4. Functions of Carbohydrates
● Carbohydrates have four important functions in living organisms:
▪ To provide energy through their oxidation.
▪ To supply carbon for the synthesis of cell components.
▪ To serve as a stored form of chemical energy.
▪ To form a part of the structural elements of some cells and tissues.
5. Definition
● A carbohydrates is a polyhydroxy aldehyde or polyhydroxy ketone or
compound that yield such derivatives upon hydrolysis.
6. Classification of Carbohydrates
● Carbohydrates are classified on the basis of molecular size as:
1. Monosaccharides
2. Disaccharides
3. Oligosaccharides
4. Polysaccharides
7. Monosaccharides
“mon-oh-SACKuh-ride”
● A monosaccharide is a carbohydrate that contains a single
polyhydroxy aldehyde or polyhydroxy ketone unit.
● Monosaccharides cannot be hydrolyzed further into simpler
sugars.
● Naturally occurring monosaccharides have from three to seven
carbon atoms; five- and six-carbon species are especially
common.
8. Nomenclature & Classification of
Monosaccharides
● Monosaccharides with an aldehydic
carbonyl or potential aldehydic
carbonyl group are called aldoses.
● Monosaccharides with a ketonic
carbonyl or potential ketonic carbonyl
group are called ketoses.
9. Nomenclature & Classification of
Monosaccharides
● Monosaccharides are further classified according to the
number of carbon atoms they contain:
11. “
”
Stereochemistry of Carbohydrates
❖ Optical isomers, also called stereoisomers, are compounds that have the
same molecular and structural formula but differ in the orientation of the
atoms in space.
12. D & L Isomerism
● D & L designations are based on the configuration about the single
asymmetric carbon in glyceraldehyde.
13. D & L Isomerism
● For sugars with more than one chiral
center, D or L refers to the
asymmetric carbon farthest from the
aldehyde or keto group.
● Most naturally occurring sugars are D
isomers.
15. Diastereomer
“dye-a-STEER-ee-o-mer”
● Stereoisomers that are not mirror images of each other are
described as diastereomers .
● Molecules that contain more than one chiral center can also exist
in diastereomeric as well as enantiomeric forms.
16. Van’t Hoff’s Rule of ‘n’
● In general, a compound that has n chiral centers may exist in a maximum of 2n
stereoisomeric forms.
● For example, when two chiral centers are present, at most four stereoisomers (22
=
4) are possible (two pairs of enantiomers).
19. Optical Activity
● Enantiomers are said to be optically active because of the way they
interact with plane-polarized light.
● An enantiomer that rotates plane-polarized light in a clockwise direction
(to the right) is said to be dextrorotatory (+) enantiomer.
● An enantiomer that rotates plane-polarized light in a counterclockwise
direction (to the left) is said to be levorotatory (-) enantiomer.
20. Optical Activity
● Not all D enantiomers rotate plane-polarized light in the same
direction, nor do all L enantiomers rotate plane-polarized light in
the same direction.
● Some D enantiomers are dextrorotatory (+); others are levorotatory
(-).
● There is no way of knowing which way an enantiomer will rotate
light until it is examined with a polarimeter.
22. Epimers
● Diastereomers that differ from each other in the configuration at
only one chiral carbon are called Epimers.
● Examples
▪ D-glucose and D-mannose are epimers with respect to C2.
▪ D-glucose and D-galactose are epimers with respect to C4.
24. Cyclization of Monosaccharides
● Alcohols react readily with aldehydes to form hemiacetals:
● Likewise, alcohols react with ketones to produce hemiketals:
25. Cyclization of Monosaccharides
● Monosaccharides have hydroxyl and carbonyl groups in the same molecule.
● Therefore, they can undergo intermolecular reaction to form cyclic hemiacetals (from
aldoses) and hemiketals (from ketoses).
