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Transcript
Phosphoacylglycerols (Phospholipids) • Phosphoacylglycerols are fatty acid esters of glycerol which also contain a phosphate group and other specific groups • The phosphate group replaces the fatty acid on C number 3 of a triacylglycerol molecule Phosphate group O CH 2 R C O CH CH 2 X O O C R O O P O X OH The groups which can bind to the phosphate are all alcohols. They are very polar molecules and are often charged HO CH 2 CH 2 NH 3 + Ethanolamine CO O HO CH 2 CH NH 3 + Serine HO CH 2CH 2 Choline CH 3 N + CH 3 CH 3 H H H H OH OH OH H H Inositol OH OH H When choline is part of the phosphoacylglycerol it is called phosphatidyl choline A Phosphoacylglycerol X Non-polar “tail” Polar “head” • Molecules which have one polar end and one nonpolar end are called amphipathic molecules • Phosphoacylglycerols are amphipathic and it is that property on which their major biological function is based Triacylglycerol From: Lehninger Principles of Biochemistry 4th edition. Figures 10-2 and 10-13 Phosphoacylglycerol Amphipathic molecules spontaneously form a number of specific structures including: Monolayers at a liquid-air interface Micelles Bilayer From: Molecular Cell Biology by Lodish 5th Edition. Based on Figure 2-20 • The phosphoacylglycerol or phospholipid bilayer is the basis of biological membrane structure From:Stryer Biochemistry 5th edition. Figures 12.11 and 12.12 • Membrane structure due to the amphipathic nature of the phosphoacylglycerols • Acylglycerols cannot form bilayers because they are not amphipathic • Sterols are also found in membranes. They are amphipathic Cholesterol Stigmasterol HO Polar HO Non-Polar Polar Non-Polar Proteins and carbohydrates are also components of membranes Described as a fluid mosaic with movement of molecules within the plane of the membrane From: Lehninger principles of Biochemistry 4th Edition. Figure 11-3 • Integral Proteins (a, b, c) – span the membrane (may have multiple transmembrane segments) or partially immersed in lipid layer • Peripheral Proteins (d, e) – loosely attached: electrostatic interaction, bonding to integral protein, hydro-phobic anchor, bonding to phosphoacylglycerol via carbohydrate chain From:Stryer Biochemistry 5th Edition. Figure 12.17 Why have proteins and carbohydrates in membranes? • Cell recognition • Extracellular enzyme activities • Transport of compounds across the membrane Three types of transport of solutes across membranes: • Diffusion • Passive transport (facilitated diffusion) • Active transport Diffusion • Solute passes across membrane from area of high concentration to area of low concentration until concentration equalised • Driven by concentration gradient • Small polar molecules diffuse through small gaps in hydrophobic environment. • Larger polar molecules (particularly if charged) do not diffuse • Uncharged, lipophilic molecules diffuse readily Passive transport (facilitated diffusion) • Depends on the presence of specific proteins which transport the molecule from one side of the membrane to the other • Driven by concentration gradient Active Transport • Depends of the presence of specific transport proteins • Transport requires energy • Transport can be against a concentration gradient Osmosis and Tonicity Osmosis • The process by which a solvent passes across a semi-permeable membrane from a solution with a low osmotic pressure to one with a high osmotic pressure • Osmotic pressure is directly proportional to the molar concentration of all the solutes and ions which cannot pass across the membrane • A semi-permeable membrane is one which allows passage of solvent but not solute Semi-permeable membrane High osmotic pressure Low osmotic pressure Both sides have equal osmotic pressure Tonicity • Biological membranes are selectively impermeable not semi-permeable • Allow free passage of solvent and some solutes but not all • Tonicity is due to the osmotic pressure exerted only by the solutes which cannot pass across the selectively permeable membrane Urea passes across the membrane, sucrose cannot 0.5 M urea 1.0 M sucrose 0.5 M sucrose Lower tonicity Flow of water 0.5 M urea Higher tonicity 0.5 M sucrose 0.5 M sucrose Equal tonicity No net flow of water Equal tonicity