Difference between revisions of "Calvin cycle" - New World Encyclopedia

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Overview of the Calvin cycle and carbon fixation

The Calvin cycle is an important and complicated series of biochemical reactions that takes place in photosynthetic organisms and results in the conversion of carbon from carbon dioxide into organic molecules. This metabolic pathway is It one of the light-independent (dark) reactions used for carbon fixation. It involves using the energy stored in ATP and using NADPH as a source of electrons (reduction protential), both provided during light-dependent reactions, to reduce carbon dioxide and fix it into the higher energy organic molecules. It is a "cycle" in that some of the product is recycled into the ccyle. Takes place in the stroma of chloroplasts. Calvin–Benson-Bassham cycle or reductive pentose phosphate cycle or C3 cycle or CBB cycle

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Overview

Redox reactions. The Calvin cycle, and the associated process known as photosynthesis, involve what are known as "redox reactions" or "oxidation-reduction reactions." Oxidation occurs when an atom or molecule loses an electron (with oxygen being the most common electron acceptor). Reduction occurs when an atom or molecule gains an electron. A reduced molecule or atom, with the extra electron, has a higher level of energy than an oxidized form of the atom or molecule. A redox reaction is a chemical reaction where electrons lost by one atom through oxidation are gained by another atom through reduction.

Photosynthesis. The process of photosynthesis is the source of the carbon found in the organic compounds within the bodies of living organisms, as well as a means for capturing the energy from sunlight. Some of the light energy is stored in the form of adenosine triphosphate (ATP) and some of the energy is used to remove electrons from a substance such as water, with the electrons then used in the reactions to turn carbon dioxide into organic compounds. In plants, algae, and cyanobacteria, these reactions to produce organic compounds comprise the metabolic pathway known as the Calvin cycle.

The overall process of photosynthesis can be described in terms of three stages: two-light dependent reactions and one light-independent reaction. The two light-dependent reactions are (1) capturing energy from sunlight; and (2) creating ATP and reducing NADP⁺ to NADPH. The light-independent reaction, which can take place in the presence or absence of light and is known as "carbon fixation," involves utilizing ATP and NADPH to synthesize organic molecules from carbon dioxide. Carbon fixation is part of the Calvin cycle.

Calvin cycle. The Calvin cycle is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms. Essentially, the light-independent Calvin cycle, also known (erroneously) as the "dark reaction" or "dark stage," uses the energy from short-lived electronically excited carriers to convert carbon dioxide and water into organic compounds (Campbell et al. 2006). It also can be described in terms of three phases: (1) carbon fixation, (2) reduction, and (3) regeneration of RuBP.

File:RuBP-2D-skeletal.png

Ribulose-1,5-bisphosphate (RuBP)

Phase I: Carbon fixation. In the carbon fixation phase of the Calvin cycle, inorganic carbon in the form of carbon dioxide becomes incorporated into organic form. Essentially, carbon dioxide is reduced to provide a higher energy molecule. During this phase, a five-carbon, energy-rich sugar RuBP (ribulose 1,5-biphosphate) is produced. It is produced by reassembling two products of glycolysis: fructose 6-phosphate and glyceraldehyde 3-phosphate (G3P). After RuBP is produced, carbon dioxide reacts with RuBP to form a transient 6-carbon intermediate. This 6-carbon intermediate immediately splits into two molecules of three carbon 3-phosphoglycerate (PGA). It uses the enzyme ribulose biphosphate carboxylase oxygenase (called RuBisCo).

Phase II: Reduction. In the second phase, reduction, PGA is reduced to G3P (glyceraldehye 3-phosphate) using ATP and NADPH. Some G3P (which has three carbons) leaves the Calvin cycle and is converted to glucose and other sugars. Note: two molecules of G3P (the output of carbon fixation and reduction) is needed to produce a six-carbon glucose molecule.

Phase III: Regeneration. While some G3P is shunted out of the Calvin cycle to produce glucose and other sugars, much of it is recycled in order to regenerate RuBP, to keep the cycle going.

The enzymes in the Calvin cycle are functionally equivalent to many enzymes used in other metabolic pathways such as gluconeogenesis and the pentose phosphate pathway, but they are to be found in the chloroplast stroma instead of the cell cytoplasm, separating the reactions. They are activated in the light (which is why the name "dark reaction" is misleading), and also by products of the light-dependent reaction. These regulatory functions prevent the Calvin cycle from being respired to carbon dioxide. Energy (in the form of ATP) would be wasted in carrying out these reactions that have no net productivity.

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

3 CO₂ + 6 NADPH + 5 H₂O + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H⁺ + 6 NADP⁺ + 9 ADP + 8 P_i

or

3 CO₂ + 6 C₂₁H₂₉N₇O₁₇P₃ + 5 H₂O + 9 C₁₀H₁₆N₅O₁₃P₃ → C₃H₅O₃-PO₃2- + 2 H₊ + 6 NADP⁺ + 9 C₁₀H₁₅N₅O₁₀P₂ + 8 P_i

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," namely, glyceraldehyde-3-phosphate (G3P).

