Final answer:
Single-celled organisms with cell walls lack contractile vacuoles due to the structural support provided by the cell wall and alternative mechanisms for water regulation.
Explanation:
Single-celled organisms with cell walls do not have contractile vacuoles because their cell walls provide structural support that helps maintain cell shape and prevent excessive water intake.
Unlike animal cells that use contractile vacuoles to regulate water balance, organisms with cell walls rely on other mechanisms like cell wall permeability to control water content within the cell.
For example, plant cells have large central vacuoles that assist in maintaining turgor pressure, a function not needed in single-celled organisms with rigid cell walls.
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