2.3.6Inquiry Process
Membrane Fluidity — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 52 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Fatty Acid Composition
The properties of the fatty acid tails decide how easily phospholipids move.
Saturated tails
- Saturated tails Straight chains pack closely, giving a membrane that is more rigid and stable at higher temperatures.
Unsaturated tails
- Unsaturated tails Double bonds create bends that prevent tight packing and keep the membrane fluid at lower temperatures.
Chain length
- Chain length Longer chains have more contact with neighbours, making the membrane less fluid than short chains.
Balance
- Balance Cells maintain a mixture that gives suitable fluidity.
| Feature | Effect on fluidity |
|---|---|
| More saturated | Decreases |
| More unsaturated | Increases |
| Longer chains | Decreases |
| Shorter chains | Increases |
Part 2 of 6Cholesterol
Cholesterol acts as a buffer of fluidity in animal cell membranes.
Cholesterol
Position
- Position Cholesterol lies between phospholipids with its small polar end near the heads.
At high temperature
- At high temperature It restrains movement of the phospholipids and reduces fluidity.
At low temperature
- At low temperature It prevents tails from packing closely, avoiding the membrane becoming rigid.
Result
- 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
- Vesicle formation Membranes must bend and pinch off to form vesicles, which requires flexibility.
Fusion
- Fusion Vesicles fuse with membranes during exocytosis only if the bilayers can merge.
Protein movement
- Protein movement Proteins need to diffuse in the membrane to cooperate and carry out reactions.
Repair
- 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.
- Lower temperature reduces the kinetic energy of phospholipids, so they pack more closely and the membrane becomes stiffer.
- Reduced fluidity slows protein movement and transport.
- 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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