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Published byJoella Scott Modified over 9 years ago
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Today Review membrane potential What establishes the ion distributions? What confers selective permeability? Ionic basis of membrane potential Next Lectures Action Potentials
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Membrane Potential –Inside of cell is negative compared to outside –Depends on: –High concentration K+ inside –Selective permeability of membrane
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What causes the different ion distributions in cells? 1.Passive distribution – Donnan equilibrium 2.Active Transport
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Passive distribution – Donnan equilibrium The ratio of positively charged permeable ions equals the ratio of negatively charged permeable ions II I K+K+ Cl - II I [K + ] = [K + ] [Cl - ] = [Cl - ] Start Equilibrium
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Donnan Equilibrium Mathematically expressed: Another way of saying the number of positive charges must equal the number of negative charges on each side of the membrane
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Passive Distribution BUT, in real cells there are a large number of negatively charged, impermeable molecules (proteins, nucleic acids, other ions) call them A - II I K+K+ Cl - Start A- II I [K + ] > [K + ] [Cl - ] < [Cl - ] Equilibrium A-
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Passive Distribution II I [K + ] > [K + ] [Cl - ] < [Cl - ] Equilibrium A- [K + ] I = [A - ] I + [Cl - ] I [K + ] II = [Cl - ] II If [A-] I is large, [K+] I must also be large +’ve = -’ve space-charge neutrality
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The presence of impermeable negatively charged molecules requires more positively charged molecules inside the cell.
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Donnan Equilibrium Example A - = 100 K + = 150 Cl - = 50 A - = 0 K + = 150 Cl - = 150 III Initial Concentrations Are these ions in electrochemical equilibrium? No, E K + = 0 mV E Cl - = -27 mV
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Solve for X, 7500 + 200X + X 2 = 22500 - 300X + X 2 X=30 Let X be the amount of K+ and Cl- that moves
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A - = 100 K + = 180 Cl - = 80 A - = 0 K + = 120 Cl - = 120 III Final Concentrations Are these ions in electrochemical equilibrium? Yes, E K + = -10 mV E Cl - = -10 mV space-charge neutrality
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What causes the different ion distributions in cells? 1.Passive distribution – Donnan equilibrium 2.Active Transport
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Active Transport ATP-powered pumps –Proteins that are capable of pumping ions from one side of the cell membrane to the other –Use energy
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Active Transport Na + - K + pump outside inside 3 Na + 2 K + ATPADP + Pi
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Active Transport Na + - K + pump –3 Na + move out –2 K + move in –Hydrolyzes ATP Maintains the concentration gradient
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Active Transport Na + - K + pump outside inside 3 Na + 2 K + Electrogenic net loss of 1 positive charge from inside Inside becomes more negative contributes a few mV to resting potential
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Na+/K+ pump is required –Due to low permeability for Na+ to leak into the cell –Without pump, Gradual accumulation of +’ve charge inside Eventually lose the membrane potential Active ingredient in rodent poison, ouabain, poisons the Na/K pump
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What causes the different ion distributions in cells? 1.Passive distribution – Donnan equilibrium 2.Active Transport
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What confers selective permeability? Ion channels –Non-gated –Leakage channels –Open at rest – allow K+ to flow out along its concentration gradient K+K+
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Membrane Potential Summary 1.Selective permeability Ion channel – K + leak channel 2.Unequal distribution of ions Passive distribution (Donnan) Active transport – Na+/K+ pump 3.The equilibrium potential of each ion is described by the Nernst equation 4.The total membrane potential is described by the Goldman equation
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Ionic basis of membrane potential
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K+1405-84 mV Na+10120+63 mV Cl-4110-83 mV Ca 2+ 0.0015+107 mV Outside (mM) Equilibrium (Nernst) Potential Inside (mM) Mammalian Cell
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Is the resting membrane potential controlled by one ion, or several? Do an experiment –Measure membrane potential (Vm) of a cell –Change extracellular concentration of an ion in the bathing solution –If Vm really depends on E ion than Vm should change as E ion changes
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Measuring Membrane Potential cell microelectrode amplifier 0 mV -80 mV time Resting potential Reference electrode
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Expt #1 vary extracellular Na Assume [Na] in = 10 mM 1-58 mV 5-17 100 2017 10058 20075 [Na] out prediction
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152 10 20 50 100 200 External Na+ concentration (mM) Membrane Potential (mV) Prediction of E Na From Nernst equation -120 -100 -80 -60 -40 20 0 40 -20 Measured Vm
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Therefore, –Conclude that Vm does not follow E Na
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Expt #2 vary extracellular K Assume intracellular [K] = 140 mM 1-124mV 5-84 10-66 20-49 100-8 2009 Extracell [K] prediction
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152102050100200 External K+ concentration (mM) Membrane Potential (mV) Prediction of E K From Nernst equation Deviation at low [K+] due to slight permeability of Na+ -120 -100 -80 -60 -40 -20 0 Measured Vm
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Therefore, the resting membrane potential (Vm) is very close to the equilibrium potential for K+ (E K )
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Summary 1.At rest P K >>P Na, P Cl, P Ca 2.Therefore, at rest, the membrane potential is close to E K 3.In general, the membrane potential will be dominated by the equilibrium (Nernst) potential of the most permeable ion
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Sample Question K = 140 Na = 10 Cl = 30 K = 5 Na = 145 Cl = 110 At rest Vm of this typical cell is -75 mV. What would Vm be if PNa >> Pk,PCl? Answer: Calculate E Na using Nernst equation. Assume Vm E Na = +67 mV
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The resting membrane potential is the basis for all electrical signaling Next Lecture: Action Potentials
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