Explain how the number of receptors on a target cell determines its sensitivity to a hormone signal.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cells of the same organism can respond very differently to the same circulating hormone concentration, depending on their receptor expression levels.
Model answer (4 marks)
Receptors are specific proteins that bind a particular hormone, and only cells that express those receptors can respond to that hormone.
A greater number of receptors on the cell surface (or intracellularly for lipid‑soluble hormones) means more hormone–receptor complexes can form at a given hormone concentration.
More hormone–receptor complexes lead to a greater intracellular signal (e.g. more second messenger produced, or more gene transcription activated), amplifying the cellular response.
Cells can up‑regulate or down‑regulate receptor numbers, allowing sensitivity to be altered; down‑regulation (e.g. following prolonged high hormone levels) reduces the response, and up‑regulation increases it.
A greater number of receptors on the cell surface (or intracellularly for lipid‑soluble hormones) means more hormone–receptor complexes can form at a given hormone concentration.
More hormone–receptor complexes lead to a greater intracellular signal (e.g. more second messenger produced, or more gene transcription activated), amplifying the cellular response.
Cells can up‑regulate or down‑regulate receptor numbers, allowing sensitivity to be altered; down‑regulation (e.g. following prolonged high hormone levels) reduces the response, and up‑regulation increases it.
Examiner tips
- Use the word ‘receptor’ and explain its role in binding the hormone.
- Show the link between receptor number, complex formation and signal amplification.
- Mention regulation of receptor numbers to explain changes in sensitivity.
- Keep the answer concise – 4 marks can be achieved with 4 short points.
Common mistakes
- Confusing hormone concentration with receptor number; the question asks about receptors, not hormone levels.
- Failing to mention that only receptor‑expressing cells respond.
- Using vague terms like ‘more receptors’ without linking to signal strength.
Mark scheme (4 marks)
- Receptors are specific proteins that bind a particular hormone, and only cells that express those receptors can respond to that hormone.
- A greater number of receptors on the cell surface (or intracellularly for lipid-soluble hormones) means more hormone–receptor complexes can form at a given hormone concentration.
- More hormone–receptor complexes lead to a greater intracellular signal (e.g. more second messenger produced, or more gene transcription activated), amplifying the cellular response.
- Cells can up-regulate or down-regulate receptor numbers, allowing sensitivity to be altered; down-regulation (e.g. following prolonged high hormone levels) reduces the response, and up-regulation increases it.
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 →
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