My Science Box

Teacher Background
Making models of cells is a fun, meaningful activity for students to help them visualize the 3 dimensional nature of cells. See the background section of the Seeing Cells activity for a description of cell parts and their functions. This puts a slightly different twist on the standard shoebox cell model by using PVA slime to suspend the various organelles much like a real cell's cytoplasm does. It is also much less messy than the often-used jello cell models since everything stays contained within a ziplock bag (and is not sticky if spilled on the floor - though avoid getting slime on carpet).

The PVA slime recipe used in this activity is:

• 180 ml 4% PVA solution
• 30-35 ml 5% Borax solution

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.

Standard notation	Scientific notation	Common prefix	Common symbol	Example
1000	1 x 103	kilo-	k	kilometer (km)
100	1 x 102
10	1 x 101
1	1 x 100	none	none	meter (m)
0.1	1 x 10-1	deci-	d	decimeter (dm)
0.01	1 x 10-2	centi-	c	centimeter (cm)
0.001	1 x 10-3	milli-	m	millimeter (mm)
0.0001	1 x 10-4
0.00001	1 x 10-5
0.000001	1 x 10-6	micro-	u	micrometer (um)
0.0000001	1 x 10-7
0.00000001	1 x 10-8
0.000000001	1 x 10-9	nano-	n	nanometer (nm)
0.0000000001	1 x 10-10

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:
__x__ = __0.16 m__
1.6 m     0.000058 m
Solving for x you get 4414 meters or 4.4 kilometers. Thus, a human made of cells as big as a ziplock bag would be 4.4 kilometers tall (over 2.7 miles)! Just imagine how many slimy cell models it would take to fill a statue over 4 kilometers tall (around 10 trillion, that's 1 x 1013). A blood cell takes around 30 seconds to circulate around the human body - in our enlarged model, that's comparable to a ziplock bag blood cell completing a 3 mile round-trip journey in 30 seconds, at 360 miles an hour!

Student Prerequisites
Students need a good background in cell structure, parts of a cell, and their functions before undertaking this activity. It is helpful if students have experience with ratios and proportions in math class and if they have had some exposure to the metric system of measurement though not required.