Explain why the products of carbohydrate digestion can be absorbed into the blood in the ileum but starch cannot.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Starch is a large polysaccharide and cannot pass through the intestinal wall of the ileum. In the small intestine it is first broken down by amylase into the disaccharide maltose, and maltase then hydrolyses maltose into the monosaccharide glucose. Glucose is a small, soluble molecule that can diffuse (or be actively transported) across the epithelial cells of the villi into the capillaries of the ileum, where it enters the blood.
Examiner tips
- Use the word "large" to explain why starch is not absorbed; mention "amylase" and "maltase" for digestion steps; state that glucose is a small, soluble molecule; note that absorption occurs by diffusion or active transport into capillaries.
- Keep the answer concise – 4 points can be covered in 3–4 sentences.
Mark scheme (4 marks)
- Starch is a large molecule / too large to pass through the wall of the ileum / too large to be absorbed
- Starch is broken down / digested by amylase into maltose, and maltase breaks maltose into glucose
- Glucose is a small / soluble molecule that can pass through the wall of the ileum
- Glucose is absorbed by diffusion (or active transport) into the blood / into the capillaries in the villi
Key terms in this question
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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