H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 68
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We have alluded to many functions of the plasmamembrane in the preceding discussion and briefly considerits major functions here.In all cells, the plasma membrane acts as a permeabilitybarrier that prevents the entry of unwanted materials fromthe extracellular milieu and the exit of needed metabolites.Specific membrane transport proteins in the plasma membrane permit the passage of nutrients into the cell and metabolic wastes out of it; others function to maintain the properionic composition and pH (≈7.2) of the cytosol.
The structure and function of proteins that make the plasma membrane selectively permeable to different molecules arediscussed in Chapter 7.The plasma membrane is highly permeable to water butpoorly permeable to salts and small molecules such as sugars and amino acids. Owing to osmosis, water moves acrosssuch a semipermeable membrane from a solution of lowsolute (high water) concentration to one of high solute (lowwater) concentration until the total solute concentrationsand thus the water concentrations on both sides are equal.Figure 5-18 illustrates the effect on animal cells of differentexternal ion concentrations.
When most animal cells areplaced in an isotonic solution (i.e., one with total concentration of solutes equal to that of the cell interior), there isno net movement of water into or out of cells. However,when cells are placed in a hypotonic solution (i.e., one witha lower solute concentration than that of the cell interior),water flows into the cells, causing them to swell.
Conversely,in a hypertonic solution (i.e., one with a higher solute concentration than that of the cell interior), water flows out ofcells, causing them to shrink. Under normal in vivo conditions, ion channels in the plasma membrane controlthe movement of ions into and out of cells so that there isno net movement of water and the usual cell volume ismaintained.Unlike animal cells, bacterial, fungal, and plant cellsare surrounded by a rigid cell wall and lack the extracellular matrix found in animal tissues. The plasma membraneis intimately engaged in the assembly of cell walls, whichin plants are built primarily of cellulose.
The cell wallprevents the swelling or shrinking of a cell that would otherwise occur when it is placed in a hypotonic or hyper-(a) Isotonic medium0.15 M KCl0.15 M NaCl(b) Hypotonic medium0.15 M KCl0.075 M NaCl(c) Hypertonic medium0.15 M KCl0.30 M NaCl▲ FIGURE 5-18 Effect of external ion concentration onwater flow across the plasma membrane of an animal cell.Sodium, potassium, and chloride ions do not move freely acrossthe plasma membrane, but water channels (aquaporins) in themembrane permit the flow of water in the direction dictated bythe ion concentration of the surrounding medium. (a) When themedium is isotonic, there is no net flux of water into or out ofthe cell.
(b) When the medium is hypotonic, water flows into thecell (red arrow) until the ion concentration inside and outside thecell is the same. Because of the influx of water, the cell volumeincreases. (c) When the medium is hypertonic, water flows outof the cell until the ion concentration inside and outside the cellis the same. Because water is lost, the cell volume decreases.tonic medium, respectively. For this reason, cells surroundedby a wall can grow in media having an osmotic strengthmuch less than that of the cytosol. The properties, function, and formation of the plant cell wall are covered inChapter 6.In addition to these universal functions, the plasmamembrane has other crucial roles in multicellular organisms.
Few of the cells in multicellular plants and animalsexist as isolated entities; rather, groups of cells with relatedspecializations combine to form tissues. In animal cells, specialized areas of the plasma membrane contain proteins andglycolipids that form specific junctions between cells tostrengthen tissues and to allow the exchange of metabolites5.3 • Organelles of the Eukaryotic Cellbetween cells. Certain plasma-membrane proteins anchorcells to components of the extracellular matrix, the mixtureof fibrous proteins and polysaccharides that provides a bedding on which most sheets of epithelial cells or small glandslie.
We examine both of these membrane functions in Chapter 6. Still other proteins in the plasma membrane act as anchoring points for many of the cytoskeletal fibers thatpermeate the cytosol, imparting shape and strength to cells(see Section 5.4).The plasma membranes of many types of eukaryotic cellsalso contain receptor proteins that bind specific signalingmolecules (e.g., hormones, growth factors, neurotransmitters), leading to various cellular responses. These proteins,which are critical for cell development and functioning, aredescribed in several later chapters. Finally, peripheral cytosolic proteins that are recruited to the membrane surfacefunction as enzymes, intracellular signal transducers, andstructural proteins for stabilizing the membrane.Like the plasma membrane, the membrane surrounding each organelle in eukaryotic cells contains a unique setof proteins essential for its proper functioning.
In the nextsection, we provide a brief overview of the main eukaryoticorganelles.KEY CONCEPTS OF SECTION 5.2Biomembranes: Protein Componentsand Basic Functions■ Biological membranes usually contain both integral (transmembrane) and peripheral membrane proteins, which do notenter the hydrophobic core of the bilayer (see Figure 5-11).Most integral membrane proteins contain one or moremembrane-spanning hydrophobic helices and hydrophilic domains that extend from the cytosolic and exoplasmic faces of the membrane (see Figure 5-12).■The porins, unlike other integral proteins, contain membranespanning sheets that form a barrel-like channel through thebilayer.■Long-chain lipids attached to certain amino acids anchor some proteins to one or the other membrane leaflet(see Figure 5-15).■Some peripheral proteins associate with the membraneby interactions with integral proteins.
