KS3SCI4 · Reproduction | KS3 Science

Revise the key ideas

Human reproduction

Gametes — Sexual reproduction involves gametes, or sex cells. Sperm are produced in the testes and eggs mature in the ovaries. Fertilisation joins the nuclei of a sperm and egg, forming a fertilised egg with genetic information from both parents.
Gametes carry information — Each gamete contributes genetic information. Fertilisation combines the two contributions in one cell, which then divides during development. Fertilisation is not simply the arrival of sperm near an egg.
Sperm structure — A sperm has a tail for movement and many mitochondria that support the energy demands of movement. Its nucleus carries genetic information. Structure supports reaching and fertilising an egg.
Male system — Testes produce sperm; sperm ducts carry them; glands add fluid to form semen. The urethra carries semen through the penis during ejaculation. It also carries urine at different times. Reproductive structures have distinct functions.
Sperm and semen — Sperm are cells made in the testes. Semen contains sperm plus fluids from glands; the two words do not name the same thing.
The male reproductive route — Sperm pass from the testes through sperm ducts. Fluids are added and semen leaves through the urethra in the penis. The urethra serves urinary and reproductive functions at different times.
Female system — Ovaries release eggs into oviducts, where fertilisation usually occurs. Implantation is when the early developing embryo attaches to and embeds in the lining of the uterus. The uterus has a lining suitable for implantation and development; the cervix connects it to the vagina. The vagina receives sperm and is the birth canal.
Female reproductive system. Original schematic; see the explanation for its limits. Full-size diagram
Egg and embryo route — An ovary releases an egg into an oviduct. Fertilisation usually occurs there; the developing cell group travels towards the uterus, where implantation may occur. The uterus is not the usual fertilisation site.
Locations in human reproduction. Original schematic; see the explanation for its limits. Full-size diagram
Uterus and cervix — The uterus has muscular walls and a lining that can support implantation. The cervix is its opening into the vagina; it is not another name for the whole uterus.
Menstrual cycle — The uterine lining builds up; an egg is released at ovulation. If pregnancy does not begin, the lining breaks down and leaves through the vagina during menstruation. Cycles vary between people and over time; a 28-day model is illustrative, not a reliable fertility prediction.
Ovulation and menstruation — Ovulation releases an egg from an ovary. Menstruation sheds uterine lining when pregnancy has not begun. They are different events in a cycle and do not normally happen simultaneously.
Cycle models have limits — A cycle diagram shows a typical sequence, not a timetable that applies to everyone. Timing and duration vary; the simplified model should not be used as a dependable contraception method.
Gestation — After implantation an embryo develops in the uterus and later is called a foetus. Human pregnancy lasts about nine months, with variation. During birth, uterine muscle contractions help deliver the baby through the cervix and vagina.
Development after fertilisation — The fertilised egg divides into more cells. After implantation, cells grow and become specialised, forming tissues and organs. Development involves increasing organisation as well as increasing size.
Placenta — The placenta allows exchange between the mother’s (maternal) and developing foetus’s (foetal) blood supplies without their normal direct mixing. Oxygen and nutrients pass to the foetus; carbon dioxide and other wastes pass away. Alcohol, nicotine-related exposures and some drugs can affect development, so professional pregnancy advice matters.
Exchange at the placenta — Oxygen and nutrients move towards the foetus, while carbon dioxide and other wastes move away. Maternal and foetal blood normally remain in separate circulations; materials cross an exchange barrier.
Placental exchange directions. Original schematic; see the explanation for its limits. Full-size diagram
Umbilical cord — Blood vessels in the umbilical cord connect the foetus to the placenta. The foetus obtains oxygen through this exchange system rather than breathing air through its lungs in the uterus.

Flowering plants

Flower structure — Anthers produce pollen containing male gametes; the filament supports the anther. The stigma receives pollen, the style connects to the ovary, and ovules contain female gametes. Petals may attract insects and sepals protect the developing flower.
Male flower structures — The anther produces pollen, and the filament holds it in a suitable position for transfer. Together they form a stamen. Pollen carries the male reproductive cells; it is not a seed.
Female flower structures — The stigma receives compatible pollen; the style connects it to the ovary. Ovules lie inside the ovary. These positions allow a pollen tube to grow towards an ovule.
Female structures within a flower. Original schematic; see the explanation for its limits. Full-size diagram
Pollination — Pollination transfers pollen from anther to stigma. Insect-pollinated flowers often have conspicuous petals, nectar and sticky pollen; wind-pollinated flowers often have exposed anthers and feathery stigmas. These are adaptations, not rules applying to every flower.
Insect pollination — Conspicuous petals and nectar can attract insects. Sticky pollen may attach to them and reach another flower's stigma. The insect transports pollen; it does not join the sex cells. That joining is called gamete fusion and happens during fertilisation.
Wind pollination — Exposed anthers release pollen into moving air; large or feathery stigmas can intercept it. Many wind-pollinated plants produce large quantities because much pollen never reaches a suitable stigma.
Plant fertilisation — After compatible pollen reaches the stigma, a pollen tube grows towards an ovule. A male gamete joins a female gamete. Pollination is transfer; fertilisation is gamete fusion. A fertilised ovule becomes a seed and the ovary develops into a fruit.
From flower to new plant. Original schematic; see the explanation for its limits. Full-size diagram
From pollination to a seed — Compatible pollen germinates on the stigma, forming a tube towards an ovule. A male gamete reaches and fuses with a female gamete. After fertilisation the ovule develops into a seed.
Seeds and fruit — A seed contains an embryo and stored reserves protected by a coat. A fruit develops from the flower's ovary and surrounds seeds. A dry pod is a fruit even though it is not sweet or fleshy.
Dispersal — Seeds disperse away from the parent, reducing competition. Wings or hairs aid wind dispersal; fleshy fruits may be eaten by animals; hooks attach to fur; some fruits burst. A fruit need not be sweet: it is a mature flower ovary.
Germination conditions — Many seeds need water, oxygen and a suitable temperature to germinate. Stored food supports early respiration and growth before leaves can photosynthesise effectively. Requirements vary between species.
Dispersal and competition — A seed germinating away from its parent may face less competition for light, water and minerals. Dispersal increases opportunities but does not guarantee survival in the new location.
Investigation — To compare model winged seeds, vary wing area, control mass and drop height, time the fall and repeat. Measure distances in controlled airflow if investigating travel. Models simplify real seeds; avoid claiming a short indoor test proves natural dispersal distances.
Testing a seed model — Change one feature such as wing area while controlling mass and release height. Repeat timed falls and compare averages. A classroom paper model tests selected features, not the complete behaviour of a living seed.