PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 39 1 



oxygen evolution is a component step in the "aerobic" photophosphoryla- 

 tion catalyzed by FAIN or vitamin K, and that molecular oxygen, when 

 present, acts as an electron acceptor in photosynthetic phosphorylation. 

 This conclusion is supported by the observed effect of chloride on cyclic 

 photophosphorylation with vitamin K and FMN in air and in nitrogen 

 (Table XVI). The omission of chloride had scarcely an effect on photo- 

 phosphorylation in nitrogen, but it se^■erely inhibited photophosphoryla- 

 tion in air, which depends on the photochemical evolution of oxygen. 



TABLE XVI 

 Effect of Chloride on Cyclic Photophosphorylation in Nitrogen or Air 



(Tsujimoto, Hall, and Arnon [92]) 



/Ltmoles P esterified 

 Treatment 



— chloride + chloride 



Nitrogen, FMN 5-1 5-7 



Nitrogen, vit. K3 9-7 9-9 



Air, FMN o • 5 6 ■ i 



Air, vit. K3 04 . 5-5 



Experimental conditions as in Table XV, except that chloroplasts were 

 prepared in 0-5 M sucrose and chloride-free reagents were used. 0-2 mg. chloro- 

 phyll was used in the air series, and 2 • 5 mg. chlorophyll per vessel was used in 

 the nitrogen series. The reaction was run for 30 min. 



The similarity in the effects of chloride, o-phenanthroline, and CMU, 

 either in air or in nitrogen, on the FMN and vitamin K pathways, under 

 the modified experimental conditions which we now use, has blurred the 

 distinction between the two pathways that was made on the basis of earlier 

 inhibitor experiments [89]. Apart perhaps, from the greater dependence 

 of the FMN pathway on TPN [89] (a dependence that has not yet been 

 reinvestigated under the new experimental conditions), what seems now 

 to distinguish the two anaerobic pathways is the greater requirement, in 

 the case of FMN, for a higher concentration of chloroplast material. 



The participation of oxygen in cyclic photophosphorylation may 

 increase the overall rate of ATP formation but only when light is abundant 

 and phosphorylation is limited by a low concentration of cofactors. 

 However, present evidence indicates (Fig. 28) that, in contrast to oxidative 

 phosphorylation, the intervention of molecular oxygen in photosynthetic 

 phosphorylation is an energy-wasteful step that lowers the efficiency of the 

 anaerobic cyclic photophosphorylation process when light is limiting. 



On the basis of evidence now available, the participation of oxygen as 

 a catalyst in cyclic photophosphorylation may be represented by the 



