View source for Electrochemical gradient

{{Short description|Gradient of electrochemical potential, usually for an ion that can move across a membrane}}
[[File:Membrane potential ions en.svg|thumb|Diagram of ion concentrations and charge across a semi-permeable cellular membrane.]]

An '''electrochemical gradient''' is a gradient of [[electrochemical potential]], usually for an [[ion]] that can move across a [[membrane]]. The gradient consists of two parts:

* The chemical gradient, or difference in [[Concentration|solute concentration]] across a membrane.
* The electrical gradient, or difference in [[Electric charge|charge]] across a membrane. 

When there are unequal concentrations of an ion across a permeable membrane, the ion will move across the membrane from the area of higher concentration to the area of lower concentration through [[Molecular diffusion|simple diffusion]]. Ions also carry an electric charge that forms an [[electric potential]] across a membrane. If there is an unequal distribution of charges across the membrane, then the difference in electric potential generates a force that drives ion diffusion until the charges are balanced on both sides of the membrane.

Electrochemical gradients are essential to the operation of [[Electric battery|batteries]] and other [[electrochemical cell]]s, [[photosynthesis]] and [[cellular respiration]], and certain other biological processes.

==Overview==
Electrochemical energy is one of the many interchangeable forms of [[potential energy]] through which energy may be [[conservation of energy|conserved]].  It appears in [[electroanalytical chemistry]] and has industrial applications such as batteries and fuel cells. In biology, electrochemical gradients allow cells to control the direction ions move across membranes. In [[mitochondria]] and [[chloroplast]]s, [[proton-motive force|proton gradients]] generate a '''chemiosmotic potential''' used to synthesize [[Adenosine triphosphate|ATP]],<ref>{{Cite journal|last1=Nath|first1=Sunil|last2=Villadsen|first2=John|s2cid=2598635|date=2015-03-01|title=Oxidative phosphorylation revisited|journal=Biotechnology and Bioengineering|language=en|volume=112|issue=3|pages=429–437|doi=10.1002/bit.25492|pmid=25384602|issn=1097-0290}}</ref> and the [[sodium-potassium gradient]] helps [[neural synapse]]s quickly transmit information.{{Citation needed|date=December 2023}}

An electrochemical gradient has two components: a differential concentration of [[electric charge]] across a membrane and a differential concentration of [[chemical species]] across that same membrane.  In the former effect, the concentrated charge attracts charges of the opposite sign; in the latter, the concentrated species tends to diffuse across the membrane to an equalize concentrations.  The combination of these two phenomena determines the thermodynamically-preferred direction for an [[ion]]'s movement across the membrane.<ref name="LehningerBioChem">{{Cite book|title=Lehninger Principles of Biochemistry|last1=Nelson|first1=David|last2=Cox|first2=Michael|publisher=W.H. Freeman|year=2013|isbn=978-1-4292-3414-6|location=New York}}</ref>{{rp|403}}<ref>{{Cite journal|last1=Yang|first1=Huanghe|last2=Zhang|first2=Guohui|last3=Cui|first3=Jianmin|date=2015-01-01|title=BK channels: multiple sensors, one activation gate|journal= Frontiers in Physiology|volume=6|pages=29|doi=10.3389/fphys.2015.00029|pmc=4319557|pmid=25705194|doi-access=free}}</ref>

The combined effect can be quantified as a gradient in the [[thermodynamic]] [[electrochemical potential]]:{{Citation needed|date=December 2023}}<math disply="block">\nabla\overline{\mu}_i = \nabla \mu_i(\vec{r}) + z_i\mathrm{F}\nabla\varphi(\vec{r})\text{,}</math> with <ul>
<li>{{math|μ<sub>''i''</sub>}} the chemical potential of the ion species {{mvar|i}}</li>
<li>{{math|''z<sub>i</sub>''}} the charge per ion of the species {{mvar|i}}</li>
<li>{{mvar|F}}, [[Faraday constant]] (the electrochemical potential is implicitly measured on a per-[[mole (chemistry)|mole]] basis)</li>
<li>{{mvar|φ}}, the local [[electric potential]].</li>
</ul>  Sometimes, the term "electrochemical potential" is abused to describe the electric potential ''generated'' by an ionic concentration gradient; that is, {{mvar|φ}}. 

