The carbon cycle represents the circulation and recycling of the chemical element carbon in nature because of the action of living beings.

Photosynthetic beings absorb carbon as carbon dioxide available in the atmosphere and the carbon atoms become part of glucose molecules. During the cellular respiration of these beings, part of this organic material is consumed to generate ATP and in this process, carbon dioxide is returned to the atmosphere. Other part is incorporated by the photosynthetic organisms into the molecules that compose their structure. The carbon atoms incorporated into the producers are transferred to the next tropic level and again part is liberated by the cellular respiration of the consumers, part becomes constituent of the consumer body and part is excreted as uric acid or urea (excretes later recycled by decomposer bacteria). Therefore, carbon absorbed by the producers in photosynthesis returns to the atmosphere through cellular respiration along the food chain until the decomposers that also liberate carbon dioxide in their energetic metabolism. Under special conditions in a process, that takes millions of years carbon incorporated into organisms may also constitute fossil fuels stored in deposits under the surface of the planet as fossil fuels burn the carbon atoms return to the atmosphere as carbon dioxide or carbon monoxide. The burning of vegetable fuels, like wood, also returns carbon to the atmosphere.