PHOTOSYNTHESIS 



459 



four carbon atoms (C4) combines with a C3 substance to give 

 sedoheptulose (c^). By combining with glyceraldehyde phos- 

 phate (Cg), sedoheptulose monophosphate gives rise to a phos- 

 phorylated Cio carbohydrate which is broken down by the 

 enzyme transketolase into two phosphopentoses, ribulose 

 monophosphate (Cj) and ribose monophosphate (C5). 



02(905) 



hi/ (quontum) 

 ITOOOA 



polysaccharides 



g (hexose) 



Fig. 38. Proposed cycle for carbon in photosynthesis 



(after Calvin). 



Thus, in this process there takes place the transformation 

 of the various sugars characteristic of the vegetable kingdom 

 into one another. In the last analysis they are all derived 

 from phosphoglyceric acid. However, this cycle of reactions 

 can only run smoothly when there occurs the essential 

 reaction of the reduction of phosphoglyceric acid into glycer- 

 aldehyde phosphate. This requires a continuous supply of 

 reduced pyridine nucleotide and ATP, the former derived 

 from system (5) and the latter from system (6). 



The participation of reduced pyridine nucleotides in the 

 dark fixation of CO2 in heterotrophs has been extensively 

 demonstrated in the researches of S. Ochoa."* The experi- 

 mental material now available shows that hydrogen, which 

 is mobilised photochemically, is transferred to pyridine 

 nucleotides by means of pigments which act as photo- 

 sensitisers. 



