Importance of oxygen in air
    Oxygen is required for various process :
    (i) Combustion : The process of burning of fuel in air with release of energy in the form of heat and light.
    (ii) Respiration : a form of combustion in which oxygen reacts with food to release energy.
    (iii) Rusting : the corrosion of iron when exposed to air (oxygen) and moisture (water).

Importance of Carbon dioxide in Air
Carbon dioxide plays a vital role in living beings through Photosynthesis.
Photosynthesis : The process by which plants make their own food (glucose) using carbon dioxide and water in the presence of sunlight and chlorophyll, and release oxygen.

Note : Aquatic plants use carbon dioxide dissolved in water for photosynthesis.

Importance of water vapour in air

    The amount of water vapor present in air is called humidity. In absence of water vapor
    •    plant leaves become dry and crumble
    •    animals can get dehydrated
    Excess water vapor can cause discomfort, heat becomes severe even though the temperatures are not very high.