DIGESTIVE SYSTEM. Laboratory Exercise Three

Transcription

1 DIGESTIVE SYSTEM Laboratory Exercise Three INTRODUCTION: The human digestive system consists of a tube running from the mouth to the anus with specialized regions for the breakdown of food. The food is broken down both mechanically (chewing, crushing, tearing) and chemically (enzymatically) into the basic nutrients (amino acids, simple sugars, fatty acids, glycerol, etc). These nutrients are absorbed and used by the body for energy and to build new molecules. Label the diagram shown below using your lecture notes and/or textbook:

2 Part 1: Pepsin Digestion Pepsin is a protease secreted in the stomach of humans. This digestive enzyme is responsible for initiating protein breakdown. It requires a very acidic environment, one with high concentrations of hydrogen ions, in order to function. Consequently, the stomach also secretes hydrochloric acid (HCl) so that the appropriate acidity will exist in the stomach while the pepsin is acting on the proteins in your food. PROCEDURES: 1. Label 12 clean test tubes With a sharp scalpel slice 12 very thin slices of the egg white. These slices should be about the size and thickness of a fingernail. Make sure that all of the slices are approximately the same size and thickness. 3. Place a slice of egg white in each test tube. 4. Add the following solutions to the 12 test tubes: Test Tube Water (ml) 4% Pepsin Solution (ml) 1% HCl Solution (ml) 5N NaOH Solution 1, 5, , 6, , 7, , 8, drop 5. Gently mix the contents of your test tubes by tapping the side of the tube with your index finger. 6. Place test tubes 1-4 in a test tube rack and allow them to sit on your desk for 1 hr. 7. Place test tubes 5-8 in a test tube rack and allow them to sit in an ice bath for 1 hr. 8. Place test tubes 9-12 in a test tube rack and allow them to sit in a 37 degree Celsius water bath for 1 hr. 9. After 1 hr, record the appearance of the egg white in Data Table I.

3 DATA TABLE I Room Temperature Ice Water 37 degrees celsius egg white + water #1 #5 #9 egg white + pepsin + water #2 #6 #10 egg white + pepsin + HCl #3 #7 #11 egg white + pepsin + NaOH + water #4 #8 #12 Use the data you collected to answer the following questions: 1. Which test tube(s) showed the greatest digestion of the egg white? Explain why. 2. Which test tube(s) showed the least protein digestion? Explain why. 3. What do these results demonstrate about the effect of temperature on the rate of protein breakdown in your stomach?

4 Part 2: Digestion of fat by pancreatic lipase Pancreatic lipase is an enzyme secreted into the small intestine by the pancreas in humans. This digestive enzyme is the primary lipase that breaks down dietary fat molecules in the human digestive system. Pancreatic lipase converts triglycerides into glycerol and three fatty acids, both of which are monomers. PROCEDURES: 1. Clean 2 test tubes and place them in a test tube rack. Label the test tubes 1 and 2 using a wax pencil. Do not use tape. Set them up as listed below: Test Tube 1 Test Tube 2 Add 5 ml of canned milk (fat) Add 5 ml of canned milk (fat) 3 ml of distilled water 3 ml of lipase (stored on ice) 1 dropper full of litmus solution 1 dropper full of litmus solution 3 drops of 2.0% NaOH 3 drops of 2.0% NaOH 2. Cover each test tube with Parafilm, thoroughly mix the contents in each tube using the vortex mixer. 3. Observe and record in Data Table II the color(s) of the mixtures in the test tubes. Then put both test tubes in the 37 degree water bath for one hour and record color(s) on Data Table II. 4. The litmus solution is used to test for acids and bases. Determine what color litmus turns in an acid or base using the table for the previous week s lab. DATA TABLE II Before Incubation After Incubation Test Tube 1 Test Tube 2 Color Acid or Base? Color Acid or Base? Use the data you collected to answer the following questions: 1. Were the tubes most likely acidic or basic when you put them in the incubator? How do you know this?

5 2. What is the initial difference between tubes 1 and 2? 3. What is the function of pancreatic lipase? Show the equation for this reaction. a. Function: b. Reaction: Lipid lipase 4. Explain both the presence and absence of the acid/base changes that you observed in this experiment. + Part 3: Starch Digestion Amylase is an enzyme responsible for the breakdown of carbohydrates (i.e. starch). There are two types of amylase found in the human digestive system. Salivary amylase is produced in the mouth by the salivary glands and is responsible for initiating the breakdown of carbohydrates in the mouth. Pancreatic amylase is by the pancreas into the small intestine where it further breaks down starches into simple sugars which can then be absorbed into the blood stream. PROCEDURES: 1. Obtain two 2 cm plugs of raw potato. 2. Clean 2 test tubes and place them in a test tube rack. Label the test tubes 1 and 2 using a wax pencil. Do not use tape. Set them up as listed below: Tube 1 Tube 2 10 ml of deionized water 10 ml of amylase (stored on ice) 20 ml of potato plug 20 ml of potato plug 3. Put both test tubes in the 37 degree water bath for 20 minutes.

6 4. After 20 minutes, remove both test tubes from the water bath. Add 2 ml of Benedict s solution to each tube after incubation. 5. Place both test tubes in the beaker of boiling water sitting on the hot plate. 6. Check your results in 2-3 minutes. Benedict s solution us used to test for glucose. If the tube changes from a blue color to an orange or yellow color, the test is positive and glucose is present. Record your results in Data Table III. Starch Digestion Questions: 1. How do tubes 1 and 2 differ? Would you expect this to have an effect on the amount of starch digestion in each tube? Explain. 2. Color after Benedict s Test Tube 1 Tube 2 3. Based on the results of this experiment, what did the enzyme amylase do? (i.e. on what substrate does it work?) 4. What happened to the starch molecules in test tube 2?