H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003) (796244), страница 97
Текст из файла (страница 97)
Thus at the cell, tissue, and organ levels,plants are generally less complex than most animals.Moreover, unlike animals, plants do not replace or repairold or damaged cells or tissues; they simply grow new organs. Most importantly for this chapter and in contrast withanimals, few cells in plants directly contact one anotherthrough molecules incorporated into their plasma membranes. Instead, plant cells are typically surrounded by arigid cell wall that contacts the cell walls of adjacent cells(Figure 6-33).
Also in contrast with animal cells, a plant cellrarely changes its position in the organism relative to othercells. These features of plants and their organization have determined the distinctive molecular mechanisms by whichtheir cells are incorporated into tissues. ❚232CHAPTER 6 • Integrating Cells into TissuesPrimarywallPectinCellulosemicrofibril50 nmHemicellulosePlasma membrane▲ FIGURE 6-33 Schematic representation of the cell wallof an onion. Cellulose and hemicellulose are arranged intoat least three layers in a matrix of pectin polymers. The size ofthe polymers and their separations are drawn to scale.
Tosimplify the diagram, most of the hemicellulose cross-linksand other matrix constituents (e.g., extensin, lignin) are notshown. [Adapted from M. McCann and K. R. Roberts, 1991, in C. Lloyd,ed., The Cytoskeletal Basis of Plant Growth and Form, Academic Press,p. 126.]The Plant Cell Wall Is a Laminate of CelluloseFibrils in a Matrix of GlycoproteinsThe plant cell wall is ≈0.2 m thick and completely coats theoutside of the plant cell’s plasma membrane. This structureserves some of the same functions as those of the extracellular matrix produced by animal cells, even though the twostructures are composed of entirely different macromoleculesand have a different organization. Like the extracellular matrix, the plant cell wall connects cells into tissues, signals aplant cell to grow and divide, and controls the shape of plantorgans. Just as the extracellular matrix helps define theshapes of animal cells, the cell wall defines the shapes ofplant cells.
When the cell wall is digested away from plantcells by hydrolytic enzymes, spherical cells enclosed by aplasma membrane are left. In the past, the plant cell wall wasviewed as an inanimate rigid box, but it is now recognized asa dynamic structure that plays important roles in controlling the differentiation of plant cells during embryogenesisand growth.Because a major function of a plant cell wall is to withstand the osmotic turgor pressure of the cell, the cell wall isbuilt for lateral strength. It is arranged into layers of cellulosemicrofibrils—bundles of long, linear, extensively hydrogenbonded polymers of glucose in glycosidic linkages. The cellulose microfibrils are embedded in a matrix composed ofpectin, a polymer of D-galacturonic acid and other monosaccharides, and hemicellulose, a short, highly branchedpolymer of several five- and six-carbon monosaccharides.The mechanical strength of the cell wall depends on crosslinking of the microfibrils by hemicellulose chains (see Figure6-33).
The layers of microfibrils prevent the cell wall fromstretching laterally. Cellulose microfibrils are synthesized onthe exoplasmic face of the plasma membrane from UDPglucose and ADP-glucose formed in the cytosol. The polymerizing enzyme, called cellulose synthase, moves within the planeof the plasma membrane as cellulose is formed, in directionsdetermined by the underlying microtubule cytoskeleton.Unlike cellulose, pectin and hemicellulose are synthesizedin the Golgi apparatus and transported to the cell surfacewhere they form an interlinked network that helps bind thewalls of adjacent cells to one another and cushions them.When purified, pectin binds water and forms a gel in thepresence of Ca2 and borate ions—hence the use of pectinsin many processed foods.
As much as 15 percent of the cellwall may be composed of extensin, a glycoprotein that contains abundant hydroxyproline and serine. Most of the hydroxyproline residues are linked to short chains of arabinose(a five-carbon monosaccharide), and the serine residues arelinked to galactose. Carbohydrate accounts for about 65 percent of extensin by weight, and its protein backbone formsan extended rodlike helix with the hydroxyl or O-linked carbohydrates protruding outward.
Lignin—a complex, insoluble polymer of phenolic residues—associates with celluloseand is a strengthening material. Like cartilage proteoglycans,lignin resists compression forces on the matrix.The cell wall is a selective filter whose permeability iscontrolled largely by pectins in the wall matrix. Whereaswater and ions diffuse freely across cell walls, the diffusion oflarge molecules, including proteins larger than 20 kDa, islimited.
