OXIDATION OF WATER AS PRIMARY PROCESS 155 



represents a possible elaboration of 7.V; its advantages and disadvan- 

 tages will be discussed in section 7. 



In Franck and Herzfeld's theory (as well as in several other theories 

 of the primary process), one of the participants in the photochemical 

 oxidation-reduction was identified with chlorophyll. We have eliminated 

 all reference to chlorophjdl from reaction schemes in this chapter, so as 

 not to prejudice their generality. The chemical function of chlorophyll 

 in photosynthesis, and its possible identification with one of the inter- 

 mediate oxidation-reduction catalysts in scheme 7.1, will be discussed in 

 chapter 19. 



2. Oxidation of Water as the Primary Process 



First Four Quanta Theory 



In first formulating the oxidation-reduction theory of photosynthesis, 

 van Niel (c/. van Niel and Muller, 1931, Muller 1933, van Niel 1931, 

 1935, 1941) postulated that photosynthesis involves a single photo- 

 chemical reaction and that this reaction is the decomposition of water (as 

 was suggested earlier by Bredig in 1914, Hofmann and Schumpelt in 

 1916, Thunberg and Weigert in 1923, and Wurmser in 1930). The 

 reduction of one molecule of carbon dioxide to a carbohydrate requires 

 the transfer of four hydrogen atoms. If we assume that each of these 

 atoms is contributed by a different molecule of water, that is, if we 

 select the alternative reaction (3.14) in preference to (3.13), we can 

 postulate, with van Niel, four identical primary photochemical reactions: 



(7.1) 4{H20i ^4 {HI +4 {OH} 



where hp symbolizes, in the usual way, a quantum of light energy. 



The decomposition of water has also been postulated, as the principal or only 

 photochemical reaction in photosynthesis, by Shibata and Yakushiji (1933), Dhar (1934), 

 and Gaffron (1942). 



In equation (7.1), braces again indicate that the components and 

 products of this reaction do not occur in the free state. Water is assumed 

 to be attached to a molecular complex, which probably includes the 

 sensitizer (chlorophyll), while the "primary reduction product," {H}, 

 and the "primary oxidation product," {OH}, are taken up by unknown 

 "acceptors." With free molecules, atoms and radicals, reaction (7.1) 

 would require not less than 110 kcal per mole (c/. Table 9. II), that is, 

 more than twice the energy available in one quantum of red light. In 

 order to make the primary reaction (7.1) at all possible, the energy of 

 association of the products, {H| and {OH}, with their acceptors, must 

 be larger than that of water, by at least 70 kcal per mole. (We recall 



