{"history":[],"article":{"id":"830f1b5d-4a48-4502-a1c8-231530485413","title":"Electrochemical gradient","slug":"electrochemical-gradient","content":":::summary\n:::\n\n# \nAn **electrochemical gradient** is a gradient of [electrochemical potential](https://bliptext.com/articles/electrochemical-potential), usually for an [ion](https://bliptext.com/articles/ion) that can move across a [membrane](https://bliptext.com/articles/membrane).\nThe gradient consists of two parts:\nWhen 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 [simple diffusion](https://bliptext.com/articles/molecular-diffusion).\nIons also carry an electric charge that forms an [electric potential](https://bliptext.com/articles/electric-potential) across a membrane.\nIf 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.\nElectrochemical gradients are essential to the operation of [batteries](https://bliptext.com/articles/electric-battery) and other [electrochemical cells](https://bliptext.com/articles/electrochemical-cell), [photosynthesis](https://bliptext.com/articles/photosynthesis) and [cellular respiration](https://bliptext.com/articles/cellular-respiration), and certain other biological processes.\n# [Diagram of ion concentrations and charge across a semi-permeable cellular membrane. | upload.wikimedia.org/wikipedia/commons/4/4f/Membrane///potential///ions///en.svg]\n\n# Overview\nElectrochemical energy is one of the many interchangeable forms of [potential energy](https://bliptext.com/articles/potential-energy) through which energy may be [conserved](https://bliptext.com/articles/conservation-of-energy).\nIt appears in [electroanalytical chemistry](https://bliptext.com/articles/electroanalytical-chemistry) and has industrial applications such as batteries and fuel cells.\nIn biology, electrochemical gradients allow cells to control the direction ions move across membranes.\nIn [mitochondria](https://bliptext.com/articles/mitochondria) and [chloroplasts](https://bliptext.com/articles/chloroplast), [proton gradients](https://bliptext.com/articles/proton-motive-force) generate a **chemiosmotic potential** used to synthesize [ATP](https://bliptext.com/articles/adenosine-triphosphate), and the [sodium-potassium gradient](https://bliptext.com/articles/sodium-potassium-gradient) helps [neural synapses](https://bliptext.com/articles/neural-synapse) quickly transmit information.\nAn electrochemical gradient has two components: a differential concentration of [electric charge](https://bliptext.com/articles/electric-charge) across a membrane and a differential concentration of [chemical species](https://bliptext.com/articles/chemical-species) across that same membrane.\nIn 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.\nThe combinat[ion](https://bliptext.com/articles/ion) of these two phenomena determines the thermodynamically-preferred direct[ion](https://bliptext.com/articles/ion) for an [ion](https://bliptext.com/articles/ion)'s movement across the membrane.\nThe combined effect can be quantified as a gradient in the [thermodynamic](https://bliptext.com/articles/thermodynamic) [electrochemical potential](https://bliptext.com/articles/electrochemical-potential): with

\nμi\nthe chemical potential of the ion species i
\nzi\nthe charge per ion of the species i
F, [Faraday constant](https://bliptext.com/articles/faraday-constant) (the electrochemical potential is implicitly measured on a per-[mole](https://bliptext.com/articles/mole-chemistry) basis)
φ, the local [electric potential](https://bliptext.com/articles/electric-potential).

