These are among the more complex, but also most important reactions in living systems. Oxidation reactions involve either the addition of oxygen or the removal of hydrogen, hydrogen ions, or electrons. Reduction reactions involve the removal of oxygen or the addition of hydrogens, hydrogen ions, or electrons. Often oxidation and reduction go hand in hand, as hydrogen is removed from one molecule and added to another. When both halves of the reaction are considered together, they may be referred to as redox reactions.
Oxidation reactions are exergonic and reduction reactions are endergonic.
These reactions are particularly important during the metabolic pathways of cellular respiration, where chains of oxidation-readuction reactions cause electrons to be passed from one carrier molecule to another along the electron transport system.