Download Lecture series Gastrointestinal tract

Dr Pradeep Kumar, Professor department of
physiology, KGMU, Lucknow
We need a range of
different types of food

Chemical substances found in the largest amounts in
food:
◦ Water (found inside cells in plants and animals)
◦ Carbohydrates
◦ Fats
◦ Protein
◦ Miniral
◦ vitamins
•
Understand the composition of protein
•
Understand the enzymes responsible for digestion of
proteins
•
What are sites for absorption
Molecular basis of protein transportation
•
Learn about diseases related to protein digestion

Proteins are a sequence of amino acids

One amino acids is joined to the next by a PEPTIDE
bond

Provide energy substrate for metabolism (4 kcals/g).
Protein load received by the gut is derived from two
primary sources: 70-100 g dietary protein, and
35-200 g endogenous protein,

Of the 20 amino acids that exist, 9 are essential
amino acids, and 11 are non-essential

There are also 4 amino acids that can be considered
conditionally essential:

arginine, tyrosine, glutamine, and cysteine

Dipeptide – 2 amino acids

Tripeptide – 3 amino acids

Oligopeptides – 4-10 amino acids

Polypeptide – more than 10 amino acids

Proteins in the body and diet are long polypeptides
(100s of amino acids)
Consist of a central carbon
atom bonded to: a hydrogen,
a carboxylic acid, an amino
group, and an additional
side group that is unique to
each amino acid

Acid, alkaline, heat and alcohol, can disrupt the
chemical forces that stabilize proteins and can cause
them to lose their shape (denature)

Denaturing of proteins happens during food
preparation (cooking, whipping, adding acids) or
digestion (in the stomach with hydrochloric acid)

No digestion of protein takes place in the mouth,

Hydrochloric acid denatures protein and also
converts pepsinogen to pepsin

Pepsin breaks the protein down into peptides of
various lengths and some amino acids

Pepsin act only at pH 1.6-3.2

Pepsin completes ~ 10-20% of digestion

Pepsin hydrolyses the bonds
between aromatic amino
acids(phenylanine or
tyrosine) and a second
amino acid

So the product of pepsin
hydrolysis is polypeptides
of diverse sizes

Trypsinogen and
chymotrypsinogen (proenzymes)
are secreted by pancreas in
response to protein in the small
intestine

They will be activated to trypsin
and chymotrypsin (now called
proteases)

These enzymes can either cleave
internal peptide bonds (i.e.
endopeptidases)

exopeptidases cleave off one amino
acid at a time from either the –COOH
or –NH2 terminal of the polypeptide
(i.e. they are carboxypeptidases , and
aminopeptidases, respectively)

The endopeptidases cleave the large polypeptides to
smaller oligopeptides, which can be acted upon by the
exopeptidases to produce the final products of protein
digestion, amino acids, di- and tripeptides, which are
then absorbed by the enterocytes

By the action of endo and exopeptidases some free
amino acids are liberated in the intestinal lumen,

But others are liberated at the cell surface by the
aminopeptidases, carboxypeptidases, endopeptidases,
and dipeptidases in the brush border of the mucosal
cells.

The di- and tripeptides are
actively transported into
enterocytes by a system
known as peptide
transporter 1) that requires
H + instead of Na +


The movement of any one amino acid can occur through one or
more amino acid transporters.
At least five amino acid transporters are present in the
basolateral membrane.

Three amino acid transport processes on the basolateral
membrane mediate amino acid exit from the cell into the blood

Two other amino acid transporters mediate uptake from the
blood for the purposes of cell nutrition.
Individual amino acids are
transported across the basolateral
membrane without the need for
cotransport.
Many different amino acid
transporters are located on the
basolateral membrane and provide
specificity

Absorption of amino acids is rapid in the duodenum and
jejunum. There is little absorption in the ileum in health

There is little absorption in the ileum in health, because the
majority of the free amino acids have already been
assimilated at that point.

Approximately 50% of the digested protein comes from ingested
food, 25% from proteins in digestive juices, and 25% from
desquamated mucosal cells.

