Electron transfer is the process where electrons move from one atom or molecule to another. This movement is crucial in many chemical reactions, especially in redox reactions, where one substance is oxidized (loses electrons) and another is reduced (gains electrons). In biological systems, electron transfer is essential for energy production. For example, in cellular respiration, electrons are transferred through a series of proteins in the electron transport chain, ultimately leading to the production of ATP, the energy currency of cells. This process highlights the importance of electron transfer in both chemistry and biology.