REACTIONS OF CARBON DIOXIDE III 



been found in the plant. Therefore if it is accepted that the 

 reduction of CO2 is a dark process our present knowledge 

 would indicate that it would have to involve more than the 

 steps corresponding to the direct reaction with four equiva- 

 lents of hydrogen. To suppose the minimum number of 

 steps, necessitates fresh hypotheses concerning the nature of 

 the enzyme systems concerned. A more complex model of 

 CO2 assimilation involving group transfer reactions as well 

 as H transfer requires less hypotheses of a biochemical 

 character than a simple model of direct reduction. All the 

 energy consumed in each process must ultimately come from 

 light when photosynthesis is represented by the over-all 

 process. The quantum of red light concerned in the photo- 

 chemistry of chlorophyll most probably corresponds to 

 41,500 cals. per mol. (the maximum for fluorescence of 

 chlorophyll a). This is considerably greater than the 30,000 

 cals. required for each of the H transfer steps to COg with 

 concurrent production of molecular oxygen. From several 

 lines of argument which have been discussed from a physical 

 standpoint by Franck and others it would appear impossible 

 that the whole of the energy corresponding to each quan- 

 tum is consumed in some corresponding chemical process. 

 The reduction of the coenzymes together with the oxidation 

 of water to O2 at about one atmosphere pressure would 

 correspond to rather more than half the energy of one 

 quantum for each H equivalent transferred. 



From the data collected and referred to in this chapter it 

 would appear that the reduction of one molecule of CO2 to 

 the state of carbohydrate by known dark processes would 

 require a molecule of coenzyme to be reduced and oxidized 

 more than twice which means the total transfer of more than 

 4 H equivalents. Four of these equivalents would be con- 

 cerned with reduction and the remainder with group transfer 

 reactions, which as was seen, enable the reduction stages to 

 be brought with the range of the reduction-oxidation of the 

 coenzymes. Such a hypothetical model is based on the as- 

 sumption that the difference between the dark and the light 

 assimilation of COg is simply one of intensity and not of 

 kind. This assumption could be supported by many of the 



