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

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[[Image:calvin-cycle3.png|thumb|right|300px| Overview of the Calvin cycle and carbon fixation]] The '''Calvin cycle''' (or '''Calvin-Benson cycle''' or carbon fixation) is a series of [[biochemistry|biochemical]] reactions that takes place in the [[stroma]] of [[chloroplast]]s in [[photosynthesis|photosynthetic]] [[organism]]s. It was discovered by [[Melvin Calvin]], [[Andrew Benson]] and Skiddy at the [[University of California, Berkeley]] with [[James Bassham]] also contributing.<ref>Bassham J., Benson A., Calvin M. 1950. [http://www.jbc.org/cgi/reprint/185/2/781.pdf "The path of carbon in photosynthesis"], ''Journal Biological Chemistry'' Volume 185, Issue 2, p 781-7. ISSN 0021-9258</ref> It is one of the [[light-independent reaction]]s or dark reactions. It´s main purpose is the assimilation of carbon.
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[[Image:Calvin-cycle4.svg|thumb|right|400px| Overview of the Calvin cycle and carbon fixation]]
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The '''Calvin cycle''' or '''Calvin–Benson-Bassham cycle''' or '''reductive pentose phosphate cycle''' or '''C3 cycle''' or '''CBB cycle''' is a series of [[biochemistry|biochemical]] [[redox]] reactions that take place in the [[Stroma (fluid)|stroma]] of [[chloroplast]]s in [[photosynthesis|photosynthetic]] [[organism]]s. It was discovered by [[Melvin Calvin]], [[James Bassham]], and [[Andrew Benson]] at the [[University of California, Berkeley]]<ref>{{cite journal |author=Bassham J, Benson A, Calvin M |title=The path of carbon in photosynthesis |url=http://www.jbc.org/cgi/reprint/185/2/781.pdf |journal=J Biol Chem |volume=185 |issue=2 |pages=781–7 |year=1950 |pmid=14774424}}</ref> by using the [[radioactive]] [[isotope]] [[carbon-14]]. It is one of the [[light-independent reaction|light-independent (dark) reactions]], used for [[carbon fixation]].
  
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==Steps of the Calvin cycle==
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==Overview==
  
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*The enzyme [[RuBisCO]] catalyzes the carboxylation of [[Ribulose-1,5-bisphosphate]], a 5 carbon compound, by carbon dioxide (a total of 6 carbons). Rubisco is a large, slow enzyme averaging 3 substrate per second compared to 1000/s of most other enzyme in the Calvin cycle. Two molecules of [[glycerate 3-phosphate]], a 3-carbon compound, are created. (also: 3-phosphoglycerate, 3-phosphoglyceric acid, 3PGA)  
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During photosynthesis, [[light]] [[energy]] is used in generating chemical [[free energy]], stored in glucose. 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 compound]]s<ref>{{cite book | last = Campbell | first = Neil A. | authorlink = | coauthors = Brad Williamson; Robin J. Heyden | title = Biology: Exploring Life | publisher = Pearson Prentice Hall | year = 2006 | location = Boston, Massachusetts | url = http://www.phschool.com/el_marketing.html | isbn = 0-13-250882-6 }}</ref> 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]].
  
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*The enzyme phosphoglycerate kinase catalyzes the phosphorylation of 3PGA by 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 CO<sub>2</sub> that enters the cycle, so this step happens twice.)
+
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 [[primary production|net productivity]].
  
