Vacuoles are small or large-sized organelles which are filled with liquid or sap and are membrane-bound. In animal cells, vacuoles are smaller in size and lesser in number as compared to plant cells. Major portion of a plant cell is occupied by vacuoles. The cell sap present inside the vacuoles is bounded by a thin membrane, called tonoplast. The cell sap is a watery solution rich in sugars, proteins, amino acids, metabolic wastes and minerals.
Vacuoles may be contractile (can contract) or non-contractile. When the pressure of the contents of a contractile vacuole increases, it contracts and releases its contents, i.e. contractile vacuoles are osmoregulatory. In some plant cells, only one large, central vacuole is present.
1. In animals, vacuoles are often associated with the maintenance of water balance.
2. They work in osmoregulation in the aquatic protozoans.
3. They help the cells to remain turgid.
4. They are also helpful in separation of a variety of substances present in the cell sap from the cytoplasm.