PHOTOCHEMICAL REACTION CENTERS IN 

 PHOTOSYNTHETIC TISSUES^ 



RODERICK K. CLAYTON 



C. F. Kettering Research Laboratory, 



Yellow Springs, Ohio 



Current theories for the mechanism of photosynthesis are grounded 

 in two major developments that began about thirty years ago. One of 

 these was the discovery of the photosynthetic unit (1,2); the other was 

 the emergence of the concept that photosynthesis begins with a photo- 

 chemical separation of oxidizing and reducing power (3), 



In modern terms the photosynthetic unit is a set of Chl2 molecules 

 that cooperate in harvesting the energy of light quanta and in channel- 

 ing this energy to a photochemical reaction center. Its existence in 

 photosynthetic tissues was established through Emerson and Arnold's 

 studies of the photosynthetic yield of short flashes of light in Chlorella 

 (1), and independently through Gaffron and Wohl's analysis of initial 

 photosynthetic rates at the onset of illumination (2). The reaction cen- 

 ter is regarded as a site where quantaof excitation energy bring about 

 electron transfer events that lead to the storage of stable chemical 

 potential. The stable chemical entities thus formed can serve as start- 

 ing points for the many chemical reactions of photosynthesis. 



In green plants and algae the characterization of photosynthetic 

 units and reaction centers meets with a complication. Study of the 

 Emerson red-drop and enhancement phenomena (4) has revealed the 

 presence of two distinct photochemical systems (5-10). System I (the 

 "far red" system) generates strong reductants such as NADPH2; it 

 receives electrons from the "short wave" System II and raises these 

 electrons to a higher energy. System II mediates the evolution of 

 oxygen and feeds electrons to System I. The partial reactions occurring 

 in System II remain obscure, whereas the makeup of a reaction center 

 in System I is beginning to emerge. This article will be restricted to a 

 consideration of System I and its apparent counterpart in the photo- 

 synthetic bacteria. 



It has not been settled whether the primary photochemical reactions 

 (in green plants, algae, and photosynthetic bacteria) are of a "one 



1 Conti'ibution No. 108 from the Charles F. Kettering Research Laboratory. 



2 In addition to the standard abbreviations, the following is used in this article: 

 BPh, bacteriopheophytin. 



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