● Cyclic forms with a:
● three-membered ring are called oxiroses,
● four-membered ring oxetoses,
● five-membered ring furanoses,
● six-membered ring pyranoses,
● seven-membered ring septanoses,
● eight-membered ring octanoses, and so on.
26. Cyclization of Monosaccharides
● Five- and six-membered cyclic hemiacetals are relatively strain-
free and particularly stable.
● Glucose, for instance, exists in aqueous solution primarily in the
six-membered, pyranose form resulting from intermolecular
nucleophilic addition of the –OH group at C5 to the C1 carbonyl
group.
● Some monosaccharides also exist in a five-membered cyclic
hemiacetal form, called a furanose form.
27.
28. Anomers
● Regardless of whether hemiacetal or hemiketal is formed, the carbonyl
carbon (of open-chain form) becomes chiral in this process, and is
referred to as anomeric carbon.
● The two stereoisomers are referred to as anomers, designated α or β
according to the configurational relationship between the anomeric
centre and a specified anomeric reference atom.
● For example, Glucose forms a six-membered ring with two anomeric
forms:
● α−D-Glucoyranose
● β−D-Glucopyranose
29. Anomers
(Fischer Projection Formula)
● The Fischer projection of the α-anomer of a D sugar (e.g D-
Glucose) has the anomeric hydroxyl group to the right of the
anomeric carbon, and the β-anomer of a D Sugar has the
anomeric hydroxyl group to the left of the anomeric carbon.
31. Anomers
(Haworth Projection Formula)
● In glucose, the α-anomers has the —OH group of C-1 trans to the CH2
OH
group attached to carbon 5.
● In glucose, the β-anomers has the —OH group of C-1 cis to the CH2
OH
group attached to carbon 5.
32. Mutarotation
● Mutarotation is the change in specific rotation that accompanies the equilibration
of α and β anomers in aqueous solution.
● For example, a solution prepared by dissolving crystalline α-D-glucopyranose in
water has a specific rotation of +112°, which gradually decreases to an
equilibrium value of +52.7° as α-D-glucopyranose reaches equilibrium with β-D-
glucopyranose.
● A solution of β-D-glucopyranose also undergoes mutarotation, during which the
specific rotation changes from +18.7° to the same equilibrium value of +52.7°.
33. Mutarotation
● The equilibrium mixture of glucose consists of:
● 64% (two-third) β-D-glucopyranose ,
● 36% (one-third) α-D-glucopyranose and
● only a trace (0.003%) of the open-chain form.
● Mutarotation is common to all carbohydrates that exist in
hemiacetal forms.
● Some cells contains mutarotases that acceletrate the
interconversion of anomeric sugars.
35. BIOCHEMICALLY IMPORTANT MONOSACCHARIDES
D-Ribose and Deoxyribose
● Two pentoses, ribose and deoxyribose, are extremely important because
they are used in the synthesis of nucleic acids (DNA and RNA).
● D-Ribose is a component of a variety of complex molecules, including
ribonucleic acids (RNAs) and energy-rich compounds such as adenosine
triphosphate (ATP).
● 2-Deoxy-D-ribose (along with phosphate groups) forms the long chains
of deoxyribonucleic acid (DNA).
36. BIOCHEMICALLY IMPORTANT MONOSACCHARIDES
● 2-deoxyribose differs from ribose by the absence of one oxygen
atom, that in the –OH group at C2.
● Both ribose and 2-deoxyribose exist in the usual mixture of open-
chain and cyclic hemiacetal forms.
37. BIOCHEMICALLY IMPORTANT MONOSACCHARIDES
D-Glucose
● Of the monosaccharides, the hexose glucose is the most important
nutritionally and the most abundant in nature.
● Glucose is present in honey and fruits such as grapes, figs, and dates.
● Ripe fruits, particularly ripe grapes (20%–30% glucose by mass), are a
good source of glucose, which is often referred to as grape sugar.
● Glucose is also known as blood sugar because it is the sugar
transported by the blood to body tissues to satisfy energy requirements.
