Yeast Is a Simple Eucaryotic Cell 26

Single-celled Organisms Can Be Large，Complex，and Fierce：The Protozoans 27

Arabidopsis Has Been Chosen Out of 300，000 Species as a Model Plant 28

The World of Animals Is Represented by a Fly，a Worm，a Mouse，and Homo Sapiens 29

Cells in the Same Multicellular Organism Can Be Spectacularly Different 31

Essential Concepts 34

Questions 35

Chapter 2 Chemical Components of Cells 37

Chemical Bonds 37

Cells Are Made of Relatively Few Types of Atoms 38

The Outermost Electrons Determine How Atoms Interact 39

Ionic Bonds Form by the Gain and Loss of Electrons 42

Covalent Bonds Form by the Sharing of Electrons 43

There Are Different Types of Covalent Bonds 45

Water Is the Most Abundant Substance in Cells 48

Some Polar Molecules Form Acids and Bases in Water 49

Molecules in Cells 52

A Cell Is Formed from Carbon Compounds 52

Cells Contain Four Major Families of Small Organic Molecules 52

Sugars Are Energy Sources for Cells and Subunits of Polysaccharides 53

Fatty Acids Are Components of Cell Membranes 55

Amino Acids Are the Subunits of Proteins 60

Nucleotides Are the Subunits of DNA and RNA 61

Macromolecules Contain a Specific Sequence of Subunits 65

Noncovalent Bonds Specify the Precise Shape of a Macromolecule 69

Noncovalent Bonds Allow a Macromolecule to Bind Other Selected Molecules 72

Essential Concepts 73

Questions 74

Chapter 3 Energy，Catalysis，and Biosynthesis 79

Catalysis and the Use of Energy by Cells 79

Biological Order Is Made Possible by the Release of Heat Energy from Cells 79

Photosynthetic Organisms Use Sunlight to Synthesize Organic Molecules 82

Cells Obtain Energy by the Oxidation of Biological Molecules 83

Oxidation and Reduction Involve Electron Transfers 84

Enzymes Lower the Barriers That Block Chemical Reactions 85

How Enzymes Find Their Substrates：The Importance of Rapid Diffusion 86

The Free-Energy Change for a Reaction Determines Whether It Can Occur 89

The Concentration of Reactants Influences △G 89

For Sequential Reactions，△G° Values Are Additive 93

Activated Carrier Molecules and Biosynthesis 94

The Formation of an Activated Carrier Is Coupled to an Energetically Favorable Reaction 95

ATP Is the Most Widely Used Activated Carrier Molecule 96

Energy Stored in ATP Is Often Harnessed to Join Two Molecules Together 97

NADH and NADPH Are Important Electron Carriers 98

There Are Many Other Activated Carrier Molecules in Cells 100

The Synthesis of Biological Polymers Requires an Energy Input 103

Essential Concepts 105

Questions 106

Chapter 4 How Cells Obtain Energy from Food 108

The Breakdown of Sugars and Fats 108

Food Molecules Are Broken Down in Three Stages to Produce ATP 108

Glycolysis Is a Central ATP-producing Pathway 110

Fermentations Allow ATP to Be Produced in the Absence of Oxygen 114

Glycolysis Illustrates How Enzymes Couple Oxidation to Energy Storage 114

Sugars and Fats Are Both Degraded to Acetyl CoA in Mitochondria 118

The Citric Acid Cycle Generates NADH by Oxidizing Acetyl Groups to CO2 119

Electron Transport Drives the Synthesis of the Majority of the ATP in Most Cells 124

