A scientist investigates the effect of temperature on the permeability of cell membranes. Pieces of beetroot are placed into water at different temperatures. At higher temperatures, more red pigment leaks out of the beetroot cells into the surrounding water. Explain why increasing temperature causes more pigment to leak out of the beetroot cells.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Beetroot cells contain a red pigment stored in the vacuole. The pigment is normally retained inside the cell by the cell membrane. When the membrane is damaged, the pigment can pass through into the surrounding water.
Model answer (5 marks)
The cell membrane is a phospholipid bilayer that controls the movement of substances.
1. Increasing temperature gives the phospholipid molecules more kinetic energy.
2. This causes the phospholipids to move more rapidly and the bilayer becomes more fluid.
3. A more fluid membrane is less stable and its packing is looser, so it becomes more permeable.
4. At very high temperatures the membrane proteins can denature, further damaging the barrier.
5. Consequently the red pigment molecules can diffuse through the damaged membrane and leak into the surrounding water.
1. Increasing temperature gives the phospholipid molecules more kinetic energy.
2. This causes the phospholipids to move more rapidly and the bilayer becomes more fluid.
3. A more fluid membrane is less stable and its packing is looser, so it becomes more permeable.
4. At very high temperatures the membrane proteins can denature, further damaging the barrier.
5. Consequently the red pigment molecules can diffuse through the damaged membrane and leak into the surrounding water.
Examiner tips
- Use the term ‘phospholipid bilayer’ and ‘membrane fluidity’. Explain the link between temperature, kinetic energy and membrane permeability. Mention protein denaturation as a high‑temperature effect. Show the final step: pigment diffusion out of the cell.
Common mistakes
- Failing to mention the phospholipid bilayer. Confusing membrane permeability with active transport. Omitting the effect of protein denaturation at high temperatures.
Mark scheme (5 marks)
- The cell membrane is made of a phospholipid bilayer (which controls what enters and leaves the cell)
- Increasing temperature provides more energy / causes phospholipid molecules to move more
- The membrane becomes more fluid / less stable / more permeable
- At very high temperatures, proteins in the membrane are denatured
- The pigment molecules can therefore pass through / diffuse out of the damaged membrane into the surrounding water
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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