Human sperm and egg cells have contrasting adaptations that enable them to meet, fuse and form a diploid zygote. Sperm are specialized for movement and penetration, whereas eggs are specialized for sperm recognition, fusion and supporting early development.
This is Additional Higher Level (AHL) content in B2.3 Cell specialization and is limited to human gametes.
| Cell | Adaptation | Functional significance |
|---|---|---|
| Sperm | Haploid nucleus containing 23 chromosomes | Fusion with the egg nucleus restores the diploid chromosome number of 46. |
| Sperm | Long flagellum | Propels the sperm through fluid toward the egg. |
| Sperm | Midpiece containing many mitochondria | Produces ATP to supply energy for flagellar movement. |
| Sperm | Acrosome containing hydrolytic enzymes | Helps the sperm penetrate the protective layers surrounding the egg, including the zona pellucida. |
| Sperm | Streamlined shape with little cytoplasm | Reduces mass and resistance during movement. |
| Egg | Large cell with abundant cytoplasm and organelles | Supports metabolism and the first stages of development after fertilization. Human eggs do not contain a large yolk reserve. |
| Egg | Haploid nucleus containing 23 chromosomes | Combines with the sperm nucleus to produce a genetically unique diploid zygote. |
| Egg | Zona pellucida surrounding the plasma membrane | Participates in sperm binding and is modified after fertilization to help prevent polyspermy. |
| Egg | Cortical granules beneath the plasma membrane | Release their contents during the cortical reaction, altering the zona pellucida so additional sperm cannot enter. |
A common misconception is that the egg is merely passive. It actively participates in sperm recognition and membrane fusion, and its cortical reaction prevents polyspermy, which would produce an abnormal chromosome number.
In an IB response to “explain” adaptations, link each named structure directly to its function. Avoid simply listing features: write linked statements such as “many mitochondria produce ATP for flagellar movement.”
