2.2 Disaccharides

General formula: C₁₂H₂₂O₁₁

• • Disaccharides are formed when two monosaccharide molecules combine to form one single larger disaccharide molecule, together with the loss of a water molecule. Reaction is known as condensation reaction.

Figure 3 Condensation of two monosaccharide molecules to form a disaccharide molecule.

Define ‘condensation reaction’.

* Condensation reaction is a chemical reaction in which two simpler molecules combine to form a larger molecule, together with the loss of a smaller molecule, eg. water.

Other examples of condensation reactions

* Molecule that is highlighted in bold and asterisk is the disaccharide.

(a) Maltose – A type of reducing sugar found in malt and germinating seeds.

Figure 4 Condensation of two glucose molecules to form a maltose molecule.

(b) Lactose – A type of reducing sugar found in milk.

Figure 5 Condensation of a glucose and galactose molecule to form a lactose molecule.

(c) Sucrose – A non-reducing sugar found in sugar-cane. Used as a sweetener.

Figure 6 Condensation of a glucose and fructose molecule to form a sucrose molecule.

Define ‘hydrolysis reaction’.

* Hydrolysis is the breakdown of a large complex molecule into two or more smaller molecules, with the help of water.

* This reaction is the direct opposite of condensation.

*Occur in the presence or acid, alkali or enzymes.

Examples of hydrolysis using specific enzymes.

(a) Hydrolysis of maltose

Figure 7 Hydrolysis of a maltose molecule to form two molecules of glucose.

(b) Hydrolysis of lactose

Figure 8 Hydrolysis of a lactose molecule to form a glucose and a galactose molecule.

(c) Hydrolysis of sucrose

Figure 9 Hydrolysis of a sucrose molecule to form a glucose and fructose molecule.

Watch the following video to find out more about disaccharides. The structure of the disaccharide in this video is simplified to remove the oxygen atom linking the two monosaccharides.