Question

Which ion is most responsible for the resting membrane potential (RMP)?

• A. Potassium (K+)
• B. Calcium (Ca2+)
• C. Sodium (Na+)
• D. Chloride (Cl-)

Hint:

The resting membrane potential (RMP) is primarily maintained by potassium (K+) ions, with the Na+/K+ ATPase pump helping to maintain the ion gradient.

Answer 1

The Correct Option is A

Answer: Potassium (K+).

A cell's resting voltage settles close to the equilibrium voltage of whichever ion the membrane lets through most easily. At rest the membrane is far more leaky to potassium than to sodium, calcium or chloride, because many potassium leak channels stay open. So potassium sets the resting value.

We can check this with the Nernst equation for potassium:
\[ E_{K} = -61 \times \log_{10}\frac{[K^+]_{out}}{[K^+]_{in}} \]
Potassium is high inside the cell and low outside, which gives about $-90$ mV. The measured resting membrane potential is about $-70$ to $-90$ mV, very near $E_{K}$. That closeness is the proof that potassium is in charge.

Why not the others? Sodium ($Na^+$) and calcium ($Ca^{2+}$) are high outside and their equilibrium voltages are positive, but at rest the membrane barely lets them in, so they have little say. Chloride ($Cl^-$) mostly follows the voltage that potassium has already set. So potassium (option 1) is the main ion behind the resting membrane potential.