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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
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
Fatty acids – long chain hydrocarbonsVary in length between 12 to 20 carbons
Glycerolipid composition of cellular membranes – ER and mitochondrial primarily phospholipids, plastids - glactolipids
Storage lipids in oil seeds
Triacylgercerol - energy source during germination
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
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
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
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)
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
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
Primary walls – extensible for elastic and plastic growth Secondary walls – internal to primary wall, not extensible Middle lamella joins adjacent cells
Cell wall layers
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
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