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*The enzyme G3P dehydrogenase catalyzes the [[redox|reduction]] of 1,3BPGA by [[NADPH]] (which was another product of the light-dependent stage). [[Glyceraldehyde 3-phosphate]] (also G3P, GP) is produced, and the NADPH itself was oxidised and hence becomes NADP<sup>+</sup>.
+
The sum of reactions in the Calvin cycle is the following:
 +
:3 {{chem|CO|2}} + 6 [[Nicotinamide adenine dinucleotide phosphate|NADPH]] + 5 {{chem|H|2|O}} + 9 [[Adenosine triphosphate|ATP]] → [[glyceraldehyde-3-phosphate]] (G3P) + 2 H<sup>+</sup> + 6 [[Nicotinamide adenine dinucleotide phosphate|NADP<sup>+</sup>]] + 9 [[Adenosine diphosphate|ADP]] + 8 P<sub>i</sub>
 +
::or
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:3 {{chem|CO|2}} + 6 {{chem|C|21|H|29|N|7|O|17|P|3}} + 5 {{chem|H|2|O}} + 9 {{chem|C|10|H|16|N|5|O|13|P|3}} → {{chem|C|3|H|5|O|3}}-{{chem|PO|3|2-}} + 2 {{chem|H|+}} + 6 [[Nicotinamide adenine dinucleotide phosphate|NADP<sup>+</sup>]] + 9 {{chem|C|10|H|15|N|5|O|10|P|2}} + 8 P<sub>i</sub>
 +
<!--NADP+ from above,should be 6 C21H29N7O17P3 + H12O32 = 6 C21H31N7O22.333P3 OR 6 C21H31N7O(67/3)P3
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As this author does not yet believe that this oxidation reaction should cause such a large change in composition, can anyone else verify this formula and insert either 6 C21H31N7O22.333P3 OR 6 C21H31N7O(67/3)P3 in place of NADP+. Note:C21H29N7O17P3 is taken from the wiki article on NADPH. I may have reversed the data, such that NADP+ = C21H29N7O17P3, and NADPH = C21H29N7O17P3 - H12O32 = 6 C21H27N7O11.666P3 OR 6 C21H31N7O(35/3)P3.
 +
—>
  
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(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.)
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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).
  
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*[[Triose phosphate isomerase]] converts some G3P reversibly into [[dihydroxyacetone phosphate]] (DHAP), also a 3-carbon molecule.
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==Steps of the Calvin cycle==
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*Aldolase and Fructose-[[1,6-bisphosphatase]] convert some of these two into [[fructose-6-phosphate]] (6C). A phosphate ion is lost into solution.
 
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Up to this point, as per the overall equation given above, 6 carbon dioxide molecules would have been converted, with the use of 6 RuBP, 12 ATP and 12 NADPH, to 12 G3P molecules. One F6P, (= 2 G3P) then exits the cycle, while 10 of these G3P molecules continue, giving a ratio of 1:5 G3P. Obviously, the ratio of carbon dioxide entering the cycle to RuBP already present is also 1:5.
 
  
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*F6P is then combined with another G3P (total 9C) and then cleaved into [[xylulose-5-phosphate]] (X5P) and [[erythrose-4-phosphate]] by [[transketolase]].
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# 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).
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# 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.)
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# 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.
 +
(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.)
  
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*E4P and DHAP are converted into [[sedoheptulose-1,7-bisphosphate]] (7C) by a transaldolase enzyme.
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# [[Triose phosphate isomerase]] converts all of the G3P reversibly into [[dihydroxyacetone phosphate]] (DHAP), also a 3-carbon molecule.
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# [[Aldolase]] and [[fructose-1,6-bisphosphatase]] convert a G3P and a DHAP into [[fructose 6-phosphate]] (6C). A phosphate ion is lost into solution.
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# Then fixation of another {{chem|CO|2}} generates two more G3P.
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# 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).
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# 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.
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# 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.
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# 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.
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# R5P is converted into [[ribulose-5-phosphate]] (Ru5P, RuP) by [[phosphopentose isomerase]]. Xu5P is  converted into RuP by [[phosphopentose epimerase]].
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# 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.
  
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*S17BPase cleaves [[sedoheptulose-1,7-bisphosphate]] into [[sedoheptulose-7-phosphate]], releasing an inorganic phosphate ion into solution.
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Thus, of 6 G3P produced, three RuBP (5C) are made, totaling 15 carbons, with only one available for subsequent conversion to hexose. This required 9 ATPs and 6 NADPH per 3 {{chem|CO|2}}.
  
