These reduced electron carriers transfer their electrons to oxygen to make water and ATP in a process called oxidative phosphorylation.
All spontaneous reactions are exergonic.
An endergonic reaction is one that proceeds only with the addition of free energy. In biochemistry, endergonic reactions are driven by energy from ATP or by coupling the reactions with exergonic ones. The equation above gives the free energy change for any process at constant pressure and temperature. If DG is less than 0, the process is spontaneous. If DG is greater than 0, the process is not spontaneous.