Let us make an in-depth study of the three stages of Calvin cycle. The three stages are: (a) Carboxylation (b) Reduction and (c) Formation of Hexose Sugar and Regeneration of RuBP.
The Calvin cycle (C3-cycle) or PCR-cycle can be divided into three stages:
(a) Car-boxylation, during which atmospheric CO2 combines with 5-C acceptor molecule ribulose 1, 5-bisphosphate (RuBP) and converts it into 3-phosphoglyceric acid (3-PGA);
(b) Reduction, which consumes ATP + NADPH (produced during primary photochemical reaction) and converts 3-PGA into 3-phosphoglyceraldehyde (3PGAld) or triose phosphate (TRI- OSE-P); and
(c) Formation of hexose sugar and regeneration of RuBP which consumes additional ATP, so that the cycle continues (Fig. 11.18).
Detailed steps of Calvin-cycle (C3-cycle) or PCR-cycle which have also been shown in Fig. 11.18A, are as follows:
(a) Carboxylation:
(i) The CO2 is accepted by ribulose 1, 5-bisphosphate (RuBP) already present in the cells and a 6-carbon addition compound is formed which is unstable. It soon gets hydrolysed into 2 molecules of 3-phosphoglyceric acid (3PGA). Both these reactions take place in the presence of ribulose bisphosphate carboxylase (Rubisco). 3-Phosphoglyceric acid is the first stable product of dark reaction of photosynthesis.
(b) Reduction:
(ii) 3-Phosphoglyceric acid is reduced to 3-phosphoglyceraldehyde by the assimilatory power (generated in light reaction) in the presence of triose phosphate dehydrogenase.
This reaction takes place in two steps:
(c) Formation of Hexose Sugar and Regeneration of RuBP:
(iii) Some of the molecules of 3-phosphoglyceraldehyde isomerise into dihydroxyaeetone phosphate, both of which then unite in the presence of the enzyme aldolase to form fructose 1, 6-bisphophate.
(iv) Fructose 1, 6-bisphosphate is converted into fructose 6-phosphate in the presence of phosphatase.
(v) Some of the fructose-6-phosphate (hexose sugar) is tapped off from the Calvin cycle and is converted into glucose, sucrose, and starch. Sucrose is synthesized in cytosol while starch is synthesized in chloroplast.
(vi) Some of the molecules of 3-phosphoglyceraldehyde produced in step (ii) instead of forming hexose sugars, are diverted to regenerate ribulose 1, 5-bisphosphate in the system as follows:
(vii) 3-Phosphoglyceraldehyde reacts with fructose-6-phosphate in the presence of enzyme transketolase to form erythrose-4-phosphate (4-C atoms sugar) and xylulose 5-phosphate (5-C atoms sugar).
(viii) Erythrose-4-phosphate combines with dihydroxyaceotone phosphate in the presence of the enzyme aldolase to form sedoheptulose 1, 7-bisphosphate (7-C atoms sugar).
(ix) Sedoheptulose 1, 7-bisphosphate loses one phosphate group in the presence of phosphatase to form sedoheptulose-7-phosphate.
(x) Sedoheptulose-7 phosphate reacts with 3-phosphoglyceraldehyde in the presence of transketolase to form xylulose-5-phosphate and ribose-5-phosphate (both 5-carbon atoms sugars).
(xi) Xylulose-5-phosphate is converted into another 5-C atoms sugar ribulose-5-phosphate in the presence of the enzyme phosphoketopentose epimerase.
(xii) Ribose-5-phosphate is also converted into ribulose-5-phosphate. The reaction is catalysed by phosphopentose isomerase.
(xiii) Ribulose-5-phosphate is finally converted into ribulose 1, 5-bisphosphate in the presence of phosphopentose kinase and ATP, thus completing the Calvin cycle.
Structural formulae of various 4, 5 and 7-C atoms sugars involved in the Calvin cycle are given Fig. 11.19.
Because first visible product of this cycle is 3-phosphoglyceric acid which is a 3-C compound, Calvin cycle is also known as C3-pathway. (Recent studies with algal cells, leaves and isolated chloroplasts have shown that ‘dark reactions’ of photosynthesis are not completely independent of light.
Several critical enzymes in the carbon reduction cycle are light activated; in the dark, they are either inactive or exhibit low activity. Activity of the enzyme Rubisco declines rapidly when light is turned off and regain slowly when light is turned on. At least four other enzymes of the PCR-cycle are known to be stimulated by light viz., 3-PGAld dehydrogenase (reaction ii), fructose 1, 6-bis phosphatase (reaction iv), Sedoheptulose 1, 7- bisphosphatase, and Ribulose 5-phosphate kinase (reaction xiii). Therefore, the designation “dark reaction” to the photosynthetic carbon reduction reactions is now considered as unappropriate).