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Plant Cell Borders: Membranes and WallHORT 301 – Plant Physiology
August 27, 2010Taiz and Zeiger – Chapter 1, Chapter 11 (p330-342), Chapter 15
Membranes – delimit the cell (plasma membrane) and organellesCompartmentalize functions – specialized organellar functionOrganellar sub compartments - separate reaction centers
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Cell membranes have similar basic molecular organization - principally lipids and proteins
LipidsGlycerol backbone – conjugated to fatty acids
Triacylglycerols – storage lipids, fatty acids at all positions, hydrophobicGlycerolipids – membrane lipids, polar group at the third positionPhopho and galactolipids are amphipathic molecules – lipid bilayer structure
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Fatty acids – long chain hydrocarbonsVary in length between 12 to 20 carbons
Saturated fatty acids – w/o double bondsUnsaturated fatty acids – w/double bonds
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Membrane lipid bilayer arrangement
Glycerolipid composition of cellular membranes – ER and mitochondrial primarily phospholipids, plastids - glactolipids
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Storage lipids in oil seeds
Triacylgercerol - energy source during germination
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Cutin, waxes and suberin – culticle (cutin and waxes) on stems and leaves, suberin in roots
Culticle prevents water loss directly from epidermal cellsSeals wounds – fruit crackingDefenseSignaling molecules
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Fatty acid synthesis in plastidsFirst cycle - condensation of malonyl-ACP to acetyl-CoA = butyryl-ACPSubsequent cycles - condensation of malonyl-ACP to butyryl-ACP, etc. = chain elongation
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Lipid biosynthesis in plastids and ERPlastids - fatty acid conjugation to glycerol-3-phosphate (G3P) to form phosphatidic acid → phospholipids and galactolipids
ER – fatty acids transported from plastid → conjugation to G3P (phosphatidic acid → phospholipids and triacylglyerol lipids
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Storage lipid conversion to sucrose
Oil body – lipid hydrolyzed (lipases) to fatty acids
Glyoxysomes - fatty acids oxidized to acetyl-CoA Acetyl-CoA - converted to succinate (glyoxylate cycle)
Mitochondria - succinate transported to mitochondria and converted to malate
Cytosol – malate transported to cytosol and converted to sucrose (gluconeogenesis)
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Lipid functionsPrimary components of membranesStorage molecules for energy productionProtective polymers Compartmentalization of reaction centersRegulate lipid and protein trafficking, and mineral ion and molecular uptake and effluxSignaling molecules
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Cell Wall – polysaccharide framework of the cell borderMost abundant carbon source in nature
Mechanical rigidity of plantsControls cell volume and water statusDetermines cell shapeProtects cells
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Primary walls – extensible for elastic and plastic growth Secondary walls – internal to primary wall, not extensible Middle lamella joins adjacent cells
Cell wall layers
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Primary cell walls – cellulose microfibrils embedded in a hydrated matrix of non-cellulosic polysaccharides and structural proteins
Arranged in microfibrils of several glucan chains (glucose polymers)Intramolecular hydrogen bonding
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Hemicellulose, pectin and structural proteins in the primary cell wall
Hemicellulose – glycan polymers that crosslink cellulose
Pectin – galacturonic acid, hydrated gel phase of the wall Structural proteins - hydroxyproline/proline rich & glycoproteins cross-link the wallsWall loosening proteins – expansins & glucosylases/hydrolases
Lignin – phenolic polymer, increases mechanical strength
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Cellulose synthesis – cellulose synthase complex located in the plasma membrane
Hemicellulose and pectins – synthesized in Golgi body, secreted in vesicles to the plasma membrane and delivered to cell wall
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Cell expansion patterns – microtubule orientation directs microfibril distributionLocalized growth or diffuse growth along an axis
Random or directed growth
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Microtubules (cytoskeleton) direct orientation of cell wall – cellulose microfibrils
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Smith et al. (2010)
Cell expansion – turgor pressure, cell wall loosening and water uptake
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Secondary wall deposition – inwards from primary wall
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Smith et al. (2010) Plant Biology
Formation of woody tissue – perennialsSecondary xylem (inside cambium) and secondary phloem (outside cambium)