FIRST HETEROTROPHS AND ANAEROBES 4OI 



compounds from very small original molecules ; the carbon 

 skeletons of the various amino acids, including the aromatic 

 and heterocyclic ones, are built up by contemporary organ- 

 isms from acetic acid and other simple breakdown products of 

 monosaccharides. ^ 



Ammonia, oxaloacetic acid, glycine and formyl residues 

 serve as the material for the synthesis of purine and pyrimi- 

 dine bases, ^ while glycine and succinic acids serve for por- 

 phyrins,^ etc. However, as we have mentioned above, such 

 syntheses require the presence of a very highly developed 

 organisation of protoplasm. If the very simple starting ma- 

 terials are to be transformed into complicated organic com- 

 pounds the biosynthesis must occur by means of a long series 

 of intermediate stages. These must be very well co-ordinated 

 in time so that the intermediate product which is formed 

 as a result of one reaction will be completely transformed 

 by the next reaction into a new and more complicated 

 compound. The greater the number of links in a metabolic 

 chain the more its realisation will depend on specific enzymes 

 or even on whole complexes of enzymes, and the more accurate 

 must be the co-ordination of the velocities of the separate 

 reactions, both those whereby the small molecules are con- 

 verted into larger formations and those supplying the energy 

 required for these syntheses. 



In 1945 N. H. Horowitz,* on the basis of studies of the 

 fungus Neurospora, gave a very interesting schematic accoimt 

 of the way in which the synthetic abilities of the primary 

 living things became more complicated during their evolu- 

 tion, though this scheme still requires some biochemical 

 particularisation. The gist of Horo^vitz' scheme is as follo^vs : 

 Let us assume that some very simple organism required the 

 rather complicated compound A for its vital processes. If 

 this compound were present, ready-made, in the surroimding 

 medium the organism could assimilate it directly without 

 possessing any chemical ability to synthesise the substance. 

 However, if there should arise a deficiency of the material 

 in the outside medium, or if it should vanish altogether, the 

 only organisms which could continue to exist would be 

 those in which there had somehow arisen a new chemical 

 mechanism enabling them to synthesise substance A from 



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