The organelle that helps to control the water balance in one-celled organisms is called the contractile vacuole.

Here's how it works:

* Water Regulation: Contractile vacuoles act like tiny pumps, collecting excess water from the cytoplasm and expelling it outside the cell. This is crucial for maintaining the correct internal water concentration and preventing the cell from bursting due to osmotic pressure.

* Osmosis: Water moves across the cell membrane from areas of high concentration to areas of low concentration. In freshwater environments, water tends to flow into the cell due to the higher concentration of dissolved solutes inside. The contractile vacuole helps to remove this excess water.

* Location: Contractile vacuoles are found in many freshwater protists, such as amoebas, paramecia, and euglena. They are absent in organisms that live in saltwater or have other mechanisms for regulating water balance.

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