C*02 FIXATION IN DARKNESS WITH AND WITHOUT PREILLUMINATION 1639 



that the two kinetically different mechanisms of dark C* uptake lead to 

 different tagged compounds. The slow mechanism, which is the only one 

 operative in starved, not preilhiminated cells, produces predominantly 

 tagged fatty acids (whether succinic, mahc, or others, is a secondary ques- 

 tion) and also some tagged amino acids. The fast mechanism, active 

 after preiUumination in absence of carbon dioxide, leads to large amounts of 

 tagged anions of ether-insoluble organic acids (e. g., glyceric acid or other 

 polyhydroxy acids which are much more hydrophilic than simple fatty 

 acids, such as succinic, and the monohydroxy acids, such as malic acid). 



Table 36.1V 



C(14) Tracer Distribution in Chlorella and Scenedesmus after Exposure to 0*0^ 



IN Dark and in Light (Calvin and Benson, 1947) 



C* fixation in darkness C* fixation in light<= 



Chlorella'^ Scenedesmus b Chlorella" Scenedesmusb 



PreiUumination time 60 min. 10 min.-'^ — — 



Fixation time 1 min. 1 min. 30 sec. 30 sec. 



Total C" fixed (millions c.p.m.) 0.97 0.98 3.1 6.2 



Proportion of C(14) in: 



I. Ether-extractable acids 13% 12% 2.5% 10% 



II. Cationic groups (amino acids). . 53% 39% 14% 11% 



III. A. Anionic groups, ammonia 



elutable'' 2.4% 4.2% 38% 44% 



III. B. Anionic groups, not am- 



monia elutable' 31% 42% 36% 27% 



IV. Nonionic compounds (sugars). . 0.3% 0.1% 4.5% 4.7% 



" Chlorella pyrenoidosa, one day old. '' Scenedesmus D-3, two day old. "= Cells, 

 rapidly photosynthesizing, given HC*Oi' and shaken, then killed. '' Adsorbed on 

 Duolite A-3, eluted with 1.5 A^ NH4OH. « Eluted with NaOH following ammonia 

 elution. ^ "Time needed to ensure maximum C*02 uptake" (Calvin and Benson). 



These conclusions are confirmed by Figure 36.2, taken from a subsequent 

 paper by Benson, Bassham et al. (1950). It shows the distribution of 

 tagged compounds obtained in the dark (with and without preiUumination), 

 and in brief photosynthesis, as revealed by paper chromatography and 

 radioautography. We note the prevalence of C3 and C4 acids in the first 

 radiogram (dark uptake without preiUumination), and the absence in it of 

 tagged phosphate esters. The other two radiograms show the prevalence 

 of phosphate esters of glyceric acid (PGA), of hexoses (HMP and HDP), 

 and of pyruvic acid (PPA) ; they also have in common the appearance of 

 tagged sucrose and of large amounts of tagged alanine. Malic acid and 

 alanine contain considerable quantities of tracer carbon after all three tag- 

 ging procedures. 



As mentioned above, Calvin and co-workers considered the similarity 



