Mitochondria (Greek, mitos = thread, khondros = granule) are organelles specific for eukaryotic cells with varying shapes and sizes (0.5 - 10 µm). Richard Altman was the first researcher to recognize the presence of these organelles. These organelles divide independently of the cell nucleus, similarly to a bacterial cell (budding). Energy metabolism is the main function of mitochondria, but these organelles are associated with: cellular signalling and differentiation, cell cycle, cell death and aging process, mitochondrial disorders (1, 2, 3).

Mitochondria and chloroplast are large organelles that can be seen with the optical microscope, but their structures have been described after the invention of electron microscopy. Structurally, a mitochondrion is composed of two membranes, which delimit a central cavity (mitochondrial matrix). The outer mitochondrial membrane contains proteins and lipids in the same quantity and has the porins, channels structure that facilitate the transmembrane transport of large molecules. The inner mitochondrial membrane is the cell membrane with the highest percentage of protein. Inner membrane surface is high due to the internal folds that delimit the small cavities called cristae. Between the two membrane is located the intermembrane space. Mitochondrial matrix is in the central region, where they are enzymes and metabolites, mitochondrial ribosomes, tRNA, rRNA and several copies of the mitochondrial DNA (1, 4).

Mitochondrial enzymes catalyze the reactions from metabolism of carbohydrates, proteins, lipids and urea (5).

In the mitochondria is carried out the final stages of biological oxidation: citric acid cycle (Szent-Györgyi and Krebs cycle) and the transfer of protons and electrons along the electron transport chain. Krebs cycle is a metabolic pathway, where the acetyl-CoA (C2) is completely oxidized to CO2. For each molecule of acetyl-CoA, in cycle Krebs (10 reactions) are two decarboxylation reactions with the formation of 2CO2 (reactions 5 and 6), four oxidation (reduction) reactions (reactions 4, 6, 8 and 10) with the formation of 3 NADH+H+ and FADH2 and a substrate level phosphorylation reaction with generate ATP (reaction 7). The electron transport chain transferred the electrons from NADH+H+, FADH2 and succinate to oxygen, which is reduced to water. The transport of electrons is achieved of four complex: complex I (NADPH dehydrogenase), complex II (succinate dehydrogenase), complex III (cytochrome bc1 complex) and complex IV (cytochrome c oxidase). Three complexes (I, III and IV) are proton pumps, that create a proton gradient by translocation the protons from the mitochondrial matrix to intermembrane space. This gradient generates the phosphorylation of ADP to ATP (oxidative phosphorylation) (6, 7, 8).

1. Electron transport/Oxidative Phosphorylation

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