INTERPRETATION OF CARBON DIOXIDE CURVES 919 



If the carboxylation mechanism is as simple as postulated in (27.1), the 

 equilibrium constant Kg, is equal to the ratio of the two rate constants ka 



and h\. 



In equation (27.4) it is assumed that the acceptor, A, is either free or 

 occupied by CO2. This may not be the complete description for two rea- 

 sons: In the first place, the first reduction product of ACO2, designated by 

 us as AHCO2, may require time for its dissociation into A and HCO2; part 

 of the acceptor is then "blockaded," during photosynthesis, by this reduction 

 product {cf. section / below). In the second place, the photochemical re- 

 duction may have to be repeated several times, e. g. : 



(27.5A) ACO2 — ^^ AHCO2 — — > AH2CO2 ^ AH3CO2 ^ 



AH4CO2 > A + H2O + {CHoO! 



before the reduction product can separate itself from the carrier A (as 

 in the Franck-Herzfeld mechanism discussed in section d below.) If 

 AHCO2 is assumed to be the only product of photochemical reduction, the 

 completion of its reduction to AH4CO2 — i. e., to the carbohydrate level- 

 must be ascribed to dismutations : 



4 AHCO2 > AH4CO2 + 3 ACO2 



cj. Vol. I, p. 158. 



The simplest assumption that can be made in the interpretation of the 

 carbon dioxide curves of photosynthesis is that they are, at least basically, 

 saturation curves of the acceptor A. This means that one assumes (a) 

 that equilibrium (27.3) is not strongly dislocated during photosynthesis, 

 at least under moderate conditions, and (6) that the rate of photosynthesis 

 is given by the rate of reduction of the compound ACO2, and the latter is 

 proportional to the concentration [ACO2]: 



(27.6) P = nkf X [ACO2] 



where the constant k* depends on the intensity of illumination (as indi- 

 cated by the asterisk). We will deal with the possible limitations of as- 

 sumption h later (see, e. g., section e). The condition for the correctness of 

 a (i. e., for the maintenance of equilibrium 27.3 in light) is (cf. formula 

 27.2) : 



(27.7) k* < K 



That this condition is not always satisfied is demonstrated by the "pick 

 up" phenomena, described in chapter 8 (Vol. I). These observations show 

 that in very intense light (i. e., when k* is very large) or in the presence of 

 certain poisons (when k'a is very small) the acceptor A becomes "denuded" 

 of carbon dioxide and afterward "picks it up" in the dark. 



