1632 CHEMICAL PATH OF CARBON DIOXIDE REDUCTION CHAP. 36 



Similar results were obtained by Calvin and Benson (1948). After one hour of 

 photosynthesis, tagged glucose contained 61% of its C* in positions 3 and 4, 24% in posi- 

 tions 2 and 5, and 15% in positions 1 and 6; after 2 hours of photosynthesis, the distri- 

 bution was 37, 36, 27%— obviously approaching equilibrium. A similar spread of 

 C(14) from position 1 (carboxyl) to positions 2 and 3 could be observed in alanine. 



Gibbs (1949) reported results quite different from those of Calvin and Benson. 

 After 1 hour photosynthesis in C(14)-labelled CO2, he found glucose to be labelled prefer- 

 entially in positions 1 and 6. 



All these early results appear to be in some contradiction to the more recent deg- 

 radation experiments in Berkeley, where all preferential tagging (up to 90% C* in glu- 

 cose in positions 3 and 4 after 30 seconds exposure, cf. table 36. VII) was found to yield 

 to practically uniform tagging in a matter of minutes, not hours. 



In the first Russian experiments with C(14) as tracer, reported by Nez- 

 govorova (1951, 1952^, long exposures to C*02 in light (of the order of one 

 hour) were used, and the tracer was found largely in proteins. Doman, 

 Kuzin, Mamul and Khudjakova (1952) went over to exposures of 1-2 

 seconds, following them by 0-300 seconds of photosynthesis in nontagged 

 carbon dioxide. They used young leaves of 17 species, and obtained, by a 

 ionophoretic method of fractionation, a large variety of results. For ex- 

 ample, Phaseolus was found to fix C*, after 1 second exposure, only in 

 anionic substances; with other plants, it was found also in cathionic sub- 

 stances, with still others, in neutral substances. 



Nezgovorova (1952^) observed the dark uptake of C*02 by leaves, and found no ef- 

 fect of preillumination. 



The insoluble tagged compounds synthesized by barley in 5-minutes 

 exposure to C*02 are 95% polysaccharides; in Scenedesmus, they are 50% 

 polysaccharides and 50% proteins, the latter containing mainly tagged 

 alanine and aspartic acid (Calvin et al. 1951). 



This result may have some bearing on the observations of Smith et al. 

 (Vol. I, page 36) that the product of photosynthesis in sunflower is almost 

 100% carbohydrates, and on the failure of other observers to obtain similar 

 simple results with other plants, particularly with algae. 



The second complication is due to the fact that, as we now know, there 

 occurs (in dark as well as in light), in addition to C* assimilation by photo- 

 synthesis, also a C* uptake by exchange of C*02 with the carboxyl groups of 

 certain respiration (and fermentation) intermediates. 



We mentioned in Vol. I (page 208) that, since 1936, many observations 

 had revealed the capacity of bacteria (and of certain animal tissues) to 

 assimilate carbon dioxide in the dark. This fixation often can be attributed 

 to the reversibility of certain respiratory decarboxylations, such as: 



(36.1) COOHCH2COCOOH (oxalacetate) ;^=i 



CH3COCOOH + CO2 (pyruvate + carbon dioxide) 



