Answer:
A. A person descending a flight of stairs one step at a time.
Explanation:
Electron transport chain consists of electron carrier proteins through which movement of electrons from electron donor to the electron acceptor occurs. The energy released by downhill electron transport is conserved in form of electrochemical potential by uphill transport of protons across the impermeable membrane. The downhill flow of protons serve as energy source for ATP synthesis. Hence, the electron transport chain harness the energy of reduced NADH and FADH2 in step by step manner.
