1036 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



(28.15). In this case, the primary back reaction (detautomerization) itself 

 becomes a competitor to finisliing dark reactions, and it can therefore be 

 assumed that the role of the catalyst Eb is to prevent this reaction from de- 

 stroying the photoproducts. This point of view was used in the elaboration 

 of kinetic equations of photosynthesis by Franck and Herzfeld (1937). 

 Their derivations are comphcated by the assumption of four successive 

 (different) photochemical steps "on the reduction side," alternating with 

 four (identical) photochemical steps "on the oxidation side"; the product 

 of each of these steps was supposed to require stabilization by the same 

 "catalyst B," in order to prevent this step from being reversed by a dark 

 reaction. Instead of trying to present here the derivations of Franck and 

 Herzfeld we will use a simpler mechanism embodying the same basic con- 

 cept of primary back reaction as cause of Ught saturation. This mechanism 

 is similar to the one given in equation (7.13) in Volume I, but is further 

 simplified by the substitution of a single primary photochemical step for 

 the two steps (7.13a) and (7.13b). The reaction scheme is: 



k*I 



(28.41a, a') ACOs-Chl-A'H.O ^=± AHCOa-Chl-A'HO Primary forward and 



k' back reaction 



k 

 (28.41b) AHCOa-Chl-A'HO + Eb ^— > EbHCO. + A-Chl-A'HO 



Catalytic 

 stabiliza- 

 k' \ tion of re- 



(28.41c) EbHCOo '- — >Eb + {HC021 duction 



product 



(28.41d) A-Chl-A'HO + H2O > A-Clil-A'HjO + (HO) V'Reloading" of 



(28.41e) A-Chl-A'HaO + CO2 > ACO.-Chl-A'HoO Jchlorophyll 



We assume that Eb is needed only for "stabilization" of the reduction 

 product, AHCO2, and that this stabilization is achieved, in reaction 

 (28.41b), by taking the reduced group, HCO2, away from the chlorophyll 

 complex and thus preventing its back reaction with the oxidized group, 

 HO. (It is not suggested that the radicals HCO2 or OH occur in the free 

 state; these symbols can stand for corresponding functional groups in 

 larger molecules, as indicated by braces in 28.41c and d.) 



In order to simplify the derivations still more and to elaborate only the 

 effect due to the back reaction, we further assume that the reactions (28.41d 

 and e), by which the photosensitive complex is supplied with fresh oxi- 

 dant and fresh reductant, respectively, are practically instantaneous. 

 Under these conditions, only two factors can cause light saturation: (a) 

 accumulation of the photosensitive complex in the tautomeric form 

 AHCOg-Chl- A'HO; and (5) accumulation of the catalyst Eg in the bound 



