The mitochondrion is enveloped by two membranes viz, inner membrane and outer membrane. The outer membrane is 6nm thick; internal to this is the inner membrane. Separating these two is a space (outer chamber) filled with a watery fluid.
This space is 40-70A thick. The inner membrane is folded into finger like projections called cristae.
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This membrane encloses the inner chamber or matrix, usually filled by dense protinaceous material.
The matrix may be homogenous or may have a granular substance. The cristae of the inner membrane facing matrix are called the M side, while the other side is called the C side.
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The cristae have an inner space called the intercristae space which is in continuity with the intermembrane space.
Within the mitochondrial matrix are small Ribosomes and a circular DNA. The Ribosomes are of 70s type and similar to those of bacteria.
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These can synthesise proteins (Tyier 1973). The DNA called mDNA may be circular as in most of the animal cells or linear as in plant cells. Rabinowitch (1968) has demonstrated that the mitochondrial DNA has more G-C content than the nuclear DNA.
The mitochondrial DNA can replicate itself as it has DNA ploymerase. It is due to this, that mitochondria can replicate themselves.
The inner membrane:
The inner membrane has numerous nail like or tennis racquet like projections protruding into the matrix.
Each particle is about 70-100A°in diameter. The particles are placed apart a distance of 100A°and each is attached to the inner membrane by a short stalk,of 35-50A0 long.
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The number of these particles per mitochondrion ranges from 104-105 these are called elementary particles, Particles or submits it was formerly believed that these particles contain all the enzymes necessary for electron transport and oxidative phosphorylation.
Hence they were named electron transport panicles (ETP). Each particle was thought to have four complexes, with complex I and II in base, III in stalk and IV in the head piece.
According to current thinking, the head piece has ATPase, the stalk consists of a special protein (oligomycin sensitivity confering protein) and the piece contains the proton channel.
The inner membrane has the respiratory chain consisting of five complexes. Of these, four are involved in the electron transport, while the fifth one is involved in the synthesis of ATP.
Mitochondria play a vital role in respiration. Glycolysis takes place in the cytosol (cytoplasm) while the enzymes of the Kerbs cycle are found in the matrix; those of oxidative phosphorylation are found in the inner membrane.
Biogensis of mitochondria:
Each mitochondrion lasts for about a week or little more. There are three views regarding the origin of mitochondria-a) origin from cell membranes, b) Division of pre existing mitochondria and c) de novo origin.
According to Robertson (1964), mitochondria in muscle cells arise by the infloding of the plasma membrane.
This infolding joins another piece and forms a vesicular structure. In the nerve fibres of Cray fish, endoplasmic reticulum may form mitochondria.
According to Luck and Rich (1964), mitochondria are capable of self reproduction. DNA, protein synthesis, DNA polymerase and the other entire essential, features for self replication are present in mitochondria.
Harvey (1951) supports a de novo origin for mitochondria. Centrifugal separation of sea-urchin eggs into two halves, a nucleated half (without mitochondria) and a non nucleated half (with mitochondria) showed that the nucleated half produced the mitochondria.
Are mitochondria symbolic organisms? The many similarities between a bacterial cell and mitochondria made many people (Lloyd 1974; Borst 1977, O’Brien 1977 believe that the mitochondrion was once an autonomous entity which later entered into a symbolic association with an anaerobically respiring host cell. The host cell derived its energy only from glycolysis. Subsequently with the association, the host cell started respiring aerobically. De Robertis (1970) opines that in evolutionary terms it is possible that a symbiotic relationship could have evolved into the present semiautonomous condition of the mitochondria.
The same theory has been proposed for chloroplasts also, as they also have all (the independent attributes of mitochondria.