Disaccharides (Edexcel A (SNAB) A Level Biology)

• Two monosaccharides can join together via condensation reactions to form disaccharides
  • A condensation reaction is one in which two molecules join together via the formation of a new chemical bond, with a molecule of water being released in the process
  • The new chemical bond that forms between two monosaccharides is known as a glycosidic bond

• Common examples of disaccharides include
  • Maltose 
    • Contains two molecules of glucose linked by a 1,4 glycosidic bond
      • This means that the glycosidic bond is located between carbon 1 of one monosaccharide and carbon 4 of the other
  • Sucrose 
    • Contains a molecule of glucose and a molecule of fructose linked by a 1,2 glycosidic bond
      • This means that the glycosidic bond is located between carbon 1 of one monosaccharide and carbon 2 of the other
  • Lactose 
    • Contains a molecule of glucose and a molecule of galactose linked by a 1,4 glycosidic bond

Sucrose is a disaccharide formed from a molecule of glucose (left) and a molecule of fructose (right) joined together by a 1,2 glycosidic bond

• The function of disaccharides is to provide the body with a quick-release source of energy
  • Disaccharides are made up of two sugar molecules so they're easily broken down by enzymes in the digestive system into their respective monosaccharides and then absorbed into the bloodstream
• Due to the presence of a large number of hydroxyl groups, disaccharides are easily soluble in water
  • These hydroxyl groups form hydrogen bonds with the water molecules when dissolved in aqueous solutions
• Just like monosaccharides they are sweet in taste
  
  • Sucrose, also known as table sugar, is an example