In this chemical reaction large food molecules are broken down, giving off energy. So we must have oxygen in order to get energy from food. Without oxygen, we would get little or no energy, and soon life would stop. Many organisms function in this way. Even the very tiniest of organisms, e.g., protozoa and bacteria, depend on respiration and oxygen to release the energy contained in large food molecules.
Plants get their energy from the sun through a chemical process called photosynthesis.
In this process, green leaves capture the energy from the sun and trap it in large molecules of sugar. The plants also have to break down these sugar molecules in order to release the energy needed for growth just like animals, fungi, many bacteria and protozoa. So plants use both the carbon dioxide for photosynthesis and oxygen for respiration. Because of this the individual gases in the air are very important to the life on earth. We can now see that air is very important for living things, and animals have a variety of means through which they can get the air they need.