An electrochemical gradient is analogous to the water [[pressure]] across a [[hydroelectric dam]]. Routes unblocked by the membrane (e.g. [[membrane transport protein]] or [[electrode]]s) correspond to turbines that convert the water's potential energy to other forms of physical or chemical energy, and the ions that pass through the membrane correspond to water traveling into the lower river.{{Tone inline|date=December 2023|reason=Somewhat textbook-like.}} Conversely, energy can be used to [[pumped storage|pump water up into the lake above the dam]], and chemical energy can be used to create electrochemical gradients.<ref>{{Cite journal|last1=Shattock|first1=Michael J.|last2=Ottolia|first2=Michela|last3=Bers|first3=Donald M.|last4=Blaustein|first4=Mordecai P.|last5=Boguslavskyi|first5=Andrii|last6=Bossuyt|first6=Julie|last7=Bridge|first7=John H. B.|last8=Chen-Izu|first8=Ye|last9=Clancy|first9=Colleen E.|date=2015-03-15|title=Na+/Ca2+ exchange and Na+/K+-ATPase in the heart|journal=The Journal of Physiology|language=en|volume=593|issue=6|pages=1361–1382|doi=10.1113/jphysiol.2014.282319|issn=1469-7793|pmc=4376416|pmid=25772291}}</ref><ref name="ReviewWithBadTitle" />

==Chemistry==
{{Unsourced|section|date=December 2023}}{{See also|concentration cell|electrode potential|table of standard electrode potentials}}
The term typically applies in [[electrochemistry]], when [[electrical energy]] in the form of an applied voltage is used to modulate the [[Thermodynamic control|thermodynamic favorability]] of a [[chemical reaction]]. In a battery, an electrochemical potential arising from the movement of ions balances the reaction energy of the electrodes. The maximum voltage that a battery reaction can produce is sometimes called the '''standard electrochemical potential''' of that reaction.

==Biological context==
The generation of a transmembrane electrical potential through ion movement across a [[cell membrane]] drives [[biological process]]es like [[nerve]] conduction, [[muscle contraction]], [[hormone]] [[secretion]], and [[Sensory system|sensation]].  By convention, physiological voltages are measured [[Gauge transformation|relative]] to the extracellular region; a typical animal cell has an [[transmembrane potential difference|internal electrical potential]] of <!-- {{val|(-70)|-|(-50)|u=mV}} doesn't parenthesize and it looks terrible-->(&minus;70)&ndash;(&minus;50)&nbsp;mV.<ref name="LehningerBioChem" />{{rp|464}}

An electrochemical gradient is essential to [[mitochondrial]] [[oxidative phosphorylation]]. The final step of [[cellular respiration]] is the [[electron transport chain]], composed of four complexes embedded in the inner mitochondrial membrane. Complexes I, III, and IV pump protons from the [[Mitochondrial matrix|matrix]] to the [[Mitochondrial intermembrane space|intermembrane space]] (IMS); for every [[electron pair]] entering the chain, ten protons translocate into the IMS.  The result is an electric potential of more than {{val|200|u=mV}}.  The resulting flux of protons back into the matrix powers the efforts of [[ATP synthase]] to combine inorganic [[phosphate]] and [[Adenosine diphosphate|ADP]].<ref>{{Cite journal|last1=Poburko|first1=Damon|last2=Demaurex|first2=Nicolas|date=2012-04-24|title=Regulation of the mitochondrial proton gradient by cytosolic Ca2+ signals|journal=Pflügers Archiv: European Journal of Physiology|language=en|volume=464|issue=1|pages=19–26|doi=10.1007/s00424-012-1106-y|pmid=22526460|s2cid=18133149|issn=0031-6768|url=http://doc.rero.ch/record/321929/files/424_2012_Article_1106.pdf}}</ref><ref name=LehningerBioChem />{{rp|743–745}}

