336 ORGANISATION IN SPACE AND TIME 



Thus, it is a peculiarity of chain reactions that a large 

 number of short cycles of reactions can be carried out by 

 means of alternating active foci, free atoms or elements, when 

 the sequence of cycles is initiated by a reaction giving rise 

 to any of the active particles. At the end of each elementary 

 cycle there are just the same number of free radicals as there 

 were at the beginning, which constitutes the essential condi- 

 tions for the perpetuation of the chain. If a larger number 

 of radicals is formed at the end of the cycle than were present 

 at the beginning, there will be a branching of the chains, 

 the number of elementary cycles will increase with a co- 

 efficient of multiplication of Km and the rate of progress will 

 quickly increase. Conversely, if the number of radicals is 

 less at the end of the cycle than at the beginning, the chains 

 will be broken and the reaction will get slower or stop. 



Unlike the chain reactions based on ions or radicals, the 

 biologically important elementary cycles based on catalysis 

 arise in another way. According to the most generally accepted 

 theory of contact catalysis the reaction occurs directly between 

 adsorbed molecules and either leads straight to the formation 

 of the final products or first to the formation of an inter- 

 mediate compound. This then breaks down to form the final 

 product of the reaction, leaving the original molecule of the 

 catalyst (e.g. the enzyme) free. 



It is true that N. Semenov*" has recently suggested that 

 heterogeneous catalytic reactions are also based on an inter- 

 mediate ionic or radical mechanism, but, however this may 

 be, the elementary cycles of catalysis end with the formation 

 of thermodynamically stable molecules, and not with that 

 of free radicals like chain reactions. We must here lay special 

 stress on the fact that the chain reactions which form the 

 basis for biological metabolism are different in principle 

 from the chain reactions described above, which undoubtedly 

 played an important part in the early stages of the evolution 

 of organic substances. The separate links in biological chains 

 are not free radicals but stable molecules, the transformation 

 of which takes place, in the great majority of cases, without 

 the regeneration of one or more of the original components, 

 while the products arising as a result of one reaction enter 

 into a new chemical transformation, which is different from 



