Answer:
Oxygen
Explanation:
The electron transport chain is the final step of the cellular respiration. This is an aerobic process (requires oxygen) that from glucose it generates energy in the form of ATP molecules.
The first step is glycolysis, in which, glucose is split into two molecules of pyruvate, generating ATP and NADH (electron carrier). Pyruvate is oxidized into acetyl-CoA, and this molecule entries in the Krebs Cycle (or Citric Acid Cycle). This is a series of oxidation reactions that produce CO2, electron carriers (FADH2, NADH), and ATP. Finally, the electron transport chain is the last step, which, through electron movements in the inner membrane of mitochondria, oxidize NADH and FADH2. Oxygen accepts the electrons and protons, and water is formed too.
The cellular respiration is not carried if there is not oxygen because after glycolysis Krebs cycle is initiated in the absence of O2.
