CourseCourseQ BankQuestion BankDocsDocuments

D3.1.15 (HL)—Preventing polyspermy

Polyspermy is prevented after one sperm enters because the cortical reaction alters the zona pellucida and blocks further sperm binding.

Syllabus
First assessment 2025
Objective
D3.1.15
Level
HL

Exam analysis

Chance of appearing9%of analysed past papers
Latest appearanceNovember 2023
Most common paperPaper1
Typical marks1

Common command terms

  • Identify
  • Explain

Scoring notes

Common mistake
Mixing up the acrosome reaction that allows sperm entry with the cortical reaction that blocks other sperm.

Recent exam appearances

November 2023Paper1 ["HL"] · TZ240[ 1 ]D3.1.15 (HL)—Preventing polyspermy
May 2023Paper1 ["HL"] · TZ140[ 1 ]D3.1.15 (HL)—Preventing polyspermy
May 2022Paper1 ["HL"] · TZ137[ 1 ]D3.1.15 (HL)—Preventing polyspermy
November 2021Paper2 ["HL"] · TZ02(d)[ 2 ]D3.1.15 (HL)—Preventing polyspermy
May 2017Paper1 ["HL"] · TZ240[ 1 ]D3.1.15 (HL)—Preventing polyspermy
Practice this objective

Coverage 2012–2023 · Updated 16 Jul 2026

Polyspermy Is Blocked after the First Sperm Fuses

HL only

Two linked reactions allow one sperm to enter an egg and then prevent additional sperm from causing polyspermy.

Reaction Trigger and effect
Acrosome reaction Enzymes released from the sperm acrosome digest a path through the zona pellucida, allowing penetration
Cortical reaction Fusion of the first sperm triggers cortical-granule release; the zona pellucida changes so other sperm cannot pass through

Blocking additional sperm preserves the normal paternal chromosome contribution and prevents an abnormal polyploid zygote.

The acrosome reaction enables penetration; the cortical reaction creates the later block. Reversing these roles gives the wrong mechanism.

Preventing polyspermy

HL only

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / Explain.

Command terms

Identify / Explain

What earns marks

Build the answer around this relationship: The acrosome reaction helps one sperm penetrate the zona pellucida.

Watch for

Mixing up the acrosome reaction that allows sperm entry with the cortical reaction that blocks other sperm.

Representative question

Question 1

[Maximum number: 2]

Explain the mechanism that prevents polyspermy during fertilization.

Retrieve the HL Reproduction Route

HL only

HL D3.1 is about ordered mechanisms and evidence judgment: endocrine control, gametogenesis contrast, one-sperm fertilization, early embryo stages, hCG detection, placental exchange, birth feedback, and HRT evaluation.

  • GnRH drives pituitary FSH/LH and sex-hormone production
  • spermatogenesis gives four sperm; oogenesis gives one ovum and polar bodies
  • acrosome entry, cortical block, cleavage, blastocyst, endometrium
  • hCG, placenta, childbirth feedback, and HRT risk-benefit evaluation

HL Human Reproduction

HL only

HL D3.1 moves from reproductive events into control and evidence. The strongest answers keep sequences in order: endocrine axis at puberty, gametogenesis outcomes, fertilization blocks, blastocyst implantation, hCG testing, placental exchange, childbirth feedback, and HRT evaluation.

  • Explain HL mechanisms in order, especially hormone pathways and early-development stages.
  • Compare gametogenesis and fertilization mechanisms using precise structural terms.
  • Evaluate HRT by weighing symptom benefits against risks and evidence quality.

Concept essentials

  • The acrosome reaction helps one sperm penetrate the zona pellucida.
  • The cortical reaction follows sperm entry and blocks additional sperm.
  • The zona pellucida is altered so further sperm cannot bind or enter.