154 



PROBLEMS OF PHOTOSYNTHESIS 



SUCROSE 



STARCH 



RIBULOSE-1,5- _ 

 DIPHOSPHATE "*" 



-CO, LABILE 



» INTERMEDIARY 

 COMPOUND 



FRUCTOSE-1,6- 

 ■ DIPHOSPHATE 



HAMAMELONIC 

 ACID 



OXALOACETIC 

 ACID 



^CO, 



3-PHOSPHOGLYCERIC 

 ACID 



PHOSPHO-ENOL- PYRUVIC 

 ACID 



ASPARAGINIC 

 ACID 



MALIC 

 ACID 



ALANINE 



TRICARBOXYLIC 

 ACID CYCLE 



Fig. 58. Kandler's scheme. Separation of photosynthesis and respiration. 



for the loss of inorganic phosphate due to the conversion of fructose- 1,6-di- 

 phosphate to saccharides. The ATP turnover in the hght at 40-fold over- 

 compensation is thus not increased 20-fold but remains practically constant. 



In the light, the labile intermediate compound is reduced, via hamamelonic 

 acid, to fructose-l,6-diphosphate when sufficient reducing power is avail- 

 able. If not, the labile compound is slowly split to 3-phosphoglyceric acid 

 (respiration). Kandler compares this clear separation of photosynthesis and 

 respiration with Warburg's induced respiration. It is difficult however to 

 find anything common to the mechanisms. Thus, according to Kandler, 

 the production of 3-phosphogiyceric acid from ribulose-l,5-diphosphate is 

 a step in the direction not of photosynthesis, but of respiration. The decisive, 

 photochemical reduction mechanism in the direction of photosynthesis starts 

 with the conversion of the labile compound to hamamelonic acid. 



Bassham et al. (8) found, after extrapolation to very small times, the ac- 

 tivity of 3-phosphoglyceric acid to be only 75% of the theoretical value of 

 100%. The sugar phosphates showed an activity not of 0% but of about 

 17%. These figures justify the assumption that 3-phosphogiyceric acid and 

 sugar phosphates are simultaneously produced from a common, not yet iden- 

 tified precursor substance. There are two possible explanations for this devi- 

 ation from the theoretical values of 100% 3-phosphoglyceric acid and 0% 

 sugar phosphates. Either the alcohol added to the extracts stops carboxyla- 

 tion more rapidly than reduction, or the enzymatic processes occurring after 

 the production of 3-phosphoglyceric acid are much faster than the formation 

 of 3-phosphoglyceric acid itself. The latter explanation would imply the 

 dependence of the activity distribution upon the interactions of substrates and 

 enzymes, so that it would be independent of the sequences in the carbon cycle. 



