Class 11 Biology Notes Chapter 15 (Chapter 15) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 15 from your Biology Lab Manual. While the manual might just label it 'Chapter 15', the core experiment here is the "Study of Osmosis by Potato Osmometer". This is a classic experiment and understanding the principles behind it is crucial, not just for your practical exams but also for theoretical concepts often tested in government exams.

Here are the detailed notes covering this experiment:

Chapter 15: Study of Osmosis by Potato Osmometer

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

2. Principle:
This experiment works on the principle of Osmosis. Let's break down the key concepts:

• Osmosis: This is the net movement or diffusion of solvent molecules (usually water in biological systems) through a selectively permeable or semipermeable membrane from a region of high solvent concentration (high water potential) to a region of low solvent concentration (low water potential). It continues until equilibrium is reached.
• Semipermeable Membrane: A membrane that permits the passage of solvent molecules (like water) but restricts the movement of larger solute molecules (like sugar or salt). In the potato osmometer, the plasma membranes and tonoplasts (vacuolar membranes) of the potato cells collectively function as the semipermeable membrane.
• Water Potential (Ψw): Represents the free energy of water molecules in a system and determines the direction of water flow. Pure water has the highest water potential, conventionally set to zero (Ψw = 0) at standard temperature and pressure.
• Solute Potential (Ψs): The reduction in water potential due to the presence of dissolved solutes. Adding solutes makes the water potential negative. The more solute, the lower (more negative) the solute potential (Ψs).
• Pressure Potential (Ψp): The hydrostatic pressure exerted on water in a system. It can be positive (like turgor pressure in a plant cell) or negative (like tension in xylem).
• Water Movement: Water always moves from an area of higher water potential to an area of lower water potential. In this experiment:
  • The pure water in the beaker has high water potential (Ψw ≈ 0).
  • The concentrated sugar solution in the potato cavity has dissolved solutes, hence a lower (negative) water potential (Ψw < 0).
  • Therefore, water moves from the beaker into the potato cavity.
• Solution Types:
  • Hypertonic: A solution with a higher solute concentration (lower water potential) than another solution. (The sugar solution in the cavity is hypertonic relative to the beaker water).
  • Hypotonic: A solution with a lower solute concentration (higher water potential) than another solution. (The beaker water is hypotonic relative to the sugar solution).
  • Isotonic: Solutions with equal solute concentrations (equal water potential). No net osmosis occurs.

3. Requirements:

• One large, fresh, and firm potato tuber
• Beaker (250 ml) or Petri dish
• Concentrated sugar solution (approx. 20-30%) or salt solution
• Plain water
• Scalpel or sharp knife
• Common pins or All-pins
• Blotting paper

4. Procedure:

1. Prepare the Potato: Select a large potato. Peel its skin using the scalpel. Cut one end flat to form a stable base.
2. Make the Cavity: From the opposite (top) end, carefully scoop out the potato's flesh to create a deep, cup-like cavity in the centre. Ensure the base and walls of the cavity are reasonably thin (a few mm) but remain intact, without any cracks or punctures reaching the outside.
3. Fill the Solution: Pour the concentrated sugar solution into the potato cavity, filling it about half to two-thirds full.
4. Mark the Level: Carefully insert a pin into the wall of the potato cavity such that the pinhead just touches the surface of the sugar solution. This marks the initial level (L1).
5. Set up the Osmometer: Place the prepared potato into the beaker containing plain water. Ensure the water level in the beaker is sufficient to submerge the base of the potato but is well below the initial level (L1) of the sugar solution inside the cavity.
6. Observe: Leave the entire setup undisturbed for a period ranging from 30 minutes to 2 hours. Observe the level of the sugar solution in the cavity.
7. Record Final Level: After the observation period, note the final level of the solution in the cavity. If it has risen, you can mark it with another pin (L2).
8. Control Setup (Important for validation): It's good practice to set up a control. This could be:
   • A similar potato osmometer filled with water (instead of sugar solution) placed in a beaker of water.
   • A potato osmometer prepared using a boiled potato, filled with sugar solution, and placed in water. Boiling denatures proteins and destroys the semipermeable nature of the cell membranes.