Storing and Utilizing Food 125

Organisms Store Food Molecules in Special Reservoirs 125

Many Biosynthetic Pathways Begin with Glycolysis or the Citric Acid Cycle 127

Metabolism Is Organized and Regulated 128

Essential Concepts 129

Questions 130

Chapter 5 Protein Structure and Function 134

The Shape and Structure of Proteins 134

The Shape of a Protein Is Specified by Its Amino Acid Sequence 134

Proteins Fold into a Conformation of Lowest Energy 139

Proteins Come in a Wide Variety of Complicated Shapes 140

The α Helix and the β Sheet Are Common Folding Patterns 141

Proteins Have Several Levels of Organization 145

Few of the Many Possible Polypeptide Chains Will Be Useful 147

Proteins Can Be Classified into Families 147

Larger Protein Molecules Often Contain More Than One Polypeptide Chain 148

Proteins Can Assemble into Filaments，Sheets，or Spheres 149

A Helix Is a Common Structural Motif in Biological Structures 152

Some Types of Proteins Have Elongated Fibrous Shapes 152

Extracellular Proteins Are Often Stabilized by Covalent Cross-Linkages 154

How Proteins Work 154

Proteins Bind to Other Molecules 155

The Binding Sites of Antibodies Are Especially Versatile 156

Binding Strength Is Measured by the Equilibrium Constant 157

Enzymes Are Powerful and Highly Specific Catalysts 167

Lysozyme Illustrates How an Enzyme Works 167

Vmax and KM Measure Enzyme Performance 169

Tightly Bound Small Molecules Add Extra Functions to Proteins 171

The Catalytic Activities of Enzymes Are Regulated 172

Allosteric Enzymes Have Two Binding Sites That Interact 173

A Conformational Change Can Be Driven by Protein Phosphorylation 174

GTP-binding Proteins Can Undergo Dramatic Conformational Changes 176

Motor Proteins Produce Large Movements in Cells 176

Proteins Often Form Large Complexes That Function as Protein Machines 178

Essential Concepts 179

Questions 180

Chapter 6 DNA 184

The Structure and Function of DNA 184

Genes Are Made of DNA 185

A DNA Molecule Consists of Two Complementary Chains of Nucleotides 185

The Structure of DNA Provides a Mechanism for Heredity 188

DNA Replication 189

DNA Synthesis Begins at Replication Origins 190

New DNA Synthesis Occurs at Replication Forks 191

The Replication Fork Is Asymmetrical 193

DNA Polymerase Is Self-correcting 194

Short Lengths of RNA Act as Primers for DNA Synthesis 194

Proteins at a Replication Fork Cooperate to Form a Replication Machine 196

DNA Repair 198

Changes in DNA Are the Cause of Mutations 198

A DNA Mismatch Repair System Removes Replication Errors That Escape from the Replication Machine 200

DNA Is Continually Suffering Damage in Cells 201

The Stability of Genes Depends on DNA Repair 202

The High Fidelity with Which DNA Is Maintained Means That Closely Related Species Have Proteins with Very Similar Sequences 205

Essential Concepts 206

Questions 207

Chapter 7 From DNA to Protein 212

From DNA to RNA 212

Portions of DNA Sequence Are Transcribed into RNA 212

Transcription Produces RNA Complementary to One Strand of DNA 213

Several Types of RNA Are Produced in Cells 215

Signals in DNA Tell RNA Polymerase Where to Start and Finish 216

Eucaryotic RNAs Undergo Processing in the Nucleus 218

Eucaryotic Genes Are Interrupted by Noncoding Sequences 219

Introns Are Removed by RNA Splicing 220

mRNA Molecules Are Eventually Degraded by the Cell 222

The Earliest Cells May Have Had Introns in Their Genes 223

From RNA to Protein 224

An mRNA Sequence Is Decoded in Sets of Three Nucleotides 224

tRNA Molecules Match Amino Acids to Codons in mRNA 225

Specific Enzymes Couple tRNAs to the Correct Amino Acid 227

The RNA Message Is Decoded on Ribosomes 227

Codons in mRNA Signal Where to Start and to Stop Protein Synthesis 230

Proteins Are Made on Polyribosomes 232

Carefully Controlled Protein Breakdown Helps Regulate the Amount of Each Protein in a Cell 232

