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Discovered in 1890 by R. Altmann, the mitochondria are the most complex organs inside the cytoplasm. They are present in all the animal cells and vegetable aerobes. The mitochondria have got the ability of self-breeding and they are in wide-measure autonomous from the cell. The mitochondria are visible to the optic microscope as little sticks or lengthened granules, with a quite constant diameter. In thin sections examined at the electronic microscope the mitochondria show a very particular ultra-structural organization. In fact, they appear to be delimited by two membranes, the outer membrane and the inner one; (this last one folds on itself forming complex folds). These two membranes are separated by a space of ca. 10 nm. They delimit two compartments (or rooms): the outside one, that is the space between the two membranes, and the inside one, delimited by the inner membrane and containing the mitochondrial matrix. The external compartment generally appears «empty», because it is not opaque to the electrons; the matrix has instead a granular aspect, and in it there are granules, ribosomes and a circular filament constituted by DNA. The functions of the mitochondria mainly concern the production of molecules with high energetic content (for example the ATP - adenosinetriphosphate) through a process of cellular breathing.
When the mitochondria were discovered in 1890 by Altmann, the technologies of the time didn't allow observing them in the detail, therefore they were accepted for a long time by biologists as organelles always present inside the cell. With the advent of modern technologies concerning the scanning and the transmission microscopes, the mitochondria started to be studied and observed much more accurately, and above all internally. This important turn in the field of the observation of the cellular micro organelles involved the discovery of the mitochondrial matrix and division in compartments but above all of the presence of the phenomena of heredity (due to the presence of DNA - deoxyribonucleic acid) independent from those of the cell. This discovery has led to the supposition that in ancient times they were free prokaryotes, similar to today's bacteria, that had found refuge inside bigger heterotrophic cells and subsequently entered in symbiosis with such cells up to lose independent life. Particularly the mitochondria can be considered similar to aerobic organisms analogous to bacteria, called Infective Endosymbionts. An endosymbiont is a micro organism which lives inside a guest cell, establishing with it a particular form of symbiosis, which results reciprocally more advantageous, but however never harmful either for the endosymbiont or for the guest2.
The endosymbiont can be a virus, a bacterium or a single-celled alga, and the guest can be a single-celled organism or a cell which belongs to a more complex organism.
Infective Endosymbionts are organisms (viruses or bacteria) penetrating in the guest for infection, which subsequently have developed a relationship of total dependence to that they become stable and necessary elements to the life of the eukaryotic cell.
Some of the tests that confirm this theory there are the following:
the fact that the Mitochondria are only formed through autonomous division by equal organelles
The fact that, as the prokaryotes, they contain only an annular DNA.
The fact that their external membrane is of eukaryotic type, while the inside is of prokaryotic type.
In favour of the Endosymbiontic Theory there is also the fact that the evolution of the eukaryotic cells has involved a notable evolutionary breakout, with the passage from simple prokaryotic cells to cells divided into a variety of functional parts, all of this without passing through some intermediary forms.
The dilemma on this lack of forms has been explained with the Theory of the Endosymbiosis: based on the strong similarities among the bacteria on one side and mitochondria and chloroplasts of the eukaryotic cells on the other. The conclusion has been reached that they had to be autonomous organisms which became united, through processes of phagocytosis and endosymbiosis, to the mother cell. In fact all the autotrophic eukaryotes contain chloroplasts, and all the eukaryotes contain mitochondria: both the organelles seem to have been acquired through different symbiotic events.
Written by: Agatino Bara, Giuseppe Gentile,Gabriele Roro, Marco Seminara