The carbon cycle describes the exchange of carbon among Earth’s biosphere (life), atmosphere (air), hydrosphere (water), pedosphere (soil) and lithosphere (rocks, crust, and mantle). It is one of several biogeochemical cycles on Earth that play a key role in making life possible and in regulating many planetary systems.
Exchanges between these spheres take many forms. Atmospheric carbon dioxide can readily dissolve into surface waters, and both atmospheric and carbon dioxide dissolved in the ocean are easily and frequently taken up living organisms. Transfer of carbon into the lithosphere takes much longer. Carbon in the lithosphere is also less mobile, often remaining stored there for millions of years, but large amounts can be released in an instant during a volcanic eruption. Human use of fossil fuels and other activities is also releasing an increasing amount stored in hydrocarbons back to the atmosphere as carbon dioxide.
Some organisms—such as photosynthetic plants and microbes and chemosynthetic bacteria—are able to take inorganic carbon, primarily in the form of carbon dioxide, and combine it with water to form simple carbohydrates (sugars). These carbohydrates formed by photosynthesis or chemosynthesis serve as the basic building blocks of all organic (carbon-containing) molecules that are necessary for life. Carbon dioxide dissolved in water is likewise readily incorporated into the marine food chain and into the carbonate minerals that make up the shells or skeletons of many marine organisms.