STEM ED/CHM Nanotechnology 2011
Diffusion of Food Coloring Through Gelatin Lab: A Model for Diffusion of Nanoscale Particles Through Cells—Teacher Pages
Part A: Diffusion and Teaching Standards
This lab has content which is applicable to various disciplines/standards
- Physical Science/Chemistry: particle motion theory; pH; temperature; mixtures and solutions; acids and bases; color change as an indicator of a change in physical properties/chemical composition; acid/base indicators and protonation
- Biology: passive transport; cellular structure, etc.
- Ecology/Environmental Science: environmental effects on living systems
- Math: rates; relationships; data collection, organization, analysis
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:
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)/x where Cout and Cin are the substrate concentrations inside and outside the cell, and x is the width of the cell membrane. When the concentration outside the 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
- Perfume sprayed in one area diffuses to another.
- A drop of food coloring diffuses in water. Hot and cold water can be compared to show temperature effects.
- 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
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 GelatinDisks
- Obtain a non-reactive, non-stick baking pan/cookie sheet.
- Determine how many gel disks (each 6.5 cm in diameter) that you will need. Each group needs 4.
- Determine how many cups of water are needed to fill the pan so the water is 1 cm deep.
- 200 mL = 1 cup
- Add 2 envelopes of plain gelatin for every 1 cup or 200 mL of cold water.
- 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
- Change the temperature and compare rate of diffusion.
- Change the density of the gel and compare.
- 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.
Procedure
1.Make gelatin double strength (as in gel diffusion lab) using a small volume of bromothymol solution in the water. The gelatin is slightly acidic so the BTB will turn yellow.
2.Pour the yellow gelatin into a 8” x 8” pan coated with Vaseline. The gelatin should be about 3 cm thick.
3.From the large block of gelatin, cut cubes of various volumes such as 1 x 1 x 1 cm and 2 x 2 x 2 cm, etc. The cubes represent various sizes of cells/tissues.
4.Put the various gelatin cubes in different containers, each having the same volume of baking soda solution. Measure and photograph diffusion through the cubes over time.
5.Have students calculate surface area to volume ratio of the different cubes. Recreating the cubes using graph paper may help with this.
6.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.
7.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?