Life from Non-Life? A Quantitative Determination
Students consider the origin of life. Students carry out Pasteur's
nutrient broth flash experiments, looking at data both quantitatively and
qualitatively and use their data to attempt to define and recognize "life".
Form of Lesson:
Type of Activity:
- Life Science
- Advanced Biology
Target Questions this activity helps students answer:
- What is the origin of life?
- Is spontaneous generation possible in early geologic time? in today's time?
- Can you quantify those features that characterize "life"?
- What is the basic unit of structure and function of all life?
Teacher Introduction: Early in my Senior Biology course, I ask my students to seriously consider the origin of life and, in particular, if they were transported back around 3.6 billion years ago on Earth, how they could qualitatively and quantitatively identify living material and distinguish it from non-living material. I lead them to some great material written by Louis Pasteur during the mid 1800's about this subject. Pasteur writes, "There is a question...about which perhaps I can dare speak to you, because it is accessible to experiment and because I have made it the object of serious and conscientious studies. It is the question of generation, so-called spontaneous. Can matter organize itself? In other words, can beings come into the world without parents, without ancestors? Here is a question to resolve."(1)
Wow, pretty weighty stuff to consider! And so, without discussions or readings that involve, or even mention, the word "cell," I ask students in groups of three (they name and become a biotechnology company) to perform experiments (Pasteur's now-classical experiments with open and closed flasks of nutrient broth, only they don't know it) exploring this important question! Specifically, how does one recognize life from non-life... how does one go about determining if changes in a non-living chemical system are the result of "life" being present? During the class discussions, students reveal that it's easy to say a horse or a person or a plant is alive, but what can one know about forms and substances that they've never seen before?
Structure: The structure of this two or three week investigation is mainly determined by each group. I am simply the facilitator and "gopher" for equipment. Students must do the initial set up, including a control, bend the glass tubing for their Erlenmeyer flasks, learn how to autoclave the nutrient broth within the flasks, etc. Students keep a detailed logbook and begin to design further experiments that will quantify any changes observed (they must get numbered information!)(2). Obviously, changes do occur; however, they again must collect qualitative and quantitative data about whether these changes are related to the possibility of living material being present.
I will have given them a series of possible protocols that may or may not help them in this endeavor; these procedures come without titles. Some examples are as follows: "Make a microburet by pulling on the middle of a plastic pipet and cutting to desired length. Using water, determine the number of drops that will form the "lens" on the photocell. You must use this number of drops each time you use the photocell. Using water, practice making uniform drops before you begin with the liquid from your flasks. Label and prepare nine microplate wells of liquid by using a ratio of 9 drops of flask liquid: 1 drop of ethanol; make a control well. Place a "lens" of liquid on the photo cell and read the resistance value on the ohm meter provided. Repeat in a like manner for all wells." Besides this test to determine turbidity (and, therefore, population growth curves), others involve pH determination, Hanging-drop slides, agar plate transfers, vital staining, and others that they develop.(3)
Again, students research what these tests and others will tell them; students must also dig out information on how to use microscopes and other equipment if these pieces of equipment will be utilized. At the conclusion of this project, students must submit a written document that supports their position, create a "science-fair" poster board that clearly illustrates their lab work and research, and present orally (lab coats and all) their Biotechnology Company's position to the class.
- (1) from Great Experiments in Biology, Gabriel and Fogel Editors. Prentice-Hall. Eighth
- (2) At the outset, students are given a grading rubric that includes an evaluation of their
daily work (this could apply to individuals or to the group effort), i.e. quality of in
class work, quality of researched information, laboratory design, qualitative and
quantitative data collection, statistical analysis, error analyses, written presentation, oral
- (3) Turbidity determination: Inexpensive photocells ($2.99/5 cadium sulfide photocells) and ohm meters that replace a spectrophotometer can be obtained from Radio Shack (Photocells: Cat.#276-1657). For a good "work stand" place a paper or wax cup upside down on the lab table and push the photocell leads through the bottom so that the photocell is flush with the bottom. Many different protocols (hanging drop, vital stains, bacterial counts, agar transfers, chemical analyses for organic materials, microscope use including oil immersion, etc. can be found in a great resource book, A Source book for the Biological Sciences by Evelyn Morholt, Paul Brandwein. Harcort, Brace Jovanovich, Pub., 1986, ISBN: 0-15-582852-5.
Students have been very successful here for many reasons:
- They explore an open-ended and important question
- They apply their individual and collective talents and backgrounds They plan and design their experiments
- They organize and decide who will accomplish what
- They solve problems (is this biological movement or simply Brownian movement, is this purification, or chemical oxidation, or Bill's not working hard enough, or...)
- They find, develop, and learn the vocabulary necessary to communicate their ideas ("Wow, I found this idea called anaerobic respiration! I wonder if it applies here?")
- They learn to support their position with reproducible evidence, and they become confident learners!
- Moreover, this project encourages diverse approaches and calls for a variety of skills and talents, so all students have opportunities to contribute meaningfully.
Extensions/Reinforcement: Extensions and reinforcements can come with further readings and discussions from selected works of A.I. Oparin, J.B.S. Haldane, H.C. Urey, S.W. Fox, S.L. Miller, C. Sagan, T.R. Cech, and others. A great resource here is: Origins of Life: The Central Concepts by David Deamer and Gail Fleischaker, Jones and Bartlett Pub., 1994, ISBN: 0-86720-184-9.