^g^ Balance among 

 "^!^ synthetic pathways 



We have seen that in each complete turn of the carbon- 

 reduction cycle 3 molecules of RuDP (15 carbon atoms) are 

 carboxylated by 3 molecules of CO2 to give 6 three-carbon 

 compounds (18 carbon atoms). Thus there is a net gain of 3 

 reduced carbon atoms. These atoms are withdrawn from the 

 cycle for further synthesis. They may be withdrawn from 

 the cycle as PGA or as any of the sugar phosphates in the 

 cycle. Before the photosynthetic reactions had been mapped, 

 it was commonly believed that photosynthesis leads first to 

 carbohydrates only and that these carbohydrates are then 

 converted via nonphotosynthetic reactions to other com- 

 pounds such as amino acids and fatty acids. We now know 

 that pathways leading from the carbon reduction cycle to 

 amino acids and fatty acids and other substances can be just 

 as important quantitatively as those leading to carbohydrates. 

 This is particularly true in a unicellular algae, as exempli- 

 fied by Chlorella pyrenoidosa, where under some conditions 

 less than half of the assimilated carbon is directly converted 

 into carbohydrate. This carbohydrate synthesis draws its 

 carbon from the cycle in the form of sugar phosphates. Con- 

 sequently, more than half of the carbon drained from the 



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