Mitochondrion

Mitochondria are usually elongated, ≈0.5–1 µm wide and 1–10 µm long, and form a dynamic network that constantly fuses and divides. They descend from an α-proteobacterium engulfed by an ancestral cell (endosymbiotic theory), which explains their two membranes, circular DNA, 55S ribosomes, division by fission and the bacterial-like sensitivity of their protein synthesis to some antibiotics.

The outer membrane is permeable to molecules < ≈5 kDa through porins (VDAC). The inner membrane is folded into cristae that greatly enlarge its surface; it is rich in the phospholipid cardiolipin, is impermeable to ions (including H⁺), and carries the electron transport chain (complexes I–IV), ATP synthase (complex V, F₀F₁) and specific transporters (e.g. ADP/ATP translocase). Electron flow pumps protons into the intermembrane space, and their return through ATP synthase — whose F₀ rotor turns like a turbine — drives ATP synthesis (chemiosmotic theory, Mitchell). The matrix contains the pyruvate dehydrogenase complex, citric-acid-cycle and β-oxidation enzymes, mtDNA, mitochondrial ribosomes and matrix granules (Ca²⁺).

Human mtDNA is a 16 569-bp circle encoding 37 genes — 13 polypeptides of the respiratory chain, 22 tRNAs and 2 rRNAs; the other ≈1000–1500 mitochondrial proteins are nuclear-encoded and imported through the TOM/TIM translocases. Each cell holds hundreds to thousands of mtDNA copies. Mitochondria are inherited only from the mother (paternal sperm mitochondria are destroyed after fertilisation), and mtDNA mutates faster than nuclear DNA. Mitochondria cluster where ATP demand is highest: between myofibrils, in the basal folds of renal tubule cells, around the sperm axoneme (midpiece).