Digestion & absorption of carbohydrate, protein and lipid following the curriculum of medical biochemistry of MBBB course

1. Digestion
&
Absorption
Dr. Farhana Atia
Assistant Professor, Biochemistry
Nilphamari Medical College, Nilphamari

2. Learning objectives
Define digestion & absorption
Enumerate digestive juices, their composition & functions
Enumerate local hormones of GIT, their source & functions
State the names & sources of digestive enzymes & their
location
Process of digestion & absorption of carbohydrate
Process of digestion & absorption of protein
Process of digestion & absorption of lipids (TAG, phospholipids,
cholesterol esters)

3. Digestion
It is defined as a physiological
process by which complex
food particles are broken
down into simple forms which
can be absorbed from the gut
for utilization by the body.

4. Absorption
A process by which end
product of digestion pass
through the intestinal
epithelium to enter into
blood or lymph

5. Digestive juices
Mainly secreted from the glands of the digestive tract
Help in the digestion of food
They are-
Saliva- 1000 ml/day
Gastric juice- 1500 ml/day
Pancreatic juice- 1000 ml/day
Succus entericus- 1800 ml/day
Bile- 1000ml/day

6. Saliva: Composition
Electrolytes (ions): K⁺, HCO₃⁻, Na⁺, Cl⁻, Thiocyanate
Proteins
 Enzymes: Ptyalin
Lingual lipase
Maltase
Lysozyme
 Mucin (α-glycoprotein)
 Protein antibodies
Water: 99.5%

7. Gastric Juice: Composition
Water
97-99%
Solid
Organic
Enzyme
Pepsinogen
Gastric lipase
Gelatinase
Rennin
Intrinsic
factor
Mucous Gastrin
Inorganic
Na⁺, K⁺,
Mg⁺⁺, H⁺
Cl⁻, HPO₄²⁻,
SO₄²⁻, free
HCL

8. Gastric glands

9. Pancreatic juice: Composition
Water-98.5%
Solid
1. Inorganic
Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺
HCO₃⁻, Cl⁻, HPO₄²⁻, SO₄²⁻
2. Organic
Enzymes
Trypsin inhibitor

10. Pancreatic Enzymes
Proteolytic
Trypsin
Chymotrypsin
Carboxypeptidase
Elastase
Nuclease
Carbohydrate splitting
Pancreatic α-amylase
Fat splitting
Pancreatic lipase,
colipase
Cholesterol Esterase
Phospholipase

11. Bile
 Water
 Bile salt
 Bilirubin
 Cholesterol
 Fatty acid
 Lecithin
 Na⁺, K⁺, Ca⁺⁺
 HCO₃⁻, Cl⁻
 Bile salt
Na-taurocholate
K-taurocholate
Na-glycocholate
K-glycocholate
 Bile acid
Cholic acid
Cheno-deoxycholic acid
Deoxycholic acid
Lithocholic acid

12. Succus Entericus: Composition
Water: 98.5%
Solid
1. Inorganic
Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺
HCO₃⁻
2. Organic
Enzymes
Activator- Enterokinase
Mucus

13. Succus Entericus: Enzymes
Proteolytic enzyme
Peptidases -Aminopeptidase
-Dipeptidase
Nuclease
Carbohydrate splitting Enzyme
Sucrase
Maltase, Isomaltase
Lactase
Fat Splitting Enzyme: Lipase

14. Local Hormones of GIT
Biologically active polypeptide
Secreted by nerve cells & gland cells in the mucosa
Act in paracrine fashion
But also enter the circulation

15. Types
According to structural & functional similarity
Others
Gastrin Family Secretin Family
Gastrin
Cholecystokinin
Secretin
Glucagon
Glicentin
VIP- Vasoactive Inhibitory Polypeptide
GIP- Gastric Inhibitory Polypeptide
Motilin
Neurotensin
Substance P
Gastrin releasing PP
Bombesin
Somatostatin
Guanylin
Enkephalin
Villikinin

16. Gastrin
Source Site of action Actions
G cell of
gastric
mucosa
• Stomach
• Lower esophageal
sphincter
• Small intestine
• Gall bladder
• Stimulate gastric acid &
pepsin secretion
• Stimulate growth of
mucosa of stomach, small
& large intestine
• Increase gastric motility

