Vacuoles are large, liquid-filled organelles found only in plant cells. A vacuole is surrounded by a membrane called the tonoplast and filled with cell sap. Vacuoles can occupy up to 90% of a cell’s volume and have a single membrane. Their main function is as a space-filler in the cell, but they can also fill digestive functions similar to lysosomes (which are also present in plant cells). Vacuoles contain a number of enzymes that perform diverse functions. Transport of protons from the cytosol to the vacuole stabilizes cytoplasmic pH, while making the vacuolar interior more acidic creating a proton motive force which the cell can use to transport nutrients into or out of the vacuole. The low pH of the vacuole also allows degradative enzymes to act.
The function and significance of vacuoles varies greatly according to the type of cell in which they are present. In general, the functions of the vacuole include:
- Isolating materials that might be harmful or a threat to the cell
- Containing waste products
- Containing water in plant cells
- Maintaining internal hydrostatic pressure or turgor within the cell
- Maintaining an acidic internal pH
- Containing small molecules
- Exporting unwanted substances from the cell
- Allows plants to support structures such as leaves and flowers due to the pressure of the central vacuole
- By increasing in size, allows the germinating plant or its organs (such as leaves) to grow very quickly and using up mostly just water.
- In seeds, stored proteins needed for germination are kept in ‘protein bodies’, which are modified vacuoles.