Reproduction
Reproduction is one of the seven characteristics of living organisms. Its purpose is the continuity of the species — that phrase is worth memorising, because a one-mark definition question usually wants both halves:
Reproduction is the production of new organisms, ensuring the continuity of the species.
There are two routes: asexual and sexual. In biology, a word beginning with a- usually means “without” — so asexual means without sex cells fusing.
1. Asexual reproduction
Definition: Asexual reproduction is the process resulting in the production of genetically identical offspring from one parent.
There is no fusion of gametes, no mixing of genetic information, and only one parent is involved. New cells are produced by mitosis, which is why the offspring are identical to the parent and to each other. Genetically identical offspring are called clones.
Examples you need to know
Binary fission in bacteria. A bacterium is a single prokaryotic cell with one circular chromosome (the two ends are joined, not free). Before dividing:
- The circular DNA replicates
- The two copies separate to opposite ends of the cell
- The cell membrane pinches inwards and the cell splits into two genetically identical daughter cells
Under good conditions a bacterium can divide roughly every 20 minutes — which is why bacterial populations grow so fast.
Bulbs (e.g. onion). An underground food-storage organ made of swollen leaf bases. A lateral bud grows out and develops into a new, identical plant.
Tubers (e.g. potato). A swollen underground stem. The “eyes” of a potato are nodes, and shoots grow from them into new plants. A useful proof that a potato is a stem rather than a root: shoots grow from it.
Runners (e.g. strawberry). Side shoots that grow along the ground carrying small plantlets. Where a plantlet touches soil it develops roots, and can then be separated from the parent plant.
Advantages and disadvantages
| Advantages | Disadvantages |
|---|---|
| Only one parent needed — no time or energy spent finding a mate | No genetic variation in the population |
| Faster than sexual reproduction; population grows rapidly | Population is vulnerable to environmental change (temperature, drought, new predators) |
| No dependence on pollinators | If one individual is susceptible to a disease, all are — disease can wipe out the whole population |
| Beneficial characteristics of the parent are passed on unchanged | Evolution and adaptation are slow |
| Good for crops: uniform, predictable, commercially useful | If the parent plant is diseased, all offspring are diseased |
The historical example worth quoting
The Irish potato famine is the textbook illustration. Potatoes were propagated asexually from tubers, so the entire crop was genetically identical. When potato blight arrived, there was no variation — no plants happened to be resistant — and the whole crop failed at once. Grown from seed, some individuals would very likely have survived.
2. Sexual reproduction
Definition: Sexual reproduction is the process involving the fusion of the nuclei of two gametes to form a zygote, and the production of offspring that are genetically different from each other.
Fertilisation is the fusion of gamete nuclei.
Gametes and chromosome number
Gametes are sex cells. They differ from normal body cells in one crucial way: they have half the number of chromosomes.
| Male gamete | Female gamete | |
|---|---|---|
| Animals | Sperm | Egg cell (ovum) |
| Plants | Pollen grain (contains the male nucleus) | Ovule (contains the egg cell) |
- A haploid nucleus contains one set of chromosomes — in humans, 23
- A diploid nucleus contains two sets — in humans, 46
Gametes are haploid. When two haploid gametes fuse at fertilisation, the zygote is diploid (23 + 23 = 46). The zygote then divides by mitosis to form an embryo.
Gametes are made by a different type of division, meiosis, which halves the chromosome number and shuffles the combinations. This is the source of genetic variation: no two egg cells are identical and no two sperm are identical, which is why siblings from the same two parents differ from each other.
Why variation matters
Because offspring differ, some will happen to have characteristics that suit a changed environment — resistance to a disease, tolerance of drought or heat. Those individuals survive and reproduce. A genetically uniform population has no such insurance.
The trade-off is cost: sexual reproduction takes more time and energy, requires two parents, and in plants depends on pollination happening at all.
3. Sexual reproduction in plants
Flower structure
The flower is the reproductive organ of the plant. Its job is to bring the plant’s gametes together.
Male part — the stamen, made of two structures:
| Structure | Function |
|---|---|
| Anther | Produces and releases pollen grains |
| Filament | The stalk that supports and holds up the anther |
Female part — the carpel, made of three structures:
| Structure | Function |
|---|---|
| Stigma | The sticky top surface where pollen grains land |
| Style | The tube connecting the stigma to the ovary |
| Ovary | Contains the ovules, which contain the female gametes |
Other parts:
| Structure | Function |
|---|---|
| Petals | Large and brightly coloured, to attract insects |
| Sepals | Protect the flower bud before it opens |
| Nectary | Produces nectar, which attracts insects |
A common exam slip: the sepals do not attract insects — that’s the petals. Sepals protect the unopened bud.
