THE QUEST TO DIGEST

Knight Foundation Summer Institute

Kaye Edwards, Haverford College

Liane D'Alessandro, Haverford College

Introduction:

The purpose of the series of experiments within this lesson is to introduce the students to their digestive system. Digestion is essential to keep us alive, but many students may not even be aware of why we need the digestive system or how it works. Begin the lesson with a discussion about eating -- an activity that the students are all familiar with -- so that they can start to think critically about the digestive system. The questions below serve to organize the information about the digestive system for the teacher. With some assistance from the teacher, the questions could also serve as discussion topics for the students.

Why do we eat? Starting out from this general question can lead to the concepts that we eat because our bodies need raw materials and energy to perform our daily activities. Ask the students for examples of such activities. They may come up with walking, breathing, talking, playing, thinking. But in addition to these activities, there are a multitude of jobs our bodies do inside of us that we may not even be aware of. For example, our bodies are constantly in the process of growing and repairing: we make bigger muscles, grow longer hair and stronger bones, replace the cells that slough off from our skin and our digestive system, make more red and white blood cells, etc.

What is food? Foods can be classified by the large molecules that it contains: carbohydrates, fats, proteins. These large molecules are polymers (long chains) of simpler molecules (monomers). For example, sugars (monomers) linked together make complex carbohydrates such as starch (a polymer). Likewise, amino acids (monomers) linked together make proteins (polymers). The fruits, vegetables, grains and meats that we eat contain different amounts and different varieties of carbohydrates, fats and proteins.

What is digestion? Digestion is the process of breaking down these large molecules found in food into smaller molecules. The digestive system continues to break down these smaller molecules into smaller and smaller molecules that the body can use to do its many jobs.

How is energy stored in food? Chemical bonds that link the atoms together within those small molecules hold energy. So do the chemical bonds that link the small molecules into large polymers. When these bonds are broken, energy is released and can be used by our bodies for all the activities mentioned in the answer to the first question.

How are raw materials stored in food? When the chemical bonds are broken and smaller molecules are formed, the small molecules can be reused to build up new larger molecules. (It's similar to playing with Legos: you can take apart a space ship, break it down into a pile of pieces, and use those pieces to make something new, like an airplane!) We might eat beef and vegetables, but we are humans, not cows or corn. We eat cow proteins, digest them into amino acids and other small molecules, and reuse them to build the human proteins we need. We eat corn carbohydrates, digest them into sugars and smaller molecules, and reuse them to build the human carbohydrates we need.

Objectives:

  1. To learn the advantages of breaking down a scientific question into smaller parts.
  2. To discover properties of complex carbohydrates (such as starch) and sugar.
  3. To discover where carbohydrates are digested into sugars.
  4. To reason why digestion is essential for life.

Vocabulary:

food pyramid/carbohydrates/fats/proteins

energy and raw materials

molecules and chemical bonds

polymers and monomers

digestion enzymes

absorption and osmosis

cells

Materials:

Procedure:

** These procedures are for a series of experiments. The first step is a demonstration by the teacher. The students will note the results of this demonstration and perform their own experiment in step 2 to explain what happened in the demonstration. Later, in step 3 the understanding of what happened in the demonstration will help the students to understand one aspect of the digestive system (i.e., does digestion occur in the mouth?). Finally, step 4 provides an experiment to illustrate another aspect of the digestive system (i.e., why we need digestion).

