KINETIC STUDIES 



1683 



nent to the problem of initial fixation. Figure 36.25 shows that extrapolat- 

 ing the percentage of fixed C* to zero time, gives 75% for PGA and 17% 

 for sugar phosphates, thus leaving 8% unaccounted for by compounds of 

 these two types. This residue was distributed between malic acid (3%), 

 free GA (2%) and phosphopyruvic acid (3%). In the sugar phosphate 

 fraction, no single component shows a trend to become predominant at 

 zero time (c/. fig. 36.21). 



9 PGA 



O GLUCOSE PHOSPHATE 



© SEDOHEPTULOSE PHOSPHATE 



® FRUCTOSE MONOPHOSPHATE 



• DIHYDROXYACETONE PHOSPHATE 



8 10 12 



TIME (SECONDS) 



Fig. 36.26. Tagging of several compounds in the first seconds of e.xposure to C*02 of 

 steadily photosynthesizing Scenedesmus (after Bassham et al. 1954). This figure cor- 

 responds to an enlargement of the left corner of Fig. 36.20. 



Using the estimates of photostationary concentrations given in table 

 36.VIII, the rate of increase in specific activity of the several intermediates 

 could be calculated from curves of the type of those in figs. 36.21 and 36.26. 

 (Figure 36.26 is an enlarged representation of the first seconds of tagging.) 

 For the period between ^ = 2 and t = 10 sec, these rates are 0.3 for glucose 

 monophosphate and 1.0 for PGA, with the values for FMP, DHAP, 

 RDP and SMP falling between these two limits (after division by 2 for 

 PGA and DHAP, and by 3 for SMP, to account for the 2 or 3 equally 

 labelled C* atoms in these compounds). This illustrates well the rapidity 

 with which the C(14) spreads over a multitude of cellular "reservoirs." 



