1. Hypotonic
This type of solution is less concentrated than the cytoplasm of the amoeba. Therefore water moves into the amoeba cell from the solution through osmosis. The amoeba will, therefore, need to remove excess water for osmoregulation purposes. The contractile vacuole serves this purpose. It moves from inside of the cell, fuses with the cell membrane and expels the excess water.
2. In the other two solutions, water will not enter the amoeba cell through osmosis. An isotonic solution has same concentration as amoeba cytoplasm while hypertonic is more concentration meaning water will move from the amoeba cell into the solution. Therefore the contractile vacuole is not needed in both instances since there is no excess water in the amoeba to be expelled.
3. Plants also have a central vacuole in their cells. However, unlike the vacuole of amoeba, the main function of the vacuole is to store nutrients with a secondary function of osmoregulation. The vacuole also holds waste products from the cell. The cell regulates its osmotic pressure by regulating the osmoticlaly active molecules in its vacuole such as glucose. The more the molecules (hence making the vacuole hypertonic) the more the plants take in water while the less the molecules (hypotonic) the more water it loses to its environment.