  1. Pour some Basic 4 into a plastic cup. Add water and stir. Add a few drops of iodine and note the blue color. Why did it turn blue? What's in Basic 4 that reacted with the iodine to make a blue color? This question will be answered by the students in step 9.
  2. Look at the ingredients label and note the presence of carbohydrates, fats, protein and salt in the cereal. Ask them to design an experiment to determine which ingredient turns blue in the presence of iodine. Students can break the problem down into several simple experiments and test each component individually, using the cornstarch or flour (complex carbohydrates), table sugar (simple carbohydrates). vegetable oil (fats), gelatin (protein), or table salt. Remind them that iodine is poisonous to eat, although it will not hurt their skin.
  3. Compile the data from the whole class and discuss the results. After the students discover that only the complex carbohydrates stain blue with iodine, ask them to figure out what saliva does to starch, using the experiment "Size Change" in Janice Van Cleave's Biology for Every Kid (instructions attached). You may want to review the relationship between starch and sugars, i.e. starch is a polymer of sugar monomers. Ask them to hypothesize what happens to starch in their mouths. (Hint: What is the purpose of digestion?) You could introduce the concept of enzymes (special proteins in our bodies that speed up chemical reactions). The enzyme in our saliva is called "amylase" and it helps break down starch (which can stain blue) into sugars (which do not stain blue).
  4. An experiment to help students visualize one reason why our bodies need to break down starches into sugars is described in "In But Not Out" from Van Cleave's book (instructions attached). If you don't want the students to use this much iodine, you can set the experiment up before class as a demonstration. Doing it ahead of time gives the iodine time to move from the outer solution through the bag and react with the starch solution inside. After they've completed the earlier experiments, show them this set up. They should be able to reason why the water outside the bag is golden (it contains iodine) and what's in the bag (starch and iodine; it turned blue). Ask them why the water outside the bag did not turn blue also. This can lead to a discussion of membranes and how our cells, just like this bag, let some molecules pass through. It is the smaller molecules, not the polymers, that typically pass from the outside of a cell into the inside. Therefore, if we didn't break down our food into smaller molecules, we couldn't easily get the food inside our bodies. Digestion is essential in helping nutrients be absorbed by our intestinal cells.

Assessments:

Ask the students to write up the experiment they designed in Step 2 in scientific format (i.e., writing the purpose, procedure, results, and conclusion).

Hold a discussion about the lessons learned from this series of experiments to make sure that each student understands the concepts.

Extensions:

A great follow-up to this lesson is another experiment in this booklet entitled, "The Eggciting Egg-speriment" which teaches the concept of osmosis in a very fun way. This would further illustrate what is happening in Step 4.

Having a concrete example to look at is often very helpful in learning a new concept. Therefore, build a life size model of the digestive system to illustrate the relationships of the various organs. The students will be amazed at how long their intestines are!

Another great way to visualize the digestive system is to take advantage of software packages about the human body. These CD-ROM's contain very exciting graphics and interesting facts for the students to explore. Some examples are The Ultimate Human Body by Dorling and Kindersley Multimedia and Body Works 5.0 by Softkey International Inc.

Philadelphia Science Content Standards:

SCIENCE CONTENT STANDARD #1: NATURE OF SCIENCE

This experiment satisfies Benchmark 1 for grades 5-8 which states that students should be able to "design, modify, and conduct and investigation through testing, revising, and occasionally discarding ideas all of which lead to a better understanding of how things work." The lesson satisfies Benchmark 3: "collect, and summarize data from an experiment and interpret the results in terms of the data."

SCIENCE CONTENT STANDARD #3: LIVING ENVIRONMENT

Benchmark 4 states that students should be able to "explain how cells carry on the many functions needed to sustain life." This lesson illustrates that one way cells are able to do their jobs is by getting energy and raw materials from food. Furthermore, this lesson satisfies Benchmark 8 which states that students should "understand how energy can change from one form to another in living things."

SCIENCE CONTENT STANDARD #4: HUMAN ORGANISM

Learning about the digestion of food satisfies Benchmark 1 since it states that students should be able to "illustrate how human beings like other animals, have body systems for obtaining and providing energy." Furthermore, this lesson partially satisfies Benchmark 2 which states that students should know how to "compare and contrast systems for the digestion, respiration, and reproduction of various animals." Finally, Benchmark 4 asks students to "demonstrate how organs and organ system work together (e.g. lungs and circulatory system work together to distribute oxygen)."

Cross-references:

This lesson fits perfectly into a life science unit on the human body in which digestion would be discussed. However, it also brings in chemistry topics such as chemical reactions, chemical bonds, and polymers.