How cells become specialised

1. Every cell in a multicellular organism carries the same genes, because they all came from one fertilised egg.
2. A cell becomes specialised by differentiation, switching on only the genes it needs and switching the rest off.
   1. Different genes being used produces different proteins, and different proteins give the cell a different shape and different contents.
3. An adaptation is a feature that helps a cell carry out its particular job well.
4. Because structure follows function, you can usually work out what an unfamiliar cell does from its shape and contents alone.

Sperm cells

1. A sperm cell is the male gamete, and its job is to reach an egg cell and fuse with it.
2. The acrosome at the tip stores digestive enzymes.
   1. The enzymes break down the outer layers of the egg cell so the sperm can push through and reach the egg membrane.
3. The haploid nucleus holds one set of chromosomes, which is $23$ in a human rather than the usual $46$.
   1. Fusing a haploid sperm nucleus with a haploid egg nucleus restores the full number of chromosomes in the fertilised egg.
4. Many mitochondria are packed into the middle section of the sperm.
   1. They release energy by aerobic respiration, and that energy drives the tail.
5. The tail beats from side to side so the sperm can swim towards the egg.

Egg cells

1. An egg cell is the female gamete, and its job is to be fertilised and then supply the early embryo with what it needs.
2. The cytoplasm is large and packed with nutrients.
   1. These nutrients feed the embryo while it divides and travels down the oviduct, before it implants and gains a blood supply.
3. The haploid nucleus holds one set of chromosomes, matching the single set brought by the sperm.
4. As soon as one sperm has entered, the cell membrane changes so that it becomes impossible for any other sperm to get in.
   1. This keeps the chromosome number correct, because a second sperm would give the zygote too many chromosomes and it would not develop.

Ciliated epithelial cells

1. Ciliated epithelial cells line the trachea and bronchi, and also the oviducts.
2. The surface facing into the tube is covered with cilia, tiny hair-like projections that beat together in a wave.
3. In the airways, goblet cells release mucus that traps dust, pollen and bacteria in the air you breathe in.
   1. The cilia then sweep that mucus up towards the throat, where it is swallowed, so the trapped microorganisms never reach the lungs.
4. In the oviduct the same beating action moves an egg cell along towards the uterus.
5. The cells contain many mitochondria, because beating cilia continuously needs a steady supply of energy from respiration.