This document discusses carbohydrate chemistry and digestion. It defines carbohydrates and describes their functions in the body. It outlines the nomenclature of monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Key monosaccharides like glucose, fructose and galactose are explained. Important disaccharides like sucrose, lactose and maltose are defined. Polysaccharides discussed include starch, glycogen, cellulose, and heteroglycans. The process of carbohydrate digestion by salivary and pancreatic amylases and intestinal disaccharide-hydrolyzing enzymes is summarized. Lactose intolerance is also briefly explained, along with monosaccharide absorption mechanisms.

1. Dental Biochemistry 1- (5)
Chemistry and digestion of
carbohydrates
1

2. DEFINITION
Carbohydrates are organic substances
composed of carbon, hydrogen and
oxygen.
2

3. Function of carbohydrates:
• Carbohydrates are the main sources of
energy in the body. Brain cells and RBCs are
almost wholly dependent on carbohydrates as
the energy source. Energy production from
carbohydrates will be 4 Kcal/g.
• Storage form of energy (starch and glycogen).
• Excess carbohydrate is converted to fat.
• Glycoproteins and glycolipids are components
of cell membranes and receptors,
3

4. • Structural unit of many organisms: Cellulose of
plants; exoskeleton of insects, cell wall of
microorganisms, mucopolysaccharides as ground
substance in higher organisms.
• Important part of nucleic acids and free
nucleotides and coenzyme.
• Major antigens are carbohydrates in nature, e.g.,
blood group substance.
• Has a biological role as a part of hormones and
their receptors and enzymes.
4

5. Nomenclature
• Molecules having only one actual or potential sugar
group [(CH2O)n] are called monosaccharides (e.g.
C6H12O6); they cannot be further hydrolyzed into
smaller units.
• When two monosaccharides are combined
together with elimination of a water molecule, it is
called a disaccharide (e.g. C12H22O11).
• Trisaccharides contain three sugar groups. Further
addition of sugar groups will correspondingly
produce tetrasaccharides, pentasaccharides and so
on, commonly known as oligosaccharides.5

6. • When more than 10 sugar units are combined, they
are generally named as polysaccharides.
• Polysaccharides having only one type of
monosaccharide units are called
homopolysaccharides and those having different
monosaccharide units are called
heteropolysaccharides.
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7. Monosaccharides
They have the common formula
(CH2O)n, where n = 3 or some
larger number
7

8. CLASSIFICATION OF MONOSACCHARIDES
Can be carried out by one of two methods:
1) According to the number of carbon atoms
:
Trioses, Tetroses, Pentoses,
Hexoses, Heptoses, Octoses.
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9. • 2) According to the characteristic
carbonyl group:
Aldehyde group or ketone group
a) Aldo sugars: Aldoses :
Monosaccharides containing aldehyde group e.g.
glucose, ribose, erythrose and glyceraldehyde.
b) Keto sugars: Ketoses :
Monosaccharides containing ketone group e.g.
fructose, ribulose and dihydroxyacetone.
9

10. 10

11. STEREOISOMERS
• Compounds having same structural formula, but
differ in spatial configuration are known as
stereoisomers.
• All monosaccharides can be considered as
molecules derived from glyceraldehyde.
• Depending on the configuration of Hand OH
around the reference carbon atom, the two
mirror forms are designated as Land D forms.
• All naturally occurring sugars are D sugars.
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12. 12

13. of AldosesEpimerism
• When sugars are different from one another,
only in configuration with regard to a single
carbon atom (other than the reference carbon
atom), they are called epimers.
• For example, glucose and mannose
13

14. Anomerism of Sugars
• This is explained by the fact that D-glucose has
two anomers, alpha and beta varieties.
14

15. Monosaccharides of
physiologic importance
1-Pentoses:
• They are sugars containing 5 carbon
atoms. Ribose is a constituent of RNA
while deoxyribose is seen in DNA.
• Ribose is also seen in co-enzymes
such as ATP and NAD.
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16. 16

17. Hexoses
Galactose, Mannose andGlucose
• They are the common aldohexoses.
• Glucose is the sugar in human blood. It is the
major source of energy.
• Mannose is a constituent of many
glycoproteins. Mannose was isolated from
plant mannans; hence the name.
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18. b) D-Fructose :
( Fruit sugar = Levulose). It is found in fruits, honey
and obtained from sucrose and inulin by hydrolysis.
c)D-Galactose :
It is obtained from hydrolysis of lactose (milk sugar). .
It is a constituent of galactolipids, glycoprotein.
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20. DlSACCHARIDES
• When two monosaccharides are
combined together by glycosidic
linkage, a disaccharide is formed.
The important disaccharides are
sucrose, maltose, isomaltose and
lactose.
20

21. . Sucrose1
• It is the sweetening agent known as cane sugar. It is
present in sugarcane and various fruits. Hydrolysis
of sucrose will produce one molecule of glucose
and one molecule of fructose.
• ii. The enzyme producing hydrolysis of sucrose is
called sucrase.
• Sucrose is not
a reducing sugar.
21

22. . Lactose2
• Reducing disaccharide.
• Capable of forming osazone.
• It is only found in milk.
• By acid or lactase enzyme in the intestine,
it yields D-galactose and D-glucose
22

