Reproduction, but bitesize.

Part 1It takes two

To make a new human, you need two parents — one male and one female. This is called sexual reproduction. Each parent makes a special sex cell, and these two cells join together to start a new life.

Those special sex cells have a name: gametes. The male gamete is the sperm. The female gamete is the ovum (also called the egg). When a sperm joins with an egg, that is fertilisation.

Because the new individual gets some instructions from the mother and some from the father, it is never an exact copy of either parent. That mixing is why brothers and sisters look different from each other — it produces variation.

Part 2What happens at fertilisation

Every ordinary body cell has a full set of genetic information (the instructions for building you), carried inside its nucleus. Gametes are different: each gamete carries only half the genetic information.

That is the clever part. When the sperm meets the egg, the nucleus of the sperm joins with the nucleus of the egg. Half plus half makes a full set again. The single cell this creates is called a zygote, and it has a complete set of instructions — half from the mother, half from the father.

Part 1The male reproductive system

The job of the male system is to make sperm and deliver them to the female. Here are the parts you need to know:

Part 2The female reproductive system

The female system makes eggs, and provides a place for a baby to develop. The parts you need to know:

Part 1The sperm — built to travel

A sperm cell has one job: reach the egg. Everything about it is built for that journey, and to break in once it arrives.

It has a long tail (flagellum) that whips from side to side so it can swim. Packed into the middle section are lots of mitochondria, which release the energy the tail needs for that long swim. The head is streamlined to slip through fluid easily. At the very tip is the acrosome, a store of enzymes that digest a way through the egg's outer coat. And sperm are made in huge numbers — millions at a time — because only one needs to succeed.

Part 2The egg — built to nourish

The egg has the opposite job. It doesn't move; it waits, and it carries supplies for the new life that begins inside it.

The egg is a large cell with a food store (yolk) to feed the embryo in its first few days, before it gets nutrients from the mother. It is surrounded by a jelly coat that changes straight after fertilisation so that no other sperm can get in. Eggs are made in small numbers compared with sperm.

Part 1From fertilisation to implantation

Fertilisation happens in the oviduct. The single cell (the zygote) then divides again and again as it travels down towards the uterus, becoming a tiny ball of cells called an embryo.

When the embryo reaches the uterus, it sinks into the soft, thick uterus lining. This is called implantation. Once it has implanted, the embryo can develop and grow. After about eight weeks, when its main organs have formed, it is called a foetus.

Part 2How the foetus is fed and protected

A growing foetus needs oxygen and nutrients, and it needs to get rid of waste — but it can't eat or breathe yet. The placenta solves this. It is an organ that grows in the uterus wall, where the mother's blood and the foetus's blood flow very close together.

At the placenta, oxygen and nutrients pass from the mother's blood to the foetus, and waste (like carbon dioxide) passes from the foetus back to the mother, who removes it. The foetus is joined to the placenta by the umbilical cord, which carries these substances to and from it.

The foetus floats in a bag of amniotic fluid, which cushions it and protects it from bumps. All this takes time: a full pregnancy (the gestation period) lasts about nine months.

Part 1A monthly cycle

Each month the female body gets ready in case an egg is fertilised. This repeating sequence is the menstrual cycle, and it takes about 28 days (it varies from person to person).

The cycle is usually counted from the first day of a period. Roughly, it goes like this:

Days 1–5 — menstruation. The thick lining of the uterus breaks down and leaves the body through the vagina. This is the "period".
Days 5–14 — the lining builds up. The uterus lining thickens again, getting ready in case a fertilised egg arrives.
Day 14 — ovulation. An egg is released from an ovary into the oviduct.
Days 14–28 — ready and waiting. The lining stays thick, ready for a fertilised egg to implant.

Part 2What if there's no fertilisation?

After ovulation, the egg only survives for a day or two. If sperm are present and one fertilises the egg, the early embryo can implant in the thick lining, and the cycle pauses — pregnancy has begun.

If the egg is not fertilised, it breaks down. The thick uterus lining is no longer needed, so it breaks down too and leaves the body. That breakdown is the next period — and the cycle starts again from day 1.

Part 1The parts of a flower

Flowers are the reproductive organs of a plant. Most flowers have both male and female parts.

The male part is the stamen, which is made of the anther (where pollen is made) on a stalk called the filament. The female part is the carpel, made of the stigma (the sticky top that catches pollen), the style (the stalk below it), and the ovary (which holds the ovules — the female sex cells). Around these are the petals (often brightly coloured to attract insects) and the sepals (which protected the flower while it was a bud).

Part 2Pollination and fertilisation

Pollination is the transfer of pollen from an anther to a stigma. There are two main ways it happens:

Insect pollination. The flower has large, brightly coloured, scented petals and sticky pollen and nectar to attract insects. Pollen sticks to the insect and is carried to the next flower.
Wind pollination. The flower has small, dull petals (no need to attract insects), anthers that dangle outside to release pollen into the air, large feathery stigmas to catch it, and lots of light pollen.

Once pollen lands on a stigma, a tube grows down the style to the ovary, and a pollen nucleus joins with an ovule nucleus. That joining is fertilisation. After fertilisation, each fertilised ovule becomes a seed, and the ovary around them grows into a fruit.

Part 3Seed dispersal

If seeds all dropped straight below the parent plant, they would crowd each other and compete for light, water and space. So plants spread their seeds away from the parent — this is seed dispersal. There are four main methods:

Wind — light seeds with wings or parachutes (sycamore, dandelion) are carried on the breeze.
Animal — juicy fruits are eaten and the seeds pass out elsewhere, or hooked seeds (burdock) catch on fur.
Water — seeds that float (coconut) are carried away by rivers or the sea.
Explosion — some seed pods (pea, gorse) dry out and burst, flinging seeds away.