The Calvin cycle is a series of biochemical reactions that take place in the chloroplasts of plants.
Role of the Calvin cycle
The Calvin cycle makes it possible to produce glucose, an energy molecule, from carbon dioxide. This is called carbon fixation.
Principle of the Calvin cycle
This biochemical cycle constitutes the second phase of photosynthesis: after photochemical reactions convert light energy into chemical energy, in the form of ATP and reducing potential, the Calvin cycle uses this energy to fix carbon in new organic molecules.
The three reactions in this cycle take place in the stroma of chloroplasts:
- fixation of carbon dioxide: incorporation of carbon from CO2 in the organic molecules that are already present through the key enzyme ribulose bisphosphate carboxylase/oxygenase ( RuBisCO). It fixes this carbon with three moles of glucide to five carbon atoms (the bisphosphate ribulose), producing 3 moles of an intermediary of six unstable carbons, which gives six moles of a three-carbon molecule (3-phosphoglyceric acid) ;
- reduction of the 3-phosphoglyceric acid: the energy of the ATP and the reducing potential is then used to form 6 moles of a glucide with three carbon atoms, glyceraldehyde-3-phosphate;
- regeneration of ribulose: one mole of glyceraldehyde-3-phosphate is exported for the plant's biosynthesis, while the five remaining moles are recycled into three moles of ribulose bisphosphate.
- The Calvin cycle can then begin again.
Diagram of the Calvin cycle. © Mike Jones, Wikimedia CC by-sa 3.0