38. BIOCHEMICALLY IMPORTANT MONOSACCHARIDES
● The normal concentration of glucose in human blood is in the
range of 70 –100 mg/dL.
● All tissues use glucose as a primary source of energy.
● Erythrocytes and brains cells utilize glucose solely for energy.
39. BIOCHEMICALLY IMPORTANT MONOSACCHARIDES
D-Galactose
● D-Galactose is seldom encountered as a free monosaccharide.
● In the human body, galactose is synthesized from glucose in the
mammary glands to produce lactose (milk sugar).
● D-Galactose is sometimes called brain sugar because it is a
component of glycoproteins (protein–carbohydrate compounds)
found in brain and nerve tissue.
40. BIOCHEMICALLY IMPORTANT MONOSACCHARIDES
● Like glucose, galactose is an aldohexose; it differs from
glucose only in the spatial orientation of the – OH group at
carbon 4.
41. BIOCHEMICALLY IMPORTANT MONOSACCHARIDES
D-Fructose
● D-Fructose, often called levulose or fruit
sugar, occurs in honey and many fruits.
● Aqueous solutions of naturally occurring
D-fructose rotate plane-polarized light to
the left; hence the name levulose.
● D-fructose is present in honey in equal
amounts with glucose.
42. BIOCHEMICALLY IMPORTANT MONOSACCHARIDES
● Like glucose and galactose, fructose is a 6-carbon sugar.
● However, it is a ketohexose rather than an aldohexose.
● In solution, fructose forms five-membered rings:
43. BIOCHEMICALLY IMPORTANT MONOSACCHARIDES
● Fructose is sweeter than sucrose and is an ingredient in many
sweetened beverages and prepared foods.
● As a phosphate, it is an intermediate in glucose metabolism.
● Seminal fluid is rich in fructose and sperms utilize fructose for
energy.
44. IMPORTANT REACTIONS OF MONOSACCHARIDES
1. Oxidation to Produce Acidic Sugars
● Monosaccharide oxidation can yield three different types of acidic
sugars.
a) Aldonic acids
b) Aldaric acids
c) Alduronic acid
● The oxidizing agent used determines the product.
45. IMPORTANT REACTIONS OF MONOSACCHARIDES
● Oxidation of the aldehyde end of an
aldose with mild oxidizing agent such
as bromine gives an aldonic acid.
● For example, the oxidation of the
aldehyde end of D-glucose with
bromine produces D-gluconic acid.
46. IMPORTANT REACTIONS OF MONOSACCHARIDES
● Oxidation of both ends of an aldose (the
aldehyde and the terminal primary
alcohol group) with Strong oxidizing agent
such as nitric acid produces a
dicarboxylic acid.
● Such polyhydroxy dicarboxylic acids
are known as aldaric or saccharic acid.
● For glucose, this oxidation produces
glucaric acid.
47. IMPORTANT REACTIONS OF MONOSACCHARIDES
● In biochemical systems, enzymes can
oxidize the primary alcohol end of an
aldose such as glucose, without
oxidation of the aldehyde group, to
produce an alduronic acid.
● For D-glucose, such an oxidation
produces D-glucuronic acid.
48. IMPORTANT REACTIONS OF MONOSACCHARIDES
● Oxidation reactions are often used as a test for the presence of an
aldehyde group in a carbohydrate.
● Benedict’s test and Fehling’s test both employ a solution of Cu2+
ions in aqueous base.
● When the carbohydrate is oxidized, the blue Cu2+
ion is reduced to
Cu2
O, which forms a brick red precipitate.
49. IMPORTANT REACTIONS OF MONOSACCHARIDES
2. Reduction to Produce Sugar Alcohols
● The carbonyl group present in a monosaccharide (either an aldose
or a ketose) can be reduced to a hydroxyl group, using hydrogen as
the reducing agent.
● The resulting compound is one of the polyhydroxy alcohols known
as alditols or sugar alcohols.
