1022 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



(28.15) ACOs-Chl-A'HoO , " ^ AHCOz-Chl-A'HO 



In considering this alternative, it is not necessary also to adopt Franck 

 and Herzfeld's complex mechanism, which involves eight consecutive 

 photochemical steps; essentially the same conclusions can be reached also 

 by considering a single photochemical step, such as reaction (28.15), 

 and leaving the completion of the process to nonphotochemical reactions, 

 such as dismutations and coupled oxido-reductions, as described in chapter 



9, Volume I. 



The second alternative — for which certain arguments were adduced in 

 Volume I (page 166)— is that the compound ACO2 (and perhaps A'H20 

 as well, although the two assumptions are separable) is kinetically inde- 

 pendent of chlorophyll; its reduction is then a secondary process, a dark 

 reaction brought about by the products of the primary photochemical re- 

 action. 



The analysis is simpler if the first alternative is chosen. If we assume 

 that the acceptor. A, is part of the chlorophyll complex, and that it takes 

 up or loses carbon dioxide without separating itself from this complex 

 (and that consequently, Ao = Chlo, and [ACO2] < Chlo), then all quanta 

 absorbed by the chlorophyll molecules carrying ACO2 must be effective 

 (as far as the primary photochemical process is concerned) — while all 

 ciuanta absorbed by chlorophyll carrying "bare" A are lost. The rate 

 of reduction of ACO2 is then k*I[AC02], as required by equation (28.14); 

 ^■*/[Chl] being the rate of absorption of quanta by chlorophyll in light of 

 intensity I (assuming that the absorbing capacity is not affected by asso- 

 ciation of chlorophyll with either ACO2 or A). This equation already was 

 used in chapter 27, section 7d (c/. equations 27.58-27.66). 



If ACO2 is kinetically independent of the photosensitive complex, the 

 concentration [ACO2] cannot be limited to Chlo; equation (28.14) now 

 appears to indicate that the quantum yield of photosynthesis, 7 = P/h 

 (/a = absorbed light energy), can increase indefinitely with increasing 

 [ACO2]. At least, this would be so if one would assume, as usual, that I^ 

 is proportional to 7, I a, = al, so that : 



(28.16) 7 = P/h = ank* [ACO2] 



Of course, a certain limit to the increase of 7 is set by the fact that ACO2 

 must be <Ao, the total number of available acceptor molecules. How- 

 ever, there is no reason why this limit could not be higher than the maxi- 

 mum yield possible under Einstein's equivalency law. Therefore, a limi- 



t Or 2 hu, if it is assumed that one quantum is used to transfer one hydrogen atom 

 from A'HoO to Chi, and another one to transfer the same atom from Chi to ACO2. 

 In (28.15), it is also assumed that the hydrogen donor is "bound water," A'HaO; and 

 that, like the hydrogen acceptor, ACO2, this donor is stably associated with Chi. 



