Mitocondrii

Mitocondrii are double-membrane-bound organelles found in most eukaryotic cells. They generate most of a cell's ATP through oxidative phosphorylation, using energy from nutrients to drive the electron transport chain housed in the inner membrane. They also participate in other essential processes, including metabolic regulation, apoptosis, and heat production in brown fat.

Structurally, mitochondria comprise an outer membrane, an inner membrane folded into cristae, and a central matrix. Their genome is a small, circular DNA molecule (mtDNA) that encodes a subset of essential respiratory chain components, plus rRNAs and tRNAs. The majority of mitochondrial proteins are encoded in the nucleus and imported via specialized translocases.

Originating from an ancient endosymbiotic event, mitochondria are believed to descend from an ancestral alpha-proteobacterium. They replicate independently of the cell cycle by fission and import many proteins from the cytosol, reflecting their semi-autonomous status.

Mitochondrial dynamics—fission, fusion, and mitophagy—maintain organelle function and distribution according to cellular energy demands. Biogenesis regulates replication of mtDNA and production of respiratory machinery in response to cellular cues.

In addition to energy metabolism, mitochondria regulate intracellular calcium, produce signaling molecules, and participate in programmed cell death via the intrinsic pathway. Some tissues use mitochondria to generate heat through the protein thermogenin (UCP1).

Mutations in mtDNA or nuclear genes encoding mitochondrial proteins cause mitochondrial diseases, such as Leber hereditary optic neuropathy, MELAS, MERRF, and Leigh syndrome. Symptoms vary with tissue energy needs and mutation load.