1.2 - Specialised cells
Explain how sperm and egg cells are adapted for fertilisation and how ciliated epithelial cells clear the airways. Connect each feature to the biological job it makes possible.
1.2 — Cells specialised for fertilisation
A specialised cell has features that make it effective at a particular function. Sperm and egg cells are gametes: their shared function is to bring genetic material together at fertilisation, but their structures prepare them for different parts of that process.
Sperm cell
| Feature | Consequence | Contribution to fertilisation |
|---|---|---|
| Acrosome | This cap in the head contains enzymes that help digest a route through the layers around the egg. | The sperm can reach and fuse with the egg cell membrane. |
| Haploid nucleus | It contains one set of chromosomes. | It contributes one set when the sperm and egg nuclei come together. |
| Many mitochondria | Aerobic respiration releases energy close to the tail. | Energy is available for tail movement. |
| Tail | It beats to propel the sperm through fluid. | The sperm can move towards the egg. |
The mitochondria and tail therefore form one linked adaptation: respiration releases energy, the tail uses that energy to move, and movement allows the sperm to reach the egg. The acrosome solves a different problem by helping the sperm penetrate the egg's surrounding layers.
Egg cell
| Feature | Consequence | Contribution to fertilisation or early development |
|---|---|---|
| Nutrients in the cytoplasm | Stored nutrient molecules can be used in respiration and as raw materials. | They support the earliest growth and cell divisions after fertilisation. |
| Haploid nucleus | It contains one set of chromosomes. | It contributes the second set, so the fertilised cell has two sets of chromosomes. |
| Cell membrane changes after fertilisation | Once one sperm has fused with the egg, the changed membrane prevents further sperm from entering. | Only one sperm contributes a nucleus, avoiding extra sets of chromosomes. |
Put together, the sequence is: a moving sperm reaches the egg, acrosomal enzymes help it penetrate the surrounding layers, the cell membranes fuse, and the two haploid nuclear contributions form a fertilised cell with two sets of chromosomes. Changes triggered as fertilisation begins block additional sperm, while nutrients in the egg cytoplasm support the first stages of development.
1.2 — Cilia clear the airways
Epithelial cells form sheets that line surfaces. Ciliated epithelial cells line parts of the airways, including the trachea and bronchi, and have many tiny hair-like cilia on the surface facing the airway.
Mucus over this surface traps dust and microorganisms from inhaled air. The cilia beat together in a coordinated direction, moving the mucus towards the throat so it can be removed from the airways. The structural adaptation is therefore many beating cilia -> directed mucus movement -> removal of trapped material, which helps protect the lungs.
Ciliated epithelial cells move mucus; they do not produce it. Other cells in the airway lining secrete the mucus, while the cilia provide the movement.