mitocondrial

Mitochondria are double-membrane-bound organelles found in most eukaryotic cells. They are best known as the primary producers of cellular energy, generating adenosine triphosphate (ATP) through oxidative phosphorylation. Beyond energy metabolism, mitochondria regulate apoptosis, calcium homeostasis, and metabolism of reactive oxygen species, and they contribute to heat generation in brown fat via thermogenic proteins.

Structure and genome: The outer membrane encloses the organelle, while the highly folded inner membrane forms cristae that increase surface area for the electron transport chain. The space inside the inner membrane is the matrix, which houses enzymes of the tricarboxylic acid (TCA) cycle, mitochondrial DNA (mtDNA) and ribosomes. Many organisms carry a circular mtDNA genome encoding a small set of essential subunits of the electron transport chain and ATP synthase; human mtDNA consists of about 16,500 base pairs and 37 genes.

Energy production: The TCA cycle generates reduced electron carriers that feed the electron transport chain on the inner membrane, creating a proton-motive force used by ATP synthase to convert ADP to ATP. Mitochondria also participate in heme and lipid metabolism and in signaling pathways related to cell fate. Mitochondria can fuse and divide to respond to cellular demands; damaged organelles are targeted by mitophagy.

Genetics and disease: Mitochondria are inherited maternally in most species; heteroplasmy refers to mixed mtDNA within a cell. Mutations in mtDNA or nuclear genes encoding mitochondrial proteins can cause mitochondrial diseases (e.g., LHON, MELAS).

Evolution and significance: Mitochondria are widely accepted to originate from an ancestral symbiotic alpha-proteobacterium via endosymbiosis; their features include double membranes, bacterial-type ribosomes, and own genome.