Laboratory Exercise 2: Physiology of Digestion
Digestion of food makes it simple enough to pass across cell membranes. The food molecules,
carbohydrates, fats, proteins and nucleic acids are reduced in molecular size and chemical structure by
physical (mechanical) and chemical (enzymatic) processes. Then these molecules pass by absorption
through the cells of the alimentary canal lining into the circulation, which distributes the molecules to
the cells of the body.
A. Absorption Time of a Salt from the GI Tract (Stomach)
This procedure demonstrates the time required for a salt (KI) to be absorbed from the stomach and then
secreted. Once swallowed the salt is absorbed by the stomach’s mucosa and then diffuses into the
underlying capillaries, taken up by the salivary gland cells to become part of the saliva.
If absorption of the salt (KI) has occurred, test the saliva for the iodine.
A small amount of saliva is combined with acidified starch solution. The acidic medium converts the
iodide (I-) to iodine (I2) and starch-I2 complex forms which turns blue.
Absorption time of salt is dependent upon food volume, fat-protein-carbohydrate ratio, and time since
last meal was consumed. A recent large meal accumulates in layers next to the gastric mucosa retards
absorption of the salt. If the meal was high in fat, layering would persist and also retard absorption of
salt.
B. Digestion of Starch
Digestion of starch occurs in the mouth, requires a neutral pH for the enzymatic reaction.
Salivary amylase hydrolyzes starch to dextrins and maltose. The starch-I2 complex breaks down, with
the disappearance of the blue color indicating the digestion of the complex carbohydrate, starch.
C. Protein Digestion
Protein digestion occurs in the stomach requires an acidic environment.
Protein digestion begins in the stomach where pepsin hydrolyzes protein into polypeptides called
peptones. The acidity for the protein digestion in vivo is maintained by secretion of HC1 by the
parietal cells of the gastric glands in the gastric mucosa.
D. Emulsifying Action of Bile
Bile produced in the liver is stored and concentrated in the gallbladder until fats arrive in the
duodenum. Bile contains bile salts, which reduces large fat globules to smaller one by emulsification.
This is a physical process not a chemical one. The bile salts attach to the fat particles and disperse
them within the intestinal fluid as small fat droplets. The fat droplets are kept apart by the bile salts
creates a stable emulsion of smaller particles. Then the enzymatic digestion of the fats occurs by the
lipase.
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E. Digestion of Triglycerides (Neutral Fat)
The enzymatic digestion of fats occurs in the small intestine by pancreatic lipase in a slightly basic
environment. The lipolysis is indicated as the triglycerides break down into fatty acid and glycerol the
pH is lowered and the indicator dye changes from blue to pink.
Physiology of Digestion
Hydrolysis Reactions
Digestion of Starch - occurs in the mouth, requires neutral pH for the enzymatic reaction to occur
Salivary
I2-Starch + H2O
Dextrins + Maltose + I2
Amylase
blue
red to colorless
Digestion of Protein - occurs in the stomach, requires acidic pH for the enzymatic reaction to occur.
Pepsin
Protein + H2O
Peptones
H+
Cloudy
clear
Emulsification - Bile salts by a physical process attaches to fat particles and disperses them into
smaller fat droplets to be acted on by the lipase. Emulsification occurs in the small intestine.
Digestion of Triglycerides (Neutral Fat) - occurs in the small intestine, requires a slightly alkaline pH
for the enzymatic reaction to occur.
Pancreatic
Triglyceride + 3H2O
Fatty Acids + Glycerol
(Fat)
Lipase
H
OH
|
|| |
H–C-O–C–C|
H
OH
|| |
HO-C-C|
H
OH
|| |
H –C-O-C-C|
H
O H
|| |
HO-C-C|
H
O H
|| |
H -C-O–C-C|
|
H
H
Blue litmus
alkaline
+3HOH
H
|
H-C-OH
+
O H
|| |
HO-C-C|
H
Red to pink litmus
acidic
2
H-C-OH
H-C-OH
|
H