H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 61
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The structural basis of the unique design of each cell type lies in thecytoskeleton, a dense network of three classes of protein filaments that permeate the cytosol and mechanically support cellular membranes. Cytoskeletal proteins are among the mostabundant proteins in a cell, and the enormous surface area ofthe cytoskeleton (see Figure 5-1) constitutes a scaffold towhich particular sets of proteins and membranes are bound.We begin our examination of cell architecture by considering the basic structure of biomembranes.
The lipid components of membranes not only affect their shape andOUTLINE5.1 Biomembranes: Lipid Compositionand Structural Organization5.2 Biomembranes: Protein Componentsand Basic Functions5.3 Organelles of the Eukaryotic Cell5.4 The Cytoskeleton: Componentsand Structural Functions5.5 Purification of Cells and Their Parts5.6 Visualizing Cell Architecture147148CHAPTER 5 • Biomembranes and Cell ArchitecturePlasma membrane(700 µm2)Internalmembranes(7000 µm2)GolgiCytoskeleton(94,000 µm2)NucleusERMitochondrion▲ FIGURE 5-1 Schematic overview of the majorcomponents of eukaryotic cell architecture.
The plasmamembrane (red) defines the exterior of the cell and controls themovement of molecules between the cytosol and theextracellular medium. Different types of organelles and smallervesicles enclosed within their own distinctive membranes (black)carry out special functions such as gene expression, energyproduction, membrane synthesis, and intracellular transport.function but also play important roles in anchoring proteinsto the membrane, modifying membrane protein activities,and transducing signals to the cytoplasm. We then considerthe general structure of membrane proteins and how theycan relate to different membranes. The unique function ofeach membrane is determined largely by the complement ofproteins within and adjacent to it.
The theme of membranelimited compartments is continued with a review of the functions of various organelles. We then introduce the structureand function of the cytoskeleton, which is intimately associated with all biomembranes; changes in the organization ofthis filamentous network affect the structure and functionof the attached membranes. In the remainder of the chapter,we describe common methods for isolating particular typesof cells and subcellular structures and various microscopictechniques for studying cell structure and function.
FIGURE 5-2 The bilayer structure of biomembranes.(a) Electron micrograph of a thin section through an erythrocytemembrane stained with osmium tetroxide. The characteristic“railroad track” appearance of the membrane indicates thepresence of two polar layers, consistent with the bilayerstructure for phospholipid membranes. (b) Schematicinterpretation of the phospholipid bilayer in which polar groupsface outward to shield the hydrophobic fatty acyl tails fromwater. The hydrophobic effect and van der Waals interactionsbetween the fatty acyl tails drive the assembly of the bilayer(Chapter 2).
[Part (a) courtesy of J. D. Robertson.]Fibers of the cytoskeleton (green) provide structural support forthe cell and its internal compartments. The internal membranesof organelles and vesicles possess more surface area than thatof the plasma membrane but less area than that of thecytoskeleton, as schematically represented by the red, black, andgreen boxes. The enormous surface area of the cytoskeletonallows it to function as a scaffold on which cellular reactions cantake place.(a)Membrane bilayerExteriorCytosol(b)Polar headgroupsHydrophobictailsPolar headgroups5.1 • Biomembranes: Lipid Composition and Structural Organization5.1 Biomembranes: Lipid Compositionand Structural OrganizationPhospholipids of the composition present in cells spontaneously form sheetlike phospholipid bilayers, which are twomolecules thick.
The hydrocarbon chains of the phospholipids in each layer, or leaflet, form a hydrophobic core thatis 3–4 nm thick in most biomembranes. Electron microscopyof thin membrane sections stained with osmium tetroxide,which binds strongly to the polar head groups of phospholipids, reveals the bilayer structure (Figure 5-2). A cross section of all single membranes stained with osmium tetroxidelooks like a railroad track: two thin dark lines (the stain–head group complexes) with a uniform light space of about 2nm (the hydrophobic tails) between them.The lipid bilayer has two important properties.
First, thehydrophobic core is an impermeable barrier that prevents thediffusion of water-soluble (hydrophilic) solutes across themembrane. Importantly, this simple barrier function is modulated by the presence of membrane proteins that mediatethe transport of specific molecules across this otherwise impermeable bilayer. The second property of the bilayer is itsstability. The bilayer structure is maintained by hydrophobic and van der Waals interactions between the lipid chains.Even though the exterior aqueous environment can varywidely in ionic strength and pH, the bilayer has the strengthto retain its characteristic architecture.Natural membranes from different cell types exhibit a variety of shapes, which complement a cell’s function (Figure5-3).
