The Composition of Biological Membranes

The main components of biological membranes are proteins, lipids, and carbohydrates in variable proportions. Carbohydrates account for less than 10% of the mass of most membranes and are generally bound either to the lipid or protein components. Myelin has few functions and is made up almost entirely of lipids. In plasma membranes, the weight ratio of lipid to protein is close to 1; in several specialized membranes (ie, mitochondrion and bacterial cells) this ratio is near 2 or 3. Thus, there appears to be a correlation between the number of activities performed by and the amount of protein in a membrane. The main membrane lipids are phospholipids, cholesterol, and glycolipids. Glycolipids seem to be cell antigens, and they, together with glycoproteins, may determine surface characteristics of a cell which distinguish it from other cells. Approximately ten polypeptide chains of different molecular weights make up most of the mass of protein in plasma membranes.