90 



PHOTOSYNTHESIS 



are made and the glycogen isolated it is found that the hexose 

 units are labelled in the 3- and 4-positions. This suggests 

 that the hexose is formed from a three carbon acid labelled 

 in the carboxyl group by a reversal of the process of 

 glycolysis. In such a scheme a key substance would be 

 3-phosphoglyceric acid labelled in the carboxyl group. 



CH2OPO3H2 



1 

 CHOH 



I 

 *COOH 



*COOH 



I 

 CHOH 



CH2OPO3H2 



3-Phosphoglyceric 

 acid 



CHoOPOgHa 



I 

 CHOH 



*CHO 



*CH20H 



1 

 CO 



I 



CH20P03H2 



Triose 

 phosphates 



FIG. 6.3 



CH2OPO3H2 



I 

 CHOH 



I 

 *CHOH 



I 

 *CHOH 



I 

 CO 



I 



CH20P03H2 



Fructose 

 diphosphate 



While the basic knowledge of COo metabolism has been 

 mainly derived from heterotrophic organisms (animal tissues 

 and micro-organisms) the same processes are presumed to 

 hold in the respiration of green plants. For example Millerd 

 and Bonner (1953) have shown with in vitro preparations 

 from tissues of Phaseolus that all the component acids of the 

 carboxylic cycle can be oxidized. 



In summary, assimilation of CO2 has been shown in the 

 dark with representatives of all the main groups of living 

 organisms including plants. In both plants and animals the 

 carbon is found to appear to some extent in the carbo- 

 hydrates. The isotope experiments show that the carbon is 

 first incorporated in the 3- and 4-positions of the hexose 

 molecule. This suggests that the process of respiration is 

 partly reversible; the assumption being that a two-carbon 

 compound is carboxylated and converted into triose phos- 

 phate via 3-phosphoglyceric acid. 



While a priori there might seem to be no necessity to 

 relate this dark fixation of COg to the process of photo- 



