Carbohydrates - Disaccharides and polysaccharides

Glucose linked to fructose forms sucrose
Glucose linked to galactose forms lactose
Glucose linked to glucose forms maltose
When monosaccharides join together, water is removed, this is known as a condensation reaction
1. The subsequent bond that is formed is known as a glycosidic bond
When water is added to a disaccharide it breaks the glycosidic bond releasing the monosaccharides. This is known as hydrolysis.
Test for non-reducing sugars
1. some disaccharides are non-reducing sugars (e.g. sucrose)
2. They don't make Benedict's regent change colour
3. The sample must be in liquid form
  1. If it is not, then grind it up with water
  2. Add 2cm3 of the food to 2cm3 of Benedict's reagent in a test tube
    1. Place in test tube in a gently boiling water bath for 5 minutes
      1. If the solution does not change colour (remans blue) then a reducing sugar is NOT present
      2. Add another 3cm3 of the food to 2cm3 of hydrochloric acid in a test tube and place in a gently boiling water bath for 5 minutes
        The hydrochloric acid will hydrolyse any disaccharides present into its monosaccharides
        Slowly add sodium hydrogencarbonate solution to the test tube in order to neutralise the hydrochloric acid, check with pH paper to ensure the solution is alkaline
        Re-test the resulting solution by heating it with 2cm3 of Benedict's reagent in a gently boiling water bath for 5 minutes
        If a non-reducing sugar is present in the original sample, the Benedict's reagent will now turn orange-brown.
        This is due to the the reducing sugars that were produced from the hydrolysis of the non-reducing sugar
Polysaccharides are polymers formed by combining together many monosaccharides
1. Monosaccharides joined together by glycosidic bonds by condensation reaction
2. Very large molecules and so are insoluble
  1. This makes them ideal for storage
  2. When hydrolysed polysaccharides break down into disaccharides and monosaccharides
  3. Polysaccharides, such as cellulose, are not used for storage, but for structural support
3. Starch is a polysaccharide
  1. This is found in small grains
    1. E.g chloroplasts
  2. Test for starch
    1. carried out at room temperature
    2. Place 2cm3 of the sample being tested into a test tube
      1. Or a few drops can be placed on a depression of a spotting tile
      2. Add two drops of iodine and shake/stir
        The presence of starch is indicated by a blue-black coloration

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Carbohydrates - Disaccharides and polysaccharides

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