Which gas do living organisms primarily need for respiration?

Living organisms primarily require oxygen for respiration because it is essential for the process of cellular respiration, which occurs in the cells. During this process, cells convert glucose and oxygen into adenosine triphosphate (ATP), the energy currency of the cell, along with carbon dioxide and water as byproducts. Oxygen serves as the final electron acceptor in the electron transport chain, which is a crucial step in aerobic respiration, allowing for the efficient production of ATP.

While other gases such as nitrogen, hydrogen, and carbon dioxide play roles in the broader context of biological processes, none are used as directly in respiration as oxygen is. Nitrogen is abundant in the atmosphere but is not utilized in the cellular respiration process. Carbon dioxide is produced as a waste product of respiration rather than being consumed directly for energy production. Hydrogen, although part of glucose and other biological molecules, does not serve as a gas needed for respiration. Thus, oxygen is the specific gas that living organisms need for the crucial process of extracting energy from food.