Because electron transfer is so important in bioenergetics and other processes, electron transfer proteins are required for life. They have redox-active prosthetic groups, or "redox sites," where oxidation and reduction take place Single domain one-electron carriers are the most basic electron transfer proteins. During electron transfer, an electron is received by an iron atom in the pigment section of a cytochrome molecule, causing it to be reduced; the electron is then transferred to the iron atom in the next cytochrome carrier in the electron transfer chain, oxidizing the first. In the mitochondria, the electron transport chain is found. In the inner membrane of the mitochondria, there are five primary protein complexes that make up the electron transport chain.