4. Carbohydrates present in the
diet
Polysaccharides
Disaccharides Monosaccharides
Starch
Glycogen
Lactose
Maltose
Sucrose
Glucose
Fructose
Pentose
In GIT, all complex carbohydrates are
converted to simpler monosaccharide form
which is the absorbable form.
5. Details of digestion of carbohydrates
2 Types of enzymes are important for the digestion of
carbohydrates
Amylases Disaccharidases
Salivary
Amylase
Pancreatic
Amylase
convert polysaccharides to disaccharides
Convert disaccharides to
monosaccharides which are
finally absorbed
Maltase
Sucrase-Isomaltase
Lactase
Trehalase
7. Digestion of Carbohydrate starts in the
mouth, upon contact with saliva during mastication.
Saliva contains a carbohydrate splitting enzyme called
salivary amylase , also known as ptylin.
Digestion in the Mouth
8. Action of ptylin (salivary
amylase)
• Location: mouth
• It is α-amylase and requires Cl− ion for
activation with an optimum pH of 6.7 (Range
6.6 to 6.8).
• The enzyme hydrolyses α-1→ 4 glycosidic
linkages deep inside polysaccharide
molecules.
• However, ptylin action stops in the stomach
when the pH falls to 3.0.
9. Starch, Glycogen and dextrins
(Large polysaccharide molecules)
α- Amylase
Glucose,Maltose and Maltotriose.
(Smaller molecules)
10. Drawback
• Shorter duration of food in mouth.
• Thus it is incomplete digestion of starch or
glycogen in the mouth
11. Digestion in the Stomach
• There is no enzyme to break the glycosidic
bonds in gastric juice.
• However, HCl present in the stomach causes
hydrolysis of sucrose to fructose and glucose.
Sucrose Fructose + Glucose
HCl
12. Digestion in Duodenum
• Food bolus reaches the duodenum from the
stomach where it meets the pancreatic juice.
• Pancreatic juice contains a carbohydrate
splitting enzyme,
pancreatic amylase
(amylopsin) similar
to salivary amylase.
13. Action of pancreatic amylase
• It is an α- Amylase
• Optimum pH=7.1
• Like ptylin, it requires Cl− ion for its activity.
• It hydrolyses α-1→ 4 glycosidic linkages
situated well inside polysaccharide molecules.
• Note: Pancreatic amylase, an isoenzyme of salivary
amylase, differs only in the optimum pH of action. Both
the enzymes require Chloride ions for their actions (Ion
activated enzymes).
14. Reaction catalyzed by pancreatic amylase
Starch/Glycogen
Maltose/ Isomaltose
+
Dextrins and
oligosaccharides
Pancreatic
Amylase
15. Digestion in Small Intestine
Note:
• Main digestion takes place in the small
intestine by pancreatic amylase
• Digestion is completed by pancreatic amylase
because food stays for a longer time in the
intestine.
16. What are Disaccharidases?
• They are present in the brush border epithelium of
intestinal mucosal cells where the resultant
monosaccharides and others arising from the diet
are absorbed.
• The different disaccharidases are :
1) Maltase,
2) Sucrase-Isomaltase (a bifunctional enzyme
catalyzing hydrolysis of sucrose and isomaltose)
3) Lactase
18. Clinical significance of Digestion
• Lactose intolerance is the inability to digest
lactose due to the deficiency of Lactase
enzyme.
• Causes
Congenital Acquired during lifetime
Primary Secondary
19. Congenital Lactose intolerance
• It is a congenital disorder
• There is complete absence or deficiency of
lactase enzyme.
• The child develops intolerance to lactose
immediately after birth.
• It is diagnosed in early infancy.
• Milk feed precipitates symptoms.
21. Primary Lactase deficiency
• Primary lactase deficiency develops over time
• There is no congenital absence of lactase but
the deficiency is precipitated during adulthood.
• The gene for lactose is normally expressed upto
RNA level but it is not translated to form
enzyme.
• It is very common in Asian population.
• There is intolerance to milk + dairy products.
23. Secondary lactase deficiency
• It may develop in a person with a healthy small intestine during
episodes of acute illness.
• This occurs because of mucosal damage or from medications
resulting from certain gastrointestinal diseases, including
exposure to intestinal parasites such as Giardia lamblia.
• In such cases the production of lactase may be permanently
disrupted.
• A very common cause of temporary lactose intolerance is
gastroenteritis, particularly when the gastroenteritis is caused
by rotavirus.
• Another form of temporary lactose intolerance is lactose
overload in infants. Secondary lactase deficiency also results
from injury to the small intestine that occurs with celiac
disease, Crohn’s disease, or chemotherapy.
