Metabolic electron transfer reactions are of crucial importance in biological cells. These oxidation-reduction reactions involve the loss of electrons by one chemical species (which is thereby oxidized), and the gain by another (which is reduced). This electron flow is "downhill", from higher to lower electrochemical potential; as such, it is analogous to the flow of electrons in an electric circuit driven by a battery. All of the reactions in a cell involving electron flow are oxidation-reduction reactions. This flow is responsible for all of the work done by living organisms, since it provides the cell with the chemical energy necessary to synthesize ATP.