Vacuole

Vacuoles are large membrane-bound compartments within some eukaryotic cells where they serve a variety of different functions: capturing food materials or unwanted structural debris surrounding the cell, sequestering materials that might be toxic to the cell, maintaining fluid balance (called turgor) within the cell, exporting unwanted substances from the cell, or even determining relative cell size. The cavity that is the vacuole is considered nonprotoplasmic and the contents classified as ergastic according to some authors (Esau, 1965). Vacuoles are especially conspicuous in most plant cells.

Vacuoles are typically filled with a liquid called cell sap, the composition of which can vary (even between vacuoles in the same cell), but is principally water. Water tends to move along concentration gradients into vacuoles.

Vacuole functions

Vacuoles have the following functions:

1. capture food
2. maintain internal hydrostatic pressure
3. contain waste products
4. maintain an acidic internal pH
5. Store small molecules
6. Enable a cell to elongate rapidly

Examples of vacuoles that perform each of these functions are described below.

Vacuoles in protists

Some protists and macrophages use food vacuoles in phagocytosis—the intake of large molecules, particles, or even other cells, by the cell for digestion.

A contractile vacuole is used to pump excess water out of the cell to reduce osmotic pressure and keep the cell from bursting. Contractile vacuoles are found in some freshwater protozoa.

Vacuoles in plant cells

Most mature plant cells have a central vacuole, which often takes up more than 90% of the cell interior. It is surrounded by a membrane and is called the tonoplast. The tonoplast actually serves several different purposes:

• Storage of organic compounds, proteins (in seeds), and inorganic ions (e.g., K⁺ and Cl^-) and salts (e.g., salts of calcium).
• Separation of toxic byproducts from cell metabolism.
• Storage of pigments (e.g., red and blue pigments in flowers).
• Protection of the plant tissue from predation by storing toxic compounds.
• Contributing to cell growth by absorbing water (e.g., cell elongation).
• Allowing some plant cells to reach considerable size.

The tonoplast stores water, with the primary purpose of regulating turgor pressure. The cell controls the flow of water into and out of the tonoplast by using active transport to pump ions of potassium (K⁺) into and out of the interior liquid. Because of osmosis, wherever solutes go, water follows.

References

• Esau, K. 1965. Plant Anatomy, 2nd Edition. John Wiley & Sons. 767 pp.