Here's why:

* Cellular respiration is the process by which cells break down glucose (a sugar) to release energy in the form of ATP.

* Oxygen acts as the final electron acceptor in the electron transport chain, the last stage of cellular respiration. This process is essential for generating the majority of ATP.

Without oxygen, cellular respiration cannot proceed efficiently, and cells will have to rely on less efficient processes like anaerobic respiration (which produces much less ATP).