B1.2 Reproduction · KS3 Science

Sexual and asexual reproduction

Sexual reproduction needs two parents. Sex cells (gametes) join at fertilisation: a sperm and an egg in animals; pollen and an ovule in plants. The offspring vary.
Asexual reproduction needs only one parent, and the offspring are identical copies (clones). Examples: bacteria splitting, strawberry runners, potato tubers and spider plant plantlets.

Male: the testes make sperm and the hormone testosterone; sperm travel along the sperm duct, glands add fluid to make semen, and it leaves through the urethra in the penis.
Female: the ovaries contain eggs; an oviduct (fallopian tube) carries an egg to the uterus (womb), where a baby develops; the cervix is a ring of muscle at the entrance to the uterus, which leads to the vagina.
Puberty is when hormones cause the body to change. Both sexes: growth spurt, body hair, spots. Boys: voice deepens, testes start making sperm, shoulders widen. Girls: breasts develop, hips widen, periods start.

The menstrual cycle lasts about 28 days. Days 1 to 5: the uterus lining breaks down and leaves the body (a period). The lining then builds up again, and around day 14 an egg is released from an ovary (ovulation).
Fertilisation happens in the oviduct when a sperm nucleus joins with an egg nucleus. The fertilised egg divides to form an embryo, which settles into the uterus lining.
After about 8 weeks, the embryo is called a fetus. The placenta passes oxygen and nutrients from the mother's blood to the fetus and removes waste such as carbon dioxide. The umbilical cord joins the fetus to the placenta, and the fluid in the amniotic sac cushions it.
Pregnancy lasts about 9 months (40 weeks). Smoking, alcohol and drugs can harm the developing fetus.

The stamen is the male part: the anther makes pollen and the filament holds it up. The carpel is the female part: the stigma is sticky to catch pollen, the style joins it to the ovary, and the ovary contains ovules.
Pollination is the transfer of pollen from an anther to a stigma. Insect-pollinated flowers have bright petals, scent, nectar and sticky pollen. Wind-pollinated flowers have small dull petals, lots of light pollen, and anthers and feathery stigmas that hang outside the flower.
Fertilisation: a pollen tube grows down the style to an ovule, and the nuclei join. The ovule becomes a seed and the ovary becomes the fruit.
Seed dispersal spreads seeds away from the parent to reduce competition for light, water and nutrients: by wind (sycamore, dandelion), animals (berries that are eaten, burrs with hooks), explosion (pea pods) or water (coconut).

Gamete: A sex cell, such as a sperm, an egg or a pollen grain.
Fertilisation: The joining of the nuclei of a male and a female sex cell.
Asexual reproduction: Reproduction with one parent, making identical offspring.
Ovary: Where eggs are stored in humans (or ovules are made in plants).
Testes: Where sperm are made.
Uterus: The womb, where a baby develops.
Ovulation: The release of an egg from an ovary.
Menstruation: The breakdown of the uterus lining, known as a period.
Placenta: Passes oxygen and nutrients to a fetus and removes its waste.
Pollination: The transfer of pollen from an anther to a stigma.
Anther: The part of a flower that makes pollen.
Stigma: The sticky part of a flower that catches pollen.
Seed dispersal: Spreading seeds away from the parent plant.