EFFECT OF CYANIDE ON FLASH YIELD 



1457 



of the finishing catalyst, Eb, since obviously the yield in the presence of 

 poisons, cannot be higher than in the noninhibited state.) 



Since the maximum yield per flash corresponds to the liberation of 

 about one molecule of oxygen per several thousand molecules of chlorophyll, 

 and since about 8 quanta are probably required for the production of one 

 molecule of oxygen, only one chlorophyll molecule in several hundred 

 needs to adsorb a quantum during the flash for flash satiu'ation to be 

 reached. (Kohn, 1936, estimated that 99% flash saturation is reached 

 when 1 quantum is absorbed during each flash by one out of a hundred 

 chlorophyll molecules.) Since the concentration of the acceptor mole- 

 cules, A, appears to be roughly equivalent to that of chlorophyll, it further 



Fig. 34.12. Yield per flash with and without cyanide (after Weller and Franck 1941). 



follows that to compensate for the consumption of A-C02 complexes by 

 photosynthesis in a saturating flash, about one acceptor molecule in a 

 hundred has to be recarboxylated during each dark interval. This is 

 the reason it was legitimate to consider recarboxylation a zero-order reac- 

 tion. If the intervals were to become sufficiently long to allow a large part 

 of the acceptor to be recarboxylated, the rate of carboxylation would finally 

 slow do^vn. However, complete recarboxylation requires as much as 20 

 seconds in the nonpoisoned state, and considerably more time in the pres- 

 ence of cyanide (as witnessed by the duration of the carbon dioxide "pick- 

 up," described in chapters 8 and 33). Therefore, in experiments with 

 dark intervals of less than 1 second, we always deal only with the initial, 

 Unear part of the carboxylation curve. If we accept Franck's suggestion 

 that, in the case of Eb hmitation, the light quanta not used for photosynthe- 

 sis nevertheless cause decomposition of the complex A-C02 (by back reac- 

 tions; c/. page 167), we have to conclude that the number of acceptor 

 molecules that need recarboxylation after each flash increases with the 

 flash energy not merely up to flash saturation, but beyond it. Let us im- 

 agine that flashing illumination of a cyanide-poisoned cell begins after a 



