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NOVA scienceNOW: Marathon Mouse

Classroom Activity

Activity Summary
Student teams analyze pictures of magnified muscle cells from a hypothetical experiment to determine the effects of exercise and performance-enhancing drugs.

Learning Objectives
Students will be able to:

  • describe the function of mitochondria

  • describe cellular changes caused by exercise

  • analyze a cause-and-effect relationship between behavior and physiology

  • make predictions based on data

  • explain the importance of a control group in an experiment

  • debate the pros and cons of performance-enhancing substances

Suggested Time
One class period


Multimedia Resources

Additional Materials


Many effects of regular exercise are obvious, such as well-defined muscles, weight loss, and less huffing and puffing through workouts. But few people consider the cellular changes underlying noticeable physical results. Improved endurance is one aspect of exercise training that has been studied at the cellular level.

Eukaryotic cells get energy thorugh respiration
All cells require energy to simply survive, and cells obtain that energy from food molecules, such as glucose. Food molecules are broken down in several chemical reactions that release usable energy, which is stored in high-energy adenosine triphosphate molecules (ATP). ATP is then used by cells to carry out life processes. One method eukaryotic cells use to make ATP from glucose is known as aerobic respiration , which occurs in the mitochondria. Overall, the process can be summarized in the following reaction:

Glucose + Oxygen → Water + Carbon Dioxide + ATP

Exercise changes cells
The show describes two types of muscle cells, slow twitch and fast twitch. Slow-twitch muscles are important for endurance. They have lots of mitochondria and efficiently burn both fat and glucose. Fast-twitch muscles, which are used in quick bursts of activity, have fewer mitochondria and primarily burn glucose. The number of slow- and fast-twitch fibers in any particular muscle appears to be genetically determined. However, athletes can gear their training to especially develop one kind of muscle. For example, endurance training will develop more slow-twitch muscle, on a percent-total-mass basis. These slow-twitch muscles will contain more and larger mitochondria and have an improved ability to convert fats into usable energy.

Endurance in a pill
The show describes a genetically engineered mouse that could run longer and farther than normal mice. How? Researchers turned a gene, called PPAR delta, on overdrive. From birth, the mouse's muscle cells made more mitochondria and burned more fat than the muscle cells of normal mice would.

Additional research on mice showed that a drug known as AICAR had the same influence as endurance training. The drug works by triggering the cell's fuel gauge, making it behave as if energy is scarce, even when it's not. The cell responds by increasing the amount of slow-twitch muscle, on a percent-total-mass basis. The cells even burn fat better. Could the drug work in humans? No one knows—yet.

In this activity
In this activity, students will analyze pictures of magnified muscle cells from a hypothetical experiment to determine the effects of exercise and performance-enhancing drugs.

Procedure Before the Lesson
  • Make copies of the Marathon Mouse handout (one per student).
  • Obtain envelopes (one envelope per student pair).
  • Make copies of the Teacher Black Master, cut out the images, and place the images from two different mice in each envelope. If you plan to reuse the materials, consider laminating the images.
  • If possible, provide computer access for each pair of students. Alternatively, as a class, watch the two videos mentioned in the student handout.
  • Bookmark the following Web sites:
The Lesson
  1. As a class, watch the NOVA scienceNOW segment Marathon Mouse (11 minutes). You can stream it from the NOVA scienceNOW Web site at Discuss any questions students have about the segment.

  2. Ask students where people get their energy. (Food) To illustrate how food is digested and distributed to various tissues for storage or energy, show the class the two-minute video titled "The Fate of Fat" (scroll down to find it):

  3. Discuss the video and tell students that the fat entering muscle cells will be broken down and used by the mitochondria. Remind them that mitochondria convert food molecules into usable energy and that a simplified reaction for aerobic respiration is:

    Glucose + Oxygen → Water + Carbon Dioxide + ATP
  4. Tell students that they will analyze data from a hypothetical experiment similar to the ones presented in the segment. Divide the class into pairs, and give each pair a copy of the handout. Also give each pair an envelope containing data from two of the four experimental mice. Tell students that the darker areas in the images are the mitochondria-rich muscle cells.

  5. Have each pair read the handout's opening statement and complete the procedure for analyzing their cell samples, including viewing the Flash animation Where do you get your energy? and the How the Body Responds to Exercise video.

  6. Write a chart on the board to organize the class data, and have students fill in their responses once pairs have completed Step 3 of the handout. Here is an example: (Note: The numbers in the chart are hypothetical numbers.)

    Mouse Student Pair Number of mitochondria-rich cells at the beginning of the experiement Number of mitochondria-rich cells at the end of the experiement
    1 Bob and Sue 4 5
    Maria and Olivia 5 5
    2 Carlos and Andre 4 7
  7. Ask students to look at the class data and predict which mouse received which treatment. Then, reveal the answers:

    • Mouse 1 is the control mouse (no exercise, no drugs, mitochondria/cells stayed the same).
    • Mouse 2 exercised daily (increased number of mitochondria-rich cells).
    • Mouse 3 received Drug x (same effect as exercise).
    • Mouse 4 received Drug y (no effect).

  8. Ask students to compare the effects of each treatment. For example, both Drug X and Exercise created more mitochondria. On the other hand, "no treatment" led to no change in the number of mitochondria-rich cells, just like the ineffective Drug Y. Ask students to explain any incorrect predictions. For example, they didn't know which drugs were effective in this hypothetical experiment until all the available information was compared.