Similar to the electron transport chain, the [[light-dependent reactions]] of photosynthesis pump protons into the [[thylakoid]] [[Lumen (anatomy)|lumen]] of chloroplasts to drive the synthesis of ATP. The proton gradient can be generated through either noncyclic or cyclic photophosphorylation. Of the proteins that participate in noncyclic photophosphorylation, [[photosystem II]] (PSII), [[Plastoquinone|plastiquinone]], and [[Cytochrome b6f complex|cytochrome b<sub>6</sub>f complex]] directly contribute to generating the proton gradient. For each four photons absorbed by PSII, eight protons are pumped into the lumen.<ref name=LehningerBioChem />{{rp|769–770}}

Several other transporters and ion channels play a role in generating a proton electrochemical gradient. One is TPK<sub>3</sub>, a [[potassium channel]] that is activated by Ca<sup>2+</sup> and conducts K<sup>+</sup> from the thylakoid lumen to the [[Stroma (fluid)|stroma]], which helps establish the [[electric field]]. On the other hand, the electro-neutral K<sup>+</sup> efflux [[antiporter]] (KEA<sub>3</sub>) transports K<sup>+</sup> into the thylakoid lumen and H<sup>+</sup> into the stroma, which helps establish the [[pH]] gradient.<ref>{{Cite journal|last1=Höhner|first1=Ricarda|last2=Aboukila|first2=Ali|last3=Kunz|first3=Hans-Henning|last4=Venema|first4=Kees|date=2016-01-01|title= Proton Gradients and Proton-Dependent Transport Processes in the Chloroplast|journal= Frontiers in Plant Science|pages=218|doi=10.3389/fpls.2016.00218|pmc=4770017|pmid=26973667|volume=7|doi-access=free}}</ref>

==Ion gradients==
[[File:Scheme sodium-potassium pump-en.svg|thumb|Diagram of the Na<sup>+</sup>-K<sup>+</sup>-ATPase.]]
Since the ions are charged, they cannot pass through cellular membranes via simple diffusion. Two different mechanisms can transport the ions across the membrane: [[Active transport|active]] or [[Passive transport|passive]] transport.{{Citation needed|date=December 2023}}

An example of active transport of ions is the [[Na+/K+-ATPase|Na<sup>+</sup>-K<sup>+</sup>-ATPase]] (NKA). NKA is powered by the [[hydrolysis]] of ATP into ADP and an inorganic phosphate; for every molecule of ATP hydrolized, three Na<sup>+</sup> are transported outside and two K<sup>+</sup> are transported inside the cell. This makes the inside of the cell more negative than the outside and more specifically generates a membrane potential ''V''<sub>membrane</sub> of about {{val|-60|u=mV}}.<ref name="ReviewWithBadTitle">{{Cite journal|last1=Aperia|first1=Anita|last2=Akkuratov|first2=Evgeny E.|last3=Fontana|first3=Jacopo Maria|last4=Brismar|first4=Hjalmar|date=2016-04-01|title=Na+-K+-ATPase, a new class of plasma membrane receptors|journal=American Journal of Physiology. Cell Physiology|language=en|volume=310|issue=7|pages=C491–C495|doi=10.1152/ajpcell.00359.2015|issn=0363-6143|pmid=26791490|url=https://zenodo.org/record/1065636|doi-access=free}}</ref>

An example of passive transport is ion fluxes through Na<sup>+</sup>, K<sup>+</sup>, Ca<sup>2+</sup>, and Cl<sup>−</sup> channels. Unlike active transport, passive transport is powered by the [[arithmetic sum]] of [[osmosis]] (a concentration gradient) and an [[electric field]] (the transmembrane potential).  Formally, the [[Mole (chemistry)|molar]] [[Gibbs free energy]] change associated with successful transport is{{Citation needed|date=December 2023}} <math display=block> \Delta G = RT\ln{\!\left(\frac{c_{\rm in}}{c_{\rm out}}\right)} + (Fz)V_{\rm membrane}</math> where {{mvar|R}} represents the [[gas constant]], {{mvar|T}} represents [[Thermodynamic temperature|absolute temperature]], {{mvar|z}} is the charge per ion, and {{mvar|F}} represents the [[Faraday constant]].<ref name="LehningerBioChem" />{{rp|464–465}}

