ReproductionEdexcel IGCSE Biology: Revision notes
Section 1
What is the difference between sexual and asexual reproduction?
Sexual reproduction involves the fusion of a male gamete and a female gamete (fertilisation) to produce a zygote, which divides and develops into an embryo. It produces offspring genetically different from either parent.
Asexual reproduction involves only one parent, producing offspring genetically identical to the parent (no fertilisation, no fusion of gametes). Plants can reproduce asexually by:
- Natural methods — such as runners (horizontal stems that grow along the ground and produce new plantlets)
- Artificial methods — such as taking cuttings (a piece of stem is cut and grown to produce a new, genetically identical plant)
Section 2
How are flowers adapted for pollination, and how does fertilisation lead to seed formation?
Flowers are adapted for pollination in two main ways:
| Feature | Insect-pollinated | Wind-pollinated |
|---|---|---|
| Petals | Large, brightly coloured, often scented | Small, dull, no scent |
| Nectar | Present, to attract insects | Absent |
| Pollen | Small amount, sticky/spiky to attach to insects | Large amount, light and smooth, easily carried by wind |
| Anthers | Inside the flower, firmly fixed | Hang outside the flower, loosely attached |
| Stigma | Inside the flower, sticky | Feathery, hangs outside to catch pollen |
After pollination, a pollen grain lands on the stigma and grows a pollen tube down through the style to the ovary. The male gamete travels down the tube and fuses with the female gamete (fertilisation) inside the ovule, leading to seed formation; the ovary develops into a fruit.
Section 3
What conditions are needed for seed germination, and how do seedlings use food reserves?
Seeds require three conditions to germinate: water (to activate enzymes and allow cell expansion), oxygen (for aerobic respiration to release energy) and a suitable temperature (for enzyme activity).
Before a seedling can photosynthesise for itself (once it has grown leaves and emerged into the light), it relies on stored food reserves within the seed (such as starch) to provide energy and building materials for growth via respiration.
Light is NOT required for germination itself — it only becomes necessary once the seedling needs to photosynthesise.
Section 4
How are the male and female reproductive systems adapted for their functions?
The male reproductive system produces sperm (in the testes) and delivers them to the female; structures include the testes, sperm ducts, prostate gland (adds fluid to sperm) and penis.
The female reproductive system produces eggs (in the ovaries) and supports development of an embryo/fetus; structures include the ovaries, oviducts (where fertilisation usually occurs), uterus (where the embryo implants and develops) and vagina.
Section 5
How do hormones control the menstrual cycle and support pregnancy?
The menstrual cycle is controlled by four hormones:
- FSH (follicle-stimulating hormone) — stimulates an egg to mature in the ovary and stimulates the ovary to produce oestrogen
- Oestrogen — repairs and thickens the uterus lining; stimulates the release of LH
- LH (luteinising hormone) — triggers ovulation (release of the egg)
- Progesterone — maintains the thickened uterus lining, ready for a fertilised egg to implant
During pregnancy, the placenta allows nutrients and oxygen to pass from the mother's blood to the developing embryo/fetus, and allows waste products to pass back, without the two blood supplies mixing. The developing embryo is protected and cushioned by amniotic fluid, contained within the amniotic sac.
Oestrogen and testosterone are also responsible for the development of secondary sexual characteristics at puberty (e.g. oestrogen: breast development, widening of hips; testosterone: facial hair, deepening voice, increased muscle mass).
Must Know
- Sexual reproduction fuses male and female gametes (fertilisation) to form a zygote; asexual reproduction needs only one parent and produces identical offspring
- Insect-pollinated flowers have large, scented, colourful petals and nectar; wind-pollinated flowers have small, dull flowers with exposed anthers/stigmas and lots of light pollen
- A pollen tube delivers the male gamete to the ovule for fertilisation, leading to seed and fruit formation
- Seeds need water, oxygen and a suitable temperature to germinate, and use stored food reserves until they can photosynthesise
- FSH matures the egg and stimulates oestrogen; oestrogen thickens the uterus lining and triggers LH; LH triggers ovulation; progesterone maintains the uterus lining
- The placenta exchanges substances between mother and embryo; amniotic fluid protects the developing embryo
That's the notes covered.
Carry on to the next subtopic.