Carotenoids and steroids

The next two lipid classes we’ll discuss—the carotenoids and the steroids—have chemical structures very different from those of triglycerides and phospholipids and from each other. Both carotenoids and steroids are synthesized by covalent linking and chemical modification of isoprene to form a series of isoprene units.

Carotenoids trap light energy

The carotenoidsare a family of light-absorbing pigments found in plants and animals. Beta-carotene is one of the pigments that traps light energy in leaves during photosynthesis. In humans, a molecule of beta-carotene can be broken down into two vitamin A molecules, from which we make the pigment rhodopsin, which is required for vision. Carotenoids are responsible for the colors of carrots, tomatoes, pumpkins, egg yolks and butter.

Steroids are signal molecules

The steroidsare a family of organic compounds whose multiple rings share carbons. The steroid cholesterol is an important constituent of membranes. Other steroids function as hormones, chemical signals that carry messages from one part of the body to another. Testosterone and the estrogens are steroid hormones that regulate sexual development in vertebrates. Cortisol and related hormones play many regulatory roles in the digestion of carbohydrates and proteins, in the maintenance of salt balance and water balance and in sexual development. Cholesterol is synthesized in the liver and is the starting material for making testosterone and other steroid hormones, as well as the bile salts that help break down dietary fats so that they can be digested. Cholesterol is absorbed from foods such as milk, butter and animal fats.

Some lipids are vitamins

Vitaminsare small molecules that are not synthesized by the body, but are necessary for its normal functioning. Vitamins must be acquired from dietary sources.

Vitamin A is formed from the beta-carotene found in green and yellow vegetables. In humans, a deficiency of vitamin A leads to dry skin, eyes and internal body surfaces, retarded growth and development and night blindness, which is a diagnostic symptom for the deficiency.

Vitamin D regulates the absorption of calcium from theintestines. It is necessary for the proper deposition of calcium in bones; a deficiency of vitamin D can lead to rickets, a bone-softening disease.

Vitamin E seems to protect cells from the damaging effects of oxidation–reduction reactions. For example, it has an important role in preventing unhealthy changes in the double bonds in the unsaturated fatty acids of membrane phospholipids. Commercially, vitamin E is added to some foods to slow spoilage.

Vitamin K is found in green leafy plants and is also synthesized by bacteria normally present in the human intestine. This vitamin is essential to the formation of blood clots. Inadequate vitamin intake can lead to deficiency diseases.

Wax coatings repel water

The sheen on human hair is not there only for cosmetic purposes. Glands in the skin secrete a waxy coating that repels water and keeps the hair pliable. Birds that live near water have a similar waxy coating on their feathers. The shiny leaves of holly plants, familiar during winter holidays, also have a waxy coating. Finally, bees make their honeycombs out of wax. All waxes have the same basic structure: They are formed by an ester linkage between a saturated, long-chain fatty acid and a saturated, long-chain alcohol. The result is a very long molecule, with 40–60 CH₂ groups. For example, here is the structure of beeswax: This highly nonpolar structure accounts for the impermeability of wax to water.