Evidence 

 ^<f for the carbon 

 'T^ reduction cycle 



The carbon reduction cycle in essentially the form 

 shown in Figure 2 was mapped during the period between 

 1946 and 1953 (11-17). The experiments, results, and inter- 

 pretations leading to its formulation have been extensively 

 discussed elsewhere (2). They will be briefly reviewed here, 

 not necessarily in chronological order. 



The carbon that enters the plants' metabolism has been 

 followed through the various intermediate compounds by 

 labeling the carbon dioxide with radiocarbon, C^*. The 

 analysis of the labeled compounds has been carried out by 

 paper chromatography and radioautography. The interpre- 

 tation of results leading to the cycle formulation has been 

 based on the kinetics of the appearance of C^* in various 

 identified compounds as a function of time of photosynthesis 

 with C^*02 and other variables. 



The methods are best described by an illustration. Con- 

 sider a simple experiment with a suspension of the algae 

 Chlorella pyrenoidosa, very extensively used in these studies. 

 These green unicellular plants, suspended in water contain- 

 ing the necessary inorganic ions (nitrate, phosphate, etc.) 

 and aerated with a stream of C^-Oo (ordinary carbon dioxide), 



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