What Are Mitochondria?

Mitochondria, singular mitochondrion, are double-membrane organelles found in nearly all eukaryotic cells that generate most of the cell's supply of adenosine triphosphate through oxidative phosphorylation. Each organelle is typically one to several micrometers long, and a single cell may contain anywhere from a few dozen to several thousand, with the count roughly tracking energy demand: cardiac muscle and neurons carry far more than skin fibroblasts. Mitochondria are not static compartments. They fuse and divide continuously, forming networks whose morphology responds to metabolic state, and damaged segments are removed by a selective autophagy pathway called mitophagy.

The organelle descends from a free-living alphaproteobacterium taken up by an ancestral host cell, an origin recorded in its retained circular genome, its bacterial-type ribosomes, and its cardiolipin-rich inner membrane. Most of the original bacterial genes have since migrated to the nuclear genome, leaving the organelle dependent on protein import for the great majority of its components.

Membrane Architecture

The organelle is divided into four subcompartments: the outer membrane, the intermembrane space, the inner membrane, and the matrix. The outer membrane is permeable to molecules up to about five kilodaltons through voltage-dependent anion channels, so the intermembrane space is chemically close to the cytosol. The inner membrane is the functional boundary: it is impermeable to ions and small solutes except through specific transporters, and it is folded into cristae that multiply its surface area several fold. Cristae junctions are shaped by the mitochondrial contact site and cristae organizing system, and their geometry constrains how respiratory complexes cluster and how protons distribute. The matrix enclosed by the inner membrane holds the citric acid cycle enzymes, the fatty acid beta-oxidation machinery, mitochondrial ribosomes, and multiple copies of the genome packaged into nucleoids.

Oxidative Phosphorylation

Electrons stripped from nutrients and carried by NADH and reduced flavin adenine dinucleotide pass along four inner-membrane complexes to molecular oxygen. Complexes I, III, and IV pump protons into the intermembrane space as they do so, creating an electrochemical gradient of roughly 150 to 180 millivolts across the membrane. ATP synthase, sometimes designated complex V, allows those protons back into the matrix through a rotary mechanism that couples proton flux to the phosphorylation of ADP. The system comprises roughly eighty nuclear-encoded subunits together with thirteen encoded by the organelle itself, and the regulation of flux through this pathway responds to substrate supply, ADP availability, and membrane potential. Assembly of the five complexes requires coordinated expression from two genomes and a large set of chaperones, a process detailed in reviews of mammalian oxidative phosphorylation complex assembly.

Genome, Inheritance, and Additional Roles

Human mitochondrial DNA is a circular molecule of 16,569 base pairs encoding thirteen respiratory chain proteins, two ribosomal RNAs, and twenty-two transfer RNAs. It is inherited maternally, uses a slightly modified genetic code, and is present in many copies per cell, so a pathogenic variant may occupy only a fraction of them. That mixture, called heteroplasmy, produces a threshold effect in which symptoms appear only once the mutant fraction exceeds a tissue-specific level. Beyond energy production, mitochondria buffer cytosolic calcium, synthesize iron-sulfur clusters and heme, contribute to steroid and pyrimidine synthesis, generate and respond to reactive oxygen species, and initiate the intrinsic apoptotic pathway by releasing cytochrome c. Failure of these functions underlies the tissue-selective damage seen in mitochondrial neurodegeneration.

Applications

Mitochondrial research has applications in a range of fields, including:

• Diagnosis and management of inherited mitochondrial disease
• Neurodegenerative disease research on Parkinson and Alzheimer pathology
• Cancer metabolism and drug development targeting metabolic reprogramming
• Forensic and population genetics using maternally inherited markers
• Exercise physiology and studies of metabolic adaptation
• Biosensor and imaging technology for membrane potential and oxygen consumption measurement