A level-readyBiology ยท WBI11 (Unit 1)
Carbohydrates: from sugars to starch
Carbohydrates are built from monosaccharides joined by glycosidic bonds. Their structure explains how they store energy.
Monosaccharides and disaccharides
Monosaccharides include glucose, fructose and galactose. Two join in a condensation reaction, releasing a water molecule and forming a glycosidic bond:
- glucose + glucose = maltose
- glucose + fructose = sucrose
- glucose + galactose = lactose
The reverse, splitting with water, is hydrolysis.
Polysaccharides
- Amylose (part of starch): 1,4-glycosidic bonds, coils into a compact helix.
- Amylopectin (part of starch): 1,4 and 1,6 bonds, branched.
- Glycogen (animals): like amylopectin but more branched, so enzymes can release glucose quickly.
Why they suit storage
Starch and glycogen are insoluble, so they do not affect water potential, and they are compact. Branches give many ends where glucose can be added or removed.
Worked example
Explain why glycogen suits an animal's need for fast energy release.
- Glycogen is highly branched.
- Many branch ends let enzymes hydrolyse glucose from many points at once.
Answer: Its many branches give many ends for rapid hydrolysis to glucose.
Check yourself
- Which two monosaccharides form sucrose?
- Glucose and glucose
- Glucose and galactose
- Glucose and fructose
- Fructose and galactose
Show answer
Glucose and fructose. Sucrose = glucose + fructose.
- What type of reaction joins two monosaccharides?
- Condensation
- Hydrolysis
- Oxidation
- Neutralisation
Show answer
Condensation. Condensation releases water as the glycosidic bond forms.
- Which bonds make amylopectin branched?
- Peptide bonds
- Ester bonds
- 1,6-glycosidic bonds
- Hydrogen bonds only
Show answer
1,6-glycosidic bonds. 1,6-glycosidic bonds form the branch points.
- Why is starch a good storage molecule?
- It dissolves easily in cytoplasm
- It is insoluble and compact
- It contains nitrogen
- It is a single sugar
Show answer
It is insoluble and compact. Insoluble starch does not change the cell's water potential, and it packs tightly.
- Glycogen is found in
- plant cell walls
- bacteria only
- plant chloroplasts
- animals
Show answer
animals. Glycogen is the storage polysaccharide of animals (and fungi).
Sources
Last updated 7 November 2026. Spotted a mistake? Email alevel@lifereadysociety.com.