ABSTRACT

The mitochondrion is an essential organelle for all eukaryotic cells. The number of mitochondria per single cell depends on its energy demand. Metabolically active organs such as the liver, the brain, and cardiac and skeletal muscle tissues may contain up to several thousands of mitochondria per cell, whereas somatic tissues with a lower demand for energy contain only a few dozen of these organelles. Each mitochondrion is composed of two membranes, which together create two separate compartments, the matrix space and the intermembrane space. The outer membrane is permeable to molecules smaller than 5 kDa, whereas the inner membrane is highly impermeable and characterized by an unusually high content of membrane proteins as well as a unique lipid composition. The mitochondrial inner membrane proteins are components of the respiratory chain and a large number of

transport proteins. The impermeability of the inner membrane is a prerequisite for the establishment of an imbalance in the distribution of protons between the mitochondrial matrix and the cytosol, which in turn is the driving force for the synthesis of adenosine triphosphate. Mitochondria are unique in comparison to all other organelles as they contain their own genome (mtDNA) and the necessary transcription and translation systems. Human mtDNA is a circular molecule 16,569 bp in size; it encodes two ribosomal RNAs, all 22 tRNAs necessary for protein synthesis at mitochondrial ribosomes, and 13 polypeptides that are all subunits of the oxidative phosphorylation (OXPHOS) enzyme complexes.