CARBON DIOXIDE CONCENTRATION AND FLUORESCENCE 939 



consequently : 



(27-8^) [ACCOO-l.a.. - [A(CO.)] - ^"^^^^J 



One may use this derivation as starting point for speculations in which 

 the "condensation" of carbon dioxide into a (7„ compound (perhaps, with 

 n = 3 or 6) is assumed to be achieved, not after photochemical reduc- 

 tion (or between successive photochemical reduction steps), but already 

 in the preliminary, nonphotochemical carboxylation stage. One could 

 quote in this connection the observations of Van Rysselberghe and co- 

 workers (1946), who found indications that carbon dioxide is reduced, at 

 the cathode of a polarograph, after preliminary formation of a poljaneric 

 adsorption complex containing six CO2 molecules. 



Studies of the Hill reaction (chapter 35) and of the C(14) uptake in 

 light (chapter 36) indicate three possible additional complications of the 

 kinetics of carbon dioxide uptake in photosynthesis: 



(i) There may be several carhoxylations involved, e.g., one of the 

 C2 -^ Cs and one of the C3 -^ C4 type. Since, in the steady state, both 

 must proceed at the same rate, this complication may not change the 

 kinetic derivations too radically. When, for some reason, one carboxyla- 

 tion becomes a rate-limiting step, the other must be slowed doAVTi too, by 

 the blocking action of its accumulated products. 



{2) Carboxylation may he coupled with hydrogenation ("reductive 

 carboxylation," typified by direct conversion of pyruvic to malic acid, and 

 already mentioned on p. 937). In other words, reaction (27.2), instead 

 of being kinetically independent, may be combined with the first step in 

 (27.5A). This, too, should not necessarily change the kinetic derivations 

 in a radical way, since "coupling" probably means a fast sequence of two 

 reaction steps, perhaps catalyzed by two active groups in the same protein 

 molecule. 



(3) The acceptor. A, 7nay he a product of photosynthesis, disappearing in 

 the dark. This would make Ao a function of the rate of photosynthesis, 

 P, and would necessitate reconsideration of the kinetic equations based on 

 a constant value of Ao. 



B. Carbon Dioxide Concentration and Fluorescence* 



In chapter 24, when discussing the fluorescence of chlorophyll in the 

 Uving cell, we mentioned the close relationship often found between the in- 

 tensity of fluorescence and the rate of photosynthesis; we stated that, 

 because of this relationship, the measurement of fluorescence has become an 

 important tool in the study of the kinetics of photosj^nthesis. Conse- 



* Bibliography, page "JGS. 



