Gametes, fertilisation and pregnancy
Sperm and egg cells
- Fertilisation is the fusion of a sperm nucleus and an egg-cell nucleus.
- Each gamete is adapted for its job:
| Sperm | Egg cell | |
|---|---|---|
| size / number | tiny, millions | large, few |
| moves? | yes — swims with a flagellum | no — carried along |
| extras | many mitochondria for energy; acrosome enzymes to enter the egg | food store; a jelly coat that blocks other sperm after fertilisation |
Practice
Which describes a sperm cell?
Sperm are tiny and numerous and swim using a flagellum; the egg is large, few, and stores food.
Pregnancy and the fetus
- The zygote divides into an embryo that implants in the uterus lining, then grows into a fetus.
- placenta — exchanges nutrients, gases and wastes between mother and fetus (the bloods never mix).
- umbilical cord — joins the fetus to the placenta.
- amniotic sac — holds amniotic fluid that cushions the fetus.
- (Supplement) Some pathogens and toxins can cross the placenta and harm the fetus.
Practice
The placenta:
The placenta is where materials are exchanged between the mother's and the fetus's blood (which never mix).
Practice
Put these stages of development in order.
The zygote divides into an embryo, which implants and grows into a fetus.
You've got it
Key idea
- sperm = small, many, swims (flagellum, acrosome); egg = large, few, food store + jelly coat
- zygote → embryo (implants) → fetus
- placenta exchanges materials (bloods don't mix); cord connects; amniotic sac/fluid cushions