Most mature
plant cells have one or several
vacuoles that typically occupy more than 30% of the cell's volume, and that can occupy as much as 90% of the volume for certain cell types and conditions.
[2] A vacuole is surrounded by a membrane called the
tonoplast.
This vacuole houses large amounts of a liquid called
cell sap, composed of water,
enzymes, inorganic ions (like K+ and Cl-), salts (such as
calcium), and other substances, including toxic byproducts removed from the cytosol to avoid interference with
metabolism. Toxins present in the vacuole may also help to protect some plants from predators. Transport of
protons from cytosol to vacuole aids in keeping cytoplasmic
pH stable, while making the vacuolar interior more acidic, allowing degradative enzymes to act. Although having a large central vacuole is the most common case, the size and number of vacuoles may vary in different tissues and stages of development. Cells of the
vascular cambium, for example, have many small vacuoles in winter, and one large one in summer.