Gametes & Fertilisation (Edexcel IGCSE Biology)

The Gametes

• Gametes are sex cells produced by meiosis

  • The sperm and ovum in animals

  • The pollen nucleus and ovum in plants

• They contain half the number of chromosomes (23 in human gametes) compared to normal body cells

• Gametes have adaptations to improve the chances of successful fertilisation and embryo development, for example:
  

  • sperm cells have a tail to propel them towards the egg and mitochondria to provide energy for this movement

  • egg cells have energy stores within the cytoplasm to support early embryo development

Human gametes diagram

Comparing sperm and egg cells

Fertilisation

• Fertilisation can be described as:

the fusion of a male and female gamete to produce a zygote

• The zygote then divides by mitosis to develop into an embryo

• Cells start to become specialised to perform specific functions, forming all the body tissues of the offspring

Fertilisation in humans

• During sexual intercourse, semen is ejaculated into the female's vagina near the cervix, and sperm travel through the cervix into the uterus.

• Fertilisation occurs in the oviduct if a sperm meets an egg, typically 1-2 days after ovulation

• A human zygote contains the full 46 chromosomes (23 pairs of chromosomes)

  • half of which came from the father and half from the mother

The process of fertilisation in humans

Fertilisation in plants

• In plants, fertilisation occurs when a pollen tube grows down from a pollen grain to deliver the male nucleus into the ovary

• Here the male and female gametes fuse to form the embryo

• More detailed notes on this process can be found here

The process of fertilisation in plants.