DANIEL I. ARNON 545 



likely, therefore, that as was concluded earlier by Wessels (IfiO) 

 and Nakanioto et al. (Ill) oxygen evolution is a component step 

 in the "aerobic" photophosphorylation catalyzed by FMN or vitamin 

 K, and that molecular oxygen, when present, acts as an electron ac- 

 ceptor in photosynthetic phosphorylation. This conclusion is sup- 

 ported by the observed effect of chloride on cyclic photophosphoryla- 

 tion with vitamin K and FMN in air and in nitrogen (Table 12) . 

 (Chloride, it will be recalled, is required for the photochemical pro- 

 duction of oxygen.) The omission of chloride had scarcely an effect 

 on photophosphorylation in nitrogen, but it inhibited severely photo- 

 phosphorylation in air. 



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

 CMU, either in air or in nitrogen, on the FMN and vitamin K path- 

 ways, 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 (172) . Apart per- 

 haps, from the greater dependence of the FMN pathway on TPN 

 (172) (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. 20) 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 diagram in Fig. 21. Here the electron flow mechanism (marked 

 by a heavy line) is composed of two parts. The first part is com- 

 pleted when electrons from excited chlorophyll are accepted by O2 

 and, in combination with protons, form water. In the second part, 

 these electrons are replaced by those donated by OH, with a con- 

 comitant evolution of oxygen, as was described for the non-cyclic 

 electron flow pathway for chloroplasts (Fig. 19) . The proposed 

 mechanism, in which oxygen acts catalytically, provides for an ex- 

 change between molecular oxygen and the oxygen of water and is 



