Mitochondria are distinct cytoplasmic organelles of the cell which participate in various functions of the cell metabolism. For their existence, cells require energy to perform various
functions. The importance of mitochondria lies precisely in the fact that they supply practically all of the necessary biological energy, and they do so primarily by oxidizing the
substrates of the Krebs cycle. A cell gets its energy from the enzymatic oxidation of chemical compounds within the mitochondria, hence they are generally referred to as ‘power houses’ of the cell.
Mitochondria are found in all eukaryotic cells, but may be lost in later stages of development of cells such as red blood cells or phloem sieve tube elements. A mitochondrion is a structure, bounded by a limiting membrane, found in all cells except the bacteria and the mature red blood cells of the multicellular organisms. It is found in plant cells, in algae, and in protozoans. In all these cells its appearance is very similar: an outer membrane limits the structure, while just inside is another membrane that has invaginations, called cristae. This typical structure occurs again and again in the cell so that it is easy to recognize a mitochondrion simply by its morphology.
Mitochondria are universally present in all eukaryotic aerobic cells. They are uniformly distributed in the cytoplasm. Total absence of mitochondria in prokaryotes is indicative of their anaerobic life. The number of mitochondria per cell remains constant, but may change sometimes depending upon their functional activity. Generally the number of mitochondria is related to the energy demand of the cell. In a normal rat liver cell the number of mitochondria may vary between 1000 and 1500. The number, however, diminishes in cancer cells.
Shape and Size
Within the cell mitochondria may assume different shapes ranging from granular to filamentous depending upon the functional state of the cell. They are spherical in yeast cells, elliptical in kidney cells, elongated in liver cells, and filamentous in fibroblasts. In some cases a swelling may appear at one end to give it an appearance of club shape.
Their size also varies, but in most cells they are about 0.5 n wide and about 0.5 to 0.7 n long, approximating the size of the bacterium, Escherichia coli. It has been noticed that the shape and the size may change with the metabolic state of the mitochondrion.
Although mitochondria vary in shape and size, they have a common organization pattern. Detailed structure of mitochondria has been given by Palade, according to whom they consist of a smooth outer membrane, separated by a space from an inner membrane. The inner membrane is thrown into folds or invagination called cristae which extend into the matrix, the mitochondrial lumen. Both the membranes are continuous, about 70A thick and resemble the unit membrane structure. Each one of them is separated by a clear intermembranal space about 60-70 A wide and the lumen of the outer sac does not communicate with the inner one. The cristae are irregularly shaped, some are villose while others are finger-like.
It is possible to separate the inner and outer membranes from the internal matrix, and to analyse these components for activity of various enzymes. The matrix is filled with a dense proteinaceous material packed with dense granules. Until recently no function was assigned to these granules, but now they are considered to be the sites for binding divalent cations. The cristae may extend partly into the lumen or may extend wall to wall (septa-like). The septa may again be simple, tubular, forked, criss-cross or fenestrated.
The inner surface of the cristae is packed with lollipop-like particles attached through short stalks. These particles are called elementary particles.
An elementary particle has a hexagonal basal plate, a stalk and a knob-like (F1) structure. Each of these particles contains four electron transfer complexes, of which complexes I and II are located in the basal plate, complex III is situated in the stalk and complex IV in the F1 portion. The outer surface of the outer membrane also bears elementary particles, but they differ from the elementary particles of the inner membrane lacking basal plate and the stalk. The knob-like particle is directly attached to the membrane. The particles associated with the outer membrane harbour NAD+ -linked dehydrogenases, while the particles of the inner membrane are rich in the enzymes of the respiratory chain and electron transport system.
Cross section of a mitochondrion showing elementary particles.