H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 71
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The outer membrane, composed of about half lipid and half protein, contains porins(see Figure 5-14) that render the membrane permeable tomolecules having molecular weights as high as 10,000. Inthis respect, the outer membrane is similar to the outer membrane of gram-negative bacteria. The inner membrane,which is much less permeable, is about 20 percent lipid and80 percent protein—a higher proportion of protein than exists in other cellular membranes.
The surface area of theinner membrane is greatly increased by a large number ofinfoldings, or cristae, that protrude into the matrix, or central space (Figure 5-26).In nonphotosynthetic cells, the principal fuels for ATPsynthesis are fatty acids and glucose. The complete aerobicdegradation of glucose to CO2 and H2O is coupled to thesynthesis of as many as 30 molecules of ATP. In eukaryoticcells, the initial stages of glucose degradation take place in172CHAPTER 5 • Biomembranes and Cell ArchitectureInner membraneCristaeOutermembraneVideo: Three-Dimensional Model of a MitochondrionMEDIA CONNECTIONSPlasma membraneGranaThylakoidmembraneStromaChloroplastmembranes(outer and inner)StarchgranuleIntermembranespaceMatrixgranulesMatrix1 m▲ FIGURE 5-26 Electron micrograph of amitochondrion.
Most ATP production in nonphotosyntheticcells takes place in mitochondria. The inner membrane,which surrounds the matrix space, has many infoldings,called cristae. Small calcium-containing matrix granules alsoare evident. [From D. W. Fawcett, 1981, The Cell, 2d ed.,▲ FIGURE 5-27 Electron micrograph of a plant chloroplast.The internal membrane vesicles (thylakoids) are fused into stacks(grana), which reside in a matrix (the stroma). All the chlorophyll inthe cell is contained in the thylakoid membranes, where the lightinduced production of ATP takes place during photosynthesis.Saunders, p.
421.][Courtesy of Biophoto Associates/M. C. Ledbetter/Brookhaven NationalLaboratory.]the cytosol, where 2 ATP molecules per glucose molecule aregenerated. The terminal stages of oxidation and the coupledsynthesis of ATP are carried out by enzymes in the mitochondrial matrix and inner membrane (Chapter 8). As manyas 28 ATP molecules per glucose molecule are generated inmitochondria. Similarly, virtually all the ATP formed in theoxidation of fatty acids to CO2 is generated in mitochondria.Thus mitochondria can be regarded as the “power plants” ofthe cell.Chloroplasts Contain Internal Compartmentsin Which Photosynthesis Takes PlaceExcept for vacuoles, chloroplasts are the largest andthe most characteristic organelles in the cells ofplants and green algae. They can be as long as10 m and are typically 0.5–2 m thick, but they vary in sizeand shape in different cells, especially among the algae.
In addition to the double membrane that bounds a chloroplast, thisorganelle also contains an extensive internal system of interconnected membrane-limited sacs called thylakoids, whichare flattened to form disks (Figure 5-27). Thylakoids oftenform stacks called grana and are embedded in a matrix, thestroma. The thylakoid membranes contain green pigments(chlorophylls) and other pigments that absorb light, as well asenzymes that generate ATP during photosynthesis. Some ofthe ATP is used to convert CO2 into three-carbon intermediates by enzymes located in the stroma; the intermediates arethen exported to the cytosol and converted into sugars. ❚The molecular mechanisms by which ATP is formed inmitochondria and chloroplasts are very similar, as explainedin Chapter 8.
Chloroplasts and mitochondria have other features in common: both often migrate from place to placewithin cells, and they contain their own DNA, which encodes some of the key organellar proteins (Chapter 10). Theproteins encoded by mitochondrial or chloroplast DNA aresynthesized on ribosomes within the organelles. However,most of the proteins in each organelle are encoded in nuclearDNA and are synthesized in the cytosol; these proteins arethen incorporated into the organelles by processes describedin Chapter 16.KEY CONCEPTS OF SECTION 5.3Organelles of the Eukaryotic CellAll eukaryotic cells contain a nucleus and numerousother organelles in their cytosols (see Figure 5-19).■5.4 • The Cytoskeleton: Components and Structural FunctionsThe nucleus, mitochondrion, and chloroplast arebounded by two bilayer membranes separated by an intermembrane space. All other organelles are surrounded bya single membrane.■Endosomes internalize plasma-membrane proteins andsoluble materials from the extracellular medium, and theysort them back to the membranes or to lysosomes fordegradation.■Lysosomes have an acidic interior and contain varioushydrolases that degrade worn-out or unneeded cellularcomponents and some ingested materials (see Figure 5-20).■173tigators believe that the cytosol is highly organized, withmost soluble proteins either bound to filaments or otherwiselocalized in specific regions.
