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Cell Membranes and Transport: Question 1

Syllabus 4.1

Multiple choice AS 1 mark

Cholesterol molecules sit between the phospholipids of an animal cell-surface membrane, with their hydroxyl group close to the phospholipid heads and their hydrocarbon ring system among the fatty acid tails.

Which statement correctly describes the effect of cholesterol on the fluidity of the membrane?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

The fluid mosaic model and cholesterol’s position

In the fluid mosaic model, phospholipids form a bilayer with hydrophilic phosphate heads facing the surrounding water and hydrophobic fatty acid tails facing inward, away from water. This bilayer is not rigid. Individual phospholipids and proteins can move sideways within their own layer, which is why the model is described as “fluid.” Cholesterol molecules are wedged between the fatty acid tails throughout the bilayer, with their small hydroxyl group oriented towards the phospholipid heads.

Cholesterol’s dual, temperature-dependent effect

Cholesterol does not simply add or remove fluidity. It moderates fluidity so the membrane stays functional across a range of temperatures:

  • At high temperatures, phospholipids would otherwise move too freely and the membrane would become too fluid and leaky. Cholesterol’s rigid ring system restricts the movement of the fatty acid tails around it, reducing fluidity and stabilising the membrane.
  • At low temperatures, phospholipids would otherwise pack tightly together and the membrane would become too rigid and prone to cracking. Cholesterol disrupts this close, regular packing, maintaining fluidity so the membrane does not solidify.

This dual role is why option B is correct: cholesterol reduces fluidity at high temperature and maintains/increases fluidity at low temperature, keeping the membrane’s properties relatively stable despite changes in temperature.

Why the other options are wrong

  • A describes the opposite temperature relationship to cholesterol’s real effect.
  • C describes the function of a water channel (an aquaporin), a channel protein, not a lipid molecule like cholesterol, which does not form pores.
  • D wrongly classifies cholesterol as a glycoprotein and as a hormone receptor; cholesterol is a lipid, and receptor functions belong to glycoproteins embedded in the membrane, not to cholesterol.

Final answer

B, cholesterol restricts phospholipid movement (reducing fluidity) at high temperatures, and prevents close packing (maintaining fluidity) at low temperatures.