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Synthesis of organic carbon molecules from inorganic CO2 requires energy and chemical reducing power, as the carbon atoms in organic molecules are in reduced form. For a review of oxidation-reduction (redox) reactions from a biological perspective, see this Khan Academy video. Briefly, atoms such as carbon or oxygen are said to be reduced if they form covalent bonds with an atom with lesser electronegativity, such as hydrogen. Conversely, carbon is oxidized when it forms a covalent bond with an