Class 11 Biology Notes Chapter 7 (Chapter 7) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 7 from your Lab Manual, which deals with a fundamental biological process: Osmosis, demonstrated using a potato osmometer. This is a classic experiment and understanding its principles is crucial.

Chapter 7: Study of Osmosis by Potato Osmometer

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

2. Introduction & Principle:

• Osmosis: This is a special type of diffusion involving the movement of solvent molecules (typically water in biological systems) across a semi-permeable membrane. The movement occurs from a region where the solvent concentration is high (which means solute concentration is low, or water potential is high) to a region where the solvent concentration is low (solute concentration is high, or water potential is low).
• Semi-permeable Membrane: A membrane that allows the passage of certain molecules (like water) but restricts the passage of other molecules (like larger solute molecules, e.g., sugar). In this experiment, the plasma membranes of the living potato cells collectively act as the semi-permeable membrane.
• Potato Osmometer: A simple device prepared from a potato tuber where a cavity is created to hold a solution, and the potato tissue itself acts as the membrane system separating this solution from the external environment (water in the beaker).
• Water Potential (Ψw): This is a measure of the free energy of water molecules and predicts the direction in which water will flow. Pure water has the highest water potential (defined as zero at standard temperature and pressure). Adding solutes lowers the water potential (makes it negative). Water always moves from a region of higher water potential to a region of lower water potential.
• Principle of the Experiment: When a potato cavity containing a concentrated solution (like sugar solution, having low water potential) is placed in pure water (having high water potential), water molecules will move from the beaker into the potato cavity through the semi-permeable potato cell membranes, driven by the difference in water potential. This influx of water causes the level of the solution in the cavity to rise.

3. Materials Required:

• A large, fresh potato tuber
• Concentrated sugar solution (e.g., 20-30%) or salt solution
• Beaker or Petri dish
• Plain water
• Sharp knife or scalpel
• Peeler
• A common pin or All pin

4. Procedure:

1. Preparation: Select a large, firm potato. Peel off its skin using a peeler. Cut one end flat to create a stable base.
2. Cavity Creation: From the opposite end, carefully scoop out a deep cavity using a knife/scalpel. Ensure the walls and base of the cavity are relatively thin but remain intact (no punctures).
3. Filling: Pour the concentrated sugar solution into the potato cavity, filling it about half to two-thirds full.
4. Marking: Mark the initial level of the sugar solution in the cavity by inserting a pin horizontally into the potato wall at the solution's surface.
5. Setup: Place the prepared potato osmometer into a beaker or Petri dish containing plain water. Ensure the water level in the beaker is below the rim of the potato cavity but high enough to cover a significant portion of the potato base and sides.
6. Control Setup (Crucial): Prepare another potato exactly the same way, but fill its cavity with plain water instead of sugar solution. Place this control potato in another beaker containing plain water.
7. Observation Period: Leave both setups undisturbed for about 1-2 hours.
8. Record Observations: Observe the level of the liquid inside the cavities of both the experimental (sugar solution) and control (water) setups relative to the initial pin mark.

5. Observations:

• Experimental Setup (Sugar Solution): The level of the liquid (sugar solution) inside the potato cavity rises significantly above the initial pin mark. The potato tissue around the cavity might feel slightly firmer.
• Control Setup (Water): There is no significant change, or only a negligible rise, in the water level inside the potato cavity.

6. Explanation & Inference:

• In the experimental setup, the water in the beaker is hypotonic (lower solute concentration, higher water potential) compared to the sugar solution inside the cavity, which is hypertonic (higher solute concentration, lower water potential).
• The living potato cells act as a semi-permeable membrane system.
• Driven by the water potential gradient, water molecules move from the beaker (higher Ψw) into the potato cavity (lower Ψw) through the potato cells. This net influx of water into the cavity is called endosmosis.
• The accumulation of water inside the cavity causes the solution level to rise.
• In the control setup, the water inside the cavity and the water outside in the beaker are essentially isotonic (similar water potential). Therefore, there is no significant water potential gradient, and hence, no net movement of water occurs, resulting in no noticeable change in the water level inside the cavity.
• Conclusion: This experiment successfully demonstrates the phenomenon of osmosis – the movement of water across a semi-permeable membrane from a region of high water potential to a region of low water potential. It also highlights that living cells with intact plasma membranes are necessary for this process.

7. Precautions:

1. Use a fresh, turgid potato; older, softer potatoes may not work well.
2. The cavity walls should be thin enough to allow osmosis but thick enough to be structurally sound and without any holes.
3. The base must be flat for stability.
4. The sugar solution must be sufficiently concentrated to create a noticeable water potential gradient.
5. Mark the initial level accurately with the pin.
6. The water level in the beaker must be below the cavity rim but sufficient to immerse the lower part of the potato.
7. A control setup is absolutely essential for comparing results and validating the conclusion.
8. Ensure the potato cells are living (if the potato were boiled first, the cell membranes would be destroyed, denaturing proteins and making them fully permeable, thus preventing osmosis).

Multiple Choice Questions (MCQs):

1. Osmosis is the movement of:
   a) Solute molecules from low to high concentration across a semi-permeable membrane.
   b) Solvent molecules from high to low solvent concentration across a semi-permeable membrane.
   c) Both solute and solvent molecules across any membrane.
   d) Water molecules from low water potential to high water potential.

2. In the potato osmometer experiment, what acts as the semi-permeable membrane?
   a) The potato skin
   b) The cell walls of potato cells
   c) The plasma membranes of potato cells
   d) The sugar solution itself

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

4. The movement of water into the potato cavity in this experiment is specifically termed:
   a) Exosmosis
   b) Plasmolysis
   c) Imbibition
   d) Endosmosis

5. What is the primary purpose of the control setup (potato cavity filled with water)?
   a) To show that the potato absorbs water regardless of the solution.
   b) To demonstrate that osmosis requires a living membrane.
   c) To prove that water movement only occurs if there is a concentration gradient (difference in water potential).
   d) To check if the pin mark was accurate.

6. Compared to the pure water in the beaker, the sugar solution inside the experimental potato cavity is:
   a) Isotonic
   b) Hypotonic
   c) Hypertonic
   d) Impermeable

7. What would likely happen if the potato used in the experiment was boiled before setting up the osmometer?
   a) Osmosis would occur much faster.
   b) The sugar level would rise even higher.
   c) No significant rise in the sugar solution level would occur.
   d) The sugar solution would leak out through the base.

8. Water moves across the semi-permeable membrane primarily due to a difference in:
   a) Temperature
   b) Pressure potential only
   c) Water potential
   d) Solute size

9. Which of the following is a crucial precaution for the potato osmometer experiment?
   a) Using distilled water only in the beaker.
   b) Ensuring the cavity walls are intact and not punctured.
   c) Keeping the setup in direct sunlight.
   d) Using a very dilute sugar solution.

10. The net direction of water movement during osmosis is always from a region of:
    a) Higher solute concentration to lower solute concentration.
    b) Lower water potential to higher water potential.
    c) Higher water potential to lower water potential.
    d) Lower solvent concentration to higher solvent concentration.

Answer Key for MCQs:

1. b
2. c
3. b
4. d
5. c
6. c
7. c
8. c
9. b
10. c

Study these notes carefully, focusing on the definitions, the experimental setup, why the results are observed, and the importance of the control. Good luck with your preparation!