Describe the roles of carbohydrates, fats, and fibre in the human body, and explain why a person who eats too little fibre may experience constipation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Carbohydrates are the main source of energy for the body, being oxidised in cellular respiration to give ATP.
Fats act mainly as a reserve of energy, provide insulation, protect organs and form the structure of cell membranes.
Fibre is not digested by the body; it adds bulk to the faeces and helps food to move through the intestines.
If fibre intake is too low, the faeces become hard and lack bulk, so the intestine cannot move food efficiently, leading to constipation.
Fats act mainly as a reserve of energy, provide insulation, protect organs and form the structure of cell membranes.
Fibre is not digested by the body; it adds bulk to the faeces and helps food to move through the intestines.
If fibre intake is too low, the faeces become hard and lack bulk, so the intestine cannot move food efficiently, leading to constipation.
Examiner tips
- Use the exact terms ‘source of energy’, ‘insulation/energy storage/organ protection/structure’, ‘not digested’, ‘adds bulk’, ‘helps food move’, ‘harder faeces’, ‘less bulk’, ‘inefficient movement’ to match the mark scheme.
- Show the causal chain: low fibre → less bulk → harder faeces → reduced intestinal movement → constipation.
- Keep the answer concise – 4 marks, so one sentence per point is sufficient.
Common mistakes
- Confusing fibre with other nutrients or saying it is digested; examiners will not credit this.
- Using vague phrases like ‘good for health’ instead of the specific roles listed in the mark scheme.
Mark scheme (4 marks)
- Carbohydrates are used as a source of energy (in respiration in cells).
- Fats have a role in insulation / energy storage / protection of delicate organs / structure (any one acceptable).
- Fibre is not digested and adds bulk to the faeces / helps food move through the intestines.
- Without sufficient fibre, faeces become harder / less bulk means the intestine cannot move food along efficiently, resulting in constipation.
Key terms in this question
carbohydrates · fats · fibre · constipation
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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