Explain how the structure of lactose differs from that of glucose, and outline one consequence of this difference for digestion in humans.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Lactose is the principal carbohydrate found in mammalian milk. Some adult humans produce insufficient quantities of the enzyme required to digest it, leading to digestive discomfort after consuming dairy products.
Model answer (4 marks)
Lactose is a disaccharide, whereas glucose is a monosaccharide.
Lactose consists of a glucose and a galactose unit joined by a β‑1,4‑glycosidic bond.
Because of this disaccharide structure lactose cannot be absorbed directly; it must first be hydrolysed by the enzyme lactase.
If lactase is insufficient, lactose reaches the large intestine undigested, where bacterial fermentation produces gas and causes discomfort or diarrhoea.
Lactose consists of a glucose and a galactose unit joined by a β‑1,4‑glycosidic bond.
Because of this disaccharide structure lactose cannot be absorbed directly; it must first be hydrolysed by the enzyme lactase.
If lactase is insufficient, lactose reaches the large intestine undigested, where bacterial fermentation produces gas and causes discomfort or diarrhoea.
Examiner tips
- State the disaccharide vs monosaccharide difference first.
- Mention the β‑1,4‑glycosidic bond and the two sugars involved.
- Explain the need for lactase and the consequence of its deficiency.
- Use the exact terminology: disaccharide, glycosidic bond, lactase, fermentation, gas.
Mark scheme (4 marks)
- Lactose is a disaccharide, whereas glucose is a monosaccharide
- Lactose is formed from glucose and galactose joined by a glycosidic bond
- Lactose cannot be absorbed directly and must first be hydrolysed by the enzyme lactase
- Insufficient lactase means lactose passes undigested to the large intestine, where bacterial fermentation produces gas and causes discomfort / diarrhoea
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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