Class 11 Biology Notes Chapter 6 (Chapter 6) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 6 from your Biology Lab Manual, which deals with a fundamental biological process: Osmosis, demonstrated using a potato osmometer. This is an important experiment often tested, so pay close attention to the details for your exam preparation.

Chapter 6: Study of Osmosis by Potato Osmometer

1. Aim:
To study and demonstrate the process of osmosis using a potato tuber as an osmometer.

2. Principle:
This experiment is based on the principle of Osmosis.

• Osmosis: It is the movement of solvent molecules (usually water in biological systems) from a region of their higher concentration (or higher water potential/lower solute concentration) to a region of their lower concentration (or lower water potential/higher solute concentration) through a semipermeable membrane.
• Semipermeable Membrane: A membrane that allows the passage of solvent molecules (like water) but restricts the movement of solute molecules (like sugar or salt). In this experiment, the plasma membranes of the potato cells collectively act as the semipermeable membrane.
• Water Potential (Ψw): It is the measure of the relative tendency of water to move from one area to another. Pure water has the highest water potential (defined as zero under standard conditions). Adding solutes lowers the water potential (making it negative). Water always moves from a region of higher water potential to a region of lower water potential.
• Solution Types:
  • Hypotonic Solution: A solution with lower solute concentration (higher water potential) compared to another solution (e.g., cell sap). If a cell is placed in it, water enters the cell (endosmosis).
  • Hypertonic Solution: A solution with higher solute concentration (lower water potential) compared to another solution. If a cell is placed in it, water leaves the cell (exosmosis).
  • Isotonic Solution: A solution with the same solute concentration (same water potential) as another solution. There is no net movement of water.

In this setup, the concentrated sugar solution inside the potato cavity has a lower water potential than the pure water in the beaker. The potato tissue acts as the semipermeable membrane, allowing water to move from the beaker into the cavity.

3. Materials Required:

• A large, fresh potato tuber
• Beaker or Petri dish
• Concentrated sugar solution (e.g., 20-30% sucrose solution)
• Plain water
• Scalpel or knife
• Pin (preferably with a coloured head)
• Blotting paper
• Measuring cylinder (for preparing solution, optional)

4. Procedure:

• Prepare the Potato Osmometer:
  • Take a large potato and peel off its outer skin using a scalpel.
  • Cut one end flat to create a stable base.
  • From the opposite end, carefully scoop out the central part of the potato using the scalpel to create a deep, hollow cavity. Ensure the base of the cavity is thin but not punctured. The walls should be reasonably thick.
• Prepare the Solutions:
  • Prepare a concentrated sugar solution (e.g., dissolve 20-30g of sugar in enough water to make 100ml solution).
  • Have plain water ready in the beaker.
• Set up the Experiment:
  • Fill the potato cavity about half to two-thirds full with the concentrated sugar solution.
  • Carefully mark the initial level of the sugar solution inside the cavity by inserting a pin horizontally into the potato wall at the solution's surface.
  • Place the prepared potato osmometer in a beaker or petri dish containing plain water. Ensure the water level in the beaker is below the level of the sugar solution in the cavity, but high enough to cover a significant portion of the potato base.
• Observation Period: Leave the setup undisturbed for about 1-2 hours.
• Record Observations: Observe the level of the solution inside the potato cavity relative to the pin marker.

(Optional but recommended: Set up a control - a potato osmometer prepared similarly but filled with water instead of sugar solution, placed in a beaker of water. Another control could be an empty cavity placed in water.)

5. Observations:

• After 1-2 hours, the level of the sugar solution inside the potato cavity is observed to have risen above the initial mark indicated by the pin.
• The volume of water in the beaker may decrease slightly.
• In the control setup (if used, with water in the cavity), no significant rise in the water level inside the cavity is observed.

