Biological Membrane Lipids

The three principal classes of lipids that form the bilayer matrix of biological membranes are glycerophospholipids, sphingolipids, and sterols (principally cholesterol). The most important characteristic of molecules in the first two groups is their amphipathic structure—well separated hydrophilic (polar) and hydrophobic (nonpolar) regions. Generally, their shape is elongated, with a hydrophilic end or head attached to a hydrophobic moiety by a short intervening region of intermediate polarity. Because of the segregation of polarity and nonpolarity, amphipathic molecules in any solvent will spontaneously form aggregates that minimize energetically unfavourable contacts (by keeping unlike regions of molecules apart) and maximize favourable contacts with the solvent (by keeping similar regions of molecules together). The molecular arrangement of the aggregate depends upon the solvent and the details of the amphipathic structure of the lipid.

In water, micelles formed by soaps (the sodium or potassium salts of fatty acids) are one such aggregate. The polar or hydrophilic portion of the soap molecules forms the surface of the micelle, while the hydrocarbon chains form its interior and are thus completely protected from the energetically unfavourable contact with water, as described in the section Fatty acids: Physical properties. Biological membrane lipids, however, do not form spherical micelles in water but instead form topologically closed lamellar (layered) structures. The polar heads of the component molecules form the two faces of the lamella, while the hydrophobic moieties form its interior. Each lamella is thus two molecules in thickness, with the long axis of the component molecules perpendicular to the plane of the bilayer.

Other types of aggregates are also formed in water by certain amphipathic lipids. For example, liposomes are artificial collections of lipids arranged in a bilayer, having an inside and an outside surface. The lipid bilayers form a sphere that can trap a molecule inside. The liposome structure can be useful for protecting sensitive molecules that are to be delivered orally.