Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 72
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Transportvesicles from the rough ER fuse with the cis region of theGolgi complex, where they deposit their protein contents. Asdetailed in Chapter 17, these proteins then progress from thecis to the medial to the trans region. Within each region aredifferent enzymes that modify proteins to be secreted andmembrane proteins differently, depending on their structuresand their final destinations.After proteins to be secreted and membrane proteins aremodified in the Golgi complex, they are transported out ofthe complex by a second set of vesicles, which seem to budfrom the trans side of the Golgi complex.
Some vesicles carrymembrane proteins destined for the plasma membrane orsoluble proteins to be released from the cell surface; othersSeveral minutes after proteins are synthesized in the roughER, most of them leave the organelle within small membranebounded transport vesicles. These vesicles, which bud fromregions of the rough ER not coated with ribosomes, carry theproteins to another membrane-limited organelle, the Golgicomplex (see Figure 5-22).Three-dimensional reconstructions from serial sections ofa Golgi complex reveal this organelle to be a series of flattened membrane vesicles or sacs (cisternae), surrounded bya number of more or less spherical membrane-limited vesicles (Figure 5-23).
The stack of Golgi cisternae has three de-MEDIA CONNECTIONS2 µmVideo: Three-Dimensional Model of a Golgi ComplexMEDIA CONNECTIONS170CHAPTER 5 • Biomembranes and Cell Architecturefunction similar to that of lysosomes in animal cells. Similarstorage vacuoles are found in green algae and many microorganisms such as fungi.Like most cellular membranes, the vacuolar membrane ispermeable to water but is poorly permeable to the small molecules stored within it. Because the solute concentration ismuch higher in the vacuole lumen than in the cytosol or extracellular fluids, water tends to move by osmotic flow intovacuoles, just as it moves into cells placed in a hypotonicmedium (see Figure 5-18).
This influx of water causes boththe vacuole to expand and water to move into the cell, creating hydrostatic pressure, or turgor, inside the cell. Thispressure is balanced by the mechanical resistance of thecellulose-containing cell walls that surround plant cells. Mostplant cells have a turgor of 5–20 atmospheres (atm); their cellwalls must be strong enough to react to this pressure in acontrolled way. Unlike animal cells, plant cells can elongateextremely rapidly, at rates of 20–75 m/h. This elongation,▲ FIGURE 5-23 Model of the Golgi complex based onthree-dimensional reconstruction of electronmicroscopy images.
Transport vesicles (white spheres)that have budded off the rough ER fuse with the cismembranes (light blue) of the Golgi complex. Bymechanisms described in Chapter 17, proteins move fromthe cis region to the medial region and finally to the transregion of the Golgi complex. Eventually, vesicles bud offthe trans-Golgi membranes (orange and red); some moveto the cell surface and others move to lysosomes. TheGolgi complex, like the rough endoplasmic reticulum, isespecially prominent in secretory cells. [From B. J.
Marsh(a)Vacuoleet al., 2001, Proc Nat’l. Acad. Sci USA 98:2399.]Chloroplastcarry soluble or membrane proteins to lysosomes or other organelles. How intracellular transport vesicles “know” withwhich membranes to fuse and where to deliver their contentsis also discussed in Chapter 17.Plant Vacuoles Store Small Molecules andEnable a Cell to Elongate RapidlyMost plant cells contain at least one membranelimited internal vacuole. The number and size ofvacuoles depend on both the type of cell and itsstage of development; a single vacuole may occupy as muchas 80 percent of a mature plant cell (Figure 5-24).
A varietyof transport proteins in the vacuolar membrane allow plantcells to accumulate and store water, ions, and nutrients (e.g.,sucrose, amino acids) within vacuoles (Chapter 7). Like alysosome, the lumen of a vacuole contains a battery ofdegradative enzymes and has an acidic pH, which is maintained by similar transport proteins in the vacuolar membrane.
Thus plant vacuoles may also have a degradativeGranumCell wall2 m▲ FIGURE 5-24 Electron micrograph of a thin section of aleaf cell. In this cell, a single large vacuole occupies much of thecell volume. Parts of five chloroplasts and the cell wall also arevisible. Note the internal subcompartments in the chloroplasts.[Courtesy of Biophoto Associates/Myron C. Ledbetter/Brookhaven NationalLaboratory.]5.3 • Organelles of the Eukaryotic Cellwhich usually accompanies plant growth, occurs when a segment of the somewhat elastic cell wall stretches under thepressure created by water taken into the vacuole. ❚The Nucleus Contains the DNA Genome, RNASynthetic Apparatus, and a Fibrous MatrixThe nucleus, the largest organelle in animal cells, is surrounded by two membranes, each one a phospholipid bilayercontaining many different types of proteins.
The inner nuclear membrane defines the nucleus itself. In most cells, theouter nuclear membrane is continuous with the rough endoplasmic reticulum, and the space between the inner and outernuclear membranes is continuous with the lumen of therough endoplasmic reticulum (see Figure 5-19). The two nuclear membranes appear to fuse at nuclear pores, the ringlikecomplexes composed of specific membrane proteins throughwhich material moves between the nucleus and the cytosol.The structure of nuclear pores and the regulated transportof material through them are detailed in Chapter 12.In a growing or differentiating cell, the nucleus is metabolically active, replicating DNA and synthesizing rRNA,N171tRNA, and mRNA. Within the nucleus mRNA binds to specific proteins, forming ribonucleoprotein particles.
Most ofthe cell’s ribosomal RNA is synthesized in the nucleolus, asubcompartment of the nucleus that is not bounded by aphospholipid membrane (Figure 5-25). Some ribosomal proteins are added to ribosomal RNAs within the nucleolus aswell. The finished or partly finished ribosomal subunits, aswell as tRNAs and mRNA-containing particles, pass througha nuclear pore into the cytosol for use in protein synthesis(Chapter 4). In mature erythrocytes from nonmammalianvertebrates and other types of “resting” cells, the nucleus isinactive or dormant and minimal synthesis of DNA andRNA takes place.How nuclear DNA is packaged into chromosomes is described in Chapter 10.
In a nucleus that is not dividing, thechromosomes are dispersed and not dense enough to be observed in the light microscope. Only during cell division areindividual chromosomes visible by light microscopy. In theelectron microscope, the nonnucleolar regions of the nucleus,called the nucleoplasm, can be seen to have dark- and lightstaining areas. The dark areas, which are often closely associated with the nuclear membrane, contain condensedconcentrated DNA, called heterochromatin (see Figure5-25).
Fibrous proteins called lamins form a two-dimensionalnetwork along the inner surface of the inner membrane, giving it shape and apparently binding DNA to it. The breakdown of this network occurs early in cell division, as wedetail in Chapter 21.Mitochondria Are the Principal Sites of ATPProduction in Aerobic CellsnHeterochromatin1 m▲ FIGURE 5-25 Electron micrograph of a thin section of abone marrow stem cell. The nucleolus (n) is a subcompartmentof the nucleus (N) and is not surrounded by a membrane. Mostribosomal RNA is produced in the nucleolus. Darkly stainingareas in the nucleus outside the nucleolus are regions ofheterochromatin.
[From P. C. Cross and K. L. Mercer, 1993, Cell andTissue Ultrastructure, W. H. Freeman and Company, p. 165.]Most eukaryotic cells contain many mitochondria, which occupy up to 25 percent of the volume of the cytoplasm. Thesecomplex organelles, the main sites of ATP production duringaerobic metabolism, are generally exceeded in size only bythe nucleus, vacuoles, and chloroplasts.The two membranes that bound a mitochondrion differin composition and function. 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.
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