THE MECHANISM OF ORGANIC SYNTHESIS 117 



formation of the generative glands, the other amino-acids being oxidised 

 and utilised for the ordinary energy requirements of the animal. 



THE SYNTHESIS OF FATS 



In some plants fat globules have been stated to appear as the first products 

 of the assimilation of carbon dioxide under the influence of sunlight, but 

 there is no doubt that as a rule the formation of fats as reserve material in 

 seeds or fruits occurs at the expense of carbohydrates. 'In the higher animals 

 too, although a certain amount of the fat of the body is derived from the fat 

 taken up with the food, the organism can also manufacture neutral fat out 

 of the carbohydrates presented to it in its food. The problem therefore 

 of the synthesis of the fats is the problem of the conversion of a sugar such 

 as glucose into glycerin and the fatty acids. Although this conversion 

 is apparently so easily effected by the living organism, it is one which from the 

 chemical standpoint involves considerable difficulties. On account of the 

 fact that the higher fatty acids consist largely of oleic and -stearic acids, i.e. 

 acids containing eighteen carbon atoms in their chain, it has been thought 

 that the synthesis might be brought about by the linking together of three 

 molecules of a hexose. Such a change, would involve a series of difficult 

 chemical transformations. For instance, no less than sixteen out of the 

 eighteen oxygen atoms present in the three glucose molecules would have 

 to be dislodged in order to convert the chain into stearic acid. Moreover, 

 although these two acids contain a multiple of six carbon atoms, a whole 

 array of fats are found both in plants and animals which could not be derived 

 by a simple aggregation of glucose molecules ; and it is worthy of note that, of 

 all the fatty acids which occur in nature, all those with more than five carbon 

 atoms contain an even number of carbon atoms. Thus in milk, in addition 

 to the three common fats, tristearin, tripalmitin, and triolein, we find the 

 glycerides of caproic, caprylic, capric, lauric, and myristic acids, i.e. acids 

 with 6, 8, 10, 12, and 14 carbon atoms. In all cases these acids are the 

 normal acids with straight unbranched chains. It seems probable that in 

 the transformation of carbohydrate into fatty acid the latter is built up, not 

 by six carbon atoms, but by two carbon atoms at a time. It has been 

 suggested by Magnus Levy and by Leathes that the transformation may 

 occur by way of lactic acid. We have seen already that glucose and the 

 sugars of analogous composition may be converted under the influence either 

 of sunlight or of micro-organisms into lactic acid. Lactic acid breaks down 

 with readiness into aldehyde and formic acid. 



CH 3 CH 3 



I I 

 CHOH = CHO + H 



I I 



COOH COOH 



Aldehyde undergoes condensation to form aldol. 



