Class 11 Biology Notes Chapter 12 (Chapter 12) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 12 from your Biology Lab Manual, which deals with a fundamental biological process: Osmosis, demonstrated using a simple yet effective setup – the potato osmometer. This experiment is important not just for your practical exams but also conceptually for various government exams where basic biology principles are tested.

Chapter 12: Study of Osmosis by Potato Osmometer

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

2. Principle:

• Osmosis: Osmosis is a special type of diffusion involving the movement of solvent molecules (usually water in biological systems) from a region of their higher concentration (or higher water potential/lower solute concentration) to a region of their lower concentration (or lower water potential/higher solute concentration) through a selectively or semi-permeable membrane.
• Semi-permeable Membrane: This is a membrane that 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 cells collectively act as the semi-permeable membrane.
• Concentration Gradient: Osmosis occurs down a water potential gradient or across a difference in solute concentration. Water moves from a hypotonic solution (lower solute concentration, higher water concentration) to a hypertonic solution (higher solute concentration, lower water concentration). If the solutions on both sides have equal concentrations, they are isotonic, and there is no net movement of water.
• Potato Osmometer: A cavity made in a potato tuber, filled with a concentrated solution (like sugar solution) and placed in water, acts as an osmometer. The potato tissue serves as the semi-permeable membrane separating the inner concentrated solution from the outer pure water (or dilute solution).

3. Materials Required:

• A large, fresh potato tuber
• Knife or scalpel
• Peeler
• Beaker or Petri dish
• Concentrated sugar solution (e.g., 20-30% sucrose solution) or strong salt solution
• Water
• Common pins/All pins
• Measuring scale (optional, for quantitative measurement)

4. Procedure:

• Preparation of Potato: Take a fresh potato tuber and peel off its skin using a peeler. Cut one end flat to create a stable base.
• Making the Cavity: From the opposite end (top), carefully scoop out the potato tissue using a knife or scalpel to create a deep, hollow cavity. Ensure the base and walls of the cavity are reasonably thin but without any holes, leaving sufficient thickness to hold the solution and act as the membrane.
• Filling the Cavity: Pour the concentrated sugar solution into the potato cavity, filling it about halfway or two-thirds full.
• Marking the Level: Carefully insert a pin into the wall of the cavity such that its head just touches the surface of the sugar solution. This marks the initial level (L1).
• Setting up the Osmometer: Place the prepared potato osmometer in a beaker or petri dish containing pure water. Ensure the level of water outside is below the initial level of the sugar solution inside the cavity, but high enough to completely immerse the base of the potato.
• Observation Period: Leave the setup undisturbed for a period ranging from 30 minutes to a few hours (e.g., 1-2 hours).
• Control Setup (Optional but Recommended): A control can be set up simultaneously. For example:
  • A potato osmometer prepared similarly but filled with water instead of sugar solution, placed in a beaker of water. (Expected result: No significant change in water level inside).
  • An empty potato osmometer placed in water.
  • A boiled potato osmometer (boiling destroys the cell membranes) filled with sugar solution and placed in water. (Expected result: Little to no rise in level, or perhaps leakage, as membranes are damaged).

5. Observations:

• After the designated time, observe the level of the solution inside the potato cavity.
• It will be observed that the level of the sugar solution inside the cavity has risen above the initial mark (pin L1). Mark the new level with another pin (L2).
• The volume of liquid inside the cavity has increased.
• The potato wall surrounding the cavity might feel firmer or more turgid.
• In the control setup (water inside, water outside), no significant rise in the water level inside the cavity is observed.

6. Results and Inference:

• The rise in the level of sugar solution inside the potato cavity indicates that water has moved from the beaker (containing pure water – higher water concentration) into the potato cavity (containing sugar solution – lower water concentration).
• This movement occurred through the living cells of the potato tuber, whose cell membranes acted as a semi-permeable membrane.
• The process responsible for this movement of water across the semi-permeable membrane is osmosis (specifically, endosmosis, as water entered the cavity).
• The absence of a significant level change in the control setup confirms that the concentration difference is necessary for osmosis to occur.

