DIFFUSION AND CHEMICAL-REACTIONS IN PHASE-SEPARATED MEMBRANES

The biological membrane may be viewed as a two-dimensional solvent sys tem, the lipid bilayer, in which the membrane components are either di ssolved (intrinsic) or to the surface of which they are adsorbed (extr insic). The solvent bilayer is made up of a large number of lipid chem ical species derived from a few lipid classes. Experience with model s ystems has shown that in mixed lipid bilayers immiscibility of compone nts is the rule rather than the exception. This suggests that the bila yer in a biological membrane is not a homogenous two-dimensional fluid but rather a heterogenous system consisting of a mosaic of co-existin g phase domains in which the phases differ both chemically and physica lly from each other. A consequence of this is the physical separation of membrane components, including proteins, based on their phase solub ility. The percolation in such a phase-separated system then determine s the range over which free lateral diffusion is possible and bimolecu lar reactions can occur. Phase percolation and long-range translationa l diffusion have been studied in model systems using the fluorescence recovery after photobleaching (FRAP) technique, and theoretical work s hows that bimolecular reaction yields can be seriously reduced in phas e-separated membranes. Transitions between percolating and non-percola ting states in biomembranes is proposed as a potential trigger mechani sm in the control of membrane physiology.