C4 carbon fixation
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2008-06-26T23:48:49Z
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{{Citations missing|date=February 2008}}[[Image:HatchSlackpathway.png|thumb|right|300px| Overview of C<sub>4</sub> carbon fixation]]
'''C<sub>4</sub> carbon fixation''' is one of three biochemical mechanisms, along with [[C3 carbon fixation|C<sub>3</sub>]] and [[CAM photosynthesis]], functioning in land [[plants]] to "fix" [[carbon dioxide]] (binding the gaseous molecules to dissolved compounds inside the plant) for [[sugar]] production through [[photosynthesis]]. Along with CAM photosynthesis, C<sub>4</sub> fixation is considered an advancement over the simpler and more ancient C<sub>3</sub> carbon fixation mechanism operating in most plants. Both mechanisms overcome the tendency of [[RuBisCO]] (the first enzyme in the [[Calvin cycle]]) to [[photorespiration|photorespire]], or waste energy by using oxygen to break down carbon compounds to CO<sub>2</sub>. However C<sub>4</sub> fixation requires more energy input than C<sub>3</sub> in the form of [[Adenosine triphosphate|ATP]]. C<sub>4</sub> plants separate rubisco from atmospheric oxygen, fixing carbon in the [[mesophyll cell]]s and using [[oxaloacetate]] and [[malate]] to ferry the fixed carbon to rubisco and the rest of the Calvin cycle enzymes isolated in the [[bundle-sheath]] cells. The intermediate compounds both contain four carbon atoms, hence the name C<sub>4</sub>.
==The pathway==
The C<sub>4</sub> pathway was discovered by [[Marshall Davidson Hatch|M. D. Hatch]] and C. R. Slack, in Australia, in 1966, so it is sometimes called the Hatch-Slack pathway.
In [[C3 plants|C<sub>3</sub> plants]], the first step in the [[light-independent reaction]]s of photosynthesis involves the fixation of CO<sub>2</sub> by the enzyme [[RuBisCo]] into [[3-phosphoglycerate]]. However, due to the dual [[carboxylase]] / [[oxygenase]] activity of [[RuBisCo]], an amount of the substrate is oxidized rather than carboxylated resulting in loss of substrate and consumption of energy, in what is known as [[photorespiration]].
In order to bypass the [[photorespiration]] pathway , C<sub>4</sub> plants have developed a mechanism to efficiently deliver CO<sub>2</sub> to the [[RuBisCO]] enzyme. They utilize their specific leaf anatomy where chloroplasts exist not only in the [[mesophyll]] cells in the outer part of their leaves but in the [[bundle sheath]] cells as well. Instead of direct fixation in the [[Calvin cycle]], CO<sub>2</sub> is converted to a 4-carbon [[organic acid]] which has the ability to regenerate CO<sub>2</sub> in the chloroplasts of the bundle sheath cells. Bundle sheath cells can then utilize this CO<sub>2</sub> to generate carbohydrates by the conventional [[C3 pathway|C<sub>3</sub> pathway]].
The first step in the pathway is the conversion of [[pyruvate]] to PEP by the enzyme pyruvate-phosphate dikinase ([[pyruvate, orthophosphate dikinase]]); this reaction requires inorganic phosphate and [[Adenosine triphosphate|ATP]] plus [[pyruvate]], giving [[phosphoenolpyruvate]], [[AMP]], and PPi (inorganic pyrophosphate) as products. The next step is the fixation of CO<sub>2</sub> by the enzyme [[phosphoenolpyruvate carboxylase]]. Both of these steps occur in the mesophyll cells:
:pyruvate + Pi + ATP → PEP + AMP + PPi
:PEP carboxylase + PEP + CO<sub>2</sub> → oxaloacetate
PEP carboxylase has a lower [[Michaelis-Menten kinetics|Km]] for CO<sub>2</sub>—and hence higher affinity—than Rubisco. Furthermore, O<sub>2</sub> is a very poor substrate for this enzyme. Thus, at relatively low concentrations of CO<sub>2</sub>, most CO<sub>2</sub> will be fixed by this pathway.
The product is usually converted to [[malate]], a simple [[organic compound]] that is transported to the bundle-sheath cells surrounding a nearby [[vein]], where it is decarboxylated to release CO<sub>2</sub>, which enters [[Calvin cycle]]. The decarboxylation leaves [[pyruvate]], which is transported back to the [[mesophyll]] cell.
Since every CO<sub>2</sub> molecule has to be fixed twice, the C<sub>4</sub> pathway is more energy-consuming than the C<sub>3</sub> pathway. The C<sub>3</sub> pathway requires 18 ATP for the synthesis of one molecule of glucose while the C<sub>4</sub> pathway requires 30 ATP. But since otherwise tropical plants lose more than half of photosynthetic carbon in [[photorespiration]], the C<sub>4</sub> pathway is an adaptive mechanism for minimizing the loss.
There are several variants of this pathway:
#The 4-carbon acid transported from mesophyll cells may be malate as above, or may be [[aspartate]].
#The 3-carbon acid transported back from bundle-sheath cells may be pyruvate as above, or [[alanine]].
#The enzyme which catalyses decarboxylation in bundle-sheath cells differs. In maize and sugarcane, the enzyme is NADP-malic enzyme, in millet, it is NAD-malic enzyme, and in ''[[Panicum]] maximum'' it is PEP carboxykinase.
