Final answer:
Paramecium have contractile vacuoles that function to expel excess water from the cell to prevent bursting. In saltier water, less water enters the cell due to higher osmolarity, thus reducing the need for contractile vacuoles to expel water. This is based on the principle of osmosis, where water moves towards a higher concentration of solutes.
Step-by-step explanation:
Paramecium are unicellular organisms that need to osmoregulate to survive in different environments. These organisms reside in freshwater, which is a hypotonic environment where the tendency is for water to move into the cell via osmosis. The contractile vacuoles in the Paramecium function to expel excess water in order to prevent the cell from bursting. In saltier, or hypertonic, environments, less water enters the Paramecium due to the higher osmolarity, which means that there is less excess water for the vacuoles to eject. This difference is due to the nature of osmosis, where water moves from areas of lower solute concentration to areas of higher solute concentration.
When Paramecium are placed in saltier water, meaning the external environment is more concentrated compared to the cell's internal environment, the concentration gradient for water is reduced. Consequently, contractile vacuoles will eject less fluid in saltier water than they would in fresh water because there is less excess water to remove. The rate of contraction of vacuoles can decrease with the increasing osmolarity of the environment. Contractile vacuoles remove water by exocytosis, merging with the cell membrane and expelling excess water and waste into the environment.