LAB EXERCISE 7
DIGESTION
(Martini p. 40,41, 45-57, 915-918)
Work in groups of 4
Objectives:
 Identify the types of carbohydrates, fats and proteins present in the diet, state their general formulas,
and outline the reactions by which they are digested to absorbable end-products.
 Identify the enzymes of the GI tract which act on each group of food substances, state their origin,
activators (if present), substrates and end-products.
 State the pH in each part of the GI tract where digestion occurs, outline the mechanisms whereby it is
established, and state its significance.
 Explain the role of bile salts in digestion, and state their origin.
SAFETY (for Exercise C):
 Do not burn yourself with boiling water or the reagent when you perform the Benedict test.
 Use the test tube clamps to pick up test tubes from the beakers on the hot plates.
 Put the test tubes inside a test tube holder before carrying them to your bench.
I. DIGESTION OF FATS
A. THE DIGESTION OF MILK FATS BY PANCREATIC LIPASE
EXERCISE A
Equipment:
 bile salts (powder)
 pancreatin solution (0.5% - contains pancreatic lipase)
 cream
 litmus solution (1%)
 distilled water
 pipettes
 pipette suction pump
 test tubes
 water bath at 37 oC
1. Label four test tubes #1A - 2A - 3A - 4A, and add solutions as designated in Table 1. Add the enzyme
last!
Table 1. Fat Digestion Tests with Pancreatic Lipase and Bile Salts
Test
SUBSTRATE
BILE
DISTILL
INDICATOR
Tube
(Cream)
SALTS
(Litmus Solution)
ED
WATER
ENZYME
(Pancreatic Lipase)
Add last
1A
3.0 ml
pinch
-
3.0 ml
3.0 ml
2A
3.0 ml
pinch
3.0 ml
3.0 ml
-
3A
3.0 ml
-
-
3.0 ml
3.0 ml
4A
-
pinch
3.0 ml
3.0 ml
3.0 ml
1
2
Notes:
i) Litmus is a pH indicator which changes colour from red (below pH 4.5) through various shades of violet
to blue (above pH 8.3). Try it yourself: take a test tube, put approximately 1 ml of litmus solution. The
colour in the test tube is blue. Add a few drops of 2N HCl. Is there a change of colour? If yes what is the
colour?
.
ii) Enzymatic digestion of fat (triglyceride) releases fatty acids (e.g. butyric acid) which react with litmus
and change its color to red.
iii) Pancreatin is a commercial preparation of dried pancreatic enzymes.
2. Shake the tubes to mix thoroughly, make note of the colour of each tube, and place in a water bath at
37oC. Incubate for 1 to 1 1/2 hours.
3. Shake tubes and record their colour in Table 2a every 5 minutes for the first 15, then for every 15
minutes until no colour change is observed between colour tests.
Choose terms to describe the colours such as: milky blue, milky violet, milky purple, pinkish purple,
reddish purple, transparent blue, transparent red… Write your results in Table 2a.
Table 2a. Colour observations during digestion tests of milk fat by pancreatic lipase
Colours in test tubes during incubation
Test
tube
0min
5min
10min
15min
30min
1A
2A
3A
4A
3
45min
1hour
1 hr 15
min
1.5 hr
4. Complete the table below:
Table 2b: Summary of results of milk fat digestion tests
Test
tube
Presence of
fatty acids
(yes/no)
Is the digestion of cream,
fast, slow or is there no
digestion at all?
Brief explanation of each result
1A
2A
3A
4A
B. DEMONSTRATION OF THE EMULSIFYING PROPERTIES OF BILE SALTS
EXERCISE B
Equipment:
 bile salts (powder)
 distilled water
 oil
1. Prepare two test tubes: In the first one, put 3.0 ml. olive oil and 3.0 ml. distilled water. In the second
test tube, put 3.0 ml water and add a pinch of bile salts. Shake vigorously. Then add 3.0 ml of oil.
2. When both test tubes are ready, take one in each hand and shake them at the same time. Put them back
in your tray and keep both test tubes until the end of the laboratory period. Do NOT shake them again.
Describe the appearance of each test tube after first shaking and changes in the appearance over time.
Note generally how long it takes for the oil and water to separate.
