Diffusion Inquiry Materials
SANDWICH BAGS
Is a sandwich bag like a cell? Does it have a selectively permeable membrane – one that will let
certain molecules pass through? Hmm… Let’s ask a few questions to get us thinking. If you put
chips or cookies in a sandwich bag, will they pass though? The answer to that question is an
obvious NO! What about water? If you fill a sandwich bag with water and seal it shut, will it
pass through? The answer is still no. But, what about molecules smaller than water? The
answer to that questions requires us to think about the properties of sandwich bags.
Many sandwich bags have a 0.4 µm pore size membrane. What in the world is a µm? The
symbol µm represents a micrometer. A micrometer is one millionth of a meter, or one
thousandth of a millimeter (0.001 mm), or about 0.000039 inch. That’s extremely tiny! This
means that even though the bag looks and feels solid and holds in liquids like water, any
molecules smaller than 0.4 µm can fit through the pores – or tiny microscopic holes – in the
plastic membrane.
INDICATORS AND IODINE-CORNSTARCH INTERACTIONS
Color sometimes acts as an indicator. What is an indicator? Let’s start with a very simple and
obvious example. Water is typically clear. If we take a bucket of clear water and throw a
handful of dirt into it, brown is an indicator that the water is no longer clean because it has mud
in it. Green could be an indicator that yellow and blue were combined. But, what else can color
show us?
Starch and iodine provide a good example. Starch is a common molecule found in a lot of foods.
Corn, potatoes, and pasta are a few examples of starchy foods. Cornstarch is a common
ingredient found in grocery stores. It comes in a box and is a white and powdery. Iodine is a
brown liquid solution. Iodine is commonly used to clean the surface of skin before surgery or
stitches. When brown iodine comes in contact with white starch, the molecules bond together
and we see the color purple. If you mix cornstarch in water, it will be a pale white liquid, kind of
like thin milk. If you add a few drops of iodine, the solution turns purple. Pretty awesome, huh?
WATER AND THE CELL ENVIRONMENT
Another concept that is helpful to think about before you design your own diffusion lab is the
important role of water in the cell’s environment. The human body is approximately 60% water.
Consider the following:
According to H.H. Mitchell, Journal of Biological Chemistry 158, the brain and heart are composed of 73%
water, and the lungs are about 83% water. The skin contains 64% water, muscles and kidneys are 79%,
and even the bones are watery: 31%. From: http://water.usgs.gov/edu/propertyyou.html
So, cells live in a very fluid environment. Here’s another way to think about this. Even if you eat
something like a dry cracker, your saliva (spit) moistens it and then your stomach acids and
digestive juices break it down into a mixture of fluids and tiny molecules. So, all molecules that
enter and exit the cell – even those from a dry cracker – do so in a wet environment.
Size of Three Different Atoms/Molecules
Cornstarch =
C27H48O20 (appx.)
Iodine = I
Water = H2O