Although known since the early days of microscopy, mitochondria continue to attract extraordinary interest because of their unique functions in both cell life and death. Dimensions, shapes, and locations of mitochondria are strikingly different in diverse types of cells in relation to the specific cellular functions. Because of their obligatory double membranes and characteristic inner compartmentalization, it is easy to discriminate mitochondria from other membrane-bound organelles, such as peroxisomes (cf. Fig. 79), under the electron microscope.

Panel A shows a mitochondrion of the crista type in a rat pancreatic acinar cell, surrounded by cytoplasm with densely packed cisternae of the rough endoplasmic reticulum. Mitochondria are located mainly at sites where energy is needed and, according to the requirements, change their locations and undergo temporary alterations in shape, forming long “filamentous” organelles and extended intracellular networks. A mitochondrial network with a ring-like architecture and branching arms is shown in panel c. Outer and inner mitochondrial membranes enclose the intermembrane space. The outer membrane contains voltage-dependent anion channels, the mitochondrial “porins” that allow ions and small molecules to enter the intermembrane space, creating a milieu resembling that of the cytoplasm. The inner membrane is impermeable to ions because of its enrichment in the phospholipid cardiolipin. It surrounds the mitochondrial matrix and contains the proteins for the oxidation reactions of the respiratory electron-transport chain, adenosine triphosphate (ATP) synthesis, and regulation of the metabolite transport into and out of the matrix. The inner membrane consists of two domains: the inner boundary membrane residing adjacent to the outer membrane and invaginations, which in most cells have the form of cristae (arrowheads in panel a), although other forms, such as tubular projections, occur as well. The latter are typical for steroid hormone-producing cells and are shown in panel B in an endocrine cell of the ovary (T-tubular projections). Inner boundary membranes and cristae membranes are connected by tubular openings, the crista junctions. For their maintenance and formation of contact sites to the outer membrane, a large heterooligomeric protein complex of the inner membrane, termed MICOS (mitochondrial contact site and cristae organizing system), has a pivotal role.

The matrix contains the enzymes of the citric acid cycle and enzymes engaged in fatty acid β-oxidation. In panel a, dense matrix granules are visible, being important for the storage of Ca²⁺ and other divalent cations. Furthermore, the mitochondrial DNA and the machineries for protein synthesis, ribosomes, and tRNAs are contained in the matrix. Only some of the mitochondrial proteins are encoded by the mitochondrial genome and synthesized in the matrix. Most of the mitochondrial proteins are synthesized on free ribosomes in the cytoplasm and are posttranslationally translocated across the mitochondrial membranes to reach their functional destinations inside the mitochondria. Three membrane protein complexes, the translocase of the outer membrane (TOM), the presequence translocase of the inner membrane (TIM 23), and the protein insertion complex of the inner membrane (TIM 22), build up the central machineries for recognition and translocation of mitochondrial precursor proteins.