portion of the molecule is usually
based on glycerol (see glyceride) or
sphingosine and the sugar is typically
galactose, glucose, or inositol. Glycolipids are components of biological
membranes. In animal plasma membranes they are found in the outer
layer of the lipid bilayer; the simplest
animal glycolipids are the *cerebrosides. Plant glycolipids are glycerides
in which the sugar group is most
commonly galactose. They are the
principal lipid constituents of chloroplasts.
glycolysis (Embden–Meyerhof pathway) The series of biochemical reactions 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 triosephosphate molecules. Each of these
is converted to pyruvate. The net energy yield is two ATP molecules per
glucose molecule. In aerobic respiration pyruvate then enters the *Krebs
cycle. Alternatively, when oxygen is
in short supply or absent, the pyruvate is converted to various products
by anaerobic respiration. Other simple sugars, e.g. fructose and galactose, and glycerol (from fats) enter
the glycolysis pathway at intermediate stages.
glycoprotein A carbohydrate
linked covalently to a protein.
Formed in the Golgi apparatus in the
process of *glycosylation, glycoproteins are important components of
cell membranes. They are also constituents of body Ûuids, such as
mucus, that are involved in lubrication. Many of the hormone receptors
on the surfaces of cells have been
identiÜed as glycoproteins. Glycoproteins produced by viruses attach
themselves to the surface of the host
cell, where they act as markers for
the receptors of leucocytes. Viral glycoproteins can also act as target molecules and help viruses to detect
certain types of host cell; for example, a glycoprotein on the surface of
HIV (the AIDS virus) enables the virus
to Ünd and infect white blood cells.
glycosaminoglycan Any one of a
group of polysaccharides that contain
amino sugars (such as glucosamine).
Formerly known as mucopolysaccharides, they include *hyaluronic acid
and chondroitin, which provide lubrication in joints and form part of
the matrix of cartilage. The threedimensional structure of these molecules enables them to trap water,
which forms a gel and gives glycosaminoglycans their elastic properties.
glycoside Any 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 replaced 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 medicinally for their stimulant effects on
the heart.
glycosidic bond (glycosidic link)
The type of chemical linkage between 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-glycosidic 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-glycosidic
249
glycosidic bond
g
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based on glycerol (see glyceride) or
sphingosine and the sugar is typically
galactose, glucose, or inositol. Glycolipids are components of biological
membranes. In animal plasma membranes they are found in the outer
layer of the lipid bilayer; the simplest
animal glycolipids are the *cerebrosides. Plant glycolipids are glycerides
in which the sugar group is most
commonly galactose. They are the
principal lipid constituents of chloroplasts.
glycolysis (Embden–Meyerhof pathway) The series of biochemical reactions 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 triosephosphate molecules. Each of these
is converted to pyruvate. The net energy yield is two ATP molecules per
glucose molecule. In aerobic respiration pyruvate then enters the *Krebs
cycle. Alternatively, when oxygen is
in short supply or absent, the pyruvate is converted to various products
by anaerobic respiration. Other simple sugars, e.g. fructose and galactose, and glycerol (from fats) enter
the glycolysis pathway at intermediate stages.
glycoprotein A carbohydrate
linked covalently to a protein.
Formed in the Golgi apparatus in the
process of *glycosylation, glycoproteins are important components of
cell membranes. They are also constituents of body Ûuids, such as
mucus, that are involved in lubrication. Many of the hormone receptors
on the surfaces of cells have been
identiÜed as glycoproteins. Glycoproteins produced by viruses attach
themselves to the surface of the host
cell, where they act as markers for
the receptors of leucocytes. Viral glycoproteins can also act as target molecules and help viruses to detect
certain types of host cell; for example, a glycoprotein on the surface of
HIV (the AIDS virus) enables the virus
to Ünd and infect white blood cells.
glycosaminoglycan Any one of a
group of polysaccharides that contain
amino sugars (such as glucosamine).
Formerly known as mucopolysaccharides, they include *hyaluronic acid
and chondroitin, which provide lubrication in joints and form part of
the matrix of cartilage. The threedimensional structure of these molecules enables them to trap water,
which forms a gel and gives glycosaminoglycans their elastic properties.
glycoside Any 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 replaced 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 medicinally for their stimulant effects on
the heart.
glycosidic bond (glycosidic link)
The type of chemical linkage between 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-glycosidic 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-glycosidic
249
glycosidic bond
g
www.AzShimi.ir www.AzShimi.com