1.1.5 The Glycosidic Bond

What is a glycosidic bond?

• To make monosaccharides more suitable for transport, storage and to have less influence on a cell’s osmolarity, they are bonded together to form disaccharides and polysaccharides
• Disaccharides and polysaccharides are formed when two hydroxyl (-OH) groups (on different saccharides) interact to form a strong covalent bond called the glycosidic bond (the oxygen link that holds the two molecules together)
• Every glycosidic bond results in one water molecule being removed, thus glycosidic bonds are formed by condensation

Glycosidic bond in Maltose

The formation of a glycosidic bond by condensation between two monosaccharides (glucose) to form a disaccharide (maltose)

• Each glycosidic bond is catalysed by enzymes specific to which OH groups are interacting
• As there are many different monosaccharides this results in different types of glycosidic bonds forming (e.g maltose has a α-1,4 glycosidic bond and sucrose has a α-1,2 glycosidic bond)

Glycosidic bond in Sucrose

The formation of a glycosidic bond by condensation between α-glucose and β-fructose to form a disaccharide (sucrose)

Glycosidic bond in a Polysaccharide

The formation of glycosidic bonds to create a polysaccharide (amylopectin)

Types of Glycosidic Bonds Table

• The glycosidic bond is broken when water is added in a hydrolysis (meaning ‘hydro’ - with water and ‘lyse’ - to break) reaction
• Disaccharides and polysaccharides are broken down in hydrolysis reactions
• Hydrolytic reactions are catalysed by enzymes, these are different to those present in condensation reactions
• Examples of hydrolytic reactions include the digestion of food in the alimentary tract and the breakdown of stored carbohydrates in muscle and liver cells for use in cellular respiration

Hydrolysis Reaction

Glycosidic bonds are broken by the addition of water in a hydrolysis reaction

• Sucrose is a non-reducing sugar which gives a negative result in a Benedict’s test. When sucrose is heated with hydrochloric acid this provides the water that hydrolyses the glycosidic bond resulting in two monosaccharides that will produce a positive Benedict's test

Breaking the Glycosidic Bond

A molecule of glucose and a molecule of fructose are formed when one molecule of sucrose is hydrolysed; the addition of water to the glycosidic bond breaks it.