Edexcel International A Level Biology

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3.11 Mammalian Gametes

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Gamete Specialisation


  • Gametes are the sex cells of an organism
    • For example, the sperm and egg (ovum) cells in humans
  • Gametes fuse during fertilisation to form a zygote
    • Fertilisation is the fusion of the nuclei from a male gamete (sperm cell) and a female gamete (egg cell)
  • These sex cells are formed during meiosis and only have one copy of each chromosome, so they are haploid cells
    • For humans, that means the sperm and egg cells contain 23 single chromosomes in their nucleus

Sexual reproduction in humans

Sexual reproduction involves the fusing of two gametes to form a zygote that contains DNA from both parents

Mammalian gametes are specialised for their functions

  • Mammalian gametes have adaptations to increase the chances of fertilisation and successful development of an embryo
  • Sperm cells:
    • Have a flagellum (tail) that allows them to swim towards the egg cell
    • Contain many mitochondria that provide energy for movement of the flagellum (swimming)
    • An acrosome that contains digestive enzymes to break down the protective glycoprotein layer (a jelly-like coating known as the zona pellucida) surrounding the egg cell - sperm cells must penetrate this layer in order to fertilise the egg
  • Egg cells:
    • Are much larger than sperm cells as most of their internal space contains food to nourish a growing embryo
    • Have follicle cells that form a protective coating
    • Have a jelly-like glycoprotein layer, known as the zona pellucida, that forms an impenetrable barrier after fertilisation by a sperm cell has occurred, to prevent other sperm nuclei from entering the egg
  • These features are summarised in the diagrams and tables below 

Sperm Cell

Structure of a mammalian sperm cell

Egg Cell

Structure of a mammalian egg cell

Adaptations of Mammalian Gametes Table

Adaptations of Gametes Explained table

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Author: Alistair

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.