The smooth flexible surface of the erythrocyte plasmamembrane allows the cell to squeeze through narrow bloodcapillaries. Some cells have a long, slender extension of theplasma membrane, called a cilium or flagellum, which beatsin a whiplike manner. This motion causes fluid to flow acrossthe surface of an epithelium or a sperm cell to swim throughthe medium. The axons of many neurons are encased bymultiple layers of modified plasma membrane called themyelin sheath. This membranous structure is elaborated by(a)10 m(b)(c)AX FIGURE 5-3 Variation in biomembranes in different cellSNtypes. (a) A smooth, flexible membrane covers the surface ofthe discoid erythrocyte cell. (b) Tufts of cilia (Ci) project from theependymal cells that line the brain ventricles.
(c) Many nerveaxons are enveloped in a myelin sheath composed of multiplelayers of modified plasma membrane. The individual myelin layerscan be seen in this electron micrograph of a cross section of anaxon (AX). The myelin sheath is formed by an adjacent supportive(glial) cell (SC). [Parts (a) and (b) from R. G. Kessel and R. H. Kardon,1979, Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy,W.
H. Freeman and Company. Part (c) from P. C. Cross and K. L. Mercer,1993, Cell and Tissue Ultrastructure: A Functional Perspective, W. H.Freeman and Company, p. 137.]Myelinsheath0.3 m149150CHAPTER 5 • Biomembranes and Cell Architecture FIGURE 5-4 The faces of cellularmembranes. The plasma membrane, asingle bilayer membrane, encloses thecell. In this highly schematicrepresentation, internal cytosol (greenstipple) and external environment (purple)define the cytosolic (red) and exoplasmic(black) faces of the bilayer. Vesicles andsome organelles have a single membraneand their internal aqueous space (purple)is topologically equivalent to the outsideof the cell. Three organelles—the nucleus,mitochondrion, and chloroplast (which isnot shown)—are enclosed by twomembranes separated by a smallintermembrane space. The exoplasmicfaces of the inner and outer membranesaround these organelles border theintermembrane space between them.For simplicity, the hydrophobic membraneinterior is not indicated in this diagram.MitochondrionVesicleOuter MitochondrialInner membranesMatrixIntermembrane spaceExoplasmicfaceGolgiLysosomeCytosolicfaceEndoplasmic reticulumPlasma membraneNucleusCytosolExteriorInner NuclearOutermembranesIntermembrane spacean adjacent supportive cell and facilitates the conduction ofnerve impulses over long distances (Chapter 7).
Despite theirdiverse shapes and functions, these biomembranes and allother biomembranes have a common bilayer structure.Because all cellular membranes enclose an entire cell oran internal compartment, they have an internal face (the surface oriented toward the interior of the compartment) and anexternal face (the surface presented to the environment).More commonly, the surfaces of a cellular membrane aredesignated as the cytosolic face and the exoplasmic face.
Thisnomenclature is useful in highlighting the topological equivalence of the faces in different membranes, as diagrammed inFigure 5-4. For example, the exoplasmic face of the plasmamembrane is directed away from the cytosol, toward the extracellular space or external environment, and defines theouter limit of the cell. For organelles and vesicles surroundedby a single membrane, however, the face directed away fromthe cytosol—the exoplasmic face—is on the inside in contact with an internal aqueous space equivalent to the extracellular space. This equivalence is most easily understood forvesicles that arise by invagination of the plasma membrane;this process results in the external face of the plasma membrane becoming the internal face of the vesicle membrane.Three organelles—the nucleus, mitochondrion, and chloroplast—are surrounded by two membranes; the exoplasmicsurface of each membrane faces the space between the twomembranes.Three Classes of Lipids Are Foundin BiomembranesA typical biomembrane is assembled from phosphoglycerides, sphingolipids, and steroids.
All three classes of lipidsare amphipathic molecules having a polar (hydrophilic) headgroup and hydrophobic tail. The hydrophobic effect and vander Waals interactions, discussed in Chapter 2, cause the tailgroups to self-associate into a bilayer with the polar headgroups oriented toward water (see Figure 5-2). Although thecommon membrane lipids have this amphipathic character incommon, they differ in their chemical structures, abundance,and functions in the membrane.Phosphoglycerides, the most abundant class of lipids inmost membranes, are derivatives of glycerol 3-phosphate(Figure 5-5a). A typical phosphoglyceride molecule consistsof a hydrophobic tail composed of two fatty acyl chains esterified to the two hydroxyl groups in glycerol phosphateand a polar head group attached to the phosphate group.The two fatty acyl chains may differ in the number of carbons that they contain (commonly 16 or 18) and their degreeof saturation (0, 1, or 2 double bonds).
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