Pollination
Pollination is the transfer of pollen grains from the anther to the stigma.
Self-pollination — pollen is transferred to the stigma of the same flower, or of another flower on the same plant.
Cross-pollination — pollen is transferred to a flower on a different plant of the same species.
Cross-pollination produces more genetic variation, so offspring are better able to adapt to environmental change and more resistant to disease. Its drawback is that it depends on a pollinator or on the wind — if these are absent, it fails. Self-pollination is more reliable but produces little variation.
Insect-pollinated vs wind-pollinated flowers
This comparison comes up constantly.
| Feature | Insect-pollinated | Wind-pollinated |
|---|---|---|
| Petals | Large, brightly coloured, conspicuous | Small, dull, often green or brown |
| Scent and nectar | Present — attracts insects | Absent — would be a waste of energy |
| Anthers | Held inside the flower on stiff filaments, so insects brush against them | Hang outside the flower, so pollen is easily blown away |
| Stigma | Sticky, inside the flower, to hold pollen brought by insects | Feathery and hangs outside, to catch airborne pollen |
| Pollen grains | Larger, heavier, often with spikes or hooks to stick to insects | Small, light and smooth, easily carried by wind |
| Amount of pollen | Less | Much more (most is wasted) |
Two exam traps in this topic:
- If a question asks for visible features, “it has no scent” will not score — you cannot see a scent, even though the statement is true.
- If the question gives you “insects are attracted by colour, scent and nectar” and then asks for other ways the flower is adapted, do not repeat colour, scent and nectar. Give the sticky stigma, the sticky/spiky pollen, the position of the anthers.
Pollination is not fertilisation
Students lose marks by using these interchangeably. They are two separate events:
- Pollination = pollen moves from anther to stigma
- Fertilisation = the pollen nucleus fuses with the nucleus of the egg cell in the ovule
From pollination to embryo — the five-mark answer
This sequence is one of the most reliably examined answers in the whole topic:
- The pollen grain lands on the stigma and germinates
- A pollen tube grows down through the style towards the ovary
- The pollen tube enters the ovule (through the micropyle)
- The male nucleus travels down the pollen tube and fuses with the female nucleus — this is fertilisation
- The resulting zygote divides by mitosis to form the embryo
Say “nucleus”, not “gamete”. It is the male nucleus that travels down the pollen tube. Writing “the male gamete travels down the tube” is imprecise and can cost the mark.
After fertilisation
- The ovule develops into the seed
- The ovary develops into the fruit
- The petals wither and fall off (they are no longer needed)
Germination
Germination is the start of growth in the seed.
Three conditions are required:
| Condition | Why |
|---|---|
| Water | The seed absorbs water and swells; this activates the enzymes in the embryo |
| Oxygen | Needed for respiration, which releases the energy for growth |
| Suitable temperature | The reactions are controlled by enzymes, which need a suitable temperature to work |
Carbon dioxide is not required for germination — and it does not inhibit it either. Light is also not required for germination itself.
The standard investigation uses boiling tubes of cress seeds on cotton wool:
| Tube | Conditions | Germinates? |
|---|---|---|
| A | Dry cotton wool, 20 °C | No — no water |
| B | Moist cotton wool, 20 °C | Yes — all three conditions present |
| C | Boiled and cooled water with a layer of oil on top, 20 °C | No — the oil layer excludes oxygen |
| D | Moist cotton wool, 4 °C (fridge) | No — temperature too low |
Boiling the water removes dissolved oxygen; the oil layer stops more dissolving back in.
4. Sexual reproduction in humans
The male reproductive system
| Structure | Function |
|---|---|
| Testis | Produces sperm and the hormone testosterone |
| Scrotum | Sac holding the testes outside the body, because sperm production needs a temperature slightly below body temperature |
| Sperm duct | Carries sperm from the testis towards the urethra |
| Prostate gland | Produces fluid that mixes with sperm to form semen |
| Urethra | Tube running through the penis, carrying either urine or semen — never both at once |
| Penis | Passes urine out of the body, and transfers semen into the vagina |
The female reproductive system
| Structure | Function |
|---|---|
| Ovary | Where egg cells mature and are released; produces oestrogen and progesterone |
| Oviduct (fallopian tube) | Lined with ciliated cells that move the egg towards the uterus; fertilisation happens here |
| Uterus | Muscular bag with a soft lining where the embryo implants and develops |
| Cervix | Ring of muscle at the base of the uterus that holds the developing fetus in place; it dilates at birth |
| Vagina | Muscular tube where semen is deposited; the birth canal |
A correction worth noting: the cervix is often described as “protecting the entry to the uterus”. A better answer is that it holds the developing fetus in place during pregnancy and dilates during birth.
