carbon reduction cycle as PGA or sugar phosphates may be 

 used in fat and protein synthesis. 



It is interesting to consider an extreme case in which 

 all the carbon assimilated by the carbon reduction cycle 

 would be withdrawn from the cycle as PGA, converted to 

 PEPA, and then carboxylated to give four-carbon compounds. 

 In this case, 75 per cent of the assimilated carbon would 

 enter the photosynthetic pathways via the carbon reduction 

 cycle, while the remaining 25 per cent would enter via the 

 carboxylation of PEPA. 



With normal conditions of steady-state growth under 

 high light intensity, the ratios of various fixation pathways 

 must be determined to a large extent by the requirements 

 of the plant for the small molecules from which the protein, 

 carbohydrate, fat, and other substances of the plant are syn- 

 thesized. 



26 



