Final answer:
The contractile vacuole in single-celled organisms regulates the cell's internal environment by pumping excess water out, which is crucial for preventing the cell from swelling and bursting in a hypotonic environment.
Step-by-step explanation:
A single-celled organism in fresh water will find itself in a hypotonic environment, where the concentration of solutes is greater inside the cell than in the surrounding water. This discrepancy leads to water moving into the cell by osmosis, which can cause the cell to swell and potentially burst. To prevent this, organisms such as Paramecia utilize a specialized organelle called a contractile vacuole to pump excess water out of the cell.
The contractile vacuole actively collects and expels water, maintaining homeostasis by controlling the cell's internal environment. The vacuole also aids in the excretion of wastes from the cell via exocytosis. The expulsion of water is an energy-intensive process, but it is essential for the survival of the cell, as it prevents the cell from bursting due to excess internal pressure.