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The mitochondria or chondriosomes are sac like, granular or filamentous organelles, conspicuous in the cytoplasm of all eukaryotic cells except mature mammalian RBCs and bacteria. They are the largest cytoplasmic organelles in animal cells and the chief energy transducing organoids in living organisms.mitochndria another know as powerhouse of the cells.
Mitochondria was first reported as granules in muscle cells by Kolliker (1850) who named it as sarcosomes. Fleming (1852) observed many thread like filamentous bodies in the cytoplasm of nerve cellsand gave the name filia. Altmann (1894) renamed these structures as bioplast. The name mitochondria ty (Gr., Mito-thread, chondrion-granule) was proposed by Benda in 1898.
In general, mitochondria are uniformly distributed throughout the cytoplasm though in some cells they are restricted to certain zones. During cell division, mitochondria aggregate around the nuclear spindle. They are also concentrated at the base of cilia and flagella, in the light band of muscle fibres, around the nucleus and beneath the plasma membrane. When necessary mitochondria can change its form and move towards intracellular active sites where energy requirement is very high. Such movement occurs by the streaming movement of cytoplasm.
Though the number and density of mitochondria varies from cell to cell and organism to organism, it is relatively fixed for a particular cell type. Basically the density of mitochondria depends upon the energy requirement of the cell. They are numerous in cells with a high rate of metabolic activities. The following table shows the number of mitochondria in different types of cells and organisms..
Shape and size:-
Each mitochondrion has a typical shape, characteristic for a particular kind of cell. But the shape can vary according to the physiological and environmental conditions. It is usually rod-like or filamentous in appearance. It may also be elongated, cup-shaped, club-shaped, sausage-shaped etc. The size of mitochondria varies from cell to cell; the length ranges from 0.7 μm to 10 um, the width ranges from 0.25 um to 1.0 μm and the diameter ranges from 0.5 μm to 1.0 μm.
Chemically, mitochondria is composed of proteins and lipids. Proteins constitute 70 % and lipids constitute 30 % of dry weight. The lipid components include phospholipids (90 %), carotenoids (10 %), cholesterol, tocopherol etc. In addition, trace amounts of sulphur, copper and iron are also present. The semi-autonomous status of mitochondria is due to the presence of mitochondrial DNA, rRNA, mRNA, ribosomes, enzymes etc.
Electron microscopic observation has revealed that mitochondria consists of two membranes and two compartments. Each mitochondrion is enclosed by a double-membrane envelop which consists of an outer membrane and an inner membrane. The space between the two membranes is called the outer chamber of the inter membrane space. Its width varies from 40 Aº to 70 A°. It is filled with a jelly like fluid. The inner membrane encloses a central cavity called the inner chamber. The inner chamber is filled with a fluid which constitutes the miochondrial matrix. The matrix contains ribosomes. DNA, enzymes of krebs cycle, tricarboxylic acid cycle, fatty acid synthesis etc. It may also contain småll, dense granules for binding with divalent ions such as Mg and Ca. The inner surface of the inner membrane is folded inwards to form a number of crests called cristae mitochondrialis. These crests increase the surface area for enzymatic reactions. The cristae bear numerous club-shaped particles. These particles are variously named such as the elementary particles, subunits of Fernandez-Moran, oxysomes, Fo-F, particles etc. The number of these particles roughly correspond to the degree of oxidative events taking place in the mitochondrion. A mitochondrion may have as many as 10,000 to 100,000 particles. Each particle consists of three parts: (i) a cuboid base (40 x 110 A” wide) attached to the membrane, (ii) a projecting stalk (30-40 Aº wide, 50 A” long), and (iii) a mushroom-shaped, knob-like, polyhedral head(80-100 Aº in diameter) projecting into the matrix. The head piece or Fo subunit is connected to the base piece or F, subunit by a cylindrical stalk. The base piece is part of the inner membrane. It carries components of the respiratory chain. The head piececarries enzyme complexes for oxidative phosphorylation and synthesis of ATP. The head piece and the base piece (F-F, combination) act as the enzyme, ATP-synthetase that catalyzes ATP synthesis.
Sometimes mitochondria degenerate into small fluid-filled structures called cyto-lysosomes. On the otherhand, several mitochondria may be fused to form complex,composite structures called chondriospheres.
Mitochondria is appropriately called the ‘Power plant’ or the Power house of the cell. In animal cells they release energy from the fuel molecules. In plant cells this function is performed by the chloroplasts. The battery of enzymes and coenzymes present in mitochondria work in a stepwise manner to produce and pack energy into molecules of ATP. The following table shows various enzymes in different parts of mitochondria.
1. Mitochondria is the respiratory centre of the living cell. The fuel molecules are oxidised through a series of steps (glucolysis, pyruvic acid oxidation, Krebs cycle, electron transfer chain) on the inner membrane to release energy in the form of high energy packages called ATP. Thus, mitochondria is the chief energy transforming system in the cell.
2. Mitochondria is associated with fat metabolism. Steroid hormones are synthesized in the membrane of mitochondria.
3. Some of the proteins required by mitochondria are manufactured in its own protein factory.
4. Some proteolytic enzymes regulate lytic and synthetic activities.
5. They regulate their own density by moving freely in the cytoplasm to the sites of high metabolic activities.
6. They take part in yolk formation.
7. Mitochondria store and regulate cytosolic concentration of calcium ions. (more…)