Adaptive features of the gametes
An adaptive feature is a structure that helps the cell do its job. Questions using the word explain want the structure and what it is for.
Sperm cell:
| Feature | Function |
|---|---|
| Flagellum (tail) | Allows it to swim towards the egg cell |
| Acrosome (at the tip of the head) | Contains enzymes that digest the jelly coat of the egg so the sperm can penetrate it |
| Many mitochondria in the midpiece | Release energy from respiration for the movement of the flagellum |
| Haploid nucleus | Carries one set of chromosomes |
Egg cell:
| Feature | Function |
|---|---|
| Cytoplasm containing a store of nutrients (yolk) | Provides energy and nutrition for the developing embryo after fertilisation |
| Jelly coat | Changes/hardens immediately after fertilisation to prevent any other sperm entering, so only one zygote forms |
| Haploid nucleus | Carries one set of chromosomes |
Never write that mitochondria “produce energy” or are “the powerhouse of the cell.” Examiners reject both. The correct wording is that mitochondria release energy from respiration. This is one of the most commonly lost marks in the topic.
Fertilisation and implantation
The sperm swims through the uterus into the oviduct. On reaching the egg, the acrosome releases enzymes that digest the jelly coat, and the sperm penetrates. The nucleus of the sperm fuses with the nucleus of the egg cell — 23 chromosomes plus 23 chromosomes — forming a diploid zygote with 46.
The zygote divides by mitosis as it travels down the oviduct, forming a ball of cells, and implants in the lining of the uterus.
The placenta
The placenta develops in the uterus and connects the fetus to the mother via the umbilical cord.
Its functions:
- Exchange of gases — oxygen passes to the fetus, carbon dioxide passes back to the mother
- Transfer of nutrients to the fetus
- Removal of excretory products (such as urea) from the fetus
- Produces progesterone during pregnancy
Do not call the placenta a “protective barrier.” It isn’t one: small molecules diffuse across it freely, including some harmful substances such as nicotine, alcohol and some pathogens. Describing it as a barrier is a factual error, not just imprecise wording.
The mother’s and the fetus’s blood do not mix — substances pass across by diffusion.
5. The menstrual cycle and its hormones
Four hormones are examined. The simplest way to hold them is in two pairs.
The pair that acts on the egg
| Hormone | Made in | Role |
|---|---|---|
| FSH (follicle stimulating hormone) | Pituitary gland | Stimulates the maturation of the follicle containing the egg; stimulates the ovary to produce oestrogen |
| LH (luteinising hormone) | Pituitary gland | Stimulates ovulation — the release of the egg — around day 14 |
The pair that acts on the uterus lining
| Hormone | Made in | Role |
|---|---|---|
| Oestrogen | Ovaries | Stimulates the growth / thickening of the uterus lining; inhibits FSH |
| Progesterone | Ovaries (and the placenta during pregnancy) | Maintains the thickened uterus lining |
Oestrogen stimulates, progesterone maintains. Writing that progesterone “prepares” the lining is marked wrong. This distinction appeared in nearly every lesson reviewed for these notes and is the single most repeated correction in the topic.
Use the word “ovulation” for LH. “Releases the egg” is weaker and may not score.
The 28-day cycle
| Days | What happens |
|---|---|
| 1–5 (approx.) | Menstruation — the uterus lining breaks down and is shed |
| ~5–7 | Lining is at its thinnest |
| ~5–14 | FSH causes a follicle to mature; oestrogen rises and rebuilds the lining |
| ~14 | LH surges, causing ovulation |
| ~14–21 | Progesterone rises and maintains the thickened lining |
| ~21–28 | If there is no fertilisation, progesterone falls, and the lining breaks down again |
If fertilisation does occur, progesterone stays high — first from the ovary, then from the placenta — which is why the lining is not shed during pregnancy. LH is not high during pregnancy: there is no need for further ovulation.
(You may meet the term corpus luteum for the structure remaining in the ovary after ovulation. It is useful background but is not required by the 0610 syllabus.)
Secondary sexual characteristics
Puberty questions often specify “at puberty”, which means they want secondary sexual characteristics — not the hormone’s role in the menstrual cycle. Read the question carefully; this is a frequent misread.
