Mitochondrium

Mitochondrion, often called the powerhouse of the cell, is a double-m membrane-bound organelle found in most eukaryotic cells. It generates most of the cell’s ATP through oxidative phosphorylation and contains its own circular DNA and ribosomes, reflecting its evolutionary origin from an ancestral bacterium.

Structurally, a mitochondrion has an outer membrane that is permeable to small molecules and an inner membrane folded into cristae, which increase the surface area available for reactions. The space inside the inner membrane is the matrix, which hosts enzymes of the citric acid cycle, mitochondrial DNA, ribosomes, and tRNAs. Between the outer and inner membranes lies the intermembrane space.

Functionally, mitochondria produce ATP by coupling the electron transport chain to ATP synthase. Electrons pass through protein complexes in the inner membrane, pumping protons and creating a proton gradient that drives ATP synthesis from adenosine diphosphate and inorganic phosphate. In addition to energy production, mitochondria regulate metabolic pathways, calcium storage, and apoptosis through controlled release of cytochrome c. They also contribute to heat generation in brown adipose tissue via uncoupling protein 1 and can generate reactive oxygen species as byproducts of respiration.

Genetically, mitochondria contain their own circular DNA and ribosomes, and they replicate by fission. In most animals, mtDNA is inherited maternally. Mitochondrial gene expression involves both mitochondrial and nuclear genomes, and mitochondrial dynamics include fission, fusion, and mitophagy for quality control.

Clinical relevance includes mitochondrial diseases caused by mtDNA or nuclear DNA mutations, often exhibiting heteroplasmy and affecting tissues with high energy demands. Examples include Leber hereditary optic neuropathy and MELAS. M mitochondrial function is also studied in aging and neurodegenerative contexts.