Transcript Carbohydrates - cpprashanths Chemistry

Carbohydrates
Carbohydrates-sugars
• Made of C, H,O
• Carb = Carbon hydr = water Carbohydrate = carbon +
water
• general formula = CH2O 2-1 ratio of hydrogen to
oxygen like water H2O
ribose
C5H10O5
glucose C6H12O6
sucrose C12H22O11
• many carbohydrate names end in -ose
More carbohydrate basics
• Monomer: monosaccharide – one sugar
• Functions of carbohydrates:
– Energy for metabolism (glucose)
– Short term energy storage (glycogen/starch)
– Structure: plants – cell wall animals –
exoskeleton
– Source of carbon for other molecules
– Cell surface markers – cell identification
Monosaccharides
Aldoses (e.g., glucose) have an
aldehyde group at one end.
H
Ketoses (e.g., fructose) have
a keto group, usually at C2.
O
CH2OH
C
C
O
HO
C
H
OH
H
C
OH
OH
H
C
OH
H
C
OH
HO
C
H
H
C
H
C
CH2OH
CH2OH
D-glucose
D-fructose
Monosaccharide: Simple Sugars
• Monossaccharides like glucose are the main
source of energy in living things
D vs L Designation
CHO
CHO
D & L designations are
H C OH
based on the
CH2OH
configuration about
the single asymmetric D-glyceraldehyde
C in glyceraldehyde.
HO
H
C
OH
CH2OH
D-glyceraldehyde
H
CH2OH
L-glyceraldehyde
CHO
The lower
representations are
Fischer Projections.
C
CHO
HO
C
H
CH2OH
L-glyceraldehyde
Sugar Nomenclature
For sugars with more
than one chiral center,
D or L refers to the
asymmetric C farthest
from the aldehyde or
keto group.
Most naturally occurring
sugars are D isomers.
O
H
C
H – C – OH
HO – C – H
H – C – OH
H – C – OH
CH2OH
D-glucose
O
H
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
L-glucose
D & L sugars are mirror
images of one another.
They have the same
name, e.g., D-glucose
& L-glucose.
Other stereoisomers
have unique names,
e.g., glucose, mannose,
galactose, etc.
O
H
C
H – C – OH
HO – C – H
H – C – OH
H – C – OH
CH2OH
D-glucose
O
H
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
L-glucose
The number of stereoisomers is 2n, where n is the
number of asymmetric centers.
The 6-C aldoses have 4 asymmetric centers. Thus
there are 16 stereoisomers (8 D-sugars and 8 Lsugars).
Hemiacetal & hemiketal formation
An aldehyde can
react with an
alcohol to form
a hemiacetal.
A ketone can
react with an
alcohol to form
a hemiketal.
H
C
H
O
+
R'
OH
R'
O
R
OH
R
aldehyde
alcohol
hemiacetal
R
C
C
R
O
+
"R
OH
R'
ketone
"R
O
C
R'
alcohol
hemiketal
OH
Pentoses and
hexoses can cyclize
as the ketone or
aldehyde reacts
with a distal OH.
Glucose forms an
intra-molecular
hemiacetal, as the
C1 aldehyde & C5
OH react, to form
a 6-member
pyranose ring,
named after pyran.
1
H
HO
H
H
2
3
4
5
6
CHO
C
OH
C
H
C
OH (linear form)
C
OH
D-glucose
CH2OH
6 CH2OH
6 CH2OH
5
H
4
OH
H
OH
3
H
O
H
H
1
2
OH
-D-glucose
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
OH
-D-glucose
These representations of the cyclic sugars are called
Haworth projections.
H
CH2OH
1
HO
H
H
2C
O
C
H
C
OH
C
OH
3
4
5
6
HOH2C 6
CH2OH
D-fructose (linear)
H
5
H
1 CH2OH
O
4
OH
HO
2
3
OH
H
-D-fructofuranose
Fructose forms either
 a 6-member pyranose ring, by reaction of the C2 keto
group with the OH on C6, or
 a 5-member furanose ring, by reaction of the C2 keto
group with the OH on C5.
6 CH2OH
6 CH2OH
5
H
4
OH
O
H
OH
3
H
H
2
OH
-D-glucose
H
1
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
H
OH
-D-glucose
Cyclization of glucose produces a new asymmetric center
at C1. The 2 stereoisomers are called anomers,  & .
