Lipids contain more energy per gram than other biological molecules, and some function as long-term energy storage molecules in organisms. Others form a membrane that separates a cell from its environment and has inner compartments as well. Steroids are a large class of lipids that includes, among other molecules, the sex hormones. Lipids are diverse in structure and function, but they have a common characteristic: They do not dissolve in water. Their low solubility in water is due to an absence of polar groups. They contain little oxygen and consist mostly of carbon and hydrogen atoms.
Fats and Oils
The most familiar lipids are those found in fats and oils. Fats, which are usually of animal origin (e.g., lard and butter), are solid at room temperature. Oils, which are usually of plant origin (e.g., corn oil and soybean oil), are liquid at room temperature. Fat has several functions in the body: It is used for long-term energy storage, it insulates against heat loss, and it forms a protective cushion around major organs. Fats and oils form when one glycerol molecule reacts with three fatty acid molecules (Fig. 2.12). A fat is sometimes called a triglyceride, because of its three-part structure, or a neutral fat, because the molecule is nonpolar and carries no charge.
Figure 2.12 Synthesis and degradation of a fatmolecule. Fatty acids can be saturated (no double bonds between carbon atoms) or unsaturated (have double bonds, colored yellow, between carbon atoms). When a fatmolecule forms, three fatty acids combine with glycerol, and three water molecules are produced.
Emulsifiers can cause fats to mix with water. They contain molecules with a nonpolar end and a polar end. The molecules position themselves about an oil droplet so that their nonpolar ends project. Now the droplet disperses in water, which means that emulsification has occurred.
Emulsification takes place when dirty clothes are washed with soaps or detergents. Also, prior to the digestion of fatty foods, fats are emulsified by bile. The gallbladder stores bile for emulsifying fats prior to the digestive process.
Saturated and Unsaturated Fatty Acids
A fatty acid is a carbon-hydrogen chain that ends with the acidic group -COOH (Fig. 2.12). Most of the fatty acids in cells contain 16 or 18 carbon atoms per molecule, although smaller ones with fewer carbons are also known. Fatty acids are either saturated or unsaturated. Saturated fatty acids have only single covalent bonds because the carbon chain is saturated, so to speak, with all the hydrogens it can hold. Saturated fatty acids account for the solid nature at room temperature of fats such as lard and butter. Unsaturated fatty acids have double bonds between carbon atoms wherever fewer than two hydrogens are bonded to a carbon atom. Unsaturated fatty acids account for the liquid nature of vegetable oils at room temperature. Hydrogenation of vegetable oils can convert them to margarine and products such as Crisco.
Phospholipids, as their name implies, contain a phosphate group (Fig. 2.13). Essentially, they are constructed like fats, except that in place of the third fatty acid, there is a phosphate group or a grouping that contains both phosphate and nitrogen. Phospholipid molecules are not electrically neutral, as are fats, because the phosphate and nitrogencontaining groups are ionized. They form the so-called hydrophilic head of the molecule, while the rest of the molecule becomes the hydrophobic tails. Phospholipids are the backbone of cellular membranes; they spontaneously form a bilayer in which the hydrophilic heads face outward toward watery solutions and the tails form the hydrophobic interior.
Figure 2.13 Phospholipid structure and function. a. Phospholipids are structured like fats, but one fatty acid is replaced by a polar phosphate group. b. Therefore, the head is polar while the tails are nonpolar. c. This causes the molecule to arrange itself as shown when exposed to water.
Steroids are lipids that have an entirely different structure from those of fats. Steroid molecules have a backbone of four fused carbon rings. Each one differs primarily by the functional groups attached to the rings. Cholesterol is a component of an animal cell’s outer membrane and is the precursor of several other steroids, such as the sex hormones estrogen and testosterone. The male sex hormone, testosterone, is formed primarily in the testes, and the female sex hormone, estrogen, is formed primarily in the ovaries. Testosterone and estrogen differ only by the functional groups attached to the same carbon backbone, yet they have a profound effect on the body and on our sexuality (Fig. 2.14a,b). Testosterone is a steroid that causes males to have greater muscle strength than females. Taking synthetic testosterone for this purpose, however, is dangerous to your health. We know that a diet high in saturated fats and cholesterol can cause fatty material to accumulate inside the lining of blood vessels, thereby reducing blood flow. Nutrition labels are now required to list the calories from fat per serving and the percent daily value from saturated fat and cholesterol.
Figure 2.14 Steroids. All steroids have four rings, but they differ by attached groups. The effects of (a) testosterone and (b) estrogen on the body largely depend on the difference in the attached groups shown in red.