186 LIGHT, VEGETATION AND CHLOROPHYLL 



the presence of ATP, by manganese salts and a number of 

 diastases — transphosphorylase, dehydrogenase, isomerase and 

 aldolase. The transphosphorylase causes a molecule of phos- 

 phoric acid of the TPN to pass on to the phosphoglycerate : 



phosphoglycerate + ATP — > 



diphosphoglycerate + ADP 



The luminous energy having reduced the DPN to DPNH2, 

 the dehydrogenase makes this hydrogen pass on to the 

 diphosphoglycerate, which loses a molecule of phosphoric acid 

 and becomes phosphoglyceraldehyde. 



The isomerase transforms a part of this substance into 

 dihydroxyacetone phosphate which, by means of another 

 diastase, aldolase, combines molecule to molecule with the 

 phosphoglyceraldehyde to give rise to a molecule of fructose 

 diphosphate. 



Two of the most delicate parts of this complex process of 

 photosynthesis have thus been carried out in the laboratory 

 by means of the diastases which are seen to play an essential 

 part in plant physiology and particularly in photosynthesis. 

 They facihtate the passage of hydrogen, or of electrons, from 

 one substance to another and enable the available energy to 

 be completely utiUzed. 



THE PATH TAKEN BY THE CARBON 



It was thought that carbon dioxide entered living material 

 only by means of photosynthesis and that animals, since they 

 lacked chlorophyll, were incapable of fixing it. 



But, in 1942, Wood and Werkmann discovered that micro- 

 organisms are capable of lengthening the carbon chain of 

 certain acids by fixing on it a molecule of carbon dioxide 

 according to the reaction: 



-COOH+C02+H2-> 



-CO-COOH+H2O 



Thus, tetracarbonate succinic acid changes into penta- 

 carbonate ketoglutaric acid, tricarbonate pyruvic acid becomes 

 tetracarbonate oxalo-acetic acid, etc. 



