The five major classes of prostaglandins are designated as PGA, PGB, PGE, PGF, and PGI. It is now known that they are synthesized in nearly all mammalian tissues and affect almost all organs in the body. They are called prostaglandins because they were originally isolated from semen found in the prostate gland. They are unsaturated fatty acids containing 20 carbon atoms and are synthesized from arachidonic acid-a polyunsaturated fatty acid-when needed by a particular cell. Prostaglandins are chemical messengers synthesized in the cells in which their physiological activity is expressed. The average daily diet should contain about 4–6 g of the essential fatty acids. In addition, the essential fatty acids are necessary for the efficient transport and metabolism of cholesterol. The body uses linoleic acid to synthesize many of the other unsaturated fatty acids, such as arachidonic acid, a precursor for the synthesis of prostaglandins. Both substances are required for normal growth and development, but the human body does not synthesize them. because humans must obtain them from their diets. Two polyunsaturated fatty acids-linoleic and α-linolenic acids-are termed essential fatty acids A fatty acid that must be obtained from the diet because it cannot be synthesized by the human body. Table 17.1 Some Common Fatty Acids Found in Natural Fats Name (For more information about cis-trans isomerism, see Chapter 13 "Unsaturated and Aromatic Hydrocarbons", Section 13.2 "Cis-Trans Isomers (Geometric Isomers)".) Naturally occurring fatty acids are generally in the cis configuration. The atoms or groups around the double bonds in unsaturated fatty acids can be arranged in either the cis or trans isomeric form. Table 17.1 "Some Common Fatty Acids Found in Natural Fats" lists some common fatty acids and one important source for each. contain two or more carbon-to-carbon double bonds. contain one carbon-to-carbon double bond, and polyunsaturated fatty acids A fatty acid that has two or more carbon-to-carbon double bonds. contain no carbon-to-carbon double bonds, monounsaturated fatty acids A fatty acid that has one carbon-to-carbon double bond. Thus, saturated fatty acids A fatty acid that has no carbon-to-carbon double bonds. They usually contain an even number of carbon atoms (typically 12–20), are generally unbranched, and can be classified by the presence and number of carbon-to-carbon double bonds. More than 70 have been identified in nature. are carboxylic acids that are structural components of fats, oils, and all other categories of lipids, except steroids. Recognize the structures of common fatty acids and classify them as saturated, monounsaturated, or polyunsaturated.įatty acids A carboxylic acid that is a structural component of fats and oils.
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