136 PROBLEMS OF PHOTOSYNTHESIS 



nected with phosphorylation. He considers back reactions unlikely since 

 chloroplasts would not be able to carry out oxidative phosphorylation when 

 supplied with O2 in the dark with appropriate substrates and TPNH. It 

 has already been pointed out that there is no photosynthesis without respira- 

 tion. As there is no respiration without phosphorylation, we logically arrive 

 at the conclusion that there is no photosynthesis without phosphorylation, 

 i.e., oxidative phosphorylation. 



As a matter of fact, the phosphate turnover in photosynthesis is still a sub- 

 ject of many contradictory investigations. As yet, anaerobic phosphoryla- 

 tion is the only kind of phosphoryladon the chemistry of which is known (see 

 the reaction with the oxidizing fermentation enzyme discovered by Warburg 

 and Christian discussed in § 44) . This is still not the case with oxidative phos- 

 phorylation and it is far less the case with phosphorylations proceeding upon 

 illumination. The hypothesis of ATP formation by means of light energy 

 may be an interesting oudet, but, for the time being, it belongs to the same 

 category as water photolysis which was at the time an equally interesting out- 

 let for which no evidence was available. 



On the other hand there is evidence that reduction of TPN+ takes place 

 under the influence of light. However, we may readily admit here the same 

 mechanism as found in the Hill reaction with ferric ions. 



Dark- 2 TPN+ + C + 2 H.O -^ 2 TPNH + 2 H+ + CO2 



Dark: CO2 - CO.3* 



Ught: CO.,* - C + O2 



Over-all- 2 TPN+ + 2 H2O -^ 2 TPNH -f 2 H+ + O2 



Traces of reactive C may be sufficient to drive the reaction. In the over-all 

 reaction only TPN+ determines the amount of O2 produced. Until now 

 this mechanism has not been proved. 



Parts of Arnon's work are of importance, though the reduction of TPN + 

 with simultaneous formation of ATP is rather unconventional. It does not 

 seem impossible to bring Arnon's findings into accord with Warburg's theory. 

 The lioht reacdon COo* -^ C + O. could be accompanied by phosphoryl- 

 ation (See § 69). 



REFERENCES 



1. Allen, M. B., Whatley, F. R. and Arnon, D. I. : Bioclnm. Bwphys. Acta, 27:18, 



1958. 



2. Anderson, I. C. and Fuller, R. C: Arch. Biochem. Biophys., 7(5:168, 1958. 



3. Arnon, D. I.: Nature, 767:1008, 1951. 



4. Arnon, D. I.: Science, 722:9, 1955. 



5. Arnon, D. I.: Nature, 184-AO, 1959. 



6. Arnon, D. I., Allen, M. B. and Whatley, F. R.: Nature, 174:394, 1954. 



