Mitochondrion

A mitochondrion (plural: mitochondria) is an organelle enclosed by two membranes. The inner membrane folds inward, increasing the surface available for energy-producing processes. Mitochondria occur in almost every human cell, although their number varies: cells with high energy demands, including many muscle and liver cells, tend to contain more.

Mitochondria convert energy from nutrients into ATP, a molecule that supplies energy for many biochemical reactions. This is why they are often described as cellular powerhouses, though energy production is not their only function. Research also links mitochondria to cellular signalling and processes such as apoptosis, the controlled removal of damaged or unneeded cells.

Mitochondria contain their own small genome in addition to the cell’s nuclear genome. Human mitochondrial DNA contains 37 genes; only a small subset of mitochondrial proteins is encoded there, while most mitochondrial proteins are encoded by nuclear genes and imported into the organelle. In humans, mitochondrial DNA is generally inherited from the mother through the egg, although inheritance patterns should not be taken as universal across every organism.

The endosymbiotic theory explains mitochondria as descendants of bacteria that became integrated into the ancestors of eukaryotic cells. Evidence includes the organelle’s own genome and its distinct translation machinery; comparisons of mitochondrial and bacterial genes point to an origin within the bacterial group Alphaproteobacteria. The broad bacterial ancestry is strongly supported, while researchers continue to investigate the details and timing of the transition.