All cells contain carbon, because they all contain proteins, fats and carbohydrates, which themselves contain carbon. For example, plant cell walls are made of cellulose, a carbohydrate.
Carbon dioxide also contains carbon. Carbon dioxide is one of the greenhouse gases. It is present in the atmosphere in small concentrations, about 0.038%. Carbon dioxide is recycled constantly through various processes that form the carbon cycle.
Processes that remove carbon dioxide from the air:
Processes that return carbon dioxide to the air:
Decomposers, such as microbes and fungi, meanwhile play an important role in the carbon cycle. They break down the remains of dead plants and animals and, in doing so, release carbon dioxide through respiration.
The diagram below should give you an idea how the carbon cycle works, though it's very simplified.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science