DANIEL I. ARNON 547 



sulfate patlnvay remains an "anaerobic" one even when molecular 

 oxygen is present. 



As far as efficiency of conversion of light energy into ATP is con- 

 cerned, it appears from experiments at limiting light intensities that 

 the anaerobic cyclic photophosphorylation with FMN or vitamin K is 

 more efficient than with phenazinc methosulfate (Figs. 9 and 10) . 

 Also, the anaerobic FMN and vitamin K cyclic pathways are more 

 efficient than their oxygen-catalyzed*' counterparts (Fig. 20) . These 

 findings suggest the participation of more than one phosphorylation 

 site in the anaerobic FMN and vitamin K pathways (compare Fig. 

 5 with Figs. 4 and 21) . 



17. Relation of Cyclic to Non-cyclic Photophosphorylation 



IN Chloroplasts 



The ability of isolated chloroplasts to carry out both cyclic (Fig. 

 5) and non-cyclic photophosphorylation (Fig. 19) raises the question 

 of the mutual relation of these two processes. Specifically, what effect 

 would the addition of one of the cof actors of cyclic photophosphory- 

 lation have on the reduction of TPN and evolution of oxygen which 

 accompany ATP formation in non-cyclic photophosphorylation? 



As shown in Figs 22 and 23, the addition of either FMN or vitamin 

 K altered non-cyclic photophosphorylation profoundly. ATP for- 

 mation \\2LS sharply increased, whereas oxygen evolution and the ac- 

 cumulation of reduced TPN were abolished. It appears, therefore, 

 that cyclic photophosphorylation is a more "tightly coupled" mechan- 

 ism for converting light energy into ATP than non-cyclic photophos- 

 phorylation. In the presence of the requisite cofactors, cyclic photo- 

 phosphorylation is capable of diverting all of the absorbed light energy 

 for the formation of ATP, and suppressing TPN reduction and O2 



* Distinct from the oxygen-catalyzed cyclic photophosphorylation discussed here 

 is the "oxidative photosvnthetic phosphorylation" (86) by chloroplasts in which 

 oxygen consumption was induced by a joint use of a dye (trichlorophenol indo- 

 phenol), an inhibitor (o-phenanthroline or CMU), and DPNHo. The correspondence 

 of the term "oxidative photosynthetic phosphorylation" to oxidative phosphoryla- 

 tion by mitochondria appears to be fortuitous. The role of DPNHo in this chloro- 

 plast system was not that of a physiological electron donor but that of a non- 

 specific reducing agent for the dye, as one of several reducing agents that were 

 effective. That the consumption of oxygen was artiricially induced and was onlv 

 a feature of the special system used, is made clear by the authors' observations 

 that "there was ample energy released by the dark oxidation of the DPNH to 

 form the high energy phosphate bonds. Nevertheless, the reaction gave no phos- 

 phorylation unless the system was illuminated, even though the light caused no 

 increase in the rate of oxygen consumption" (86). 



