Which molecule serves as the final electron acceptor in the electron transport chain?

In the electron transport chain (ETC), the final electron acceptor is oxygen (O2). This crucial role is essential for the production of ATP through oxidative phosphorylation. As electrons are passed along the chain, they release energy which is used to pump protons across the inner mitochondrial membrane, creating an electrochemical gradient.

At the end of the chain, the electrons are transferred to molecular oxygen, which combines with protons to form water (H2O). This reaction is vital for maintaining the flow of electrons through the chain, as it ensures that there is a continual acceptor available for the electrons. If oxygen were not available, the entire process would slow down dramatically, leading to a backup of electrons and a decrease in ATP production.

The other choices—NAD+, FAD, and CO2—do not serve this role. NAD+ and FAD are electron carriers that transport electrons to the ETC but do not accept them at the end. Carbon dioxide (CO2) is a waste product of cellular respiration and does not function in the electron transport chain. This makes oxygen the indispensable final electron acceptor, completing the process and supporting aerobic respiration effectively.