Sometimes, the term \"electrochemical potential\" is abused to describe the electric potential generated by an ionic concentration gradient; that is, φ.\nAn electrochemical gradient is analogous to the water [pressure](https://bliptext.com/articles/pressure) across a [hydroelectric dam](https://bliptext.com/articles/hydroelectric-dam).\nRoutes unblocked by the membrane (e.g. [membrane transport protein](https://bliptext.com/articles/membrane-transport-protein) or [electrodes](https://bliptext.com/articles/electrode)) 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.\nConversely, energy can be used to [pump water up into the lake above the dam](https://bliptext.com/articles/pumped-storage), and chemical energy can be used to create electrochemical gradients.\n\n# Chemistry\nThe term typically applies in [electrochemistry](https://bliptext.com/articles/electrochemistry), when [electrical energy](https://bliptext.com/articles/electrical-energy) in the form of an applied voltage is used to modulate the [thermodynamic favorability](https://bliptext.com/articles/thermodynamic-control) of a [chemical reaction](https://bliptext.com/articles/chemical-reaction).\nIn a battery, an electrochemical potential arising from the movement of ions balances the reaction energy of the electrodes.\nThe maximum voltage that a battery reaction can produce is sometimes called the **standard electrochemical potential** of that reaction.\n\n# Biological context\nThe generation of a transmembrane electrical potential through ion movement across a [cell membrane](https://bliptext.com/articles/cell-membrane) drives [biological processes](https://bliptext.com/articles/biological-process) like [nerve](https://bliptext.com/articles/nerve) conduction, [muscle contraction](https://bliptext.com/articles/muscle-contraction), [hormone](https://bliptext.com/articles/hormone) [secretion](https://bliptext.com/articles/secretion), and [sensation](https://bliptext.com/articles/sensory-system).\nBy convention, physiological voltages are measured [relative](https://bliptext.com/articles/gauge-transformation) to the extracellular region; a typical animal cell has an [internal electrical potential](https://bliptext.com/articles/transmembrane-potential-difference) of (−70)–(−50) mV.\nAn electrochemical gradient is essential to [mitochondrial](https://bliptext.com/articles/mitochondrial) [oxidative phosphorylation](https://bliptext.com/articles/oxidative-phosphorylation).\nThe final step of [cellular respiration](https://bliptext.com/articles/cellular-respiration) is the [electron transport chain](https://bliptext.com/articles/electron-transport-chain), composed of four complexes embedded in the inner mitochondrial membrane.\nComplexes I, III, and IV pump protons from the [matrix](https://bliptext.com/articles/mitochondrial-matrix) to the [intermembrane space](https://bliptext.com/articles/mitochondrial-intermembrane-space) (IMS); for every [electron pair](https://bliptext.com/articles/electron-pair) entering the chain, ten protons translocate into the IMS.\nThe result is an electric potential of more than 200 mV.\nThe energy resulting from the flux of protons back into the matrix is used by [ATP synthase](https://bliptext.com/articles/atp-synthase) to combine inorganic [phosphate](https://bliptext.com/articles/phosphate) and [ADP](https://bliptext.com/articles/adenosine-diphosphate).\nSimilar to the electron transport chain, the [light-dependent reactions](https://bliptext.com/articles/light-dependent-reactions) of photosynthesis pump protons into the [thylakoid](https://bliptext.com/articles/thylakoid) [lumen](https://bliptext.com/articles/lumen-anatomy) of chloroplasts to drive the synthesis of ATP.\nThe proton gradient can be generated through either noncyclic or cyclic photophosphorylation.\nOf the proteins that participate in noncyclic photophosphorylation, [photosystem II](https://bliptext.com/articles/photosystem-ii) (PSII), [plastiquinone](https://bliptext.com/articles/plastoquinone), and [cytochrome b6f complex](https://bliptext.com/articles/cytochrome-b6f-complex) directly contribute to generating the proton gradient.\nFor each four photons absorbed by PSII, eight protons are pumped into the lumen.\nSeveral other transporters and ion channels play a role in generating a proton electrochemical gradient.