Question

Resting membrane potential (RMP) is predominantly affected by which ion?

• A. K\(^+\)
• B. Ca
• C. Na
• D. Cl

Hint:

At rest the membrane is most permeable to one ion via leak channels, so RMP sits near that ion's Nernst potential. Which ion?

Answer 1

The Correct Option is A

Step 1: Define the resting membrane potential (RMP).
The RMP (about \(-70\) to \(-90\) mV in most cells) is the voltage across the cell membrane at rest, set by the relative permeability of the membrane to different ions and their concentration gradients.

Step 2: Identify the dominant ion.
At rest the membrane is most permeable to potassium (K\(^+\)) because of open K\(^+\) "leak" channels. Since permeability to K\(^+\) greatly exceeds that to other ions, the resting potential lies close to the potassium equilibrium (Nernst) potential, \[E_K=\frac{RT}{zF}\ln\frac{[K^+]_o}{[K^+]_i}\] which is roughly \(-90\) mV. Therefore K\(^+\) predominantly determines the RMP.

Step 3: Eliminate the distractors.
(B) Ca\(^{2+}\) and (C) Na\(^+\) have very low resting permeability; Na\(^+\) influx is the main driver of the action-potential UPSTROKE, not the resting state. (D) Cl\(^-\) usually distributes passively and only fine-tunes the potential in most cells; it is not the predominant determinant. The membrane's high resting K\(^+\) permeability dominates.

Final answer: A - K\(^+\).