There Are Many Steps Between DNA and Protein 234

RNA and the Origins of Life 234

Simple Biological Molecules Can Form Under Prebiotic Conditions 235

RNA Can Both Store Information and Catalyze Chemical Reactions 237

RNA Is Thought to Predate DNA in Evolution 239

Essential Concepts 240

Questions 241

Chapter 8 Chromosomes and Gene Regulation 246

The Structure of Eucaryotic Chromosomes 246

Eucaryotic DNA Is Packaged into Chromosomes 246

Chromosomes Exist in Different States Throughout the Life of a Cell 247

Specialized DNA Sequences Ensure That Chromosomes Replicate Efficiently 249

Nucleosomes Are the Basic Units of Chromatin Structure 250

Chromosomes Have Several Levels of DNA Packing 252

Interphase Chromosomes Contain Both Condensed and More Extended Forms of Chromatin 253

Position Effects on Gene Expression Reveal Differences in Interphase Chromosome Packing 256

Interphase Chromosomes Are Organized Within the Nucleus 256

Gene Regulation 257

Cells Regulate the Expression of Their Genes 258

Transcription Is Controlled by Proteins Binding to Regulatory DNA Sequences 259

Repressors Turn Genes Off and Activators Turn Them On 261

Initiation of Eucaryotic Gene Transcription Is a Complex Process 263

Eucaryotic RNA Polymerase Requires General Transcription Factors 264

Eucaryotic Gene Regulatory Proteins Control Gene Expression from a Distance 265

Packing of Promoter DNA into Nucleosomes Can Affect Initiation of Transcription 266

Eucaryotic Genes Are Regulated by Combinations of Proteins 267

The Expression of Different Genes Can Be Coordinated by a Single Protein 268

Combinatorial Control Can Create Different Cell Types 269

Stable Patterns of Gene Expression Can Be Transmitted to Daughter Cells 271

The Formation of an Entire Organ Can Be Triggered by a Single Gene Regulatory Protein 273

Essential Concepts 274

Questions 275

Chapter 9 Genetic Variation 278

Genetic Variation in Bacteria 278

The Rapid Rate of Bacterial Division Means That Mutation Will Occur Over a Short Time Period 279

Mutation in Bacteria Can Be Selected by a Change in Environmental Conditions 280

Bacterial Cells Can Acquire Genes from Other Bacteria 281

Bacterial Genes Can Be Transferred by a Process Called Bacterial Mating 282

Some Bacteria Can Take Up DNA from Their Surroundings 284

Gene Exchange Occurs by Homologous Recombination Between Two DNA Molecules of Similar Nucleotide Sequence 285

Genes Can Be Transferred Between Bacteria by Bacterial Viruses 288

Transposable Elements Create Genetic Diversity 289

Sources of Genetic Change in Eucaryotic Genomes 291

Random DNA Duplications Create Families of Related Genes 292

Genes Encoding New Proteins Can Be Created by the Recombination of Exons 293

A Large Part of the DNA of Multicellular Eucaryotes Consists of Repeated，Noncoding Sequences 294

About 105551300f the Human Genome Consists of Two Families of Transposable Sequences 295

The Evolution of Genomes Has Been Accelerated by Transposable Elements 296

Viruses Are Fully Mobile Genetic Elements That Can Escape from Cells 297

Retroviruses Reverse the Normal Flow of Genetic Information 300

Retroviruses That Have Picked Up Host Genes Can Make Cells Cancerous 302

Sexual Reproduction and the Reassortment of Genes 304

Sexual Reproduction Gives a Competitive Advantage to Organisms in an Unpredictably Variable Environment 304

