General physiology: Gibbs-Donnan effect: 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: Gibbs-Donnan effect

Sample item

Animal cells contain large, negatively charged proteins that cannot cross the plasma membrane. If the Na+-K+ ATPase of a cell were completely blocked for several hours, what would happen to cell volume, and why?

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

The answer and the reasoning

A. It would swell, because impermeant intracellular anions draw in diffusible ions and water

Impermeant intracellular anions attract diffusible cations (the Gibbs-Donnan effect), so the cell interior holds more osmotically active particles than the outside. Water would enter and the cell would swell. The pump counteracts this by extruding 3 Na+ for every 2 K+ taken in, keeping effective intracellular osmolality equal to that outside.

Option by option

  • A. It would swell, because impermeant intracellular anions draw in diffusible ions and water (correct)

    Correct. Without the pump the net particle load inside rises and water follows, producing swelling.

  • B. It would shrink, because water follows sodium out of the cell

    Sodium would move into, not out of, the cell when the pump is blocked, so water would follow it inward.

  • C. It would stay constant, because impermeant anions balance the charge of cations

    Electrical neutrality is achieved at the cost of extra osmotic particles inside, so volume does not stay constant.

  • D. It would shrink, because potassium would leak out down its gradient

    Potassium leak alone does not dominate; without the pump sodium and chloride accumulate and the net result is swelling, not shrinkage.

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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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is A, It would swell, because impermeant intracellular anions draw in diffusible ions and water. Impermeant intracellular anions attract diffusible cations (the Gibbs-Donnan effect), so the cell interior holds more osmotically active particles than the outside.

Why is "It would shrink, because water follows sodium out of the cell" not the answer?

Sodium would move into, not out of, the cell when the pump is blocked, so water would follow it inward.

Why is "It would stay constant, because impermeant anions balance the charge…" not the answer?

Electrical neutrality is achieved at the cost of extra osmotic particles inside, so volume does not stay constant.

Why is "It would shrink, because potassium would leak out down its gradient" not the answer?

Potassium leak alone does not dominate; without the pump sodium and chloride accumulate and the net result is swelling, not shrinkage.

What concept does this question test?

It tests General physiology: Gibbs-Donnan effect in Physiology. Water would enter and the cell would swell. The pump counteracts this by extruding 3 Na+ for every 2 K+ taken in, keeping effective intracellular osmolality equal to that outside.

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: Gibbs-Donnan effect?

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.