ATP, a higher-energy molecule than ADP, is synthesized from ADP in the electron transport chain. Given that molecules cannot go from a lower energy to a higher energy on their own without an external input of energy, how does the energy for this reaction enter the system?

the relative standard reduction potential of the half-cell in which reduction occurs; more positive than the other half-cell. reduction is a chemical process in which electrons are added to an atom or an ion; it always occurs accompanied by oxidation of the reducing agent.