23. . Maltose3
• Maltose contains two glucose residues with
alpha1,4 linkage.
• It is found in germinating cereals and malt.
• Intermediate product of the action of amylases on
starch. It is reducing disaccharide
• Hydrolysis: By acid or by maltase enzyme into
two D-glucose units.
23

24. Isomaltose.4
• It is also a reducing sugar. It contains 2 glucose
units combined in alpha -1, 6 linkage.
• Partial hydrolysis of glycogen and starch
produces isomaltose.
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25. POLYSACCHARIDES
• These are polymerized products of many
monosaccharide units.
• They may be homoglycans composed of
single kind of monosaccharides, e.g. starch.
glycogen and cellulose.
• Heteroglycans are composed of two or more
different monosaccharides.
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26. Starch.1
• It is the reserve carbohydrate of plant as in
potatoes, rice, wheat. 10-20% is soluble part called
amylose. The insoluble part is called amylopectin.
• Amylose is made up of glucose units with alpha-
1,4 glycosidic linkages to form an unbranched long
chain.
• Amylopectin is also made up of glucose units, but
is highly branched with molecular weigh more
than 1 million. The branching points are made by
alpha-1,6 linkage (similar to isomaltose) .
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27. 2. Glycogen
• It is the reserve carbohydrate in animals. It is
stored in liver and muscle. About 5% of weight of
liver is made up by glycogen.
• Glycogen is composed of glucose units joined by
alpha-1 ,4 and alpha-1 ,6 glycosidic linkages.
3. Cellulose
• It is the chief carbohydrate in plants.
• The enzyme act on hydrolysis of cellulose is absent
in animal and human digestive system, and hence
cellulose cannot be digested.
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28. 4. Inulin
• It is the reserve carbohydrate present in onion,
garlic, etc.
• 5. Chitin
• It is present in exoskeletons of crustacea and
insects
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29. B- Heteroglycan:
These are polysaccharides containing more than one
type of sugar residues.
1.Agar:
• It is prepared from sea weeds and contains
galactose, glucose and other sugars.
• It is used as a supporting medium for
electrophoresis.
• 2. Mucopolysaccharides:
• Mucopolysaccharides or glycosaminoglycans (GAG)
are carbohydrates containing uronic acid and amino
sugars.29

30. 3. Hyaluronic acid:
• It is present in connective tissues, tendons,
4. Heparin:
• It is an anticoagulant
• 5. Keratan sulphate:
• It is found in cornea and tendons.
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31. Glycoproteins and Mucoproteins
• When the carbohydrate chains are attached to a
polypeptide chain it is called a proteoglycan.
• If the carbohydrate content is less than 10%, it is
generally named as a glycoprotein.
• If the carbohydrate content is more than 10% it is a
mucoprotein.
• They are seen in almost all tissues and cell
membranes.
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32. Digestion of carbohydrates
• Cooking helps in breaking of glycosidic
linkages in polysaccharides and thus makes
the digestion process easier.
• In the diet carbohydrates are available as
polysaccharides (starch, glycogen), and to a
minor extent, as disaccharides (sucrose and
lactose).
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33. • This process of digestion starts in mouth by
the salivary alpha-amylase. However, the
time available for digestion in the mouth is
limited. The gastric hydrochloric acid will
inhibit the action of salivary amylase.
• In the pancreatic juice another alpha-amylase
is available which will hydrolyze the alpha-1,4
glycosidic linkages randomly, so as to produce
smaller subunits like maltose, isomaltose,
dextrin and oligosaccharides.
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34. • The intestinal juice (succus entericus)
and brush border of intestinal cells
contain enzymes, which will hydrolyze
disaccharides into component
monosaccharaides.
• These enzymes are sucrase, maltase,
isomaltase and lactase.
• The monosaccharides are then
absorbed.
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35. Lactose intolerance
• This is produced by the deficiency of lactase.
This enzyme hydrolyses lactose to glucose and
galactose.
• In this condition, lactose accumulate in the
gut produce irritant diarrhea.
• If milk is withdrawn temporarily, the diarrhea
will be limited. Curd is also an effective
treatment, because the lactobacilli present in
curd contains the enzyme lactase.
• Lactase activity is high during infancy and it
decreases to adult levels by 5-7 years of age.35

36. Absorption of carbohydrates
• Only monosaccharides are absorbed by the
intestine. Minute quantities of disaccharides that
may be absorbed, are immediately eliminated
through kidneys.
• The duodenum and upper jejunum absorb the
bulk of the dietary sugars.
• Insulin is not required for the uptake of glucose by
intestinal cells.
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37. • However, different sugars have different
mechanisms of absorption.
1. galactose and glucose are transported into the
mucosal cells by an active, energy-requiring
process that involves a specific transport protein
and requires a concurrent uptake of sodium ions
(sodium-dependent monosaccharide
transporter SGluT).
2. Fructose uptake requires a sodium-independent
monosaccharide transporter (GLUT-5) for its
absorption.
3. All three monosaccharides are transported from
the intestinal mucosal cell into the portal circulation
by yet another transporter, GLUT-2.
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