122 PHYSIOLOGY 



lactic acid, and so from carbohydrate, is fermented by yeast with the prcduclicn of 

 acetaldehyde and carbon dioxide, by means of a ferment carboxylase. If we assume 

 the existence of a similar ferment in the cells of the body, it would split this acid into 

 aldehyde and CO 2 . Aldehyde, however, combines with a molecule of pyruvic acid to 

 form a higher keto=acid, which might either be oxidised to the fatty acid containing one 

 carbon atom less, or might be again transformed by enzymes into an aldehyde capable 

 of reacting with another molecule of pyruvic acid. These changes are represented in the 

 following equations : 



CH 3 CO.COOH = CH 3 CHO + CO 2 



CH 3 CHO + CH 3 CO.COOH = CH 3 CHOH . CH 2 . CO . COOH 

 CH 3 CHOH.CH 2 .CO.COOH + O = CH 3 CHOH . CH 2 COOH + CO 2 



/3-oxyacids would thus be a normal stage in the building up as well as in the breaking 

 down of fatty acids. 



The glycerin which enters into the formation of the ordinary 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 conversion 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 



CHO CHO 



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

 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 absorp- 

 tion 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 transport 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. 



