STEM ED/CHM Nanotechnology 2012
Teacher Resource Pages
Diffusion of Food Dye Through Gelatin Lab: A Model
for Diffusion of Nanoscale Particles Through Cells
Part A: Diffusion and Teaching Standards
This lab has content which is applicable to various disciplines/standards
 Physical Science/Chemistry: particle motion theory; temperature;
mixtures and solutions
 Biology: passive transport; cellular structure, etc.
 Ecology/Environmental Science: environmental effects on living
systems
 Math: rates; relationships; data collection, proportions, ratios
Part B: Background Readings About Diffusion With
Applications in Biology, Physics, Chemistry and Math
The physical processes involving movement of materials in and out of a cell
are diffusion and osmosis. Both these movements involve movement along the
concentration gradient. Hence, there is no expenditure of energy.
Diffusion
It is a process, which involves movement of a substance from a region of its
higher concentration to a region of its lower concentration.
fig. 15.3 - Diffusion
Molecules of any substance are in constant random movement in all fluids.
This movement is called Brownian movement. Apart from the three states of
matter, diffusion can also occur through semipermeable membranes. The
molecules of the substance undergoing diffusion exert a pressure in the
available space. This pressure is known as diffusion pressure. Higher the
diffusion pressure, higher is the rate of diffusion. The rate of diffusion is
decided by factors like concentration of the molecules undergoing diffusion,
space available for diffusion and temperature of the medium.

Small molecules of non-polar substances (like uncharged polar carbon
dioxide) rapidly diffuse through the lipid bilayer.

Some essential charged particles may diffuse into the cell through
specific channels or process formed temporarily by tunnel proteins.

Some times a carrier molecule, almost always a protein, facilitates the
movement of certain substances like amino acids or nucleotides, across
the membrane. This process is called facilitated diffusion
The Rate of Diffusion
The rate of diffusion signifies how fast a molecule or ion diffuses in a given
time. The rate of diffusion of gases is generally fast while liquid and solids
have slow rate of diffusion. As diffusion of molecules occurs from a region
of their higher free energy or chemical potential to a region of their lower
free energy, the rate of diffusion will be influenced by all those factors
which bring about the changes in free energy.
Factors Influencing the Rate of Diffusion

Temperature: The increase in temperature increases the kinetic
energy of the diffusing molecule or ions. So with increase of
temperature, the rate of diffusion also increases.

Medium of diffusion: The rate of diffusion is also influenced by the
medium in which diffusion occurs and it is inversely proportional to the
concentration of the molecules of the medium. So increase in the
number of “medium” molecules will decrease the rate of diffusion. The
medium molecules cause collision with the diffusing molecules.

Density of the diffusing substance: The rate of diffusion is
inversely proportional to the square root of the density of the
diffusing substance i.e. r=1/√d where r = rate of diffusion and d=
density of the diffusing substance. This law of diffusion is called
Graham’s law of diffusion. In general, the size and solubility of
diffusing substance affect the rate of diffusion of solutes in solvent,
liquids in liquids and gases in liquids. Larger the size of the diffusing
molecules, slower is the rate of diffusion.

Diffusion concentration gradient: The diffusion concentration
gradient which is the difference in the concentrations of the diffusing
substance between two areas over a specific distance. In general,
greater the diffusion concentration gradient, the greater will be the
rate of diffusion.
Significance of the Rate of Diffusion

The exchange of gases like the intake of oxygen and output of carbon
dioxide in respiration and the intake of carbon dioxide and the output
of oxygen in photosynthesis occurs by the principle of diffusion and
the faster rate of diffusion means faster processes.

During passive update of salts in plants, the faster rate of diffusion
will lead to the faster absorption of ions. The same is the case in
translocation of food materials.

The increased rate of diffusion is related to faster rate of
transpiration through stomata in plants.

