1 / 41

Electrochemical Potential

Electrochemical Potential. Δ E i (ek) = E m – E i ( electrochemical potential ) E.g. for Na + ; -80-(+60)=-140 mV For Ca 2+ ; -80-(+129)=-209 mV For K + ; -80-(-94)=+14 mV. GIBBS-DONNAN EQUILIBRIUM POTENTIALS . GIBBS-DONNAN POTENTIAL.

nikkos
Download Presentation

Electrochemical Potential

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. ElectrochemicalPotential • ΔEi(ek) = Em – Ei(electrochemicalpotential) • E.g. for Na+ ; -80-(+60)=-140 mV • For Ca2+ ; -80-(+129)=-209 mV • For K+ ; -80-(-94)=+14 mV

  2. GIBBS-DONNAN EQUILIBRIUM POTENTIALS

  3. GIBBS-DONNAN POTENTIAL • is passive; that is, energy is not necessarytoitsestablishment. • Incontrast, therestingpotential is activelygeneratedbytheaction of theNa+-K+pump.

  4. Membrane Membrane Equilibrium Initial The fluxcontinuetobecomepartialmolarfreeenergies at equilibriumforNa+andfor Cl- in bothtwosides.

  5. Inequilibrium • G0Na + RT ln[Na+]1 + FV1 = G0Na + RT ln[Na+]2 + FV2 • G0Cl + RT ln[Cl-]1 - FV1 = G0Cl + RT ln[Cl-]2 - FV2 • [Na+]1/[Na+]2 = [Cl-]2/[Cl-]1 = r • (Donnanequation, r: Donnanratio)

  6. Because of electroneutrality • [Na+]1 = [Cl-]1 = c1 • [Na+]2 = [Cl-]2+ [A-]

  7. Thepotentialdifferences at equilibrium; • V = V2 – V1 =RT/zFln([Na+]1/[Na+]2) =-RT/zFln([Cl-]2/[Cl-]1) =-RT/zFlnr

  8. At 37 °C RT/zF = -61 mV [Na+2] = [Cl-2]

  9. [Na+]1 = 100mM + x • [Cl-]1 = x • [Na+]2 = 100mM – x • [Cl-]2 = 100mM – x ise; • [Na+]1[Cl-]1 = [Na+]2[Cl-]2 • There is a flux as itsquantity is “x”fromside 1 toside 2 andfromside 2 toside 1;

  10. As a result; • Donnanequilibriumshowsthatthepotentialdifferencesmayarisealsobypassivemechanisms (orforces).

  11. DIFFUSION POTENTIAL

  12. DiffusionPotential • It is similartoNernstPotential. • The restingpotentialarisesfromthediffusionpotential of K+.

  13. It is alsoexplainedbyusingterm of mobility. • Mobility(μ)is a movement of an ionthrough a cellmembrane.

  14. IonMobility(μ): • When an electricalfield is appliedacross a cellmembrane (E), ionsgain a velocitytomovethroughthiselectricalfield.

  15. Mobilitydepends on temperatureandelectricalfield. • Furthermore, thecharactheristics of environmentandconcentration.

  16. V = μ.E (ionmobility)

  17. There is a potentialdifferencesbetweentwosides of membranebecause of thedifferences of ion’smobilities. • E.g.; …

  18. membrane

  19. Infigure, themobility of Cl-is higherthanthemobility of Na+. • Therefore, rightside of membrane is morenegativethanleftside of membrane.

  20. Incase, thePotentialdifference is; RT(μNa – μCl) (c1) F (μNa + μCl) (c2) • (volt) • c1: concentration of leftside • c2: concentration of rightside ln V =

  21. GOLDMAN-HODGKIN-KATZ EQUATION

  22. By far the most commonly used formalism for describing ion permeability and selectivity of membranes has been described by Goldman (1943) and Hodgkin & Katz (1949). • Its name is ‘constant field theory’.

  23. Two important assumptions are that • ions cross the membrane independently (without interaction with each other), • and that the electric field in the membrane is constant (the potential drops linearly across the membrane).

  24. Thesesimplifyingassumptionsleadtotwowidelyusedexpressions: • 1. GHK currentequation, • 2. GHK voltageequation.

  25. 1. GHK currentequation • says that the current carried by ‘ion S’ is equal to the permeability PS multiplied by a nonlinear function of voltage. • This equationallowsonetocalculatetheabsoluteionpermeabilityfrom a currentmeasurementifconcentrationandmembranepotentialareknown.

  26. 2. GHK voltageequation • Itgivesthemembranepotential at whichno net currentflows. • Forexample, it givestherestingmembranepotential in a cellwithoutelectrogenicpumps.

  27. Thisequation is a functionthat • it contains ion fluxes, membrane potential and the ion concentrations of intracellular and extracellular fluid.

  28. Goldman-Hodgkin-Katz, • Firstly thought that at least three ions (Na+, K+, Cl-) contribute to the membrane potential. • Then, they admit the electrical field of cell membrane as a linear and constant.

  29. By the way, • they consider the resting membrane potential as a diffusion potential of three ions. • Instead of Mobility, • Pi = Di / δ = μi RT / zi Fδ

  30. RT [(PK[K+]out+PNa[Na+]out+PCl[Cl-]in)] F [(PK[K+]in+PNa[Na+]in+PCl[Cl-]out)] (Goldman-Hodgkin-KatzEquation) ln EM=ERev=

  31. If the permeability of cell membrane to one ion is higher than the others, the GHK equation is reduced to Nernst equilibrium potential for this ion.

  32. Forexample, • the permeability of cell membrane to potassium is higher than sodium and chlor. So, • Em~ EK

  33. As a result, • Themembranepotential of anycell is neartheNernstpotential of anyionorionswhich has orhavehigherconcentrationorhigherpermeabilityformembrane.

  34. E.g., • Thepermeability of cellmembrane’s of squidaxonforions is: • PK:1 PNa:0.04 PCl:0.45

  35. REVERSAL POTENTIAL • Zero-currentpotential • EM in GHK equation is alsoErev.

  36. Also, it equals Nernst potential for one ion. • For more than one ion, it equals the average of Nernst Potentials of ions.

  37. But, • If only one permeant ion is on each side of the membrane and both ions have the valance z, Erev is given by the simple biionicequation: RT (PA [A]0) zF (PB [B]i) Erev= ln

  38. REFERENCES • Pehlivan F. (5. Baskı). Biyofizik. Pelikan Kitabevi. • Sperelakis N. (1995). Cell Physiology. AcademicPress. Inc. • Hille B. (2001). IonChannels of ExcitableMembranes.SinauerAssociates. Inc.

More Related