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*S7P is then combined with another G3P (total 10C) and then cleaved into another X5P and [[ribose-5-phosphate]] (R5P) again by transketolase.
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[[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 3PGA 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 3PGA. 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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*X5P is converted into [[ribulose-5-phosphate]] (Ru5P, RuP) by [[phosphopentose epimerase]]. R5P is also converted into RuP by ribose isomerase.
 
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*Finally, phosphoribulokinase phosphorylates RuP into RuBP, ribulose-1,5-bisphosphate, completing the Calvin ''cycle''. This requires the input of one ATP.
 
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All the G3P produced earlier is converted into RuBP (5C), so 10 G3Ps (30C, 10 phosphates) were needed to produce 6 RuBPs (30C, 6 phosphates). 6 ATPs were also needed in the last step, giving a total of 18 ATPs used up per 6 CO<sub>2</sub>s. However, four phosphate ions are lost and these also form ATP. The energy in those ATPs is used to drive some of the reactions.
 
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At high temperatures, RuBisCO will react with O<sub>2</sub> instead of CO<sub>2</sub> in ''[[photorespiration]]''. This turns RuBP into 3PGA and 2-phosphoglycolate, a 2-carbon molecule which can be converted into 3PGA, some of which will exit the Calvin cycle. However, if this continues the RuBP will eventually be depleted, which slows down the cycle if electrons are entering from the light-dependent reaction too quickly.
 
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The cycle has to be repeated for six times, because each time the cycle is done one atom of [[carbon]] is produced, so six carbon atoms are needed for the production of [[fructose]] and other similar plant compounds that are consisted of exactly six carbon atoms.
 
  
 
==Products of the Calvin cycle==
 
==Products of the Calvin cycle==
  
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The immediate product of the Calvin cycle is glyceraldehyde-3-phosphate (G3P). 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.
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The immediate products of one turn of the Calvin cycle are 2 glyceraldehyde-3-phosphate (G3P) molecules, 3 ADP, and 2 NADP<sup>+</sup> (ADP and NADP<sup>+</sup> are regenerated 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, 5/6 carbon 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.<ref>Russell, Wolfe et al.''Biology: Exploring the Diversity of Life''.Toronto:Nelson College Indigenous,1st ed, Vol. 1, 2010, pg 151</ref>
  
 
==See also==
 
==See also==
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*[[Krebs Cycle]]
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*[[Light-independent reaction]]
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*[[Light-dependent reactions]]
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*[[Citric Acid Cycle]]
 
*[[Photorespiration]]
 
*[[Photorespiration]]
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*[[C4 carbon fixation]]
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*[[C4 carbon fixation|C<sub>4</sub> carbon fixation]]
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*[[Nitrogen Fixation]]
  
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==Notes==
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==References==
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;Citations
 
{{reflist}}
 
{{reflist}}
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;Bibliography
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*{{cite journal |author=Bassham JA |title=Mapping the carbon reduction cycle: a personal retrospective |journal=Photosyn. Res. |volume=76 |issue=1-3 |pages=35–52 |year=2003 |pmid=16228564 |doi=10.1023/A:1024929725022 }}
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*{{cite web |author=Diwan, Joyce J. |title=Photosynthetic Dark Reaction |date=2005 |work= |publisher=Biochemistry and Biophysics, Rensselaer Polytechnic Institute |url=http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/dark.htm}}
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*{{Cite journal | last1=Portis | first1=Archie | last2=Parry | first2=Martin | title=Discoveries in Rubisco (Ribulose 1,5-bisphosphate carboxylase/oxygenase): a historical perspective | url=http://ddr.nal.usda.gov/dspace/bitstream/10113/3976/1/IND43944177.pdf | doi=10.1007/s11120-007-9225-6 | year=2007 | journal=Photosynthesis Research | volume=94 | issue=1 | pages=121–143 | pmid=17665149 | postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. —>{{inconsistent citations}}}}
  
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==References==
 
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* Bassham, J.A. 2003. "Mapping the carbon reduction cycle: a personal retrospective." ''Photosynthesis Research'', volume 76, pages 25-52. ISSN 01668595  (see: {{Entrez Pubmed|16228564}}). Mario Otmman (1998)
 
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==External links==
 
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* Diwan, Joyce J. (2005). at [http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/dark.htm Calvin Cycle - Photosynthetic Carbon Reactions], Retrieved October 11, 2007.
 
