CORE PRINCIPLES
Biological molecules
CARBOHYDRATES:
Learning objective
to be able to:
• Identify the elements that make up
carbohydrates
• To describe how monosacharides are
the basic molecular units of
carbohydrates.
• Explain how the condensation of
monosacharides forms disacharides
• Which elements make up
carbohydrates?
• Carbon, hydrogen and oxygen
• What are carbohydrates needed for?
• Energy
• Where can we find carbohydrates?
• Pasta, potatoes, bread etc
• Give the chemical formula of a well
known carbohydrate
• C6H12O6 - glucose
Carbohydrates include monomers,
dimers and polymers.
Carbohydrates
sugars
Monosaccharides
(monomers)
Disaccharides
(dimers)
Polysaccharides
(polymers)
eg
glucose
fructose
ribose
eg
sucrose
maltose
lactose
eg
starch
cellulose
glycogen
Monosaccharides
• These all have the formula (CH2O)n, where n can
be 3-7.
• The most common and important monosaccharide
is glucose, where n=6
• This is a six-carbon or hexose sugar, so has the
formula C6H12O6. Its structure is:
6
CH OH
2
C5
O
H
H
C4
HO OH
C3
H
CH2OH
O
H
C1
H
OH
C2
OH
Or more
simply:
HO
OH
Isomers of glucose
• They have the same molecular formula
(C6H12O6), but different structural
formulae.
• These isomers include
• Fructose
• Galactose
• Mannose
αβ-- Glucose
CH2OH
H
H
C
O
C
OH
H
C
OH
C
C
OH
H
OH
α- Glucose
Galactose
CH2OH
OH
H
H
C
O
C
OH
H
C
OH
H
C
C
OH
H
OH
Fructose
Pentose monosaccharides
• Common five-carbon, or pentose
sugars (where n = 5, C5H10O5) include
• Ribose
• Deoxyribose
–(found in nucleic acids and ATP)
• Ribulose
Disaccharides
• Disaccharides are formed when two monosaccharides
are joined together by a glycosidic bond.
• The reaction involves the formation of a molecule of
water (H2O)
CH2 OH
CH2 OH
O
OH
O
OH OH
glucose + glucose = maltose
OH
CH2OH
C5
O H
H
H
C4
C1
OH
H
HO
OH
C3
C2
H
OH
6
CH2OH
C5
O H
H
H
C4
C1
OH
H
HO
OH
C3
C2
H
OH
6
CH2OH
CH2OH
6
C5
O H
C5
O H
H
H
H
H
C4
C1 C4
C1
H
OH
H
HO OH
O
OH
C3
C2
C3
C2
H
H
OH
OH
glycosidic bond
6
+
H 2O
•This shows two glucose molecules joining
together to form the disaccharide maltose.
•Because this bond is between carbon 1 of one
molecule and carbon 4 of the other molecule it is
called a 1-4 glycosidic bond.
•Bonds between other carbon atoms are possible,
leading to different shapes, and branched chains.
Polymerisation
• When molecules are joined together
this is called polymerisation
• Because a small molecule is lost,
(usually water) this is called a
condensation reaction
• The reaction that joins two
monosaccharides together is therefore
called a
– condensation polymerisation reaction
Hydrolysis
• Breaking apart the disaccharide would
require a water molecule
• This is called a hydrolysis reaction
In general:
•polymerisation reactions are condensations
•breakdown reactions are hydrolysis
Three common disaccharides
• Maltose (or malt sugar) is glucose 1-4 glucose.
• Sucrose (or cane sugar) is glucose 1-2 fructose.
• Lactose (or milk sugar) is galactose 1-4 glucose.
Summary of Carbohydrates
• Carbon atoms bond strongly to each other forming
chains and rings.
• Carbohydrates include monosaccharides,
disaccharides and polysaccharides. They can act as
an energy source or provide structural support.
• Monosaccharides include glucose, fructose and
galactose. They have the same chemical formula but
different chemical structures ~ we call them isomers.
• Monosaccharides can become linked together by a
glycosidic bond to form disaccharides such as
maltose, sucrose and lactose, and polysaccharides,
such as starch and cellulose.
• Polysaccharides can perform structural roles, for
example cellulose in plant cells, and energy storage
roles, for example starch in plants and glycogen in
animals.