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5. Slimy cells - Background
The PVA slime recipe used in this activity is:
This makes a wonderful, viscous, oozing slime that is wet to the touch but holds together well even if removed from the ziplock bag. Polyvinyl alcohol exists in water as a long polymer of (C2H4O)n units. Each chain is up to 2,000 units long. When Borax is combined with the PVA solution, the PVA chains crosslink, forming a highly viscous gel. Since the crosslinks are weak, they continually break and reform as the slime is handled.
PVA slime is quite safe to touch and handle, although you don't want to eat any since the Borax is toxic in large doses. It is easy to clean up with soap and water. Unadulterated slime can be stored for several weeks in a ziplock bag.
I also use this activity to introduce students to the metric system of measurement and the use of ratios to see the relative size of things. Although students realize that cells are tiny, especially after looking through the microscope at them, it is often hard for them to imagine just how tiny cells really are. By going through all the steps of calculating how big a human would be if one of their ziplock bag cell models was really a cell, they are better able to recognize just how tiny a cell is.
For your reference, below is a table showing standard versus scientific notation as well as the common metric prefixes for each.
A human cheek cell is approximately 58 micrometers (um) or 0.000058 meters (m) wide. A typical seventh grader is approximately 1.6 meters (m) tall. A standard ziplock sandwich bag is approximately 16 centimeters (cm) or 0.16 meters (m) wide. Thus you can set up a proportion to figure out how big a human being would be (x) if the ziplock bag represented a cheek cell: