234 REDUCTION OF CARBON DIOXIDE CHAP. 9 



how eight quanta can be utiHzed in photosynthesis for the reduction of 

 one molecule of carbon dioxide. In chapter 7, two alternative hypotheses 

 were suggested for the mechanism by which eight quanta could be used to 

 move four hydrogen atoms from water to carbon dioxide: a twice repeated 

 photochemical activation of the same four hydrogen atoms (as in Schemes 

 7. V and 7. VA) ; and the transfer of energy, initially conferred upon eight 

 hydrogen atoms, to four of them, as illustrated by equation system 

 (7.14) and scheme 7. VI. The introduction of a second intermediate 

 system, Y-H2Y, between the systems H-H2X and Z-HZ, permits an 

 interpretation of the second alternative. For this, we have to assume 

 that the average oxidation-reduction potential of system Y-H2Y is not 

 very different from that of system X-II2X, thus making reaction (9.10c), 

 possible, but that the radical HX is much less stable than the radical 

 HY thus making possible the "energy dismutation" by reactions (O.lOd) 

 and(9.10e): 



(9.10a) 8HZ + 8Y ^ 8 Z + 8 HY 



(9. 10b) 8 H Y > 4 H2Y + 4 Y 



(9.10c) 4H2Y + 4X >4H2X + 4Y 



(9.10d) 4H2X + 4Z >4HX + 4HZ 



(9.10e) 4HX + 4{C02} >4{HC02}+4X 



(9.10f) 4 JHCO2} > ICH2OI + 3 CO2 + H2O 



(9.10g) 4 Z + 4 H2O > 4 HZ + O2 + 2 H2O 



(9.10) 4 H2O + 4 ICO2} > {CH2O} + O2 + 3 {COj} + 3 H2O 



Reaction mechanism (9.10), represented in scheme 9. Ill, provides 

 the desired elaboration of the hypothesis of "energy dismutation" (c/. 

 equations 7.14, and Scheme 7. VI). The "coupled" reaction (7.14b), by 

 which the "energy dismutation" was originally represented, is dissolved 

 in (9.10), into the reaction sequence (9.10b, c, d, e), marked by double 

 arrows in scheme 9. III. It is based on the assumption that the removal 

 of one hydrogen atom from the intermediate H2X by recombination 

 with the oxidation intermediate, Z, leaves a radical, (semiquinone) HX, 

 which is sufficiently unstable to react with the carboxyl group in {CO2}. 

 The radicals HY are produced by single quanta of light, but two quanta 

 must cooperate to produce a single radical HX. 



As mentioned on page 166, the "energy dismutation" theory is 

 supported mainly by the analogy between the photosynthesis of green 

 plants and the chemosynthesis of hydrogen bacteria, for which it offers 

 a simple explanation. 



To give this explanation, it is sufficient to assume that, in organisms 

 which contain an active hydrogenase and an active oxidase, the system 

 X-H2X can be hydrogenated by molecular hydrogen, and "half-oxidized" 



