Reproduction: Concepts & Plants
Sexual vs asexual reproduction; meiosis producing haploid gametes; Extended advantages/disadvantages; flower structure and functions; insect vs wind pollination; fertilisation and seed/fruit formation; seed dispersal; Extended germination conditions; human reproductive systems; sperm and egg adaptations; fertilisation in the oviduct; Extended menstrual cycle and hormones.
Reproduction
CORE EXTENDEDSexual vs Asexual Reproduction
| Feature | Sexual reproduction | Asexual reproduction |
|---|---|---|
| Number of parents | Two | One |
| Gametes involved? | Yes — male and female gametes fuse (fertilisation) | No gametes — offspring produced by mitosis |
| Genetic variation in offspring | Yes — offspring are genetically different from each other and from parents | No — offspring are genetically identical to parent (clones) |
| Type of cell division | Meiosis (to produce gametes) + mitosis (for growth) | Mitosis only |
| Examples | Humans, flowering plants, most animals | Bacteria (binary fission), yeast (budding), strawberry runners, potato tubers, Hydra |
Advantages and Disadvantages — Extended
| Sexual reproduction | Asexual reproduction | |
|---|---|---|
| Advantages | Genetic variation — offspring better able to adapt to changing environments; natural selection can operate; disease less likely to wipe out entire population | Only one parent needed; faster and more energy-efficient; all offspring can reproduce; useful in stable environments; allows rapid population increase |
| Disadvantages | Requires two parents; slower; more energy-intensive; fewer offspring may be produced in a given time compared to asexual reproduction | No genetic variation — all offspring vulnerable to same disease or environmental change; cannot adapt to changing conditions; harmful mutations accumulate |
Meiosis and Gametes
Meiosis is the type of cell division that produces gametes (sex cells). It occurs in the reproductive organs (testes and ovaries in animals; anthers and ovules in flowering plants).
Key features of meiosis:
- Produces four genetically different daughter cells (unlike mitosis which produces two identical cells)
- Each daughter cell has half the chromosome number of the parent cell (haploid, n) — essential so that fertilisation restores the diploid (2n) number
- Genetic variation arises through crossing over (exchange of chromosome segments) and independent assortment (random distribution of chromosomes)
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Production of gametes (sex cells) |
| Daughter cells produced | 2 | 4 |
| Chromosome number | Same as parent (diploid, 2n) | Half parent number (haploid, n) |
| Genetic variation? | None — identical to parent | Yes — crossing over + independent assortment |
Which statement correctly describes asexual reproduction?
- A. It involves the fusion of two gametes to produce a zygote
- B. It produces offspring with greater genetic variation than sexual reproduction
- C. It requires only one parent and produces genetically identical offspring
- D. It always involves meiosis to produce daughter cells
Sexual Reproduction in Plants
CORE EXTENDEDStructure of a Flower
| Part | Function | Contains / produces |
|---|---|---|
| Sepal | Encloses and protects the flower bud before it opens | — |
| Petal | Attracts insect pollinators (by colour, shape, scent) | May have nectar guides |
| Stamen (male) | Produces pollen | Anther (makes pollen) + filament (supports anther) |
| Anther | Produces and releases pollen grains (contain male gametes) | Pollen grains |
| Carpel (female) | Contains the female reproductive structures | Stigma + style + ovary + ovule(s) |
| Stigma | Receives pollen grains during pollination; sticky surface | Top of carpel |
| Style | Connects stigma to ovary; pollen tube grows through it | — |
| Ovary | Contains the ovules; becomes the fruit after fertilisation | Ovule(s) |
| Ovule | Contains the female gamete (egg cell); becomes the seed after fertilisation | Egg cell |
| Nectary | Produces nectar to attract pollinators | Sugary solution |
Pollination
Pollination is the transfer of pollen from an anther to a stigma.
Self-pollination: pollen transferred to the stigma of the same flower or another flower on the same plant.
