Final answer:
The contractile vacuole in freshwater protozoans like Paramecia removes excess water from the cell by contracting and expelling it, a process essential for osmoregulation and maintaining osmotic balance.
Step-by-step explanation:
The contractile vacuole in freshwater protozoans, like Paramecia, is an organelle responsible for removing excess water from the cell. Due to a hypotonic environment, water tends to enter the cell continuously because the solute concentration inside the protozoan is higher than in the surrounding freshwater.
To prevent the cell from bursting, the contractile vacuole collects this excess water and then contracts to forcefully expel it through exocytosis. This helps to maintain the proper water balance and osmotic pressure within the protozoan, ensuring its survival.
As visualized under bright field light microscopy at 480x magnification, a Paramecium's contractile vacuole is continuously active. More specifically, the contractile vacuoles expel not just excess water, but also waste products, such as uric acid. They are essential for the osmoregulation process, which is a critical aspect of homeostasis in these freshwater organisms.