Class 11 Biology Notes Chapter 13 (Chapter 13) – Lab Manual (English) Book

Detailed Notes with MCQs of a fundamental biological process today, demonstrated through a classic experiment from your lab manual – the study of osmosis using a potato osmometer. This is Chapter 13, and understanding it well is crucial not just for your practical exams but also because osmosis is a key concept frequently tested in various government examinations involving biology.

Chapter 13: 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:

• Osmosis: This is the core concept. Osmosis is defined as the net movement or diffusion of solvent molecules (usually water in biological systems) across a selectively permeable (or semipermeable) membrane from a region of higher solvent potential (lower solute concentration) to a region of lower solvent potential (higher solute concentration).
• Semipermeable Membrane: A membrane that allows the passage of certain molecules (like water) but restricts the passage of other molecules (like sugar or salt solutes). In this experiment, the plasma membranes of the potato cells collectively act as the semipermeable membrane.
• Concentration Gradient: Osmosis occurs down a water potential gradient, which is influenced by solute concentration and pressure. Water moves to equalize the concentration on both sides of the membrane.
• Types of Solutions:
  • Hypotonic Solution: A solution with a lower solute concentration (higher water potential) compared to another solution (e.g., pure water compared to cell sap).
  • Hypertonic Solution: A solution with a higher solute concentration (lower water potential) compared to another solution (e.g., concentrated sugar solution compared to cell sap).
  • Isotonic Solution: A solution with the same solute concentration (equal water potential) as another solution.

3. Materials Required:

• A large, fresh potato tuber
• Peeler/Scalpel/Knife
• Petri dish or Beaker
• Concentrated sugar solution (e.g., 20-30%) or salt solution
• Distilled water
• Pin (preferably with a coloured head)
• Measuring cylinder (optional, for preparing solutions)
• Marker pen (optional)

4. Procedure:

• Preparation of Potato Osmometer:
  1. Take a large, firm potato and wash it thoroughly.
  2. Peel the skin off the potato using a peeler.
  3. Cut the potato tuber flat at one end (the base) so it can rest stably in the Petri dish/beaker.
  4. From the opposite (top) end, carefully scoop out the central part of the potato using a scalpel or knife to create a hollow cavity. The cavity should be deep, but care must be taken not to puncture the base or make the walls too thin. The walls should ideally be uniformly thick.
• Setting up the Experiment: Usually, multiple setups are prepared for comparison:
  • Setup A (Demonstrating Endosmosis):
    1. Pour distilled water into the Petri dish/beaker, enough to submerge about one-third of the potato base.
    2. Carefully fill the potato cavity about half-full with the concentrated sugar solution.
    3. 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.
  • Setup B (Control - Isotonic/No Gradient):
    1. Pour distilled water into the Petri dish/beaker.
    2. Fill the potato cavity about half-full with distilled water.
    3. Mark the initial level of the water inside the cavity with a pin.
  • Setup C (Demonstrating Exosmosis - Optional but useful):
    1. Pour concentrated sugar solution into the Petri dish/beaker.
    2. Fill the potato cavity about half-full with distilled water.
    3. Mark the initial level of the water inside the cavity with a pin.
  • Setup D (Control - Integrity Check):
    1. Keep a prepared potato osmometer (with cavity) empty and place it in a Petri dish filled with water. This checks if the cavity itself collects water mechanically.
• Observation Period: Leave the setups undisturbed for a period ranging from 30 minutes to a few hours (e.g., 1-2 hours). Observe the level of the liquid inside the potato cavity in each setup relative to the initial pin mark.

5. Observations:

• Setup A (Sugar solution in cavity, water outside): The level of the sugar solution inside the potato cavity rises significantly above the initial pin mark. The volume of liquid inside has increased.
• Setup B (Water in cavity, water outside): There is no significant change, or only a very slight negligible rise, in the water level inside the cavity.
• Setup C (Water in cavity, sugar solution outside): The level of the water inside the potato cavity decreases, falling below the initial pin mark. The volume of liquid inside has decreased.
• Setup D (Empty cavity, water outside): The cavity remains empty or shows negligible water collection.

6. Inference/Conclusion:

• Setup A: The concentration of water molecules (water potential) was higher in the beaker (distilled water) than inside the potato cavity (sugar solution). The potato tissue acted as a semipermeable membrane, allowing water molecules to move from the beaker into the cavity via osmosis (specifically, endosmosis - movement of water into the cells/cavity). This caused the level of the solution in the cavity to rise.
• Setup B: The water potential inside the cavity and outside in the beaker was roughly equal. Therefore, there was no significant net movement of water across the potato tissue, and the water level remained largely unchanged. This acts as a control.
• Setup C: The concentration of water molecules (water potential) was lower in the beaker (sugar solution) than inside the potato cavity (distilled water). Water moved from the cavity (higher water potential) across the semipermeable potato tissue into the surrounding hypertonic solution via osmosis (specifically, exosmosis - movement of water out of the cells/cavity). This caused the level of water in the cavity to fall.
• Setup D: Shows that the rise in liquid level in Setup A is not due to simple absorption or capillary action but due to the osmotic process driven by a concentration gradient across a semipermeable membrane.
• Overall Conclusion: The experiment successfully demonstrates the phenomenon of osmosis, where water moves across a semipermeable membrane (potato tissue) from a region of high water potential to a region of low water potential.

