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Unformatted text preview: ss the cell, one will
increase/decrease (circle one) the equilibrium potential. Why?
One would decrease the equilibrium potential. Remember that at the electrochemical
equilibrium potential, the concentration gradient is balanced by the electrical gradient. If
we decrease the magnitude of the concentration, the electrical gradient will also necessarily
decrease. Another way to think about it, is as follows: by increasing extracellular K+, we
are decreasing the flow of K+ out of the cell because the concentration gradient that makes
this process energetically favourable has decreased. Assuming the Na+ concentration
remains the same, its flow in the cell remains the same. So, we have decreased the flow
(current) of K+ out of the cell while not changing the flow of Na+ into the cell. As a result,
the cell should depolarize.
d) Assume that the resting membrane of a cell is determined primarily by the equilibrium
potential of potassium. If you insert an electrode into this cell and clamp the membrane voltage
to +20 mV, would K+ be moving into or out of the cell through the potassium channels (K+out = 3
mM, K+in = 90 mM)?
From above, the equilibrium potential of potassium is –87.2 mV. The ratio of K+ (out/in) is
1/30. At +20 mV, the ratio of K out/in would be:
E = 0.02V, E = 0.059log (out/in), (out/in) = 10(E/0.059) = 891.3.
Since the present K+ concentration ratio of 1/30 is less than that required at +20 mV, the
potassium will leave the cell until the concentration outside is nearly 3 orders of magnitude
greater than the one on the inside. 7.06 Spring 2004 PS 2 KEY 6 of 7 e) You isolate cells that have only K+ channels. You also have a drug that causes this channel to
open. With a voltage clamp apparatus, you find, as expected, that in the presence of the drug the
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- Spring '11