CHAPTER LXXXI 



FAT METABOLISM (Cont'd) 



Two very important questions of fatty-acid metabolism may now be 

 considered: namely, (1) how is fatty acid formed from carbohydrate? 

 and (2) what becomes of the fragments into which the fatty-acid molecule 

 is split as the result of the desaturation process? Although these prob- 

 lems involve chemical details of a somewhat complex nature, we must 

 not on this account fail to consider them; for, as we shall see, much of 

 what is known has an important practical application depending on the 

 fact that certain of the intermediary substances may accumulate in the 

 organism and develop a toxic action. 



The Production of Fatty Acid out of Carbohydrate. If we place the 

 formulas for glucose and palmitic acid side by side, thus: 



CH 2 OH-(CHOH) 4 -CHO (glucose), and 

 CH 3 -(CH 2 ) 14 -COOH (palmitic acid); 



we shall see that this transformation must involve: (1) a considerable 

 alteration in the structure of the molecule, (2) the removal of oxygen, 

 and (3) the fusion of several glucose molecules into one molecule of fatty 

 acid. 



The conversion of carbohydrate to fat therefore involves a process of 

 reduction, and the resulting molecule must be capable of yielding more 

 energy when it is oxidized than the original one of carbohydrate, for 

 obviously the system 2 -CH 2 (which corresponds to fat) will develop 

 more energy than that of 2 - CHO (which corresponds to carbohydrate) ; 

 just as a piece of wood when it is burned will develop more heat than a 

 piece of charcoal. This explains why one gram of fat yields 9.3 calories 

 of heat, and one gram of carbohydrate, only 4.1 (page 571). Fatty 

 acid therefore contains more potential energy than sugar, and in explain- 

 ing its synthesis from sugar in the animal body we must indicate the 

 source of the extra energy. This is dependent on oxidation of some sugar 

 molecules which do not themselves become changed to fatty acid 

 proceeding side by side with the reduction which affects the others and 

 is represented in the outcome of the reaction by the combustion products 

 C0 2 and H 2 0, thus: 



6C 6 H 12 6 + 13 2 = 20 C0 2 + C 16 H 32 2 + 20 H 2 0. 

 (glucose) (fatty acid) 



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