Unit 2 Section 1.4 Biochemical Tests for Molecules

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  • Biochemical tests for Molecules
    • The Biuret Test for proteins
      • if you need to find out if a substance contained proteins you would use the biuret test there are two stages to this test
      • 1) - The test solution meeds to be alkaline, so first you add a few drops of sodium hydroxide solution
        • 2) - Then you add some Copper Sulfate solution
          • if protein is present, the solution turns purple.
          • if nmo protein is present then the solution will stay blue
      • 2) - Then you add some Copper Sulfate solution
        • if protein is present, the solution turns purple.
        • if nmo protein is present then the solution will stay blue
    • The Benedicts Test for Sugars
      • all sugars can be classified as reducing or non reducing, to test for sugars you use the benedicts Test.
        • the test differs depending on the type of sugar you are testing for
          • Reducing Sugars
            • include all monosaccharides and some disaccharides
              • You add benedicts reagent to a sample and heat it
                • if it tests positive it will form a coloured precipitate
          • Non Reducing Sugars
            • if the result of the reducing sugars test is negative, there could still be a non reducing sugar present
              • to test for non reducing sugars, first you have to break them down into monosaccharides.
                • to do this you take a new sample and boil it with dilute hydrochloric acid,
                  • then you neutralise it by adding sodium hydrogencarbonate
                    • Finally just carry out the benedicts test as you would for reducing sugar
    • The Iodine Test for Starch
      • if you want to test for the presence of starch in a sample, you'll need to do the iodine test
      • to test add iodine dissolved in potassium iodide to the test sample
        • if there is starch present the sample changes from browny - orange to a dark, blue black colour
        • if there is no starch present the sample stays browny - orange
    • Emulsion Test for Lipids
      • if you want to test for the presence of lipids in a sample, you will need to do an emulsion test
        • To use this test, you shake the substance with ethanol for about a minute, then pour solution into water
          • any lipid will show up as a milky emulsion
          • if there is no lipid, the solution will stay clear

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