BYU PWS 440 - chapter 15_ Cell walls (1) (14 pages)

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chapter 15_ Cell walls (1)



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chapter 15_ Cell walls (1)

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Pages:
14
School:
Brigham Young University
Course:
Pws 440 - Plant Physiology

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Chapter 15 Cell walls Cell wall structure Primary cell wall 1 produced early in cell development 2 thin and expandable Secondary cell wall 1 thick does not expand therefore they form after cell growth ceases can make up to 90 weight in large trees Middle lamella interspace between cell walls Primary wall composition cellulose microfibrils Cellulose microfibrils glucan chains glucose polymer woven into strands extremely stable with high tensile strength Synthesized on the plasma membrane by a rosette unit cell loom Rosette composed of 6 subunits composed of 6 cellulose synthase enzymes Cellulose synthase produces glucan chains by linking glucose monomers derived from the hydrolysis of sucrose Primary wall composition matrix polymers Hemicelluloses flexible polysaccharides that form tethers that bind microfibrils into a cohesive network Pectins hydrated gel in which microfibril hemicellulose network is embedded Hemicellulose pectin matrix is 80 water giving primary cell walls extensibility Hemicelluloses and pectins are sythesized by glycosyltranferases in Golgi apparatus and delivered to the cell wall via exocytosis Evidence for self assembly and enzyme mediated cell wall assembly Non covalent bonding contributes to cell wall linkages Groups of neighboring pectin chains are linked together via Ca2 Negative carboxyl groups bind to calcium s positive charge Patterns of cell expansion Microfibril orientation influences growth directionality Because microfibrils resist expansion cell growth occurs perpindicular to microfibril orientation Secondary wall composition Secondary walls form after cell expansion ceases cellulose microfribrils impregnated with lignin Function of lignin 1 increase structural strength in stems and branches 2 prevents the collapse of xylem during periods of high water tension 3 hydrophobic which reduces susceptibility to pathogen enzymes 4 decreases palatability of plant tissue to herbivores Cell wall function Provides protection insects pathogens



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