High School Biology- Chemistry SMILE Meeting
04 December 2001
Notes Prepared by Porter Johnson

Tyrethis Penrice (Oak Park Elementary) Behavior of Matter: Handouts on Adhesion and Cohesion
Tyrethis led our discussion of these questions:

  1. Q: What happens when drops of water are placed on talcum powder.
    We suggested "beading up", absorption, and "clumping" of talcum powder
  2. Q: What happens to the surface of water sprinkled with pepper when detergent is dropped in?
    We thought the pepper grains would move away from the detergent layer.
Tyrethis then handed out two worksheets, corresponding to each of these questions, and we proceeded to investigate in groups.  We noticed these things:
  1. Drops did form beads which, after a few minutes, had a skin of talcum powder on them. The beads formed only where talc was fairly thickly spread on the paper. We may have been seeing cohesion of H2O to H2O, talc to talc, or adhesion of talc to H2O---or perhaps all of them. We would not expect the talc to H2O interaction to be present, since beading occurs.
  2. Initially the pepper floats uniformly spread on the surface of the H2O .  The addition of detergent initially causes the pepper grains to move away from one another suddenly (similar to "like charge" repulsion?) and then to sink below the surface (loss of surface tension).
  3. In a third activity a glass was filled to the top with H2O so that there was a "surface bulge" over the edge of the glass.  A cork was then floated on the surface.  The surface tension kept the cork near the center of the glass, far away from the edges.
Nice job, Tyrethis!

Karlene Joseph (Lane Tech HS) Handout: Cell Diversity
Karlene started by asking "What types of cells are there?"
  mentioned animal and plant cells.  Inside the human body we have these types of cells:

brain, blood, muscle, skin, nerve, follicle, ...

Activity: Karlene distributed clumps of clay of the same volume.  Each person took a clump and molded it into shapes corresponding to the shape of a given type of cell.  Here are some examples:

This led to a discussion of sickle cell anemia, thallassemias, etc, including "balanced polymorphism" of the sickle cell allele.  Even though the rr individuals have sickle cell disease, rR individuals are more resistant to Malaria than RR.   In regions where Malaria is prevalent, the rR individuals are the most fit of the three possibilities [rr, rR, and RR], and this "balances" the presence of both r and R alleles in the population.

This also led to a discussion of the sizes of various types of cells, which vary (generally) from  a few to a few dozen microns in diameter (1 micron = 1 mm = 10-6 meters).  We considered intrinsic limitations in sizes of cells in terms of the surface area/volume ratio.  We made "clay cells" of various shapes using our clumps of clay.  Then we measured the dimensions and calculated the surface/volume ratios.  [For example, a cube of side 2.5 cm has a surface area of 37.5 cm2 and volume of 15.625 cm3, so that the surface to volume ratio is 2.40 cm-1.]  Cells need to be small so that the surface/volume ratio is large,  so that cells are able to absorb O2, food, and other nutrients at rates required to support life.  By contrast, large egg cells already have the nutrients inside them, so that they are not limited so much in size.  (For example, consider ostrich eggs.)

Very interesting, Karlene!.  

Mary Scott (Williams School) Handout:  Air Power
Mary used a small amount of air to lift a heavy load, to demonstrate the power of air.  She inserted a drinking straw into a 4-liter (gallon) size "zip-lock" bag, and carefully sealed the opening with heavy-duty clear tape.  She put the deflated bag on the table, and put a rather heavy book on top of it.  When she inflated the bag through the straw, the book was lifted.  She repeated the experiments with several books on the bag, demonstrating the effect of pressure of compressed air. The air blown into the bag becomes compressed, and exerts enough (additional) pressure to lift and then support the books.

Good work, Mary!

Notes taken by Ben Stark.