IGCSE Biology 0610 · Chapter 12 notes · ~21 min read
Reproduction
Asexual vs sexual reproduction, the parts of a flower, pollination and fertilisation, germination, the human reproductive systems, the placenta, the menstrual cycle and how HIV and other STIs are spread and controlled.
Asexual & sexualFlowers & pollinationGerminationHuman reproductive systemsMenstrual cycleSTIs
ASby Aiza Ma'am
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No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end
In 30 seconds
- Asexual reproduction needs one parent and makes identical clones. Sexual reproduction fuses the nuclei of two gametes, so the offspring are genetically different.
- In a flower, pollination (pollen reaches the stigma) comes first, then fertilisation (two nuclei fuse). The ovule becomes the seed. A seed needs water, oxygen and warmth to germinate.
- In humans, a sperm fertilises an egg in the oviduct, the embryo implants in the uterus lining and is fed through the placenta. Hormones control puberty and the menstrual cycle.
By the end you can
- Define asexual and sexual reproduction and give examples of each.
- Identify the parts of a flower and state what each part does.
- Compare insect-pollinated and wind-pollinated flowers, including their pollen.
- Define pollination and fertilisation in plants.
- State the conditions needed for seed germination and plan an investigation.
- Identify the male and female reproductive organs and state their functions.
- Compare sperm and egg and explain how each is adapted for its job.
- Describe fertilisation and implantation, and the roles of the umbilical cord, placenta, amniotic sac and amniotic fluid.
- State the roles of testosterone and oestrogen at puberty and describe the menstrual cycle.
- Define an STI, and describe how HIV is spread, how it leads to AIDS and how its spread is controlled.
- Discuss the advantages and disadvantages of asexual and sexual reproduction in wild populations and crops. EXT
- Explain the effects of self-pollination and cross-pollination, and describe pollen tube growth. EXT
- Explain how the placenta works and which harmful substances can cross it. EXT
- Describe the roles of FSH, LH, oestrogen and progesterone in the menstrual cycle. EXT
1Two ways to reproduce
Asexual reproduction
Asexual reproduction — a process resulting in the production of genetically identical offspring from one parent.
- No gametes and no fertilisation are needed. The parent divides by mitosis (see mitosis), so the offspring are clones with the same genes as the parent.
- Examples: bacteria split in two, a potato plant grows new plants from its tubers, a strawberry plant sends out runners that root and become new plants.
Sexual reproduction
Sexual reproduction — a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote, and the production of offspring that are genetically different from each other.
- Gametes: sperm and egg in animals. In flowering plants, the male gamete is in the pollen grain and the female gamete is the egg cell in the ovule.
- Offspring get half their genes from each parent in a new mix, so they vary.
- EXT Gametes are haploid (one set of chromosomes). Fertilisation makes a diploid zygote (two sets), which divides by mitosis to grow into the new organism. (chromosomes)
Which is better?EXTENDED
| Asexual | Sexual | |
|---|---|---|
| advantages | only one parent, no mate needed; fast, many offspring; good features are kept; no energy spent on gametes or flowers | variation: some offspring may survive a new disease or a changed environment; the species can adapt over time |
| disadvantages | no variation: one disease or one change in conditions can kill all of them; offspring crowd each other | needs two parents or a pollinator; slower; uses more energy; fewer offspring |
| in crops | all plants are identical, so uniform yield, but all are equally at risk from the same disease | farmers can breed new varieties (more disease resistance, higher yield), but plants are less uniform |
| in wild populations | quick way to fill a stable habitat | variation is vital when conditions change, so the population can survive and evolve |
Exam tipDo not just write "asexual is faster". Say why it matters: no variation means a single disease can wipe out the whole population.
2Reproduction in flowering plants
Parts of a flower
| Part | Function |
|---|---|
| sepal | green; protects the flower bud |
| petal | bright colour in insect-pollinated flowers, to attract insects |
| stamen (male) | anther + filament |
| anther | makes pollen grains, which contain the male gametes |
| filament | holds the anther up |
| carpel (female) | stigma + style + ovary |
| stigma | sticky top that catches pollen |
| style | holds the stigma up |
| ovary | contains the ovules; it becomes the fruit |
| ovule | contains the female gamete (egg cell); it becomes the seed |
Memory trickAnther Adds pollen. Stigma is Sticky and Sticks pollen on.
Exam tipDrawing a flower: sharp pencil, large drawing (use over half the space), single clear lines, no shading or colour, straight label lines drawn with a ruler that never cross. Use a hand lens to see the small parts.
