KS3 · Biology

Food as a source of building materials and energy

Two bowls of cereal, and one has less energy per 100 g. Which bowl gives you more energy? The answer is not the obvious one.

Build me, fuel me, or both?

Which job does each nutrient do?

Food has two jobs: it is a building material for growth and repair, and it is where your energy mainly comes from. Some nutrients do one job, some do both. For each nutrient, tick the set (or both sets), then check.

  • A Building materials for growth and repair
  • B Main source of energy
  1. Carbohydrates
  2. Fats (lipids)
  3. Proteins
  4. Vitamins
  5. Minerals

From your dinner to your muscles

?

Reason it through

Why does eating food give you the energy to move?

Link 1 of 4

First link · your turn

Food is broken down in the digestive system. What does that give your cells?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Biomass

What is biomass, and how do you measure it?

Your class wants to find the biomass of a small plant. Three students have three different plans.

Which plan is closest to how biomass is really found?
How sure are you?

Why do people need different amounts of energy?

Who needs the most energy each day?

How much energy each person needs per day

1 · Needs the most energy per day5 · Needs the least energy per day

  1. An active teenage girl

  2. A sedentary woman over 60

  3. A very active teenage boy

    Very active means moving around the house, school or work, plus at least 60 minutes of gentle exercise such as walking and 60 minutes of vigorous exercise such as running or swimming.

  4. An active woman over 60

  5. An active teenage boy

Predict, then check: two cereal labels

These numbers are made up to show the idea. A label gives energy per 100 g and per recommended portion, and both matter.

Cereal A has 1,600 kJ per 100 g and a recommended portion of 30 g. Cereal B has 1,540 kJ per 100 g and a recommended portion of 45 g. Someone eats one recommended portion of each. Which bowl gives more energy?

Reading food labels

What has to be on the label?

Imagine you are checking a food label. Which information must be on it by law, and which is chosen by the manufacturer?

Still to sort

Must be on the label by law (0)

Information about the nutritional value of the food.

Chosen by the manufacturer (0)

Other information, not required by law.

8 of 8 still to sort.

Watch out: Whole foods such as an apple or a banana do not carry food labels at all.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Food builds you. Food fuels you. Some of it does both.

What you need to know

  • Food does two jobs: it is the building material for growth and repair, and it is the fuel for cellular respiration.
  • Carbohydrates and fats are the main sources of energy in our diets. Protein is not the main source of energy.
  • Have a goZara is totally sure that protein is the main source of energy in our diets. Which two nutrients would you point her towards instead?

    Carbohydrates and fats.

    Zara's mix-up is common, but protein is not the main source of energy; carbohydrates and fats are.

  • Glucose and oxygen provide energy in cellular respiration, in the mitochondria, for life processes like building proteins and muscle contraction.
  • Biomass is dry mass: dry the organism in an oven, then reweigh after further drying until the mass is completely stable.
  • Have a goA sample of a plant is dried in an oven and weighs 9.2 g. Dried for longer, it weighs 8.1 g. After more drying it still weighs 8.1 g. Which reading is its biomass?

    8.1 g.

    The 9.2 g reading was not stable yet, because more drying changed it; 8.1 g is the first completely stable mass.

  • In a food chain the producer makes the food, and it is passed along every time the next consumer eats it.
  • Different people need different amounts of food and energy each day, depending on how active they are, and on their sex and age.
  • Labels must show energy, fat, saturated fat, carbohydrate, sugars, protein and salt by law. Vitamin content is the manufacturer's choice.
  • Labels give energy per 100 g and per recommended portion, in kilojoules (1 kJ = 1000 J) or kilocalories.
  • Have a goA label says a snack gives 2,000 kJ. How many joules is that?

    2,000,000 J.

    One kilojoule is 1000 joules, so you multiply the kilojoules by 1000.

  • A food with less energy per 100 g can give more energy per recommended portion, if its portion is larger.

The big picture

Food does two jobs at once. It is the building material your body uses for growth and repair, and it is the fuel your cells use in respiration. Carbohydrates and fats are the main energy sources; protein is not. You'll also meet biomass, why people need different amounts of energy, and how to read a food label properly.

Key points

1Ask of every nutrient: does it build you, fuel you, or both? Carbohydrates and fats do both; protein is mainly a building material, not the main energy source.
2Biomass is the dry mass of an organism, found by drying until its mass is completely stable. Its food is made by the producer and passed along the food chain.
3Cells get energy when glucose and oxygen are used in cellular respiration in the mitochondria, and all living things need that energy to stay alive.
4More activity always means more energy needed per day. Males need more than females at a similar activity level, and teenagers need more than people over 60 in all levels of activity.
5Per 100 g shows which food contains more energy. Per recommended portion shows what a person actually takes in.
6Whole foods such as an apple or a banana do not carry food labels.

Worked example

Problem

A class dries a small plant sample to find its biomass. Fresh, it has a mass of 18.0 g. After each spell in the oven they reweigh it and get 7.4 g, then 6.9 g, then 6.8 g, then 6.8 g. What is the biomass of the sample?

⚠ Watch out

Thinking protein is the main source of energy. It is a building material for growth and repair. Carbohydrates and fats are the main energy sources.

🧠

Memory hook

Ask every nutrient: build me, fuel me, or both? Carbohydrates and fats say 'both'. Protein says 'build'.

✓

Check yourself

Without looking back: which nutrients are the main energy sources? Then explain, in your own words, why a cereal with less energy per 100 g can still give you more energy in the bowl.

Flashcards

(15)
What two jobs does food do?
It is a source of building materials (for growth and repair) and it is the fuel for cellular respiration.
Which nutrients are the main source of energy in our diets?
Carbohydrates and fats. Protein is not the main source of energy.
Which nutrients do children need to build biomass for growth?
Carbohydrates, proteins, lipids, vitamins and minerals.
Who needs food for repair?
Everybody: damaged tissues need repairing.
What is biomass?
The dry mass of a living organism.
How is biomass found?
Dry the organism in an oven to remove all the water, and weigh it again after further drying until the mass is completely stable.
In a food chain, who makes the food and how does it pass on?
The producer always makes it, and it is passed along every time the next consumer eats it. Organisms use it to build biomass.
What fuels cellular respiration, and where does it happen?
Glucose, with oxygen, in the mitochondria of both animal and plant cells.
What do cells use the energy from respiration for?
Life processes, such as chemical reactions (building up molecules like proteins), muscle contraction and movement.
What is 1 kilojoule in joules?
1000 joules. Labels may also give energy in kilocalories.
How does activity affect the energy a person needs per day?
More activity means more energy needed, no matter which age or sex.
Males and females: who needs more energy per day?
Males, when the activity level is similar.
Teenagers and people over 60: who needs more energy per day?
Teenagers, in all levels of activity.
What must a food label include by law?
The amount of energy, fat, saturated fat, carbohydrate, sugars, protein and salt.
Why can a food with less energy per 100 g give more energy per portion?
Its recommended portion may be larger, and the energy you take in depends on the portion size.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Biologylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 9 October 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.