1642 



CHEMICAL PATH OF CARBON DIOXIDE REDUCTION 



CHAP. 36 



between the C* distribution obtained by the "fast" dark fixation after 

 preilkimination, and that obtained by fixation in light, as argument for the 

 assumption of a "photosynthetic" rather than "respiratory" nature of the 

 initial dark C*02 fixation by preilluminated cells. They further assumed 

 that the occurrence of "photosynthetic" carbon fixation in preilluminated 

 algae means that illumination leaves in the cells, in addition to a carbon 

 dioxide acceptor, also a strong reductant ("reducing power") capable of 

 reducing carbon dioxide in the dark; this reductant survives for a period 

 of several minutes. An alternative h3T)othesis (Gaffron ei at., cf. section 4) 



DARK 



O 



* 



o 



30 60 30 30 30 

 TIME, min 



20 20 



TIME, min. 



(a) (b) 



Fig. 36.3. Effect of duration of preillumination. (a) On five min. dark C*02 fixation 

 by Chlorella; (b) on one min. dark fixation in Scenedesmus (after Calvin et al. 1948). 



is that preillumination leads merely to the accumulation of a carbon dioxide 

 acceptor, capable of being carboxylated in the dark, mthout reduction, 

 forming phosphoglyceric acid or substances of a similar reduction level. 

 The presence in Calvin's radiograms of preilluminated cells of tagged su- 

 crose and hexose phosphates (fig. 36.2(5)) cannot be explained in this way; 

 but Gaffron et al. found practically nothing but phosphoglyceric and 

 phosphopyruvic acids among the tagged compounds produced by dark 

 fixation after preillumination. This disagreement remains to be resolved. 

 Calvin and Benson (1947) measured C*02 fixation by preilluminated 

 algae, as function of the duration of preillumination. The cells (in a thin, 

 lollipop-shaped vessel in C02-free atmosphere) were illuminated from two 

 sides with 17 klux, then dropped into a darkened HC^Os" solution, and 

 killed 1 or 5 min. later. The resulting curves (fig. 36.3) indicate gradual 

 saturation of cells with a C*02 acceptor (and a reductant?) in light, and 

 their disappearance in the dark. Calvin and Benson stated that the build- 

 up was 80-100% complete after 20 seconds of preillumination, but figure 

 36.3a indicates that in Chlorella, at least, it may require as much as one 

 hour. In Scenedesmus (fig. 36.3b), saturation was reached sooner, but 



