Lysosomes are organelles that digest and dispose of unwanted protein, DNA, RNA, carbohydrates, and lipids in the cell. The inside of the lysosome is acidic and contains many enzymes that break down molecules. It is referred to as the cell’s recycling center, but that does not it mean only plays a passive role in the cell. Aside from breaking down unwanted molecules, and even other organelles, its recycling function is at the center of a process called autophagy, in which the cell digests itself. Autophagy is triggered when the cell is under stress and is one way in which a cell undergoes senescence, or growth arrest, to preserve energy. Lysosomes are all essential components of macrophages, which defend the body against pathogens.
The lysosome is a membrane pouch that pumps protons, or hydrogen ions, into its center, causing its insides to have an acidic pH of 5. It contains 50 different types of enzymes, called hydrolases, that break the chemical bonds that hold molecules together. Lysosomal enzymes are unique in that they only function in an acidic pH, as opposed to the relatively neutral 7.2 pH of the cytoplasm. This is a safeguard for the cell, in case the lysosome pouch breaks and the enzymes are released.
Lysosomes form from small pouches, called vesicles, that bud off from the Golgi complex -- the "post office" that sends pouches throughout the cell. The lysosome pouch then fuses with endosomes, which are pouches that pinched in from the cell surface membrane. The new pouch resulting from this fusion becomes the mature lysosome. Lysosomes digest whatever is inside them, which can be particles engulfed from the cell's external environment or organelles that are inside of the cell. The bits and pieces that result from the digestion of molecules can then be recycled to make new protein, DNA, sugars and fats, or broken down even further. Immune cells, such as macrophages, which engulf foreign particles and pathogens, have many lysosomes.
Autophagy and Senescence
When cells are stressed because of a chemical imbalance, such as too many dangerous oxygen radicals produced by the day-to-day chemical reactions in the cell, it undergoes a form of growth arrest called senescence. Oxygen radicals are unstable molecules that break chemical bonds in other molecules, and can cause mutations. Senescence is a process in which the cell stops growing and becomes dormant. Part of what happens in senescence is a process called autophagy, or self-eating, during which the cell begins to digest its own organelles. Lysosomes are the main organelles that perform autophagy.
There are 30 different human diseases that result from mutations of genes that encode for enzymes in a lysosome -- they are called lysosomal storage diseases. One such disease is Tay-Sach’s disease, which causes mental retardation and other nerve problems. This disease is caused by a mutation in a gene that is responsible for digesting a fat molecule that is found in brain cells. Lysosomes in Tay-Sach’s patients are clogged with this fat molecule, called a GM2 ganglioside, which causes them to swell and disrupt the function of the brain cell.