REACTION CENTERS IN PHOTOSYNTHESIS 



379 



ratio of P700 to Chi a is about 1:400, in harmony with the size of a 

 photosynthetic unit as determined by flashing light experiments. There 

 is one molecule of Cyt/ present for each molecule of P700 (approxi- 

 mately) in chloroplasts (16). The Cyt/ (potential 365 mv) is also oxi- 

 dized reversibly by illumination (19), Partially purified preparations 

 of P700, obtained by extracting chloroplasts with acetone, retain the 

 Cyt/ in a 1:1 ratio to P700. In these preparations the light reaction of 

 P700 persists, but that of Cyt/ is absent (16). The reversible bleaching 

 at 700 m^, in partially purified P700 preparations, is accompanied by 

 a bleaching at 432 mfi. This has led Kok (16) and others (7,20) to 

 suggest that P700 is a chlorophyll- like pigment, perhaps Chi a in a 

 specialized environment. 



The light-induced oxidations of P700 and Cyt occur irreversibly 

 at liquid nitrogen temperature (7). At room temperature a brief flash 

 of light elicits a transitory oxidation of P700, the re- reduction requir- 

 ing about 1 msec, Witt et al. (7) report that under these conditions the 

 appearance of oxidized Cyt parallels the disappearance of oxidized 



Electrons to 

 PPNR, etc. 



Electron flow 

 for cyclic 

 phosptiorylotion 



Electrons from 

 System n 



Chi 



P700 



P700 



PQ PQ PQ 



P700 — ► P700— ►P700 

 Cyt. Cyt. Cyt."*" 



Fig. 1. HyiDothetical mechanism for the operation of the far-red photochemical 

 system (System I) in green plants and algae. 



