Notes on the meaning, structure and functions of Mitochondria

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Mitochondria are present in the living eucaryotic cells as membrane bound organelles essential for cellular respi­ration. Although quite variable in shape, mitochondria are generally sau­sage-shaped, spherical, oval, pear­shaped, cylindrical or filamentous mitochondrion are not uncommon. A typical mitochondrion is about 0.5u in diameter with a maximum of 7u in length. Their number increases with the increase in metabolic activity of the cell.

The cells of germinating seeds show a much larger number of mito­chondria than those of dormant seeds. There are fewer mitochondria in plant cells than in animal cells. Some yeast cells contain one or a few highly branched mitochondria. These organelles are lost secondarily in mammalian RBC and are completely absent from prokaryotic cells. Nor­mally, mitochondria are evenly distrib­uted throughout the cytoplasm. But the density of their presence goes up in cells involved in energy dependent ac­tivities. Mitochondria are aggregated at the base of cilia or flagella to provide energy for their movement.

A mitochondrion is enclosed by a double-membrane envelope. The two membranes- outer and in­ner are separated by a narrow, about 100 A fluid-filled space called the perimitochondrial space, inter­membrane space or outer compartment. The outer membrane is smooth and completely encloses the organelle. It is more permeable to small mol­ecules; so, when an isolated mitochon­drion is placed in sucrose solution, the sucrose penetrates the outer membrane but not the inner one and the perimitochondrial space enlarges by drawing more amount of water from the matrix.

The .membrane bears a number of enzymes but lacks some proteins in comparison to the inner membrane. The inner membrane surrounds the inner compartment or central cav­ity filled with a fluid called matrix. It is selectively permeable, enzyme-rich and contains more carrier proteins. The inner membrane has many incomplete partitions which extend into the ma­trix. These folds, called cristae (sing crista), enormously increase the sur­face area of the inner membrane and provide space for a much larger number of compounds responsible for oxidative phosphorylation.

The fluid-filled matrix contains a variety of materials, including enzymes of the citric acid cycle, ribosomes,enzymes and factors needed for protein synthesis and a small circular double stranded DNA. The inner membrane restricts the pas­sage of molecules and ions between the two compart­ments and plays an impor­tant role in ATP production in mitochondria.

The inner membrane and cristae bear numerous tennis-racket-like particles with a spherical head and a stalk. These par­ticles have variously been named as subunits of Fernades-Moran, el­ementary particles, F1 particles, electron trans­port particles or the oxysomes and are about 100 Aº apart from each other. Their number varies from 104-105 per mitochondria.

Each particle consists of a spheri­cal head called F1 subunit, approxi­mately 100 Aº in diameter and is con­nected to a base piece called F0 subunit which is 35-50 Aº in length. The F1 subunit is an integral protein embed­ded in the inner membrane. The F0-F1 combination has got the special ATPase enzyme that catalyses ATP synthesis.

Mitochondria are semi-autonomous organelles. They have got their own protein synthetic machinery. However, many of their proteins come from the cytoplasm. Like chloroplasts, mito­chondria increase in number by binary fission. During cell division the daugh­ter cells inherit mitochondria from their mother cells. These mitochondria subsequently replicate to restore the normal number of mitochondria in the cell.

Mitochondria are known as the power house of the cell because they associated with synthesis and stor­age of energy rich compound, ATP. They mainly function as sites for aerobic res­piration. Besides oxidation of carbohy­drates, mitochondria are also involved in the oxidation of protein and fats.


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