Edexcel A (SNAB) A Level Biology:复习笔记1.2.6 The Glycosidic Bond

• To make monosaccharides more suitable for storage they are bonded together to form disaccharides and polysaccharides
  • Polysaccharides are insoluble so have less influence on the process of osmosis
• Disaccharides and polysaccharides are formed when two hydroxyl (OH) groups on different monosaccharides interact to form a strong covalent bond called a glycosidic bond
• The name of the glycosidic bond that forms depends on the location of the OH groups on the monosaccharides concerned, e.g.
  • If the OH groups are located on carbon 1 of one monosaccharide and carbon 4 of the other, a 1,4 glycosidic bond forms
  • If the OH groups are located on carbon 1 of one monosaccharide and carbon 6 of the other, a 1,6 glycosidic bond forms
• Every glycosidic bond results in one water molecule being released, thus glycosidic bonds are formed by a condensation reaction

Glycosidic bonds form through condensation reactions, during which a water molecule is released. When two glucose molecules are joined by a glycosidic bond, the resulting disaccharide is maltose.

Glycosidic bonds can link monosaccharides together to form polysaccharides such as amylopectin, a form of starch. Amylopectin contains 1,4 and 1,6 glycosidic bonds.

• There are many different types of monosaccharide, which join together to form different disaccharides, e.g.
  • Two molecules of glucose join to form maltose
  • Glucose joins with the monosaccharide fructose to form sucrose
  • Glucose joins with the monosaccharide galactose to form lactose

Breaking the glycosidic bond

• The glycosidic bond is broken when water is added in a hydrolysis reaction
  • Hydro = water
  • Lysis = to break
• 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

Glycosidic bonds are broken when water is added in a hydrolysis reaction