Here's why:

* Electron Transport Chain: This process occurs within the mitochondria of eukaryotic cells. It involves a series of protein complexes that pass electrons from one to another, releasing energy along the way. This energy is used to pump protons across the mitochondrial membrane, creating a proton gradient.

* Proton Gradient: The proton gradient drives the production of ATP, the cell's energy currency, through a process called oxidative phosphorylation.

* Oxygen's Role: Oxygen acts as the final electron acceptor, combining with electrons and protons to form water (H2O). This step is crucial because it removes electrons from the transport chain, allowing the process to continue.

Without oxygen, the electron transport chain would come to a halt, and ATP production would cease, leading to cell death.