334 ORGANISATION IN SPACE AND TIME 



is only half that of the reaction A -> C. Naturally, after a 

 certain time, when all of substance A has disappeared, the 

 resulting mixture will be found to contain 70 per cent B, 

 20 per cent C and 10 per cent D. Thus, under the given 

 conditions the bulk of substance A will have been converted 

 to substance B, that is to say, it will have followed the path 

 along which the reaction proceeds fastest. 



If we apply to such a system any influence which will 

 increase the rates of all possible reactions equally (e.g., raising 

 the temperature) then the ratio of the end products will 

 not be changed in any way. If, however, we add to the 

 original mixture a catalyst which specifically increases, by 

 perhaps a million times, the rate of the reaction A -> D alone 

 and does not alter the rates of the reactions A ^ B and 

 A -> C, the effect produced will be quite different. Under 

 these circumstances substance A will be converted almost 

 entirely to substance D while B and C will be present in 

 barely perceptible or imperceptible traces. 



The substance D which is formed in this way, like sub- 

 stance A or any other organic compound, has many chemical 

 potentialities and also follows the fastest course in its chemical 

 transformations. The compound N which is formed from it 

 may similarly form the starting point for further chemical 

 transformations. In this way there arises a chain of successive 

 reactions, related to one another in time, the co-ordination 

 of which is based upon the relative reaction velocities. 



--> etc. 



Such chains of successive transformations form the basis of 

 biological metabolism, in particular the synthesis of the most 

 complicated components of protoplasm. For example, as we 

 saw in Chapter V, porphyrin is formed in living cells from 

 the relatively simple compounds glycine and succinic acid. 

 This, however, can only occur as a result of a long series 

 of strictly co-ordinated chemical transformations. First the 

 succinic acid forms succinyl coenzyme A, by means of which 

 it condenses with the a-carbon atom of glycine. This reaction 

 gives rise to a-amino-^-oxoadipic acid, which is converted to 



