Vacuole

A vacuole is a fluid-filled compartment enclosed by a single membrane. Vacuoles are found in plant and fungal cells, in many protists and, usually as small sacs, in some animal cells. They are larger than ordinary vesicles and, unlike them, do not normally fuse with other membranes in the cell. What they hold varies: water, ions, sugars, pigments, stored proteins, digestive enzymes or waste.

In a mature plant cell, a single central vacuole can fill most of the cell's volume, and its membrane, the tonoplast, controls what passes in and out. As the vacuole takes up water, it presses the cytoplasm outward against the cell wall, creating turgor pressure, which keeps soft stems and leaves firm. When a plant runs short of water, its vacuoles shrink and it wilts. A full vacuole also lets a plant cell grow larger cheaply, without making much new cytoplasm. Plant vacuoles break down large molecules, a task done by lysosomes in animal cells, and can store bitter or toxic compounds that discourage animals from eating the plant.

Protists show other kinds. Paramecium digests the bacteria it captures inside food vacuoles, and freshwater protists use contractile vacuoles to collect water that floods in by osmosis and squeeze it out, so the cell does not burst.