Key Stage 3 science · Biology – Structure and function of living organisms

KS3 diet and digestion: nutrients, enzymes and the gut

In Key Stage 3 pupils study what the body needs from food and what happens to it after it is eaten. They learn the seven components of a healthy diet and why each is needed, work out the energy in food, find out what happens when a diet is unbalanced, follow food through the digestive system, and see how plants make their own food. The topic links diet, chemistry and cells, and sets up GCSE biology.

Diet and digestion, explained

A healthy diet contains seven components. Carbohydrates are the main source of energy. Lipids (fats and oils) store energy, insulate the body and are part of cell membranes. Proteins are used for growth and repair. Vitamins and minerals are needed in small amounts: vitamin C helps keep skin and gums healthy, vitamin D helps the body use calcium for bones, iron is needed for the red pigment in blood, and calcium builds bones and teeth. Dietary fibre cannot be digested by our own enzymes but adds bulk that keeps food moving through the gut and helps prevent constipation, and water is needed for almost every process in the body.

Energy needs vary with age, body size, sex and how active a person is, so there is no single correct figure. Pupils practise calculating energy from food labels, which give energy in kilojoules (kJ) and often in kilocalories (kcal), where 1 kcal is about 4.2 kJ, and comparing it with an energy requirement they are given. Some schools measure the energy in a food by burning a small sample to heat water, in a carefully supervised demonstration. An unbalanced diet has consequences. Too much energy over a long time can lead to obesity, which raises the risk of conditions such as heart disease and type 2 diabetes. Far too little food over a long time leads to starvation, with weight loss, weakness and a lowered ability to fight infection. A shortage of a single nutrient causes a deficiency disease: scurvy from too little vitamin C, rickets from too little vitamin D or calcium, and anaemia from too little iron.

Digestion breaks large food molecules into small soluble ones that can pass into the blood. Food is chewed in the mouth, where saliva begins breaking down starch, and is moved down the oesophagus (gullet) by waves of muscle contraction called peristalsis. In the stomach, muscles churn the food, an enzyme starts digesting protein, and acid kills many bacteria. The liver makes bile, which is stored in the gall bladder and breaks fats into small droplets so that enzymes can work on them faster. The pancreas makes several enzymes and releases them into the small intestine, whose own lining adds more.Digestion finishes in the small intestine, which is also where most absorption happens. It is adapted for this: it is long, its lining is covered in millions of tiny finger-like villi that give a very large surface area, and each villus has a thin wall and a rich blood supply. The large intestine absorbs water, and the remains leave as faeces.

Enzymes are biological catalysts: proteins that speed up chemical reactions without being used up. Different enzymes do different jobs, for example amylase breaks down starch to sugars, proteases break down proteins to amino acids, and lipases break down fats to fatty acids and glycerol. Huge numbers of bacteria live in the gut, mostly in the large intestine, and many are helpful. They help break down some fibre and make some vitamins, and they crowd out harmful microbes. Finally, pupils see that plants make their own food. In their leaves they make carbohydrate (glucose) by photosynthesis, using carbon dioxide, water and light energy absorbed by chlorophyll, and they release oxygen. They take in water and mineral nutrients, such as nitrates, from the soil through their roots, mostly through root hair cells.

Words your child will hear

Carbohydrate: A nutrient, such as starch or sugar, that provides the body with most of its energy.
Lipid: The scientific word for fats and oils, which store energy and insulate the body.
Deficiency disease: An illness caused by too little of a nutrient, such as scurvy (vitamin C).
Enzyme: A biological catalyst that speeds up a reaction, such as breaking down food, without being used up.
Peristalsis: Waves of muscle contraction that push food along the gut.
Absorption: Digested food passing through the gut wall into the blood.
Photosynthesis: The process by which plants use light energy to make glucose from carbon dioxide and water.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about the content of a healthy human diet: carbohydrates, lipids (fats and oils), proteins, vitamins, minerals, dietary fibre and water, and why each is needed
Pupils should be taught about calculations of energy requirements in a healthy daily diet
Pupils should be taught about the consequences of imbalances in the diet, including obesity, starvation and deficiency diseases
Pupils should be taught about the tissues and organs of the human digestive system, including adaptations to function and how the digestive system digests food (enzymes simply as biological catalysts)
Pupils should be taught about the importance of bacteria in the human digestive system
Pupils should be taught about plants making carbohydrates in their leaves by photosynthesis and gaining mineral nutrients and water from the soil via their roots

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: working out the energy in a breakfast

The labels say that a breakfast cereal contains 1,550 kJ of energy per 100 g, and the milk contains 200 kJ per 100 ml. A pupil eats 40 g of cereal with 200 ml of milk. The pupil’s energy requirement for the day is 9,000 kJ. What proportion of the day’s energy does this breakfast provide?

