918 CONCENTRATION FACTORS CHAP. 27 



pound ACO2 reacts must be a — direct or indirect — product of the primary 

 photochemical process. 



Two different premises can be used in the kinetic analysis of the effect 

 of carboxylation. One alternative (indicated by arrows in equation 27.1, 

 is to assume that the carboxylation is markedly reversible, i. e., that k^ is 

 of the same order of magnitude as A-a[C02]a. In this case, the association 

 of the acceptor A with carbon dioxide is not complete even without any 

 dislocation of the equilibrium by the consumption of ACO2 in light. The 

 other alternative, preferred by Frank and Herzfeld, is to assume that the 

 carboxylation equilibrium lies entirely on the side of association (meaning 

 A-a[C02] » A-a), so that in the dark practically all acceptor is "saturated" 

 by carbon dioxide molecules (at all practically significant partial pressures 

 of carbon dioxide), unless the carbon dioxide molecules are displaced by 

 other association partners, such as reduction intermediates, narcotics, 

 etc. Free molecules A occur in this picture only during (or immediately 

 after) intense photosynthesis, when reduction of ACO2 is (or has been) too 

 rapid for the recarboxylation to keep step with it. 



As described in chapter 8 (vol. I) the known equilibria of carboxylation 

 in vitro correspond to practically complete dissociation. Only few cases 

 are known in which the carboxyl group is thermodynamically stable with 

 respect to decarboxylation (at least, under sufficiently high carbon dioxide 

 pressures). The "saturation" of photosynthesis with carbon dioxide, which 

 occurs under partial pressures as low as 0.1%, indicates that conditions may 

 be different here, perhaps in consequence of a coupling of carboxylation 

 with another reaction, such as degradation of a "high energy phosphate," 

 or an "endergonic" oxidation-reduction (c/. Vol. I, page 201). However, 

 there is no experimental or theoretical reason — except convenience in ana- 

 lytical formulation— to postulate that in photosynthesis the carboxylation 

 equilibrium lies completely on the side of synthesis, even at the lowest practi- 

 cally significant carbon dioxide pressures. We will therefore begin our 

 analysis by assuming that the degree of saturation of the acceptor with car- 

 bon dioxide does depend on the external concentration of carbon dioxide. 



If one molecule of carbon dioxide is taken up by one molecule of ac- 

 ceptor, the carboxylation equilibrium is determined by the equation: 



(27.3) [ACO2] = (A'aAo[C02]a)/(l + i^a[C02]a) 



where ^0 is the total available concentration of the acceptor: 



(27.4) Ao = [A] + [ACO2] 



and iCa is the equilibrium constant of carboxylation: 



(27.5) Ka[C02]a[A] = [ACO2] 



