The electron transport chain requires a varying supply of electrons in order to properly function and generate ATP. However, the electrons that have entered the electron transport chain would eventually pile up like cars traveling down a blocked one-way street. Those electrons are finally accepted by oxygen (O2). As a result, they form two molecules of water (H2O). By accepting the electrons, oxygen allows the electron transport chain to continue functioning. The chain is organized in the cristae lumen membrane, i.e. the membrane inside the junction.