Vacuoles are characteristic of matured plant cells. A young dividing plant cell is generally filled with cytoplasm and has no vacuoles. Similarly, animal cells have either very small vacuoles or these may be absent. In plant cell small vacuoles appear with the growth of the cell which ultimately coalesce to form a large central vacuole. About 90% of the cell is likely to be occupied by the vacuole with the cytoplasm and the nucleus being adpressed against the plasmamembrane and cell wall.
The vacuole is delimited by a single membrane called vacuolar membrane or tonoplast keeping an aqueous solution called cell sap or vacuolar sap inside. The sap contains minerals, sugars, amino acids, proteins, water-soluble pigments and waste products in soluble crystalline form.
Like plasmamembrane, the tonoplast is differentially permeable and hence, can maintain concentrations of nutrients and waste materials. High concentration of salts and sugars and many water-soluble pigments such as red pigments of beets and many flower colour pigments are found in vacuoles.
The vacuole is originated from fusion and enlargement of small vacuoles present in meristematic cells which are believed to originate from the endoplasmic reticulum. The vacuoles maintain the turgor of plant cells and provide an aqueous environment for the accumulation of water-soluble compounds.