PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 393 



Once the electrons are accepted by oxygen and form water, the cycHc 

 pathway can be maintained only by a release of electrons in the oxygen- 

 forming reaction of non-cyclic photophosphorylation in chloroplasts 

 (Section 13). By contrast, phenazine methosulphate catalyzes the transfer 

 of electrons to cytochrome so effectively [90] that it is able to prevent their 

 "escape" to oxygen and hence the phenazine methosulphate pathway 

 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 FAIN or vitamin K is more 

 efficient than with phenazine methosulphate (Figs. 9 and 10). Also, the 

 anaerobic FMN and vitamin K cyclic pathways are more efficient than 

 their oxygen-dependent* counterparts (Fig. 28). 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 29). 



15. 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. 27) raises the question of the 

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

 addition of one of the cefaclors of cyclic photophosphorylation have on 

 the reduction of TPN and evolution of oxygen which accompany ATP 

 formation in non-cyclic photophosphorylation ? 



As shown in Figs. 30 and 31, the addition of either FMN or vitamin K 

 altered non-cyclic photophosphorylation profoundly. ATP formation was 

 sharply increased, whereas oxygen evolution and the accumulation of 

 reduced TPN were abolished. It appears, therefore, that cyclic photo- 

 phosphorylation is a more "tightly coupled" mechanism for converting 

 light energy into ATP than non-cyclic photophosphorylation. In the 



* Distinct from the oxygen-dependent cyclic photophosphorylation discussed 

 here is the "oxidative photosynthetic phosphorylation" [165] by chloroplasts in 

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

 indophenol), an inhibitor (o-phenanthroline or CMU) and DPXHo. The corres- 

 pondence of the term "oxidative photosynthetic phosphorylation' to oxidative 

 phosphorylation by mitochondria appears to be fortuitous. The role of DPNH., 

 in this chloroplast system, was not that of a physiological electron donor but that 

 of a non-specific reducing agent for the dye, one of several reducing agents that 

 were effective. That the consumption of oxygen was artificially induced and was 

 only 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" [165]. 



