Class 11 Biology - Biomolecules

Activity 9.1: Test for the Presence of Carbohydrates (Reducing Sugars)

Aim: To detect the presence of reducing sugars (like glucose) using Benedict's Reagent.

Materials Required:

Glucose solution (1% w/v)
Benedict’s Qualitative Reagent (containing Copper sulfate, Sodium carbonate, and Sodium citrate)
Distilled water
Test tubes and test tube holder
Water bath (maintained at 95°C - 100°C)
Graduated pipettes (2 mL and 5 mL)

Procedure:

Take 2 mL of the test solution (1% Glucose) in a clean test tube.
Add 2 mL of Benedict’s Reagent to the tube.
Mix the contents thoroughly.
Place the test tube in a boiling water bath for exactly 5 minutes.
Observe the color change and formation of precipitate.

Scientific Note Benedict's reagent contains $Cu^{2+}$ ions (blue) which are reduced to $Cu^{+}$ ions by the free aldehyde or ketone group of reducing sugars. These ions precipitate as cuprous oxide ( $Cu_2O$ ), which is brick-red. The color intensity (Green -> Yellow -> Orange -> Brick Red) provides a semi-quantitative estimate of sugar concentration.

Observation: The blue solution turns green, then yellow, and finally forms a brick-red precipitate.

Lab Best Practice Always use a water bath instead of direct heating to ensure uniform temperature and prevent the solution from bumping out of the test tube.

Activity 9.2: Test for the Presence of Proteins (Biuret Test)

Aim: To detect the presence of proteins (peptide bonds) in a given sample.

Materials Required:

Egg albumin solution (freshly prepared 5% v/v)
1% Copper sulfate ( $CuSO_4 \cdot 5H_2O$ )
40% Sodium hydroxide ( $NaOH$ )
Test tubes and droppers

Procedure:

Take 2 mL of the protein solution in a test tube.
Add 2 mL of 40% $NaOH$ to make the solution strongly alkaline.
Add 2-3 drops of 1% $CuSO_4$ solution down the side of the tube.
Shake gently and allow it to stand for 2 minutes.

Scientific Note The Biuret test relies on the formation of a coordination complex between $Cu^{2+}$ ions and the nitrogen atoms of the peptide bonds in an alkaline medium. At least two peptide bonds are required for a positive result, which appears as a violet or purple coloration.

Safety First 40% Sodium Hydroxide is highly caustic. In case of skin contact, wash immediately with plenty of running water and notify the instructor. Use safety goggles.

Observation: A distinct violet or purplish color develops in the solution.

Activity 9.3: Test for the Presence of Fats (Emulsification Test)

Aim: To detect the presence of lipids in a given sample.

Materials Required:

Vegetable oil or Ghee
Absolute Ethanol (99%)
Distilled water
Test tubes

Procedure:

Add 2 mL of the lipid sample to a test tube containing 5 mL of absolute ethanol.
Shake vigorously to dissolve the lipid.
In a separate test tube containing 5 mL of distilled water, carefully pour the ethanol-lipid mixture.

Observation: A cloudy, white "milky" emulsion forms at the top of the water layer.

Lab Best Practice Ensure the test tube used for dissolving the lipid in ethanol is completely dry, as any water will cause premature precipitation/emulsification.

Activity 9.4: Effect of Temperature on Salivary Amylase Activity

Aim: To study the effect of varying temperatures (5°C, 37°C, 80°C) on the digestion of starch by salivary amylase.

Materials Required:

1% Starch solution (prepared by boiling 1g soluble starch in 100mL distilled water)
Freshly collected Saliva (diluted 1:20 with distilled water)
Iodine solution (Lugol's Iodine)
Ice cubes (for 5°C), Incubator (for 37°C), and Boiling water bath (for 80°C)
White porcelain spotting plate
Stop clock

Procedure:

Prepare three sets of test tubes, each containing 5 mL of 1% starch solution.
Label them A (5°C), B (37°C), and C (80°C).
Place the tubes in their respective temperature environments for 10 minutes to equilibrate.
Add 1 mL of diluted saliva to each tube and start the stop clock immediately.
Every 2 minutes, take a drop from each tube and place it on a spotting plate containing a drop of Iodine.
Record the time taken for the "achromic point" (the point where iodine no longer turns blue-black).

Scientific Note Salivary amylase (ptyalin) is an $\alpha$ -amylase that hydrolyzes starch into maltose. At 37°C (human body temperature), the enzyme is at its kinetic optimum. At 5°C, kinetic energy is too low for frequent collisions. At 80°C, the enzyme's tertiary structure is irreversibly denatured due to the breaking of hydrogen and disulfide bonds.

Observation:

Tube B (37°C): Reaches achromic point fastest (usually 6-10 mins).
Tube A (5°C): Slow reaction; takes a very long time.
Tube C (80°C): No reaction; starch remains blue-black indefinitely.

