General physiology: Goldman-Hodgkin-Katz equation: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · General physiology: Goldman-Hodgkin-Katz equation

Sample item

During the upstroke of a nerve action potential, sodium permeability rises sharply while potassium permeability is unchanged. A student wants one equation that predicts the membrane potential from the concentration gradients and relative permeabilities of several ions together. Which equation is it?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Goldman-Hodgkin-Katz equation

The Goldman-Hodgkin-Katz equation sums the permeability-weighted gradients of Na+, K+ and Cl-, so the membrane potential moves toward the ion with the highest permeability. The Nernst equation gives the equilibrium potential of one ion only. Remember GHK for membrane potential and Nernst for a single ion.

Option by option

  • A. Henderson-Hasselbalch equation

    Henderson-Hasselbalch relates pH to bicarbonate and CO2, not membrane potential.

  • B. Goldman-Hodgkin-Katz equation (correct)

    Correct. It weights each ion's gradient by its permeability to give the membrane potential.

  • C. Nernst equation

    The Nernst equation applies to a single ion at equilibrium and cannot combine several permeabilities.

  • D. Gibbs-Donnan relationship

    Gibbs-Donnan describes unequal distribution of diffusible ions due to impermeant protein, not dynamic permeability changes.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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What is the answer to this Physiology question?

The best answer is B, Goldman-Hodgkin-Katz equation. The Goldman-Hodgkin-Katz equation sums the permeability-weighted gradients of Na+, K+ and Cl-, so the membrane potential moves toward the ion with the highest permeability.

Why is "Henderson-Hasselbalch equation" not the answer?

Henderson-Hasselbalch relates pH to bicarbonate and CO2, not membrane potential.

Why is "Nernst equation" not the answer?

The Nernst equation applies to a single ion at equilibrium and cannot combine several permeabilities.

Why is "Gibbs-Donnan relationship" not the answer?

Gibbs-Donnan describes unequal distribution of diffusible ions due to impermeant protein, not dynamic permeability changes.

What concept does this question test?

It tests General physiology: Goldman-Hodgkin-Katz equation in Physiology. The Nernst equation gives the equilibrium potential of one ion only. Remember GHK for membrane potential and Nernst for a single ion.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study General physiology: Goldman-Hodgkin-Katz equation?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.