lecture_09_28

lecture_09_28 - I.C.1. The Nernst equation Equilibrium...

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I.C.1. The Nernst equation Equilibrium potential for potassium (E k ) E K = RT ln [K + ] out 17 ° E K = +57.5 log 10 [K + ] out zF [K + ] in [K + ] in This equation (the Nernst equation) relates the equilibrium potential of a particular ion to the concentration difference across a membrane ASSUMING the membrane is only permeable to that single ion. This equation can easily be used to calculate the equilibrium potential for ANY ion. For example, the Nernst equation could also be used to calculate the equilibrium potential for a membrane only permeable to sodium or chloride ions. If we didn't have a voltmeter to measure the -83 mV equilibrium potential recorded in the three-beaker example. we could use the Nernst equation to calculate the equilibrium potential. Let's do it. For a more specific example to calculate the K + equilibrium potential of the inside of a cell relative to the outside at 17 o C you would use the more specific equation E K = 57.5 * log ([K + ] outside /[K + ] inside ) Remember the potential depends on Z which is the charge (including the sign) of the ion. So Cl - has a charge of –1 (that is Z= -1 if you are using Cl-) and Ca ++ has a charge of +2 (that is Z= +2 if you are using Ca 2+ ) Remember the potential depends on the temperature & reference_side is the side with the ground wire. More generally it is the reference side, which is usually the outside of a cell unless otherwise stated For example to calculate an equilibrium potential of the inside of a cell relative to the outside @ 17 o C we would use the more specific equation E Ion = (57.5/Z) * log ([Ion] outside /[Ion]) inside An important description of this equation and how to use it is in your computer exercise. This includes the values of R and F. @17 o C Æ E Ion = (57.5/Z) * log ([Ion] reference_side /[Ion]) electrode_side E Ion = (RT/ZF)*ln([Ion] reference_side /[Ion] electrode_side ) & electrode_side is the side with the electrode in the beaker example. More generally electrode side is the side whose voltage you want to predict. This usually is the inside of the cell, unless otherwise stated. (Remember voltage is always relative! When we talk about the voltage of the cell we usually are referring to the voltage of the inside relative to the outside) Where [Ion] is the concentration of the ion of interest. General Equation You want to know left relative to right
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I.C.2. One ion hypothesis Anatomy indicates that neurons are mostly permeable to potassium ions. Given this fact, the following hypothesis was proposed: Hypothesis: The resting membrane potential recorded across a neuron is solely due to the cell membrane being permeable to potassium ions. Question. How would you test this hypothesis? A: Well, we have already performed the essential experiments!
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This note was uploaded on 11/22/2009 for the course NPB 100 taught by Professor Chapman during the Fall '08 term at UC Davis.

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lecture_09_28 - I.C.1. The Nernst equation Equilibrium...

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