1648 



CHEMICAL PATH OF CARBON DIOXIDE REDUCTION 



CHAP. 36 



made only a few measurements of the C*02 uptake in darkness, and studied 

 mainly the tracer uptake in brief periods of illumination. 



The C* fixation in the dark was found by them to be fastest in the first 

 hour of exposure to C*02, and to settle to an approximately constant rate 

 (0.01 to 0.02 cc./C*02, per cc. of cells per hour), after the second hour (fig. 

 36.5). No significant disappearance of carbon dioxide from the medium 

 could be observed during this period: C* uptake appeared therefore to be 

 due entirely to an exchange of CO2 and C*02 (Fig. 36.5 is to be compared 

 with Calvin's Fig. 36.1). 



100 



10 15 



TIME, min. 



Fig. 36.7. Time change of tracer dis- 

 tribution between fractions (A), (B), 

 and (C) during photosynthesis (after 

 Brown, Fager, and Gaffron 1949). 



o 



DARK 

 WITH C02 



LIGHT 

 WITHOUT COg 



20 



40 

 TIME, min 



60 



80 



Fig. 36.8. Effect of light and CO2 on 

 transfer of tracer from fraction (B) 

 (after Brown, Fager, and Gaffron 

 1949). 



Under anaerobic conditions, the C*02 fixation by Scenedesmus in the dark 

 was only about one-half of that in air, indicating that the capacity for re- 

 versible carboxylation is smaller for the intermediates of anaerobic than 

 for those of aerobic metabolism. Furthermore, the tagged products of 

 anaerobic fixation were found only in the fraction "B" (soluble in aqueous 

 alcohol, insoluble in benzene), while the products of aerobic fixation were 

 scattered over all three fractions (A, B and C) into which the cell material 

 was divided {cf. below). It seems likely (cf. fig. 36.6) that under aerobic 

 conditions, too, all C* was initially taken up in fraction B, but passed 

 rapidly into the other fractions (while under anaerobic conditions it stayed 

 in fraction B). If, after a period of dark fixation, C*02 was removed from 

 the medium, the tracer content of fraction B declined, confirming that this 

 fraction contained carbon in a reversibly bound form (e. g., that of a dissoci- 

 able carboxyl group). The transfer of C* from the water-soluble fraction B 



