is reduced to triose phosphate. If the carboxylation product 

 is reductively split (dashed line in Figure 2) the requirement 

 for TPNH would probably be the same, that is, 6 molecules 

 per complete turn of the cycle. In this case, however, the 

 cycle might require either 9 molecules of ATP or only 6. 



Figure 2. Carbon reduction pathways in photosynthesis. Com- 

 pounds: (1) 2-carboxy-3-keto-l,5-diphosphoribitol, (2) 3-phospho- 

 glyceric acid (3-PGA), (3) glyceraldehyde-3-phosphate, (4) dihy- 

 droxyacetone phosphate, (5) fructose- 1,6-diphosphate, (6) ery- 

 throse-4-phosphate, (7) sedoheptulose-l,7-diphosphate, (8) xylu- 

 lose-5-phosphate, (9) ribose-5-phosphate, (10) ribuIose-5-phosphate, 

 (11) ribulose-l,5-diphosphate, (12) 2-phosphoglyceric acid (2- 

 PGA), (13) phosphoenolpyruvic acid (PEPA), (14) oxalacetic acid. 

 _@: fructose diphosphate and sedoheptulose diphosphate lose 

 one phosphate group before transketolase reaction occurs. 



11 