  9. Have students debate the pros and cons of a pill that simulates exercise. For example, such a pill could benefit the elderly, hospital patients, and people with muscular conditions. On the other hand, a pill that simulates exercise could be abused in sports and might have adverse side effects. Also, remind students that, currently, the only guaranteed way for people to increase their fitness is to exercise.

  • Examine and discuss the differences between light and dark meat in chicken. Dark meat contains more fat, has a richer blood supply, and is composed primarily of slow-twitch muscle fibers, which are full of cells rich in mitochondria. This muscle is well adapted to low-intensity, long- duration activities. Ask students to consider why dark meat is concentrated in specific locations in the chicken. (Dark meat is full of mitochondria-rich cells and is found in muscles used in activities requiring endurance, such as flying and walking.) Ask students to hypothesize where dark meat might be found in other animals, such as those that travel long distances (e.g., ducks, caribou, and certain turtles and fish).

  • For advanced students, show the animation PPAR-delta Activation in the Muscle Cell at: (scroll down to find it). It illustrates how the PPAR-delta receptor activates certain genes in a muscle cell, resulting in the burning of fat. Show the animation several times and have students summarize what is happening: PPAR-delta is a type of receptor related to the metabolism and storing of fat. It activates certain genes that signal the muscle cell to burn fat for energy. Also, relate this animation to the original NOVA scienceNOW segment: the Marathon Mouse was genetically engineered to have PPAR-delta in overdrive, allowing it to run significantly longer distances than regular mice could.


Student Handout

  1. Summarize the function of mitochondria. (Mitochondria convert food energy, such as glucose, into usable energy known as ATP. Because of this, mitochondria are often referred to as the "powerhouses" of the cell.)

  2. Predict how the number of mitochondria-rich muscle cells would change in each of the situations below. (Answers will vary.)

  3. In the table below, enter the number of mitochondria-rich muscle cells shown in each of your images. (Answers will vary slightly, but should reflect the approximate numbers below:)

    Mouse Number Number of mitochondria-rich cells at beginning of experiment Number of mitochondria-rich cells at end of experiment
    1 4 4
    2 4 10
    3 4 8
    4 5 5

Student Handout Questions

  1. Explain your conclusions about how and why each treatment affected the number of mitochondria-rich cells for each of the four mice. (The number of mitochondria-rich cells in Mouse 1 and 4 did not change, whereas the number of mitochondria-rich cells in Mouse 2 and 3 increased. No treatment resulted in no change in the number of mitochondria-rich cells. Exercise caused the number of mitochondria-rich cells to increase because mitochondria supply energy required for activity. Drug X appears to mimic exercise, since Drug X caused an increase in mitochondria-rich cells also. Drug Y, however, appears to have no effect on the number of mitochondria-rich cells.)

  2. In both the Marathon Mouse experiment and the hypothetical experiment above, one mouse received no special treatment. What was the purpose of having this mouse in the experiment? (The mouse that was left in its cage served as the experimental control. The mouse was included in the experiment as a standard for comparison, to provide a baseline against which to compare the other mice.)

  3. List some pros and cons of a drug that can produce the same effects as exercise. (Pros: maintain muscle mass of chronically ill or hospitalized patients and people unable to get sufficient exercise. Cons: potential for abuse by athletes; side effects; potential for addiction; potential for entering the human food chain through animals treated with the drug.)

  4. Imagine that you could design an animal with great endurance, either flying, running, or swimming. Draw the animal and label its endurance characteristics. (If possible, watch the video How the Body Responds to Exercise at: (Answers will vary. Endurance characteristics could include: more slow-twitch muscle, increased number of mitochondria in each muscle cell, increased blood flow to the muscles, and increased ability to use oxygen efficiently.)

Use the following rubric to assess each team's work.

Excellent Satisfactory Needs improvement
Complete the Marathon Muscles student handout and graph the hypothetical data.
  • Students use Web resources effectively to answer questions.
  • Students show ability to obtain, graph, and interpret data.
  • Students use information effectively to justify predictions.
  • Students need assistance while using resources.
  • Students have difficulty obtaining, graphing, and interpreting data.
  • Students make predictions but cannot justify their reasoning.
  • Students have difficulty using Web resources to answer questions.
  • Students cannot obtain, graph, or interpret the experimental data.
  • Students cannot make predictions based on information.


The Marathon Mouse activity aligns with the following National Science Education Standards (see

Grades 9-12
Life Science Standard C

  • The Cell
  • Matter, energy, and organization in living systems

Life Science Standard F

  • Science in Personal and Social Perspectives

Life Science Standard G

  • Science as a human endeavor
  • Nature of scientific knowledge

Classroom Activity Author

Alison Fromme and WGBH Educational Outreach staff

Teacher's Guide
NOVA scienceNOW: Marathon Mouse

WebsiteMarathon Mouse QuickTime or Windows Media Video
WebsiteThe Fate of Fat QuickTime or Windows Media Video
WebsiteWhere Do You Get Your Energy? Flash Interactive
WebsiteMarathon Mouse QuickTime or Windows Media Video
WebsiteHow the Body Response to Exercise QuickTime Video