4.6.4Additional lessons
Fertilisation — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 119 of 142
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.
Part 1 of 5Types of Fertilisation
External
- External Gametes meet outside the body in water, as in many fish and frogs, so large numbers of gametes must be released together.
Internal
- Internal Sperm are placed inside the female body, as in mammals, birds and reptiles, which protects gametes from drying out.
Why it matters
- Why it matters Terrestrial animals cannot release gametes into the open air, so internal fertilisation is an adaptation to life on land.
Parental care
- Parental care Internal fertilisation usually gives fewer, better-protected offspring with greater parental care.
| Feature | External | Internal |
|---|---|---|
| Gamete number | Very many | Fewer |
| Survival chance per gamete | Low | Higher |
| Typical habitat | Aquatic | Often terrestrial |
Part 2 of 5Events at the Egg
Blocking extra sperm matters because an embryo with too many chromosome sets would not develop normally.
1
Sperm swim through the female tract and arrive at the egg in the oviduct, where many surround the egg.
2
The acrosome reaction releases digestive enzymes from the sperm head, which break down the zona pellucida.
3
The membrane of one sperm fuses with the egg cell membrane, and the sperm nucleus enters the egg.
4
The cortical reaction follows: granules release their contents, which change the zona pellucida and prevent other sperm from entering.
5
The haploid nuclei fuse to form a diploid zygote, which holds a new combination of genes.
Part 3 of 5Cleavage and Implantation
Cleavage and Implantation
Cleavage
- Cleavage The zygote divides repeatedly by mitosis as it travels down the oviduct, forming a ball of cells without increasing in overall size.
Early embryo
- Early embryo After several divisions it forms a hollow ball of cells called a blastocyst.
Implantation
- Implantation The blastocyst sinks into the endometrium of the uterus about a week after fertilisation, where it gets nutrients.
Hormone support
- Hormone support The embryo secretes hCG, which keeps the corpus luteum active so progesterone stays high and menstruation does not occur.
Placenta
- Placenta Embryonic and maternal tissues later form the placenta, which exchanges nutrients, oxygen and wastes.
Part 4 of 5Why Fertilisation Matters
Restores chromosome number
- Restores chromosome number Two haploid nuclei combine, so the zygote has the diploid number characteristic of the species.
Creates variation
- Creates variation Random fusion of particular gametes gives a new genotype, adding to variation from meiosis.
Triggers development
- Triggers development Fertilisation activates the egg to begin cell division and embryonic development.
Part 5 of 5Exam Checklist & Tips
4.6.4 – Key Points for the Exam
- Fertilisation The fusion of a male and a female gamete nucleus.
- Acrosome reaction Release of enzymes from the sperm head that digest the zona pellucida.
- Cortical reaction Release of granule contents by the egg that blocks entry of more sperm.
- Blastocyst A hollow ball of cells formed after cleavage that implants in the uterus.
- Implantation Attachment of the embryo to the endometrium.
Common Mistakes
- Saying many sperm enter the egg; the cortical reaction ensures only one does.
- Writing that the zygote is haploid; it is diploid.
- Stating that implantation and fertilisation are the same event;
- implantation happens days later in the uterus.
- Describing the acrosome as a source of energy; it holds digestive enzymes.
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