Chapter Fourteen
Carbohydrates
Biochemistry: An Overview
Copyright © Houghton Mifflin Company. All rights reserved.
14–2
Biochemistry is the study of the chemical substances
found in living systems and the chemical interactions
of these substances with each other.
A biochemical substance is a chemical substance
found within a living organism. They are divided into
two groups: bioorganic and bioinorganic substances.
Bioinorganic substances include water and inorganic
salts (substances not containing carbon).
Bioorganic substances include carbohydrates, lipids,
proteins, and nucleic acids.
Copyright © Houghton Mifflin Company. All rights reserved.
14–3
Mass composition data for the human body in
terms of major types of biochemical substances.
Data for the human body
Copyright © Houghton Mifflin Company. All rights reserved.
14–4
THERE ARE FOUR MAIN CLASSES
OF BIOMOLECULES
Carbohydrates
Lipids
Proteins
Nucleic Acids
Copyright © Houghton Mifflin Company. All rights reserved.
14–5
Occurrence and Functions of
Carbohydrates
Copyright © Houghton Mifflin Company. All rights reserved.
14–6
Carbohydrates are the most abundant class of
bioorganic molecules overall. They constitute
about 75% by mass of dried plant materials.
Recall that proteins are the most abundant
bioorganic molecules in the human body.
In green (chlorophyll-containing) plants
carbohydrates are produced via photosynthesis.
CO2 + H2O + solar energy chlorophyll carbohydrates + O2
--In the form of cellulose, carbohydrates serve as
structural elements.
--In the form of starch, carbohydrates provide energy
reserves for plants.
Copyright © Houghton Mifflin Company. All rights reserved.
14–7
In humans carbohydrates have the following functions:
• Oxidation of carbohydrates provides energy.
• Storage of carbohydrates as glycogen provides a
Short-term energy reserve.
• Carbohydrates provide carbon atoms for the synthesis
of proteins, lipids, and nucleic acids.
• Carbohydrates form part of the structural framework of
DNA and RNA biomolecules.
• Carbohydrate markers on cell surfaces play key roles in
cell-cell recognition processes.
Copyright © Houghton Mifflin Company. All rights reserved.
14–8
General Types of Carbohydrates
Copyright © Houghton Mifflin Company. All rights reserved.
14–9
A carbohydrate is a polyhydroxy aldehyde, a polyhydroxy ketone, or a compound that yields such a
substance upon hydrolysis. The carbohydrate, glucose
is a polyhydroxy aldehyde and the carbohydrate,
fructose is a polyhydroxy ketone.
aldehyde
H
HO
H
H
CHO
C OH
C H
C OH
C OH
CH2OH
glucose
Copyright © Houghton Mifflin Company. All rights reserved.
CH2OH
C O
HO C H
H C OH
H C OH
CH2OH
ketone
fructose
14–10
CLASSES OF CARBOHYDRATES
Monosaccharides
Disaccarides
Polysaccarides
Copyright © Houghton Mifflin Company. All rights reserved.
14–11
A monosaccharide is a carbohydrate that contains a
single polyhydroxy aldehyde or polyhydroxy ketone
unit such as glucose or fructose.
aldehyde
H
HO
H
H
CHO
C OH
C H
C OH
C OH
CH2OH
glucose
Copyright © Houghton Mifflin Company. All rights reserved.
CH2OH
C O
HO C H
H C OH
H C OH
CH2OH
ketone
fructose
14–12
An oligosaccharide is a carbohydrate that contains from
two to ten monosaccharide units covalently bonded to
each other.
e.g. glucose-glucose-glucose-glucose-glucose
The most common type of oligosaccharides are
disaccharides. A disaccharide is a carbohydrate
that contains two monosaccharide units covalently
bonded to each other. Sucrose and lactose are
disaccharides.
Sucrose = Glucose + Fructose
Copyright © Houghton Mifflin Company. All rights reserved.
14–13
A polysaccharide is a carbohydrate that contains many
monosaccharide units covalently bonded to each other.
They can consist of tens of thousands of monosaccharide
units. For example, cellulose and starch are polysaccharides.
Paper, cotton, and wood are primarily cellulose.
Starch is a component of bread, pasta, rice, corn, beans, etc.
Copyright © Houghton Mifflin Company. All rights reserved.
14–14
Classification of
Monosaccharides
Copyright © Houghton Mifflin Company. All rights reserved.
14–15
Introduction to Carbohydrates
• Carbohydrates are a large class of naturally
occurring polyhydroxy aldehydes and
ketones.
• Monosaccharides also known as simple
sugars, are the simplest carbohydrates
containing 3-7 carbon atoms.
• A sugar containing an aldehyde is known as
an aldose.
