Answer: Oxygen
The cells use oxygen molecules during oxidative phosphorylation. Oxidative phosphorylation is the final stage of cellular respiration which is made up of two closely connected components: 1) the electron transport chain and 2)chemiosmosis.
Oxygen sits at the end of this electron transport chain, where it accepts electrons and picks up protons to form water. If oxygen is not available to accept electrons, the electron transport chain will cease running, and ATP Adenosine triphosphate) will no longer be produced by chemiosmosis. Without enough ATP, cells can’t carry out the reactions they need to function, causing death.