Cell membranes

Cell membranes form the boundary between a cell's interior environment and its exterior environment. They consist of a 5 nm thick bilayer of lipids which interact in a hydrophobic manner to transmembrane proteins embedded within the membrane. Cell membranes contain 500-1000 lipid types which give rise to the permeability barrier to water-soluble molecules. The embedded proteins function as either substrate transporters, receptors, enzymes or provide links to the cytoskeleton. Cell membranes also contain sterols, glycolipids, and glycoproteins.

The lipids that constitute a cell membrane are synthesised in the cytosolic monolayer of the endoplasmic reticulum. They are amphiphilic in nature; they have a hydrophilic head group and two hydrophobic tails. It is this amphiphilic property that leads to a natural tendency to form bilayers. In a bilayer, the tails face inwards and interact through van der Waals forces whereas the head groups interact with water molecules either side of the membrane. Phospholipids are the most abundant lipid type and mainly feature phosphoglycerides of which there are three: phosphatidylserine, phosphatidylethanolamine, and phosphatidylcholine. The tails of phosphoglycerides are fatty acids containing 14-24 carbon atoms. Two fatty acids form ester bonds with glycerol which also binds to a polar head group featuring a phosphate group bound to either choline, ethanolamine, or serine. The length and saturation of the fatty acids affects the fluidity of the cell membrane. Microorganisms which have temperatures dependent on the environment, like bacteria and yeast, are able to change the properties of their membrane lipids. For example, when temperatures fall, lipids with short chain length and highly unsaturated fatty acids are produced which lowers the temperature at which a phase transition between a liquid to crystalline state occurs. The lipids within a membrane are able to rotate, move laterally within a monolayer but movement between monolayers, known as 'flip-flop', rarely occurs. However, there is great asymmetry in the lipid compositions of the two monolayers. Phospholipid translocators catalyse the 'flip-flop' of specific lipids. In red blood cells, choline-containing lipids are concentrated in the outer monolayer whereas amino acid-containing lipids are in the cytosolic monolayer. Another membrane lipid is sphingomyelin. It is derived form shingosine, an acyl chain which features an amine group and two hydroxyl groups at one end of the molecule. Sphingomyelin is made through addition of a fatty acid to the amine group and a phosphocholine group to a hydroxyl group in sphingosine. Sphingomyelin is involved in lipid rafts, membrane domains which are thicker and better accommodate some transmembrane proteins. Lipid rafts are involved in localising proteins for vesicular transport or in the formation of protein assemblies.

Cholesterol and glycolipids are also cell membrane constituents. Cholesterol is a sterol that is present in membranes. They reduce the permeability of the membrane to water-soluble substances and prevent crystallization between the highly concentrated hydrocarbon tails of lipids. It is also important in stabilising lipid rafts. Bacterial cell membranes have no cholesterol. Glycolipids are exposed to the extracellular environment due to glycosylation within the lumen of the Golgi apparatus. They are involved in cell-cell recognition. Complex glycolipids called gangliosides provide entry points for bacterial toxins like the cholera toxin .