COMPARISON OF DIFFERENT PRIMARY PROCESSES 167 



photochemical reduction of carbon dioxide can occur in the same organ- 

 isms (anaerobically-adapted Scenedesmus and similar green algae). 



As to the second question, that of the direct participation of the 

 reaction components in the primary photochemical process, the non- 

 photochemical reduction of carbon dioxide in autotrophic bacteria and 

 hydrogen-adapted algae constitutes a strong, even if indirect, argument 

 against the association of carbon dioxide with the photochemical reaction 

 proper. One is tempted to attribute photosynthesis and chemosynthesis 

 to a reaction of carbon dioxide with the same reducing agents, formed in 

 one case by a photochemical reaction and in the other case by the catalytic 

 oxidation of hydrogen, hydrogen sulfide, or another inorganic or organic 

 reductant. 



Against this argument, one must weigh several observations which 

 speak in favor of a closer association of carbon dioxide with the photo- 

 chemical apparatus, and which caused Franck and Herzfeld to assume 

 such an association in their scheme 7.VA. 



One such observation is the light-induced liberation of carbon dioxide, 

 which occurs occasionally (c/. Emerson and Lewis, page 207) during the 

 induction period of photosynthesis, and which ma^^ be attributed to a 

 photochemical decomposition of the complex, {CO2}, into acceptor and free 

 carbon dioxide. HoM'ever, Franck (1942), in a discussion of this "CO2 

 gush," decided that it is caused, not by a direct photochemical interaction 

 between {CO2} and excited chlorophyll, but by back reactions of the 

 first intermediate ({HCO2} in scheme 7.VA), in which so much energy 

 is released that the regenerated complex, {CO2}, dissociates immediately 

 into free acceptor and carbon dioxide. This mechanism does not require 

 that {HCO2} be formed by a direct photochemical interaction of {CO2I 

 with chlorophyll, but can equally well be fitted into a scheme in which 

 the complex {CO2} is reduced by an intermediate reductant. 



A second argument in support of a photochemical interaction between 

 chlorophyll and carbon dioxide is the relationship between the photosyn- 

 thesis and chlorophyll fluorescence in vivo. This phenomenon will be dis- 

 cussed in detail in volume II, chapters 24 and 32. The essential point is 

 that the yield of fluorescence sometimes increases at high light intensities 

 and that, according to Franck, French, and Puck (1941), this occurs 

 whenever the stationary concentration of the complexes, {CO2}, is de- 

 pleted. The simplest explanation of the quenching effect, which the 

 complex {CO2} apparently exercises on chlorophyll fluorescence, is the 

 assumption of a direct photochemical interaction of this complex with 

 excited cidorophyll molecules. 



However, in this case, too, an indirect interaction may suffice to 

 produce the observed results. For example, if chlorophyll reacts photo- 

 chemically with an intermediate oxidant X, and the reduced intermediate 



