The Plant Cell: Cell Membranes and WallHORT 301 – Plant Physiology

August 28, 2009Taiz and Zeiger – Chapter 1

paul.m.hasegawa.1@purdue.edu

Cell MembranesLipids and proteins are principal components of cell membranes

LipidsGlycerol backboneTriacylglycerols – storage lipids, fatty acids at all positions, hydrophobicGlycerolipids - membrane lipids, polar group at the third position, amphipathic

Fatty acids – long chain hydrocarbons, acidic group is conjugated to glycerol, ester linkage Fatty acids vary in length between 12 to 20 carbonsSaturated fatty acids – w/o double bondsUnsaturated fatty acids – w/double bonds

Saturated fatty acidslauric acid (12:0) CH3(CH2)10CO2Hmyristic acid (14:0) CH3(CH2)12CO2H palmitic acid (16:0) CH3(CH2)14CO2Hstearic acid (18:0) CH3(CH2)16CO2Harachidic acid (20:0) CH3(CH2)18CO2H

Unsaturated fatty acidsoleic acid (18:1)linoleic acid (18:2)linolenic acid (18:3)

Lipid bilayer arrangement

Glycerol lipid composition of cellular membranes

Storage lipids in oil seedsBean, sunflower, oil palm, canolaEnergy source during germination

Cutin, waxes and suberin – cutin on stems, suberin in roots, waxes associated with cutin and waxesCutin and suberin - long chain fatty acid polymersWaxes – complex mixtures of lipids

Cuticle – cutin and wax combination on surface of stemsPrevents water loss directly from epidermal cellsSeals wounds and protection

Signaling molecules

Fatty acids – synthesized in plastidsFirst step – acetyl CoA + CO2 forming malonyl-CoA, condensed to ACP (malonyl-ACP)First cycle – acetyl-CoA + malony-ACP – acetylacetyl-ACP to butyryl-ACP (4 carbon)Further cycles – condensation of malonyl-ACP to fatty acid chain, 2 carbon additions (acetyl-CoA)

Glycerol lipid synthesis in plastids (storage lipids) and the ER

Formation of phosphatidic acid, diacylglycerol phosphate

Formation of glycerolipids

Storage lipids conversion to sucrose (translocation)Lipases – hydrolyze fatty acids from the glycerol moietyFatty acids are oxidized in peroxisomes (glyoxysomes) to acetyl-CoAAcetyl-CoA converted to succinate in the glyoxysomeSuccinate transported to mitochondria – converted to oxaloacetate, then malateMalate transported to the cytosol – converted to glucose by gluconeogenesis

Cell WallCell walls are the most abundant source of carbonMany organisms, including humans, access carbon from cell wallsMajor carbon load to the soil

Rigid structure of plantsControls cell volume and water statusDetermines cell shapeProtects cells

Primary walls – extensible for elastic and plastic growth, actively growing cells Secondary walls – internal to primary wall, not extensible, cells after growth (enlargement) has ceased, cell specialization Middle lamella joins adjacent cells

Primary cell walls

Cellulose is the major component of primary and secondary plant cell walls

Cellulose has tensile strength of steel

Arranged in microfibrils of several glucan chains (glucose polymers)

Hemicellulose, pectin and proteins comprise the cell wall matrix Hemicellulose – glycan polymers, flexible polysaccharides, tether (interlock) cellulose by noncovalent and covalent linkages (cross-linking)

Pectin – acidic sugar (galacturonic acid) and neutral sugar (rhamnose, galactose, arabinose) polymers, hydrated gel phase of the wall

Cell wall proteins Structural - hydroxyproline/proline rich and gylcoproteins cross-link the wallsWall loosening – expansions (break cross-linking hydrogen bonds), glucosylases/hydrolases – hydrolyze glucosidic bonds and rejoins ends during loosening and growth

Hemicellulose and pectins – synthesized in Golgi body and secreted to the plasma membrane and cell wall

Cellulose – synthesized by cellulose synthase complex located in the plasma membrane

Synthesis → secretion → assembly → cell expansion → cross-linking → growth cessation → secondary wall formation

Secondary walls – strengthen stem (tree trunk), xylem vessels

Lignin – phenolic polymer secreted to wall binds tightly to cellulosePrevents cell expansion and provides mechanical strength to walls

Cell Expansion - cell enlargement is a turgor pressure driven process that requires wall loosening, water uptake and secretion of cell wall materials

Direction of cell growth – determined by orientation of the cell wall microfibrils