The cell membranes of all organisms contain ion channels that permit ions to pass into or out of the cell, and these channels play extremely important roles in fundamental physiological processes such as heartbeats and the rapid conduction of signals along neurons. An important property of these ion channels is their selective conductivity—they selectively permit the passage of particular ions. For example, potassium channels more readily permit the passage of potassium ions than the passage of sodium ions, despite the fact that potassium ions are larger.