Cells tap energy from electrons transferred from organic fuels to oxygen.
These movements of hydrogens in the reaction above represent movements of high-energy electrons. At each step, electrons move from a chemical bond in a molecule where they have more energy to a bond where they have less energy. Cellular respiration involves a gradual series of steps, each coupling an exergonic with an endergonic reaction. When these oxygen atoms bind with the hydrogen atoms carrying the electrons, they form water molecules with relatively low-energy covalent bonds.