160 



PRIMARY PHOTOCHEMICAL PROCESS 



CHAP. 7 



water, it may be useful to write down also a less specific scheme, in 

 which both the reduction of carbon dioxide and the oxidation of water 

 are assigned to secondary catalytic reactions, and the primary photo- 

 chemical process is thought of as an oxidation-reduction reaction between 

 two intermediates, e. g., X and Z in scheme 7.1: 



(7.10a) 



(7.10b) 



(7.10b') 



(7.10c) 



(7.10c') 



(7.10d) 



(7.10e) 



(7.10) 



4HZ + 4X 



4 Ay 



-» 4 Z + 4 HX 



^4 HZ + 4 {0H| 



).4HZ+ {O2I 



or 



4Z + 4 {H2OI — 

 4Z + 2 IH2O) — 

 4 {OH! > 2 H2O + O2 



{O2} >02 



4 HX + 4 {CO2! > 4 {HCO2! + 4 X 



or 



4 {HCO2 



-> {CH2OI +H2O + 3CO2 



4{C02l +4{H20i 



ihy 



-^ { CH2O) + O2 + 3 HoO + 3 CO2 



Reaction system (7.10) is represented graphically in scheme 7.1 V. 

 It also was used, because of its unspecific character, in the construction 

 of schemes 6.1 and 6.III in the preceding chapter. 



2H,0 



jCHaO} +H2O + 3C0; 



Scheme 7.IV. — Photosynthesis, with an oxidation-reduction reaction between two inter- 

 mediate catalysts as the primary photochemical process (third four quanta theory). 



5. Two Different Primary Four Quanta Processes 

 First Eight Quanta Theory 



Schemes 7. II, 7. Ill, and 7. IV have in common the assumption oi four 

 identical photochemical reactions for each reduced molecule of carbon 

 dioxide. But after a controversy which lasted for several years (Vol. II, 

 Chapter 29) it now seems probable that the maximum quantum yield of 

 photosynthesis is not 1/4, but 1/8 (perhaps even 1/10 or 1/12), that is, 

 that at least eight quanta must be absorbed for each reduced molecule 

 of carbon dioxide. This larger number of available quanta is welcome, 

 because the energy contained in four quanta of red light (about 160 kcal 



