This document discusses digestion and absorption of carbohydrates, proteins, and lipids in the gastrointestinal tract. It describes how carbohydrates, proteins, and lipids are broken down by digestive enzymes into smaller molecules that can be absorbed. Carbohydrates are broken into monosaccharides like glucose and fructose. Proteins are broken into amino acids, dipeptides and tripeptides. Lipids are emulsified, formed into micelles, and broken into fatty acids and monoglycerides which are absorbed and resynthesized into triglycerides for transport. The key sites of digestion and enzymes involved are also outlined.

2. Faculty of Medicine
Department of Physiology

4. Digestion and Absorption
 Introduction
 CARBOHYDRATES
 PROTEINS
 LIPIDS

5. Introduction
 Digestion and absorption are the ultimate functions of the
gastrointestinal tract
 Digestion is the chemical breakdown of ingested foods
into absorbable molecules
 The digestive enzymes are secreted in:-
1. Saliva
2. gastric juices
3. pancreatic juices
 Also they are found in apical membrane of intestinal
epithelial cells (Brush border(.

6. Introduction
 Absorption is the movement of nutrients, water, and
electrolytes from the lumen of the intestine into the
blood .
Absorption
cellular path paracellular path

7. basolateral membrane
lumen
apical (luminal) membrane
blood
Transporters
lateral intercellular spaces
tight junctions
intestinal epithelial cell

8. intestinal epithelial cell
cellular path
Para-cellular path

9. fold
3 times
villus
10 times
Microvilli
(brush border)
20 times
The surface area is increased=3X10X20=600 times
Total surface area is about 300 m2
tennis court

11. CARBOHYDRATES
 Ingested carbohydrates are :-
 Monosaccharides are those carbohydrates that cannot
be hydrolyzed into simpler carbohydrates (glucose and
fructose).
 Disaccharides are condensation products of two
monosaccharide units. (sucrose, lactose, maltose,,)
 Oligosaccharides are condensation products of two to
ten monosaccharides; maltotriose is an example.
 Polysaccharides are condensation products of more than
ten monosaccharide units (starch & Glycogen)

12. Digestion of Carbohydrates
 General rule
 Only monosaccharides are absorbed by the intestinal
epithelial cells
 CHO + Digestive enzyme = Monosaccharides→ absorbed
 Digestion of CHO:-
1. Starts → mouth (Salivary α-amylase)
2. Ends → small intestine (Pancreatic α-amylase +
Enzyme “Brush boarder” )
 Action of α-amylase
 The amylase enzymes target internal α-1,4 bonds of
both molecules

13. Digestion of Carbohydrates
 Action of α-amylase
 The amylase enzymes target internal α-1,4 bonds
of both molecules
 α-amylase is not acted on:-
1. terminal α-1,4 bonds
2. α-1,4 bonds near from branched-chain
3. α-1,6 bonds (branched-chain )
4. β-1,4 bonds

14. G
G G
G G
1. terminal α-1,4 bonds
2. α-1,4 bonds near from branched-chain
3. α-1,6 bonds (branched-chain )
4. β-1,4 bonds
G G
α-amylase
maltose
maltotriose
α-limit dextrins
X X
X
X
X
maltose
14
(Mohand Hassan Malla)

15. Digestion of Carbohydrates
 Starch and glycogen are converted into:-
1. Maltose
2. Maltotriose
3. α-limit dextrins
 These cannot be absorbed until converted
into monosaccharides
G
G
G G
G
G G
G

16. Digestion of Carbohydrates
 Enzyme present as part of brush border membrane
 These are :-
1. Maltase (Break maltose and maltotriose)
2. Isomaltas (digestion of dextrin)
3. Sucrase (Breaks sucrose = glucose & fructose)
4. Lactase (Breaks lactose = glucose & galactose)
 So that monosaccharides that able for absorption are
:- (glucose – fructose- galactose)
 Digestion by enzymes in brush border is known as
contact digestion

17. brush border
absorbable molecules
(monosaccharides)

18. Absorption of Carbohydrates
 The mechanism of monosaccharide
absorption by intestinal epithelial cells:-
1. Glucose and galactose are absorbed by
Secondary active transport (mechanisms
involving Na+-dependent cotransport)
2. Fructose is absorbed by facilitated diffusion

19. intestinal lumen
3Na
2K
3Na
2K
3Na
2K
3Na
2K
3Na
3Na
Na
gradient
Na
gradient
Na
gradient
Na
gradient
Na
Na
gradient
SGLT 1
GLUT2
intestinal epithelial cell
Blood

20. Absorption of Carbohydrates
 Both glucose and galactose move from the intestinal
lumen into the cell on the Na+-glucose cotransporter
(SGLT 1), against an electrochemical gradient
 The energy for this step does not come directly from
(ATP) but from the Na+ gradient.
 The Na+ gradient is, of course, created and maintained
by the Na+-K+ ATPase
 Glucose and galactose are extruded from the cell into
the blood, across the basolateral membrane, by
facilitated diffusion (GLUT 2).

