Часть 2 (1121000), страница 62
Текст из файла (страница 62)
In evolving relatively rigid wails, which can be up to manymicrometers thick, ear$ plant cells forfeited the ability to crawl about andadopted a sedentary lifestyle that has persisted in all present-day plants.of the CellWallDependson the CellTypeTheCompositionAll cell walls in plants have their origin in dividing cells, as the cell plate formsduring cltokinesis to create a new partition wall between the daughter cells (discussed in Chapter 17). The new cells are usually produced in special regionscalled merisfems(discussedin Chapter 22), and they are generally small in comparison with their final size.To accommodate subsequent cell growth, the wallsof the newborn cells, called primary cell walls, are thin and extensible, althoughtough.
Once growth stops, the wall no longer needs to be extensible: sometimesthe primary wall is retained without major modification, but, more commonly,a rigid, secondary cell wall is produced by depositing new layers of matrix insidethe old ones. These new layers generally have a composition that is significantlydifferent from that of the primarywall. The most common additional polymer insecondarywalls is lignin, a complex network of covalently linked phenolic compounds found in the walls of the xylem vesselsand fiber cells of woody tissues.Although the cell walls of higher plants vary in both composition and organization, theV are all constructed, Iike animal extracellular matrices, using a"1196 chapter19:cell Junctions,cell Adhesion,and the ExtracellularMatrixFigurel9-76 Plantcellwalls.(A)Electronmicrographof the root tip of a rush,showingthe organizedpatternof cellsthat resultsfrom an orderedseouenceofcelldivisionsin cellswith relativelyrigidcellwalls.In this growingtissue,the cellwallsarestillrelativelythin,appearingasfine blacklinesbetweenthe cellsin the(B)Sectionof a typicalcellmicrograph.wall separatingtwo adjacentplantcells.Thetwo darktransversebandscorrespondto plasmodesmatathat spanthe wall (seeFigure19-38).(A,courtesyof C.
BusbyandB.Gunning,Eur.J. CellBiol.21:214-233,1980.With permissionfrom Elsevier;B,courtesyof JeremyBurgess.)(B)1 0p mstructural principle common to all fiber-composites, including fiberglass andreinforced concrete. one component provides tensile strength, while another, inwhich the first is embedded, provides resistanceto compression.\A/hilethe principle is the same in plants and animals, the chemistry is different.
Unlike the animal extracellular matrix, which is rich in protein and other nitrogen-containingpolymers, the plant cell wall is made almost entirely of polymers that contain nonitrogen, including celluloseand lignin. For a sedentary organism that dependson CO2,H2O, and sunlight, these two abundant biopolymers represent,,cheap,,,carbon-based, structural materials, helping to conserve the scarce fixed nitrogen available in the soil that generally limits plant growth. Thus trees, for example, make a huge investment in the cellulose and lignin that comprise the bulkof their biomass.In the cell walls of higher plants, the tensile fibers are made from thepolysaccharide cellulose, the most abundant organic macromolecule on Earth,type.The plant cell wall thus has a "skeletal" role in supporting the structure ofthe plant as a whole, a protective role as an enclosure for each cell individually,and a transport role, helping to form channels for the movement of fluid in theplant.
\Mhen plant cells become specialized, they generally adopt a specificshape and produce specially adapted types of walls, according to which the different types of cells in a plant can be recognized and classified (Figure lg-zz;1197THEPLANTCELLWALL(c)(B)1 0 0p m50pm50pmFigure 19-77 Some specializedplant celltypes with appropriatelymodified cellwalls.(A)A trichome,or hair,on the upperleaf.Thisspiky,surfaceofan Arabidopsisprotectivesinglecellis shapedby thelocaldepositionof a tough,cellulose-richwall.
(B)Surfaceview of tomato leafepidermalcells.Thecellsfit togethersnuglylikethe piecesof a jigsawpuzzle,providinga strongouter coveringfor theleaf.The outer cell wall is reinforcedwith acuticleand with waxesthat waterproofthe leafand helpdefendit against(C)Thisview into youngxylempathogens.elementsshowsthe thick,lignified,hoopreinforcedsecondarycell wall that createsrobusttubes for the transportof waterthroughoutthe plant.(A,courtesyof PaulB and C,courtesyof Kim Findlay.)Linstead;see also Panel22-2, pp. 1404-f 405).Wefocus here, however, on the primary cellwall and the molecular architecture that underlies its remarkable combinationof strength, resilience, and plasticity, as seen in the growing parts of a plant.TheTensileStrengthof the CellWallAllowsPlantCellsto DevelopTurgorPressureThe aqueous extracellular environment of a plant cell consists of the fluid contained in the walls that surround the cell.
