1684 



CHEMICAL PATH OF CARBON DIOXIDE REDUCTION 



CHAP. 36 



In one set of experiments, the volumes of the several C* reservoirs 

 during steady photosynthesis in 1% CO2 were compared with those after a 

 sudden decrease of carbon dioxide pressure to 3 X 10~^ %. Figure 36.27 

 shows the result (which is opposite of that caused by darkening, cf. Fig. 

 36.22). The RDP reservoir increased sharply, reached a peak, and then 

 settled to a new, higher steady level; the PGA reservoir, to the contrary, 

 decreased, within 2 min. to a much lower steady level. The change in 



20 



O 



> 



UJ 



10 



UJ 

 IT 



-1 1 1 r- 



1% COj 



T (^ o 



r' PGA 



RIBULOSE Dl© 

 AREA 



^^00 



} J =200" 



-100 



SCENEDESMUS 6* C 



0.003% CO2 



300 



TIME IN SECONDS 



Fig. 36.27. Effect of a sudden decline in CO2 concentration from 1 to 3 X 10~'% on the 

 volume of tagged reservoirs in Scenedesmus (after Bassham et al. 1954). 



steady levels is in agreement with the concept that PGA is a product of 

 carboxylation of RDP; but the transient peak of [RDP] requires interpre- 

 tation (as do all the peaks and troughs observed in the transitional states). 



10. The Sequence of Sugars 



In chapter 3 (p. 44) we discussed the evidence concerning the "first 

 sugar" derived from chemical analysis of the products of photosynthesis. 

 Some of this evidence favored the disaccharide, sucrose, as a precursor of 

 free monosaccharides, glucose and fructose; other observations contra- 

 dicted this hypothesis. I^owing now from tracer studies how brief must 