Class 11 Biology - Biomolecules

Activity 9.1: Test for the Presence of Carbohydrates (Reducing Sugars)

Aim: To detect the presence of reducing sugars (like glucose) using Benedict's Reagent.

Materials Required:

Glucose solution (1% w/v)
Benedict’s Qualitative Reagent (containing Copper sulfate, Sodium carbonate, and Sodium citrate)
Distilled water
Test tubes and test tube holder
Water bath (maintained at 95°C - 100°C)
Graduated pipettes (2 mL and 5 mL)

Procedure:

Take 2 mL of the test solution (1% Glucose) in a clean test tube.
Add 2 mL of Benedict’s Reagent to the tube.
Mix the contents thoroughly.
Place the test tube in a boiling water bath for exactly 5 minutes.
Observe the color change and formation of precipitate.

Observation: The blue solution turns green, then yellow, and finally forms a brick-red precipitate.

Lab Best Practice Always use a water bath instead of direct heating to ensure uniform temperature and prevent the solution from bumping out of the test tube.

Activity 9.2: Test for the Presence of Proteins (Biuret Test)

Aim: To detect the presence of proteins (peptide bonds) in a given sample.

Materials Required:

Egg albumin solution (freshly prepared 5% v/v)
1% Copper sulfate ( $CuSO_4 \cdot 5H_2O$ )
40% Sodium hydroxide ( $NaOH$ )
Test tubes and droppers

Procedure:

Take 2 mL of the protein solution in a test tube.
Add 2 mL of 40% $NaOH$ to make the solution strongly alkaline.
Add 2-3 drops of 1% $CuSO_4$ solution down the side of the tube.
Shake gently and allow it to stand for 2 minutes.

Safety First 40% Sodium Hydroxide is highly caustic. In case of skin contact, wash immediately with plenty of running water and notify the instructor. Use safety goggles.

Observation: A distinct violet or purplish color develops in the solution.

Activity 9.3: Test for the Presence of Fats (Emulsification Test)

Aim: To detect the presence of lipids in a given sample.

Materials Required:

Vegetable oil or Ghee
Absolute Ethanol (99%)
Distilled water
Test tubes

Procedure:

Add 2 mL of the lipid sample to a test tube containing 5 mL of absolute ethanol.
Shake vigorously to dissolve the lipid.
In a separate test tube containing 5 mL of distilled water, carefully pour the ethanol-lipid mixture.

Observation: A cloudy, white "milky" emulsion forms at the top of the water layer.

Lab Best Practice Ensure the test tube used for dissolving the lipid in ethanol is completely dry, as any water will cause premature precipitation/emulsification.

Activity 9.4: Effect of Temperature on Salivary Amylase Activity

Aim: To study the effect of varying temperatures (5°C, 37°C, 80°C) on the digestion of starch by salivary amylase.

Materials Required:

1% Starch solution (prepared by boiling 1g soluble starch in 100mL distilled water)
Freshly collected Saliva (diluted 1:20 with distilled water)
Iodine solution (Lugol's Iodine)
Ice cubes (for 5°C), Incubator (for 37°C), and Boiling water bath (for 80°C)
White porcelain spotting plate
Stop clock

Procedure:

Prepare three sets of test tubes, each containing 5 mL of 1% starch solution.
Label them A (5°C), B (37°C), and C (80°C).
Place the tubes in their respective temperature environments for 10 minutes to equilibrate.
Add 1 mL of diluted saliva to each tube and start the stop clock immediately.
Every 2 minutes, take a drop from each tube and place it on a spotting plate containing a drop of Iodine.
Record the time taken for the "achromic point" (the point where iodine no longer turns blue-black).

Observation:

Tube B (37°C): Reaches achromic point fastest (usually 6-10 mins).
Tube A (5°C): Slow reaction; takes a very long time.
Tube C (80°C): No reaction; starch remains blue-black indefinitely.

Class 11 Biology - Biomolecules (Activities)

Class 11 Biology - Biomolecules

Activity 9.1: Test for the Presence of Carbohydrates (Reducing Sugars)

Activity 9.2: Test for the Presence of Proteins (Biuret Test)

Activity 9.3: Test for the Presence of Fats (Emulsification Test)

Activity 9.4: Effect of Temperature on Salivary Amylase Activity

On this page

Class 11 Biology - Biomolecules (Activities)

Class 11 Biology - Biomolecules

Activity 9.1: Test for the Presence of Carbohydrates (Reducing Sugars)

Activity 9.2: Test for the Presence of Proteins (Biuret Test)

Activity 9.3: Test for the Presence of Fats (Emulsification Test)

Activity 9.4: Effect of Temperature on Salivary Amylase Activity

On this page