PHOTOSYNTHETIC REACTIONS OP AUTOTROPHIC BACTERIA 105 



In confirmation of this hypothesis, Eymers and Wassink quoted the 

 observation that, for each molecule of carbon dioxide assimilated by the 

 bacteria, four were taken up by the medium — obviously to neutralize the 

 four hydroxyl ions. However, most of the energy accumulation in (5.7) 

 is associated with the formation of the four hydroxyl ions. If this re- 

 action occurs in an acid medium, and the hydroxyl ions are neutralized, 

 the heat effect is only —12.5 kcal. 



Altogether, it appears that bacterial photosynthesis is not necessarily 

 inefficient as far as energy conversion is concerned, but can lead to the 

 conversion into chemical energy of up to one-third or one-half the 

 amount which is accumulated in the photosynthesis of the higher plants. 



Because electrolytes are involved in many forms of bacterial photo- 

 synthesis, the free energies of these reactions often differ considerably 

 from their total energies (while the free energy of normal photosynthesis 

 is almost equal to its total energy; c/. Table 3.V). Calculated for 

 standard conditions (atmospheric pressures of gases and one-molar 

 solutions of the solutes), the gains in free energy in different forms of 

 bacterial photosynthesis, generally are larger than those in total energy, 

 by as much as 20 or 30 kcal per mole. For example, the free energy of 

 reaction (5.7) is AF = 97 kcal per mole in alkaline, and 20 kcal in acid 

 solution. 



In reactions (5.1) to (5.6), carbon dioxide is reduced to carbohydrate 

 by means of different inorganic reductants. In chapter 3, we have 

 interpreted normal photosynthesis as a transfer of hydrogen atoms from 

 water to carbon dioxide. Van Niel (1931, 1935) generalized this concept 

 by describing all forms of bacterial photosynthesis as hydrogen transfers 

 from various hydrogen donors (reductants) to carbon dioxide as the 

 common hydrogen acceptor. 



In analogy to the two alternatives, (3.13) and (3.14), in normal 

 photosynthesis, the generalized equation of photosynthesis can be 

 written in two forms: 



light 

 (5.8a) CO2 + 4 R'H > {CH2O) + HoO + 4 R' or 



light 



(5.8b) CO2 + 2 R"H2 > i CH2O I + H2O + 2 R" 



In the first formulation, each molecule of the reductant contributes one, 

 and in the second formulation two, hydrogen atoms, towards the reduction 

 of one molecule of carbon dioxide. 



In the case of normal photosynthesis, R' is OH, or — if formulation 

 (5.8b) is preferred — R" is 0. In the case of photoreduction with sulfide, 

 R' is SH or R" is S, and in that of photoreduction with hydrogen, R' is 

 H or R" is nothing. The application of equations (5.8) when the 

 reductant contains no hydrogen at all (as in the case of elementary 



