Figure 2.26 Formation of a Fat The R group of the fatty acids commonly contains 16 or 18 carbon atoms bonded to hydrogen atoms.
Simple lipids contain only carbon, hydrogen, and oxygen. The most common are the fats, a combination of fatty acids and glycerol that are solid at room temperature (figure 2.26). A molecule that contains only carbon and hydrogen is a hydrocarbon. Fatty acids are molecules with long chains of C atoms bonded to H atoms with an acidic group (—COOH) on one end. Since glycerol has three hydroxyl groups, a maximum of three fatty acid molecules, either the same or different, can be linked through covalent bonds between the —OH group of glycerol and the —COOH group of the fatty acid. If only one fatty acid is bound to glyc-erol, the fat is called a monoglyceride; when two fatty acids are joined, it is a diglyceride; when three fatty acids are bound, a triglyceride is formed. Fatty acids are stored in the body as an energy reserve by forming triglycerides.
Unlike carbohydrates, fatty acids do not have a ratio of approximately 1:2:1 (C: H : O)—since they have few oxygen atoms. Although hundreds of different fatty acids exist, they can be divided into two groups based on whether or not any double bonds are present in the hydrocarbon portion. If there are no double bonds, the fatty acid is termed saturated with H atoms. If it contains one or more double bonds, it is unsaturated. Unsaturated fats tend to be liquid and are then called oils. Oils are liquid because these unsaturated fatty acids develop kinks in their long tails that prevent tight packing. The saturated fats can pack their straight long tails tightly together and therefore are solid (figure 2.27a). Two of the most common saturated fatty acids in nature are palmitic acid (16 carbon atoms), and stearic acid (18 carbon atoms). A common unsaturated fatty acid is oleic acid (18 carbon atoms and one double bond in the molecule). Because oleic acid has only one double bond, it is a monounsaturated fatty acid (figure 2.27b). Other fatty acids contain numerous double bonds that make them polyunsaturated. Different lipids are called highly saturated or highly unsaturated when they contain highly saturated or unsaturated fatty acids.
Another very important group of simple lipids is the steroids. All members of this group have the four-membered ring structure shown in figure 2.28a. These compounds differ from the fats in chemical structure, but both are classified as lipids because they are both insoluble in water. If a hydroxyl group is attached to one of the rings, the steroid is called a sterol, an example being cholesterol (figure 2.28b). Sterols are
Saturated fatty acid
Unsaturated fatty acid
Saturated fatty acid
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