All living organisms contain carbon. Most of the molecules in our body are based on carbon atoms.

Most of the food we eat like carbohydrate contain carbon.

Whenever carbon (or a substance containing carbon like coal, wood or wax) is burnt then carbon dioxide gas is formed.

Important reactions involving carbon and carbon dioxide:

Burning coal or carbon:
carbon +  oxygen → carbon dioxide

Respiration:
glucose + oxygen → carbon dioxide + water

Photosynthesis:
carbon dioxide + water → glucose + oxygen

The reactions above can all be summarized in a diagram called the carbon cycle