Every living thing is made of cells

The cell is the fundamental unit of all living organisms — the smallest part that can be called "alive". You look at them using a light microscope.

• Some organisms are unicellular (one cell only) — like bacteria and amoeba. That single cell does everything the organism needs.
• Larger organisms are multicellular — billions of cells working together, with different jobs.

Structure fits function: the whole topic keeps coming back to this idea — a part is shaped so it can do its job well.

Animal and plant cells

Both cell types share four parts. Plant cells then have three extra parts on top:

• Nucleus — controls the cell; holds the genetic instructions (DNA).
• Cytoplasm — jelly where the cell's reactions happen.
• Cell membrane — controls what goes in and out.
• Mitochondria — release energy by respiration.
• Cell wall (plants) — made of cellulose, gives support and shape.
• Vacuole (plants) — holds cell sap and keeps the cell firm.
• Chloroplasts (plants) — contain chlorophyll for photosynthesis. Only cells in green parts have them; root cells do not.

Common mistake: people say plant cells "don't have" the parts animal cells have. Not true — plant cells have everything an animal cell has, plus three extras (wall, vacuole, chloroplasts).

Which parts are plant-only?

Specialised cells

In a multicellular body, cells become specialised — their structure is adapted to a particular job:

• Red blood cell — no nucleus, so it can pack in more haemoglobin and carry more oxygen. Its dimpled disc shape gives a big surface area for oxygen to move in and out.
• Nerve cell (neurone) — very long and thin to carry electrical signals over distances.
• Sperm cell — has a tail to swim to the egg.
• Root hair cell — a long "hair" that gives a big surface area to absorb water.

Structure fits function again: each cell's shape is a clue to the job it does.

Cell part ↔ its job

Tap a cell part on the left, then tap the job it does on the right.

From cells to a whole organism

In a multicellular organism, cells are organised in levels, from small to big:

• A tissue is a group of similar cells that do the same job.
• An organ is made of different tissues working together.
• An organ system is a group of organs working together.

Common mistake: mixing up the order. From smallest to biggest: cell → tissue → organ → organ system → organism.

Diffusion: in and out of cells

Oxygen and carbon dioxide cross the cell membrane by diffusion.

Diffusion is when particles spread from where there are lots of them (a high concentration) to where there are fewer (a low concentration). It happens by itself: the cell does not have to push.

• Oxygen diffuses out of red blood cells and into body cells, where respiration uses it up.
• Carbon dioxide, made by respiration, diffuses out of body cells into the blood.

Common mistake: particles do not "choose" where to go. They move at random, and overall they spread from high to low concentration.

Which way does oxygen go?

Why does a red blood cell have no nucleus?

How do you straighten your arm?

What the skeleton does

Your skeleton is more than a frame. It has four main functions:

• Support — holds the body up and gives it shape.
• Protection — the skull guards the brain; the ribs guard the heart and lungs.
• Movement — bones act as levers that muscles pull on.
• Making blood cells — new blood cells are made inside bone marrow.

Bones meet at joints (like the knee and elbow). Joints let the skeleton bend.

Muscles work in pairs

A muscle can only pull (contract) — it can't push. So muscles are arranged in antagonistic pairs that pull the bone in opposite directions.

Watch out: when one muscle contracts to move the joint, its partner relaxes — that is what "antagonistic" means.

How do you straighten your arm?

How does a sandwich reach your muscles?

A balanced diet

A healthy diet contains the right amounts of seven things, each with a job:

• Carbohydrates — the body's main energy source.
• Lipids (fats & oils) — energy store and insulation.
• Proteins — for growth and repair.
• Vitamins and minerals — small amounts keep the body working (e.g. iron for blood).
• Dietary fibre — keeps food moving through the gut.
• Water — needed for nearly every process.

Imbalance has consequences: too much energy in the diet can lead to obesity; too little food causes starvation; missing a vitamin or mineral causes a deficiency disease.

How much energy do you need?

The energy in food is measured in kilojoules (kJ). A typical 12-year-old needs about 9,000 kJ a day. Older or more active people need more.

Food labels give the energy in 100 g (or 100 ml for drinks). For your portion:

energy = energy per 100 g × mass in g ÷ 100

Bread: 1,000 × 80 ÷ 100 = 800 kJ.

