Redox reactions involve the transfer of electrons between chemical species. They are essential in biological systems, as many metabolic pathways involve the transfer of electrons from one compound to another. In redox reactions, the species that donates electrons is said to undergo oxidation, while the species that accepts electrons is said to undergo reduction. The process of oxidation involves the loss of electrons or an increase in oxidation state, while the process of reduction involves the gain of electrons or a decrease in oxidation state. Redox reactions can be balanced using the half-reaction method, which involves balancing the oxidation and reduction half-reactions separately before combining them. The tendency of a species to undergo reduction can be quantified using its reduction potential, which is measured relative to a standard hydrogen electrode.