# Explain why a water-soluble hormone, such as adrenaline, cannot enter its target cell and must rely on a second messenger system to produce a cellular response.

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

## Mark scheme (4 marks)

1. Water-soluble hormones are polar / hydrophilic and therefore cannot pass through the hydrophobic core of the phospholipid bilayer.
2. The hormone binds to a specific receptor protein located on the outer surface of the plasma membrane (extracellular domain).
3. Hormone–receptor binding activates a G-protein or adenylyl cyclase, leading to the production of a second messenger such as cyclic AMP (cAMP) inside the cell.
4. The second messenger (cAMP) relays and amplifies the signal within the cell, activating protein kinases that phosphorylate target proteins to bring about the cellular response.

## Key terms

- [water-soluble hormone](https://www.gradenine.co.uk/glossary/water-soluble-hormone)
- [second messenger](https://www.gradenine.co.uk/glossary/second-messenger)

## 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-why-a-water-soluble-hormone-such-3f301b3e) · Published by Druglandscape Ltd.