Common mistake: using the label value as it is. It is for 100 g, not for your portion.

Energy in a portion

Testing food (the basics)

In the lab you can test which nutrients a food contains using colour-change tests:

• Starch — add iodine: orange-brown turns blue-black.
• Sugar (glucose) — add Benedict's and heat: blue turns brick-red / orange.
• Protein — add Biuret solution: blue turns purple / lilac.
• Lipid (fat) — the emulsion test: a cloudy white layer forms.

Tip: remember starch → iodine → blue-black. It's the one you'll be asked about most.

Reading a food test

The digestive system

Food is too big to enter your blood, so the digestive system breaks it down. Follow the journey:

The liver makes bile and the pancreas makes enzymes that help digestion happen in the small intestine. There, digested food is absorbed into the blood, which carries it to every cell in the body.

Enzymes and helpful bacteria

Enzymes are biological catalysts — they speed up the breakdown of big food molecules into small ones that can be absorbed. Different enzymes work on different foods (e.g. one for starch, one for protein).

Your gut also contains huge numbers of helpful bacteria. They help digest food, make some vitamins, and crowd out harmful microbes.

Common misconception: enzymes are not alive. They are special protein molecules made by living cells — but the molecule itself is not a living thing and cannot "die". Very high temperatures or the wrong pH denature (change the shape of) an enzyme so it stops working.

What is an enzyme?

Organ ↔ its function

Tap an organ on the left, then tap the job it does on the right.

How does a sandwich reach your muscles?

Where does your body release energy?

The breathing system

Your lungs take in the oxygen your cells need and get rid of carbon dioxide. Air travels down the trachea, branches into bronchi and bronchioles, and ends at tiny air sacs called alveoli.

Breathing in: the diaphragm and rib muscles contract, the chest gets bigger, pressure inside drops, and air rushes in. Breathing out is the reverse.

A model of breathing: the bell jar

A model shows how breathing moves air. The jar is the chest, the balloons are the lungs, the tube is the trachea and bronchi, and the rubber sheet is the diaphragm.

Pull the sheet down: the space inside the jar gets bigger, so the pressure drops. Air rushes in and the balloons inflate. Push it up: the space gets smaller, the pressure rises, and air is pushed out. Air always moves from higher to lower pressure.

Limits of the model: in a real chest, the ribs also move up and out. The jar's walls cannot move.

Pulling the sheet

Measuring lung volume

How much air can your lungs push out in one breath? You can measure it by displacing water.

1. Fill a bottle marked in litres with water. Stand it upside down in a trough of water.
2. Take a deep breath, then breathe out fully through the tube.
3. Your air pushes water out of the bottle. Read the volume of air on the scale.

Repeat three times and work out the mean.

Stay safe: use a clean mouthpiece. Only breathe out through the tube, never in.

Breathing is not respiration

These two words are easy to mix up, but they mean different things:

• Breathing — moving air in and out of the lungs (a physical action of muscles).
• Respiration — the chemical reaction inside every cell that releases energy from glucose, using oxygen.

Common misconception: "breathing = respiration" is wrong. Breathing just supplies the oxygen that respiration uses inside the cells (in the mitochondria). Smoking damages the alveoli, and asthma narrows the airways — both make it harder to get oxygen into the blood.

Explain the difference

Diet, exercise, drugs and smoking

Your lifestyle choices affect how well your body systems work:

• Balanced diet + exercise keep the heart, muscles and skeleton healthy and help control body mass.
• Smoking damages the alveoli and reduces how much oxygen the blood can carry.
• Recreational drugs and alcohol can change behaviour and damage organs; some are addictive.
• During pregnancy, harmful substances can pass through the placenta (where food and oxygen pass from mother to baby) and harm the foetus.

Big picture: scientists study the links between diet, exercise, drugs and health to give advice — this is real, evidence-based science.

Smoking and the lungs

Where does your body release energy?

Exercise and asthma

Exercise: your muscles need more oxygen, so you breathe faster and deeper. More carbon dioxide is removed too. With regular exercise you get fitter: the same activity leaves you less out of breath.

