KS3 Biology topics

Cells & microscopy

Plant and animal cells; using a microscope well.

Tissues → organs → systems

How cells team up into working bodies.

Builds on: Cells & microscopy — “Organs are teams of cells — meet the players first.” · Specialised cells

Animal cell structure

The parts every animal cell shares — nucleus, cytoplasm, membrane, mitochondria.

Builds on: Cells & microscopy — “You meet the cell's parts by first looking down the microscope.”

Plant cell extras

The three parts a plant cell adds: cell wall, chloroplast, vacuole.

Builds on: Animal cell structure — “A plant cell is the animal plan plus a few extras — learn the plan first.”

Specialised cells

How a cell's shape follows its job — a red blood cell, a nerve cell, a root hair.

Builds on: Animal cell structure — “A specialised cell is the basic cell, reshaped for a job.”

Diffusion

Particles spread from where they are crowded to where they are not.

Builds on: Animal cell structure

Photosynthesis — the word equation

Plants make food from carbon dioxide and water using light.

Builds on: Plant cell extras — “Photosynthesis happens in the chloroplast you found in the plant cell.”

Magnification & scale

Work out how much bigger the image is than the real thing.

Builds on: Cells & microscopy — “Scale only means something once you've used the microscope.”

The human skeleton

A frame that supports, protects and lets us move.

Antagonistic muscles

Muscles work in pairs — one pulls while the other relaxes.

Builds on: The human skeleton — “Muscles pull on the skeleton — you need the frame first.”

A balanced diet

The right nutrients in the right amounts for a healthy body.

Builds on: Nutrients & their jobs — “A balanced diet balances the nutrients you've named.”

Nutrients & their jobs

Carbohydrate, protein, fat, vitamins, minerals, fibre, water — what each is for.

Microbes: friend and foe

Not all microbes harm — some make bread, yoghurt and medicine.

Builds on: Cells & microscopy — “You can't study microbes without the microscope that reveals them.”

Puberty & adolescence

The physical and emotional changes that ready the body for reproduction.

Builds on: The human skeleton — “Puberty reshapes the growing body you've mapped.”

Flower structure

The parts of a flower and the job each does in reproduction.

Food chains

Who eats whom — and which way the energy flows.

Builds on: Habitats & adaptation — “A food chain links the organisms sharing a habitat.”

Habitats & adaptation

Where an organism lives, and the features that fit it there.

Life cycles

The loop from seed or egg to adult and back again.

Builds on: Flower structure

Identification keys

Sort living things by answering yes/no questions.

Builds on: Habitats & adaptation — “A key sorts the organisms you find in a habitat.”

Digestion & nutrition

Your first full system story, from plate to cell.

Builds on: Tissues → organs → systems — “Digestion is your first full system story.”

Respiration & breathing

Releasing energy in cells; moving gases in and out.

Builds on: Tissues → organs → systems — “Gas exchange needs the organ map.” · The gas exchange system · Chemical reactions & equations (CHEM)

Reproduction

Gametes, fertilisation and development.

Builds on: Cells & microscopy · Gametes & fertilisation — “Reproduction is the gamete story, told in full.”

Ecosystems & food webs

Interdependence: who eats whom, and what breaks.

Builds on: Food chains — “A food web is many food chains, interlinked.”

The leaf as an organ

How a leaf's layers are built for catching light and swapping gases.

Builds on: Photosynthesis — the word equation — “The leaf's design only makes sense once you know what it's for.” · Tissues → organs → systems — “A leaf is an organ — see it as tissues with a job.”

Limiting factors for photosynthesis

Light, carbon dioxide and warmth each cap how fast a plant makes food.

Builds on: Photosynthesis — the word equation — “You can only limit a process you can already describe.”

Plant mineral nutrition

Why plants still need nitrates and magnesium from the soil.

Builds on: Photosynthesis — the word equation · The periodic table (CHEM)

Transport in plants

Xylem carries water up; phloem carries food around.

