IGCSE Biology / Reproduction in humans
Sperm, egg and fertilisation
Edexcel International GCSE Biology (4BI1), chapter 9: Reproduction in humans, specification point 3.1, 3.2. Free lesson.
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Sperm and egg cells are adapted for their different roles in reproduction. Fertilisation is the fusion of their haploid nuclei to form a diploid zygote.
How a sperm cell is adapted
A sperm is very small and can swim to reach the egg. Its structures help it travel and enter the egg. The head contains a haploid nucleus with 23 chromosomes. The acrosome is a sac of enzymes at the tip of the head. These enzymes digest through the egg's outer layers. The mid-piece contains many mitochondria, which release energy for swimming. The tail propels the sperm.
How an egg cell is adapted
An egg is much larger than a sperm. Its cytoplasm contains food reserves that supply the early embryo before implantation. The egg nucleus is haploid, with 23 chromosomes. The jelly coat changes chemically after one sperm enters. This stops any other sperm from getting through. Egg cells have mitochondria, but these are not for movement.
How sperm and eggs differ
The differences follow from what each cell needs to do. Sperm need to travel to the egg, while the egg supplies the early embryo with food. Sperm are very small and produced in hundreds of millions continuously. An egg is much larger, and about one is produced every 28 days. Sperm swim using their tails. An egg cannot move itself, but cilia in the oviduct move it along. Sperm have almost no food store because they only need to travel. Eggs have a large food store for the early embryo. Sperm have many mitochondria in the mid-piece to provide energy for swimming.
What happens during fertilisation
Fertilisation is the fusion of a male and a female gamete nucleus to produce a diploid zygote. It takes place in the oviduct.
Haploid and diploid chromosome numbers
Haploid means one set of chromosomes. In humans, that is 23. Diploid means two sets, or 46 chromosomes. So when the haploid nuclei fuse during fertilisation, they form a diploid zygote. Haploid gamete + haploid gamete → diploid zygote 23 chromosomes + 23 chromosomes = 46 chromosomes