• A sugar containing a ketone is known as a
ketose.
Copyright © Houghton Mifflin Company. All rights reserved.
14–16
CARBOHYDRATE TERMINOLOGY

The family name ending -ose indicates a
carbohydrate.
• A sugar containing an aldehyde and five
carbons is called an aldopentose.
• A sugar containing an aldehyde and six
carbons is called an aldohexose.
• Simple sugars are known by common names
such as glucose, ribose, fructose, etc.
• The term sugar is also associated with
sweetness and many mono and disaccharides
have a sweet taste.
Copyright © Houghton Mifflin Company. All rights reserved.
14–17
• The number of carbon atoms in an aldose
or ketose may be specified by tri, tetr,
pent, hex, or hept. For example, glucose
is an aldohexose and fructose is a
ketohexose.
• Monosaccharides react with each other to
form disaccharides and polysaccharides
(splitting out water – a condensation
reaction).
• Monosaccharides are chiral molecules
and exist mainly in cyclic forms rather
than the straight chain.
Copyright © Houghton Mifflin Company. All rights reserved.
14–18
An Historical Aside: Most sugars have the suffix -ose
Albert Szent-Györgyi (1893-1986) was an Hungarian-American
chemist who won the 1937 Nobel Prize in Physiology or Medicine
for his isolation of vitamin C (ascorbic acid). He thought it might
be a sugar.
He isolated the compound, but didn’t know its structure.
“Originally I called ascorbic acid “ignose,” not knowing what it was
(ignosco meaning, I don’t know). The editor of the Biochemistry
Journal reprimanded me for making jokes about science. My
proposition “Godnose” was no more successful.”
How he came to identify vitamin C: He had tried to isolate it from
a number of plant sources “One night my wife gave me paprika for
supper and I had no desire to eat it, but had no courage to say no.
So I told my wife that instead of eating it I would take it to the
laboratory. By midnight I knew that it was a treasure trove of
ascorbic acid.”
Copyright © Houghton Mifflin Company. All rights reserved.
14–19
Handedness (“Chirality”)
in Monosaccharides
Copyright © Houghton Mifflin Company. All rights reserved.
14–20
Most monosaccharides exist in two forms; i.e. a
left-handed and a right-handed form. They are
related in the same way that your left hand and right
hand are related to each other; i.e. as mirror
images. The hands are similar, but not identical
because they are not “superimposable.”
All objects have mirror images, but in some cases
the mirror images are superimposable on the
object, and in other cases the mirror image is not
superimposable on the object.
For example, a flat dinner plate has a
superimposable mirror image, but your hands do
not.
Copyright © Houghton Mifflin Company. All rights reserved.
14–21
The mirror image of the right hand is the left hand.
Conversely, the mirror image of the left hand is the
right hand.
Hands are not
superimposable.
Copyright © Houghton Mifflin Company. All rights reserved.
14–22
A person’s left and right hands are not
superimposable upon each other.
The fingers and
the thumbs do
not match up,
and therefore
your hands are
mirror images of
each other, but
they are not
superimposable.
Copyright © Houghton Mifflin Company. All rights reserved.
14–23
A simple example of a molecule that possesses
handedness (also called chirality) is the trisubstituted
methane molecule, bromochloroiodomethane
Br
Cl-C-H
I
Notice that this molecule, just like your hands, is not
superimposable upon its mirror image.
I
H
C Cl
Br
Cl
H
C I
Br
mirror
Copyright © Houghton Mifflin Company. All rights reserved.
14–24
Examples of simple molecules that have
nonsuperimposable mirror images. (a) The molecule
bromochloroiodomethane. (b) The molecule
glyceraldehyde.
models
Copyright © Houghton Mifflin Company. All rights reserved.
models
14–25
A carbon atom containing four different groups, as in
bromochloroiodomethane and glyceraldehyde, is called
a chiral center. A chiral center is a carbon atom in a
molecule that has four different groups attached to it.
A molecule that contains a chiral center is said to be chiral.
A chiral molecule is a molecule whose mirror image is not
superimposable upon itself. Most chiral molecules contain
carbon atoms that have four different groups attached to
them.
An achiral molecule is a molecule whose mirror image
is superimposable upon itself. Achiral molecules generally
do not possess carbon atoms containing four different
groups.
Each of the two nonsuperimposable forms (the left handed
and the right handed forms) are called enantiomers.
Copyright © Houghton Mifflin Company. All rights reserved.
14–26
Handedness in Carbohydrates
• Carbohydrates are chiral molecules
since they have carbon atoms carrying
four different groups.
• The simplest three-carbon, naturally
occurring carbohydrate, glyceraldehyde,
has a chiral carbon atom and exists as a
pair of enantiomers – a “right-handed” D
form and a “left-handed” L form.
