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| '''Oxygen''' '''evolution''' is the process of generating molecular [[oxygen]] through [[chemical reaction]]. Mechanisms of oxygen evolution include the [[oxidation]] of [[water]] during oxygenic [[photosynthesis]], [[electrolysis]] of water into oxygen and [[hydrogen]], and electrocatalytic oxygen evolution from [[oxide]]s and [[oxoacid]]s.
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| ==Oxygen evolution in nature==
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| Photosynthetic oxygen evolution is the fundamental process by which breathable oxygen is generated in earth's [[biosphere]]. The reaction is part of the [[light-dependent reaction]]s of photosynthesis in [[cyanobacteria]] and the [[chloroplast]]s of [[green algae]] and [[plant]]s. It utilizes the energy of [[light]] to split a water molecule into its [[proton]]s and [[electron]]s for photosynthesis. Free oxygen is generated as a by-product of this reaction, and is released into the [[Earth's atmosphere|atmosphere]].
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| ===Biochemical reaction===
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| Photosynthetic oxygen evolution occurs via the light-dependent oxidation of water to molecular oxygen and can be written as the following simplified chemical reaction:
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| 2H<sub>2</sub>O <math>\longrightarrow</math> 4e<sup>-</sup> + 4H<sup>+</sup> + O<sub>2</sub>
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| The reaction requires the energy of four [[photon]]s. The electrons from the oxidized water molecules replace electrons in the P<sub>680</sub> component of [[photosystem|photosystem II]] that have been removed into an [[electron transport chain]] via light-dependent [[excited state|excitation]] and [[resonance energy transfer]] onto [[plastoquinone]].<ref name="Raven">{{cite book | last = Raven | first = Peter H. | coauthors = Ray F. Evert, Susan E. Eichhorn | title = Biology of Plants, 7th Edition | publisher = W.H. Freeman and Company Publishers | date = 2005 | location = New York | pages = 115–127 | isbn = 0-7167-1007-2}}</ref> Photosytem II, therefore, has also been referred to as water-plastoquinone oxido-reductase.<ref name="Raval">{{cite journal |author=Raval M, Biswal B, Biswal U |title=The mystery of oxygen evolution: analysis of structure and function of photosystem II, the water-plastoquinone oxido-reductase |journal=Photosynthesis Research |volume=85 |issue=3 |pages=267–93 |year=2005 |pmid=16170631| doi=10.1007/s11120-005-8163-4}}</ref> The protons are released into the [[thylakoid lumen]], thus contributing to the generation of a proton gradient across the thylakoid membrane. This proton gradient is the driving force for [[Adenosine triphosphate|ATP]] synthesis via [[photophosphorylation]] and coupling the absorption of light energy and oxidation of water to the creation of chemical energy during photosynthesis.<ref name="Raven"/>
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| ===Oxygen-evolving complex===
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| Water oxidation is catalyzed by a [[manganese]]-containing [[Cofactor (biochemistry)|cofactor]] contained in [[photosystem II]] known as the [[oxygen-evolving complex]] (OEC) or water-splitting complex. Manganese is an important [[Cofactor (biochemistry)|cofactor]], and [[calcium]] and [[chloride]] are also required for the reaction to occur.<ref name="Raven"/>
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| [[X-ray crystallography|X-ray crystallogra]] is structure of the oxygen-evolving complex and its manganese cluster.<ref name="Ferreira">{{cite journal |author=Ferreira K, Iverson T, Maghlaoui K, Barber J, Iwata S |title=Architecture of the photosynthetic oxygen-evolving center |journal=Science |volume=303 |issue=5665 |pages=1831–8 |year=2004 |pmid=14764885| doi=10.1126/science.1093087}}</ref> Based on structural and [[spectroscopy|spectroscopic]] experiments, oxygen evolution involves a core three-plus-one cluster of three manganese [[ion]]s and one calcium ion, with one additional manganese, which are oxidized via intermediate states called ''S-states''. The O-O bond of molecular oxygen is formed between manganese-ligated oxygen atoms at the most oxidized, or S4, state.<ref name="Robblee">{{cite journal |author=Robblee J, Cinco R, Yachandra V |title=X-ray spectroscopy-based structure of the Mn cluster and mechanism of photosynthetic oxygen evolution |journal=Biochimica et Biophysica Acta |volume=1503 |issue=1-2 |pages=7–23 |year=2001 |pmid=11115621 |doi=10.1016/S0005-2728(00)00217-6 }}</ref>
