Vacuoles are compartments within the protoplast bounded by membrane tonoplast containing sap (aqueous solution). Vacuoles are present in both plant and animal cells. But they are most abundant in plant cells. In plants, they occupy major part of the cell. Thus the remaining intracellular structures form a thin peripheral layer. The numbers of vacuoles are different in different types of cells. They occupy 90% space of cell in parenchymatous cell. But large vacuoles is absent in meristematic cells. These cells contain a large number of smaller vacuoles.
The vacuoles are bound by single membrane. It has thickness like plasma membrane. But membrane of vacuoles has ability to absorb substances by active transport. They are formed by the fusion of small vacuoles during plant growth and development. Vacuoles are formed by endoplasmic reticulum and dictoysomes. Endoplasmic reticulum forms protein part of the membrane of vacuole. These proteins are transferred to dictoysomes by transport vesicles. Vacuoles are budded off from the surface of dictoysomes as small vesicles.
Different vacuoles have different contents it its sap. Most part of vacuoles are occupied by water. All other contents are dissolved in it. The solutes are IC, Na., Ca., PO4–, SO4, Cac. The sap of vacuoles also contain sucrose, glucose, organic acids, phenolic compounds like tannins and nitrogenous compounds like amino acids, peptides, proteins etc.
I. They expands the plant cells and do not dilute its cytoplasm.
- They store water, cell products or metabolic intermediates. It also stores food reserves like protein, flavonoids, and alkaloids.
- They cause turgor in plant cell. This turgor provides support to individual plant cells. It creates rigidity in leaves and young parts of plants.
- Vacuoles also contain pigments.