Diffusion Lab

Diffusion Through Membranes

IB Biology 11

Diffusion Through Membranes

OBJECTIVES
In this experiment, you will
Use a Conductivity Probe to measure the ionic concentration of various solutions.
Study the effect of concentration gradients on the rate of diffusion.
Determine if the diffusion rate for a molecule is affected by the presence of a second molecule.

BACKGROUND
Diffusion is a process that allows ions or molecules to move from where they are more concentrated to where they are less concentrated. This process accounts for the movement of many small molecules across a cell membrane. Diffusion allows cells to acquire food and exchange waste products. Oxygen, for instance, might diffuse in pond water for use by fish and other aquatic animals. When animals use oxygen, more oxygen will diffuse to replace it from the neighboring environment. Waste products released by aquatic animals are diluted by diffusion and dispersed throughout the pond.
Questions:
It is important to consider how the rate of diffusion of particles may be affected or altered.
Diffusion may be affected by how steep the concentration gradient is. The direction that a diffusing molecule or ion might travel is random. While the particles are diffusing, is there a net movement from where they are concentrated to where they are less concentrated?
Yes particles diffuse from higher concentration to lower concentration
Diffusion may be affected by other different, neighboring particles. For instance, if oxygen diffuses towards a single-celled pond organism at a certain rate, will that rate be altered by the presence of another type of molecule? Would the presence of other molecules block or enhance the diffusion of a molecule? Would the molecule’s rate be independent of particles that do not alter the concentration gradient?
Yes the diffusion would be effected because the particles could block the diffusion of the oxygen. The rate isn’t independent of the other molecules because the presence of other molecules may slow the rate of diffusion.
One way to measure the rate of diffusion of ions is to monitor their concentration in solution over a period of time. Since ions are electrically charged, water solutions containing ions will conduct electricity. A Conductivity Probe measures the concentration of ions in a solution, but not the concentration of electrically neutral molecules. Salts, such as sodium chloride, produce ions when they dissolve in water. If you place a salt solution inside a selectively permeable membrane such as dialysis tubing, the salt ions can diffuse out of the tubing and into the surrounding water.

MATERIALS
LabQuest
LabQuest App
Vernier Conductivity Probe three 18 × 150 mm test tubes with rack
1%, 5%, and 10% salt water
400 mL beaker ring stand and utility clamp dialysis tubing, 2.5 cm × 12 cm scissors dropper pipet or Beral pipet stirring rod dental floss or clamp

ABSTRACT
In this experiment we tested the effect of salt concentration on the rate of diffusion. We made a model cell using dialysis tubing containing different salt solutions; 1%, 5%, and 10%. Each tube was tied off and placed in a container (not at the same time) with distilled water, and with a LabQuest, the rate of diffusion was measured. We predicted the higher the percentage of salt, the faster the “cell” would diffuse; and we found our predictions to be correct. The data showed that the 10% solution diffused the fastest then the 5% then the 1%.

PROCEDURE
1. Set up the utility clamp, and ring stand as shown in Figure 1.
2. Set the selector switch on the side of the Conductivity Probe to the 0–2000 µS/cm range. Connect the Conductivity Probe to LabQuest and choose New from the File menu. If you have an older sensor that does not auto-ID, manually set up the sensor.
3. On the Meter screen, tap Rate. Change the data-collection rate to 0.2 samples/second and the data-collection length to 60 seconds.
4. Test whether different concentration gradients affect the rate of diffusion. To do this, three solutions of differing salt concentrations (1%, 5%, and 10%) will be placed in distilled water. Each salt solution will be placed in a dialysis tube and allowed to diffuse into the surrounding water. When salt diffuses, the conductivity of the water in the beaker will increase.
5. In Table 1, predict what you believe will happen in this set of experiments. How will the rate of diffusion change when a 10% salt solution is placed in contact with pure water compared to when a 1% salt solution is placed in contact with pure water?
6. Prepare the dialysis tubing. Obtain a wet dialysis tube and a dialysis tube clamp or a short length of dental floss. Using the clamp or floss, tie one end of the tube closed about 1 cm from the end, as in Figure 2.
7. Place a 1% salt solution into a section of dialysis tubing. To do this,
a. Obtain about 15 mL of a 1% salt water solution in a test tube.
b. Using a funnel or Beral pipet, transfer about 10 mL of the 1% salt water into the dialysis tube, as in Figure 2. Note: To open the tube, you may need to rub the tubing between your fingers.
c. Tie off the top of the dialysis tube with a clamp or a new length of dental floss. Try not to allow any air into the dialysis tube. The tube should be very firm after it is tied or clamped. Trim off any excess dental floss extending more than 1 cm from either knot.
d. Wash the outside of the tubing with tap water thoroughly, so that there is no salt water adhering to the tubing.
8. Place 300 mL of distilled water into a 400 mL beaker. Secure the Conductivity Probe with the utility clamp in the water filled beaker as shown in Figure 1.
9. Place the dialysis tube into the water. Be sure the tubing is submerged completely under the water. Important: Position the Conductivity Probe and dialysis tubing the same distance apart in each trial.
10. After stirring the solution for 15 seconds, start data collection. Stir the solution slowly and continuously throughout the one-minute data collection period.
11. Data collection will stop after 60 seconds. Analyze the graph to determine the rate of diffusion for the curve of conductivity vs. time:
a. Examine the graph and identify the most linear region.
b. Tap and drag your stylus across the most linear region to select these data points.
c. Choose Curve Fit from the Analyze menu. Select Linear as the Fit Equation.
d. Record the slope, m, as the rate of diffusion in (µS/cm/s) in Table 2. Select OK.
12. Remove one of the clamps. If the dialysis tubing is tied off with floss, use a pair of scissors and carefully cut one of the dental floss knots and discard the floss. If you accidentally make a cut in the tubing, replace it.
13. Empty all of the liquid out of the dialysis tube. Squeeze the excess liquid out with your fingers.
14. Rinse the Conductivity Probe with distilled water.
15. Store the data from the first run by tapping the File Cabinet icon.
16. Obtain 15 mL of a 5% salt solution in a test tube. Repeat Steps 7–15, substituting this 5% salt solution for the 1% solution.
17. Obtain 15 mL of a 10% salt solution in a test tube. Repeat Steps 7–15, substituting this 10% salt solution for the 1% solution.

DATA/RESULTS
Table 1
Prediction
Conclusion
The 10% solution will diffuse the fastest because there is less water in the solution than the other solutions, and the 1% solution will diffuse the slowest.
The higher the percentage of the solution the faster it diffused. Therefore our prediction was right, the 10% diffused the fastest and the 1% the slowest.

Table 2: Summary of Data
Salt concentration (%)
Rate of Diffusion (µS/cm/s)
1
1.6624
5
3.4205
10
5.4700

CONCLUSION
1. From Table 2, I can conclude that as the salt concentration increases the rate of diffusion increases.
2. My conclusion was the same as my prediction, so my prediction was correct.
3. The rate of diffusion for the 10% solution was (3.29/1) about 3.29 the rate of the 1% salt solution. The rate of diffusion for the 5% solution was (2.05/1) about 2.05 the rate of the 1% salt solution.
4. The rate of diffusion of a 3% salt solution, according to our graph, would be about 2.5µS/cm/s
5. Temperature, size of particles, charge of molecules are other variables that could be tested.