Mitochondria
Mitochondria are double-membrane organelles in eukaryotic cells that help convert energy from food into ATP while also supporting metabolism, signaling, calcium balance, and cell stress responses.
What mitochondria are
Mitochondria are organelles found in most eukaryotic cells. They are best known for their role in cellular respiration, but they are not only power supplies. They are active metabolic compartments that help cells manage fuel use, chemical signals, stress, and survival decisions.
Structure inside the organelle
A mitochondrion has an outer membrane, an inner membrane, an intermembrane space, and an internal matrix. The inner membrane folds into cristae, increasing surface area for the protein complexes that move electrons, pump protons, and help make ATP. The matrix contains enzymes, mitochondrial DNA, ribosomes, and metabolites used in energy pathways.
How mitochondria make ATP
During aerobic respiration, fuel molecules are broken down through linked pathways. Pyruvate from glycolysis can be converted into acetyl-CoA, which enters the citric acid cycle in the mitochondrial matrix. Electron carriers then feed high-energy electrons into the electron transport chain on the inner membrane. The resulting proton gradient powers ATP synthase.
More than energy conversion
Mitochondria also take part in fatty acid breakdown, amino acid metabolism, calcium handling, heat production in some tissues, reactive oxygen species signaling, and programmed cell death. Their exact job depends on the cell type: a muscle cell, neuron, liver cell, and egg cell all place different demands on mitochondrial function.
Mitochondrial DNA
Mitochondria carry a small genome separate from the chromosomes in the nucleus. In humans, mitochondrial DNA is usually inherited from the mother and encodes a small set of components needed for oxidative phosphorylation. Most mitochondrial proteins, however, are made from nuclear genes and imported into the organelle after translation.
Endosymbiotic origin
The leading explanation for mitochondrial origin is endosymbiosis: an ancient archaeal or eukaryotic host formed a lasting partnership with a bacterium related to modern alphaproteobacteria. Over evolutionary time, that partner became an organelle, transferred many genes to the nucleus, and remained essential to eukaryotic cell biology.
Health and disease
Because mitochondria support energy-intensive tissues, mitochondrial problems can affect muscles, nerves, the heart, vision, hearing, metabolism, and development. Disease can arise from mutations in mitochondrial DNA, mutations in nuclear genes that support mitochondria, environmental damage, or broader cell stress that overwhelms repair and quality-control systems.
Why it matters
Mitochondria matter because they connect cell structure, food energy, oxygen use, inheritance, evolution, and health. Understanding them helps explain why eukaryotic cells can sustain complex work, why oxygen is useful but chemically risky, and why tiny compartments inside cells can influence the fate of whole tissues.