EXPERIMENTS WITH HEAVY WATER 



297 



The influence of light intensit}'', and the results obtained in flashing 

 light, point to the influence of heavy water on the rate of a dark catalytic 

 reaction, which limits the rate in strong light, and can be brought to 

 completion during dark intervals between the flashes. (The unchanged 

 temperature coefficient of photosynthesis in heavy water may mean that 

 the substitution of D for H affects the "collision factor" in the rate 

 equation, rather than the activation energy.) 



E 



O 



5 6 



O 4 

 «. 



E 

 E 



o 2 



0.01 0.03 0.05 0.07 



Length of dark period, seconds 



0.09 



Fig. 26. — Yield of photosynthesis of Chlorella in flashing hght per unit total time 

 as a function of the length of the dark intervals, in ordinary water and heavy water 

 (after Pratt and Trelease 1938). 



What catalytic reaction is reduced in rate by the substitution of 

 deuterium oxide remains to be elucidated. Of the three groups of 

 catalj^tic reactions defined on page 172, one — the formation of a complex 

 from an acceptor and free carbon dioxide — is not likely to be affected by 

 this change ; while the other two — the conversion of the primary oxidation 

 product into free oxygen, and the transformation of the primary reduc- 

 tion product into carbohydrate — should be so affected because they are 

 dismutations or oxidation-reductions, and as such probably involve 

 transfers of hydrogen atoms. In the theory of Franck and Herzfeld, 

 it is assumed that, normally, a reaction of the third group limits the 

 rate of photosynthesis in strong light {cf. Vol. II, Chapter 28). It is 

 thus natural to assume that the participation of deuterium in this reac- 

 tion is responsible for the effect of heavy water on the maximum rate of 

 photosynthesis in strong light (unless we assume that some other reac- 

 tion, which is not limiting in ordinary water, is slowed down by heavy 

 water so strongly as to become the limiting one). 



