Contents
1. Types of Reproduction
| Asexual reproduction | Sexual reproduction | |
|---|---|---|
| Parents involved | One | Two (usually) |
| Gametes | Not required | Required (egg and sperm) |
| Genetic variation | None — offspring are clones | Yes — offspring are genetically different |
| Speed | Rapid | Slower |
| Examples | Binary fission (bacteria), budding (yeast), runners (strawberry) | Humans, most animals, flowering plants |
Genetic variation gives populations the ability to adapt to changing environments through natural selection. Asexual reproduction is faster but produces no variation — if the environment changes, all individuals may be equally vulnerable.
2. Human Reproductive System
Male reproductive system
| Structure | Function |
|---|---|
| Testes | Produce sperm by meiosis; produce testosterone |
| Epididymis | Sperm mature and are stored here |
| Sperm duct (vas deferens) | Carries sperm from epididymis to urethra |
| Seminal vesicle / prostate | Produce fluid that nourishes and activates sperm (semen) |
| Urethra | Carries semen (or urine) out of body |
Female reproductive system
| Structure | Function |
|---|---|
| Ovaries | Produce eggs (ova) by meiosis; produce oestrogen and progesterone |
| Oviduct (Fallopian tube) | Carries egg from ovary to uterus; site of fertilisation |
| Uterus | Site of implantation and development of embryo/foetus |
| Cervix | Lower part of uterus; dilates during birth |
| Vagina | Receives penis during intercourse; birth canal |
3. Menstrual Cycle
The menstrual cycle averages 28 days and is controlled by four hormones: FSH, LH, oestrogen and progesterone.
| Days | Events | Hormones involved |
|---|---|---|
| 1–5 | Menstruation — uterine lining breaks down and is shed | Low oestrogen and progesterone |
| 6–13 | Follicle develops in ovary; uterine lining rebuilds | FSH (from pituitary) stimulates follicle; follicle produces oestrogen |
| ~Day 14 | Ovulation — egg released from ovary | High oestrogen triggers LH surge (from pituitary) |
| 15–28 | Corpus luteum forms; uterine lining maintained | Progesterone (from corpus luteum) maintains lining |
If fertilisation does not occur: corpus luteum degenerates → progesterone falls → lining breaks down → menstruation begins again.
If fertilisation occurs: embryo produces hCG (human chorionic gonadotropin) → maintains corpus luteum → progesterone stays high → lining maintained → no menstruation.
4. Fertilisation and Development
Fertilisation occurs in the oviduct (Fallopian tube) when a sperm fuses with an egg. One sperm penetrates the egg membrane; the nuclei fuse forming a zygote (2n = 46 chromosomes).
Implantation and placenta
The zygote divides repeatedly (cleavage) forming a ball of cells (embryo) which implants into the uterine lining. The placenta develops — it allows exchange of substances between maternal and foetal blood without them mixing:
- Oxygen and glucose → from mother to foetus (diffusion).
- CO₂ and urea → from foetus to mother (diffusion).
- Antibodies → from mother to foetus (passive immunity).
- Harmful substances (alcohol, nicotine, some drugs, rubella virus) → can cross and damage foetus.
Adaptations of the placenta
- Large surface area of villi for efficient diffusion.
- Thin walls — short diffusion distance.
- Rich blood supply to maintain steep concentration gradients.
- Sexual reproduction: involves meiosis + fertilisation. Genetic variation through crossing over and random assortment.
- Asexual reproduction: mitosis only. No variation. Advantage: fast, no mate needed.
- Menstrual cycle: FSH (follicle growth) -> oestrogen peak (lining thickens, LH triggered) -> LH surge (ovulation, day 14) -> progesterone (lining maintained) -> if no fertilisation: progesterone falls, lining sheds
- Placenta: large surface area (villi), thin walls, huge blood supply. Functions: exchange O2/nutrients/waste, produces oestrogen and progesterone
- Foetal and maternal blood NEVER mix in the placenta
5. Common Exam Traps
Fertilisation occurs in the oviduct (Fallopian tube). The fertilised egg (zygote) then travels to the uterus and implants in the uterine lining. Students often incorrectly state the uterus as the site of fertilisation.
The placenta allows exchange of substances, but the two blood supplies remain separate. Exchange occurs by diffusion across the thin walls of the villi — not by direct mixing of blood.
Oestrogen rebuilds the uterine lining after menstruation. Progesterone maintains it in the second half of the cycle. If progesterone falls (no pregnancy), the lining breaks down. These roles are frequently swapped by students.
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.