Class 11 Biology Notes Chapter 9 (Chapter 9) – Lab Manual (English) Book

Detailed Notes with MCQs of the practical aspects covered around Chapter 9 in your Lab Manual. These experiments are crucial not just for your practical exams but often form the basis for conceptual questions in various government exams. We'll delve into the key experiments typically found here, focusing on the principles and observations you need to know.

Core Experiments (Often around #9 in sequence): Plant Physiology

These experiments demonstrate fundamental physiological processes in plants.

1. Experiment: To Study Osmosis by Potato Osmometer

• Aim: To demonstrate the process of osmosis using a potato tuber.
• Principle:
  • Osmosis: 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 semi-permeable membrane.
  • Semi-permeable Membrane: A membrane that allows the passage of solvent molecules but restricts the movement of solute molecules. In this experiment, the cell membranes of the potato tuber cells act as the semi-permeable membrane.
  • Water Potential (Ψw): The potential energy of water per unit volume relative to pure water in reference conditions. Water moves from a region of higher water potential to lower water potential. Pure water has the highest water potential (zero under standard conditions). Adding solutes lowers the water potential (makes it negative).
  • Hypertonic Solution: A solution with a lower water potential (higher solute concentration) compared to another solution or cell sap.
  • Hypotonic Solution: A solution with a higher water potential (lower solute concentration) compared to another solution or cell sap.
  • Isotonic Solution: A solution with the same water potential (same solute concentration) as another solution or cell sap.
• Materials: Large potato tuber, knife/scalpel, beaker, water, concentrated sugar solution, pin.
• Procedure Summary:
  1. Peel a large potato and cut its base flat.
  2. Scoop out a cavity from the top, creating a cup-like structure. Ensure the walls are reasonably thin but intact.
  3. Fill the potato cavity about halfway with the concentrated sugar solution. Mark the initial level with a pin.
  4. Place the potato osmometer in a beaker containing pure water, ensuring the water level outside is below the pin mark but covers a significant portion of the potato base.
  5. Set up a control: A similar potato osmometer filled with water placed in a beaker of water, OR an empty potato osmometer in water, OR a boiled potato osmometer with sugar solution in water (to show living cells are necessary).
  6. Observe the level of the sugar solution in the cavity after some time (e.g., 1-2 hours).
• Observations:
  • The level of the sugar solution inside the potato cavity rises.
  • In the control setup (e.g., boiled potato), no significant rise in the level is observed.
• Inference/Conclusion:
  • Water moved from the beaker (higher water potential) into the potato cavity containing sugar solution (lower water potential) through the potato cells acting as a semi-permeable membrane. This demonstrates osmosis (specifically, endosmosis into the cavity).
  • The control shows that osmosis requires living, intact cell membranes (boiling destroys the membranes).
• Significance: Demonstrates a fundamental process for water absorption by root hairs and cell-to-cell water movement in plants.

2. Experiment: To Study Plasmolysis in Plant Cells (e.g., Epidermal Peels of Rhoeo leaf)

• Aim: To observe and study the phenomenon of plasmolysis in plant cells.
• Principle:
  • Plasmolysis: The shrinkage of the protoplast (the living part of the cell including the plasma membrane, cytoplasm, and nucleus) away from the cell wall due to exosmosis when a plant cell is placed in a hypertonic solution.
  • Exosmosis: The outward movement of water from a cell into the surrounding solution when the solution is hypertonic to the cell sap.
  • Turgor Pressure (Ψp): The pressure exerted by the protoplast against the cell wall due to the entry of water. In a flaccid cell, it's zero. In a turgid cell, it's positive.
  • Wall Pressure: The counter-pressure exerted by the elastic cell wall on the expanding protoplast. Normally equals turgor pressure.
  • Incipient Plasmolysis: The stage where the protoplast just begins to separate from the cell wall, usually at the corners. Turgor pressure is zero.
  • Evident Plasmolysis: The stage where the protoplast has shrunk considerably and is visibly detached from the cell wall.
  • Deplasmolysis: The recovery of the plasmolysed cell (protoplast swells and presses against the cell wall again) when placed back in a hypotonic solution or pure water, due to endosmosis.
• Materials: Rhoeo leaf (or onion bulb), slide, coverslip, microscope, forceps, needle, water, concentrated salt or sugar solution (hypertonic).
• Procedure Summary:
  1. Peel a thin layer of the lower epidermis from the Rhoeo leaf (it's coloured, making observation easier).
  2. Mount the peel in a drop of water on a slide and observe under the microscope. Note the turgid cells with the protoplast pressing against the cell wall.
  3. Prepare another peel and mount it directly in a drop of the hypertonic (salt/sugar) solution.
  4. Observe under the microscope immediately and then periodically. Note the changes in the cells.
  5. (Optional) To observe deplasmolysis: Replace the hypertonic solution surrounding a plasmolysed peel with pure water using blotting paper and observe again.
• Observations:
  • In water (hypotonic/isotonic): Cells are turgid, protoplast fills the cell, pressing against the cell wall. The coloured cell sap fills the vacuole.
  • In hypertonic solution: Water moves out (exosmosis). The protoplast shrinks and pulls away from the cell wall, starting at the corners (incipient plasmolysis) and then becoming more pronounced (evident plasmolysis). The space between the cell wall and shrunk protoplast is filled with the external hypertonic solution.
  • During deplasmolysis: If placed back in water, water re-enters the cell (endosmosis), the protoplast swells and presses against the cell wall again.
• Inference/Conclusion:
  • Plant cells undergo plasmolysis when placed in a hypertonic solution due to water loss via exosmosis.
  • The cell wall is fully permeable (allows both solute and solvent), while the plasma membrane acts as the selectively permeable membrane.
  • Plasmolysis demonstrates the osmotic behaviour of plant cells and the presence of turgor pressure in normal conditions. Deplasmolysis shows the process is reversible if the cell is not damaged.
• Significance: Plasmolysis is used in preserving foods (jams, pickles) by preventing microbial growth. It also explains wilting in plants under water stress.

