Biology_Unit_5 (1110837), страница 10
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(Collagens and their roles in body structures are described in further detail in Chapter 36.)The consistency of the matrix, which mayrange from soft and jellylike to hard and elastic,depends on a network of proteoglycans that surrounds the collagen fibers. Proteoglycans are glycoproteins that consist of small proteins noncovalently attached to long polysaccharide molecules.Matrix consistency depends on the number of interlinks in this network, which determines howmuch water can be trapped in it.
For example, cartilage, which contains a high proportion of interlinked glycoproteins, is relatively soft . Tendons,which are almost pure collagen, are tough andelastic. In bone, the glycoprotein network that sur-PolysaccharidemoleculeProteoglycansCollagen fibersFibronectinPlasma membraneIntegrin(receptor protein)MicrofilamentsFIGURE 5.30Components of the extracellular matrix in an animal cell.CHAPTER 5THE CELL: AN OVERVIEW111Having laid the groundwork for cell structure and functionin this chapter, we next take up further details of individual cellstructures, beginning with the roles of cell membranes in transport in the next chapter.rounds collagen fibers is impregnated with mineral crystals,producing a dense and hard—but still elastic—structure that isabout as strong as fiberglass or reinforced concrete.Yet another class of glycoproteins is fibronectins, which aidin organizing the ECM and help cells attach to it.
Fibronectinsbind to receptor proteins called integrins that span the plasmamembrane. On the cytoplasmic side of the plasma membrane,the integrins bind to microfilaments of the cytoskeleton. Integrins integrate changes outside and inside the cell by communicating changes in the ECM to the cytoskeleton.STUDY BREAK 5.5<1.
Distinguish between anchoring junctions, tight junctions, andgap junctions.2. What is the structure and function of the extracellular matrix?UNANSWERED QUESTIONScells spatially organized, how is their organization achieved, and how doesthis organization contribute to cell growth and division? Because prokaryotes are small, answering these questions will rely in part on advances inmicroscopy technologies. The answers will shed light on the evolutionaryorigins of cellular architecture.The field of cell biology seeks to understand the properties and behaviorsof cells, including their growth and division, shape and movement, subcellular organization and transport systems, and interactions and communication with each other and the environment. Although cell biology research has dramatically enhanced our basic understanding of cells, manyfundamental questions remain to be answered.How do the complex properties of cells arise from the functions andinteractions of their molecular components?Over the past few decades, inspired by advances in molecular biology andgenetics, most cell biologists have pursued a reductionist approach bydetermining the detailed role of individual genes and proteins in cell structure and behavior.
This approach has been very successful, and togetherwith recent advances in genome sequencing technology, it has enabledcell biologists to catalog many molecules that perform key roles in thecell. In the past few years, cell biologists, including Marc Kirschner atHarvard University and others, have embraced a more holistic approachcalled systems biology, which is focused on understanding how the complex properties of cells and subcellular systems arise from the propertiesand interactions of their individual parts.
Systems cell biologists work withmathematicians and computer scientists to develop models of cellularprocesses that can be refined by experimental validation. Which approach,reductionist or holistic, represents the future of cell biology? Both do, aseach relies on the other to generate insights and hypotheses that movethe field forward.How do eukaryotic cells balance assembly and disassembly pathwaysto maintain a complex cellular and subcellular structure?Eukaryotic cells have a high degree of organizational complexity in theform of numerous subcellular organelles and structures. Maintenance ofthis complexity depends on a delicate balance between the pathways thatpromote the formation and disassembly of each structure. How is thisbalance achieved? At the whole cell level, researchers such as Paul Nurseat Rockefeller University have worked to understand how the balance between growth and division is coordinated to control cell size.
At the subcellular level, work from Peter Walter’s lab at the University of California,San Francisco aims to understand how organelle biogenesis and disassembly are balanced to determine organelle abundance, and my own labstudies how the assembly and disassembly of cytoskeletal polymers arebalanced to coordinate cell movement and division. As with all questionsin cell biology, answers will come from a combination of experiments oncells and experiments aimed at reconstructing subcellular processes outside of the cell.112UNIT ONEMOLECULES AND CELLSThink CriticallyEven the simplest cells are structurally and functionally complex, so howcould such complexity arise through the process of evolution? After thinking about this, read Chapter 24 for some potential answers.Matthew WelchHow intricate is the subcellular organization of prokaryotic cells?Despite a historical focus on eukaryotic cells, cell biologists are becomingincreasingly interested in prokaryotic cells (Bacteria and Archaea), whichevolved more than a billion years before eukaryotes and hold clues to theevolutionary origins of basic cellular properties.
