2010 Bio 317 Lecture 4

2010 Bio 317 Lecture 4 - BIO 317 Lecture 4 Prof William...

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1 Prof. William Collins Office: 534 Life Sciences Building Office Hours: Mondays, 4:00 – 5:00 PM or by appointment Recommended Reading: Matthews, Chapters 6 & 7 BIO 317: Lecture 4 The membrane permeability is determined by the properties of the aqueous ion channels in the plasma membrane 1. Ion selectivity of channel 2. Channel conductance ( g s ) - index of ion flux through the membrane (opposite of resistance) 3. The average proportion of the time individual channels are open ( P o ) 4. Channel number ( N ) or density (number per area) - functional density vs. anatomical density Thus, the membrane permeability for an ion ( p ion ) is determined by the density and conductance of the channels in the membrane that are selective for that ion. Membrane Permeability p ion g ion = Ng s P o
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2 The membrane potential is determined by: 1. The intracellular and extracellular concentrations of the permeant ions - Determines equilibrium potential (Nernst Potential) 2. The relative permeability ( p ion ) of the membrane for each ion - Determines relative influence of each ion on E m In cells in the resting state, P K > P Na Therefore, K + is the dominant ion and the membrane potential is close to (E K ) Membrane Potential Membrane Potential (E m ) Since the permeability of the membrane for K + is much greater than for Na + (P K > P Na ), K + is the dominant ion and the membrane potential is close to the equilibrium potential for K + . E K = -81 mV E m -70 mV Question: Why is E m not equal to E K ? ECF ICF 3 Na + 2 K + E m -70 mV 12 mM Na+ 120 mM Na+ 125 mM K+ 5 mM K+ 5 mM Cl - 125 mM Cl - 108 mM A -1.2
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3 Goldman Equation Quantitative description of steady state membrane potential. Simplify equation: 1) assume T= 20 o C 2) convert Ln to Log 3) express permeabilities in relative terms 4) express as mV E m = [K+] o + b [Na+] o [K+] i + b [Na+] i ( ) 58 mV log How Does b Influence E m ?
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