What is the function of contractile vacuole in paramecium?

Analyze the question: It asks about the main function of the contractile vacuole in Paramecium, a freshwater unicellular organism.

Concept: Paramecium lives in freshwater, which is hypotonic to its cytoplasm (the surrounding water has a lower solute concentration than the cell). Water therefore enters the cell continuously by osmosis. If this water were not removed, the cell would swell and burst. The contractile vacuole collects the excess water and periodically contracts, expelling it from the cell. This process is called osmoregulation.

Solution, option by option:

• A. Produces enzymes: Wrong. Enzymes are made by ribosomes and the rough endoplasmic reticulum, not by the contractile vacuole.
• B. Gets rid of excreta: This is partly tempting, since some small metabolic wastes may leave with the water. But the main function is osmoregulation. Most nitrogenous waste (ammonia) simply diffuses out across the cell membrane.
• C. Stores and digests food: Wrong. This is the job of the food vacuole.
• D. Gets rid of excess water: Correct. This is the primary function of the contractile vacuole, which regulates the water balance of the cell.

Exam tip: Remember these pairings:

• Contractile vacuole → water balance (osmoregulation)
• Food vacuole → digestion

If you see "freshwater protozoan + contractile vacuole," think "expels excess water."