ROUEHT HILL AND WALTER I). liONSER, JR. 



133 



the system would result in decreased rates of (H) acceptor reduction 

 and ot oxygen production, il pliosphorylation was obligatory, or no 

 change if phos])horyIation was not necessary. However, such addi- 

 tions result in greatly increased rates of both reactions (I), and the 

 above simple hypothesis has to be abandoned. It was indicated above 

 that such a series of reactions was unlikely in chlonjplasts where some 

 of the reactions of (H) transport must be against the thermochemical 

 gradient. 



A second hypothesis would involve three light-driven reactions, and 

 ones that proceed against the thermochemical gradient as first out- 

 lined by Davenport and Hill (4) . This hypothesis is illustrated 

 diagrammatically in the left-hand portion of Fig. 8. 



Recently Hill and F. Bendall (8) have suggested that most of the 

 experimental observations relating to ferricyanide reduction, oxygen 

 evolution, etc., are best explained in a sequence of reactions illus- 

 trated diagrammatically in the right-hand portion of Fig. 8. Involved 

 in this suggestion are two light-driven reactions, against the thermo- 

 chemical gradient. The two light-driven reactions are considered to 



oxygen electrode. 



cytochro me f 



z 



^cytochrome be 



hydrogen electrode 



-OB 



0.6 



-04 



-0.2 



-0 



—02 



--04 



Fig. 8. Hypothetical sequences in the chloroplast reaction. Downward pointing 

 arrows indicate endergonic reactions, against the thermochemical gradient; upward 

 pointing arrows indicate exergonic reactions. On the left, a sequence involving 

 three light-driven reactions; on the right, two light-driven reactions connected by 

 an exergonic reaction between cytochromes / and feg. 



