Vacuole

Vacuoles are large membrane-bound compartments within some eukaryotic cells and can serve different purposes, such as capturing food materials or unwanted structural debris surrounding the cell, sequestering materials that might be toxic to cells, maintaining fluid balance (called turgor) within the cell, exporting unwanted substances from the cell, and even determining relative cell size. Examples of vacuoles that perform these functions are described below.

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 used by some freshwater protozoa.

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 many different purposes; these are:

• Storage of organic compounds, proteins (in seeds), and inorganic ions (e.g., K⁺ and Cl^-).
• 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.