Class 11 Biology Notes Chapter 14 (Chapter 14) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 14 from your Class 11 Biology Lab Manual, which deals with the Study of Osmosis by Potato Osmometer. This is a fundamental experiment often tested in various government exams, especially those with a biology component, as it demonstrates a key physiological process.

Chapter 14: Study of Osmosis by Potato Osmometer

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

2. Introduction to Osmosis:
Osmosis is a special type of diffusion. It 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 concentration (or higher water potential) to a region of lower solvent concentration (or lower water potential). This movement continues until equilibrium is reached.

3. Principle:

• A semipermeable membrane 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 tuber cells collectively act as a semipermeable membrane.
• Water Potential (Ψw): It is a measure of the free energy of water molecules per unit volume. Pure water has the highest water potential, conventionally set to zero at standard temperature and pressure. Adding solutes lowers the water potential (makes it negative).
• Solute Potential (Ψs): The magnitude of lowering of water potential due to the dissolution of solute. It is always negative.
• Pressure Potential (Ψp): The hydrostatic pressure exerted on the solution. It can be positive (turgor pressure) or negative (tension).
• Water always moves from an area of higher water potential to an area of lower water potential.
• In this setup, the concentrated sugar solution inside the potato cavity has a lower water potential (more negative Ψs) compared to the pure water or dilute solution outside in the beaker (higher water potential, closer to zero). Therefore, water molecules move from the beaker into the potato cavity across the cell membranes.

4. Materials Required:

• A large, fresh potato tuber
• Peeler/Scalpel/Knife
• Beaker or Petri dish
• Concentrated sugar solution (e.g., 20-30%)
• Distilled water
• Common pins/All pins
• Measuring cylinder (optional, for preparing solution)
• Marker pen

5. Procedure:

• Preparation of Potato Osmometer:
  • Take a large, firm potato tuber and wash it thoroughly.
  • Peel off the skin from the potato.
  • Cut the potato flat at one end (base) so it can rest stably in the beaker/Petri dish.
  • From the other end, scoop out a cavity in the centre using a scalpel or knife, leaving a reasonably thick wall (at least 0.5 cm to 1 cm) at the base and sides. Ensure the cavity does not puncture the base. This hollowed potato acts as the osmometer.
• Setting up the Experiment:
  • Pour distilled water into the beaker or Petri dish, enough to submerge about one-third of the potato base.
  • Carefully fill the potato cavity about half to two-thirds full with the concentrated sugar solution.
  • Mark the initial level of the sugar solution inside the cavity by inserting a common pin horizontally into the potato wall at the solution's surface.
  • Carefully place the potato osmometer in the beaker containing distilled water. Ensure the water level outside is lower than the sugar solution level inside the cavity.
• Control Setup (Optional but Recommended):
  • Prepare another potato osmometer similarly.
  • Fill the cavity with distilled water instead of sugar solution.
  • Place it in another beaker containing distilled water. Mark the initial water level inside the cavity with a pin.
• Observation:
  • Leave the setup undisturbed for about 1-2 hours (or longer, depending on concentration and potato).
  • Observe the level of the sugar solution in the cavity of the experimental setup and the water level in the cavity of the control setup.

6. Observations:

• Experimental Setup: The level of the sugar solution inside the potato cavity rises significantly above the initial mark (pin). Water may also collect in the cavity if left for too long. The potato tissue around the cavity might feel firmer.
• Control Setup: There is no significant change, or only a very slight change, in the water level inside the potato cavity.
• Reason: Water molecules from the beaker (higher water potential) moved into the potato cavity containing sugar solution (lower water potential) through the potato cells acting as a semipermeable membrane. This influx of water increased the volume of the solution inside the cavity, causing the level to rise. In the control setup, the water potential inside and outside the cavity was nearly equal, hence negligible net movement of water occurred.

7. Inference/Conclusion:

• The experiment demonstrates the process of osmosis.
• The living cells of the potato tuber act as a selectively permeable membrane system.
• Water moves from a region of its higher concentration (or higher water potential) to a region of its lower concentration (or lower water potential) across this membrane.

