Editing Redox (section)

==Redox reactions in biology==
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|[[File:Ascorbic acid structure.svg|150px|ascorbic acid]]
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|[[File:Dehydroascorbic acid 2.svg|150px|dehydroascorbic acid]]
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<div style="border: none; width:150px;"><div class="thumbcaption"><small>Top: [[ascorbic acid]] ([[reducing agent|reduced form]] of [[Vitamin C]])<br />Bottom: [[dehydroascorbic acid]] ([[oxidizing agent|oxidized form]] of [[Vitamin C]])</small></div></div></div>
[[File:Extremely overripe banana.jpg|thumb|upright|[[Food browning#Enzymatic browning|Enzymatic browning]] is an example of a redox reaction that takes place in most fruits and vegetables.]]
Many essential [[biology|biological]] processes involve redox reactions. Before some of these processes can begin, iron must be [[Assimilation (biology)|assimilated]] from the environment.<ref>{{Cite book|chapter-url=https://www.degruyter.com/document/doi/10.1515/9783110589771-005|doi = 10.1515/9783110589771-005|chapter = Titles of Volumes 1–44 in the Metal Ions in Biological Systems Series|title = Metals, Microbes, and Minerals - the Biogeochemical Side of Life|year = 2021|pages = xxiii-xxiv|publisher = De Gruyter|isbn = 9783110588903|s2cid = 242013948}}</ref>

[[Cellular respiration]], for instance, is the oxidation of [[glucose]] (C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>) to [[carbon dioxide|CO<sub>2</sub>]] and the reduction of [[oxygen]] to [[water]]. The summary equation for cellular respiration is:

:{{chem2|C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O + Energy}}

The process of cellular respiration also depends heavily on the reduction of [[Nicotinamide adenine dinucleotide|NAD<sup>+</sup>]] to NADH and the reverse reaction (the oxidation of NADH to NAD<sup>+</sup>). [[Photosynthesis]] and cellular respiration are complementary, but photosynthesis is not the reverse of the redox reaction in cellular respiration:

:{{chem2|6 CO2 + 6 H2O + [[photon|light energy]] -> C6H12O6 + 6 O2}}

[[Biological energy]] is frequently stored and released using redox reactions. Photosynthesis involves the reduction of [[carbon dioxide]] into [[sugar]]s and the oxidation of [[water (molecule)|water]] into molecular oxygen. The reverse reaction, respiration, oxidizes sugars to produce carbon dioxide and water. As intermediate steps, the reduced carbon compounds are used to reduce [[nicotinamide adenine dinucleotide]] (NAD<sup>+</sup>) to NADH, which then contributes to the creation of a [[proton gradient]], which drives the synthesis of [[adenosine triphosphate]] (ATP) and is maintained by the reduction of oxygen. In animal cells, [[mitochondria]] perform similar functions.

{{See also|Membrane potential}}

The term ''redox state'' is often used to describe the balance of [[Glutathione|GSH/GSSG]], NAD<sup>+</sup>/NADH and [[Nicotinamide adenine dinucleotide phosphate|NADP<sup>+</sup>/NADPH]] in a biological system such as a [[Cell (biology)|cell]] or [[Organ (biology)|organ]]. The redox state is reflected in the balance of several sets of metabolites (e.g., [[lactic acid|lactate]] and [[pyruvate]], [[beta-hydroxybutyrate]] and [[acetoacetate]]), whose interconversion is dependent on these ratios. Redox mechanisms also control some cellular processes. Redox proteins and their genes must be co-located for redox regulation according to the [[CoRR hypothesis]] for the function of [[DNA]] in [[Mitochondrion|mitochondria]] and [[chloroplast]]s.

===Redox cycling===
Wide varieties of [[aromaticity|aromatic compounds]] are [[enzyme|enzymatically]] reduced to form [[Radical (chemistry)|free radicals]] that contain one more electron than their parent compounds. In general, the electron donor is any of a wide variety of [[flavoenzyme]]s and their [[coenzyme]]s. Once formed, these anion free radicals reduce molecular oxygen to [[superoxide]] and regenerate the unchanged parent compound. The net reaction is the oxidation of the flavoenzyme's coenzymes and the reduction of molecular oxygen to form superoxide. This catalytic behavior has been described as a [[futile cycle]] or redox cycling.