THE PHOTOCHEMICAL REACTIONS 



145 



between the cellular hydrogen donors, H2R', and cellular hydrogen 

 acceptors, R, on the one side, and the primary photochemical products, 

 Z and HX, on the other, which provide parallel photochemical channels 

 to dark reactions (6.6b) and (6.6b'). (This relation between the paths 

 of dark and photochemical reaction explains, incidentally, why they are 

 not necessarily inhibited by the same poisons.) 



The assumption of reaction (6.14) leads to a question which must for 

 the time being be left open: Why are nonadapted green plants — in which 



20 



10 



c 

 o 



U 



3 — 



S.-IO 



E 

 E 



-20 



-30 



6 9 



Time, minuiss 



12 



IS 



Fig. 14. — Time course of gas exchange of anaerobically incubated Scenedesmus 

 (after Gaffron 1942). 



Downward trend — absorption of hydrogen; upward trend — liberation of oxygen. 

 1— Hydrogen— 560 lux. 3— Hydrogen— 200 lux. 



2— Hydrogen— 1020 lux. 4— Hydrogen— 6000 lux. 



5— Air— 6000 lux. 



allegedly only the system H2Eh/Eh is ehminated by oxidation to EhO — 

 incapable of using organic compounds as hydrogen donors in photosyn- 

 thesis (instead of water)? One may suggest that the enzyme which 

 catalyzes reaction (6.14) is de-activated simultaneously with the hydro- 

 genase, or that the rate of reaction (6.14) is so slow as to make its com- 

 petition with reaction (6.7a,b) impossible, as long as the "deoxidase" 

 which catalyzes the latter reaction is fully active. (The de-activation 

 of this enzyme was postulated, on page 134, to be the second feature of 

 the adaptation process, supplementing the activation of the hydrogenase.) 

 Reaction (6.15) poses a similar question as to why plants are unable 



