3.2.3Unit 3: Practical Skills in Biology I
Mammalian Gametes — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 17 of 27 Unit quiz
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.
Section 1 of 4Gametes
Mammalian gametes have different adaptations that increase the chance of fertilisation.
Haploid cells
- Haploid cells Gametes are formed by meiosis and have one copy of each chromosome, so human gametes have 23 chromosomes.
Fertilisation
- Fertilisation Fusion of the nuclei of a male gamete and a female gamete forms a zygote.
Zygote
- Zygote The fertilised egg has 46 chromosomes, 23 from each parent, and divides by mitosis to form an embryo.
Sperm flagellum
- Sperm flagellum The tail moves in a whip-like way so the sperm can swim towards the egg cell.
Mitochondria in sperm
- Mitochondria in sperm Many mitochondria in the mid-piece supply the ATP that powers the movement of the flagellum.
Acrosome
- Acrosome A lysosome-like vesicle in the sperm head containing digestive enzymes which break down the layer around the egg.
Section 2 of 4Egg Cell Adaptations
Egg Cell Adaptations
Large size
- Large size The egg is much larger than the sperm because it contains a food store to nourish the early embryo.
Zona pellucida
- Zona pellucida A jelly-like glycoprotein layer around the egg that sperm must digest a path through to reach the membrane.
Follicle cells
- Follicle cells A protective coat of cells surrounds the egg and also helps to protect it before fertilisation.
Cortical granules
- Cortical granules Vesicles beneath the egg membrane that release their contents after a sperm enters, preventing further sperm from entering.
| Feature | Sperm | Egg |
|---|---|---|
| Size | Small | Much larger |
| Movement | Swims using flagellum | Does not swim |
| Food store | Little | Large |
| Chromosomes | 23 | 23 |
Section 3 of 4Fertilisation Steps
1
Sperm follow a chemical trail from the egg and swim through the cervix and uterus into the oviduct.
2
The sperm head reaches the zona pellucida and undergoes the acrosome reaction, releasing enzymes that digest a path through the layer.
3
The sperm membrane fuses with the egg cell membrane, and the sperm nucleus enters the egg.
4
The egg releases the contents of its cortical granules, which causes the zona pellucida to thicken and harden, so no more sperm can enter (the cortical reaction).
5
The haploid nuclei of the sperm and egg fuse to form a diploid zygote nucleus with 46 chromosomes.
Section 4 of 4Exam Checklist & Tips
3.2.3 – Key Points for the Exam
- Gamete A haploid sex cell, such as a sperm or egg.
- Acrosome reaction Release of digestive enzymes from the sperm head to break down the zona pellucida.
- Cortical reaction Release of cortical granule contents that hardens the zona pellucida after a sperm has entered.
- Zygote The diploid cell formed when the nuclei of two gametes fuse.
- Zona pellucida The glycoprotein layer surrounding a mammalian egg.
Common Mistakes
- Saying that the sperm contains no mitochondria, when its many mitochondria power the flagellum.
- Saying that the cortical reaction helps the first sperm enter, when it prevents extra sperm from entering.
- Stating that gametes are diploid, when they are haploid.
- Confusing the acrosome reaction in the sperm with the cortical reaction in the egg.
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