Protozoa in the Pool. Paramecium swimming in fresh water have to deal with constant streaming-in of water by osmosis. If unchecked, this could make them swell and explode. Instead, water is captured by radiating canals into contractile vacuoles that pulse, expelling water with each beat.
Aquaporins (water channels) and Proton Pumps work together to drive water into the contractile vacuoles. Where ions go, water follows!