Physiology Box

3. Testing for Life - Logistics

10-20 min introduction (depending on how deeply you want to talk about the biochemistry)
35-50 min to conduct tests (10-15 min per station)
20-30 min to discuss results

Teams of 3 students

General materials for students and test stations:

  • Copy of the lab handout for each student
  • Copy of test station directions at each test station
  • Biuret solution (see Sources section for ordering information)
  • Benedict’s solution (see Sources)
  • Iodine tincture
  • 3 beakers or cups for every group of students
  • 4 test tubes for every group of students
  • 1 test tube rack for every group of students
  • Permanent markers and labeling tape for the beakers and test tubes
  • 30-45 eye droppers
  • 6-9 trays or bins to keep the materials for each test station
  • 6-9 small 100 ml beakers or squeeze bottles to contain test reagents
  • Optional: 6-9 large squeeze bottles of water (500 ml disposable plastic water bottles are fine) for rinsing test tubes at test stations rather than going to a sink
  • 6-9 large beakers or cups to dump waste materials
  • hot plate or source of boiling water
  • 2-3 insulated containers such as a thermos or Styrofoam cup for creating a hot water bath
  • Optional: 2-3 thermometers to monitor the temperature in the hot water bath
  • disposable latex gloves

3. Testing for Life's Molecules

All known life is made out of a small group of chemical compounds called organic molecules. Common organic molecules include proteins, glucose, starch, lipids, and nucleic acids. This lesson plan asks students to conduct tests for proteins, glucose, and starch. At the beginning of the activity, they choose 3 items to test: one known to be “never alive”, one known to be “once was alive”, and one mystery item. In addition, each station includes a positive control. By the end of the experiment, students should be familiar with some of the major organic molecules and should recognize that living things, and substances derived from them, are made of organic molecules. In addition, this is a chance to bring in topic surrounding nutrition, health, and digestion. Since our bodies are made up of organic molecules, we need each of these molecules as nutrients in our food.

2. Life Traps - Sources

The inspiration for this lesson was Mission 11 from the Life in the Universe curriculum, published by the SETI Institute. The recipe for the nutrient agar came from Biology F.A.Q. at Flinn Scientific. Sterilization tips were found at and Wikipedia. Finally, many teaching tips were discovered from Leslie Hathaway’s Bacteria Gathering lesson plan.

2. Life Traps - Assessments


  1. Collect students’ lab notebooks with their observations and conclusions.
  2. Revisit the characteristics of life list from the Is It Alive? activity. Revise the criteria as necessary.

Going Further

  1. Test the effects of various antiseptics. After growth has taken place, add a piece of filter paper soaked in an antiseptic cleaning agent (Lysol®, bleach, 409, rubbing alcohol, Neosporin®) to the plate. Be very careful when opening the agar plates. Wear a dust mask and stay in a well ventilated area since the high concentrations of spores can cause lung distress.
  2. Make new life traps and test the effect of different environmental factors. For example, with similarly seeded plates, place one at room temperature, one in a warm place like near the water heater, and one in the refrigerator. Or investigate the effect of sunlight versus darkness.
  3. Make new life traps and compare the effectiveness of various manipulations thought to disinfect surfaces. Compare plates seeded with unwashed versus washed hands. Compare a table top before and after cleaning.
  4. Trap wild yeast from the air and use it to make sourdough bread! A mixture of water and flour provides the nutrient base for the yeast to establish itself. There are hundreds of recipes and different ways to create a sourdough culture. Before store bought yeast, sourdough cultures were the primary means of leavening bread. Since each culture has a slightly different population of yeast, every culture will produce its own distinctive flavor. Starting a sourdough culture is very simple to do and extremely fun. The Exploratorium website provides one way to start a culture using a lump of dough. Other sourdough starters use a more liquid culture with a consistency more similar to pancake batter than bread dough. For instance, the How Things Work website describes the procedure for creating a wild yeast starter with this more liquid consistency. I started my own sourdough starter using a procedure found in the cookbook, The Cheeseboard Collective Works, published by my all time favorite Berkeley bakery and cheese shop, The Cheeseboard.

2. Life Traps - Lesson Plan

Fish tank microbes: Fish tank microbes collected by Woody, 6th grader, February 2006.Fish tank microbes: Fish tank microbes collected by Woody, 6th grader, February 2006. Air-borne microbes: Air-borne microbes collected by Irene Salter from her classroom with a 30 minute exposure.Air-borne microbes: Air-borne microbes collected by Irene Salter from her classroom with a 30 minute exposure.