Although the fluid in the plant cell wallcontains more solutes than does the water in the plant's external milieu (forexample, soil), it is still hlpotonic in comparison with the cell interior. Thisosmotic imbalance causesthe cell to develop a large internal hydrostatic pressure, or turgor pressure, which pushes outward on the cell wall, just as an innertube pushes outward on a tire. The turgor pressure increasesjust to the pointwhere the cell is in osmotic equilibrium, with no net inflrx of water despite thesalt imbalance (seePanel I 1-1, p. 664). The turgor pressuregenerated in this waymay reach 10 or more atmospheres,about five times that in the averagecar tire.This pressureis vital to plants becauseit is the main driving force for cell expansion during growth, and it provides much of the mechanical rigidity of livingplant tissues.Compare the wilted leaf of a dehydrated plant, for example, withthe turgid leaf of a well-watered one.
It is the mechanical strength of the cell wallthat allows plant cells to sustain this internal pressure...\celIulosemicrofibrilMicrofibrilsThePrimaryCellWallls Builtfrom CelluloseInterwovenwith a Networkof PecticPolysaccharidesCellulose gives the primary cell wall tensile strength. Each cellulose moleculeconsists of a linear chain of at least 500 glucose residues that are covalentlylinked to one another to form a ribbonlike structure, which is stabilized byhydrogen bonds within the chain (Figure f 9-78).
In addition, hydrogen bondsbetween adjacent cellulose molecules cause them to stick together in overlapping parallel arrays, forming bundles of about 40 cellulose chains, all of whichhave the same polarity. These highly ordered crystalline aggregates, manymicrometers long, are called cellulose microfibrils, and they have a tensilestrength comparable to steel (see Figure 19-78). Sets of microfibrils arearranged in layers, or lamellae, with each microfibril about 20-40 nm from itsneighbors and connected to them by long cross-linking glycan molecules thatare attached by hydrogen bonds to the surface of the microfibrils.
The primarycell wall consists of several such lamellae arranged in a plywoodlike network(Figure f 9-79).CelluloseFigure19-78 Cellulose.arelong,unbranchedchainsofmoleculesp1,4-linkedglucoseunits.Eachglucoseresidueis invertedwith respectto itsand the resultingdisaccharideneighbors,reoeatoccurshundredsof timesin aAbout40singlecellulosemolecule.assembleindividualcellulosemoleculesto form a strong,hydrogen-bondedcellulosemicrofibril.1 198chapter 19:cell Junctions,cell Adhesion,and the ExtracellularMatrixThe cross-linking glycans are a heterogeneous group of branched polysaccharides that bind tightly to the surface of each cellulose microfibril and therebyhelp to cross-link the microfibrils into a complex network. Their function isanalogous to that ofthe fibril-associated collagens discussedearlier (seeFigure19-68). There are many classesof cross-linking glycans,but they all have a longlinear backbone composed of one type of sugar (glucose,xylose, or mannose]from which short side chains of other sugars protrude.
It is the backbone sugarmolecules that form hydrogen bonds with the surface of cellulose microfibrils,cross-linking them in the process.Both the backbone and the side-chain sugarsvary according to the plant speciesand its stage of development.coextensive with this network of cellulose microfibrils and cross-linking glycans is another cross-linked polysaccharide network based on pectins (seeFigurel9-79). Pectins are a heterogeneousgroup ofbranched polysaccharidesthatcontain many negatively charged galacturonic acid units. Becauseof their negative charge,pectins are highly hydrated and associatedwith a cloud of cations,resembling the glycosaminoglycansof animal cells in the large amount of spacethey occupy (see Figure 19-56).
\A/hen ca2* is added to a solution of pectinmolecules, it cross-links them to produce a semirigid gel (it is pectin that isadded to fruit juice to make jam set). certain pectins are particularly abundantinthe middle lamella, the specialized region that cements together the walls ofadjacent cells (seeFigure 1g-79); here, ca2* cross-links are thought to help holdcell-wall components together.Although covalent bonds also play a part in linking the components, very little is knonm about their nature.
Regulated separation of cells at the middle lamella underlies such processesas the ripening oftomatoes and the abscission (detachment) of leaves in the fall.In addition to the two polysaccharide-basednetworks that form the bulk ofall plant primary cell walls, proteins are present, contributing up to about 5% ofthe wall's dry mass. Many of these proteins are enzyrnes, responsible for wallturnover and remodeling, particularly during growth.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
