Reproduction
Asexual and sexual reproduction, flowers and pollination, and human reproduction, pregnancy and hormones.
📌 Key points
- Asexual reproduction involves one parent and produces genetically identical offspring. Sexual reproduction involves the fusion of the nuclei of two gametes (fertilisation) to form a zygote, and produces genetically different offspring.
- Gametes are haploid (one set of chromosomes); the zygote is diploid (two sets). Variation from sexual reproduction helps a species adapt to a changing environment.
- Flower parts: sepals, petals, stamens (anther and filament) that make pollen, and carpels (stigma, style and ovary containing ovules).
- Pollination is the transfer of pollen grains from an anther to a stigma. Insect-pollinated flowers have large, brightly coloured petals, nectar and sticky pollen; wind-pollinated flowers have small or no petals, feathery stigmas and anthers hanging outside, and lots of light, smooth pollen.
- After pollination a pollen tube grows down the style to the ovule and the nuclei fuse. The ovule becomes a seed and the ovary becomes a fruit. Germination needs water, oxygen and a suitable temperature.
- Male: testes make sperm and testosterone; sperm pass along the sperm duct, gain fluid from the prostate gland and leave through the urethra in the penis. Female: ovaries release eggs into the oviducts, where fertilisation happens; the embryo develops in the uterus.
- Sperm have a flagellum to swim, many mitochondria to release energy and an acrosome containing enzymes. Eggs have energy stores and a jelly coat that changes after fertilisation.
- The embryo implants in the uterus lining. The placenta and umbilical cord pass oxygen, glucose and amino acids to the fetus and remove carbon dioxide and urea. Amniotic fluid protects the fetus.
- Menstrual cycle: FSH and LH come from the pituitary gland; oestrogen and progesterone come from the ovaries. LH causes ovulation (around day 14); oestrogen builds up the uterus lining and progesterone maintains it.
- HIV is a sexually transmitted infection that can lead to AIDS. It spreads by sexual intercourse and contact with infected blood; condoms, limiting partners and testing reduce its spread.
📖 Key terms
- Asexual reproduction
- Reproduction by one parent, giving genetically identical offspring.
- Sexual reproduction
- Fusion of the nuclei of two gametes to form a zygote; offspring are genetically different.
- Gamete
- A sex cell, such as a sperm, egg or pollen nucleus; it is haploid.
- Fertilisation
- The fusion of the nuclei of two gametes.
- Zygote
- The diploid cell formed at fertilisation.
- Pollination
- Transfer of pollen grains from an anther to a stigma.
- Implantation
- The embryo becoming attached to the lining of the uterus.
✏️ Worked example
A flower has small green petals, no nectar, large feathery stigmas and anthers that hang outside the flower. Is it insect-pollinated or wind-pollinated? Explain.
- Small green petals and no nectar: it does not need to attract insects.
- Feathery stigmas outside the flower: a large surface area to catch pollen from the air.
- Anthers hanging outside: pollen can be blown away by the wind.
- Answer: wind-pollinated.
Pollination is not fertilisation. Pollination is the transfer of pollen; fertilisation is the fusion of the male and female nuclei in the ovule.
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