Chapter 9 



THE NONPHOTOCHEMICAL PARTIAL PROCESS 

 IN PHOTOSYNTHESIS 



II. REDUCTION OF CARBON DIOXIDE* 



This chapter is based on the assumption that the reduction of the 

 complex, {CO2}, is a nonphotochemical process involving an intermediate 

 reductant (designated as {H} or HX in Chapter 7), and not a direct 

 photochemical reaction with reduced and excited chlorophyll, as postu- 

 lated in the theory of Franck and Herzfeld. 



In chapter 4, we stated that carbon dioxide, as well as the carboxyl 

 group, can be reduced in vitro only by means of strong reductants which 

 are unhkely to occur in living cells. However, the example of autotrophic 

 bacteria shows that organisms can produce agents capable of reducing 

 carbon dioxide, even without the help of light. We have as yet no 

 knowledge of their nature, but we may assume as a possibility — perhaps 

 even as a probability — that the same reductants are responsible for the 

 reduction of carbon dioxide in the photosynthesizing higher plants as well. 



To understand the thermodynamic difficulties of the reduction of 

 carbon dioxide (or of the carboxyl group), it is useful to review some 

 fundamental facts of the thermochemistry of organic compounds. 



1. The Standard Bond Energies 



The energy of a homopolar single bond is, to a first approximation, 

 independent of the nature of other groups to which the bonded atoms 

 are attached, and the energy content of a nonionic molecule is, to the 

 same approximation, the sum of these bond energies. However, second- 

 ary influences often cause considerable deviations from additivity. For 

 example, the standard energy of an oxygen-hydrogen bond is 110 kcal, 

 one-half the total energy of the water molecule. However, the energies 

 of dissociation of OH into O and H and of H2O into OH and H, are 

 known separately; and it turns out that 117 kcal is liberated in the 

 formation of the first OH bond, and only 104 kcal in that of the second 

 one (probably because of the mutual repulsion of dipoles associated with 

 the oxygen-hydrogen bonds). 



♦Bibliography, page 244. 



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