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Topic 4 extracellular connections.odt

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Topic 4A Extracellular InteractionsPlant Cell Wallprovides rigidity and protection regulates permeabilityprevents cell swelling from osmotic pressure Instead turgor pressure builds up that is responsible for rigidity of plant tissueStructure involves 3 classes of molecules1Structural Fibrescellulose microfibrils2Matrixpolysaccharides3Adhesive moleculespectinscomposed principally of polysaccharidesBasic structural polysaccharide is cellulose a linear polymer of glucose residuesjoined by beta 14 linkages form cellulose microfibrilsthese microfibrils are embedded in a matrix consisting of proteins and two other polysaccharids hemicellulose and pectinshemicellulose are highly branched polysaccharides that are hydrogen bonded to cellulose microfibrils This crosslinks the cellulose microfibrils into a network which gives the cell wall strengthPectins are branched polysaccharides containing a lot of negatively charged calagturonic acid which makes it trap water molecules to form gels Cell walls also contain a variety of glycoproteins that are incorporated into the matrix to provide further structural supportPrimary vs Secondary Cell wallprimary cell wall consists of a loose network of cellulose fibrilsonce cells cease growth they may lay down a secondary cell wall between the plasma membrane and primary cell wall These are thicker more rigid highly organized cellulose fibrils secondary cell wall lacks pectin is multilayered and may contain lignin which has extreme strength The wall is laid down in layers where the cellulose fibers differ in orientation for each layer Cell wall is not synthesized in the cell but on the cell surface because cellulose is large and rigidExtracellular surface of cell membranes have enzymes rosettes which synthesize itIOn the intracellular side these rosettes link to microtubules by means of motor proteinsThe cell controls the pattern of microfibrils by controlling which way the enzyme rosettes move Through that mechanism you can get loose networks of microfibrils primary cell wall or you can find organized networks arrays of microfibrils secondary cell wallsinitial layer middle lamella layer of pectines That is the layer that allows cell walls of
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