Cambridge A‑Level Biology 9700 – Carbohydrates and Lipids
Carbohydrates and Lipids – Focus on Glycosidic Bond Hydrolysis
Learning Objective
Describe how a glycosidic bond in polysaccharides and disaccharides is broken by hydrolysis, and explain how the non‑reducing sugar test is used to demonstrate this process.
1. Basic Concepts
Monosaccharides – the simplest carbohydrates (e.g., glucose, fructose).
Disaccharides – two monosaccharide units linked by a glycosidic bond (e.g., sucrose, maltose).
Polysaccharides – long chains of monosaccharide units (e.g., starch, glycogen, cellulose).
Glycosidic bond – a covalent linkage formed between the anomeric carbon of one sugar and a hydroxyl group of another.
2. Types of Glycosidic Bonds
Bond Position
Example
Reducing or Non‑reducing?
α‑1,4
Maltose (Glc‑α‑1,4‑Glc)
Reducing (free anomeric carbon on second glucose)
β‑1,4
Cellobiose (Glc‑β‑1,4‑Glc)
Reducing
α‑1,2 (sucrose)
Sucrose (Glc‑α‑1,2‑Fru)
Non‑reducing (both anomeric carbons involved)
3. Hydrolysis of Glycosidic Bonds
Hydrolysis is the addition of a water molecule to break the bond:
Acid‑catalysed hydrolysis – uses dilute mineral acid (e.g., HCl) to protonate the glycosidic oxygen, making it a better leaving group.
Enzymatic hydrolysis – specific enzymes (e.g., amylase, sucrase) lower the activation energy and operate under mild conditions.
During hydrolysis the anomeric carbon is regenerated as a free aldehyde (or ketone) group, which can act as a reducing agent.
4. Reducing vs Non‑Reducing Sugars
A sugar is termed “reducing” if it possesses a free hemiacetal or hemiketal group capable of being oxidised. In a disaccharide, if either monosaccharide retains a free anomeric carbon, the molecule is reducing.
5. Non‑Reducing Sugar Test (Hydrolysis Followed by Benedict’s Test)
Sample preparation – dissolve a known amount of the disaccharide (e.g., sucrose) in water.
Hydrolysis step
Heat the solution with a few drops of dilute HCl for 5–10 min, or add the appropriate enzyme (sucrase) at its optimum temperature.
Neutralise the acid with NaOH if acid hydrolysis was used.
Benedict’s test
Add an equal volume of Benedict’s reagent to the hydrolysed sample.
Heat in a boiling water bath for 2–3 min.
Observe the colour change: blue → green → yellow → orange → brick‑red precipitate indicates the presence of reducing sugars.
Interpretation
Before hydrolysis, a non‑reducing sugar (e.g., sucrose) gives no colour change (remains blue).
After hydrolysis, the appearance of a coloured precipitate confirms that the glycosidic bond has been cleaved, producing glucose and fructose, both of which are reducing sugars.
6. Example: Hydrolysis of Sucrose
Sucrose (a non‑reducing disaccharide) is hydrolysed as follows:
Suggested diagram: schematic of acid‑catalysed hydrolysis of a glycosidic bond, showing protonation of the oxygen, cleavage of the bond, and formation of two monosaccharides.
Key Points to Remember
Hydrolysis adds water across the glycosidic bond, regenerating free carbonyl groups.
Acid catalysis and enzymes achieve the same net reaction but under different conditions.
The non‑reducing sugar test demonstrates that a previously non‑reducing disaccharide becomes reducing after hydrolysis.
Understanding this reaction is fundamental for topics such as digestion, metabolism, and the industrial processing of carbohydrates.