Oestrogen in females: start of the menstrual cycle, widening of the hips, development of the breasts, growth of pubic and underarm hair.
Testosterone in males: deepening of the voice, growth of facial, pubic and body hair, growth spurt, broadening of the shoulders, and the production of sperm.
6. Sexually transmitted infections — HIV
HIV (human immunodeficiency virus) is the STI named in the syllabus.
How it is transmitted: through the exchange of body fluids, specifically via
- unprotected sexual intercourse
- sharing of needles
- infected blood transfusions
- from mother to baby during childbirth or breastfeeding
Do not write “physical contact” or “touching.” It is not transmitted by touching, saliva, or sweat. The answer examiners want is exchange of body fluids.
What it does: HIV attacks lymphocytes, the white blood cells that produce antibodies. With fewer functioning lymphocytes, the immune system is weakened, so the person becomes vulnerable to other infections (such as tuberculosis) that a healthy immune system would normally control.
HIV and AIDS are not the same thing. HIV is the virus. AIDS (acquired immune deficiency syndrome) is the condition that may develop years later, once the virus has damaged the immune system enough that serious secondary infections take hold. A person living with HIV — particularly with modern treatment — may have no symptoms for many years; AIDS is the advanced stage.
Related recall: lymphocytes produce antibodies; phagocytes engulf pathogens (from phagocytosis). These get swapped around constantly.
7. Mistakes that cost marks
Confusing pollination with fertilisation. Pollination is anther → stigma. Fertilisation is the fusion of nuclei.
Writing that progesterone “prepares” the uterus lining. Oestrogen stimulates its growth; progesterone maintains it.
Saying mitochondria “produce energy” or are “the powerhouse of the cell.” They release energy from respiration.
Saying “the male gamete travels down the pollen tube.” It is the male nucleus.
Calling the placenta a protective barrier. Small molecules diffuse across it.
Writing “no scent” when asked for visible features.
Repeating information the question already gave you when it asks for other adaptations.
Confusing glycogen and glucagon. Different topic, same papers, endless confusion. Glycogen is stored carbohydrate; glucagon is a hormone.
Answering “describe” when the question says “explain.” For asexual reproduction, describing the method (cutting a runner and planting it) is not explaining it. The explanation is that new cells are produced by mitosis, so the offspring are genetically identical.
Giving a number as an example of discontinuous variation. Number of leaves is a range of values, so it is continuous. Use shape or colour instead.
Frequently asked questions
What is the difference between pollination and fertilisation? Pollination is the transfer of pollen from the anther to the stigma. Fertilisation is the fusion of the male nucleus with the female nucleus inside the ovule. Pollination comes first; fertilisation may follow.
Why is cross-pollination better than self-pollination? It produces more genetic variation, so offspring are better able to adapt to environmental change and are less likely to all be wiped out by one disease. The drawback is that it depends on a pollinator or on wind.
Why are gametes haploid? So that when two gametes fuse, the zygote has the correct diploid number. If gametes were diploid, the chromosome number would double every generation.
Where does fertilisation happen in humans? In the oviduct (fallopian tube). Implantation then happens in the uterus lining — a different place, and a common mix-up.
Which hormone causes ovulation? LH, which surges around day 14 of a 28-day cycle.
Does a seed need light to germinate? No. Germination requires water, oxygen and a suitable temperature. Light is needed for photosynthesis once the seedling emerges, not for germination itself.
Why is the scrotum outside the body? Sperm production requires a temperature slightly lower than normal body temperature.
Quick revision checklist
- I can define reproduction, asexual reproduction, sexual reproduction and fertilisation
- I can give the advantages and disadvantages of both types
- I can describe binary fission and give plant examples of asexual reproduction
- I can label a flower and give the function of every part
- I can compare insect- and wind-pollinated flowers across all six features
- I can distinguish pollination from fertilisation
- I can write the full pollen tube → embryo sequence, using the word nucleus
- I know that the ovule becomes the seed and the ovary becomes the fruit
- I can state the three conditions for germination and explain each
- I can label both human reproductive systems and give functions
- I can explain three adaptive features of sperm and two of the egg cell
- I never write that mitochondria “produce energy”
- I know all four menstrual cycle hormones, where each is made, and what each does
- I know that oestrogen stimulates and progesterone maintains
- I can describe the functions of the placenta without calling it a barrier
- I can explain how HIV is transmitted and how it affects the immune system
These notes cover topic 16 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
Finished this topic?
Saved on this device — no account needed.