Copyright © Houghton Mifflin Company. All rights reserved.
14–27
By convention the D-enantiomer has the OH group on the
right hand side of the chiral carbon atom. And therefore by
convention the L-enantiomer has the OH group on the left
hand side of the chiral carbon atom. D derives from the
Latin, dextro = right and L derives from the Latin, levo = left.
Copyright © Houghton Mifflin Company. All rights reserved.
14–28
• Two forms of glyceraldehyde (D and L)
have the same physical properties except
they behave differently in the presence of
a polarized light.
• The two forms of glyceraldehyde rotate
the plane of a polarized light in opposite
directions by the same amount.
• An instrument known as a polarimeter
can be used to measure the degree of
rotation of the plane of a polarized light.
Copyright © Houghton Mifflin Company. All rights reserved.
14–29
Principles of a polarimeter, used to determine
optical activity. A solution of an optically active
isomer rotates the plane of the polarized light by a
characteristic amount.
Copyright © Houghton Mifflin Company. All rights reserved.
14–30
As the number of carbon atoms in a
monosaccharide increases, the number of
chiral centers present in the molecule
increases. For example, glucose has four
chiral centers.
• In general, compounds with n chiral
carbon atoms have a maximum of 2n
possible stereoisomers and half that
many pairs of enantiomers.
• Glucose, has four chiral carbon atoms
and a total of 24 = 16 possible
stereoisomers (8 pairs of enantiomers).
Copyright © Houghton Mifflin Company. All rights reserved.
14–31
In a multi-chiral-center molecule such as glucose,
handedness is determined by the chiral center most
distant from the carbonyl group.
D
H
HO
H
H
CHO
C OH
C H
C OH
C OH
CH2OH
HO
H
HO
HO
CHO
C H
C OH
C H
C H
CH2OH
L
mirror
D-glucose
Copyright © Houghton Mifflin Company. All rights reserved.
L-glucose
14–32
D
L
D
L
Two pairs of enantiomers: the four isomeric
aldotetroses, 2,3,4-trihydroxybutanals.
Copyright © Houghton Mifflin Company. All rights reserved.
14–33
Why this is Important:
Right-handed and left-handed molecules elicit different
biological responses.
For example, D-epinephrine (adrenaline) is 20 times more
potent in its biological effect than is its L-isomer.
All naturally-occurring carbohydrate molecules are
made up of right-handed (D) enantiomers.
Many drugs are active only in one of their two chiral
forms: Lipitor, the cholesterol-lowering drug, is only
active in its left-handed form.
Copyright © Houghton Mifflin Company. All rights reserved.
14–34
Thalidomide - A Case in Point
• This drug comes in two mirror-image forms. It was used in Europe in the
1950s to relieve morning sickness in pregnant women, but was found to cause
severe birth defects (deformed arms and legs) in some cases.
• It was not distributed in the U.S. because an alert inspector, Dr. Francis
Kelsey, at the U.S. FDA delayed the drug’s release.
• Originally it was believed that one enantiomer was effective and the other
caused birth defects.
• Later studies have shown that the body rapidly converts one form into the
other (racimizes the drug) so that the given form doesn’t matter.
• Ironic twist: Recently thalidomide has been found to reduce the immune
system’s inflammatory response in a host of illnesses, including arthritis, lupus,
cancer, leprosy, and AIDs.
Note: A racimic mixture contains both forms of a chiral compound. (The
mixture is sometimes called a “racemate.”)
Copyright © Houghton Mifflin Company. All rights reserved.
14–35
Historical Note
• Louis Pasteur first separated two optical isomers (enatiomers)
of the sodium ammonium salt of tartaric acid in 1848. He
mechanically separated crystals of the d- and l- forms of the
compound by hand.
• Tartaric acid, H2C4H4O6, is also called 2,3-dihydroxybutanedioic
acid:
HOOC--CHOH-CHOH-COOH
• Pasteur was lucky!
--Many, if not most, such salts crystallize as racemic crystals,
not as separate d- and l- crystals
--He worked in the cool Paris climate; above 26° C the salt he
worked with itself crystallizes as a racemate.
Ref. Kauffman & Myers, J. Chem. Educ. 52, 777-781 (1975)
Copyright © Houghton Mifflin Company. All rights reserved.
14–36
The D and L Families of Sugars:
Drawing Sugar Molecules
• Fisher Projection: represent threedimensional structures of stereoisomers
on a flat page.
• A chiral carbon atom is represented in
the Fisher projection as the intersection
of two crossed lines. Bonds that point
up out of the page are shown as
horizontal lines, and bonds that point
behind the page are shown as vertical
lines, see the following scheme.
