# Explain how the structure of lactose differs from that of glucose, and outline one consequence of this difference for digestion in humans.

> IB DP Biology Standard Level (2023 syllabus) — B1.1 Carbohydrates and lipids · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Lactose is a disaccharide, whereas glucose is a monosaccharide
2. Lactose is formed from glucose and galactose joined by a glycosidic bond
3. Lactose cannot be absorbed directly and must first be hydrolysed by the enzyme lactase
4. Insufficient lactase means lactose passes undigested to the large intestine, where bacterial fermentation produces gas and causes discomfort / diarrhoea

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