Biomolecules and enzymes: cholesterol and membrane fluidity: practice question

A Biochemistry 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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Biochemistry · Biomolecules and enzymes: cholesterol and membrane fluidity

Sample item

Cholesterol is intercalated among phospholipids of a plasma membrane. At body temperature, what is the main effect of the cholesterol on the bilayer?

AIt makes the bilayer less fluid and less permeable to small polar moleculesBIt makes the bilayer more fluid and more permeable to small polar moleculesCIt anchors proteins covalently to the outer leaflet of the membraneDIt prevents lateral movement of lipids completely, creating a gel

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

The answer and the reasoning

A. It makes the bilayer less fluid and less permeable to small polar molecules

Cholesterol's rigid steroid ring restricts the movement of fatty acyl chains near the phospholipid head groups, which reduces fluidity and permeability at body temperature. At low temperature it prevents tight packing, so it acts as a buffer of fluidity rather than creating a gel.

Option by option

A. It makes the bilayer less fluid and less permeable to small polar molecules (correct)
Correct. Cholesterol stiffens the bilayer at body temperature and lowers permeability.
B. It makes the bilayer more fluid and more permeable to small polar molecules
Cholesterol does the opposite at body temperature; it decreases fluidity and permeability.
C. It anchors proteins covalently to the outer leaflet of the membrane
Covalent anchoring of proteins is done by lipid modifications such as GPI or prenyl groups, not by free cholesterol.
D. It prevents lateral movement of lipids completely, creating a gel
Cholesterol limits movement but does not stop lateral diffusion, and at low temperature it actually prevents gel formation.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, It makes the bilayer less fluid and less permeable to small polar molecules. Cholesterol's rigid steroid ring restricts the movement of fatty acyl chains near the phospholipid head groups, which reduces fluidity and permeability at body temperature.

Why is "It makes the bilayer more fluid and more permeable to small polar mol…" not the answer?

Cholesterol does the opposite at body temperature; it decreases fluidity and permeability.

Why is "It anchors proteins covalently to the outer leaflet of the membrane" not the answer?

Covalent anchoring of proteins is done by lipid modifications such as GPI or prenyl groups, not by free cholesterol.

Why is "It prevents lateral movement of lipids completely, creating a gel" not the answer?

Cholesterol limits movement but does not stop lateral diffusion, and at low temperature it actually prevents gel formation.

What concept does this question test?

It tests Biomolecules and enzymes: cholesterol and membrane fluidity in Biochemistry. At low temperature it prevents tight packing, so it acts as a buffer of fluidity rather than creating a gel.

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 Biomolecules and enzymes: cholesterol and membrane fluidity?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.