Which cellular structure is associated with osmosis in Amoeba and Paramecium?

• A. nucleus
• B. mitochondria
• C. ribosome
• D. contractile vacuole

Answer: (D)

Osmosis in these freshwater protists creates a constant influx of water due to the surrounding hypotonic environment, so they must actively manage their water balance. The contractile vacuole collects excess water from the cytoplasm and periodically contracts to expel it outside the cell, preventing too much water from accumulating and causing lysis. This function directly controls osmoregulation, which is why it’s the structure tied to osmosis. The nucleus, mitochondria, and ribosomes handle genetic information, energy production, and protein synthesis, respectively, but they don’t regulate water balance.