Vacuole
A vacuole is a membrane-bound storage sac in a cell; plant cells have one large central vacuole that stores water and keeps the cell firm.
Definition
A vacuole is a storage bag inside a cell that can hold water, food, or waste, and plant cells usually have one very large vacuole that takes up most of the cell. It is a membrane-bound organelle that stores water, nutrients, pigments, or waste products; the plant central vacuole can occupy up to $90$ percent of the cell's volume and maintains turgor pressure against the cell wall, while animal cells have several small vacuoles and some protists use contractile vacuoles to pump out excess water. The central vacuole is bounded by a membrane called the tonoplast, which holds proton pumps (V-type ATPases and pyrophosphatases) that acidify the vacuolar sap to about pH $5.5$ and drive the accumulation of ions, sugars, organic acids, and secondary metabolites. The resulting osmotic potential draws water inward and generates the turgor pressure that supports non-woody tissue and drives cell expansion. Vacuoles also act as lytic compartments comparable to lysosomes, store defensive compounds and pigments such as anthocyanins, and sequester toxic ions.
Example
When a plant wilts, its vacuoles have lost water and the cells go soft; after watering, the vacuoles fill back up and the leaves stand up again. A freshwater paramecium constantly takes in water by osmosis, and its contractile vacuole fills and empties several times per minute to keep the cell from swelling and bursting. Stomatal guard cells open the pore by pumping K$^+$ and other solutes into their vacuoles, drawing in water and raising turgor, and reversing the flow closes the pore, so a leaf can regulate water loss within minutes.
Key Insight
The big vacuole in a plant cell works like the air in a bike tire: when it is full the cell is stiff, and when it is low the cell is floppy. It also lets a plant cell grow large cheaply, because the cell expands by filling with water instead of making expensive cytoplasm. The flower colors of many plants come from anthocyanins dissolved in vacuolar sap, and the same pigment can look red or blue depending on the vacuole's pH, an everyday example of pH indicators in nature.