Biological Occurrences
Group 6 is notable in that it contains some of the only elements in periods 5 and 6 with a known role in the biological chemistry of living organisms: molybdenum is common in enzymes of many organisms, and tungsten has been identified in an analogous role in enzymes from some archaea, such as Pyrococcus furiosus. In contrast, and unusually for a first-row d-block transition metal, chromium appears to have few biological roles, although it is thought to form part of the glucose metabolism enzyme in some mammals.
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—The Columbia Encyclopedia, Fifth Edition, the first sentence of the article on “life” (based on wording in the First Edition, 1935)
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—George Washington (1732–1799)