ABSTRACT
In the fluid mosaic model, the prevailing model for membrane structure, phospholipids form a bilayer into which proteins are partially or fully immersed. Phospholipids, as proteins, are not, however, static molecules and are able to perform various types of movements in the bilayer. Individual species of biological membrane phospholipids adopt in their liquid crystalline state one of three structures: micellar, hexagonal, or bilayer. A simple shape concept model has been proposed to predict the type of structure that phospholipids will adopt upon hydration. In the fluid state, individual phospholipid molecules undergo rapid rotational diffusion around the axis of the phospholipid and rocking motions. In addition, phospholipids can migrate laterally in the plane of the lipid crystalline bilayer. Enhanced transbilayer mobility of the four major phospholipids are phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin has been observed using electron spin resonance in Plasmodium knowlesi-infected monkey erythrocytes.