17. Cholecystokinin- Pancreozymin (CCK-PZ)
Source Site of action Actions
I cell of
mucosa of
duodenum
& jejunum
• Gall bladder  Stimulates
• Pancreatic enzyme secretion
• Pancreatic HCO₃⁻ secretion
• Gall bladder contraction
• Growth of exocrine pancreas
 Inhibit
• Gastric emptying
• Gastric motility

18. Secretin
Source Site of action Actions
S cell of
mucosa of
duodenum
• Pancreatic
duct cell
 Stimulates
• Pepsin secretion
• Pancreatic HCO₃⁻ secretion
• Biliary HCO₃⁻ secretion
• Growth of exocrine pancreas
 Inhibit
• Gastric acid secretion
• Effect of gastrin on growth of
gastric mucosa

19. GIP- Gastric inhibitory polypeptide
(43 amino acids)
Source Site of action Actions
S cell of
mucosa of
duodenum
• Stimulate Insulin secretion
• Inhibit gastric acid secretion
• Decrease gastric motility

20. Digestive enzymes
Carbohydrate splitting
enzyme
Proteolytic enzyme
• Ptyalin
• Pancreatic amylase
• Sucrase
• Maltase
• Lactase
• α-dextrinase
• Pepsinogen
• Trypsin
• Chymotrypsin
• Carboxypeptidase
• Aminopeptidase
• Dipeptidase

21. Digestive enzymes
Lipolytic enzyme Others
• Gastric lipase
• Lingual lipase
• Pancreatic lipase
• Phospholipase A2
• Cholesterol esterase
• Colipase
• Enteric lipase
• Phosphatase
• Rennin
• Lysozyme
• Lecithinase

22. Digestion
In mouth cavity
In stomach
In small intestine

23. Carbohydrates in diet
Polysaccharide
• Starch
• Glycogen
Disaccharide
• Lactose
• Maltose
• Sucrose
Monosaccharide
• Glucose
• Fructose
In GIT, all complex
carbohydrates are
converted to simpler
monosaccharide

25. Digestion in mouth
Salivary α-amylase
Location: mouth
Hydrolyses α-1→ 4 glycosidic linkages
Ptyalin action stops in the stomach (pH falls to 3.0)
Food present in shorter duration, so
Incomplete digestion

26. 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
Digestion in the Stomach
HCl

27. Digestion in Duodenum
Food bolus reaches the
duodenum
Meets the pancreatic juice
Pancreatic juice contains
carbohydrate splitting
enzyme,
Pancreatic amylase
similar to salivary amylase.
 Main digestion takes place
in the small intestine by
pancreatic amylase
 Disaccharidases present in
the brush border epithelium
of intestinal mucosal cell
Maltase
Sucrase-Isomaltase
Lactase
Digestion in Small Intestine

28. Pancreatic amylase
An α- Amylase
Optimum pH=7.1
It hydrolyses α-1→ 4
glycosidic linkages
Note: Pancreatic amylase,
an isoenzyme of salivary
amylase, differs only in the
optimum pH of action.
Starch/Glycogen
Maltose/ Isomaltose
+
Dextrins and
oligosaccharides
Amylase

29. Absorption of carbohydrates
Glucose transporter
1. Na+ dependent
transporter [SGLT]
2. Na+ independent
transporter [GLUT]

30. Lactose intolerance
Inability to digest lactose due to the deficiency of Lactase
enzyme.
Presents as abdominal cramps, distensions, diarrhea,
constipation, flatulence upon ingestion of milk or dairy
products
Undigested lactose in intestine is converted to CO2, H2, 2 &
3 carbon compounds [by bacteria]
CO2 & H2 causes distensions and flatulence
Lactose + 2C + 3C are osmotically active
Cause osmotic diarrhea or constipation because of
undigested bulk

31. 30% fat digested
Digestion of Lipids

32. Emulsification
Emulsification is a prerequisite for digestion of lipids
The lipids are dispersed into
smaller droplets
surface tension is reduced
surface area of droplets is ↑
This process is favoured by:
1. Bile salts (detergent action)
2. Peristalsis (mechanical mixing)
3. Phospholipids