\nOne is TPK3, a [potassium channel](https://bliptext.com/articles/potassium-channel) that is activated by Ca2+ and conducts K+ from the thylakoid lumen to the [stroma](https://bliptext.com/articles/stroma-fluid), which helps establish the [electric field](https://bliptext.com/articles/electric-field).\nOn the other hand, the electro-neutral K+ efflux [antiporter](https://bliptext.com/articles/antiporter) (KEA3) transports K+ into the thylakoid lumen and H+ into the stroma, which helps establish the [pH](https://bliptext.com/articles/ph) gradient.\n\n# Ion gradients\nSince the ions are charged, they cannot pass through cellular membranes via simple diffusion.\nTwo different mechanisms can transport the ions across the membrane: [active](https://bliptext.com/articles/active-transport) or [passive](https://bliptext.com/articles/passive-transport) transport.\nAn example of active transport of ions is the [Na+-K+-ATPase](https://bliptext.com/articles/na-k-atpase) (NKA).\nNKA is powered by the [hydrolysis](https://bliptext.com/articles/hydrolysis) of ATP into ADP and an inorganic phosphate; for every molecule of ATP hydrolized, three Na+ are transported outside and two K+ are transported inside the cell.\nThis makes the inside of the cell more negative than the outside and more specifically generates a membrane potential Vmembrane of about -60 mV.\nAn example of passive transport is ion fluxes through Na+, K+, Ca2+, and Cl− channels.\nUnlike active transport, passive transport is powered by the [arithmetic sum](https://bliptext.com/articles/arithmetic-sum) of [osmosis](https://bliptext.com/articles/osmosis) (a concentration gradient) and an [electric field](https://bliptext.com/articles/electric-field) (the transmembrane potential).\nFormally, the [molar](https://bliptext.com/articles/mole-chemistry) [Gibbs free energy](https://bliptext.com/articles/gibbs-free-energy) change associated with successful transport is where R represents the [gas constant](https://bliptext.com/articles/gas-constant), T represents [absolute temperature](https://bliptext.com/articles/thermodynamic-temperature), z is the charge per ion, and F represents the [Faraday constant](https://bliptext.com/articles/faraday-constant).\nIn the example of Na+, both terms tend to support transport: the negative electric potential inside the cell attracts the positive ion and since Na+ is concentrated outside the cell, osmosis supports diffusion through the Na+ channel into the cell.\nIn the case of K+, 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.\nThe converse phenomenon (osmosis supports transport, electric potential opposes it) can be achieved for Na+ in cells with abnormal transmembrane potentials: at 70 mV, the Na+ influx halts; at higher potentials, it becomes an efflux.\n# [Diagram of the Na+-K+-ATPase. | upload.wikimedia.org/wikipedia/commons/a/a5/Scheme///sodium-potassium///pump-en.svg]\n\n# Proton gradients\nProton gradients in particular are important in many types of cells as a form of energy storage.\nThe gradient is usually used to drive ATP synthase, [flagellar](https://bliptext.com/articles/flagellum) rotation, or [metabolite](https://bliptext.com/articles/metabolite) transport.\nThis section will focus on three processes that help establish proton gradients in their respective cells: [bacteriorhodopsin](https://bliptext.com/articles/bacteriorhodopsin) and noncyclic photophosphorylation and oxidative phosphorylation.\n\n## Bacteriorhodopsin\nThe way [bacteriorhodopsin](https://bliptext.com/articles/bacteriorhodopsin) generates a proton gradient in [Archaea](https://bliptext.com/articles/archaea) is through a [proton pump](https://bliptext.com/articles/proton-pump).\nThe proton pump relies on proton carriers to drive protons from the side of the membrane with a low H+ concentration to the side of the membrane with a high H+ concentration.\nIn bacteriorhodopsin, the proton pump is activated by absorption of [photons](https://bliptext.com/articles/photon) of 568nm [wavelength](https://bliptext.com/articles/wavelength), which leads to [isomerization](https://bliptext.com/articles/isomerization) of the [Schiff base](https://bliptext.com/articles/schiff-base) (SB) in [retinal](https://bliptext.com/articles/retinal) forming the K state.