A vacuole is a fluid-filled space with a membrane found in the cytoplasm of a cell.
	Vacuoles are often found in animal cells but only appear when they are needed, then disappear.
	Single-celled organisms (such as amoeba) form food vacuoles around their prey and use vacuoles to control their water content. White blood cells form similar vacuoles around pathogens to engulf them.
	In plant cells, vacuoles are permanent. They are made of a membrane called the tonoplast and are filled with cell sap.
	The current theory of stomatal opening suggests that the guard cells of a leaf are made turgid when their water content increases through the movement of water by osmosis into the cell vacuole.