Lipid polymorphism and the functional roles of lipids in biological membranes

The reasons for the great variety of lipids found in biological membranes, and the relations between lipid composition and membrane function pose major unsolved problems in membrane biology. Perhaps the only major functional role of lipids which may be regarded as firmly established involves the bilayer structure of biological membranes. The observations that biological membranes contain regions of bilayer structure, and that model systems consisting of naturally occurring and synthetic phospholipids often spontaneously adopt such a configuration on hydration, provide strong evidence that the lipid component is responsible for the basic biomembrane structure. The fact remains however, that a single phospholipid species such as phosphatidylcholine could satisfy such structural requirements. In this context, the observation that a typical mammalian cell membrane contains one hundred or more distinctly different tipids implicitly suggests that lipids play other functional roles. In this review we shall indicate alternative functional roles arising from a property of lipids which has not received the serious attention it deserves in recent years, namely the ability of lipids to adopt nonbilayer configurations in addition to the bilayer phase. This ability implies a view of biological membranes which differs from previous models such as the fluid mosaic model of Singer and Nicolson or the earlier unit membrane model. It is therefore appropriate to briefly review the possible functional roles of lipids in terms of these earlier models so that the requirement for an alternative approach becomes apparent.