1680 



CHEMICAL PATH OF CARBON DIOXIDE REDUCTION 



CHAP. 36 



In contrast to the compounds listed in table 36. VIII, the reservoirs of 

 malic acid, glutamic acid, and sucrose showed constant growth of activity 

 during photosynthesis over periods of the order of 30 min. (fig. 36.22). 

 These compounds must therefore form large, slowly labelled reservoirs. 



Figure 36.22 shows peculiar changes in the "postphotosynthesis" period, 

 including a sudden "wave" of [PGA] (which reaches a peak in about one 

 minute, and then subsides), and an almost instantaneous depletion of di- 

 phosphate esters. Analysis of the monophosphate area indicated also a 



LIGHT 



TIME (mm.) OF EXPOSURE TO C^*0, 



30 

 DARK 2 



Fig. 36.22. Change in radioactivity of several intermediates in Scene- 

 desmus with time during photosynthesis and after cessation of illumina- 

 tion (after Calvin and Massini 1952). 



depletion of sedoheptulose phosphate, while the reservoirs of tagged hexoses 

 (and of sucrose) increased. The malic acid reservoir dropped in darkness, 

 while that of glutamic acid increased rapidly. The citric acid reservoir, 

 negligible in light, increased suddenly to a low, but measurable steady 

 value (fig. 36.23). 



In another run, the concentration of ribulose diphosphate was much 

 smaller in light, and the increase in PGA, after light had been switched off, 

 was much less pronounced. While the reasons for the difference between 

 the two experiments are unknown, the parallelism of the two changes may 

 be significant (confirming that the C5 sugar is the precursor of PGA). 



If light was switched on again after 10 min. of darkness, the stores of 

 diphosphates, PGA, and malic acid grew again. 



These results bear obvious relation to the manometric (and other) ob- 

 servations of transient gas exchange phenomena at the beginning and at the 



