Fertilisation
Fertilisation is where male and female gametes fuse.
In mammals fertilisation occurs in the oviduct, and in flowering plants it occurs in the embryo sac.
- Sperm is deposited high up in the vagina, near the cervix. Once there the sperm makes its way up the cervix and uterus and into one of the two oviducts fertilisation occurs.
- The sperm swims towards the egg cell in the oviduct.
- Once the sperm contacts the zona pellucida of the egg cell, the acrosome swells and bursts, releasing digestive enzymes, these digestive enzymes digest the zona pellucida, so it can reach the cell membrane of the egg cell.
- Once the sperm head fuses with the cell membrane, this causes a cortical reaction occurs, which is when the egg cell releases cortical granules in a space between the cell membrane and the zona pellucid.
- The chemicals from the cortical granules make the zona pellucid thicken, making it impenetrable to other sperm cells.
- Once the nuclei fuse the tall is discarded.
A flowering plant:
- A pollen grain lands on the stigma of a flower. The grain absorbs water and splits open.
- A pollen tube grows out of the pollen grain down the style. There are three nuclei in the pollen tube. One tube nucleus at the tube tip and two male gamete nuclei behind it that digest surrounding cells, making a way through for the pollen tube to go.
- When the tube reaches the ovary, it grows through the micropyle, and into the embryo sac within the ovule.
- In the embryo sac, the tube nucleus disintegrates and the tip of the pollen tube bursts releasing two male nuclei
- One male nucleus fuses with the egg nucleus to make a zygote. This divides by mitosis.
- The other male nucleus fuses with two other nuclei called polar nuclei, which is at the centre of the embryo sac. This produces a ell with a large nucleus. This provides a large food store, called the endosperm.
- So a double fertilisation has taken place.