Class 11 Biology Notes Chapter 4 (Chapter 4) – Lab Manual (English) Book

Alright class, let's delve into the practical aspects of osmosis as detailed in Chapter 4 of your Lab Manual. This experiment, using a potato osmoscope, is fundamental to understanding how water moves across biological membranes. Pay close attention, as these concepts are frequently tested.

Chapter 4: Study of Osmosis by Potato Osmoscope

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

2. Principle:

• Osmosis: Osmosis is the net movement or diffusion of solvent molecules (usually water) across a selectively permeable or semi-permeable membrane from a region of higher solvent concentration (lower solute concentration) to a region of lower solvent concentration (higher solute concentration). It continues until equilibrium is reached.
• Semi-permeable Membrane: A membrane that allows the passage of certain molecules (like solvent molecules) but restricts the movement of other molecules (like solute molecules). In this experiment, the plasma membranes of the potato cells collectively act as a semi-permeable membrane.
• Concentration Gradient: The difference in the concentration of solutes between two solutions separated by a semi-permeable membrane drives osmosis. Water moves down its water potential gradient.
• Solutions:
  • Hypotonic Solution: A solution having lower solute concentration (higher water concentration) compared to another solution.
  • Hypertonic Solution: A solution having higher solute concentration (lower water concentration) compared to another solution.
  • Isotonic Solution: Solutions having equal solute concentrations.
• In this experiment: The sugar solution inside the potato cavity is hypertonic (high solute, low water concentration) compared to the pure water in the beaker, which is hypotonic (low solute, high water concentration). Therefore, water molecules will move from the beaker into the potato cavity through the potato cells (acting as a semi-permeable membrane).

3. Materials Required:

• A large, fresh potato tuber
• Beaker or Petri dish
• Concentrated sugar solution (e.g., 20-30%)
• Plain water
• Scalpel or knife
• Pins
• Measuring cylinder (optional, for preparing solution)
• Pen/Marker

4. Procedure:

• Preparation of Potato Osmoscope:
  • Take a large, firm potato and peel off its skin.
  • Cut one end flat to create a stable base.
  • From the opposite end, carefully scoop out the central part of the potato using a scalpel to create a deep cavity. Ensure the base of the cavity is thin but not punctured. The walls should be reasonably thick.
• Setting up the Experiment:
  • Pour concentrated sugar solution into the potato cavity, filling it about half to two-thirds full.
  • Carefully mark the initial level of the sugar solution inside the cavity using a pin.
  • Place the potato osmoscope in a beaker or Petri dish containing plain water. Ensure the water level outside is below the rim of the cavity but covers a significant portion of the potato base.
• Setting up the Control:
  • Prepare another potato osmoscope similarly.
  • Fill its cavity with plain water instead of sugar solution.
  • Mark the initial level with a pin.
  • Place it in another beaker containing plain water. (Alternatively, a control can be set up with an empty cavity, or with sugar solution inside and outside). The water-water setup is a common control to show that the level rise isn't due to absorption by potato tissue alone but requires a concentration difference.
• Observation Period: Leave both setups undisturbed for 1-2 hours.
• Recording Observations: Observe the level of the liquid inside the cavities of both the experimental and control setups. Note any changes in the firmness of the potato.

5. Observations:

• Experimental Setup (Sugar Solution inside):
  • The level of the sugar solution inside the potato cavity rises significantly above the initial mark (pin).
  • The potato wall may feel more turgid (firm).
  • Water is seen collected in the cavity.
• Control Setup (Water inside):
  • There is no significant rise (or only a very negligible rise) in the water level inside the potato cavity.
  • The potato firmness remains relatively unchanged.

6. Inference/Conclusion:

• The rise in the level of sugar solution in the experimental setup is due to the entry of water molecules from the beaker into the potato cavity through the potato tissues.
• This movement occurred because the concentration of water molecules was higher in the beaker (hypotonic solution) compared to the sugar solution inside the cavity (hypertonic solution).
• The potato cells' plasma membranes acted collectively as a semi-permeable membrane, allowing water molecules to pass through but restricting the larger sugar molecules.
• This demonstrates the process of osmosis.
• The control setup shows that without a significant concentration gradient across the semi-permeable membrane, substantial movement of water (osmosis) does not occur.

7. Precautions:

• Use a fresh, firm potato.
• The cavity should be deep, but care must be taken not to puncture the bottom wall.
• The base of the potato must be cut flat for stability.
• The sugar solution should be sufficiently concentrated to create a noticeable osmotic gradient.
• Mark the initial level of the solution accurately with a pin.
• The outer level of water in the beaker should not reach the brim of the cavity.
• A control setup is essential for valid comparison.
• Ensure the potato skin is completely removed as it can be impermeable.

Multiple Choice Questions (MCQs):

1. Osmosis is the movement of:
   a) Solute molecules from lower to higher concentration across a semi-permeable membrane.
   b) Solvent molecules from higher to lower solvent concentration across a semi-permeable membrane.
   c) Both solute and solvent molecules across any membrane.
   d) Solvent molecules from lower to higher solvent concentration across a semi-permeable membrane.
   Answer: b)

2. In the potato osmoscope experiment, what acts as the semi-permeable membrane?
   a) The potato skin
   b) The cell walls of potato cells
   c) The plasma membranes of potato cells
   d) The sugar solution
   Answer: c)

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) Potato cells absorb sugar actively.
   d) Water molecules move from the cavity into the beaker.
   Answer: b)

4. The water in the beaker surrounding the potato osmoscope (with sugar solution inside) is considered:
   a) Hypertonic compared to the cavity solution
   b) Isotonic compared to the cavity solution
   c) Hypotonic compared to the cavity solution
   d) A non-polar solvent
   Answer: c)

5. What is the primary purpose of the control setup (potato cavity filled with water, placed in water)?
   a) To show that potato absorbs water anyway.
   b) To demonstrate that a concentration gradient is necessary for osmosis.
   c) To check if the potato leaks.
   d) To measure the rate of evaporation.
   Answer: b)

6. What would likely happen if the potato used for the osmoscope was boiled before the experiment?
   a) Osmosis would occur faster.
   b) The sugar level would rise much higher.
   c) No significant osmosis would occur.
   d) Sugar would leak out rapidly.
   Answer: c) (Boiling destroys the semi-permeable nature of cell membranes)

7. A crucial precaution while making the potato cavity is:
   a) To make it very shallow.
   b) To ensure the base is not punctured.
   c) To use a blunt knife.
   d) To leave the skin intact at the base.
   Answer: b)

8. If the concentration of sugar solution inside the cavity was equal to the concentration of a solution in the beaker, the level inside the cavity would:
   a) Rise significantly
   b) Fall significantly
   c) Remain almost unchanged
   d) First rise and then fall
   Answer: c) (Isotonic conditions)

9. The movement of water during osmosis occurs down the:
   a) Solute potential gradient
   b) Water potential gradient
   c) Pressure gradient
   d) Temperature gradient
   Answer: b)

10. In the experimental setup, the net movement of water stops (or slows drastically) when:
    a) All the sugar has moved out.
    b) All the water from the beaker has moved in.
    c) The water potential inside the cavity equals the water potential outside.
    d) The potato cells burst.
    Answer: c) (Equilibrium is approached)

Study these notes thoroughly. Understanding the principle and procedure of this experiment is key. Good luck with your preparation!