1654 CHEMICAL PATH OF CARBON DIOXIDE REDUCTION CHAP. 36 



trated into the reduction products of phosphoglyceric acid. When preillu- 

 minated cells were tagged for 60 sec. in the dark, freed from C*02 by washing, 

 and then illuminated for 40 sec, the total amount of C* in the cells was 

 found to be the same as before washing, but only 35% of the original quan- 

 tity of phosphoglyceric acid was found; this, too, indicated that, in light, 

 phosphoglyceric acid underwent further transformation (reduction) . When 

 cells preilluminated in absence of CO2 and then exposed to CO2 in light for 

 10 sec. were left in the dark in C*02 for 60 sec. before killing, an additional 

 tracer uptake was observed, again concentrated practically exclusively in 

 PGA and PA (bar C). 



These results are in disagreement with the paper chromatograms ob- 

 tained at Berkeley (see, e. g., fig. 36.2b), which indicated the formation 

 in preilluminated algae of tagged products (such as sucrose) that could 

 not be obtained from phosphoglyceric acid without further reduction. 



In the Berkeley and Chicago experiments, preillumination was carried 

 out in an atmosphere of N2, H2, or He2, free of CO2 or O2 (the latter to avoid 

 C02-production by respiration). Chlorella was used in Berkeley, Scenedes- 

 mus in Chicago; but it is unlikely that the survival of the "reducing power" 

 after illumination should occur only in certain species. 



Gaffron, Fager, and Rosenberg (1951) described also the "pick-up" of 

 C*02 carbon after a period of illumination in abundant carbon dioxide. It 

 differs, in several respects, from the C*02 uptake after preillumination with- 

 out CO2. Fig. 36.12 illustrates the findings. Bar a indicates the residual 

 "dark fixation" as it occurs after the effect of preillumination has worn out 

 (z. e., after 2 minutes in darkness) ; bar 6 shows the much larger C* fixation 

 after a period of photosynthesis in ordinary CO2; bars e and / show the 

 fixation during 10 and 20 sec. photosynthesis in C*02, respectively; bars 

 d, e, and g, the "pick-up" of C* after these pretreatments. It is seen that 

 the pick-up is completely developed after 10 sec. photosynthesis in satu- 

 rating light, and is then equivalent to C02-consumptionin 10 sec. of photo- 

 synthesis. The pick-up is completed in less than 10 sec. (while the dark 

 C*02 uptake after illumination in C02-free medium requires several min- 

 utes for completion according to both the Berkeley and the Chicago meas- 

 urements). 



The much larger shaded area in d, compared to that in h, could perhaps 

 be taken as an indication of slow penetration of C*02 to the site of the pick- 

 up. An alternative explanation, suggested by Gaffron et al., is that after 

 photosynthesis with ample CO2, all acceptor is present in the cells in the 

 form of a loose complex (corresponding to A.CO2 in Franck and Herzf eld's 

 theory) . If the last ten seconds of illumination had been carried out in tagged 

 C*02, this loose complex is tagged, and its transformation into the stable 

 complex, ACO2 (which is supposed to follow in the dark), will be registered 

 as a "C*02 pick-up." (It being assumed that without time allowance for 

 a dark stabilization period, the loose complex A.C*02 will dissociate when 



