Editing Oxidative phosphorylation (section)

== Chemiosmosis ==
{{further|Chemiosmosis|Bioenergetics}}
Oxidative phosphorylation works by using [[energy]]-releasing chemical reactions to drive energy-requiring reactions. The two sets of reactions are said to be ''coupled''. This means one cannot occur without the other. The chain of redox reactions driving the flow of electrons through the electron transport chain, from electron donors such as [[NADH]] to [[electron acceptor]]s such as [[oxygen]] and hydrogen (protons), is an [[exergonic]] process&nbsp;– it releases energy, whereas the synthesis of ATP is an [[endergonic]] process, which requires an input of energy. Both the electron transport chain and the ATP synthase are embedded in a membrane, and energy is transferred from the electron transport chain to the ATP synthase by movements of protons across this membrane, in a process called ''[[chemiosmosis]]''.<ref>{{cite journal | vauthors = Mitchell P, Moyle J | title = Chemiosmotic hypothesis of oxidative phosphorylation | journal = Nature | volume = 213 | issue = 5072 | pages = 137–139 | date = January 1967 | pmid = 4291593 | doi = 10.1038/213137a0 | s2cid = 4149605 | bibcode = 1967Natur.213..137M }}</ref> A current of protons is driven from the negative N-side of the membrane to the positive P-side through the proton-pumping enzymes of the electron transport chain. The movement of protons creates an [[electrochemical gradient]] across the membrane, is called the [[proton-motive force]]. It has two components: a difference in proton concentration (a H<sup>+</sup> gradient, Δ[[pH]]) and a difference in [[electric potential]], with the N-side having a negative charge.<ref name=Dimroth2000>{{cite journal | vauthors = Dimroth P, Kaim G, Matthey U | title = Crucial role of the membrane potential for ATP synthesis by F(1)F(o) ATP synthases | journal = The Journal of Experimental Biology | volume = 203 | issue = Pt 1 | pages = 51–59 | date = January 2000 | pmid = 10600673 | doi = 10.1242/jeb.203.1.51 | bibcode = 2000JExpB.203...51D | url = http://jeb.biologists.org/cgi/reprint/203/1/51 | url-status = live | archive-url = https://web.archive.org/web/20070930044752/http://jeb.biologists.org/cgi/reprint/203/1/51 | archive-date = 30 September 2007 }}</ref>

ATP synthase releases this stored energy by completing the circuit and allowing protons to flow down the electrochemical gradient, back to the N-side of the membrane.<ref name=Schultz>{{cite journal | vauthors = Schultz BE, Chan SI | title = Structures and proton-pumping strategies of mitochondrial respiratory enzymes | journal = Annual Review of Biophysics and Biomolecular Structure | volume = 30 | pages = 23–65 | year = 2001 | pmid = 11340051 | doi = 10.1146/annurev.biophys.30.1.23 | url = https://resolver.caltech.edu/CaltechAUTHORS:SCHarbbs01 | author-link2 = Sunney Chan }}</ref> The electrochemical gradient drives the rotation of part of the enzyme's structure and couples this motion to the synthesis of ATP.

The two components of the proton-motive force are [[thermodynamic]]ally equivalent: In mitochondria, the largest part of energy is provided by the potential; in [[alkaliphile]] bacteria the electrical energy even has to compensate for a counteracting inverse pH difference. Inversely, [[chloroplast]]s operate mainly on ΔpH. However, they also require a small membrane potential for the kinetics of ATP synthesis. In the case of the [[Fusobacteriota|fusobacterium]] ''[[Propionigenium modestum]]'' it drives the counter-rotation of subunits a and c of the F<sub>O</sub> motor of ATP synthase.<ref name=Dimroth2000 />

The amount of energy released by oxidative phosphorylation is high, compared with the amount produced by [[anaerobic fermentation]]. [[Glycolysis]] produces only 2 ATP molecules, but somewhere between 30 and 36 ATPs are produced by the oxidative phosphorylation of the 10 NADH and 2 succinate molecules made by converting one molecule of [[glucose]] to carbon dioxide and water,<ref>{{cite journal | vauthors = Rich PR | title = The molecular machinery of Keilin's respiratory chain | journal = Biochemical Society Transactions | volume = 31 | issue = Pt 6 | pages = 1095–1105 | date = December 2003 | pmid = 14641005 | doi = 10.1042/bst0311095 }}</ref> while each cycle of [[beta oxidation]] of a [[fatty acid]] yields about 14 ATPs. These ATP yields are theoretical maximum values; in practice, some protons leak across the membrane, lowering the yield of ATP.<ref>{{cite journal | vauthors = Porter RK, Brand MD | title = Mitochondrial proton conductance and H+/O ratio are independent of electron transport rate in isolated hepatocytes | journal = The Biochemical Journal | volume = 310 | issue = Pt 2 | pages = 379–382 | date = September 1995 | pmid = 7654171 | pmc = 1135905 | doi = 10.1042/bj3100379 }}</ref>