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Overview of C4 carbon fixation

C4 carbon fixation is one of three biochemical mechanisms, along with C3 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. C4 fixation is an elaboration of C3 carbon fixation (which operates in most plants), and is believed to have evolved more recently. C4 and CAM overcome the tendency of RuBisCO (the first enzyme in the Calvin cycle) to fix oxygen rather than carbon dioxide, which leads to a loss of energy and carbon in a process called photorespiration. This is achieved by using a more efficient enzyme to fix CO2 in mesophyll cells and shuttling the fixed carbon via malate or oxaloacetate to bundle-sheath cells, where Rubisco is sequestered from atmospheric oxygen and can be saturated with CO2 released by decarboxylation of the malate or oxaloacetate. However, these additional steps require energy in the form of ATP. Because of these tradeoffs, no one of these three photosynthetic pathways is considered superior to the others -- rather, each is best suited to a different set of conditions. The name "C4" comes from the fact that the first product of CO2 fixation in these plants has four carbon atoms, rather than three, as is the case in C3 plants.

Contents

The pathway

The C4 pathway was discovered by M. D. Hatch and C. R. Slack, in Australia, in 1966, so it is sometimes called the Hatch-Slack pathway.[1]

In C3 plants, the first step in the light-independent reactions of photosynthesis involves the fixation of CO2 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, C4 plants have developed a mechanism to efficiently deliver CO2 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, CO2 is converted to a 4-carbon organic acid which has the ability to regenerate CO2 in the chloroplasts of the bundle sheath cells. Bundle sheath cells can then utilize this CO2 to generate carbohydrates by the conventional C3 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 ATP plus pyruvate, giving phosphoenolpyruvate, AMP, and PPi (inorganic pyrophosphate) as products. The next step is the fixation of CO2 by the enzyme phosphoenolpyruvate carboxylase. Both of these steps occur in the mesophyll cells:

pyruvate + Pi + ATP → PEP + AMP + PPi
PEP carboxylase + PEP + CO2 → oxaloacetate

PEP carboxylase has a lower Km for CO2—and hence higher affinity—than Rubisco. Furthermore, O2 is a very poor substrate for this enzyme. Thus, at relatively low concentrations of CO2, most CO2 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 CO2, which enters the Calvin cycle. The decarboxylation leaves pyruvate, which is transported back to the mesophyll cell.

Since every CO2 molecule has to be fixed twice, the C4 pathway is more energy-consuming than the C3 pathway. The C3 pathway requires 18 ATP for the synthesis of one molecule of glucose while the C4 pathway requires 30 ATP. But since otherwise tropical plants lose more than half of photosynthetic carbon in photorespiration, the C4 pathway is an adaptive mechanism for minimizing the loss.

There are several variants of this pathway:

  1. The 4-carbon acid transported from mesophyll cells may be malate as above, or may be aspartate.
  2. The 3-carbon acid transported back from bundle-sheath cells may be pyruvate as above, or alanine.
  3. 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.

C4 leaf anatomy

The C4 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 chloroplasts 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, German for "wreath"). The primary function of kranz anatomy is to provide a site in which carbon dioxide can be concentrated around RuBisCO, thereby 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.[2]

Although most C4 plants exhibit kranz anatomy, there are a number of species which operate a limited C4 cycle without any distinct bundle sheath tissue. Suaeda aralocaspica (formerly known as Borszczowia aralocaspica), Bienertia cycloptera and Bienertia sinuspersici (all 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 C4 carbon dioxide concentrating mechanisms which are unique amongst the known C4 mechanisms. Although the cytology of both species differ slightly, the basic principle is that fluid filled vacuoles are employed to divide the cell into two 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 C3 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 C4 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 C4 pathway

C4 plants have a competitive advantage over plants possessing the more common C3 carbon fixation pathway under conditions of drought, high temperatures and nitrogen or carbon dioxide limitation. 97% of the water taken up by C3 plants is lost through transpiration,[3] compared to a much lower proportion in C4 plants, demonstrating their advantage in a dry environment.

C4 carbon fixation has evolved on up to 40 independent occasions in different groups of plants, making it a prime example of convergent evolution.[4] C4 plants arose around 25 to 32 million years ago[4] during the Oligocene (precisely when is difficult to determine) and did not become ecologically significant until around 6 to 7 million years ago, in the Miocene Period.[4] C4 metabolism originated when grasses migrated from the shady forest undercanopy to more open environments,[5] where the high sunlight gave it an advantage over the C3 pathway.[6] Drought was not necessary for its innovation - rather, the increased resistance to water stress was a by-product of the pathway and allowed C4 plants to more readily colonise arid environments.[6]

Today, C4 plants represent about 5% of Earth's plant biomass and 1% of its known plant species.[7] Despite this scarcity, they account for around 30% of terrestrial carbon fixation.[4] Increasing the proportion of C4 plants on earth could assist biosequestration of CO2 and represent an important climate change strategy. Present-day C4 plants 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 C3 plants.

Examples of plants that use C4 carbon fixation

See also

References

  1. ^ http://www.biochemj.org/bj/103/0660/1030660.pdf Biochem. J. (1967) 103, 660 Comparative Studies on the Activity of Carboxylases and Other Enzymes in Relation to the New Pathway of Photosynthetic Carbon Dioxide Fixation in Tropical Grasses
  2. ^ Laetsch (1971) Photosynthesis and Photorespiration, eds Hatch, Osmond and Slatyer
  3. ^ Raven, J.A.; Edwards, D. (2001). "Roots: evolutionary origins and biogeochemical significance". Journal of Experimental Botany 52 (90001): 381–401. doi:10.1093/jexbot/52.suppl_1.381 (inactive 2008-06-21). 
  4. ^ a b c d Osborne, C.P.; Beerling, D.J. (2006). "Review. Nature's green revolution: the remarkable evolutionary rise of C4 plants". Philosophical Transactions of the Royal Society B: Biological Sciences 361 (1465): 173–194. doi:10.1098/rstb.2005.1737. http://rstb.royalsocietypublishing.org/content/361/1465/173.full.html. Retrieved 2008-02-11. 
  5. ^ Edwards, E.; Smith, S. (2010). "Phylogenetic analyses reveal the shady history of C4 grasses.". Proceedings of the National Academy of Sciences of the United States of America 107: 2532. doi:10.1073/pnas.0909672107. PMID 20142480.  edit
  6. ^ a b Osborne, P.; Freckleton, P. (Feb 2009). "Ecological selection pressures for C4 photosynthesis in the grasses". Proceedings. Biological sciences / the Royal Society 276 (1663): 1753. doi:10.1098/rspb.2008.1762. ISSN 0962-8452. PMID 19324795.  edit
  7. ^ Bond, W.J.; Woodward, F.I.; Midgley, G.F. (2005). "The global distribution of ecosystems in a world without fire". New Phytologist 165 (2): 525–538. doi:10.1111/j.1469-8137.2004.01252.x. 
  8. ^ van der Maarel, E. (2005). Vegetation ecology. Wiley-Blackwell. p. 363. ISBN 0632057610. 
  9. ^ Zhu, Xin-Guang,Long, Stephen P;Ort, R Donald (2008). "What is the maximum efficiency with which photoysynthesis can convert solar energy into biomass?". Current Opinion in Biotechnology 19: 153–159. doi:10.1016/j.copbio.2008.02.004. 
  10. ^ Kadereit, G; Borsch,T; Weising,K; Freitag, H (2003). "Phylogeny of Amaranthaceae and Chenopodiaceae and the Evolution of C4 Photosynthesis". International Journal of Plant Sciences 164 (6): 959–86. doi:10.1086/378649. 
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