Practice Test 5

A researcher adds a chemical uncoupler to actively respiring mitochondria provided with glucose-derived pyruvate, oxygen, and abundant ADP + Pi. The uncoupler allows protons to cross the inner mitochondrial membrane without passing through ATP synthase. Electron carriers still deliver electrons to the electron transport chain, and oxygen remains available as the terminal electron acceptor. The citric acid cycle continues generating NADH and FADH2 while substrates are available. The uncoupler does not directly inhibit any electron transport proteins. Which outcome is most likely after adding the uncoupler?

A researcher inhibits ATP synthase in mitochondria while providing oxygen and a constant supply of NADH. Electron transport chain components can still accept and donate electrons, but protons pumped into the intermembrane space cannot return through ATP synthase. As protons accumulate, the electrochemical gradient becomes very steep, which increasingly opposes additional proton pumping by the electron transport chain. Over time, electron flow through the chain slows because it is coupled to proton pumping. Which outcome is most likely after ATP synthase inhibition?