Editing Oxygen (section)

==Biological production and role of O<sub>2</sub>==
{{Main|Dioxygen in biological reactions}}
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===Photosynthesis and respiration===
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[[File:Simple photosynthesis overview.svg|thumb|Photosynthesis splits water to liberate {{chem|O|2}} and fixes {{chem|CO|2}} into sugar in what is called a [[Calvin cycle]].|alt=A diagram of photosynthesis processes, including income of water and carbon dioxide, illumination and release of oxygen. Reactions produce ATP and NADPH in a Calvin cycle with a sugar as a by product.]]
In nature, free oxygen is produced as a [[byproduct]] of [[photolysis|light-driven splitting]] of water during [[chlorophyll]]ic [[photosynthesis]]. According to some estimates, marine [[photoautotroph]]s such as [[red algae|red]]/[[green algae]] and [[cyanobacteria]] provide about 70% of the free oxygen produced on Earth, and the rest is produced in terrestrial environments by plants.<ref>{{cite book|chapter-url=https://books.google.com/books?id=g6RfkqCUQyQC&pg=PA147|title=Plants: the potentials for extracting protein, medicines, and other useful chemicals (workshop proceedings)|date=September 1983|chapter=Marine Plants: A Unique and Unexplored Resource|last=Fenical|first=William|page=147|isbn=978-1-4289-2397-3|publisher=DianePublishing|access-date=August 23, 2020|archive-date=March 25, 2015|archive-url=https://web.archive.org/web/20150325221600/http://books.google.com/books?id=g6RfkqCUQyQC&pg=PA147|url-status=live}}</ref> Other estimates of the oceanic contribution to atmospheric oxygen are higher, while some estimates are lower, suggesting oceans produce ~45% of Earth's atmospheric oxygen each year.<ref>{{cite book|last=Walker|first=J. C. G.|date=1980|title=The oxygen cycle in the natural environment and the biogeochemical cycles|publisher=Springer-Verlag|location=Berlin}}</ref>

A simplified overall formula for photosynthesis is<ref>{{cite book|last1=Brown|first1=Theodore L. |last2=LeMay|first2=Burslen|title=Chemistry: The Central Science|url=https://archive.org/details/studentlectureno00theo|url-access=registration|isbn=978-0-13-048450-5|page=958|date=2003|publisher=Prentice Hall/Pearson Education}}</ref>

: 6 {{CO2}} + 6 {{chem|H|2|O}} + photons → {{chem|C|6|H|12|O|6}} + 6 {{chem|O|2}}

or simply

: carbon dioxide + water + sunlight → [[glucose]] + dioxygen

Photolytic [[oxygen evolution]] occurs in the [[thylakoid membrane]]s of photosynthetic organisms and requires the energy of four [[photon]]s.<ref group=lower-alpha>Thylakoid membranes are part of [[chloroplast]]s in algae and plants while they simply are one of many membrane structures in cyanobacteria. In fact, chloroplasts are thought to have evolved from [[cyanobacteria]] that were once symbiotic partners with the progenitors of plants and algae.</ref> Many steps are involved, but the result is the formation of a [[proton]] gradient across the thylakoid membrane, which is used to synthesize [[adenosine triphosphate]] (ATP) via [[photophosphorylation]].<ref name="Raven">[[#Reference-idRaven2005|Raven 2005]], 115–27</ref> The {{chem|O|2}} remaining (after production of the water molecule) is released into the atmosphere.<ref group=lower-alpha>Water oxidation is catalyzed by a [[manganese]]-containing [[enzyme]] complex known as the [[oxygen evolving complex]] (OEC) or water-splitting complex found associated with the lumenal side of thylakoid membranes. Manganese is an important [[Cofactor (biochemistry)|cofactor]], and [[calcium]] and [[chloride]] are also required for the reaction to occur. (Raven 2005)</ref>

Oxygen is used in [[mitochondria]] of [[eukaryote]]s to generate ATP during [[oxidative phosphorylation]]. The reaction for aerobic respiration is essentially the reverse of photosynthesis and is simplified as

: {{chem|C|6|H|12|O|6}} + 6 {{chem|O|2}} → 6 {{CO2}} + 6 {{chem|H|2|O}} + 2880 kJ/mol

