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Overview of the Calvin cycle and carbon fixation

The Calvin cycle, Calvin–Benson-Bassham (CBB) cycle, reductive pentose phosphate cycle or C3 cycle is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It is also known as the light-independent reactions.

The cycle was discovered by Melvin Calvin, James Bassham, and Andrew Benson at the University of California, Berkeley^[1] by using the radioactive isotope carbon-14. It is one of the light-independent reactions used for carbon fixation.

Overview

Photosynthesis occurs in two stages in a cell. In the first stage, light-dependent reactions capture the energy of light and use it to make the energy-storage molecules ATP and NADPH. The light-independent Calvin cycle uses the energy from short-lived electronically excited carriers to convert carbon dioxide and water into organic compounds^[2] that can be used by the organism (and by animals that feed on it). This set of reactions is also called carbon fixation. The key enzyme of the cycle is called RuBisCO. In the following biochemical 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 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 CO
₂ + 6 NADPH + 5 H
₂O + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H⁺ + 6 NADP⁺ + 9 ADP + 8 P_i (P_i = inorganic phosphate)

Hexose (six-carbon) sugars are not a product of the Calvin cycle. Although many texts list a product of photosynthesis as C
₆H
₁₂O
₆, 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).

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Products

The immediate products of one turn of the Calvin cycle are 2 glyceraldehyde-3-phosphate (G3P) molecules, 3 ADP, and 2 NADP⁺. (ADP and NADP⁺ are not really "products." They are regenerated and later used again in the Light-dependent reactions). Each G3P molecule is composed of 3 carbons. In order for the Calvin cycle to continue, RuBP (ribulose 1,5-bisphosphate) must be regenerated. So, 5 out of 6 carbons from the 2 G3P molecules are used for this purpose. Therefore, there is only 1 net carbon produced to play with for each turn. To create 1 surplus G3P requires 3 carbons, and therefore 3 turns of the Calvin cycle. To make one glucose molecule (which can be created from 2 G3P molecules) would require 6 turns of the Calvin cycle. Surplus G3P can also be used to form other carbohydrates such as starch, sucrose, and cellulose, depending on what the plant needs.^[3]

References

Citations

^ Bassham J, Benson A, Calvin M (1950). "The path of carbon in photosynthesis" (PDF). J Biol Chem. 185 (2): 781–7. PMID 14774424.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Campbell, Neil A. (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 0-13-250882-6. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Russell, Wolfe et al.Biology: Exploring the Diversity of Life'.Toronto:Nelson College Indigenous,1st ed, Vol. 1, 2010, pg 151

Bibliography

Bassham JA (2003). "Mapping the carbon reduction cycle: a personal retrospective". Photosyn. Res. 76 (1–3): 35–52. doi:10.1023/A:1024929725022. PMID 16228564.
Diwan, Joyce J. (2005). "Photosynthetic Dark Reaction". Biochemistry and Biophysics, Rensselaer Polytechnic Institute.
Portis, Archie; Parry, Martin (2007). "Discoveries in Rubisco (Ribulose 1,5-bisphosphate carboxylase/oxygenase): a historical perspective" (PDF). Photosynthesis Research. 94 (1): 121–143. doi:10.1007/s11120-007-9225-6. PMID 17665149 Template:Inconsistent citations{{cite journal}}: CS1 maint: postscript (link)

External links

Khan Academy, video introduction

[1] Bassham J, Benson A, Calvin M (1950). "The path of carbon in photosynthesis" (PDF). J Biol Chem. 185 (2): 781–7. PMID 14774424.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[2] Campbell, Neil A. (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 0-13-250882-6. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[3] Russell, Wolfe et al.Biology: Exploring the Diversity of Life'.Toronto:Nelson College Indigenous,1st ed, Vol. 1, 2010, pg 151

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[2]

