A pathway for cellular respiration occurring in many plants that permits electron transport and reduction of oxygen to water in the presence of cyanide and other substances that completely inhibit respiration in animals. It thus acts as an alternative to the electron transport pathways common to both plants and animals. Like the components of the normal electron transport chain, those of the alternative pathway reside in the mitochondrial inner membrane. The principal enzyme is the so-called alternative oxidase, which transfers electrons directly to oxygen and in so doing bypasses at least two sites for 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. One hypothesis is that 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. Another interesting observation is that the alternative pathway may act as a short-term heat-generating system in certain tissues. 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. See also uncoupling protein.
Subjects: Biological Sciences.