In [[aquatic animal]]s, [[gas exchange]] of dissolved oxygen occurs via diffusion [[cutaneous respiration|across the skin]], [[enteral respiration|through the gut mucosae]] or via specialized respiratory organs known as [[gill]]s. In [[tetrapod]] [[vertebrate]]s, which are predominantly a terrestrial clade, atmospheric {{chem|O|2}} is inhaled into the [[lung]]s and diffuses through [[alveolar]] membranes into the blood stream. [[Hemoglobin]] in [[red blood cell]]s binds {{chem|O|2}}, changing color from bluish red to bright red<ref name="GuideElem48" /> ({{chem|CO|2}} is released from another part of hemoglobin through the [[Bohr effect]]). Other terrestrial [[invertebrate]]s use [[hemocyanin]] ([[mollusc]]s and some [[arthropod]]s) or [[hemerythrin]] (spiders and lobsters) instead.<ref name="NBB298" /> A liter of blood can dissolve up to 200&nbsp;cm<sup>3</sup> of {{chem|O|2}}.<ref name="NBB298" />

Until the discovery of [[anaerobic organism|anaerobic]] [[animal|metazoa]],<ref name="pmid20370908">{{cite journal |display-authors=4 |author=Danovaro R |author2=Dell'anno A |author3=Pusceddu A|author4=Gambi C |author5=Heiner I|author6=Kristensen RM |title=The first metazoa living in permanently anoxic conditions |journal=BMC Biology |volume=8 |issue=1 |pages=30 |date=April 2010 |pmid=20370908 |pmc=2907586 |doi=10.1186/1741-7007-8-30 |doi-access=free}}</ref> oxygen was thought to be a requirement for all complex life.<ref>{{cite book |last1=Ward |first1=Peter D. |last2=Brownlee |first2=Donald |title=Rare Earth: Why Complex Life is Uncommon in the Universe |publisher=Copernicus Books (Springer Verlag) |date=2000 |isbn=978-0-387-98701-9 |page=217}}</ref>

[[Reactive oxygen species]], such as [[superoxide]] ion ({{chem|O|2|-}}) and [[hydrogen peroxide]] ({{chem|H|2|O|2}}), are reactive by-products of oxygen use in organisms.<ref name="NBB298" /> Parts of the [[immune system]] of higher organisms create peroxide, superoxide, and singlet oxygen to destroy invading microbes. Reactive oxygen species also play an important role in the [[hypersensitive response]] of plants against pathogen attack.<ref name="Raven" /> Oxygen is damaging to [[Obligate anaerobe|obligately anaerobic organisms]], which were the dominant form of [[Evolutionary history of life|early life]] on Earth until {{chem|O|2}} began to accumulate in the atmosphere about 2.5 billion years ago during the [[Great Oxygenation Event]], about a billion years after the first appearance of these organisms.<ref>{{cite press release |title=NASA Research Indicates Oxygen on Earth 2.5 Billion Years ago |url=http://www.nasa.gov/home/hqnews/2007/sep/HQ_07215_Timeline_of_Oxygen_on_Earth.html |publisher=[[NASA]] |date=September 27, 2007 |access-date=March 13, 2008 |archive-date=March 13, 2008 |archive-url=https://web.archive.org/web/20080313063940/http://www.nasa.gov/home/hqnews/2007/sep/HQ_07215_Timeline_of_Oxygen_on_Earth.html |url-status=live }}</ref><ref name="NYT-20131003">{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=Earth's Oxygen: A Mystery Easy to Take for Granted |url=https://www.nytimes.com/2013/10/03/science/earths-oxygen-a-mystery-easy-to-take-for-granted.html |date=October 3, 2013 |work=[[The New York Times]] |access-date=October 3, 2013 |archive-date=May 16, 2020 |archive-url=https://web.archive.org/web/20200516083101/https://www.nytimes.com/2013/10/03/science/earths-oxygen-a-mystery-easy-to-take-for-granted.html |url-status=live }}</ref>

