Why is the resting membrane potential negative?

Why is the resting membrane potential negative?

• A. There are slightly more cations inside the cell than anions.
• B. There are slightly more anions inside the cell than cations.
• C. The membrane is permeable to water.
• D. Ion pumps maintain equal numbers on both sides.

Answer: (B)

The resting membrane potential is negative because the cell interior carries a net negative charge compared with the outside. This arises from impermeant anions inside the cell (like certain proteins and phosphates) and the membrane’s greater permeability to potassium, which tends to diffuse out of the cell. As K+ leaves, positive charge exits more readily than it enters, leaving behind more negative charges inside. The Na+/K+ pump helps maintain the ion gradients but does not make the sides equal; it supports the conditions that allow K+ efflux to dominate. The other statements don’t fit: more cations inside would make the interior positive, water permeability doesn’t establish the voltage, and pumps maintaining equal numbers would erase the gradient that generates the negative potential.

The resting membrane potential is negative because the cell interior carries a net negative charge compared with the outside. This arises from impermeant anions inside the cell (like certain proteins and phosphates) and the membrane’s greater permeability to potassium, which tends to diffuse out of the cell. As K+ leaves, positive charge exits more readily than it enters, leaving behind more negative charges inside. The Na+/K+ pump helps maintain the ion gradients but does not make the sides equal; it supports the conditions that allow K+ efflux to dominate. The other statements don’t fit: more cations inside would make the interior positive, water permeability doesn’t establish the voltage, and pumps maintaining equal numbers would erase the gradient that generates the negative potential.