... are hydrophobic ("water hating") and non-polar. The hydrophilic head bears electrical charges contributed by the phosphate and some bases. These charges are responsible for the hydrophilicity. No lipid bears a positive charge they are all negative or neutral. Therefore all membranes are negatively charged. They form a bilayer structure with the hydrophilic heads to the external and internal surface and the hydrophobic tails towards each other. Lipid molecules make up between 30 and 80% of biological membranes by mass. The remainder is protein (20 to 60%) and sometimes carbohydrate (0 to 10%). Within the lipid structure there is also a certain amount of cholesterol embedded between the phospholipids. This serves to increase the strength of the membrane and makes it a little less fluid. Andrea McCaffery HEFC Biological Science The protein molecules are very much larger than the lipid molecules. Proteins are responsible for moving molecules and messages across the membrane and they are ...

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