Find the energy in the cereal. The label is per 100 g, and the portion is 40 g, which is 0.4 of 100 g. So 1,550 × 0.4 = 620 kJ.
```
1,550 kJ per 100 g
40 g is 0.4 × 100 g
1,550 × 0.4 = 620 kJ
```
Find the energy in the milk. The portion is 200 ml, which is 2 × 100 ml. So 200 × 2 = 400 kJ.
Add them together: 620 + 400 = 1,020 kJ for the whole breakfast.
Compare with the daily requirement. 1,020 ÷ 9,000 = 0.1133, which is about 11%.
```
1,020 ÷ 9,000 = 0.1133
0.1133 × 100 = 11.3%
```
Sense check. One tenth of 9,000 kJ is 900 kJ, and 1,020 kJ is a little over that, so about 11% is reasonable. Only a small part of the day’s energy has come from this breakfast, so the rest comes from the other meals and snacks, which should also supply protein, vitamins and minerals.

The breakfast provides 1,020 kJ, about 11% of the 9,000 kJ the pupil needs in a day.

Common mistakes and how to help

Thinking all digestion, and all absorption, happens in the stomach

The stomach is the best known part of the gut, and “my tummy” is where people feel food.

What helps: Trace food from mouth to anus on a drawing. Show that digestion starts in the mouth and finishes in the small intestine, where most nutrients are absorbed.

Thinking enzymes are living things or that they get used up

Enzymes seem to “eat” food and the word sounds like an organism, so pupils expect them to be consumed.

What helps: Describe an enzyme as a molecule, a protein, that helps a reaction happen and then is free to help again. One enzyme can break down a lot of food.

Saying plants take in food through their roots

We feed plants with “plant food” and watering, and soil looks like the plant’s dinner.

What helps: Plants make their own food, glucose, in the leaves. From the soil they take water and mineral nutrients such as nitrates, which they need to build other materials, but these are not their source of energy.

Believing a “healthy diet” is the same for everyone, or that fats and fibre are simply bad or useless

Headlines and labels often sort foods into good and bad, and fibre, which we do not digest, looks like waste.

What helps: Talk about balance and proportion. Fats are needed, and fibre keeps the gut working, but amounts and types matter. Energy needs differ by age, size and activity.

How to help at home

Read a label together. Pick two foods and compare the energy per 100 g and per portion. Ask your child to work out the energy in the portion actually eaten, as in the worked example. Keep it factual and curious, rather than about counting or judging what people eat.
Spot the seven. Look at a meal and name which food provides which nutrient group: carbohydrate, lipid, protein, vitamins, minerals, fibre and water. Which groups would be short if a meal was only chips, and what could be added?
Watch an enzyme at work. Chew a plain cracker or a piece of bread slowly, without swallowing, for a minute. It begins to taste slightly sweet as amylase in saliva breaks starch down into sugars. This is a good starting point for talking about digestion.
Trace a food through the gut. Draw an outline body and ask your child to trace a mouthful of pasta from the mouth to the end of the gut, labelling what happens at each stage. Check the order of the organs together.
Plants in the kitchen. Compare a pot of cress or herbs grown on a sunny windowsill with one kept in the dark. Ask what the plant needs to make its food.

Builds on

Year 6: Classifying living things

More on biology – Structure and function of living organisms

Key Stage 3: Cells and organisation
Key Stage 3: Breathing, gas exchange and drugs
Key Stage 3: Reproduction in humans and plants
Key Stage 3: Skeleton and muscles

Everything in Key Stage 3 science

Updated 20 October 2026.