Short notes on the ultra structure of mitochondria

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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 40-70 A0 thick. The inner membrane is folded into finger like projections called cristae.

This membrane encloses the inner chambers or matrix, usually filled by dense proteinaceous material. The matrix may be homogenous or may have a granular substance. The cristae of the inner membrane incompletely divide the matrix into chambers. The side of the inner membrane facing matrix is called the M side, while the other side is called the C side.

The cristae have an inner space called the inter cristae space which is in continuity with the intermembrane space. Within the mitochondrial matrix are small ribosomes and the circular DNA- The ribosomes are of 70s type and similar to those of bacteria. These can synthesise proteins (Tyler 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.

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The mitochondinal DNA can replicate itself as it has DNA polymerase. It is due to this, that mitochondria can replicate themselves. The inner membrane: The inner membrane has mumerous nail like or tennis racquet like projections protruding into the matrix. Each particle is about 70-100 A0 in diametre. The particles are placed apart a distance of 100 A0 and each is attached to the inner membrane b/a short stalk of 35-50 A0 long.

The number of these particles per mitochondrion ranges from 104 -105. These are called elementary particles, F1 particles or subunits. It was formerly believed that these particles contain all the enzymes necessary for electron transport and oxidative phosphorylation. Hence they are named elecĀ­tron transport particles (ETP).

Each particle was thought to have four cornplexes, 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 conferring protein) and the base 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.

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Mitochondria play a vital role in respiration. Glycolysis takes place in the cytosol (cytoplasm) while the enzymes of the Krebs cycle are found in the matrix, those of oxidative phosphorylation are found in the inner membrane.

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