The end reaction of the electron transport chain (ETC) occurs in the inner mitochondrial membrane and culminates in the reduction of molecular oxygen (O₂) to water (H₂O). In the ETC, electrons are transferred through a series of protein complexes and mobile electron carriers, ultimately reaching Complex IV (cytochrome c oxidase).
At this final step, electrons are transferred to oxygen, which is the terminal electron acceptor. This reaction also involves protons (H⁺) being added to oxygen, resulting in water formation. The overall reaction can be summarized as follows:
\[ \text{4 e}^- + \text{4 H}^+ + \text{O}_2 \rightarrow \text{2 H}_2\text{O} \]
This process is crucial for aerobic respiration as it contributes to the proton gradient used in ATP synthesis through oxidative phosphorylation.