166 PRIMARY PHOTOCHEMICAL PROCESS CHAP. 7 



molecules of a comparatively mild reductant is utilized for the production 

 of one molecule (or radical) able to react with carbon dioxide. This 

 analogy with chemosynthesis is the main reason for the introduction of 

 the concept of "energy dismutation" into the discussion of the mechanism 

 of photosynthesis. This concept enables one to postulate only one kind 

 of primary photochemical processes even if the number of these processes 

 is much larger than the number of elementary oxidation-reduction acts 

 (hydrogen transfers or electron transfers) required for the completion of 

 the overall reaction. 



A possible mechanism of "energy dismutation" in photosynthesis 

 and chemosynthesis will be discussed in chapter 9, and the results 

 presented in schemes 9. Ill and 9. IV. The assumption on which these 

 schemes are based is that, after a compound, RH2, has been first oxidized 

 by a strong oxidant (oxygen, for example) to a radical, RH, the latter 

 may be able to yield its remaining hydrogen atom to a much weaker 

 second oxidant (carbon dioxide, for example). 



7. Comparison of Different Primary Processes 



Comparing critically the various schemes of photosynthesis presented 

 in this chapter, we can discard the four quantum schemes as contradicting 

 recent quantum yield determinations, as well as straining dangerously 

 the thermochemical possibilities. As between the alternative eight 

 quanta theories, no final decision is possible at present. Two questions 

 remain to be decided: Is the assumption of eight identical photochemical 

 processes (as in 7.14) more probable than that of two different kinds of 

 primary processes (as in 7.11 or 7.13) or of five such processes (as in 

 7.12)? Does carbon dioxide or water (or both or neither) participate 

 (directly or as complexes) in the primary photochemical process? 



Although the hypothesis of two sets of primary photochemical 

 processes — photoxidations and photoreductions — does not require the 

 direct chemical participation of chlorophyll in both of them, an experi- 

 mental proof of the existence of two interconvertible colored forms of 

 chlorophyll belonging to different reduction levels and capable of using 

 light energy for photoxidations and photoreductions, respectively, would 

 strengthen this hypothesis almost to the point of certainty. We shall 

 see, in chapter 18, that experiments with extracted chlorophyll make the 

 existence of two such chlorophyll modifications plausible but do not 

 prove it. The main argument in favor of the alternative theory of eight 

 identical photochemical reactions (beside the greater simplicity of this 

 scheme) is the analogy which it enables to establish between the mechan- 

 isms of photosynthesis and chemosynthesis. This appeals to our desire 

 for a unified conception of all forms of organic synthesis, and receives 

 support from the discovery of Gaffron that photochemical and non- 



