'Powerhouse of the cell' comes from a 1957 magazine article
Every biology student learns that mitochondria are the powerhouse of the cell. The phrase was popularised by Philip Siekevitz in a 1957 Scientific American article with that title. The organelles themselves were first seen in 1857 in insect muscle, and they still carry their own small genome, a relic of a bacterial past.
Albert von Kolliker spotted mitochondria in the flight muscles of insects in 1857, and in 1898 Carl Benda named them from Greek roots meaning thread-like granule. They occur in most eukaryotes, including animals, plants and fungi, and generate ATP, the cell's energy currency, through aerobic respiration. Numbers vary enormously: a mature red blood cell has none, while a liver cell can contain more than 2,000. Typically they measure 0.75 to 3 square micrometres in cross-section and are invisible without staining.
Their genome resembles those of bacteria, which is the main reason scientists broadly accept symbiogenesis: the idea that free-living prokaryotes merged permanently with ancestral eukaryotic cells long ago. The partnership became so complete that animals, plants and fungi now depend on these descendants for respiration.
Two membranes create distinct zones. The outer membrane, 60 to 75 angstroms thick, is studded with pore proteins; the main channel, called VDAC, is a barrel structure much like those in the outer membranes of certain bacteria and lets small molecules pass freely. As a result, ions and sugars in the gap between membranes match the cytosol, but larger proteins need a signal tag and an escort complex called a translocase to get in, so the protein mix differs; cytochrome c is one resident. The inner membrane folds into cristae, which multiply its surface and boost ATP output, and encloses a fluid matrix. A mitochondrion stripped of its outer layer is called a mitoplast.
Energy is only part of the job. Mitochondria take part in signalling, cell differentiation, control of the cell cycle and growth, and cell death; if the outer membrane ruptures, proteins spill into the cytosol and the cell dies. Contact points with the endoplasmic reticulum pass calcium signals and lipids between the two. Faults in these organelles are linked to mitochondrial diseases and to heart failure.
Source: Mitochondria
