Due to the sessile nature of plants, it is crucial for their survival and growth that they can handle a constantly changing, and thus stressful, ambient environment by modifying their structure and metabolism. The central metabolism of plants is characterized by many alternative options for metabolic pathways, which allow a wide range of adjustments of metabolic processes in response to environmental variations. Many of the metabolic pathways in plants involve the processing of redox compounds and the use of adenylates. They converge at the mitochondrial electron transport chain (ETC) where redox compounds from carbon degradation are used for powering ATP synthesis. The standard ETC contains three sites of energy conservation in complexes I, III, and IV, which are in common with most other eukaryotes. However, the complexity of the plant metabolic system is mirrored in the ETC. In addition to the standard enzymes, plants have a large set of supplementary electron transport enzymes. Many of these, such as the external and internal NAD(P)H dehydrogenases, proline dehydrogenase, and glycerol-3-phosphate dehydrogenase, feed into the ubiquinone pool and they therefore bypass the first site of energy conservation in the ETC. The alternative oxidase provides a non-energy-conserving alternative to electron transport through complexes III and IV. There also appears to be a special coupling between specific NAD(P)H dehydrogenases and specific members of the alternative oxidase family. These additional enzymes therefore give a great flexibility in the type and origin of the substrate, the electron transport route(s) used, and the energy yield. At the same time special reactions, such as ascorbate biosynthesis, can take place. In this way, the mitochondrial ETC can mediate major adjustments in cellular metabolism that is important for cellular function under a great variety of stress conditions such as low temperature and drought.