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Birgit Vennesland 



for the Calvin-Benson cycle. The photosynthetic unit of several hundred 

 chlorophyll molecules is regarded as a necessary postulate to provide for the 

 Intensification and transfer of the reducing power by way of the catalytic 

 components of the respiratory chain present in the chloroplasts. 



If this prevailing view is right, there is no such thing as functional acti- 

 vated CO^ in the isolated chloroplasts of leaves, and one must conclude 

 that the experiments demonstrating activated CO_ in Chlorella are incor- 

 rectly interpreted, or that Chlorella and leaves do photosynthesis by funda- 

 mentally different mechanisms. This is the reason that the demonstrated 

 stimulation of the Hill reaction by CO^ has such great theoretical Impor- 

 tance. It is the fact which reconciles the Hill reaction in grana with the 

 behavior of intact Chlorella^^"^^^ 



We think of the Hill reaction as starting with an elimination of O^ from 

 CO*, and proceeding with a complete reoxidatlon of the reduced carbon by 

 the Hill reagent. What we are seeing in the Hill reaction is a modified form 

 of the light-induced respiration. This back reaction is going too far, how- 

 ever. The reduced carbon is reoxidized completely. The mechanism for 

 conserving a portion of the reduced carbon Is not operating in the grana. 



Of the various naturally occurring Hill reagents, TPN is certainly one of 

 the more interesting. Both in Warburg's view and in Arnon's view, the pyri- 

 dine nucleotide plays an important role. But the sequence of events Is pic- 

 tured quite differently. Arnon and many others think of the TPN as the re- 

 agent which Is primarily responsible for transmitting reducing power from 

 grana to the carbon substrates, specifically by way of the triose-phosphate 

 dehydrogenase reaction. Warburg thinks of the reduced carbon (left behind 

 by O^ elimination) as the primary photo product, which then can reduce 

 TPN to TPNH. As evidence for this view I submit the effect of CO- on 

 TPN photoreductlon (Fig. 2) (9, 10). 



The effect of CO„ on O^ evolution with TPN is not difficult to demon- 

 strate. Even Intact spinach chloroplasts may show a stimulatory effect of 

 CO^ amounting to 20%, after a preliminary incubation of one hour. Fig. 2 

 (left) shows the effect obtained by using broken chloroplasts without a pro- 

 longed depletion treatment. A better material demonstrating the CO- effect 

 on TPN photoreductlon is sonerated Chlorel la(9). Fig. 2 (right) shows two 

 sets of measurements which I made In Berlin with this material. The lower 

 part of the graph shows a repetition of an experiment which Warburg and 



