Carbon
Carbon
The seventeenth most abundant element on earth, carbon occurs in at least six different allotropic forms, the best known of which are diamond and graphite. It is a major component of all biochemical compounds that occur in living organisms: carbohydrates, proteins, lipids, and nucleic acids. Carbon-rich rocks and minerals such as limestone, gypsum, and marble often are created by accumulated bodies of aquatic organisms. Plants, animals, and microorganisms cycle carbon through the environment , converting it from simple compounds like carbon dioxide and methane to more complex compounds like sugars and starches, and then, by the action of decomposers , back again to simpler compounds. One of the most important fossil fuels , coal , is composed chiefly of carbon.