cellsmitochondria

Mitochondria are double-membrane-bound organelles found in the cells of most eukaryotes. They generate most of the cell’s ATP through oxidative phosphorylation and also participate in a range of other essential processes, including metabolic regulation, apoptosis, and calcium storage. Mitochondria are dynamic organelles that continually change through fission and fusion in response to cellular needs.

Structure and genome: The organelle consists of an outer membrane and an extensively folded inner membrane that forms cristae, increasing surface area for the respiratory machinery. The matrix contains enzymes of the citric acid cycle, as well as mitochondrial DNA (mtDNA) and ribosomes. In humans, mtDNA is a circular genome of about 16,569 base pairs that encodes 37 genes (13 proteins of the respiratory chain, 22 tRNAs, and 2 rRNAs). Mitochondrial ribosomes are distinct from cytosolic ribosomes and are adapted to mitochondrial translation.

Energy production and metabolism: NADH and FADH2 transfer electrons through protein complexes in the inner membrane electron transport chain. The resulting proton gradient drives ATP synthase to convert ADP to ATP. The citric acid cycle, occurring in the matrix, provides reducing equivalents to the respiratory chain and integrates various metabolic pathways, including fatty acid oxidation and amino acid metabolism.

Dynamics and inheritance: Mitochondria are inherited maternally in many organisms and exhibit genetic variation through heteroplasmy. They continually undergo fission and fusion, a process known as mitochondrial dynamics, and are subject to quality control mechanisms such as mitophagy that remove damaged organelles.

Clinical significance: Mitochondrial dysfunction is associated with a variety of diseases and aging. Mutations in mtDNA or nuclear genes encoding mitochondrial proteins can lead to mitochondrial diseases, particularly affecting tissues with high energy demands such as the nervous system and muscles.