The Calvin cycle was discovered by Melvin Calvin, James Bassham, and Andrew Benson at the University of California, Berkeley by using the radioactive isotope carbon-14 (Bassham et al. 1950).

Steps of the Calvin cycle

The enzyme RuBisCO catalyses the carboxylation of ribulose-1,5-bisphosphate (RuBP), a 5-carbon compound, by carbon dioxide (giving a total of 6 carbons) in a two-step reaction (Farazdaghi 2009).Cite error: Closing </ref> missing for <ref> tag

References
ISBN links support NWE through referral fees

Bassham, J., A. Benson, and M. Calvin. 1950. The path of carbon in photosynthesis. J Biol Chem 185(2): 781–7. Retrieved July 26, 2011.

Campbell, N. A., B. Williamson, and R. J. Heyden. 2006. Biology: Exploring Life. Boston, MA: Pearson Prentice Hall. ISBN 0132508826.

Raven, P. H., G. B. Johnson, J. B. Losos, K. A. Mason, and S. R. Singer. 2008. Biology, 8th edition. Boston: McGraw Hill. ISBN 9780073337234.

Citations

Bibliography

Bassham JA (2003). Mapping the carbon reduction cycle: a personal retrospective. Photosyn. Res. 76 (1-3): 35–52.
Diwan, Joyce J. (2005). Photosynthetic Dark Reaction. Biochemistry and Biophysics, Rensselaer Polytechnic Institute.
(2007). Discoveries in Rubisco (Ribulose 1,5-bisphosphate carboxylase/oxygenase): a historical perspective. Photosynthesis Research 94 (1): 121–143.

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 ==Steps of the Calvin cycle==
-# The enzyme [[RuBisCO]] catalyses the carboxylation of [[ribulose-1,5-bisphosphate]], RuBP, a 5-carbon compound, by carbon dioxide (a total of 6 carbons) in a two-step reaction.<ref>{{cite journal |author=Farazdaghi H |title=Modeling the Kinetics of Activation and Reaction of Rubisco from Gas Exchange |journal=Advances in Photosynthesis and Respiration |volume=29 |issue=IV |pages=275–294 |year=2009 |doi=10.1007/978-1-4020-9237-4_12 |url=http://www.springerlink.com/content/qu357422246r8870/}}</ref> The product of the first step is enediol-enzyme complex that can capture {{chem|CO|2}} or {{chem|O|2}}. Thus, enediol-enzyme complex is the real carboxylase/oxygenase. The {{chem|CO|2}} that is captured by enediol in second step produces a six-carbon intermediate initially that immediately splits in half, forming two molecules of [[3-phosphoglycerate]], or PGA, a 3-carbon compound<ref>Campbell, and Reece Biology: 8th Edition, page 198. Benjamin Cummings, December 7, 2007.</ref> (also: 3-phosphoglycerate, 3-phosphoglyceric acid, 3PGA).
+# The enzyme [[RuBisCO]] catalyses the carboxylation of [[ribulose-1,5-bisphosphate]] (RuBP), a 5-carbon compound, by carbon dioxide (giving a total of 6 carbons) in a two-step reaction (Farazdaghi 2009).<ref> The product of the first step is enediol-enzyme complex that can capture {{chem|CO|2}} or {{chem|O|2}}. Thus, enediol-enzyme complex is the real carboxylase/oxygenase. The {{chem|CO|2}} that is captured by enediol in second step produces a six-carbon intermediate initially that immediately splits in half, forming two molecules of [[3-phosphoglycerate]], or PGA, a 3-carbon compound (Campbell and Reece 2007). (also: 3-phosphoglycerate, 3-phosphoglyceric acid, 3PGA).
 # The enzyme phosphoglycerate kinase catalyses the phosphorylation of 3PGA by [[Adenosine triphosphate|ATP]] (which was produced in the light-dependent stage). [[1,3-Bisphosphoglycerate]] (glycerate-1,3-bisphosphate) and [[Adenosine diphosphate|ADP]] are the products. (However, note that two PGAs are produced for every {{chem|CO|2}} that enters the cycle, so this step utilizes two [[Adenosine triphosphate|ATP]] per {{chem|CO|2}} fixed.)
 # The enzyme G3P dehydrogenase catalyses the [[redox|reduction]] of 1,3BPGA by [[NADPH]] (which is another product of the light-dependent stage). [[Glyceraldehyde 3-phosphate]] (also G3P, GP, TP, PGAL) is produced, and the NADPH itself was oxidized and becomes NADP<sup>+</sup>. Again, two NADPH are utilized per {{chem|CO|2}} fixed.