Anelectrochemical gradient is a gradient ofelectrochemical potential, usually for anion that can move across amembrane. The gradient consists of two parts:
If 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 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 ofbatteries and otherelectrochemical cells,photosynthesis andcellular respiration, and certain otherbiological processes.
Electrochemical energy is one of the many interchangeable forms ofpotential energy through which energy may beconserved. It appears inelectroanalytical 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. Inmitochondria andchloroplasts,proton gradients generate achemiosmotic potential used to synthesizeATP,[1] and thesodium-potassium gradient helpsneural synapses quickly transmit information.[citation needed]
An electrochemical gradient has two components: a differential concentration ofelectric charge across a membrane and a differential concentration ofchemical 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 anion's movement across the membrane.[2]: 403 [3]
The combined effect can be quantified as a gradient in thethermodynamicelectrochemical potential:[citation needed]
with
Sometimes, the term "electrochemical potential" is abused to describe the electric potentialgenerated by an ionic concentration gradient; that is,φ.
An electrochemical gradient is analogous to the waterpressure across ahydroelectric dam. Routes unblocked by the membrane (e.g.membrane transport protein orelectrodes) 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] Conversely, energy can be used topump water up into the lake above the dam, and chemical energy can be used to create electrochemical gradients.[4][5]
The term typically applies inelectrochemistry, whenelectrical energy in the form of an applied voltage is used to modulate thethermodynamic favorability of 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 potential of that reaction.[citation needed]
The generation of a transmembrane electrical potential through ion movement across acell membrane drivesbiological processes likenerve conduction,muscle contraction,hormonesecretion, andsensation. By convention, physiological voltages are measuredrelative to the extracellular region; a typical animal cell has aninternal electrical potential of (−70)–(−50) mV.[2]: 464
An electrochemical gradient is essential tomitochondrialoxidative phosphorylation. The final step ofcellular respiration is theelectron transport chain, composed of four complexes embedded in the inner mitochondrial membrane. Complexes I, III, and IV pump protons from thematrix to theintermembrane space (IMS); for everyelectron pair entering 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 synthase to combine inorganicphosphate andADP.[6][2]: 743–745
Similar to the electron transport chain, thelight-dependent reactions of photosynthesis pump protons into thethylakoidlumen 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),plastiquinone, andcytochrome b6f complex directly contribute to generating the proton gradient. For each four photons absorbed by PSII, eight protons are pumped into the lumen.[2]: 769–770
Several other transporters and ion channels play a role in generating a proton electrochemical gradient. One is TPK3, apotassium channel that 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 thepH gradient.[7]
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 orpassive transport.[citation needed]
An example of active transport of ions is theNa+-K+-ATPase (NKA). NKA is powered by thehydrolysis 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. This makes the inside of the cell more negative than the outside and more specifically generates a membrane potentialVmembrane of about−60 mV.[5]
An example of passive transport is ion fluxes through Na+, K+, Ca2+, and Cl− channels. Unlike active transport, passive transport is powered by thearithmetic sum ofosmosis (a concentration gradient) and anelectric field (the transmembrane potential). Formally, themolarGibbs free energy change associated with successful transport is[citation needed] whereR represents thegas constant,T representsabsolute temperature,z is the charge per ion, andF represents theFaraday constant.[2]: 464–465
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.[citation needed]
| 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[a] 0.5 - 0.8[b] | 1 - 1.5 | 50 | 30 - 100[a] 0.01 - 1[b] | 50 | ||
| Ca2+ | 10−4 | 2.2 - 2.6[c] 1.3 - 1.5[d] | 10−4 - 3×10−4 | 10 | 2 | 3[a] 10−4[b] | 10 |
| Cl− | 4 | 110 | 40 - 150 | 560 | 10 - 200[e] | 500 | |
| X− (negatively charged proteins) | 138 | 9 | 300 - 400 | 5-10 | |||
| HCO3− | 12 | 29 | |||||
| pH | 7.1 - 7.3[12] | 7.35 to 7.45[12] (normal arterial blood pH) 6.9 - 7.8[12] (overall range) | 7.2 - 7.8[13] | 8.1 - 8.2[14] | |||
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,flagellar rotation, ormetabolite transport.[15] 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]
The waybacteriorhodopsin generates a proton gradient inArchaea is 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 ofphotons of 568nmwavelength, which leads toisomerization of theSchiff base (SB) inretinal forming 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 isreprotonated by Asp96 which forms the N state. It is important that the second proton comes from Asp96 since itsdeprotonated state 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.[15][16]
PSII also relies onlight 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 inP680 to a higherenergy level. These higher energy electrons are transferred to protein-boundplastoquinone (PQA) and then to unbound plastoquinone (PQB). This reduces plastoquinone (PQ) to plastoquinol (PQH2) which is released from PSII after gaining two protons from the stroma. The electrons in P680 are replenished by oxidizingwater through theoxygen-evolving complex (OEC). This results in release of O2 and H+ into the lumen, for a total reaction of[15]
After being released from PSII, PQH2 travels to thecytochrome b6f complex, which then transfers two electrons from PQH2 toplastocyanin 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, PQH2 binds to the complex on the lumen side and one electron is transferred to theiron-sulfur center which then transfers it tocytochrome f which then transfers it to plastocyanin. The second electron is transferred toheme bL which then transfers it to heme bH which then transfers it to PQ. In the second reaction, a second PQH2 gets oxidized, adding an electron to another plastocyanin and PQ. Both reactions together transfer four protons into the lumen.[2]: 782–783 [17]
Main article:Oxidative phosphorylation
In the electron transport chain,complex I (CI)catalyzes thereduction ofubiquinone (UQ) toubiquinol (UQH2) by the transfer of twoelectrons from reducednicotinamide adenine dinucleotide (NADH) which translocates four protons from the mitochondrial matrix to the IMS:[18]
Complex III (CIII) catalyzes theQ-cycle. The first step involving the transfer of two electrons from the UQH2 reduced by CI to two molecules of oxidizedcytochrome c at the Qo site. In the second step, two more electrons reduce UQ to UQH2 at the Qi site. The total reaction is:[18]
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[18]
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