What gas, making up about 21% of the atmosphere, is used for respiration?

Oxygen is the gas that constitutes about 21% of the Earth's atmosphere and is essential for respiration in most living organisms. During the process of respiration, organisms inhale oxygen, which is then utilized by cells to produce energy through cellular respiration. This energy production involves a series of biochemical reactions that convert glucose and oxygen into carbon dioxide, water, and energy in the form of adenosine triphosphate (ATP).

Oxygen is not just vital for humans but also for animals and many forms of life, facilitating the energy needs necessary for growth, maintenance, and overall functioning. The role of oxygen in cellular processes highlights its critical importance in ecosystems, where it also contributes to various biochemical cycles, such as the carbon cycle.

In contrast, nitrogen, which makes up about 78% of the atmosphere, does not play a direct role in the respiration process for most organisms. Argon, making up around 0.93% of the atmosphere, is an inert gas with no biological function, and helium is a trace gas that has little relevance in biological processes. Thus, oxygen stands out as the correct answer, being key to the energy-producing processes of respiration.