Mitochondria are often described as the powerhouses of the cell, but their functions extend far beyond ATP production. These dynamic organelles are central to metabolism, calcium signaling, reactive oxygen species balance, and programmed cell death.
Mitochondrial Structure
Mitochondria possess two specialized membranes that create distinct compartments. The outer mitochondrial membrane (OMM) is permeable to small molecules through porin channels (VDAC) and contains proteins that regulate mitochondrial fusion, fission, and apoptosis. The intermembrane space contains proteins such as cytochrome c and procaspases that are released during apoptosis. The inner mitochondrial membrane (IMM) is highly folded into cristae that dramatically increase surface area, and it is impermeable to ions and metabolites without specific transporters. The IMM houses the electron transport chain complexes, ATP synthase, and metabolite carriers such as the adenine nucleotide translocase (ANT) that exchanges ADP and ATP across the membrane. The mitochondrial matrix contains the citric acid cycle enzymes, mitochondrial DNA (mtDNA), ribosomes, and enzymes for fatty acid oxidation and amino acid metabolism.
The Electron Transport Chain
The electron transport chain (ETC) consists of four protein complexes embedded in the inner mitochondrial membrane. Complex I (NADH:ubiquinone oxidoreductase) accepts electrons from NADH and transfers them to coenzyme Q (ubiquinone), pumping four protons across the IMM. Complex II (succinate dehydrogenase) accepts electrons from FADH₂ generated by the citric acid cycle and transfers them to coenzyme Q without proton pumping. Complex III (cytochrome bc1 complex) transfers electrons from reduced coenzyme Q to cytochrome c, pumping four protons. Complex IV (cytochrome c oxidase) transfers electrons from cytochrome c to molecular oxygen, the final electron acceptor, reducing it to water and pumping two protons. The proton gradient generated by complexes I, III, and IV creates both a pH gradient and a membrane potential that drives ATP synthesis.
Chemiosmotic Coupling and ATP Synthesis
ATP synthase (complex V) is a molecular turbine composed of a membrane-bound F₀ domain and a catalytic F₁ domain. Protons flow back into the matrix through the F₀ domain, causing rotation of the central stalk that drives conformational changes in the F₁ domain, allowing ADP and inorganic phosphate to bind and form ATP. Under normal conditions, approximately 2.5 ATP molecules are generated per NADH and 1.5 ATP per FADH₂. The chemiosmotic theory, proposed by Peter Mitchell, explains how the proton motive force couples electron transport to ATP synthesis. Uncoupling proteins (UCP1 in brown adipose tissue) dissipate the proton gradient, generating heat instead of ATP in a process called non-shivering thermogenesis.
Mitochondrial DNA and Genetics
Mitochondrial DNA is a circular double-stranded molecule of approximately 16.6 kb in humans, encoding 13 proteins of the ETC, 22 tRNAs, and 2 rRNAs. The remaining mitochondrial proteins (approximately 1,500) are nuclear-encoded, synthesized in the cytosol, and imported through the TOM/TIM translocase complexes. mtDNA is maternally inherited, lacks introns, and has a higher mutation rate than nuclear DNA due to the oxidative environment and limited repair capacity. Heteroplasmy refers to the co-existence of wild-type and mutant mtDNA within a cell, and symptoms of mitochondrial disease emerge only when the proportion of mutant mtDNA exceeds a critical threshold.
Mitochondrial Dynamics
Mitochondria constantly undergo fusion and fission, collectively called mitochondrial dynamics, which regulate organelle morphology, distribution, and quality control. Fusion is mediated by mitofusins 1 and 2 (MFN1, MFN2) on the OMM and optic atrophy protein 1 (OPA1) on the IMM, allowing mixing of mtDNA and protein contents between mitochondria. Fission is mediated by dynamin-related protein 1 (DRP1), which is recruited to the OMM by receptors such as MFF and Fis1 and constricts the membrane using GTP hydrolysis. Imbalanced dynamics are linked to neurodegenerative diseases: excessive fission in Parkinson disease and defective fusion in Charcot-Marie-Tooth disease type 2A and dominant optic atrophy.
Mitochondria in Calcium Homeostasis and Signaling
Mitochondria take up Ca²⁺ from the cytosol through the mitochondrial calcium uniporter (MCU), driven by the negative membrane potential of the inner membrane. This uptake buffers cytosolic Ca²⁺ signals, shapes IP₃ receptor-mediated Ca²⁺ release from the ER, and provides Ca²⁺ to stimulate the activity of three key mitochondrial dehydrogenases (pyruvate dehydrogenase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase), linking Ca²⁺ signaling to increased ATP production. Excessive mitochondrial Ca²⁺ overload can trigger opening of the mitochondrial permeability transition pore (mPTP), leading to dissipation of the membrane potential, swelling, and cell death.
Reactive Oxygen Species and Antioxidant Defense
During electron transport, approximately 0.1–1% of electrons leak from complexes I and III to generate superoxide (O₂⁻), the primary mitochondrial reactive oxygen species (ROS). Superoxide is converted to hydrogen peroxide (H₂O₂) by manganese superoxide dismutase (MnSOD/SOD2) in the matrix, and H₂O₂ is further detoxified to water by glutathione peroxidase and peroxiredoxin. Moderate ROS levels serve as signaling molecules, activating pathways involved in cell proliferation, adaptation to stress (mitohormesis), and immune responses. Excessive ROS, however, causes oxidative damage to mtDNA, proteins, and lipids, contributing to aging, neurodegenerative diseases, cardiovascular disease, and cancer.