Haworth projections represent the cyclic sugars as having
essentially planar rings, with the OH at the anomeric C1:
  (OH below the ring)
  (OH above the ring).
Sugar derivatives
CHO
COOH
CH2OH
H
C
OH
H
C
OH
H
C
OH
CH2OH
D-ribitol
H
C
OH
HO
C
H
OH
H
C
OH
OH
H
C
OH
H
C
OH
HO
C
H
H
C
H
C
CH2OH
D-gluconic acid
COOH
D-glucuronic acid
 sugar alcohol - lacks an aldehyde or ketone; e.g., ribitol.
 sugar acid - the aldehyde at C1, or OH at C6, is oxidized
to a carboxylic acid; e.g., gluconic acid, glucuronic acid.
Glycosidic Bonds
The anomeric hydroxyl and a hydroxyl of another sugar
or some other compound can join together, splitting out
water to form a glycosidic bond:
R-OH + HO-R'  R-O-R' + H2O
E.g., methanol reacts with the anomeric OH on glucose
to form methyl glucoside (methyl-glucopyranose).
H OH
H OH
H2O
H O
HO
HO
H
H
H
+
CH3-OH
H O
HO
HO
H
OH
H
OH
-D-glucopyranose
methanol
H
OH
OCH3
methyl--D-glucopyranose
Disaccharides-2 sugars
• 2 monosaccharides linked together by a
condensation reaction
- form a glycosidic bond
Examples:
Sucrose – Table Sugar
glucose + fructose
Lactose – Milk Sugar
glucose + galactose
Maltose –
glucose + glucose
Disaccharides:
Maltose, a cleavage
product of starch
(e.g., amylose), is a
disaccharide with an
(1 4) glycosidic
link between C1 - C4
OH of 2 glucoses.
It is the  anomer
(C1 O points down).
6 CH2OH
6 CH2OH
H
5
O
H
OH
4
OH
3
H
H
H
1
H
4
4
maltose
OH
H
H
1
OH
2
H
OH
2OH
H
H
1
O
4
5
O
H
OH
H
H
3
H
6 CH
O
H
OH
H
OH
3
OH
5
O
O
2
6 CH2OH
H
5
2
OH
3
cellobiose
H
2
OH
OH
1
H
Other disaccharides include:
 Sucrose, common table sugar, has a glycosidic bond
linking the anomeric hydroxyls of glucose & fructose.
Because the configuration at the anomeric C of glucose
is  (O points down from ring), the linkage is (12).
The full name of sucrose is -D-glucopyranosyl-(12)-D-fructopyranose.)
 Lactose, milk sugar, is composed of galactose & glucose,
with (14) linkage from the anomeric OH of galactose.
Its full name is -D-galactopyranosyl-(1 4)--Dglucopyranose
Polysaccharides-Many Sugars
• Polysaccharides are polymers composed of
large numbers of monosaccharides.
- the monosaccharides are joined by
condensation reactions.
- form glycosidic bonds
• Used for short term
energy storage and
structure
CH 2OH
H
O
H
OH
H
H
H
1
O
OH
6CH OH
2
5
O
H
4 OH
3
H
OH
H
H
H
H 1
O
H
OH
CH 2OH
CH 2OH
CH 2OH
H
H
H
O
H
OH
H
O
O
H
H
O
H
OH
H
O
OH
2
OH
H
OH
H
OH
H
H
OH
amylose
Polysaccharides:
Plants store glucose as amylose or amylopectin, glucose
polymers collectively called starch.
Glucose storage in polymeric form minimizes osmotic
effects.
Amylose is a glucose polymer with (14) linkages.
The end of the polysaccharide with an anomeric C1 not
involved in a glycosidic bond is called the reducing end.
CH 2OH
CH 2OH
O
H
H
OH
H
H
OH
H
O
OH
CH 2OH
H
OH
H
OH
H
H
OH
H
H
OH
CH 2OH
O
H
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
H
O
4
amylopectin
H
1
O
6 CH 2
5
H
OH
3
H
CH 2OH
O
H
2
OH
H
H
1
O
CH 2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
H
OH
H
OH
Amylopectin is a glucose polymer with mainly (14)
linkages, but it also has branches formed by (16)
linkages. Branches are generally longer than shown above.
The branches produce a compact structure & provide
multiple chain ends at which enzymatic cleavage can occur.
