The carbon cycle is very important to all ecosystems, and ultimately life on earth. The carbon cycle is critical to the food chain.
Living tissue contains carbon, because they contain proteins, fats and carbohydrates. The carbon in these (living or dead) tissues is recycled in various processes.
Let's see how this cycle works using the simple sketch below:
Human activities like heating homes and cars burning fuels (combustion) give off carbon into the atmosphere. During respiration, animals also introduce carbon into the atmosphere in the form of carbon dioxide.
The Carbon dioxide in the atmosphere is absorbed by green plants (producers) to make food in photosynthesis.
When animals feed on green plants, they pass on carbon compounds unto other animals in the upper levels of their food chains. Animals give off carbon dioxide into the atmosphere during respiration.
Carbon dioxide is also given off when plants and animals die. This occurs when decomposers (bacteria and fungi) break down dead plants and animals (decomposition) and release the carbon compounds stored in them.
Very often, energy trapped in the dead materials becomes fossil fuels which is used as combustion again at a later time.