Body cells are workshops that continuously operate to produce and process substances to maintain metabolism. When a substance enters a cell for processing, it is surrounded by a portion of the cell's outer membrane to form a sac. The sac then buds off into the cell and becomes a vesicle containing the substance. This ingestion of substances by the cell is called endocytosis. The vesicle is then quickly merged with an endosome, an organelle also frequently referred to as a "sorting station." From the endosome, the substance is either recycled back to the cell membrane (for exiting the cell) or forwarded to a lysosome—a cell organelle containing enzymes for the breakdown of substances for degradation. The substances entering a cell—and thereby an endosome—could be nutrients or signal molecules for processing, or even pathogenic viruses that cause disease. It is therefore extremely important to fully understand the molecular basis of how endosomes are formed and maintained.