Living organisms break down organic molecules in a series of reactions to generate energy. Many of these reactions are oxidation-reduction reactions or redox reactions. Oxidation is the removal of electrons from an atom, while reduction is the addition of electrons. Because the number of electrons in a reaction is conserved, oxidation and reduction half-reactions always occur in pairs. Inside cells, when a molecule gains an electron, it often accepts a proton from its surroundings. This addition of hydrogen is called hydrogenation, and the molecule is reduced. Conversely, when a molecule loses hydrogens, this is a dehydrogenation, and the molecule is oxidized. Protons and electrons can be transferred to electron-carrying molecules, including coenzymes. For example, in the dehydrogenation of succinate to fumarate, electrons and protons are transferred to the coenzyme FAD, reducing it to FADH2. The reduced FADH2 further transfers the electrons through the electron transport chain and is oxidized back to FAD.