INITIAL. SYSTEMS ?^37 



the preceding one. Thus biological chains are formed of 

 different links succeeding one another in a definite sequence 

 of different reactions and do not consist of a continual repeti- 

 tion of one and the same chemical act, as do the chain 

 reactions of free radicals. This may be illustrated by the 

 example of the biosynthesis of porphyrin which we have 

 already adduced, or by alcoholic fermentation, in the course 

 of which a molecule of sugar successively enters into reactions 

 of phosphorylation, enolisation, the breakdown of the carbon 

 chain, oxido-reduction, decarboxylation, etc., giving rise to 

 new products each time, right up to the final products, 

 carbon dioxide and alcohol, which are discharged from the 

 cell into the external medium. 



Biological chains of chemical transformations may branch, 

 but this phenomenon is fundamentally different from the 

 branching of chain reactions (of the radical or ionic type) 

 based on an increase in the number of radicals formed and 

 hence an increase in the number of identical cycles of 

 reactions. The branching of biological chains, on the other 

 hand, consists in the occurrence of reactions going in different 

 directions. For example, in a chain of transformations of 

 organic acids, fumaric acid may give rise to succinic acid but 

 it may also be converted to aspartic acid" : Pyruvic acid -^ 



^ aspartic acid 

 oxaloacetic acid -^ malic acid -> fumaric acid 



"^ succinic acid. 

 After a long series of reactions biological chains may join up 

 to form cycles, (e.g. the tricarboxylic acid cycle of Krebs, 

 which we discuss in more detail below) but these cycles have 

 nothing in common with the elementary cycles of chain 

 reactions. They are always associated with irreversible 

 branchings and therefore biological metabolism as a whole 

 always proceeds in the same direction^^ and is a flowing 

 system such as those described during our discussion of open 

 systems. 



This difference in principle between radical chain re- 

 actions and biological chains must be kept in mind because 

 there have recently appeared in the scientific literature 

 attempts to explain the origin of the organisation of proto- 



22 



