Lipid Polymorphism and Membrane Function

One of the fundamental problems in membrane biology is that of lipid diversity. The number of chemically different membrane lipids is much larger than other key biological building blocks such as nucleotides, amino acids, and carbohydrates. For instance, a relatively simple biomembrane such as that of the red blood cell contains well over a hundred different lipid species. Except for some metabolic and receptor-type functions, current models of biological membranes do not take into account this lipid diversity. In the fluid mosaic model (Singer and Nicolson, 1972), the lipids are thought to form a fluid semipermeable bilayer which acts as a matrix for the functional membrane proteins. On the basis of our present understanding, a single unsaturated phosphatidylcholine (PC) species could easily satisfy such demands. The incomplete nature of such membrane models is reinforced by two basic membrane phenomena. First, biological membranes are highly dynamic structures which are continuously involved in a variety of biochemical processes during which transient departures from bilayer structure must occur. Examples include the transbilayer transport of lipids and proteins as well as membrane fusion, among others. Second, although the ability of lipids to adopt a variety of phases has been recognized