Plant Cell Borders: Membranes and WallHORT 301 – Plant Physiology

August 27, 2010Taiz and Zeiger – Chapter 1, Chapter 11 (p330-342), Chapter 15

paul.m.hasegawa.1@purdue.edu

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

Saturated fatty acids – w/o double bondsUnsaturated fatty acids – w/double bonds

Membrane lipid bilayer arrangement

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

Lignin – phenolic polymer, increases mechanical strength

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

Cell expansion patterns – microtubule orientation directs microfibril distributionLocalized growth or diffuse growth along an axis

Random or directed growth

Microtubules (cytoskeleton) direct orientation of cell wall – cellulose microfibrils

Smith et al. (2010)

Cell expansion – turgor pressure, cell wall loosening and water uptake

Secondary wall deposition – inwards from primary wall

Smith et al. (2010) Plant Biology

Formation of woody tissue – perennialsSecondary xylem (inside cambium) and secondary phloem (outside cambium)