Understanding Plant Cells in a Hypertonic Solution

This question explores what happens when a plant cell, specifically from an epidermal leaf peel, is placed in a hypertonic solution. Understanding the concept of osmosis and cell membranes is key to answering this.

What is a Hypertonic Solution?

A hypertonic solution is a solution that has a higher solute concentration, and therefore a lower water potential, compared to the cell it surrounds. In the context of a plant cell, this means the solution outside the cell has less free water molecules than the cell's cytoplasm.

Osmosis and Plant Cells

Osmosis is the movement of water molecules across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration). Plant cells have both a cell membrane (selectively permeable) and a cell wall (fully permeable).

When a plant cell is placed in a hypertonic solution:

• Water moves out of the cell's cytoplasm, across the cell membrane, into the surrounding hypertonic solution.
• This movement occurs because the water potential is higher inside the cell than outside in the hypertonic solution.
• The cell wall provides structural support but is fully permeable to water and solutes. It does not prevent water loss.

Observing Plasmolysis

As water leaves the cell, the volume of the cytoplasm shrinks. The cell membrane pulls away from the rigid cell wall. This phenomenon is called plasmolysis.

When the student examines the epidermal leaf peel cells under the microscope after immersion in the hypertonic solution, they would observe that:

• The cytoplasm and cell membrane have shrunk.
• The cell membrane has detached from the cell wall.
• The cell appears 'shrunken' inside its cell wall. This is the state of plasmolysis.

Analyzing the Options

Let's consider the given options based on our understanding of plasmolysis in a hypertonic solution:

• Option 1: 'the cells swelled.' Swelling of cells happens when water enters the cell, typically in a hypotonic solution, not a hypertonic one. So, this is incorrect.
• Option 2: 'the cells were plasmolysed.' This describes the exact phenomenon observed when a plant cell loses water to a hypertonic solution, causing the cell membrane and cytoplasm to shrink away from the cell wall. This aligns with our understanding.
• Option 3: 'the cells built up turgor pressure.' Turgor pressure is the pressure exerted by the cell contents against the cell wall when the cell swells due to water intake in a hypotonic solution. In a hypertonic solution, water leaves the cell, turgor pressure decreases and eventually becomes negative. So, this is incorrect.
• Option 4: 'the cells size was unaffected.' The cell size (specifically the volume of the protoplast, which includes the cytoplasm and cell membrane) is significantly affected as water leaves the cell. The cell wall maintains the overall shape, but the living part shrinks. So, this is incorrect.

Therefore, the observation in a hypertonic solution is that the cells become plasmolysed.

Revision Table: Cell Behavior in Different Solutions

Additional Information: Understanding Plasmolysis Stages

Plasmolysis is a process that occurs in stages:

• Incipient Plasmolysis: This is the initial stage where the cell just begins to lose water, and the protoplast starts pulling away from the cell wall at the corners. Turgor pressure is zero.
• Evident Plasmolysis: More water is lost, and the protoplast significantly shrinks and detaches from the cell wall throughout the cell.
• Complete Plasmolysis: The protoplast has shrunk maximally, and the cell wall is no longer under any pressure from the protoplast.

Plasmolysis is often reversible if the plasmolysed cell is placed back into a hypotonic solution before it sustains irreversible damage, allowing water to re-enter and the protoplast to swell back against the cell wall (deplasmolysis).