In the example of Na<sup>+</sup>, both terms tend to support transport: the negative electric potential inside the cell attracts the positive ion and since Na<sup>+</sup> is concentrated outside the cell, osmosis supports diffusion through the Na<sup>+</sup> channel into the cell.  In the case of K<sup>+</sup>, the effect of osmosis is reversed: although external ions are attracted by the negative intracellular potential, entropy seeks to diffuse the ions already concentrated inside the cell.  The converse phenomenon (osmosis supports transport, electric potential opposes it) can be achieved for Na<sup>+</sup> in cells with abnormal transmembrane potentials: at {{val|+70|u=mV}}, the Na<sup>+</sup> influx halts; at higher potentials, it becomes an efflux.{{Citation needed|date=December 2023}}

{| class="wikitable"
|+
Common cellular ion concentrations ([[Molar concentration|millimolar]])<ref>{{Cite web|url=http://book.bionumbers.org/what-are-the-concentrations-of-different-ions-in-cells/|title=» What are the concentrations of different ions in cells?|last=Philips|first=Ron Milo & Ron|language=en|access-date=2019-06-07}}</ref><ref>{{Cite web|url=https://www.ncbi.nlm.nih.gov/books/NBK21627/table/A4057/|title=Table 15-1, Typical Ion Concentrations in Invertebrates and Vertebrates|last1=Lodish|first1=Harvey|last2=Berk|first2=Arnold|date=2000|website=www.ncbi.nlm.nih.gov|language=en|access-date=2019-06-07|last3=Zipursky|first3=S. Lawrence|last4=Matsudaira|first4=Paul|last5=Baltimore|first5=David|last6=Darnell|first6=James}}</ref><ref>{{Cite web|url=http://www.chm.bris.ac.uk/webprojects2001/riis/ionconc.htm|title=The following table gives an idea of the intra and extra cellular ion concentrations in a squid axon and a mammalian cell|website=www.chm.bris.ac.uk|access-date=2019-06-07}}</ref><ref name="diem">{{cite book|title=Scientific Tables|vauthors=Diem K, Lenter C|publisher=Ciba-Geigy Limited|isbn=978-3-9801244-0-9|edition=Seventh|volume=565|location=Basel|pages=653–654}}</ref>
! rowspan="2" |Ion
! colspan="2" |[[Mammal]]
! colspan="2" |[[Squid giant axon|Squid axon]]
! rowspan="2" |[[Saccharomyces cerevisiae|S. cerevisiae]]
! rowspan="2" |[[Escherichia coli|E. coli]]
! rowspan="2" |[[Seawater|Sea water]]
|-
!Cell
!Blood
!Cell
!Blood
|-
|[[Potassium in biology|K<sup>+</sup>]] || 100 - 140 || 4-5 || 400 || 10 - 20 || 300 || 30 - 300 || 10
|-
|[[Sodium in biology|Na<sup>+</sup>]] || 5-15 || 145 || 50 || 440 || 30 || 10 || 500
|-
|[[Magnesium in biology|Mg<sup>2+</sup>]] || 10 {{efn|name=bound|Bound}}<br>0.5 - 0.8 {{efn|name=free|Free}} || 1 - 1.5 || || || 50 || 30 - 100 {{efn|name=bound}}<br>0.01 - 1 {{efn|name=free}} || 50
|-
|[[Calcium in biology|Ca<sup>2+</sup>]] || 10<sup>−4</sup> || 2.2 - 2.6 {{efn|name=total|Total}}<br>1.3 - 1.5 {{efn|name=ionised|Ionised}} || 10<sup>−4</sup> - 3×10<sup>−4</sup> || 10 || 2 || 3 {{efn|name=bound}}<br>10<sup>−4</sup> {{efn|name=free}} || 10
|-
|[[Chloride|Cl<sup>−</sup>]] || 4 || 110 || 40 - 150 || 560
| colspan="2" |10 - 200 {{efn|Medium dependent}} || 500
|-
|[[Protein|X<sup>−</sup> (negatively charged proteins)]] || 138 || 9 || 300 - 400 || 5-10 || || ||
|-
|[[Bicarbonate|HCO<sub>3</sub><sup>−</sup>]] || 12 || 29 || || || || ||
|-
|pH || 7.1 - 7.3<ref name=":7">{{Citation|last1=Spitzer|first1=Kenneth W.|title=Regulation of Intracellular pH in Mammalian Cells|date=2003|work=The Sodium-Hydrogen Exchanger: From Molecule to its Role in Disease|pages=1–15|editor-last=Karmazyn|editor-first=Morris|publisher=Springer US|language=en|doi=10.1007/978-1-4615-0427-6_1|isbn=9781461504276|last2=Vaughan-Jones|first2=Richard D.|editor2-last=Avkiran|editor2-first=Metin|editor3-last=Fliegel|editor3-first=Larry}}</ref> || 7.35 to 7.45 <ref name=":7" /> (normal arterial blood pH)<br />6.9 - 7.8 <ref name=":7" /> (overall range) || || || ||7.2 - 7.8<ref>{{Cite journal|last1=Slonczewski|first1=Joan L.|last2=Wilks|first2=Jessica C.|date=2007-08-01|title=pH of the Cytoplasm and Periplasm of Escherichia coli: Rapid Measurement by Green Fluorescent Protein Fluorimetry|journal=Journal of Bacteriology|language=en|volume=189|issue=15|pages=5601–5607|doi=10.1128/JB.00615-07|issn=0021-9193|pmid=17545292|pmc=1951819}}</ref>
|8.1 - 8.2<ref>{{Cite journal|url=https://www.scientificamerican.com/article/rising-acidity-in-the-ocean/|title=Rising Acidity in the Ocean: The Other CO2 Problem|last=Brewer|first=Peter G.|date=September 1, 2008|doi=10.1038/scientificamericanearth0908-22}}</ref>
|}