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II. DIGESTION OF CARBOHYDRATES
EXERCISE C
Equipment:
 starch solution (1%)
 distilled water
 2N HCl
 pancreatin solution (2.5% - contains pancreatic amylase)
 test tubes
 paraffin
 pH paper
 water bath at 37oC
 iodine solution
 glucose solution (1%)
 maltose solution (1%)
 Benedict's solution
 hot plate
 test tube clamp
THE DIGESTION OF STARCH BY AMYLASE
1. Label four test tubes #1C - 2C - 3C - 4C and add solutions as described in Table 3. Add the enzyme
last.
Table 3. Carbohydrate Digestion Tests with Pancreatic Amylase
Test
Tube
OTHER REAGENTS
SUBSTRATE
(starch solution)
ENZYME (Pancreatic
Amylase)
Add last
1C
5.O ml
---
5.0 ml
2C
5.0 ml
5.0 ml dist. Water
---
3C
---
5.0 ml dist. Water
5.0 ml
4C
5.0 ml
5 drops 2N HCl
5.0 ml
2. Shake the tubes and determine and record the pH of each reaction mixture in table 4.
3. Place the test tubes in a water bath at 37oC and incubate for at least 45 minutes. Shake the tubes every
15 minutes. (Complete the iodine test and Benedict’s test control test tubes as instructed below while
waiting for these tubes to incubate for the 45 minutes)
4. Divide each of the 4 samples by pouring half of it into a clean test tube (label).
5. Test one set of 4 test tubes for starch by adding 5 drops of iodine solution. Shake and record the colour
of the iodine solution in table 4. See the control tests below to analyze results of iodine test.
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6. Test the second set of 4 test tubes for reducing sugar by adding 5 ml. of Benedict's reagent to each, and
placing in a boiling water bath for 5 min. Remove and rate the precipitate for colour according to the
following scale:
Blue Green +
Yellow ++
Orange +++
Red ++++
Record the colour of the precipitate in Table 4. See the control tests below to analyze results of Benedict’s
test.
Iodine solution indicates the presence of starch by turning blue. Try it yourself!
- Prepare three test tubes. Place 2ml of 1% starch solution in one test tube, 2ml of 1% glucose solution
in a second, and 2ml of 1% maltose in the third.
- Add 5 drops of iodine solution to each test tube and shake.
Iodine solution is yellow. Is there any change in colour after it is added to the starch solution? If yes, what
colour is formed?
Is there any change in colour after it is added to the glucose solution? If yes, what colour is formed?
Is there any change in colour after it is added to the maltose solution? If yes, what colour is formed?
Keep these test tubes as controls to compare to the test tubes prepared for starch digestion.
Benedict’s reagent tests for the presence of glucose and other reducing sugars. Reducing sugars
(glucose, fructose, maltose, lactose, etc.) react with Cu+2 ions (blue, soluble) in Benedict's reagent to
form Cu+, which combines with O2 to form Cu2O (red, insoluble). Non-reducing sugars (starch,
galactose, sucrose) do not react with Cu+2. Try it yourself!
- Prepare three test tubes by adding 5 ml of Benedict's reagent to each test tube
- Place 4ml of 1% starch solution in one test tube, 4ml of 1% glucose solution in a second, and 4ml of
1% maltose in the third.
- Heat in a boiling water bath for 5 min.
- Cool.
A green, yellow or red precipitate should form if glucose is present. The extent of the colour change is
related to the amount of red Cu+ formed in relation to the amount of blue Cu++ remaining. This is
determined by the amount of glucose present.
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Record colour of the precipitate for starch:
Record colour of the precipitate for glucose:
Record colour of the precipitate for maltose:
Keep these test tubes as controls to compare to the test tubes prepared for starch digestion.
Table 4. Results of iodine and Benedicts tests for starch digestion by amylase.
Tes
t
tub
e
pH
Iodine
test
(colour)
Benedict
test
(colour)
Presence
of starch
(yes/no)
Presence of
reducing
sugar (yes/no)
1C
2C
3C
4C
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Brief explanation of each result
III. DIGESTION OF PROTEINS
EXERCISE D
Equipment:
 Capillary tubes which have been completely filled with coagulated egg white and incubated at 37 oC for
24 hours in the following solutions:
(1) pepsin in buffer, pH 2.0
(2) pepsin in buffer, pH 4.0
(3) pepsin in buffer, pH 8.0
(4) buffer, pH 2.0
(5) buffer, pH 4.0
(6) buffer, pH 8.0
(7) trypsin in buffer, pH 2.0
(8) trypsin in buffer, pH 4.0
(9) trypsin in buffer, pH 8.0
 Ruler
1. Each of these tubes was totally filled with coagulated egg white. Determine the amount of egg white
that has disappeared by measuring the length of empty tube at each end in mm, summing them and
expressing them in percentage of the total length of the capillary tube (tubes were completely filled
with egg white at the start of the experiment).
