The term reduction refers to the addition of an electron (e-); conversely, the term oxidation refers to the removal of an electron. In the process of reduction, the addition of an electron results in stored energy in the reduced compound. In the process of oxidation, the removal of an electron liberates energy from the oxidized compound. Whenever one substance is reduced, another is oxidized.  


For clarification, consider the following example of two molecules, A and B.

When molecules A and B come into contact, here is what happens:


  • B grabs an electron from molecule A.
  • Molecule A has been oxidized because it has lost an electron.
  • The charge of B has been reduced because it has gained a negative electron (e-).


In biological systems, removal or addition of an electron constitutes the most frequent mechanism of oxidation-reduction reactions. These oxidation-reduction reactions are frequently called redox reactions.