Explain how the structure of a phospholipid makes it suitable for forming a bilayer in cell membranes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A phospholipid has a hydrophilic phosphate head group and two hydrophobic fatty acid tails, giving it an amphipathic nature.
In an aqueous environment the hydrophobic tails orientate away from water and the hydrophilic heads orientate towards water, driving self‑assembly of the bilayer.
The hydrophobic interior of the bilayer acts as a barrier to the passage of polar/charged molecules and ions, maintaining selective permeability.
The fluid nature of the bilayer arises because phospholipids are not covalently bonded to each other, allowing lateral movement, which maintains membrane flexibility and integrity.
In an aqueous environment the hydrophobic tails orientate away from water and the hydrophilic heads orientate towards water, driving self‑assembly of the bilayer.
The hydrophobic interior of the bilayer acts as a barrier to the passage of polar/charged molecules and ions, maintaining selective permeability.
The fluid nature of the bilayer arises because phospholipids are not covalently bonded to each other, allowing lateral movement, which maintains membrane flexibility and integrity.
Examiner tips
- Use the word ‘amphipathic’ to link structure to function. Show the two‑step orientation of heads/tails in water. Mention selective permeability and fluidity as separate points. Keep the answer concise – 4 points, one sentence each.
Common mistakes
- Confusing the phospholipid head with the tail; forgetting the phosphate group. Describing the bilayer as a rigid sheet instead of a fluid one. Failing to mention the barrier function for polar molecules.
Mark scheme (4 marks)
- A phospholipid has a hydrophilic phosphate head group and two hydrophobic fatty acid tails, giving it an amphipathic nature.
- In an aqueous environment, the hydrophobic tails orientate away from water and the hydrophilic heads orientate towards water, driving self-assembly of the bilayer.
- The hydrophobic interior of the bilayer acts as a barrier to the passage of polar/charged molecules and ions, maintaining selective permeability.
- The fluid nature of the bilayer arises because phospholipids are not covalently bonded to each other, allowing lateral movement, which maintains membrane flexibility and integrity.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Carbohydrates and lipids questions
- Explain how the molecular structure of cellulose makes it suitable for its role …
- Explain how the structural differences between saturated and unsaturated fatty a…
- Explain how the structure of a triglyceride is related to its function as an ene…
- Explain how the structures of starch and glycogen make them suitable for energy …
- Explain how the structure of chitin makes it suitable for its role as a structur…
- Explain how the structure of a triglyceride makes it suitable for energy storage…