Mitochondrion

Mitochondria are membrane-bound organelles found in most eukaryotic organisms, including animals, plants, fungi, and many protists. Best known for generating adenosine triphosphate (ATP) through aerobic respiration, they provide the chemical energy required for numerous cellular processes. Beyond energy production, mitochondria participate in cell signalling, metabolic regulation, differentiation, growth control, and programmed cell death. Their widespread presence and biochemical versatility make them essential contributors to life in complex cells.

Discovery and Evolutionary Background

Mitochondria were first identified in 1857 by Albert von Kölliker and were later named by Carl Benda in 1898, the term combining Greek roots meaning “thread” and “granule”. The popular characterisation of mitochondria as the powerhouses of the cell emerged in the mid-twentieth century.
The presence of mitochondrial DNA—structurally similar to bacterial genomes—supports the theory of symbiogenesis, which proposes that mitochondria evolved from free-living prokaryotes that entered into an endosymbiotic relationship with ancestral eukaryotic cells. Over evolutionary timescales, this partnership became permanent, with most mitochondrial genes transferring to the nuclear genome.
While mitochondria are nearly universal among eukaryotes, notable exceptions exist. Certain unicellular lineages, such as microsporidia and diplomonads, possess reduced organelles known as hydrogenosomes or mitosomes. A few organisms, such as Monocercomonoides, have lost mitochondria entirely, whereas some multicellular species, including mature mammalian red blood cells, naturally lack them.

Structural Organisation

Mitochondria typically measure between 0.75 and 3 micrometres in diameter, though their size and morphology can vary significantly. They are composed of two membranes that create distinct internal regions, each with specialised functions.
Outer MembraneThe outer membrane forms the organelle’s boundary and contains porins such as the voltage-dependent anion channel (VDAC), which allow the passage of metabolites, ions, and small molecules. Larger molecules require targeted transport through complexes such as the translocase of the outer membrane (TOM). The outer membrane also houses enzymes involved in lipid metabolism and amino acid oxidation. It can associate with the endoplasmic reticulum via mitochondria-associated membranes (MAMs), facilitating calcium signalling and lipid exchange.
Intermembrane SpaceLocated between the inner and outer membranes, this compartment shares many small-molecule characteristics with the cytosol, owing to the permeability of the outer membrane. However, its protein composition is specialised, including factors such as cytochrome c, which plays a role in apoptosis when released into the cytosol.
Inner MembraneThe inner mitochondrial membrane is highly specialised, impermeable to ions and most molecules unless they are transported by carrier proteins. It contains a high proportion of proteins responsible for oxidative phosphorylation, including components of the electron transport chain and ATP synthase. Cardiolipin, a distinctive phospholipid associated with mitochondrial and bacterial membranes, contributes to membrane integrity and the optimal functioning of respiratory enzymes. Proteins enter this compartment through complexes such as TIM and OXA1L. The membrane’s tight regulation and electrical potential are crucial for ATP generation.
CristaeThe inner membrane forms extensive folds known as cristae, increasing surface area and thereby enhancing ATP production. Cristae density can vary within a single cell, with energy-demanding tissues such as muscle containing mitochondria rich in cristae. The cristae contain ATP synthase complexes and other respiratory proteins arranged to maximise efficiency.
MatrixThe matrix, enclosed by the inner membrane, contains a concentrated mixture of enzymes responsible for the citric acid cycle, fatty-acid oxidation, and other metabolic pathways. It also houses mitochondrial ribosomes, transfer RNAs, and mitochondrial DNA, supporting the organelle’s partial genetic autonomy.

Functional Roles

Although ATP production is the core function, mitochondria support numerous physiological processes.
Energy ProductionOxidative phosphorylation within the inner membrane uses electrons derived from nutrients to drive proton pumping and generate ATP. The matrix hosts enzymes that feed the respiratory chain through the oxidation of pyruvate, fatty acids, and amino acids.
Cell Signalling and HomeostasisMitochondria regulate cellular calcium levels, contributing to signal transduction and metabolic control. They also mediate apoptosis through the release of cytochrome c and other pro-apoptotic factors.
Development and Growth ControlMitochondrial biogenesis is tightly integrated with the cell cycle and with developmental signalling pathways. Changes in mitochondrial dynamics—fusion and fission—affect differentiation and tissue function.
Biosynthesis and MetabolismThe organelles contribute to the synthesis of steroid hormones, haem groups, and certain amino acids, linking them to diverse biochemical pathways.

Mitochondrial Diversity and Abundance

The number of mitochondria varies widely across cell types. High-energy tissues such as liver and muscle contain thousands of mitochondria per cell, while others may contain only a few. Their structure and metabolic capacity adapt to the specific functional demands of different tissues.
In some species, mitochondria undergo extreme modification or reduction. Hydrogenosomes and mitosomes represent organelles derived from mitochondria that have lost canonical respiratory functions. The loss of mitochondrial DNA in some parasites and the absence of mitochondria in certain animal cells illustrate the evolutionary flexibility of these organelles.

Medical and Biological Significance

Defects in mitochondrial function can lead to a range of disorders. Mitochondrial diseases arise from mutations in either mitochondrial or nuclear genes affecting oxidative phosphorylation. Mitochondrial dysfunction is also implicated in conditions such as heart failure and various neurodevelopmental disorders.
Altered mitochondrial dynamics, disrupted membrane integrity, and abnormalities in ATP production contribute to degenerative and metabolic diseases. Because mitochondria influence both energy metabolism and programmed cell death, disturbances in their function can have widespread physiological consequences.
Mitochondria therefore occupy a central position in cell biology, linking cellular energy supply with signalling networks, metabolic pathways, and evolutionary heritage. Their complex structure and multifaceted roles underscore their importance in maintaining the health, viability, and adaptability of eukaryotic cells.