Several chemical compounds in food and in the body are classified as lipids. They include (1) neutral fat, also known as triglycerides;(2) phospholipids; (3) cholesterol; and (4) a few others of less importance. Chemically, the basic lipid moiety of the triglycerides and the phospholipids is fatty acids, which are simply long-chain hydrocarbon organic acids. A typical fatty acid, palmitic acid, is the following: CH3(CH2)14COOH.
Although cholesterol does not contain fatty acid, its sterol nucleus is synthesized from portions of fatty acid molecules, thus giving it many of the physical and chemical properties of other lipid substances.
The triglycerides are used in the body mainly to provide energy for the different metabolic processes, a function they share almost equally with the carbohydrates. However, some lipids, especially cholesterol, the phospholipids, and small amounts of triglycerides, are used to form the membranes of all cells of the body and to perform other cellular functions.
Basic Chemical Structure of Triglycerides (Neutral Fat). Because most of this dealswith the utilization of triglycerides for energy, the following typical structure of the triglyceride molecule must be understood.
Note that three long-chain fatty acid molecules are bound with one molecule of glycerol. The three fatty acids most commonly present in the triglycerides of the human body are (1) stearic acid (shown in the tristearin example above), which has an 18-carbon chain and is fully saturated with hydrogen atoms; (2) oleic acid, which also has an 18-carbon chain but has one double bond in the middle of the chain; and (3) palmitic acid, which has 16 carbon atoms and is fully saturated.