The document provides a comprehensive overview of digestion and absorption processes, including the roles of digestive juices, enzymes, and hormones involved in breaking down carbohydrates, proteins, and lipids. It details the specific composition and functions of various digestive fluids such as saliva, gastric juice, pancreatic juice, and bile, as well as the mechanisms of nutrient absorption in the intestines. Key points include the importance of enzymatic activity and the physiological processes necessary for effective digestion and assimilation of nutrients into the body.

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