Plant Cell Borders: Membranes and Wall HORT 301 – Plant Physiology August 27, 2010 Taiz and Zeiger – Chapter 1, Chapter 11 (p ), Chapter 15 Membranes – delimit the cell (plasma membrane) and organelles Compartmentalize functions – specialized organellar function Organellar sub compartments - separate reaction centers

Cell membranes have similar basic molecular organization - principally lipids and proteins Lipids Glycerol backbone – conjugated to fatty acids Triacylglycerols – storage lipids, fatty acids at all positions, hydrophobic Glycerolipids – membrane lipids, polar group at the third position Phopho and galactolipids are amphipathic molecules – lipid bilayer structure

Fatty acids – long chain hydrocarbons Vary in length between 12 to 20 carbons Saturated fatty acids – w/o double bonds Unsaturated 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 cells Seals wounds – fruit cracking Defense Signaling molecules

Fatty acid synthesis in plastids First cycle - condensation of malonyl-ACP to acetyl-CoA = butyryl-ACP Subsequent cycles - condensation of malonyl-ACP to butyryl-ACP, etc. = chain elongation

Lipid biosynthesis in plastids and ER Plastids - 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 functions Primary components of membranes Storage molecules for energy production Protective polymers Compartmentalization of reaction centers Regulate lipid and protein trafficking, and mineral ion and molecular uptake and efflux Signaling molecules

Cell Wall – polysaccharide framework of the cell border Most abundant carbon source in nature Mechanical rigidity of plants Controls cell volume and water status Determines cell shape Protects 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 walls Wall 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 distribution Localized 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 – perennials Secondary xylem (inside cambium) and secondary phloem (outside cambium)