General physiology: diffusion across membranes: 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: diffusion across membranes

Sample item

Two uncharged drugs of similar molecular size are given at the same plasma concentration, and both cross the cell membrane only by simple diffusion. Drug A has an oil-water partition coefficient 100 times that of drug B. How do their rates of entry into cells compare?

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

The answer and the reasoning

A. Drug A enters faster, because diffusion rate rises with lipid solubility

In Fick law the flux across a membrane is proportional to the area, the concentration gradient and the permeability, and permeability rises with the lipid-water partition coefficient. Drug A dissolves far more readily in the lipid bilayer, so it diffuses faster at an identical gradient. Simple diffusion needs neither energy nor a carrier.

Option by option

  • A. Drug A enters faster, because diffusion rate rises with lipid solubility (correct)

    Correct. Higher partition coefficient means higher membrane permeability and greater flux.

  • B. Drug B enters faster, because water-soluble drugs cross the membrane through pores

    Water solubility does not speed passage through the lipid core of the bilayer; polar molecules cross slowly.

  • C. Drug A enters faster only if a carrier protein is available

    Simple diffusion does not depend on carriers, which belong to facilitated diffusion.

  • D. Both enter at the same rate, because the concentration gradient is identical

    The gradient is only one determinant; the partition coefficient also sets the permeability.

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, Drug A enters faster, because diffusion rate rises with lipid solubility. In Fick law the flux across a membrane is proportional to the area, the concentration gradient and the permeability, and permeability rises with the lipid-water partition coefficient.

Why is "Drug B enters faster, because water-soluble drugs cross the membrane…" not the answer?

Water solubility does not speed passage through the lipid core of the bilayer; polar molecules cross slowly.

Why is "Drug A enters faster only if a carrier protein is available" not the answer?

Simple diffusion does not depend on carriers, which belong to facilitated diffusion.

Why is "Both enter at the same rate, because the concentration gradient is id…" not the answer?

The gradient is only one determinant; the partition coefficient also sets the permeability.

What concept does this question test?

It tests General physiology: diffusion across membranes in Physiology. Drug A dissolves far more readily in the lipid bilayer, so it diffuses faster at an identical gradient. Simple diffusion needs neither energy nor a carrier.

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: diffusion across membranes?

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.