Reduction

Reduction is a chemical process in which a substance gains electrons or in which the oxidation state of one or more of its atoms decreases. In organic chemistry, reduction is also commonly associated with gaining hydrogen or losing oxygen, although these changes reflect altered electron distribution rather than the fundamental definition of reduction .¹

Reduction occurs together with oxidation. When one chemical species gains electrons and is reduced, another species loses electrons and is oxidized. Reactions involving these paired changes are called oxidation-reduction, or redox, reactions. A substance that donates electrons and thereby causes another substance to be reduced is called a reducing agent .^1,2

In biological systems, reduction frequently involves transfer of electrons together with hydrogen. Electron carriers such as NAD+ and NADP+ become reduced by accepting reducing equivalents, forming NADH and NADPH, respectively. These reduced carriers can later donate electrons in other reactions. NADH is particularly important in energy-yielding metabolism, whereas NADPH commonly supplies reducing power for biosynthetic reactions .²

Reduction is therefore not synonymous with adding hydrogen, although hydrogen addition often accompanies reduction in biological and organic chemistry. The defining feature is a gain in electrons or a decrease in oxidation state. Reduction and oxidation are complementary processes within redox reactions .^1,2