The faster rate of diffusion of scent in some plants lead to efficient
pollination by certain insects.
Source:
http://www.tutorvista.com/content/biology/biologyiii/biomembranes/physical-processes.php#section1
Part C: Additional Background Reading with Math Application
DIFFUSION THROUGH A CELL MEMBRANE
Introduction: Substances, such as water, ions, and molecules needed for
cellular processes, can enter and leave cells by a passive process such as
diffusion. Diffusion is random movement of molecules but has a net direction
toward regions of lower concentration in order to reach an equilibrium.
Simple passive diffusion occurs when small molecules pass through the lipid
bilayer of a cell membrane.
Importance: The rate of diffusion is affected by properties of the cell, the
diffusing molecule, and the surrounding solution. We can use simple equations
and graphs to examine how particular molecules and their concentration
affect the rate of diffusion. We can also compare simple and facilitated
diffusion.
Question: How do rates of simple and facilitated diffusion differ in
response to a concentration gradient?
Simple Diffusion
Variables:
n
number of molecules inside cell (mol)
t
time (seconds)
P
permeability constant for a particular
molecule (cm/sec)
A
surface area of the cell membrane (cm2)
C
Concentration of diffusing molecule
(mol/cm3)
x
width of cell membrane (cm)
Method: The rate of simple diffusion can be expressed by a modification of
Fick's Law for small, nonpolar molecules. The rate of diffusion, dn/dt, is the
change in the number of diffusing molecules inside the cell over time.
Since the net movement of diffusing molecules depends on the concentration
gradient, the rate of diffusion is directly proportional to the concentration
gradient (dC/dx) across the membrane. The concentration gradient, dC/dx,
is the difference in molecule concentration inside and outside of the cell
across a cell membrane of width dx. This is equivalent to (Cout - Cin)/
Cout and Cin are the substrate concentrations inside and outside the cell, and
cell (Cout) is larger than inside the cell (Cin), the concentration gradient
(dC/dx) will be positive, and net movement will be into the cell (positive value
of dn/dt).
We can describe the rate of diffusion as directly proportional to the
concentration gradient by the following equation:
where A is the membrane area and P is the permeability constant. P is a
constant relating the ease of entry of a molecule into the cell depending on
the molecule's size and lipid solubility.
Notice that when A and P are constants, this equation simply describes a line
where dn/dt is a function of dC/dx. If we graph the rate of diffusion as a
function of the concentration gradient, we get a simple linear function.
Interpretation: Notice the rate of diffusion increases as the concentration
gradient increases. If the concentration of molecules outside the cell is very
high relative to the internal cell concentration, the rate of diffusion will also
be high. If the internal and external concentrations are similar (low
concentration gradient) the rate of diffusion will be low.
Part D: Diffusion Demonstrations
1. Perfume sprayed in one area diffuses to another.
2. A drop of food coloring diffuses in water. Hot and cold water can be
compared to show temperature effects.
3. Vanilla extract inside of a balloon—vanilla scent particles can be
detected outside the balloon (diffused through the balloon like
particles diffuse through cell membranes). The balloon is selectively
permeable like a cell. It only lets some kinds of particles diffuse
through it.
Part E: Simulations
http://www.echalk.co.uk/Science/biologyContent.htm
http://www.starkdesign.com
Part F: Materials Needed for the Diffusion of Food Coloring
Through Gelatin Lab
Red, yellow and blue food coloring – baking section of the grocery store
Gelatin—unflavored Knox gelatin at the grocery store
Biscuit cutter (6.5 cm diameter)—grocery store
Petri dishes (9 cm diameter)—can be ordered at Carolina Biological Supply
10 mL syringes—ordered from any scientific supply company.
Part G: Directions for Making the Gelatin Disks
1. Obtain a non-reactive, non-stick baking pan/cookie sheet.
2. Determine how many gel disks (each 6.5 cm in diameter) that you will
need. Each group needs 4.
3. Determine how many cups of water are needed to fill the pan so the
water is 1 cm deep.
4. 200 mL = 1 cup
5. Add 2 envelopes of plain gelatin for every 1 cup or 200 mL of cold
water. Microwave for 90 seconds.
7. Pour the gelatin solution into pan. Let set in refrigerator. Using a 6.5
cm (2.5 inches) diameter cookie cutter, cut out and remove desired
number of gelatin disks. Place in 9 cm petri dishes. Cover.
Part H: Ideas for Additional Diffusion Investigations
1. Change the temperature and compare rate of diffusion.
2. Change the density of the gel and compare.
3. Change the concentrations of the food dye solutions and
compare.
Part I: Lab Extension
Diffusion is the random movement of particles from an area of high
concentration to an area of low concentration until equilibrium. The
diffusion of water across a semi-permeable membrane, such as a cell
membrane, is called osmosis. Diffusion and osmosis are two primary ways in
which materials move into and out of cells. The processes of osmosis and
diffusion enable cells to get nutrients and water and get rid of waste. Cell
size is limited because of diffusion and osmosis. A simple experiment using
gelatin, bromothymol blue (or other acid/base indicator), and a baking soda
solution illustrates this.
Materials Needed for The CO2 Diffusion Through Gel Lab
 Bromothymol .04% Blue Indicator Solution – can be ordered from
Flinn Scientific
 Gelatin—unflavored Knox gelatin at the grocery store
 9 x 13” baking pan (other sizes will do)
 Centimeter rulers
 Plastic knives
 Seltzer water—grocery store
 Balance
 Baking soda
 Graduated cylinders, 250 ml beakers
 Baby food jars with lids ( 3 per group)
Teacher Directions for Making The Bromothymol Blue Gelatin
Cubes
1. Obtain one or more non-reactive, non-stick baking pans.
2. Determine how many gel cubes (each 2.5 cm on an edge) that you will
need. Each group needs 3.
3. Determine how many cups of water are needed to fill the pan so the
water is 2.54 cm (1 inch) deep.
4. 200 mL = 1 cup
5. Use 2 envelopes of plain gelatin for every 1 cup or 200 mL of cold
water. Instead of cold water, you will be substituting a diluted BTB
solution. The directions for this solution are in Step 6.
6. Make a diluted BTB solution: Put 20 ml of 0.04% aqueous
Bromothymol Blue indicator solution into a beaker. Fill the rest of
the beaker with cold tap water up to the 200 mL mark.
7. Stir gelatin into the cold BTB solution. Gelatin is slightly acidic so the
BTB will turn yellow.
8. To make the BTB become basic (and turn blue again), add 2.5 grams of
baking soda (NaHCO3) for every 200 ml of BTB solution. Mix to
dissolve. Heat in microwave until warm. Cool in fridge. Cut into 2.5 cm
X 2.5 cm X 2.5 cm cubes ahead of time if you have younger students.
If you have older students, you may want to let them cut their own
cubes.
EXTENSION
1. Make different sized gelatin cubes. Have students calculate surface
area to volume ratio of the different cubes. Recreating the cubes
using graph paper may help with this.
2. Have students create a table showing surface area/volume ratio and
diffusion distance for various times. Graph the results using time as
the independent variable and diffusion distance as the dependent
variable. Different colors can be used to show different size cubes.
3. For fun, show students a trailer of The Blob and discuss why it is
science fiction.
Question To Consider
What are the connections between the results of this activity and the
transport of drugs in the extra-vascular space?