state that glucose, fructose and maltose are reducing sugars and that sucrose is a non-reducing sugar

Published by Patrick Mutisya · 14 days ago

Cambridge A‑Level Biology 9700 – Carbohydrates and Lipids

Carbohydrates and Lipids

Learning Objective

State that glucose, fructose and maltose are reducing sugars and that sucrose is a non‑reducing sugar.

What is a Reducing Sugar?

A reducing sugar is any carbohydrate that possesses a free carbonyl group (an aldehyde or a ketone) capable of acting as a reducing agent. In aqueous solution the carbonyl group can be oxidised, while the sugar itself reduces metal ions such as Cu²⁺ in Benedict’s or Fehling’s reagents.

Chemical Basis

In the open‑chain form a monosaccharide contains a carbonyl group:

\$\text{R–C(=O)–H} \quad \text{(aldehyde)}\$

or

\$\text{R–C(=O)–R'} \quad \text{(ketone)}\$

If this carbonyl carbon is not involved in a glycosidic bond, the sugar can act as a reducing agent. When the carbonyl carbon participates in a glycosidic linkage, the reducing ability is blocked, giving a non‑reducing sugar.

Common Reducing and Non‑Reducing Sugars

SugarTypeReducing AbilityKey Test Result
GlucoseMonosaccharide (aldose)ReducingPositive (brick‑red precipitate)
FructoseMonosaccharide (ketose)ReducingPositive (after isomerisation to aldehyde form)
MaltoseDisaccharide (α‑1,4‑linked glucose)ReducingPositive
SucroseDisaccharide (glucose‑fructose)Non‑reducingNegative

Why Sucrose Is Non‑Reducing

In sucrose the glycosidic bond joins the anomeric carbon of glucose (C1) to the anomeric carbon of fructose (C2). Both carbonyl carbons are therefore involved in the bond, eliminating any free aldehyde or ketone group. Consequently sucrose cannot reduce Cu²⁺ ions and gives a negative result in Benedict’s or Fehling’s tests.

Typical Laboratory Tests

  1. Prepare a solution of the carbohydrate sample.

  2. Add an equal volume of Benedict’s reagent.

  3. Heat the mixture in a boiling water bath for 2–3 minutes.

  4. Observe the colour change:

    • Blue → green → yellow → orange → brick‑red precipitate indicates a reducing sugar.

    • No colour change (remains blue) indicates a non‑reducing sugar such as sucrose.

Suggested Diagram

Suggested diagram: Structural formula of sucrose showing the glycosidic bond between the anomeric carbons of glucose and fructose, illustrating why it is a non‑reducing sugar.

Key Points to Remember

  • Glucose, fructose and maltose contain a free carbonyl group → they are reducing sugars.

  • Sucrose has both anomeric carbons involved in the glycosidic bond → it is a non‑reducing sugar.

  • Reducing sugars give a positive Benedict’s/Fehling’s test; non‑reducing sugars do not.