464 FURTHER EVOLUTION 



possible. This was made possible by the complete oxidation 

 of the nutrients by means of atmospheric oxygen. 



The origin of respiration. 



The accomplishment of this task required the organisation 

 of two new systems. First, a system for the mobilisation of 

 the hydrogen which, under anaerobic conditions, had gone 

 to waste, being given off from the organism in the form of 

 reduced organic compounds which could not be used any 

 more (e.g. acids and alcohols), or even in the form of gaseous 

 products such as hydrogen. Secondly, a system for the activa- 

 tion of oxygen so that it might be possible to oxidise hydro- 

 gen to water, to carry out the reaction which occurs when a 

 mixture of hydrogen and oxygen gases is exploded. 



The individual mechanisms of the first system are very 

 ancient. They were mostly present in the anaerobic organ- 

 isms. They are the pyridine nucleotides, coenzyme A, etc., 

 with which we are already quite familiar. On the develop- 

 ment of aerobiosis their activity was merely extended to a 

 number of new products which were absent from the general 

 scheme of glycolysis. In its essence this scheme of the initial 

 transformation of carbohydrates was retained unchanged in 

 aerobes but, at particular places in this scheme, there were 

 embodied new chains and cycles of reactions the individual 

 components of which give up their hydrogen to a pyridine 

 nucleotide or some other analogous acceptor (e.g. a flavine 

 derivative). 



Such places where new cycles have been embodied have 

 been located exactly in rather primitive facultative anaerobes. 

 In our earlier discussion of Strep, faecalis and other analogous 

 microbes, we saw how aerobiosis first arose. Here we were 

 concerned with the pyruvic acid which was formed in the 

 process of glycolysis and which, in the absence of oxygen, 

 underwent anaerobic dehydrogenation and decarboxylation 

 to form acetic acid, alcohol and lactic acid. In the presence 

 of oxygen, however, the pyruvic acid was decarboxylated 

 oxidatively so that the formation of acetic acid was not 

 necessarily accompanied by the appearance of reduced pro- 

 ducts (alcohol and lactic acid). In propionic acid bacteria, 



