Final answer:
Contractile vacuoles are organelles in some unicellular eukaryotes that help regulate the cell's water and salt balance by excreting excess water and wastes via exocytosis.
Step-by-step explanation:
Contractile vacuoles are specialized organelles found in certain unicellular eukaryotic organisms, such as amoebas and paramecia. These organelles play a crucial role in osmoregulation, which is the maintenance of an appropriate water and salt balance within the cell. Contractile vacuoles work by taking up excess water and cellular wastes and then, through a process called exocytosis, merging with the cell membrane to expel these substances into the external environment.
While endocytosis is a process used by cells to internalize substances, it is not the process through which contractile vacuoles release their contents. It is the process of exocytosis that is essential for the role of contractile vacuoles in maintaining osmotic balance. This function is particularly vital in a hypotonic medium, where the tendency of water to enter the cell can lead to swelling and potentially bursting.