Sexual Reproduction Involves Both Diploid and Haploid Cells 305

Meiosis Generates Haploid Cells from Diploid Cells 306

Meiosis Generates Enormous Genetic Variation 307

Essential Concepts 309

Questions 310

Chapter 10 DNA Technology 315

How DNA Molecules Are Analyzed 315

Restriction Nucleases Cut DNA Molecules at Specific Sites 315

Gel Electrophoresis Separates DNA Fragments of Different Sizes 317

The Nucleotide Sequence of DNA Fragments Can Be Determined 320

Nucleic Acid Hybridization 320

DNA Hybridization Facilitates the Prenatal Diagnosis of Genetic Diseases 321

In Situ Hybridization Locates Nucleic Acid Sequences in Cells or on Chromosomes 323

DNA Cloning 324

DNA Ligase Joins DNA Fragments Together to Produce a Recombinant DNA Molecule 325

Bacterial Plasmids Can Be Used to Clone DNA 326

Human Genes Are Isolated by DNA Cloning 327

cDNA Libraries Represent the mRNA Produced by a Particular Tissue 329

Hybridization Allows Even Distantly Related Genes to Be Identified 331

The Polymerase Chain Reaction Amplifies Selected DNA Sequences 332

DNA Engineering 335

Completely Novel DNA Molecules Can Be Constructed 335

Rare Cellular Proteins Can Be Made in Large Amounts Using Cloned DNA 337

RNAs Can Be Produced by Transcription in Vitro 338

Mutant Organisms Best Reveal the Function of a Gene 339

Transgenic Animals Carry Engineered Genes 340

Essential Concepts 342

Questions 343

Chapter 11 Membrane Structure 348

The Lipid Bilayer 348

Membrane Lipids Form Bilayers in Water 349

The Lipid Bilayer Is a Two-dimensional Fluid 352

The Fluidity of a Lipid Bilayer Depends on Its Composition 353

The Lipid Bilayer Is Asymmetrical 354

Lipid Asymmetry Is Generated Inside the Cell 355

Lipid Bilayers Are Impermeable to Solutes and Ions 356

Membrane Proteins 357

Membrane Proteins Associate with the Lipid Bilayer in Various Ways 358

A Polypeptide Chain Usually Crosses the Bilayer as an α Helix 358

Membrane Proteins Can Be Solubilized in Detergents and Purified 360

The Complete Structure Is Known for Very Few Membrane Proteins 361

The Plasma Membrane Is Reinforced by the Cell Cortex 363

The Cell Surface Is Coated with Carbohydrate 364

Cells Can Restrict the Movement of Membrane Proteins 366

Essential Concepts 368

Questions 368

Chapter 12 Membrane Transport 372

The Ion Concentrations Inside a Cell Are Very Different from Those Outside 372

Carrier Proteins and Their Functions 373

Solutes Cross Membranes by Passive or ActiveTransport 375

Electrical Forces as Well as Concentration Gradients Can Drive Passive Transport 375

Active Transport Moves Solutes Against Their Electrochemical Gradients 377

Animal Cells Use the Energy of ATP Hydrolysis to Pump Out Na＋ 378

The Na＋-K＋ Pump Is Driven by the Transient Addition of a Phosphate Group 379

Animal Cells Use the Na＋ Gradient to Take Up Nutrients Actively 380

The Na＋-K＋ Pump Helps Maintain the Osmotic Balance of Animal Cells 381

Intracellular Ca 2＋ Concentrations Are Kept Low by Ca2＋ Pumps 383

H＋ Gradients Are Used to Drive Membrane Transport in Plants，Fungi，and Bacteria 384

Ion Channels and the Membrane Potential 385

Ion Channels Are Ion Selective and Gated 386

Ion Channels Randomly Snap Between Open and Closed States 388

Voltage-gated Ion Channels Respond to the Membrane Potential 390

The Membrane Potential Is Governed by Membrane Permeability to Specific Ions 391

Ion Channels and Signaling in Nerve Cells 394

Action Potentials Provide for Rapid Long-Distance Communication 395

Action Potentials Are Usually Mediated by Voltage-gated Na＋ Channels 395

Voltage-gated Ca2＋ Channels Convert Electrical Signals into Chemical Signals at Nerve Terminals 397

