ENTRY INTO THE BIOSPHERE 



357 



This division into two stages is confirmed by the Hill reaction, i.e. the 

 production of oxygen by isolated chloroplasts in the presence of a hydrogen 

 acceptor such as benzoquinone or 2,6-dichlorophenol. By means of 

 labelled oxygen it has been proved that the oxygen liberated in the Hill 

 reaction is derived from the water. 



Fig. 95. — Racker's reaction scheme showing the formation of a molecule of triosephosphate 

 from three molecules of CO2 in the pentose phosphate cycle acting as a reductive cycle 



during photosynthesis. 



{d) The Reduction of Carbon Dioxide 

 Secotid Stage of Photosynthesis 



Carbon dioxide is first fixed in a carboxyl group, in the general way, 

 the reverse of decarboxylation. This CO2 appears in the carboxyl of 

 3-phosphoglyceric acid. Although the role of 3-phosphoglyceric acid is 

 usually admitted, there is still much discussion as to the nature of the 

 substance which first combines with the COg. 



Calvin and his school believe that ribulose diphosphate is the immediate 

 precursor of 3-phosphoglyceric acid. Thus the photosynthetic cycle 

 would begin by a carboxylation of Ru-PP with formation (C5 + C = 2C3) 

 of two molecules of phosphoglyceric acid (PGA). Starting with PGA, all 



