Explain why the phospholipid bilayer acts as a selective barrier, allowing some molecules to cross freely whilst preventing others from crossing without the assistance of membrane proteins.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cell membranes are described as selectively permeable structures. The phospholipid bilayer forms the fundamental barrier of all cell membranes, yet cells must be able to exchange a wide variety of substances with their surroundings to maintain homeostasis.
Model answer (4 marks)
The phospholipid bilayer has a hydrophobic core formed by the fatty‑acid tails, which repels polar and charged molecules.
Small non‑polar or lipid‑soluble molecules such as O₂, CO₂ and steroid hormones can dissolve in this core and diffuse freely across the membrane.
Polar molecules and ions cannot cross the hydrophobic core unaided because they are hydrophilic and are repelled by the non‑polar interior.
Membrane proteins (channel or carrier proteins) provide a hydrophilic pathway, allowing polar molecules and ions to cross the bilayer.
Small non‑polar or lipid‑soluble molecules such as O₂, CO₂ and steroid hormones can dissolve in this core and diffuse freely across the membrane.
Polar molecules and ions cannot cross the hydrophobic core unaided because they are hydrophilic and are repelled by the non‑polar interior.
Membrane proteins (channel or carrier proteins) provide a hydrophilic pathway, allowing polar molecules and ions to cross the bilayer.
Examiner tips
- Use the word "hydrophobic core" and mention fatty‑acid tails. Include examples of non‑polar molecules. Explain why polar molecules are repelled. State the role of channel/carrier proteins.
Mark scheme (4 marks)
- The hydrophobic core of the bilayer is formed by the non-polar fatty acid tails, which repel polar and charged molecules.
- Small non-polar / lipid-soluble molecules (e.g. oxygen, carbon dioxide, steroid hormones) can dissolve in and diffuse freely across the hydrophobic core.
- Polar molecules and ions cannot cross the hydrophobic core unaided because they are hydrophilic / repelled by the non-polar interior.
- Membrane proteins (channel or carrier proteins) provide a hydrophilic pathway, allowing polar molecules and ions to cross the bilayer.
Key terms in this question
phospholipid bilayer · selective barrier · membrane proteins
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →