Here's why:

* Cellular Respiration: Humans, like most organisms, obtain energy through cellular respiration. This process breaks down glucose to generate ATP, the cell's energy currency.

* Electron Transport Chain: A key part of cellular respiration is the electron transport chain. This chain of proteins embedded in the mitochondrial membrane passes electrons from one molecule to the next, releasing energy along the way.

* Oxygen as the Final Acceptor: The final step of the electron transport chain involves oxygen accepting the electrons. This reaction forms water (H2O) as a byproduct.

Without oxygen as the final electron acceptor, the electron transport chain would halt, and the cell would be unable to generate the majority of its ATP. This is why we need to breathe oxygen!