Explain how the structure of the plasma membrane enables it to act as a selectively permeable barrier.

IB DP Biology Higher Level (2023 syllabus) — A2.2 Cell structure · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The plasma membrane is a phospholipid bilayer with hydrophilic heads outward and hydrophobic tails inward, forming a barrier that blocks polar/charged molecules and ions.

Embedded in this bilayer are channel and carrier proteins that allow specific polar molecules, ions or other hydrophilic substances to cross, giving selective permeability.

Small, non‑polar or uncharged molecules such as O₂, CO₂ and steroid hormones dissolve in the hydrophobic core and diffuse freely across the membrane.

Because the membrane is fluid, proteins can move laterally, allowing the membrane to adjust its permeability dynamically and providing specificity for different substances.

Examiner tips

  • Use the phrase "phospholipid bilayer" and describe head/tail orientation. Mention both channel and carrier proteins. Explain simple diffusion for non‑polar molecules. Highlight the fluid mosaic model for dynamic regulation.

Common mistakes

  • Confusing the bilayer with a single lipid layer. Forgetting to mention the role of proteins in selective permeability. Failing to note that non‑polar molecules diffuse freely.

Mark scheme (4 marks)

  1. The phospholipid bilayer consists of hydrophilic heads facing outward and hydrophobic tails forming the interior, creating a barrier that restricts the passage of polar/charged molecules and ions.
  2. Channel proteins and/or carrier proteins span the membrane and allow specific polar molecules, ions, or other hydrophilic substances to cross, contributing to selective permeability.
  3. Small, non-polar/uncharged molecules (e.g. O₂, CO₂, steroid hormones) can dissolve in and pass freely through the hydrophobic core by simple diffusion.
  4. The fluid nature of the membrane (fluid mosaic model) allows lateral movement of proteins, enabling dynamic regulation of permeability, and the mosaic of different proteins provides specificity for different substances.

Key terms in this question

plasma membrane · selectively permeable

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