Explain how a disaccharide is formed from two monosaccharides, and describe what happens when a disaccharide is broken down during digestion.

AQA A-Level Biology (7402) — 3.1.2 Carbohydrates · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Two monosaccharides join to form a disaccharide.
A condensation reaction occurs, releasing a molecule of water.
The reaction creates a glycosidic bond between the two sugars.
During digestion the disaccharide is hydrolysed.
Water is added in the hydrolysis, giving two monosaccharides.

Examiner tips

Use the word ‘condensation’ and ‘hydrolysis’ to show understanding of the reactions.
Mention the release or addition of water to link the two key processes.
Show the glycosidic bond as the structural link between the sugars.
Keep the answer concise – one sentence per point.

Common mistakes

Forgetting to state that water is released in condensation or added in hydrolysis.
Using the wrong term (e.g. ‘synthesis’ instead of ‘condensation’).
Not naming the glycosidic bond or the type of bond formed.

Mark scheme (5 marks)

Two monosaccharides join together to form a disaccharide
A condensation reaction occurs / water is released during the formation
A glycosidic bond is formed between the two monosaccharides
During digestion, hydrolysis breaks down the disaccharide
Water is added / used during hydrolysis to produce monosaccharides

Key terms in this question

monosaccharide · disaccharide