1618 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



oxygen amount was obtained which corresponded to 75 ± 5% of the theo- 

 retical equivalent of the amount of quinone added. Since white light was 

 used, this low yield might have been caused by direct photochemical de- 

 composition of quinone. An increase in the amount of cells did not change 

 the yield. At the lower concentration of quinone (0.25 mg. or 0.5 mg. in 3 

 cc), the initial rate and the fuial yield were the same as with 1.0 mg. and 

 exhaustion was reached correspondingly sooner — in 30 min. and 15 min,, 



20 40 60 80 100 120 



PHOTOCHEMICAL REACTION TIME, minutes 



20 40 60 80 100 

 REACTION TIME, minutes 



Fig. 35.25. Effect of quinone concentration on photochemical oxygen production by 

 Chlorella (concentrations in milligrams in 3 cc.) (after Clendenning and Ehrmantraut, 

 1951). (A) In white light (fluorescent Ught); (B) in red-orange light (X > 520 m/n). 



respectively. With 2 mg. quinone the initial rate and the total yield were 

 markedly smaller, and with 4 mg. only very little oxygen was produced 

 (fig. 35.25A) . Two phenomena seem to be involved in the failure to obtain 

 a stoichiometric oxygen equivalent of the added quinone. One is the de- 

 struction of quinone (by reactions with cell constituents; in blue- violet 

 light, quinone is decomposed even in the absence of cells). That quinone 

 actually is lost is indicated by the observation that if the reaction with 0.5 

 mg. quinone is continued until all oxygen production ceases, the addition 

 of another 0.5 mg. revives it, and approximately the same oxygen amount 

 (equivalent to ~75% of the extra quinone added) can be produced again. 

 The results were improved by using red-orange light, only slightly absorbed 



