Case Teaching Notes for
“Life—The Final Frontier: A Case Study on the Characteristics of Life”

Introduction / Background

This case study is a “clicker case.” It combines the use of student personal response systems (clickers) with case teaching methods and formats. The case is presented in class using a series of PowerPoint slides punctuated by questions (called “clicker questions”) that students respond to before moving on to the next slide. In this way, students work through the material to understand (and solve) the problem presented in the case. Specifically designed for use in large introductory science classes, the method integrates lecture material, case storylines, student discussion, (clicker) questions, the clarification of answers to those questions, more lecture, and data.

The case involves a fictionalized story of the President of the United States having to deal with a sample on its way back from Mars. The President would like to make an announcement regarding the discovery of extra-terrestrial life. However, NASA has difficulties in determining exactly what is to be considered life. Students take part in the decision process and help determine if the sample contains enough evidence of life from Mars. The inspiration and many of the ideas for the case were drawn from the NASA Astrobiology Institute—see their website at http://nai.arc.nasa.gov/.

This case is designed for introductory biology courses and can be used with either majors and non-majors. It can be used in a class of any size.

Objectives

• List and define with examples the necessary characteristics of life.
• Given an unfamiliar structure, be able to determine which life properties are observable.
• Categorize objects as alive or not alive.
• Generate an example of why it is important to be able to determine if something is alive or represents life.
• Understand that many nonliving things can show several characteristics of living things, but cannot show them all.

In addition, by participating in this case study, students will:

• Become more familiar with the use of case studies in biology.
• Be introduced to various areas of study within biology.
• Begin to realize that biological “facts” can change as scientists learn more.
• Engage in a critical thinking exercise.

Misconceptions

Students may think “life” means intelligent, human-like life and/or familiar forms of life such as plants and animals. According to Benchmarks for Science Literacy (page 341):

Elementary and middle school students typically use criteria such as “movement,” “breathing,” “reproduction,” and “death” to decide whether things are alive. Thus, some believe fire, clouds, and the Sun are alive, but others think plants and certain animals are nonliving … High school and college students also mainly use obvious criteria (e.g., “movement,” “growth”) to distinguish between “living” and “nonliving” and rarely mention structural criteria (“cells”) or biochemical characteristics (“DNA”).

Classroom Management

Because of the topic, this case will likely be used very early in a course. The case is set up by having the students read a few introductory PowerPoint slides. The instructor uses clicker questions to engage students in decisions about the data being presented in the case. Students are asked to generate and evaluate characteristics of life and apply their criteria to new (potential) life forms. The story is brought to a close by a final decision about whether life has indeed been discovered.

In-Class Case Presentation

Introduction and Characteristics of Life (Slides 1–10)

The case begins with Slide 2, which is intended to engage students by asking them whether they think life exists elsewhere than on Planet Earth. Students have likely thought about and discussed this informally at some point in their lives. The question is designed to lead students toward understanding the importance of being able to determine if something is alive. It also introduces the next set of slides.

Slide 3, Slide 4, Slide 5, and Slide 6 describe at least one reason NASA is interested in life elsewhere: the small, but real, possibility of introducing a pathogen or organism from outer space that disrupts our already fragile ecology. This slide also sets a realistic tone to the case. Adding to the realism, NASA has a Planetary Protection Policy that is designed to protect solar system bodies (i.e., planets, moons, comets, and asteroids) from contamination by Earth life, and to protect Earth from possible alien life forms that may be returned from other solar system bodies. The policy protects planets from living organisms, not fossil or chemical evidence of life.

The class discussion at this point should get into the basic characteristics of life—often in textbooks these include reproduction, growth, DNA, metabolism, homeostasis, adaptation, movement, order, and evolutionary history. We envision instructors allowing students to develop a list of characteristics students feel all life or living organisms share. This may require some blackboard (overhead, etc.) work. Use the class discussion to eliminate some of the criteria. For example, students may include “uses energy” and “needs oxygen to get energy out of its food” as characteristics. Another student may bring up the point that some living organisms are anaerobic. Or photosynthesis may be brought up in discussion. All of these might be placed under the broader characteristic of “metabolism.” Note: This discussion can be lengthy; instructors may wish to assign this part of the case as a preparatory assignment to be completed before class.

Special Use of Clickers—The class can be polled to see if they want a certain characteristic to be included on the list. For example, if the characteristic “eats other animals’ is on their initial list, students can be asked to press “A” if they want it included, “B” if they do not. Because this will be based upon a list the students develop, specific clicker questions should not be developed in advance. Rather, using the “verbal” questioning mode is appropriate.

Keep the list displayed or remind students to record the characteristics in their notes because it will be useful in Part II. Instructors should emphasize that life is a combination of many characteristics. Many objects that are not considered life do exhibit some of the characteristics, which is the point of Slide 8 and Slide 9. For example:

• Flame—grow, reproduce
• Furnace—homeostasis
• Lawnmower—metabolism, moves

Other things that are not alive, do indicate that life exists:

• Fossils—some have DNA, but not all
• Oil—no characteristics, but we know how it was formed
• A fallen leaf—DNA, but no homeostasis or reproduction

Clicker Question 2 (Slide 10) is designed to get them thinking about the case again by asking about one specific component of life: the use of water.

