156 PRIMARY PHOTOCHEMICAL PROCESS CHAP. 7 



that, on page 73, the adsorption of hydrogen and hydroxjd radicals by 

 zinc oxide was suggested as an explanation of the zinc oxide-sensitized 

 decomposition of water in ultraviolet light.) 



Instead of representing the elementary photochemical process as a 

 decomposition of water (as it appears in equation 7.1), it may be useful 

 to emphasize its character as an oxidation-reduction, with water (or a 

 water-acceptor complex) in the part of the reductant and an inter- 

 mediary hydrogen acceptor in the part of the oxidant. In this case, 

 we may write, using the symbols introduced in scheme 7.1: 



(7.2) 4{H20)4-4Z ^4 {OH) +4 HZ 



The completion of photosynthesis, initiated by reaction (7.2), calls 

 for a nonphotochemical reduction of carbon dioxide (or, more probably, 

 of an association product, {CO2}), by the primary reduction product 

 HZ, perhaps through the intermediary of other catalysts (Y and X in 

 scheme 7.1): 



catalysts 



(7.3) {C021+4HZ > {CH2O! +H2O + 4Z 



We have further to assume the dismutation of the oxidation product, 

 {OH}, into water and oxygen. The latter can occur either directly, 



catalysts 



(7.4) 4 {OH} > 2 H2O + O2 



or through the intermediary of biradicals (peroxides { OH } 2 or moloxides 

 {Oohc/. Chapter 11): 



(7.4a) 4 |0H1 ^2{OHl2 or 



(7.4a') 4 10H| > 2 H2O + {O2) 



(7.4b) 2 {0H!2 >2H20 + 02 or 



(7.4b') {O2I ^02 



The alternative formulation of equations (7.4a) and (7.4b) is the one 

 used previously in chapter 6, e. g., in scheme 6.1. 



By the summation of (7.2) and (7.4), one obtains, for the oxidation 

 of water in photosynthesis, the equation: 



(7.5) 4{H20}+4Z ^4HZ + 2H20 + 02 



By further addition of equation (7.3), the over-all reaction of photosynthesis 

 becomes 



(7.6) 4 {H2O! + {CO2I ) {CH2OI + 3 H2O + O2 



Thus, in van Niel's theory, four water molecules and one carbon dioxide 

 molecule participate in the formation of one {CH2O} group, and three 

 water molecules are recovered in the end — two by the dismutation of the 



