2.14: Cholesterol and phospholipids are used to build sex hormones and membranes.

Not all lipids are fats, nor do lipids necessarily function in energy storage. A second group of lipids, called the sterols, plays an important role in regulating growth and development (FIGURE 2-33). This group includes some very familiar lipids: cholesterol and the steroid hormones such as testosterone and estrogen. All of these molecules are variations on one basic structure formed from four interlinked rings of carbon atoms.

Cholesterol is an important component of most cell membranes. For this reason, it is an essential molecule for living organisms. Cells in our liver produce almost 90% of the circulating cholesterol by transforming the saturated fats in our diet. Cholesterol is also present in foods and has a bad reputation in most Western cultures that is mostly well deserved. When we ingest too much cholesterol (present in animal-based foods such as egg yolks, red meat, and cream) and high levels of cholesterol circulate in our bloodstream, the cholesterol can attach to blood vessel walls and cause them to thicken. In turn, this thickening can lead to high blood pressure, a major contributor to strokes and heart attacks. For these reasons, nutritionists advise limiting the consumption of foods high in cholesterol and saturated fats.

The steroid hormones estrogen and testosterone are built through slight chemical modifications to cholesterol. These hormones are among the primary molecules that direct and regulate sexual development, maturation, and sperm and egg production. In both males and females, estrogen influences memory and mood, among other traits. Testosterone has numerous effects, one of which is to stimulate muscle growth. As a consequence, athletes (particularly bodybuilders) have often been found to take synthetic variants of testosterone to increase their muscularity (FIGURE 2-34). But the use of these supplements is often accompanied by dangerous side effects, including extreme aggressiveness (“’roid rage”), high cholesterol, and, following long-term use, cancer. As a consequence, nearly all athletic organizations have banned their use.

Phospholipids and waxes are also lipids. Phospholipids are the major component of the membrane that surrounds the contents of a cell and controls the flow of chemicals into and out of the cell (FIGURE 2-35). They have a structure similar to fats, but with two differences: they contain a phosphorus atom (hence phospholipids) and they have two fatty acid chains rather than three. We explore the significant role of phospholipids in cell membranes in the next chapter.

Waxes resemble fats but have only one long-chain fatty acid linked to the glycerol head of the molecule. Because the fatty acid chain is highly nonpolar, waxes are strongly hydrophobic; that is, these molecules do not mix with water but repel it. Their water resistance accounts for their presence as a natural coating on the surface of many plants and in the outer coverings of many insects. In both cases, the waxes prevent the plants and animals from losing the water essential to their life processes. Many birds, too, have a waxy coating on their wings, keeping them from becoming water-logged when they get wet.