Plant Cell Vacuoles

The vacuole is the largest compartment of a mature plant cell and may occupy up to 95% of the total cell volume. In such mature cells, the cytosol is visible only as a thin layer, which is separated from the cell wall by the plasma membrane, and from the vacuolar sap (cell sap) by the vacuolar membrane (tonoplast). The constituents of the cell sap are mainly inorganic salts and water. The vacuole, therefore, enables the plant to reach a large size and a large surface area by accumulating salts from the environment, which drive further water uptake osmotically. Thus only a minimumof energy is diverted from the energy-consuming synthesis of metabolites. The requirement for cytosolic ions to be in homeostasis is also met by the vacuole as it serves as an internal reservoir of metabolites and nutrients (Matile, 1978; Boller and Wiemken, 1986; Martinoia, 1992). It can also be used for storage of other compounds that would be problematic in the cytosol, such as ions, pigments, storage compounds or toxic substances.