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 since they have a hydrophilic head group and two hydrophobic tails. It is this amphiphilic property that leads to a natural tendency of lipids 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 membrane lipid and feature a group called phosphoglycerides of which there are three types: phosphatidylserine, phosphatidylethanolamine, and phosphatidylcholine. The tails of phosphoglycerides are fatty acids containing between 14-24 carbon atoms. Two fatty acids form ester bonds with glycerol, which also binds to a polar head group consisting of a phosphate group bound to either choline, ethanolamine, or serine. The chain length and saturation of 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 the membrane lipids change phase from a liquid to a crystalline state. This maintains the cell membrane at a relatively constant fluidity. The lipids within a membrane are able to rotate and 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 (ethanolamine- or serine-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 the addition of a fatty acid and phosphocholine group to an amine group and hydroxyl group in sphingosine, respectively. Sphingomyelin is involved in lipid rafts which are membrane domains that are thicker than that normal of a cell membrane and can 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 and its presence in membranes reduces the permeability of the membrane to water-soluble substances and prevents crystallization occuring 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 can provide entry points for bacterial toxins like the cholera toxin .