H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 69
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Cells also differ considerably in shape and in theprominence of various organelles and substructures.5.3 • Organelles of the Eukaryotic Cell(a)167(b)PhagosomePlasmamembraneBacteriumPrimarylysosome2 Phagocytosis1EndocytosisPrimary lysosome0.1 µmSecondarylysosome(c)EarlyendosomeLateendosomePrimarylysosomeMitochondrionERAutophagosome3AutophagyMPSL1 µm▲ FIGURE 5-20 Cellular structures that participate indelivering materials to lysosomes. (a) Schematic overview ofthree pathways by which materials are moved to lysosomes.Soluble macromolecules are taken into the cell by invagination ofcoated pits in the plasma membrane and delivered to lysosomesthrough the endocytic pathway ( 1 ).
Whole cells and other large,insoluble particles move from the cell surface to lysosomesthrough the phagocytic pathway ( 2 ). Worn-out organelles andbulk cytoplasm are delivered to lysosomes through theautophagic pathway ( 3 ). Within the acidic lumen of lysosomes,hydrolytic enzymes degrade proteins, nucleic acids, and otherlarge molecules.
(b) An electron micrograph of a section of acultured mammalian cell that had taken up small gold particlescoated with the egg protein ovalbumin. Gold-labeled ovalbumin(black spots) is found in early endosomes (EE) and lateendosomes (LE), but very little is present in autophagosomes(AV). (c) Electron micrograph of a section of a rat liver cellshowing a secondary lysosome containing fragments of amitochondrion (M) and a peroxisome (P). [Part (b) from T.
E. Tjellelysosomes are responsible for degrading certain componentsthat have become obsolete for the cell or organism. Theprocess by which an aged organelle is degraded in a lysosomeis called autophagy (“eating oneself”). Materials taken into acell by endocytosis or phagocytosis also may be degraded inlysosomes (see Figure 5-20a). In phagocytosis, large, insoluble particles (e.g., bacteria) are enveloped by the plasmamembrane and internalized.Lysosomes contain a group of enzymes that degradepolymers into their monomeric subunits.
For example, nucleases degrade RNA and DNA into their mononucleotidebuilding blocks; proteases degrade a variety of proteins andpeptides; phosphatases remove phosphate groups frommononucleotides, phospholipids, and other compounds; stillother enzymes degrade complex polysaccharides and glycolipids into smaller units. All the lysosomal enzymes workmost efficiently at acid pH values and collectively are termedacid hydrolases. Two types of transport proteins in the lysosomal membrane work together to pump H and Cl ions(HCl) from the cytosol across the membrane, thereby acidifying the lumen (see Figure 7-10b).
The acid pH helps to denature proteins, making them accessible to the action of thelysosomal hydrolases, which themselves are resistant to aciddenaturation. Lysosomal enzymes are poorly active at theneutral pH of cells and most extracellular fluids. Thus, if alysosome releases its enzymes into the cytosol, where the pHis between 7.0 and 7.3, they cause little degradation of cytosolic components. Cytosolic and nuclear proteins generallyare not degraded in lysosomes but rather in proteasomes,large multiprotein complexes in the cytosol (see Figure 3-13).Lysosomes vary in size and shape, and several hundredmay be present in a typical animal cell. In effect, they function as sites where various materials to be degraded collect.Primary lysosomes are roughly spherical and do not containobvious particulate or membrane debris.
Secondary lysosomes, which are larger and irregularly shaped, appear to result from the fusion of primary lysosomes with othermembrane-bounded organelles and vesicles. They containparticles or membranes in the process of being digested(Figure 5-20c).et al., 1996, J. Cell Sci. 109:2905. Part (c) courtesy of D. Friend.]Tay-Sachs disease is caused by a defect in one enzyme catalyzing a step in the lysosomal breakdownof gangliosides.
The resulting accumulation ofthese glycolipids, especially in nerve cells, has devastatingconsequences. The symptoms of this inherited disease areusually evident before the age of 1. Affected children commonly become demented and blind by age 2 and die beforetheir third birthday. Nerve cells from such children aregreatly enlarged with swollen lipid-filled lysosomes. ❚168CHAPTER 5 • Biomembranes and Cell ArchitecturePports into peroxisomes an enzyme required for the oxidationof these fatty acids. Persons with the severe form of ADL areunaffected until midchildhood, when severe neurological disorders appear, followed by death within a few years.