50. IMPORTANT REACTIONS OF MONOSACCHARIDES
● For example, the reduction of D-
glucose gives D-glucitol.
● D-Glucitol, also known by the
common name D-sorbitol, is used
as the sweetener.
● D-Sorbitol accumulation in the eye
is a major factor in the formation
of cataracts due to diabetes.
51. IMPORTANT REACTIONS OF MONOSACCHARIDES
● The sugar alcohols formed from mannose, fructose and galactose
are:
▪ D-Mannose D-Mannitol
▪ D-Fructose D-Mannitol + D-Sorbitol
▪ D-Galactose D-Dulcitol
▪ D-Xylose D-Xylitol
● Mannitol is frequently used medically as an osmotic diuretic to
reduce cerebral edema.
52. IMPORTANT REACTIONS OF MONOSACCHARIDES
3. Glycoside Formation
● Cyclic monosaccharides (hemiacetals and hemiketals) readily
react with alcohols in the presence of acid solution to form acetals
and ketals, which are called glycosides.
● A glycoside produced from glucose is called a glucoside, that from
galactose is called a galactoside, and so on.
53. IMPORTANT REACTIONS OF MONOSACCHARIDES
● For example, glucose reacts with methanol to produce methyl
glucoside.
54. IMPORTANT REACTIONS OF MONOSACCHARIDES
● The bond between the anomeric carbon atom of the
monosaccharide and the oxygen atom of the –OR group is called
a glycosidic bond.
● Disaccharides and polysaccharides form as a result of glycosidic
bonds between monosaccharide units.
55. IMPORTANT REACTIONS OF MONOSACCHARIDES
4. Phosphate Ester Formation
● The hydroxyl groups of a monosaccharide can react with acids
and derivatives of acids to form esters.
● The phosphate esters are particularly important because they are
the usual intermediates in the breakdown of carbohydrates to
provide energy.
● Phosphate esters are frequently formed by transfer of a phosphate
group from ATP to give the phosphorylated sugar and ADP.
56. IMPORTANT REACTIONS OF MONOSACCHARIDES
● For example, specific enzymes in
the human body catalyze the
esterification of the hemiacetal group
(carbon 1) and the primary alcohol
group (carbon 6) in glucose to
produce the compounds glucose 1-
phosphate and glucose 6-phosphate,
respectively.
57. Lesson No: 2
Biochemistry
Dr. Usman Saleem
Pharm.D, R.Ph
LEARNING OBJECTIVES
▪ Draw the structures and list
sources and properties for
important disaccharides.
▪ Write reactions for the
hydrolysis of disaccharides.
58. Disaccharides
● Disaccharides are sugars composed of two monosaccharide units
linked together by the acetal or ketal linkages.
● They can be hydrolyzed to yield their monosaccharide building
blocks by boiling with dilute acid or reacting them with appropriate
enzymes.
● In disaccharide formation, one of the monosaccharide reactants
functions as a hemiacetal, and the other functions as an alcohol.
60. Disaccharides
● Three important naturally occurring disaccharides are:
1. Maltose
2. Lactose
3. Sucrose
● They illustrate the three different ways monosaccharides are linked:
▪ By a glycosidic bond in the α orientation (maltose),
▪ A glycosidic bond in the β orientation (lactose), or
▪ a bond that connects two anomeric carbon atoms (sucrose).
61. Maltose
(Malt Sugar)
Occurrence
● Maltose, often called malt sugar, is present in fermenting grains
and can be prepared by enzyme-catalyzed degradation of
starch.
● In the body, it is produced during starch digestion by α-amylase
in the small intestine and then hydrolyzed to glucose by a
second enzyme, maltase.
62. Maltose
(Malt Sugar)
Chemistry
● Chemically, maltose consists of
two D-glucose units, one of which
must be α-D-glucose.
● The glycosidic linkage between
the two glucose units is called an
(1 → 4) linkage.
