Examples include sugars (monosaccharide, di- and
polysaccharides), starches, and cellulose. Carbohydrates
are used as an energy source by organisms, and most
are formed by green plants and are obtained by animals
via food intake.
carbonates Chemical compounds derived from car-
bonic acid or carbon dioxide.
carbon cycle All parts (reservoirs) and fluxes of car-
bon. The cycle is usually thought of as four main reser-
voirs of carbon interconnected by pathways of
exchange. The reservoirs are the atmosphere, terrestrial
biosphere (usually includes freshwater systems), oceans,
and sediments (includes fossil fuels). The annual move-
ments of carbon, the carbon exchanges between reser-
voirs, occur because of various chemical, physical,
geological, and biological processes. The ocean contains
the largest pool of carbon near the surface of the Earth,
but most of that pool is not involved with rapid
exchange with the atmosphere.
carbon dioxide (CO 2 ) A colorless, odorless gas that
makes up the fourth most-abundant gas in the atmo-
sphere, used by plants in carbon fixation. Atmospheric
CO 2 has increased about 25 percent since the early
1800s due to burning of fossil fuels and deforestation.
Increased amounts of CO 2 in the atmosphere enhance
the greenhouse effect by blocking heat from escaping
into space, thus contributing to the warming of Earth’s
lower atmosphere and having an effect on the world’s
biota. This is a major issue currently being debated by
scientists around the world.
See alsoGREENHOUSE EFFECT.
carbon fixation The process by which carbon atoms
from CO 2 gas are incorporated into sugars. Carbon
fixation occurs in the chloroplasts of green plants or
any photosynthetic or chemoautotrophic organism.
See alsoPHOTOSYNTHESIS.
carbon-14 dating Estimating the age of once-living
material by measuring the amount of radioactive iso-
tope of carbon present in the material tested.
carbonic anhydrase A zinc-containing ENZYME(car-
bonate hydrolyase, carbonate dehydratase) that cat-
alyzes the reversible decomposition of carbonic acid to
carbon dioxide and water.carbonium ion The term should be used with great
care, since several incompatible meanings are currently
in use. It is not acceptable as the root for systematic
nomenclature for CARBOCATIONs.- In most of the existing literature the term is used in
its traditional sense for what is here defined as CAR-
BENIUM ION. - A carbocation, real or hypothetical, that contains at
least one five-coordinate carbon atom. - A carbocation, real or hypothetical, whose structure
cannot adequately be described by two-electron
two-center BONDs only. (The structure may involve
carbon atoms with a COORDINATION NUMBER
greater than five.)
40 carbonates
Molecular models of assorted greenhouse gases, including
carbon dioxide (right, with double bonds), methane (top, center),
water (top, left), and several unidentified chlorofluorocarbons
(CFCs). The buildup of these gases in the Earth’s atmosphere traps
an increased amount of solar radiation and leads to a gradual
warming of the whole planet. The gases are generated by indus-
try, the burning of fossil fuels, modern agricultural methods, and
deforestation. Global warming is expected to cause massive
changes in weather patterns and a rise in sea level due to polar
ice melting that could flood coastal regions.(Courtesy of Adam
Hart-Davis/Science Photo Library)