Chapter 18. Summary

Classification of Lipids. Lipids are a family of naturally occurring compounds grouped together on the basis of their relative insolubility in water and solubility in nonpolar solvents. Lipids are classified as saponifiable (ester-containing) or nonsaponifiable. Saponifiable lipids are further classified as simple or complex, depending on the number of structural components.

Fatty Acids. A fatty acid consists of a long nonpolar chain of carbon atoms, with a polar carboxylic acid group at one end. Most natural fatty acids contain an even number of carbon atoms. They may be saturated, unsaturated, or polyunsaturated (containing two or more double bonds). Fats and oils are simple lipids that are esters of glycerol and fatty acids. The difference between fats and oils is the melting point, which is essentially a function of the fatty acids in the compound.

Chemical Properties of Fats and Oils. Fats and oils can be hydrolyzed in the presence of acid to produce glycerol and fatty acids. When the hydrolysis reaction is carried out in the presence of a strong base, salts of the fatty acids (soaps) are produced. During hydrogenation, some multiple bonds of unsaturated fatty acids contained in fats or oils are reacted with hydrogen and converted to single bonds.

Waxes. Waxes are simple lipids composed of a fatty acid esterified with a long-chain alcohol. Waxes are insoluble in water and serve as protective coatings in nature.

Phosphoglycerides. Phosphoglycerides consist of glycerol esterified to two fatty acids and phosphoric acid. The phosphoric acid is further esterified to choline (in the lecithins) and to ethanolamine or serine (in the cephalins). The phosphoglycerides are particularly important in membrane formation.

Sphingolipids. These complex lipids contain a backbone of sphingosine rather than glycerol and only one fatty acid component. They are abundant in brain and nerve tissue.

Biological Membranes. Membranes surround tissue cells as a selective barrier, and they encase the organelles found in eucaryotic cells. Membranes contain both proteins and lipids. According to the fluid mosaic model, the lipids are arranged in a bilayer fashion with the hydrophobic portions on the insider of the bilayer. Proteins float in the bilayer.

Steroids. Steroids are compounds that have four rings fused together in a specific way. The most abundant steroid in humans is cholesterol, which serves as a starting material for other important steroids such as the bile salts, adrenocorticoid hormones, and the sex hormones.

Steroid Hormones. Hormones are chemical messengers, synthesized by specific glands, that affect various target tissues in the body. The adrenal cortex produces a number of steroid hormones that regulate carbohydrate utilization (the glucocorticoids) and electrolyte balance (the mineralocorticoids). The testes and ovaries produce steroid hormones that determine secondary sexual characteristics and regulate the menstrual cycle in females.

Prostaglandins. These compounds are synthesized from the 20-carbon fatty acid ai:achidonic acid. They exert many hormone-like effects on the body and are used therapeutically to induce. labor, treat asthma, and control gastric secretions.