Final answer:
The contractile vacuole is the structure that collects and expels water in the cells of protozoans, helping to maintain osmotic balance through exocytosis.
Step-by-step explanation:
The structure that collects water and expels it in the cells of protozoans is the contractile vacuole. This organelle plays a crucial role in maintaining osmotic balance by removing excess water from the cell. Contractile vacuoles function by collecting water from the cytoplasm through canals and then pumping the water into the vacuole, which expands. Once the vacuole is full, it contracts to expel the water out of the cell via a pore. This process, known as exocytosis, is essential for protozoans living in hypotonic environments like freshwater, where the risk of water influx is high. An example of this is observed in the paramecium, where the contractile vacuole can be seen working to prevent the organism from bursting due to excess water intake.
It is important to note that contractile vacuoles are distinct from other types of vacuoles that may store food or water. Contractile vacuoles exclusively deal with the excretion of water and wastes to help the cell regulate its internal pressure and volume against the osmotic gradient.