Which of the following molecules is the final electron acceptor in cellular respiration?

In aerobic cellular respiration, glucose is broken down and the electrons removed from it are passed along the electron transport chain in the mitochondria. Each carrier passes the electrons on, releasing energy that is used to make ATP. At the very end of the chain something must collect those electrons, and that final electron acceptor is oxygen.

Oxygen picks up the electrons and joins with hydrogen ions to form water. This is exactly why we breathe in oxygen: without it the chain would jam, electrons would pile up and ATP production would stop.

Water is not the acceptor — it is the product formed after oxygen accepts the electrons. Glucose is the fuel at the start that gives up the electrons, so it cannot also accept them. Carbon dioxide is a waste product released during glycolysis and the Krebs cycle. So oxygen is the correct answer.