Digestion and absorbtion of
carbohydrates
Cellular Biochemistry
and Metabolism
Dr. Samah Kotb
Lecturer of Biochemistry
2015
Digestion
• Digestion is the breakdown of food into smaller
components that can be more easily absorbed and
assimilated by the body. In certain organisms, these
smaller substances are absorbed through the small
intestine into the blood stream.
Digestion
Digestion
 Digestion involves hydrolyzing food molecules into
smaller molecules for absorption through the
gastrointestinal epithelium.
 Polysaccharides
monosaccharides,
are
absorbed
triacylglycerols
as
as
2-
monoacylglycerols, fatty acids, glycerol and proteins
as amino acids.
Digestion
 The diet must provide:
 metabolic fuels (carbohydrate and fat) for bodily
growth and activity.
 protein for synthesis of tissue proteins.
 fiber for roughage.
 minerals for specific metabolic functions.
 vitamins,
organic
compounds
needed
in
amounts for many varied essential functions.
small
Functions of Carbohydrates
 Provide energy source:
• Living things use carbohydrates as their main source
of energy.
• The breakdown of sugar supplies immediate energy for
all cell activities.
 Provide energy storage:
• Plants store energy in a complex carbohydrate form
called starch.
• Animals store energy in a complex carbohydrate in
their muscle tissue and liver in the called glycogen.
Body’s Need
 Major source of energy:
 The body breaks carbohydrates into
simple sugars.
 These sugars are absorbed into the
bloodstream.
 As the sugar level rises in your body, the
pancreas releases a hormone called
insulin.
 Insulin is needed to move sugar from
the blood into the cells, where the sugar
can be used as a source of energy.
Body’s Need
•When this process goes fast as with simple sugars - you're
more likely to feel hungry
again soon.
•When it occurs more slowly,
as with a whole-grain food,
you'll be satisfied longer.
•These types of complex
carbohydrates give you energy
Carbohydrates are chains (polymers)
made of monomers.
The most common monomer of
carbohydrates is…
THERE ARE 2 TYPES OF
CARBOHYDRATES
Simple
Complex
Classes of Carbohydrates
Classifications based on number of sugar
units in total chain:
Monosaccharides - single sugar unit
Disaccharides - two sugar units
Oligosaccharides- 3 to 10 sugar units
Polysaccharides- more than 10 units
Monosaccharides
The basic building blocks (monomers) of carbohydrates.
(cannot be further hydrolyzed into smaller units).
Known as simple sugars.
Readily soluble in water.
Same no. of C as O atoms.
Have the general formula (CH2O)n
They contain:
– a carbonyl group (C=O)
1
2
– either at C atom or at the C atom
– multiple hydroxyl groups (-OH)
White crystalline solids.
e.g. Glucose is C6H12O6
Some important monosaccharides
 Glucose:
•
•
•
•
In plants and fruits.
Mild sweet flavor.
known as blood sugar.
Essential energy source.
 Fructose:
• Sweetest sugar
• Found in fruits and honey.
• Added to soft drinks, cereals, deserts.
 Galactose:
• Part of milk sugar.
• Hardly tastes sweet.
• Rarely found naturally as a single sugar.
Some important monosaccharides
 Glyceraldehyde
Simplest sugar
 Ribose
Found in RNA
 Deoxyribose
Found in DNA
Glyceraldehyde
Disaccharides
These are formed when two monosaccharide molecules join
together with the elimination of one molecule of water.
They have the general formula C12H22O11.
C6H12O6 + C6H12O6 =
Glucose + Glucose =
Sweet tasting.
Water soluble.
C12H22O11 + H2O
Maltose + Water
EXAMPLE:
Maltose
Sucrose
Lactose
Disaccharide Formation
Disaccharides are formed when two monosaccharides are joined by
dehydration synthesis reaction.
