ee We'll see numerous examples of both reaction types in the following Keep in mind that in vivo reactions (reactions in living systems) are enzyme-ca nd occur at rates that are far greater than when the same transformations are ca in vitro ("in glass") in the absence of enzymes. In spite of the rapidity with which enzyme-catalyzed reactions take place, the nature of these transformations is essentiall the same as the fundamental processes of organic chemistry described throughout this text Fats are one type of lipid. They have a number of functions in living systems, luding that of energy storage. Although carbohydrates serve as a source of readily more efficient for an organism to store energy in the form of fat because it requireS Rr available energy, an equal weight of fat delivers over twice the amount of energy. It mass than storing the same amount of energy in carbohydrates or proteins. How living systems convert acetate to fats is an exceedingly complex story, one that is well understood in broad outline and becoming increasingly clear in detail as well We will examine several aspects of this topic in the next few sections, focusing mostly on its structural and chemical features 26.2 FATS, OILS, AND FATTY ACIDS Fats and oils are naturally occurring mixtures of triacylglycerols, also called triglyc- An experiment describing generaly ignore this distinction and refer to both groups as lie d oils are liquids. We the analysis of the trioeral brides. They differ in that fats are solids at room temperature Triacylglycerols are built on a glycerol framework vegetable oils is described in the May 1988 issue of the ournal of chemical educa. on(pp.464-466) HOCH, CHCH,OH RCOCHCHCH,OCR OCR Glycerol A triacylglycerol All three acyl groups in a triacylglycerol may be the same, all three may be different or one may be different from the other two Figure 26.2 shows the structures of two typical triacylglycerols, 2-oleyl-1, 3 distearylglycerol(Figure 26. 2a) and tristearin(Figure 26.2b). Both occur naturally--in cocoa butter, for example. All three acyl groups in tristearin are stearyl(octadecanoyl) groups. In 2-oleyl-1, 3-distearylglycerol, two of the acyl groups are stearyl, but the one in the middle is oleyl(cis-9-octadecenoyl). As the figure shows, tristearin can be pre pared by catalytic hydrogenation of the carbon-carbon double bond of 2-oleyl-1, 3 distearylglycerol. Hydrogenation raises the melting point from 43C in 2-oleyl-1, 3 distearylglycerol to 72C in tristearin and is a standard technique in the food industry for converting liquid vegetable oils to solid"shortenings. "The space-filling models of Strictly speaking, the te the two show the flatter structure of tristearin, which allows it to pack better in a crys se carboxylic acids that tal lattice than the more irregular shape of 2-oleyl-1, 3-distearylglycerol permits. This ur naturally in triacylglyc. irregular shape is a direct result of the cis double bond in the side chain. erols. Many chemists and Hydrolysis of fats yields glycerol and long-chain fatty acids. Thus, tristearin gives to all unbranched carboxylic glycerol and three molecules of stearic acid on hydrolysis. Table 26.1 lists a few repre- acids, irrespective of their sentative fatty acids. As these examples indicate, most naturally occurring fatty acids possess an even number of carbon atoms and an unbranched carbon chain. The carbon fatty acids. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website26.2 Fats, Oils, and Fatty Acids 1017 We’ll see numerous examples of both reaction types in the following sections. Keep in mind that in vivo reactions (reactions in living systems) are enzyme-catalyzed and occur at rates that are far greater than when the same transformations are carried out in vitro (“in glass”) in the absence of enzymes. In spite of the rapidity with which enzyme-catalyzed reactions take place, the nature of these transformations is essentially the same as the fundamental processes of organic chemistry described throughout this text. Fats are one type of lipid. They have a number of functions in living systems, including that of energy storage. Although carbohydrates serve as a source of readily available energy, an equal weight of fat delivers over twice the amount of energy. It is more efficient for an organism to store energy in the form of fat because it requires less mass than storing the same amount of energy in carbohydrates or proteins. How living systems convert acetate to fats is an exceedingly complex story, one that is well understood in broad outline and becoming increasingly clear in detail as well. We will examine several aspects of this topic in the next few sections, focusing mostly on its structural and chemical features. 26.2 FATS, OILS, AND FATTY ACIDS Fats and oils are naturally occurring mixtures of triacylglycerols, also called triglyc￾erides. They differ in that fats are solids at room temperature and oils are liquids. We generally ignore this distinction and refer to both groups as fats. Triacylglycerols are built on a glycerol framework. All three acyl groups in a triacylglycerol may be the same, all three may be different, or one may be different from the other two. Figure 26.2 shows the structures of two typical triacylglycerols, 2-oleyl-1,3- distearylglycerol (Figure 26.2a) and tristearin (Figure 26.2b). Both occur naturally—in cocoa butter, for example. All three acyl groups in tristearin are stearyl (octadecanoyl) groups. In 2-oleyl-1,3-distearylglycerol, two of the acyl groups are stearyl, but the one in the middle is oleyl (cis-9-octadecenoyl). As the figure shows, tristearin can be pre￾pared by catalytic hydrogenation of the carbon–carbon double bond of 2-oleyl-1,3- distearylglycerol. Hydrogenation raises the melting point from 43°C in 2-oleyl-1,3- distearylglycerol to 72°C in tristearin and is a standard technique in the food industry for converting liquid vegetable oils to solid “shortenings.” The space-filling models of the two show the flatter structure of tristearin, which allows it to pack better in a crys￾tal lattice than the more irregular shape of 2-oleyl-1,3-distearylglycerol permits. This irregular shape is a direct result of the cis double bond in the side chain. Hydrolysis of fats yields glycerol and long-chain fatty acids. Thus, tristearin gives glycerol and three molecules of stearic acid on hydrolysis. Table 26.1 lists a few repre￾sentative fatty acids. As these examples indicate, most naturally occurring fatty acids possess an even number of carbon atoms and an unbranched carbon chain. The carbon HOCH2CHCH2OH OH Glycerol OCR RCOCH2CHCH2OCR O O O A triacylglycerol An experiment describing the analysis of the triglyc￾eride composition of several vegetable oils is described in the May 1988 issue of the Journal of Chemical Educa￾tion (pp. 464–466). Strictly speaking, the term “fatty acid” is restricted to those carboxylic acids that occur naturally in triacylglyc￾erols. Many chemists and biochemists, however, refer to all unbranched carboxylic acids, irrespective of their origin and chain length, as fatty acids. 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