Plant vacuoles are single-layered membrane-bound organelles found in plant cells and are filled with water. Some mature plant cells have a large central vacuole, which is one of the distinguishing features between plant and animal cells. Young plant cells produced by meristematic tissues have many small and scattered vacuoles that originate from the endoplasmic reticulum. As the cell grows and differentiates, these vacuoles grow and fuse to form a large central vacuole.
The vacuole is separated from the cytoplasm by a selectively permeable lipid bilayer membrane. The lipid membrane contains transport proteins that regulate the movement of molecules in and out of the vacuole. The fluid inside the vacuole is called cell sap, which contains inorganic salts, sugars, amino acids, proteins, alkaloids, organic acids, pigments, and more. These substances stored in the vacuole provide nutrition to the plant during drought or serve as food for animals. The sucrose and fructose in the vacuoles of certain fruits give them a palatable sweetness. The pigments in the vacuole result in vibrant colors of flowers and fruits, attracting animals for pollination or seed dispersal.
The ions and molecules in the vacuole increase their osmotic pressure to allow plants to absorb more water from the external environment. Water channels or aquaporins on the vacuolar membrane facilitate the rapid entry of water molecules. A fully hydrated plant vacuole can occupy up to 90% of the cell volume. The cytoplasm and other organelles are compressed against the cell wall to form a thin layer. The cell wall exerts strong pressure to prevent the vacuole from further expanding by absorbing water. This pressure from the cell walls also play an important role in supporting the plant to remain stiff and straight. In times of water deficiency, the vacuoles release water into the cytoplasm to sustain cellular activities. The vesicles also control the entry or exit of ions to create a stable intracellular environment.
Vacuoles also contain hydrolytic enzymes that are similar to lysosomes in function. They can dissolve invading bacteria and viruses and also destroy damaged organelles or proteins during autophagy.