Mitochondria

Mitochondria are among the most important organelles in a eukaryotic cell, popularly and accurately described as the "powerhouse of the cell" because they are the principal site at which aerobic respiration takes place and the great majority of the cell's usable chemical energy, in the form of ATP (adenosine triphosphate), is generated. First observed by Kolliker as early as 1880 and later given their modern name by Benda, mitochondria are generally granular or filamentous, rod-like structures distributed through the cytoplasm, though their exact number, shape and distribution within a cell vary considerably depending on the cell's particular energy requirements -- a single cell may contain anywhere from a few hundred to a few thousand mitochondria, with metabolically very active cells, such as muscle and liver cells, generally containing far more than relatively inactive ones.

Structurally, a mitochondrion is bound by two separate membranes rather than one. The outer membrane forms a smooth, continuous envelope around the entire organelle. The inner membrane, lying just within the outer one, is far more elaborately organised: rather than forming a simple smooth layer, it is thrown into a series of finger-like or shelf-like folds called cristae, which project inward into the central cavity of the organelle. This folding dramatically increases the total surface area of the inner membrane available within a fixed volume, and this is functionally significant because the inner mitochondrial membrane is where the key membrane-bound reactions of aerobic respiration, culminating in ATP synthesis, actually occur -- the more folded the inner membrane, generally speaking, the greater a mitochondrion's capacity for ATP production. Studded along the inner face of the cristae are small, distinctive particles called oxysomes, or F1 particles, which are the actual sites of ATP synthesis.

The narrow, fluid-filled gap between the outer and inner mitochondrial membranes is called the perimitochondrial space (or intermembrane space), and is rich in certain enzymes involved in respiration. The large central cavity enclosed by the inner membrane, meanwhile, is filled with a semi-fluid substance called the matrix, which contains not only the enzymes needed for many of the biochemical reactions of respiration, but also, remarkably, its own genetic material -- a single, small, circular DNA molecule distinct from the cell's nuclear DNA -- together with a small number of RNA molecules and its own set of 70S ribosomes, matching the ribosome size typical of prokaryotic cells rather than the 80S ribosomes found in the surrounding eukaryotic cytoplasm.

Because a mitochondrion possesses its own DNA and its own protein-synthesising machinery, it is able to divide independently of the cell as a whole (by a process of fission broadly similar to bacterial binary fission) and can synthesise some, though by no means all, of its own structural proteins -- for this reason mitochondria are described as semi-autonomous organelles: partly self-sufficient, but still dependent on the cell's nucleus for the great majority of the proteins they need, which must be manufactured elsewhere in the cell and imported into the organelle. …