An adult human at rest inhales<!--simply inhales (most is exhaled again) or takes up and respires?--> 1.8 to 2.4&nbsp;grams of oxygen per minute.<ref>{{Cite web|url=https://patents.google.com/patent/US6224560B1/en|title=Flow restrictor for measuring respiratory parameters|access-date=August 4, 2019|archive-date=May 8, 2020|archive-url=https://web.archive.org/web/20200508103811/https://patents.google.com/patent/US6224560B1/en|url-status=live}}</ref> This amounts to more than 6 billion tonnes of oxygen inhaled by humanity per year.<ref group=lower-alpha>(1.8 grams/min/person)×(60 min/h)×(24 h/day)×(365 days/year)×(6.6 billion people)/1,000,000 g/t=6.24 billion tonnes</ref>

===Living organisms===
{{anchor|partial pressure}}
{|class="wikitable" style="float:right; margin-left:25px"
|+Partial pressures of oxygen in the human body (PO<sub>2</sub>)
|-
! Unit !! Alveolar pulmonary<br /> gas pressures !! Arterial blood oxygen !! Venous blood gas
|-
| [[kPa]] || 14.2 || 11{{efn|name=mmHg|Derived from mmHg values using 0.133322 kPa/mmHg}}-13{{efn|name=mmHg}} || 4.0{{efn|name=mmHg}}-5.3{{efn|name=mmHg}}
|-
| [[mmHg]] || 107 || 75<ref name="southwest">
[http://pathcuric1.swmed.edu/PathDemo/nrrt.htm Normal Reference Range Table] {{Webarchive|url=https://web.archive.org/web/20111225185659/http://pathcuric1.swmed.edu/PathDemo/nrrt.htm |date=December 25, 2011 }} from The University of Texas Southwestern Medical Center at Dallas. Used in Interactive Case Study Companion to Pathologic basis of disease.
</ref>-100<ref name="southwest" /> || 30<ref name="brookside" />-40<ref name="brookside">[http://www.brooksidepress.org/Products/OperationalMedicine/DATA/operationalmed/Lab/ABG_ArterialBloodGas.htm The Medical Education Division of the Brookside Associates--> ABG (Arterial Blood Gas)] {{Webarchive|url=https://web.archive.org/web/20170812201558/http://www.brooksidepress.org/Products/OperationalMedicine/DATA/operationalmed/Lab/ABG_ArterialBloodGas.htm |date=August 12, 2017 }} Retrieved on December 6, 2009</ref>
|-
|}
The free oxygen [[partial pressure]] in the body of a living vertebrate organism is highest in the [[respiratory system]], and decreases along any [[arterial system]], peripheral tissues, and [[venous system]], respectively. Partial pressure is the pressure that oxygen would have if it alone occupied the volume.<ref>{{cite book|author=Charles Henrickson|title=Chemistry|publisher=Cliffs Notes|date=2005|isbn=978-0-7645-7419-1|url=https://archive.org/details/chemistry00henr}}</ref>

===Build-up in the atmosphere===
{{Main|Geological history of oxygen}}
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[[File:Oxygenation-atm.svg|thumb|left|upright=1.35|{{chem|O|2}} build-up in Earth's atmosphere: 1) no {{chem|O|2}} produced; 2) {{chem|O|2}} produced, but absorbed in oceans & seabed rock; 3) {{chem|O|2}} starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4–5) {{chem|O|2}} sinks filled and the gas accumulates|alt=A graph showing time evolution of oxygen pressure on Earth; the pressure increases from zero to 0.2 atmospheres.]]

