" what happens if we take a cyanide pill: it jams up the final proton pump of the respiratory chain in our mitochondria. If the respiratory pumps are impeded in this way, protons can continue to flow in through the ATP synthase for a few seconds before the proton concentration equilibrates across the membrane, and net flow ceases. It is almost as hard to define death as life, but the irrevocable collapse of membrane potential comes pretty close. So "

― Nick Lane , The Vital Question: Energy, Evolution, and the Origins of Complex Life