7. Precautions:

• Use a fresh, firm potato tuber.
• The cavity should be deep and have relatively thin, intact walls, but the base must not be punctured.
• The base of the potato should be cut flat for stability and proper contact with the surrounding water.
• The initial level of the solution/water inside the cavity must be marked accurately.
• The outer surface of the potato osmometer (base) should be in contact with the water/solution in the beaker/Petri dish.
• Ensure the sugar solution is sufficiently concentrated for a noticeable effect.
• Leave the setup undisturbed during the observation period.

Key Concepts Recap for Exams:

• Osmosis vs. Diffusion: Diffusion is the movement of any substance from high to low concentration, while osmosis specifically refers to the movement of solvent (usually water) across a semipermeable membrane.
• Water Potential (Ψw): The potential energy of water per unit volume relative to pure water in reference conditions. Pure water has the highest water potential (zero). Adding solutes lowers the water potential (makes it negative). Water always moves from higher water potential to lower water potential.
• Solute Potential (Ψs): The effect of dissolved solutes on water potential; it's always negative.
• Pressure Potential (Ψp): The effect of pressure on water potential; can be positive (turgor pressure) or negative (tension).
• Semipermeable Membrane: Essential for osmosis. In plants, the cell membrane (plasma membrane) and the tonoplast (vacuolar membrane) are key examples.
• Endosmosis: Movement of water into a cell/system.
• Exosmosis: Movement of water out of a cell/system.
• Turgor Pressure: The pressure exerted by the protoplast against the cell wall due to the entry of water.
• Plasmolysis: Shrinkage of protoplast away from the cell wall due to excessive exosmosis when placed in a hypertonic solution.

Multiple Choice Questions (MCQs):

1. In the potato osmometer experiment, what acts as the semipermeable membrane?
   a) The potato skin
   b) The sugar solution
   c) The cell walls of potato cells
   d) The plasma membranes of potato cells

2. If the potato cavity is filled with concentrated sugar solution and placed in a beaker of pure water, what phenomenon will be observed?
   a) Exosmosis, level in cavity decreases
   b) Endosmosis, level in cavity increases
   c) Plasmolysis, potato shrinks
   d) No change in water level

3. Osmosis is the movement of:
   a) Solute molecules from lower to higher concentration across a semipermeable membrane.
   b) Solvent molecules from higher water potential to lower water potential across a semipermeable membrane.
   c) Both solute and solvent molecules across any membrane.
   d) Ions against a concentration gradient using energy.

4. Why is the potato peeled before making the osmometer?
   a) To make it easier to cut
   b) The skin is impermeable to water
   c) To expose the living cells whose membranes act semipermeably
   d) To prevent fungal growth

5. A control setup in this experiment could involve:
   a) Filling the cavity with sugar solution and placing it in sugar solution.
   b) Filling the cavity with water and placing it in pure water.
   c) Using a boiled potato instead of a fresh one.
   d) Placing an empty potato cavity in water.

6. What is the primary driving force for osmosis?
   a) Temperature difference
   b) Difference in solute concentration
   c) Difference in water potential
   d) Atmospheric pressure

7. If the level of solution inside the potato cavity decreases, it indicates that:
   a) The solution inside the cavity was hypotonic to the surrounding solution.
   b) The solution inside the cavity was hypertonic to the surrounding solution.
   c) The solution inside the cavity was isotonic to the surrounding solution.
   d) The potato tissue was damaged.

8. Which of the following has the highest water potential?
   a) 10% Salt solution
   b) 20% Sugar solution
   c) Distilled water
   d) Cytoplasm of a root hair cell

9. The function of the pin inserted at the initial liquid level in the cavity is:
   a) To prevent evaporation
   b) To measure the temperature
   c) To serve as a reference point for observing changes in volume
   d) To hold the potato in place

10. If a boiled potato is used to set up the osmometer (cavity filled with sugar solution, placed in water), what result would you expect?
    a) The level in the cavity will rise rapidly.
    b) The level in the cavity will fall rapidly.
    c) No significant change in the level will occur.
    d) The sugar solution will leak out immediately.

Answer Key for MCQs:

1. d) The plasma membranes of potato cells
2. b) Endosmosis, level in cavity increases
3. b) Solvent molecules from higher water potential to lower water potential across a semipermeable membrane.
4. c) To expose the living cells whose membranes act semipermeably (The skin is relatively impermeable compared to the cell membranes).
5. b) Filling the cavity with water and placing it in pure water. (Also d is a valid control). Note: Option b is a better control for comparing osmotic potential differences.
6. c) Difference in water potential
7. a) The solution inside the cavity was hypotonic to the surrounding solution. (Meaning the surrounding solution was hypertonic, causing water to leave the cavity - Exosmosis).
8. c) Distilled water
9. c) To serve as a reference point for observing changes in volume
10. c) No significant change in the level will occur. (Boiling destroys the semipermeable nature of the cell membranes).

Study these notes carefully. Understand the principle behind each step and observation. Good luck with your preparation!