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3.2.4Unit 3: Practical Skills in Biology I

Fertilisation in Mammals — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 4 parts Lesson 18 of 27 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 4Journey to the Egg

Fertilisation normally takes place in the oviduct, so the sperm must complete a long journey first.

1
Semen is deposited high in the vagina close to the cervix, and the sperm cells begin swimming using their flagella.
2
Sperm are attracted by chemicals released from the egg cell and swim up through the cervix and into the uterus.
3
They continue along the oviduct, which is where the egg cell waits after ovulation from one of the ovaries.
4
Fertilisation is most likely when sperm meet an egg within a day or two of ovulation, because the egg survives only a short time.

Part 2 of 4Gamete Fusion

Several structures and reactions make sure that only one sperm fuses with each egg.

Zona pellucida
  • Zona pellucida A protective glycoprotein layer around the egg cell that sperm must digest a path through before reaching the membrane.
Acrosome reaction
  • Acrosome reaction The acrosome at the sperm head releases digestive enzymes that break down the zona pellucida so the sperm can reach the egg membrane.
Cortical reaction
  • Cortical reaction Once one sperm enters, the egg releases the contents of cortical granules, which harden the zona pellucida and block other sperm.
Nuclear fusion
  • Nuclear fusion The haploid sperm nucleus enters the egg and fuses with the haploid egg nucleus, which is the actual moment of fertilisation.
Polyspermy block
  • Polyspermy block Preventing a second sperm entering protects the zygote from having an abnormal extra set of chromosomes.

Part 3 of 4After Fertilisation

The fertilised egg is the starting point of a new individual.

After Fertilisation
Zygote
  • Zygote The cell formed when the two nuclei fuse;
    • it is diploid and contains 46 chromosomes, 23 from each parent in humans.
Early division
  • Early division The zygote divides by mitosis into two cells and then keeps dividing to form a ball of cells and later an embryo.
Differentiation
  • Differentiation As division continues, many of the new cells become specialised and form all the tissues of the offspring.
Genetic origin
  • Genetic origin Half of the zygote genes come from the father and half from the mother, giving a new genetic combination.

Part 4 of 4Exam Checklist & Tips

3.2.4 – Key Points for the Exam

  • Fertilisation The fusion of the nucleus of a male gamete with the nucleus of a female gamete.
  • Acrosome reaction Release of enzymes from the sperm head that digest the zona pellucida.
  • Cortical reaction Release of cortical granule contents that harden the zona pellucida and stop further sperm entering.
  • Zygote The diploid cell formed when the gamete nuclei fuse.
  • Zona pellucida The glycoprotein coat surrounding the mammalian egg cell.

Common Mistakes

  • Saying that many sperm fuse with the egg;
    • the cortical reaction ensures only one sperm nucleus fuses with the egg nucleus.
  • Describing the acrosome reaction as the sperm and egg fusing, when it is the release of digestive enzymes that open a path.
  • Stating that the zygote is haploid; it is diploid because two haploid nuclei have fused.
  • Mixing up ovulation (release of the egg from the ovary) with fertilisation (fusion of the gamete nuclei).

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