Class 11 Biology Notes Chapter 17 (Chapter 17) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 17 from your Biology Lab Manual, which details the experiment to study osmosis using a potato osmometer. This is a fundamental concept often tested in various government exams, especially those with a general science or biology component. Pay close attention to the principles and procedures.

Chapter 17: Study of Osmosis by Potato Osmometer

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

2. Principle:

• Osmosis: Osmosis is the movement of solvent molecules (usually water) from a region of their higher concentration (dilute solution or pure solvent) to a region of their lower concentration (concentrated solution) through a semipermeable membrane.
• Semipermeable Membrane: A membrane that allows the passage of solvent molecules but restricts the movement of solute molecules. In this experiment, the plasma membranes of the potato cells collectively act as a semipermeable membrane.
• Osmotic Pressure: The pressure that needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane. It's a measure of the tendency of a solution to take in water by osmosis.
• Potato Osmometer: A cavity is made in a peeled potato tuber. This cavity is filled with a concentrated solution (like sugar or salt solution). The potato tissue surrounding the cavity acts as the semipermeable membrane. When this setup is placed in water (hypotonic solution relative to the cavity's contents), water moves from the beaker into the potato cavity due to the concentration gradient, demonstrating osmosis.

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
• Pins
• Measuring cylinder (optional, for preparing solution)
• Bell pin needle/Common pin

4. Procedure:

• Preparation of Potato:
  • Take a large, healthy potato tuber and peel off its skin using a scalpel or knife.
  • Cut the potato flat at one end (the base) so it can rest stably in the beaker/petri dish.
  • From the opposite end, carefully scoop out the central part of the potato using the scalpel to create a deep, hollow cavity. Ensure the base of the cavity is thin but intact (not punctured). The walls should be reasonably thick.
• Setting up the Osmometer:
  • Fill the beaker or petri dish with plain water to about half its volume.
  • Carefully pour the concentrated sugar solution into the potato cavity, filling it about halfway or two-thirds full.
  • 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.
  • Carefully place the potato osmometer setup into the beaker/petri dish containing water. Ensure the water level outside is below the level of the sugar solution inside the cavity and also below the rim of the cavity.
• Control Setup (Optional but Recommended):
  • Prepare another potato osmometer similarly, but fill the cavity with plain water instead of sugar solution. Place it in another beaker containing plain water. This acts as a control to show that the level change is due to osmosis and not just absorption.
• Observation:
  • Leave the setup undisturbed for about 1-2 hours.
  • Observe the level of the sugar solution inside the potato cavity.

5. Observations:

• In the experimental setup (potato with sugar solution placed in water), the level of the sugar solution inside the cavity rises above the initial mark (indicated by the pin).
• The potato cavity may contain more liquid than initially filled.
• In the control setup (potato with water in the cavity placed in water), there is no significant change in the water level inside the cavity.

6. Inference/Conclusion:

• The potato tissue (specifically the cell membranes) acts as a semipermeable membrane.
• The concentration of water molecules is higher in the beaker (pure water) compared to the concentration of water molecules inside the potato cavity (sugar solution).
• Therefore, water molecules move from the beaker (region of higher water concentration) into the potato cavity (region of lower water concentration) through the semipermeable potato cell membranes.
• This influx of water causes the level of the solution inside the cavity to rise.
• This demonstrates the process of osmosis. The control setup confirms that the level rise is due to the difference in solute concentration across the membrane.

7. Precautions:

• Use a fresh, firm potato tuber.
• The cavity should be deep and have a thin base, but without any punctures.
• The outer surface of the potato (base) should be flat for stability.
• The initial level of the solution in the cavity must be marked accurately.
• The water level in the beaker should not cover the rim of the potato cavity.
• Ensure the sugar solution is sufficiently concentrated.
• Allow adequate time (at least an hour) for observable results.

Relevance for Exams: Understanding osmosis is crucial for explaining various biological phenomena like water absorption by plant roots, maintenance of turgor pressure in cells, water reabsorption in kidneys, etc. Questions might test the definition, the conditions required, the direction of water movement based on solution concentrations (hypotonic, hypertonic, isotonic), and the role of the semipermeable membrane.

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 membranes of the potato tuber cells
   d) The water in the beaker

2. Osmosis is the movement of:
   a) Solute molecules from dilute to concentrated solution
   b) Solvent molecules from concentrated to dilute solution
   c) Solvent molecules from dilute to concentrated solution through a semipermeable membrane
   d) Solute molecules from concentrated to dilute solution through a semipermeable membrane

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) Potato cells absorb sugar from the solution.
   d) Water molecules move from the cavity into the beaker.

4. What is the purpose of using a concentrated sugar solution in the potato cavity?
   a) To make the potato cells turgid initially.
   b) To create a lower water potential (lower water concentration) inside the cavity compared to the beaker.
   c) To provide food for the potato cells.
   d) To prevent water from entering the cavity.

5. If the potato osmometer (with sugar solution) is placed in a highly concentrated salt solution instead of plain water, what would likely be observed?
   a) The level in the cavity would rise significantly.
   b) The level in the cavity would fall.
   c) There would be no change in the level.
   d) The potato would dissolve.

6. A control setup in this experiment typically involves:
   a) Placing a potato with sugar solution in an empty beaker.
   b) Placing a potato with water in the cavity into a beaker of water.
   c) Placing a boiled potato with sugar solution into a beaker of water.
   d) Placing a plastic cup with sugar solution into a beaker of water.

7. The term 'water potential' refers to the:
   a) Amount of water present in a solution.
   b) Kinetic energy of water molecules.
   c) Tendency of water to move from one area to another.
   d) Purity of water.

8. Which of the following is NOT a necessary precaution for the potato osmometer experiment?
   a) Ensuring the potato cavity base is thin but intact.
   b) Using distilled water in the beaker.
   c) Marking the initial solution level accurately.
   d) Using a sufficiently concentrated solution in the cavity.

9. If the potato tuber used is boiled before setting up the osmometer, what will happen?
   a) Osmosis will occur faster.
   b) The level of solution in the cavity will rise higher.
   c) No osmosis will occur because the cell membranes are destroyed.
   d) The sugar solution will leak out immediately.

10. The movement of water across the semipermeable membrane continues until:
    a) All the sugar has moved out.
    b) The concentration of water is equal on both sides (equilibrium is reached).
    c) The potato cavity overflows.
    d) The water in the beaker is used up.

Answer Key:

1. c) The cell membranes of the potato tuber cells
2. c) Solvent molecules from dilute to concentrated solution through a semipermeable membrane
3. b) Water molecules move from the beaker into the cavity.
4. b) To create a lower water potential (lower water concentration) inside the cavity compared to the beaker.
5. b) The level in the cavity would fall (water would move out of the cavity into the more concentrated salt solution).
6. b) Placing a potato with water in the cavity into a beaker of water.
7. c) Tendency of water to move from one area to another. (Related to free energy/concentration of water)
8. b) Using distilled water in the beaker. (Tap water usually works fine, though distilled water is purer).
9. c) No osmosis will occur because the cell membranes are destroyed. (Boiling denatures proteins and destroys membrane integrity).
10. b) The concentration of water is equal on both sides (equilibrium is reached). (Or until limited by opposing pressure).

Study these notes thoroughly. Understanding the 'why' behind each step and observation is key for tackling exam questions effectively. Let me know if any part needs further clarification.