446 FURTHER EVOLUTION 



specific supplementary mechanisms in green plants they are 

 transformed into hydrogen peroxide which is broken down 

 by catalase to liberate molecular oxygen, thus enriching 

 the surrounding atmosphere with this gas. Van Niel"" gives 

 the following scheme to clarify this difference between photo- 

 synthesis in bacteria and in green plants (Fig. 37). 



Fig. 37. Scheme of the reactions involved in photo- 

 synthesis by bacteria and green plants (after van Niel). 



In this diagram the symbols e' and e" denote factors 

 preventing the recombination of the hydrogen and hydroxyl 

 radicals formed during photolysis. Factor e' directs the 

 hydrogen to the reduction of CO2 (to CHgO) to an equal extent 

 in both the bacteria and green plants. 



In the bacteria factor e" transfers the hydroxyl radicals 

 to the appropriate hydrogen donor, indicated by the symbol 

 AH2;, which may be H2S, thereby splitting off water from it 

 and leading to the formation of the product of oxidation, a, 

 e.g. sulphur. In green plants the factor e" transfers the 

 hydroxyl radicals to a special mechanism which transforms 

 them to O2 and water via H2O2. 



Certain doubts have been expressed by photochemists as 

 to whether water can be photolysed directly by visible light. 

 Nobody has succeeded in bringing about this photolysis in 

 model experiments although various pigments have been 

 used as photosensitisers. 



However, even if we admit that, in van Niel's scheme, the 

 photolysis of water is a somewhat speculative explanation 

 of the reversible, photochemical transfer of hydrogen or an 

 electron by means of the energy of the light absorbed by the 



