Final answer:
The contractile vacuole in Euglena and other freshwater protozoa expels excess water from the cell to prevent bursting from osmotic pressure. Without this organelle, Euglena would be unable to maintain osmotic balance and would likely lyse due to the hypotonic surroundings.
Step-by-step explanation:
The contractile vacuole is an important organelle in organisms such as the Euglena, which lives in a hypotonic environment like freshwater. Its primary function is to remove excess water from the cell, thereby preventing the cell from lysing due to the influx of water via osmosis. If the Euglena did not have a contractile vacuole, it would likely take in too much water and burst, since the vacuole's role is crucial in maintaining osmotic balance within the cell.
Specifically, the contractile vacuole, which can be seen in organisms like the Paramecium as well, collects water from the cytoplasm through a network of canals, then pumps this water out of the cell via exocytosis when the vacuole merges with the cell membrane. This action effectively controls osmosis and protects the cell from osmotic stress. In the absence of the contractile vacuole, the cell would lack this protective mechanism against the challenges posed by a hypotonic external environment.