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