Pollination: insect or wind?
Pollination — the transfer of pollen grains from the anther to the stigma.
| Feature | Insect-pollinated | Wind-pollinated |
|---|---|---|
| petals | large, brightly coloured | small and dull (or none) |
| scent and nectar | yes: they attract insects | none |
| anthers | inside the flower, where an insect brushes them | hang outside on long filaments, so wind shakes pollen out |
| stigma | small and sticky, inside the flower | large and feathery, hangs outside to catch pollen from the air |
| pollen | larger, sticky or spiky, so it clings to the insect | smaller, lighter, smooth, made in huge amounts |
- Examples: insect-pollinated: hibiscus, sunflower. Wind-pollinated: grasses, maize, wheat.
Worked example
A drawing of an insect-pollinated pollen grain is 36 mm across. The real grain is 0.04 mm across. Calculate the magnification.
- magnification = drawing size ÷ real size
- = 36 ÷ 0.04
- = × 900
- both sizes must be in the same unit (here mm), so no conversion is needed
Common mistake"Pollination and fertilisation are the same thing." Pollination = pollen lands on the stigma. Fertilisation = nuclei fuse (it happens later).
Self- and cross-pollinationEXTENDED
Self-pollination — EXT transfer of pollen from an anther to a stigma of the same flower or another flower on the same plant.
Cross-pollination — EXT transfer of pollen from an anther to the stigma of a flower on a different plant of the same species.
| Effect on the population | Self-pollination | Cross-pollination |
|---|---|---|
| variation | low: offspring are similar to the parent | high: genes from two plants are mixed |
| response to change | poor: a new disease or climate change may harm all | better: some offspring are likely to survive and adapt |
| cost | needs no pollinator or second plant | needs wind or insects and another plant nearby |
Fertilisation and seeds
Fertilisation (plants) — the fusion of a pollen nucleus (male) with the nucleus of an ovule (female).
- EXT Pollen tube: a pollen grain that lands on a sticky stigma grows a pollen tube down through the style to the ovary. The tube enters the ovule through a tiny opening (the micropyle) and the male nucleus fuses with the egg nucleus to make the zygote.
- After fertilisation: the zygote becomes the embryo, the ovule becomes the seed and the ovary becomes the fruit.
Seed germination
Germination — the start of growth of a seed into a young plant (a seedling).
- Three conditions: water, oxygen and a suitable temperature.
- Water switches on the enzymes that start growth. Oxygen is needed for aerobic respiration, which releases the energy for growth (respiration). A suitable (warm) temperature lets enzymes work well (enzymes).
- Light and soil are not needed. The seed has its own food store until the leaves can make food (photosynthesis).
- How to investigate: keep one condition missing in each tube: dry cotton wool (no water), boiled water with a layer of oil (boiling drives out oxygen and the oil keeps air out), a fridge (too cold). Tube A is the control.
Exam tipFair test: same kind and number of seeds, same volume of water, same temperature (except in tube D). Use about 10 seeds per tube, because a few may be dead.
Worked example
In an experiment, 18 of 20 seeds germinated in tube A and 3 of 20 in tube D. Calculate the percentage germination in each tube.
- tube A: 18 ÷ 20 × 100 = 90%
- tube D: 3 ÷ 20 × 100 = 15%
- the difference is 90 − 15 = 75 percentage points
- conclusion: low temperature reduced germination
3Human reproduction
The male reproductive system
| Part | Function |
|---|---|
| testes | make sperm and the hormone testosterone |
| scrotum | sac that holds the testes outside the body, where it is a little cooler than the body: better for sperm |
| sperm duct | carries sperm from the testes to the urethra |
| prostate gland | adds fluid to the sperm, making semen |
| urethra | tube through the penis that carries semen (or urine, but never both at once) |
| penis | puts sperm into the vagina during sexual intercourse |
- Extra detail: the epididymis (coiled tube on each testis) stores sperm, and the seminal vesicles also add fluid to the semen.
The female reproductive system
| Part | Function |
|---|---|
| ovary | makes eggs and releases one about every month; also makes the hormones oestrogen and progesterone |
| oviduct (egg tube) | carries the egg to the uterus; fertilisation happens here |
| uterus (womb) | where the embryo implants and the fetus develops; its lining thickens each month |
| cervix | ring of muscle at the bottom of the uterus; it holds the fetus in during pregnancy |
| vagina | receives the penis and sperm; the birth canal |
Common mistake"Fertilisation happens in the uterus." Fertilisation is in the oviduct. The uterus is where the embryo implants and grows.
