1644 CHEMICAL PATH OF CARBON DIOXIDE REDUCTION CHAP. 36 



photosynthesis when external carbon dioxide pressure drops below 1.0 mm. — a phe- 

 nomenon which, however, may be due only to slow penetration of carbon dioxide into the 

 cells, cf. chapter 27, section A7.) 



Calvin and Benson also noted that the "carbon dioxide curves" of dark fixation in 

 preilluminated cells (fig. 36.4) are similar to those of photosynthesis {cf., for example, 

 fig. 27.4), and asserted — without further elaboration— that this similarity can best 

 be explained by the assumption that fixation is caused by the formation, during the pre- 

 illumination period and subsequent survival in the dark, of both a reductant and a car- 

 bon dioxide acceptor. 



Stepka, Benson, and Calvin (1948) found by paper chromatography that in the ma- 

 terial from preilluminated Scenedesmus, exposed to C*02 in the dark, the predominant 

 tagged amino acid was aspartic acid, while Chlorella yielded mainly tagged alanine. 

 The most abundant — but entirely untagged— amino acid was, in both cases, glutamic 

 acid. 



The question of the "surviving reductant" is important for the interpre- 

 tation of the primary photochemical process in photosynthesis. It was 

 mentioned in chapter 28 (p. 941) that Franck considered the effect of [CO2] 

 on chlorophyll fluorescence as proof of a direct association of the oxidant 

 (be it CO2 itself, or a carboxylation product such as PGA) with the sensi- 

 tizing pigment. Other arguments — e. g., the nonphotochemical reduction 

 of carbon dioxide by chemosynthetic organisms — have been adduced in 

 favor of the reduction process being coupled to the primary photochemical 

 process by the intermediary of a kinetically independent "primary reduc- 

 tion product" (designated by X in many of our schemes); X could be the 

 same in true photosynthesis, Hill reaction, bacterial photosynthesis, and 

 perhaps even chemosynthesis. (Reduced pyridine nucleotide is one such 

 possible mediator between the photochemical apparatus and the enzymatic 

 reduction system.) 



The second view is the more generally accepted one at the present time ; 

 and if the observations of Benson, Calvin et al. of the survival of the reduc- 

 ing power in preilluminated cells could be definitely confirmed, a strong 

 argument would be won against Franck's hypothesis. 



Apart, however, from the uncertain "survival of the reductant," none 

 of the tracer observations is altogether irreconcilable with the hypothesis 

 that the reduction of carbon dioxide, from the formation of the acceptor to 

 triose synthesis, or even beyond this stage, takes place in molecular associa- 

 tion with chlorophyll, so that H atoms for reduction can be supplied to the 

 acceptor (phosphoglyceric acid) directly by the photochemical process. 



On p. 1615 we reported the divergeiit findings of Mehler and of Kras- 

 novsky concerning the survival of a reductant in preilluminated chloro- 

 plast suspensions. 



3. C*02 Fixation in Light: Phosphoglyceric Acid as First Intermediate 



More conclusive proved the observations which Calvin and Benson 

 (1947) made on the tracer distribution in cell material obtained by ex- 



