Every cell contains hundreds of mitochondria, a population of complex organelles that evolved from an ancient lineage of symbiotic bacteria that merged with early forms of cell to form the first eukaryotic cells. Mitochondria still act like bacteria in many ways, retaining a fragment of their original circular DNA, replicating by division, fusing together and passing around component parts, but are nonetheless now tightly integrated into cellular metabolism. Most mitochondrial genes have migrated into the cell nucleus, and a complex process of quality control known as mitophagy operates to recycle worn and damaged mitochondria.
The primary function of mitochondria is the manufacture of adenosine triphosphate (ATP), a chemical energy store molecule used to power the cell. The core of the protein machinery inside a mitochondrion that carries out this manufacture is the electron transport chain, also known as the respiratory chain. Collectively the structures of the chain are capable of building up the necessary energy to form ATP by, as one might guess from the name, reductive and oxidative chemical reactions that transport electrons along the chain. The electron transport chain consists of many distinct proteins that join together to form four protein complexes. These complexes themselves can also assemble in a number of ways to form supercomplexes. Indeed, researchers have shown that supercomplex formation is necessary for normal levels of ATP production.
