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 offspring are. One parent = identical copies. 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 MethodC16.1
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.
> 🔑 Key idea: asexual = one parent + no gametes → genetically identical clones.
Worked Example: The Potato Family TreeC16.1
Problem A potato tuber grew from the stem of a parent potato plant, and three shoots are now growing from that tuber. How do the genotypes of the shoots compare with the genotype of the tuber and with the genotype of the parent plant?
Step 1 The parent made the tuber by asexual reproduction, so the tuber is genetically identical to the parent.
Step 2 The tuber made the shoots by asexual reproduction too, so the shoots are genetically identical to the tuber.
Step 3 Identical-to-identical is still identical, so the shoots are also genetically identical to the original parent. Every step of asexual reproduction is a straight copy — there's no mixing anywhere in the chain.
Sexual Reproduction and Fertilisation: The ShuffleC16.2
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.
> 🔑 Key idea: fertilisation = gamete nuclei fuse, giving genetically different offspring.
Worked Example: Naming FertilisationC16.2
Problem What is meant by the term fertilisation?
Step 1 Recall what a gamete is: a sex cell, like a sperm or an egg.
Step 2 Fertilisation is when two gametes come together.
Step 3 More precisely, it's the fusion of the nuclei of the two gametes. Write "nuclei," not "cells" or "gametes join" — that precision is the mark.
Flower Power: The Plant's Reproductive KitC16.3
Plants do sexual reproduction with flowers, and you need to label an insect-pollinated one. 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).
> 🔑 Key idea: stamen (anther + filament) = male; carpel (stigma + style + ovary) = female. Pollination = pollen anther → stigma. NOT fertilisation.
Worked Example: Pollen's Pit StopC16.3
Problem A flower has an anther where pollen is made, a stigma where pollen lands, a style and an ovary containing ovules. After pollination has taken place, where in the flower would pollen grains be found?
Step 1 Pollen starts out in the anther, where it's made — some grains are still there.
Step 2 Pollination moves some of that pollen onto the stigma.
Step 3 It hasn't gone any further yet, so the answer is: in the anther, and on the stigma. Nothing has reached the style or ovary at this stage — that's the next chapter of the story.
Insect vs Wind: Two Delivery ServicesC16.3
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.
> 🔑 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: Wind or Bugs?C16.3
Problem Which feature is an adaptation of a wind-pollinated flower?
Step 1 Ask what job the feature does — attract a visitor, or catch a breeze?
Step 2 Anthers hanging outside on long filaments let the wind shake pollen loose easily.
Step 3 So that's the answer. Sticky stigmas held inside, big scented petals, and nectar are all insect-pollination features — a wind-pollinated flower has zero use for any of them.
Self, Cross, and GerminationC16.3
Two flavours of pollination. Self-pollination = pollen lands on a stigma of the same plant. Cross-pollination = pollen goes to a different plant of the same species. 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's fertilisation.
After fertilisation: the ovule becomes the seed and the ovary becomes the fruit. And if you're growing seeds, they only germinate with water, oxygen and warmth. 🌱
> 🔑 Key idea: self = same plant, cross = different plant. Pollen tube → male nucleus fuses with female nucleus in the ovule = fertilisation. Ovule → seed, ovary → fruit. Seeds need water + oxygen + warmth.
Worked Example: The Germination LineupC16.3
Problem Four test-tubes of seeds are set up with different conditions and placed in the dark. Tube 1 has seeds on moist cotton wool at 25 °C with air present. Tube 2 has seeds on moist cotton wool at 0 °C. Tube 3 has seeds on dry cotton wool at 25 °C. Tube 4 has seeds in water with the oxygen removed, covered by oil, at 25 °C. In which tube will the seeds germinate first?
Step 1 Germination needs three things: water, oxygen and a suitable warm temperature.
Step 2 Check each tube: tube 2 is too cold, tube 3 has no water, tube 4 has no oxygen.
Step 3 Only tube 1 has all three, so its seeds germinate first. Run every condition against the checklist rather than guessing from one missing factor.