Gametes: sperm and egg
Gamete — a sex cell (sperm or egg) with half the normal number of chromosomes (haploid).
| Feature | Sperm | Egg |
|---|---|---|
| size | very small | much larger (about 0.1 mm, just visible) |
| movement | swims using its tail | does not move: it is carried along the oviduct |
| numbers | millions made | usually one released a month |
| food store | very little | lots in the cytoplasm |
- Sperm are adapted: a tail to swim, many mitochondria to release energy for swimming, and enzymes in the head (acrosome) that digest the outer layer of the egg.
- The egg is adapted: a large food store for the early embryo, and an outer layer that changes after one sperm enters, so no other sperm can get in.
Memory trickSperm Swim. The Egg is Enormous and Edible (full of food).
Fertilisation and implantation
Fertilisation (animals) — the fusion of the nuclei of a sperm and an egg to form a zygote.
- The zygote divides by mitosis as it is moved down the oviduct, forming a ball of cells: the embryo.
- Implantation: the embryo sinks into the thick uterus lining. This is when pregnancy begins.
- After about 8 weeks the embryo has all its organs and is called a fetus. A pregnancy lasts about 9 months.
The fetus and the placenta
| Structure | Function |
|---|---|
| amniotic sac | a membrane bag around the fetus that holds the amniotic fluid |
| amniotic fluid | cushions the fetus against bumps and shocks, helps keep a steady temperature and lets the fetus move |
| umbilical cord | joins the fetus to the placenta; carries blood vessels to and from the fetus |
| placenta | disc-shaped organ on the uterus wall where substances are exchanged between mother's blood and fetus's blood |
- From mother to fetus: oxygen, glucose, amino acids, vitamins and minerals, water and antibodies. From fetus to mother: carbon dioxide and urea.
Common mistake"The mother's blood flows into the baby." No. Blood stays in separate vessels; only dissolved substances cross the thin barrier.
How the placenta worksEXTENDED
- EXT Adapted for exchange: the placenta has a large surface area (many finger-like villi), thin walls (short diffusion distance) and a rich blood supply that keeps the concentration gradients steep.
- EXT The two bloods do not mix. Substances cross by diffusion and active transport (diffusion).
- EXT Harmful things can cross too: toxins such as alcohol and nicotine, and pathogens such as the rubella and HIV viruses. That is why pregnant women are advised not to smoke or drink alcohol. (diseases)
Puberty
- Puberty is when the body becomes able to reproduce. It usually starts between about 10 and 15 years.
| Hormone | Made in | Changes |
|---|---|---|
| testosterone | the testes | facial and body hair, deeper voice, more muscle, sperm production starts |
| oestrogen | the ovaries | breasts develop, hips widen, body hair, the menstrual cycle starts |
- Both sexes also have a growth spurt, pubic hair and emotional changes.
The menstrual cycle
- Day 1 is the first day of the period. A typical cycle lasts about 28 days (it varies between women).
- Days 1 to 5, menstruation (the period): the lining of the uterus breaks down and leaves through the vagina with some blood.
- Days 5 to 14: the lining repairs and thickens again.
- About day 14, ovulation: an egg is released from an ovary into the oviduct.
- Days 14 to 28: the lining stays thick and spongy, ready for an embryo. If no embryo implants, the lining breaks down and the next period starts. If an embryo implants, the lining is kept and there are no periods.
Worked example
A woman's period starts on 3 March (day 1) and her cycle is 28 days. Find the date of ovulation and the date her next period should start.
- ovulation is on day 14
- day 1 = 3 March, so day 14 = 3 + 13 = 16 March
- next period starts on day 29, which is day 1 of the next cycle
- 3 March + 28 days = 31 March
Common mistake"Ovulation and menstruation are the same." Ovulation = an egg is released (about day 14). Menstruation = the lining breaks down (the period, days 1 to 5).
Hormones of the cycleEXTENDED
| Hormone | Made in | Main roles |
|---|---|---|
| FSH | pituitary gland | makes a follicle (egg) develop in the ovary; stimulates the ovary to make oestrogen |
| LH | pituitary gland | a surge about day 14 causes ovulation |
| oestrogen | ovaries | repairs and thickens the lining; high levels trigger the LH surge and inhibit FSH |
| progesterone | ovaries | keeps the lining thick; if it falls, the lining breaks down (period) |
- EXT If the woman becomes pregnant, progesterone stays high, so the lining is maintained. (hormones)
Memory trickFSH = Follicle-Stimulating: it Fires up the egg. LH = Launches the egg.
