A vacuole membrane, or tonoplast, is a lipid structure that surrounds and encases the vacuole. The membrane allows certain substances to flow in and out of the vacuole from areas of low concentration to high concentration. When necessary, the tonoplast stretches, allowing a great deal of fluid inside the vacuole.
Vacuoles are found in plants and fungi. These organelles have three primary jobs. When they are filled with water, vacuoles provide support to help make plants rigid. The acidic environment of vacuoles breaks down certain large molecules that have been discarded. In addition, vacuoles are storage areas for materials, such as pigments, alkaloids, latex, resins, proteins, fats and carbohydrates.