CHEMICAL KINETICS OF THE HILL REACTION o80 



hibitors for the Hill reaction attack chlorophyll dircctl.y — a conclusion 

 long since established for the whole photosynthetic process. This con- 

 clusion is not, however, trivial, for more and more differences between 

 the two processes are becoming apparent. The latest of these mani- 

 fests itself as a difference of activation energy for the A-/, process. We 

 have found that quantity to be considerably lower than the many 

 values reported for the dark-limiting step of photosynthesis. 



CONCLUSION 



A wide variety of experiments on the kinetics of the Hill reaction 

 has provided large numbers of data, some of which, it must be ad- 

 mitted, may be shown to apply only to the Hill reaction and not to 

 the overall process of photosynthesis. However, Hill reaction data 

 can be obtained under much more thoroughly controlled conditions 

 so that in many cases they are less confused by artifacts than in- 

 formation of the type presently available on whole-cell photosyn- 

 thesis. Not all the essential control \'ariables are known or understood 

 in the Hill reaction of isolated chloroplast fragments, but there appears 

 no reason to doubt that the remaining difficulties, which are largely 

 associated with variables in plant-growth conditions and chloroplast 

 preparative techniques, will be worked out in the near future. In 

 almost every respect, results with the chloroplast Hill reaction have 

 demonstrated simple basic forms which are already well known in 

 small-molecule reactions or in enzyme reactions. As a result it seems 

 probable that most problems in Hill reaction kinetics will be suscepti- 

 ble to explanation on the basis of mechanisms already understood. 

 Certain essential experiments have not yet been successfully com- 

 pleted in our studies, nor do we have sufficient data of the necessary 

 precision on the flashing-light effects, pH, deuterium oxide effects, 

 etc., fully to establish the mechanisms involved. The data and ob- 

 servations summarized in this paper can, however, be partially inte- 

 grated. These interpretations along with complete details of the ex- 

 periments will be published shortly. 



Discussion 



Spikes: The chloroplasts in the system contained about a half milHgram of 

 chlorophyll which would be about half a millimole in 3 ml. The oxidant was ef- 

 lective at concentrations as low as 10 ~* M. 



Lumry : May I say just one thing about the thioctic acid study of the group at 



