Mitochondria are believed to have evolved from an aerobic prokaryote (Eubacterium) that lived symbiotically within primitive eukaryotic cells. This hypothesis received support with the demonstration that mitochondria possess their own genome, increase their numbers by division, and synthesize some of their structural (constituent) proteins. Mitochondrial DNA is a closed circular molecule that encodes 13 enzymes involved in the oxidative phosphorylation pathway, two rRNAs, and 22 transfer RNAs (tRNAs) used in the translation of the mitochondrial mRNA.
Mitochondria possess a complete system for protein synthesis, including the synthesis of their own ribosomes. The remainder of the mitochondrial proteins is encoded by nuclear DNA; new polypeptides are synthesized by free ribosomes in the cytoplasm and then imported into mitochondria with the help of two protein complexes. These include translocase of the outer mitochondrial membrane (TOM complexes) and translocase of the inner mitochondrial membrane (TIM complexes). Translocation of proteins through mitochondrial membranes requires energy and assistance from several specialized chaperone proteins.
Mitochondria are present in all cells except red blood cells and terminal keratinocytes.
The number, shape, and internal structure of mitochondria are often characteristic for specific cell types. When present in large numbers, mitochondria contribute to the acidophilia of the cytoplasm because of the large amount of membrane they contain. Mitochondria may be stained specifically by histochemical procedures that demonstrate some of their constituent enzymes, such as those involved in ATP synthesis and electron transport.
Mitochondria possess two membranes that delineate distinct compartments.
Mitochondria display a variety of shapes, including spheres, rods, elongated filaments, and even coiled structures. All mitochondria, unlike other organelles described above, possess two membranes (Fig. 2.37). The inner mitochondrial membrane surrounds a space called the matrix. The outer mitochondrial membrane is in close contact with the cytoplasm. The space between the two membranes is called the intermembrane space. The following structural components of mitochondria possess specific characteristics related to their functions.
 Outer mitochondrial membrane. This 6to 7-nm-thick smooth membrane contains many voltage-dependent anion channels (also called mitochondrial porins). These large channels (approximately 3 nm in diameter) are permeable to uncharged molecules as large as 5,000 daltons. Thus, small molecules, ions, and metabolites can enter the intermembrane space but cannot penetrate the inner membrane. The environment of the intermembrane space is therefore similar to that of cytoplasm with respect to ions and small molecules. The outer membrane possesses receptors for proteins and polypeptides that translocate into the intermembrane space. It also contains several enzymes, including phospholipase A2, monoamine oxidase, and acetyl coenzyme A (CoA) synthase.
 Inner mitochondrial membrane. The TEM reveals that this membrane is thinner than the outer mitochondrial membrane. It is arranged into numerous cristae (folds) that significantly increase the inner membrane surface area (see Fig. 2.37). These folds project into the matrix that constitutes the inner compartment of the organelle. In some cells involved in steroid metabolism, the inner membrane may form tubular or vesicular projections into the matrix. The inner membrane is rich in the phospholipid cardiolipin,
FIGURE 2.37 • Structure of the mitochondrion. a. This electron micrograph shows a mitochondrion in a pancreatic acinar cell. Note that the inner mitochondrial membrane forms the cristae (C) through a series of infoldings, as is evident in the region of the arrow. The outer mitochondrial membrane is a smooth continuous envelope that is separate and distinct from the inner membrane. 200,000.