What are the main food molecules made of, and what does the body use each one for?
Describe the elements and building blocks of carbohydrates, proteins and fats, and state the role of each in living organisms
A scaffolded answer to the N(A)-Level Biology outcome on the main food molecules. The elements and building blocks of carbohydrates, proteins and fats, and what each is used for in the body, with simple examples.
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What this dot point is asking
This outcome wants you to know the three main food molecules (carbohydrates, proteins and fats), the chemical elements they contain, the smaller building blocks they are made from, and what the body uses each one for. You do not need complicated chemistry, just the key facts stated clearly. The most tested point is that proteins contain nitrogen, which the other two do not.
The answer
Carbohydrates
Carbohydrates contain the elements carbon, hydrogen and oxygen. They are made of simple sugars (such as glucose) as their building blocks. A single sugar like glucose is a simple carbohydrate; many sugars joined together make a complex carbohydrate such as starch (in plants) or glycogen (stored in animals). The main job of carbohydrates is to provide energy, which is released by respiration.
Proteins
Proteins contain carbon, hydrogen, oxygen and nitrogen. They are made of amino acids joined together in long chains. There are many different amino acids, and the order in which they are joined decides which protein is made. Proteins are used for growth and the repair of cells and tissues. They also form important substances such as enzymes, antibodies and some hormones.
Fats (lipids)
Fats contain carbon, hydrogen and oxygen, like carbohydrates, but with much less oxygen. They are made of fatty acids and glycerol as their building blocks. Fats are used as a store of energy (they contain more energy per gram than carbohydrates), as insulation to keep the body warm, and to protect organs.
Examples in context
Example 1. Why athletes eat carbohydrates before a race. Carbohydrates are the body's main fuel, broken down to glucose and used in respiration to release energy. Eating pasta or rice before exercise tops up the body's stores, so there is plenty of glucose ready to power the muscles.
Example 2. Why growing children need protein. Children are building new cells and tissues as they grow. Because proteins supply the amino acids needed for growth and repair, a diet with enough protein (from foods such as eggs, fish and beans) is important during the growing years.
Try this
Q1. State the building blocks of proteins. [1 mark]
- Cue. Amino acids.
Q2. Give the main use of carbohydrates in the body. [1 mark]
- Cue. To provide energy (released by respiration).
Q3. Explain how you could tell, from its elements, that a molecule is a protein rather than a carbohydrate. [2 marks]
- Cue. A protein contains nitrogen as well as carbon, hydrogen and oxygen, while a carbohydrate contains only carbon, hydrogen and oxygen; so the nitrogen shows it is a protein.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksName the building blocks (smaller units) that make up (a) carbohydrates such as starch, and (b) proteins. For each, state one use of that food group in the body.Show worked answer →
(a) Carbohydrates such as starch are made of many simple sugars (glucose units) joined together. One use: carbohydrates are the main source of energy for the body.
(b) Proteins are made of amino acids joined together. One use: proteins are used for growth and repair of cells and tissues (they also make up enzymes).
What markers reward: glucose (or simple sugars) for carbohydrates and amino acids for proteins, plus one correct use each. Energy for carbohydrates and growth and repair for proteins are the standard marking points. Naming the wrong building block (for example, saying proteins are made of sugars) scores nothing.
Original3 marksState the chemical elements found in (a) carbohydrates, (b) fats, and (c) proteins.Show worked answer →
(a) Carbohydrates contain carbon, hydrogen and oxygen.
(b) Fats contain carbon, hydrogen and oxygen.
(c) Proteins contain carbon, hydrogen, oxygen and nitrogen.
What markers reward: the three elements for carbohydrates and fats, and the same three plus nitrogen for proteins. The presence of nitrogen is what makes proteins different from the other two, so it is the key marking point.
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