By displacing water molecules from the interface, amphipathic molecules such as lipids lower the free
energy cost of creating the air/water interface. The surface tension drops and, under the effect of
gravity, the droplet sags. Lipids, and other amphipathic molecules, are therefore said to be surfaceactive compounds or surfactants. Nearly all biological molecules, including proteins, comprise
hydrophobic and hydrophilic regions and, as a consequence, are surface-active. In the present book,
however, the term “surfactant” will be restricted to relatively small molecules, such as lipids,
detergents, or amphipols (APols), and not used for biological macromolecules.
Fig. 1.1 Chemical structures of some of the polar lipids commonly found in biological membranes. Chol
cholesterol, CL cardiolipin, Ergo ergosterol, Gan ganglioside, Glu glucolipid, LPS lipopolysaccharide, PA
phosphatidic acid, PC phosphatidylcholine, PE phosphatidylethanolamine, PG phosphatidylglycerol, PI,
phosphatidylinositol, PS phosphatidylserine. The nature of the acyl chains is conveniently indicated by the
notation C m:n , where m stands for the number of carbons and n for that of double bonds.
1.1 Introduction
3