5. Observation:

• In the experimental setup, the level of the sugar solution inside the potato cavity (L2) is observed to rise significantly above the initial level marked by the pin (L1).
• The volume of water in the beaker may decrease slightly.
• In the control setup with water in the cavity, no significant rise in the water level inside the cavity is observed.
• In the control setup with the boiled potato, no rise (or negligible rise) in the sugar solution level is observed. The membranes are no longer selectively permeable.

6. Inference/Conclusion:

• The rise in the level of sugar solution inside the potato cavity demonstrates that water molecules have moved from the beaker (region of higher water potential) into the cavity (region of lower water potential).
• This movement occurred across the living cells of the potato tuber, whose cell membranes acted as a selectively permeable membrane, allowing water to pass but restricting the larger sugar molecules.
• This process of water movement across a semipermeable membrane driven by a water potential gradient is Osmosis.
• The control experiments confirm that a concentration gradient (difference in water potential) and intact, living semipermeable membranes are necessary for osmosis to occur.

7. Precautions:

• The potato must be fresh and firm.
• The potato must be peeled.
• The cavity walls should be thin but completely intact (no leakage).
• The base must be flat for stability.
• The initial level of the solution must be marked accurately.
• The sugar solution should be concentrated enough to create a noticeable osmotic effect.
• The water level in the beaker must be below the level of the solution in the cavity initially.
• The setup should remain undisturbed during the experiment.

Multiple Choice Questions (MCQs):

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

2. In the potato osmometer, which part acts as the semipermeable membrane?
   a) The outer skin of the potato
   b) The cell walls of the potato cells
   c) The sugar solution itself
   d) The plasma membranes and tonoplasts of the potato cells

3. Why does the level of sugar solution rise in the potato cavity?
   a) Sugar moves out of the cavity into the beaker.
   b) Water moves from the beaker (higher Ψw) into the cavity (lower Ψw).
   c) Potato cells actively pump water into the cavity.
   d) Water moves from the cavity (higher Ψw) into the beaker (lower Ψw).

4. If a boiled potato is used to set up the osmometer, the level of sugar solution in the cavity will likely:
   a) Rise much faster
   b) Rise significantly after a longer time
   c) Remain unchanged or decrease due to leakage
   d) Cause the potato to swell excessively

5. The water in the beaker is considered ______ relative to the sugar solution in the cavity.
   a) Hypertonic
   b) Isotonic
   c) Hypotonic
   d) Impermeable

6. What is the purpose of peeling the potato?
   a) To make it easier to cut
   b) To expose the living cells directly to the water
   c) To remove the starch
   d) To prevent the potato from floating

7. Water potential (Ψw) is highest in:
   a) The concentrated sugar solution
   b) The cytoplasm of potato cells
   c) Pure water in the beaker
   d) A highly concentrated salt solution

8. Which of the following is NOT a necessary precaution for this experiment?
   a) Using a freshly cut potato
   b) Ensuring the cavity wall is intact
   c) Using distilled water only in the beaker
   d) Marking the initial solution level accurately

9. Osmosis involves the movement of:
   a) Solute molecules from low to high concentration
   b) Solvent molecules across a semipermeable membrane
   c) Both solute and solvent molecules equally
   d) Ions against a concentration gradient

10. If the potato cavity was filled with pure water and the beaker contained a concentrated sugar solution, what would be the expected result?
    a) The water level in the cavity would rise.
    b) The water level in the cavity would decrease.
    c) There would be no change in the water level.
    d) Sugar would move into the cavity, increasing the level.

Remember to connect this practical understanding to the theoretical concepts of transport in plants and water relations in cells. Good luck with your preparation!