C*02 FIXATION IN DARKNESS WITH AND WITHOUT PREILLUMINATION 1637 



groups), 15% in amino acids, and 9% in "anionic substances" {i. e., sub- 

 stances whose active component could be adsorbed on anion-exchange res- 

 ins), while in (preilluminated) sample II, the largest fraction of total activ- 

 ity (30%) was found in amino acids (mostly alanine), 25% in an unidenti- 

 fied fraction (described as extractable from water by ether at pH 1), and 

 10% in anionic substances; only small amounts were found in succinic 

 acid (6%), mahc acid (6%), and fumaric acid (1%); and 1.5% in sugars. 



c 



> 



O 



01 



w 



O 

 UJ 

 X 





TIME, min 



Fig. 36.1. Rate of dark fixation of radiocarbon by Scenedesmus. Curve A, 

 one-day-old Scenedesmus cultures after one hour dark incubation in 4% CO2 in 

 nitrogen. Curve B, same cells immediately after 10 min. preillumination (after 

 Calvin and Benson 1947). 



Calvin and Benson (1947) gave the curves (reproduced in fig. 36.1) for 

 the time course of the C* uptake in the dark without preliminary illumina- 

 tion (curve A), and with 10-min. preillumination (curve B). In answer to 

 suggestions that much if not all of the dark C*02 uptake previously reported 

 by them, may have been due to the reversal of decarboxylations in the 

 respiratory system (and thus bear no direct relation to photosynthesis), 

 Calvin and Benson pointed out that figure 36.1 indicates the superposition 

 of two processes: a slow uptake (curve A, and second part of curve B), 

 which admittedly may be due to reversible reactions in the respiratory 

 system, and afast uptake (initial part of curve B), which occurs only after 

 preillumination and which (it was argued) must be related to photosyn- 

 thesis. (This uptake could be considered as radiometric equivalent of the 

 manometi'ically observed fast CO2 "pick-up.") 



