Introductory Crop Physiology

Calvin cycle or C₃ cycle

Calvin Cycle
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 in chlorella, unicellular green algae. Calvin was awarded Nobel Prize for this work in 1961. Since the first stable compound in Calvin cycle is a 3 carbon compound (3 phosphoglyceric acid), the cycle is also called as C₃ cycle.
The reactions of Calvin’s cycle occur in three phases.

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 diphosphate to form three molecules of an unstable intermediate 6C compound. This reaction is catalysed by the enzyme, carboxydimutase



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 catalysed 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.

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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 diphosphoglyceric acid and 6 molecules of ADP. This reaction is catalysed by the enzyme, Kinase




Six molecules of 1,3 diphosphoglyceric 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 diphosphate is regenerated. The regenerative phase is called as pentose phosphate pathway or hexose monophophate shunt. It involves the following steps.
1. Some of the molecules of 3 phosphoglyceraldehyde isomerise into dihydroxy acetone phosphate. Both 3 phosphoglyceraldehyde and dihydroxy acetone phosphate then unite in the presence of the enzyme, aldolase to form fructose, 1-6 diphosphate.



2. Fructose 6 phosphate is converted into fructose 6 phosphate in the presence of phosphorylase



3. Some of the molecules of 3 phosphoglyceraldehyde instead of forming hexose sugars are diverted to regenerate ribulose 1-5 diphosphate



4. 3 phosphoglyceraldehyde reacts with fructose 6 phosphate in the presence of enzyme transketolase to form erythrose 4 phosphate ( 4C sugar) and xylulose 5 phosphate(5C sugar).



5. Erythrose 4 phosphate combines with dihydroxy acetone phosphate in the presence of the enzyme aldolase to form sedoheptulose 1,7 diphosphate(7C sugar).



6. Sedoheptulose 1, 7 diphosphate loses one phosphate group in the presence of the enzyme phosphatase to form sedoheptulose 7 phosphate.



7. Sedoheptulose phosphate reacts with 3 phosphoglyceraldehyde in the presence of transketolase to form xylulose 5 phosphate and ribose 5 phosphate ( both % c sugars)



8. Ribose 5 phosphate is converted into ribulose 1, 5 diphosphate in the presence of enzyme, phosphopentose kinase and ATP. Two molecules of xylulose phosphate are also converted into one molecule of ribulose monophosphate. The ribulose monophosphate is phosphorylated by ATP to form ribulose diphosphate and ADP, 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).

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