Essentials of Anatomy & Physiology Lab Manual
Prof. Atsma 2004
Exercise 15 - The Digestive System
Overview
The digestive system is divided into organs of the digestive tract and accessory organs.
The Mouth and Salivary Glands
The salivary glands secrete water, mucous, and amylase into the mouth to aid chewing and swallowing.
Pharynx, Stomach and Duodenum
Swallowing starts in the pharynx (throat) and continues down the esophagus. Food is liquefied in the stomach before passing out into the first part of the small intestine - the duodenum. The stomach protects itself from the acid and protease needed to start protein breakdown by having a layer of mucous, and lining cells that divide rapidly.
Small Intestine
The small intestine is divided into 3 parts: the first several inches (duodenum), the first half after the duodenum (jejunum) and the last half (ileum). Most digestion and absorption occurs in the small intestine.
The large intestine has several parts: the blind pouch or cecum; the colon (further named for the direction waste moves through); and the rectum. Mainly water absorption and some vitamin production absorption occur in the large intestine.
Digestive Tract Tissues
Serosa: outer connective tissue layer
Muscularis: middle muscle layer
Submucosa: connective tissue layer
Mucosa: inner layer with absorptive epithelial cells
The villus is modified
for absorption (note capillaries and lacteals).
The cells even have microvilli to further increase surface area.
The muscles move the food
through the digestive tract two ways. Peristalsis is wave-like “milking”
motion. Segmentation pushes small compartments along.
The liver produces bile to
emulsify fats, and processes absorbed nutrients. The gallbladder stores and
concentrates bile, and injects it into the small intestine when food enters the
intestine. The pancreas produces several enzymes and bicarbonate ions to
neutralize acid.
1. Learn the digestive system structures on the models using the figure on pg 141 as your guide, except... you do not need to know the specific names of the salivary glands (just “salivary glands”).
2. Use the model of the wall of the digestive tract to learn the 4 tissue layers and key structures (serosa, muscularis, submucosa, mucosa, villi, capillaries, and lacteals).
3. On the fetal pig, just learn the bulleted, bold print structures on pages 142-144. The pig figures in the lab book are tough to follow at times, so use the color photos on your tables whenever possible.
4. Do the Peanut Test, and use one peanut without the shell. Use the Enzyme Action handout instead of the procedure in the book.
Embedded Questions
p. 140
Mechanical digestion is performed primarily in the mouth by chewing, but also in the stomach with its churning action.
Chemical digestion is performed in nearly all parts of the digestive tract except the esophagus and large intestine. However, the stomach and small intestine are the main sites of chemical digestion.
HCl in the stomach is also a nonspecific defense against infectious agents.
The category of food molecule not broken down by enzymes until it reaches the small intestine would be the lipids. Lipase molecules are only effective at the pH we find in the small intestine.
p. 141
Two non enzymes would be sodium bicarbonate (NaHCO3) which neutralizes acid, and bile, which is important for lipid emulsification.
Pepsin, a protease produced by the stomach, works best in an acidic environment.
The pancreatic proteases, which are delivered to the small intestine, work best in an alkaline environment.
p. 142
The esophagus and large intestine are the two organs not involved in digestion or absorption of the food we eat. However the large intestine absorbs water and vitamins.
The esophagus is involved solely in transport of food to the stomach. The large intestine absorbs water and vitamins made by bacteria, which are technically not food.
Activity Questions
p. 144
The small intestine is longer than the large intestine.
The large intestine is considerably larger in diameter than the small intestine.
The small intestine is named for the fact that it is smaller in diameter compared to the large intestine.
p. 145
Yes, the experimental results should be different from the theoretical estimate.
Although your results may vary considerably, it is not unusual for your experimental results to be less than half of the 850 calorie estimate.
p. 146
The main reason for the difference is that the human body is far more efficient than fire at extracting the chemical energy in a peanut. Another reason for the difference is experimental error. Some of the heat was lost to the air, so not all of the heat energy was captured by the water in the test tube.
