EFFECT OF FINISHING DARK REACTIONS 1035 



of fluorescence. To explain saturation not accompanied by changes in 

 fluorescence intensity, the deficient catalyst must be placed, not between 

 one of the two reactants (ACO2 or A'HsO) and the photosansitive complex, 

 but between the primary and the finished products of photosynthesis. In 

 other words, the back reactions caused by the catalytic deficiency must be 

 "secondary" rather than "primary." A back reaction of this kind was 

 therefore added as reaction d in mechanisms (28.20) and (28.21). We can 

 postulate, e. g., that reaction (28.20d) comes into play because the transfor- 

 mation of the first photoproduct, AHCO2, into a more stable intermediate 

 requires a catalyst, Eb (perhaps a "mutase"), which is present in limited 

 quantity. A similar postulate could be made for the effect of the catalyst 

 Ec, which is required for the first step in the conversion of A'HO into free 

 oxygen. Because of the symmetry we have assumed between the right and 

 left sides in schemes 28.IA and B, a limitation in the utilization of the oxida- 

 tion products will have the same effect on the kinetics of the process as a 

 whole as a limitation of the utilization of the reduction products. In the 

 first case, the secondary back reaction will be accelerated by the accumula- 

 tion of the primary oxidation product, A'HO; in the second case, by the ac- 

 cumulation of the primary reduction product, AHCO2. 



Using reactions (28.20) we can tentatively assume that Eb acts on the 

 first reduction product, AHCO2, according to the scheme; 



(28.38a) AHCO2 + Eb ^ ^ ^ EbAHCO. 



(28.38b) EBAHCO2 > Eb + A + {HCO,} 



where { HCO2 } designates a stabilized reduction product. 



(If reaction 28.38b is a dismutation, it may require the participation of 

 two AHCO2 radicals, but we ^nll use here the simplest possible mechanism.) 



The rate of photosynthesis is, according to this scheme : 



(28.39) P = nfcelEBAHCOo] 

 and the "absolute maximum rate" is: 



(28.40) PZ^: = nkeEl 



where Eg is the total available quantity of the "limiting" enzyme. 



An equation for P as a function of / and [CO2] can be derived from this 

 mechanism; but it is complicated, even if all the possible simplifying 

 assumptions are made, and little could be achieved by writing it do^^^l here. 



The situation is simplified if we again make use of Franck and Herzf eld's 

 mechanism (scheme 7.VA, Vol. I), in which the oxidant, ACO2, and the re- 

 ductant, A'H20, belong to the photosensitive complex, and take part in 

 the primary photochemical process, e. g., in the way indicated in equation 