7. Precautions:

• Use a fresh, firm potato tuber.
• The cavity walls and base should be uniformly thin but without any punctures.
• The base of the potato must be flat for stability.
• The initial level of the solution inside the cavity must be marked accurately.
• The level of water in the beaker should be lower than the initial solution level inside the cavity but sufficient to cover the osmotic surface (base).
• Ensure the sugar solution is sufficiently concentrated for a noticeable effect.
• The setup should remain undisturbed during the experiment.

Key Concepts Recap for Exams:

• Osmosis: Definition, requirements (semi-permeable membrane, concentration gradient).
• Semi-permeable membrane: Function, examples (cell membrane, parchment paper, egg membrane). In this case, potato cell membranes.
• Solutions: Hypotonic (less solute), Hypertonic (more solute), Isotonic (equal solute). Water moves from hypotonic to hypertonic.
• Endosmosis: Movement of water into a cell or system (observed here).
• Exosmosis: Movement of water out of a cell or system (would happen if the potato cavity contained water and the beaker contained concentrated sugar solution).
• Turgor Pressure: The pressure exerted by the cell contents against the cell wall (relevant to why the potato might feel firmer).

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 semi-permeable membrane?
   a) The potato skin
   b) The sugar solution
   c) The cell walls of potato cells
   d) The plasma membranes of potato cells

3. Why is the level of the sugar solution expected to rise inside the potato cavity?
   a) Sugar molecules move out into the water.
   b) Water molecules move from the beaker into the cavity.
   c) Potato cells absorb the sugar solution.
   d) Water molecules move from the cavity into the beaker.

4. If the potato tuber used was boiled before setting up the osmometer, what would likely be observed?
   a) A much faster rise in the solution level.
   b) No significant change or a very slight rise in the solution level.
   c) The sugar solution level would decrease.
   d) The potato would dissolve in the water.

5. The movement of water in osmosis occurs from a region of:
   a) Higher solute concentration to lower solute concentration.
   b) Lower water potential to higher water potential.
   c) Higher water concentration to lower water concentration.
   d) Lower osmotic pressure to higher osmotic pressure.

6. What term describes the solution in the beaker (pure water) relative to the sugar solution inside the potato cavity?
   a) Hypertonic
   b) Isotonic
   c) Hypotonic
   d) Saturated

7. Which of the following is a crucial precaution for the potato osmometer experiment?
   a) Using a very thick-walled cavity.
   b) Filling the cavity completely to the brim.
   c) Ensuring the outer water level is higher than the solution inside.
   d) Making sure the base of the cavity is intact and not punctured.

8. The specific type of osmosis occurring when water enters the potato cavity is called:
   a) Exosmosis
   b) Endosmosis
   c) Reverse Osmosis
   d) Plasmolysis

9. What is the purpose of setting up a control experiment with water inside the cavity and water outside?
   a) To show that sugar is necessary for the process.
   b) To prove that a concentration gradient is required for osmosis.
   c) To measure the rate of evaporation.
   d) To demonstrate diffusion instead of osmosis.

10. If the beaker contained a sugar solution of the same concentration as that inside the potato cavity, the solution level inside the cavity would:
    a) Rise significantly
    b) Fall significantly
    c) Remain unchanged
    d) First fall, then rise

Answer Key for MCQs:

1. c) Osmosis
2. d) The plasma membranes of potato cells
3. b) Water molecules move from the beaker into the cavity.
4. b) No significant change or a very slight rise in the solution level. (Boiling destroys the semi-permeable nature of the membranes).
5. c) Higher water concentration to lower water concentration.
6. c) Hypotonic
7. d) Making sure the base of the cavity is intact and not punctured.
8. b) Endosmosis
9. b) To prove that a concentration gradient is required for osmosis.
10. c) Remain unchanged (The solutions are isotonic, so no net osmosis occurs).

Study these notes carefully. Understanding the principle and procedure of this experiment builds a strong foundation for comprehending water transport in biological systems. Good luck with your preparation!