Although these smallercells were once thought to be simple in their organization and behaviors,recent advances from Lucy Shapiro’s lab at Stanford University and othersindicate that prokaryotic cells exhibit a high degree of spatial organization.Moreover, work from Harold Erickson’s lab at Duke University and othershas shown that prokaryotes have components that contribute to subcellular architecture, such as a cytoskeleton, that were initially thought to beunique to eukaryotic cells.
To what extent are components of prokaryoticMatthew Welch is a professor of Molecular and Cell Biologyat the University of California, Berkeley. His research interests include cytoskeleton dynamics and microbial pathogenesis. Learn more about his work at http://mcb.berkeley.edu/labs/welch.review key conceptsGo to CENGAGENOW at www.cengage.com/login to access quizzing,animations, exercises, articles, and personalized homework help.5.1 Basic Features of Cell Structure and Function• According to the cell theory: (1) all living organisms are composedof cells; (2) cells are the structural and functional units of life; and(3) cells arise only from the division of preexisting cells.• Cells of all kinds are divided internally into a central region containing the genetic material and the cytoplasm, which consists ofthe cytosol, the cytoskeleton, and organelles and is bounded by theplasma membrane.• The plasma membrane is a lipid bilayer in which transport proteinsare embedded (Figure 5.6).• In the cytoplasm, proteins are made, most of the other moleculesrequired for growth and reproduction are assembled, and energyabsorbed from the surroundings is converted into energy usable bythe cell.Animation: Overview of cellsAnimation: Surface-to-volume ratioAnimation: Cell membranes5.2 Prokaryotic Cells• Prokaryotic cells are surrounded by a plasma membrane and, inmost groups, are enclosed by a cell wall.
The genetic material,typically a single, circular DNA molecule, is located in thenucleoid. The cytoplasm contains masses of ribosomes (Figure 5.7).Animation: Typical prokaryotic cell5.3 Eukaryotic Cells• Eukaryotic cells have a true nucleus, which is separated from thecytoplasm by the nuclear envelope perforated by nuclear pores. Aplasma membrane forms the outer boundary of the cell. Othermembrane systems enclose specialized compartments as organellesin the cytoplasm (Figures 5.9 and 5.10).• The eukaryotic nucleus contains chromatin, a combination of DNAand proteins.
A specialized segment of the chromatin forms thenucleolus, where ribosomal RNA molecules are made and combined with ribosomal proteins to make ribosomes. The nuclearenvelope contains nuclear pore complexes with pores that allowpassive or assisted transport of molecules between the nucleus andthe cytoplasm. Proteins destined for the nucleus contain a shortamino acid sequence called a nuclear localization signal (Figures5.11 and 5.12).• Eukaryotic cytoplasm contains ribosomes (Figure 5.13), an endomembrane system, mitochondria, microbodies, the cytoskeleton,and some organelles specific to certain organisms.
The endomembrane system includes the nuclear envelope, ER, Golgi complex,lysosomes, vesicles, and plasma membrane.• The endoplasmic reticulum (ER) occurs in two forms, as rough andsmooth ER. The ribosome-studded rough ER makes proteins thatbecome part of cell membranes or are released from the cell.Smooth ER synthesizes lipids and breaks down toxic substances(Figure 5.14).• The Golgi complex chemically modifies proteins made in the roughER and sorts finished proteins to be secreted from the cell,embedded in the plasma membrane, or included in lysosomes(Figures 5.15, 5.16, and 5.18).• Lysosomes, specialized vesicles that contain hydrolytic enzymes,digest complex molecules such as food molecules that enter the cellby endocytosis, cellular organelles that are no longer functioningcorrectly, and engulfed bacteria and cell debris (Figure 5.17).• Mitochondria carry out cellular respiration, the conversion of fuelmolecules into the energy of ATP (Figure 5.19).• Microbodies conduct the initial steps in fat breakdown and otherreactions that link major biochemical pathways in the cytoplasm(Figure 5.20).• The cytoskeleton is a supportive structure built from microtubules,intermediate filaments, and microfilaments.
Motor proteinswalking along microtubules and microfilaments produce mostmovements of animal cells (Figures 5.21–5.23).• Motor protein-controlled sliding of microtubules generates themovements of flagella and cilia. Flagella and cilia arise from centrioles (Figures 5.24–5.26).Animation: Common eukaryotic organellesAnimation: Nuclear envelopeAnimation: The endomembrane systemPractice: Structure of a mitochondrionAnimation: Cytoskeletal componentsAnimation: Motor proteinsAnimation: Flagellar structure5.4 Specialized Structures of Plant Cells• Plant cells contain all the eukaryotic structures found in animalcells except for lysosomes. They also contain three structures notfound in animal cells: chloroplasts, a central vacuole, and a cell wall(Figure 5.10).• Chloroplasts contain pigments and molecular systems that absorblight energy and convert it to chemical energy.
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