Qualitative chemical tests for biological molecules

• describe the test for starch

  • iodine test
  • positive test - purple/black
  • negative test - yellow/brown
• describe the test for reducing sugars

  • add benedict’s and place in boiling water bath
  • positive test - brick red
  • negative test - blue
• describe the test for proteins

  • add a few drops of sodium hydroxide solution
  • add copper sulphate solution
  • positive test - purple solution
  • negative test - blue solution
  • BIURET SOLUTION
• describe the test for lipids

  • mix with ethanol
  • mix with water and shake
  • positive result - formation of white emulsion layer on top
  • negative result - colourless solution
• what are reducing sugars?

  • sugars that react with benedicts reagent
  • they can reduce, give electrons to other molecules
  • all monosaccharides and some disaccharides are reducing sugars
• how does the test for reducing sugars work?

  • benedict’s reagent - react with the Cu2+ ions in the copper sulphate solution
  • sugar becomes oxidised and the Cu2+ is reduced to Cu+
  • a brick red precipitate of Copper oxide is formed
• why will glucose react with benedict’s reagent?

  • the Cu2+ ions in the benedict’s will only react with a sugar that has an -OH group in the Carbon-1 position
  • electrons reduce the Cu2+ to Cu causing colour change
• what is colour change of sugars affected by?

  the concentration
• what are non-reducing sugars?

  • don’t react with benedict’s
  • some disaccharides - the reducing groups are not present as they have formed part of the glycosidic bond - don’t react with Cu2+ in benedict’s reagent
• what is the test for non-reducing sugars?

  • disaccharide first must be hydrolysed into 2 monosaccharides by heating with hydrochloric acid (all monosaccharides are reducing sugars)
  • sodium carbonate is used the neutralise the acid
  • the monosaccharides produced are reducing sugars, so they can react with benedict’s
• what are the colour results for non-reducing sugars?

  positive - brick-red

  negative - blue
• why will sucrose not react with benedict’s reagent?

  is a non-reducing sugar and has no free OH group
• what are reagent test strips?

  • manufactured reagent test strips used to test for the presence of reducing sugars, most commonly glucose
  • the advantage is that with the use of a colour-coded chart, the concentration of the sugar can be determined