\nThis moves SB away from Asp85 and Asp212, causing H+ transfer from the SB to Asp85 forming the M1 state.\nThe protein then shifts to the M2 state by separating Glu204 from Glu194 which releases a proton from Glu204 into the external medium.\nThe SB is [reprotonated](https://bliptext.com/articles/protonation) by Asp96 which forms the N state.\nIt is important that the second proton comes from Asp96 since its [deprotonated](https://bliptext.com/articles/deprotonation) state is unstable and rapidly reprotonated with a proton from the [cytosol](https://bliptext.com/articles/cytosol).\nThe protonation of Asp85 and Asp96 causes re-isomerization of the SB, forming the O state.\nFinally, bacteriorhodopsin returns to its resting state when Asp85 releases its proton to Glu204.\n# [Diagram of the conformational shift in retinal that initiates proton pumping in bacteriorhodopsin. | upload.wikimedia.org/wikipedia/commons/5/57/Bacteriorhodopsin///retinal.png]\n\n## Photophosphorylation\nPSII also relies on [light](https://bliptext.com/articles/light) to drive the formation of proton gradients in chloroplasts, however, PSII utilizes vectorial redox chemistry to achieve this goal.\nRather 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.\nAbsorption of photons of 680nm wavelength is used to excite two electrons in [P680](https://bliptext.com/articles/p680) to a higher [energy level](https://bliptext.com/articles/energy-level).\nThese higher energy electrons are transferred to protein-bound [plastoquinone](https://bliptext.com/articles/plastoquinone) (PQA) and then to unbound [plastoquinone](https://bliptext.com/articles/plastoquinone) (PQB).\nThis reduces plastoquinone (PQ) to plastoquinol (PQH2) which is released from PSII after gaining two protons from the stroma.\nThe electrons in P680 are replenished by oxidizing [water](https://bliptext.com/articles/water) through the [oxygen-evolving complex](https://bliptext.com/articles/oxygen-evolving-complex) (OEC).\nThis results in release of O2 and H+ into the lumen, for a total reaction of\nAfter being released from PSII, PQH2 travels to the [cytochrome b6f complex](https://bliptext.com/articles/cytochrome-b6f-complex), which then transfers two electrons from PQH2 to [plastocyanin](https://bliptext.com/articles/plastocyanin) in two separate reactions.\nThe process that occurs is similar to the Q-cycle in Complex III of the electron transport chain.\nIn the first reaction, PQH2 binds to the complex on the lumen side and one electron is transferred to the [iron-sulfur center](https://bliptext.com/articles/iron-sulfur-protein) which then transfers it to [cytochrome f](https://bliptext.com/articles/cytochrome-f) which then transfers it to plastocyanin.\nThe second electron is transferred to [heme bL](https://bliptext.com/articles/heme-b) which then transfers it to heme bH which then transfers it to PQ.\nIn the second reaction, a second PQH2 gets oxidized, adding an electron to another plastocyanin and PQ.\nBoth reactions together transfer four protons into the lumen.\n# [Simplified diagram of photophosphorylation. | upload.wikimedia.org/wikipedia/commons/2/2f/Cyclic///Photophosphorylation.svg]\n\n## Oxidative phosphorylation\nIn the electron transport chain, [complex I](https://bliptext.com/articles/complex-i) (CI) [catalyzes](https://bliptext.com/articles/catalysis) the [reduction](https://bliptext.com/articles/redox) of [ubiquinone](https://bliptext.com/articles/coenzyme-q10) (UQ) to [ubiquinol](https://bliptext.com/articles/ubiquinol) (UQH2) by the transfer of two [electrons](https://bliptext.com/articles/electron) from reduced [nicotinamide adenine dinucleotide](https://bliptext.com/articles/nicotinamide-adenine-dinucleotide) (NADH) which translocates four protons from the mitochondrial matrix to the IMS:\n[Complex III](https://bliptext.com/articles/complex-iii) (CIII) catalyzes the [Q-cycle](https://bliptext.com/articles/q-cycle).\nThe first step involving the transfer of two electrons from the UQH2 reduced by CI to two molecules of oxidized [cytochrome c](https://bliptext.com/articles/cytochrome-c) at the Qo site.\nIn the second step, two more electrons reduce UQ to UQH2 at the Qi site.\nThe total reaction is:\nComplex IV (CIV) catalyzes the transfer of two electrons from the cytochrome c reduced by CIII to one half of a full oxygen.