In humans, sexual reproduction involves the fusion of a male gamete (sperm) with a female gamete (egg cell, or ovum) to form a zygote. The reproductive systems are adapted for producing gametes, fertilisation and development of the fetus.
Male and female systems
The testes produce sperm and the hormone testosterone. Sperm are small, have a tail (flagellum) for swimming and many mitochondria for energy. The ovaries release egg cells and produce oestrogen and progesterone. The egg cell is much larger and contains a food store (cytoplasm).
Key idea
Fertilisation is the fusion of the nuclei of a sperm and an egg cell to form a diploid zygote. It normally occurs in the oviduct (fallopian tube).
Fertilisation to birth
After fertilisation the zygote divides by mitosis to form an embryo, which implants in the wall of the uterus. The placenta allows exchange of substances between the mother's blood and the fetus's blood without the two mixing: oxygen, glucose and amino acids pass to the fetus, while carbon dioxide and urea pass to the mother. The umbilical cord connects the fetus to the placenta, and the amniotic fluid protects the fetus.
Worked example
Why does a fetus rely on the placenta for oxygen? Its lungs are not yet functioning and it is surrounded by amniotic fluid. Oxygen diffuses from the mother's blood, across the placenta, into the fetal blood, moving down a concentration gradient.
Remember
- Sperm: small, motile, many mitochondria.
- Egg: large, non-motile, food store.
- Placenta exchanges materials; blood does not mix.