Electrochemical gradient

Anelectrochemical gradientis a gradient ofelectrochemical potential,usually for anionthat can move across amembrane.The gradient consists of two parts:

• The chemical gradient, or difference insolute concentrationacross a membrane.
• The electrical gradient, or difference inchargeacross a membrane.

Diagram of ion concentrations and charge across a semi-permeable cellular 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 throughsimple diffusion.Ions also carry an electric charge that forms anelectric potentialacross 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 ofbatteriesand otherelectrochemical cells,photosynthesisandcellular respiration,and certain other biological processes.

Overview

Electrochemical energy is one of the many interchangeable forms ofpotential energythrough which energy may beconserved.It appears inelectroanalytical chemistryand has industrial applications such as batteries and fuel cells. In biology, electrochemical gradients allow cells to control the direction ions move across membranes. Inmitochondriaandchloroplasts,proton gradientsgenerate achemiosmotic potentialused to synthesizeATP,and thesodium-potassium gradienthelpsneural synapsesquickly transmit information.

An electrochemical gradient has two components: a differential concentration ofelectric chargeacross a membrane and a differential concentration ofchemical speciesacross 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 anion's movement across the membrane.

The combined effect can be quantified as a gradient in thethermodynamicelectrochemical potential:$\nabla {\overline {\mu }}_{i}=\nabla \mu _{i}({\vec {r}})+z_{i}\mathrm {F} \nabla \varphi ({\vec {r}}){\text{,}}$with

• μ_ithe chemical potential of the ion speciesi
• z_ithe charge per ion of the speciesi
• F,Faraday constant(the electrochemical potential is implicitly measured on a per-molebasis)
• φ,the localelectric potential.

Sometimes, the term "electrochemical potential" is abused to describe the electric potentialgeneratedby an ionic concentration gradient; that is,φ.

An electrochemical gradient is analogous to the waterpressureacross ahydroelectric dam.Routes unblocked by the membrane (e.g.membrane transport proteinorelectrodes) 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.Conversely, energy can be used topump water up into the lake above the dam,and chemical energy can be used to create electrochemical gradients.

Chemistry

See also:concentration cell,electrode potential,andtable of standard electrode potentials

The term typically applies inelectrochemistry,whenelectrical energyin the form of an applied voltage is used to modulate thethermodynamic favorabilityof achemical 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 thestandard electrochemical potentialof that reaction.

Biological context

The generation of a transmembrane electrical potential through ion movement across acell membranedrivesbiological processeslikenerveconduction,muscle contraction,hormonesecretion,andsensation.By convention, physiological voltages are measuredrelativeto the extracellular region; a typical animal cell has aninternal electrical potentialof (−70)–(−50) mV.

An electrochemical gradient is essential tomitochondrialoxidative phosphorylation.The final step ofcellular respirationis theelectron transport chain,composed of four complexes embedded in the inner mitochondrial membrane. Complexes I, III, and IV pump protons from thematrixto theintermembrane space(IMS); for everyelectron pairentering the chain, ten protons translocate into the IMS. The result is an electric potential of more than200 mV.The energy resulting from the flux of protons back into the matrix is used byATP synthaseto combine inorganicphosphateandADP.

Similar to the electron transport chain, thelight-dependent reactionsof photosynthesis pump protons into thethylakoidlumenof 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),plastiquinone,andcytochrome b₆f complexdirectly contribute to generating the proton gradient. For each four photons absorbed by PSII, eight protons are pumped into the lumen.

Several other transporters and ion channels play a role in generating a proton electrochemical gradient. One is TPK₃,apotassium channelthat is activated by Ca²⁺and conducts K⁺from the thylakoid lumen to thestroma,which helps establish theelectric field.On the other hand, the electro-neutral K⁺effluxantiporter(KEA₃) transports K⁺into the thylakoid lumen and H⁺into the stroma, which helps establish thepHgradient.

Ion gradients

Diagram of the Na⁺-K⁺-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:activeorpassivetransport.

An example of active transport of ions is theNa⁺-K⁺-ATPase(NKA). NKA is powered by thehydrolysisof 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. This makes the inside of the cell more negative than the outside and more specifically generates a membrane potentialV_membraneof about−60 mV.

An example of passive transport is ion fluxes through Na⁺,K⁺,Ca²⁺,and Cl⁻channels. Unlike active transport, passive transport is powered by thearithmetic sumofosmosis(a concentration gradient) and anelectric field(the transmembrane potential). Formally, themolarGibbs free energychange associated with successful transport is$$\Delta G=RT\ln {\!\left({\frac {c_{\rm {in}}}{c_{\rm {out}}}}\right)}+(Fz)V_{\rm {membrane}}$$whereRrepresents thegas constant,Trepresentsabsolute temperature,zis the charge per ion, andFrepresents theFaraday constant.

In 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. In 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. The 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.

