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Digestive System

Each of the first four volumes provides 25 probes with easy-to-follow steps for uncovering and addressing students’ ideas by promoting learning through conceptual change instruction. Probes cover topics such as physical, life, and Earth and space science; the nature of science; and unifying themes. Each volume on page 23 provides topic-specific probes. These invaluable books include teacher materials that explain content, identify links to standards, and suggest grade-appropriate ways to present materials so students learn the concepts accurately. Teachers, professional development coordinators, and college science and preservice faculty will find these resources essential and exciting.

 

Digestive System

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Purpose

The purpose of this assessment probe is to elicit students’ ideas about the digestive system. The probe is designed to find out whether students realize a main function of the digestive system is to break food down into molecules that can be used by cells.

Related Concepts

Cells, digestive system, food, nutrients

Explanation

The best answer is Sasha’s: “I think the main function is to break food down into molecules that can be absorbed by cells.” There is no single purpose of the digestive system; rather, it has two major purposes: (1) to break down food and (2) to prepare nutrients for absorption by cells. The digestive system carries out six basic functions: taking food in (ingestion), secretion, movement of food and wastes, breakdown of food, absorption from gastrointestinal tract to cells, and removal of wastes. In regard to Mina’s response, the digestive system does not release energy from food. Instead it breaks food down into molecules that are absorbed by cells and that can then be used to release energy within the cell. Harriet’s response is partially correct. The stomach does break ingested food down into smaller pieces of food. However, there is more to digestion than what happens in the stomach. These small pieces of food are not used directly by the body but are further broken down into small particles (molecules) absorbed by cells as they pass through the intestines. The difference between Sasha’s and Harriet’s responses is that the broken down food must be small enough to be taken in by and used by cells. Even though the mouth and stomach break food down into small pieces of food, it is not until the food is broken down into the molecular units that make up the food that it can be used by the body to carry out the life processes that happen within cells. Todd is partially correct in that the digestive system does move food and nutrients; however, it moves these things through the digestive tract and not through different parts of the body. It is the circulatory system that moves nutrients throughout the body to cells.

Curricular and Instructional Considerations

Elementary Students

In the elementary grades, students learn basic ideas about the human body and body structures that help us take in and digest food, such as the mouth, teeth, and stomach. Young children primarily equate the stomach as the organ responsible for digesting food as they haven’t yet learned how all the parts work together. By third grade, students begin to view the body as a system that works together and they can further explore what happens to food when it is taken into the body. At the upper-elementary level, it is important for students to know that food is broken down to obtain energy and materials for growth and repair, but the molecular aspect can wait until middle school.

Middle School Students

In the middle grades, students develop a more sophisticated understanding of the human body and the organs and systems that work together to enable humans and other organisms to carry out their life processes. This is the time for them to understand that when food is broken down, it must be digested into molecules that can be absorbed and transported to different parts of the body. At this level, they are ready to understand the link between the digestive system and the circulatory system for breaking down food into molecules and transporting nutrients and to understand the role of the digestive and excretory system in eliminating the parts of food that are not used.

High School Students

Students at the high school level expand their understanding to encompass molecular energy release and the biochemical details related to metabolism. Their growing knowledge of cells helps them understand how molecules are taken into cells and used to carry out life processes.

Administering the Probe

If this probe is used with elementary students, consider substituting the word molecules with tiny particles.

Related Ideas in National Science Education Standards (NRC 1996)

K–4 Characteristics of Organisms

  • Organisms have basic needs. For example, animals need air, water, and food; plants require air, water, nutrients, and light.

5–8 Structure and Function in Living Systems

  • Cells carry on the many functions needed to sustain life. This requires that they take in nutrients, which they use to provide energy for the work that cells do and to make the materials that a cell or an organism needs.
  • The human organism has systems for digestion, respiration, reproduction, circulation, excretion, movement, control and coordination, and protection from diseases. These systems interact with one another.

