Biology_Unit_5 (1110837), страница 6
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Cells that are highly active in making proteins tobe released outside the cell, such as pancreatic cells that make digestive enzymes, are packed with rough ER but have relativelylittle smooth ER. By contrast, cells that primarily synthesize lipids or break down toxic substances are packed with smooth ERbut contain little rough ER.FIGURE 5.15The Golgi complex.0.25 μmthe proteins, which tags them for sorting to their final destinations.
For instance, proteins secreted from the cell are transportedto the plasma membrane in secretory vesicles, which release theircontents to the exterior by exocytosis (Figure 5.16A). In this process, a secretory vesicle fuses with the plasma membrane andspills the vesicle contents to the outside. The contents of secretoryvesicles vary, including signaling molecules such as hormonesand neurotransmitters (see Chapter 7), waste products or toxicsubstances, and enzymes (such as from cells lining the intestine).The membrane of a vesicle that fuses with the plasma membranebecomes part of the plasma membrane.
In fact, this process isused to expand the surface of the cell during cell growth.Vesicles also may form by the reverse process, called endocytosis, which brings molecules into the cell from the exterior(Figure 5.16B). In this process, the plasma membrane forms apocket, which bulges inward and pinches off into the cytoplasmas an endocytic vesicle. Once in the cytoplasm, endocytic vesicles, which contain segments of the plasma membrane as well asproteins and other molecules, are carried to the Golgi complexor to other destinations such as lysosomes in animal cells. Thesubstances carried to the Golgi complex are sorted and placedinto vesicles for routing to other locations, which may include lysosomes. Those routed to lysosomes are digested into molecularsubunits that may be recycled as building blocks for the biological molecules of the cell.
Exocytosis and endocytosis are discussed in more detail in Chapter 6.A. Exocytosis: A secretory vesicle fuses with the plasmamembrane, releasing the vesicle contents to the cell exterior. Thevesicle membrane becomes part of the plasma membrane.Outside cellLYSOSOMES Lysosomes (lys breakdown; some body) aresmall, membrane-bound vesicles that contain more than 30 hydrolytic enzymes for the digestion of many complex molecules,including proteins, lipids, nucleic acids, and polysaccharides (Figure 5.17).
The cell recycles the subunits of these molecules. Lysosomes are found in animals, but not in plants. The functions of lysosomes in plants are carried out by the central vacuole (seeSection 5.4). Depending on the contents they are digesting, lysosomes assume a variety of sizes and shapes instead of a uniformstructure as is characteristic of other organelles. Most commonly,lysosomes are small (0.1–0.5 m in diameter) oval or sphericalbodies.
A human cell contains about 300 lysosomes.Lysosomes are formed by budding from the Golgi complex.Their hydrolytic enzymes are synthesized in the rough ER, modified in the lumen of the ER to identify them as being bound fora lysosome, transported to the Golgi complex in a vesicle, andthen packaged in the budding lysosome.The pH within lysosomes is acidic (pH ⬃5) and is significantly lower than the pH of the cytosol (pH ⬃7.2).
The hydrolyticenzymes in the lysosomes function optimally at the acidic pHwithin the organelle, but they do not function well at the pH ofthe cytosol; this difference reduces the risk to the viability of thecell should the enzymes be released from the vesicle.Lysosomal enzymes can digest several types of materials.They digest food molecules entering the cell by endocytosis whenan endocytic vesicle fuses with a lysosome.
In a process calledautophagy, they digest organelles that are not functioning correctly. A membrane surrounds the defective organelle, forming alarge vesicle that fuses with one or more lysosomes; the organellethen is degraded by the hydrolytic enzymes. They also play a rolein phagocytosis, a process in which some types of cells engulfbacteria or other cellular debris to break them down. These cellsPlasma membranePlasmamembraneSecretory vesicleCytoplasmLysosomeB. Endocytosis: Materials from the cell exterior are enclosed in aEndocytic vesicleDon W.
Fawcett/Photo Researchers, Inc.segment of the plasma membrane that pockets inward and pinchesoff as an endocytic vesicle.LysosomecontainingingestedmaterialFIGURE 5.16FIGURE 5.17Exocytosis and endocytosis.A lysosome.CHAPTER 5THE CELL: AN OVERVIEW101include the white blood cells known as phagocytes, which play animportant role in the immune system (see Chapter 43).
