A-Level Biology (OCR) – Carbohydrates: monosaccharides & disaccharides

2.5 - Carbohydrates: Monosaccharides & Disaccharides

1The different types of monosaccharides
2The difference between alpha and beta glucose
3The different types of disaccharides
4The reactions which form and break down disaccharides

10 min

Monosaccharides

Monosaccharides are the simplest form of carbohydrates, also known as 'simple sugars'. Monosaccharides are soluble, sweet-tasting and are found in many foods such as fruits, vegetables, and grains.

They have the general formula (CH₂O)_n where 'n' can be any number from 3 to 7.

Monosaccharides are classified according to the number of carbon atoms in each molecule:

Pentose sugars (5 carbon atoms)

Ribose
Deoxyribose

Hexose sugars (6 carbon atoms)

Glucose
Fructose
Galactose

Alpha-glucose and beta-glucose

Glucose is a hexose (6-carbon) sugar with the formula C₆H₁₂O₆. The atoms in glucose can be arranged in two different ways.

This means that there are two isomers of glucose:

Alpha-glucose (α-glucose)
Beta-glucose (β-glucose)

Diagram showing the structure of alpha-glucose and beta-glucose molecules with the hydroxyl group orientation.

The only difference between the two forms is the orientation of the hydroxyl group (OH) on carbon 1 (the first carbon atom in the ring).

Diagram showing the structures of alpha-glucose and beta-glucose with the difference in the orientation of the hydroxyl group on carbon 1.

This diagram is showing the same molecules, but shows all the atoms.

Properties and uses of glucose

Glucose is used as the primary energy source in animals and plants.

The following features of glucose help it to function as an energy source:

It is soluble - The hydroxyl groups can form hydrogen bonds with water, so it can be transported around organisms.
Its bonds store lots of energy - This energy is released when the bonds are broken.

Disaccharides

Disaccharides are formed when two monosaccharides join together. Examples of disaccharides include maltose (found in grains and cereals), sucrose (used as a transport sugar in plants), and lactose (the main carbohydrate found in milk).

Maltose is made up of glucose joined to glucose.

Diagram showing the structure of maltose, a disaccharide made of two glucose molecules.

Sucrose is made up of glucose joined to fructose.

Diagram showing the structure of sucrose with glucose and fructose molecules.

Lactose is made up of glucose joined to galactose.

Diagram showing the formation of lactose from glucose and galactose.

Disaccharide formation and breakdown

Disaccharides are created via condensation reactions, and broken down via hydrolysis reactions. These reactions involve the formation or the breakdown of a covalent bond known as a glycosidic bond.

Condensation reaction

Diagram showing disaccharide formation and breakdown with glycosidic bond formation and water release.

When two monosaccharides join, a hydroxyl group (OH) of one monosaccharide reacts with a hydroxyl group (OH) of another monosaccharide. This forms a glycosidic bond, and a water molecule (H₂O) is released.

In the image above, the hydroxyl groups on carbons 1 and 4 are reacting together, so a 1-4 glycosidic bond forms, but they can also form between other carbons.

Hydrolysis reaction

Diagram showing the formation and breakdown of a disaccharide. A glycosidic bond is formed between two glucose molecules, releasing water. The bond is broken by adding water, producing two glucose molecules.

When a water molecule (H₂O) is added to a disaccharide, the glycosidic bond is broken to release the 2 monosaccharides.