What is the role of oxygen in the electron transport chain?

Oxygen plays a key role as the final electron acceptor in the electron transport chain (ETC). Toward the end of the electron transport chain, molecular oxygen (O2) combines with hydrogen ions (H2) and electrons to form water. This process is crucial in synthesizing ATP (adenosine triphosphate) through oxidative phosphorylation. ATP is the energy currency of the cell. It is essential for all cellular activities including protein synthesis, muscle contraction, and active transport among others. 

Without oxygen, the electron transport chain is unable to generate ATP efficiently, resulting in insufficient ATP production and a subsequent decrease in cellular energy.