is an important process that explains the passive movement of materials into
and out of cells as well as the traffic of certain molecules within the cell.
The motive force driving diffusion is generated by the presence of a
concentration (or energy) difference operating in the following manner:
molecules always move (unless there is an obstruction) from regions of higher
concentration (or higher energy) to regions of lower concentration (or lower
lab will allow you to investigate the effects of solute concentration on
diffusion rate using two plexi-glass chambers separated by a semi-permeable
membrane: one chamber to mimic the shape of a cell and the other, its external
environment. The object of the experiments will be to place differing solutions
of sodium chloride in the environment chamber and measure the diffusion rate of
salts into the cell chamber.
is diffusion rate?
rate is a quantity expressed per unit of time. For example, the speed you
travel in your automobile is a rate, e.g., 80 miles/hour. So, diffusion
rate is expressed as the amount of salt moved per time, e.g., 0.1
do you determine diffusion rates?
this experiment, diffusion rates are determined by measuring the increase in
salt concentrations in the cell chamber over a fixed time period. If the salt
concentrations (dependent variable) are plotted against the time they were
measured (independent variable), the slope of the resulting line is the
do you measure salt concentrations?
measuring the molar concentration of salt in a solution is beyond the scope of
our lab. Instead, we will measure salt concentrations indirectly by determining
how much electric current is carried by the solution using a special instrument
(linked to a computer workstation) called a conductivity probe. The
units for conductivity are μS/cm.
What to do today:
Set-up your computer workstation and calibrate the conductivity probe
DO NOT USE THE COMPUTERS FOR ANY OTHER REASON EXCEPT TO
DO TODAY’S LAB.
FOLLOW ALL DIRECTIONS CAREFULLY TO AVOID MISTAKES.
- Connect your conductivity
probe to the computer using the LabPro interface (it looks like a
calculator with ports on the side).
- Make sure the tip of the
probe is in air before you continue.
- On the right hand side of
the screen, click on the “Logger Pro” Icon.
- On the horizontal menu
(towards the right of the screen) click on the “Lab Pro” button.
- Click on the “channel 1”
box that indicates the conductivity probe. Select “calibrate” on that
- Click on “calibrate now”.
Note the voltage in the display: it should read 0 volts. Click on “keep”.
- Place the probe in the
standard salt solution available at your table. It is in a small plastic
bottle. It should read 1000 μS/cm.
- Type in “1000” in the
appropriate box on the calibration display (right hand box).
- Check the voltage. It
should read ~1.3 volts. Click on “keep”and "done"
- Close the LabPro menu
window (use the bubble in the upper left-hand corner).
- Take the probe out of the
standard salt solution and clean off the probe in distilled water. Dry it
carefully with a Kimwipe. DO NOT RUN THE WIPER THROUGH THE HOLE IN THE
- To check if you have
calibrated the probe correctly, place the probe back into the standard
salt solution and confirm that the display on the computer screen reads
approximately 1000μS/cm. Repeat step 11 before you continue.
Set-up the artificial cell
- The cell should be
positioned such that the cell side (the lower of the two chambers) is
directly in the center of the stir plate. Carefully place the stir bar in
the cell side and turn on the stir plate to get a moderate stirring rate.
Both sides of the cell should contain distilled water.
- Lower the conductivity
probe into the cell side of the chamber using the probe holder and note
the conductivity. It should read approximately 0 μS/cm. If everything
is working correctly, proceed to the experiments.
Experiment 1: When is the maximum rate of diffusion?
- Construct a data collection
table (similar as found in the online lab manual) to record diffusion
rates at 60-second intervals for 300 seconds.
- Using a large plastic syringe,
remove the water from the environment cell and replace it with
approximately 30ml of 0.15 M salt solution. Make sure the stir bar is
- Quickly click on
"collect" on the computer. It has been preset to collect
conductivity data for 300 seconds at 2 samples per second. The computer
will plot the change in conductivity per time on the screen as you watch
and will automatically stop collecting data when the sampling period is
- Once the data gathering is
complete, hold down the left hand button on the mouse and select the
interval on the line representing conductivity change for the first 60
seconds. The selected region should be highlighted. Select “analyze” on
the menu across the top of the screen and then chose “linear fit”. A box should
appear on the graph with the equation of the line and the correlation
coefficient (goodness of fit for the data to the line). Record the slope
in your data collection table. What are the units?
- Repeat step 5 for four
additional 60-second intervals.
- Analyze your results. What
interval gave the highest diffusion rate? Why did the diffusion rates
change with each measurement interval?
How to save data after a run:
"experiment" on the menu across the top of the screen and then
chose "store latest run".
- If you want to hide the
data from a run, select “data” from the top menu and chose "hide data
set". NOTE: Always choose
“run 1” (or whatever number is appropriate) on the menu that appears. DO
NOT CHOOSE “LATEST RUN”. If you do, on the next assay, this will cause the
program to lock up and give the message, “waiting for data”. (If you make
this mistake anyway, go to the “data” menu and select “show data set” and
the problem will be resolved.)
Experiment 2: Does concentration have an effect on the rate of
Pose a hypothesis from this RQ and design an experiment to
test your hypothesis. Remember to construct an appropriate data collection
table in your lab notebook before you proceed. Note that each lab group will
provide the data for one replication of the class experiment, so everyone must
use the salt concentrations listed on the board as your experimental groups.
What is the control group?
- Carefully clean out the
both chambers of the artificial cell every time you re-use them. MAKE SURE
YOU DO NOT LOSE THE STIR BAR!!
- Make up 30 ml of each salt
solution using the mass balance equation to calculate the amount of 1.2 M
salt stock solution needed for each dilution.
- Determine the diffusion
rates of each solution into the cell chamber as described above.
- As your diffusion rates
are determined, write the results in the summary table on the front board.
- Using the class data,
calculate the average diffusion rate for each experimental group and the
control group. Also, note the range for each treatment.
- Construct a final figure
of the results and bring it with you next week.
How to save your Logger Pro file and close the Logger Pro
program when the lab is completed:
- Go to the "file"
menu and select "save as".
- Make sure to re-name your
file and save it in the appropriate student folder.
- Using the horizontal menu
(at the top left of the screen) select "Logger Pro" and choose
"Quit Logger Pro"