Common cellular ion concentrations (millimolar)

Ion	Mammal		Squid axon		S. cerevisiae	E. coli	Sea water
	Cell	Blood	Cell	Blood
K+	100 - 140	4-5	400	10 - 20	300	30 - 300	10
Na+	5-15	145	50	440	30	10	500
Mg2+	10 0.5 - 0.8	1 - 1.5			50	30 - 100 0.01 - 1	50
Ca2+	10−4	2.2 - 2.6 1.3 - 1.5	10−4- 3×10−4	10	2	3 10−4	10
Cl−	4	110	40 - 150	560	10 - 200		500
X−(negatively charged proteins)	138	9	300 - 400	5-10
HCO3−	12	29
pH	7.1 - 7.3	7.35 to 7.45(normal arterial blood pH) 6.9 - 7.8(overall range)				7.2 - 7.8	8.1 - 8.2

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,flagellarrotation, ormetabolitetransport.This section will focus on three processes that help establish proton gradients in their respective cells:bacteriorhodopsinand noncyclic photophosphorylation and oxidative phosphorylation.

Bacteriorhodopsin

Diagram of the conformational shift in retinal that initiates proton pumping in bacteriorhodopsin.

The waybacteriorhodopsingenerates a proton gradient inArchaeais through aproton pump.The 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. In bacteriorhodopsin, the proton pump is activated by absorption ofphotonsof 568nmwavelength,which leads toisomerizationof theSchiff base(SB) inretinalforming the K state. This moves SB away from Asp85 and Asp212, causing H⁺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 isreprotonatedby Asp96 which forms the N state. It is important that the second proton comes from Asp96 since itsdeprotonatedstate is unstable and rapidly reprotonated with a proton from thecytosol.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.

Photophosphorylation

Simplified diagram of photophosphorylation.

PSII also relies onlightto 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 inP₆₈₀to a higherenergy level.These higher energy electrons are transferred to protein-boundplastoquinone(PQ_A) and then to unbound plastoquinone (PQ_B). This reduces plastoquinone (PQ) to plastoquinol (PQH₂) which is released from PSII after gaining two protons from the stroma. The electrons in P₆₈₀are replenished by oxidizingwaterthrough theoxygen-evolving complex(OEC). This results in release of O₂and H⁺into the lumen, for a total reaction of $$4h\nu +2{\ce {H2O}}+2{\ce {PQ}}+4{\ce {H+}}({\text{stroma}})\longrightarrow {\ce {O2}}+2{\ce {PQH2}}+4{\ce {H+}}({\text{lumen}})$$

After being released from PSII, PQH₂travels to thecytochrome b₆f complex,which then transfers two electrons from PQH₂toplastocyaninin 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, PQH₂binds to the complex on the lumen side and one electron is transferred to theiron-sulfur centerwhich then transfers it tocytochrome fwhich then transfers it to plastocyanin. The second electron is transferred toheme b_Lwhich then transfers it to heme b_Hwhich then transfers it to PQ. In the second reaction, a second PQH₂gets oxidized, adding an electron to another plastocyanin and PQ. Both reactions together transfer four protons into the lumen.

Oxidative phosphorylation

Detailed diagram of the electron transport chain in mitochondria.

In the electron transport chain,complex I(CI)catalyzesthereductionofubiquinone(UQ) toubiquinol(UQH₂) by the transfer of twoelectronsfrom reducednicotinamide adenine dinucleotide(NADH) which translocates four protons from the mitochondrial matrix to the IMS: $${\ce {NADH}}+{\ce {H^+}}+{\ce {UQ}}+4\underbrace {{\ce {H^+}}} _{\mathrm {matrix} }\longrightarrow {\ce {NAD^+}}+{\ce {UQH_2}}+4\underbrace {{\ce {H^+}}} _{\mathrm {IMS} }$$

Complex III(CIII) catalyzes theQ-cycle.The first step involving the transfer of two electrons from the UQH₂reduced by CI to two molecules of oxidizedcytochrome cat the Q_osite. In the second step, two more electrons reduce UQ to UQH₂at the Q_isite. The total reaction is: $$2\underbrace {\text{cytochrome c}} _{\text{oxidized}}+{\ce {UQH_2}}+2\underbrace {{\ce {H^+}}} _{\text{matrix}}\longrightarrow 2\underbrace {\text{cytochrome c}} _{\text{reduced}}+{\ce {UQ}}+4\underbrace {{\ce {H^+}}} _{\text{IMS}}$$

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 $$2{\text{cytochrome c}}({\text{reduced}})+4{\ce {H+}}({\text{matrix}})+{\frac {1}{2}}{\ce {O2}}\longrightarrow 2{\text{cytochrome c}}({\text{oxidized}})+2{\ce {H+}}({\text{IMS}})+{\ce {H2O}}$$

See also

• Concentration cell
• Transmembrane potential difference
• Action potential
• Cell potential
• Electrodiffusion
• Galvanic cell
• Electrochemical cell
• Proton exchange membrane
• Reversal potential

References

• Campbell & Reece (2005).Biology.Pearson Benjamin Cummings.ISBN978-0-8053-7146-8.
• Stephen T. Abedon, "Important words and concepts from Chapter 8, Campbell & Reece, 2002 (1/14/2005)", for Biology 113 at the Ohio State University