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| ===History of discovery===
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| It was not until the end of the 18th century that [[Joseph Priestley]] discovered by accident the ability of plants to "restore" air that had been "injured" by the burning of a candle. He followed up on the experiment by showing that air "restored" by [[vegetation]] was ''"not at all inconvenient to a [[mouse]]."'' He was later awarded a medal for his discoveries that: ''"...no vegetable grows in vain... but cleanses and purifies our atmosphere."'' Priestley's experiments were followed up by [[Jan Ingenhousz]], a Dutch physician, who showed that "restoration" of air only worked in the presence of light and green plant parts.<ref name="Raven"/>
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| Ingenhousz suggested in 1796 that CO<sub>2</sub> ([[carbon dioxide]]) is split during photosynthesis to release oxygen, while the [[carbon]] combined with water to form [[carbohydrate]]s. While this hypothesis was attractive and reasonable and thus widely accepted for a long time, it was later proven incorrect. Graduate student [[C.B. Van Niel]] at [[Stanford University]] found that [[purple sulfur bacteria]] reduce carbon to carbohydrates, but accumulate [[sulfur]] instead of releasing oxygen. He boldly proposed that, in analogy to the sulfur bacteria's forming elemental sulfur from H<sub>2</sub>S ([[hydrogen sulfide]]), plants would form oxygen from H<sub>2</sub>O (water). In 1937, this hypothesis was corroborated by the discovery that plants are capable of producing oxygen in the absence of CO<sub>2</sub>. This discovery was made by [[Robin Hill (biochemist)|Robin Hill]], and subsequently the light-driven release of oxygen in the absence of CO<sub>2</sub> was called the ''Hill reaction''. Our current knowledge of the mechanism of oxygen evolution during photosynthesis was further established in experiments tracing [[isotopes of oxygen]] from water to oxygen gas.<ref name="Raven"/>
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| ==Technological oxygen evolution==
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| <!-- Deleted image removed: [[Image:Hoffman voltameter.jpg|thumb|300px|Hoffman electrolysis apparatus used in electrolysis of water{{puic|Image:Hoffman voltameter.jpg|log=2008 April 26}}]] -->
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| Oxygen evolution occurs as a byproduct of [[hydrogen production]] via [[electrolysis of water]]. While oxygen production is not the main focus of industrial applications of water electrolysis, it becomes essential for [[life support system]]s in situations that require the generation of oxygen for air revitalization. Human exploration of regions that lack breathable oxygen, such as the deep sea or outer space, requires means of reliably generating oxygen apart from earth's atmosphere. [[Submarine]]s and [[spacecraft]] utilize either an [[electrolysis|electrolytic]] mechanism (water or solid oxide electrolysis) or [[chemical oxygen generator]]s as part of their life support equipment.
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| ==References==
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| {{reflist}}
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| ==External links==
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| *Plant Physiology Online, 4th edition: [http://4e.plantphys.net/article.php?ch=7&id=72 Topic 7.7 - Oxygen Evolution]
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| *[http://www.life.uiuc.edu/crofts/bioph354/lect25.html Oxygen evolution] - Lecture notes by Antony Crofts, UIUC
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| *[http://www.globalchange.umich.edu/globalchange1/current/lectures/samson/evolution_atm/ Evolution of the atmosphere] – Lecture notes, Regents of the University of Michigan
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| *[http://www.wikihow.com/Make-Oxygen-and-Hydrogen-from-Water-Using-Electrolysis How to make oxygen and hydrogen from water using electrolysis]
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| [[Category:Photosynthesis]]
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| [[Category:Breathing gases]]
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| [[Category:Oxygen]]
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| [[Category:Evolution]]
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| [[Category:Geological history of Earth]]
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| [[Category:Biological evolution]]
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