Biol 1440, Principles of Biology I
Cellular Processes
Diffusion  across cell membranes

Experimental Set-up using a Conductivity Probe

	Equipment & Materials 1.  Diffusion/osmosis chamber 2.  Conductivity probe set to 0-2000 mS/cm 3.  Calibration solution 4.  Pipetter and pipettes 5.  Graduated cylinders: 10 & 50 mL 6.  30cc syringe with plastic tubing 7.  100 mL beakers 8.  marking pen  9.  magnetic stir plate with small stir bar.
	The chamber is divided into two parts: Cell - This side will be where the conductivity probe will be inserted.  You will place distilled H2O as a simulated cytosol in this side. A small stir bar should be placed in this side and set to stir gently. Environment-  this side will be where the salt (NaCl) solution is placed.
	Fill each side of the chamber with the appropriate solution using the 30cc syringe.         Place the tubing in a bottom corner of the cell away from the membrane.   The Conductivity probe will be placed into the Cell side. Calibrate the probe before collecting any data
	Click on the LoggerPro icon the computer desktop to set-up your data collection . Data collection will run a for 300 sec at 2 samples/sec. Ions such as Na+ will diffuse across the membrane and be detected by the conductivity probe.
	Data will automatically be graphed.     Discuss with your lab partners the results.     Decide which parts of the graph to analyze for the rate of diffusion. Write down the results in YOUR Data Collection Table, so you have a copy.  Don't rely on your partners to do it!!

Equipment Set-Up
Use a computer and conductivity probe to measure the conductivity of salt solutions.

Data Acquisition System
1. Check to be sure the conductivity probe is set to the intermediate setting, 0-2000 microS/cm (equivalent to a concentration of 1,000 mg/L).

The Graph Window should look like this:

Calibration of Conductivity probe
Steps to calibrate the Conductivity probe will be given by your instructor.
Check switch on probe box is in the 0-2000 position.
From Setup Menu, select <Calibrate>
1^st Value in air w/ dry probe: Input 1 should be at or near 0.0, select <Keep>

Place the probe in the Vernier calibration salt solution
Change 2^nd Value to 1000 microS/cm or uS/cm Input 1 should be at or near 1.30mV <Keep>.  

If the Input is too high or low, notify your instructor.

2. Test whether different concentration gradients affect the rate of diffusion. You will place a minimum of three solutions of differing salt concentrations (prepared by your group from a stock solution) in distilled water. Each salt solution will be placed in a Environment side and allowed to diffuse into the Cell side of the Diffusion/osmosis chamber. When salt diffuses, the conductivity of water in the beaker will increase.

3.   After stirring the solution slowly for 20 seconds, begin data collection by clicking on the Collect button. Stir the solution slowly and continuously throughout the data collection period.

Before analyzing data, store data of your latest run as follows:
   i.  Select <Experiment>
  ii.  Select <Store Latest Run>.  Line of current graph should change from bold to regular.

4. Determine the rate of diffusion of the salt solution. To do this:

• Select a region of the graph to analyze
• Choose Linear Fit from the Analyze menu. 
• A box appears with the values for the equation of a line y = mx + b, where m is the slope and is equal to the rate of diffusion.  Record the value of the slope.

5. Repeat with each salt concentration.  Be sure to have a data collection table to record your results.