Mitochondrial Diseases

The origins of mitochondria are unknown, but the likely explanation, called the endosymbiont hypothesis, holds that they arose as free-living bacteria that colonized proto-eukaryotic cells, thereby establishing a symbiotic relationship. Primitive eukaryotic cells with intracellular mitochondria capable of metabolizing oxygen would have had an advantage in an oxygen-rich environment. The electron transport chain produces far more energy for each molecule of glucose consumed than is produced by anaerobic respiration. The oxidative phosphorylation process conducted by the mitochondria produces thirty-eight molecules of ATP, compared to two molecules of ATP produced by anaerobic glycolysis. Oxidative phosphorylation allows the conversion of toxic oxygen to water, a protective biological advantage.

A disadvantage of oxidative phosphorylation, however, is the formation of reactive oxygen species, such as singlet oxygen and hydroxyl radicals, which damage such cellular components as lipids, proteins, and DNA. A normally functioning electron transport chain produces reactive oxygen species from about 2 percent of the electrons that it transports. In disease states and in aging, larger quantities of reactive oxygen species are generated, and this may be a significant factor in cellular deterioration as well as a major contributor to the aging process.