Digestion and absorption of carbohydrates and proteins

1. DIGESTION AND
ABSORPTION OF
NUTRITIONAL
CONSTITUENTS
Dr Ifat Ara Begum
Assistant Professor
Dept Of Biochemistry
Dhaka Medical College
Dhaka

2. DAY 2
Digestion and
absorption of
carbohydrates &
proteins

3. DIETARY CARBOHYDRATES
Major source of carbohydrate is
found in plants.
Monosaccharides: Glucose,
Fructose, Galactose (in fruits,
honey etc): Don’t need digestion
Disaccharides: Sucrose, Lactose,
Maltose
Polysaccharides: Starch (potato,
rice, corn, wheat), Cellulose (in cell
wall of plants)

4. ENZYMES FOR CARBOHYDRATES
DIGESTION

5. CONTD

6. STEPS OF DIGESTION OF
CARBOHYDRATES

7. A) DIGESTION IN MOUTH
Food undergoes mastication. During
mastication, food comes in contact with
saliva. Saliva contains salivary amylase
(ptyalin)
Action of ptyalin:
 It needs Cl ion for activation
 Optimum pH: 6.7 (6.6-6.8).
 Stops activity when pH is 3 (that is in
stomach)
 Accounts for 20-40% of starch digestion

8. CONTD
 Hydrolyzes α (1→4) glycosidic
linkage at random deep inside
polysaccharide molecules, producing
maltose, maltotriose, unbranched
oligosaccharides and dextrins

9. CONTD
 Draw back:
 Shorter duration of food in mouth
 Thus it is incomplete digestion of
starch /carbohydrates in mouth

10. B) DIGESTION IN STOMACH
Digestion of carbohydrates temporarily
stops here.
The action of ptyalin stops here due to high
acidity of stomach
No CHO splitting enzymes in gastric juice
However HCl present in the stomach causes
hydrolysis of sucrose to glucose and
fructose

11. C) DIGESTION IN INTESTINE
Food bolus reach the small intestine from
stomach where it meets the pancreatic
juice. Further digestion of CHO occurs in
small intestine by pancreatic enzyme
(pancreatic amylase/ amylopsin)

12. CONTD
Main digestion occurs in small
intestine.
2 phases of intestinal digestion:
 Digestion due to pancreatic amylase
(It accounts for 50-80% of starch
digestion)
 Digestion due to intestinal brush
border enzyme

13. CONTD

14. CONTD
Pancreatic amylase hydrolyzes the
dextrins to maltose, isomaltose, α-
limit dextrins.

18. END PRODUCTS OF
CARBOHYDRATE DIGESTION
Glucose
Fructose
Galactose

19. ABSORPTION OF END PRODUCTS OF
CARBOHYDRATE DIGESTION
The end products of carbohydrates
digestion (glucose, fructose, galactose)
are readily absorbed through the
intestinal mucosal cells in to the blood
stream.
2 mechanisms are responsible for the
absorption of these sugars:
I. Facilitated transport with conc.
gradient
II.Active transport against conc. gradient

21. CONTD

23. TYPE OF GLUCOSE
TRANSPORTERS

28. CLINICAL SIGNIFICANCE OF
CARBOHYDRATE DIGESTION

29. CONGENITAL LACTOSE
INTOLERANCE
Complete absence/deficiency of lactase
enzyme
Child develops intolerance to lactose
immediately after birth
It is diagnosed in early infancy
Milk feed precipitates symptoms:
diarrhoea, flatulence, abdominal cramps
& distension.

31. SUCRASE DEFICIENCY

32. DIGESTION OF PROTEIN
Degradation of proteins by
cellular enzymes in a process
called hydrolysis.
Dietary source of protein: Animal
& plant sources

33. CONTD
Takes place in 2 different phases:
I. In the stomach (gastric digestion)
II.In the small intestine (intestinal
digestion)
Both of these phases of digestion
are based on several types of
enzymes called proteinases/
proteases.

38. END PRODUCTS OF PROTEIN
DIGESTION
A mixture of free AA, di and
tripeptides, all of which are
absorbed actively from lumen to
cell