\nUtilizing one full oxygen in oxidative phosphorylation requires the transfer of four electrons.\nThe 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\n# [Detailed diagram of the electron transport chain in mitochondria. | upload.wikimedia.org/wikipedia/commons/c/c4/ETC///electron///transport///chain.svg]","current_revision":null,"created_by":"kACnuQmH5k9Nr5VNVEpVeg7XpvUmMctD","created_at":"2025-01-25T22:29:40.027Z","updated_at":"2025-01-25T22:29:40.027Z","search_vector":"'electrochem':1 'gradient':2","search_content":" \n \n\n# \nAn **electrochemical gradient** is a gradient of [electrochemical potential](https://bliptext.com/articles/electrochemical-potential), usually for an [ion](https://bliptext.com/articles/ion) that can move across a [membrane](https://bliptext.com/articles/membrane).\nThe gradient consists of two parts:\nWhen 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 [simple diffusion](https://bliptext.com/articles/molecular-diffusion).\nIons also carry an electric charge that forms an [electric potential](https://bliptext.com/articles/electric-potential) across a membrane.\nIf 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.\nElectrochemical gradients are essential to the operation of [batteries](https://bliptext.com/articles/electric-battery) and other [electrochemical cells](https://bliptext.com/articles/electrochemical-cell), [photosynthesis](https://bliptext.com/articles/photosynthesis) and [cellular respiration](https://bliptext.com/articles/cellular-respiration), and certain other biological processes.\n# [Diagram of ion concentrations and charge across a semi-permeable cellular membrane. | upload.wikimedia.org/wikipedia/commons/4/4f/Membrane///potential///ions///en.svg]\n\n# Overview\nElectrochemical energy is one of the many interchangeable forms of [potential energy](https://bliptext.com/articles/potential-energy) through which energy may be [conserved](https://bliptext.com/articles/conservation-of-energy).\nIt appears in [electroanalytical chemistry](https://bliptext.com/articles/electroanalytical-chemistry) and has industrial applications such as batteries and fuel cells.\nIn biology, electrochemical gradients allow cells to control the direction ions move across membranes.\nIn [mitochondria](https://bliptext.com/articles/mitochondria) and [chloroplasts](https://bliptext.com/articles/chloroplast), [proton gradients](https://bliptext.com/articles/proton-motive-force) generate a **chemiosmotic potential** used to synthesize [ATP](https://bliptext.com/articles/adenosine-triphosphate), and the [sodium-potassium gradient](https://bliptext.com/articles/sodium-potassium-gradient) helps [neural synapses](https://bliptext.com/articles/neural-synapse) quickly transmit information.\nAn electrochemical gradient has two components: a differential concentration of [electric charge](https://bliptext.com/articles/electric-charge) across a membrane and a differential concentration of [chemical species](https://bliptext.com/articles/chemical-species) across that same membrane.\nIn 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.\nThe combinat[ion](https://bliptext.com/articles/ion) of these two phenomena determines the thermodynamically-preferred direct[ion](https://bliptext.com/articles/ion) for an [ion](https://bliptext.com/articles/ion)'s movement across the membrane.\nThe combined effect can be quantified as a gradient in the [thermodynamic](https://bliptext.com/articles/thermodynamic) [electrochemical potential](https://bliptext.com/articles/electrochemical-potential): with \n \nμi\nthe chemical potential of the ion species i \n \nzi\nthe charge per ion of the species i \n F, [Faraday constant](https://bliptext.com/articles/faraday-constant) (the electrochemical potential is implicitly measured on a per-[mole](https://bliptext.com/articles/mole-chemistry) basis) \n φ, the local [electric potential](https://bliptext.com/articles/electric-potential). \n Sometimes, the term \"electrochemical potential\" is abused to describe the electric potential generated by an ionic concentration gradient; that is, φ.\nAn electrochemical gradient is analogous to the water [pressure](https://bliptext.com/articles/pressure) across a [hydroelectric dam](https://bliptext.com/articles/hydroelectric-dam).