436 



PHOTOMECHANICAL CONSIDERATIONS 



difference between the "cytochrome response" and the "carotenoid re- 

 sponse" is clearly demonstrated. The steady- state carotenoid shifts 

 (solid curve) plotted versus logarithm of intensity fit the symmetrical 

 sigmoid curve for a rectangular hyberbola, whereas the steady- state 

 cytochrome change (dashed curve) shows four inflections suggesting 

 separate electron transfer systems having distinctly different sensi- 

 tivities to light. The two inflection points for the most sensitive cyto- 

 chrome reactions occur at intensities well below the inflection point 

 for the carotenoid reaction and are consistent with the observation in 

 Fig, 1 that the light-induced carotenoid changes disappear before the 

 light-induced cytochrome oxidation. 



2 3 4 5 



INTENSITY IN lO'" EINSTEIN cm'^SEC"' 



Fig. 3, Initial rate of absorbancy decrease at cytochrome Soret peak versus 

 quantum intensity of actinic light {0.80 jU). Anaerobic bacteria suspended in 0.1 

 M succinate. 



