Monosaccharides and Disaccharides Revision Notes | A Level AQA Biology

1. Why Carbohydrates Start Here

Every complex carbohydrate in biology — from the starch in your food to the cellulose in a plant cell wall — is built from the same small units. To understand polysaccharides, you first need to understand the units they are made from: monosaccharides.

Think of monosaccharides as individual LEGO bricks. On their own they are functional molecules. Snap two together and you get a disaccharide. Keep building and you get a polysaccharide. The snap mechanism is always the same: a condensation reaction that forms a glycosidic bond.

2. Monosaccharides: The Monomers of Carbohydrates

Monosaccharides are the monomers from which larger carbohydrates are made. The three you need to know for AQA are glucose, galactose, and fructose. All three share the molecular formula $C_6H_{12}O_6$, which makes them structural isomers of one another — same atoms, different arrangement.

Glucose is the most biologically significant of the three. It is the primary respiratory substrate — the molecule cells break down to release ATP. Galactose is found in milk sugar, and fructose is the sugar found in fruit. Despite sharing the same formula, their different structures mean they behave differently in the body and form different disaccharides.

2a. The Two Faces of Glucose: α and β

Glucose itself has two isomers: α-glucose and β-glucose. The difference is subtle but biologically enormous. In both structures, glucose forms a six-membered ring. The only difference is the position of the hydroxyl group (–OH) on carbon-1.

• In α-glucose: the –OH group on carbon-1 points downward (below the plane of the ring)
• In β-glucose: the –OH group on carbon-1 points upward (above the plane of the ring)
• All other –OH and –H positions remain the same in both isomers

This single positional difference has profound consequences. α-glucose monomers link together to form starch and glycogen (compact, coiled storage molecules). β-glucose monomers link together to form cellulose (straight, rigid chains that provide structural support in plant cell walls). The same monomer, one positional difference, two completely different functions.

3. Condensation Reactions and Glycosidic Bonds

Now that you know what monosaccharides look like, here is how they join together. When two monosaccharides come together, a reaction occurs between a hydroxyl group on one molecule and a hydroxyl group on the other. A molecule of water is released, and a new covalent bond — the glycosidic bond — forms between the two sugars.

The reverse reaction is called hydrolysis. In hydrolysis, a water molecule is added across the glycosidic bond, breaking it and regenerating the two original monosaccharides. This is how your digestive system breaks down disaccharides — enzymes catalyse hydrolysis reactions in the small intestine.

Worked example: Tracing a Condensation Reaction Between Two Glucose Molecules

Describe, step by step, what happens when two α-glucose molecules undergo a condensation reaction to form maltose.

1. Each α-glucose molecule has –OH groups at multiple positions. The –OH on carbon-1 of one glucose and the –OH on carbon-4 of the second glucose are involved. — The bond that forms is specifically called a 1,4-glycosidic bond, named after the carbon positions involved. AQA does not require you to number the carbons, but knowing this helps you visualise the reaction.
2. –OH (from C-1) + H– (from –OH on C-4) → $H_2O$ is released — This is the defining feature of a condensation reaction: a water molecule is produced. One H comes from one –OH, and the –OH itself comes from the other molecule.
3. The oxygen that remains bridges carbon-1 of the first glucose to carbon-4 of the second glucose, forming the glycosidic bond: Glucose–O–Glucose — The glycosidic bond is a covalent bond. It is strong and requires enzyme-catalysed hydrolysis (or acid hydrolysis) to break.
4. Two α-glucose molecules → maltose + $H_2O$ — Maltose is a disaccharide. For every glycosidic bond formed, exactly one water molecule is released.

Answer: Two α-glucose molecules undergo a condensation reaction. A water molecule is released and a glycosidic bond forms between carbon-1 of one glucose and carbon-4 of the other, producing the disaccharide maltose.

4. The Three Disaccharides You Must Know

Three disaccharides appear repeatedly in AQA Biology: maltose, sucrose, and lactose. Each is formed by the condensation of a specific pair of monosaccharides. You need to know both the name of the disaccharide and the monosaccharides it is made from.

Notice that glucose appears in all three disaccharides — it is the most common monosaccharide in biology. The distinguishing factor is the second monomer: fructose gives you sucrose, galactose gives you lactose, and a second glucose gives you maltose.

Sucrose is particularly important in plants. It is the form in which carbohydrate is transported through the phloem from leaves (where it is made by photosynthesis) to other parts of the plant. Lactose is significant in mammalian biology — it is the sugar in breast milk and requires the enzyme lactase to be hydrolysed. Individuals who do not produce sufficient lactase experience lactose intolerance.

5. Comparing the Disaccharides

Exam questions often ask you to compare disaccharides or to explain why a particular disaccharide is formed. The key is to be precise about which monomers are involved and to use the term 'condensation reaction' alongside 'glycosidic bond' in your answer.

All three disaccharides share the same type of bond — a glycosidic bond — and all three are formed by condensation. The differences lie entirely in the identity of the monosaccharide monomers. This means that different enzymes are needed to hydrolyse each disaccharide: maltase hydrolyses maltose, sucrase hydrolyses sucrose, and lactase hydrolyses lactose.

Questions on this topic tend to be short-answer or extended-response, and the marks are often lost through imprecise language rather than a lack of knowledge. The following guidance targets the most common errors seen in mark schemes.

6. Exam Technique and Common Mistakes

The three disaccharides and the α/β-glucose distinction are high-frequency exam topics. Maltose = glucose + glucose; sucrose = glucose + fructose; lactose = glucose + galactose. Anchor these pairings firmly and you have covered the core of this specification section.