Transmitter-gated Channels in Target Cells Convert Chemical Signals Back into Electrical Signals 399

Neurons Receive Both Excitatory and Inhibitory Inputs 400

Synaptic Connections Enable You to Think，Act，and Remember 401

Essential Concepts 404

Questions 405

Chapter 13 Energy Generation in Mitochondria and Chloroplasts 409

Cells Obtain Most of Their Energy by a Membrane-based Mechanism 409

Mitochondria and Oxidative Phosphorylation 410

A Mitochondrion Contains Two Membrane-bounded Compartments 411

High-Energy Electrons Are Generated via the Citric Acid Cycle 413

Electrons Are Transferred Along a Chain of Proteins in the Inner Mitochondrial Membrane 414

Electron Transport Generates a Proton Gradient Across the Membrane 415

The Proton Gradient Drives ATP Synthesis 417

Coupled Transport Across the Inner Mitochondrial Membrane Is Driven by the Electrochemical Proton Gradient 419

Proton Gradients Produce Most of the Cell’s ATP 419

The Rapid Conversion of ADP to ATP in Mitochondria Maintains a High ATP：ADP Ratio in Cells 421

Electron-Transport Chains and Proton Pumping 421

Protons Are Readily Moved by the Transfer of Electrons 422

The Redox Potential Is a Measure of Electron Affinities 422

Electron Transfers Release Large Amounts of Energy 423

Metals Tightly Bound to Proteins Form Versatile Electron Carriers 425

Protons Are Pumped Across the Membrane by the Three Respiratory Enzyme Complexes 427

Respiration Is Amazingly Efficient 429

Chloroplasts and Photosynthesis 430

Chloroplasts Resemble Mitochondria but Have an Extra Compartment 430

Chloroplasts Capture Energy from Sunlight and Use It to Fix Carbon 432

Excited Chlorophyll Molecules Funnel Energy into a Reaction Center 433

Light Energy Drives the Synthesis of ATP and NADPH 434

Carbon Fixation Is Catalyzed by Ribulose Bisphosphate Carboxylase 436

Carbon Fixation in Chloroplasts Generates Sucrose and Starch 438

The Genetic Systems of Mitochondria and Chloroplasts Reflect Their Procaryotic Origin 438

Our Single-celled Ancestors 439

RNA Sequences Reveal Evolutionary History 439

Ancient Cells Probably Arose in Hot Environments 440

Methanococcus Lives in the Dark，Using Only Inorganic Materials as Food 441

Essential Concepts 443

Questions 444

Chapter 14 Intracellular Compartments and Transport 448

Membrane-bounded Organelles 448

Eucaryotic Cells Contain a Basic Set of Membrane-bounded Organelles 449

Membrane-bounded Organelles Evolved in Different Ways 450

Protein Sorting 452

Proteins Are Imported into Organelles by Three Mechanisms 453

Signal Sequences Direct Proteins to the Correct Compartment 453

Proteins Enter the Nucleus Through Nuclear Pores 455

Proteins Unfold to Enter Mitochondria and Chloroplasts 457

Proteins Enter the Endoplasmic Reticulum While Being Synthesized 458

Soluble Proteins Are Released into the ER Lumen 459

Start and Stop Signals Determine the Arrangement of a Transmembrane Protein in the Lipid Bilayer 461