63. Maltose
(Malt Sugar)
● Maltose has one free hemiacetal group. Consequently, maltose
exists in three forms:
▪ α-maltose,
▪ β-maltose, and
▪ open-chain form.
▪ In solution, maltose exists as an equilibrium mixture of all the three
forms.
▪ In the solid state, the β form is dominant.
64. Maltose
(Malt Sugar)
Properties
● Maltose is a white crystalline solid, with a melting point 160–165 °C.
● It is soluble in water and is dextrorotatory.
● Maltose is a reducing sugar because the hemiacetal group on the
right unit of D-glucose is in equilibrium with the free aldehyde and
can be oxidized to a carboxylic acid.
● It is also capable of exhibiting mutarotation.
65. Maltose
(Malt Sugar)
● Hydrolysis of D-maltose produces two molecules of D-glucose.
● Acidic conditions or the enzyme maltase is needed for the
hydrolysis to occur.
66. Lactose
(Milk Sugar)
● Lactose, or milk sugar, is the principal
carbohydrate in milk.
● Human mother’s milk obtained by
nursing infants contains 7%–8%
lactose, almost double the 4%–5%
lactose found in cow’s milk.
● Lactose is an important ingredient in
commercially produced infant
formulas that are designed to
simulate mother’s milk.
Cow’s milk is about 5% lactose.
68. Lactose
(Milk Sugar)
Properties
● Lactose is a white, crystalline solid with a melting point 203°C and
is also dextrorotatory.
● The equilibrium mixture has a specific rotation +52.5°.
● It is less soluble in water and much less sweet than sucrose.
● The presence of a hemiacetal group in the glucose unit makes
lactose a reducing sugar.
69. Lactose
(Milk Sugar)
● Lactose can be hydrolyzed by acid or by the enzyme lactase,
forming an equimolar mixture of galactose and glucose.
● In the human body, the galactose so produced is then converted to
glucose by other enzymes.
70. Lactose
(Milk Sugar)
Lactose Intolerance
● The genetic condition lactose intolerance is
an inability of the human digestive system
to hydrolyze lactose.
● In lactose-intolerant individuals, lactose
remains in the intestines undigested rather
than being absorbed, causing fullness,
discomfort, cramping, nausea, and
diarrhea.
71. Sucrose or Saccharose
(Table sugar, Cane sugar, Beet sugar)
Occurrence
● Sucrose is common table sugar,
which is principally extracted
from sugarcane and sugar beets.
● Sugar cane contains up to 20%
by mass sucrose, and sugar
beets contain up to 17% by mass
sucrose.
Hummingbirds depend on the
sucrose and other carbohydrates
of nectar for their energy.
72. Sucrose or Saccharose
(Table sugar, Cane sugar, Beet sugar)
Chemistry
● The monosaccharide units that make
up sucrose are α-D-glucose and β-D-
fructose.
● The α C-1 carbon of the glucose is
linked to the β C-2 carbon of the
fructose in a glycosidic linkage that
has the notation α,β(1 → 2).
73. Sucrose or Saccharose
(Table sugar, Cane sugar, Beet sugar)
Properties:
● Sucrose is a white crystalline solid, soluble in water and with a
melting point 180°C.
● When heated above its melting point, it forms a brown substance
known as caramel.
● It is dextrorotatory and has a specific rotation of + 66.7°.
● Sucrose does not exhibit mutarotation.
74. Sucrose or Saccharose
(Table sugar, Cane sugar, Beet sugar)
● Sucrose is not a reducing sugar because both anomeric groups are
involved in the glycosidic linkage.
● Sucrase, the enzyme needed to break the α, β(1 → 2) linkage in
sucrose, is present in the human body. Hence sucrose is an easily
digested substance.
● Sucrose hydrolysis (digestion) produces an equimolar mixture of D-
glucose and D-fructose called invert sugar.
75. Sucrose or Saccharose
(Table sugar, Cane sugar, Beet sugar)
● Honeybees and many other insects
possess an enzyme called invertase that
hydrolyzes sucrose to invert sugar.
