1.1.2 - Sugars

• sugar is a general term for monosaccharides and disaccharides
• all carbohydrates contain the elements Carbon (C), Hydrogen (H) and Oxygen (O)
• the monomers that make up carbohydrates are called monosaccharides
• examples of monosaccharides include glucose, fructose and galactose
• glucose is a hexose sugar (a monosaccharide with 6 carbon atoms in each molecule)
• there are 2 isomers of glucose - alpha glucose (a-glucose) and beta glucose (b-glucose)

  isomers are molecules with the same molecular formula but different structural formula (arrangement of atoms)
• structures of a-glucose and b-glucose

  
• disaccharides are formed when 2 monosaccharides join together by a condensation reaction
• the bond formed between 2 monosaccharides is called a glycosidic bond, as well as a molecule of water being released as a by-product
• Need To Know Equation:

  a-glucose + a-glucose ——> maltose
• need to know equation:

  a-glucose + galactose ——> lactose
• need to know equation:

  a-glucose + fructose ——> sucrose
• the Benedict’s test is used when testing for sugars
• all sugars can be classified as reducing or non-reducing, and the Benedict’s test differs slightly depending on which type of sugar is being tested for
• reducing sugars includes all monosaccharides (glucose, galactose, fructose) and some disaccharides (maltose and lactose)
• non-reducing sugars include some disaccharides (sucrose)
• reducing sugar test

  • add (blue) Benedict’s reagent to a sample
  • heat in a water bath that has been brought to the boil

  positive test - colour change from blue to anywhere from green (weak sugar solution) to brick red (strong sugar solution)

  negative test - no colour change
• non-reducing sugar test:

  • add dilute hydrochloric acid to a new sample
  • heat in a water bath that has been brought to the boil
  • neutralise the solution by adding sodium hydrogencarbonate
  • add (blue) Benedict’s reagent
  • heat in a water bath that has been brought to the boil

  positive result - colour change from blue to anywhere from green (weak sugar solution) to brick red (strong sugar solution)

  negative result - no colour change