Explain why large food molecules must be broken down before they can be used by the body.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Large food molecules are too big and insoluble to pass through the gut wall into the bloodstream.
Digestion breaks these large molecules into smaller, soluble ones.
The small soluble molecules can then pass through the wall of the small intestine and be absorbed into the blood.
Once in the blood they are transported to cells where they are used for energy, growth or repair.
Digestion breaks these large molecules into smaller, soluble ones.
The small soluble molecules can then pass through the wall of the small intestine and be absorbed into the blood.
Once in the blood they are transported to cells where they are used for energy, growth or repair.
Examiner tips
- Use the exact wording from the mark scheme (e.g. ‘large food molecules are insoluble’).
- Show the logical sequence: size → digestion → absorption → utilisation.
- Keep each point brief and separate with a line break or bullet.
Common mistakes
- Confusing ‘large’ with ‘high‑calorie’ or ‘fatty’ – the key is size and solubility.
- Failing to mention that absorption occurs in the small intestine.
- Using vague terms like ‘broken down’ without linking to solubility and transport.
Mark scheme (4 marks)
- Large food molecules are insoluble / too large to pass through the gut wall / into the bloodstream
- Digestion breaks large insoluble molecules into smaller soluble molecules
- Small soluble molecules can pass through the wall of the small intestine / be absorbed into the bloodstream
- The absorbed molecules are then transported to cells / used for energy or growth or repair
Related
- All WJEC GCSE Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →