THE PHOTOCHEMICAL REACTIONS 



143 



time to reach the absorption vessel. Similarly to the hydrogen fermenta- 

 tion in the dark, the hydrogen production in light can be sustained by an 

 added hydrogen donor, e. g., glucose. 



The hydrogen evolution in light is not ajffected by some poisons 

 (e. g., dinitrophenol) which inhibit the hydrogen fermentation in the dark. 

 It thus seems that light permits the by-passing of an enzymatic step 

 necessary for the hydrogen fermentation in the dark. 



60 80 100 



Tims, minutes 



Fig. 13. — Photochemical absorption and evolution of hydrogen by Scenedesmus in 

 absence of carbon dioxide at different light intensities (after Gaffron 1942). 



0.074 ml. of cells of Scenedesmus D3 in 0.05 M phosphate buffer of pH 6.2 at 25° C. 

 Side arm contains KOH. Preceding dark periods, 20 hours; during this time the algae 

 have formed hydrogen up to 0.2% of the gas phase. 



The explanation of the photochemical evolution of hydrogen requires 

 the assumption that the photochemical hydrogen transfer can be inter- 

 posed between the hydrogen donor R'H2 and the hydrogenase system 



H 



E 



H 



H2A 



H 



H2E 



H, 



H 



as represented in scheme 6. Ill, where the photochemical reaction se^ 

 quence (6.14), (7.10a), (6.13), achieves the same result as the dark 

 reaction (6.6b')- The method of representation used in scheme 6. Ill is 

 different from that in schemes 6.1 and 6. II. Instead of giving a complete 

 representation of each partial reaction, we have merely written down 

 the oxidation-reduction systems which participate in the process, and 

 indicated by arrows the direction of hydrogen transfer between them. 



The photochemical absorption of hydrogen can be interpreted in two 

 ways: either as a light-accelerated hydrogenation of organic acceptors, 

 R, or (as mentioned on page 129) as a photoreduction of carbon dioxide 

 produced by fermentation. The first alternative is represented in 

 scheme 6. Ill by the reaction sequence (6.6c), (7.10a), (6.15), whose 



