Sexual reproduction in humans
Gametes, fertilisation, pregnancy and the role of hormones.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 16.3.1Describe fertilisation, the development of the fetus and the function of the placenta.
- 16.3.2Describe the menstrual cycle and the roles of oestrogen and progesterone.Supplement
6 minute read
Sexual reproduction in humans
The gametes
- Sperm — small, motile, with a tail for swimming and many mitochondria for energy. Produced in vast numbers.
- Egg (ovum) — much larger, non-motile, containing food stores (yolk) for the early embryo.
The size difference is the point: the egg supplies resources, the sperm supplies mobility.
Fertilisation
Fertilisation is the fusion of the sperm nucleus with the egg nucleus, forming a zygote. It occurs in the oviduct (fallopian tube), not the uterus. The zygote divides by mitosis to form an embryo, which travels to the uterus and implants in the thickened uterus lining.
The placenta
The organ of exchange between mother and fetus. Their blood does not mix; instead materials diffuse across a thin barrier with a very large surface area.
- To the fetus: oxygen, glucose, amino acids, antibodies.
- From the fetus: carbon dioxide and urea.
Why blood must not mix: the mother's blood pressure would damage fetal vessels, blood groups may differ, and it limits pathogen transfer. The umbilical cord connects fetus to placenta, and the amniotic fluid cushions the fetus against physical damage.
The menstrual cycle
Roughly 28 days, controlled by hormones:
- Oestrogen — from the ovaries, repairs and thickens the uterus lining, and triggers ovulation.
- Progesterone — maintains the uterus lining. When it falls, the lining breaks down and menstruation begins.
- Ovulation — release of an egg, around day 14.
The pattern to remember: oestrogen builds, progesterone maintains, and withdrawal of progesterone triggers the period.
Think of it like this
The placenta is a service interface, not a merger: two separate blood supplies run close enough to trade goods across a membrane, but never join — much like passing parcels through a hatch rather than opening the door.
Worked examples
Method, step by step
Explain how the placenta allows the fetus to receive oxygen without the two blood supplies mixing.
- 1The placenta has villi giving a very large surface area for exchange.
- 2The barrier between maternal and fetal blood is very thin, giving a short diffusion distance.
- 3Oxygen concentration is higher in the mother's blood, so oxygen diffuses down its concentration gradient into fetal blood.
- 4The two circulations remain separate throughout, so only dissolved substances cross.
Oxygen diffuses across a thin, large-surface-area barrier from the mother's blood, where its concentration is higher, into the fetal blood — the two blood supplies stay separate.
Common misconceptions
- Thinking fertilisation happens in the uterus. It occurs in the **oviduct**; implantation happens in the uterus.
- Believing maternal and fetal blood mix in the placenta. They stay separate and exchange by diffusion.
- Confusing the hormone roles — oestrogen builds the lining, progesterone maintains it.
In the exam
- For placenta questions, always state "large surface area and thin barrier for diffusion" plus "blood does not mix".
- For hormone questions, link the falling level of progesterone to the start of menstruation — that causal step is a mark.