THE MECHANISM OF ORGANIC SYNTHESIS 113 



possibility of synthesis of the different amino-acids in animals. We need 

 therefore at present deal only with the possible methods by which, from the 

 glucose or substances produced in the assimilation of carbon and 

 from the ammonia or nitrates derived from the soil, the plant is able 

 to make the different groups which go to the building up of the protein 

 molecule. 



All the amino-acids contain the NH 2 group in the a position. We can 

 therefore consider them as formed by the interaction of an a-oxyacid and 

 ammonia. Thus : 



CH 3 CH 3 



! I 



CH.OH + NH 3 CH.NH 2 + H 2 O 



I I 



COOH COOH 



lactic acid alanine 



This particular example, namely, the formation of 'alanine, may occur 

 at the expense of the glucose produced as the first product of assimilation 

 of carbon dioxide. If a solution of glucose together with lime be exposed to 

 sunlight for a considerable time it undergoes decomposition with the forma- 

 tion of lactic acid. Thus : 



C 6 H 12 6 2C 3 H 6 3 



glucose lactic acid 



This change of glucose to lactic acid under the catalytic influence of the 

 alkaline calcium hydrate probably occurs by means of a shifting of the 

 elements of the water, a process which in many long chains seems to occur 

 with considerable facility, and is dependent on the spatial configuration of 

 the molecule involved. Thus the change of sugar to lactic acid is readily 

 effected by means of many micro-organisms in the case of glucose, fructose, 

 and mannose, but with considerable difficulty in the case of galactose. In 

 the three former sugars the atoms round the two middle carbon atoms of the 

 chain are disposed thus : 



I I 



OH.C.H H.C.OH 



I or | 



H.C.OH OH.C.H 



I I 



When either of these arrangements reacts with water, thus : 



CH 2 OH CH 2 OH 



I I 



CHOH CHOH 



I I 



OH.C.H OH COH + H 2 O 



HCOH H CHoOH 



I I 



CHOH CHOH 



I I 



COH COH 



