Chemical and Physical Processes of Digestion

Objectives:

1. To list the digestive system enzymes involved in the digestion of proteins, fats, and carbohydrates; to state their site of origin; and to summarize the environmental conditions promoting their optimal functioning.
2. To recognize the variation between different types of enzyme assays.
3. To name the end products of digestion of proteins, fats, and carbohydrates.
4. To perform the appropriate chemical tests to determine if digestion of a particular foodstuff has occurred.
5. To cite the function(s) of bile in the digestive process.
6. To discuss the possible role of temperature and pH in the regulation of enzyme activity.
7. To define enzyme, catalyst, control, substrate, and hydrolase.
8. To explain why swallowing is both a voluntary and a reflex activity.
9. To discuss the role of the tongue, larynx, and gastro-esophageal (lower esophageal) sphincter in swallowing.
10. To compare and contrast segmentation and peristalsis as mechanisms of propulsion.

 

Enzyme Action

Materials:

General supply area:

• Test tubes and test tube rack
• Wax markers
• Large beakers
• Water bath set at 37C (if not available, incubate at room temperature and double the time)
• Ice water bath
• Hot plates
• Chart on chalkboard for recording class results

Supply area 1:

• Spot plates
• Dropper bottles of distilled water
• 250-mi beakers
• Boiling chips
• Dropper bottles of
• 1% alpha-amylase solution (The alpha-amylase must be a low-maltose preparation for good results.)
• 1% boiled starch solution, freshly prepared (Prepare by adding 1 g starch to 100 mi distilled water; boil and cool; add a pinch of salt (NaC1).)
• 1% maltose solution
• Lugol's IKI (Lugol's iodine)
• Benedict's solution

Supply area 2:

• Dropper bottles of

• I% trypsin
• 1% BAPNA solution

Supply area 3:

• Dropper bottles of
• 1% pancreatin solution
• Litmus cream (fresh cream to which powdered litmus is added to achieve a blue color)
• O.1NHCI
• Vegetable oil
• Bile salts (sodium taurocholate)
• Parafilm (small squares to cover the test tubes)

 

Starch Digestion by Salivary Amylase

Methods:

Work in groups of 3 or 4, with each group taking responsibility for setting up and conducting one of the following experiments. Each group should then communicate its results to the rest of the class by recording them in a chart on the chalkboard. All members of the class should observe the controls as well as the positive and negative examples of all experimental results. Additionally, all members of the class should be able to explain the tests used and the results observed and anticipated for each experiment. Note that water baths and hot plates are at the general supply area.

1. From the general supply area, obtain a test tube rack, 10 test tubes, and a wax marking pencil. From supply area 1, obtain a dropper bottle of distilled water and dropper bottles of maltose, amylase, and starch solutions.
2. Since in this experiment you will investigate the hydrolysis of starch to maltose by salivary amylase (the enzyme produced by the salivary glands and secreted into the mouth), it is important to be able to identify the presence of these substances to determine to what extent the enzymatic activity has occurred. Thus controls must be prepared to provide a known standard against which comparisons can be made. Starch decreases and sugar increases as digestion occurs, according to the following equation:

3.     
Chart 1 below outlines the general procedures. Two students should prepare the controls (tubes 1A to 3A) while the other two prepare the experimental samples (tubes 4A to 6A).

