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.

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 and a dimpled disc shape, so it can pack in more haemoglobin and carry more oxygen.
• 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.

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: both muscles never contract at once. When one pulls, its partner relaxes — that is what "antagonistic" means.

Bending your arm

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.

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.

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.

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.

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

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

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.

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 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

The big ideas to remember

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