Types of Carbohydrates
Section 17.1
Four Types of Carbohydrates
Monosaccharides
Contain a single sugar unit
Examples: glucose and fructose
Disaccharides
Contain two monosaccharides units joined
through bridging oxygen atoms AKA glycosidic
bond
Examples: sucrose and lactose
Four Types, continued
Oligosaccharides
Three to ten monosaccharide units joined by
glycosidic bonds
Polysaccharides
Long, often highly branched, chains of
monosaccharides
Examples: starch, glycogen, and cellulose
Monosaccharides
Composed of carbon, hydrogen and
oxygen
Most follow the general formula (CH2O)n
All names end in –ose
Two types
Ketose – monosaccharide that contains a
ketone carbonyl group
Aldose – monosaccharide that contains an
aldehyde carbonyl group
Ketose OR Aldose?
O
O
CH
CH2OH
CH
CH2OH
C
O
HO
C
H
OH
H
C
OH
OH
H
C
OH
H
C
OH
C
O
H
C
OH
H
C
OH HO
C
H
HO
C
H
H
C
OH H
C
OH H
C
H
C
OH H
C
OH H
C
CH2OH
CH2OH
CH2OH
CH2OH
Ketose OR Aldose
O
O
CH
CH2OH
CH
CH2OH
C
O
HO
C
H
OH
H
C
OH
OH
H
C
OH
H
C
OH
C
O
H
C
OH
H
C
OH HO
C
H
HO
C
H
H
C
OH H
C
OH H
C
H
C
OH H
C
OH H
C
CH2OH
CH2OH
CH2OH
CH2OH
Another system – number of carbons
Another system of nomenclature tells us
the number of carbon atoms in the main
skeleton.
3 carbon triose
4 carbon tetrose
5 carbon pentose
6 carbon hexose
Etc.
Triose, tetrose, pentose, etc?
O
O
CH
O
CH
CH
H
C
OH
CH2OH
H
C
OH
HO
C
H
H
C
OH
H
C
OH
H
C
OH
H
C
OH
H
C
OH
CH2OH
CH2OH
Answers
O
O
CH
O
CH
CH
H
C
OH
CH2OH
C
OH
HO
C
H
H
C
OH
H
C
OH
H
C
OH
H
C
OH
H
C
OH
CH2OH
Triose
H
Pentose
CH2OH
Hexose
Combining the two systems
O
CH2OH
O
CH
CH
H
C
H
OH
CH2OH
HO
H
C
C
C
C
O
HO
C
H
H
C
OH
H
C
OH
OH
H
OH
H
C
OH
Aldose
Triose
CH2OH
Aldotriose
Aldose
D-Glyceraldehyde Hexose
Aldohexose
D-Glucose
CH2OH
Ketose
Hexose
Ketohexose
D-Fructose
Stereochemistry:
different spatial arrangements of atoms
Enantiomers
Chiral molecules
2 stereoisomers
Molecules that exist
as enantiomers
that are
(they have a right &
nonsuperimposable
left version)
mirror images of one Chiral
carbon
another
a carbon that has 4
different groups
attached to it
Chiral Objects
Chiral compounds have the same number
of atoms arranged differently in space.
A chiral carbon atom has four different
groups attached.
Mirror Images
The three-dimensional structure of a chiral
compound has a mirror image.
Your hands are chiral. Try to superimpose your
thumbs, palms, back of hands, and little fingers.
Is it possible? Why or why not?
Important in biochemistry because our
biochemical processes will recognize &
act on only ONE of the chiral compounds.
The other compound is foreign to us.
Chiral or NOT?
Determine if there is a chiral carbon in each
compound.
Cl
C CH3
H
CH2CH3
A
Cl
H
C CH3
H
B
Solution
Cl
Cl
H
C CH3
CH2CH3
A Yes, 4 different
groups are attached
to the second C atom
H
C CH3
H
B No, the
2 H atoms
are identical
D and L Notation
D,L tells to which of the two chiral
isomers we are referring.