{{Notelist|30em}}

==Proton gradients==
Proton gradients in particular are important in many types of cells as a form of energy storage. The gradient is usually used to drive ATP synthase, [[Flagellum|flagellar]] rotation, or [[metabolite]] transport.<ref name="H+PumpReview">{{Cite journal|last1=Gunner|first1=M. R.|last2=Amin|first2=Muhamed|last3=Zhu|first3=Xuyu|last4=Lu|first4=Jianxun|date=2013-08-01|title=Molecular mechanisms for generating transmembrane proton gradients|journal=Biochimica et Biophysica Acta (BBA) - Bioenergetics|series=Metals in Bioenergetics and Biomimetics Systems|volume=1827|issue=8–9|pages=892–913|doi=10.1016/j.bbabio.2013.03.001|pmc=3714358|pmid=23507617}}</ref> This section will focus on three processes that help establish proton gradients in their respective cells: [[bacteriorhodopsin]] and noncyclic photophosphorylation and oxidative phosphorylation.{{Citation needed|date=December 2023}}

=== Bacteriorhodopsin ===
[[File:Bacteriorhodopsin retinal.png|thumb|Diagram of the conformational shift in retinal that initiates proton pumping in bacteriorhodopsin.]]
The way [[bacteriorhodopsin]] generates a proton gradient in [[Archaea]] is through a [[proton pump]]. The proton pump relies on proton carriers to drive protons from the side of the membrane with a low H<sup>+</sup> concentration to the side of the membrane with a high H<sup>+</sup> concentration. In bacteriorhodopsin, the proton pump is activated by absorption of [[photon]]s of 568nm [[wavelength]], which leads to [[isomerization]] of the [[Schiff base]] (SB) in [[retinal]] forming the K state. This moves SB away from Asp85 and Asp212, causing H<sup>+</sup> transfer from the SB to Asp85 forming the M1 state. The protein then shifts to the M2 state by separating Glu204 from Glu194 which releases a proton from Glu204 into the external medium. The SB is [[Protonation|reprotonated]] by Asp96 which forms the N state. It is important that the second proton comes from Asp96 since its [[Deprotonation|deprotonated]] state is unstable and rapidly reprotonated with a proton from the [[cytosol]]. The protonation of Asp85 and Asp96 causes re-isomerization of the SB, forming the O state. Finally, bacteriorhodopsin returns to its resting state when Asp85 releases its proton to Glu204.<ref name="H+PumpReview" /><ref>{{Cite journal|last1=Wickstrand|first1=Cecilia|last2=Dods|first2=Robert|last3=Royant|first3=Antoine|last4=Neutze|first4=Richard|date=2015-03-01|title=Bacteriorhodopsin: Would the real structural intermediates please stand up?|journal=Biochimica et Biophysica Acta (BBA) - General Subjects|series=Structural biochemistry and biophysics of membrane proteins|volume=1850|issue=3|pages=536–553|doi=10.1016/j.bbagen.2014.05.021|pmid=24918316|doi-access=free}}</ref>

