Class 11 Biology Notes Chapter 10 (Chapter 10) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 10 from your Lab Manual, which deals with a fundamental biological process: Osmosis. This experiment, using a potato osmoscope, is not just important for your practical exams but also helps solidify concepts frequently tested in various government exams where Biology is a component. Pay close attention to the principles involved.

Chapter 10: Study of Osmosis by Potato Osmometer

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

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 membrane (also called semi-permeable membrane) from a region of their higher concentration (or higher water potential/hypotonic solution) to a region of their lower concentration (or lower water potential/hypertonic solution).
• Semi-permeable Membrane: A membrane that permits the passage of solvent molecules (like water) but restricts the movement of larger solute molecules (like sugar or salt). In this experiment, the plasma membranes of the potato cells collectively act as the semi-permeable membrane.
• Water Potential (Ψw): This is the potential energy of water per unit volume relative to pure water in reference conditions. It determines the direction of water movement. Water always moves from an area of higher water potential to an area of lower water potential.
  • Pure water has the highest water potential, conventionally set to zero (Ψw = 0) at standard temperature and pressure.
  • Adding solutes to water lowers its free energy and thus decreases (makes more negative) the water potential. This component is called Solute Potential (Ψs), and it's always negative.
  • Pressure greater than atmospheric pressure applied to a solution increases its water potential. This component is called Pressure Potential (Ψp). It is usually positive (e.g., turgor pressure in plant cells) but can be negative (e.g., tension in xylem).
  • Water Potential equation: Ψw = Ψs + Ψp
• Solutions:
  • Hypertonic Solution: A solution with a lower water concentration (higher solute concentration) and hence lower water potential compared to another solution or cell cytoplasm.
  • Hypotonic Solution: A solution with a higher water concentration (lower solute concentration) and hence higher water potential compared to another solution or cell cytoplasm.
  • Isotonic Solution: A solution with the same water concentration and water potential as another solution or cell cytoplasm. There is no net movement of water across the membrane.
• Potato Osmoscope: The potato tuber, with its living cells containing cell sap (a solution) and surrounded by cell membranes (acting as semi-permeable membranes), can be fashioned into a simple osmoscope to demonstrate osmosis.

3. Materials Required:

• A large, firm potato tuber
• Beaker or Petri dish
• Concentrated sugar solution (e.g., 20-30%) or salt solution
• Plain water
• Scalpel or knife
• Peeler
• A common pin or marker pen

4. Procedure:

• Preparation: Peel the skin off the potato tuber. Cut one end flat to create a stable base. From the opposite end, carefully scoop out a cavity in the centre, leaving a reasonably thin but intact wall and base (like a cup).
• Setup: Pour the concentrated sugar solution into the potato cavity, filling it about half to two-thirds full. Carefully mark the initial level of the sugar solution using a pin inserted into the potato wall at the solution's surface, or with a marker.
• Immersion: Place the prepared potato osmoscope in a beaker or Petri dish containing pure water. Ensure the water level outside is high enough to cover the base of the potato but significantly lower than the initial level of the sugar solution inside the cavity.
• Observation: Leave the setup undisturbed for about 1-2 hours. Observe any change in the level of the sugar solution inside the potato cavity relative to the marked initial level.

5. Observation:

• After the designated time, the level of the sugar solution inside the potato cavity will be observed to have risen above the initial mark.
• The volume of water in the beaker might decrease slightly.
• The potato tissue around the cavity might feel firmer.

6. Inference:

• The sugar solution inside the cavity is hypertonic (lower water potential) compared to the pure water in the beaker, which is hypotonic (higher water potential, Ψw ≈ 0).
• The living cells of the potato tuber act as a semi-permeable membrane.
• Water molecules moved from the region of higher water potential (beaker) to the region of lower water potential (cavity) across the potato cell membranes via osmosis.
• This net influx of water into the cavity caused the level of the solution inside to rise. This specific type of osmosis, where water enters the system/cell, is called endosmosis (relative to the cavity).