• This type of lactase deficiency can occur at any age but is more
common in infancy.
25. Clinical manifestations
• In the form of abdominal
cramps, distensions, diarrhea, constipation, flatulenc
e upon ingestion of milk or dairy products
Biochemical basis
• Undigested lactose in
intestinal lumen is
acted upon by bacteria
and is converted to
CO2 , H2 , 2 carbon
compounds and 3 carbon
compounds or it may
remain undigested.
26. • CO2 and H2 causes Distensions and flatulence
• Lactose + 2C + 3C are osmotically active.
• They withdraw H2O from intestinal mucosal
cell and cause osmotic diarrhea or
constipation because of undigested bulk.
Abdominal distension Flatulence
27. Diagnosis
• Two tests are commonly
used: -
• Hydrogen Breath Test
• The person drinks a lactose-
loaded beverage and then
the breath is analyzed at
regular intervals to measure
the amount of hydrogen.
Normally, very little
hydrogen is detectable in
the breath, but undigested
lactose produces high levels
of hydrogen. The test takes
about 2 to 3 hours.
28. • Stool Acidity Test
• The stool acidity test is used for
infants and young children to
measure the amount of acid in
the stool. Undigested lactose
creates lactic acid and other
short chain fatty acids that can be
detected in a stool sample.
Glucose may also be present in
the stool as a result of undigested
lactose.
• Besides these tests, urine shows-
positive test with Benedict’s
test, since lactose is a reducing
sugar and a small amount of
lactose is absorbed in the
intestinal cell by pinocytosis and
is rapidly eliminated through
kidneys in to urine.(Lactosuria)
• Mucosal biopsy confirms the
diagnosis.
29. Management of lactose intolerance
• Avoidance of dairy products.
• Although the body’s ability to produce lactase
cannot be changed, the symptoms of lactose
intolerance can be managed with dietary changes.
• Most people with lactose intolerance can tolerate
some amount of lactose in their diet. Gradually
introducing small amounts of milk or milk products
may help some people adapt to them with fewer
symptoms.
• Partly digested dairy products can also be given.
30. • Lactose-free, lactose-reduced milk,
Soy milk and other products may be
recommended.
• Lactase enzyme drops or tablets(Yeast tablets)
can also be consumed.
• Getting enough calcium is important for people
with lactose intolerance when the intake of milk
and milk products is limited.
• A balanced diet that provides an adequate
amount of nutrients—including calcium
and vitamin D—and minimizes discomfort is to
be planned for the patients of lactose
intolerance.
31. Sucrase-Isomaltase deficiency
• These 2 enzymes are synthesized on a single polypeptide
chain,hence , their deficiencies coexist.
• Signs and symptoms
• Same as that of lactose intolerance.
• Urine does not give +ve test with Benedict’s test because of
sucrose(Non reducing sugar).
• History confirms the diagnosis.
• Most confirmatory test is mucosal
biopsy.
33. Features Passive diffusion Facilitated diffusion Active transport
Concentration
gradient
Down the
concentration
gradient from high to
low.
Down the
concentration
gradient from high to
low.
Against a
concentration
gradient from low to
high
Energy expenditure none none Energy expenditure is
in the form of ATP
Carrier protein/
transporter
Not required required required
Speed Slowest mode Fast Fastest mode
Note: Glucose is a polar molecule. It cannot
pass through lipid bilayer of cell.
35. Na+ dependent transporter
• Type of co-transport
• 2 binding sites on the transporter, one for Na+ and other
for glucose.
• Na+ binding is important because after Na+
binding, conformational changes occurs so that glucose
can bind.
• Na+ is transported across cell membrane, down the
concentration gradient and glucose goes against a
concentration gradient.
• ATP is spent at the level of Na-K ATPase pump to expel
Na out.
• Both glucose and galactose are absorbed by a sodium-
dependent process.
• They are carried by the same transport protein (SGLT
36. Figure- Showing the co transport of Glucose, mediated by SGLT-
1/2. SGLT-1 are present on the intestinal cells while SGLT-2
are present on the proximal renal tubular cells.
37. 3 reasons for expulsion of sodium
1) Na + is osmotically active, causes osmotic
flow to cells, leading to osmolysis.
2) Na + concentration has to be kept minimal to
maintain the downward gradient.
3) Na + is inhibitory to many enzyme actions.
38. Downward
gradient of
Na+
releases
energy
Na expelled
out through
Na-K
ATPase
pump.
