day, store it as organic acid (malic), and release CO 2 by
decarboxylation of the acids for fixing into sugar. This
reduces transpirational water loss during photosynthe-
sis. The CALVIN CYCLEoccurs during the day.
Cambrian explosion A period about 530 million
years ago (Cambrian age) when a large explosion of
species, both in number and diversity, appeared on
Earth. It lasted about 10 million years, and it is the first
recorded evidence through the fossil record of larger
and more complex life forms appearing.
Canadian shield A geographic area of Canada
centered around Hudson Bay and composed of 2- to 3-
billion-year-old igneous and metamorphic shield rock.
It covers much of northern Canada.
cancer Diseases in which abnormal cells divide and
grow unchecked and can spread from the original site
to other parts of the body; often fatal.
capillary The smallest blood vessels in the circulato-
rysystem. Capillaries have thin walls that facilitate the
transfer of oxygen and glucose into a cell and the
removal of waste products such as carbon dioxide back
out into the blood stream, to be carried away and
taken out of the body via the lungs. They act as the
bridge between the arteries, which carry blood away
from the heart, and the veins, which carry blood back
to the heart.
See alsoBLOOD.
capsid The outer protein coat or shell of a virus sur-
rounding its genetic material. Also capsid bugs (capsi-
dae), which number over 6,000 species and live on
plants, sucking juice and damaging cultivated plants.
carbohydrate A large class of compounds that con-
tain carbon, hydrogen, and oxygen in a general formu-
la of Cn(H 2 O)n. Classified from simple to complex,
they form mono-, di-, tri-, poly-, and heterosaccha-
rides. Examples include sugars (monosaccharide, di-
and polysaccharides), starches, and cellulose. Carbohy-
drates are used as an energy source by organisms, and
most are formed by green plants and are obtained by
animals via food intake.
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 fossil fuels and deforestation.
Increased amounts of CO 2 in the atmosphere enhance
the greenhouse effect, blocking heat from escaping into
space and contributing to the warming of Earth’s lower
atmosphere and affecting 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.
carbonic anhydrase A zinc-containing ENZYME
(carbonate hydrolyase, carbonate dehydratase) that
catalyzes the reversible decomposition of carbonic acid
to carbon dioxide and water.
Carboniferous period A geological time period
(360 to 280 millions of years ago) during the middle-
to-late Paleozoic era. It is divided into the Pennsylvani-
an period (325 to 280 millions of years ago) and the
Mississippian period (360 to 325 millions of years
ago).
See alsoGEOLOGIC TIME.
carbon monoxide(CO) A colorless, odorless gas
that is toxic.
carbon monoxide dehydrogenases ENZYMEs that
catalyze the oxidation of carbon monoxide to carbon
dioxide. They contain IRON-SULFUR CLUSTERs and
52 Cambrian explosion