458 FURTHER EVOLUTION 



tion. It would seem that there is formed, as an intermediate 

 product during the process of photo-oxidation of water, 

 either hydrogen peroxide or else an organic peroxide which, 

 on breaking down, gives rise to molecular oxygen. The oxy- 

 gen which is formed in this way is mainly given off into the 

 atmosphere, but part of it is used in the process of photo- 

 synthesis, especially in system (6). 



In green plants the dark fixation of CO2 (system 2) is mainly 

 carried out by the same mechanisms which operate in 

 ordinary heterotrophs, namely coenzyme A and phosphate 

 dehydrogenases. According to M. Calvin the primary accep- 

 tor of CO2 is ribulose disphosphate, which is obtained by the 

 phosphorylation of ribulose monophosphate at the expense 

 of ATP. The formation of ATP occurs in system (6) which 

 will be discussed below, while the initial ribulose mono- 

 phosphate is formed in system (4). 



Ribulose disphosphate is carboxylated by CO2 with the 

 help of the enzyme carboxydismutase and the intermediate 

 product thus obtained, which now contains six carbon atoms, 

 is broken down to two molecules of phosphoglyceric acid. 

 This acid is the primary product of the fixation of CO2 by 

 green plants and is later transformed, in systems (3) and (4), 

 into various sugars. 



In system (3) there takes place the reduction of phospho- 

 glyceric acid to triose phosphates (glyceraldehyde phosphate 

 and dihydroxyacetone phosphate). The carrying out of this 

 reaction requires, in the first place, ' active hydrogen ' which 

 is supplied in the form of reduced di- and triphosphopyridine 

 nucleotides which are elaborated in the special system (5). 

 In the second place it requires ATP which, as we shall see, 

 is obtained from system (6). 



The later transformation of triose phosphates takes place 

 in system (4) and comprises, in part, their condensation to 

 hexose disphosphate by means of aldolase (Fig. 38) and partly 

 the formation of a number of phosphoric esters of various 

 sugars having four, five, six, seven and ten carbon atoms. 

 This leads, in particular, to the formation of ribulose mono- 

 phosphate. According to Calvin this process takes place in 

 the following order: The hexose (Cg), formed from triose- 

 phosphates, is broken down (c, and C4). The sugar with the 



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