All living things today depend on the source of carbon cycle energy for their survival, especially heterotrophs. Heterotrophs cannot make their own nutrients, but must rely on external carbon sources to survive.

External carbon sources, there are many kinds. Or organic things, such as sugars, lipids, proteins and the like. Heterotrophs ingest such things, through complex biochemical processes, decompose and transform, and release energy for personal use. For example, animals eat plants, fruits, fungi rot, branches and leaves, all of which are the way to obtain carbon sources.

In the ecological system, heterotrophs are closely related to other things. Green plants photosynthesize, sequester carbon into organic, and supply heterotrophs. Heterotrophs eat it, carbon enters its body, and through respiration, carbon dioxide is returned to the atmosphere, and then enters the carbon cycle. In this way, the ecological balance is maintained and the life of all things is orderly.