The biosphere operates on a carbon cycle, consisting mostly of two processes: photosynthesis, in which CO2 is reduced to organic molecules by using light energy, and cellular respiration, in which organic molecules (especially glucose) are oxidized to CO2. The reactions of respiration in eucaryotic cells occur in mitochondria. Each mitochondrion has a relatively permeable outer membrane and an inner membrane, containing many enzymes of the Krebs cycle, the carriers of the electron transport system, and the ATP synthases that convert ADP to ATP. Some bacteria carry out different forms of respiration in which they reduce nitrate to nitrite or sulfate to sulfide instead of reducing oxygen to water. The central metabolic pathways are called amphibolic because they serve both a catabolic function and an anabolic function.

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