Determining Life (Slides 11–21)

The scenario presented here is fictional. Although NASA does have a Planetary Protection program, and there is strong evidence of ice on Mars, as of the writing of this case no evidence of Martian life forms has been found.

At this point, the instructor can set up role playing scenarios. As outlined in Slide 11, the students are part of the NASA Planetary Protection Team, and the Team has been given a chunk of ice collected on Mars by one of the rovers. Their task is to consider the data that follows to determine if the particles associated with the ice chunk constitute evidence of life.

Slide 12 provides some information about the Mars samples.

Clicker Question 3 (Slide 13) asks students what they would do to determine if the particles are alive. This slide allows the instructor to initiate a discussion about what are common characteristics of living organisms on Earth. Follow-up discussion can focus on why choices were ignored and why choices were made in relation to determining what is alive. It should relate back to the list the class developed in Part I of the case.

Students are next given results from a series of tests (Slide 14). Electron microscope images of the nanoparticles revealed a cellular membrane surrounding them, but no membrane-bound structures on the inside. The membrane appears to be composed of calcium phosphate rather than of phospholipids, as in other known cells (some discussion on membrane chemistry is possible here). Discuss the results, and then use Clicker Question 4 (Slide 15) to involve the students and allow them to apply their criteria for determining life.

The next slide (Slide 16) presents evidence that the particles are viral. The basic biology of viruses should be introduced here so that students have the background information they need to complete the reminader of the case. Instructors are encouraged to find their own suitable image(s) for Slide 17 for this purpose. A sub-sample of the objects was extracted and preserved. Electron microscope images of the objects revealed a phospholipid cellular membrane, but also virus-like particles attached to some of the objects (some viruses, called phages, infect bacteria cells). Upon closer analyses, these virus-like particles also contained DNA. Again, discuss the results (Slide 18), and then use Clicker Question 5 (Slide 19) to involve the students and allow them to apply their criteria for determining life. This question introduces the conflict in biology of how to define viruses.

Finally, students vote (Clicker Question 6 on Slide 20) as to whether the President should announce to the world that life outside the Planet Earth has been discovered.

Wrap-up

We expect that some variety will be seen in student responses for Clicker Questions 4 and 5. This should allow the instructor to point out that “biological” entities important on Earth are difficult to define as life, and not just for students. Examples:

1. Instructors may want to take time to introduce the idea that some biologists consider viruses to be living entities, while other biologists, and likely the class textbook, consider viruses to be non-living particles.
2. Instructors may take time to introduce the controversy in biology as to whether Earth nanobacteria are considered life—see the case study entitled “Nanobacteria: Are They or Aren’t They Alive?” on the National Center for Case Study Teaching in Science website at http://www.sciencecases.org/nanobacteria/nanobacteria.asp.
3. Prions are also difficult to define. They reproduce and infect, and possibly evolve, but don’t contain DNA or RNA.

Assessment

This case was developed as part of an NSF-sponsored grant (# DUE 0618570) to determine whether clicker cases such as this one produced greater learning than the traditional lecture approach. As part of that project, the clicker cases had questions that were asked of students both before and after the class in which the material was presented. The questions were also used again during the final exam. These additional pre- and post-case questions are presented in the Answer Key.

Answer Key

Answers to the questions posed in the case study are provided in a separate answer key to the case. Those answers are password-protected. To access the answers for this case, go to the key. You will be prompted for a username and password. If you have not yet registered with us, you can see whether you are eligible for an account by reviewing our password policy and then apply online or write to answerkey@sciencecases.org.

References

Websites

The NASA Planetary Protection Policy website helped generate the idea for the case and offers realism to the case. This is an actual national policy.

http://planetaryprotection.nasa.gov/pp/about/index.htm Last accessed: October 21, 2008

There seems to be a great deal of interest in our society about the existence of life forms other than on Earth. The following are links may be of interest for further discussion:

NASA’s Mars Exploration Program

http://marsprogram.jpl.nasa.gov/science/life/index.html

Did NASA accidentally kill life on Mars?

http://www.cnn.com/2007/TECH/01/07/mars.life.ap/index.html

Life on Mars?

http://www.cnn.com/2006/TECH/space/12/07/tbr.martianlife/index.html

Mysterious red cells might be aliens

http://www.cnn.com/2006/TECH/science/06/02/red.rain/index.html

National Geographic Channel: Extraterrestrial

http://channel.nationalgeographic.com/channel/extraterrestrial/

Alien Infection

http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=570

Slide Credits

Acknowledgements: This material is based upon work supported by the NSF Grant No. DUE-0618570. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of NSF. The authors are thankful for the copious amounts of input from the “clickers in case studies group,” especially Terry Platt, Eric Ribbens, Jon Shaver, and BJ Wolters.

Date Posted: November 26, 2008.

Copyright © 1999–2010 by the National Center for Case Study Teaching in Science.  Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.