388 DANIEL I. ARNON 



The proposed reactions from OH " to oxygen evolution appear to be 

 thermodynamically feasible. The energy contribution of one einstein of red 

 light, about 43 Kcal., is equivalent to a potential of i -9 V. per faraday, 

 and is sufficiently large, after making allowances for TPN reduction and 

 ATP formation, to endow a chlorophyll-linked cytochrome with a redox 

 potential more oxidizing than 0-815 V., as is needed for oxygen evolution. 



,'--, Reductase ^ ^. , 

 ; e-.i » PN 



Chi r + 'j^mmfmm Cyt 



^■T^ 





HA 



LIGHT 02'^0H-< — H2O 



Non- cyclic photophosphorylation (chloroplasts) 

 Fig. 27. Scheme for non-cyclic photophosphorylation in chloroplasts. 

 Details in the text. Chloride is required for oxygen evolution. 



It must be emphasized that, in our present state of knowledge, the 

 proposed mechanism for oxygen evolution must remain tentative. The 

 possibility exists that the transfer of electrons from OH ^ to cytochromes 

 requires an auxiliary input of light energy via a photosynthetic pigment.* 



14. Oxygen-dependent cyclic photophosphorylation 



The mechanisms of photosynthetic phosphorylation in chloroplasts 

 discussed thus far include anaerobic cyclic photophosphorylation (Figs. 4 

 and 5) and non-cyclic photophosphorylation (Fig. 27). Recent work by 

 Tsujimoto et al. [92] suggests the operation in chloroplasts of a third 

 mechanism, an oxygen-dependent cyclic photophosphorylation. 



As was already discussed in Section 4, a catalytic role for oxygen was 

 envisaged in explaining the first experiments on photosynthetic phos- 

 phorylation, in which the presence of oxygen was required but no oxygen 

 consumption was observed [13]. Interest in the role of oxygen was 

 heightened when several laboratories reported that at low, "micro- 

 catalytic", concentrations of FMN or vitamin K (Fig. 3), photophos- 

 phorylation remained dependent on oxygen [54-56]. 



* Note added in proof. Experimental evidence for a separate light reaction 

 responsible for oxygen evolution has now been obtained, (cf. M. Losada, F. R. 

 Whatley and D. I. Arnon, Nature, Land. 190, 606-610, 1961.) 



