electron carrier

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various molecules, such as cytochromes and quinonoid compounds, that accept one or two electrons from one molecule and donate them to another in the process of electron transport, resulting in the release of energy that is used to produce ATP.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biochemistry Any of several classes of molecule that acts as both an acceptor and donator of electrons and functions in an energy-producing system such as photosynthesis