In making a fat, three fatty acid molecules each join to glycerol by an ester linkage, a bond between a hydroxyl group and a carboxyl group. The resulting fat, also called a triacylglycerol , thus consists of three fatty acids linked to one glycerol molecule. (Still another name for a fat is triglyceride, a word often found in the list of ingredients on packaged foods.) The fatty acids in a fat can be the same, as in Figure 5.11b, or they can be of two or three different kinds.
A fat made from saturated fatty acids is called a saturated fat. Most animal fats are saturated: The hydrocarbon chains of their fatty acids—the “tails” of the fat molecules—lack double bonds, and the molecules can pack tightly, side by side. Saturated animal fats—such as lard and butter—are solid at room temperature. In contrast, the fats of plants and fishes are generally unsaturated, meaning that they are built of one or more types of unsaturated fatty acids. Usually liquid at room temperature, plant and fish fats are referred to as oils—olive oil and cod liver oil are examples. The kinks where the cis double bonds are located prevent the molecules from packing together closely enough to solidify at room temperature. The phrase “hydrogenated vegetable oils” on food labels means that unsaturated fats have been synthetically converted to saturated fats by adding hydrogen. Peanut butter, margarine, and many other products are hydrogenated to prevent lipids from separating out in liquid (oil) form.
Steroids are lipids characterized by a carbon skeleton consisting of four fused rings.