A vacuole contains a fluid-filled sac that stores salts, water, minerals, nutrients, pigments and proteins within a membrane barrier called a tonoplast. Vacuoles can also contain waste products that make a plant taste bitter to animals.
Vacuoles use water to develop hydrostatic pressure, which helps keep the plant rigid. They store nutrient and non-nutrient chemicals, and breakdown complex molecules. Water can freely pass in and out of vacuoles, but other small molecules are stored within them. Vacuoles breakdown waste products, protecting the rest of the plant cell from contamination. Storage cells in seeds can store proteins, fats and carbohydrates in their vacuoles for many years, until germination takes place. A single plant cell can contain more than one kind of vacuole.
Vacuoles in plant cells are much larger than those in animal cells. One very large vacuole usually indicates that the plant cell has stopped growing. The environment inside a vacuole is slightly acidic, while the rest of the plant cell is slightly alkaline. When a vacuole in a plant cell contains all the water it needs, the plant stands up in a turgid state. A wilting plant retains most of its shape because of the cell walls, even though its vacuoles are shrinking. When the plant receives water again, the vacuoles fill up and the plant regains its shape.