What are the main food molecules made of, and what are they used for?
Describe the elements, building blocks and roles of carbohydrates, fats and proteins
A focused answer to the O-Level Biology outcome on biological molecules. The elements and building blocks of carbohydrates, fats and proteins, and the role each plays in the body.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
SEAB wants you to know the three main groups of food molecules (carbohydrates, fats and proteins), the chemical elements each contains, the smaller units they are built from, and the main roles each plays in the body. This underpins the food tests, digestion, and the work of enzymes.
The answer
Carbohydrates
Carbohydrates contain the elements carbon, hydrogen and oxygen. Their building blocks are simple sugars such as glucose. Many glucose units joined together form larger carbohydrates such as starch (the plant store) and glycogen (the animal store), or the structural carbohydrate cellulose in plant cell walls.
Main role: to provide energy. Glucose is the fuel for respiration, which releases energy for the cell.
Fats (lipids)
Fats also contain carbon, hydrogen and oxygen, but with far less oxygen than carbohydrates. Their building blocks are fatty acids and glycerol: one glycerol joined to three fatty acids makes a fat molecule.
Main roles: to store energy (fats store more energy per gram than carbohydrates), to insulate the body against heat loss, and to protect organs. Fats also form part of the cell membrane.
Proteins
Proteins contain carbon, hydrogen, oxygen and nitrogen (and sometimes sulfur). The nitrogen is the key extra element that distinguishes proteins from the other two groups. Their building blocks are amino acids, joined in long chains and folded into a particular shape.
Main roles: growth and repair of tissues (building new cells), and making important molecules such as enzymes, antibodies and some hormones. Muscle, skin and hair are rich in protein.
Why the building blocks matter
During digestion, these large molecules are broken down into their building blocks (glucose, fatty acids and glycerol, amino acids) so they are small enough to be absorbed into the blood. Knowing the building blocks links directly to digestion.
Examples in context
Example 1. Energy stores compared. A seed stores food as oil (a fat) because fat packs the most energy into the least mass, useful for a small seed. A potato stores food as starch, a carbohydrate, ready for quick release as glucose.
Example 2. Building a new tissue. When a wound heals, the body needs amino acids to make new proteins for the new skin and muscle. This is why a diet short of protein slows growth and the repair of injuries.
Try this
Q1. Name the elements found in a carbohydrate. [1 mark]
- Cue. Carbon, hydrogen and oxygen.
Q2. State the building blocks of a protein and a fat. [2 marks]
- Cue. A protein is built from amino acids; a fat is built from fatty acids and glycerol.
Q3. Explain why proteins are needed for growth. [2 marks]
- Cue. Proteins are made of amino acids and are used to build new cells and tissues, so they are needed to make the body grow and to repair damage.
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.
Original6 marksComplete a comparison of carbohydrates, fats and proteins by stating, for each: (a) the chemical elements they contain, (b) their basic building block(s), and (c) one role in the body.Show worked answer →
Carbohydrates: contain carbon, hydrogen and oxygen; built from simple sugars such as glucose; main role is to provide energy.
Fats (lipids): contain carbon, hydrogen and oxygen; built from fatty acids and glycerol; roles include energy storage and insulation (also accepted: protecting organs).
Proteins: contain carbon, hydrogen, oxygen and nitrogen (and sometimes sulfur); built from amino acids; roles include growth and repair, making enzymes (also accepted: antibodies, some hormones).
Markers reward the correct elements, building blocks and a valid role for each of the three molecules. The key distinguishing fact is that proteins also contain nitrogen.
Original3 marksExplain why proteins are described as essential for growth, and name one part of the body that is largely made of protein.Show worked answer →
Proteins are built from amino acids and are used to make new cells and tissues, so they are needed for growth and for the repair of damaged tissue. They also build important molecules such as enzymes.
A part of the body largely made of protein is muscle (also accepted: skin, hair, or the structural protein in these).
Markers reward proteins building new cells and tissues for growth and repair, and a correct protein-rich body part such as muscle.
Related dot points
- Carry out and interpret the standard food tests for starch, reducing sugar, protein and fat
A focused answer to the O-Level Biology practical outcome on food tests. The reagent, method and colour change for starch, reducing sugar, protein and fat, and how to write up a result for full marks.
- Explain that enzymes are biological catalysts and describe their action using the lock and key model
A focused answer to the O-Level Biology outcome on enzymes. What a biological catalyst is, the lock and key model, the meaning of substrate, active site and specificity, and why enzymes matter in the body.
- Describe and explain the effects of temperature and pH on the rate of enzyme-controlled reactions
A focused answer to the O-Level Biology outcome on enzyme activity. The effect of temperature and pH on the rate of reaction, the meaning of the optimum, and what denaturing does to an enzyme's active site.
- Describe the human digestive system and the role of mechanical and chemical digestion
A focused answer to the O-Level Biology outcome on human digestion. The path of food through the gut, the difference between mechanical and chemical digestion, and the main digestive enzymes and what they break down.
- Describe how the small intestine is adapted for absorption and explain assimilation
A focused answer to the O-Level Biology outcome on absorption and assimilation. How the small intestine is adapted with villi, what each digested product is used for, and the difference between absorption and assimilation.