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 within the cell, exporting unwanted substances from the cell, and even determining relative cell size. Some examples of such vacuoles are described below.

Some protists and macrophages use food vacuoles in phagocytosis, which is the intake of large molecules, 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 apart. 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. It is surrounded by a membrane which is called the tonoplast. The central vacuole serves many different purposes. They 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 from predators by storing toxic compounds.
• Determining cell growth by absorbing water (e.g., elongation).
• Allowing plant cells to reach considerable size.