Technology Idea
Osmosis Demonstration
Probeware Lab

Background:

Water moves into plant roots by a special type of diffusion called osmosis. Water will move into the root cells from an area where there are more water molecules to an area where there are fewer molecules.

Demonstrating osmosis can be difficult because it is hard to reproduce the membrane of the plant cells. In this lab, you will witness water movement across a membrane that mimics the plant cell membrane. Your teacher will use dialysis tubing as the membrane, and use solutions with different concentrations of water molecules to illustrate osmosis.

Dialysis tubing is a semi-permeable membrane - this means that it allows some substances to pass, while others cannot get through. Water molecules move easily through dialysis tubing, but larger molecules such as complex sugars have a more difficult time getting through.

By using a pressure sensor connected to an osmotic system, you can detect that pressure changes indicate the movement of water associated with osmosis.

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The Question:

How does water move 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.

Materials:

  • thistle tube or pipet and rubber stopper with hole
  • rubber bands, string
  • Ring stand and two clamps
  • soaked dialysis tubing
  • sugar
  • distilled water (or substitute tap water if not available)
  • low pressure sensor, computer interface or USB link
  • 1000 ml beakers

Procedure:

This activity is meant as a teacher demonstration, but may be performed by student groups if desired.

Step 1:

Add several tablespoons of sugar to approximately 500 ml of water. Tie off one end of 12 cm long piece of dialysis tubing. Open the other end of the tubing to form a small baggie, then pour the sugar water into the baggie. Fit the open end of the baggie tightly over the end of a thistle tube (use rubber band if necessary). If a thistle tube is not available, place an appropriate size single-hole rubber stopper into the open end of the baggie, and use a rubber band to secure it. Stick the end of a pipet into the hole in the rubber stopper.

Step 2:

Connect the top of the thistle tube or pipet to a low pressure sensor using a plastic tube (usually provided with the sensor). Connect the low pressure sensor to the computer, and configure the data collection software to display a graph of pressure over time.

Suspend the dialysis baggie in a 1000 ml beaker of distilled water using the clamps and a ring stand. Your setup should look similar to this (your sensor may differ):

Step 3:

Begin collecting data soon after the baggie is suspended in the distilled water. Allow data to collect for several minutes. Your data may appear similar to these sample experimental results:

Step 4 (optional):

Continue to collect data.

Supersaturate a sugar solution in another 1000 ml beaker. Move the apparatus to the edge of the counter, then slip out the beaker of distilled water and replace it with the supersaturated solution. If you cannot perform the switch, simply add sugar to the surrounding distilled water. Allow data collect for another couple of minutes.

Observe the data that has been collecting. In the following sample data, the distilled water beaker was replaced with concentrated sugar solution, then after a brief period of time the sugar solution was again replaced by the original distilled water:

Step 5:

Stop collecting data and print out the resulting graph. Copy and distribute to students.

Forming Conclusions:

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

1. How does water move across a semi-permeable membrane?
2. What is the relationship between the amount of water molecules on either side of a semi-permeable membrane and osmosis?
3. How can osmosis account for the shape of the graph you have observed?

Extension:

  • Try repeating this experiment with the initial solutions reversed - that is, with distilled water in the baggie and a sugar solution surrounding. Can you predict the results? Perform the experiment to test your prediction.

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