# Explain how the number of receptors on a target cell determines its sensitivity to a hormone signal.

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

> Cells of the same organism can respond very differently to the same circulating hormone concentration, depending on their receptor expression levels.

## Mark scheme (4 marks)

1. Receptors are specific proteins that bind a particular hormone, and only cells that express those receptors can respond to that hormone.
2. 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.
3. 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.
4. 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

- [receptor](https://www.gradenine.co.uk/glossary/receptor)
- [sensitivity](https://www.gradenine.co.uk/glossary/sensitivity)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-number-of-receptors-d46abd20) · Published by Druglandscape Ltd.