Cell membranes are currently thought to consist of different lipid phases and domains at the nanometer scale. [1] These domains (also known as "lipid rafts") differ in composition, structure, and stability and consequently in properties and function. They selectively incorporate or exclude specific proteins, and thereby fulfill an important function in cell activity and signaling. [2] The compositions of the different membranes in the cell organelles differ greatly according to their function. [3] Membrane compositions also vary substantially among different types of cells and among organelles within the same cell. To date, thousands of different lipid molecules have been found to exist in the cell membranes, often differing one from the other only in chain length or other parameters not associated with functional-group interaction. [4] The possible advantage of this multiplicity and redundancy may be explained at least in part by the different interactions between the lipid chains, which in turn regulate the composition, organization, and functional parameters of the membrane. Understanding the rules that govern membrane structure is thus crucial to understanding cell biology.
Grazing-incidence X-ray diffraction (GIXD) [5] can provide direct information on the organization of laterally