A single tree absorbs tons of carbon dioxide in its 30-year life cycle and it releases a ton of oxygen. The free nitrogen from the atmosphere is captured by the nitrogen-fixing bacteria and it converts nitrogen into nitrates and nitrites which is then absorbed by the plants. Trees, like all organisms, grow by adding mass (biomass). Carbon is the central ingredient in making that new biomass. Tree biomass is comprised of all parts of the tree; leaves, stems, branches, roots, tree trunks. The biomass of the woody tissue in the tree pictured on the right is made mostly of cellulose, a carbon compound. In a process called carbon fixation, plants transform CO2, an inorganic carbon compound into organic carbon compounds.