The Plant Cell: Cell Membranes and Wall HORT 301 – Plant Physiology August 28, 2009 Taiz and Zeiger – Chapter 1 Cell Membranes Lipids and proteins are principal components of cell membranes Lipids Glycerol backbone Triacylglycerols – storage lipids, fatty acids at all positions, hydrophobic Glycerolipids - 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 carbons Saturated fatty acids – w/o double bonds Unsaturated fatty acids – w/double bonds Saturated fatty acids lauric acid (12:0) CH 3 (CH 2 ) 10 CO 2 H myristic acid (14:0) CH 3 (CH 2 ) 12 CO 2 H palmitic acid (16:0) CH 3 (CH 2 ) 14 CO 2 H stearic acid (18:0) CH 3 (CH 2 ) 16 CO 2 H arachidic acid (20:0) CH 3 (CH 2 ) 18 CO 2 H Unsaturated fatty acids oleic acid (18:1) linoleic acid (18:2) linolenic acid (18:3)

Lipid bilayer arrangement Glycerol lipid composition of cellular membranes

Storage lipids in oil seeds Bean, sunflower, oil palm, canola Energy source during germination

Cutin, waxes and suberin – cutin on stems, suberin in roots, waxes associated with cutin and waxes Cutin and suberin - long chain fatty acid polymers Waxes – complex mixtures of lipids Cuticle – cutin and wax combination on surface of stems Prevents water loss directly from epidermal cells Seals wounds and protection Signaling molecules

Fatty acids – synthesized in plastids First step – acetyl CoA + CO 2 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 moiety Fatty acids are oxidized in peroxisomes (glyoxysomes) to acetyl-CoA Acetyl-CoA converted to succinate in the glyoxysome Succinate transported to mitochondria – converted to oxaloacetate, then malate Malate transported to the cytosol – converted to glucose by gluconeogenesis

Cell Wall Cell walls are the most abundant source of carbon Many organisms, including humans, access carbon from cell walls Major carbon load to the soil Rigid structure of plants Controls cell volume and water status Determines cell shape Protects 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 walls Wall 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 cellulose Prevents 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