Disaccharides

A disaccharide is a sugar made of two monosaccharides joined by a glycosidic bond. The bond forms in a condensation reaction, which releases one molecule of water, and it is broken again by hydrolysis. The three disaccharides you need are maltose (glucose + glucose), sucrose (glucose + fructose) and lactose (glucose + galactose). All three have the formula C₁₂H₂₂O₁₁.

What is a disaccharide?

Carbohydrates are built from single sugar units called monosaccharides, such as glucose. "Di" means two: a disaccharide is two of these units joined together. Many units joined in a long chain make a polysaccharide, such as starch or cellulose. Disaccharides are sweet, soluble in water, and small enough to taste, which is why table sugar (sucrose) is one.

Examples of disaccharides

AQA A-level Biology describes each one in the same way: maltose forms by condensation of two glucose molecules, sucrose by condensation of a glucose and a fructose, and lactose by condensation of a glucose and a galactose. Glucose is in all three.

How a disaccharide forms: condensation

A condensation reaction joins two molecules and releases water. An OH group on one monosaccharide and an H from an OH on the other are removed as H₂O, and the oxygen left behind links the two sugars. That C-O-C link is the glycosidic bond.

The bond is named after the carbons it joins. In maltose it links carbon 1 of one α-glucose to carbon 4 of the other, so it is an α-1,4 glycosidic bond. The difference between α and β glucose is explained in alpha and beta glucose.

Breaking a disaccharide: hydrolysis

AQA defines hydrolysis as breaking a chemical bond between two molecules using a water molecule. Adding water across the glycosidic bond splits a disaccharide back into its two monosaccharides:

In the body this is done by enzymes. Each disaccharide has its own, named by changing the ending to "-ase":

These are membrane-bound disaccharidases: AQA notes that carbohydrates are digested by amylases and by disaccharidases bound to the membranes of the cells lining the ileum. The monosaccharides are then absorbed, glucose by co-transport with sodium ions. Digestion is covered step by step in the Human Physiology course.

Testing for disaccharides

AQA expects you to test for reducing and non-reducing sugars with Benedict's solution. Maltose and lactose are reducing sugars; sucrose is a non-reducing sugar.

Reducing sugars

Heat the sample with Benedict's solution in a water bath. A reducing sugar turns the blue solution green, yellow, orange or brick red, depending on how much is present.

Non-reducing sugars (sucrose)

1. Do the reducing sugar test first. If it stays blue, there is no reducing sugar.
2. Boil a fresh sample with dilute hydrochloric acid. This hydrolyses sucrose into glucose and fructose.
3. Neutralise with sodium hydrogencarbonate, because Benedict's does not work in acid.
4. Heat with Benedict's solution again. A colour change now shows a non-reducing sugar was present.

Check your understanding.

Worked example: a mystery sugar

A solution stays blue when heated with Benedict's solution. A second sample is boiled with dilute acid, neutralised, and heated with Benedict's again: it turns orange. When the original solution is treated with the enzyme sucrase, the products are glucose and fructose. What is the sugar?

• No colour change at first, so it is not a reducing sugar. That rules out maltose and lactose.
• A colour change after acid hydrolysis shows it was a non-reducing sugar that broke into reducing sugars.
• Sucrase hydrolysed it into glucose and fructose, so it is sucrose.

Common mistakes

• Mixing up condensation and hydrolysis. Condensation joins and releases water; hydrolysis splits and uses water.
• Saying sucrose is a reducing sugar. It gives no colour change until it has been hydrolysed.
• Getting the monosaccharides wrong. Maltose: glucose + glucose. Sucrose: glucose + fructose. Lactose: glucose + galactose.
• Writing the formula as C₁₂H₂₄O₁₂. One water is lost, so it is C₁₂H₂₂O₁₁.
• Forgetting to neutralise in the non-reducing sugar test.

Practise with a short timed paper

Carbohydrates and the sugar tests are in the first year of A-level Biology. The daily A-level Biology: Year 1 paper is a short timed paper of 8 questions, with a new one every day.

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