Mitochondria

Mitochondria are membrane-bound organelles present in the cells of most eukaryotes, such as animals, plants, and fungi. They are primarily responsible for producing adenosine triphosphate (ATP) via aerobic respiration, supplying the cell with chemical energy. Albert von Kölliker first observed them in 1857 within the voluntary muscles of insects. The name "mitochondrion," meaning "a thread-like granule," was introduced by Carl Benda in 1898. The popular nickname "powerhouse of the cell" was popularized by Philip Siekevitz in a 1957 Scientific American article.

In cross section, mitochondria typically measure between 0.75 and 3 μm², though their size and shape vary. They are not visible without specific staining. A mitochondrion has a double-membrane structure, creating five distinct compartments: the outer membrane, the intermembrane space, the inner membrane, the cristae (folds of the inner membrane), and the matrix. The inner membrane’s folds increase its surface area, boosting ATP production. When the outer membrane is removed, the remaining structure is called a mitoplast.

Beyond energy production, mitochondria participate in signaling, cellular differentiation, cell death, and regulating the cell cycle and growth. Their biogenesis is coordinated with these processes. Mitochondrial dysfunction is linked to human disorders including mitochondrial diseases, cardiac dysfunction, heart failure, and autism.

The number of mitochondria per cell varies widely by organism, tissue, and cell type. For instance, mature red blood cells have none, while a liver cell may contain over 2,000.

Mitochondria possess their own genome, called the mitogenome, which resembles bacterial genomes. This supports the endosymbiotic theory, which holds that free-living prokaryotic ancestors of mitochondria permanently fused with early eukaryotic cells, eventually evolving so that modern eukaryotes respire to generate energy.

The outer membrane is 60 to 75 angstroms thick, with a protein-to-phospholipid ratio similar to the cell membrane. It contains porins, including the voltage-dependent anion channel (VDAC), which transports nucleotides, ions, and metabolites between the cytosol and intermembrane space. Larger proteins enter via a signaling sequence that binds to a translocase complex. The outer membrane also hosts enzymes for fatty acid elongation, epinephrine oxidation, and tryptophan degradation, such as monoamine oxidase and fatty acid-CoA ligase. Damage to this membrane allows intermembrane proteins to leak into the cytosol, triggering cell death. The outer membrane can associate with the endoplasmic reticulum in a structure called MAM, important for calcium signaling and lipid transfer. Outside it are small particles called subunits of Parson.

The intermembrane space, or perimitochondrial space, lies between the outer and inner membranes. Because the outer membrane is permeable to small molecules, the concentrations of ions and sugars here match those in the cytosol. Large proteins, however, require a signaling sequence to cross the outer membrane, giving this space a distinct protein composition. Cytochrome c is one such protein found here.

The inner membrane contains proteins for electron transport chain redox reactions, ATP synthase, and transport proteins that regulate metabolite passage. It holds over 151 different polypeptides and has a high protein-to-phospholipid ratio (more than 3:1 by weight). About one-fifth of all mitochondrial protein resides here. The inner membrane is also rich in cardiolipin, a phospholipid originally discovered in cow hearts.