Builds on: The leaf as an organ — “Transport tissues are part of the leaf-and-stem plan.” · Osmosis

Joints as levers

Where bones meet and turn — the body's hinges and pivots.

Builds on: The human skeleton — “A joint is where two bones meet and move.” · Antagonistic muscles

Enzymes in digestion

Biological tools that break big food molecules into small ones.

Builds on: Digestion & nutrition — “Enzymes are the tools digestion uses — meet digestion first.”

Absorption in the gut

How digested food crosses the gut wall into the blood.

Builds on: Diffusion · Enzymes in digestion — “Only broken-down food is small enough to absorb.”

The gas exchange system

Lungs built to swap oxygen for carbon dioxide.

Builds on: Tissues → organs → systems — “The lungs are an organ system — you need the organisation idea.”

Aerobic respiration

Glucose plus oxygen releases energy — the word equation.

Builds on: Photosynthesis — the word equation · Respiration & breathing — “Aerobic respiration is the equation behind 'releasing energy'.” · Chemical reactions & equations (CHEM)

The circulatory system

Heart and vessels moving blood on a double loop. Read the full explainer →

Builds on: Tissues → organs → systems — “The heart and vessels are an organ system too.”

The breathing mechanism

How ribs and the diaphragm pump air in and out.

Builds on: The gas exchange system — “Breathing is the mechanism that ventilates the gas-exchange surface.”

Testing a leaf for starch

Use iodine to show photosynthesis really made food.

Builds on: Photosynthesis — the word equation — “Testing for starch checks that photosynthesis really happened.”

Deficiency diseases

What goes wrong when a nutrient is missing — scurvy, rickets, anaemia. Read the full explainer →

Builds on: A balanced diet — “A deficiency is a diet missing one balanced part.” · Nutrients & their jobs

Effects of smoking

How tobacco smoke damages the gas-exchange surface and more.

Builds on: The gas exchange system — “Smoking damages the gas-exchange surface first.”

Effects of alcohol

How alcohol affects the brain, liver and behaviour.

Builds on: Digestion & nutrition

Unicellular organisms

Life that is a single cell — bacteria, yeast, amoeba.

Builds on: Cells & microscopy — “A unicellular organism IS one cell — start there.”

Human reproductive systems

The organs that make and deliver gametes.

Builds on: Tissues → organs → systems — “The reproductive system is an organ system.” · Puberty & adolescence

Gametes & fertilisation

Sperm meets egg — two half-sets of instructions become one.

Builds on: Specialised cells · Human reproductive systems — “Gametes are made by the reproductive organs.”

Pollination

Moving pollen from one flower to another — by wind or animal.

Builds on: Flower structure — “Pollination moves pollen between the flower parts you named.”

Plant fertilisation & seeds

How a pollinated flower becomes a seed and fruit.

Builds on: Pollination — “Fertilisation follows once pollen arrives.”

Seed dispersal

Why plants send seeds away — and the clever ways they travel.

Builds on: Plant fertilisation & seeds — “Seeds must form before they can be dispersed.”

Energy transfer & pyramids

Why there is less energy at each step up a food chain.

Builds on: Ecosystems & food webs — “Energy pyramids quantify the web's flow.”

Producers & photosynthesis

Plants capture the sun's energy that every food chain then spends.

Builds on: Photosynthesis — the word equation — “Producers are the plants doing photosynthesis.” · Food chains

Decomposers & recycling

The microbes and fungi that return nutrients to the soil.

Builds on: Ecosystems & food webs — “Decomposers close the loop the food web leaves open.”

Sampling with quadrats

Estimate a population by counting inside random squares.

Builds on: Habitats & adaptation — “Quadrats sample the organisms in a habitat.” · Averages & range (MATH)

Osmosis

Diffusion's special case: water crossing a membrane towards the crowded side.

Builds on: Diffusion — “Osmosis is diffusion's special case — water only.”

