888 LIGHT AND LIFE 



out non-cyclic electron flow which might be coupled to phosphoryla- 

 tion, electrons from the photoreductant being used to reduce DPN, 

 while reduced flavin mononucleotide, FMNH2, donates electrons to 

 the cytochrome of the system. Frenkel also protests that light-induced 

 reduction of TPN in green plants and of DPN in photosynthetic bac- 

 teria does not require the simultaneous generation of ATP. In 

 bacteria, no coupling of phosphorylation and reduction of pyridine 

 nucleotide has in fact been shown to date, although in Rhodospir- 

 illum rubrum a coupling of photophosphorylation and oxidation of 

 cytochrome c^ has been demonstrated (Smith and Baltchewsky) . 

 Frenkel agrees with Arnon's view that cyclic photophosphorylation 

 is a sort of "short-circuit" electron transport system to which one 

 or more sites of phosphorylation are coupled; but he still prefers 

 van Niel's interpretation of photosynthesis as being initially a forma- 

 tion of a photoreductant and a photooxidant. Thus, in the meta- 

 bolism of acetate by R. rubrum, where Stanier et al. have suggested 

 that photophosphorylation is important, it is still quite possible to 

 hold that the electron flow is initiated by light and that the forma- 

 tion of high-energy phosphate, though important, is a secondary 

 phenomenon which may or may not accompany the light-induced 

 flow of electrons. The photoreductant (XH) may be sufficient to 

 generate all the electrons required for the reduction of pyridine 

 nucleotide, and the photooxidant (FO) to oxidize the acetate to 

 CO2 and HoO. Alternatively, the electrons from the succinate-fuma- 

 rate pair may be sufficiently upgraded by means of energy from ATP 

 to reduce pyridine nucleotide. 



Andre T. Jagendorf and G. Forti also differ with Arnon's views in 

 some respects. They summarize the possible reactions resulting from 

 the absorption of light in five sequential equations, as follows: 



X + y + H,>0 + hv -^ XHo + YO (1) 



The initial reaction involves the absorption of light. As van Niel 

 proposed, it may involve the participation of water, just as written; 

 but whether it does or not is really irrelevant. Formation of the 

 photoreductant and photooxidant as products will require that, if 



