A lysosome (derived from the Greek words lysis, meaning "to loosen", and soma, "body") is a membrane-bound cell organelle found in most animal cells. They are spherical vesicles which contain hydrolytic enzymes that can break down virtually all kinds of biomolecules. Simply stated, a lysosome is a type of vesicle with specific composition, of both its membrane proteins, and proteins of its lumen. The lumen's pH (4.5 - 5.0) is optimal for the enzymes involved in hydrolysis, analogous to the activity of the stomach. Besides degradation of polymers, the lysosome is involved in various cell processes, including secretion, plasma membrane repair, cell signalling and energy metabolism.