6. Inference/Conclusion:

• The rise in the level of sugar solution inside the potato cavity indicates that water molecules have moved from the beaker (region of higher water potential - pure water) into the potato cavity (region of lower water potential - sugar solution).
• This movement occurred across the potato cells, whose plasma membranes acted collectively as a semipermeable membrane.
• Therefore, the experiment successfully demonstrates the phenomenon of osmosis (specifically, endosmosis into the cavity).

7. Precautions:

• The potato must be fresh and firm.
• The outer skin must be completely removed as it is impermeable to water.
• The cavity should be deep enough to hold a sufficient amount of solution.
• The base of the cavity must remain intact; it should not be punctured.
• The base of the potato should be cut flat so it rests evenly in the beaker.
• The sugar solution must be concentrated enough to create a significant water potential gradient.
• The initial level of the solution in the cavity must be marked accurately.
• The water level in the beaker should be sufficient to immerse the base but should not reach the brim of the cavity initially.

Multiple Choice Questions (MCQs):

1. The phenomenon demonstrated by the potato osmometer experiment is:
   a) Diffusion
   b) Active Transport
   c) Osmosis
   d) Imbibition

2. In the potato osmometer experiment, the semipermeable membrane is represented by:
   a) The cell wall of potato cells
   b) The plasma membrane of potato cells
   c) The sugar solution
   d) The peel of the potato

3. Water moves into the potato cavity because:
   a) The water potential inside the cavity is higher than outside.
   b) The solute concentration inside the cavity is lower than outside.
   c) The water potential in the beaker (pure water) is higher than inside the cavity (sugar solution).
   d) The potato cells actively pump water into the cavity.

4. What would happen if the potato osmometer cavity was filled with pure water and placed in a beaker containing concentrated sugar solution?
   a) The water level inside the cavity would rise.
   b) The water level inside the cavity would fall.
   c) There would be no change in the water level.
   d) The potato would disintegrate immediately.

5. Why is the skin of the potato peeled off before making the osmometer?
   a) To make it easier to cut.
   b) The skin is impermeable to water.
   c) The skin contains toxic substances.
   d) To allow sugar to leak out.

6. The purpose of marking the initial level of the sugar solution with a pin is:
   a) To prevent leakage.
   b) To measure the initial volume accurately.
   c) To provide a reference point for observing changes in the solution level.
   d) To hold the potato in place.

7. If you forget to add sugar to the water inside the potato cavity (i.e., fill it with pure water) and place it in a beaker of pure water, what would you observe after 2 hours?
   a) The water level in the cavity would rise significantly.
   b) The water level in the cavity would fall significantly.
   c) There would be little to no change in the water level inside the cavity.
   d) The potato would swell and burst.

8. Osmosis is a special type of diffusion involving the movement of:
   a) Solute molecules across any membrane.
   b) Solvent molecules across a semipermeable membrane.
   c) Gas molecules from high to low concentration.
   d) Ions against a concentration gradient.

9. Which of the following has the highest water potential?
   a) 10% Sugar solution
   b) 20% Sugar solution
   c) Pure water
   d) Cytoplasm of potato cells

10. A key precaution while making the cavity in the potato is:
    a) To make it as wide as possible.
    b) To ensure the base of the cavity is not punctured.
    c) To use boiled potato.
    d) To add salt instead of sugar.

Answer Key for MCQs:

1. c) Osmosis
2. b) The plasma membrane of potato cells
3. c) The water potential in the beaker (pure water) is higher than inside the cavity (sugar solution).
4. b) The water level inside the cavity would fall (exosmosis).
5. b) The skin is impermeable to water.
6. c) To provide a reference point for observing changes in the solution level.
7. c) There would be little to no change in the water level inside the cavity.
8. b) Solvent molecules across a semipermeable membrane.
9. c) Pure water
10. b) To ensure the base of the cavity is not punctured.

Study these notes carefully. Understanding the principle of osmosis and the specifics of this experiment is crucial. Good luck with your preparation!