==C<sub>4</sub> Leaf Anatomy==
The C<sub>4</sub> plants possess a characteristic [[leaf]] anatomy. Their vascular bundles are surrounded by two rings of cells. The inner ring, called Bundle Sheath Cells, contain [[starch]]-rich [[chloroplast]]s '''lacking grana''' which differ from those in [[mesophyll]] cells present as the outer ring. Hence, the chloroplasts are called dimorphic. This peculiar anatomy is called [[Kranz Anatomy]] (Kranz-Crown/Halo). The primary function of the Kranz is to provide a site in which carbon dioxide can be concentrated around RuBisCO, thus reducing photorespiration. In order to facilitate the maintenance of a significantly higher carbon dioxide concentration in the bundle sheath compared to the mesophyll, the boundary layer of the Kranz has a low conductance to carbon dioxide, a property which may be enhanced by the presence of suberin.
Although most C<sub>4</sub> plants exhibit Kranz anatomy, there are a number of species which operate a limited C<sub>4</sub> cycle without any distinct bundle sheath tissue. ''[[Suaeda]] aralocaspica'' (formerly known as ''Borszczowia aralocaspica''), ''[[Bienertia]] cycloptera'' and ''Bienertia sinuspersici'' (all [[Chenopodioideae|chenopods]]) are terrestrial plants which inhabit dry, salty depressions in the deserts of south-east Asia. These plants have been shown to operate single-cell C<sub>4</sub> carbon dioxide concentrating mechanisms which are unique amongst the known C<sub>4</sub> mechanisms. Although the cytology of both species differ slightly, the basic principle is that fluid filled vacuoles are employed to divide the cell into to separate areas. Carboxylation enzymes in the cytosol can therefore be kept separate from decarboxylase enzymes and RuBisCo in the chloroplasts, and a diffusive barrier can be established between the chloroplasts (which contain RuBisCO) and the cytosol. This enables a bundle-sheath type area and a mesophyll type area to be established within a single cell. Although this does allow a limited C<sub>3</sub> cycle to operate, it is relatively inefficient, with much leakage of CO2 from around RuBisCO occurring. There is also evidence for the non-Kranz aquatic macrophyte Hydrilla verticillata exhibiting inducible C<sub>4</sub> photosynthesis under warm conditions, although the mechanism by which CO2 leakage from around RuBisCO is minimised is currently uncertain.
==The Evolution and Advantages of the C<sub>4</sub> Pathway==
{{see|Evolutionary history of plants#Advances in metabolism}}
C<sub>4</sub> plants have a competitive advantage over plants possessing the more common [[C3 carbon fixation|C<sub>3</sub> carbon fixation]] pathway under conditions of [[drought]], high [[temperature]]s and [[nitrogen]] or [[carbon dioxide]] limitation. 97% of the water taken up by {{c3}} plants is lost through transpiration,<ref name=Raven2001>{{cite journal
| author = Raven, J.A.
| coauthors = Edwards, D.
| year = 2001
| title = Roots: evolutionary origins and biogeochemical significance
| journal = Journal of Experimental Botany
| volume = 52
| issue = 90001
| pages = 381–401
| doi = 10.1093/jexbot/52.suppl_1.381
| doi_brokendate = 2008-06-21
}}</ref
> compared to a much lower{{Quantify|date=April 2008}} proportion in {{c4}} plants, demonstrating their advantage in a dry environment.
C<sub>4</sub> carbon fixation has [[evolution|evolved]] on up to 40 independent occasions in different groups of plants, making it an example of [[convergent evolution]]<ref name=Osborne2006/>. Plants which use C<sub>4</sub> metabolism include [[sugarcane]], [[maize]], [[sorghum]], [[finger millet]], [[amaranth]], and [[switchgrass]]. C<sub>4</sub> plants arose around {{Ma|25|32}}<ref name=Osborne2006/> during the [[Oligocene]] (precisely when is difficult to determine) and did not become ecologically significant until around {{Ma|6|7}}, in the [[Miocene|Miocene Period]].<ref name=Osborne2006>{{cite journal
| author = Osborne, C.P.
| coauthors = Beerling, D.J.
| year = 2006
| title = Review. Nature's green revolution: the remarkable evolutionary rise of C 4 plants
| journal = Philosophical Transactions: Biological Sciences
| volume = 361
| issue = 1465
| pages = 173–194
| url = http://www.journals.royalsoc.ac.uk/index/YTH8204514044972.pdf
| accessdate = 2008-02-11
| doi = 10.1098/rstb.2005.1737
}}</ref> Today they represent about 5% of Earth's plant biomass and 1% of its known plant species.<ref name=Bond2005>{{cite journal
| author = Bond, W.J.
| coauthors = Woodward, F.I.; Midgley, G.F.
| year = 2005
| title = The global distribution of ecosystems in a world without fire
| journal = New Phytologist
| volume = 165
| issue = 2
| pages = 525–538
| doi = 10.1111/j.1469-8137.2004.01252.x
}}</ref> However, they account for around 30% of terrestrial carbon fixation.<ref name=Osborne2006/> These species are concentrated in the tropics (below latitudes of 45°) where the high air temperature contributes to higher possible levels of oxygenase activity by RuBisCO, which increases rates of photorespiration in C<sub>3</sub> plants.
==See also==
*[[C3 carbon fixation|C<sub>3</sub> carbon fixation]]
*[[Crassulacean acid metabolism|CAM Photosynthesis]]
==References==
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[[Category:Photosynthesis|C<sub>4</sub> carbon fixation]]
[[cs:Hatch-Slackův cyklus]]
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