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2.3.6Inquiry Process

Membrane Fluidity — IB Biology HL Revision Notes & Topical Questions

5 min read 6 parts Lesson 52 of 160

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Part 1 of 6Fatty Acid Composition

2.3.6 Membrane Fluidity

The properties of the fatty acid tails decide how easily phospholipids move.

Saturated tails
  • Straight chains pack closely, giving a membrane that is more rigid and stable at higher temperatures.
Unsaturated tails
  • Double bonds create bends that prevent tight packing and keep the membrane fluid at lower temperatures.
Chain length
  • Longer chains have more contact with neighbours, making the membrane less fluid than short chains.
Balance
  • Cells maintain a mixture that gives suitable fluidity.
FeatureEffect on fluidity
More saturatedDecreases
More unsaturatedIncreases
Longer chainsDecreases
Shorter chainsIncreases

Part 2 of 6Cholesterol

Cholesterol acts as a buffer of fluidity in animal cell membranes.

Cholesterol
Position
  • Cholesterol lies between phospholipids with its small polar end near the heads.
At high temperature
  • It restrains movement of the phospholipids and reduces fluidity.
At low temperature
  • It prevents tails from packing closely, avoiding the membrane becoming rigid.
Result
  • Cholesterol makes membrane fluidity less affected by temperature changes.

Part 3 of 6Adapting to Temperature

1
A drop in temperature slows phospholipid movement and makes the membrane stiffer.
2
Some bacteria make fatty acid desaturases that add double bonds to fatty acids.
3
More unsaturated tails restore fluidity in the cold.
4
Organisms living in cold water tend to have more unsaturated fatty acids in their membranes.
5
In hot conditions more saturated fats help keep the membrane stable.

Part 4 of 6Why Fluidity Matters

Vesicle formation
  • Membranes must bend and pinch off to form vesicles, which requires flexibility.
Fusion
  • Vesicles fuse with membranes during exocytosis only if the bilayers can merge.
Protein movement
  • Proteins need to diffuse in the membrane to cooperate and carry out reactions.
Repair
  • Fluid membranes can reseal if damaged.

Part 5 of 6Worked Example

Question

  • A bacterium grown at 37 °C is moved to 15 °C.
  • Predict what happens to its membrane and describe a response that restores function.

  1. Lower temperature reduces the kinetic energy of phospholipids, so they pack more closely and the membrane becomes stiffer.
  2. Reduced fluidity slows protein movement and transport.
  3. The bacterium can make desaturase enzymes that add double bonds to fatty acid tails, creating kinks.

Answer

  • The membrane becomes less fluid at first;
    • increased unsaturation of fatty acids restores fluidity.

Part 6 of 6Exam Checklist & Tips

2.3.6 – Key Points for the Exam

  • Membrane fluidity The ease with which phospholipids and proteins move within the membrane.
  • Saturated fatty acid A fatty acid with no double bonds, producing a straight chain.
  • Cholesterol A lipid that regulates fluidity in animal cell membranes.
  • Desaturase An enzyme that introduces double bonds into fatty acids.

Common Mistakes

  • Saying saturated fats increase fluidity.
  • Thinking cholesterol only makes membranes more rigid.
  • Forgetting that very long chains reduce fluidity.
  • Believing fluidity does not matter to cell function.

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