Structural and Functional Aspects of the Chemical and Physical Diversity of Membrane Lipids

Abstract

Current concepts on the structure of biological membranes depict such a membrane as a liquid crystalline bilayer of lipid molecules in which penetrating protein structures are embedded. It is often assumed that the proteins are responsible for the specific properties of the membrane such as signal transduction and membrane transport, whereas the bilayer is there to provide a fluid matrix and to form an impermeable barrier. In view of such concepts the complexity of the lipid composition of a membrane is a striking fact. Although one suitable type of lipid could accomplish the requirements of a fluid matrix, most biological membranes appear to be built with complex mixtures of lipid species, showing many variations in size and charge of the polar headgroups, in length and unsaturation of the paraffin chains, and in cholesterol content [1]. For a given membrane the lipid composition is characteristic, but between membranes with different functions, large differences in composition can be noticed. This raises the question whether such a complex lipid composition is a functional requirement and provokes the hypothesis that lipids contribute in more specific ways to membrane functions than by just forming a fluid impermeable matrix.