Describe how membrane potential is established in animal cells and the role of Na+/K+ ATPase in maintaining the resting potential.

• A. Ion gradients arise from random diffusion; Na+/K+ ATPase is not involved.
• B. The Na+/K+ ATPase pumps Na+ out and K+ in, contributing to the resting membrane potential.
• C. The Na+/K+ ATPase pumps equal numbers of Na+ and K+ with no net charge change.
• D. Membrane potential is solely due to passive membrane permeability with no active pumps.

Answer: (B)

Membrane potential at rest is shaped by ions crossing the membrane and by the work that maintains the ion gradients themselves. The cell membrane is more permeable to potassium than to other ions, so K+ tends to leak out. As K+ leaves, the inside becomes more negative, setting the resting potential close to the potassium equilibrium potential. The Na+/K+ ATPase keeps these gradients strong by actively moving Na+ out and K+ in, using ATP. Its pumping is electrogenic: for every cycle, 3 Na+ are pumped out and 2 K+ are pumped in, producing a net outward movement of positive charge. This small outward current helps reinforce the negative interior and prevents the gradients from dissipating due to leaks. So, while the resting potential mainly reflects passive K+ permeability, the Na+/K+ ATPase is crucial for maintaining the gradients that allow that resting potential to be sustained.