Lesson Plan

2. Life Traps - Getting Ready

Getting Ready
To prepare agar plates:

  1. Thoroughly wash your hands with soap and water.
  2. Set the pot (or pressure cooker) on the stove.
  3. In the pot, mix together 1 liter of distilled water, 15 g of agar agar, and 2 beef bouillon cubes.
  4. Heat to boiling, stirring occasionally to fully dissolve the agar agar and bouillon.
  5. If using a regular pot, simmer on medium-low for at least 30 minutes to get as sterilize as possible. (It is impossible to completely sterilize a solution simply by boiling but it’s good enough for a rough classroom experiment such as this one – 25% of the plates I made were contaminated, but that also means more microbes for the students to observe). To ensure a sterile starting solution, use a pressure cooker and cook for 15-20 minutes at 15 psi.
  6. While the nutrient agar is boiling, clean your work surface with warm water, soap, and perhaps even an antiseptic cleaning solution.
  7. Open the packages of Petri dishes and line them up on your work surface. Keep the lids on!
  8. When the nutrient agar has finished cooking, open the lid of each Petri dish as little as possible while carefully pouring 15-20 ml of the solution into the dish – each dish should end up between a third to a half full. If possible, hold the lid directly above the dish while pouring to prevent air-borne particles from settling onto the surface of the agar. Replace the lid immediately. This homemade nutrient agar will contain flecks of protein and fat from the beef bouillon, it will not disturb the experiment but students should carefully note the location of these flecks so that they are not mistaken with microbe colonies.
  9. Once the nutrient agar has solidified (10-15 minutes) it will turn cloudy and opaque as opposed to translucent. At that time, turn the plates upside-down to prevent condensation from pooling on the surface of the agar.
  10. Store them stacked upside-down in the refrigerator in their original plastic sleeves until ready to use.

2. Life Traps - Background

Teacher Background
When living things are provided with the proper nutrients, water, and environmental factors (temperature, humidity, etc.) they will grow and reproduce, often explosively and in surprising ways. To kids, microbes are abstract, invisible germs that mysteriously spread disease, but otherwise have little relevance to their daily lives. However, microbes in the form of bacteria, fungi and viruses are prolific and exist all around us and even inside us. Of the 100 trillion cells found within your skin, only 10 trillion of these (a measly 10%) are human cells! The rest are primarily bacteria living on the surface of your skin, around your eyes, mouth, reproductive organs, and digestive tract, comprising between 500-1,000 different species. Since bacterial cells are generally much smaller than human cells, a great many more bacteria fit into the borders of the human body than human cells.

2. Life Traps - Logistics

30 min to introduce the activity and seed the plates
5-10 min to make observations every other day over the next 2 weeks

Individual or in pairs.

For approximately 50 plates you need:

  • 50 sterile disposable plastic 15 mm x 100 mm Petri dishes (can be purchased from Ward’s Natural Science, item number 18 V 7101, approximately $4 for a package of 20 dishes)
  • 15 g agar agar powder (a gelatin substitute made from red seaweed, available at most Asian grocery stores and health food stores, ideally purchase the unsweetened variety but pre-sweetened is OK as long as you ensure that you subtract out the weight of the sugar when measuring out 15 g of agar agar)
  • 2 beef bouillon cubes
  • 1 liter distilled water
  • stove and large pot for preparing nutrient agar and steam sterilizing the Q tips (better yet, if you have a pressure cooker, you will be far more assured of initially sterile conditions in your agar plates)
  • 50 Q tips
  • paper towels
  • permanent markers for labeling plates
  • masking tape
  • bleach

2. Life Traps

Life trapsLife trapsAs part of recognizing the characteristics of life that all organisms share, students grow microbes on nutrient agar plates. Students swipe surfaces with a sterile Q tip swab and seed plates resulting in a wide range of colorful and prolific bacteria and fungi colonies. Other plates may be simply opened to the air to catch life floating in the air. Through these experiences, students learn that all living things, even those so small and invisible as to be floating in the air, grow and reproduce when provided with the proper nutrients and water. Teachable moments abound since the “dirtiest places”, like the toilet rim, often result in the least bacterial growth while presumably “clean” places, like the surface of your skin, have the most. A fun extension of this activity (see the Going Further section) is to start a sourdough culture from wild yeast in the air and make sourdough bread.

1. Is it alive? - Sources and Standards

I discovered the “glue monster” or “scooting glue” demo from Flinn Scientific (click on “Glue Monsters” to download the pdf file). Kitchen chemistry also provides a write up for the same activity with a better description of the chemistry behind the demo. For a quicktime movie of a paper boat “fleeing” from a dish soap coated toothpick, see the University of Iowa Physics and Astronomy Lecture Demonstrations.