3.3.5 Visking Tubing Practical

Aim:

To investigate how large molecules are broken down into smaller molecules that can pass through a partially permeable membrane.

Equipment:

• Visking tubing (acts as the gut wall).
• Starch solution.
• Amylase solution.
• Beakers.
• Distilled water.
• Iodine solution (for starch detection).
• Benedict's reagent (for sugar detection).
• Pipettes and test tubes.

Method:

1. Set Up the Model:

• Soak the Visking tubing in water to soften it.
• Tie one end of the tubing securely and open the other end.

2. Prepare the Mixture:

• Mix starch solution with amylase solution and pour into the Visking tubing.

3. Submerge the Tubing:

• Place the filled tubing into a beaker of distilled water, ensuring the tubing is fully submerged.

4. Leave to React:

• Allow the setup to stand for 20–30 minutes to give the enzyme time to hydrolyse starch into maltose.

5. Test the Contents:

• Take samples of the water outside the tubing and test for:
• Starch: Add iodine solution. If no colour change occurs (stays orange-brown), starch has not passed through.
• Sugar: Add Benedict's reagent and heat. A brick-red precipitate indicates the presence of reducing sugars (e.g., maltose).

Results:

• Starch:
  • Starch molecules are too large to pass through the partially permeable membrane, so no starch is found outside the tubing.
• Maltose:
  • Maltose (produced by amylase breaking down starch) is small enough to diffuse through the membrane, so reducing sugars are detected in the water.

Conclusion:

• The Visking tubing shows that large molecules (starch) cannot pass through a partially permeable membrane, while small molecules (maltose) can.
• This models the digestion of carbohydrates in the small intestine, where enzymes break down large molecules into smaller ones for absorption.

Limitations of the Model:

1. No Active Transport:

• Unlike real intestinal absorption, the model does not demonstrate active transport, which requires energy and carrier proteins.

2. No Blood Flow:

• The model does not account for the role of blood in maintaining a concentration gradient by removing absorbed molecules.

3. Simplified Structure:

• The tubing lacks adaptations like villi and microvilli that increase the surface area for absorption.

Applications:

• Demonstrates the importance of enzymes in digestion.
• Shows how molecular size affects diffusion through a membrane.
• Highlights the role of the small intestine in breaking down and absorbing nutrients.