biochemical tests

Question 1

describe how to test for proteins in a sample

Answer 1

• biuret test confirms presence of peptide bond
  1. add equal volume of sodium hydroxide to sample at room temperature
  2. add drops of dilute copper (II) sulphate solution. swirl to mix (steps 1 and 2 make Biuret reagent)
  3. positive result = colour changes from blue to purple
  negative result = solution remains blue

Question 2

describe how to test for lipids in a sample

Answer 2

1. dissolve solid samples in ethanol
2. add an equal volume of water and shake
3. positive result = milky white emulsion forms

Question 3

describe how to test for reducing sugars

Answer 3

1. add an equal volume of Benedict’s reagent to a sample
2. heat the mixture in an electric water bath at 100°C for 5 minutes
3. positive result = colour changes from blue to orange and brick red precipitate forms (may also change to green or yellow if concentration of sugar is lower)
   - or use a test strip coated in a reagent that changes colour if reducing sugar is present

Question 4

describe how to test for non reducing sugars

Answer 4

1. repeat steps for reducing: add Benedict’s to sample, heat in water bath at 100°C for 5 mins. result will be negative so will remains blue
2. hydrolyse the non reducing sugar by adding 1cm^3 HCl (hydrochloric acid) and heat in water bath for 5 mins - this reduces the sugar into its monomers
3. neutralise the mixture using sodium carbonate solution
4. proceed with Benedict’s test as usual

Question 5

describe the test for starch

Answer 5

1. add iodine solution to sample
2. positive result: colour changes from orange to blue-black