1. Chapter 3 Carbohydrates 2- Disaccharides 3- Polysaccharides
1- Monosaccharides
2- Disaccharides
3- Polysaccharides
4- Digestion of carbohydrates

2. Carbohydrates
1. Chemically, carbohydrates are the most abundant organic molecules in nature in which carbon, hydrogen and oxygen bond together in the ratio: C x (H2O)y
2. Their functions:
a) Providing the energy in the diet of most organisms.
Humans break down carbohydrates during the process of metabolism to release energy.
For example, the chemical metabolism of the sugar
glucose is shown below:
C6H12O6 + 6 O CO2 + 6 H2O + energy

3. Human obtain carbohydrates by eating foods that contain them, for example potatoes, rice, breads,ect.
Carbohydrates are manufactured by plants during the process of photosynthesis is a metabolic pathway that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight.
Plants harvest (collect) energy from sunlight to run the reaction described above in reverse
6 CO2 + 6 H2O + energy C6H12O6 + 6 O2  
from
sunlight

4. Carbohydrates Acting as a storage form of energy in the body
Serving as cell membrane components.
Serve as a structural component of many organisms such as:
1- Cell wall of bacteria.
2- Exoskeleton of insects.
3-Fibrous cellulose of plants.
The empiric formula for simple Carbohydrates is:
(CH2O) n , hence the name “ Hydrate of Carbon “
C6 H12 O6 , (CH2 O)6 or C6 (H2O)6

5. II. Classification and structure of Carbohydrates
There are three classes of Carbohydrates, Based on the number of sugar units:
Monosaccharides
Disaccharides
Polysaccharides:

6. Monosaccharides: (simple sugar)
Definition:
Monosaccharides are colorless crystalline solids
Freely soluble in water
Insoluble in non polar solvents
It is the basic units of Carbohydrates
Most have a sweet taste.

7. Many of Monosaccaharides are synthesized from simpler (non-carbohydrate) substances in a process named gluconeogenesis (is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids).
Monosaccaharides are aldehyde or ketone derivatives of straight - chain polyhydroxy alcohols.

8. Classification
1) Monosaccharides are classified according to the chemical nature of their carbonyl group, which (is the carbon atom that is doubled- bounded to an oxygen atom to form a carbonyl group ) while each of other carbon atoms has a hydroxyl group :
If the carbonyl group is an aldehyde the sugar is an aldose as in glyceraldehyde and glucose.
If the carbonyl group is a ketone the sugar is an ketose as in Dihydroxyacetone and fructose

10. The smallest monosaccharide, those with three carbon atoms are trioses
2. Monosaccharides can be classified according to the number of carbon atoms they contain:
The smallest monosaccharide, those with three carbon atoms are trioses
Those with 4,5,6 and 7 carbon atoms are called tetroses, pentoses, hexoses, and heptoses respectively.

11. 2. Each of these monosaccharides exists in two series :
aldo trioses and keto trioses
aldo tetroses and keto tetroses
aldo pentoses and keto pentoses
aldo hexoses and keto hexoses
3.The simplest monosaccharide are the two 3- carbon trioses:
Glyceraldehydes, an aldose
Dihydroxyacetone, a ketose.
4.The most abundant monosaccharides in nature are the hexoses, which
Include the aldohexose D- glucose and the ketohexose D- fructose.

14. C) Isomers and epimers Isomers: Are compounds that have Example :
The same chemical formula
Have different structures
Example :
Fructose, glucose, mannose, and galactose.
Are all isomers of each other having the same chemical formula C6 H12 O6

15. Epimers: They are also isomers
Are compounds that differ in their configuration around only one specific carbon atom ( with the exception of the carbonyl carbon ).
They are also isomers
1) Glucose and galactose are C4 epimers
Their structures differ only in the position of the - OH group at carbon 4
2) Glucose and mannose are C2 epimers.
Their structures differ only in the position of
the - OH group at carbon 2

17. Enantiomers :
A special type of isomers is found in the pairs of structures that are mirror images of each other.
The two members of the pair are designated as a D- and L- sugar .
The vast majority of the sugars in humans are D- sugars.