Free oxygen gas was almost nonexistent in [[Earth's atmosphere]] before photosynthetic [[archaea]] and [[bacteria]] evolved, probably about 3.5 billion years ago. Free oxygen first appeared in significant quantities during the [[Paleoproterozoic]] era (between 3.0 and 2.3 billion years ago).<ref name="Crowe2013">{{Cite journal | last1 = Crowe | first1 = S. A. | last2 = Døssing | first2 = L. N. | last3 = Beukes | first3 = N. J. | last4 = Bau | first4 = M. | last5 = Kruger | first5 = S. J. | last6 = Frei | first6 = R. | last7 = Canfield | first7 = D. E. | title = Atmospheric oxygenation three billion years ago | journal = Nature | volume = 501 | issue = 7468 | pages = 535–38 | year = 2013 | pmid = 24067713 | doi = 10.1038/nature12426 | bibcode = 2013Natur.501..535C | s2cid = 4464710 }}</ref> Even if there was much dissolved [[iron]] in the oceans when oxygenic photosynthesis was getting more common, it appears the [[banded iron formation]]s were created by anoxyenic or micro-aerophilic iron-oxidizing bacteria which dominated the deeper areas of the [[photic zone]], while oxygen-producing cyanobacteria covered the shallows.<ref>[https://www.sciencedaily.com/releases/2013/04/130423110750.htm Iron in primeval seas rusted by bacteria] {{Webarchive|url=https://web.archive.org/web/20200311023339/https://www.sciencedaily.com/releases/2013/04/130423110750.htm |date=March 11, 2020 }}, ScienceDaily, April 23, 2013</ref> Free oxygen began to [[Outgassing|outgas]] from the oceans 3–2.7&nbsp;billion years ago, reaching 10% of its present level around 1.7&nbsp;billion years ago.<ref name="Crowe2013" /><ref name="Campbell">{{cite book|last1 = Campbell|first1 = Neil A.|last2=Reece|first2=Jane B.|title = Biology|edition = 7th|publisher = Pearson&nbsp;– Benjamin Cummings |date=2005|location = San Francisco|pages = 522–23|isbn = 978-0-8053-7171-0}}</ref>

The presence of large amounts of dissolved and free oxygen in the oceans and atmosphere may have driven most of the extant [[anaerobic organism]]s to [[extinction]] during the [[Great Oxygenation Event]] (''oxygen catastrophe'') about 2.4 billion years ago. [[Cellular respiration]] using {{chem|O|2}} enables [[aerobic organism]]s to produce much more [[Adenosine triphosphate|ATP]] than anaerobic organisms.<ref name="Freeman">{{cite book|last = Freeman|first = Scott|title = Biological Science, 2nd|publisher = Pearson&nbsp;– Prentice Hall|date = 2005|location = Upper Saddle River, NJ|pages = [https://archive.org/details/biologicalscienc00scot/page/214 214, 586]|isbn = 978-0-13-140941-5|url = https://archive.org/details/biologicalscienc00scot/page/214}}</ref> Cellular respiration of {{chem|O|2}} occurs in all [[eukaryote]]s, including all complex multicellular organisms such as plants and animals.

Since the beginning of the [[Cambrian]] period 540 million years ago, atmospheric {{chem|O|2}} levels have fluctuated between 15% and 30% by volume.<ref name="geologic">{{cite journal |title=Atmospheric oxygen over Phanerozoic time |first=Robert A. |last=Berner |issue=20 |pages=10955–57 |date=1999|journal=Proceedings of the National Academy of Sciences of the USA |pmid=10500106 |doi=10.1073/pnas.96.20.10955 |volume=96 |pmc=34224 |bibcode=1999PNAS...9610955B|doi-access=free }}</ref> Towards the end of the [[Carboniferous]] period (about 300&nbsp;million years ago) atmospheric {{chem|O|2}} levels reached a maximum of 35% by volume,<ref name="geologic" /> which may have contributed to the large size of insects and amphibians at this time.<ref name="Butterfield2009">{{Cite journal | last1 = Butterfield | first1 = N. J. | title = Oxygen, animals and oceanic ventilation: An alternative view | doi = 10.1111/j.1472-4669.2009.00188.x | journal = Geobiology | volume = 7 | issue = 1 | pages = 1–7 | year = 2009 | pmid = 19200141 | bibcode = 2009Gbio....7....1B | s2cid = 31074331 }}</ref>

