PHOTOSYNTHESIS AS A SENSITIZED OXIDATION-REDUCTION 53 



Kluyver (1930) interpreted different anaerobic fermentations as similar 

 transfers of hydrogen to acceptors other than oxygen. 



The hypothesis that photosynthesis can be placed alongside with 

 other biological oxidation-reductions and interpreted as an intermolecular 

 exchange of hydrogen atoms between water and carbon dioxide, was first 

 discussed by Thunberg (1923), but the credit for its clear formulation, 

 based on the analaysis of the metabolism of sulfur bacteria (which will 

 be discussed in chapter V), belongs to van Niel (1931). Starting from 

 Kluyver and Donker's generalization of Wieland's ideas, van Niel pro- 

 claimed photosynthesis to be a hydrogen transfer from water to carbon 

 dioxide in the higher plants, and from other hydrogen "donors" to carbon 

 dioxide in bacteria. 



In spontaneous metabolic proce.sses, the transfer of hydrogen atoms 

 (or electrons) always occurs "downhill," that is, in the direction of 

 decreasing oxidation-reduction potentials. The substance with higher 

 (more positive) potential yields its hydrogen to the substance with the 

 lower (more negative) potential. In photosynthesis, which is the reverse 

 of respiration, the hydrogen atoms must be moved "uphill," from a 

 system with a lower potential — O2/H2O — to the system with a higher 

 potential — C02/{CH20} — light being relied upon to give the necessary 

 "push." 



The reduction of carbon dioxide to the carbohydrate level requires 

 the hydrogenation of two C=0 double bonds, and thus the transfer of 

 four hydrogen atoms: 



H 



(3. 10) 0=C=0 + 4 H > HO— C— OH 



H 



The primary product of photosynthesis, according to (3.10) is formal- 

 dehyde hydrate. However, van Kiel's theory does not require the forma- 

 tion of this compound as an intermediate in photosynthesis, since it can 

 equally well be applied to the reduction of a larger molecule (e. g., of a 

 carboxylic acid, R-COOH), into which CO2 has been incorporated in a 

 preliminary reaction step. 



The four hydrogen atoms required in (3.10) can be provided by either 

 two or four water molecules : 



(3.11) 2H2O ^02 + 4H or 



(3.12) 4 H2O > 2 H2O2 + 4 H > O2 + 2 H2O + 4 H 



The over-all reaction of normal photosynthesis becomes, with (3.11): 



(3.13) CO2 + 2 H2O > H2C(OH)2 + O2 > {CH2OI + H2O + O2 



and with (3.12): 



(3.14) CO2 + 4H2O >H2C(OH)2 + 2H20 + 02 



> {CH2O! +3H2O + O2 



