Study of plasmolysis in epidermal peels (e.g. Rhoeo or lily leaves or onion bulb scale leaves)

Understanding the fascinating world of plant cell behavior through plasmolysis experiments using onion bulb scale leaves and Rhoeo leaves

Introduction

Plasmolysis represents one of the most fundamental concepts in plant cell biology, demonstrating how plant cells respond to hypertonic environments. This laboratory experiment focuses on observing plasmolysis in epidermal peels from easily accessible plant materials like onion bulb scale leaves and Rhoeo leaves, providing students with hands-on experience in understanding cellular osmosis and water relations.

Aim

To study and demonstrate the phenomenon of plasmolysis in epidermal peels of onion bulb scale leaves or Rhoeo leaves by observing the effects of hypertonic salt solution on plant cells under a compound microscope.

Apparatus Required

Essential Equipment:

• Compound microscope
• Glass slides and cover slips
• Dropping pipettes
• Forceps
• Scalpel or sharp blade
• Absorbent paper or filter paper
• Watch glass or petri dish

Chemical Solutions:

• Distilled water
• 10% sodium chloride (NaCl) solution or 5-10% sucrose solution
• Dilute safranin solution (optional stain)

Specimen Materials:

• Fresh onion bulb scale leaves
• Fresh Rhoeo leaves
• Lily bulb scale leaves (alternative)

Theory

What is Plasmolysis?

Plasmolysis is the process where the protoplast (cytoplasm and nucleus) of a plant cell shrinks away from the cell wall due to water loss through osmosis when placed in a hypertonic solution. This occurs because the concentration of solutes outside the cell is higher than inside, causing water to move out of the cell.

Types of Plasmolysis:

1. Incipient Plasmolysis: Initial stage where the protoplast just begins to detach from the cell wall
2. Evident Plasmolysis: Advanced stage where clear shrinkage of protoplast is visible

Important Terms:

• Hypertonic Solution: Higher solute concentration outside the cell
• Hypotonic Solution: Lower solute concentration outside the cell
• Isotonic Solution: Equal solute concentration on both sides
• Plasmolysis Reversal (Deplasmolysis): Restoration of normal cell structure when transferred to hypotonic solution

Biological Significance:

Understanding plasmolysis helps explain:

• Plant wilting mechanism
• Osmoregulation in plants
• Cell turgor pressure maintenance
• Plant adaptation to saline environments

Procedure

Preparation of Epidermal Peel:

1. Onion Bulb Scale Leaves:

   • Take a fresh onion bulb scale leaf
   • Peel off the thin, transparent membrane from the inner concave surface using forceps
   • Cut a small, thin piece (1-2 cm²) for observation

2. Rhoeo Leaves:

   • Select a fresh Rhoeo leaf
   • Make a small transverse section and carefully peel off the lower epidermis
   • Ensure the peel is thin and transparent

Microscopic Observation:

Step 1: Normal Cell Observation (Control)

1. Place the epidermal peel on a clean glass slide
2. Add 1-2 drops of distilled water
3. Cover with a cover slip, avoiding air bubbles
4. Observe under low power first, then high power objective
5. Note the normal turgid condition of cells
6. Record observations

Step 2: Inducing Plasmolysis

1. Remove the slide from the microscope stage
2. Add 2-3 drops of 10% NaCl solution at one edge of the cover slip
3. Place absorbent paper at the opposite edge to draw the solution underneath
4. Allow 5-10 minutes for the process to occur
5. Observe under the microscope again

Step 3: Reversal of Plasmolysis (Optional)

1. Remove the slide and add distilled water at one edge
2. Use absorbent paper at the opposite edge to draw water underneath
3. Observe the gradual return of normal cell condition

Observation Table

Detailed Observations:

Normal Condition (Control):

• Cells appear turgid and swollen
• Protoplast pressed firmly against cell wall
• Large, prominent central vacuole
• Clear nucleus visible
• Regular cell shape (rectangular in onion)

Plasmolysed Condition:

• Protoplast visibly shrunk and rounded
• Clear space between protoplast and cell wall
• Cell contents concentrated in center
• Cell walls appear more prominent
• Irregular cell shape

Result

The experiment successfully demonstrates plasmolysis in plant cells. When epidermal peels from onion or Rhoeo leaves are placed in hypertonic NaCl solution:

1. Water moves out of the cells due to osmosis
2. Protoplast shrinks and detaches from the cell wall
3. Clear evidence of plasmolysis is observed under the microscope
4. Upon returning to distilled water, cells regain their normal turgid condition (deplasmolysis)

Conclusion: The experiment confirms that plant cells in hypertonic solutions lose water and undergo plasmolysis, proving the osmotic nature of water movement in plant cells.