Variations in atmospheric oxygen concentration have shaped past climates. When oxygen declined, atmospheric density dropped, which in turn increased surface evaporation, causing precipitation increases and warmer temperatures.<ref>{{cite journal|url=http://ns.umich.edu/new/releases/22942-variations-in-atmospheric-oxygen-levels-shaped-earth-s-climate-through-the-ages|doi=10.1126/science.1260670|pmid=26068848|journal=Science|title=Long-term climate forcing by atmospheric oxygen concentrations|author1=Poulsen, Christopher J.|author2=Tabor, Clay|author3=White, Joseph D.|volume=348|issue=6240|pages=1238–41|bibcode=2015Sci...348.1238P|year=2015|s2cid=206562386|access-date=June 12, 2015|archive-date=July 13, 2017|archive-url=https://web.archive.org/web/20170713125418/http://ns.umich.edu/new/releases/22942-variations-in-atmospheric-oxygen-levels-shaped-earth-s-climate-through-the-ages|url-status=live}}</ref>

At the current rate of photosynthesis it would take about 2,000&nbsp;years to regenerate the entire {{chem|O|2}} in the present atmosphere.<ref>{{cite journal|title=The Natural History of Oxygen|first=Malcolm|last=Dole |journal=The Journal of General Physiology|volume=49|pages=5–27|date=1965|doi=10.1085/jgp.49.1.5|pmid=5859927|issue=1|pmc=2195461}}</ref>
{{clear}}

It is estimated that oxygen on Earth will last for about one billion years.<ref>{{Cite journal|url=https://www.nature.com/articles/s41561-021-00693-5|title=The future lifespan of Earth's oxygenated atmosphere|first1=Kazumi|last1=Ozaki|first2=Christopher T.|last2=Reinhard|date=March 9, 2021|journal=Nature Geoscience|volume=14|issue=3|pages=138–142|via=www.nature.com|doi=10.1038/s41561-021-00693-5|arxiv=2103.02694|bibcode=2021NatGe..14..138O |s2cid=232083548 }}</ref><ref>{{Cite web|url=https://www.eurekalert.org/news-releases/825455|title=How much longer will the oxygen-rich atmosphere be sustained on Earth?|website=EurekAlert!}}</ref>

===Extraterrestrial free oxygen===
{{Main|Extraterrestrial atmosphere}}
In the field of [[astrobiology]] and in the search for [[extraterrestrial life]] oxygen is a strong [[biosignature]]. That said it might not be a definite biosignature, being [[Extraterrestrial atmosphere#Abiotic oxygen|possibly produced abiotically]] on [[celestial bodies]] with processes and conditions (such as a peculiar [[hydrosphere]]) which allow free oxygen,<ref>{{cite web|url=https://earthsky.org/space/oxygen-exoplanets-not-always-indicator-of-life|title=Oxygen and life: a cautionary tale|date=3 January 2019|author=Paul Scott Anderson|access-date=29 December 2020|archive-date=January 22, 2021|archive-url=https://web.archive.org/web/20210122134654/https://earthsky.org/space/oxygen-exoplanets-not-always-indicator-of-life|url-status=live}}</ref><ref>{{cite journal | vauthors = Luger R, Barnes R | title = Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs | journal = Astrobiology | volume = 15 | issue = 2 | pages = 119–43 | date = February 2015 | pmid = 25629240 | pmc = 4323125 | doi = 10.1089/ast.2014.1231 | bibcode = 2015AsBio..15..119L | arxiv = 1411.7412 }}</ref><ref>{{cite journal |last1=Wordsworth |first1=Robin |last2=Pierrehumbert |first2=Raymond |title=Abiotic oxygen-dominated atmospheres on terrestrial habitable zone planets |journal=The Astrophysical Journal |date=1 April 2014 |volume=785 |issue=2 |pages=L20 |doi=10.1088/2041-8205/785/2/L20 |bibcode=2014ApJ...785L..20W |arxiv=1403.2713 |s2cid=17414970 }}</ref> like with [[Europa (moon)|Europa's]] and [[Ganymede (moon)|Ganymede's]] thin oxygen atmospheres.<ref name="Hall1998">{{cite journal |last1=Hall |first1=D.T. |last2=Feldman |first2=P.D. |last3=McGrath |first3=M.A. |last4=Strobel |first4=D. F. |display-authors=2 |title=The Far-Ultraviolet Oxygen Airglow of Europa and Ganymede |journal=The Astrophysical Journal |date=1998 |volume=499 |issue=1 |pages=475–81 |doi=10.1086/305604 |bibcode=1998ApJ...499..475H |doi-access=free }}</ref>