18. Enantiomers :

19. An aldehyde can react with an alcohol to form a hemiacetal
A ketone can react with an alcohol to form a hemiketal.

20. Cyclization of monosaccharides
Monosaccharides are predominantly found in a ring form:
In which the aldehyde ( or ketone ) group has reacted with an alcohol group on the same sugar, and form a covalent bond
While less than 1 % exists in the open - chain ( acyclic ) form.

21. Cyclic Fischer Projection of Haworth Projection of
a-D-Glucose a-D-Glucose

22. 1)Anomeric carbon
Formation of a ring results in the creation of an anomeric carbon:
At carbon 1 of an aldose or
At carbon 2 of a ketoses.
Many simple sugars can exist in a chain form or a ring form.
The ring form is favored in aqueous solutions, and the mechanism of ring formation is similar for most sugars.

24. The glucose ring form is created when:
The oxygen on carbon number 5 links with the carbon comprising the carbonyl group (carbon number 1)
And transfers its hydrogen to the carbonyl oxygen to create a hydroxyl group.
The rearrangement produces alpha glucose when the hydroxyl group is on the opposite side of the -CH2OH group,
or beta glucose when the hydroxyl group is on the same side as the -CH2OH group.

25. Anomers: α or β
Are isomers which differ only in their configuration about their carbonyl carbon atom.
These structures are designated the α or β configuration of the sugar. For example:
α - D - glucose and β - D – glucose. These two sugars are both glucose, but they are anomers of each other.

27. Fructose can form either:
A six-member pyranose ring,
By reaction of the C2 keto group with the hydroxyl on C6
A 5-member furanose ring,
by reaction of the C2 keto group with the hydroxyl on C5.

29. 2) Mutarotation process:
Is when cyclic α and β anomers of a sugar in solution are in equilibrium with each other, and can be spontaneously interconverted.
The cyclic form of glucose known as glucopyranose.
The cyclic form of fructose known as
fructofuranose.

30. Haworth projections:
Represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1 extending either:
Below the ring (a)
Above the ring (b).

31. Reducing sugars
Is when the oxygen on the anomeric carbon (the carbonyl group) of a sugar is not attached to any other structure.
Reducing agents are electron donors
Oxidizing agents are electron acceptors.
A reducing sugar can react with chemical reagents. For example,
(Benedict’s solution) and reduce the reactive component, with the anomeric carbon becoming oxidized.

32. II. Disaccharides
Consist of two monosaccharides covalently bound to each other.
In disaccharides the chemical bond that joins the two monosaccharide units is called a glycosidic bond.
Glycosidic bond :
Is formed when the hydroxyl group on one of the sugars reacts with the anomeric carbon on the second sugar.
Is readily hydrolyzed by acid.
It resists cleavage by base.

33. Disaccharides can be hydrolyzed to yield their free monosaccharide components by boiling with dilute acid.
They are also abundant in nature.
The most common disaccharides are:
Maltose
Lactose
Sucrose

34. Maltose

35. Maltose
Contain two D- glucose residues joined by a glycosidic linkage between carbon atom 1 ( the anomeric carbon ) of the first glucose residue and carbon atom 4 of the second glucose.
The configuration of the anomeric carbon atom in the glycosidic linkage between the two D-glucose residues is α
The linkage is thus symbolized α (1- 4)
The monosaccharide unit bearing the Anomeric carbon is designated by the first number in this symbol.

36. Both the glucose residues of maltose are in pyranose form
Maltose is a reducing sugar since it has one free carbonyl group that can be oxidized.
Maltose is hydrolyzed to two molecules of D-glucose by the intestinal enzyme Maltase that is specific for the α (1- 4) bond

37. Lactose

38. Lactose
Found in milk
Contain D-galactose and D- glucose joined by a glycosidics linkage between carbon atom 1 (the anomeric carbon )of galactose residue and carbon atom 4 of the glucose.
The configuration of the anomeric carbon atom in the glycosidic linkage between the D-galactose residues and the glucose residue is
β and the linkage is thus symbolized β (1- 4)

39. Both the galactose and glucose residues of lactose are in pyranose form
Lactose is a reducing sugar since it has one free carbonyl group on the glucose residue that can be oxidized.
During digestion lactose undergoes enzymatic hydrolysis by the Lactase enzyme of the intestinal mucosal cells.

