884 LIGHT AND LIFE 



The fact that the rate ol evokition of oxygen is greatly increased 

 when it is coupled with phosphorylation inclines these workers to 

 view the Hill reaction as an uncoupled photophosphorylation. The 

 mechanism of the reaction above is different from the anaerobic 

 cyclic photophosphorylation occurring in isolated chloroplasts and 

 bacterial chromatophores, for ferricyanide in the presence of chloride 

 behaves here not as an inhibitor but as an electron acceptor that 

 promotes the formation of ATP. The reaction scheme is therefore 

 likened to the non-cyclic photophosphorylation in Chromatium, with 

 water playing the role of electron donor in place of organic sub- 

 strates. Reduced pyridine nucleotide is produced in place of hydro- 

 gen; and the donation of electrons from OH- ions to the chloroplast 

 cytochromes will lead to the evolution of oxygen (Fig. 11) . 





Non- cyclic photophosphorylation (ctiloroplosts) 

 Fig. 11. Scheme for non-cyclic photophosphorylation in chloroplasts. 



Still a third mechanism is postulated to occur in chloroplasts, a 

 cyclic photophosphorylation. It arises from the previously mentioned 

 catalytic effect of oxygen on photophosphorylation. At low light 

 intensity, photophosphorylation is more efficient in an atmosphere of 

 nitrogen than in air, but requires larger amounts of FMN or vitamni 

 K, and high concentrations of chloroplast material. The action of 

 the inhibitors o-phenanthroline and o-chlorophenyl dimethylurea is 

 much less effective on the chloroplast system in nitrogen than in air. 

 Their strong inhibition of oxygen evoluticm by illiniiinated chloro- 

 plasts is therefore taken to signify that the evolution of oxygen is a 

 step in an aerobic type of photophosphorylation, catalyzed like the 

 anaerobic cycle by FMN and vitamin K. It is likely that the par- 