CH 2OH
CH 2OH
O
H
H
OH
H
H
OH
H
O
OH
CH 2OH
H
H
OH
H
H
OH
H
H
OH
CH 2OH
O
H
OH
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
H
O
4
glycogen
H
1
O
6 CH 2
5
H
OH
3
H
CH 2OH
O
H
2
OH
H
H
1
O
CH 2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
H
OH
H
OH
Glycogen, the glucose storage polymer in animals, is
similar in structure to amylopectin.
But glycogen has more (16) branches.
The highly branched structure permits rapid glucose
release from glycogen stores, e.g., in muscle during
exercise.
The ability to rapidly mobilize glucose is more essential to
animals than to plants.
CH 2OH
O
H
H
OH
H
OH
H
1
O
H
H
OH
6CH OH
2
5
O
H
4 OH
3
H
H
H 1
2
OH
O
O
H
OH
CH 2OH
CH 2OH
CH 2OH
H
H
O
O
H
OH
H
OH
O
H
O
H
OH
H
OH
OH
H
H
H
H
H
H
H
OH
cellulose
Cellulose, a major constituent of plant cell walls, consists
of long linear chains of glucose with (14) linkages.
Every other glucose is flipped over, due to  linkages.
This promotes intra-chain and inter-chain H-bonds and
van der Waals interactions, that cause cellulose chains
to be straight & rigid, and
pack with a crystalline arrangement in thick bundles -
Energy Storage Polysaccharides
• Starch
– polymer made
up of glucose
monomers
– Stores glucose
in plants
Chloroplast
Starch
1 m
Starch: a plant polysaccharide
• Glycogen
– Polymer of glucose monomers
– Is the major storage form of glucose in
Mitochondria Glycogen
animals
granules
– Stored in liver and muscle
– More highly branched than
starch – contains more
stored energy
0.5 m
Glycogen
Glycogen: an animal polysaccharide
Starch
Easily
• Allows
the and
storedGlycogen
glucose to beare
easily
used
Broken Apart by Hydrolysis
Structural Polysaccharides
Cellulose
 Is a polymer of glucose – connected in a
straight unbranched chain
 Multiple strands of cellulose are held
together by hydrogen bonds – makes a
rigid structure
 Is a major component of the tough walls
that enclose plant cells
Cell walls
Cellulose microfibrils
in a plant cell wall
Microfibril
About 80 cellulose
molecules associate
to form a microfibril, the
main architectural unit
of the plant cell wall.
0.5 m
Plant cells
Parallel cellulose molecules are
held together by hydrogen
bonds between hydroxyl
groups attached to carbon
atoms 3 and 6.
Figure 5.8
OH CH2OH
OH
CH2OH
O O
O O
OH
OH
OH
OH
O
O O
O O
O CH OH
OH
CH
2
2OH
H
CH2OH
OH CH2OH
OH
O O
O O
OH
OH
OH
OH
O
O O
O O
O CH OH
OH
CH
2
2OH
H
CH2OH
OH
OH CH2OH
O O
O O
OH
OH
OH O
O OH
O O
O
O CH OH
OH CH2OH
2
H
 Glucose
monomer
Cellulose
molecules
A cellulose molecule
is an unbranched 
glucose polymer.
• Cellulose and starch are both polymers of
glucose,
but the bonds which hold them together are
different
• Cellulose is difficult to digest
– Animals can’t break the bonds between the
glucose molecules –dietary fiber
– Animals that eat plants have bacteria in their
stomachs that can break the bonds of celluloseallow their hosts to digest plants
Figure 5.9
• Chitin, another important structural
polysaccharide
– Is a polymer of a form of glucose with an
attached functional group
– Is found in the exoskeleton of arthropods
CH2O
H
O OH
H
H
OH H
OH
H
H
NH
C
O
CH3
(a) The structure of the
chitin monomer.
Figure 5.10 A–C
(b) Chitin forms the exoskeleton (c) Chitin is used to make a
of arthropods. This cicada
strong and flexible surgical
is molting, shedding its old
thread that decomposes after
exoskeleton and emerging
the wound or incision heals.
in adult form.
3 Molecules Made From Glucose –
What’s the Difference?
• Differences in bonding
and shape give the
molecules different
functions
Other Uses for Carbohydrates
• Cell surface markers – carbohydrates attached
to parts on the cell membrane where they act
to identify the cell
• ABO blood groups
are identified by
carbohydrates on
their surface