=== Photophosphorylation ===
[[File:Cyclic Photophosphorylation.svg|thumb|Simplified diagram of photophosphorylation.]]
PSII also relies on [[light]] to drive the formation of proton gradients in chloroplasts, however, PSII utilizes vectorial redox chemistry to achieve this goal. Rather than physically transporting protons through the protein, reactions requiring the binding of protons will occur on the extracellular side while reactions requiring the release of protons will occur on the intracellular side. Absorption of photons of 680nm wavelength is used to excite two electrons in [[P680|P<sub>680</sub>]] to a higher [[energy level]]. These higher energy electrons are transferred to protein-bound [[plastoquinone]] (PQ<sub>A</sub>)  and then to unbound plastoquinone (PQ<sub>B</sub>). This reduces plastoquinone (PQ) to plastoquinol (PQH<sub>2</sub>) which is released from PSII after gaining two protons from the stroma. The electrons in P<sub>680</sub> are replenished by oxidizing [[water]] through the [[oxygen-evolving complex]] (OEC). This results in release of O<sub>2</sub> and H<sup>+</sup> into the lumen, for a total reaction of<ref name="H+PumpReview" />
<math chem display=block>4h\nu+2\ce{H2O}+2\ce{PQ}+4\ce{H+}(\text{stroma})\longrightarrow\ce{O2}+2\ce{PQH2}+4\ce{H+}(\text{lumen})</math>

After being released from PSII, PQH<sub>2</sub> travels to the [[Cytochrome b6f complex|cytochrome b<sub>6</sub>f complex]], which then transfers two electrons from PQH<sub>2</sub> to [[plastocyanin]] in two separate reactions. The process that occurs is similar to the Q-cycle in Complex III of the electron transport chain. In the first reaction, PQH<sub>2</sub> binds to the complex on the lumen side and one electron is transferred to the [[Iron–sulfur protein|iron-sulfur center]] which then transfers it to [[cytochrome f]] which then transfers it to plastocyanin. The second electron is transferred to [[Heme B|heme b<sub>L</sub>]] which then transfers it to heme b<sub>H</sub> which then transfers it to PQ. In the second reaction, a second PQH<sub>2</sub> gets oxidized, adding an electron to another plastocyanin and PQ. Both reactions together transfer four protons into the lumen.<ref name=LehningerBioChem />{{rp|782–783}}<ref>{{Cite journal|last1=Schöttler|first1=Mark Aurel|last2=Tóth|first2=Szilvia Z.|last3=Boulouis|first3=Alix|last4=Kahlau|first4=Sabine|date=2015-05-01|title=Photosynthetic complex stoichiometry dynamics in higher plants: biogenesis, function, and turnover of ATP synthase and the cytochrome b 6 f complex|journal=Journal of Experimental Botany|language=en|volume=66|issue=9|pages=2373–2400|doi=10.1093/jxb/eru495|issn=0022-0957|pmid=25540437|doi-access=free}}</ref>

