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TEACHER BACKGROUND:
CARBON DIOXIDE AND THE CARBON CYCLE
All life is based on the element carbon. Carbon is the major chemical
constituent of most organic matter, from fossil fuels to the complex molecules ( DNA and RNA ) that control genetic reproduction in organisms. B By weight however, carbon is not one of the most abundant elements within the Earth's crust. In fact, the lithosphere is only 0.032% carbon by weight. In comparison, oxygen and silicon respectively make up 45.2% and 29.4% of the Earth's surface rocks.
Carbon is stored on our planet in the following major sinks (1) as organic molecules in living and dead organisms found in the biosphere; (2) as the gas carbon dioxide in the atmosphere; (3) as organic matter in soils; (4) in the lithosphere as fossil fuels and sedimentary rock deposits such as limestone, dolomite and chalk; and (5) in the oceans as dissolved atmospheric carbon dioxide and as calcium carbonate shells in marine organisms.
The Carbon Cycle.
Estimated major stores of carbon on the Earth
Ecosystems gain most of their carbon dioxide from
the atmosphere. Producers (plants) have specialized
systems that allow for absorption of this gas into their cells. With the addition of water and energy from solar radiation, these organisms use
photosynthesis to chemically convert the carbon
dioxide to carbon-based sugar molecules. These molecules can then be chemically modified by these organisms through the metabolic addition of other elements to produce more complex compounds like proteins, cellulose, and amino acids. Some of the organic matter
produced in plants is passed down to consumers (animals) through
consumption.
Carbon dioxide enters the waters of the ocean by
simple diffusion. Once dissolved in seawater, the
carbon dioxide can remain as is or can be
converted into carbonate (CO 3 -2) or bicarbonate
(HCO 3 - ). Certain forms of sea life biologically fix
bicarbonate with calcium (Ca+2) to produce calcium
carbonate (CaCO 3 ). This substance is used to
produce shells and other body parts by organisms such as coral, clams, oysters, some protozoa, and some algae. When these organisms die, their shells and body parts sink to the ocean floor where they accumulate as carbonate-rich deposits. After long periods of time, these deposits are physically and chemically altered into sedimentary rocks. Ocean deposits are by far the biggest sinks of carbon on the planet.
Sink Billions of Metric Tons
Atmosphere
578 (as of 1700) - 766 (as of
Soil Organic Matter 1500 to 1600 Ocean 38,000 to 40, Marine Sediments and Sedimentary Rocks
66,000,000 to 100,000,
Terrestrial Plants 540 to 610 Fossil Fuel Deposits 4000
