Introduction
Okay, real talk: every living thing eventually dies, so every species that's still around has cracked one problem — how to make more of itself. There are literally only two ways to do it on the entire planet, and this whole chapter is those two ways plus the gear each one needs.
Here's the cheat code that makes the whole topic click: the number of parents decides how similar the babies are. One parent = identical copies (clones). Two = everyone's a little different. Hold that, and potatoes, bees, flowers, placentas and periods all suddenly make sense. Let's go 🌱
Asexual Reproduction: The Copy-Paste MethodE16.1
Asexual reproduction makes genetically identical offspring from one parent. No mate, no gametes, no fusion — just one organism making copies of itself. Think Ctrl+C, Ctrl+V. Bacteria split in two, potatoes grow new potatoes, strawberries send out runners, and Hydra grow little buds that drop off. All clones, all identical to the parent.
The upside: it's fast and you only need one parent, so you can flood an area with offspring quickly. The downside is the big one examiners love: no variation. If a disease shows up that can kill one, it can kill all of them, because they're all identical. That's why a whole field of cloned crops is a risky bet.
> 🔑 Key idea: asexual = one parent + no gametes → genetically identical clones. Fast, but zero variation means one weakness = everyone's weakness.
Worked Example: The Potato DetectiveE16.1
Problem A potato was planted, a plant grew, and that plant made several new potatoes underground. What type of reproduction is this, and how do you know?
Step 1 Ask the golden question: how many parents? Just one. And did gametes fuse? Nope — the new potatoes grew straight from the plant, not from seeds.
Step 2 One parent + no gametes = asexual reproduction.
Step 3 So the new potatoes are genetically identical to the original. The examiner wants you to name it AND justify it: asexual, because one parent, no gametes, identical clones.
Sexual Reproduction: The ShuffleE16.2
Sexual reproduction is the fusion of the nuclei of two gametes (sex cells) to form a zygote, and the offspring come out genetically different from each other. That fusion event has a name: fertilisation = the fusion of gamete nuclei. Say nuclei, not "the cells join" — that's a mark right there.
Here's the clever bit: gametes are haploid (one set of chromosomes), so when two fuse, the zygote is diploid (two sets) — and the chromosome number stays constant every generation instead of doubling forever. The payoff of all this shuffling is variation, which means if the environment changes, some offspring might just have the right combo of alleles to survive. 🧬
> 🔑 Key idea: fertilisation = gamete nuclei fuse. Haploid + haploid → diploid zygote. Variation is the whole point of sexual reproduction.
Worked Example: Add the ChromosomesE16.2
Problem In a plant, a pollen nucleus with 7 chromosomes fuses with an egg-cell nucleus with 7 chromosomes. How many chromosomes does the zygote have, and what's its ploidy?
Step 1 Each gamete nucleus is haploid — 7 chromosomes each.
Step 2 At fertilisation the nuclei fuse, so you just add them. $$7 + 7 = 14$$
Step 3 The zygote has 14 chromosomes and is diploid. The rule to lock in: haploid + haploid → diploid, and fusion adds the two nuclei together.
Flower Power: The Plant's Reproductive KitE16.3
Plants do sexual reproduction with flowers, and you need to label an insect-pollinated one. Split it into male and female. Male = stamen = anther + filament (the anther makes the pollen, the filament holds it up). Female = carpel = stigma + style + ovary (the sticky stigma catches pollen, the style connects down, and the ovary holds the ovules that become seeds). Plus the sepals (protect the bud) and petals (big and colourful to pull in insects).
Then there's pollination: the transfer of pollen grains from an anther to a stigma. That's the whole definition — anther to stigma. Do not confuse it with fertilisation (that's the nuclei fusing later, down in the ovule). This is the single most-tested trap in the plant topic.
> 🔑 Key idea: stamen (anther + filament) = male; carpel (stigma + style + ovary) = female. Pollination = pollen from anther → stigma. NOT the same as fertilisation.
Worked Example: Name That PartE16.3
Problem (a) Which structure produces pollen grains? (b) What does the stigma do? (c) Where does fertilisation actually happen in a flowering plant?
Step 1 (a) The anther makes the pollen grains — that's the top of the male stamen.
Step 2 (b) The stigma is the sticky surface that receives (catches) pollen grains during pollination.
Step 3 (c) Fertilisation happens in the ovule (inside the ovary), NOT on the stigma. The stigma is just where pollen lands — the male gamete still has to travel down to the ovule. Being precise about location wins the mark here.
Insect vs Wind: Two Delivery ServicesE16.3
Same job (get pollen from anther to stigma), two totally different couriers. Insect-pollinated flowers show off: big coloured scented petals, nectar as a reward, sticky or spiky pollen (only a little needed), and anthers/stigmas tucked inside so the insect brushes past. Wind-pollinated flowers don't bother impressing anyone: dull or no petals, no nectar, masses of tiny smooth light pollen, and anthers and feathery stigmas dangling outside to catch the breeze.
