Topic 16.4D · Core
Sperm and Egg Cells
Sperm and egg cells are built to solve completely different problems. A sperm cell's job is to travel and get in; an egg cell's job is to sit still, be found, and already have what a new organism needs to start growing. Every adaptation on this page exists to solve one of those two problems.
Sperm cell adaptations
A sperm cell has a flagellum, which propels it and enables it to swim towards an egg. It contains large numbers of mitochondria, which carry out aerobic respiration to release the energy that powers movement of the flagellum. And its head carries an acrosome, which contains enzymes that help the sperm penetrate the outer coverings of the egg during fertilisation.
Notice the pattern in how each of those three adaptations is best explained: feature, then function, then the reproductive advantage it actually delivers. “Sperm have lots of mitochondria because they need energy” is a weak answer on its own — the stronger version names exactly what the energy is used for: many mitochondria release energy by aerobic respiration, and that energy powers movement of the flagellum toward the egg.
Egg cell adaptations
An egg cell carries energy stores, providing the nutrients that support the very earliest stages of development immediately after fertilisation, before the embryo has any other way of obtaining nutrients. It’s also surrounded by a jelly coat, which changes structurally the moment fertilisation happens — a change that helps prevent any additional sperm cells from entering and fertilising the same egg.
Comparing the two gametes
| Feature | Sperm cell | Egg cell |
|---|---|---|
| Size | Very small | Much larger |
| Structure | Flagellum, many mitochondria, acrosome | Large cell with energy stores and a jelly coat |
| Motility | Motile — able to swim using its flagellum | Non-motile |
| Numbers produced | Very large numbers | Far fewer released |
That size and number difference isn’t incidental — it reflects the different strategies each gamete is built around. A sperm cell is small, cheap to produce in bulk, and disposable in huge numbers because only one needs to succeed. An egg cell is large and metabolically expensive precisely because it has to arrive already carrying the resources a new organism will need before it can feed itself in any other way.
Where this leads next
These two cells exist to meet at one precise event — covered on fertilisation, implantation and pregnancy, which follows the zygote that fertilisation produces all the way through to the structures that support a developing fetus.