CARBOCISTEINE
C
carbocisteine
A type of
mucolytic drug
that is used to
thin sputum, making it easier to cough
up. Carbocisteine may be helpful in
chronic obstructive pulmonary disease
(see
pulmonary disease, chronic obstruc-
tive
)
and
cystic fibrosis
.
carbohydrates
A
group
of
compounds,
including
starches (complex carbohydrates) and
sugars (simple carbohydrates), that are
composed of carbon, hydrogen, and
oxygen and supply the body w ith its
main source of energy.
TYPES AND SOURCES
Carbohydrates are found in fruits, cer-
eals, and root vegetables. They fall into
two main groups: available carbohy-
drates and unavailable carbohydrates.
The main available carbohydrates are
starches and sugars, w hich are meta-
bolized into glucose for the body’s use.
Unavailable carbohydrates include cellu-
lose, w hich cannot be broken down by
digestive
enzymes
and makes up the
bulk of dietary fibre (see
fibre, dietary
) .
CARBOHYDRATE METABOLISM
The different types of carbohydrates are
processed by
the
body
in
different
ways. Monosaccharides (also known as
simple sugars), w hich include glucose
(grape sugar), galactose (a m ilk sugar),
and fructose (fruit sugar), can all be
absorbed, unchanged, into the blood-
stream,
whereas
disaccharides
(also
known as “double” sugars), including
sucrose, maltose, and lactose (a m ilk
sugar), need to be broken down into
simple sugars before they are absorbed.
Starches also have to be broken down
TYPES OF CARBOHYDRATE
Monosaccharides are the simplest,
consisting of a single saccharide
molecule. Disaccharides consist of
two linked saccharide molecules.
Polysaccharides consist of a long
chain of many saccharide molecules.
Starch is an important carbohydrate
and a major constituent in the diet.
Monosaccharides
glucose, galactose, fructose
Disaccharides
sucrose, lactose, maltose
Polysaccharides
starches, cellulose
into simple sugars before they can be
absorbed by the body This process is
carried out by enzymes (chemical cata-
lysts) in the digestive tract.
The simple sugars (mainly glucose)
are then absorbed through the intes-
tinal w all and into the bloodstream
for distribution throughout the body
Some
glucose is burned up
im m e-
diately
(see
metabolism
)
in order to
generate energy for cells that need a
constant supply, such as brain cells and
red blood cells. Galactose and fructose
are converted to glucose in the liver to
be used by body cells. Surplus glucose
is transported to the liver, muscles, and
fat cells where it is converted into
glyco-
gen
and fat for storage.
W hen blood glucose levels are high,
glucose storage is stimulated by
insulin
,
a hormone
that is secreted by the
pancreas
.
W hen blood glucose levels
become low, insulin secretion dim in-
ishes and
glucagon
,
another hormone
produced by the pancreas, stimulates
the conversion of stored glycogen to
glucose
for release
into
the blood-
stream.
Fat
cannot
be
converted to
glucose, although it can be used as a
fuel to conserve glucose.
In
the
disorder
diabetes mellitus
,
carbohydrate metabolism is disturbed
by a deficiency of insulin.
carbolic acid
See
phenol
.
carbon
A nonmetallic element that is present
in all the fundamental molecules of
living organisms, such as
proteins
,
fats
,
and
carbohydrates
,
and well as in some
inorganic molecules,
such as
carbon
dioxide
,
carbon monoxide
,
and
sodium
bicarbonate
.
Pure carbon is the major
constituent of diamond, coal, charcoal,
and graphite.
carbon dioxide (CO
2
)
A
colourless,
odourless
gas.
Carbon
dioxide consists of one carbon atom
linked to two oxygen atoms and has
the chemical formula CO2. The gas is
present in small amounts in the air and
is an important by-product of
metabo-
lism
in
cells. It is produced by the
breakdown of substances such as car-
bohydrates and fats to produce energy.
It is then carried in the bloodstream to
the lungs, where it is exhaled.
Carbon dioxide helps to control the
rate of respiration: when a person exer-
CARBOHYDRATE METABOLISM
Glucose is absorbed into the bloodstream for
distribution throughout the body or is used
directly by body cells.
cises, CO2 levels in the blood rise, caus-
ing the person to breathe more rapidly
in order to expel carbon dioxide and to
take in more
oxygen
.
Carbon dioxide that is compressed
and cooled to -
7 5
°C becomes solid
dry
ice
,
w hich is used in
cryosurgery
(des-
truction of diseased tissues by freezing).
carbon monoxide (CO)
A colourless, odourless, poisonous gas.
Carbon monoxide consists of one car-
bon atom linked to one oxygen atom
and has the chemical formula CO. It
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