mitochondrion

Mitochondrion is an organelle found in most eukaryotic cells. It is bounded by two membranes: an outer membrane that is relatively permissive and an inner membrane that is highly selective and folded into cristae, which increase the surface area for biochemical reactions. The space enclosed by the inner membrane is the mitochondrial matrix, which houses enzymes of the tricarboxylic acid (TCA) cycle, mitochondrial DNA, and ribosomes.

The primary function of mitochondria is the production of adenosine triphosphate (ATP), the cell’s main energy currency. In the process of oxidative phosphorylation, electrons are transferred through a series of protein complexes (the electron transport chain) embedded in the inner membrane, creating a proton gradient that drives ATP synthase to generate ATP from adenosine diphosphate (ADP). Other important roles include regulation of metabolic pathways such as fatty acid oxidation and the TCA cycle, calcium storage, and the initiation of programmed cell death (apoptosis). Mitochondria can also generate reactive oxygen species that participate in signaling.

Genetically, mitochondria contain their own circular genome of about 16.5 kilobases in humans, encoding 37 genes (13 protein subunits of the respiratory chain, 22 transfer RNAs, and 2 ribosomal RNAs). Most mitochondrial proteins are encoded by nuclear DNA and imported. Mitochondrial DNA is typically inherited maternally, and cells can contain a mixture of normal and mutated genomes (heteroplasmy), influencing disease expression. Mitochondria continually divide, fuse, and are degraded by mitophagy as part of cellular quality control. Their endosymbiotic origin is supported by their bacterial-like features and genome.