Explain how down-regulation of receptors allows a target cell to reduce its sensitivity to a hormone signal.

IB DP Biology Higher Level (2023 syllabus) — C2.1 Chemical signalling (HL only) · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Down‑regulation reduces the number of receptors on the target cell surface, so fewer hormone–receptor complexes can form.
Receptors are internalised (endocytosed) after hormone binding, removing them from the membrane.
With fewer receptors available, less hormone can bind, so the magnitude of the cellular response is lowered.
This negative‑feedback mechanism desensitises the cell to sustained or excessive hormone levels.

Examiner tips

  • Use the word ‘reduction’ and mention ‘receptors on the surface’ for the first point. Show the sequence: hormone binds → receptor internalised → fewer receptors → weaker response. Highlight that it is a negative‑feedback mechanism to prevent over‑stimulation.

Common mistakes

  • Confusing down‑regulation with up‑regulation. Forgetting to mention receptor internalisation. Using vague terms like ‘less sensitivity’ without explaining the receptor number change.

Mark scheme (4 marks)

  1. Down-regulation involves a reduction in the number of receptors present on the surface of (or within) the target cell
  2. Receptors are internalised (endocytosed) into the cell when hormone–receptor complexes form, removing them from the cell surface
  3. With fewer receptors available, less hormone can bind, so the magnitude/intensity of the cellular response is reduced
  4. Down-regulation is a form of negative feedback that prevents over-stimulation of / desensitises the target cell to sustained or excessive hormone levels

Key terms in this question

down-regulation

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