Vesicular Transport 462

Transport Vesicles Carry Soluble Proteins and Membrane Between Compartments 463

Vesicle Budding Is Driven by the Assembly of a Protein Coat 463

The Specificity of Vesicle Docking Depends on SNAREs 465

Secretory Pathways 467

Most Proteins Are Covalently Modified in the ER 467

Exit from the ER Is Controlled to Ensure Protein Quality 468

Proteins Are Further Modified and Sorted in the Golgi Apparatus 469

Secretory Proteins Are Released from the Cell by Exocytosis 470

Endocytic Pathways 472

Specialized Phagocytic Cells Ingest Large Particles 472

Fluid and Macromolecules Are Taken Up by Pinocytosis 473

Receptor-mediated Endocytosis Provides a Specific Route into Animal Cells 474

Endocytosed Macromolecules Are Sorted in Endosomes 475

Lysosomes Are the Principal Sites of Intracellular Digestion 476

Essential Concepts 478

Questions 479

Chapter 15 Cell Communication 482

General Principles of Cell Signaling 482

Signals Can Act over Long or Short Range 482

Each Cell Responds to a Limited Set of Signals 484

Receptors Relay Signals via Intracellular Signaling Pathways 486

Some Signal Molecules Can Cross the Plasma Membrane 488

Nitric Oxide Can Enter Cells to Activate Enzymes Directly 489

There Are Three Main Classes of Cell-Surface Receptors 490

Ion-Channel-linked Receptors Convert Chemical Signals into Electrical Ones 491

Intracellular Signaling Cascades Act as a Series of Molecular Switches 492

G-Protein-linked Receptors 493

Stimulation of G-Protein-linked Receptors Activates G-Protein Subunits 493

Some G Proteins Regulate Ion Channels 495

Some G Proteins Activate Membrane-bound Enzymes 496

The Cyclic AMP Pathway Can Activate Enzymes and Turn On Genes 497

The Pathway Through Phospholipase C Results in a Rise in Intracellular Ca 2＋ 499

A Ca 2＋ Signal Triggers Many Biological Processes 501

Intracellular Signaling Cascades Can Achieve Astonishing Speed，Sensitivity，and Adaptability：Photoreceptors in the Eye 502

Enzyme-linked Receptors 504

Activated Receptor Tyrosine Kinases Assemble a Complex of Intracellular Signaling Proteins 505

Receptor Tyrosine Kinases Activate the GTP-binding Protein Ras 506

Protein Kinase Networks Integrate Information to Control Complex Cell Behaviors 508