GLUCOSE + GLUCOSE -> Maltose (malt sugar)
GLUCOSE + FRUCTOSE -> Sucrose (cane sugar)
GLUCOSE + GALACTOSE -> Lactose (milk sugar)
CH2OH
H
CH2OH
O
H
H
CH2OH
O
H
H
CH2OH
O
H
H
O
H
H20
+
O
OH
α- GLUCOSE
OH
HO
α- GLUCOSE
OH
OH
OH
MALTOSE
Disaccharide Formation
• Building reaction.
• H2O is removed in order to form a new bond.
Hydrolysis Reaction
• Breaking reaction.
• H2O is required to break a bond.
Glycosidic linkage
• The bond between monosaccharides.
• What type of reaction would form this bond?
– Dehydration synthesis reaction.
• What kind of bond is a glycosidic linkage?
– Polar covalent bond.
OH
OH
H2 C
H 2C
O
OH
O
C
HO
OH
OH
OH
HO
HO
HO
OH
OH
H2C
H2C
O
OH
HO
O
C
OH
O
HO
HO
+ H2O
OH
Oligosaccharides
• Oligosaccharide is a few linked monosaccharides and
are at time associated with proteins (glycoproteins) or
lipids (glycolipids).
Polysaccharides
Polysaccharides are chains of monosaccharides that have
been joined by many dehydration synthesis reactions.
Do not taste sweet and do not crystallize.
Insoluble in water.
Form colloidal solutions when
added to water.
Starch
 Energy storage used by plants.
– Storage form of glucose in plants.
– Found in grains, tubers, and
legumes.
 Body hydrolyzes plant starch to
glucose.
 Long repeating chain of α-D-glucose.
 Chains up to 4000 units.
Glycogen
 Energy storage of animals:
– Storage form of glucose in the body.
– Provides a rapid release of energy when needed.
 Structure is similar to amylopectin but more branches.
 Made from α-glucose.
Glycogen
 Found mainly in liver and muscle cells.
 When the level of glucose in your blood runs low, glycogen is
released from your liver.
 The
glycogen stored in your muscles supplies the energy for
muscle contraction and thus, for movement.
DIGESTION
OF
CARBOHYDRATES
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Digestion & Absorption of
carbohydrates
The digestion of complex carbohydrates is by hydrolysis to
liberate oligosaccharides, then free mono- and disaccharides.
 Amylases Catalyze the hydrolysis of Starch
The hydrolysis of starch by salivary and pancreatic amylases
catalyze random hydrolysis of α(1→4) glycoside bonds,
yielding dextrins, then a mixture of glucose, maltose, and
isomaltose.
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Digestion & Absorption of
carbohydrates
 Disaccharidases (Brush Border Enzymes)
The disaccharidases—maltase, sucrase-isomaltase, lactase, are
located on the brush border of the intestinal mucosal cells where the
resultant monosaccharides and others arising from the diet are
absorbed.
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MECHANISMS FOR THE
ABSORPTION OF
MONOSACCHARIDES IN THE
SMALL INTESTINE
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Two Separate Mechanisms for the
Absorption of Monosaccharides in
the Small Intestine
 Glucose
and galactose are absorbed by a sodium-
dependent process. They are carried by the same
transport protein sodium-glucose linked transporter
(SGLT 1) and compete with each other for intestinal
absorption.
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Two Separate Mechanisms for the
Absorption of Monosaccharides in
the Small Intestine
 Other monosaccharides are absorbed by carriermediated diffusion. Because they are not actively
transported, fructose and sugar alcohols are only
absorbed down their concentration gradient.
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Transport of glucose, fructose, and galactose across
the intestinal epithelium.
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Two Separate Mechanisms for the
Absorption of Monosaccharides in
the Small Intestine
 The
SGLT 1 transporter is coupled to the Na+-K+
pump, allowing glucose and galactose to be
transported against their concentration gradients.
 The GLUT 5 Na+-independent facilitative
transporter allows fructose as well as glucose and
galactose to be transported with their concentration
gradients.
 Exit from the cell for all the sugars is via the GLUT
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2 facilitative transporter. Dr Samah Kotb
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