Which of the following statements about the electron transport chain is true? a. the proton gradient established during electron transport is a form of potential energy. b. protons are pumped out of the mitochondria by the complexes of the electron transport chain. c. the electron transport chain can be found in the mitochondria of aerobic bacteria and other cells. d. atp synthesis associated with the electron transport chain is an example of substrate level phosphorylation. e. the movement of protons down a concentration gradient is an endergonic process.