Calvin cycle

The Calvin cycle (or Calvin-Benson cycle or carbon fixation) is a series of biochemical reactions that takes place in the chloroplasts of photosynthetic organisms. It was discovered by Melvin Calvin and Andrew Benson at the University of California, Berkeley. James Bassham also made important contributions to elucidating this pathway. It is one of the light-independent reactions and occurs in the stroma.

During photosynthesis, light energy is used to generate chemical free energy, stored in ATP and NADPH. The light-independent Calvin ("dark") cycle uses the energy from short-lived electronically-excited carriers to convert carbon dioxide and water into organic compounds that can be used by the organism (and by animals which feed on it). This set of reactions is also called carbon fixation. The key enzyme of the cycle is called RuBisCO. In the following equations, the chemical species (phosphates and carboxylic acids) exist in equilibria among their various ionized states as governed by the pH.

The sum of reactions in the Calvin cycle is the following:

6 CO₂ + 12 NADPH + 12 H⁺ + 18 ATP → C₆H₁₂O₆ + 6 H₂O + 12 NADP⁺ + 18 ADP + 18 P_i

• The steps in the Calvin cycle are:
  
  • Ribulose-1,5-bisphosphate is combined with CO₂,with the help of an enzyme rubisco, creating a 3-carbon compound, PGA.
    
  • One ATP from the light reactions is used, producing an ADP and a P_i (inorganic phosphate).
    
  • An NADPH from the light reactions combines with an H⁺ and becomes NADP⁺.
    
  • PGAL, a 3-carbon compound, is produced, which stores free energy.
    
  • Another ATP is consumed, yielding an ADP and a P_i.
    
  • RuBP is produced, which is a 5-carbon compound.
    

At high temperatures, RuBisCO will react with O₂ instead of CO₂ in photorespiration, an apparently-puzzling process, since it seems to throw away captured energy. However it may be a mechanism for preventing overload during periods of high light flux. C4 plants use the enzyme PEP initially, which has a higher affinity for CO₂. The process first makes a 4-carbon intermediate compound; hence the name C4 plants.

CAM plants keep their stomata (on the underside of the leaf) closed during the day, which conserves water but prevents photosynthesis, which requires CO₂ to pass by gas exchange through these openings. Evaporation through the upper side of a leaf is prevented by a layer of wax.