BIOCHEMICALLY IMPORTANT COMPOUNDS 197 



polymerisation, ring formation and the migration of radicals. 

 In living things, however, these reactions are strictly co- 

 ordinated in respect of their velocities so that they form a 

 long chain of processes in which one reaction follows the 

 other in a strictly determined sequence. As a result of this, 

 it is a general rule that not all the transformations which 

 are thermodynamically possible in the organism actually 

 occur there. Only strictly determined synthetic pathways are 

 followed and therefore highly specialised compounds are 

 formed. The reactions are also so completely harmonious 

 that they can be combined in such a way that the energy 

 liberated by one reaction can be used for another which 

 could not take place spontaneously without it. 



Such co-ordination can, however, only occur in very highly 

 developed and well-organised systems (such as Organisms) 

 and not simply in a solution of various carbon compounds. 

 Any co-ordination which may occur in these, if indeed any 

 does, is a purely temporary and fortuitous phenomenon and, 

 as a rule, the only reactions which take place are those in 

 which the compounds participating are themselves rich in 

 free energy or receive supplementary energy from quanta 

 of light, electric discharges, increased pressure, etc. Con- 

 sequently, in the chaos of different and often mutually 

 independent transformations, there is a predominance (some- 

 times temporary and short-lived) of those reactions which, 

 under the given physico-chemical conditions and, above all, 

 in the presence of particular catalysts, occur fastest. 



Unfortunately we cannot bring direct observation to bear 

 on processes of this sort under natural conditions. This is 

 prevented, not only by the oxidised conditions of the present 

 age, but even more by the ubiquitous distribution of living 

 things on the surface of the Earth. In their presence it is 

 very hard to differentiate between the abiogenic processes 

 which were possible in the primitive hydrosphere and the 

 biogenic ones which only occur at the present time. Organ- 

 isms confuse the whole issue in this respect. They discharge 

 into the surrounding inorganic medium large amounts of 

 specific substances which can only be formed in the course of 

 highly organised metabolic processes and which are most 

 unlikely to have been formed under primaeval conditions. 



