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

Oxygen is the final electron acceptor in the electron transport chain, which is a crucial step in the process of cellular respiration. During this stage, electrons that have been passed down through a series of protein complexes are ultimately transferred to molecular oxygen. This transfer is essential because it helps maintain the flow of electrons through the chain, allowing for the continued production of ATP.

When oxygen accepts these electrons, it also combines with protons (hydrogen ions) to form water. This reaction is vital for preventing a backlog of electrons in the chain, which could halt the process and prevent ATP synthesis. The consumption of oxygen and the production of water are significant components of aerobic respiration, highlighting the importance of oxygen in metabolic processes.

In contrast, the other choices do not serve the role of the final electron acceptor in the electron transport chain. NAD+ and FADH2 are both involved earlier in cellular respiration as carriers of electrons, but they do not accept electrons at the end of the process. Carbon dioxide, while a byproduct of cellular respiration, is not involved in the electron transport chain and does not function as an electron acceptor. Hence, oxygen's role as the ultimate acceptor is foundational to the efficiency and functionality of aerobic respiration.