If the –OH group on the next to the
bottom carbon atom points to the right,
the isomer is a D-isomer; if it points
left, the isomer is L.
The D form is usually the isomer found
in nature.
D notation
H C
O
H C OH
H C OH
CH2OH
Right = D
Glucose
H
C O
H C OH
HO C H
H C OH
H C OH
CH 2OH
D-Glucose
D or L?
Homework
Complete problems 17.3 and 17.4 on page
472; 17.5 and 17.6 on page 474-5; 17.24,
17.25, and 17.26 on page 493
Importance of Glucose
Most important sugar in the human body.
Glucose is broken down in glycolysis and
other pathways to release energy for body
functions
The concentration of glucose in the blood
is carefully controlled by insulin and
glucagon.
Normal blood glucose levels are 100-120
mg/100mL
Importance of Glucose
Insulin stimulates the uptake of the excess
glucose by most of the cells in the body.
1-2 hrs after eating the glucose levels
return to normal.
If glucose concentrations drop too low, the
individual feels lightheaded and shaky.
When this happens, glucagon stimulates
the liver to release glucose in the blood.
Diabetes / Hypoglycemia and Glucose
 Type I diabetes or diabetes mellitus – caused by
the inability to produce the hormone insulin; If
untreated, end up with high blood sugar
 Type II diabetes or adult-onset diabetes –
caused by insulin resistance (body fails to
properly use insulin) combined with a relative
insulin deficiency; If untreated, end up with high
blood sugar
 Hypoglycemia – caused by the over excretion of
insulin in response to a rise in blood sugar; If
untreated, end up with low blood sugar
Structure of glucose
In reality, the open-chain form of glucose
is present in very small concentrations in
cells.
In most cases, the cyclic intramolecular
hemiacetal is formed.
The hydroxyl group on C-5 reacts with the
aldehyde group to form a hemiacetal.
Two isomers are formed because a new
chiral carbon is created.
6 CH2OH
5
O
1
H
HO
H
H
CH
2
3
4
α-D-glucose
O
H
H
1
4
OH
H
OH
OH
3
H
H
OH
6
CH2OH
2
5
O
OH
H
OH
OH
H
4
H
5
OH
1
OH
6 CH2OH
D-glucose
OH
3
β-D-glucose
H
H
H
2
OH
Steps for drawing
Haworth Structures for D-Glucose
1. Break the double bond
2. Move the H from the –OH group on C5 to
the O on C1
3. Connect the ring structure by bonding the
O on C5 directly to C1
4. Write –OH groups on the right (C2, C4)
down
5. Write –OH groups on the left (C3) up
6. The new –OH on C1 has two possibilites:
down for  form, up for  form
α-D-glucose
CH2OH
H
C
OH
O
H
C
OH
HO
C
H
H
HO
C
OH
H2
C
C
H
H
O
OH
H
H
OH
H
H
OH
OH
β-D-glucose
CH2OH
HO
C
H
O
H
C
H
OH
OH
H
O
HO
C
H
OH
H
HO
H2
C
C
C
OH
H
H
OH
OH
H
H
You Try This One!
Write the cyclic form of -D-galactose
H
O
C
H C OH
HO C H
HO C H
H C OH
CH 2OH
Solution -D-galactose
H
C
OH
H
C
OH
CH2OH
O
HO
OH
C
H
OH
C
H
CH2
C
H
o
OH
OH
OH
OH
Benedict’s reagent
Benedict’s reagent – a basic buffer
solution that contains Cu2+ ions
It readily oxidizes the aldehyde group of
aldoses to form a brick red Cu2O
precipitate.
It will also oxidize ketoses because of the
–OH group on the carbon next to the
carbonyl group.
Reducing Sugars
Reducing sugar – a sugar that can be
oxidized by Benedict’s reagent
All monosaccharides and the common
disaccharides (except sucrose) are
reducing sugars.
Use of Benedict’s reagent
Benedict’s reagent was commonly used to
qualitatively monitor excess glucose in the
urine by diabetics to insure proper dosage
of insulin.
These have been replaced by blood
glucose tests that are more accurate.