190 LIGHT, VEGETATION AND CHLOROPHYLL 



Phosphoglycerate by reducing its carboxyl easily changes 

 to a triose (tricarbonate glucoside) which, by uniting two of its 

 molecules, forms a hexose (hexacarbonate glucoside), such as 

 fructose or glucose, which can unite to form saccharose or be 

 polymerized to inulin, starch or cellulose: 



Fhosphoglycerate->tnosQ->hQxosQ->ssLCchsLTOSQ, etc. 

 Phosphopyruvate directs us towards lipides through the 

 intermediary of fatty acids, or towards albumins if the phos- 

 phoric acid is replaced by an amine radical which thus forms 

 alanine : 



Lipides-<- fatty SLcids<- phosphopyruvate^ 

 amino-acids->albumins 



Malic acid, which is one of the essential elements of the 

 citric acid cycle, can serve as a pivot towards the albumins by 

 way of aspartic acid, or, by reduction and fission of its 

 molecule, towards the dicarbonate substance on which carbon 

 dioxide is first fixed, if indeed it is always fixed on the same 

 substance: 



Dicarbonate substances<-m<3//c acid->- 

 aspartic acid->albumins 



The radioactive carbon can provide some further infor- 

 mation. When the assimilation has lasted only a short time, 

 this carbon is not equally distributed among the six hnks of 

 the glucose; more than half of it is found in the third and 

 fourth in the centre. Fixing on the dicarbonate molecules, the 

 radioactive carbon dioxide forms glycerate the carboxyl of 

 which is "labelled." To form glucose, the two tricarbonate 

 molecules unite by their carboxyl and that is why the third 

 and fourth atoms are "labelled" first. But how can the other 

 atoms of carbon be "labelled"? 



It is obvious that here various cycles must intervene through 

 which glucose can be degraded to provide finally these dicar- 

 bonate molecules on which the carbon dioxide will be fixed. 



The malic acid "labelled" after a very brief exposure 

 contains radioactive carbon only in one of its carboxyls; this 

 shows that carbon dioxide is not fixed only on the dicarbonate 



