Biological Molecules - Tests

Question 1

Semi-quantitative use of Benedict’s test?

Answer 1

It can be used to estimate the amount of reducing sugar present in a sample.

Question 2

Test for a non-reducing sugar?

Answer 2

In order to detect a NRS it must first be hydrolysed into it’s monosaccharide components through hydrolysis.

Question 3

Test for reducing sugars?

Answer 3

Add sample being tested into a test tube.
Add Benedict’s reagent.
Heat the mixture in a water bath for five minutes.
If the Benedict’s reagent turns from blue to an orange/brown then a reducing sugar is present. This is because the reducing sugar transfers electrons to the Benedict’s solution.

Question 4

Test for starch?

Answer 4

Place 2cm cubed of the sample being tested into a depression in a spotting tile.
Add two drops of iodine solution.
The presence of starch is indicated by a blue-black coloration.

Question 5

Test for lipids?

Answer 5

It is known as an emulsion test. Take a dry test tube. Add ethanol to the sample being tested. Shake the tube thoroughly to dissolve any lipid in the sample. Then add water and shake gently. A cloudy-white colour indicates the presence of a lipid. Repeat the test but use water instead of ethanol to see if the acid actually affects the emulsion.

Question 6

Test for proteins

Answer 6

Buiret test: detects peptide bonds
Place a sample of the solution to be tested in a test tube and add an equal volume of sodium hydroxide solution at tom temperature.
Add a few drops of dilute copper sulfate solution and mix gently.
A purple coloration indicates the presence of peptide bonds, and therefore a protein. If the solution is blue then there is no protein present.