Paramecium and other single-celled freshwater organisms have difficulty since they are usually hypertonic relative to their outside environment. Thus water will tend to flow across the cell membrane, swelling the cell and eventually bursting it. The contractile vacuole is the Paramecium's response to this problem. The pumping of water out of the cell by this method requires energy since the water is moving against the concentration gradient. One commonly employed mechanism is a contractile vacuole.
Water is collected into the central ring of the vacuole and actively transported from the cell.
The functioning of a contractile vacuole in Paramecium . Image from W.H. Freeman and Sinauer Associates, used by permission. Examples include the diffusion of oxygen and carbon dioxide, osmosis of water, and facilitated diffusion. Image from W.H. Freeman and Sinauer Associates, used by permission. Active transport requires the cell to spend energy, usually in the form of ATP .