Only 2–5% of the protein in the small intestine escapes
digestion and absorption. Some of this is eventually digested by
bacterial action in the colon.

During the postnatal period, intestinal epithelial cells
absorb protein by endocytosis,

a process that provides a mechanism for transfer of
passive immunity from mother to child.

The uptake of intact protein by the epithelial cell
ceases by the sixth month

The adult intestine can absorb some amounts of
intact protein and polypeptides by the process of
endocytosis , may act as an allergens, most of this
protein is degraded in lysosomes ,

Acute Pancreatitis
Premature activation of pancreatic proteolytic enzymes in the
pancreas itself causes digestion of the secretory mucosa causing
Acute pancreatitis. It is a life-threatening condition.

In conditions of deficient pancreatic secretions like cystic
Fibrosis, chronic pancreatitis or surgical removal of pancreas, the
digestion and absorption of fats and proteins is left incomplete
with the resultant appearance of lipids and undigested proteins in
the feces. This condition is called Steatorrhea

Hartnup disease and cystinuria are hereditary disorders of
amino acid transport across the apical membrane.

These autosomal recessive disorders are associated with both
small intestine and renal tubule abnormalities

the absorption of neutral amino acids in the case of Hartnup
disease and of cationic (i.e., basic) amino acids and cystine in
the case of cystinuria.




Lippincott’s Illustrated Reviews: Physiology (2013)
Medical Physiology, UPDATED SECOND EDITION
(Walter F. Boron, MD, PhD)
BERNE & LEVY, PHYSIOLOGY, SIXTH EDITION,
UPDATED EDITION
Ganong’s Review of Medical Physiology, T W E N T Y -F O U R T H
EDITION
a. Alkaline PH
b. Enterokinase
c. Bile salts
D. Biliverdin
a. Alkaline PH
b. Enterokinase
c. Bile salts
D. Biliverdin
a. Enterokinase
b. Alkaline PH
c. Trypsin
d. Bile salts
a. Enterokinase
b. Alkaline PH
c. Trypsin
d. Bile salts
a. gastrin
B. secretin
b. Bile salts
c. Enterokinase
a. gastrin
B. secretin
b. Bile salts
c. Enterokinase
(A) Trypsinogen is activated by enterokinase.
(B) Chymotrypsinogen is activated by trypsin
(C) Pancreatic lipase is activated by alkaline pH
(D) Bile salts causes emulsification of lipids
(A) Trypsinogen is activated by enterokinase.
(B) Chymotrypsinogen is activated by trypsin
(C) Pancreatic lipase is activated by alkaline pH
(D) Bile salts causes emulsification of lipids
(A) Bile salts.
(B) Secretin.
(c) Acetylcholine.
(D) Bile pigments
(A) Bile salts.
(B) Secretin.
(c) Acetylcholine.
(D) Bile pigments
(A) Aminopeptidase.
(B) Carboxypeptidase.
(C) Nucleases.
(D) Dipeptidase.
(A) Aminopeptidase.
(B) Carboxypeptidase.
(C) Nucleases.
(D) Dipeptidase.

A. Apical fructose uptake

B. Basolateral glucose transport

C. Apical glucose uptake

D. Basolateral amino acid transport

A. Apical fructose uptake

B. Basolateral glucose transport

C. Apical glucose uptake

D. Basolateral amino acid transport
a)
brunner’s gland
b)
Crypts of lieberkuhns
c)
Peyer’s patches
d)
Gut associated lymphoid tissues
a)
brunner’s gland
b)
Crypts of lieberkuhns
c)
Peyer’s patches
d)
Gut associated lymphoid tissues
a)
Bile salts
b)
CCK
c)
Secretin
d)
Gastrin
a)
Bile salts
b)
CCK
c)
Secretin
d)
Gastrin
a)
Secretin
b)
CCK
c)
Gastrin
d)
GIP
a)
Secretin
b)
CCK
c)
Gastrin
d)
GIP
Thank you
Dr Pradeep Kumar, Professor department of
physiology, KGMU, Lucknow