392 DANIEL I. ARNON 



diagram in Fig. 29. Here the electron flow mechanism (marked by a heavy 

 Hne) is composed of two parts. The first part is completed when electrons 

 expelled from 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 concomitant evolution of oxygen, as was 

 described for the non-cyclic electron flow pathway for chloroplasts (Fig. 

 27). The proposed mechanism, in which oxygen participates, provides 

 for an exchange between molecular oxygen and the oxygen of water and is 

 in agreement with the ^^O exchange data recently reported by Nakamoto 

 and Vennesland [163] and Jagendorf [164]. 



le 



— ^ 



COfox— COfred. 



I 



O2 ^,"2 



— H2O 



I 



Chi 



+:'^ 



- Cytox.^Cyt.ed. 



LIGHT ~P-^DP [^ 



02-dependent cyclic photophosphorylation 



Fig. 29. Scheme for oxygen-dependent cyclic photophosphorylation in chloro- 

 plasts. Details in text. 



In summary then, FMN and vitamin K seem to catalyze two pathways 

 of cyclic photophosphorylation, one anaerobic and one catalyzed by 

 molecular oxygen (cf. [62]). The anaerobic pathway, when investigated in 

 an atmosphere of nitrogen, requires appreciable, although still catalytic, 

 concentrations of cofactors and, particularly in the case of FMN, high 

 concentrations of chloroplast material that evidently supply the additional 

 factor(s) needed for the efficient conversion of light energy into ATP under 

 anaerobic conditions. The ox3^gen-dependent pathway for FMN or vitamin 

 K is catalyzed by very low, "microcatalytic", concentrations of these 

 cofactors and is much less dependent on additional chloroplast material 

 than the anaerobic pathway. 



These findings are interpreted to mean that oxygen, when present in a 

 system catalyzed by either FMN or vitamin K, is able to compete effec- 

 tively with cytochromes for the electrons of cyclic photophosphorylation. 



