The atmosphere over one hectare of land contains fifty tons of carbon dioxide (CO2
), and living organisms return fifty tons to the atmosphere each year. Carbon dioxide can be transformed into organic compounds by green plants, algae, cyanobacteria, purple and green phototrophic bacteria, and chemo-autotrophic bacteria.
These organisms use carbohydrates, which they produce by fixation of CO2
, to build complex organic compounds such as cellulose. Bacteria and fungi use two enzymes to degrade cellulose, which is present in dead plants, into glucose. This glucose can then be used by many types of microorganisms. Complete oxidation of glucose produces H2
O and CO2
. Carbon dioxide is not produced solely by carbohydrate decomposition. It is also generated by decomposition of amino acids from proteolysis and decomposition of fatty acids from the decomposition of lipids.
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