The phosphate group replaces the fatty acid on C number 3 of a triacylglycerol molecule O O CH 2 O C R CH 2 O P O X OH.

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1 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 R O C O X O CH 2 O C R CH O CH 2 O P O X OH

2 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 COO - + HO CH 2 CH NH 3 Serine H H H H OH OH OH H H OH OH H Inositol CH 3 HO CH 2 CH 2 N + CH 3 CH 3 Choline When choline is part of the phosphoacylglycerol it is called phosphatidyl choline

3 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

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5 Triacylglycerol Phosphoacylglycerol From: Lehninger Principles of Biochemistry 4 th edition. Figures 10-2 and 10-13

6 Amphipathic molecules spontaneously form a number of specific structures including: Monolayers at a liquid-air interface Micelles Bilayer From: Molecular Cell Biology by Lodish 5 th Edition. Based on Figure 2-20

7 The phosphoacylglycerol or phospholipid bilayer is the basis of biological membrane structure From:Stryer Biochemistry 5 th edition. Figures and 12.12

8 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 HO Polar Non-Polar Polar Non-Polar

9 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 4 th Edition. Figure 11-3

10 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 5 th Edition. Figure 12.17

11 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

12 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

13 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

14 Osmosis Osmosis and Tonicity 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

15 Semi-permeable membrane High osmotic pressure Low osmotic pressure Both sides have equal osmotic pressure

16 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

17 Urea passes across the membrane, sucrose cannot 0.5 M urea 0.5 M sucrose Lower tonicity Flow of water 1.0 M sucrose Higher tonicity 0.5 M urea 0.5 M sucrose Equal tonicity No net flow of water 0.5 M sucrose Equal tonicity