PROCESSES IN THE PHOTOSENSITIVE COMPLEX 1033 



the assumption that Hght saturation is usually caused by non-photochemical 

 hack reactions, which compete with the forward dark reaction catalyzed by 

 the catalyst "B.' 



>> 



(f) Analytical Formulation: Effect of "Finishing^^ Dark Reactions 



The necessity of considering, in addition to the "preparatory" dark re- 

 actions and the reversible changes in the chlorophyll complex, the "finish- 

 ing" dark reactions as possible sources of light saturation phenomena in 

 photosynthesis arises from several observations. It was mentioned above 

 that experiments in flashing light (to be described in chapter 34) demon- 

 strated directly the existence of a "finishing" catalyst with a "working 

 period" of the order of lO^''^ sec. at room temperature. The maximum 

 yield obtainable per single flash shows that this catalyst is present in a con- 

 centration equivalent to 0.05% of that of chlorophyll. Consequently, as 

 indicated above (page 1031 it imposes a "ceiling" on the over-all rate of 

 photosynthesis of about one molecule carbon dioxide reduced per chloro- 

 phyll molecule every 40 seconds — which is close to the actually observed 

 maximum rate of photosynthesis at room temperature. A second relevant 

 observation is made by comparing the light curves of photosynthesis of 

 various plants with the light curves of their fluorescence (cf. part B of this 

 chapter). If the light saturation of photosynthesis were due to a slow 

 supply reaction (i. e., to the depletion of one of the reactants, ACO2 or 

 A'Il20, in intense light), or to slow regeneration of the photosensitive form 

 of the chlorophyll complex, in both cases, the light saturation would be 

 associated with accumulation of the photosensitive complex in a chemically 

 changed form (such as HX-Chl-Z or HX-Chl-HZ), and should therefore 

 reveal itself by simultaneous changes in the fluorescence yield of the com- 

 plex. This is actually the case sometimes, but not always. Figure 28.24, 

 for example, shows light saturation of photosjoithesis of Chlorella without 

 any change in the yield of fluorescence; and even in figure 28.26, satura- 

 tion is almost completed before fluorescence begins to change its yield. 

 In all such cases, saturation must be due to the failure of a finishing dark 

 reaction to keep pace with the primary photochemical process — a failure 

 that produces no change in the composition of the photosensitive complex, 

 but leads to the loss of a large part of primary photoproducts by back reac- 

 tions. 



It was stated before that the distinction between preparatory and 

 finishing dark reactions is not so clear-cut in Franck's theory of "narcotic 

 regulation" of photosynthesis. According to this theory, accumulation of 

 primary oxidation products ("photoperoxides") leads not (or not only) to 

 back reactions between these peroxides and the primary reduction products 



