140 



PROBLEMS OF PHOTOSYNTHESIS 



glyceraldehyde joins with the CHoOH group of phosphodihydroxyacetone. 

 Therefore, the labeled C atoms in fructose-l,6-diphosphate should be sym- 

 metrically distributed (see § 57). 



Further research (18) showed, however, that ribulose-1, 5-diphosphate was 

 a precursor of 3-phosphoglyceric acid. This pentose diphosphate is car- 

 boxylated, producing 3-phosphoglyceric acid. 



A labile ketoacid is intermediately produced (40). With this carboxylation 

 reaction proceeding much faster than the carboxylation reactions discussed in 

 § 46, the cycle represented in Figure 50 is started. The ribulose-1, 5-diphos- 

 phate is regenerated, according to the Horecker cycle, thus enabling renewed 

 uptake of COo. 



Ribulose-1, 5-diphosphate is produced from ribulose-5-phosphate. Calvin 

 assumed that one molecule 3-phosphoglyceraldehyde and one molecule sedo- 

 heptulose-7-phosphate are converted to two molecules ribose-5-phosphate. 

 However, more recent investigations (7) have shown that in this conversion 

 only one molecule ribose-5-phosphate and one molecule xylulose-5-phosphate 

 are produced instead of two molecules ribose-5-phosphate. By the action 

 of phosphoketopentose epimerase, xylulose-5-phosphate is changed into ri- 

 bulose-5-phosphate. In the conversion of fructose- 1,6-diphosphate + 3- 

 phosphoglyceraldehyde to erythrose-4-phosphate + ribulose-5-phosphate, 

 xylulose-5-phosphate occurs as an intermediate (see Fig. 50). The enzymes 

 indicated in Figure 50 are needed for the various reactions of the carbon cycle. 

 Their presence in cell-free extracts of green plants has been determined by 

 Racker (47). When DPN+, ATP, labeled bicarbonate and small amounts of 

 ribose-5-phosphate are added to such extracts, paper chromatography shows, 

 after some time, the presence of labeled sugars and sugar phosphates. 



The carbon balance of Calvin's cycle can be written as follows : 



3 A TP 



3 C5 + 3 CO2 > 6 3-phosphoglyceric acid 



12 H 



6 3-pho?phogivceric acid > 6 Cn 



6 ATP 