33. End products of lipid digestion
TAG Monoacylglycerol + 2FFA
Cholesterol ester Cholesterol + FA
Phospholipid Lysophospholipid + FA
Pancreatic lipase
Cholesterol esterase
Phospholipase A2

34. Lipid absorption
1. Short & medium chain FA (<10 carbons) directly enter
the portal circulation  liver (albumin bound form)
2. LCFA (>12 carbons) are absorbed to the lymph
Portal
blood

35. Micelle Formation
The products of digestion are incorporated into molecular
aggregates to form micelle
They are spherical particles with a hydrophilic exterior &
hydrophobic interior core
Bile salts help to form micelle
Essential for the absorption of fat-soluble vitamins
Aligned at the microvillus surface of the mucosa
Fatty acids, 2-MAG and other digested products passively
diffuse into the mucosal cell

36. Chylomicrons
Inside the intestinal mucosal cell, the long chain fatty acids are re-
esterified to form triglycerides
They are surrounded by a thin layer of apolipoproteins (A1 and B-
48) & phospholipids. These particles are chylomicrons
Released into lymphatic vessels by exocytosis
Enter into thoracic duct  systemic circulation  peripheral
tissues (muscle, adipose tissue)  liver

37. Portal
blood
Chylomicron

38. Physiologically important lipases
Lipase Site of action Preferred substrate Product(s)
Lingual / acid stable
lipase
Mouth, stomach TAG with medium chain
FA
FFA+DAG
Pancreatic lipase +
co-lipase
Small intestine TAG with long chain FA FFA+2-MAG
Intestinal lipase Small intestine TAG with medium chain
FA
FFA + glycerol
Phospholipase A2 Small intestine Phospholipids FFA +
Lysolecithin
Lipoprotein lipase Capillary wall TAG in CM or VLDL FFA+ glycerol
Hormone sensitive
lipase
Adipose cell TAG stored in adipose cells FFA+ glycerol

39. Digestion of
protein

40. Proteolytic enzymes are secreted as
inactive form (Zymozen). It has to be
activated before action.

41. Absorption of amino acids
By Na+ dependent active transport system (Na+
amino acid cotransport)
An energy requiring process

42. Carbohydrate Lipid Protein
In diet Starch
Lactose
Sucrose
Cellulose
TAG (90%)
Cholesterol
Cholesterol ester
Phospholipids
Free fatty acid
Protein
Mouth [Salivary α-amylase]
Starch
Dextrins
Isomaltose
Maltose
Maltotriose
Lactose
Sucrose
Cellulose

43. Carbohydrate Lipid Protein
Stomach Low pH stop enzyme
action
[Lingual lipase]
[Gastric lipase]
TAG with short & medium
chain fatty acid
[HCl]+[Pepsin]
Denaturation
Polypeptides
Amino acids
Small
intestine
[Pancreatic α-amylase]
Isomaltose
Maltose
Maltotriose
Lactose
Sucrose
Cellulose
[Disaccharidase]
Emulsification
[Pancreatic lipase]
[Colipase]
[Cholesterol esterase]
[Phospholipase A2]
TAG
Cholesterol ester
Phospholipid
[Trypsin]
[Chymotrypsin]
[Elastase]
[Carboxypeptidase]
Oligopeptides
Amino acids
[Aminopeptidase]
[Di & tripeptidase]

44. Carbohydrate Lipid Protein
End
products
Glucose
Fructose
Galactose
Cellulose  colon &
excreted
FFA
2-Monoacyl glycerol
Cholesterol
Lysophospholipid  not
absorbed
Amino acids
Absorption Glucose
Galactose
[SGLT1]
↓
Enterocyte
[GLUT2]
↓
Portal blood
Fructose
[GLUT5]
↓
Enterocyte
[GLUT2]
↓
Portal
blood
SCFA(12)
↓
Portal blood
LCFA
Micelle(+bile salt
+ FSV)
↓
enterocyte
Chylomicron
↓
Lymphatics
↓
Blood
Seven different
active transport
process