Precautions

1. Specimen Preparation:

   • Use very thin, transparent peels for better observation
   • Handle peels gently to avoid tearing
   • Use fresh materials for optimal results

2. Microscopic Technique:

   • Avoid air bubbles under the cover slip
   • Use proper focusing techniques to prevent crushing cells
   • Adjust light intensity appropriately

3. Solution Handling:

   • Use freshly prepared solutions
   • Apply solutions carefully to avoid washout
   • Allow sufficient time for the process to occur

4. General Precautions:

   • Clean all apparatus properly before use
   • Handle microscope with care
   • Record observations immediately

Viva Question Answers

Q1: What is plasmolysis?

A: Plasmolysis is the process where the protoplast of a plant cell shrinks away from the cell wall when placed in a hypertonic solution due to water loss through osmosis.

Q2: Why don't animal cells show plasmolysis?

A: Animal cells lack a rigid cell wall, so they simply shrink or may even burst when placed in hypertonic solutions. The cell wall in plant cells prevents bursting and allows observation of plasmolysis.

Q3: What is the difference between plasmolysis and cytolysis?

A: Plasmolysis involves shrinkage of protoplast from cell wall in hypertonic conditions, while cytolysis is the bursting of cells in hypotonic conditions due to excessive water intake.

Q4: Why are onion peels commonly used for this experiment?

A: Onion peels are:

• Thin and transparent for microscopic observation
• Easily available and inexpensive
• Have large, easily visible cells
• Cell wall and nucleus are clearly visible

Q5: What happens if you put plasmolysed cells in distilled water?

A: The cells undergo deplasmolysis - water enters the cells, protoplast swells and comes back in contact with the cell wall, restoring normal turgidity.

Q6: What is the significance of the cell wall in plasmolysis?

A: The rigid cell wall provides structural support and prevents the cell from bursting. It also makes plasmolysis visible by maintaining cell shape while protoplast shrinks.

Q7: Can plasmolysis occur in living cells?

A: Yes, plasmolysis can occur in living cells as long as the cell membrane is intact and functional for osmosis to take place.

Q8: What is incipient plasmolysis?

A: Incipient plasmolysis is the initial stage where the protoplast just begins to pull away from the cell wall corners, indicating the start of plasmolysis process.

Q9: Why is NaCl solution used instead of plain water?

A: NaCl solution creates a hypertonic environment necessary for plasmolysis. Plain water would be hypotonic and cause swelling (cytolysis) instead of shrinkage.

Q10: What is the role of osmosis in plasmolysis?

A: Osmosis drives the movement of water molecules from inside the cell (lower solute concentration) to outside the cell (higher solute concentration), causing water loss and subsequent protoplast shrinkage.

Applications and Extensions

This fundamental experiment has broader applications in:

• Understanding plant water relations
• Studying salt tolerance in crops
• Investigating osmoregulation mechanisms
• Plant breeding for drought resistance

Safety Considerations

• Handle all chemicals with care
• Wash hands after handling specimens
• Dispose of biological waste properly
• Use microscope according to manufacturer guidelines

Conclusion

The study of plasmolysis using epidermal peels provides an excellent hands-on approach to understanding osmosis and water relations in plant cells. Onion bulb scale leaves and Rhoeo leaves serve as ideal specimens due to their readily available, thin, and transparent epidermal layers. This experiment not only demonstrates a fundamental biological process but also helps students grasp the practical applications of osmosis in plant physiology and environmental adaptations.

Through careful observation and systematic methodology, students can clearly visualize how plant cells respond to different osmotic conditions, laying a strong foundation for advanced studies in plant biology and biophysics.