A pathway for cellular respiration occurring in many plants as well as some fungi and protists that permits electron transport and reduction of oxygen to water with greatly reduced ATP formation. It involves activity of the alternative oxidase (AOX) enzyme in the electron transport chain (ETC) in mitochondria. This effectively uncouples electron flow through the ETC from ATP production and allows the cell to regulate the balance between energy generation and utilization. The alternative pathway also has important roles in stress tolerance and in protecting the photosynthetic apparatus. It is not affected by cyanide and other substances that completely inhibit respiration in animals. Alternative oxidase catalyses the oxidation of reduced ubiquinone and transfers electrons directly to oxygen; in so doing it bypasses major sites for proton pumping into the mitochondrial intermembrane space and hence ATP formation by phosphorylation. Thus, instead of being conserved as ATP, energy flowing through the alternative pathway (in the form of electrons) is converted mainly to heat. This provides a mechanism for ‘burning off’ energy—produced by photosynthesis in the form of carbohydrate—that is temporarily more than the plant can cope with. Some plants exploit this heat-generating system. For example, before pollination the spadix of the skunk cabbage (Sympocarpus foetidus) undergoes a temperature rise of about 10°C, which causes the emission of volatile chemicals that attract insect pollinators. The expression of genes encoding alternative oxidase and its enzymatic activity are regulated by numerous factors, including stresses such as drought, cold, salinity, and infection by pathogens. See also uncoupling protein.