Explain why the phospholipid bilayer is described as selectively permeable, referring to the different properties of the molecules that cross it.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cell membranes control the movement of substances into and out of cells. Experimental evidence shows that small non-polar molecules cross the membrane rapidly, while large polar molecules and ions cross very slowly or require assistance.
Model answer (4 marks)
The phospholipid bilayer has a hydrophobic core that blocks the passage of charged or polar molecules and ions.
Small non‑polar or lipid‑soluble molecules can diffuse freely through this core.
Large or polar molecules and ions need protein channels or carrier proteins to cross.
Thus only certain molecules can cross, making the membrane selectively permeable rather than freely permeable.
Small non‑polar or lipid‑soluble molecules can diffuse freely through this core.
Large or polar molecules and ions need protein channels or carrier proteins to cross.
Thus only certain molecules can cross, making the membrane selectively permeable rather than freely permeable.
Examiner tips
- Use the word ‘selectively permeable’ early. Show the link between hydrophobic core and blocking of polar/charged species. Mention diffusion for small non‑polar molecules. Include protein channels/carriers for large or polar species.
Common mistakes
- Confusing ‘selectively permeable’ with ‘freely permeable’. Forgetting to mention the hydrophobic core. Omitting the role of protein channels or carriers for large/polar molecules.
Mark scheme (4 marks)
- The hydrophobic core of the bilayer prevents passage of charged or polar molecules/ions
- Small non-polar / lipid-soluble molecules can diffuse freely through the hydrophobic core
- Large or polar molecules / ions require protein channels or carrier proteins to cross the membrane
- This selective control means only certain molecules can cross, making the membrane selectively permeable rather than freely permeable
Key terms in this question
phospholipid bilayer · selectively permeable
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →