192 METABOLISM AND PHYSIOLOGY 



DPIP. In our experiments ascorbate was present in such great excess 

 that it served not only as an inhibitor of cyclic photophosphorylation 

 but also as an electron donor for noncyclic photophosphorylation. 



As discussed previously (18), photosynthesis in plants and bacteria 

 is now seen as having in common two photochemical processes, cyclic 

 and noncyclic photophosphorylation. However, the bacterial noncyclic 

 photophosphorylation never produces oxygen— a consequence of the 

 inability of the bacterial system to use water as the electron donor. 



It seems reasonable to consider that photosynthesis in plants and 

 bacteria depends to a different degree on cyclic and noncyclic photo- 

 phosphorylation. In conventional plant photosynthesis, noncyclic photo- 

 phosphorylation would appear to be the dominant photochemical process 

 since, apart from its contribution of ATP, it is the exclusive mecha- 

 nism for bringing about a hydrogen transfer (via TPN) from water to 

 CO2. The role of cyclic photophosphorylation in plants would thus 

 appear to be that of supplementing the ATP needs for carbon assimila- 

 tion which are not fully met by noncyclic photophosphorylation. 



In bacterial photosynthesis, on the other hand, cyclic photophos- 

 phorylation seems to be the dominant photochemical process, because 

 it provides a most effective anaerobic mechanism for generating ATP 

 for biosynthetic purposes, Photosynthetic bacteria, unlike plants, have 

 no exclusive dependence on a photochemical reaction for the generation 

 of reduced pyridine nucleotide. With some bacterial electron or hydro- 

 gen donors, as for example, with hydrogen gas or malate, the reduc- 

 tion of pyridine nucleotide requires no input of light energy; it can 

 proceed with the aid of appropriate enzyme systems in the dark (5,11), 

 However, with certain other electron donors, such as thiosulfate or 

 succinate, an input of light energy becomes necessary for the reduc- 

 tion of pyridine nucleotide, and in such cases a noncyclic electron 

 flow with an accompanying ATP formation would become a component 

 of bacterial photosynthesis. 



SUMMARY 



Features of cyclic and noncyclic photophosphorylation in chromato- 

 phore preparations of R. ruhnim were investigated with special refer- 

 ence to the experimental conditions needed to demonstrate a distinc- 

 tion between these two electron pathways in bacterial photophosphory- 

 lations. 



Noncyclic photophosphorylation was demonstrated in a system in 

 which the ascorbate-dichlorophenolindophenol couple served as the 

 electron donor and DPN as the electron acceptor. Cyclic photophos- 

 phorylation under these conditions was suppressed by the presence of 

 antimycin A and an excess of ascorbate. 