Copyright © Houghton Mifflin Company. All rights reserved.
14–37
Copyright © Houghton Mifflin Company. All rights reserved.
14–38
In a Fisher projection, the aldehyde or
ketone carbonyl group of a monosaccharide
is always placed at the top.
Copyright © Houghton Mifflin Company. All rights reserved.
14–39
Note
There is no correlation between the D
and L designation and the direction of
rotation of the plane of polarized light.
The D and L terms relate only to the
position of the –OH group on the bottom
carbon in a Fisher projection.
Copyright © Houghton Mifflin Company. All rights reserved.
14–40
Elephant Pheromones
Copyright © Houghton Mifflin Company. All rights reserved.
14–41
Names and Structures of
Biochemically Important
Monosaccharides
Copyright © Houghton Mifflin Company. All rights reserved.
14–42
Structure of Glucose and Other Monosaccharides
• D-Glucose, sometimes called dextrose or blood
sugar, is the most widely occurring of all
monosaccharides.
• In nearly all living organisms, D-glucose serves
as a source of energy for all biochemical
reactions.
• D-glucose is stored in polymeric form as starch
in plants and as glycogen in animals.
Copyright © Houghton Mifflin Company. All rights reserved.
14–43
H
HO
H
H
CHO
C OH
C H
C OH
C OH
CH2OH
D-glucose a.k.a. dextrose
Most dietary carbohydrates are
polymers of D-glucose. Upon
digestion the D-glucose units
are released, which become
the major energy source to
fuel daily activities.
Copyright © Houghton Mifflin Company. All rights reserved.
14–44
H
HO
HO
H
CHO
C OH
C H
C H
C OH
CH2OH
D-galactose
H
HO
H
H
CHO
C OH
C H
C OH
C OH
CH2OH
D-glucose
D-galactose differs from D-glucose in the configuration of
only one carbon atom. The chiral carbon atom at C-4 in
D-galactose has the opposite chirality from that in D-glucose.
D-galactose and D-glucose are isomers because these two
aldohexoses have the same molecular formula, C 6H12O6. This
very small difference in chemical structure is enough to give
these two molecules very different biological properties.
Copyright © Houghton Mifflin Company. All rights reserved.
14–45
H
HO
HO
H
CHO
C OH
C H
C H
C OH
CH2OH
H
HO
H
H
CHO
C OH
C H
C OH
C OH
CH2OH
CH2OH
C O
HO C H
H C OH
H C OH
CH2OH
D-galactose D-glucose D-fructose
D-fructose is also a hexose, but it is not an aldohexose.
It is a ketohexose. The only difference in chemical
structure between D-glucose and D-fructose is at
carbons 1 and 2. All three of these monosaccharides
are structural isomers because they have the same
molecular formula, C6 H12O6.
Copyright © Houghton Mifflin Company. All rights reserved.
14–46
Chemical Portraits:
Three Important Isomeric Monosaccharides
Copyright © Houghton Mifflin Company. All rights reserved.
14–47
H
HO
H
H
CHO
C OH
C H
C OH
C OH
CH2OH
D-glucose
CHO
H C OH
H C OH
H C OH
CH2OH
D-ribose
CHO
H C H
H C OH
H C OH
CH2OH
2-deoxy-D-ribose
D-glucose is an aldohexose, but D-ribose is an
aldopentose. D-ribose is a component of RNA. 2Deoxy-D-ribose is also an important monosaccharide
found in DNA. The prefix deoxy indicates that it is the
same as D-ribose except that it is missing an oxygen
atom at the 2 position.
Copyright © Houghton Mifflin Company. All rights reserved.
14–48
Cyclic Forms of
Monosaccharides
Copyright © Houghton Mifflin Company. All rights reserved.
14–49
Up until now the structures of monosaccharides have been
depicted as open-chain polyhydroxy aldehydes and ketones.
Experimental studies indicate however, that in solution
Monosaccharides containing five or more carbons exist
predominantly as cyclic structures rather than open-chain
structures.
The cyclic forms of monosaccharides result from the tendency
of the carbonyl group to react with a neighboring hydroxyl
group to form a hemiacetal (aldoses) or a hemiketal (ketoses).
H
H
HO
H
H
O
C
C OH
C H
C OH
C OH
CH2OH
Open-chain form
Copyright © Houghton Mifflin Company. All rights reserved.
H OH
HO
O
HO
CH2OH
OH
Cyclic hemiacetal form
14–50
Figure 14.7 The cyclic hemiacetal forms of dglucose result from the intramolecular reaction
between the carbonyl group and the hydroxyl group
on carbon 5.