● Thus honey is predominantly a mixture
of D-glucose and D-fructose with some
unhydrolyzed sucrose.
76. Invert Sugar
● The term invert sugar comes from the observation that the direction
of rotation of plane-polarized light changes from positive
(clockwise) to negative (counterclockwise) when sucrose is
hydrolyzed to invert sugar.
● The rotation is +66o
for sucrose. The net rotation for the invert
sugar mixture of fructose (-92o
) and glucose (+52o
)is -40o
.
77. Summery of Important Disaccharides
● Following table summarizes some features of the three disaccharides we
have discussed.
Name
Monosaccharide
Constituents
Glycoside
Linkage Source
Maltose Two glucose units α(1 → 4) Hydrolysis of starch
Lactose Galactose and glucose β(1 → 4) Mammalian milk
Sucrose Glucose and fructose α-1 → β−2 Sugar cane and sugar
beet juices
78. Lesson No: 3
Biochemistry
Dr. Usman Saleem
Pharm.D, R.Ph
LEARNING OBJECTIVES
▪ Describe the structures and list
sources and uses for important
polysaccharides.
79. Polysaccharides
● A polysaccharide is a polymer that contains many
monosaccharide units bonded to each other by glycosidic linkages.
● Polysaccharides are often also called glycans.
● Glycan is an alternate name for a polysaccharide.
80. Polysaccharides
● The important parameters that distinguish various
polysaccharides (or glycans) from each other are:
1. The identity of the monosaccharide repeating unit(s) in the polymer
chain.
▪ The more abundant polysaccharides in nature contain only one
type of monosaccharide repeating unit.
▪ Such polysaccharides, including starch, glycogen, cellulose, and
chitin, are examples of homopolysaccharides.
81. Polysaccharides
▪ Polysaccharides whose structures contain two or more types of
monosaccharide monomers, including hyaluronic acid and heparin,
are called heteropolysaccharides.
3. The length of the polymer chain.
▪ Polysaccharide chain length can vary from less than a hundred
monomer units to up to a million monomer units.
4. The type of glycosidic linkage between monomer units.
▪ As with disaccharides, several different types of glycosidic linkages
are encountered in polysaccharide structures.
82. Polysaccharides
4. The degree of branching of the polymer chain.
▪ The ability to form branched-chain structures distinguishes
polysaccharides from the other two major types of biochemical
polymers: proteins and nucleic acids, which occur only as linear
(unbranched) polymers.
83.
84. Homopolysaccharides
● A homopolysaccharide is a polysaccharide in which only one type of
monosaccharide monomer is present.
● The most important Homopolysaccharides are:
❖ Starch
❖ Glycogen
❖ Cellulose
● Starch and glycogen are examples of storage polysaccharides,
cellulose and chitin are structural polysaccharides.
85. Starch
Over View
● Starch is energy-storage polysaccharide in plants.
● It occur in plant cells usually as starch granules in the cytosol.
● Starch is fully digestible and is an essential part of the human diet.
● The major sources of starch are beans, the grains wheat and rice,
and potatoes.
86. Starch
Chemistry
● Starch is a homopolysaccharide containing only α-D-glucose
monosaccharide units.
● Two different polyglucose polysaccharides can be isolated from
most starches:
a. Amylose
b. Amylopectin.
87. Starch
a. Amylose
● Amylose, a linear glucose polymer, usually accounts for 15%
–20% of the starch.
● In amylose’s non-branched structure, the glucose units are
connected by (α1→4) glycosidic linkages.
● The number of glucose units present in an amylose chain
depends on the source of the starch; 300–500 monomer units
are usually present.
89. Starch
b. Amylopectin
● Amylopectin, a branched glucose polymer, accounts for the
remaining 80%–85% of the starch.
● Amylopectin has a high degree of branching in its polyglucose
structure.
● The branch points involve α(1→6) glycosidic linkages.