Cross-pollination: pollen transferred to the stigma of a flower on a different plant of the same species. This produces greater genetic variation and is generally favoured.
| Feature | Insect-pollinated flower | Wind-pollinated flower |
|---|---|---|
| Petals | Large, brightly coloured, scented | Small, green, or absent |
| Nectaries | Present (attracts insects) | Absent |
| Pollen | Sticky, heavy, spiky — clings to insect bodies | Smooth, light, dry — easily carried by wind |
| Anthers | Inside flower, not exposed | Hanging outside flower, exposed to wind |
| Stigma | Sticky, inside flower | Feathery, large, exposed outside flower — maximises pollen capture |
| Pollen quantity | Small amount produced | Very large amounts produced (most is wasted) |
Fertilisation in Plants
1. Pollen grain lands on the sticky stigma.
2. Pollen grain germinates — grows a pollen tube down through the style toward the ovary.
3. The pollen tube enters the ovule through the micropyle.
4. The male gamete (nucleus from pollen) travels down the tube and fuses with the egg cell nucleus in the ovule — this is fertilisation.
5. The fertilised egg cell (zygote) develops into an embryo.
6. The ovule develops into a seed; the ovary wall develops into the fruit.
Seed Dispersal
Seeds must be dispersed away from the parent plant to reduce competition for space, light, and nutrients. Different mechanisms are adapted for different dispersal agents:
| Method | Adaptations | Examples |
|---|---|---|
| Wind dispersal | Light seeds; wing-like extensions; feathery plumes | Dandelion (parachute hairs); sycamore (wings/helicopter) |
| Animal (external) | Hooks or burrs that attach to animal fur or clothing | Burdock; goosegrass |
| Animal (internal) | Fleshy, nutritious fruit; seeds pass through gut undigested | Berries, cherries, tomatoes |
| Water dispersal | Buoyant, waterproof seed coat | Coconut; water lily |
| Self-dispersal (explosive) | Pod dries and twists, expelling seeds forcefully | Peas; gorse; squirting cucumber |
Germination — Extended
A seed will germinate (begin to grow) when three conditions are met simultaneously:
1. Water: activates enzymes; mobilises food reserves (starch hydrolysed to glucose for respiration); provides turgor for cell expansion.
2. Oxygen: required for aerobic respiration to release energy for growth of the embryo.
3. Suitable temperature: enzyme-controlled reactions require a temperature above the minimum for those enzymes to function; most seeds germinate best at 15–25°C.
Note: Light is NOT required for germination (seeds germinate underground); it is only needed for photosynthesis once the seedling emerges.
Experimental design: Set up several groups of seeds with one variable changed at a time:
Group A (control): water + air + 20°C → seeds germinate.
Group B: dry cotton wool (no water) + air + 20°C → no germination (water absent).
Group C: water + boiled/cooled water (no O₂) + 20°C → no germination (oxygen absent).
Group D: water + air + 4°C (fridge) → no germination or very slow (temperature too low).
Variables controlled: same species of seed, same number per group, same volume of water where applicable, same observation period.
In a wind-pollinated flower, the stigma is feathery and positioned outside the flower. What is the advantage of this adaptation?
- A. It produces a large surface area to attract insect pollinators
- B. It prevents self-pollination by keeping the stigma away from the anthers
- C. It maximises the chance of airborne pollen grains landing on the stigma
- D. It protects the pollen grains from being damaged by wind
Sexual Reproduction in Humans
CORE EXTENDEDMale Reproductive System
| Structure | Function |
|---|---|
| Testes | Produce sperm by meiosis; produce testosterone |
| Sperm duct (vas deferens) | Carries sperm from testes to urethra |
| Urethra | Carries sperm (and urine separately) out of the body through the penis |
| Penis | Organ of copulation; delivers sperm into the vagina during sexual intercourse |
| Prostate gland / seminal vesicles | Produce seminal fluid — nourishes sperm and provides a medium for swimming |
Female Reproductive System
| Structure | Function |
|---|---|
| Ovaries | Produce eggs (oocytes) by meiosis; produce oestrogen and progesterone |
| Oviduct (Fallopian tube) | Carries egg from ovary to uterus; site of fertilisation |
| Uterus (womb) | Muscular organ where the embryo implants and develops during pregnancy |
| Cervix | Lower opening of uterus; dilates during labour |
| Vagina | Receives penis during copulation; birth canal during delivery |
Sperm and Egg — Structure and Adaptations
| Feature | Sperm cell | Egg cell (oocyte) |
|---|---|---|
| Size | Very small | Much larger — largest human cell |
| Motility | Motile — has a tail (flagellum) for swimming | Non-motile |
| Mitochondria | Many in midpiece — provide ATP for flagellum movement | Many — provide energy for early embryo development |
| Acrosome | Contains digestive enzymes to penetrate the egg’s outer layers | — |
| Cytoplasm | Very little — streamlined for swimming | Large amount — stores food reserves / nutrients for early embryo development |
| Nucleus | Haploid (n) — carries paternal chromosomes | Haploid (n) — carries maternal chromosomes |
Fertilisation
Fertilisation is the fusion of a male gamete (sperm) with a female gamete (egg) to form a diploid zygote.
