To survive, every organism must have access to carbon atoms. Carbon makes up about 49 percent of the dry weight of organisms. The carbon cycle includes movement of carbon from the gaseous phase (carbon dioxide in the atmosphere) to solid phase (carbon-containing compounds in living organisms) and then back to the atmosphere via decomposers. The atmosphere is the largest reservoir of carbon, containing 32 percent CO₂. Biological processes on land shuttle carbon between atmospheric and terrestrial compartments, with photosynthesis removing CO₂ from the atmosphere and cell respiration returning CO₂ to the atmosphere.