Purpose

This simulation helps students understand what happens to food as it passes through the digestive system. The student has a card representing food, which he/she carries to stations around the room. Each station represents one of the organs of the digestive system. At each station, the student is given a specific instruction to "digest" the index card and then instructed to "proceed to the next station". At the end of the simulation, students form cooperative learning groups to analyze and apply the results of the simulation to the human digestive system.

Introduction

Function

The digestive tract is a continuous tube starting with the oral cavity, pharynx, esophagus, stomach, small intestine and large intestine, ending with the anus. It is the site of digestion and absorption of nutrients from the foods we eat. In general, the "Upper GI" refers to mouth through stomach, and "Lower GI" refers to intestines to anus. However, the digestive system cannot fully accomplish the task of digestion and absorption without proper functioning of the accessory organs: teeth, tongue, salivary glands, liver, gall bladder and pancreas.

Digestion refers to mechanical (chewing, muscular contractions/churning and mixing) and chemical (acid hydrolysis, digestive enzyme activity) processes that break down food. The chemical composition of food is very similar to the chemical composition of our bodies:

Carbohydrates -polymers of glucose

Fats - triglycerides

Proteins - polymers of amino acids

The only way these macromolecules (Carbs, proteins and fats) get into our bodies is by breaking down into their monmeric units and absorbing them in these smaller units. This optimizes our usage of the fuels we ingest because at the end of the digestion/absorption process, the body now has the raw materials to make the proteins and chemical substances it needs from moment to moment. This also poses a problem: the digestive enzymes that we use to digest food can also potentially digest us.

There are two strategies employed to overcome this problem. The first is to place the digestive enzymes and acid in the lumen of the gi tract. The lumen is the surface of the GI tract that is exposed to the food (it is technically on the outside of the body). This protects the contents of the cells that line the lumenal side of the GI tract. The second strategy is to store digestive enzymes within cells as inactive precursors (zymogens). The precursors, once outside of the cell (in the lumen) of the intestine require an activation step in order to get enzyme function.

Overview of digestion

Food is delivered to the stomach after it is chewed and swallowed in the mouth. The main mechanism of digestion here is mechanical, however, the smaller bits of starch released from the process of chewing are digested to smaller pieces by amylase. Amylase is an enzyme secreted in saliva from the salivary glands. Saliva is secreted in response to food in the mouth (or the mere thought of food in the mouth - think Pavlov).

Sherwood figure 16-5

Once swallowed, food is in the esophagus. The distention of the esophagus tube is the stimulus for muscle contractions called peristalsis. Peristalsis is caused by contraction of the muscle layer of the esophagus. Food is then delivered to the stomach.

Figure 16-8 and Figure 16-9

The food delivered to the stomach is relatively undigested. The stomach is filled with a highly acidic fluid and once food is in the stomach, muscle contractions in the stomach, churn the food, mixing it well with acid. Both mechanical (churning) and chemical (acid and pepsin, see next paragraph) digestion take place here. The acidic enviroment denatures proteins, exposing their peptide backbones. Acid is contained withing the stomach by the sphincters present at the junction of the esophagus and the stomach and the junction of the stomach and the duodenum. The sensation of heartburn is from acid reflux into the esophagus.

Aside from the acid (HCl), an inactive precursor enzyme called pepsinogen is secreted in the stomach. Pepsinogen, under the highly acidic conditions of the stomach, has weak activity, but enough so that it can catalyze the digestion of itself to the active form called pepsin. Pepsin is an protein digesting enzyme. Furthermore, pepsin's pH optimum is at stomach pH. Therefore, the low pH of stomach acid serves two purposes: denaturing proteins and exposing them to the newly activated pepsin. Starch digestion does not progress much here, in fact, salivary amylase whose pH optimum is close to 7, is inactivated here. Up to this point, fat digestion has not progressed much either. However, the processes described so far have served the purpose of preparing the macromolecules to be digested with enzymes, which mainly takes place in the intestine.

Figure 16-13

How does the stomach keep from digesting itself? Cells lining the stomach also secrete large amounts of bicarbonate (a base) containing mucous, this forms a protective layer of mucous for the cells lining the stomach. In addition, cells lining the stomach are very tightly associated with each other, forming a barrier. Finally, the stomach replaces its entire lining every 3 days.

The food stuff that leaves the stomach is now called chyme. Peristaltic waves move chyme down the digestive tract. It moves into the small intestine. Structurally and functionally the small intestine is divided into three parts (in order of proximity to the stomach) duodenum, jejunum, and ileum. The pH of the chyme is very low. Furthermore, food particles in the chyme are too large to be absorbed by the intestine. To further digestion, the pancreas secretes juices containing digestive enzymes and a high amount of bicarbonate. While in the bicarbonate juice, the digestive enzymes have no activity. When secreted into the duodenum, the bicarbonate neutralizes the chyme, allowing for activation of trypsin by an enzyme on the surface of the cell in the duodenum. Trypsin, then activates other digestive enzymes.

The digestive enzymes in the pancreatic juices can digest protein, lipids and carbohydrates. Fat is broken down by breaking off the fatty acids attached to the glycerol backbone. However, the problem with fat digestion is that fat is not water soluble and digestion is a water based process. Fat needs to be solubilized in water before it can be digested by the enzymes secreted from the pancreas. This is accomplished by bile salts, (See Figure 16-24) which are secreted by the gall bladder into the duodenum. Once solubilized by bile salts fats can be broken down into fatty acids and monoglycerides.

Figure 16-24

Jejunum is largely the site of nutrient absorption. Digestive enzymes are present at the cell surface of jejunum cells and there they act on the smallest carbohydrate and protein pieces (in essence dimers of glucose and dimers of amino acids). Absorption of these nutrient (now broken down into monomers) occurs mainly by carrier mediated transport. This means that the cells of the intestine have proteins carriers/transporters (see Membrane transport lab for review) at their cell surface that recognize this monomers and carry them across the cell membrane. Absorption of fatty acids and monoglycerides occurs by diffusion through the cell membrane (because it is mainly lipid based). See Figures 16-30, 16-31, 16-32

In the ileum, bile salts are reabsorbed (and recycled), vitamin B12 is absorbed along with electrolytes (Na+ and K+ etc…) and water. The chyme that passes from ileum goes to the large intestine. The large intestine has no digestive function, however, it continues to absorb water, electrolytes and some B vitamins. Resident bacteria in the large intestine produce vitamin K and folic acid that are absorbed in turn. Finally, the waste products are excreted out the anus.

Method

As stated above, the purpose of this lab is to introduce you to the components of the digestive system and to understand their roles in digestion. Each student should go through the digestive system in groups with their "meal". Each of the four benches will be a station that represents a portion of the digestive system. Read through the information about each organ and follow the directions on how to "digest" your meal. Start at the mouth!. Discuss the following questions in the link to the write up. Answer them and then we will discuss the questions as a group.

Link to WRITE UP

This activity was adapted from " A Walk Through the Gut" by VivianLee Ward, 1994.