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Biomolecules
Part II: Carbohydrates
Carbohydrates
• Are made from just three elements:
carbon, hydrogen and oxygen
• Have a characteristic ratio of these
atoms – for every one carbon atom
there are two hydrogen atoms and
one oxygen atom.
• Carbohydrates perform many
important biological functions.
Functions of Carbohydrates
• Source of energy and a means of storing
chemical energy.
• Involved in many chemical reactions, as
components of molecules such as DNA
and RNA, and can be chemically bonded
to lipids and proteins.
• Form structural elements in the cell
walls of plants, fungi and bacteria. Also
make up the exoskeleton of insects and
crustaceans.
Classes of Carbohydrates
• Carbohydrates are classified into classes
depending upon the number of linked sugar
molecules they contain.
• Classes of Carbohydrates
–
–
–
–
Monosaccharides
Disaccharides
Oligosaccharides
Polysaccharides
• Carbohydrates containing one or two sugar units are
often referred to as simple carbohydrates; those
containing many sugar molecules are called complex
carbohydrates
Simple Carbohydrates
Monosaccharides
General formula:
(CH2O)n
n=3, triose
n=5, pentose
n=6, hexose
Glucose
Fuel molecule
Fructose
Fuel molecule
Ribose
Component of the
nucleotide for RNA
Disaccharides
(common dietary
components)
Sucrose
(a glucose + b fructose)
Transport sugar in
vascular plants
Lactose
Component of milk
(b galactose + a glucose)
Maltose
(a glucose + a glucose)
Obtained in the
breakdown of
starch
Complex Carbohydrates
Polysaccharides
(insoluble
macromolecules)
Starch
Storage molecule in
plants
Glycogen
Storage molecule in
animals
Cellulose
Component of the
plant cell wall
Chitin
Component of the
exoskeletons of
insects and
crustaceans
Peptidoglycan
Component of
bacterial cell wall
More about monosaccharides
• They are white crystalline solids
that readily dissolve in water.
• Solubility is due to their OH groups
which readily form hydrogen bonds
with water molecules.
• All monosaccharides are sweet
tasting to varying degrees.
• The most common and biologically
important simple sugar is glucose.
What’s so good about glucose?
• Fuel molecule for the cell.
• Building block for many
other important
carbohydrates.
• Has a large number of
isomeric forms. (e.g. L and D
forms)
• When in solution it forms a
ring structure.
• There are two forms called
alpha and beta glucose
depending on the position of
the OH group on carbon 1.
• The properties of many
biologically important
polysaccharides depend
upon whether the alpha or
beta form is present.
Linking monosaccharides
• Linking two monosaccharides
results in a disaccharide molecule.
• Repetitive linking of many
monosaccharides results in a
polysaccharide.
• The process of linking
monosaccharide monomers is
referred to as condensation
polymerisation.
Condensation polymerisation
• The polymerisation reaction involves an OH group at C1
with another OH group on an adjacent monosaccharide.
• During the reaction a water molecule is eliminated, hence
the name condensation polymerisation.
• The bond formed between the monomers is referred to as
a glycosidic bond.
Polysaccharides
• The different
properties of
polysaccharides can
be explained by:
– The length of the
polymer chain
– The extent of any
branching chains
– Whether the polymer
chain is straight, or
folded, or coiled.
More about Polysaccharides
• Polysaccharides are divided into
two categories:
– Storage polysaccharides
•Starch
•Glycogen
– Structural polysaccharides
•Cellulose
•Chitin
•peptidoglycan
More about storage
polysaccharides
• Starch
– Fuel storage
polysaccharide in plants.
– Exists in two forms:
amylose and
amylopectin.
– Both are made by linking
alpha glucose monomers.
– Amylose is an
unbranched polymer
that coils into a helical
structure and can form a
suspension in hot water.
– Amylopectin is a
branched polymer that is
completely insoluble in
water.
More about storage
polysaccharides
• Glycogen
– Fuel storage
polysaccharide in animals.
– Found in the liver and
muscle cells.
– Excess glucose will be
polymerised to glycogen
for storage.
– It is amylopectin with
short distances between
the branching side chains.
– Glycogen molecules tend
to be larger and more
branched than starch
molecules.
– Glycogen is insoluble in
water.
More about structural
polysaccharides
• Cellulose
–
–
–
–
Major component of plant cell walls.
Most abundant organic substance on Earth.
Made from linking of b-glucose monomers
In the plant cell wall, hydrogen bonding between the long
parallel polymer chins holds them together to form strong
microfibrils.
More about structural
polysaccharides
• Chitin
– Formed by linking glucose monomers that have a
nitrogen-containing group of atoms attached.
– Translucent, pliable but tough compound.
– Found in the exoskeletons of insects and crustaceans.
– Also in the cell walls of many fungi, moulds and
yeasts.
More about structural
polysaccharides
• Peptidoglycan
– Made up of two
alternatively
repeating monomers
are Nacetylglucosamine (the
monomer for chitin)
and N-acetylmuramic
acid.
– Both monomers based
upon glucosamine – a
glucose molecule with
an NH2 (amino group)
substituted for an OH
group on C2.