The electron leak in the substrate-side of respiratory chain causes the generation of O 2 -· and H 2O 2 . The electron leak in the oxygen-side at the cytochrome c of the respiratory chain causes the reduction of H 2O 2. Both the electron leaks of the respiratory chain mentioned above make up a metabolic path of O 2 -· →H 2O 2 → H 2O in mitochondria. The reactive pathways initiated by O 2 -· have been defined as the radical metabolism of mitochondria, because many different kinds of reactive oxygen species (ROS) can be generated in the process of the reactions of O 2 -· . The level of electron leak of respiratory chain changes with the variation of the physiological state of the animal, it is always higher when the animals are in pathological states. In this article a hypothesis of mitochondria having a metabolism of superoxide anion is suggested. The efficiency of mitochondria working on ATP production is defined as the ratio of transferred electrons to the total electrons, and an equation of E = F×P/O is used to calculate the efficiency. The factors causing decrease of mitochondrial efficiency are discussed on the topic of exercise induced fatigue. Based on the H + additive reactions involved in the reactions of radical metabolism, an explanation on H + leak of mitochondria is presented as H + spent in radical metabolism instead of H + leaking back.
