Carbon in the environment form the carbon cycle. For example, photosynthetic plants draw carbon dioxide from the atmosphere (or seawater) and build it into biomass, as in the Calvin cycle, a process of carbon fixation. Some of this biomass is eaten by animals, while some carbon is exhaled by animals as carbon dioxide. The carbon cycle is considerably more complicated than this short loop; for example, some carbon dioxide is dissolved in the oceans; if bacteria do not consume it, dead plant or animal matter may become petroleum or coal, which releases carbon when burned. Carbon can form very long chains
Carbon dioxide standard hydrogen electrode. The nickel-containing enzyme carbon monoxide dehydrogenase catalyses this process. Carbon dioxide is colorless. At low concentrations the gas is odorless; however, at sufficiently-high concentrations, it has a sharp, acidic odor. At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m, about 1.67 times that of air. Carbon dioxide has no liquid state at pressures below . At 1 atmosphere (near mean sea level pressure), the gas deposits directly to a solid at temperatures below and the solid sublimes directly to a gas above −78.5 °C. In its solid state, carbon dioxide is commonly