2.1.10Reproduction and Inheritance
Role of the Placenta — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 2 Revision Notes & Topical Questions
5 min read 7 parts Lesson 11 of 28
Section 1 of 7Forming the placenta
1
After fertilisation in the oviduct the zygote divides repeatedly to make an embryo as it travels to the uterus.
2
The embryo implants in the soft lining of the uterus, where it continues to develop.
3
A placenta forms at the site of implantation.
4
The umbilical cord links the fetus's blood supply to the placenta.
Section 2 of 7Exchange across the placenta
| Direction | Substances |
|---|---|
| Mother to fetus | Oxygen; glucose, amino acids, vitamins and mineral ions; antibodies |
| Fetus to mother | Carbon dioxide and urea (waste) |
Section 3 of 7Why it works well
Efficient exchange surface
Large surface area
- Large surface area The lining is folded into many projections, giving a large area for substances to move across.
Thin barrier
- Thin barrier Maternal and fetal blood lie very close together, so the diffusion distance is short.
Blood stays separate
- Blood stays separate The two blood supplies come close but do not mix,
- so substances cross by diffusion and active transport and the mother's higher blood pressure cannot damage the fetus's delicate vessels.
Section 4 of 7Moving substances across
1Oxygen and carbon dioxide
- Oxygen and carbon dioxide Oxygen is at a higher concentration in the mother's blood than in the fetal blood,
- so it diffuses across the thin barrier into the fetus.
- Carbon dioxide moves the opposite way for the same reason, down its concentration gradient.
2Nutrients
- Nutrients Glucose, amino acids, vitamins and mineral ions pass from the mother to the fetus.
- The fetus uses them for respiration, growth and building new tissue.
3Wastes
- Wastes Urea made by the fetus and carbon dioxide from respiration pass into the mother's blood.
- Her kidneys and lungs then remove them from the body.
4Antibodies
- Antibodies Some of the mother's antibodies cross into the fetus, giving it temporary protection against certain diseases after birth.
Section 5 of 7A partial barrier
What it can block
- What it can block The placenta stops many harmful substances and some pathogens from reaching the fetus.
What gets through
- What gets through Not everything is stopped.
- Nicotine, alcohol and other drugs can cross, and so can virus particles.
- This is why pregnant women are advised not to smoke or drink alcohol.
Other roles
- Other roles The placenta also secretes hormones such as progesterone, which help maintain the pregnancy.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Placenta | An organ that forms in the uterus wall and allows exchange between mother and fetus. |
| Umbilical cord | The cord carrying fetal blood to and from the placenta. |
| Diffusion | Net movement of particles from high to low concentration. |
| Implantation | Embedding of the embryo in the uterus lining. |
| Urea | A nitrogen-containing waste made in the liver. |
| Pathogen | A microorganism that causes disease. |
Section 7 of 7Exam technique
1
Saying the mother's and baby's blood mix in the placenta. They stay separate; substances cross a thin barrier.
2
Describing the role without giving direction, for example writing 'oxygen is exchanged'. State oxygen goes to the fetus and carbon dioxide leaves it.
3
Claiming the placenta blocks all harmful substances. Nicotine, alcohol and viruses can pass.
4
Forgetting the adaptationslarge surface area and thin wall for efficient diffusion.
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