Electron transfer reactions are important in many areas. In biological systems, processes like photosynthesis, nitrogen fixation, and aerobic respiration (the process your body uses to make energy using oxygen) all rely heavily on electron transfer reactions. Electron transfer reactions are also frequently used to obtain pure metals from ore. Electrochemical cells (see “Analytical Chemistry”) also rely on electron transfer reactions; the batteries that power your cellular phone and other devices use electron transfer reactions to do so.