40. Sucrose

41. Sucrose In sucrose: Is a disaccharide of glucose and fructose joined
by a glycosidic linkage between carbon atom 1
(the anomeric carbon )of glucose residue and
carbon atom 2 of the fructose.
In sucrose:
The anomeric hydroxyl group of glucose in
the α configuration is Glycosidically linked
To the anomeric hydroxyl group of fructose
in the β configuration.
The linkage is thus symbolized (α1- β2).

42. Sucrose is Not a reducing sugar
Because it contains No free anomeric carbon
Since the anomeric carbons of both glucose and fructose residues are linked to each other.
Sucrose is hydrolyzed in the small intestine to glucose and fructose by action of the enzyme sucrase (invertase).
It is the sweetest of the three common disaccharides.

43. III. Polysaccharides
Polysaccharides are polymer of twenty or more monosaccharide units.
The polysaccharides can be:
Homopolymers:
Composed of a single type of monosaccharide
Glucans : are homopolymers of glucose
Mannans: are homopolymers of mannose
Levans : are homopolymers of fructose
Hetropolymers:
Made up from a variety of different monosaccharides.

44. In addition they can have
Linear
Branched
The glycosidic bonds between the monosaccharide units can be
1 - 2
1 - 3
1 - 4
1 - 6
2 -1
2-1
2-3
2- 4
2- 6
with the anomeric carbon in either the α of β configuration.

45. CELLULOSE Is the main structural element of plant cells.
It is part of the cell wall and
Is a major component of wood.
Cellulose is a linear polymer of glucose linked by β (1- 4) glycosidic bonds.
The glucose β (1 - 4) glucose glycosidic bond in cellulose results in a rigid linear polymer of glucose.
The rigid nature of this polymer makes it an excellent structural element.

46. The glucose β (1- 4) glucose bond are
Very stable
Difficult to hydrolyzed by chemical means.
A very few species of bacteria contain an enzyme that can hydrolyzed the β (1- 4) bond.
Animals, such as cow that derive most of their nutrients from plant material it:
Contain bacteria in their gut that can hydrolyze the glucose β (1- 4) glucose bond of cellulose.

47. STARCH Is the storage form of glucose units in plant cells
When a plant seed, tuber, and / or bulb is in an energy poor state:
The starch is broken down and used for energy and / or precursors.
Starch as isolated from plants is a mixture of two polymers :
AMYLOSE
AMYLOPECTIN.

48. AMYLOSE: AMYLOPECTIN: Is a linear polymer of glucose units liked
α (1- 4) glycosidic bond.
AMYLOPECTIN:
Is a branched polymer.
It has a backbone of glucose units linked in α (1- 4) glycosidic bond.
Every 24 to 30 residues along a backbone there is a branch point.
At the branch points the glucose residues are linked in and α (1- 6 )configuration.

50. GLYCOGEN It is a homopolymer of glucose.
Is the storage form of glucose in animal cells.
In men, glycogen is stored in the liver and skeletal muscle.
When energy poor :
Glycogen is broken down to glucose and used for
Energy
Precursors

51. Glycogen is a branched polymer
Similar to amylopectin:
It has backbone of glucose residues linked in an α (1- 4) configuration and
Branches that are linked α (1- 6).
Different from amylopectin:
In that it is more highly branched.
It contains α (1- 6) branch every 8-13 residues along a backbone.

52. The branched polysaccharides (Amylopectin and Glycogen:
Have only one reducing end and numerous non reducing ends

53. DEXTRANS Are an important family of storage polysaccharides in
Bacteria and yeast.
The glycosidic linkage along the backbone of these polysaccharides is α (1- 6)
Dextrans are usually branched polysaccharides
Dextrans produced by oral bacteria are usually glucans

54. Heteropolysaccharides
The heteropolysaccharides can be divided into two major types:
1) There are heteropolysaccharides of repeating modified disaccharides units. Example:
Glycosaminoglycans
2) The other type of heteropolysaccharides are composed of
A wide assortment of monosaccharide
Modified monosaccharide units arranged in random order this group is often branched.

55. Complex carbohydrates
Glycosidic bonds to non-carbohydrate structures can attach carbohydrates to:
Purines and Pyrimidines (found in nucleic acids)
Aromatic rings (such as those found in steroids and bilirubin)
Proteins (found in glycoproteins and glycosaminoglycans)
Lipids (found in glycolipids)

56. The aldose, the carbon 1 of which (or ketose, the carbon 2 of which ) participates in the glycosidic link, is called a glycosyl residue. For example:
If the anomeric carbon of glucose participates in such a bond, that sugar is called a glucosyl residue
Thus. The disaccharide lactose is galactosyl- glucose.

