Biochemical tests

• How do you test for proteins?

  Biuret test
• describe the steps of the biurets test

  1) mix 3cm³ liquid sample with sodium hydroxide solution

  2) add a few drops of copper sulfate until the sample solution turns blue

  3) if proteins are present, it will turn purple
• what is the test for non-reducing and reducing sugars

  benedict's test
• describe the steps of the benedict's test for reducing sugars

  1) place the sample in a boiling tube and add an equal amount of benedict's reagent

  2) heat the mixture in a boiling water bath for 5 mins

  3) if a reducing sugar is present, it will turn from blue to brick red
• describe the steps of the benedict's test for non-reducing sugars

  1) add dilute HCl to the solution and heat in a water bath that's been brought to boil

  2) add sodium hydrocarbonate to neutralise the acid

  3) carry out the benedict's test --> if the test is negative, the solution will stay blue, if not it will turn up to brick red
• what is used to test for starch

  Iodine
• describe the iodine test

  drops of iodine are added to the sample and it will turn from browny-orange to blue-black
• describe the lipid emulsion test

  lipids are insoluble in water but soluble in ethanol

  1) add the substance and a small amount of ethanol and shake

  2) add water drop by drop down the side of the tube

  3) if a white cloudy emulsion appears - it contains lipid
• what is a more accurate method than the benedict's test?

  filter the solution and weigh the precipitate
• what is a quantitative method to determine the concentration of a chemical substance in a solution?

  colorimetry
• what does a colorimeter do?

  measures the strength of a coloured solution by seeing how much light passes through it (% absorbance)
• how to make up several glucose solutions of different concentrations for colorimetry?

  serial dilution
• how to make five serial dilutions with a dilution factor of 2 of different known concentrations?

  start with an initial concentration of 40mM:

  1. line up 5 test tubes
  2. add 10cm³ of the initial 40mM solution to the first test tube and 5cm³ of distilled water to the other 4 test tubes
  3. using a pipette, add 5cm³ of the solution from the first test tube to the second test tube and mix
  4. repeat this 3 more times to get 10mM, 5mM and 2.5mM
• steps for colourimetry?

  make serial dilutions of glucose

  do a benedict's test on each solution and a negative control of water

  remove any precipitate

  use a colorimeter with a red filter to measure % absorbance

  plot a calibration curve with concentration against % absorbance and use the curve to find the concentrations of the unknown substances