456 



J. A. BASSHAM, M. KIRK 



experimental error, (b) During the period between lo and 30 sec exposure to "COj, 

 the appearance of ^^C in individual compounds which can be isolated by our methods 

 of paper chromatography, is equal to at least 85 % of the rate of total uptake, (c) If 

 there is a pool of COj, HCOJ or other unstable or volatile compound lying between 

 administered COj and stable compounds in the fixation pathway, its amount is not 

 more than i.o to 1.5 /xmoles (4 to 6 sec fixation) and it is essentially saturated after 

 10 sec. 



Let us next consider the question of how much of this fixed ^*C must pass through 

 the PGA pool. 



In Fig. 6 are shown the labeling curves of some of the more rapidly labeled 

 compounds and groups of compounds. By 3 min, compounds of the carbon reduction 

 cycle are essentially saturated with radiocarbon. Secondary intermediates such as 

 sucrose, malic acid, and several amino acids are not saturated until longer times 

 (5 to 30 min). In order to evaluate the importance of the fixation pathway leading 

 through PGA, we have tabulated the actual measurements of "C found in compounds 



Fig. 6. Appearance of '*C in PGA and sugar phosphates in Chlorella pyrenoidosa vs. time of photo- 

 synthesis with '^COj. 



during the first minute (Table V). The "C found in all those compounds derived 

 from PGA without further carboxylation (see Fig. i) is added together (Tj). Com- 

 pounds labeled by C3-C1 carboxylation are totaled separately (Tj). Since three of 

 the carbon atoms in these compounds are derived from PGA, their total radioactivity 

 is multiplied by a factor which is 3/4 times the degree of saturation of the PGA, 

 which is presumed to be the same as that of their immediate precursor, namely, PEPA. 

 (The saturation curves for PGA and PEPA are in fact very similar in this and other 

 experiments.) The sum of Tj and Tjf, representing measured '^C derived from the 

 primary reaction which forms PGA, is plotted in Fig. 7. Again the "pool" of HCO3 

 or other volatile or unstable compound is about i /^mole and in this case it must 

 precede PGA in the chain of reactions. Where one draws the curve of maximum 

 slope through these points is somewhat arbitrary, but the maximum rate of ap- 

 pearance of '^C in these compounds falls somewhere between 11 and 13 /xmoles/min. 

 Thus on the basis of the appearance of "C in these extractable, stable compounds 

 alone, at least 70 to 85 % of all carbon fixed during photosynthesis (measured ex- 

 ternally) is incorporated via the carbon reduction cycle. It must be emphasized 



112 



