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A LevelBiologyOCRRevision Notes2. Foundations in BiologyBiological MoleculesThe Glycosidic Bond

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

Lára Marie McIvor

Last updated

The Glycosidic Bond

Forming the glycosidic bond

  • To make monosaccharides more suitable for transport and storage, 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 reactions

Two glucose molecules undergo condensation to form maltose, a disaccharide, releasing water. The resulting bond is an α-1,4-glycosidic bond.
The formation of a glycosidic bond by condensation between two monosaccharides (glucose) to form a disaccharide (maltose)
Chemical reaction of α-glucose and β-fructose forming sucrose and water via condensation, highlighting the glycosidic bond formed.
The formation of a glycosidic bond by condensation between α-glucose and β-fructose to form a disaccharide (sucrose)

Breaking the glycosidic bond

  • The glycosidic bond is broken when water is added in a hydrolysis 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

Diagram illustrating hydrolysis of maltose into two alpha-glucose molecules, highlighting the glycosidic bond and addition of water molecule.
Glycosidic bonds are broken by the addition of water in a hydrolysis reaction
Diagram showing sucrose hydrolysis into α-glucose and β-fructose. Highlights glycosidic bond breakage and water addition, with numbered carbon atoms.
A molecule of glucose and a molecule of fructose are formed when one molecule of sucrose is hydrolysed; the addition of water breaks the glycosidic bond

Common disaccharides

  • Monosaccharides can join together via condensation reactions to form disaccharides

  • Common examples of disaccharides include:

    • maltose = the sugar formed in the production and breakdown of starch

    • sucrose = the main sugar produced in plants

    • lactose = a sugar found only in milk

Disaccharide

Monosaccharides from which it is made

Maltose

glucose + glucose

Sucrose

glucose + fructose

Lactose

glucose + galactose

Examiner Tips and Tricks

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

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

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    Lára Marie McIvor

    Author: Lára Marie McIvor

    Expertise: Biology Lead

    Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.