The Human Systems and GametesC16.4
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), 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 and an acrosome to break into the egg. Eggs are built to provide: big, made rarely, packed with food stores, with a jelly coat that changes the instant one sperm gets in to block the rest.
> 🔑 Key idea: sperm = small, many, tail (built to travel). Egg = big, few, food store, jelly coat locks after one sperm (built to provide).
Worked Example: Which Two Facts About Eggs?C16.4
Problem Egg cells are adapted for their function in reproduction. Which option gives two correct adaptive features of an egg cell?
Step 1 Think about what an egg cell needs to do: feed the early embryo, and stop extra sperm getting in.
Step 2 It contains energy stores in its cytoplasm, and it has a jelly coat that changes after fertilisation.
Step 3 A tail and lots of mitochondria for movement belong to a sperm cell, not an egg — that's the swap this question is testing for.
Pregnancy HQ: The PlacentaC16.4
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 — a disc attached to the uterus wall where the mother's blood and the fetus's blood swap materials without ever mixing. Useful stuff (food, oxygen) goes mother → fetus; waste goes fetus → mother. The umbilical cord connects the fetus to the placenta, and the amniotic sac + fluid cushion the fetus like bubble wrap.
One catch: the placenta isn't a perfect wall. Some toxins (like nicotine and alcohol) can cross from mother to fetus — exactly why pregnant people are told not to smoke or drink. 🚭
> 🔑 Key idea: placenta = exchange without blood mixing. Good stuff goes TO the fetus, waste comes BACK. Some toxins sneak across too.
Worked Example: What's the Cord's Job?C16.4
Problem The umbilical cord contains blood vessels but no muscles for pumping and no air tubes. Using this information, which is the best description of what the umbilical cord does?
Step 1 No muscles means it can't pump blood itself — that's the fetus's own heart's job.
Step 2 No air tubes means it isn't carrying air from anywhere.
Step 3 Blood vessels only, so its job is carrying the fetus's blood to and from the placenta, where the actual exchange with the mother's blood happens.
Puberty and the Menstrual Cycle: 28 Days of PrepC16.5
At puberty, testosterone (from the testes) and oestrogen (from the ovaries) trigger the secondary sexual characteristics — deeper voice and facial hair for testosterone, breast growth and wider hips for oestrogen, with "sexual organs grow" shared by both. 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.
Two hormones run the show here: oestrogen thickens the lining, and progesterone keeps it thick. The one relationship examiners test to death: progesterone maintains the lining, and when progesterone drops, the lining breaks down → a period.
> 🔑 Key idea: oestrogen thickens the lining, progesterone maintains it. Progesterone drops = lining breaks down = period.
Worked Example: Reading the Cycle GraphC16.5
Problem The diagram shows how the thickness of the uterus lining changes during one 28-day menstrual cycle. On which days is the lining being lost during menstruation?
Step 1 Menstruation is when the thick lining breaks down and is lost, so the lining is at its thinnest.
Step 2 On the graph, that's the dip right at the start of the cycle.
Step 3 So the answer is days 1 to 5 — the lining then rebuilds from about day 6 onward.
STIs: HIV, AIDS, and Staying SafeC16.6
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 plates, or sitting nearby — 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: Is This Action Risky?C16.6
Problem A person is infected with HIV. Which everyday action carries a real risk of passing HIV to another person?
Step 1 Check for an exchange of blood, sexual fluids or breast milk.
Step 2 Sharing a needle with the infected person exchanges blood directly — that's a real risk.
Step 3 Hugging, sharing a plate, or sitting nearby involve no exchange of those fluids, so they're safe. It always comes back to the route.
Key Points to Remember
- Asexual reproduction: one parent, no gamete fusion → genetically identical clones.
- Sexual reproduction: fusion of gamete nuclei → genetically different 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.
- Self-pollination = same plant. Cross-pollination = different plant. Pollen tube carries the male nucleus to the ovule = fertilisation. Ovule → seed, ovary → fruit. Germination needs water, oxygen, warmth.
- 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 — useful stuff to the fetus, waste back; some toxins cross too.
- Testosterone and oestrogen drive puberty. Oestrogen thickens the uterus lining; progesterone maintains it; 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.