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
glucose + oxygen → carbon dioxide + water
carbon dioxide + water → glucose + oxygen