Editing Electron transport chain (section)

{{Short description|Energy-producing metabolic pathway}}
An '''electron transport chain''' ('''ETC'''<ref>{{cite book |doi=10.1016/B978-0-443-10281-3.00013-0 |chapter=Biochemistry |title=Basic Science in Obstetrics and Gynaecology |year=2010 |last1=Lyall |first1=Fiona |pages=143–171 |isbn=978-0-443-10281-3 }}</ref>) is a series of [[protein complex]]es and other molecules which [[electron transfer|transfer]] [[electron]]s from [[electron donor]]s to [[electron acceptor]]s via [[redox]] reactions (both reduction and oxidation occurring simultaneously) and couples this electron transfer with the transfer of [[proton]]s (H<sup>+</sup> ions) across a [[biological membrane|membrane]]. Many of the [[enzyme]]s in the electron transport chain are embedded within the [[membrane]].

The flow of electrons through the electron transport chain is an [[exergonic process]]. The energy from the redox reactions creates an [[electrochemical gradient|electrochemical proton gradient]] that drives the synthesis of [[adenosine triphosphate]] (ATP). In [[Cellular respiration#Aerobic respiration|aerobic respiration]], the flow of electrons terminates with molecular [[oxygen]] as the final electron acceptor. In [[anaerobic respiration]], other electron acceptors are used, such as [[sulfate]].

In an electron transport chain, the redox reactions are driven by the difference in the [[Gibbs free energy]] of reactants and products. The free energy released when a higher-energy electron donor and acceptor convert to lower-energy products, while electrons are transferred from a lower to a higher [[redox potential]], is used by the complexes in the electron transport chain to create an electrochemical gradient of ions. It is this electrochemical gradient that drives the synthesis of ATP via coupling with [[oxidative phosphorylation]] with [[ATP synthase]].<ref name="Anraku 101–132">{{cite journal | vauthors = Anraku Y | title = Bacterial electron transport chains | journal = Annual Review of Biochemistry | volume = 57 | issue = 1 | pages = 101–32 | date = June 1988 | pmid = 3052268 | doi = 10.1146/annurev.bi.57.070188.000533 }}</ref>

In [[Eukaryotes|eukaryotic organisms]], the electron transport chain, and site of [[oxidative phosphorylation]], is found on the [[inner mitochondrial membrane]]. The energy released by reactions of oxygen and reduced compounds such as [[cytochrome]] ''c'' and (indirectly) [[Nicotinamide adenine dinucleotide|NADH]] and [[Flavin adenine dinucleotide|FADH{{sub|2}}]] is used by the electron transport chain to pump protons into the [[intermembrane space]], generating the [[electrochemical gradient]] over the inner [[mitochondrial membrane]]. In [[Photosynthesis|photosynthetic]] eukaryotes, the electron transport chain is found on the [[thylakoid]] membrane. Here, light energy drives electron transport through a [[proton pump]] and the resulting proton gradient causes subsequent synthesis of ATP. In [[bacteria]], the electron transport chain can vary between species but it always constitutes a set of redox reactions that are coupled to the synthesis of ATP through the generation of an electrochemical gradient and oxidative phosphorylation through ATP synthase.<ref>{{cite journal | vauthors = Kracke F, Vassilev I, Krömer JO | title = Microbial electron transport and energy conservation - the foundation for optimizing bioelectrochemical systems | language = en | journal = Frontiers in Microbiology | volume = 6 | pages = 575 | date = 2015 | pmid = 26124754 | pmc = 4463002 | doi = 10.3389/fmicb.2015.00575 | doi-access = free }} – This source shows four ETCs (''Geobacter'', ''Shewanella'', ''Moorella '', ''Acetobacterium'') in figures 1 and 2.</ref>