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@@ Line 19: / Line 19: @@
 Hexose (six-carbon) sugars are not a product of the Calvin cycle. Although many texts list a product of photosynthesis as {{chem|C|6|H|12|O|6}}, 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).
+I love my baby Lorenzo <3
-==Steps==
+He is truly amazing. 5.2.11 forever & always <3 mwah baby ko ! :-*
-In the first stage of the Calvin cycle, a {{CO2}} molecule is incorporated into one of two three-carbon molecules ([[glyceraldehyde 3-phosphate]] or G3P), using up two molecules of [[Adenosine triphosphate|ATP]] and two molecules of [[NADPH]], both of which were produced in the light-dependent stage. Three steps are involved.
-[[Image:Calvin cycle step 1.svg|thumb|450px|Calvin cycle step 1 (black circles represent carbon atoms)]]
-[[Image:Calvin cycle step 2 (doubled).svg|thumb|532px|Calvin cycle steps 2 and 3 combined]]
-# 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 3-PGA, a 3-carbon compound<ref>Campbell, and Reece Biology: 8th Edition, page 198. Benjamin Cummings, December 7, 2007.</ref> (also: 3-phosphoglyceric acid, PGA, 3PGA).
-# The enzyme [[phosphoglycerate kinase]] catalyses the phosphorylation of 3-PGA by [[Adenosine triphosphate|ATP]] (which was produced in the light-dependent stage). [[1,3-Bisphosphoglycerate]] (1,3BPGA, glycerate-1,3-bisphosphate) and [[Adenosine diphosphate|ADP]] are the products. (However, note that two 3-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 [[Glyceraldehyde-3-phosphate dehydrogenase (NADP+) (phosphorylating)|glyceraldehyde 3-phosphate 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.
-[[Image:Calvin cycle step 3.svg|thumb|535px|Regeneration stage of the Calvin cycle]]
-The next stage in the Calvin cycle is to regenerate RuBP. Five G3P molecules produce three RuBP molecules, using up three molecules of ATP. Since each {{CO2}} molecule produces two G3P molecules, three {{CO2}} molecules produce six G3P molecules, of which five are used to regenerate RuBP, leaving a net gain of one G3P molecule per three {{CO2}} molecules (as would be expected from the number of carbon atoms involved).
-The regeneration stage can be broken down into steps.
-# [[Triose phosphate isomerase]] converts all of the G3P reversibly into [[dihydroxyacetone phosphate]] (DHAP), also a 3-carbon molecule.
-# [[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 {{chem|CO|2}} generates two more G3P.
-# F6P has two carbons removed by [[transketolase]], giving [[erythrose-4-phosphate]]. The two carbons on [[transketolase]] are added to a G3P, giving the ketose [[xylulose-5-phosphate]] (Xu5P).
-# E4P and a DHAP (formed from one of the G3P from the second {{chem|CO|2}} fixation) are converted into [[sedoheptulose-1,7-bisphosphate]] (7C) by aldolase enzyme.
-# Sedoheptulose-1,7-bisphosphatase (one of only three enzymes of the Calvin cycle that are unique to plants) cleaves [[sedoheptulose-1,7-bisphosphate]] into [[sedoheptulose-7-phosphate]], releasing an inorganic phosphate ion into solution.
-# Fixation of a third {{chem|CO|2}} generates two more G3P. The ketose S7P has two carbons removed by [[transketolase]], giving [[ribose-5-phosphate]] (R5P), and the two carbons remaining on [[transketolase]] are transferred to one of the G3P, giving another Xu5P. This leaves one G3P as the product of fixation of 3 {{chem|CO|2}}, with generation of three pentoses that can be converted to Ru5P.
-# R5P is converted into [[ribulose-5-phosphate]] (Ru5P, RuP) by [[phosphopentose isomerase]]. Xu5P is  converted into RuP by [[phosphopentose epimerase]].
-# Finally, phosphoribulokinase (another plant-unique enzyme of the pathway) phosphorylates RuP into RuBP, ribulose-1,5-bisphosphate, completing the Calvin ''cycle''. This requires the input of one ATP.
-Thus, of six G3P produced, five are used to make three RuBP (5C) molecules (totaling 15 carbons), with only one G3P available for subsequent conversion to hexose. This requires nine ATP molecules and six NADPH molecules per three {{chem|CO|2}} molecules. The equation of the overall Calvin cycle is shown diagrammatically below.
-[[Image:Calvin cycle overall.svg|thumb|center|580px|The overall equation of the Calvin cycle (black circles represent carbon atoms)]]
-[[RuBisCO]] also reacts competitively with {{chem|O|2}} instead of {{chem|CO|2}} in [[photorespiration]]. The rate of photorespiration is higher at high temperatures. Photorespiration turns RuBP into 3-PGA and 2-phosphoglycolate, a 2-carbon molecule that can be converted via glycolate and glyoxalate to glycine. Via the glycine cleavage system and tetrahydrofolate, two glycines are converted into serine +{{chem|CO|2}}. Serine can be converted back to 3-phosphoglycerate. Thus, only 3 of 4 carbons from two phosphoglycolates can be converted back to 3-PGA. It can be seen that photorespiration has very negative consequences for the plant, because, rather than fixing {{chem|CO|2}}, this process leads to loss of {{chem|CO|2}}. [[C4 carbon fixation]] evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates, for example, corn.
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Revision as of 02:28, 30 November 2012

Overview

Products

See also

References

External links