A cavity within the cytoplasm of a cell, surrounded by a single membrane and containing fluid, food, or metabolic waste. Vacuoles are found in the cells of plants, protists, and some primitive animals. In mature plant cells, there is usually one large vacuole which occupies a large part of the cell's volume and is filled with a liquid called cell sap. The cell sap stores food reserves, pigments, defensive toxins, and waste products to be expelled or broken down. In the cells of protists, however, there may be many small specialized vacuoles, such as digestive vacuoles for the absorption of captured food and contractile vacuoles for the expulsion of excess water or wastes.