[Chop. XVI SYNTHESIS OF FATS AND PROTEINS 141 



coconut, cottonseed, linseed, castor, pea, peanut, tung, olive, soybean, 

 and cocoa. 



Summary. Fats that melt at ordinary room temperatures are liquid 

 fats or oils. Fats are made from sugar in the cells of both plants and 

 animals. By chemical reduction some of the sugar is transformed to 

 glycerin and fattv acids. The energy required for this process comes from 

 the oxidation of sugar, or its derivatives, in the cells. Then 3 molecules 

 of fatty acid unite with 1 molecule of glycerin by condensation, and a 

 fat or oil is the result. A pound of fat, therefore, contains much more 

 potential energy than a pound of sugar — about 2}4 times as much. 



When a fat is digested, glycerin and fattv acids are set free. These fatty 

 acids mav then recombine with the glycerin in a different order or they 

 may be altered and then recombine with glycerin, depending upon 

 enzymes and certain other conditions within the cells. As a consequence, 

 man}' different kinds of fats are present in both the plant and animal 

 kingdoms. Fats may also be retransfomied to sugar in li\'ing cells. The 

 protoplasm of all cells is composed in part of certain kinds of fats and 

 fat-like derivatives. Fats are extracted from plants and used commer- 

 cially in many ways. 



Protein Synthesis 



The proteins are a third class of foods. One sometimes hears that they 

 are the most important foods. But when three things are essential, no one 

 of them is more important than another. In a later chapter we shall see 

 that one may truthfull>' say that the cell walls of plants are constructed 

 mainlv from carbohvdrate foods, and that the larger part of the organic 

 matter in protoplasm is made from protein foods. But enzymes, pig- 

 ments, vitamins, hormones, and all three classes of foods are essential 

 in the construction of protoplasm. All of the foods contain chemically 

 bound energy that was bound first of all in sugar. This energy again 

 becomes free and actixe in the cells through oxidation, mostly of carbo- 

 hvdrate foods and least of all of proteins. Evidently no part of the plant 

 will grow much unless it is well supplied with all three of these foods, 

 or with all the conditions necessary for making them. By numerous ex- 

 periments this statement has been shown to be true particularly of 

 sugar and proteins, as we shall see in later chapters. 



Protein molecules. The proteins, unlike the complex carbohydrates 

 and fats, are made onlv in part from sugar. A molecule of protein is 

 composed of nitrogen and sulfur in addition to the three elements 