The cyclic structures
can exist in two
forms called
anomers.
alpha = OH and CH2OH opposite sides, beta = same side
Copyright © Houghton Mifflin Company. All rights reserved.
14–51
• Ordinarily, crystalline glucose is entirely in the a-form.
• When dissolved in water, an equilibrium becomes
established between the open chain and the two cyclic
forms of glucose. The optical rotation of a fresh solution
of a (or b) glucose gradually changes from its original
value until it reaches a value representing the equilibrium
mixture.
• The equilibrium solution has 37% alpha and 63% beta,
with only about 0.01% of the straight chain.
• Monosaccharides form white crystalline water-soluble
solids.
Most monosaccharides are sweet-tasting,
digestible, and non-toxic
Copyright © Houghton Mifflin Company. All rights reserved.
14–52
•The structure of D-galactose: Just like Dglucose it can also exist as an open chain
hydroxy aldehyde or as a pair of cyclic
hemiacetals.
Copyright © Houghton Mifflin Company. All rights reserved.
14–53
• Anomers: Cyclic sugars that differs only in
positions of substituents at the hemiacetal
carbon; the a-form has the –OH group on the
opposite side from the –CH2OH; the b-form has
the –OH group on the same side as the –CH2OH
group.
Copyright © Houghton Mifflin Company. All rights reserved.
14–54
D-fructose is a ketohexose and it too
can exist in cyclic forms, but it has mostly
five membered rings instead of six
membered rings.
CH2OH
C O
HO C H
H C OH
H C OH
CH2OH
D-fructose
Copyright © Houghton Mifflin Company. All rights reserved.
14–55
D-ribose is an aldopentose and it too cyclizes into
a cyclic five membered ring structure.
CHO
H C OH
H C OH
H C OH
CH2OH
D-ribose
Copyright © Houghton Mifflin Company. All rights reserved.
HO
O
OH
OHOH
beta-D-ribose
14–56
14.7 Some Important Monosaccharides
Monosaccharides are generally high-melting,
white, crystalline solids that are soluble in water
and insoluble in nonpolar solvents.
Most
monosaccharides are sweet tasting, digestible, and
nontoxic.
Copyright © Houghton Mifflin Company. All rights reserved.
14–57
Corrections to the text –
Question 12.59, parts (c) and (d) The answers given in the back of
the book (Appendix 5) for these questions label the compounds as
“chlorides.” They should be called “bromides.” Thanks to M. Crabtrey.
Question 14.17. Skip this question, it is not well defined.
Thanks to C. Hager
Page 399: The compound on the left in the middle of the page is
galactose, not glucose. Thanks to G. Bradds
Question 15.5. The answers to (a) and (d) in the back of the book are
incorrect. They should read “saturated.” Thanks to G. Bradds
Question 13.17. The answer in the back of the book omits a –CH2group. Thanks to J. Scarlett.
Copyright © Houghton Mifflin Company. All rights reserved.
14–58
14.8 Reactions of
Monosaccharides
Copyright © Houghton Mifflin Company. All rights reserved.
14–59
14.8 Reactions of Monosaccharides
• Reactions with Oxidizing Agents: Reducing
Sugars
• When an open chain aldehyde form of an
aldose monosaccharide, is oxidized, its
equilibrium with the cyclic form is displaced.
The aldehyde group of the monosaccharide is
ultimately oxidized to a carboxylic acid
group.
Copyright © Houghton Mifflin Company. All rights reserved.
14–60
H
H
HO
H
H
O
C
C OH
C H
C OH
C OH
CH2OH
D-glucose
O
weak oxidizing
agent
H
HO
H
H
OH
C
C OH
C H
C OH
C OH
CH2OH
D- gluconic acid
Since the sugar reduces the oxidizing agent, the sugar
is called a reducing sugar. A reducing sugar is a
carbohydrate that gives a positive test with Tollens
reagent (Ag+) and Benedict’s reagent (Cu++). Tollens
and Benedict’s reagents can be used to check for
D-glucose in the urine (a symptom of diabetes).
Copyright © Houghton Mifflin Company. All rights reserved.
14–61
Benedict’s: Cu++
Cu+ red precipitate
Tollens: Ag+
Ag silver mirror
With Benedict’s reagent the solution remains blue if
the urine is normal (contains no D-glucose) and it gives
a red precipitate if it contains D-glucose. This is such a
common laboratory test that it has been simplified by
the use of plastic dip strips.
Copyright © Houghton Mifflin Company. All rights reserved.
14–62
The glucose content of urine can be determined by
dipping a plastic strip treated with oxidizing agents
into the urine sample and comparing the color change
of the strip to a color chart that indicates glucose
concentration.
Copyright © Houghton Mifflin Company. All rights reserved.
