2.7. Vacuole

Mature plant cells appear to consist mostly of empty space when viewed under a microscope, but these “empty spaces” are in fact the fluid-filled organelles known as vacuoles. A vacuole is a large reservoir of water and salts that is surrounded by a single lipid bilayer called the tonoplast. The liquid content, called cell sap, is highly acidic and is responsible for the sour taste of citrus fruits such as lemons and oranges. Young cells, such as those in growing meristems, may contain several smaller vacuoles that, upon maturation, merge into a single, central vacuole that can fill as much as 90% of the cell volume. By filling the majority of the protoplast with cell sap, which requires relatively little energy to make, it allows a plant cell to be quite large without wasting energy synthesizing many proteins and cytoskeletal elements. The vacuole is also a storage container that holds salts, sugars, and organic molecules. Some specialized vacuoles also store pigments, defense compounds, toxic chemicals and are often involved in breaking down and recycling molecules within a plant cell.