Final answer:
Unicellular freshwater organisms maintain osmotic balance by using contractile vacuoles to expel excess water, ensuring their survival in a hypotonic environment.
Step-by-step explanation:
Unicellular freshwater organisms such as protists face a challenge because water tends to enter their cells through osmosis. The environment they live in is hypotonic, meaning the concentration of solutes is lower outside the cell than inside. To prevent the cells from bursting due to the incessant water intake, these organisms have evolved specialized structures such as contractile vacuoles. These vacuoles collect the excess water and then contract to expel it out of the cell, maintaining proper osmotic pressure. While this process is energy-intensive, it is crucial for the survival of organisms like Paramecia, which constantly pump water out of their cells. Fish and other larger organisms rely on different strategies, such as excreting dilute urine or adjusting salt levels through their gills, to cope with their hypotonic freshwater environments.