Class 11 Biology Notes Chapter 16 (Chapter 16) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 16 from your Lab Manual, which deals with the study of osmosis using a potato osmometer. This is a fundamental experiment often asked about in various exams, so pay close attention.

Chapter 16: Study of Osmosis by Potato Osmometer

1. Aim:
To 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 solvent molecules (like water) but restricts the movement of solute molecules (like sugar or salt).
• Potato Tuber: In this experiment, the living cells of the potato tuber collectively act as a semipermeable membrane. The cell membranes (plasma membrane) and the tonoplast (vacuolar membrane) within the potato cells are selectively permeable.
• Water Potential (Ψw): Water moves from an area of higher water potential to an area of lower water potential. Pure water has the highest water potential (defined as zero under standard conditions). Adding solutes lowers the water potential (makes it more negative).
• Setup: A cavity is made in a potato tuber, which is filled with a concentrated solution (like sugar or salt solution, having low water potential). This potato is then placed in a beaker of pure water (having high water potential). Water is expected to move from the beaker into the potato cavity across the potato cells.

3. Materials Required:

• A large, fresh potato tuber
• Beaker or Petri dish
• Concentrated sugar solution (e.g., 20-30%) or salt solution
• Plain water
• Scalpel or knife
• Pin (preferably with a coloured head)
• Peeler

4. Procedure:

• Preparation of Potato Osmometer:
  • Take a large, healthy potato tuber and peel off its skin using a peeler.
  • Cut one end of the potato flat to create a stable base.
  • From the opposite end, carefully scoop out the central part of the potato using a scalpel or knife to create a deep, hollow cavity. Ensure the base of the cavity is thin but intact (no holes). The walls should also be reasonably thick.
• Filling the Osmometer:
  • Pour the concentrated sugar solution into the potato cavity, filling it about half to two-thirds full.
• Marking the Initial Level:
  • Carefully insert a pin into the wall of the cavity such that the pinhead marks the initial level of the sugar solution.
• Setting up the Experiment:
  • Place the potato osmometer carefully into a beaker or petri dish containing plain water.
  • Ensure that the level of water in the beaker is below the level of the sugar solution inside the cavity, but high enough to cover the base and lower sides of the potato that are in contact with the water.
• Observation Period:
  • Leave the setup undisturbed for a period ranging from 30 minutes to a few hours (observation time may vary).
• Control Setup (Optional but Recommended):
  • A control can be set up using a boiled potato prepared similarly, or a potato osmometer filled with water instead of sugar solution and placed in water, or a potato osmometer with sugar solution placed in an empty beaker (no external water). This helps confirm that the observed phenomenon is indeed osmosis due to living, semipermeable cells.

5. Observation:

• After the observation period, you will notice that the level of the sugar solution inside the potato cavity has risen above the initial mark indicated by the pin.
• The volume of water in the beaker might decrease slightly.
• The potato tissue might feel firmer.
• Explanation: Water molecules move from the beaker (higher water potential) into the potato cavity (lower water potential due to dissolved sugar) through the potato cells acting as a semipermeable membrane. This influx of water causes the level of the solution in the cavity to rise.
• Control Observation: In a control setup using a boiled potato (where cells are dead and membranes lose semipermeability) or one with water in the cavity, no significant rise in the liquid level inside the cavity will be observed.

6. Result:
The rise in the level of sugar solution within the potato cavity demonstrates that water has moved from the beaker into the cavity across the potato tissue. This confirms the process of osmosis.

7. Precautions:

• Use a fresh, large potato tuber.
• The cavity should be deep and have a thin, intact base.
• The outer surface of the potato (base) should be flat for stability and proper contact with water.
• The initial level of the solution in the cavity must be marked accurately.
• The concentration of the sugar/salt solution should be sufficiently high to create a significant water potential gradient.
• Ensure the potato cavity is watertight.
• The level of water in the beaker should not be higher than the initial level of the solution in the cavity.

8. Significance for Exams:

• This experiment provides a clear, visual demonstration of osmosis.
• Understand the definitions: Osmosis, semipermeable membrane, water potential, hypotonic (water in beaker relative to solution), hypertonic (solution in cavity relative to water).
• Know the role of potato cells as the semipermeable membrane.
• Be able to explain why the water level rises (movement down the water potential gradient).
• Understand the importance of a control setup.
• Relate this to biological processes like water absorption by plant roots, maintenance of turgor pressure in cells, and water movement between cells.

Multiple Choice Questions (MCQs):

1. The process 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 potato skin
   b) The sugar solution
   c) The cell membranes of potato tuber cells
   d) The beaker wall

3. Water moves into the potato cavity because the water potential inside the cavity (containing sugar solution) is:
   a) Higher than the water in the beaker
   b) Lower than the water in the beaker
   c) Equal to the water in the beaker
   d) Equal to zero

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

5. What is the primary driving force for the movement of water in osmosis?
   a) Solute concentration gradient
   b) Temperature difference
   c) Difference in water potential
   d) Hydrostatic pressure

6. If a boiled potato is used instead of a fresh one in the setup, what observation is expected?
   a) The level of solution in the cavity will rise much faster.
   b) The level of solution in the cavity will fall.
   c) No significant change or only a very slight rise in the level will occur.
   d) The potato will dissolve in the water.

7. What is the purpose of peeling the potato?
   a) To make it look clean
   b) To remove the impermeable corky layer (skin) allowing better contact with water
   c) To reduce the potato's weight
   d) To make it easier to cut

8. The pin is inserted into the potato cavity to:
   a) Prevent leakage
   b) Stir the solution
   c) Mark the initial level of the solution
   d) Provide support to the potato

9. Which of the following is NOT a necessary precaution for this experiment?
   a) Ensuring the potato cavity base is intact
   b) Using a highly concentrated sugar solution
   c) Keeping the setup in direct sunlight for faster results
   d) Marking the initial solution level accurately

10. Osmosis is biologically important for which of the following processes in plants?
    a) Absorption of minerals
    b) Absorption of water by root hairs
    c) Translocation of food
    d) Photosynthesis

Answer Key for MCQs:

1. c) Osmosis
2. c) The cell membranes of potato tuber cells
3. b) Lower than the water in the beaker
4. c) Hypertonic
5. c) Difference in water potential
6. c) No significant change or only a very slight rise in the level will occur. (Boiling destroys the semipermeable nature of cell membranes)
7. b) To remove the impermeable corky layer (skin) allowing better contact with water
8. c) Mark the initial level of the solution
9. c) Keeping the setup in direct sunlight for faster results (Temperature affects rate, but direct sunlight isn't a necessary precaution and could cause other issues)
10. b) Absorption of water by root hairs

Study these notes carefully. Understanding the principle and the steps involved is crucial. Good luck with your preparation!