Lab on Digestion
In this exercise, you will study the role of enzymes in digestion. Follow the instructions below, observe the outcome, and then answer the questions that follow.
Warning: Use caution when handling hydrochloric acid and artificial gastric juice. Biuret reagent can stain skin and/or clothing.
Part A: Gastric Digestion
Food travels from the mouth to the stomach for gastric digestion. During gastric digestion, glands in the stomach wall secrete gastric juice, which mixes with food particles. Gastric juice is a mixture of hydrochloric acid and the enzyme pepsin. At body temperature (37°C) and in the presence of hydrochloric acid, pepsin reduces proteins into polypeptides. The food particles are converted to a semi-liquid state in preparation for intestinal digestion, where enzymes break down the particles even more.
With labeling tape, label three test tubes with your initials and number them 1, 2, and 3. Place one scoop of albumin (rich in protein) in each. Add the following using the graduated cups:
Test tube 1:
5 mL of pepsin solution
Test tube 2
5 mL of hydrochloric acid
Test tube 3
5 mL of artificial gastric juices (pepsin and hydrochloric acid)
Stir each test tube to mix the contents well washing the stirring rod between each test tube mixing. Place the test tubes in a water bath set at 37°C (99°F) for 45 minutes. Remove the test tubes from the water bath at the end of 45 minutes. Add 6 - 7 drops of Biuret reagent to each test tube and shake to mix the contents. Record the color changes below. For the best view, look downward through the top of each test tube.
- Test tube 1 - ______
- Test tube 2 - ______
- Test tube 3 - ______
Biuret reagent reacts with the products of gastric protein digestion (amino acids) to give a pink color. Whole proteins give a purple color.
4. In which test tubes did protein digestion occur? ______
5. Why is the incubator at 37 – 40°C? ______
6. What effect does pH have on the effectiveness of pepsin? ______
______
Part B. Intestinal Digestion
The semi-liquid produced during gastric digestion passes into the small intestine, stimulating the production of alkaline intestinal fluids. When the semi-liquid becomes alkaline, gastric digestion ceases and intestinal digestion begins. Pancreatic juice is secreted by the pancreas into the duodenum. This juice contains the enzyme amylase, which breaks starch down into glucose, which can be absorbed into the body through the lining of the small intestine.
A. With labeling tape, label three test tubes with your initials and number them 1, 2, and 3. Using a clean graduated cup, add 5mL of starch solution to each test tube. Using a clean graduated cup, add 5 mL of pancreatic juice to each test tube. For 30 minutes, maintain the test tubes separately under the following conditions:
Test tube 1: room temperature
Test tube 2: 37°C water bath
Test tube 3: 75°C water bath
Remove test tube 2 and test tube 3 from the water baths. Add 3 drops of iodine-potassium-iodide solution to all three test tubes. Gently shake to mix the contents. Record the colors below. For the best view, look downward through the top of each test tube.
- Test tube 1 (room temperature) - ______
- Test tube 2 (37°C) - ______
- Test tube 3 (75°C) - ______
In the presence of starches, iodine-potassium iodide changes from bright yellow to blue-black. In the test tubes, the presence of starch is indicated by a purple-black color. The absence of starch is indicated by a light brown color.
10. What happened to the starch in test tube 1? ______
Has digestion occurred? ______
11. What happened to the starch in test tube 2? ______
Has digestion occurred? ______
12. What happened to the starch in test tube 3? ______
Has digestion occurred? ______
13. What is the product of starch breakdown? ______
14. Which temperature was best for starch digestion? ______
Why do you think this is so? ______
The semi-liquid produced during gastric digestion passes into the small intestine, stimulating the production of alkaline intestinal fluids. When the semi-liquid becomes alkaline, gastric digestion ceases and intestinal digestion begins. Pancreatic juice is secreted by the pancreas into the duodenum. This juice contains the enzymes trypsin and lipase. Proteins are broken down into amino acids by trypsin. Fats are broken down into glycerol and fatty acids by lipase. Amino acids and fatty acids can be absorbed into the body through the lining of the small intestine.
B. With labeling tape, label two test tubes with your initials and number them 1 and 2, respectively. To each add 10 mL of litmus milk solution. Then add the following:
Test tube 1
Add one scoop of pancreatin powder.
Test tube 2
Add a few drops of distilled water.
Shake gently to mix the contents. Place the test tubes in the water bath at 37 - 40°C for 45 minutes. Then remove the test tubes and observe the color changes. Record the colors below. For the best view, look downward through the top of each test tube.
15. Test tube 1 - ______
16. Test tube 2 - ______
The milk turns blue when basic and red when acidic. The litmus in the milk indicates the presence of acid by a color change from light gray to dark pink-purple.
17. What happened to the proteins and lipids (fats and oils) in test tube 1? ______
Has digestion occurred? ______
18. What happened to the proteins and lipids (fats and oils) in test tube 2? ______
Has digestion occurred? ______
19. Milk contains proteins and lipids (fats and oils). Why would the digestion of these nutrients cause an increase of acidity? ______
(Hint: What are the products of protein and lipid digestion?)
20. Why was the water bath temperature set to 37°C? ______
Part C. Absorption in the Small Intestine
To provide nourishment useful to the body cells, absorption must follow gastric and intestinal digestion. Gastric digestion reduces food particles to a semi-liquid state. Intestinal digestion breaks down starches into glucose, and proteins and fats into amino acids and fatty acids. Absorption is the passage of digestion byproducts through the intestinal mucosa into the blood or lymph. If molecules are not broken down adequately, they are not absorbed through the lining of the small intestine.
Soak a 6-inch piece of dialysis tubing under running water for 30 seconds, then rub it between your thumb and index finger to open it. Knot one end of the dialysis tubing to create a “bag.” Half-fill the dialysis tubing with an equal mixture of starch solution and glucose solution. Tie off the other end of the tubing. Rinse the outside of the tubing in tap water and carefully wipe dry the dialysis tubing to remove traces of nutrients.
Place it in a 250 mL beaker containing 50mL of distilled water. Wait for 30 minutes. During this time, make sure that the dialysis tubing remains completely submerged in the distilled water.
With labeling tape, label two test tubes with your initials and number them 1 and 2. Remove the dialysis tubing from the beaker. Using the larger of the 2 droppers in your basket, transfer 5 mL of the water in the beaker into each test tube.
Add 20 drops of Benedict’s solution to test tube 1. Place the test tube in a boiling water bath for 5 minutes. After 5 minutes, observe the color.
When heated, Benedict’s solution reacts with glucose to produce a red-orange precipitate.
21. From your observation of the test tube, did glucose diffuse into the water in the beaker? ______
How do you know? ______
22. Do you think that glucose can be absorbed by the small intestine? ______
Why or why not? ______
Add 3 drops of iodine-potassium-iodide to the second test tube 2. Observe the color. The presence of starch is indicated by a blue-black color.
23. From your observation of the test tube, did starch diffuse into the water in the beaker? ______24. Do you think starch can be absorbed by the small intestine? ______
Why or why not? ______