Chapter Eighteen
Carbohydrates
Carbohydrates cont’d
Ch 18 | # 2 of 52
Carbohydrates
• Major source of ______ from our diet
• Composed of the elements _, _ and _
• Produced by photosynthesis in plants
• Also called saccharides (sugars)
Ch 18 | # 3 of 52
Carbohydrates, Chemically
• Polyhydroxy aldehydes or polyhydroxy ketones
O
H2C
OH
C
O
CH
HC
HO
OH
CH
HC
HC
HC
OH
HC
CH3
OH
OH
HC
HC
OH
OH
OH
CH3
Ch 18 | # 4 of 52
Monosacchrides
• Three Carbons =
________
• Four Carbons =
________
• Five Carbons =
________
• Six Carbons =
________
Ch 18 | # 5 of 52
Monosacchrides
• Aldoses are monosacchrides with an _________________
group and many hydroxyl (-OH) groups.
• Ketoses are monosacchrides with a __________________
group and many hydroxyl (-OH) groups.
Ch 18 | # 6 of 52
Glucose
Ch 18 | # 7 of 52
Galactose
D-galactose
Ch 18 | # 8 of 52
Fructose
Ch 18 | # 9 of 52
Types of Carbohydrates
• Monosacchrides
A carbohydrate that contains a single
polyhydroxy aldehyde or polyhydroxy ketone
unit.
• Disaccharides
Contain 2 monosacchride units
• Polysacchrides
Contain many monosacchride units
Ch 18 | # 10 of 52
Different Forms of Monosaccharides
• Most monosaccharides exist in two different forms
– “left-handed” and
– “right-handed”
• The two forms are related to each other the same
way that mirror images are related to each other.
Ch 18 | # 11 of 52
Carbohydrates cont’d
← Fig. 18.3
The mirror image of the
right hand is the left
hand. Conversely, the
mirror image of the left
hand is the right hand.
Ch 18 | # 12 of 52
Carbohydrates cont’d
→ Fig. 18.4
A person’s left and right
hands are not
superimposable upon
each other.
Ch 18 | # 13 of 52
Mirror Images
• Mirror images
– The reflection of an object in a mirror
• Superimposable mirror images
– Images that coincide at all points when the images are laid upon
each other
• Nonsuperimposable mirror images
– Images where not all points coincide when the images are laid
upon each other
• The easiest way to determine if two mirror images are
superimposable or not, is to make models
Ch 18 | # 14 of 52
Chiral Objects
• Chiral molecules have mirror images that are not
superimposable
• Chiral molecules contain at least one chiral center
• A chiral carbon atom (“chiral center”) has four
different groups attached to it (typically marked
with *)
Ch 18 | # 15 of 52
Carbohydrates cont’d
Fig. 18.5
Examples of simple molecules that are chiral.
Ch 18 | # 16 of 52
Stereoisomerism
• Structural isomers
– Isomers in which atoms are connected to each other in
different ways
• Stereoisomers
– Isomers whose atoms are connected in the same way
but which differ in the orientation of these atoms in
space
• Stereoisomers always have a chiral center and structural rigidity
• Cis-trans isomers are one form of stereoisomerism
Ch 18 | # 17 of 52
Stereoisomers
• 2 different kinds of stereoisomers
– Enantiomers
• Stereoisomers whose molecules are nonsuperimposable mirror
images of each other (the left- and right-hand versions of a
single molecule)
– Diastereomers
• Stereoisomers whose molecules are not mirror images of each
other
Ch 18 | # 18 of 52
Carbohydrates cont’d
→ Fig. 18.6
Enantiomers are
stereoisomers whose
molecules are
nonsuperimposable mirror
images of each other.
a) Enantiomers
b) Diastereomers –
molecules are not
mirror images
Ch 18 | # 19 of 52
Carbohydrates cont’d
← Fig. 18.7
The “thought process”
used in classifying
molecules as
enantiomers or
diastereomers.
Ch 18 | # 20 of 52
Ch 18 | # 21 of 52
Carbohydrates cont’d
→ Fig. 18.8
Emil Fischer was one of
the early greats in
organic chemistry. He is
credited with the
development and use of
the Fischer Projection
Formulas. We have
already used this notation
in the tetrahedral drawing
in a two dimensional
form. See page 519 in
your text.
Edgar Fahs Smith Collection, University of
Pennsylvania Library
Ch 18 | # 22 of 52
Fischer Projections
• A two-dimensional structural notation for showing the
spatial arrangement of groups about chiral centers in
molecules
– Vertical lines = bonds to groups directed into the page (away
from you)
– Horizontal lines = bonds to groups directed out of the page
(towards you)
H
O
O
CH
CH
C
CH3
OH
HO
C
H
CH3
Ch 18 | # 23 of 52
Properties of Chiral Centers
• Most important property is a solution of a
stereoisomer is its ability to rotate plane polaraized
light.
