When the membrane potential (Vm) is greater than the equilibrium potential (Ei) of an ion, what direction will the ion move?

• A. Toward the electrical force
• B. Against the concentration gradient
• C. Toward the chemical force
• D. Not move at all

Answer: (A)

When the membrane potential (Vm) is greater than the equilibrium potential (Ei) of an ion, the ion will move in the direction of the electrical force. This is because the electrical force acts to move positively charged ions toward areas of lower positive charge or higher negative charge and negatively charged ions toward areas of higher positive charge.

In this scenario, when Vm exceeds Ei, it suggests that the inside of the cell is more positive than the equilibrium state for that particular ion. Therefore, if the ion is positively charged, it will experience a driving force that pushes it into the cell, where the concentration of the ion may be lower, trying to restore the balance that is represented by Ei. Conversely, if the ion is negatively charged, it will be pushed out of the cell due to the positive Vm.

Thus, the direction of movement aligns with the electrical force in response to the existing membrane potential relative to the ion's equilibrium potential. This principle is fundamental in understanding ion movement and membrane dynamics in physiology.