Vacuoles are membrane-bound molecular systems that perform certain storage and removal functions within someeukaryotic cells .
As an organelle of the cell, the vacuole is where food materials are captured and stored and where unwanted and sometimes toxic substances can be sequestered for decomposition and removal. Vacuoles also help prevent cellular self-poisoning by breaking down and exporting old cellular parts.
The primary functions that vacuoles perform are maintaining fluid balance (or internal hydrostatic pressure), acting as cellular pumps; exporting unwanted substances; maintaining an acidic internal pH for the cell; and determining relative cell size and even shape, as they can help the cell elongate rapidly. The largest and most useful vacuoles are generally found in plant cells.
Instead of protoplasm, vacuoles are filled with a different liquid called cell sap. The composition of this sap is primarily water, but can vary from cell to cell, and even from vacuole to vacuole.
Vacuoles in different cells have differing purposes. Protists and macrophages may use food vacuoles in phagocytosis, or the capture and digestion of food particles. They also use contractile vacuoles to pump out excess water so that the cell does not burst; these are most often found in freshwater protozoa like paramecium.
Plant cells have large central vacuoles, often as large as 80% of the cell interior. This vacuole holds water, enzymes, inorganic ions like calcium, and toxic byproducts that are being eliminated. The size and number of these vacuoles may change according to type of plant, life stage, and even season; that's partly because the central vacuole also performs the function of maintaining cell pressure through water storage.
As with most internal organelles, vacuoles are not possessed by prokaryotes such as bacteria.
Web Resources On Vacuole
The Vacuole Organelle
Book Resources On Vacuole
Plant Cell Vacuoles: An Introduction by Deepesh N.
The Plant Vacuole by Roger A. Leigh (Editor)