1456 PHOTOSYNTHESIS IN INTERMITTENT LIGHT CHAP. 34 



suggested by Weller and Franck (1941), is that the reaction catalyzed by 

 Eb and usually responsible for the rate hmitation in strong continuous 

 light, as well as for the maximum yield per flash, is practically insensitive 

 to cyanide. The effect of cyanide on the flash yield is attributed, in this 

 theory, to a second reaction, not rate-determining in the absence of cy- 

 anide, but assuming this role when a large fraction of its catalyst is in- 

 hibited by cyanide. In chapter 12, evidence was presented that the main 

 source of cyanide sensitivity of photosynthesis is the carboxylation of the 

 acceptor. A, catalyzed by the carboxylase, Ea; and we also concluded that 

 this reaction is not usually rate-determining, but can become limiting in 

 the presence of an inhibitor. (In this and subsequent discussions, we 

 A\dll use the concept of "rate-limiting" reactions in the crudest qualitative 

 form. In any quantitative analysis of the data, the relationships derived 

 in chapter 26, and used in analytical derivations in chapters 27 and 28, 

 mil have to be taken into account. It was shown there that the rate 

 never actually "hits the ceiling," and that if several "rate ceilings" are pro- 

 vided by several physical or chemical reactions, of not too different maxi- 

 mum capacities, the rate is affected by all of them, and not only by the 

 lowest one.) 



According to Franck's theory, the maximum flash yield in the non- 

 poisoned state is essentially determined by the deficiency of the finishing 

 catalyst, Eb, while the maximum flash yield in the cyanide-poisoned 

 state is effectively limited by the deficiency of the reduction substrate, 

 caused by inhibition of a preparatory catalyst, Ea- 



Weller and Franck obtained confirmation of this hypothesis in the 

 shape of the curves showing flash yield vs. dark time. Figure 34.12 presents 

 two pairs of such curves — one determined mth nonpoisoned cells and the 

 other, with cells inhibited by 3 X lO"" M cyanide. The first curve is 

 exponential; the second begins as a straight line (as can best be seen in 

 curves (A)), but later bends and approaches the same limit, P'"^"-, as the 

 first curve (best seen in curves (B)). This difference in shape is precisely 

 what one would expect if the flash yield in the noninhibited state were 

 determined by the rate of a first-order transformation of a limited quantity 

 of an intermediate, while the yield in the inhibited state were dependent on 

 the rate of a zero-order transformation of a practically unlimited quantity 

 of a material. In the first case, the number of working molecules of the 

 limiting catalyst declines exponentially during the dark interval (as shown 

 by the peaks in figs. 34.7 and 34.9) ; in the second case, all the available 

 catalyst molecules can work uniformly throughout the dark periods, so 

 that their production increases linearly with the duration of these periods. 

 (The linear increase in P"'""^- \vith ta in poisoned cells obviously must cease 

 with the approach to the limit imposed on the flash yield by the deficiency 



