CHAPTER 6 



MECHANISMS OF THE FIXATION AND 

 REDUCTION OF CARBON DIOXIDE 



HISTORICAL 



During the earlier studies of photosynthesis carbon dioxide 

 had been considered as a relatively unreactive molecule. 

 From the purely chemical standpoint there were, for ex- 

 ample, three well-known reactions: the formation of carb- 

 oxylic acids from phenols (which proceeds under relatively 

 mild conditions with phloroglucinol), the reduction to form- 

 aldehyde by magnesium metal in acid solution (Fenton), and 

 the ready reaction of certain organic derivatives of magnesium 

 (Grignard reagents) with carbon dioxide to give a carboxylic 

 acid. Such chemical knowledge combined with the fact 

 established by Willstatter that chlorophyll was an organic 

 compound of magnesium gave plausibility to the view that 

 the first attack on the CO2 molecule by the green plant was 

 through a photochemical reaction directly with chlorophyll. 

 Although Willstatter and StoU were unable to find with 

 pure chlorophyll preparations evidence of any specific 

 photochemical reactions with COg the idea of an intimate 

 relation between chlorophyll and CO2 in the plant held an 

 important place for many years. At this moment there is a 

 tendency to the extreme opposite view, namely that chloro- 

 phyll and the light reaction have no direct relation to the 

 uptake of CO2 in photosynthesis. The main reasons for this 

 change of outlook are, firstly, that the photochemical step 

 in photosynthesis is to be regarded as a process of hydrogen 

 transfer in which a molecule such as water participates, 

 the reduction of carbon dioxide being an indirect process; 

 and, secondly, that carbon dioxide must be regarded as 

 a normal metabolite for perhaps the majority of living 

 organisms. 



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