A mitochondrion (plural mitochondria) is a double-membrane organelle found in most eukaryotic cells. It converts chemical energy from nutrients into adenosine triphosphate (ATP), chiefly through oxidative phosphorylation. Mitochondria also contribute to biosynthesis, calcium regulation, cellular signaling, and programmed cell death. They contain their own genetic material but depend on the rest of the cell for most of their components. Their number, shape, and organization vary with cell type and physiological conditions. (genome.gov)
Structure and compartments
A mitochondrion has an outer membrane, an inner membrane, an intermembrane space, and an internal compartment called the mitochondrial matrix. The outer membrane contains channels that allow many small molecules to pass. The inner membrane is much more selective: transport proteins regulate the movement of metabolites, while its low permeability to ions helps maintain the electrochemical conditions needed for ATP synthesis. (ncbi.nlm.nih.gov)
The inner membrane forms folds called cristae, which increase its surface area and organize the machinery of respiration. These folds vary in architecture rather than forming a uniform set of partitions. The matrix contains metabolic enzymes, mitochondrial DNA, and the machinery needed to express mitochondrial genes. This compartmentalization separates reactions while allowing controlled exchange between the matrix, membranes, and surrounding cytoplasm. (ncbi.nlm.nih.gov)
Mitochondria are not invariably isolated, bean-shaped bodies. They can form elongated tubules and interconnected networks that continually change through fusion, division, and membrane remodeling. Consequently, their appearance in a fixed microscopic image represents only one state of a dynamic system. (nature.com)
Respiration and ATP production
Mitochondria carry out major stages of aerobic cellular respiration. Glycolysis, which occurs outside mitochondria in the cytosol, produces pyruvate from sugars. After entering mitochondria, pyruvate can be converted into acetyl coenzyme A. Fatty-acid breakdown also supplies acetyl coenzyme A, which enters the citric acid cycle. These reactions generate reduced electron carriers, principally NADH and FADH₂. (ncbi.nlm.nih.gov)
The electron transport chain in the inner membrane transfers electrons from these carriers through a series of complexes and mobile carriers. Oxygen acts as the final electron acceptor and is reduced to water. Energy released during electron transfer drives the movement of protons from the matrix toward the intermembrane space, establishing both a concentration difference and an electrical potential across the membrane. (ncbi.nlm.nih.gov)
Protons return to the matrix through ATP synthase, which couples their movement to ATP formation from ADP and inorganic phosphate. This coupling of a membrane gradient to chemical work is called chemiosmosis. ATP is then exchanged with the cytosol to support cellular processes. Mitochondria therefore transform energy rather than create it; ATP production depends on substrates, electron transport, membrane integrity, and cellular demand. (ncbi.nlm.nih.gov)
Genome and protein assembly
Mitochondrial DNA constitutes a small genome distinct from the nuclear genome. Human mitochondrial DNA encodes 13 respiratory-chain and ATP-synthase subunits, two ribosomal RNAs, and 22 transfer RNAs. Mitochondrial genomes differ substantially among organisms in size, organization, and coding content, so the compact human genome is not a universal model. (ncbi.nlm.nih.gov)
Mitochondrial ribosomes synthesize the proteins encoded by this genome. However, most mitochondrial proteins are encoded in the cell nucleus, synthesized in the cytosol, and imported through specialized membrane machinery. Organelle assembly consequently requires coordination between two genetic systems. Protein targeting, transport, folding, and incorporation into larger complexes are essential parts of mitochondrial biogenesis. (ncbi.nlm.nih.gov)
In humans, mitochondrial DNA is generally inherited maternally. This inheritance pattern concerns the mitochondrial genome, not all proteins or functions associated with mitochondria: nuclear genes supplying mitochondrial components follow nuclear inheritance patterns. (genome.gov)
Dynamics and quality control
Mitochondrial fusion joins membranes and allows mixing of internal components; fission divides mitochondria. In mammals, mitofusins help fuse outer membranes, OPA1 participates in inner-membrane fusion and organization, and the dynamin-related protein DRP1 is central to division. These processes enable the mitochondrial network to respond to metabolic changes and cellular stress. (nature.com)
Quality control includes mitophagy, the selective removal of mitochondria through autophagic degradation. Together with biogenesis and molecular maintenance, this process helps regulate mitochondrial abundance and eliminate damaged components. Contacts with the endoplasmic reticulum and other organelles also coordinate membrane dynamics and integrate mitochondrial activity with wider cellular functions. (nature.com)
Functions beyond ATP synthesis
Mitochondria are integrated into metabolism and signaling rather than serving solely as ATP-producing compartments. Their uptake and release of calcium contribute to calcium homeostasis, linking cellular signals with metabolic activity. Mitochondrial metabolites and reactive oxygen species can also participate in communication with other cellular compartments. (nature.com)
Mitochondria have a central role in intrinsic apoptosis. Regulated release of mitochondrial factors can activate cell-death pathways. Other mitochondrial signals influence innate immune responses and inflammation, although these effects depend on context and the nature of the released material. (nature.com)
Evolutionary origin
The endosymbiotic theory explains mitochondria as descendants of bacteria that became permanently integrated into an ancestral host cell. Their bacterial ancestry is supported by molecular comparisons, including relationships between mitochondrial genes and bacterial counterparts. During integration, many ancestral genes were lost or transferred to the host nuclear genome, leaving modern mitochondria dependent on imported proteins. (ncbi.nlm.nih.gov)
Mitochondria are closely related evolutionarily to Alphaproteobacteria, but their precise placement relative to living bacterial groups remains unresolved. Different phylogenetic analyses recover different positions, partly because ancient divergence, rapid sequence change, and compositional biases complicate reconstruction. This uncertainty concerns the identity and relationships of the ancestral lineage, not the established bacterial origin of mitochondria. (nature.com)