Edexcel A Level (IAL) Biology -3.11-3.12 Fertilization: Mammals- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -3.11-3.12 Fertilization: Mammals- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -3.11-3.12 Fertilization: Mammals- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 3.11 understand how mammalian gametes are specialised for their functions (including the acrosome in sperm and the zona pellucida in the egg cell)
- 3.12 know the process of fertilisation in mammals, including the acrosome reaction, the cortical reaction and the fusion of nuclei
Specialisation of Mammalian Gametes (Sperm & Egg Cells)
🌱 Introduction
Mammalian gametes sperm (male) and egg/ovum (female) – are highly specialised cells designed for sexual reproduction. Each has unique structures and adaptations to meet, fuse, and form a zygote.
1. Sperm Cell (Male Gamete)
Function: To deliver haploid male nucleus (DNA) to the egg for fertilisation.
✴️ Key Structural Adaptations & Functions
| Structure | Description | Function |
|---|---|---|
| Head | Contains nucleus with haploid chromosomes (23) | Carries male genetic material |
| Acrosome | Cap-like vesicle at tip, from Golgi apparatus | Contains enzymes to penetrate egg’s zona pellucida |
| Midpiece | Packed with mitochondria | Provides ATP for tail movement |
| Tail / Flagellum | Long whip-like structure | Enables swimming toward egg |
| Small cytoplasm | Minimal cell contents | Light, streamlined for faster movement |
📘 Summary: Sperm is built for speed, energy efficiency, and egg penetration.
2. Egg Cell / Ovum (Female Gamete)
Function: To provide half the genetic material, nutrients, and a protective environment for early embryo development.
✴️ Key Structural Adaptations & Functions
| Structure | Description | Function |
|---|---|---|
| Nucleus | Large haploid nucleus (23 chromosomes) | Carries female genetic information |
| Cytoplasm | Rich in proteins, lipids, organelles | Provides nutrients for zygote |
| Zona Pellucida | Thick glycoprotein layer around plasma membrane | Protective barrier; ensures species-specific fertilisation |
| Corona Radiata | Layer of follicle cells surrounding zona pellucida | Supplies nutrients and support before/after ovulation |
| Cell membrane receptors | Bind specifically to proteins on sperm head | Allows fusion of only one sperm → prevents polyspermy |
📘 Summary: Egg is large, nutrient-rich, and selective – designed to support and protect the zygote.
3. Interaction During Fertilisation
- Sperm binds to receptors on zona pellucida.
- Acrosome reaction → enzymes digest zona pellucida.
- Sperm fuses with egg membrane → nuclei combine to form diploid zygote (46 chromosomes).
- Zona pellucida hardens → prevents polyspermy.
📊 Summary Table
| Gamete | Key Feature | Function |
|---|---|---|
| Sperm | Acrosome | Contains enzymes to digest egg membrane |
| Mitochondria | Provide energy for movement | |
| Flagellum | Enables swimming toward egg | |
| Egg (Ovum) | Zona Pellucida | Protects egg, allows only one sperm entry |
| Cytoplasm | Provides nutrients for zygote | |
| Corona Radiata | Supplies nourishment & protection |
⚡ Quick Recap
Sperm → small, mobile, enzyme-rich → delivers DNA to egg.
Egg → large, nutrient-rich, protective → supports zygote formation.
Acrosome → enzyme sac that digests zona pellucida.
Zona Pellucida → protective glycoprotein coat, species-specific fertilisation.
After fertilisation → zona pellucida hardens → blocks polyspermy.
Fertilisation in Mammals
🌱 Introduction
Fertilisation is the fusion of a haploid sperm and haploid egg to form a diploid zygote. It occurs in the oviduct (fallopian tube) and involves highly specialised processes to ensure that only one sperm fertilises the egg.
1. Sperm-Egg Recognition
- Sperm binds to receptors on the zona pellucida of the egg.
- Ensures species-specific fertilisation.
- Only sperm with correct membrane proteins can bind.
2. Acrosome Reaction
- Purpose: Allows sperm to penetrate the egg.
- Acrosome (cap on sperm head) releases digestive enzymes (e.g., acrosin).
- Enzymes locally digest the zona pellucida.
- Sperm moves through zona pellucida to reach egg plasma membrane.
- Key Point: Only one sperm reaches the egg cytoplasm at a time.
3. Cortical Reaction
- Purpose: Prevents polyspermy (entry of multiple sperms).
- After sperm fuses with egg membrane, cortical granules in egg cytoplasm release enzymes.
- Enzymes modify zona pellucida → becomes impermeable to other sperms.
- Key Point: Cortical reaction → permanent block to polyspermy.
4. Fusion of Nuclei (Karyogamy)
- Sperm nucleus enters egg cytoplasm.
- Nuclear membranes dissolve → haploid sperm nucleus fuses with haploid egg nucleus.
- Diploid zygote forms (46 chromosomes in humans).
- Key Point: Restores diploid chromosome number and combines parental genetic material.
5. Summary of Steps
- Sperm binds zona pellucida → species-specific recognition.
- Acrosome reaction → enzymes digest zona → sperm reaches egg membrane.
- Cortical reaction → zona hardens → prevents polyspermy.
- Fusion of nuclei → haploid sperm + haploid egg → diploid zygote.
📊 Summary Table
| Step | Location / Structure | Function |
|---|---|---|
| Sperm-egg recognition | Zona pellucida | Ensures species-specific binding |
| Acrosome reaction | Sperm head | Releases enzymes to digest zona pellucida |
| Cortical reaction | Egg cytoplasm (cortical granules) | Prevents entry of additional sperms (polyspermy) |
| Fusion of nuclei | Egg cytoplasm | Combines maternal & paternal DNA → diploid zygote |
⚡ Quick Recap
Fertilisation occurs in oviduct → diploid zygote forms.
Acrosome reaction → sperm digests zona pellucida.
Cortical reaction → prevents polyspermy.
Nuclear fusion (karyogamy) → combines maternal + paternal DNA.
Result → genetically unique zygote, ready for development.