mitochondrials

Mitochondrials, usually referred to as mitochondria, are double-membrane–bound organelles present in most eukaryotic cells. Their primary function is to convert chemical energy from nutrients into ATP, the cell’s main energy currency. They also participate in metabolism, calcium storage, and regulation of programmed cell death.

A mitochondrion comprises an outer membrane, a highly folded inner membrane with cristae, an intermembrane space, and a gel-like matrix. The inner membrane houses the electron transport chain and ATP synthase, while the matrix contains enzymes of the citric acid cycle.

Mitochondria contain their own circular DNA and ribosomes, reflecting their bacterial ancestry. Most mitochondrial proteins are encoded in the nuclear genome and imported into the organelle via specialized translocons. In many animals, mtDNA is inherited maternally.

Mitochondria are dynamic, undergoing fission and fusion to adapt to cellular needs. They import most proteins through the TOM and TIM complexes, and they maintain quality by mitophagy.

The prevailing endosymbiotic theory posits that mitochondria originated from an ancestral α-proteobacterium. Comparative genomics and the presence of mitochondrially encoded ribosomes support this view.

Defects in mitochondrial DNA or nuclear genes affecting mitochondria can cause mitochondrial diseases, often with energy failure in high-demand tissues. Heteroplasmy—mixed mtDNA variants within a cell—modulates disease severity; examples include Leber hereditary optic neuropathy and MELAS.

Mitochondria remain a focus of medical and aging research, with approaches like mitochondrial replacement therapy and investigations into mitochondrial dysfunction in neurodegenerative diseases.