Carbon (C) is the third or fourth most abundant element in the universe. Though it is not especially abundant on the whole of the Earth, it is a major component element of life. C is a highly mobile element in ecosystems, with common forms in gaseous, liquid, as well as solid phases. C has a valence state of 4 and can form four strong bonds with four other atoms; this allows it to produce virtually limitless arrangements of chains, rings, and other highly complex 3-D shapes. Its importance in biological systems is indicated by the fact that the soft tissue of living organisms is generally 40-55% C by dry mass. This large amount and generally tightly bounded range reflects the constant, ubiquitous, structural role of C throughout living systems.

Most biochemicals fall within a narrow range of C contents. Carbohydrates are similar to many other types of molecules in being almost 50% C. Of all common biological molecules, fats have the highest carbon content, with about 75% of their mass contributed by C. Organic structural matter that may make up large amounts of organism biomass includes the cellulose and lignin of plants and the chitin of arthropod exoskeletons. Even most inorganic structural matter used in living things contains considerable carbon, for example, the calcium carbonate of mollusk shells and the apatite of vertebrate bones both have large amounts of C. An exception is the silicon-containing shells of diatoms; these are nearly carbon free.

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