Explain how the fluid mosaic model accounts for the selective permeability of the plasma membrane.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The phospholipid bilayer has a hydrophobic core that blocks polar or charged molecules such as ions and glucose from passing through.
Small non‑polar or lipid‑soluble molecules (e.g. O₂, CO₂, steroid hormones) can diffuse freely across the bilayer.
Integral proteins – channel or carrier proteins – span the membrane and provide specific routes for polar molecules or ions to cross.
Because the membrane is fluid, phospholipids and proteins move laterally; this allows the cell to change the number or type of transport proteins in the membrane, maintaining selective permeability under different conditions.
Small non‑polar or lipid‑soluble molecules (e.g. O₂, CO₂, steroid hormones) can diffuse freely across the bilayer.
Integral proteins – channel or carrier proteins – span the membrane and provide specific routes for polar molecules or ions to cross.
Because the membrane is fluid, phospholipids and proteins move laterally; this allows the cell to change the number or type of transport proteins in the membrane, maintaining selective permeability under different conditions.
Examiner tips
- Use the phrase "hydrophobic core" to show understanding of the bilayer’s barrier. Mention both passive diffusion and protein‑mediated transport. Explain how fluidity allows regulation of protein composition. Keep answer concise to fit 4 marks.
Common mistakes
- Confusing the bilayer as a solid barrier; it is a fluid matrix. Forgetting that only non‑polar molecules diffuse freely. Not linking fluidity to the ability to insert or remove transport proteins.
Mark scheme (4 marks)
- The phospholipid bilayer forms a hydrophobic core that restricts the passage of polar/charged molecules (e.g. ions, glucose)
- Small, nonpolar/lipid-soluble molecules (e.g. O₂, CO₂, steroid hormones) can diffuse freely through the phospholipid bilayer
- Intrinsic/integral proteins (channel or carrier proteins) span the membrane and allow specific polar molecules or ions to cross
- The 'fluid' nature of the membrane (lateral movement of phospholipids and proteins) allows the membrane to adjust its protein composition, maintaining selectivity under varying conditions / proteins can be inserted or removed to regulate transport
Key terms in this question
fluid mosaic model · selective permeability
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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