7. Precautions:

• Use a fresh, firm potato.
• The cavity walls should be uniformly thin but not punctured.
• The base must be flat for stability.
• Mark the initial level accurately.
• Ensure the sugar solution is sufficiently concentrated.
• The external water level should not reach the brim of the cavity.
• A control setup (e.g., potato osmoscope with water inside and outside, or using a boiled potato which kills cells and denatures membranes) can be used to confirm that the process requires living cells and a concentration gradient. In a boiled potato, membranes lose their semi-permeability, and osmosis will not occur significantly.

8. Significance & Related Concepts (Important for Exams):

• Demonstrates a fundamental process of water transport in living systems.
• Crucial for understanding water absorption by root hairs, cell-to-cell water movement, maintenance of turgor pressure in plant cells, opening/closing of stomata.
• Plasmolysis: If a plant cell is placed in a hypertonic solution, water moves out via exosmosis, causing the protoplast (cell membrane and contents) to shrink away from the cell wall.
• Turgor Pressure: The pressure exerted by the protoplast against the cell wall due to water entry via endosmosis. It keeps plant cells firm or turgid.

Multiple Choice Questions (MCQs):

1. In the potato osmoscope experiment, the semi-permeable membrane is represented by:
   a) The potato peel
   b) The cell walls of potato cells
   c) The plasma membranes of potato cells
   d) The sugar solution

2. Osmosis is the movement of:
   a) Solute molecules from lower to higher concentration across a semi-permeable membrane.
   b) Solvent molecules from lower to higher concentration across a semi-permeable membrane.
   c) Solute molecules from higher to lower concentration across a semi-permeable membrane.
   d) Solvent molecules from higher to lower solvent concentration across a semi-permeable membrane.

3. Why does the level of sugar solution rise in the potato cavity?
   a) Sugar molecules move out 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 diffuse within the cavity.

4. Which of the following has the highest water potential (Ψw)?
   a) 10% salt solution
   b) Pure water
   c) 20% sugar solution
   d) Cytoplasm of a root hair cell

5. If a boiled potato is used for the osmoscope experiment, what result would be expected?
   a) The level of sugar solution will rise much faster.
   b) The level of sugar solution will fall.
   c) No significant change or very slow rise in the level will occur.
   d) The potato will dissolve in the water.

6. The net direction of water movement between two solutions separated by a semi-permeable membrane depends on:
   a) Only the solute potential (Ψs)
   b) Only the pressure potential (Ψp)
   c) The difference in water potential (Ψw)
   d) The temperature of the solutions

7. Placing a plant cell in a hypertonic solution results in:
   a) Turgidity
   b) Plasmolysis
   c) Deplasmolysis
   d) Imbibition

8. In the potato osmoscope setup, the water in the beaker is _______ compared to the sugar solution in the cavity.
   a) Hypertonic
   b) Isotonic
   c) Hypotonic
   d) Viscous

9. Which factor is NOT directly required for osmosis to occur in this experiment?
   a) A concentration gradient
   b) A semi-permeable membrane
   c) Light energy
   d) Water (solvent)

10. Turgor pressure develops in plant cells due to:
    a) Exosmosis
    b) Active transport of salts
    c) Endosmosis
    d) Imbibition

Answer Key for MCQs:

1. c) The plasma membranes of potato cells
2. d) Solvent molecules from higher to lower solvent concentration across a semi-permeable membrane. (Note: Higher solvent concentration = Lower solute concentration = Higher water potential)
3. b) Water molecules move from the beaker into the cavity.
4. b) Pure water
5. c) No significant change or very slow rise in the level will occur. (Boiling destroys the semi-permeability of membranes)
6. c) The difference in water potential (Ψw)
7. b) Plasmolysis
8. c) Hypotonic
9. c) Light energy
10. c) Endosmosis

Study these notes carefully, focusing on the definitions and the 'why' behind the observations. Good luck with your preparation!