Experiment 2: Concentration Gradients and Membrane Permeability
In this experiment, you will dialyze a solution of glucose and starch to observe:
The directional movement of glucose and starch.
The effect of a selectively permeable membrane on the diffusion of these molecules.
An indicator is a substance that changes color when in the presence of a specific substance. In this experiment, IKI will be used as an indicator to test for the presence of starch
Materials
(5) 100 mL Beakers
10 mL 1% Glucose Solution, C6H12O6
4 Glucose Test Strips
(1) 100 mL Graduated Cylinder
4 mL 1% Iodine-Potassium Iodide, IKI
5 mL Liquid Starch, C6H10O5
3 Pipettes
4 Rubber Bands (Small; contain latex, handle with gloves on if allergic
Permanent Marker
*Stopwatch
*Water
*Scissors
*15.0 cm Dialysis Tubing
*You Must Provide
*Be sure to measure and cut only the length you need for this experiment. Reserve the remainder for later experiments.
Attention!
Do not allow the open end of the dialysis tubing to fall into the beaker. If it does, remove the tube and rinse thoroughly with water before refilling it with the starch/glucose solution and replacing it in the beaker.
Note:
If you make a mistake, the dialysis tubing can be rinsed and used again.
Dialysis tubing must be soaked in water before you will be able to open it up to create the dialysis “bag.” Follow these directions for this experiment:
Procedure
Note:The color results of these controls determine the indicator reagent key. You must use these results to interpret the rest of your results.
Using the same pipette that was used to mix the solution in Step 3, transfer 8 mL of the dialysis bag solution to the prepared dialysis bag.
Figure 4: Step 9 reference.
Figure 4:Step 9 reference.
10.Allow the solution to sit for 60 minutes. Clean and dry all materials except the beaker holding the dialysis bag.
11.After the solution has diffused for 60 minutes, remove the dialysis bag from the beaker and empty the contents of the bag into a clean, dry beaker. Label the beaker “final dialysis bag solution.”
12.Test the final dialysis bag solution for the presence of glucose by dipping one glucose test strip into the dialysis bag. Wait one minute before reading the results of the test strip. Record your results for the presence of glucose in Table 4.
13.Test for the presence of starch by adding 2 mL IKI. After one minute has passed, record the final color in Table 4.
14.Use a pipette to transfer 8 mL of the water in the beaker to a clean beaker. Test the beaker water for the presence of glucose by dipping one glucose test strip into the beaker. Wait one minute before reading the results of the test strip, and record the results in Table 4.
15.Test for the presence of starch by adding 2 mL of IKI to the beaker water. Record the final color of the beaker solution in Table
Table 3: Indicator Reagent Data
Indicator
Starch Positive
Control (Color)
Starch Negative
Control (Color)
Glucose Positive
Control (Color)
Glucose Negative
Control (Color)
Glucose Test Strip
n/a
n/a
IKI Solution
n/
n/a
Table 4: Diffusion of Starch and Glucose Over Time
Indicator
Dialysis Bag After 60 Minutes
Beaker Water After 60 Minutes
IKI Solution
Glucose Test Strip
Post-Lab Questions
Experiment 1: Diffusion through a Liquid
In this experiment, you will observe the effect that different molecular weights have on the ability of dye to travel through a viscous medium.
Materials
1 60 mL Corn Syrup Bottle, C12H22O11
Red and Blue Dye Solutions (Blue molecular weight = 793 g/mole; red molecular weight = 496 g/mole)
(1) 9 cm Petri Dish (top and bottom halves)
Ruler
*Stopwatch
*Clear Tape
*You Must Provide
Procedure
Carefully fill the half of the petri dish with corn syrup until the entire surface is covered.
Develop a hypothesis regarding which color dye you believe will diffuse faster across the corn syrup and why. Record this in the post-lab questions.
Place a single drop of blue dye in the middle of the corn syrup. Note the position where the dye fell by reading the location of its outside edge on the ruler.
Record the location of the outside edge of the dye (the distance it has traveled) every ten seconds for a total of two minutes. Record your data in Table 1 and use your results to perform the calculations in Table 2.
Repeat the procedure using the red dye, the unused half of the petri dish, and fresh corn syrup.
Table 1: Rate of Diffusion in Corn Syrup
Time (sec
Blue Dye
Red Dye
Time (sec)
Blue Dye
Red Dye
10
70
20
80
30
90
40
100
50
110
60
120
Table 2: Speed of Diffusion of Different Molecular Weight Dyes
Structure
Molecular Weight
Total Distance
Traveled (mm)
Speed of Diffusion
(mm/hr)*
Blue Dye
Red Dye
*Multiply the total distance diffused by 30 to get the hourly diffusion rate
Post-Lab Questions
Record your hypothesis from Step 3 here. Be sure to validate your predictions with scientific reasoning.
Which dye diffused the fastest?
Does the rate of diffusion correspond with the molecular weight of the dye?
Does the rate of diffusion change over time? Why or why not?
Examine the graph below. Does it match the data you recorded in Table 2? Explain why, or why not. Submit your own plot if necessary.
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