Metal ions play essential roles in biological processes. Ions such as K(+) and Na(+) are important in ion transport, and Mg(2+), Ca(2+), and Zn(2+) are important chelators in many processes, including phosphotransfer and harvesting of light for energy metabolism. The transition metals readily undergo one-electron chemistry, and in this capacity they function uniquely in biological processes such as long-range and inner sphere electron transfer. They also facilitate many one-electron chemical reactions involving free radical intermediates. Iron, being the dominant element in the earth and a transition metal, most frequently participates in biological one-electron chemistry.