Heart rate & exercise

Measure how the heart answers a bigger demand for energy.

Builds on: Aerobic respiration · The circulatory system — “Heart rate is the circulatory system, measured.”

Energy balance & health

Taking in more or less energy than you use, and what follows.

Builds on: Aerobic respiration · A balanced diet — “Energy balance is the diet idea put on a scale.”

Root hair cells

A cell stretched into a hair so a root can drink more water by osmosis.

Builds on: Specialised cells · Osmosis — “A root hair drinks by osmosis — that's its whole trick.”

Inheritance & variation

Why offspring resemble — and differ from — parents.

Builds on: Reproduction — “Inheritance starts where reproduction ends.” · DNA, genes & chromosomes — “Inheritance is genes passed on — meet genes first.” · Variation

Anaerobic respiration

Releasing energy without oxygen — and why it can't last.

Builds on: Aerobic respiration — “Anaerobic is what happens when the aerobic route runs short of oxygen.”

Recreational drugs

How drugs change the nervous system, and the risks that follow.

Exercise & fitness

How training helps the heart, muscles and lungs meet demand.

Builds on: Aerobic respiration — “Fitness is your body meeting a bigger respiration demand.” · Heart rate & exercise

The menstrual cycle

The monthly cycle that prepares the body for pregnancy. Read the full explainer →

Builds on: Human reproductive systems — “The cycle is the reproductive system on a monthly clock.”

Gestation & the placenta

How a fetus is fed and protected before birth.

Builds on: Gametes & fertilisation — “Gestation begins the moment gametes join.” · The menstrual cycle

DNA, genes & chromosomes

The instructions inside the nucleus, and how they nest together.

Builds on: Gametes & fertilisation · Animal cell structure — “Genes live in the nucleus you found in the cell.”

Variation

Why no two living things are exactly alike.

Builds on: DNA, genes & chromosomes — “Variation comes from the genes you've just met.”

Predator–prey cycles

How hunter and hunted numbers rise and fall together.

Builds on: Ecosystems & food webs — “Predator–prey cycles are a food web over time.”

Competition & interdependence

Living things compete for what a habitat can only partly supply.

Builds on: Ecosystems & food webs — “Competition is interdependence with limited resources.” · Habitats & adaptation

The carbon cycle

Carbon moving through air, plants, animals and fuels.

Builds on: Aerobic respiration · Producers & photosynthesis — “The carbon cycle runs through photosynthesis.” · Chemical reactions & equations (CHEM)

Bioaccumulation

Why toxins build up in the animals at the top of a chain.

Builds on: Energy transfer & pyramids — “Toxins concentrate up the pyramid you drew.”

Human impact on ecosystems

How pollution, land use and hunting reshape food webs.

Builds on: The carbon cycle · Ecosystems & food webs — “Human impact ripples through the food web.”

Natural selection

Better-suited individuals survive and pass on their traits.

Builds on: Variation — “Selection acts on the variation you've described.” · Inheritance & variation

Extinction

When a species dies out — the causes and the finality.

Builds on: Predator–prey cycles · Natural selection — “Extinction is selection with no survivors.”

Biodiversity

The variety of life, and why it makes ecosystems resilient.

Builds on: Ecosystems & food webs — “Biodiversity counts the variety a web contains.”

Darwin's theory

How Darwin explained the fit between living things and their world.

Builds on: Variation · Natural selection — “Darwin's theory is natural selection, named.”

Continuous & discontinuous variation

Some traits form a smooth range; others fall into groups.

Builds on: Variation — “Continuous vs discontinuous sorts the variation you've described.”

Conservation

Protecting habitats and species from the harm we cause.

Builds on: Human impact on ecosystems — “Conservation answers the human impact you've measured.” · Biodiversity

The nitrogen cycle

How nitrogen passes from air to soil to living things and back, via microbes. Read the full explainer →

Builds on: Decomposers & recycling — “The nitrogen cycle runs on decomposer microbes.”