Anaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen.
In aerobic organisms undergoing respiration, electrons are shuttled to an electron transport chain, and the final electron acceptor is oxygen. Molecular oxygen is a highly oxidizing agent and, therefore, is an excellent acceptor. In anaerobes, other less-oxidizing substances such as sulfate (SO42−), nitrate (NO3−), sulphur (S), or fumarate are used. These terminal electron acceptors have smaller reduction potentials than O2, meaning that less energy is released per oxidized molecule. Anaerobic respiration is, therefore, in general energetically less efficient than aerobic respiration.