In the composition of Earth atmosphere, which gas is essential for aerobic respiration in humans and most other animals?

Introduction / Context:
Respiration is the process by which living cells release energy from food. In aerobic respiration, oxygen acts as the final electron acceptor in the energy releasing reactions. This question asks which gas in the atmosphere is essential for respiration in humans and most other animals.

Given Data / Assumptions:

• Earth atmosphere is made mainly of nitrogen and oxygen, with smaller amounts of other gases.
• Humans and most animals rely on aerobic respiration.
• The options include carbon dioxide, oxygen, nitrogen, and argon.

Concept / Approach:
In aerobic respiration, cells use oxygen to break down glucose and other fuel molecules, releasing energy in the form of ATP. Oxygen is inhaled into the lungs, diffuses into the bloodstream, and is transported to cells. Carbon dioxide, on the other hand, is a waste product of this process and is exhaled. Nitrogen is abundant in the atmosphere but is relatively inert and is not directly used in respiration by most organisms. Argon is a noble gas with no biological role in respiration. Therefore, oxygen is the gas essential for respiration.

Step-by-Step Solution:

Verification / Alternative check:
Biology and physiology textbooks describe how red blood cells carry oxygen bound to hemoglobin from the lungs to tissues and carry carbon dioxide back to the lungs for exhalation. Any disruption in oxygen supply leads quickly to fatigue, organ damage, or death, showing its essential role. These sources consistently identify oxygen as the key gas for respiration, confirming the choice.

Why Other Options Are Wrong:

• Carbon dioxide is wrong as the essential gas for respiration because it is mainly a waste product, though it is important for plants in photosynthesis.
• Nitrogen is wrong because, although abundant in air, it is not used directly by human cells in respiration.
• Argon is wrong because it is chemically inert and plays no role in biological energy production.

Common Pitfalls:
Some learners may confuse the roles of oxygen and carbon dioxide because both are exchanged in the lungs. Remember the direction of movement: oxygen moves into the body for use in respiration, while carbon dioxide moves out as a waste product. The simplest way to avoid confusion is to memorise that oxygen is needed to burn food in cells, and carbon dioxide is the exhaust gas that must be removed.

Final Answer:
Oxygen, required for aerobic respiration in living cells