K224 / K225 / 2027

Lesson 1 of 8 / Biological molecules

Carbohydrates, fats and proteins

What does each group contribute to a living organism?

I can state the main roles of carbohydrates, fats and proteins.

  • I can state the main roles of carbohydrates, fats and proteins.Syllabus

    Syllabus K224 / K225, Topic 3(a). State the main roles of carbohydrates, fats and proteins.

About 4 min

Make a guess. You are not marked.

Carbohydrates and fats are both made of carbon, hydrogen and oxygen. Which food group contains nitrogen as well?

  1. Carbohydrates, because they are eaten most.
  2. Proteins, which are built from amino acids.
  3. Fats, because they store the most energy.
Show the answer

Proteins, which are built from amino acids.

Every amino acid contains nitrogen, so every protein does. Some proteins contain sulfur too.

The key idea

Carbohydrates provide an immediate energy source, fats provide insulation and long-term energy storage, and proteins support cell growth and repair.

A food can contain several groups of biological molecules. Name the molecule group and its role, rather than treating one food as a pure substance.

Explore the idea

Which role does the molecule perform?

Immediate energy source

Immediate source of energy. Glucose can be broken down in respiration, releasing energy for cellular activities.

Original role symbols, not molecular structures. Energy is released during respiration; insulation reduces heat loss.

Explanation

Carbohydrates give us an immediate source of energy. Glucose is a carbohydrate. Cells break glucose down in respiration, which releases energy for things such as moving muscles.

Fats are used for long-term energy storage. Fat under the skin also works as insulation, which means it slows down heat loss to cold surroundings. Storing energy and keeping warm are two different jobs.

Proteins are needed for growth and repair. Growth means building new cells; repair means fixing or replacing damaged ones. Many parts of cells are made of protein. Enzymes and haemoglobin are proteins too.

These are the main jobs of each group. A food can contain more than one group, so use food tests to find out what a sample really contains.

Step by step
  1. 1

    Name the group

    Choose carbohydrate, fat or protein.

  2. 2

    Read the context

    Distinguish immediate energy, longer-term storage, insulation, growth and repair.

  3. 3

    Connect the role

    Explain how the group contributes to the stated need.

Worked example

A seed begins to grow

A germinating seed uses stored food for respiration and makes new cells. Explain the roles of carbohydrate and protein in these two processes.

One way to explain it

Carbohydrate can supply glucose that is broken down in respiration to release energy. Proteins are needed to build and repair the structures of the growing cells.

Why this answer works
  • Link carbohydrate to respiration and energy release.
  • Link protein to building cell structures.
  • Do not describe energy as a substance that becomes a cell.

Watch out for this

A student says: "Insulation means fat produces heat continuously." What is wrong with this?

Show the answer

Insulation reduces heat loss. Energy release in respiration and reduced transfer of heat to surroundings are different processes.

Check your understanding

Which pairing correctly gives a main role?

  1. Carbohydrates are used mainly for cell repair.
  2. Proteins form the insulating layer under the skin.
  3. Fats provide long-term energy storage and insulation.
Show the answer

Fats provide long-term energy storage and insulation.

The two functions are storing chemical energy and reducing heat loss.

Quick recall

Card 1 of 3. Answer in your head, then check.

What does each group contribute to a living organism?

You can return to this lesson any time.