Explain how the structure of the cell membrane allows it to control the movement of substances in and out of the cell.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The cell membrane is semi‑permeable, allowing only certain substances to pass.
The phospholipid bilayer, with hydrophilic heads and hydrophobic tails, creates a barrier that blocks ions and large polar molecules.
Embedded proteins – such as channel, carrier and pump proteins – provide specific routes for substances to cross the bilayer.
By controlling which molecules enter or leave, the membrane maintains the cell’s internal environment, essential for cell function.
The phospholipid bilayer, with hydrophilic heads and hydrophobic tails, creates a barrier that blocks ions and large polar molecules.
Embedded proteins – such as channel, carrier and pump proteins – provide specific routes for substances to cross the bilayer.
By controlling which molecules enter or leave, the membrane maintains the cell’s internal environment, essential for cell function.
Examiner tips
- Use the word ‘semi‑permeable’ early. Mention the phospholipid bilayer and its hydrophilic/hydrophobic nature. Name at least one type of membrane protein and its role. Show how this control is vital for cell function.
Common mistakes
- Confusing the membrane with the cytoplasm. Omitting the role of proteins or the bilayer. Using vague terms like ‘selective’ without explaining the mechanism.
Mark scheme (4 marks)
- The cell membrane is semi-permeable
- Phospholipid bilayer structure contributes to its function
- Embedded proteins facilitate transport across the membrane
- Regulation of substance movement is crucial for cell function
Related
- All OCR GCSE Biology A: Gateway Science (J247) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →