Welcome to class of
Carbohydrates
Dr. Meera Kaur
University of Winnipeg
Learning objectives
• To understand carbohydrates as an important
biomolecule
• To learn the classifications of carbohydrates
• To understand the basic structures and
properties of carbohydrates
• To learn the functions of carbohydrates
• To know the deficiency and excess of
carbohydrates in human body
Introduction
Carbohydrates are the most abundant
biomolecules.
They serve as:
• the major source of energy for plant kingdom
and humankind (4kcal/g)
• supportive structural components in plant
(cellulose)
• essential components in the mechanism of
genetic control of growth and development in
living cells (ribose and deoxyribose)
Electron micrograph of the erythrocyte surface. Its thick (up to 1400 A)
carbohydrate coat, called the glycocalyx, consists of closely packed
oligosaccharides attached to cell-surface proteins and lipids.
Definitions
• Carbohydrates are polyhydroxy aldehydes or ketones
or substances that yield such compounds on
hydrolysis (hydrates of carbon)
– Some carbohydrates also contain nitrogen,
phosphorus or sulfur.
– They are referred as saccharides from the latin
word saccharum which means ‘sugar’.
– The simpler members of the carbohydrtaes family
often give a sweet taste. They have a suffix -ose
Empirical formula: (CH2O)n
Classifications
Monosaccharides
-Glucose
(blood sugar)
-Fructose
(honey)
Galactose
Disaccharides
- Sucrose
(table sugar)
- Lactose
(milk sugar)
-Maltose
(malt sugar)
Polysaccharides
- Starch
(Corn starch)
- Glycogen
(liver)
- Cellulose
(cotton fibre)
- Hemi-cellulose
(dietary fibre)
Monosaccharides…
• They are the simplest carbohydrates
• The general formula is CnH2nOn
Classifications
Triose (C3H6O3)
Tetrose(C4H8O4)
Pentose(C5H10O5)
Hexose(C6H12O6)
Heptose(C7H14O7)
Octose(C8H16O8)
Properties of monosaccharides…
– Colorless crystaline solids that are freely soluble in
water but insoluble in non polar solvents( alcohol,
ether etc.)
– Backbone is an unbranched carbon chain where all
the carbon atoms are linked by single bond
– Consist of a single polyhydroxy aldehyde or ketone
unit
– One of the carbon atom is double bonded to to
form a carbonyl group; each of the other carbon
atom has a hydroxyl group
– Depending upon the position of the carbonyl group
they are either aldose or ketose.
Triose
Aldoses
Triose
Tetrosse
Ketoses
Examples of aldopentoses (5-carbon aldoses)
Pentoses
Examples of ketopentoses (5-carbon ketoses)
Hexoses
Examples of aldohexoses (6-carbon aldoses)
Examples of ketohexoses (6-carbon ketoses)
The D-aldoses with 3-6 carbon atom
The ketoses with 3-6 carbon atoms
Properties of monosaccharides…
•
Mono Saccharides have asymmetric (chiral
centre): means in a carbon atom all the groups are
different. The exception is dehydroxyacetone
•
Compounds whose molecules contain one
asymmetric carbon have handedness ie there is
two kind of molecules with same structural formula
•
Also known as stereoisomers ie molecules of the
same structural formula with different arrangement
of groups
Fischer projections
• Fischer projections allow us to represent three
dimensional organic moleculer structure in in two
dimensions. They are a convenient means of
differentiating sterioisomers
• In Fischer projections:
– Functional group is always at the top
– Carbon chain is written vertically with substituent groups to
the left and right
– The groups to the right and left of the asymmetric carbon
are above the plane, the chain carbon are below the plane
– The hydroxyl group to the right of the carbon chain is D form
and to the left is L form of the sugar. Virtually all sugars
found in nature are of the D family
Properties of monosaccharides…
•
Common monosaccharides (pentoses and
hexoses) occur in cyclic forms(closed chain)
•
The five and the six membered sugar ring system
is known as furanose and pyranose respectively
•
The stereochemistry of cyclic forms of sugar is
depicted by Haworth projections, the standardized
ways of depicting the positions of the hydroxyl
groups in space (Pl. refer to the hand out for
additional knowledge)
Properties of monosaccharides…
•
Simple monosaccharides are reducing agent
•
They are oxidised by mild oxiding agent like ferric
(Fe3+) or cupric(Cu) ions
•
This property is the basis for Fehling’s reaction, a
qualitative test for the presence of reducing sugars
eg. glucose
Aldoses can be oxidized (while something else gets reduced) and so are
known as “reducing sugars”.
Properties of monosaccharides…
Optical activity
The D and L from of handed pairs of stereoisomers are
identical in their physical and chemical properties, except
that they rotate plane-polarized light in opposite direction
Plane-polarized light is the light in which vibrations are in
only one direction. So, handed pairs of stereoisomers are
often called optical isomers
Optical isomers are handed stereoisomers that rotate
plane- polarized light in opposite directions
e.g. D-glyceraldehyde and L- glyceraldehyde rotate planepolarized light to the right and left respectively
Properties of monosaccharides…
-The - and the - forms of monosaccharides are
readily interconvertable in aquous solution
-The changes in specific rotation that accompanies this
interconversion is known as mutarotation
Examples: a freshly prepared solution of -D-glucose
shows an initial rotation of +1120 to +520. Similarly,
a solution of -D glucose undergoes
mutaroatation from +190 to +520
Mutarotation
Glucose family
Disaccharides
When two monosaccharides join together, they form a
disaccharide.eg.Lactose
How monosaccharides are joined?
Two monosaccharides are covalently joined to form a
glycosidic bond
How a glycosidic bond is formed?
When the hydroxyl group of one sugar reacts with the
anomeric carbon of the other sugar, one water
molecule is eliminated and a glycosidic bond is
formed.
Structures of some disaccharides
Polysaccharides
Definitions: carbohydrates containing many
monosaccharides units connected by a
glycisidic bond are called polysaccharides
Storage polysaccharides Structural Polysaccharides
Starch (plant)
Glycogen (liver)
Cellulose (bark)
Chitin(crabs, lobster)
Hyaluronic acids (connective
tissues)
Chitin (shells of crabs, lobsters)
Storage Polysaccharides
Starch
Amylose
Amylopectin
glucose units: 60-300
300-6000
Mol.Wt.10,000-50,000
50,000-1,000,000
Chain: Straight structure
Highly branched structure
Joined by: -1,4-glycosidic
1,6linkage
-1,4-glycosidic + glycosidic linkage
Cellulose
Structural Polysaccharides
The important structural polysaccharide is cellulose
- it is the material of plant cell walls, as in wood
- is a linear polymer of D- glucose units (300,
15,000)
- They are linked by beta 1-4 linkage
Blood group antigens: notice difference in oligosaccharide structure
Castor bean plant
From http://www.portfolio.mvm.ed.ac.uk/studentwebs/session2/group12/ricin.htm
Ricin
from http://www.portfolio.mvm.ed.ac.uk/studentwebs/session2/group12/ricin.htm