1. Sperm deposited in vagina swim through the cervix and uterus into the oviduct.
2. One sperm penetrates the egg cell in the oviduct — fertilisation occurs.
3. The zygote undergoes repeated mitotic divisions as it travels down the oviduct — becomes an embryo (ball of cells).
4. The embryo implants into the thickened uterus lining (endometrium) approximately 6–10 days after fertilisation.
5. The embryo develops into a foetus; the placenta forms.
The Menstrual Cycle — Extended
The menstrual cycle is a ~28-day hormone-driven cycle that prepares the uterus for possible implantation of an embryo each month.
| Phase | Days (approx.) | Events | Hormones |
|---|---|---|---|
| Menstruation | 1–5 | Uterus lining (endometrium) breaks down and is shed as menstrual blood | Low oestrogen and progesterone |
| Follicular phase | 1–13 | FSH stimulates follicle development in ovary; rising oestrogen causes uterus lining to thicken | FSH rising; oestrogen rising |
| Ovulation | ~14 | LH surge triggers release of egg from follicle (ovulation) | LH surge; peak oestrogen |
| Luteal phase | 15–28 | Empty follicle becomes corpus luteum, secreting progesterone; uterus lining maintained; if no fertilisation, corpus luteum degenerates → lining sheds | Progesterone high; then drops |
FSH (follicle-stimulating hormone) — secreted by pituitary; stimulates follicle development and oestrogen production.
Oestrogen — secreted by ovary; causes uterus lining to thicken; triggers LH surge at high concentration.
LH (luteinising hormone) — secreted by pituitary; triggers ovulation.
Progesterone — secreted by corpus luteum; maintains uterus lining; inhibits FSH and LH (preventing another cycle while pregnant).
Where does fertilisation normally occur in the human female reproductive system?
- A. In the ovary, immediately after ovulation
- B. In the oviduct (Fallopian tube)
- C. In the uterus, after implantation
- D. In the cervix, during copulation
Describe the roles of oestrogen and progesterone in the menstrual cycle, including what happens to uterus lining thickness during each phase. [4 marks]
- Oestrogen is produced by the developing follicle (days 1–13); rising oestrogen causes the uterus lining to thicken / endometrium to proliferate [1 mark]
- At peak concentration, oestrogen triggers an LH surge leading to ovulation (~day 14) [1 mark]
- Progesterone is produced by the corpus luteum (days 15–28); it maintains the thickened uterus lining, keeping it ready for implantation [1 mark]
- If no fertilisation occurs, the corpus luteum degenerates; progesterone (and oestrogen) levels fall; the uterus lining breaks down and is shed (menstruation) [1 mark]
Comprehensive Practice Questions
Mixed questions across Topics 16.1, 16.2, and 16.3.
Which statement correctly describes meiosis?
