142 PROBLEMS OF PHOTOSYNTHESIS 



2 C3 —*■ Ce 

 Ce + 2 C3 -^ C5 + C7 



Cy -\- C3 —*■ 2 C5 



9 ATP 

 12 H + 3 CO2 ^ C3 + 3 HoO 



According to Figure 50, the following reactions must also be considered 



Ce + C3 — »- C4 + C5 

 C4 -|- C3 — *■ C7 



§ 54 Is Photosynthesis the Reversal of Respiration? 



The main pathway of respiration is constituted by the sequence glycoly- 

 sis, tricarboxylic acid cycle and biological oxidation. Ochoa (43) assumed 

 that in photosynthesis the tricarboxylic acid cycle may proceed in the oppo- 

 site direction. The reductive carboxylation of a-ketoglutaric acid to iso- 

 citric acid, which is assumed to occur in photosynthesis, is indeed the rever- 

 sal of the oxidative decarboxylation of /50-citric acid to a-ketoglutaric acid 

 occurring in respiration. The same could be said for the reductive carboxyl- 

 ation of pyruvic acid and the oxidative decarboxylation of malic acid (44, 45), 

 all the reactions of the tricarboxylic acid cycle being reversible. With the 

 exception of the conversion of glucose to glucose-6-phosphate, all the re- 

 actions of glycolysis are also reversible. 



Ochoa and Vishniac (46) reduced 3-phosphoglyceric acid to fructose-1,6- 

 diphosphate upon illumination in vivo. This process goes as follows: 



light 



1. 2 DPN+ + 2 H2O ^ 2 DPNH + 2 H+ + O2 



Mg ions 



2. 2 3-phosphoglyceric acid + 2 ATP ^ * 



2 1,3-diphosphoglyceric acid + 2 ADP 



3. 2 1,3-diphosphoglyceric acid + 2 DPNH + 2H+ ^ 



2 3-phosphoglyceraldehyde + 2 DPN+ + 2 ph 



4. 2 3-phosphoglyceraldehyde ;i^ fructose-l,6-diphosphate 



In these experiments the chloroplasts were suspended in a solution of 3- 

 phosphoglyceric acid containing ATP, DPN+, Mg ions and the necessary 

 enzymes. Only reaction 1 is photochemical. No fructose- 1,6-diphosphate 

 was obtained without illumination. The over-all reaction 



light Mg ions DPN+ 



2 H2O + 2 3-phosphoglyceric acid + 2 ATP ^ 



fructose-l,6-diphosphate + 2 ADP + 2 ph + O2 



is the reversal of the degradation reaction of hexose phosphate to triose phos- 

 phate, in which reaction ATP is produced and DPN+ reduced (see § 44). 

 It was assumed that in the photochemical reaction light energy produces 

 water photolysis. ATP serves as an additional energy source. 



The investigations with labeled CO2 carried out by Calvin (§ 53), Kandler 



