238 



REDUCTION OF CARBON DIOXIDE 



CHAP. 9 



The reason why only the second step in the reduction of oxygen is 

 coupled with carbon dioxide reduction may be that much more energy 

 is Hberated in this step than in the reduction of oxygen to peroxide (c/. 

 Table 11. 1). In other words, only the peroxide, {H2O2}, and not oxygen 

 may be able to oxidize H2X to a free HX radical. 



Reduction of 

 Carbon Dioxide 



4CO2 



40a 



E' 



4(002} 



6X 



Combustion of 

 Hydrogen 



6H2 



(9.11a, 9.12a) 



{o.y 



rr, 



9.11 b) 



4HX 

 _J 



4 HO, 



(9.11c) 



4{hC02} 



-B 



4. 

 4X 



2X 



(9.12b) 



4{h20j} 



(9.12c) 



{CHjOl+aCOa+HjO 



4H,0 + 2 0, 



Scheme 9.IVA. — Chemosynthesis of hydrogen bacteria according to reaction sys 

 terns (9.11) and (9.12). Ea: "carboxylase"; E'o: oxidase (page 135); En: hydrogenase. 

 Double arrows represent the energy-dismuting reactions. 



Scheme 9.IVB.— Chemosynthesis of hydrogen bacteria according to equations (9.16). 

 Double arrows represent the energy-dismuting reactions. 



Schemes 9.III and 9.IV describe the photosynthesis of green plants 

 and the chemosynthesis of hydrogen bacteria respectively. Scenedesmus 

 and other green algae, whose metabolism was discussed in chapter 6, 

 can, under appropriate conditions, carry out both normal photosynthesis 