\nRoutes unblocked by the membrane (e.g. [membrane transport protein](https://bliptext.com/articles/membrane-transport-protein) or [electrodes](https://bliptext.com/articles/electrode)) 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.\nConversely, energy can be used to [pump water up into the lake above the dam](https://bliptext.com/articles/pumped-storage), and chemical energy can be used to create electrochemical gradients.\n\n# Chemistry\nThe term typically applies in [electrochemistry](https://bliptext.com/articles/electrochemistry), when [electrical energy](https://bliptext.com/articles/electrical-energy) in the form of an applied voltage is used to modulate the [thermodynamic favorability](https://bliptext.com/articles/thermodynamic-control) of a [chemical reaction](https://bliptext.com/articles/chemical-reaction).\nIn a battery, an electrochemical potential arising from the movement of ions balances the reaction energy of the electrodes.\nThe maximum voltage that a battery reaction can produce is sometimes called the **standard electrochemical potential** of that reaction.\n\n# Biological context\nThe generation of a transmembrane electrical potential through ion movement across a [cell membrane](https://bliptext.com/articles/cell-membrane) drives [biological processes](https://bliptext.com/articles/biological-process) like [nerve](https://bliptext.com/articles/nerve) conduction, [muscle contraction](https://bliptext.com/articles/muscle-contraction), [hormone](https://bliptext.com/articles/hormone) [secretion](https://bliptext.com/articles/secretion), and [sensation](https://bliptext.com/articles/sensory-system).\nBy convention, physiological voltages are measured [relative](https://bliptext.com/articles/gauge-transformation) to the extracellular region; a typical animal cell has an [internal electrical potential](https://bliptext.com/articles/transmembrane-potential-difference) of (−70)–(−50) mV.\nAn electrochemical gradient is essential to [mitochondrial](https://bliptext.com/articles/mitochondrial) [oxidative phosphorylation](https://bliptext.com/articles/oxidative-phosphorylation).\nThe final step of [cellular respiration](https://bliptext.com/articles/cellular-respiration) is the [electron transport chain](https://bliptext.com/articles/electron-transport-chain), composed of four complexes embedded in the inner mitochondrial membrane.\nComplexes I, III, and IV pump protons from the [matrix](https://bliptext.com/articles/mitochondrial-matrix) to the [intermembrane space](https://bliptext.com/articles/mitochondrial-intermembrane-space) (IMS); for every [electron pair](https://bliptext.com/articles/electron-pair) entering the chain, ten protons translocate into the IMS.\nThe result is an electric potential of more than 200 mV.\nThe energy resulting from the flux of protons back into the matrix is used by [ATP synthase](https://bliptext.com/articles/atp-synthase) to combine inorganic [phosphate](https://bliptext.com/articles/phosphate) and [ADP](https://bliptext.com/articles/adenosine-diphosphate).\nSimilar to the electron transport chain, the [light-dependent reactions](https://bliptext.com/articles/light-dependent-reactions) of photosynthesis pump protons into the [thylakoid](https://bliptext.com/articles/thylakoid) [lumen](https://bliptext.com/articles/lumen-anatomy) of chloroplasts to drive the synthesis of ATP.\nThe proton gradient can be generated through either noncyclic or cyclic photophosphorylation.\nOf the proteins that participate in noncyclic photophosphorylation, [photosystem II](https://bliptext.com/articles/photosystem-ii) (PSII), [plastiquinone](https://bliptext.com/articles/plastoquinone), and [cytochrome b6f complex](https://bliptext.com/articles/cytochrome-b6f-complex) directly contribute to generating the proton gradient.\nFor each four photons absorbed by PSII, eight protons are pumped into the lumen.\nSeveral other transporters and ion channels play a role in generating a proton electrochemical gradient.\nOne is TPK3, a [potassium channel](https://bliptext.com/articles/potassium-channel) that is activated by Ca2+ and conducts K+ from the thylakoid lumen to the [stroma](https://bliptext.com/articles/stroma-fluid), which helps establish the [electric field](https://bliptext.com/articles/electric-field).