14–63
The carbonyl group in monosaccharides
can be reduced to an hydroxy group.
H
H
HO
H
H
O
C
C OH
C H
C OH
C OH
CH2OH
hydride reducing
D-glucose
Copyright © Houghton Mifflin Company. All rights reserved.
agent
H
HO
H
H
CH2OH
C OH
C H
C OH
C OH
CH2OH
D- glucitol
14–64
D-glucitol is also known by its common name of
sorbitol. It is used in chewing gum because
bacteria that cause tooth decay do not use sorbitol
as a food source. It is also used in other foods and it
is used in cosmetics as a moisturizing agent due to its
high affinity for water. Sorbitol accumulation in the
eye is a major factor in the formation of cataracts due
to diabetes.
H
H
HO
H
H
O
C
C OH
C H
C OH
C OH
CH2OH
D-glucose
Copyright © Houghton Mifflin Company. All rights reserved.
hydride reducing
agent
H
HO
H
H
CH2OH
C OH
C H
C OH
C OH
CH2OH
D- glucitol
14–65
CONDENSATION REACTIONS
--Momosaccharides can be linked to each
other through condensation reactions, i.e.,
reactions that split out water molecules and
join parts.
--In disaccharides and polysaccharides,
monosaccharides are connected to each
other through glycosidic bonds.
--Hydrolysis is the reverse of
condensation
reaction;
digestion
carbohydrates involves hydrolysis.
Copyright © Houghton Mifflin Company. All rights reserved.
a
of
14–66
PHOSPHATE ESTERS
--The –OH group of a sugar can add a –PO3H2
group to form phosphate esters.
--Phosphate esters of monosaccharides appear
as reactants and products throughout the
metabolism of carbohydrates.
CH2OH
O
OH
OH
O P OH
HO
OH O
alpha-D-glucose-1-phosphate
Copyright © Houghton Mifflin Company. All rights reserved.
OH
CH2O P OH
O O
OH
OH
HO
OH
alpha-D-glucose-6-phosphate
14–67
Disaccharides
Copyright © Houghton Mifflin Company. All rights reserved.
14–68
Disaccharides are made up of two monosaccharides. For example, sucrose, table sugar,
is a disaccharide made up of one glucose and one
fructose.
Sucrose = D-glucose-D-fructose
Lactose = D-galactose-D-glucose
Maltose = D-glucose-D-glucose
Copyright © Houghton Mifflin Company. All rights reserved.
14–69
Most fruits and fresh vegetables contain
mono and disaccharides.
The glycosidic link is formed as a
condensation reaction between two –OH
groups:
R-OH + HO-R’  R-O-R’ + H2O
Copyright © Houghton Mifflin Company. All rights reserved.
14–70
CH2OH
O
OH
OH
HO
OH
CH2OH
O
OH
OH
HO
OH
CH2OH
O
OH
O
HO
OH
CH2OH
O
OH
OH
OH
+ H2O
Two monosaccharides can be coupled to form a
disaccharide. They are connected by a glycosidic
linkage. The glycosidic linkage is the carbonoxygen-carbon bond that joins the two components
of a glycoside together.
Copyright © Houghton Mifflin Company. All rights reserved.
14–71
The three naturally occurring common
disaccharides are:
--Maltose: Two a-glucoses are joined by
an a-1,4-link.
CH2OH
O
OH 1
O
HO
OH
CH2OH
O
4 OH
OH
OH
maltose = a disaccharide
maltose = D-glucose-D-glucose
Copyright © Houghton Mifflin Company. All rights reserved.
14–72
The three forms of maltose present
in aqueous solution.
Copyright © Houghton Mifflin Company. All rights reserved.
14–73
Lactose, also known as milk sugar: It is the major
carbohydrate found in mammalian milk. Two
monosaccharides are joined by a beta-1,4-link. The
enzyme lactase can hydrolyze lactose to form an
equal mixture of D-galactose and D-glucose. Some
adults lack lactase and are “lactose intolerant.”
CH2OH
HO
O OH
OH
OH
CH2OH
O
OH
OH
HO
OH
CH2OH
HO
O O
OH
CH2OH
O
OH
OH
OH
OH
lactose = a disaccharide
Lactose = D-galactose-D-glucose
Copyright © Houghton Mifflin Company. All rights reserved.
14–74
Sucrose, table sugar: Sugar beets and sugar cane
are the most common sources of sucrose. One
molecule of D-fructose and one molecule of
D-glucose are joined together by a 1,2-link between
the anomeric carbons.
CH2OH
O
OH
sucrose = a non-reducing sugar
HO
OH
HO
O
O
HO
CH2OH
sucrose = D-glucose-D-fructose
OH
The enzyme sucrase is present in the human body
to break down sucrose into equal amounts of
glucose and fructose.
