Class 9 Science Notes Chapter 5 (Chapter 5) – Lab Manual (English) Book

Alright class, let's focus on Chapter 5 from your Science Lab Manual. This chapter is crucial as it deals with fundamental biological techniques and observations, often forming the basis for questions in various government exams. We'll cover the key experiments, observations, and concepts you need to know.

Chapter 5: Study of Cells and Osmosis

This chapter primarily covers three important experiments:

1. Preparation of a temporary mount of an onion peel.
2. Preparation of a temporary mount of human cheek cells.
3. Demonstration of osmosis using raisins or apricots.

Let's break down each one.

Experiment 1: Preparation of Temporary Mount of Onion Peel

• Aim: To prepare a stained temporary mount of an onion peel and observe its cells under a microscope.
• Materials Required: Onion bulb, glass slide, coverslip, forceps, needle, watch glass, dropper, water, safranin stain, blotting paper, glycerine, compound microscope.
• Procedure Highlights:
  1. Take a fleshy leaf from an onion bulb.
  2. Break it and carefully peel off a thin, transparent layer (epidermis) from the inner concave surface using forceps.
  3. Immediately place the peel in a watch glass containing water to prevent drying.
  4. Transfer the peel to another watch glass containing a few drops of Safranin stain. Leave for 1-2 minutes. (Safranin is used because it stains the nucleus and cell wall, making them clearly visible).
  5. Briefly dip the stained peel in water to remove excess stain.
  6. Place a drop of glycerine on a clean glass slide. (Glycerine prevents the specimen from drying out quickly and provides a clearer view).
  7. Carefully transfer the stained peel onto the glycerine on the slide using a needle or brush. Ensure the peel is flat and not folded.
  8. Gently lower a coverslip onto the peel using a needle, avoiding air bubbles. (Lower it at an angle).
  9. Remove any excess glycerine/stain from the edges of the coverslip using blotting paper.
  10. Observe under the low power and then high power of the compound microscope.
• Observations:
  • Numerous cells are seen, tightly packed together.
  • Cells are generally rectangular or polygonal in shape.
  • A distinct cell wall is visible surrounding each cell.
  • Inside the cell wall is the cell membrane (often pressed against the cell wall).
  • A prominent, darkly stained nucleus is visible, usually towards the periphery (side) of the cell.
  • Lightly stained cytoplasm fills the cell.
  • A large central vacuole occupies most of the cell volume, pushing the nucleus and cytoplasm to the periphery.
• Inference: The onion peel shows typical plant cell structure with a definite shape, cell wall, large central vacuole, and peripheral nucleus.
• Precautions:
  • Use a thin, transparent peel.
  • Avoid folding the peel.
  • Do not over-stain or under-stain the peel.
  • Avoid trapping air bubbles under the coverslip.
  • Handle the slide and coverslip carefully.

Experiment 2: Preparation of Temporary Mount of Human Cheek Cells

• Aim: To prepare a stained temporary mount of human cheek cells and observe them under a microscope.
• Materials Required: Toothpick (blunt, sterile) or ice cream spoon, glass slide, coverslip, dropper, water, methylene blue stain, blotting paper, glycerine, compound microscope.
• Procedure Highlights:
  1. Rinse your mouth thoroughly with water.
  2. Gently scrape the inner surface of your cheek with the blunt end of a clean toothpick or spoon. (Avoid injury).
  3. Smear the collected material in a drop of water placed on a clean glass slide. Spread it thinly.
  4. Add a drop of Methylene Blue stain. Leave for 1-2 minutes. (Methylene blue stains the nucleus and cytoplasm of animal cells).
  5. Add a drop of glycerine.
  6. Carefully lower a coverslip, avoiding air bubbles.
  7. Remove excess liquid with blotting paper.
  8. Observe under low power and then high power.
• Observations:
  • Cells are scattered, not tightly packed like onion cells.
  • Cells are irregular in shape (no fixed shape).
  • No cell wall is present. Only a thin outer boundary, the cell membrane (plasma membrane), is visible.
  • A distinct, darkly stained nucleus is usually located near the center of the cell.
  • Lightly stained, granular cytoplasm fills the cell.
  • No large central vacuole is seen. Small vacuoles, if present, are not prominent.
• Inference: Cheek cells show typical animal cell structure with an irregular shape, absence of a cell wall and large central vacuole, and a centrally located nucleus.
• Precautions:
  • Use a sterile toothpick/spoon and scrape gently.
  • Spread the cells thinly on the slide.
  • Avoid air bubbles.
  • Do not over-stain.

