Many freshwater one celled organisms, like paramecium, have contractile vacuoles. there's structures collect and pump out excess water that accumulates in the cell. explain why these organisms needs such a structure.

This is because their cells are usually hypertonic to the freshwater they swim in. Their cells will always have dissolved osmotically-active molecules that are critical for cell function.  Therefore, osmotically, water will move from the freshwater to the inside of the cells. If not regulated, the cells would swell and lyse. Therefore excess water has to be actively pumped out to maintain cell homeostasis.    

when a reversible reaction is occurring in both directions at the same rate; the products and reactants do not change in concentration. a chemical reaction where the compound is broken into its constituent parts.

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