PHOSPHOLIPIDS 395 



catalyzed by the enzyme lipase. In the fat palmitin, represented in the above 

 equation, the three fatty acid radicals are all of the same species. As men- 

 tioned previously, however, the three fatty acid radicals in a fat may be all 

 different, or tvvo may be alike and the third different. Naturally occurring 

 fats are usually mixtures of a number of chemically different kinds of fats of 

 which palmitin is simply one common example. 



Evidence that this is the final step in the synthesis of fats may be regarded 

 as conclusive. This is indicated on the one hand by the fact that when fats 

 are hydrolyzed in the laboratory by the action of acids, alkalis, or extracts of 

 the enzyme lipase the products of the reaction are fatty acids and glycerol, 

 suggesting that these are also the compounds from which the fats are syn- 

 thesized. Digestion of fats to fatty acids and glycerol in plant cells is also 

 accomplished by lipase (Chap. XXVII). 



Furthermore synthesis of fats from fatty acids and glycerol under the 

 influence of lipase can actually be demonstrated in the laboratory. If gly- 

 cerol, a fatty acid, and an extract of lipase be mixed in the proper propor- 

 tions, precautions being taken to keep the mixture sterile, and incubated 

 for a suitable period of time, the disappearance of fatty acids from the mix- 

 ture can be demonstrated. A part of the fatty acids present is removed from 

 the mixture under such conditions and tied up in the formation of fats. 



The synthesis of fats from fatty acids and glycerol, like other condensa- 

 tion processes, involves only a negligible energv' change. 



Phospholipids. — The constitution of these compounds is indicated in a 

 general way by their hydrolytic products which are fatty acids, a nitrogenous 

 base, phosphoric acid, and glycerol. The best known of the phospholipids 

 are the lecithins, which are believed to occur in all plant and animal cells. 

 The widespread occurrence of these compounds suggests that they play fun- 

 damental roles in cell metabolism, but it is by no means certain what these 

 are. The following is the probable structural formula of the lecithins, in 

 which Ri and R2 represent fatty acid radicals: 



CH200CR1 



CHOOCR2 HTT 



I 5==^^ / PPT 



CH2 — 0— P :— -0 CH2-CH2— NC r5^ 



OH XcHa 



Lecithin molecules are similar to fattj^ acid molecules, the essential dif- 

 ference being that one fatty acid radical is replaced by radicals of phosphoric 

 acid and the nitrogenous base choline. Since various kinds of fatty acid 

 radicals can be combined in the R^ and Ro positions as shown in the above 

 structural formula, a number of different lecithins are possible. 



