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THE WILD WEST: Model Planet

A critical problem inside Biosphere 2 was the unexpected and dangerous buildup of CO2; trees and other vegetation should have absorbed the gas, as they do on Earth. Some theories to explain the rise of carbon dioxide include: a smaller ratio of trees and vegetation grew in the Biosphere than on Earth; microorganisms in the very rich soil contributed to excessive levels; the concrete walls of the Biosphere itself may have produced outgassing; or, decaying materials broke down faster than expected, thus releasing more CO2 into the atmosphere of Biosphere 2.

On Earth, excess CO2 is taken up by green plants during photosynthesis. This activity on carbon uptake lets you observe the connection between CO2 depletion and photosynthetic activity.

Materials: test tubes
  • 3 test tubes
  • test tube rack
  • marker pen
  • phenol red (CO2 indicator)
  • sprig of Elodea
  • drinking straw
  • water (enough for test tubes)
  • boiling apparatus
  • notebook for recording observations
  1. Before starting this lab, boil the water, then cool to room temperature.

  2. Fill three test tubes with the room-temperature water.

  3. Number the test tubes 1, 2 and 3.

  4. Add a very small amount of phenol red to each tube (powder or liquid). Note the color of each solution in your observation log.

  5. Using a clean straw, exhale briefly into tubes 1 and 2 until the color just changes to yellow.

  6. Place a generous sprig of Elodea in tube 1.

  7. Cover all three tubes and set them aside in bright light.

  8. At the end of your class period (30 minutes in the tubes is ideal, but color change will occur in less time), examine the color of each tube.
  1. Why did you boil the water, then cool it?
    (to remove dissolved air)

  2. What color does phenol red appear when in the presence of CO2?

  3. What color changes occurred at the end of the time period?
    (tube 1 turned back to red)

  4. What do your observations suggest?
    (the sprig of Elodea in tube 1 used up the CO2)

  5. Do you think the results would be different if the tubes were placed in the dark? Explain.
    (Yes. Photosynthesis would not have occurred; therefore the CO2 level in tube 1 would not have dropped.)

  6. Is tube 3 an experimental control? Explain.
    (Tube 3 is not a control; tube 2 is the control setting. Tube 3 is kept as a color standard from which a color change back to the original red can be measured.)

  7. What might this experiment tell us about the role of plants and/or vegetation in the environment? What about in Biosphere 2? If plants and vegetation absorb CO2 in the atmosphere, why might the levels of CO2 continue to rise inside Biosphere 2? What are some of the factors causing carbon dioxide levels to increase on Earth?


Scientific American Frontiers
Fall 1990 to Spring 2000
Sponsored by GTE Corporation,
now a part of Verizon Communications Inc.