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What is a mitochondrion? Definition:

The fine structure and the exact functions of mitochondria have only been known for around 60 years. At that time, mitochondria were first examined and visualized using an electron microscope (see picture on the right).
Mitochondria are found exclusively in the cells of eukaryotes; in prokaryotic cells, however, they are completely absent. Mitochondria are those cell organelles that act as power plants ("power plants of the cells") and are therefore responsible for energy production. These particular organelles are found in most eukaryotic cells. Depending on the cell type, there may be a single, very large mitochondrion or up to 2000 small mitochondria. For example, in humans the red blood cells do not contain any mitochondria, whereas in the metabolically active liver cells up to 2000 mitochondria per cell may be present.

Structure of mitochondria

Mitochondria are about 1μm (microns) long. The shape of mitochondria is variable, the cells move in the cytoplasm and can take different forms. Like chloroplasts and the nucleus, mitochondria are surrounded by two cell membranes. The two cell membranes result in a matrix space in the interior of the mitochondrion and an intermembrane space. The outer mitochondrial membrane is responsible for the exchange of substances and the protection of the organelle. They contain so-called porins, special transmembrane proteins, through which substances can be absorbed and released. The inner mitochondrial membrane folds, resulting in a strong surface enlargement. Here are the proteins of the respiratory chain.
In the matrix space lies the genetic information. Mitochondria have their own DNA, the mtDNA. They function as an autonomous organelle and share independently of the cell cycle of the cell. The mitochondrial DNA is ring-shaped and evolutionarily likely originates from the invasion of a bacterium into a precursor of the eukaryotic cell. This theory is also supported by the fact that mitochondria contain 70S ribosomes, which are typical for prokaryotic cells. In eukaryotic cells, however, there are always 80S ribosomes.

Function of the mitochondria

The main task of mitochondria is the production of energy in the form of ATP (adenosine triphosphate); this happens via the respiratory chain. The respiratory chain consists of a series of enzymes that lie in the inner mitochondrial membrane. These enzymes lie side by side and form a chain through which electrons are transported. This results in a side effect of a concentration of protons, which is used by the ATP synthetase for the production of ATP.
In addition to energy production, the mitochondria, through their subdivision into different compartments, provide space for the course of chemical processes of assembly and disassembly, which must be spatially separate from each other. So in the mitochondrial matrix there are enzymes which are important for a citric acid cycle and degradation of fatty acids. In addition, the mitochondria serve as calcium storage. You can save calcium ions for a certain period of time. When calcium is needed, mitochondria release the absorbed calcium ions and thus contribute to the maintenance of the cell (homeostasis).


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