Osmosis Lab Planning Center

As you and your group prepare for the Osmosis Inquiry Investigation, there are several things you need to consider before you begin. This page has been designed to help you collaborate with your fellow group members to prepare for and document your experiment.

Background Information

Diffusion is the spontaneous tendency of molecules to spread apart and move from areas of high concentration to low concentration. Passive transport occurs when substances diffuse across biological membranes. Biological membranes are selectively permeable, which means that some substances can pass through the membrane and some cannot. Passive transport does not require energy, distinguishing it from active transport, which does require energy.

2625 Aquaporin Water Channel
Aquaporins allow water molecules to cross a phospholipid bilayer
Image from Wikimedia Commons

Osmosis is a special type of passive transport involving the diffusion of water molecules down a concentration gradient across a selectively permeable membrane. Osmosis occurs across the plasma membrane that surrounds all living cells. Most of the diffusion of water through the plasma membrane occurs through special proteins called “aquaporins.” Therefore, osmosis through a living membrane is actually an example of facilitated diffusion.

In this investigation, dialysis tubing serves as an excellent representation of the plasma membrane. The enclosed tube may, for the purpose of this experiment, be considered a single living cell in greatly enlarged form. Dialysis tubing membranes are made of purified cellulose containing microscopic pores. The pore size is controlled during manufacturing. The pore size determines the membrane’s permeability to molecules of different sizes. Typically, dialysis tubing will only allow smaller molecules to pass through the membrane, while larger molecules will remain in the dialysis tubing. For example, you learned from the Membrane Function Inquiry Investigation that starch molecules are too large to fit through the pores in the dialysis tubing and therefore will not diffuse through the membrane.

But what about water? Is the dialysis tubing permeable to water? In other words, will osmosis occur through the dialysis tubing?

Planning Your Experiment

Your group will need to develop an experiment or series of experiments that will allow you to answer the following research question:

Does osmosis occur across the dialysis tubing membrane?

Remember that a controlled experiment should test a single independent variable. In order to draw a meaningful conclusion, you need to be able to compare the outcome of your experiment with what would happen if you did not manipulate the variable you have chosen to test.

With all of this in mind, consider the following questions to help you get started:

  • What variable will you be testing (independent variable)?
  • What variable(s) will you be measuring (dependent variable)?
  • What variables will you hold constant (constant variable)?
  • What evidence would confirm that water has crossed the membrane?
  • How will you be confident in the validity of your results?
  • What will you use as a standard of comparison (control group)?

Navigate to your group’s page below to plan and document your experiment.

Select Your Page Below

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