3a Reproduction¶
Part of 3 Reproduction and Inheritance.
Reproduction combines the production of new individuals with the early stages of development. The topic brings together plant reproduction, human reproduction, germination and the contrast between sexual and asexual methods.
Learning Objectives¶
| ID | Official specification wording | Main teaching sections |
|---|---|---|
3a-lo-1 |
3.1 understand the differences between sexual and asexual reproduction 3.2 understand that fertilisation involves the fusion of a male and female gamete to produce a zygote that undergoes cell division and develops into an embryo Flowering plants |
Sexual and Asexual Reproduction |
3a-lo-2 |
3.3 describe the structures of an insect-pollinated and a wind-pollinated flower and explain how each is adapted for pollination 3.4 understand that the growth of the pollen tube followed by fertilisation leads to seed and fruit formation 3.5 practical: investigate the conditions needed for seed germination 3.6 understand how germinating seeds utilise food reserves until the seedling can carry out photosynthesis 3.7 understand that plants can reproduce asexually by natural methods (illustrated by runners) and by artificial methods (illustrated by cuttings) Humans |
Plant Reproduction and Germination |
3a-lo-3 |
3.8 understand how the structure of the male and female reproductive systems are adapted for their functions 3.9 understand the roles of oestrogen and progesterone in the menstrual cycle 3.10B understand the roles of FSH and LH in the menstrual cycle 3.11 describe the role of the placenta in the nutrition of the developing embryo 3.12 understand how the developing embryo is protected by amniotic fluid 3.13 understand the roles of oestrogen and testosterone in the development of secondary sexual characteristics |
Human Reproduction, Hormones and Pregnancy |
Sexual and Asexual Reproduction¶
| Feature | Sexual reproduction | Asexual reproduction |
|---|---|---|
| Number of parents | Two | One |
| Offspring variation | Yes — variation is produced | No — offspring are genetically identical clones |
| Need to find a mate | Yes | No |
| Speed and energy | Slower; requires more energy | Faster; less energy needed |
| Example advantage | Variation gives some offspring a survival advantage if conditions change | In favourable conditions, large numbers of identical offspring can be produced quickly |
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:
| Feature | Insect-pollinated | Wind-pollinated |
|---|---|---|
| Petals | Large and brightly coloured to attract insects | Small, dull and often green or brown |
| Nectar/scent | Present — attracts insects | Absent |
| Pollen | Sticky and produced in moderate amounts | Smooth, light and produced in large amounts |
| Anthers | Inside the flower; stiff to allow insects to brush past | Hanging outside the flower on long filaments for easy release |
| Stigma | Inside the flower; sticky so pollen grains adhere | Feathery and outside the flower to catch airborne pollen |
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:
| Structure | Role |
|---|---|
| Testes (in scrotum) | Produce sperm and testosterone |
| Sperm duct | Carries sperm from the testes |
| Sex gland | Produces semen that contains and nourishes sperm |
| Urethra | Carries urine or semen out of the body through the penis |
| Penis | Delivers semen into the female reproductive tract |
Female reproductive system:
| Structure | Role |
|---|---|
| Ovaries | Contain developing eggs (ova); produce oestrogen and progesterone |
| Oviducts (Fallopian tubes) | Connect ovaries to the uterus; ciliated cells move the ovum towards the uterus |
| Uterus | Has a thick lining (endometrium) for implantation of a fertilised egg |
| Cervix | Ring of muscle at the lower end of the uterus; keeps the foetus in place during pregnancy |
| Vagina | Muscular tube leading from the uterus to the outside |
Secondary sexual characteristics develop during puberty under the influence of sex hormones:
| Females (oestrogen) | Males (testosterone) |
|---|---|
| Breast development | Growth of penis and testes |
| Onset of menstrual cycle | Start of sperm production |
| Growth of body hair | Growth of facial and body hair |
| Widening of hips | Muscle development |
| Increased height | Voice deepening |
The menstrual cycle (28 days): - Day 1–5: menstruation — the uterus lining breaks down as progesterone levels fall. - Oestrogen causes thickening of the uterus lining; levels peak around day 10 then fall. - FSH causes maturation of the egg in the follicle; LH triggers ovulation around day 14. - Progesterone maintains the thick lining after ovulation; levels peak around day 17. - If the egg is not fertilised, progesterone levels fall 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.
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 (egg or sperm) 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.