132 



PROBLEMS OF PHOTOSYNTHESIS 



quired for the vitamin K pathway which is the only type of bacterial photo- 

 synthesis. Thus, in the absence of CI ions chloroplasts behave like chromato- 

 phores in being able only to follow the vitamin K pathway. 



Fig. 49. Similarities and difierences in photosynthesis of green plants and bacteria (Arnon) 



Arnon (5, 13) considers that the Hill reactions are non-physiological vari- 

 ants of the over-all reaction, e.g.: 



4Fe3+ + 2H2O + 2ADP + 2ph > 4Fe2+ + O2 + 2ATP + 4H + 



TPN+ does not participate. It is replaced by the Hill reagents, e.g. ferric 

 ions. This non-physiological non-cyclic photophosphorylation also requires 

 CI ions. According to Arnon's recent investigations, the reduction of TPN"*" 

 does not result from water photolysis. In his earlier work it was the mysteri- 

 ous product [H] which elicited the reduction of TPN+. Today he claims 

 that H ions from water, together with electrons expelled from excited chloro- 

 phyll molecules are sufficient for this reduction in the presence of a light- 

 dependent reductase. Thus, water is split into H ions and OH ions, the latter 

 being converted into Oo. In photosynthetic bacteria the hydrogen donor 

 which is still needed does not reduce COo, as had been claimed by van Niel, 

 but gives off hydrogen to reduce phosphopyridine nucleotides. In terms of 

 comparative biochemistry, the common denominator of photosynthesis in 

 green plants and in bacteria is, according to Arnon (5), the conversion of light 

 energy absorbed by chlorophyll into phosphate bond energy. It is not the 

 photochemical formation of a COo reductant. 



Thomas et al. (54) succeeded in preparing chloroplast fragments of Spiro- 

 gyra which show evolution of Oo as well as uptake of CO2 without the addi- 



