A suitable terpene chain can be condensed into a four-ring framework that is typical of another class of lipids: steroids. Cholesterol (right) is a steroid molecule that is an important part of many membranes, but which can cause trouble when it occurs as a fatty deposit in blood vessels and chokes off blood flow. Cortisone, testosterone, and other steroids are hormones - chemical messengers that are released in minute amounts at one location in the body, and have profound regulatory effects at a number of other distant locations.

Cortisone's main function is to adjust the level of glucose in the blood by conversion of liver glycogen (see Page 23) to glucose. Testosterone is a male sex hormone; and other steroid hormones control the levels of Na⁺, Cl^-, and water in the body, influence sexual development, and control inflammation and allergic response.

Steroid and polypeptide (protein) hormones seem to be the regulatory messengers in all plants and animals above the one-celled level, carrying information about the state of one group of cells to another group of cells that are capable of taking appropriate action.