The logic is easy to rebuild: an insect is a precise courier, so the flower attracts and rewards it and sticks a little pollen on it. Wind is random, so the flower makes tonnes of light pollen and hangs its bits in the air. 🌾
> 🔑 Key idea: insect flowers = bright + scented + nectar + little sticky pollen. Wind flowers = dull + no nectar + loads of small light pollen + parts hang outside.
Worked Example: Reading the Flower's CluesE16.3
Problem A flower is pollinated by bees. Give three features that adapt it for insect pollination, and say how each helps.
Step 1 Pick three: large brightly coloured petals; scent/nectar; sticky or spiky pollen.
Step 2 Now the "how it helps" for each — coloured petals attract insects, nectar and scent attract and reward them so they visit, and sticky pollen grabs onto the insect to hitch a ride to the next flower.
Step 3 The examiner wants feature plus how it helps for each one. "Coloured petals" alone is weak; "coloured petals to attract insects" is the full mark.
Self, Cross, and the Pollen TubeE16.3
Two flavours of pollination. Self-pollination = pollen lands on a stigma of the same plant (less variation). Cross-pollination = pollen goes to a different plant of the same species (more variation, because you're mixing two parents' alleles). But pollination only gets pollen onto the stigma — the male gamete still has to reach the ovule. It does that by growing a pollen tube down through the style into the ovary and into the ovule, where the male gamete nucleus fuses with the female gamete nucleus. That fusion is fertilisation.
And the payoff line examiners love: after fertilisation, the ovule becomes the seed and the ovary becomes the fruit. Meanwhile, if you're growing seeds, they only germinate with water, oxygen and warmth. 🌱
> 🔑 Key idea: self = same plant (less variation), cross = different plant (more variation). Pollen tube grows down the style → male nucleus fuses with female nucleus in the ovule = fertilisation. After that: ovule → seed, ovary → fruit.
Worked Example: The Pollen Tube's JourneyE16.3
Problem After a pollen grain lands on the stigma, a long narrow tube grows downwards. Name this tube, say where it grows to, and explain what happens when it arrives.
Step 1 The tube is the pollen tube, and it grows down through the style towards the ovary.
Step 2 It carries the male gamete nucleus down and enters an ovule.
Step 3 When it arrives, the male gamete nucleus fuses with the female gamete nucleus inside the ovule — that's fertilisation, forming a zygote. Keep the order straight: pollination → pollen tube → fertilisation.
The Human Systems: Male + Female BlueprintsE16.4
Two systems to label. Male: testes (make sperm + testosterone), scrotum (holds testes outside, keeping them cooler for sperm), sperm duct (carries sperm), prostate gland (adds fluid to make semen), urethra (carries semen and urine out), penis. Female: ovaries (make eggs + oestrogen/progesterone), oviduct (carries the egg, and where fertilisation happens), uterus (where the fetus grows), cervix (holds the fetus in), vagina.
The gametes are adapted for opposite jobs. Sperm are built to travel: tiny, made in millions, with a tail to swim, loads of mitochondria for energy, and an acrosome to break into the egg. Eggs are built to provide: big, made rarely, packed with food stores, with a membrane that changes the instant one sperm gets in to block the rest.
> 🔑 Key idea: sperm = small, many, tail, mitochondria, acrosome (built to travel). Egg = big, few, food store, membrane locks after one sperm (built to provide).
Worked Example: Built to SwimE16.4
Problem Sperm have adaptive features. Explain how the tail and the many mitochondria each help the sperm do its job.
Step 1 The tail lets the sperm swim towards the egg cell in the oviduct.
Step 2 The many mitochondria carry out respiration to release energy.
Step 3 Here's the link examiners want: that energy powers the tail, so the sperm can keep swimming. Don't just write "mitochondria for energy" — connect it to the movement of the tail.
Pregnancy HQ: The PlacentaE16.4
After fertilisation in the oviduct, the zygote divides into an embryo, implants in the thick uterus lining, and grows into a fetus. It's kept alive by the placenta — the exchange organ where the mother's blood and the fetus's blood swap materials without ever mixing. Oxygen, glucose and nutrients diffuse mother → fetus; carbon dioxide and urea diffuse fetus → mother. The umbilical cord connects them, and the amniotic sac + fluid cushion the fetus like bubble wrap.
One catch: the placenta isn't a perfect wall. Some pathogens (like HIV) and toxins (like nicotine and alcohol) can cross from mother to fetus — which is exactly why pregnant people are told not to smoke or drink. 🚭
> 🔑 Key idea: placenta = exchange without blood mixing. Good stuff (O₂, glucose) goes TO the fetus; waste (CO₂, urea) comes BACK. Some toxins/pathogens sneak across too.
