Triglycerides, Cholesterol Esters and a Tip from Sherlock Holmes.

The most common lipid in our bodies is triglyceride, and cholesterol often occurs as a fatty acid ester. (And in some of us these compounds are more common than we would like!) Yet neither is found in biological membranes. Why not? Why are diglycerides and cholesterol necessary components of plasma membranes, but the closely related triglycerides and cholesterol esters never present?

Like Sherlock Holmes, who solved one of his more famous cases thinking about a hound who didn't bark in the night, scientists often gain very useful information analysing patterns of absent elements as well as ones that are present. And we can too! Exactly how do triglycerides and cholesterol esters, illustrated on the facing page, differ from diglycerides and cholesterol?

In contrast with membrane lipids, triglycerides and cholesterol esters are entirely hydrophobic; for this reason they are often called neutral fats. They readily associate with each other, or other hydrophobic molecules, and often form large droplets or spherical bodies inside cells. They are not soluble in cytoplasm or other aqueous solutions, although they will dissove in organic solvents. In contrast, diglycerides and cholesterol interact with water just like polar solute molecules and they readily associate with each other (like their neutral kin). Their hydrophobic property prevents them from dissolving in aqueous solutions, and their polarity prevents them from forming large, amorphous fat droplets. Such seemingly contradictory behavior is exactly what is meant by "amphipathic" and is crucial for the ability of phospholipids to form membranes, as we shall investigate in greater detail in <Chapter 3> [html link].