• Light passing through polarized filters has waves
only in one direction, i.e., the light is polarized.
Ch 18 | # 24 of 52
Carbohydrates cont’d
← Fig. 18.9
Vibrational
characteristics of
ordinary and polarized
light.
Ch 18 | # 25 of 52
Carbohydrates cont’d
Fig. 18.10
Schematic depiction of how a polarimeter works.
Ch 18 | # 26 of 52
• Rotation of light:
– Levorotatory: Rotation of light to the left
– Dextrorotatory: Rotation of light to the right
• Not to be confused with D and L forms or the
“handedness” of molecules. They are different.
Ch 18 | # 27 of 52
D notation
Ch 18 | # 28 of 52
Chiral Centers – Why Do We Care?
• Monosaccharides often contain more than one
chiral center
– This means that there are at least two different forms of
each monosaccharide, a left-handed form and a righthanded form
– Each form elicits a different chemical response
• Biologically active monosaccharides are the righthanded versions of molecules
Ch 18 | # 29 of 52
Carbohydrates cont’d
← Fig. 18.11
The distinctly different
natural flavors of
spearmint and caraway
are caused by
enantiomeric
molecules.
Ch 18 | # 30 of 52
Intermolecular Interactions
• Interactions where there is binding or contact at
three points.
• Look at enantiomeric pair and see how each
would bind differently.
Ch 18 | # 31 of 52
Carbohydrates cont’d
→ Fig. 18.12
Epinephrine binds to
the receptor at three
points.
Ch 18 | # 32 of 52
Ch 18 | # 33 of 52
Ch 18 | # 34 of 52
Cyclic Structures
• Monosaccharides with 5-6 carbon atoms
form cyclic structures
• The hydroxyl group on C-5 reacts with the
aldehyde group or ketone group (to form a
hemiacetal or a hemiketal)
Ch 18 | # 35 of 52
Haworth Structure for D-Isomers
• Two-dimensional structural notations that
specifies the 3-dimensional structure of a cyclic
form of a monosaccharide
– The cyclic structure of a D-isomer has the final CH2OH
group located above the ring.
Ch 18 | # 36 of 52
→ Fig. 18.16
The cyclic
hemiacetal
forms of Dglucose result
from the
intramolecular
reaction
between the
carbonyl group
and the
hydroxyl group
on carbon 5.
Ch 18 | # 37 of 52
Haworth Structure for D-Glucose


-D-Glucose
-D-Glucose
Ch 18 | # 38 of 52
Carbohydrates cont’d
← Fig. 18.17
Walter Norman
Haworth was a British
carbohydrate chemist.
© Hulton-Deutsch Collection / CORBIS
Ch 18 | # 39 of 52
→ Fig. 18.19
The three forms of maltose present in aqueous solution.
Ch 18 | # 40 of 52
Disaccharides
• A disaccharide consists of two monosaccharides
• Glucose + Glucose  Maltose + water
• Glucose + Galactose  Lactose + water
• Glucose + Fructose  Sucrose + water
Ch 18 | # 41 of 52
Ch 18 | # 42 of 52
← Fig. 18.22 The polymer chain of a polysaccaride may be unbranched or branched.
Ch 18 | # 43 of 52
Amylose, Amylopectin, and Glycogen
• Amylose is a continuous chain of glucose
molecules linked by -1,4 glycosidic bonds
• Amylopectin is a branched chain of glucose
molecules linked by a -1,4- and -1,6-glycosidic
bonds.
• Glycogen is similar to amylopectin, but more
highly branched
Ch 18 | # 44 of 52
Fig. 18.24 Two perspectives on the structure of the polysaccaride amylopectin.
Ch 18 | # 45 of 52
Cellulose
• Cellulose is a polymer of glucose molecules linked
by -1,4-glycosidic bonds
• Enzymes in saliva can hydrolyze -1,4 glycosidic
bonds in starch, but not -1,4 glycosidic bonds in
cellulose
Ch 18 | # 46 of 52
Carbohydrates cont’d
Ch 18 | # 47 of 52
Carbohydrates cont’d
Fig. 18.28 The structures of cellulose (a) chitin (b).
Ch 18 | # 48 of 52
Phosphate Ester Formation
• When a cyclic monosaccharide reacts with
phosphoric acid, a phosphate ester is produced
• Phosphodiesters link the monomers in DNA
Ch 18 | # 49 of 52
Amino Sugar Formation
• When a cyclic monosaccharide reacts with an
amine, an amine sugar is produced
Ch 18 | # 50 of 52
Glycolipids and Glycoproteins
• A glycolipid is a lipid molecule that has one or
more carbohydrate (or carbohydrate derivative)
units covalently bonded to it.
• A glycoprotein is a protein molecule that has one
or more carbohydrate (or carbohydrate derivative)
units covalently bonded to it.
Ch 18 | # 51 of 52
Ch 18 | # 52 of 52