THE MECHANISM OF ORGANIC SYNTHESIS 135 



neutral fats can be synthetised by both plants and animals, and there 

 is every ground for believing that it, like the fatty acids, may be 

 derived from carbohydrates. We have already seen that in the con- 

 version of glucose into lactic acid the first step is the formation of 

 glyceric aldehyde, 



CH 2 OH CH 2 OH 



CHOH CHOH 



CHOH CHO 



CHOH CH 2 OH 



CHOH CHOH 



I I 



CHO CHO 



and it is easy to understand how by a process of reduction the alde- 

 hyde is converted into the corresponding alcohol, namely, glycerin. 

 The synthesis of the neutral fat from glycerin and fatty acid is a change 

 which can be accomplished by many ferments. It is one involving 

 practically no absorption or expenditure of energy. The change is a 

 reversible one, and we find both in plants and animals that a hydrolysis 

 of neutral fat into fatty acid and glycerin always occurs when a trans- 

 port of the fat is required, while the laying down of fat as a store of 

 energy is always preceded by a resynthesis of the neutral fat. We 

 shall have occasion to deal in greater detail with these questions when 

 we have to discuss the formation and fate of the fat in the animal 

 body. 



