Testing For Biological Molecules



Testing For Biological Molecules

Testing For Biological Molecules

Testing For Biological Molecules

There are various tests that can help in finding out the biological molecules that are present in a mixture. This can also help in finding out the digestive activity of different enzymes.

This unit contains a different test that is performed for different biomolecules.

A Test For Carbohydrates:

For Reducing Sugar: (Benedict Test)

All monosaccharides and disaccharides except sucrose are reducing sugar. They have a hydroxyl group at the anomeric carbon.


Reducing sugar reduces CuSO4 (blue) and produces precipitates of Cu2O(red).


Benedict reagent is a mixture of Cuso4, Na2Co3 and sodium citrate.

Take 2cm3 of the mixture and add 2 cm3 of Benedict reagent.
        Mix them both and heat it at 95 ͦ C in a water bath.

The change in the color of the solution shows the amount of sugar present in the solution.

For Non Reducing Sugar: (Benedict Test)


Sucrose is itself non reducing sugar. But, if it is hydrolyzed the result monosaccharides ( fructose and glucose) are reducing sugars. Thus, before the hydrolysis benedict test will be negative after that it will give positive results.


Test the solution for reducing sugar first.

Then add a few drops of HCL and heat it. This will hydrolyze sucrose.

Now, test the solution again for reducing sugar.


The first benedict test will be negative. After hydrolysis, the result will be positive. Hence sucrose is present.

Semi Quantitative Benedict Test On Reducing Sugar

The color of the solution varies with the amount of reducing sugar present. Thus, a benedict solution can help in approximating the amount of sugar present in a solution with unknown concentration. Here are steps on how the Benedict test can be used semi-quantitatively.

Here are the steps to asses the relation between color and concentration of the solution.

Take 6 test tubes.

Add 2 cm3 of 1% glucose in tube 1.

Add 2 cm3 of water in tubes 2 to 6.

Now transfer 2 cm3 of solution from tube 1 to tube 2.

Mix it well.

Again transfer 2 cm3 of solution form tube 2 to 3 and mix it.

Repeat the same for tubes 4, 5 and 6.

Place the test tube in a water bath heated at 100 ͦ C.

Now add 2 cm3 of Benedict solution to each test tube.

Mix them thoroughly.

Place the test tubes back in the rack.

Observe the color change in each test tube.

The color change will occur in the same way in a glucose solution of unknown concentration.

In step 3-7 serial dilution of the solution has occurred. Due to this, the solution has decreased in concentration from test tube 1 to 6. Thus, the intensity of color change will also decrease. In this way, the concentration can be estimated as an unknown solution by looking at the intensity of color change.

Testing For Starch: (Iodine Test)


Starch reacts with iodine and changes color to blue/black.


1) Take 2 cm3 of the solution and add 2 drops of iodine.


If starch is present the color will change to blue/black due to the formation of starch polyiodide complex.

Testing For Lipids: ( Emulsion Test)


Lipids being nonpolar will dissolve in ethanol.


Add ethanol to the test solution.

Then, decant the solution in water.


If lipid is present an emulsion will be formed.

Testing For Proteins: (Biuret Test)


It tests for the presence of peptide bonds.


Add biuret reagent to the test solution.


If the color of reagent changes from blue to purple test is positive.

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