Every living thing must eventually make new individuals of its kind, or the species dies out. This is reproduction, and there are two main types: asexual (one parent, identical offspring) and sexual (two parents, and offspring that vary).
Asexual vs sexual reproduction
| Feature | Asexual | Sexual |
|---|---|---|
| Parents | One | Two |
| Gametes (sex cells) | None | Yes — they fuse |
| Offspring | Genetically identical (clones) | Genetically varied |
| Example | Bacteria dividing; potato tubers | Humans; flowering plants |
In sexual reproduction, two gametes (sex cells) join in fertilisation to form a zygote, which grows into a new individual. Because it mixes genes from two parents, sexual reproduction produces variation.
Reproduction in flowering plants
A flower is the plant's reproductive organ. Its main parts:
- Stamen (male) = anther (makes pollen) + filament.
- Carpel (female) = stigma (catches pollen) + style + ovary (contains ovules).
- Petals — often bright and scented in insect-pollinated flowers to attract insects.
- Pollination — pollen is transferred from an anther to a stigma, by insects or wind.
- Fertilisation — a pollen grain grows a tube down the style to an ovule; the male nucleus fuses with the female nucleus.
- The fertilised ovule becomes a seed, and the ovary becomes a fruit that helps spread the seeds.
Human reproduction and the menstrual cycle
Humans reproduce sexually. The male system makes sperm in the testes; the female system releases an egg cell (ovum) from an ovary into the oviduct, where fertilisation can occur. A fertilised egg implants in the uterus lining and develops into a baby.
The menstrual cycle is a roughly 28-day cycle in which the uterus lining is prepared for a possible pregnancy:
| Days (approx.) | What happens |
|---|---|
| 1–5 | Menstruation — the uterus lining breaks down and is lost |
| 6–13 | The lining rebuilds (repairs and thickens) |
| ~14 | Ovulation — an egg is released from an ovary |
| 15–28 | The lining is maintained; if no fertilisation, it breaks down and the cycle repeats |
Two key hormones: oestrogen helps rebuild the uterus lining and triggers ovulation, while progesterone maintains the lining. If the egg is not fertilised, hormone levels fall and menstruation begins again.