Asthma: a trigger, such as dust, pollen or smoke, makes the airways narrow. The muscle around them tightens, the lining swells and extra mucus is made.

With narrow airways, less air reaches the alveoli. The person may cough, wheeze and feel short of breath. A reliever inhaler relaxes the muscle and opens the airways.

How leaves exchange gases

Plants exchange gases through tiny pores in their leaves called stomata (one is a stoma). Most are on the lower surface. Two guard cells around each stoma open and close it.

By day, in the light:

• carbon dioxide diffuses in, for photosynthesis;
• oxygen diffuses out;
• water vapour escapes too.

Common mistake: thinking plants do not respire. They respire all the time. In bright light, photosynthesis uses carbon dioxide faster than respiration makes it, so overall carbon dioxide goes in.

How does pollen get from flower to flower?

Human reproduction

Humans reproduce sexually. Special sex cells called gametes join together:

• The male gamete is the sperm (made in the testes).
• The female gamete is the egg / ovum (made in the ovaries).
• Fertilisation is when a sperm joins an egg. The fertilised egg grows into an embryo.

The menstrual cycle is a roughly monthly cycle in females: the lining of the uterus builds up ready for a fertilised egg. If none arrives, the lining breaks down (a period) and the cycle starts again.

Gestation is pregnancy — in humans it lasts about 9 months. The developing baby gets food and oxygen from the mother through the placenta, which is why the mother's lifestyle (e.g. smoking or alcohol) can affect the foetus.

Gametes and fertilisation

The reproductive systems

Female: about once a month, an ovary releases an egg. It moves along an oviduct towards the uterus (womb).

Male: the testes make sperm. Sperm travel along the sperm ducts. Glands add a fluid, making semen, which leaves through the urethra in the penis.

Sperm released into the vagina swim through the cervix and uterus to the oviducts. If a sperm meets an egg there, fertilisation can happen.

The menstrual cycle

From puberty, the menstrual cycle repeats about every 28 days. The length varies from person to person.

• The uterus lining builds up, ready for a fertilised egg.
• About halfway through, an ovary releases an egg: ovulation.
• If the egg is not fertilised, the lining breaks down and leaves through the vagina: a period. Then the cycle starts again.

Gestation, the placenta and birth

The embryo settles into the thick uterus lining and grows into a foetus. At the placenta, food and oxygen pass from the mother's blood to the foetus. Carbon dioxide and waste pass back.

The mother's lifestyle affects the foetus:

• Smoking: less oxygen reaches it, so it is more likely to be born early and small.
• Alcohol can damage its developing brain.
• Drugs: some cross the placenta; a baby can be born dependent on them.
• Diet: a balanced diet gives it what it needs to grow.

Birth: after about 9 months, the uterus muscles contract, the cervix widens, and the baby is pushed out through the vagina.

Smoking in pregnancy

Plant reproduction and pollination

Flowers are the reproductive organs of a plant. Their key parts:

• Stamen (anther + filament) — the male part; the anther makes pollen.
• Carpel (stigma, style, ovary) — the female part; the ovary holds the ovules.

Pollination is the transfer of pollen from an anther to a stigma. It can be carried by insects (bright petals, nectar, sticky pollen) or by the wind (small dull flowers, lots of light pollen). After pollination, fertilisation forms seeds and fruits, which are then dispersed.

Why it matters: insect pollination of crops is vital for human food security — many of the foods we eat rely on pollinators like bees.

Seed dispersal

A plant can't move from place to place, so its seeds need help to get away from the parent plant. This is seed dispersal. Far away, a seedling does not compete with its parent for light and water.

• Wind: light seeds with wings (sycamore) or parachutes (dandelion).
• Animals: seeds in eaten fruits pass out in droppings; hooked fruits catch on fur.
• Bursting pods: some pods dry out and split open, flinging the seeds out.

Investigate it: time how long paper sycamore models with different wing sizes take to fall.

Common mistake: pollination is not dispersal. Pollination moves pollen to a stigma; dispersal moves seeds away from the plant.

How does pollen get from flower to flower?

The big ideas to remember

You've covered the key ideas in the KS3 topic — Structure and function of living organisms. Press Finish to see your score.