Sexual reproduction joins genetic material from two reproductive cells, or gametes. In humans, testes produce sperm and ovaries contain eggs. Sperm are small, mobile cells adapted to reaching an egg; an egg is larger and supplies cell contents for early development. Fertilisation joins a sperm and egg to form a zygote. Human fertilisation usually occurs in an oviduct, not in the uterus.
The developing embryo divides as it travels towards the uterus. Implantation is its attachment to the uterine lining. The uterus supports development, while the placenta allows exchange of substances between maternal and developing offspring circulations without their blood normally mixing directly. Internal and external fertilisation occur in different animals; the location and protection of gametes affect reproductive success.
Example 1
Put these human events in order: implantation, fertilisation, sperm reaches egg.
Sperm reaches egg → fertilisation forms a zygote → cell divisions and movement towards the uterus → implantation in the lining.
Example 2
Explain one structure–function link in a sperm cell.
Its flagellum helps propel it through fluid. This supports movement towards an egg; it does not mean every sperm will fertilise an egg.
Watch out: Fertilisation forms a zygote by joining gametes. Implantation happens later when an embryo attaches to the uterine lining.