FREE RADICALS IN PHOTOSYNTHESIS 237 



(9.12a) 2H2 + 2X >'2H2X 



(9.12b) 4HO2 + 2H2X >4{H202l+2X 



(9.12c) 4 {H2O2) > 4 H2O + 2 O2 



(9.12) 4 HO2 + 2 H2 > 4 H2O + 2 O2 



Alternatively, it may be first dismuted to oxygen and a peroxide and then 

 reduced: 



(9.13a) 2H2 + 2X >2H2X 



(9.13b) 4 HO2 > 2 {H2O2) + 2 O2 



(9.13c) 2{H202)+2H2X )• 4 H.O + 2 X 



(9.13) 4 HO2 + 2 H2 y 4 H2O + 2 O2 



If we substitute (9.12) or (9.13) for (9.11), the over-all reaction of 

 chemosynthesis becomes : 



(9.14) 6 H2 + 2 O2 + {CO2} y {CH2O) + 5 H2O 



with the correct ratio AH2 : AO2 = 3. 



Reaction (9.11), with the variation (9.12), is represented in scheme 

 9.IVA, whose similarity to scheme 9. Ill is easily recognizable. 



Another possible explanation of the relation, AH2 : AO2 > 2, is that 

 not all HX radicals react with (CO2}, as assumed in (9.11c), but that 

 some are lost by autoxidation: 



(9.15a) 2HX + 2O2 >2H02 + 2X 



or by dismutation: 



(9.15b) 2 HX > H2X + X 



Finally, we can explain the ratio, AH2 : AO2 = 3 also by recalling 

 the conclusion reached in chapter 6 (page 141), that the reduction of 

 carbon dioxide is coupled (at least in adapted algae) only ^^-ith the second 

 step of oxygen reduction, the step which leads from a peroxide to water 

 (cf. Eqs. 6.12). We may thus assume that, in the first stage of reaction 

 between H2X and oxygen and carbon dioxide, both hydrogen atoms go to 

 oxygen, while in the second one they are shared between oxygen and 

 carbon dioxide: 



(9.16a) 6H2 + 6X >6H2X 



(9.16b) 2H2X + 2O2 )-2{H202}+2X 



(9.16c) 4H2X + 2{H202| )■ 4 H2O + 4 HX 



(9.16d) 4HX + 4{C02l >4!HC02!+4X 



(9.16e) 4 {HCO2I > ICH2OI + H2O + 3 CO2 



(9.16) 6H2 + 2O2+ {CO2) >{CH201+5H20 



This mechanism is represented in scheme 9.IVB. 