3 sodium
are expelled
out and 2 K
are
internalised
The Na removed
to paracellular
spaces exerts
osmotic pressure
that causes flow of
water to
intracellular
spaces
Water
carries
dissolved
glucose with
it
This type of
absorption
is called
solvent drag
Energy released is
captured for transport of
glucose against a conc.
Gradient.
Energy is
consumed
at the level
of ATP
Solvent
drag
absorption
39. Clinical significance
• In deficiency of SGLT- 1, glucose is left
unabsorbed and is excreted in feces.
Galactose is also malabsorbed.
• In deficiency of SGLT- 2, the filtered glucose is
not reabsorbed back, it is lost in urine, causing
glycosuria.
40. • Solvent drag is not the main mechanism of
glucose absorption but is important after a
carbohydrate rich diet.
• Absorption of galactose is faster than glucose.
• In kidney, reabsorption of filtered glucose
takes place by a similar mechanism, i.e, it is
also a co-transport with Na. The transporter is
SGLT- 2.
• In intestine, it is SGLT- 1.
41. Na+ independent transporters
• Used for facilitated transport.
• These transporters are numbered from 1 to
14 GLUT.
• In the intestine, GLUT 2 are present towards
the serosal surface of intestinal epithelial
cells and GLUT 5 are present towards the
luminal surface.
44. Purpose of GLUT 5 and GLUT 2
> For the transportation of
fructose.(least affinity for glucose).
>Fructose is mainly transported by
facilitated diffusion.
>By GLUT 5, fructose moves down the
gradient and the process is faster than
passive diffusion.
>Energy is not consumed.
>Mainly responsible for pouring all the
absorbed glucose into blood.
>GLUT 2 transporter transports all
absorbed glucose to blood. It is down a
concentration gradient.
>Transporter is present but there is no
energy expenditure.
>Pentoses are absorbed by passive
diffusion which is a very slow process.
45. Factors affecting rate of absorption of
Monosaccharides
• The absorption is faster through intact mucosa.
The absorption is decreased if there is some
inflammation or injury to the mucosa.
• Thyroid hormones ↑ the rate of absorption of
glucose.
• Mineralocorticoid,i.e Aldosterone ↑ the rate of
absorption.
46. • Vitamin B6,B12, pantothenic acid, folic acid are
required for absorption of glucose.
• With advancing age, rate of absorption declines.
• Note: Insulin has no role in the absorption of
monosaccharide like glucose.
47. Uptake of glucose in peripheral cells
• Mechanism: facilitated diffusion.
• There are 7 important glucose transporter for
uptake of glucose into special cells.
• They have been numbered from 1 to 7 (GLUT
1 to GLUT 7).
• They are biologically important.
48. Tissue specific
glucose transporter
Tissue distribution Functions Clinical significance
GLUT-1
(great affinity for
glucose)
Present in almost all
cells with an
abundance in RBC.
Na-independent Cancer cells express
high level of GLUT-1,
so they can
internalize more of
glucose, which is
used as a source of
energy for rapidly
dividing cells.
GLUT-2
(low affinity for
glucose, it can
transport only when
there is glucose load
in the body)
Present in intestine,
liver and pancreas.
Releases insulin by
movement of glucose
into β-cells of
pancreas.
(Acts as a sensor for
the release of insulin
by pancreas.)
Promotes uptake of
glucose in liver cells,
lowering down blood
glucose.
Diabetes Mellitus.
49. GLUT 3 Brain cells, all other
cells of body
Cancer cells express
high level of GLUT-
3, so they can
internalize more of
glucose, which is
used as a source of
energy for rapidly
dividing cells
GLUT 4 Adipose tissue,
skeletal muscles,
cardiac muscles
The only
transporters which
are under the
influence of insulin.
Insulin promotes
uptake of glucose in
the tissues by
mobilizing the
transporters to the
cell surface
whenever there is
high glucose
concentration in the
blood.
50. GLUT 5
(least affinity for
glucose)
Intestine at the
luminal surface,
testicles, seminal
vesicles
Mainly for the
transport of fructose
GLUT 6 Non-functional
transporter product of
a pseudogene.
No role in absorption
of glucose in
peripheral cells.
GLUT 7 Surface of
endoplasmic
reticulum
Transportation of
glucose across the
membrane of
endoplasmic
reticulum.
SGLT 1 Kidney, intestine For the absorption of
glucose.
In cases where SGLT 1
is deficient, glucose is
left unabsorbed and is
excreted in faeces.
SGLT 2 Kidney For the re-absorption
of glucose.
If deficient, filtered
glucose is not re-
absorbed and is lost in
urine.
