The lysosomes have sometimes been likened to "The Police Force of the Cell". Even in a place as small as a cell, we need someone to keep things in order. But unlike the police, these lysosomes literally eat things which disturb the natural order of the cell. Lysosomes pick up foreign invaders such as bacteria, food and old organelles and break them into small pieces that can hopefully be used again. If they pick up a really harmful invader, they will eat it up and expel what is left of it out of the cell so that the debris can be removed from the body. \n\nThe lysosome is able to do this because it is filled with enzymes. These enzymes are specially made for the lysosome by the rough endoplasmic reticulum and work only at low pH (highly acidic) levels. The reason for this is that the enzymes are so strong that they could eat the whole cell if the lysosome ever let them out. However because they can only work at low pH levels and the rest of the cell has a neutral pH level, they can be neutralized if they accidentally escape from the lysosome. \n\nElectron micrograph of lysosomes. \n\nLysosomes are produced in the Golgi apparatus \n\nFrom here they are dispersed throughout the cytosol. Despite the fact that the range of enzymes found in lysosomes are fairly uniform throughout many types of cells they take many forms. For example some are comparatively large (up to 1 um in diameter) whereas other present as small vesicles (25 to 50 nm in diameter). This variation is most evident in macrophages which are cells specialised in ingesting particles and organisms which have invaded the body. Along with similar organelles called peroxisomes they form what is known as the acid vesicle system . \n\nWithin the human acid vesicle system there are more than 30 defined acid hydrolases present in various types of lysosome. \n\nAnother function of lysosome in the human occurs during fertilization of the egg by the sperm. The head of the sperm cell contains a package of lysosomal material called the acrosome. The enzymes from this are released when the sperm makes contact with an egg and they effectively bore a hole through the cell membrane of the egg allowing the sperm to enter. \n\nLysosomes also play a key role in destroying old organelles within the cell and thus allow them to be replaced with fresher, more effective ones. \nThis process is known as autophagy and is accomplished in two stages. Firstly a membrane is donated by the endoplasmic reticulum. This membrane then surrounds the old organelle. Secondly a lysosome fuses with this membrane to form an autophagic vacuole. The lysosome can safely enter it's deadly enzyme contents into this vacuole and destroy the old organelle. \nIn human liver cells it is calculated that at least one mitochondria undergoes autophagy every 10 minutes. This reflects the highly active nature of the liver as an organ and it's need for a constant supply of fresh organelles. \nThe electron micrograph below shows a lysosome in the process of destroying a membrane bound mitochondria.