Explain how the structure of a phospholipid bilayer enables it to act as a barrier to the movement of ions and polar molecules across the plasma membrane.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Phospholipids are amphipathic – the hydrophilic phosphate head faces the aqueous extracellular and cytoplasmic sides, while the hydrophobic fatty‑acid tails face each other in the interior of the bilayer.
The interior of the bilayer is therefore a non‑polar, hydrophobic core. Charged ions and polar molecules cannot enter this region because they would have to shed their hydration shell, which is energetically unfavourable. Consequently the bilayer is essentially impermeable to ions and polar species.
Only when channel or carrier proteins are present can ions or polar molecules cross the membrane, showing that the bilayer’s structure creates a selective barrier.
The interior of the bilayer is therefore a non‑polar, hydrophobic core. Charged ions and polar molecules cannot enter this region because they would have to shed their hydration shell, which is energetically unfavourable. Consequently the bilayer is essentially impermeable to ions and polar species.
Only when channel or carrier proteins are present can ions or polar molecules cross the membrane, showing that the bilayer’s structure creates a selective barrier.
Examiner tips
- Use the word ‘amphipathic’ and describe head vs tail orientation. Mention the hydrophobic core repelling ions. Explain the energy cost of shedding hydration. Show that transport proteins are required for ions/polar molecules.
Common mistakes
- Saying the bilayer is a ‘solid barrier’ – it is semi‑fluid. Forgetting to mention the hydration shell of ions. Confusing the role of proteins with the bilayer itself.
Mark scheme (4 marks)
- Phospholipids are amphipathic, with hydrophilic phosphate heads facing outward (toward aqueous environments) and hydrophobic fatty acid tails facing inward.
- The hydrophobic core of the bilayer (formed by the non-polar fatty acid tails) repels / is impermeable to charged or polar species.
- Ions carry a net charge and are surrounded by a hydration shell of water molecules, making their passage into the non-polar core energetically unfavourable / requiring a very large energy input.
- Therefore ions and polar molecules require channel proteins or carrier proteins (or pumps) to cross the membrane, demonstrating how bilayer structure underpins selective/controlled transport.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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