9–12 The Cell  

  • Most cell functions involve chemical reactions. Food molecules taken into cells react to provide the chemical constituents needed to synthesize other molecules. Both breakdown and synthesis are made possible by a large set of protein catalysts, called enzymes. The breakdown of some of the food molecules enables the cell to store energy in specific chemicals that are used to carry out the many functions of the cell.
Related Ideas in Benchmarks for Science Literacy (AAAS 1993 and 2008)
Related Ideas in Benchmarks for Science Literacy (AAAS 1993 and 2008)

K–2 Basic Functions

  • The human body has parts that help it seek, find, and take in food when it feels hunger—eyes and noses for detecting food, legs to get to it, arms to carry it away, and a mouth to eat it.

3–5 Basic Functions

  • From food, people obtain fuel and materials for body repair and growth. The indigestible parts of food are eliminated. (R)

6–8 Basic Functions

  • Organs and organ systems are composed of cells and help to provide all cells with basic needs.
  • For the body to use food for energy and building materials, the food must first be digested into molecules that are absorbed and transported to cells.

Related Research

  • Some upper-elementary students have a primitive notion of the digestive system as the place where “lumps of food” are broken down, juices or acids dissolve the food, and “goodness” is somehow extracted from it. Few children ages 9–12 know that after food is changed in the stomach it is then broken down into even simpler substances that are carried to tissues throughout the body (Driver et al. 1994).
  • Lower-elementary students know food is related to growing and being strong and healthy, but they are not aware of the physiological mechanisms (AAAS 1993).
  • In a study conducted by Mintzes (1984), the youngest children up to age seven related the stomach to breathing, blood, strength, and energy. When they are about seven years old, children begin to understand that the stomach helps to break down or digest food and that later the food is transferred somewhere else after being in the stomach (Driver et al. 1994).

Suggestions for Instruction and Assessment

  • Students know that humans need food for energy and that the digestive system breaks down food so it can be used, but students often think that the food is just broken into smaller pieces. At the middle level, students should be explicitly taught the idea that food must be broken down into molecules in order for the body to use it.
  • Show the connections among the digestive, circulatory, and respiratory systems in terms of breaking down food into molecules, transporting the molecules to cells, and releasing energy in the cells.
  • Help students understand that the energy we get from food is released inside a cell. To get into the cell, the food must be broken down into molecules in order to pass through the cell membrane. Teaching students that food must eventually get into the cell in a form the cell can use will help them understand that food must be broken down into molecules.
  • Use models and diagrams so that younger students can see that food continues to pass through the digestive system after the stomach. Some students think food is broken down in the stomach and the rest passes out as waste. Few students comprehend the role of the intestines in terms of absorption.

Related NSTA Science Store Publications, NSTA Journal Articles, NSTA SciGuides, NSTA SciPacks, and NSTA Science Objects

Crowley, J. 2004. Nutritional chemistry. The Science Teacher (Apr.): 49–51.

Robertson, W. 2006. Science 101: How does the human body turn food into useful energy? Science & Children (Mar.): 60–61.

Schroeder, C. 2007. Inquiring into the digestive system. Science Scope (Nov.): 30–34.

Texley, J. 2001. Anatomy by logic. Science Scope (Sept.): 56–59.

Related Curriculum Topic Study Guides (Keeley 2005) “Human Body Systems” “Food and Nutrition”

References

American Association for the Advancement of Science (AAAS). 1993. Benchmarks for science literacy. New York: Oxford University Press.

American Association for the Advancement of Science (AAAS). 2008. Benchmarks for science literacy online. www.project2061.org/publications/bsl/online

Driver, R., A. Squires, P. Rushworth, and V. Wood Robinson. 1994. Making sense of secondary science: Research into children’s ideas. London: RoutledgeFalmer.

Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press.

Mintzes, J. 1984. Naive theories in biology: Children’s concepts of the human body. School Science and Mathematics 84 (7): 548–555.

National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academy Press.

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