B io Factsheet
www.curriculum-press.co.uk Number 159
The Structure and Function of
Polysaccharides
This Factsheet summarises:
Typical exam questions on this topic
• The structure of the polysaccharides starch, glycogen and
• Outline what is meant by a condensation reaction or draw/
cellulose.
label a diagram showing the reaction.
• The functions of polysaccharides.
• Explain how structure fits function in eg starch, glycogen
• How structure is related to function.
and cellulose.
Structure • Describe in extended prose the significance of
The key points are: polysaccharides in animals and plants.
• Polysaccharides are organic molecules made up of carbon (C), Occasionally, you might be asked to comment on the biological
Hydrogen (H) and Oxygen (O). significance of other polysaccharides – chitin, for example, but
• They are polymers ie they are large molecules (macromolecules) because these aren’t specifically mentioned in any specification
made up of simple units (building blocks) repeated many times. you can usually just make up something which is biologically
• Glucose is the commonest monomer from which the sensible –we’ll explain this later on in the Factsheet.
macromolecule or polymer is formed.
• These simple units –or monomers – are held together by bonds Starch
formed in condensation reactions in which a molecule of water Starch is a polymer of α-glucose.
is formed as two monomers are joined . It is a mixture of two substances: amylose and amylopectin.
• These bonds are called glycosidic bonds (Fig1). Typically, starch = 20% amylose, 79% amylopectin and 1%
phosphates/fatty acids but the precise composition varies between
Fig 1 species (remember starch is only found in plants).
Amylose and amylopectin have very different structures (Fig 2).
α- glucose α- glucose
6
6 Fig 2.
5 O 5 O Amylose
H H H H
Numbers are
4 labelling the 1 4 1
carbon atoms
HO OH HO OH
3 2 3 2
amylose helix (6
α-glucose molecule glucose units in
each turn)
condensation
CH 2OH CH 2OH CH2OH CH 2OH
H2O O O O O
H H H H H H H H H H H H
OH OH OH OH
H O H O H O H
H H H H
O OH OH OH OH
O
H H H α 1-4 glycosidic bond
H
Amylopectin
HO O OH CH 2OH CH 2OH CH 2OH
O O O
1-4 glycosidic bond H H H H H H H H H
OH OH OH
maltose O H O H O H
H H H O 1-6 glycosidic bond
OH OH OH
CH2OH CH2OH CH2 CH 2OH
The polysaccharides that come up repeatedly in the exams are
O O O O
starch, glycogen and cellulose. H H H H H H H H H H H H
OH OH OH OH
O H O H O H O H O
H H H H
OH OH OH OH
1
www.curriculum-press.co.uk Number 159
The Structure and Function of
Polysaccharides
This Factsheet summarises:
Typical exam questions on this topic
• The structure of the polysaccharides starch, glycogen and
• Outline what is meant by a condensation reaction or draw/
cellulose.
label a diagram showing the reaction.
• The functions of polysaccharides.
• Explain how structure fits function in eg starch, glycogen
• How structure is related to function.
and cellulose.
Structure • Describe in extended prose the significance of
The key points are: polysaccharides in animals and plants.
• Polysaccharides are organic molecules made up of carbon (C), Occasionally, you might be asked to comment on the biological
Hydrogen (H) and Oxygen (O). significance of other polysaccharides – chitin, for example, but
• They are polymers ie they are large molecules (macromolecules) because these aren’t specifically mentioned in any specification
made up of simple units (building blocks) repeated many times. you can usually just make up something which is biologically
• Glucose is the commonest monomer from which the sensible –we’ll explain this later on in the Factsheet.
macromolecule or polymer is formed.
• These simple units –or monomers – are held together by bonds Starch
formed in condensation reactions in which a molecule of water Starch is a polymer of α-glucose.
is formed as two monomers are joined . It is a mixture of two substances: amylose and amylopectin.
• These bonds are called glycosidic bonds (Fig1). Typically, starch = 20% amylose, 79% amylopectin and 1%
phosphates/fatty acids but the precise composition varies between
Fig 1 species (remember starch is only found in plants).
Amylose and amylopectin have very different structures (Fig 2).
α- glucose α- glucose
6
6 Fig 2.
5 O 5 O Amylose
H H H H
Numbers are
4 labelling the 1 4 1
carbon atoms
HO OH HO OH
3 2 3 2
amylose helix (6
α-glucose molecule glucose units in
each turn)
condensation
CH 2OH CH 2OH CH2OH CH 2OH
H2O O O O O
H H H H H H H H H H H H
OH OH OH OH
H O H O H O H
H H H H
O OH OH OH OH
O
H H H α 1-4 glycosidic bond
H
Amylopectin
HO O OH CH 2OH CH 2OH CH 2OH
O O O
1-4 glycosidic bond H H H H H H H H H
OH OH OH
maltose O H O H O H
H H H O 1-6 glycosidic bond
OH OH OH
CH2OH CH2OH CH2 CH 2OH
The polysaccharides that come up repeatedly in the exams are
O O O O
starch, glycogen and cellulose. H H H H H H H H H H H H
OH OH OH OH
O H O H O H O H O
H H H H
OH OH OH OH
1
