The Glycosidic Bond (Cambridge International A-Level Biology)

Forming the Glycosidic Bond

Spec point — describe the formation of a glycosidic bond by condensation, with reference to disaccharides, including sucrose, and polysaccharides

• To make monosaccharides more suitable for transport and 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

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)

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

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

Exam Hint Make sure you can identify where the glycosidic bond is in a carbohydrate.

Breaking the Glycosidic Bond

Spec point — 2.2.6 · describe the breakage of a glycosidic bond in polysaccharides and disaccharides by hydrolysis, with reference to the non-reducing sugar test

• 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 stomach and intestines
  • The breakdown of stored carbohydrates in muscle and liver cells for use in cellular respiration

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

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

Exam Hint Remember that disaccharides hydrolyse to two monosaccharides whereas polysaccharides must undergo many hydrolytic reactions until they form monosaccharides.