THE ELECTRON TRANSPORT SYSTEM OF PHOTOSYNTHESIS 

 DEDUCED FROM EXPERIMENTS WITH 

 MUTANTS OF CHLAMYDOMONAS REINHARDI 



R. P. Levine 



INTRODUCTION 



Six years have elapsed since Emerson and his coworkers^ ' described two 

 different types of light effects in green plant photosynthesis, and it has been only 

 three years since Hill and Bendall suggested that the electron transport system 

 of photosynthesis could be interpreted in terms of two distinct light-dependent 

 reactions coupled by at least one light-independent reaction^^). Subsequently, 

 there has been almost a surfeit of publications on the two light effects in photo- 

 synthesis; merely a brief reference to the papers presented in this and other 

 recent symposia is sufficient to emphasize the impact that Emerson's original 

 contribution has had on contemporary research into the mechanism of photo- 

 synthesis^"^"^). 



It is indeed noteworthy that at present there is a fair extent of uniformity 

 among the several schemes proposed for the electron transport system of photo- 

 synthesis. However, the general nature of the schemes, and the lack of suffi- 

 cient data for their support, makes it virtually impossible to accept or deny any 

 one of them. The uniformity of most of the popular schemes is a mixed blessing, 

 for it suggests on the one hand that diverse experimental approaches are leading 

 to a set of final and general conclusions. On the other hand, the specific and 

 significant details by which these schemes differ suggest that there is much to 

 learn before we have a complete understanding of the mechanism of photo- 

 synthesis. 



The purpose of this contribution to the symposium is to present data con- 

 cerning reactions in the electron transport system of photosynthesis of the 

 unicellular green alga, Chlamydomonas reinhardi , as studied with the wild type 

 strain and mutant strains that are unable to carry out normal photosynthesis. 

 Furthermore, these data will be used as the basis of a model for a sequence of 

 reactions in the electron transport system. 



Three years ago we became interested in the genetic control of photo- 

 synthesis. The investigation, however, soon turned from one having an orienta- 

 tion primarily of a genetic sort to one more directly concerned with the mecha- 

 nism of photosynthesis per se . The work was begun with the aid of a mutant 

 strain of C . reinhardi that was unable to fix carbon dioxide by photosynthesis^^). 

 It was assumed that this, and other mutant strains isolated subsequently^ '' ^>, 



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