The term redox is a combination of the words reduction and oxidation. It refers to all reactions that involve the transport of electrons from an electron donor to an electron acceptor. A molecule that gets reduced gains electrons, and loses electrons when it gets oxidized (see Figure 1). For a redox reaction to occur, a molecule has to be reduced by another molecule that gets oxidized.

A reaction occurs. The molecules are represented as spheres. There are two reactants: the first one is negatively charged,  meaning that it carries an electron; and the second one is neutral. The charged reactant, labeled as electron donor, undergoes an oxidation reaction and it loses an electron. At the same time, the neutral reactant, the electron acceptor, undergoes a reduction reaction and gains an electron. So, as a result, the products are two spheres, one neutral and one charged negatively.

Figure 1: Redox reactions

OIL RIG is a common mnemonic to remember which is the oxidation and which is the reduction: Oxidation Is Loss (of electrons) and Reduction Is Gain (of electrons).

In biological systems, redox reactions are critical for cellular respiration. In the cell, electrons are often transferred via electron carriers, molecules that accept electrons from one molecule and transfer them to another.