57. O- and N-glycosides
If the group on the non-carbohydrate molecule to which the sugar is attached
Is an - OH group, the structure is an O-glycoside
Is an NH2, the structure is an N-glycoside

58. Naming glycosidic bonds
Glycosidic bonds between sugars are named According:
To the numbers of the connected carbons
To the position of the anomeric hydroxyl group of the sugar involved in the bond.
If this anomeric hydroxyl group is in the α configuration, the linkage is an α-bond.
If it is in the β configuration, the linkage is a β-bond.

59. Lactose, for example:
Is synthesized by forming a glycosidic bond between
Carbon 1 of β-galactose and
Carbon 4 of glucose.
The linkage is, therefore, β (1- 4) glycosidic bond

60. Other Biologically Important Sugar Derivatives
Monosaccharide units, which an OH group is replaced by H are known as deoxy sugars.
The biologically most important of these is
β-D-2-deoxyribose, the sugar component of DNAs sugar-phosphate backbone.

61. D-Ribose
2-Deoxy-D-Ribose
L-Ribose
α-D-deoxyribose

62. are components of numerous biologically important polysaccharide.
In amino sugars:
One or more OH groups are replaced by an
Acetylated amino group:
D-Glucosamine
D-galactosamine
are components of numerous biologically important polysaccharide.

63. DIGESTION OF CARBOHYDRATES
The principal sites of dietary carbohydrate digestion are
The mouth
Intestinal lumen
This digestion is rapid and is generally completed by the time:
The stomach contents reach the junction of the duodenum and jejunum
There is little monosaccharide present in diets of mixed animal and plant origin.

64. The enzymes needed for degradation of most dietary carbohydrates are primarily:
Disaccharidases
Endoglycosidases:
That break oligosaccharides and polysaccharides).

65. Hydrolysis of glycosidic bond is catalyzed by a family of glycosidases that
Degrade carbohydrates into reducing sugar components.
These enzymes are usually specific for the:
Structure and configuration of the glycosyl residue to be removed
Type of bond to be broken.

66. A. Digestion of carbohydrates begins in the mouth
The major dietary polysaccharides are of :
Animal (glycogen)
Plant origin (starch, composed of amylose and amylopectin).
During mastication, salivary α-amylase acts briefly on dietary starch in a random manner, breaking some α (1-4) bonds.
There are both α (1-4)- and β (1-4)-endoglucosidases in nature.

67. But humans Do not produce and secret β (1-4)-endoglucosidases in digestive juices. Therefore,
They are unable to digest cellulose - a carbohydrate of plant origin containing β (1-4) glycosidic bonds between glucose residues.
Because branched amylopectin and glycogen also contain α (1-6) bonds,
The digest resulting from the action of α-amylase contains a mixture of smaller, branched oligosaccharide molecules.

68. Carbohydrate digestion halts (stop) temporarily in the stomach,
Because the high acidity
Inactivates the salivary α-amylase

69. When the acidic stomach contents reach the small intestine:
B) Further digestion of carbohydrates by pancreatic enzymes occurs in the small intestine
When the acidic stomach contents reach the small intestine:
They are neutralized by bicarbonate secreted by the pancreas
Pancreatic α-amylase continues the process of starch digestion.

70. C. Final carbohydrate digestion by enzymes synthesized by the intestinal mucosal Cells
The final digestive processes occur at the mucosal lining of the upper jejunum declining as they proceed down the small intestine, and include the action of several:
Disaccharidases
Oligosaccharidases. For example:
1)Isomaltase cleaves the α (1-6) bond in isomaltose
2)Maltase cleaves maltose
Both producing glucose

71. 3) Sucrase cleaves sucrose producing glucose and fructose
4) Lactase (β-galactosidase) cleaves lactose producing galactose and glucose.

72. D. Absorption of monosaccharides by intestinal mucosal cells
The duodenum and upper jejunum
Absorb the bulk of the dietary sugars.
Insulin is not required for the uptake of glucose by intestinal cells
Different sugars have different mechanisms of absorption.