Cellular Respiration Practice Test

In an experiment, isolated mitochondria are supplied with NADH and ADP + Pi. When oxygen is removed from the chamber, electrons can no longer be transferred to the final electron acceptor in the electron transport chain. As a result, proton pumping across the inner mitochondrial membrane decreases, reducing the proton-motive force that drives ATP synthase. The citric acid cycle enzymes remain present, but their ability to regenerate NAD+ depends on electron flow through the chain. No inhibitors of ATP synthase are added. Which outcome is most likely when oxygen is absent?

A lab measures oxygen consumption and ATP synthesis in mitochondria supplied with ADP + Pi and a steady source of NADH. When ADP becomes depleted, oxygen consumption drops sharply even though oxygen is still present. Electron transport proteins remain intact, and no inhibitors are added. The inner membrane remains impermeable to protons except through ATP synthase. Because ATP synthase lacks its substrate, protons accumulate outside the matrix, strengthening the gradient and opposing further proton pumping by the electron transport chain. Which explanation best accounts for the decreased oxygen consumption when ADP is depleted?