1. A protein; such as, flavoprotein or cytochrome, which can gain and lose electrons in either direction and can transport electrons from one compound to another compound or to oxygen.
2. A molecule that accepts electrons from electron donors and donates them to electron acceptors, creating an energy-producing electron transport chain; such as, that which occurs in respiration and photosynthesis.
3. A molecule associated with membrane-bound proteins that accepts and transfers electrons.
4. Any of various molecules that are capable of accepting one or two electrons from one molecule and donating them to another in the process of electron transport.

As the electrons are transferred from one electron carrier to another carrier, their energy level decreases, and energy is released.