Explain how the structure of a sperm cell is adapted for its function in fertilisation.

IB DP Biology Standard Level (2023 syllabus) — B2.3 Cell specialisation · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The sperm head contains an acrosome that releases hydrolytic enzymes to digest the zona pellucida, allowing penetration of the egg.
The nucleus in the head is haploid, providing half the genetic material so that fertilisation restores the diploid number.
The midpiece is densely packed with mitochondria that generate ATP by aerobic respiration, supplying energy for motility.
The flagellum (tail) propels the sperm through the female tract by beating, enabling it to reach and fuse with the egg.

Examiner tips

  • Use the word ‘acrosome’ and link it to zona pellucida digestion. Mention haploidy of the nucleus. Explain mitochondria in the midpiece as the energy source. Describe flagellum movement as propulsion.

Common mistakes

  • Confusing the acrosome with the nucleus. Forgetting to state the sperm is haploid. Omitting the role of mitochondria. Describing the flagellum as a static structure rather than a propulsive organ.

Mark scheme (4 marks)

  1. The acrosome (at the tip of the head) contains hydrolytic enzymes that digest the zona pellucida / egg coat, allowing the sperm to penetrate the egg.
  2. The nucleus (in the head) is haploid, carrying half the genetic information so that fertilisation restores the diploid number.
  3. The midpiece is packed with mitochondria that produce ATP by aerobic respiration to power the movement of the flagellum.
  4. The flagellum (tail) propels the sperm cell towards the egg by beating/rotating, enabling it to travel through the female reproductive tract.

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