Copyright © Houghton Mifflin Company. All rights reserved.
14–75
Chemical Portraits:
Biologically Imported Disaccharides
Copyright © Houghton Mifflin Company. All rights reserved.
14–76
Polysaccharides
Copyright © Houghton Mifflin Company. All rights reserved.
14–77
Some Important Polysaccharides
Polysaccharides are polymers of many
monosaccharides linked together through
glycosidic bonds. The three most important
polysaccharides are cellulose, starch, and
glycogen.
• Cellulose is a fibrous substance that
provides structure in plants. It consists
entirely of several thousand β-units joined
together in a long straight chain by β -1,4links (it is unbranched).
Copyright © Houghton Mifflin Company. All rights reserved.
14–78
IT’S IN THE BONDS!
Cotton is almost pure cellulose and wood is about
50% cellulose. Even though cellulose is a polymer
of glucose units attached in a long chain, it is not
nutritional for humans because we cannot
hydrolyze the beta (14) linkage. We lack the
proper enzymes.
Horses, cows, and sheep contain bacteria in their
digestive systems that have special enzymes
(cellulases) and therefore can digest cellulose. In
this way termites can use wood as a source of
glucose for nutritional needs.
Copyright © Houghton Mifflin Company. All rights reserved.
14–79
The structures of cellulose (a) and chitin (b).
In both substances, all glycosidic linkages
are of the b(1  4) type.
These polymers are made of long linear, unbranched chains.
Copyright © Houghton Mifflin Company. All rights reserved.
14–80
Starch
--Starch, like cellulose, is a polymer of
glucose.
--Starch is fully digestible and is an
essential part of the human diet.
--In starch, the glucose units are joined by
a-1,4-links. (We can digest these.)
Copyright © Houghton Mifflin Company. All rights reserved.
14–81
Amylose and Amylopectin
--Two different glucose polysaccharides can be isolated from
most starches: amylose and amylopectin.
--Amylose is a straight chain glucose polymer and usually
accounts for 15-20% of the starch.
--Amylopectin is a highly branched glucose polymer and
accounts for about 80-85% of the starch.
--The glycosidic linkages in amylose are alpha 14, but in
amylopectin they are alpha 14 and alpha 16.
--These alpha linkages can be broken down in the
human digestive tract by the enzyme amylase. Starches
contained in potatoes, rice, corn, wheat, etc. are major
food sources.
Copyright © Houghton Mifflin Company. All rights reserved.
14–82
Some polysaccharides have a linear chain
structure (a); others have a branched chain
structure (b).
amylose
Copyright © Houghton Mifflin Company. All rights reserved.
amylopectin
14–83
Two perspectives on the structure of the polysaccharide
amylopectin. (a) Molecular structure of amylopectin. (b) An
overview of the branching that occurs in the amylopectin
structure. Each circle is a glucose unit.
Copyright © Houghton Mifflin Company. All rights reserved.
14–84
GLYCOGEN
--Glycogen, also called animal starch, serves as the energy
storage role that starch serves in plants. Some of the
glucose from starches we eat is used immediately as fuel,
and some is stored as glycogen for later use. It is stored in
muscle cells and in liver cells.
--Glycogen has a structure similar to amylopectin in that it
contains both alpha (14) and alpha (16) glycosidic
linkages. Glycogen is about three times more highly
branched than amylopectin and it has a much larger
molecular weight.
--When excess glucose is present in the blood from eating
too much starch, it is converted to glycogen and stored for
future use. When needed, it is converted back to glucose.
Copyright © Houghton Mifflin Company. All rights reserved.
14–85
Variations on the Carbohydrate Theme
--Monosaccharides with modified functional groups
are components of a wide variety of biomolecules.
--Short chains of monosaccharides, known as
oligosaccharides, enhance the function of proteins
and lipids to which they are bonded.
--In
some
cases,
oligosaccharides
recognition sites on cell surfaces.
Copyright © Houghton Mifflin Company. All rights reserved.
form
14–86
EXAMPLES
--Chitin: the hard shells of lobsters, beetles,
and spiders.
--Heparin: an agent that prevents or retards
the clotting of blood.
--Glycoproteins: perform important function
at the cell surface. They can function as
receptors for molecular messengers or
drugs. They are also responsible for the
familiar A, B, O system of typing blood.
Copyright © Houghton Mifflin Company. All rights reserved.
14–87
Chemistry at a
Glance:
Biochemically
Important
Carbohydrates
Copyright © Houghton Mifflin Company. All rights reserved.
