NCERT Class 9 Science — Chapter 1: The fundamental unit of life

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Which one of the following best explains why a plant cell does not burst when placed in a hypotonic solution?

Official previous-year question

Verified 1 Aug 2026.

Options

AThe plasma membrane becomes impermeable to water.
BThe central vacuole actively pumps out excess water.
CThe cytoplasm solidifies to prevent expansion.
DThe rigid cell wall exerts an equal and opposite pressure against the swelling cell.

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Solution

Correct Option

The correct option is

The rigid cell wall exerts an equal and opposite pressure against the swelling cell.

Explanation

When a plant cell is placed in a hypotonic solution (a solution with higher water potential than the cell sap), water enters the cell via endosmosis. This influx of water causes the protoplast to swell and exert outward pressure, known as Turgor Pressure, against the cell wall.

Analysis of Options

The plasma membrane becomes impermeable to water. is Incorrect: The plasma membrane remains selectively permeable. It does not become impermeable to water; rather, the net influx of water stops only when the internal pressure equals the osmotic potential, not because the membrane changes its properties.
The rigid cell wall exerts an equal and opposite pressure against the swelling cell. is Correct: The plant cell wall is a rigid, non-elastic structure made primarily of cellulose. As the cell swells, the wall exerts an inward pressure called Wall Pressure that is equal and opposite to the Turgor Pressure. This mechanical resistance prevents the cell from expanding beyond its limit and bursting (lysis).
The central vacuole actively pumps out excess water. is Incorrect: While the central vacuole stores water and contributes to turgidity, it does not actively pump out excess water to prevent bursting in this context. Contractile vacuoles perform this function in certain unicellular organisms (like Amoeba), but not in typical plant cells.
The cytoplasm solidifies to prevent expansion. is Incorrect: The cytoplasm does not solidify. The structural integrity is maintained by the external cell wall, not by a phase change in the internal cytoplasm.

Key Takeaway
Animal cells burst in hypotonic solutions because they lack a rigid boundary. Plant cells survive and become turgid because the rigid cell wall exerts Wall Pressure to counteract the osmotic entry of water.

Your note

More from Chapter 1: The fundamental unit of life

With reference to Plastids, consider the following statements: 1. They are present only in plant cells. 2. Chromoplasts are colourless plastids, while leucoplasts contain chlorophyll. 3. Unlike mitochondria, plastids lack their own DNA and ribosomes. Which of the statements given above is/are correct?

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Which of the following pairs is correctly matched?

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Consider the following statements about Vacuoles: 1. They are small-sized in animal cells while plant cells have very large vacuoles. 2. In Amoeba, the food vacuole is absent. 3. They can play a role in expelling excess water from the cell. Which of the statements given above is/are correct?

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With reference to osmosis, consider the following statements: 1. Osmosis is the net diffusion of water across a selectively permeable membrane. 2. If a cell is placed in a hypotonic solution, it is likely to shrink. 3. Water molecules pass across the membrane in only one direction during osmosis. Which of the statements given above is/are correct?

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A plant cell is placed in a hypertonic solution. Which of the following phenomena is most likely to occur?

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Correct Option

The correct option is

The rigid cell wall exerts an equal and opposite pressure against the swelling cell.

Explanation

Analysis of Options

The plasma membrane becomes impermeable to water. is Incorrect: The plasma membrane remains selectively permeable. It does not become impermeable to water; rather, the net influx of water stops only when the internal pressure equals the osmotic potential, not because the membrane changes its properties.
The rigid cell wall exerts an equal and opposite pressure against the swelling cell. is Correct: The plant cell wall is a rigid, non-elastic structure made primarily of cellulose. As the cell swells, the wall exerts an inward pressure called Wall Pressure that is equal and opposite to the Turgor Pressure. This mechanical resistance prevents the cell from expanding beyond its limit and bursting (lysis).
The central vacuole actively pumps out excess water. is Incorrect: While the central vacuole stores water and contributes to turgidity, it does not actively pump out excess water to prevent bursting in this context. Contractile vacuoles perform this function in certain unicellular organisms (like Amoeba), but not in typical plant cells.
The cytoplasm solidifies to prevent expansion. is Incorrect: The cytoplasm does not solidify. The structural integrity is maintained by the external cell wall, not by a phase change in the internal cytoplasm.