CHLOROPHYLL AND ENERGY 191 



molecules, but also on tricarbonate molecules, with a strong 

 preference for the latter when the Hght diminishes. It is true 

 that in these conditions the relatively stronger respiration 

 provides a large quantity of pyruvic acid. Nevertheless, malic 

 acid is not a necessary intermediary in photosynthesis, for if 

 it is prevented from forming, glucosides are still synthesized; 

 it is a means of storage. 



This leaves unidentified the dicarbonate substance which 

 is found on the normal path of photosynthesis, or at least 

 on that of the rapid cycle which furnishes by the scission of a 

 tetracarbonate or hexacarbonate molecule dicarbonate mole- 

 cules already "labelled" by C^^. Ochoa suggests that it could 

 well be glycolic acid, CHgOH-COOH. Whatever it may be, 

 this scission must give symmetrical elements, since the second 

 and fifth atoms of carbon of the glucose are ordinarily of the 

 same activity as the first and sixth. A cycle which would 

 fulfil these conditions is given on page 192. 



It is therefore possible, at least provisionally, to form a 

 fairly accurate, if not complete, idea of the cycle followed by 

 the carbon in the course of photosynthesis. In the presence of 

 hydrogen from the water decomposed by solar energy, carbon 

 dioxide is fixed on a dicarbonate molecule to give, with 

 phosphoric acid, phosphopyruvic acid, unless, the solar energy 

 being lower and the respiration stronger, pyruvic acid is 

 predominant and directs the synthesis towards maUc acid as a 

 result of the abundance of tricarbonate molecules. 



Phosphoglyceric acid and phosphopyruvic acid, which can 

 easily be changed into each other, constitute the pivot of 

 photosynthesis. Three directions are possible: one towards 

 fatty acids and lipides from phosphopyruvic acid ; the second, 

 through the intervention of nitrogen, towards amino-acids and 

 albumins from phosphopyruvic acid or mafic acid; finally, 

 the third towards the oses — trioses and hexoses such as 

 fructose and glucose from phosphoglyceric acid. And the 

 cycle is closed by the formation of dicarbonate molecules 

 from malic acid or hexoses. 



Thus the essential lines of photosynthesis become per- 



