The big idea: Cholesterol keeps an animal cell's membrane at the right fluidity whatever the temperature: it is a modulator of fluidity.
The points to remember
- Cholesterol sits between the phospholipids in animal cell membranes.
- Its small OH group lies by the phosphate heads; its rigid rings lie among the fatty acid tails.
- At higher temperatures it restrains the phospholipids, so the membrane does not become too fluid or leaky: it stabilises the membrane.
- At lower temperatures it keeps the tails apart, so the membrane does not stiffen.
- So it modulates (adjusts) fluidity in both directions.
Remember it as: Cholesterol: a brake when warm, a spacer when cold.
Real example: red blood cells: Nearly half of the lipid molecules in a red blood cell's membrane are cholesterol. The membrane stays flexible enough to squeeze through capillaries, yet strong enough not to burst.
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How it is asked: Paper 2 asks for the effect of cholesterol on fluidity [1], often next to a question on fatty acids, or as part of how membranes are adapted to temperature.
IB-style questionOutline[1 mark]
Outline the effect of cholesterol on the fluidity of the membranes of a lizard's cells.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
And a two-mark Explain:
IB-style questionExplain[2 marks]
Explain why plant cell membranes are not described as containing cholesterol as a modulator.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.