Explain how the structure of a monosaccharide enables it to be used in the synthesis of disaccharides and polysaccharides.

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

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

Model answer (4 marks)

Monosaccharides have several –OH groups on their carbon skeletons. When two monosaccharides join, a –OH from one reacts with a –OH from the other, releasing H₂O in a condensation reaction. This forms a covalent glycosidic bond between the two sugars. Because each monosaccharide contains multiple –OH groups, the same process can repeat, linking many units together to give disaccharides or longer polysaccharide chains.

Examiner tips

Use the term ‘hydroxyl group’ and ‘condensation reaction’ to show understanding of the chemistry.
Mention the release of water and the formation of a glycosidic bond to hit the key points.
Explain that multiple –OH groups allow chain elongation for polysaccharides.
Keep the answer concise – 4 marks can be earned with 3–4 short sentences.

Common mistakes

Forgetting to state that water is released in the condensation reaction.
Using ‘hydrogen bond’ instead of ‘glycosidic bond’.
Not mentioning that multiple –OH groups enable chain elongation.

Mark scheme (4 marks)

Monosaccharides contain hydroxyl (–OH) groups on their carbon skeleton
Two hydroxyl groups (one from each monosaccharide) react together, releasing a water molecule in a condensation reaction
A covalent glycosidic bond is formed between the two sugar units
This process can repeat many times because each monosaccharide has multiple –OH groups, allowing chain elongation to produce polysaccharides

Key terms in this question

monosaccharide · polysaccharide