DANIEL I. ARNON 



537 



re-.- 



Chl [ + ;.^ 



LIGHT 



Hydrogenost .^^ / i, \ 



~T. — ^yy 



t 



HA 



•Cyf 



ADP 



-> A 



->U\TP) 



Non-cvclic photophosphorylation (bacteria) 



I'ig. 17. .Sclieinc lor noncxclic photopli()spho))lation in Chioinatiuin. 



Details in the text. 



(95) , account for the i^hotoproduction of hydrogen from thiosulfate 

 in accordance with the overall reaction: 



S203= + 5H2O 



light 



^ 4H2 + 2S0r + 2H 



+ 



To recapitulate, then, the scheme shown in Fig. 17 is supported by 

 experimental evidence for the photoproduction of hydrogen from 

 succinate (Fig. 15 and Table 8) or thiosulfate (95) and for the re- 

 duction of Chromatiiim cytochromes by succinate (Fig. 16) or thiosul- 

 fate (95) . By analogy with non-cyclic photophosphorylation in 

 chloroplasts (9) , the non-cylic electron flow mechanism in bacteria 

 should also result in a phosphorylation, as is indicated by preliminary 

 experiments (94) . We further conclude, that hydrogen gas is evolved 

 in light when the "reducing" electrons are not consumed in metabolic 

 reactions, as, for example, in the reduction of COo via pyridine 

 nucleotide (114). 



15. Photoproduction of Reductant in Plants: Non-cyclic 

 Photophosphorylation 



Photosynthetic bacteria can reduce pyridine nucleotide in photo- 

 synthesis, either with molecular hydrogen in the dark or with a less 

 reduced electron donor, organic or inorganic, in the light. Green 

 plants that do not contain hydrogenase depend unconditionally on 

 light for reducing pyridine nucleotide in photosynthesis, the more 

 so since they use water as the electron donor. The reduction of pyri- 



