mitochondria

Mitochondria are double-membrane–bound organelles found in most eukaryotic cells. They are best known as the cell’s energy producers, generating most of the adenosine triphosphate (ATP) needed for cellular work through oxidative phosphorylation.

They consist of an outer membrane, an inner membrane folded into cristae, an aqueous intermembrane space, and a central matrix. The inner membrane houses the electron transport chain and ATP synthase, while the matrix contains enzymes of the citric acid cycle, mitochondrial DNA, and ribosomes.

Most ATP is produced by oxidative phosphorylation, which uses high-energy electrons carried by NADH and FADH2 to pump protons across the inner membrane and drive ATP synthesis. The Krebs cycle occurs in the matrix, and the mitochondrial genome encodes a small set of essential components of the respiratory chain.

Beyond energy production, mitochondria regulate apoptosis, buffer intracellular calcium, and generate heat in brown adipose tissue through protein UCP1. They also participate in reactive oxygen species signaling and various metabolic pathways. Mitochondria are dynamic, constantly undergoing fission and fusion, which shapes their number, shape, and distribution.

Most mitochondria contain their own circular DNA and ribosomes, supporting some protein synthesis independently of the nucleus. In humans, mtDNA is inherited almost exclusively from the mother, and heteroplasmy can influence disease expression. Mitochondrial diseases arise from mutations in mtDNA or in nuclear genes that encode mitochondrial proteins.

Origin theories posit that mitochondria originated from ancestral alpha-proteobacteria through endosymbiosis. In plants and other organisms, mitochondria operate in concert with chloroplasts to supply cellular energy.