=== Oxidative phosphorylation ===
[[File:ETC electron transport chain.svg|thumb|Detailed diagram of the electron transport chain in mitochondria.]]
In the electron transport chain, [[complex I]] (CI) [[Catalysis|catalyzes]] the [[Redox|reduction]] of [[Coenzyme Q10|ubiquinone]] (UQ) to [[ubiquinol]] (UQH<sub>2</sub>) by the transfer of two [[electron]]s from reduced [[nicotinamide adenine dinucleotide]] (NADH) which translocates four protons from the mitochondrial matrix to the IMS:<ref name="PumpStructures">{{Cite journal|last1=Sun|first1=Fei|last2=Zhou|first2=Qiangjun|last3=Pang|first3=Xiaoyun|last4=Xu|first4=Yingzhi|last5=Rao|first5=Zihe|date=2013-08-01|title=Revealing various coupling of electron transfer and proton pumping in mitochondrial respiratory chain|journal=Current Opinion in Structural Biology|volume=23|issue=4|pages=526–538|doi=10.1016/j.sbi.2013.06.013|pmid=23867107}}</ref>
<math chem display=block>\ce{NADH} + \ce{H^+} + \ce{UQ} + 4\underbrace{\ce{H^+}}_{\mathrm{matrix}} \longrightarrow \ce{NAD^+} + \ce{UQH_2} + 4\underbrace{\ce{H^+}}_{\mathrm{IMS}}</math>

[[Complex III]] (CIII) catalyzes the [[Q cycle|Q-cycle]]. The first step involving the transfer of two electrons from the UQH<sub>2</sub> reduced by CI to two molecules of oxidized [[cytochrome c]] at the Q<sub>o</sub> site. In the second step, two more electrons reduce UQ to UQH<sub>2</sub> at the Q<sub>i</sub> site. The total reaction is:<ref name="PumpStructures" />
<math chem display=block>2\underbrace{\text{cytochrome c}}_{\text{oxidized}}+\ce{UQH_2}+2\underbrace{\ce{H^+}}_{\text{matrix}}\longrightarrow2\underbrace{\text{cytochrome c}}_{\text{reduced}}+\ce{UQ}+4\underbrace{\ce{H^+}}_{\text{IMS}}</math>

Complex IV (CIV) catalyzes the transfer of two electrons from the cytochrome c reduced by CIII to one half of a full oxygen. Utilizing one full oxygen in oxidative phosphorylation requires the transfer of four electrons. The oxygen will then consume four protons from the matrix to form water while another four protons are pumped into the IMS, to give a total reaction<ref name="PumpStructures" />

== <math chem="" display="block">2\text{cytochrome c}(\text{reduced})+4\ce{H+}(\text{matrix})+\frac{1}{2}\ce{O2}\longrightarrow2\text{cytochrome c}(\text{oxidized})+2\ce{H+}(\text{IMS})+\ce{H2O}</math>See also ==
{{Div col|colwidth=20em}}
*[[Concentration cell]]
*[[Transmembrane potential difference]]
*[[Action potential]]
*[[Membrane potential|Cell potential]]
*[[Electrodiffusion]]
*[[Galvanic cell]]
*[[Electrochemical cell]]
*[[Proton exchange membrane]]
*[[Reversal potential]]
{{Div col end}}

==References==
{{Reflist|2}}
*{{cite book | author=Campbell & Reece | title=Biology | publisher=Pearson Benjamin Cummings | year=2005 | isbn=978-0-8053-7146-8}}
*[https://web.archive.org/web/20051029161309/http://www.mansfield.ohio-state.edu/~sabedon/campbl08.htm Stephen T. Abedon, "Important words and concepts from Chapter 8, Campbell & Reece, 2002 (1/14/2005)", for Biology 113 at the Ohio State University]
{{Membrane transport}}

{{DEFAULTSORT:Electrochemical Gradient}}
[[Category:Cellular respiration]]
[[Category:Electrochemical concepts]]
[[Category:Electrophysiology]]
[[Category:Membrane biology]]
[[Category:Physical quantities]]
[[Category:Thermodynamics]]