A KINETIC ANALYSIS OF THE LIGHT RESPONSES OF 

 PHOTOSYNTHETIC BACTERIA AND PLANTS 



BRITTON CHANCE 



Johnson Research Foundation, University of Pennsylvania, 

 Philadelphia 4, Pennsylvania 



For some time, physical methods have been employed to record 

 light-induced changes in photosynthetic bacteria (1,2), yet an inter- 

 pretation of the reaction kinetics has only been made in the case of the 

 "light-on" reaction in Chromatium. In this case, the cytochrome rate 

 has been compared with the quantum rate to calculate the efficiency 

 of the intracellular electron transfer reaction (3). 



The basic assumption of such studies is that the initial rate of cyto- 

 chrome oxidation is unimpeded by reducing reactions— an assumption 

 that appears to be justified by the high quantum efficiencies that are 

 computed on this basis. 



This paper treats reaction kinetics of light-induced cytochrome 

 oxidation in models for cyclic and noncyclic electron flow. 



Cyclic electron flow. 



As an example of cyclic electron flow, we shall use one of the 

 current hypotheses of electron flow in Rhodospirillum ruhnim and in 

 their chromatophores (4) (Fig. 1). 



Fig. 1. Possible pathway of cyclic electron flow(ME-116). 



Chlorophyll activated by light (hv) causes electron flow through 

 the chain of electron carriers, cyt C2 the primary electron donor, X 

 the primary acceptor, fp, rhp, and cyt b being intermediate carriers, 



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