Background:

The contents of a cell are separated from the outside environment by a membrane. A biological membrane is the cellular organelle which isolates biochemical reactions, enzymes and genetic material essential to the vitality of the individual cell from the outside world. In some cases, the cell membrane acts as a passive barrier. When the membrane acts passively, the materials migrate in or out due to a difference in concentration (or osmotic gradient) between the inside and outside of the membrane.

In other cases, the membrane can be very selective to what passes from one side to the other. It is through the membrane that essential nutrients pass into and waste products pass out of the cell. An active membrane is also the anchorage site for many enzymes and coenzymes. The cell membrane is NOT a simple bag holding cell parts . The active nature of a biological membrane makes it a living part of the cell.

While membranes are selectively permeable, this experiment investigates the role of the cell as a passive mediator of diffusion of materials from one side of the membrane to the other.

In this activity you will use piece of dialysis tubing as a model of the cell membrane, then compare the rate of change of pH around the dialysis tubing when the tubing contains an acid to the rate of change of pH around the dialysis tubing when the tubing contains a base.

At what rate do different ions diffuse across a semi-permeable membrane?

Variables:

Identify the type of data you will collect to support your hypothesis and state the manipulated, responding and controlled variables in this investigation.

SAFETY REMINDER: Wear protective equipment while handling chemicals.

Step 4:

Equipment Setup:

• Put a spin bar in a 250-mL beaker and place the beaker on a magnetic stirrer.
• Use a base and support rod and a clamp to mount the pH electrode so the end of the sensor is in the beaker.
• Position the sensor so the electrode cannot hit the spin bar.
• Use a clamp to suspend the dialysis tubing bag containing hydrochloric acid above the beaker:

Step 5a:

Data collection - movement of hydrogen ions through the membrane:

1. Put 100 mL of distilled water into the beaker. Turn on the magnetic stirrer. Get ready to lower the dialysis tubing into the water.
2. When you are ready, start recording data. Allow the sensor/computer to record for about 5 seconds.
3. After about 5 seconds, lower the dialysis tubing bag containing the hydrochloric acid into the water.
4. The pH Sensor will measure the pH of the water bath for 200 seconds and then data recording will stop automatically.
5. Dispose of the dialysis tubing bag and hydrochloric acid as directed.
6. Turn off the magnetic stirrer. Raise the pH electrode out of the water. Use the wash bottle to thoroughly rinse the pH electrode. Catch the rinse water in the beaker.
7. Remove the spin bar and dispose of the rinse water in the beaker as directed. Clean and dry the beaker.

Step 5b:

Data collection - movement of hydroxide ions through the membrane:

1. Put 100 mL of distilled water in the beaker. Add the spin bar and place the beaker on the magnetic stirrer. Turn on the stirrer.
2. Position the pH electrode so it is in the water in the beaker, but does not hit the spin bar in the beaker.
3. Suspend the second dialysis tubing bag (with sodium hydroxide) above the water in the beaker.
4. Start recording data. After about 5 seconds, lower the dialysis tubing bag containing the sodium hydroxide into the water.
5. After data recording stops automatically, dispose of the dialysis tubing bag and sodium hydroxide as directed.
6. Turn off the magnetic stirrer. Raise the pH electrode out of the water. Use the wash bottle to thoroughly rinse the pH electrode. Catch the rinse water in the beaker. Remove the spin bar and dispose of the rinse water in the beaker as directed.

Analyzing and Interpreting:

Step 6:

• Use the graph’s built-in analysis tools to find the rate of change of pH for each run of data - click the 'Smart Tool' button in the graph, then move the Smart Tool to the point on Run #1 where the pH begins to change. Move the cursor to a corner of the Smart Tool. Notice that the cursor changes to a delta shape:

• Click-and-drag the delta cursor to the end of Run #1. The Smart Tool shows the change in time and the change in pH between the starting and ending points:

• Record the change of pH and the change of time between the beginning and end points for the first run of data.
• Repeat the process to find the change of pH and time for the second run of data.

Step 7:

Convert the amount of time to minutes. Calculate the 'Rate of change of pH' by dividing the change in pH by the amount of time. Record the 'Rate of change of pH':

Forming Conclusions:

Based on the data you have collected, write a summary statement for the following questions:

1.	At what rate do different ions diffuse across a semi-permeable membrane?
2.	Which ion appears to have migrated across the membrane the fastest?
3.	How could you use a series of ions to determine the size of the pores in the membrane?

Extension:

• If you have access to dissolved ion detectors, consider repeating this experiment with different ionic solutions in the dialysis tubing. A ion-specific detector should be able to record any ions that get through the dialysis tubing into the surrounding distilled water.