98 PHOTOSYNTHESIS 



reversal of the part of the hexose monophosphate cycle con- 

 cerned with CO2 is important in photosynthesis; an early 

 formation of the i -labelled hexose would result as against 

 the 3- and 4-labelling actually found. The transketolase and 

 transaldolase reactions, however, are of fundamental impor- 

 tance in Calvin's interpretation of the formation of phospho- 

 glyceric acid. As these reactions may be considered rever- 

 sible, the following scheme may be set out: 



transaldolase 



Ce + C3 = (C4) + C5- 

 C3 + (C4) = C, 



^ ^^^ ~ ^J^^ ^V transketolase 

 C3 + (C2) = C5 / 



2^^6 T V_-3 = 3^5 



This represents the formation of 3 equivalents of ribose-5- 

 phosphate from two equivalents of hexose phosphate and 

 one of triose phosphate. 



The carboxylation of ribulose diphosphate. The reaction of 

 CO2 with the pentulose 1-5-diphosphate to give two mole- 

 cules of phosphoglyceric acid was first postulated by Calvin 

 for the interpretation of the tracer carbon results. 



H2COPO3H2 H2COPO3H2 



I I 



CO HCOH 



HCOH + CO2 = COOH + COOH 



1 1 



HCOH HCOH 



1 1 



H2COPO3H2 H2COPO3H 



In recent work, developed from the results of Fager 

 (1952), Calvin (1954) has obtained an enzyme preparation 

 from green cell extract which produces phosphoglyceric 

 acid from COg in the presence of ribulose diphosphate. If 

 this enzyme system can be purified and a phosphokinase 

 isolated which will convert ribulose monophosphate into the 

 diphosphate, then it may be that all the enzymes necessary 

 for the formation of carboxyl-labelled phosphoglyceric acid 

 from hexose and labelled CO2 would be known. 



