Plants and other creatures inhale oxygen, from the atmosphere, and exhale carbondioxide.
Plants, in the presence of sun light receive Carbondioxide, through **Photosynthesis**, and their chlorophyll separates the carbondioxide into carbon and oxygen.
Plants use carbon in the preparation of their food, and discharge oxygen for the other creatures.
Such a cycle always operates in this nature, which helps to maintain definite amount of carbondioxide in the air, called **Carboncycle.**