Q.61 If protoplasts are placed in distilled water, they swell and burst as a result of endosmosis.
The plot representing the kinetics of burst is

Introduction

When protoplasts (plant cells without a cell wall) are placed in distilled water, they experience endosmosis—the inward movement of water due to a hypotonic environment. Because protoplasts lack the rigid cell wall, excessive water entry causes them to swell and eventually burst.

A common exam question asks: Which graph correctly represents the kinetics of protoplast bursting over time?
Let’s analyze the correct answer and clearly explain all four options.

Understanding the Axes of the Graph

• Y-axis: Number of intact protoplasts

• X-axis: Time

As time increases in distilled water:

• Protoplasts initially remain intact

• Then rapidly burst due to osmotic pressure

• Finally, very few or no intact protoplasts remain

This produces a sigmoidal (S-shaped) decreasing curve.

✅ Correct Answer: Option (A)

Why Option (A) Is Correct

• Shows a high initial number of intact protoplasts

• Followed by a lag phase where swelling occurs

• Then a rapid decline as many protoplasts burst

• Finally reaches a plateau near zero, when most protoplasts have burst

This perfectly matches the kinetics of bursting due to endosmosis.

📌 Key Concept: Bursting is not instantaneous; it occurs progressively, giving a sigmoid decrease in intact cells.

❌ Explanation of Incorrect Options

Option (B): Increasing Sigmoidal Curve

• Shows the number of intact protoplasts increasing with time

• This is biologically incorrect because:

  • Protoplasts do not regenerate or increase in distilled water

  • Endosmosis causes bursting, not survival or multiplication

🚫 Incorrect trend direction

Option (C): Linear Increase

• Indicates a steady increase in intact protoplasts over time

• This contradicts osmotic principles:

  • Distilled water is hypotonic

  • Protoplasts should lose integrity, not gain it

🚫 No biological relevance

Option (D): Linear Decrease

• Shows a constant, straight-line decline

• Bursting does not occur at a constant rate

• Real biological systems show:

  • Lag phase

  • Rapid bursting phase

  • Plateau phase

🚫 Oversimplified and ignores kinetics

Final Conclusion

✔️ Correct Answer: Option (A)

Option (A) accurately represents the kinetics of protoplast bursting in distilled water, showing a sigmoidal decrease in the number of intact protoplasts due to endosmosis.

Exam Tip (NEET / CSIR / GATE Biology)

Whenever you see:

• Protoplast + distilled water

• Think endosmosis → swelling → burst

• Choose a sigmoid decreasing curve, not a straight line or increasing graph