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 digerides and cholesterol necessary components of plasma membranes, but the closely related triglycerides and cholesterol esters never present? Their structures are presented on the facing page. How do they differ from the corresponding digelyceride and cholesterol molecules?

Like Sherlock Holmes, who solved one of his most famous cases from a hound who didn't bark in the night, scientists often gain very useful information about patterns by analysing elements that are absent as well as those that are present. And we can too!

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. Although they are soluble in many organic solvents they will not dissove in aqueous solutions. 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 apolar property prevents them from dissolving in aqueous solutions, and their polarity prevents them from forming large, amorphous fat droplets. Such dual, seemingly contradictory behavior is exactly what is meant by "amphipathic" and is crucial for their ability to form membranes, as we shall investigate in greater detail in the next chapter.