Development of organisms and continuity of life
Reproduction in plants
Asexual reproduction needs one parent and produces genetically identical offspring (clones), e.g. runners, tubers, cuttings: fast, but no variation. Sexual reproduction involves the fusion of male and female gametes (fertilisation), giving variation. In flowers, pollen (male gametes) from the anther lands on the stigma (pollination). A pollen tube grows down to the ovule in the ovary, where fertilisation happens; the ovule becomes a seed and the ovary a fruit. Insect-pollinated flowers are colourful with nectar and sticky pollen; wind-pollinated flowers are small with feathery stigmas and light pollen. Germination needs water, oxygen and warmth.
≈ 40 minHuman reproduction
Sperm are made in the testes; eggs are released from the ovaries. Fertilisation happens in the oviduct; the zygote divides into an embryo that implants in the uterus lining. The placenta lets oxygen and nutrients diffuse from mother to fetus and CO₂ and urea back, without the bloods mixing; amniotic fluid cushions the fetus. The menstrual cycle (about 28 days) is controlled by hormones: oestrogen rebuilds the uterus lining, LH triggers ovulation around day 14, progesterone maintains the lining. STIs such as HIV spread through sexual contact and can be prevented with condoms.
≈ 45 min