We know the chemical composition of air and this composition exists only on earth which makes life possible on this planet. As we know, oxygen plays a vital role in many processes involved with biotic components.
Fact:  In planets such as Venus and Mars, carbon dioxide (95% - 97%) is the primary component that makes life impossible on those planets. 
As a source of energy, the air is renewable in nature and cannot be exhausted by any means. It also enables life in plants and animals by supplying the necessary gases vital for survival. Air, in general, is a medium of gases suspended on the earth's surface. It is colourless, odourless, tasteless but can be weighed.
Importance of air:
1. To sustain life and growth:
All living things use this air present in the atmosphere for respiration or photosynthesis. Oxygen present in the atmosphere is involved in the respiration process (we breathe in oxygen) and breathe out carbon dioxide.
Air also acts as an energy source for all living things. As we know, living things are made up of cells, and these cells take oxygen from the blood to produce energy for various processes in our human body.
Likewise, there is a need for oxygen in prokaryotic (e.g. bacteria, paramecium) and eukaryotic (e.g. plants, animals, fungi, protists). Oxygen is utilised to break down food molecules to get chemical energy for cellular activities by organisms which are known as cellular respiration.
Glycolysis refers breakdown of complex sugar molecules(glucose) into simple sugar molecules like (pyruvic acid).
Fact: If carbon dioxide becomes excessive, it leads to a condition known as acidosis.
2. Combustion process: For any substance to burn, we need oxygen which is present in the atmosphere. This process is essential in converting many natural resources to our convenient form; for example, natural fuels are converted to synthetic fuels like petrol, diesel, etc., and used in heat and electricity generation.
Fact: In the absence of air combustion process ceases.
3. Photosynthesis: We know green plants take in carbon dioxide present in the atmosphere and give out oxygen, and they produce their food with the help of sunlight.
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