General physiology: Nernst potential: 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: Nernst potential

Sample item

In a mammalian neurone at 37 degrees C the intracellular K+ concentration is 150 mmol/L and the extracellular concentration is 5 mmol/L. Using the Nernst equation in the form 61 x log10 (outside concentration / inside concentration), what is the equilibrium potential for K+?

A-122 mVB-61 mVC+90 mVD-90 mV

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

The answer and the reasoning

D. -90 mV

The Nernst potential is the membrane voltage at which the electrical force exactly balances the chemical gradient for one ion. For K+ the ratio outside to inside is 5/150, or 1/30, and log10 of 1/30 is about -1.48. Multiplying by 61 gives about -90 mV, close to the resting potential of a neurone, which is why resting potential is dominated by K+ permeability.

Option by option

A. -122 mV
-122 mV would result from a 100-fold ratio (log10 of 1/100 = -2), which is larger than the 30-fold gradient given.
B. -61 mV
-61 mV would result from a tenfold concentration ratio (log10 of 1/10 = -1), but the ratio here is 30-fold.
C. +90 mV
The sign is wrong; K+ leaves the cell down its gradient, so the equilibrium potential is negative, not positive. A positive value would be seen if the ratio were inverted.
D. -90 mV (correct)
61 x log10 (1/30) = 61 x (-1.48), which is about -90 mV.

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.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is D, -90 mV. The Nernst potential is the membrane voltage at which the electrical force exactly balances the chemical gradient for one ion.

Why is "-122 mV" not the answer?

-122 mV would result from a 100-fold ratio (log10 of 1/100 = -2), which is larger than the 30-fold gradient given.

Why is "-61 mV" not the answer?

-61 mV would result from a tenfold concentration ratio (log10 of 1/10 = -1), but the ratio here is 30-fold.

Why is "+90 mV" not the answer?

The sign is wrong; K+ leaves the cell down its gradient, so the equilibrium potential is negative, not positive. A positive value would be seen if the ratio were inverted.

What concept does this question test?

It tests General physiology: Nernst potential in Physiology. For K+ the ratio outside to inside is 5/150, or 1/30, and log10 of 1/30 is about -1.48. Multiplying by 61 gives about -90 mV, close to the resting potential of a neurone, which is why resting potential is dominated by K+ permeability.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: Nernst potential?

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.