Carbohydrates

• Benedict's Test

  Reducing Sugars

  • Heat mixture with Benedict's reagent gently
  • Reducing sugars give positive test
  • Reduce copper (II) sulphate to copper (I)
  • Change from blue to brick red, green, or orange-brown (depend on sugar conc)

  Non-reducing Sugars

  • Boil w/ dilute hydrochloric acid to hydrolyse sugar to constituent monosaccharides
  • Neutralise by mixing with sodium hydroxide
  • Retest with Benedict's
• Key Carb Info


  • “Carb” - carbon, “hydr” - hydrogen, “ates” - oxygen

  Mono-/di- saccharides:

  • Small molecules
  • Simple sugars
  • Water-soluble (polar; -OH groups)
  • Crystalline
  • Sweet (e.g. sugar)

  Polysaccharides:

  • Macromolecules
  • Many monosaccharide monomers bonded by condensation
  • Form long chains/polymers
  • Insoluble
  • Not sweet (e.g. flour)
• Disaccharides

  • 2 monosaccharides, glycosidic bonded (by condensation)
  • 2(CH_2O)n - H_2O
  • Hydrolysed to monosaccharide monomers by adding water

  Maltose:

  • 2 α-glucoses (C 1 + 4; α 1-4 glycosidic bond)
  • Formed in starch breakdown

  Sucrose:

  • Glucose + Fructose
  • Sugar transported in green plants (in phloem)
  • Not reducing; carbonyl group lost forming glycosidic bond

  Lactose:

  • Glucose + Galactose
  • Milk Sugar
• Monosaccharide

  • Monomers of single sugar
  • -OH group is hydroxyl group (polar so water-soluble)
  • Straight-chain form
  • All but 1 C attached -OH
  • Other carbonyl group (C=O); ketone or aldehyde
  • Reducing sugars
  • Hexose: C_6H_12O_6; Glucose, Fructose or Galactose
  • Pentose: C_5H_10O_5; Ribose or Deoxyribose

  Glucose:

  • Ring structure + straight form in solution
  • Equilibrium but ring dominant
  • Isomers: Same molecular formla but vaired structural formula; slightly diff properties
  • 2 isomers; α-glucose + β-glucose
  • Slight structural variance + diff properties
  • Plants + animals respire alpha
  • 'Stereoisomers'