878 LIGHT AND LIFE 



when illuminated (Williams) . Although at first the rates in the 

 cell-free reactions were low, rates equal to those of maximinn car- 

 bon assimilation in intact leaves were later obtained (Jagendorf and 

 Avron) . 



In addition to FMN and vitamin K, magnesium ions and ascorbate 

 serve as cofactors in the process of photophosphorylation. It was 

 thought that FMN and vitamin K might provide alternative pathways 

 in an electron transport chain, since they exhibit a differential de- 

 pendence upon TPN and a differential sensitivity to the inhibitors 

 dinitrophenol and o-phenanthroline. Further insight came with the 

 discovery by a number of workers that the dye phenazine metho- 

 sulfate, a strong reducing agent for cytochromes, will strongly stim- 

 ulate photosynthetic phosphorylation. The phenazine methosulfate 

 system more closely resemble the vitamin K pathway, although dif- 

 fering from it in certain respects. In bacterial photophosphoryla- 

 tion, FMN proved entirely ineffective, but vitamin K and ascorbate 

 were active cofactors, and phenazine methosulfate also. 



These observations have led Arnon to a hypothesis about the 

 electron flow in photo synthetic phosphorylation. The simplest view 

 would be to assume that formation of the high-energy pyrophosphate 

 bond of ATP is coupled with a release of free energy during electron 

 transport, as in oxidative phosphorylation. Since the photosynthetic 

 system does not utilize external substrate as an electron donor, the 

 source of electrons must be sought elsewhere; and there must be a 

 suitable electron acceptor available, too. Arnon proposes that the 

 excitation of chlorophyll by light leads to an expulsion of an elec- 

 tron that has been raised to some energy level above the ground 

 state. The resulting oxidized state of the chlorophyll [Chl]+, leaves 

 it in position also to be the electron acceptor, the expelled electron 

 returning stepwise via an electron carrier system and releasing free 

 energy as it does so. The intermediate electron carriers terminate 

 with a cytochrome which is coupled with the enzyme of the photo- 

 jjhosphorylative process. Earlier electron carriers might be FMN 

 or vitamin K, the place of which could be taken non-physiologically 

 by phenazine methosulfate. Arnon proposes the name of "cyclic 

 photojjhosphorylation" to describe this system, which is illustrated 

 in Figs. 8 and 9. 



The first phase of this cycle, namely, the production by light of 

 both a high-energy electron and also of the ultimate electron acceptor, 

 [Chl]+, is postulated on the basis of a finding by Lewis and Lip- 

 kin that in rigid media, and for a variety of substances, one of the 



