Editing Reactive oxygen species (section)

==Inventory of ROS==
ROS are not uniformly defined.  All sources include superoxide, singlet oxygen, and hydroxyl radical. Hydrogen peroxide is not nearly as reactive as these species, but is readily activated and is thus included.<ref>{{Cite journal |vauthors=Nosaka Y, Nosaka AY |date=September 2017 |title=Generation and Detection of Reactive Oxygen Species in Photocatalysis |journal=Chemical Reviews |volume=117 |issue=17 |pages=11302–11336 |doi=10.1021/acs.chemrev.7b00161 |pmid=28777548}}</ref>  Peroxynitrite and [[nitric oxide]] are reactive oxygen-containing species as well. 
* [[Hydroxyl radical]] ({{chem2|HO*}}) is generated by [[Fenton reaction]] of hydrogen peroxide with ferrous compounds and related reducing agents:
:{{chem2|Fe(II) + H2O2 ->  Fe(III)OH  +  HO*}}
In its fleeting existence, the hydroxyl radical reacts rapidly irreversibly with all organic compounds.
* [[superoxide]] ({{chem2|O2-}}) is produced by reduction of O<sub>2</sub>.<ref name="Turrens_2003">{{Cite journal |vauthors=Turrens JF |date=October 2003 |title=Mitochondrial formation of reactive oxygen species |journal=The Journal of Physiology |volume=552 |issue=Pt 2 |pages=335–44 |doi=10.1113/jphysiol.2003.049478 |pmc=2343396 |pmid=14561818}}</ref> Several grams are produced per day in the human body within the mitochondria.<ref>{{Cite journal |last1=Hayyan |first1=M. |last2=Hashim |first2=M.A. |last3=AlNashef |first3=I.M. |year=2016 |title=Superoxide Ion: Generation and Chemical Implications |journal=Chem. Rev. |volume=116 |issue=5 |pages=3029–3085 |doi=10.1021/acs.chemrev.5b00407 |pmid=26875845 |doi-access=free}}</ref>
:{{chem2|O2 + e-  ->  O2-}}
Competing with its formation, superoxide is destroyed by the action of [[superoxide dismutase]]s, enzymes that catalyze its disproportionation:
:{{chem2|2 O2-  +  2H+ ->  O2  +  H2O2}}
* [[hydrogen peroxide]] ({{chem2|H2O2}}) is also produced as a side product of respiration.<ref name="Turrens_2003" />
* [[Peroxynitrite]] ({{chem2|ONO2-}}) results from the reaction of [[superoxide]] and [[nitric oxide]].
* [[Singlet oxygen]] ({{chem2|^{1}O2}}) is sometimes included as an ROS. [[Photosensitizer]]s such as [[chlorophyll]] may convert [[Triplet oxygen|triplet]] ({{chem2|^{3}O2}}) to singlet oxygen:<ref name="Laloi_2015">{{Cite journal |vauthors=Laloi C, Havaux M |year=2015 |title=Key players of singlet oxygen-induced cell death in plants |journal=Frontiers in Plant Science |volume=6 |pages=39 |doi=10.3389/fpls.2015.00039 |pmc=4316694 |pmid=25699067 |doi-access=free|bibcode=2015FrPS....6...39L }}</ref> Singlet oxygen is highly reactive with unsaturated organic compounds. [[Carotenoid]]s, [[tocopherol]]s, and [[plastoquinone]]s contained in chloroplasts quench singlet oxygen and protect against its toxic effects. Oxidized products of [[β-carotene]] arising from the presence of singlet oxygen act as [[Second messenger system|second messenger]]s that can either protect against singlet oxygen induced toxicity or initiate programmed cell death. Levels of [[jasmonate]] play a key role in the decision between cell acclimation or cell death in response to elevated levels of this reactive oxygen species.<ref name="Laloi_2015" />