<Li> Cytosol (fluid that contains organelles, comprising the cytoplasm) </Li> <P> A lysosome is a membrane - bound organelle found in nearly all animal cells . They are spherical vesicles which contain hydrolytic enzymes that can break down many 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 signaling, and energy metabolism . </P> <P> The lysosomes also act as the waste disposal system of the cell by digesting unwanted materials in the cytoplasm, both from outside the cell and obsolete components inside the cell . Material from outside the cell is taken - up through endocytosis, while material from the inside of the cell is digested through autophagy . Their sizes can be very different--the biggest ones can be more than 10 times bigger than the smallest ones . They were discovered and named by Belgian biologist Christian de Duve, who eventually received the Nobel Prize in Physiology or Medicine in 1974 . </P> <P> Lysosomes are known to contain more than 60 different enzymes . Enzymes of the lysosomes are synthesised in the rough endoplasmic reticulum . The enzymes are imported from the Golgi apparatus in small vesicles, which fuse with larger acidic vesicles . Enzymes destined for a lysosome are specifically tagged with the molecule mannose 6 - phosphate, so that they are properly sorted into acidified vesicles . </P>

The process of lysosomes digesting materials is called