Essential Concepts 510

Questions 511

Chapter 16 Cytoskeleton 514

Intermediate Filaments 514

Intermediate Filaments Are Strong and Durable 515

Intermediate Filaments Strengthen’ Cells Against Mechanical Stress 516

Microtubules 518

Microtubules Are Hollow Tubes with Structurally Distinct Ends 519

Microtubules Are Maintained by a Balance of Assembly and Disassembly 519

The Centrosome Is the Major Microtubule-organizing Center in Animal Cells 521

Growing Microtubules Show Dynamic Instability 522

Microtubules Organize the Interior of the Cell 523

Motor Proteins Drive Intracellular Transport 525

Organelles Move Along Microtubules 526

Cilia and Flagella Contain Stable Microtubules Moved by Dynein 527

Actin Filaments 529

Actin Filaments Are Thin and Flexible 530

Actin and Tubulin Polymerize by Similar Mechanisms 531

Many Proteins Bind to Actin and Modify Its Properties 532

An Actin-rich Cortex Underlies the Plasma Membrane of Most Eucaryotic Cells 533

Cell Crawling Depends on Actin 533

Actin Associates with Myosin to Form Contractile Structures 536

During Muscle Contraction Actin Filaments Slide Against Myosin Filaments 538

Muscle Contraction Is Triggered by a Sudden Rise in Ca 2＋ 539

Essential Concepts 543

Questions 544

Chapter 17 Cell Division 549

Overview of the Cell Cycle 549

The Eucaryotic Cell Cycle Is Divided into Four Phases 549

The Cytoskeleton Carries Out Both Mitosis and Cytokinesis 551

Some Organelles Fragment at Mitosis 551

Mitosis 552

The Mitotic Spindle Starts to Assemble in Prophase 552

Chromosomes Attach to the Mitotic Spindle at Prometaphase 553

Chromosomes Line Up at the Spindle Equator at Metaphase 557

Daughter Chromosomes Segregate at Anaphase 557

The Nuclear Envelope Re-forms at Telophase 559

Cytokinesis 560

The Mitotic Spindle Determines the Plane of Cytoplasmic Cleavage 560

The Contractile Ring of Animal Cells Is Made of Actin and Myosin 561

Cytokinesis in Plant Cells Involves New Cell-Wall Formation 562

Meiosis 563

Homologous Chromosomes Pair Off During Meiosis 563

Meiosis Involves Two Cell Divisions Rather Than One 564

Essential Concepts 567

Questions 568

Chapter 18 Cell-Cycle Control and Cell Death 572

The Cell-Cycle Control System 572

A Central Control System Triggers the Major Processes of the Cell Cycle 572

The Cell-Cycle Control System Is Based on Cyclically Activated Protein Kinases 574

MPF Is the Cyclin-Cdk Complex That Controls Entry into M Phase 575

Cyclin-dependent Protein Kinases Are Regulated by the Accumulation and Destruction of Cyclin 576

The Activity of Cdks Is Further Regulated by Their Phosphorylation and Dephosphorylation 578

Different Cyclin-Cdk Complexes Trigger Different Steps in the Cell Cycle 578

The Cell Cycle Can Be Halted in G1 by Cdk Inhibitor Proteins 580

Cells Can Dismantle Their Control System and Withdraw from the Cell Cycle 581

Control of Cell Numbers in Multicellular Organisms 582

Cell Proliferation Depends on Signals from Other Cells 582

Animal Cells Have a Built-in Limitation on the Number of Times They Will Divide 584

Animal Cells Require Signals from Other Cells to Avoid Programmed Cell Death 584

Programmed Cell Death Is Mediated by an Intracellular Proteolytic Cascade 585

Cancer Cells Disobey the Social Controls on Cell Proliferation and Survival 587

Essential Concepts 589

Questions 590

Chapter 19 Tissues 594

Extracellular Matrix and Connective Tissues 594

Plant Cells Have Tough External Walls 594

Cellulose Fibers Give the Plant Cell Wall Its Tensile Strength 596

Animal Connective Tissues Consist Largely of Extracellular Matrix 600

Collagen Provides Tensile Strength in Animal Connective Tissues 600

Cells Organize the Collagen That They Secrete 602

Integrins Couple the Matrix Outside a Cell to the Cytoskeleton Inside It 603

Gels of Polysaccharide and Protein Fill Spaces and Resist Compression 604

Epithelial Sheets and Cell-Cell Junctions 605

Epithelial Sheets Are Polarized and Rest on a Basal Lamina 606

Tight Junctions Make an Epithelium Leak-proof and Separate Its Apical and Basal Surfaces 607

Cytoskeleton-linked Junctions Bind Epithelial Cells Robustly to One Another and to the Basal Lamina 609

Gap Junctions Allow Ions and Small Molecules to Pass from Cell to Cell 612

Tissue Maintenance and Renewal，and Its Disruption by Cancer 613

Different Tissues Are Renewed at Different Rates 615

Stem Cells Generate a Continuous Supply of Terminally Differentiated Cells 615

Mutations in a Single Dividing Cell Can Cause It and Its Progeny to Violate the Normal Controls 618

Cancer Is a Consequence of Mutation and Natural Selection Within the Population of Cells That Form the Body 619

Cancer Requires an Accumulation of Mutations 620

Development 621

Programmed Cell Movements Create the Animal Body Plan 622

Cells Switch On Different Sets of Genes According to Their Position and Their History 622

Diffusible Signals Can Provide Cells with Positional Information 624

Studies in Drosophila Have Given a Key to Vertebrate Development 626

Similar Genes Are Used Throughout the Animal Kingdom to Give Cells an Internal Record of Their Position 627

Essential Concepts 628

Questions 629