The Glycosidic Bond (AQA A Level Biology): Revision Note

Forming the 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 create a strong covalent bond called the glycosidic bond

  • Each glycosidic bond is catalysed by enzymes specific to which OH groups are interacting

• Every glycosidic bond results in one water molecule being removed, thus, glycosidic bonds are formed by condensation

• As there are many different monosaccharides, this results in different types of glycosidic bonds forming (e.g. maltose has an α-1,4 glycosidic bond and sucrose has an α-1,2 glycosidic bond)

Glycosidic bond in maltose

• Maltose is a disaccharide formed by the condensation reaction of two glucose molecules

Glycosidic bond in sucrose

• Sucrose is a disaccharide formed by the condensation of a glucose molecule and a fructose molecule

Glycosidic bond in lactose

• Lactose is a disaccharide formed by the condensation of a glucose molecule and a galactose molecule

Glycosidic bond in a polysaccharide

• Polysaccharides are formed by the condensation of many glucose units

  • E.g. Glycogen and starch are formed by the condensation of α-glucose

  • E.g. Cellulose is formed by the condensation of β-glucose

Breaking the glycosidic bond

• The glycosidic bond is broken when water is added in a hydrolysis (meaning ‘hydro’ - with water and ‘lyse’ - to break) reaction

  • Hydrolytic reactions are catalysed by enzymes, these are different to those present in condensation reactions

• Disaccharides and polysaccharides are broken down into smaller molecules in hydrolysis 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