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The contractile vacuole allows free-living protists to expel extra water. <b>The contractile vacuole allows free-living protists to expel extra water <br /></b>.
The vacuole is surrounded by a membrane (tonoplast). Back to the water balloon in the box model-imagine the vacuole to be analogous to another water balloon inside our protoplast balloon. This water balloon is a separate entity that can be physically removed from the cell.. The Box This part of the cell is analogous to the cell wall. Like a cardboard box, the cell wall is relatively rigid, it is non-living and highly structured. Its more obvious functions are to support and protect the cell. It is produced by the protoplast (see below).