Comparison: Onion Peel Cell vs. Human Cheek Cell

Experiment 3: Osmosis using Raisins/Apricots

• Aim: To demonstrate osmosis by observing the effect of water and concentrated solution on raisins or dried apricots.
• Materials Required: Dried raisins/apricots, beakers/bowls, water, concentrated sugar or salt solution, weighing balance (optional but good for quantitative results).
• Procedure Highlights:
  1. Take a few dried raisins/apricots. Note their appearance and weight (if using a balance).
  2. Place some in a beaker containing plain water (hypotonic solution relative to raisin cells).
  3. Place some in another beaker containing a concentrated sugar or salt solution (hypertonic solution relative to raisin cells).
  4. Leave them undisturbed for a few hours (e.g., 2-4 hours or overnight).
  5. Observe the changes in size, texture, and weight (if measured).
• Observations:
  • In Water: Raisins/apricots swell up, become turgid (firm), and increase in weight. This is due to endosmosis – water moves from the beaker (higher water concentration) into the raisin cells (lower water concentration) through their cell membranes (acting as semi-permeable membranes).
  • In Concentrated Solution: Raisins/apricots shrink, become flaccid (limp), and decrease in weight (or show less swelling than those in water if initially placed in water then transferred). This is due to exosmosis – water moves out from the raisin cells (higher water concentration) into the surrounding solution (lower water concentration).
• Inference: This experiment demonstrates osmosis – the movement of water molecules from a region of higher water concentration to a region of lower water concentration through a semi-permeable membrane. Cell membranes act as semi-permeable membranes.
• Key Terms:
  • Osmosis: Movement of solvent (usually water) across a semi-permeable membrane from a dilute solution to a concentrated solution.
  • Semi-permeable Membrane: A membrane that allows certain molecules (like water) to pass through but restricts others (like sugar solutes).
  • Hypotonic Solution: A solution with a lower solute concentration (higher water concentration) than the cell. Cells placed in it gain water (Endosmosis).
  • Hypertonic Solution: A solution with a higher solute concentration (lower water concentration) than the cell. Cells placed in it lose water (Exosmosis).
  • Isotonic Solution: A solution with the same solute concentration as the cell. No net movement of water occurs.
  • Endosmosis: Movement of water into the cell.
  • Exosmosis: Movement of water out of the cell.
  • Plasmolysis: (Observed in plant cells like Rhoeo leaf in hypertonic solution) The shrinking of the protoplast (cell membrane + cytoplasm + nucleus) away from the cell wall due to excessive water loss via exosmosis.

Multiple Choice Questions (MCQs)

1. Which stain is commonly used to prepare a temporary mount of onion peel cells?
   a) Methylene Blue
   b) Iodine Solution
   c) Safranin
   d) Eosin

2. In the temporary mount of an onion peel, the nucleus is typically observed:
   a) In the center of the cell
   b) Attached to the cell membrane
   c) Near the periphery of the cell
   d) Inside the large central vacuole

3. Why is glycerine added to the slide before placing the coverslip in preparing temporary mounts?
   a) To stain the cells
   b) To prevent the specimen from drying out
   c) To kill the cells
   d) To dissolve the cell wall

4. A student observed cells under a microscope that were irregular in shape and lacked a cell wall. These cells are most likely:
   a) Onion peel cells
   b) Bacterial cells
   c) Human cheek cells
   d) Yeast cells

5. When preparing a temporary mount of human cheek cells, which stain is generally used?
   a) Safranin
   b) Methylene Blue
   c) Iodine Solution
   d) Cotton Blue

6. A key difference observed between onion peel cells and human cheek cells under a microscope is the presence of:
   a) Cytoplasm
   b) Nucleus
   c) Cell membrane
   d) Cell wall

7. Raisins swell when placed in plain water because:
   a) Water moves out of the raisins (Exosmosis)
   b) Water moves into the raisins (Endosmosis)
   c) Sugar from raisins dissolves in water
   d) The skin of the raisin dissolves

8. The process responsible for the swelling of raisins in water is:
   a) Diffusion
   b) Active Transport
   c) Osmosis
   d) Imbibition

9. If a plant cell (like Rhoeo leaf cell) is placed in a highly concentrated salt solution, what phenomenon will be observed?
   a) Turgidity
   b) Plasmolysis
   c) Endosmosis
   d) Cell bursting

10. To avoid air bubbles while preparing a temporary mount, the coverslip should be:
    a) Dropped quickly onto the slide
    b) Placed vertically on the slide
    c) Lowered gently at an angle using a needle
    d) Heated slightly before placing

Answer Key:

1. c) Safranin
2. c) Near the periphery of the cell
3. b) To prevent the specimen from drying out
4. c) Human cheek cells
5. b) Methylene Blue
6. d) Cell wall
7. b) Water moves into the raisins (Endosmosis)
8. c) Osmosis
9. b) Plasmolysis
10. c) Lowered gently at an angle using a needle

Study these notes carefully, focusing on the procedures, observations, and the reasons behind them. Understanding these basics is key for your exams. Good luck!