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Why is oxygen needed for aerobic cellular respiration?

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Fluorescent confocal image of HeLa cells with green microtubules outlining cell shapes, red mitochondria, and blue nuclei. — mitochondrion Mitochondria (red) are found throughout the cytoplasm of almost all eukaryotic cells (cell nucleus is shown in blue; cytoskeleton is shown in yellow).

In aerobic cellular respiration, oxygen is essential because it serves as the final electron acceptor in the electron transport chain, a series of reactions occurring within the mitochondria.123

During the earlier stages of respiration, such as the tricarboxylic acid cycle, energy from food molecules is captured in the form of high-energy electron carriers, specifically NADH and FADH2. These carriers donate their electrons to the respiratory chain, where the electrons are passed through a series of protein complexes.142

Oxygen’s role is to accept these electrons at the end of the chain, combining with them and protons to form water. This process is critical for two reasons:12
* Energy Production: The flow of electrons to oxygen releases energy that is used to pump protons across the mitochondrial membrane, creating an electrochemical gradient that drives the synthesis of adenosine triphosphate (ATP).256
* Efficiency: By acting as the final acceptor, oxygen allows for the complete oxidation of food molecules, yielding significantly more ATP than anaerobic processes like fermentation.537