AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
A condensation reaction between two monosaccharides forms a glycosidic bond.
Full specification wording
Carbohydrates: A condensation reaction between two monosaccharides forms a glycosidic bond.
When two monosaccharides are brought together, a hydroxyl group on each reacts, one molecule of water is eliminated and a glycosidic bond forms - a single oxygen atom bridging the two sugar rings.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 2 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When two monosaccharides are brought together, a hydroxyl group on each reacts, one molecule of water is eliminated and a glycosidic bond forms - a single oxygen atom bridging the two sugar rings. Name the carbons involved: most glycosidic bonds run from carbon 1 of one residue to carbon 4 of the next, written as a 1,4 bond, while the branch points in amylopectin and glycogen are 1,6 bonds between carbon 1 and carbon 6. Straight, unbranched molecules such as cellulose contain only 1,4 bonds, and claiming a 1,6 bond in cellulose is treated as an error. Because beta-glucose carries its hydroxyl group on carbon 1 above the ring, alternate beta-glucose molecules must be rotated through 180 degrees before their hydroxyl groups align and the bond can form - which is exactly what produces the straight cellulose chain.
What earns the marks
- Stating that the bond forms by a condensation reaction between two monosaccharides.
- Stating that a molecule of water is released as the bond forms.
- Naming the glycosidic bond, and identifying it as a 1,4 linkage where the diagram shows a straight chain.
- Stating that alternate beta-glucose molecules are rotated 180 degrees so the bond can form.
- Stating that the bond is broken by hydrolysis, using a water molecule.
Common mistakes and how to put them right
Referring to 1,6 linkages in cellulose, which is rejected because cellulose chains are unbranched.
Put it right: Cellulose contains only 1,4 glycosidic bonds.
Calling the bond a peptide or ester bond in a carbohydrate question.
Put it right: The bond between monosaccharides is a glycosidic bond.
Describing the bond as forming 'between two hydroxyl groups' without saying that water is released.
Put it right: Condensation releases water as the bond forms.
Writing glycogenic or glucosidic instead of glycosidic.
Put it right: The correct spelling is glycosidic.
Saying the bond is broken by heat rather than by hydrolysis.
Put it right: Hydrolysis uses water to break the bond.
Exam tips
- If you quote carbon numbers, check the diagram first - 1,4 for a straight chain, 1,6 only at a branch point.
- Pair the two reactions in your answer: condensation forms the glycosidic bond and releases water, hydrolysis breaks it and uses water.
- Spell glycosidic correctly; it is a technical term the mark scheme expects to see in full.
Try the lesson
The start of the Carbohydrates: A condensation reaction between two monosaccharides forms a glycosidic bond. lesson, made from checked teaching notes for this specification point.
Have a go first
What type of bond is formed when two monosaccharides join together?
Listen
When two monosaccharides are brought together, a hydroxyl group on each reacts, one molecule of water is eliminated and a glycosidic bond forms - a single oxygen atom bridging the two sugar rings. Name the carbons involved: most glycosidic bonds run from carbon 1 of one residue to carbon 4 of the next, written as a 1,4 bond, while the branch points in amylopectin and glycogen are 1,6 bonds between carbon 1 and carbon 6. Straight, unbranched molecules such as cellulose contain only 1,4 bonds, and claiming a 1,6 bond in cellulose is treated as an error. Because beta-glucose carries its hydroxyl group on carbon 1 above the ring, alternate beta-glucose molecules must be rotated through 180 degrees before their hydroxyl groups align and the bond can form - which is exactly what produces the straight cellulose chain. A strong answer earns marks for these points. stating that the bond forms by a condensation reaction between two monosaccharides. stating that a molecule of water is released as the bond forms. naming the glycosidic bond, and identifying it as a 1,4 linkage where the diagram shows a straight chain. stating that alternate beta-glucose molecules are rotated 180 degrees so the bond can form. A common slip: referring to 1,6 linkages in cellulose, which is rejected because cellulose chains are unbranched. Instead, cellulose contains only 1,4 glycosidic bonds. Exam tip: If you quote carbon numbers, check the diagram first - 1,4 for a straight chain, 1,6 only at a branch point.
See it
Key words
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glycosidic bond
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condensation and water loss
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1,4 and 1,6 linkages
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beta-glucose rotation
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See every definition
- glycosidic bond
- A bond formed by a condensation reaction between two monosaccharides, in which a single oxygen atom bridges the two sugar rings.
- condensation and water loss
- A reaction between two monosaccharides in which a hydroxyl group on each reacts and one molecule of water is eliminated as the bond forms.
- 1,4 and 1,6 linkages
- Bonds running from carbon 1 of one residue to carbon 4 of the next, or, at branch points, from carbon 1 to carbon 6.
- beta-glucose rotation
- Alternate beta-glucose molecules must be rotated through 180 degrees before their hydroxyl groups align and the bond can form, producing the straight cellulose chain.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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