Human and plant reproductionIB MYP Sciences: Revision notes

The male system makes sperm and delivers them to the female.

• Testes: make sperm cells and the hormone testosterone
• Scrotum: a bag of skin holding the testes outside the body, where it is slightly cooler than body temperature, which suits sperm production
• Sperm duct: carries sperm from each testis towards the urethra
• Prostate gland and seminal vesicles: add fluid that feeds the sperm and helps them swim; sperm plus this fluid is called semen
• Urethra: tube through the penis that carries semen (and, at other times, urine)
• Penis: places semen inside the vagina

The female system makes eggs, and provides a place for a baby to grow.

• Ovaries: make egg cells and female hormones; about once a month an egg is released (ovulation)
• Oviduct (fallopian tube): carries the egg towards the uterus; fertilisation normally happens here
• Uterus (womb): a muscular organ with a soft lining where the embryo implants and the foetus grows
• Cervix: a ring of muscle at the base of the uterus
• Vagina: receives the penis during sex and is the birth canal

Fertilisation is the fusion of the nucleus of a sperm with the nucleus of an egg, forming a zygote. The zygote divides again and again to form a ball of cells, the embryo, which moves down the oviduct and sinks into the lining of the uterus. This is implantation.

The placenta grows where the embryo implants, joined to the baby by the umbilical cord. Across its thin wall, by diffusion:

• oxygen and nutrients pass from the mother's blood to the baby's blood
• carbon dioxide and urea pass from the baby to the mother

The two blood supplies do not mix. The baby is cushioned by amniotic fluid in a bag called the amnion. Pregnancy lasts about 40 weeks. Harmful substances such as nicotine, carbon monoxide and alcohol can cross the placenta.

Most flowers have both male and female parts.

• Sepals: protect the flower in bud
• Petals: often large and colourful in insect-pollinated flowers, to attract insects
• Stamen (male): the anther makes pollen, held up by the filament
• Carpel (female): the stigma catches pollen, the style joins it to the ovary, which contains ovules; each ovule holds an egg cell
• Nectary: makes sugary nectar to attract insects

Pollination is the transfer of pollen from an anther to a stigma. Pollen is carried by insects or by the wind.

• Insect-pollinated flowers: large, bright petals, scent and nectar; small amounts of sticky or spiky pollen that clings to insects
• Wind-pollinated flowers: small, dull petals, no scent or nectar; anthers hang outside the flower; large feathery stigmas to catch pollen; huge amounts of light, smooth pollen

If pollen goes to a different plant of the same species this is cross-pollination, which mixes genes and increases variety.

After pollination a pollen tube grows from the pollen grain down the style into the ovary and into an ovule. The male nucleus travels down the tube and fuses with the nucleus of the egg cell: this is fertilisation in plants. The zygote grows into an embryo plant.

Afterwards the ovule becomes the seed (containing the embryo and a food store, inside a protective coat) and the ovary becomes the fruit. Petals and other parts usually wither.

Seed dispersal is the spreading of seeds away from the parent plant. This reduces competition for light, water and minerals and lets plants colonise new places.

• Wind: light seeds with wings or parachutes, e.g. sycamore, dandelion
• Animals: hooked fruits catch on fur (e.g. burdock); fleshy fruits are eaten and the seeds pass out in the faeces
• Water: buoyant fruits float, e.g. coconut
• Explosive: pods dry and burst open, flinging seeds out, e.g. peas and gorse