no PHOTOSYNTHESIS 



that of the 3 C decreases proportionately. Also immediately 

 after the light is turned off there is a marked decrease in 

 labelling of the 5 C compound with an increase in the 3 C 

 compound. 



The results with tracer carbon are beginning to give clear 

 indications as to the enzyme systems likely to be concerned 

 in the reduction of carbon dioxide to sugars. There is little 

 evidence that incorporation of carbon dioxide occurs via 

 reversal of the reactions of the Krebs cycle; greater impor- 

 tance is indicated for the enzymes concerned with 5 C 

 rather than 4 C compounds. Calvin and Massini (1952) have 

 suggested that during illumination lipoic acid, a co-factor 

 for oxidative decarboxylation of pyruvic acid, is reduced and 

 made inactive. All the work with radioactive carbon has 

 been confined to a few species of plants, mainly Chlorella, 

 Scenedesmus, and barley. This should perhaps suggest some 

 hesitancy in accepting broad generalizations too readily. 

 Nevertheless tracer carbon has already proved itself a 

 powerful tool. 



ENERGETICS OF THE PROCESSES INVOLVED IN 

 THE REDUCTION OF COg 



The formation of a hexose sugar from carbon dioxide 

 (gas) and water (liquid) by the over-all reactions, 



6CO2+6H2O ^ C6H12O6+6O2, AF°=69o,ooo cals. 



would require a very strong reducing agent. The transfer of 

 24 equivalents of hydrogen is concerned. Each monovalent 

 step therefore requires 28,800 cals. This represents a 

 potential of -0-02 vohs, a potential more reducing than the 

 hydrogen electrode at one atmosphere pressure. In the meta- 

 bolism of carbohydrates in living cells the main pathway for 

 hydrogen transport is through the coenzymes DPN and 

 TPN. Their characteristic potentials are about o-i volt more 

 positive than the hydrogen electrode at a physiological pH 

 (see Table 7.1). Thus they are only to be regarded as effec- 

 tive in hydrogen transfer at potentials less reducing than 

 ordinary hydrogen. So far no coenzyme or redox-catalyst of 

 a potential more reducing than the hydrogen electrode has 



