636 



J. A. Bassham 



TPP 



Fig. 1. The photosynthetic carbon reduction cycle (PSCR cycle) . Abbreviations 

 include: FDP, fructose-l,6-diphosphate; SDP, sedoheptulose-l,7-diphosphate; 

 RuDP, ribulose-l,5-diphosphate; TPP, thiamine pyrophosphate; E, E' , unspecified 

 enzymes or proteins. The E'(FeSH)2 symbolizes a reduced ferredoxin-type 

 electron carrier with a potential of -0.4 v. The asterisks denote the order of 

 labeling (not the accurate magnitude) of various carbon atoms of sugar phos- 

 phates, and other intermediates, following a short period (such as 10 sec) of 

 photosynthesis with 1^002 . Depending on pool sizes, particularly that of triose 

 phosphates, asymmetry of labeling in hexose and heptose phosphates may be 

 more or less than shown. 



