# Explain the significance of the acrosome reaction and the cortical reaction in ensuring that only one sperm fertilises a secondary oocyte.

> IB DP Biology Higher Level (2023 syllabus) — D3.1 Reproduction · Explain · 4 marks

## Mark scheme (4 marks)

1. The acrosome is a lysosome-like vesicle at the tip of the sperm head that releases hydrolytic enzymes upon contact with the zona pellucida.
2. The enzymes from the acrosome digest the zona pellucida, allowing the sperm plasma membrane to fuse with the oocyte plasma membrane so the sperm nucleus can enter.
3. Sperm entry triggers the cortical reaction, in which cortical granules fuse with the oocyte plasma membrane and release their contents into the perivitelline space by exocytosis.
4. The contents of the cortical granules chemically modify / harden the zona pellucida (forming the fertilisation membrane / slow block to polyspermy), preventing additional sperm from penetrating and thereby ensuring monospermy.

## Key terms

- [acrosome reaction](https://www.gradenine.co.uk/glossary/acrosome-reaction)
- [cortical reaction](https://www.gradenine.co.uk/glossary/cortical-reaction)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-the-significance-of-the-acrosome-5651c575) · Published by Druglandscape Ltd.