Explain how the structure of lactose and the enzyme lactase relate to the specificity of enzyme–substrate interactions in carbohydrate digestion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Lactose is a disaccharide found in mammalian milk. Individuals who do not produce sufficient lactase experience digestive discomfort after consuming dairy products.
Model answer (4 marks)
Lactose is a disaccharide consisting of a glucose and a galactose linked by a β‑1,4‑glycosidic bond.
The active site of lactase has a three‑dimensional shape that is complementary to the size, shape and orientation of the glucose–galactose unit, allowing only lactose to bind.
Because of this complementarity, only lactose can form a stable enzyme–substrate complex; other disaccharides such as sucrose or maltose cannot fit into the lactase active site and are not hydrolysed.
Lactase catalyses the hydrolysis of the β‑1,4‑glycosidic bond, producing free glucose and galactose that are absorbed; without sufficient lactase the lactose remains in the gut, leading to osmotic water influx and fermentation, causing discomfort.
The active site of lactase has a three‑dimensional shape that is complementary to the size, shape and orientation of the glucose–galactose unit, allowing only lactose to bind.
Because of this complementarity, only lactose can form a stable enzyme–substrate complex; other disaccharides such as sucrose or maltose cannot fit into the lactase active site and are not hydrolysed.
Lactase catalyses the hydrolysis of the β‑1,4‑glycosidic bond, producing free glucose and galactose that are absorbed; without sufficient lactase the lactose remains in the gut, leading to osmotic water influx and fermentation, causing discomfort.
Examiner tips
- Use the term ‘active site’ and ‘complementary shape’ to show understanding of specificity.
- Explain the consequence of no binding for other sugars to justify why only lactose is hydrolysed.
- Mention the products of hydrolysis to demonstrate the reaction’s outcome.
- Keep the answer concise – 4 points can be covered in 4–5 sentences.
Common mistakes
- Confusing the bond type (β‑1,4 vs α‑1,4) or omitting the bond name.
- Saying ‘any sugar’ can be hydrolysed, ignoring specificity.
- Failing to mention the products (glucose and galactose) or the symptoms caused by accumulation.
Mark scheme (4 marks)
- Lactose is a disaccharide formed by a glycosidic bond between glucose and galactose.
- The active site of lactase has a specific three-dimensional shape complementary to the structure of lactose.
- Only lactose can form an enzyme–substrate complex with lactase, so other disaccharides (e.g. sucrose or maltose) are not hydrolysed by lactase.
- Lactase catalyses hydrolysis of the glycosidic bond in lactose, releasing glucose and galactose for absorption; without sufficient lactase the disaccharide accumulates and causes osmotic/fermentation-related symptoms.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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