880 LIGHT AND LIFE 



to support the scheme is still indirect, and rests upon the effects 

 of chloride and ferricyanide, upon light-induced oxidations of cyto- 

 chromes, and upon the effects of vitamin K on the reduction of the 

 cytochromes. 



With respect to chloride, it has been established that it is es- 

 sential for those photosynthetic steps in which oxygen is liberated, 

 but is not necessary for photophosphorylation. The anaerobic photo- 

 phosphorylation cycle of bacteria and isolated chloroplasts proceeds 

 without chloride ions. The experiments with ferricyanide were done 

 to see whether an outside electron acceptor that would trap the 

 electrons flowing in the postulated closed circuit of the cycle would 

 in fact interfere with photophosphorylation. This was demonstrated 

 to be the case. In both isolated chloroplasts and bacterial chroma- 

 tophores, very low concentrations of ferricyanide were sufficient to 

 abolish cyclic photophosphorylation. The effect could be reversed 

 by reducing the ferricyanide with ascorbate; and ferrocyanide had 

 no effect upon the system. 



Both cytochrome / and cytochrome Co have been found to be oxi- 

 dized by light, the latter independently of temperature (Chance and 

 Nishimura) and in cell-free preparations of Chromatiiim (Arnon et 

 al.) . The addition of vitamin K to a bacterial chromatophore prep- 

 aration has been found, as expected on the basis of the hypothesis, 

 greatly to accelerate the reduction of the oxidized cytochromes in 

 the dark (but only when the preparation was aged) . 



At limiting light intensities, vitamin K is much more efficient in 

 i:)hotophosphorylation than is phenazine methosulfate. These ob- 

 servations are taken by Arnon to mean that there are at least two 

 sites of phosphorylation in the vitamin K (and FMN) pathways; that 

 one of them at high light intensity becomes rate-limiting; and that 

 this part of the pathway is bypassed by phenazine methosulfate. Fig. 

 9 also illustrates this conception of the existence of nuiltiple sites 

 of photoj)hosphorylation in the usual anaerobic cyclic system. Other 

 interpretations of these findings ajjpear not yet to have i)een ruled 

 out. 



When chloroplasts were disrupted by sonication and jjurified prep- 

 arations of grana were obtained, it was found that the photoj^hos- 

 phorylating system (especially clearly when catalyzed by phenazine 

 methosulfate) was located in the grana, together with the chloro- 

 plast cytochromes. Since the enzymes concerned with CO2 assimila- 

 tion are readily dissociable during isolation of the grana and the 

 bacterial chromatophores, Arnon has drawn the conclusion that in 