- A. It produces two genetically identical diploid cells
- B. It occurs during wound healing and tissue repair
- C. It produces four genetically different haploid cells
- D. It does not involve chromosome separation
(a) State the definition of pollination. [1 mark]
(b) Give three differences between insect-pollinated and wind-pollinated flowers. [3 marks]
(c) Describe the sequence of events from pollination to the formation of a seed. [3 marks]
- (a) Transfer of pollen from an anther to a stigma [1 mark]
- (b) Any three differences from the comparison table — e.g. insect: large/coloured petals vs wind: small/absent; insect: sticky pollen vs wind: smooth/light; insect: nectaries present vs wind: absent; insect: stigma inside vs wind: feathery/exposed [3 marks]
- (c) Pollen grain lands on stigma and germinates; pollen tube grows down through style and ovary to ovule [1 mark]; male gamete travels down pollen tube and fuses with egg cell in ovule (fertilisation) [1 mark]; fertilised egg (zygote) develops into embryo; ovule develops into seed; ovary wall develops into fruit [1 mark]
(a) State the three conditions required for germination and explain the role of each. [3 marks]
(b) A farmer can choose to grow strawberries either from seeds (sexual) or from runners (asexual). Evaluate the advantages and disadvantages of each method for the farmer. [4 marks]
- Water: activates enzymes, enables respiration, provides turgor for cell expansion [1 mark]
- Oxygen: required for aerobic respiration to provide energy for embryo growth [1 mark]
- Suitable temperature: enzymes controlling germination reactions need a temperature above the minimum for activity [1 mark]
- Seeds (sexual): genetic variation means some offspring may have better characteristics; but offspring are unpredictable and may not match the parent quality; slower; requires pollination [2 marks]
- Runners (asexual): all offspring are genetically identical to the parent plant — guaranteed to have the same desirable traits (flavour, yield, disease resistance); faster; no pollinator needed; but all plants equally vulnerable to the same disease [2 marks]
High-Frequency Mistakes — Topics 16.1, 16.2 & 16.3
- 🔄Confusing pollination and fertilisationPollination = transfer of pollen from anther to stigma (no gamete fusion). Fertilisation = fusion of male and female gametes inside the ovule. Pollination must occur before fertilisation, but they are two separate events. Saying "fertilisation occurs when pollen lands on the stigma" is wrong.
- 🔄Confusing ovule and ovary — seed and fruitThe ovule develops into the seed after fertilisation. The ovary (containing the ovule) develops into the fruit. Saying "the ovule becomes the fruit" reverses this. Remember: ovAry = frAit (fruit); ovUle = seed (think of the U as inside the fruit).
- 🌿Saying light is needed for germinationLight is NOT one of the three conditions for germination. Seeds germinate underground in the dark. Light is only needed once the seedling has emerged and begins photosynthesising. The three conditions are: water, oxygen, and suitable temperature — no light.
- ⚡Saying meiosis produces two cellsMeiosis produces FOUR haploid daughter cells. Mitosis produces two diploid daughter cells. The number matters: meiosis involves two rounds of division — the first separates homologous chromosome pairs, the second separates sister chromatids — producing four cells total.
- 👶Saying fertilisation occurs in the uterusIn humans, fertilisation normally occurs in the oviduct (Fallopian tube) — not in the uterus. The zygote then travels to the uterus for implantation. If fertilisation occurs in the uterus it would be too late for normal implantation. Ectopic pregnancy = implantation in the oviduct (very dangerous).
- 📈Ext: Saying oestrogen maintains the uterus lining throughout the cycleOestrogen causes the uterus lining to thicken (proliferative phase). It is progesterone (from the corpus luteum after ovulation) that maintains the thickened lining ready for implantation. If asked what maintains the lining after ovulation, the answer is progesterone, not oestrogen.
- 🔁Ext: Saying asexual reproduction produces variationAsexual reproduction produces genetically identical offspring (clones) — there is no genetic variation between offspring. All variation in asexual offspring arises only from random mutation (which is rare). This is why a disease that affects one clone plant in an asexual crop will affect all of them equally.
Highest-yield Core items: sexual vs asexual comparison table (genetic variation, gametes, parents, type of division); meiosis produces 4 haploid genetically different cells (vs mitosis 2 diploid identical); flower parts and their functions (especially stamen/carpel/anther/stigma/ovule — learn all 10); insect vs wind pollination comparison; pollination definition; fertilisation pathway (pollen tube → ovule → gamete fusion → seed/fruit formation); site of human fertilisation (oviduct); germination conditions (water, oxygen, temperature — not light). For Extended: advantages/disadvantages of each reproduction type; menstrual cycle four phases with hormone roles (FSH, LH, oestrogen, progesterone) — the hormone table is a reliable Paper 4 target.