Phagocytosis produces a large vesicle that contains the engulfed materialsuntil lysosomes fuse with the vesicle and release the hydrolyticenzymes necessary for degrading them.In certain human genetic diseases known as lysosomal storage diseases, one of the hydrolytic enzymes normally found in thelysosome is absent. As a result, the substrate of that enzyme accumulates in the lysosomes, and this accumulation eventually interferes with normal cellular activities.
An example is Tay–Sachsdisease, which is a fatal disease of the central nervous systemcaused by the failure to synthesize the enzyme needed for hydrolysis of fatty acid derivatives found in brain and nerve cells.Proteins (greenand yellow) areassembled fromamino acids byribosomes attachedto the ER or free inthe cytosol.Instructions for buildingproteins leave the nucleusand enter the cytoplasm.NucleusRibosomesRoughERDamagedorganelleEndocyticvesicle10 2UNIT ONEMOLECULES AND CELLSMitochondria Are the Organellesin Which Cellular Respiration OccursMitochondria (singular, mitochondrion) are the membrane-boundorganelles in which cellular respiration occurs. Cellular respiration isthe process by which energy-rich molecules such as sugars, fats,and other fuels are broken down to water and carbon dioxide bymitochondrial reactions, with the release of energy. Much ofthe energy released by the breakdown is captured in ATP.
Infact, mitochondria generate most of the ATP of the cell. Mitochondria require oxygen for cellular respiration—whenyou breathe, you are taking in oxygen primarily for yourmitochondrial reactions (see Chapter 8).1 Proteins made by ERribosomes enter ER membranesor the space inside ER cisternae.Chemical modification of someproteins begins.
Membrane lipidsare also made in the ER.Vesicles2 Vesicles bud fromthe ER membrane andthen transport unfinishedproteins and lipids to theGolgi complex.Golgicomplex3 Protein and lipidmodification is completedin the Golgi complex, andproducts are sorted intovesicles that bud fromthe complex.SecretoryvesiclesLysosomesSUMMARY In summary, the endomembrane system is a majortraffic network for proteins and other substances within the cell.The Golgi complex in particular is a key distribution station formembranes and proteins (Figure 5.18).
From the Golgi complex,lipids and proteins may move to storage or secretory vesicles, andfrom the secretory vesicles, they may move to the cell exterior byexocytosis. Membranes and proteins may also move between thenuclear envelope and the endomembrane system. Proteins andother materials that enter cells by endocytosis also enter the endomembrane system to travel to the Golgi complex for sorting anddistribution to other locations.4 Secretory vesicles buddingfrom the Golgi membranestransport finished products to theplasma membrane. The productsare released by exocytosis.
Othervesicles remain in storage inthe cytoplasm.5 Lysosomes budding from theGolgi membranes containhydrolytic enzymes that digestdamaged organelles or thecontents of endocytic vesicles thatfuse with them. Endocytic vesiclesform at the plasma membraneand move into the cytoplasm.FIGURE 5.18Vesicle traffic in thecytoplasm. The ER andGolgi complex are part ofthe endomembranesystem, which releasesproteins and other substances to the cell exteriorand gathers materialsfrom outside the cell.Mitochondria are enclosed by two membranes (Figure 5.19).The outer mitochondrial membrane is smooth and covers theoutside of the organelle.
The surface area of the inner mitochondrial membrane is expanded by folds called cristae (singular,crista). Both membranes surround the innermost compartmentof the mitochondrion, called the mitochondrial matrix. TheATP-generating reactions of mitochondria occur in the cristaeand matrix.The mitochondrial matrix also contains DNA and ribosomes that resemble the equivalent structures in bacteria. Theseand other similarities suggest that mitochondria originated fromancient bacteria that became permanent residents of the cytoplasm during the evolution of eukaryotic cells (see Chapter 24for further discussion).Microbodies Carry Out Vital Reactions ThatLink Metabolic PathwaysMicrobodies are small, relatively simple membrane-bound organelles found in various forms in essentially all eukaryotic cells.They consist of a single boundary membrane that encloses a collection of enzymes and other proteins (Figure 5.20).
Recent research has shown that the ER is involved in microbody production. Proteins and phospholipids are continuously imported intomicrobodies. The phospholipids are used for new membrane synthesis, leading to growth of the microbody. Division of a microbody then produces new microbodies.Microbodies have various functions that are often specific toan organism or cell type. Commonly, they contain enzymes thatconduct preparatory or intermediate reactions linking majorbiochemical pathways. For example, the series of reactions thatallows cells to use fats as an energy source begins in microbodiesand continues in mitochondria.
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