4Sexually transmitted infections
HIV and AIDS
Sexually transmitted infection (STI) — an infection that is passed on by sexual contact.
- HIV (human immunodeficiency virus) is the pathogen. It infects and destroys white blood cells, so the immune system weakens (immunity).
- AIDS (acquired immune deficiency syndrome) develops when the immune system is too weak to fight off infections and some cancers.
- How HIV is spread: unprotected sexual intercourse, sharing needles (blood), infected blood in transfusions, and from mother to baby (across the placenta, at birth or in breast milk).
- How it is controlled: condoms, never sharing needles, testing donated blood, anti-viral drugs (these slow the virus but do not cure it) and testing and treating pregnant women.
- Other STIs, such as gonorrhoea and chlamydia, are caused by bacteria and treated with antibiotics. Antibiotics do not work on HIV because it is a virus.
Common mistake"HIV and AIDS are the same." HIV is the virus. AIDS is the condition that can develop later.
Common mistake"You can catch HIV from hugging, sharing food or a toilet seat." No. HIV passes only through blood and body fluids.
5Test yourself
Test yourself — cover the answers first
Quick check
- Define asexual reproduction.
- Define sexual reproduction.
- Give two examples of asexual reproduction.
- What does the anther do?
- Name the female parts of a flower.
- Define pollination.
- Give two features of a wind-pollinated flower.
- State the three conditions needed for germination.
- Where does fertilisation take place in humans?
- What is implantation?
- What is the function of the amniotic fluid?
- What joins the fetus to the placenta?
- Which hormone causes the changes in boys at puberty?
- On about which day of a 28-day cycle does ovulation happen?
- What virus causes AIDS?
- EXT Give one advantage of sexual reproduction.
- EXT What does the pollen tube do?
- EXT Which hormone causes ovulation?
- EXT Which hormone keeps the uterus lining thick?
- EXT Name two harmful things that can cross the placenta.
Exam-style
- Describe the differences between insect-pollinated and wind-pollinated flowers. [4]
- Describe how you would investigate whether oxygen is needed for germination. [4]
- Describe what happens from fertilisation to implantation. [4]
- EXT Explain how the placenta is adapted for exchange. [3]
- EXT Explain why a pregnant woman should not smoke or drink alcohol. [3]
- EXT Describe the roles of FSH and LH in the menstrual cycle. [4]
- Describe how HIV is spread and how its spread can be controlled. [4]
Show answers
- production of genetically identical offspring from one parent
- fusion of the nuclei of two gametes to form a zygote; the offspring are genetically different
- bacteria dividing; potato tubers; strawberry runners
- makes pollen grains
- stigma, style and ovary (the carpel)
- the transfer of pollen grains from the anther to the stigma
- small dull petals; anthers hanging outside; large feathery stigma; light smooth pollen
- water, oxygen, a suitable temperature
- in the oviduct
- the embryo sinks into the lining of the uterus
- cushions and protects the fetus
- the umbilical cord
- testosterone
- day 14
- HIV (human immunodeficiency virus)
- variation, so some offspring may survive a change or a new disease
- carries the male nucleus down the style to the ovule
- LH (luteinising hormone)
- progesterone
- alcohol, nicotine, viruses such as rubella or HIV
- insect: large bright petals; wind: small dull petals
insect: scent and nectar; wind: none
insect: sticky or spiky pollen; wind: light smooth pollen in large amounts
insect: anthers and stigma inside; wind: anthers hang out and the stigma is feathery - put the same number of seeds in each of two tubes
one tube has boiled (cooled) water with a layer of oil, so there is no oxygen
the other (control) tube has moist cotton wool with air
keep the temperature and water the same and count the seeds that germinate after several days - sperm nucleus fuses with egg nucleus in the oviduct
forming a zygote
the zygote divides by mitosis into a ball of cells (embryo)
the embryo moves to the uterus and sinks into the lining - large surface area from villi
thin walls so a short diffusion distance
rich blood supply keeps concentration gradients steep - toxins (nicotine, alcohol) cross the placenta into the fetal blood
they can harm the development of the fetus
smoking can lead to a low birth mass - both are made by the pituitary gland
FSH makes a follicle and egg develop in the ovary
FSH stimulates the ovary to make oestrogen
LH causes ovulation (release of the egg) about day 14 - spread by unprotected sex and sharing needles
spread by infected blood and from mother to baby
control by using condoms
control by not sharing needles, testing donated blood or using anti-viral drugs
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