What Happens when an Ion Channel is Permeable to only One Species of Ion?
Generation of a Membrane Potential
In this demo, a membrane (in yellow) separates two aqueous compartments. A channel runs through the membrane, permeable to potassium (K+) ions only. The compartment on the left contains a potassium chloride solution (K+) ions in blue, Cl- ions in maize). The right-hand compartment contains sodium (Na+ in red) ions and chloride ions.
Notice that, when the demo starts, the voltage everywhere is the same except for transient deviations from electroneutrality caused by thermal motions of the ions. As the simulation runs, watch for K+ ions crossing through the channel into the right compartment. This creates a separation of charge (the K+ ion moved to the right, but its corresponding Cl- ion had to stay behind. As a result, you will note that now the right-hand compartment is more positive than the left (notice that the center part of the voltage trace, corresponding to the charge-free membrane, is a straight line sloping up and to the right). Wait to see if you can catch two or even three K+ on the right side, and notice how the voltage difference across the membrane increases with each additional K+ that crosses to the right.
Notice also that this same voltage difference affects the K+ ions in the channel, tending to move them back to the left side. An equilibrium is reached when the tendency of K+ to move from its high concentration to low concentration is exactly balanced by the electrical force pulling in the opposite direction. The voltage at which this happens is given by the Nernst equation.
This page last updated 15.IV.1999 by Robert Stephenson.
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