4.7.4Additional lessons
Fertilisation — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 58 of 101
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 5Sequence of Fertilisation
1
Sperm swim through the female tract and reach the egg surrounded by a jelly-like layer called the zona pellucida.
2
On contact the acrosome reaction occurs: enzymes released from the acrosome digest a path through the zona pellucida.
3
The plasma membranes of the sperm and egg fuse, and the sperm nucleus enters the egg.
4
The cortical reaction follows: vesicles beneath the egg membrane release enzymes that harden the zona pellucida.
5
The egg completes meiosis II, and the sperm and egg nuclei fuse to produce a diploid zygote.
Section 2 of 5Preventing Polyspermy
Ensuring that exactly one sperm nucleus enters the egg is essential for a normal chromosome number.
Problem
- Problem If more than one sperm entered the egg, the zygote would have too many chromosome sets and could not develop normally.
Cortical granules
- Cortical granules Vesicles in the egg's cortex fuse with the membrane and release their contents outside the cell.
Zona pellucida change
- Zona pellucida change The released enzymes alter the zona so that it no longer binds sperm, creating a block to extra sperm.
Timing
- Timing The reaction spreads quickly from the entry point across the egg.
Section 3 of 5Gametes in Fertilisation
Sperm head
- Sperm head Contains the haploid nucleus and the acrosome, a vesicle full of digestive enzymes.
Mitochondria
- Mitochondria Packed into the sperm midpiece to supply ATP for swimming.
Egg
- Egg Contains food reserves and cortical granules, and is surrounded by the zona pellucida.
Zygote
- Zygote Has 46 chromosomes in humans, 23 from each gamete.
| Structure | Function |
|---|---|
| Acrosome | Releases enzymes to digest the zona pellucida |
| Cortical granules | Release enzymes to block other sperm |
| Zona pellucida | Protective layer where sperm bind |
Section 4 of 5Where and When
Location
- Location In humans, fertilisation takes place in the oviduct.
Window
- Window The egg can be fertilised for about a day after ovulation, although sperm may survive in the tract for several days.
Next step
- Next step The zygote starts dividing by mitosis as it moves towards the uterus.
Section 5 of 5Exam Checklist & Tips
4.7.4 – Key Points for the Exam
- Acrosome A vesicle at the tip of the sperm that contains digestive enzymes.
- Zona pellucida The glycoprotein layer surrounding a mammalian egg.
- Polyspermy Entry of more than one sperm into an egg.
- Cortical reaction Release of enzymes from the egg's vesicles that blocks other sperm.
Common Mistakes
- Saying the whole sperm enters the egg; mainly the nucleus does.
- Mixing up the acrosome and cortical reactions.
- Stating that the zygote is haploid.
- Describing the zona pellucida as part of the sperm.
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