\nOn the other hand, the electro-neutral K+ efflux [antiporter](https://bliptext.com/articles/antiporter) (KEA3) transports K+ into the thylakoid lumen and H+ into the stroma, which helps establish the [pH](https://bliptext.com/articles/ph) gradient.\n\n# Ion gradients\nSince the ions are charged, they cannot pass through cellular membranes via simple diffusion.\nTwo different mechanisms can transport the ions across the membrane: [active](https://bliptext.com/articles/active-transport) or [passive](https://bliptext.com/articles/passive-transport) transport.\nAn example of active transport of ions is the [Na+-K+-ATPase](https://bliptext.com/articles/na-k-atpase) (NKA).\nNKA is powered by the [hydrolysis](https://bliptext.com/articles/hydrolysis) of ATP into ADP and an inorganic phosphate; for every molecule of ATP hydrolized, three Na+ are transported outside and two K+ are transported inside the cell.\nThis makes the inside of the cell more negative than the outside and more specifically generates a membrane potential Vmembrane of about -60 mV.\nAn example of passive transport is ion fluxes through Na+, K+, Ca2+, and Cl− channels.\nUnlike active transport, passive transport is powered by the [arithmetic sum](https://bliptext.com/articles/arithmetic-sum) of [osmosis](https://bliptext.com/articles/osmosis) (a concentration gradient) and an [electric field](https://bliptext.com/articles/electric-field) (the transmembrane potential).\nFormally, the [molar](https://bliptext.com/articles/mole-chemistry) [Gibbs free energy](https://bliptext.com/articles/gibbs-free-energy) change associated with successful transport is where R represents the [gas constant](https://bliptext.com/articles/gas-constant), T represents [absolute temperature](https://bliptext.com/articles/thermodynamic-temperature), z is the charge per ion, and F represents the [Faraday constant](https://bliptext.com/articles/faraday-constant).\nIn the example of Na+, both terms tend to support transport: the negative electric potential inside the cell attracts the positive ion and since Na+ is concentrated outside the cell, osmosis supports diffusion through the Na+ channel into the cell.\nIn the case of K+, 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.\nThe converse phenomenon (osmosis supports transport, electric potential opposes it) can be achieved for Na+ in cells with abnormal transmembrane potentials: at 70 mV, the Na+ influx halts; at higher potentials, it becomes an efflux.\n# [Diagram of the Na+-K+-ATPase. | upload.wikimedia.org/wikipedia/commons/a/a5/Scheme///sodium-potassium///pump-en.svg]\n\n# Proton gradients\nProton gradients in particular are important in many types of cells as a form of energy storage.\nThe gradient is usually used to drive ATP synthase, [flagellar](https://bliptext.com/articles/flagellum) rotation, or [metabolite](https://bliptext.com/articles/metabolite) transport.\nThis section will focus on three processes that help establish proton gradients in their respective cells: [bacteriorhodopsin](https://bliptext.com/articles/bacteriorhodopsin) and noncyclic photophosphorylation and oxidative phosphorylation.\n\n## Bacteriorhodopsin\nThe way [bacteriorhodopsin](https://bliptext.com/articles/bacteriorhodopsin) generates a proton gradient in [Archaea](https://bliptext.com/articles/archaea) is through a [proton pump](https://bliptext.com/articles/proton-pump).\nThe proton pump relies on proton carriers to drive protons from the side of the membrane with a low H+ concentration to the side of the membrane with a high H+ concentration.\nIn bacteriorhodopsin, the proton pump is activated by absorption of [photons](https://bliptext.com/articles/photon) of 568nm [wavelength](https://bliptext.com/articles/wavelength), which leads to [isomerization](https://bliptext.com/articles/isomerization) of the [Schiff base](https://bliptext.com/articles/schiff-base) (SB) in [retinal](https://bliptext.com/articles/retinal) forming the K state.\nThis moves SB away from Asp85 and Asp212, causing H+ transfer from the SB to Asp85 forming the M1 state.\nThe protein then shifts to the M2 state by separating Glu204 from Glu194 which releases a proton from Glu204 into the external medium.\nThe SB is [reprotonated](https://bliptext.com/articles/protonation) by Asp96 which forms the N state.