Photorespiration
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Typo fix.
'''Photorespiration''' (or "photo-respiration") is the alternate pathway for production of [[glyceraldehyde 3-phosphate]] (G3P) by [[RuBisCO]], the main enzyme of the light-independent reactions of [[photosynthesis]] (also known as the [[Calvin cycle]] or the Primary Carbon Reduction (PCR) cycle). Although RuBisCO favors [[carbon dioxide]] to [[oxygen]],(approximately 3 carboxylations per oxygenation), oxygenation of [[RuBisCO]] occurs frequently, producing a [[glycolate]] and a glycerate. This usually occurs when oxygen levels are high; for example, when the [[stomata]] (tiny pores on the [[leaf]]) are closed to prevent water loss on dry days. It involves three cellular organelles: chloroplasts, peroxisomes, and mitochondria. Photorespiration produces no ATP.
==Biochemistry of photorespiration==
[[Image:Photorespiration eng.png|thumb|left|200px|Photorespiration]] The oxidative photosynthetic carbon cycle reaction is [[catalyst|catalyzed]] by [[RuBisCO|RuBP oxygenase]] activity:
:RuBP + O<sub>2</sub> → Phosphoglycolate + [[Glycerate 3-phosphate|3-phosphoglycerate]]
The phosphoglycolate is salvaged by a series of reactions in the [[peroxisome]], [[mitochondria]], and again in the [[peroxisome]] where it is converted into [[serine]] and later glycerate. Glycerate reenters the chloroplast and subsequently the [[Calvin cycle]] by the same transporter that exports glycolate. A cost of 1 [[Adenosine triphosphate|ATP]] is associated with conversion to 3-phosphoglycerate (PGA) ([[Phosphorylation]]), within the [[chloroplast]], which is then free to reenter the PCR cycle. One carbon dioxide molecule is produced for every 2 molecules of O<sub>2</sub> that are taken up by RuBisCO.
Photorespiration is a wasteful process because [[G3P]] is created at a reduced rate and higher metabolic cost (2ATP and one [[NAD(P)H]]) compared with [[Calvin cycle|RuBP carboxylase activity]]. G3P produced in the chloroplast is used to create "nearly all" of the food and structures in the plant. While photorespiratory carbon cycling results in G3P eventually, it also produces waste [[ammonia]] that must be detoxified at a substantial cost to the cell in ATP and reducing equivalents.
==Role of photorespiration==
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Photorespiration is said to be an [[Vestigial structure|evolutionary relic]]. Photorespiration lowers the efficiency of [[photosynthesis]] by removing carbon molecules from the [[Calvin Cycle]]. The early atmosphere in which primitive plants originated contained very little oxygen, so it is hypothesized that the early evolution of [[RuBisCO]] was not influenced by its lack of discrimination between O<sub>2</sub> and carbon dioxide.
Another theory postulates that it may function as a "safety valve", preventing excess [[NADPH]] and [[Adenosine triphosphate|ATP]] from reacting with oxygen and producing [[Radical (Chemistry)|free radicals]], as these can damage the metabolic functions of the cell by subsequent reactions with lipids or metabolites of alternate pathways.
== Minimization of photorespiration (C4 and CAM plants)==
[[Image:Field, corn, Liechtenstein, Mountains, Alps, Vaduz, sky, clouds, landscape.jpg|right|thumb|Maize uses the C4 pathway, minimizing photorespiration.]]
Since photorespiration requires additional energy from the light reactions of photosynthesis, some plants have mechanisms to reduce uptake of molecular oxygen by [[RuBisCO]].
They increase the concentration of [[CO2|CO<sub>2</sub>]] in the leaves so that [[Rubisco]] is less likely to produce glycolate through reaction with O<sub>2</sub>.
[[C4_carbon_fixation|C4]] plants capture carbon dioxide in cells of their mesophyll (using an enzyme called PEP carboxylase), and oxaloacete is formed. this oxaloacetate is then converted to malate and is release into the bundle sheath cells (site of carbon dioxide fixation by [[RuBisCO]]) where [[oxygen]] concentration is low to avoid photorespiration. Here Carbon dioxide is removed from the malate and combined with RuBP in the usual way. The Calvin cycle then proceeds as normal.
The enzyme PEP carboxylase (which catalyzes the combination of carbon dioxide with a compound called Phosphoenolpruvate or PEP) is also found in other plants such as cacti and succulents who use a mechanism called [[CAM_photosynthesis|Crassulacean acid metabolism]] or CAM in which PEP carboxylase sequesters carbon at night and releases it to the photosynthesizing cells during the day. This provides a mechanism for reducing high rates of water loss ([[transpiration]]) by stomata during the day.
This ability to avoid photorespiration makes these plants more hardy than other plants in dry conditions where stomata are closed and oxygen concentrations rise. C4 plants include [[sugar cane]], [[maize|corn (maize)]], and [[sorghum]].
==References==
*Stern, Kingsley R., Shelley Jansky, James E Bidlack. Introductory Plant Biology. Mc Graw Hill. 2003 ISBN 0-07-290941-2
*Siedow, James N., David Day. Chapter 14 "Respiration and Photorespiration". Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists. 2000.
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