Explain the significance of the acrosome reaction and the cortical reaction in ensuring that only one sperm fertilises a secondary oocyte.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The acrosome is a lysosome‑like vesicle at the tip of the sperm head that releases hydrolytic enzymes when the sperm contacts the zona pellucida. These enzymes digest the zona, allowing the sperm plasma membrane to fuse with the oocyte membrane so the sperm nucleus can enter.
Sperm entry triggers the cortical reaction: cortical granules fuse with the oocyte membrane and exocytose their contents into the perivitelline space. The released material chemically modifies and hardens the zona pellucida (forming the fertilisation membrane/slow block to polyspermy), preventing further sperm from penetrating and ensuring monospermy.
Sperm entry triggers the cortical reaction: cortical granules fuse with the oocyte membrane and exocytose their contents into the perivitelline space. The released material chemically modifies and hardens the zona pellucida (forming the fertilisation membrane/slow block to polyspermy), preventing further sperm from penetrating and ensuring monospermy.
Examiner tips
- Use the exact terms ‘acrosome reaction’, ‘cortical reaction’, ‘zona pellucida’, ‘fertilisation membrane’, ‘slow block to polyspermy’.
- Show the sequence: acrosome → zona digestion → sperm fusion → cortical reaction → zona hardening.
- Mention that the cortical reaction is triggered by sperm entry and blocks polyspermy.
- Keep the answer concise – 4 marks allow a short paragraph, not a long essay.
Common mistakes
- Confusing the acrosome reaction with the cortical reaction or vice versa.
- Omitting the role of the cortical granules in hardening the zona pellucida.
- Using the wrong terminology (e.g., ‘fast block’ instead of ‘slow block’).
Mark scheme (4 marks)
- The acrosome is a lysosome-like vesicle at the tip of the sperm head that releases hydrolytic enzymes upon contact with the zona pellucida.
- 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.
- 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.
- 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 in this question
acrosome reaction · cortical reaction
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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