This limitation may account for why many planthormones are small, water-soluble molecules, which can diffuse across the cell wall and interact with receptors in theplasma membrane of plant cells.Loosening of the Cell Wall Permits Elongationof Plant CellsBecause the cell wall surrounding a plant cell prevents thecell from expanding, its structure must be loosened when thecell grows. The amount, type, and direction of plant cellgrowth are regulated by small-molecule hormones (e.g., indoleacetic acid) called auxins.
The auxin-induced weakeningof the cell wall permits the expansion of the intracellularvacuole by uptake of water, leading to elongation of the cell.We can grasp the magnitude of this phenomenon by considering that, if all cells in a redwood tree were reduced to thesize of a typical liver cell, the tree would have a maximumheight of only 1 meter.The cell wall undergoes its greatest changes at the meristem of a root or shoot tip. These sites are where cells divideand expand.
Young meristematic cells are connected by thinprimary cell walls, which can be loosened and stretched to6.6 • Plant Tissuesallow subsequent cell elongation. After cell elongationceases, the cell wall is generally thickened, either by the secretion of additional macromolecules into the primary wallor, more usually, by the formation of a secondary cell wallcomposed of several layers. Most of the cell eventually degenerates, leaving only the cell wall in mature tissues suchas the xylem—the tubes that conduct salts and water fromthe roots through the stems to the leaves (see Figure 8-45).The unique properties of wood and of plant fibers such ascotton are due to the molecular properties of the cell wallsin the tissues of origin.Plasmodesmata Directly Connect the Cytosolsof Adjacent Cells in Higher PlantsPlant cells can communicate directly through specializedcell–cell junctions called plasmodesmata, which extendthrough the cell wall.
Like gap junctions, plasmodesmata areopen channels that connect the cytosol of a cell with that ofan adjacent cell. The diameter of the cytosol-filled channelis about 30–60 nm, and plasmodesmata can traverse cellwalls as much as 90 nm thick. The density of plasmodesmatavaries depending on the plant and cell type, and even thesmallest meristematic cells have more than 1000 interconnections with their neighbors.Molecules smaller than about 1000 Da, including a variety of metabolic and signaling compounds, generally candiffuse through plasmodesmata. However, the size of thechannel through which molecules pass is highly regulated.In some circumstances, the channel is clamped shut; in others, it is dilated sufficiently to permit the passage of molecules larger than 10,000 Da. Among the factors that affectthe permeability of plasmodesmata is the cytosolic Ca2concentration, with an increase in cytosolic Ca2 reversiblyinhibiting movement of molecules through these structures.Although plasmodesmata and gap junctions resembleeach other functionally, their structures differ in twosignificant ways (Figure 6-34).
The plasma membranesof the adjacent plant cells merge to form a continuouschannel, the annulus, at each plasmodesma, whereas themembranes of cells at a gap junction are not continuouswith each other. In addition, an extension of the endoplasmic reticulum called a desmotubule passes through theannulus, which connects the cytosols of adjacent plantcells. Many types of molecules spread from cell to cellthrough plasmodesmata, including proteins, nucleic acids,metabolic products, and plant viruses. Soluble moleculespass through the cytosolic annulus, whereas membranebound molecules can pass from cell to cell through thedesmotubule.(b)(a)233PlasmodesmataPlasma membraneCell wallEndoplasmic reticulumCell 1PlasmamembraneCell 2Cell wallERAnnulusPlasmodesmaERDesmotubule▲ FIGURE 6-34 Structure of a plasmodesma.(a) Schematic model of a plasmodesma showing thedesmotubule, an extension of the endoplasmic reticulum, andthe annulus, a plasma membrane–lined channel filled withcytosol that interconnects the cytosols of adjacent cells.
Notshown is a gating complex that fills the channel and controls thetransport of materials through the plasmodesma.(b) Electron micrograph of thin section of plant cell and cell wallcontaining multiple plasmodesmata. [E. H. Newcomb and W. P.Wergin/Biological Photo Service.]234CHAPTER 6 • Integrating Cells into TissuesOnly a Few Adhesive Molecules Have BeenIdentified in PlantsSystematic analysis of the Arabidopsis genome and biochemical analysis of other plant species provide no evidence for theexistence of plant homologs of most animal CAMs, adhesionreceptors, and ECM components. This finding is not surprising, given the dramatically different nature of cell–cell andcell–matrix/wall interactions in animals and plants.Among the adhesive-type proteins apparently unique toplants are five wall-associated kinases (WAKs) and WAK-likeproteins expressed in the plasma membrane of Arabidopsiscells.
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