384 PHOTOMECHANICAL CONSIDERATIONS 



BOTH SCHEMES, IF CYT OXIDIZED 

 IN DARK STEADY STATE 



Fig. 2. Alternative schemes for the initial photo- 

 chemical reaction and subsequent events in bacterial 

 chromatophores . 



An elucidation of the correct sequence of primary photochemical 

 events should result from kinetic observations. Thus far the kinetic 

 relationships between the reactions of P870-890 and Cyt have been 

 made only with instruments that respond too slowly to measure rapid 

 transient phenomena. What has been observed is that the P870-890 

 reaction, as well as the reactions of Types E and F, are maximal 

 when the Cyt reaction is absent and become truncated when the light- 

 induced oxidation of Cyt can occur (29). This observation is not suffi- 

 cient to establish the reaction sequence. If oxidation of Cyt is a sequel 

 to oxidation of P870-890, a brief flash of light should elicit an imme- 

 diate oxidation of P870 or P890, and the appearance of oxidized Cyt 

 should parallel the return of P870 or P890 to its reduced form. This 

 important test, as applied by Witt et al. (7) for the reactions of P700 

 and Cyt/, has not been attempted with bacterial photosynthetic tissues. 



A PHOTOCHEMICAL REACTION CENTER 

 IN PHOTOSYNTHETIC BACTERIA 



For the present, a working hypothesis will be entertained that the 

 reaction center in photosynthetic bacteria parallels that of System I in 