Worked Example: Which Way Does It Go?E16.4
Problem State the net direction of movement of oxygen and of carbon dioxide across the human placenta, and name the process.
Step 1 Oxygen moves by diffusion from mother to fetus. The fetus is respiring and using oxygen, so its blood has the lower oxygen level.
Step 2 Carbon dioxide moves by diffusion from fetus to mother. The fetus makes CO₂, so its blood has more, and the mother clears it through her lungs.
Step 3 Both move by diffusion, down their concentration gradients. Linking direction to "high → low concentration" is what earns full marks.
The Menstrual Cycle: 28 Days of PrepE16.5
At puberty, testosterone (from the testes) and oestrogen (from the ovaries) trigger the secondary sexual characteristics. Then, monthly, the menstrual cycle (about 28 days) preps the uterus. Days 1–5: the lining breaks down (a period). Days 6–13: it rebuilds and thickens. Day 14-ish: ovulation — an egg is released. Days 15–28: the lining is kept thick, waiting.
Four hormones run the show: FSH matures an egg and triggers oestrogen; oestrogen thickens the lining; LH causes ovulation; progesterone keeps the lining thick. The one relationship examiners test to death: progesterone maintains the lining, and when progesterone drops, the lining breaks down → a period. If you're pregnant, progesterone stays high, so no period.
> 🔑 Key idea: FSH matures egg → oestrogen thickens lining → LH triggers ovulation → progesterone maintains lining. Progesterone drops = lining breaks down = period.
Worked Example: The FSH InjectionE16.5
Problem A doctor gives a woman an injection of FSH as part of a fertility treatment. Explain what FSH does and why this could help a woman who isn't producing eggs.
Step 1 FSH causes an egg to mature in the ovary (and makes the ovary produce oestrogen).
Step 2 If her own FSH is too low, no egg matures and she can't ovulate.
Step 3 So an FSH injection stimulates an egg to mature, making ovulation — and therefore fertilisation — possible. The chain: FSH → egg matures → ovulation → pregnancy possible.
STIs: HIV, AIDS, and Staying SafeE16.6
A sexually transmitted infection (STI) is an infection passed on through sexual contact. HIV is a virus (a pathogen) that causes an STI, and it can lead to AIDS — because HIV attacks the lymphocytes (white blood cells) your immune system needs, so eventually you can't fight off other infections. HIV spreads through unprotected sex, infected blood (shared needles, untested transfusions), and mother-to-child (placenta, birth, breast milk). It does not spread by hugging, sharing cups, or toilet seats — you need an exchange of blood, sexual fluids or breast milk.
Because it spreads by set routes, you block those routes: condoms (barrier), not sharing needles, screening donated blood, testing and treating infected people, and education. Name the method AND the route it blocks for full marks. 🛡️
> 🔑 Key idea: HIV = virus, attacks lymphocytes → AIDS. Spread by sexual fluids, infected blood, mother-to-child. Stop it by breaking a route: condoms, no shared needles, blood screening, treatment, education.
Worked Example: Which Actions Spread HIV?E16.6
Problem Three actions involving an HIV-infected person: (1) hugging them, (2) sharing a needle they used, (3) receiving a transfusion of their untested blood. Which could transmit HIV, and why?
Step 1 Look for an exchange of blood, sexual fluids or breast milk.
Step 2 Sharing a needle (2) and an untested blood transfusion (3) both exchange infected blood → both can transmit HIV.
Step 3 Hugging (1) has no exchange of those fluids → it cannot transmit HIV. Answer: 2 and 3 only. It's always about the route.
Key Points to Remember
- Asexual reproduction: one parent, no gamete fusion → genetically identical clones. Fast, but no variation.
- Sexual reproduction: fusion of haploid gamete nuclei → diploid zygote → genetically varied offspring. Fertilisation = nuclei fuse.
- Flowers: stamen (anther + filament) = male; carpel (stigma + style + ovary) = female. Pollination = pollen anther → stigma (not fertilisation).
- Insect flowers: bright, scented, nectar, little sticky pollen. Wind flowers: dull, no nectar, lots of light pollen, parts hang out.
- Male: testes, sperm duct, prostate, urethra, penis. Female: ovaries, oviduct (fertilisation site), uterus, cervix, vagina. Sperm = small/many/tail; egg = big/few/food store.
- Placenta: exchange without blood mixing — O₂ and nutrients to fetus, CO₂ and urea back; some toxins/pathogens cross.
- Menstrual cycle: FSH → oestrogen → LH → progesterone. Progesterone maintains the lining; its drop causes a period.
- HIV is a viral STI that destroys lymphocytes → AIDS. Spread by sexual fluids, infected blood, mother-to-child; controlled by condoms, no shared needles, blood screening, treatment, education.