● There are usually 24 to 30 D-glucose units, all connected by α
(1→4) linkages, between each branch point of amylopectin.
91. Starch
Amylopectin
● Because of the branching,
amylopectin has a larger average
molecular mass than the linear
amylose.
● Up to 100,000 glucose units may
be present in an amylopectin
polymer chain. ❖ An α(1→ 6) linkage is present in
the amylopectin structure at
each branch point.
92. Starch
● All of the glycosidic linkages in starch (both amylose and amylopectin)
are of the α type.
● In amylose, they are all α(1→4); in amylopectin, both α(1→4) and α
(1→6) linkages are present.
● Because starch molecules are digested mainly in the small intestine
by α-amylase, which catalyzes hydrolysis of the α(1→4) linkages,
starch has nutritional value for humans.
● Complete hydrolysis of both amylose and amylopectin yields only D-
glucose.
93. Starch
● The most usual conformation of amylose
is a helix with six residues per turn.
● Iodine molecules can fit inside the helix to
form a starch–iodine complex, which has a
characteristic dark-blue color.
● The formation of this complex is a well-
known test for the presence of starch.
94. Activity No. 1
❖ An individual starch molecule contains thousands of
glucose units but has only a single hemiacetal group
at the end of the long polymer chain. Would you
expect starch to be a reducing carbohydrate?
Explain.
95. Glycogen
Over-View
● Glycogen is the glucose storage polysaccharide in humans and
animals.
● Glycogen, sometimes called animal starch, serves the same energy
storage role in animals that starch serves in plants.
● The largest amounts of glycogen are stored in the liver and
muscles.
● The total amount of glycogen in the body of a well-nourished adult
human is about 350 g.
96. Glycogen
Chemistry
● Glycogen is a branched-chain polymer of α-D-glucose, and in
this respect it is similar to the amylopectin fraction of starch.
● Like amylopectin, glycogen consists of a chain of α(1→4)
linkages with α(1→6) linkages at the branch points.
● Branch points occur about every 8-12 residues in glycogen.
97. Glycogen
● Glycogen is about three times more highly branched than
amylopectin, and is much larger—up to one million glucose units per
molecule.
98.
99. Cellulose
Over-View
● Cellulose is the major structural
component of plants, especially of
wood and plant fibers.
● Cotton is almost pure cellulose(95%),
and wood is about 50% cellulose.
100. Cellulose
Chemistry
● It is a linear homopolysaccharide of β-D-glucose, and all residues
are linked in β(1→4) glycosidic bonds.
● Typically, cellulose chains contain about 5000 glucose units.
101. Fig: Structure of Cellulose
Repeating disaccharide in cellulose is β-cellobiose
102. Cellulose
● Humans and other animals lack the
enzymes capable of catalyzing the
hydrolysis of β-linkages in cellulose.
● However, the intestinal tracts of grazing
animals such as horses, cows, and sheep
contain bacteria that produce cellulase, an
enzyme that can hydrolyze β(1→4) linkages
and produce free glucose from cellulose.
103. Cellulose
● Despite its nondigestibility, cellulose is
still an important component of a
balanced diet.
● It serves as dietary fiber.
● Dietary fiber provides the digestive
tract with “bulk” that helps move food
through the intestinal tract and
facilitates the excretion of solid
wastes.
104. Common Polysaccharides
Type
Made
From
Glycosidic
Linkage
Function Digestible by Humans
Amylose Glucose α(1→4) Fuel storage in plant Yes, requires amylase
Amylopectin Glucose
α(1→4)
α(1→6) at
branch points
Fuel storage in plant Yes, requires amylase
and debranching enzyme
Glycogen Glucose
α(1→4)
α(1→6) at
branch points
Fuel storage in plant
(Sometimes, called
“animal starch”).
Yes, requires amylase
and debranching enzyme
Cellulose Glucose β(1→4)
Structural material in
plant
No, requires cellulase
