Describe the two forces that drive an ion across the plasma membrane and explain how the Nernst equation takes into account both of these forces

Question

Use the components of the equation to support your explanation and be sure to specify the assumptions being made when using the Nernst equation to calculate membrane potential.

Answer 1

The forces that drive the movement of an ion across the plasma membrane include a concentration gradient (that is, there is a negative change in free energy associated with an increase in entropy for ions in solution) and an electrical component (the force resulting from the attraction between molecules of opposite charges). The Nernst equation expresses the change in voltage across the membrane as it relates to a change in the ratio of ions on either side of the plasma membrane. As written below, the voltage changes by 62 millivolts with every tenfold change in the ion concentration ratio across the membrane.

V = 62 log10(Co/Ci)

As written in this simplified form, the equation assumes that the flow of ions has reached an equilibrium at 37°C and that the ions that are moving are positive ions. Because the ions that are moving across the membrane are typically K+ ions (resting membrane potential) or Na+ ions (action potentials) these assumptions hold true for most of the biological systems being examined.