PROBLEM OF THE PRIMARY PROCESS 151 



In the present chapter, we will disregard these hypotheses and consider 

 the primary photochemical process as a stage in the main oxidation- 

 reduction reaction between water (or a substitute reductant) and carbon 



dioxide. 



In green plants, and in many bacteria as well, the hydrogen transfer 

 must occur against the gradient of chemical potential, i. e., from an 

 oxidation-reduction system (O2-H2O) with a more negative potential to 

 a system (C02-{CH20}) with a more positive potential. This "uphill 

 flow" is possible only with the assistance of external energy; here, light 

 is called upon to play its part. However, the exact location of this 

 "lift" in the reaction sequence cannot be predicted a priori. When a 

 canal is built between two bodies of water situated at different levels, 

 the provision of locks cannot be avoided; but whether these locks are 

 constructed at the upper or lower end of the waterway is a purely practical 

 problem. Similarly, the photochemical processes, which serve as "locks " 

 in the flow of hydrogen from water to carbon dioxide, can be located 

 either at the beginning of the transfer (in the oxidation of water) or at 

 its end (in the reduction of carbon dioxide), or somewhere in the middle, 

 or even in several different places. 



The description of the photochemical process in photosynthesis as 

 a transfer of hydrogen atoms, which will be used throughout this chapter, 

 does not exclude the possibiUty that it may be primarily an electron 

 transfer. As described elsewhere (c/. Chapter 9, page 219), electron 

 transfers coupled with acid-base equilibria, are equivalent to hydrogen 

 transfers (and, if coupled with hydrations and dehydrations, may be 

 equivalent to oxygenations). 



Franck (1935) and Stoll (1936) once made the suggestion that the 

 primary photochemical process in photosynthesis may be an exchange of 

 hydrogen for hydroxyl (cf. Chapter 19, page 555). However, the assump- 

 tion of a transfer of hydroxyl radicals from carbonic acid to water is 

 equivalent to the postulate that one part of the liberated oxygen originates 

 in carbon dioxide, a concept which was found in chapter 3 (page 55) 

 to be in conflict with experimental evidence. 



Looking for analogies to the postulated primary photochemical 

 reaction in the realm of ordinary photochemistry, we find them in certain 

 phenomena discussed in sections 4 and 5 of chapter 4. It was suggested 

 there, that light absorption by inorganic ions in solution often leads to 

 the oxidation of water, even though this effect remains "hidden" be- 

 cause of the high rate of back reactions. In certain dyestuff solutions, a 

 similar photochemical electron transfer takes place in the presence of 

 added reductants, for example, ferrous ions, and may perhaps occur also 

 in their absence. In the system thionine-ferrous ions, the back reaction 

 is so slow that the mixture can lose all its color in light (as described 



