Carbohydrates
B.3
Introduction
• most abundant class of biological molecules
• range from simple sugars (glucose) to
complex carbohydrates (starch)
• major functions: (B.3.4)
– energy source: glucose converted into ATP
– energy storage/reserves: glycogen
– precursor for molecules such as DNA
Monosaccharides
•
(B.3.1 and 2)
simplest sugars (single sugars)
–
–
–
•
•
empirical formula is (CH2O)
contain a ketone, C=O (fructose) or aldehyde,
H-C=O (glucose)
contain at least two hydroxyl groups (-OH)
can be straight chains or cyclic form
two common monosaccharides
–
both C6H12O6
• glucose
• fructose
Glucose
both on same
side of ring
on different
sides of ring
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
H
Fructose
Condensation of monosaccharides to form
disaccharides and polysaccharides (B.3.3 and B.3.5)
• example of a typical condensation reaction
A-OH + B-OH ===> H2O + A-O-B
(2 monosaccharides =>
water + 1 disaccharide)
• requires enzymes
• the hydroxyl (-OH) of two monomers are brought
together and the H of one and the OH of the other
come together to make H2O
• the remaining O from one of the monomers bonds
the two together in a bond called a glycosidic
linkage
Hydrolysis
• the reverse of condensation
• the decomposition of a substance by the
insertion of water molecules between certain
of its bonds.
• food is digested by hydrolysis
Disaccharides
• double sugar (contains 2 monosaccharides)
• three common disaccharides:
1. sucrose - common table sugar
glucose + fructose
2. lactose - major sugar in milk
glucose + galactose
3. maltose - product of starch digestion
glucose + glucose
+
+
Polysaccharides (3 types)
• longer chains of simple sugars made of glucose
• serve principally as food storage and structural
molecules in plants
• 3 Types of Polysaccharides
1. Starches (plants)
– polymers of glucose molecules
– serve as storage depots of glucose
– two forms
• amylose- water soluble, straight chains, thousands of
glucoses
• amylopectin- insoluble, branched chains, up to
millions of glucoses
glucose
polymer with
(14)
linkages, and
branches
formed by 
(16)
linkages
glucose
polymer
with only 
(14)
linkages.
2. Glycogen (animals)
• a short-term storage
polysaccharide for
animals
• highly branched
glucose units put
together that are
broken down to meet
energy demands of
the body
3. Cellulose (plants)
• most abundant polysaccharide on Earth
• the major structural material of which plants are made
(wood and plant fibers)
• insoluble and has great tensile strength because the
hydroxyls (-OH) are reversed on successive glucoses
compared to other polysaccharides
• plant cell walls are among the strongest of biological
structures
• most organisms can’t break cellulose down into
simple sugars because they don’t have the enzyme
cellulase which is necessary to hydrolyze the
glycosidic linkages
Dietary fiber
B.3.6-7
• dietary fiber is plant material that we ingest
but are not able to digest
• passes through the gut relatively intact, as
we do not possess cellulase enzymes
capable of hydrolysing it
– bacteria in our gut can digest it somewhat
– example: cellulose
• importance in our diet
– helps “bulk” move through the large intestine
more eaisly
• prevent constipation and diverticulosis (bulges in the
colon at weak places leading to pain)
• may prevent irritable bowel syndrome (IBS)
• may prevent hemorrhoids
– may cause a “full feeling” and therefore
decrease chance of obesity
– may help prevent Crohn’s desease