14–88
Chapter Summary
• Monosaccharides are compounds with 3 to 7
carbon atoms, an aldehyde group on carbon 1
or a ketone on carbon 2, and hydroxyl group on
all other carbon atom.
• Monosaccharides are chiral molecules.
A
monosaccharide with n chiral carbon atom may
have 2n stereoisomers, half that number of pair
of enantiomers.
• Fisher projection formula represent open-chain
structures of monosaccharides.
Copyright © Houghton Mifflin Company. All rights reserved.
14–89
Chapter Summary Continued
• In solution, open-chain monosaccharides with
five or six carbons establish equilibria with
cyclic forms that are hemiacetals.
• The hemiacetal carbon is refereed to as the
anomeric carbon.
• Oxidation of a monosaccharide can results in a
carboxyl group on the carbon 1.
• Reaction with an alcohol converts the –OH
group on the anomeric carbon to a –OR group
through a bond known as glycosidic bond.
Copyright © Houghton Mifflin Company. All rights reserved.
14–90
Chapter Summary Continued
• Disaccharides result from glycosidic bond
formation between two monosaccharides.
Polysaccharides result from glycosidic bond
formation between many monosaccharides.
• Chitin is a hard structural polysaccharide found
in the shells of lobster and insects.
• Heparin is a polysaccharide that plays a role in
blood clotting.
• Glycoproteins have short carbohydrate chains
bonded to proteins; the carbohydrate segments
function as receptors at cell surfaces.
Copyright © Houghton Mifflin Company. All rights reserved.
14–91
Chapter Summary Continued
• Cellulose is a fibrous substance that provides
structure in plants. They consist entirely of
several thousand b-units joined together in a
long straight chain by b-1,4-links.
• Starch, like cellulose, is a polymer of glucose.
Starch is fully digestible and is an essential part
of the human diet. In starch, glucose units are
joined by a-1,4-links.
• Glycogen, also called animal starch, serves as
the energy storage role as starch serves in
plants.
Copyright © Houghton Mifflin Company. All rights reserved.
14–92
What you absolutely must know from this chapter:
• Carbohydrates are the most abundant bioorganic compounds in
the living world.
• They are formed by photosynthesis:
6CO2 + 6H2O  C6H12O6 + 6O2
(light)
• You should know what roles carbohydrates play in plants and
animals.
• There are five important monosaccarides (glucose, galactose,
fructose, ribose, and deoxyribose). Know whether each is an
aldose or a ketose, and how many carbon atoms each has.
• Know what “chirality” and “optical activity” refer to, and how
these terms apply to sugars. Know what a “chiral center” is.
• Understand why chirality is important in biochemistry and for the
chemistry of life in general.
Copyright © Houghton Mifflin Company. All rights reserved.
14–93
• Understand that in water monosaccarides exist predominantly in
cyclic forms, rather than the linear forms pictured early in the
chapter.
• Appreciate that aldose sugars can be oxidized to carboxylic
acids (in the same way that aldehydes react in general - see
chapter 13).
• Understand that monosaccharides can be joined together by
condensation reactions between their OH groups, splitting out
water:
R-OH + HO-R’  R-O-R’ +H2O
The links are called a or b glycosidic bonds, depending on their
geometry.
• Know the three most important disaccharides:
sucrose = (glucose)-(fructose)
lactose = (galactose)-(glucose)
maltose = (glucose)-(glucose)
Copyright © Houghton Mifflin Company. All rights reserved.
14–94
• Understand that the glycosidic bonds between these units can
be broken by hydrolysis--addition of a water molecule. Note
that this is the reverse of a condensation reaction.
• Note the following:
--Sucrose is common table sugar.
--Fructose is the most common sweetening agent in colas,
etc., because it is sweeter than sucrose and cheaper.
--Many people become “lactose intolerant” as they age
because they lose the ability to produce the enzyme lactase,
which digests lactose.
--Blood glucose levels are regulated by the hormone insulin.
• Appreciate that polysaccharides are polymers composed of
chains (possibly branched) of monosaccharides (their monomer
units).
• Know the distinctive characteristics of the principal
polysaccharides: cellulose, starch, glycogen, and chitin, and
where they are found.
Copyright © Houghton Mifflin Company. All rights reserved.
14–95
• Understand why we can’t digest cellulose (or chitin, for that
matter) and how grazing animals can digest it by using bacteria
in their guts.
• Understand the differences between amylose and
amylopectin, the two forms of polysaccharides found in starch.
Copyright © Houghton Mifflin Company. All rights reserved.
14–96
To Do List
• Read chapter 14!!
• Do additional problems
• Do practice test T/F
• Do practice test MC
• Review Lecture notes for
Chapter Fourteen
Copyright © Houghton Mifflin Company. All rights reserved.
14–97