AQA A Level Biology复习笔记1.1.5 The Glycosidic Bond

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 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

 

Forming-a-glycosidic-bond

 

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)

 

 

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

 

 

Glycosidic-bonds-in-a-Polysaccharide

 

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

 

Types of Glycosidic Bonds Table

Types-of-glycosidic-bonds-table

Exam Tip

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

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
  • 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

Breaking-of-a-glycosidic-bond

 

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

 

Hydrolysis-of-Sucrose

 

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 Tip

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

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