# 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 every tissue.

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

> Adrenaline is released from the adrenal medulla directly into the bloodstream during the 'fight or flight' response. Despite circulating throughout the body, its effects are highly specific — for example, it causes increased heart rate and dilation of the pupils, but does not affect all cell types equally.

## Mark scheme (4 marks)

1. Adrenaline only affects cells that possess the specific complementary receptor for adrenaline
2. The receptor is specific to adrenaline due to its complementary shape / tertiary structure
3. Cells lacking this receptor cannot bind adrenaline, so no intracellular response is triggered
4. Adrenaline is a non-steroid / hydrophilic hormone so it cannot enter cells; it binds to a cell-surface receptor, triggering a second messenger cascade inside target cells only

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
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