One would need to eat about 587 peanuts per day to get 2000 calories. Although, this would not be a good idea since peanuts do not contain all of the valuable nutrients, vitamins, and minerals that one needs to be healthy.
Enzyme Action:
Note: these answers are for the new handout activity, not the experiment in the lab book.
Step 4. Five minutes after the time you recorded in step 2, add 5 drops of IKI solution to test tube #3. Write your description of the color below: The liquid should be dark, perhaps a gray or bluish gray.
Compare test tube #3 with the other test tubes. Describe the color of test tube #3 in comparison to the others: Although it is much closer in color to the starch control in test tube 2, it should be at least somewhat lighter than test tube 2.
Step 5. Twenty minutes after the time you recorded in step 2, add 5 drops of IKI solution to test tube #4. Write your description of the color below: The liquid should be gold or perhaps a dark amber.
Compare test tube #4 with the other test tubes. Describe the color of test tube #4 in comparison to the others: It should be much closer in color to test tube 1.
Using the introduction information on page 1 of this supplement (or that found in your lab book or textbook) explain what happened in tubes 3 & 4 during this experiment: Amylase breaks starch molecules into sugar molecules. Because it was closer in color to test tube 2, which had starch, there clearly was plenty of starch left in test tube #3. But test tube #4 had no starch left, as its color was closer to that of test tube #1, which had no starch. Thus, the enzyme seemed to break down much more starch in test tube #4.
If you obtained different colors in tubes 3 & 4 (no matter how slight) propose a reason for this difference: Test tubes 3 & 4 had exactly the same ingredients, and the only variable was time. Enzymes were present in much smaller numbers than starch molecules (similar to the typical situation in the human body). Thus, they need more time to work their way though all the starch molecules. That is why starch breakdown was incomplete after 5 minutes, but there were at most trace amounts left in test tube #4 after 20 minutes.
Consider that the amount of starch used in test tubes 3 & 4 was less than 1/1000 the amount that would be found in one french fry (or freedom fry if you prefer). What does this suggest to you about the length of the small intestine and the digestive process?: The small intestine needs to be long enough to give the enzymes that breakdown starch and other molecules enough time to do their work. The length of the small intestine allows the several hours its enzymes may need to break down the millions of starch (and other food) molecules, and also leave enough time/distance for absorption of these molecules.
Study Questions
p. 147
1. Mouth
Pharynx
Esophagus
Stomach
small intestine (duodenum-jejunum-ileum)
large intestine
2. Mechanical digestion causes a physical change (usually breaking of large food particles into smaller pieces), but no chemical change. Chemical digestion breaks up food molecules into different smaller molecules (a chemical change).
3. The three main macromolecules are: starches or polysaccharides which are made of sugars, proteins or polypeptides which are made of amino acids, and lipids which are made of glycerol and fatty acids.
4. The muscularis (externa) causes peristalsis, the wavelike motion that pushes food through the digestive tract.
5. Most digestion occurs in the small intestine.
The liver (by producing bile) & pancreas (by producing NaHCO3, amylase, lipase, and proteases) assist with digestion without being part of the digestive tract.
6. Most absorption takes place in the small intestine.
Microvilli increase the surface area of the small intestine for greater absorption.
7. Amylases break down starch molecules, proteases break down proteins, and lipases break down lipids.
8. Most fat digestion occurs in the small intestine. Pancreatic lipase is delivered to the small intestine where it is the main enzyme used for lipid digestion. Prior to entry into the small intestine, no active enzyme was present to break down lipid molecules.
9. Bile is a surfactant or emulsifier that breaks up large fat globules into smaller ones (a physical change), and thus increases the surface area for lipase attack.
10. Both the human and fetal pig have nearly all of the same digestive structures, including a gallbladder, which is not found in all mammals.
The pig has a larger cecum and no appendix. The pig's large intestine is coiled ("spiral colon"), but the large intestine in humans forms a square shape.