❚MPlant seeds contain glyoxisomes, small organellesthat oxidize stored lipids as a source of carbon andenergy for growth. They are similar to peroxisomes and contain many of the same types of enzymes aswell as additional ones used to convert fatty acids into glucose precursors. ❚Smooth ERRough ERPThe Endoplasmic Reticulum Is a Networkof Interconnected Internal MembranesM1 m▲ FIGURE 5-21 Electron micrograph showing variousorganelles in a rat liver cell. Two peroxisomes (P) lie in closeproximity to mitochondria (M) and the rough and smoothendoplasmic reticulum (ER).
Also visible are accumulations ofglycogen, a polysaccharide that is the primary glucose-storagemolecule in animals. [Courtesy of P. Lazarow.]Generally, the largest membrane in a eukaryotic cell enclosesthe endoplasmic reticulum (ER)—an extensive network ofclosed, flattened membrane-bounded sacs called cisternae(see Figure 5-19). The endoplasmic reticulum has a numberof functions in the cell but is particularly important in thesynthesis of lipids, membrane proteins, and secreted proteins. The smooth endoplasmic reticulum is smooth becauseit lacks ribosomes.
In contrast, the cytosolic face of the roughendoplasmic reticulum is studded with ribosomes.Peroxisomes Degrade Fatty Acidsand Toxic CompoundsThe Smooth Endoplasmic Reticulum The synthesis of fattyGlycogenAll animal cells (except erythrocytes) and many plant cellscontain peroxisomes, a class of roughly spherical organelles,0.2–1.0 m in diameter (Figure 5-21). Peroxisomes containseveral oxidases—enzymes that use molecular oxygen to oxidize organic substances, in the process forming hydrogenperoxide (H2O2), a corrosive substance.
Peroxisomes alsocontain copious amounts of the enzyme catalase, which degrades hydrogen peroxide to yield water and oxygen:2 H2O2Catalase_____→ 2 H2O + O2In contrast with the oxidation of fatty acids in mitochondria, which produces CO2 and is coupled to the generation of ATP, peroxisomal oxidation of fatty acids yieldsacetyl groups and is not linked to ATP formation (see Figure 8-11). The energy released during peroxisomal oxidationis converted into heat, and the acetyl groups are transportedinto the cytosol, where they are used in the synthesis of cholesterol and other metabolites.
In most eukaryotic cells, theperoxisome is the principal organelle in which fatty acids areoxidized, thereby generating precursors for importantbiosynthetic pathways. Particularly in liver and kidney cells,various toxic molecules that enter the bloodstream also aredegraded in peroxisomes, producing harmless products.In the human genetic disease X-linked adrenoleukodystrophy (ADL), peroxisomal oxidation ofvery long chain fatty acids is defective.
The ADLgene encodes the peroxisomal membrane protein that trans-acids and phospholipids takes place in the smooth ER. Although many cells have very little smooth ER, this organelleis abundant in hepatocytes. Enzymes in the smooth ER of theliver also modify or detoxify hydrophobic chemicals such aspesticides and carcinogens by chemically converting theminto more water-soluble, conjugated products that can be excreted from the body. High doses of such compounds resultin a large proliferation of the smooth ER in liver cells.The Rough Endoplasmic Reticulum Ribosomes bound to therough ER synthesize certain membrane and organelle proteinsand virtually all proteins to be secreted from the cell (Chapter16). A ribosome that fabricates such a protein is bound to therough ER by the nascent polypeptide chain of the protein. Asthe growing polypeptide emerges from the ribosome, it passesthrough the rough ER membrane, with the help of specificproteins in the membrane.
Newly made membrane proteinsremain associated with the rough ER membrane, and proteinsto be secreted accumulate in the lumen of the organelle.All eukaryotic cells contain a discernible amount ofrough ER because it is needed for the synthesis of plasmamembrane proteins and proteins of the extracellular matrix.Rough ER is particularly abundant in specialized cells thatproduce an abundance of specific proteins to be secreted.
Forexample, plasma cells produce antibodies, pancreatic acinarcells synthesize digestive enzymes, and cells in the pancreatic islets of Langerhans produce the polypeptide hormonesinsulin and glucagon. In these secretory cells and others, alarge part of the cytosol is filled with rough ER and secretoryvesicles (Figure 5-22).5.3 • Organelles of the Eukaryotic Cell1694Overview Animation: Protein Secretion(a)MitochondrionNucleusNuclearmembraneSecretoryvesicle(b)Secreted protein32Secretory vesicleGolgi vesicles1NucleusRough ERGolgivesiclesEndoplasmicreticulumPlasmamembraneIntercellularspace▲ FIGURE 5-22 Charateristic features of cells specialized tosecrete large amounts of particular proteins (e.g., hormones,antibodies).
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