4.7.12Additional lessons
The Placenta — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 66 of 101
Section 1 of 5Structure
The placenta is an organ that connects the fetus to the mother for the whole of pregnancy.
Origin
- The placenta forms from embryonic tissue, the trophoblast, together with maternal uterine tissue.
Chorionic villi
- Finger-like projections of fetal tissue containing capillaries extend into pools of maternal blood.
Umbilical cord
- Contains two umbilical arteries carrying deoxygenated blood to the placenta and one umbilical vein returning oxygenated blood.
Separate circulations
- Maternal and fetal blood normally do not mix, with substances crossing a thin barrier by diffusion and active transport.
Section 2 of 5Exchange Across the Placenta
Oxygen
- Diffuses from maternal blood to fetal blood, helped by fetal haemoglobin which has a higher oxygen affinity.
Nutrients
- Glucose, amino acids and minerals pass to the fetus by facilitated diffusion and active transport.
Waste
- Carbon dioxide and urea diffuse from the fetus into maternal blood for removal.
Antibodies
- IgG antibodies cross the placenta and give the baby passive immunity to diseases the mother has had.
| Substance | Direction | Mechanism |
|---|---|---|
| Oxygen | Mother to fetus | Diffusion |
| Glucose | Mother to fetus | Facilitated diffusion |
| Urea | Fetus to mother | Diffusion |
| IgG antibodies | Mother to fetus | Endocytosis |
Section 3 of 5Adaptations for Exchange
Large surface area
- Many villi greatly increase the area available for exchange.
Thin barrier
- Only a few cell layers separate the circulations, so diffusion distance is short.
Rich blood supply
- Continuous blood flow maintains steep concentration gradients.
Hormone production
- The placenta secretes progesterone and oestradiol, which maintain pregnancy.
Section 4 of 5Limits of the Barrier
Harmful substances
- Alcohol, nicotine, some drugs and some viruses, such as rubella and Zika, can cross to the fetus.
Pathogens
- Many bacteria and larger pathogens are kept out, but protection is not complete.
Advice
- Pregnant women are advised to avoid alcohol, smoking and certain medicines.
Section 5 of 5Exam Checklist & Tips
4.7.12 – Key Points for the Exam
- Placenta An organ of fetal and maternal tissue where exchange between mother and fetus occurs.
- Chorionic villus A projection of fetal tissue that increases the surface area for exchange.
- Umbilical cord The structure linking the fetus to the placenta.
- Passive immunity Short-term protection by antibodies received from another individual.
Common Mistakes
- Saying that maternal and fetal blood mix freely.
- Stating that the umbilical vein carries deoxygenated blood to the fetus.
- Claiming that the placenta is a complete barrier to pathogens and drugs.
- Forgetting that the placenta also secretes hormones.
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