21. Absorption of Carbohydrates
 Fructose
 Its absorption does not involve an energy-requiring step or a
cotransporter in the apical membrane
 fructose is transported across both the apical and basolateral
membranes by facilitated diffusion
1. (apical membrane( fructose-specific transporter → GLUT 5
2. (basolateral membrane( fructose-specific transporter → GLUT 2

22. GLUT2
intestinal lumen
intestinal epithelial cell
GLUT5
facilitated diffusion
Blood

24. Digestion of protein
 Dietary proteins are digested to absorbable forms:-
1. Amino acids
2. Dipeptides
3. Tripeptides
 Digestion of protein:-
1. Starts → Stomach (Pepsin)
2. Ends → Small intestine (pancreatic and brush-border
proteases )
aa
aa
aa
aa
aa aa

25. Digestion of protein
 Proteolytic enzymes (proteases) are:-
1. Pepsin
2. Trypsin
3. Chymotrypsin
4. Carboxypeptidase
5. Aminopeptidases

26. Pepsin
 Source of Enzyme
 Stomach chief cells
 Site of Action of Enzymes
 Stomach antrum
 Action of Enzymes
1. Pepsin hydrolyzes protein to form smaller
polypeptides
2. classified as an endopeptidase because it attacks
specific peptide bonds (at phenylalanine)
 Activation of enzyme (Pepsinogen)
1. HCL (pH = 1-3)
2. active pepsin (autocatalysis )

27. Trypsin
 Source of Enzyme
 Exocrine pancreas (acinar cell)
 Site of Action of Enzymes
 Small-intestine lumen
 Action of Enzymes
 Attack different peptide fragments
 classified as an endopeptidase
 Activation of enzyme (Trypsinogen)
1. Enteropeptidase (secreted by small intestine cell)
2. active trypsin (autocatalysis )

28. Chymotrypsin
 Source of Enzyme
 Exocrine pancreas (acinar cell)
 Site of Action of Enzymes
 Small-intestine lumen
 Action of Enzymes
1. Attack different peptide fragments
2. classified as an endopeptidase
 Activation of enzyme (Trypsinogen)
1. active trypsin

29. Carboxypeptidase
 Source of Enzyme
 Exocrine pancreas (acinar cell)
 Site of Action of Enzymes
 Small-intestine lumen
 Action of Enzymes
1. Attack different peptide fragments
2. classified as an Exopeptidases
 Activation of enzyme (Trypsinogen)
1. active trypsin

30. Aminopeptidases
 Source of Enzyme
 Small-intestine epithelial cells
 Site of Action of Enzymes
 Small-intestine (brush border)
 Small-intestine (Lumen)
 Small-intestine cell (Cytoplasm)
 Action of Enzymes
 Hydrolyze peptide fragments to amino Acids

31. absorption of amino acids
 The amino acids are transported from the lumen into the
cell by Na+-amino acid cotransporters in the apical
membrane
 energized by the Na+ gradient.
 There are four separate cotransporters: one each for
1. For neutral amino acids
2. For acidic amino acids
3. For basic amino acids
4. For proline , hydroxyproline and glycine
 The amino acids then are transported across the basolateral
membrane into the blood by facilitated diffusion, again by
separate mechanisms for neutral, acidic, basic, and proline ,
hydroxyproline and glycine amino acids

32. neutral
intestinal lumen
intestinal epithelial cell
acidic
facilitated diffusion
Blood
basic
glycine
neutral
acidic
basic
glycine
Na
Neutral
Na
acidic
Na
basic
Na
glyci

33. absorption of peptides

34. 3Na
2K
3Na
2K
3Na
2K
3Na
2K
Blood
Na
H
intestinal lumen
Na
H
Na
H
aa
Peptidase
facilitated diffusion
Peptides

36. Lipids
 Digestion and absorption of lipids occur in four
stages:-
1. Emulsification
2. Fromation of micellles
3. Absorption of FFA and monglyceride
4. Resynthesis and delivery of fat to lacteals

37. Emulsification
 Definition:- division of the large lipid droplets into
a number of much smaller droplets increasing their
surface area and action of Pancreatic lipase .
 The emulsification of fat requires:-
1. Mechanical mixing in stomach and intestine
2. emulsifying agent (bile acid) which acts to prevent
the smaller droplets from reaggregating back into
large droplets
 bile acid has two groups:-
1. Sterol group that dissolves in the fat (nonpolar head)
2. Carboxyl group which is water soluble (polar head)

38. intestinal lumen
large lipid droplets
Small lipid droplets

39. intestinal lumen

40. intestinal lumen
Bile
acid

41. intestinal lumen
colipase
Pancreatic lipase
FFA Monoglyceride

42.  Mixed micelles are cylindrically shaped disks with an
average diameter of 50 Å
 Micelles consist of bile acids, fatty acids,
monoglycerides, phospholipids and Lipid soluble
vitamin
 Core of a micelle contains products of lipid digestion
i.e. nonpolar portions forming the micelle’s core
(hydrophobic portion )
 Exterior of a micelle is lined with bile acids i.e. polar
ends of each molecule oriented toward the micelle’s
surface (hydrophilic portion )
Fromation of micellles

43. Core
Exterior

44. Fromation of micellles
 Purpose (micelles):-
 Making products of lipid digestion water
soluble for easy diffuse in aqueous medium
 enhances the delivery of lipid to the brush
border membrane

45. Absorption of FFA and
monglyceride

46. H+
H+
H+
H+
H+
H+
H+
Acid microclimate
Na
H
Na
H
Na
H

47. Resynthesis and delivery of fat to lacteals
 TAGs are formed inside intestinal cells from
monoglycerides and FFA
 FFA with less than 12 carbon chains (short and
medium chain) are directly absorbed in portal blood.
 TAG + phospholipids +cholesterol+ protein forming
chylomicron
 The chylomicrons are too large to enter vascular
capillaries
 they can enter the lymphatic capillaries (lacteals)