Organization of Lipid Molecules within Biomembranes

In this review, we report on the organization of lipid molecules forming biomembranes. More precisely, the question is how these amphiphiles phase separate under a change of a suitable parameter, like temperature, pressure or membrane environment. The mixture may undergo a lateral or transversal phase separations. This essentially depends on two principal factors, which are the structure (length) of hydrocarbon chains of lipid molecules and the curvature asymmetry of membranes. When the former dominates, a lateral separation is then expected. In contrary, for those biomembranes of high curvature asymmetry, a vertical separation is rather observed. We examine the problem from a static (phase diagrams) and kinetics (relaxation in time) point of view. Finally, the discussion is also extended to the phase separation between lipid molecules and cholesterol in biomembranes (formation of rafts), between phospholipids and grafted polymers on liposomes, or between surfactant and its co-surfactant in bilayer systems.