# Explain why a hormone such as adrenaline can only affect certain cells in the body, even though it travels in the blood and reaches virtually all tissues.

> OCR A-Level Biology A (H420) — 5.4 Hormonal communication · Explain · 4 marks

> Adrenaline is released from the adrenal medulla during a stress response and circulates throughout the body via the bloodstream. Despite this widespread distribution, only specific cells respond to adrenaline.

## Mark scheme (4 marks)

1. Only target cells possess the specific complementary receptor for adrenaline
2. The receptor has a complementary shape to adrenaline, so only adrenaline can bind to it (shape specificity)
3. Binding of adrenaline to its receptor initiates a response / activates a second messenger cascade inside the cell
4. Non-target cells lack this receptor and so cannot respond, even when adrenaline is present

## Key terms

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

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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-why-a-hormone-such-as-1037b4f0) · Published by Druglandscape Ltd.