Mitochondria are membrane bound organelles found in most eukaryotic cells and are major sites of aerobic energy metabolism. They are enclosed by an outer membrane and a highly specialised inner membrane, with an intermembrane space between them and a matrix enclosed by the inner membrane. The inner membrane forms folds called cristae that greatly increase its surface area and contain components of the respiratory chain and ATP synthase.
Mitochondria are commonly described as the principal sites of oxidative phosphorylation, through which energy released from reduced metabolic substrates is converted into ATP. Their functions extend considerably beyond ATP production and include metabolic integration, calcium handling, regulation of apoptosis, biosynthesis of certain molecules and cellular signalling. Mitochondrial number, morphology and activity vary according to cell type and physiological demand.
Mitochondria possess their own DNA and ribosomes, which are important evidence for the endosymbiotic origin of mitochondria. The widely accepted evolutionary model proposes that an ancestral eukaryotic cell established a stable association with an aerobic bacterial lineage, eventually giving rise to mitochondria. Mitochondria retain a small circular genome in many organisms, although the majority of genes originally associated with the ancestral endosymbiont have been lost or transferred to the nuclear genome during evolution. Consequently, mitochondrial proteins are encoded by both mitochondrial and nuclear DNA. Mitochondria therefore depend on extensive communication between two genetic systems. Their double membrane, bacterial-like genetic features and division by fission are among the characteristics supporting their evolutionary history.
Aerobic respiration within mitochondria involves a sequence of linked processes. Pyruvate produced by glycolysis can enter mitochondria and be converted to acetyl coenzyme A, which enters the citric acid cycle. The citric acid cycle generates reduced electron carriers including NADH and FADH2. These donate electrons to respiratory chain complexes embedded in the inner mitochondrial membrane. Electron transfer is coupled to proton pumping, creating an electrochemical proton gradient across the inner membrane. ATP synthase uses the energy stored in this gradient to phosphorylate ADP and produce ATP. Oxygen acts as the terminal electron acceptor in the respiratory chain, ultimately being reduced to water. This mechanism is known as chemiosmotic oxidative phosphorylation.
Mitochondrial metabolism is broader than oxidative phosphorylation. The organelle participates in the metabolism of fatty acids, amino acids and other substrates and contributes intermediates used elsewhere in the cell. Fatty acid beta oxidation generates acetyl coenzyme A, NADH and FADH2, providing substrates and reducing equivalents for energy production. Mitochondria also participate in pathways involved in haem synthesis, steroid production in specialised cells and regulation of metabolic intermediates. Because mitochondria can redistribute metabolites according to nutritional and energetic conditions, they function as metabolic hubs rather than simple ATP generators. Their activity is influenced by hormones, nutrient availability, cellular energy demand and signals originating from other organelles.
Mitochondria are involved in programmed cell death, particularly intrinsic apoptosis. In response to severe cellular stress, mitochondrial outer membrane permeabilisation can lead to the release of proteins such as cytochrome c into the cytosol. Cytochrome c participates in formation of the apoptosome and activation of initiator caspases, which can subsequently activate executioner caspases. Mitochondria also regulate intracellular calcium and can influence reactive oxygen species signalling. Controlled production of reactive oxygen species can contribute to normal signalling, whereas excessive oxidative stress can damage proteins, membrane lipids and nucleic acids. Mitochondrial function is therefore closely linked to cell survival, stress responses and the balance between adaptive signalling and cellular damage.
Mitochondrial morphology is dynamic. Mitochondria undergo fusion and fission, allowing the network to change according to energetic requirements, developmental state and cellular stress. Fusion can promote exchange of mitochondrial contents and may help maintain functional populations of mitochondria, while fission contributes to distribution during cell division and can facilitate removal of damaged mitochondrial fragments. Selective mitochondrial degradation through mitophagy is an important quality control process. Defects in mitochondrial dynamics or mitophagy can lead to accumulation of dysfunctional mitochondria. Mitochondrial movement through the cytoplasm is also regulated by motor proteins and cytoskeletal structures, allowing mitochondria to be positioned near regions with high energy or calcium requirements.
Mitochondrial genomes are inherited differently from nuclear genomes in many organisms. In humans, mitochondrial DNA is generally inherited maternally because the egg contributes most of the cytoplasmic organelles to the embryo. Mitochondrial DNA encodes a limited number of proteins involved in oxidative phosphorylation as well as ribosomal RNAs and transfer RNAs required for mitochondrial protein synthesis. Cells may contain mixtures of mitochondrial genomes with different sequence variants, a condition called heteroplasmy.
The proportion and distribution of different mitochondrial genomes can influence cellular effects because mitochondrial function depends on the aggregate behaviour of many organelles. Mitochondrial genetics is therefore distinct from conventional Mendelian inheritance and is important in studies of evolution, population history and inherited mitochondrial disorders.
Mitochondrial dysfunction can impair tissues with high energetic requirements, including nervous tissue, skeletal muscle and cardiac muscle. Genetic defects can affect mitochondrial DNA, nuclear genes encoding mitochondrial proteins, respiratory chain complexes, mitochondrial transport systems or quality control pathways. Acquired mitochondrial dysfunction can also arise through ageing, toxins, oxidative stress and other cellular insults. However, mitochondria are not universally identical, and their functions differ among tissues and physiological conditions.
A common misconception is that every mitochondrion continuously performs exactly the same metabolic role. In reality, mitochondrial populations can be heterogeneous in membrane potential, morphology, metabolic activity and location. Their integration with the nucleus, endoplasmic reticulum, lysosomes and cytoskeleton makes mitochondrial biology a central part of cellular physiology.
