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Membrane Permeability with Beets

Phil Vavala



ABSTRACT

I use this exercise at the beginning of the school year because it is so simple and open-ended. I think it gets the students into the frame of mind that they won't be given an elaborate procedure every time they do a lab, and that they will have to try alternate approaches to any problem. By the time they do this lab, students know about diffusion and cell membranes. Before performing the lab, I sometimes purposefully discuss diffusion and particle size. Invariably they predict that the smaller alcohols will permeate more quickly, and that higher concentrations will permeate more quickly than lower. As the data is recorded, seemingly contradictory data lead students to begin to critically think about the nature of the composition of the membrane.

PROCEDURE:

  1. Obtain water-washed beet cubes and three alcohols. Set up nine test tubes so as to obtain data for the chart below. (Serially dilute your alcohols: 8M, 4M, 2M).

  2. Let stand for 20 minutes, then shake the tubes.

  3. Record qualitative observations as to the color (dark red to colorless) of the liquid in the tubes.
   Alcohol         2M       4M       8M        Remarks    
   Methyl    
   Ethyl 
   n-propyl 
   n-butyl

DATA ANALYSIS

  1. What trend do you see? What prediction would you make for n-butyl alcohol? Its structure can be found below with the structural formulas of the three alcohols you used.

  2. Devise and conduct quantitative tests using the same materials and any equipment available to you. Some ideas:
  3. a. Rate of diffusion and alcohol size.
  4. b. Rate of diffusion and alcohol molarity.
  5. c. Rate of diffusion and beet volume and/or surface areas.
  6. d. Rate of diffusion and temperature.
  7. How will you measure your data? By what standards that another student could use?


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