While vacuoles in animal cells are small and temporary, plant vacuoles are big and practically permanent. The vacuoles in plant cells usually occupy about 30% of the cell’s volume but can sometimes become as much as 90%. Why is this so? Well, plant vacuoles have many functions.
First, it can act a storage area of many different things ranging from water, minerals, enzymes, ions, and even toxic substances. The toxin helps protect the plant from being eaten by predators. If we cut a plant, the contents of the vacuoles, generally called cell sap, spill out.
Vacuoles also store pigments. These pigments are responsible for giving us the beautiful colors of flowers and fruits.
The second and even more important function of vacuoles in plant cells is to maintain turgor pressure against the cell wall. Turgor pressure makes the plant firm and rigid.
Vacuoles are surrounded by a special membrane called tonoplast. The tonoplast is selectively permeable and has pumps that actively pump ions in order to maintain the water content of the vacuoles. If you remember, I mentioned before that water moves by osmosis. It always moves to were the solutes are. So if the tonoplast for example pumps in potassium ions, then water follows the ions and moves inside the vacuole. If there is enough water available, then turgor pressure is easily maintained by the plant through this pumping action. The plant then remains firm and rigid.
If there is not enough water however, the vacuole shrinks and moves away from the cell wall. Turgor pressure then decreases. When this happens, the cells plasmolyse and the plant wilts. So vacuoles really help maintain the structural integrity of plant cells.
Oh by the way, some of the other materials stored in plant vacuoles are: opium, rubber, and garlic flavoring.