Biochemical Tests

Question 1

Reducing sugars test

Answer 1

Benedicts reagent is added. This is a bright blue liquid (due to it containing copper sulphate). The name reducing sugar is given to sugars that can reduce cu2+ ions in benedicts reagent to cu+ ions in the form of copper(1)oxide, which forms a brick red precipitate.

Question 2

Reducing sugars test- chemical procedure

Answer 2

1.Add Benedict’s reagent to the sample you are testing
2. Heat
3. If a colour change of blue to yellow/green/red is observed, then this is confirmation that a reducing sugar is present.
4. If the solution remains blue, there is no reducing sugar present.

Question 3

Non reducing sugar test

Answer 3

Sucrose is a non reducing sugar because it cannot reduce cu2+, this is because the chemicak group needed for this reduction reaction is involved in the glycosidic bonds between the monosaccharides. This bond must be hydrolysed to expose the reducing group to prove they are sugars

Question 4

Non-reducing sugar test chemical procedure

Answer 4

1. Mix sucrose with HCL and boil- this is acid hydrolysis and it breaks the glycosidic bond so that sucrose is hydrolysed back into glucose and fructose.
2. Cool the solution and then add sodium hydroxide to make the solution alkaline. Benedicts reagent only works in alkaline solutions. You must cool the solution first to prevent excessive, dangerous fizzing.
3. Add a few drops of Benedicts reagent and heat. If a colour change of blue to yellow/green/red is observed then this is confirmation that a non reducing sugar is present.
   The rustier red the precipitate that forms after a reducing or non- reducing sugar test, the higher the concentration of sugar present. This is because more cu2+ has been reduced to cu+, which forms copper oxide/rust

Question 5

Starch test

Answer 5

The present of starch can be confirmed by adding a few drops of iodine. Iodine is orange/brown in colour when no starch is present, but it turns blue/black if starch is present.

Question 6

Protein test

Answer 6

To test for proteins you add biuret reagent which is blue but will turn purple when added to protein.

Question 7

Lipid test

Answer 7

To test for lipids your sample must first be dissolved in ethanol. This is acheived by shaking the sample you are testing in ethanol. Once the sample is dissolved, add distilled water and shake again. If lipids are present, you will then observe a white emulsion.