Oxygen comprises approximately $21\%$ of the Earth’s atmosphere.

The atmosphere surrounding our planet is a complex mixture of various gases, with nitrogen and oxygen being the most prominent. Oxygen, denoted by the chemical formula $O_2$, makes up around $21\%$ of the total volume of gases in the atmosphere. This proportion remains relatively stable in the lower atmosphere, where humans and other living organisms exist and breathe.

Oxygen is an essential element for life on Earth. It is crucial for the process of respiration in most organisms, including humans. During respiration, oxygen is utilized to break down glucose, resulting in the release of energy that powers various bodily functions. Without oxygen, this vital process could not take place, and life as we know it would be unsustainable.

The oxygen present in our atmosphere is primarily produced through photosynthesis, a process performed by plants, algae, and certain bacteria. In photosynthesis, these organisms harness sunlight to convert carbon dioxide and water into glucose and oxygen. The oxygen generated is subsequently released into the atmosphere, contributing to the $21\%$ that we rely on for survival.

It’s important to understand that while $21\%$ may not seem substantial, it is a carefully regulated concentration. If the oxygen level were significantly higher, fires could easily become uncontrollable, as oxygen is a key supporter of combustion. Conversely, if the concentration were much lower, most organisms would struggle to survive due to insufficient oxygen for respiration.

In conclusion, oxygen, which constitutes about $21\%$ of the Earth’s atmosphere, is vital for sustaining life on our planet. It plays an integral role in essential biological processes such as respiration and photosynthesis, and its atmospheric concentration is maintained at a level that supports life while mitigating the risk of excessive combustion.