Lipid-binding motifs in other peripheral proteins interact with the polar headgroups of membrane phospholipids (see Table 5-3).■The binding of a water-soluble enzyme (e.g., a phospholipase, kinase, or phosphatase) to a membrane surfacebrings the enzyme close to its substrate and in some casesactivates it. Such interfacial binding is due to the attraction between positive charges on basic residues in the protein and negative charges on phospholipid head groups inthe bilayer.■1655.3 Organelles of the Eukaryotic CellThe cell is in a dynamic flux.
In the light microscope, a livecell exhibits myriad movements ranging from the translocation of chromosomes and vesicles to the changes in shape associated with cell crawling and swimming. Investigation ofintracellular structures begins with micrographs of fixed, sectioned cells in which all cell movements are frozen. Suchstatic pictures of the cell reveal the organization of the cytoplasm into compartments and the stereotypic location ofeach type of organelle within the cell. In this section, we describe the basic structures and functions of the major organelles in animal and plant cells (Figure 5-19).
Plant andfungal cells contain most of the organelles found in an animal cell but lack lysosomes. Instead, they contain a large central vacuole that subserves many of the functions of alysosome. A plant cell also contains chloroplasts, and itsmembrane is strengthened by a rigid cell wall. Unique proteins in the interior and membranes of each type of organellelargely determine its specific functional characteristics, whichare examined in more detail in later chapters. Thoseorganelles bounded by a single membrane are covered first,followed by the three types that have a double membrane—the nucleus, mitochondrion, and chloroplast.Endosomes Take Up Soluble Macromoleculesfrom the Cell ExteriorAlthough transport proteins in the plasma membrane mediate the movement of ions and small molecules across thelipid bilayer, proteins and some other soluble macromolecules in the extracellular milieu are internalized by endocytosis.
In this process, a segment of the plasma membraneinvaginates into a “coated pit,” whose cytosolic face is linedby a specific set of proteins including clathrin. The pitpinches from the membrane into a small membrane-boundedvesicle that contains extracellular material and is delivered toan early endosome, a sorting station of membrane-limitedtubules and vesicles (Figure 5-20a, b). From this compartment,some membrane proteins are recycled back to the plasmamembrane; other membrane proteins are transported to alate endosome where further sorting takes place.
The endocytic pathway ends when a late endosome delivers its membrane and internal contents to lysosomes for degradation.The entire endocytic pathway is described in some detail inChapter 17.Lysosomes Are Acidic Organelles That Containa Battery of Degradative EnzymesLysosomes provide an excellent example of the ability of intracellular membranes to form closed compartments inwhich the composition of the lumen (the aqueous interiorof the compartment) differs substantially from that of thesurrounding cytosol. Found exclusively in animal cells,1237 814456159 1011 12 13161Plasma membrane controls movement of molecules in andout of the cell and functions in cell-cell signaling and celladhesion.92Mitochondria, which are surrounded by a double membrane,generate ATP by oxidation of glucose and fatty acids.10 Secretory vesicles store secreted proteins and fuse with theplasma membrane to release their contents.3Lysosomes, which have an acidic lumen, degrade materialinternalized by the cell and worn-out cellular membranes andorganelles.11 Peroxisomes detoxify various molecules and also break downfatty acids to produce acetyl groups for biosynthesis.4Nuclear envelope, a double membrane, encloses the contentsof the nucleus; the outer nuclear membrane is continuouswith the rough ER.5Nucleolus is a nuclear subcompartment where most of thecell's rRNA is synthesized.6Nucleus is filled with chromatin composed of DNA andproteins; in dividing cells is site of mRNA and tRNA synthesis.7Smooth endoplasmic reticulum (ER) synthesizes lipids anddetoxifies certain hydrophobic compounds.8Rough endoplasmic reticulum (ER) functions in the synthesis,processing, and sorting of secreted proteins, lysosomalproteins, and certain membrane.▲ FIGURE 5-19 Schematic overview of a “typical” animalcell and plant cell and their major substructures.
Not everycell will contain all the organelles, granules, and fibrous166Golgi complex processes and sorts secreted proteins,lysosomal proteins, and membrane proteins synthesized onthe rough ER.12 Cytoskeletal fibers form networks and bundles that supportcellular membranes, help organize organelles, and participatein cell movement.13 Microvilli increase surface area for absorption of nutrientsfrom surrounding medium.14 Cell wall, composed largely of cellulose, helps maintain thecell's shape and provides protection against mechanicalstress.15 Vacuole stores water, ions, and nutrients, degradesmacromolecules, and functions in cell elongation duringgrowth.16 Chloroplasts, which carry out photosynthesis, are surroundedby a double membrane and contain a network of internalmembrane-bounded sacs.structures shown here, and other substructures can be presentin some.
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