REDUCTION OF CARBON DIOXIDE AS PRIMARY PROCESS 



157 



first oxidation product, according to (7.4), and one by the dehydration 

 of an intermediate reduction product, as described by equations (3.11) 

 and (3.12). 



CO2 



' 1 



^{HzO) 



{CH20}*H20+4Z 



(7.4a) 



(74 b) 



2H2O + O2 



Scheme 7.II.— Photosynthesis, with water oxidation by an intermediate catalyst as the 

 primary photochemical process (first four quanta theory). 



Scheme 7. II may help to visuaUze the mechanism of photosynthesis 

 according to van Niel. The heavy arrow in this and all subsequent 

 schemes designates the primary photochemical process, while figures in 

 parentheses refer to equations in text. 



Wislicenus (1918), Thunberg (1923) and Weigert (1923, 1924) suggested that the 

 primary photochemical process in photosynthesis is the decomposition of water into 

 hydrogen and hydrogen peroxide, and that it is followed by a nonphotochemical reduction 

 of carbon dioxide by hydrogen peroxide, either alone (Eq. 4.18), or in cooperation with 

 hydrogen (Eq. 4.19b). As stated in chapter 4 (page 79), the nonphotochemical re- 

 actions postulated in this theory require too much energy to occur spontaneously at 

 the low temperatures prevailing in living organisms. There is therefore no need to 

 consider the Thunberg-Weigert theory in more detail here. 



Experiments to be described in chapter 11 (page 295) prove that the substitution 

 of heavy water for ordinary water affects the rate of a nonphotochemical reaction in 

 photosynthesis. This does not furnish, however, an argument against the participation 

 of water in the primary photochemical process, because hydrogen (or deuterium) atoms 

 transferred by hght from water to an intermediate acceptor must afterwards take part 

 in a number of catalytic reactions. In fact, the only partial reaction in photosynthesis 

 whose rate is Ukely to be left unaffected by the substitution of heavy water for ordinary 

 water, is the fixation of carbon dioxide in the {CO2I complex. 



3. Reduction of Carbon Dioxide as the Primary Process 



Second Four Quanta Theory 



The oldest theory, according to which the primary process in photo- 

 synthesis was thought to be the decomposition of carbon dioxide, had to be 

 discarded when hydrogen transfer was proved to be the main mechanism 

 of biochemical oxidation-reductions, and when all oxygen in photosynthe- 

 sis was shown to originate in water. We can nevertheless associate the 

 primary photochemical process in photosynthesis with a transformation 

 of carbon dioxide, if we consider this process as a photochemical hydro- 

 genation rather than a decomposition of this compound (cf. Scheme 7.1). 



