sample_exam1b_Sol

sample_exam1b_Sol - Exam 1 2002 Key pg 1 of 4 EXAM 1 KEY...

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Exam 1 2002 Key pg. 1 of 4 EXAM 1 KEY Color code: Orange the answer: blue extra explanatory comments Part 1 (3.4 points). You record from a special cell that has a resting membrane potential of 9 mV . When the cell is subjected to stretch special ion channels open and the membrane voltage goes to 73 mV. Through a series of experiments you determine that when open, these special ion channels are permeable to, Ca 2+ , I - , or Li + , but only ONE of these ions. Given the resting concentrations and permeabilities below, which type of ion (Ca 2+ I - or Li + , choose only ONE) is the special stretch channel permeable to and why ? The concentrations and permeabilities at rest are as follows. [Ca 2+ ] outside = 351 mM [I - ] outside = 117 mM [Li + ] outside = 421 mM [Ca 2+ ] inside = 217 mM [I - ] inside = 20 mM [Li + ] inside = 14 mM P Ca = 65 P I = 18 P Li = 8 Show your calculations and provide and explain the reasoning behind your choice of answer. Show Calculations: (1.7 points) Nernst equation: E ion = (57.5/z)*log 10 ([ion] out /[ion] in ) E Ca = (57.5/2)log 10 (351/217) = 6 mv; E I = (57.5/-1)log 10 (117/20) = -44.1 mV E Li = (57.5/1)*log 10 (421/14) = 85.0 mV Why? The special ion channels are permeable to Li when stretched. To explain this we need to use the rule “ If the permeability of the cell membrane to an ion is increased & all other variables are held constant, the membrane potential will always shift towards the equilibrium potential of that ion.” (1) Because E Ca and E I are more negative than the RMP, opening of Ca or I channels would have to lead to hyperpolarization by the rule (2) lithium’s equilibrium potential is above the RMP and therefore if the channels were permeable to Li, the cell would depolarize (3) the cell depolarized so it must be lithium. A common mistake was to say that the depolarization to 73 mV was closes to E Li . Therefore the channel is permeable to Li. This logic is flawed. We can show this with two exceptions. Exception 1: two depolarizing synapses. Say the membrane is permeable to ion2. Both E ion1 (10 mV) and E ion2 (100 mV) are more positive than the RMP (-67 mV). Say when the channels open we got a depolarization that took the membrane potential to –30 mV. Eion1 is closer to the depolarized voltage BUT the channel could be permeable to EITHER ion1 or ion2. Therefore this logic does not work. Exception 2: The cell depolarizes and the membrane voltage is very close To, but overshoots the nearest equilibrium potential. Say the membrane is permeable to ion 3 whose equilibrium potential is 100 mV, and not to ion 4 whose equilibrium potential is 10 mV. Both are above the RMP (-73 mV), and the cells membrane voltage depolarizes to 15 mV when an ion channel opens.
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