Q.(a) With the help of well-labelled diagrams, describe the process of plasmolysis in plants, giving appropriate examples.

(a) When a living plant cell (for example, an onion epidermal cell) is placed in a solution whose water potential is much lower than that of the cell's own cell sap — i.e., a concentrated (hypertonic) solution of salt or sugar — water moves out of the vacuole and cytoplasm by osmosis. As water is lost, the protoplast (cell membrane + cytoplasm + vacuole) shrinks and gradually pulls away from the rigid cell wall, first at the corners (incipient plasmolysis) and then further as more water is lost (evident/full plasmolysis), leaving a visible gap between the shrunken protoplast and the wall. This process, called plasmolysis, is reversible — placing the plasmolysed cell back in water or a dilute solution causes water to re-enter and the protoplast to swell back against the wall (deplasmolysis). …

(a) Plasmolysis

Plasmolysis is the phenomenon in which a living plant cell loses water by osmosis and its protoplast shrinks away from the surrounding cell wall, when the cell is placed in a solution whose water potential is considerably lower (more negative) than that of the cell's own cell sap — i.e., a strongly hypertonic solution (for example, a concentrated solution of salt, sugar, or another osmoticum such as sodium chloride or sucrose).

The process can be followed through distinct visible stages, classically demonstrated using onion peel (epidermal) cells mounted in a strong salt or sugar solution and viewed under a microscope:

• Normal/turgid cell: In water or a dilute medium, the protoplast is pressed fully against the cell wall, with the wall exerting an inward wall pressure that balances the tendency of water to enter.
• Incipient plasmolysis: As the cell is exposed to the hypertonic solution, water begins to leave the vacuole and cytoplasm; the protoplast first starts to pull away from the wall at the corners of the cell, since the wall itself does not shrink (being rigid and freely permeable), while the protoplast, being elastic, does.
• Evident (full) plasmolysis: With continued water loss, the protoplast shrinks further and rounds off, pulling away from the wall over most or all of its surface, leaving a clear, fluid-filled gap between the shrunken protoplast and the wall.

Plasmolysis is a reversible process: if the plasmolysed cell is returned to pure water or a solution of higher water potential, water re-enters the cell by osmosis, the protoplast swells, and it gradually presses back against the wall — a process called deplasmolysis. This reversibility is used experimentally (e.g., to estimate a tissue's osmotic/water potential) and also demonstrates that the cell membrane and protoplast remain alive and intact through the process, provided plasmolysis is not too prolonged or severe.

(b) A plant cell placed in a solution of higher water potential

If a plant cell is instead placed in a solution whose water potential is higher than that of the cell (i.e., a hypotonic solution, or pure water, which has the maximum possible water potential), the water potential gradient now favours net entry of water into the cell by osmosis.

• As water enters the vacuole and cytoplasm, the protoplast swells in volume.
• Because the protoplast is enclosed by a semi-rigid but strong cell wall, its outward swelling is resisted by the wall, which pushes back on the protoplast — this reaction is called wall pressure, and its counterpart acting outward from the protoplast against the wall raises the cell's pressure potential (ψp). …

For (a), describe plasmolysis as osmotic water loss in a hypertonic medium and walk through its stages (turgid → incipient → full plasmolysis), noting reversibility via deplasmolysis, using onion peel as the standard example. For (b), apply the water-potential-gradient logic in the oppo …

• Describing plasmolysis and the cell simply 'shrinking' without distinguishing the protoplast (which shrinks/detaches) from the cell wall (which stays essentially the same size).
• Forgetting to state that plasmolysis is reversible (deplasmolysis) if the cell is returned to a hypotonic medium. …