Explain how the structure of a phospholipid bilayer allows water molecules to move across the plasma membrane by osmosis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Phospholipids form a bilayer with their hydrophobic fatty‑acid tails facing inward and their hydrophilic heads facing the aqueous surroundings. The hydrophobic core therefore blocks most polar or charged solutes from passing freely.
Water molecules are small and polar; they can slip slowly through the bilayer either directly between phospholipids or via aquaporins. Because the membrane is partially permeable to water, water moves by osmosis from the side of higher water potential (lower solute concentration) to the side of lower water potential (higher solute concentration).
Water molecules are small and polar; they can slip slowly through the bilayer either directly between phospholipids or via aquaporins. Because the membrane is partially permeable to water, water moves by osmosis from the side of higher water potential (lower solute concentration) to the side of lower water potential (higher solute concentration).
Examiner tips
- Use the phrase "hydrophobic core" to show understanding of the bilayer structure. Explain that water is polar but small enough to cross the core. Mention osmosis as movement from high to low water potential. Keep the answer concise and use correct terminology.
Common mistakes
- Confusing the direction of water movement (high to low solute concentration). Failing to mention the hydrophobic core or the bilayer arrangement. Using vague terms like "pass through" without explaining how water crosses the membrane.
Mark scheme (4 marks)
- Phospholipids are arranged in two layers (bilayer) with hydrophobic fatty acid tails facing inward / towards each other
- The hydrophobic core restricts / prevents free passage of most polar or charged solutes across the membrane
- Water molecules, being small and polar, can pass slowly through the phospholipid bilayer / directly between phospholipid molecules
- Water moves by osmosis from a region of lower solute concentration (higher water potential) to a region of higher solute concentration (lower water potential) across the partially permeable membrane
Key terms in this question
phospholipid bilayer · osmosis
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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