As was true of most nonpolarDescribes a molecule with no net permanent dipole; this can occur when there is no separation of centers of positive and negative electrical charge or because there are bond dipoles that cancel each others' effects. A polar molecule will assume certain orientations more than others in an electric field. lipids, the structures of polar lipids are based on condensationThe process in which a liquid forms from gas or vapor of the same substance. A chemical reaction in which two molecules combine to form a very small molecule and a larger molecule than either of the two reactants. of fatty acids with glycerol. The main difference is that only two of the three OH groups on glycerol are involved. The third is combined with a highly polar moleculeA set of atoms joined by covalent bonds and having no net charge.:
An example of a common biological phospholipid is phosphatidylserine, seen in Figure 1. The hydroxyl end of serine has been added onto the phosphate group. The general structure is , where X can be a number of functional groups, such as choline, glycerol, ethanolamine, and serine, the example we have given. This give a structure where there is a highly polar head group, with two longer, non-polar fatty acid chain tails. This structure is often generalized in a cartoon form also shown in Fig. 1.
In one sense the polar lipids are like the anions of fatty acids, only more so. They contain two hydrophobicWater-hating; not attracted to water molecules or polar molecules. hydrocarbonA compound containing only the elements carbon and hydrogen. tails and a head which may have several electrically charged sites. As in the case of soapA salt of a fatty acid produced by the saponification of fat. and detergent molecules, the tails of polar lipids tend to avoid water and other polar substances, but the heads are quite compatible with such environments.
The polar lipids are most commonly found as components of cell walls and other membranes. Nearly all hypotheses regarding membrane structure take as a fundamental component a lipid bilayer (Fig. 2). Bilayers made in the laboratory have many properties in common with membranes. Ions such as Na+, K+, and Cl– cannot pass through them, but water molecules can. The hydrocarbon core of such a bilayer should have large electrical resistance, as does a membrane. Certain carrier molecules can transport K+ and other ions across a bilayer, apparently by wrapping a hydrophobic cloak around them to disguise their charges. Membrane proteins in a bilayer also allow for transport of ions and other molecules across the bilayer which could not cross otherwise.