1. Mark each tube with a wax pencil and load the tubes as indicated in Chart 1, using 3 drops of each indicated substance.
2. Place all tubes in a rack in the appropriate water bath for approximately 1 hour.
3. While these tubes are incubating, proceed to the next section.
3. From supply area I obtain a 250-mL beaker, boiling chips, a spot plate, and dropper bottles of Lugol's Iodine and Benedict's solution. Use a hot plate as necessary from the general supply area.
4. Place a few boiling chips and about 125 mL water into the beaker and bring to a boil. While the water is heating, mark the spot plate A (for amylase) and number six of its depressions 1-6 for sample identification.
5. Pour about a drop of the sample from each of the tubes into the appropriately numbered spot. Into each sample droplet, place a drop of Lugol's Iodine solution. A blue-black color indicates the presence of starch and is referred to as a positive starch test. If starch is not present, the mixture will not turn blue, which is referred to as a negative starch test. Record your results (+ for positive, - for negative) in Chart 1 and on the chalkboard.
6. Into the remaining mixture in each tube, place 3 drops of Benedict's solution. Put each tube into the beaker of boiling water for about 5 minutes. If a green to orange precipitate forms, maltose is present; this is a positive sugar test. A negative sugar test is indicated by no color change. Record your results in Chart I and on the chalkboard.

Results:

Chart 1 Salivary Amylase Digestion of Starch

 

Protein Digestion by Trypsin

Trypsin, an enzyme produced by the pancreas, hydrolyzes proteins to small fragments (proteoses, peptones, and peptides). BAPNA is a synthetic (man-made) protein substrate consisting of a dye covalently bound to an amino acid. Trypsin hydrolysis of BAPNA cleaves the dye molecule from the amino acid, causing the solution to change from colorless to bright yellow. Since the covalent bond between the dye molecule and the amino acid is the same as the peptide bonds that link amino acids together, the appearance of a yellow color indicates the presence and activity of an enzyme that is capable of peptide bond hydrolysis. Because the color change from clear to yellow is direct evidence of hydrolysis, additional tests are not required when determining trypsin activity using BAPNA.

Methods:

1. From the general supply area, obtain 5 test tubes and a test tube rack, and from supply area 2 get a dropper bottle of trypsin and one of BAPNA and bring them to your bench.
2. Chart 2 below outlines the general procedures. Two students should prepare the controls (tubes 1T and 2T) while the other two prepare the experimental samples (tubes 3T to 5T).
1. Mark each tube with a wax pencil and load the tubes as indicated in Chart 2, using 3 drops of each indicated substance.
2. Place all tubes in a rack in the appropriate water bath for approximately 1 hour.
3. While these tubes are incubating, proceed to the next section.
3. Since BAPNA is a synthetic colorigenic (color-producing) substrate, the presence of yellow color indicates a positive hydrolysis test; the dye molecule has been cleaved from the amino acid. If the sample mixture remains clear, no detectable hydrolysis has occurred. 1. Record the color of the experimental tubes in Chart 2 and on the chalkboard.

Results:

Chart 2 Trypsin Digestion of Protein

 

Pancreatic Lipase Digestion of Fats and the Action of Bile

Methods:

The fact that some of the end products of fat digestion (fatty acids) are organic acids that decrease the pH provides an easy way to recognize that digestion is ongoing or completed. You will be using a pH indicator called litmus blue to follow these changes; it changes from blue to pink as the test tube contents become acid.

1. From the general supply area, obtain 9 test tubes and a test tube rack, plus one dropper bottle of each of the solutions in supply area 3.
2. Although bile, a secretory product of the liver, is not an enzyme, it is important to fat digestion because of its emulsifying action (the physical breakdown of larger particles into smaller ones) on fats. Emulsified fats provide a larger surface area for enzymatic activity. To demonstrate the action of bile on fats, prepare two test tubes and mark them 1E and 2E.
1. To tube 1E add 10 drops of water and 2 drops of vegetable oil.
2. To tube 2E add 10 drops of water, 2 drops of vegetable oil, and a pinch of bile salts.
3. Cover each tube with a small square of Parafilm, shake vigorously, and allow the tubes to stand at room temperature.
3. After 10-15 minutes, observe both tubes. If emulsification has not occurred, the oil will be floating on the surface of the water. If emulsification has occurred, the fat droplets will be suspended throughout the water, forming an emulsion. In which tube has emulsification occurred?
4. Two students should prepare the controls (1L, and 2L,), while the other two students in the group set up the experimental samples (3L to 5L, 4B, and 5B) as indicated in Chart 3.

