I. INTRODUCTION A. LIPID BILAYERS AS MODEL MEMBRANES A common structural motif of all biological membranes is the fluid lipid bilayer12 as illustrated in Figure 1. The lipid bilayer is the core of the membrane, and it provides the cell with an appropriate permeability barrier and hence supports a distinction between the inside and outside of the cell. Whereas the lipid bilayer provides the chemical seal of the cell incapsulation, it is other structural elements such as the cytoskeleton of eukaryotic plasma membranes which control the shape of the cell and give the total membrane aggregate the necessary mechanical strength.2 The lipid bilayer component of cell membranes contains a large number of different lipid species in addition to sterols, as well as integral and peripheral membrane proteins and polypeptides.