According to fluid mosaic model there is rapid internal motion involving flexing within each lipid molecule a rapid lateral diffusion of the lipids is possible and a slow ‘flip-flop’ motion, i.e., a transfer of lipid molecules from one side of the bilayer to the other, is also possible. The lipid molecules might also rotate about their axes. The proteins of the membrane are concerned with the enzymatic activity of the membrane, with transport of molecules, and with a receptor function whereas, the lipid bilayer provides the permeability barrier.