"Food vacuoles enter and exit the cell through processes called phagocytosis and exocytosis, respectively. The cell membrane actually surrounds the substance entering until it pinches off and forms a membranous vacuole. This type of vacuole is then ready to meet up with a hungry lysosome which will digest the contents. The food is then broken down to nutrient particles small enough to diffuse across the lysosome membrane to be absorbed by the cell.
"The lysosome is also capable of digesting possibly threatening substances which enter the cell. Examples of this are macrophages such as white, bacteria engulfing, blood cells, whose primary function of destroying harmful, foreign bacteria and viruses, is carried out primarily by lysosomes. Inherited disorders caused by missing lysosomic enzymes occur when the lysosome is unable to digest substances which accumulate to dangerous levels.
"The "interchangeability" of the vacuole and lysosome membranes allows the digestion of macromolecules to safely occur within one compartmentalized, area so that the rest of the cell remains protected. The waste which remains after this digestion will exit the cell through the reversed process - the vacuole will fuse with the cell membrane and open on the exterior, expelling the waste."