In an electron micrograph of atypical animal cell, soluble proteins packing the cell interiorconceal much of the internal structure. If a cell is pretreatedwith a nonionic detergent (e.g., Triton X-100), which permeabilizes the membrane, soluble cytosolic proteins diffuseaway. In micrographs of detergent-extracted animal cells,two types of structures stand out—membrane-limited organelles and the filaments of the cytoskeleton, which fill thecytosol (Figure 5-28).Peroxisomes are small organelles containing enzymesthat oxidize various organic compounds without the production of ATP.
By-products of oxidation are used inbiosynthetic reactions.■Secreted proteins and membrane proteins are synthesizedon the rough endoplasmic reticulum, a network of flattened membrane-bounded sacs studded with ribosomes.■Proteins synthesized on the rough ER first move to theGolgi complex, where they are processed and sorted for transport to the cell surface or other destination (see Figure 5-22).MembranemicrofilamentlinkagesPlant cells contain one or more large vacuoles, whichare storage sites for ions and nutrients. Osmotic flow ofwater into vacuoles generates turgor pressure that pushesthe plasma membrane against the cell wall.Core actinfilaments■■The nucleus houses the genome of a cell.
The inner andouter nuclear membranes are fused at numerous nuclearpores, through which materials pass between the nucleusand the cytosol. The outer nuclear membrane is continuous with that of the rough endoplasmic reticulum.■Mitochondria have a highly permeable outer membraneand a protein-enriched inner membrane that is extensivelyfolded. Enzymes in the inner mitochondrial membrane andcentral matrix carry out the terminal stages of sugar andlipid oxidation coupled to ATP synthesis.■Chloroplasts contain a complex system of thylakoidmembranes in their interiors. These membranes contain thepigments and enzymes that absorb light and produce ATPduring photosynthesis.Actin filaments(rootlets)SpectrinconnectingfibersKeratin intermediatefilaments■5.4 The Cytoskeleton: Componentsand Structural FunctionsThe cytosol is a major site of cellular metabolism and contains a large number of different enzymes.
Proteins constituteabout 20–30 percent of the cytosol by weight, and from aquarter to half of the total protein within cells is in the cytosol. Estimates of the protein concentration in the cytosolrange from 200 to 400 mg/ml. Because of the high concentration of cytosolic proteins, complexes of proteins can formeven if the energy that stabilizes them is weak. Many inves-▲ FIGURE 5-28 Electron micrograph of the apical part of adetergent-extracted intestinal epithelial cell. Microvilli,fingerlike projections of the plasma membrane, cover the apicalsurface of an intestinal epithelial cell.
A bundle of microfilamentsin the core of each microvillus stabilizes the structure. Theplasma membrane surrounding a microvillus is attached to thesides of the bundle by evenly spaced membrane–microfilamentlinkages (yellow). The bundle continues into the cell as a shortrootlet. The rootlets of multiple microvilli are cross-braced byconnecting fibers (red) composed of an intestinal isoform ofspectrin. This fibrous actin-binding protein is found in a narrowband just below the plasma membrane in many animal cells.
Thebases of the rootlets are attached to keratin intermediatefilaments. These numerous connections anchor the rootlets in ameshwork of filaments and thereby support the uprightorientation of the microvilli. [Courtesy of N. Hirokawa.]174CHAPTER 5 • Biomembranes and Cell ArchitectureIn this section, we introduce the protein filaments thatcompose the cytoskeleton and then describe how they support the plasma and nuclear membranes and organize thecontents of the cell. Later chapters will deal with the dynamicproperties of the cytoskeleton—its assembly and disassemblyand its role in cellular movements.Three Types of Filaments Composethe CytoskeletonThe cytosol of a eukaryotic cell contains three types of filaments that can be distinguished on the bases of their diameter,type of subunit, and subunit arrangment (Figure 5-29).
Actinfilaments, also called microfilaments, are 8–9 nm in diameter and have a twisted two-stranded structure. Microtubulesare hollow tubelike structures, 24 nm in diameter, whosewalls are formed by adjacent protofilaments. Intermediatefilaments (IFs) have the structure of a 10-nm-diameter rope.Each type of cytoskeletal filament is a polymer of protein subunits (Table 5-4).
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