3. Experiment: To Study Imbibition in Seeds/Raisins

• Aim: To demonstrate the phenomenon of imbibition using seeds (like gram or pea) or raisins.
• Principle:
  • Imbibition: A special type of diffusion where water is absorbed by solid particles (colloids) causing them to enormously increase in volume. It involves both adsorption onto the surface and absorption into the material.
  • Imbibant: The solid substance that imbibes water (e.g., dry seeds, wood, proteins).
  • Imbibate: The liquid (usually water) that is imbibed.
  • Water Potential Gradient: Water moves from its higher potential (pure water) to the very low (highly negative) water potential of the dry imbibant.
  • Prerequisites: A water potential gradient between the imbibant and the liquid, and affinity between the imbibant and the liquid are necessary.
  • Imbibition Pressure: The pressure generated by the swelling imbibant due to water uptake. Can be very high (e.g., splitting rocks).
• Materials: Dry seeds (gram, pea) or raisins, beaker, water, balance (optional).
• Procedure Summary:
  1. Take a known weight or number of dry seeds/raisins. Note their initial size/volume.
  2. Place them in a beaker and add sufficient water to completely submerge them.
  3. Leave for a few hours or overnight.
  4. Observe the change in size/volume and texture. Measure the final weight (optional, after gently blotting dry).
• Observations:
  • The seeds/raisins swell up considerably.
  • Their volume and weight increase.
  • They become softer.
• Inference/Conclusion:
  • The dry seeds/raisins absorbed water by the process of imbibition, due to the presence of hydrophilic colloids (proteins, carbohydrates) in them.
• Significance: Imbibition is crucial for the initial stage of seed germination (rupturing the seed coat) and the rehydration of dry plant tissues.

Key Takeaways for Exams:

• Definitions: Be crystal clear on Osmosis, Diffusion, Plasmolysis, Imbibition, Turgor Pressure, Water Potential, Hypo/Hyper/Isotonic solutions.
• Differences: Osmosis vs Diffusion (semi-permeable membrane is key for osmosis), Plasmolysis vs Flaccidity vs Turgidity.
• Requirements: What is needed for each process? (e.g., semi-permeable membrane for osmosis, colloids & water potential gradient for imbibition).
• Examples & Significance: Know the biological importance of each process (water absorption, wilting, food preservation, seed germination).
• Potato Osmometer: Understand why the level rises and the role of the control.
• Plasmolysis Stages: Recognize incipient and evident plasmolysis. Understand deplasmolysis. Know that the cell wall is permeable.

Multiple Choice Questions (MCQs):

1. In the potato osmometer experiment, the potato tuber cavity is filled with concentrated sugar solution and placed in water. What process primarily causes the rise in the solution level inside the cavity?
   a) Diffusion
   b) Imbibition
   c) Endosmosis
   d) Active Transport

2. A plant cell placed in a hypertonic solution will undergo:
   a) Plasmolysis due to exosmosis
   b) Deplasmolysis due to endosmosis
   c) Turgidity due to endosmosis
   d) No change in shape or size

3. Which of the following acts as the semi-permeable membrane in the potato osmometer experiment?
   a) Potato peel
   b) Cell wall of potato cells
   c) Plasma membrane of potato cells
   d) Sugar solution

4. Imbibition, the absorption of water by solid colloids, requires:
   a) A semi-permeable membrane
   b) A difference in water potential and affinity between adsorbent and liquid
   c) Only a difference in solute concentration
   d) Active expenditure of energy (ATP)

5. During incipient plasmolysis, the turgor pressure (Ψp) of the cell is:
   a) Positive
   b) Negative
   c) Zero
   d) Equal to osmotic potential

6. If a plasmolysed cell (e.g., Rhoeo epidermal cell) is placed in pure water, it regains turgidity. This process is called:
   a) Imbibition
   b) Exosmosis
   c) Deplasmolysis
   d) Reverse Osmosis

7. Which part of the plant cell is primarily responsible for maintaining turgor?
   a) Cell wall
   b) Plasma membrane
   c) Large central vacuole
   d) Nucleus

8. The space between the cell wall and the shrunken protoplast in a plasmolysed cell is filled with:
   a) Air
   b) Pure water
   c) The external hypertonic solution
   d) Cell sap that leaked out

9. Why does a boiled potato fail to function as an osmometer?
   a) Boiling removes starch.
   b) Boiling makes the cell walls impermeable.
   c) Boiling destroys the semi-permeable nature of the cell membranes.
   d) Boiling increases the solute concentration inside the cells.

10. Which phenomenon is responsible for the initial swelling of dry seeds when soaked in water, leading to the rupture of the seed coat?
    a) Osmosis
    b) Diffusion
    c) Active transport
    d) Imbibition

Answer Key:

1. c) Endosmosis
2. a) Plasmolysis due to exosmosis
3. c) Plasma membrane of potato cells
4. b) A difference in water potential and affinity between adsorbent and liquid
5. c) Zero
6. c) Deplasmolysis
7. c) Large central vacuole (as it holds the cell sap whose osmotic entry of water generates turgor)
8. c) The external hypertonic solution
9. c) Boiling destroys the semi-permeable nature of the cell membranes.
10. d) Imbibition

Study these concepts thoroughly. Understanding the 'why' behind the observations is key for competitive exams. Good luck!