1622 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



20 40 60 80 



PHOTOCHEMICAL REACTION TIME, minutes 



100 



Fig. 35.27. Effect of preilluminating Chlorella cells and quinone on subsequent 

 photochemical oxygen production by Chlorella (after Clendenning and Ehrman- 

 traut 1951). 



total yield of oxygen liberated from a given amount of quinone. 



It would be interesting to laiow whether inhibition by preillumination 

 affects equally the maximum rate of the Hill reaction in strong light and 

 its quantum efficiency on weak light ; no such observations have been made 



The Hill reaction with quinone also is inhibited by the preillumination 

 of quinone without the cells (usmg the blue-violet light absorbed by qui- 

 none) . This illumination causes a darkening of the quinone solution, and 

 apparently produces a strongly poisonous substance that inhibits the Hill 

 reaction. The two preillumination effects, one due to the deactivation of 

 cells, and the other caused by photochemical decomposition of quinone, are 

 independent and additive. 



When the Hill reaction is measured with quinone as oxidant, in light 

 from which blue-violet rays have not been excluded, one must e.xpect that 

 the decomposition of quinone and the production of the poison will pro- 

 ceed simultaneously with the main reaction (unless all quinone added is 

 taken up by the cells and bound there in such a way as to be protected 

 from photodecomposition). Available evidence does not show to what ex- 



