Answer: Oxygen (O2)
Explanation: Oxygen ions combines with hydrogen ions to form water molecules.
Cellular aerobic respiration occurs in the mitochondria. Aerobic respiration requires oxygen and glucose and releases carbon dioxide, water, and energy as by products.
The chemical equation is; C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (ATP).
(glucose + oxygen -> carbon dioxide + water + energy).