ORIGIN OF ENZYMES 365 



sugar and oxygen, carbon dioxide and water, are only the 

 first and last links ; between them there are long chains of 

 chemical transformations. In these chains the intermediate 

 product which is produced by one reaction immediately 

 enters into the next reaction, which is strictly determinate 

 for the vital process in question. If these sequences are 

 changed, if any single link in the chain of transformations is 

 removed or altered, then the whole process will become quite 

 different or even be thrown right out of action. As we have 

 seen, these organisational features, which are characteristic 

 of everything living, are exactly analogous, in principle, to 

 the network of chemical transformations which forms the 

 basis of any more or less complicated chemical open system. 

 As in these systems, so in living things, the characteristic 

 order of phenomena which has been described is based on 

 a close co-ordination of the rates of the chemical reactions 

 which form the individual links of the long and labyrinthine 

 chain of metabolism. 



Organic substances, which are the essential components of 

 living systems, seem to be the only material which can form 

 the basis of such chains of reactions. It is characteristic of 

 these substances that they can react in the most diverse ways. 

 Although they have tremendous chemical potentialities, these 

 are only realised extremely slowly under ordinary conditions 

 and in isolation. This very slow rate of reaction depends 

 essentially on the gi'eat amount of energy of activation, i.e. 

 the high energy -barrier "^\^hich molecules of organic substances 

 must surmount before they can participate in any chemical 

 reaction. However, depending on all the combinations of 

 circumstances under which any given reaction takes place, 

 its velocity may vary within very wide limits. 



If the conditions are such that only one of the reactions 

 possible for any particular organic substance occurs very fast 

 while all the rest of the possible reactions proceed compara- 

 tively slowly then, naturally, the practical significance of the 

 latter will be quite negligible in the over-all result. In other 

 words, there lie before each organic substance in protoplasm 

 many routes of chemical transformation which are thermo- 

 dynamically open to it. In fact, however, each compound 

 \vhich enters the protoplasm from the environment, and any 



