1638 CHEMICAL PATH OF CARBON DIOXIDE REDUCTION CHAP. 36 



This hypothesis is plausible; however, a period of intense illumination 

 in the absence of external CO2 supply (the pretreatment used in these ex- 

 periments) could lead to extensive decarboxylation not only of the carbon 

 dioxide acceptor in photosynthesis but also of respiratory intermediates. 

 Consequently, the fact that a "C02-gulp" was observed after such a period 

 is not in itself convincing evidence that the C*, taken up in this gulp, 

 enters the photosyiithetic reaction sequence. Calvin and co-workers saw 

 an additional proof of their hypothesis in the distribution of tracer carbon 

 among different compounds. This distribution was qualitatively similar, 

 after the fast dark uptake by preilluminated cells, to that after a short 

 period of photosynthesis in C*02, but quite different after the slow dark 

 uptake without preillumination (c/. fig. 36.2). 



Tables 36. Ill and 36. IV show the fractionation of the tagged material 

 obtained under different conditions. We note (table 36. Ill) thsitwithout pre- 

 illumination a large part of total activity was in ether-soluble organic acids. 

 (Only 5% of total C* was now found in succinic acid — as against 70% re- 

 ported in 1947 — ^16% in malic acid, and 31% in other ether-soluble car- 

 boxylic acids; 31% in amino acids, and only 16% in "anionic groups not 

 extractable with ether.") 



In preilluminated cells, the proportion of tagged carbon present in 

 ether-soluble fatty acids was much smaller. It decreased with the duration 

 of preillumination, while the relative activity of amino acids and of ether- 

 insoluble, anionically tagged substances increased. The distribution was 

 similar to that obtained after brief photosynthesis in C*02 (table 36. IV). 



Tables 36. Ill and 36. IV are thus in agreement with the hypothesis 



Table 36.III 



C(14) Tracer Distribution in Chlorella pyrenoidosa'^ 

 AFTER A Dark Fixation Time of Five Minutes (Calvin and Benson, 1947) 



No 



preillumina- 

 tion Preillumination 

 (total (total fixation, 10 rel. units) 

 fixation, 



Fraction 1 rel. unit) 5 min. 60 min. 120 min. 



I. Carboxylic acids in ether ex- 



tract^ 52% 21% 14% 11% 



Including malic acid'= 16 11.5 7.4 — 



Including succinic acid'' 5.2 3.1 0.5 — 



II. Amino acids'' (adsorbed on cat- 



ion resin) 31 41 64 74 



III. Anionic substances* (adsorbed 



on anion resin) 16 29 21 — 



IV. Sugars (nonionized compounds )-'^. . 0.45 1.0 0.96 1.0 



" One-day-old cultures. '' Rapid, continuous 15-hr. extraction. "^ Separated 



by partition chromatography on silica gel column. '' Eluted from Duolite C-3 resin 



with 2.5 N HCl. " Eluted from Duolite A-3 resin with 1.5 N NaOH. / Effluate 

 from both resins. 



