PHOTOSYNTHESIS 



carbon reduction would be anywhere near as far advanced had 

 either of these tools been missing. In any event, it is now possible 

 to trace the entire path of carbon reduction from the entry of 

 carbon dioxide into the plant cell to the formation of sugars and 

 other end products (4). The essential steps in this process are 

 ( 7) the carboxylation of a sugar phosphate, ribulose diphosphate, 

 to give two three-carbon molecules, both of which are probably 

 phosphoglyceric acid (see discussion later in this section) ; (2) 

 the reduction (with the aid of ATP) of phosphoglyceric acid by 

 reducing agents formed from water by the photochemical re- 

 actions ; and (3) the rearrangement of most of the molecules of 

 reduction product, phosphoglyceraldehyde, to give (with the aid 

 of ATP) more ribulose diphosphate for continued carboxylation, 

 with a smaller part of the sugar phosphates being drained 

 ofT as end products. The individual steps in this process are 

 shown in Figure 1, together with the enzymes believed to be 

 involved. The carbon balance of this system is indicated in the 

 following scheme. 



3C5 + 3CO2 J^^I^ 6PGA (phosphoglycerate) 



6PGA -^^^ 6C3 



6ATP 



2C3 > Ce 



Ce + 2C3 > C5 + C7 



Cv + C3 > 2C5 



Q A.TP 



12 [H] + 3C0.2 — > C3 + 3H2O 



A slightly different version of the sugar rearrangement might 

 be proposed in which different reactions replace those between 

 fructose-6-phosphate and glyceraldehyde phosphate by trans- 

 ketolase to give ribulose-5-phosphate and a four-carbon aldose 

 which then combines with dihydroxyacetone phosphate (aldo- 

 lase?) to give sedoheptulose-l,7-diphosphate. The modified 

 version would postulate that two molecules of fructose-6- 

 phosphate are split and recombined by transketolase and trans- 

 aldolase to give directly sedoheptulose-7-phosphate and ribulose- 



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