CYO Summative Exam Practice 2026 - Free CYO Exam Practice Questions and Study Guide

Oxygen is the gas involved in releasing energy during cellular respiration. In aerobic respiration, electrons are passed along the electron transport chain in the mitochondria, and oxygen acts as the final electron acceptor. It grabs these electrons (and combines with protons) to form water, a step that helps maintain the flow of electrons and sustains the proton gradient used by ATP synthase to generate most of the cell’s ATP. Without oxygen, this chain backs up, NADH and FADH2 can’t be reoxidized, and cells rely on far less efficient anaerobic pathways.

The other gases listed don’t play this role in typical cellular respiration: nitrogen and argon aren’t used as electron acceptors in energy production, and hydrogen isn’t the terminal electron acceptor in aerobic respiration (though some microbes use hydrogen in other pathways).