Calvin cycle



  The Calvin cycle (or Calvin-Benson cycle or carbon fixation) is a series of light-independent reactions or dark reactions.


During photosynthesis, 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 enzymes in the Calvin cycle are functionally equivalent to many enzymes used in other metabolic pathways such as net productivity.

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

3 CO2 + 6 NADPH + 5 H2O + 9 ATP → C3H5O3-PO32- + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi

It should be noted that hexose (six carbon) sugars are not a product of the Calvin cycle. Although many texts list a product of photosynthesis as C6H12O6, this is mainly a convenience to counter the equation of respiration, where six-carbon sugars are oxidized in mitochondria. The carbohydrate products of the Calvin Cycle are three-carbon sugar phosphate molecules, or "triose phosphates," specifically, glyceraldehyde-3-phosphate.

Steps of the Calvin cycle

  • The enzyme glycerate 3-phosphate, a 3-carbon compound, are created. (also: 3-phosphoglycerate, 3-phosphoglyceric acid, 3PGA)
  • The enzyme phosphoglycerate kinase catalyses the phosphorylation of 3PGA by ATP (which was produced in the light-dependent stage). 1,3-bisphosphoglycerate (glycerate-1,3-bisphosphate) and ADP are the products. (However, note that two PGAs are produced for every CO2 that enters the cycle, so this step utilizes 2ATP per CO2 fixed.
  • The enzyme G3P dehydrogenase catalyses the Glyceraldehyde 3-phosphate (also G3P, GP) is produced, and the NADPH itself was oxidized and becomes NADP+. Again, two NADPH are utilized per CO2 fixed.

(Simplified versions of the Calvin cycle integrate the remaining steps, except for the last one, into one general step - the regeneration of RuBP - also, one G3P would exit here.)

  • Aldolase and fructose-1,6-bisphosphatase convert a G3P and a DHAP into fructose-6-phosphate (6C). A phosphate ion is lost into solution.
  • Then fixation of another CO2 generates two more G3P.
  • F6P then has two carbons removed by transketolase are added to a G3P, giving the ketose xylulose-5-phosphate (Xu5P).
  • E4P and a DHAP are converted into sedoheptulose-1,7-bisphosphate (7C) by aldolase enzyme.
  • Sedoheptulose-1,7-bisphosphatase (one of only three enzymes of the Calvin cycle which are unique to plants) cleaves sedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate, releasing an inorganic phosphate ion into solution.
  • Fixation of a third CO2 generates two more G3P. The ketose S7P has two carbons removed by transketolase are transferred to one of the G3P, giving another Xu5P. This leaves one G3P as the product of fixation of 3 CO2, with generation of three pentoses which can be converted to Ru5P.
  • R5P is converted into ribulose-5-phosphate (Ru5P, RuP) by phosphopentose epimerase. R5P is also converted into RuP by ribose isomerase.
  • Finally, phosphoribulokinase (another plant unique enzyme of the pathway) phosphorylates RuP into g9, ribulose-1,5-bisphosphate, completing the Calvin cycle. This requires the input of one ATP.

Thus, of 6 G3P produced, three RuBP (5C) are made totalling 15 carbons, with only one available for subsequent conversion to hexose. This required 9 ATPs and 6 NADPH per 3 CO2.

C4 carbon fixation evolved to circumvent photorespiration, but can only occur in certain plants living in very warm or tropical climates.

Products of the Calvin cycle

The immediate product of the Calvin cycle is glyceraldehyde-3-phosphate (G3P) and water. Two G3P molecules (or one F6P molecule) that have exited the cycle are used to make larger carbohydrates. In simplified versions of the Calvin cycle they may be converted to F6P or F5P after exit, but this conversion is also part of the cycle.

Hexose isomerase converts about half of the F6P molecules into sucrose, a non-reducing sugar which is a stable storage sugar, unlike glucose.

See also

References

  1. ^ Bassham J, Benson A, Calvin M (1950). "The path of carbon in photosynthesis". J Biol Chem 185 (2): 781-7. PMID 14774424.
  • Bassham, J.A. (2003). Mapping the carbon reduction cycle: a personal retrospective. Photosynthesis Research, volume 76, pages 25-52 (see: Entrez PubMed 16228564).Mario Otmman (1998)
  • Diwan, Joyce J. (2005). Photosynthetic Dark Reaction at [2]
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