Which statement regarding membrane potential and equilibrium potential is true?

Question

BlackTom AI · Accepted Answer

Equilibrium potential for an ion is the voltage at which that ion’s chemical (concentration) gradient and electrical gradient exactly balance; the Nernst equation quantifies this. Membrane potential is the actual electrical potential across the membrane produced by the combined effects of multiple ions and their relative permeabilities (described by the Goldman-Hodgkin-Katz equation).

Relevant concepts:
- Nernst equation (single ion): E_ion = (RT/zF) ln([outside]/[inside]). At 37°C for a monovalent ion, E_ion ≈ 61.5 mV · log10([outside]/[inside]).
- Membrane potential (V_m) depends on multiple ions (e.g., K+, Na+, Cl−) and their permeabilities; V_m is a weighted average of E_ions (GHK equation).

Step-by-step reasoning and calculation example:
1. For K+ with typical concentrations ([K+]in ≈ 140 mM, [K+]out ≈ 4 mM), use the Nernst form:
   E_K ≈ 61.5 mV · log10(4 / 140).
2. Compute ratio: 4 / 140 = 0.02857.
3. log10(0.02857) ≈ −1.544.
4. E_K ≈ 61.5 × (−1.544) ≈ −95 mV.
This E_K is the equilibrium potential for K+ alone. The resting membrane potential (~ −70 mV) differs because Na+ and other ions (with different E_ions and permeabilities) also contribute.

Why the correct answer is correct:
- It states equilibrium potential is determined by the concentration and electrical gradients of one ion (true; Nernst describes a single ion’s balance), while membrane potential is affected by gradients of all ions (true; GHK shows combined contributions weighted by permeability).

Why the other options are incorrect:
- Option 2: Equilibrium potential depends on concentrations both inside and outside, not only inside.
- Option 3: Equilibrium potentials are independent of membrane permeability (they are properties of a single ion’s gradient); membrane potentials are affected by permeability.
- Option 4: During an action potential the membrane potential changes substantially; equilibrium potentials for major ions change minimally because bulk ion concentrations don’t change much.
- Option 5: Equilibrium potentials depend on ionic concentration ratios and thus are not intrinsically different between neurons with similar concentrations; the statement is misleading and not the general principle.

Conclusion: The correct choice is “Equilibrium potential is affected by the concentration and electrical gradients of one ion; membrane potential is affected by gradients of all ions.” This matches the provided answer.

Which statement regarding membrane potential and equilibrium potential is true?单项选择题

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