Vacuoles

small, usually spherical cavities in animal and plant cells or in unicellular organisms. In the cells of a number of multicellular invertebrates (sponges, coelenterates, turbellarians, and some mollusks) that are capable of intracellular digestion and in the bodies of certain unicellular organisms (Protozoa), digestive vacuoles containing digestive enzymes are formed. In higher animals digestive vacuoles are formed in special cells called phagocytes. In other cells vacuoles contain salts, enzymes, and metabolic products (such as fats). In many unicellular organisms there are also contractile, or pulsating vacuoles, which periodically discharge their contents into the external environment. In protozoans, contractile vacuoles are the principal apparatus for regulating osmotic pressure; they also serve to remove decomposition products from the organism.

Plant vacuoles are filled with colorless or pigmented cell fluid. The cytoplasm is separated from the vacuoles by a lipoid-protein semipermeable membrane. Substances dissolved in the cell fluid of plant vacuoles (such as sugars, polysaccharides, alkaloids, tannin, pigments, and certain salts) cause nutritive substances and water to enter the cell by virtue of osmosis and create mechanical tension of cells and tissues, or turgor. In very young cells there are no vacuoles or they are almost unnoticeable; as the cell grows and ages, vacuoles appear in its various sectors, then gradually enlarge and unite into one large vacuole—a so-called vacuolar system.