Vacuoles serve as the cell's storage centers of food and other necessary materials. The vacuole further functions in the removal of unwanted structural materials, the containing of several waste products and small molecules (which could involve the isolation of potentially harmful products in the cell), the exportation of unwanted materials from the cell, and the maintenance of an acidic internal pH and constant internal pressure. In plants, the central vacuole is typically the largest compartment of all the cell's organelles, occupying in itself the majority of the cell volume. This large compartment is enclosed by a membrane called the tonoplast, which is selectively permeable to certain solutes within the cytosol. The solution inside the vacuole is referred to as cell sap, which is of different composition from the cytosol. Some plant vacuoles contain pigments that color the cells, such as during pollination season by which the plant must attract different organisms to carry out its fertilization. Plants maintain its structure through the maintenance of internal pressure through the manipulation of water into and out of the vacuole. Through water osmosis, water diffuses into the vacuole, which places pressure onto the cell wall. If too much water were to be lost, this pressure against the cell wall would be lacking, and the cell would collapse onto itself. Thus, cells also serve to maintain the cell's size. Another important feature of the vacuole in plants is that an enlarged central vacuole may add a specific amount of pressure against the other compartments in the cell and push them towards the cell membrane, thereby giving a type of conformation that permits the absorbance of more solar energy.