\nIt is important that the second proton comes from Asp96 since its [deprotonated](https://bliptext.com/articles/deprotonation) state is unstable and rapidly reprotonated with a proton from the [cytosol](https://bliptext.com/articles/cytosol).\nThe protonation of Asp85 and Asp96 causes re-isomerization of the SB, forming the O state.\nFinally, bacteriorhodopsin returns to its resting state when Asp85 releases its proton to Glu204.\n# [Diagram of the conformational shift in retinal that initiates proton pumping in bacteriorhodopsin. | upload.wikimedia.org/wikipedia/commons/5/57/Bacteriorhodopsin///retinal.png]\n\n## Photophosphorylation\nPSII also relies on [light](https://bliptext.com/articles/light) to drive the formation of proton gradients in chloroplasts, however, PSII utilizes vectorial redox chemistry to achieve this goal.\nRather 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.\nAbsorption of photons of 680nm wavelength is used to excite two electrons in [P680](https://bliptext.com/articles/p680) to a higher [energy level](https://bliptext.com/articles/energy-level).\nThese higher energy electrons are transferred to protein-bound [plastoquinone](https://bliptext.com/articles/plastoquinone) (PQA) and then to unbound [plastoquinone](https://bliptext.com/articles/plastoquinone) (PQB).\nThis reduces plastoquinone (PQ) to plastoquinol (PQH2) which is released from PSII after gaining two protons from the stroma.\nThe electrons in P680 are replenished by oxidizing [water](https://bliptext.com/articles/water) through the [oxygen-evolving complex](https://bliptext.com/articles/oxygen-evolving-complex) (OEC).\nThis results in release of O2 and H+ into the lumen, for a total reaction of\nAfter being released from PSII, PQH2 travels to the [cytochrome b6f complex](https://bliptext.com/articles/cytochrome-b6f-complex), which then transfers two electrons from PQH2 to [plastocyanin](https://bliptext.com/articles/plastocyanin) in two separate reactions.\nThe process that occurs is similar to the Q-cycle in Complex III of the electron transport chain.\nIn the first reaction, PQH2 binds to the complex on the lumen side and one electron is transferred to the [iron-sulfur center](https://bliptext.com/articles/iron-sulfur-protein) which then transfers it to [cytochrome f](https://bliptext.com/articles/cytochrome-f) which then transfers it to plastocyanin.\nThe second electron is transferred to [heme bL](https://bliptext.com/articles/heme-b) which then transfers it to heme bH which then transfers it to PQ.\nIn the second reaction, a second PQH2 gets oxidized, adding an electron to another plastocyanin and PQ.\nBoth reactions together transfer four protons into the lumen.\n# [Simplified diagram of photophosphorylation. | upload.wikimedia.org/wikipedia/commons/2/2f/Cyclic///Photophosphorylation.svg]\n\n## Oxidative phosphorylation\nIn the electron transport chain, [complex I](https://bliptext.com/articles/complex-i) (CI) [catalyzes](https://bliptext.com/articles/catalysis) the [reduction](https://bliptext.com/articles/redox) of [ubiquinone](https://bliptext.com/articles/coenzyme-q10) (UQ) to [ubiquinol](https://bliptext.com/articles/ubiquinol) (UQH2) by the transfer of two [electrons](https://bliptext.com/articles/electron) from reduced [nicotinamide adenine dinucleotide](https://bliptext.com/articles/nicotinamide-adenine-dinucleotide) (NADH) which translocates four protons from the mitochondrial matrix to the IMS:\n[Complex III](https://bliptext.com/articles/complex-iii) (CIII) catalyzes the [Q-cycle](https://bliptext.com/articles/q-cycle).\nThe first step involving the transfer of two electrons from the UQH2 reduced by CI to two molecules of oxidized [cytochrome c](https://bliptext.com/articles/cytochrome-c) at the Qo site.\nIn the second step, two more electrons reduce UQ to UQH2 at the Qi site.\nThe total reaction is:\nComplex IV (CIV) catalyzes the transfer of two electrons from the cytochrome c reduced by CIII to one half of a full oxygen.\nUtilizing one full oxygen in oxidative phosphorylation requires the transfer of four electrons.\nThe 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\n# [Detailed diagram of the electron transport chain in mitochondria. | upload.wikimedia.org/wikipedia/commons/c/c4/ETC///electron///transport///chain.svg]","revisionCount":0},"hasMore":false,"nextCursor":null}

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