What occurs when substances move against the concentration gradient requires and the aid of carrier proteins?

Carrier molecules are molecules that play a role in transporting electrons through the electron transport chain. Carrier molecules are usually proteins bound to a nonprotein group; they can undergo oxidation and reduction relatively easily, thus allowing electrons to flow through the system. There are four types of carrier: flavoproteins (e.g. FAD), cytochromes, iron-sulphur proteins (e.g. ferredoxin), and ubiquinone.