1. Mark each tube with a wax pencil and load the tubes using 5 drops of each indicated solution.
2. A pinch of bile salts should be placed in tubes 4B and 5B.
3. Cover each tube with a small square of Parafilm and shake to mix the contents of the tube.
4. Remove the Parafilm and place all tubes in a rack in the appropriate water bath for approximately 1 hour.
4. The basis of this assay is a pH change that is detected by a litmus powder indicator. Alkaline or neutral solutions containing litmus are blue but will turn reddish in the presence of acid. Since fats are digested to fatty acids (organic acids) during hydrolysis, they lower the pH of the sample they are in. Litmus cream (fresh cream providing the fat substrate to which litmus powder was added) will turn from a bluish color to pink if the solution is acid. Because the effect of hydrolysis is directly seen, additional assay reagents are not necessary.
5. To prepare a color control, add 0.1 N HC1 drop by drop to tubes 1L and 2L (covering the tubes with a square of Parafilm after each addition and shaking to mix) until the cream turns pink.
6. Record the color of the tubes in Chart 3 and on the chalkboard.

Results:

Chart 3 Pancreatic Lipase Digestion of Fats

 

 

Physical Processes: Mechanisms of Food Propulsion and Mixing

Although enzyme activity is a very important part of the overall digestion process, foods must also be processed physically (churning and chewing), and moved by mechanical means along the tract if digestion and absorption are to be completed. Just about any time organs exhibit mobility, muscles are involved, and movements of and in the gastrointestinal tract are no exception. Although we tend to think only of smooth muscles when visceral activities are involved, both skeletal and smooth muscles are involved in digestion. This fact is amply demonstrated by the simple demonstrations that follow.

Materials:

Supply area 4:

• Water pitcher
• Paper cups
• Stethoscope
• Alcohol swabs
• Disposable autoclave bag

 

Deglutition (Swallowing)

Swallowing, or deglutition, which is largely the result of skeletal muscle activity, occurs in two phases: buccal (mouth) and pharyngeal-esophagea1. The initial phase --the buccal--is voluntarily controlled and initiated by the tongue. Once begun, the process continues involuntarily in the pharynx and esophagus, through peristalsis, resulting in the delivery of the swallowed contents to the stomach.

Methods:

1. Obtain a pitcher of water, a stethoscope, a paper cup, an alcohol swab, and an autoclave bag in preparation for making the following observations.
2. While swallowing a mouthful of water, consciously note the movement of your tongue during the process. Record your observations.
3. Repeat the swallowing process while your laboratory partner watches the externally visible movements of your larynx. (This movement is more obvious in a male, who has a larger Adam's apple.) Record your observations. What do these movements accomplish?
4. Before using the stethoscope your lab partner should clean the earpieces with an alcohol swab. Then, he or she should place the diaphragm of the stethoscope over your abdominal wall, approximately 1 inch below the xiphoid process and slightly to the left, to listen for sounds as you again take two or three swallows of water. There should be two audible sounds--one when the water splashes against the gastro-esophageal (lower esophageal) sphincter and the second when the peristaltic wave of the esophagus arrives at the sphincter and the sphincter opens, allowing water to gurgle into the stomach. Determine, as accurately as possible, the time interval between these two sounds and record it below.
   This interval gives a fair indication of the time it takes for the peristaltic wave to travel down the l0-inch-long esophagus. (Actually the time interval is slightly less than it seems, because pressure causes the sphincter to relax before the peristaltic wave reaches it.) Dispose of the used paper cup in the autoclave bag.

 

Results and Discussion:

Describe the movement of your tongue.

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Describe the movements of your larynx.

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Determine the time interval between the sound of the water splashing against the gastro-esophageal (lower esophageal) sphincter and the sound of the water gurgling into the stomach.

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© David G. Ward, Ph.D.  Last modified by wardd 23 May, 2006