8. Absorption of Carbohydrates
• The monosaccharides: glucose about 80% to 85% and a
few disaccharides are absorbed from small intestine
through small capillaries of the intestinal mucosa. The
order of absorption of monosaccharides is:
• Galactose > glucose > fructose > mannose > pentose.
• Their relative rates of absorption as compared to glucose
are given below:
9. Rates of absorption as compared to
glucose
• Galactose 110%
• Glucose 100%
• Fructose 40%
• Mannose 20%
• Xylose 15%
10. • Certain carrier systems operate for sugar absorption and
there is a competition between various sugars for
absorption.
• If large amounts of galactose are being transported the
rate of glucose transport is reduced.
• Two mechanisms are apparently involved for the
absorption of sugars from small intestine.
i. Simple diffusion
ii. Active transport.
11. • Simple diffusion: it depends upon the concentration
gradient between intestinal lumen of mucosal cells and
blood plasma. Sugar molecules tend to move from higher
glucose concentration of small intestine to lower glucose
concentration of blood plasma.
• Active transport: Active transport involves the transport of
sugar from lower concentration to higher sugar
concentration by the application of energy produced as a
result of metabolic processes.
12. FACTORS AFFECTING ABSORPTION OF
SUGAR
1. Physical factors: rate of absorption is decreased with a
hurried passage of food through the small intestine in a
condition like diarrhea.
2. Hormones: the absorption of sugars from the small
intestine is subject to the endocrine influences. The
thyroid hormones increase the rate of absorption.
Hyperthyroidism increases the rate of glucose absorption
and hypothyroidism depresses the rate of glucose
absorption.
13. • Hormones of adrenal cortex have been found to facilitate
absorption. They help in presence of Na ions. A deficiency
of these hormones results in the Addison’s disease which
is due to depressed absorption of glucose from small
intestine.
• Active transport of sugar is depressed by the agents
which inhibit respiration (CN-, N-3 etc.) and by
dinitrophenol which uncouples oxidation from
phosphorylation. Phosphorylation is involved in absorption
of sugars.
14. • Certain vitamins i.e. thiamine, pyridoxine and pantothenic
acid are also required for absorption of sugars.
Consequently the rate of absorption is decreased in the
absence of these factors.
15. Facts about mechanism of monosaccharides
transport
1. Chemical nature of most actively transported
monosaccharides has the following features in the choice
of carriers:
a) Presence of six or more carbon atoms in the molecule.
b) D-pyranose ring structure
c) An intact –OH group at C-2.
16. • 2. Glucose and galactose transport is blocked when
sodium transport is blocked. This is thought to be due to
presence of carrier in the brush cells which has receptive
sights for both glucose and sodium.it does not move when
the both sites are not filled. That’s why absorption of
glucose is blocked if sodium absorption is blocked. The
energy to cause movement of the carrier from exterior to
the interior of the membrane is derived from the difference
in sodium concentration between outside and inside. This
is called sodium gradient theory of glucose transport.
17. • The transport of fructose is somewhat different from that
of glucose. It is not inhibited by metabolic poisons
specifically by phlorizin. It is converted into glucose in the
epithelial cells before absorption into portal blood.
• The disaccharides are not hydrolyzed in the small
intestinal lumen. They are absorbed as such in the
mucosal cell lining of small intestine.