2. Tabulate results. Compare the amount of disappearance at each pH with and without enzyme. Record
your results in Table 5 and on blackboard.
Table 5: Digestion of proteins by pepsin and trypsin.
Capillar
y tube
Solutions
1
pepsin in buffer, pH 2.0
2
pepsin in buffer, pH 4.0
3
pepsin in buffer, pH 8.0
4
buffer, pH 2.0
5
buffer, pH 4.0
6
buffer, pH 8.0
7
trypsin in buffer, pH 2.0
8
trypsin in buffer, pH 4.0
9
trypsin in buffer, pH 8.0
Amount of egg white that
has disappeared (in %)
8
Why did we bother to incubate egg white in buffer solutions without enzymes?
How would you calculate the amount of egg white digested by pepsin and trypsin at different pH values?
CLEAN UP:
 Wash all glassware with soap and warm water,
 Return equipment to where you found it.
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NAME & section #:
Biology 153 LAB REPORT
EXERCISE 7: DIGESTION.
INTRODUCTION
State the purpose of this exercise
MATERIALS AND METHODS
See the lab manual.
RESULTS
I: DIGESTION OF FATS
1. Copy results in Table 2a to the table below.
Table A. Changes in colour observed during the digestion of milk fat by pancreatin.
Colours in test tubes during incubation
Test
tube
0min
5min
10min
15min
30min
45min
1A
2A
3A
4A
10
1hr
1h15mi
n
1h30mi
n
2. Briefly describe the distribution of oil with and without bile salts (Exercise B). Describe the appearance
of each test tube after first shaking and changes in the appearance over time. Note generally how long it
takes for the oil and water to separate.
II: DIGESTION OF CARBOHYDRATES
In the following table, copy results for the digestion of starch by amylase.
Table B. Benedict’s and iodine test results for starch digestion by amylase.
Test
tube
pH
iodine test
(colour)
Benedict
test (colour)
Presence of
starch (yes/no)
Presence of
reducing sugar
(yes/no)
1C
2C
3C
4C
III: DIGESTION OF PROTEINS
1. Complete the following table (Exercise D).
Table C. Digestion of egg white protein by pepsin and trypsin.
Capillary
tubes
Solutions
1
pepsin in buffer, pH 2.0
2
pepsin in buffer, pH 4.0
3
pepsin in buffer, pH 8.0
4
buffer, pH 2.0
5
buffer, pH 4.0
6
buffer, pH 8.0
7
trypsin in buffer, pH 2.0
8
trypsin in buffer, pH 4.0
Amount of egg
white that has
disappeared (in
%)
11
Amount of egg
white that has
been digested (in
%)
9
trypsin in buffer, pH 8.0
2. Prepare one graph showing the percent digestion of egg white by pepsin and trypsin. Percent of
digestion should be on the Y-axis and pH on the X-axis. Describe your results in no more than 5 lines and
explain the significance of the capillary tubes filled with egg white incubated in buffers pH 2, 4 and 8.
DISCUSSION
Answer in the spaces provided below. Do not use extra pages.
I: DIGESTION OF FATS
Explain what happened in each tube used in Exercise A by completing the following table.
Table D: Discussion of results of milk fat digestion by pancreatic lipase
Test
tube #
Is the digestion of
cream, fast, slow or is
there no digestion at all?
Explain. Be sure to include: function of components of the test
tube, products of digestion, explanation for colour change or no
colour change, explanation for changes in rate of digestion, etc.
1A
2A
3A
4A
What is the significance of using this test tube?
12
13
II: DIGESTION OF CARBOHYDRATES
Explain what happened in each tube used in Exercise C by completing the following table.
Table E: Discussion of results of carbohydrate digestion by pancreatic amylase
Test
tube
Presence
of starch
(yes/no)
Presence of
reducing sugar
(yes/no)
Explain your results.
1C
2C
3C
4C
III:DIGESTION OF PROTEINS
Explain the results you graphed for the digestion of proteins (Exercise D).
Explain how optimum pH is created for these enzymes in the gastrointestinal tract (consider the organs
secreting these enzymes, the non-enzyme secretions at these locations, and the importance of pH in the
activity of each enzyme).
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