cells, vacuoles perform mostly subordinate roles, assisting in larger processes of exocytosis and endocytosis. Animal vacuoles are smaller than their plant counterparts but also usually greater in number. There are also animal cells that do not have any vacuoles. Exocytosis is the extrusion process of proteins and lipids from the cell. These materials are absorbed into secretory granules within the Golgi apparatus before being transported to the cell membrane and secreted into the extracellular environment. In this capacity, vacuoles are simply storage vesicles which allow for the containment, transport and disposal of selected proteins and lipids to the extracellular environment of the cell. Endocytosis is the reverse of exocytosis and can occur in a variety of forms. Phagocytosis ("cell eating") is the process by which bacteria, dead tissue, or other bits of material visible under the microscope are engulfed by cells. The material makes contact with the cell membrane, which then invaginates. The invagination is pinched off, leaving the engulfed material in the membrane-enclosed vacuole and the cell membrane intact. Pinocytosis ("cell drinking") is essentially the same process, the difference being that the substances ingested are in solution and not visible under the microscope. Phagocytosis and pinocytosis are both undertaken in association with lysosomes which complete the breakdown of the material which has been engulfed. Salmonella is able to survive and reproduce in the vacuoles of several mammal species after being engulfed. The vacuole probably evolved several times independently, even within the Viridiplantae. Vacuole types Gas vacuoles Gas vesicles, also known as gas vacuoles, are nanocompartments which are freely permeable to gas, and occur mainly in Cyanobacteria, but are also found in other bacteria species and some archaea. Gas vesicles allow the bacteria to control their buoyancy. They are formed when small biconical structures grow to form spindles. The vesicle walls are composed of a hydrophobic gas vesicle protein A (GvpA) which form a cylindrical hollow, proteinaceous structure that fills with gas. Small variances in the amino acid sequence produce changes in morphology of the gas vesicle, for example, GvpC, is a larger protein. Central vacuoles Most mature plant cells have one large vacuole that typically occupies more than 30% of the cell's volume, and that can occupy as much as 80% of the volume for certain cell types and conditions. Strands of cytoplasm often run through the vacuole. A vacuole is surrounded by a membrane called the tonoplast (word