3a Reproduction¶

Part of 3 Reproduction and Inheritance.

Reproduction makes new individuals. Sexual reproduction mixes the genes of two parents and so produces variation, whereas asexual reproduction copies one parent and produces clones. This topic follows both methods through plants and humans, from pollination and germination to the menstrual cycle and pregnancy.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

The differences between sexual and asexual reproduction, how flowering plants are pollinated and how seeds germinate, the human reproductive systems and menstrual cycle, and how the placenta supports a pregnancy.

Sexual and Asexual Reproduction¶

Fertilisation is the fusion of a male and female gamete (reproductive cell), each with half the full chromosome number (haploid). The resulting cell is a zygote, which has the full chromosome number (diploid) and divides repeatedly to form an embryo.

Plant Reproduction and Germination¶

Pollination is the transfer of pollen grains from an anther to a stigma. Two different mechanisms produce contrasting adaptations:

Seed and fruit formation:

1. A pollen grain lands on a stigma.
2. A pollen tube grows down through the style and into the ovary, reaching the ovule.
3. The male nucleus travels down the pollen tube and fuses with the female egg nucleus in the ovule, forming a zygote.
4. The zygote divides by mitosis to form a seed (embryo).
5. The ovule wall becomes the seed coat; the ovule becomes the seed.
6. The ovary wall becomes the fruit.

Germination is the process by which a seed begins to develop into a seedling. Three conditions are required:

• Water: activates the enzymes needed to break down starch food reserves in the seed.
• Oxygen: required for aerobic respiration to release energy for growth.
• Warmth: sufficient temperature for enzymes to work at a useful rate.

Parts of a germinating seed:

• Embryo: the young root and shoot that will develop into the adult plant.
• Food store: contains starch to supply energy until the seedling can photosynthesise.
• Seed coat: a protective outer covering.

Asexual reproduction in plants:

• Natural — runners (e.g. strawberry plants) grow horizontally over the soil surface, root at nodes and form new plants.
• Artificial — cuttings: a stem section is removed from the parent plant and placed in a growth medium containing hormones and nutrients. The cutting develops into a plantlet that is then transferred to compost.

Human Reproduction¶

Male reproductive system:

Female reproductive system:

Secondary sexual characteristics develop during puberty under the influence of sex hormones:

The menstrual cycle lasts about 28 days. Each stage is driven by a hormone, and each hormone acts on the next stage:

If the egg is not fertilised, progesterone levels fall, the lining breaks down and the cycle restarts.

Hormones and Pregnancy¶

The placenta is a temporary organ that develops in the uterus during pregnancy. It allows exchange of materials between the mother's blood and the embryo's blood without the two mixing directly:

• From mother to embryo: glucose, oxygen and amino acids diffuse across for growth.
• From embryo to mother: carbon dioxide and urea pass across to be removed by the mother.
• The placenta also produces progesterone to maintain the uterus lining.

Amniotic fluid is produced inside the amnion (a membrane surrounding the embryo in the uterus). It cushions and protects the developing foetus from mechanical damage. At the start of labour, the amnion ruptures and the fluid is released ('waters breaking').

Common Confusions¶

• Pollination vs fertilisation: Pollination is the transfer of pollen to the stigma. Fertilisation happens later when the male nucleus fuses with the egg nucleus inside the ovule.
• Runners vs cuttings: Runners are a natural asexual method. Cuttings require human intervention and are an artificial method.
• Clones vs variation: Asexual reproduction involves one parent and only mitosis, so offspring are genetically identical. Sexual reproduction mixes the genes of two gametes, so offspring vary.
• Pregnancy exchange: The mother's blood and the embryo's blood do not mix. Substances cross the placenta by diffusion.

Check Yourself¶

1. State two differences between sexual and asexual reproduction.
2. Describe two features of a wind-pollinated flower and explain how each helps.
3. Distinguish between pollination and fertilisation.
4. A seed is kept in a warm, dry cupboard and does not germinate. Explain why.
5. Name the hormone that triggers ovulation and the hormone that maintains the uterus lining after ovulation.
6. Explain why the mother's blood and the embryo's blood do not mix, but the embryo still receives oxygen.

Key Terms¶

• Fertilisation: fusion of male and female gametes to form a diploid zygote.
• Zygote: the cell formed immediately after fertilisation.
• Gamete: a reproductive cell containing half the full chromosome number.
• Pollination: transfer of pollen from an anther to a stigma.
• Pollen tube: the tube that grows from a pollen grain down to the ovule to allow fertilisation.
• Germination: the development of a seed into a seedling when conditions are suitable.
• Placenta: an organ that allows exchange of substances between the mother and the developing embryo.
• Amniotic fluid: the fluid that surrounds and cushions the embryo in the uterus.
• Oestrogen: a hormone that thickens the uterus lining and triggers secondary sexual characteristics in females.
• Progesterone: a hormone that maintains the uterus lining during pregnancy.
• FSH (follicle-stimulating hormone): triggers egg maturation in the ovary.
• LH (luteinising hormone): triggers ovulation.
• Ovulation: the release of a mature egg from the ovary.

Connected Pages¶

• 2j Co-ordination and Response
• 3b Inheritance
• 5b Selective Breeding
• IGCSE Biology Key Terms Glossary
• Practical Investigation Reference