Different types of mitochondria

Three different types of mitochondria are known: the sacculus type, the cristal type and the tubule type. The classification is made on the basis of the indentations of the inner membrane in the interior of the mitochondria. Depending on how these indents look, the type can be determined.

These folds serve as surface enlargement (more space for the respiratory chain). The cristae type has thin strip-shaped inversions. The tubule type has tubular indents and the saccule type has tubular indents with small bulges.

The critae type is the most common. The tubule type mainly occurs in cells that produce steroids. The sacculus type is found only in the zona fasciculata of the adrenal cortex. Occasionally, a fourth type is also called the prism type. The protuberances of this type appear triangular and it is found only in special cells (astrocytes) of the liver.

Mitochondrial DNA

In addition to the cell nucleus, mitochondria contain their own DNA as the main storage location. This makes them unique compared to other cell organelles. Another special feature is that this DNA is present in a ring-shaped form as a so-called plasmid and not, as in the cell nucleus, in the form of chromosomes.

This phenomenon can be explained by the so-called endosymbiont theory, which states that mitochondria were own living cells in primeval times. These primeval mitochondria were eventually swallowed by larger unicellular organisms and henceforth placed their work in the service of the other organism. This cooperation worked so well that the mitochondria lost the characteristics that made them an independent life form and integrated themselves into the cell life.

A further argument for this theory is that mitochondria divide and grow independently without requiring information from the cell nucleus. Also with their DNA, mitochondria are an exception to the rest of the body, because mitochondrial DNA is strictly inherited from the mother. They are delivered with the maternal egg cell, so to speak, and divide during embryonic development until each body cell has sufficient mitochondria.Their DNA is identical, which means that maternal hereditary lines can be traced back a long time. Of course, there are also genetic diseases of the mitochondrial DNA, so-called mitochondropathies. However, these can only be inherited from mother to child and are generally extremely rare.