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Revision as of 18:18, 26 July 2011

Overview of the Calvin cycle and carbon fixation

The Calvin cycle or Calvin–Benson-Bassham cycle or reductive pentose phosphate cycle or C3 cycle or CBB cycle is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It 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 (dark) reactions, used for carbon fixation.

Overview

During photosynthesis, light energy is used in generating chemical free energy, stored in glucose. 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[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 CO2 + 6 NADPH + 5 H2O + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi
or
3 CO2 + 6 C21H29N7O17P3 + 5 H2O + 9 C10H16N5O13P3 → C3H5O3-PO32- + 2 H+ + 6 NADP+ + 9 C10H15N5O10P2 + 8 Pi

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).

Steps of the Calvin cycle

  1. 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.[3] The product of the first step is enediol-enzyme complex that can capture CO2 or O2. Thus, enediol-enzyme complex is the real carboxylase/oxygenase. The CO2 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[4] (also: 3-phosphoglycerate, 3-phosphoglyceric acid, 3PGA).
  2. 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 two ATP per CO2 fixed.)
  3. The enzyme G3P dehydrogenase catalyses the 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+. 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.)

  1. Triose phosphate isomerase converts all of the G3P reversibly into dihydroxyacetone phosphate (DHAP), also a 3-carbon molecule.
  2. Aldolase and fructose-1,6-bisphosphatase convert a G3P and a DHAP into fructose 6-phosphate (6C). A phosphate ion is lost into solution.
  3. Then fixation of another CO2 generates two more G3P.
  4. 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).
  5. E4P and a DHAP (formed from one of the G3P from the second CO2 fixation) are converted into sedoheptulose-1,7-bisphosphate (7C) by aldolase enzyme.
  6. 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.
  7. Fixation of a third CO2 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 CO2, with generation of three pentoses that can be converted to Ru5P.
  8. R5P is converted into ribulose-5-phosphate (Ru5P, RuP) by phosphopentose isomerase. Xu5P is converted into RuP by phosphopentose epimerase.
  9. 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 6 G3P produced, three RuBP (5C) are made, totaling 15 carbons, with only one available for subsequent conversion to hexose. This required 9 ATPs and 6 NADPH per 3 CO2.

RuBisCO also reacts competitively with O2 instead of CO2 in photorespiration. The rate of photorespiration is higher at high temperatures. Photorespiration turns RuBP into 3PGA 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 +CO2. Serine can be converted back to 3-phosphoglycerate. Thus, only 3 of 4 carbons from two phosphoglycolates can be converted back to 3PGA. It can be seen that photorespiration has very negative consequences for the plant, because, rather than fixing CO2, this process leads to loss of CO2. C4 carbon fixation evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates, for example, corn.

Products of the Calvin cycle

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 regenerated 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, 5/6 carbon 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.[5]

See also

  • Light-independent reaction
  • Light-dependent reactions
  • Citric Acid Cycle
  • Photorespiration
  • C4 carbon fixation
  • Nitrogen Fixation

References
ISBN links support NWE through referral fees

Citations
  1. ↑ Bassham J, Benson A, Calvin M (1950). The path of carbon in photosynthesis. J Biol Chem 185 (2): 781–7.
  2. ↑ Campbell, Neil A. and Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 0-13-250882-6. 
  3. ↑ Farazdaghi H (2009). Modeling the Kinetics of Activation and Reaction of Rubisco from Gas Exchange. Advances in Photosynthesis and Respiration 29 (IV): 275–294.
  4. ↑ Campbell, and Reece Biology: 8th Edition, page 198. Benjamin Cummings, December 7, 2007.
  5. ↑ Russell, Wolfe et al.Biology: Exploring the Diversity of Life.Toronto:Nelson College Indigenous,1st ed, Vol. 1, 2010, pg 151
Bibliography

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