Which statement about membrane potential is true?

• A. It is the measure of the chemical gradient only.
• B. It is the flow rate of ions through channels.
• C. It is the negative value of the cell.
• D. It is the voltage difference across the membrane, with the inside typically negative relative to the outside.

Answer: (D)

Membrane potential is the voltage difference across the cell membrane. It isn’t simply the chemical gradient or a measure of how fast ions flow through channels. The inside of a resting cell is usually negative relative to the outside because the membrane is more permeable to potassium than to other ions, which allows positive charge to leave more readily than it enters. In addition, many negatively charged proteins stay inside, contributing to the negative interior. The Na+/K+ pump helps maintain the ion gradients that underlie this potential, but the resting potential mainly reflects the balance of leak currents and impermeant anions, commonly around −70 mV in neurons. So the statement that membrane potential is the voltage difference across the membrane with the inside typically negative relative to the outside best captures its meaning.