Available carbohydrates are predomi-
nantly starches (complex carbohydrates)
and sugars (simple carbohydrates). In
carbohydrate metabolism, the monosac-
charides (simple sugars) glucose (grape
sugar), galactose (a milk sugar), and
fructose (fruit sugar) are absorbed into
the bloodstream unchanged. The disac-
charides (double sugars) sucrose, maltose
and lactose (a milk sugar) are broken
down into simple sugars before they are
absorbed. Starches also have to be bro-
ken down into simple sugars.
Some glucose is burned up immedi-
ately (see metabolism) in order to generate
energy for cells, such as brain cells, that
need a constant supply. Galactose and
fructose have to be converted to glu-
cose in the liver before they can be used
by body cells. Surplus glucose is con-
veyed to the liver, muscles, and fat cells
where it is converted into glycogenand
fat for storage. When blood glucose lev-
els are high, glucose storage is stimulated
by insulin, a hormone that is secreted by
the pancreas. When the blood glucose
level becomes low, insulin secretion
diminishes and glucagon, which is anoth-
er hormone produced by the pancreas,
stimulates the conversion of stored gly-
cogen to glucose for release into the
bloodstream. Although fat cannot be
converted to glucose, it can be burned
as a fuel in order to conserve glucose.
In the disorder diabetes mellitus, carbo-
hydrate metabolism is disturbed by a
deficiency of insulin.
carbonA nonmetallic element present
in all the fundamental molecules of liv-
ing organisms, such as proteins, fats,
and carbohydrates, and in some inor-
ganic molecules such as carbon dioxide,
carbon monoxide, and sodium bicarbon-
ate. Pure carbon is the major constituent
of diamond, coal, charcoal, and graphite.
carbon dioxide (CO 2 )A colourless,
odourless gas. Carbon dioxide is pre-
sent in small amounts in the air and is
an important by-product of metabolism
in cells. It is produced by the breakdown
of substances such as carbohydrates
and fats to produce energy, and is car-
ried in the blood to the lungs and
exhaled. Carbon dioxide helps to control
the rate of respiration: when a personexercises, CO 2 levels in the blood rise,
causing the person to breathe more
rapidly in order to expel carbon dioxide
and to take in more oxygen.
When it is compressed and cooled to
-75ºC, carbon dioxide becomes solid
dry ice, which is used in cryosurgery.
carbon monoxide (CO)A colourless,
odourless, poisonous gas present in
motor exhaust fumes and produced by
inefficient burning of coal, gas, or oil.
Carbon monoxide binds with haemo-
globinand prevents the transportation
of oxygen to body tissues. The initial
symptoms of acute high-level carbon
monoxide poisoning are dizziness, head-
ache, nausea, and faintness. Continued
inhalation of the gas may lead to loss of
consciousness, permanent brain damage,
and even death. Low-level exposure to
carbon monoxide over a period of time
may cause fatigue, nausea, diarrhoea,
abdominal pain, and general malaise.
carbon tetrachloride (CCl 4 )A colour-
less, poisonous, volatile chemical with
a characteristic odour that is present in
some home dry-cleaning fluids and in-
dustrial solvents. It can cause dizziness,
confusion, and liver and kidney damage
if it is inhaled or swallowed.
carbuncleA cluster of interconnected
boils, usually caused by the bacterium
STAPHYLOCOCCUS AUREUS. The back of the
neck and the buttocks are common sites.
Carbuncles mainly affect people with
reduced immunity, particularly those with
diabetes mellitus. Treatment is usually
with an antibioticand hot compresses.
Incision and drainage may be necessary
if a carbuncle is persistent.
carcinogenAny agent capable of caus-
ing cancer. Chemicals are the largest
group of carcinogens. Major types in-
clude polycyclic aromatic hydrocarbons
(PAHs), which occur in tobacco smoke,
pitch, tar fumes, and soot. Exposure to
PAHs may lead to cancer of the respira-
tory system or skin. Certain aromatic
amines used in the chemical and rubber
industries may cause bladder cancer
after prolonged exposure.
The best-known physical carcinogen is
high-energy radiation, such as nuclear
radiation and X-rays. Exposure may cause
cancerous changes in cells, especially inCARBON CARCINOGENC