BSCI 426/Biol 622                         Fall '14                          Problem Set 10

assigned:     11/20/14
due:           12/02/14

1. The electrophysiological properties of a single channel were recorded in the presence of a salt gradient. Given that the reversal potential of the channel was -30 mV and the current at zero voltage was 20 pA,

a) what is its conductance?
b) what assumption did you have to make in order to estimate the conductance?

2. This time the membrane contained two conductances. One of these was an ohmic conductance due to current traveling through pathways other than the channel being recorded (i.e. the "leak"). The other was current flowing through the channel. The reversal potential of the total current was -40 mV. When the channel closed, the reversal potential became -10 mV. When zero voltage was applied, the current varied from +6 pA to 0.5 pA as the channel gated between the open and closed state.

a) What are the conductances of the channel and the "leak"?
b) What is the reversal potential of the channel?
c) What are the permeability ratios (Pcation/Panion) for the channel and the "leak" using the constant field approximation given the following conditions:  The salt concentration on the ground side (i.e. defined as 0 mV) was 10 mM KCl while that on the side the voltage was measured was 100 mM KCl.  No other ions were present.