It is a cyclic reaction occurring in the dark phase of photosynthesis. In this reaction, CO2 is converted into sugars and hence it is a process of carbon fixation. The Calvin cycle was first observed by Melvin Calvin and his colleagues in the 1950s. Calvin was awarded Nobel Prize for this work in 1961. In this cycle, the first stable compound in Calvin cycle is a 3 carbon compound (3-phosphoglyceric acid), the cycle is also called as C3 cycle or PCR (Photosynthetic Carbon Reduction).
The Calvin cycle is also referred to as the reductive pentose phosphate (RPP) cycle.
The reactions of Calvin’s cycle occur in three distinct phases. These phases are :-
1. Carboxylative phase
2. Reductive phase
3. Regenerative phase
1). Carboxylative phase :
Three molecules of CO2 are accepted by 3 molecules of 5C compound viz., ribulose bisphosphate to form three molecules of an unstable intermediate 6C compound. This reaction is catalyzed by the enzyme, carboxy dismutase.
The three molecules of the unstable 6 carbon compound are converted by the addition of 3 molecules of water into six molecules of 3 phosphoglyceric acid. This reaction is also catalyzed by the enzyme carboxy mutase.
3-phosphoglyceric acid (PGA) is the first stable product of dark reaction of photosynthesis and since it is a 3-carbon compound, this cycle is known as C3 cycle.
2). Reductive phase :
Six molecules of 3PGA are phosphorylated by 6 molecules of ATP (produced in the light reaction) to yield 6 molecules of 1,3-biphosphoglyceric acid and 6 molecules of ADP. This reaction is catalyzed by the enzyme, Kinase.
Six molecules of 1, 3 bisphosphoglyceric acid are reduced with the use of 6 molecules of NADPH2 (produced in light reaction) to form 6 molecules of 3- phospho glyceraldehyde. This reaction is catalysed by the enzyme, triose phosphate dehydrogenase.
3). Regenerative phase :
In the regenerative phase, the ribose biphosphate is regenerated. The regenerative phase is called as pentose phosphate pathway or hexose monophophate shunt.
It involves the following steps :
i). Some of the molecules of 3-phosphoglyceraldehyde into dihydroxyacetone phosphate in the presence of enzyme triose phosphate isomerase. Both 3-phospho glyceraldehyde and dihydroxy acetone phosphate then unite in the presence of the enzyme, aldolase to form fructose 1,6- bisphosphate.
ii). Fructose 1,6-phosphate is converted into fructose 6-phosphate in the presence of phosphatase.
Some of the fructose 6-phosphate (hexose suger) 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 the chloroplast.
iii). Some of the molecules of 3-phosphoglyceraldehyde instead of forming hexose sugars are diverted to regenerate ribulose 1,5-bisphosphate.
iv). 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).
v). Erythrose 4-phosphate combines with dihydroxyacetone phosphate in the presence of the enzyme aldolase to form sedoheptulose 1, 7-bisphosphate(7-C atoms sugar).
vi). Sedoheptulose 1, 7-bisphosphate loses one phosphate group in the presence of the enzyme phosphatase to form sedoheptulose 7-phosphate.
vii). Sedoheptulose 7-phosphate reacts with 3-phosphoglyceraldehyde in the presence of transketolase to form xylulose 5-phosphate and ribose 5- phosphate ( both 5-C atoms sugars).
viii). Xylulose 5-phosphate is converted into another 5-C atoms suger ribulose 5-phosphate in the presence of the enzyme phosphoketopentose epimerase.
ix). Ribose 5-phosphate is also converted into ribulose 5-phosphate. The reaction is catalysed by the enzyme phosphopentose isomerase.
x). Ribulose 5-phosphate is finally converted into ribulose 1,5-bisphosphate in the presence of enzyme, phosphopentose kinase and ATP.
Thus completing Calvin cycle.
In the dark reaction, CO2 is fixed to carbohydrates and the CO2 acceptor ribulose diphosphate is regenerated. In Calvin cycle, 12 NADPH2 and 18 ATPs are required to fix 6 CO2 molecules into one hexose sugar molecule (fructose 6 phosphate).
The net reaction of the calvin cycle is:
6 CO2 + 18 ATP + 12 NADPH + 12 H+ + 12 H2O fructose 6 phosphate +
18 ADP + 18Pi + 12NADP+
Fig: Calvin cycle (after Taiz L., Zeiger E. (2010): Plant Physiology. p. 203).
Stoichiometry of the calvin cycle :
Note that 6molecules of CO2 combine with 6 molecules of RuBP leading to the formation of 12 molecules of PGA (12×3). 3 molecules of hexose phosphates are formed out of which one is removed as heoxse gain. 2 molecules of hexose react with 2 molecules of PGA to form 2 molecules of xylulose-phosphate (2X5C) and 2 molecules of erythrose-phosphate (2x4C). 2molecules of erthrose-phosphate further combine with 3-PGA to form 2 molecules of seduheptulose-phosphate (2X7). Two molecules of seduheptulose-phosphate combine with 2molecules of PGA to form 2 molecules of xylulose (2X5C) and 2 molecules of ribose phosphate (2X5C). The 5C [ribose phosphate (2X5C) and xylulose phosphate (4X5C)] are converted to 6 molecules of ribulose 1,5 bisphosphate (6x6C)