portion of the molecule is usually
based on glycerol (see glyceride) or
sphingosine and the sugar is typically
galactose, glucose, or inositol. Glycol-
ipids are components of biological
membranes. In animal plasma mem-
branes they are found in the outer
layer of the lipid bilayer; the simplest
animal glycolipids are the *cerebro-
sides. Plant glycolipids are glycerides
in which the sugar group is most
commonly galactose. They are the
principal lipid constituents of chloro-
plasts.
glycolysis(Embden–Meyerhof path-
way)The series of biochemical reac-
tions in which glucose is broken
down to pyruvate with the release of
usable energy in the form of ATP.
One molecule of glucose undergoes
two phosphorylation reactions
and is then split to form two triose-
phosphate molecules. Each of these
is converted to pyruvate. The net en-
ergy yield is two ATP molecules per
glucose molecule. In aerobic respira-
tion pyruvate then enters the Krebs
cycle. Alternatively, when oxygen is
in short supply or absent, the pyru-
vate is converted to various products
by anaerobic respiration. Other sim-
ple sugars, e.g. fructose and galac-
tose, and glycerol (from fats) enter
the glycolysis pathway at intermedi-
ate stages.
glycoprotein A carbohydrate
linked covalently to a protein.
Formed in the Golgi apparatus in the
process of *glycosylation, glycopro-
teins are important components of
cell membranes. They are also con-
stituents of bodyÛuids, such as
mucus, that are involved in lubrica-
tion. Many of the hormone receptors
on the surfaces of cells have been
identiÜed as glycoproteins. Glycopro-
teins produced by viruses attach
themselves to the surface of the host
cell, where they act as markers for
the receptors of leucocytes. Viral gly-
coproteins can also act as target mol-
ecules and help viruses to detect
certain types of host cell; for exam-
ple, a glycoprotein on the surface of
HIV (the AIDS virus) enables the virus
toÜnd and infect white blood cells.glycosaminoglycanAny one of a
group of polysaccharides that contain
amino sugars (such as glucosamine).
Formerly known as mucopolysaccha-
rides, they include *hyaluronic acid
and chondroitin, which provide lu-
brication in joints and form part of
the matrix of cartilage. The three-
dimensional structure of these mol-
ecules enables them to trap water,
which forms a gel and gives gly-
cosaminoglycans their elastic proper-
ties.
glycosideAny one of a group of
compounds consisting of a pyranose
sugar residue, such as glucose, linked
to a noncarbohydrate residue (R) by a
*glycosidic bond: the hydroxyl group
(–OH) on carbon-1 of the sugar is re-
placed by –OR. Glycosides are widely
distributed in plants; examples are
the *anthocyanin pigments and the
cardiac glycosides, such as digoxin
and ouabain, which are used medici-
nally for their stimulant effects on
the heart.glycosidic bond (glycosidic link)
The type of chemical linkage be-
tween the monosaccharide units of
disaccharides, oligosaccharides, and
polysaccharides, which is formed by
the removal of a molecule of water
(i.e. a *condensation reaction). The
bond is normally formed between
the carbon-1 on one sugar and the
carbon-4 on the other. An α-glyco-
sidic bond is formed when the –OH
group on carbon-1 is below the plane
of the glucose ring and a β-glycosidic
bond is formed when it is above the
plane. Cellulose is formed of glucose
molecules linked by 1-4 β-glycosidic249 glycosidic bond
g