8. Precautions:

• Use a fresh, firm potato tuber.
• The cavity should be deep enough, but the base must not be punctured.
• The walls of the cavity should be sufficiently thick and intact.
• The sugar solution should be concentrated enough for a noticeable effect.
• The initial level of the sugar solution should be marked carefully.
• The outer surface of the potato base should be in contact with the water in the beaker.
• The water level in the beaker should be below the level of the sugar solution in the cavity initially.
• The control setup is essential to confirm that the level rise is due to osmosis and not just absorption by dead tissue or evaporation.

Key Concepts for Government Exams:

• Osmosis: Definition, direction of water movement.
• Semipermeable Membrane: Definition, examples (cell membrane, parchment paper, egg membrane).
• Solutions:
  • Hypertonic: Higher solute concentration (lower water potential). Cells placed in it lose water (plasmolysis).
  • Hypotonic: Lower solute concentration (higher water potential). Cells placed in it gain water (become turgid).
  • Isotonic: Same solute concentration (equal water potential). No net movement of water.
• Water Potential (Ψw): Concept, components (Ψs + Ψp), unit (Pascals/Bars), significance in water transport. Pure water has Ψw = 0.
• Plasmolysis: Shrinkage of protoplast away from the cell wall due to exosmosis when a plant cell is placed in a hypertonic solution.
• Turgor Pressure: The pressure exerted by the protoplast against the cell wall due to the entry of water. It prevents excessive water entry and cell bursting in plant cells.

Multiple Choice Questions (MCQs):

1. The primary aim of the potato osmometer experiment is to demonstrate:
   a) Diffusion
   b) Active Transport
   c) Osmosis
   d) Imbibition

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

3. Why is the level of sugar solution observed to rise in the potato cavity?
   a) Sugar molecules move out of the cavity into the beaker.
   b) Water molecules move from the beaker into the cavity.
   c) Water molecules move from the cavity into the beaker.
   d) Sugar molecules actively pull water into the cavity.

4. If the potato cavity was filled with distilled water and placed in a beaker of concentrated salt solution, what would be observed?
   a) The water level in the cavity would rise.
   b) The water level in the cavity would fall.
   c) The water level in the cavity would remain unchanged.
   d) The potato would dissolve.

5. A control setup is used in this experiment primarily to:
   a) Speed up the process of osmosis.
   b) Ensure the potato used is fresh.
   c) Compare the results and rule out other factors causing level change.
   d) Measure the exact concentration of the sugar solution.

6. Water potential is highest for:
   a) 10% salt solution
   b) Pure water
   c) 20% sugar solution
   d) Cytoplasm of root hair cell

7. The net direction of water movement between two solutions separated by a semipermeable membrane is always from:
   a) Higher solute potential to lower solute potential
   b) Lower water potential to higher water potential
   c) Higher water potential to lower water potential
   d) Lower solute concentration to higher solute concentration

8. Which of the following precautions is essential for the potato osmometer experiment?
   a) Using boiled potato
   b) Making the cavity puncture the base
   c) Ensuring the potato base is submerged in water
   d) Filling the cavity completely with sugar solution

9. If a plant cell is placed in a hypertonic solution, it undergoes:
   a) Turgidity
   b) Plasmolysis
   c) Imbibition
   d) Deplasmolysis

10. The water potential (Ψw) of a solution is determined by:
    a) Solute potential (Ψs) only
    b) Pressure potential (Ψp) only
    c) Both solute potential (Ψs) and pressure potential (Ψp)
    d) Temperature only

Answer Key for MCQs:

1. c) Osmosis
2. c) The plasma membranes of potato cells
3. b) Water molecules move from the beaker into the cavity.
4. b) The water level in the cavity would fall. (Water moves out into the hypertonic salt solution)
5. c) Compare the results and rule out other factors causing level change.
6. b) Pure water
7. c) Higher water potential to lower water potential
8. c) Ensuring the potato base is submerged in water
9. b) Plasmolysis
10. c) Both solute potential (Ψs) and pressure potential (Ψp)

Study these notes thoroughly. Understanding the concept of water potential and how osmosis works is crucial. Good luck with your preparation!