Question

Osmotic expansion of a cell kept in water is chiefly regulated by an organelle

• A. Mitochondria
• B. Vacuoles
• C. Plastids
• D. Ribosomes

Hint:

The tonoplast (vacuolar membrane) contains active transport proteins (proton pumps) that pump ions into the vacuole against their concentration gradient, maintaining the high osmotic pressure needed to draw water into the cell.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
When a plant cell is placed in water (a hypotonic environment), water moves into the cell down its water potential gradient. The regulation of this osmotic water influx and the resulting turgor pressure is managed by specialized cellular compartments.

Step 2: Detailed Explanation:
Let us examine the roles of the listed organelles in cellular water relations:

Vacuoles: Mature plant cells contain a large, central vacuole bound by a selectively permeable membrane called the tonoplast.
The vacuole contains "cell sap," which is a highly concentrated solution of inorganic ions, organic acids, sugars, and pigments.
Because of this high solute concentration, the vacuole has a highly negative solute potential (\(\psi_s\)), which lowers the overall water potential (\(\psi_w\)) of the cell.
When the cell is kept in water, water enters the cytoplasm and is quickly pumped into the vacuole.
This causes the vacuole to expand, pushing the cytoplasm against the cell wall, which generates turgor pressure (pressure potential, \(\psi_p\)).
Thus, the central vacuole is the primary organelle regulating osmotic expansion and cell turgidity.

Mitochondria, Plastids, and Ribosomes: Mitochondria are involved in respiration, plastids (such as chloroplasts) are involved in photosynthesis or storage, and ribosomes are involved in protein synthesis. None of these organelles directly regulate cellular osmotic expansion.
Therefore, vacuoles are the chief regulators of osmotic expansion.

Step 2: Final Answer:
The correct option is (B).