Editing Oxygen (section)

===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>