THE MECHANISM OF ORGANIC SYNTHESIS Jii7 



of these constituent groups. Just as in digestion the protein mole- 

 cule is taken to pieces with the formation of the different amino- acids, 

 so in the synthetic action of protoplasm the reverse process of dehy- 

 dration occurs, resulting in a coupling up of the different groups, as has 

 been effected by Fischer in the case of the polypeptides. Wherever 

 transport of protein from one part of the organism to another is 

 necessary the protein is carried, not in its original form, but in the 

 hydrolysed condition of amino- acids. Thus the germination of seeds 

 which contain rich stores of protein is accompanied by a liberation 

 of proteolytic ferments within the cells of the seeds, and the break- 

 down of the reserve protein into its constituent amino- acids. As 

 amino-acids it is transported into the growing tip and leaves of the 

 seedling, analysis of the latter showing a very large percentage of 

 nitrogen in the form of amino-acids. This is especially the case if 

 the synthetic functions of the growing tip are hindered by inter- 

 ference with assimilation, as, e.g. by keeping the plant in the dark. 

 Under these circumstances, asparagine may form as much as 

 25 per cent, of the total dried weight of the seedling. In animals 

 the greater part of the protein of the food is broken down into its 

 constituent amino-acids in the intestine. These are absorbed and 

 probably carried to the different organs of the body, where they 

 are resynthetised, generally in different proportions from those that 

 obtained in the original protein, into the protein specific for the organ 

 or tissue. The same process of hydrolysis and subsequent synthesis 

 occurs whenever the transport of protein is necessary from one organ 

 to another. We shall later on have to discuss the possibility of 

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

 at present only deal 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 



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



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 assimila- 

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



