Explain how the concentration of cholesterol in the plasma membrane affects membrane fluidity and why this is important for normal cell function.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Cholesterol molecules are interspersed among the phospholipid tails in the bilayer.
At high temperatures cholesterol limits the lateral movement of phospholipids, thereby reducing fluidity.
At low temperatures cholesterol prevents the phospholipids from packing tightly, keeping the membrane from solidifying or gelling.
Maintaining an appropriate fluidity allows membrane proteins such as receptors, transporters and enzymes to move laterally, preserves selective permeability and membrane integrity, and is essential for processes such as vesicle formation, cell signalling and cell–cell recognition.
At high temperatures cholesterol limits the lateral movement of phospholipids, thereby reducing fluidity.
At low temperatures cholesterol prevents the phospholipids from packing tightly, keeping the membrane from solidifying or gelling.
Maintaining an appropriate fluidity allows membrane proteins such as receptors, transporters and enzymes to move laterally, preserves selective permeability and membrane integrity, and is essential for processes such as vesicle formation, cell signalling and cell–cell recognition.
Examiner tips
- Use the phrase "interspersed among phospholipid tails" to show understanding of structure.
- Explain both temperature effects in one sentence to cover two marks.
- Mention the functional consequence (protein movement, permeability) to secure the final mark.
Common mistakes
- Confusing cholesterol’s effect at high temperatures as increasing fluidity.
- Forgetting that cholesterol prevents solidification at low temperatures.
- Failing to link fluidity to a specific cell function such as signalling or vesicle formation.
Mark scheme (4 marks)
- Cholesterol molecules are interspersed among phospholipid tails in the bilayer
- At high temperatures, cholesterol restricts phospholipid movement and reduces fluidity; at low temperatures, cholesterol prevents the membrane from solidifying / gelling by spacing phospholipids apart
- Appropriate membrane fluidity is necessary for the lateral movement of membrane proteins such as receptor proteins, transport proteins, or enzymes
- Correct fluidity also maintains selective permeability / integrity of the membrane, allowing normal vesicle formation, cell signalling, or cell recognition
Key terms in this question
cholesterol · membrane fluidity
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Membranes and membrane transport questions
- Explain how the structure of a phospholipid bilayer enables it to act as a barri…
- Explain how the fluid mosaic model accounts for the selective permeability of th…
- Explain how the process of endocytosis allows a cell to take up large molecules …
- Explain how the sodium–potassium pump (Na⁺/K⁺-ATPase) maintains the electrochemi…
- Explain how the process of simple diffusion differs from facilitated diffusion i…
- Explain how the fluid mosaic model accounts for the lateral movement of membrane…